A photothermal evaporation ion separation membrane, a preparation method and use thereof

Abstract

The application provides a photothermal evaporation ion separation membrane, a preparation method and an application. The separation membrane comprises a mass transfer layer and an ion separation layer. The ion separation layer can separate ions in a solution by a pore structure or charge property, so that required ions pass through the ion separation layer and the remaining ions are intercepted. The mass transfer layer is provided with nanoscale porous channels and has good solvent compatibility and can generate a megapascal pressure, so as to drive water to drive the ions to be separated to pass through the ion separation layer to the mass transfer layer, and then ion separation is realized. The separation membrane can not only be used for separating lithium ions and magnesium ions in a salt lake solution to realize lithium extraction, but also can be used for separating other ions with different radii or different charges.

Description

TECHNICAL FIELD

[0001] The present application relates to a photothermal evaporation ion separation membrane, a preparation method and a use, and belongs to the field of ion separation. BACKGROUND

[0002] Lithium (Li) has become one of the most valuable resources in the 21st century, with a sharp increase in demand due to its application in electric transportation batteries and sustainable energy storage. It is essential to procure lithium from various sources, of which lithium obtained from salt lake brine accounts for 70% of the recoverable lithium on land, and it has become an important supply route to ensure the healthy development of the lithium battery market.

[0003] Lithium ions (Li + ) are abundant in salt lake brine, but the concentration of Li + is relatively low compared to other coexisting cations, among which the concentration of magnesium (Mg) is often one to three orders of magnitude higher than that of Li. In the sequence of typical processing technology for extracting Li (from brine), Li + is recovered in the form of hydroxide or carbonate compound in the final precipitation step. Since Mg(OH)2 or MgCO3 also has a low solubility, co-precipitation ((LiOH or Li2CO3 precipitation)) occurs at the same time as the precipitation treatment. Therefore, the key to achieving a high-purity final product lies in the selective and efficient separation of Li + and Mg 2 + before the precipitation treatment.

[0004] The traditional solar evaporation / precipitation-based lithium extraction process requires the use of a large amount of chemicals and cannot effectively handle brine with a high Mg 2+ / Li + mass ratio (MLR). Nanofiltration (NF) can selectively separate monovalent and divalent ions, and due to its separation efficiency, modularity and process sustainability, it has been widely used for the separation of Li + / Mg 2+ . However, due to the high ionic strength of salt lake water, the complexity of raw material composition, and the problem of easy scaling, the use of nanofiltration process for direct extraction of lithium is highly challenging, so NF is usually integrated with other pretreatment and post-treatment unit processes to form a complete processing technology. The typical integrated nanofiltration processing technology is: before the NF module processing, the typical pretreatment process is to pre-enrich Li in the original salt lake water through a sodium-potassium precipitation process module. The high magnesium-lithium ratio brine is diluted by a certain proportion before entering the NF process module, and the magnesium-lithium ratio of the brine obtained is greatly reduced, and Li + is highly enriched. Subsequently, a reverse osmosis membrane processing module is used as a post-treatment process to concentrate Li +Used for the last precipitation step and to recover clean water for dilution of the high concentration brine before the NF process module.

[0005] Such a multi-unit synergistic treatment process is complex, and the operation is cumbersome, involving high equipment facility investment cost and great energy consumption in the pressure-driven membrane treatment process section (nanofiltration membrane and reverse osmosis membrane treatment units). SUMMARY

[0006] The present application provides a photothermal evaporation ion separation membrane to solve the problem of separating lithium and magnesium ions in the existing solution, which can not only be used to separate lithium and magnesium ions in salt lake solution to realize lithium extraction, but also be used to separate other ions with different radii or different charges, and the solution can be an aqueous solution or an organic solution. The separation membrane includes a mass transfer layer and an ion separation layer, the ion separation layer can separate ions in the solution through pore structure or charge properties, so that the required ions pass through the ion separation layer and the remaining ions are retained, the mass transfer layer is provided with nanoscale porous channels, and has good solvent compatibility. If the solution is an aqueous solution, the mass transfer layer is a hydrophilic material, and if the solution is an organic solution, the mass transfer layer is an organic solvent. The porous channels in the mass transfer layer can generate a capillary force of more than 7 MPa to drive water to carry the ions to be separated through the ion separation layer to the mass transfer layer. In use, the lower surface of the ion separation layer (the side facing away from the mass transfer layer) is in contact with the solution, and the mass transfer layer is placed above the solution surface to absorb light and convert it into heat. The heated water entering the mass transfer layer is converted into water vapor or the heated organic solvent escapes into the air in a gaseous state, leaving the required ions, which are deposited in the porous channels of the mass transfer layer or on the upper surface of the mass transfer layer. When the ions accumulate to a certain amount, they are removed from the mass transfer layer by soaking, scraping or other methods.

