rexresearch.com


Arun WAGH

Ceramicrete / Grancrete







http://www.world-science.net : "Spray-on Homes" Invented (December 26, 2004, Argonne National Laboratory)


www.davinciinstitute.com:  Grancrete


WAGH Arun, & ANTINK, A.: US Patent # 7,001,860 -- Construction Material & Method


WAGH, A. US Patent # 6,776,837 -- Formation of Chemically Bonded Ceramics with Magnesium Dihydrogen Phosphate Binder


WAGH, A.: US Patent # 6,518,212 -- Chemically Bonded Phospho-Silicate Ceramics


Wagh's Patents (List)


WAGH, et al.: US Patent # 5,830,815 -- Method of Waste Stabilization via Chemically Bonded Phosphate Ceramics


WAGH : US Patent # 5,645,518 -- Method for Stabilizing Low-Level Mixed Wastes at Room Temperature


SINGH & WAGH, A.: US Patent # 5,846,894 -- Phosphate Bonded Structural Products from High Volume Wastes


WAGH, et al.: United States Patent # 6,133,498 -- Method for Producing Chemically Bonded Phosphate Ceramics...


SINGH, et al.: US Patent # 6,153,809 -- Polymer Coating for Immobilizing Soluble Ions in a Phosphate Ceramic Product


DILEEP, et al.: US Patent # 6,204,214 -- Pumpable/injectable Phosphate-Bonded Ceramics


http://www.world-science.net/

"Spray-on Homes" Invented

Technology may help the poor, its developers say

December 26, 2004
Argonne National Laboratory and World Science staff

Researchers say they have found a way to build cheap, sturdy homes in one day by spraying a quick-drying ceramic onto flimsy frames. The technology could help the world's poor, of which the United Nations estimates there are 1.3 billion, they say.

Scientists at Argonne National Laboratory, a U.S. government facility in Argonne, Illinois, and Casa Grande LLC, a Mechanicsville, Virginia-based company, developed the technology. They say they will make it available worldwide after testing whether the homes are earthquake and hurricane resistant.

The ceramic is called Grancrete. The researchers say that when sprayed onto a crude frame made of Styrofoam, Grancrete dries to form a light, hard surface. This creates a dwelling much better than the flimsy structures in which many poor people live.

Grancrete is based on an Argonne-developed material called Ceramicrete, developed in 1996 to encase nuclear waste, according to Argonne's Explorer Magazine.

Ceramicrete thus prevents pollutants from leaking into the environment,  the magazine reported. Grancrete also netted its developers an award from R&D Magazine as one of the "100 most technologically significant new products" of 2004.

Casa Grande president Jim Paul told Explorer that his company became involved with the technology because initially, it was was looking for a concrete substitute for American industry. The need arose because concrete erodes in acidic conditions. "But as I traveled in Venezuela, I recognized the demand for cheap housing, and I thought about how to use our material for that", he told the magazine.

Paul then collaborated with Argonne's Arun Wagh to create Grancrete.

Grancrete is stronger than concrete, is fire resistant and withstands both tropical and below-freezing temperatures, the developers said; it keeps homes in arid regions cool, and those in frigid regions warm.

To build a home, Grancrete is sprayed onto Styrofoam walls, to which it adheres and dries, according to the developers. The Styrofoam remains in place as an effective insulator, although Wagh suggests simpler walls, such as woven fiber mats, also would work well and further reduce the raw materials required.

Using Grancrete in developing countries has additional advantages, says Wagh. "When you build houses in these poor villages, the materials you use should be indigenous, and the labor should be indigenous", he told the magazine. "Every village has soil and ash, and the labor and training requirements are so minimal that two local people can build a house in two days".

Workers only need two days of training to learn how to operate the machinery, Paul told the publication. Casa Grande typically assembles a team of five people who can start in the morning and create a home that residents can move into that evening, he asserted. The material dries in minutes, he added, whereas concrete can take hours or days.

Grancrete is made from an environmentally friendly mix of locally available chemicals, according to the developers.

It consists of sand or sandy soil, ash, magnesium oxide and potassium phosphate, which is a biodegradable element in fertilizer. So even if Grancrete were to decompose, Wagh told the magazine, it would revitalize the soil.

It costs about $6,000 U.S. dollars to build a Grancrete home, Paul told Explorer, several times cheaper than a conventionally built home. The homes are more than four simple walls, the developers added; for less than $10,000 U.S., laborers can produce Grancrete dwellings twice of 800 square feet, twice the size of a typical apartment in Bombay, India.

Wagh said he aims to see Grancrete used throughout his native India and the world to produce housing for the poor.

Born in the Indian state of Karnataka, Wagh grew up in a neighborhood where even to this day the homes have walls and ceilings made from knitted mats of palm leaves, and the floors are made of dried cow dung, according to Explorer magazine.

"These homes are regularly subjected to hundreds of inches of monsoon rains and cyclone winds and therefore often have to be repaired or even entirely rebuilt", Wagh told the publication. "Obviously such conditions can have a great impact on the health, well-being, and longevity of the children and adults living there".

The spray-on cement now offers hundreds of millions of people such as these the opportunity to have adequate housing and live longer, healthier lives, he told the publication.

Argonne and Casa Grande have extensively tested Grancrete for structural properties, post-application behavior and production costs, the developers said.

Their next step will be to test it for earthquake and hurricane resistance, after which they will make the product available worldwide. Wagh hopes the United Nations and other international organizations will subsidize mass-scale production around the world.




Impact Lab // DaVinci Institute, PO Box 270315, Louisville, CO 80027 // www.davinciinstitute.com // Phone :303-666-4133
 
Arun Wagh spent a decade at the Argonne National Laboratory here working on a ceramic material that offered the toughness of concrete. He finally developed a substance called 'grancrete', which can be used to quickly build houses at minimal expense.
"I was asked to create a material that could safely encase nuclear waste so that the waste did not get into ground water," said Wagh. The substance Wagh developed combined magnesium oxide and potassium phosphate with water and ashes.
The promising new technology may lead to affordable housing for the world's poorest. Houses can be built by spraying grancrete on to a simple frame of Styrofoam and it hardens quickly and will not crack easily.
Experiments have proved that grancrete is stronger than concrete, is fire resistant and can withstand both tropical and sub-freezing temperatures, making it ideal for a broad range of geographic locations. It insulates so well that it keeps dwellings in arid regions cool and those in frigid regions warm.
"Grancrete is 50 percent sand or sandy soil, 25 percent ash and 25 percent binding material," Wagh said.
"Binding material is composed of magnesium oxide and potassium phosphate, the latter of which is a biodegradable element in fertilizer. So even if grancrete were to decompose, it would revitalize the soil," said the scientist.
"For every tonne of conventional concrete, you get a tonne of greenhouse gases. With one tonne of grancrete, you get one-tenth of the greenhouse gases."
According to an estimate by Casa Grande, the company that is collaborating with Argonne in making grancrete, the cost of building a grancrete home is about $6,000.
"Casa Grande made this estimate for building a house in Venezuela. In India, it would be much cheaper," said Wagh, whose goal is to see grancrete used throughout India, and the world, to produce housing for the poor.
In fact, a test house using grancrete is being built in India.
Wagh is familiar with the housing the poor live in. He grew up in a village in Karnataka where, even to this day, the homes have walls and ceilings made from knitted mats of palm leaves and the floors are made up of dried cow dung.
Grancrete is so versatile that Wagh even paints using it. "It becomes like a paste and you can add any colour to it... It is a little more difficult to use than oil paint.
"Every day I come to the office, I get a call from people telling me it can be used for something else. You can do anything with it. The only thing you cannot do is eat it," Wagh said.
Argonne and Casa Grande have extensively field-tested grancrete for structural properties, post application behaviour and production costs. Their next step will be to test it for both earthquake and hurricane resistance, after which they will make the product available worldwide.
According to Jim Paul, president of Casa Grande, workers need only two days of training to learn how to calibrate the machinery.
Casa Grande typically assembles a team of five people who can start in the morning and create a home that residents can move into that evening. Grancrete cures in 15 minutes, while conventional concrete can take hours, or even days, to dry.
Wagh completed his Masters in Mumbai and got a doctorate from the State University in New York. He returned to India, taught in Goa, and then spent 12 years in Jamaica.
In Jamaica, Wagh changed tracks from physics to materials science. Returning to the US, he joined Argonne as a materials scientist.



US Patent # 7,001,860

Construction Material & Method

WAGH ARUN S (US); ANTINK ALLISON L

Classification:- international: C08J9/00; C08J9/00; - european:
Application number: US20020335462 20021230
Priority number(s): US20020335462 20021230
Also published as:  US 2005288175 // US 2005288174
Abstract: A structural material of a polystyrene base and the reaction product of the polystyrene base and a solid phosphate ceramic. The ceramic is applied as a slurry which includes one or more of a metal oxide or a metal hydroxide with a source of phosphate to produce a phosphate ceramic and a poly(acrylic acid or acrylate) or combinations or salts thereof and polystyrene or MgO applied to the polystyrene base and allowed to cure so that the dried aqueous slurry chemically bonds to the polystyrene base. A method is also disclosed of applying the slurry to the polystyrene base.

US Cl. 501/111; Intl. Cl. C04B 35/447 (20060101)
References Cited:
U.S. Patent Documents: 5234754 ~ 5645518 ~ 5830815 ~ 5846894 ~ 6133498 ~ 6153809 ~ 6518212 ~ 2002/0123422
Foreign Patent Documents: JP 2001-231848
Other References:
Bohner et al., Gentamicin Release from a Hydraulic Calcium Phosphate Cement . . . , 3.sup.rd General Meeting of the Swiss Society of Biomaterials, May 1997. cited by examiner
Government Interests
The United States Government has rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the U.S. Department of Energy and The University of Chicago representing Argonne National Laboratory.

Description
TECHNICAL FIELD

This invention relates to forming polymer modified chemically bonded phosphate ceramics. In particular, this invention addresses a need to form a room-temperature-setting ceramic based on the conventional Ceramicrete.RTM. and Ferroceramicrete technology that will bond polymeric surfaces such as Styrofoam.

BACKGROUND OF THE INVENTION

Haematite, having the chemical formula Fe.sub.2O.sub.3, is one of the most abundant minerals in nature. It exists as iron ore, in other minerals such as bauxite, and is also a component in clay minerals. It is the major component in laeritic soils (red soils found in the tropics). Similarly, manganese oxide, having a formula Mn.sub.2O.sub.3 is also a very common component in several laeritic soils and also exists as a mineral of manganese in the tropics.

U.S. Pat. Nos. 5,645,518 and 5,830,815 issued to Wagh et al. on Jul. 8, 1997 and Nov. 3, 1998, respectively, disclose processes for utilizing phosphate ceramics to encapsulate waste. U.S. Pat. No. 5,846,894 issued to Singh et al. on Dec. 8, 1998 discloses a method to produce phosphate bonded structural products from high volume benign wastes. None of these patents provides a method for utilizing the waste materials of iron and manganese.

U.S. Pat. No. 6,153,809 issued to Singh et al. Nov. 28, 2000 and U.S. patent application Ser. No. 09/751,655 filed Dec. 29, 2000, publication no. U.S. 2002/0123422 to Wagh et al. represent additional development of the use of chemically bonded phosphate ceramics to useful materials. Each of the aforementioned patents, that is U.S. Pat. No. 5,645,518 issued to Wagh et al., U.S. Pat. No. 5,846,894 issued to Singh et al., U.S. Pat. No. 5,830,815 issued to Wagh et al., U.S. Pat. No. 6,153,809 issued to Singh et al., U.S. Pat. No. 6,133,498 issued to Singh et al. and the above-identified publication no. U.S. 2002/0123422 (patent application Ser. No. 09/751,655) is incorporated herein in their entireties.

The phosphate ceramics disclosed in the various patents and publication hereinbefore mentioned illustrate a continuing effort to use the chemically bonded phosphate ceramics disclosed therein for a variety of purposes including the encapsulation of hazardous or radioactive waste as seen in the aforementioned publication, as well as the production of low cost structural materials. Accordingly, therefore, a need exists in the art for a low cost structural material which combines with synthetic organic resin based structures, for particular usage in the construction industry. Typically, in warm weather climates, low cost housing may be constructed using styrofoam as a base material onto which is sprayed a concrete-like material as a finish coating to seal the styrofoam base material against the elements and to provide a satisfactory looking structure. Heretofore, the phosphate ceramics disclosed in the above-captioned patents and publication were used as a finish coating in warm temperature climates but have not been satisfactory because the bond between styrofoam and the phosphate ceramics herein above disclosed is physical and peelable such that durable coatings have not been able to be provided with the extant material.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a structural material and method for chemically bonding the phosphate ceramics hereinbefore disclosed to foam material and particularly to polystyrene foam.

Another object of the present invention is to provide a method to coat styrofoam structures with a material which cures or sets at room temperature and is easy to apply in the field.

Yet another object of the present invention is to provide an aqueous based material which may be applied to a styrofoam or other synthetic organic resin in the field at low cost and with high efficiency.

Another object of the invention is to provide a method for preparing and chemically bonding a phosphate ceramic to a polymer foam.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.

FIG. 1 is a schematic representation of the dissolution of poly (acrylic acid) sodium salt;

FIG. 2 is a schematic representation of the bonding of the dissolved poly (acrylic acid) with ceramicrete; and

FIG. 3 is a schematic representation of the stabilization of acrylic acid with magnesium and styrene.

Although the invention has been described particularly with respect to polyacrylic acid sodium salt, as hereinbefore stated, other acrylates and the salts thereof are also applicable to the present invention and the invention is not limited to the disclosed materials of polyacrylic acid salt, polymethylmethacrylate, polyacryl amide and polyacrylnitryl. However, while the above description is particularly suited to providing a material which chemically bonds with polystyrene foam by the incorporation of styrene into the aqueous slurry, other systems may be used with the present invention wherein the ceramicrete or ferroceramicrete is combined with water soluble powders and a stabilizing or cross-linking polymer in an aqueous solution in order to chemically bond the resultant material to the synthetic organic resin structure.

DETAILED DESCRIPTION OF THE INVENTION

The process and product disclosed herein provides an inexpensive construction material, particularly adapted for use in warm weather climates where styrofoam or other synthetic organic resin foams are used as construction materials and require a coating of a hard, dense material for a surface finish. There are a large variety of materials which may be used to form the slurry which is thereafter chemically adhered to the synthetic organic resin foam base or surface. One such material is haematite which may be used in combination with sand, fly ash, and a variety of other materials hereinbefore described, combined with a reducing agent and magnesium oxide or other metal oxides with phosphoric acid or monopotassium phosphate and acrylate to form the ceramic phosphate formulations hereinbefore described. The reducing agents to be used in ferroceramicrete may be a variety of materials including elemental metals, tin chloride, ferric sulfate or other typical low costs moities.

An aqueous slurry of the ceramic has a compressive strength similar to that of Portland cement, approximately 4000 psi. However, the slurry frequently sets very rapidly and can be retarded in the rapidity with which it sets, as set forth in the previously incorporated '498 patent. In general, as previously stated, the ceramicrete and ferroceramicrete disclosed in the above-mentioned applications can be used in solid particulate form at the construction site and either premixed with solid styrofoam and acrylate prior to arrival at the construction site or mixed at the construction site. As previously disclosed, the phosphate ceramic of the invention may be made from a source of phosphate and one or more of an oxide, hydroxide or carbonate of one or more of Si, Fe, Mg, Al, Mn, Ca, Zr or mixtures thereof.

The inventors have discovered that by adding an acrylate such as polyacrylic acid sodium salt, or any other suitable salt, polymethylmethacrylate or polyacryl amide or other suitable acrylates such as polyacrylnitrile or others in combination with polystyrene to form an aqueous slurry of the Ceramicrete.RTM. or Ferroceramicrete binder with the acrylate and styrofoam, the resultant material when applied to a styrofoam base forms not merely a physical bond as previously occurred in the art, but a chemical bond which is firmly adhered to the styrofoam base thereby providing a inexpensive and easy mechanism by which at styrofoam based forms on site. Ceramicrete.RTM. and Ferroceramicrete are trademarks of Argonne National Laboratory but are used herein to denote the phosphate ceramics made by the processes disclosed in the above-incorporated patents and publication.

By way of note, the '809 patent teaches a surface coating material used to reduce the leaching of soluble salts from ceramicrete waste forms. However, the resin there disclosed cannot be mixed with the Ceramicrete.RTM. aqueous slurry because the resins are not water based. Moreover, the materials there disclosed are not suitable for use with polystyrene foam because it collapses the foam structure. The materials used in the '809 patent are styrene solvent and benzoyl peroxide, both of these components being toxic and not useful in the construction industry.

The present invention fulfills a significant requirement in the construction industry in warm climates in that the present invention provides polymers which with Ceramicrete.RTM. (will bond chemically to a polystyrene foam surface. The invention consists of water soluble materials that can be applied in an aqueous Ceramicrete.RTM. slurry and do not adversely affect the setting properties of the Ceramicrete.RTM. slurry. Moreover, when set, the added polymers do not significantly alter the mechanical and physical properties of the Ceramicrete.RTM. or Ferroceramicrete material, and more particularly, the polymers do not introduce porosity in the Ceramicrete.RTM. or Ferroceramicrete material and do not render the set material water soluble.

As previously stated, there are a number of polymers which are water soluble and compatible with the aqueous slurry of Ceramicrete.RTM. or Ferroceramicrete particles. These water soluble polymers include the acrylates such as polyacrylic acid (AA) salt, preferably the sodium salt, polymethylmethacrylate (PMMA), polyacryl amide and others such as polyacrylnitryl. The acrylate salts when dissolved in water produce carboxylate anions COO.sup.- by releasing sodium ions into the solution. The dissolution may be written as COONa--.fwdarw.COO.sup.-+Na.sup.+. The dissolution is illustrated in FIG. 1. In addition to dissolution of AA in aqueous solution of Ceramicrete slurry, dissolution of MgO in the acidic Ceramicrete slurry forms Mg(aq).sup.++. The two ions will react to produce COOMg.sup.+ complexes. The complex may be of the type COO.sup.---Mg --OOC, in which case, one Mg cation will satisfy two carboxylate ion. In another mechanism, only one carboxylate ion may be bonded to Mg cation and the cation in turn bonds to one of the anions from the Ceramicrete matrix. The first possibility stabilizes AA partially (FIG. 3) and the second possibility will provide a bonding between Ceramicrete matrix and the polymer (FIG. 2). These reactions may be written as follows:

Dissolution of MgO: MgO+2H.sup.+=Mg(aq).sup.+++H.sub.2O

Complete complexation of AA and Mg: COO--+Mg:COO.sup.-+Mg(aq).sup.+++.sup.- -OOC.dbd.COO--Mg--OOC Partial complexation of AA and Mg::--COO--+Mg(aq).sup.++.dbd.--COO--Mg.sup.+

AA by itself, however, needs to be polymerized to form a stable component in the matrix. To form a copolymer, styrene (C.sub.6H.sub.5CH.dbd.CH.sub.- 2) is added to the composition. Styrene will bond to AA as shown in FIG. 3. This reaction forms a stable polymer within the Ceramicrete.RTM. matrix.

The bonding between the Ceramicrete composite matrix and Styrofoam is facilitated by the reaction of styrene and AA in the same manner as above again as shown in FIG. 3. A similar complexation also occurs if an amide is used instead of styrene and also with use of PMMA, or other suitable acrylate.

Table 1 lists various attempts to bind Ceramicrete.RTM. and styrofoam. In each case, Ceramicrete.RTM. slurry was made in a conventional way, with 7 9 wt. % MgO, 18 et,. % KH.sub.2PO.sub.4, 50 wt. % sand and the rest Class F fly ash. To this as added 12 wt. % water. The slurry was mixed for 25 minutes. Each batch was approximately 500 Grams. AA and styrene beads were added at different times as shown in Table 1. The slurry was then poured over a surface of dense styrofoam and was allowed to set. Typical thickness of the Ceramicrete.RTM. layer was 0.5 cm. The following criteria were used to test if the product was acceptable as a structural material to be sprayed on styrofoam walls. 1. The slurry should warm up in 25 minutes 2. It should set into a hard ceramic within another hour, 3. The bond between Styrofoam and Ceramicrete composite should be chemical.

The last criterion was tested by inserting a spatula between the Ceramicrete.RTM. and styrofoam and lifting it up to open the interface. If the whole cast of Ceramicrete.RTM. separated from the Styrofoam, then it was considered to be only a physical bond. If on the other hand, the Ceramicrete.RTM. cast or styrofoam broke at the tip of the spatula and the rest of the material retained good adhesion, then it was considered to be a good chemical bond.

As seen in Table 1, the bonding was chemical only when styrene or excess MgO were used along with AA. In the first case, it shows that Ceramicrete.RTM. itself with styrofoam sheet. In the second case, adding of only AA did not achieve the desired result. In fact, it adversely affected the setting of Ceramicrete.RTM.. In the third case, Ceramicrete.RTM. reacted with AA and excess MgO to provide a chemical bond. Although 10% excess MgO is reported in Table 1, excess MgO may be present in the range of from 1 to about 20% by weight, more preferably 1 1 to about 10% by weight, and most preferably about 10% by weight. In the last two cases, however, styrene reacted with AA and provided the necessary chemical bonding between styrofoam and Ceramicrete.RTM., and also stabilized AA within Ceramicrete.RTM..

TABLE-US-00001 TABLE 1 Various admixtures of Poly-ceramicrete and the results Ceramicrete and Mode of Heat generation at Nature of Polymer composition Application 25 min and setting time Bonding Ceramicrete .RTM. only Poured on Styrofoam Warmed up, One hour Physical setting Ceramicrete .RTM. with Same as above, AA Less warming, Physical, Ceramicrete .RTM. AA only added to slurry, Long time to set was set but slightly wet Ceramicrete .RTM. with Same as above, AA More warming, Chemical AA and 10% added to slurry short time to set additional magnesium oxide Styrene beads The solution was Warmed up, one Chemical dissolved in hot AA mixed with Ceramicrete .RTM. hour setting solution powder and slurry was formed, mixed for 25 min. and poured Mixture of styrene Mixed slurry for 25 Warmed up, One Chemical and AA added to min. and poured on hour setting Ceramicrete .RTM. powder Styrofoam

As may be seen therefore, there has been disclosed a structural material and a method of making same in which the aqueous slurry of particles of a solid phosphate ceramic composite and a polyacrylic acid or acrylate or combinations or salts thereof are combined with either polystyrene or excess MgO to form a reaction product which chemically bonds to a polystyrene base. More particularly, the solid phosphate ceramic composites may include the reaction product of a source of phosphate such as phosphoric acid or monopotassium phosphate and an acrylate. Further, the ceramic component may be one or more of a metal oxide or hydroxide. The structural material disclosed herein may include the oxide wherein the oxide or hydroxide is one or more of Si, Fe, Mg, Al, Mn, Ca, Zr or various mixtures or combinations thereof. As before stated in the incorporated patents, the solid phosphate ceramic generally includes alkali metal ions and more particularly and preferably alkali metal potassium ions. Various polyacrylates may be used including polyacrylic acid or polymethymethacrylate or the sodium salt of polyacrylic acid. Additionally, polyacrylamide may also be employed.

In general, the acrylic acid or acrylate or combinations of salts thereof may be present in the aqueous solution in the range of from about 3% by weight to about 8% by weight. More preferably, in the range of from about 4% by weight to about 6% by weight. More preferably, the polyacrylic acid or polyacrylate or combinations of salts thereof is present in the aqueous slurry at a concentration of about 5% by weight. The polystyrene which may be used to form the chemical bond in combination with the other materials hereinbefore set forth may be generally present in the aqueous solution in the range of from about 1% by weight to about 10% by weight.

The structural material disclosed in the above specification is particularly useful, as hereinbefore stated, in combination with polystyrene base materials in warm climate construction. As before stated, the slurry of particles of solid phosphate ceramic component along with a suitable acrylate or salt thereof in combination with either or both of styrene and excess magnesium oxide will provide the chemical bond required to obtain the benefits of the present invention. When excess MgO over and above the stoichiometric amount are needed in the slurry is used, it may be present in a range of from about 1% to about 20% by weight, more preferably in the range of from about 1% to about 10% by weight and most preferably about 10% by weight. The method of chemically bonding a structural material to a polystyrene base has been disclosed in which an aqueous solution of particulate solid phosphate ceramic composite particles and either polystyrene particles or an excess of MgO or both and a polyacrylate or a polyacrylic acid or salt thereof has been used to form a aqueous reaction product which when applied to polystyrene base reacts to form a dried reaction product chemically bound to the polystyrene base, all as hereinbefore disclosed.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the relevant art that changes and modifications may be made without departing from the invention in its broader aspects. Therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention. The matter set forth in the foregoing description and accompanying drawings is offered by way of illustration only and not as a limitation. The actual scope of the invention is intended to be defined in the following claims when viewed in their proper perspective based on the prior art.


US Patent # 6,776,837

Formation of Chemically Bonded Ceramics with Magnesium Dihydrogen Phosphate Binder

US Cl. 106/690 // Intl Cl. C04B 012/02

Abstract ~ A new method for combining magnesium oxide, MgO, and magnesium dihydrogen phosphate to form an inexpensive compactible ceramic to stabilize very low solubility metal oxides, ashes, swarfs, and other iron or metal-based additives, to create products and waste forms which can be poured or dye cast, and to reinforce and strengthen the ceramics formed by the addition of fibers to the initial ceramic mixture.

References Cited
U.S. Patent Documents: 5645518 ~ 6133498

CONTRACTUAL ORIGIN OF INVENTION

The United States Government has rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the U.S. Department of Energy and the University of Chicago, representing Argonne National Laboratory.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for forming compactible ceramics and for forming ceramics with improved compression, flexural and fracture strength, and more specifically, this invention relates to a method for using a phosphate binder with enhanced binding characteristics in high waste loading scenarios which is compactible in the paste stage and which can be used with fibers to improve the ceramics' strength.

2. Background of the Invention

The effective sequestration and disposition of waste oils, bulky waste forms and other unwieldy objects continues to elude disposal researchers. Typical concretes and ceramics, the later of which are described in U.S. Pat. Nos. 5,846,894, 5,830,815, 5,645,518, and 6,204,214, and incorporated herein by reference, have high compression strengths. However, these materials exhibit comparatively poor flexural and fracture properties. This leads to crack propagation, particularly when attempting to macroencapsulate large-size objects. Compactibility of these materials also is lacking.

To macroencapsulate large-size objects, better fracture toughness is needed to avoid crack propagation. For cementing lateral junctures in multilateral oil well completions, improved flexural strength is needed. To arrest surface crack propagation, fiber reinforcement is needed.

The use of glass fiber as a strengthening additive in cement causes problems. Cement is very alkaline and glass fibers deteriorate in such alkaline environments. This leads to weakening of the composite structure. Some common solutions are to over-engineer the composite to compensate for the eventual degradation and loss of strength, to use alkali resistant glass fibers, and to use coatings such as polyvinyl chloride (PVC) over the glass fiber to protect it from the alkaline environment. All of these solutions lead to higher costs.

Ceramic systems leading to highly ductile waste forms remain elusive. For example, in the ceramic formation reaction disclosed in U.S. Pat. No. 5,830,815, and given by Equation 1, infra.

MgO+H.sub.3 PO.sub.4 +2H.sub.2 O.fwdarw.MgHPO.sub.4.multidot.3H.sub.2 O Eq. 1

102.44 Kilojoules per mole of heat are released. This high amount of heat results in too rapid ceramic product formation (leading to brittleness) for any practical use. Also, some materials for disposal prove too soluble in the very low pH environs in which the phosphate ceramics disclosed in the '815 patent operate. Alternatively, solids of low solubility (pK.sub.s.about.15 to 25) are not soluble enough in the solutions utilized in the '815 patent.

U.S. Pat. No. 5,846,894 issued to Singh et al. on Dec. 8, 1998 discloses a method to produce phosphate bonded structural products from high volume benign wastes.

U.S. Pat. No. 5,678,234 issued to Colombo, et al. on Oct. 14, 1997 discloses an encapsulation method utilizing a modified sulfur cement at elevated temperatures, and glass or other fibers for enhancement of the compressive and tensile strength.

None of the aforementioned patents teaches a method for the reduction of volume during stabilization of solid powdered wastes. In addition, none of these patents provides a method for the stabilization of near insoluble oxides.

None of the aforementioned patents teaches a method for enhancing the flexural and fracture toughness of the structural products via a truly homogeneous ceramic-fiber composite.

None of the aforementioned patents even contemplates using a dispersant for fiber additives for strength enhancement.

A need exists in the art for a method to produce superior compactible structural products with enhanced flexural and fracture toughness. The method should result in a ceramic which can be compressed while it is still putty-like, i.e., before it sets completely. The method also should result in the formation of a durable and chemically stable ceramic which can be utilized to sequester hard-to-contain wastes. Finally, the method should utilize inexpensive and commonly available reactants at ambient temperatures to produce low cost ceramics.

SUMMARY OF INVENTION

An object of the present invention is to provide a method for producing chemically bonded phosphate ceramics (CBPCs) that overcomes many of the disadvantages of the prior art.

Another object of the present invention is to provide a ceramic capable of encapsulating very low solubility metal oxides. A feature of the invention is that the oxides do not need to be calcined prior to encapsulation. An advantage is that an encapsulation process utilizing a ceramic uses much less energy and, accordingly, is less expensive.

Still another object of the present invention to provide a method for producing ceramics which have enhanced flexural and fracture toughness. A feature of the invention is that fibers being evenly dispersed throughout the ceramic binder enhances the flexural and fracture toughness of the binder. An advantage of this feature is that it minimizes leakage of encapsulated hazardous wastes.

Yet another object of the present invention is to provide a method for producing ceramics which can encapsulate wastes that contain nonpolar, or oil-based fluids. A feature of this invention is that a magnesium dihydrogen phosphate (MHP)-based binder can effectively encapsulate wastes having trace amounts of oil. An advantage of this feature is that, at present, there is not any effective means for encapsulating petroleum fluid-tainted wastes.

Another object of the present invention is to provide a method for producing glass fiber-reinforced ceramics. A feature of the invention is that phosphate-based ceramics are homogeneously mixed with the fibers to produce a ceramic structure containing fibers dispersed throughout the structure. An advantage of the method is that it provides an acidic- to neutral-pH environ favorable to the glass fibers. Therefore, corrosion of the glass fibers is minimized, and structural integrity of the resulting structures is maximized for periods of time heretofore not attainable.

Still another object of the present invention is to provide a method for the effective dispersal of fiber additives in a ceramic binder. A feature of the invention is that monopotassium phosphate is utilized as a dispersant to prevent aggregation of fibers into strands and bunches. An advantage of this feature is that the flexural strength of the resulting structure is typically twice that of cements.

Yet another object of the present invention is to provide a method which produces compactible ceramics. A feature of the invention is the use of magnesium dihydrogen phosphate as a binder. An advantage of the method is that it allows for compression of the ceramic to a volume 40% less than the starting volume of the reaction slurry.

Briefly, the invention provides a room temperature process for producing a compactible ceramic from powders, the process comprising combining MgO and magnesium dihydrogen phosphate dihydrate to create a dry homogeneous mixture; and contacting the mixture with water to form a slurry.

Also provided is a process for strengthening phosphate ceramics, the process comprising adding fibers to the initial ceramic mixture to create a homogeneous composite substrate.

