Composite nanofiltration membrane based on PTFE (Polytetrafluoroethylene) as well as preparation method and application of composite nanofiltration membrane

Abstract

The invention relates to a PTFE-based composite nanofiltration membrane as well as a preparation method and application thereof, and relates to the technical field of nanofiltration membranes and membrane assemblies. The PTFE-based composite nanofiltration membrane comprises a PTFE substrate, a modification structure, cross-linked molecules and a nanopore layer, the modification structure is attached to the surface of the PTFE substrate, one end of each cross-linked molecule is connected with the modification structure, and the other end of each cross-linked molecule is connected with the nanopore layer. The composite nanofiltration membrane has good compactness, effectively solves the aperture defect of the existing nanofiltration membrane, can significantly improve the selective recovery efficiency of lithium, and provides a new thought for efficient and environment-friendly lithium battery recovery.

Description

Technical Field

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

[0002] In recent years, the world has paid more and more attention to the development of green and sustainable renewable energy to cope with problems such as energy shortages and climate warming. Lithium-ion batteries (LIBs) are widely used in portable electronic devices and new energy vehicles (EVs) due to their high energy density, green, renewable and stable cycles, becoming a new quality productivity for efficient power storage. However, the service life of LIBs is limited, and generally more than 8-10 years is not enough to support the normal use of electric vehicle power cycles. It is worth noting that LIBs positive electrode materials contain a variety of precious metals, such as LiNi x Co y Mn 1-x Mn x O 2 The cathode material, of which cobalt (Co) accounts for 5-20wt%, nickel (Ni) accounts for 5-12wt%, manganese (Mn) accounts for 7-10wt%, and lithium (Li) accounts for 2-5wt%, has a metal content that exceeds that of natural ores. If the valuable metal elements in spent LIBs cannot be effectively recovered, it may be challenging to meet future market demand solely by relying on natural mineral resources.

[0003] As shown in Table 1, existing recycling technologies include direct recycling, pyrometallurgy and hydrometallurgy. However, direct recycling and pyrometallurgy cannot be widely used in commercial industries due to their complex operation, high energy consumption and low recovery rate; although hydrometallurgy has a high recovery rate and is relatively widely used, this method is prone to produce a large amount of waste and consumes a large amount of chemical raw materials such as extractants, adsorbents, acid and alkali solutions. At the same time, the leachate of this method is strongly acidic, so the pH of the acidic leachate needs to be adjusted. Alkali neutralization will inevitably lose some valuable metals, which increases the leaching cost and complexity of the leaching process. Among them, the acidic leachate is mainly obtained by discharging and disassembling the battery, dissolving the positive electrode material in alkali to remove the Al current collector, and then calcining the remaining precipitate at high temperature to remove adhesives, organic matter and impurities, etc., and the metal oxides can be reduced by high-temperature calcination. Finally, the calcined mixture is dissolved in acid, and subsequent separation and enrichment are carried out in the form of a solution.

[0004] Table 1 Summary of lithium-ion battery recycling processes used by many companies around the world

[0005]

[0006]

[0007] However, in the actual recycling process, based on the requirements of large recycling volume, limited operating technology and no secondary pollution, finding a method that meets the stringent conditions of simple operation, high recovery rate and resistance to acidic solvents is of great research significance for the efficient recycling of waste LIBs. Summary of the invention

[0008] In view of the above problems, the present invention provides a PTFE-based composite nanofiltration membrane, which has good density and effectively solves the pore size defects of existing nanofiltration membranes. The composite nanofiltration membrane can significantly improve the selective recovery efficiency of lithium, providing a new idea for efficient and environmentally friendly lithium battery recycling.

[0009] In order to achieve the above-mentioned purpose, the present invention provides a PTFE-based composite nanofiltration membrane, comprising a PTFE substrate, a modified structure, a cross-linked molecule and a nanoporous layer, wherein the modified structure is attached to the surface of the PTFE substrate, one end of the cross-linked molecule is connected to the modified structure, and the other end is connected to the nanoporous layer.

[0010] Nanofiltration membrane (NF) separation mainly relies on two separation principles: charge effect and size screening. Therefore, it is widely used in the fields of efficient and selective separation of low molecular weight organic matter and high-valent / monovalent metal ions in water samples. At the same time, nanofiltration membrane separation is the only membrane technology used for large-scale recovery of lithium ions. This technology is widely used in the field of water treatment due to its advantages such as simple practical operation, large processing capacity and high selectivity. The nanofiltration membrane is mainly composed of a supporting base membrane and a charged selective separation membrane. Therefore, in order to meet the stringent requirements of the acidic leachate of waste lithium batteries, it is necessary to ensure that both the base membrane and the selective layer have strong acid resistance.

[0011] Polytetrafluoroethylene (PTFE), an organic polymer material known as the "King of Plastics", has strong acid and alkali resistance, organic solvent resistance and other characteristics that traditional PES and PSF organic substrate membranes cannot meet due to the strong stability of the CF bond. It can even be stored dry without a protective agent, thereby reducing the storage cost and transportation difficulty of the equipment. However, due to technical limitations, the commercial PTFE membrane only has a pore size of 0.1μm, so it has larger pores than PES and PSF, which are the support layers of Da-level nanofiltration membranes. Therefore, if the nanofiltration membrane with macroporous PTFE as the support layer is used, the ultra-thin polyamide (PA) selective layer synthesized on its surface will reduce the stability of the nanofiltration membrane due to the lack of support caused by the macropores of the base membrane. Secondly, since the F element of the strong CF bond in PTFE has strong electronegativity, the membrane surface is strongly negatively charged, but the target ion is a cation, and the purpose of positive charge repulsion by the Donnan effect cannot be achieved. Therefore, if you want to make a nanofiltration membrane suitable for lithium ion separation and enrichment based on PTFE, the key to solving the pore problem of PTFE and the modification of the positive charge on the membrane surface is to solve the problem of PTFE porosity and the modification of the positive charge on the membrane surface. The above-mentioned PTFE-based composite nanofiltration membrane combines the nanoporous layer with the PTFE substrate through cross-linking molecules and modified structures attached to the PTFE substrate, so that the composite nanofiltration membrane has better density, effectively solving the pore size defects of existing nanofiltration membranes, and the use of the composite nanofiltration membrane can significantly improve the selective recovery efficiency of lithium, providing new ideas for efficient and environmentally friendly lithium battery recycling.

[0012] In one embodiment, the modified structure is mainly obtained by interfacial polymerization of PEI and TMC;

[0013] The cross-linking molecules include GA;

[0014] The nanoporous layer is mainly obtained by co-deposition of TA and PEI through Schiff base and Michael addition reaction.

