Composite nanofiltration membrane for magnesium-lithium separation and preparation method and application thereof

Abstract

The application discloses a composite nanofiltration membrane for magnesium-lithium separation and a preparation method and application thereof. The composite nanofiltration membrane comprises a base film, an intermediate layer formed on the base film, and a separation layer formed on the intermediate layer; the intermediate layer is formed by co-deposition of polydopamine and iron molecular cages on the surface of the base film; and the separation layer comprises a polyamide layer. The intermediate layer is prepared by co-deposition of hydrochloric acid dopamine and iron molecular cages, the release and diffusion rate of polyethylene imine monomers in the interfacial polymerization process can be controlled, the thickness of the selective separation layer is reduced, the mass transfer resistance is reduced, the permeation flux of the nanofiltration membrane is improved, the magnesium ion rejection rate is improved, and the problems of low separation selectivity, low flux and difficult preparation of the existing magnesium-lithium separation nanofiltration membrane are solved.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, and in particular to a composite nanofiltration membrane for magnesium-lithium separation, its preparation method, and its application. Background Technology

[0002] The rapid development and large-scale application of rechargeable lithium-ion battery electric vehicles have led to a surge in market demand for lithium resources. Therefore, effective lithium extraction technologies are essential. Lithium resources are abundant on Earth, with approximately 70% found in seawater, brines, and salt lakes; however, high magnesium content limits their utilization. Currently, common lithium extraction methods include chemical precipitation, adsorption, solvent extraction, evaporation, electrochemical methods, and membrane separation.

[0003] Membrane separation technology, especially nanofiltration technology, has attracted much attention due to its advantages such as high efficiency, low energy consumption, and simple operation. Composite nanofiltration membranes prepared through interfacial polymerization exhibit excellent performance in separating monovalent and multivalent ions due to their charged surfaces and nanoscale pore sizes, and can also separate high-Mg ions. 2+ / Li + Lithium extraction from brine at a mass ratio is considered the most promising method for extracting Mg. 2+ / Li + One of the separation technologies. However, traditional piperazine-pyromellitic trichloroisocyanurate nanofiltration membranes exhibit a relatively negative surface charge, which is not conducive to the separation of monovalent / divalent cations. Therefore, researchers have developed many nanofiltration membranes with positively charged surfaces using polyamines for magnesium-lithium separation. Polyethyleneimine (PEI) contains a large number of positively charged amine (NH2) groups. Therefore, PEI is often used as an aqueous monomer to prepare positively charged nanofiltration membranes through interfacial polymerization. However, the relatively thick selective separation layer results in low permeation flux, limited mass transfer separation, and the separation effect needs to be improved.

[0004] In addition, some researchers have modified polyethyleneimine-based nanofiltration membranes to address the aforementioned defects, such as through surface grafting or interfacial polymerization using comonomers. The results show that grafting and other methods can significantly increase the flux of nanofiltration membranes. However, the introduction of grafting targets incompletely reacted acyl chloride groups, making the process difficult to control and increasing the complexity of the reaction system. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a composite nanofiltration membrane for magnesium-lithium separation, its preparation method, and its applications. The composite nanofiltration membrane of this invention comprises a porous base membrane, an intermediate layer, and a selective separation layer formed on the intermediate layer. The porous base membrane is a polymer ultrafiltration membrane; the intermediate layer is a co-deposited layer composed of dopamine and iron molecular cages; and the selective layer is a polyamide layer formed by the interfacial polymerization of polyethyleneimine and polyacrylamide chlorides. This invention solves the problems of low separation selectivity, low flux, and difficult preparation of existing magnesium-lithium separation nanofiltration membranes.

[0006] The technical solution of the present invention is as follows:

[0007] A first aspect of the present invention provides a composite nanofiltration membrane, comprising a base membrane, an intermediate layer formed on the base membrane, and a separation layer formed on the intermediate layer; the intermediate layer is formed by co-depositing polydopamine and iron molecular cages on the surface of the base membrane; the separation layer comprises a polyamide layer.

