Accelerated dissolution mechanisms of rare earth elements in waste permanent magnet with oxygen vacancy.

Slow dissolution rate of magnet oxides is an important barrier to the separation of rare earth elements. We can combine mechanochemical processing with extraction using deep eutectic solvents (DES) to achieve an accelerated dissolution of iron (Fe) and neodymium (Nd) in permanent magnet oxides. The dissolution rate of Nd was found to increase to a greater extent compared to that of Fe, which was partly because iron oxide (Fe2O3) possessed a higher internal energy reserve than neodymium oxide (Nd2O3). The emergence of oxygen vacancies played a primary role for the enhanced dissolution and rate differentiation of Fe and Nd. Density functional theory calculations indicated that Nd2O3 was more likely to develop oxygen vacancies than Fe2O3. The process of Nd2O3 dissolution in the DES system (guanidine hydrochloride and lactic acid) occurred through a two-step reaction mechanism involving the adsorption of lactic acid molecules on oxygen vacancies and electronic interaction with lactic acid molecules. The defective oxygen vacancies in Nd2O3 (001) exhibited greater adsorption and binding affinity towards hydrogen ions and lactic acid molecules when compared to Fe2O3 (001). Our work provides a mechanistic understanding of the accelerated dissolution of rare earth elements in waste permanent magnets in DES systems.