A composite nanofiltration membrane based on PTFE and a preparation method and application thereof

Abstract

The application relates to a PTFE-based composite nanofiltration membrane and 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 base, a modification structure, cross-linking molecules and a nanopore layer, the modification structure is attached to the surface of the PTFE base, one end of the cross-linking molecules is connected to the modification structure, and the other end of the cross-linking molecules is connected to the nanopore layer. The composite nanofiltration membrane has good compactness, effectively solves the aperture defect of an existing nanofiltration membrane, and can significantly improve the selective recovery efficiency of lithium by using the composite nanofiltration membrane, thereby providing a new idea for efficient and environment-friendly lithium battery recovery.

Description

Technical Field

[0001] This invention relates to the field of nanofiltration membrane and membrane module technology, and in particular to a PTFE-based composite nanofiltration membrane, its preparation method, and its application. Background Technology

[0002] In recent years, the world has increasingly emphasized the development of green and sustainable renewable energy to address issues such as energy shortages and climate change. Lithium-ion batteries (LIBs), with their advantages of high energy density, being green, renewable, and having stable cycle life, have been widely used in portable electronic devices and new energy vehicles (EVs), becoming a new type of high-efficiency energy storage technology. However, LIBs have a limited lifespan; generally, after 8-10 years, they are insufficient to support the normal operation of electric vehicles' electric cycles. It is worth noting that LIB cathode materials contain various precious metals, such as LiNi. x Co y Mn 1-x Mn x O2 cathode materials contain 5-20 wt% cobalt (Co), 5-12 wt% nickel (Ni), 7-10 wt% manganese (Mn), and 2-5 wt% lithium (Li), with metal content exceeding that of natural ores. If valuable metal elements in waste LIBs cannot be effectively recovered, relying solely on natural mineral resources to meet future market demands may be challenging.

[0003] As shown in Table 1, existing recycling technologies include direct recycling, pyrometallurgy, and hydrometallurgy. However, direct recycling and pyrometallurgy are not 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 relatively wide application, this method easily generates a large amount of waste and consumes a large amount of chemical raw materials such as extractants, adsorbents, and 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 inevitably results in the loss of some valuable metals, which increases the leaching cost and the complexity of the leaching process. Among them, acidic leachate mainly involves 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. Furthermore, high-temperature calcination can reduce metal oxides. Finally, the calcined mixture is dissolved in acid, and subsequent separation and enrichment are carried out in solution form.

[0004] Table 1 Summary of Lithium-ion Battery Recycling Processes Used by Multiple Companies Worldwide

[0005]

[0006]

[0007] However, in actual recycling processes, given the requirements of large recycling volume, limited operational technology, and no secondary pollution, finding a method that meets stringent conditions such as simple operation, high recovery rate, and resistance to acid solvents is of great research significance for the efficient recycling of waste LIB. Summary of the Invention

[0008] To address the aforementioned problems, this invention provides a PTFE-based composite nanofiltration membrane. This composite nanofiltration membrane exhibits good compactness, effectively solving the pore size defects of existing nanofiltration membranes. Furthermore, the use of this composite nanofiltration membrane can significantly improve the selective recovery efficiency of lithium, providing a new approach for efficient and environmentally friendly lithium battery recycling.

[0009] To achieve the above objectives, the present invention provides a PTFE-based composite nanofiltration membrane, comprising a PTFE substrate, a modified structure, crosslinked molecules, and a nanoporous layer. The modified structure is attached to the surface of the PTFE substrate, and one end of the crosslinked molecules is connected to the modified structure, and the other end is connected to the nanoporous layer.

[0010] Nanofiltration (NF) separation primarily relies on two separation principles: charge effect and size sieving. Therefore, it is widely used in the efficient and selective separation of low molecular weight organic matter and high / monovalent metal ions in water samples. Furthermore, NF separation is the only membrane technology used for large-scale lithium-ion recovery. This technology is widely applied in water treatment due to its advantages of simple operation, high throughput, and high selectivity. A NF membrane mainly consists of a supporting substrate membrane and a charged selective separation membrane. Therefore, to meet the stringent requirements of acidic leachates from spent lithium batteries, both the substrate membrane and the selective layer must possess strong acid resistance.

[0011] Polytetrafluoroethylene (PTFE), an organic polymer material known as the "King of Plastics," possesses unique properties due to the strong stability of its CF bonds. PTFE membranes exhibit resistance to strong acids and alkalis, as well as organic solvents, characteristics unmatched by traditional PES and PSF organic substrate membranes. They can even be stored dry without preservatives, reducing equipment storage costs and transportation difficulties. However, due to technological limitations, commercially available PTFE membranes only achieve a pore size of 0.1 μm. Therefore, compared to PES and PSF, which form the support layer of large-scale nanofiltration membranes, PTFE has significantly larger pores. Consequently, if a nanofiltration membrane with macroporous PTFE as the support layer is used, the synthesized ultrathin polyamide (PA) selective layer on its surface will lack sufficient support due to the large pores of the substrate membrane, leading to reduced nanofiltration membrane stability. Secondly, the strong electronegativity of the fluorine element in the strong CF bonds of PTFE results in a strongly negatively charged membrane surface. However, since the target ion is a cation, the positive repulsion effect of the Donnan effect cannot be achieved. Therefore, to fabricate nanofiltration membranes suitable for lithium-ion separation and enrichment based on PTFE, the key issues to address are the porosity of PTFE and the modification of the membrane surface's positive charge. The aforementioned PTFE-based composite nanofiltration membrane combines the nanoporous layer with the PTFE substrate through cross-linked molecules and modified structures attached to the PTFE substrate. This results in a composite nanofiltration membrane with good density, effectively solving the pore size defects of existing nanofiltration membranes. Furthermore, this composite nanofiltration membrane can significantly improve the selective recovery efficiency of lithium, providing a new approach for efficient and environmentally friendly lithium battery recycling.

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

[0013] The cross-linked 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 using interfacial polymerization (IP), a method with easily controllable reactions and mild synthesis conditions. Traditional IP uses m-phenylenediamine (MPD) and piperazine (PIP) as aqueous active monomers to interfacially polymerize with trimesoyl chloride (TMC) to generate ultrathin, negatively charged nanofiltration membranes. However, since most recycled waste lithium-ion batteries contain polyvalent metal cations, positively charged nanofiltration membranes are the research target. Long-chain branched amino-based polyethyleneimine (PEI) is a water-soluble polymer rich in amino groups. It is cationic in aqueous solution and has a high positive charge density, making it a suitable alternative to traditional aqueous monomers in the reaction with the TMC organic phase. Figure 1This method aims to modify membrane materials with positive charges to enhance the charge effect. Meanwhile, tannic acid (TA) is a common plant polyphenol, also known as tannic acid. Tannic acid is widely used due to its advantages such as hydrophilicity, adhesion, strong metal chelating ability, and low price. Under alkaline conditions, its self-oxidation can modify materials through Michael addition or Schiff base principle co-deposition with amino groups (-NH2), such as… Figure 2 .

