Effects of Hydrothermal Treatment on the Major Heavy Metals in Fly Ash from Municipal Solid Waste Incineration

Circulating fluidized bed (CFB) and stoker grate are the two major municipal solid waste incineration (MSWI) technologies currently being used in China. In this work, the hydrothermal stabilization of heavy metals in fly ash collected from two types of MSWI plants is investigated. The components and mineralogical phases of the fly ash are studied by means of X-ray fluorescence and X-ray diffraction spectrometry. The distribution patterns of Cr, Cu, Zn, and Pb in MSWI fly ash before and after the hydrothermal process are evaluated using a modified Bureau Communautaire de Reference sequential extraction procedure. In addition, the environmental risk is assessed using individual contamination factors and risk assessment codes. The results indicate the following: (1) In comparison to raw fly ash, the speciation distributions of Cr, Cu, Zn, and Pb from a hydrothermally treated CFB fly ash sample are more stable than those from a hydrothermally treated stoker grate fly ash sample. (2) Co-firing municipal solid waste and coal changes the characteristics of the fly ash and is beneficial to heavy metal stabilization using the hydrothermal process. (3) The two types of hydrothermally treated fly ash samples in this study are considered to pose little or no contamination risk to the environment.