Positive charge composite nanofiltration membrane based on ptf, and preparation method and application thereof

Abstract

The application relates to a PTFE-based positive charge composite nanofiltration membrane and a preparation method and application thereof, and relates to the technical field of nanofiltration membranes and membrane assemblies. The positive charge composite nanofiltration membrane comprises a PTFE base, a deposition layer, a selective layer and a crosslinking layer which are sequentially covered. The positive charge composite nanofiltration membrane takes PTFE as the basis, makes up the macropore defects, improves the stability and hydrophilicity of the base supporting membrane, retains the porosity while increasing the permeability of the base, simultaneously, the surface of the composite nanofiltration membrane is positively charged, the charge effect can be utilized to efficiently separate and enrich monovalent Li and divalent ions in an acid leaching solution, and the base surface is deposited with TA-PEI, so that the macropore defects of the membrane surface are filled, and the stability and smoothness of the selective layer of the membrane surface are optimized, thereby reducing the water passage resistance of the membrane channel.

Description

Technical Field

[0001] The present invention relates to the technical field of nanofiltration membranes and membrane components, in particular to a PTFE-based positively charged composite nanofiltration membrane and a preparation method and application thereof. Background Art

[0002] Lithium-ion batteries (LIBs), the primary power storage system for electronic devices and new energy vehicles, are experiencing explosive growth in demand for replacement. Recycling spent LIBs is of dual importance: it recovers valuable metal resources (primarily Co and Li) and effectively mitigates environmental pollution. However, traditional recycling methods have numerous limitations.

[0003] Membrane separation technology is widely used in the field of water treatment due to its advantages such as convenient operation, low energy consumption, large processing capacity, no secondary pollution, and environmental protection. At the same time, nanofiltration membrane (NF) separation technology has attracted much attention due to its lower operating pressure than reverse osmosis and its advantages of selective separation of ions with different valence states. Excellent nanofiltration membrane separation materials not only need to meet a high retention rate, but their permeation flux is also an intuitive parameter for judging the performance of the membrane. Methods to improve the permeation flux of membrane materials include: changing the thickness of the separation layer, increasing the porosity of the membrane, and improving the hydrophilicity of the membrane surface. Among them, hydrophilic modification can be achieved through plasma treatment, ultraviolet irradiation, surface coating, atomic deposition (ALD) and chemical treatment (introduction of polar hydrophilic groups: such as hydroxyl, carboxyl and amine groups, etc.).

[0004] PTFE, often called the "King of Plastics," is widely used in various fields due to its excellent performance. However, due to technical limitations, the smallest pore size of PTFE used as a base membrane material is only 0.1 micron. While large-pore membranes can improve permeation flux, the severe macropore defects result in poor support for the selective layer. Summary of the Invention

[0005] In response to the above problems, the present invention provides a PTFE-based positively charged composite nanofiltration membrane. The positively charged composite nanofiltration membrane is based on PTFE, which makes up for the macropore defects and improves the stability and hydrophilicity of the substrate support membrane. It retains the porosity while increasing the permeability of the substrate. At the same time, the surface of the composite nanofiltration membrane is positively charged, and the charge effect can be used to efficiently separate and enrich monovalent Li and divalent ions in the acidic leachate. TA-PEI is deposited on the surface of the substrate to fill the pore defects on the membrane surface, and the stability and smoothness of the selective layer on the membrane surface are optimized, so that the resistance of water passing through the membrane channel is reduced, which is more conducive to its application in the recycling of waste batteries.

[0006] In order to achieve the above object, the present invention provides a positive charge composite nanofiltration membrane based on PTFE, comprising a PTFE substrate, a deposition layer, a selective layer, and a cross-linked layer covered in sequence; the deposition layer is composed of TA-Fe Ⅲ The selective layer is mainly obtained by interfacial polymerization of PEI and TMC, and the cross-linked layer is mainly obtained by strong cross-linking of cross-linking agent, TA and PEI.

[0007] In view of the macroporous defects of PTFE, the inventors proposed to use TA-Fe Ⅲ Tannic acid TA, also known as tannic acid, is a polyphenolic high molecular compound. It is widely used due to its strong hydrophilicity and substrate-independent adhesiveness. At the same time, due to the large number of phenolic hydroxyl groups in TA forming complexes with metal ions, the reaction speed is fast and the efficiency is high, which is often used for material modification. Ⅲ The preparation of the method is mainly divided into one step of self-assembly (TA and Fe Ⅲ Simultaneously placed on the substrate surface for mixed reaction) and layer-by-layer self-assembly (TA and Fe Ⅲ Alternately immersed in the substrate surface layer by layer mixed reaction), the reaction mainly exists in the form of a three-ligand complex under pH>7 conditions (such as Figure 1 ), the complex formed under this condition is relatively stable. Ⅲ The complex deposition modified macroporous PTFE substrate support membrane can achieve the dual purpose of macropore filling and improving the hydrophilicity of the material.

[0008] like Figure 2 The present invention is based on the TA / Fe Ⅲ Coating improves macroporous substrate defects and enhances the hydrophilicity of the substrate supporting membrane, further improving the permeation flux of the membrane material; relying on the interfacial polymerization of polyamine-based long-chain PEI and TMC to form a selective layer with a positive charge on the surface; the cross-linked layer formed by strong cross-linking of TA and PEI has dense porosity and forms a covalent bond to enhance the stability of the membrane. By introducing TA-Fe ⅢCoordination and complexation are used to compensate for the macroporous defects of polytetrafluoroethylene (PTFE). The large number of hydrophilic groups (phenolic hydroxyl groups) in tannic acid (TA) further enhance the stability and hydrophilicity of the base support membrane. The modified PTFE base membrane increases the permeability of the base membrane while retaining the porosity. In addition, the positive charge on the membrane surface is modified by a single interfacial polymerization (IP) of polyethyleneimine (PEI) and trimesoyl chloride (TMC), and TA-PEI is deposited on the surface, so that the pore defects on the membrane surface can be filled. The self-oxidation of TA under alkaline conditions to generate quinones and PEI form a stronger chemical bond (CN\C=N), which optimizes the stability and smoothness of the selective layer on the membrane surface and reduces the resistance of water passing through the membrane channel. This provides a reference for further improving the feasibility of using PTFE substrates as nanofiltration membrane support layers in nanofiltration membrane separation technology.

[0009] In one embodiment, the deposited layer is mainly composed of TA and Fe in a molar ratio of 1: (1.8-2.2). 3+ Prepared.

[0010] In one embodiment, the selective layer is mainly prepared from PEI and TMC in a mass percentage of 1 wt%:(0.3-0.5) wt%.

[0011] In one embodiment, the cross-linking layer is mainly prepared by a cross-linking agent, TA, and PEI in a mass percentage of (1.8-2.2) wt%: (1.3-1.7) wt%: (2.2-2.6) wt%, and the cross-linking agent includes GA.

[0012] The present invention also provides a method for preparing the positively charged composite nanofiltration membrane, comprising the following steps:

[0013] Preparation of deposition layer: Place TA solution on the surface of PTFE substrate, let it stand, add Fe 3+ The aqueous solution reacts to make TA-Fe Ⅲ Layer by layer self-assembly to form a deposition layer, which is then heated and solidified to obtain PTFE-TA / Fe Ⅲ Modified membrane;

[0014] Preparation of selective layer: PEI solution was placed on PTFE-TA / Fe Ⅲ Modify the membrane surface, let it stand, add the organic phase containing TMC to react, obtain the selective layer through interfacial polymerization, heat and solidify to obtain PTFE-TA / Fe Ⅲ -PA modified membrane;

[0015] Preparation of cross-linking layer: GA and TA are placed on PTFE-TA / Fe Ⅲ -PA modified membrane surface, reacted, heated and solidified, and PEI solution was placed on PTFE-TA / Fe ⅢThe surface of the PA modified membrane is reacted, heated and solidified to obtain the positive charge composite nanofiltration membrane.

