Recent progress in impurity removal during rare earth element processing: A review

Supply of rare earth elements (REEs) critical to the development and maintenance of industrialized economies has become uncertain in recent years owing to the limited number of REE producers. In order to develop alternative REE sources, new mines are being explored and end-of-life secondary sources have been targeted for REE recovery. However, the composition of ores and secondary sources often differ significantly from one source to another and hydrometallurgical practices are not rigorously established or matured for many alternative REE sources. This presents a challenge in processing these materials while minimizing the entrainment of impurities. This work presents a comprehensive and systematic review on recent progress in impurity removal during REE processing. A large number of original research articles covering techniques including solvent extraction, ion exchange, precipitation, and other emerging technologies have been reviewed in detail and critically assessed to understand the role and behaviour of specific impurities in REE processing (including Al, Ca, Mg, Fe, Si, Th, U, Ti, Zr, Hf, Cr, Mo, Mn, Co, Ni, Cu, Zn, Sn, Pb, and Bi). Control of the most troublesome impurities for industry (Al, Fe, Th, U) depends on their concentration, redox ratio, and the degree of purity required. Most aluminum, ferric iron, thorium, and uranium(IV) selectively precipitate as hydroxides prior to REEs. Ferrous iron does not usually co-extract with REEs in their ion exchange or solvent extraction circuits. Precipitation of REE oxalates is effective to partition REEs and uranyl (UO22+) species. To remove thorium and uranium to the highest degree requires separate solvent extraction or ion exchange steps designed to specifically extract these impurities by anion exchange.