Extracting Lithium from the High Concentration Ratio of Magnesium and Lithium Brine Using Imidazolium-Based Ionic Liquids with Varying Alkyl Chain Lengths

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The present study investigates the extraction of lithium ion from the high concentration ratio of magnesium and lithium salt lake brine in a series of 1-alkyl-3-methylimidazolium-based ionic liquids (ILs), in which the alkyl chain lengths were n-butyl (C₄), n-pentyl (C₅), n-hexyl (C₆), n-heptyl (C₇), n-octyl (C₈) or n-nonyl (C₉), in the presence of triisobutyl phosphate (TIBP)+kerosene systems. The results showed that with shorter alkyl chain length of imidazolium-based ILs there was higher extraction efficiency of lithium ion. The optimal novel FeCl₃-free extraction system of [C₄mim][PF₆]+TIBP+kerosene was established, and the single-pass extraction and back extraction efficiencies for lithium ion were 74.14% and 86.37% under the optimal conditions of [C₄mim][PF₆] : TIBP : kerosene=1 : 8 : 1 (v/v), pH=5.0, R(O/A)=2.0 in the extraction step, as well as 1 mol/L HCl eluent, R(O/A)=3 in the back extraction step. A comparison of extraction behaviors in a multilevel cross-flow and multistage counter-current extraction process for lithium ion was also carried out. On the basis of infrared spectroscopy and slope analysis methods, a cation-exchange extraction mechanism in the system of [C₄mim][PF₆]+TIBP+kerosene for lithium ion extraction was demonstrated.