An acid-resistant positively charged nanofiltration composite membrane and its preparation method

Abstract

This invention belongs to the field of polymer separation membrane technology for water treatment, specifically relating to an acid-resistant positively charged nanofiltration composite membrane and its preparation method. A polysulfone ultrafiltration membrane is immersed in an aqueous solution containing polyamines and urea compounds, and after standing, excess aqueous phase is removed. Then, it is coated with an oil solution containing polyacrylamide chlorides, and after standing, dried. The resulting membrane is immersed in a coating solution, stood, rinsed, and then dried to obtain the nanofiltration membrane. The total mass concentration of the polyamines and urea compounds is 1%–4%, and the polyamines are one or more of diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and polyethyleneimine, with a mass concentration of 0.9%–3.9%. The urea compounds are one or more of methanesulfonylbicyclourea, 4-chlorophenylurea, 1,3-bis(4-chlorophenyl)thiourea, and benzylthiourea, with a mass concentration of 0.1%–1%. This nanofiltration membrane exhibits excellent acid resistance and salt separation effect, making it suitable for industrial applications, especially lithium extraction from acidic salt solutions, and has high practical value.

Description

Technical Field

[0001] This invention belongs to the field of polymer separation membrane technology for water treatment, specifically relating to an acid-resistant positively charged nanofiltration composite membrane and its preparation method. Background Technology

[0002] With the continuous development of new energy technologies, lithium-ion batteries, as an indispensable part of them, are in increasing demand. Lithium, the cathode material of lithium-ion batteries, cannot be directly obtained from nature and is generally found in mineral lithium mines and brine lithium mines in salt lakes. The commonly used process for extracting lithium carbonate from mineral lithium mines is the sulfuric acid process. This method first involves roasting spodumene with sulfuric acid, adding water, and then leaching to obtain a lithium-containing acid solution. This solution is then neutralized and filtered to remove impurities such as calcium and magnesium from the leachate.

[0003] Nanofiltration membranes are a novel type of separation membrane with pore sizes between reverse osmosis and ultrafiltration membranes. They possess nanoscale pore sizes and multiple charged surfaces, allowing low-molecular-weight salts to pass through while retaining higher-molecular-weight organic matter and multivalent ions, resulting in unique separation performance and higher separation precision. Currently, commercially produced and industrially applied nanofiltration composite membranes are mostly negatively charged membranes, composed of a porous support layer and a surface functional layer. They are prepared through interfacial polymerization between polyamines and polyacrylamide chlorides, significantly reducing selectivity. Furthermore, the desalination functional layer of nanofiltration composite membranes is made of polyamide materials. Since amide bonds hydrolyze under low pH conditions, leading to a severe decline in membrane performance, this limits the application of nanofiltration composite membranes under certain acidic conditions.

[0004] The separation mechanism of nanofiltration membranes mainly includes size sieving and electrostatic repulsion. Ion separation primarily depends on the ion size and the pore size of the nanofiltration membrane. Another separation pathway utilizes the Donnan effect to separate polyvalent and monovalent ions. Divalent ions have a higher charge intensity than monovalent ions, so positively charged nanofiltration membranes exhibit significantly greater repulsion for divalent ions than monovalent ions, which is beneficial for magnesium-lithium separation. Literature reports that polyethyleneimine, as an aqueous monomer, undergoes interfacial polymerization with trimesoyl chloride on the surface of polyethersulfone ultrafiltration membranes to design amino-rich positively charged nanofiltration membranes, improving magnesium-lithium separation efficiency. Patent 106621841A improves divalent / monovalent salt separation by coating a pre-crosslinked polyvinyl alcohol and cationic polyelectrolyte mixed copolymer positively charged coating solution onto a base membrane. Therefore, designing separation materials to achieve a positively charged separation layer is currently the mainstream research direction for magnesium-lithium ion separation.

