WO2000007195A1

WO2000007195A1 - Method for encapsulation and stabilization of hazardous and radioactive mixed wastes using polysiloxane

Info

Publication number: WO2000007195A1
Application number: PCT/US1999/016904
Authority: WO
Inventors: Stephan V. Prewett
Original assignee: Orbit Technologies, Inc.
Priority date: 1998-07-29
Filing date: 1999-07-27
Publication date: 2000-02-10
Also published as: KR20010030797A; AU5770699A

Abstract

A process for encapsulating dry solid waste material and the waste form produced thereby. Dry, granular waste material is mixed in with a polysiloxane and appropriate catalyst and cured, preferably under ambient conditions. The waste form exhibits a low leaching index and other characteristics necessary for long-term stability.

Description

METHOD FOR ENCAPSULATION AND STABILIZATION OF HAZARDOUS AND RADIOACTIVE MIXED WASTES USING POLYSILOXANE

Technical Field

This invention pertains to a method for encapsulation and stabilization of mixed wastes containing hazardous and/or radioactive components. More particularly, the invention pertains to a method for encapsulation and stabilization of mixed wastes within a polysiloxane matrix. Background Art

Methods have been developed for treatment of waste material in an effort to keep them from damaging the environment. Wastes that cannot easily or economically be rendered harmless to the environment pose great disposal problems. An example of potential harmful waste is the mixed waste salts generated by the United States Department of Energy (DOE). There have been many attempts to provide storage/disposal methods for such wastes. Some methods attempt to stabilize the waste in various forms and then to prevent the wastes from leaching or otherwise returning to the environment.

The present best technology for stabilizing this waste results in a large volume increase or production of secondary waste streams. One method of waste containment provides a 20% waste loading using Portland cement. Another option for treatment of the waste salts is thermal treatment. These thermal techniques involve expensive and questionable off-gas systems which are especially difficult for transuranic materials. There exists a need in the art to increase the waste loading while adhering to Resource Conservation and Recovery Act (RCRA) standards, including passing toxicity characteristic leaching procedure (TCLP) testing. In addition, the final waste form must not exhibit ingitability, corrosivity, or reactivity. The present invention is directed to a method of stabilizing dry granular salts that may be generated by solidifying neutralized acidic solutions used to recover and reformulate weapons material. The waste material may include soluble chromium compounds, sodium nitrate, potassium nitrate, traces of plutonium 238 and 239, traces of uranium 238, as well as other harmful contaminants. Disclosure of the Invention One object of the present invention is to provide a method to encapsulate and stabilize the waste material and form a cohesive material suitable for disposal.

It is another object of the invention to provide a waste form with a low leaching index or potential. It is a further object of the invention to provide a method for waste encapsulation able to perform at greater than 20% waste loading.

Briefly, the invention provides a method to stabilize a mixture of dry granulated salts by grinding the solid waste, combining the ground solid waste with polysiloxane to create a slurry, adding a catalyst to the slurry, molding the slurry - catalyst mixture, and curing the molded mixture.

According to one aspect of the invention, a solid waste encapsulating system is provided. The system comprises, before cure, from 10% to 60% by weight of a dry solid waste, from 40% to 90% by weight of a polysiloxane, and a catalyst in an amount effective to cure the polysiloxane. According to another aspect of the invention the solid waste comprises a granular salt mixture of soluble chromium compounds, sodium nitrate, potassium nitrate, and/or sodium chloride.

According to another aspect of the invention, the dry waste may have a high nitrate content. According to another aspect of the invention, dry waste may have a high chloride content.

According to another aspect of the invention, the polysiloxane system cures under ambient conditions.

According to another aspect of the invention, a waste form comprising a solid waste encapsulated in a cured polysiloxane system is provided. The waste form is produced by grinding the solid waste to a predetermined particle size, combining the ground solid waste with a polysiloxane to obtain a slurry, mixing the slurry for a predetermined time, adding a catalyst to the slurry, mixing the slurry-catalyst mixture for a predetermined time, molding the slurry -catalyst mixture, and curing the molded mixture.

One advantage of the present invention is the ability to stabilize DOE salt wastes in a form suitable for shallow land burial. The waste form has a low leaching index and passes current Department of Transportation (DOT) oxidizer standards.

Another advantage of the present invention is the ease with which the invention can be practiced due to the workability and availability of the necessary materials.

Another advantage of the present invention is the use of polysiloxane systems which have been shown to resist severe environments, including radiation.

Yet another advantage of the present invention is that the method may be carried out under ambient conditions.

Still other benefits and advantages of the invention will become apparent to those skilled in the art to which it pertains upon a reading and understanding of the following detailed specification.

