Effect of TiO₂ Addition and Cooling Rate on Crystallization Behavior of Separated Slag Containing Low-grade RE

With the continued exploitation and utilization of high-grade rare earth ores, it is increasingly important to extract rare earths from separated slag containing low-grade rare earth. The X-ray powder diffraction, scanning electron microscopy, electron probe micro-analyzer and confocal laser scanning microscopy were used to explore the influence of TiO2 and cooling rate on the crystallization of CaO-SiO2-TiO2-10 wt% P2O5-8 wt% Nb2O5-5 wt% CeO2-5 wt% CaF2 slag system. In this study, the britholite was precipitated selectively as the Ce-enriched phase. When TiO2 was added at less than 12 wt%, the britholite was promoted to crystallize meanwhile the Ca2Nb2O7 was suppressed. However, CaTiSiO5 inhibited the growth of britholite when the TiO2 content exceeded 15 wt%. The non-isothermal crystallization kinetics had also been investigated for the TiO2 content and cooling rate varied from 0-18 wt% and 10-40 degrees C/min, respectively. The continuous cooling transformation diagram and the relative crystallinity of the primary crystals were also constructed. Based on the observation and measurement of crystallization process, the modified Avrami model was applied to determine the crystallization mode of britholite with 9 wt% TiO2 addition. It was constant nucleation rate and one-dimensional growth with diffusion controlled. Considering the nucleation and growth of crystals, 20-30 degrees C/min was preferred to be the reasonable parameter during cooling stage.