A nanofiltration membrane for extracting lithium from salt lakes and a preparation method thereof

Abstract

The application relates to a nanofiltration membrane for extracting lithium from salt lakes and a preparation method thereof, which comprises the steps of preparing a nanofiltration membrane base film, preparing a water phase solution, preparing an oil phase solution and preparing a nanofiltration functional layer. The application further coats or crosslinks an intermediate layer on the base film in advance, further reduces the pore size of the support layer, makes the pore size distribution more uniform, makes the pore size distribution of the finally formed membrane sheet more uniform, improves the ion selectivity of the nanofiltration membrane, the water flux of the nanofiltration membrane is 20.3-35.4 LMH, the desalination rate of the nanofiltration membrane is 97.7-99.4%, the separation coefficient of the nanofiltration membrane is 62.2-220.4, and the Li 2+ desalination rate of the nanofiltration membrane is -22.2--42.3%. + ​

Description

[Technical Field]

[0001] This invention belongs to the field of chemical technology. More specifically, this invention relates to a nanofiltration membrane for lithium extraction from salt lakes, and also to a method for preparing the nanofiltration membrane for lithium extraction from salt lakes. [Background Technology]

[0002] Lithium, the lightest metallic element in nature, is widely used in various fields such as medicine and aerospace. Its development and application are particularly important, especially in lithium-ion batteries that power electric vehicles. Globally proven lithium resources amount to 80 million tons, with approximately 59% of these resources found in salt lake brines. The cost of producing lithium from brine is typically 30% to 50% lower than that from hard rock. Considering the high energy consumption, high cost, and pollution associated with mineral resource development, lithium extraction from salt lake brines is expected to become a major trend in the industry. Nanofiltration (NF) technology, a novel membrane separation technology developed in the late 1980s, effectively retains divalent magnesium ions and allows monovalent lithium ions to pass through, thanks to the combined effects of sieving, Donnan, and dielectric repulsion. Therefore, nanofiltration membranes show promising application prospects in lithium extraction from salt lakes. Currently, nanofiltration membranes used for salt separation mainly achieve their effect by altering the pore size of the membrane through changes in the concentration and reaction time of polyamines and acyl chlorides, or by directly adding inorganic particles and polar solvents to the aqueous or oil phase. However, this method causes a unidirectional change in the rejection of monovalent and divalent ions, without improving the membrane's ion-selective permeability. Some researchers have modified the hydrophilicity and surface pore structure of the base membrane by adding additives to obtain a functional layer with superior performance, or by introducing different functional groups onto the base membrane surface to influence the interfacial polymerization reaction. In their paper titled "Anionic covalent organic framework as an interlayer to fabricate negatively charged polyamide composite nanofiltration membrane featuring ionssieving" (Chemical Engineering Journal, Vol. 427, No. 23, January 2022), Fu Liu et al. described the preparation of a PA composite membrane by constructing an anionic covalent organic framework rich in sulfonic acid groups as an interlayer on the base membrane surface through a liquid-liquid interface Schiff reaction. This interlayer forms a defect-free nanofiltration membrane by regulating the diffusion of amine monomers. CN112999898A discloses a method for depositing HAP (hydroxyapatite) onto the surface of a polysulfone-based membrane via vacuum filtration to improve the pore size distribution and hydrophilicity of the membrane, thereby uniformly storing the aqueous monomer in the intermediate layer, slowing down the monomer release rate, and effectively controlling the formation of the polyamide layer. However, the HAP used in this method is an inorganic material with weak interaction and poor binding to the membrane, making it difficult to guarantee its stability and safety during long-term use. Furthermore, the deposition process is time-consuming, thus hindering industrial-scale production.

[0003] Therefore, based on a summary of existing technologies and addressing their technical shortcomings, the inventors conducted extensive experimental research and analysis, and finally completed this invention. [Summary of the Invention]

[0004] The purpose of this invention is to provide a nanofiltration membrane for lithium extraction from salt lakes, and also relates to a method for preparing the nanofiltration membrane for lithium extraction from salt lakes.

[0005] Another object of the present invention is to provide a method for preparing the nanofiltration membrane for lithium extraction from salt lakes.

[0006] The present invention is achieved through the following technical solution.

[0007] This invention relates to a method for preparing a nanofiltration membrane for lithium extraction from salt lakes.

[0008] The preparation steps of this method are as follows:

[0009] A. Preparation of nanofiltration membrane base membrane:

[0010] A water-soluble polymer, crosslinking agent, catalyst, and humectant are added to water in a weight ratio of 1:0.1-3.5:0.1-3.0:5.0-30.0 to make up a total of 100 parts by weight. The resulting solution is coated onto the surface of the support layer and held for 0.1-10.0 min. Then it is dried to form an intermediate layer on the surface of the support layer, thus obtaining the nanofiltration membrane base membrane.

[0011] B. Preparation of aqueous solution

[0012] Polyamine, camphor sulfonic acid and triethylamine are dissolved in water to obtain an aqueous solution containing 0.8-1.2% polyamine, 2.0-2.2% camphor sulfonic acid and 0.3-5.0% triethylamine by weight.

[0013] C. Preparation of oil phase solution

[0014] Organic acyl chlorides are dissolved in an organic solvent to obtain an oil-phase solution of organic acyl chlorides with a concentration of 0.4–0.8% by weight.

[0015] D. Preparation of nanofiltration functional layer

[0016] The nanofiltration membrane base obtained in step A is coated with the aqueous solution obtained in step B and kept for 0.4 to 0.6 minutes to remove excess aqueous solution from its surface. Then, it is coated with the oil solution obtained in step C and kept for 0.8 to 1.2 minutes to remove excess oil solution from its surface. After drying, the nanofiltration membrane is obtained.

[0017] According to a preferred embodiment of the present invention, in step A, the support layer is a polysulfone, polyacrylonitrile, polyvinylidene fluoride, polyether ether ketone, polytetrafluoroethylene, or polyethylene membrane layer, with a molecular weight cutoff Mw of 40,000 to 200,000; the molecular weight cutoff Mw of the nanofiltration membrane base membrane is 1,000 to 10,000.

