The role of water in extraction and separation of rare earth elements in supercritical carbon dioxide

Rare earth elements can be complexed with tributyl phosphate, thus forming a metal-ligand complex which is soluble in supercritical CO₂. In this study, neodymium and holmium nitrate were extracted into supercritical CO₂ with varying amounts of tributyl phosphate (from 8 to 80 mM) and water (from 0 to 200 mM). Temperature was held at 60 °C and pressure ranged from 17 to 28 MPa. UV–vis spectroscopy was used to measure supercritical metal concentrations, and near-IR spectroscopy was used to measure water concentrations. It was found that holmium is preferentially extracted over neodymium, with the greatest separation occurring when the molar ratio of tributyl phosphate to lanthanides was approximately 1–2, and when no water was added. Affinity constants were fit to the data. The results suggest that a soluble Ho(NO₃)₃·2TBP·H₂O complex accounts for the preferential extraction of holmium, especially at lower water and tributyl phosphate concentrations.