High-flux and high-selectivity nanofiltration membrane for extracting lithium from salt lake and preparation method of nanofiltration membrane

Abstract

The invention provides a high-flux and high-selectivity salt lake lithium extraction nanofiltration membrane and a preparation method thereof, and belongs to the technical field of nanofiltration membrane preparation. The method comprises the following steps: (1) immersing a support layer into a water phase solution of carboxylated polyethyleneimine, carrying out first immersion treatment, and taking out and drying the support layer after the first immersion treatment is finished; (2) immersing the membrane into an oil phase solution A, carrying out second immersion treatment, taking out the membrane after the second immersion treatment is finished, and drying the membrane to obtain a PEIa-COOHb TFC nanofiltration membrane; and (3) immersing the PEIa-COOHb TFC nanofiltration membrane into an oil phase solution B, carrying out third immersion treatment, taking out the membrane after the third immersion treatment is finished, and drying the membrane to obtain the PEIa-COOHb-X TFC nanofiltration membrane. The prepared high-flux and high-selectivity nanofiltration membrane for extracting lithium from the salt lake has high flux and high selectivity, can be effectively applied to extracting lithium ions from the salt lake, and has a relatively good application prospect in the aspect of extracting lithium from the salt lake.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of nanofiltration membrane preparation, and particularly relates to a high-flux and high-selectivity nanofiltration membrane for extracting lithium from a salt lake and a preparation method thereof. BACKGROUND

[0002] In recent years, the vigorous rise of the global new energy automobile industry has presented an explosive growth trend in the demand for lithium resources as a core raw material. Survey results show that the global lithium resources are about 85 million tons, of which the lithium resources in salt lake brine account for about 62%, and the lithium resources in China are about 5 million tons, of which more than 80% of the lithium resources are stored in salt lake brine. Therefore, the development of salt lake lithium resources becomes particularly important.

[0003] The membrane separation method has the advantages of environmental protection, energy saving, simple operation, good removal effect on other impurities in the salt lake brine and the like, and thus is outstanding in the technology for extracting lithium from a salt lake. However, the nanofiltration membranes currently applied to the extraction of lithium from a salt lake on the market are usually negatively charged nanofiltration membranes, such as a polyamide nanofiltration membrane preparation method combining piperazine (PIP) pre-diffusion and trimesoyl chloride (TMC) controlled release proposed by a Liu Fu research group of the Chinese Academy of Sciences. When separating, the negative charge on the surface of the nanofiltration membrane can cause adsorption of part of the cations, thereby causing serious concentration polarization in a high-concentration salt lake, leading to a reduction in separation efficiency. The positively charged nanofiltration membrane is usually synthesized by taking PEI as the water-phase monomer, such as a positively charged nanofiltration membrane rich in and designed and prepared by an interface polymerization method by using polyethyleneimine (PEI) and trimesoyl chloride (TMC) on the surface of a polyethersulfone ultrafiltration membrane by a Xu Zhiwei research group of Tianjin University of Technology. A large number of aminos contained in the nanofiltration membrane are easy to form a high-crosslinking-density and high-density separation layer in the interface polymerization due to high activity. The dense structure is beneficial to separation performance, but also significantly increases the permeation resistance of water molecules, leading to a reduction in water flux. In addition, the positively charged nanofiltration membrane is easy to adsorb negative pollutants in the brine due to the positive charge on the surface, thereby leading to serious attenuation of the membrane performance.

[0004] Therefore, the development of a nanofiltration membrane with high selectivity and high flux has broad application prospects at the present stage. SUMMARY

[0005] Therefore, the application provides a preparation method of a high-flux and high-selectivity nanofiltration membrane for extracting lithium from a salt lake.

[0006] (1) The support layer is immersed in a water-phase solution of carboxylated polyethyleneimine to perform first immersion treatment, and the support layer after drying is obtained after being taken out and dried.

