A method for separating lithium and magnesium based on a cationic polyelectrolyte nanofiltration membrane

Abstract

The application discloses a lithium-magnesium separation method based on a cationic polyelectrolyte nanofiltration membrane, and belongs to the technical field of membrane separation. The preparation method comprises the following steps: (1) adding the cationic polyelectrolyte into part or all of lithium-magnesium mixture feed liquid to prepare a ternary mixed solution; (2) adopting a nanofiltration membrane with a cutting molecular weight of 100-2000 Da and a membrane pore size of 0.2-1 nm to perform cross-flow filtration separation on the ternary mixed solution or sequentially on the ternary mixed solution and the remaining lithium-magnesium mixture feed liquid, and a high-lithium low-magnesium solution is separated. The method is simple, the separation selectivity of the lithium-magnesium mixture can be improved by 1.7-10 times, and the method has a wide application prospect in the fields of lithium extraction from salt lakes and other single / multiple valence cation selective separation.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, and specifically to a method for lithium-magnesium separation using a nanofiltration membrane based on a cationic polyelectrolyte. Background Technology

[0002] With the increasing global demand for lithium resources, lithium extraction from salt lakes has become an important component of the sustainable supply of lithium resources. Among these, mono / divalent Li... + and Mg 2+ Ion separation is a key step in lithium extraction from salt lakes. However, the Mg content in my country's salt lake brines... 2+ / Li + It has a relatively high specific gravity (up to 500) and contains a large number of associated ions. High Mg 2+ / Li + The complex ionic composition of lithium makes traditional lithium extraction processes cumbersome, costly, and with low recovery rates. Therefore, developing high-Li content lithium extraction technologies is crucial. + / Mg 2 + Selective and low-cost monovalent / multivalent ion separation technology is of great significance for ensuring the supply of lithium resources.

[0003] Nanofiltration (NFL) is a green and efficient separation technology that falls between ultrafiltration and reverse osmosis. Based on separation mechanisms such as pore size sieving, the Donnan effect, and dielectric repulsion, NFL membranes can effectively retain divalent magnesium ions while allowing monovalent lithium ions to pass through, showing promising application prospects in lithium extraction from salt lakes. However, most commercially available NFL membranes currently on the market have negatively charged surfaces, resulting in poor separation of monovalent / polyvalent cations due to the Donnan effect, making it difficult to achieve efficient separation of lithium and magnesium ions. Preparing positively charged NFL membranes can improve lithium-magnesium separation efficiency. Many studies have attempted to use positively charged aqueous monomers in interfacial polymerization to prepare positively charged NF membranes. However, from the perspective of interfacial polymerization membrane formation mechanisms, the reaction tends to occur on the oil phase side, where acyl chloride monomers are usually in excess. Furthermore, the nascent membrane formed inhibits the further diffusion of aqueous monomers into the organic phase to react with acyl chloride monomers, resulting in a large number of unreacted acyl groups remaining on the membrane surface, which further hydrolyze into carboxyl groups in aqueous solution. Therefore, it is difficult to obtain a positively charged surface with high charge density simply by controlling the positive charge of the aqueous monomers. By utilizing residual acyl chloride and carboxyl groups, grafting positively charged molecules onto the membrane surface can achieve surface charge reversal. Many methods have significantly improved the positive charge of membrane surfaces by grafting functional molecules containing positively charged amine, quaternary ammonium, and imidazole groups. However, due to the limited density of groups at grafting sites and the non-uniform spatial distribution of these groups, conventional membrane surface modification strategies struggle to further improve the positive charge density and uniformity of charge distribution. Furthermore, methods such as developing novel monomers and secondary membrane surface modification suffer from high monomer development costs, complex operations, long modification times, and low grafting efficiency, hindering large-scale fabrication. Summary of the Invention

[0004] This invention provides a method for lithium-magnesium separation using a nanofiltration membrane based on a cationic polyelectrolyte, from the perspective of membrane processes. The process is simple: a cationic polyelectrolyte is added to a portion of a lithium-magnesium mixed solution with a high magnesium-to-lithium ratio. The cationic polyelectrolyte then self-assembles on the membrane surface to form a highly positively charged polyelectrolyte active layer, which can improve the selectivity of the nanofiltration membrane for lithium-magnesium ion separation by 1.7-10 times. It can also be used for the separation of lithium-magnesium mixed solutions in the future, and has good application prospects in the field of lithium extraction from salt lakes.

