Polyamide nanofiltration membrane as well as preparation method and application thereof

Abstract

The invention provides a polyamide nanofiltration membrane as well as a preparation method and application thereof. The preparation method comprises the following steps: a) mixing cycleanine, cetylpyridinium chloride and water, and dissolving to obtain an amine solution; b) soaking a PAN base membrane in the amine solution to carry out a first reaction, taking out and drying the PAN base membrane, and carrying out a second reaction on the PAN base membrane and a normal hexane solution of paraphthaloyl chloride to obtain a PA membrane; and c) washing the PA membrane with an n-hexane solution, and carrying out heat treatment to obtain the polyamide nanofiltration membrane. Compared with the prior art, the preparation method provided by the invention has the advantages that specific raw materials are matched with specific process steps and conditions to realize better overall interaction: cyclic molecule cycleanine and paraphthaloyl chloride are cross-linked, cetylpyridinium chloride is introduced into a water phase, and the temperature of an organic phase is controlled at 0 DEG C, so that the interfacial polymerization process is synergistically regulated and controlled, and the molecular weight is reduced; the prepared polyamide membrane has high water flux and can efficiently separate lithium and magnesium at the same time.

Description

Technical Field

[0001] The present invention relates to the field of membrane separation technology, and more specifically to a polyamide nanofiltration membrane and a preparation method and application thereof. Background Art

[0002] In recent years, the increasing emphasis on renewable energy has led to a rapid expansion of the lithium battery industry, exacerbating the global demand for lithium resources. Natural lithium exists mainly in two sources: ores and salt lakes. Among them, salt lakes stand out because they have larger reserves and the lithium extraction process involved is more energy-efficient than ore mining. Therefore, in recent years, salt lake water has become the main source of lithium worldwide. However, the lithium concentration in salt lakes is low and there are many high-concentration co-existing ions. The key challenge in extracting lithium from salt lakes is to effectively separate lithium from the co-existing high-concentration magnesium in salt lake water. Li + and Mg 2+ With similar chemical properties, their ionic hydration radii differ by less than 0.1 nm. In addition, several known salt lakes in the world are classified as high magnesium-lithium mass ratio salt lakes (Mg 2+ / Li + >8), which makes it difficult to obtain 2+ Separation and extraction of Li from brine + The key step to achieving lithium extraction from salt lake water is to design a method that can effectively separate lithium and magnesium.

[0003] NF membranes usually have charged surfaces with nanoscale pores, which enable them to separate monovalent and multivalent ions. However, precise control of membrane pore size variation within the range of 0.1 nm has made it possible to + Through, while hindering Mg 2+ Passing it is still extremely challenging.

[0004] NF membranes are usually composed of a base membrane and a thin polyamide layer. The separation performance of the membrane is mainly determined by the polyamide layer, which is mainly formed by interfacial polymerization (IP) between amine and acyl chloride monomers. However, the polyamide membranes obtained by traditional interfacial polymerization have uneven pore size distribution and often have low lithium-magnesium separation capabilities. At the same time, there is a trade-off effect between permeability and selectivity in the separation membrane. Therefore, how to achieve the simultaneous improvement of membrane permeability and selectivity has become a technical problem that needs to be solved urgently by technicians in this field. Summary of the invention

[0005] In view of this, the purpose of the present invention is to provide a polyamide nanofiltration membrane and a preparation method and application thereof, by regulating the interfacial polymerization process to achieve a simultaneous improvement in membrane permeability and selectivity, and to prepare a polyamide nanofiltration membrane with a narrow pore size distribution, which is suitable for the efficient separation of lithium and magnesium.

[0006] The present invention provides a method for preparing a polyamide nanofiltration membrane, comprising the following steps:

[0007] a) Mix cyclen, cetylpyridinium chloride and water, and obtain an amine solution after dissolution;

[0008] b) Immerse the PAN-based membrane in the above amine solution for the first reaction, take it out and dry it, and then carry out the second reaction with a n-hexane solution of terephthaloyl chloride to obtain a PA membrane;

[0009] c) Wash the PA membrane with a n-hexane solution and then carry out heat treatment to obtain a polyamide nanofiltration membrane.

