Effects of composite fluxing agents on the melting characteristics and mineral transformations of MSWI fly ash

Municipal solid waste incineration (MSWI) fly ash is rich in hazardous substances such as heavy metals and dioxins, posing significant challenges to environmental safety and disposal. High-temperature vitrification effectively immobilizes these pollutants, but the process requires high temperatures and substantial energy consumption. Therefore, developing efficient fluxing systems to lower the melting temperature of fly ash and promote its resource utilization has become an important area of research. This study aims to explore the effects of composite fluxing agents on the melting characteristics and mineral transformations of MSWI fly ash, and to develop an optimized fluxing system suitable for low-temperature sintering and resource recovery. Using thermodynamic simulations and repeated experiments, The study examines the impact of primary fluxing agents (SiO₂ and Al₂O₃) and auxiliary fluxes (B₂O₃, P₂O₅, and CaF₂) on the melting temperature, mineral composition, and structural evolution of sintered fly ash ceramsites. The results indicate that SiO₂ significantly lowers the melting temperature and narrows the melting range, demonstrating the strongest fluxing effect. Al₂O₃ lowers the initial softening temperature at high additions but expands the melting range and limits flow. B₂O₃ promotes early liquid phase formation and the generation of low-melting calcium borosilicates. P₂O₅ aids early softening but forms stable calcium phosphate crystals, limiting flow. CaF₂ improves flowability at low additions and enhances densification. This study clarifies the mechanisms of different fluxing agents, providing compositional design guidelines for low-temperature sintering of fly ash and offering new approaches for the resource utilization of fly ash.