Complete heavy metal removal from fly ash by heat treatment: Influence of chlorides an evaporation rates

Thermal treatment is a promising way for the decontamination and inertization of residues from waste incineration. The evaporation of heavy metal compounds thereby is of great significance. It is the goal of this work to identify, by analyzing evaporation rates, the predominant thermochemical reactions of the heavy metals with other constituents of fly ash, with respect to volatilization. To this end, experiments were performed with fly ash from a municipal solid waste (MSW) incineration plant as well as with synthetic powder mixtures in the temperature range of 670-1000 degrees C. The rates of Cd, Cu, Pb, and Zn evaporation can be described accurately by a simple first-order rate law and a rate coefficient which itself follows an exponential temperature dependence analogous to the Arrhenius equation. The degrees (completeness) as well as the rates of evaporation of the heavy metals are markedly influenced by chlorides contained in the fly ash, largely as NaCl. Experiments with model substrates indicate that the heavy metals Zn and Cu in fly ash, which are the least volatile among the group investigated, are predominantly present as chlorides. Their evaporation is completed by shifting the oxide/chloride equilibrium if surplus chlorine, e.g., in the form of NaCl, is available. The heavy metal evaporations are probably limited by reactions that form heavy metal silica/alumina compounds.