[0005] The specific technical solution adopted is as follows:

[0006] A method for lithium-magnesium separation using a nanofiltration membrane based on a cationic polyelectrolyte includes the following steps:

[0007] (1) Add cationic polyelectrolytes to part or all of the lithium-magnesium mixture feed liquid to prepare a ternary mixed solution;

[0008] (2) A high-lithium, low-magnesium solution is obtained by cross-flow filtration separation of the ternary mixed solution or the ternary mixed solution and the remaining lithium-magnesium mixture feed liquid in sequence using a nanofiltration membrane with a molecular weight of 100-2000 Da and a membrane pore size of 0.2-1 nm.

[0009] This invention utilizes the self-assembly of cationic polyelectrolytes in aqueous solutions to form an active polyelectrolyte layer with positively charged groups in its molecules, resulting in a polyelectrolyte layer with extremely high potential strength. A cationic polyelectrolyte is added to a mixed solution of lithium chloride and magnesium chloride to prepare a ternary mixed solution of polyelectrolyte and lithium-magnesium. A nanofiltration membrane with appropriate molecular weight and pore size (larger pores may cause the polyelectrolyte to permeate the membrane without remaining on the surface) is used as the membrane filtration medium for cross-flow filtration separation of the ternary mixed solution. Based on electrostatic and hydrophilic-hydrophobic interactions, the polyelectrolyte can self-assemble on the membrane surface to form a polyelectrolyte active layer with high potential strength. The formation of a polyelectrolyte layer with high positive charge strength significantly improves the retention performance for high-valence magnesium ions, while low-valence lithium ions, due to weaker electrostatic repulsion, can permeate the membrane, thus achieving highly efficient lithium-magnesium separation.

[0010] Preferably, after preparing the cationic polyelectrolyte and the lithium-magnesium ternary mixed solution, the lithium-magnesium ternary mixed solution is first subjected to a nanofiltration membrane separation step, and then the remaining lithium-magnesium mixture feed liquid is subjected to a nanofiltration membrane separation step.

[0011] The cationic polyelectrolytes include at least one of the following: polyamine polyelectrolytes (such as polyethyleneamine, polyethyleneimine), quaternary ammonium salt polyelectrolytes (such as polydimethyldiallylammonium chloride, polyacrylamide chloride, polyquaternary ammonium salt 1-40), quaternary phosphate salt polyelectrolytes, quaternary sulfonium salt polyelectrolytes, polypyridine polyelectrolytes (such as poly(4-vinylpyridine)), and polyamino acid polyelectrolytes (such as poly-L-lysine).

[0012] In step (1), the concentration of the cationic polyelectrolyte in the ternary mixed solution is 10-10000 ppm. When the polyelectrolyte concentration is low, it is difficult to form a tight polyelectrolyte layer structure, while when the polyelectrolyte concentration increases, the polyelectrolyte can self-assemble in the membrane surface to form a tight polyelectrolyte layer structure.

[0013] Preferably, in step (1), the concentration of magnesium chloride in the ternary mixed solution is 500-10000 ppm, the concentration of lithium chloride is 10-10000 ppm, and the mass ratio of magnesium to lithium in the feed solution is 0.05-1000. More preferably, the mass ratio of magnesium to lithium in the feed solution is 150, and the concentration of the mixed salt is 2000 ppm.

[0014] Preferably, in step (1), the pH of the ternary mixed solution is 2-12.

[0015] The electrostatic and hydrophilic-hydrophobic interactions between cationic polyelectrolytes and the nanofiltration membrane surface are beneficial to the self-assembly and conformational stability of the polyelectrolytes at the interface. For example... Figure 1 As shown, when a negatively charged nanofiltration membrane is used, the cationic polyelectrolyte can bind more tightly under electrostatic interactions, achieving charge reversal on the membrane surface. When a positively charged nanofiltration membrane is used, the cationic polyelectrolyte can bind to the membrane surface under hydrophobic interactions, increasing the positive charge density and charge distribution uniformity on the nanofiltration membrane surface, thereby greatly improving the lithium-magnesium separation selectivity. However, due to the electrostatic repulsion between the membrane surface and the polyelectrolyte active layer, the binding ability of the polyelectrolyte layer on the membrane surface is relatively weak. Under vertical transmembrane pressure, a stable polyelectrolyte active layer can be formed for lithium-magnesium separation. Furthermore, this invention allows for the continuous addition of cationic polyelectrolytes to the lithium-magnesium mixed solution, ensuring high lithium-magnesium separation selectivity.