[0018] According to another preferred embodiment of the present invention, in step A, the water-soluble polymer is one or more polymers selected from polyvinyl alcohol, polyethylene glycol, polyvinylpyrrolidone, polymethacrylic acid, gelatin, acrylic acid, polyurethane, chitosan, or cyclodextrin; the crosslinking agent is one or more crosslinking agents selected from glyoxal, malondialdehyde, succinal, glutaraldehyde, or terephthalaldehyde; the catalyst is one or more catalysts selected from hydrochloric acid, sulfuric acid, nitric acid, or phosphoric acid; and the humectant is glycerin or sodium camphor sulfonate.

[0019] According to another preferred embodiment of the present invention, in steps A and D, the drying is performed at a temperature of 60 to 100°C for 1 to 20 minutes.

[0020] According to another preferred embodiment of the present invention, in step B, the polyamine is one or more polyamines selected from piperazine, 2-methylpiperazine, m-phenylenediamine, p-phenylenediamine, o-phenylenediamine, mesonemine, benzylamine, N,N-dimethylm-phenylenediamine, 1,4-cyclohexanediamine, N,N-dimethylcyclohexanediamine, triethanolamine, polyethyleneimine, or 2,5-diaminobenzenesulfonic acid.

[0021] According to another preferred embodiment of the present invention, in step C, the organic acyl chloride is one or more organic acyl chlorides selected from trimesoyl chloride, phthaloyl chloride, terephthaloyl chloride, dimethylphosphoryl chloride, or pyrophosphoryl chloride; the organic solvent is one or more organic acyl chlorides selected from Isopar E, Isopar G, Isopar H, Isopar I, C 1-8 Straight-chain alkanes, C 1-8 Branched alkanes or C 6-10 Cycloalkanes.

[0022] According to another preferred embodiment of the present invention, in step A, a solvent-based polymer and a co-solvent are added to a solvent to dissolve and obtain a solution containing 0.1 to 5.0% solvent-based polymer and 5.0 to 30.0% co-solvent by weight. The solution is coated onto the surface of the support layer and held for 0.1 to 10.0 min, and then dried, thereby forming an intermediate layer on the surface of the support layer to obtain the nanofiltration membrane base membrane.

[0023] According to another preferred embodiment of the present invention, the solvent-based polymer is one or more polymers selected from polysulfone, sulfonated polysulfone, sulfonated polyarylene ether sulfone, polyether sulfone, or polyarylene ether sulfone; the co-solvent is one or more co-solvents selected from dimethylformamide, dimethylacetamide, dimethyl sulfoxide, or N-methylpyrrolidone; and the solvent is one or more solvents selected from ethylene glycol methyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, m-xylene, chlorobenzene, dichlorobenzene, or dichloromethane.

[0024] The present invention also relates to a nanofiltration membrane for lithium extraction from salt lakes prepared by the aforementioned preparation method.

[0025] According to another preferred embodiment of the present invention, the water flux of the nanofiltration membrane used for lithium extraction from salt lakes is 20.3–35.4 LMH; its Mg... 2+ The desalination rate is 97.7–99.4%; its Li + The desalination rate is -22.2% to -42.3%; its magnesium-lithium separation coefficient is 62.2% to 220.4%.

[0026] The invention will now be described in more detail.

[0027] This invention relates to a method for preparing a nanofiltration membrane for lithium extraction from salt lakes.

[0028] Prior to existing interfacial polymerization reactions, this invention employs crosslinking and coating methods to form an intermediate layer on the base membrane surface. This intermediate layer has a uniform pore size distribution, which is conducive to the diffusion and distribution of aqueous amine monomers. This intermediate layer enables the pore size of the four-layer structure of the overall nanofiltration membrane to exhibit a gradient distribution, resulting in a defect-free nanofiltration layer with more uniform performance. Simultaneously, this intermediate layer can improve the hydrophilicity and resistance to organic fouling of the nanofiltration membrane to a certain extent, and extend the cleaning cycle, thereby effectively reducing operating costs.

[0029] The preparation steps of this method are as follows:

[0030] A. Preparation of nanofiltration membrane base membrane:

[0031] A water-soluble polymer, crosslinking agent, catalyst, and humectant are added to water in a weight ratio of 1:0.1-3.5:0.1-3.0:5.0-30.0 to make up a total of 100 parts by weight. The resulting solution is coated onto the surface of the support layer and held for 0.1-10.0 min. Then it is dried to form an intermediate layer on the surface of the support layer, thus obtaining the nanofiltration membrane base membrane.

[0032] In this invention, the water-soluble polymer should be understood as a polymer with a solubility of 2 to 50 g / L in distilled water under conditions of room temperature or heating at 90°C. Its main role in the preparation of nanofiltration membrane base membrane is to form an intermediate layer on the surface of the support layer.

[0033] The water-soluble polymer used in this invention is one or more polymers selected from polyvinyl alcohol, polyethylene glycol, polyvinylpyrrolidone, polymethacrylic acid, gelatin, acrylic acid, polyurethane, chitosan, or cyclodextrin. These are all commercially available products, for example, those marketed by Shanghai Aladdin Biochemical Technology Co., Ltd. under the trade name... PVA-203 is the polyvinyl alcohol sold by Sigma-Aldrich (Shanghai) Trading Co., Ltd. under the trade name polyvinylpyrrolidone.

[0034] In this invention, the crosslinking agent should be understood as a compound having two aldehyde groups, whose main function in preparing the nanofiltration membrane base is to crosslink the intermediate layer onto the support layer.

[0035] The crosslinking agent used in this invention is one or more crosslinking agents selected from glyoxal, malondialdehyde, succinaldehyde, glutaraldehyde or terephthalaldehyde. These are all products currently sold on the market, such as glyoxal sold by Shanghai Maclean Biochemical Technology Co., Ltd. under the trade name glyoxal solution.

