An economical and eco-friendly magnet recycling method was developed based on the extraction of hard magnetic Nd2Fe14B grains by selective leaching of Nd or rare-earth (RE) rich phase. At first electrochemical extraction of grains was carried out but yield of grain extraction and leaching rate of the RE-rich phase was low, so the magnet was pulverized by hydrogen decrepitation and grains were extracted by treating the powder with citric acid. For comparison, nitric acid was used to investigate selectivity towards the leaching of the RE-rich phase.
However, the nitric acid was found to have no selectivity for the Nd-rich phase, and exposure to nitric acid results in a structurally damaged Nd2Fe14B matrix phase as evidenced by the SEM (Fig. 1)
The yield obtained by this process was comparatively higher than the electrochemical process and the process time was 15 minutes which was further lowered to 5 minutes by ultrasonication. Results of the SEM (Fig. 1 and 2) microstructural investigation and ICP-MS compositional analysis (Fig. 3 and 4) and magnetic measurements (Fig. 5) of the leached Nd-Fe-B powders confirm that citric acid can indeed selectively leach away the RE-rich phase while preserving the Nd2Fe14B matrix phase concerning its magnetic moment/mass values. The work was published in MDPI Materials in 2023.
The extracted Nd2Fe14B grains were blended with Nd70Cu30 eutectic alloy and consolidated with spark plasma sintering (SPS). The low-eutectic alloy with the composition Nd70Cu30 was prepared through arc melting of stoichiometric amounts of the metal neodymium and copper, followed by melt spinning and milling. The Nd-Cu was added (0-30 wt.% )to study its impact on the microstructure and magnetic properties of magnets re-engineered via Spark Plasma Sintering (SPS). Notably, Nd-Cu prevented the decomposition of the Nd2Fe14B matrix phase due to Joule overheating at particle contacts during SPS. A theoretical density of 7.6 g/cm³ was achieved with a 10 wt.% Nd-Cu addition.
Discussions are being carried out with industrial parties like KOLEKTOR through Boris Saje and Magneti Ljubljana through Matej Zaplotnik to test this protocol and to make novel magnets from recovered grains