[0015] The inventors propose that the composite nanofiltration membrane is prepared by an interfacial polymerization (IP) method with easy-to-control reactions and mild synthesis conditions. Traditional IP uses metaphenylenediamine (MPD) and piperazine (PIP) as water-phase active monomers to interfacially polymerize with trimesoyl chloride (TMC) to generate an ultra-thin negatively charged nanofiltration membrane. However, since most of the waste lithium-ion batteries actually recycled are multivalent metal cations, positively charged nanofiltration membranes are the research target. Long-chain branched amino polyethyleneimine (PEI) is a water-soluble high molecular polymer rich in amino groups. It is cationic in aqueous solution and has a high positive charge density. It can replace traditional aqueous monomers to react with TMC organic phase, such as Figure 1, to achieve the purpose of positive charge modification of membrane materials to strengthen the charge effect. At the same time, tannic acid (TA) is a common plant polyphenol, also known as tannic acid. Tannic acid is widely used due to its hydrophilicity, adhesion, strong metal chelating ability and low price. Under alkaline conditions, it can react with amino groups (-NH 2 ) Modification of materials by Michael addition or Schiff base co-deposition, such as Figure 2 .

[0016] Therefore, the present invention adopts an interfacial polymerization (IP) strategy and utilizes polyamine-based long-chain PEI to enhance the positive charge of the nanofiltration membrane surface; at the same time, GA is used as a cross-linking agent to achieve a strong cross-linking effect through the deposition of PEI and TA, thereby enhancing the compactness of the nanofiltration membrane, and finally successfully preparing a positively charged composite nanofiltration membrane. In addition, the present invention also systematically characterizes the preparation conditions, structural characteristics and performance of the synthetic materials, and applies the membrane materials prepared under the optimal conditions to the recovery experiment of waste lithium-ion batteries (LIB). Composite nanofiltration membrane preparation and separation working principle as shown in the figure. Figure 3 shown.

[0017] In one embodiment, one end of the cross-linking molecule is connected to the amide group of the modified structure, and the other end is connected to the phenol group of the nanoporous layer.

[0018] The present invention also provides a method for preparing the composite nanofiltration membrane, comprising the following steps:

[0019] Preparation of a PTFE membrane with a modified structure: Based on a PTFE substrate, PEI and TMC are used for polymerization reaction to obtain a PTFE membrane with a modified structure;

[0020] Preparation of composite nanofiltration membrane: GA was used as a cross-linking agent, and TA and PEI were used for co-deposition of Schiff base and Michael addition reaction to obtain a composite nanofiltration membrane.

[0021] In one embodiment, in the step of preparing the PTFE membrane with a modified structure, the polymerization reaction includes two interfacial polymerizations; the interfacial polymerization includes: using a solution containing PEI, a surfactant, and an acid acceptor to react with TMC, and heating and curing.

[0022] In one embodiment, the surfactant comprises SDS, and the acid acceptor comprises TEA;

[0023] In the interfacial polymerization, the mass percentages of PEI, surfactant, acid acceptor and TMC are (2.2-2.6wt%): (0.2-0.6wt%): (2.2-2.6wt%): (1-1.4wt%), the reaction time is 70-90s, the heating curing temperature is 55-65°C, and the heating curing time is 8-12min.

[0024] In one embodiment, in the step of preparing the composite nanofiltration membrane, the co-deposition of the Schiff base and Michael addition reaction includes: placing GA and TA on the surface of the PTFE membrane with a modified structure, reacting, heating and curing, and placing PEI on the surface of the PTFE membrane with a modified structure, reacting, and heating and curing.

[0025] In one embodiment, in the co-deposition of the Schiff base and Michael addition reaction, the mass percentages of GA, TA, and PEI are (1.8-2.2wt%): (1.3-1.8wt%): (2.3-2.8wt%);

[0026] The co-deposition of Schiff base and Michael addition reaction includes: placing GA and TA on the surface of the PTFE membrane with modified structure, reacting for 15-25 minutes, heating and curing at 45-55° C. for 3-8 minutes, placing PEI on the surface of the PTFE membrane with modified structure, reacting for 0.8-1.2 hours, and heating and curing at 45-55° C. for 13-18 minutes.

[0027] The present invention also provides the use of the composite nanofiltration membrane in magnesium-lithium separation, lithium recovery from waste lithium-ion batteries, heavy metal removal, or dye removal.

[0028] The present invention also provides a method for recovering lithium from waste lithium-ion batteries, comprising the following steps: using the composite nanofiltration membrane to separate and enrich lithium in the acidic leaching solution of the positive electrode material of the waste lithium-ion batteries.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] The present invention discloses a PTFE-based composite nanofiltration membrane and a preparation method and application thereof. The composite nanofiltration membrane has good hydrophilicity, acid resistance, compactness and positive charge properties, effectively solving the pore size defects of existing nanofiltration membranes. The composite nanofiltration membrane can significantly improve the selective recovery efficiency of lithium, providing a new idea for efficient and environmentally friendly lithium battery recycling. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Schematic diagram of the interfacial polymerization reaction between PEI and TMC;

[0032] Figure 2This is a schematic diagram of the principle of TA self-oxidation reaction under alkaline conditions;

[0033] Figure 3 The schematic diagram of the preparation and separation mechanism of composite nanofiltration membrane based on PTFE;

[0034] Figure 4 Results of the experiment to optimize the synthesis of PA selective layers for interfacial polymerization; test conditions: 2 g / L CoCl 2 6H 2 O aqueous solution, 0.4Mpa, 25°C, pH=7;

[0035] Figure 5 Results of an experiment to optimize the synthesis of a dense selective layer; test conditions: 2g / L CoCl 2 6H 2 O aqueous solution, 0.4Mpa, 25°C, pH=7;

[0036] Figure 6 is the SEM image of the original PTFE support membrane and the modified membrane surface, where: Figure 6 a 1 -c 1 :7.00KX; Figure 6 a 2 -c 2 :10.00KX; Figure 6 a 3 -c 3 :20.00KX; a: PTFE membrane; b: PTFE-PA membrane; c: PTFE-2PA-TA / PEI membrane;

[0037] Figure 7 AFM images of the original PTFE support membrane and the modified membrane surface: a: PTFE membrane; b: PTFE-PA membrane; c: PTFE-2PA-TA / PEI membrane;

[0038] Figure 8 ATR-FTIR graphs of original PTFE membrane and modified membrane;

[0039] Fig. 9 The full XPS spectra of the original membrane and the modified membrane;

[0040] Fig.10 Schematic diagram of the cross-linking reaction principle of the dense layer;

[0041] Fig.11 The XPS fine peak spectra of C1s and N1s of PTFE-PA and PTFE-2PA-TA / PEI modified membranes, where the left side is the fine peak spectra of C1s of different membranes, and the right side is the fine peak spectra of N1s of different membranes;

[0042] Fig.12 The results of water contact angle (WCA) on the surface of original PTFE membrane and modified membrane are shown in the figure;

[0043] Fig.13 The figure shows the retention rate of different polyethylene glycol (PEG) molecules of PTFE-PA membrane and PTFE-2PA-TA / PEI membrane;