[0007] (2) The dried support layer is immersed in oil phase solution A for a second impregnation treatment. After the treatment, it is removed and dried to obtain PEI. a-COOHb TFC nanofiltration membrane;

[0008] (3) The PEI a-COOHb The TFC nanofiltration membrane was immersed in oil phase solution B for a third impregnation treatment. After the treatment, it was removed and dried to obtain PEI. a-COOHb -X TFC nanofiltration membrane.

[0009] Furthermore, the support layer in step (1) includes a porous support layer.

[0010] Furthermore, the porous support layer is a hydrophilic porous support layer.

[0011] Furthermore, the preparation method of the carboxylated polyethyleneimine in step (1) includes: introducing carboxyl groups into the branched PEI structure by covalent grafting.

[0012] Furthermore, the carboxyl donor includes succinic anhydride.

[0013] Furthermore, the average molecular weight of the branched PEI is 600, 1800, or 25000.

[0014] Furthermore, the degree of carboxylation of the carboxylated polyethyleneimine is 10%, 30%, or 50%.

[0015] Furthermore, in step (1), the mass concentration of carboxylated polyethyleneimine in the aqueous solution is 1% to 3%, and the mass concentration of surfactant in the aqueous solution is 0.05% to 0.1%; the surfactant includes at least one of ammonium dodecyl sulfate, sodium dodecylbenzene sulfonate, and sodium dodecyl sulfate.

[0016] Furthermore, in step (1), the drying process is carried out until there is no moisture residue on the surface of the support layer. The preferred drying method is natural air drying.

[0017] Furthermore, the oil phase solution A in step (2) comprises polyacrylamide chloride and organic solvent.

[0018] Furthermore, the polyacrylamide chloride monomer is at least one of pyromellitic tricarboxylic acid chloride, terephthaloyl chloride, phthaloyl chloride, and 1,2,4,5-benzenetetra(formyl chloride).

[0019] Furthermore, the mass concentration of the polyacrylamide chloride is 0.1% to 0.3%.

[0020] Furthermore, the oil phase solution B in step (3) comprises a polyisocyanate and an organic solvent.

[0021] Furthermore, the polyisocyanate monomer is at least one selected from toluene diisocyanate, hexamethylene diisocyanate, and diphenylmethane diisocyanate.

[0022] Furthermore, the mass concentration of the polyisocyanate is 0.1% to 0.3%.

[0023] Furthermore, the organic solvent includes at least one of n-hexane, cyclohexane, and n-heptane.

[0024] Furthermore, the immersion time for the first immersion treatment, the second immersion treatment, and the third immersion treatment is 30-120 seconds.

[0025] Furthermore, in steps (2) and (3), the drying process is carried out until there is no solvent residue on the surface of the support layer. The drying method is preferably baking, and the baking temperature is 50-80℃ and the baking time is 1-5min.

[0026] The present invention also provides a high-throughput and high-selectivity nanofiltration membrane for lithium extraction from salt lakes prepared according to the method.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The high-throughput and high-selectivity nanofiltration membrane for lithium extraction from salt lakes prepared by this invention combines high throughput and high selectivity, and can be effectively applied to the extraction of lithium ions from salt lakes, showing good application prospects in lithium extraction from salt lakes. Detailed Implementation

[0029] This invention provides a method for preparing a high-flux and high-selectivity lithium extraction nanofiltration membrane from salt lakes, comprising the following steps:

[0030] (1) The support layer is immersed in an aqueous solution of carboxylated polyethyleneimine for the first impregnation treatment. After the first impregnation treatment, it is taken out and dried to obtain the dried support layer.

[0031] (2) The dried support layer is immersed in oil phase solution A for a second impregnation treatment. After the treatment, it is removed and dried to obtain PEI. a-COOHb TFC nanofiltration membrane;

[0032] (3) The PEI a-COOHb The TFC nanofiltration membrane was immersed in oil phase solution B for a third impregnation treatment. After the treatment, it was removed and dried to obtain PEI. a-COOHb -X TFC nanofiltration membrane.

[0033] In some specific embodiments of the present invention, the support layer in step (1) includes a porous support layer.

[0034] In some specific embodiments of the present invention, the porous support layer is a hydrophilic porous support layer.

