A nanofiltration membrane with high lithium-magnesium separation performance and a preparation method thereof

Abstract

The application discloses a nanofiltration membrane with high lithium-magnesium separation performance and a preparation method thereof, and comprises the following steps: selecting an ultrafiltration membrane as a base membrane, preparing water phase solutions with different doping ratios by taking piperazine and polyethylene imine as basic water phase monomers, preparing an organic phase solution by taking n-hexane as a solvent and trimesoyl chloride as a solute, immersing the base membrane in a pure piperazine water phase solution for one-time interfacial polymerization, immersing the base membrane in a water phase solution doped with piperazine and polyethylene imine for two-time interfacial polymerization, and immersing the base membrane in a pure polyethylene imine water phase solution for three-time interfacial polymerization to obtain the nanofiltration membrane with high lithium-magnesium separation performance. The application realizes the differential directional construction of vertical gradient charges on the membrane surface by progressively performing multiple interfacial polymerization reactions, constructs a uniform strong positive electric layer on the surface to strengthen the electrostatic repulsion of magnesium ions, and simultaneously improves magnesium retention and lithium permeation, so that the lithium-magnesium separation performance of the nanofiltration membrane is greatly improved.

Description

Technical Field

[0001] This invention relates to the field of nanofiltration membrane wastewater treatment technology, specifically to a nanofiltration membrane with high lithium-magnesium separation performance and its preparation method. Background Technology

[0002] Lithium resources are an extremely important strategic resource and a core strategic mineral for ensuring the new energy industry and energy security. Nanofiltration membranes, as a green, energy-saving, and highly efficient separation technology, are widely used in lithium extraction projects from salt lakes. However, the low magnesium-lithium separation efficiency remains a core challenge in nanofiltration-based lithium extraction from salt lakes. Although existing research has employed various membrane modification methods to improve the lithium-magnesium separation performance of positively charged nanofiltration membranes to achieve more efficient lithium extraction from salt lakes, these methods are still limited to the overall modification of two-dimensional planar charges, greatly neglecting the intrinsic distribution and synergistic effect of charges in the horizontal and vertical directions. This makes it difficult for the prepared positively charged nanofiltration membranes to truly achieve both high magnesium retention and high lithium permeability.

[0003] Therefore, how to break through the limitations of traditional two-dimensional planar modification, utilize the distribution regulation and accessibility differences of acyl chloride sites in the separation layer to achieve differentiated directional construction of surface and depth charges, construct a uniform positive charge layer on the surface to enhance magnesium electrostatic repulsion, and construct gradient charge channels in the depth to reduce the lithium transport energy barrier, thereby simultaneously improving magnesium retention and lithium permeation, is the key scientific problem and technical bottleneck currently faced in nanofiltration lithium extraction projects in salt lakes.

[0004] In summary, this study explores a simple, controllable, and effective method for the differentiated directional construction of surface and depth charges on a membrane, aiming to enhance the electrostatic repulsion of magnesium ions and reduce the transport resistance of lithium ions, thereby simultaneously achieving a significant improvement in the selective separation performance of lithium and magnesium in the membrane. This approach has clear scientific value and broad application prospects. Summary of the Invention

[0005] Based on the above, the present invention provides a nanofiltration membrane with high lithium-magnesium separation performance and its preparation method. The method achieves differentiated directional construction of longitudinal vertical gradient charge on the membrane surface by changing the type and ratio of aqueous monomers and progressively carrying out multiple interfacial polymerization reactions. This constructs a uniform and strongly positively charged layer on the surface to enhance the electrostatic repulsion of magnesium ions and constructs a multi-level vertical gradient charge channel to reduce the lithium transport energy barrier. Thus, it simultaneously improves magnesium retention and lithium permeation, greatly enhancing the lithium-magnesium separation performance of the nanofiltration membrane.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention discloses a method for preparing a nanofiltration membrane with high lithium-magnesium separation performance, comprising the following steps: Step 1: Select an ultrafiltration membrane as the base membrane and pretreat it by immersing it in deionized water; Step 2: Using piperazine (PIP) and polyethyleneimine (PEI) as basic aqueous monomers, prepare aqueous solutions with different PIP / PEI doping ratios but the same total mass concentration; wherein, (according to the PIP:PEI percentage, they are divided into: 100% : 0%, 50% : 50%, 0% : 100%). Step 3: Prepare an organic phase solution using n-hexane as solvent and trimesoyl chloride (TMC) as solute; Step 4: Immerse the pretreated substrate film from Step 1 in an aqueous solution of pure piperazine. After removing it, use a rubber roller to remove any residual aqueous solution from the surface and immerse it in an organic solution for interfacial polymerization. After removing it, air dry it and then perform a heat curing treatment. Step 5: Immerse the membrane after the thermosetting treatment in Step 4 in an aqueous solution containing both piperazine and polyethyleneimine. After removing it, use an air knife to scrape off the residual aqueous solution on the surface. Place it flat in a dry and clean petri dish and use an airbrush to spray the organic phase solution onto the membrane surface to carry out a secondary interfacial polymerization reaction. After the membrane surface solution has dried, perform thermosetting treatment. Step 6: Immerse the membrane after the thermosetting treatment in Step 5 in an aqueous solution of pure polyethyleneimine. After taking it out, repeat the subsequent operations of Step 5. After three interfacial polymerization reactions, a nanofiltration membrane with high lithium-magnesium separation performance is obtained.

