High-selectivity anti-swelling magnesium-lithium separation composite membrane, preparation method and application thereof

Abstract

This invention provides a method for preparing a magnesium-lithium separation composite membrane, comprising: fibrillating powder A and powder B to obtain mixture C; dry-forming mixture C to obtain a self-supporting dry-forming base membrane; adding PVDF and organic matter D to solvent E to obtain an impregnation solution; immersing the self-supporting dry-forming base membrane in the impregnation solution, and subjecting it to impregnation and drying to obtain the magnesium-lithium separation composite membrane. This invention utilizes a step-by-step synergistic process of fibrillation pretreatment – ​​dry-forming – impregnation composite, combined with multi-component coupling of inorganic powder A – organic framework – impregnated functional layer, to both fully preserve the fibrillated structure and enhance membrane toughness, while achieving a seamless and robust bond between the base membrane and the functional layer. The synergistic effect of PVDF and sulfonated ion-conductive polymers suppresses swelling while constructing a high conductivity pathway. The dry-forming base membrane physically confines and activates the intrinsic sieving ability of powder A, resulting in a membrane with high strength, low swelling, and high magnesium-lithium selectivity, exhibiting excellent overall performance.

Description

Technical Field

[0001] This invention belongs to the field of ion separation membrane technology, specifically relating to a highly selective anti-swelling magnesium-lithium separation composite membrane, its preparation method and application, and is particularly suitable for electrodialysis treatment of high magnesium-lithium ratio salt lake brine and lithium-containing industrial wastewater. Background Technology

[0002] Lithium, as a key strategic metal, is widely used in power batteries, energy storage systems, and electronic devices. my country's salt lakes account for over 70% of the country's total lithium resources and are a crucial source of future lithium resources. However, most salt lakes exhibit extremely high magnesium-to-lithium ratios (Mg2+). 2+ / Li + The ratio can far exceed 50:1, and in some extreme salt lakes it even exceeds 1500:1. Furthermore, the brine contains various impurity ions, plus Mg... 2+ With Li + Similar ionic radii and chemical properties significantly increase the difficulty of lithium extraction. Furthermore, my country's dependence on imported lithium resources still exceeds 50%, making magnesium-lithium separation a core technological bottleneck in the industrialization of lithium extraction from salt lakes. Developing efficient magnesium-lithium separation technology is crucial for ensuring national lithium industry security. Ion exchange membrane separation technology has gained widespread attention in the field of magnesium-lithium separation due to its advantages of high efficiency, energy saving, and environmental friendliness. Among these technologies, LATP ceramic materials, with their unique lithium-ion sieving effect, have become the preferred substrate for magnesium-lithium separation membranes.

[0003] However, existing magnesium-lithium separation membrane technologies still face several bottlenecks: First, traditional LATP composite membrane preparation often employs wet casting processes, requiring large amounts of organic solvents, resulting in high energy consumption, difficulty in solvent recovery, and while high addition levels can improve separation efficiency, they easily lead to pore defects in the membrane layer, making it difficult to balance the separation coefficient and mechanical properties. Second, traditional LATP ceramic membranes have poor mechanical properties and are brittle, easily breaking during transportation and use, making them unsuitable for industrial production. Third, existing commercial nanofiltration membranes are mostly negatively charged, limited by the Donnan effect in their ability to separate magnesium-containing membranes. 2+ With Li + The separation coefficient is generally below 5, which is difficult to meet the separation requirements of high magnesium-to-lithium ratio brine. Even the positively charged composite nanofiltration membranes developed in recent years involve complex interfacial polymerization or multilayer self-assembly processes, such as the composite modification of tannic acid and imidazole cation donors (CN117504623A). The process is cumbersome and costly, which limits its large-scale application.

[0004] Furthermore, most existing membrane technologies are susceptible to concentration polarization and ion interference when treating brines with high magnesium-to-lithium ratios, further reducing separation efficiency. Therefore, developing a LATP-based magnesium-lithium separation composite membrane with a simple preparation process, low solvent consumption, excellent mechanical properties, high separation efficiency, and strong resistance to swelling is of significant practical importance for overcoming the technological bottleneck of lithium extraction from salt lakes in my country and reducing dependence on imported lithium resources. Summary of the Invention

[0005] The present invention aims to solve the above-mentioned problems of the prior art, and its purpose is to provide a magnesium-lithium separation composite membrane, its preparation method and application.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: Firstly, a method for preparing a magnesium-lithium separation composite membrane is provided, including: S1. Powder A and powder B are subjected to fibrillation treatment to obtain mixture C; powder A includes one or more of LATP (lithium aluminum titanium phosphate), LAGP (lithium aluminum germanium phosphate), and LLZTO (lithium lanthanum zirconium tantalum oxide); powder B includes one or more of PTFE (polytetrafluoroethylene), UHMWPE (ultra-high molecular weight polyethylene), and Sericin. S2 and mixture C are dried to form a film, resulting in a self-supporting dry base film; S3. Dissolve PVDF and organic material D in solvent E, and stir until completely dissolved and a homogeneous solution is formed to obtain an impregnation solution; the organic material D includes one or more of Nafion (perfluorosulfonic acid resin), sulfonated polyether ether ketone, and sulfonated polyphenylene ether. S4. Immerse the self-supporting dry-process base membrane in the impregnation solution, and after impregnation and drying, obtain the magnesium-lithium separation composite membrane.

[0007] Secondly, a magnesium-lithium separation composite membrane is provided, which is prepared using the preparation method described in the first aspect.

