Investigation on the Lithium Extraction Process with the TBP–FeCl3 Solvent System Using Experimental and DFT Methods

Lithium has been a high-demand element with the rapid development of lithium-ion batteries. The recovery of lithium from salt lake brine via solvent extraction is considered a promising technique. In this study, the solvent extraction process in the tributyl phosphate (TBP)–FeCl3/brine system was investigated. The order of metal ions competitively combining with TBP–FeCl3 was Li+ > Na+ > K+ > Mg2+, and the compositions of main cationic extraction complexes were Li(TBP)2(H2O)2+, Na(TBP)2(H2O)4+, K(TBP)2(H2O)4+, and Mg(TBP)2(H2O)42+, respectively. The energy decomposition analysis showed that the electrostatics was the major interaction between the metal ions and the solvent (TBP and water). Compared with water, TBP bound more competitively with the metal ions. The calculated species distribution results showed that the ferric ion reacted with the chloride ion to form FeCl2+, FeCl3, and FeCl4– in high Cl– concentration brine. Co-extractant FeCl3 was found to exist in the organic phase in the form of FeCl4– upon comparing the experimental and simulated spectra. Furthermore, the length of the Fe–Cl bond increased and the charge on the iron element was distracted when the FeCln3–n complexes coordinated with the water molecules. This indicated that FeCl4– was more stable in the TBP solvent environment and it can be extracted from the aqueous phase. The results help us understand the extraction mechanism in essence and develop a high-efficiency extraction solvent for industrial lithium recovery.