PTFE multilayer composite filter membrane with gradient aperture structure

Abstract

The invention relates to a PTFE-PAN multilayer composite filter membrane with a gradient aperture structure and application thereof, and belongs to the technical field of water treatment. The composite membrane is formed by alternately compounding polytetrafluoroethylene (PTFE) membrane layers and polyacrylonitrile (PAN) filling membranes, and a three-stage gradient structure of a large-aperture layer (50 microns), a medium-aperture layer (10 microns) and a small-aperture layer (1 microns) is formed. The pore size distribution of each layer is accurately controlled by regulating and controlling the molecular weight ratio of the PTFE resin and two-way stretching process parameters; carboxymethyl cellulose (CMC) is added into the PAN filling membrane, so that the PAN filling membrane has a chemical adsorption function. The composite membrane can realize graded filtration: the large-aperture layer intercepts silt and suspended solids (gt, 50 [mu] m), the medium-aperture layer removes colloidal substances (1-10 [mu] m), and the small-aperture layer adsorbs heavy metal ions (lt, 1 [mu] m). The retention rate of the membrane to silt / suspended solids is greater than or equal to 99.5%, the removal rate of colloidal substances is greater than or equal to 98%, the retention rates of the membrane to Cd, Pb and Cu are greater than or equal to 95.0%, 97.0% and 98.0% respectively, and the membrane is suitable for efficient purification treatment of industrial wastewater.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of water treatment, in particular to a PTFE-PAN multi-layer composite filter membrane with gradient pore size structure and application. The composite membrane can realize the graded interception of pollutants of different particle sizes by constructing a three-level gradient pore size structure, and is particularly suitable for industrial wastewater treatment containing heavy metal ions. BACKGROUND

[0002] With the continuous expansion of industrial scale, heavy metal pollution in industrial wastewater poses a serious threat to the ecological environment and human health due to its high toxicity and non-biodegradability. Membrane separation technology has become a research hotspot in the field of water treatment due to its advantages such as high efficiency and low energy consumption. Among them, polytetrafluoroethylene (PTFE) membrane has shown good application prospects in wastewater treatment due to its excellent mechanical strength, chemical stability and controllable pore size distribution. Compared with traditional polymer membranes such as polysulfone and cellulose, PTFE membrane can maintain structural stability under extreme conditions such as strong acid and strong base, avoiding the problem of decreased interception performance caused by degradation of membrane materials.

[0003] Currently, the preparation methods of PTFE membrane mainly include stretching method, electrospinning method and phase separation method, etc. Studies have shown that PTFE membranes prepared by stretching method have uniform microporous structure and good removal effect on heavy metal ions. Stretching method can be divided into unidirectional stretching and biaxial stretching according to the stretching direction: unidirectional stretching process is simple, but stretching in a single direction will lead to insufficient transverse strength of the film, which is easy to tear; biaxial stretching process can significantly improve the mechanical properties and dimensional stability of the film through longitudinal and transverse stretching, and can accurately control the microporous structure, so it has become the mainstream preparation technology of high-performance PTFE membrane.

[0004] However, the PTFE membranes prepared by biaxial stretching in the prior art are mostly single-layer structures, and have not been combined with functional materials (such as thermoplastic filled membranes) to construct a multifunctional system, making it difficult to balance mechanical strength and flexibility. For example: the PTFE microporous membrane disclosed in CN09608794A has a pore size distribution of 0.2-1 μm, which has basic filtration performance, but due to the homogenization of the pore structure, it cannot realize the graded interception of large particles (>10 μm), colloids (1-10 μm) and heavy metal ions (<1 μm); CN115449105A modifies the PTFE membrane by introducing AOI monomers, but the modified monomers are easy to fall off, have poor corrosion resistance, and the modification process may damage the PTFE microporous structure, resulting in a decrease in membrane strength.

