Evaluating citric acid as a selective leaching agent to extract Nd2Fe14B matrix phase from end-of-life magnets

A single-phase Nd 2 Fe 14 B powder was prepared from end-of-life (EOL) wind turbine magnets by a combination of hydrogen processing of magnetic scrap (HPMS) and selective leaching with citric acid. The impact of leaching time on chemical composition, particle size and magnetic properties was investigated. Due to the low reduction potential of rare earth element (REE), the Nd-rich phase was preferentially leached. The use of an acid concentration of 0.5 mol/L, a solid to liquid ratio of 1/10 and a leaching time of 30 min was sufficient to leach the Nd-rich phase. Atomic resolution transmission electron microscopy was employed to examine the surface structure and chemistry of the leached Nd 2 Fe 14 B powder. It is revealed that the leaching process affects not only the Nd-rich phase but also the matrix grains, resulting in the formation of a predominant oxygen-rich amorphous reaction layer, 25 nm thick. However, the oxygen content is reduced from 3500 to 2500 ppm and the magnetic saturation is increased by 8%. This method is a promising addition to the HPMS process, as the powder can be mixed with fresh, unoxidized grain boundary phase to produce recycled magnets with high remanence. A combination of hydrogen processing of magnetic scrap (HPMS) and selective leaching with citric acid was employed to remove the partially oxidised grain boundary phase from end of life magnets. This process yields a single-phased Nd 2 Fe 14 B with a reduced oxygen content, which is then suitable for further processing.