Separation of heavy rare-earth elements by non-aqueous solvent extraction: Flowsheet development and mixer-settler tests

The separation of rare-earth elements (REEs) is considered one of the most challenging processes in solvent extraction. In recent years, non-aqueous solvent extraction, a unit operation within solvometallurgy, has stepped into the limelight as one of the promising techniques for efficient REEs separation. In this paper, a rare-earth hydroxide concentrate, originating from mining waste and containing mainly heavy rare-earth elements (HREEs), was redissolved in ethylene glycol + 10 vol% water, 0.43 mol L-1 HCl and 0.8 mol L-1 NaCl. Based on batch experiments, a conceptual flowsheet was proposed for the separation of the HREEs into 2 groups: a thulium group (Tm, Yb and Lu) and a dysprosium group (Dy, Ho, Er and Y). Continuous solvent extraction tests in lab-scale mixer-settlers were performed to confirm the technical feasibility of the developed system, as well as to identify and resolve possible bottleneck points. Eventually, using only 16 stages of lab-scale mixer-settlers, the purity of the thulium group and dysprosium group elements, originally 34% and 54%, respectively, reached 99.8% and 98.7%, respectively. Further optimization remains necessary for the separation and purification into highly pure single REEs.