Preparation method for forming hierarchical pore filter material through secondary coating

Abstract

The invention discloses a preparation method for forming a hierarchical pore filter material through secondary coating. In order to solve the problems that an existing filtering material is single in pore diameter, and filtering efficiency and pressure drop are difficult to consider at the same time, the hierarchical pore filtering material is formed through the following steps that 1, first-time coating is conducted, specifically, a NaCl template agent and a PTFE solution are mixed according to the ratio of 7: 3, 7%-16% of a solvent is added, a NaCl-coated PTFE foaming solution is prepared, a polyphenylene sulfide needled felt is coated with the NaCl-coated PTFE foaming solution, pre-drying is conducted, and then washing is conducted to form a macroporous structure; 2, secondary coating: coating the macroporous layer with a PTFE foaming liquid containing an AC foaming agent, and carrying out stepped sintering to form micropores, so that efficient interception of particles of 0.5-2.5 microns is realized, and the pressure drop is reduced to 116-251Pa. The multi-stage hole filtering material is suitable for high-temperature flue gas treatment such as waste incineration, has high filtering performance and low running resistance, and has remarkable environmental protection and economic value.

Description

Technical Field

[0001] The invention belongs to the field of high-temperature smoke filtration, and in particular relates to a preparation method of a filter material formed by secondary coating to form multi-level pores. Background Art

[0002] With the expansion of industrialization, global atmospheric particulate matter (PM) emissions have increased significantly, causing serious harm to public health and ecosystems. In this context, the development of filter media with high-efficiency interception performance for high-temperature flue gases emitted by industrial processes such as waste incineration, cement calcination, and metallurgical smelting has become an important topic in the field of environmental engineering. The bag filter system based on the surface filtration mechanism can achieve PM2.5 removal through the synergistic effect of gradient pore structure and surface modification technology. 2.5 -PM 10 Multi-stage deep capture of particulate matter can achieve a dust removal efficiency of over 99.9%, making it the most cost-effective particulate matter control solution in industrial flue gas treatment. However, the filter pore size obtained by existing preparation methods is relatively single, either mainly micropores, mesopores, or mainly macropores. This single pore structure filter has some problems in practical applications: a pore size that is too small will increase the resistance to fluid passage and reduce filtration efficiency; while a pore size that is too large may result in poor filtration effect and fail to effectively intercept tiny particles. To solve these problems, researchers are committed to developing filters with a multi-stage pore structure to meet the requirements of high filtration efficiency and low resistance.

[0003] Currently, traditional fiber materials (such as aramid, PTFE, and glass fiber needled felt) are difficult to meet ultra-low emission standards and are often used as the substrate for composite filters. Industrial bag dust removal systems often use a post-finishing process to treat the filter substrate, and polytetrafluoroethylene (PTFE) foam coating is widely used in this system due to its high thermal stability. For example, Chinese patent CN 110629560 A proposes a method for preparing aramid needled felt filter material. First, PTFE solution, modifier, foaming agent, foam stabilizer, adhesive, dispersant, flame retardant particles, and deionized water are mixed in proportion to form a coating solution. Then, the solution is applied to the surface of the aramid needled felt using expansion foaming and single-sided coating technology. After being treated with the PTFE foam coating finishing solution, the average pore size of the PTFE / aramid composite needled felt becomes smaller, and the filtration performance, temperature resistance, acid and alkali corrosion resistance, and wear resistance are all improved. However, the filtration efficiency of this patented product is only 65%, which is difficult to meet the current strict low emission standards. Chinese patent CN 104529505 A uses diatomaceous earth material containing a large number of micropores or mesopores as raw material, tert-butyl alcohol as solvent, citric acid and polyvinyl butyral as dispersant and binder respectively, and adopts freeze sublimation method combined with ball milling and sintering process to prepare a hierarchical pore filter with a large number of micropores, mesopores and macropores. The prepared filter has macropores with straight pore morphology and micropores and mesopores distributed on the walls of the macropores. This hierarchical pore structure can effectively improve the filtration efficiency and air permeability of the filter. However, the entire preparation process is relatively complicated and time-consuming, which makes it difficult to meet the needs of large-scale industrial production for fast and efficient production, which is not conducive to the large-scale promotion and application of this technology.

