Formation of Friedel’s salt in simulated municipal solid waste incineration bottom ash

To recycle incineration bottom ash as a raw material in cement production, it is important to reduce the chlorides in the ash. Due to Friedel’s salt, washing only is insufficient to remove the chlorides. However, limited research has examined the mechanism of Friedel’s salt formation in ash. In particular, the effect of heating in an incinerator has not been reported. In this study, we investigated the formation of Friedel’s salt using reagents. Friedel’s salt was sufficiently formed upon heating at 1350 °C for 24 h and curing for 30 days. Higher temperatures than actual incineration conditions were required to produce pure Friedel’s salt, but a small amount of chlorine was also fixed in samples closer to actual incineration conditions. XRD and TG/DTA showed that the fixed chlorine most likely contained Friedel’s salt analogues. Washing experiments conducted to understand the leachability of chlorides from samples showed that 20–69% of the fixed chlorine did not dissolve during washing. The leaching behavior of the coexisting materials suggested that some of the fixed chlorine was present not in the form of Ca4Al2O6Cl2·10H2O but in chlorides or mixtures of chlorides with Ca and Cl bound at ratios of 1:1–2, which are more easily dissolved.