[0005] Currently, acid-resistant nanofiltration membrane materials mainly include polysulfonamides, triazine rings, sulfonated polymers, and polyelectrolytes. Polysulfonamide nanofiltration membranes are effectively acid-resistant, but without the addition of other monomers, the sulfonyl chloride monomers have the disadvantages of large molecular radii or few active groups, resulting in thicker polymer membranes with larger and less dense pores, leading to lower retention rates and fluxes compared to polyamide membranes. Triazine rings are polymers with triazine rings as the main chain. Using triazine ring-containing monomers (usually cyanuric chloride, CC) to replace traditional acyl chloride monomers in membrane preparation imparts good acid stability. However, CC has low reactivity, resulting in lower membrane retention rates, necessitating the addition of other organic phase monomers or the preparation of amine monomer membranes containing triazine ring structures. Polyelectrolytes are assembled on a supporting membrane using an alternating layer-by-layer self-assembly method of polycations and anions to obtain the separation layer, improving membrane stability through electrostatic interactions. However, due to its complex and time-consuming operation, it is not suitable for large-scale industrial applications. Therefore, existing technologies struggle to achieve both good acid resistance and salt separation performance, requiring further improvement.

[0006] Due to the specific structural requirements of acid-resistant nanofiltration membrane materials in lithium extraction industrial applications, and the need for good long-term operational stability, it is necessary to develop a new nanofiltration composite membrane that can not only improve the membrane's acid resistance and stability but also achieve better salt separation and lithium extraction effects. This is of great practical significance for expanding the industrial application of nanofiltration membrane technology in lithium extraction. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides an acid-resistant positively charged nanofiltration composite membrane and its preparation method. The acid-resistant positively charged nanofiltration composite membrane has good acid resistance and salt separation effect, and its performance can maintain good continuous stability under acidic conditions.

[0008] The primary objective of this invention is to provide a method for preparing an acid-resistant positively charged nanofiltration composite membrane. The method uses a polysulfone ultrafiltration membrane as the base membrane and employs interfacial polymerization to prepare the acid-resistant positively charged nanofiltration membrane.

[0009] The present invention achieves the above objectives through the following technical solutions:

[0010] A method for preparing an acid-resistant positively charged nanofiltration composite membrane includes the following steps:

[0011] S1. Immerse the surface of the polysulfone ultrafiltration membrane in an aqueous solution containing polyamines and urea compounds, and after standing for a certain period of time, remove the excess aqueous solution from the surface of the membrane.

[0012] S2. The surface of the polysulfone ultrafiltration base membrane obtained in step S1 is coated with an oil phase solution containing polyacrylamide chloride. After standing for a certain period of time, the base membrane is dried to obtain the nascent nanofiltration membrane.

[0013] S3. Immerse the nanofiltration membrane obtained in step S2 into a coating solution containing cationic polyelectrolytes. After standing for a certain period of time, rinse the membrane clean and dry it again to obtain an acid-resistant nanofiltration membrane with a positively charged surface.

[0014] In step S1, the total mass concentration of the polyamine and urea compound is 1%-4%; the polyamine is one or more of diethylenetriamine, triethylenetetramine, tetraethylenepentamine and polyethyleneimine, with a mass concentration of 0.9%-3.9%; the urea compound is one or more of methanesulfonylbicyclourea, 4-chlorophenylurea, 1,3-bis(4-chlorophenyl)thiourea and benzylthiourea, with a mass concentration of 0.1%-1%.

[0015] Currently, the amines commonly used in the aqueous phase of products are mostly piperazines. Under low pH conditions, polyamides decompose via an acid-catalyzed pathway as follows: In the protonation step, water molecules approach carbon and form peptide bonds. When the water molecule splits, a proton is released. Subsequently, a hydroxyl group approaches the carbon atom of the amide, while the alkaline intermediate adsorbs the proton, breaking the CN bond. Therefore, most commercial nanofiltration membranes are unsuitable for long-term use due to their sensitivity to strong acids.