[0004] The current post-finishing process of polytetrafluoroethylene (PTFE) foam coating still has the following deep-seated problems: First, there is a lack of precise control over the thickness ratio of the coating layer and the ratio of the foaming agent and the template agent, resulting in a wide pore size distribution range or an unreasonable gradient design, making it difficult to simultaneously achieve a balance between high-precision filtration (especially for 0.5-2.5μm particles) and low fluid resistance; second, the residues of the template agent (such as NaCl) and the foaming agent (such as azodicarbonamide) in the traditional process are not completely removed during the high-temperature sintering process, which can easily clog the pores or form a non-uniform pore structure, reducing the effective filtration area and air permeability; third, the existing methods mostly rely on complex processes (such as freeze-sublimation, multi-step sintering, etc.), resulting in low production efficiency and high energy consumption, which is difficult to meet the needs of industrial large-scale production. These problems seriously restrict the application effect of multi-level porous filter materials in actual high-temperature smoke control. The innovative process of the present invention aims to systematically solve the above problems by optimizing the layered coating strategy, precisely controlling the component ratio and simplifying the process flow. Summary of the Invention

[0005] To further improve the performance of bag filter systems, especially the collection efficiency of particles with a diameter of 0.5 to 2.5 μm, in-depth research and optimization of the post-processing of the polytetrafluoroethylene foam coating is required. This is expected to reduce the average pore size of the coating, increase the density of the coating, and improve the surface properties of the coating, thereby achieving higher filtration efficiency for particles with a diameter of 0.5 to 2.5 μm.

[0006] The present invention provides a preparation method for a multi-level porous filter material formed by secondary coating. The process uses polytetrafluoroethylene (PTFE) as the main body, and by preparing a foam coating finishing liquid, it is evenly coated on the surface of the filter substrate to form a dense and stable PTFE film layer. On this basis, by precisely controlling the thickness of the PTFE foam layer doped with the NaCl template agent and the PTFE foam layer doped with the foaming agent azodicarbonamide (AC), the two layers of film are sequentially coated on the polyphenylene sulfide needle-punched felt. After precise heating and drying treatment, the layers are firmly bonded to each other, thereby constructing a foam pore layer with a multi-level porous structure. This process not only realizes the precise design of the pore size of the filter material, but also successfully prepares a composite filter material with both high-efficiency filtration performance and excellent high-temperature resistance, opening up new possibilities in the field of filtration technology.

[0007] In order to solve the above problems, the present invention provides a preparation method of a multi-level porous filter material formed by secondary coating, comprising the following steps:

[0008] (1) The template NaCl particles and PTFE solution were mixed in a ratio of 7:3, ultrasonically dispersed for 30 minutes, stirred at 300-600 r / min for 10-15 minutes using a magnetic stirrer, and then 7%-16% of a solvent was added to prepare a PTFE foaming coating solution containing NaCl particles. The NaCl@PTFE foaming liquid was prepared by a high-speed multidimensional stirring method at a speed of 1000-3000 r / min for 15-35 minutes.

[0009] (2) Use an automatic coating machine to coat NaCl@PTFE foam on the filter substrate and pre-dry it in an oven at 80°C for 3 to 5 minutes. Immerse the pre-dried NaCl@PTFE foam layer in 1% dilute hydrochloric acid and ultrasonically treat it for 10 to 15 minutes to dissolve the remaining NaCl particles. Then rinse it with deionized water until it is neutral, ensuring that the residual template is less than 0.1wt%, solving the problem of residual template (NaCl) and foaming agent (azodicarbonamide) in traditional processes, which leads to pore blockage or uneven structure.

[0010] (3) A chemical foaming agent azodicarbonamide (AC) solution was mixed with a PTFE solution and stirred evenly. A magnetic stirrer was used to stir the mixture at a speed of 300 to 600 r / min for 10 to 15 minutes. Then, 7% to 16% of a solvent was added to prepare a PTFE foam coating solution containing an AC solution. The AC@PTFE foam was prepared by a high-speed multidimensional stirring method at a speed of 1000 to 3000 r / min for 15 to 35 minutes.

[0011] (4) The AC@PTFE foam is coated on the NaCl@PTFE foam film layer, placed in an oven and heated at 180°C for 3 to 5 minutes, taken out, and then placed in an oven at 350°C for sintering for 3 to 5 minutes to form an AC@NaCl@PTFE composite foam film layer structure.