BRIEF DESCRIPTION OF THE DRAWING

The invention together with the above and other objects and advantages will be best understood from the following detailed description of the preferred embodiment of the invention shown in the accompanying drawing, wherein:

FIG. 1. is a schematic representation of the X-ray diffraction of a swarf before encapsulation, in accordance with features of the present invention;

FIG. 2 is a schematic representation of the X-ray diffraction of a swarf after encapsulation, in accordance with features of the present invention;

FIG. 3 is a schematic representation of the X-ray diffraction of yttrium phosphate ceramic, in accordance with features of the present invention;

FIG. 4 is a schematic representation of the X-ray diffraction of magnetite phosphate ceramic, in accordance with features of the present invention;

FIGS. 5(a-b) is a schematic representation of compressive strength of fiber-reinforced ceramicrete as a function of fiber content for 40 wt. % ash and 60 wt. % ash composites, in accordance with features of the present invention;

FIGS. 6(a-b). is a schematic representation of flexural strength of fiber-reinforced ceramicrete as a function of fiber content for 40 wt. % ash and 60 wt. % ash composites, in accordance with features of the present invention;

FIGS. 7(a-b) is a schematic representation of fracture toughness of fiber-reinforced ceramicrete as a function of fiber content for 40 wt. % ash and 60 wt. % ash composites, in accordance with features of the present invention; and

FIG. 8 is a schematic representation of a scanning electron micrograph of a fractured surface of a fiber-reinforced MKP-based ceramicrete, in accordance with features of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A process is provided to facilitate macro-encapsulation of bulk-waste, oil waste, and similar unwieldy types of waste forms using ceramic materials and fiber-enforced ceramic materials. The same compactible phosphate binder may be used in "neat" formulations to make ceramics.

The inventors also have developed a ceramic-based waste binder for utilization with compaction technologies. Use of the binder in a compaction mode also accommodates very high waste loadings. The binder has enhanced binding characteristics which allow treatment of more difficult wastes such as those containing oils.

The invented method and binder enable the production of structural products such as conventional bricks and blocks and at lower costs than typical processes and materials.

The inventors have determined that phosphate-based ceramics, such as magnesium dihydrogen phosphate (MHP), homogeneously mixed with reinforcing fibers form compactible, high strength waste sequestration matrices. The phosphate ceramics utilized in the reinforcing fiber-encapsulation method include, but are not limited to magnesium potassium phosphate hexahydrate, magnesium ammonium phosphate, magnesium sodium phosphate, magnesium phosphate, aluminum phosphate, iron phosphate, zinc phosphate, and phosphates of all rare earths such as, but not limited to, phosphates of lanthanum, cerium, yttrium, and neodymium.

The resulting ceramics have wide-ranging utility, including the ability to encapsulate metal wastes that require a more durable ceramic which will withstand the shock of impacts and will not crack over time. Such metal wastes include hazardous materials (e.g. chromium and arsenic), and fission products such as technetium wastes and low-level radioactive materials. Low-level radioactive materials suitable for encapsulation in the instant ceramic include pyrophoric uranium chips that are stored in mineral oils. It is these oil-tainted materials cannot be easily encapsulated using state-of-the-art technology.

The general process for formulating the ceramic starting material comprises mixing MgO, and magnesium dihydrogen phosphate dihydrate (MHP) to produce a dry mixture; and then combining the mixture with water at room temperature to produce Newberyite, MgHPO.sub.4.3H.sub.2 O. That reaction is illustrated in Equation 2, infra.

MgO+Mg(H.sub.2 PO.sub.4).sub.2.multidot.2H.sub.2 O+3H.sub.2 O.fwdarw.2MgHPO.sub.4.multidot.3H.sub.2 O Eq. 2

MHP can be produced by reacting MgO and phosphoric acid according to Equation 3:

MgO+H.sub.3 PO.sub.4 +H.sub.2 O.fwdarw.Mg(H.sub.2 PO.sub.4).sub.2.2H.sub.2 O Eq. 3

Both reactions given by Eqs. 2 and 3 are exothermic and produce 56.5 and 108.07 Kilojoules of energy per mole, respectively. As such, the addition of heat is not necessary to produce the binder or resulting composite.

The magnesium oxide, MgO (calcined), and MHP are present in a molar ratio of MgO to MHP which varies from 3:7 to 1:1. An equimolar mixture of MgO and MHP is the preferred mode used in all MHP binder work mentioned herein. The MgO/MHP mixture to water weight ratio varies from 10:1.85 (stoichiometric composition) to 10:3.7. The inventors found that MgO should be calcined as described in U.S. Pat. No. 6,204,214, issued to Singh, et al. on Mar. 20, 2001 incorporated herein by reference, as the preferred mode for the instant invention.

Crystalline MHP is made by mixing 16 wt. % of MgO, 77 wt. % of H.sub.3 PO.sub.4, and 7 wt. % H.sub.2 O. Those weight percentages reflect the stoichiometry of the reaction. Only the MgO can be added in excess. The H.sub.3 PO.sub.4 is dissolved in water with subsequent slow addition of the MgO. The MgO must be added slowly as the reaction is very exothermic and to maintain a temperature increase of approximately 5-10.degree. C. from the ambient temperature.

The mixture described supra, without the addition of any other materials (i.e., "neat MHP), provides a compactible and durable ceramic. The fiber enhanced ceramic made with the MHP process is also compactible.

Waste Detail

For optimal utility, the MHP binder mixture is loaded with any one of a myriad of waste powders derived from high volume wastes. These wastes include, but are not limited to, the group consisting of any inorganic oxides and metals, such as hazardous and radioactive wastes, low solubility metal oxides, ceramic powders, ashes, red mud, sand, swarfs, lateritic soils, mineral wastes, and the same with traces of oils or greases, or combinations thereof. The waste powder loading into the ceramic mixture comprises up to 85 wt. % of the final ceramic.

The hazardous and radioactive waste is material selected from the group consisting of high level radioactive wastes, low-level radioactive wastes, low-level radioactive and hazardous waste called "mixed waste," heavy metals, fission products, uranium and any other radioactive and pyrophoric metals stored in mineral oil, or combinations thereof. Aside from products tainted with mineral oil, other oils or greases also can be accommodated by the instant binder, including petroleum-based or vegetable-based nonpolar compounds, and any other hydrocarbons.

The low solubility metal oxides (pK.sub.sp in the range of .about.15 to 25) as waste encapsulation candidates include, but are not limited to, oxides of cobalt, copper, dysprosium, erbium, europium, holmium, neodymium, palladium, samarium, tellurium, ytterbium, yttrium, and zinc.

The inventors found that the MHP formulations can be compacted or compressed to a smaller volume as much as 40% of the original volume. The slurry is compacted at temperatures between 0 and 30.degree. C. Compression process temperatures above 30.degree. C. cause the formulation to lose water by evaporation and the ceramic loses strength. Compaction methods employed can include, but are not limited to, uniaxial presses, harmonic presses, adobe presses, and cold or hot isostatic presses. The ceramic mixture also facilitates dye casting.

Monopotassium phosphate can be used as the sole initial phosphate reactant to produce a ceramic mixture. Alternatively, monopotassium phosphate can be added to any initial phosphate binder-waste mixture up to 20 wt. % of the initial mixture as a dispersant.

Fiber Detail

The phosphate-based ceramic systems utilized herein have acidic to neutral environments. Those pH ranges are favorable for glass fibers which deteriorate in environments of high alkalinity (i.e., high pH), as noted supra. As such, the inventors have found that the alkaline degradation problem of glass fiber in concrete systems, is resolved when cement is supplanted by ceramic material, and particularly low-pH ceramic formation systems.

Glass has a very low solubility in moderately acidic and neutral environments. The invented ceramic paste, of the type derived from equation 3, has an initial pH of 4.3, and when setting is complete, it has a pH of 8. This is much lower than the pH of cement, which is typically 12 to 13.

Fibers are added to the initial binder mixture to reinforce phosphate-based ceramics and arrest cracks and crack propagation. The addition of fibers increases the flexural strength up to 2000 psi and the fracture toughness up to 0.8 Megapascal.multidot. meter (Mpa.multidot.m.sup.1/2). The compressive strength of the product is up to five times that of conventional bricks and blocks for which the compressive strength is 2000 psi.

To effectively enhance the strength characteristics of the ceramicrete, the fibers are dispersed evenly throughout the ceramic binder to create a composite mixture that is homogenous throughout. The inventors found that monopotassium phosphate is a good dispersant of cut fibers. This is advantageous in that fibers can be added as bunches and strands, or any combination thereof, but they disperse throughout the binder to be encapsulated as individual fibers by MKP. Monopotassium phosphate can be added to any initial phosphate binder-waste mixture as a dispersant up to 10 wt. % of the initial mixture of any phosphate binder.

Fibers are comprised of materials selected from the group consisting of ceramics, glass fibers, organic polymers, carbon, metal fibers, and natural substances. Fibers come from natural substances selected from the group consisting of coconut, corn, bagasse, jute, sisal, wood, and any cellulosic material. Polymers are organic compounds selected from the group consisting of nylon, polyethylene, polypropylene, and polyvinyl chloride (PVC).

The fibers can be added to the ceramic mixture as weaved mat, short cut fibers, long cut fibers, oriented strands, or simply as cut fibers that are not oriented in any way, or any combination thereof, and are added as 1 to 10 wt. % of the substrate. To obtain fiber loadings above 2 wt. %, fiber to the extent of 2 wt. % must first be added to the binder mixture with subsequent stirring to allow the binder mixture to dissolve, then addition of fiber up to the desired higher wt. % when the mixing slurry becomes thin due to dissolution of the phosphate binder.

The fiber-reinforced ceramic paste can be cast, molded, and used to dip-coat, paint, or spray surfaces, and to cement lateral junctures in oil and gas wells.

Addition of fibers increases the viscosity of the setting slurry making it difficult to pour the slurry into suitable molds. It is easier to dye cast the forms, but dye casting needs a modified Ceramicrete binder. MHP serves this need very well.

Process Detail

A salient feature of the invented process is the ability to produce sequestration matrices, up to 80 weight percent of which is comprised of waste, and without the addition of heat.

In one instance, as much as 73 wt. % powder with 18 wt. % MgO/MHP binder (equimolar amounts of MgO and MHP) and 9 wt. % water is utilized to form a slurry of putty-like consistency. The resultant paste can then be pressed at a pressure of .about.1000 pounds per square inch (psi) to form a dense monolith. Initial setting takes place within ten minutes. Alternatively, more water can be added to make a thinner paste or slurry and pour this slurry into a mold. Within an hour, the slurry sets into a hard ceramic.

Boric acid can be added to retard the reaction and reduce the reaction rate. This provides more time to transfer the mixture into the mold and apply pressure for the purpose of compacting the slurry. The iron examples infra are present to serve as illustrations. Other metals can be encapsulated by the invention disclosed herein.

EXAMPLE 1

Pelletizing Steel Industry Waste or Swarfs

Swarfs are machining wastes containing iron in them. These wastes also contain oils and as such, conventional cements cannot be used to solidify them. The presence of metal also makes swarfs unsuitable for encapsulation/incorporation into ceramics generated from a phosphoric acid solution, as disclosed in U.S. Pat. No. 5,830,815, because reaction of metal with phosphoric acid generates large amounts of heat and boils the slurry.

In storage, swarfs oxidize and form magnetite and haematite. While they are pyrophoric wastes and hence are a liability, they are ideal raw materials for forming iron phosphate ceramics. This is because the wastes contain a significant amount of elemental iron that has not rusted, and they also contain different iron oxide forms that include haematite and magnetite. To recover metal values from these wastes, it is necessary that they be pelletized.

Swarf waste powder was pelletized by encapsulation pursuant to the procedure given supra in the "Process Detail" with a swarf waste loading of 73 wt. % in the final dry mixture, the remaining 23 wt. % being the dry mixture described on p. 6. During mixing and pressing, the mixture did not generate heat. Each sample formed was a briquette of dimensions 2".times.2".times.1", and was placed under pressure for 2 minutes at a pressure of 1000 psi, thus reducing its volume 40%. Each briquette was stored in a polyethylene bag for 3 weeks for complete curing. Within a day all samples appeared hard and were unscratchable, but continued to release water in the bag indicating a continued reaction.

FIG. 2 shows an X-ray diffraction pattern of a typical swarf before encapsulation in ceramic. The diffraction pattern shows that the swarf contains iron (Fe), carbonized iron (Fe.sub.3 C) and haematite (Fe.sub.2 O.sub.3) as its main constituents. FIG. 3 shows a typical X-ray diffraction pattern of the encapsulated swarf. Apart from the unreacted haematite ("#"), unreacted iron (".star-solid."), and unreacted carbonized iron ("+"), FIG. 3 discloses the presence of additional compound, magnetite (Fe.sub.3 O.sub.4), designated as @. Magnetite may form via the reactions according to equations 4 and 5:

Fe.sub.2 O.sub.3 +Fe+2e.sup.-.fwdarw.3FeO Eq. 4

FeO+Fe.sub.2 O.sub.3.fwdarw.Fe.sub.3 O.sub.4 Eq. 5

Some of the FeO that forms via the reaction given in Equation 4 may react with MHP to form FeHPO.sub.4 according to Equation 6.

FeO+Mg(H.sub.2 PO.sub.4).sub.2.multidot.2H.sub.2 O.fwdarw.MgHPO.sub.4.multidot.3H.sub.2 O+FeHPO.sub.4 Eq. 6

The water that was added to dissolve the MHP binder does not participate in the reaction, but is released in the polyethylene bag. Thus at the end, 80 wt. % swarf and 20 wt. % binder all reacted to form a hydrophosphate compound.

The briquettes' properties are given in TABLE 1.
 

                             TABLE 1
             Properties of solidified swarf samples.
                   Property              Measured values
             Density (g/cm.sup.3)               2.16
            Open Porosity (vol. %)              10
            Compressive strength (psi)        2345 .+-. 345


Given the fact that the iron content was high in the original swarf material, the density of the briquette is relatively low. The lower than anticipated density is most likely due to the formation of hydrated iron compounds that generally have lower densities. The open porosity of 10% is lower than found in solidified concrete and hence lower water absorption.

The compressive strength of the iron-encapsulating ceramic is very similar to that of conventional bricks (.about.2000 psi). The samples are hard enough to withstand being dropped from a height of 12 feet onto a hard floor. The data in TABLE 1 clearly shows that the swarf pellets can withstand rough transportation and rough handling. The pellets are also light weight and therefore can easily be picked up by an electromagnet for feeding into blast furnaces, without being broken into pieces.

These observations show that the MHP binder can be very suitable in recycling metal wastes and for stabilizing uranium chips, which have been stored in mineral oils.

The binder metal wastes and radioactive materials are easily incorporated into monolithic waste forms for long-term storage.

EXAMPLE 2

Compacting Ashes

A number of different ashes were used which included high chloride content fly ash, bottom ash and low chloride content Class F fly ash. These ashes were radioactively contaminated chloride-containing ashes from various U.S. Department of Energy (DOE) sites. In each case, a small amount of water was added to moisten the mixture. Briquettes of dimensions 2".times.2".times.1" were made using these ashes along with the MHP binder. As with the swarf, each sample formed was a briquette of dimensions 2".times.2".times.1", and was placed under pressure for 2 minutes at a pressure of 1000 psi, thus reducing its volume. Each briquette was stored in a polyethylene bag for 3 weeks. The samples were cured for one full week and were then cut into 1".times.1".times.1" cubes for which compression strengths were measured. The results of the strength measurements are given in TABLE 2 which also contains results for briquettes made with MKP.
 

                             TABLE 2
    Compressive strengths of ash briquettes produced with MHP and MKP.
         Wt. % of ash components
       High Cl    High Cl Low Cl Class  Wt. % of    Compressive
     Bottom Ash   Fly Ash    Fly Ash     Binder   Strength (psi)
         35         --         35        MHP, 30       4421
         50         --         20        MHP, 30       3038
         27.5       7.5        35        MHP, 30       2495
         35         --         35        MKP, 30       2402
         50         --         20        MKP, 30       2327
         27.5       --         35        MKP, 30       1843
         --         --         80        MHP, 20       4056
         --         --         85        MHP, 15       2059
         --         --         90        MHP, 10        600


As noted in the last three rows of the table, high waste loadings can be attained with ash of low Cl content. The compressive strengths values are 4056 psi, 2059 psi, and 600 psi for low Cl waste loadings of 80%, 85%, and 90%, respectively. The minimum strength requirement for land disposal of hazardous and radioactive waste is 500 psi.

As a result of compressing the MHP binder briquettes, their volumes decreased by as much as .about.40%. The MKP binder briquettes do not undergo any volume reduction whatsoever when compressed. For waste treatment, this compaction reduces disposal costs by 40% which is a distinct advantage over waste encapsulation processes utilizing MKP, and is estimated to be the lowest cost process for treating high volume radioactive waste.

EXAMPLE 3

Yttrium Oxide Containing Waste Forms

Yttrium oxide was thoroughly mixed with pre-mixed MHP dry mixture in a weight ration of 1:2. Water was then added to the powder mixture at a weight ratio of powder to water of 3:1. The resultant slurry, viscosity of 200 cp, was mixed for 15 minutes until it warmed slightly to .about.40.degree. C. due to the exothermicity of the process, and subsequently set into a hard ceramic within 10 minutes. As such, the slurry provides an exothermic reaction, which aids in the setting reaction.

The ceramic formed in this Y.sub.2 O.sub.3 protocol has a density of 1.78 grams per cubic centimeter (g/cc) and its open porosity is .about.5% giving the same density-porosity characteristics of MKP binder with various encapsulated wastes. However, because Y.sub.2 O.sub.3 is less than sparsely soluble, the invented MHP process is the best means to form a ceramic of this oxide and other rare earth oxides with similar solubilities.

FIG. 3 shows a X-ray diffraction pattern of the yttrium phosphate ceramic. In addition to Newberyite ("*"), Y.sub.2 O.sub.3 ("o"), and yttrium phosphate trihydrate, YPO.sub.4.multidot.3H.sub.2 O (".circle-solid.") are present.

EXAMPLE 4

Magnetite-Containing Ceramic Waste Forms

Magnetite based ceramics have been made in the past by the direct reaction of magnetite and aqueous phosphoric acid solution. The reaction is rapid and very exothermic which creates difficulties when attempting to form large monoliths. This problem may be overcome by using MHP as the binder.

Magnetite and MHP powder were mixed thoroughly in a weight ratio of 1:2 of magnetite to MHP. Water was added in a weight ratio of 1:3 of water to mixture. The resultant slurry, viscosity of 200 cp, was mixed for .about.15 minutes until the mixture's temperature began to increase. The slurry was then poured into molds after which it set within an hour and formed a dense and hard ceramic. The ceramic has a density of 1.71 g/cc and an open porosity of 4.6%. This particular magnetite ceramic is a lightweight material.

FIG. 4 shows the X-ray diffraction pattern of the ceramic formed. Peaks are directly observed only for Fe.sub.3 O.sub.4 ("x") and MgHPO.sub.4.3H.sub.2 O ("T") are seen to be present. Although no peaks of iron phosphate are visible, the large humps indicate a significant amount of a glassy phase. The magnetite ceramic sample also looked very glassy and scanning electron microscopy showed large portions of featureless or glassy material. Thus, the iron phosphate formed an amorphous or glassy solid structure. The solid is free of open porosity with almost zero water absorption.

EXAMPLE 5

Strength Enhancement by Fiber Addition

As taught supra, reinforcing substrates such as fibers may be directly incorporated into the phosphate powders. In this example, the binder powder mixtures comprised magnesium potassium phosphate hexahydrate, and Class F fly ash. Two different chopped glass fiber lengths were used, 0.25 inch and 0.5 inch. Fibers were added into the powder blend in a proportion of 1 to 3 wt. % of the total mixture. Water was added and as the powder dissolved, more fibers could be added, if desired or needed to obtain a particular wt. %. A dye-casting process can be used to form composites with a greater amount of fiber, up to 10 wt. %.

FIGS. 5(a-b) shows the compressive strength as a function of fiber content in composites of 40 and 60 wt. % Class F fly ash with two different fiber lengths. As shown in FIG. 5a, with a waste loading of 40 wt. % ash and loading with 0.25 inch chopped glass fiber, the compressive strength increased, from a value of 6500 psi for MHP binder ceramic without fibers, to 10,800 psi when fiber was added to the extent of 1 and 2 wt. % of the total composite. With a fiber loading of 3 wt. %, the compressive strength decreased to 9,400 psi. The results for 0.5 inch chopped glass fiber were somewhat lower than those for 0.25 inch fiber and dropped to 9800 psi at 3 wt. % fiber.

In the case of 60 wt. % ash, the compressive strength profiles are exactly the opposite of those for 40 wt. % ash, as shown in FIG. 5b. The compressive strength without fiber is 10,600 psi. With 1 wt % of 0.25 inch fiber, the compressive strength is 12,000 psi; then inversely, the compressive strength of the composite with 0.5 inch fibers consistently decreases with increasing fiber content.

FIGS. 6(a-b) shows the flexural strength as a function of fiber content in composites of 40 wt. % ash and 60 wt. % ash. FIG. 6a shows the results for 40 wt. % ash and FIG. 6b the results for 60 wt. % ash. For both ash wt. %'s and both fiber lengths, flexural strength increases as the fiber wt. % increases.

FIGS. 7(a-b) shows the fracture toughness of these same composites. FIG. 7a shows the results for 40 wt. % fiber and FIG. 7b the results for 60 wt. % fiber. The fracture toughness of the ceramicrete samples without any fibers is 0.22 and 0.35 Mpa.multidot.m.sup.1/2. Adding fiber to the extent of 3 wt. % for both 0.25 and 0.5 inch fibers increases the fracture toughness of 40 wt. % ash composite to 0.65 Mpa.multidot.m.sup.1/2. That figure is approximately twice the fracture toughness of MKP formulations, neat, i.e., without any fiber additive.

FIG. 8 shows the scanning electron micrograph of a fractured surface of a MKP-Class F fly ash composite sample, 60 wt. % Class F fly ash and 2 wt. % of 0.25 inch glass fiber. The fibers were added as strands or bunches of fibers; yet, the fibers dispersed and became encapsulated as individual fibers in the ceramicrete matrix. The micrograph clearly shows each fiber is surrounded by the matrix. MKP serves as an effective dispersant. As is to be expected in an acidic or neutral pH environment, there is no corrosion on the fibers' surfaces. This indicates a compatibility between the glass fibers and the matrix.

MKP is a rugged binder, applicable to a wide variety of wastes and for specialized structural products where the strength requirements are very high. On the other hand, MHP allows for high waste loading, is much less expensive, and may be useful for development of structural products of high volume waste streams.

A myriad of wastes can be encapsulated by the invented methods and ceramics. As such, hazardous metals are good candidates, including, but not limited to arsenic, cadmium, chromium, lead, nickel, and zinc. Low solubility-oxides are also good waste substrate candidates, as are fission products, including technetium, strontium, barium and cesium. Low level wastes, such as biomedical materials and other slightly radioactive substrates are suitable encapsulation candidates. Wastes containing difficult to encapsulate oils are particularly good candidates for the instant invention. Even heterogeneous wastes and mixed phase wastes are suitable.

While the invention has been described with reference to details of the illustrated embodiment, these details are not intended to limit the scope of the invention as defined in the appended claims.



US Patent # 6,518,212

Chemically Bonded Phospho-Silicate Ceramics

Arun WAGH, et. al.

US Cl. 501/111; Intl Cl. C04B 035/447

Abstract:  A chemically bonded phospho-silicate ceramic formed by chemically reacting a monovalent alkali metal phosphate (or ammonium hydrogen phosphate) and a sparsely soluble oxide, with a sparsely soluble silicate in an aqueous solution. The monovalent alkali metal phosphate (or ammonium hydrogen phosphate) and sparsely soluble oxide are both in powder form and combined in a stochiometric molar ratio range of (0.5-1.5):1 to form a binder powder. Similarly, the sparsely soluble silicate is also in powder form and mixed with the binder powder to form a mixture. Water is added to the mixture to form a slurry. The water comprises 50% by weight of the powder mixture in said slurry. The slurry is allowed to harden. The resulting chemically bonded phospho-silicate ceramic exhibits high flexural strength, high compression strength, low porosity and permeability to water, has a definable and bio-compatible chemical composition, and is readily and easily colored to almost any desired shade or hue.

References Cited
U.S. Patent Documents:
2687967 ~ 3078186 ~ 3821006 ~ 3960580 ~ 4036655 ~ 4066471 ~ 4375516 ~ 4504555 ~ 4792359 ~ 4872912 ~
4956321 ~ RE33366 ~ 4978642 ~ 5002610 ~ 5518541 ~ 5645518 ~ 5718757 ~

Other References:

Semler, Charles "A Quick-Setting Wollastonite Phosphate Cement" Ceramic Bulletin vol. 55, No. 11 (1976).
Sugama and Allan "Calcium Phosphate Cements Prepared by Acid-Base Reaction" J. Am. Ceram. Soc. (Aug. 1992).
Fukase et al. "Setting Reactions and Compressive Strengths of Calcium Phosphate Cements" J. Dent. Res. vol. 69 No. No. 12 (Dec. 1990).
Brown and Chow "A New Calcium Phosphate, Water-Setting Cement" pp. 352-379 (1986).
C.E. Semler "A Quick-Setting Wollastonite Phosphate Cement" American Ceramic Society Bulletin, vol. 55, No. 11, (No Date Available).

Goverment Interests

The United States Government has rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the U.S. Department of Energy and University of Chicago operators of Argonne National Laboratory.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to ceramics, and more particularly to a chemically bonded phospho-silicate ceramic that exhibits high flexural strength, high compression strength, low porosity and permeability to water, sets rapidly at room temperature, has a definable and bio-compatible chemical composition, is easily colored, and a method of producing the same.

2. Description of the Related Art

There is an acute need for a rapid setting pore-free high strength binding material for use in the construction and waste management industries. Traditional cements and ceramics used in these industries have many drawbacks that make those traditional materials less than ideal. For example, traditional cement, such as Portland cement, lacks fracture toughness, is extremely porous and permeable to water, and is very slow in setting. The open porosity of these traditional cement materials makes these materials susceptible to deterioration during the freezing and thawing which occurs in many climates across the United States, Europe and beyond. The expansion and contraction of water within the open pores of these traditional cement materials causes them to break down as they are exposed to extreme temperature fluctuations. Additionally, traditional cements, such as Portland cement, are slow in setting, requiring continuous hydration and attention until the cement material has been properly set, thus adding considerable labor costs to any given project.

The open porosity and thus high water permeability of traditional cement materials also limits the practical use of these materials in waste management and waste encapsulation projects. Highly porous cements are permeable to ground water and allow wastes and toxins to leach out from the encapsulated cement material.

Slightly soluble silicate minerals such as Wollastenite (CaSiO.sub.3) and serpentinite (Mg.sub.6 Si.sub.4 O.sub.10 (OH).sub.8), have been used to develop phosphate cements. These phosphate cements are produced by using phosphoric acid, partially neutralized with zinc and aluminum, and then reacted with Wollastenite or serpentine. In spite of the neutralization step, the acid solutions are highly acidic, making them hard to transport to a construction site as a raw material and requiring rigorous safety training for employees in the construction industry, who are used to just adding water to powdered cement. Additionally, the high acidity of these phosphate cements corrodes conventional construction and concrete equipment.

Ceramics are typically less porous than traditional cement, however, traditional ceramics must be fired at extremely high temperatures in order to solidify and cure the ceramic material for practical use. Fired ceramic construction products are expensive, especially if there are large size components. The firing process is not suitable for waste management purposes because waste components volatilize during firing. Resins and other polymer products used as binding materials also provide a less porous product than traditional ceramic materials, however resins are typically expensive to manufacture, their fumes are toxic, and resulting resin products are flammable.

One ceramic material that has had some success as a binding material is the ceramicrete binder. Ceramicrete binders disclosed in our U.S. Pat. Nos. 5,645,518; 5,830,815 and 5,846,894, include compounds such as magnesium potassium phosphate (MgKPO.sub.4.6H.sub.2 O) and newberyite (MgHPO.sub.4.3H.sub.2 O) ceramics. These ceramicrete binders are considerably less porous than conventional cements, are not toxic or flammable, set at a controllable rate, and are a low cost alternative to polymer resins. These ceramicrete binders provide a compression strength comparable to the compression strength exhibited by Portland cement.

It is also known to combine ash with ceramicrete binders, as disclosed in our U.S. Pat. No. 5,830,815, to increase the compression strength to a level two to three times that of the compression strength of Portland cement, The porosity of the ceramicrete ash product is quite low reducing its susceptibility to freeze thaw deterioration and increasing its practical usefulness as a suitable waste encapsulation material that resists permeation of ground water and the leaching of wastes out of the encapsulated ceramicrete ash product. The ceramicrete ash product, however, is not often suitable for architectural uses where many true and subtle colors and shades are desired because the ash product cannot be easily dyed or colored. The ceramicrete ash product is gray or beige depending upon whether fly ash or bottom ash is used. This gray or beige starting color prevents many common architectural colors such as red, yellow, blue, etc., from being achieved, regardless of how much dye or pigment is added to the ash-containing product. Additionally, ash is a mixture of many oxides and silicates and may contain components that are not bio-compatible.

The lack of bio-compatibility in ash containing products, limits the use of those products in the bio-material industries which also have a great need for rapid setting, pore free, high strength binding materials which are also bio-compatible. Only bio-compatible components can make up the binding materials used in dentistry and orthopedics etc. For example, zinc phosphate cements have been used as dental cements because they are dense, hard and also bio-compatible. Zinc phosphate cements, however, are expensive to manufacture and set rapidly, within minutes, making them difficult to work with and produce in any sort of large quantity. For these reasons zinc phosphate cements are not practical for use in construction or waste encapsulation projects as well. Zinc phosphate cements also do not contain calcium phosphates or hydroxyapatite, which are desirable elements for bone tissue growth.

None of the previous binding materials provide a high strength, low porosity, rapid setting, easily colored, bio-compatible chemical composition needed for use in the construction, waste-management, and bio-material industries.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a chemically bonded phospho-silicate ceramic that exhibits high flexural and compression strength. The high strength phospho-silicate ceramic of the present invention can reduce the size of load-bearing structures in the construction industry, provide very strong waste encapsuation matrix and provide a high strength biomaterial for use in prosthetics and dentistry.

Another object of the present invention is to provide the new chemically bonded phospho-silicate ceramic exhibiting low porosity and permeability to water, providing a desirable construction material that is resistant to freeze-thaw deterioration during temperature fluctuations, as well as providing an excellent material for waste encapsulation that is resistant to permeation of ground water and leaching from the encapsulation material.

It is another object of the present invention to provide a phospho-silicate ceramic that sets rapidly at room temperature, without the continuous hydration and attention required by traditional cement materials, thus reducing labor costs. Additionally, the low temperature manufacture of the present invention makes the ceramic suitable for the construction, waste management and bio-material industries.

It is another object of the present invention to provide a new chemically bonded phospho-silicate ceramic made from non-toxic, readily available, easily transportable and inexpensive compounds.

It is another object of the present invention to provide a phospho-silicate ceramic having a definite and definable chemical composition, suitable in the bio-material industry where the chemical components must be known to ensure their bio-compatibility before introduction into the human body.

It is yet another object of the present invention to provide a phospho-silicate ceramic that is easily and readily colored to true colors in any variety of shades or hues.

It is yet another object of the present invention to provide a kit for the simple and easy manufacture of the new chemically bonded phospho-silicate ceramic at an industrial site or for home use.

Yet another object of the present invention is to provide a simple and quick method for manufacturing the new chemically bonded phospho-silicate ceramic of the present invention.

According to one aspect of the present invention, the above objects are realized in a phospho-silicate ceramic formed by chemically reacting a monovalent alkali metal phosphate and a sparsely soluble oxide, with a sparsely soluble silicate in an aqueous solution. The preferred sparsely soluble oxide is magnesium oxide, and the preferred sparsely soluble silicate is calcium silicate.

In one embodiment, the monovalent alkali metal phosphate, the sparsely soluble oxide and the sparsely soluble silicate are all in powder form and are combined to form a mixture. The mixture is comprised of 60% sparsely soluble silicate.

According to one aspect of the invention, the above objects are realized in a method of producing a phospho-silicate ceramic comprising the steps of (a) combining a monovalent alkali metal phosphate powder with a sparsely soluble oxide powder in a stochiometric molar ratio of 1:1 to form a binder powder; (b) adding a sparsely soluble silicate powder in a range of 1-80% by weight to the binder powder, to form a mixture;

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an X-ray diffraction pattern of phospho-silicate ceramic of Example 1.

FIG. 2 is a scanning electron microphotograph of a phospho-silicate ceramic sample.

