A positively charged nanofiltration membrane for lithium extraction from salt lakes and its preparation method

Abstract

The present invention relates to the field of nanofiltration membrane technology, specifically a positively charged nanofiltration membrane for lithium extraction from salt lakes and a preparation method thereof. The method comprises the following steps: Step 1: applying a sulfonated polyetherethersulfone casting solution to the surface of a non-woven fabric and drying to obtain a rigid base membrane; Step 2: depositing a modified composite nanofluid on the surface of the rigid base membrane and drying to obtain a nano-deposition intermediate layer; Step 3: applying a polyethyleneimine aqueous solution to the surface of the nano-deposition intermediate layer and quaternizing it to form a permeation layer; thereby obtaining a positively charged nanofiltration membrane for lithium extraction from salt lakes.

Description

Technical Field

[0001] The present invention relates to the technical field of nanofiltration membranes, in particular to a positively charged nanofiltration membrane for extracting lithium from salt lakes and a preparation method thereof. Background Art

[0002] With the development of the new energy industry and the growing demand for lithium batteries, lithium extraction from salt lakes is gaining increasing attention. Nanofiltration technology, with its advantages of low energy consumption, environmental friendliness, and ease of use, has become a key technology for lithium extraction from salt lakes.

[0003] Among them, positively charged nanofiltration membranes can achieve magnesium-lithium separation due to their high charge density; however, at present, positively charged nanofiltration membranes generally face the key problem of balancing lithium ion selectivity and permeability. On the one hand, some positively charged separation layers are formed through the interfacial polymerization of polyethyleneimine and trimesoyl chloride. Although this can increase water flux, the pore size distribution of such separation layers is uneven and they are easily affected by the charge shielding effect of the high magnesium-lithium ratio system in salt lakes, resulting in poor lithium ion selectivity and unable to meet the needs of lithium extraction from salt lakes. On the other hand, increasing the thickness of the functional layer to improve selectivity will significantly increase the ion transmission resistance, and the water flux is generally reduced, making it impossible to achieve both.

[0004] Currently, porous fillers that provide conductive pathways for lithium ions are often introduced to form nano-deposits. However, in actual production, the following problems often arise: First, the interfacial bonding between the nano-deposits and the base film is poor, leading to interlayer peeling in practical applications. Second, insufficient compatibility and weak interfacial bonding between the composite fillers can lead to aggregation, significantly reducing permeability and lithium ion selectivity.

[0005] In summary, it is of great significance to solve the above problems and prepare a positively charged nanofiltration membrane for lithium extraction from salt lakes. Summary of the Invention

[0006] The object of the present invention is to provide a positively charged nanofiltration membrane for lithium extraction from salt lakes and a preparation method thereof, so as to solve the problems raised in the above background technology.

[0007] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0008] A method for preparing a positively charged nanofiltration membrane for extracting lithium from salt lakes comprises the following steps:

[0009] Step 1: applying the sulfonated polyetherethersulfone casting solution to the surface of the non-woven fabric and drying it to obtain a rigid base membrane;

[0010] Step 2: depositing the modified composite nanofluid on the surface of the rigid base film and drying it to obtain a nano-deposition intermediate layer;

[0011] Step 3: coating the polyethyleneimine aqueous solution on the surface of the nano-deposition intermediate layer, quaternizing it to form a permeation layer; and obtaining a positively charged nanofiltration membrane.

[0012] More optimally, the thickness of the rigid base film is 2.4-4.6 μm; the thickness of the nano-deposition intermediate layer is 10-15 nm; and the thickness of the permeation layer is 2-6 nm.

[0013] More optimally, the preparation process of the modified composite nanofluid is as follows: S1-1: under a nitrogen atmosphere, sodium polystyrene sulfonate, 3-bromopropylene, and potassium carbonate are added to N,N-dimethylformamide, refluxed and stirred at 50-70° C. for 10-14 hours, filtered, purified, and dried to obtain vinyl styrene sulfonic acid;

[0014] S1-2: Under a nitrogen atmosphere, 6-mercapto-1-hexanol, vinylstyrene sulfonic acid, and azobisisobutyronitrile were added to anhydrous ethanol, reacted at 50-60°C for 3-4 hours, and purified and dried; then, 4'-carboxybenzo-15-crown-5-ether, tannic acid, 4-dimethylaminopyridine, N,N'-dicyclohexylcarbodiimide, and N,N-dimethylformamide were added, reacted at 30-50°C for 20-28 hours, washed, purified, and dried to obtain modified tannic acid;

[0015] S1-3: Activate the MOF in a hydrochloric acid solution at 85-95°C for 10-15 hours, then add modified tannic acid and stir at 20-40°C for 10-16 hours to obtain a modified MOF solution;

[0016] S1-4: Ultrasonic dispersion of MXene, modified MOF solution, and crosslinker for 1 to 2 hours to obtain a modified composite nanoliquid.

