Since its discovery in 1983, Nd-Fe-B has become the permanent magnet (PM) material with the highest energy product at ambient temperature [1]. Today Nd-Fe-B is used in many key technologies and the demand for high quality PMs will increase significantly in the near future. The required rare earths (RE) elements are considered as highly critical and the metallurgical processes to gain the RE oxides from ores have a large environmental footprint [2]. The usage of recycled material would lower the criticality and increases the sustainability of RE PMs [3]. For a viable and efficient industrial recycling process a circular economy will be necessary in which a material can be recycled multiple times. To enhance such a recycling process with reproducible outcomes the knowledge of the material behavior through every processing step or recycling cycle is mandatory.

With this in mind, a Nd-Fe-B PM alloy from magnetic resonance imaging (MRI) application was multiple recycled with the so-called functional recycling approach [4] using hydrogen decrepitation. Different material properties like chemical composition, particle size, density, microstructure, magnetic values or the degree of alignment were analyzed in detail over three recycling cycles. For the determination of the degree of alignment different methods like electron backscatter diffraction (EBSD) or magnetometry were used and compared. The multiple processing and recycling of the alloy leads to a decrease in texture, orientation and the resulting magnetic properties of the recycled magnets, respectively. Meanwhile impurities and particle size of the material increase through several milling and sintering processes. Different amounts of NdH2 were mixed with the recyclate to improve the properties. With 4 wt.% NdH2 the density of recycled magnet can be fully restored. It could be shown, that magnetic properties of multiple recycled magnets meet the specification of several applications such as loudspeakers or hoverboards, however they outperform clearly magnets produced from primary materials in terms of sustainability.

References:
[1] O. Gutfleisch, M.A. Willard, E. Brück, C.H. Chen, S.G. Sankar, J.P. Liu, Magnetic materials and devices for the 21st century: stronger, lighter, and more energy efficient, Advanced materials (Deerfield Beach, Fla.) 23 (2011) 821–842. https://doi.org/10.1002/adma.201002180.
[2] European Commission, Study on the EU’s list of Critical Raw Materials: Final Report (2020), 2020.
[3] O. Diehl, M. Schönfeldt, E. Brouwer, A. Dirks, K. Rachut, J. Gassmann, K. Güth, A. Buckow, R. Gauß, R. Stauber, O. Gutfleisch, Towards an Alloy Recycling of Nd–Fe–B Permanent Magnets in a Circular Economy, J. Sustain. Metall. 4 (2018) 163–175. https://doi.org/10.1007/s40831-018-0171-7.
[4] A. Walton, H. Yi, N.A. Rowson, J.D. Speight, V. Mann, R.S. Sheridan, A. Bradshaw, I.R. Harris, A.J. Williams, The use of hydrogen to separate and recycle neodymium–iron–boron-type magnets from electronic waste, Journal of Cleaner Production 104 (2015) 236–241. https://doi.org/10.1016/j.jclepro.2015.05.033.