Process for preparing hollow fiber nanofiltration membrane through multi-step crosslinking and gradient pore structure regulation

Abstract

The invention provides a process for preparing a hollow fiber nanofiltration membrane through multi-step crosslinking and gradient pore regulation. The process comprises the following steps: S1, pretreating raw materials; s2, preparing a membrane casting solution; s3, debugging a spinning nozzle; s4, dry-wet spinning forming; s5, preliminary regulation and control of a gradient pore structure; s6, carrying out multi-step crosslinking treatment; s7, carrying out post-treatment modification; and S8, carrying out performance stabilization treatment. In the first step of crosslinking, an epoxy crosslinking agent reacts with the fiber at a specific temperature and pH value; the method comprises the following steps: step 1, preparing a membrane structure, step 2, reacting with an amine cross-linking agent, and step 3, treating with an aldehyde cross-linking agent, so as to gradually construct the stable membrane structure with specific performance, step 2, preliminarily regulating and controlling a gradient pore structure, and controlling the temperature gradient of a coagulating bath, solution composition and the running path and time of fibers in the gradient pore structure. The construction of a precise gradient pore structure with gradually increased pore diameter from the outer layer to the inner layer of the membrane is realized. Through precise membrane structure regulation and control, the membrane has better selective separation capability on substances with different molecular weights and properties, and the separation performance of the membrane is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of nanofiltration membrane production, and in particular relates to a process for preparing hollow fiber nanofiltration membranes by multi-step cross-linking and gradient pore structure regulation. Background Art

[0002] Nanofiltration membrane technology is widely used in current water treatment and related separation fields due to its unique separation properties, such as its ability to effectively remove most multivalent ions and some small organic molecules from water, with a molecular weight cutoff between that of reverse osmosis membranes and ultrafiltration membranes. Hollow fiber nanofiltration membranes, with their high packing density per unit volume, relatively low influent quality requirements, and resistance to fouling, have become a hot research and application area.

[0003] However, there are many problems with the existing hollow fiber nanofiltration membrane preparation process. Traditional preparation methods, such as phase inversion, interfacial polymerization, chemical cross-linking, surface grafting, etc., have exposed many defects in practical applications. For example, the rolled nanofiltration membrane prepared by interfacial polymerization, although mature in technology, has problems such as high operating pressure requirements, complex pretreatment requirements and relatively low water flux, which not only increases operating costs and energy consumption, but also increases operational complexity. In the preparation of hollow fiber nanofiltration membranes, some processes are difficult to accurately control the pore structure and cross-linking degree of the membrane, resulting in unstable membrane performance. The separation layer of some membranes has weak bonding with the support layer, and the separation layer is prone to fall off during use, resulting in deterioration of the separation performance of the membrane and shortened service life. Moreover, existing hollow fiber nanofiltration membrane products generally have the problem of low water flux, usually less than 10L / m 2 ・hr・bar, which greatly limits its efficiency and applicability in applications such as large-scale water treatment.

[0004] In addition, most preparation processes have numerous process control points, making it difficult to precisely control the coating thickness or cross-linking reaction. Furthermore, many are limited to the preparation of specific membrane types, lacking versatility and flexibility. In terms of cost, some preparation processes use expensive raw materials and complex preparation processes, further limiting the large-scale promotion and application of hollow fiber nanofiltration membranes. Therefore, there is an urgent need to develop a hollow fiber nanofiltration membrane preparation process that can precisely control membrane structure, improve membrane performance, simplify the preparation process, and reduce costs. Summary of the Invention

[0005] The purpose of the present invention is to provide a process for preparing hollow fiber nanofiltration membranes by multi-step cross-linking and gradient pore structure regulation, so as to solve the problems raised in the above background technology.

[0006] In view of this, the present invention provides a process for preparing a hollow fiber nanofiltration membrane by multi-step cross-linking and gradient pore control, comprising the following steps: S1, raw material pretreatment; S2, preparation of casting solution; S3, spinneret debugging; S4, dry and wet spinning; S5, preliminary regulation of gradient pore structure; S6, multi-step cross-linking treatment; S7, post-processing modification; S8, performance stabilization processing.

[0007] In the present invention, a further embodiment is that the step S1 comprises: drying the polymer material at 80-100°C and a vacuum degree of -0.08 to -0.06MPa for 4-6 hours, stirring the polymer material every hour during the drying process to ensure uniform heating, the solvent is pressure-filtered using a 0.22μm polytetrafluoroethylene filter membrane, and the filtration pressure is controlled at 0.2-0.3MPa. The additive is ground by a planetary ball mill, the grinding medium is zirconia balls, the ball-to-material ratio is 5:1, the grinding speed is 300-400r / min, and the grinding time is 2-3 hours until the particle size is less than 50μm, and then sieved for use.

[0008] In the present invention, a further embodiment is that the step S2 comprises: adding 25-32wt% of the pretreated polymer material, 58-70wt% of the pretreated solvent, and 3-12wt% of the pretreated additive in a double-layer glass reactor with a stirring paddle and a thermometer in a mass ratio, wherein the stirring paddle is an anchor structure and the blade diameter is 0.6-0.8 times the inner diameter of the reactor, mixing at 60-75°C and a stirring rate of 300-500r / min for 4-6 hours, sampling and observing every 30 minutes until a uniform solution without visible particles is formed, transferring the solution to a vacuum degassing tank, and degassing for 2-4 hours under a vacuum environment of -0.09 to -0.06MPa. During the degassing process, the temperature in the tank is maintained at 50-60°C, and stirring is performed intermittently to prevent the solution from stratifying.