[0035] In some specific embodiments of the present invention, the preparation method of carboxylated polyethyleneimine in step (1) includes: introducing carboxyl groups into the branched PEI structure by covalent grafting.

[0036] In some specific embodiments of the present invention, the carboxyl donor includes succinic anhydride.

[0037] In some specific embodiments of the present invention, the preparation method of carboxylated polyethyleneimine in step (1) includes: adding a succinic anhydride solution of a certain mass concentration to a polyethyleneimine solution for reaction, and purifying by dialysis to obtain carboxylated polyethyleneimine (PEI). a-COOHb (where a is the average molecular weight of PEI, and b is the degree of carboxylation).

[0038] In some specific embodiments of the present invention, the reaction time of the method for preparing carboxylated polyethyleneimine is 10h-15h.

[0039] In some specific embodiments of the present invention, the succinic anhydride solution in the method for preparing carboxylated polyethyleneimine refers to a DMSO solution of succinic anhydride; the polyethyleneimine solution refers to a DMSO solution of branched PEI, and the method for preparing the polyethyleneimine solution includes: dissolving branched PEI in anhydrous DMSO to obtain a polyethyleneimine solution; the dialysis is performed using a dialysis bag, and the dialysis bag is any one of MWCO 500 Da, MWCO 1000 Da, and MWCO 7000 Da.

[0040] In some specific embodiments of the present invention, the average molecular weight of the branched PEI is 600, 1800 or 25000.

[0041] In some specific embodiments of the present invention, the degree of carboxylation of the carboxylated polyethyleneimine is 10%, 30%, or 50%.

[0042] In some specific embodiments of the present invention, the carboxylated polyethyleneimine includes PEI. 600-COOH10% PEI 600-COOH30% PEI 600-COOH50% PEI 1800-COOH10% PEI 1800-COOH30% PEI 1800 - COOH50% PEI 25000-COOH10% PEI 25000-COOH30% PEI 25000-COOH50% At least one of them.

[0043] In some specific embodiments of the present invention, the mass concentration of carboxylated polyethyleneimine in the aqueous solution in step (1) is 1% to 3%, and the mass concentration of surfactant in the aqueous solution is 0.05% to 0.1%; the surfactant includes at least one of ammonium dodecyl sulfate, sodium dodecylbenzene sulfonate, and sodium dodecyl sulfate.

[0044] In some specific embodiments of the present invention, the drying process in step (1) until there is no moisture residue on the surface of the support layer is preferably natural air drying.

[0045] In some specific embodiments of the present invention, the oil phase solution A in step (2) includes polyacrylamide chloride and organic solvent.

[0046] In some specific embodiments of the present invention, the polyacrylamide chloride monomer is at least one of pyromellitic tricarboxylic acid chloride, terephthaloyl chloride, phthaloyl chloride, and 1,2,4,5-benzenetetra(formyl chloride).

[0047] In some specific embodiments of the present invention, the mass concentration of the polyacrylamide chloride is 0.1% to 0.3%.

[0048] In some specific embodiments of the present invention, the oil phase solution B in step (3) includes polyisocyanate and organic solvent.

[0049] In some specific embodiments of the present invention, the polyisocyanate monomer is at least one of toluene diisocyanate, hexamethylene diisocyanate, and diphenylmethane diisocyanate.

[0050] In some specific embodiments of the present invention, the mass concentration of the polyisocyanate is 0.1% to 0.3%.

[0051] In some specific embodiments of the present invention, the organic solvent includes at least one of n-hexane, cyclohexane, and n-heptane.

[0052] In some specific embodiments of the present invention, the immersion time for the first immersion treatment, the second immersion treatment and the third immersion treatment is 30-120 seconds.

[0053] In some specific embodiments of the present invention, the drying process in steps (2) and (3) is to dry until there is no solvent residue on the surface of the support layer. The drying method is preferably baking, the drying temperature is 50-80°C, and the drying time is 1-5 min.

