A two-dimensional clay mineral lithium extraction film for enhanced lithium-magnesium separation and its preparation method

Abstract

This invention belongs to the field of membrane separation technology and lithium extraction technology. Specifically, it relates to a two-dimensional clay mineral lithium extraction membrane for enhanced lithium-magnesium separation. The membrane is obtained by cross-linking and modifying a two-dimensional clay mineral nanosheet suspension with a strongly positively charged functional organic compound to form a strongly positively modified two-dimensional clay nanosheet functional layer. A sulfonated polymer is then introduced for cross-linking. The strongly positively charged functional organic compound contains two or more positively charged groups, which are modified and connected to the nanosheet end faces and surfaces through hydrogen bonding, focusing on strengthening the positively charged environment at the nanosheet end faces. This invention can enhance the lithium-magnesium ion separation effect and the structural strength of the membrane, thereby improving the stability of the membrane in brine environments and showing great application potential.

Description

Technical Field

[0001] This invention belongs to the field of membrane separation technology and lithium extraction technology, specifically relating to a two-dimensional clay mineral lithium extraction film with enhanced lithium-magnesium separation and film stability, and its preparation method. Background Technology

[0002] Salt lake brine is the main form of lithium occurrence, but the development and utilization of salt lake resources in my country has been at a low level, failing to fully realize their due value. Membrane separation, due to its green and environmentally friendly characteristics, low cost, and high selectivity, has become the mainstream process for lithium extraction from salt lakes.

[0003] Two-dimensional channel films are a novel type of separation film with layered regularity and nano / angstrom-level channels, and have wide applications in the field of ion separation. However, two-dimensional materials generally have negatively charged surfaces, and the assembled films exhibit negative charge characteristics, which is not conducive to the retention of magnesium ions. At present, most positive charge modification of two-dimensional films is carried out on the surface, such as manufacturing positively charged MXene (P-MXene) nanosheets from negatively charged (N-MXene) surfaces, and selecting polydimethylammonium diene (PDDA) to change the surface charge type of MXene through electrostatic interaction (Angew Chem Int Ed Engl, 2020, 59(22): 8720-8726.). However, existing film technology lacks modification of the end face direction. Moreover, there have been reports that when clay materials are placed in electrolytes, the intrinsic cations in the interlayer (usually K and Mg²⁺) + The magnesium ions can be replaced by other ions in the electrolyte (Nat. Mater. 20, 1677–1682 (2021).), so the problem of poor membrane stability or deterioration of separation performance still exists due to high magnesium ion concentrations in salt lake brine or different brine environments (different lithium-magnesium ratios).

[0004] Therefore, there is a need for a method to prepare two-dimensional clay mineral lithium extraction films that enhance lithium-magnesium separation and film stability. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a two-dimensional clay mineral lithium extraction film that enhances lithium-magnesium separation, which not only enhances lithium-magnesium separation, but also improves stability in brine environment, and achieves efficient retention of magnesium ions and highly selective separation of lithium-magnesium ions.

[0006] The technical solution adopted by the present invention to solve the above-mentioned problems is as follows: A two-dimensional clay mineral lithium extraction film for enhancing lithium-magnesium separation is obtained by cross-linking and modifying a two-dimensional clay mineral nanosheet suspension with a strongly positively charged functional organic compound to form a strongly positively modified two-dimensional clay nanosheet functional layer, and then simultaneously introducing a sulfonated polymer for cross-linking reaction.

[0007] According to the above scheme, the end face of the two-dimensional clay mineral nanosheet contains aluminum hydroxyl groups, and the surface of the sheet contains silicon-oxygen bonds; the average sheet diameter of the two-dimensional clay mineral nanosheet is 100-1500 nm, and the average sheet thickness is 1-15 nm.

[0008] According to the above scheme, the two-dimensional clay mineral is one or more of montmorillonite, vermiculite, soapstone, illite, etc., mixed in any proportion.

