A PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance, and preparation method and application thereof

Abstract

The present invention belongs to the technical field of solid-liquid separation, and specifically relates to a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance, and a preparation method and application thereof. The method comprises: (1) taking PTFE resin, heating and melting it into a fluid, and then forming it on a net to prepare a precursor; (2) uniformly spraying molten fibers on the surface of the precursor by a melt-blowing process, and hot-pressing to prepare a PTFE separation membrane; (3) modifying the PTFE separation membrane to obtain a PTFE modified separation membrane; (4) stacking and folding the PTFE modified separation membrane into fins to obtain a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance. The scheme of the present invention prepares a "ladder" structure to reduce pressure loss during filtration and prevent fine particles from clogging the deep layer of the filter element. In addition, it adopts a special post-processing technology to improve its filtration efficiency. It can also increase the dirt holding capacity of the foldable filter element and reversely flush the filter element by water flow.

Description

Technical Field

[0001] The invention belongs to the technical field of solid-liquid separation, and specifically relates to a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance and a preparation method thereof. Background Art

[0002] In the existing technology, the life of flushable filter elements generally does not exceed 1 year (PP cotton, activated carbon), and some can reach 2-3 years (external pressure ultrafiltration membrane, reverse osmosis membrane), etc. The flushable filter element is generally a combined filter element, composed of ultrafiltration membrane + copper-zinc alloy + activated carbon. The advantages are easy installation and large water output. The first disadvantage is that the flushable filter element is only suitable for water bodies with a total dissolved solid content of less than 500. It is not suitable for river water with serious water pollution, groundwater sources with a total dissolved solid content of more than 1000, colloids in the water, and water bodies with more impurities. The filter membrane is generally an internal pressure type, the membrane tube is very thin, and the filter element is easily blocked. Flushing can only flush out the inside of the tube. The colloids adhering to the tube wall are very sticky and extremely difficult to flush out, thereby reducing the service life of the filter element; second, the combined filter element contains activated carbon, which is located after the filter membrane. Since the activated carbon has a large surface area, it mainly plays the role of adsorbing discoloration, odor, and impurities in the combined filter element. The service life of activated carbon is generally 1 year. If it exceeds the service life, it will not only fail to filter, but also release harmful substances. Because the activated carbon adsorbs impurities saturated, the harmful substances can no longer be adsorbed, and some harmful substances will be released with the water flow. At this time, the filter element does not have a water purification effect; third, the filter element has limited filtering capacity for impurities in the water body. It has a high filtering capacity for general impurities, viruses and bacteria, but a poor filtering capacity for metal ions.

[0003] In the prior art, pleated membrane filter elements are widely used in the field of filtration, and the materials of the pleated membranes are mainly PTFE, PES, PVDF, Nylon, etc. However, during the backwashing process of the existing pleated membrane filter elements, the pleated membranes are easily damaged by the fluid, and the filter cake on the surface of the pleated membrane is not backwashed thoroughly. Therefore, the existing pleated membrane filter element structure cannot meet the requirements of the backwashing process. In addition, in the traditional chemical industry, the accuracy of traditional non-woven pleated filter elements is not high, and cannot meet the filtration accuracy requirements of particle diameters between 0.1 and 1 μm. Moreover, ordinary non-woven pleated filter elements cannot continue to be used after the dirt holding capacity reaches saturation, so there are technical problems such as poor filtration accuracy, low filtration accuracy, and short service life. Summary of the invention

[0004] In order to solve the existing problems of poor dirt holding capacity, low filtering accuracy and short service life, the present invention provides a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance, and a preparation method of the filter element.

[0005] The purpose of the present invention is: First, to provide a foldable filter element capable of high-efficiency filtration and low flow resistance.

[0006] 2. Improve the dirt holding capacity of the foldable filter element.

[0007] 3. Prepare a filter element that can be backwashed using water flow.

[0008] 4. The prepared filter element can effectively filter metal ions in water.

[0009] To achieve the above objectives, the present invention adopts the following technical solutions.

[0010] A method for preparing a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance, the method comprising: (1) taking PTFE resin, heating and melting it into a fluid, and then forming it on a net to prepare a precursor.

[0011] (2) The molten fibers are uniformly sprayed on the surface of the precursor by melt-blowing technology, and the PTFE separation membrane is prepared by hot pressing.

[0012] (3) The PTFE separation membrane is modified to obtain a PTFE modified separation membrane.

[0013] (4) The PTFE modified separation membrane is stacked and folded into fins to obtain a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance.

[0014] Preferably, the heating temperature in step (1) is 330-340°C, and stirring is maintained during the heating process for 15-30 minutes.

[0015] Preferably, the web forming in step (1) is dried to form a film under the conditions of an ambient temperature of 20 to 60°C and an air flow rate of 25 to 30 km / h, and the film thickness is controlled to be 0.1 to 0.3 mm.

[0016] Preferably, in the melt-blowing process of step (2), the melt-blowing temperature is controlled to be 325-330°C, the spinneret holes are arranged in a single row, with a diameter of 0.2-0.4 mm, an aspect ratio of ≥10, and a hole pitch of 0.6-1.0 mm, and single-sided coating is performed.

[0017] Preferably, the fiber coating in step (2) is a composite fiber, whose components are 20-25 wt% polypropylene, 12-17 wt% silica powder, 22-27 wt% polyvinylidene fluoride, and the balance is polytetrafluoroethylene resin, wherein the silica powder has a particle size of 100-300 nm.

[0018] Preferably, in step (2), the membrane thickness is hot-pressed to a target thickness of the PTFE separation membrane at 110-115° C., and the target thickness of the PTFE separation membrane is 0.15-0.35 mm.

[0019] Preferably, the organic solvent in step (3) is a mixed solvent uniformly mixed with triethylenetetramine, acrylonitrile and titanium chloride in a mass ratio of 1: (1.2-1.7): (0.8-1.2).