[0007] The technical solution adopted is: a photothermal evaporation separation membrane, including a mass transfer layer and an ion separation layer, the mass transfer layer is provided with dense porous channels and can absorb sunlight and convert it into heat, the ion separation layer is a porous structure and can allow the required ions and solvent molecules to pass through, so as to separate the required ions from other ions, the average pore size of the ion separation layer is 0.4-0.5 nm, the porous channels on the mass transfer layer are beneficial to generating capillary force to drive ions and solvent to pass through the ion separation membrane to the mass transfer layer, and on the other hand, provide transmission channels for ions and solvent. The mass transfer layer is a hydrophilic material, the porous channels on it are preferably nanoscale channels, and more preferably channels with an average pore size of 8-12 nm and a contact angle of 60-80°, preferably 72°. Further preferably, the average pore size is 10 nm, which not only provides a megapascal osmotic pressure, but also facilitates the transmission of ions and water, avoiding the deposition of ions in the pore channel to block the pore channel. The porosity is 50-80%, preferably 60-70%, and the thickness of the mass transfer layer is preferably 240 μm.

[0008] As a preferred solution, the mass transfer layer and the ion separation layer are an integral structure, and at least one of the two is directly formed on another membrane layer. This arrangement is conducive to the capillary force of the mass transfer layer fully acting on the ion separation layer, thereby achieving effective transport of water and ions. As one implementation, the ion separation membrane is formed on one side of the mass transfer layer, and then an integral structure is formed.

[0009] As a preferred solution, the photo-thermal evaporation ion separation membrane is used to separate ions in an aqueous solution, and more preferably to separate ions of different valence states, and further preferably to separate monovalent ions and divalent ions.

[0010] As a preferred solution, the mass transfer layer is a mixed material of an organic polymer material and a light-absorbing material. The organic polymer material is conducive to forming porous channels, and the pore size can be adjusted, and has the advantages of hydrophilic performance and adjustable hydrophilic performance. After the light-absorbing material is compounded into the organic polymer material, the mass transfer layer can have the function of light absorption and heat conversion. As a preferred solution, the organic polymer material is one of polyvinylidene fluoride, polyimide, polyether sulfone, polyester, polytetrafluoroethylene, acetate fiber, polysulfone, and polyacrylonitrile. The light-absorbing material can be an organic or inorganic material, such as carbon nanotubes (CNTs), carbon black, carbon powder, graphene, and polyaniline. As a preferred combination, the organic polymer material is polyether sulfone, and the light-absorbing material is polyaniline. The two have good compatibility and uniform dispersion, and are conducive to forming a stable pore structure during phase inversion.

[0011] The application also provides a preparation method of the above-mentioned photo-thermal evaporation separation membrane. The preparation method can be one of the following two methods: (1) first preparing the mass transfer layer, and then preparing the ion separation layer on the mass transfer layer; and (2) first preparing the ion separation layer, and then preparing the mass transfer layer on the ion separation layer. Preferably, the mass transfer layer is prepared first, and then the ion separation layer is prepared in situ on one side of the mass transfer layer.

[0012] As a preferred solution, the preparation method of the mass transfer layer is as follows: an organic polymer is selected as the main body for forming a porous membrane, and a light-absorbing material is selected to be mixed with the organic polymer to form a porous membrane. The following steps are performed: (1) the organic polymer, the light-absorbing material, and an additive are blended in a solvent in a set amount to form a mixed solution; (2) the mixed solution is mechanically stirred, and after the stirring is completed, the solution is left to stand to allow the solution to be fully degassed. Preferably, the standing time is 24 hours; (3) after the standing is completed, the solution is scraped to form a film. Preferably, a 250-μm solution scraper is used to scrape a square liquid film with a side length of 10 cm on a smooth glass plate in a casting manner; and (4) then the liquid film is quickly immersed in deionized water for non-solvent induced phase inversion, and the immersion is continuously performed for 24 hours to complete the phase inversion. Finally, the mass transfer layer is prepared.

