Enabling sustainable green close-loop membrane lithium extraction by acid and solvent resistant poly (ether ether ketone) membrane

Membrane extraction (ME) for separating lithium ions from salt-lake brine is a green process comparing to conventional liquid-liquid extraction. Previous research has identified suitable organic extractant and membrane materials for extracting lithium from salt-lake brine. However, stripping solution, a concentrated hydrochloride acid (HCl), dissolves current solvent resistant membrane. We presents here a novel application of acid and solvent resistant poly (ether ether ketone) (PEEK) membrane for membrane stripping. PEEK membranes were prepared via immersion precipitation using a mixture solvent consisting of methanesulphonic acid (MSA) and sulfuric acid (SA). A linear cloud point curve of PEEK/MSA/SA/H2O quaternary system was determined and the phase separation kinetics was analyzed. Competition of solid-liquid and liquid-liquid demixing was proposed as the mechanism for the morphology change varying with PEEK concentration. Increase of PEEK concentration lead to high crystallinity in membrane, but comparably high mass transfer resistance; this lead to similar trend in the water permeability, lithium ion diffusion and stripping fluxes as well. Successful stripping of lithium ions from lithium-loaded extractant using 7 mol/L HCl solution was demonstrated. The resistance of PEEK membrane in HCl solution and organic extractant (tributyl phosphate (TBP)/kerosene) was demonstrated by monitoring the weight change and mechanical strength of PEEK membrane. Long-term stable stripping performance for 504 h further confirmed the stability of PEEK membrane. An extraction-stripping close-loop is thus available by combining with the solvent resistant nanoporous poly (ethylene vinyl alcohol, EVAL) in extraction and PEEK membrane in the stripping steps. Besides lithium extraction, the membrane based extraction/stripping process is suitable for a wider ion separation and purification.