| Membrane extraction(ME)is a new separation technology combining the advantages of membrane process and liquid-liquid extraction.Because asqueous phase is not direct contact with organic phase,it dose not suffer from the problems such as entrainment pollution,back mixing and loss of liquid membrane phase.Thereby ME has shown great application potentials in metal ions purification,organic extraction and biodegradable membrane contactor due to its advantages of simplify and high efficiency without secondary pollution.The stability of membrane materials in organic solution and ion transport flux are of key important to the development of membrane extraction.In this thesis,based on TBP/kerosene extraction system,amphipathic block copolymer poly(ethylene-co-vinyl alcohol)(EVAL)and polyethersulfone(PES)ultrafiltration membrane coated by sulfonated poly ether ether ketone(SPEEK)were used as membrane materials to extract lithium from salt lake brine water.Ion permeability,selectivity and solvent-resistant properties of membrane were investigated to provide a new idea for the development of solvent-resistant membrane materials in membrane extraction technology and purification of substances difficult to separate.The content of ethylene in EVAL polymer has an important impact on crystallization temperature of polymer in membrane prepration process.Intercompetition exists between solid-liquid phase separation and liquid-liquid phase separation when EVAL membranes with different ethylene content were prepared via immersion precipitation.Crystallization temperature(Tc)of EVAL polymer was measured by DSC.EVAL membrane morphology was regulated by Tc and coagulation temperature.The as-prepared EVAL membranes were characterized by SEM,mechanical strength,water uptake,pure water flux and Li+diffusion flux.The results showed that the EVAL membrane with 44 mol%ethylene content showed Li+extraction flux of 2.93 10-8 mol/(cm2·s)and was chosed for membrane extraction experiment.Based on EVAL poltmer with 44 mol%ethylene,membrane preparation conditions such as polymer content,casting knife thickness and coagulation temperature were investigated to optimize membrane structure,ion transfer properties,mechanical strength and oil and water barrier properties.Ultimately,polymer concentration of 30 wt.%,casting knife thickness of 150?m and coagulation bath temperature of 1±0.5?were decided as optimal EVAL membrane preparation conditions.SEM,water uptake,Li+flux and pure water flux were investigated.EVAL membrane has a dense skin and relatively loose structure with interconnected nano-pore structure,exhibiting high Li+flux,low pure water flux and excellent mechanical property.The elongation of membrane reached to 418%.Lithium ion extraction flux was almost no change in long-term dynamic stability tests of 1037 h and FTIR result also showed no change in material structure,demonstrating EVAL membrane material is stable in TBP/kerosene extraction system.Charged property of membrane surface makes a negligible impact on ion diffusion flux in membrane.Composite hollow fiber membrane prepared via SPEEK coated on PES ultrafiltration membrane inner surface exhibited excellent negatively charged property due to a lot of negatively charged groups on SPEEK,improving the efficiency of lithium ion flux greatly.SPEEK composite layer could avoid leakage of organic to aqueous phases.Diffusion experiments showed that the lithium ion diffusion flux ascended with test temperature and concentration of lithium in feed solution increasing.In membrane extraction experiment,lithium ion extraction flux was 2.53 10-8 mol/(cm2·s)from salt lake with SPEEK/PES composite hollow fiber membrane.Excellent stability of lithium ion flux in long-term solvent-resistant membrane extraction tests approved the application of SPEEK/PES composite hollow fiber membrane in lithium extraction from brine water by TBP/kerosene system.The Li+extraction flux of membrane contactor extraction technology from high Mg2+/Li+is grearer than that in a supported liquid membrane separation process.These results demonstrated that amphipathic block copolymer poly(ethylene-co-vinyl alcohol)(EVAL)and SPEEK/PES composite hollow fiber membrane have important potentials in membrane extraction filed. |
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