[0013] As a preferred solution, the organic polymer is polyether sulfone, the content of which in the mixed solution is 10-15wt%, the light-absorbing material is polyaniline, the content of which in the mixed solution ranges from 3-8wt%, and the additives include non-solvent additives and liquid solvent additives, wherein the non-solvent additives are lithium chloride, polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP), which are beneficial to improving the hydrophilicity of the mass transfer layer on the one hand, and forming pores with different sizes after reacting with polyether sulfone on the other hand, which is beneficial to regulating the pore size of the final mass transfer layer and making it generate a pressure of megapascal level. In the present application, preferably, the mass ratio of lithium chloride, polyethylene glycol and polyvinylpyrrolidone is (1-5):(1-5):(1-5); more preferably, the content of the three in the polymer solution is 1-5wt%, lithium chloride can form pores with a size of 10-50nm after reacting with polyether sulfone, PEG can form pores with a size of 20-220nm after reacting with polyether sulfone, and PVP can form pores with a size of 50-220nm after reacting with polyether sulfone. Under this mixing ratio, the final pore size is 8-12nm, and a nanoscale pore is formed. Further preferably, the liquid solvent additive is glycerol and isopropyl alcohol, which makes the polymer solution close to its cloud point, the hydroxyl groups in the molecule have strong hydrogen bonding ability with water during the phase inversion film forming process, promote the exchange of solvent and water, instantaneous liquid-liquid phase separation occurs, a porous membrane with higher porosity is formed, and the pore size distribution of the polymer membrane is narrowed (the pore size distribution is more uniform). Preferably, the content of glycerol and isopropyl alcohol in the mixed solution ranges from 3-5wt%.

[0014] As a preferred solution, the solvent is N,N-dimethylformamide (DMF), the content of which in the mixed solution ranges from 52-78wt%, DMF has a relatively low boiling point and higher polymer solubility, and has more solid content ratio regulation options for PES polymer. In addition, DMF can be replaced with water in the coagulation bath more quickly during the phase inversion of the polymer, and the porosity of the formed polymer membrane is higher.

[0015] Under the synergistic effect of polyether sulfone, polyaniline, lithium chloride, polyethylene glycol, polyvinylpyrrolidone, glycerol, isopropyl alcohol and N,N-dimethylformamide, a black polymer membrane with an average pore size of 10nm is finally formed.

[0016] As a preferred solution, the preparation method of the ion separation layer is: S01 configuring an aqueous solution of piperazine and sodium dodecyl benzene sulfonate as a first solution; configuring a n-hexane solution of trimesoyl chloride as a second solution; S02 immersing one side of the mass transfer layer in the first solution for a set time, then taking it out and immersing it in the second solution for polymerization reaction, generating a polymer on the second surface, and obtaining the ion separation layer. As a preferred way, the mass fraction of piperazine in the first solution is 0.1-5wt%, the mass fraction of sodium dodecyl benzene sulfonate is 0.1-3.6mmol / L, and the mass fraction of trimesoyl chloride in the second solution is 0.05-0.3wt%. Further preferably, the preparation method of the ion separation layer is: (1) configuring an aqueous solution containing 0.1-5wt% of piperazine (PIP) and 0.1-3.6mmol / L of sodium dodecyl benzene sulfonate (SDBS), magnetically stirring at 20-40℃ for 1-5h, to obtain a completely dissolved PIP aqueous solution; (2) configuring a n-hexane (n-Hexane) solution containing 0.05-0.3wt% of trimesoyl chloride (TMC), magnetically stirring at 20-40℃ for 1-5h, to obtain a completely dissolved TMC n-hexane solution; (3) immersing one side of the mass transfer layer in the PIP aqueous solution for 0.5-10min; (4) taking the mass transfer layer out of the PIP aqueous solution, removing the excess water on the surface, and then immersing it in the TMC-containing n-hexane solution for 10-360s for interfacial polymerization reaction, and the specific reaction formula is as follows:

[0017] wherein represents the whole piperazine or trimesoyl chloride monomer; the polyamide is a high molecular chain structure, which is an integral whole of two monomers connected together by alternating reaction; the PIP monomer in the water phase and the TMC monomer in the n-hexane solution perform irreversible and rapid condensation polymerization on the surface of the mass transfer layer, and finally a dense membrane layer is formed in situ on the surface of the mass transfer layer; (5) the mass transfer layer is taken out from the n-hexane solution of TMC, and is slowly flushed with pure n-hexane for 1-5 min to remove unreacted chemicals; (6) the mass transfer layer with the polymer formed in situ is placed in an oven at 50-80℃ for 1-3 h, preferably at 50℃ for 2 h, to obtain an ion separation membrane layer formed integrally with the mass transfer layer, which is a nanofiltration membrane layer; (7) the ion separation membrane is taken out from the oven and soaked in deionized water at room temperature for use. In order to improve the retention rate of the ion separation layer and the extraction efficiency of ions, in step (1), the mass fraction of PIP in the PIP & SDBS aqueous solution is 2 wt%, the concentration of SDBS is 1.2 mmol / L, and the concentration of TMC in the n-hexane solution is 0.15 wt%, and the monomer solutions of the water phase and the oil phase are obtained by stirring for 2 h. The immersion time of the mass transfer layer in the water phase solution and the oil phase solution is 1 min and 30 s, respectively. The ion separation layer prepared by the method is an integral structure with the mass transfer layer, has specific nanopores and surface charges, and can enable it to efficiently retain Mg 2+ while allowing most of Li + As the water flows through the internal nanopores and migrates upward, the Li + is eventually enriched inside and on the surface of the evaporation / mass transfer mixed layer, preferably, the pore size of the ion separation layer is 0.4-0.8 nm, and the surface charge is -50 to -20 mV at pH = 6.5.