[0010] Preferably, in step a), the concentration of cyclen in the amine solution is 0.25 wt% to 2 wt%, and the concentration of cetylpyridinium chloride is 0.1 mmol / L to 1 mmol / L.

[0011] Preferably, before the PAN-based membrane is immersed in step b), it further includes:

[0012] Pre-treat the PAN-based membrane; the specific process of the pre-treatment is:

[0013] Immerse the commercial PAN membrane in isopropanol, take it out after 20 min to 40 min and rinse it with water until there is no residual isopropanol, and dry it at 20°C to 30°C to obtain the pre-treated PAN-based membrane.

[0014] Preferably, the time of the first reaction in step b) is 1 min to 2 min.

[0015] Preferably, the concentration of the n-hexane solution of terephthaloyl chloride in step b) is 0.05 wt% to 0.25 wt%, and the temperature is 0°C to 5°C.

[0016] Preferably, the time of the second reaction in step b) is 2 min to 5 min.

[0017] Preferably, the time of washing with the n-hexane solution in step c) is 0.1 min to 1 min.

[0018] Preferably, the temperature of the heat treatment in step c) is 50°C to 70°C, and the time is 10 min to 30 min.

[0019] The present invention also provides a polyamide nanofiltration membrane prepared by using the preparation method described in the above technical solution.

[0020] The present invention also provides an application of the polyamide nanofiltration membrane described in the above technical solution in lithium-magnesium separation.

[0021] The present invention provides a polyamide nanofiltration membrane, a preparation method thereof, and an application thereof; the preparation method includes the following steps: a) Mix cyclen, cetylpyridinium chloride, and water, and dissolve to obtain an amine solution; b) Immerse the PAN-based membrane in the above amine solution for the first reaction, take it out and dry it, and then perform a second reaction with a n-hexane solution of terephthaloyl chloride to obtain a PA membrane; c) Wash the PA membrane with a n-hexane solution and then perform heat treatment to obtain a polyamide nanofiltration membrane. Compared with the prior art, the preparation method provided by the present invention uses specific raw materials in combination with specific process steps and conditions to achieve good overall interaction: crosslinking with the cyclic molecule cyclen and terephthaloyl chloride, introducing cetylpyridinium chloride in the aqueous phase, and controlling the temperature at 0 °C in the organic phase, and the two cooperate to regulate the interfacial polymerization process. The prepared polyamide membrane can efficiently separate lithium and magnesium while having a high water flux. Experimental results show that the polyamide nanofiltration membrane provided by the present invention has a lithium-magnesium selectivity of 10-60 when the mass ratio of Mg 2+ to Li + is 20, and the pure water flux is 13.5-19.68 L·m -2 ·h -1 ·bar -1 , which are significantly better than the control group.

[0022] Meanwhile, the preparation method provided by the present invention has a simple process, mild and easy-to-control conditions, and has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 FIGS. (A) and (B) are SEM images of the surface morphology of the polyamide membrane before and after regulation, respectively. FIG. (A) shows the surface morphology of the membrane before regulation, corresponding to the control group; FIG. (B) shows the surface morphology of the membrane after regulation, corresponding to Example 1;

[0024] Figure 2 FIGS. (A) and (B) are AFM images of the surface roughness of the polyamide membrane before and after regulation, respectively. FIG. (A) shows the surface morphology of the membrane before regulation, corresponding to the control group; FIG. (B) shows the surface morphology of the membrane after regulation, corresponding to Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0025] The technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art without making creative efforts based on the embodiments of the present invention fall within the protection scope of the present invention.

[0026] The present invention provides a preparation method of a polyamide nanofiltration membrane, including the following steps:

[0027] a) Mix cyclen, cetylpyridinium chloride and water, and dissolve them to obtain an amine solution;

[0028] b) Immerse the PAN-based membrane in the above amine solution for the first reaction, take it out and dry it, then react it with a n-hexane solution of terephthaloyl chloride for the second reaction to obtain a PA membrane;

[0029] c) Wash the PA membrane with a n-hexane solution and then perform heat treatment to obtain a polyamide nanofiltration membrane.

[0030] In the present invention, cyclen, cetylpyridinium chloride and water are first mixed and dissolved to obtain an amine solution. There are no special restrictions on the sources of cyclen and cetylpyridinium chloride in the present invention, and commercially available products well-known to those skilled in the art can be used. In the present invention, the water is preferably deionized water.