[0036] In this invention, the catalyst should be understood as a compound that provides protons. Its main role in the preparation of nanofiltration membrane base membrane is that the polymer undergoes a crosslinking reaction with the crosslinking agent under the action of the catalyst, thereby forming an intermediate layer on the surface of the support layer.

[0037] The catalyst used in this invention is one or more catalysts selected from hydrochloric acid, sulfuric acid, nitric acid or phosphoric acid, all of which are currently commercially available products.

[0038] In this invention, the humectant should be understood as a compound with moisture-absorbing and pore-preserving functions. Its main role in the preparation of nanofiltration membrane base membrane is to ensure that the membrane pores do not close during the drying process.

[0039] The humectant used in this invention is glycerin or sodium camphor sulfonate, both of which are currently commercially available products, such as sodium camphor sulfonate sold by Beijing Bailingwei Technology Co., Ltd. under the trade name 10-Sodium Camphor Sulfonate Salt.

[0040] In this invention, when the amounts of water-soluble polymer, catalyst, and humectant are within the specified range, if the amount of crosslinking agent is less than 0.1, the crosslinked intermediate layer formed will be incomplete and have low strength, affecting the performance of the subsequent nanofiltration layer; if the amount of crosslinking agent is greater than 3.5, the formed intermediate layer will be too dense, affecting the water phase carrying capacity; therefore, an amount of crosslinking agent of 0.1 to 3.5 is reasonable, preferably 0.6 to 3.0; more preferably 1.0 to 2.5.

[0041] When the amounts of water-soluble polymer, crosslinking agent, and humectant are within the specified range, if the amount of catalyst is less than 0.1, it will affect the crosslinking process and lead to incomplete crosslinking; if the amount of catalyst is greater than 3.0, it will lead to excessive crosslinking and affect the carrying capacity of the aqueous solution. Therefore, the amount of catalyst is suitable to be 0.1 to 3.0, preferably 0.5 to 2.6, and more preferably 0.9 to 2.2.

[0042] When the amounts of water-soluble polymer, crosslinking agent, and catalyst are within the specified range, if the amount of humectant is less than 5.0, the membrane pores of the support layer are prone to closure during the drying process in step A; if the amount of humectant is greater than 30.0, a large amount of humectant will remain on the surface after drying in step A, affecting subsequent experiments; therefore, an amount of humectant of 5.0 to 30.0 is appropriate, preferably 8.0 to 26.0; more preferably 12.0 to 20.0.

[0043] In this invention, the main function of the support layer in the nanofiltration membrane base is to provide a carrier for the formation of the intermediate layer.

[0044] The supporting layer is a polysulfone, polyacrylonitrile, polyvinylidene fluoride, polyether ether ketone, polytetrafluoroethylene, or polyethylene membrane layer, with a molecular weight cutoff Mw of 40,000 to 200,000; the nanofiltration membrane base membrane has a molecular weight cutoff Mw of 1,000 to 10,000.

[0045] In this invention, the molecular weight cutoff (Mw) of the support layer and the nanofiltration membrane base membrane is determined according to the standard method for determining the molecular weight cutoff of ultrafiltration membranes (GB / T 32360-2015). The molecular weight cutoff (Mw) of the support layer is 40,000 to 200,000. If its molecular weight cutoff (Mw) is less than 40,000, it is not conducive to the formation of the intermediate layer; if its molecular weight cutoff (Mw) is greater than 200,000, the support layer is prone to compaction, leading to unstable nanofiltration membrane performance. Therefore, a molecular weight cutoff (Mw) of 40,000 to 200,000 is appropriate.

[0046] The molecular weight cutoff (Mw) of the nanofiltration membrane base is 1,000 to 10,000. If its molecular weight cutoff (Mw) is less than 1,000, it is not conducive to the carrying of the aqueous phase; if its molecular weight cutoff (Mw) is greater than 10,000, it will affect the pore size distribution of the formed nanofiltration functional layer. Therefore, a molecular weight cutoff (Mw) of 1,000 to 10,000 is appropriate.

[0047] The polysulfone, polyacrylonitrile, polyvinylidene fluoride, polyetheretherketone, polytetrafluoroethylene, or polyethylene used in this invention are all products currently sold on the market, such as polysulfone sold by Shanghai McLean Biochemical Technology Co., Ltd. under the trade name polysulfone, and polyetheretherketone sold by Beijing Merida Technology Co., Ltd. under the trade name polyetheretherketone.

[0048] The thickness of the support layer is typically 20–200 μm.

[0049] According to the present invention, the main function of keeping the solution coated on the surface of the support layer for 0.1 to 10.0 min by the coating method is to ensure that the coating solution has sufficient contact and diffusion on the surface of the support layer.

[0050] According to the present invention, the support layer coated with the solution is dried at a temperature of 60–100°C for 1–20 minutes. When the drying time is within this range, if the drying temperature is below 60°C, the drying will be incomplete or the intermediate layer will not be fully cured; if the drying temperature is above 100°C, it will easily lead to closure of the pores in the support layer membrane. Therefore, a drying temperature of 60–100°C is suitable. When the drying temperature is within this range, if the drying time is less than 1 minute, the intermediate layer will not be fully cured; if the drying time is longer than 20 minutes, over-drying will occur, affecting the bonding strength between the intermediate layer and the support layer. Therefore, a drying time of 1–20 minutes is appropriate.

[0051] The basic performance parameters of the dried nanofiltration membrane base obtained in this step are pure water permeability and molecular weight cutoff. These performance parameters are determined using the methods specified in the ultrafiltration membrane test method (GB / T 32360-2015) standard.

[0052] B. Preparation of aqueous solution

[0053] Polyamine, camphor sulfonic acid and triethylamine are dissolved in water to obtain an aqueous solution containing 0.8-1.2% polyamine, 2.0-2.2% camphor sulfonic acid and 0.3-5.0% triethylamine by weight.