[0005] In actual industrial wastewater treatment, the above-mentioned technologies have obvious defects: the single-layer homogeneous structure is easy to be blocked by large-particle impurities, resulting in a decrease in filtration flux; the lack of grading treatment capability makes it difficult to simultaneously and efficiently remove different particle size pollutants; and the chemical stability of the modified PTFE membrane is insufficient, affecting the service life of the membrane module. Therefore, developing a PTFE composite membrane with a gradient pore size structure, capable of grading interception of pollutants and having excellent mechanical properties and chemical stability, has become a technical problem to be solved in the field. SUMMARY

[0006] The present application provides a PTFE-PAN composite filtration membrane with a gradient pore size structure and a preparation method thereof, mainly solving the problem that the existing filtration materials are difficult to balance the interception efficiency of different particle size pollutants and the filtration resistance.

[0007] 1. Composite filtration membrane structure: the composite filtration membrane is composed of alternating stacking of PTFE membrane layers and PAN filled membranes, forming a three-level gradient pore size structure: PTFE membrane large pore size layer (50 μm): prepared by using PTFE resins with molecular weights of 2 million and 10 million at a mass ratio of 2:1; medium pore size layer (10 μm): prepared by using the above-mentioned PTFE resins at a mass ratio of 1:1; small pore size layer (1 μm): prepared by using the above-mentioned PTFE resins at a mass ratio of 1:2.

[0008] The PAN filled membrane contains 2-10 wt% carboxymethyl cellulose (CMC) to enhance the membrane layer bonding force and stability.

[0009] 2. Preparation method including the following steps: (1) mixing and pretreatment: mixing different molecular weight PTFE dispersion resins with a proportion of 15-25 wt% and a co-extrusion agent (liquid paraffin or petroleum ether), ball milling for 30-60 min (40-50 r / min), and aging at 30-45℃ for 24-36 h; (2) extrusion and calendering to form a film: the blend is extruded through a double screw extruder at 250-280℃, and is calendered through a flat die at 30-50℃ to form a 200-800 μm thick initial film sheet; (3) solvent removal: the film sheet is heated at 150-200℃ for 2-4 h to remove the co-extrusion agent; (4) two-way stretching: longitudinally stretching 5-10 times and transversely stretching 10-15 times (rate 5-15 m / min) at 250-300℃ to obtain PTFE membranes with different pore sizes; (5) PAN filled membrane preparation: 10-20wt% PAN resin is dissolved in DMF, 2-10wt% CMC is added, and a 50-200μm thick film is scraped, dried at 60-100℃; (6) Hot pressing composite: PTFE film and PAN film are stacked in gradient, hot pressed at 200℃, 0.3 MPa pressure, and 8 m / min.

[0010] 3. Performance and application The composite membrane shows excellent performance in water treatment: Graded filtration performance: retention rate of > 50μm particulate matter ≥ 99.5%, effluent turbidity ≤ 0.25NTU; removal rate of 1-10μm colloidal matter ≥ 98%; retention rate of Cd²⁺, Pb²⁺, Cu²⁺ ≥ 95.0%, 97.0%, 98.0% respectively.

[0011] Running stability: pressure loss < 0.05MPa during filtration process, initial flux ≥ 80 L・m⁻²・h⁻¹, flux decay rate ≤ 5% after 10 hours of continuous operation, membrane body mechanical strength is stable, no damage or deformation phenomenon.