[0054] The present invention also provides a high-throughput and high-selectivity nanofiltration membrane for lithium extraction from salt lakes prepared according to the method.

[0055] Mechanism of action of the present invention:

[0056] This invention first employs a nucleophilic substitution reaction between branched polyethyleneimine (PEI) and succinic anhydride. The primary amino groups abundant in the PEI molecular chain act as nucleophiles, attacking the partially positively charged carbonyl carbons in the succinic anhydride molecule to form an unstable tetrahedral intermediate. Subsequently, the tetrahedral structure rapidly collapses, leading to the breakage of the anhydride bond. One carbonyl group is restored to a carboxyl group, while the other forms an amide bond with the amino group, thus realizing the carboxylation process of PEI.

[0057] This invention reduces the positive charge of the PEI molecule and the surface potential of the film by introducing carboxyl groups onto the PEI monomer, thereby improving the prepared PEI... a-COOHb TFC tends to be electrically neutral, reducing the deposition of pollutants in positively charged membranes in brine and the concentration polarization problem on the surface of negatively charged membranes. In addition, the introduction of a large number of carboxyl groups can also improve the hydrophilicity of the membrane, thereby increasing the water flux. At the same time, the amino and carboxyl groups remaining after the polymerization of PEI molecules can form intramolecular hydrogen bonds, reducing the internal pore size of the membrane and improving the magnesium ion rejection rate.

[0058] Secondly, this invention uses diisocyanates for secondary interfacial polymerization. The flexible structure of diisocyanates penetrates the internal pores of the membrane, where they can undergo nucleophilic addition reactions with residual amino groups to form urea bonds. At the same time, some groups will form amide bonds with residual carboxyl groups, forming covalent connections between molecules and creating a stable three-dimensional network structure. This reduces the average internal pore size of the membrane, further improving the selectivity of magnesium-lithium separation, while also improving the rigidity and pressure resistance of the nanofiltration membrane.

[0059] The high-flux and high-selectivity nanofiltration membrane for lithium extraction from salt lakes provided by this invention can solve the problem of balancing selectivity and flux in existing nanofiltration membranes for lithium extraction from salt lakes.

[0060] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0061] Unless otherwise specified, the test methods or experimental methods described in the following examples are all conventional methods; unless otherwise specified, the raw materials and additives are obtained from conventional commercial sources or prepared by conventional methods.

[0062] The pretreatment steps for the dialysis bags in the following examples or comparative examples are as follows: Rinse the inner and outer surfaces of the dialysis bag with deionized water, then completely immerse the dialysis bag in a boiling mixture of 2% sodium bicarbonate + 1 mmol / L EDTA for 15 minutes. After removing the bag, rinse it repeatedly with hot deionized water 3 times, and then soak it in room temperature ultrapure water for 30 minutes, changing the water twice during this period, to obtain the pretreated dialysis bag.

[0063] Example 1

[0064] A method for preparing a high-flux and high-selectivity lithium extraction nanofiltration membrane from salt lakes, comprising the following specific steps:

[0065] (1) Immerse the support layer in an aqueous solution of carboxylated polyethyleneimine for 60 seconds. After immersion, remove the support layer and air dry the water droplets on its surface to obtain the dried support layer.

[0066] (2) The dried support layer is immersed in oil phase solution A for 60s, and then dried at 70℃ for 5min to obtain PEI. a-COOHb TFC nanofiltration membrane;

[0067] (3) The PEI a-COOHb The TFC nanofiltration membrane was immersed in oil phase solution B for 60 seconds, and then dried at 70°C for 5 minutes to obtain PEI. a-COOHb -X TFC nanofiltration membrane.

[0068] The support layer in step (1) is a hydrophilic porous support layer. The preparation method of the hydrophilic porous support layer is as follows: weigh 200g of polysulfone and 20g of PEG-600 and disperse them in 780g of... In the process, the mixture is stirred at 70°C until completely dissolved, then degassed under vacuum to obtain a polymer solution. The polymer solution is then coated onto a nonwoven fabric using a scraper and solidified in a coagulation bath to form a polysulfone support layer, i.e., a hydrophilic porous support layer. The solution is then refrigerated in a freezer at 5°C for later use.