[0009] In the present invention, a further embodiment is that the step S3 comprises: selecting a stainless steel annular spinneret with an outer diameter of 0.6-1.8 mm, an inner diameter of 0.2-0.9 mm, and a concentricity deviation of the inner and outer channels ≤0.02 mm. During debugging, hot water at 60-70 ° C is first introduced to preheat the spinneret for 30-60 minutes to make the overall temperature of the spinneret reach 40-60 ° C and uniform, and compressed air is introduced from the inner and outer channels respectively at a pressure of 0.1-0.2 MPa for 10-15 seconds to check whether the channel is unobstructed and observe the spinneret outlet with an optical microscope.

[0010] In the present invention, a further implementation scheme is that the step S4 includes: delivering the casting liquid to the spinneret through a metering pump, the metering pump is a gear pump with an accuracy of ±0.5% and a delivery rate of 8-15 mL / min, the core liquid introduced into the inner tube of the spinneret is a solvent aqueous solution with a mass fraction of 10-20%, and the core liquid is delivered by a constant flow pump with a flow rate of 3-8 mL / min. The core liquid temperature is controlled at 20-30°C and is delivered to the spinneret through an insulation pipeline. The extruded casting liquid runs in a clean environment for 0.3-8s, a windproof baffle is set on the running path, the wind speed is controlled at ≤0.1 m / s, and it enters the coagulation bath vertically, and the angle between the nascent fiber and the coagulation bath liquid surface is 90°±5.

[0011] In the present invention, a further embodiment is that step S5 includes: the coagulation bath is a rectangular trough with a length of 2-4m and a depth of 0.5-0.8m, and is composed of 60-80% water, 15-30% solvent and 2-8% non-solvent by mass fraction. A circulating pump is used to form a unidirectional flow of the coagulation bath liquid from the inlet to the outlet in the trough, with a flow rate of 0.1-0.5m / s. A segmented temperature control device is provided on the outside of the coagulation bath trough, dividing the trough into 3-5 sections, and the temperature of each section is independently controlled so that the temperature is gradually reduced by 2-10°C from the inlet to the outlet, and the temperature difference between adjacent sections is ≤3°C. The spun fiber runs in an S-shaped path in the coagulation bath through guide rollers, with a residence time of 15-40 minutes, a guide roller spacing of 50-80cm, and a synchronized and adjustable speed to ensure smooth fiber operation.

[0012] In a further embodiment of the present invention, step S6 includes: a first crosslinking step, wherein the preliminarily coagulated fiber is placed in a 2-6% by mass epoxy crosslinker solution, the solution is heated in a constant temperature water bath at a temperature of 20-45°C, and the pH value is adjusted to 8-10 by dropwise addition of a 10% by mass sodium hydroxide solution, the fiber is completely immersed and relaxed in the solution, and the fiber is immersed for 1-4 hours, with the fiber being turned over every 30 minutes; The second step is cross-linking. After removing the fiber, rinse the surface with deionized water to remove the residual cross-linking agent. Then put it into a 4-9% by mass amine cross-linking agent solution at a temperature of 30-65°C. Adjust the pH value to 9-11 by adding a 10% by mass hydrochloric acid solution. Let it react for 2-5 hours. The third step is cross-linking. The fiber is transferred to a 1-3% by mass aldehyde cross-linking agent solution and treated at 40-55°C for 0.5-2 hours. The solution stirring rate is maintained at 50-100 r / min during the cross-linking process.

[0013] In a further embodiment of the present invention, step S7 comprises: soaking the cross-linked fibers in a 5-10% by mass ethanol solution at 25-30° C. for 1-2 hours, wherein the volume of the ethanol solution is 10-15 times the volume of the fibers; Rinse with deionized water 3-5 times, placing the fiber in flowing deionized water each time, with a water temperature of 20-30°C, a flushing flow rate of 500-1000 mL / min, and each flushing time of 10-20 minutes, until the conductivity of the flushing liquid is consistent with that of deionized water; Place the fiber in a vacuum drying oven and dry it for 4-6 hours at 30-40°C and a vacuum degree of -0.07 to -0.06 MPa. During the drying process, open the oven door every 2 hours to release water vapor. After drying, the fiber is placed in a hydrophilic modifier solution with a mass fraction of 0.5-2%, the solution temperature is 25-35°C, and soaked for 0.5-1.5 hours. The modifier is polyvinyl alcohol or sodium alginate. The fiber is completely stretched in the solution without entanglement.

[0014] In a further embodiment of the present invention, step S8 comprises: placing the modified fiber in deionized water at a temperature of 50-70° C., completely immersing the fiber, stirring the water to allow it to flow slightly, and treating for 2-4 hours; The fiber is taken out, and after draining the surface moisture, it is placed in a hot air drying oven. A multi-layer tray is set in the drying oven, and the fibers are evenly laid on the tray without overlapping. The hot air temperature is 80-100°C, and the wind speed is 1-3m / s. The relative humidity of the drying environment is controlled at 30-50% by a dehumidifier. Dry for 2-3 hours, and record the fiber weight every 30 minutes until the difference between the two weights is ≤0.5%. The finished hollow fiber nanofiltration membrane is obtained.

[0015] The beneficial effects of the present invention are: Through a unique multi-step cross-linking process, the degree of cross-linking and the cross-linking level of the membrane can be precisely controlled. In the first cross-linking step, the epoxy cross-linker reacts with the fiber at a specific temperature and pH value, laying the foundation for subsequent cross-linking; the reaction of the amine cross-linker in the second step and the treatment of the aldehyde cross-linker in the third step gradually build a stable membrane structure with specific properties. At the same time, the initial control step of the gradient pore structure utilizes the temperature gradient of the coagulation bath, the solution composition, and the fiber's running path and time control to achieve the construction of a precise gradient pore structure with gradually increasing pore size from the outer layer to the inner layer of the membrane. This precise membrane structure control gives the membrane better selective separation capabilities for substances of different molecular weights and properties, greatly improving the separation performance of the membrane.

