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

Abstract

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.