Leaching kinetic study of sulfuric acid roasted mixed-type rare earth concentrate for reducing the solid-waste production and chemical consumption

Abstract

The Baotou bastnaesite and monazite mixed rare earth ore is one of the most significant rare earth ore in the world and is mainly processed by the sulfuric acid roasting method. However, the high content of Fe and P in the leaching solution contributes to a large amount of radioactive precipitate waste produced in the purification process, and that causes the primary loss of rare-earth elements (REEs) and the environmental issue and safety concern. The goal of this paper is to reduce the solid-waste by improving the selective leaching efficiency of REEs based on a leaching kinetics study of REEs, Fe and P in the H2SO4 system. The experiments were carried out focusing on the effect of acidity, roasted concentrate particle size, temperature, and liquid-solid ratio on the leaching kinetics. The results indicate that the equilibrium leaching time of REEs (<15 min) and P (<10 min) is far less than that of the Fe (60 min to more than 180 min) which is highly depended on temperature. The leaching kinetics model of REEs, Fe and P follows the shrinking core model but is governed by different limiting step: both REEs and P are controlled by diffusion, while Fe is controlled by chemical reaction. A possible leaching mechanism is proposed. Based on the kinetic study, the dissolution of Fe is inhibited and its content in the leaching liquid is reduced by controlling the leaching time. Meanwhile, the result of neutralization for purifying the leaching solution shows that the amount of solid waste generated in the neutralization process is reduced by 19% with improving 3% REEs recovery and reducing 15% MgO consumption. This study shows a good prospect for improving the sulfuric acid roasting method to be a cleaner and lower chemical consumption process in dealing with the bastnaesite and monazite mixed rare earth ore.