Description of the Preferred Embodiment

In accordance with the present invention, a method for encapsulating and stabilizing waste material in a polysiloxane matrix is provided. For the purposes of the present invention, three surrogate waste materials are presented. Surrogate Waste #1 (SW#1) is intended to mimic the nitrate salt form found at the Idaho National Engineering and Environmental Laboratory (INEEL) Subsurface Disposal Area referred to as "Pad-A" . Two additional surrogate wastes are intended to mimic Mixed Waste Focus Area (MWFA) wastes including a high chloride surrogate waste (SW#2) and a high nitrate surrogate waste (SW#3) with RCRA metals as the contaminants. The composition of the surrogate wastes are presented below in Tables 1 and 2.

The wastes are in a dry granular form. In the inventive method, the dry waste is mixed with a polysiloxane encapsulating agent. After addition of an appropriate catalyst, the polysiloxane system is molded and cured. In the preferred embodiment, the polysiloxane system is room temperature curable. The cured system is durable and has a low leaching index. These characteristics provide for long-term stability of the final waste form and suitability for disposal or transportation to a disposal facility.

Polysiloxane systems have a variety of unique properties that make them attractive as potential waste encapsulation systems. They are extremely chemically resistant to normal solvents, acids and bases in the appropriate formulations. Polysiloxane systems are environmentally friendly. The systems generally operate at ambient or slightly elevated temperatures and as such have no associated off-gas treatment processes. The system viscosity can be modified from a very free-flowing liquid to an extremely viscous material depending on the application. Set times can also be controlled from a few seconds up to several days, depending on the catalyst used and the temperature of the materials undergoing curing.

For the purposes of the present invention, it is found that the actual curing process is very tolerant of a number of different chemicals in the system. Also, it is found that polysiloxane systems generally do not undergo disruption with a large variety of different contaminants present. Further, for the purposes of the present invention, all surrogate wastes used in testing were technical-grade rather than reagent-grade materials to more closely approximate the trace contaminants that would be expected in any typical waste form. In the preferred embodiment of the present invention, the viscosity of the polysiloxane systems are low enough to allow ease of mixing, and high enough to hold the contaminants in uniform distribution during cure. Example I: Sample Preparation

A general formulation for a waste encapsulation system is given below in Table 3. For purposes of experimental studies, the mixing took place in 1 gallon plastic buckets using a hand-held drill equipped with a stainless steel paddle. The dry waste was first ground to a particle size less than 1 mm, with the preferred particle size being less than 0.6mm. The polymer base and the dry waste were mixed together for a minimum of five minutes. The catalyst was added. The mixture was then mixed for an additional five minutes and then transferred to plastic cylinder molds. The samples were placed under a hood and allowed to cure for a minimum of 24 hours prior to testing. TABLE 1: SW #1

TABLE 3: Sample Preparation

Example II: Representative Formulations

Representative formulations for use with nitrate salt waste (SW#1) are given in Table 4. The preferred polymer base materials include room temperature curable polysiloxanes such as RTV664 (marketed by General Electric) and ELECTROGUARD™ 2100 (marketed by Wacker Silicones Corp.). Product data for the preferred polysiloxanes is available from the manufacturers' Technical Data Sheets which are incorporated herein by reference. The weight of the catalyst is based on the weight of the polysiloxane and is preferably 10 wt% for the GE RTV-664 and 0.3 wt% for Wacker-2100. The waste forms were prepared according to the procedure given above and allowed to cure for a minimum of 24 hours prior to testing. Other samples having varied waste loadings were also prepared and tested.

TABLE 4: Encapsulated Waste Formulations

Example III: Leachabilitv Studies

Leachability is the ability of a waste form to retard the release of hazardous constituents to the environment. Samples of waste forms produced by the inventive methods using the GE RTV664 polysiloxane system were subjected to TCLP testing in accordance with USEPA SW-846. Table 5 shows the results of SW#1 at both 30% and 50% waste loadings. Table 6 shows the results of SW#2 and SW#3 at both 30% and 50% waste loadings.

TABLE 5: TCLP Results (SW#1)

TABLE 6: TCLP Results (SW#2 and SW#3)

ND = not detected TCE = trichloroethylene

Table 7 below provides leaching data as a function of waste load using total chromium as the measured species. The samples were prepared using the GE RTV664 polysiloxane system. The testing conformed to ASTM C-1308.

TABLE 7: Leach Index

Example IV: Compressive Strength Studies

The waste form's mechanical integrity and ability to withstand loading pressures in a disposal environment is directly related to compressive strength. Compressive strength is also used as a gauge of the resistance of samples that have undergone durability tests. Compressive strength testing showed that much greater than 60 psi compressive strength could be achieved for waste loadings as high as 50% . It is expected that the waste forms produced by the methods of the present invention can also withstand significant shear such as that generated during an earthquake or differential settling during a shallow land burial application.

