Metal behavior during fluidized bed thermal treatment of soil

The Superfund dumpsites are frequently composed of soils contaminated with hazardous organic constituents and toxic heavy metals. While thermal treatment is an effective method of remediating the contaminated soils, the major environmental concerns are the emissions of toxic metal fumes during the treatment and the leaching of metals from the treated soil. The US EPA has reported that metals can account for almost all of the identified cancer risks from waste incineration systems. Research leading to better understanding of their behavior and better controlling of their emissions is urgently needed. In this study, the behavior of metals during the fluidized bed thermal treatment of artificially prepared metal-contaminated clay was experimentally and theoretically investigated. The objective of the study was to evaluate the effects of operating conditions on metal volatilization and metal leachability associated with the process. Metal experiments were carried out in a well instrumented 76 mm (3 inch) i.d. fluidized bed incinerator. The metals involved were compounds of lead and cadmium and the operating parameters included metal concentration, air flow rate, treatment temperature and treatment duration. The observed results indicated that metal volatilization is mainly a function of treatment temperature and treatment duration. The degree of volatilization was observed to range from 5 to 40% depending on the operating conditions. Cadmium leachability was observed to be relatively high compared to that of lead. In addition to the experimental study, a theoretical model based on the laws of heat and mass transfer operations and reaction kinetics was derived to simulate the metal volatilization process. The derived model was found to predict reasonably well the experimental observations.