[0016] In one embodiment, the TA solution in the step of preparing the deposition layer is a Tris-HCl buffer solution containing TA, the Fe 3+ aqueous solution is an FeCl3·6H2O aqueous solution; the molar ratio of TA in the TA solution to Fe 3+ in the Fe 3+ aqueous solution is 1:(1.8-2.2);

[0017] The step of preparing the deposition layer comprises: placing the TA solution on the surface of the PTFE substrate, standing for 2-5 min, adding the Fe 3+ aqueous solution to react for 0.5-1.5 min, and making the TA-Fe Ⅲ layer self-assemble, wherein the layer self-assembling is performed at least twice to form the deposition layer, and the deposition layer is heated and solidified to obtain the PTFE-TA / Fe Ⅲ modified membrane.

[0018] In one embodiment, the PEI aqueous solution in the step of preparing the selective layer is a Tris-HCl buffer solution containing PEI and SDS, and the mass percentage of PEI and SDS is 1wt%:(0.02-0.06)wt%.

[0019] The step of preparing the selective layer comprises: placing the PEI solution on the surface of the PTFE-TA / Fe Ⅲ modified membrane, standing for 30-50 min, adding an organic phase containing TMC to react for 90-110 s, and heating and solidifying to obtain the PTFE-TA / Fe Ⅲ -PA modified membrane.

[0020] In one embodiment, the PEI solution in the step of preparing the cross-linking layer is a Tris-HCl buffer solution containing PEI.

[0021] The step of preparing the cross-linking layer comprises: placing GA and TA on the surface of the PTFE-TA / Fe Ⅲ -PA modified membrane to react for 15-25 min, heating and solidifying, placing the PEI solution on the surface of the PTFE-TA / Fe Ⅲ -PA modified membrane to react for 0.8-1.2 h, heating and solidifying to obtain the positive charge composite nanofiltration membrane.

[0022] The application also provides applications of the positive charge composite nanofiltration membrane in magnesium-lithium separation, lithium recovery from waste lithium ion batteries, heavy metal removal, or dye removal.

[0023] The application further provides a method for recycling lithium from waste lithium ion batteries, comprising the following steps: using the positive charge composite nanofiltration membrane to separate and enrich lithium ions and divalent ions in an acidic leaching solution of a positive electrode material of the waste lithium ion batteries.

[0024] Compared with the prior art, the application has the following beneficial effects:

[0025] The PTFE-based positive charge composite nanofiltration membrane, the preparation method and the application thereof make up for the macropore defects, improve the stability and hydrophilicity of the base support membrane, retain the porosity and increase the permeability of the base, the surface of the composite nanofiltration membrane is positively charged, the charge effect can be used to efficiently separate and enrich monovalent Li and divalent ions in the acidic leaching solution, and the TA-PEI is deposited on the surface of the base, so that the macropore defects of the membrane surface are filled, and the stability and smoothness of the selective layer on the membrane surface are optimized, the water passing through the membrane channel is reduced in resistance, and the application in the recycling of waste batteries is more favorable. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 TA / Fe under different pH conditions Ⅲ Complex form schematic diagram;

[0027] Figure 2 PTFE-TA / Fe Ⅲ Synthesis and separation schematic diagram of PA-TA / PEI composite nanofiltration membrane;

[0028] Figure 3 PTFE-TA / Fe Ⅲ Retention rate and permeation flux result graph of the composite nanofiltration membrane under different conditions, wherein the test conditions are as follows: 2g / L CoCl2·6H2O aqueous solution, 0.4Mpa, 25 DEG C, and pH=7;

[0029] Figure 4 PTFE-TA / Fe Ⅲ Retention rate and permeation flux result graph of the PA composite nanofiltration membrane under different conditions, wherein the test conditions are as follows: 2g / L CoCl2·6H2O aqueous solution, 0.4Mpa, 25 DEG C, and pH=7;

[0030] Figure 5 PTFE-TA / Fe Ⅲ Retention rate and permeation flux result graph of the PA-TA / PEI composite nanofiltration membrane under different conditions, wherein the test conditions are as follows: 2g / L CoCl2·6H2O aqueous solution, 0.4Mpa, 25 DEG C, and pH=7;

[0031] Figure 6SEM images of the original PTFE membrane and the modified membrane, where: a1-d1: 7.00KX; a2-d2: 10.00KX; a3-d3: 20.00KX; a: PTFE membrane; b: PTFE-TA / Fe Ⅲ Membrane; c: PTFE-TA / Fe Ⅲ -PA membrane; d: PTFE-TA / Fe Ⅲ -PA-TA / PEI;

[0032] Figure 7 EDS and element mapping analysis diagrams of the original membrane and modified membrane surface, where: (a) PTFE-TA / Fe Ⅲ (b) PTFE-TA / Fe Ⅲ -PA modified membrane; (c) PTFE-TA / Fe Ⅲ -PA-TAGA-PEI modified membrane;

[0033] Figure 8 AFM images of the original PTFE membrane and the modified membrane, where ab is the original PTFE membrane; cd is the PTFE-TA / Fe Ⅲ Modified membrane; ef is: PTFE-TA / Fe Ⅲ -PA modified membrane; gh is: PTFE-TA / Fe Ⅲ -PA-TA / PEI modified membrane;

[0034] Figure 9 ATR-FTIR images of the original PTFE membrane and the modified membrane, where a is the full spectrum; bc are local images;

[0035] Figure 10 The XPS spectra are shown in Figure 1, where the left figure shows the full spectrum of the original PTFE membrane and the modified membrane; the right figure shows the full spectrum of the PTFE-TA / Fe membrane. Ⅲ Fitting peak diagram of Fe element in modified membrane;

[0036] Figure 11 The XPS peak fitting spectrum of C1s, where the left picture is the original PTFE membrane; the right picture is PTFE-TA / Fe Ⅲ Modified membrane;

[0037] Figure 12 The XPS peak fitting spectrum of N1s, where the left figure is PTFE-TA / Fe Ⅲ -PA modified membrane; the right picture is PTFE-TA / Fe Ⅲ -PA-TA / PEI modified membrane;

[0038] Figure 13 TA and Fe Ⅲ Schematic diagram of coordination reaction;

[0039] Figure 14 Schematic diagram of TA autooxidation and Schiff-base and Michael reactions under alkaline conditions;

[0040] Figure 15 Schematic diagram of the polyamide (PA) selective layer reaction;

[0041] Figure 16 Schematic diagram of selective dense layer reaction;

[0042] Figure 17 The water contact angle (WCA) result diagram of the membrane surface;

[0043] Figure 18 The results of the solid Zeta potential test and pore size analysis test on the membrane surface are shown in the figure below. The left figure is PTFE-TA / Fe Ⅲ -PA-TA / PEI composite nanofiltration membrane PEG molecule retention rate results, the right figure is PTFE-TA / Fe Ⅲ -Pore size distribution results of PA-TA / PEI composite nanofiltration membrane;

[0044] Figure 19 PTFE membrane, PTFE-TA / Fe under different pH conditions Ⅲ Membrane and PTFE-TA / Fe Ⅲ -Plot of the Zeta potential of PA-TA / PEI membrane as a function of pH;

[0045] Figure 20 The results of the single ion retention experiment are shown in Figure 1, where the left figure is the NF membrane separation retention rate and the right figure is the permeation flux;

[0046] Figure 21 The left figure shows the rejection rate of each ion separated by the actual sample (NCM 811) NF membrane, and the right figure shows the permeation flux and ion concentration ratio;

[0047] Figure 22 Schematic diagram of the operating steps for recycling lithium-ion batteries using NF membrane separation technology. DETAILED DESCRIPTION

[0048] To facilitate understanding of the present invention, the present invention will be described more fully below with reference to the accompanying drawings. Preferred embodiments of the present invention are shown in the accompanying drawings. However, the present invention may be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and comprehensive understanding of the present disclosure.

