Effects and mechanism of the conditions of sintering on heavy metal leaching characteristic in municipal solid waste incineration fly ash

Municipal solid waste incineration (MSWI) fly ash treated with toxicity metals holds enormous potential for constructure use to economize on resources and protect environment. To reach the goal, this study investigated the effects of sintering conditions on leaching characteristic of heavy metals for MSWI fly ash, especially Cr, Cr6+, Ag, and Ba, with the orthogonal and Box-Benhnken design experiment, which considered grain size (D-50 = 30, 45, and 60 mu m), fluxing agent (CaO = 0, 2.5, and 5%), setting temperature (1000, 1050, and 1100 degrees C), and setting time (120, 180, and 240 min). The mechanism of immobilization for heavy metals was also discussed through the analyses of morphological characterizations, mineral phases, chemical composition, and leaching values of metals. The results indicated that changing grain size and adding fluxing agent of CaO have positive influence on reducing the leaching of heavy metals compared with direct sintering. The leaching values of As, Pb, Cd, Cu, Ni, Zn, Mn, Hg, Be, Se, and fluoride are not detected after sintering. Ideal sintering condition with desirability of 1.00 was predicted and optimized by the Box-Benhnken response method in grain size of D-50 = 30 mu m, fluxing agent of CaO = 5%, setting temperature = 1050 degrees C, and setting time = 180 min, which immobilized Cr, Cr6+, Ag, and Ba lower than the limitation of standards. Actual experiment was consistent with numerical optimization. Furthermore, the model of leaching characteristic for heavy metals in MSWI fly ash was established with the discussion on species distribution of heavy metals to better explain the mechanism during sintering.