[0016] Therefore, this invention employs an interfacial polymerization (IP) strategy, utilizing long-chain polyamine PEI to enhance the positive charge on the nanofiltration membrane surface; simultaneously, GA is used as a crosslinking agent, achieving a strong crosslinking effect through the deposition of PEI and TA, thereby enhancing the compactness of the nanofiltration membrane, ultimately successfully preparing a positively charged composite nanofiltration membrane. Furthermore, this invention systematically characterizes the preparation conditions, structural properties, and performance of the synthesized material, and applies the membrane material prepared under optimal conditions to a recycling experiment of spent lithium-ion batteries (LIBs). The preparation and separation working principle of the composite nanofiltration membrane are as follows: Figure 3 As shown.

[0017] In one embodiment, the crosslinked molecule is connected to an amide group of the modified structure at one end and a phenol group of the nanoporous layer at the other end.

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

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

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

[0021] In one embodiment, in the step of preparing the PTFE membrane with the modified structure, the polymerization reaction includes two interfacial polymerizations; the interfacial polymerization includes reacting TMC with a solution containing PEI, surfactant, and acid acceptor, followed by 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 (2.2-2.6 wt%): (0.2-0.6 wt%): (2.2-2.6 wt%): (1-1.4 wt%), the reaction time is 70-90 s, the heating and curing temperature is 55-65 ℃, and the heating and curing time is 8-12 min.

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

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

[0026] The co-deposition of the Schiff base and Michael addition reaction includes: placing GA and TA on the surface of the modified PTFE membrane, reacting for 15-25 min, and curing at 45-55℃ for 3-8 min; placing PEI on the surface of the modified PTFE membrane, reacting for 0.8-1.2 h, and curing at 45-55℃ for 13-18 min.

[0027] The present invention also provides the application 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 recycling lithium from waste lithium-ion batteries, comprising the following steps: using the composite nanofiltration membrane to separate and enrich lithium in the acidic leachate of the positive electrode material of waste lithium-ion batteries.

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

[0030] This invention discloses a PTFE-based composite nanofiltration membrane, its preparation method, and its application. The composite nanofiltration membrane possesses good hydrophilicity, acid resistance, density, and positive charge properties, effectively solving the pore size defects of existing nanofiltration membranes. Furthermore, the use of this composite nanofiltration membrane can significantly improve the selective recovery efficiency of lithium, providing a new approach for efficient and environmentally friendly lithium battery recycling. Attached Figure Description

[0031] Figure 1 This is a schematic diagram illustrating the principle of the interfacial polymerization reaction between PEI and TMC.

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

[0033] Figure 3 This is a schematic diagram illustrating the preparation and separation mechanism of a PTFE-based composite nanofiltration membrane.

[0034] Figure 4 Figure showing the results of the experimental study on the synthesis optimization of PA selective layer preparation for interfacial polymerization reaction; test conditions: 2 g / L CoCl2·6H2O aqueous solution, 0.4 MPa, 25℃, pH=7;

[0035] Figure 5 Figure showing the results of the experiment to optimize the synthesis of dense selective layers; test conditions: 2 g / L CoCl2·6H2O aqueous solution, 0.4 MPa, 25℃, pH=7;

[0036] Figure 6 Here are SEM images of the original PTFE support membrane and the modified membrane surface, where, Figure 6 a1-c1: 7.00KX; Figure 6 a2-c2: 10.00KX; Figure 6 a3-c3: 20.00KX; a: PTFE membrane; b: PTFE-PA membrane; c: PTFE-2PA-TA / PEI membrane;

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

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

[0039] Figure 9 XPS full spectra of the original membrane and the modified membrane;

[0040] Figure 10 This is a schematic diagram illustrating the cross-linking reaction principle of the dense layer;

[0041] Figure 11 The XPS spectra of PTFE-PA and PTFE-2PA-TA / PEI modified membranes are finely divided for C1s and N1s. The left side shows the finely divided C1s spectra of different membranes, and the right side shows the finely divided N1s spectra of different membranes.

[0042] Figure 12 Figure 1 shows the water contact angle (WCA) results for the original and modified PTFE membrane surfaces.

[0043] Figure 13The graph shows the results of different polyethylene glycol (PEG) molecule rejection rates for PTFE-PA membrane and PTFE-2PA-TA / PEI membrane;

[0044] Figure 14 Pore ​​size distribution diagrams for PTFE-PA membranes and PTFE-2PA-TA / PEI membranes;

[0045] Figure 15 The zeta potential results of PTFE membrane and PTFE-2PA-TA / PEI membrane under different pH conditions are shown in the figure.

[0046] Figure 16 Figures showing the rejection rate and permeation flux under different single ion conditions for primary (M1) and secondary (M1+M2, M1+M3) rejection: M1: PTFE-PA; M2: PTFE-2PA; M3: PTFE-2PA-TA / PEI;

[0047] Figure 17 The graph shows the retention rate and permeation flux results for the primary and secondary retention of the simulated sample, where M1: PTFE-PA; M2: PTFE-2PA; M3: PTFE-2PA-TA / PEI;

[0048] Figure 18 The graph shows the retention rate and permeate flux of the permeate in the primary and secondary stages of the actual sample (NCM: 811), where M1: PTFE-PA; M2: PTFE-2PA; M3: PTFE-2PA-TA / PEI. Detailed Implementation

[0049] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0050] 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 invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0051] source:

[0052] Experimental apparatus:

[0053] Spiral flat-plate membrane pilot-scale 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), inductively coupled plasma atomic emission spectrometer (SPECTRO ARCOSMV, SPECTRO GmbH, Germany), pH meter (CT-6021A, Qingdao Juchuang Times Environmental Protection Technology Co., Ltd.), magnetic conductivity meter (DDS-307A, Shanghai Instrument & Electronics Scientific Instruments Co., Ltd.), attenuated total reflection Fourier transform infrared spectrometer (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 GmbH, Germany), analytical electronic balance (A200S, Sartorius GmbH, Germany), atomic force microscope (Dimenson). ICON (Bruker Corporation, USA) and ultrasonic instrument (SK250H, Shanghai Keda Instrument Co., Ltd.);