[0008] Thirdly, the application of the magnesium-lithium separation composite membrane described in the second aspect in lithium extraction from salt lake brine and treatment of lithium-containing industrial wastewater is provided.

[0009] Fourthly, an electrodialysis stack is provided, comprising a magnesium-lithium separation composite membrane used alone or stacked in a flow cell, the magnesium-lithium separation composite membrane being the magnesium-lithium separation composite membrane described in the second aspect.

[0010] Compared with the prior art, one or more of the above technical solutions can achieve at least one of the following beneficial effects: This invention employs a step-by-step synergistic process of fibrillation pretreatment, dry film formation, and impregnation composite. First, powder A and powder B are fibrillated to pre-form a primary composite structure in which organic fibers constrain inorganic particles. This structure is completely preserved during dry film formation, forming a self-supporting base film with high structural strength. Finally, a composite impregnation solution of PVDF and sulfonated ion-conductive polymer is used to achieve deep wetting and interface anchoring through the porous structure of the dry base film. This process not only preserves the good fibrillation structure but also achieves good membrane interface bonding, enabling seamless composite of the base film and functional layer without peeling during long-term operation.

[0011] This invention utilizes a triple synergy between inorganic powder A, an organic framework, and an impregnation functional layer. Powder B, through fibrillation, forms a three-dimensional network, which, in conjunction with inorganic powder A, addresses the defects of easy particle agglomeration and high membrane brittleness, significantly improving mechanical strength. PVDF in the impregnation solution synergizes with organic matter D, inhibiting swelling while constructing a highly conductive pathway. The dry-process base membrane and the impregnation layer work together, utilizing the submicron-level pores provided by the dry-process base membrane to physically confine the swelling of the sulfonated ion-conductive polymer sulfonate groups. Simultaneously, chemical modification is achieved through PVDF-ion-conductive polymer impregnation treatment. The PVDF-sulfonated ion-conductive polymer composite layer in the pores, even with slight swelling, ensures that the solution cannot freely shuttle between the two sides of the separation membrane. This constructs continuous lithium-ion transport channels on the surface and in the gaps between inorganic powder A particles, guiding lithium ions to preferentially and selectively pass through the ceramic lattice, thereby fully leveraging the intrinsic magnesium-lithium sieving function of the ceramic powder. The synergistic effect of multiple components enables the membrane to simultaneously possess high toughness, low swelling rate, high ion conductivity, and high magnesium-lithium selectivity, breaking through the technical bottleneck of traditional membrane mechanics and separation performance being difficult to balance.

[0012] Under the combined effect of step synergy and component synergy, the resulting composite membrane has a tensile strength ≥5MPa, swelling rate ≤10%, and magnesium-lithium separation coefficient ≥50, with simultaneous optimization of mechanical properties, structural stability, and separation efficiency. At the same time, it adopts a green process with low solvent dry method as the main method and impregnation modification as the auxiliary method. The solvent consumption is low, the energy consumption is low, the process is simple and controllable, and it is highly compatible with existing dry membrane production lines. The membrane is flat and erosion resistant, can be stacked and assembled, and is precisely adapted to existing separation equipment. It efficiently solves the core technical problem of lithium extraction from salt lakes with high magnesium-lithium ratio and is suitable for large-scale production. Attached Figure Description

[0013] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0014] Figure 1This is a SEM image of the LATP-PTFE mixture prepared in Example 1 of the present invention; Figure 2 This is a cross-sectional SEM image of the composite membrane prepared in Example 1 of the present invention; Figure 3 These are optical photographs of the composite film prepared in Example 1 of the present invention, wherein (a) is an initial photograph of the composite film, (b) is a photograph of the composite film when it is bent, and (c) is a photograph of the composite film after it has recovered from bending. Figure 4 This is a comparison diagram of the mechanical properties of the composite membranes prepared in Example 1 and Comparative Example 2 of the present invention; Figure 5 The electrodialysis apparatus used in this invention; Figure 6 This is a comparison chart of the swelling rates of the composite membrane of Example 2 of the present invention, samples of Comparative Examples 1, 2, and 4, and the commercial Nafion membrane. Detailed Implementation

[0015] Some embodiments provide a method for preparing a magnesium-lithium separation composite membrane, including: S1. Powder A and powder B are subjected to fibrillation treatment to obtain mixture C; powder A includes one or more of LATP powder, LAGP powder and LLZTO powder; powder B includes one or more of PTFE, UHMWPE (ultra-high molecular weight polyethylene) and Sericin. S2 and mixture C are dried to form a film, resulting in a self-supporting dry base film; S3. Dissolve PVDF and organic material D in solvent E, and stir until completely dissolved and a homogeneous solution is formed to obtain an impregnation solution; the organic material D includes one or more of Nafion, sulfonated polyether ether ketone, and sulfonated polyphenylene ether. S4. The self-supporting dry-process base membrane is immersed in an impregnation solution. After impregnation and drying, the solvent evaporates, thereby forming a stable ion-selective functional layer within and on the surface of the base membrane, ultimately obtaining a magnesium-lithium separation composite membrane. The ion-selective functional layer is composed of a composite material formed by blending organic compound D with the fluoropolymer polyvinylidene fluoride (PVDF), which is filled into the pores of the porous base membrane through an impregnation process.

[0016] In some preferred embodiments, the particle size of powder A is 200~400nm, such as 200nm, 300nm, 400nm, etc.

