Preparation method of magnesium-lithium separation nanofiltration membrane and application thereof in magnesium-lithium separation

Abstract

The application discloses a preparation method of a magnesium-lithium separation nanofiltration membrane and application of the magnesium-lithium separation nanofiltration membrane in magnesium-lithium separation. The magnesium-lithium separation nanofiltration membrane is composed of an ultrafiltration membrane supporting base membrane and a magnesium-lithium separation polyamide active layer which is combined on the surface of the ultrafiltration membrane supporting base membrane. In the application, the polyelectrolyte modified ultrafiltration base is used to control interface polymerization to prepare the polyamide nanofiltration membrane, and then the polyamide nanofiltration membrane is post-modified by using piperazine, so that the electronegativity of the membrane surface is weakened, and a nano Turing structure which can increase the water area on the membrane surface and thus improve the membrane flux is generated. The magnesium-lithium separation nanofiltration membrane disclosed in the application has the characteristics of weak surface negative electricity, small membrane hole and narrow pore size distribution, so that the magnesium-lithium separation nanofiltration membrane has high magnesium-lithium separation performance while ensuring the water flux and Na2SO4 retention performance.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation technology, specifically relating to a method for preparing a magnesium-lithium separation nanofiltration membrane and its application in magnesium-lithium separation. Background Technology

[0002] Lithium-ion batteries have become a major advancement in the field of new energy development due to their impressive energy storage performance. Lithium ions are a crucial component of these batteries, and market demand is substantial. Natural lithium is primarily found in the ores and brines of salt lakes. Given the abundant lithium reserves in salt lakes and their high energy efficiency, researchers are actively advancing technologies for lithium extraction from them. Considering the generally high proportion of magnesium-lithium ions in salt lakes, effective magnesium-lithium separation technology has become crucial. Nanofiltration (NF) membrane separation technology is considered a very promising method for magnesium-lithium separation due to its cost-effectiveness, ease of operation, environmental friendliness, and especially its superior ion separation capabilities.

[0003] NF membranes typically consist of a support layer and an active layer, the latter primarily determining the membrane's separation performance. Currently, polyamide (PA) NF membranes, produced via interfacial polymerization (IP) of piperazine (PIP) and trimesoyl chloride (TMC), are the most common type of NF membrane. PANF membranes possess a strong negative surface potential due to the carboxyl groups formed by the hydrolysis of TMC. However, this negatively charged surface gives rise to Mg based on the Donnan effect. 2+ Effective retention of magnesium and lithium presents challenges, and the Donnan effect is crucial for magnesium-lithium separation. Therefore, researchers explored developing positively charged NF membranes using IP technology to improve magnesium-lithium separation. However, the electrostatic interaction of the positive charge on the NF membrane surface with anions negatively impacts their retention.

[0004] The substrate is a key element of the NF membrane, significantly influencing the interfacial polymerization process and the properties of the resulting PA layer. Major substrate characteristics such as zeta potential, porosity, and hydrophilicity play crucial roles in determining monomer diffusion and PA layer properties. Therefore, substrate modification is an effective method for preparing magnesium-lithium separation NF membranes. Furthermore, post-modification of NF membranes has been considered an effective strategy to reduce surface electronegativity. This is mainly achieved by consuming residual acyl chloride groups after interfacial polymerization and reducing carboxyl groups generated by hydrolysis, thereby reducing the surface electronegativity of the NF membrane. In view of this, our strategy involves modifying the substrate during interfacial polymerization to alter the properties of the PA layer. Subsequently, post-modification is used to shield the surface negative charge, thereby achieving efficient separation of magnesium and lithium by the NF membrane. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing a magnesium-lithium separation NF membrane and its application in magnesium-lithium separation. This invention achieves a high-efficiency magnesium-lithium separation NF membrane through a two-step modification method combining polyelectrolyte regulation and NF membrane surface modification. The membrane exhibits high permeability and high retention capacity, ensuring the general performance of NF membranes (water flux and Na2SO4 retention, etc.) while also providing highly efficient magnesium-lithium separation performance.

[0006] The magnesium-lithium separation NF membrane of the present invention includes an ultrafiltration (UF) support substrate and a magnesium-lithium separation polyamide active layer composited on the surface of the UF support substrate. The UF support substrate consists of a substrate and a UF active layer composited on the surface of the substrate.

[0007] The method for preparing the magnesium-lithium separation NF membrane of the present invention firstly modifies the UF support substrate with polyelectrolyte to obtain the modified UF support substrate; then, the modified UF support substrate is soaked in an aqueous phase solution to obtain a modified UF support substrate wetted with aqueous phase monomers, and then soaked in an organic phase solution for interfacial polymerization; finally, the prepared NF membrane is post-modified with a post-modified amine solution to obtain the magnesium-lithium separation NF membrane.