[0016] In summary, the surface charge properties, charge density, and pore size of nanofiltration membranes influence the interfacial conformation and binding capacity of polyelectrolytes. Adjusting the concentration of the polyelectrolyte and the inorganic salt concentration also affects the conformation and interfacial stability of the polyelectrolyte on the membrane surface. Furthermore, operating conditions such as temperature, pressure, and membrane flow rate also affect the conformation of the polyelectrolyte layer and the separation efficiency.

[0017] Preferably, the nanofiltration membrane used in step (2) has a molecular weight cut of 100 to 2000 Da and a pore size of 0.2 to 1 nm.

[0018] Preferably, the nanofiltration membrane used in step (2) has a magnesium chloride rejection rate of 5%-100% and a lithium chloride rejection rate of 5%-95%.

[0019] Preferably, the nanofiltration membrane used in step (2) is of at least one type selected from fully aromatic polyamide, semi-aromatic polyamide, polyester, and polyelectrolyte.

[0020] Preferably, the nanofiltration membrane used in step (2) is at least one of a positively charged nanofiltration membrane or a negatively charged nanofiltration membrane.

[0021] Preferably, in step (2), during the cross-flow filtration separation of the cationic polyelectrolyte and lithium-magnesium mixed solution by nanofiltration membrane, the separation process temperature is 5–45°C.

[0022] Preferably, in step (2), during the cross-flow filtration separation of the cationic polyelectrolyte and lithium magnesium mixed solution by the nanofiltration membrane, the filtration pressure is 0.4 to 1.5 MPa.

[0023] Preferably, in step (2), during the cross-flow filtration separation of the cationic polyelectrolyte and lithium magnesium mixed solution by the nanofiltration membrane, the membrane surface flow rate is 20-100 LMH.

[0024] Thanks to the presence of a cationic polyelectrolyte active layer, the nanofiltration membrane exhibits a 1.7-10 times higher selectivity for separating lithium-magnesium mixtures, and maintains good operational stability in terms of permeability and separation selectivity during long-term operation.

[0025] The lithium-magnesium separation method coupled with a cationic polyelectrolyte and a nanofiltration membrane described in this invention can be applied in the field of membrane separation technology, especially in the field of lithium extraction from salt lakes.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] (1) This invention can be directly coupled with existing nanofiltration membranes without changing the existing nanofiltration membrane preparation process. The method is simple, low-cost, and the equipment is easy to obtain. The operating conditions are mild. By adding an appropriate amount of cationic polyelectrolyte to the feed liquid, the separation selectivity of the nanofiltration membrane for lithium-magnesium mixtures can be significantly improved, which is convenient for the industrial application of lithium extraction from salt lakes.

[0028] (2) The present invention develops a method for enhancing the lithium-magnesium selectivity of nanofiltration membranes based on cationic polyelectrolytes. The cationic polyelectrolytes have a stable structure on the membrane surface and can significantly increase the positive charge density on the membrane surface, thereby improving the separation selectivity of lithium-magnesium mixtures by 1.7-10 times. During the stability test, the membrane permeability and separation selectivity maintain good stability. It has broad application prospects in lithium extraction from salt lakes and other fields of selective separation of monovalent / polyvalent cations. Attached Figure Description

[0029] Figure 1 The diagrams are schematic diagrams of the lithium-magnesium separation process, wherein (a) is a schematic diagram of the lithium-magnesium separation process of a negatively charged PA membrane, (b) is a schematic diagram of the lithium-magnesium separation process of a negatively charged PA membrane based on a cationic polyelectrolyte according to the present invention, (c) is a schematic diagram of the lithium-magnesium separation process of a positively charged PA / PEI membrane, and (d) is a schematic diagram of the lithium-magnesium separation process of a positively charged PA / PEI membrane based on a cationic polyelectrolyte according to the present invention.

[0030] Figure 2 The diagram shows the preparation process of PA and PA / PEI membranes, where (a) is a schematic diagram of the preparation process of PA and PA / PEI membranes, and (b) is a schematic diagram of the molecular structure.

