HYDROTHERMAL STABILIZATION OF FLY ASH FROM A FLUIDIZED BED INCINERATOR CO-FIRING REFUSE AND COAL

Hydrothermal stabilization of heavy metals in fly ash from a municipal solid waste and coal co-firing fluidized bed incinerator was experimentally investigated, with alkali concentration, liquid/solids ratio, and temperature as major reaction parameters. The properties of fly ash were studied by means of X-ray fluorescence (XRF) spectrometry, Xray diffraction (XRD), and scanning electron microscopy (SEM); its hydrothermal stabilization was examined by using Toxicity Characteristic Leaching Procedure (TCLP) tests. The stabilization of Cr, Cu, Pb, and Zn is less effective than that of Mn, Ni and Cd. The optimal alkali concentration, liquid/solids ratio, and temperature are found to be 0.5 mol/L, 4:1 ml/g and 150 degrees C, respectively, and the stabilization efficacy of all heavy metals is more than 95 %. XRD analysis shows that zeolite-like minerals, including sodalite and amorphous geopolymer, are synthesized during the hydrothermal process. The stabilization of heavy metals is mainly attributed to absorption, ion exchange, and physical encapsulation of heavy metals in the zeolite-like minerals synthesized. Under optimal conditions the quality of the treated fly ash and of the residual hydrothermal liquid attains the corresponding national standards. Overall results indicate that the hydrothermal technique tested is a promising and seemingly effective way for stabilizing heavy metals in fly ash.