[0020] Preferably, the soaking time in step (3) is 20 to 30 minutes.

[0021] Preferably, in step (4), two PTFE modified separation membranes are stacked in sequence and the two unmelt-sprayed surfaces thereof are bonded together to form a filter matrix layer.

[0022] A PTFE pleated backwashable filter element with high dirt holding capacity and low flow resistance.

[0023] The invention discloses an application of a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance.

[0024] The PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance is used in a fixed backwashable filtration system. Due to the use characteristics of the fixed backwashable filtration system, it has backwashing capabilities but the process of replacing the filter element is extremely cumbersome. The backwashable filter elements currently on the market usually lose most of their filtration performance after multiple backwashings, and the single backwashing time needs to reach about 5 minutes, and some even need to be backwashed for 150 minutes, which is an extremely long backwashing time. The PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance of the present invention can give full play to its performance, has a short backwashing time, and does not require the use of a specific filter element cleaner. Direct backwashing with clean water can achieve a very excellent backwashing and cleaning effect on the filter element, greatly reducing the maintenance cost of the filtration and decontamination system.

[0025] In actual application, PTFE folded filter element can have good filtration efficiency, excellent chemical stability, heat resistance, cold resistance and chemical corrosion resistance, and also has good electrical insulation, low surface tension and low friction coefficient, non-flammability, high and low temperature adaptability and high mechanical properties. However, if the content of macromolecules in the water increases, the pores of the filter matrix will be blocked, resulting in a decrease in fluid flux, a small amount of dirt holding, and inability to backwash. In order to solve the above problems, the present invention modifies the PTFE base membrane and makes structural changes.

[0026] The core of the technical solution of the present invention lies in that a network structure formed by the coordination of metal ions or metal clusters with organic ligands is used to construct an internal and external skeleton with two-way flow guidance. Due to the particularity of its pore structure, it can be used as an absorber and can also use the addition of air to improve the backwashing effect. At the same time, it can achieve the following: improve the pollution holding capacity, progressive filtration, graded filtration, intercept large particles at the outermost side of the filter material, improve the backwashing capacity of the filter material, and avoid all pollutants being concentrated on the filter material to cause insufficient recoil force and incomplete recoil.

[0027] In the technical solution of the present invention, melt-blown coating is used to modify the surface of the separation membrane for the first time. After this modification, the surface of the separation membrane undergoes structural changes, forming a short fiber bundle composed of polytetrafluoroethylene resin as a skeleton and externally connected polyvinylidene fluoride resin, forming an external skeleton for two-way flow guidance. The porosity of the short fiber bundle layer decreases from large to small along the filtration direction, and it is a variable porosity filter layer. The free end of the short fiber bundle can swing and sway freely with the water body. This type of structure is conducive to backwashing the filter element. The short fiber bundles that swing freely on both sides of the folded membrane can assist the water flow to wash the filter element. The short fiber bundle layer can form an upper fluffy layer with a larger porosity and a lower compacted layer with a small porosity along the flow direction of the water body during filtration. Combined with the filtration effect of the PTFE separation membrane, the variable porosity of the forward filtration can be realized along the filtration direction from large to small and then from large to small pore size changes. During backwashing, the short fiber bundle layer is fully stretched to maintain a large porosity, which is conducive to the swing of the short fiber bundle.

[0028] In the prior art, backwashing is performed from the inside of the filter to the outside. The backwash filter element is generally composed of two sections, with a small section of filter on the top. During backwashing, the spring may be pressed downward by water pressure, so that the water can only flow into the filter element from the small section of the filter on the top, and then flow out from the inside of the filter element. The backwash filter element is also equipped with a rotary spray assembly, which is also driven by water flow to spray water from the inside of the filter element to the outside, so that the water flow can be better used to clean the filter. Because water flows from the outside to the inside during filtration, impurities are all on the outer surface of the filter, and smaller impurities may be embedded in the holes of the filter. At this time, reverse flushing from the inside to the outside may not be able to flush them clean, while the technology of the present invention The foldable filter element provided in the solution, during filtration, due to the weight of the PTFE separation membrane and the filter material itself, and the influence of the water flow direction, naturally compresses the short fiber layer, so that the density of the upper fluffy layer of the short fiber layer increases, and the density of the short fiber layer gradually increases from top to bottom, realizing a gradient change in the gaps, and increasing the dirt holding capacity of the filter element; during backwashing, the washing liquid flows from the inner layer to the outer layer of the filter element, and the flow rate of the inner layer is greater. The filter residue is first washed off and pushed to the outer layer, causing the pore size of the outer layer to decrease and the flow rate to increase, thereby enhancing the ability to strip the filter residue, so that the filter residue is discharged from the outermost layer. The pore size that increases from the inner layer to the outer layer in the filter layer is particularly conducive to removing the filter residue during backwashing of the filter element.

[0029] Polyvinylidene fluoride has good mechanical properties, chemical corrosion resistance, and stable electrical properties. It is one of the widely used membrane materials, but its thermal stability is not high and its electrical conductivity is low. For this reason, the technical solution of the present invention adds inorganic nanoparticles for improvement. Silicon dioxide has good biocompatibility and high mechanical strength. It is evenly dispersed in polyvinylidene fluoride and polytetrafluoroethylene resin fibers to form a uniform microporous structure, which can improve the thermal stability of the composite material at high temperatures. Silicon dioxide nanoparticles are added to prepare composite microporous fibers. Since the composite polymer structure is conducive to contacting more water and absorbing more impurities in the water, the grafting of short fiber bundles makes the separation membrane have a more complex filtration structure system as a whole, so that the pores of the composite membrane become larger and the pollution-receiving capacity of the separation membrane is enhanced. The localized growth of nanoparticles in the resin fiber matrix can effectively prevent the aggregation of nanoparticles and control the final particle size, which makes the silicon dioxide particles in the composite membrane evenly dispersed in the resin fiber matrix, and significantly improves the mechanical properties, thermal stability and anti-fouling properties of the separation membrane.

