Efficient separation of transition metals from rare earths by an undiluted phosphonium thiocyanate ionic liquid

Abstract

The ionic liquid trihexyl(tetradecyl)phosphonium thiocyanate has been used for the extraction of the transition metal ions Co(II), Ni(II), Zn(II), and the rare-earth ions La(III), Sm(III) and Eu(III) from aqueous solutions containing nitrate or chloride salts. The transition metal ions showed a high affinity for the ionic liquid phase and were efficiently extracted, while the extraction efficiency of the rare-earth ions was low. This difference in extraction behavior enabled separation of the pairs Co(II)/Sm(III), Ni(II)/La(III) and Zn(II)/Eu(III). These separations are relevant for the recycling of rare earths and transition metals from samarium cobalt permanent magnets, nickel metal hydride batteries and lamp phosphors, respectively. The extraction of metal ions from a chloride or nitrate solution with a thiocyanate ionic liquid is an example of “split-anion extraction”, where different anions are present in the aqueous and ionic liquid phase. Close to 100% loading was possible for Co(II) and Zn(II) up to a concentration of 40 g L⁻¹ of the transition metal salt in the initial aqueous feed solution, whereas the extraction efficiency for Ni(II) gradually decreased with increase in the initial feed concentration. Stripping of Co(II), Zn(II) and Ni(II) from the loaded ionic liquid phase was possible by a 15 wt% NH₃ solution. The ionic liquid could reused after extraction and stripping.