[0012] Pollution resistance: the hydrophilic network structure of CMC in PAN filled membrane can reduce the adsorption of pollutants, the chemical inertness of PTFE substrate inhibits the growth of microorganisms, and the deposition amount of membrane surface pollutants is reduced by more than 40% compared with traditional membranes. Beneficial effects

[0013] The present application realizes the gradient distribution of PTFE membrane pore size by precise control of bidirectional stretching process parameters, and combines the stacking of PAN filled membrane to construct a "coarse filtration-fine filtration-chelation" three-level synergistic filtration system: 1. Improved graded filtration performance Innovative design of 50 μm→10 μm→1 μm three-level gradient pore size structure, through the synergistic effect of physical screening and chemical adsorption. Through the large pore size layer (50 μm) to intercept suspended solids such as silt, the medium pore size layer (10 μm) to remove colloidal substances, and the small pore size layer (1 μm) to block small particles, forming a step-by-step filtration; 2. Material synergistic effect optimization Different molecular weight PTFE resins are used for compounding, high molecular weight resin (10 million) ensures the mechanical strength of the membrane body, and low molecular weight resin (2 million) improves the processing fluidity, and the two are gradient ratio controlled for flexibility.

[0014] 3. Synergistically enhanced mechanical-chemical stability The PTFE substrate provides high-strength support through a biaxial stretching structure, combined with an interfacial interlocking structure formed by hot-pressing, significantly improving mechanical durability. The CMC in the PAN-filled membrane selectively adsorbs heavy metal ions (such as Cd²⁺ and Pb²⁺) through carboxyl chelation, while its hydrogen bond network forms an antifouling coating, combining chemical stability and long-term operation stability.

[0015] 4 High-efficiency anti-pollution and selective separation function The hydrophilic segment of CMC forms a hydration layer on the membrane surface, combined with the low surface energy property of PTFE, synergistically inhibiting the adsorption of suspended solids and organic matter. The chemical-physical dual anti-fouling mechanism (carboxyl chelation + hydration layer barrier) realizes selective adsorption of heavy metals and simultaneous barrier of pollutants, suitable for high-precision separation requirements in complex environments. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 Structure diagram of PTFE-PAN composite filter membrane in the present application DETAILED DESCRIPTION

[0017] The present application will be further described below in conjunction with specific examples.

[0018] The composite filter membrane is composed of PTFE membrane layers and PAN-filled membranes with the same pore size, alternately stacked to form a large-middle-small three-level gradient pore size structure. EMBODIMENT

[0019] The preparation steps of the PTFE membrane layer are as follows: (1) Mixing and pretreatment Large pore size layer: 2 million and 10 million molecular weight PTFE dispersion resins are mixed in a mass ratio of 2:1, mixed with liquid paraffin (extrusion aid) at a proportion of 25wt%, ball milled at 40 r / min for 30 min, and aged at 30℃ for 24 h.

[0020] Medium pore size layer: 2 million and 10 million molecular weight PTFE resins are mixed in a ratio of 1:1, mixed with liquid paraffin (extrusion aid) at a proportion of 20wt%, ball milled at 45 r / min for 30 min, and aged at 35℃ for 28 h.

[0021] Small pore size layer: 2 million and 10 million molecular weight PTFE resins are mixed in a ratio of 1:2, mixed with liquid paraffin (extrusion aid) at a proportion of 15wt%, ball milled at 50 r / min for 60 min, and aged at 40℃ for 28 h.

[0022] (2) Extrusion and calendering to form a film Large pore size layer: The blend is extruded at 280℃ through a double-screw extruder (50 rpm), with a compression ratio of 100, and is calendered through a 50℃ flat die to form an 800 μm thick initial film.

[0023] Medium pore size layer: The blend was extruded through a twin-screw extruder at 260°C (60 rpm), compression ratio 50, and calendered through a 50°C flat die into an initial 400 μm thick film.

[0024] Small pore size layer: The blend was extruded through a twin-screw extruder at 250°C (65 rpm), compression ratio 30, and calendered through a 50°C flat die into an initial 200 μm thick film.

[0025] (3) Solvent removal Large pore size layer: The film was heated at 150°C for 2 h to remove the extrusion aid.

[0026] Medium pore size layer: The film was heated at 180°C for 3 h to remove the extrusion aid.

[0027] Small pore size layer: The film was heated at 200°C for 4 h to remove the extrusion aid.