[0030] In the technical solution of the present invention, from the perspective of the filter element's capture efficiency of ultrafine particles, only a few fine particles penetrate the short fiber layer, and most of the solid suspended matter is blocked in the short fiber layer. During backwashing, the short fiber layer is subjected to a change in pressure direction, and the intercepted solid suspended matter is separated from the filter element as the washing liquid and the short fibers swing, thereby maintaining the smoothness of the filter element surface and facilitating dust cleaning. This type of structure can significantly reduce operating resistance.

[0031] In the technical solution of the present invention, the PTFE composite separation membrane is also modified for the second time to produce Duoan chelating fiber to enhance the filtering performance of the filter element. It can be cross-linked with the resin and the fiber, and externally connected to metallocene to form a network structure composed of metal ions or metal clusters coordinated with organic ligands. The organism can produce an organic-inorganic skeleton with a continuous structure and flexible morphology to form a two-way guiding inner skeleton. The skeleton openings are combined with the short fiber bundle structure to achieve better fluid performance, with two-way guiding effects for filtration and backwashing, and the backwashing capacity of the filter element is enhanced. The metallocene catalytic performance is as follows.

[0032]

[0033] Wherein: R, R' are functional groups containing -COOH and / or -NH2, and the number of carbon atoms in the functional group is ≤15.

[0034] Inorganic materials, especially minerals and silica, play an important role in modern society, but their bulk manufacturing is a huge challenge. In practice, many inorganic materials are produced in powder form and then consolidated by pressing and sintering. However, through the sintering process, the mass transport between particles is often insufficient, and complete particle-particle fusion cannot be achieved within the bulk material, and particle boundaries still exist. Due to internal discontinuities, the performance of sintered inorganic blocks is not ideal, especially in terms of mechanical strength. In nature, biological organisms can produce inorganic skeletons with continuous structures and flexible morphology, which are superior to artificial skeletons due to their structural integration. There is increasing evidence that these biological organisms use amorphous particles as precursors to produce skeletons through particle-particle fusion. In the technical solution of the present invention, silica nanoparticles are used as metal carriers, and a polymer is formed with silica as the core, which forms a stable bidentate anchor with the metallocene and combines with the polytetrafluoroethylene resin in the matrix to form an isotactic center. Due to the steric hindrance, it will no longer coordinate with other polymer materials, so that the catalytic activity of the metallocene will not be lost. One titanium atom group in the bidentate anchor coordinates with the polytetrafluoroethylene resin, and the other titanium atom group becomes a high isotactic center, which serves as a catalytic active center, increasing the H + The exchange capacity with heavy metal ions forms a polymer. The multi-coordinated functional groups on the polymer form a complex with the metal ions. The atoms on the functional groups react with the metal ions to produce coordinated covalent bonds and form a chelate with a stable structure. This process has a strong adsorption effect on heavy metal ions such as chromium, nickel, and zinc in the environment. The bidentate anchor structure is as follows.

[0035]

[0036] Wherein: P represents the polymer chain, and the number of carbon atoms in the polymer is ≥2.

[0037] In the existing technical solutions, the precise control of crystal structure, molecular weight distribution and the addition of comonomers to the polymer molecular chain is the characteristic of metallocene catalysts with a single active center. The products produced by using this characteristic have low crystallinity, narrow molecular weight distribution, excellent transparency and gloss, strong impact resistance and excellent toughness, small crystallites and good compatibility with other resins. Most importantly, it can break the defects of traditional catalysts and synthesize new propylene copolymers.

[0038] In the technical solution of the present invention, after the metallocene catalyst is loaded on the polytetrafluoroethylene resin, the relative molecular weight and isotacticity of the polytetrafluoroethylene are greatly improved, and the internal structure is changed during the polymerization with silica, so that it has a firm honeycomb grid structure, which reduces the weight of the filter unit monomer in the filter element, but the filtering effect will not be weakened. The grid structure is filled with chelated fiber filter units, and under the action of coordination, the filter unit has high activity, maximizes the catalytic activity of the metallocene and the chelated fiber, and becomes a catalyst of a bidirectional electron donor. The metallocene catalyst is an internal electron donor catalyst, has a wide relative molecular weight distribution, high stereospecificity, and the filter element exhibits a homopolymer and a multiphase copolymer with high rigidity, which to a certain extent expands the performance of the filter layer composed of the polytetrafluoroethylene resin copolymer; the metallocene catalyst is an external electron donor catalyst, which is added during the polymerization process, has a unique structure, can be easily coordinated with the transition metal, and promotes the formation of a bidentate anchor structure.

[0039] The advantages of the present invention are: (1) The present invention prepares a new type of composite filter membrane using a "ladder" structure, which reduces the pressure loss of the filter element during the filtration process.

[0040] (2) The "ladder" structure of the filter element prepared in the scheme of the present invention prevents fine particles from clogging the deep layer of the filter element during the filtration process, and the special post-treatment technology is used to improve the filtration efficiency.

[0041] (3) The "ladder" structure of the filter element prepared in the scheme of the present invention can also increase the dirt holding capacity of the folded filter element.

[0042] (4) The filter element prepared in the scheme of the present invention can be backwashed using water flow. DETAILED DESCRIPTION

[0043] The present invention is further described in detail below in conjunction with specific embodiments. Those of ordinary skill in the art will be able to implement the present invention based on these descriptions. In addition, the embodiments of the present invention involved in the following description are generally only embodiments of a part of the present invention, rather than all embodiments. Therefore, based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work should fall within the scope of protection of the present invention.

[0044] Unless otherwise specified, the raw materials used in the examples of the present invention are all commercially available or available to those skilled in the art; unless otherwise specified, the methods used in the examples of the present invention are all methods known to those skilled in the art.