[0016] The aqueous phase components used in this invention are complexes. One type is an aliphatic polyamine, which has strong molecular chain motion and a large free volume. In addition, it has a higher diffusion rate and lower reactivity than PIP (piperazine), and the active layer tends to form a thick film, which helps to prepare a membrane with strong acid resistance. The other type is a urea compound, which itself has strong hydrogen bonds. The aliphatic amine also has strong hydrogen bonds, which can effectively improve nucleophilic attack and ensure the acid resistance of the membrane. At the same time, the presence of hydrogen bonds makes the molecular structure of the separation layer more compact and improves the desalination rate.

[0017] Preferably, the settling time in step S1 is 0.5-5 minutes, and the removal is performed by air knife purging with an air knife pressure of 15-50 Psi.

[0018] Preferably, the polyacrylamide chloride monomer mentioned in step S2 is one of isophthaloyl chloride, biphenyltetracarboxylate chloride, pyromellitic tricarboxylate chloride, or phthaloyl chloride.

[0019] Preferably, the oil phase solvent in step S2 is one or more of cyclohexane, n-hexane, ISOPAR E, or ISOPAR G.

[0020] Preferably, the settling time in step S2 is 5-60 seconds.

[0021] Preferably, the drying temperature in step S2 is 40-80℃ and the drying time is 1-5 minutes.

[0022] Coating with cationic polyelectrolyte solutions can effectively reduce the attack of hydrogen ions and improve the acid resistance of the membrane when the membrane is positively charged. At the same time, it makes the nanofiltration membrane surface positively charged. During the separation of ionic salts of different valence states, the membrane's repulsion of divalent cations is significantly higher than that of monovalent cations. Therefore, monovalent cations can more easily permeate the membrane into the permeate, while divalent cations are retained. Consequently, the retention rate of divalent cations is higher than that of monovalent cations, which is beneficial for magnesium-lithium separation and achieves a better effect of salt separation and lithium extraction.

[0023] Preferably, the coating solution in step S3 contains sodium chloride and polydimethylammonium chloride, with the sodium chloride mass concentration being 2%-10% and the polydimethylammonium chloride mass concentration being 0.5%-5%, and the solvent being deionized water.

[0024] Preferably, the resting time in step S3 is 2-10 minutes, the drying temperature is 70-90℃, and the drying time is 1-5 minutes.

[0025] The second objective of this invention is to provide an acid-resistant positively charged nanofiltration composite membrane prepared by the above method.

[0026] The present invention also provides the application of the above-mentioned nanofiltration composite membrane in industrial lithium extraction by salt separation, which is particularly suitable for acidic conditions.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] This invention utilizes a combination of aliphatic polyamines and urea compounds as the aqueous phase. Aliphatic polyamines possess strong molecular chain motion and a large free volume, exhibiting a higher diffusion rate and lower reactivity than PIPs, and the active layer tends to form a thick film, facilitating the preparation of membranes with strong acid resistance. Simultaneously, the urea compounds and aliphatic polyamines possess strong hydrogen bonds, effectively enhancing nucleophilic attack, increasing acid resistance, and improving salt separation efficiency. Furthermore, the use of a monolayer coating with a polycationic electrolyte solution, where the membrane is positively charged, effectively weakens hydrogen ion attack, improving acid resistance, and further enhancing the salt separation effect by giving the nanofiltration membrane a positive charge. The membrane preparation process in this invention can be integrated with existing production lines, allowing for rapid production after simple adjustments. The prepared acid-resistant positively charged nanofiltration composite membrane has significant implications for industrial applications in lithium extraction. Detailed Implementation

[0029] The technical solution of the present invention will be clearly and completely described below with reference to embodiments and comparative examples. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; the materials and reagents used are commercially available unless otherwise specified.

