Energy efficient process for recovery of rare earths from spent NdFeB magnet by chlorination roasting and water leaching

Abstract

Rare earth elements are one of the most essential ingredient of modern technological applications. The extraction and recovery of rare earths from primary resources generates huge overburden, which adversely affect the environment. The spent Neodymium-Iron-Boron (NdFeB) magnet can be a potential secondary resource of rare earths through urban mining and recycling. In the earlier studied hydrometallurgical processes for the recovery of rare earths from spent NdFeB magnet, use of concentrated acids during leaching, discharge of acidic effluents and involvement of energy intensive oxidation roasting operations (500-950 degrees C) found as the major drawbacks of the processes. In view of high energy consumption and environmental concern, the present paper is focused on the development of an energy efficient process for the recovery of rare earths from spent NdFeB magnet. In this study, conventional oxidation roasting-acid leaching method was adapted to a lower temperature chlorination roasting-water leaching process by reducing the roasting temperature to 300 degrees C. In the process, ammonium chloride (NH4Cl) was used as chloridizing agent. The thermodynamic feasibility for selective chlorination of rare earths in the presence of NH4Cl was studied using FactSage and Thermo Gravimetric-Differential Thermal Analysis (TG-DTA). In order to determine the most suitable chlorination roasting condition, the effect of different parameters such as temperature, NH4Cl dosage and roasting time were studied in detail. The roasted products were characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy - Energy Dispersive X-Ray Spectroscopy (SEM-EDS) and chemical analysis. The rare earths were quantitatively and selectively recovered at the most suitable chlorination roasting condition (300 degrees C, 3 times of stoichiometric amount, 3 h) followed by water leaching. From the leach solution, rare earth oxide of 99.2% purity was produced. The leach residue containing 96.4% Fe2O3 was obtained as a by-product of the process. The lower chlorination roasting temperature and shorter roasting time (300 degrees C, 3 h) make the process more energy-efficient. The process with zero effluent discharge is easily scalable and environment-friendly.