Cyclone separation of hydrogen processed NdFeB magnets for improved properties

The importance of NdFeB magnets is growing in modern technology, and with increasing demand, the development of advanced recycling methods is becoming inevitable [1]. The recovery of NdFeB magnets in the form of an alloy powder have been demonstrated successfully through the ‘Hydrogen processing of magnet scrap’ (HPMS) for a range of end-of-life magnet applications including for example hard disk drives (HDD), automotive scrap and loudspeakers [2, 3]. HPMS uses hydrogen decrepitation combined with mechanical separation to produce a friable alloy powder containing two hydrides (Nd2Fe14BHx and NdH2.7), without having to fully disassemble the original application [4]. The resulting powder contains more oxygen than the primary alloys, and it is predominantly picked up by the grain boundary phase (GBP), while the matrix hydride phase remains largely unaffected. It has been demonstrated that due to this oxidation the recycled re-sintered magnets cannot achieve the same grade as the original magnets [5-7]. To separate the matrix hydride phase and the grain boundary hydride, a spiral jet mill equipped with a dry cyclone and classifier wheel was used. The HPMS powders, obtained from magnets of hard disk drives (HDD) and electronic waste (EW), were jet milled to below 10 µm and separated into coarse and fine fractions. Their compositions were analysed by Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), Oxygen/Nitrogen (O/N) and Carbon/Sulphur (C/S) analysers. Significant differences in oxygen and total rare earth (TRE) were detected, with a high concentration of rare-earth-rich GBP in the fine fraction. The jet-milled powders with and without separating the GBP were re-sintered after blending with 5 wt.% primary NdH2.7 which was added to overcome the negative effects of Nd-oxide and to ensure full densification on sintering [6]. Their magnetic properties were investigated. Sintering the coarse fraction alone resulted in higher remanence and coercivity compared to when blended with fine GBP fraction in the powder. The coercivity (HcJ) was 1182 kA/m (Coarse) and 1028 kA/m (Coarse + Fine) for HDD magnets, and it was 1466 kA/m (C) and 1385 kA/m (C+F) for EW magnets. The remanence (Br) was 1.28 T (C) and 1.25 T (C+F) for HDD magnets, and it was 1.28 T (C) and 1.23 T (C+F) for EW magnets. This improvement was due to the elimination of Nd-oxide which was significantly concentrated in the fine fraction. This work demonstrates that it is possible to upgrade the properties of HPMS alloy powders by removing unwanted oxide phases from the powder.