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 CaCl 2 , which is important for applications in molten salt electrolysis. The dissolution mechanism of solid CaO and mass transfer characteristics of O 2– in molten CaCl 2 were investigated through comprehensive chemical analysis. The results revealed that the incongruently melting compound in the CaCl 2 –CaO system is Ca 4 OCl 6 (CaO·3CaCl 2 ), with a peritectic point observed at 19 mol pct of CaO and 1104 K. The previously reported compounds Ca 3 OCl 4 (CaO·2CaCl 2 ) and Ca 5 OCl 8 (CaO·4CaCl 2 ) were never detected, while only Ca 4 OCl 6 (CaO·3CaCl 2 ) was observed under air-tight conditions. The dissolution behavior of solid CaO in molten CaCl 2 is highly temperature dependent, with a critical threshold identified at 1104 K. Below this temperature, CaO reacts with CaCl 2 to form Ca 4 OCl 6 , which subsequently dissociates into simple ions (Ca 2+ , Cl – , and O 2– ). Above 1104 K, CaO directly dissociates to produce O 2– . Although temperature influences the formation and dissociation of Ca 4 OCl 6 , it has minimal impact on the dissociation of CaO and the mass transfer of O 2– in the melt.