[0007] As a further improvement of the present invention, in step 1, the ultrafiltration membrane used as the base membrane is made of one or more of polyethersulfone, polyacrylonitrile, polyvinylidene fluoride, polysulfone, and polyvinyl chloride, with a molecular weight cutoff of 50-100 kDa and a size of 70-70 mm. The pretreatment operation parameters are as follows: the ultrafiltration membrane is immersed in deionized water at 15-30 °C for 48-72 h, and the deionized water is replaced every 4-24 h.

[0008] As a further improvement of the present invention, in step 2, the total mass concentration of the aqueous solution is 1.0~2.0 g / L, and the mass concentrations of the two aqueous monomers under different aqueous phase ratio scenarios are as follows: when piperazine:polyethyleneimine is 100%:0%, the concentration of piperazine is 1.0~2.0 g / L; when piperazine:polyethyleneimine is 50%:50%, the mass concentrations of piperazine and polyethyleneimine are the same, both 0.5~1.0 g / L; when piperazine:polyethyleneimine is 0%:100%, the concentration of polyethyleneimine is 1.0~2.0 g / L, and the molecular weight of polyethyleneimine is 3000.0, 5000.0 or 7000.0 Da.

[0009] As a further improvement of the present invention, in step 3, the concentration of pyromellitic acid chloride in the organic phase solution is 0.5~1.0 g / L.

[0010] As a further improvement of the present invention, in step 4, the immersion time of the substrate in the aqueous solution of pure piperazine is 2-6 min, and the temperature is 20-30 ℃; wherein, the mass concentration ratio of piperazine to polyethyleneimine in the aqueous solution of pure piperazine is 100% : 0%, and the concentration of piperazine is 1.0-2.0 g / L.

[0011] As a further improvement of the present invention, in step 4, The rubber roller with residual aqueous solution on the surface refers to the rubber roller method used to remove residual piperazine from the membrane surface. The roller is made of polyurethane, with a roller width of 8~11 cm and an operating temperature of 25~30 ℃. The operation method is as follows: place the membrane flat on the absorbent paper with the active layer facing up, and gently roll the roller along the edge of the membrane from left to right to remove the residual piperazine solution on the membrane surface until there are no obvious droplets on the membrane surface. After the rubber roller is completed, it is immersed in the organic phase solution for 1-3 minutes at a temperature of 20-30 °C. The first interfacial polymerization refers to the formation of a negatively charged polyamide separation layer on the surface of the bottom film to facilitate subsequent second and third interfacial polymerization. The air-drying and heat curing treatment refers to the evaporation of the residual n-hexane solution on the film surface after the interfacial polymerization is completed at a temperature of 25-30 °C. The heat curing treatment refers to placing the air-dried film in a clean petri dish and then drying it in an oven at a temperature of 60-80 °C for 1-3 minutes.

