Soil vitrification can be performed both in-situ and ex-situ. In-situ vitrification (ISV) uses electrical power to heat and melt soil, sludge or sediments. Then the molten material is cooled to form the vitrified product. The vitrified monolith is usually left in place after treatment. However, if onsite disposal is not allowed, the vitrified monolith should be excavated and removed. A gas-effluent treatment system is required to collect and treat the gas produced by high temperature. Two electrodes need to be inserted into the soil to perform vitrification. To allow the current to flow through the soil, there has to be sufficient monovalent alkali cations to provide needed electrical conductivity. If the soil does not meet the requirement, fluxing materials containing cations should be added to the base material. Since dry soil has high electric resistance, the applied voltage needs to be very high (as much as several thousand volts) at the beginning to overcome the resistance. Once the soil is molten, the mobilization of ions will increase the conductivity and reduce the resistance of soil. Therefore, at steady state, the voltage will reduce to several hundred volts. The current will be increased to maintain steady power supply. The ISV device is illustrated in Figure. 3.
Figure.3 In-situ Vitrification (EPA, 1994)
An alternative technique of in-situ vitrification is called in-situ plasma vitrification (ISPV). Instead of using traditional electrodes, ISPV uses a plasma torch to melt the contaminated soil. The plasma torch is able to create temperatures ranging from 4000 to 7000 degrees Celsius. It is rapid, inexpensive and efficient. The procedures of ISPV are similar to ISV. The only difference is the method of melting the soil. The ISPV device is shown in Figure.4. (Circeo and Martin, 1997)
Figure.4 In-situ Plasma Vitrification (Circeo and Martin, 1997)
Soil vitrification process is seldom studied quantitatively. However, there is a mathematical model to predict vitrification time, depth, width and electrical consumption for ISV (Koegler and Kindle, 1991). The model has been developed at Pacific Northwest Laboratory as a predictive tool to assist engineers and researchers in the application of ISV to different sites. The model is based on energy balance, which means that electrical energy is converted into heat in the molten glass through resistance. The heat loss is taken into account by empirical measurements. With certain increment in depth, the model is able to calculate the corresponding increment in the width of the vitrified product. Figure. 5 shows a sample result of the model. The model output reaches a high agreement with the actual shape of the vitrified product.
Figure.5 Comparison of Model Output with Actual Melt Shape (Koegler and Kindle, 1991)
Ex-situ vitrification treats contaminated soil in special reactors. The theory is similar to in-situ vitrification. The difference is that the waste needs to be excavated prior to treatment. The process of ex-situ vitrification varies for different technologies used. Several technologies have been developed as described in (Staley, 1995).
Figure.6 is an advanced multifuel-capable combustion and melting system developed by Vortec Corporation, which is an example of the ex-situ vitrification equipment. It uses high efficiency combustion of fossil fuels in cyclonic flow to rapidly heat and melt contaminated soil. Contaminated soil and, if needed, glass-forming additives, are preheated in a cyclonic flow precombustor. Combustion and melting are completed in a counter rotating vortex combustion unit. Melting is completed in a cyclone melter. Slag is separated from combustion gasses at the exhaust of the cyclone melter and is removed from the CMS and allowed to cool. Exhaust gasses are treated via electrostatic precipitation to remove particulate prior to being released to the atmosphere. The addition of glass-forming additives to the process enhances the products of the vitrified product (Staley, 1995).
Figure.6 Vortec Vitrification System (Staley, 1995)