Tailoring hydrophobic deep eutectic solvent for selective lithium recovery from the mother liquor of Li2CO3

Abstract

• HDES synthesized by oleic acid and tetrabutyl ammonium chloride are more environmentally benign co-extractant. • The green synergic system TBP-HDES showed high lithium selectivity extraction. • The extraction mechanism were mainly cation exchange and Li + coordination. Recovering Li + from the mother liquor obtained during the process of Li 2 CO 3 production has been considered as a promising solution to the shortage of lithium resources. However, as the mother liquor is a complex saline system with high concentration of Li + (1.42 g/L) and a large amount of Na + (55.62 g/L), there is no competent method to carry out for the effective utilization of Li + . This work reports a class of hydrophobic deep eutectic solvents (HDESs) formed by tetrabutylammonium chloride (TBAC) and oleic acid (OA), which is a novel co-extraction reagent for Li + extraction. In addition, tributyl phosphate (TBP) is used as extractant and ammonia is applied to regulate pH. TBAC/3OA showed the highest extraction efficiency in single extraction (76.8%) with the ammonia concentration, the volume ratio of organic phase and aqueous phase (O/A phase ratio), and volume fraction of HDES as 1.5 mol/L, 1, and 30%, respectively. Mechanism studies through the UV–vis and FT-IR proved that cation exchange between Li + and HDES, and Li + coordination was the dominant force in the separation process. The complexing ability of P = O in TBP led to the selective extraction of Li + . The reusability experiments of organic phase demonstrated that the extraction efficiency was still above 60% after five cycles, indicating the favorable stability of the extraction system. Finally, the separation factor ( β Li/Na ) of Li/Na in the mother liquor was 20.5, indicating that the TBP-HDES system has the capability to selectively extract Li + with high Na/Li ratio. This will help to design possible green HDES for lithium recovery.