Strong-alkali-resistant composite nanofiltration membrane as well as preparation method and application thereof

Abstract

The invention discloses a strong-alkali-resistant composite nanofiltration membrane as well as a preparation method and application thereof. The strong-alkali-resistant composite nanofiltration membrane comprises a porous support layer and a compact separation layer which are sequentially stacked, wherein the dense separation layer is formed on the surface of the porous supporting layer through interfacial polymerization of a water-phase monomer and an oil-phase monomer, the water-phase monomer comprises an amine monomer, the oil-phase monomer comprises a benzyl bromide monomer, and the benzyl bromide monomer has a plurality of benzyl bromide groups. The strong-alkali-resistant composite nanofiltration membrane is soaked in a 3-5 mol / L NaOH solution for a long time, so that the structural performance is stable, and the retention rate of the strong-alkali-resistant composite nanofiltration membrane on lignin is only 10%-15% while the retention rate of the strong-alkali-resistant composite nanofiltration membrane on NaOH is 95% or above; meanwhile, the composite nanofiltration membrane disclosed by the invention has excellent strong alkali resistance, not only shows excellent separation selectivity to lignin / alkali liquor, but also has great application potential in the aspects of separation of other substances and purification and recovery of alkali liquor in a strong alkali environment.

Description

Technical Field

[0001] The present invention relates to a strong alkali-resistant composite nanofiltration membrane and a preparation method and application thereof, and in particular to a strong alkali-resistant composite nanofiltration membrane and a preparation method thereof and application thereof in purifying and recovering alkali liquor in an alkaline lignin extraction process, belonging to the field of materials and water treatment technology. Background Art

[0002] Lignin, a natural fiber material, is widely found in woody plants. It is the second largest bioresource after cellulose and plays a vital role in industrial production, food processing, and pharmaceutical processing. Currently, the main methods for extracting lignin include organic solvents, alkaline extraction, supercritical carbon dioxide extraction, and microbial extraction. With the exception of alkaline extraction, other extraction methods are complex or highly polluting. While alkaline extraction is simple and efficient, it also consumes large amounts of acid and alkali during neutralization and precipitation, resulting in a waste of resources. Therefore, if the alkali solution and lignin in the alkaline pulping solution can be directly separated, the consumption of acid and alkali resources would be greatly reduced, greatly optimizing the extraction process. Nanofiltration, a popular membrane separation technology in recent years, offers advantages such as low energy consumption, high efficiency, and ease of operation. It can separate different solutes in a mixed solution through pore size differences and the Donnan effect. Using nanofiltration to separate lignin and alkaline solution can reduce acid and alkali consumption during the extraction process. However, the current problem is that most commercial nanofiltration membranes lack good alkaline resistance and are easily hydrolyzed and destroyed in alkaline environments, resulting in a loss of separation performance.

[0003] Therefore, it is of great practical significance to optimize the chemical structure of the separation layer of the composite membrane, give the membrane material excellent alkali resistance and selectivity, and apply it to the purification and recovery of alkali liquor in alkaline lignin extraction and even other alkaline water treatment fields. Summary of the Invention

[0004] The main purpose of the present invention is to provide a strong alkali-resistant composite nanofiltration membrane and its preparation method and application, so as to overcome the deficiencies in the prior art.

[0005] To achieve the aforementioned object of the invention, the technical solutions adopted by the present invention include:

[0006] An embodiment of the present invention provides a strong alkali-resistant composite nanofiltration membrane, which includes: a porous support layer and a dense separation layer stacked in sequence; wherein the dense separation layer is formed on the surface of the porous support layer by interfacial polymerization of water phase monomers and oil phase monomers, the water phase monomers include amine monomers, the amine monomers have a plurality of amine reactive groups, and the oil phase monomers include benzyl bromide monomers, the benzyl bromide monomers have a plurality of benzyl bromide groups.

