Phase Equilibrium Relationship of CaO-SiO2-La2O3 System and Crystallization Behaviors of RE Phases

Recently, the recovery and utilization of rare earth elements (REEs) from secondary resources have become increasingly important with the rapid consumption of high-grade rare earth resources. There is a certain content of REEs in the blast furnace slag, which mostly are light rare earth elements (LREEs). However, the lack of basic thermodynamic data has caused difficulty for the recovery and utilization. In this study, the thermodynamic equilibria of CaO-SiO 2 -La 2 O 3 system were systematically investigated with lanthanum (La) as the representative LREE. Three rare earth equilibrium phases of Ca x La 4.67-x (SiO 4 ) 3 O 1-0.5 (70 wt.% La 2 O 3 ), La 2 Ca 3 (SiO 3 ) 6 (40 wt.% La 2 O 3 ) and La x Ca 2-x (SiO 4 )O 0.5x (20 wt.% La 2 O 3 ) were identified in this system, and Ca x La 4.67-x (SiO 4 ) 3 O 1-0.5× was considered the optimal RE phase for the separation of REEs by comparing the microstructures and chemical compositions. Isothermal phase diagram of CaO-SiO 2 -La 2 O 3 system at 1300 °C was constructed and the crystallization behavior of Ca x La 4.67-x (SiO 4 ) 3 O 1-0.5× phase studied to provide a theoretical basis for the enrichment of REEs and separation of RE phases from the blast furnace slag.