DETAILED DESCRIPTION OF THE INVENTION

The present invention teaches a new chemically bonded phospho-silicate ceramic that will benefit the construction, waste management, and biomaterial industries, as well as a method for producing the new chemically bonded phospho-silicate ceramic. The phospho-silicate ceramic of the present invention exhibits high flexural strength, high compression strength, low porosity and permeability to water, has a definable and bio-compatible chemical composition and is readily and easily colored to almost any desired shade or hue. The phospho-silicate ceramic of the present invention is simply manufactured in large or small quantities, sets rapidly at room temperatures in only a few hours, and continues to cure over a period of time, The phospho-silicate ceramic of the present invention can also be easily made on site away from the manufacturing plant as its separate components are very safely and easily transportable.

The phospho-silicate ceramic of the present invention is formed by chemically reacting a monovalent alkali metal phosphate and a sparsely soluble divalent oxide, with a sparsely soluble silicate in an aqueous solution. The phospho-silicate ceramic of the present invention can alternatively be formed by replacing the monovalent alkali metal phosphate with ammonium hydrogen phosphate, aluminum hydrophosphate or a phosphoric acid solution. The monovalent alkali metal phosphate (or ammonium hydrogen phosphate, aluminum hydrophosphate or phosphoric acid solution) and sparsely soluble oxide form a ceramicrete binder as disclosed in our earlier patents, namely U.S. Pat. Nos. 5,645,518, 5,830,815 and 5,846,894, incorporated herein by reference. U.S. Pat. Nos. 5,645,518, 5,830,815 and 5,846,894, disclose ceramicrete binders such as magnesium potassium phosphate (MgKPO.sub.4.6H.sub.2 O) and newberylite (MgHPO.sub.4.3H.sub.2 O), etc. Ceramicrete binders are inexpensive to manufacture in large scale, because their components are widely available and generally inexpensive.

Monovalent alkali metal phosphates suitable for forming ceramicrete binders include dihydrogen phosphates of all Group 1A elements in the periodic table and suitable ammonium hydrogen phosphates include, ammonium dihydrogen phosphate ((NH.sub.4)H.sub.2 PO.sub.4) and diammonium hydrogen phosphate ((NH.sub.4).sub.2 HPO.sub.4). Sodium dihydrogen phosphate (NaH.sub.2 PO.sub.4); lithium dihydrogen phosphate (LiH.sub.2 PO.sub.4); and potassium dihydrogen phosphate (KH.sub.2 PO.sub.4) are preferable monovalent alkali metals for forming the ceramicrete binders. In addition, aluminum dihydrogen phosphate may also be used.

Suitable sparsely soluble oxides include oxides of Group IIA elements that have a solubility constant between 5 and 25. Preferably, solubility constants between 5 and 12 are desired, and magnesium oxide (MgO); Barium oxide (BaO); and Calcium oxide (CaO) are the most preferred oxides for reacting with the monovalent alkali metals to form ceramicrete binders. Zinc oxide (ZnO) of group IIB elements may also be used.

Sparsely soluble silicates suitable for forming the phospho-silicate ceramics of the present invention include silicates of Group IIA and IA elements that have a solubility constant between 5 and 25. Once again silicates with a solubility constant between 5-12 are more preferred and silicates such as calcium silicate (CaSiO.sub.3); talc or magnesium silicate (MgSiO.sub.3) barium silicate (BaSiO.sub.3); sodium silicate (NaSiO.sub.3); lithium silicate (LiSiO.sub.3); and serpentinite (Mg.sub.6 Si.sub.4 O.sub.10 (OH.sub.8) are preferred. Calcium silicate has a solubility product constant of approximately 8, which is similar to magnesium oxide, and thus would be a compatible material with the ceramicrete binder, and the most preferred sparsely soluble silicate for forming the phospho-silicate ceramics of the present invention. Calcium silicate, or Wollastonite as it is known in its mineral form, is an inexpensive product, typically 5-10 cents per pound, and is widely available in large amounts. Wollastonite is also available in powder form, is not toxic, and is thus easy to handle and transport. Wollastonite as referred to in the present patent application is defined according to Dana's Manual of Mineralogy, Revised by Cornells Klein and Cornelius S. Hurlbut, Jr. 20.sup.th ed, pub. John Wiley and Sons, New York (1977) pp. 406-408.

The addition of Wollastonite to the ceramicrete binders produced exciting and unexpected results, as the Wollastonite unexpectedly modified the ceramicrete binders significantly. The Wollastonite crystals greatly increased the fracture toughness of the ceramicrete matrix, and amorphous silicate released from the Wollastonite in solution, greatly increased the compression and flexural strength of the matrix while at the same time reducing its porosity and permeability to water.

Individual Wollastonite crystals are acicular or elongated in structure, and when combined within the ceramicrete matrix, the Wollastonite crystals act as whiskers to resist crack propagation. The Wollastonite crystals either stop or divert propagating cracks requiring more energy for the crack to continue, and thus increasing the fracture toughness of the resulting product. Wollastonite also increases the viscosity of the mixture, and overall strength of the resulting phospho-silicate ceramic.

Amorphous silica released from Wollastonite in an aqueous solution chemically reacts with the phosphates of the ceramicrete binder to form a glassy phase within the ceramicrete matrix. This chemical reaction was completely unexpected because typically silicates and silicas, i.e. sand, are stable materials. Silicates and silicas, even do not dissolve in acidic solutions and do not react in an aqueous environment. However, it was observed that the addition of a sparsely soluble silicate such as Wollastonite to phosphate ceramicrete binders provided the very unexpected chemical reactions as outlined below.

Sparsely soluble silicates such as Wollastonite, talc, and serpentinite are slightly alkaline and when combined with water they become ionized, releasing the metal cations. For example, Wollastonite dissolved in acidic water such as solutions of H.sub.3 PO.sub.4, KH.sub.2 PO.sub.4, Al(H.sub.2 PO.sub.4).sub.3 etc., released cations Ca.sup.++ and silicate SiO.sub.3.sup.--. The calcium cations reacted with the phosphates to form calcium phosphates. The silicate anion formed metasilicic acid (H.sub.2 SiO.sub.3) which further reacted with available cations to form K.sub.2 SiO.sub.3 as seen in equation 5. ##EQU1##

The first and third equations demonstrate that the addition of a sparsely soluble silicate such as calcium silicate is a good method for introducing metasilicic acid to a phosphate slurry. The acid reacts subsequently with other available cations, such as 2K.sup.+ as shown in equation 5 to form silicate glass. For example, if sodium dihydrogen phosphate or potassium dihydrogen phosphate was used instead of phosphoric acid water as the provider of phosphate anions, the metasilicic acid will react with either Na.sup.+ or K.sup.+ ions to form alkali metal glass. This alkali metal glass formed within the phospho-silicate ceramic of the present invention is believed to fill the voids between particles of the ceramic and produce a dense solidified non-porous ceramic product. Additionally, the glassy phase within the ceramic product is also believed to bind particles of the product together to produce a hard ceramic, thus increasing both the compression and flexural strength of the resulting product.

After the chemical reaction between the silicate and the ceramicrete binder, at least three products are produced, namely, magnesium potassium phosphate binder (MgKPO.sub.4.6H.sub.2 O), calcium hydrophosphate (CaHPO.sub.4.2H.sub.2 O), and potassium silicate (K.sub.2 SiO.sub.3). The magnesium potassium phosphate provides the bulk strength for the new phospho-silicate ceramic, and the potassium silicate produces a glassy phase that fills the voids between the bulk compounds, resulting in a product that is almost completely dense. This glassy phase provides the benefits of reducing or even eliminating the porosity of the resulting ceramic and smoothing its surface.

The compounds of the phospho-silicate ceramic of the present invention are not as acidic as phosphate cement, nor as alkaline as Portland cement. The compounds are more neutral, less corrosive, and weather better over time. However, similar to Portland cement, the phospho-silicate ceramic of the present invention can be easily made on site by just adding water to a blend of powders. Thus, current construction equipment can be easily used, without extensive modifications, to make the phospho-silicate ceramic of the present invention on a construction site.

The phospho-silicate ceramic of the present invention is manufactured through a unique but relatively simple process of combining a monovalent alkali metal phosphate powder and a sparsely soluble oxide powder, with the sparsely soluble silicate powder in an aqueous solution. The powders are simply blended together. The sparsely soluble silicate powder comprises 1-80% of the powder mixture and preferably 50-60% of the powder mixture by weight. The monovalent alkali metal phosphate and the sparsely soluble oxide powder are combined in the molar ratio range of (0.5-1.5):1 in the widest range, (0.8-1.2):1 in a preferred range, and 1:1 in the most preferred ratio range. Alternatively, an ammonium hydrogen phosphate powder, aluminum hydrophosphate powder or phosphoric acid solution can replace the monovalent alkali metal phosphate powder in the same concentration.

Water is then stirred into the powder blend to form a slurry. A suitable water to powder weight ratio is (1-1.5):2 in the widest range, (1-1.2): 2 in a preferred range and 1:2 in the most preferred range. The slurry is stirred for 10 to 25 minutes at room temperature and left to harden, or alternatively, poured into molds and left to harden. The ceramic material will harden within two hours, and then continue to cure for at least 3 weeks.

Coloring the phospho-silicate ceramic of the present invention can also be achieved by simply adding an inorganic powder pigment to the powder blend before the water is added. Since Wollastonite crystals are white, the phospho-silicate ceramic of the present invention can easily be colored to any desired shade or hue. This attribute can be particularly useful in the construction industry where colored binding materials can provide substantial cost savings and provide great architectural freedom and creativity in designing structures utilizing binding materials of various color schemes.

EXAMPLE 1

In a preferred method, 60% by weight Wollastonite powder, 10% by weight magnesium oxide powder, and 30% by weight potassium dihydrogen phosphate powder were combined to form a powder blend. Water was added to the powder blend in a weight ratio of 1:5 respectively, and stirred for 15-25 minutes at room temperature to form a slurry. The slurry was then poured into plastic syringes of one inch diameter and filled to a volume of 60 cc's and left to cure. The slurry set into a hard ceramic within two hours. The resulting chemically bonded phospho-silicate ceramic was dense, non-porous and homogenous.

EXAMPLE 2

In another embodiment of the invention, 12.5% by weight magnesium oxide powder, 37.5% by weight potassium dihydrogen phosphate powder, and 50% by weight Wollastonite power was mixed to form a powder blend. Water was added to the powder blend in a weight ratio of 1:4 respectively, and stirred for 15-25 minutes at room temperature to form a slurry. The slurry was then poured into plastic syringes having a one inch diameter and filled to a volume of 60 cc's, and left to cure. The slurry set into a hard ceramic within two hours. The resulting chemically bonded phospho-silicate ceramic was dense, non-porous and homogenous.

The following Table compares the mechanical properties of Examples 1 and 2, with a phospho-silicate ceramic/sand sample, a ceramicrete binder sample, a ceramicrete/ash sample, and high strength concrete.
 

             TABLE 1
                               Cure                    Fracture
       Composition (wt. %)     Time   Strengths (psi)  Toughness Water
     Absorption
    Binder* Wollastonite Other   Days   Compres  Flexural  MPa.  m       Wt. %
      40        60      Nil      21     8,426    1,474    0.66           2
      50        50      Nil      14     7,755    1,236    0.63           2
      30        30      Sand 40  11     6,264    1,255    0.63           3.0
      100       Nil     Nil      21   .apprxeq.3,500 .apprxeq.1,100    n/a
      .about.15
      40        Nil     Ash 60   14    11,507    1,474    0.19           1.78
    High Strength concentrate   28     8,000     940      n/a    .apprxeq.10-20
        (literature value)
    *Binder is defined as the mixture of potassium dihydrogen phosphate
     (monopotassium phosphate) and magnesium oxide.


As shown in Table 1, the phospho-silicate ceramics of Examples 1 and 2 have a low water absorption, and thus reduced porosity, compared to high strength concrete and ceramicrete binder. The weight percent water absorption of the phospho-silicate ceramics of Examples 1 and 2 is 1/5-1/10 the weight percent water absorption of high strength concrete, and 1/2-1/8 the weight percent water absorption of ceramicrete binder. It appears that the addition of Wollastonite to the ceramicrete binder reduced the porosity of the resulting phospho-silicate ceramic by a significant margin, thus resulting in a much denser end product. Typically, water absorption is a direct indication of the porosity of a material, however, in the phospho-silicate ceramics as shown in Examples 1 and 2, it is believed that at least some of the water absorption is due to the formation of hydroxides of magnesium and calcium rather than due to the porosity of the ceramic. The actual porosity of the phospho-silicate ceramic of the present invention is believed to be less than that indicated by the water absorption test in Table 1. As a result, it is expected that the phospho-silicate ceramics of the present invention would not experience freeze/thaw deterioration during the temperature fluctuations experienced in cold climates because the phospho-silicate ceramics of the present invention appear fully dense.

As shown in Table 1 above, the phospho-silicate ceramic of Examples 1 and 2 have a flexural strength far superior to the flexural strength of high strength concrete and ceramicrete binder. The phospho-silicate ceramics of Examples 1 and 2 had a flexural strength 30-50% higher than the flexural strength of high strength concrete. The phospho-silicate ceramics of Examples 1 and 2 have a flexural strength 20-40% greater than the flexural strength of ceramicrete binder. Additionally, as shown in Table 1, the phospho-silicate ceramics of Examples 1 and 2 have a compression strength two times the compression strength of ceramicrete binder.

Table 1 also compares the fracture toughness of the phospho-silicate ceramics of Examples 1 and 2, with the ceramicrete binder/ash product. The MPa.m fracture toughness values for phospho-silicate ceramic Examples 1 and 2 are 0.66 and 0.63 respectively, and the fracture toughness value for ceramicrete binder/ash is 0.19. Both phospho-silicate ceramic Examples 1 and 2 exhibit a fracture toughness 3 times greater than the fracture toughness of the ceramicrete/ash product. This result further supports Applicant's theory that the acicular or elongated crystals of Wollastenite act as whiskers to resist crack propagation and thus increase the overall fracture toughness of the resulting product. Ash does not contain elongated crystal structures, and as a result, ash is poor in resisting crack propagation and increasing the overall fracture toughness of the resulting product.

The X-ray diffraction pattern of the phospho-silicate ceramic of Example 1 shown in FIG. 1 exhibited a broad hump in the center of the pattern. This broad hump is believed to be due to the formation of potassium silicate glass (K.sub.2 SiO.sub.3) formed by the chemical reaction between potassium cations and metasilic acid, as shown in equation 5 above. It is believed that this glassy phase fills in the voids and pores in the resulting ceramic of the present invention, and is important for both reducing the open porosity of the ceramic of the present invention, which appears very dense and has a smooth surface, and for increasing the compression and flexural strength of the ceramic of the present invention.

Also shown in the X-ray diffraction analysis, are major peaks that were identified as unreacted calcium silicate, magnesium potassium phosphate binder (MgKPO.sub.4.6H.sub.2 O) and calcium hydrophosphate (CaHPO.sub.4.2H.sub.2 O). Calcium hydrophosphate is likely to be absorbed into the human body and can regenerate body tissues when the phospho-silicate ceramic of the present invention is used in bio-materials. For this reason, the phospho-silicate ceramic of the present invention may be one of the most suitable materials for bio-material purposes such as orthopedic and dental applications.

A scanning electron micrograph of fracture surface of samples of the phospho-silicate ceramic of the present invention is shown in FIG. 2. The micrograph shows the crack propagation of the ceramic is intergranular, with the cracks running around the elongated crystals of the Wollastonite. Such crack deflection by the Wollastonite crystals increases the fracture energy and improves the fracture toughness of the phospho-silicate ceramic of the present invention.

In another embodiment, the phospho-silicate ceramic of the present invention can be sold as separate components grouped into kits for forming phospho-silicate ceramic structures for home as well as industrial use. For example, a typical kit would include a bag or drum, depending on the volume needed, of suitable monovalent alkali metal phosphate powder, suitable sparsely soluble oxide, and suitable sparsely soluble silicate. Each powder is combined in an appropriate amount as described above. The kit could also include an optional bag or drum of inorganic pigment powder to dye the final product to a desired color, or a bag or drum of an aggregate, such as granite, if a particular texture is desired in the final product. Easy to follow instructions would direct the user to combine the powders, add the appropriate amount of water to form a slurry, and add any desired aggregate. The slurry is then poured into a mold, for example a countertop mold for home use, and allowed to set and cure.

Alternatively, the phospho-silicate ceramic can be sprayed onto the surface of a structure for fireproofing, water proofing, etc. The phospho-silicate ceramic will chemically bond to the substrate, making this product far superior to a laminate applied with an adhesive to a substrate.

Other ways of making Wollastonite containing phospho-silicate ceramics include reacting Wollastonite with phosphoric acid solution, or aluminum hydro phosphate solution, but a small amount of boric acid needs to be added as a retardant because phosphoric acid is too reactive. It is also possible to neutralize the phosphoric add with hydroxides or carbonates of an alkali metal such as sodium or potassium, and react it with Wollastonite.

EXAMPLE 3

1. 40 g of Wollastonite, and 67 g of 50 wt. % concentrated phosphoric acid solution neutralized with 15% of sodium carbonate were reacted for 20 min. The slurry warmed up and set into a hard ceramic. We believe, sodium carbonate reacted with phosphoric acid solution to form sodium dihydrogen phosphate which reacted with Wollastonite to form the ceramic. The ceramic contained some glassy phase, probably sodium phosphate and sodium silicate, unreacted Wollastonite, and calcium hydrophosphate.

2. 40 g of Wollastonite, 67 g of 50% concentrated phosphoric acid solution when mixed together reacted instantaneously and formed a precipitate. With addition of 3 g of boric acid, however, the slurry set into a hard ceramic. Thus, it is possible to produce ceramics of Wollastonite without neutralizing phosphoric acid or adding magnesium oxide as done in the previous case studies. The product contained calcium hydrophosphate and unreacted Wollastonite.

3. 80 g of Wollastonite, 100 g of 50% concentrated phosphoric acid solution and 15 g of potassium carbonate were mixed for 10 min. Initial setting was in one hour and complete setting was in 3 days.

4. 100 g of Wollastonite was mixed with 100 g of sodium dihydrogen phosphate and 100 g of water. The slurry was mixed for 30 min. It warmed up and set in two days.

5. 25 g of Wollastonite, 50 g of sodium dihydrogen phosphate, 112.5 g of ash and 80 g of water were mixed for 10 min. The slurry set in two days. The set product contained unidentifiable glassy phase.

All these tests demonstrate that phosphates may be added in different forms to produce Wollastonite containing chemically bonded phospho-silicate ceramics.

The foregoing description has been provided to clearly define and completely describe the present invention. Various modifications may be made without departing from the scope and spirit of the invention which is defined in the following claims.



WAGH PATENTS

COMPOSITION AND APPLICATION OF NOVEL SPRAYABLE PHOSPHATE CEMENT THAT BONDS TO STYROFOAM
WO 2006001891

1-05-2006
WAGH ARUN S (US); PAUL JAMES W JR (US)
Classification: - international: B05D3/02; B05D7/00; C04B2/00; C04B7/00; C04B9/00; C04B12/02; C04B14/00; C04B14/38; C04B18/06; C04B28/30; C04B28/34; B05D3/02; B05D7/00; C04B2/00; C04B7/00; C04B9/00; C04B12/00; C04B14/00; C04B14/38; C04B18/04; C04B28/00; (IPC1-7): C04B28/34; C04B14/06; C04B14/30; C04B18/08; C04B22/00; C04B28/34; - european: C04B28/34
Application number: WO2005US13451 20050418
Priority number(s): US20040868062 20040615
Also published as:   US2005274290 (A1)
Cited documents: WO03031367 // WO0066878 // WO9734848 // EP0203658 // WO0006519

Abstract: A dry mix particulate composition of a calcined oxide of Mg and/or Ca, an acid phosphate, and fly ash or equivalent, wherein the calcined oxide is present in the range of from about 17% to about 40% by weight and the acid phosphate is present in the range of from about 29% to about 52% by weight and the fly ash or equivalent is present in the range of from about 24% to about 39% by weight when sand is added to the dry mix, it is present in the range of from about 39% to about 61 % by weight of the combined dry mix and sand. A method of forming a structural member is also disclosed wherein an aqueous slurry of about 8-12 pounds of water is added to dry mix and sand.


PERMAFROST CERAMICRETE
WO 2005073145

8-11-2005
WAGH ARUN S (US); NATARAJAN RAMKUMAR (US); FISHER BRANDON
Classification:- international: C04B28/34; C09K8/46; C04B28/00; C09K8/42; (IPC1-7): C04B28/34; - european: C04B28/34; C09K8/46
Application number: WO2005US00485 20050107
Priority number(s): US20040538818P 20040123; US20040941592 20040914
Also published as: WO 2005073145 //  US 2005160944
Cited documents: US6136088 // WO0066878 // WO0006519 // US2003131759 // EP0203658

Abstract: A dry mix of a calcined oxide of Ca and/or Mg and an acid phosphate and fly ash with or without insulating extenders useful in permafrost conditions. Calcined oxide is present at about 12% to about 40% by weight and the acid phosphate is present at about 35% to about 45% by weight. The fly ash is present at about 10% to about 50% by weight with the fly ash being between about 50% to about 100% class F with the remainder class C. Insulating extenders are present in the range from 0% to about 15% by weight of the combined calcined oxide and acid phosphate and fly ash. 0.1% to about 0.5% boric acid and/or borate by weight of the dry mix is present.


METHOD OF WASTE STABILIZATION WITH DEWATERED CHEMICALLY BONDED PHOSPHATE CERAMICS
WAGH ARUN S (US); MALONEY MARTIN
EC:   IPC: A62D3/00; B09B3/00; G21F9/00 (+7)
CA 2540293
9-10-2004

CERAMICRETE STABILIZATION OF U-AND PU-BEARING MATERIALS
WAGH ARUN S (US); MALONEY DAVID D
EC:  C04B28/34  IPC: G21F9/04; C04B28/34; G21F9/16 (+5)
CA 2540287
9-02-2004

METHOD AND PRODUCT FOR PHOSPHOSILICATE SLURRY FOR USE IN DENTISTRY AND RELATED BONE CEMENTS
WAGH ARUN S (US); PRIMUS CAROLYN
EC:  A61K6/00  IPC: A61K6/00; A61K6/00; (IPC1-7): A61K6/06 (+1)
EP 1651172
5-03-2006

CHEMICALLY BONDED PHOSPHATE CERAMIC SEALANT FORMULATIONS FOR OIL FIELD APPLICATIONS
WAGH ARUN S (US); JEONG SEUNG-YOUNG
EC:   IPC: C04B28/00; C04B28/00
US 2006048682
3-09-2006

CONSTRUCTION MATERIAL AND METHOD
WAGH ARUN S (US); ANTINK ALLISON L
US 2006003886
1-05-2006

METHOD & PRODUCT FOR PHOSPHOSILICATE SLURRY FOR USE IN DENTISTRY...
WAGH ARUN S (US); PRIMUS CAROLYN
EC:  A61K6/00  IPC: A61K6/00; A61K6/00; (IPC1-7): A61C13/08
US 2005028705
2-10-2005

CANISTER- SEALING METHOD & COMPOSITION FOR SEALING A BOREHOLE
BROWN DONALD W (US); WAGH ARUN S
EC:  C04B28/34; C09K8/46; (+2)  IPC: C04B28/34; C09K8/46; E21B27/02 (+12)
US 2003150614
8-14-2003

CHEMICALLY BONDED PHOSPHATE CERAMICS OF TRIVALENT OXIDES OF IRON AND MANGANESE
WAGH ARUN S (US); JEONG SEUNG-YOUNG
EC:  C04B28/34A; C04B32/00; (+1)  IPC: C04B28/34; C04B32/00; G21F9/16 (+4)
WO 02058077
7-25-2202

FORMATION OF CHEMICALLY BONDED CERAMIC...
WAGH ARUN S (US); JEONG SEUNG-YOUNG
EC:  C04B28/34A  IPC: C04B12/02; C04B35/447; C04B12/00 (+2)
US 2003092554
5-15-2003

CORROSION PROTECTION
BROWN DONALD W (US); WAGH ARUN
EC:  C23C22/73; C23C22/74  IPC: C23C22/73; C23C22/74; C23C22/73 (+1)
US 2002179190
12-05-2002

DOWNHOLE SEALING METHOD AND COMPOSITION
BROWN DONALD W; WAGH ARUN
EC:  C04B28/34; C09K8/46; (+2)  IPC: C04B28/34; C09K8/46; E21B27/02 (+7)
WO 0066878
11-09-2000

PUMPABLE/INJECTABLE PHOSPHATE-BONDED CERAMICS
SINGH DILEEP; WAGH ARUN
EC:  B09B1/00; B09C1/08; (+2)  IPC: B09B1/00; B09C1/08; C04B28/34 (+9)
WO 0006519
2-10-2000

METHOD FOR PRODUCING CHEMICALLY BONDED PHOSPHATE CERAMICS...
SINGH DILEEP (US); WAGH ARUN S
EC:  A62D3/00M10D; A62D3/00K4; (+9)  IPC: A62D3/00; C03C1/00; C03C10/00 (+14)
US 6133498
10-17-2000

POLYMER COATING FOR IMMOBILIZING SOLUBLE IONS...
SINGH DILEEP (US); WAGH ARUN
EC:  A62D3/00; A62D3/00E4; (+5)  IPC: A62D3/00; B09B3/00; C04B41/48
US 6153809
11-28-2000

PUMPABLE/INJECTABLE PHOSPHATE-BONDED CERAMICS
SINGH DILEEP (US); WAGH ARUN
EC:  B09B1/00; C04B28/00; (+3)  IPC: B09B1/00; C04B28/00; C04B28/34 (+9)
US 6204214
3-20-2001

METHOD OF WASTE STABILIZATION...
WAGH ARUN S; JEONG SEUNG-YOUNG
EC:   IPC: B09B; C02F; C04B (+9)
ZA 9708254
6-10-1998

METHOD OF WASTE STABILIZATION VIA CHEMICALLY BONDED PHOSPHATE CERAMICS, STRUCTURAL MATERIALS INCORPORATING POTASSIUM PHOSPHATE CERAMICS
WAGH ARUN S (US); SINGH DILEEP
EC:  B09B1/00; C04B28/00; (+3)  IPC: B09B1/00; C04B28/00; C04B28/34 (+10)
WO 9734848
9-25-1997

PHOSPHATE-BONDED STRUCTURAL PRODUCTS FROM HIGH VOLUME WASTES
SINGH DILEEP (US); WAGH ARUN S
EC:  C04B28/00; C04B28/34; (+1)  IPC: C04B28/00; C04B28/34; C04B35/63 US 5846894
12-08-1998

QUICK-SETTING CONCRETE & METHOD FOR MAKING
WAGH ARUN S (US); SINGH DILEEP
EC:  C04B7/36; C04B22/00H; (+1)  IPC: C04B7/36; C04B22/00; C04B40/00
US 5624493
4-29-1997

METHOD FOR STABILIZING LOW-LEVEL MIXED WASTES AT ROM TEMPEPRATURE
WAGH ARUN S (US); SINGH DILEEP
EC:  A62D3/00; C04B28/34; (+3)  IPC: A62D3/00; C04B28/34; G21F9/16
US 5645518
7-08-1997

CERAMICRETE STABILIZATION OF U-AND PU-BEARING MATERIALS
CA 2540287
9-02-2004
WAGH ARUN S (US); MALONEY DAVID D (US); THOMPSON GARY H (US)
Classification: - international: G21F9/04; C04B28/34; G21F9/16; G21F9/28; C04B28/00; G21F9/04; G21F9/16; G21F9/28;- european: C04B28/34
Application number: CA20042540287 20040218
Priority number(s): US20030448792P 20030218; WO2004US04885 20040218
Also published as: WO2004075207 // WO2004075207 // EP1597736 // EP1597736 //

 


US Patent # 5,830,815

Method of Waste Stabilization via Chemically Bonded Phosphate Ceramics
Arun S. WAGH, et al.
US Cl. 501/155

Abstract: A method for regulating the reaction temperature of a ceramic formulation process is provided comprising supplying a solution containing a monovalent alkali metal; mixing said solution with an oxide powder to create a binder; contacting said binder with bulk material to form a slurry; and allowing the slurry to cure. A highly crystalline waste form is also provided consisting of a binder containing potassium and waste substrate encapsulated by the binder.

References Cited:
U.S. Patent Documents
3093593 ~ 3383228 ~ 3879211 ~ 3985567 ~ 5302565 ~ 5502268 ~ 5645518

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for stabilizing large volumes of waste, and more specifically, this invention relates to a ceramic material to stabilize large volumes of low-level radioactive and mixed wastes and a method for producing the ceramic material.

2. Background of the Invention

Low-level mixed wastes contain hazardous chemical and low-level radioactive materials. Generally, mixed waste streams contain aqueous liquids, heterogeneous debris, inorganic sludges and particulates, organic liquids and soils. The projected volume over the next five years of the mixed waste generated by the U.S. Department of Energy alone is estimated at approximately 1.2 million cubic meters.

Stabilization of these mixed wastes requires that both phases of contaminants are stabilized effectively.

Typical approaches to stabilization and storage of these mixed wastes include vitrification. For example, one process (Crowe, U.S. Pat. No. 5,302,565) requires firing temperatures of at least 1,850.degree. C. for at least 12 hours to produce ceramic containers. However, such processes, associated with high temperatures are costly. In addition, vitrification of waste streams often result in the lighting off of volatile components contained in the waste stream. This lighting off results in the unwanted generation of secondary waste streams.

One system for producing cements having ceramic type properties does not require high temperatures for final crystallization (Sugama et al. U.S. Pat. No. 4,436,555, assigned to the instant assignee). However, that process results in ammonia being liberated during processing and storage, which leads to container corrosion, and also explosive compositions if wastes contain nitrates.

The inventors also have developed ceramic fabrication methods to both stabilize and encapsulate waste. These methods offer a number of advantages over typical portland cement grout-, polymer- and ceramic-encapsulation techniques. Ceramic encapsulation systems are particularly attractive given that the bonds formed in these systems are either ionic or covalent, and hence stronger than the hydration bonds in portland cement. Since waste stabilization using ceramics is due to chemical stabilization as well as physical encapsulation, the leaching characteristics of these final waste forms are superior to the above-identified waste forms which are mainly dependent on physical encapsulation. Also, unlike prior vitrification requirements, the exothermic ceramic formulation process needs no thermal treatment or heat input, resulting in waste stabilization being done economically on site and without capital intensive equipment and transportation procedures.

However, exothermic ceramic formulation processes are not suitable for the economic encapsulation of large amounts of waste. The inventors have found that the production of large amounts of heat during reaction causes the reacting solution to boil, leading to flaws (i.e. pores) in the final ceramic form, short workability time, and fast, uneven curing. While reaction temperatures may be partially controlled by circulating cold water around the slurry container or mold in which the sample is setting, sufficient heat conduction is not present as sample sizes increase.

Another drawback to typical ceramic waste production processes is that such systems foster low pH conditions. For example, acid-base ceramic encapsulation reactions begin in severe acidic conditions, near pH 0. Such severe conditions destabilize HgS to a leachable form prior to its physical encapsulation. Low pH conditions also lead to CaCO.sub.3 decomposition.

A need exists in the art for a high volume waste stabilization and solidification method that does not generate high amounts of heat during the encapsulation process. The process must also be operational at moderate pH conditions so as to facilitate stabilization of wastes which are unstable at low pH. The final product must exhibit low leachability and high durability in aqueous systems.

SUMMARY OF THE INVENTION

An object of the present invention is to overcome many of the disadvantages of the prior art in the encapsulation and stabilization of low-level, radioactive, mixed and other wastes.

Another object of the present invention is to provide a temperature-controlled ceramics formation process to encapsulate and stabilize wastes. A feature of the invention is the utilization of readily available compounds to regulate the acid-base reactions associated with the formation of ceramics waste forms. An advantage of the invention is maintaining a low temperature during the formation process.

Yet another object of the present invention is to provide a low temperature reaction liquor in a process to stabilize mixed waste using chemically bonded phosphate ceramics. A feature of the present invention is the moderation of the pH of the reaction liquor. An advantage of the present invention is that the lower reaction temperatures facilitate the formation of more dense waste forms. Another advantage is that certain waste materials, which decompose or destabilize in low-pH environs, are more completely stabilized.