[0012] As a further improvement of the present invention, in step 5, The membrane after the thermosetting treatment in step 4 was immersed in an aqueous solution simultaneously doped with piperazine and polyethyleneimine to construct a weakly positively charged separation layer on top of the negatively charged separation layer. The immersion time was 1-3 min and the temperature was 25-30℃. The mass concentration ratio of piperazine to polyethyleneimine in the aqueous solution simultaneously doped with piperazine and polyethyleneimine was 50%:50%, and the mass concentrations of piperazine and polyethyleneimine were the same, both 0.5-1.0 g / L. Using an air knife to scrape off residual aqueous solution on the surface refers to using an ultra-thin air knife to remove droplets from the surface of a glass membrane until no obvious water droplets remain on the membrane surface or deep within it, leaving only a damp mark. Under strong light, the mark appears uniform, continuous, and extremely faintly moist. The operating parameters are as follows: air knife width 8.0~10.0 cm, air curtain thickness 0.1~0.2 mm, air supply pressure 0.1~0.2 MPa, distance between air knife tip and membrane surface 3.0~5.5 mm, angle between air knife and membrane surface (incident angle) 15.0~30.0°, reverse blowing mode, and membrane speed 0.2~2.0 m / min. The procedure for spraying the organic phase solution using an airbrush is as follows: Place the membrane, after being purged by an air knife, with the wet side facing upwards, into a dry, clean petri dish (inner diameter 9.0–11.0 cm). Manually and evenly spray the organic phase solution, using a laboratory gravity-type double-action airbrush containing a 0.5–1.0 g / L concentration of organic phase solution, aiming at the center of the membrane. Specific operating parameters are: nozzle orifice diameter 0.1–0.5 mm, atomizing air pressure 0.1–0.3 MPa, liquid flow rate 0.3–0.9 ml / min, nozzle-to-membrane distance 15.0–20.0 cm, and manual, even spraying 2.0–3.0 times.

[0013] As a further improvement of the present invention, in step 5, the secondary interfacial polymerization refers to the vertical construction of a weakly positively charged separation layer on the negatively charged separation layer constructed by the primary interfacial polymerization. The reaction conditions are: reaction time of 120~150s, air drying time of 10~15s, and temperature of 25~30℃; the thermosetting treatment conditions are: oven temperature of 75~85℃ and time of 3~5min.

[0014] As a further improvement of the present invention, in step 6, The membrane, after secondary interfacial polymerization and thermosetting, is immersed again in an aqueous solution of pure polyethyleneimine for 3-5 minutes at a temperature of 25-30 °C. The mass concentration ratio of piperazine to polyethyleneimine in the aqueous solution of pure polyethyleneimine is 0%:100%, and the concentration of polyethyleneimine is 1.0-2.0 g / L. Repeating the subsequent operations in step 5, obtaining a nanofiltration membrane with high lithium-magnesium separation performance through three interfacial polymerization reactions refers to the following steps: after removing the membrane immersed in a pure PEI aqueous solution, scraping off the residual aqueous solution on the surface with an air knife, placing it flat in a dry and clean petri dish, and spraying an organic phase solution onto the membrane surface with an airbrush to carry out three interfacial polymerization reactions. A strongly positively charged separation layer is constructed on the basis of the existing two separation layers. After the solution on the membrane surface dries, a heat curing treatment is performed to finally obtain a nanofiltration membrane with high lithium-magnesium separation performance based on a multi-level charge distribution constructed through multiple interfacial polymerization reactions.

[0015] The present invention also discloses a nanofiltration membrane with high lithium-magnesium separation performance, which is prepared by the above preparation method.

[0016] The beneficial effects of this invention are as follows: This invention differs from traditional methods for modifying positively charged nanofiltration membranes. It is not limited to charge modification on the two-dimensional plane of the membrane, but creatively improves the intrinsic distribution and synergistic effect of charge in the nanofiltration membrane in both horizontal and vertical gradient directions. By utilizing the distribution and accessibility differences of acyl chloride sites within the separation layer, it achieves the directional construction of differentiated charges on the membrane surface and in the vertical direction. A uniform, strongly positively charged layer is constructed on the outermost layer to enhance magnesium electrostatic repulsion. In the vertical direction, a multi-level gradient charge channel of "strong positive point - weak positive charge - negative charge" is constructed from top to bottom, which further enhances the enrichment efficiency of lithium ions in the channel and reduces the energy barrier for their transport in the vertical channel. This invention achieves charge modification in two dimensions: the membrane surface and the vertical transport channels, by changing the type and ratio of aqueous monomers and combining them with a multi-stage progressive interfacial polymerization reaction method. At the same time, it directionally constructs a multi-level differentiated charge transport channel with a vertical gradient, which greatly improves the lithium-magnesium separation performance of nanofiltration membranes. Attached Figure Description