[0007] The present invention also provides a method for preparing the aforementioned strong alkali-resistant composite nanofiltration membrane, which comprises:

[0008] providing a polyethersulfone ultrafiltration membrane as a porous support layer;

[0009] Furthermore, an aqueous solution containing amine monomers is added dropwise to the surface of the polyethersulfone ultrafiltration membrane for infiltration treatment, and then the obtained polyethersulfone ultrafiltration membrane is placed in an oil phase solution containing benzene bromide monomers for interfacial polymerization reaction to obtain a strong alkali resistant composite nanofiltration membrane.

[0010] The embodiment of the present invention also provides the use of the aforementioned strong alkali-resistant composite nanofiltration membrane in purifying and recovering alkali liquor in a lignin / alkali liquor mixed system.

[0011] The embodiment of the present invention further provides a method for purifying and recovering alkali liquor in a lignin / alkali liquor mixture system, comprising:

[0012] Providing a lignin / alkali solution mixed system and the aforementioned strong alkali-resistant composite nanofiltration membrane;

[0013] Furthermore, the alkali-resistant composite nanofiltration membrane is used to separate and treat the lignin / alkali solution mixed system, thereby realizing the recovery of the alkali solution.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] (1) The strong alkali-resistant composite nanofiltration membrane of the present invention has excellent alkali resistance, which solves the problem of poor alkali resistance of most commercial nanofiltration membranes. Moreover, a series of membrane materials with different molecular weight cut-offs can be prepared by regulating the reaction conditions;

[0016] (2) The strong alkali-resistant composite nanofiltration membrane provided in the present invention can separate lignin and alkali solution in the alkaline lignin extraction process, optimize the process flow, and recover high-purity alkali solution. Moreover, the strong alkali-resistant composite nanofiltration membrane has the potential to be promoted to more applications in alkaline waste liquid treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments recorded in the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0018] Figure 1 is a SEM image of a cross section of the strong alkali-resistant composite nanofiltration membrane prepared in Example 3 of the present invention;

[0019] Figure 2This is a graph showing changes in alkali recovery and lignin retention during filtration of a lignin / alkali solution mixture using the strong alkali-resistant composite nanofiltration membrane prepared in Example 3 of the present invention;

[0020] Figure 3 These are optical photographs of the original feed solution, the concentrated solution, and the permeate before and after filtration of a lignin / alkaline solution mixed solution using the strong alkali-resistant composite nanofiltration membrane prepared in Example 3 of the present invention. DETAILED DESCRIPTION

[0021] In light of the shortcomings of the prior art, the inventors of this case, after extensive research and extensive practice, were able to propose the technical solution of the present invention. This invention designs and prepares a series of alkali-resistant composite nanofiltration membranes formed by interfacial polymerization of amine monomers and benzyl bromide monomers. The monomers in the separation layer of the composite nanofiltration membranes are connected by CNC bonds. This chemical structure effectively protects against attack by hydroxide ions, resulting in excellent alkali resistance. After immersion in a 3-5 mol / L NaOH solution for 30 days, the composite nanofiltration membranes show little change in retention and flux performance. The composite nanofiltration membranes prepared under optimal conditions can achieve up to 95% retention of lignin in a mixed solution of alkali and lignin, while the retention rate for alkali is only 10%-15%. Using this composite nanofiltration membrane to treat the mixed solution of alkali and lignin, nearly 80% of the alkali can be efficiently recovered, with a purity exceeding 98%. This optimizes the alkaline lignin extraction process and significantly reduces the consumption of acid and alkali resources.