[0028] (4) Biaxial stretching: Large pore size layer: Biaxial stretching was performed at 250°C, 5x in the machine direction and 10x in the transverse direction at a rate of 5 m / min to obtain a large pore size PTFE film.

[0029] Medium pore size layer: Biaxial stretching was performed at 280°C, 7x in the machine direction and 12x in the transverse direction at a rate of 10 m / min to obtain a medium pore size PTFE film.

[0030] Small pore size layer: Biaxial stretching was performed at 300°C, 10x in the machine direction and 15x in the transverse direction at a rate of 15 m / min to obtain a small pore size PTFE film.

[0031] PAN filled film preparation Large pore size layer: 10 wt% PAN resin was dissolved in DMF with 3 wt% CMC and cast into a 200 μm thick film, which was dried at 60°C.

[0032] Medium pore size layer: 15 wt% PAN resin was dissolved in DMF with 5 wt% CMC and cast into a 100 μm thick film, which was dried at 80°C.

[0033] Small pore size layer: 20 wt% PAN resin was dissolved in DMF with 7 wt% CMC and cast into a 50 μm thick film, which was dried at 100°C.

[0034] Hot press lamination The PTFE films and PAN films were stacked in the order large -> medium -> small pore size (PTFE-PAN order) and hot pressed at 200°C, 0.3 MPa pressure at a speed of 8 m / min.

[0035] The composite membrane rejection rate: ≥99.5% for silt and suspended solids (>50 μm), effluent turbidity ≤0.25 NTU; colloidal particles (1-10 μm) ≥98.2%; heavy metal rejection: Cd 2+ ≥95.8%, Pb 2+ ≥97.8%, Cu 2+ ≥98.5%, pressure loss <0.05 MPa, initial flux 80 L·m⁻²·h⁻¹, flux decay rate ≤3% after 10 hours of continuous operation, and membrane surface pollutant deposition amount reduced by 42% compared with traditional membranes. Example

[0036] The PTFE membrane layer preparation steps are as follows: (1) Mixing and pretreatment Large pore size layer: 2 million and 10 million molecular weight PTFE dispersion resins were mixed in a mass ratio of 2:1, mixed with petroleum ether (co-extrusion agent) at a proportion of 24wt%, ball milled at a speed of 38 r / min for 35 min, and aged at 40℃ for 24 h.

[0037] Medium pore size layer: 2 million and 10 million molecular weight PTFE resins were mixed in a ratio of 1:1, mixed with petroleum ether (co-extrusion agent) at a proportion of 19wt%, ball milled at a speed of 45 r / min for 30 min, and aged at 35℃ for 28 h.

[0038] Small pore size layer: 2 million and 10 million molecular weight PTFE resins were mixed in a ratio of 1:2, mixed with petroleum ether (co-extrusion agent) at a proportion of 16wt%, ball milled at a speed of 50 r / min for 60 min, and aged at 40℃ for 30 h.

[0039] (2) Extrusion and film formation by calendering Large pore size layer: The blend was extruded at 275℃ through a double screw extruder (55 rpm), with a compression ratio of 90, and calendered through a flat die at 55℃ to form an initial film sheet with a thickness of 750 μm.

[0040] Medium pore size layer: The blend was extruded at 255℃ through a double screw extruder (60 rpm), with a compression ratio of 55, and calendered through a flat die at 50℃ to form an initial film sheet with a thickness of 400 μm.

[0041] Small pore size layer: The blend was extruded at 250℃ through a double screw extruder (65 rpm), with a compression ratio of 35, and calendered through a flat die at 50℃ to form an initial film sheet with a thickness of 200 μm.

[0042] (3) Solvent removal Large pore size layer: The film sheet was heated at 150℃ for 2 h to remove the co-extrusion agent.

[0043] Medium pore size layer: The film was heated at 180°C for 3 h to remove the extrusion aid.

[0044] Small pore size layer: The film was heated at 200°C for 4 h to remove the extrusion aid.