[0017] Figure 1 This is a flowchart of a method for preparing a nanofiltration membrane with high lithium-magnesium separation performance disclosed in this invention. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] The present invention will now be described in further detail with reference to the accompanying drawings: like Figure 1 As shown, the present invention provides a nanofiltration membrane with high lithium-magnesium separation performance and its preparation method, comprising the following steps: Step 1: Select a polyethersulfone (PES) ultrafiltration membrane with a molecular weight cutoff of 100 kDa as the base membrane, cut multiple sheets to a size of 7 cm × 7 cm, soak them in deionized water for 48 h, and change the deionized water every 8 h for subsequent use. Step 2: Dissolve 0.2 g of anhydrous piperazine in 100.0 ml of deionized water to prepare a 2.0 g / L pure PIP aqueous solution; dissolve 0.2 g of polyethyleneimine with a molecular weight of 3000.0 Da in 100.0 ml of deionized water to prepare a 2.0 g / L pure PEI aqueous solution; dissolve 0.1 g of anhydrous piperazine and 0.1 g of polyethyleneimine with a molecular weight of 3000.0 Da in 100 ml of deionized water to prepare a 2.0 g / L PIP:PEI 50% : 50% aqueous solution. Step 3: Dissolve 0.2 g of pyromellitic acid chloride in 200.0 ml of n-hexane to prepare an organic phase solution of 1.0 g / L; Step 4: Remove the pretreated substrate and immerse it in an aqueous solution of pure PIP for 5.0 min. Remove it and place it on absorbent paper with the active layer facing upwards. Use a 9.0 cm wide polyurethane roller to gently remove any remaining PIP solution from the membrane surface from left to right along the membrane edge until no obvious droplets remain. After roller removal, immerse the substrate in an organic phase solution for 1.0 min to perform the first interfacial polymerization reaction. After completion, remove the substrate, air dry any remaining n-hexane solution on the surface, and then place it in a petri dish. Cure the curing reaction in an oven at 60.0 ℃ for 1.0 min. Step 5: Immerse the thermosetting membrane from Step 4 in an aqueous solution with a PIP / PEI doping ratio of 50%:50% for 5.0 min. After removal, place the membrane with the active layer facing upwards in a petri dish lined with absorbent paper and fix it in place. Set the air knife airflow pressure to 0.1 MPa, the air knife width to 8.0 cm, the air curtain thickness to 0.1 mm, the purging mode to reverse purging, and the membrane speed to 1.0 m / min. Purge at an incident angle of 20.0° at a distance of 4.0 mm from the membrane surface until there are no obvious water droplets or deep water droplets remaining on the membrane surface, leaving only a wet mark. Under strong light, the membrane exhibits a uniform, continuous, and extremely weak watery feel. Further, the membrane (wet side up) after being purged by an air knife was placed flat in a dry and clean petri dish. The organic phase solution was manually and evenly sprayed using a laboratory gravity-type double-action spray gun (0.3 mm orifice) containing a 1.0 g / L organic phase solution. The spray gun atomization pressure was set to 0.2 MPa and the liquid flow rate to 0.6 ml / min. The solution was sprayed evenly 3.0 times vertically to the center of the membrane at a distance of 15.0 cm from the membrane surface. After spraying, the membrane was allowed to stand for 120.0 s to carry out the second interfacial polymerization reaction. After completion, the membrane was air-dried for 15.0 s and then heat-cured in an oven at 80.0 ℃ for 3.0 min. Step 6: Immerse the thermosetting membrane from Step 5 in an aqueous solution of pure PEI for 5.0 min, then remove it. Repeat the subsequent operations of Step 5. After three interfacial polymerization reactions, a nanofiltration membrane with high lithium-magnesium separation performance is obtained. The thermosetting nanofiltration membrane with high lithium-magnesium separation performance is then removed and sealed in deionized water for 24.0 h for subsequent measurements. Example 1

[0020] A method for preparing a nanofiltration membrane with high lithium-magnesium separation performance, wherein all steps below without specified temperature are performed at room temperature.