[0022] To facilitate understanding of the present application, the present application will be described in more detail below. However, it should be understood that the present invention can be implemented in many different forms and is not limited to the embodiments or examples described herein. On the contrary, the purpose of providing these embodiments or examples is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0023] Specifically, as one aspect of the technical solution of the present invention, a strong alkali-resistant composite nanofiltration membrane involves: a porous support layer and a dense separation layer stacked in sequence; wherein, the dense separation layer is formed on the surface of the porous support layer by interfacial polymerization of water phase monomers and oil phase monomers, the water phase monomers include amine monomers, the amine monomers have a plurality of amine reactive groups, and the oil phase monomers include benzyl bromide monomers, the benzyl bromide monomers have a plurality of benzyl bromide groups.

[0024] In some preferred embodiments, the porous support layer includes any one or more combinations of polyethersulfone ultrafiltration membrane, polysulfone ultrafiltration membrane, and polyethersulfone microfiltration membrane, but is not limited thereto.

[0025] In some preferred embodiments, the thickness of the dense separation layer is 20-80 nm.

[0026] In some preferred embodiments, the alkali-resistant composite nanofiltration membrane has a retention rate of lignin of more than 70%, and a retention rate of alkali liquor of less than 30%.

[0027] Furthermore, the alkali-resistant composite nanofiltration membrane has a retention rate of lignin of more than 95%, and a retention rate of alkali liquor of less than 15%.

[0028] In some preferred embodiments, the alkali-resistant composite nanofiltration membrane has a molecular weight cutoff of 800Da-2500Da.

[0029] As another aspect of the technical solution of the present invention, the method for preparing the aforementioned strong alkali-resistant composite nanofiltration membrane involves:

[0030] providing a polyethersulfone ultrafiltration membrane as a porous support layer;

[0031] Furthermore, an aqueous solution containing amine monomers is added dropwise to the surface of the polyethersulfone ultrafiltration membrane for infiltration treatment, and then the obtained polyethersulfone ultrafiltration membrane is placed in an oil phase solution containing benzene bromide monomers for interfacial polymerization reaction to obtain a strong alkali resistant composite nanofiltration membrane.

[0032] In some preferred embodiments, the preparation method specifically includes: in an environment with a temperature of 25-45°C and a relative humidity of 40%-60%, an aqueous phase solution containing an amine monomer is added dropwise to the surface of the polyethersulfone ultrafiltration membrane for infiltration treatment for 1-5 minutes, excess aqueous phase solution is removed, and then the obtained polyethersulfone ultrafiltration membrane is placed in an oil phase solution containing a benzene bromide monomer and kept at an ambient temperature of 25°C-45°C for interfacial polymerization reaction for 120-390 minutes, and then dried at 40-70°C for 5-30 minutes to obtain a strong alkali-resistant composite nanofiltration membrane.

[0033] Furthermore, the amine monomer includes piperazine, but is not limited thereto.

[0034] Furthermore, the benzene bromide monomer includes 1,3,5-tris(bromomethyl)benzene, but is not limited thereto.

[0035] Furthermore, the oil phase solvent used in the oil phase solution includes a single alkane solvent or a mixed alkane solvent.

[0036] Furthermore, the alkane solvent includes n-hexane, but is not limited thereto.

[0037] Furthermore, the concentration of the amine monomer in the aqueous solution is 1-10 wt%.

[0038] Furthermore, the concentration of the benzyl bromide monomer in the oil phase solution is 0.1-0.3 wt %.

[0039] As another aspect of the technical solution of the present invention, it also relates to the use of the aforementioned strong alkali-resistant composite nanofiltration membrane in purifying and recovering alkali liquor in a lignin / alkali liquor mixed system.

[0040] As another aspect of the technical solution of the present invention, it also relates to a method for purifying and recovering alkali liquor in a lignin / alkali liquor mixed system, which comprises:

[0041] Providing a lignin / alkali solution mixed system and the aforementioned strong alkali-resistant composite nanofiltration membrane;

[0042] Furthermore, the alkali-resistant composite nanofiltration membrane is used to separate and treat the lignin / alkali solution mixed system, thereby realizing the recovery of the alkali solution.

[0043] In some preferred embodiments, the lignin / alkali solution mixed system is an alkali solution / lignin mixed solution in a process of extracting lignin by an alkali solution extraction method.

