Extraction equilibrium of molybdenum(VI) and tungsten(VI) in aqueous solutions containing hydrogen peroxide by synergistic solvent extraction with TRPO and TBP

The separation of molybdenum from peroxotungstate solution with neutral phosphorus extractant such as trialkyl phosphine oxide (TRPO), tri-butyl phosphate (TBP) is often accompanied by extraction loss of tungsten, which greatly reduces the separation efficiency. To solve the above problems and optimize process, clarifying the reaction equilibrium of extraction is necessary. However, relevant research, especially the TRPO-TBP synergistic system, has not been reported. Here, based on the effect of extractant concentration and equilibrium pH on Mo and W extraction with three extraction systems (TRPO, TBP and TRPO-TBP), the Mo and W distribution equations in the two phases and the stoichiometry of chemical reactions were determined via slope analysis. The apparent extraction equilibrium constants and cumulative protonation constant of [Mo2O3(O2)4]2− were calculated by successive-approximation method. The order of the extraction equilibrium constants is as follows: logKMo-TRPO-H (5.8) > logKMo-TRPO-TBP-H (4.3) > logKMo-TRPO (3.8) > logKW-TRPO (3.7) > logKMo-TRPO-TBP (2.3). The synergistic mechanism of TBP for Mo and W extraction was revealed through Raman, FT-IR and 31P NMR spectroscopy, proposing a synergistic extraction compound TBP·[H2Mo2O3(O2)4·2H2O]·TRPO. The experimental results showed that keeping a higher equilibrium pH (2–2.5) can improve the separation efficiency of W and Mo, which verified the feasibility of optimization measure proposed based on equilibrium data. This study reveals the extraction mechanism of W and Mo in H+-H2O2/TRPO-TBP system, which can provide some reference for the optimization of separation process.