THE STATE OF THE ART IN SEPARATING AND PURIFYING THE HEAVY RARE EARTHS: SOLVENT EXCHANGE, ION EXCHANGE, AND SOLID PHASE EXTRACTION, WHICH IS THE OPTIMAL PROCESS?

The purification of rare earth elements (REE) at mining operations is traditionally accomplished by multiple chemical phase-transfer processes including precipitation, solvent extraction, and ion exchange. We describe REE purification results with Solid Phase Extraction (SPE) columns that were developed to improve the speed and economy of REE purification. The SPE columns have the properties of enhanced equilibration kinetics, so that residence time of a few seconds will accomplish selective chemical transfers from solution phase to solid phase. We describe the separation of REE’s as a class from leach solutions and the further purification of REE salts. The rapid rate of chemical equilibration of lanthanide salts in solution with selective chelating agents tethered to the solid support material of SPE columns enable the production of purified REE salts in minutes of residence time. In the example of cerium purification, the purification of 3500 tons Ce per year in a 10,000 ton REE plant can be accomplished with columns having a total volume of 0.7 cubic meters. Likewise, the separation of lanthanum and its nearest chemical neighbor, praseodymium, is effected in one stage to give >99% purity. We believe that the rapid phase transfer kinetics of Solid Phase Extraction columns will be helpful in the evolution of rare earth element separation.