A hollow fiber supported ionic liquid membrane contactor for continuous extraction of lithium from high magnesium/lithium ratio brine

Abstract

High-efficiency lithium (Li⁺) extraction from a salt-lake brine with a low Li concentration and a high Mg/Li mass ratio poses a great challenge owing to the great physical and chemical similarities between Mg²⁺ and Li⁺. In this study, a hollow fiber (HF) membrane with an inside diameter of 0.872 mm and an outside diameter of 1.228 mm was fabricated using nonsolvent induce phase separation method with 14-crown-4 ether functionalized polyimide (14C4PI) as a polymer matrix. The organic phase, a solution of tributyl phosphate and sodium bis(trifluoromethylsulfonyl)imide (NaNTf₂) was filled into porous membranes as the solid phase using an impregnation method to construct a supported ionic liquid membranes (SILMs) contactor for lithium extraction from simulated salt-lake brine. The feed and stripping phases of the contactor were a mixed Mg/Li solution and 0.5 mol/L HCl, respectively. The contactor operated continuously for 120 h. The results showed an HF membrane with an average pore size of 20.1 nm, porosity of 73.6 % and a breaking strength of 5.64 MPa. When the Mg/Li mass ratio in the feed was approximately 3.5, the mass transfer rate of Li⁺ and the separation factor (SF_Li-Mg) obtained from the contactor with a packing density of 1.5% were 0.468 μm/s and 15.86, respectively. After further increasing the mass ratio of the Mg/Li solution to 35 in the feed and the membrane packing density to 15%, the mass transfer rate of Li⁺ and the SF_Li-Mg increased to 0.623 μm/s and 30.8, respectively. This implies that the SILMs contactor with a high packing density of HF membrane showed good operating stability and enhanced ion extraction efficiency. The high performance was ascribed to the crown ether groups in 14C4PI achieving specific recognition of Li⁺ through the size-sieving effect. Specifically, the inter-molecular interaction between crown ether and ionic liquids (ILs) improved the stability of the filled ILs. Meanwhile, ILs provided ionic transfer channels and promoted the dehydration process of Li⁺, leading to a high SF_Li-Mg. In addition, NaNTf₂ adopted to replace traditional ILs prevented cation loss and provided an efficient and continuous method for extracting lithium from salt-lake. The above advantages are expected to achieve large-scale extraction of lithium ions.