Multi-electric-layer nanofiltration membrane for extracting lithium from salt lake and preparation method of multi-electric-layer nanofiltration membrane

Abstract

The invention provides a multi-electric-layer nanofiltration membrane for extracting lithium from a salt lake and a preparation method of the multi-electric-layer nanofiltration membrane, and belongs to the technical field of nanofiltration membranes. Firstly, polysulfone and dimethylformamide are put into a mixing kettle, and a membrane casting solution is obtained; coating a non-woven fabric with the membrane casting solution, and then preparing a non-woven fabric and polysulfone combined base membrane through phase inversion; soaking the base membrane in an isopropanol solution, and removing redundant solution on the surface by using a rubber roller; the preparation method comprises the following steps: coating the surface of a base membrane with a solution of a water phase 2, then removing the redundant water phase on the surface, then coating the surface with an oil phase 2, drying a membrane, sequentially and repeatedly coating the membrane with the water phase 2 and the oil phase 2 for multiple times to obtain a nanofiltration membrane with multiple middle layers, then coating the membrane with a solution of a water phase 1, removing the redundant water phase on the surface, then coating the surface with an oil phase 1, and drying to obtain the nanofiltration membrane with multiple electric layers. According to the invention, a plurality of middle layers are positively charged, so that the electrostatic repulsion effect on magnesium ions can be fully exerted, and the magnesium ion interception is obviously improved.

Description

Technical Field

[0001] This invention relates to the field of nanofiltration membrane technology, and specifically to a multi-layered nanofiltration membrane for lithium extraction from salt lakes and its preparation method. Background Technology

[0002] The rise of nanofiltration membrane technology in the brine lithium extraction industry stems from the significant imbalance between global lithium resource supply and demand, as well as the inherent bottlenecks of traditional lithium extraction technologies. Traditional brine lithium extraction technologies, such as precipitation, adsorption, and extraction, each have their limitations. Precipitation: Originally used in brine lithium extraction, it removes impurities such as magnesium through evaporation concentration and chemical precipitation. However, this method is only suitable for brine lakes with low magnesium-to-lithium ratios. For brine lakes with high magnesium-to-lithium ratios, it requires large amounts of precipitant, resulting in significant lithium loss, low yield, and the generation of substantial waste. Adsorption: Utilizes the selective adsorption of lithium ions by adsorbents for lithium extraction. This technology is advanced, but the preparation cost of adsorbents is high, the solubility rate is high, and the desorption process generates large amounts of wastewater. Extraction: Uses organic solvents to extract lithium. This method is highly efficient, but the solvents are volatile and prone to emulsification, posing safety and environmental risks and requiring sophisticated equipment.

[0003] In summary, traditional technologies face challenges such as high cost, low efficiency, environmental unfriendliness, and poor adaptability when treating brine from salt lakes with high magnesium-to-lithium ratios. Therefore, the industry urgently needs a new separation technology that is efficient, energy-saving, low-cost, and highly selective. Nanofiltration membrane technology has emerged as a solution in this context.

[0004] Nanofiltration composite membranes are a very important type of nanofiltration separation membrane. Due to their stable structure and operational performance, they have become the most commercially available type of nanofiltration membrane. The structure of the composite membrane consists of a lower porous support layer and an upper ultrathin selective skin layer. It is generally believed that the lower support layer provides the mechanical strength of the composite membrane, while the upper selective skin layer determines the separation performance. Currently, most composite nanofiltration membranes are prepared using interfacial polymerization.