[0054] According to the present invention, the basic purpose of preparing an aqueous solution from polyamine, camphor sulfonic acid and triethylamine is that polyamine is the reactant monomer, triethylamine is the acid-binding agent, and camphor sulfonic acid is conducive to the formation of nanofiltration membrane pores in the aqueous solution.

[0055] In this invention, when the contents of camphor sulfonic acid and triethylamine are within the specified range, if the content of polyamine is less than 0.8%, the concentration of the reactant monomer is too low, resulting in low strength of the formed nanofiltration functional layer; if the content of polyamine is greater than 1.2%, the permeation performance of the nanofiltration membrane is poor; therefore, a polyamine content of 0.8% to 1.2% is reasonable.

[0056] When the polyamine and triethylamine contents are within the specified range, if the camphor sulfonic acid content is below 2.0%, the nanofiltration membrane permeability is low; if the camphor sulfonic acid content is above 2.2%, the nanofiltration membrane selectivity deteriorates; therefore, a camphor sulfonic acid content of 2.0–2.2% is appropriate.

[0057] When the content of polyamines and camphor sulfonic acid is within the specified range, if the content of triethylamine is less than 0.3%, the interfacial polymerization reaction is limited, which is not conducive to the formation of the nanofiltration functional layer; if the content of triethylamine is higher than 5.0%, it may lead to a decrease in the permeation performance of the nanofiltration membrane; therefore, a triethylamine content of 0.3% to 5.0% is appropriate.

[0058] The polyamine used in this invention is one or more selected from piperazine, 2-methylpiperazine, m-phenylenediamine, p-phenylenediamine, o-phenylenediamine, mesopranin, benzylamine, N,N-dimethylm-phenylenediamine, 1,4-cyclohexanediamine, N,N-dimethylcyclohexanediamine, triethanolamine, polyethyleneimine, or 2,5-diaminobenzenesulfonic acid. These are all products currently sold on the market, such as piperazine sold by Shanghai Aladdin Biochemical Technology Co., Ltd., N,N-dimethylm-phenylenediamine sold by Beijing Bailingwei Technology Co., Ltd. under the trade name N,N-dimethylm-phenylenediamine, and 2,5-diaminobenzenesulfonic acid sold by Hubei Nuona Technology Co., Ltd. under the trade name 2,5-diaminobenzenesulfonic acid.

[0059] The camphor sulfonic acid and triethylamine used in this invention are products currently sold on the market, such as camphor sulfonic acid sold by Shanghai Maclean Biochemical Technology Co., Ltd. under the trade name camphor sulfonic acid, and triethylamine sold by Shanghai Aladdin Biochemical Technology Co., Ltd. under the trade name triethylamine.

[0060] C. Preparation of oil phase solution

[0061] Organic acyl chlorides are dissolved in an organic solvent to obtain an oil-phase solution of organic acyl chlorides with a concentration of 0.4–0.8% by weight.

[0062] In this invention, the basic purpose of preparing an oil-phase solution of organic acyl chloride is to form a nanofiltration functional layer through an interfacial reaction with the aqueous phase.

[0063] The organic acyl chloride used in this invention is one or more organic acyl chlorides selected from trimesoyl chloride, phthaloyl chloride, terephthaloyl chloride, dimethylphosphoryl chloride, or pyrophosphoryl chloride. These are all products currently sold on the market, such as trimesoyl chloride sold by Sigma-Aldrich (Shanghai) Trading Co., Ltd. under the trade name trimesoyl chloride, and dimethylphosphoryl chloride sold by Beijing Bailingwei Technology Co., Ltd. under the trade name dimethylphosphoryl chloride.

[0064] The organic solvent used in this invention is one or more selected from Isopar E, Isopar G, Isopar H, Isopar I, C 1-8 Straight-chain alkanes, C 1-8 Branched alkanes or C 6-10Cycloalkanes are products currently sold on the market, such as Isopar E sold by Jiangsu Enmoasai Biotechnology Co., Ltd. under the trade name Isopar E, and n-hexane sold by Shanghai Aladdin Biochemical Technology Co., Ltd. under the trade name n-hexane.

[0065] D. Preparation of nanofiltration functional layer

[0066] The nanofiltration membrane base obtained in step A is coated with the aqueous solution obtained in step B and kept for 0.4 to 0.6 minutes to remove excess aqueous solution from its surface. Then, it is coated with the oil solution obtained in step C and kept for 0.8 to 1.2 minutes to remove excess oil solution from its surface. After drying, the nanofiltration membrane is obtained.

[0067] The basic function of coating the nanofiltration membrane base with the aqueous solution obtained in step A is to ensure that the surface of the nanofiltration membrane base is fully coated with the aqueous solution. The holding time is 0.4 to 0.6 minutes. It is not advisable to hold the solution for a longer time than the specified range, because carrying too much aqueous solution will affect the interfacial polymerization reaction.

[0068] The nanofiltration membrane base membrane coated with the aqueous phase solution is then coated with the oil phase solution and kept for 0.8 to 1.2 minutes. It is not advisable to keep it for longer than the specified time, because the interfacial polymerization reaction time is too long and can easily reduce the permeability of the nanofiltration layer.

[0069] The nanofiltration membrane substrate soaked in the oil phase solution is dried at a temperature of 60–100°C for 1–20 minutes. The purpose of this drying is to improve the cross-linking degree of the nanofiltration functional layer and to dry the oil phase solvent. Within this drying time range, if the drying temperature is below 60°C, the oil phase solvent will not evaporate completely; if the drying temperature is above 100°C, it will lead to a decrease in the permeability and selectivity of the nanofiltration membrane. Therefore, a drying temperature of 60–100°C is appropriate.