[0044] In some preferred embodiments, the pH value of the lignin / alkali solution mixed system is 11-14.

[0045] The strong alkali-resistant composite nanofiltration membrane of the present invention has stable structural properties after long-term immersion in a 3-5 mol / L NaOH solution, and has a retention rate of more than 95% for lignin while only retaining 10%-15% for NaOH. At the same time, the composite nanofiltration membrane of the present invention has excellent strong alkali resistance and lignin / alkali solution selectivity, and has great application value in the purification and recovery of alkali solution in a lignin / alkali solution mixed system.

[0046] In the present invention, the reaction between piperazine and TBMB is a polymerization reaction. While forming a chemical bond connecting the two, a hydrogen bromide is released. This hydrogen ion comes from piperazine, and thus becomes a tertiary amine after the reaction. The tertiary amine bond is formed by a Hofmann alkylation reaction. Unlike the most common amide bond in existing nanofiltration membranes, the tertiary amine bond has no carbonyl group, thus avoiding attack by hydroxide, and thus exhibiting excellent alkali resistance and stability.

[0047] The present invention is further illustrated by the following examples. The present invention can be better understood according to the following examples. However, it will be readily understood by those skilled in the art that the specific material ratios, process conditions, and results described in the examples are merely illustrative of the present invention and should not, and do not, limit the present invention as described in detail in the claims.

[0048] Unless otherwise specified, the various raw materials, reaction equipment, testing equipment and testing methods used in the following examples are all well known in the art.

[0049] Example 1

[0050] The preparation of strong alkali-resistant composite nanofiltration membrane should be carried out indoors at a temperature of 25°C and a relative humidity of 50%. First, prepare a 7wt% piperazine (PIP) aqueous solution and a 0.3wt% 1,3,5-tris(bromomethyl)benzene (TBMB) n-hexane solution. Drop the PIP solution onto the polyethersulfone ultrafiltration base membrane and soak it for 2 minutes. Then remove the solution to ensure that there are no visible water stains on the surface but the base membrane remains wet. Then place the wet base membrane in the TBMB solution to react for 120 minutes, during which the room temperature is kept constant at 25°C, but the humidity can no longer be controlled. After the reaction ends, place the composite membrane in a 60°C oven for heat treatment for 15 minutes to ensure that the layers of the composite membrane do not peel off. The prepared strong alkali-resistant composite nanofiltration membrane should be stored in deionized water at 4°C for later use.

[0051] The test showed that the strong alkali-resistant composite nanofiltration membrane prepared in this embodiment was stable in structure and performance after long-term immersion in 3-5 mol / L NaOH solution, with a lignin retention of 67.5%, a sodium hydroxide retention of 4.5%, and a flux of 43.4 Lm- 2 h- 1 bar- 1 Test conditions: test temperature is 25°C, operating pressure is 6 bar, lignin concentration is 100 ppm, and sodium hydroxide concentration is 1000 ppm.

[0052] Example 2

[0053] Preparation of the strong-alkali-resistant composite nanofiltration membrane should be performed indoors at a temperature of 25°C and a relative humidity of 50%. First, prepare a 7wt% piperazine (PIP) aqueous solution and a 0.3wt% 1,3,5-tris(bromomethyl)benzene (TBMB) n-hexane solution. The PIP solution is dripped onto the polyethersulfone ultrafiltration membrane and allowed to soak for 2 minutes. The solution is then removed, ensuring that no visible water stains remain on the surface but the membrane remains moist. The moistened membrane is then placed in the TBMB solution for a reaction of 2-10 minutes, maintaining a constant room temperature of 25°C during this time. However, humidity control is not required. After the reaction is complete, the composite membrane is heat-treated in a 60°C oven for 15 minutes to prevent delamination between the membrane layers. The prepared strong-alkali-resistant composite nanofiltration membrane should be stored in deionized water at 4°C until ready for use.