[0005] Traditional nanofiltration membranes typically consist of a polyester nonwoven fabric, a polysulfone ultrafiltration support layer, and a polyamide desalination layer. The nanofiltration membrane involved in this patent is a polypiperazine amide nanofiltration membrane obtained by interfacial polymerization of piperazine and trimesoyl chloride on a nonwoven fabric / polysulfone ultrafiltration base membrane. Conventional polypiperazine amide nanofiltration membranes have a generally negatively charged surface with locally positively charged areas, but the positively charged parts are easily shielded by the charge. Summary of the Invention

[0006] The purpose of this invention is to provide a multi-layered nanofiltration membrane for lithium extraction from salt lakes and its preparation method. By inserting multiple positively charged coatings between the polysulfone-based membrane and the polyamide layer, the electrostatic repulsion of the nanofiltration membrane is greatly enhanced. Since the positively charged coatings are located in the middle, they can effectively avoid being shielded by charged particles in the bulk solution, and can continuously and stably exert electrostatic repulsion. It has extremely high magnesium-lithium separation efficiency and operational stability, and solves at least one of the technical problems existing in the background art.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a multi-layered nanofiltration membrane for lithium extraction from salt lakes, wherein multiple uniform positively charged coating layers are inserted between the polyamide layer and the polysulfone-based membrane of a conventional nanofiltration membrane.

[0009] In a second aspect, the present invention provides a method for preparing a multi-layer nanofiltration membrane for lithium extraction from salt lakes as described in the first aspect, comprising:

[0010] Preparation of casting solution: First, polysulfone and dimethylformamide are placed in a mixing tank, stirred, and then allowed to stand to remove bubbles to obtain the casting solution;

[0011] Base film preparation: The above-prepared casting solution is coated on nonwoven fabric, and then a base film combining nonwoven fabric and polysulfone is prepared through phase inversion.

[0012] The prepared base film was immersed in isopropanol solution, and excess solution on the surface was removed using a rubber roller.

[0013] Aqueous phase 2 solution is coated onto the surface of the base membrane, and then excess aqueous phase is removed. Oil phase 2 is then coated onto the surface, and the membrane is dried. The coating process of aqueous phase 2 and oil phase 2 is repeated multiple times to obtain a nanofiltration membrane with multiple intermediate layers. Aqueous phase 1 solution is then coated onto the membrane, and excess aqueous phase is removed. Oil phase 1 is then coated onto the surface, and the membrane is dried to obtain a multi-electric-layer nanofiltration membrane.

[0014] As a further limitation of the second aspect of the present invention, the aqueous phase 1 solution is prepared by sequentially adding 1-2 wt% piperazine, 2.2-2.5 wt% camphor sulfonic acid, and 0.2-0.5% polyvinyl alcohol to pure water, stirring at 35-45°C for 1-2 hours, and then letting it stand for 1-2 hours.

[0015] As a further limitation of the second aspect of the present invention, the aqueous phase 2 solution is prepared by adding 1-2 wt% PEI (polyethyleneimine) to pure water, stirring at 35-45°C for 1-2 hours, and then letting it stand for 1-2 hours.

[0016] As a further limitation of the second aspect of the present invention, the oil phase 1 solution is prepared by adding 0.5-1.0% trimesoyl chloride to a pure hexane solution, stirring magnetically at room temperature for 1 hour, and then letting it stand for 1 hour.

[0017] As a further limitation of the second aspect of the present invention, the oil phase 2 solution is prepared by adding 0.5-1.0% cyanuric chloride to a pure hexane solution, stirring magnetically at room temperature for 1 hour, and then letting it stand for 1 hour.

[0018] As a further limitation of the second aspect of the present invention, the surface of the base membrane is coated with an aqueous phase 1 solution, and after two minutes, the excess aqueous phase on the surface is removed using a nitrogen knife. Then, an oil phase 1 is coated on the surface. Piperazine in the aqueous phase and trimesoyl chloride in the oil phase undergo an interfacial polymerization reaction to generate a high molecular weight polyamide material. This material adheres to the base membrane, thus forming a nanofiltration membrane with a surface Turing morphology.

[0019] The beneficial effects of this invention are as follows: The positively charged intermediate layer fully exerts its electrostatic repulsion effect on magnesium ions, significantly improving magnesium ion retention; screening of monovalent ions is mainly achieved through pore size screening, followed by charge screening. Due to the precise control of the intermediate layer pore size, lithium ions can easily pass through, and because of the low charge density of lithium ions, combined with the water production ion balance effect, lithium ion permeation is actually promoted; the electrostatic repulsion effect of the nanofiltration membrane is greatly enhanced, and because the positively charged coating is located in the middle, it can effectively avoid being shielded by charged particles in the bulk solution, allowing for a continuous and stable electrostatic repulsion effect.