[0017] In some preferred embodiments, the particle size of powder B is 500~1000nm, such as 500nm, 550nm, 600nm, 700nm, 800nm, 900nm, 1000nm, etc.; the mass of powder B is 1~10% of the mass of powder A, preferably 2~8%, such as 2%, 3%, 4%, 5%, 6%, 7%, 8%, etc.

[0018] In some preferred embodiments, when the powder B is PTFE, the number-average molecular weight Mn of the PTFE is >10. 6 Da; When the powder B is UHMWPE, the number-average molecular weight of UHMWPE is >3 million g / mol; when the powder B is Sericin, the number-average molecular weight of Sericin is >30 kDa.

[0019] The chemical formula of LATP can be Li 1+x Al x Ti 2-x (PO4)3, 0 ≤ x ≤ 0.5.

[0020] In some preferred embodiments, in step S1, the method for fibrillating powder A and powder B is method (a) or method (b): Method (a): Powder A and powder B are successively dried, mixed and fibrillated and ground to obtain mixture C; Method (b): Powder A and powder B are subjected to deagglomeration dry mixing, preliminary adhesion, deep fibrillation and low temperature homogenization treatment to obtain mixture C.

[0021] For method (a), the following steps are performed: ① Drying: Powder A and powder B are dried separately (e.g., at 40-60℃ for 9-15 hours) to remove moisture and residual impurities from the powders; ② Manual mixing: The two dried powders are added to a mortar and manually ground and mixed (e.g., for 5-20 minutes) to achieve preliminary uniform mixing; ③ Acoustic resonance mixing: The pre-mixed powders are added to an acoustic resonance mixer (200-400W, 45-55Hz) for blending (3-20 minutes) to further improve the uniformity of mixing; ④ Deep fibrillation of PTFE: The powders after acoustic resonance mixing are added to a mortar and ground again (e.g., at 60-100℃ for 5-20 minutes) so that PTFE forms a continuous three-dimensional fiber network under the combined action of heat and mechanical force, which coats and constrains LATP particles.

[0022] For method (b), for example, the following steps are performed: ① De-agglomeration: Powder A and powder B are added to a high-speed mixer and first dry-mixed at high speed (e.g., dry-mixed at 1000~2500 rpm for 10~20 min) to remove the original agglomerates; ② Initial adhesion: Heating (e.g., raising the temperature to 70~90℃) and first mixing in the same direction (e.g., at a speed of 5~8 m / s) (e.g., mixing for 1~3 min) to allow PTFE to initially adhere to the LATP powder; ③ Deep fibrillation: Increasing the speed (e.g., increasing to 20~25 m / s) and mixing in the same direction (e.g., mixing for 12~15 min) to achieve deep fibrillation of PTFE; ④ Low-speed homogenization: Cooling to below 10℃ and homogenizing at low speed (e.g., for 5~10 min) to obtain mixture C. In this method, if step ② is omitted, the degree of fibrillation may be insufficient, and a fibrillated network may not be formed. If step ① is omitted, powder A particles will form larger secondary particles due to the rapid formation of a fibrillated network, which may affect the mechanical properties of the material.

[0023] In some preferred embodiments, in step S2, the temperature of the dry film formation is 60~150℃, for example, 60℃, 70℃, 80℃, 90℃, 100℃, 110℃, 120℃, 130℃, 140℃, 150℃, etc., preferably 80~150℃; the pressure of the dry film formation is 5~15MPa, for example, 5MPa, 6MPa, 7MPa, 8MPa, 9MPa, 10MPa, 11MPa, 12MPa, 13MPa, 14MPa, 15MPa, etc.; the dry film formation is carried out using a roller press or a rolling mill, for example, adding the obtained mixture C to a roller press and rolling it at 60~150℃ and 5~15MPa to obtain a self-supporting dry base film; or using a rolling mill to roll the mixture C at 60~150℃ to obtain a self-supporting dry base film; the roller pressing is preferably bidirectional roller pressing. The rolling process achieves the stretching and heat treatment of the film.

[0024] In some preferred embodiments, the thickness of the self-supporting dry substrate is 60~90μm; the porosity of the self-supporting dry substrate is 20~40%, for example 20%, 25%, 30%, 35%, 40%, etc.

[0025] In some preferred embodiments, the total mass of PVDF and organic matter D is 3 to 10% of the mass of solvent E, for example, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc.

[0026] In some preferred embodiments, the molecular weight of the PVDF is 50,000 to 100,000 Da.

[0027] In some preferred embodiments, the solvent E is NMP, or a mixture of NMP with water and / or ethanol.

[0028] In some preferred embodiments, the mass ratio of PVDF to organic compound D is 1:9 to 10:1, preferably 1:1 to 10:1, such as 1:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, etc., and more preferably 5:1 to 10:1. In some preferred embodiments, organic compound D is introduced in the form of a solution, specifically, a solution of organic compound D is introduced.

[0029] In some preferred embodiments, in step S3, the stirring speed is 300~1500 rpm, such as 300 rpm, 500 rpm, 800 rpm, 1000 rpm, 1200 rpm, 1500 rpm, etc.; the stirring time is 6~12 hours.

[0030] In some preferred embodiments, in step S4, the temperature of the impregnation solution is 25~40°C; the impregnation time is 10~30 min. The porous base membrane is completely immersed in the impregnation solution and kept for 10 to 30 minutes (preferably 1 to 10 minutes) to allow the impregnation solution to fully wet and fill the pores of the base membrane.

