Patent Number: 061538094
Section: description

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. FNT (ND indicates None Detected; * indicates test in progress). 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.