[0044] Fig.14 The pore size distribution diagram of PTFE-PA membrane and PTFE-2PA-TA / PEI membrane;

[0045] Fig.15 The Zeta potential results of PTFE membrane and PTFE-2PA-TA / PEI membrane under different pH conditions;

[0046] Fig.16 is the first interception under different single ion conditions (M 1 ) and secondary interception (M 1 +M 2 、M 1 +M 3 ) retention rate and permeation flux results: M 1 :PTFE-PA; M 2 :PTFE-2PA;M 3 :PTFE-2PA-TA / PEI;

[0047] Fig.17 The interception rate and permeation flux result diagram of the simulated sample primary interception and secondary interception, where M 1 :PTFE-PA; M 2 :PTFE-2PA;M 3 :PTFE-2PA-TA / PEI;

[0048] Fig.18 The figure shows the results of the first and second interception and permeate flux of the actual sample (NCM: 811), where M 1 :PTFE-PA; M 2 :PTFE-2PA;M 3 :PTFE-2PA-TA / PEI. DETAILED DESCRIPTION

[0049] In order to facilitate the understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. The preferred embodiments of the present invention are given in the drawings. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0050] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0051] source:

[0052] Experimental instruments:

[0053] Spiral flat membrane pilot 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, Austria), plasma emission spectrometer (SPECTRO ARCOSMV, Spike, Germany), pH meter (CT-6021A, Qingdao Juchuang Times Environmental Protection Technology Co., Ltd.), thunder magnetic conductivity meter (DDS-307A, Shanghai Yidian Scientific Instrument Co., Ltd.), attenuated total reflection Fourier transform infrared spectroscopy (Nicolet 6700, PerkinElmer, USA), contact angle meter (JC2000D3P, Shanghai Zhongchen Digital Technology Equipment Co., Ltd.), field emission scanning electron microscope (ZEISS Ultra 55, Carl Zeiss, Germany), analytical electronic balance (A200S, Sartorius, Germany), atomic force microscope (Dimenson ICON, Bruker Corporation, USA), ultrasonic instrument (SK250H, Shanghai Kedao Instrument Co., Ltd.);

[0054] Experimental reagents:

[0055] 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 = 70,000Da, 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.) Hexahydrated sodium chloride Cobalt (analytical grade, Shanghai McLean Biochemical Company) Nickel chloride hexahydrate (analytical grade, Shanghai McLean Biochemical Company) Manganese chloride tetrahydrate (analytical grade, Shanghai McLean Biochemical Company) Polyethylene glycols of different molecular weights (PEG, analytical grade, Shanghai McLean Biochemical Company) Tannic acid (TA, analytical grade, Tianjin Fuchen Chemical Reagent Factory) Glutaraldehyde (GA, analytical grade, Tianjin Komiou Chemical Reagent Co., Ltd.) Hydrochloric acid (HCl, analytical grade, Guangdong Guangzhou Reagent Technology Co., Ltd.) Hydrophilic PTFE membrane (pore size 0.1um, hydrophilic, Longjin Membrane Technology Co., Ltd.)

[0056] Unless otherwise specified, the reagents, materials, and equipment used in this example are all commercially available; the experimental methods are all conventional experimental methods in the art unless otherwise specified.

[0057] Example

[0058] 1. Preparation of a PTFE-based composite nanofiltration membrane.

[0059] 1. Preparation of PTFE-PA and PTFE-2PA composite nanofiltration membranes by interfacial polymerization (IP) reaction.

[0060] The PTFE membrane was immersed in an ethanol solution for 12 hours and dried in an oven at 60°C to remove the adhesive and impurities on the membrane surface. A PEI aqueous solution (containing 0.1wt% SDS as a surfactant and 1.5wt% TEA as an acid acceptor) was added to the surface of the hydrophilic PTFE membrane fixed by a circular membrane pool clamp and immersed for 40 minutes so that it could evenly penetrate the membrane surface. Then, after removing the excess aqueous solution on the membrane surface with a roller, the membrane was covered with TMC dissolved in n-hexane at room temperature for a certain period of time to allow interfacial polymerization to occur. The membrane surface was washed with n-hexane to remove excess unreacted TMC organic solution to prevent the positive charge on the membrane surface from being weakened due to the hydrolysis of unreacted TMC. Finally, the membrane was placed in a 60°C oven for heating and curing for 10 minutes. The prepared composite NF membrane was stored in deionized water for subsequent testing and characterization. The prepared membrane was named M 1 (PTFE-PA).

[0061] Compared with the traditional nanofiltration membrane PES, the PTFE membrane has a larger pore size in the PSF support layer and a thinner PA selectivity layer, which makes the synthesized M1 (PTFE-PA) modified membrane less stable and easily ruptured by high pressure, resulting in a decrease in the retention rate. Therefore, this experiment designed two PEI and TMC interfacial polymerization IPs to prepare a modified nanofiltration membrane, and the membrane synthesized after the second IP was named M2 (PTFE-2PA) to enhance stability while increasing its positive charge.

[0062] 2. PTFE-2PA-TA / PEI is strongly cross-linked to prepare a dense composite nanofiltration membrane (i.e., a composite nanofiltration membrane based on PTFE).

[0063] Under the M2 membrane conditions, the membrane performance is still poor in selectivity due to the large pore size, so it is considered to strengthen the pore size screening effect and increase the pore size density on the membrane surface. The specific synthesis steps are as follows Figure 3 The prepared M2 membrane was fixed on a circular membrane pool clamp device, glutaraldehyde (GA) was selected as a cross-linking agent, GA and TA solutions were soaked on the membrane surface for 10 minutes respectively, and then heated in a 50°C oven for a certain period of time to solidify, and the unreacted GA and TA solutions on the surface were washed off with deionized water. Finally, PEI solution (containing 2.4wt% TEA) was added to its surface to react at room temperature for 1 hour, and the reacted membrane was placed in a 50°C oven and heated for 15 minutes to solidify the film. The obtained composite NF membrane was stored in deionized water for subsequent testing and characterization. The prepared membrane was named M3 (PTFE-2PA-TA / PEI) membrane.

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

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

[0066] Fourier transform infrared spectroscopy (FTIR) is an important method for qualitative and structural analysis of organic compounds and high molecular polymers. Traditional testing methods such as tableting and paste methods have difficulty testing special samples that are difficult to dissolve, difficult to crush or thick. For this reason, the attenuated total reflection (ATR) technology combined with FTIR came into being, significantly expanding the application range of infrared spectroscopy. ATR-FTIR analyzes the structure of organic matter on the surface of the sample through the penetration and reflection of infrared light within a certain depth on the surface of the sample. The specific experimental steps are as follows: Take the synthesized membrane material out of deionized water, cut it into 1cm×1cm square samples and dry it. Subsequently, use ATR-FTIR at 4000cm -1 Up to 600cm -1 Scanning is performed within a certain range to analyze the chemical structure characteristics of the membrane surface.

