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 (O2-) in molten CaCl2, which is important for applications in molten salt electrolysis. The dissolution mechanism of solid CaO and mass transfer characteristics of O2- 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 center dot 3CaCl(2)), with a peritectic point observed at 19 mol pct of CaO and 1104 K. The previously reported compounds Ca3OCl4 (CaO center dot 2CaCl(2)) and Ca5OCl8 (CaO center dot 4CaCl(2)) were never detected, while only Ca4OCl6 (CaO center dot 3CaCl(2)) 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 O2-). Above 1104 K, CaO directly dissociates to produce O2-. Although temperature influences the formation and dissociation of Ca4OCl6, it has minimal impact on the dissociation of CaO and the mass transfer of O2- in the melt.a