[0045] Example 1: A method for preparing a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance, the method comprising: (1) taking PTFE resin, heating and stirring at a temperature of 330°C for 15 minutes to melt it into a fluid, and then drying it to form a film under the conditions of an ambient temperature of 20°C and an air flow rate of 25 km / h, wherein the film thickness is controlled to be 0.1 mm, and forming it on a screen to prepare a precursor.

[0046] (2) At a temperature of 325 °C, the spinneret holes are arranged in a single row with a diameter of 0.2 mm, an aspect ratio of ≥10, and a hole pitch of 0.6 mm. Single-sided melt-blowing is performed to evenly spray molten fiber coating on the surface of the precursor, and the membrane thickness is hot pressed to 0.15 mm at 110 °C to prepare a PTFE separation membrane, wherein the fiber coating comprises 20 wt% polypropylene, 12 wt% silica powder, 22 wt% polyvinylidene fluoride, and the remainder is polytetrafluoroethylene resin, wherein the silica powder has a particle size of 100 nm.

[0047] (3) Triethylenetetramine, acrylonitrile and titanium chloride were uniformly mixed in a mass ratio of 1:1.2:0.8, and after the PTFE separation membrane was cooled, the mixture was immersed in the mixture for 20 min to obtain a PTFE modified separation membrane.

[0048] (4) The two surfaces of the two PTFE modified separation membranes that have been soaked and not melt-sprayed are bonded together to form a filter matrix layer, and the filter matrix layer is folded into fins to obtain a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance. The PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance is surrounded by a cylinder to form an inner support cylinder, and then an outer support cylinder is sleeved outside the filter matrix layer to obtain a cartridge filter based on the PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance.

[0049] The prepared filter element was assembled into a cylindrical filter and then subjected to the following performance tests. The specific characterization results are as follows.

[0050] Structural integrity test: FBP-II porous material tester is used to test the first bubble point pressure p1 of PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance. The maximum pore size value can be calculated from the bubble point pressure. If the gas pressure continues to increase, boiling bubbles will appear in the liquid, which is called group bubble point pressure p2. The difference between p2 and p1 should be as small as possible. If the difference is less than 10% of p1, it is considered that the filter element is not damaged during the preparation process and the joints are reliably sealed.

[0051] Water flux test: Place the cartridge filter in the filtration device, add 20 mL of water, and circulate until the amount of water filtered within 5 minutes is stable. Then add 20 mL of water to the filtration device, measure the amount of water filtered naturally without external pressure, and calculate the volume of water passing through per unit area per unit time, that is, the water flux [L / (m 2 ·h)].

[0052] Unit area dirt holding capacity test: When the test liquid passes through the tested filter element at a flow rate of 8 mm / s (effective filter area is 10 cm 2 ), add test powder with a particle size of 10 μm to the upstream of the filter at a rate of 10 mg / min until the ultimate pressure difference of the filter element reaches 0.4 MPa. Stop the test and calculate the cumulative amount of test dust added. The calculation formula is as follows: M=G×q×t÷S÷1000.

[0053] Where: M——pollution holding capacity (g / cm 2 ).

[0054] G——Average mass contamination in the dust adding device (mg / L).

[0055] q——average injection flow rate (L / min).

[0056] t——actual test time when the final pressure difference is reached (min).

[0057] S——Effective filtration area of ​​filter element (cm 2 ).

[0058] Filtration efficiency test: 5 μm standard particles are used, and the coefficient of variation of particle size distribution is controlled within 5%. An appropriate amount of standard particles and clean water are used to prepare the test solution. The prepared PTFE folded filter element is connected to the detection system, and the detection system is started. The test solution is passed through the filter element at a pressure difference of 0.15 MPa. The filter is filtered for 2 h, and the liquid samples before and after filtration are taken to characterize the weight of standard particles in the liquid sample and calculate the particle retention rate. The formula is as follows: Particle retention rate = (1-N0 / N) × 100.

[0059] Where: N0 – weight of standard particles per unit volume of filtrate (g).

[0060] N——weight of standard particles per unit volume of test fluid (g).

[0061] Pressure loss detection: △P = (32 × η × L × Q) / (π 2 d 4 ).

[0062] In the formula: △P——pressure difference, the unit is Pa.

[0063] η——Viscosity of the filter medium, in Pa·s.

[0064] L——Filter element length, in cm.

[0065] Q——Flow rate of medium, in L / s.

[0066] d——filter element radius, in cm.

[0067] Backwashing capacity: Weigh the weight of the dried folded filter element prepared in the example and the folded filter element after the dirt holding capacity per unit area test, use pure water to rinse the folded filter element prepared in the example at a flow rate of 20 mm / s, so that the pressure difference between the inside and outside of the filter element reaches 0.05 MPa, and continue for 100 s. Take samples every 5 s to characterize the backwash water. When the backwash water reaches the standard of suspended particle concentration ≤3 mg / L, it indicates that there is no stain. After drying, compare it with the original weight of the filter element to calculate the dirt removal rate. The formula is as follows.

[0068] Dirt removal rate = 1-(M-M0) / (M1-M0)×100%.

[0069] Wherein: M0 is the weight of the dry folded filter element prepared in the embodiment, in g.

[0070] M1 - the weight of the folded filter element after the dirt holding capacity per unit area is tested, in grams.

[0071] M——weight of folded filter element after drying, in g.

[0072]

[0073] Metal ion removal rate: Collect metal smelting industrial wastewater, especially steel plant production wastewater, which is characterized by high suspended matter content and high heavy metal concentration including harmful substances such as iron, copper, cadmium, zinc, etc. The steel plant production wastewater is used as the standard test liquid, and the pollutant content in the test liquid at this time is determined as the liquid A data. The prepared folded filter element is connected to the detection system, and the detection system is started to pass the standard test liquid through the prepared folded filter element at a flow rate of 8 mm / s for 2 hours. The pollutant content of the filtered solution is characterized and the liquid B data is recorded. The specific characterization results are as follows.