Still another object of the present invention is to provide a ceramic waste form high in potassium. A feature of the invention is a high amount of crystalline phase in the final waste form. An advantage of the invention is a more dense, less porous waste form.

Briefly, the present invention provides for a method for regulating the reaction temperature of a ceramic formulation process comprising supplying a solution containing a monovalent alkali metal; mixing said solution with an oxide powder to create a binder; contacting said binder with bulk material to form a slurry; and allowing the slurry to cure.

The invention also provides for a ceramic waste form comprising a potassium containing ceramic binder and waste substrate encapsulated by the binder.

BRIEF DESCRIPTION OF THE DRAWING

These and other objects and advantages of the present invention will become readily apparent upon consideration of the following detailed description and attached drawing, wherein:

FIG. 1 is a temperature graph showing the effects of the addition of a carbonate solution to the ceramic processing liquor, in accordance with the features of the present invention;

FIG. 2 is a graph showing the compression strength of an exemplary waste form, in accordance with the features of the present invention; and

FIG. 3 is a graph depicting the porosity of an exemplary ceramic form, in accordance with the features of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention teaches two processes for chemically controlling the reaction temperature in ceramic formulation processes. These two processes allow for the formation of large final waste forms for a wide variety of waste streams, said waste streams containing ash, cement, silica, Bayer process wastes (red mud), potliner residue, pyrophorics, salt mixtures, volatiles, such as mercury, lead, cadmium, chromium, and nickel, and unstable compounds which cannot be treated by conventional high temperature techniques such as vitrification. The invention is also applicable to stabilize secondary waste streams resulting from thermal treatment processes, such as vitrification and plasma hearth processes.

Radioactive materials are also stabilized by this method, such materials including uranium, plutonium, thorium, americium, fission products, and any other radioactive isotopes. Irradiated lead, hazardous metals, flue-gas desulfurization residues are also stabilized and/or encapsulated by the invented method.

The invention also can be used to stabilize certain RCRA organics. The inventors have found that certain of these organics do not retard the setting of phosphate ceramics. In one scenario, organics such as naphthalene and dichlorobenzene are trapped in activated carbon which in turn is stabilized in the phosphate matrix by the method claimed herein. This method of stabilization can be utilized in situations wherein mixed waste contains trace amounts of organics such as polychlorinated biphenyls, dioxin, dichlorobenzene, naphthalene, among others. As such, the invented method is superior to encapsulation methods wherein cement is utilized, in that cement cannot stabilize in the presence of organics.

The method may also be used to stabilize and solidify wastes containing salts, such as chlorides, nitrates, nitrides, sulfites and sulfates. Conventional cement technology cannot stabilize these waste streams.

Ash waste may be consolidated by this process to 80 volume percent of its original volume. Experiments by the inventors show good reaction and bonding between amorphous and reactive silica from fly ash and bottom ash with phosphate matrix. Formation of hard silico-phosphate bonds via this reaction can be used for the stabilization of hazardous silica compounds such as asbestos. The invention also encapsulates and stabilizes silica based filter aids, such as vermiculites and perlites, which are used in the removal of contaminants from liquid waste streams.

The two invented temperature control processes yield superior-strength final forms having uniform high density throughout and improved microstructure compared to typical methods of ceramics formation.

A salient feature of the low-temperature ceramic-waste formulation processes is an acid-base reaction, such as that depicted in Equation 1, below. Typically, the reaction produces phosphate of MgO (Newberyite).

The acid base reaction results in the reaction of the waste components with the acid or acid-phosphates. These reactions lead to chemical stabilization of the waste. In addition, encapsulation of the waste in the phosphate ceramics formed by the reaction products results in physical encapsulation of the waste components.

As noted supra, a problem with the above-disclosed reaction sequence is the extremely low pH that exists in the reaction liquor as a result of the presence of the phosphoric acid. This low pH leads to destabilization of some waste materials during encapsulation, and higher reaction temperatures which ultimately renders weak final waste forms.

The two processes for minimizing the exothermicity of the acid-base reactions are disclosed as follows: Process #1 deals with pretreating phosphoric acid with a carbonate, bicarbonate or hydroxide of a monovalent metal prior to mixing with an oxide or hydroxide powder so as to buffer the acid. An exemplary reaction for process #1 is illustrated in Equation 2, below:

whereby M is a monovalent metal which can be selected from the group consisting of potassium, sodium, lithium. M'oxide designates the oxide powder, whereby M' is a metal which can be selected from the group consisting of Mg, Al, Ca, and Fe. As noted above, M' also can be supplied as an hydroxide.

Process #2 discloses a method for bypassing the use of acid altogether and mixing the oxide powder with a dihydrogen phosphate to form a ceramic at a higher pH. Illustrations of process #2 are Equations 3-5, below:

Solid Waste Preparation Detail

Solid wastes first can be manipulated in powder form by grinding the waste to an average, preferable approximate particle size of 8 to 10 micrometers (.mu.m). However, particles can range in size from between approximately 5 .mu.m to several millimeters.

Ash and cement wastes can be first mixed with the starter oxide or hydroxide powders using a vibratory shaker, or any conventional agitator. Weight percentages of the mixture varies at this juncture, but can range from between approximately 15 percent oxide to 50 percent oxide. Typically, an even weight percent (50:50) of oxide to solid waste is sought. However, the inventors have successfully encapsulated and stabilized single-component fly ash at weight percents as high as 85 percent ash to 15 percent MgO powder, which makes this technique particularly attractive for utilities where single-component fly ash is a major land-filling problem.

The above mixture of powders is then added to pretreated phosphoric acid solution (process #1) or to the dihydrogen phosphate solution (process #2) to form a reaction slurry. The slurry is mixed using a mixer for 10 minutes to 30 minutes during which it forms a viscous paste. The paste sets in a few hours once poured into a mold. Typically, no pressure is applied to the now-molded slurry. The slurry gains full strength in approximately one day.

Mold shapes can vary, depending on the configuration of the ultimate deposition site, and can be selected from a myriad of geometrical shapes including cuboid, pyramidal, spherical, planar, conical, cylindrical, trapezoidal, rectangular, and the like. Generally, molds having the shape and size of a typical 55 gallon drum are used for waste management applications.

Liquid Waste Processing Detail

In dealing with liquid waste, the invented temperature regulated encapsulation method provides a simplified approach for an end user compared to more typical encapsulation methods. For example, acid phosphates systems are made by adding said phosphate to the liquid on site, a process similar to that practiced in the cement industry. As such, liquid wastes, such as tritiated water, are easily and economically encapsulated with this procedure.

Either process #1 or process #2 can be used if solely liquid is being encapsulated and stabilized. In process #1, the waste liquid is first combined with acid to form a pH modified solution. This modified solution is then mixed with oxide powder. Alternatively, the waste liquid can be added to oxide powder, to form a slurry, and then mix the slurry with acid.

In process #2, the liquid waste is mixed with dihydrogen phosphate solution. Then, oxide powder is added. As above, an alternative procedure is to first combine the liquid waste with oxide powder and then add the dihydrogen solution.

The inventors have found that the ratio of acid to water, selected from a range of between approximately 37:63 to 50:50, produces good results. An acid:water ratio of 50:50 is most preferred. If the liquid waste contains more than the required amount of water, then correspondingly less water is added to the acid to bring the water weight percent of the liquid waste-acid mixture up to 50 percent.

In situations involving liquid-solid waste streams, the liquid fraction of the waste stream can be prepared as outlined directly above. The resulting liquid waste-acid mixture is then mixed with a mixture of solid waste and oxide powder in weight percent ranges similar to those outlined above for solid waste processing. When using powder mixtures containing MgO and dibasic phosphate, weight percent ratios of the oxide to the phosphate selected from the range of approximately 87:13 to 77:23 produce good results.

Phosphate and Oxide Reactant Detail

Several phosphate systems can be used for the stabilization of the target chemical, radioactive and mixed waste streams. Some final phosphate-ceramic forms include, but are not limited to phosphates of Mg, Mg--Na, Mg--K, Al, Zn and Fe, whereby the metals are derived from starter oxide powders and hydroxide powders (such as in process #1). In process #2, the metals in the final phosphate ceramic forms are derived from both the starter powders and the dihydrogen phosphates. Exemplary dihydrogen phosphates used in process #2 include, but are not limited to, phosphates of potassium, sodium and lithium. The acid component may be concentrated or dilute phosphoric acid or acid phosphate solutions such as dibasic or tribasic sodium or potassium, or aluminum phosphates. The setting times for the pastes formed by the reaction ranges from a few hours to a week. The phosphates attain their full strength in approximately three weeks.

Oxide powders can be pretreated for better reactions with the acids. One technique includes calcining the powders to a typical temperature of between approximately 1,200.degree. C. and 1,500.degree. C. and more typically 1,300.degree. C. The inventors have found that the calcining process modifies the surface of oxide particles in a myriad of ways to facilitate ceramic formation. Calcining causes particles to stick together and also form crystals; this leads to the slower reaction rates that foster ceramic formation. Fast reactions tend to form undesired powdery precipitates.

Another reaction enhancement technique is washing the powders with dilute nitric acid and then water.

A myriad of oxide and hydroxide powders can be utilized to produce the ceramic system, including but not limited to MgO, Al(OH).sub.3, CaO, FeO, Fe.sub.2 O.sub.3, and Fe.sub.3 O.sub.4.

MgO and Al(OH).sub.3 powders are available through any commercial supply house, such as Baxter Scientific Products, McGaw Park, Ill. The myriad iron oxides enumerated above could actually be supplied as part of some waste streams such as those generated in conjunction with soil and also in low-temperature oxidation systems which destroy organics using iron compounds.

Process #1--pH Modification of Acid Solution

Surprisingly and unexpectedly, the inventors have found that when carbonate, bicarbonate, or hydroxides of monovalent metals (such as K, Na, Li, and Rb) are used to pretreat the acid prior to the acid-base reaction, a decrease in reaction temperature results. Also unexpectedly, the inventors have found that the addition of potassium containing alkali compounds (such as K.sub.2 OO.sub.3) result in a more crystalline waste form that is impervious to weathering, compressive forces and leaching.

Furthermore, and as can be determined in FIGS. 1-3, the higher the concentration of potassium containing compounds (such as K.sub.2 OO.sub.3, KHCO.sub.3, and KOH) in the pre-reaction mixture, the more crystalline the final product. This high crystallinity correlates to higher compression strength and lower porosity.

The carbonate in the pretreatment process decomposes into hydroxide, with an evolution of CO.sub.2. This results in a partial neutralization of the acid, which in turn reduces the rate of reaction and the rate of heat evolution. Typically, pH of the reaction slurry is raised from zero to between approximately 0.4 and 1.

Overheating of the slurry is thus avoided by this pH adjustment mechanism. Second, and as more thoroughly disclosed infra, the use of potassium carbonate generates more crystalline, and therefore more stable, phosphate complexes.

EXAMPLE 1

K.sub.2 OO.sub.3 Buffer

5, 10 and 15 weight percent of potassium carbonate K.sub.2 OO.sub.3 was added to a 50 weight percent dilute solution of phosphoric acid. The resulting solution was allowed to equilibrate for several hours. In the equilibration process, the pH of the solution raised from near zero to 0.4, 0.6 and 0.9, respectively. After equilibration, 100 grams of the solution was mixed with 50 grams of an oxide powder. The oxide powder was a combination of calcined MgO and boric acid in a 85 weight percent MgO to 15 weight percent boric acid ratio.

While adding the MgO and boric acid mixture to the acid solution, the temperature of the slurry, for phosphate concentrations ranging from 0 to 10 weight percent, was monitored. FIG. 1 depicts the temperature rise in each case. System A was a simulation of a process wherein no K.sub.2 OO.sub.3 was added. The maximum temperature reached in this system was 45.degree. C. in a 50 cc volume sample. For samples B and C made with 5 and 10 weight percent of K.sub.2 OO.sub.3, the temperature rise was 8.degree. C. and 2.degree. C., respectively. No temperature increase was noted when 15 weight percent of K.sub.2 OO.sub.3 was added to the acid prior to reaction.

X-ray diffraction analysis of the samples showed high crystallinity with samples made with 15 weight percent of K.sub.2 OO.sub.3. Samples made with 5 and 10 weight percent of K.sub.2 OO.sub.3 were more glassy. As can be noted in Table 1, below, the X-ray diffraction studies of the samples identified unique mineral phases that are responsible for the desired low solubility product constant of the final product. This superior final product, a chemically bonded composite ceramic, is designated hereafter as MKHP.

TABLE 1 ______________________________________ Mineral composition of Ceramic Developed Via K.sub.2 CO.sub.3 Addition Mineral Phase Chemical formula weight % ______________________________________ Magnesium potassium phosphate MgKPO.sub.4.6H.sub.2 O 52 Lunebergite Mg.sub.3 B.sub.2 (PO.sub.4).sub.2 (OH).sub.6.6H.sub.2 24 Newberyite MgHPO.sub.4.3H.sub.2 O 14 Residual Magnesium Oxide MgO 10 ______________________________________

Surprisingly and unexpectedly, magnesium potassium phosphate (MKP) is a new component in the material that formed exclusively by the addition of K.sub.2 OO.sub.3. MKP represents a superior phase for waste form matrix materials, given its solubility constant of 10.sup.-11, which is five magnitudes lower than that of newberyite which is 10.sup.-6. All of the phases depicted in Table 1 have very low solubilities in ground water, and lunebergite and newberyite are natural minerals which are hence stable in ground water environments.

Porosity characteristics of the samples varied widely. In the K.sub.2 OO.sub.3 5- and 10-weight percent samples, the glass phase of the samples was abundant, with a concomitant higher amount of cracking and therefore porosity. By comparison, the K.sub.2 OO.sub.3 15 weight percent samples showed an open porosity of approximately 6.1 percent. Density was 1.77 g/cc, and closed porosity was 10.2 volume percent. Compression strength was approximately 3,700 psi.

EXAMPLE 2

K.sub.2 OO.sub.3 Buffer+Fly Ash

The matrix material disclosed in Example 1 was used in Example 2. Starter powder composition was 70 weight percent fly ash, 25.5 weight percent calcined MgO, and 4.5 weight percent boric acid. The solution used was a 50 weight percent diluted H.sub.3 PO.sub.4 buffered with K.sub.2 OO.sub.3. The solution was poured into a mixer, such as a cement mixer, and the powder was slowly added until all the powder was mixed with the solution in approximately 48 minutes. A cylindrical sample of 1,000 ml was made.

The maximum temperature during mixing and setting ranged from between approximately 50.degree. C. and 60.degree. C. These temperatures did not increase, even when smaller weight percents of K.sub.2 OO.sub.3 were used. For example, when 10 weight percent of K.sub.2 OO.sub.3 was used, even at higher volumes (1,200 cc), maximum temperatures attained were between 56.degree. C. and 58.degree. C.

The inventors have found that in the absence of K.sub.2 OO.sub.3, the concentration of MgO in the final product is high, with Newberyite as the main crystalline phase in the material.

Data on compression strength and porosity of the materials made in Example 2 are shown in FIGS. 2 and 3. These figures show that as the content of K.sub.2 OO.sub.3 increases, the strength increases and the porosity drops. When K.sub.2 OO.sub.3 is 15 weight percent in the solution, the compression strength is 8,750 psi (which is more than twice that of portland cement) while porosity is reduced to 7.5 percent.

EXAMPLE 3

Sodium Carbonate

5, 10, and 15 weight percent of sodium carbonate (Na.sub.2 OO.sub.3) was added to 50 weight percent of a dilute solution of phosphoric acid and the resulting solution was allowed to equilibrate for several hours. The pH of the solution was raised in the process from near zero to approximately 2.3. 100 grams of this solution was reacted with 30 grams of a mixture of calcined MgO and boric acid (85 weight percent MgO and 15 weight percent boric acid) and 70 grams of fly ash.

The properties of the 5 weight percent Na.sub.2 OO.sub.3 sample were measured. Density was 1.7 g/cc and its open porosity was 8.6 volume percent. Microstructural analysis of the samples revealed that the sample was primarily glassy except for the fly ash particles. This process shows that completely glassy phase material can be made by the process described above.

EXAMPLE 4

Hazardous Material+MKHP

Two different hazardous material waste streams were treated. An iron oxide-iron chloride waste stream (95 weight percent Fe.sub.2 O.sub.3 +5 weight percent FeCl.sub.3) was spiked with 0.5 weight percent of Ce.sup.3+ and Ce.sup.4+ as surrogates of U.sup.3+,.sup.4+ and Pu.sup.3+,.sup.4+, incorporated as oxide. Also added was 0.5 weight percent of Ce.sup.4+ as a surrogate of U.sup.4+ and Pu.sup.4+, incorporated as oxide.

The second waste stream was iron phosphate waste stream (FePO.sub.4) spiked with 0.5 weight percent of Pb to represent hazardous component, introduced as soluble nitrates.

Both waste streams were stabilized via the carbonate modification method of Process #1. Containment of Ce.sup.3+, Ce.sup.4+ and Pb was 8.7 ppm, <0.09 ppm and <0.2 ppm, respectively. In as much as the 5 ppm regulatory limit on Pb is due to be revised downward to 0.37 ppm, the results show that the invented encapsulation procedure provides an acceptable method of containment.

Process #2--Dihydrogen Phosphate

Instead of adding carbonate to reactants to reduce reaction temperatures, the inventors have devised a simplified method to achieve the same results. This second process reacts dihydrogen phosphates of potassium, sodium, lithium, or any other monovalent alkali metal with an oxide to form a phosphate ceramic. This method forms a ceramic at higher pH while minimizing heat generation. An exemplary ceramic formed via this process is magnesium potassium phosphate hexahydrate (MKP), which is formed via the reaction mechanism depicted in Equation 5, above.

The inventors found that with the avoidance of acid in the initial reaction slurry, initial pH values are approximately 6.2. Consistent with the fact that the dissolution of KH.sub.2 PO.sub.4 is an endothermic process, the inventors found that at initial mixing, the temperature of the slurry slightly decreases. As the dissolution and reaction of MgO progresses, however, slurry temperatures increase to approximately 30.degree. C.

EXAMPLE 5

MKP Ceramic Fabrication

One mole of calcined and ground MgO was mixed with one mole of ground potassium dihydrophosphate (KH.sub.2 PO.sub.4) crystals. The mixture was slowly added to 5 moles of water to form a paste. When the paste was well mixed, it was poured into cylindrical molds, of 1 cm in diameter and 20 cc volume. Hard ceramic forms developed in approximately 1 hour.

X-ray diffraction analysis revealed that all major peaks were MKP. No peaks of the potassium dihydrophosphate were noted, indicating that it all reacted.

Open porosity, measured by the water intrusion method, was calculated as 2.87 volume percent. Density was 1.73 g/cc. Given a theoretical density of 1.88 g/cc, the total porosity is calculated to be 8.19 volume percent. Thus, closed porosity (i.e., that porosity that is not accessible from outside the sample) was 5.33 g/cc.

These values show that MKP is much denser than Mg-phosphate ceramic, wherein total porosity is approximately 30 percent.

EXAMPLE 6

MKP+Fly Ash

MKP ceramic synthesized in Example 5 was used to develop waste forms of fly ash. Samples were made using three different powders which are mixtures of calcined MgO and KH.sub.2 PO.sub.4 in mole ratios of 1:1, 1.5:1 and 2:1. These powders were mixed with fly ash in equal weight proportions using a hopper and feeder mechanism. The final mixtures of the powders were combined at a slow but constant rate with 5 moles of water in a cement mixer to form a slurry.

The slurry was poured into 1.5 gallon molds as well as 1 liter molds. Smaller samples were made by stirring the powders in water and using 1 cm diameter, 20 cc cylindrical molds. All samples set in approximately 1 hour and hardened fully after one week.

Unlike the material described in Examples 1 and 2, the temperature of the slurry does not rise during mixing, but only during setting. This eliminates the prior art problem of evaporation of contaminants that occurs as a result of heat generation during the mixing stage. The inventors found that temperatures of the slurry before setting generally do not exceed 30.degree. C. As such, no evaporation of either the water fraction or the components of the waste occur. Once the slurry starts setting, the temperature rises. However, maximum temperatures (approximately 75.degree. C.) are reached after the sample sets into a hard monolith, thereby not resulting in any detrimental effect on the final waste form. Furthermore, the inventors have found that the temperature rise is not proportional to the size of specimens but in fact tapers off as the specimen size is increased. This facilitates the target waste encapsulation sizes of 55 gallons.

The invented process utilizing MKP generates superior final ceramic forms. Open porosity values of the waste forms was found to be approximately 4.18 volume percent. Measured density was 1.8 g/cc. Given the estimated theoretical density of 2.05 g/cc, the total porosity is 8.9 volume percent, which is much lower than Mg-phosphate ceramic found in the prior art. Closed porosity was calculated as 4.72 volume percent. Compression strength of the sample was 6,734 psi, which is more than 50 percent stronger than portland cement concrete.

EXAMPLE 7

Boric Acid+MKP+Ash

Samples of fly ash waste forms were made with MKP matrix and calcined MgO powder in which from 0-5 weight percent boric acid was added. The addition of boric acid delayed the temperature rise of the reacting slurry. Therefore, the addition of boric acid facilitates the large scale processing of waste streams where more time is needed to mix and pour the slurry.

EXAMPLE 8

CaCO.sub.3 Stabilization

As noted supra, CaCO.sub.3 decomposes in low pH environs. As a result, if waste streams contain this compound, carbon dioxide is produced which bubbles from the reaction slurry. Such bubbling makes the set product porous and hence permeable to ground water. Strength is also compromised.

Cement sludge, typical of cement-containing waste streams was prepared. The composition of the waste stream is depicted in Table 2, below:

TABLE 2 ______________________________________ Cement Waste Stream Composition Component Weight Percent ______________________________________ Activated Carbon 10 Fly Ash 10 Water 10 Concrete 50 Plaster of Paris 10 Haematite (Fe2O3) 3 Alumina 3 Perlite 1.5 ______________________________________

Samples were made by two methods. In the first method, slurry was formed with H.sub.3 PO.sub.4 as the reacting acid and waste forms containing approximately 30 volume percent of waste were fabricated. During this first process, the slurry formed tiny bubbles of CO.sub.2, which made the samples porous.

When samples were made with the MKP process disclosed in Examples 5 and 6, supra, wherein KH.sub.2 PO.sub.4 was used as the acid phosphate, no evolution of CO.sub.2 occurred. Comparison of the sample values are presented in Table 3, below:

TABLE 3 ______________________________________ Physical properties of chemically bonded waste forms. Parameter H.sub.3 PO.sub.4 Stabilization KH.sub.2 PO.sub.4 Stabilization ______________________________________ pH of acid soln. 0.2 4 Open porosity 28-33 6.2 (volume percent) Density 1.2-1.3 1.77 ______________________________________

Table 3 shows that the waste forms generated via the MKP process are denser and contain relatively small amounts of open porosity, thereby illustrating the superiority of the invented process compared to processes whereby large amounts of acid are utilized.

EXAMPLE 9

Red Mud+MKP

The refining of bauxite to produce aluminum oxides results in the production of large amounts of residue, known as red mud. Red mud consists of 50 percent inorganic oxides, other compounds and hazardous metals. Tremendous amounts of red mud are generated annually.

Large volumes of red mud are easily stabilized when combined with the invented phosphate ceramic binder. If the reaction slurry, loaded with red mud, is poured as a barrier layer, it not only bonds with the substrate soil but also enters fissures in the soil and quickly hardens to form a nonporous ceramic layer. For example, red mud ceramics produced by the invented process exhibit low porosities (.apprxeq.2 volume percent) and high compression strengths (4,944 psi). The materials display a low porosity and high durability in a range of acid and basic environments, thereby making them ideal for mining industry applications, pond liners, tailing liners, waste pond dikes, and quick-setting grouts.

A myriad of red mud waste can be utilized in producing final structural forms. Red mud waste used by the inventors was produced from gibbsitic bauxite. Essentially, it was dry mud collected from the periphery of a red mud waste pond. Its contents were .apprxeq.50 weight percent iron oxide (Fe.sub.2 O.sub.3), .apprxeq.16.5 weight percent alumina (Al.sub.2 O.sub.3), .apprxeq.3 weight percent silica (SiO.sub.2), .apprxeq.5.7 weight percent calcium oxide (CaO), and .apprxeq.6.8 weight percent titania (TiO.sub.2). X-ray diffraction analysis identified haematite (.alpha.-Fe.sub.2 O.sub.3), goethite (.alpha.-FeOOH), calcite (CaCO.sub.3), boehmite (.gamma.-AlOOH), anatase (TiO.sub.2), and bayerite (.beta.-AlOOH) as the major crystalline phases. Surprisingly and unexpectedly, the inventors have learned that the hydrated phases, i.e., boehmite, bayerite and goethite, facilitate the development of phosphate bonds in the binding process.

The amorphous characteristics of the alumina and silica components of red mud, which is discussed above, plays a major role in the ceramic bonding mechanisms. The inventors have found that it is the characteristically smaller particles of amorphous material that readily participates in the acid-base reaction and therefore facilitates the setting reaction during ceramic formation.

As discussed supra, MgO, when reacted with phosphoric acid or an acid phosphate solution, forms magnesium phosphate precipitate in an exothermic reaction. This reaction can be controlled by use of calcined MgO and also by adjusting the feed rate of the solid phase (i.e., the red mud powder+oxide+boric acid) to the solution.

In one embodiment, calcined MgO first is mixed with red mud powder in a specific weight percent, disclosed in Table 4, below. Crushed dry red mud is a super-fine material with more than 60 weight percent of the particles finer than 10 mm. Particle sizes ranging from between 1 and 5 mm provide good results, so that grinding of the mud may not always be necessary prior to combining with the MgO.

TABLE 4 ______________________________________ Physical Properties of Red Mud Ceramics Waste Maximum Open Compression loading particle size Density Porosity Strength (wt %) (mm) (g/cm.sup.3) (%) (psi) ______________________________________ 40 5 2.19 0.82 4944 40 1 2.1 1.09 4294 50 5 2.26 2.98 2698 55 5 2.29 1.94 2310 ______________________________________

The dry mixture is then reacted with the phosphoric acid or an acid phosphate solution via constant stirring. This results in a low-viscosity paste which thickens as the reaction proceeds. The paste is then poured into cylindrical molds of 1.9 cm in diameter. Dense ceramics form in approximately 15 minutes, with complete hardening occurring in 2 to 24 hours. Prior to testing, the samples were stored for three weeks.

Density was measured by weighing the samples and measuring the dimensions and determining the volume. Open porosity was determined by water immersion in which the pre-weighed samples were immersed in water at 70.degree. C. for 2 hours. The samples were then cooled in the water and then removed from the water. Excess water was wiped from the surface of the samples and the samples were weighed again to determine the amount of water that filled the open pores. This higher weight (compared to pre-immersion weights) yielded the volume of the open pores in the samples, thereby allowing for calculation of the open porosity.

Compression strength was measured with an Instron.TM. machine used in compression mode.

Waste loadings of the samples ranged from 40 to 55 weight percent. As an example, a 40 weight percent waste loading means that 40 grams of a sample is red mud and 60 grams is both binder and water. Densities of samples with red mud are slightly higher than that of pure matrix (binder) material, which is 1.73 gm/cm.sup.3. Red mud density is approximately 3.3 g/cm.sup.3.

As can be determined in Table 2, the open porosity of the red-mud-loaded samples was low compared to the .apprxeq.20 percent value seen in cement. Furthermore, the compression strength of the samples with 40 weight percent loading was found to be higher than the 4,000 psi value for portland cement concrete.

Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses of a fractured red-mud sample revealed a glassy region and a granular region. Both of these regions are well bonded. The glassy phase was cracked everywhere while the granular phase displayed only those cracks emanating from the glassy phase. Table 5, below, provides the general elemental composition of each phase. The values contained therein are averages of three measurements taken at three different locations of each phase.

The granular phase is attributable to the red mud and the glassy phase is mostly the phosphate matrix. As shown, the granular phase was found rich in red-mud elements such as Fe and Al, while the glassy phase is rich in Mg phosphate elements such as Mg and P.

TABLE 5 ______________________________________ Elemental distributions of glassy and granular phases of red-mud ceramics Elements Phase Fe Al Mg P Other ______________________________________ Glassy 5.78 4.7 23.53 34.23 31.76 Granular 23.66 18.6 5.3 16.2 36.2 ______________________________________

That significant amounts of phosphate and some magnesium are also in the granular phase indicates that phosphate binding occurred here with Fe and Al as the cations.

The relatively few cracks seen in the granular phase (red mud) portion of the samples indicates that the strength of the invented material is due to this phase. Improving the strength of the ceramic therefore entails reducing the amount of the glassy phase or reinforcing the glassy phase with particulates. One method for such reinforcement is to incorporate finer red mud in the starter powder so as to facilitate more consistent distribution and better particle reinforcement. Grinding the red mud prior to mixing with the oxide powder is one way to obtain this finer red mud material.

While the invention has been described with reference to details of the illustrated embodiment, these details are not intended to limit the scope of the invention as defined in the appended claims.

For example, in as much as process #2 utilizes KH.sub.2 PO.sub.4, and in as much as KH.sub.2 PO.sub.4 has components of common fertilizer, i.e. potash (K.sub.2 O) and phosphate (P.sub.2 O.sub.5), process #2 makes it possible to use high potash and high phosphate fertilizer to stabilize soils containing contaminants.

Also, given that red mud contains high concentrations of oxides, it is feasible to mix red mud with MKP to generate the stable waste forms otherwise generated using process #2. This modification precludes the need for supplying and pretreating oxides, such as MgO in red mud stabilization procedures.


US Patent # 5,645518

Method for Stabilizing Low-Level Mixed Wastes at Room Temperature

July 8, 1997

Abstract

A method to stabilize solid and liquid waste at room temperature is provided comprising combining solid waste with a starter oxide to obtain a powder, contacting the powder with an acid solution to create a slurry, said acid solution containing the liquid waste, shaping the now-mixed slurry into a predetermined form, and allowing the now-formed slurry to set. The invention also provides for a method to encapsulate and stabilize waste containing cesium comprising combining the waste with Zr(OH).sub.4 to create a solid-phase mixture, mixing phosphoric acid with the solid-phase mixture to create a slurry, subjecting the slurry to pressure; and allowing the now pressurized slurry to set. Lastly, the invention provides for a method to stabilize liquid waste, comprising supplying a powder containing magnesium, sodium and phosphate in predetermined proportions, mixing said powder with the liquid waste, such as tritium, and allowing the resulting slurry to set.

US Cl. 588/318; 501/155; 588/18; 588/256; 588/404; 588/405; 588/406; 588/408; 588/409
International Class: A62D 3/00 (20060101); C04B 28/34 (20060101); C04B 28/00 (20060101); G21F 9/30 (20060101); G21F 9/16 (20060101

References Cited
U.S. Patent Documents
4049462 ~ 4351749 ~ 4436555 ~ 4460500 ~ 4620947 ~ 5198190 ~ 5202033 ~ 5246496 ~ 5302565
Other References:
 

Low-Temperature-Setting Phosphate Ceramics for Stabilizing DOE Problem Low-Level Mixed Waste (Part I -Material & Waste Form Development -Dileep Singh, Arun S. Wagh and Lerry Knox) and Part II Low-Temperature-Setting Phosphate Ceramics for Stabilizing DOE Problem Low-Level Mixed Waste (Performance Studies on Final Waste Foms) -Arun S. Wagh, Dileep Singh, Manish Sutaria, and Sara Kurokawa -Proceedings of Waste Management 94 Conference-Tucson, AZ -Feb. 17-Mar. 3 1994-26 pages. .
Stabilization of Low Level Mixed Waste In Chemically Bonded Phosphate Ceramics -Arun s. Wagh; Dileep singh and J. Cunnane -Spectrum 1994 "Nuclear and Hazardous Waste Management International Topical Meeting"-Atlanta, GA.-Aug. 14-18 -4 pages. .
Phosphate-Bonded Ceramics as Candidate Final-Waste-Form Materials -D. Singh, A.S. Wagh, J. Cunnane, M. Sutaria & S. Kurokawa -Proceedings of 96th Annual Meeting of the American Ceramic Society, Indianapolis, IN, -April 24-28, 1994 -11 pages. .
Low-Temperature-Setting Phosphate Ceramics for Mixed Waste Stabilization -Arun S. Wagh & Dileep Singh -Proceedings of Second International Symposium and Exhibition on Environmental Contamination in Central and Eastern Europe, Budapest, Hungary, Sep. 20-23 1994 -12 pages. .
Conner, Jesse R., Chemical Fixation And Solidification of Hazardous Wastes, Van Nostrand Reinhold, 1990, p. 299-303.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a material to stabilize waste and a method of producing the material, and more specifically, this invention relates to a ceramic material to stabilize low-level mixed wastes and a method for producing the ceramic material.