[0049] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in the description herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0050] Experimental instruments:

[0051] Spiral flat membrane pilot plant equipment (Flowmem-0016, Guangzhou Yuekuang Machinery Equipment Co., Ltd.), X-ray photoelectron spectrometer (AXIS SUPRA, Shimadzu Corporation, Japan), total organic carbon analyzer (TOC-L, Shimadzu Corporation, Japan), solid surface Zeta potential analyzer (Anton Paar surpass 3, Anton Paar GmbH, Austria), plasma emission spectrometer (SPECTRO ARCOS MV, SPECTRO, Germany), pH meter (CT-6021A, Qingdao Juyue Times Environmental Protection Technology Co., Ltd.), conductivity meter (DDS-307A, Shanghai Yilian Scientific Instrument Co., Ltd.), attenuated total reflection Fourier transform infrared spectroscopy (Nicolet 6700, Perkinelmer Corporation, USA), contact angle measuring instrument (JC2000D3P, Shanghai Zhongchen Digital Technology Equipment Co., Ltd.), field emission scanning electron microscope (ZEISS Ultra 55, Carl Zeiss Company, Germany), analytical electronic balance (A200S, Sartorius, Germany), atomic force microscope (Dimenson ICON, Bruker Corporation, USA), ultrasonic cleaner (SK250H, Shanghai Kedao Instrument Co., Ltd.).

[0052] Experimental reagents:

[0053] 1,3,5-Benzenetricarboxylic acid chloride (TMC) (analytical grade, Shanghai McLean Biochemical Co., Ltd.), n-hexane (analytical grade, Shanghai McLean Biochemical Co., Ltd.), triethylamine (TEA) (analytical grade, Shanghai McLean Biochemical Co., Ltd.), polyethyleneimine (PEI) (Mw = 70,000 Da, 30% aqueous solution, analytical grade, Shanghai McLean Biochemical Co., Ltd.), sodium hydroxide (NaOH) (analytical grade, Shanghai McLean Biochemical Co., Ltd.), sodium dodecyl sulfate (SDS) (analytical grade, Shanghai McLean Biochemical Co., Ltd.), anhydrous lithium chloride (analytical grade, Shanghai McLean Biochemical Co., Ltd.), cobalt chloride hexahydrate (analytical grade, Shanghai McLean Biochemical Co., Ltd.) Co., Ltd.), nickel chloride hexahydrate (analytical grade, Shanghai Maclean Biochemical Co., Ltd.), manganese chloride tetrahydrate (analytical grade, Shanghai Maclean Biochemical Co., Ltd.), polyethylene glycol (PVA) of different molecular weight (analytical grade, Shanghai Maclean Biochemical Co., Ltd.), tannic acid (TA) (analytical grade, Tianjin Fuchen Chemical Reagent Factory), glutaraldehyde (GA) (analytical grade, Tianjin Komiou Chemical Reagent Co., Ltd.), ferric chloride hexahydrate (analytical grade, Shanghai Maclean Biochemical Co., Ltd.), hydrochloric acid (HCl) (analytical grade, Guangdong Guangzhou Reagent Technology Co., Ltd.), hydrophilic PTFE membrane (pore size 0.1 μm, hydrophilic, Longjin Membrane Technology Co., Ltd.).

[0054] Unless otherwise specified, the reagents, materials, and equipment used in this example are all commercially available; and the experimental methods, unless otherwise specified, are all conventional experimental methods in the art.

[0055] Example

[0056] 1. Positively charged composite nanofiltration membrane based on PTFE and its preparation method.

[0057] 1. PTFE-TA / Fe Ⅲ Synthesis steps of modified membranes.

[0058] Dissolve 0.1 g of TA in 50 ml of Tris-HCl (pH = 8, 50 mM) aqueous solution (avoid light) to prepare a molar ratio of TA: Fe 3+ = 1:2 FeCl3·6H2O aqueous solution 50ml, put the TA solution on the PTFE bare membrane and let it stand for 3min, pour and remove the excess TA solution on the membrane surface, and add Fe 3+ The aqueous solution reacts for 1 min. Similarly, TA-Fe Ⅲ Repeat the layer-by-layer self-assembly (LBL) for 3 times, and place the modified membrane in an oven for 12 hours to heat and solidify. Ⅲ membrane.

[0059] 2. PTFE-TA / Fe Ⅲ -Synthesis steps of PA modified membrane.

[0060] Prepare 1wt% PEI (SDS 0.04wt%, Tris-HCl, pH = 8.5, 50mM) aqueous solution and place it on the membrane modified support membrane (PTFE-TA / Fe Ⅲ ) surface for 40 minutes, and then the excess water phase on the membrane surface was poured out and removed, and then the interfacial polymerization reaction with 0.4wt% TMC in the organic phase was carried out for 100 seconds, and then heated and cured at 60°C for 10 minutes. Ⅲ -PA modified membrane.

[0061] 3. PTFE-TA / Fe Ⅲ -Synthesis steps of PA-TA / PEI modified membrane.

[0062] GA (2wt%, pH=2) and TA (1.5wt%, pH=3) were placed on the membrane surface for reaction for 20 min, and the membrane was placed in a 50°C oven for 5 min to solidify. Finally, PEI (2.4wt%, Tris-HCl, pH=8.5, 50mM) was immersed on the membrane surface for 1 h and then thermally cured in an oven at 60°C for 15 min to obtain a nanofiltration membrane PTFE-TA / Fe with a dense membrane surface and positive charge. Ⅲ -PA-TA / PEI.

[0063] Note: Except for the TMC solution which is n-hexane, the other solvents are deionized water.

[0064] 2. Structural characterization of original membrane and modified composite nanofiltration membrane.

[0065] 1. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) analysis.

[0066] The original membrane and modified membrane materials were taken out of the deionized water and dried, and 1 cm × 1 cm square pieces were cut. ATR-FTIR was used to analyze the membrane at 4000 cm -1 -600cm -1 Scan and analyze the chemical structure of the membrane surface.

[0067] 2. Field emission scanning electron microscope (SEM).

[0068] The original membrane and modified membrane materials were taken out of the deionized water and dried, and small pieces of 1 cm × 1 cm were cut out. After gold spraying, they were placed in an electron microscope room under vacuum conditions to observe the surface changes.

[0069] 3. Atomic force microscope (AFM).

[0070] The original membrane and synthetic membrane samples were first rinsed with deionized water and dried at 40°C under vacuum. The test membrane sample was prepared by cutting the membrane to the appropriate size and fixing it to the sample stage with conductive double-sided tape. The test area was 10 μm x 10 μm using a tapping mode. The composite nanofiltration membrane surface roughness was expressed in terms of the root mean square roughness (Rms).

[0071] 4. X-ray photoelectron spectroscopy (XPS).