[0054] Experimental reagents:

[0055] 1,3,5-Benzotricarboxylic acid chloride (TMC, analytical grade, Shanghai Maclean's Biochemical Co., Ltd.) n-Hexane (analytical grade, Shanghai Maclean's Biochemical Co., Ltd.) Triethylamine (TEA, analytical grade, Shanghai Maclean's Biochemical Co., Ltd.) Polyethyleneimine (PEI, Mw = 70,000 Da, 30% aqueous solution, analytical grade, Shanghai Maclean's Biochemical Co., Ltd.) Sodium hydroxide (NaOH, analytical grade, Shanghai Maclean's Biochemical Co., Ltd.) Sodium dodecyl sulfate (SDS, analytical grade, Shanghai Maclean's Biochemical Co., Ltd.) Anhydrous lithium chloride (analytical grade, Shanghai Maclean's Biochemical Co., Ltd.) Hexahydrate chloride Cobalt (analytical grade, Shanghai Maclean's Biochemical Co., Ltd.), Nickel chloride hexahydrate (analytical grade, Shanghai Maclean's Biochemical Co., Ltd.), Manganese chloride tetrahydrate (analytical grade, Shanghai Maclean's Biochemical Co., Ltd.), Polyethylene glycol of different molecular weights (PEG, analytical grade, Shanghai Maclean's Biochemical Co., Ltd.), Tannic acid (TA, analytical grade, Tianjin Fuchen Chemical Reagent Factory), Glutaraldehyde (GA, analytical grade, Tianjin Kemeio Chemical Reagent Co., Ltd.), Hydrochloric acid (HCl, analytical grade, Guangzhou Reagent Technology Co., Ltd., Guangdong), Hydrophilic PTFE membrane (0.1µm pore size, hydrophilic, Longjin Membrane Technology Co., Ltd.)

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

[0057] Example

[0058] I. 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 adhesives and impurities from the membrane surface. An aqueous PEI solution (containing 0.1 wt% SDS as a surfactant and 1.5 wt% TEA as an acid acceptor) was added to the surface of the hydrophilic PTFE membrane, which was fixed by a circular membrane pool clamp, and immersed for 40 minutes to ensure uniform penetration. Then, after removing excess aqueous solution from the membrane surface with a roller, the membrane was covered with TMC dissolved in n-hexane at room temperature for a certain period to allow interfacial polymerization. The membrane surface was washed with n-hexane to remove excess unreacted TMC organic solution and prevent the weakening of the positive charge on the membrane surface due to hydrolysis of unreacted TMC. Finally, the membrane was cured in a 60°C oven for 10 minutes. The prepared composite NF membrane was stored in deionized water for subsequent testing and characterization. The prepared membrane was named M1 (PTFE-PA).

[0061] Compared to traditional PES nanofiltration membranes, PTFE membranes have larger pore sizes in the PSF support layer and thinner PA selective layer, resulting in weaker stability of the synthesized M1 (PTFE-PA) modified membrane. The membrane is susceptible to rupture under high pressure, leading to a decrease in retention rate. Therefore, this experiment designed a two-stage interfacial polymerization (IP) process between PEI and TMC to prepare a modified nanofiltration membrane. The membrane synthesized after the two IP processes was named M2 (PTFE-2PA) to enhance stability and increase its positive charge.

[0062] 2. Dense composite nanofiltration membranes (i.e., PTFE-based composite nanofiltration membranes) are prepared by strong cross-linking of PTFE-2PA-TA / PEI.

[0063] Under M2 membrane conditions, the membrane performance still suffers from poor selectivity due to the large pore size. Therefore, it is considered to enhance the pore size sieving effect and increase the pore density on the membrane surface. Specific synthesis steps are as follows: Figure 3 The prepared M2 membrane was fixed in a circular membrane pool clamping device. Glutaraldehyde (GA) was used as the crosslinking agent. GA and TA solutions were immersed in the membrane surface for 10 min each, followed by curing in a 50℃ oven for a certain time. Unreacted GA and TA solutions were washed away with deionized water. Finally, PEI solution (containing 2.4 wt% TEA) was added to the surface and reacted at room temperature for 1 h. The reacted membrane was then placed in a 50℃ oven for 15 min to cure. The resulting 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] II. Structural characterization of the original membrane and the modified composite nanofiltration membrane.

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

[0066] Fourier transform infrared spectroscopy (FTIR) is an important method for the qualitative and structural analysis of organic compounds and polymers. Traditional testing methods, such as pellet and paste methods, present difficulties for special samples that are difficult to dissolve, pulverize, or have large thicknesses. To address this, attenuated total reflectance (ATR) technology combined with FTIR has emerged, significantly expanding the application range of infrared spectroscopy. ATR-FTIR analyzes the structure of organic matter on the sample surface by observing the penetration and reflection of infrared light within a certain depth. The specific experimental steps are as follows: The synthesized membrane material is removed from deionized water, cut into 1cm × 1cm square samples, and dried. Subsequently, ATR-FTIR is used at 4000 cm⁻¹. -1 Up to 600cm -1 A scan was performed within the range to analyze the chemical structural features of the membrane surface.

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

[0068] Scanning electron microscopy (SEM) is one of the important methods for characterizing 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 was as follows: the original film and the modified film were removed from deionized water and dried, cut into small samples of 1cm × 1cm, sputter-coated with gold, and then placed in a vacuum electron microscope chamber to observe the changes in their surface morphology.

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

[0070] Atomic force microscopy (AFM) is an important method for characterizing the surface morphology of materials by measuring the surface roughness of samples. The experimental procedure is as follows: First, the synthesized membrane sample is thoroughly rinsed with deionized water and then vacuum-dried at 40°C. Subsequently, an appropriately sized sample is cut and fixed to the sample stage using conductive double-sided tape to prepare the test sample. A tapping mode is used during the test, with a scanning area of ​​10 μm × 10 μm. The surface roughness of the composite nanofiltration membrane is quantitatively characterized using root mean square roughness (Rms).

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

[0072] XPS is a highly sensitive analytical method for accurately analyzing the elemental composition and valence state of a sample. However, this characterization method is limited to detecting the surface chemical composition of samples within 10 nm and requires carbon correction. Specifically, the membrane is removed from deionized water, cut into small pieces, and vacuum-dried at 40°C. A 1cm × 1cm piece is then cut and fixed to the sample stage with conductive adhesive to test the surface chemical composition and valence state of the membrane.

