Ionic liquid-assisted highly selective lithium extraction from magnesium-rich brines using phenyl phosphate

Effective lithium extraction from magnesium-rich brines is crucial to meet the increasing lithium demand. Traditional solvent extraction using tributyl phosphate (TBP) as the main extractant exhibits limited Li/Mg separation and dissolution losses during long-term operation. Herein, a novel ionic liquid-assisted lithium extraction system (2-Ethylhexyl diphenyl phosphate (DPOP)/1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide [Emim][NTf2]) using organophosphate containing rigid phenyl groups was proposed. The phenyl group increased the steric hindrance of the extractant, enhancing the system efficiency for separating Li+ from impurities. Results demonstrated that introducing the phenyl group into phosphate considerably improved the Li/Mg and Li/Ca separation factors, reaching 4,641 and 93, respectively, under optimal conditions, with a single-stage Li+ extraction efficiency of 67%. After washing and stripping, a LiCl solution (purity = 99.68%) was obtained. Electrospray ionization mass spectrometry confirmed that the molecular formula of the extracted Li+ complex was [Li & sdot;2DPOP][NTf2]. Subsequent density functional theory calculations on the complexes revealed shorter bond lengths for the DPOP/[Emim][NTf2] system with Li+ than those with Mg2+ and more negative binding energy and Gibbs free energy change during Li+ extraction. Therefore, this system preferentially extracts Li+ with tighter and more stable binding than Mg2+, exhibiting excellent Li selectivity.