[0017] More optimally, in the raw materials of the modified composite nanofluid, the mass ratio of MXene, modified MOF solution, and cross-linking agent is 1-1.3:12-16:0.1-0.2.

[0018] More optimally, the size of the MXene is 2-6 μm; the average particle size of the MOF is 1-4 μm.

[0019] More optimally, the raw materials of the modified MOF solution include the following components: 3 to 4 parts by mass of MOF, 20 to 40 parts by mass of hydrochloric acid solution, and 2 to 3 parts by mass of modified tannic acid;

[0020] The raw materials of the modified tannic acid include the following components: 0.5-1.2 parts of sodium polystyrene sulfonate, 1-2 parts of 3-bromopropylene, 0.01-0.03 parts of potassium carbonate, 1-2.5 parts of 6-mercapto-1-hexanol, 0.04-0.07 parts of azobisisobutyronitrile, 4-9 parts of 4'-carboxybenzo-15-crown 5-ether, 2-4 parts of tannic acid, 0.1-0.3 parts of 4-dimethylaminopyridine, and 0.3-0.5 parts of N,N'-dicyclohexylcarbodiimide;

[0021] The concentration of the hydrochloric acid solution is 0.8-1.2 mol / L.

[0022] More optimally, the cross-linking agent includes one or both of glutaraldehyde and glyoxal.

[0023] More optimally, the quaternization process is as follows: coating the surface of the nano-deposition intermediate layer with a polyethyleneimine aqueous solution, drying it, immersing it in an acetic anhydride-pyridine solution at 20-30°C for 10-20 minutes, coating it with a trimesoyl chloride-n-hexane solution, washing and drying it, acidifying it at 20-40°C for 10-25 minutes, washing and drying it, and then immersing it in iodomethane-isopropanol at 20-30°C for 25-40 minutes, washing and drying it to form a permeation layer; and obtaining a positively charged nanofiltration membrane.

[0024] More optimally, in the raw materials of the polyethyleneimine aqueous solution, the mass ratio of polyethyleneimine to deionized water is 1:95~98; in the raw materials of the acetic anhydride-pyridine solution, the mass ratio of acetic anhydride, pyridine, and tetrahydrofuran is 5~10:1~3:85~95; in the raw materials of the trimesoyl chloride-n-hexane solution, the mass ratio of trimesoyl chloride to n-hexane is 0.1~0.5:99~99.5; in the raw materials of the iodomethane-isopropyl alcohol, the mass ratio of iodomethane to isopropyl alcohol is 1~3:17~20.

[0025] In a further embodiment, the MOF is UiO-66.

[0026] In a further embodiment, the preparation process of the sulfonated polyether ether sulfone casting solution is as follows: S2-1: acidifying the lithium ion sieve precursor, washing and drying; adding it to 3-methacryloxypropyltrimethoxysilane and 70-80wt% ethanol aqueous solution under a nitrogen atmosphere, ultrasonically dispersing it, reacting it at 75-85°C for 5-7h, washing and drying it to obtain a vinyl lithium ion sieve;

[0027] S2-2: Add vinyl lithium ion sieve, 2-acrylamido-2-methylpropanesulfonic acid, sodium p-styrenesulfonate, methyl methacrylate, and azobisisobutyronitrile to deionized water, react at 60-80°C for 8-12 hours, add hydrochloric acid for 1-2 hours to adjust the pH to 2.5-3.5, wash, and dry to obtain a sulfonic acid polymer;

[0028] S2-3: Add sulfonated polyether ether sulfone and sulfonic acid polymer to N,N-dimethylformamide, and stir at 50-60° C. for 16-20 hours to obtain a sulfonated polyether ether sulfone casting solution.