[0021] Step 1: Select a polyethersulfone (PES) ultrafiltration membrane with a molecular weight cutoff of 100 kDa as the base membrane, cut multiple sheets to a size of 7 cm × 7 cm, soak them in deionized water for 48 hours, and change the deionized water every 8 hours for subsequent use. Step 2: Dissolve 0.2g of anhydrous piperazine in 100.0ml of deionized water to prepare a 2.0g / L pure PIP aqueous solution; dissolve 0.2g of polyethyleneimine with a molecular weight of 3000.0 Da in 100.0ml of deionized water to prepare a 2.0g / L pure PEI aqueous solution; dissolve 0.1g of anhydrous piperazine and 0.1g of polyethyleneimine with a molecular weight of 3000.0 Da in 100ml of deionized water to prepare a 2.0g / L PIP:PEI 50% : 50% aqueous solution. Step 3: Dissolve 0.2g of trimesoyl chloride in 200.0ml of n-hexane to prepare an organic phase solution of 1.0g / L; Step 4: Remove the pretreated substrate and immerse it in an aqueous solution of pure PIP for 5.0 min. Remove it and place it on absorbent paper with the active layer facing upwards. Use a 9.0 cm wide polyurethane roller to gently remove any remaining PIP solution from the membrane surface from left to right along the membrane edge until no obvious droplets remain. After roller removal, immerse the substrate in an organic phase solution for 1.0 min to perform the first interfacial polymerization reaction. After completion, remove the substrate, air dry any remaining n-hexane solution on the surface, and then place it in a petri dish. Cure the curing reaction in an oven at 60.0 ℃ for 1.0 min. Step 5: Immerse the thermo-cured membrane from Step 4 in an aqueous solution with a PIP / PEI doping ratio of 50%:50% for 5.0 min. After removal, place the membrane with the active layer facing upwards in a petri dish lined with absorbent paper and fix it in place. Set the air knife airflow pressure to 0.1 MPa, the air knife width to 8.0 cm, the air curtain thickness to 0.1 mm, the purging mode to reverse purging, the membrane speed to 1.0 m / min, and the purging to 4.0 mm from the membrane surface at an incident angle of 20.0°. Purging continues until there are no obvious water droplets or deep water droplets remaining on the membrane surface, leaving only a wet mark. Under strong light, the membrane exhibits a uniform, continuous, and extremely weak watery feel. Further, the membrane (wet side up) after being purged by an air knife was placed flat in a dry and clean petri dish. The organic phase solution was manually and evenly sprayed using a laboratory gravity-type double-action spray gun (0.3 mm orifice) containing a 1.0 g / L organic phase solution. The spray gun atomization pressure was set to 0.2 MPa and the liquid flow rate to 0.6 ml / min. The solution was sprayed evenly 3.0 times vertically to the center of the membrane at a distance of 15.0 cm from the membrane surface. After spraying, the membrane was allowed to stand for 120.0 s to carry out the second interfacial polymerization reaction. After completion, the membrane was air-dried for 15.0 s and then heat-cured in an oven at 80.0 ℃ for 3.0 min. Step 6: Immerse the thermosetting membrane from Step 5 in an aqueous solution of pure PEI for 5.0 min, then remove it. Repeat the subsequent operations of Step 5. After three interfacial polymerization reactions, a nanofiltration membrane with high lithium-magnesium separation performance is obtained. The thermosetting nanofiltration membrane with high lithium-magnesium separation performance is then removed and sealed in deionized water for 24.0 h for subsequent measurements.

[0022] Step 7: The dried nanofiltration membrane with high lithium-magnesium separation performance (M1) was taken out and pre-pressed at 0.6 MPa for 30 min. Then, at 0.4 MPa, the performance of the prepared nanofiltration membrane with high lithium-magnesium selective separation performance was tested using pure water, a 2000 ppm MgCl2 aqueous solution, a 2000 ppm LiCl solution, and a mixed aqueous solution of MgCl2 and LiCl with a total concentration of 2000 ppm. The test results showed that the pure water permeability of the highly selective nanofiltration membrane was 42.33 L·m. −2 ·h −1 ·bar −1 The retention rate was 97.45% for pure MgCl2 aqueous solution and 21.39% for pure LiCl aqueous solution. For the MgCl2 and LiCl mixed aqueous solution... 2+ The retention rate was 99.55%, Li + The retention rate was 15.66%, and its lithium-magnesium separation factor S Li,Mg=187.42. In addition, based on the Zeta potential characterization test of the longitudinal etching depth of the membrane surface and membrane channels, the properties and intensity of the charge on the membrane surface and in the channels were determined. The results showed that under neutral conditions of pH=7, the Zeta potential of the M1 membrane surface was +53.16 mV, the Zeta potential at the median membrane thickness was +20.13 mV, and the Zeta potential at the junction of the support layer and the separation layer (the bottommost layer) was -9.77 mV.