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

[0068] Scanning electron microscopy (SEM) is one of the important methods to characterize the macroscopic morphology of materials. A ZEISS Ultra55 field emission scanning electron microscope was used to characterize the morphological changes of the materials. The specific operation is as follows: the original membrane and the modified membrane were taken out of the deionized water and dried, cut into small pieces of 1cm×1cm, and placed in an electron microscope chamber under vacuum conditions after gold spraying to observe the changes in their surface morphology.

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

[0070] Atomic force microscopy (AFM) is an important method to characterize the surface morphology of materials by measuring the surface roughness of samples. The experimental steps are as follows: First, the synthesized membrane sample is thoroughly rinsed with deionized water and vacuum dried at 40°C. Subsequently, the membrane sample to be tested is cut to an appropriate size and fixed to the sample table using a conductive double-sided tape to prepare the test sample. The tapping mode is used during the test, and the scanning area is 10μm×10μm. Among them, the surface roughness of the composite nanofiltration membrane is quantitatively characterized by the root mean square roughness (Root Mean Square Roughness, Rms).

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

[0072] XPS is a highly sensitive analytical method for accurate analysis of sample element types and valence states. This characterization is limited to detecting the surface chemical composition of samples within 10nm and requires carbon correction. Specifically, the membrane in deionized water is taken out, small pieces are cut and vacuum dried at 40℃, 1cm×1cm pieces are cut, and they are fixed to the sample stand with conductive glue to test the surface chemical composition and valence state of the membrane.

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

[0074] Hydrophilicity is an important performance requirement of membrane materials. The contact angle test can directly characterize the hydrophilicity of materials. When the contact angle is closer to 180°, the material is more hydrophobic / oleophilic; when the contact angle is closer to 0°, the material is more hydrophilic / oleophobic, and the water permeability of the material is enhanced. In this test, the membrane to be tested is dried and cut into 1cm×2cm specimens. One drop of deionized water is used as the test solution, and the contact angle size is saved and analyzed using digital images. Multiple tests are performed to calculate the average value.

[0075] 6. Zeta potential test of membrane solid surface.

[0076] The surface Zeta potential of the modified nanofiltration membrane was measured using a solid surface potential analyzer. The specific experimental steps are as follows: First, the membrane to be tested was fully infiltrated in a 0.001M KCl solution, and then the membrane sample was fixed in the sample pool using a conductive glue. A 0.001M KCl solution was used as the test solution, and the pH value of the solution was adjusted by HCl and NaOH to measure the surface potential of the membrane under different pH conditions. To ensure the accuracy of the data, each group of experiments was measured multiple times and the average value was taken as the final result.

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

[0078] The molecular weight of a certain solute with a retention rate of 90.0% is defined as the membrane molecular weight cut-off (MWCO). The larger the MWCO, the larger the pore size of the membrane material. In this embodiment, different molecular weights (200, 400, 600, 1000, 2000 Da) of the electrically neutral polymer polyethylene glycol (PEG) are used as the test molecules, and the feed liquid is configured with a concentration of 0.1 g / L. The permeate after the test of the membrane to be tested is the inlet and outlet sample liquid, and the change in its carbon content is tested by a total organic carbon analysis (TOC) instrument. Finally, the PEG retention rate is calculated by formula (1) to characterize the membrane pore size.

[0079]

[0080] Among them, R PEG It 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 total organic carbon concentrations of the feed solution and the permeate, respectively.

[0081] By performing nonlinear curve fitting of the LogNormal function on the retention rate data of the above-mentioned PEG tests with different molecular weights, the relationship equation between the retention rate of the membrane for PEG molecules and the molecular weight of PEG was explored.

[0082] 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):

[0083]

[0084] The pore size distribution is calculated by the following probability density function (3):

[0085]

[0086] When R PEG =50%, corresponding to the Stokes diameter μ of the PEG molecule s The average diameter of the membranep 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

[0087] 3. Performance test of composite nanofiltration membrane.

[0088] In this example, a 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 tests. The effective area of ​​the membrane used for the measurement was 69.36 cm 2 The feed concentration was 2000ppm, the test pressure was 0.4Mpa, and the pre-pressure was 30mim at room temperature and atmospheric pressure to ensure stable membrane performance. At least 3 identical membranes were tested for each test for comparison to eliminate errors.

[0089] 1. Penetration performance.

[0090] Permeation flux refers to the volume of liquid passing through a unit effective area per unit time under unit pressure, which directly characterizes the separation speed of the membrane. Therefore, the larger the permeation flux, the faster the separation rate. It is calculated as follows:

[0091]

[0092] Where J is the permeation flux (L m -2 h -1 ), V represents the liquid volume (L) passing through the operating conditions, and A is the effective membrane area (m 2 ), t is the sampling time (h).

[0093] 2. Interception performance.

[0094] During the membrane separation test, the divalent ions in the feed solution are retained in the permeate, and the single ions pass through the membrane into the permeate, thereby achieving efficient separation of divalent / monovalent ions. The retention rate (R) refers to the ratio of the concentration of the retained solute to the concentration of the feed solution solute, calculated as follows (5):

[0095]

[0096] Where R is the retention efficiency of the membrane for the solute (%), C f is the solute concentration of the feed solution, C p is the solute concentration in the permeate.

[0097] Note: In this embodiment, the single ion solution is tested by a conductivity meter (Raymag, DDS-307A), and the mixed ion solution is tested by inductively coupled plasma (ICP).

[0098] 3. Configuration of single ion and actual sample feed and membrane separation steps.

[0099] Deionized water was used as solvent and 2 g / L (LiCl, NiCl 2 6H 2 O,CoCl 2 6H 2 O,MnCl 2 ·4H 2 O) as a single ion test feed solution.

[0100] Laboratory waste lithium-ion battery NCM (811) was taken 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℃, 5h). Acetylene black and polyvinylidene fluoride (PVDF) were removed by high-temperature pyrolysis. After cooling and grinding, the positive electrode powder was obtained. 2g of the powder was dissolved in 1L of deionized water solvent, and the pH was adjusted to 2 to prepare a feed solution concentration of 2g / L as the feed sample.

[0101] Specific NF membrane separation steps: the above-mentioned single ion solution or actual sample leaching liquid is used as the feed liquid to pass through the PTFE-PA membrane for a first nanofiltration (1st), and the permeate after the first nanofiltration is used as the feed liquid for the second nanofiltration for secondary nanofiltration separation (2nd) to obtain the final permeate. The single ion inlet and outlet samples are tested using a conductivity meter; the actual mixed ion sample liquid is measured using inductively coupled plasma (ICP) to characterize the membrane separation performance.

[0102] 4. Results and discussion.

[0103] 1. Optimization of synthesis conditions of composite nanofiltration membrane.

