Selective precipitation and non-isothermal crystallization kinetics of britholite from low grade REE-bearing slag

In order to improve the recovery rate of rare earth, the effect of P2O5 and cooling rate on the selective precipitation of CaO–SiO2–TiO2–P2O5–Nb2O5–CeO2–CaF2 slag system was explored using X-ray powder diffraction, a scanning electron microscope, an electron probe micro-analyzer and a confocal laser scanning microscope. The apatite (Ca2.02Ce7.98Si6.00O26.00) is tiny and difficult to be separated from the slag without P2O5. When the addition of P2O5 is 6 wt%–10 wt%, the cerium and phosphorus are found to be enriched in britholite (Ca5–xCex[(SiO4)x(PO4)3–x]F) phase while x is negatively correlated with P2O5 content in the slag. The influence of P2O5 and cooling rate on the non-isothermal crystallization kinetics was also investigated. For the cooling rate between 5 and 40 °C/min, continuous cooling transformation diagram of britholite was constructed when the P2O5 content is between 6 wt%–10 wt%. The modified Avrami model was applied to determine the crystallization mode of slag with 10 wt% P2O5. It is shown that the crystallization mode is diffusion-controlled with constant nucleation rate and one-dimensional growth. In consideration of quantity and volume of crystals, the reasonable cooling rate is between 10 and 30 °C/min.