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

[0074] Hydrophilicity is a crucial performance requirement for membrane materials, and contact angle testing can directly characterize its hydrophilicity. The closer the contact angle is to 180°, the more hydrophobic / oleophilic the material; conversely, the closer the contact angle is to 0°, the more hydrophilic / oleophobic the material, and the greater its water permeability. In this test, the membrane sample was dried and cut into 1cm × 2cm specimens. One drop of deionized water was used as the test solution, and the contact angle was digitally captured and analyzed. Multiple tests were conducted, and the average value was calculated.

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

[0076] The surface zeta potential of modified nanofiltration membranes was determined using a solid surface potential analyzer. The specific experimental procedures are as follows: First, the membrane to be tested was thoroughly immersed in a 0.001M KCl solution. Then, conductive adhesive was used to fix the membrane sample in the sample cell. Using the 0.001M KCl solution as the test solution, the pH value was adjusted by adding HCl and NaOH to measure the surface potential of the membrane under different pH conditions. To ensure data accuracy, multiple measurements were performed for each experiment, and the average value was taken as the final result.

[0077] 7. Molecular weight cutoff and pore size distribution.

[0078] The molecular weight cut-off (MWCO) of a solute with a rejection rate of 90.0% is defined as the membrane molecular weight cut-off. A higher MWCO indicates a larger pore size in the membrane material. In this embodiment, different molecular weights (200, 400, 600, 1000, 2000, Da) of the electrically neutral polymer polyethylene glycol (PEG) were used as test molecules. A feed solution of 0.1 g / L was prepared, and the permeate after passing through the membrane was used as the influent and effluent. The carbon content change was measured using a total organic carbon (TOC) analyzer. Finally, the membrane pore size was characterized by calculating the PEG rejection rate using formula (1).

[0079]

[0080] Among them, R PEG The pore size of the membrane material is characterized by the PEG molecule rejection rate. (TOC) permeate and TOCfeed These represent the total organic carbon concentrations of the feed liquid and the permeate, respectively.

[0081] By fitting the nonlinear curves of the LogNormal function to the retention rate data of PEG molecules of different molecular weights as described above, we explored the relationship equation between the membrane's retention rate of PEG molecules and the molecular weight of PEG.

[0082] The Stokes diameter (ds, nm) of a PEG molecule can be determined by its molecular weight (M). w The calculation is performed on Da, and the relationship is as follows (2):

[0083]

[0084] The aperture distribution is calculated using the following probability density function (3):

[0085]

[0086] When R PEG When the content is 50%, it corresponds to the Stokes diameter μ of the PEG molecule. s With the average diameter μ of the membrane p The same, σ p Defined as the average effective pore size μ of the membrane p The geometric standard deviation, corresponding to R PEG =84.1% of the Stokes diameter (d) of polyethylene glycol molecules s ,nm) and μ s ratio

[0087] III. Performance testing of composite nanofiltration membranes.

[0088] In this embodiment, a commercially available PTFE membrane (0.1 μm pore size) was used, and its performance was tested using a cross-flow spiral flat-sheet membrane pilot-scale test apparatus. This included tests on membrane permeate flux, single valuable metal rejection rate, and mixed ion rejection rate. The effective membrane area used for measurement was 69.36 cm². 2 The feed concentration was 2000 ppm, the test pressure was 0.4 MPa, and the pre-compression was 30 min at room temperature and atmospheric pressure to ensure stable membrane performance. At least three identical membranes were tested for each test for comparison to eliminate errors.

[0089] 1. Permeability.

[0090] Permeation flux refers to the volume of liquid passing through a unit effective area per unit time under unit pressure, directly characterizing the membrane separation rate. Therefore, the larger the permeation flux, the faster the separation rate. It is calculated using the following formula (4):

[0091]

[0092] Where J is the permeation flux (L m) -2 h -1 V represents the volume of liquid passing through under these operating conditions (L), and A is the effective membrane area for the membrane separation test (m²). 2 ), where t is the sampling time (h).

[0093] 2. Retention performance.

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

[0095]

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

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

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

[0099] Using deionized water as the solvent, 2 g / L solutions of (LiCl, NiCl2·6H2O, CoCl2·6H2O, MnCl2·4H2O) were prepared as single-ion test feed solutions.

[0100] Using laboratory waste lithium-ion batteries NCM(811) as the actual test object, the waste batteries were discharged and then disassembled. The positive electrode material was dissolved in NaOH for 10 hours to ensure that the current collector Al of the positive electrode material was dissolved and removed. The undissolved solids were calcined at high temperature (500℃, 5 hours). The high-temperature calcination removed acetylene black and polyvinylidene fluoride (PVDF). 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] The specific NF membrane separation steps are as follows: The above-mentioned single-ion solution or actual sample leaching solution is used as the feed liquid and passed through a PTFE-PA membrane for primary nanofiltration (1st). The permeate after primary nanofiltration is used as the feed liquid for secondary nanofiltration (2nd) to obtain the final permeate. The single-ion inlet and outlet samples are tested using a conductivity meter; the actual mixed-ion sample solution is tested using inductively coupled plasma (ICP) to characterize the membrane separation performance.

[0102] IV. Results and Discussion.

[0103] 1. Optimization of synthesis conditions for composite nanofiltration membranes.

[0104] The synthesis steps for synthesizing a polyamide (PA) selective layer (i.e., PTFE-PA membrane) on the surface of the original PTFE membrane were optimized. For example... Figure 4 (a) As the PEI concentration increases, the rejection rate of the composite nanofiltration membrane first increases and then decreases with increasing concentration. That is, when the PEI concentration is 2.4 wt%, the rejection rate RCo2+ is the highest at 72.58%. The trade-off effect between rejection rate and permeation flux shows that the flux of the ion solution first decreases and then increases. This is because the composite nanofiltration membrane with low PEI concentration has a low degree of cross-linking and certain defects, forming a loose polyamide layer with low cross-linking degree. However, as the concentration increases, the cross-linking density of the selective layer increases, making its pore structure more compact, thus reducing the permeation flux. The rejection rate increases due to the reduced pore size and the positive charge effect of PEI. When the PEI concentration exceeds 2.4 wt%, the excessively high concentration leads to a faster reaction and a lower degree of polymerization. Within a certain time, it is difficult to form a dense network structure, resulting in defects and inhomogeneity in the ultrathin selective layer, ultimately increasing the permeation flux and decreasing the rejection rate.