[0029] In a further embodiment, in the raw materials of the sulfonated polyether ether sulfone casting solution, the mass ratio of sulfonated polyether ether sulfone to sulfonic acid polymer is 6-7:1-1.3;

[0030] The raw materials of the sulfonic acid polymer include the following components: 3 to 5 parts by mass of lithium ion sieve precursor, 2 to 3 parts of 3-methacryloxypropyltrimethoxysilane, 3 to 5 parts of 2-acrylamido-2-methylpropanesulfonic acid, 3 to 5 parts of sodium p-styrenesulfonate, 1.5 to 2.5 parts of methyl methacrylate, and 0.5 to 0.8 parts of azobisisobutyronitrile.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] This solution achieves both improved lithium ion selectivity and permeability through multi-layer structure synergy, deposited filler composite, and quaternary ammonium penetration.

[0033] The rigid base membrane is made of sulfonated polyetherethersulfone and non-woven fabric, which improves the rigidity of the base membrane while providing a lithium ion sieve, providing a rigid substrate for the positively charged nanofiltration membrane; the nano-deposition intermediate layer is obtained by modified composite nano-liquid deposition, in which tannic acid is grafted with crown ether and sulfonic acid groups to form a modified MOF, and the chelation of tannic acid is used to further functionalize the MOF, and the crown ether is used to improve the specific complexation of lithium ions and improve the lithium ion selectivity; the modified MOF and MXene form an intercalation structure, optimize the size, and further improve the lithium ion selectivity; the permeation layer is formed by quaternization of polyethyleneimine to form a multifunctional layer. On the one hand, after quaternization, a magnesium ion charge barrier is formed through the Donnan effect, and a channel that promotes the selective transmission of lithium ions is constructed through coordination; on the other hand, after quaternization, the water flux and anti-fouling performance of the membrane can also be improved.

[0034] However, the interfacial bonding between the nano-deposition interlayer and the remaining layers remains weak, and the compatibility of the composite fillers in the nano-deposition interlayer is poor, leading to easy agglomeration. Therefore, to address these issues, a crosslinker, polystyrene sulfonic acid groups, and tannic acid were introduced into the nano-deposition interlayer. On the one hand, the polystyrene sulfonic acid groups provide a synergistic size screening effect with the MOF, improving lithium ion selectivity while also acting as a bridge to enhance the interfacial bonding with the remaining layers and prevent flaking. On the other hand, the covalent crosslinked network constructed by tannic acid and the crosslinker improves the compatibility and interfacial bonding between the fillers, thereby inhibiting the agglomeration of the composite fillers, maintaining unobstructed channels, and further improving permeability and lithium ion selectivity. DETAILED DESCRIPTION

[0035] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0036] It should be noted that the following parts are by mass, and all raw materials involved in the present invention are purchased from manufacturers without any special restrictions. Examples include: In the following examples, the product number of sulfonated polyether ether sulfone is PES, purchased from Dongguan Jinshixiang Plastic Raw Materials Co., Ltd.; the lithium ion sieve precursor is purchased from East China University of Science and Technology; the relative molecular mass of sodium polystyrene sulfonate is 50,000; the CAS number of 3-bromopropylene is 106-95-6; the CAS number of 6-mercapto-1-hexanol is 1633-78-9; the CAS number of 4'-carboxybenzo-15-crown 5-ether is 56683-55-7; the CAS number of tannic acid is 1401-55-4; The CAS number of -methacryloyloxypropyltrimethoxysilane is 2530-85-0; the CAS number of 2-acrylamido-2-methylpropanesulfonic acid is 15214-89-8; the CAS number of sodium p-styrenesulfonate is 2695-37-6; the CAS number of methyl methacrylate is 80-62-6; the relative molecular mass of polyethyleneimine is 70,000; the CAS number of trimesoyl chloride is 4422-95-1; the size of MXene is 5 μm, the product number is ZKK01, and it was purchased from Beijing Dekedaojin Technology Co., Ltd.; the average particle size of UiO-66 is 3 μm, and it was purchased from Nanochemazone.