In the preferred embodiment of the present invention, the polysiloxane systems achieve greater than the 20% waste loadings or do not generate the secondary waste streams of prior art waste disposal systems. For the purposes of the present invention, waste loadings of from 30% to 90% were tested. Cohesive monolith waste forms were achieved with up to 50% waste loadings while maintaining at least 60 psi of compressive strength. Table 3 provides data on various samples of waste forms prepared according to the present invention. Samples 1 and 2 utilized the Wacker2100 polysiloxane system. Samples 3 and 4 utilized the GE RTV664 polysiloxane system.

TABLE 8: Compressive Strength (SW#1)

Example 4: Incorporation of Fillers It is possible to improve the leachability levels of the encapsulated wastes by incorporating fillers into the waste form. For example, scavengers such as iron oxides, phosphates, or other anti-leaching agents may be utilized. Table 9 provides simulated TCLP results for iron oxide addition to GE RTV664 samples at pH 3 and pH7. The sample weight is 10 grams, waste load is 37 wt% , and iron oxide addition is based on total weight of the sample.

TABLE 9: Addition of Iron Oxide

Other fillers may be added to the polysiloxane matrix in order to provide economic benefits. Such fillers include calcium carbonate, fumed silica, silicon dioxide, magnesium silicate, calcium silicate and others known to those skilled in the art. Filler materials should ideally be chosen so as to not adversely effect the physical characteristics of the final waste form produced according to the invention. The invention has been described with reference to preferred embodiment. Obviously, modifications and alterations will occur to others upon a reading and understanding of this specification. It is intended to include all such modifications and alternations in so far as they come within the scope of the appended claims or the equivalence thereof.

Having thus described the invention, it is now claimed:

Claims

CLAIM

1. A solid waste encapsulating system, before cure, characterized by: from 10% to 60% by weight of a dry solid waste; from 40% to 90% by weight of a polysiloxane; and, a catalyst in an amount effective to cure the polysiloxane.

2. The solid waste encapsulating system of claim 1 wherein the dry waste is characterized by a granular salt mixture.

3. The solid waste encapsulating system of claim 1 wherein the dry waste is characterized by at least one member of the group consisting of soluble chromium compounds, cadmium, mercury, lead, sodium nitrate, potassium nitrate, and sodium chloride.

4. The solid waste encapsulating system of claim 1 wherein the dry waste is characterized by approximately from 50% to 60% by weight sodium nitrate.

5. The solid waste encapsulating system of claim 1 wherein the dry waste is characterized by up to approximately 35 % by weight potassium nitrate.

6. The solid waste encapsulating system of claim 1 wherein the dry waste is characterized by up to approximately 10% by weight sodium chloride.

7. The solid waste encapsulating system of claim 1 wherein the polysiloxane is curable at ambient conditions.

8. The solid waste encapsulating system of claim 1 is further characterized by: up to 10 % by weight of a filler material.

9. The solid waste encapsulating system of claim 8 wherein the filler material is characterized by at least one member of the group consisting of iron oxide, calcium carbonate, magnesium silicate, and calcium silicate.

10. The solid waste encapsulating system of claim 8 wherein the filler material is an anti-leaching agent.

11. A process for producing a waste form comprising a solid waste encapsulated in a cured polysiloxane system, the process characterized by: grinding the solid waste to a predetermined particle size; combining the ground solid waste with a polysiloxane to obtain a slurry; mixing the slurry for a predetermined time; adding a catalyst to the slurry; mixing the slurry-catalyst mixture for a predetermined time; molding the slurry-catalyst mixture; and, curing the molded mixture.

12. The process of claim 11 further characterized by: adding a filler material to the slurry before adding the catalyst.

13. The process of claim 12 wherein adding a filler material to the slurry before adding the catalyst comprises the step of: providing the filler material from the group consisting of iron oxide, calcium carbonate, magnesium silicate, and calcium silicate.

14. The process of claim 11 wherein the process is further characterized by: providing the solid waste from the group consisting of soluble chromium compounds, cadmium, mercury, lead, sodium nitrate, potassium nitrate, and sodium chloride.

15. The process of claim 11 wherein curing the molded mixture comprises the step of curing the polysiloxane system at ambient conditions.

16. The process of claim 11 wherein the process further comprises the step of: providing the solid waste at between 10% and 60% by weight.

17. The process of claim 11 wherein the process further comprises the step of: providing the polysiloxane at between 40% and 90% by weight.

18. A waste form produced by the process of claim 11.

19. A waste form produced by the process of claim 13.