Structural and Magnetic Characterization of Nd-Pr-Fe-B Sintered Magnet Machining Wastes.

Nd-Pr-Fe-B sintered magnets are considered important for emerging technologies. They are fundamental to the energy matrix transition, such as electric and hybrid vehicles and wind turbines. The production of these magnets generates tons of residues in the machining process step. Since China dominates the rare-earth (RE) market, leading to supply shortages, processing wastes are a promising alternative for recycling or reusing RE materials. Due to the amount generated and the chemical composition, containing up to 30 wt % of critical rare-earth elements, the studies of RE magnets are expanding in the current circular economy scenario. In this work, Nd-Pr-Fe-B machining wastes from two different machining processes (diamond cutting and grinding) were characterized by X-ray diffraction, Mössbauer spectroscopy, vibrating sample magnetometer with first-order-reversal-curves, scanning electron microscopy, X-ray fluorescence, elemental analysis, and X-ray photoelectron spectroscopy. The results showed that the degradation of the phases in both wastes is relatively strong. The phases of the magnets are decomposed into oxides, hydroxides, and hydrated oxides such as Nd(OH), ferrihydrite, and metallic iron. In addition, the machining process provokes a change in the iron vicinity of the NdFeB phase. The presence of impurities and the wide dispersion of particle sizes resulted in low magnetic properties and affected the magnetization behavior of the machining waste. Using different characterization techniques, it was found that the oxides formed during the machining processes are located on the surfaces of the particles, while the center consists of a nondegraded NdFeB phase. It was also found that the Nd-Pr-Fe-B wastes have similarities, indicating that it is possible to mix wastes from different machining processes before recycling. The complete characterization of the Nd-Pr-Fe-B machining residues indicated that different reuse and recycling strategies can be evaluated to improve the efficiency of reusing these machining wastes as secondary sources.