[0074]

[0075] Observing the above characterization results, the filter element prepared by the present invention has a high pollution holding capacity, can also meet the national regulations on the existing enterprise water pollutant emission concentration limit, and also has a high pollution removal rate during backwashing.

[0076] Example 2: A method for preparing a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance, the method comprising: (1) taking PTFE resin, heating and stirring at a temperature of 335°C for 25 minutes to melt it into a fluid, and then drying it at an ambient temperature of 45°C and an air flow rate of 27 km / h to form a film, controlling the film thickness to 0.15 mm, and forming it on a screen to prepare a precursor.

[0077] (2) At a temperature of 327 °C, the spinneret holes are arranged in a single row with a diameter of 0.3 mm, an aspect ratio of ≥10, and a hole pitch of 0.8 mm. Single-sided melt-blowing is performed to evenly spray molten fiber coating on the surface of the precursor, and the membrane is hot-pressed to a thickness of 0.20 mm at 113 °C to prepare a PTFE separation membrane, wherein the fiber coating comprises 23 wt% polypropylene, 15 wt% silica powder, 25 wt% polyvinylidene fluoride, and the remainder is polytetrafluoroethylene resin, wherein the silica powder has a particle size of 200 nm.

[0078] (3) Triethylenetetramine, acrylonitrile and titanium chloride were uniformly mixed in a mass ratio of 1:1.5:1, and after the PTFE separation membrane was cooled, the mixture was immersed in the mixture for 25 min to obtain a PTFE modified separation membrane.

[0079] (4) The two surfaces of the two PTFE modified separation membranes that have been soaked and not melt-sprayed are bonded together to form a filter matrix layer, and the filter matrix layer is folded into fins to obtain a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance. The PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance is surrounded by a cylinder to form an inner support cylinder, and then an outer support cylinder is sleeved outside the filter matrix layer to obtain a cartridge filter based on the PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance.

[0080] The prepared filter element was assembled into a cylindrical filter and then subjected to the following performance tests. The specific characterization results are as follows.

[0081] Structural integrity test: FBP-II porous material tester is used to test the first bubble point pressure p1 of PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance. The maximum pore size value can be calculated from the bubble point pressure. If the gas pressure continues to increase, boiling bubbles will appear in the liquid, which is called group bubble point pressure p2. The difference between p2 and p1 should be as small as possible. If the difference is less than 10% of p1, it is considered that the filter element is not damaged during the preparation process and the joints are reliably sealed.

[0082] Water flux test: Place the cartridge filter in the filtration device, add 20 mL of water, and circulate until the amount of water filtered within 5 minutes is stable. Then add 20 mL of water to the filtration device, measure the amount of water filtered naturally without external pressure, and calculate the volume of water passing through per unit area per unit time, that is, the water flux [L / (m 2 ·h)].

[0083] Unit area dirt holding capacity test: When the test liquid passes through the tested filter element at a flow rate of 8 mm / s (effective filter area is 10 cm 2 ), add test powder with a particle size of 10 μm to the upstream of the filter at a rate of 10 mg / min until the ultimate pressure difference of the filter element reaches 0.4 MPa. Stop the test and calculate the cumulative amount of test dust added. The calculation formula is as follows: M=G×q×t÷S÷1000.

[0084] Where: M——pollution holding capacity (g / cm 2 ).

[0085] G——Average mass contamination in the dust adding device (mg / L).

[0086] q——average injection flow rate (L / min).

[0087] t——actual test time when the final pressure difference is reached (min).

[0088] S——Effective filtration area of ​​filter element (cm 2 ).

[0089] Filtration efficiency test: 5 μm standard particles are used, and the coefficient of variation of particle size distribution is controlled within 5%. An appropriate amount of standard particles and clean water are used to prepare the test solution. The prepared PTFE folded filter element is connected to the detection system, and the detection system is started. The test solution is passed through the filter element at a pressure difference of 0.15 MPa. The filter is filtered for 2 h, and the liquid samples before and after filtration are taken to characterize the weight of standard particles in the liquid sample and calculate the particle retention rate. The formula is as follows: Particle retention rate = (1-N0 / N) × 100.

[0090] Where: N0 – weight of standard particles per unit volume of filtrate (g).

[0091] N——weight of standard particles per unit volume of test fluid (g).

[0092] Pressure loss detection: △P = (32 × η × L × Q) / (π 2 d 4 ).

[0093] In the formula: △P——pressure difference, the unit is Pa.

[0094] η——Viscosity of the filter medium, in Pa·s.

[0095] L——Filter element length, in cm.

[0096] Q——Flow rate of medium, in L / s.

[0097] d——filter element radius, in cm.

[0098] Backwashing capacity: Weigh the weight of the dried folded filter element prepared in the example and the folded filter element after the dirt holding capacity per unit area test, use pure water to rinse the folded filter element prepared in the example at a flow rate of 20 mm / s, so that the pressure difference between the inside and outside of the filter element reaches 0.05 MPa, and continue for 100 s. Take samples every 5 s to characterize the backwash water. When the backwash water reaches the standard of suspended particle concentration ≤3 mg / L, it indicates that there is no stain. After drying, compare it with the original weight of the filter element to calculate the dirt removal rate. The formula is as follows.

[0099] Dirt removal rate = 1-(M-M0) / (M1-M0)×100%.

[0100] Wherein: M0 is the weight of the dry folded filter element prepared in the embodiment, in g.

[0101] M1 - the weight of the folded filter element after the dirt holding capacity per unit area is tested, in grams.

[0102] M——weight of folded filter element after drying, in g.