[0072] XPS explores the chemical composition of the sample surface and requires carbon correction analysis. The membrane in deionized water was taken out, cut into small pieces and dried at 40°C under vacuum, cut into 1 cm x 1 cm squares, and fixed to the sample stage with conductive glue to test the surface chemical composition and valence state of the membrane.

[0073] 5. Membrane surface hydrophilicity / contact angle test.

[0074] In this test, the test sample membrane was dried and cut into 1 cm x 2 cm samples, 1 drop of deionized water was used as the test solution, and the contact angle was saved and analyzed using a digital picture. Multiple tests were performed to obtain the average value.

[0075] 6. Membrane solid surface Zeta potential test.

[0076] The test membrane was immersed in a 0.001 M KCl solution and soaked thoroughly, then fixed in the sample cell using conductive glue. The 0.001 M KCl solution was used as the test medium, and the pH value of the solution was adjusted by adding HCl or NaOH to determine the surface potential of the membrane under different pH conditions.

[0077] 7. Molecular weight cut-off and pore size distribution.

[0078] The molecular weight of a solute with a rejection rate of 90.0% is defined as the molecular weight cut-off (MWCO) of the membrane. The larger the MWCO, the larger the pore size of the membrane material. In this study, different molecular weights (200, 400, 600, 1000, 2000 Da) of the electrically neutral polymer polyethylene glycol (PEG) were used as test molecules, and a concentration of 0.1 g / L was used as the feed liquid. The permeate after passing through the test membrane was used as the inlet and outlet sample liquid, and the total organic carbon analyzer (TOC) was used to test the change in carbon content.

[0079] Finally, the PEG rejection rate was calculated using formula (1) to characterize the pore size of the membrane.

[0080]

[0081] Among them, RPEG is based on the retention rate of PEG molecules to characterize the pore size of the membrane material, TOC permeate and TOC feed are the organic carbon concentrations of the feed solution and the permeate, respectively.

[0082] By performing nonlinear curve fitting on the retention rate data of PEG with different molecular weights tested above using the LogNormal function, the relationship equation between the retention rate of the membrane for PEG molecules and the molecular weight of PEG was explored.

[0083] The Stokes diameter (ds, nm) of a PEG molecule can be determined by its molecular weight (M w , Da) is calculated, and the relationship is as follows (2):

[0084]

[0085] The pore size distribution is calculated using the following probability density function (3):

[0086]

[0087] When R PEG =50%, corresponding to the Stokes diameter μ of the PEG molecule s The average diameter of the membrane μ p Same, σ p Defined as the average effective pore size μ of the membrane p The geometric standard deviation corresponds to R PEG = 84.1% of the Stokes diameter of polyethylene glycol molecules (d s , nm) and μ s ratio.

[0088] 3. Performance test of composite nanofiltration membrane.

[0089] In the present invention, commercial PTFE membrane (pore size 0.1 μm) was purchased and the membrane performance was tested using a cross-flow spiral flat membrane test device, including membrane permeation flux, single valuable metal retention rate and mixed ion retention rate performance test. The effective area of ​​the membrane used for the measurement was 69.36 cm 2 The feed concentration is 2000ppm, the test pressure is 0.4Mpa, and the pre-pressure is 30min at room temperature and atmospheric pressure to ensure stable membrane performance. At least 3 identical membranes are tested for each test for comparison to eliminate errors.

[0090] 1. Penetration performance.

[0091] Permeation flux refers to the volume of liquid passing through a unit effective area per unit time under unit pressure, which directly reflects the separation speed of the membrane. Therefore, the greater the permeation flux, the faster the separation rate. It is calculated as follows:

[0092]

[0093] Where J is the permeation flux (L m -2 h -1 ), V represents the volume of liquid passing through the operating conditions (L), A is the effective membrane area of ​​the membrane separation test (m 2 ), t is the sampling time (h).

[0094] 2. Retention performance.

[0095] During the membrane separation test, divalent ions in the feed solution are retained in the permeate, and single ions pass through the membrane into the permeate, thereby achieving efficient separation of divalent and monovalent ions. The retention rate (R) refers to the ratio of the concentration of the retained solute to the concentration of the feed solution, and is calculated as follows:

[0096]

[0097] Among them, R is the retention efficiency of the membrane for solute (%), C f is the solute concentration of the feed solution, C p is the solute concentration in the permeate.

[0098] Note: In this work, single ion solutions were tested using a conductivity meter (Raymag, DDS-307A), and mixed ion solutions were tested using inductively coupled plasma (ICP).

[0099] 3. Single ion test and actual sample performance test.

[0100] (1) Single sample preparation: Deionized water was used as the solvent, and 2 g / L (LiCl, NiCl2·6H2O, CoCl2·6H2O, MnCl2·4H2O) was prepared as the single ion test feed solution.

[0101] (2) Preparation of actual sample leaching solution: The laboratory waste lithium-ion battery NCM (811) was used as the actual test object. After the waste battery was discharged, it was disassembled and the positive electrode material was dissolved in NaOH for 10 hours to ensure that the positive electrode material current collector Al was dissolved and removed. The undissolved solid was calcined at high temperature (500℃, 5 hours), and 4g of the residue was dissolved in 1L of deionized water solvent, and the pH was adjusted to 4. The feed solution concentration was configured to be 4g / L.

[0102] (3) Test steps: The above single ion solution or actual sample leaching solution is used as the feed solution to pass through the PTFE-TA / Fe ⅢThe permeate is obtained by performing a single nanofiltration run through a PA-TA / PEI composite nanofiltration membrane. Single ion inlet and outlet samples are tested using a conductivity meter; mixed ion samples are tested using inductively coupled plasma (ICP). Finally, the retention rate and permeate flux are calculated using a formula to characterize membrane separation performance.

[0103] 4. Results and discussion.

[0104] 1. Optimization conditions for synthesis of composite nanofiltration membrane.

[0105] (1)PTFE-TA / Fe Ⅲ Optimized preparation of composite nanofiltration membrane.

[0106] With the strong adhesion of TA and the 3+ Rapid self-assembly to generate inorganic metal complexes can achieve the purpose of filling the macropores of the substrate support membrane PTFE and optimizing the hydrophilicity. Ⅲ Modified membrane properties mainly include TA / Fe 3+ Layer-by-layer self-assembly (LBL) layer number, TA and Fe 3+ concentration ratio, self-assembly reaction time, etc. Ⅲ The modification conditions were optimized, such as Figure 3 Among them, the number of self-assembled layers has a great influence on the membrane performance. That is, the higher the number of membrane layers, the more uniform the deposition, the more stable the support layer, and the higher the membrane rejection rate. However, the corresponding permeation flux will decrease due to the increase in water channel resistance. Therefore, under the premise of ensuring the support of the substrate without losing the permeation flux, Figure 3 (a) It can be seen that the condition of selecting three layers of layer-by-layer self-assembly as the appropriate modification of the basement membrane. Figure 3 As shown in (b), when TA / Fe Ⅲ When the concentration is relatively low, the high Fe concentration leads to over-coordination to form granular deposits and even agglomeration, resulting in uneven deposition; when TA / Fe Ⅲ When the concentration is relatively high, the Fe concentration is too low, resulting in insufficient coordination sites for TA, which exists in the form of double coordination and single coordination. This state is extremely unstable. 3+ When the concentration ratio is 1:2, the stable three-coordinate complex is achieved. 3+ The reaction conditions are mild and rapid, so the membrane thickness can be controlled by controlling the self-assembly time to improve the membrane properties. Figure 3 (c) It can be seen that when the self-assembly reaction time is 1.5 min, better retention rate and permeation flux can be achieved.

