A hydro/solvothermal process for recycling Nd-Fe-B magnets

Sintered Nd-Fe-B permanent magnets were subjected to a hydro/solvothermal pulverization process developed for recycling rare-earth permanent magnets. This process, with a low environmental footprint, consists of heating the magnets in a closed reactor in the presence of water (hydrothermal treatment) or ethanol (solvothermal treatment). Different parameters such as preliminary grinding, processing time and nature of the solvent have been investigated. The pulverization process is confirmed by the degradation of the intergranular Nd-rich phase. The magnetic phase is hydrogenated and then decomposed by the reaction of the rare earth with oxygen and hydrogen to form oxides and hydroxides. In order to stop the degradation of the hard magnetic phase and to produce recycled powders with good magnetic properties, the influence of the hydro/solvothermal process parameters (solvent composition and treatment time) was investigated. The results show an important lowering of the hard magnetic phase decomposition for powders subjected to preliminary mechanical grinding, and then treated in pure ethanol for short durations.