[0008] Preferably, the base membrane is a polymer ultrafiltration membrane; the separation layer is a polyamide layer formed by interfacial polymerization of polyethyleneimine and polyacrylamide chloride on the intermediate layer.

[0009] Preferably, the thickness of the intermediate layer is 10-100 nm; the thickness of the separation layer is 10-200 nm.

[0010] Preferably, the iron molecular cage is constructed by coordination self-assembly using ferrous ions as nodes and 4,4'-diaminobiphenyl-2,2'-disulfonic acid hydrate and pyridine-2-carboxaldehyde as ligands, and has a tetrahedral cavity structure.

[0011] A second aspect of the present invention provides a method for preparing the composite nanofiltration membrane described in the first aspect, comprising the following steps:

[0012] S1: Add dopamine hydrochloride and iron molecular cages to Tris buffer solution at pH=8.5, sonicate to obtain a mixture, pour the mixture onto the surface of the base membrane, shake, and wash to obtain a composite membrane containing an intermediate layer.

[0013] S2: The composite membrane containing the intermediate layer prepared in step S1 is placed in a polyethyleneimine solution for reaction, then a polyacrylamide chloride solution is added. After interfacial polymerization, excess solution is removed, and heat treatment is performed to obtain a composite nanofiltration membrane.

[0014] Preferably, the base membrane comprises one of polyacrylonitrile ultrafiltration membrane, polyvinylidene fluoride ultrafiltration membrane, polyethersulfone ultrafiltration membrane, and polysulfone ultrafiltration membrane; the pore size of the base membrane is 5-50 nm.

[0015] Preferably, in step S1, the concentration of the dopamine hydrochloride solution in the mixed solution is 0.02-0.5 wt%, the concentration of the iron molecular cage is 0.001-0.1 wt%, the concentration of the Tris buffer solution is 10-20 mM, and the oscillation time is 0.5-5 h.

[0016] Preferably, in step S2, the molecular weight of the polyethyleneimine is 1000-70000; the concentration of the polyethyleneimine is 0.1-5wt%; the reaction temperature is 25℃ and the reaction time is 5-10min.

[0017] Preferably, in step S2, the polyacrylamide chloride solution is obtained by dissolving the polyacrylamide chloride in an organic solvent; the organic solvent includes at least one of n-hexane, n-heptane, benzene, and toluene; the concentration of the polyacrylamide chloride is 0.1–1 wt%; the polyacrylamide chloride includes at least one of trimesoyl chloride, isophthaloyl chloride, and terephthaloyl chloride; the temperature of the interfacial polymerization reaction is 25°C, and the time is 0.5–3 min; the temperature of the heat treatment is 30–60°C, and the time is 3–10 min.

[0018] A third aspect of the present invention provides an application of the composite nanofiltration membrane described in the first aspect above or the composite nanofiltration membrane prepared by the preparation method described in the second aspect above, wherein the composite nanofiltration membrane is used for magnesium-lithium separation.

[0019] The beneficial technical effects of this invention are as follows:

[0020] This invention utilizes the co-deposition of dopamine hydrochloride and iron molecular cages to prepare an intermediate layer that can control the release and diffusion rate of polyethyleneimine monomers during interfacial polymerization, thereby reducing the thickness of the selective separation layer, decreasing mass transfer resistance, and increasing the permeation flux of the nanofiltration membrane.

[0021] Furthermore, the intermediate layer prepared by co-deposition of dopamine hydrochloride and iron molecular cages in this invention improves the hydrophilicity of the ultrafiltration membrane. The introduction of the intermediate layer increases the roughness of the selective separation layer and the effective filtration area, which is beneficial to improving the flux of the nanofiltration membrane.

[0022] The intermediate layer prepared by co-deposition of dopamine hydrochloride and iron molecular cages in this invention can store more polyethyleneimine aqueous monomers due to electrostatic adsorption, forming a defect-free selective separation layer, which is beneficial for Mg. 2+ The retention rate has been improved, which is beneficial to improving the magnesium-lithium separation performance.