[0104] The synthesis steps of the polyamide (PA) selective layer on the surface of the original PTFE membrane (i.e., PTFE-PA membrane) were optimized. Figure 4(a), with the increase of PEI concentration, the rejection rate of the composite nanofiltration membrane first increases and then decreases with the increase of concentration, that is, when the PEI concentration is 2.4wt%, the rejection rate RCo2+ is the highest at 72.58%. From the trade-off effect of the rejection rate and the permeation flux, it can be seen that the flux of the ion solution first decreases and then increases. This is because the composite nanofiltration membrane with low concentration PEI has a low degree of cross-linking and has certain defects, forming a loose polyamide layer with a low degree of cross-linking. However, as its concentration increases, the cross-linking density of the selective layer increases, making its pore structure more dense, so the permeation flux decreases, and the rejection rate increases due to the decrease in pore size and the positive charge effect of PEI; when the PEI concentration exceeds 2.4wt%, the PEI concentration is too high, and the reaction is too fast, resulting in a decrease in the degree of polymerization. Within a certain period of time, it is difficult to form a dense network structure, resulting in defects and unevenness in the ultra-thin selective layer, which ultimately increases the permeation flux and reduces the rejection rate.

[0105] When the PEI concentration (2.4 wt%) is constant, the relationship curve between the permeation flux and retention rate of the composite nanofiltration membrane and the TMC organic phase monomer concentration is as follows: Figure 4 (b). As can be seen from the figure, as the TMC concentration increases, its retention rate first increases and then decreases. At 1.2wt%, the maximum retention rate is 74.77%. At first, the TMC concentration is too small, and the selectivity layer of the composite nanofiltration membrane has serious defects. Then, as the TMC concentration increases, the cross-linking density increases, causing the network structure of the selective layer to become denser, resulting in an increase in the retention rate and a decrease in the permeation flux. However, if the TMC concentration is too high, the interfacial polymerization reaction occurs quickly, causing the PEI to be consumed, so that the unreacted TMC acyl chloride groups are hydrolyzed into carboxyl groups, resulting in a decrease in the positive charge on the surface of the composite nanofiltration membrane, and then a decrease in the retention rate.

[0106] Figure 4 (c) Optimization of the PA interfacial polymerization reaction time. The interfacial polymerization reaction is a rapid reaction based on the migration of the water phase to the oil phase. Since the PEI in the water phase has a fast migration rate and the interfacial reaction occurs on the organic phase side, the IP process is actually a non-isomeric polycondensation. Therefore, the longer the reaction time, the thicker the selective thin layer of the reaction, and the higher the retention rate. However, the thicker selective layer will increase the permeation resistance of water on the membrane surface, resulting in a decrease in the permeation flux. Based on the premise of not compromising its retention rate and ensuring its permeation flux, the reaction time of 80s was selected as the optimal reaction time.

[0107] Figure 4(d) shows the relationship curve of the retention rate and permeation flux of the composite nanofiltration membrane with the change of heat treatment temperature under the condition of heat treatment time of 10 minutes. It can be seen from the figure that with the increase of heat curing temperature, the retention first increases and then decreases, while the permeation flux increases from decrease to increase. Heat curing can stabilize the selective layer on the membrane surface to a certain extent, but when the degree of heat treatment is too large, it will cause the film to shrink and rupture, resulting in a decrease in the membrane retention rate and an increase in the permeation flux due to the large pore size caused by membrane defects.

[0108] The final modified membrane M 1 The optimal synthesis scheme of (PTFE-PA) is: 2.4wt% PEI (0.4wt% SDS, 2.4wt% TEA) and 1.2wt% TMC in n-hexane as solvent, interfacial polymerization for 80s, and heating and curing at 60°C.

[0109] Since the selective layer is a thin film and its support strength is weak and insufficient to meet the practical application of the subsequent modified film, a secondary interfacial polymerization reaction is selected to strengthen the support of the selective layer on the membrane surface. The modified membrane after the second interfacial polymerization (IP) is named M 2 (PTFE-2PA).

[0110] Due to the unevenness of the polyamide layer reaction, the nanofiltration membrane size screening effect cannot meet the separation of monovalent and divalent ions, so it is necessary to further enhance the density of the nanofiltration membrane. Using glutaraldehyde as a cross-linking agent can bond TA and PEI deposition to increase the density of the membrane. Figure 5 As shown in (ab), when the GA concentration is 2.0 wt%, GA acts as a crosslinker between the free amine groups of the polyamide layer and TA, making the crosslinks formed on the membrane surface denser, making Co 2+ The interception rate is the largest. According to the trade-off effect, the permeation flux of the modified membrane also decreases. When the concentration of TA is 1.5wt%, under the condition of PEI reaction for 1h, TA self-oxidizes under alkaline conditions to generate quinone, which reacts with the long-chain amine of PEI to undergo Michael addition and Schiff base reaction, so that part of the electrostatic force and secondary bonds such as hydrogen bonds are replaced by part of the covalent bonds (CN / C=N). Therefore, TA / PEI co-deposition can achieve the purpose of enhancing the cross-linking degree of the membrane material and increasing the stability of its membrane surface.

[0111] like Figure 5(c) This study investigated the effect of heat treatment time on the performance of membrane materials. Heat curing can increase the degree of bonding reaction between aldehyde and amide, and between aldehyde and phenol, so that one end of GA is connected to the amide in PTFE-2PA and the other end is connected to the phenol in TA to achieve the purpose of preparing a dense membrane. However, too long heating time will cause the selective membrane layer to shrink and rupture, resulting in a decrease in performance. Therefore, the optimal heating curing time was selected to be 5 minutes at 50°C. At this time, the selectivity of PTFE-2PA-TA / PEI was 95.01%, and the permeation flux was 18.03 L m -2 h -1 Mpa -1 .

[0112] Finally, the optimal synthesis scheme of the nanofiltration membrane of the entire system is as follows:

[0113] (1) PTFE-PA membrane: 2.4 wt% PEI (0.4 wt% SDS, 2.4 wt% TEA) and 1.2 wt% TMC in n-hexane, interfacial polymerization for 80 s, cured by heating at 60 °C for 10 min;

[0114] (2) PTFE-2PA membrane: secondary interfacial polymerization (IP) reaction based on PTFE-PA membrane;

[0115] (3) PTFE-2PA-TA / PEI membrane: Based on the PTFE-2PA membrane, 2.0wt% GA (pH=2) and 1.5wt% TA (pH=3) were placed on the surface of the reaction membrane for 20 minutes and then placed in a 50°C oven for thermal curing for 5 minutes to remove the unreacted GA and TA on the surface. Then, 2.5wt% (2.5wt% TEA) PEI was placed on the membrane surface for reaction for 1 hour and then placed in a 50°C oven for thermal curing for 15 minutes. The prepared composite nanofiltration membrane was placed in deionized water for use.

[0116] 2. Structural characterization of materials.

