Vitrification uses heat to melt and then solidify harmful chemicals in a solid mass of glasslike material. It can be applied both in-situ (in-situ vitrification or ISV) and above ground in a treatment unit (ex-situ). These are described below.
ISV and Planar-ISV
ISV uses electric power to create the heat needed to melt soil. Electrodes are inserted in the polluted area and an electric current is passed between them, melting the soil between them. ISV uses extremely high temperatures (1,600 to 2,000 ¡C or 2,900 to 3,650 ¡F). Melting starts near the ground surface and moves down. As the soil melts, the electrodes sink further into the ground causing deeper soil to melt. When the power is turned off, the melted soil cools and vitrifies, which means it turns into a solid block of glass-like material. The electrodes become part of the block. This causes the ground surface in the area to sink slightly. To level it, the sunken area is filled with clean soil. Any harmful chemicals that remain underground become trapped in the vitrified block, which is left in place. When the soil hardens, it forms a glass-like substance that retards migration of compounds encapsulated in the glass.
The vitrification product is a chemically stable, leach-resistant, glass and crystalline material similar to obsidian or basalt rock. ISV destroys or volatizes most organic pollutants by pyrolysis (i.e., application of heat without oxygen). A vacuum hood is often placed over the treated area to collect off-gases, which are treated before release. Radionuclides and heavy metals are retained within the molten soil. The conventional method of top-down melting in ISV typically results in substantial over-melting of the remediation area. Planar-ISV involves starting the melting process in specific areas of the subsurface. Consequently, the melting process can be focused directly on the region requiring treatment, and it can attain greater melt depths.
Ex-situ vitrification is much like ISV, except that it is done inside a chamber. Heating devices include plasma torches and electric arc furnaces. With plasma torch technology, waste is fed into a rotating hearth; the waste and molten material are held against the side by centrifugal force. During the rotation, the waste moves through plasma generated by a stationary torch. To remove the molten material from the furnace, the hearthÕs rotation slows and the slag flows through a bottom opening. Effluent gases are generally kept in a separate container where high temperatures combust/oxidize the contents. The arc furnace contains carbon electrodes, cooled sidewalls, a continuous feed system, and an off-gas treatment system. In this process, waste is fed into a chamber where it is heated to temperatures greater than 1500¡C. The melt exits the vitrification unit and cools to form a glassy solid that immobilizes inorganics. The Department of Energy has developed a Transportable Vitrification System (TVS) for treating mixed waste (radioactive and toxic) so the process can be used at several sites.
Limitations and Concerns
With most Stabilization/Solidification processes, there is potential for a substantial increase in waste volume.
Concerns include the durability of the waste form. Glass waste forms, as compared to a grouted or cemented waste form, are expected to be more stable over longer periods due to the corrosion resistance of glass. However, de-vitrification of glass can occur over time periods involving thousands of years. While the heat used to melt the soil can destroy some of the harmful chemicals, it may cause others to evaporate. The evaporated chemicals must be captured and treated.
Complete characterization of the candidate waste stream is essential, before initiating either in-situ or ex-situ vitrification, to determine what glass forms are already present in the waste and what additional glass stabilizers and fluxes need to be added. Onsite analytical services with quick turnaround are required to determine glass product characteristics and to validate melter performance.
There are specific limitations to ISV and ex-situ vitrification. These are described below.
ISV cannot be used with buried pipes or drums and rubble exceeding 20% by weight.
Heating the soil may cause the subsurface migration of contaminants into clean areas.
ISV cannot be used where there are large accumulations of flammable or explosive materials.
The solidified material may hinder future site use.
ISV rapidly volatilizes some organic compounds and volatile radionuclides, including Cs-137, Sr-90, and tritium. Control of these off-gases, as well as the high voltage used, present potential health and safety risks.
ISV reduces the volume and mobility of radionuclides, but it does not reduce their radioactivity. Therefore, protective barriers that limit exposure to radioactive emissions may still be required at some sites.
ISV is most effective for near-surface contamination, although new approaches may increase treatment depths to 10 meters.
Use, storage, or disposal of the vitrified slag is required. Vitrification does not reduce the wasteÕs radioactivity. Vitrified wastes containing radioactive contamination must be stored in facilities that protect the public from radiation exposure.
Debris greater than 60 mm in diameter typically must be removed prior to processing.
Excavation of radioactively contaminated soils could cause radiation exposure to workers from fugitive gas and dust emissions, and it may increase the risk to nearby populations.
There is potential for the accumulation of volatile radionuclides in the melter off-gas system.
Heat loads associated with cesium loading in the glass waste form should be assessed for any applicable limits or relationships to disposal site requirements.
If plutonium is being vitrified, provisions need to be made for preventing its theft, as it can be re-extracted from the glass.
Ex-situ vitrification and ISV can destroy or remove organics and immobilize most inorganics in contaminated soils, sludges, or other earthen materials. Vitrification applies to a broad range of solid media (e.g., debris, soil, etc.). The process has been tested on a broad range of volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs), other organics including dioxins and polychlorinated biphenyls (PCBs), and on most priority pollutant metals and radionuclides.
Technology Development Status
ISV is a proven, commercially available technology. An electrical distribution system, off-gas treatment system, and process components are developed. Ex-situ vitrification technologies generally involve applying existing technologies (e.g., metals processing) to new purposes. Demonstrations and studies at several sites (including Oak Ridge, TN and Washington, DC) indicate that ex-situ vitrification technologies can be implemented without significant difficulties. The TVS technology is now available to users at DOE sites.
Other Resources and Demonstrations
See http://www.osti.gov/bridge/servlets/purl/610197-wQKYkr/webviewable/ for a description of vitrification projects initiated by teh Energy DepartmentÕs Savannah River Site. Also see http://www.osti.gov/bridge/servlets/purl/10159052-bABKfq/native/ for a paper on the vitrification of hazardous wastes.
See http://www.osti.gov/bridge/servlets/purl/899770-b42JCN/ for a description of the demonstration of a bulk vitrification system at Hanford, where 53 million gallons of radioactive liquid are planned to be vitrified.
See http://www.ieer.org/sdafiles/vol_5/5-4/deararj.html for a description of vitrification and some of its problems. See www.ieer.org for numerous articles on vitrifying fissile materials.