The solubility and stability of organophosphoric acid extractants in H2SO4 and HCl media

The solubility and stability of several organophosphoric acid extractants is investigated in light of the selection of suitable extractants for iron(III) solvent extraction as a potential iron rejection route in zinc processing. Specifically, the effects were studied of acid medium (HCl and H2SO4), metal-salt presence (Fe2(SO4)3 and FeCl2,3), and temperature (20 and 50 °C) on extractant solubility and degradation. Aqueous entrainment was not considered. Tests were performed on three organophosphoric acid extractant candidates: OPAP (equimolar mixture of mono- and di-octylphenyl acid phosphates); D2EHPA (di-(2-ethylhexyl) phosphoric acid); and M2EHPA (mono-2-ethylhexyl phosphoric acid). With regard to simple H2SO4 and HCl acid solutions, the order of solubility and degradation followed was M2EHPA≫OPAP>D2EHPA. The presence of metal-salts reversed the observed order of degradation, H2SO4(aq)>HCl(aq), increasing degradation in the latter medium. To explain the degradation behaviour in HCl strip medium, a mechanism that considers iron(III) metal-catalyzed ester hydrolysis is postulated.