Vitrification processes

Vitrification processes are being developed in several countries in which the dried waste is calcined and heated with ground glass frit to produce a borosilicate glass which can be stored or disposed of more permanently if there is agreement on suitable sites. 

FIGURE 16.15 During the vitrification process and after vitrification. (Taken from U.S. EPA, A Citizen s Guide to Vitrification, Technical Report EPA-542-F-01-017, U.S. EPA, Washington, DC, 2001.)... 

The next stage, baking, begins at about 600°C, when small amounts of flux within the clay mixture melt and induce incipient melting of the surrounding clay, which is therefore converted to glass (see Chapter 3). The melting, or vitrification process, starts at isolated sites within the clay mixture... 

HARVEST [Highly Active Residue Vitrification Experimental Studies] A process for immobilizing nuclear waste by incorporation in a borosilicate glass. Developed from FIN-GAL. Piloted by the UK Atomic Energy Authority at Sellafield, in the late 1970s, but abandoned in 1981 in favor of AVM, the French vitrification process. 

Vitriflx A vitrification process for converting asbestos to a harmless glassy substance, suitable for use as a construction material. Developed in the United Kingdom. 

Waste treatment processes. See also Hazardous waste management Radioactive waste management Solid waste management for radioactive waste, 25 853-854 titanium-related, 25 64-65 Waste vitrification process, 12 616 Wastewater, 9 443. See also Effluent treatment... 

Geotech Development Corporation offers a proprietary Cold Top ex situ vitrification process for the treatment of contaminated soil. The system melts the soil using an electric resistance furnace that can operate at temperatures of up to 5200°F. The vendor claims that wastes are transformed into an essentially monolithic, vitrified mass. The process is termed cold top vitrification because soil is added to the top of the melt to act as an insulator and to minimize the loss of volatile metals into the off-gas treatment system. The technology has been evaluated in a pilot-scale facility and is commercially available. 

Fellinger, A. P., Baich, M. A. et al. 1999. Americium-curium vitrification process development (I). Materials Research Society Symposium Proceedings, 556, 367-374. 

Petitiean, V., Fillet, C., Boen, R., Veyer, C. Flament, T. 2002. Development of vitrification process and glass formulation for nuclear waste conditioning. In Waste Management 02 Conference. Laser Options, Inc., Tucson, CD-ROM. 

Timmons, D. M. Thompson, L. E. 1996. Geochemical and petrographic studies and the relationships to durability and leach resistance of vitrified products from the in-situ vitrification process. In Proceedings of the International Topical Meeting on Nuclear and Hazardous Waste Management SPECTRUM 96. American Nuclear Society, Inc., La Grange Park, 1026-1029. 

The procedure to obtain glass-ceramics does not differ very much from the vitrification process, as it typically only requires an additional carefully controlled thermal treatment, which induces devitrification. In particular, a partly... 

The use of inorganic ion exchangers to solidify liquid radioactive waste followed by pressure sintering to produce a ceramic waste form appears to be a viable alternative to calcina-tion/vitrification processes. Both the process and waste form are relatively insensitive to changes in the composition of the waste feed. The stability of the ceramic waste form has been shown to be superior to vitrified wastes in leaching studies at elevated temperatures. Further studies on the effects of radiation and associated transmutation and the influence of temperature regimes associated with potential geologic repositories are needed for a more definitive comparison of crystalline and amorphous waste forms. 

Mendel, J. E., Palmer, C. R., and Eschback, E. A., "Preliminary Assessment of Potential Effects of Alternate Fuel Cycles on High-Level Waste Vitrification Processing," Symposium on Waste Management and Fuel Cycles 78," Edited by R.G. Post and M. E. Wacks, Tucson, AZ, March (1978). 

In this chapter we discuss preparative routes for inorganic materials in three basic types of systems involving the presence of a distinct solid-liquid interface those in which the liquid and solid phases are of the same chemical identity (solidification and vitrification processes), those in which the liquid and solid phases are not of the same chemical identity (crystallization, precipitation), and the special case in which the liquid phase is a pure ionic liquid or molten salt. Ionic liquids can serve as the solvent as well as a templating agent, and the liquid components may or may not become incorporated into the final solid product. We also discuss two areas where the distinct solid-liquid interface becomes somewhat blurred namely, sol-gel and solvothermal processes. 

The first theoretician of the vitrification process was Simon (1930), who pointed out that it can be interpreted as a "freezing-in" process. Simon measured specific heats and entropies of glycerol in the liquid, crystalline and glassy state below Tg the entropy of the supercooled liquid could, as a matter of fact, only be estimated. Linear extrapolation would lead to a negative entropy at zero temperature (paradox of Kauzmann, 1948) which would be in contradiction with Nernst s theorem. So one has to assume a sharp change in the slope of the entropy, which suggested a second order transition as defined by Ehrenfest. 

Important work on the kinetically interpreted vitrification process was done by Volkenshtein and Ptitsyn (1957), Wunderlich and coworkers (1964-1974) and Moynihan (1974-1978). They considered the vitrification process as a "chemical reaction" involving the passage of "kinetic units" (e.g. "holes") from one energy level to another. 

When an amorphous polymer is gradually cooled from above the glass transition temperature Tg its volume decreases (see Fig. 13.32) according to its thermal expansion coefficient aj. In the region around the Tg the volume decrease will lag behind, starting at temperature Tel because the rate of reorganisation process becomes too small. The polymer starts to vitrify and a temperature Tel will be reached where the reorganisation completely stops and where the vitrification process is completed. Decrease of volume is only the result of normal volume contraction with expansion coefficient ag. The relationship between both thermal expansion coefficients is... 

Fig. 1. Second order phase transition versus the vitrification process. They have different temperature dependence of the volume (F) and enthalpy (H), and their derivatives a (thermal expansion) and C (specific heat)...

Also gaining acceptance as alternative routes for immobilizing metals are processes such as soil vitrification, and the addition of cement-like (pozzolonic) agents whereby undesirable soil components are entrapped in a vitrified matrix. Vitrification of radioactive waste materials is currently being used as well. Examples of vitrifying media include borosilicate glasses and iron phosphates. Whether a specific vitrification process is chemical or physical in nature is not always clear. Table 10.2 lists the main selection criteria for this application. 

Vitrification processes are becoming increasingly important for the incorporation of pollutants into solid materials. See Section 10.1.2.1. 

Although different aspects of the isothermal TTT cure diagram have been presented in this review from an experimental point of view, this section will present some recent work that has attempted to model the cure process. Only the gelation and vitrification processes are examined, and the complicating effects of thermal degrada-... 

The model was also applied to the reaction of a tetrafunctional amine with a trifunctional epoxy, denoted A4 + 4/3Bj, and was compared with available data (Fig. 18). An approximate value of k was obtained from the times to gelation. This model appears to provide a reasonable framework within which the vitrification process for nonlinear systems can be discussed. 

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