Kinetics and mechanism of hydrochloric acid leaching of rare earths from Bayan Obo slag and recovery of rare earth oxalate and high purity oxides

Abstract

Recovery of rare earth elements (REEs) from tailings is the solution to the waste of valuable resource and hazard of tailing dam. In order to lower the cost and increase the rate of REEs recovery, we investigated leaching kinetics and dissolution mechanisms of REE slag in hydrochloric acid (HCl) solution, and present a method for selectively recovering REEs directly from the leachate. Experiments were performed to elucidate the effects of the initial concentration of HCl, liquid–solid ratio, and reaction temperature on the leaching efficiency of REEs. The maximum leaching efficiency of REEs in this work exceeded 99%. The leaching kinetics and dissolution mechanism have also been analyzed by model fitting and microstructural observation. The leaching reaction was controlled by both of the interfacial transfer and diffusion across the product layer. The pitting corrosion and the colloidal silicic acid generated at the interface of REE-rich phase showed positive and negative effects on the leaching reaction rate, respectively. Finally, the selective recovery of REEs from the leachate was developed, and both the recovery of REEs and purity of the REE precipitate exceeded 99%. This study is expected to provide a feasible method for the recovery of REEs from the silicates containing slag.