[0031] Figure 3 The images are SEM images of the PA film, where (a) shows the surface morphology and (b) shows the cross-sectional morphology.

[0032] Figure 4The images are SEM images of the PA / PEI film, where (a) shows the surface morphology and (b) shows the cross-sectional morphology.

[0033] Figure 5 The zeta potential curves for PA and PA / PEI membranes in the pH range of 3-10 are shown.

[0034] Figure 6 The figures show the stability test results of Comparative Examples 1-2 and Examples 3 and 5 after 9 days. (a) shows the stability test results of PA and PA / PEI films of Comparative Examples 1-2 after 9 days, and (b) shows the stability test results of PA and PA / PEI films of Examples 3 and 5 after 9 days. Detailed Implementation

[0035] The present invention will be further illustrated below with reference to the embodiments and accompanying drawings. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0036] Example 1

[0037] Preparation of negatively charged polyamide nanofiltration membranes: such as Figure 2 As shown, an aqueous solution of 0.25 wt% piperazine and 1.5 wt% trisodium phosphate was used as the aqueous monomer, and a hexane solution of 0.15 wt% tricresyl chloride was used as the oil monomer. 25 mL of the aqueous monomer was poured onto the surface of the polysulfone support layer, allowed to stand for 1 min, then the reaction solution was discarded, and excess solution was removed using a squeeze roller. An equal amount of the oil monomer was then slowly poured onto the bottom membrane. After reacting for 40 s, the nascent membrane was heat-treated in an 80°C oven for 10 min to obtain a negatively charged polyamide nanofiltration membrane (PA membrane). Figure 3 The images are SEM images of the PA film, where (a) shows the surface morphology and (b) shows the cross-sectional morphology. Figure 5 The diagram shows the zeta potential of the PA membrane at pH 7. It can be seen that the PA membrane surface exhibits a negatively charged characteristic under neutral conditions (zeta potential of -28.5 mV). The PA membrane has a molecular weight cutoff of 200 Da and a pore radius of 0.281 nm.

[0038] Example 2

[0039] Positively charged polyamide nanofiltration membranes are prepared by grafting polyethyleneimine (PEI) onto the surface of negatively charged nanofiltration membranes (denoted as PA / PEI membranes). The preparation method is as follows: Figure 2 As shown, a positively charged PA / PEI membrane was prepared using the PA membrane obtained in Example 1. The preparation process included:

[0040] (1) Preparation of reaction solution:

[0041] Carboxyl activation solution: Dissolve 0.25 wt% EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride) and 0.4 wt% NHS (N-hydroxysuccinimide) in MES (morpholinoethanesulfonic acid) buffer (containing 0.1 M MES and 0.5 M NaCl) to prepare an EDC / NHS carboxyl activation solution, and adjust the pH of the solution to 5.5–5.6.

[0042] PEI reaction solution: Dissolve 3 wt% PEI in PBS buffer (containing 8 g / L NaCl, 0.2 g / L KCl, 1.97 g / L Na2HPO4, and 0.22 g / L KH2PO4) and adjust the pH of the solution to 7.3–7.4.

[0043] (2) EDC / NHS amidation reaction

[0044] Carboxyl activation: First, fix the PA membrane onto a glass plate using a Telflon plate frame, and pour 25 mL of carboxyl activation solution onto the surface of the PA membrane. Place the membrane in a shaker at 39℃ and 60 r / min for 1 h to activate the carboxyl groups on the PA membrane surface. After activation, discard the excess activation solution and set aside for later use. Amide reaction and PEI grafting: Pour 25 mL of PEI reaction solution onto the surface of the carboxyl-activated PA membrane, and place the membrane in a shaker at 39℃ and 60 r / min for 2 h. After the reaction, discard the reaction solution and rinse the membrane surface several times with deionized water to remove unreacted PEI, thus obtaining the EDC / NHS amidation modified membrane (i.e., PA / PEI membrane).

[0045] The PA / PEI membrane has a molecular weight cutoff of 198 Da and a pore radius of 0.28 nm.

[0046] Figure 4 The images are SEM images of the PA / PEI film, where (a) shows the surface morphology and (b) shows the cross-sectional morphology. Figure 5 The diagram shows the zeta potential of the PA / PEI membrane at pH=7, indicating that the PA / PEI membrane exhibits a positively charged characteristic (zeta potential of +7.2mV).