2. Background of the Invention

Low-level mixed wastes contain hazardous chemical and low-level radioactive materials. Generally, mixed waste streams contain aqueous liquids, heterogeneous debris, inorganic sludges and particulates, organic liquids and soils. The projected volume over the next five years of the mixed waste generated by the U.S. Department of Energy alone is estimated at approximately 1.2 million cubic meters.

Stabilization of these mixed wastes requires that both phases of contaminants are stabilized effectively.

Typical approaches to stabilization and storage of these mixed wastes include vitrification. For example, one process (Crowe, U.S. Pat. No. 5,302,565) requires firing temperatures of at least 1850.degree. C. for at least 12 hours to produce ceramic containers. However, such processes, associated with high temperatures are costly. In addition, vitrification of waste streams often result in the lighting off of volatile components which often are contained in the waste stream. This lighting off results in the unwanted generation of secondary waste streams.

One system for producing cements having ceramic type properties, does not require high temperatures for final crystallization (Sugama et al. U.S. Pat. No. 4,436,555, assigned to the instant assignee). However, that process results in ammonia being liberated during processing and storage, which leads to container corrosion.

A need exists in the art for a low level waste encapsulation technology that connotes relatively high strength and low porosity to the final product, and which also sets up at low temperatures. The final product must exhibit low leachability and high durability in aqueous systems.

SUMMARY OF THE INVENTION

An object of the present invention is to overcome many of the disadvantages of the prior art in the encapsulation and stabilization of mixed low-level wastes.

Another object of the present invention is to provide a ceramic to encapsulate and stabilize mixed low-level wastes. A feature of the invention is the utilization of readily available materials. An advantage of the invention is the low cost of production of these ceramics.

Yet another object of the present invention is to provide a method for using chemically bonded phosphate ceramics to stabilize mixed waste forms. A feature of the present invention is the fabrication of these ceramics at room temperatures. An advantage of the present invention is that the low temperature setting characteristics of these ceramics makes them suitable for stabilization of mixed wastes containing volatile compounds, without the generation of secondary waste streams.

Still another object of the present invention is to provide a method for producing phosphate ceramics for use as waste stabilizers. A feature of the present invention is the fabrication of these ceramics via acid-base reactions between an inorganic oxide and a phosphate-containing acid solution. An advantage of the present invention is the ability to treat both solid and liquid wastes, while also obviating the need for high temperature vitrification processes, and therefore reducing the costs of final waste forms production.

Another object of the present invention is to provide for a method to minimize bulk of final waste forms. A feature of the present invention is the utilization of liquid and/or solid waste fractions to produce chemically bonded ceramic forms. An advantage of the invention is the economic and environmental savings of bulk reduction.

Briefly, the present invention provides for a method to stabilize solid and liquid waste at room temperature comprising grinding the solid waste to a predetermined particle size, combining the now ground solid waste with a starter oxide to obtain a powder, contacting the powder with an acid solution to create a slurry, said acid solution containing the liquid waste, mixing the slurry while maintaining the slurry below a predetermined temperature, shaping the now-mixed slurry into a predetermined form, and allowing the now-formed slurry to set.

The invention also provides for a method to encapsulate and stabilize waste containing cesium comprising combining the waste with Zr(OH).sub.4 to create a solid-phase mixture, grinding said solid-phase mixture, mixing phosphoric acid with the solid-phase mixture to create a slurry, shaping the now-mixed slurry into a predetermined form, subjecting the now-shaped slurry to pressure; and allowing the now pressurized slurry to set.

In addition, the invention provides for a method to stabilize liquid waste comprising supplying a powder containing magnesium, sodium and phosphate in predetermined proportions, mixing said powder with the liquid waste to produce a slurry, forming the slurry into a predetermined shape, and allowing the now-shaped slurry to set.

BRIEF DESCRIPTION OF THE DRAWING

These and other objects and advantages of the present invention will become readily apparent upon consideration of the following detailed description and attached drawing, wherein:

FIG. 1 is a schematic diagram of a method for producing ceramic waste forms, in accordance with the features of the present invention;

FIG. 2 is a graph depicting weight changes in an exemplary waste form during immersion, in accordance with the features of the present invention;

FIG. 3 is a graph depicting variation of pH in immersion water for an exemplary waste form, in accordance with the features of the present invention; and

FIG. 4 is a graph depicting variation in compression strength for an exemplary waste form before and after immersion, in accordance with the features of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

This invention addresses the need to develop benign final waste forms for a wide variety of waste streams, said waste streams containing pyrophorics, volatiles, such as mercury, lead, cadmium, chromium, and nickel, and other unstable compounds which cannot be treated by conventional high temperature techniques such as vitrification. The invention is also applicable to stabilize secondary waste streams resulting from thermal treatment processes, such as vitrification and plasma hearth processes.

The invention also can be used to stabilize certain RCRA organics. The inventors have found that certain of these organics do not retard the setting of phosphate ceramics. In one scenario, organics such as naphthalene and dichlorobenzene are trapped in activated carbon which in turn is stabilized in the phosphate matrix by the method claimed herein. This method of stabilization can be utilized in situations wherein mixed waste contains trace amounts of organics such as polychlorinated biphenyls, dioxin, dichlorobenzene, naphthalene, among others. As such, the invented method is superior to encapsulation methods wherein cement is utilized, in that cement cannot stabilize in the presence of organics.

Ash waste may be consolidated by this process to 80 volume percent of its original volume. Experiments by the inventors show good reaction and bonding between amorphous and reactive silica from fly ash and bottom ash with phosphate matrix. Formation of hard silico-phosphate bonds via this reaction can be used for the stabilization of hazardous silica compounds such as asbestos. The invention also encapsulates and stabilizes silica based filter aids, such as vermiculites and perlites, which are used in the removal of contaminants from liquid waste streams.

The inventors have identified a number of phosphate systems, which form into hard ceramics via chemical bonding, that can stabilize contaminants by both chemical and physical means. The process needs no thermal treatment, resulting in waste stabilization being done economically on site and without capital intensive equipment and transportation costs.

The room-temperature setting phosphate ceramic waste forms are formulated by using a route of acid-base reactions. Oxides or hydroxides of various elements are used as starter powders for this purpose. Said oxides and hydroxides chemically react with phosphoric acid or soluble acid phosphates to form ceramics. When waste is mixed with these powders or acid components, the waste also participates to form various stable phases in the final reaction product which then may be set into ceramic waste forms.

In one instance, the acid-base reaction results in the formation of the phosphate of MgO (Newberyite) via the following equation:

The acid base reaction also results in the reaction of the waste components with the acid or acid-phosphates. These reactions lead to chemical solidification of the waste. In addition, encapsulation of the waste in the phosphate ceramics formed by the reaction products results in physical encapsulation of the waste components.

Several advantages in phosphate waste forms exist viz portland cement grout or polymer encapsulation techniques. Since the stabilization in chemically bonded ceramics is due to chemical solidification as well as physical encapsulation, the leaching characteristics of these final waste forms is superior to the above-identified waste forms which are dependent on only physical encapsulation. The setting in chemically bonded ceramics can occur in a wide pH range and hence is not very sensitive to pH of the waste. Further, setting reactions are not sensitive to ambient temperatures and hence open-field stabilization in cold climates is possible.

The bonds are either ionic or covalent in phosphate ceramics, and hence they are stronger than the hydration bonds in portland cement thereby providing better strength to the final product.

The results on chemical stabilization presented infra are very general, given that in these acid-base systems, the acid phosphates seem to react with the contaminants irrespective of the bulk composition of the waste stream. This translates into a wide application in the stabilization of different types of waste streams.

Several phosphate systems can be used for the stabilization of chemical, radioactive and mixed waste streams. These include, but are not limited to, phosphates of Mg, Mg--Na, Al, Ca, Fe and Zr. The acid component may be concentrated or dilute phosphoric acid or acid phosphate solutions such as dibasic or tribasic sodium or aluminum phosphates. The reactions are exothermic and require no external heat treatment. However, the paste-setting reactions can be controlled either by the addition of boric acid to reduce the reaction rate, or by controlling the rate of addition of powder to the acid while concomitantly controlling the temperature of the reaction vessel. The setting times for the pastes formed by the reaction ranges from a few hours to a week. The phosphates attain their full strength in approximately three weeks.

The various invented systems include those outlined in Table 1, below:

TABLE 1 ______________________________________ Phosphate Systems and Processing Details Starting System Materials Solution Curing Time ______________________________________ Mg phosphate Calcined MgO Phosphoric >8 days acid-water (50/50) Mg--NH.sub.4 phos- Crushed dibasic Water 21 days phate NH.sub.4 phos- phate crystals mixed w. cal- cined MgO Mg--Na phos- Crushed dibasic Water 21 days phate Na phosphate crystals mixed with calcined MgO Al phosphate Al(OH).sub.3 Phosphoric acid Reacted pow- powder (.apprxeq.60.degree. C.) der, pressed Zr phosphate Zr(OH).sub.4 Phosphoric acid 21 days powder ______________________________________

The five systems disclosed in Table 1 were chosen for ready availability of materials, low cost, and availability of literature on the materials.

Three waste streams were selected for treatment: ash waste, contaminated cement sludge, and salts dominated by carbonates. Each of the waste streams differs in their bulk composition, with the bulk compositions forming nearly 91 weight percent of the total waste streams. Representative bulk constituents for the ash waste stream sample include activated carbon, fly ash, coal ash and vermiculite. 3Representative bulk constituents for the salt waste include activated carbon, Na.sub.2 (CO.sub.3).sub.2, widely used cation or anion exchange resins (such as Purolite), water, NaCl, Na(NO.sub.3).sub.2, Na.sub.3 PO.sub.4, Na.sub.2 SO.sub.4. Representative bulk constituents for the cement sludge waste include activated carbon, fly ash, water, concrete, Plaster of Paris, Haematite (Fe.sub.2 O.sub.3), alumina and perlite.

Each of these waste streams was spiked to a level of approximately 0.5 weight percent, with heavy metals, said metals added in the form of soluble nitrates. Total nitrate content was approximately 7 weight percent of the total waste. The heavy metals included Cr, Ni, Pb, and Cd. The invented method can stabilize and encapsulate high heavy metal concentrations (for example, exceeding 2 percent); however, most metal concentrations encountered are one percent or less, with economics dictating the reclamation of anything above one percent.

While a myriad of salts are encountered during waste processing, the above identified metals were added for demonstration purposes via solubilization to the following RCRA nitrates:

and

RCRA organics added to the three waste streams included naphthalene (C.sub.10 H.sub.8), and dichlorobenzene (C.sub.6 H.sub.4 Cl.sub.2). These organics were added to see their effect on the stabilization process. A radionuclide surrogate CsCl was also added to each of the three waste mixtures.

The heavy metals, organics and radionuclide examples disclosed above are meant to serve merely as representative of the variety of such compounds that could be encapsulated and stabilized, given the myriad of mixed waste scenarios to which the invented product and method could be applied.

Oxide and Hydroxide Preparation

Oxide powders can be pretreated for better reactions with acids. One technique includes calcining the powders to a typical temperature of between approximately 1,200.degree. C. and 1,500.degree. C. and more typically 1,300.degree. C. Another reaction enhancement technique is washing the powders with dilute nitric acid and then water. A myriad of oxide and hydroxide powders can be utilized to produce the ceramic system, including but not limited to MgO, Al(OH).sub.3, CaO, FeO, Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4 and Zr(OH).sub.4.

MgO and Al(OH).sub.3 powders are available through any commercial supply house, such as Baxter Scientific Products, McGaw Park, Ill. Zr(OH).sub.4 is obtained through Atomergic Chemetals Corporation, Farmingdale, N.Y. The myriad iron oxides enumerated above could actually be supplied as part of the waste stream to be stabilized and encapsulated.

Waste Processing Detail

Both solid and liquid wastes are treated by the invented process of using acid-base reactions. An exemplary process is depicted in FIG. 1., designated generally as numeral 10. Solid waste 12, such as contaminated ash, cement, medical waste, may first be crushed and mixed with the starter powder, 14, (Table 1, "Starting Materials"), via a vibratory mixer, 16. The resulting mixture is then reacted with the solution, 18, (Table 1, "Solution") to form a slurry or paste. If the waste is a liquid, 20, said liquid 20 is mixed with the solution 18 in a standard splash mixer 22 and then the mixture is reacted with the powder-mix in a reaction chamber 24.

After reaction, the reaction liquor is allowed to stabilize in a stabilization unit 26. The setting or stabilization step 26 takes from a few hours to a week. Final waste forms 28 are obtained in approximately 3 weeks.

Solid Waste Processing:

In the solid waste processing scenarios depicted in Table 1, the waste streams were manipulated in powder form by grinding the waste to an average, preferable approximate particle size of 8 to 10 micrometers (.mu.m). However, particles can range in size from between approximately 4-75 .mu.m.

Ash and cement wastes can be first mixed with the starter oxide or hydroxide powders using a vibratory shaker, or any conventional agitator. Weight percentages of the mixture varies at this juncture, but can range from between approximately 15 percent oxide to 50 percent oxide. Typically, an even weight percent (50:50) of oxide to solid waste is sought. However, the inventors have successfully encapsulated and stabilized single-component fly ash at weight percents as high as 85 percent ash to 15 percent MgO powder, which makes this technique particularly attractive for utilities where single-component fly ash is a major land-filling problem.

Unlike ash and cement waste, salt waste is reacted first with phosphoric acid to consume the CO.sub.2 that is formed via the reaction depicted in equation 2, below, and then mixed with the starting powder. Otherwise, the evolution of CO.sub.2 gas during stabilization results in very porous ceramic waste forms being produced.

In all three processing scenarios, the resulting powders are slowly added to the respective acid solution and thoroughly mixed using a mixer. Mixing can occur at ambient temperatures.

Typically, the rate of powder addition to the acid solution should result in the reaction liquor being maintained at less than 100.degree. C. Typical times required for controlled mixing range from 30 minutes to 1 hour. Mixing times can be shortened if the heat from resulting exothermic reactions, associated with the above method, is dissipated via reaction vessel cooling. The inventors found that reaction vessel cooling is more likely to be necessary when the resulting oxide powder-solid waste mixture contains less than 50 weight percent of waste.

Alternatively, the reaction can be slowed with the addition of from 5 to 25 weight percent of boric acid in the powder, and preferably 15 weight percent.

Upon temperature equilibration, the reacted paste is a liquid slurry which sets in a few hours once poured into a mold. Typically, no pressure is applied to the now-molded slurry. However, in some processing scenarios, such as when zirconium-based powders are utilized, discussed infra, pressures in the range of 1,000 to 2,000 pounds per square inch are used. The slurry gains full strength in approximately one week.

Mold shapes can vary, depending on the configuration of the ultimate deposition site, and can be selected from a myriad of geometrical shapes including cuboid, pyramidal, spherical, planar, conical, cylindrical, trapezoidal, rectangular, and the like. Generally, molds having the shape and size of a typical 55 gallon drum are used.

In the case of zirconium phosphate, the slurry is allowed to thicken first and then pressed into uniform shapes in a mold at low pressures. Set times for zirconium systems are approximately 3 weeks and requires initial pressure to achieve a dense form. While this additional processing time may seem to detract from using zirconium phosphate in the invented encapsulation method, surprisingly and unexpectedly the inventors have found that this phosphate provides orders of magnitude better encapsulation of cesium viz-a-viz the other phosphates. For example, while cesium leaching results from Mg-Phosphate and Mg--Na phosphate waste forms yielded low levels of 11.1 parts per million and 23 parts per million respectively, leaching levels obtained when using Zr phosphate waste forms were 0.26 parts per million.

Liquid Waste Forms:

The invented liquid encapsulation method provides a simplified approach for an end user compared to more typical encapsulation methods. For example, Mg--Na phosphates systems are made by adding said phosphate to the liquid on site, a process similar to that practiced in the cement industry. As such, liquid wastes, such as tritiated water, are easily and economically encapsulated with this procedure.

If solely liquid is being encapsulated and stabilized by the invented method and product, the liquid is first combined with an acid, such as phosphoric acid. The inventors have found that the ratio of acid to water, selected from a range of between approximately 37:63 to 50:50, produces good results. An acid:water ratio of 50:50 is most preferred. If the liquid waste contains water, then correspondingly less water is added to the acid to bring the water weight percent of the liquid waste-acid mixture up to 50 percent. The resulting liquid waste-acid mixture is then mixed with oxide powder in weight percent ranges similar to those outlined above for solid waste processing. When using powder mixtures containing MgO and dibasic phosphate, weight percent ratios of the oxide to the phosphate selected from the range of approximately 87:13 to 77:23 produce good results.

Physical and Mechanical Properties

The physical properties most relevant to the final waste forms are density, porosity and compression strength. Density of the samples was measured by determining the mass and geometrical volume. Porosity values were obtained by water immersion. Compression strength was measured using an Instron.TM. machine in compression mode, or other similar compression strength machine. The results of these measurements are depicted in Table 2, below.

Overall, the product waste forms are light weight materials having very low density. Very low porosity in Mg phosphate waste forms with ash waste was attained. High loading, in the range of approximately 50-80 percent was observed for the Magnesium systems.

Strength of the waste forms in the Mg phosphate system with ash waste does not depend on waste loading, and in fact is higher than other room temperature setting materials, such as Portland Cement (approx. 6,500 vs. 5,750 psi at 50 percent weight loading and 7000 vs. 2000 psi at 70 percent). All of these values well exceed the minimum statutory land disposal compression value of 500 psi for final waste forms.

TABLE 2 ______________________________________ Physical Properties and Compression Strength of Waste Forms Waste Matrix Loading Density Porosity Compression Material (Wt %) (g/cm.sup.3) (Vol %) Strength (psi) ______________________________________ Mg phosphate 50-70 1.706-1.756 <5 6223-6787 w. ash waste Mg phosphate 50-70 1.26-1.32 29.4-38.7 2224-5809 w. cement waste Mg phosphate 50-70 1.239-1.319 29.4-34.3 2224-5809 w. salt waste Mg--Na phos- 50-70 1.285-1.436 36.8-49.6 1172-1573 phate w. ash waste Zr phosphate 20 16 7572 w. ash waste ______________________________________

Leaching Detail

All of the invented phosphate systems are very effective in stabilizing heavy metal contaminants. As Table 3, depicts below, a comparison between the leaching levels for the untreated waste and the stabilized samples shows that the leaching levels in the stabilized waste are by an order of magnitude lower than those for the untreated waste. The data also show that the leaching values, even at a high loading of 70 weight percent, are from one to two orders of magnitude below the regulatory limits established for these metals.

The superior immobilization is due to the chemical stabilization of the contaminants in the matrix. This chemical stabilization results from the reaction between contaminant metal salts and the acid solution, followed by the physical encapsulation within the dense phosphate matrix. The nitrates of heavy metals are converted to insoluble phosphates by chemical reactions and hence they do not leach into the acidic leachate water used in the Toxicity Characteristics Leaching Procedure (TCLP) of the EPA. The physical encapsulation also immobilizes the contaminants in the matrix, thereby providing an excellent final waste form for long term storage.

TABLE 3 ______________________________________ TCLP Data on Waste Streams Treated With Invented Ceramic Materials Contamination levels (ppm) Sample Specification Cd Cr Ni Pb ______________________________________ Ash waste (neat) 40.4 196 186 99.7 Ash w H.sub.3 PO.sub.4 1.5 0.12 57.5 <0.5 MgP w. 50% ash 0.09 <0.05 0.21 <0.2 MgP w. 60% ash 0.12 <0.05 1.27 <0.2 MgP w. 70% ash 0.06 <0.05 3.71 <0.2 MNP w. 50% ash 0.03 0.12 0.04 <0.2 MNP w. 60% ash 0.06 0.11 0.05 <0.2 MNP w. 70% ash 0.13 0.12 0.08 <0.2 MgP w. 50% cement 0.03 <0.05 0.13 <0.2 MgP w. 60% cement 0.04 <0.05 0.26 <0.2 MgP w. 70% cement 0.06 <0.05 0.74 <0.2 MgP w. 60% salt <0.01 <0.02 <0.05 <0.5 MgP w. 70% salt <0.01 <0.02 <0.06 <0.5 ZrP w. 20% ash <0.02 0.04 0.55 <0.1 Regulatory Limits 1 5 -- 5 ______________________________________

Immersion Detail

Immersion studies indicate that the phosphate waste forms are durable in aqueous environments. Samples were immersed in distilled water and periodically the water was replenished to compensate for evaporation loss. Periodic measurements were made whereby the samples were removed from the water, dried and weighed to observe any weight loss. The pH of the water was also measured. At the end of 90 days, specimens were tested for compression strength. Results of weight change, pH and compression strength of Mg-phosphate ash waste forms at the end of 90 days are shown in FIGS. 2-4, respectively.

FIG. 2 shows the weight change of Mg-phosphate specimens with 70 weight percent ash waste loading after 90 days immersion compared to the weight of the waste form at the beginning of the immersion study. After an initial weight loss, probably due to the release of free phosphoric acid and unreacted MgO, the waste form stabilized.

FIG. 3 depicts pH changes, with the curve depicted therein similar to that of FIG. 2. The initial drop in pH is again due to phosphoric acid release, with a later rebound due to the slower release of MgO. Once the excess acid and unreacted MgO were completely released, the pH and weight of the waste form stabilized.

FIG. 4 shows the variation in the compression strength of the Mg-Phosphate waste forms as a function of waste loading. For comparison purposes, the strength of just-fabricated Mg-phosphate waste forms that were not exposed to water is presented. FIG. 4 shows that all of the Mg-phosphate waste forms exhibited excellent strength viz the 500 psi requirements for land-fill final forms. The resultant strength of all of the waste forms was exceptionally good and satisfied the regulatory requirements after a 90-day exposure in an aqueous environment.

Microstructure and Reaction Mechanism

The structure of the final waste forms is complex. Scanning Electron Microscope (SEM) analysis reveals that the room-tempera-ture stabilized Mg-phosphate ash waste forms are both crystalline and noncrystalline. Both crystalline and noncrystalline phases contain Mg phosphates and silicates, perhaps evidence of the presence of phospho-silicate structures.

X-ray diffraction analysis done on ash waste forms of Mg and Zr phosphate suggest that Mg phosphate waste forms contain unreacted MgO while the reaction in Zr phosphate is complete. The formation of phosphate phases is due to a MgO surface reaction with phosphoric acid, wherein the core portion of the MgO grains remains unscathed.

The major crystalline phase observed in the Mg phosphate system is Newberyite (MgHPO.sub.4.3H.sub.2 O), an insoluble stable phase. The major phase in Zr phosphate is either zirconium phosphate or a hydrophosphate of zirconium.

The fact that the compression strength of the ash waste forms in Mg phosphate systems does not depend on the extent of ash loading indicates that ash waste itself participates in the stabilization process. NMR studies reveal that ash loading modifies the mineralogy of the Mg phosphate matrix. Such a result makes the Mg phosphate system very suitable for stabilization of ash waste streams.

SEM analysis of Mg phosphate ash waste forms indicates that contaminates in the final waste forms are well dispersed both in the crystalline and well as noncrystalline phases. These data coincide with the results obtained by the inventors whereby any variations in the formulations of crystalline or noncrystalline phases due to variations in waste stream composition will not effect contaminant distribution. Generally, the inventors found that the phosphate waste resins microencapsulate contaminants very effectively in a homogeneous distribution in a complex matrix.

EXAMPLE 1

Magnesium Phosphate Ceramic

Magnesium oxide powder of approximately 8 micron size was first calcined for one hour at 1,300.degree. C. 15 weight percent of boric acid was added to this powder and mixed well. The mixture was then reacted with 50 weight percent dilute phosphoric acid. The addition was done slowly by constantly stirring the powder into the solution. The resulting paste was either put in a mold to set or was pressed into a hard ceramic during its setting.

To make a waste form, ash surrogate waste in powder form (with particle size of approximately 8 microns) was added to the oxide powder. The ratio of powder to the waste was 65:35 by weight in this test. The surrogate waste used in these experiments was a mixture of fly ash (Class F), coal cinder ash and vermiculite as the major component, and activated carbon as a minor component. The fly ash content was 40 weight percent, coal cinder ash 33 weight percent and vermiculite content was 20 weight percent. Thus, these three constituents formed 93 weight percent of the total surrogate composition. Trace contaminants used were RCRA metal nitrates and organics. The concentrations of the metal nitrates were such that the metal concentration in the surrogate waste was 0.5 weight percent of each metal. Each of the organics was also 0.5 weight percent of the final waste form.

The samples were cured for at least one week. They were subjected to various tests. They were found stable in water immersion tests. Their strength was 2,923 psi and concentrations of heavy metals in TCLP test leachates were 1.64 ppm of Cd, 0.05 ppm of Cr, 9.63 ppm of Ni and <0.1 ppm of Pb. These values are well below the EPA's pass/fail tests for Cd, Cr and Pb of 1 ppm, 5 ppm and 5 ppm, respectively. The levels for organics were below the detection limit of 5 ppm for both naphthalene and dichlorobenzene. In as much as the minimum compression strength for a waste form is 500 psi, the magnesium phosphate waste forms generated herewith the invented method exceeds such requirement.

EXAMPLE 2

Magnesium Sodium Phosphate

The starter MgO powder was washed with 0.28M nitric acid solution, then by distilled water and dried. Considerable amounts of hydroxide was formed in the powder as a result of the washings and this hydroxide reacted with acids to form phosphates. Beyond this, the procedure followed herein was the same as that in Example 1. The compressive strength observed was 2561.4 psi and TCLP results were 0.03 ppm for Cd, 0.05 ppm for Cr. 0.05 ppm for Ni and <0.1 ppm for Pb. The levels of organics in the leachate were below <5 ppm for both dichlorobenzene and naphthalene. These results show that the magnesium sodium phosphate waste form meets regulatory requirements.

EXAMPLE 3

Zirconium Phosphate

Zirconium hydroxide was the starter powder used. It was reacted with 90 wt % concentrated (or 10 wt % diluted) phosphoric acid. The reaction yielded a paste which took approximately 3 weeks to set into a hard ceramic. The waste form was prepared by the method given in Example 1 and the same test procedures were followed. Compression strength was 6781.6 psi, the levels in the leachate for Cd, Cr, Ni and Pb were 0.07 ppm, 0.16 ppm, 11 ppm and <0.1 ppm. The levels for organics were <5 ppm. This shows that Zr-phosphate waste forms meet regulatory requirements.

While the invention has been described with reference to details of the illustrated embodiment, these details are not intended to limit the scope of the invention as defined in the appended claims.



US Patent # 5,846,894

Phosphate Bonded Structural Products from High Volume Wastes

Dileep SINGH & Arun WAGH

Abstract: A method to produce structural products from benign waste is provided comprising mixing pretreated oxide with phosphoric acid to produce an acid solution, mixing the acid solution with waste particles to produce a slurry, and allowing the slurry to cure. The invention also provides for a structural material comprising waste particles enveloped by an inorganic binder.

US Cl. 501/155; 252/62; 588/10; 588/15; 588/249; 588/252; 588/256; 588/901
References Cited
U.S. Patent Documents: 4432666 ~ 5482550 ~ 5502268 ~ 5580378 ~ 5645518
Other References
CA 78:88360, "Waste solidification program, Evaluation of solidified waste products", nuclear Science abstracts, 26(23). (No Month) 1972. .
Low-Temperature-Setting Phosphate Ceramics for Stabilizing DOE Problem Low-Level Mixed Waste (Part I--Material & Waste Form Department--Dileep Singh, Arun S. Wagh and Lerry Knox) and Part II Low-Temperature-Setting Phosphate Ceramics for Stabilizing DOE Problem Low-Level Mixed Waste (Performance Studies on Final Waste Forms)--Arun S. Wagh, Dileep Singh, Manish Sutaria, and Sara Kurokawa--Proceedings of Waste Management 94 Conference-Tucson, AZ--26 pages, Mar. 1994..

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for producing structural materials and, more specifically, this invention relates to a method for producing structural products by binding benign wastes with a ceramic binder. The method is also applicable to producing a near term containment material.

2. Background of the Invention

The amount of available landfill space continues to dwindle. To preserve remaining space, recycling programs have been implemented to separate out reusable waste materials from materials that cannot be recycled, often at considerable expense. Despite these efforts, tipping fees continue to escalate, particularly because tremendous amounts of unrecyclable waste are still generated. Some examples of this waste include lumber waste, styrofoam, various kinds of cellulose fiber, automobile tires, ashes, used carpet backing, mineral wastes, and plastics. Ash, typically generated from incinerators, has extremely high disposal costs, partially due to the presence of heavy metals. Inked substrates, such as colored paper, colored fabrics, and synthetic fabrics pose recyclability problems. In addition to not being recyclable, many waste forms, such as plastics, or polymeric materials, also are not biodegradable.

A myriad of applications exist to convert many of these waste forms into usable products. However, many of the current methods incorporate organic compounds, such as formaldehyde, in polymeric binders. For example, organic binders are flammable, give off noxious fumes during setting, and have limited long-term stability. They are also expensive. Such methods are therefore unsuitable for housing applications.

Methods for encapsulating small amounts of low-level mixed wastes using ceramic binders also has been considered. However, those processes also are not suitable, primarily because they require high weight ratios of ceramic binder to waste forms. The inventors have found that high concentrations of binder leads to undesirably fast curing times, and therefore reduces the flowability characteristics of the slurry that are required for application in structural component and insulation substrate applications. Also, a high weight ratio of binder to waste particles connotes higher costs in that the space and economic advantages of disposing large volumes of benign waste with small amounts of binder are not realized.

A method for encapsulating waste using phosphate-containing material also is known in the art (U.S. Pat. No. Re. 32,329 to Paszner et al.). However, that process is relegated to porous vegetable matter, such as sugarcane, plant stalks and wood. The process also is designed to facilitate rapid setting of the final product, which is the antithesis of rendering a blowable or flowable mixture for use as a structural support or insulation product.

A need exists in the art for a method to utilize or otherwise dispose of nonrecyclable and non-biodegradable, benign waste without generating secondary waste streams. The method must be economical in providing structural materials for use in housing. A need also exists for an inexpensive structural product which is partially comprised of benign waste.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome many of the disadvantages of the prior art in the utilization of benign waste forms.

It is another object of the present invention to provide a method for utilizing benign waste. A feature of the invention is encapsulating and shaping the benign waste using a binder material. An advantage of the invention is conserving valuable landfill space.

Still another object of the present invention is to provide a method for producing a structural material. A feature of the invention is encapsulating benign waste using a nontoxic binder material. An advantage of the invention is utilizing the now encapsulated benign waste as safe insulative material and fire-proof material in housing and other structures. Another advantage of the invention is that the method does not emit noxious materials and therefore is safe for operators and end users.

Another object of the present invention is to provide a method for producing light-weight structural materials. A feature of the invention is the room-temperature encapsulation of large amounts of widely available wastes with relatively smaller amounts of an inorganic binder. An advantage of the invention is that it is an inexpensive process to utilize nonrecyclable waste material in blowable or pumpable preparations for ultimate use as housing materials.

Yet another object of the present invention is to provide a structural material partially comprised of benign waste. A feature of the invention is a high volume percent of waste to binder material. An advantage of the invention is the production of light weight, strong structural materials that can supplant traditional materials.