[0023] This invention utilizes a combination of co-deposition and interfacial polymerization to prepare magnesium-lithium separation composite nanofiltration membranes. The preparation method is convenient, simple to operate, and has good operational stability. Attached Figure Description

[0024] Figure 1The infrared spectrum of the composite nanofiltration membrane prepared in Example 2 of this invention.

[0025] Figure 2 This is a schematic diagram of a composite nanofiltration membrane structure with an intermediate layer in one embodiment of the present invention.

[0026] Figure 3 This is a scanning electron microscope image of an intermediate layer formed on an ultrafiltration membrane in one embodiment of the present invention.

[0027] Figure 4 This is a scanning electron microscope image of a composite nanofiltration membrane with an intermediate layer, according to one embodiment of the present invention. Detailed Implementation

[0028] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0029] This invention provides a composite nanofiltration membrane for magnesium-lithium separation and its preparation method. On one hand, the negatively charged sulfonate groups on the surface of the iron molecular cage can generate electrostatic interactions with the amine groups of polyethyleneimine, thereby slowing down the diffusion rate of polyethyleneimine from the aqueous phase to the organic phase, ultimately reducing the thickness of the polyamide separation layer. On the other hand, the co-deposited layer formed by dopamine and the iron molecular cage has a rough surface morphology, which can provide a template for the subsequent interfacial polymerization to form polyamide wrinkles, thus providing a larger effective filtration area. The reduced separation layer thickness and rough surface morphology significantly improve the water permeability of the composite nanofiltration membrane.

[0030] A first aspect of the present invention provides a composite nanofiltration membrane, comprising a base membrane, an intermediate layer formed on the base membrane, and a separation layer formed on the intermediate layer; the intermediate layer is formed by co-depositing polydopamine and iron molecular cages on the surface of the base membrane; the separation layer comprises a polyamide layer.

[0031] In some embodiments, the base membrane is a polymer ultrafiltration membrane; the separation layer is a polyamide layer formed by interfacial polymerization of polyethyleneimine and polyacrylamide chloride on an intermediate layer.

[0032] In some embodiments, the thickness of the intermediate layer is 10-100 nm; the thickness of the separation layer is 10-200 nm.

[0033] In some embodiments, the iron molecular cage is constructed by coordination self-assembly using ferrous ions as nodes and 4,4'-diaminobiphenyl-2,2'-disulfonic acid hydrate and pyridine-2-carboxaldehyde as ligands, and has a tetrahedral cavity structure.

[0034] In this invention, the iron molecular cage belongs to a metal-organic polyhedron (MOP) and is formed through a self-assembly reaction driven by metal-ligand bond formation. Typically, it is between 1-5 nm in size, with the sulfonate groups of the molecular cage symmetrically arranged outwards, and exhibits good solubility in water. It is understood that the iron molecular cage of this invention uses ferrous ions as nodes, an imine component formed by 4,4'-diaminobiphenyl-2,2'-disulfonic acid and 2-formylpyridine as the organic chain backbone, and has a hollow tetrahedral structure with an internal cavity volume of [missing information]. The strong binding and mutual stabilization between iron(II) and the imine ligand play a crucial role in its stability. The iron molecular cage described in this invention has corresponding crystal structure data in the Cambridge Analytica database, CCDC-688687.

[0035] A second aspect of the present invention provides a method for preparing the composite nanofiltration membrane described in the first aspect, comprising the following steps:

[0036] S1: Add dopamine hydrochloride and iron molecular cages to Tris buffer solution at pH=8.5, sonicate to obtain a mixture, pour the mixture onto the surface of the base membrane, shake, and wash to obtain a composite membrane containing an intermediate layer.

[0037] S2: The composite membrane containing the intermediate layer prepared in step S1 is placed in a polyethyleneimine solution for reaction, then a polyacrylamide chloride solution is added. After interfacial polymerization, excess solution is removed, and heat treatment is performed to obtain a composite nanofiltration membrane.

