Effect of pretreatment methods on the selective leaching of rare earth elements from NdFeB permanent magnets using deep eutectic solvents

Abstract

In this study, deep eutectic solvents (DESs) were employed as environmentally friendly alternatives of inorganic acid to selectively leach REEs from waste NdFeB magnets. Three DESs were tested, effective for light REE leaching, and preliminary experiments were conducted to study the selective leaching of REEs, using synthetic materials, Nd₂O₃, Fe, Fe₃O₄, and Fe₂O₃. The findings suggest that neodymium and iron should exist as Nd₂O₃ and Fe₂O₃, respectively, for selective leaching. To enhance the selective leaching of the NdFeB magnet, three pretreatment methods were applied: oxidative roasting, NaOH digestion, and NaOH digestion-oxidative roasting. During the oxidative roasting, NdFeO₃ was formed, and it hindered REE leaching, achieving the highest REE leaching efficiency of 22.5% in the guanidine hydrochloride (GUC)-lactic acid (LA) DES. To counteract NdFeO₃ formation, the NaOH digestion was introduced, yielding Nd(OH)₃ and Fe₃O₄. These were then converted to Nd₂O₃ and Fe₂O₃ through oxidative roasting. With the NaOH-digested product, selective REE leaching was achieved solely with the ethylene glycol (EG)-maleic acid (MA) DES, while it was feasible in all three DESs with the NaOH digestion-oxidative roasting. The EG-MA DES displayed the highest selectivity, with a leaching efficiency of 97.3% Nd and 0.8% Fe. Additionally, the solvent could be reused at least twice, and the leaching efficiencies of 97% for Nd and 0.7% for Fe were maintained. This selective leaching technique benefits from using environmentally friendly solvents compared to the traditional inorganic acid leaching and demonstrates high REE selectivity and solvent reusability, suggesting a novel method for REE recovery via DESs.