[0016] Optimized process parameters, such as appropriate casting solution formula, precise spinning conditions and reasonable post-processing steps, make the prepared hollow fiber nanofiltration membrane have higher pure water flux. Under the operating pressure of 0.5-1.5MPa, the pure water flux can reach 15-60L / (m 2 ・h・bar), which is significantly improved compared with the generally low water flux in existing technologies and can meet the high flux requirements of large-scale water treatment.

[0017] The synergistic effect of multi-step cross-linking and the gradient pore structure gives the membrane excellent retention of target substances. For organic compounds with a molecular weight of 300-1000 Da, the retention rate is ≥90%, and the retention rate increases with molecular weight, with each 100 Da increase in molecular weight resulting in a 3-5% increase in retention. This allows for the effective separation and retention of harmful substances in water, ensuring effective water purification.

[0018] A series of rigorous steps, from raw material pretreatment to performance stabilization, ensure the stability of the membrane structure. After 72 hours of continuous operation, the pure water flux retention rate is ≥90%, and the rejection rate retention rate is ≥95%. This effectively solves the problem of rapid performance degradation of existing membranes during long-term use, extends the membrane life, and reduces replacement costs.

[0019] The entire preparation process is clear and organized, with each step closely coordinated to reduce unnecessary complex operations. During the preparation of the casting solution, clear raw material ratios, mixing temperature, stirring rate, and degassing conditions ensure efficient and stable preparation of the casting solution. The spinneret debugging steps are simple and targeted, ensuring a smooth spinning process. Compared with the numerous process control points and difficult-to-control coating layer thickness and cross-linking reactions in traditional preparation processes, the process of the present invention is easier to operate and control, making it more suitable for large-scale industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a schematic diagram of the process of the present invention. DETAILED DESCRIPTION

[0021] The following will be combined with the accompanying drawings in the embodiments of the present application to clearly describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field are within the scope of protection of this application.

[0022] In the description of this application, it should be noted that the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the exemplary embodiments according to this application. For ease of description, the dimensions of the various parts shown in the drawings are not drawn according to the actual proportional relationship. Technologies, methods and equipment known to ordinary technicians in the relevant fields may not be discussed in detail, but where appropriate, the technologies, methods and equipment should be considered as part of the authorization specification. In all examples shown and discussed here, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of the exemplary embodiments may have different values. It should be noted that similar numbers and letters represent similar items in the following figures, so once an item is defined in one figure, it does not need to be further discussed in subsequent figures.

[0023] It should be noted that the terms "first," "second," etc. in the specification and claims of this application are used to distinguish similar objects, and are not used to describe a specific order or sequence. It should be understood that the data used in this way are interchangeable where appropriate, so that the embodiments of this application can be implemented in an order other than those illustrated or described herein, and that the objects distinguished by "first," "second," etc. are generally of the same type, and do not limit the number of objects. For example, the first object can be one or more. In addition, "and / or" in the specification and claims represents at least one of the connected objects, and the character " / " generally indicates that the objects associated with each other are in an "or" relationship.

[0024] It should be noted that, in the description of this application, the directions or positional relationships indicated by directional terms such as "front, back, up, down, left, right", "horizontal, vertical, vertical, horizontal" and "top, bottom" are usually based on the directions or positional relationships shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description. Unless otherwise specified, these directional terms do not indicate or imply that the device or element referred to must have a specific direction or be constructed and operated in a specific direction. Therefore, they cannot be understood as limiting the scope of protection of this application; the directional terms "inside and outside" refer to the inside and outside relative to the outline of each component itself.

[0025] It should be noted that, in the present application, the terms "comprise", "include" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the statement "comprises a ..." does not exclude the presence of other identical elements in the process, method, article or device comprising the element. In addition, it should be pointed out that the scope of the methods and devices in the embodiments of the present application is not limited to performing functions in the order shown or discussed, and may also include performing functions in a substantially simultaneous manner or in the opposite order according to the functions involved. For example, the described method may be performed in an order different from that described, and various steps may also be added, omitted, or combined. In addition, the features described with reference to certain examples may be combined in other examples.

[0026] This embodiment provides a process for preparing a hollow fiber nanofiltration membrane by multi-step cross-linking and gradient pore control, comprising the following steps: S1, raw material pretreatment; S2, preparation of casting solution; S3, spinneret debugging; S4, dry and wet spinning; S5, preliminary regulation of gradient pore structure; S6, multi-step cross-linking treatment; S7, post-processing modification; S8, performance stabilization processing.

[0027] Furthermore, step S1 includes drying the polymer material at 80-100°C and a vacuum of -0.08 to -0.06 MPa for 4-6 hours. During the drying process, the polymer material is stirred every hour to ensure uniform heating. The solvent is pressure-filtered using a 0.22μm polytetrafluoroethylene filter at a pressure of 0.2-0.3 MPa. The additive is ground using a planetary ball mill with zirconium oxide balls as the grinding medium, a ball-to-material ratio of 5:1, a grinding speed of 300-400 rpm, and a grinding time of 2-3 hours until the particle size is less than 50μm. The material is then sieved and set aside. Specifically, to ensure the stability of the polymer material and the uniformity of subsequent processing, residual moisture and volatile components must first be removed from the material. The polymer is placed in a constant temperature vacuum drying oven and heat treated within a temperature range of 80–100°C while maintaining a system vacuum of -0.08 to -0.06 MPa to promote the desorption of deep-seated moisture. The drying process lasts 4–6 hours, with mechanical stirring every hour to ensure even heating and avoid local overheating that could lead to material degradation. After drying, the moisture content is measured to ensure it is below 0.1 wt% to meet subsequent processing requirements.