[0023] Comparative Example 1 Nanofiltration membrane M2 was prepared according to the method of Example 1, except that steps 5 and 6 were not performed. After step 4, the thermo-cured membrane was sealed in deionized water for 24.0 h for subsequent measurement.

[0024] The nanofiltration membrane (M2) prepared in Comparative Example 1 was subjected to performance testing using the same method as in Example 1. The test results showed that the pure water permeability of the nanofiltration membrane prepared in Comparative Example 1 was 19.15 L·m⁻¹. −2 ·h −1 ·bar −1 The retention rate for pure MgCl2 aqueous solution was 45.43%, and the retention rate for pure LiCl aqueous solution was 27.36%. For the MgCl2 and LiCl mixed aqueous solution... 2+ The retention rate was 47.69%, Li + The retention rate was 29.35%, and its lithium-magnesium separation factor S Li,Mg =1.35. In addition, based on the Zeta potential characterization test of the membrane surface, the results showed that the Zeta potential of the M2 membrane surface was -19.55mV under neutral conditions of pH=7.

[0025] Comparative Example 2 Nanofiltration membrane M3 was prepared according to the method of Example 1. The difference from Example 1 is that the related operations of steps 4 and 6 are not performed. After completing step 1-(3), step 4 is skipped. The bottom membrane is directly immersed in an aqueous solution with a PIP / PEI doping ratio of 50%:50% for 5.0 min. After taking it out, the subsequent operation of step 5 is performed. After completion, the thermo-cured membrane is sealed in deionized water for 24.0 h for subsequent measurement.

[0026] The nanofiltration membrane (M3) prepared in Comparative Example 2 was tested for performance using the same method as in Example 1. The test results showed that the pure water permeability of the nanofiltration membrane (M3) prepared in Comparative Example 2 was 22.64 L·m⁻¹. −2 ·h −1 ·bar −1The rejection rate for pure MgCl2 aqueous solution was 69.21%, and the rejection rate for pure LiCl aqueous solution was 33.15%. For the MgCl2 and LiCl mixed aqueous solution, the rejection rate was... 2+ The retention rate was 74.52%, Li + The retention rate was 40.19%, and its lithium-magnesium separation factor S Li,Mg =2.17. In addition, based on the Zeta potential characterization test of the membrane surface, the results showed that the Zeta potential of the M3 membrane surface was +23.69 mV under neutral conditions of pH=7.

[0027] Comparative Example 3 Nanofiltration membrane M4 was prepared according to the method of Example 1, except that steps 4 and 5 were not performed. The bottom membrane was directly immersed in a pure PEI aqueous solution for 5.0 min. After removal, the subsequent operation of step 6 was performed. After completion, the thermo-cured membrane was sealed in deionized water for 24.0 h for subsequent measurement.

[0028] The nanofiltration membrane (M4) prepared in Comparative Example 3 was subjected to performance testing using the same method as in Example 1. The test results showed that the pure water permeability of the nanofiltration membrane (M4) prepared in Comparative Example 3 was 20.98 L·m⁻¹. −2 ·h −1 ·bar −1 The retention rate was 95.63% for pure MgCl2 aqueous solution and 41.66% for pure LiCl aqueous solution. For the MgCl2 and LiCl mixed aqueous solution... 2+ The retention rate was 96.28%, Li + The retention rate was 42.11%, and its lithium-magnesium separation factor S Li,Mg =15.56. In addition, based on the Zeta potential characterization test of the membrane surface, the results showed that the Zeta potential of the M4 membrane surface was +36.99 mV under neutral conditions of pH=7.