[0103]

[0104] Metal ion removal rate: Collect metal smelting industrial wastewater, especially steel plant production wastewater, which is characterized by high suspended matter content and high heavy metal concentration including harmful substances such as iron, copper, cadmium, zinc, etc. The steel plant production wastewater is used as the standard test liquid, and the pollutant content in the test liquid at this time is determined as the liquid A data. The prepared folded filter element is connected to the detection system, and the detection system is started to pass the standard test liquid through the prepared folded filter element at a flow rate of 8 mm / s for 2 hours. The pollutant content of the filtered solution is characterized and the liquid B data is recorded. The specific characterization results are as follows.

[0105]

[0106] Observing the above characterization results, the filter element prepared by the present invention has a high pollution holding capacity, can also meet the national regulations on the existing enterprise water pollutant emission concentration limit, and also has a high pollution removal rate during backwashing.

[0107] Example 3: A method for preparing a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance, the method comprising: (1) taking PTFE resin, heating and stirring at a temperature of 340°C for 30 minutes to melt it into a fluid, and then drying it at an ambient temperature of 60°C and an air flow rate of 30 km / h to form a film, controlling the film thickness to 0.2 mm, and forming it on a screen to prepare a precursor.

[0108] (2) At a temperature of 330 °C, the spinneret holes are arranged in a single row with a diameter of 0.4 mm, an aspect ratio of ≥10, and a hole pitch of 1.0 mm. Single-sided melt-blowing is performed to evenly spray molten fiber coating on the surface of the precursor, and the membrane is hot-pressed to a thickness of 0.25 mm at 115 °C to prepare a PTFE separation membrane, wherein the fiber coating comprises 25 wt% polypropylene, 17 wt% silica powder, 27 wt% polyvinylidene fluoride, and the remainder is polytetrafluoroethylene resin, wherein the silica powder has a particle size of 300 nm.

[0109] (3) Triethylenetetramine, acrylonitrile and titanium chloride were uniformly mixed in a mass ratio of 1:1.7:1.2, and after the PTFE separation membrane was cooled, the mixture was immersed in the mixture for 30 min to obtain a PTFE modified separation membrane.

[0110] (4) The two surfaces of the two PTFE modified separation membranes that have been soaked and not melt-sprayed are bonded together to form a filter matrix layer, and the filter matrix layer is folded into fins to obtain a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance. The PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance is surrounded by a cylinder to form an inner support cylinder, and then an outer support cylinder is sleeved outside the filter matrix layer to obtain a cartridge filter based on the PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance.

[0111] The prepared filter element was assembled into a cylindrical filter and then subjected to the following performance tests. The specific characterization results are as follows.

[0112] Structural integrity test: FBP-II porous material tester is used to test the first bubble point pressure p1 of PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance. The maximum pore size value can be calculated from the bubble point pressure. If the gas pressure continues to increase, boiling bubbles will appear in the liquid, which is called group bubble point pressure p2. The difference between p2 and p1 should be as small as possible. If the difference is less than 10% of p1, it is considered that the filter element is not damaged during the preparation process and the joints are reliably sealed.

[0113] Water flux test: Place the cartridge filter in the filtration device, add 20 mL of water, and circulate until the amount of water filtered within 5 minutes is stable. Then add 20 mL of water to the filtration device, measure the amount of water filtered naturally without external pressure, and calculate the volume of water passing through per unit area per unit time, that is, the water flux [L / (m 2 ·h)].

[0114] Unit area dirt holding capacity test: When the test liquid passes through the tested filter element at a flow rate of 8 mm / s (effective filter area is 10 cm 2 ), add test powder with a particle size of 10 μm to the upstream of the filter at a rate of 10 mg / min until the ultimate pressure difference of the filter element reaches 0.4 MPa. Stop the test and calculate the cumulative amount of test dust added. The calculation formula is as follows: M=G×q×t÷S÷1000.

[0115] Where: M——pollution holding capacity (g / cm 2 ).

[0116] G——Average mass contamination in the dust adding device (mg / L).

[0117] q——average injection flow rate (L / min).

[0118] t——actual test time when the final pressure difference is reached (min).

[0119] S——Effective filtration area of ​​filter element (cm 2 ).

[0120] Filtration efficiency test: 5 μm standard particles are used, and the coefficient of variation of particle size distribution is controlled within 5%. An appropriate amount of standard particles and clean water are used to prepare the test solution. The prepared PTFE folded filter element is connected to the detection system, and the detection system is started. The test solution is passed through the filter element at a pressure difference of 0.15 MPa. The filter is filtered for 2 h, and the liquid samples before and after filtration are taken to characterize the weight of standard particles in the liquid sample and calculate the particle retention rate. The formula is as follows: Particle retention rate = (1-N0 / N) × 100.

[0121] Where: N0 – weight of standard particles per unit volume of filtrate (g).

[0122] N——weight of standard particles per unit volume of test fluid (g).

[0123] Pressure loss detection: △P = (32 × η × L × Q) / (π 2 d 4 ).

[0124] In the formula: △P——pressure difference, the unit is Pa.

[0125] η——Viscosity of the filter medium, in Pa·s.

[0126] L——Filter element length, in cm.

[0127] Q——Flow rate of medium, in L / s.

[0128] d——filter element radius, in cm.

[0129] Backwashing capacity: Weigh the weight of the dried folded filter element prepared in the example and the folded filter element after the dirt holding capacity per unit area test, use pure water to rinse the folded filter element prepared in the example at a flow rate of 20 mm / s, so that the pressure difference between the inside and outside of the filter element reaches 0.05 MPa, and continue for 100 s. Take samples every 5 s to characterize the backwash water. When the backwash water reaches the standard of suspended particle concentration ≤3 mg / L, it indicates that there is no stain. After drying, compare it with the original weight of the filter element to calculate the dirt removal rate. The formula is as follows.

[0130] Dirt removal rate = 1-(M-M0) / (M1-M0)×100%.

[0131] Wherein: M0 is the weight of the dry folded filter element prepared in the embodiment, in g.

[0132] M1 - the weight of the folded filter element after the dirt holding capacity per unit area is tested, in grams.

[0133] M——weight of folded filter element after drying, in g.