[0008] Specifically, the steps include the following:

[0009] Step 1: Dissolve the polyelectrolyte in a sodium chloride solution to obtain a polyelectrolyte solution;

[0010] Step 2: Dissolve the amine monomer in water to obtain an aqueous solution, which is also used as the post-modified amine solution;

[0011] Step 3: Dissolve the acyl chloride monomer in an organic solvent to obtain an organic phase solution;

[0012] Step 4: Immerse the UF support substrate in the polyelectrolyte solution prepared in Step 1 to obtain the modified UF support substrate;

[0013] Step 5: Immerse the modified UF support substrate in the aqueous solution prepared in step 2 to obtain a modified UF support substrate wetted with aqueous monomers;

[0014] Step 6: Immerse the modified UF support substrate obtained in Step 5 with the aqueous phase monomer in the organic phase solution prepared in Step 3 to carry out interfacial polymerization reaction to obtain a one-step modified polyamide film composite NF film.

[0015] Step 7: The polyamide film composite NF membrane is post-modified using the post-modified amine solution prepared in step 2 to obtain a magnesium-lithium separation NF membrane.

[0016] The polyelectrolyte is a cationic polyelectrolyte.

[0017] Furthermore, the polyelectrolyte is polydimethyldiallylammonium chloride (PDADMAC), with the following general structural formula:

[0018]

[0019] The amine monomer is PIP, and the acyl chloride monomer is TMC.

[0020] In the polyelectrolyte solution prepared in step 1, the concentration of the polyelectrolyte is 0.1–10 g / L, and the concentration of the NaCl solution is 0.1–1 mol / L.

[0021] The concentration of amine monomers in the aqueous solution prepared in step 2 is 0.1–8.0 wt%; the concentration of amine monomers in the post-modified amine solution is 0.1–8.0 wt%.

[0022] In the organic phase solution prepared in step 3, the concentration of acyl chloride monomer is 0.05–0.5 wt%.

[0023] In step 4, the soaking time is 1 to 60 minutes.

[0024] Step 4 further includes cleaning the UF support substrate before it comes into contact with the polyelectrolyte solution.

[0025] In step 5, the soaking time is 10 to 300 seconds.

[0026] In step 6, the reaction time for the interfacial polymerization reaction is 10–300 s.

[0027] In step 7, the post-modification time is 1 to 15 minutes.

[0028] The magnesium-lithium separation polyamide active layer of this invention has a thickness of 50-200 nm, a molecular weight cutoff of 150-500 Da, and an average membrane pore size of 0.1-1.2 nm.

[0029] The present invention relates to the application of the magnesium-lithium separation NF membrane in magnesium-lithium separation.

[0030] In the preparation process of the high-efficiency magnesium-lithium separation NF membrane of this invention, the polyelectrolyte is used to modify the substrate UF membrane, increasing the substrate's capacity for aqueous monomers. Furthermore, the post-modification of the PA layer involves reacting unreacted acyl chloride groups from interfacial polymerization with amino groups on its surface, consuming the residual acyl chloride groups and achieving charge shielding on the membrane surface.

[0031] This invention employs a combination of polyelectrolyte regulation and surface modification to prepare magnesium-lithium separation NF membranes. The UF support substrate is modified using a PDADMAC solution, commonly used in layer-by-layer self-assembly of NF membranes. The hydrophilicity and positive charge of PDADMAC alter the surface properties of the UF support substrate. The modified substrate exhibits higher PIP capacity; during the IP process, more PIPs react with TMC, forming denser crosslinks. Furthermore, the more PIPs involved in the reaction, the fewer residual acyl chloride groups remain, resulting in a weaker electronegativity in the PA layer. Finally, the PA layer surface of the NF membrane is post-modified using a PIP solution commonly used in interfacial polymerization for NF membrane preparation. This consumes unreacted acyl chloride groups from the IPs, achieving shielding of the negative charge on the membrane surface and enhancing the magnesium-lithium separation performance of the NF membrane. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the preparation process of the present invention.

[0033] Figure 2 The degree of crosslinking of the PA layer and the fine O1s spectrum of the control membrane in Comparative Example 1, the one-step modified membrane in Example 1, and the two-step modified membrane in Example 2. Figure 2 (ac) represents the degree of PA layer crosslinking of the control membrane in Comparative Example 1, the one-step modified membrane in Example 1, and the two-step modified membrane in Example 2; (df) represents the O1s spectrum of the control membrane in Comparative Example 1, the one-step modified membrane in Example 1, and the two-step modified membrane in Example 2.