[0105] When the PEI concentration (2.4 wt%) is constant, the relationship curves between the permeation flux and rejection rate of the composite nanofiltration membrane and the concentration of TMC organic phase monomers are as follows: Figure 4 (b) As shown in the figure, the retention rate initially increases and then decreases with increasing TMC concentration. The maximum retention rate of 74.77% is achieved at 1.2 wt%. Initially, the TMC concentration is too low, resulting in severe defects in the selective layer of the composite nanofiltration membrane. Subsequently, as the TMC concentration increases, the crosslinking density increases, leading to a denser network structure in the selective layer, which in turn increases the retention rate and decreases the permeate flux. However, if the TMC concentration is too high, rapid interfacial polymerization occurs, causing the PEI to be completely consumed. This results in the hydrolysis of the acyl chloride groups of unreacted TMC into carboxyl groups, leading to a decrease in the positive charge on the surface of the composite nanofiltration membrane, and consequently, a decrease in the retention rate.

[0106] Figure 4 (c) Optimization of the reaction time for PA interfacial polymerization. Interfacial polymerization is a rapid reaction based on the migration from the aqueous phase to the oil phase. Due to the high migration rate of PEI in the aqueous phase, the interfacial reaction occurs on the organic phase side, so the IP process is actually a non-equilibrium polycondensation. Therefore, a longer reaction time results in a thicker selective layer and an increased rejection rate. However, a thicker selective layer increases the permeation resistance of water on the membrane surface, leading to a decrease in permeation flux. Based on the principle of ensuring permeation flux without compromising rejection rate, a reaction time of 80 s was selected as the optimal reaction time.

[0107] Figure 4 (d) shows the relationship between the rejection rate and permeation flux of the composite nanofiltration membrane and the heat treatment temperature under a heat treatment time of 10 min. The figure shows that with increasing thermosetting temperature, the rejection rate initially increases and then decreases, while the permeation flux increases after decreasing. Thermosetting can stabilize the selective layer on the membrane surface to a certain extent, but excessive heat treatment can lead to thermal shrinkage and cracking of the membrane, resulting in a decrease in rejection rate and an increase in permeation flux due to the large pore size caused by membrane defects.

[0108] The optimal synthesis scheme for the final modified membrane M1 (PTFE-PA) is: 2.4wt% PEI (0.4wt% SDS, 2.4wt% TEA) and 1.2wt% TMC in hexane as solvent, interfacial polymerization for 80s, followed by curing at 60℃.

[0109] Since the selective layer is a thin film with insufficient support strength to meet the practical application requirements of subsequent modified films, a secondary interfacial polymerization reaction was chosen to enhance the support of the selective layer on the film surface. The modified film after two interfacial polymerizations (IP) was named M2 (PTFE-2PA).

[0110] The non-uniformity of the polyamide layer reaction prevents the nanofiltration membrane from achieving the desired separation of monovalent and divalent ions, necessitating further enhancement of membrane density. Using glutaraldehyde as a crosslinking agent can bond TA and PEI deposits, thereby increasing membrane density. Figure 5 As shown in (ab), when the GA concentration is 2.0 wt%, GA acts as a crosslinking agent between the free amine groups of the polyamide layer and TA, resulting in a denser crosslinking on the film surface, which in turn makes Co... 2+ The retention rate is the highest, and according to the trade-off effect, the permeate flux of the modified membrane also decreases. When the TA concentration is 1.5 wt%, under PEI reaction conditions of 1 h, TA undergoes auto-oxidation to generate quinones, which then undergo Michael addition and Schiff base reactions with the long-chain amines of PEI. This results in some electrostatic forces and secondary bonds such as hydrogen bonds being replaced by some 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 its surface stability.

[0111] like Figure 5(c) This study investigated the effect of heat treatment time on the performance of membrane materials. Thermal curing can enhance the bonding reaction between aldehydes and amides, and between aldehydes and phenols, enabling GA to connect one end to the amide in PTFE-2PA and the other end to the phenol in TA, thus achieving the purpose of preparing a dense membrane. However, excessive heating time leads to selective membrane shrinkage and rupture, resulting in performance degradation. Therefore, a heat treatment time of 50℃ for 5 minutes was selected as the optimal curing time. At this time, the selectivity of PTFE-2PA-TA / PEI was 95.01%, and the permeation flux was 18.03 L / m². -2 h -1 Mpa -1 .

[0112] The optimal synthesis scheme for the nanofiltration membrane in the entire system is as follows:

[0113] (1) PTFE-PA film: 2.4wt% PEI (0.4wt% SDS, 2.4wt% TEA) and 1.2wt% TMC in hexane as solvent, interfacial polymerization for 80s, and curing at 60℃ for 10min;

[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 min and then heat-cured in an oven at 50℃ for 5 min to remove the unreacted GA and TA on the surface. Then, 2.5wt% (2.5wt% TEA) PEI was placed on the membrane surface and reacted for 1 h and then heat-cured in an oven at 50℃ for 15 min. The prepared composite nanofiltration membrane was placed in deionized water for later use.

[0116] 2. Structural characterization of materials.

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

[0118] like Figure 6 As shown in (a1-a3), the original PTFE-supported membrane exhibits a large-pore network structure composed of nodally connected fibers. In the first step, due to the interfacial polymerization reaction between PEI and TMC, some nanoscale / microscale aggregates form on the membrane surface network structure. Furthermore, due to the non-uniformity of the PA reaction, some "valley and ridge" shaped network wrinkles appear on the membrane surface, such as... Figure 6(b1-b3). After PA interfacial polymerization, the macropores on the membrane surface are covered; however, the unevenness of the interfacial polymerization reaction leads to membrane defects, resulting in poor membrane retention performance. Therefore, GA is further used as a crosslinking agent. TA and PEI are co-deposited through Schiff base and Michael addition reactions, forming a dense, defect-free nanoporous layer on the membrane surface, such as... Figure 6 (c1-c3), the PTFE-PA membrane surface nodes and mesh structure are filled. Figure 6 As shown in (c3), the phenolic hydroxyl groups in the TA molecule are more easily oxidized to highly reactive quinones under alkaline conditions, resulting in a nano-aggregate state on the deposited membrane surface. Furthermore, the self-crosslinking reaction of quinones promotes covalent bonding between aromatic rings, further driving the formation of nano-aggregates. Due to the accelerated shrinkage of the TA / PEI composite by intermolecular forces (hydrogen bonds and electrostatic interactions), a smooth, dense, and uniform crosslinked coating ultimately forms on the membrane surface, completely covering the porous substrate and defects—a phenomenon consistent with AFM characterization. This method is advantageous for satisfying the size effect of nanofiltration membranes to achieve high selectivity.