[0031] In some preferred embodiments, the drying in step S4 is gradient drying. Gradient drying ensures better release of intramembrane stress, preventing membrane cracking and deformation. Preferably, the gradient drying includes: drying at room temperature for 12-30 hours, then pre-drying at 60-80°C for 2-8 hours, and then drying at 100-120°C for 4-6 hours; preferably, the 100-120°C drying is vacuum drying; the vacuum degree of the vacuum drying is -0.08 to -0.1 MPa.

[0032] In step S4, the impregnated membrane is removed from the solution, excess liquid on the surface is removed, and the solvent is dried to evaporate, forming a stable ion-selective functional layer in the pores and on the surface of the base membrane, thus finally obtaining the composite electrodialysis membrane material.

[0033] One embodiment provides a magnesium-lithium separation composite membrane, which is prepared using the aforementioned preparation method; the magnesium-lithium separation composite membrane includes a self-supporting dry substrate membrane and a composite layer of PVDF-organic D coated on the surface and within the pores of the self-supporting dry substrate membrane.

[0034] It is understood that the core of the magnesium-lithium separation composite membrane of the present invention lies in a composite electrodialysis membrane material with a selective ion transport function constructed by a specific method. Based on this core structure, the material can take, but is not limited to, the following physical forms or product forms: 1. Self-supporting membrane: an independent membrane material with a complete double-layer structure, which is directly obtained by the above-mentioned preparation method; 2. Composite membrane: A densified membrane made from the composite membrane material obtained by the above method through a compression molding process; 3. Reinforced composite membrane: A membrane product with higher mechanical strength obtained by mixing the above composite membrane material with additional fibers, fabrics or polymer adhesives and then molding it. 4. Composite layer form: A selective separation coating is formed by pressing or bonding the composite membrane material onto the surface of a support (such as non-woven fabric, porous ceramic plate, or highly selective oxide ceramic solid electrolyte) and then curing it. 5. Membrane module integration form: Plate-and-frame, spiral wound, or hollow fiber electrodialysis membrane modules are formed by directly assembling or encapsulating membranes of any of the above-mentioned shapes as functional units.

[0035] Specifically, the core structure of the composite electrodialysis membrane material includes: a porous base membrane as a supporting framework, and an ion-selective functional layer loaded inside and on the surface of its pores.

[0036] The high magnesium-lithium separation coefficient (up to 72) of the composite membrane is achieved through the synergistic effect of two parts. First, the LATP dry-process base membrane constructed by PTFE fibrillation provides a stable ion channel and a highly selective substrate, laying the foundation for rapid lithium-ion penetration and efficient magnesium-ion retention. Second, the PVDF-Nafion composite layer optimizes ion conduction efficiency and selectivity. The anti-swelling properties of PVDF and the high lithium-ion conduction performance of Nafion work together to further improve separation accuracy and conduction efficiency. The synergistic effect of the two achieves a highly efficient separation effect, solving the technical pain point that existing membranes cannot achieve both selectivity and conduction efficiency.

[0037] In some preferred embodiments, the magnesium-lithium separation composite membrane in a two-component solution of magnesium chloride and lithium chloride has a magnesium-lithium separation coefficient ≥50, and Li + Flux ≥ 2.0 × 10 -6 mol·cm -2 ·s -1 .

[0038] In some preferred embodiments, the tensile strength of the magnesium-lithium separation composite membrane is ≥5MPa.

[0039] In some preferred embodiments, the swelling ratio of the magnesium-lithium separation composite membrane is ≤10%.

[0040] In some preferred embodiments, the total thickness of the magnesium-lithium separation composite membrane is 65~95μm.

[0041] Some embodiments of the present invention provide the application of a magnesium-lithium separation composite membrane in lithium extraction from salt lake brine and treatment of lithium-containing industrial wastewater. The magnesium-lithium separation composite membrane is used as a selective ion exchange membrane in electrodialysis processes, and is particularly suitable for extracting lithium from brine containing magnesium ions (Mg²⁺). + ) and lithium ion (Li + This method selectively separates and enriches lithium ions in mixed aqueous solutions; it is particularly suitable for extracting lithium from resources with a high magnesium-to-lithium ratio, such as salt lake brine or leachate from spent lithium-ion batteries. Lithium-ion electrodialysis technology is preferred, with electrodialysis treatment of high magnesium-to-lithium ratio salt lake brine and lithium-containing industrial wastewater being the ultimate preferred method.

[0042] A third aspect of the present invention provides a composite electrodialysis membrane material prepared by the above method.

[0043] The fourth aspect of the present invention provides the application of the above-mentioned composite electrodialysis membrane material.

[0044] The fifth aspect of the present invention provides an electrodialysis component or apparatus, which includes at least one composite electrodialysis membrane material as described above as a selective exchange membrane; the electrodialysis component may be in the form of a single membrane laid flat and fixed or multiple membranes stacked and assembled.

[0045] This invention provides a magnesium-lithium separation device, such as... Figure 5 As shown, the magnesium-lithium separation equipment includes the aforementioned electrodialysis components, a feeding unit, a separation unit, a collection unit, and a control system. The separation unit contains at least one set of the aforementioned electrodialysis components. The feeding unit is equipped with a pretreatment filtration module (pore size 0.45~1μm, used to remove suspended impurities from the feed solution). The collection unit has two independent storage tanks to collect the lithium-rich solution and lithium-lean solution after membrane separation, respectively. The control system is used to regulate the feed rate, separation temperature, and current density. The preferred operating parameters of the equipment are: feed rate 30~100mL / min, separation temperature 25~40℃, and operating pressure 0~0.5MPa. The number of membrane components can be adjusted according to the magnesium-lithium ratio of the feed solution. 2+ / Li + When the ratio is greater than 300:1, 3 to 5 membrane modules are connected in series to improve the lithium recovery rate.