[0037] The following embodiments are particularly described:

[0038] (1) The preparation process of sulfonated polyether ether sulfone casting solution is as follows: S1: After 4 parts of lithium ion sieve precursor are acidified, washed and dried; under nitrogen atmosphere, added to 2.5 parts of 3-methacryloyloxypropyltrimethoxysilane and 75wt% ethanol aqueous solution, ultrasonically dispersed, reacted at 80℃ for 6h, washed and dried to obtain vinyl lithium ion sieve; S2: vinyl lithium ion sieve, 4 parts of 2-acrylamido-2-methylpropanesulfonic acid, 4 parts of sodium p-styrenesulfonate, 2 parts of methyl methacrylate, and 0.6 parts of azobisisobutyronitrile are added to deionized water, reacted at 70℃ for 10h, added hydrochloric acid for 1.5h to adjust the pH to 3.0, washed and dried to obtain sulfonic acid polymer; S3: Sulfonated polyether ether sulfone and sulfonic acid polymer are added to N,N-dimethylformamide in a mass ratio of 6.5:1.2, stirred at 55℃ for 18h to obtain sulfonated polyether ether sulfone casting solution.

[0039] (2) The polyethyleneimine aqueous solution includes polyethyleneimine and deionized water in a mass ratio of 1:96; the acetic anhydride-pyridine solution includes acetic anhydride, pyridine, and tetrahydrofuran in a mass ratio of 7:2:90; the trimesoyl chloride-n-hexane solution includes trimesoyl chloride and n-hexane in a mass ratio of 0.3:99.2; and the iodomethane-isopropanol solution includes iodomethane and isopropanol in a mass ratio of 2:18.5.

[0040] (3) The concentration of hydrochloric acid solution is 1 mol / L.

[0041] Example 1: A method for preparing a positively charged nanofiltration membrane for lithium extraction from salt lakes, comprising the following steps:

[0042] Step 1: S1-1: Under a nitrogen atmosphere, 0.9 parts of sodium polystyrene sulfonate, 1.5 parts of 3-bromopropylene, and 0.02 parts of potassium carbonate were added to N,N-dimethylformamide, refluxed and stirred at 60°C for 12 hours, filtered, purified, and dried to obtain vinyl styrene sulfonic acid; S1-2: Under a nitrogen atmosphere, 2 parts of 6-mercapto-1-hexanol, vinyl styrene sulfonic acid, and 0.06 parts of azobisisobutyronitrile were added to anhydrous ethanol, reacted at 55°C for 3.5 hours, purified, and dried; then 8 parts of 4'-carboxybenzo-15-crown 5-ether, 3 parts of tannic acid, and 0.2 parts of 4-dimethylaminopyridine were added thereto. , 0.5 parts of N,N'-dicyclohexylcarbodiimide and N,N-dimethylformamide, react at 40°C for 24 hours, wash, purify and dry to obtain modified tannic acid; S1-3: 3.5 parts of UiO-66 are activated in 30 parts of hydrochloric acid solution at 90°C for 12.5 hours, and then 2.5 parts of modified tannic acid are added and stirred at 30°C for 13 hours to obtain a modified MOF solution; S1-4: MXene, modified MOF solution and glutaraldehyde are ultrasonically dispersed at a power of 300 W and a frequency of 40 kHz in a mass ratio of 1.2:14:0.15 for 1.5 hours to obtain a modified composite nanofluid;

[0043] Step 2: applying the sulfonated polyetherethersulfone casting solution to the surface of the non-woven fabric and drying it to obtain a rigid base membrane;

[0044] Step 3: depositing the modified composite nanofluid on the surface of the rigid base film and drying it to obtain a nano-deposition intermediate layer;

[0045] Step 4: Coat the surface of the nano-deposition intermediate layer with a polyethyleneimine aqueous solution, and after drying, immerse it in an acetic anhydride-pyridine solution at 25°C for 15 minutes, coat it with a trimesoyl chloride-n-hexane solution, wash and dry it, acidify it at 30°C for 18 minutes, wash and dry it, and then immerse it in iodomethane-isopropanol at 25°C for 32 minutes, wash and dry it to form a permeation layer; and obtain a positively charged nanofiltration membrane.

[0046] In the above embodiment, the thickness of the rigid base film is 3.2 μm; the thickness of the nano-deposition intermediate layer is 12.5 nm; the thickness of the permeation layer is 4 nm; the size of the MXene is 5 μm; and the average particle size of the UiO-66 is 3 μm.