[0054] The test showed that the strong alkali-resistant composite nanofiltration membrane prepared in this embodiment was stable in structure and performance after long-term immersion in 3-5 mol / L NaOH solution, with a lignin retention of 87.6%, a sodium hydroxide retention of 8.9%, and a flux of 20.8 Lm- 2 h- 1 bar- 1Test conditions: test temperature is 25°C, operating pressure is 6 bar, lignin concentration is 100 ppm, and sodium hydroxide concentration is 1000 ppm.

[0055] Example 3

[0056] The preparation of strong alkali-resistant composite nanofiltration membrane should be carried out indoors at a temperature of 25°C and a relative humidity of 50%. First, prepare a 7wt% piperazine (PIP) aqueous solution and a 0.3wt% 1,3,5-tris(bromomethyl)benzene (TBMB) n-hexane solution. Drop the PIP solution onto the polyethersulfone ultrafiltration base membrane and soak it for 2 minutes. Then remove the solution to ensure that there are no visible water stains on the surface but the base membrane remains wet. Then place the wet base membrane in the TBMB solution to react for 300 minutes, during which the room temperature is kept constant at 25°C, but the humidity can no longer be controlled. After the reaction ends, place the composite membrane in a 60°C oven for heat treatment for 15 minutes to ensure that the layers of the composite membrane do not peel off. The prepared strong alkali-resistant composite nanofiltration membrane should be stored in deionized water at 4°C for later use.

[0057] The SEM image of the cross section of the strong alkali resistant composite nanofiltration membrane prepared in this embodiment is as follows: Figure 1 As shown; the changes in alkali recovery rate and lignin retention during filtration of lignin / alkali solution by the alkali-resistant composite nanofiltration membrane prepared in this embodiment are shown in FIG. Figure 2 As shown; optical photographs of the original feed liquid, concentrate and permeate before and after filtration of the lignin / alkali solution by the alkali-resistant composite nanofiltration membrane prepared in this embodiment are as follows: Figure 3 shown.

[0058] The test showed that the strong alkali-resistant composite nanofiltration membrane prepared in this embodiment was stable in structure and performance after long-term immersion in 3-5 mol / L NaOH solution, with a lignin retention of 95%, a sodium hydroxide retention of 12.7%, and a flux of 14.3 Lm- 2 h- 1 bar- 1 Test conditions: test temperature is 25°C, operating pressure is 6 bar, lignin concentration is 100 ppm, and sodium hydroxide concentration is 1000 ppm.

[0059] Example 4

[0060] The preparation of strong alkali-resistant composite nanofiltration membrane should be carried out indoors at a temperature of 25°C and a relative humidity of 50%. First, prepare a 7wt% piperazine (PIP) aqueous solution and a 0.3wt% 1,3,5-tris(bromomethyl)benzene (TBMB) n-hexane solution. Drop the PIP solution onto the polyethersulfone ultrafiltration base membrane and soak it for 2 minutes. Then remove the solution to ensure that there are no visible water stains on the surface but the base membrane remains wet. Then place the wet base membrane in the TBMB solution to react for 390 minutes, during which the room temperature is kept constant at 25°C, but the humidity can no longer be controlled. After the reaction ends, place the composite membrane in a 60°C oven for heat treatment for 15 minutes to ensure that the layers of the composite membrane do not peel off. The prepared strong alkali-resistant composite nanofiltration membrane should be stored in deionized water at 4°C for later use.

[0061] The test showed that the strong alkali-resistant composite nanofiltration membrane prepared in this embodiment was stable in structure and performance after long-term immersion in 3-5 mol / L NaOH solution, with a lignin retention of 96.3%, a sodium hydroxide retention of 29.1%, and a flux of 10.1 Lm- 2 h- 1 bar- 1 Test conditions: test temperature is 25°C, operating pressure is 6 bar, lignin concentration is 100 ppm, and sodium hydroxide concentration is 1000 ppm.

