Insights into the Dissolution Behavior of CaO and Mass Transfer of Oxide Ions in Molten CaCl₂

This study focuses on understanding the behavior of oxide ions (O^2–) in molten CaCl₂, which is important for applications in molten salt electrolysis. The dissolution mechanism of solid CaO and mass transfer characteristics of O^2– in molten CaCl₂ were investigated through comprehensive chemical analysis. The results revealed that the incongruently melting compound in the CaCl₂–CaO system is Ca₄OCl₆ (CaO·3CaCl₂), with a peritectic point observed at 19 mol pct of CaO and 1104 K. The previously reported compounds Ca₃OCl₄ (CaO·2CaCl₂) and Ca₅OCl₈ (CaO·4CaCl₂) were never detected, while only Ca₄OCl₆ (CaO·3CaCl₂) was observed under air-tight conditions. The dissolution behavior of solid CaO in molten CaCl₂ is highly temperature dependent, with a critical threshold identified at 1104 K. Below this temperature, CaO reacts with CaCl₂ to form Ca₄OCl₆, which subsequently dissociates into simple ions (Ca²⁺, Cl^–, and O^2–). Above 1104 K, CaO directly dissociates to produce O^2–. Although temperature influences the formation and dissociation of Ca₄OCl₆, it has minimal impact on the dissociation of CaO and the mass transfer of O^2– in the melt.