[0038] In some embodiments, the base membrane includes one of polyacrylonitrile ultrafiltration membrane, polyvinylidene fluoride ultrafiltration membrane, polyethersulfone ultrafiltration membrane, and polysulfone ultrafiltration membrane; the pore size of the base membrane is 5-50 nm.

[0039] In some embodiments, in step S1, the concentration of the dopamine hydrochloride solution in the mixed solution is 0.02-0.5 wt%, including but not limited to 0.02 wt%, 0.06 wt%, 0.08 wt%, 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, and 0.5 wt%; the concentration of the iron molecular cage is 0.001-0.1 wt%, including but not limited to 0.001 wt%, 0.005 wt%, 0.01 wt%, 0.05 wt%, 0.08 wt%, and 0.1 wt%; the concentration of the Tris buffer solution is 10-20 mM; and the oscillation time is 0.5-5 h, including but not limited to 0.5 h, 1 h, 2 h, 3 h, 4 h, and 5 h.

[0040] In some embodiments, in step S2, the molecular weight of the polyethyleneimine is 1000-70000; the concentration of the polyethyleneimine is 0.1-5 wt%, including but not limited to 0.1 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, and 5 wt%; the reaction temperature is 25°C, and the reaction time is 5-10 min, including but not limited to 5 min, 6 min, 7 min, 8 min, and 10 min.

[0041] In some embodiments, in step S2, the polyacrylamide chloride solution is obtained by dissolving the polyacrylamide chloride in an organic solvent; the organic solvent includes at least one selected from n-hexane, n-heptane, benzene, and toluene; the concentration of the polyacrylamide chloride is 0.1-1 wt%, including but not limited to 0.1 wt%, 0.4 wt%, 0.6 wt%, 0.8 wt%, and 1 wt%; the polyacrylamide chloride includes at least one selected from trimesoyl chloride, isophthaloyl chloride, and terephthaloyl chloride; the temperature of the interfacial polymerization reaction is 25°C, and the time is 0.5-3 min; the temperature of the heat treatment is 30-60°C, including but not limited to 30°C, 40°C, 50°C, and 60°C, and the time is 3-10 min, including but not limited to 3 min, 4 min, 5 min, 6 min, 7 min, 8 min, and 10 min.

[0042] A third aspect of the present invention provides an application of the composite nanofiltration membrane described in the first aspect above or the composite nanofiltration membrane prepared by the preparation method described in the second aspect above, wherein the composite nanofiltration membrane is used for magnesium-lithium separation.

[0043] The present invention will be further illustrated below through examples and comparative examples.

[0044] Example 1

[0045] A composite nanofiltration membrane for magnesium-lithium separation is prepared by the following steps:

[0046] (1) Preparation of the intermediate layer: After cleaning the polyvinylidene fluoride ultrafiltration membrane, 0.02 wt% dopamine hydrochloride and 0.002 wt% iron molecular cages were dispersed in a 10 mM Tris buffer solution with pH = 8.5. After ultrasonic treatment, the solution was poured onto the surface of the polyvinylidene fluoride ultrafiltration membrane, placed in a shaker, and shaken for 0.5 h before cleaning to form a composite membrane including the intermediate layer.

[0047] (2) Preparation of the separation layer: The composite membrane including the intermediate layer prepared in step (1) was placed in a polyethyleneimine aqueous solution with a concentration of 0.15 wt% and a MW of 1800 for 5 min, and the excess aqueous solution on the surface was removed; another 0.1 wt% terephthaloyl chloride n-heptane solution was taken and subjected to interfacial polymerization reaction on the above membrane surface for 0.5 min, the residual organic phase solution was removed and the membrane was heat-treated at 35°C for 3 min to obtain a composite nanofiltration membrane for magnesium-lithium separation.

