Isothermal Phase Relations in the CaO-Al2O3-CeO2-MgO Quaternary Oxide System at 1500 °C

Investigation of the isothermal phase diagram of the CaO-Al2O3-CeO2-MgO slag system is of considerable importance for the enrichment and recovery of secondary rare-earth resources. However, studies on the thermodynamic properties of this quaternary slag system remain limited, and essential thermodynamic data, particularly those concerning phase-equilibrium relationships, are still lacking. In the present study, the phase equilibria of the CaO-Al2O3-CeO2-MgO slag system were systematically investigated through high-temperature equilibration experiments at 1500 °C in an air atmosphere, combined with scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and an isothermal phase diagram of the system was constructed. The results indicate that 14 phase equilibrium relationships exist within the investigated composition range. A liquid-phase region was identified at 1500 °C over the composition ranges of w(CaO)/w(Al2O3) = 0.52~1.39, w(CeO2) = 0~25.75% and w(MgO) = 0~10.00%. Based on the constructed isothermal phase diagram, the formation of Ce-enriched solid phases during the recovery of secondary rare-earth resources was further analyzed. These results provide an important thermodynamic basis for the efficient recovery of secondary rare-earth resources and the optimization of related metallurgical processes.