| It is still focus on exploring a simple process, low energy consumtion and environmentally friendly technology of extracting lithium from brine with high concentration ratio of Mg/Li to develop brine resource. Among methods of extracting lithium from brine, the membrane separation is worthy of developing and promoting technology due to its relatively simple process, low energy consumption and environmental protection. Because the properties of magnesium and lithium are very similar, and coexist in salt lake brine, so to extract lithium from brine, the problem of the separation of magnesium and lithium must be solved. The membrane of selection to lithium should be solved by synthesizing the membrane which has a high ability to saprate lithium and magnesium. With the similar lumen diameter of dibenzo-14-crown-4 (DB14C4) and lithium, four oxygen atoms of DB14C4 ring into four ligands atomic meeting "hard affinity hard" principle and two benzene rings which can be modified, thereby DB14C4 is chosen as a small separated functional molecules.However, the experiment shows that the extraction rate of lithium is not high when the sepration ability of DB14C4 to lithium is evaluated by the experiment of extraction and dynamic migration. The reason is that DB14C4 cannot satisfy the the preferred lithium coordination number of 5 or 6. Therefore, it is essential to seek coordination system of DB14C4 and organ ophosphate which has a coordination number of 5.The system of a synergistic coordination constituting with DB14C4 and organophosphate by the extraction and dynamic migration experiments, it shows the extraction rate of lithium and the separation coefficient of magnesium lithium had been effectively improved. However, if the system is used to the complete separation of lithium and magnesium, it will appear that the operation cost is high, the solvent consumption is huge, the recovery of the extraction agent is easily emulsified, and the environmental treatment of the medicament is not easy. Membrane materials which have a high selectivity of lithium and magnesium are prepared by the macromolecular system and fixing in a hydrophilic framework of the polyurethane polymer. Strong hydrophilic membrane is prepared by polyurethane as polymer skeleton material, which contained a variety of hydrophilic groups and cooperated with DB14C4 by carbamate and ether-bond on molecular chain. Study on the separation membrane, this major results obtained as following.(1)With ultrasonic synthetic method, under the condition of normal temperature and short reaction time, DB14C4 was synthesized with the yield of 42.22%. The structure of DB14C4 was obtainedby comparing its infrared spectrum and the gaussian frequency. MS and 1H NMR spectrometry also proved its successful synthesis.(2)The effects of lithium magnesium separation was examined by the DB14C4 and its coordination with organic phosphate, the result of experiment showed that lithium ion extraction rate was only 2% using the single DB14C4, the selective coefficient of lithium ion extracting from mixture of lithium and magnesium was only 1.2. When the synergistic coordination system composed of DB14C4 and Bis-(2-ethylhexyl) phosphate (B2EHPA), the extraction rate of lithium ion increased to 36%, the selectivity coefficient increased to 6.17. Dynamic migration of liquid membrane extraction experiments had confirmed that the synergistic extraction system can increase the lithium ion selective, when DB14C4 and B2EHPA molar ratio of 2:1, the selective migration coefficient is the highest of all, it’s 9.7.(3)The DB14C4 and formaldehyde reacted under acid-catalyzed conditions to get the DB14C4 hydroxymethyl product, then mixed it with B2EHPA and polyurethane raw materials, polyurethane-type lithium ion membrane material was successfully prepared, and the membrane was modified by glycerol.(4)The polyurethane-type lithium ion membrane was evaluated by infrared spectroscopy (IR), thermal analysis (TG-DTA), SEM, water absorption and mechanical properties of the membrane on the overall. The optimum ratio among the various synthetic materials, DB14C4 hydroxymethylated product 0.76 g, B2EHPA 0.45 mL, TDI 2.0 mL, polyether polyols 4.0 mL, glycerol 0.80 mL (GL/PPG= 20%).(5)The experiment of pure water flux was investigated by ultrafiltrate cup. The pure water flux of the separation membrane of lithium ion retention was more than 125 mL·m-2·min-1 when the glycerol added amount was more than or equal to 20%. In the penetration experiment of salt solution, the average retention rate of lithium ion in separation membrane was 94.49%, while the rate of magnesium ion was 0. After the separation membrane of lithium ion was regenerated, in the selective experiment of the lithium/magnesium mixed solution, the retention rate of Li+was decreased by 1%, while all magnesium ions were almost able to pass through the multiple regenerations. The performance of the membrane was stable after multiple elutions, indicating that the lithium ion membrane could be recycled for many times, and it was a better membrane material. |
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