[0048] Example 2

[0049] A composite nanofiltration membrane for magnesium-lithium separation is prepared by the following steps:

[0050] (1) Preparation of the intermediate layer: After cleaning the polyacrylonitrile ultrafiltration membrane, 0.2 wt% dopamine hydrochloride and 0.02 wt% iron molecular cages were dispersed in a Tris buffer solution with a concentration of 15 mM and pH = 8.5. After ultrasonic treatment, the solution was poured onto the surface of the polyacrylonitrile ultrafiltration membrane, placed in a shaker, and washed after shaking for 1 h to form a composite membrane including the intermediate layer.

[0051] (2) Preparation of the separation layer: The composite membrane including the intermediate layer prepared in step (1) is placed in a polyethyleneimine aqueous solution with a concentration of 1wt% and a MW of 10000 for 10 min, and the excess aqueous solution on the surface is removed; a 0.3wt% pyromellitic chlorohexane solution is then placed on the surface of the above membrane for interfacial polymerization reaction for 1 min, the residual organic phase solution is removed, and the membrane is heat-treated at 50℃ for 5 min to obtain a composite nanofiltration membrane for magnesium-lithium separation.

[0052] Example 3

[0053] A composite nanofiltration membrane for magnesium-lithium separation is prepared by the following steps:

[0054] (1) Preparation of the intermediate layer: After cleaning the polyethersulfone ultrafiltration membrane, 0.5 wt% dopamine hydrochloride and 0.1 wt% iron molecular cages were dispersed in a Tris buffer solution with a concentration of 20 mM and pH = 8.5. After ultrasonic treatment, the solution was poured onto the surface of the polyethersulfone ultrafiltration membrane, placed in a shaker, and washed after shaking for 5 h to form a composite membrane including the intermediate layer.

[0055] (2) Preparation of the separation layer: The composite membrane including the intermediate layer prepared in step (1) is placed in a 5 wt% polyethyleneimine aqueous solution with a MW of 70000 for 10 min, and the excess aqueous solution on the surface is removed; another 1 wt% isophthaloyl chloride n-hexane solution is taken and subjected to interfacial polymerization reaction on the above membrane surface for 3 min, the residual organic phase solution is removed and the membrane is heat-treated at 60℃ for 10 min to obtain a magnesium-lithium separation composite nanofiltration membrane.

[0056] Comparative Example 1

[0057] A polyethyleneimine-based nanofiltration membrane, the preparation method of which includes the following steps:

[0058] After cleaning the polyacrylonitrile ultrafiltration membrane, it was placed in a 1 wt% polyethyleneimine aqueous solution with a MW of 70,000 for 5 min, and then the excess aqueous solution on the surface was removed. Separately, a 0.3 wt% pyromellitic chlorohexane solution was used to carry out an interfacial polymerization reaction on the above membrane surface for 1 min, and then the residual organic phase solution was removed. The membrane was then heat-treated at 50°C for 10 min to obtain a polyethyleneimine nanofiltration membrane, which was stored in deionized water for later use.

[0059] Comparative Example 2

[0060] A composite nanofiltration membrane for magnesium-lithium separation is prepared by the following steps:

[0061] (1) Preparation of the intermediate layer: After cleaning the polyethersulfone ultrafiltration membrane, 0.2 wt% dopamine hydrochloride was dispersed in a Tris buffer solution with a concentration of 15 mM and pH = 8.5. After ultrasonic treatment, the solution was poured onto the surface of the polyethersulfone ultrafiltration membrane, placed in a shaker, and washed after shaking for 1 h to form a composite membrane including the intermediate layer.

[0062] (2) Preparation of the separation layer: The composite membrane including the intermediate layer prepared in step (1) is placed in a polyethyleneimine aqueous solution with a concentration of 1 wt% and a MW of 70000 for 5 min, and the excess aqueous solution on the surface is removed; a 0.3 wt% pyromellitic chlorohexane solution is then placed on the surface of the above membrane for interfacial polymerization reaction for 1 min, the residual organic phase solution is removed, and the membrane is heat-treated at 60℃ for 5 min to obtain a composite nanofiltration membrane for magnesium-lithium separation.

