Effect of Atmospheres on Transformation of Heavy Metals during Thermal Treatment of MSWI Fly Ash: By Thermodynamic Equilibrium Calculation

The vaporization behaviors of eight heavy metals (Pb, Zn, Cu, Cd, Cr, Co, Mn, and Ni) in municipal solid wastes incineration (MSWI) fly ash during thermal treatment under air atmosphere (21% O(2)/79% N(2)), an inert atmosphere (100% N(2)), and a reducing atmosphere (50% CO/50% N(2)) were evaluated based on a thermodynamic equilibrium calculation by FactSage 8.1. The results show that the reducing atmosphere promotes the melting of MSWI fly ash, resulting in a more liquid phase than in air or an inert atmosphere. Except for Cd, the formation of liquids can dissolve heavy metals and reduce their vaporization ratio. In the air and inert atmospheres, Pb, Zn, Cu, Co, Mn, and Ni vaporize mainly in the form of metallic chlorides, while Cd volatilizes in the form of metallic Cd (g) and CdO (g). In the reducing atmosphere, Co, Mn, and Ni still vaporize as chlorides. Zn and Cd mainly vaporize in the form of Zn (g) and Cd (g), respectively. In terms of Pb, in addition to its chlorides, the volatiles of Pb contain some Pb (g) and PbS (g). Cr has a low vaporization ratio, accounting for 2.4% of the air atmosphere. Cr, on the other hand, readily reacts with Ca to form water-soluble CrCaO(4), potentially increasing Cr leaching. Except for Cd, the results of this study suggest that the reducing atmosphere is used for the thermal treatment of MSWI fly ash because it promotes the melting of fly ash and thus prevents heavy metal vaporization.