Multi-step Wet Enhanced Removal of Chlorine and Heavy Metals from Municipal Solid Waste Incineration Fly Ash
Municipal solid waste incineration (MSWI) fly ash contains high levels of soluble chlorine and heavy metals, posing environmental risks. This study employed a sequential wet treatment process (water washing–water washing–acid washing–water washing) at a low liquid-to-solid ratio of 2:1 L·kg⁻¹ to enhance the removal of chlorine and heavy metals. Acetic acid and a mixed acid (acetic acid:sulfuric acid molar ratio 1:1) were used as acid washing agents. Results showed that the soluble chlorine content decreased from 23.96% in the raw ash to approximately 0.6%, achieving a removal efficiency of 97.5%. The 3 mol·L⁻¹ acetic acid group exhibited high removal efficiencies for Pb, Cu, and Cd at 52.97%, 29.60%, and 61.54%, respectively, while increasing the stable fraction of heavy metals. However, excessive dissolution of Ca and Al occurred. The mixed acid group demonstrated a 6.9-fold higher 'calcium retention' capacity compared to acetic acid alone, attributed to the presence of sulfate. After treatment, the leaching concentrations of Pb and Zn were significantly reduced to 0.001 mg·L⁻¹ and 0.05 mg·L⁻¹, respectively, meeting the limits of the 'Technical Specification for Pollution Control of MSWI Fly Ash' (HJ 1134—2020). The treated ash exhibited a CaO-SiO₂-MgO-Al₂O₃ system, suitable for building material applications. This study provides technical support for on-site, building-material-oriented disposal of MSWI fly ash.