[0063] Comparative Example 3

[0064] A composite nanofiltration membrane for magnesium-lithium separation is prepared by the following steps:

[0065] (1) Preparation of the intermediate layer: After cleaning the polyvinylidene fluoride ultrafiltration membrane, 0.2 wt% dopamine hydrochloride and 0.12 wt% iron molecular cages were dispersed in a Tris buffer solution with a concentration of 15 mM and pH = 8.5. After ultrasonic treatment, the solution was poured onto the surface of the polyvinylidene fluoride ultrafiltration membrane, placed in a shaker, and shaken for 2 hours before cleaning to form a composite membrane including the intermediate layer.

[0066] (2) Preparation of the separation layer: The composite membrane including the intermediate layer prepared in step (1) is placed in a polyethyleneimine aqueous solution with a concentration of 1 wt% and a MW of 70000 for 5 min, and the excess aqueous solution on the surface is removed; a 0.3 wt% pyromellitic chlorohexane solution is then placed on the surface of the above membrane for interfacial polymerization reaction for 1 min, the residual organic phase solution is removed, and the membrane is heat-treated at 60℃ for 5 min to obtain a composite nanofiltration membrane for magnesium-lithium separation.

[0067] Test example:

[0068] (1) Membrane structure characterization

[0069] The composite nanofiltration membrane prepared in Example 2 of this invention was subjected to infrared spectroscopy, and the results are as follows: Figure 1 As shown in the figure. It can be seen from the figure that at 1170cm... -1 1064cm -1 The characteristic peak appears at this point, which is attributed to SO3 on the iron molecule cage. - Extensive vibration; 3500~3300cm -1 The characteristic peak at 1645 cm⁻¹ is attributed to the stretching vibrations of OH and NH. -1 The characteristic peak at the location is due to the stretching vibration of C=O, which indicates that molecular cages are deposited on the base film and the upper polyamide separation and selection layer is formed.

[0070] Figure 3 , Figure 4 The images are scanning electron microscope (SEM) images of the intermediate layer composite membrane and the composite nanofiltration membrane prepared in Example 2.

[0071] (2) Nanofiltration performance measurement

[0072] The pure water flux (F) and rejection ratio (R) of the membranes prepared in the embodiments and comparative examples of the present invention were determined. The specific methods are as follows:

[0073] The flux of the composite nanofiltration membranes in the embodiments and comparative examples of the present invention was tested using a cross-flow membrane performance evaluation instrument at a test temperature of 25°C.

[0074] The method for testing pure water flux is as follows: First, using deionized water as feed, the nanofiltration membranes prepared in the examples or comparative examples are pre-pressed at a pressure of 0.6 MPa for 30 min. Then, the pressure is adjusted to 0.5 MPa and stabilized for 30 min. A certain volume of permeate is taken and the permeation time is recorded. The pure water flux (L / m³) is calculated using formula (1). -2 h -1 ).

[0075] The method for testing the salt rejection rate is as follows: First, the nanofiltration membrane prepared in the example or comparative example is pre-pressed at a pressure of 0.6 MPa for 30 min with deionized water as feed. Then, the pressure is adjusted to 0.5 MPa and stabilized for 30 min. Then, the deionized water is replaced with a 1 g / L MgCl2 solution or LiCl solution. After stabilizing for 30 min, the permeate and feed solution are sampled and the concentration is measured. The rejection rate R (%) of magnesium chloride or lithium chloride solution is calculated according to formula (2). The rejection rate is tested by a conductivity meter.

[0076]

[0077]

[0078] In equation (1), F is the pure water flux, in L / m³. -2 h -1 V is the volume of the permeate in liters (L), and S is the filtration area of ​​the membrane in square meters (m²). 2 ; t is the time required to measure a certain volume of permeate, in h; in formula (2), R is the rejection rate, in %; Cp is the permeate concentration, in g / L; Cf is the feed concentration, in g / L.

