Improved molten salt thermal treatment process for municipal waste incineration fly ash by liquid-liquid phase separation: Chloride form transition and release behavior

Previously, we proposed a molten salt thermal treatment process for municipal solid waste incineration fly ash (IFA), aimed at extracting heavy metals into the molten salt while inhibiting their volatilization. However, the use of molten chloride generates chloride residues, which limits the resource utilization of the treated IFA. These chloride residues are found in two main forms: (a) the insoluble chlorides formed during thermal treatment and (b) the molten chlorides remained in the interstices of the ash particles. In this study, the formation mechanism of the insoluble chlorides in molten salt was identified, and a method for the separation of molten chlorides from treated IFA was proposed. The findings indicated that CaCl₂ not only promoted the reaction between CaO, SiO₂, and Al₂O₃ but also displaced CaO in the Ca/Si/Al/Mg compounds forming Ca₁₂Al₁₄O₃₂Cl₂ and Ca₈Mg(SiO₄)₄Cl₂. Moreover, in situ melting IFA in molten salt to form the liquid-liquid phase was proposed to separate the molten chlorides from ash particles. The results showed that approximately 95 wt% of the chlorine in the IFA was transferred into the molten salt at 1300 °C for 10 min. The residual chlorides were suspended as the salt bubbles or dissolved in the molten IFA. Finally, a molten salt-assisted IFA melting and vitrification process was proposed. The extraction of heavy metals and the separation of salts from IFA could be achieved in a single unit without pre-treatment, allowing for the further utilization of the molten IFA.