[0020] The advantages of additional aspects of the invention will be set forth more clearly in the following description or will be learned by practice of the invention. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the base film preparation process according to an embodiment of the present invention.

[0023] Figure 2 This is a schematic diagram of the process of adding and drying the aqueous and oil phases according to an embodiment of the present invention.

[0024] Figure 3 This is a schematic diagram of the reaction process of piperazine in the aqueous phase and pyromellitic trimethylol chloride in the oil phase according to an embodiment of the present invention.

[0025] Figure 4This is a schematic diagram of the special nanofiltration membrane preparation process described in an embodiment of the present invention.

[0026] Figure 5 This is a schematic diagram of the Zeta potential curve of the positively charged intermediate layer according to an embodiment of the present invention.

[0027] Figure 6 This is a SEM image of the special nanofiltration membrane described in an embodiment of the present invention.

[0028] Figure 7 This is a SEM image of a conventional nanofiltration membrane as described in an embodiment of the present invention. Detailed Implementation

[0029] Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0030] It will be understood by those skilled in the art that, unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0031] It should also be understood that terms such as those defined in general dictionaries should be understood to have meanings consistent with their meanings in the context of the prior art, and should not be interpreted in an idealized or overly formal sense unless defined as here.

[0032] Those skilled in the art will understand that, unless specifically stated otherwise, the singular forms “a,” “an,” “the,” and “the” used herein may also include the plural forms. It should be further understood that the term “comprising” as used in this specification means the presence of the stated features, integers, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, and / or groups thereof.

[0033] In the description of this specification, references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of those different embodiments or examples.

[0034] To facilitate understanding of the present invention, the present invention will be further explained and described below with reference to the accompanying drawings and specific embodiments. However, the specific embodiments do not constitute a limitation on the embodiments of the present invention.

[0035] Those skilled in the art should understand that the accompanying drawings are merely schematic diagrams of embodiments, and the components in the drawings are not necessarily essential for implementing the present invention.

[0036] Nanofiltration membranes exhibit excellent selective separation of multivalent and monovalent ions in water. Salt lakes contain high levels of magnesium ions, which significantly impact the purity of lithium extraction. Therefore, the selective separation of magnesium and lithium in salt lakes using nanofiltration membranes is currently the mainstream technology. With continuous technological advancements, the industry demands even higher separation efficiency for magnesium and lithium. Conventional nanofiltration membranes, due to their overall negatively charged surface and the shielding effect of localized positive charges by negative ions under high influent salinity, experience increased magnesium permeation during separation. This invention employs a novel membrane preparation method and a multi-layer coating process, breaking through traditional coating methods. The multi-layer intermediate layer significantly improves magnesium-lithium separation performance. By introducing multiple positively charged intermediate layers, the positively charged intermediate layers exert strong electrostatic repulsion on magnesium ions during transmembrane transport, greatly increasing the transmembrane transport energy barrier and effectively improving magnesium ion retention. Simultaneously, since the charge density of lithium ions is much lower than that of magnesium ions, the electrostatic repulsion is less significant, resulting in almost unchanged permeability. Furthermore, because the positive charge in the intermediate layer is less easily shielded by negatively charged ions in the bulk solution, the magnesium-lithium separation effect is more stable during long-term operation.

[0037] Example 1

[0038] In this embodiment 1, a method for preparing a lithium extraction nanofiltration membrane from a salt lake is provided. Multiple layers of uniform positively charged coatings are inserted between the polyamide layer and the polysulfone base membrane of a conventional nanofiltration membrane to manufacture a highly stable electrostatically repulsive lithium extraction nanofiltration membrane from a salt lake.