Another object of the present invention is to provide a method for producing a near-term containment material. A feature of the invention is using high weight ratios of the containment material to the invented binder. An advantage of the invented method is the ability to confine the near-term containment material to desired mold shapes or structures until most activity is reduced or dissipated.

Briefly, the invention provides for a method to produce structural products from benign waste comprising calcining an inorganic oxide, mixing the now-calcined oxide with a powdered acid to produce a mixture, contacting the mixture with phosphoric acid to produce an acid solution, mixing the acid solution with waste particles to produce a slurry or a wet mix, and allowing the slurry or mix to cure.

The invention also provides for a structural material comprising waste particles, and an inorganic binder enveloping the waste particles.

BRIEF DESCRIPTION OF THE DRAWING

These and other objects and advantages of the present invention will become readily apparent upon consideration of the following detailed description and attached drawing, wherein:

Fig. 1  is a schematic diagram of a method for producing ceramic bonded waste forms using benign material.

DETAILED DESCRIPTION OF THE INVENTION

The method described herein produces ceramically bound benign wastes for use as structural materials. Unlike previous attempts to produce stable structural materials from bulk waste, the instant process produces an amorphous, more flowable material by utilizing lower volume percents of binder to formulate the final forms. The resulting ceramic formulation is sufficiently amorphous and low in crystalline properties to insure good flow of the material and extended work time. The amorphous phase mimics polymeric formulations by facilitating the encapsulation of waste particles with binder during formation of the final slurry.

A myriad of benign wastes are utilized to produce the structural materials, including, but not limited to, lumber wastes, styrofoam, various cellulose fibers (including those fibers having colored ink), tires, textile wastes, ashes, carpet backing, mineral wastes, plastics and other solid materials that cease to be useful. These wastes are used in powder or shredded forms and are solidified by using a phosphate binder to form desired shapes for use in the construction industry. Products produced from the method include blowable insulation, particle boards, packaging materials, bricks, tiles, wall-forms and engineered barrier and shield systems.

The room-temperature setting phosphate ceramic waste forms are formulated by using a route of acid-base reactions. Oxides or hydroxides of various elements are used as starter powders for this purpose. Said oxides and hydroxides chemically react with phosphoric acid or soluble acid phosphates to form ceramics. When waste is mixed with these powders or acid components, the waste also may participate to form various stable phases in the final reaction product which then may be set into ceramic waste forms.

In one instance, the acid-base reaction results in the formation of the phosphate of MgO (Newberyite) via the following equation:

The acid base reaction also results in the reaction of the waste components with the acid or acid-phosphates. These reactions lead to chemical solidification of the waste. In addition, encapsulation of the waste in the phosphate ceramics formed by the reaction products results in physical encapsulation of the waste components.

Oxide and Hydroxide Preparation

Oxide powders can be pretreated for better reactions with acids. One technique includes calcining the powders to a typical temperature of between approximately 1,200.degree. C. and 1,500.degree. C. and more typically 1,300.degree. C. Another reaction enhancement technique is washing the powders with dilute nitric acid and then water. A myriad of oxide and hydroxide powders can be utilized to produce the ceramic system, including but not limited to MgO, Al(OH).sub.3, CaO, FeO, Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4 and Zr(OH).sub.4, ZrO, and TiO.sub.2 and crushed dibasic sodium phosphate crystals mixed with MgO.

MgO and Al(OH).sub.3 powders are available through any commercial supply house, such as Baxter Scientific Products, McGaw Park, Ill. Zr(OH).sub.4 is obtained through Atomergic Chemetals Corporation, Farmingdale, N.Y.

A generic embodiment of the invented method is depicted in FIG. 1 as numeral 10. First, a supply of oxide 12 is subjected to either or both a calcining pretreatment step 14 and a boric acid addition step 16. Both steps serve to slow down the reaction mechanism. Generally, the boric acid is incorporated when a slower reaction is required, for example when extended workability of the material is desired. The inventors have found that the boric acid forms a glassy phase that coats the oxide particles so that the oxide cannot as readily react with phosphoric acid.

As discussed supra, a myriad of oxides can be used in the invented method. Any mixing of the oxide with the boric acid is strictly controlled to maintain an optimum weight percent of constituents of the resulting dry mixture. This weight percent is selected from a range of between approximately 5 weight percent boric acid to the oxide to 15 percent boric acid to the oxide. A preferable weight percent is 10 percent boric acid to oxide, e.g., 10 grams of boric acid for every 90 grams of oxide, to form the dry mixture.

The resulting dry mixture 18 is then mixed with between approximately 50 to 60 weight percent dilute phosphoric acid 20 to form an acid solution or binder 22. A preferable weight percent is at or below 55 percent, i.e., 55 grams of dry mixture to 45 grams of 50 percent dilute phosphoric acid, so as to facilitate flowability of the resulting solution. Concentrated acid tends to make the reaction more intense. This results in a thick slurry developing which is not conducive to coating the particles.

Waste particles, 24, which may be subjected to a pretreatment sizing process 23, are then thoroughly mixed with the binder. The mixing step 26 ensures that the waste particles are completely encapsulated or coated with binder. The resulting slurry is molded into desired shapes 28 under no pressure or under small pressure (approximately 1,000 pounds per square inch), depending on the waste material being bonded. For example, processes for encapsulating wood waste often requires the aid of pressurization, primarily because wood surfaces participate less in the ceramic formation reaction. Rather, bonding in such cases is purely from the phosphate phase encapsulating the wood particles. Compression also may be required to attain desired strengths of final products, such as in particle board manufacture.

Solid Waste Processing:

In solid waste processing scenarios, the waste streams are often manipulated in powder form by grinding the waste to an average, preferable approximate particle size of 8 to 10 micrometers (.mu.m). However, particles can range in size from between approximately 4-75 .mu.m.

Ash and cement wastes can be first mixed with the starter oxide or hydroxide powders using a vibratory shaker, or any conventional agitator. Weight percentages of the mixture varies at this juncture, but can range from between approximately 15 percent oxide to 50 percent oxide. Typically, an even weight percent (50:50) of oxide to solid waste is sought. However, the inventors have successfully encapsulated and stabilized single-component fly ash at weight percents as high as 85 percent ash to 15 percent MgO powder, which makes this technique particularly attractive for utilities where single-component fly ash is a major land-filling problem.

Typically, the rate of powder addition to the acid solution should result in the reaction liquor being maintained at less than 100.degree. C. Typical times required for controlled mixing range from 30 minutes to 1 hour. Mixing times can be shortened if the heat from resulting exothermic reactions, associated with the above method, is dissipated via reaction vessel cooling. The inventors found that reaction vessel cooling is more likely to be necessary when the resulting oxide powder-solid waste mixture contains less than 50 weight percent of waste.

Alternatively, the reaction can be slowed with the addition of from 5 to 25 weight percent of boric acid in the powder, and preferably 10 weight percent.

Upon temperature equilibration, the reacted paste is a liquid slurry which sets in a few hours once poured into a mold. Typically, his slurry is mixed with shredded waste and put into a mold.

Mold shapes can vary, depending on the configuration of the ultimate deposition site, and can be selected from a myriad of geometrical shapes including cuboid, pyramidal, spherical, planar, conical, cylindrical, trapezoidal, rectangular, and the like.

Liquid Waste Forms

Sludges and benign wastes with high water content can be incorporated or bound using this binder system by appropriate modification to the water content in the acid.

The inventors have found that the volume of the loading of the wastes in the final product can range from between approximately 50-90 volume percent. For insulation and building reinforcement applications, the composition of the mixture is adjusted to convert it into a pumpable slurry (50 volume percent waste) or a blowable particle mixture (80-90 volume percent waste) so as to facilitate the filling of cavities.

The acid-base reaction between the oxide and phosphoric acid results in the formation of phosphates on the surface of the particles thereby encapsulating individual particles with a thin layer of impermeable phosphate binder. This results in a structural product in which particles of the waste are protected by the binder to provide not only product strength but also confers resistance to fire, chemical attack, humidity and other weathering conditions.

Several advantages of the resulting embodiment exist over commercially available polymer-based binders. Unlike polymer binders, phosphate binders are nonflammable. Also, several polymer ingredients are occupational hazards, whereas inorganic phosphate binders are comparatively safe. No toxic chemicals or vapors are released during production of phosphate bonded products. Lastly, phosphate based binder improves the rigidity and long-term stability to the structural product, compared to currently used organic binders.

EXAMPLE 1

Styrofoam Insulation

The inventors have found that utilizing the above-disclosed method, styrofoam particles are completely coated with a thin, impermeable layer of the phosphate binder phase. The uniform coating of the styrofoam particles not only provides structural stability but also confers resistance to fire, chemical attack, humidity and other weathering conditions. As shown in Table 1, below, these characteristics are superior to more typical insulation materials.

TABLE 1 ______________________________________ Comparison of ceramic-bonded Styrofoam insulation to Fiber Glass- and Cellulose-insulation Bonded Key Features Styrofoam Fiberglass Cellulose ______________________________________ Density (lb/ft.sup.3) 2.0 0.4-1.0 2-3.5 R Values 4.5 2-3 3-3.5 (1 in. thicknesses) Fire Resistance noncombustible noncombustible noncombustible Water absorption <4% 1-2% 5-20% Dimensional Stability .apprxeq.2% settling noted .apprxeq.20% Health Hazards minimum high minimum Material Costs Low/blown or high low pumped ______________________________________

As depicted in Table 1, the resulting binder-covered styrofoam material provided superior R values. For example, thermal conductivity measurements, utilizing a modified radial hot-wire technique (established by Anter Laboratories, Pittsburgh, Pa.) showed that the thermal resistance of the material produced via the invented method was approximately 4.5 hour square foot degrees Fahrenheit per British Thermal Unit (h.ft..sup.2 .degree.F./BTU, compared to 2-3 h.ft..sup.2 .degree.F./BTU for fiber glass and 3-3.5 h.ft..sup.2 .degree.F./BTU for cellulose. This superior R value indicates that phosphate ceramic binder-covered styrofoam provides superior energy savings when used as an insulation product.

Insulation products often are susceptible to humidity and tend to sag, thereby loosing their structural integrity over time. The invented material was subjected to an aging test pursuant to ASTM D 2126 (ASTM=American Society for Testing and Materials), whereby the material is exposed to severe environments for extended periods of time with dimension changes of the material closely monitored. The material was exposed to 38.degree. C. temperatures at 98 percent humidity for 3 weeks. Specimen volume change was shown to be approximately two percent after a two week period. This compares to 20 percent for cellulose insulation material and is also superior to that seen in fiberglass material.

Generally, a wide range of waste particle sizes can be utilized when producing structural products using the invented method. When using styrofoam materials, optimal results are obtained when particle sizes ranging from 2 millimeters to 5 millimeters are used, and when the particles are mixed with binder in a weight ratio of 1:2. Optimal weight loading of the styrofoam in the final product is approximately 7.5 weight percent, which corresponds to approximately 85 to 90 volume percent of the final product.

EXAMPLE 2

Wood Waste

The inventors have found that when wood waste is subjected to the invented method, particle board having superior flexural strength values is produced. For example, samples containing 50 weight percent of wood and 50 weight percent of binder display approximately 1,500 psi in flexural strength. Samples containing 60 weight percent and 70 weight percent of wood exhibit flexural strengths of 400 psi and 300 psi, respectively.

Generally, suitably sized wood particles range from between approximately 1 and 5 millimeters (mm) long, 1 mm thick and 2 to 3 mm wide.

In addition, once the wood and binder is thoroughly mixed, the samples are subjected to pressurized molding on the order of approximately 2650 psi, and for approximately 30 to 90 minutes.

The disclosed process is not be construed as limited to the above examples. Also, aside from the myriad of wastes listed above for which this process can be used to encapsulate, other waste streams are also sufficiently stabilized herewith. For example, potliner residue, Bayer sands, ashes generated at plant sites and any other mining refuse can be stabilized by this process and utilized as structural components. Potliner residue, when combined with magnesium phosphate hexahydrate in a 50:50 weight proportion, yields a ceramic form having a density of 2.9 grams per cubic centimeter, a porosity of 2.17 percent and a compressive strength of 4,210 psi, the last of which is comparable to portland cement forms. Consistent with data disclosed supra, when higher proportions (60 weight percent) of waste material (in this instance potliner residue) is used, desired values degraded slightly from those values obtained when 50 weight percent loadings were used. Density decreased to 2.0 grams per cubic centimeter, porosity increased to 2.6 percent and compression strength decreased to 3,402 psi.

While the invention has been described with reference to details of the illustrated embodiment, these details are not intended to limit the scope of the invention as defined in the appended claims.



`
United States Patent # 6,133,498

Method for Producing Chemically Bonded Phosphate Ceramics and for Stabilizing Contaminants Encapsulated therein Utilizing Reducing Agents

A. Wagh, et al.

October 17, 2000

Abstract: Known phosphate ceramic formulations are improved and the ability to produce iron-based phosphate ceramic systems is enabled by the addition of an oxidizing or reducing step during the acid-base reactions that form the phosphate ceramic products. The additives allow control of the rate of the acid-base reactions and concomitant heat generation. In an alternate embodiment, waste containing metal anions are stabilized in phosphate ceramic products by the addition of a reducing agent to the phosphate ceramic mixture. The reduced metal ions are more stable and/or reactive with the phosphate ions, resulting in the formation of insoluble metal species within the phosphate ceramic matrix, such that the resulting chemically bonded phosphate ceramic product has greater leach resistance.

Current U.S. Class:588/319 ; 264/.5; 501/111; 501/112; 501/155; 588/10; 588/252; 588/320; 588/4; 588/408; 588/413

Current International Class:A62D 3/00 (20060101); C03C 10/00 (20060101); C03C 14/00 (20060101); C03C 1/00 (20060101); C04B 20/02 (20060101); C04B 18/04 (20060101); C04B 20/00 (20060101); C04B 28/34 (20060101); C04B 28/00 (20060101); G21F 9/16 (20060101)

References Cited

U.S. Patent Documents:5645518 ~ 5830815 ~ 5846894 ~ 5939039

Other References

Modified Phosphate Ceramics for Stabilization and Solidification of Salt Mixed Wastes, authored by Dileep Singh, Kartikey Patel, Arun S. Wagh, Seung-Young Jeong, published in the Proceedings of Spectrum '98, International Conference on Decommissioning and Decontamination and on Nuclear and Hazardous Waste Management, Denver, Colorado, Sep. 13-18, 1998. .
Modified Phosphate Ceramics for Stabilization of Salt Mixed Wastes, thesis authored by Kartikey Patel, submitted for review on or after May 7, 1998 and defended on Jun. 26, 1998. .
U.S. Patent Application Ser. No..sub.-- /124,822, Continuation-in-part of U.S. Patent Nos. 5,846,894 and 5,830,815. .
U.S. Patent Application Serial No..sub.-- /617,284, Continuation-in-part of U.S. Patent No. 5,830,815. .
PCT Patent Application No. PCT/US97/04132.

Description

TECHNICAL FIELD

This invention relates to the use of chemically bonded phosphate ceramic (CBPC) waste forms for immobilizing large volumes of low-level, radioactive and/or hazardous waste, and, in particular, to an improved process and CBPC product.

BACKGROUND OF INVENTION

Low-level mixed wastes contain hazardous chemical and low-level radioactive materials. Of particular concern are low-level mixed waste streams that contain heavy metals, such as lead, cadmium, copper, zinc, nickel, and iron, among others, and waste streams from nuclear materials processing applications that contain technetium-99, chromium, and antimony. The U.S. Environmental Protection Agency classifies waste as hazardous, under the Resource Conservation and Recovery Act (RCRA), if excessive amounts of heavy metals leach from the waste during the Toxicity Characteristic Leaching Procedure (TCLP). Land disposal of leachable heavy metal waste is very expensive and strictly regulated, and therefore cost-effective, safe, leach resistant methods for encapsulating heavy metal waste is of current environmental importance.

Stabilization of low-level mixed waste requires that the contaminants, including soluble heavy metals ions, are effectively immobilized. No single solidification and stabilization technology is known to successfully treat and dispose of low-level mixed waste, due to the physical and chemical diversity of the waste streams. Conventional high-temperature waste treatment methods (e.g., incineration, vitrification) are largely unsuitable for the treatment of low-level mixed waste streams, because their reliance on high temperature risks the release of volatile contaminants and they generate undesirable secondary waste streams. A low-temperature approach is to stabilize hazardous waste by using inorganic (e.g., pozzolanic) binders, such as cement, lime, kiln dust, and/or fly ash. Disadvantages of this approach include a high sensitivity to the presence of impurities, high porosity, and low waste loading volume. Organic binders (e.g., thermosetting polymers) are used even less frequently, because of cost and greater complexity of application. Organic binders are not compatible with water-based wastes, unless the waste is first pre-treated and converted to an emulsion or solid, and organic binders are subject to deterioration from environmental factors, including biological action and exposure to ultraviolet light.

Recently, an alternative low-temperature approach has been developed at Argonne National Laboratory for stabilizing and solidifying low-level mixed waste by incorporating or loading the waste into a phosphate ceramic waste form. This technique immobilizes the waste by solidification, such that the waste is physically micro-encapsulated within the dense matrix of the phosphate ceramic waste form, and stabilizes the waste by converting the waste into their insoluble phosphate forms. Ceramic encapsulation systems are particularly attractive given that the bonds formed in these systems are ether ionic or covalent, and hence stronger than the hydration bonds in cement systems. Also, the ceramic formulation process is exothermic and economical.

Phosphates are particularly good candidates for stabilization of radioactive and hazardous waste, because phosphates of radio nuclides and hazardous metals are essentially insoluble in groundwater. A salient feature of the low-temperature ceramic phosphate formulation process is an acid-base reaction. For example, magnesium phosphate ceramic waste forms have been produced by reacting magnesium oxide (MgO) with phosphoric acid to form a phosphate of magnesium oxide, Newberyite, as represented in Equation (1), below.

The acid-base reaction results in the reaction of the waste components with the acid or acid-phosphates, leading to chemical stabilization of the waste. In addition, encapsulation of the waste in the phosphate ceramic results in physical containment of the waste components. The reaction represented by Equation (1) above occurs rapidly and generates heat, and upon evaporation of the water, a porous ceramic product results.

U.S. Pat. No. 5,645,518 issued to Wagh, et al., incorporated herein by reference, describes in detail the process steps for setting liquid or solid waste in CBPC products using acid-base reactions. Accordingly, the process involves mixing ground solid waste, including salt waste spiked with heavy metals, with a starter powder of oxide and hydroxide powders of various elements; slowly adding the waste-powder mixture to an acid solution of phosphoric acid or soluble acid phosphates; thoroughly mixing the waste-powder-acid mixture for about a half hour to an hour at ambient temperatures (less than 100.degree. C.), such that the components of the

mixture chemically react to form stable phases and a reacted viscous slurry or paste results; and allowing the slurry or paste to set for a few hours into the final CBPC product. Liquid waste is similarly stabilized by mixing the liquid waste with the acid solution (preferably 50:50), and then reacting the waste-acid mixture with the starting powders. The maximum temperature for the process is about 80.degree. C. The CBPC products attain full strength in about three weeks, and exhibit a complex structure, including a major crystalline phase, e.g., Newberyite (MgHPO.sub.4.3H.sub.2 O), and an insoluble, stable phase. The waste components are generally homogeneously distributed within the phosphate ceramic matrix. Unfortunately, however, the porous product (Newberyite) is unsuitable for waste treatment on a large scale.

U.S. Pat. No. 5,830,815 issued to Wagh, et al., incorporated herein by reference, describes improving the CBPC fabrication process by incorporating two temperature control processes for both reducing heat generation during the encapsulation (reaction) steps and moderating pH conditions (some wastes are unstable at a low pH). The first temperature control process involves pre-treating the phosphoric acid with a carbonate, bicarbonate or hydroxide of a monovalent metal (e.g., K, Na, Li, Rb) prior to mixing with an oxide or hydroxide powder so as to buffer the acid. In particular, potassium containing alkali compounds (e.g., K.sub.2 O, KHCO.sub.3, KOH) result in a more crystalline waste form, and the higher the concentration of potassium in the potassium containing compound, the more crystalline the final product, resulting in a higher compression strength, lower porosity, and greater resistance to weathering, compressive forces, and leaching. The second temperature control process involves bypassing the use of the acid and mixing the oxide powder with dihydrogen phosphates of potassium, sodium, lithium, or other monovalent alkali metal, to form a ceramic at a higher pH.

Neutralizing the phosphoric acid solution in equation (1) by adding potassium hydroxide (KOH), as represented in the chemical equation (2) below, reduces the reaction rate and heat generation, and results in the formation of a superior magnesium potassium phosphate (MKP) mineral product, MgKPO.sub.4.6H.sub.2 O (magnesium potassium phosphate hexahydrate), as represented in chemical equation (3) below.

The chemically bonded ceramic phosphate (CBPC) waste form (e.g, MgKPO.sub.4.6H.sub.2 O) is a dense, hard material with excellent durability and a high resistance to fire, chemicals, humidity, and weather. The low-temperature (e.g., room-temperature) process encapsulates chloride and nitrate salts, along with hazardous metals, in magnesium potassium phosphate (MKP) ceramics, with salt waste loadings of up to between approximately 70 weight percent and approximately 80 weight percent. This durable MKP ceramic product has been extensively developed and used in U.S. Department of Energy waste treatment projects.

Phosphates in general are able to bind with hazardous metals in insoluble complexes over a relatively wide pH range and most metal hydroxides have a higher solubility than their corresponding phosphate forms. In addition to the magnesium and magnesium-potassium phosphate waste products discussed above, known waste encapsulating phosphate systems include, but not limited to, phosphates of magnesium-ammonium, magnesium-sodium, aluminum, calcium, iron, zinc, and zirconium (zirconium is preferred for cesium encapsulation). A non-exclusive summary of known phosphate systems and processing details is provided in Table I below, selected according to the ready availability of materials and low cost. It is also known to add other materials to either the waste or ceramic binder ingredients, such as fly ash.

TABLE I ______________________________________ Phosphate Systems and Processing Details Curing System Starting Materials Solution Time ______________________________________ MKP Ground MgO, ground K Water 1 hour dihydrophosphate crystals Mg phosphate Calcined MgO Phosphoric acid- >8 days water (50/50) Mg-NH.sub.4 Crushed dibasic NH.sub.4 Water 21 days phosphate phosphate crystals mixed with calcined MgO Mg-Na phosphate Crushed dibasic Na Water 21 days phosphate crystals mixed with calcined MgO Al phosphate Al(OH).sub.3 powder Phosphoric acid Reacted (.apprxeq. 60.degree. C.) powder, pressed Zr phosphate Zr(OH).sub.4 Phosphoric acid 21 days ______________________________________

Iron oxides including either iron oxide (FeO) itself or magnetite (Fe.sub.3 O.sub.4) have also been used in the formation of phosphate ceramic products, however, these materials are uncommon and expensive. Haematite (Fe.sub.2 O.sub.3) is a very unreactive powder and efforts to form a chemically bonded phosphate ceramic (CBPC) product using haematite have been unsuccessful. When mixed with phosphoric acid, and even highly concentrated phosphoric acid, the haematite either does not react or reacts at such a slow rate that the reaction is impractical for the development of CBPC products. The slow rate of reaction is due to the insolubility of haematite, which is in its highest oxidation state.

Appropriate oxide powders include, but are not limited to, oxides or hydroxides of aluminum, calcium, iron, magnesium, titanium, and zirconium, and combinations thereof. The oxide powders may be pre-treated (e.g., heated, calcined, washed) for better reactions with the acids. While no pressure is typically applied to the reacted paste, about 1,000 to 2,000 pounds per square inch may be applied when zirconium-based powders are used.

The acid component may be dilute or concentrated phosphoric acid or acid phosphate solutions, such as dibasic or tribasic sodium, potassium, or aluminum phosphates, and the paste-setting reactions are controllable either by the addition of boric acid to reduce the reaction rate, or by adding powder to the acid while concomitantly controlling the temperature. Examples of appropriate phosphates include phosphates of aluminum, beryllium, calcium, iron, lanthanum, magnesium, magnesium-sodium, magnesium-potassium, yttrium, zinc, and zirconium, and combinations thereof. Salt waste may be reacted with phosphoric acid to consume any carbon dioxide (CO.sub.2) present, prior to mixing the salt waste with the oxide powders or binding powders, as the evolution of CO.sub.2 results in very porous final ceramic products.

Unfortunately, the acid-base reactions involved in the phosphate ceramic systems described above occur very rapidly, resulting in the generation of considerable exothermic heat that prevents the formation of homogeneous large scale phosphate ceramic monoliths. The rapid setting of the CBPC products also hinders the proper conversion of hazardous or radioactive contaminants into stabilized phosphate forms. As a result, the CBPC products formed by methods known in the art have very poor density and strength.

Encapsulation of waste containing heavy metals in known CBPC systems is also of limited practical use. Although heavy metals in the form of soluble nitrates (e.g., Cr(NO.sub.3).sub.3.9H.sub.2 O, Ni(NO.sub.3).sub.2.6H.sub.2 O, Pb(NO.sub.3).sub.2, and Cd(NO.sub.3).sub.2.4H.sub.2 O) are reportedly converted to insoluble phosphates by the CBPC forming chemical reactions, there is a critical need to improve their leach resistance and to provide greater stabilization for the metal anions of technetium-99, chromium, and antimony. Efforts to encapsulate heavy metal waste in phosphate ceramic products are further hampered by low maximum waste loading capacities, because of interference of the metal anions with ceramic-setting reactions, leaching of soluble metal anions from the resulting highly porous ceramic product (especially in aqueous environments), and rapid structural degradation of the ceramic product caused by the high leach rates. Also, environmental stresses degrade the integrity of known CBPC waste forms over time. For example, exposure to repeated cycles of wetting, drying and/or freezing, or acidic or other conditions conducive to leaching may affect the long term effectiveness of waste encapsulated CBPC waste forms.

A need exists for improved phosphate ceramic systems and improved methods for disposing of wastes containing metal anions in phosphate ceramic products.

The present invention is a surprisingly effective process step that significantly improves known phosphate ceramic formulations, enables the production of iron-based phosphate ceramic systems, and critically increases the stabilization of wastes containing heavy metals within CBPC composites. The invention involves adding oxidants or reductants to the ceramic phosphate formulations to retard or accelerate the acid-base reactions and thereby control the exothermic temperature of the reactions. In addition, the use of reducing agents significantly improves the stabilization of the metal anions within the phosphate ceramic composition, and thus the leach resistance of the encapsulated metals, by changing the valence of the metal to a lower oxidation state, such that the metal is more stable in the presence of the phosphate ions and/or the metal is more reactive with the phosphate ions.

Therefore, in view of the above, a basic object of the present invention is to control the reactions rates and heat generation in phosphate ceramic processes to allow homogeneous large scale phosphate ceramic monoliths.

Another object of the present invention is to significantly improve the density and strength of phosphate ceramic products formulated from methods known in the art.

Another object of the present invention is to form chemically bonded phosphate ceramic products from inexpensive iron-based materials, such as haematite.

Yet another object of the present invention is to provide an improved method for stabilizing waste containing metal anions in a phosphate ceramic composite having increased loading capacity and improved leach resistance.

A further object of the invention is to provide an improved, safe, low temperature, economical method for stabilizing large volumes of waste containing metal anions in a durable, long term storage phosphate ceramic product.

Additional objects, advantages, and novel features of the invention are set forth in the description below and/or will become apparent to those skilled in the art upon examination of the description below and/or by practice of the invention. The objects, advantages, and novel features of the invention may be realized and attained by means of instrumentation and combinations particularly pointed out in the appended claims.

BRIEF SUMMARY OF THE INVENTION

Briefly, this invention is a surprisingly effective method for significantly improving phosphate ceramic formulations and enabling the production of iron-based phosphate ceramic systems. The invention involves the addition of an oxidizing or reducing step to known phosphate ceramic formulations during the acid-base reactions between the oxide powders and phosphoric acid or acid phosphate solutions. The additives allow control of the rate of the acid-base reactions and concomitant heat generation. As a result, phosphate ceramic systems incorporating iron-based materials are practical, including the formation of iron phosphate ceramic products from haematite, a readily available, inexpensive material. The CBPC products may be crystalline ceramics and/or glass (non-crystalline).

In an alternate embodiment, the addition of reducing agents to the ceramic phosphate system significantly improves the stabilization of heavy metal waste encapsulated within chemically bonded phosphate ceramic (CBPC) waste forms. Addition of the reducing agent, preferably a stannous salt, changes the valence of the metal to a lower oxidation state, such that the metal is more stable in the presence of the phosphate ions and/or the metal is more reactive with the phosphate ions. Importantly, the reduced metal ions are more stable and/or more reactive with the phosphate ions, resulting in the formation of insoluble metal species within the final phosphate ceramic matrix.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended claims set forth those novel features which characterize the invention, however, the invention itself, as well as further objects and advantages thereof, will best be understood with reference to the following detailed description of a preferred embodiment, in conjunction with the accompanying drawings, where like reference characters identify like elements throughout the various figures, in which:

FIG. 1 shows an scanning electron microscopy (SEM) micrograph of an iron-based CBPC product made from haematite;

FIG. 2 shows an SEM micrograph of an iron-based CBPC product made from magnetite; and

FIG. 3 graphically illustrates Eh-pH values for magnesium potassium phosphate (MKP) ceramic products fabricated by the present improved method .

DETAILED DESCRIPTION OF THE INVENTION

The present invention modifies known methods for encapsulating waste in chemically bonded phosphate ceramic (CBPC) products, described in detail in the Background Section above, by incorporating a new and unique oxidation or reducing step that controls the rate of the acid-base reaction in the formation of phosphate ceramic systems. The addition of the oxidation or reducing agent to the CBPC binder mix aids in controlling the rate of the acid-base reactions and, importantly, changes the oxidation state and/or reactivity of CBPC ingredients, e.g., starter oxide powders. Altering the oxidation state of the compound may allow CBPC compounds to become more reactive and encapsulated waste to become more reactive and less soluble.

Iron Phosphate Ceramic Products Formed from Haematite

As discussed above, haematite (Fe.sub.2 O.sub.3), an inexpensive and very common ingredient in lateritic soils and several mineral wastes, including iron waste tailing and red mud, has not been successfully used to form a chemically bonded phosphate ceramic (CBPC) product, because of its high oxidation state, insolubility, and slow reaction rate with phosphoric acid. Using known CBPC formulations, the haematite remains unaltered in the oxidation environment of the phosphate solution during the acid-base reactions.

The present invention enables the formation of an iron phosphate ceramic product made from haematite by converting the haematite into a slightly lower oxidation state, thereby increasing its reactivity. For example, adding reductants, such as tin chloride or iron sulfide, during the acid-base reaction results in the conversion of Fe.sub.2 O.sub.3 to Fe.sub.2 O.sub.3-.delta.. Preferably, the haematite is reduced prior to contacting the haematite with the acid phosphate solution. The reduction of haematite is alternatively accomplished by heating haematite powder in a reducing atmosphere that contains, but is not limited to, nitrogen, high carbon, low oxygen, carbon monoxide, and/or iron, or by calcining the haematite powder in a vacuum.

EXAMPLE 1

An iron-based chemically bonded phosphate ceramic product was formed using haematite and tin chloride as a reductant. First, 100 g of haematite having a particle size of between 10-50 .mu.m was mixed with 5 wt % of tin chloride, in a powder form. The mixture was thoroughly stirred in 50 wt %

dilute phosphoric acid solution, in a solution to powder ratio of 1:1, for 30 minutes, forming a thick, pourable paste. The paste hardened after 24 hours and was completely set after 3 weeks. The resulting iron-based CBPC product was dense with negligible porosity, and had the red color of haematite. The surface appeared very glassy, and upon fracturing, the fracture surfaces were smooth and glass-like with no visible pores. X-ray diffraction showed the iron-based CBPC product to be mainly glassy, and, therefore, the newly discovered iron-based CBPC product is identified as a glass-crystalline ceramic. FIG. 1 shows an SEM micrograph of the iron-based CBPC product made from haematite.