[0031] The polysulfone ultrafiltration membrane used in this invention can be obtained by conventional preparation methods: polysulfone (16wt%) is added to an N,N-dimethylformamide (DMF) solution, stirred and dissolved at 75°C and degassed under vacuum, the dissolved polysulfone is poured onto a nonwoven fabric, and the membrane is scraped using a doctor blade with a gap of 100 micrometers. After scraping, the membrane is immersed in deionized water to solidify the membrane, and the solidified membrane is taken out and stored in deionized water for later use.

[0032] Example 1

[0033] A method for preparing an acid-resistant positively charged nanofiltration composite membrane includes the following steps:

[0034] S1. Immerse the surface of the polysulfone ultrafiltration membrane in an aqueous solution of 1.9wt% diethylenetriamine and 0.1wt% dimethyl sulfonylurea. After standing for 1 minute, use an air knife to blow away the excess aqueous solution on the surface of the polysulfone ultrafiltration membrane. The air knife pressure is 30Psi.

[0035] S2. The surface of the above polysulfone ultrafiltration base membrane is immersed in a hexane solution containing 0.2 wt% trimesoyl chloride. After standing for 15 seconds, the polysulfone ultrafiltration base membrane is placed in a 60°C oven and dried for 1.5 minutes to obtain a nascent nanofiltration membrane, which is then stored in deionized water.

[0036] S3. Immerse the nanofiltration membrane in an aqueous solution containing 5 wt% sodium chloride and 1 wt% polydimethylammonium chloride coating, let it stand for 5 minutes, remove the membrane and rinse it with deionized water, and then put the membrane into an 80°C oven to dry for 2 minutes to obtain an acid-resistant nanofiltration membrane with a positively charged surface.

[0037] Example 2

[0038] A method for preparing an acid-resistant positively charged nanofiltration composite membrane includes the following steps:

[0039] S1. Immerse the surface of the polysulfone ultrafiltration membrane in an aqueous solution of 1.9wt% diethylenetriamine and 0.1wt% dimethyl sulfonylurea. After standing for 1 minute, use an air knife to blow away the excess aqueous solution on the surface of the polysulfone ultrafiltration membrane. The air knife pressure is 30Psi.

[0040] S2. The surface of the above polysulfone ultrafiltration base membrane is immersed in a hexane solution containing 0.2 wt% trimesoyl chloride. After standing for a certain time of 15 seconds, the polysulfone ultrafiltration base membrane is placed in a 60°C oven and dried for 1.5 minutes to obtain the nascent nanofiltration membrane, which is then stored in deionized water.

[0041] S3. Immerse the nanofiltration membrane in an aqueous solution containing 5 wt% sodium chloride and 1.5 wt% polydimethylammonium chloride coating. After standing for 5 minutes, remove the membrane and rinse it with deionized water. Then, put the membrane into an 80°C oven and dry it for 2 minutes to obtain an acid-resistant nanofiltration membrane with a positively charged surface.

[0042] Example 3

[0043] A method for preparing an acid-resistant positively charged nanofiltration composite membrane includes the following steps:

[0044] S1. Immerse the surface of the polysulfone ultrafiltration membrane in an aqueous solution of 2.5wt% diethylenetriamine and 0.1wt% dimethoxyurethane. After standing for 1 minute, use an air knife to blow away the excess aqueous solution on the surface of the polysulfone ultrafiltration membrane. The air knife pressure is 30Psi.

[0045] S2. The surface of the above polysulfone ultrafiltration base membrane is immersed in a hexane solution containing 0.2 wt% trimesoyl chloride. After standing for a certain time of 15 seconds, the polysulfone ultrafiltration base membrane is placed in a 60°C oven and dried for 1.5 minutes to obtain the nascent nanofiltration membrane, which is then stored in deionized water.

[0046] S3. Immerse the nanofiltration membrane in an aqueous solution containing 5 wt% sodium chloride and 1 wt% polydimethylammonium chloride coating. After standing for 5 minutes, remove the membrane and rinse it with deionized water. Then, dry the membrane again in an 80°C oven for 2 minutes. This yields an acid-resistant nanofiltration membrane with a positively charged surface.