[0117] (1) SEM and AFM characterization of original membrane and modified membrane.

[0118] like Figure 6 (a 1 -a 3 ), the original PTFE support membrane presents a macroporous network structure composed of node-connected fibers with large pores. In the first step, due to the interfacial polymerization reaction between PEI and TMC, some nano / micro-scale aggregates are formed on the network structure of the membrane surface, and due to the unevenness of the PA reaction, there are some "valley and ridge" network wrinkles on the membrane surface, such as Figure 6 (b 1 -b 3After PA interfacial polymerization, the macropores on the membrane surface are covered. However, due to the unevenness of the interfacial polymerization reaction, the membrane defects lead to poor membrane retention performance. Therefore, GA is further used as a cross-linking agent. TA and PEI are co-deposited through Schiff base and Michael addition reactions to form a dense, defect-free nanoporous layer on the membrane surface. Figure 6 (c 1 -c 3 ), the nodes and network structure on the surface of the PTFE-PA membrane are filled. Figure 6 (c 3 ) It can be seen that the surface of the deposited film presents a nano-aggregate state because the phenolic hydroxyl group in the TA molecule is more easily oxidized to a highly reactive quinone under alkaline conditions. In addition, the self-crosslinking reaction of the quinone promotes the covalent bonding between the aromatic rings, thereby further promoting the formation of nano-aggregates. Since the intermolecular forces (hydrogen bonds and electrostatic effects) accelerate the shrinkage of the TA / PEI complex, a smooth, dense and uniform cross-linked coating is finally formed on the membrane surface, completely covering the porous substrate and defects. This phenomenon is consistent with the AFM characterization. This method is conducive to satisfying the size effect of the nanofiltration membrane to achieve high selectivity.

[0119] Table 2 Surface roughness of original PTFE membrane and modified membrane

[0120]

[0121] Figure 7 Table 2 shows the 3D AFM images and the root mean square roughness (Rq) and arithmetic mean roughness (Ra) of the original PTFE membrane, PTFE-PA membrane and PTFE-2PA-TA / PEI membrane, where a 1 -a 2 is PTFE membrane; b 1 -b 2 is PTFE-PA membrane; c 1 -c 2 It is a PTFE-2PA-TA / PEI membrane. The Ra and Rq values ​​of the unmodified PTFE-based sheet are 153nm and 187nm, respectively. After PA co-deposition, the membrane surface is obviously filled with nodules ( Figure 6 (b)), resulting in a decrease in roughness, which is 121nm and 151nm respectively. Furthermore, the uneven areas on the substrate surface are filled by the cross-linked deposition of TA / PEI, which increases the uniformity of the membrane surface and makes the membrane surface smoother. Therefore, both Ra=100nm and Rq=129nm are lower than those of the original PTFE and PTFE-PA membranes. The change in membrane roughness (AFM) is consistent with the change in membrane morphology (SEM).

[0122] (2) ATR-FTIR and XPS characterization of original membrane and modified membrane.

[0123] ATR-FTIR and XPS were used to characterize the main functional groups, chemical bonds and chemical composition of the membrane surface. Figure 8 The original PTFE support layer is at 1203cm -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). Compared with the PTFE membrane, the modified membrane has a strong CF characteristic peak at 3000-3500 cm -1 The peaks of hydrophilic groups -OH and NH are shown at 1615 cm -1 The amide bond (O=CN) and 1435 cm -1 The new peak of the CN bond characteristic peak at the bottom indicates that PEI and TMC are successfully polymerized. However, after cross-linking GA and TA / PEI, the NH vibration peak of the primary amine is weakened, and the stretching vibration peak of the imine -C=N- is generated. Due to the peak overlap, the peak of PTFE-2PA-TA / PEI is obscured and broadened compared with PTFE-PA membrane.

[0124] XPS analysis characterized the change of chemical composition on the membrane surface. Fig. 9 The XPS elemental spectra of the original membrane and different modified membranes are shown in Figure 2. Due to the addition of PEI, the N element is also present in the subsequent membrane surface characterization. On the PTFE substrate, F1s (*CF 2 The typical characteristic peak of 292eV) does not exist on the PTFE-2PA-TA / PEI composite film because the F1s characteristic peak becomes less and less obvious with the deposition of multiple layers. Based on the XPS detection depth of no more than 10nm, F1s cannot be detected. This is attributed to the successful deposition of TA / PEI, which increases the film thickness.

[0125] Table 3 shows the elemental composition and percentage of the membrane surface. Compared with the original PTFE membrane, the N content in PTFE-PA and PTFE-2PA increased to 10.38% and 12.25% respectively, indicating that the amount of N element introduced into the membrane surface increased. And the representative characteristic peaks of O and N are complete and the degree of cross-linking increases (generally speaking, a lower O / N value means a larger degree of cross-linking (DNC), thereby achieving a dense PA layer), which indicates that the nanofiltration membrane prepared by the IP strategy has a complete structure and no obvious defects. The PTFE-2PA-TA / PEI membrane shows an increase in the content of O, which is attributed to the introduction of GA. TA contains a large number of aldehyde groups and hydroxyl groups, which increases the O content compared to N, so the O / N ratio also increases.

[0126] Table 3 Atomic content of original PTFE membrane and modified membrane

[0127]

[0128] The chemical bonds on the membrane surface were further characterized by high-resolution C1s and N1s XPS spectra.

[0129] like Fig.11 It can be seen that the formation of C1s peaks at O=C*-N (287 eV) and C*-N (285.4 eV) indicates the synthesis of the polyamide (PA) layer; N1s has two main peaks at 399.5 eV and 401.0 eV, representing N*-C=O bond and N*-H bond, respectively, indicating the synthesis of amide bond and protonation of PEI terminal primary amine to generate -NH 3 + , making the membrane surface positively charged. The above characterizations are all successful synthesis of the surface PA layer.

[0130] Depend on Figure 7 , the C1 and N1s peaks of the PTFE-2PA-TA / PEI modified membrane can be obtained, and the new peak C*-OC / C*-OH (286.0eV) is attributed to the reaction of glutaraldehyde (GA) with phenolic hydroxyl groups in tannic acid (TA). And the new peak C=N* (399.2eV) is attributed to the Schiff base reaction of TA and co-deposition on the surface of PEI. The increase of CN* (398.7eV) peak is attributed to the excess PEI. Similar to the PTFE-PA layer, -N*H 3 + The presence of indicates that the membrane surface remains positively charged due to the protonation of the residual primary amines in PEI. Fig.10 The reaction principle is the same.

[0131] (3) Contact angle of membrane surface.