[0018] In order to make the ion separation membrane float on the liquid surface, the ion separation membrane is packaged between two O-shaped rubber gaskets, the gaskets play a supporting role and also provide floating force, so that the whole device floats on the liquid surface and ensures that the ion separation layer is in contact with the solution.

[0019] The beneficial effects produced by the present application include:

[0020] (1) The ion separation membrane in the present application utilizes interface photothermal evaporation to provide driving force for the transport of ions or solutes, can utilize the capillary negative pressure of up to hundreds of megapascals generated by the photothermal evaporation of liquid in the capillary pores to drive the selective ion separation of the underlying nanofiltration membrane, realize green and environmental protection while reducing operating costs, and directly collect lithium salt crystals on the surface of the integrated photothermal evaporation-separation membrane by using the photothermal effect, which greatly improves the salt concentration capacity compared with traditional membrane separation processing technology, and realizes green, clean, cheap and renewable lithium extraction from salt lakes.

[0021] (2) The ion separation membrane in the present application can directly process high-concentration and high-magnesium-lithium-ratio salt lake brine, and collect high-purity lithium salt crystals on the surface of the separation membrane. By adjusting the pore size and surface charge properties of the nanofiltration layer, the separation factor can reach more than 20, and the lithium chloride crystal rate can reach more than 600 mg / m 2 / h. Moreover, it can realize long-term stable operation, cyclic collection of lithium salt crystals, and economic efficiency.

[0022] (3) The mass transfer layer in the present application has the functions of light-heat conversion, evaporation, and transmission, and can precisely control the appropriate light-heat conversion rate, pore size, and hydrophilic-hydrophobic properties according to the properties of the specific feed solution (such as the total concentration of salt ions in the solution and the lithium-magnesium content ratio) to adapt to solutions of various properties.

[0023] (4) The ion separation layer in the present application can be prepared in situ on either side of the mass transfer layer, forming an integrated structure with the mass transfer layer without loss of capillary force transmission.

[0024] (5) The ion separation membrane in the present application can realize large-scale production and has portability (light weight and thin thickness), with a thickness of less than 0.5 mm. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 Structure diagram of the light-heat evaporation ion separation membrane in the present application;

[0026] Figure 2 Water evaporation rate curve of the light-heat evaporation ion separation membrane in Example 1 in salt solutions of various concentrations;

[0027] Figure 3 Lithium-magnesium separation factor of the light-heat evaporation ion separation membrane in Example 1 in salt solutions of various concentrations;

[0028] Figure 4 Lithium chloride crystal rate of the light-heat evaporation ion separation membrane in Example 1 in salt solutions of various concentrations;

[0029] Figure 5 Water evaporation rate curve of the light-heat evaporation ion separation membrane in Example 1 in salt solutions with different lithium-magnesium ion mass ratios;

[0030] Figure 6 Lithium-magnesium separation factor column chart of the light-heat evaporation ion separation membrane in Example 1 in salt solutions with different lithium-magnesium ion mass ratios;

[0031] Figure 7 Lithium chloride crystal rate curve of the light-heat evaporation ion separation membrane in Example 1 in salt solutions with different lithium-magnesium ion mass ratios;

[0032] In the figure, 1 is the mass transfer layer, and 2 is the ion separation layer. DETAILED DESCRIPTION

[0033] The application will be further described in conjunction with the accompanying drawings and specific embodiments, but it should be understood that the scope of protection of the application is not limited by the specific embodiments.

[0034] Example 1

[0035] The photo-thermal evaporation ion separation membrane in the application, such as Figure 1 , comprises a mass transfer layer 1 and an ion separation layer 2, the mass transfer layer 1 is provided with densely arranged pores, the pores are nanoscale, can generate capillary action force, and the mass transfer layer 1 is a hydrophilic material or the inner wall of the pores is a hydrophilic material, under the capillary action force, water flows upward along the pores.

[0036] The ion separation layer can play the role of ion separation through pore size or / and charge quantity, the required ions are passed through the channel, and the remaining ions are intercepted. The ion separation layer in the embodiment passes lithium ions and intercepts divalent ions such as magnesium ions.