[0034] Figure 3 The NF performance of the control membrane in Comparative Example 1 and the modified membranes in Examples 1, 3 and 4 of this invention is shown. Figure 3 (a) represents the flux of the control membrane in Comparative Example 1 and the modified membranes in Examples 1, 3 and 4; (b) represents the salt rejection performance of the control membrane in Comparative Example 1 and the modified membranes in Examples 1, 3 and 4.

[0035] Figure 4 The NF performance of the control membranes in Comparative Examples 1 and 2 and the modified membranes in Examples 1, 2 and 5 of this invention is shown. Figure 4 (a) represents the flux of the control membrane in Comparative Examples 1 and 2 and the modified membrane in Examples 1, 2 and 5; (b) represents the salt rejection performance of the control membrane in Comparative Example 1 and the modified membrane in Examples 1, 2 and 5.

[0036] Figure 5 The magnesium-lithium separation performance of the control membrane in Comparative Example 1 and the modified membranes in Examples 1 and 2 of this invention is shown. Figure 5 (a) shows the retention performance of the control membrane in Comparative Example 1 and the modified membranes in Examples 1 and 2 for MgCl2 and LiCl; (b) shows the magnesium-lithium separation factor of the modified membranes in Examples 1 and 2. Detailed Implementation

[0037] To illustrate the present invention in more detail, a detailed description will be provided below, but it is not limited thereto. All matters not described in detail in the present invention are based on conventional techniques in the art.

[0038] Research has revealed that the properties of the substrate, a crucial component of the NF membrane, significantly influence the interfacial polymerization process and the PA layer. For instance, the zeta potential, porosity, and hydrophilicity of the substrate surface affect monomer diffusion, thereby altering the properties of the PA layer. Therefore, substrate modification can serve as a method for designing magnesium-lithium separation NF membranes. Furthermore, post-modification of the NF membrane is considered an effective strategy for reducing the electronegativity of the membrane surface. Post-modification can consume residual acyl chloride groups after IP (Integrated Polymerization), leading to a reduction in carboxyl groups generated by hydrolysis, thus weakening the electronegativity of the NF membrane surface. Therefore, we modulate the IP process through substrate modification to alter the properties of the PA layer, and then achieve efficient magnesium-lithium separation using the NF membrane through post-modification to shield the charge.

[0039] In this application, the magnesium-lithium separation NF membrane includes a UF support substrate and a magnesium-lithium separation polyamide active layer composited on the surface of the UF support substrate.

[0040] The UF support substrate is preferably a UF film with a nonwoven fabric as the base and a UF active layer as the top layer. In this application, the UF support substrate is a polyethersulfone UF (PES UF) support substrate, with a nonwoven fabric as the base and a PES active layer as the active layer. The thickness of the PES active layer is 200–400 μm.

[0041] The thickness of the magnesium-lithium separated polyamide active layer is 50–200 nm, the molecular weight cutoff is 150–500 Da, and the average membrane pore size is 0.1–1.2 nm; further, the thickness of the magnesium-lithium separated polyamide active layer is 100–200 nm, the molecular weight cutoff is 200–400 Da, and the average membrane pore size is 0.30–0.60 nm; even further, the thickness of the magnesium-lithium separated polyamide active layer is 150–200 nm, the molecular weight cutoff is 200–300 Da, and the average membrane pore size is 0.35–0.45 nm.

[0042] The method for preparing the magnesium-lithium separation NF membrane of the present invention includes the following steps:

[0043] Step 1: Dissolve the polyelectrolyte in a sodium chloride solution to obtain a polyelectrolyte solution;

[0044] Step 2: Dissolve the amine monomer in water to obtain an aqueous solution, which is also used as the post-modified amine solution;

[0045] Step 3: Dissolve the acyl chloride monomer in an organic solution to obtain an organic phase solution;

[0046] Step 4: Immerse the UF support substrate in the polyelectrolyte solution prepared in Step 1 to obtain a modified UF support substrate wetted with polyelectrolyte;

[0047] Step 5: Immerse the modified UF support substrate membrane wetted with polyelectrolyte in the aqueous solution prepared in step 2 to obtain the modified UF support substrate membrane wetted with aqueous monomer.

[0048] Step 6: Immerse the modified UF support substrate obtained in Step 5 with the aqueous phase monomer in the organic phase solution prepared in Step 3 to carry out interfacial polymerization reaction to obtain a one-step modified polyamide film composite NF film.

[0049] Step 7: Immerse the polyamide film composite NF membrane in the post-modified amine solution prepared in Step 2 for post-modification to obtain a magnesium-lithium separation NF membrane.