[0046] This invention provides a method for separating and recovering lithium from a magnesium-containing solution. The method employs an electrodialysis device containing the composite electrodialysis membrane material described above to treat the solution. The specific application process is as follows: 1) Pretreatment of the solution: removing suspended impurities (such as silt and organic matter) and adjusting the pH of the solution to 5.5-7.5; 2) Equipment commissioning: starting the equipment and setting and preheating to the target parameters; 3) Separation operation: passing the pretreated solution into the feed unit, where it passes through the electrodialysis components to achieve lithium recovery. + With Mg 2+Selective separation; 4) Post-processing: The lithium-rich liquid is concentrated and purified to obtain high-purity lithium salt products, and the lithium-poor liquid can be further recycled.

[0047] To ensure the stability of the preparation process and the uniformity of membrane performance, the preparation of the impregnation solution, the impregnation of the base membrane, and the initial stage of gradient drying after impregnation must be carried out in a low-humidity environment with controllable relative humidity, with the relative humidity controlled at 30%~45%. This humidity control requirement is specifically designed to address process defects related to NMP solvent: on the one hand, it prevents excessively low ambient humidity from causing NMP to evaporate too quickly, resulting in rapid crusting and skinning on the membrane surface, which would block the uniform evaporation of residual solvent inside and lead to defects such as pinholes, cracks, and structural inhomogeneity in the membrane layer; on the other hand, it avoids excessively high ambient humidity, which would cause the NMP solvent to absorb moisture from the air during the entire impregnation and drying process, thereby disrupting the uniformity of the impregnation solution system and inducing phase separation and pore disorder inside the membrane. This ensures, from the process end, that the composite membrane has a dense structure and stable performance that meets the standards.

[0048] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0049] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0050] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0051] The raw materials used in this invention include LATP powder, PTFE powder, PVDF powder, Nafion solution, NMP, and ethanol, all of which are commercially available chemicals with a purity suitable for the preparation of electrochemical materials. Specific specifications are as follows: LATP powder: particle size 200~400nm, purity ≥99%; PTFE particles: particle size 500~1000nm, number average molecular weight Mn=1.2×10⁻⁶. 6 ~1.5×10 6 Da: purity ≥99%; PVDF: molecular weight 50,000~100,000 Da, crystallinity 60~70%; Nafion: 5% (w / w) alcohol-water solution, equivalent weight EW=1100; NMP solvent: purity ≥99.5%, moisture content ≤0.05%, industrial grade solvent. All raw materials were not subjected to additional purification treatment before use, only targeted drying pretreatment according to the requirements of each embodiment.

[0052] Example 1 The highly selective, anti-swelling magnesium-lithium separation composite membrane was prepared according to the following steps: 1) Mixture preparation: Take 4.85g of LATP powder with a particle size of 200~400nm and 0.15g of PTFE powder with a particle size of 500~1000nm, and dry them separately in a 60℃ forced-air drying oven for 12h; after taking them out, put them in an agate mortar and grind them manually for 10min, with the grinding speed controlled at 60~80 revolutions / minute; transfer the above mixed powder to the zirconia jar of the acoustic resonance mixer and mix for 8min at 300W and 50Hz; then transfer it to a 100℃ constant-temperature agate mortar and grind for 15min to complete the deep fibrillation of PTFE, and obtain the LATP-PTFE mixture. Its SEM image is shown below. Figure 1 As shown, by Figure 1 It can be clearly observed that after deep fibrillation, PTFE forms a continuous three-dimensional fiber network, which tightly encapsulates LATP powder without obvious particle agglomeration. This proves that the drying-manual mixing-acoustic resonance mixing-deep fibrillation mixture preparation process described in this invention can effectively improve the mixing uniformity of LATP and PTFE.

[0053] 2) Dry base film preparation: Take 1g of the mixture and spread it evenly on the surface of the stainless steel substrate of the biaxial roller press. Heat it to 120℃ at a heating rate of 5℃ / min and keep it at the temperature for 30min. Set the roller pressure to 12MPa and the roller speed to 5r / min and roll the mixture biaxially (roll 3 times in each direction). After cooling to room temperature, peel off the substrate to obtain a LATP dry base film with a thickness of 65μm.

[0054] 3) Preparation of impregnation solution: Weigh 0.315g of PVDF and 0.7g of 5% Nafion alcohol aqueous solution according to the PVDF to Nafion mass ratio of 9:1, add 6.705g of NMP solvent; stir at 450rpm and 60℃ for 4h, and then degas under vacuum at -0.09MPa and 25℃ for 30min to obtain a uniform PVDF-Nafion / NMP impregnation solution.