[0047] Example 4

[0048] A method for preparing an acid-resistant positively charged nanofiltration composite membrane includes the following steps:

[0049] S1. Immerse the surface of the polysulfone ultrafiltration membrane in an aqueous solution of 1.9 wt% diethylenetriamine and 0.1 wt% 4-chlorophenylurea. After standing for 1 minute, use an air knife to blow away the excess aqueous solution on the surface of the polysulfone ultrafiltration membrane. The air knife pressure is 30 Psi.

[0050] S2. The surface of the above polysulfone ultrafiltration base membrane is immersed in a hexane solution containing 0.2 wt% trimesoyl chloride. After standing for a certain time of 15 seconds, the polysulfone ultrafiltration base membrane is placed in a 60°C oven and dried for 1.5 minutes to obtain the nascent nanofiltration membrane, which is then stored in deionized water.

[0051] S3. Immerse the nanofiltration membrane in an aqueous solution containing 5 wt% sodium chloride and 1 wt% polydimethylammonium chloride coating, let it stand for 5 minutes, remove the membrane and rinse it with deionized water, and then put the membrane into an 80°C oven to dry for 2 minutes to obtain an acid-resistant nanofiltration membrane with a positively charged surface.

[0052] Example 5

[0053] A method for preparing an acid-resistant positively charged nanofiltration composite membrane includes the following steps:

[0054] S1. Immerse the surface of the polysulfone ultrafiltration membrane in an aqueous solution of 1.9 wt% diethylenetriamine and 0.1 wt% 1,3-bis(4-chlorophenyl)thiourea. After standing for 1 minute, use an air knife to blow away the excess aqueous solution on the surface of the polysulfone ultrafiltration membrane. The air knife pressure is 30 Psi.

[0055] S2. The surface of the above polysulfone ultrafiltration base membrane is immersed in a hexane solution containing 0.2 wt% trimesoyl chloride. After standing for a certain time of 15 seconds, the polysulfone ultrafiltration base membrane is placed in a 60°C oven and dried for 1.5 minutes to obtain the nascent nanofiltration membrane, which is then stored in deionized water.

[0056] S3. Immerse the nanofiltration membrane in an aqueous solution containing 5 wt% sodium chloride and 1 wt% polydimethylammonium chloride coating, let it stand for 5 minutes, remove the membrane and rinse it with deionized water, and then put the membrane into an 80°C oven to dry for 2 minutes to obtain an acid-resistant nanofiltration membrane with a positively charged surface.

[0057] Example 6

[0058] A method for preparing an acid-resistant positively charged nanofiltration composite membrane includes the following steps:

[0059] S1. Immerse the surface of the polysulfone ultrafiltration membrane in an aqueous solution of 1.9wt% diethylenetriamine and 0.1wt% dimethyl sulfonylurea. After standing for 1 minute, use an air knife to blow away the excess aqueous solution on the surface of the polysulfone ultrafiltration membrane. The air knife pressure is 30Psi.

[0060] S2. The surface of the above polysulfone ultrafiltration base membrane is immersed in a hexane solution containing 0.2 wt% isophthaloyl chloride. After standing for a certain time of 15 seconds, the polysulfone ultrafiltration base membrane is placed in a 60°C oven and dried for 1.5 minutes to obtain the nascent nanofiltration membrane, which is then stored in deionized water.

[0061] S3. Immerse the nanofiltration membrane in an aqueous solution containing 5 wt% sodium chloride and 1 wt% polydimethylammonium chloride coating, let it stand for 5 minutes, remove the membrane and rinse it with deionized water, and then put the membrane into an 80°C oven to dry for 2 minutes to obtain an acid-resistant nanofiltration membrane with a positively charged surface.

