Phase Equilibrium Study of Rare Earth Oxide-Fluoride Salt System: A Review

The applications of rare earth metals and alloys are becoming increasingly widespread and there is a strong market demand. Currently, most of the production enterprises adopt the fluoride-oxide system for electrolytic preparation of rare earth metals and alloys. The solubility of rare earth oxides in molten salt directly affects the selection of operational parameters in the electrolysis process. When the added amount of RE2O3 is less than its solubility, it leads to a decreased electrolytic efficiency. Conversely, an excessive amount of oxide is prone to settle at the bottom of the electrolytic cell, impeding smooth production. The RE2O3 solubility in the fluoride salt can be represented by the phase equilibrium of the RE2O3-REF3-LiF system. The isothermal lines in the primary phase field of rare earth oxide represent the solubility of the oxide in the fluoride salt at the corresponding temperature. This paper outlines the research methods and experimental results on the phase equilibria of the RE2O3-REF3-LiF system. The characteristics and existing problems in the current phase equilibrium study are analyzed. The solubility data of RE2O3 are expressed in the forms of ternary and pseudo-binary phase diagrams of the RE2O3-REF3-LiF system, providing theoretical guidance for the establishment of an accurate and reliable rare earth electrolysis system database and the optimization of electrolytic processes.