[0079] The results showed that the composite nanofiltration membrane for magnesium-lithium separation prepared in Example 1 had an intermediate layer thickness of 18 nm, a polyamide layer thickness of 69 nm, and a pure water flux of 52.2 L / m³. 2 The rejection rates of MgCl2 solution and LiCl solution were 88.2% and 44.2%, respectively; the composite nanofiltration membrane for magnesium-lithium separation prepared in Example 2 had an intermediate layer thickness of 27 nm, a polyamide layer thickness of 37 nm, and a pure water flux of 67.6 L / m³. 2 The rejection rates of MgCl2 solution and LiCl solution were 93.2% and 50.2%, respectively; the composite nanofiltration membrane for magnesium-lithium separation prepared in Example 3 had an intermediate layer thickness of 85 nm, a polyamide layer thickness of 108 nm, and a pure water flux of 28.8 L / m³. 2 The retention rates of MgCl2 solution and LiCl solution were 94.2% and 57.6%, respectively, at h.

[0080] The polyamide layer of the magnesium-lithium separation composite nanofiltration membrane prepared in Comparative Example 1 had a thickness of 132 nm and a pure water flux of 24.7 L / m³. 2 The rejection rates of MgCl2 solution and LiCl solution were 86.2% and 41.2%, respectively, in Comparative Example 2. The pure water flux of the magnesium-lithium separation composite nanofiltration membrane prepared with an intermediate layer thickness of 30 nm and a polyamide layer thickness of 95 nm was 29.6 L / m³. 2 The rejection rates of MgCl2 solution and LiCl solution were 90.6% and 47.8%, respectively; the composite nanofiltration membrane for magnesium-lithium separation prepared in Comparative Example 3 had an intermediate layer thickness of 46 nm, a polyamide layer thickness of 76 nm, and a pure water flux of 35.2 L / m³. 2 The retention rates of MgCl2 solution and LiCl solution were 91.4% and 43.8%, respectively, at h.

[0081] (3) Magnesium-Lithium Separation Test

[0082] Magnesium-lithium separation was performed on the composite nanofiltration membranes prepared in the embodiments and comparative examples of this invention using a cross-flow membrane performance evaluation instrument and an atomic absorption spectrophotometer. The test temperature was 25℃. The specific method was as follows: First, the nanofiltration membranes prepared in the embodiments and comparative examples were pre-pressed at a pressure of 0.6 MPa for 30 min using deionized water as feed. Then, the pressure was adjusted to 0.5 MPa and stabilized for 30 min. After that, the feed solution was replaced with a solution with a total salt concentration of 2 g / L and Mg... 2+ :Li + A mixed salt solution of lithium chloride and magnesium chloride (20:1) was prepared and run for 30 minutes. A certain volume of permeate was collected, and the Mg content in the feed solution and permeate was analyzed using an atomic absorption spectrophotometer. 2+ Li + Concentration was tested. The magnesium-lithium separation factor was calculated using formula (3).

[0083]

[0084] In the formula: SF(Li + / Mg 2+ ) represents the magnesium-lithium separation factor, C f，Mg2+ Indicates the amount of Mg in the feed solution 2+ Concentration of C; f，Li+ Indicates Li in the feed liquid + Concentration of C; p,Mg2+ Indicates Mg in the permeate 2+ Concentration of C; p,Li+ Indicates Li in the permeate + The concentrations are all expressed in g / L.

[0085] The results showed that for a concentration of 2 g / L, Mg 2+ :Li +A 20:1 MgCl2 and LiCl mixed solution was used. The separation factor of the membrane prepared in Example 1 was 6.83; the separation factor of the composite nanofiltration membrane prepared in Example 2 was 9.23; the separation factor of the composite nanofiltration membrane prepared in Example 3 was 9.47; the separation factor of the membrane prepared in Comparative Example 1 was 5.35; the separation factor of the membrane prepared in Comparative Example 2 was 6.33; and the separation factor of the membrane prepared in Comparative Example 3 was 6.45.

[0086] It is evident that the membrane of this application has a superior separation factor when performing magnesium-lithium separation, and can achieve magnesium-lithium separation more effectively.