[0047] Example 2: A method for preparing a positively charged nanofiltration membrane for lithium extraction from salt lakes, comprising the following steps:

[0048] Step 1: S1-1: Under a nitrogen atmosphere, 0.9 parts of sodium polystyrene sulfonate, 1.5 parts of 3-bromopropylene, and 0.02 parts of potassium carbonate were added to N,N-dimethylformamide, refluxed and stirred at 60°C for 12 hours, filtered, purified, and dried to obtain vinyl styrene sulfonic acid; S1-2: Under a nitrogen atmosphere, 2 parts of 6-mercapto-1-hexanol, vinyl styrene sulfonic acid, and 0.06 parts of azobisisobutyronitrile were added to anhydrous ethanol, reacted at 55°C for 3.5 hours, purified, and dried; then 8 parts of 4'-carboxybenzo-15-crown 5-ether, 3 parts of tannic acid, and 0.2 parts of 4-dimethylamino Pyridine, 0.5 parts of N,N'-dicyclohexylcarbodiimide, and N,N-dimethylformamide were reacted at 40°C for 24 hours, washed, purified, and dried to obtain modified tannic acid; S1-3: 3.5 parts of UiO-66 were activated in 30 parts of hydrochloric acid solution at 90°C for 12.5 hours, and then 2.5 parts of modified tannic acid were added and stirred at 30°C for 13 hours to obtain a modified MOF solution; S1-4: MXene, modified MOF solution, and glutaraldehyde were ultrasonically dispersed at a power of 300 W and a frequency of 40 kHz in a mass ratio of 1:12:0.1 for 1.5 hours to obtain a modified composite nanofluid;

[0049] Step 2: applying the sulfonated polyetherethersulfone casting solution to the surface of the non-woven fabric and drying it to obtain a rigid base membrane;

[0050] Step 3: depositing the modified composite nanofluid on the surface of the rigid base film and drying it to obtain a nano-deposition intermediate layer;

[0051] Step 4: Coat the surface of the nano-deposition intermediate layer with a polyethyleneimine aqueous solution, and after drying, immerse it in an acetic anhydride-pyridine solution at 25°C for 15 minutes, coat it with a trimesoyl chloride-n-hexane solution, wash and dry it, acidify it at 30°C for 18 minutes, wash and dry it, and then immerse it in iodomethane-isopropanol at 25°C for 32 minutes, wash and dry it to form a permeation layer; and obtain a positively charged nanofiltration membrane.

[0052] In the above embodiment, the thickness of the rigid base film is 2.4 μm; the thickness of the nano-deposition intermediate layer is 10 nm; the thickness of the permeation layer is 2 nm; the size of the MXene is 5 μm; and the average particle size of the UiO-66 is 3 μm.

[0053] Example 3: A method for preparing a positively charged nanofiltration membrane for lithium extraction from salt lakes, comprising the following steps:

[0054] Step 1: S1-1: Under a nitrogen atmosphere, 0.9 parts of sodium polystyrene sulfonate, 1.5 parts of 3-bromopropylene, and 0.02 parts of potassium carbonate were added to N,N-dimethylformamide, and the mixture was refluxed and stirred at 60°C for 12 hours, filtered, purified, and dried to obtain vinyl styrene sulfonic acid; S1-2: Under a nitrogen atmosphere, 2 parts of 6-mercapto-1-hexanol, vinyl styrene sulfonic acid, and 0.06 parts of azobisisobutyronitrile were added to anhydrous ethanol, reacted at 55°C for 3.5 hours, purified, and dried; then 8 parts of 4'-carboxybenzo-15-crown 5-ether, 3 parts of tannic acid, and 0.2 parts of 4-dimethylaminopyridine were added thereto. pyridine, 0.5 parts of N,N'-dicyclohexylcarbodiimide, and N,N-dimethylformamide, reacted at 40°C for 24 hours, washed, purified, and dried to obtain modified tannic acid; S1-3: 3.5 parts of UiO-66 were activated in 30 parts of hydrochloric acid solution at 90°C for 12.5 hours, and then 2.5 parts of modified tannic acid were added and stirred at 30°C for 13 hours to obtain a modified MOF solution; S1-4: MXene, modified MOF solution, and glutaraldehyde were ultrasonically dispersed at a power of 300 W and a frequency of 40 kHz in a mass ratio of 1.3:16:0.2 for 1.5 hours to obtain a modified composite nanofluid;

[0055] Step 2: applying the sulfonated polyetherethersulfone casting solution to the surface of the non-woven fabric and drying it to obtain a rigid base membrane;

[0056] Step 3: depositing the modified composite nanofluid on the surface of the rigid base film and drying it to obtain a nano-deposition intermediate layer;

[0057] Step 4: Coat the surface of the nano-deposition intermediate layer with a polyethyleneimine aqueous solution, and after drying, immerse it in an acetic anhydride-pyridine solution at 25°C for 15 minutes, coat it with a trimesoyl chloride-n-hexane solution, wash and dry it, acidify it at 30°C for 18 minutes, wash and dry it, and then immerse it in iodomethane-isopropanol at 25°C for 32 minutes, wash and dry it to form a permeation layer; and obtain a positively charged nanofiltration membrane.