[0069] The method for preparing the aqueous solution of carboxylated polyethyleneimine in step (1) is as follows:

[0070] S1. Weigh 1g of PEI 600 Dissolve PEI in 50 mL of anhydrous DMSO and stir at room temperature until completely dissolved; weigh 88.4 mg of succinic anhydride and dissolve it in 10 mL of anhydrous DMSO; using a constant pressure dropping funnel, add the succinic anhydride DMSO solution dropwise to the PEI solution, and continue stirring for 12 hours after the addition is complete. After the reaction is complete, dialysis is performed using a pretreated MWCO 500Da dialysis bag to obtain PEI with 10% carboxylation. 600-COOH10% Store in a sealed container until needed.

[0071] S2. Carboxylated polyethyleneimine, surfactant sodium dodecyl sulfate, and water are mixed evenly to obtain an aqueous solution of carboxylated polyethyleneimine; the mass concentration of carboxylated polyethyleneimine in the aqueous solution is 2%; the mass concentration of surfactant in the aqueous solution is 0.05%.

[0072] The preparation method of the oil phase solution A in step (2) is as follows: dissolve pyromellitic chloride in n-hexane to obtain oil phase solution A; the mass concentration of pyromellitic chloride in oil phase solution A is 0.1%.

[0073] The preparation method of the oil phase solution B in step (3) is as follows: hexamethylene diisocyanate is dissolved in n-hexane to obtain oil phase solution B; the mass concentration of hexamethylene diisocyanate in the oil phase solution B is 0.1%.

[0074] Example 2

[0075] The difference from Example 1 is that the mass of succinic anhydride is 265 mg, while the other steps are the same as in Example 1.

[0076] Example 3

[0077] The difference from Example 1 is that the mass of succinic anhydride is 442 mg, while the other steps are the same as in Example 1.

[0078] Example 4

[0079] The difference from Example 1 is that the average molecular weight of PEI is 1800, the mass of succinic anhydride is 65 mg, and a MWCO 1000Da dialysis bag is used. The other steps are the same as in Example 1.

[0080] Example 5

[0081] The difference from Example 4 is that the mass of succinic anhydride is 195 mg, while the other steps are the same as in Example 1.

[0082] Example 6

[0083] The difference from Example 4 is that the mass of succinic anhydride is 325 mg, while the other steps are the same as in Example 1.

[0084] Example 7

[0085] The difference from Example 1 is that the average molecular weight of PEI is 25,000, the mass of succinic anhydride is 60 mg, and an MWCO 7000Da dialysis bag is used. The other steps are the same as in Example 1.

[0086] Example 8

[0087] The difference from Example 7 is that the mass of succinic anhydride is 180 mg, while the other steps are the same as in Example 1.

[0088] Example 9

[0089] The difference from Example 7 is that the mass of succinic anhydride is 300 mg, while the other steps are the same as in Example 1.

[0090] Example 10

[0091] The difference from Example 1 is that the mass concentration of carboxylated polyethyleneimine in the aqueous solution is 1%, while the other steps are the same as in Example 1.

[0092] Example 11

[0093] The difference from Example 1 is that the mass concentration of carboxylated polyethyleneimine in the aqueous solution is 3%, while the other steps are the same as in Example 1.

[0094] Comparative Example 1

[0095] The difference from Example 4 is that the aqueous monomer is PEI. 1800 No PEI 1800 Carboxylation modification was performed, and the other steps were the same as in Example 4.

[0096] Comparative Example 2

[0097] The difference from Example 1 is that no secondary interface aggregation is performed, i.e. step (3) is omitted, while the other steps are the same as in Example 1.

[0098] Comparative Example 3

[0099] The difference from Example 5 is that the aqueous monomer is piperazine and it has not undergone carboxylation modification; the other steps are the same as in Example 5.