[0062] Example 5

[0063] The preparation of strong alkali-resistant composite nanofiltration membrane should be carried out indoors at a temperature of 25°C and a relative humidity of 50%. First, prepare a 1wt% piperazine (PIP) aqueous solution and a 0.3wt% 1,3,5-tris(bromomethyl)benzene (TBMB) n-hexane solution. Drop the PIP solution onto the polyethersulfone ultrafiltration base membrane and soak it for 2 minutes. Then remove the solution to ensure that there are no visible water stains on the surface but the base membrane remains wet. Then place the wet base membrane in the TBMB solution to react for 300 minutes, during which the room temperature is kept constant at 25°C, but the humidity can no longer be controlled. After the reaction ends, place the composite membrane in a 60°C oven for heat treatment for 15 minutes to ensure that the layers of the composite membrane do not peel off. The prepared strong alkali-resistant composite nanofiltration membrane should be stored in deionized water at 4°C for later use.

[0064] The test showed that the strong alkali-resistant composite nanofiltration membrane prepared in this embodiment was stable in structure and performance after long-term immersion in 3-5 mol / L NaOH solution, with a lignin retention of 68.2%, a sodium hydroxide retention of 4.3%, and a flux of 25.4 Lm- 2 h- 1 bar- 1Test conditions: test temperature is 25°C, operating pressure is 6 bar, lignin concentration is 100 ppm, and sodium hydroxide concentration is 1000 ppm.

[0065] Example 6

[0066] The preparation of strong alkali-resistant composite nanofiltration membrane should be carried out indoors at a temperature of 25°C and a relative humidity of 50%. First, prepare a 4wt% piperazine (PIP) aqueous solution and a 0.3wt% 1,3,5-tris(bromomethyl)benzene (TBMB) n-hexane solution. Drop the PIP solution onto the polyethersulfone ultrafiltration base membrane and soak it for 2 minutes. Then remove the solution to ensure that there are no visible water stains on the surface but the base membrane remains wet. Then place the wet base membrane in the TBMB solution to react for 300 minutes, during which the room temperature is kept constant at 25°C, but the humidity can no longer be controlled. After the reaction ends, place the composite membrane in a 60°C oven for heat treatment for 15 minutes to ensure that the layers of the composite membrane do not peel off. The prepared strong alkali-resistant composite nanofiltration membrane should be placed in deionized water at 4°C for storage.

[0067] The test showed that the strong alkali-resistant composite nanofiltration membrane prepared in this embodiment was stable in structure and performance after long-term immersion in 3-5 mol / L NaOH solution, with a lignin retention of 74.6%, a sodium hydroxide retention of 5.9%, and a flux of 23.7 Lm- 2 h- 1 bar- 1 Test conditions: test temperature is 25°C, operating pressure is 6 bar, lignin concentration is 100 ppm, and sodium hydroxide concentration is 1000 ppm.

[0068] Example 7

[0069] Preparation of the strong-alkali-resistant composite nanofiltration membrane should be performed indoors at a temperature of 25°C and a relative humidity of 50%. First, prepare a 10 wt% piperazine (PIP) aqueous solution and a 0.3 wt% 1,3,5-tris(bromomethyl)benzene (TBMB) n-hexane solution. The PIP solution is dripped onto the polyethersulfone ultrafiltration membrane and allowed to soak for 2 minutes. The solution is then removed, ensuring that no visible water stains remain on the surface but that the membrane remains moist. The moistened membrane is then placed in the TBMB solution for a reaction of 300 minutes, maintaining a constant room temperature of 25°C during this time. However, humidity control is not required. After the reaction is complete, the composite membrane is heat-treated in a 60°C oven for 15 minutes to ensure that the layers of the composite membrane do not delaminate. The prepared strong-alkali-resistant composite nanofiltration membrane should be stored in deionized water at 4°C until ready for use.