[0050] In the process of preparing the magnesium-lithium separation NF membrane, the present invention first prepares a polyelectrolyte solution for modifying the substrate UF membrane, that is, the polyelectrolyte is added to a NaCl salt solution; then, an aqueous phase solution (post-modified amine solution) and an organic phase solution are prepared; that is, the amine monomer is dissolved in water to obtain an aqueous phase solution, which is also used as a post-modified amine solution, and the acyl chloride monomer is dissolved in an organic solution to obtain an organic phase solution.

[0051] In this application, the polyelectrolyte is a cationic polyelectrolyte well known to those skilled in the art, which, in addition to being positively charged, also possesses a certain degree of hydrophilicity. Specifically, the amine monomer can be selected from PIP, the acyl chloride monomer can be selected from TMC, and the post-modified amine solution can be selected from PIP. Furthermore, the concentration of the polyelectrolyte solution is 0.1–10.0 g / L, the concentration of the NaCl solution is 0.1–1 mol / L, the concentration of the PIP monomer in the aqueous phase is 0.1–8.0 wt%, the concentration of the acyl chloride monomer in the organic phase is 0.05–0.5 wt%, and the concentration of PIP in the post-modified solution is 0.1–8.0 wt%. Further, the concentration of the polyelectrolyte solution is 0.1–5.0 g / L, the concentration of the NaCl solution is 0.1–0.8 mol / L, the concentration of the PIP monomer in the aqueous phase is 0.5–2.0 wt%, the concentration of the acyl chloride monomer in the organic phase is 0.1–0.3 wt%, and the concentration of the PIP in the post-modification solution is 0.5–5.0 wt%. Even further, the concentration of the polyelectrolyte solution is 0.1–5.0 g / L, the concentration of the NaCl solution is 0.2–0.5 mol / L, the concentration of the PIP monomer in the aqueous phase is 0.8–1.0 wt%, the concentration of the acyl chloride monomer in the organic phase is 0.1–0.2 wt%, and the concentration of the PIP in the post-modification solution is 1.0–2.0 wt%.

[0052] In this application, the UF supporting substrate is modified with a polyelectrolyte to obtain a modified UF supporting substrate. The modified UF supporting substrate is then immersed in an aqueous solution to obtain a modified UF supporting substrate wetted with aqueous monomers. This membrane is then immersed in an organic solution for interfacial polymerization. Finally, the prepared membrane is modified with a post-modified amine solution to obtain a magnesium-lithium separation NF membrane. During this process, the UF supporting substrate is preferentially cleaned before contacting the polyelectrolyte solution. Specifically, the cleaning involves immersing the UF supporting substrate in 25 wt% isopropanol and then rinsing it with deionized water. The modification time is 1–100 min, the immersion time is 10–300 s, the interfacial polymerization time is 10–300 s, and the post-modification time is 1–15 min. Alternatively, the modification time may be 5–50 min, the immersion time 50–200 s, the interfacial polymerization time 10–100 s, and the post-modification time 1–10 min. Furthermore, the modification time is 30 min, the soaking time is 90 s, the interfacial polymerization reaction time is 30 s, and the post-modification time is 5 min. The active layer in the magnesium-lithium separation NF membrane of this application undergoes a two-step modification treatment. In the first step, the concentrations of the modified polyelectrolytes are 0.2 g / L, 1.0 g / L, and 4.0 g / L, respectively, and in the second step, the concentrations of the post-modification solutions are 1.0 wt% and 2.0 wt%, respectively.

[0053] To further explain the present invention, the following detailed description of the magnesium-lithium separation NF membrane, its preparation method, and its application is provided in conjunction with embodiments. The scope of protection of the present invention is not limited by the following embodiments.

[0054] Example 1:

[0055] The preparation steps of the magnesium-lithium separation NF membrane in this embodiment are as follows:

[0056] 1. Soak the commercial PES UF film in 25wt% isopropanol for 30 minutes to wash away the preservative on the film surface; then rinse it several times with deionized water to obtain a usable PES UF film; prepare several glass plates and wash them.

[0057] 2. Place the PES UF membrane from step 1 with the active layer facing up onto the clean glass plate from step 1 for later use;

[0058] 3. Weigh out a certain amount of polyelectrolyte and dissolve it in NaCl solution. The concentration of polyelectrolyte is 0.2 g / L and the concentration of NaCl solution is 0.5 mol / L. Weigh out a certain amount of PIP monomer and dissolve it in deionized water as the aqueous phase solution for interfacial polymerization. The monomer mass fraction is 1 wt%. Weigh out a certain amount of TMC and dissolve it in n-hexane solution. The mass fraction of TMC is 0.15 wt%. This is used as the organic phase solution for interfacial polymerization.

