Effect of phase transition during roasting of Mountain Pass rare earth concentrate on leaching efficiency of rare earths

Abstract

To find a greener and more economical approach for treating the Mountain Pass rare earth concentrate (MPREC), a novel “Combination Method”, including the processes of oxidizing roasting, HCl leaching and sulfuric acid roasting, is proposed in this research. In this paper, the effect of phase transition behavior in hydrochloric acid leaching during oxidative roasting at 450–600 °C was studied. During roasting, the bastnasite in MPREC is decomposed into CeLa2O3F3 and (Ce0.33,La0.33,Ca0.33)O1.5. As the CeLa2O3F3 phase has the fluorine-fixation effect, the generated CeLa2O3F3 phase can not be dissolved by HCl after roasting at 500 and 550 °C, but it can be decomposed by HCl after roasting at over 600 °C, resulting in the improvement of the leaching rate of total F element. The maximum leaching efficiency of rare earth elements (REEs) can reach 70.32 wt% at the roasting temperature of 550 °C. After roasting at over 600 °C, some F– ions in the solution combined with RE3+ to form REF3 precipitate and enter the leaching residue, and the others are in the form of [CeF3]+ in the leaching solution during leaching, therefore, the leaching efficiency of non-cerium REEs decreases while that of cerium element increases. This work provides basic research for optimizing the actual production process of MPREC.