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 P_(2)O_(5) and cooling rate on the selective precipitation of CaO-SiO_(2)-TiO_(2)-P_(2)O_(5)-Nb_(2)O_(5)-CeO_(2)-CaF_(2) 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(Ca_(2.02)Ce_(7.98)Si_(6.00)O_(26.00))is tiny and difficult to be separated from the slag without P_(2)O_(5).When the addition of P_(2)O_(5) is 6 wt%-10 wt%,the cerium and phosphorus are found to be enriched in britholite(Ca_(5-x)Ce_(x)[(SiO_(4))_(x)(PO_(4))_(3-x)]F)phase while x is negatively correlated with P_(2)O_(5) content in the slag.The influence of P_(2)O_(5) and cooling rate on the non-isothermal crystallization kinetics was also investigated.For the cooling rate between 5 and 40℃/min,continuous cooling transformation diagram of britholite was constructed when the P205 content is between 6 wt%-10 wt%.The modified Avrami model was applied to determine the crystallization mode of slag with 10 wt%P_(2)O_(5).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 10and 30℃/min.