[0037] In use, the double-layer structure is fixed and connected around the membrane through rubber washers up and down, so that the ion separation membrane floats on the solution surface, the mass transfer layer is the upper layer, the ion separation layer is the lower layer, the ion separation layer contacts the solution, and can be used for separating lithium ions and magnesium ions in the solution. When separating the two ions, lithium ions pass through the ion separation layer and are deposited in the form of salt crystals at the top of the mass transfer layer, while most of the magnesium ions cannot pass through the ion separation layer. Finally, high-purity lithium ions are obtained by scraping or flushing the evaporation / mass transfer mixed layer at the top, and the device can be reused for cyclic extraction of lithium salt crystals.

[0038] The device has simple structure and excellent ion selective separation performance. With the aid of the small pore diameter channel of the mass transfer layer and the photo-thermal conversion capability, concentrated lithium salt crystals can be collected at the top of the mass transfer layer under the driving of solar energy.

[0039] The preparation method of the photo-thermal evaporation ion separation membrane comprises the following steps

[0040] S01 Preparation of mass transfer layer

[0041] A A thin film main body is prepared by mixing polyether sulfone (PES) and polyaniline (PANI) as the main solidified polymer, lithium chloride, polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP) as non-solvent additives, glycerol and isopropyl alcohol as liquid solvent additives, and N,N-dimethylformamide (DMF) as a solvent. The thin film main body material, non-solvent additives and liquid solvent additives are mixed in the solvent to form a mixed solution. The mass fraction of each component in the mixed solution is: 12wt% PES, 8wt% PANI, 2wt% lithium chloride, 4wt% PEG, 2wt% PVP, 2wt% glycerol, 5wt% isopropyl alcohol, and 65wt% DMF.

[0042] B The mixed solution is mechanically stirred at 50°C for 15h, and then left to stand for 24h to complete the solution degassing.

[0043] C A 10cm square liquid film is cast on a smooth glass plate using a 250μm solution doctor blade.

[0044] D Then the liquid film is quickly immersed in deionized water for non-solvent induced phase inversion, and the immersion is continued for 24h to complete the phase inversion. A black mass transfer layer is prepared, the average pore size of the channel is 10nm, and the porosity tested by the weight method is 62%.

[0045] S02Preparation of ion separation layer

[0046] (1) An aqueous solution containing 2wt% piperazine (PIP) and 0.15mmol / L sodium dodecylbenzenesulfonate (SDBS) is prepared, and the solution is magnetically stirred at 35°C for 2h to obtain a completely dissolved PIP aqueous solution;

[0047] (2) A n-hexane (n-Hexane) solution containing 0.15wt% trimesoyl chloride (TMC) is prepared, and the solution is magnetically stirred at 35°C for 2h to obtain a completely dissolved TMC n-hexane solution;

[0048] (3) The upper surface of the mass transfer layer is the first surface, and the lower surface is the second surface. The second surface of the mass transfer layer is immersed in the PIP aqueous solution for 1min, and then taken out to remove the excess water on the surface;

[0049] (4) The second surface is immersed in the TMC-containing n-hexane solution for 30s to perform interfacial polymerization, and the polymerization reaction formula is:

[0050]

[0051] The PIP monomers in the water phase and the TMC monomers in the n-hexane solution in the reaction irreversibly and rapidly undergo polycondensation reaction on the second surface of the mass transfer layer, and finally a dense ion separation layer is formed in situ on the surface of the mass transfer layer. The average pore size of the ion separation layer is 0.42 nm by using neutral molecule interception method.

[0052] (5) The mass transfer layer with surface polymerization completed is taken out from the n-hexane solution containing TMC, and is slowly rinsed with pure n-hexane for 1 min to remove unreacted chemicals, to obtain a photo-thermal evaporation ion separation membrane;

[0053] (6) The photo-thermal evaporation ion separation membrane is placed in an oven at 50°C for 2 h for drying;

[0054] (7) The ion separation membrane is taken out from the oven and soaked in deionized water at room temperature, taken out for use, packaged, and then floated on the solution.

[0055] Test results

[0056] The following solutions are configured:

[0057] Solution one: the salt solution is a mixed salt aqueous solution of magnesium chloride and lithium chloride with a total concentration of 80 g / L, and the mass ratio of lithium to magnesium ions is 1:1.

[0058] Solution two:

[0059] The salt solution is a mixed salt aqueous solution of magnesium chloride and lithium chloride with a total concentration of 10 g / L, and the mass ratio of lithium to magnesium ions is 1:1.

[0060] Solution three:

[0061] The salt solution is a mixed salt aqueous solution of magnesium chloride and lithium chloride with a total concentration of 20 g / L, and the mass ratio of lithium to magnesium ions is 1:1.

[0062] Solution four:

[0063] The salt solution is a mixed salt aqueous solution of magnesium chloride and lithium chloride with a total concentration of 30 g / L, and the mass ratio of lithium to magnesium ions is 1:1.

