Mechanistic study of solvent extraction of rare earths with sterically hindered alkyl phosphonic acids.

The rare-earth elements (REEs) are a group of 17 chemically similar metallic elements (15 lanthanides, plus Sc and Y). Although REE minerals contain most of these elements simultaneously, high-tech and cleantech applications such as permanent magnets and lamp phosphors typically require only one specific, high-purity REE. To provide the individual REEs on an industrial scale, REE separation is performed by solvent extraction. Solvent extraction involves two immiscible liquid phases: an aqueous and an organic phase containing organic molecules (extractant). The REE with the highest affinity for the extractant preferentially moves to the organic phase, whereas the REE with the lowest affinity tends to remain in the aqueous phase. Due to the very similar chemical properties of REEs, the separation needs to be performed in 30-100 steps until the desired purity is obtained. Therefore, state-of-the art rare-earth separation is a costly, energy and resource intensive process. In this project, the structure of typical extractants used in industry for REE separations are modified. The reaction between the REEs and the extractants is studied on a molecular scale by various techniques. These techniques allow to modify the structure of the extractants and to drastically improve the REE separation efficiency. Hence, the number of separation steps and the environmental footprint of the process significantly decrease.