[0039] Preparation of casting solution: First, put 20-25wt% polysulfone and 75-80wt% dimethylformamide (DMF) into a mixing tank, heat the oil furnace to a temperature of 80-95℃; stir at a speed of 40-60Hz; stir for 6-8 hours, then degas and cool with water at a temperature of 25-35℃; stir rapidly at 30-50Hz for the first 10 minutes; then stir slowly at a negative pressure for 5-10Hz for more than 6 hours; when the material temperature reaches 25℃, let it stand for degassing for 12-15 hours.

[0040] Base film preparation: such as Figure 1 As shown, the above-prepared casting solution is applied to a nonwoven fabric, and then a base film combining nonwoven fabric and polysulfone is prepared through phase inversion.

[0041] The prepared base film was immersed in a 30% isopropanol solution for 30 minutes, then removed and the excess solution on the surface was removed using a rubber roller for later use.

[0042] To prepare aqueous phase 1 solution: add 1-2 wt% piperazine, 2.2-2.5 wt% camphor sulfonic acid, and 0.2-0.5% polyvinyl alcohol sequentially to pure water. Stir at 35-45℃ for 1-2 hours, then let stand for 1-2 hours for later use.

[0043] Preparation of aqueous phase 2 solution: Add 1-2 wt% PEI (polyethyleneimine) to pure water, stir at 35-45℃ for 1-2 hours, and then let stand for 1-2 hours for later use.

[0044] Preparation of oil phase 1 solution: Add 0.5-1.0% trimesoyl chloride to the pure hexane solution, stir magnetically at room temperature for 1 hour, and then let stand for 1 hour for later use.

[0045] Preparation of oil phase 2 solution: Add 0.5-1.0% cyanuric chloride to the pure hexane solution, stir magnetically at room temperature for 1 hour, and then let stand for 1 hour for later use.

[0046] In this embodiment, the preparation process of a conventional nanofiltration membrane is provided: as follows Figure 2 As shown, the surface of the above-mentioned base membrane is coated with aqueous phase 1 solution. After two minutes, the excess aqueous phase on the surface is removed using a nitrogen knife. Then, the above-mentioned oil phase 1 is coated on the surface. After one minute, it is placed in an oven at 90 degrees Celsius and dried for one minute. Piperazine in the aqueous phase and trimesoyl chloride in the oil phase will undergo an interfacial polymerization reaction to generate a high molecular weight polyamide material. This material adheres to the base membrane, thus forming a nanofiltration membrane with a surface Turing morphology.

[0047] The reaction process of piperazine in the aqueous phase and pyromellitic trimethylol chloride in the oil phase is as follows: Figure 3 As shown. Combined with Figure 3 As shown, PIP (aqueous phase) and TMC (oil phase) contact at the interface of hexane and water, rapidly forming a cross-linked polyamide film through an amidation reaction. This process utilizes the immiscibility of the two monomer phases, confining the reaction to the interface, thereby preparing an ultrathin, uniform separation functional layer with nanoscale pores.

[0048] This embodiment also provides a process for preparing a special nanofiltration membrane: such as Figure 4 As shown, the surface of the above-mentioned base membrane is coated with aqueous phase 2 solution. After two minutes, the excess aqueous phase on the surface is removed using a nitrogen knife. Then, the above-mentioned oil phase 2 is coated on the surface. After one minute, it is placed in an oven at 90 degrees Celsius and dried for one minute. After drying, the membrane is coated with aqueous phase 2 and oil phase 2 again to obtain two positively charged intermediate layers. If you want to prepare a multi-layer intermediate layer, the aqueous phase 2 and oil phase 2 are coated in sequence. The nanofiltration membrane with the obtained multi-layer intermediate layer is coated with aqueous phase 1 solution. After two minutes, the excess aqueous phase on the surface is removed using a nitrogen knife. Then, the above-mentioned oil phase 1 is coated on the surface. After one minute, it is placed in an oven at 90 degrees Celsius and dried for one minute to obtain a special nanofiltration membrane.