[0047] Example 3

[0048] A method for lithium-magnesium separation using a nanofiltration membrane based on a cationic polyelectrolyte, comprising:

[0049] 100 ppm of polyethyleneimine (molecular weight 70,000 Da) was added to a mixed solution with a salt concentration of 2000 ppm and a magnesium-to-lithium mass ratio of 150 to form a ternary mixed solution. Under experimental conditions of 0.6 MPa pressure, solution pH of 6.5, temperature of 25 °C and membrane flow rate of 60 LMH, lithium-magnesium separation was performed using the negatively charged polyamide nanofiltration membrane prepared in Example 1.

[0050] Example 4

[0051] In this embodiment, the only difference between the nanofiltration membrane lithium-magnesium separation method based on cationic polyelectrolytes and Example 3 is that 100 ppm of polydimethyldiallyl ammonium chloride (molecular weight of 100,000 Da) is added to the lithium-magnesium mixed salt.

[0052] Example 5

[0053] In this embodiment, the only difference between the nanofiltration membrane lithium-magnesium separation method based on cationic polyelectrolytes and Example 3 is that the PA-PEI positively charged nanofiltration membrane prepared in Example 2 is used, and 100 ppm of polyethyleneimine is added to the lithium-magnesium mixed salt.

[0054] Example 6

[0055] In this embodiment, the only difference between the nanofiltration membrane lithium-magnesium separation method based on cationic polyelectrolytes and Example 5 is that 100 ppm of polydimethyldiallyl ammonium chloride is added to the lithium-magnesium mixed salt.

[0056] Example 7

[0057] A method for lithium-magnesium separation using a nanofiltration membrane based on a cationic polyelectrolyte, comprising:

[0058] A ternary mixed solution was prepared by adding 10,000 ppm of poly(4-vinylpyridine) (molecular weight of 10,000 Da) to a mixed solution with a salt concentration of 20,000 ppm and a magnesium-to-lithium mass ratio of 0.05. Under experimental conditions of 0.4 MPa pressure, solution pH of 2, temperature of 45 °C, and membrane flow rate of 100 LMH, lithium-magnesium separation was performed using a negatively charged fully aromatic polyamide nanofiltration membrane (cut molecular weight of 100 Da and membrane pore size of 0.2 nm).

[0059] Example 8

[0060] A method for lithium-magnesium separation using a nanofiltration membrane based on a cationic polyelectrolyte, comprising:

[0061] A ternary mixed solution was prepared by adding 1 ppm of poly-L-lysine (molecular weight 2,000,000 Da) to a mixed solution with a salt concentration of 20 ppm and a magnesium-to-lithium mass ratio of 1000. Lithium and magnesium were separated using a negatively charged polyester nanofiltration membrane (cut molecular weight 2,000 Da and pore size 1 nm) under experimental conditions of 1.5 MPa pressure, pH 12, temperature 5 °C, and membrane flow rate of 20 LMH.

[0062] Comparative Example 1

[0063] The negatively charged polyamide nanofiltration membrane (PA membrane) prepared in Example 1 was used to separate lithium and magnesium using a mixed solution with a salt concentration of 2000 ppm and a magnesium-to-lithium mass ratio of 150 as the feed liquid, under experimental conditions of 0.6 MPa pressure, solution pH of 6.5, temperature of 25 °C and membrane surface flow rate of 60 LMH.

[0064] Comparative Example 2

[0065] The positively charged polyamide nanofiltration membrane (PA / PEI membrane) prepared in Example 2 was used to separate lithium and magnesium using a mixed solution with a mixed salt concentration of 2000 ppm and a magnesium-to-lithium mass ratio of 150 as the feed liquid under experimental conditions of 0.6 MPa pressure, solution pH of 6.5, temperature of 25 °C and membrane surface flow rate of 60 LMH.

[0066] In the lithium-magnesium separation tests of the above embodiments and comparative examples, the membrane was pre-pressurized for 60 min before the test. The salt rejection rate (%) and separation factor of the membrane can be calculated by equations (1-1) and (1-2):

[0067]

[0068] Where: R(%) is the retention rate, C f C represents the salt ion concentration on the feed side. p The concentration of salt ions permeated through the liquid side.