[0045] (4) Biaxial stretching Large pore size layer: Biaxial stretching was performed at 250°C, 6 times in the longitudinal direction and 10 times in the transverse direction, at a stretching rate of 5 m / min, to obtain a PTFE film with large pore size.

[0046] Medium pore size layer: Biaxial stretching was performed at 250°C, 8 times in the longitudinal direction and 13 times in the transverse direction, at a stretching rate of 10 m / min, to obtain a PTFE film with medium pore size.

[0047] Small pore size layer: Biaxial stretching was performed at 250°C, 10 times in the longitudinal direction and 15 times in the transverse direction, at a stretching rate of 14 m / min, to obtain a PTFE film with small pore size.

[0048] PAN filled membrane preparation Large pore size layer: 12 wt% PAN resin was dissolved in DMF, 3.5 wt% CMC was added, and a 180 μm thick film was scraped, dried at 60°C.

[0049] Medium pore size layer: 16 wt% PAN resin was dissolved in DMF, 5 wt% CMC was added, and a 150 μm thick film was scraped, dried at 80°C.

[0050] Small pore size layer: 20 wt% PAN resin was dissolved in DMF, 8 wt% CMC was added, and a 50 μm thick film was scraped, dried at 100°C.

[0051] Hot-pressing compounding PTFE membranes and PAN membranes were alternately stacked in the order of large → medium → small pore size (PTFE-PAN sequence) and hot-pressed at 200°C, 0.3 MPa pressure, and a speed of 8 m / min.

[0052] The rejection rate of the composite membrane: ≥99.6% for silt and suspended solids (>50 μm), water turbidity ≤0.25 NTU; ≥98.5% for colloids (1-10 μm); heavy metal rejection: Cd 2+ ≥96.1%, Pb 2+ ≥98.2%, Cu 2+ ≥98.3%; pressure loss <0.05 MPa, initial flux 82 L・m⁻²・h⁻¹, flux decay rate ≤2% after 10 hours of continuous operation, and the amount of pollutants deposited on the membrane surface was reduced by 45% compared to traditional membranes. The flux of the membrane remained stable during the filtration process and did not decrease significantly. Example

[0053] PTFE film layer preparation steps are as follows: (1) Mixing and pretreatment Large pore size layer: PTFE dispersion resin with molecular weight of 2 million and 10 million was mixed at a mass ratio of 2:1, mixed with liquid paraffin (extrusion aid) at a proportion of 23 wt%, ball-milled at 42 r / min for 30 min, and aged at 35°C for 26 h.

[0054] Medium pore size layer: PTFE resin with molecular weight of 2 million and 10 million was mixed at a ratio of 1:1, mixed with petroleum ether (extrusion aid) at a proportion of 18 wt%, ball-milled at 45 r / min for 40 min, and aged at 35°C for 28 h.

[0055] Small pore size layer: PTFE resin with molecular weight of 2 million and 10 million was mixed at a ratio of 1:2, mixed with petroleum ether (extrusion aid) at a proportion of 15 wt%, ball-milled at 50 r / min for 60 min, and aged at 40°C for 32 h.

[0056] (2) Extrusion and film formation by calendering Large pore size layer: The blend was extruded at 275°C through a twin-screw extruder (55 rpm) with a compression ratio of 80, and calendered through a flat die at 55°C to form an initial film sheet with a thickness of 600 μm.

[0057] Medium pore size layer: The blend was extruded at 255°C through a twin-screw extruder (60 rpm) with a compression ratio of 50, and calendered through a flat die at 50°C to form an initial film sheet with a thickness of 400 μm.

[0058] Small pore size layer: The blend was extruded at 250°C through a twin-screw extruder (65 rpm) with a compression ratio of 40, and calendered through a flat die at 50°C to form an initial film sheet with a thickness of 200 μm.