[0012] Preferably, the foaming coating solution in step (1) contains, by mass percentage, 62% to 69% of PTFE solution as a solid content, 22% to 29% of NaCl particles with a particle size of 100-300 μm, 1% to 2% of sodium sulfate as a foaming agent, 2% to 6% of alkyl alcohol amide as a foam stabilizer, and 4% to 8% of hydroxyethyl cellulose as a thickener.

[0013] Preferably, the filter substrate in step (2) is polyphenylene sulfide needle-punched felt.

[0014] Preferably, the thickness of the coated foam layer in step (2) and step (4) is 0.3 mm to 1 mm, wherein the thickness ratio of the NaCl@PTFE and AC@PTFE foam layers is NaCl@PTFE:AC@PTFE=(3:7) to (7:3).

[0015] Preferably, the PTFE foam coating solution containing AC solution in step (3) contains, by mass percentage, 72% to 79% of PTFE solution with a solid content of 60wt%, 12% to 14% of AC solution with a solid content of 5wt%, 1% to 2% of sodium fatty alcohol polyoxyethylene ether sulfate as a foaming agent, 2% to 6% of alkyl alcohol amide as a foam stabilizer, and 4% to 8% of hydroxyethyl cellulose as a thickener.

[0016] Preferably, the AC@PTFE foam coating in step (4) is performed by superimposing the coating on the NaCl@PTFE foam layer that has been dried and cooled to room temperature.

[0017] Compared with the prior art, the present invention has the following beneficial effects:

[0018] This invention enables precise control of pore size: By precisely adjusting the thickness ratio of the NaCl@PTFE foam layer and the AC@PTFE foam layer, the pore size of the filter material is precisely controlled. This process involves precisely adjusting the solid content of the PTFE and AC solutions, the NaCl particles, the foaming agent, the foam stabilizer, and the thickener, as well as strictly controlling the coating and curing process parameters. This precise reagent ratio and process control enable the filter material to effectively adsorb particles of varying sizes, improving filtration efficiency and accuracy.

[0019] The present invention can optimize the filtration structure: using a layered coating process, first coating a NaCl@PTFE foam layer, and then coating an AC@PTFE foam layer thereon, to form a multi-level porous foam layer structure. The layered structure design enables the filter material to maintain efficient filtration performance and good mechanical stability when facing different filtration conditions.

[0020] This invention reduces pressure drop by optimizing the thickness and structure of the foam layer, adjusting the thickness and density of the coating foam layer, and ensuring a reasonable pore size distribution within the filter material to reduce resistance to the passage of air or other fluids. This effectively controls pressure drop during the filtration process. Furthermore, precise control of the heating and drying process ensures a uniform pore structure within the foam layer, thereby reducing the overall resistance of the filter material. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of the multi-level pore filter material structure.

[0022] Figure 2 This is a diagram of the multi-level pore structure.

[0023] Figure 3 This is the coating process flow chart. DETAILED DESCRIPTION

[0024] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the specific implementation of the present invention will be further described below in conjunction with the accompanying drawings and specific implementation examples, but the implementation of the present invention is not limited thereto.

[0025] Example 1

[0026] The specific preparation method comprises the following steps:

[0027] (1) Preparation of foaming liquid:

[0028] (1-1) Preparation of NaCl@PTFE solution: Template NaCl particles and PTFE solution were mixed in a ratio of 7:3, ultrasonically dispersed for 30 min, and stirred at 300 r / min using a magnetic stirrer for 15 min to prepare a NaCl@PTFE solution.

[0029] (1-2) NaCl@PTFE Foaming Solution: Add 93 g of NaCl@PTFE solution to a stirring vessel, along with 1 g of sodium alcohol polyoxyethylene ether sulfate as a foaming agent, 2 g of alkylolamide as a foam stabilizer, and 4 g of hydroxyethyl cellulose as a thickener. Stir at 3000 rpm for 30 min to prepare the NaCl@PTFE foaming solution.