EXAMPLE 2

An iron-based chemically bonded phosphate ceramic product was formed using haematite and iron sulfide as a reductant. 100 g of haematite having a particle size of between 10-50 .mu.m was mixed with 5 wt % of iron sulfide, in a powder form, and the mixture was thoroughly stirred in 60 wt % dilute phosphoric acid solution, in a solution to powder ratio of 1:1, for 30 minutes, forming a thick, pourable paste. The paste hardened after 24 hours and was completely set after 1 week. The resulting iron-based CBPC product was red in color, dense with negligible porosity, and was also identified as a glass-crystalline ceramic.

In an alternate embodiment, the rate of reaction in the formation of iron phosphate ceramics from iron oxide or magnetite is retarded by the addition of boric acid. It is known to control paste-setting reactions by the addition of boric acid.

EXAMPLE 3

An iron-based chemically bonded phosphate ceramic product was formed from magnetite. First, 100 g of magnetite having a particles size of between 10-50 .mu.m was mixed with 10 wt % of boric acid (retardant). The resultant mixture was thoroughly stirred in 40 to 50 wt % dilute phosphoric acid solution in a 1:1 ratio of solution to powder for 30 minutes, forming a thick, pourable paste. The paste hardened after 1 hour and was completely set in 24 hours. The iron-based CBPC product had a black color, and was dense with negligible porosity. X-ray diffraction indicated the existence of glassy and crystalline phases. FIG. 2 shows an SEM micrograph of the iron-based CBPC product made from magnetite. A similar material was also made using 50 wt % class F fly ash in the binder powder. Analysis of the iron-based CBPC products made from magnetite indicated that a considerable amount of unreacted magnetite remained in the CBPC product.

In another embodiment, the reaction rate of the acid-base reaction in the formation of an iron-based CBPC product made from magnetite was controlled by adjusting the concentration of the phosphoric acid solution and the pH. Although it is known to use phosphoric acid (H.sub.3 PO.sub.4) having a concentration of 50 wt %, magnetite or FeO may be reacted with a dilute H.sub.3 PO.sub.4 solution that is between about 30 and about 40 wt %. The H.sub.3 PO.sub.4 solution may also be partially neutralized using oxides, hydroxides, carbonates, or anhydrous phosphates prior to the acid-base reaction with FeO or Fe.sub.3 O.sub.4 to reduce the rate of the reactions for forming the iron-based CBPC products.

EXAMPLE 4

An iron-based chemically bonded phosphate ceramic product was formed from magnetite and a pH adjusted phosphoric acid. 50 wt % concentrated phosphoric acid was mixed with 5 to 15 wt % potassium carbonate (K.sub.2 CO.sub.3). 100 g of magnetite having a particle size of between 10-50 .mu.m was thoroughly stirred in the pH adjusted phosphoric acid solution, in a ratio of powder to solution of 1:2, forming a thick, pourable paste. The paste hardened after 1 hour and was completely set in 24 hours. The iron-based CBPC product had a black color, and was dense with negligible porosity. X-ray diffraction indicated the existence of glassy and crystalline phases. A similar material was also made using 50 wt % class F fly ash in the binder powder. Analysis of the iron-based CBPC products made from magnetite indicated that a considerable amount of unreacted magnetite remained in the CBPC product.

Stabilization of Metal Anions

In this embodiment, radioactive and/or hazardous waste materials containing metal anions are stabilized in CBPC products by the addition of a reducing agent to the waste and/or phosphate ceramic ingredients. The waste is generally waste containing nitrates, chlorides, sulfates, silicates, salts, heavy metals, any type of inorganic waste, and/or combinations thereof. Addition of the reducing agent to the metal anions, oxide or hydroxide powders, and/or phosphoric acid or soluble acid phosphates reduces the valency of the metal anions to a lower oxidation state during formation of the CBPC product. Incorporation of the reducing agent into CBPC formulations solves the problems experienced in the art due to the presence of metal anions in the waste stream by stabilizing the metal anions within the CBPC product in an insoluble form.

The reducing agent is preferably selected from a group including, but is not limited to, sodium monosulfide (Na.sub.2 S), potassium monosulfide (K.sub.2 S), calcium sulfide (CaS), iron sulfide (FeS), iron sulfate (FeSO.sub.4.7H.sub.2 O), sodium thiosulfate (Na.sub.2 S.sub.2 O.sub.5), sulfur dioxide (SO.sub.2), sodium borohydride (NaBH.sub.4), hydrazine, sodium bisulfite (NaHSO.sub.3), calcium hydroxide (Ca(OH).sub.2), sodium hydroxide (NaOH), sodium carbonate (Na.sub.2 CO.sub.3), sulfuric acid (H.sub.2 SO.sub.4), and formic acid (HCOOH), among others. Preferably, the reducing agent is a stannous salt, such as tin chloride (SnCl.sub.2). Table II below provides a summary of preferred reducing agents depending upon the content of the metal waste. More than one reducing agent may be used where the waste is known to contain various metal contaminants.

TABLE 11 ______________________________________ Appropriate Reducing Agents for Heavy Metal Waste Metal Waste Reducing Agent ______________________________________ Arsenic SnCl.sub.2 Chromate SnCl.sub.2, Na.sub.2 S Mercury Na.sub.2 S Selenium SnCl.sub.2 Technetium SnCl.sub.2 ______________________________________

The reducing agent may be added to the heavy metal waste, starter powder, and/or acid solution, in any combination. Preferably, the reducing agent is initially added to the heavy metal waste, resulting in the precipitation of the hazardous metals, and subsequently mixed with the CBPC powder and acid solution. The addition of the reducing agent to the waste and/or ceramic binder ingredients is largely dependent upon the type of reducing agent and its reactivity with the phosphates. For example, if a reducing agent is very strong, it is preferably to add the reducing agent to the waste-ceramic slurry early in the mixing step. In general, two to three times more than the stochiometric amount of the reducing agent is used, depending on the amount of metal present in the waste. The addition of the reducing agent results in reduction of the metal anions to their lower oxidation states, and in some cases to cations, such that the reduced metal ions are more stable and/or more readily react with the phosphate ions to form insoluble metal species, including oxides and hydroxides of the metals.

EXAMPLE 5

In this non-limiting example, the stabilization of chromium anions was improved in magnesium potassium phosphate (MKP) ceramics by incorporating waste containing chromate (Cr.sub.2 O.sub.7.sup.2-) into an MKP binder powder including a tin chloride (SnCl.sub.2) reducing agent. Addition of the reducing agent results in the reduction of the valency of the chromium anions from +6 to +3, thus decreasing the leachability of the chromate from the MKP ceramic product. In addition, the reduction of the chromate may increase the reactivity of the chromium ions with the phosphate ions in the acid solution, promoting the formation of insoluble chromium phosphate.

MKP ceramic composites loaded with 58 wt % and 70 wt % nitrate waste were fabricated by incorporating nitrate waste containing chromium into MKP binder materials, both with and without the presence of a tin chloride reducing agent.

The 58 wt % waste loaded MKP ceramic product was fabricated by first adding 50 g of the waste to 0.17 g of the reducing agent sodium monosulfide (Na.sub.2 S) and 12 g of water. About 0.5 g of boric acid may also be added as a retarder. Next, 0.86 g of a second reducing agent, tin chloride (SnCl.sub.2), was added to the waste slurry and mixed for 5 minutes. Fractions of the ceramic binder ingredients, including 4.67 g of water and a mixture of 7.69 g of magnesium oxide (MgO) and 25.64 g of potassium dihydrogen phosphate (K.sub.2 HPO.sub.4), were then added to the waste slurry in 5 minute intervals.

The 70 wt % loaded MKP ceramic waste product was fabricated by the same steps, except the amounts of ceramic binder ingredients used were 12 g water, 4.95 g MgO, and 16.48 g KH.sub.2 PO.sub.4.

Table III below provides the results of the Toxicity Characteristic Leaching Procedure (TCLP) applied to the fabricated MKP ceramic products. The results show dramatic improvements in leach resistance in MKP ceramic products fabricated in accordance with the present invention. Without the reducing agent, the MKP ceramic product failed the leaching test, while addition of the reducing agent produced an MKP ceramic product that is well below allowable EPA regulatory limits. Thus, addition of the SnCl.sub.2 reducing agent was critical to the successful stabilization and containment of the chromium.

TABLE III ______________________________________ Phosphate Systems and Processing Details CBPC Product Fabricated Nitrate Leach Resistance Waste Chromium EPA TCLP Loading Concentration Addition of Regulatory Results (wt %) (ppm) SnCl.sub.2 Limit (ppm) (ppm) ______________________________________ 58 300 NO 0.86 10.3 58 300 YES 0.86 0.02 70 360 NO 0.86 16.3 70 610 YES 0.86 0.04 ______________________________________

EXAMPLE 6

Technetium-99 (.sup.99 Tc) is present in some high-level wastes (HLW), in addition to other volatile fission products, including cesium-137 (.sup.137 Cs) and strontium-90 (.sup.90 Sr). Under oxygen-containing conditions, the predominant form of technetium is the pertechnetate anion, TcO.sub.4.sup.-, which is highly soluble in water and readily mobile in the environment. Immobilization of technetium-99 is of critical concern, because of its high leachability and long half life (e.g., 2.13.times.10.sup.5 years). Technetium-99 was successfully stabilized in the MKP ceramic product, in accordance with the present invention, in that the addition of stannous chloride reduced the oxidation state of technetium-99 from +7 to +4. The waste solution used in this example was a stripping solution generated by a complexation and elution process developed at the Los Alamos National Laboratory (LANL) to separate technetium-99 from HLW, and contained approximately 20 ppm to 150 ppm of technetium-99.

In a first approach, eluted aqueous waste was directly stabilized, such that the water in the waste was used in the CBPC fabrication process. CBPC products were fabricated with and without the addition of the reducing agent, tin chloride (SnCl.sub.2). MKP ceramic products were formed from 19.973 g of LANL stripping solution containing about 40 ppm .sup.99 Tc by adding the stripping solution to a binder mixture including 2.48 g SiO.sub.2, 8.38 g of MgO, and 28.28 g of KH.sub.2 PO.sub.4, and mixing the mixture for 20 minutes, resulting in a fine slurry. The reducing agent, 1.16 g of tin chloride (SnCl.sub.2) was added to the mixtures after 18 minutes of mixing. No water was added during the process. MKP ceramic products were similarly formed, by the same process, from 1.712 g of LANL stripping solution that did not contain any .sup.99 Tc.

The resulting fine slurries were transferred into molds, and allowed to set. The typical temperature rise during setting was between about 55.degree. C. to about 70.degree. C. The slurries hardened into a dense monolithic MKP ceramic products in about 2 hours. After at least 14 days of curing, the resulting MKP ceramic products fabricated directly from the LANL elution solution had a density of 1.8 g/cm.sup.3, a very low open porosity of 4%, and a compression strength of 30.+-.6.7 MPa, a compression strength significantly higher than the land disposal compression strength requirement of 3.4 MPa, demonstrating the MKP ceramic product's superior dense, hard, high-strength structure. The MKP ceramic products were tested for strength, leaching, and water immersion, evidencing that the addition of the reducing agents helped to maintain the .sup.99 Tc in its relatively insoluble cationic form, Tc.sup.+4. The optimal loading of the elution solution in the MKP ceramic product was 35%, and the concentrations of the .sup.99 Tc in the MKP ceramic products were in the range of between about 20 to about 150 ppm.

The leachability of .sup.99 Tc is highly dependent upon its oxidation state, and, therefore, it is important to establish redox conditions in the phosphate slurry during fabrication of the MKP ceramic product. FIG. 1 is a graphical illustration of an Eh/pH diagram for both a Re--O--H and a Tc--O--H system in the pH range of 5 to 10. Also shown are experimentally determined Eh values as a function of pH for the MKP slurries containing eluted waste, measured at various elution solution loadings, wherein rhenium was added to the elution solution as a substitute for .sup.99 Tc. For example, FIG. 3 shows that the Eh and pH values of the slurry with 36% .sup.99 Tc elution solution loading under the normal setting conditions are +225 mV and 6.5, respectively. In the pH range of 5 to 10 pH, Eh values of Re are in the highly soluble heptavalent oxidation state. For a pH of less than 7, Eh values of .sup.99 Tc are in the insoluble TcO.sub.2 (Tc.sup.+4) oxidation state, while for a pH of greater than 7, .sup.99 Tc is present as TcO.sub.4.sup.- (Tc.sup.+7). The MKP slurry and setting conditions prescribed by the present invention (e.g., 36% loading, 6 to 7 pH, Eh +225 mV) are highly conducive to maintaining .sup.99 Tc in the insoluble +4 oxidation state. Importantly, the addition of a reducing agent to the slurry critically aids in the reduction of TcO.sub.4.sup.- (Tc.sup.+7) to its stable and insoluble +4 oxidation state.

In a second approach, technetium-99 was precipitated from the eluted solution by heating in the presence of zinc and 4 M hydrochloric acid, and the precipitated technetium-99 (TcO.sub.2.2H.sub.2 O) was incorporated into the MKP ceramic product. The loadings of the technetium-99 in the MKP ceramic products were as high as 900 ppm. Since it is well known that TcO.sub.2.2H.sub.2 O is highly insoluble, with a solubility of 10.sup.-7 to 10.sup.-8 mol/L in water under mildly reducing conditions, precipitation of the technetium-99 as the highly insoluble TcO.sub.2.2H.sub.2 O, followed by encapsulation in the MKP matrix, yields a superior, stabilized phosphate ceramic product, with a higher loading than that accomplished by the direct elution method of the first approach.

Technetium-99 is generally precipitated from LANL stripping solutions by adding zinc to the LANL stripping solution, adding HCl to the mixture, and boiling the mixture at about 70.degree. C. for about 45 minutes. This process results in the precipitation of TcO.sub.2.2H.sub.2 O with about a 40% recovery of .sup.99 Tc.

Table IV below provides results of diffusivity and leachability testing of the MKP ceramic products loaded with .sup.99 Tc, fabricated with and without the reducing agent step. These results demonstrate that the MKP ceramic products fabricated with the addition of the reducing agent (SnCl.sub.2) provide significantly improved retention of contaminants within the MKP matrix.

TABLE IV ______________________________________ ANS 16.1 Results for MKP Ceramic Products Containing .sup.99 Tc CBPC Product Fabricated Test Results .sup.99 Tc Effective Concentration Diffusivity Leachability Composition (ppm) (cm.sup.2 /s) Index ______________________________________ MKP + Eluted Waste 20 1.20E-09 8.92 MKP + Eluted Waste 40 2.95E-09 8.53 MKP + SnCl.sub.2 + Eluted Waste 20 2.9E-12 11.54 MKP + SnCl.sub.2 + Eluted Waste 40 5.4E-12 11.27 MKP + SnCl.sub.2 + Eluted Waste 124 3.8E-15 14.42 MKP + SnCl.sub.2 + Precipitated 41 2.2E-14 14.6 .sup.99 Tc MKP + SnCl.sub.2 + Precipitated 16 4 2.3E-13 13.3 .sup.99 Tc MKP + SnCl.sub.2 + Precipitated 903 7.2E-15 14.6 .sup.99 Tc ______________________________________ * The Nuclear Regulatory Commission (NRC) requires a leachability index o at least 6.0.

Table V below provides results of Product Consistency Test (PCT) conducted on the MKP ceramics fabricated according to the second, precipitation approach. Normalized leaching rates of .sup.99 Tc, after a 7-day test period at room temperatures, e.g., 25.degree. C., were reported as low as 1.times.10-3 g/m.sup.2 -d. At an elevated temperature, e.g., 90.degree. C., the dissolution rate of the matrix increases, and, therefore, the normalized leaching rate for .sup.99 Tc also increased to the 10.sup.-2 to 10.sup.-1 g/m.sup.2 -d range. Significantly, for both the room temperature and elevated temperature testes, the MKP ceramics with the highest .sup.99 Tc loadings demonstrated the lowest normalized leaching rate. The PCT test was initially designed to evaluate chemical durability of crushed borosilicate glass. A comparison between PCT test results at 90.degree. C. for high-temperature encapsulation of .sup.99 Tc in borosilicate glass, resulting in a leach rate as low as 10.sup.-2 g/m.sup.2 -d, versus the low-temperature encapsulation of .sup.99 Tc within MKP ceramics stabilized with a reducing agent, as reported in Table IV, show that the present invented low-temperature encapsulation is a viable and competitive approach.

TABLE V ______________________________________ PCT Results for MKP Ceramic Products Containing .sup.99 Tc Test .sup.99 Tc Temper- Concen- Normalized ature tration Leaching Composition (.degree. C.) (ppm) Rate (g/m.sup.2 -d) ______________________________________ MKP + SnCl.sub.2 + Precipitated .sup.99 Tc 25 40 3.9E-3 MKP + SnCl.sub.2 + Precipitated .sup.99 Tc 25 164 8.5E-3 MKP + SnCl.sub.2 + Precipitated .sup.99 Tc 25 903 1.1E-3 MKP + SnCl.sub.2 + Precipitated .sup.99 Tc 90 40 7.2E-2 MKP + SnCl.sub.2 + Precipitated .sup.99 Tc 90 164 1.1E-1 MKP + SnCl.sub.2 + Precipitated .sup.99 Tc 90 903 3.6E-2 ______________________________________

The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments described explain the principles of the invention and practical applications and should enable others skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. While the invention has been described with reference to details of the illustrated embodiment, these details are not intended to limit the scope of the invention, rather the scope of the invention is to be defined by the claims appended hereto.



US Patent # 6,153,809

Polymer Coating for Immobilizing Soluble Ions in a Phosphate Ceramic Product

Singh, et al.

Abstract: A polymer coating is applied to the surface of a phosphate ceramic composite to effectively immobilize soluble salt anions encapsulated within the phosphate ceramic composite. The polymer coating is made from ceramic materials, including at least one inorganic metal compound, that wet and adhere to the surface structure of the phosphate ceramic composite, thereby isolating the soluble salt anions from the environment and ensuring long-term integrity of the phosphate ceramic composite.

References Cited

U.S. Patent Documents: 4315831 ~ 5318730 ~ 5402455 ~ 5645518 ~ 5830815 ~ 5846894
Other References: Modified Phosphate Ceramics for Stabilization and Solidification of Salt Mixed Wastes, authored by Dileep Singh, Kartikey Patel, Arun S. Wagh, Seung-Young Jeong, published in the Proceedings of Spectrum '98, International Conference on Decommissioning and Decontamination and on Nuclear and Hazardous Waste Management, Denver, Colorado, Sep. 13-18, 1998. .
Modified Phosphate Ceramics for Stabilization of Salt Mixed Wastes, thesis authored by Kartikey Patel, submitted for review on or after May 7, 1998 and defended on Jun. 26, 1998. .
U.S. Patent Application Serial No. .sub.-- /124,822, Continuation-in-part of U.S. Patent Nos. 5,846,894 and 5,830,815. .
U.S. Patent Application Serial No. .sub.-- /617,284, Continuation-in-part of U.S. Patent No. 5,830,815. .
PCT Patent Application No. PCT/US97/04132.

Description

TECHNICAL FIELD

This invention relates to the use of chemically bonded phosphate ceramics (CBPCs) for immobilizing large volumes of low-level mixed waste material, and, in particular, to a polymeric coating that increases the leach resistance in CBPCs encapsulating waste containing salt anions.

BACKGROUND OF INVENTION

Low-level mixed waste streams are composed of aqueous liquids, heterogeneous debris, inorganic sludge and particulates, organic liquids, and soils. Of particular concern are low-level mixed waste streams that are high in salt content, especially those salt waste streams generated as sludge and solid effluents in nuclear processing applications. For example, the extraction of plutonium and uranium from their ore matrices by the use of strong acids or precipitation techniques produces nitrate salt and heavy metal waste. Chemical compositions typically found in salt waste streams, either high in chloride or high in nitrate, include aluminum trihydroxide, sodium phosphate, MicroCel E (CaSiO.sub.3), water, sodium chloride, trichloroethylene, calcium sulfate, sodium nitrate, and oxides of lead, chromium, mercury, iron, cadmium, and nickel, among other compounds.

Stabilization of salt waste requires that the contaminants and soluble salt anions are effectively immobilized. No single stabilization and solidification technology is known to successfully treat and dispose of salt waste, due to the physical and chemical diversity of salt waste streams. Generally, stabilization refers to the conversion of the waste into a less soluble form, while solidification refers to the micro-encapsulation of the waste in a monolithic solid of high structure integrity. Conventional thermal waste treatment methods, such as incineration or vitrification, are expensive and largely unsuitable for the treatment of salt waste streams because of their reliance on high temperature steps that risk the release of volatile contaminants and the generation of undesirable (e.g., pyrophoric) secondary waste streams. In addition, thermal treatments produce hot spots that affect the quality of a solidified final waste form.

A low-temperature approach is to stabilize hazardous waste by using inorganic (e.g., pozzolanic) binders, such as cement, lime, kiln dust, and/or fly ash. Disadvantages of this approach include a high sensitivity to the presence of impurities, high porosity, and low waste loading volume. Organic binders (e.g., thermosetting polymers) are used even less frequently, because of cost and greater complexity of application. Organic binders are not compatible with water-based wastes, unless the waste is first pre-treated and converted to an emulsion or solid, and organic binders are subject to deterioration from environmental factors, including biological action and exposure to ultraviolet light.

Recently, an alternative non-thermal, low-temperature approach has been developed at Argonne National Laboratory for stabilizing and solidifying low-level mixed waste by incorporating or loading the waste into a phosphate ceramic waste form having a high structural integrity. This technique immobilizes the waste by solidification, such that the waste is physically micro-encapsulated within the dense matrix of the phosphate ceramic waste form, and/or stabilizes the waste by converting the waste into their insoluble phosphate forms. Ceramic encapsulation systems are particularly attractive given that the bonds formed in these systems are ether ionic or covalent, and hence stronger than the hydration bonds in cement systems. Also, the ceramic formulation process is exothermic and economical.

Phosphates are particularly good candidates for stabilization of radioactive and hazardous waste, because phosphates of radio nuclides and hazardous metals are essentially insoluble in groundwater. A salient feature of the low-temperature ceramic phosphate formulation process is an acid-base reaction. For example, magnesium phosphate ceramic waste forms have been produced by reacting magnesium oxide (MgO) with phosphoric acid to form a phosphate of magnesium oxide, Newberyite, as represented in the chemical equation (1), below.

The acid-base reaction results in the reaction of the waste components with the acid or acid-phosphates, leading to chemical stabilization of the waste. In addition, encapsulation of the waste in the phosphate ceramic results in physical containment of the waste components. The reaction represented above in Equation (1) occurs rapidly and generates heat, and upon evaporation of the water, a porous ceramic product results.

U.S. Pat. No. 5,645,518 issued to Wagh, et al., incorporated herein by reference, describes in detail the process steps for setting liquid or solid waste in CBPC products using acid-base reactions. Accordingly, the process involves mixing ground solid waste, including salt waste spiked with heavy metals, with a starter powder of oxide and hydroxide powders of various elements; slowly adding the waste-powder mixture to an acid solution of phosphoric acid or soluble acid phosphates; thoroughly mixing the waste-powder-acid mixture for about a half hour to an hour at ambient temperatures (less than 100.degree. C.), such that the components of the mixture chemically react to form stable phases and a reacted viscous slurry or paste results; and allowing the slurry or paste to set for a few hours into the final CBPC product. Liquid waste is similarly stabilized by mixing the liquid waste with the acid solution (preferably 50:50), and then reacting the waste-acid mixture with the starting powders. The maximum temperature for the process is about 80.degree. C. The CBPC products attain full strength in about three weeks, and exhibit a complex structure, including a major crystalline phase, e.g., Newberyite (MgHPO.sub.4.3H.sub.2 O), and an insoluble, stable phase. The waste components are generally homogeneously distributed within the phosphate ceramic matrix. Unfortunately, however, the porous product (Newberyite) is unsuitable for waste treatment on a large scale.

U.S. Pat. No. 5,830,815 issued to Wagh, et al., incorporated herein by reference, describes improving the CBPC fabrication process by incorporating two temperature control processes for both reducing heat generation during the encapsulation (reaction) steps and moderating pH conditions (some wastes are unstable at a low pH). The first temperature control process involves pre-treating the phosphoric acid with a carbonate, bicarbonate or hydroxide of a monovalent metal (e.g., K, Na, Li, Rb) prior to mixing with an oxide or hydroxide powder so as to buffer the acid. In particular, potassium containing alkali compounds (e.g., K.sub.2, KHCO.sub.3, KOH) result in a more crystalline waste form, and the higher the concentration of potassium in the potassium containing compound, the more crystalline the final product, resulting in a higher compression strength, lower porosity, and greater resistance to weathering, compressive forces, and leaching. The second temperature control process involves bypassing the use of the acid and mixing the oxide powder with dihydrogen phosphates of potassium, sodium, lithium, or other monovalent alkali metal, to form a ceramic at a higher pH.

Neutralizing the phosphoric acid solution in equation (1) by adding potassium hydroxide (KOH), as represented in the chemical equation (2) below, reduces the reaction rate and heat generation, and results in the formation of a superior magnesium potassium phosphate (MKP) mineral product, MgKPO.sub.4.6H.sub.2 O (magnesium potassium phosphate hexahydrate), as represented in chemical equation (3) below.

The chemically bonded ceramic phosphate (CBPC) waste form (e.g, MgKPO.sub.4.6H.sub.2 O) is a dense, hard material with excellent durability and a high resistance to fire, chemicals, humidity, and weather. The low-temperature (e.g., room-temperature) process encapsulates chloride and nitrate salts, along with hazardous metals, in magnesium potassium phosphate (MKP) ceramics, with salt waste loadings of up to between approximately 70 weight percent and approximately 80 weight percent. This durable MKP ceramic product has been extensively developed and used in U.S. Department of Energy waste treatment projects.

Phosphates in general are able to bind with hazardous metals in insoluble complexes over a relatively wide pH range and most metal hydroxides have a higher solubility than their corresponding phosphate forms. In addition to the magnesium and magnesium-potassium phosphate waste products discussed above, known waste encapsulating phosphate systems include, but not limited to, phosphates of magnesium-ammonium, magnesium-sodium, aluminum, calcium, iron, zinc, and zirconium (zirconium is preferred for cesium encapsulation). A non-exclusive summary of known phosphate systems and processing details is provided in Table I below, selected according to ready availability of materials and literature about the materials, in addition to low cost.

TABLE I ______________________________________ Phosphate Systems and Processing Details Curing System Starting Materials Solution Time ______________________________________ MKP Ground MgO, ground K Water 1 hour dihydrophosphate crystals Mg phosphate Calcined MgO Phosphoric >8 days acid-water (50/50) Mg--NH.sub.4 phosphate Crushed dibasic NH.sub.4 Water 21 days phosphate crystals mixed w. calcined MgO Mg--Na phosphate Crushed dibasic Na Water 21 days phosphate crystals mixed w. calcined MgO Al phosphate Al(OH).sub.3 powder Phosphoric Reacted acid powder, (.apprxeq.60.degree. C.) pressed Zr phosphate Zr(OH).sub.4 Phosphoric 21 days acid ______________________________________

Appropriate oxide powders include, but are not limited to, MgO, Al(OH).sub.3, CaO, FeO, Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Zr(OH).sub.4, ZrO, and TiO.sub.2, and combinations thereof. The oxide powders may be pre-treated (e.g., heated, calcined, washed) for better reactions with the acids. While no pressure is typically applied to the reacted paste, about 1,000 to 2,000 pounds per square inch may be applied when zirconium-based powders are used.

The acid component may be dilute or concentrated phosphoric acid or acid phosphate solutions, such as dibasic or tribasic sodium, potassium, or aluminum phosphates, and the paste-setting reactions are controllable either by the addition of boric acid to reduce the reaction rate, or by adding powder to the acid while concomitantly controlling the temperature.

Representative bulk constituents for salt waste include, but are not limited to, activated carbon, Na.sub.2 (CO.sub.3).sub.2, widely used cation or anion exchange resins, water, NaCl, Na(NO.sub.3).sub.2, Na.sub.3 PO.sub.4, and Na.sub.2 SO.sub.4. The salt waste may be reacted with phosphoric acid to any consume carbon dioxide (CO.sub.2) present, prior to mixing the salt waste with the oxide powders or binding powders, as the evolution of CO.sub.2 results in very porous final ceramic products.

Unfortunately, however, encapsulation of low-level mixed waste into CBPC products is currently of limited practical use for waste that is predominantly comprised of salts, such as chlorides, nitrates, and sulfates. Efforts to encapsulate salt waste in phosphate ceramic products are hampered by low maximum waste loading capacities, because of interference of the salt anions with ceramic-setting reactions, leaching of soluble salt anions from the resulting highly porous ceramic product (especially in aqueous environments), and rapid structural degradation of the ceramic product caused by the high leach rates. Also, environmental stresses degrade the integrity of known CBPC waste forms over time. For example, exposure to repeated cycles of wetting, drying and/or freezing, or acidic or other conditions conducive to leaching may affect the long term effectiveness of waste encapsulated CBPC waste forms.

A need in the art exists for a method for disposing of salt waste that involves a low-temperature stabilization process and improves resistance to leaching, without degrading the integrity of the ceramic phosphate product.

The present invention is a process and product for safely containing radioactive and/or hazardous waste comprised of salt anions in a phosphate ceramic product, involving a new and surprisingly effective immobilization technique. The invented process and product involves the application of a specific polymer coating to the exterior surface of a phosphate ceramic composite encapsulating waste, such that the polymer coating infiltrates the surface structure and adheres to and/or bonds to the phosphate ceramic composite matrix, effectively isolating the waste from the environment and improving the leach resistance of the phosphate ceramic composite. The polymer coating contains at least one inorganic metal compound, preferably an inorganic metal oxide of magnesium or silicon.

Therefore, in view of the above, a basic object of the present invention is to provide an improved process and product for immobilizing hazardous, radioactive, and/or mixed salt waste in phosphate ceramic composites.

Another object of the invention is to provide a safe, low temperature, economical process and product for immobilizing salt waste in a phosphate ceramic product that increases the loading capacity and improves the leach resistance of the salt waste within the phosphate ceramic product.

A further object of the invention is to provide process and product for immobilizing large volumes of salt waste in a durable, long term storage phosphate ceramic product.

Additional objects, advantages, and novel features of the invention are set forth in the description below and/or will become apparent to those skilled in the art upon examination of the description below and/or by practice of the invention. The objects, advantages, and novel features of the invention may be realized and attained by means of instrumentation and combinations particularly pointed out in the appended claims.

BRIEF SUMMARY OF THE INVENTION

Briefly, the present invention is a surprisingly effective process and product for immobilizing waste having a high concentration of salt in chemically bonded phosphate ceramic (CBPC) products. The invention involves a new coating step, wherein a select polymer coating is applied to the surface of a fabricated salt waste loaded CBPC product, such that the coating infiltrates the surface structure of the CBPC product and adheres to the phosphate ceramic matrix, thereby isolating soluble salt anions from the environment and ensuring long-term integrity of the phosphate ceramic system. The fabricated salt waste loaded CBPC product is formulated by methods known in the art.