[0059] 4. Pour the polyelectrolyte solution prepared in step 3 onto the surface of the PES UF membrane prepared in step 2, soak it at room temperature for 5 minutes, and then pour off the polyelectrolyte solution on the membrane surface to obtain the polyelectrolyte-modified PES UF membrane.

[0060] 5. Pour the aqueous solution prepared in step 3 onto the surface of the modified PES UF membrane obtained in step 4. After soaking at room temperature for 90 seconds, pour off the aqueous solution on the membrane surface. After standing for 120 seconds, use a rubber roller to remove the residual aqueous solution on the membrane surface to obtain a modified PES UF membrane impregnated with aqueous monomers.

[0061] 6. Immerse the modified PES UF membrane with the aqueous monomer obtained in step 5 in the organic phase solution prepared in step 3, and carry out the interfacial polymerization reaction at room temperature for 30 seconds, and then take it out to obtain a PANF membrane with a polyamide active layer formed on the PES UF membrane.

[0062] 7. Air dry the PANF membrane prepared in step 6 at room temperature for 3 minutes, wash it several times with deionized water, and then store it in deionized water.

[0063] Experimental testing showed that the pure water flux of the magnesium-lithium separation NF membrane prepared in this embodiment was 7.1 L·m⁻¹. -2 ·h -1 ·bar -1 The retention rates were as follows: Na₂SO₄ 99.4%, MgCl₂ 86.6%, CaCl₂ 72.4%, NaCl 54.3%, and LiCl 37.7%. The magnesium-lithium separation factors were 7.2, 9.3, 12.6, and 24.5 for magnesium-lithium mass ratios of 5, 10, 20, and 50, respectively.

[0064] Example 2:

[0065] The preparation steps of the magnesium-lithium separation NF membrane in this embodiment are as follows:

[0066] 1. Soak the commercial PES UF film in 25wt% isopropanol for 30 minutes to wash away the preservative on the film surface; then rinse it several times with deionized water to obtain a usable PES UF film; prepare several glass plates and wash them.

[0067] 2. Place the PES UF membrane from step 1 with the active layer facing up onto the clean glass plate from step 1 for later use;

[0068] 3. Weigh out a certain amount of polyelectrolyte and dissolve it in NaCl solution, with a polyelectrolyte concentration of 0.2 g / L and a NaCl solution concentration of 0.5 mol / L; weigh out a certain amount of PIP monomer and dissolve it in deionized water as the aqueous phase solution for interfacial polymerization, with a monomer mass fraction of 1 wt%; weigh out a certain amount of TMC and dissolve it in n-hexane solution, with a TMC mass fraction of 0.15 wt%, as the organic phase solution for interfacial polymerization; weigh out a certain amount of PIP monomer and dissolve it in deionized water as the post-modification solution, with a monomer mass fraction of 1 wt%.

[0069] 4. Pour the polyelectrolyte solution prepared in step 3 onto the surface of the PES UF membrane prepared in step 2, soak it at room temperature for 5 minutes, and then pour off the polyelectrolyte solution on the membrane surface to obtain the polyelectrolyte-modified PES UF membrane.

[0070] 5. Pour the aqueous solution prepared in step 3 onto the surface of the modified PES UF membrane obtained in step 4. After soaking at room temperature for 90 seconds, pour off the aqueous solution on the membrane surface. After standing for 120 seconds, use a rubber roller to remove the residual aqueous solution on the membrane surface to obtain a PES UF membrane wetted with aqueous monomers.

[0071] 6. Immerse the PES UF membrane with aqueous monomers obtained in step 5 in the organic phase solution prepared in step 3, and carry out interfacial polymerization reaction at room temperature for 30 seconds, and then take it out to obtain a PANF membrane with a polyamide active layer formed on the PES UF membrane.

[0072] 7. Pour the post-modified amine solution prepared in step 3 onto the surface of the PANF membrane prepared in step 6, treat for 5 minutes, and then pour off the post-modified solution on the surface to obtain the post-modified PANF membrane.

[0073] 8. The post-modified PANF membrane prepared in step 7 is air-dried at room temperature for 3 minutes, washed multiple times with deionized water, and then stored in deionized water.

[0074] Experimental testing showed that the pure water flux of the magnesium-lithium separation NF membrane prepared in this embodiment was 7.3 L·m⁻¹. -2 ·h -1 ·bar -1 The retention rates were as follows: Na2SO4 98.5%, MgCl2 94.7%, CaCl2 87.8%, NaCl 43.7%, and LiCl 38.3%. The magnesium-lithium separation factors were 15.3, 37.8, 44.3, and 63.7 when the magnesium-lithium mass ratio was 5, 10, 20, and 50, respectively.