[0028] A high-purity organic solvent is used as the reaction medium. To remove any possible particles and impurities, pressure filtration is performed using a 0.22 μm polytetrafluoroethylene (PTFE) membrane. The filtration system is equipped with a constant pressure device to precisely control the pressure at 0.2–0.3 MPa, ensuring filtration efficiency while preventing membrane damage. The filtered solvent undergoes transmittance testing to ensure a turbidity below 1 NTU, ensuring purity for subsequent polymerization reactions.

[0029] To improve the dispersion of the additive in the polymer matrix, ultrafine grinding is performed in a planetary ball mill. High-hardness zirconia balls are used as the grinding media, and a ball-to-batch ratio of 5:1 is set to optimize grinding efficiency. Milling is continued for 2–3 hours at a speed of 300–400 r / min, utilizing high-energy collisions and shear forces to gradually refine the additive particles. After milling, the particle size distribution is measured using a laser particle size analyzer to ensure a D90 value of <50 μm. The particles are then graded using a 200-mesh vibrating screen to remove insufficiently refined agglomerates, resulting in a highly uniform additive powder for future use.

[0030] In the present invention, a further embodiment is that the step S2 comprises: adding 25-32wt% of the pretreated polymer material, 58-70wt% of the pretreated solvent, and 3-12wt% of the pretreated additive in a double-layer glass reactor with a stirring paddle and a thermometer in a mass ratio, wherein the stirring paddle is an anchor structure and the blade diameter is 0.6-0.8 times the inner diameter of the reactor, mixing at 60-75°C and a stirring rate of 300-500r / min for 4-6 hours, sampling and observing every 30 minutes until a uniform solution without visible particles is formed, transferring the solution to a vacuum degassing tank, and degassing for 2-4 hours under a vacuum environment of -0.09 to -0.06MPa. During the degassing process, the temperature in the tank is maintained at 50-60°C, and stirring is performed intermittently to prevent the solution from stratifying.

[0031] Specifically, according to the optimized ratio, accurately weigh 25–32 wt% of the pretreated polymer material, 58–70 wt% of the purified solvent, and 3–12 wt% of the ultrafine additive. Weighing is performed using a high-precision electronic balance (accuracy ±0.001 g), ensuring a composition error of less than ±0.5%. The materials are added in a step-by-step manner (solvent → polymer → additive) to reduce local concentration gradients and improve dissolution efficiency.

[0032] A double-layer glass reactor was used as the mixing container, and a constant temperature circulating medium (such as silicone oil) was introduced into the interlayer to achieve precise temperature control. The stirring system used an anchor-type stirring paddle, and the paddle diameter was designed to be 0.6–0.8 times the inner diameter of the reactor to ensure that the gap between the reactor wall and the paddle was ≤5 mm to avoid material dead zones. The reaction was continued for 4–6 hours at 60–75°C and a stirring rate of 300–500 r / min. The shear-convection synergy of the paddle promoted the disentanglement of the polymer chains and the uniform dispersion of the additives. Samples were taken every 30 minutes, and the particle distribution was detected using a laser scattering particle size analyzer (target Dv(90) < 10 μm), supplemented by visual observation of the solution transparency. The mixing endpoint was determined when the solution was homogeneous (no gel clusters, no crystal points) and the transmittance was ≥95% (λ=600 nm).

[0033] Transfer the mixture to a vacuum degassing tank and degas at a negative pressure of -0.09 to -0.06 MPa for 2–4 hours. Maintain the tank temperature at 50–60°C (below the boiling point of the solvent but sufficient to reduce viscosity) during degassing. Initiate low-speed stirring (50–100 rpm for 2 minutes) every 20 minutes to break the surface tension of the bubbles and prevent separation of the solution. After degassing, determine the residual bubble content by specific gravity.

[0034] In the present invention, a further embodiment is that the step S3 comprises: selecting a stainless steel annular spinneret with an outer diameter of 0.6-1.8 mm, an inner diameter of 0.2-0.9 mm, and a concentricity deviation of the inner and outer channels ≤0.02 mm. During debugging, hot water at 60-70 ° C is first introduced to preheat the spinneret for 30-60 minutes to make the overall temperature of the spinneret reach 40-60 ° C and uniform, and compressed air is introduced from the inner and outer channels respectively at a pressure of 0.1-0.2 MPa for 10-15 seconds to check whether the channel is unobstructed and observe the spinneret outlet with an optical microscope.

[0035] In this example, a 316L stainless steel annular spinneret with an outer diameter of 0.6–1.8 mm and an inner diameter of 0.2–0.9 mm was used. CNC precision machining was used to ensure concentricity deviation of the inner and outer channels within 0.02 mm. The inner wall of the spinneret orifice was electropolished to a surface roughness Ra ≤ 0.1 μm to reduce melt flow resistance and prevent material buildup. The spinneret module features a quick-release design, allowing for easy replacement of different aperture combinations to accommodate fiber production requirements of various specifications.

[0036] The spinneret is installed in a constant temperature chamber and preheated with circulating hot water at 60–70°C. The heating rate is controlled at 2°C / min to avoid thermal stress and deformation. After 30–60 minutes of preheating, the surface temperature distribution of the spinneret is monitored using an infrared thermal imager to ensure that the overall temperature reaches 40–60°C and the temperature difference between different areas is ≤±1°C. During this stage, an auxiliary hot air system (50–80°C) can be activated to maintain the outer heat of the spinneret and further improve temperature uniformity.

[0037] Finally, introduce compressed air at 0.1–0.2 MPa for 10–15 seconds. Monitor the gas flow rate with a flowmeter to determine if there is any blockage and perform an inner channel test. At the same pressure, test the annular gap for airflow. If the pressure drop exceeds 10% of the initial value, this indicates a structural abnormality and prompts an outer channel test. Finally, observe the spinneret exit morphology using a 500x industrial microscope. Ensure the orifice edge is free of burrs and metal debris, and that the airflow trajectory forms a symmetrical cone. If defects are found, immediately clean them with a microfiber brush and ultrasonic cleaning.

