Phosphate stabilization ceramics

In the phosphate washing discussed above, only a small amount of acid phosphate is used to convert contaminants into their insoluble phosphate forms. To fabricate CBPC waste forms, however, a larger amount of binder is needed, so compared to the phosphate washing, the cost of the binder is high. This condition does not mean that the volume of the stabilized waste will increase. Typically, the washed waste is loosely packed, but the fully stabilized ceramic matrix is dense. As a result, the volume does not increase and, hence, the disposal cost will remain the same. Depending on the nature of the waste and amount of the phosphate binder used, the binder cost may be the only higher cost in the CBPC treatment compared to simple acid washing. 

The development of calcium phosphate-based ceramics at high temperature requires taking into account the thermal stability of these compounds. We can distinguish two schemes of decomposition according to the temperature irreversible decompositions (condensation of hydrogenophosphate ions, decomposition of carbonate ions, of hydroxide ions, etc.) at low temperature (150-1,000°C) and reversible decomposition (decomposition of the apatite into TCP, TTCP and lime) at high temperatures (T > 1,000°C). 

Uses. Commercial monocalcium phosphate is available as both the anhydrous and the monohydrate salts. Most uses are based on acidic properties. Monocalcium phosphate is used to control acidity in powdered drink mixes, as an ingredient in effervescent tablets, as a plastics stabilizer, and in ceramics. Its single largest appHcation is as a leavening agent in bread, cake mixes, and self-rising flour. 

Ceramicrete is an ex situ stabilization technology that uses chemically bonded phosphate ceramics to stabilize low-level radioactive waste and hazardous waste containing radionuclides and heavy metals. The technology mixes phosphates with acidic solution, causing an exothermic reaction similar to that used in forming concrete. But while concrete is based on relatively weak hydrogen and van der Waals bonding, Ceramicrete uses a combination of ionic, covalenf and van der Waals bonds to stabilize contaminants. 

Low-temperature treatment of low-level mixed wastes has also been accomplished by solidification/stabilization with chemically bonded phosphate ceramics (CBPC). These are made by hydrothermal chemical reaction rather than by sintering. Chemical bonding develops when acid phosphates react with oxides to form crystalline orthophosphate (Singh et al. 1997). The ceramic matrix stabilizes the wastes by microencapsulation. The low temperature of the reaction allows volatile radionuclides to be treated (Singh et al. 1997). 

Singh, D., Wagh, A. S., Cunnane, J. C. Mayberry, J. L. 1997. Chemically bonded phosphate ceramics for low-level mixed-waste stabilization. Journal of Environmental Science and Health, Part A Environmental Science and Engineering Toxic and Hazardous Substance Control, A32, 527-541. 

Wagh, A. S., Strain, R., Jeong, S. Y., Reed, D., Krause, T. Singh, D. 1999. Stabilization of Rocky Flats Pu-contaminated ash within chemically bonded phosphate ceramics. Journal of Nuclear Materials, 265, 295-307. 

A.S. Wagh, D. Singh, and S.Y. Jeong, Chemically bonded phosphate ceramics for stabilization and solidification of mixed wastes, Hazardous and Radioactive Waste Treatment Technologies Handbook (CRC Press, Boca Raton, FL, 2001), pp. 6.3.1-6.3.18. 

Development of superior CBPC products for the wide-ranging applications shown in Fig. 2.1 requires a fundamental understanding of their kinetics of formation and their properties. This topic is extensively addressed in Chapters 4-6. The dissolution model described in these chapters also helps in understanding the role of individual components in formation of ceramics and the end performance of the ceramics. In addition, the dissolution model explains how hazardous and radioactive components are stabilized in a phosphate matrix. The stabilization mechanisms are discussed in Chapters 16 and 17. 

A. Wagh, D. Singh, andS. Jeong, Method of waste stabilization via chemically bonded phosphate ceramics, US Patent No. 5,830,815, 1998. 

A.S. Wagh, S.Y. Jeong, D. Singh, A.S. Aloy, T.I. Kolytcheva, E.N. Kovarskaya, and Y.J. Macharet, Iron-phosphate-based chemically bonded phosphate ceramics for mixed waste stabilization, Proceedings on Waste Management 97, March 2—6 1997, (1997). 

Some hazardous metals such as chromium (Cr) and radioactive fission products such as technetium (Tc) exhibit exactly opposite solubility characteristics as compared to the metals discussed above. These metals in higher oxidation states, e.g., chromates (Cr ) and pertechnetate (Tc ), are more soluble than their counterparts, e.g., chromium and technetium oxide (Cr and Tc " "). Chromium is a hazardous metal and technetium ( Tc) is a radioactive isotope. As we shall see in Chapters 16 and 17, one way to reduce their dispersibility is to reduce their solubility in ground water and reduce them into their lower oxidation state, and then encapsulate them in the phosphate ceramic. Thus, the reduction approach is also useful in stabilization of hazardous metal oxides of high oxidation states. Because of these reasons, a good understanding of the reduction mechanism of oxides... 

