Mechanism of Ferric Chloride Facilitating Efficient Lithium Extraction from Magnesium-Rich Brine with Tri-n-butyl Phosphate

The synergistic solvent extraction system comprising tri-n-butyl phosphate (TBP) and FeCl3 has been intensively studied for selective extraction of Li(I) from Mg(II)-rich brine. The extraction occurs via the formation of an ion-pair complex [Li(TBP)(x)][FeCl4] in which the negatively charged [FeCl4](-) neutralizes the positively charged [Li(TBP)(x)](+). Counterintuitively, many other metal chlorides give much lower Li(I) extraction efficiency than FeCl3, although they can also form chlorometallate anions similar to [FeCl4](-). In this study, the capabilities of CuCl2, AlCl3, InCl3, and SnCl4 for Li(I) extraction with TBP were examined and compared with that of FeCl3, accompanied by speciation studies. It was found that (1) AlCl3 does not form [AlCl4](-) while other metal chlorides (CuCl2, InCl3, FeCl3, and SnCl4) can form chlorometallate anions that could facilitate Li(I) extraction and (2) CuCl2, InCl3, and SnCl4 form neutral solvation complexes with TBP strongly, which hinders the formation of chlorometallate anions, but FeCl3 does not form neutral complexes with TBP. Concurrently being able to form [FeCl4](-) and to avoid forming neutral complexes facilitates the efficient Li(I) extraction by FeCl3 with TBP.