[0030] (1-3) AC@PTFE Foaming Solution: 79 g of 60 wt% solids PTFE solution and 12 g of 5 wt% solids AC solution were added to a stirring vessel. 2 g of sodium fatty alcohol polyoxyethylene ether sulfate as a foaming agent, 2 g of alkylolamide as a foam stabilizer, and 5 g of hydroxyethyl cellulose as a thickener were also added. A high-speed stirrer was used at 3000 rpm for 30 minutes to prepare the AC@PTFE foaming solution.

[0031] (2) Preparation of the foaming film layer: Use an automatic coating machine to coat, first set the first stage coating thickness to 0.5mm, coat the NaCl@PTFE foam to the polyphenylene sulfide needled felt filter substrate, and place it in an oven for pre-drying at 80℃ for 5 minutes. Immerse the pre-dried NaCl@PTFE foam layer in 1% dilute hydrochloric acid and ultrasonically treat it for 10 minutes to dissolve the NaCl particles that have not been completely removed. Then rinse it with deionized water until it is neutral, ensuring that the residual amount of the template agent is less than 0.1wt%, and obtain a filter material with a single-sided NaCl@PTFE foam layer; then set the second stage coating thickness to 0.5mm, coat the AC@PTFE foam to the PTFE foam layer filter material, place it in an oven for heating and drying at 180℃ for 5 minutes, then take it out, and then place it in an oven at 350℃ for sintering for 3 minutes to obtain a filter material with a composite foam film layer.

[0032] Example 2

[0033] The specific preparation method comprises the following steps:

[0034] (1) Preparation of foaming liquid:

[0035] (1-1) Preparation of NaCl@PTFE solution: Template NaCl particles and PTFE solution were mixed in a ratio of 7:3, ultrasonically dispersed for 30 min, and stirred at 300 r / min using a magnetic stirrer for 15 min to prepare a NaCl@PTFE solution.

[0036] (1-2) Preparation of NaCl@PTFE foaming solution: Add 84 g of NaCl@PTFE solution to a stirring vessel, along with 2 g of sodium alcohol polyoxyethylene ether sulfate as a foaming agent, 6 g of alkylolamide as a foam stabilizer, and 8 g of hydroxyethyl cellulose as a thickener. Stir at 3000 rpm for 30 minutes to prepare the NaCl@PTFE foaming solution.

[0037] (1-3) Preparation of AC@PTFE Foaming Solution: 79 g of 60 wt% solids PTFE solution and 13 g of 5 wt% solids AC solution were added to a stirring vessel. 1 g of sodium fatty alcohol polyoxyethylene ether sulfate was added as a foaming agent, 2 g of alkylolamide was added as a foam stabilizer, and 5 g of hydroxyethyl cellulose was added as a thickener. A high-speed stirrer was used at 3000 rpm for 30 minutes to prepare the AC@PTFE foaming solution.

[0038] (2) Preparation of the foaming film layer: Use an automatic coating machine to coat, first set the first stage coating thickness to 0.7mm, coat the NaCl@PTFE foam to the polyphenylene sulfide needled felt filter substrate, and place it in an oven for pre-drying at 80℃ for 5 minutes. Immerse the pre-dried NaCl@PTFE foam layer in 1% dilute hydrochloric acid and ultrasonically treat it for 12 minutes to dissolve the NaCl particles that have not been completely removed. Then rinse it with deionized water until it is neutral, ensuring that the residual amount of the template agent is less than 0.1wt%, and obtain a filter material with a single-sided NaCl@PTFE foam layer; then set the second stage coating thickness to 0.3mm, coat the AC@PTFE foam to the PTFE foam layer filter material, place it in an oven for heating and drying at 180℃ for 5 minutes, then take it out, and then place it in an oven at 350℃ for sintering for 3 minutes to obtain a filter material with a composite foam film layer.

[0039] Example 3

[0040] The specific preparation method comprises the following steps:

[0041] (1) Preparation of foaming liquid:

[0042] (1-1) Preparation of NaCl@PTFE solution: Template NaCl particles and PTFE solution were mixed in a ratio of 7:3, ultrasonically dispersed for 30 min, and stirred at 300 r / min using a magnetic stirrer for 15 min to prepare a NaCl@PTFE solution.

