Effects of phosphoric acid and ageing time on solvent extraction behavior of phosphotungstic acid

A sulfuric-phosphoric acid leaching process for scheelite was developed. In the process, tungsten exists as a soluble phosphotungstic acid in the leachate. Thus, extracting tungsten from the solution is necessary. Considering the advantages of solvent extraction with oxygen-containing solvents, 2-octanol and tri-butyl phosphate (TBP) were employed to extract tungsten from phosphotungstic-sulfuric acid solutions. The impacts of phosphoric acid and ageing time on the extraction were primarily investigated. It was determined that for H3PW12O40 solution with high H2SO4 (49.71 g/L S) content, the WO3 content in raffinates remained as low as 0.01 g/L after a single stage extraction even if the ageing time increased. However, if such solution contained high H3PO4 (113.95 g/L P), the remaining WO3 content increased to approximately 0.29 g/L after aged for 0.5 h. And, it further increased to 1.06 and 2.16 g/L after ageing for 24 h and 14 days. It indicated that phosphoric acid and ageing time played an anti-synergistic effect on the extraction. Furthermore, for the solution with abundant H3PO4 aged for 0.5 h, the 31P NMR results showed there were only the characteristic peak of H3PW12O40 (δ = − 16.24 ppm) and the peak of H3PO4 (δ = − 0.96 ppm). However, an unknown peak (δ = − 2.80 ppm) appeared in the spectrum of such solution aged for two weeks and that of its raffinate. This finding implies that the combination reaction between phosphoric and phosphotungstic acid creates another species that is difficult to extract. Therefore, to obtain satisfactory extraction efficiency, the dosage of phosphoric acid should be controlled during the mineral decomposition stage.