[0134]

[0135] Metal ion removal rate: Collect metal smelting industrial wastewater, especially steel plant production wastewater, which is characterized by high suspended matter content and high heavy metal concentration including harmful substances such as iron, copper, cadmium, zinc, etc. The steel plant production wastewater is used as the standard test liquid, and the pollutant content in the test liquid at this time is determined as the liquid A data. The prepared folded filter element is connected to the detection system, and the detection system is started to pass the standard test liquid through the prepared folded filter element at a flow rate of 8 mm / s for 2 hours. The pollutant content of the filtered solution is characterized and the liquid B data is recorded. The specific characterization results are as follows.

[0136]

[0137] Observing the above characterization results, the filter element prepared by the present invention has a high pollution holding capacity, can also meet the national regulations on the existing enterprise water pollutant emission concentration limit, and also has a high pollution removal rate during backwashing.

[0138] Comparative Example 1: A method for preparing a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance. The specific preparation method is the same as Example 2, except that the organic solvent unique to the present invention is not used to prepare the folded filter element. The specific operation is as follows: A method for preparing a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance, the method comprising: (1) taking PTFE resin, heating and stirring at a temperature of 335°C for 25 minutes to melt it into a fluid, and then drying it to form a film under an ambient temperature of 45°C and an air flow rate of 27 km / h, and controlling the film thickness to be 0.15 mm for online molding to prepare a precursor.

[0139] (2) At a temperature of 327 °C, the spinneret holes are arranged in a single row with a diameter of 0.3 mm, an aspect ratio of ≥10, and a hole pitch of 0.8 mm. Single-sided melt-blowing is performed to evenly spray molten fiber coating on the surface of the precursor, and the membrane is hot-pressed to a thickness of 0.20 mm at 113 °C to prepare a PTFE separation membrane, wherein the fiber coating comprises 23 wt% polypropylene, 15 wt% silica powder, 25 wt% polyvinylidene fluoride, and the remainder is polytetrafluoroethylene resin, wherein the silica powder has a particle size of 200 nm.

[0140] (3) The two surfaces of two PTFE separation membranes that are not melt-sprayed are bonded together to form a filter matrix layer. The filter matrix layer is folded into fins. The filter matrix layer is wrapped around a cylinder to form an inner support cylinder. Subsequently, an outer support cylinder is placed outside the filter matrix layer to obtain a cartridge filter.

[0141] The prepared filter element was characterized in the same manner as in Example 2, and the specific characterization results are as follows.

[0142]

[0143]

[0144] By comparing the above characterization results with those of Example 2, it is found that the filter element prepared in the comparative example has a higher dirt holding capacity, but the filter element prepared in the comparative example has a poor filtering ability for metal ions. The invention team observed that the organic modifier will perform a second modification on the PTFE composite separation membrane to manufacture Duoan chelating fiber and externally connect metallocene to form a two-way flow-guiding internal skeleton, thereby enhancing the filtering performance of the filter element, having a two-way flow-guiding effect for filtration and backwashing, and enhancing the backwashing ability of the filter element.

[0145] Comparative Example 2: A method for preparing a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance. The specific preparation method is the same as Example 2, except that the fiber coating unique to the present invention is not used to prepare the folded filter element. The specific operation is as follows: A method for preparing a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance, the method comprising: (1) taking PTFE resin, heating and stirring at a temperature of 335°C for 25 minutes to melt it into a fluid, and then drying it to form a film under an ambient temperature of 45°C and an air flow rate of 27 km / h, and controlling the film thickness to be 0.15 mm for online molding to prepare a precursor.

[0146] (2) Triethylenetetramine, acrylonitrile and titanium chloride were uniformly mixed in a mass ratio of 1:1.5:1, and the precursor was immersed in the mixture for 25 min to obtain a PTFE modified separation membrane.

[0147] (3) The two soaked PTFE modified separation membranes are bonded together to form a filter matrix layer, the filter matrix layer is folded into fins, the filter matrix layer is wrapped around a cylinder to form an inner support cylinder, and then an outer support cylinder is placed outside the filter matrix layer to obtain a cartridge filter.

[0148] The prepared filter element was characterized in the same manner as in Example 2, and the specific characterization results are as follows.

[0149]

[0150]

[0151] From the comparison between the above characterization results and Example 2, it is found that the filtering capacity of the filter element prepared in the comparative example for metal ions is not much different from that in Example 2, but the amount of dirt holding is significantly reduced. Through the observation of the invention team, it is speculated that the fiber coating can be used for the first modification of the surface of the separation membrane. After this modification, the surface of the separation membrane undergoes structural changes, forming short fiber bundles composed of polytetrafluoroethylene resin as the skeleton and polyvinylidene fluoride resin as the external part. This structure is conducive to backwashing the filter element. The short fiber bundles that swing freely on both sides of the folded membrane can assist the water flow in backwashing the filter element. During the flushing and filtering, the short fiber layer is naturally compressed by the influence of the water flow direction, so that the density of the fluffy layer on the upper part of the short fiber layer is increased, the gradient change of the gap is realized, and the dirt holding capacity of the filter element is increased; during backwashing, the washing liquid flows from the inner layer to the outer layer of the filter element, and the flow rate of the inner layer is larger. The filter residue is first washed off and pushed to the outer layer, causing the pore size of the outer layer to decrease and the flow rate to increase, thereby enhancing the ability to strip off the filter residue, so that the filter residue is discharged from the outermost layer. The pore size increasing from the inner layer to the outer layer in the filter layer is conducive to the removal of filter residue when the filter element is backwashed.

[0152] Comparative Example 3: A method for preparing a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance. The specific preparation method is the same as Example 2, except that the components of the fiber coating unique to the present invention are used, and an equal amount of obsidian powder is used instead of silica powder to prepare the folded filter element. The specific operation is as follows: A method for preparing a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance, the method comprising: (1) taking PTFE resin, heating and stirring at a temperature of 335°C for 25 minutes to melt it into a fluid, and then drying it to form a film under an ambient temperature of 45°C and an air flow rate of 27 km / h, and controlling the film thickness to be 0.15 mm for online molding to prepare a precursor.

