Scaling high-speed counter-current chromatography for preparative neodymium purification: Insights and challenges

Efficient rare earth element (REE) separations are becoming increasingly important to technologies ranging from renewable energy and high-performance magnets to applied radioisotope separations. These separations are made challenging by the extremely similar chemical and physical characteristics of the individual elements, which almost always occupy the 3+ oxidation state under ambient conditions. Herein, we discuss the development of a novel REE separation aimed at obtaining purified samples of neodymium (Nd) on a multi-milligram scale using high-speed counter-current chromatography (HSCCC). The method takes advantage of the subtle differences in ionic radii between neighboring REEs to tune elution rates in dilute acid through implementation of the di-(2-ethylhexyl)phosphoric acid (HDEHP)-infused stationary phase (SP) of the column. A La/Ce/Nd/Sm separation was demonstrated at a significantly higher metal loading than previously accomplished by HSCCC (15 mg, R_Nd/REE > 0.85), while the Pr/Nd separation was achieved at lower metal loadings (0.3 mg, R_Nd/Pr = 0.75 - 0.83). The challenges associated with scaling REE separations via HSCCC are presented and discussed within.