[0070] The nanofiltration membrane prepared by this invention was tested for its magnesium-lithium separation performance. The specific test method is as follows:

[0071] I. Preparation of mixed magnesium lithium salt solution

[0072] Chemically pure magnesium chloride and lithium chloride were dissolved separately in distilled water to obtain Mg 2+ Concentration of 3000 mg / L with Li + A magnesium-lithium mixed salt solution with a concentration of 500 mg / L;

[0073] II. Separation Equipment and Conditions

[0074] The test was conducted using a membrane testing device manufactured by Shanghai Jingchun Water Treatment Technology Co., Ltd., at a pressure of 150 psi.

[0075] III. Magnesium-Lithium Ion Concentration Detection

[0076] Magnesium-lithium ion concentration was detected under normal conditions using an inductively coupled plasma spectrometer sold by PerkinElmer Enterprise Management (Shanghai) Co., Ltd.

[0077] IV. Experimental Data Processing

[0078] (i) Formula for calculating water flux:

[0079]

[0080] In the formula:

[0081] F represents water flux (LMH);

[0082] V represents the volume of water (L);

[0083] A represents the membrane area (m²) 2 );

[0084] t represents time (h)

[0085] (ii) Desalination rate calculation formula:

[0086]

[0087] In the formula:

[0088] R represents the desalination rate (%);

[0089] C p This represents the concentration of magnesium or lithium ions in the produced water;

[0090] C f This represents the concentration of magnesium or lithium ions in the raw water.

[0091] (iii) Formula for calculating magnesium-lithium separation coefficient:

[0092]

[0093] In the formula:

[0094] SF represents the magnesium-lithium separation factor;

[0095] This represents the ratio of magnesium to lithium concentration in the raw water;

[0096] The ratio of magnesium to lithium concentration in the produced water;

[0097] The present invention also relates to another method for preparing a nanofiltration membrane for lithium extraction from salt lakes.

[0098] The preparation method is the same as the preparation method using water-soluble polymers described above, except that step A is implemented as follows: the solvent-based polymer and the co-solvent are added to the solvent to dissolve and obtain a solution containing 0.1 to 5.0% solvent-based polymer and 5.0 to 30.0% co-solvent by weight. The solution is coated onto the surface of the support layer and held for 0.1 to 10 minutes. Then it is dried to obtain the nanofiltration membrane base membrane.

[0099] According to the present invention, the solvent-based polymer should be understood as a polymer with good film-forming properties.

[0100] The solvent-based polymer used in this invention is one or more polymers selected from polysulfone, sulfonated polysulfone, sulfonated polyarylene ether sulfone, polyether sulfone or polyarylene ether sulfone. These are all products currently sold on the market, such as polysulfone sold by Shanghai Maclean Biochemical Technology Co., Ltd. under the trade name polysulfone, and sulfonated polyarylene ether sulfone sold by Dechi Technology (Linyi) Co., Ltd. under the trade name sulfonated polysulfone.

[0101] According to the present invention, the cosolvent should be understood as a cosolvent compound capable of dissolving the aforementioned polymer.

[0102] The cosolvent used in this invention is one or more cosolvents selected from dimethylformamide, dimethylacetamide, dimethyl sulfoxide or N-methylpyrrolidone. These are all products currently sold on the market, such as dimethylformamide sold by Shanghai Mairui Biochemical Technology Co., Ltd. under the trade name N,N-dimethylformamide.

[0103] According to the present invention, the solvent should be understood to be a solvent compound miscible with the aforementioned cosolvent.

[0104] The solvent used in this invention is one or more solvents selected from ethylene glycol methyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, m-xylene, chlorobenzene, dichlorobenzene, or dichloromethane. These are all products currently sold on the market, such as ethylene glycol methyl ether sold by Shanghai Maclean Biochemical Technology Co., Ltd. under the trade name ethylene glycol methyl ether, and ethylene glycol monoethyl ether sold by Shanghai Aladdin Biochemical Technology Co., Ltd. under the trade name ethylene glycol monoethyl ether.

[0105] According to the present invention, when a solution is obtained containing 5.0% to 30.0% by weight of a co-solvent within the specified range, if the concentration of the solvent-based polymer is less than 0.1%, the resulting intermediate layer is too thin, discontinuous, and has low strength; if the concentration of the solvent-based polymer is greater than 5.0%, the resulting intermediate layer is too dense, which is not conducive to the carrying of the aqueous phase, thereby affecting the formation of the nanofiltration functional layer; therefore, a solvent-based polymer concentration of 0.1% to 5.0% is reasonable. When the solvent-based polymer concentration is within the specified range, if the concentration of the co-solvent is less than 5.0%, the polymer cannot be completely dispersed and dissolved in the solvent; if the concentration of the co-solvent is greater than 30.0%, it affects the subsequent drying efficiency; therefore, a co-solvent concentration of 5.0% to 30.0% is preferable.

[0106] The other steps and related contents of this preparation method are the same as those of the previous preparation method using water-soluble polymers, so they will not be repeated here.

[0107] The present invention also relates to nanofiltration membranes prepared by the above-described preparation method.

[0108] According to the above-mentioned test method for magnesium-lithium separation performance of nanofiltration membranes, the water flux of the nanofiltration membrane is determined to be 20.3–35.4 LMH; its Mg... 2+ The desalination rate is 97.7–99.4%; its Li + The desalination rate is -22.2% to -42.3%; its separation coefficient is 62.2% to 220.4%.

[0109] The beneficial effects of this invention are: by pre-coating or cross-linking an intermediate layer onto the base membrane, the pore size of the support layer is further reduced, and the pore size distribution is more uniform, thereby affecting the diffusion and carrying of the aqueous phase in the support layer. This results in a more uniform charge and pore size distribution in the final membrane, improving the ion selectivity of the nanofiltration membrane. The water flux of the nanofiltration membrane is 20.3–35.4 LMH; its Mg... 2+ The desalination rate is 97.7–99.4%; its Li + The desalination rate is -22.2% to -42.3%; its separation coefficient is 62.2% to 220.4%.

Detailed Implementation Methods

[0110] The invention will be better understood through the following examples.

