Electrochemical leaching of rare-earth elements from spent NdFeB magnets

Abstract

Spent NdFeB magnets are highly appealing secondary resources for rare earth elements (REEs), especially for Nd. The goal of this study was to investigate electrochemical leaching of Fe and rare earths from NdFeB magnets with sulfuric and oxalic acids at different acid concentrations and current densities and reveal the mechanism of the leaching in varying conditions. The metals were leached from polished magnet surface and from crushed magnets using a 3D-printed Ti basket. Electron and atomic force microscopic analysis showed a highly heterogeneous structure of the magnets containing on average 60 wt-% of Fe and 25 wt-% of Nd. According to Kelvin probe force microscopy, the Volta potential difference between the matrix and the REE-rich grains was over 500 mV. Addition of oxalic acid (H2C2O4) in the sulfuric acid leach solution allows leaching at lower cell voltage and separation of the REE-oxalate precipitates. Some incorporation of rare-earth element oxalates in the cathodic sediment was also observed. Finally, the leaching mechanism is discussed.