[0058] In the above embodiment, the thickness of the rigid base film is 4.6 μm; the thickness of the nano-deposition intermediate layer is 15 nm; the thickness of the permeation layer is 6 nm; the size of the MXene is 5 μm; and the average particle size of the UiO-66 is 3 μm.

[0059] Comparative Example 1: Based on Example 1, the nano-deposition intermediate layer does not use a cross-linking agent, and the other processes remain unchanged, and are adjusted as follows:

[0060] Step 1: S1-1: Under a nitrogen atmosphere, 0.9 parts of sodium polystyrene sulfonate, 1.5 parts of 3-bromopropylene, and 0.02 parts of potassium carbonate were added to N,N-dimethylformamide, and the mixture was refluxed and stirred at 60°C for 12 hours, filtered, purified, and dried to obtain vinyl styrene sulfonic acid; S1-2: Under a nitrogen atmosphere, 2 parts of 6-mercapto-1-hexanol, vinyl styrene sulfonic acid, and 0.06 parts of azobisisobutyronitrile were added to anhydrous ethanol, reacted at 55°C for 3.5 hours, purified, and dried; then 8 parts of 4'-carboxybenzo-15-crown 5-ether, 3 parts of tannic acid, and 0.2 parts of 4-dimethylformamide were added thereto. The reaction mixture was added with methylaminopyridine, 0.5 parts of N,N'-dicyclohexylcarbodiimide and N,N-dimethylformamide, and reacted at 40°C for 24 hours, and then washed, purified and dried to obtain modified tannic acid; S1-3: 3.5 parts of UiO-66 were activated in 30 parts of hydrochloric acid solution at 90°C for 12.5 hours, and then 2.5 parts of modified tannic acid were added and stirred at 30°C for 13 hours to obtain a modified MOF solution; S1-4: MXene and modified MOF solution were ultrasonically dispersed at a power of 300 W and a frequency of 40 kHz at a mass ratio of 1.2:14 for 1.5 hours to obtain a modified composite nanoliquid.

[0061] Comparative Example 2: Based on Example 1, no MOF is added to the nano-deposition intermediate layer, and the other processes remain unchanged, and are adjusted as follows:

[0062] Step 1: S1-1: Under a nitrogen atmosphere, 0.9 parts of sodium polystyrene sulfonate, 1.5 parts of 3-bromopropylene, and 0.02 parts of potassium carbonate were added to N,N-dimethylformamide, and the mixture was refluxed and stirred at 60°C for 12 hours, filtered, purified, and dried to obtain vinyl styrene sulfonic acid; S1-2: Under a nitrogen atmosphere, 2 parts of 6-mercapto-1-hexanol, vinyl styrene sulfonic acid, and 0.06 parts of azobisisobutyronitrile were added to anhydrous ethanol, reacted at 55°C for 3.5 hours, purified, and dried; then 8 parts of 4'-carboxybenzo-15-crown 5-ether, 3 parts of mono- The modified tannic acid was added to 0.2 parts of 4-dimethylaminopyridine, 0.5 parts of N,N'-dicyclohexylcarbodiimide and N,N-dimethylformamide, reacted at 40 ° C for 24 hours, washed, purified and dried to obtain modified tannic acid; S1-3: the modified tannic acid was added to 25wt% ethanol aqueous solution, stirred at 30 ° C for 12 hours to obtain a modified tannic acid solution; MXene, modified tannic acid solution and glutaraldehyde were ultrasonically dispersed at a power of 300 W and a frequency of 40 kHz in a mass ratio of 1.2:14:0.15 for 1.5 hours to obtain a modified composite nanoliquid.

