Fate of heavy metals during molten salts thermal treatment of municipal solid waste incineration fly ashes

• Published in: Waste management (Elmsford) Vol. 103; pp. 334 - 341
• Main Authors: Xie, Kang, Hu, Hongyun, Xu, Sihua, Chen, Tongzhou, Huang, Yongda, Yang, Yuhan, Yang, Fu, Yao, Hong
• Format: Journal Article
• Language: English
• Published: United States Elsevier Ltd 15.02.2020
• Subjects: Carbon; Chlorination; Coal Ash; Heavy metals; Incineration; Metals, Heavy; Molten salts thermal treatment; MSWI fly ash; Particulate Matter; Refuse Disposal; Salts; Silicon Dioxide; Solid Waste; Stabilization; MSWI fly ash; Chlorination; Stabilization; Molten salts thermal treatment; Heavy metals
• ISSN: 0956-053X; 1879-2456
• DOI: 10.1016/j.wasman.2019.12.047

[Display omitted] •The ionic chlorine in molten salts enhanced the direct chlorination of PbO/CdO.•Ash containing SiO2/Al2O3 favored heavy metal oxides’ indirect chlorination.•Some Zn2+ was precipitated from molten salts as Si/Al composite oxides.•Dissolved heavy metals were thermally stable in molten salts and few evaporated.•Heavy metals in ash slag became more stable after molten salts thermal treatment. Thermal treatment could effectively realize the detoxification of heavy metals in municipal solid waste incineration (MSWI) fly ash through the approach of removal or stabilization process. To lower the operating temperature and suppress the evaporation of heavy metals, a molten salts (NaCl-CaCl2) thermal treatment method was proposed for the detoxification of heavy metals from MSWI fly ash at a relatively mild condition (600/800 °C). The fate of heavy metals during the heating process and their stabilization properties in the remained ash slag after molten salts thermal treatment were investigated. The results showed that, compared with the traditional thermal treatment, heavy metals were more easily chlorinated by the means of molten salts thermal treatment. The well distributed chloride in molten salts facilitated the direct chlorination of PbO/CdO. Furthermore, Al2O3 in ash enhanced the indirect chlorination of CuO/PbO/CdO, except for ZnO. In contrast, SiO2 showed better performance in promoting the indirect chlorination of heavy metal oxides. Meanwhile, some Zn2+ was precipitated from molten salts as Si/Al-Zn composite oxides through the interactions with ash containing Si/Al oxides. On the other hand, the dissolved heavy metals in molten salts showed a good thermal stability during the thermal treatment. The volatilization fractions of all detected heavy metals were less than 5%. After the molten salts thermal treatment, heavy metals in the ash slag were well stabilized and the amount of heavy metals leached was significantly lower than that from the raw fly ash.