Vitrification is a process of melting contaminated soil, buried hazardous wastes, or toxic sludges at a temperature as high as 1600-2000°C, in an electric furnace or in place at a contaminated site, to render the materials nonhazardous. The final nonhazardous product is a glassy and/or crystalline solid matrix that is resistant to leaching and more durable than natural granite or marble. If the vitrification process is carried out in an electric furnace, it is called ex situ vitrification (ESV). If it is carried in place at a contaminated site, it is called in situ vitrification (ISV).
The technology is based on the concept of joule heating to melt the contaminated soil or sludges electrically in order to destroy toxic organic and inorganic contaminants by pyrolysis. It was initially developed by the US Department of Energy (USDOE) to provide enhanced isolation of previously disposed radioactive wastes. Today over 160 bench-scale (10 kW, 5-10 kg), engineering-scale (30 kW, 0.05-1 ton), pilot-scale (500 kW, 10-50 ton), and large-scale (3755 kW, 400-1000 tons) vitrification tests have been conducted and have demonstrated the general feasibility and its widespread applications in treating or containing hazardous wastes: contaminated soil sites, burial grounds, and storage tanks that contain hazardous materials in the form of either sludge or salt cakes, process sludges, and many others.
A case history of ex situ vitrification using electric furnace vitrification is presented first. The ex situ vitrification technology uses an electric furnace to convert contaminated soils, sediments, and sludges into oxide glasses at over 1500°C, chemically rendering them nontoxic and suitable for landfilling as nonhazardous materials. Successful vitrification of soils, sediments, and sludges requires: (a) development of glass compositions tailored to a specific waste, and (b) glass melting technology that can convert the waste and additives into a stable glass without producing toxic emissions. There are two types of melters:
• Electric melter. In an electric melter, glass, which is an ionic conductor of relatively high electrical resistivity, stays molten with joule heating. Such melters process waste under a relatively thick blanket of feed material, which forms a counterflow scrubber that limits volatile emissions. Commercial electric melters have significantly reduced the loss of inorganic volatile constituents such as boric anhydride (B2O3) or lead oxide (PbO). Because of its low emission rate and small volume of exhaust gases, electric melting is a promising technology for incorporating waste into a stable glass.
• Fossil fuel melter. In contrast, fossil fuel melters have large, exposed molten glass surface areas from which hazardous constituents can volatilize. Because of its high toxic emission rate, a fossil fuel melter may not be more beneficial than an electric melter for vitrifying toxic wastes.
Ex situ vitrification using an electric melter and furnace (Fig. 5) stabilizes inorganic components found in hazardous waste. In addition, the high temperature involved in glass production (over 1500°C) decomposes anthracene, bis (2-ethylhexyl phthalate), and
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