Patent Number: 061334987
Section: description

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.