[0049] Example 2

[0050] In this embodiment 2, a method for preparing a special nanofiltration membrane is provided. The specific preparation process includes the following steps:

[0051] Preparation of casting solution: First, put 20wt% polysulfone and 80wt% dimethylformamide (DMF) into a mixing tank, heat the oil furnace to a temperature of 95℃, stir at 60Hz, stir for 8 hours, then degas and cool with water at 35℃, stir rapidly at 50Hz for the first 10 minutes, then stir slowly at 10Hz under negative pressure for 6 hours, and let stand for 12 hours after the material temperature reaches 25℃ for degassing.

[0052] Base film preparation: The above-prepared casting solution is applied to nonwoven fabric, and then a base film combining nonwoven fabric and polysulfone is prepared through phase inversion. The prepared base film is immersed in 30% isopropanol solution for 30 minutes and then removed. Excess solution on the surface is removed using a rubber roller for later use.

[0053] Preparation of aqueous phase 1 solution: Add 1 wt% piperazine, 2.2 wt% camphor sulfonic acid, and 0.5% polyvinyl alcohol sequentially to pure water, stir for 2 hours at 35-45℃, and then let stand for 2 hours for later use.

[0054] Preparation of aqueous phase 2 solution: Add 2wt% PEI (polyethyleneimine) to pure water, stir at 35-45℃ for 2 hours, then let stand for 2 hours for later use.

[0055] Preparation of oil phase 1 solution: Add 1.0% trimesoyl chloride to the pure hexane solution, stir magnetically at room temperature for 1 hour, and then let stand for 1 hour for later use.

[0056] Preparation of oil phase 2 solution: Add 1.0% cyanuric chloride to the pure hexane solution, stir magnetically at room temperature for 1 hour, and then let stand for 1 hour for later use.

[0057] The surface of the above-mentioned base membrane is coated with aqueous phase 2 solution. After two minutes, the excess aqueous phase on the surface is removed using a nitrogen knife. Then, the above-mentioned oil phase 2 is coated on the surface. After one minute, it is placed in an oven at 90 degrees Celsius and dried for one minute to obtain a special nanofiltration membrane.

[0058] Electron micrograph of a special nanofiltration membrane Figure 5 and potential diagram Figure 6 As shown in the figure, the isoelectric point of the middle layer is 7.74, and it carries a positive charge under neutral conditions (pH=7), which is beneficial for exerting an electrostatic repulsion effect on magnesium ions and improving the retention effect; the SEM image shows that the surface is relatively smooth and Figure 7The morphology of the conventional nanofiltration membranes shown is consistent.

[0059] Example 3

[0060] In this embodiment 3, another method for preparing a special nanofiltration membrane is provided. The specific preparation process includes the following steps:

[0061] The preparation process is modified by adding one intermediate layer coating process, and the rest are the same as in Example 2.

[0062] The surface of the base membrane was coated with aqueous phase 2 solution. After two minutes, excess aqueous phase was removed from the surface using a nitrogen knife. Then, oil phase 2 was coated onto the surface. After one minute, the membrane was placed in a 90°C oven and dried for one minute. The surface was then coated with aqueous phase 2 solution again. After two minutes, excess aqueous phase was removed from the surface using a nitrogen knife. Then, oil phase 2 was coated onto the surface. After one minute, the membrane was placed in a 90°C oven and dried for one minute. The surface was then coated with aqueous phase 1 solution. After two minutes, excess aqueous phase was removed from the surface using a nitrogen knife. Then, oil phase 1 was coated onto the surface. After one minute, the membrane was placed in a 90°C oven and dried for one minute to obtain a special nanofiltration membrane.

[0063] Example 4

[0064] In this embodiment 4, another method for preparing a special nanofiltration membrane is provided. The specific preparation process includes the following steps:

[0065] The preparation process is modified by adding one intermediate layer coating process, and the rest are the same as in Example 2.