[0009] According to the above scheme, the strongly positively charged functional organic compound contains two or more positively charged groups, and the positively charged groups can form hydrogen bonds with the aluminum hydroxyl groups on the end face of the nanosheet and the silicon-oxygen bonds on the surface of the nanosheet. Preferably, in the strongly positively charged functional organic compound, the positively charged groups that can form hydrogen bonds with the aluminum hydroxyl groups on the end face of the nanosheet and the silicon-oxygen bonds on the surface of the nanosheet are one or more of amino, guanidine, quaternary ammonium, etc.

[0010] Furthermore, when the positively charged functional organic compound includes multiple amino groups, the positively charged functional organic compound is preferably one of polylysine, polyetheramine, decanediamine, polydopamine, etc.

[0011] Furthermore, when the positively charged groups in the strongly positively charged functional organic compound include amino and guanidine groups, the strongly positively charged functional organic compound is preferably one of arginine, polyhexamethylene guanidine, etc.

[0012] Furthermore, when the positively charged functional organic compound includes multiple quaternary ammonium groups, the positively charged functional organic compound is preferably one of polydiallyldimethylammonium chloride, quaternized polysiloxane, etc.

[0013] According to the above scheme, the sulfonated polymer is a functional polymer containing sulfonic acid groups, and can be at least one of polyvinyl sulfonate, polystyrene sulfonate, sulfonated polyimide, lignin sulfonate, etc.

[0014] Based on the above, the present invention also provides a method for preparing the above-mentioned enhanced lithium-magnesium separation two-dimensional clay mineral lithium extraction film, specifically including the following steps: (1) Two-dimensional clay mineral nanosheets and strongly positively charged functional organic matter (as crosslinking agent) were added to water and stirred for a period of time to obtain a strongly positively modified two-dimensional clay mineral nanosheet casting suspension. (2) Cast the suspension obtained in step (1) onto the base film to obtain the film precursor, namely the strongly positively modified two-dimensional clay nanosheet functional layer. (3) The sulfonated polymer is uniformly coated on the surface of the film precursor in step (2) for crosslinking and dried to form a film, thereby obtaining the two-dimensional clay lithium extraction film with enhanced lithium-magnesium separation.

[0015] Preferably, in step (1), the concentration of two-dimensional clay mineral nanosheets in the suspension is 0.5-4 wt%.

[0016] Preferably, in step (1), the amount of strongly positively charged functional organic material used is 0.5%-50% of the mass of the two-dimensional clay mineral nanosheets.

[0017] Preferably, in step (1), room temperature stirring conditions are used, and the reaction time is 0.5-12 h.

[0018] Preferably, in step (2), the base film is made of one of the following materials: polyamide film, polycarbonate, polytetrafluoroethylene, polyethersulfone, polypropylene, etc., and the pore size of the base film is generally in the range of 0.1-5 micrometers.

[0019] Preferably, the casting in step (2) and the coating in step (3) can both be done by one of the following methods: scraping, spin coating, spraying, vacuum filtration, etc.

[0020] Preferably, in step (3), the sulfonated polymer is dispersed in a solvent to prepare a solution for coating, with a concentration of 1-15wt%, the amount of sulfonated polymer is 5wt%-10wt% of the two-dimensional clay mineral nanosheets, and the crosslinking time is 10-60 min.

[0021] Preferably, in step (3), the drying temperature is 10-120℃ and the drying time is 0.5-24 h.

[0022] Application of the two-dimensional clay mineral lithium extraction film prepared by the above method in lithium extraction from brine.

