CBPC treatment

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. 

As in the case of hazardous contaminants discussed in Chapter 16, CBPC treatment converts radioactive constituents of waste streams into their nonleachable phosphate mineral forms. It follows the philosophy [7] that, if nature can store radioactive minerals as phosphates (apatite, monozites, etc.) without leaching them into the environment, researchers should be capable of doing the same by converting radioactive and hazardous... 

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). 

Aluminum is the second most abundant metal on earth s crust. It is a common metal in tropical soils called laterites (red soils). It is extracted from bauxite that is a rich laterite by Bayer process that involves dissolution and separation of the oxide in caustic soda solution between 150 and 250°C and 20 atm of pressure. Though abundant and inexpensive, alumina based CBPCs are difficult to form because even in an acid solution the solubility of alumina is very low. This solubility, however, can be enhanced by a mUd thermal treatment and suitable CBPCs can be formed. Alumina is available commercially as calcined alumina called corundum, or as its hydrated forms such as aluminum hydroxide (Al(OH)3), as bohmite, (A1203-3H20), gibbsite (AI2O3 H2O) or in impure forms as in kaolin clay. These mineral forms and their use in ceramic formation are discussed in Chapter 11. 

The latter reaction can form long chain phosphates, where n is theoretically infinite. Being formed by heat treatments, these phosphates are excellent candidates for high-temperature ceramics and glasses. Because the subject of this volume is low-temperature ceramics, we will not discuss the condensed phosphates in detail, except in one case in Chapter 15, where cements for geothermal wells are discussed with sodium metaphosphate. However, bear in mind that CBPCs can be precursors to high temperature phosphates and glasses. For this reason, as we have seen in the literamre survey presented in Chapter 3, early interest in CBPCs was the formation of refractory shapes at room temperature, which were then fired to produce the final refractory components. 

This equation works best for trivalent ( = 3) and quadrivalent n = 4) oxides in the acidic region when CBPCs are formed by a thermal treatment. Substituting for ACp from Eq. 6.40 in Eqs. 6.35 and 6.36, we obtain... 

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. 

This test is the key to success of any stabilization method for treatment of hazardous waste. Because the waste is crushed and leached using acidic water, the actual leaching of the contaminants depends on their solubility. Thus, as in the case of CBPC formation discussed in Chapters 4-6, solution chemistry plays a major role in stabilization. For this reason, we review the solution chemistry of the hazardous contaminants before we proceed to the actual stabilization. 

This incremental cost increase may be justified when stabilizing mixed waste streams or waste streams containing As, Cr, and Hg, because simple acid washing will not stabilize these waste streams. To stabilize Hg, in addition to the CBPC formation, a sulfide pretreatment is used [55]. The pretreatment converts the contaminants into their most insoluble sulfide forms, then the CBPC formation produces a waste form that is far superior to any other treatment. This dual treatment has the advantage of being performed at room temperature in a one-step mixing process. 

Most waste streams contain more than one contaminant. Some may have contaminants such as Hg, whose sulfide has a higher pAQp than its phosphate, and Ba, whose sulfide is soluble, but phosphate is insoluble. Even in these cases, sulfide treatment followed by phosphate ceramic formation is very effective. The sulfide treatment will produce insoluble HgS that will be microencapsulated in the phosphate matrix, but Ba will be converted partially into soluble BaS, which subsequently will dissolve and will be converted to insoluble phosphate in the CBPC matrix. Thus, the dual treatment is very effective even when several contaminants with varying solubility of sulfides and phosphates are found in the same waste. 

Finally, some DOE sites also have stored hexavalent uranium fluoride (UFs). The approach to stabilize this compound is to calcine it to form stable uranium oxide. There is no study reported in the literature on treatment of fluorides using a CBPC matrix, but considering that fluroapatites are stable minerals, they should be applicable to stabilization of actinide compounds. 

Oxides of Ba and Sr are soluble in acidic and neutral aqueous solutions, and as seen from Fig. 17.1, like MgO, their solubility decreases as the pH increases on the alkahne side. Therefore, these two oxides may be easily stabilized in CBPC using the acid-base treatment. 

Considerable development has occurred on sintered ceramics as bone substitutes. Sintered ceramics, such as alumina-based ones, are uru eactive materials as compared to CBPCs. CBPCs, because they are chemically synthesized, should perform much better as biomaterials. Sintered ceramics are fabricated by heat treatment, which makes it difficult to manipulate their microstructure, size, and shape as compared to CBPCs. Sintered ceramics may be implanted in place but cannot be used as an adhesive that will set in situ and form a joint, or as a material to fill cavities of complicated shapes. CBPCs, on the other hand, are formed out of a paste by chemical reaction and thus have distinct advantages, such as easy delivery of the CBPC paste that fills cavities. Because CBPCs expand during hardening, albeit slightly, they take the shape of those cavities. Furthermore, some CBPCs may be resorbed by the body, due to their high solubility in the biological environment, which can be useful in some applications. CBPCs are more easily manufactured and have a relatively low cost compared to sintered ceramics such as alumina and zirconia. Of the dental cements reviewed in Chapter 2 and Ref. [1], plaster of paris and zinc phosphate... 

The data from different sources does not often match exactly, while it has been necessary to use the data on oxides from one source and phosphates of the same elements from another. To avoid any confusion resulting from this, we have used a certain order in using these sources. Pourbaix s Atlas of Electrochemical Equilibria [1] is the first source that we have used for the Gibb s free energy of oxides and ions, and for the solubility product constants. This is prompted by the fact that much of the formulation discussed in this book is hinged to Pourbaix s treatment, and to be consistent, Pourbaix s data is preferred over others. The CRC Handbook of Chemistry and Physics [2] is the next source from which much of the enthalpy and specific heat of oxides and ions are taken. The data on phosphates comes from The Phosphate Minerals [3], while the mineral formulae are from Dana s Mineralogy Handbook [4] and also from The Phosphate Minerals [3]. These detailed references and additional ones [5,6] useful for further development of CBPC materials are given below. 

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