Combining Selective Extraction and Easy Stripping of Lithium Using a Ternary Synergistic Solvent Extraction System through Regulation of Fe3+ Coordination

Lithium is becoming increasingly important due to its essential role in lithium-ion batteries. Over 70% of the global lithium resources are found in salt lake brines, where lithium is always accompanied by magnesium. The system of TBP (tributyl phosphate)/FeCl3 has been widely studied for selective lithium extraction from high Mg-containing salt lake brines. However, the system requires a high concentration of HCl for lithium stripping, which causes equipment corrosion, consumes large amounts of reagents, and requires careful neutralization of the organic phase for regeneration. In this study, we develop a novel ternary synergistic solvent extraction system of TBP/FeCl3/P507 (P507 is 2-ethylhexyl phosphonic acid mono 2-ethylhexyl) to significantly enhance the stripping of lithium while largely maintaining lithium extraction capacity. During lithium extraction from Mg-containing solutions, TBP and FeCl3 coordinate with lithium in the form of Li·2TBP·FeCl4, while P507 is not involved in the coordination. When the loaded organic phase is contacted with water, P507 and TBP synergistically coordinate with Fe3+ in the form of FeCl2L·(HL)·2TBP (HL denotes P507), leading to efficient stripping of lithium due to the breaking of the coordination structure of Li·2TBP·FeCl4, and Fe3+ is maintained in the organic phase for the next extraction cycle. The efficient stripping of lithium using only water, instead of high concentration HCl, is a key step forward toward the sustainable extraction of lithium from brines.