Towards a geochemical approach to guide hydrothermal REE recovery from NdFeB magnets

The NdFeB magnet world demand has doubled since 2005 to reach above 120 kton in 2020 [1]. The growing demand for REEs prompts their recycling. Sintered NdFeB permanent magnets are usually composed of above 20 wt.% Nd and a few wt.% of Pr and Dy. There are two main recycling ways for REE-based magnets. In the “short-loop process”, the main alloy is maintained in the valorized magnets, meaning that the microstructure and magnetic properties directly arise from the end-of-life product. In the “long loop” recycling process, one tries to extract the REEs from the alloy, which is the purpose of the present work.

The chemical separation of REEs from each other is difficult due to the similarity of their chemical properties. Following the encouraging results obtained by [2] in recovering REEs from NdFeB by hydrothermal treatment, we developed a geochemical approach of aqueous fluid – REE-compounds interactions based on the use PHREEQC software with the implementation of relevant REE-phases in the database. The database is tested against hydrothermal experiments on NdFeB powders with in-situ solution sampling. When reacted at 250°C and 100 bar, NdFeB powders transform into Nd(OH)₂, magnetite and Nd-borates along with large amounts of H₂. The low Nd solubility measured in the experiment is likely controlled by Nd-borates. The database will allow to investigate the effect of chlorine or CO₂ on the REE behaviour.

[1] Yang, Y. et al. (2017). J. Sustain. Metall., 3, 122-149.

[2] Maât, N. et al. ACS Sustain. Chem. Eng., 4, 6455-6462.