[0038] In the present invention, a further implementation scheme is that the step S4 includes: delivering the casting liquid to the spinneret through a metering pump, the metering pump is a gear pump with an accuracy of ±0.5% and a delivery rate of 8-15 mL / min, the core liquid introduced into the inner tube of the spinneret is a solvent aqueous solution with a mass fraction of 10-20%, and the core liquid is delivered by a constant flow pump with a flow rate of 3-8 mL / min. The core liquid temperature is controlled at 20-30°C and is delivered to the spinneret through an insulation pipeline. The extruded casting liquid runs in a clean environment for 0.3-8 s, a windproof baffle is set on the running path, the wind speed is controlled at ≤0.1 m / s, and it enters the coagulation bath vertically, and the angle between the spun fiber and the coagulation bath liquid surface is 90±5.

[0039] The casting solution was stably delivered to the annular spinneret at a rate of 8–15 mL / min by a high-precision gear metering pump (accuracy ±0.5%), ensuring that the flow fluctuation was ≤1%.

[0040] The core liquid is a 10–20% solvent-water solution, precisely controlled by a constant-flow pump within the range of 3–8 mL / min. The solution is delivered through a constant-temperature, insulated pipeline (20–30°C ± 0.5°C) to prevent temperature fluctuations that could affect interfacial stability. The extruded casting solution is run in a Class 1000 cleanroom for 0.3–8 s. Adjustable windshields are installed on both sides of the run path to maintain a strict wind speed of ≤0.1 m / s to prevent airflow disturbances that could cause fiber vibration or uneven diameters.

[0041] The nascent fibers enter the coagulation bath in a vertical position (90±5°) to ensure symmetrical force distribution and avoid bending deformation. The coagulation bath is equipped with an automatic liquid level balancing system to maintain liquid level fluctuations ≤±1mm, ensuring consistent fiber formation.

[0042] This process achieves precise control of fiber structure through coordinated regulation of multiple parameters, laying the foundation for subsequent high-performance fiber preparation.

[0043] In the present invention, a further embodiment is that step S5 includes: the coagulation bath is a rectangular trough with a length of 2-4m and a depth of 0.5-0.8m, and is composed of 60-80% water, 15-30% solvent and 2-8% non-solvent by mass fraction. A circulating pump is used to form a unidirectional flow of the coagulation bath liquid from the inlet to the outlet in the trough, with a flow rate of 0.1-0.5m / s. A segmented temperature control device is provided on the outside of the coagulation bath trough, dividing the trough into 3-5 sections, and the temperature of each section is independently controlled so that the temperature is gradually reduced by 2-10°C from the inlet to the outlet, and the temperature difference between adjacent sections is ≤3°C. The spun fiber runs in an S-shaped path in the coagulation bath through guide rollers, with a residence time of 15-40 minutes, a guide roller spacing of 50-80cm, and a synchronized and adjustable speed to ensure smooth fiber operation.

[0044] In this embodiment, step S6 includes: a first crosslinking step, wherein the preliminarily coagulated fibers are placed in a 2-6% by mass epoxy crosslinker solution, the solution is heated in a constant temperature water bath at 20-45° C., and the pH value is adjusted to 8-10 by dropwise addition of a 10% by mass sodium hydroxide solution. The fibers are completely immersed and relaxed in the solution for 1-4 hours, with the fibers being turned over every 30 minutes; The second step is cross-linking. After removing the fiber, rinse the surface with deionized water to remove the residual cross-linking agent. Then put it into a 4-9% by mass amine cross-linking agent solution at a temperature of 30-65°C. Adjust the pH value to 9-11 by adding a 10% by mass hydrochloric acid solution. Let it react for 2-5 hours. The third step is cross-linking. The fiber is transferred to a 1-3% by mass aldehyde cross-linking agent solution and treated at 40-55°C for 0.5-2 hours. The solution stirring rate is maintained at 50-100 r / min during the cross-linking process.

[0045] In detail, the first step is to completely immerse the initially solidified fiber in a 2% to 6% by mass epoxy crosslinker solution, and use a constant temperature water bath circulation system to accurately control the reaction temperature in the range of 20°C to 45°C. A 10% by mass sodium hydroxide solution is slowly added through a peristaltic pump to dynamically adjust the pH value of the system to a weak alkaline environment of 8 to 10 to promote the ring-opening reaction of the epoxy group. The fiber remains naturally relaxed in the solution to avoid stress concentration. The soaking time is 1 to 4 hours. During this period, a robotic arm is used to assist in turning the fiber every 30 minutes to ensure uniform penetration of the crosslinker. The reaction vessel is equipped with a pH online monitor to adjust the alkali solution dripping speed in real time.

[0046] In the second step, the fibers, having completed the first stage of crosslinking, are treated in a three-tank countercurrent deionized water cleaning system, with the water temperature maintained at 25°C to 30°C, to thoroughly remove any residual epoxy crosslinker from the surface. The fibers are then transferred to a 4% to 9% by weight amine crosslinker solution. The reaction temperature is raised to a gradient of 30°C to 65°C using an oil bath heating system. Simultaneously, a 10% by weight hydrochloric acid solution is added dropwise to finely control the pH to a strongly alkaline range of 9 to 11. This reaction lasts for 2 to 5 hours, with inert gas bubbling used to prevent amine oxidation. An ultrasonic assist device is installed in the reactor to enhance crosslinker diffusion efficiency.