[0023] This invention solves the problems of high-precision separation of lithium and magnesium ions in brine and poor film stability by enhancing the positive charge environment of the thin film to amplify the charge repulsion difference between lithium and magnesium ions. The working principle of this invention is as follows: Two-dimensional clay mineral nanosheets are first cross-linked using strongly positively charged functional organic materials. This cross-linking is not limited to charge control along the two-dimensional channel direction but also focuses on modification along the normal direction. When the strongly positively charged organic materials undergo hydrogen bonding with the aluminum hydroxyl groups on the nanosheet end faces, the positive charge environment at the end faces is enhanced. Based on the difference in charge between lithium and magnesium ions, the electrostatic forces they experience differ, resulting in a stronger repulsion force on magnesium ions. Magnesium ions are intercepted at the end face entrance between the nanosheets, improving the magnesium ion retention rate and thus achieving efficient magnesium ion retention and highly selective separation of lithium and magnesium ions. Secondly, by utilizing the cation exchange capacity of clay minerals, strongly positively charged organic materials are introduced and intercalated to replace metal ions between the clay mineral layers, thus modifying the nanosheets. This controls interlayer stability while also providing a positive repulsion effect. The silicon-oxygen bonds between the strongly positively charged organic materials and the nanosheets are connected through hydrogen bonds, preventing the replacement of the magnesium ion-intercalated positively charged organic materials. This enhances the constraint on the internal channels of the membrane and the overall structural strength, thereby improving the membrane's stability. Furthermore, due to the complexity and diversity of the brine environment, other pollutants can also affect the membrane's lifespan. By grafting surface-crosslinked sulfonated polymers with strongly positively charged organic materials, the modified nanosheets, now positively charged, can also bind to the negatively charged sulfonic acid groups. The strong hydrophilicity of the sulfonated polymer forms a hydration layer on the membrane surface, reducing pollutant adsorption and extending the membrane's lifespan.

[0024] Compared with the prior art, the beneficial effects of the present invention include: First, the present invention can enhance the positive charge environment at the inlet direction of the end face between two-dimensional clay mineral nanosheets, while increasing the strength and stability of the thin film structure, thereby achieving stable and enhanced separation of lithium and magnesium from salt lake brine.

[0025] Second, this invention introduces a strongly positively charged functional organic compound that undergoes hydrogen bonding with the aluminum hydroxyl groups on the end face of two-dimensional clay mineral nanosheets to create a strongly positively charged environment on the end face. By utilizing the different charge repulsion forces experienced by lithium and magnesium ions, magnesium ions are efficiently retained under the drive of the repulsion force, and lithium ions are successfully enriched, thereby achieving enhanced high-selectivity separation of lithium and magnesium ions.

[0026] Third, this invention utilizes the cation exchange capacity of clay minerals to replace metal ions between clay mineral layers with strongly positively charged functional organic matter, thereby enhancing the positively charged environment between layers. Furthermore, it modifies and connects with the silicon-oxygen bonds on the surface of nanosheets through hydrogen bonding, thereby strengthening the structural strength of the thin film and preventing the organic matter from being replaced when the external ion concentration is too high, thus improving the stability of the thin film in a brine environment.

[0027] Fourth, this invention introduces sulfonated polymers and strongly positively charged organic materials for grafting, and the sulfonic acid groups are negatively charged, which can coat the surface of the modified positively charged two-dimensional clay film. The strong hydrophilicity of the sulfonated polymers causes a hydration layer to form on the film surface, reducing pollutant adsorption and extending the service life of the film. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the structure of the two-dimensional montmorillonite lithium extraction film prepared in Example 1.

[0029] Figure 2 This is a schematic diagram illustrating the modification principle of two-dimensional montmorillonite nanosheets in terms of channel direction and normal direction.

[0030] Figure 3 Magnesium ion rejection rate of the two-dimensional clay lithium extraction film prepared in Example 1 during long-term testing.

