Ion-pair induced solvent extraction of lithium (I) from acidic chloride solutions with tributyl phosphate

Lithium (Li) is an important energy metal in the 21st century. However, the selective recovery of Li is still a big challenge, especially from acidic solutions with multiple metal ions existence. Herein we report a new ion pair induced mechanism for selectively extracting Li+ from acidic chloride solutions by tributyl phosphate (TBP). It is shown that the acidity and the chloride ions in the aqueous phase have great effects on the extraction of Li+. The FT-IR, UV-Vis and ESI-MS experiments provide solid evidence for the formation of ion-pair complex [Li(TBP)n(H2O)m]+[FeCl4]− (n = 1, 2, 3; m = 0, 1) in the organic phase, which brings about the effective and efficient extraction of Li+. This mechanism can overcome the Hofmeister bias and allow for the selective extraction of Li+ from the extremely hydrophilic chlorides. It has also been proved that the loaded Li in TBP can be effectively stripped by concentrated HCl solution with a Li/Fe separation factor > 500. The understanding of the ion-pair transport mechanism is helpful for optimizing the recovery process or further advancing more efficient recovery techniques for Li from acidic liquor.