Silicon-aluminum additives assisted hydrothermal process for stabilization of heavy metals in fly ash from MSW incineration

Hydrothermal treatment (HT), as a novel promising technology, still could not make fly ash (FA) from municipal solid waste incineration (MSWI) meet the standard of non-hazardous waste, due to the unsuccessful formation of aluminosilicate minerals caused by the lack of Si and Al in FA. In this research the mechanism of zeolites formation and stabilization/migration of heavy metals during hydrothermal process affected by external addition of coal fly ash (CFA), bentonite, and kaolin as silicon-aluminum source, were studied. Tobermorite could be synthesized, attributing to the proper element proportion of Ca/(Si + Al) adjusted by 30% of additives addition. Heavy metals were detected on the surface of tobermorite crystalline. The leaching toxicity of Pb, Zn, Cu, Cd, Cr in FA were significantly reduced after HT, associated with the transformation into the residue fraction. The decrease of heavy metals in liquid phase and the decrease of leaching toxicity in solid phase were observed, indicating that heavy metals were really stabilized in the treated FA, rather than transferred into liquid. Higher reaction temperature could promote the formation of tobermorite, but increase the leaching toxicity and migration of heavy metals. Addition of 30% of CFA and temperature of 150 °C would be the recommended technological parameters.