[0100] Comparative Example 4

[0101] The difference from Implementation Case 1 is that 234 mg of dodecenylsuccinic anhydride was used to treat PEI. 600 Carboxylation was performed, and the other steps were the same as in Example 1.

[0102] Comparative Example 5

[0103] The difference from Implementation Case 1 is that 130.3 mg of phthalic anhydride was used to treat PEI. 600 Carboxylation was performed, and the other steps were the same as in Example 1.

[0104] The high-flux and high-selectivity lithium extraction nanofiltration membranes prepared in the examples and comparative examples were subjected to performance tests, and the test results are shown in Table 1. The test method is as follows: the selected membranes were tested on a cross-flow membrane testing platform. First, the magnesium chloride solution was tested, i.e., the test was conducted under test condition ①; then the sodium chloride solution was tested, i.e., the test was conducted under test condition ②. Test condition ① was: 2000 ppm MgCl2 aqueous solution, operating pressure 100 psi, test temperature 25℃, and pH 7.0; test condition ② was: 2000 ppm LiCl aqueous solution, operating pressure 100 psi, test temperature 25℃, and pH 7. Three membranes were taken from each sample for testing, and the results of the magnesium chloride and sodium chloride tests were averaged.

[0105] Table 1

[0106]

[0107] As shown in Table 1, compared to Examples 4-6, Comparative Example 1, using uncarboxylated modified PEI polymer aqueous amine monomers, exhibits a high-density, highly cross-linked network structure during interfacial polymerization due to the high reactivity of its polyamine groups. This results in a high rejection rate for magnesium chloride and a relatively high rejection rate for sodium chloride, with a selectivity separation factor of only 95.1. Furthermore, the highly dense membrane structure formed in Comparative Example 1 increases its water mass transfer resistance, leading to a lower water flux. This demonstrates that using carboxylated polyethyleneimine as the aqueous monomer significantly improves the high flux and high selectivity performance of nanofiltration membranes for lithium extraction from salt lakes.

[0108] Compared to Example 1, the nanofiltration membrane prepared in Comparative Example 2 without secondary interfacial polymerization exhibited poor selectivity. This demonstrates that the present invention, in PEI... a-COOHb By performing secondary interfacial polymerization on the TFC nanofiltration membrane, the selectivity of the nanofiltration membrane for magnesium and lithium ions can be significantly improved while maintaining a nearly similar sodium chloride rejection rate.

[0109] Compared to Example 1, Comparative Example 4 uses an anhydride with a long alkyl chain for carboxylation treatment. While introducing carboxyl groups, it also imparts flexible hydrophobic segments to the PEI monomer, resulting in a lower diffusion coefficient during interfacial polymerization. This generates a PA layer structure with low crosslinking degree and poor hydrophilicity, thus exhibiting relatively low separation selectivity and water permeability.

[0110] Compared to Example 1, Comparative Example 5 uses carboxylates with hydrophobic structures to perform carboxylation treatment. While introducing carboxyl groups, it also imparts a rigid benzene ring structure to the PEI monomer, resulting in a PA layer with a relatively large internal pore size. At the same time, the hydrophobic benzene ring structure leads to poor hydrophilicity, resulting in relatively low separation selectivity and water permeability.

[0111] Compared to Example 5, the nanofiltration membrane prepared by Comparative Example 3, with piperazine as the aqueous monomer and without carboxylation modification, showed better control of magnesium ions. and lithium ions The separation selectivity is low, and the membrane performance is poor. This may be because the aqueous monomer piperazine lacks an amino structure, resulting in a significantly negatively charged surface on the prepared nanofiltration membrane, which has a low rejection rate for magnesium chloride, leading to low separation selectivity.

[0112] The experimental data from Examples 1 and 10-11 show that the desalination rate of the prepared membrane is positively correlated with the concentration of PEI polymer, but the water flux of the membrane is negatively correlated. Therefore, selecting an appropriate polymer concentration is helpful in constructing a high-flux lithium extraction nanofiltration membrane from salt lakes.