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

[0120]

[0121] Figure 7 Table 2 shows the three-dimensional AFM images, root mean square roughness (Rq), and arithmetic mean roughness (Ra) of the original PTFE film, PTFE-PA film, and PTFE-2PA-TA / PEI film, where a1-a2 are PTFE films; b1-b2 are PTFE-PA films; and c1-c2 are PTFE-2PA-TA / PEI films. The Ra and Rq values ​​of the unmodified polytetrafluoroethylene sheet are 153 nm and 187 nm, respectively. After PA co-deposition, the film surface is clearly filled with a nodular network. Figure 6 (b) This resulted in a reduction in roughness, with values ​​of 121 nm and 151 nm, respectively. Furthermore, the TA / PEI crosslinking deposited and filled the uneven areas on the substrate surface, increasing the film surface uniformity and making the film surface smoother. Therefore, both Ra = 100 nm and Rq = 129 nm were lower than those of the original PTFE and PTFE-PA films. The changes in film roughness (AFM) showed good consistency with the changes in film morphology (SEM).

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

[0123] The major functional groups, chemical bonds, and chemical composition of the film surface were characterized using ATR-FTIR and XPS. Figure 8 The original PTFE support layer was at 1203cm. -11146cm -1 It exhibits strong CF characteristic peaks (the peak values ​​correspond to the symmetric and asymmetric FCF stretching vibrations of the PTFE-supported membrane, respectively). Compared to the PTFE membrane, the modified membrane shows strong CF characteristic peaks at 3000-3500 cm⁻¹. -1 The peaks at 1615 cm⁻¹ are hydrophilic -OH and NH groups. -1 The presence of amide bonds (O=CN) and 1435 cm⁻¹ is attributed to this. -1 The presence of a new CN bond characteristic peak indicates successful polymerization of PEI and TMC. However, due to the weakening of the NH vibration peak of the primary amine after crosslinking GA and TA / PEI, a stretching vibration peak of the imine -C=N- is generated. Due to peak overlap, the peak of PTFE-2PA-TA / PEI is masked and broadened compared to PTFE-PA film.

[0124] XPS analysis characterized the changes in the chemical composition of the membrane surface. For example... Figure 9 The XPS elemental spectra of the original membrane and membranes with different modifications are shown. The addition of PEI resulted in nitrogen (N) being observed in subsequent membrane surface characterization. On the PTFE substrate, the typical characteristic peak of F1s (*CF2, 292 eV) was absent in the PTFE-2PA-TA / PEI composite membrane. This is because the F1s characteristic peak becomes increasingly less prominent with multilayer deposition. Based on XPS detection depths not exceeding 10 nm, F1s was undetectable. This is attributed to the successful deposition of TA / PEI, which increased the membrane thickness.

[0125] Table 3 shows the elemental composition and percentages of the membrane surface. Compared to the original PTFE membrane, the N content in PTFE-PA and PTFE-2PA increases to 10.38% and 12.25% respectively, indicating an increase in the amount of N introduced to the membrane surface. Furthermore, the representative characteristic peaks of O and N are complete and show increased cross-linking (generally, a lower O / N value implies a higher degree of cross-linking (DNC), resulting in a dense PA layer), indicating that the nanofiltration membrane prepared using the IP strategy has a complete structure without obvious defects. The PTFE-2PA-TA / PEI membrane shows an increased O content, attributed to the introduction of GA. TA contains a large number of aldehyde and hydroxyl groups, leading to an increase in O content relative to N, thus also increasing the O / N ratio.

[0126] Table 3. Atom content of PTFE raw membrane and modified membrane

[0127]

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

[0129] like Figure 11It can be seen that the formation of C1s peaks at O=C*-N (287eV) and C*-N (285.4eV) indicates the synthesis of the polyamide (PA) layer; the two main peaks of N1s at 399.5eV and 401.0eV, representing N*-C=O and N*-H bonds respectively, indicate the synthesis of amide bonds and the protonation of the PEI terminal primary amine to form -NH3. + This results in a positively charged membrane surface. The above characterization confirms the successful synthesis of the surface PA layer.

[0130] Depend on Figure 7 Peak separation of C1 and N1s in the PTFE-2PA-TA / PEI modified film reveals that the new peak C*-OC / C*-OH (286.0 eV) is attributed to the reaction between glutaraldehyde (GA) and the phenolic hydroxyl groups in tannic acid (TA). The new peak C=N* (399.2 eV) is attributed to the Schiff base reaction co-deposited on the TA and PEI surfaces. The increase in the CN* (398.7 eV) peak is attributed to excess PEI. Similar to the PTFE-PA layer, -N*H3... + The presence of this indicates that the membrane surface retains a positive charge due to the protonation of the primary amines remaining in the PEI. The XPS peak fitting results are consistent with... Figure 10 The reaction principle is the same.

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

[0132] Generally, a lower WCA (water content coefficient) indicates better hydrophilicity. Hydrophilic membrane surfaces readily bind with water molecules, forming a hydration layer to resist the adhesion of pollutants. Furthermore, the chemical composition and roughness of the membrane surface are the main factors influencing membrane hydrophilicity. Figure 12 It can be seen that after the PA layer is formed on the PTFE-supported membrane, the WCA decreases from 104.78° to 80.24°, which is attributed to the introduction of a large number of hydrophilic groups, including -NH2, -OH, and -COOH. The generation of the hydrophilic group -COOH is due to 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 increase accordingly due to the introduction of a large number of phenolic hydroxyl groups from TA. However, on the one hand, phenol readily undergoes auto-oxidation to aldehyde under alkaline conditions, and aldehyde reacts well with -NH2 / -COOH, leading to the consumption of some hydrophilic groups. On the other hand, the increased membrane thickness / decreased pore size increases the resistance of aqueous solution permeation through the membrane channel, resulting in a decrease in the surface hydrophilicity of the PTFE-2PA-TA / PEI membrane compared to the PTFE-PA layer.

[0133] (4) Solid Zeta potential test and pore size analysis test on membrane surface.

[0134] like Figure 13 , Figure 14The figures show the molecular weight cutoff (MWCO) and pore size distribution of the membrane. The MWCO can be indirectly characterized by the molecular weight of PEG at a 90% retention rate, while the pore size distribution can be indirectly characterized by the retention rates of PEGs with different molecular weights. The MWCO of the PTFE-PA membrane and the PTFE-2PA-TA / PEI membrane are 1396 Da and 630 Da, respectively, with average effective pore sizes of 0.47 nm and 0.17 nm, respectively. Experiments show that the monolayer PA selective layer still has some pore size defects, and further modification through TA / PEI co-deposition reduces the membrane pore size and makes it more compact.