[0059] (3) Solvent removal Large pore size layer: The film sheet was heated at 150°C for 2.5 h to remove the liquid paraffin.

[0060] Medium pore size layer: The film sheet was heated at 180°C for 3 h to remove the petroleum ether.

[0061] Small pore size layer: The film sheet was heated at 200°C for 4 h to remove the petroleum ether.

[0062] (4) Two-way stretching Large pore size layer: Two-way stretching was performed at 255°C with a longitudinal stretching of 5 times and a transverse stretching of 12 times at a stretching rate of 5 m / min.

[0063] Medium pore size layer: Two-way stretching was performed at 250°C with a longitudinal stretching of 7 times and a transverse stretching of 12 times at a stretching rate of 10 m / min.

[0064] Small pore size layer: two-way stretching at 250℃, 10 times in the longitudinal direction and 15 times in the lateral direction, stretching rate 15 m / min.

[0065] PAN filled membrane preparation Large pore size layer: 12 wt% PAN resin was dissolved in DMF, 4 wt% CMC was added, and a 185 μm thick film was scraped, dried at 70℃.

[0066] Medium pore size layer: 16 wt% PAN resin was dissolved in DMF, 7 wt% CMC was added, and a 140 μm thick film was scraped, dried at 80℃.

[0067] Small pore size layer: 20 wt% PAN resin was dissolved in DMF, 10 wt% CMC was added, and a 50 μm thick film was scraped, dried at 100℃.

[0068] Hot-pressing compounding PTFE membrane and PAN membrane were alternately stacked in the order of large → medium → small pore size gradient (PTFE-PAN sequence), and hot-pressed at 200℃, 0.3MPa pressure and 8 m / min speed.

[0069] The rejection rate of the composite membrane: the rejection rate of silt and suspended solids (>50 μm) is ≥99.7%, the water turbidity is ≤0.25 NTU; the removal rate of colloids (1-10 μm) is ≥98.8%; the heavy metal ion rejection rate: Cd²⁺≥96.2%, Pb²⁺≥98.8%, Cu²⁺≥98.6%; pressure loss <0.05 MPa, initial flux 85 L・m⁻²・h⁻¹, flux decay rate ≤1% after 10 hours of continuous operation, and the amount of membrane surface pollutant deposition is reduced by 48% compared with traditional membranes. The flux stability is high, and there is no damage or deformation after 10 hours of continuous operation.

[0070] Comparative Example 1 The difference between this comparative example and Example 1 is that a cross-pore size cross-compounding structure (large pore PTFE + small pore PAN / medium pore PTFE + large pore PAN / small pore PTFE + medium pore PAN) is used. 1. Large pore size PTFE membrane compounded with small pore size PAN membrane PTFE membrane preparation (same as the original example of large pore size layer): Resin ratio: molecular weight 2 million: molecular weight 10 million = 2:1, liquid paraffin ratio 25%, ball milling 30min, aging 24h; extrusion temperature 280℃, calendering thickness 800 μm, two-way stretching, 5 times in the longitudinal direction and 10 times in the lateral direction, pore size 50 μm.

[0071] PAN filling membrane preparation (same as the original embodiment of the small pore size layer parameters): PAN concentration 20%, CMC content 10%, blade coating thickness 50 μm, drying temperature 100℃, pore size 1 μm.

[0072] 2. Mesoporous PTFE membrane composite macroporous PAN membrane PTFE membrane preparation (same as the original embodiment of the mesoporous layer): Resin ratio 1:1, petroleum ether ratio 20%, ball milling 30 min, aging 28 h; Extrusion temperature 260℃, calendering thickness 400 μm, two-way stretching, longitudinal 7 times, transverse 12 times, pore size 10 μm.

[0073] PAN filling membrane preparation (same as the original embodiment of the large pore size layer parameters): PAN concentration 10%, CMC content 3%, blade coating thickness 200 μm, drying temperature 60℃, pore size 50 μm.

