Ultrasound-assisted leaching of rare-earth elements from coal gangue by betaine hydrochloride

Coal gangue as a mineral waste, contains a large number of valuable elements including Rare-Earth Elements (REEs), which can be used in many precision instruments and military industries. Organic ammonium salt leaching is an effective method to extract REEs for coal gangue utilization. In this study, the ultrasound-assisted leaching using an organic ammonium salt betaine hydrochloride has been first attempted to extract REEs and other elements. With the betaine hydrochloride of 0.8 mol/L and ultrasonic power of 400 W, the recoveries of Total-REEs, Light-REEs, and Heavy-REEs achieve 85.65%, 90.81%, and 68.46%, respectively. For other coexisting elements of iron (Fe) and aluminum (Al), the leaching recoveries are as low as 2.49% and 2.72%, which indicates a good selective leaching performance. Kinetics analysis shows that the activation energies for Light-REEs and Heavy-REEs are 23.51 kJ·mol−1 and 39.80 kJ·mol−1, governing a diffusion and chemical reaction process. Ultrasound-assisted betaine hydrochloride leaching with the high efficiency and good selectivity provides a good solution for environmentally friendly REEs recovery.