[0113] Experimental data from Examples 1-9 show that as the molecular weight of PEI molecules increases, the number of terminal amino groups gradually decreases. Short-chain molecules have more terminal amino structures and their reaction rate is significantly higher than that of the other two types of long-chain PEI molecules. They exhibit the highest reactivity during interfacial polymerization, forming a relatively dense polyamide layer. While achieving high selectivity, they have a lower water flux. Medium- and long-chain PEI molecules, due to their moderate carbon chain length and number of amino groups, can form appropriate membrane pore sizes and near-neutral surface charges during the interfacial process. Appropriate membrane pore sizes can ensure the penetration of polyisocyanates during secondary interfacial polymerization, further reducing the internal pore size and improving the rejection rate. Long-chain PEI molecules, due to their long and disordered alkyl chains and fewer terminal amino structures, produce polyamide layers with larger membrane pore sizes and lower surface charges, thus exhibiting relatively low selectivity in separation performance. Increasing the carboxylation degree of PEI polymers can effectively improve the hydrophilicity of the membrane to increase flux. However, excessive carboxylation will lead to a reduction in the number of amino groups in PEI molecules, resulting in a loose polyamide layer and reducing the separation selectivity of the nanofiltration membrane. As can be seen from the results of Examples 1 to 9 above, the nanofiltration membrane prepared by the method of the present invention has both excellent water flux and retention rate, and can effectively separate magnesium and lithium in salt lakes.

[0114] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a high-flux and high-selectivity nanofiltration membrane for lithium extraction from salt lakes, characterized in that, The method comprises the following steps: (1) immersing the support layer into an aqueous solution of carboxylated polyethyleneimine for a first immersion treatment, and then taking out and drying to obtain a dried support layer; (2) The dried support layer is immersed in the oil phase solution A for the second time, and after the end of the second time, it is taken out and dried to obtain PEI a-COOHb TFC nanofiltration membrane (3) The PEI a-COOHb The TFC nanofiltration membrane is immersed in the oil phase solution B for a third time, and after the end of the third time, it is taken out and dried to obtain PEI a-COOHb The TFC nanofiltration membrane.

2. The method for preparing high flux and high selectivity nanofiltration membrane for lithium extraction from salt lake according to claim 1, characterized in that, The support layer in step (1) comprises a porous support layer; preferably, the porous support layer is a hydrophilic porous support layer.

3. The method for preparing high flux and high selectivity nanofiltration membrane for lithium extraction from salt lake according to claim 1, characterized in that, The preparation method of the carboxylated polyethyleneimine in step (1) comprises introducing carboxyl groups into a branched PEI structure by covalent grafting; preferably, the carboxyl group donor comprises succinic anhydride.

4. The method of claim 3, wherein the method is characterized by, The average molecular weight of the branched PEI is 600, 1800 or 25000; preferably, the carboxylation degree of the carboxylated polyethyleneimine is 10%, 30% or 50%.

5. The method of claim 1, wherein the method is characterized by: The mass concentration of the carboxylated polyethyleneimine in the aqueous solution in step (1) is 1%-3%, and the mass concentration of the surfactant in the aqueous solution is 0.05%-0.1%.

6. The method of claim 1, wherein the method is characterized by: The oil phase solution A in step (2) comprises a polyacyl chloride and an organic solvent; preferably, the polyacyl chloride monomer is at least one of trimesoyl chloride, terephthaloyl chloride, phthaloyl chloride and 1,2,4,5-benzene tetra (formyl chloride).

7. The method of claim 6, wherein the method is characterized by, The mass concentration of the polyacyl chloride is 0.1%-0.3%.

8. The method of claim 1, wherein the method is characterized by: The oil phase solution B in step (3) comprises a polyisocyanate and an organic solvent; preferably, the polyisocyanate monomer is at least one of toluene diisocyanate, hexamethylene diisocyanate and diphenylmethane diisocyanate.

9. The method of claim 8, wherein the method is characterized by, The mass concentration of the polyisocyanate is 0.1%-0.3%.

10. A high-flux and high-selectivity lithium extraction nanofiltration membrane prepared by the method according to any one of claims 1-10.

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