Phase equilibria of CaO–SiO2–CaF2–P2O5–Ce2O3 system and formation mechanism of britholite

Recently, the sustainable utilization of REE-bearing slag for the recovery and application of rare-earth elements (REEs) has attracted considerable attention. However, a limited amount of thermodynamic data and crystal information for REE systems has been reported, which greatly limits the utilization of REE-bearing slag. In this study, the isothermal phase diagram of the CaO–SiO2–CaF2(30 wt%)-P2O5(10 wt%)-Ce2O3 system was constructed to provide phase equilibria data for the REEs in REE-bearing slag. The formation mechanism of britholite (Ca5-xCex[(Si,P)O4]3F) in the quinary system was found: it evolved from Ca5(PO4)3F, when a Ce3+ replaced a Ca2+, there would be a SiO4 4− instead of a PO4 3−. The phase equilibria and formation mechanism of REEs in the CaO–SiO2–CaF2–P2O5–Ce2O3 system are supplied to provide the data required for sustainable utilization of REE-bearing slag.