[0043] (1-2) Preparation of NaCl@PTFE foaming solution: Add 92 g of NaCl@PTFE solution to a stirring vessel, along with 1 g of sodium fatty alcohol polyoxyethylene ether sulfate as a foaming agent, 3 g of alkylolamide as a foam stabilizer, and 4 g of hydroxyethyl cellulose as a thickener. Stir at 3000 rpm for 30 minutes to prepare the NaCl@PTFE foaming solution.

[0044] (1-3) Preparation of AC@PTFE Foaming Solution: 75 g of 60 wt% solids PTFE solution and 13 g of 5 wt% solids AC solution were added to a stirring vessel. 2 g of sodium fatty alcohol polyoxyethylene ether sulfate was added as a foaming agent, 5 g of alkylolamide was added as a foam stabilizer, and 5 g of hydroxyethyl cellulose was added as a thickener. A high-speed stirrer was used at 3000 rpm for 30 minutes to prepare the AC@PTFE foaming solution.

[0045] (2) Preparation of the foaming film layer: The coating was performed using an automatic coating machine. First, the first-stage coating thickness was set to 0.3 mm. NaCl@PTFE foam was applied to the polyphenylene sulfide needled felt filter substrate. The substrate was placed in an oven at 80°C for pre-drying for 5 minutes. The pre-dried NaCl@PTFE foam layer was immersed in 1% dilute hydrochloric acid and ultrasonically treated for 14 minutes to dissolve the unremoved NaCl particles. The substrate was then rinsed with deionized water until neutral, ensuring that the residual amount of the template agent was less than 0.1 wt %, to obtain a filter material with a single-sided NaCl@PTFE foam layer. The second-stage coating thickness was then set to 0.7 mm. AC@PTFE foam was applied to the PTFE foam layer filter material. The substrate was placed in an oven at 180°C for heating and drying for 5 minutes, then taken out and placed in an oven at 350°C for sintering for 3 minutes to obtain a filter material with a composite foam film layer.

[0046] Example 4

[0047] The specific preparation method comprises the following steps:

[0048] (1) Preparation of foaming liquid:

[0049] (1-1) Preparation of NaCl@PTFE solution: Template NaCl particles and PTFE solution were mixed in a ratio of 7:3, ultrasonically dispersed for 30 min, and stirred at 300 r / min using a magnetic stirrer for 15 min to prepare a NaCl@PTFE solution.

[0050] (1-2) Preparation of NaCl@PTFE foaming solution: Add 90 g of NaCl@PTFE solution to a stirring vessel, along with 2 g of sodium alcohol polyoxyethylene ether sulfate as a foaming agent, 3 g of alkylolamide as a foam stabilizer, and 5 g of hydroxyethyl cellulose as a thickener. Stir at 3000 rpm for 30 minutes to prepare the NaCl@PTFE foaming solution.

[0051] (2) Preparation of the foamed film layer: NaCl@PTFE foam was applied to a polyphenylene sulfide needled felt filter substrate using an automatic coating machine with a coating thickness of 1 mm. The substrate was then pre-dried in an oven at 80°C for 5 min. The pre-dried NaCl@PTFE foam layer was then immersed in 1% dilute hydrochloric acid and ultrasonically treated for 15 minutes to dissolve any remaining NaCl particles. The substrate was then rinsed with deionized water until neutral, ensuring that the residual template agent content was less than 0.1 wt %. A filter material having a single-sided NaCl@PTFE foamed layer was obtained.

[0052] The filtration performance test of the multi-level porous filter material prepared by the present invention is shown in Table 1. By adjusting the coating thickness and the formula of the foaming liquid, the filtration efficiency and filtration pressure drop of the filter material can be affected. Deep filtration mechanisms such as inertia, interception, diffusion and gravity inside the filter material allow particulate matter to be captured by the fibers, even if the particle size is smaller than the pore size. Compared with Example 4 in Table 1, which only coats the first-stage NaCl@PTFE membrane layer composite filter material, the multi-level porous foam layer coated filter material of Example 3 has a higher filtration efficiency for 0.5μm particles, but the filtration pressure drop increases. Taking into account the pressure drop and filtration efficiency of the filter material, Example 2 has the best filtration performance.

