Ionic liquid containing crown ether supported liquid membrane for lithium extraction and purification

A multimodal, supported liquid membrane consisting of a lithium-selective carrier 1,4,7,10-tetraoxacyclododecane (12-crown-4), an ionic liquid diluent, (1-butyl-3-methylimadazolium bis(trifluoromethanesulfonyl)imide ([C4mim][NTf2])) and a neutral co-extractant tri-n-butyl phosphate (TBP) was immobilized within the pores of a polyvinylidene fluoride (PVDF) membrane. The effects of feed concentrations (10–60 mM) lithium chloride, mixing in a diffusion cell (70–200 rpm stirring), strip phase composition, and selectivity for Li+ over Mg2+ and Na+, were investigated. A stirring speed of 100 rpm minimized concentration polarization effects in the diffusion cell. The presence of 0.01 M HCl in the strip solution lead to only a 1.2-fold increase in Li+ extraction. Using deionized water as the strip solution, the presence of 12-crown-4 in the liquid membrane lead to a significant increase in Li+ extraction and selectivity. For feeds containing the following ratios of ions Li+:Mg2+ 1:10 and 1:20, Li+:Na+ 1:350 and Li+:Mg2+:Na+ 1:10:250 Li+ extraction was 65%, 61%, 76% and 74% over a 24-h period. In all cases, in the presence of 12-crown-4, Li+ separation factors over Mg2+ and Na+ were more than 60%, even though the Li+ concentration in the feed was much lower. A multimodal SLM could provide high selectivity for Li+ from low-concentration brines.