[0055] 4) Composite membrane molding: The LATP dry-process base membrane was placed in a 30℃ constant temperature water bath and immersed in the above impregnation solution for 20 min. After removal, the surface residual liquid was blown off with a nitrogen air gun, and dried at 25℃ and 40-50% relative humidity for 24 h. It was then pre-dried at 60℃ for 1 h, pre-dried at 70℃ for 3 h, and finally vacuum-dried at 110℃ and -0.09 MPa for 5 h to obtain a dense, highly selective, anti-swelling magnesium-lithium separation composite membrane with a total thickness of 72 μm. The cross-sectional SEM image of the composite membrane is shown below. Figure 2 As shown, by Figure 2As can be seen, the PVDF-Nafion composite layer forms a tight interfacial bond with the LATP dry-process base film, with no obvious gaps. This verifies that the impregnation modification process of this invention can achieve a strong composite between the base film and the functional layer, avoiding interfacial delamination problems during long-term operation. Optical photographs and mechanical property diagrams of the composite film are shown below. Figure 3 As shown, by Figure 3 It can be seen that the composite membrane can recover after being manually bent, indicating that the composite membrane has good toughness. Precise characterization was performed using stress-strain curves, and the characterization results are as follows: Figure 4 As shown, compared with the LATP casting film of the same proportion, its ultimate tensile strength is significantly increased from 0.55 MPa to 5.8 MPa.

[0056] Example 2 Except for the following parameter adjustments, the remaining preparation steps are the same as in Example 1: 1) Mixture ratio: 4.9g LATP powder, 0.1g PTFE powder (PTFE accounts for 2% of the mass fraction of the mixture); 2) Dry base membrane preparation: heating rate 5℃ / min, heating to 60℃ and then holding at that temperature for 30min, roller pressing pressure 12MPa, roller speed 5r / min, roller pressing in both directions 3 times each, to obtain a 60μm thick LATP dry base membrane; according to the impregnation and drying process of Example 1, a dense, highly selective, anti-swelling magnesium-lithium separation composite membrane with a total thickness of 90μm is obtained.

[0057] Example 3 Except for adjusting the mass ratio of PVDF to Nafion in the impregnation solution to 1:1 (PVDF accounts for 50%), the other preparation steps are the same as in Example 1: Specifically, the total mass of PVDF and Nafion is 0.35g, 3.5g of 5% Nafion solution is taken, 6.705g of NMP solvent is added, and the impregnation solution preparation, impregnation and drying process in Example 1 is followed to obtain a dense, highly selective, anti-swelling magnesium-lithium separation composite membrane with a total thickness of 73μm.

[0058] Example 4 Except for adjusting the mass ratio of PVDF to Nafion in the impregnation solution to 1:9 (PVDF accounts for 10%), the other preparation steps are the same as in Example 1: Specifically, the total mass of PVDF and Nafion is 0.35g, 6.3g of 5% Nafion solution is taken, 6.705g of NMP solvent is added, and the impregnation solution preparation, impregnation and drying process of Example 1 is followed to obtain a dense, highly selective, anti-swelling magnesium-lithium separation composite membrane with a total thickness of 76μm.

[0059] Example 5 Except for adjusting the mass ratio of LATP to PTFE in the impregnation mixture to 95:5 (PTFE accounts for 5% of the mass fraction of the mixture), the other preparation steps are the same as in Example 1: specifically, the total mass of LATP and PTFE is 5g, and PTFE powder is 0.25g (PTFE accounts for 5% of the mass fraction of LATP powder). The impregnation solution preparation, impregnation and drying process in Example 1 is followed to obtain a dense, highly selective, anti-swelling magnesium-lithium separation composite membrane with a total thickness of 70μm.

[0060] Comparative Example 1 The following steps were performed to prepare the film: 0.5 g of LATP powder, 0.45 g of PVDF, and 1 g of 5% Nafion solution (LATP to PVDF, Nafion mass ratio 1:0.9:0.1) were taken, and 9 g of NMP solvent was added; the mixture was stirred at 60℃ and 400 rpm for 6 h, and then vacuum degassed for 30 min to obtain the casting solution; the casting solution was inverted onto a horizontal glass substrate, and a doctor blade with a 600 μm gap was used to uniformly scrape the film at a speed of 5 cm / s; the film was then dried sequentially in a forced-air drying oven at 30℃ for 2 h, 60℃ for 2 h, and 80℃ for 2 h, and after cooling to room temperature, the substrate was peeled off to obtain a traditional LATP composite film with a thickness of 60 μm. This comparative example belongs to the traditional wet casting process.

[0061] Comparative Example 2 The following steps were followed: 4.85 g of LATP powder, 0.485 g of PVDF, and 1.08 g of 5% Nafion solution (LATP to PVDF, Nafion mass ratio 9:0.9:0.1) were taken, and 48.5 g of NMP solvent was added; the mixture was stirred at 60℃ and 400 rpm for 6 h, and then vacuum degassed for 30 min to obtain the casting solution; the remaining casting, drying, and demolding steps were the same as in Comparative Example 1, resulting in a conventional LATP composite film with a thickness of 64 μm. This comparative example also used the conventional wet casting process.

[0062] Comparative Example 3 A LATP-PTFE mixture was prepared in the same manner as in Example 1. Take 4.85g of LATP-PTFE mixture, 0.485g of PVDF, and 1.08g of 5% Nafion solution (LATP to PVDF, Nafion mass ratio 9:0.9:0.1), and add 48.5g of NMP solvent; the remaining PVDF and Nafion usage, casting, drying, and demolding steps are the same as in Comparative Example 2, to obtain an LATP composite film with a thickness of 71μm. This comparative example also uses the traditional wet casting process.

[0063] Comparative Example 4 LATP dry-process base membrane (PTFE content 3%, 100℃ / 12MPa rolling pressure) was prepared according to the steps of Example 1. Take 9g of 5% Nafion solution, add 1g of NMP solvent, stir at 400rpm for 1h, and then degas under vacuum for 30min to obtain the impregnation solution. The subsequent impregnation and drying processes are the same as in Example 2, to obtain a composite film with a thickness of 68μm. This comparative example does not use the PVDF-Nafion synergistic modification scheme of the present invention, but only uses Nafion for impregnation.