[0111] Example 1: Preparation of nanofiltration membrane for lithium extraction from salt lakes according to the present invention

[0112] The implementation steps of this embodiment are as follows:

[0113] A. Preparation of nanofiltration membrane base membrane:

[0114] A water-soluble polyvinyl alcohol polymer, glyoxal crosslinking agent, hydrochloric acid catalyst, and glycerin humectant were added to a total of 100 parts by weight of water at a weight ratio of 1:1.2:0.1:5.0. The resulting solution was coated onto the surface of a polysulfone support layer with a molecular weight cutoff (Mw) of 40,000 and held for 7.0 min. Then, it was dried at 90°C for 8 min to form an intermediate layer on the support layer surface, resulting in a nanofiltration membrane base membrane with a molecular weight cutoff (Mw) of 1,000.

[0115] B. Preparation of aqueous solution

[0116] Piperazine polyamine, camphor sulfonic acid and triethylamine were dissolved in water to obtain an aqueous solution containing 1.0% polyamine, 2.0% camphor sulfonic acid and 5.0% triethylamine by weight.

[0117] C. Preparation of oil phase solution

[0118] Phthaloyl chloride organic acyl chloride was dissolved in propane organic solvent to obtain an oil phase solution of organic acyl chloride with a concentration of 0.6% by weight;

[0119] D. Preparation of nanofiltration functional layer

[0120] The nanofiltration membrane base obtained in step A is immersed in the aqueous solution obtained in step B for 0.4 min, then removed to remove excess aqueous solution from its surface, and then immersed in the oil solution obtained in step C for 1.0 min, then removed to remove excess oil solution from its surface, and dried at 76°C for 13 min to obtain the nanofiltration membrane.

[0121] According to the method described in this application, its water flux is 27.49 LMH; its Mg... 2+ The desalination rate is 98.71%; its Li + The desalination rate is -30.89%; its magnesium-lithium separation coefficient is 101.47.

[0122] Meanwhile, a polysulfone nanofiltration membrane was prepared using the polysulfone membrane of this embodiment according to step D. Detected using the above method, its water flux was 28.14 LMH; its Mg... 2+ The desalination rate is 98.14%; its Li + The desalination rate is -15.11%; its magnesium-lithium separation coefficient is 72.4.

[0123] Example 2: Preparation of nanofiltration membrane for lithium extraction from salt lakes according to the present invention

[0124] The implementation steps of this embodiment are as follows:

[0125] A. Preparation of nanofiltration membrane base membrane:

[0126] A water-soluble polyethylene glycol polymer, glutaraldehyde crosslinking agent, sulfuric acid catalyst, and sodium camphor sulfonate humectant were added to a total of 100 parts by weight of water at a weight ratio of 1:0.1:1.0:14. The resulting solution was coated onto the surface of a polyacrylonitrile support layer with a molecular weight cutoff (Mw) of 70,000 and held for 0.1 min. Then, it was dried at 60°C for 20 min to form an intermediate layer on the support layer surface, resulting in a nanofiltration membrane base membrane with a molecular weight cutoff (Mw) of 4,000.

[0127] B. Preparation of aqueous solution

[0128] N,N-dimethyl-m-phenylenediamine polyamine, camphor sulfonic acid and triethylamine were dissolved in water to obtain an aqueous solution containing 0.8% polyamine, 2.1% camphor sulfonic acid and 1.8% triethylamine by weight.

[0129] C. Preparation of oil phase solution

[0130] The organic acyl chloride pyromellitic acid chloride was dissolved in Isopar E organic solvent to obtain an oil phase solution of organic acyl chloride with a concentration of 0.4% by weight.

[0131] D. Preparation of nanofiltration functional layer

[0132] The nanofiltration membrane base obtained in step A is immersed in the aqueous solution obtained in step B for 0.5 min, then removed to remove excess aqueous solution from its surface. Next, it is immersed in the oil solution obtained in step C for 0.8 min, then removed to remove excess oil solution from its surface. Finally, it is dried at 60°C for 20 min to obtain the nanofiltration membrane.

[0133] According to the method described in this application, its water flux is 28.82 LMH; its Mg... 2+ The desalination rate is 97.71%; its Li + The desalination rate is -42.34%; its magnesium-lithium separation coefficient is 62.16.

[0134] Meanwhile, a polyacrylonitrile nanofiltration membrane was prepared using the polyacrylonitrile membrane of this embodiment according to step D. Detected using the above method, its water flux was 30.27 LMH; its Mg... 2+ The desalination rate is 97.83%; its Li + The desalination rate is -21.11%; its magnesium-lithium separation coefficient is 55.81.

[0135] Example 3: Preparation of nanofiltration membrane for lithium extraction from salt lakes according to the present invention

[0136] The implementation steps of this embodiment are as follows:

[0137] A. Preparation of nanofiltration membrane base membrane:

[0138] A water-soluble polymer of polyvinylpyrrolidone, a terephthalaldehyde crosslinking agent, a nitric acid catalyst, and a glycerol humectant were added to a total of 100 parts by weight of water at a weight ratio of 1:2.4:3.0:22. The resulting solution was coated onto the surface of a polyvinylidene fluoride support layer with a molecular weight cutoff (Mw) of 200,000 and held for 10.0 min. Then, it was dried at 100°C for 1 min to form an intermediate layer on the surface of the support layer, thus obtaining a nanofiltration membrane base membrane with a molecular weight cutoff (Mw) of 10,000.

[0139] B. Preparation of aqueous solution

[0140] p-phenylenediamine polyamine, camphor sulfonic acid and triethylamine were dissolved in water to obtain an aqueous solution containing 0.9% polyamine, 2.2% camphor sulfonic acid and 0.3% triethylamine by weight.