[0062] Comparative Example 1

[0063] S1. Immerse the surface of the polysulfone ultrafiltration membrane with a 2wt% piperazine aqueous solution, let it stand for 1 minute, and then use an air knife to blow away the excess aqueous solution on the surface of the polysulfone ultrafiltration membrane. The air knife pressure is 30Psi.

[0064] S2. The surface of the above-mentioned polysulfone ultrafiltration base membrane is immersed in a hexane solution containing 0.2 wt% trimesoyl chloride. After standing for a certain period of time (15 seconds), the polysulfone ultrafiltration base membrane is placed in a 60°C oven and dried for 1.5 minutes to obtain the nascent nanofiltration membrane, which is then stored in deionized water.

[0065] Comparative Example 2

[0066] S1. Immerse the surface of the polysulfone ultrafiltration membrane in an aqueous solution of 1.9wt% diethylenetriamine and 0.1wt% dimethyl sulfonylurea. After standing for 1 minute, use an air knife to blow away the excess aqueous solution on the surface of the polysulfone ultrafiltration membrane. The air knife pressure is 30Psi.

[0067] S2. The surface of the above-mentioned polysulfone ultrafiltration base membrane is immersed in a hexane solution containing 0.2 wt% trimesoyl chloride. After standing for a certain period of time (15 seconds), the polysulfone ultrafiltration base membrane is placed in a 60°C oven and dried for 1.5 minutes to obtain the nascent nanofiltration membrane, which is then stored in deionized water.

[0068] Comparative Example 3

[0069] S1. Immerse the surface of the polysulfone ultrafiltration membrane in a 1.5% aqueous solution of 3-aminobenzenesulfonamide. After standing for 1 minute, use an air knife to blow away the excess aqueous solution on the surface of the polysulfone ultrafiltration membrane. The air knife pressure is 30Psi.

[0070] S2. The surface of the above-mentioned polysulfone ultrafiltration base membrane is immersed in a hexane solution containing 0.2 wt% trimesoyl chloride. After standing for a certain period of time (15 seconds), the polysulfone ultrafiltration base membrane is placed in a 60°C oven and dried for 1.5 minutes to obtain the nascent nanofiltration membrane, which is then stored in deionized water.

[0071] Comparative Example 4

[0072] S1. Immerse the surface of the polysulfone ultrafiltration membrane in a 1% aqueous solution of 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane. After standing for 1 minute, use an air knife to blow away the excess aqueous solution on the surface of the polysulfone ultrafiltration membrane. The air knife pressure is 30Psi.

[0073] S2. The surface of the above-mentioned polysulfone ultrafiltration base membrane is immersed in a hexane solution containing 0.2 wt% trimesoyl chloride. After standing for a certain period of time (15 seconds), the polysulfone ultrafiltration base membrane is placed in a 60°C oven and dried for 1.5 minutes to obtain the nascent nanofiltration membrane, which is then stored in deionized water.

[0074] Comparative Example 5

[0075] S1. Immerse the surface of the polysulfone ultrafiltration membrane in a 1.5% triethylenetetramine aqueous solution, let it stand for 1 minute, and then use an air knife to blow away the excess aqueous solution on the surface of the polysulfone ultrafiltration membrane. The air knife pressure is 30Psi.

[0076] S2. The surface of the polysulfone ultrafiltration base membrane is immersed in a hexane solution containing 0.2 wt% toluene-2,6 diisocyanate. After standing for a certain time of 15 seconds, the polysulfone ultrafiltration base membrane is placed in a 60°C oven and dried for 1.5 minutes to obtain the nascent nanofiltration membrane, which is then stored in deionized water.

[0077] Comparative Example 6

[0078] S1. Immerse the surface of the polysulfone ultrafiltration membrane in a 1.5% triethylenetetramine aqueous solution, let it stand for 1 minute, and then use an air knife to blow away the excess aqueous solution on the surface of the polysulfone ultrafiltration membrane. The air knife pressure is 30Psi.

