Recovery of rare earth metal oxides from NdFeB magnet leachate by hydrophobic deep eutectic solvent extraction, oxalate stripping and calcination

With the scarcity of resources and the growing consumer demand for rare earth elements (RE), it is imperative to develop clean and sustainable methods to recover these valuable elements. This research proposes a novel green process for the selective recovery of RE from spent NdFeB permanent magnet using hydrophobic deep eutectic solvent (HDES). A total of 13 HDES systems composed of trioctylphosphine oxide (TOPO) as hydrogen bond acceptor (HBA) and alcohols as hydrogen bond donor (HBD) were prepared and characterized. Through a simple screening process, it was determined that ternary HDES (DA:MA:TOPO = 5:5:4) exhibited the best extraction and separation performance for REs and Fe. The extraction conditions such as aqueous phase acidity, organic/aqueous phase ratio, the concentration of Fe3+ and equilibration time were further studied and optimized. Under the most suitable conditions, HDES (DA:MA:TOPO = 5:5:4) was used for batch countercurrent extraction, where the concentration of REs in the raffinate was lowered from 4390 mg/L to 34 mg/L, and the recovery reached 99%. The HDES could be stripped by 0.2 mol/L Na2C2O4, which could further separate a small amount of loaded Fe (133 mg/L) and RE, so that the purity of the final product reached >99%. The stripped HDES could be recovered after washing with deionized water. Even after five cycles, HDES still maintained good stability. This innovative process does not require the use of salting-out agents or toxic volatile solvents, providing new possibilities for efficient and clean separation.