[0066] The surface of the above-mentioned base membrane was coated with aqueous phase 2 solution. After two minutes, the excess aqueous phase on the surface was removed using a nitrogen knife. Then, the above-mentioned oil phase 2 was coated on the surface. After one minute, it was placed in an oven at 90 degrees Celsius and dried for one minute. The surface was coated with aqueous phase 2 solution again. After two minutes, the excess aqueous phase on the surface was removed using a nitrogen knife. Then, the above-mentioned oil phase 2 was coated on the surface. After one minute, it was placed in an oven at 90 degrees Celsius and dried for one minute. The surface was coated with aqueous phase 2 solution again. After two minutes, the excess aqueous phase on the surface was removed using a nitrogen knife. Then, the above-mentioned oil phase 2 was coated on the surface. After one minute, it was placed in an oven at 90 degrees Celsius and dried for one minute. The surface was coated with aqueous phase 1 solution. After two minutes, the excess aqueous phase on the surface was removed using a nitrogen knife. Then, the above-mentioned oil phase 1 was coated on the surface. After one minute, it was placed in an oven at 90 degrees Celsius and dried for one minute to obtain a special nanofiltration membrane.

[0067] Comparative experiment

[0068] In the comparative experiment, a method for preparing a conventional nanofiltration membrane was prepared. The specific preparation process includes the following steps: only the intermediate layer coating is reduced in the nanofiltration membrane preparation process, and the rest are the same as the steps in Example 2; the surface of the above base membrane is coated with aqueous phase 1 solution, and after two minutes, the excess aqueous phase on the surface is removed with a nitrogen knife. Then, the above oil phase 1 is coated on the surface. After 1 minute, it is placed in an oven at 90 degrees Celsius and dried for 1 minute to obtain a conventional nanofiltration membrane.

[0069] Testing experiment: The nanofiltration membranes prepared in Examples 2-4 and the comparative experiment were tested for performance: a composite solution of 2000 ppm Mg and 400 ppm Li was prepared using magnesium chloride and lithium chloride, and the membrane flux, magnesium rejection rate and lithium rejection rate were tested at 25°C under an operating pressure of 10 bar.

[0070] Membrane flux is calculated using the formula F = V / (A·t), where F is the flux [L / (m²]]. 2 ·h)], V is the permeate volume (L), A is the membrane area (m²) 2 ), t is the permeation time (h); the higher the membrane flux value, the higher the membrane permeability, and vice versa; the significance of magnesium rejection rate and lithium rejection rate: the higher the magnesium rejection rate and the lower the lithium rejection rate, the better the magnesium-lithium ion selectivity, and vice versa; the results are shown in Table 1.

[0071] Table 1

[0072]

[0073] As shown in Table 1, Examples 2 and 3 with intermediate coatings are significantly better than the conventional nanofiltration membranes prepared in the comparative experiment in terms of magnesium rejection rate, and the lithium ion rejection rate is relatively lower. This proves that the special intermediate layer nanofiltration membrane is better than the conventional nanofiltration membrane in terms of magnesium-lithium separation capability. Example 3 has better overall performance in terms of combined flux and magnesium-lithium rejection.

[0074] The structure and performance of the nanofiltration separation layer are closely related. Nanofiltration can selectively separate monovalent and divalent ions. From the separation mechanism perspective, due to the charged surface of the nanofiltration membrane, electrostatic repulsion / attraction is used to improve the selectivity of ions with different valence states. In summary, the multi-layered nanofiltration membrane for lithium extraction from salt lakes and its preparation method described in this invention, with its positively charged intermediate layer, can fully exert the electrostatic repulsion effect on magnesium ions, significantly improving magnesium ion retention. On the other hand, the nanofiltration membrane mainly sieves monovalent ions through pore size sieving, followed by charge sieving. Due to the precise control of the pore size of the intermediate layer, lithium ions can easily pass through, and because of the low charge density of lithium ions, combined with the ion balance effect in the produced water, lithium ion permeation is actually promoted. This enhances the magnesium-lithium separation effect from the above two aspects. Conventional polypiperazine amide nanofiltration membranes have an overall negatively charged surface and locally positively charged areas, but the positively charged parts are easily shielded by charge. The method of this invention can greatly enhance the electrostatic repulsion effect of the nanofiltration membrane, and because the positively charged coating is in the middle position, it can effectively avoid being shielded by charged particles in the bulk solution, and can continuously and stably exert the electrostatic repulsion effect.