[0107] Above PTFE-TA / Fe Ⅲ The optimal conditions for preparing composite nanofiltration membranes are: TA and Fe 3+The concentration ratio is 1:2, 3 layers are self-assembled and the self-assembly reaction time is 1.5 minutes.

[0108] (2)PTFE-TA / Fe Ⅲ -Optimized preparation of PA composite nanofiltration membrane.

[0109] like Figure 4 , fixed 0.4wt% TMC to explore the effect of PA layer aqueous phase monomer concentration and interfacial polymerization time on PTFE-TA / Fe Ⅲ -PA composite nanofiltration membrane performance. A thinner selective layer can improve the membrane permeation flux, so when PEI is 1.2wt%, one end can be connected to the modified base membrane PTFE-TA / Fe under alkaline conditions. Ⅲ Unreacted TA is self-oxidized to ketone and undergoes Schiff base or Michael addition (such as Figure 14 ) to enhance the stability of the bond with the substrate; the other end undergoes interfacial polymerization reaction with TMC (such as Figure 15 If the PEI concentration is too high and the interfacial polymerization reaction time is too long, the thickness of the selective membrane layer will increase, which will greatly reduce the permeation flux performance of the membrane material.

[0110] Therefore, the above PTFE-TA / Fe Ⅲ The optimal conditions for the preparation of -PA composite nanofiltration membrane are: PEI concentration of 1.2wt% and interfacial polymerization reaction time of 100s.

[0111] (3)PTFE-TA / Fe Ⅲ -Optimized preparation of PA-TA / PEI composite nanofiltration membrane.

[0112] Due to the unevenness of the interfacial polymerization and the thinness of the selective layer, there are some defective pores on the membrane surface, such as SEM Figure 6 (c) can be obtained, so it is necessary to further fill the pores and enhance the pore density of the selective membrane layer. GA is used as a cross-linking agent to deposit strong cross-linked TA and PEI on the surface of the defective membrane, thereby enhancing the pore density and replacing some secondary bonds (hydrogen bonds, electrostatic effects, etc.) through covalent bonds (CN or C=N) to enhance the surface stability of the membrane (such as Figure 16 ).like Figure 5 By weighing the trade-off between rejection rate and permeation flux, GA and TA were selected for deposition reaction for 20 min respectively. When the PEI concentration was 2.5wt%, a better membrane performance material could be prepared.

[0113] In summary, the optimal synthesis conditions of composite nanofiltration membrane are: the molar ratio of TA:Fe 3+ =1:2,TA / Fe 3+The coordination reaction was carried out for 1 minute, followed by layer-by-layer self-assembly into 3 layers and thermal curing at 50°C for 12 hours. 1.2wt% PEI (SDS 0.1wt%, Tris-HCl pH=8.550mM) and 0.4wt% TMC were interfacially polymerized with IPs for 100 seconds and thermally cured at 60°C for 10 minutes. GA (2wt%, pH=2) and TA (1.5wt%, pH=3) were reacted for 20 minutes respectively and heated at 50°C for 5 minutes. Finally, PEI (2.5wt%, Tris-HCl pH=8.550mM) was reacted for 1 hour and then thermally cured at 60°C for 15 minutes.

[0114] 2. Structural characterization of materials.

[0115] (1) SEM and AFM characterization of original membrane and modified membrane materials.

[0116] The macroscopic morphology of the original membrane and the modified membrane was characterized by SEM. Figure 6 It can be seen that the surface morphology of the membrane varies greatly under different conditions. Figure 6 As shown in (a1-a3), compared with traditional PSF and PES substrate support membranes, PTFE has larger pores, which greatly reduces the support strength of the ultra-thin selective layer. It is very easy to cause the membrane to rupture due to high pressure or long-term water penetration, thereby destroying the membrane performance. Figure 6 As shown in (b1-b3), based on TA-Fe Ⅲ After deposition modification, the membrane pores are significantly smaller, which can provide stable support for the subsequent selective layer. Figure 17 As shown, TA-Fe Ⅲ The complex makes the membrane surface highly hydrophilic. Ⅲ The modified membrane has small pore size and hydrophilic properties.

[0117] like Figure 6 (c1-c3), in PTFE-TA / Fe Ⅲ The modified membrane surface is subjected to interfacial polymerization (IP) reaction to prepare a positively charged selective polyamide (PA) layer, which further reduces the membrane pores. At the same time, the unreacted free amine groups (-NH2) can be protonated to form positively charged -NH3 + On the other hand, it can react with the unreacted TA to strengthen the adhesion of the PA layer on the membrane surface. However, due to the uneven distribution of polymer molecular weight or non-uniform diffusion at the interface, some wider pores still exist. At the same time, PTFE-TA / Fe Ⅲ -PA membrane surface has a lot of wrinkles, resulting in greater roughness (such as Figure 8), will increase the resistance of water to permeate the membrane, resulting in a decrease in membrane flux, and thus weakening the membrane performance. Based on GA cross-linking, the strong cross-linking reaction of TA and PEI deposited on the membrane surface can further optimize the membrane pore size and surface smoothness, such as Figure 6 (d1-d3). After modification, a composite nanofiltration membrane with nanoscale pore size and smooth membrane surface was prepared.

[0118] like Figure 7 , EDS-mapping was used to characterize the element content and distribution of the modified membrane. The XPS spectrum of the original PTFE membrane shows that it only contains three elements: C, O, and F. Ⅲ After deposition, the EDS spectra of the modified membrane surface showed a new, uniformly distributed Fe signal. Furthermore, the EDS spectrum indicated the presence of N, confirming the successful modification by the interfacial polymerization of PEI and TMC.

[0119] PTFE-TA / Fe Ⅲ -PA-TAGA-PEI modified membrane compared with PTFE-TA / Fe Ⅲ The N and O content of the -PA modified membrane increased, indicating the successful optimization of the TA and PEI dense layers. At the same time, it can be concluded from the element mapping diagram that each element is evenly distributed, indicating that the membrane modification is also relatively uniform, and its smooth and dense surface is consistent with the SEM characterization.

[0120] Combining SEM and AFM Figure 8 As shown in Table 1, the original base PTFE membrane has large pore size and large membrane nodules, which leads to larger RMS and Ra and the largest roughness. Ⅲ After modification, the complex is deposited on the macropores of the basement membrane to fill the large particle nodules. The reasonable concentration ratio of TA / Fe Ⅲ The self-assembly reaction generates a stable tri-coordinate complex, which greatly reduces the roughness. Figure 8 (cd). PTFE-TA / Fe Ⅲ The increase in the roughness of the PA-modified membrane is mainly attributed to the unevenness of the polyamide (PA) reaction, which leads to the formation of a wrinkled structure on the membrane surface, resulting in a slight increase in its roughness. Subsequently, through the modification of the TA / PEI dense layer, the TA / PEI deposits filled the uneven areas of the wrinkles, making the membrane surface smoother and ultimately reducing the overall roughness. However, the PTFE-TA / Fe Ⅲ -The roughness of PA-TA / PEI membrane is still higher than that of PTFE-TA / Fe ⅢThe reason for the cross-linking reaction between TA and PEI is that the cross-linking reaction between TA and PEI depends on the dissolved oxygen in water to realize the self-oxidation of TA to generate quinone. The uneven distribution of dissolved oxygen in water leads to a certain roughness on the surface of TA / PEI deposition layer. Ⅲ- Compared with PA membrane, PTFE-TA / Fe Ⅲ -The surface roughness of PA-TA / PEI membrane was significantly reduced, achieving the expected modification goal.

