Ball-Milling Processing of Hydrogenated NdFeB Powders from the Recycling of End-of-Life Magnets

Rare-earth-based permanent magnets are key elements for today's technologies. Their recycling is crucial for securing the supply of critical raw materials and ensuring a sustainable circular economy in magnet production. In this study, hydrogen decrepitated NdFeB powders recovered from end-of-life magnets were processed using planetary ball milling to obtain powders suitable for recycled magnet production. The magnets were sourced from a wind turbine, scooter motor, and ring magnet, and exhibit different compositions and properties. The structural, morphological, and magnetic properties of the hydrogenated and ball-milled powders were investigated considering the effects of the milling time and ball diameters. Submicron-sized powders were achieved within 10 min with 5 mm balls, while longer milling times were required to obtain similar particle size reduction with 10 mm balls. Milling resulted in structural damage to the hydrogenated powders. Furthermore, although milling decreased the magnetization of the powders, an increase in the coercive field was observed for certain milling times. Micrometric powders sourced from the wind turbine, milled for 10 min using 5 mm balls, exhibited the highest coercive field of 0.27 T. This behavior is discussed considering the effect of the particle size reduction, the structural damage, the particle agglomeration, and the local compositional changes. We conclude that the powders produced from different source magnets exhibit similar patterns of morphological, structural, and magnetic changes under milling, and only the Tc and the Hc of the powders depend on their original magnet. Ball milling represents a promising technique for particle size refining in the hydrogen processing of magnetic scraps, a crucial step in the magnet-to-magnet recycling process.