[0070] The test showed that the strong alkali-resistant composite nanofiltration membrane prepared in this embodiment was stable in structure and performance after long-term immersion in 3-5 mol / L NaOH solution, with a lignin retention of 90.4%, a sodium hydroxide retention of 10.1%, and a flux of 13.5 Lm- 2 h- 1 bar- 1 Test conditions: test temperature is 25°C, operating pressure is 6 bar, lignin concentration is 100 ppm, and sodium hydroxide concentration is 1000 ppm.

[0071] It should be noted that the strong alkali-resistant composite nanofiltration membranes obtained in the above examples were all tested using a cross-flow method. The retention rates of alkali solution and lignin were calculated based on the ratio of the permeate concentration to the feed solution concentration using the following formula:

[0072]

[0073] Flux is the volume of liquid filtered per hour per square meter of membrane area, normalized to unit atmospheric pressure:

[0074]

[0075] Comparative Example 1

[0076] The preparation of the composite nanofiltration membrane should be carried out indoors at a temperature of 25°C and a relative humidity of 50%. First, prepare a 7wt% aqueous solution of m-phenylenediamine (MPD) and a 0.3wt% n-hexane solution of 1,3,5-tris(bromomethyl)benzene (TBMB). Drop the MPD solution on the polyethersulfone ultrafiltration bottom membrane and soak it for 2 minutes, then remove the solution to ensure that there are no visible water stains on the surface but the bottom membrane remains wet. Then place the wet bottom membrane in the TBMB solution to react for 300 minutes, during which the room temperature is kept constant at 25°C, but the humidity can no longer be controlled. After the reaction ends, place the composite membrane in a 60°C oven for heat treatment for 15 minutes to ensure that the layers of the composite membrane will not peel off. The prepared composite nanofiltration membrane should be placed in deionized water at 4°C for storage.

[0077] Testing showed that after 10 days of immersion in a 3-5 mol / L NaOH solution, the composite nanofiltration membrane prepared in this comparative example retained only 19.8% of lignin. This poor alkaline resistance damaged the composite membrane, resulting in a loss of separation performance. Test conditions: 25°C, 6 bar operating pressure, 100 ppm lignin concentration, and 1000 ppm sodium hydroxide concentration.

[0078] Comparative Example 2

[0079] The preparation of the composite nanofiltration membrane should be carried out indoors at a temperature of 25°C and a relative humidity of 50%. First, prepare a 1wt% piperazine (PIP) aqueous solution and a 2wt% trimesoyl chloride (TMC) n-hexane solution. Drop the PIP solution on the polyethersulfone ultrafiltration bottom membrane and soak it for 2 minutes, then remove the solution to ensure that there are no visible water stains on the surface but the bottom membrane remains wet. Then place the wet bottom membrane in the TMC solution to react for 1 minute, during which the room temperature is kept constant at 25°C, but the humidity can no longer be controlled. After the reaction ends, place the composite membrane in a 60°C oven for heat treatment for 15 minutes to ensure that the layers of the composite membrane will not peel off. The prepared composite nanofiltration membrane should be placed in deionized water at 4°C for storage.

[0080] Testing revealed that the composite nanofiltration membrane prepared in this comparative example exhibited no retention effect on either lignin or sodium hydroxide after only one day of immersion in a 3-5 mol / L NaOH solution. This poor alkaline resistance damaged the composite membrane, resulting in no separation performance. Test conditions: 25°C, 6 bar operating pressure, 100 ppm lignin concentration, and 1000 ppm sodium hydroxide concentration.

[0081] In addition, the inventors of this case also referred to the aforementioned embodiments and conducted experiments using other raw materials, process operations, and process conditions described in this specification, and obtained relatively ideal results.

[0082] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention, and such changes and modifications fall within the scope of the invention as claimed.