[0064] Solution five:

[0065] The salt solution is a mixed salt aqueous solution of magnesium chloride and lithium chloride with a total concentration of 40 g / L, and the mass ratio of lithium to magnesium ions is 1:1.

[0066] Solution six:

[0067] The mass ratio of lithium to magnesium ions is 10:1, and the total concentration of the mixed salt aqueous solution of magnesium chloride and lithium chloride is fixed at 10 g / L.

[0068] Solution seven:

[0069] The fixed lithium magnesium ion mass ratio was 20:1, and the total concentration of the mixed salt aqueous solution of magnesium chloride and lithium chloride was 10 g / L.

[0070] Solution eight:

[0071] The fixed lithium magnesium ion mass ratio was 40:1, and the total concentration of the mixed salt aqueous solution of magnesium chloride and lithium chloride was 10 g / L.

[0072] Solution nine:

[0073] The fixed lithium magnesium ion mass ratio was 80:1, and the total concentration of the mixed salt aqueous solution of magnesium chloride and lithium chloride was 10 g / L.

[0074] Solution ten:

[0075] The fixed lithium magnesium ion mass ratio was 160:1, and the total concentration of the mixed salt aqueous solution of magnesium chloride and lithium chloride was 10 g / L.

[0076] The above solutions were respectively poured into 20 cm*20 cm*10 cm square boxes, and the square boxes corresponding to the solutions one to ten were respectively square box one to square box ten. The photo-thermal evaporation ion separation membrane obtained in Example 1 was placed on the square box corresponding to each solution, the photo-thermal evaporation ion separation membrane floated on the solution, the ion separation layer was in contact with the solution, and due to the existence of the nano-scale pore channel in the top mass transfer layer, a hundred megapascal level capillary force was generated, and the pressure was transmitted to the ion separation layer to start the membrane filtration, so as to selectively retain Mg 2+ , and Li + The part of lithium salt was finally crystallized and precipitated at the top of the mass transfer layer. It can be directly collected by a brush (or dissolved and collected by washing). The whole test time was 24 h, and under the irradiation of 3 solar intensities, the phenomenon of lithium salt crystal precipitation could be seen at the top of the mass transfer layer.

[0077] The water evaporation rate, separation factor and lithium chloride crystallization rate in the corresponding square boxes of test solutions one to five were tested, and the results were as shown in Figure 2 , Figure 3 and Figure 4 As shown by Figure 2 and Figure 3 , with the increase of the mixed salt aqueous solution of magnesium chloride and lithium chloride, the water evaporation rate and the separation factor basically remained unchanged, indicating that the photo-thermal evaporation ion separation membrane could be applied to various concentrations of salt solutions, and the separation efficiency was not affected by the concentration of the salt solution. As shown by Figure 4 , with the increase of the mixed salt aqueous solution of magnesium chloride and lithium chloride, the lithium chloride crystallization rate increased, indicating that high concentration of salt solution was beneficial to improve the lithium chloride crystallization rate.

[0078] The water evaporation rate, separation factor and lithium chloride crystallization rate in the square boxes of test solutions six to ten were tested, and the results were as shown inFigure 5 , Figure 6 and Figure 7 .Depend on Figure 5 and Figure 6 As shown, when the total concentration of the mixed salt solution of magnesium chloride and lithium chloride is fixed at 10 g / L, the water evaporation rate remains basically unchanged as the mass ratio of lithium to magnesium ions increases, while the separation factor gradually increases. This indicates that the photothermal evaporation ion separation membrane can be used for salt solutions with various magnesium-to-lithium mass ratios, and the separation efficiency is not affected by the magnesium-to-lithium mass ratio in the salt solution. Figure 7 As shown, as the mass ratio of magnesium to lithium in the mixed salt solution of magnesium chloride and lithium chloride increases, the crystallization rate of lithium chloride gradually decreases. This is because the mass fraction of lithium ions in the mixed salt solution decreases significantly, but the crystallization rate of lithium chloride remains relatively high.

[0079] The separation factor is obtained as follows: Separation factor = mass ratio of lithium to magnesium ions after separation / mass ratio of lithium to magnesium ions before separation. Taking a mass ratio of lithium to magnesium ions in the mixed solution to be separated as 1:1 as an example, in this embodiment, assuming that the mass ratio of lithium to magnesium ions entering the salt storage layer is 20:1 (the mass of magnesium and lithium ions in the salt storage layer can be measured by inductively coupled plasma spectrometry), then the separation ratio is (20:1) / (1:1) = 20.