[0075] Example 3:

[0076] The preparation method steps in this embodiment are the same as those in Example 1, except that:

[0077] In Example 1, the concentration of the polyelectrolyte solution in step 3 is 1.0 g / L, and the conditions for other steps are the same;

[0078] Experimental testing showed that the modified NF membrane prepared in this embodiment had a pure water flux of 5.7 L·m⁻¹. -2 ·h -1 ·bar -1 The rejection rates were as follows: Na₂SO₄ 95.4%, MgCl₂ 86.5%, CaCl₂ 78.3%, and NaCl 50.9%.

[0079] Example 4:

[0080] The preparation method steps in this embodiment are the same as those in Example 1, except that:

[0081] In Example 1, the concentration of the polyelectrolyte solution in step 3 was 4.0 g / L, and the conditions for other steps were the same;

[0082] Experimental testing showed that the modified NF membrane prepared in this embodiment had a pure water flux of 6.9 L·m⁻¹. -2 ·h -1 ·bar -1 The rejection rates were as follows: Na₂SO₄ 97.3%, MgCl₂ 85.7%, CaCl₂ 78.6%, and NaCl 41.4%.

[0083] Example 5:

[0084] The preparation method steps in this embodiment are the same as those in Example 2, except that:

[0085] In Example 2, the concentration of PIP in the post-modified amine solution in step 3 was 2.0 wt%, and the conditions for other steps were the same;

[0086] Experimental testing showed that the modified NF membrane prepared in this embodiment had a pure water flux of 7.1 L·m⁻¹. -2 ·h -1 ·bar -1 The rejection rates were as follows: Na₂SO₄ 98.8%, MgCl₂ 91.4%, CaCl₂ 78.8%, and NaCl 43.5%.

[0087] Comparative Example 1:

[0088] The preparation conditions for the PANF membrane, which serves as a blank control, are as follows:

[0089] 1. Soak the commercial PES UF film in 25wt% isopropanol for 30 minutes to wash away the preservative on the film surface; then rinse it several times with deionized water to obtain a usable PES UF film; prepare several glass plates and wash them.

[0090] 2. Place the PES UF membrane from step 1 with the active layer facing up onto the clean glass plate from step 1 for later use;

[0091] 3. Weigh a certain amount of PIP monomer and dissolve it in deionized water to serve as the aqueous phase solution for interfacial polymerization, with a monomer mass fraction of 1 wt%; weigh a certain amount of TMC and dissolve it in n-hexane solution, with a TMC mass fraction of 0.15 wt%, to serve as the organic phase solution for interfacial polymerization.

[0092] 4. Pour the aqueous solution prepared in step 3 onto the surface of the PES UF membrane prepared in step 2. After soaking at room temperature for 90 seconds, pour off the aqueous solution on the membrane surface. After standing for 120 seconds, use a rubber roller to remove the residual aqueous solution on the membrane surface to obtain a PES UF membrane wetted with aqueous monomers.

[0093] 5. Immerse the PES UF membrane with aqueous monomers obtained in step 4 in the organic phase solution prepared in step 3, and carry out interfacial polymerization reaction at room temperature for 30 seconds, and then take it out to obtain a PANF membrane with a polyamide active layer formed on the PES UF membrane.

[0094] 6. Air dry the PANF membrane prepared in step 5 at room temperature for 3 minutes, wash it several times with deionized water, and then store it in deionized water.

[0095] Experimental testing showed that the pure water flux of the NF membrane prepared in the comparative example was 8.1 L·m⁻¹. -2 ·h -1 ·bar -1 The retention rates were as follows: Na₂SO₄ 97.9%, MgCl₂ 46.1%, CaCl₂ 38.3%, NaCl 45.8%, and LiCl 33.8%.

[0096] Comparative Example 2:

[0097] The preparation method described in Comparative Example 1 is the same as the steps described above, except that:

[0098] The PANF membrane prepared in step 5 of Comparative Example 1 was the same as that in Example 2, and was treated with a 1 wt% PIP-modified amine solution for 5 min, with other conditions being the same.

[0099] Experimental testing showed that the modified NF membrane prepared in this comparative example had a pure water flux of 9.1 L·m⁻¹. -2 ·h -1 ·bar-1 The rejection rates were as follows: Na₂SO₄ 98.7%, MgCl₂ 63.8%, CaCl₂ 58.4%, and NaCl 44.7%.