[0079] S2. The surface of the polysulfone ultrafiltration base membrane is immersed in a hexane solution containing 0.2 wt% toluene-2,6 diisocyanate. After standing for a certain time of 15 seconds, the polysulfone ultrafiltration base membrane is placed in a 60°C oven and dried for 1.5 minutes to obtain the nascent nanofiltration membrane, which is then stored in deionized water.

[0080] S3. Immerse the nanofiltration membrane in an aqueous solution containing 5 wt% sodium chloride and 1 wt% polydimethylammonium chloride coating. After standing for 5 minutes, remove the membrane and rinse it with deionized water. Then, dry the membrane again in an 80°C oven for 2 minutes to obtain the nascent nanofiltration membrane, which is then stored in deionized water.

[0081] Comparative Example 7

[0082] A method for preparing an acid-resistant positively charged nanofiltration composite membrane includes the following steps:

[0083] S1. Immerse the surface of the polysulfone ultrafiltration membrane in a 2.0 wt% diethylenetriamine aqueous solution, let it stand for 1 minute, and then use an air knife to blow away the excess aqueous solution on the surface of the polysulfone ultrafiltration membrane. The air knife pressure is 30 Psi.

[0084] S2. The surface of the above-mentioned polysulfone ultrafiltration base membrane is immersed in a hexane solution containing 0.2 wt% trimesoyl chloride. After standing for 15 seconds, the polysulfone ultrafiltration base membrane is placed in a 60°C oven and dried for 1.5 minutes to obtain a nascent nanofiltration membrane, which is then stored in deionized water.

[0085] Performance testing and analysis

[0086] Evaluation of the acid resistance of nanofiltration membranes: The membranes prepared in the embodiments and comparative examples of this invention were immersed in 10% sulfuric acid solution at room temperature, and then rinsed with pure water before testing.

[0087] Evaluation of salt separation performance of nanofiltration membrane: The membrane was evaluated by cross-flow filtration. 2000 ppm magnesium sulfate solution and 2000 ppm sodium chloride solution were used as test solutions, with a pH of 7-8. The water flux and salt rejection rate of the membrane were tested at 100 Psi pressure.

[0088] Salt rejection rate R: Under certain test conditions, the salt concentration (C) in the influent. f ) and salt concentration in the product water (C p Divide the difference by the influent salt concentration:

[0089] R = (1-C) p / C f )*100%

[0090] Water flux: Under certain test conditions, the amount of water produced per unit membrane area per unit time (GFD).

[0091] The test results of the membranes prepared in the various embodiments and comparative examples of the present invention are shown in Table 1.

[0092] The acid resistance stability test results of the nanofiltration membranes prepared in the embodiments of the present invention are shown in Table 2.

[0093] Table 1

[0094]

[0095]

[0096] As shown in Table 1, the nanofiltration membrane prepared in Comparative Example 1 using piperazine as the aqueous phase suffered severe structural damage and a significant decrease in rejection rate after immersion in acidic conditions. Comparative Example 2, using diethylenetriamine and dicyclourea methanesulfonate as the aqueous phase, showed significantly better acid resistance and salt separation performance than Comparative Example 1 after acid-resistant immersion. However, Comparative Example 2 was not coated with a polycationic electrolyte. Compared to Comparative Example 2, the membrane coated with polycationic electrolyte in Example 1 showed a certain improvement in the rejection rate of magnesium sulfate, while the rejection rate of sodium chloride decreased significantly. This indicates that the coating of the polyoxygen electrolyte layer further improved the separation effect of monovalent and divalent cations. Comparative Examples 3, 4, and 5 used different types of monomers to prepare membranes, which showed acceptable acid resistance, but generally increased rejection rates for monovalent and divalent salts, resulting in unsatisfactory salt separation performance. Compared to Comparative Example 5, Comparative Example 6, although showing some improvement in salt separation performance after coating with a polycationic electrolyte, still fell far short of practical application requirements, exhibiting poor salt separation performance. In Comparative Example 7, the nanofiltration membrane prepared without the addition of urea compounds to the aqueous phase showed poor separation performance for monovalent / divalent salts, indicating that the introduction of urea compounds plays a crucial role in salt separation. The rational combination of aqueous and oil phase components in this invention resulted in a positively charged nanofiltration membrane that not only exhibits good acid resistance but also achieves excellent salt separation performance. As shown in Table 2, after one month of continuous acid resistance testing, the performance of the positively charged nanofiltration membrane prepared in this invention showed no significant change, demonstrating good long-term operational stability.