Claims

1. A strong alkali resistant composite nanofiltration membrane, characterized in that: include: A porous support layer and a dense separation layer are stacked in sequence; wherein the dense separation layer is formed on the surface of the porous support layer by interfacial polymerization of water-phase monomers and oil-phase monomers, the water-phase monomers include amine monomers, the amine monomers have multiple amine-reactive groups, and the oil-phase monomers include benzyl bromide monomers, the benzyl bromide monomers have multiple benzyl bromide groups.

2. The alkali-resistant composite nanofiltration membrane according to claim 1, characterized in that: The porous support layer comprises any one or more combinations of polyethersulfone ultrafiltration membrane, polysulfone ultrafiltration membrane, and polyethersulfone microfiltration membrane; and / or, the thickness of the dense separation layer is 20-80 nm; And / or, the alkali-resistant composite nanofiltration membrane has a retention rate of lignin of more than 70%, and the alkali-resistant composite nanofiltration membrane has a retention rate of alkali liquor of less than 30%.

3. The strong alkali-resistant composite nanofiltration membrane according to claim 2, characterized in that: The alkali-resistant composite nanofiltration membrane has a lignin retention rate of more than 95%, a alkali-resistant composite nanofiltration membrane has an alkali solution retention rate of less than 15%, and a molecular weight cutoff of 800Da-2500Da.

4. The method for preparing a strong alkali-resistant composite nanofiltration membrane according to any one of claims 1 to 3, characterized in that: include: providing a polyethersulfone ultrafiltration membrane as a porous support layer; Furthermore, an aqueous solution containing amine monomers is added dropwise to the surface of the polyethersulfone ultrafiltration membrane for infiltration treatment, and then the obtained polyethersulfone ultrafiltration membrane is placed in an oil phase solution containing benzene bromide monomers for interfacial polymerization reaction to obtain a strong alkali resistant composite nanofiltration membrane.

5. The preparation method according to claim 4, characterized in that Specifically include: In an environment with a temperature of 25-45°C and a relative humidity of 40%-60%, an aqueous solution containing an amine monomer is added dropwise to the surface of a polyethersulfone ultrafiltration membrane for a wetting treatment of 1-5 minutes, and excess aqueous solution is removed. The obtained polyethersulfone ultrafiltration membrane is then placed in an oil phase solution containing a benzyl bromide monomer and maintained at an ambient temperature of 25°C-45°C for an interfacial polymerization reaction of 120-390 minutes, and then dried at 40-70°C for 5-30 minutes to obtain a strong alkali-resistant composite nanofiltration membrane.

6. The preparation method according to claim 5, characterized in that: The amine monomer includes piperazine; And / or, the brominated monomer includes 1,3,5-tris(bromomethyl)benzene; And / or, the oil phase solvent used in the oil phase solution includes a single alkane solvent or a mixed alkane solvent.

7. The preparation method according to claim 6, characterized in that: The concentration of the amine monomer in the aqueous solution is 1-10 wt%; And / or, the concentration of the benzyl bromide monomer in the oil phase solution is 0.1-0.3 wt %.

8. Use of the strong alkali-resistant composite nanofiltration membrane according to any one of claims 1 to 3 in purifying and recovering alkali liquor in a lignin / alkali liquor mixture system.

9. A method for purifying and recovering alkali liquor in a lignin / alkali liquor mixture system, characterized in that: include: Providing a lignin / alkali solution mixed system and the strong alkali-resistant composite nanofiltration membrane according to any one of claims 1 to 3; Furthermore, the alkali-resistant composite nanofiltration membrane is used to separate and treat the lignin / alkali solution mixed system, thereby realizing the recovery of the alkali solution.

10. The method according to claim 9, characterized in that: The lignin / alkali solution mixed system is an alkali solution / lignin mixed solution used in the lignin extraction process using an alkali solution extraction method; And / or, the pH value of the lignin / alkali solution mixed system is 11-14.

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