Chloride-retaining co-melting of municipal solid waste incineration fly ash for network-breaking index-guided metal...

Municipal solid waste incineration (MSWI) fly ash contains high levels of soluble chlorides and exhibits strong heavy-metal leachability; therefore, conventional treatment routes rely on pre-washing. This study proposes an integrated pathway combining no-wash chlorine retention, staged thermal recovery of heavy metals, and ambient-cooled vitrification, coordinated through a network-breaking index (NBI). This approach eliminates the need for washing, thereby reducing water consumption and simplifying the treatment of chlorine-containing solid waste. Blends of municipal and coal fly ash (C30-C50, where Cx denotes x wt.% coal fly ash) were heated to 1200 °C for 1 h and cooled under ambient air. X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, and BCR sequential extraction were performed to resolve the relationships among phase evolution, glass-network structure, and environmental performance. The results show that a mid-range NBI (0.67-0.97) provides both low-temperature fusibility and robust network formation. C40-C45 achieved near-complete vitrification (∼99.9%). Under identical conditions, metal volatilization followed the order Pb/Cd > Cu > Zn > Cr. Mechanistically, thin NaCl/KCl salt films first lowered the mass-transfer barriers, and a continuous melt subsequently developed and polymerized. FTIR revealed depolymerization from C35-C40, followed by re-polymerization from C40-C45, which strongly correlated with K/Na release, indicating shared composition-process controls. These findings guided a temperature-zoned recovery strategy-800-900 °C for Pb/Cd, 900-1100 °C for Cu, and 1150-1200 °C for Zn-followed by vitrification at 1200 °C and ambient cooling to immobilize low-volatility species (e.g., Cr). This method minimizes preprocessing, reduces secondary pollution, and offers a promising route for Ca- and Cl-rich hazardous fly ash.