[0121] Table 1 Surface roughness of original PTFE membrane and modified membrane

[0122]

[0123] (2) ATR-FTIR and XPS characterization of original and modified membranes

[0124] like Figure 9 (a), the original membrane PTFE support layer at 1203 cm -1 , 1146cm -1 The modified membrane shows strong CF characteristic peaks (the peaks correspond to the FCF symmetric stretching vibration and FCF asymmetric stretching vibration of the PTFE support membrane respectively). New characteristic peaks appear on the basis of the original characteristic peaks. -1 The new peak at 1225-1060 cm corresponds to the formation of C=N, which strongly proves the Schiff base crosslinking between PEI and TA. -1 The C—O—C ether bond is formed by the chemical reaction between GA and the phenolic hydroxyl group in TA.

[0125] like Figure 9 (b) Since TA contains a large number of phenolic hydroxyl groups and the entire material is hydrophilic, the -1 There are a lot of broad peaks of hydrophilic groups -OH and NH. Even at 3347 cm -1 The peak enhancement near the membrane surface is due to the stretching vibration of the phenolic -OH group in TA, indicating that the TA / PEI dense layer is successfully cross-linked on the membrane surface.

[0126] like Figure 9 (c) Compared with the base membrane PTFE, the characteristic peaks of polyamide in modified membranes M2 and M3 are at 3500-3100 cm -1 NH stretching vibration at 1680-1630 cm -1 C=O stretching vibration (band Ⅰ), 1640-1550cm -1 The in-plane bending vibration of NH (band II) and the stretching vibration of CN (band III) are at 1420-1400 cm -1The characteristic peaks are present at all locations. This proves that the polyamide (PA) selective layer was successfully prepared. Compared with PTFE-TA / Fe Ⅲ PTFE-TA / Fe-PA membrane Ⅲ -PA-TA / PEI membrane, at 1640-1550cm -1 The enhanced CN bending vibration peak of the primary amine at the membrane surface indicates a high degree of cross-linking of TA-PEI.

[0127] XPS was used to characterize the surface structure of the original membrane and the modified membrane. Figure 10 From the atomic percentage content table, we can see that the base support membrane PTFE only contains C1s, F1s, and O1s peaks. Ⅲ After modification, the weak Fe 2p characteristic peak cannot be displayed in the overall image due to the strong F1s characteristic peak. However, based on the hundreds of atoms on the surface of the modified membrane and the Figure 10 (Right) EDS-mapping image shows Fe 3+ Evenly distributed on the membrane surface, and PTFE-Fe Ⅲ / TA modified membrane was finely fitted and peak separated, and the peak at 724.68eV (Fe 2p 1 / 2 ) and 710.58eV(Fe 2p 3 / 2 ) have two Fe 2p characteristic peaks. Secondly, since XPS can only characterize the depth within 10nm of the material surface, in TA-Fe Ⅲ The increased film thickness after modification resulted in the undetectable F characteristic peak. These characteristics indicate that TA-Fe Ⅲ As a macroporous filler, it is effectively deposited on the surface of the base PTFE membrane. Figure 10 (Left) With the addition of PEI, the modified membrane PTFE-TA / Fe Ⅲ -PA and PTFE-TA / Fe Ⅲ -PA-TA / PEI membranes both introduce N1s characteristic peaks.

[0128] Depend on Figure 11 The original PTFE membrane and PTFE-TA / Fe Ⅲ The modified membrane C1s element was finely fitted and peaked. Compared with the original PTFE membrane, the -CF2 peak disappeared due to the increase in thickness. The characteristic peaks at 283.52eV and 290.63eV correspond to CH and C=O functional groups, respectively, which can characterize the TA benzene ring functional group CH and some unreacted TA self-oxidation under alkaline conditions to generate quinones (such as Figure 14 ).

[0129] Depend on Figure 12 For PTFE-TA / Fe Ⅲ -PA and PTFE-TA / FeⅢ -PA-TA / PEI modified membrane N1s element fine fitting peak separation. PTFE-TA / Fe Ⅲ -PA modified membrane generates polyamide characteristic functional groups O=CN* with 399.84eV and CN* functional groups with 398.9eV and can characterize polyamide synthesis (reaction principle as Figure 15 ). PTFE-TA / Fe Ⅲ The increase in the CN* functional group content of PA-TA / PEI at 399 eV (from 32.52% to 64.54%) indicates that the subsequent addition of TA and PEI mainly undergoes Michael addition reaction (reaction principle as shown in Figure 16 ). And due to the 401eV -NH3 + The characteristic peaks can all indicate that the surface of the modified membrane material is positively charged. + The binding energy shifts to the left, possibly due to -NH3 + The formation of hydrogen bonds or electrostatic interactions with other functional groups (hydroxyl or carboxyl) reduces the electron cloud density of N, thereby reducing the binding energy and shifting to the left.

[0130] Table 2 Atomic content of PTFE original membrane and modified membrane

[0131]

[0132] Finally, the synthesis mechanism of the composite nanofiltration membrane can be analyzed by ATR-FTIR and XPS characterization. Figure 13 、 14 , 15, and 16.

[0133] (3) Surface contact angle / hydrophilicity characterization.

[0134] The hydrophilic modification of the membrane surface can improve the membrane's anti-fouling properties and increase the membrane permeation flux. The water contact angle of the modified membrane and the original PTFE membrane were measured. The results are as follows Figure 17 As shown in (d), the contact angles of various types of water are: M0: 105.94°; M1: 56.55°; M2: 43.92°; M3: 57.37°. Compared with the original PTFE membrane TA / Fe Ⅲ Modified PTFE-TA / Fe Ⅲ The hydrophilic angle of the membrane is reduced due to the large number of -OH hydrophilic groups in TA. At the same time, the addition of the polyamide PA selective layer maximizes the hydrophilicity of the membrane due to the introduction of a large amount of -NH2. The contact angle of the subsequent dense nanofiltration membrane is slightly improved. This can be attributed to the Michael or Schiff base reaction between GA, TA and the free amino groups of PEI, which leads to a decrease in the hydrophilic groups -OH and -NH2, resulting in an increase in the contact angle. At the same time, the increase in membrane thickness increases the resistance to water passing through, which also leads to a slight decrease in hydrophilicity.

[0135] Since TA itself has strong adhesion and is based on TA-Fe Ⅲ The rapid coordination reaction can modify the support membrane PTFE in the form of deposition. That is, TA adheres to the membrane, indicating that the phenolic hydroxyl groups are bonded to Fe 3+ Coordination, repeated cycles to achieve layer-by-layer self-assembly (LBL). Figure 17 (a) It can be seen that when the LBL is 5, the contact angle is 41.73°, indicating the highest hydrophilicity. However, as the number of layers increases, the membrane thickness increases, which lengthens the water channel, increasing the water permeation resistance and reducing the membrane permeation flux performance. As can be seen from the optimization diagram, an LBL of 3 is selected, resulting in a contact angle of 46.29°, which is the optimal number of layers.

[0136] In TA / Fe Ⅲ After modification, it can be clearly observed from the electron microscope that there is obvious deposition on the membrane surface, which leads to a decrease in the membrane pore size. The iron element EDS diagram shows that the iron element is evenly distributed, indicating that TA / Fe Ⅲ Successfully deposited on the surface of PTFE membrane, but with different TA-Fe Ⅲ The contact angle of the film surface also changes slightly with the different concentration ratios, such as Figure 17 (b), when TA-Fe Ⅲ The concentration ratio decreases, Fe 3+ Too high a concentration will result in a high degree of coordination. The reaction will produce larger precipitated particles and even agglomerate, resulting in a decrease in hydrophilicity. Ⅲ As the concentration ratio increases, Fe 3+ The concentration is too low, Fe 3+ Insufficient coordination with TA leads to TA deposition on the surface, which makes the hydrophilicity decrease continuously. Ⅲ When the concentration ratio is close to 1:2, the contact angle is 57.94°, which just realizes the tri-coordinate complex TA-Fe Ⅲ It exists in a stable form and has a large number of -OH hydrophilic groups, resulting in optimal hydrophilicity.

