Optimization of Cl and heavy metal removal during heat treatment of municipal solid waste incineration fly ash and municipal sludge after co-washing

Abstract

Heat treatment, known for its detoxification and volume reduction characteristics, is a promising technology for the management of municipal solid waste incineration fly ash (MFA) and municipal sludge (MS). This paper uses the solid residue from MFA and MS after co-washing as the raw material to study the melting properties, phase transformations, changes in Cl content, heavy metal removal efficiency, and leaching toxicity. The results indicated that co-processing of MFA and MS can effectively reduce the melting temperature. The migration of Cl elements was influenced by the CaO/(SiO₂ + Al₂O₃) ratio and the formation of Cl-containing phases. During the heat treatment process, Cr and Ni remained relatively stable, while Zn, Pb, Cu, and Cd were affected by Cl migration. Chlorine can promote the volatilization and escape of these heavy metals, but as Cl became fixed within Ca₁₀(SiO₄)₃(SO₄)₃Cl₂ and Ca₅(PO₄)₃Cl, the removal efficiency decreased. To achieve the highest removal rates for these elements, the addition of MFA should be limited to no more than 20%, and the CaO/(SiO₂ + Al₂O₃) ratio should be below 2.9. After heat treatment, the leaching concentrations of Zn, Pb, Cu, Cr, Cd, and Ni met applicable standards. The findings of this study could provide a method for the treatment of MFA and MS after co-washing and can serve as guidance for heat treatment processes.