As argued earlier, by coupling redox and dissolution reactions, it should be possible to form phosphate ceramics of certain insoluble oxides by reducing them to a lower oxidation state that is more soluble, or to convert contaminants of lower oxidation states and stabilize them as phosphates. For example, a mixture of insoluble hematite and elemental... 

A.S. Wagh, R. Strain, S.Y. Jeong, D. Reed, T. Krause, and D. Singh, Stabilization of rocky flats Pu-contaminated ash within chemically bonded phosphate ceramics, J. Nucl. Mater., 265 (1999) 295-307. M.M. Sychev, I.N. Medvedeva, V.A. Biokov, and O.S. Krylov, Effect of reaction kinetics and morphology of neoformation on the properties of phosphate cements based on magnesium titanates, Chem. Abstr., 96, 222252e. 

D. Singh, A.S. Wagh, and L. Knox, Low-temperature setting phosphate ceramics for stabilizing DOE problem low-level mixed waste, eds. M. Wacks and R. Post, Proceedings WM94 (WM Conferences, Inc., Tucson, AZ, 1994), pp. 1853-1857. 

A. Wagh, S. Jeong, D. Singh, A. Aloy, T. Kolytcheva, and Y. Macheret, Iron-Phosphate-Based Chemically Bonded Phosphate Ceramics For Mixed Waste Stabilization, Proceedings of the Waste Management Annual Meeting, Session 29, Tuscon, March 2-6, 1997). 

Inorganic contaminants are immobilized by washing the waste with soluble phosphates. This treatment uses a very small amount of phosphate, does not change other characteristics of the waste such as its granular nature or volume, and is relatively inexpensive. If the waste contains radioactive contaminants, phosphate washing is not sufficient because the dispersibility of the radioactive contaminant powders needs to be reduced, and hence, the waste needs to be solidified. Solidification requires generating phosphate ceramics of the waste in the form of a CBPC. In the case of radioactive waste, both stabilization and solidification are needed because they not only immobilize the contaminants, but also solidify the entire waste. As we will see in this and the next chapter, whether phosphate treatment is used only for stabilization or for both stabilization and solidification, it is very effective for a wide range of waste streams. 

To gain an insight into the sulfide stabilization, examine the solubility product constants for the sulfides and phosphates of hazardous metals listed in Table 16.4. In this table, except for barium sulfide, other sulfides as well as phosphates have very high pK p, indicating that their aqueous solubility is almost negligible. In particular, the pA sp of HgS and Ag2S is very high, and these two sulfides are insoluble in water. Therefore, when a waste stream contains one of these two, sulfide pretreatment followed by phosphate ceramic formation is an ideal way to treat the waste stream. 

The samples were stored for 3 weeks for curing. Each sample was then crushed and was subjected to the TCLP test. The TCLP test results on both the waste stream and the treated CBPC waste form are given in Table 16.6. The results on the untreated waste streams show that the leaching levels far exceed the regulatory limits. The results for the waste forms, on the other hand, are an order of magnitude below the EPA limit. These results indicate superior stabilization of Hg in the phosphate ceramic waste forms coupled with sulfide immobilization. 

A. Wagh, S. Jeong, and D. Singh, Mercury stabilization in chemically bonded phosphate ceramics, Ceram. Trans., 87 (1998) 63-73. 

D. Singh, A. Wagh, M. Tlustochowicz, and S. Jeong, Phosphate ceramic process for macroencapsulation and stabilization of low-level debris waste, Waste Mgmt, 18 (1998) 135-143. 

Memoire sur les arseniates et les phosphates. Ann Chim Phys 19 350-419 Molodetsky I, Navrotsky A (1998) The energetics of cubic zirconia from solution calorimetry of yttria- and calcia-stabilized zirconia. Z Physikal Chemie 207 59-65 Muller G (1995) The scientific basis. In Bach H (ed) Low Thermal Expansion Glass Ceramics. Springer-Verlag, Berlin, p 13-49... 

Use Ceramics, calcium acid phosphate, phosphorus and phosphoric acid, polishing powder, cattle foods, clarifying sugar syrups, medicine, mordant (dyeing textiles with Turkey red), fertilizers, dentifrices, stabilizer for plastics, in meat tenderizers, in foods as anticaking agent, buffer, nutrient supplement, removal of Sr from milk. 

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