[0031] Figure 4 A comparison chart showing the magnesium and lithium ion rejection rates of the two-dimensional clay lithium extraction films prepared in each embodiment. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0033] Example 1 A two-dimensional clay lithium extraction film for enhancing lithium-magnesium separation is prepared by the following steps: (1) Two-dimensional montmorillonite nanosheets with an average sheet diameter of 100 nm and an average sheet thickness of 1 nm were reacted with polydopamine at 15% of the amount of nanosheets in water and stirred for 6 h to obtain a modified two-dimensional montmorillonite nanosheet casting suspension, wherein the concentration of the two-dimensional montmorillonite nanosheets was 4 wt%; (2) The modified two-dimensional montmorillonite nanosheet casting suspension was formed on a polytetrafluoroethylene membrane (pore size of 0.22 micrometers) by vacuum filtration. Then, a 1% sodium polystyrene sulfonate solution was sprayed onto the surface of the film for 60 min, wherein the sodium polystyrene sulfonate accounted for 10% of the amount of nanosheets. After drying at 10°C for 24 h, the two-dimensional montmorillonite lithium extraction film with enhanced lithium-magnesium separation was obtained.

[0034] The two-dimensional montmorillonite lithium extraction membrane prepared in this embodiment was tested. It was installed as a filter membrane in an ion separation device, and the content of each ion after passing through the membrane from the original brine of the salt lake was measured. The magnesium-to-lithium ratio of the original brine of the salt lake was greater than 10:1. Samples were taken at equal time intervals during operation, and the content of each ion after passing through the membrane from the original brine of the salt lake was measured using atomic absorption spectrometry. The rejection rate and transmittance were calculated. Figure 3 and Figure 4 As shown, the magnesium ion rejection rate can reach 100%, the lithium ion permeability is 99%, and the film still has good performance after 240 h of operation.

[0035] Example 2 A two-dimensional clay lithium extraction film for enhancing lithium-magnesium separation is prepared by the following steps: (1) Two-dimensional vermiculite nanosheets with an average sheet diameter of 500 nm and an average sheet thickness of 7 nm were reacted with polydiallyldimethylammonium chloride (30% of the nanosheets) in water and stirred for 8 h to obtain a modified two-dimensional vermiculite nanosheet casting suspension, wherein the concentration of the two-dimensional vermiculite nanosheets was 0.5 wt%. (2) The modified two-dimensional vermiculite nanosheet casting suspension was formed on a polyethersulfone base membrane (pore size of 1.2 micrometers) by a scraping method. Then, a 15% sodium polyvinyl sulfonate solution was crosslinked on the surface of the film by a scraping method for 10 min. The sodium polyvinyl sulfonate accounted for 5% of the amount of nanosheets. After drying at 60°C for 10 h, a two-dimensional vermiculite lithium extraction film with enhanced lithium-magnesium separation was obtained.

[0036] The two-dimensional vermiculite lithium extraction film material prepared in this embodiment was tested in accordance with Example 1. The results showed that the magnesium ion rejection rate could reach 93%, the lithium ion permeability was 95%, and the film still had good performance after 220 h of operation.

[0037] Example 3 A two-dimensional clay lithium extraction film for enhancing lithium-magnesium separation is prepared by the following steps: (1) Montmorillonite and vermiculite mixed nanosheets with an average sheet diameter of 1500 nm and an average sheet thickness of 15 nm were reacted with polyhexamethylene guanidine (50% of the nanosheets) in water and stirred for 12 h to obtain a modified two-dimensional montmorillonite and vermiculite mixed nanosheet casting suspension, wherein the concentration of the two-dimensional montmorillonite and vermiculite mixed nanosheets was 2.5 wt%. (2) The modified two-dimensional montmorillonite and vermiculite mixture nanosheet casting suspension was formed on a polypropylene base film (pore size of 5 micrometers) by spraying. Then, an 8% sulfonated polyimide solution was crosslinked on the surface of the film by scraping for 30 min, wherein the sulfonated polyimide accounted for 8% of the amount of nanosheets. After drying at 120℃ for 0.5 h, a two-dimensional montmorillonite and vermiculite mixture lithium extraction film with enhanced lithium-magnesium separation was obtained.