[0087] As can be seen from the above experiments, compared with the comparative examples, the magnesium-lithium separation composite nanofiltration membranes prepared in Examples 1-3 all have higher pure water flux, with a flux increase of 15% to 200%, improved magnesium chloride retention, and higher magnesium-lithium separation factor in magnesium-lithium mixed salt solutions. This indicates that the composite nanofiltration membrane has excellent performance and can be applied to lithium extraction from salt lakes.

[0088] The above description is merely a preferred embodiment of the present invention, and the present invention is not limited to the above embodiments. It is understood that other improvements and variations that are directly derived or conceived by those skilled in the art without departing from the spirit and concept of the present invention should be considered to be included within the protection scope of the present invention.

Claims

1. A composite nanofiltration membrane, characterized by, The composite nanofiltration membrane includes a base membrane, an intermediate layer formed on the base membrane, and a separation layer formed on the intermediate layer; The intermediate layer is formed by co-depositing polydopamine and iron molecular cages on the surface of the base film; The separation layer includes a polyamide layer; The base membrane is a polymer ultrafiltration membrane; the separation layer is a polyamide layer formed by interfacial polymerization of polyethyleneimine and polyacryl chloride on an intermediate layer. The iron molecular cage is constructed by coordination self-assembly of ferrous ions as nodes and 4,4'-diaminobiphenyl-2,2'-disulfonic acid hydrate and pyridine-2-carboxaldehyde as ligands, and has a tetrahedral cavity structure.

2. The composite nanofiltration membrane according to claim 1, wherein, The thickness of the intermediate layer is 10-100 nm; the thickness of the separation layer is 10-200 nm.

3. A method for preparing the composite nanofiltration membrane according to any one of claims 1-2, characterized in that, The preparation method includes the following steps: S1: Add dopamine hydrochloride and iron molecular cages to a buffer solution, sonicate to obtain a mixture, pour the mixture onto the surface of the base membrane, oscillate, and wash to obtain a composite membrane containing an intermediate layer. S2: The composite membrane containing the intermediate layer prepared in step S1 is placed in a polyethyleneimine solution for reaction, then a polyacrylamide chloride solution is added. After interfacial polymerization, excess solution is removed, and heat treatment is performed to obtain a composite nanofiltration membrane.

4. The production method according to claim 3, characterized by, In step S1, the base membrane includes one of polyacrylonitrile ultrafiltration membrane, polyvinylidene fluoride ultrafiltration membrane, polyethersulfone ultrafiltration membrane, and polysulfone ultrafiltration membrane; the pore size of the base membrane is 5-50 nm.

5. The preparation method according to claim 3, characterized in that, In step S1, the concentration of the dopamine hydrochloride solution in the mixture is 0.02-0.5 wt%, the concentration of the iron molecular cage is 0.001-0.1 wt%, the concentration of the buffer solution is 10-20 mM, and the oscillation time is 0.5-5 h.

6. The preparation method according to claim 3, characterized in that, In step S2, the molecular weight of the polyethyleneimine is 1000~70000; the concentration of the polyethyleneimine is 0.1-5 wt%; the reaction temperature is 25℃ and the time is 5-10 min.

7. The preparation method according to claim 3, characterized in that, In step S2, the polyacrylamide chloride solution is obtained by dissolving the polyacrylamide chloride in an organic solvent; the organic solvent includes at least one of n-hexane, n-heptane, benzene, and toluene; the concentration of the polyacrylamide chloride is 0.1-1 wt%; the polyacrylamide chloride includes at least one of trimesoyl chloride, isophthaloyl chloride, and terephthaloyl chloride; the temperature of the interfacial polymerization reaction is 25°C, and the time is 0.5-3 min; the temperature of the heat treatment is 30-60°C, and the time is 3-10 min.

8. The application of a composite nanofiltration membrane according to any one of claims 1-2 or a composite nanofiltration membrane prepared by the preparation method according to any one of claims 3-7, characterized in that, The composite nanofiltration membrane is used for magnesium-lithium separation.

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