[0137] like Figure 17 (c) is TA / Fe Ⅲ Study on the effect of reaction time on the hydrophilicity of membrane surface. TA / Fe 3+ Under alkaline conditions, it is very easy to form complexes with metal chelates, and the large amount of phenolic hydroxyl groups introduced by TA enhance the hydrophilicity of the membrane surface. Ⅲ The complex can be stably deposited on the surface of the macroporous PTFE membrane. As the reaction time increases, TA preferentially reacts with Fe 3+ The rapid reaction exists in a stable state. At this time (reaction time: 1.5min), the phenol in TA is more likely to react with Fe. 3+Coordination reduces the number of hydrophilic groups on the membrane surface and increases the water contact angle. When the reaction time continues to increase, the excess unreacted phenolic hydroxyl groups of TA can improve the hydrophilicity of the membrane surface. However, as the reaction time increases, the hydrophilicity increases slightly, but the deposition thickness also increases accordingly, which will eventually increase the resistance of water passing through the membrane channel and reduce the overall permeation flux of the modified membrane. Therefore, after weighing the permeation flux and the rejection rate, TA / Fe is selected. Ⅲ The reaction time was 1 min, at which time the water contact angle was 57.99°.

[0138] (4) Membrane surface solid Zeta potential test and pore size analysis test.

[0139] The performance of composite nanofiltration membrane is mainly achieved through two major separation principles: charge effect and size screening.

[0140] The size sieving effect is based on the separation of ions of different sizes through different membrane pore sizes. The molecular weight cut-off of neutral PEG can characterize the size of the membrane pore size. Figure 18 As shown in (a), as the PEG molecular weight increases, the retention rate increases. When the PRG retention rate is 90%, the PEG molecular weight is characterized as the molecular weight cut-off of the nanofiltration membrane (MWCO = 635Da). The membrane pore size distribution is calculated by the model as shown in Figure 18 (b), Characterization of prepared PTFE-TA / Fe Ⅲ -The average pore size of the PA-TA / PEI composite nanofiltration membrane is 0.22 nm.

[0141] The charge effect of positively charged nanofiltration membranes in separating cations of different valences is primarily due to the difference in positive-positive repulsion, resulting in selective separation. That is, the more positive charge on the membrane surface, the greater the repulsion for divalent ions compared to monovalent ions, and the higher the divalent ion retention rate. Figure 19 Original PTFE membrane, PTFE-TA / Fe Ⅲ Modified membrane and PTFE-TA / Fe Ⅲ -The curve of the Zeta potential of the PA-TA / PEI modified membrane surface changing with pH. As can be seen from the figure, the strong negative charge of the F element in the strong CF bond of PTFE makes the membrane surface negatively charged in the entire pH range; PTFE-TA / Fe Ⅲ Medium Fe 3+The introduction of TA slightly increased the surface charge of the membrane, but due to the large number of phenolic hydroxyl groups in TA, the membrane surface still showed negative charge in an aqueous environment with a pH greater than 3.4. When polyamine-based long-chain branched PEI aqueous monomers were introduced, the free amine groups were protonated, and some phenolic hydroxyl groups reacted with PEI amine groups and were consumed, which greatly increased the positive charge of the membrane surface. When the pH was greater than 7.6, the surface electronegativity of the membrane was due to the presence of unreacted acyl chloride groups on the membrane surface, which hydrolyzed to form carboxyl groups. In addition, TA was easily self-oxidized under strong alkaline conditions, which could increase the negative charge of the membrane surface.

[0142] 3. Membrane performance analysis.

[0143] (1) Single ion retention.

[0144] In order to explore the separation performance of divalent / monovalent metal ions of composite nanofiltration membrane, this experiment used spiral flat membrane pilot equipment to perform nanofiltration membrane separation of different metal cations at 25 ° C, pH = 7, and 0.4 MPa pressure. Figure 20 . Among them, the single ion solution Ni 2+ The permeate flux was 58.55 L m -2 h -1 Mpa -1 , the retention rate is 97.48%; Co 2+ The permeate flux was 59.47 L m -2 h -1 Mpa -1 , the retention rate is 98.47%; Mn 2+ The permeate flux was 60.47 L m -2 h -1 Mpa -1 , the retention rate is 98.16%; Li + The permeate flux was 62.18 L m -2 h -1 Mpa -1 , the retention rate was 37.11%.

[0145] (2) Actual sample analysis.

[0146] The prepared composite nanofiltration membrane was applied to the recovery of actual waste LIB primary NF membrane separation. Its membrane performance (such as Figure 21 ) are Ni 2+ The retention rate was 98.52%; Co 2+ The retention rate was 98.53%; Mn 2+ The retention rate was 98.24%; + The retention rate was 48.86%; the feed sample (Ni 2+ +Co 2+ +Mn 2+) / Li decreased from 34.47 to 1.03. The separation flux of the mixed cation NF membrane was 55.67 L m -2 h -1 Mpa -1 The results are consistent with those expected for NF membranes. Among them, since the divalent ions themselves and their hydrated state make the ion size larger than Li, the separation of Li and other ions can be achieved by virtue of the size effect. At the same time, due to the Ni 2+ 、Co 2+ 、Mn 2+ Both are divalent, compared to the monovalent Li + Due to the positive charge of the membrane surface, the divalent ions are subject to a strong positive repulsion, resulting in a high divalent ion retention rate; since the nanofiltration membrane is relatively dense and the membrane pore size is small, some ions in the solution are retained. Therefore, it is speculated that the separation of the nanofiltration membrane is mainly based on the charge Donnan effect. Ⅲ After deposition modification, the macropores are filled, enhancing the support of the membrane substrate. Simultaneously, the introduction of tannic acid (TA) enhances the hydrophilicity of the modified substrate membrane, thereby increasing the permeation flux of the composite nanofiltration membrane. This demonstrates the feasibility of preparing composite nanofiltration membranes based on macroporous PTFE for practical applications in the recovery of lithium from spent lithium batteries.

[0147] Table 3 Comparison with commercial NF membranes and reported NF membranes

[0148]

[0149] 4. Economic and technological competitiveness.

[0150] (1) Technical solution.

[0151] The traditional methods of lithium-ion battery recycling mainly include: cascade utilization, mechanical disassembly, pyrometallurgy and hydrometallurgy. The above methods all have advantages and disadvantages, as shown in Table 4. Among the above methods, hydrometallurgy is the most widely used in the recycling of waste lithium-ion batteries due to its advantages such as high separation performance and simple operation. However, its disadvantage is that the pre-treated acidic leachate needs to be alkali neutralized or alkaline leached, which causes the loss of metal ions due to precipitation; in addition, the consumption of extractants, strong acids, precipitants, etc. is large, so the amount of waste liquid to be treated is large. Therefore, membrane separation technology is introduced to directly use the acidic leachate for acid-resistant nanofiltration membrane separation, such as Figure 22 The operation is simple, the processing capacity is large, and it is applicable to the centralized recycling of batteries of any model.

[0152] Table 4 Summary of lithium-ion battery recycling processes used by various companies around the world

[0153]

[0154] (2) Economic aspects.