[0097] Table 2

[0098]

[0099] Obviously, the above embodiments of the present invention are merely examples to clearly illustrate the technical solution of the present invention, and are not intended to limit the specific implementation of the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the claims of the present invention should be included within the protection scope of the claims of the present invention.

Claims

1. A method for preparing an acid-resistant positively charged nanofiltration composite membrane, characterized in that, Includes the following steps: S1. Immerse the surface of the polysulfone ultrafiltration membrane in an aqueous solution containing polyamines and urea compounds, and after standing, remove the excess aqueous solution from the surface of the membrane. S2. The surface of the polysulfone ultrafiltration base membrane obtained in step S1 is dipped into an oil phase solution containing polyacrylamide chlorides. After standing, the base membrane is dried to obtain the nascent nanofiltration membrane. S3. Immerse the nanofiltration membrane obtained in step S2 into a coating solution containing cationic polyelectrolytes, let it stand, rinse the membrane, and dry it again to obtain the acid-resistant positively charged nanofiltration composite membrane. In step S1, the total mass concentration of the polyamine and urea compounds is 1%-4%; the polyamine is one or more of diethylenetriamine, triethylenetetramine, tetraethylenepentamine, and polyethyleneimine, with a mass concentration of 0.9%-3.9%; the urea compounds are one or more of methanesulfonylbicyclourea, 4-chlorophenylurea, 1,3-bis(4-chlorophenyl)thiourea, and benzylthiourea, with a mass concentration of 0.1%-1%. The monomer of the polyacrylamide chloride mentioned in step S2 is one of isophthaloyl chloride, biphenyltetracarboxylic chloride, pyromellitic tricarboxylic chloride or phthaloyl chloride.

2. The method for preparing an acid-resistant positively charged nanofiltration composite membrane according to claim 1, characterized in that, The settling time in step S1 is 0.5-5 minutes, and the removal is carried out by air knife blowing with an air knife pressure of 15-50 Psi.

3. The method for preparing an acid-resistant positively charged nanofiltration composite membrane according to claim 1, characterized in that, The oil phase solvent in step S2 is one or more of cyclohexane, n-hexane, ISOPAR E, or ISOPAR G.

4. The method for preparing an acid-resistant positively charged nanofiltration composite membrane according to claim 1, characterized in that, The settling time mentioned in step S2 is 5-60 seconds.

5. The method for preparing an acid-resistant positively charged nanofiltration composite membrane according to claim 1, characterized in that, The drying temperature in step S2 is 40-80℃, and the drying time is 1-5 minutes.

6. The method for preparing an acid-resistant positively charged nanofiltration composite membrane according to claim 1, characterized in that, The coating solution in step S3 contains sodium chloride and polydienedimethylammonium chloride, with a sodium chloride mass concentration of 2%-10% and a polydienedimethylammonium chloride mass concentration of 0.5%-5%, and the solvent is deionized water.

7. The method for preparing an acid-resistant positively charged nanofiltration composite membrane according to claim 1, characterized in that, The settling time in step S3 is 2-10 minutes, the drying temperature is 70-90℃, and the drying time is 1-5 minutes.

8. An acid-resistant positively charged nanofiltration composite membrane, characterized in that, Prepared by the method described in any one of claims 1-7.

9. The application of the acid-resistant positively charged nanofiltration composite membrane according to claim 8 in the separation and extraction of lithium under acidic conditions.

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