[0063] Comparative Example 3: Based on Example 1, the permeation layer was not quaternized, and the remaining processes remained unchanged, and were adjusted to:

[0064] Step 4: Coat the surface of the nano-deposition intermediate layer with a polyethyleneimine aqueous solution, dry it, immerse it in an acetic anhydride-pyridine solution at 25°C for 15 minutes, coat it with a trimesoyl chloride-n-hexane solution, acidify it at 30°C for 18 minutes, wash it, and dry it to form a permeation layer; and obtain a positively charged nanofiltration membrane.

[0065] Comparative Example 4: Based on Example 1, the nano-deposition intermediate layer is not modified with sodium polystyrene sulfonate, and the other processes remain unchanged, and are adjusted as follows:

[0066] Step 1: S1-1: Activate 3.5 parts of UiO-66 in 30 parts of hydrochloric acid solution at 90°C for 12.5 hours, then add 2.5 parts of tannic acid and 1 part of 4'-carboxybenzo-15-crown 5-ether, and stir at 50°C for 13 hours to obtain a modified MOF solution; S1-2: Ultrasonic dispersion of MXene and modified MOF solution at a mass ratio of 1.2:14 at a power of 300 W and a frequency of 40 kHz for 1.5 hours to obtain a modified composite nanoliquid.

[0067] Testing experiment: The positively charged nanofiltration membranes prepared in Examples 1 to 3 and Comparative Examples 1 to 4 were tested for their performance: after pre-pressing with 5 MPa pure water for 240 h, a test solution containing lithium chloride and magnesium chloride was used, and the lithium ion concentration was 2×10 2 ~4×10 2 mg / L, magnesium ion concentration is 1×10 4 ~1.5×10 4 mg / L, tested membrane flux, magnesium retention rate and lithium retention rate at 25℃;

[0068] Membrane flux is calculated according to the formula F=V / (A·t), where F is the flux [L / (m 2 ·h)], V is the permeate volume (L), A is the membrane area (m 2 ), t is the permeation time (h); the higher the membrane flux value, the higher the membrane permeability, and vice versa; the significance of magnesium retention rate and lithium retention rate: the higher the magnesium retention rate and the lower the lithium retention rate, the better the lithium ion selectivity, and vice versa; the results are shown in Table 1;

[0069] Table 1

[0070]

[0071] Result analysis: According to the data analysis in Table 1, it can be seen that the lithium ion selectivity and permeability are improved by synergy between multiple layers, composite deposition fillers, and quaternization penetration. According to the data of Comparative Example 1, the nano-deposition intermediate layer does not use a cross-linking agent, the compatibility and interfacial bonding between fillers are poor, agglomeration is prone to occur, the lithium ion selectivity is reduced, and the membrane flux is reduced; According to the data of Comparative Example 2, the nano-deposition intermediate layer does not add MOF, there is no MOF size screening, and it cannot be complexed with crown ethers, the lithium ion selectivity is reduced, and the membrane flux is reduced; According to the data of Comparative Example 3, the permeation layer is not quaternized, the lithium ion selectivity is reduced, and the permeability is significantly reduced; According to the data of Comparative Example 4, the nano-deposition intermediate layer is not modified with sodium polystyrene sulfonate, the interfacial bonding between layers is reduced, the lithium ion selectivity is reduced, and the permeability is reduced.

[0072] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations that come within the meaning and range of equivalents of the claims be embraced therein.

Claims

1. A method for preparing a positively charged nanofiltration membrane for lithium extraction from salt lakes, characterized in that: The following steps are involved: Step 1: applying the sulfonated polyetherethersulfone casting solution to the surface of the non-woven fabric and drying it to obtain a rigid base membrane; Step 2: depositing the modified composite nanofluid on the surface of the rigid base film and drying it to obtain a nano-deposition intermediate layer; Step 3: coating the surface of the nano-deposition intermediate layer with a polyethyleneimine aqueous solution, quaternizing it, and forming a permeation layer; obtaining a positively charged nanofiltration membrane; The preparation process of the modified composite nanofluid is as follows: S1-1: under a nitrogen atmosphere, sodium polystyrene sulfonate, 3-bromopropylene, and potassium carbonate are added to N,N-dimethylformamide, refluxed and stirred at 50-70° C. for 10-14 hours, filtered, purified, and dried to obtain vinyl styrene sulfonic acid; S1-2: Under a nitrogen atmosphere, 6-mercapto-1-hexanol, vinylstyrene sulfonic acid, and azobisisobutyronitrile were added to anhydrous ethanol, reacted at 50-60°C for 3-4 hours, and purified and dried; then, 4'-carboxybenzo-15-crown-5-ether, tannic acid, 4-dimethylaminopyridine, N,N'-dicyclohexylcarbodiimide, and N,N-dimethylformamide were added, reacted at 30-50°C for 20-28 hours, washed, purified, and dried to obtain modified tannic acid; S1-3: Activate the MOF in a hydrochloric acid solution at 85-95°C for 10-15 hours, then add modified tannic acid and stir at 20-40°C for 10-16 hours to obtain a modified MOF solution; S1-4: Ultrasonic dispersion of MXene, modified MOF solution, and crosslinker for 1 to 2 hours to obtain a modified composite nanoliquid.