[0064] The magnesium-lithium separation performance of the composite membranes prepared in the above embodiments and comparative examples was tested according to the performance characterization test methods described in this specification: 1) Magnesium-Lithium Separation Performance Test Test method: An electrodialysis apparatus was used for testing. The test system was set up outside the glove box. The composite membranes prepared in the above examples and comparative examples were cut into square pieces with a diameter of 20 mm and assembled in the middle of the flow cell as selective separation membranes; Mg 2+ / Li + Using a simulated brine of 0.5 mol / L MgCl2-LiCl with a ratio of 250:1 as the feed solution, continuous testing was conducted at room temperature (25℃) and a constant voltage of 0.8 V. The Li content in the feed solution before and after permeation through the membrane was determined by inductively coupled plasma optical emission spectrometry (ICP-OES). + and Mg 2+ The concentration of magnesium and lithium was then used to calculate the magnesium-lithium separation coefficient α(Li) using a formula. + / Mg 2+ )=(cLi + receive / cMg 2+ receive ) / (cLi + feed / c Mg 2+ feed ), where cLi + receive cMg 2+ receive Li in the permeable side solution + Mg 2+ The concentration of cLi + feed cMg 2+ feed Li in the feed solution + Mg 2+ The concentration.

[0065] 2) Anti-swelling performance test Test method: Take the composite membrane prepared in the above examples and comparative examples and cut it into square samples of 20mm×20mm. Accurately weigh the initial mass m0. Immerse the sample in the above simulated brine and soak at 25℃ for 120h. During this period, take out the sample every 24h, blot dry the residual liquid on the surface with lint-free paper, and accurately weigh the mass m. t The swelling ratio S is calculated using the formula S = (m t - m0) / m0×100% is used to evaluate the swelling resistance of the composite membrane.

[0066] The magnesium-lithium separation coefficient and anti-swelling performance at room temperature of the membrane materials prepared in each embodiment and comparative example are shown in Table 1 below: Table 1 As shown in Table 1, the magnesium-lithium separation coefficients of the composite membranes prepared in each embodiment of the present invention are all ≥55, with the separation coefficients of Examples 1, 2, and 5 reaching 72.0, 60.2, and 64.7, respectively. In contrast, Comparative Examples 1 and 2, using the traditional wet casting process, have separation coefficients of only 8.26 and 1.44, respectively, and Comparative Example 4, lacking PVDF synergistic modification, has a separation coefficient of only 36.9, all significantly lower than those of the embodiments of the present invention. These data fully demonstrate that the technical solution of PTFE fibrillation dry film formation + PVDF-Nafion synergistic modification adopted in the present invention can significantly improve the magnesium-lithium separation performance of the composite membrane, effectively overcoming the technical bottleneck of low separation efficiency in traditional LATP composite membranes.

[0067] Comparative Example 3, prepared using a mixture combined with a traditional wet process, showed a significantly lower separation coefficient compared to Example 1, but similar to Comparative Example 2. Analysis suggests this may be due to the poor compatibility between the traditional wet membrane-forming process and the mixture obtained from the dry process. In the traditional wet process, the inorganic filler and organic phase solution require high-speed stirring to achieve uniform dispersion. This process may disrupt the original fibrillated structure of the mixture, ultimately leading to a decrease in the separation coefficient.

[0068] The comparison results of Example 1 (90% PVDF) with Example 3 (50% PVDF) and Example 4 (10% PVDF) show that the mass ratio of PVDF to Nafion has a significant impact on the separation performance. When the mass ratio of the two is in the range of 10:1 to 5:1 as described in claim 3 (corresponding to Examples 1, 3, and 5), the separation performance is optimal.

[0069] As can be seen from Table 1, the magnesium-lithium separation coefficient of the composite membrane prepared in Example 2 is much lower than that in Example 1. Analysis shows that this is mainly because the rolling temperature in Example 2 is lower, which prevents PTFE from being fully fiberized, thus failing to properly constrain LATP particles and form a regular LATP network.

[0070] Figure 6 The graph shows a comparison of the swelling rates of the composite membrane of Example 1, samples of Comparative Examples 1, 2, and 4, and the commercial Nafion membrane. It can be seen that the swelling rate of the composite membrane of the present invention is significantly lower than that of the traditional wet casting membrane, and the swelling rate after 120h immersion is ≤10%, which proves that the PVDF-Nafion synergistic modification scheme of the present invention can effectively improve the swelling resistance of the membrane.

[0071] The magnesium-lithium separation performance of the composite membrane in Example 1 in the composite solution system was also tested. The test method is as follows: The test was conducted using an electrodialysis apparatus. A test system was constructed outside a glove box. The composite membrane prepared in Example 1 was cut into square pieces with a diameter of 20 mm and assembled in the middle of the flow cell as a selective separation membrane. Mg... 2+ Li + Na + A simulated brine solution of 4 mol / L MgCl2-LiCl-NaCl (ratio 200:1:200) was used as the feed solution. Continuous testing was conducted at room temperature (25°C) and a constant voltage of 0.8 V. The Li content in the feed solution before and after permeation through the membrane was determined using inductively coupled plasma optical emission spectrometry (ICP-OES). + and Mg 2+ The concentration of magnesium and lithium was then used to calculate the magnesium-lithium separation coefficient α(Li) using a formula. + / Mg 2+ )=(cLi + receive / cMg 2+ receive ) / (cLi + feed / c Mg 2 + feed ), where cLi + receive cMg 2+ receive Li in the permeable side solution + Mg 2+ The concentration of cLi + feed cMg 2+ feed Li in the feed solution + Mg 2+ The concentration.

