Behaviour of metals in MSW incinerator fly ash during roasting with chlorinating agents

A process for removal of heavy metals from municipal solid waste (MSW) incinerator fly ash was studied on a laboratory scale. The process is characterized by the formation of volatile chlorides of heavy metals during thermal treatment at temperatures from 600{dollar}spcirc{dollar}C to 1050{dollar}spcirc{dollar}C. A quartz tube furnace was used for roasting the fly ash sample in an air stream with the presence of a chlorinating agent. Operational parameters, such as roasting temperature, roasting time, and types and quantities of chlorinating agents, were studied. The working conditions for volatilization of Pb, Cu, Zn and Cd, were established. This process not only recovers the valuable heavy metals from the fly ash, but also converts the fly ash from potentially hazardous into non-hazardous material for recycle or its safe disposal in a landfill. Material balances on the fly ash components revealed the metal contaminant distribution between the volatile matter and the ash residue. More acid resistant compounds were formed after the heat treatment. CaCl{dollar}sb2{dollar} was found to be a more selective chlorinating agent compared to Cl{dollar}sb2{dollar} for volatilizing the above heavy metals from a complex matrix such as fly ash. A linear relationship between the efficiencies of volatilization of the partially recovered metals and their standard free-energy changes for the chlorination reactions was indicated. Many of these chlorination reactions were found to be of first order. X-ray powder diffraction was used to identify various chemical compounds to confirm specific reaction mechanisms and to characterize the volatile matter and the heat-treated/untreated fly ash. The reason for the unexpected leachability of Cr and Al from the treated ash residue was investigated and resolved. The applicability of the results to other residues was determined. The techniques developed for the MSW fly ash were applied to a steel dust waste with success for removal of zinc and lead contaminants.