A critical feature of the invention is the selection of the polymer coating, which contains at least one inorganic metal compound. Preferably, the polymer coating is a polymer resin comprised of fine powders of magnesium oxide and/or silicon oxide. The powders of the coating material act as wetting agents that apparently cause mechanical and/or chemical bonding between the phosphate ester in the surface structure of the CBPC product and the polymer coating composition. The polymer coating infiltrates and macro-encapsulates the CBPC product to improve durability and leach resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

The appended claims set forth those novel features which characterize the invention, however, the invention itself, as well as further objects and advantages thereof, will best be understood with reference to the following detailed description of a preferred embodiment, in conjunction with the accompanying drawings, where like reference characters identify like elements throughout the various figures, in which:

FIG. 1 is a schematic diagram of a method for fabricating magnesium potassium phosphate (MKP) ceramic waste products loaded with surrogate salt waste;

FIG. 2 is a scanning electron microscopy (SEM) photomicrograph of a fractured surface of an MKP ceramic waste product loaded with 58% surrogate salt waste;

FIG. 3 is a high magnification (2000.times.) scanning electron microscopy (SEM) photomicrograph of the surface of a polymer coated MKP ceramic waste product loaded with surrogate salt;

FIG. 4 is a very high magnification (7500.times.) scanning electron microscopy (SEM) photomicrograph of the surface of a polymer coated MKP ceramic waste product loaded with surrogate salt waste;

FIG. 5 is a low magnification (350.times.) scanning electron microscopy (SEM) photomicrograph of the interface between a MKP ceramic waste product loaded with surrogate salt waste and a polymer coating applied thereon;

FIG. 6 is a high magnification (2000.times.) scanning electron microscopy (SEM) photomicrograph of the interface between a MKP ceramic waste product loaded with surrogate salt waste and a polymer coating applied thereon; and

FIG. 7 is a graphical illustration of cumulative nitrate leaching for MKP ceramic products with and without the invented polymer coating.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an improved process and product for immobilizing waste in a chemically bonded phosphate ceramic (CBPC) waste form. As described in detail in the background section above, although methods for fabricating CBPC products encapsulating waste materials are well known, the known CBPC encapsulation methods are ineffective for containing wastes having a high concentration of salt.

The present invention modifies known CBPC encapsulation methods and products to include a unique immobilization step that specifically addresses problems experienced in the art due to the presence of soluble salt anions in the waste stream. According to the present invention, a polymer coating is applied to the exterior surface of the CBPC product to infiltrate the complex surface structure of the CBPC product and bond and/or adhere thereto, such that salt waste is effectively macro-encapsulated with in phosphate ceramic matrix and isolated from the environment. Advantageously, the polymer surface coating protects the CBPC waste form from environmental stresses by providing a greater resistance to air, water, organic liquids, acids, and alkalis, among other conditions. The polymer surface coated CBPC waste form also has improved mechanical properties, such as greater hardness and high abrasion resistance.

The polymer coating has three main components: the binder, the pigment, and the solvent. The binder provides adhesion and cohesion between the coating and the CBPC surface, the pigment is a fine powder that provides the coating with color and hardness, and the solvent is a volatile liquid for dissolving solid or semi-solid binders. The pigment has considerable influence on the consistency of the properties of the polymer coating and contributes to its abrasion and weather resistance.

A feature of the invention is the inclusion of at least one inorganic metal compound in the binder component of the polymer coating. Preferred inorganic metal compounds are inorganic metal oxides, such as magnesium oxide (MgO) and/or silicon oxide (SiO.sub.2). These inorganic metal compounds may be in the form of magnesite (MgCO.sub.3), talc (Mg.sub.2 (Si.sub.2 O.sub.5).sub.2.Mg(OH).sub.2), or borosilicate glass (i.e., silicate glass with at last 5% boron oxide). These ceramic materials provide an excellent interface adhesion between the polymer coating and the surface and infiltrated structure of the CBPC product, apparently caused by mechanical and chemical interactions between the phosphate ester comprising the CBPC product and the ceramic coating composition. Polymer coating materials that do not contain ceramic, inorganic metal compounds peel off of the surface of the phosphate ceramic product after curing.

The most preferred polymer material is a commercially available thermoset polyester resin that is comprised of a polyester resin binder, magnesite, talc, or soda-lime glass pigment, a styrene monomer solvent, and also a benzoyl peroxide initiator. Generally preferred polymer coatings are comprised of unsaturated polyester resins that are straight-chain polymers having reactive double bonds at intervals along the chain. In their popular form, unsaturated polyester resins are supplied as solutions in vinyl monomer (e.g., styrene), and copolymerization is activated by the addition of an initiator (e.g., organic peroxides or hydroperoxides) and promoters (e.g., metallic dryers, cobalt octoate, naphthenate). Copolymerization results in the cross-linking of polyester chains by the formation of polmerized vinyl monomors.

According to the preferred method of the present invention, the polymer material is applied to the exterior surface of a phosphate ceramic product as a thin film by adding the initiator to the pigment and the binder, mixing the initiator-pigment-binder composition for a few minutes to form a slurry, uniformly coating the exterior surface of the phosphate ceramic product with the slurry, and chemically drying the coating by allowing sufficient time for the slurry to infiltrate the phosphate ceramic product surface, such that the slurry completely wets and adheres to the surface. Although the polymer coating hardens in about ten minutes, a curing time of 24 hours is preferred. The polymer coating is subjected to a chemical drying step, e.g., curing, a process in which the molecules of the binder chemically react with one other to form bonds within the film by primary valences. These bonds are very strong and not susceptible to dissolution by the action of solvents. Thus, a feature of the invention is the subjection of the surface coated CBPC product to a chemical drying step that converts the coating from a fluid to a solid state, wherein chemical reactions occur to anchor the thin film coating to the CBPC surface.

Table II below provides the results of the American Nuclear Society's ANS 16.1 Standard Test for nitrate and chloride loaded polymer coated MKP ceramic products. Generally, the ANS 16.1 Standard Test studies leachability of contaminants contained in matrices in an aqueous environment over time and evaluates retention rates by calculating a leachability index value from the test data. (The leachability index is the negative logarithm of the effective diffusivity coefficient). Sample polymer coated salt loaded MKP ceramic products were placed in the leaching solution for a fixed period of time, after which the leaching solution was analyzed for specific ions. As shown in Table II, the chloride leaching was excessively low, with the chloride ion reading below the detection limit even after a cumulative 96 hours of exposure. The nitrate leaching was relatively higher.

TABLE II ______________________________________ Cumulative Leaching of Chloride and Nitrate Ions from Polymer Coated MKP Ceramic Products Cumulative Chloride Ion (Cl.sup.-) Nitrate Ion (NO.sub.3.sup.-) Time (hours) (ppm) (ppm) ______________________________________ 2 ND 3.96 7 ND 5.28 24 ND 2.20 48 ND 3.08 72 2.64 96 ND 2.20 456 3.4 13.20 1128 * 43.12 2136 * 176.00 ______________________________________ ND indicates None Detected; *indicates test in progress.

Salt waste is generally highly reactive and therefore its flammability is of concern, in view of transportation and storage issues. Department of Transportation (DOT) oxidation tests conducted on polymer coated salt loaded phosphate ceramic products demonstrated that because phosphate ceramics are inorganic ceramic-type materials, they advantageously inhibit the spread of flames and are an excellent solidification medium for flammable salt waste.

The resulting phosphate ceramic materials may be used to produce building and construction materials, e.g., engineering barrier systems.

EXAMPLE

Nitrate Loaded Polymer Coated MKP Ceramic Product

Surrogate waste having the composition listed below in Table III was prepared in the laboratory and mixed for 72 hours using mixing rollers. The surrogate waste was chemically treated by mixing the surrogate waste first with an aqueous solution containing a small amount of sodium monosulfide (Na.sub.2 S) for about 8 to 10 minutes to efficiently convert mercury (Hg) into its most stable form of mercury sulfide (HgS), and next treating the surrogate waste with tin chloride (SnCl.sub.2) for about 5 minutes to reduce the valency of chromium from +6 to a less toxic, less water soluble oxidation state of +3.

TABLE III ______________________________________ Surrogate Waste Composition Constituent wt % Contaminant ppm ______________________________________ Fe.sub.2 O.sub.3 6.0 PbO 1000 Al.sub.2 (OH).sub.3 4.0 CrO.sub.3 1000 Na.sub.3 PO.sub.4 2.0 HgO l000 Mg(OH).sub.2 4.0 CdO 1000 CaSiO.sub.3 8.0 NiO 1000 Portland Cement 2.0 H.sub.2 O 14.0 NaNO.sub.3 (nitrate salt) 60.0 ______________________________________

Magnesium potassium phosphate (MKP) ceramic waste products incorporating the surrogate waste were fabricated by methods generally shown in FIG. 1 for waste loadings of 58% and 70%. Accordingly, a binder was formed by spontaneously reacting a stoichiometric amount of well mixed, calcined magnesium oxide (MgO) powder and monopotassium phosphate (KH.sub.2 PO.sub.4), under aqueous conditions and constant stirring, in four successive batches at one minute intervals, to produce magnesium potassium phosphate (MgKPO.sub.4.6H.sub.2 O), according to Equation (3) above. The resulting binder has a highly crystalline ceramic structure and a solubility product constant as low as 10.sup.-12.

The chemically treated surrogate waste and binder were combined to form a slurry that initially experienced a few degrees decrease in temperature due to the dissolution of the phosphate crystals in the water. Upon dissolution of the phosphate, the temperature increased to about 35.degree. C., and the slurry having a pH of about 6 to 7 was stirred thoroughly for about 18 to 20 minutes, or until the slurry started to set. The slurry was hardened in molds for about 2 to 5 hours, resulting in dense, monolithic, chemically bonded phosphate ceramic (CBPC) waste products. After 14 days of curing, the CBPC waste products were subjected to variance performance tests, including strength, leaching and characterization.

FIG. 2 is a high magnification (2000.times.) scanning electron microscopy (SEM) photomicrograph of a fractured surface of a magnesium potassium phosphate (MKP) ceramic waste product loaded with 58% surrogate salt waste. The photomicrograph shows a very dense, crystalline structure with a small amount of pores. Pores allow water to penetrate the waste form, causing nitrates to (e.g., NaNO.sub.3) to dissolve and leach into the environment.

According to the present invention, a select number of the CBPC waste products were coated with an unsaturated polyester resin system to further immobilize the surrogate waste within the CBPC waste products. FIGS. 3 and 4 show high (2000.times.) and very high (7500.times.) magnification SEM photomicrographs, respectively, of the polymer coated surface of a CBPC waste product. The photomicrographs show a very smooth, substantially pore free surface structure, demonstrating a very low possibility for water to penetrate into the polymer coated CBPC waste product through its surface structure, and the prevention of nitrate dissolution and subsequent leaching. FIGS. 5 and 6 show low (350.times.) and high (2000.times.) magnification SEM micrographs of the interface between a CBPC waste product loaded with surrogate waste and a polymer coating applied thereon. As shown in FIGS. 5 and 6, the polymer coating has completely wet and adhered to the phosphate ceramic surface, resulting in a CBPC waste product having superior leaching performance. The polymer coating-CBPC waste product interface also appears to be essentially free of cracks demonstrating high compression strength and excellent compatibility between the polymer coating and the CBPC waste product.

Table IV below provides the results of density and compression strength tests conducted on the uncoated and polymer coated magnesium potassium phosphate (MKP) ceramic products loaded with 58 weight percent and 70 weight percent nitrate salts. The compression strength of the waste forms are well above of the Nuclear Regulatory Commission (NRC) minimum requirement of 500 psi.

TABLE IV ______________________________________ Structure Properties of MKP and Nitrate Waste Products Uncoated Uncoated Polymer Coated 58 wt % Salt 70 wt % Salt 58 wt % Salt Property Waste Waste Waste ______________________________________ Density (g/cc) 1.893 2.000 1.691 Compression Strength 1400 .+-. 160 1900 .+-. 180 1970 (PSI) ______________________________________

FIG. 7 is a graphical illustration of cumulative nitrate leaching for nitrate loaded MKP ceramic products with and without the polymer (unsaturated polyester resin) coating. As depicted, the polymer coated nitrate loaded MKP ceramic product immobilized the nitrate ions significantly more effectively than the uncoated nitrate loaded MKP ceramic product. A comparison of the leachability index for the polymer coated nitrate loaded MKP ceramic product versus an uncoated nitrate loaded MKP ceramic product is provided in Table V, below. The calculated leachability index for the polymer coated nitrate loaded MKP ceramic product was greater than 12, substantially above the ANS 16.1 standard leachability index of at least 6.0. Generally, the leachability index is related to the effective diffusivity in that the higher the leachability index, the lower is the effective diffusivity, resulting in a more favorable retention of a contaminant within a matrix. These results demonstrate that the essentially pore free surface structure of the polymer coated salt waste loaded MKP ceramic product provides superior immobilization of the waste salts than uncoated salt loaded phosphate ceramic products currently known in the art.

TABLE V ______________________________________ ANS 16.1 Results for Various Waste Containment Products NO.sub.3.sup.- in Waste Fraction Waste Containment of NO.sub.3.sup.- Effective Containment Product Leached Diffusivity Leachability Product (ppm) Out (cm.sup.2 /s) Index (LI) ______________________________________ Uncoated, 58 wt % 218700 0.33 6.31 .times. 10.sup.-8 7.20 Loaded Uncoated, 70 wt % 260600 0.35 5.82 .times. 10.sup.-8 7.24 Loaded Polymer Coated 218700 0.0169 6.87 .times. 10.sup.-13 12.16 58 wt % Loaded ______________________________________

Alternative coating systems were tested, including fly ash coatings, epoxy resins, and rubber derivatives. The fly ash coating system exhibited excellent film integrity and good waste form compatibility, while the epoxy resin and rubber derivative coating systems demonstrated very poor film integrity and waste form compatibility.

The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments described explain the principles of the invention and practical applications and should enable others skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. While the invention has been described with reference to details of the illustrated embodiment, these details are not intended to limit the scope of the invention, rather the scope of the invention is to be defined by the claims appended hereto.



US Patent # 6,204,214

Pumpable/injectable phosphate-bonded ceramics

S. Dileep, et al.

Abstract -- A pumpable ceramic composition is provided comprising an inorganic oxide, potassium phosphate, and an oxide coating material. Also provided is a method for preparing pumpable ceramic-based waste forms comprising selecting inorganic oxides based on solubility, surface area and morphology criteria; mixing the selected oxides with phosphate solution and waste to form a first mixture; combining an additive to the first mixture to create a second mixture; adding water to the second mixture to create a reactive mixture; homogenizing the reactive mixture; and allowing the reactive mixture to cure.

References Cited
U.S. Patent Documents
4347325 ~2391493 ~ 3357843 ~ 3392037 ~ 3540897 ~ 3647488 ~ 3920464 ~ 3923534 ~ 3960580 ~ 4160673 ~ 275091 ~ 4298391 ~ 4444594 ~ 4459156 ~ 4836854 ~ 4843044 ~ 4921536 ~

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a pumpable/injectable ceramic, and more particularly this invention relates to a ceramic composition that maintains low viscosity for extended periods of time to allow the composition to be pumped or injected into hard to reach geologic or manmade locations.

2. Background of the Invention

Disposal of hazardous waste, low-level radioactive waste or benign waste continues to present problems. Landfill space is becoming more scarce so that only nonrecyclable material or nonbiodegradable material is often considered the only candidates for land filling.

When hazardous material is land filled, care must be taken to prevent destabilization of the material, so that leaching will not occur.

Aside from land filling, efforts have been made to combine to-be-disposed-of-material with cement so as to form solid monoliths or waste forms for burial or for use as structural products. For example, U.S. Pat. No. 4,432,666 to Frey et al., discloses using cement and other water repellant binders to dispose of waste thought to be damaging to the environment. However, cement is unstable in many situations, for example when attempts are made to encapsulate halogenated materials.

U.S. Pat. No. 3,093,593 to Arrance discloses a method for vitrifying silicate materials to encapsulate radioactive waste. However, temperatures of up to 1,400.degree. C. are required to produce final waste forms.

In U.S. Pat. No. 5,645,518, awarded to the instant Assignee, a method to stabilize low-level mixed wastes, such as radioactive medical wastes and other such materials, is provided, wherein phosphate ceramics physically and chemically stabilize the waste at ambient temperatures. However, in such endeavors, final waste forms rapidly set. This rapid-setting feature causes various degrees of unworkability to the process, particularly in situations where low viscosity and long operational times are required. Such situations include where the material is to be blown, poured or injected into deep wells or crevices, in-situ stabilization of buried wastes, remediation at nuclear-accident and waste spillage sites, and pumpable refractory applications.

In addition to the drawbacks of the above-mentioned processes, commercially supplied materials for use as components (particularly the oxides) of the above processes are in forms adverse to formulating flowable mixtures. As a result, exothermic reactions become uncontrollable, leading to thicker slurries, with the final product lacking homogeneity. Also, the resulting rapid setting time of the ceramic does not provide adequate working time.

A need exists in the art for a formulation and a waste encapsulation process to accommodate stabilization of a myriad of waste materials in a myriad of deposition scenarios. The formulation and process should provide waste liquors having protracted, workable consistencies or viscosities to accommodate currently available pump- injection-, or spray application-equipment. The process should also provide a protocol for selecting and preparing components of the formulation so as to tailor the formulation for situations requiring varying degrees of viscosity.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a formulation comprising waste and waste-encapsulation material that overcomes many of the disadvantages of the prior art.

It is another object of the present invention to provide a method for using waste as a bulk-component in pumpable hardening agents. A feature of the invention is the use of additives to slow the reaction between components of the hardening agents. An advantage of the invention is the extension of time of workability. Another advantage is the employ of heretofore unusable waste.

Still another object of the present invention is to provide a means for converting commonly available oxides into a component in pumpable hardening agents. A feature of the invention is calcining the oxides and then mixing the calcined oxides with coating agents. An advantage of the invention is the reduction in reaction rate of the calcined oxide in relation to the other components of the hardening agents, leading to extended workability time of the hardening agent prior to curing.

Yet another object of the present invention is to provide selection criteria for oxides to be used in the production of pumpable ceramic hardening formulations. A feature of the invention is the use of solubility, porosity and morphology characteristics of the oxides to differentiate acceptable oxides from unacceptable oxides. An advantage of the invention is that the characteristics are determinable prior to mixing or forming of waste forms. Another advantage is that oxides having the desired characteristics can be produced via calcination.

Briefly, the invention provides pumpable ceramic composition comprising an inorganic oxide, potassium phosphate, and an oxide coating material.

Also provided is a pumpable ceramic-based waste formulation comprising 7 to 14 weight percent MgO, 25 to 40 weight percent KH.sub.2 PO.sub.4, 15 to 50 weight percent ash, 1 to 4 weight percent boric acid, 0.5 to 2 weight percent lignosulfonate, and 15 to 25 weight percent water.

The invention also provides a method for preparing pumpable ceramic-based waste forms comprising selecting inorganic oxides based on solubility, surface area and morphology criteria; mixing the selected oxides with phosphate solution and waste to form a first mixture; combining an additive to the first mixture to create a second mixture; adding water to the second mixture to create a reactive mixture; homogenizing the reactive mixture; and allowing the reactive mixture to cure.

BRIEF DESCRIPTION OF THE DRAWING

The present invention together with the above and other objects and advantages may best be understood from the following detailed description of the embodiment of the invention illustrated in the drawings, wherein:

FIG. 1 is a schematic diagram of a process for producing pumpable ceramic waste forms, in accordance with features of the present invention;

FIG. 2 is an x-ray diffraction analysis of the invented pumpable ceramic waste form;

FIGS. 3A and 3B are photomicrographs depicting acceptable and unacceptable oxide components, respectively, of a ceramic formulation, in accordance with features of the present invention;

FIGS. 4A and 4B are x-ray diffraction analysis of acceptable and unacceptable oxide components for use in the invented pumpable formulation;

FIG. 5 is a decision tree for determining suitable oxides, in accordance with features of the present invention; and

FIG. 6 is a solubility curve of various oxides, in accordance with features of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have found a method for incorporating wastes into pumpable or sprayable mixtures which in turn are used to fixate the wastes in hard-to-reach locations. Also, solely the binders from such mixtures can be utilized and combined with waste in situ for encapsulation, particularly in situations wherein the waste is located in inaccessible or dangerous areas such as the sarcophagus of a contaminated nuclear power plant, or waste spillage sites. Generally, the method retards the setting action of phosphate ceramics and also reduces the overall viscosity of the phosphate ceramic slurry or the phosphate ceramic/waste slurry long enough to allow the slurry to be pumped, sprayed or otherwise transported to final points. Ultimately, the method produces a hard, impenetrable waste form rivaling the durability of concrete forms, and without the addition of heat.

The inventors also disclose herein a process for selecting appropriate oxides for use as a component of the free flowing binder. Also disclosed is a process for pretreating common oxides for subsequent use as a component of the free-flowing binder, as mentioned supra.

As to the first aspect of the invention, pumpable ceramic, waste-encapsulating binder has been developed. The inventors have found that the addition of certain compounds coats the components of waste binders and serves to slow the setting-reaction to the point where setting or curing is retarded long enough for the reacting ceramic-waste liquor to be pumped, sprayed, or otherwise deposited in its final resting place. Once deposited, the ceramic waste liquor is allowed to cure or set up to its very dense, final form. Despite the use of coating materials to slow reaction speeds, the final product proves to be harder than those produced using Portland cement. Compression strengths of more than 4,000 pounds per square inch are typical.

Fabrication of the pumpable waste-encapsulation material is shown schematically as numeral 10 in FIG. 1. Briefly, oxide powder 12 is mixed with phosphate powder/solution 14 and previously sized waste material 16 in a first mixing process 17 and in various proportions.

Waste material of a size less than 200 microns is suitable. A myriad of phosphate solutions can be utilized including phosphates having cation moieties selected from the group consisting of potassium, sodium, calcium, zirconium, iron, magnesium-ammonium, and combinations thereof. A suitable concentration of phosphate solution is one which creates a slurry which when combined with other ingredients enumerated below, provides a liquor with a centipoise value that facilitates easy dispersion. Once such phosphate concentration is where the weight ratio of water to KH.sub.2 PO.sub.4 is approximately 0.66.

The mixing of the oxide powder, the phosphate solution and the waste creates a first mixture or slurry 18. Optionally, the oxide powder is subjected to a pretreatment process 11 prior to mixing with the phosphate solution and waste material. Details of the oxide pretreatment process are disclosed infra.

Suitable oxide: phosphate: waste weight proportions range from 1:3.4:1.45 to 1:3.4:6.6. When MgO is utilized as the oxide, KH.sub.2 PO.sub.4 as the phosphate and ash as the waste material, a particularly suitable MgO:KH.sub.2 PO.sub.4 :Ash weight ratio is 1:3.4:1.45.

In one exemplary procedure, the slurry 18 is contacted with additives 20 comprising various coating agents and water "getters" to create a second mixture 21. However, the additives could be added at separate points in the mixing process. For example, coating agents, including but not limited to citric acid or boric acid, can be added directly to the oxide powder 12 prior to the formation of mixing of the oxide with the phosphate solution and waste material. Then, a water getter such as a lignosulphonate can be added after the above three components are thoroughly mixed together.

After integration of the additives, water 22 is added to the resulting mixture and the now-hydrated liquor is mixed in a second mixing stage 23 to create a homogenous, reacting liquor 25. Mixing times of approximately 20 minutes to 60 minutes are typical.

The addition of water begins the reaction process 24. It is during the reaction process 24 that the now-reacting liquor 25 can be pumped, jet grouted or otherwise manipulated 26 prior to final setting occurring. Setting, stabilization or curing 28 of the liquor 25 occurs no earlier than 2 hours after the water addition step 22, discussed above.

A salient feature of the invented process is that no externally-applied heat is required to effect reaction, pumping, transporting and ultimately curing of the ceramic-waste liquor.

Despite the fact that additives are used to fluidize the reaction liquor, the resulting final, cured waste form exhibits superior qualities. For example, and as depicted in FIG. 2, the process yields a waste form primarily comprising magnesium potassium phosphate crystalline phase, whereby the crystalline phase is represented by sharp peaks on the graph.

Oxide Preparation Detail

A myriad of oxides are suitable powders for the invention. Calcium oxide, sodium oxide, zirconium oxide, iron oxide, magnesium oxide and combinations thereof are all appropriate starter powders.

The inventors have found that pretreatment 11 of the oxide often enhances the flowability of the resulting ceramic binder-waste mixture. However, pretreated oxide as supplied by commercial suppliers does not provide the desired enhancements to flowability. Rather, the invented calcining process disclosed herein yields a binder with optimal properties such as high strength and low porosity. Setting rates of approximately two hours, low exothermicity (i.e., low heat generation) even during setting, and a very homogenous structure result from employing the invented oxide pretreatment process 11.

To produce suitable ceramic-binder, the oxide component should have the following properties:

1. Average oxide particle sizes should be approximately 8 to 10 microns. Each particle should be dense, crystalline, and free from any amorphous coating.

2. The oxide particles used should not result in substantial heating of the ceramic binder-waste mixture during mixing. Also, the oxide particle selection should not result in slurry thickening.

3. The final product should be a homogenous ceramic and should not result in precipitates forming in the slurry. Ultimate compression strength should be at least 3,500 pounds per square inch. Open or connected porosity of the product should be approximately less than 10 percent by volume of the entire monolith as formed and cured.

The inventors have found that oxides which do not satisfy the above enumerated parameters are not suitable for use in the stabilization of radioactive waste, or in applications for producing high volume structural products. As such, the following three test criteria have been developed for determining the suitability of oxides. The criteria can be applied to oxides provided by typical suppliers, or can be applied to determine the efficacy of oxide pretreatment methods employed on site by end users or others.

As illustrated in the decision tree depicted in FIG. 5, all three of the following criteria should be satisfied to qualify an oxide as a suitable component in a flowable ceramic binder for waste:

A.) Surface area: Surface area of the powder should be in the range of 0.3 to 0.55 m.sup.2 /g. Otherwise, the inventors have found that lower surface area results from agglomeration of the particles which in turn inhibits reaction between the oxide and other reactants. Surface areas above the desired range results in an accelerated reaction rate, leading to the slurry over heating, in some cases causing the slurry to boil. Surface areas of commercially available oxides are approximately 4 m.sup.2 /g.

B.) Morphology: Under a high resolution microscope (i.e., approximately 1500.times.), oxide particle surfaces should appear smooth and with striations indicating crystalline structure. Examples of acceptable and unacceptable morphologies are depicted as FIGS. 3A and 3B, respectively.

Aside from visual, albeit microscopic inspection, of oxide candidates, x-ray diffraction analysis also allows determination of sufficient crystalline structure in the oxides. For example, suitable oxides (FIG. 4A) exhibit sharp peaks, indicative of crystalline structure, whereas unsuitable oxides (FIG. 4B) exhibit broad peaks, which is indicative of the presence of amorphous materials. Amorphous materials react fast and generate unwanted excess heat in the slurry.

C.) Solubility: Solubility of the oxide in a solution of phosphoric acid also distinguishes between acceptable and unacceptable oxide. Generally, the less soluble the oxide is, the more suitable it is for producing a flowable ceramic material, which will solidify into an impenetrable mass. Lower oxide solubility results in more MgO particles present after reaction and therefore more nucleation sites existing in the reaction liquor. These nucleation sites help form individual centers of hardness which adds to the overall strength of the final waste form.

FIG. 6 depicts a graph showing two acceptable oxides (dotted lines) and two unacceptable oxides (solid lines). As illustrated, 20 percent more (by weight) of the acceptable oxides are necessary to bring an acid-oxide solution to pH 8 compared to the amount required of unacceptable oxides. Initial impurity levels of the suitable oxides do not effect their solubilities. For example, the more soluble oxides (i.e., the acceptable oxides) depicted in FIG. 5 had varying levels of calcium contaminants with one oxide (analytical grade) containing 0.05 weight percent calcium and the other acceptable oxide (technical grade) containing 0.45 weight percent calcium.

Solubility testing of oxide candidates are conducted as follows: A 30 milliliter solution of 5 percent H.sub.3 PO.sub.4 in deionized water is kept in constant agitation, via a magnetic stirrer or other means. One gram of MgO is added to the solution and the resulting mixture is allowed to equilibrate, with pH recorded. Additional MgO is added, one gram at a time, with equilibration allowing to occur. The procedure is continued until the pH reaches 8.

The inventors have found that one method for obtaining oxide satisfying criteria A-C stated above is through a calcining process. If commercial MgO is to be used, the oxide first must be sized to between 8 and 10 microns, as noted supra. Calcium content of the oxide should not exceed 0.5 weight percent.

Once the calcium content and particle size of the oxide is determined to be suitable, the oxide is placed in a furnace maintained at 1,300.degree. C. The oxide could be placed in a "preheated" furnace or present during temperature ramp-up. Soaking or heating time of the oxide will vary depending on the source (and therefore impurity) levels of the oxide. Highly pure (analytical grade) oxide is soaked for approximately one hour while technical grade is heated for approximately three hours.

After heating, the samples remain in the furnace during cool down. The resulting, now-cooled oxide usually presents as an agglomerated mass, which must be reground or sized to 8 to 10 micron levels.

Additive Agent Detail

As noted supra, a salient feature of the invention is the use of additives to stymie reaction and therefore extend workability of the reaction liquor 25. The additives can comprise two components: a coating agent and a water "getter" or water eliminating agent. As mentioned supra, suitable coating agents are selected from the group consisting of boric acid, citric acid, and combinations thereof. The coating component of the additives work by coating and therefore isolating the oxide particle from a complete exposure or contact by other reactants.

Suitable water getters are derived from the class of polymeric organic compounds comprising the lignophosphonates. The lignophosphonate compounds serve to keep water from quickly reacting with the phosphates. In essence, these ligno-compounds scavenge up any water to prevent rapid hydration of the phosphate. Alkaline lignophosphonate compounds are preferable getters, particularly those selected from the group consisting of lignosulphonate, lignophosphonates and other hydroxylated organic compounds and combinations thereof. A suitable group of lignosulphonate compounds consists of sodium lignosulphonate, calcium lignosulphonate, potassium lignosulphonate and combinations thereof. One exemplary lignosulphonate compound is DARATARD 17.TM., available through Grace Construction Products of Cambridge, Mass.

Weight percents of the additive agents to the final reaction liquor can vary, with values of between 0.5 percent and 3.5 percent suitable. Generally, the additive is combined with the first mixture in an additive to oxide to phosphate weight ratio of between 1.83:10.3:35 to 2.1:10.3:34

The inventors have found that the water-isolating effect of lignosulphonate compounds is enhanced with the presence of boric acid, wherein boric acid is present in weight percents similar to those utilized for the lignophosphonates.

The additive agent can be added anytime before or at the initiation of the reaction process 24, with exemplary results obtained when the additive is introduced as designated in FIG. 1, that is, prior to the addition of water 22.

In one embodiment, proportions of each ingredient of the final, pumpable product are as follows: MgO (12 weight percent), KH.sub.2 PO.sub.4 (40 weight percent), fly ash (17 weight percent), boric acid (1 weight percent), lignosulfonate (1 weight percent), and water (29 weight percent). This embodiment results in the slurry having the consistency of milk, and therefore easily pumped or grouted. After two to three hours, the slurry sets into a hard and dense ceramic. X-ray diffraction analysis of the final form, as depicted in FIG. 2. Along with hydrated magnesium potassium phosphate, also present is residual magnesium oxide.

As can be determined in Table 1 below, a myriad of different slurry consistencies result from varying concentrations of the additives. Table 1 shows the viscosities in centipoises (cp) for additive concentrations of between 0.5 and 3.5 weight percent to the total slurry weight.

TABLE 1 Viscosities of ceramic-waste liquors depending on variations in additive concentrations. Additive Initial Waste Particles Concen. Viscosity <200 .mu.m Set Time Final Hardness (Wt %) (Centipoise) Wt % (Min) (Psi) 0.0 27,000 60 20 >8000 2.0 300 15 >190 >3000 2.5 3000 28 190 >3000

Inasmuch as spraying and injection applications are enhanced at viscosity values less than 1,500 cp, formulations containing between 0.5 and 3 weight percent coating agent of additive (which comprises both a coating agent and a water "getter") are suitable.

Several different types of waste material can be incorporated with the pumpable ceramic binder, either separately or combined, to form a waste form. Such wastes include, but are not limited to ash, saw dust, clay, soils, red mud, dust from metal industries, slags, and combinations thereof. Sizing, grinding or other pretreatment of the material may be necessary. Suitable micron sizes of the waste range from 10 microns to 200 microns.

The resulting slurry, as described supra, can be utilized either neat to dispose of the already-incorporated waste particles in the pumpable liquor, or used as a sealant agent to stabilize other waste. For example, a slurry formulated with ash can be subsequently injected into contaminated soil for stabilization of the latter. Another application is the use of the invented waste-containing slurry as a pumpable refractory.

While the invention has been described with reference to details of the illustrated embodiment, these details are not intended to limit the scope of the invention as defined in the appended claims.