Interest in Rare Earth Element Separation Grows

It is not surprising that in the list of Minerals Engineering's most recently downloaded articles on ScienceDirect, papers on rare earth separation are in the top five. Rare earth elements (REE) have unique physicochemical properties that make them essential elements in many high-tech components. They are widely used for traditional sectors including metallurgy, petroleum, textiles, and agriculture but they are also becoming uniquely indispensable and critical in many other applications, such as hybrid cars, wind turbines, and compact fluorescent lights, flat screen televisions, mobile phones, disc drives, and defence technologies. Different rare earths are needed to supply the required functionality in these applications. In some cases, a single rare earth element may be required, such as La for nickel-metal hydride batteries, but other applications require a mixture of rare earths, for example Nd and Pr for rare earth magnets (see also posting of 11th February 2013) and Eu (or Tb) and Y for rare earth phosphors.

Bastnesite (La, Ce)FCO3, monazite, (Ce, La, Y, Th)PO4, and xenotime, YPO4, are the main commercial sources of rare earths. Increased demand for the different products manufactured from REE has resulted in a constriction of supply from China, which currently produces 97% of the world’s rare earths, via export quotas. Many new rare earth deposits are currently being developed to help meet the demand void created by the Chinese export quotas, however most of these developing deposits include rare earth minerals for which there is limited processing knowledge. A paper from McGill University Canada (see also posting of 1st February 2013) examines the separation techniques, usually flotation, gravity or magnetic, that are currently employed for rare earth mineral beneficiation and identifies areas in need of further research.

Rare earth mineral concentrates are leached with aqueous inorganic acids, such as HCl, H2SO4, or HNO3. After filtration or counter current decantation (CCD), solvent extraction is usually used to separate individual rare earths or produce mixed rare earth solutions or compounds. Rare earth producers follow similar principles and schemes when selecting specific solvent extraction routes. The use of cation exchangers, solvation extractants, and anion exchangers, for separating rare earths has been extensively studied. The choice of extractants and aqueous solutions is influenced by both cost considerations and requirements of technical performance. Commercially, D2EHPA, HEHEHP, Versatic 10, TBP, and Aliquat 336 have been widely used in rare earth solvent extraction processes. Up to hundreds of stages of mixers and settlers may be assembled together to achieve the necessary separations. It is a challenging process, and a paper by authors from Northeastern University, China, University of British Columbia, Canada, and University of California-Berkeley, USA, the most downloaded article in the past 3 months, reviews the chemistry of different solvent extractants and typical configurations for rare earth separations.

Expect to see many more papers on rare earths in future journal articles and conference presentations.