Distribution and chemical species transition behavior of chlorides in municipal solid waste incineration fly ash during the pressure-assisted sintering treatment

[Display omitted] •The chemical species and transformation process of Cl in fly ash were investigated;•The distribution of chloride was affected by microstructure transition;•Chloride transformed into a stable form during pressure-assisted sintering. High chloride content in municipal solid waste incineration (MSWI) fly ash is a critical restriction for the recycling and re-utilization due to the acceleration of the corrosion process in construction material. Therefore, the investigation of chloride distribution and transition behavior in fly ash is of great significance. In this study, the distribution and chemical species of chloride in fly ash were first analyzed through varying mechanical pressures (10, 30, 50, 70, and 100 MPa) and sintering temperatures (300 °C, 400 °C and 500 °C) during the pressure-assisted sintering process. Instrumental analyses of SEM-EDX, element mapping, XPS, XRD and chloride leaching concentration were conducted to explore the chloride transition behavior. The results suggested that chlorides were homogeneously distributed on the surface of fly ash particles, which was significantly influenced by the microstructure change during the pressure-assisted sintering process. In addition, the chloride migration was enhanced by the pore shrinkage under different mechanical pressures and the increased atomic mobility at various temperatures. The mineral results showed that the main crystals in raw fly ash were NaCl, KCl and CaClOH, and the minerals were transformed into more complex and stable crystal phases, such as Ca10(Si2O7)2SiO4Cl2(OH)2 and Ca10(SiO4)3(SO4)3Cl. The transformation of chloride from soluble salts to stable Cl-containing minerals is beneficial to the immobilization of chloride, which suggested that pressure-assisted sintering could effectively decrease the chloride leaching concentration. This study provides a further understanding of chloride transition and distribution behavior during pressure-assisted sintering treatment.