[0075] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that, based on the technical solutions disclosed in the present invention, various modifications or variations that can be made by those skilled in the art without creative effort should be included within the scope of protection of the present invention.

Claims

1. A multi-electrolyte nanofiltration membrane for lithium extraction from salt lakes, characterized in that: A plurality of uniform positive charge coatings are inserted between the polyamide layer and the polysulfone base film of a conventional nanofiltration membrane.

2. The method for preparing a multi-electrode nanofiltration membrane for lithium extraction from salt lakes according to claim 1, characterized in that, The application relates to a preparation method of a nanofiltration membrane with a Turing pattern surface. The preparation method comprises the following steps: Preparation of a casting solution: firstly, poly sulfone and dimethyl formamide are put into a mixing kettle and stirred, and then the casting solution is obtained after standing and defoaming; Preparation of a base film: the prepared casting solution is coated on non-woven fabric, and then a base film combined with non-woven fabric and polysulfone is prepared through phase inversion; The prepared base film is soaked in an isopropanol solution, and a rubber roller is used to remove the excessive solution on the surface; 3. The method for preparing a multi-layer nanofiltration membrane for lithium extraction from salt lakes according to claim 2, characterized in that, The surface of the base film is coated with a water phase 2 solution, the excessive water phase on the surface is removed, then an oil phase 2 is coated on the surface, and the film is dried; the water phase 2 and the oil phase 2 are repeatedly coated on the surface for multiple times to obtain a nanofiltration membrane with a plurality of intermediate layers, then a water phase 1 solution is coated on the surface, the excessive water phase on the surface is removed, then an oil phase 1 is coated on the surface, and the nanofiltration membrane with a plurality of layers is obtained after drying.

4. The method for preparing a multi-layer nanofiltration membrane for lithium extraction from salt lakes according to claim 2, characterized in that, Preparation of the water phase 1 solution: 1-2 wt% of piperazine, 2.2-2.5 wt% of camphor sulfonic acid and 0.2-0.5% of polyvinyl alcohol are sequentially added into pure water, stirring is carried out at 35-45 DEG C for 1-2 hours, and then standing is carried out for 1-2 hours.

5. The method for preparing a multi-layer nanofiltration membrane for lithium extraction from salt lakes according to claim 2, characterized in that, Preparation of the water phase 2 solution: 1-2 wt% of PEI (polyethylene imine) is added into pure water, stirring is carried out at 35-45 DEG C for 1-2 hours, and then standing is carried out for 1-2 hours.

6. The method for preparing a multi-layer nanofiltration membrane for lithium extraction from salt lakes according to claim 2, characterized in that, Preparation of the oil phase 1 solution: 0.5-1.0% of trimesoyl chloride is added into a pure n-hexane solution, magnetic stirring is carried out at room temperature for 1 hour, and then standing is carried out for 1 hour.

7. The method for preparing a multi-layer nanofiltration membrane for lithium extraction from salt lakes according to claim 2, characterized in that, Preparation of the oil phase 2 solution: 0.5-1.0% of cyanuric chloride is added into a pure n-hexane solution, magnetic stirring is carried out at room temperature for 1 hour, and then standing is carried out for 1 hour. The surface of the base film is coated with the water phase 1 solution, the excessive water phase on the surface is removed by using a nitrogen knife after two minutes, then the oil phase 1 is coated on the surface, and an interfacial polymerization reaction occurs between the piperazine in the water phase and the trimesoyl chloride in the oil phase, thereby generating a high-molecular polyamide material which is attached to the base film, so that the nanofiltration membrane with a Turing pattern surface is formed.

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