[0074] 3. Small pore size PTFE membrane composite mesoporous PAN membrane PTFE membrane preparation (same as the original embodiment of the small pore size layer): Resin ratio 1:2, liquid paraffin ratio 15%, ball milling 60 min, aging 36 h; Extrusion temperature 250℃, calendering thickness 200 μm, two-way stretching, longitudinal 10 times, transverse 15 times, pore size 1 μm.

[0075] PAN filling membrane preparation (same as the original embodiment of the mesoporous layer parameters): PAN concentration 15%, CMC content 5%, blade coating thickness 100 μm, drying temperature 80℃, pore size 10 μm.

[0076] 4. Hot pressing composite Stacking order: 50 μm PTFE-1 μm PAN-10 μm PTFE-50 μm PAN-1 μm PTFE-10 μm PAN, hot pressing composite at 200℃, 0.3 MPa, and a speed of 8 m / min.

Claims

1. A PTFE-PAN multilayer composite filter membrane with a gradient pore size structure, characterized in that: It is composed of alternating stacking of polytetrafluoroethylene (PTFE) membrane layers and polyacrylonitrile (PAN) filling membranes to form a three-level gradient structure with a large pore layer (50μm), a medium pore layer (10μm) and a small pore layer (1μm); the pore size of the PTFE membrane layer is regulated by a biaxial stretching process, and the PAN filling membrane contains 2-10wt% carboxymethyl cellulose (CMC).

2. The PTFE-PAN multilayer composite filter membrane according to claim 1, characterized in that: The large-pore layer is prepared using PTFE resins with molecular weights of 2 million and 10 million at a mass ratio of 2:1, the medium-pore layer is prepared using the above resins at a mass ratio of 1:1, and the small-pore layer is prepared using the above resins at a mass ratio of 1:

2.

3. A method for preparing the PTFE-PAN multilayer composite filter membrane according to any one of claims 1 to 2, characterized in that: The following steps are involved: (1) Mixing and pretreatment: PTFE dispersion resins with different molecular weights are mixed with extrusion aids at a ratio of 15-25 wt%, ball milled at a speed of 40-50 r / min for 30-60 min, and aged at 30-45 ° C for 24-36 h; (2) Extrusion and calendering: The blend is extruded through a twin-screw extruder at 250-280°C and calendered through a flat die at 30-50°C into an initial film with a thickness of 200-800 μm; (3) Solvent removal: Heat the membrane at 150-200 °C for 2-4 h to remove the extrusion aid; (4) Biaxial stretching: Biaxial stretching is performed at 250-300°C, with a longitudinal stretching ratio of 5-10 times and a transverse stretching ratio of 10-15 times at a stretching rate of 5-15 m / min to obtain PTFE membranes with different pore sizes; (5) Preparation of PAN-filled membrane: Dissolve 10-20 wt% PAN resin in DMF, add 2-10 wt% CMC, scrape into a 50-200 μm thick membrane, and dry at 60-100 °C; (6) Hot pressing composite: PTFE membrane and PAN membrane were stacked alternately according to the pore size gradient, and hot pressed at 200 °C, 0.3 MPa pressure and a speed of 8 m / min.

4. The preparation method according to claim 3, characterized in that The extrusion aid is liquid paraffin or petroleum ether.

5. The use of the PTFE-PAN multilayer composite filter membrane in water treatment according to claim 1, characterized in that: The retention rate for particles >50μm is ≥99.5%, and the effluent turbidity is ≤0.25NTU; the removal rate for 1-10μm colloidal substances is ≥98%; the retention rates for Cd²⁺, Pb²⁺, and Cu²⁺ are ≥95.0%, 97.0%, and 98.0%, respectively; the pressure loss is <0.05MPa, the initial flux is ≥80L・m⁻²・h⁻¹, the flux decay rate is ≤5% after 10 hours of continuous operation, and the membrane is not damaged or deformed.

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