THERMAL AND GEOCHEMICAL CHANGES IN SOILS VITRIFIED USING A PLASMA ARC TORCH

Using a plasma arc torch to vitrify soils contaminated with solid and hazardous wastes produces an extremely durable product greatly reduced in volume and surface area. Although vitrification is understood in principle, thermal and geochemical changes occurring in contaminated soils during vitrification are poorly characterized. These changes were studied by first constructing a theoretical model and then performing laboratory scale vitrification experiments. The theoretical model is founded on standard heat transfer equations which were used to create a two-dimensional finite difference heat transfer computer model. The model predictions were used to design the subsequent experiments. The model successfully predicted a 100 °C temperature plateau and the width of the vitrified zone formed using kilowatt size torches. The model indicated that most melting occurs in the first 30 minutes after the torch is turned on suggesting that powering the torch for long periods of time is inefficient. The model also showed that melting below the groundwater table is energy inefficient. Laboratory experiments were conducted by filling a 4-foot tall, 4-foot diameter cylinder with compacted soil doped with CsHCO3 and Ho2O3 and inserting a plasma torch in a centrally located borehole for three to four hours. Thermocouples installed at various distances from the torch revealed an extremely steep thermal gradient. This thermal gradient is reflected in razor sharp variations in soil color as the soil dehydrates, sinters and finally melts. Soil and glass samples were analyzed using x-ray fluorescence and inductively coupled plasma mass spectrometry. The glasses produced were more homogenous than the starting soil indicating vigorous convection within the molten zone. Water, organic compounds and silica were strongly volatilized during vitrification and lead had the highest volatility of all trace elements studied. Significantly, little cesium was volatilized during melting suggesting that the nearly instantaneous melting process minimizes this potential environmental hazard.