[0100] Experimental Example 1:

[0101] The film samples obtained in Example 1 and Comparative Examples 1 and 2 were characterized by X-ray photoelectron spectroscopy (XPS), as shown in the attached figures. Figure 2 As shown, Figure 2 XPS data analysis was performed on the membrane samples obtained in Example 1 and Comparative Examples 1 and 2. Here, (ac) represents the ratio of C, N, and O elements in the PA layer of the membrane samples obtained in Example 1 and Comparative Examples 1 and 2, and the degree of crosslinking of the PA layer calculated based on the ratio; (df) represents the fine O1s spectra of the membrane samples obtained in Example 1 and Comparative Examples 1 and 2. From... Figure 2 As can be seen from (ac), the PA layer of the film in Example 2, due to the presence of more PIP participating in IP, introduces more C element along with N element, ultimately resulting in a lower N element content; furthermore, after PDADMAC modification of the substrate, the crosslinking degree of the PA layer of the film in Example 1 increased, and after treatment with the PIP modification solution, the crosslinking degree of the PA layer of the film in Example 2 further increased. From Figure 2 As can be seen from (df), the carboxyl content on the surface of the PA layer of the film in Example 2 is reduced. The carboxyl groups generated by the hydrolysis of carboxyl groups will make the film surface negatively charged. The carboxyl groups are generated by the hydrolysis of unreacted acyl chloride groups. The reduction of acyl chloride groups is caused by the consumption of residual acyl chloride groups after PIP modification, indicating that PIP modification can achieve charge shielding.

[0102] Experimental Example 2:

[0103] The water permeability and salt rejection performance of the PANF membranes prepared in the examples and comparative examples were tested using an NF cross-flow filtration system to determine the changes in the balance between permeability and selectivity of the magnesium-lithium membrane and to optimize the best conditions for the two-step modification.

[0104] The blank control membrane of Comparative Example 1 and the modified membranes from Examples 1, 3, and 4, stored in deionized water, were removed and cut to a size suitable for the filtration system (effective membrane area 2.94 cm²). 2 The pure water flux was determined using deionized water at 25°C and a flow rate of 30 L / h. The membrane was first compacted at 10 bar until the flux stabilized, and then the pure water flux and single salt rejection performance were tested at 6 bar. The feed solutions were 10 mM solutions of Na₂SO₄, MgCl₂, CaCl₂, and NaCl, respectively. Figure 3 As shown in (a), the water flux of the modified membranes in Examples 1, 3, and 4 decreased compared to the control membrane in the comparative example; Figure 3 As shown in Figure (b), the modified membranes of Examples 1, 3, and 4 exhibit high salt rejection performance, especially with a significant improvement in the rejection performance of MgCl2 and CaCl2. Considering both membrane flux and salt rejection performance, 0.2 g / L is the optimal modification concentration for PDADMAC in Example 1.

[0105] The control membranes from Comparative Examples 1 and 2, and the modified membranes from Examples 1, 2, and 5, stored in deionized water, were removed and cut to fit the size of the filtration system (effective membrane area 2.94 cm²). 2 The pure water flux was determined using deionized water at 25°C and a flow rate of 30 L / h. The membrane was first compacted at 10 bar until the flux stabilized, and then the pure water flux and single salt rejection performance were tested at 6 bar. The feed solutions were 10 mM solutions of Na₂SO₄, MgCl₂, CaCl₂, and NaCl, respectively. Figure 4 As shown in Figure (a), the water flux of the membrane in Comparative Example 2 is higher than that of the membrane in Comparative Example 1; the water flux of the membranes in Examples 2 and 5 is also slightly higher than that of the membrane in Example 1, indicating that the modification after PIP helps to increase the membrane flux; Figure 4 As shown in (b), after PIP modification, the MgCl2 and CaCl2 retention performance of the membrane in Comparative Example 2 was improved to a certain extent. The membranes in Examples 2 and 5 had high MgCl2 retention performance, indicating that PIP modification can indeed improve the MgCl2 and CaCl2 retention performance of the NF membrane. However, PIP modification alone cannot achieve a high MgCl2 retention. Since the MgCl2 retention rates of the membranes in Examples 2 and 5 are similar, the optimal concentration of the PIP modification solution is 1 wt%.

[0106] Experimental Example 3:

[0107] The magnesium-lithium separation performance of the magnesium-lithium separation membrane was evaluated using single salt solutions of MgCl2 and LiCl, and mixed salt solutions, respectively, as feed solutions.