[0141] C. Preparation of oil phase solution

[0142] Terephthaloyl chloride organic acyl chloride was dissolved in Isopar G organic solvent to obtain an oil phase solution of organic acyl chloride with a concentration of 0.8% by weight;

[0143] D. Preparation of nanofiltration functional layer

[0144] The nanofiltration membrane base obtained in step A is immersed in the aqueous solution obtained in step B for 0.6 min, then removed to remove excess aqueous solution from its surface, and then immersed in the oil solution obtained in step C for 1.1 min, then removed to remove excess oil solution from its surface, and dried at 84°C for 9 min to obtain the nanofiltration membrane.

[0145] According to the method described in this application, its water flux is 20.30 LMH; its Mg... 2+ The desalination rate is 99.42%; its Li + The desalination rate is -27.83%; its magnesium-lithium separation coefficient is 220.4.

[0146] Meanwhile, a polyvinylidene fluoride nanofiltration membrane was prepared using the polyvinylidene fluoride membrane of this embodiment according to step D. Detected using the above method, its water flux was 26.62 LMH; its Mg... 2+ The desalination rate is 99.03%; its Li + The desalination rate is -16.11%; its magnesium-lithium separation coefficient is 119.7.

[0147] Example 4: Preparation of the nanofiltration membrane for lithium extraction from salt lakes according to the present invention

[0148] The implementation steps of this embodiment are as follows:

[0149] A. Preparation of nanofiltration membrane base membrane:

[0150] A polyurethane water-soluble polymer, succinate crosslinking agent, phosphoric acid catalyst, and sodium camphor sulfonate humectant were added to water in a weight ratio of 1:3.5:2.0:30.0 to make up a total of 100 parts by weight. The resulting solution was coated onto the surface of a polytetrafluoroethylene support layer with a molecular weight cutoff (Mw) of 140,000 and held for 4.0 min. Then, it was dried at 80°C for 14 min to form an intermediate layer on the surface of the support layer, resulting in a nanofiltration membrane base membrane with a molecular weight cutoff (Mw) of 8,000.

[0151] B. Preparation of aqueous solution

[0152] Polyethyleneimine polyamine, camphor sulfonic acid and triethylamine were dissolved in water to obtain an aqueous solution containing 1.2% polyamine, 2.0% camphor sulfonic acid and 3.4% triethylamine by weight.

[0153] C. Preparation of oil phase solution

[0154] Dimethylphosphoryl chloride organic acyl chloride was dissolved in isobutane organic solvent to obtain an oil phase solution of organic acyl chloride with a concentration of 0.5% by weight;

[0155] D. Preparation of nanofiltration functional layer

[0156] The nanofiltration membrane base obtained in step A is immersed in the aqueous solution obtained in step B for 0.6 min, then removed to remove excess aqueous solution from its surface, and then immersed in the oil solution obtained in step C for 1.2 min, then removed to remove excess oil solution from its surface, and dried at 100°C for 1 min to obtain the nanofiltration membrane.

[0157] According to the method described in this application, its water flux is 35.40 LMH; its Mg... 2+ The desalination rate is 98.64%; its Li + The desalination rate is -22.18%; its magnesium-lithium separation coefficient is 89.84.

[0158] Meanwhile, a polytetrafluoroethylene nanofiltration membrane was prepared using the polytetrafluoroethylene membrane of this embodiment according to step D. Detected using the above method, its water flux was 33.11 LMH; its Mg... 2+ The desalination rate is 97.43%; its Li + The desalination rate is -11.11%; its magnesium-lithium separation coefficient is 43.23.

[0159] Example 5: Preparation of nanofiltration membrane for lithium extraction from salt lakes according to the present invention

[0160] The implementation steps of this embodiment are as follows:

[0161] A. Preparation of nanofiltration membrane base membrane:

[0162] Sulfonated polysulfone solvent polymer and dimethylformamide co-solvent were added to ethylene glycol methyl ether solvent to dissolve and obtain a solution containing 0.1% solvent polymer and 5.0% co-solvent by weight. The solution was coated onto the surface of a polyether ether ketone support layer with a molecular weight cutoff of 50,000 Mw and held for 2.0 min. Then it was dried at 65°C for 16 min to form an intermediate layer on the surface of the support layer, resulting in a nanofiltration membrane base membrane with a molecular weight cutoff of 6,000 Mw.

[0163] B. Preparation of aqueous solution

[0164] 2-Methylpiperazine polyamine, camphor sulfonic acid and triethylamine were dissolved in water to obtain an aqueous solution containing 1.0% polyamine, 2.1% camphor sulfonic acid and 2.5% triethylamine by weight.

[0165] C. Preparation of oil phase solution

[0166] Phthaloyl chloride organic acyl chloride was dissolved in propane organic solvent to obtain an oil phase solution of organic acyl chloride with a concentration of 0.6% by weight;

[0167] D. Preparation of nanofiltration functional layer

[0168] The nanofiltration membrane base obtained in step A is immersed in the aqueous solution obtained in step B for 0.5 min, then removed to remove excess aqueous solution from its surface, and then immersed in the oil solution obtained in step C for 1.0 min, then removed to remove excess oil solution from its surface, and dried at 68°C for 17 min to obtain the nanofiltration membrane.

[0169] According to the method described in this application, its water flux is 23.12 LMH; its Mg... 2+ The desalination rate is 98.12%; its Li + The desalination rate is -33.21%; its magnesium-lithium separation coefficient is 70.86.

[0170] Meanwhile, a polyetheretherketone (PEEK) nanofiltration membrane was prepared using the PEEK of this embodiment according to step D. Detected using the above method, its water flux was 29.21 LMH; its Mg content was... 2+ The desalination rate is 95.12%; its Li + The desalination rate is -14.20%; its magnesium-lithium separation coefficient is 23.4.

