Selective leaching and recovery of neodymium from NdFeB carbonyl residues

Neodymium iron boron (NdFeB) is a widely-used magnetic material; however, the production process generates significant amounts of rare earth element-rich waste. Refining the Fe via carbonylation leads to carbonylated slag containing 0.2–2.0 % rare earth elements. The extraction of trace rare earth elements from waste materials has not been reported, although the Nd recovery would be economically and environmentally beneficial. In this work, we calcinated NdFeB carbonylation slag (with less than 0.3 % rare earth content) and identified a green deep eutectic solvent (DES) that can selectively recover Nd. The best performing DES comprised a 2:1 M ratio of ethylene glycol to maleic acid and was highly selective for Nd and inert toward Fe, thus enabling effective separation. Specifically, the Nd/Fe selectivity factor exceeded 3000, i.e., under optimal conditions, 28.07 % of the Nd was extracted, while the Fe leaching rate was less than 0.01 %. The Nd in the DES was subsequently extracted and then enriched by reverse extraction using a di-(2-ethylhexyl)phosphoric acid (P204)-sulfonated kerosene system and hydrochloric acid. DES still had recyclability after extraction. Finally, the Nd was precipitated by adding oxalic acid to form the rare earth oxalate, which was calcined to obtain a high-purity Nd2O3. The strategy applied in this work based on DES and conventional organic extraction represents a promising and sustainable approach for the selective recovery of trace rare earth elements.