[0053] Table 1 Example Example 1 Example 2 Example 3 Example 4 First stage coating thickness 0.5mm 0.7mm 0.3mm 1mm Second stage coating thickness 0.5mm 0.3mm 0.7mm none 0.5μm particle filtration efficiency 62.5％ 78.8％ 51.7％ 31.2％ 2.5μm particle filtration efficiency 99.8％ 99.9％ 99.9％ 98.9％ Filtration pressure drop 251Pa 183Pa 116Pa 90Pa

[0054] Although the present invention has been described in detail by way of preferred embodiments, the present invention is not limited thereto. Those skilled in the art may make various specific changes according to different actual needs without departing from the scope and spirit of the present invention, but they still fall within the scope of protection of this application.

Claims

1. A method for preparing a multi-level porous filter material by secondary coating, characterized in that: The following steps are involved: S1. Mix the template NaCl particles with the PTFE solution in a ratio of 7:3, ultrasonically disperse for 30 minutes, use a magnetic stirrer to stir at a speed of 300-600 r / min for 10-15 minutes, then add 7%-16% of a solvent to prepare a PTFE foam coating solution containing NaCl particles, and use a high-speed multi-dimensional stirring method to stir at a speed of 1000-3000 r / min for 15-35 minutes to prepare NaCl@PTFE foam; S2. Use an automatic coating machine to coat the NaCl@PTFE foam on the filter substrate, put it in an oven at 80°C for pre-drying for 3-5 minutes, and then immerse the pre-dried NaCl@PTFE foam layer in dilute hydrochloric acid with a concentration of 1%, ultrasonically treat for 10-15 minutes to dissolve the NaCl particles that have not been completely removed, and then rinse with deionized water until neutral, ensuring that the residual amount of the template is less than 0.1wt%; S3. Mix the chemical foaming agent AC solution with the PTFE emulsion, stir them evenly, use a magnetic stirrer to stir at a speed of 300-600 r / min for 10-15 minutes, then add 7%-16% of a solvent to prepare a PTFE foam coating solution containing the AC solution, and use a high-speed multi-dimensional stirring method to stir at a speed of 1000-3000 r / min for 15-35 minutes to prepare AC@PTFE foam; S4. Coat the AC@PTFE foam on the NaCl@PTFE foam film layer, place it in an oven for heating and drying at 180°C for 3-5 minutes, then take it out and place it in an oven at 350°C for sintering for 3-5 minutes to form an AC@NaCl@PTFE composite foam film layer structure.

2. The method for preparing a multi-level porous filter material by secondary coating according to claim 1, characterized in that: The foaming coating solution in S1 contains, by mass percentage, 62% to 69% of PTFE solution with a solid content, 22% to 29% of NaCl particles with a particle size of 100-300 μm, 1% to 2% of sodium sulfate, a foaming agent, 2% to 6% of alkyl alcohol amide, and a thickener, 4% to 8% of hydroxyethyl cellulose.

3. The method for preparing a multi-level porous filter material by secondary coating according to claim 1, characterized in that: The filter substrate in S2 is polyphenylene sulfide needle-punched felt.

4. The method for preparing a multi-level porous filter material by secondary coating according to claim 1, characterized in that: The PTFE foam coating solution containing AC solution in S3 contains, by mass percentage, 72% to 79% of PTFE emulsion with a solid content of 60wt%, 12% to 14% of AC solution with a solid content of 5wt%, 1% to 2% of sodium fatty alcohol polyoxyethylene ether sulfate as a foaming agent, 2% to 6% of alkyl alcohol amide as a foam stabilizer, and 4% to 8% of hydroxyethyl cellulose as a thickener.

5. The method for preparing a multi-level porous filter material by secondary coating according to claim 1, characterized in that: The thickness of the S2 and S4 coating foam layers is 0.3 mm to 1 mm, wherein the thickness ratio of the NaCl@PTFE and AC@PTFE foam layers is NaCl@PTFE:AC@PTFE=(3:7) to (7:3).

6. The method for preparing a multi-level porous filter material by secondary coating according to claim 1, characterized in that: The AC@PTFE foam coating in S4 is performed on the NaCl@PTFE foam layer that has been dried and cooled to room temperature.

7. The method for preparing a multi-level porous filter material by secondary coating according to claim 1, characterized in that: Compounding foam layers with different foam pore sizes to form a multi-level pore foam layer structure is beneficial to improving the filtration accuracy and filtration efficiency of the filter material.

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