[0031] In the present invention, ultrasonic treatment is preferably used for dissolution to obtain an amine solution; the concentration of cyclen in the amine solution is preferably 0.25 wt% to 2 wt%, more preferably 1 wt% to 2 wt%, and the concentration of cetylpyridinium chloride is preferably 0.1 mmol / L to 1 mmol / L, more preferably 0.5 mmol / L to 1 mmol / L.

[0032] After obtaining the amine solution, in the present invention, the PAN-based membrane is immersed in the above amine solution for the first reaction, taken out and dried, and then reacted with a n-hexane solution of terephthaloyl chloride for the second reaction to obtain a PA membrane (a composite membrane with a polyamide layer).

[0033] In the present invention, before the PAN-based membrane is immersed, it preferably further includes:

[0034] Pre-treat the PAN-based membrane; the process of the pre-treatment is preferably specifically:

[0035] Immerse a commercial PAN membrane in isopropanol, take it out after 20 min to 40 min and rinse it with water until there is no isopropanol residue, and dry it at 20 °C to 30 °C to obtain a pre-treated PAN-based membrane;

[0036] More preferably:

[0037] Immerse a commercial PAN membrane in isopropanol, take it out after 30 min and rinse it with water until there is no isopropanol residue, and dry it at 20 °C to 30 °C to obtain a pre-treated PAN-based membrane.

[0038] In the present invention, the process of immersing the PAN-based membrane in the above amine solution for the first reaction is preferably specifically:

[0039] Fix the PAN-based membrane in a frame, soak it in the amine solution for a certain period of time, and then blow it with an air gun to remove the excess aqueous solution, followed by drying at room temperature.

[0040] In the present invention, the time of the first reaction is preferably 1 min to 2 min.

[0041] The present invention has no special limitation on the source of the n-hexane solution of terephthaloyl chloride, and commercially available products well-known to those skilled in the art can be used.

[0042] In the present invention, the concentration of the n-hexane solution of terephthaloyl chloride is preferably 0.05 wt% to 0.25 wt%, more preferably 0.1 wt% to 0.2 wt%, and the temperature is preferably 0 °C to 5 °C, more preferably 0 °C.

[0043] In the present invention, the process of the second reaction with the n-hexane solution of terephthaloyl chloride is preferably specifically as follows:

[0044] Place the dried membrane in the n-hexane solution of terephthaloyl chloride for the second reaction to obtain a PA membrane; the time of the second reaction is preferably 2 min to 5 min.

[0045] After obtaining the PA membrane, the present invention washes the PA membrane with a n-hexane solution and then performs heat treatment to obtain a polyamide nanofiltration membrane, and finally transfers it to pure water at 4 °C for storage.

[0046] In the present invention, the time for washing with the n-hexane solution is preferably 0.1 min to 1 min, more preferably 0.5 min.

[0047] In the present invention, the temperature of the heat treatment is preferably 50 °C to 70 °C, more preferably 60 °C, and the time is preferably 10 min to 30 min, more preferably 15 min to 20 min.

[0048] Aiming at the problem that the diffusion rate of amine monomers does not match the reaction rate during the interfacial polymerization process, resulting in a wide pore size distribution of the polyamide membrane, the present invention adds cetylpyridinium chloride to the aqueous phase to promote the rapid and orderly diffusion of amine monomers, while controlling the temperature of the organic phase at 0 °C to slow down the reaction rate, narrowing the gap between the two, so that the reaction has continuity in time and space, and a polyamide nanofiltration membrane with a uniform pore size distribution is obtained to improve the lithium-magnesium selectivity. The preparation method of the polyamide nanofiltration membrane provided by the present invention is simple, has low energy consumption, and the preparation process is easy to scale up industrially, having extremely high industrial application value.

[0049] The present invention also provides a polyamide nanofiltration membrane prepared by using the preparation method described in the above technical solution.