[0047] In the third step, the fibers cross-linked in the first two stages are transferred to a 1% to 3% by mass aldehyde cross-linking agent solution for final shaping. The solution temperature is stabilized at 40°C to 55°C by a PID temperature control module, and the magnetic stirring system maintains uniform flow of the solution at a rate of 50 to 100 revolutions per minute. The reaction time is 0.5 to 2 hours. During the process, a UV-visible spectrophotometer is used to monitor the cross-linking agent consumption rate. The reaction endpoint is determined when the absorbance change rate of the reaction solution is less than 1% / 10 minutes. Immediately after the fiber is discharged, it enters a low-temperature quenching tank to terminate the reaction and solidify the cross-linked network structure.

[0048] In this embodiment, step S7 includes: placing the cross-linked fibers in a 5-10% by mass ethanol solution at 25-30° C. for 1-2 hours, wherein the volume of the ethanol solution is 10-15 times the volume of the fibers; Rinse with deionized water 3-5 times, placing the fiber in flowing deionized water each time, with a water temperature of 20-30°C, a flushing flow rate of 500-1000 mL / min, and each flushing time of 10-20 minutes, until the conductivity of the flushing liquid is consistent with that of deionized water; Place the fiber in a vacuum drying oven and dry it for 4-6 hours at 30-40°C and a vacuum degree of -0.07 to -0.06 MPa. During the drying process, open the oven door every 2 hours to release water vapor. After drying, the fiber is placed in a hydrophilic modifier solution with a mass fraction of 0.5-2%, the solution temperature is 25-35°C, and soaked for 0.5-1.5 hours. The modifier is polyvinyl alcohol or sodium alginate. The fiber is completely stretched in the solution without entanglement.

[0049] In detail, the first stage: the cross-linked fibers are immersed in an ethanol-water solution with a mass fraction of 5% to 10%. The solution volume is strictly controlled to be 10 to 15 times the fiber volume to ensure sufficient contact area. The reaction system is maintained at a constant temperature of 25°C to 30°C, and the immersion time is 1 to 2 hours. This stage is assisted by magnetic stirring, and the speed is controlled at 100 to 150 revolutions per minute to promote the diffusion of residual cross-linking agent and by-products from the interior of the fiber to the solvent phase. After cleaning, the fibers and waste liquid are separated by siphoning, and the waste liquid enters the recovery system for ethanol purification and reuse.

[0050] The second stage: The fibers are transferred to a continuous water washing tank, which utilizes a three-stage countercurrent rinsing design. Each stage uses high-purity deionized water at 20°C to 30°C, with a flow rate of 500 to 1000 ml / min, and each stage lasts 10 to 20 minutes. During the washing process, the fibers are fixed to a rotating bracket, which rotates at a rate of 5 revolutions per minute, achieving full rinsing. An online conductivity meter monitors the drainage water quality in real time. Cleaning is terminated when the outlet water conductivity stabilizes below 1.0 μS / cm, ensuring the complete removal of ionic impurities on the fiber surface and within its pores.

[0051] The third stage: The clean fibers are evenly spread on a specially designed porous drying tray and placed in a vacuum drying oven for staged drying. Initially, a mild temperature of 30°C to 40°C and a negative pressure of -0.07 to -0.06 MPa are set to gradually remove bound water. A rapid air break is performed every two hours, with the door open for no more than 30 seconds to promptly expel accumulated water vapor. The drying process is monitored gravimetrically, with the final drying point determined when the difference in the fiber mass change rate between two consecutive measurements is less than 0.5%. The drying process takes a total of four to six hours.

[0052] Stage 4: Prepare a 0.5% to 2% modifier solution, preferably polyvinyl alcohol with a molecular weight of 80,000 to 120,000 or sodium alginate with a viscosity of 200 to 400 mPa·s. The solution temperature is maintained at a constant 25°C to 35°C. The fibers are immersed in a separate, single-filament state, and ultrasonic vibrations assist in unwinding to ensure zero entanglement. The modification tank is equipped with a circulating spray system, which fully refreshes the solution three times per hour to maintain a uniform concentration. The treatment lasts 0.5 to 1.5 hours. After completion, an air knife is used to remove any surface residue, leaving a monolayer of modifier.

[0053] In this embodiment, step S8 includes: placing the modified fiber in deionized water at a temperature of 50-70° C., completely immersing the fiber, stirring the water to allow it to flow slightly, and treating for 2-4 hours; The fiber is taken out, and after draining the surface moisture, it is placed in a hot air drying oven. A multi-layer tray is set in the drying oven, and the fibers are evenly laid on the tray without overlapping. The hot air temperature is 80-100°C, and the wind speed is 1-3m / s. The relative humidity of the drying environment is controlled at 30-50% by a dehumidifier. Dry for 2-3 hours, and record the fiber weight every 30 minutes until the difference between the two weights is ≤0.5%. The finished hollow fiber nanofiltration membrane is obtained.

[0054] Phase 1: The surface-modified fiber is completely immersed in deionized water at 50°C to 70°C, with the water bath volume being 20 to 30 times the volume of the fiber to ensure full stretching of the fiber. A low-speed stirring system is used to maintain a water flow rate of 0.2 to 0.5 meters per second, creating a gentle convection environment. The treatment time is 2 to 4 hours, during which the water temperature fluctuation is monitored by an online temperature sensor cluster to not exceed ±0.5°C. This phase promotes the rearrangement of the polymer chain segments inside the fiber, eliminates the internal stress generated during the pre-treatment process, and stabilizes the microscopic pore structure of the hollow fiber. The water bath is equipped with an automatic water replenishment device to maintain a constant liquid level.