[0135] Figure 15 The zeta potential of PTFE membrane and PTFE-2PA-TA / PEI membrane under different pH conditions was measured. The isoelectric point of the modified PTFE-2PA-TA / PEI membrane was 9.5. Compared to the original PTFE membrane, which is strongly negatively charged, this modified membrane exhibits strong positive charge. The original PTFE membrane is mainly composed of a polymer substrate formed by CF bonds, resulting in a highly negative surface charge due to the hydrophilic -OH groups and strong CF bonds. The modified membrane material introduces a large amount of PEI amine-based long-chain branched aqueous monomers, which provide amine reaction sites for the formation of selective thin films. Simultaneously, unreacted free amines are protonated to generate positively charged -NH3. + This significantly increases the positive charge on the membrane surface. When the pH > 9.5, the membrane surface becomes noticeably negatively charged. This is because the carboxyl groups after TMC hydrolysis undergo further hydrolysis, and the free amine groups are deprotonated, causing the zeta potential on the membrane surface to gradually become negative.

[0136] 3. Membrane performance analysis.

[0137] (1) Single ion test.

[0138] Depend on Figure 16 It can be seen that after one filtration through M1, the retention rates of divalent metal ions are R. Ni2+ =66.7%, R Co2+ =74.0%, R Mn2+ =72.5%. The similarity in retention rate is due to the similarity in the charge and size of divalent ions. Lithium ions, however, have a poorer retention effect due to their smaller size and lower charge (R...). Li+ =13.70%). When two layers of PA membrane are selected as the secondary retention membrane, after secondary retention, the divalent ion retention effect only reaches about 85%, and the retention effect is still not good.

[0139] However, by choosing GA as the crosslinking agent, the M3 membrane, after strong crosslinking due to the self-oxidation of TA and the reaction with amino groups (-NH2) via Michael addition and Schiff base principle, exhibits a retention rate exceeding 97% due to its smaller pore size effect and charge repulsion (e.g., Figure 16 As shown, the secondary retention test yielded a single-ion solution Ni2+ The permeation flux is 20.26 L / m³. -2 h -1 Mpa -1 The retention rate was 98.04%; Co 2+ The permeation flux was 20.91 L / m³. -2 h -1 Mpa -1 The retention rate was 97.63%; Mn 2+ The permeation flux was 19.62 L / m². - 2 h -1 Mpa -1 The retention rate was 97.11%; Li + The permeation flux is 20.02 L / m². -2 h -1 Mpa -1 The retention rate was 49.90%; compared to single retention of M1 and double retention of M1+M2, neither of these methods could achieve a high retention rate.

[0140] However, as shown by the trade-off effect, a higher rejection rate leads to a lower permeation flux, thus M3 has a relatively smaller permeation flux than M1. This is also evident from the SEM images, which show that the reduced membrane pore size results in a lower permeation flux. Furthermore, under the same mass concentration conditions, the smaller molecular weight of anhydrous lithium chloride leads to a lower permeation flux for Li. + The concentration of Li is relatively high compared to that of divalent ions, therefore... + The permeation flux will decrease reasonably due to the increase in the osmotic pressure difference between the feed and the permeate solution.

[0141] Furthermore, the repulsive effect of membrane pore size caused by steric hindrance on ions is also noteworthy. Figure 14 It is clearly observed that the M3 membrane has a denser pore structure compared to the M1 membrane. This pore size effect results in a better retention effect for high-valence hydrated ions with large ionic radii. This trend is consistent with... Figure 16 The two membranes exhibited consistent retention performance for salt solutions, primarily attributed to the smaller pore size of the M3 membrane. The results indicate that the retention of four major metal ions in waste LiBS leachate by nanofiltration membranes is a result of the combined effects of the Donnan effect and steric hindrance. The Donnan effect is related to the membrane's surface charge, while the steric hindrance effect is related to the membrane's pore size. Therefore, further research into the surface charge characteristics and pore structure of nanofiltration membranes is of significant guiding importance for optimizing their application in recovering lithium ions from leachate.

[0142] (2) Simulated sample test.

[0143] like Figure 17As shown, this test demonstrates the retention effect of the composite nanofiltration membrane on various metal ions in 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 due to ion size sieving. The difference in retention rates between monovalent lithium ions and divalent metal ions indicates that the membrane carries a positive charge under neutral conditions of pH=7 and pH=4. Figure 15 As shown, the ion ratio (Ni) 2+ +Co 2+ +Mn 2+ Li + The pH decreased from 13.93 to 1.37 (pH=7) and 1.79 (pH=4). Specifically, this was due to the decrease in Li... + Carrying the least positive charge, this membrane experiences less Donnan repulsion during nanofiltration, resulting in a relatively low rejection rate. Consequently, it exhibits higher rejection rates for other multivalent metals, ultimately leading to the enrichment of monovalent lithium ions in the permeate while divalent metal ions are retained in the feed solution. This aligns with our expectation that nanofiltration membranes can directly treat waste LIB leachate, providing a feasibility reference for applying membrane separation to lithium recovery.

[0144] (3) Actual sample testing

[0145] In practical applications, the positive electrode of the battery is made of LiNi. x Co y Mn 1-x Mn x A mixture of O2 dissolves in acid to form a mixed ionic acidic solution (pH = 2). Therefore, we investigated the separation performance of this composite nanofiltration membrane in actual waste LiBS acidic leachate, and the rejection rates of different ions were as follows: Figure 18 As shown. The secondary filtration process resulted in the nanofiltration membrane exhibiting rejection rates of nickel, cobalt, and manganese of Ni, respectively. 2+ (R = 97.7%), Co 2+ (R = 98.1%), Mn 2+ (R = 99.96%). The lithium-rich solution obtained after treating the leachate with this membrane will have a feed concentration ratio as low as 0.6 from 13.11, with a permeation flux of 18.39 L / m³. -2 h -1 Mpa -1These results confirm that at pH=2, the positive surface charge of the membrane leads to a high rejection rate for multivalent metal ions, while allowing a large amount of lower-charged lithium ions to pass through. Furthermore, M3 exhibits a small pore size and high rejection rates for all metal ions; simultaneously, the strong covalent bond (CN / C=N) crosslinking between GA and TA / PEI ensures that the selective membrane retains good stability under acidic conditions, demonstrating acid resistance under strong acid conditions. These results further confirm that the recovery efficiency of metal ions from the leachate by nanofiltration membranes mainly depends on the membrane's surface charge and pore size.

[0146] 4. Summary.