[0038] The two-dimensional lithium extraction thin film material prepared by the mixture of montmorillonite and vermiculite in this embodiment was tested in accordance with Example 1. The results showed that the magnesium ion rejection rate could reach 95%, the lithium ion permeability was 98%, and the film still had good performance after 264 h of operation.

[0039] Comparative Example 1 (1) A modified two-dimensional hydrotalcite nanosheet with an average sheet diameter of 80 nm and an average sheet thickness of 5 nm was reacted with polydopamine at 15% of the amount of nanosheet in water for 6 h to obtain a casting suspension of the modified two-dimensional hydrotalcite nanosheet, wherein the concentration of the two-dimensional hydrotalcite nanosheet was 4 wt%. (2) The modified hydrotalcite nanosheet casting suspension was deposited on a polytetrafluoroethylene membrane (pore size 0.22 μm) by vacuum filtration. Then, a 1% sodium polystyrene sulfonate solution was sprayed onto the membrane surface for 60 min for crosslinking. The sodium polystyrene sulfonate accounted for 5% of the amount of nanosheets. After drying at 10 °C for 24 h, a two-dimensional hydrotalcite lithium extraction membrane with enhanced lithium-magnesium separation was obtained. The magnesium ion rejection rate of this two-dimensional hydrotalcite lithium extraction membrane was only 63%, and the lithium ion permeability was 75%. The membrane performance decreased significantly after 48 h of operation.

[0040] Comparative Example 2 (1) Two-dimensional montmorillonite nanosheets with an average sheet diameter of 100 nm and an average sheet thickness of 1 nm were reacted with glutaraldehyde at 15% of the nanosheet amount in water for 6 h to obtain a modified two-dimensional montmorillonite nanosheet casting suspension, wherein the concentration of the two-dimensional montmorillonite nanosheets was 4 wt%. (2) The modified two-dimensional montmorillonite nanosheet casting suspension was deposited on a polytetrafluoroethylene membrane (pore size 0.22 μm) by vacuum filtration. Then, 1% sodium polystyrene sulfonate was sprayed onto the membrane surface for 60 min for crosslinking, where sodium polystyrene sulfonate accounted for 5% of the amount of nanosheets. After drying at 10 °C for 24 h, a two-dimensional montmorillonite lithium extraction membrane with enhanced lithium-magnesium separation was obtained. The magnesium ion rejection rate of this two-dimensional montmorillonite lithium extraction membrane reached only 50%, and the lithium ion permeability was 70%. After 80 h of operation, the membrane performance decreased significantly.

[0041] In summary, the two-dimensional clay mineral lithium extraction film for enhanced lithium-magnesium separation provided by this invention constructs a stable strongly positively charged environment on the end face of the two-dimensional clay mineral nanosheets by introducing strongly positively charged functional groups. Utilizing the cation exchange properties of clay minerals, strongly positively charged organic materials are inserted into the interlayer of the nanosheets and connected to the silicon-oxygen bonds on the nanosheet surface via hydrogen bonding. This not only enhances lithium-magnesium separation but also improves the stability of the film, thereby achieving efficient magnesium ion retention and highly selective separation of lithium and magnesium ions. Furthermore, considering the complexity and diversity of brine environments, and the potential impact of other pollutants on the film's lifespan, this invention also incorporates surface-crosslinked sulfonated polymers grafted with strongly positively charged organic materials. The modified nanosheets, now positively charged, can also bind to the negatively charged sulfonic acid groups. The strong hydrophilicity of the sulfonated polymer forms a hydration layer on the film surface, reducing pollutant adsorption and extending the film's lifespan.

[0042] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the inventive concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A two-dimensional clay mineral lithium extraction film for enhanced lithium-magnesium separation, characterized in that, A strongly positively modified two-dimensional clay nanosheet functional layer was obtained by cross-linking and modifying a two-dimensional clay mineral nanosheet suspension with a strongly positively charged functional organic compound, and then a sulfonated polymer was introduced for cross-linking reaction.