[0055] The second stage: Pre-treatment begins by transferring the heat-set fibers to a centrifugal dehydrator, where they are dehydrated at 500 to 800 rpm for 30 seconds to remove free surface water. The fibers are then laid flat on specially made porous stainless steel trays in a single layer, ensuring zero overlap. The spacing between the trays is at least 5 cm to ensure airflow. Hot air drying utilizes a through-flow hot air circulation drying system, with the hot air temperature precisely controlled between 80°C and 100°C, and the wind speed adjustable from 1 to 3 meters per second. A six-point temperature and humidity monitoring system is installed within the drying oven, and a dehumidifier maintains the relative humidity within an optimized range of 30% to 50%. A gradient temperature increase strategy is implemented during the drying process, with the temperature set at 80°C for the initial 30 minutes to prevent surface crusting, and then gradually increased to 100°C to accelerate internal moisture migration.

[0056] Monitoring is performed, with automatic weighing and sampling every 30 minutes using a high-precision electronic balance. The drying endpoint is determined when the difference between two consecutive mass values ​​is ≤0.5%. The total drying time is typically 2 to 3 hours. Fiber shrinkage is monitored simultaneously during the drying process, with a laser rangefinder ensuring axial shrinkage is kept below 1%.

[0057] Through a unique multi-step cross-linking process, the degree of cross-linking and the cross-linking level of the membrane can be precisely controlled. In the first cross-linking step, the epoxy cross-linker reacts with the fiber at a specific temperature and pH value, laying the foundation for subsequent cross-linking; the reaction of the amine cross-linker in the second step and the treatment of the aldehyde cross-linker in the third step gradually build a stable membrane structure with specific properties. At the same time, the initial control step of the gradient pore structure utilizes the temperature gradient of the coagulation bath, the solution composition, and the fiber's running path and time control to achieve the construction of a precise gradient pore structure with gradually increasing pore size from the outer layer to the inner layer of the membrane. This precise membrane structure control gives the membrane better selective separation capabilities for substances of different molecular weights and properties, greatly improving the separation performance of the membrane.

[0058] Optimized process parameters, such as appropriate casting solution formula, precise spinning conditions and reasonable post-processing steps, make the prepared hollow fiber nanofiltration membrane have higher pure water flux. Under the operating pressure of 0.5-1.5MPa, the pure water flux can reach 15-60L / (m 2 ・h・bar), which is significantly improved compared with the generally low water flux in existing technologies and can meet the high flux requirements of large-scale water treatment.

[0059] The synergistic effect of multi-step cross-linking and the gradient pore structure gives the membrane excellent retention of target substances. For organic compounds with a molecular weight of 300-1000 Da, the retention rate is ≥90%, and the retention rate increases with molecular weight, with each 100 Da increase in molecular weight resulting in a 3-5% increase in retention. This allows for the effective separation and retention of harmful substances in water, ensuring effective water purification.

[0060] A series of rigorous steps, from raw material pretreatment to performance stabilization, ensure the stability of the membrane structure. After 72 hours of continuous operation, the pure water flux retention rate is ≥90%, and the rejection rate retention rate is ≥95%. This effectively solves the problem of rapid performance degradation of existing membranes during long-term use, extends the membrane life, and reduces replacement costs.

[0061] The entire preparation process is clear and organized, with each step closely coordinated to reduce unnecessary complex operations. During the preparation of the casting solution, clear raw material ratios, mixing temperature, stirring rate, and degassing conditions ensure efficient and stable preparation of the casting solution. The spinneret debugging steps are simple and targeted, ensuring a smooth spinning process. Compared with the numerous process control points and difficult-to-control coating layer thickness and cross-linking reactions in traditional preparation processes, the process of the present invention is easier to operate and control, making it more suitable for large-scale industrial production.

[0062] The embodiments of the present application are described above in conjunction with the accompanying drawings. Unless there is a conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application is not limited to the above-mentioned specific implementation methods. The above-mentioned specific implementation methods are merely illustrative and not restrictive. Under the guidance of this application, ordinary technicians in this field can also make many forms without departing from the purpose of this application and the scope of protection of the claims, all of which are within the protection of this application.

Claims

1. A process for preparing hollow fiber nanofiltration membranes by multi-step cross-linking and gradient pore control, characterized in that: The following steps are involved: S1, raw material pretreatment; S2, preparation of casting solution; S3, spinneret debugging; S4, dry and wet spinning; S5, preliminary regulation of gradient pore structure; S6, multi-step cross-linking treatment; S7, post-processing modification; S8, performance stabilization processing.

2. The process for preparing hollow fiber nanofiltration membrane by multi-step cross-linking and gradient pore control according to claim 1, characterized in that: The step S1 comprises: drying the polymer material at 80-100° C. and a vacuum degree of -0.08 to -0.06 MPa for 4-6 hours, stirring the polymer material every hour during the drying process to ensure uniform heating, pressurizing and filtering the solvent using a 0.22 μm polytetrafluoroethylene filter membrane, and controlling the filtration pressure at 0.2-0.3 MPa, grinding the additive using a planetary ball mill, using zirconium oxide balls as the grinding medium, a ball-to-material ratio of 5:1, a grinding speed of 300-400 r / min, and a grinding time of 2-3 hours until the particle size is less than 50 μm, and then sieving for later use.

3. The process for preparing hollow fiber nanofiltration membrane by multi-step cross-linking and gradient pore control according to claim 1, characterized in that: The step S2 comprises: adding 25-32 wt% of the pretreated polymer material, 58-70 wt% of the pretreated solvent, and 3-12 wt% of the pretreated additive into a double-layer glass reactor equipped with a stirring paddle and a thermometer according to a mass ratio, wherein the stirring paddle is an anchor structure and the diameter of the paddle is 0.6-0.8 times the inner diameter of the reactor; mixing at 60-75° C. and a stirring rate of 300-500 r / min for 4-6 hours, sampling and observing every 30 minutes until a uniform solution without visible particles is formed; transferring the solution to a vacuum degassing tank, and degassing for 2-4 hours under a vacuum environment of -0.09 to -0.06 MPa; maintaining the temperature in the tank at 50-60° C. during the degassing process, and intermittently stirring to prevent the solution from stratification.