[0080] The lithium chloride crystallization rate is the mass of lithium chloride powder collected on top of the mass transfer layer per unit area of ​​the separation membrane per unit time through solar-driven interfacial evaporation. In this invention, the salt crystal powder collected on top of the mass transfer layer is dissolved in a certain volume of aqueous solution. The mass concentration of lithium ions in the solution can be determined by measuring the aqueous solution using inductively coupled plasma spectrometry (ICP-S). For example, if the measured value is 1 mg / L and the volume of the aqueous solution is 0.1 L, then the mass of lithium in the salt crystals collected on top of the evaporation / mass transfer mixing layer is 0.1 mg. This mass is then converted using the molar mass ratio of lithium chloride and divided by the working area of ​​the separation membrane and the working time to obtain the final lithium chloride crystallization rate.

[0081] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A photothermal evaporation separation membrane, characterized in that: The mass transfer layer and the ion separation layer are provided with porous channels, and the mass transfer layer absorbs light to convert into heat. The ion separation layer is selectively permeable to the ions to be separated. In use, the photo-thermal evaporation separation film is placed on the liquid surface, the ion separation layer is in contact with the solution, the porous channels generate capillary force, the solvent and the ions to be separated in the solution pass through the ion separation layer to the mass transfer layer, the solvent absorbs heat in the mass transfer layer to convert into gaseous state to escape, and the ions to be separated are deposited in the pores of the mass transfer layer or on the surface of the mass transfer layer. The mass transfer layer and the ion separation layer are integrally formed, and the ion separation layer is grown on one side of the mass transfer layer or the mass transfer layer is grown on one side of the ion separation layer. The mass transfer layer is prepared by the following steps: S01, mixing organic polymer materials, light-absorbing materials and additives in a solvent to form a mixed solution; S02, mechanically stirring the mixed solution and standing to remove bubbles; and S03, forming a liquid film from the solution after the removal of bubbles, and then immersing the liquid film in water to induce phase inversion to obtain the gel-state mass transfer layer. The ion separation layer is prepared on one side of the mass transfer layer by in-situ irreversible rapid polycondensation of piperazine monomers in an aqueous solution and trimesoyl chloride monomers in a n-hexane solution on the surface of the mass transfer layer. The pore size of the porous channels on the mass transfer layer is 8-12 nm, and the contact angle of the mass transfer layer is 25-80°.

2. The photothermal evaporation separation membrane of claim 1, wherein: The average pore size of the ion separation layer is 0.4-0.8 nm. The porosity of the mass transfer layer is 50-80%.

3. The photothermal evaporation separation membrane according to claim 1 or 2, wherein: The thickness of the ion separation layer is 100-200 nm. The organic polymer material is one or a combination of polyvinylidene fluoride, polyimide, polyether sulfone, polyester, polytetrafluoroethylene, acetate fiber, polysulfone and polyacrylonitrile.

4. The photothermal evaporation separation membrane of claim 1 or 2, wherein: The light-absorbing material is an organic and / or inorganic light-absorbing material. The additives include non-solvent additives and liquid solvent additives, the non-solvent additives are lithium chloride, polyethylene glycol and polyvinylpyrrolidone, and the liquid solvent additives are glycerol and isopropyl alcohol. The organic polymer is polyether sulfone, the light-absorbing material is polyaniline, the mass content of the polyether sulfone in the mixed solution is 10-15%, and the mass content of the polyaniline is 3-8%.

5. The photothermal evaporation separation membrane of claim 4, wherein:

6. The photo-thermal evaporation separation film according to claim 4, wherein the non-solvent additives are lithium chloride, polyethylene glycol and polyvinylpyrrolidone, and the mass ratio of lithium chloride, polyethylene glycol and polyvinylpyrrolidone is (1-5):(1-5):(1-5); and the liquid solvent additives are glycerol and isopropyl alcohol, and the mass ratio of glycerol to isopropyl alcohol is (3-5):(3-5). The mass fraction of the piperazine monomers in the aqueous solution is 0.1-5 wt%, and the mass fraction of the trimesoyl chloride monomers in the n-hexane solution is 0.05-0.3 wt%. Any one of the following modes is included. ​ 7. The photothermal evaporation separation membrane of claim 4, wherein: ​ 8. A method of making the photothermal evaporation separation membrane of any one of claims 1-7, wherein: ​ The preparation method comprises the following steps: S1, preparing a mass transfer layer; S2, growing an ion separation layer on one side of the mass transfer layer; and S3, growing a mass transfer layer on one side of the ion separation layer. The preparation method of the mass transfer layer comprises the following steps: S01, mixing organic polymer materials, light-absorbing materials and additives in a solvent to form a mixed solution; S02, mechanically stirring the mixed solution and standing for defoaming; and S03, after defoaming, forming a liquid film by scraping the solution, and then immersing the liquid film in water to induce phase inversion, thereby obtaining the gel mass transfer layer. The preparation method of the ion separation layer comprises the following steps: S21, preparing an aqueous solution of piperazine monomers and a solution of trimesoyl chloride monomers in n-hexane; and S22, performing irreversible rapid polycondensation reaction of the piperazine monomers in the aqueous solution and the trimesoyl chloride monomers in the n-hexane solution on the surface of the mass transfer layer to form a dense film layer in situ. The preparation method comprises the following steps: S1, growing an ion separation layer; S2, growing a mass transfer layer on one side of the ion separation layer; and S3, growing a mass transfer layer on one side of the ion separation layer.