[0029] In summary, the high lithium-magnesium separation performance nanofiltration membrane (M1) prepared in the embodiments of the present invention exhibits significantly higher rejection rates for magnesium salt ion solutions in both single and mixed systems compared to the nanofiltration membranes (M2, M3, M4) prepared in the comparative examples. Its permeation effect on lithium salt ion solutions in both single and mixed systems is also more pronounced. Furthermore, the lithium-magnesium selective separation factor of the high lithium-magnesium separation performance nanofiltration membrane (M1) prepared in the embodiments is 12 to 139 times higher than that of the nanofiltration membranes (M2, M3, M4) prepared in the comparative examples, demonstrating extremely strong lithium-magnesium separation performance. In addition, the pure water permeation performance of the nanofiltration membrane (M1) prepared in the embodiments is also significantly higher than that of the other comparative examples. Therefore, the high lithium-magnesium separation performance nanofiltration membrane and its preparation method involved in the present invention have very good effects.

[0030] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a nanofiltration membrane with high lithium-magnesium separation performance, characterized in that, Includes the following steps: Step 1: Select an ultrafiltration membrane as the base membrane and pretreat it by immersing it in deionized water; Step 2: Using piperazine and polyethyleneimine as basic aqueous monomers, prepare aqueous solutions with different piperazine:polyethyleneimine doping ratios but the same total mass concentration. Step 3: Prepare an organic phase solution using n-hexane as solvent and trimesoyl chloride as solute; Step 4: Immerse the pretreated substrate film from Step 1 in an aqueous solution of pure piperazine. After removing it, use a rubber roller to remove any residual aqueous solution from the surface and immerse it in an organic solution for interfacial polymerization. After removing it, air dry it and then perform a heat curing treatment. Step 5: Immerse the membrane after the thermosetting treatment in Step 4 in an aqueous solution containing both piperazine and polyethyleneimine. Remove the membrane and scrape off the residual aqueous solution on the surface. Spray an organic solution onto the membrane surface to carry out a secondary interfacial polymerization reaction. After the membrane surface solution has dried, perform a thermosetting treatment. Step 6: Immerse the membrane after the thermosetting treatment in Step 5 in an aqueous solution of pure polyethyleneimine. After taking it out, repeat the subsequent operations of Step 5. After three interfacial polymerization reactions, a nanofiltration membrane with high lithium-magnesium separation performance is obtained.

2. The preparation method according to claim 1, characterized in that, In step 1, the ultrafiltration membrane used as the base membrane is made of one or more of polyethersulfone, polyacrylonitrile, polyvinylidene fluoride, polysulfone, and polyvinyl chloride, with a molecular weight cutoff of 50-100 kDa. The pretreatment operation parameters are as follows: the ultrafiltration membrane is immersed in deionized water for 48-72 h at 15-30 ℃, and the deionized water is replaced every 4-24 h.

3. The preparation method according to claim 1, characterized in that, In step 2, the total mass concentration of the aqueous solution is 1.0~2.0 g / L. The mass concentrations of the two aqueous monomers under different aqueous phase ratios are as follows: when piperazine:polyethyleneimine is 100%:0%, the piperazine concentration is 1.0~2.0 g / L; when piperazine:polyethyleneimine is 50%:50%, the mass concentrations of piperazine and polyethyleneimine are the same, both 0.5~1.0 g / L; when piperazine:polyethyleneimine is 0%:100%, the polyethyleneimine concentration is 1.0~2.0 g / L, and the molecular weight of polyethyleneimine is 3000.0, 5000.0, or 7000.0 Da.

4. The preparation method according to claim 1, characterized in that, In step 3, the concentration of pyromellitic acid chloride in the organic phase solution is 0.5~1.0 g / L.

5. The preparation method according to claim 1, characterized in that, In step 4, the immersion time of the substrate in the aqueous solution of pure piperazine is 2-6 min, and the temperature is 20-30 ℃; wherein, the mass concentration ratio of piperazine to polyethyleneimine in the aqueous solution of pure piperazine is 100% : 0%, and the concentration of piperazine is 1.0-2.0 g / L.