4. The process for preparing hollow fiber nanofiltration membrane by multi-step cross-linking and gradient pore control according to claim 1, characterized in that: The step S3 comprises: selecting a stainless steel annular spinneret with an outer diameter of 0.6-1.8 mm, an inner diameter of 0.2-0.9 mm, and a concentricity deviation of the inner and outer channels of ≤0.02 mm; during debugging, first introducing 60-70° C. hot water to preheat the spinneret for 30-60 minutes to make the overall temperature of the spinneret reach 40-60° C. and uniform; introducing compressed air from the inner and outer channels at a pressure of 0.1-0.2 MPa for 10-15 seconds, checking whether the channels are unobstructed, and observing the spinneret outlet with an optical microscope.

5. The process for preparing hollow fiber nanofiltration membrane by multi-step cross-linking and gradient pore control according to claim 1, characterized in that: The step S4 includes: delivering the casting liquid to the spinneret through a metering pump, the metering pump is a gear pump with an accuracy of ±0.5% and a delivery rate of 8-15 mL / min, the core liquid introduced into the inner tube of the spinneret is a solvent aqueous solution with a mass fraction of 10-20%, the core liquid is delivered by a constant flow pump with a flow rate of 3-8 mL / min, the core liquid temperature is controlled at 20-30°C, and the core liquid is delivered to the spinneret through an insulation pipeline. The extruded casting liquid runs in a clean environment for 0.3-8 seconds, a windproof baffle is set on the running path, the wind speed is controlled at ≤0.1 m / s, and the liquid enters the coagulation bath vertically, and the angle between the nascent fiber and the coagulation bath liquid surface is 90°±5.

6. The process for preparing hollow fiber nanofiltration membrane by multi-step cross-linking and gradient pore control according to claim 1, characterized in that: The step S5 includes: the coagulation bath is a rectangular trough with a length of 2-4 meters and a depth of 0.5-0.8 meters, and is composed of 60-80% water, 15-30% solvent, and 2-8% non-solvent by mass. A circulating pump is used to form a unidirectional flow of the coagulation bath liquid from the inlet to the outlet in the trough at a flow rate of 0.1-0.5 m / s. A segmented temperature control device is provided on the outside of the coagulation bath trough, dividing the trough into 3-5 sections. The temperature of each section is independently controlled so that the temperature is gradually reduced by 2-10°C from the inlet to the outlet, and the temperature difference between adjacent sections is ≤3°C. The spun fibers are run in an S-shaped path in the coagulation bath through guide rollers, with a residence time of 15-40 minutes, a guide roller spacing of 50-80 cm, and a synchronized and adjustable rotation speed to ensure smooth fiber operation.

7. The process for preparing hollow fiber nanofiltration membrane by multi-step cross-linking and gradient pore control according to claim 1, characterized in that: The step S6 comprises: a first crosslinking step, wherein the preliminarily coagulated fiber is placed in a 2-6% by mass epoxy crosslinking agent solution, the solution is heated in a constant temperature water bath at a temperature of 20-45° C., and the pH value is adjusted to 8-10 by dropwise addition of a 10% by mass sodium hydroxide solution, the fiber is completely immersed and relaxed in the solution, and the fiber is immersed for 1-4 hours, with the fiber being turned over every 30 minutes; The second step is cross-linking. After removing the fiber, rinse the surface with deionized water to remove the residual cross-linking agent. Then put it into a 4-9% by mass amine cross-linking agent solution at a temperature of 30-65°C. Adjust the pH value to 9-11 by adding a 10% by mass hydrochloric acid solution. Let it react for 2-5 hours. The third step is cross-linking. The fiber is transferred to a 1-3% by mass aldehyde cross-linking agent solution and treated at 40-55°C for 0.5-2 hours. The solution stirring rate is maintained at 50-100 r / min during the cross-linking process.

8. The process for preparing hollow fiber nanofiltration membrane by multi-step cross-linking and gradient pore control according to claim 1, characterized in that: The step S7 comprises: placing the cross-linked fibers in a 5-10% by mass ethanol solution at 25-30° C. for 1-2 hours, wherein the volume of the ethanol solution is 10-15 times the volume of the fibers; Rinse with deionized water 3-5 times, placing the fiber in flowing deionized water each time, with a water temperature of 20-30°C, a flushing flow rate of 500-1000 mL / min, and each flushing time of 10-20 minutes, until the conductivity of the flushing liquid is consistent with that of deionized water; Place the fiber in a vacuum drying oven and dry it for 4-6 hours at 30-40°C and a vacuum degree of -0.07 to -0.06 MPa. During the drying process, open the oven door every 2 hours to release water vapor. After drying, the fiber is placed in a hydrophilic modifier solution with a mass fraction of 0.5-2%, the solution temperature is 25-35°C, and soaked for 0.5-1.5 hours. The modifier is polyvinyl alcohol or sodium alginate. The fiber is completely stretched in the solution without entanglement.

9. The process for preparing hollow fiber nanofiltration membrane by step-crosslinking and gradient pore control according to claim 1, characterized in that: The step S8 comprises: placing the modified fiber in deionized water at a temperature of 50-70° C., completely immersing the fiber, stirring the water to allow it to flow slightly, and treating for 2-4 hours; The fiber is taken out, and after draining the surface moisture, it is placed in a hot air drying oven. A multi-layer tray is set in the drying oven, and the fibers are evenly laid on the tray without overlapping. The hot air temperature is 80-100°C, and the wind speed is 1-3m / s. The relative humidity of the drying environment is controlled at 30-50% by a dehumidifier. Dry for 2-3 hours, and record the fiber weight every 30 minutes until the difference between the two weights is ≤0.5%. The finished hollow fiber nanofiltration membrane is obtained.

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