[0171] Example 6: Preparation of the nanofiltration membrane for lithium extraction from salt lakes according to the present invention

[0172] The implementation steps of this embodiment are as follows:

[0173] A. Preparation of nanofiltration membrane base membrane:

[0174] Sulfonated polyarylether sulfone solvent polymer and N-methylpyrrolidone co-solvent were added to m-xylene solvent to obtain a solution containing 5.0% solvent polymer and 30.0% co-solvent by weight. This solution was coated onto the surface of a polyethylene membrane support layer with a molecular weight cutoff (Mw) of 100,000 and held for 6.0 min. Then, it was dried at 75°C for 5 min, forming an intermediate layer on the support layer surface, resulting in a nanofiltration membrane base membrane with a molecular weight cutoff (Mw) of 8,000.

[0175] B. Preparation of aqueous solution

[0176] Triethanolamine polyamine, camphor sulfonic acid and triethylamine were dissolved in water to obtain an aqueous solution containing 1.1% polyamine, 2.2% camphor sulfonic acid and 4.0% triethylamine by weight.

[0177] C. Preparation of oil phase solution

[0178] Pyrophosphoryl chloride organic acyl chloride was dissolved in cyclopentane organic solvent to obtain an oil phase solution of organic acyl chloride with a concentration of 0.7% by weight;

[0179] D. Preparation of nanofiltration functional layer

[0180] The nanofiltration membrane base obtained in step A is immersed in the aqueous solution obtained in step B for 0.6 min, then removed to remove excess aqueous solution from its surface, and then immersed in the oil solution obtained in step C for 1.2 min, then removed to remove excess oil solution from its surface, and dried at 92°C for 5 min to obtain the nanofiltration membrane.

[0181] According to the method described in this application, its water flux is 30.24 LMH; its Mg... 2+ The desalination rate is 99.10%; its Li + The desalination rate is -37.60%; its magnesium-lithium separation coefficient is 152.89.

[0182] Meanwhile, a polyethylene nanofiltration membrane was prepared using the polyethylene membrane of this embodiment according to step D. Detected using the above method, its water flux was 31.95 LMH; its Mg... 2+ The desalination rate is 96.23%; its Li + The desalination rate is -13.24%; its magnesium-lithium separation coefficient is 30.04.

Claims

1. A method for preparing a nanofiltration membrane for lithium extraction from salt lakes, characterized in that... The preparation steps of this method are as follows: A. Preparation of nanofiltration membrane base membrane: A water-soluble polymer, crosslinking agent, catalyst, and humectant are added to water at a weight ratio of 1:0.1–3.5:0.1–3.0:5.0–30.0 to a total volume of 100 parts by weight. The resulting solution is coated onto the surface of a support layer and held for 0.1–10.0 min. Then, it is dried at 60–100°C for 1–20 min to form an intermediate layer on the support layer surface, thus obtaining the nanofiltration membrane base membrane. The molecular weight cutoff (Mw) of the support layer is 40,000–200,000; the molecular weight cutoff (Mw) of the nanofiltration membrane base membrane is 1,000–10,000. B. Preparation of aqueous solution Polyamine, camphor sulfonic acid and triethylamine are dissolved in water to obtain an aqueous solution containing 0.8-1.2% polyamine, 2.0-2.2% camphor sulfonic acid and 0.3-5.0% triethylamine by weight. C. Preparation of oil phase solution Organic acyl chlorides are dissolved in an organic solvent to obtain an oil-phase solution of organic acyl chlorides with a concentration of 0.4–0.8% by weight. D. Preparation of nanofiltration functional layer The nanofiltration membrane base obtained in step A is coated with the aqueous solution obtained in step B and kept for 0.4 to 0.6 minutes to remove excess aqueous solution from its surface. Then, it is coated with the oil solution obtained in step C and kept for 0.8 to 1.2 minutes to remove excess oil solution from its surface. After drying, the nanofiltration membrane is obtained.

2. The preparation method according to claim 1, characterized in that... In step A, the support layer is a polysulfone, polyacrylonitrile, polyvinylidene fluoride, polyether ether ketone, polytetrafluoroethylene, or polyethylene film.

3. The preparation method according to claim 1, characterized in that... In step A, the water-soluble polymer is one or more polymers selected from polyvinyl alcohol, polyethylene glycol, polyvinylpyrrolidone, polymethacrylic acid, gelatin, acrylic acid, polyurethane, chitosan, or cyclodextrin; the crosslinking agent is one or more crosslinking agents selected from glyoxal, malondialdehyde, succinal, glutaraldehyde, or terephthalaldehyde; the catalyst is one or more catalysts selected from hydrochloric acid, sulfuric acid, nitric acid, or phosphoric acid; and the humectant is glycerin or sodium camphor sulfonate.

4. The preparation method according to claim 1, characterized in that... In step D, the drying is performed at a temperature of 60–100°C for 1–20 minutes.

5. The preparation method according to claim 1, characterized in that... In step B, the polyamine is one or more polyamines selected from piperazine, 2-methylpiperazine, m-phenylenediamine, p-phenylenediamine, o-phenylenediamine, pyromellitic triamine, benzylamine, N,N-dimethylm-phenylenediamine, 1,4-cyclohexanediamine, N,N-dimethylcyclohexanediamine, triethanolamine, polyethyleneimine, or 2,5-diaminobenzenesulfonic acid.

6. The preparation method according to claim 1, characterized in that... In step C, the organic acyl chloride is one or more organic acyl chlorides selected from trimesoyl chloride, phthaloyl chloride, terephthaloyl chloride, dimethylphosphoryl chloride, or pyrophosphoryl chloride; the organic solvent is one or more organic acyl chlorides selected from Isopar E, Isopar G, Isopar H, Isopar I, C1-8 straight-chain alkanes, C1-8 branched-chain alkanes, or C6-10 cycloalkanes.

7. A nanofiltration membrane for lithium extraction from salt lakes prepared by the preparation method according to any one of claims 1-6.

8. The nanofiltration membrane for lithium extraction from salt lakes according to claim 7, characterized in that... Its water flux is 20.3–35.4 LMH; its Mg2+ desalination rate is 97.7–99.4%; its Li+ desalination rate is -22.2–-42.3%; and its magnesium-lithium separation coefficient is 62.2–220.4.

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