[0132] Generally speaking, lower WCA means better hydrophilicity. The hydrophilic membrane surface easily combines with water molecules to form a hydration layer to resist the adhesion of pollutants. The chemical composition and roughness of the membrane surface are the main factors affecting the hydrophilicity of the membrane. Fig.12 It can be seen that after the formation of the PA layer on the PTFE support membrane, the WCA decreased from 104.78° to 80.24°, which is attributed to the introduction of a large number of hydrophilic groups, including -NH 2 , -OH and -COOH. Among them, the hydrophilic group -COOH is generated by the hydrolysis of excess unreacted acyl chloride. When TA / PEI is deposited on the defects and surface of the PA layer, the hydrophilicity of the membrane should also increase accordingly due to the introduction of a large number of phenolic hydroxyl groups in TA. However, on the other hand, phenol is easily self-oxidized to generate aldehydes under alkaline conditions, and aldehydes react with -NH 2 / -COOH has good reactivity, which causes some hydrophilic groups to react and be consumed; on the other hand, the resistance of aqueous solution passing through the membrane channel increases due to the increase in membrane thickness / decrease in membrane pore size, which reduces the hydrophilicity of the PTFE-2PA-TA / PEI membrane surface compared to the PTFE-PA layer.

[0133] (4) Membrane surface solid Zeta potential test and pore size analysis test.

[0134] like Fig.13 , Fig.14 These are the characterization diagrams of membrane molecular weight cutoff MWCO and membrane pore size distribution. The molecular weight cutoff MWCO of the membrane material can be indirectly characterized by the size of the PEG molecular weight when the membrane retention rate is 90%, and the pore size distribution of the membrane can be indirectly characterized by the retention rate of PEG with different molecular weights. The MWCO of the PTFE-PA membrane and PTFE-2PA-TA / PEI membrane are 1396Da and 630Da, respectively, and the average effective pore sizes are 0.47nm and 0.17nm, respectively. The experiment shows that there are still some pore size defects in the single-layer PA selective layer, and further modification by TA / PEI co-deposition reduces the pore size of the membrane and makes it denser.

[0135] Fig.15 The Zeta potential of PTFE membrane and PTFE-2PA-TA / PEI membrane under different pH conditions. The isoelectric point of the modified membrane PTFE-2PA-TA / PEI is 9.5. Compared with the original membrane, the strongly negatively charged PTFE membrane has a strong positive charge. The original PTFE membrane is mainly a high molecular organic polymer base membrane formed by CF bonds. Due to the hydrophilic group -OH and strong CF bonds, the surface charge of the membrane is extremely negative. The modified membrane material introduces a large amount of PEI amino long-chain branched aqueous monomers to form a selective film to provide amine reaction sites. At the same time, the unreacted free amine groups are protonated to generate positively charged -NH 3 + , which greatly increases the positive charge on the membrane surface. When pH>9.5, the membrane surface is obviously negatively charged, because the carboxyl groups after TMC hydrolysis will be further hydrolyzed, and the free amine groups will be deprotonated, making the Zeta potential of the membrane surface gradually become negative.

[0136] 3. Membrane performance analysis.

[0137] (1) Single ion test

[0138] Depend on Fig.16 Available, through M 1 After one filtration, the retention rates of divalent metal ions are R Ni2+ =66.7%, R Co2+ =74.0%, R Mn2+=72.5%. The retention effect is similar due to the similarity of the divalent ion electrical properties and ion size. However, the retention effect of lithium ions is poor due to their small size and low charge (R Li+ =13.70%). When two layers of PA membranes were selected as the secondary interception membranes, the interception effect of divalent ions only reached about 85% after the secondary interception, and the interception effect was still not good.

[0139] However, when GA is used as a crosslinker, TA can react with amino groups (-NH 2 ) M after strong cross-linking through Michael addition and Schiff base principle reaction 3 The membrane has a retention rate of more than 97% due to the small pore size effect and charge repulsion (such as Fig.16 As shown, the secondary interception test obtained a single ion solution Ni 2+ The permeate flux was 20.26 L m -2 h -1 Mpa -1 , the interception rate is 98.04%; Co 2+ The permeate flux was 20.91 L m -2 h -1 Mpa -1 , the interception rate is 97.63%; Mn 2+ The permeate flux was 19.62 L m - 2 h -1 Mpa -1 , the interception rate is 97.11%; Li + The permeate flux was 20.02 L m -2 h -1 Mpa -1 , the interception rate was 49.90%; compared with only one interception of M 1 and M 1 +M 2 Secondary interception cannot achieve the purpose of high interception rate).

[0140] However, as shown by the trade-off effect, a higher rejection rate will lead to a lower permeate flux, so M 3 The permeation flux is higher than that of M 1 It is relatively small. At the same time, it can be seen from the electron microscope SEM that the permeation flux decreases due to the decrease in membrane pore size. At the same time, under the same mass concentration conditions, due to the small relative molecular mass of anhydrous lithium chloride, Li + The concentration is higher than that of divalent ions, so Li + The permeate flux will logically decrease due to an increase in the osmotic pressure difference between the feed and permeate solutions.

[0141] In addition, the steric hindrance effect caused by the membrane pore size on the ions is also worth paying attention to. Fig.14 It can be clearly observed that M 3 Membrane compared to M 1 The membrane has a denser pore structure. This pore effect enables the membrane to have a better retention effect on high-valent hydrated ions with large ionic radius. Fig.16 The retention performance of the two membranes for saline solution is consistent, which is mainly attributed to the 3 The results show that the retention of the four main metal ions in the waste LiBS leachate by the nanofiltration membrane is the result of the combined action of the Donnan effect and the steric hindrance effect, among which the Donnan effect is related to the surface charge of the membrane, while the steric hindrance effect is related to the pore size of the membrane. Therefore, in-depth research on the surface charge characteristics and pore size structure of the nanofiltration membrane has important guiding significance for optimizing the application of nanofiltration membrane in recovering lithium ions from leachate.

[0142] (2) Simulation sample test

[0143] like Fig.17 As shown in the figure, the test is the interception effect of the composite nanofiltration membrane on various metal ions in the simulated waste LIB leachate. The selective separation of metal ions by the membrane is attributed to the electrostatic repulsion of the membrane surface charge caused by the Donnan effect between ions, and the steric hindrance effect caused by the ion size screening. The difference in the interception rate between monovalent lithium ions and divalent metal ions indicates that the membrane is positively charged under neutral pH = 7 and pH = 4. Fig.15 As shown, the ion ratio (Ni 2+ +Co 2+ +Mn 2+ :Li + ) decreased from 13.93 to 1.37 (pH = 7) and 1.79 (pH = 4). + The positive charge carried is minimal, so during the nanofiltration process, the Donnan repulsion is small, resulting in a relatively small retention rate. Therefore, the membrane exhibits a higher retention rate for other multivalent metals, and ultimately achieves the enrichment of monovalent lithium ions in the permeate through the nanofiltration membrane, while divalent metal ions are retained in the feed liquid. This is consistent with our expectations, that is, the nanofiltration membrane can directly treat the waste LIB leachate, providing a feasibility reference for the application of membrane separation in lithium recovery.