2. The method for preparing a positively charged nanofiltration membrane for lithium extraction from salt lakes according to claim 1, wherein: The thickness of the rigid base film is 2.4-4.6 μm; the thickness of the nano-deposition intermediate layer is 10-15 nm; and the thickness of the permeation layer is 2-6 nm.

3. The method for preparing a positively charged nanofiltration membrane for lithium extraction from salt lakes according to claim 1, wherein: In the raw materials of the modified composite nanofluid, the mass ratio of MXene, modified MOF solution, and cross-linking agent is 1-1.3:12-16:0.1-0.

2.

4. The method for preparing a positively charged nanofiltration membrane for lithium extraction from salt lakes according to claim 1, wherein: The size of the MXene is 2-6 μm; the average particle size of the MOF is 1-4 μm.

5. The method for preparing a positively charged nanofiltration membrane for extracting lithium from salt lakes according to claim 3, wherein: The raw materials of the modified MOF solution include the following components: 3 to 4 parts by mass of MOF, 20 to 40 parts by mass of hydrochloric acid solution, and 2 to 3 parts by mass of modified tannic acid; The raw materials of the modified tannic acid include the following components, calculated by mass: 0.5-1.2 parts of sodium polystyrene sulfonate, 1-2 parts of 3-bromopropylene, 0.01-0.03 parts of potassium carbonate, 1-2.5 parts of 6-mercapto-1-hexanol, 0.04-0.07 parts of azobisisobutyronitrile, 4-9 parts of 4'-carboxybenzo-15-crown 5-ether, 2-4 parts of tannic acid, 0.1-0.3 parts of 4-dimethylaminopyridine, and 0.3-0.5 parts of N,N'-dicyclohexylcarbodiimide; The concentration of the hydrochloric acid solution is 0.8-1.2 mol / L.

6. The method for preparing a positively charged nanofiltration membrane for extracting lithium from salt lakes according to claim 3, characterized in that: The cross-linking agent includes one or both of glutaraldehyde and glyoxal.

7. The method for preparing a positively charged nanofiltration membrane for lithium extraction from salt lakes according to claim 1, characterized in that: The quaternization process comprises: applying a polyethyleneimine aqueous solution to the surface of the nano-deposition intermediate layer, drying, immersing in an acetic anhydride-pyridine solution at 20-30° C. for 10-20 minutes, applying a trimesoyl chloride-n-hexane solution, washing and drying, acidifying at 20-40° C. for 10-25 minutes, washing and drying, and then immersing in iodomethane-isopropyl alcohol at 20-30° C. for 25-40 minutes, washing and drying to form a permeation layer; A positively charged nanofiltration membrane is obtained.

8. The method for preparing a positively charged nanofiltration membrane for extracting lithium from salt lakes according to claim 7, characterized in that: In the raw materials of the polyethyleneimine aqueous solution, the mass ratio of polyethyleneimine to deionized water is 1:95-98; in the raw materials of the acetic anhydride-pyridine solution, the mass ratio of acetic anhydride, pyridine, and tetrahydrofuran is 5-10:1-3:85-95; in the raw materials of the trimesoyl chloride-n-hexane solution, the mass ratio of trimesoyl chloride to n-hexane is 0.1-0.5:99-99.5; and in the raw materials of the iodomethane-isopropyl alcohol, the mass ratio of iodomethane to isopropyl alcohol is 1-3:17-20.

9. A positively charged nanofiltration membrane obtained according to the method for preparing a positively charged nanofiltration membrane for lithium extraction from salt lakes according to any one of claims 1 to 8.

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