Recovery and Purity Trade-Off in Nd/Ho Separation from Real NdFeB Magnet Leachates

The growing demand for electric vehicles has increased the need for rare earth elements (REEs), promoting the recycling of secondary sources such as end-of-life NdFeB permanent magnets. However, the selective separation of REEs with similar chemical properties, such as neodymium (Nd) and holmium (Ho), remains a major challenge. In this work, their separation from a real NdFeB permanent magnet leachate was investigated by liquid–liquid extraction using di-(2-ethylhexyl) phosphoric acid (D2EHPA) as an extractant. The effects of extractant concentration (0.05 M and 0.1 M), organic-to-aqueous phase ratio (1:1 and 2:1, O:A), and pH (0.6 and 2) on the extraction process were evaluated. Increasing extractant concentration and phase ratio improved Ho extraction, reaching above 90%, whereas Nd co-extraction remained relatively low at pH 0.6 (0.8–4.6%, depending on the conditions). However, pH was found to be the key parameter. At pH 2, Ho extraction exceeded 96% in all cases, while Nd co-extraction increased significantly (up to ~28%), reducing selectivity. Due to the high initial Nd:Ho concentration ratio (~12:1), even low Nd co-extraction percentages resulted in significant contamination of Ho-rich products, identifying product purity as the primary limiting factor. These results show a trade-off between recovery and selectivity, with low pH favoring selectivity and high pH favoring extraction efficiency.