[0072] Testing revealed that when a ternary solution system (NaCl, LiCl, MgCl2) was used as the simulated solution, the magnesium-lithium separation coefficient in Example 1 increased to 83.4. This indicates an improvement in the magnesium-lithium separation coefficient in the multi-component solution. Analysis suggests this is because, in this multi-component system, LiCl... +Migration resistance in selective films is lower than that in Mg 2+ and Na + Therefore, the latter two are more likely to compete for transport at the film interface or within the channel, thereby indirectly weakening their effect on Li. + The effects of migration are manifested in Li + An improvement in relative selectivity.

[0073] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a magnesium-lithium separation composite membrane, characterized in that, include: S1. Powder A and powder B are subjected to fibrillation treatment to obtain mixture C; powder A includes one or more of LATP powder, LAGP powder and LLZTO powder; powder B includes one or more of PTFE, UHMWPE and Sericin; S2 and mixture C are dried to form a film, resulting in a self-supporting dry base film; S3. Add PVDF and organic material D to solvent E, stir until completely dissolved and a homogeneous solution is formed to obtain an impregnation solution; the organic material D includes one or more of Nafion, sulfonated polyether ether ketone, and sulfonated polyphenylene ether. S4. Immerse the self-supporting dry-process base membrane in the impregnation solution, and after impregnation and drying, obtain the magnesium-lithium separation composite membrane.

2. The method for preparing the magnesium-lithium separation composite membrane as described in claim 1, characterized in that, The particle size of powder A is 200~400nm; The particle size of powder B is 500~1000 nm; the mass of powder B is 1~10% of the mass of powder A. When powder B is PTFE, the number-average molecular weight Mn of the PTFE is >10. 6 Da; When the powder B is UHMWPE, the viscosity-average molecular weight of UHMWPE is >3 million g / mol; when the powder B is Sericin, the number-average molecular weight of Sericin is >30 kDa.

3. The method for preparing the magnesium-lithium separation composite membrane as described in claim 1, characterized in that, In step S1, the method for fibrillating powder A and powder B is either method (a) or method (b): Method (a): Powder A and powder B are successively dried, mixed and fibrillated and ground to obtain mixture C; Method (b): Powder A and powder B are subjected to deagglomeration dry mixing, preliminary adhesion, deep fibrillation and low temperature homogenization treatment to obtain mixture C.

4. The method for preparing the magnesium-lithium separation composite membrane as described in claim 1, characterized in that, In step S2, the temperature of the dry film formation is 60~150℃; the pressure of the dry film formation is 5~15MPa; and the dry film formation is carried out using a roller press or a rolling mill. The thickness of the self-supporting dry substrate membrane is 60~90μm; the porosity of the self-supporting dry substrate membrane is 20~40%.

5. The method for preparing the magnesium-lithium separation composite membrane as described in claim 1, characterized in that, The mass ratio of PVDF to organic matter D is 1:9~10:1; The total mass of the PVDF and organic compound D is 3-10% of the mass of solvent E; The molecular weight of the PVDF is 50,000~100,000 Da; The solvent E is NMP, or a mixture of NMP with water and / or ethanol.

6. The method for preparing the magnesium-lithium separation composite membrane as described in claim 1, characterized in that, In step S3, the stirring speed is 300~1500 rpm; the stirring time is 6~12 h. In step S4, the temperature of the impregnation solution is 25~40℃; the impregnation time is 10~30min. In step S4, the drying process is gradient drying; the gradient drying includes: drying at room temperature for 12-30 hours, then pre-drying at 60-80°C for 2-8 hours, and then drying at 100-120°C for 4-6 hours; preferably, the drying at 100-120°C is vacuum drying; preferably, the vacuum degree of the vacuum drying is -0.08 to -0.1 MPa.

7. A magnesium-lithium separation composite membrane, characterized in that, The magnesium-lithium separation composite membrane is prepared by any one of the preparation methods described in claims 1 to 6; the magnesium-lithium separation composite membrane comprises a self-supporting dry base membrane and a composite layer of PVDF-organic D coated on the surface and within the pores of the self-supporting dry base membrane.

8. The magnesium-lithium separation composite membrane as described in claim 7, characterized in that, The magnesium-lithium separation composite membrane exhibits a magnesium-lithium separation coefficient ≥50 in a two-component solution of magnesium chloride and lithium chloride. + Flux ≥ 2.0 × 10 -6 mol·cm -2 ·s -1 ; The tensile strength of the magnesium-lithium separation composite membrane is ≥5MPa; The swelling rate of the magnesium-lithium separation composite membrane is ≤10%; The total thickness of the magnesium-lithium separation composite membrane is 65~95μm.

9. The application of the magnesium-lithium separation composite membrane as described in claim 7 or 8 in lithium extraction from salt lake brine and treatment of lithium-containing industrial wastewater.

10. An electrodialysis stack, characterized in that, A magnesium-lithium separation composite membrane is included for single or stacked use in a flow cell, wherein the magnesium-lithium separation composite membrane is the magnesium-lithium separation composite membrane according to any one of claims 7 to 8.

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