[0108] The control membrane from Comparative Example 1 and the modified membranes from Examples 1 and 2, stored in deionized water, were removed and cut to a size suitable for the filtration system (effective membrane area 2.94 cm²). 2 The pure water flux was determined using deionized water at 25°C and a flow rate of 30 L / h. The membrane was first compacted at 10 bar until the flux stabilized, and then the membrane's MgCl2 and LiCl retention performance was tested at 6 bar. The feed solutions were 10 mM MgCl2 and LiCl salt solutions, respectively. Figure 5As shown in Figure (a), the MgCl2 and LiCl rejection rates of the membrane in Comparative Example 1 are similar, while the MgCl2 and LiCl rejection rates of the membranes in Examples 1 and 2 differ significantly. Obviously, the membranes in Examples 1 and 2 have the potential for magnesium-lithium separation.

[0109] The magnesium-lithium separation performance test and the salt rejection performance test were conducted under the same conditions, except that the feed solution was a mixed solution of magnesium and lithium with different mass ratios; for example... Figure 5 As shown in Figure (b), under different magnesium-lithium mass ratios, the membrane of Example 2 exhibited a higher magnesium-lithium separation factor than the membrane of Example 1, and the membrane of Example 2 had high-efficiency magnesium-lithium separation performance.

[0110] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of these embodiments are merely to aid in understanding the method and core ideas of the present invention, including the best mode, and to enable any person skilled in the art to practice the present invention, including manufacturing and using any device or system, and implementing any combined method. It should be noted that those skilled in the art can make various improvements and modifications to the present invention without departing from its principles, and these improvements and modifications also fall within the scope of protection of the claims. The scope of protection of this patent is defined by the claims and may include other embodiments that can be conceived by those skilled in the art. If these other embodiments have structural elements similar to those expressed in the claims, or if they include equivalent structural elements that are not substantially different from those expressed in the claims, then these other embodiments should also be included within the scope of the claims.

Claims

1. A method for preparing a magnesium-lithium separation nanofiltration membrane, characterized in that: First, a polyelectrolyte-modified ultrafiltration support membrane is obtained. Then, the modified ultrafiltration support membrane is immersed in an aqueous solution containing amine monomers to obtain an aqueous monomer-wetted modified ultrafiltration support membrane. Next, it is immersed in an organic solution containing acyl chloride monomers for interfacial polymerization. Subsequently, the prepared nanofiltration membrane is post-modified using a post-modified amine solution to obtain a magnesium-lithium separation nanofiltration membrane. The polyelectrolyte is a cationic polyelectrolyte. Includes the following steps: Step 1: Dissolve the polyelectrolyte in a sodium chloride solution to obtain a polyelectrolyte solution; Step 2: Dissolve the amine monomer in water to obtain an aqueous solution, which is also used as the post-modified amine solution; Step 3: Dissolve the acyl chloride monomer in an organic solvent to obtain an organic phase solution; Step 4: Immerse the ultrafiltration support membrane in the polyelectrolyte solution prepared in Step 1 to obtain the modified ultrafiltration support membrane; Step 5: Immerse the modified ultrafiltration support membrane in the aqueous solution prepared in step 2 to obtain a modified UF support membrane wetted with aqueous monomers; Step 6: Immerse the modified ultrafiltration support membrane with aqueous phase monomers obtained in Step 5 in the organic phase solution prepared in Step 3 to carry out interfacial polymerization reaction to obtain a one-step modified polyamide film composite nanofiltration membrane. Step 7: The one-step modified polyamide film composite nanofiltration membrane is post-modified using the post-modified amine solution prepared in step 2 to obtain a magnesium-lithium separation nanofiltration membrane; The polyelectrolyte is polydimethyldiallylammonium chloride, the amine monomer is piperazine, and the acyl chloride monomer is trimesoyl chloride.

2. The preparation method according to claim 1, characterized in that: In the polyelectrolyte solution prepared in step 1, the concentration of the polyelectrolyte is 0.1~10 g / L, and the concentration of the sodium chloride solution is 0.1~1 mol / L.

3. The preparation method according to claim 1, characterized in that: The concentration of amine monomers in the aqueous phase solution prepared in step 2 is 0.1–8.0 wt%; the concentration of amine monomers in the post-modified amine solution is 0.1–8.0 wt%. In the organic phase solution prepared in step 3, the concentration of acyl chloride monomer is 0.05~0.5 wt%.

4. The preparation method according to claim 1, characterized in that: In step 4, the soaking time is 1 to 60 minutes.

5. The preparation method according to claim 1, characterized in that: In step 5, the soaking time is 10~300 s.

6. The preparation method according to claim 1, characterized in that: In step 6, the reaction time for the interfacial polymerization reaction is 10~300 s.

7. The preparation method according to claim 1, characterized in that: In step 7, the post-modification time is 1~15 min.

8. The application of the magnesium-lithium separation nanofiltration membrane prepared by any one of the preparation methods in claims 1-7 in magnesium-lithium separation.

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