[0050] The preparation method provided by the present invention uses specific raw materials in combination with specific process steps and conditions to achieve good overall interaction: crosslinking cyclen with terephthaloyl chloride, introducing cetylpyridinium chloride in the aqueous phase, and controlling the temperature of the organic phase at 0 °C. The two cooperate to regulate the interfacial polymerization process. The prepared polyamide membrane can efficiently separate lithium and magnesium while having a high water flux. Experimental results show that the polyamide nanofiltration membrane provided by the present invention has a lithium-magnesium selectivity of 10-60 and a pure water flux of 13.5-19.68 L·m 2+ and Li + when the mass ratio is 20, which are significantly better than those of the control group. -2 ·h -1 ·bar -1

[0051] The present invention also provides an application of the polyamide nanofiltration membrane described in the above technical solution in the separation of lithium and magnesium. By synergistically regulating the aqueous phase and the organic phase, the present invention reduces the gap between the monomer diffusion rate and the reaction rate, and further adjusts the pore size distribution of the formed polyamide dense layer; the formed polyamide nanofiltration membrane has a narrow pore size distribution, effectively improving the lithium-magnesium separation selectivity while ensuring a high water flux.

[0052] The polyamide nanofiltration membrane prepared by the present invention has excellent permeability and lithium-magnesium separation performance: when the feed liquid is a single-salt condition (the single-salt concentration is 1 g / L, and the single salts are MgCl2 and LiCl respectively, and the test pressure is 5 bar), the rejection rate of MgCl2 is 94%-99%, the rejection rate of LiCl is 23%-40%, and the water flux is 60-100 L·m -2 ·h -1 .

[0053] The specific operation of using the prepared high-selectivity polyamide nanofiltration membrane for lithium-magnesium separation is as follows:

[0054] First, prepare a mixed solution of MgCl2 and LiCl with a total concentration of 2 g / L and a mass ratio of Mg 2+ and Li + of 20 to simulate the composition of the salt lake. Under cross-flow conditions, at 25 °C and an operating pressure of 5 bar, continuously filter the prepared polyamide membrane for 2 h. Use ion chromatography to measure the concentrations of Li + and Mg 2+ in the permeate and the feed liquid respectively, and calculate the ion separation selectivity according to the following formula:

[0055]

[0056]

[0057] Under the above test conditions, the prepared lithium-magnesium separation polyamide nanofiltration membrane obtains Li + ​​The rejection rate is between 15% and 5%, Mg 2+ The rejection rate is above 93%, and the selectivity calculated therefrom is between 10 and 60, showing excellent lithium-magnesium separation selectivity. It is superior to the ion-selective membranes reported currently in terms of permeability and lithium-magnesium selectivity, indicating that the membrane preparation method of the present invention has significant technological progress.

[0058] The present invention provides a polyamide nanofiltration membrane for lithium-magnesium separation, its preparation method and application; the preparation method includes the following steps: a) Mix cyclen, cetylpyridinium chloride and water, and obtain an amine solution after dissolution; b) Immerse the PAN-based membrane in the above amine solution for the first reaction, take it out and dry it, and then conduct a second reaction with a n-hexane solution of terephthaloyl chloride to obtain a PA membrane; c) Wash the PA membrane with a n-hexane solution and then conduct heat treatment to obtain a polyamide nanofiltration membrane. Compared with the prior art, the preparation method provided by the present invention uses specific raw materials in combination with specific process steps and conditions to achieve good overall interaction: crosslink cyclen, a cyclic molecule, with terephthaloyl chloride, introduce cetylpyridinium chloride in the aqueous phase, and control the temperature at 0 °C in the organic phase, and the two cooperate to regulate the interfacial polymerization process. The prepared polyamide membrane can efficiently separate lithium and magnesium while having a high water flux. Experimental results show that the polyamide nanofiltration membrane provided by the present invention has a lithium-magnesium selectivity of 10 to 60 and a pure water flux of 13.5 to 19.68 L·m 2+ versus Li + at a mass ratio of 20, which are significantly better than the control group. -2 ·h -1 ·bar -1

[0059] Meanwhile, the preparation method provided by the present invention has a simple process, mild and easy-to-control conditions, and has broad application prospects.

[0060] To further illustrate the present invention, the following detailed description is given through the following examples. All raw materials used in the following examples of the present invention are commercially available.

[0061] Example 1

[0062] S1. Substrate membrane pretreatment: Immerse a commercial PAN membrane in isopropanol, take it out after 30 min and thoroughly rinse it with deionized water until there is no isopropanol residue, and dry it at room temperature to obtain a PAN-based membrane with a certain degree of swelling.