[0153] (2) At a temperature of 327 °C, the spinneret holes are arranged in a single row with a diameter of 0.3 mm, an aspect ratio of ≥10, and a hole pitch of 0.8 mm. Single-sided melt-blowing is performed to evenly spray molten fiber coating on the surface of the precursor, and the membrane is hot-pressed to a thickness of 0.20 mm at 113 °C to prepare a PTFE separation membrane, wherein the fiber coating comprises 23 wt% polypropylene, 15 wt% obsidian powder, 25 wt% polyvinylidene fluoride, and the remainder is polytetrafluoroethylene resin, wherein the obsidian powder has a particle size of 200 nm.

[0154] (3) Triethylenetetramine, acrylonitrile and titanium chloride were uniformly mixed in a mass ratio of 1:1.5:1, and after the PTFE separation membrane was cooled, the mixture was immersed in the mixture for 25 min to obtain a PTFE modified separation membrane.

[0155] (4) The two surfaces of the two PTFE modified separation membranes that have been soaked and not melt-sprayed are bonded together to form a filter matrix layer, and the filter matrix layer is folded into fins to obtain a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance. The PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance is surrounded by a cylinder to form an inner support cylinder, and then an outer support cylinder is sleeved outside the filter matrix layer to obtain a cartridge filter based on the PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance.

[0156] The prepared filter element was characterized in the same manner as in Example 2, and the specific characterization results are as follows.

[0157]

[0158]

[0159] From the comparison of the above characterization results with Example 2, it is found that the water flux and dirt holding capacity of the filter element prepared in the comparative example are reduced, the filtering ability of metal ions is deteriorated, and even the difference between the bubble point pressures p2 and p1 is greater than 10% of p1. The prepared filter element is damaged and the joints are not completely sealed, and it is impossible to prepare a foldable filter element that meets the technical requirements. After observation and speculation by the invention team, obsidian is a naturally formed silica glass obtained by rapid cooling of a molten body. When it changes from a molten state to a glassy state, the viscosity increases sharply during the cooling process, and the particles have no time to arrange in a regular manner to form crystals, and no latent heat of crystallization is released. Therefore, it contains a higher internal energy and belongs to a metastable state, which is an unstable high-energy state. There is a tendency to crystallize during the skeleton construction process, resulting in cracks inside the prepared filter element.

[0160] Comparative Example 4: A commercially available backwash folded filter element was subjected to the same performance characterization as in Example 2. The specific characterization results are as follows.

[0161]

[0162]

[0163] By comparing the above characterization results with those of Example 2, it is found that the commercially available filter element has a higher water flux and a smaller pressure loss. However, the commercially available filter element has limited filtering capacity for impurities in water and a poor filtering capacity for metal ions. At the same time, there are obvious impurity residues in the filter element, indicating that the sewage discharge time of the commercially available backwash folded filter element is greater than 100 s.

[0164] Application example: The PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance of the present invention is used to improve and upgrade the folded wing-shaped backwashable filter element in the original fixed backwashable filtration system of a chemical company in Hangzhou. After 3 months of sampling and calculation, the overall dirt removal efficiency is improved by about 31.62%, and the use and maintenance costs of the filtration system are reduced by about 71.69%.

Claims

1. A method for preparing a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance, characterized in that: The method comprises: (1) taking PTFE resin, heating and melting it into a fluid, and then putting it on a net for molding to prepare a precursor; (2) uniformly spraying fiber coating on the surface of the precursor by melt-blowing process, and hot-pressing to prepare a PTFE separation membrane; (3) modifying the PTFE separation membrane, preparing an organic solvent as a modification liquid and immersing the PTFE separation membrane in the organic solvent for modification to obtain a PTFE modified separation membrane; (4) stacking and folding the PTFE modified separation membrane into fins, so as to obtain a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance; in the melt-blowing process of step (2), the melt-blowing temperature is controlled to be 325-330°C, the spinneret spinnerets are arranged in a single row, the diameter is 0.2-0.4 mm, the aspect ratio is ≥10, the hole spacing is 0.6-1.0 mm, and single-sided coating is performed; Step 2) the fiber coating is a composite fiber, which is composed of 20-25 wt% polypropylene, 12-17 wt% silica powder, 22-27 wt% polyvinylidene fluoride, and the balance is polytetrafluoroethylene resin, wherein the silica powder has a particle size of 100-300 nm; The organic solvent in step (3) is a mixed solvent uniformly mixed with triethylenetetramine, acrylonitrile and titanium chloride in a mass ratio of 1: (1.2-1.7): (0.8-1.2); The soaking modification time in step (3) is 20 to 30 minutes.

2. The method for preparing a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance according to claim 1, characterized in that: The heating temperature in step (1) is 330-340°C, and stirring is maintained during the heating process for 15-30 minutes.

3. The method for preparing a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance according to claim 1, characterized in that: The net forming in step (1) is dried to form a film under the conditions of an ambient temperature of 20 to 60°C and an air flow rate of 25 to 30 km / h, and the film thickness is controlled to be 0.1 to 0.3 mm.

4. The method for preparing a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance according to claim 1, characterized in that: In step (2), the membrane thickness is hot-pressed at 110-115° C. to a target thickness of the PTFE separation membrane, wherein the target thickness of the PTFE separation membrane is 0.15-0.35 mm.

5. The method for preparing a PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance according to claim 1, characterized in that: In step (4), two PTFE modified separation membranes are stacked in sequence and the two surfaces not melt-spray coated are bonded together to form a filter matrix layer.

6. A PTFE foldable backwashable filter element with high dirt holding capacity and low flow resistance prepared by the method of any one of claims 1 to 5.

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