Extraction of phosphorus, arsenic and/or silica from sodium tungstate or molybdate solutions with primary amine and tributyl phosphate as solvents. II — Mechanism of extraction of phosphorus, arsenic and silica from tungstate and molybdate solutions

Abstract

The mechanism of the removal of phosphorus, arsenic and silica from tungstate or molybdate solutions by synergistic extraction with primary amine and tributyl phosphate (TBP) was investigated. Phosphorus, arsenic and silica were considered to be extracted into the organic phase as heterotungstic or heteromolybdic acids. The composition of the extracted species is strongly dependent on the concentration of tungsten and molybdenum and equilibrium pH values of the aqueous phase. An extraction mechanism based on the formation of hydrogen bonding between the primary amine, TBP and heteropoly acids is proposed. The loaded organic phase was characterized by UV and IR spectra and by molar conductance. It is proposed from the experimental results that the proton transfer from the heteropoly acids to the primary amine in the extracted species occurs intramolecularly, resulting in the formation of ion-associated species, because the primary amine is a weak base and heteropoly acids are strong acids. The mechanism of proton transfer is also proposed.