[0147] This invention develops a positively charged composite nanofiltration membrane based on acid-resistant polytetrafluoroethylene (PTFE) as the substrate support membrane, and applies it to the recovery of lithium from acidic leachate of spent lithium-ion battery cathode materials. The study uses a PTFE membrane with excellent mechanical properties and acid / alkali resistance as the support layer. Two interfacial polymerization (IP) processes are employed to reduce the high roughness caused by defects in the PTFE production process and to compensate for insufficient support due to the large pores of the PTFE substrate. Amino-based long-chain branched polyethyleneimine (PEI) is used as the aqueous monomer for the interfacial polymerization reaction, providing positive charge to the membrane surface. Simultaneously, glutaraldehyde (GA) is used as a crosslinking agent, combined with tannic acid (TA) and polyethyleneimine (PEI) co-deposition to form a stable and dense layer linked by covalent bonds (CN / C=N). This makes the membrane surface smoother, improves the hydrophilicity of the membrane material, and effectively compensates for the pore size defects of the nanofiltration membrane. Ultimately, this achieves a true balance between hydrophilicity, acid resistance, density, and positive charge properties in both the support layer and the selective layer. The final synthesized nanofiltration membrane material has a hydrophilic angle of 88.25°, a molecular weight cutoff (MWCO) of 630 Da, and exhibits positive charge on the membrane surface when the pH is less than 9.5.

[0148] Experimental results show that the nanofiltration membrane exhibits excellent performance under acidic conditions (pH=2): water permeability reaches 18.39 L / m³. -2 h -1 MPa -1 It also has a high rejection rate for divalent cations (single ion: Ni). 2+ Co 2+ Mn 2+ Li + The retention rates were 97.71%, 98.17%, 99.96%, and 54.6%, respectively. In actual mixed-ion (Ni...) 2+ +Mn 2+ +Co 2+ ) and Li +In the separation process, the concentration ratio before and after nanofiltration membrane retention decreased from 13.11 to 0.60, significantly improving the selective recovery efficiency of lithium. This invention confirms the feasibility of nanofiltration membrane separation technology in the recycling of waste lithium batteries, providing a new approach for efficient and environmentally friendly lithium battery recycling.

[0149] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above 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 embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A PTFE-based composite nanofiltration membrane, characterized by, It comprises: a PTFE base, a modification structure, a cross-linking molecule and a nanopore layer, the modification structure is attached to the surface of the PTFE base, one end of the cross-linking molecule is connected to the modification structure, and the other end is connected to the nanopore layer; The preparation method of the PTFE-based composite nanofiltration membrane comprises the following steps: Preparation of PTFE membrane with modification structure: based on PTFE base, polyethyleneimine and 1,3,5-benzene trichloride are used for polymerization reaction to obtain PTFE membrane with modification structure; the polymerization reaction includes 2 times of interfacial polymerization, the interfacial polymerization includes: using a solution containing polyethyleneimine, a surfactant and an acid acceptor to react with 1,3,5-benzene trichloride, and heating and curing; the mass percentage of polyethyleneimine, surfactant, acid acceptor and 1,3,5-benzene trichloride is (2.2-2.6wt%):(0.2-0.6wt%):(2.2-2.6wt%):(1-1.4wt%); Preparation of composite nanofiltration membrane: using glutaraldehyde as cross-linking agent, tannic acid and polyethyleneimine are used for Schiff base and Michael addition reaction co-deposition to obtain composite nanofiltration membrane; the Schiff base and Michael addition reaction co-deposition includes: placing glutaraldehyde and tannic acid on the surface of the PTFE membrane with modification structure, reacting, heating and curing, and placing polyethyleneimine on the surface of the PTFE membrane with modification structure, reacting, heating and curing; in the Schiff base and Michael addition reaction co-deposition, the mass percentage of glutaraldehyde, tannic acid and polyethyleneimine is (1.8-2.2wt%):(1.3-1.8wt%):(2.3-2.8wt%).

2. The composite nanofiltration membrane according to claim 1, wherein, The amide group at one end of the cross-linking molecule is connected to the modification structure, and the phenolic group at the other end is connected to the nanopore layer.

3. The method for preparing the composite nanofiltration membrane according to any one of claims 1-2, characterized in that, It comprises the following steps: Preparation of PTFE membrane with modification structure: based on PTFE base, polyethyleneimine and 1,3,5-benzene trichloride are used for polymerization reaction to obtain PTFE membrane with modification structure; the polymerization reaction includes 2 times of interfacial polymerization, the interfacial polymerization includes: using a solution containing polyethyleneimine, a surfactant and an acid acceptor to react with 1,3,5-benzene trichloride, and heating and curing; the mass percentage of polyethyleneimine, surfactant, acid acceptor and 1,3,5-benzene trichloride is (2.2-2.6wt%):(0.2-0.6wt%):(2.2-2.6wt%):(1-1.4wt%); Preparation of composite nanofiltration membrane: a composite nanofiltration membrane is prepared by Schiff base and Michael addition reaction co-deposition of tannic acid and polyethyleneimine using glutaraldehyde as a crosslinking agent; the Schiff base and Michael addition reaction co-deposition comprises: placing glutaraldehyde and tannic acid on the surface of a PTFE membrane with a modified structure, reacting, and heating and curing; placing polyethyleneimine on the surface of the PTFE membrane with a modified structure, reacting, and heating and curing; in the Schiff base and Michael addition reaction co-deposition, the mass percentage of glutaraldehyde, tannic acid, and polyethyleneimine is (1.8-2.2wt%):(1.3-1.8wt%):(2.3-2.8wt%).

4. The production method according to claim 3, characterized by, The surfactant comprises sodium dodecyl sulfate, and the acid acceptor comprises triethylamine. In the interfacial polymerization, the reaction time is 70-90s, the heating and curing temperature is 55-65℃, and the heating and curing time is 8-12min.

5. The preparation method according to claim 3, characterized in that, The Schiff base and Michael addition reaction co-deposition comprises: placing glutaraldehyde and tannic acid on the surface of a PTFE membrane with a modified structure, reacting for 15-25min, heating and curing at 45-55℃ for 3-8min, placing polyethyleneimine on the surface of the PTFE membrane with a modified structure, reacting for 0.8-1.2h, and heating and curing at 45-55℃ for 13-18min.

6. Use of the composite nanofiltration membrane of any one of claims 1-2 in magnesium-lithium separation, lithium recovery from waste lithium ion batteries, heavy metal removal, or dye removal.

7. A method for recycling lithium from waste lithium-ion batteries, characterized in that, The method comprises the following steps: The lithium in the acidic leaching solution of the positive electrode material of the waste lithium ion battery is separated and enriched using the composite nanofiltration membrane of any one of claims 1-2.

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