Usage of inorganic polyoxometalates of tungsten for extraction of rare earths in an aqueous two-phase system (ATPS): Preliminary results

Abstract

The extraction of rare earth elements (REEs) such as Y^III, La^III, Pr^III, Nd^III, Eu^III, Tb^III, Dy^III, Er^III and Yb^III in aqueous two-phase system (ATPS) formed by PEG/MgSO₄ and polyoxometalates (POMs) as extractants was investigated under various conditions. Increased vacant sites in the POM and higher POM concentrations enhanced the competitive effect among REEs extraction. Lower acidity also enhanced the effect, and the extraction depended on the initial REEs composition. Optimal separation was achieved at 8.8E-4 M REE mix solution, 67.5 molar ratio HCl: REE, 3:1 molar ratio of Na₁₀[SiW₉O₃₄]: REE and 25 °C, achieving 52 % extraction for Tb and an average selectivity of 2.1. The extraction trend followed the REE atomic number, except for Y. The stripping of REE from the light phase was done by precipitation using tetraethylammonium chloride. The ICP and FT-IR techniques indicate stripping of REE: POMs complexes and REE: hydroxosulfate complexes according to OH⁻ in the system. The stripping of REE in the first complexes follows the same trend as in extraction, and the second type offers selective stripping of La^III, Pr^III, Nd^III, Eu^III as a group. Furthermore, continuous flow experiments using microreactors were conducted, varying reactor lengths and flow-rates based on batch test conditions. Fluctuations were observed in all cases, likely due to mass transfer between phase-forming compounds, resulting in wavy core-annular flow. The small discrepancies with batch experiments demonstrate the feasibility of continuous extraction using ATPS and POM as extractants.