9. The method of claim 8, wherein: The organic polymer material is one or a combination of polyvinylidene fluoride, polyimide, polyether sulfone, polyester, polytetrafluoroethylene, acetate fiber, polysulfone and polyacrylonitrile. The light-absorbing material is an organic and / or inorganic light-absorbing material. The additives include non-solvent additives and liquid solvent additives, the non-solvent additives are lithium chloride, polyethylene glycol and polyvinylpyrrolidone, and the liquid solvent additives are glycerol and isopropyl alcohol.

10. The method of claim 8, wherein: The organic polymer is polyether sulfone, the light-absorbing material is polyaniline, the mass content of the polyether sulfone in the mixed solution is 10-15%, and the mass content of the polyaniline is 3-8%.

11. The preparation method of the photothermal evaporation separation film according to claim 9, wherein: The non-solvent additives are lithium chloride, polyethylene glycol and polyvinylpyrrolidone, and the mass ratio of lithium chloride, polyethylene glycol and polyvinylpyrrolidone is (1-5):(1-5):(1-5). The liquid solvent additives are glycerol and isopropyl alcohol, and the mass ratio of glycerol to isopropyl alcohol is (3-5):(3-5).

12. The method of claim 8, wherein: The mass fraction of the piperazine monomers in the aqueous solution is 0.1-5wt%, and the mass fraction of the trimesoyl chloride monomers in the n-hexane solution is 0.05-0.3wt%.

13. The method of claim 8-12, wherein: The preparation method of the mass transfer layer comprises the following steps: S1-1, mixing organic polymer materials, light-absorbing materials and additives in a solvent to form a mixed solution; wherein the additives include non-solvent additives and liquid solvent additives, the non-solvent additives are lithium chloride, polyethylene glycol and polyvinylpyrrolidone, and the liquid solvent additives are glycerol and isopropyl alcohol; S1-2, mechanically stirring the mixed solution and standing for defoaming; S1-3, after defoaming, scraping the solution to form a liquid film, and then immersing the film in water to induce phase inversion, thereby obtaining the mass transfer layer; S2-1, preparing an aqueous solution of piperazine and sodium dodecylbenzenesulfonate as a first solution, and preparing a n-hexane solution of trimesoyl chloride as a second solution; S2-2, immersing one side of the mass transfer layer in the first solution for a set time, taking out the immersed side, immersing the other side in the second solution for polymerization reaction, and generating a polymer on the second surface, thereby obtaining the ion separation layer.

14. The method of claim 13, wherein: In S1-1, the mass content of the lithium chloride, polyethylene glycol and polyvinylpyrrolidone in the mixed solution is 1-5 wt%, and the mass content of the glycerol and isopropyl alcohol is 3-5 wt%. In S2-1, the mass fraction of the piperazine in the first solution is 0.1-5 wt%, the mass fraction of the sodium dodecyl benzene sulfonate is 0.1-3.6 mmol / L, and the mass fraction of the trimesoyl chloride in the second solution is 0.05-0.3 wt%.

15. The method of claim 14, wherein: In S1-1, the organic polymer is polyether sulfone, and the light-absorbing material is polyaniline.

16. The method of claim 13, wherein: In S1-1, the mass ratio of the lithium chloride, polyethylene glycol and polyvinylpyrrolidone is (1-5):(1-5):(1-5). The mass ratio of the glycerol and isopropyl alcohol is (3-5):(3-5).

17. The method of claim 14, wherein: In S1-1, the mass ratio of the lithium chloride, polyethylene glycol and polyvinylpyrrolidone is (1-5):(1-5):(1-5). The mass ratio of the glycerol and isopropyl alcohol is (3-5):(3-5).

18. Use of a photothermal evaporative ion separation membrane, characterized in that: The photothermal evaporation ion separation film prepared by the method of any one of claims 8-17 is used for lithium extraction, the ion separation layer allows lithium ions to pass through and retains magnesium ions, the photothermal evaporation ion separation film is placed in a lithium-containing solution, the mass transfer layer is hydrophilic, under the capillary force provided by the mass transfer layer, water and lithium ions pass through the ion separation layer to the mass transfer layer, water evaporates in the mass transfer layer, and lithium ions are deposited in the pores or on the surface of the mass transfer layer.

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