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

Abstract

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 Fe 3+ in the form of FeCl2L•(HL)•2TBP (HL denotes P507), leading to efficient stripping of lithium due to the broken of the coordination structure of Li•2TBP•FeCl4, and Fe 3+ 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 towards the sustainable extraction of lithium from brines.

Opposite selectivities of tri‐n‐butyl phosphate and Cyanex 923 in solvent extraction of lithium and magnesium

“…The TBP/FeCl 3 system has been intensively studied for the selective extraction of Li from high Mg‐Li ratio brines by other researchers, 14‐20 but the C923/FeCl 3 system has not been reported yet. The two systems are compared here for their extraction performance from a synthetic brine solution containing 0.10 mol·L −1 LiCl and 4.0 mol·L −1 MgCl 2 (Figure 2), which is a typical composition of concentrated brine 11,16,19 . The extraction of Li by the TBP/FeCl 3 system increased from 62% at 0.10 mol·L −1 FeCl 3 to 81% at 0.20 mol·L −1 FeCl 3 .…”

“…13 Because of the low cost of TBP and FeCl 3 and the good Li/Mg selectivity of this solvent extraction system, it has been studied extensively. [14][15][16][17][18][19][20] In the synergic solvent extraction of Li by the TBP/FeCl 3 Cyanex 923 (C923, a commercial mixture of trialkyl phosphine oxides with mainly n-hexyl and n-octyl, shown in Figure 1) is a popular commercial neutral extractant that is similar to TBP. It has been used in the extraction of a wide range of metals, including rare earth elements (REEs), transition metals, and precious metals.…”

“…The synergic solvent extraction system containing tri‐ n ‐butyl phosphate (TBP, shown in Figure 1) in the organic phase and FeCl 3 in the aqueous phase was found to be able to selectively extract Li from Mg‐containing brine solutions 13 . Because of the low cost of TBP and FeCl 3 and the good Li/Mg selectivity of this solvent extraction system, it has been studied extensively 14‐20 . In the synergic solvent extraction of Li by the TBP/FeCl 3 system, the role of FeCl 3 is to form an anionic [FeCl 4 ] − complex that can further form an ion pair with the cation [Li(TBP) x ] + ( x = 1, 2), as shown in Equation ().…”

Technology for the Recovery of Lithium from Geothermal Brines

“…El-Eswed et al [205] investigated the organophosphorus ligands phenylphosphonic, phenylphosphinic and bis(2-ethylhexyl) phosphoric acid and observed that by adding ammonia to the aqueous phase, lithium extraction increased as high as 90% with bis(2-ethylhexyl) phosphoric acid. Neutral extraction systems containing TBP, FeCl 3 , and kerosene have been extensively investigated for lithium extraction [63,179,183,206,207]. In this solvent extraction approach, kerosene is the diluent, TBP serves as a neutral organophosphorus extractant, and FeCl 3 is the co-extraction reagent [63,179,183].…”

Selective removal of magnesium from lithium‐rich brine for lithium purification by synergic solvent extraction using β‐diketones and Cyanex 923

“…It seems that the Mg/Li selectivity of single‐extractant SX systems (Such as D2EHPA and Versatic Acid 10) is limited. Synergic SX systems (using mixtures of two or more extractants) often have much‐improved separation performance compared with single‐extractant systems 21‐24 . β‐diketones and some neutral ligands have been found to have synergism for Li extraction since the 1960s.…”