[0069]

[0070] Among them, (C) Li ) p For Li + Ion osmotic concentration measurement, (C Mg ) p Mg 2+ Ion osmotic concentration measurement, (C Li ) f For Li + Ion concentration measurement in feed, (C Mg ) f Mg 2+ Ion feed concentration measurement, R Li For Li+ Ion rejection rate, R Mg Mg 2+ Ion rejection rate. S Li / Mg It is the lithium-magnesium separation factor.

[0071] The separation performance of Comparative Examples 1-2 and Examples 3-8 in lithium-magnesium mixed solutions is shown in Table 1. The results indicate that the method of this invention can significantly improve the separation selectivity of nanofiltration membranes for lithium-magnesium mixtures, increasing the lithium-magnesium separation factor by 1.7-10 times compared to Comparative Examples 1 and 2 without the addition of cationic polyelectrolyte. This is because the cationic polyelectrolyte self-assembles on the nanofiltration membrane surface to form a polyelectrolyte-charged active layer, improving the retention performance of the nanofiltration membrane for magnesium ions. After a 9-day stability test, the PA membranes and PA / PEI membranes without the addition of cationic polyelectrolyte showed good separation performance for Mg ions. 2+ The retention rate has decreased to some extent (e.g.) Figure 6 a) However, after adding cationic polyelectrolytes, the PA membrane and PA / PEI membrane maintained good operational stability (e.g. Figure 6 b).

[0072] In addition, experiments have shown that Examples 7-8 also have excellent lithium-magnesium separation performance, with the lithium-magnesium separation factor increased by 1.7-10 times without the addition of cationic polyelectrolytes.

[0073] Table 1. Li in the films of Comparative Examples 1-2 and Examples 3-7 + Mg 2+ Ion rejection rate and lithium-magnesium separation factor

[0074]

[0075]

[0076] The embodiments described above provide a detailed explanation of the technical solutions of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, additions, or similar substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for lithium-magnesium separation using a nanofiltration membrane based on a cationic polyelectrolyte, characterized in that, Includes the following steps: (1) Add cationic polyelectrolyte to part or all of the lithium-magnesium mixture feed liquid to prepare a ternary mixed solution; the concentration of cationic polyelectrolyte in the ternary mixed solution is 10-10000 ppm; (2) A nanofiltration membrane with a molecular weight of 100~2000 Da and a pore size of 0.2~1 nm is used to perform cross-flow filtration separation on the ternary mixed solution or the ternary mixed solution and the remaining lithium-magnesium mixture feed liquid in sequence to obtain a high lithium and low magnesium solution; wherein, the cationic polyelectrolyte self-assembles on the membrane surface to form a highly positively charged polyelectrolyte active layer.

2. The method according to claim 1, characterized in that, The cationic polyelectrolyte includes at least one of polyamine polyelectrolytes, quaternary ammonium salt polyelectrolytes, quaternary phosphate salt polyelectrolytes, quaternary sulfonium salt polyelectrolytes, polypyridine polyelectrolytes, and polyamino acid polyelectrolytes.

3. The method according to claim 1, characterized in that, The molecular weight of the cationic polyelectrolyte is 10,000-2,000,000.

4. The method according to claim 1, characterized in that, In the lithium-magnesium mixed feed solution, the concentration of magnesium chloride is 500-10000 ppm, the concentration of lithium chloride is 10-10000 ppm, and the mass ratio of magnesium to lithium in the feed solution is 0.05-1000.

5. The method according to claim 1, characterized in that, The pH of the ternary mixed solution is 2-12.

6. The method according to claim 1, characterized in that, The nanofiltration membrane has a rejection rate of 5%-100% for magnesium chloride and 5%-95% for lithium chloride.

7. The method according to claim 1, characterized in that, The nanofiltration membrane is of at least one type selected from fully aromatic polyamide, semi-aromatic polyamide, polyester, and polyelectrolyte.

8. The method according to claim 1, characterized in that, The nanofiltration membrane is at least one of a positively charged nanofiltration membrane or a negatively charged nanofiltration membrane.

9. The method according to claim 1, characterized in that, The separation process temperature is 5~45℃.

10. The method according to claim 1, characterized in that, The filtration pressure is 0.4~1.5 MPa, and the membrane surface flow rate is 20~100 LMH.

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