[0063] ​S2. Film formation: Fix the above-mentioned base film in the frame. First, soak it in an aqueous solution containing 0.5 mmol / L cetylpyridinium chloride and 2 wt% cyclanoline for 2 min. After purging the excess aqueous solution with an air gun, then pour a n-hexane solution containing 0.2 wt% terephthaloyl chloride at 0 °C onto the surface of the base film. Immediately remove the excess solution after reacting for 5 min to obtain a PA film.

[0064] S3. Wash the above-mentioned film with n-hexane solution for 30 s to remove the unreacted acyl chloride on the surface. Subsequently, place it in an oven at 60 °C for heat treatment for 15 min to form a polyamide layer. Finally, test the lithium-magnesium separation performance of the obtained polyamide nanofiltration membrane (see Figures 1 - 2 and Table 1).

[0065] Example 2

[0066] Prepare a polyamide film according to the method of Example 1, except that step two is replaced with: S2. Film formation: Fix the above-mentioned base film in the frame. First, soak it in an aqueous solution containing 1 mmol / L cetylpyridinium chloride and 2 wt% cyclanoline for 2 min. After purging the excess aqueous solution with an air gun, then pour a n-hexane solution containing 0.1 wt% terephthaloyl chloride at 0 °C onto the surface of the base film. Immediately remove the excess solution after reacting for 5 min to obtain a PA film.

[0067] Example 3

[0068] Prepare a polyamide film according to the method of Example 1, except that step two is replaced with: S2. Film formation: Fix the above-mentioned base film in the frame. First, soak it in an aqueous solution containing 1 mmol / L cetylpyridinium chloride and 1 wt% cyclanoline for 2 min. After purging the excess aqueous solution with an air gun, then pour a n-hexane solution containing 0.2 wt% terephthaloyl chloride at 0 °C onto the surface of the base film. Immediately remove the excess solution after reacting for 5 min to obtain a PA film.

[0069] Example 4

[0070] Prepare a polyamide film according to the method of Example 1, except that step two is replaced with: S2. Film formation: Fix the above-mentioned base film in the frame. First, soak it in an aqueous solution containing 0.5 mmol / L cetylpyridinium chloride and 2 wt% cyclanoline for 2 min. After purging the excess aqueous solution with an air gun, then pour a n-hexane solution containing 0.1 wt% terephthaloyl chloride at 0 °C onto the surface of the base film. Immediately remove the excess solution after reacting for 2 min to obtain a PA film.

[0071] Example 5

[0072] Prepare the polyamide membrane according to the method of Example 1, except that step two is replaced with: S2. Film formation: Fix the above-mentioned base film in a frame, first soak it in an aqueous solution containing 0.5 mmol / L cetylpyridinium chloride and 1 wt% cyclanoline for 1 minute, blow off the excess aqueous solution with an air gun, and then pour a n-hexane solution containing 0.2 wt% terephthaloyl chloride at 0 °C onto the surface of the base film. After reacting for 5 minutes, immediately remove the excess solution to obtain a PA membrane.

[0073] Example 6

[0074] Prepare the polyamide membrane according to the method of Example 1, except that step two is replaced with: S2. Film formation: Fix the above-mentioned base film in a frame, first soak it in an aqueous solution containing 1 mmol / L cetylpyridinium chloride and 1 wt% cyclanoline for 1 minute, blow off the excess aqueous solution with an air gun, and then pour a n-hexane solution containing 0.1 wt% terephthaloyl chloride at 0 °C onto the surface of the base film. After reacting for 5 minutes, immediately remove the excess solution to obtain a PA membrane.

[0075] Control group

[0076] Prepare the polyamide membrane according to the method of Example 1, except that step two is replaced with: S2. Film formation: Fix the above-mentioned base film in a frame, first soak it in an aqueous solution containing 2 wt% cyclanoline for 2 minutes, blow off the excess aqueous solution with an air gun, and then pour a n-hexane solution containing 0.2 wt% terephthaloyl chloride onto the surface of the base film. After reacting for 5 minutes, immediately remove the excess solution to obtain a PA membrane.

[0077] The separation performance of the polyamide membrane prepared by the present invention is shown in Table 1.