6. The preparation method according to claim 1, characterized in that, In step 4 The rubber roller with residual aqueous solution on the surface refers to the rubber roller method used to remove residual piperazine from the membrane surface. The roller is made of polyurethane, with a roller width of 8~11 cm and an operating temperature of 25~30 ℃. The operation method is as follows: place the membrane flat on the absorbent paper with the active layer facing up, and gently roll the roller along the edge of the membrane from left to right to remove the residual piperazine solution on the membrane surface until there are no obvious droplets on the membrane surface. After the rubber roller is completed, it is immersed in the organic phase solution for 1-3 minutes at a temperature of 20-30 °C. The first interfacial polymerization refers to the formation of a negatively charged polyamide separation layer on the surface of the bottom film to facilitate subsequent second and third interfacial polymerization. The air-drying and heat curing treatment refers to the evaporation of the residual n-hexane solution on the film surface after the interfacial polymerization is completed at a temperature of 25-30 °C. The heat curing treatment refers to placing the air-dried film in a clean petri dish and then drying it in an oven at a temperature of 60-80 °C for 1-3 minutes.

7. The preparation method according to claim 1, characterized in that, In step 5 The membrane after the thermosetting treatment in step 4 was immersed in an aqueous solution simultaneously doped with piperazine and polyethyleneimine to construct a weakly positively charged separation layer on top of the negatively charged separation layer. The immersion time was 1-3 min and the temperature was 25-30 ℃. The mass concentration ratio of piperazine to polyethyleneimine in the aqueous solution simultaneously doped with piperazine and polyethyleneimine was 50%:50%, and the mass concentrations of piperazine and polyethyleneimine were the same, both 0.5-1.0 g / L. Using an air knife to scrape off residual aqueous solution on the surface refers to using an ultra-thin air knife to remove droplets from the surface of a glass membrane. The liquid is blown away until there are no obvious water droplets left on the membrane surface or deep within it, leaving only a wet mark. Under strong light, the water appears uniform, continuous, and extremely faintly moist. The operating parameters are as follows: air knife width is 8.0~10.0 cm, air curtain thickness is 0.1~0.2 mm, air supply pressure is 0.1~0.2 MPa, distance between air knife tip and membrane surface is 3.0~5.5 mm, angle between air knife and membrane surface (incident angle) is 15.0~30.0°, blowing mode is reverse blowing, and membrane speed is 0.2~2.0 m / min. The procedure for spraying the organic phase solution using an airbrush is as follows: Place the membrane, after being purged by an air knife, flat in a dry, clean petri dish with the wet side facing upwards. Use a laboratory gravity-type double-action airbrush containing an organic phase solution with a concentration of 0.5–1.0 g / L to manually and evenly spray the solution onto the center of the membrane. Specific operating parameters are: nozzle orifice diameter 0.1–0.5 mm, atomizing air pressure 0.1–0.3 MPa, liquid flow rate 0.3–0.9 ml / min, nozzle-to-membrane distance 15.0–20.0 cm, and manual and even spraying 2.0–3.0 times.

8. The preparation method according to claim 1, characterized in that, In step 5, the secondary interfacial polymerization refers to the further vertical construction of a weakly positively charged separation layer on the negatively charged separation layer constructed by the primary interfacial polymerization. The reaction conditions are: reaction time of 120-150 s, air drying time of 10-15 s, and temperature of 25-30 ℃. The thermosetting treatment conditions are: oven temperature of 75-85 ℃ and time of 3-5 min.

9. The preparation method according to claim 1, characterized in that, In step 6, The membrane, after secondary interfacial polymerization and thermosetting, is immersed again in an aqueous solution of pure polyethyleneimine for 3-5 minutes at a temperature of 25-30 °C. The mass concentration ratio of piperazine to polyethyleneimine in the aqueous solution of pure polyethyleneimine is 0%:100%, and the concentration of polyethyleneimine is 1.0-2.0 g / L. Repeating the subsequent operations in step 5, obtaining a nanofiltration membrane with high lithium-magnesium separation performance through three interfacial polymerization reactions refers to the following steps: after removing the membrane immersed in a pure PEI aqueous solution, scraping off the residual aqueous solution on the surface with an air knife, placing it flat in a dry and clean petri dish, and spraying an organic phase solution onto the membrane surface with an airbrush to carry out three interfacial polymerization reactions. A strongly positively charged separation layer is constructed on the basis of the existing two separation layers. After the solution on the membrane surface dries, a heat curing treatment is performed to finally obtain a nanofiltration membrane with high lithium-magnesium separation performance based on a multi-level charge distribution constructed through multiple interfacial polymerization reactions.

10. A nanofiltration membrane with high lithium-magnesium separation performance, characterized in that, It is prepared by any one of claims 1 to 9.

Images