2. The two-dimensional clay mineral lithium extraction film for enhanced lithium-magnesium separation according to claim 1, characterized in that, The average diameter of the two-dimensional clay mineral nanosheets is 100-1500 nm, and the average thickness of the sheets is 1-15 nm. The two-dimensional clay mineral is one or more of montmorillonite, vermiculite, soapstone, and illite in any proportion.

3. The two-dimensional clay mineral lithium extraction film for enhanced lithium-magnesium separation according to claim 1, characterized in that, The strongly positively charged functional organic compound contains two or more positively charged groups, and the positively charged groups can form hydrogen bonds with the aluminum hydroxyl groups on the end face of the nanosheet and the silicon-oxygen bonds on the surface of the sheet; the sulfonated polymer is a functional polymer containing sulfonic acid groups.

4. The two-dimensional clay mineral lithium extraction film for enhanced lithium-magnesium separation according to claim 3, characterized in that, In the strongly positively charged functional organic compound, the positively charged groups that can form hydrogen bonds with the aluminum hydroxyl groups on the end face of the nanosheet and the silicon-oxygen bonds on the surface of the nanosheet are one or more of amino, guanidine, and quaternary ammonium groups.

5. A two-dimensional clay mineral lithium extraction film for enhanced lithium-magnesium separation according to claim 3 or 4, characterized in that, When the positively charged functional organic compound contains multiple amino groups, the strong positively charged functional organic compound is one of polylysine, polyetheramine, decanediamine, and polydopamine; when the positively charged functional organic compound contains amino and guanidine groups, the strong positively charged functional organic compound is one of arginine and polyhexamethylene guanidine; when the positively charged functional organic compound contains multiple quaternary ammonium groups, the strong positively charged functional organic compound is one of polydiallyldimethylammonium chloride and quaternized polysiloxane. The sulfonated polymer is selected from at least one of polyvinyl sulfonate, polystyrene sulfonate, sulfonated polyimide, and lignin sulfonate.

6. The method for preparing the two-dimensional clay mineral lithium extraction film for enhanced lithium-magnesium separation as described in claim 1, characterized in that, Includes the following steps: (1) Two-dimensional clay mineral nanosheets and strongly positively charged functional organic matter were added to water and stirred for a period of time to obtain a strongly positively modified two-dimensional clay mineral nanosheet casting suspension. (2) Cast the suspension obtained in step (1) onto the base membrane to obtain the film precursor; (3) The sulfonated polymer is uniformly coated on the surface of the film precursor in step (2) for crosslinking and dried to form a film, thereby obtaining the two-dimensional clay lithium extraction film with enhanced lithium-magnesium separation.

7. The method for preparing a two-dimensional clay mineral lithium extraction film with enhanced lithium-magnesium separation according to claim 6, characterized in that, In step (1), the concentration of two-dimensional clay mineral nanosheets in the suspension is 0.5-4 wt%; the amount of strongly positively charged functional organic matter is 0.5%-50% of the mass of the two-dimensional clay mineral nanosheets; and the stirring reaction time is 0.5-12 h.

8. The method for preparing a two-dimensional clay mineral lithium extraction film for enhanced lithium-magnesium separation according to claim 6, characterized in that, In step (2), the base film is made of one of the following materials: polyamide film, polycarbonate, polyvinyl fluoride, polyethersulfone, and polypropylene; the casting in step (2) and the coating in step (3) are both done by one of the following methods: scraping, spin coating, spraying, and vacuum filtration.

9. The method for preparing a two-dimensional clay mineral lithium extraction film with enhanced lithium-magnesium separation according to claim 6, characterized in that, In step (3), the sulfonated polymer is prepared as a solution for coating, with a concentration of 1-15wt%; the amount of sulfonated polymer used is 5wt%-10wt% of the two-dimensional clay mineral nanosheets, and the crosslinking time is 10-60 min.

10. The application of the two-dimensional clay mineral lithium extraction film according to claim 1 in lithium extraction from brine.

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