[0078] Table 1

[0079]

[0080] a Test conditions: Use a 1 g / L MgCl2 or LiCl aqueous solution as the test solution, at an operating pressure of 5 bar, 25 °C, continuously cross-flow filter the prepared polyamide nanofiltration membrane for 2 hours to test the performance;

[0081] b Test conditions: A mixed salt solution of MgCl2 and LiCl with a total concentration of 2 g / L and a mass ratio of Mg 2+ to Li + of 20 is used as the test solution. At an operating pressure of 5 bar, 25 °C, continuously cross-flow filter the prepared polyamide nanofiltration membrane for 2 hours to test the Li + and Mg 2 + retention rate, and calculate the ion separation selectivity.

[0082] As can be seen from Table 1, under the test conditions of the mixed salt solution, the prepared polyamide nanofiltration membrane for lithium-magnesium separation has a high Mg 2+ rejection rate and a low Li + rejection rate. The calculated selectivity is between 10 and 60, indicating that the polyamide membrane obtained in the present invention has application potential in lithium-magnesium separation and can achieve high-selectivity screening of lithium and magnesium.

[0083] In summary, the advantages of the present invention are as follows:

[0084] (1) By synergistically regulating the aqueous phase and the organic phase, the present invention reduces the gap between the monomer diffusion rate and the reaction rate, and further adjusts the pore size distribution of the formed polyamide dense layer. The feature of this invention is that the formed polyamide nanofiltration membrane has a narrow pore size distribution, effectively improving the lithium-magnesium separation selectivity while ensuring a high water flux.

[0085] (2) The preparation method of the polyamide nanofiltration membrane provided by the present invention is simple, with low energy consumption, and the preparation process is easy to scale up industrially, having extremely high industrial application value.

[0086] (3) The polyamide nanofiltration membrane prepared by the present invention has excellent permeability and lithium-magnesium separation performance. When the feed liquid is under single-salt conditions (single-salt concentration is 1 g / L, single salts are MgCl2 and LiCl respectively, and the test pressure is 5 bar), the MgCl2 rejection rate is 94% - 99%, the LiCl rejection rate is 23% - 40%, and the water flux is 60 - 100 L·m -2 ·h -1 .

[0087] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A preparation method of a polyamide nanofiltration membrane, characterized in that, It includes the following steps: a) Mix cyclen, cetylpyridinium chloride and water, and obtain an amine solution after dissolution; b) Immerse the PAN-based membrane in the above amine solution for the first reaction, take it out and dry it, and then carry out the second reaction with a n-hexane solution of terephthaloyl chloride to obtain a PA membrane; c) Wash the PA membrane with a n-hexane solution and then carry out heat treatment to obtain a polyamide nanofiltration membrane.

2. The preparation method according to claim 1, characterized in that, In step a), the concentration of cyclen in the amine solution is 0.25 wt% to 2 wt%, and the concentration of cetylpyridinium chloride is 0.1 mmol / L to 1 mmol / L.

3. The preparation method according to claim 1, characterized in that, Before the PAN-based membrane is immersed in step b), it further includes: Pre-treat the PAN-based membrane; the specific process of the pre-treatment is: Immerse the commercial PAN membrane in isopropanol, take it out after 20 min to 40 min and rinse it with water until there is no residual isopropanol, and dry it at 20°C to 30°C to obtain the pre-treated PAN-based membrane.

4. The preparation method according to claim 1, characterized in that, In step b), the time of the first reaction is 1 min to 2 min.

5. The preparation method according to claim 1, wherein In step b), the concentration of the n-hexane solution of terephthaloyl chloride is 0.05 wt% to 0.25 wt%, and the temperature is 0°C to 5°C.

6. The preparation method according to claim 1, wherein, In step b), the time of the second reaction is 2 min to 5 min.

7. According to the preparation method described in claim 1, characterized in that In step c), the time of washing with the n-hexane solution is 0.1 min to 1 min.

8. The preparation method according to claim 1, characterized in that, In step c), the temperature of the heat treatment is 50°C to 70°C, and the time is 10 min to 30 min.

9. A polyamide nanofiltration membrane, characterized in that, Prepared by the preparation method described in any one of claims 1 to 8.

10. Use of the polyamide nanofiltration membrane described in claim 9 in lithium-magnesium separation.

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