[0144] (3) Actual sample test

[0145] In practical applications, the positive electrode of the battery is made of positive electrode material LiNi x Co y Mn 1-x Mn x O 2The mixture is composed of ions, which form a mixed ion acidic solution (pH = 2) after being dissolved in acid. Therefore, we investigated the separation performance of the composite nanofiltration membrane in the actual waste LiBS acidic leaching solution. The retention rates of different ions are as follows Fig.18 As shown in the figure, the retention rates of nickel, cobalt and manganese by the nanofiltration membrane after secondary interception are Ni 2+ (R = 97.7%), Co 2+ (R = 98.1%), Mn 2+ (R = 99.96%). The lithium-rich solution obtained by treating the leachate with the membrane had a feed concentration ratio of 13.11 reduced to 0.6, and its permeate flux was 18.39 L m -2 h -1 Mpa -1 These results confirm that at pH = 2, the positive surface charge of the membrane leads to a high rejection rate of multivalent metal ions while allowing a large amount of lower-charged lithium ions to pass through. 3 It has a smaller pore size and shows a higher rejection rate for all metal ions; at the same time, the strong covalent bond (CN / C=N) cross-linking between GA and TA / PEI enables the selective layer membrane to retain good stability under acidic conditions and has acid resistance under strong acid conditions. These results further confirm that the recovery efficiency of nanofiltration membrane for metal ions in leachate mainly depends on the surface charge and pore size of the membrane.

[0146] 4. Summary.

[0147] The present invention develops a positive charge composite nanofiltration membrane based on acid-resistant polytetrafluoroethylene (PTFE) as a base support membrane, and applies it to the recovery of lithium from acidic leachate of waste lithium battery positive electrode materials. The study uses a PTFE membrane with excellent mechanical properties and acid and alkali resistance as a support layer, and uses two interfacial polymerizations (IP) to reduce the high roughness caused by defects in the production process of polytetrafluoroethylene and to make up for the defect of insufficient support caused by the large pores of the PTFE substrate; amino long-chain branched polyethyleneimine (PEI) is used as an aqueous monomer for the interfacial polymerization reaction, which can provide positive charge to the surface of the membrane material; at the same time, glutaraldehyde (GA) is used as a cross-linking agent, combined with tannic acid (TA) and polyethyleneimine (PEI) to co-precipitate to form a stable dense layer bonded by covalent bonds (CN / C=N), making the membrane surface smoother, the hydrophilicity of the membrane material is improved, and the pore size defects of the nanofiltration membrane are effectively compensated, and finally the support layer and the selective layer are truly realized to take into account the hydrophilicity, acid resistance, density and positive charge properties at the same time. The hydrophilic angle of the finally synthesized nanofiltration membrane material is 88.25°, the molecular weight cutoff (MWCO) of the composite nanofiltration membrane material is 630Da, and when the pH is less than 9.5, the membrane surface is positively charged.

[0148] The experimental results show that the nanofiltration membrane exhibits excellent performance under acidic conditions (pH = 2): the water permeability reaches 18.39 L m -2 h -1 MPa -1 , and has a high retention rate for divalent cations (single ion: Ni 2+ 、Co 2+ , Mn 2+ , Li + The retention rates of Ni and Mg were 97.71%, 98.17%, 99.96% and 54.6% respectively. 2+ +Mn 2+ +Co 2+ ) and Li + In the separation, the concentration ratio before and after the nanofiltration membrane interception dropped from 13.11 to 0.60, which significantly improved the selective recovery efficiency of lithium. The present invention confirms the feasibility of nanofiltration membrane separation technology in the recycling of waste lithium batteries and provides a new idea for efficient and environmentally friendly lithium battery recycling.

[0149] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described 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.

[0150] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A composite nanofiltration membrane based on PTFE, characterized in that: include: A PTFE substrate, a modified structure, a cross-linked molecule and a nanoporous layer. The modified structure is attached to the surface of the PTFE substrate, one end of the cross-linked molecule is connected to the modified structure, and the other end is connected to the nanoporous layer.

2. The composite nanofiltration membrane according to claim 1, characterized in that The modified structure is mainly obtained by interfacial polymerization of PEI and TMC; The cross-linking molecules include GA; The nanoporous layer is mainly obtained by co-deposition of TA and PEI through Schiff base and Michael addition reaction.

3. The composite nanofiltration membrane according to claim 2, characterized in that One end of the cross-linking molecule is connected to the amide group of the modified structure, and the other end is connected to the phenol group of the nanoporous layer.

4. The method for preparing the composite nanofiltration membrane according to any one of claims 1 to 3, characterized in that: The following steps are involved: Preparation of a PTFE membrane with a modified structure: Based on a PTFE substrate, PEI and TMC are used for polymerization reaction to obtain a PTFE membrane with a modified structure; Preparation of composite nanofiltration membrane: GA was used as a cross-linking agent, and TA and PEI were used for co-deposition of Schiff base and Michael addition reaction to obtain a composite nanofiltration membrane.

5. The preparation method according to claim 4, characterized in that: In the step of preparing the PTFE membrane with a modified structure, the polymerization reaction includes two interfacial polymerizations; The interfacial polymerization comprises: using a solution containing PEI, a surfactant and an acid acceptor to react with TMC, and heating and curing.

6. The preparation method according to claim 5, characterized in that: The surfactant comprises SDS, and the acid acceptor comprises TEA; In the interfacial polymerization, the mass percentages of PEI, surfactant, acid acceptor and TMC are (2.2-2.6wt%): (0.2-0.6wt%): (2.2-2.6wt%): (1-1.4wt%), the reaction time is 70-90s, the heating curing temperature is 55-65°C, and the heating curing time is 8-12min.

7. The preparation method according to claim 4, characterized in that: In the step of preparing the composite nanofiltration membrane, the co-deposition of the Schiff base and Michael addition reaction includes: placing GA and TA on the surface of the PTFE membrane with a modified structure, reacting, heating and curing, and placing PEI on the surface of the PTFE membrane with a modified structure, reacting, and heating and curing.

8. The preparation method according to claim 7, characterized in that: In the co-deposition of the Schiff base and Michael addition reaction, the mass percentages of GA, TA and PEI are (1.8-2.2wt%): (1.3-1.8wt%): (2.3-2.8wt%); The co-deposition of Schiff base and Michael addition reaction includes: placing GA and TA on the surface of the PTFE membrane with modified structure, reacting for 15-25 minutes, heating and curing at 45-55° C. for 3-8 minutes, placing PEI on the surface of the PTFE membrane with modified structure, reacting for 0.8-1.2 hours, and heating and curing at 45-55° C. for 13-18 minutes.

9. Use of the composite nanofiltration membrane according to any one of claims 1 to 3 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 composite nanofiltration membrane described in any one of claims 1 to 3 is used to separate and enrich lithium in the acidic leaching solution of the positive electrode material of waste lithium-ion batteries.

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