Eco-Friendly Rare-Earth-Metal Recovering Process with High Versatility from Nd-Fe‑B Magnets

We investigated the recovery of rare-earth elements from three types of Nd-Fe-B sintered magnets (Nd-mags) to study the effectiveness of the wet-mechanochemical resource recovery technique. These magnets were of different chemical compositions and were studied via the wet-mechanochemical treatment using a H2SO4-(COOH)2 mixed reaction solution. A high recovery rate (R) and high purity of rare-earth elements in the collected precipitate (P) could be achieved by optimizing the H+/RE and oxalate/RE. H+/RE is defined as the molar ratio between the H+ amount in the added H2SO4 and the rare-earth content in the Nd-mag (RE); oxalate/RE is the molar ratio between the added amount of (COOH)2 and RE values. The optimum conditions were H+/RE ∼ 5–10 and oxalate/RE ∼ 10; simultaneously, the R and P values were approximately 97 and 100 mass %, respectively. We also investigated the particle size effect on the R and P values, and the optimum treatment time was determined to be more than 1 h. Furthermore, the power consumption of the wet-mechanochemical treatment under optimal conditions was about 0.2 kWh. Thus, it was found that the wet-mechanochemical technique is an effective and eco-friendly method for recovering rare-earth elements as oxalate with high purity from a Nd-mag.