Phase Equilibrium Relationship of CaO-Al2O3-Ce2O3-CaF2 Slag System at 1300~1500 °C

CaO-Al2O3-Ce2O3 is a potential new-type basic metallurgical slag system for rare earth steel. To investigate the effects of CaF2 on the melting point and equilibrium phase types of this slag system, the phase equilibrium relationships and extent of the liquid phase region of CaO-Al2O3-Ce2O3-CaF2 slag system at 1300 °C, 1400 °C, and 1500 °C in C/CO were determined by the high-temperature phase equilibrium experiment, Scanning Electron Microscope-Energy Dispersive X-ray Spectrometer (SEM-EDX) and X-ray Diffraction (XRD), and the isothermal phase diagram was plotted. The experimental results show that within the composition range in this study, the slag system has five, seven, and six liquid–solid equilibrium coexistence regions at 1300 °C, 1400 °C, and 1500 °C. The involved multiphase equilibrium regions include five two-phase regions (i.e., Liquid + CaO, Liquid + CaO·2Al2O3, Liquid + 2CaO·Al2O3·Ce2O3, Liquid + 2CaO·3Al2O3·Ce2O3, Liquid + 11CaO·7Al2O3·CaF2), 4 three-phase regions (i.e., Liquid + CaO + 2CaO·Al2O3·Ce2O3, Liquid + 11CaO·7Al2O3·CaF2 + 2CaO·Al2O3·Ce2O3, Liquid + CaO·2Al2O3 + 2CaO·3Al2O3·Ce2O3, Liquid + 11CaO·7Al2O3·CaF2 + 2CaO·3Al2O3·Ce2O3), and 1 four-phase region (i.e., Liquid + CaO + 11CaO·7Al2O3·CaF2 + 2CaO·Al2O3·Ce2O3). Meanwhile, based on liquid phase compositions under liquid–solid multiphase equilibrium, the slag system’s liquid phase ranges at the experimental temperatures were determined as follows: at 1300 °C: w(CaO)/w(Al2O3) = 0.42~0.92, w(Ce2O3) = 1.63%~8.02%, w(CaF2) = 9.17%~21.46%; 1400 °C: 0.28~1.18, 0.9%~12.62%, 1.04%~23.34%, respectively; 1500 °C: 0.23~1.21, 0~14.42%, 0~26.32%, respectively.