[0155] Cascade utilization, mechanical disassembly and fire recovery are not economical for industrial application due to their low recovery efficiency, strict operating requirements, generation of waste gas and waste liquid, and high energy consumption. Although wet recovery has a high recovery rate, it requires a large amount of reagents and complex post-processing, and needs further optimization and exploration.

[0156] Nanofiltration membrane separation technology has become a mass-produced application for separating and recovering lithium from brine with a high magnesium-to-lithium ratio. The full utilization of PTFE support membranes with strong acid and alkali resistance provides a broader application path for the development of membrane separation technology, and also provides a feasibility reference for the application of membrane separation technology in the recovery of lithium from waste lithium batteries. Secondly, the monomers (PEI, TMC, TA, GA), support membranes (PTFE) and solvents (water, trimesoyl chloride) used for interfacial polymerization in this work are all commercially available (Table 5). Secondly, there are few acid-resistant membranes available on the market, and the price is very high (retail price: Suez (GE) -DK, price 373.2rmb / m -2 ). Reported NF membranes, due to their use of PES and PSF support membranes, are limited in their application under strongly acidic conditions. Furthermore, membrane technology has emerged and proven beneficial in waste battery recycling. In summary, the use of PTFE-based composite nanofiltration membranes is economically and technically competitive and shows promising prospects for scale-up.

[0157] Table 5 Main materials and prices used in the present invention

[0158]

[0159] 5. Summary

[0160] The present invention intends to introduce TA / Fe ⅢCoordination complexation compensates for macroporous defects in polytetrafluoroethylene (PTFE), and the numerous hydrophilic groups (phenolic hydroxyl groups) in tannic acid (TA) further enhance the stability and hydrophilicity of the base support membrane. The modified PTFE base membrane retains porosity while increasing its permeability. Positive surface charge modification is achieved through a single interfacial polymerization (IP) reaction of polyethyleneimine (PEI) and trimesoyl chloride (TMC). Glutaraldehyde (GA) is used as a crosslinker to fill the pore defects in the polyamide (PA) selective layer. TA / PEI is deposited on the surface, allowing the membrane surface pore defects to be filled. The alkaline autooxidation of TA to form quinones forms a stronger chemical bond (CN / C=N) with PEI, optimizing the stability and smoothness of the selective layer and reducing the resistance to water passage through the membrane. The resulting nanofiltration membrane material has a hydrophilic angle of 57.37° and a molecular weight cut-off (MWCO) of 635 Da. The membrane surface exhibits a positive charge when the pH is below 7.6. Finally, only one NF operation can achieve the separation and enrichment of Li and other divalent metals in the leaching solution of waste LIB cathode materials. 2+ The retention rate was 98.52%; Co 2+ The retention rate was 98.53%; Mn 2+ The retention rate was 98.24%; Li + The rejection rate was 48.86%; the mixed feed sample (Ni 2+ +Co 2+ +Mn 2+ ) / Li decreased from 34.47 to 1.03, and the NF membrane separation permeation flux (55.67 L m -2 h -1 Mpa -1 These results are consistent with the expected separation mechanism of NF membranes.

[0161] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0162] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the patent for this invention shall be determined by the appended claims.

Claims

1. A positively charged composite nanofiltration membrane based on PTFE, characterized in that: It includes a PTFE substrate, a deposition layer, a selective layer, and a cross-linked layer covered in sequence; the deposition layer is composed of TA-Fe Ⅲ The selective layer is obtained by interfacial polymerization of PEI and TMC, and the cross-linked layer is obtained by strong cross-linking of a cross-linking agent, TA, and PEI.

2. The positively charged composite nanofiltration membrane according to claim 1, characterized in that The deposited layer is composed of TA and Fe in a molar ratio of 1: (1.8-2.2) 3+ Prepared.

3. The positively charged composite nanofiltration membrane according to claim 1, characterized in that The selective layer is prepared from PEI and TMC in a mass percentage of 1 wt%:(0.3-0.5) wt%.

4. The positively charged composite nanofiltration membrane according to claim 1, characterized in that The cross-linked layer is prepared from a cross-linking agent, TA, and PEI in a mass percentage of (1.8-2.2) wt%: (1.3-1.7) wt%: (2.2-2.6) wt%, wherein the cross-linking agent includes GA.

5. The method for preparing the positively charged composite nanofiltration membrane according to any one of claims 1 to 4, characterized in that: The following steps are involved: Preparation of deposition layer: Place TA solution on the surface of PTFE substrate, let it stand, add Fe 3+ The aqueous solution reacts to make TA-Fe Ⅲ Layer by layer self-assembly to form a deposition layer, which is then heated and solidified to obtain PTFE-TA / Fe Ⅲ Modified membrane; Preparation of selective layer: PEI solution was placed on PTFE-TA / Fe Ⅲ Modify the membrane surface, let it stand, add the organic phase containing TMC to react, obtain the selective layer through interfacial polymerization, heat and solidify to obtain PTFE-TA / Fe Ⅲ -PA modified membrane; Preparation of cross-linking layer: GA and TA are placed on PTFE-TA / Fe Ⅲ -PA modified membrane surface, reacted, heated and solidified, and PEI solution was placed on PTFE-TA / Fe Ⅲ -PA modified membrane surface, reacted, and heated to solidify to obtain a positively charged composite nanofiltration membrane.

6. The preparation method according to claim 5, characterized in that In the step of preparing the deposition layer, the TA solution is a Tris-HCl buffer containing TA, and the Fe 3+ The aqueous solution is FeCl3·6H2O aqueous solution; the TA in the TA solution and the Fe 3+ Fe in aqueous solution 3+ The molar ratio is 1: (1.8-2.2); The preparation of the deposition layer comprises: placing the TA solution on the surface of the PTFE substrate, letting it stand for 2-5 minutes, adding Fe 3+ The aqueous solution reacts for 0.5-1.5min to make TA-Fe Ⅲ Layer-by-layer self-assembly is performed at least twice to form a deposition layer, which is then heated and solidified to obtain PTFE-TA / Fe Ⅲ Modified membrane.

7. The preparation method according to claim 5, characterized in that In the step of preparing the selective layer, the PEI solution is a Tris-HCl buffer solution containing PEI and SDS, and the mass percentage of the PEI and SDS is 1 wt%: (0.02-0.06) wt%; The preparation of the selective layer comprises: placing the PEI solution in the PTFE-TA / Fe Ⅲ Modify the membrane surface, let it stand for 30-50 minutes, add the organic phase containing TMC and react for 90-110 seconds, heat and solidify to obtain PTFE-TA / Fe Ⅲ -PA modified membrane.

8. The preparation method according to claim 5, characterized in that In the step of preparing the cross-linking layer, the PEI solution is a Tris-HCl buffer solution containing PEI; The preparation of the cross-linked layer comprises: placing GA and TA in PTFE-TA / Fe Ⅲ -PA modified membrane surface reaction for 15-25min, heat curing, PEI solution placed on PTFE-TA / Fe Ⅲ The PA modified membrane surface reacts for 0.8-1.2h and is heated and cured to obtain a positively charged composite nanofiltration membrane.

9. Use of the positively charged composite nanofiltration membrane according to any one of claims 1 to 4 in magnesium-lithium separation, lithium recovery from waste lithium-ion batteries, heavy metal removal, or dye removal.

10. A method for recovering lithium from waste lithium-ion batteries, characterized in that: The following steps are involved: The positively charged composite nanofiltration membrane according to any one of claims 1 to 4 is used to separate and enrich lithium ions and divalent ions in the acidic leachate of the positive electrode material of waste lithium ion batteries.

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