The migration and transformation mechanisms of heavy metals during molten salt cyclic thermal treatment of MSWI fly ash.

[Display omitted] • Molten chlorides can be recycled to extract heavy metals from fly ash effectively. • The migration and reaction of Cd, Cu, Pb in molten salt differ from those of Zn. • Heavy metal oxide and chloride in molten salt can be converted into each other. • Zn2+ can react with Si 2 O 7 6- formed by O2– and SiO 2 , to be stabilized as Ca 2 ZnSi 2 O 7. • For recovery, heavy metals can be enriched in the slags of molten salt. Molten salt thermal treatment has been demonstrated to be effective in dissolving heavy metals in municipal solid waste incineration (MSWI) fly ash. However, the complex thermochemical reactions of various heavy metals during the process remain unclear. This study investigated the migration and transformation mechanisms of typical heavy metals during molten salt (NaCl-CaCl 2) cyclic thermal treatment of fly ash at 600–750 °C, as well as the leaching characteristics and recovery methods of heavy metals in the reacted products. Molten chlorides could be recycled to extract heavy metals from fly ash, which reduced the leaching toxicity of heavy metals in the reacted slags. Over 64.3% of Cd, Cu and Pb in fly ash were transferred to molten salt while the migration rate of Zn was restrained to only 32.3% during the cyclic process at 750 °C. Regarding Cd, Cu, and Pb in molten salt, the chlorination and dissolution of their oxides were significant, while the combination of their chlorides with free O2– to form oxides precipitation was weak. However, the above reaction characteristics of Zn were the opposite. Furthermore, Zn2+ could be stabilized into hardystonite (Ca 2 ZnSi 2 O 7) in molten salt by reacting with Si 2 O 7 6- generated from the combination of O2– and SiO 2. Besides, increasing the reaction temperature facilitated the chlorination and dissolution of heavy metals. For recovery, heavy metals could be concentrated on the insoluble matters of molten salt, with a maximum concentration of approximately 5 times that in fly ash. [ABSTRACT FROM AUTHOR]