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 °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.

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Fly ash is commonly deposited in special landfills as it contains toxic concentrations of heavy metals, such as Zn, Pb, Cd, and Cu. This study was inspired by our efforts to detoxify fly ash from municipal solid waste incineration by thermal treatment to produce secondary raw materials suited for reprocessing. The potential of the thermal treatment was studied by monitoring the evaporation rate of zinc from a certified fly ash (BCR176) during heating between 300 and 950 degrees C under different carrier gas compositions. Samples were quenched at different temperatures for subsequent investigation with X-ray absorption spectroscopy (XAS). The XAS spectra were analyzed using principal component analysis (PCA), target transformation (TT), and linear combination fitting (LCF) to analyze the major Zn compounds in the fly ash as a function of the temperature. The original fly ash comprised about 60% zinc oxides mainly in the form of hydrozincite (Zn5(OH)6(CO3)2) and 40% inerts like willemite (Zn2SiO4) and gahnite (ZnAl2O4) in a weight ratio of about 3:1. At intermediate temperatures (550-750 degrees C) the speciation underlines the competition between indigenous S and Cl with solid zinc oxides to form either volatile ZnCl2 or solid ZnS. ZnS then transformed into volatile species at about 200 degrees C higher temperatures. The inhibiting influence of S was found absent when oxygen was introduced to the inert carrier gas stream or chloride-donating alkali salt was added to the fly ash.

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