Wastewater softening and silicon removal method based on polytetrafluoroethylene-based ultrafiltration membrane

Abstract

The invention discloses a waste water softening and silicon removal method based on a polytetrafluoroethylene-based ultrafiltration membrane, belongs to the technical field of waste water softening and silicon removal, and aims to solve the technical problem that the waste water softening, hardness reducing and silicon removal effects and the anti-pollution performance need to be further improved in the prior art. The method specifically comprises the following steps: completely dipping a base membrane of the ultrafiltration membrane into a modification solution, carrying out ultrasonic dispersion for 40-60 minutes, taking out the base membrane of the ultrafiltration membrane, and draining. According to the invention, sulfonated siloxane modification, polyolefin interface blending, electrostatic spinning membrane forming, hot-pressing compounding and hydrophilic cross-linking modification are carried out on a polytetrafluoroethylene membrane to construct the PTFE-based ultrafiltration membrane with high hydrophilicity, low pollution and high stability, and the PTFE-based ultrafiltration membrane is used for softening and silicon removal of waste water after alkaline pretreatment. The hardness and silicon content of softened water are effectively reduced, and the membrane interception filtration is not easy to block and stable in operation.

Description

Technical Field

[0001] This invention relates to the field of wastewater softening and silicon removal technology, specifically to a wastewater softening and silicon removal method based on a polytetrafluoroethylene ultrafiltration membrane. Background Technology

[0002] With the rapid development of industry, various industrial production processes generate a large amount of wastewater containing high concentrations of hard ions and silicon compounds. Hard ions can lead to eutrophication of water bodies, affecting the survival of aquatic organisms. The accumulation of silicon compounds in natural water bodies can alter the physicochemical properties of the water and disrupt the ecological balance. At the same time, in some industrial fields with high water quality requirements, such as the electronics and pharmaceutical industries, wastewater with high hardness and high silicon content can seriously affect product quality and the normal operation of production equipment, increasing production costs. In the process of zero discharge and reuse of industrial wastewater, high-hardness wastewater needs to undergo softening treatment to remove hard ions such as calcium and magnesium, as well as silicates.

[0003] Currently, the softening and desiliconization process typically employs a combination of "high-density sedimentation tank + multi-media filter + external pressure ultrafiltration membrane". However, the pretreatment is complex, and the external pressure ultrafiltration membrane has strict requirements on the turbidity of the influent, requiring the installation of high-density sedimentation tanks and multi-media filters, which occupy a large area and have high investment costs. Moreover, to prevent fine sediments from clogging the membrane system, flocculants and coagulants need to be added, and acid needs to be added to adjust the pH, resulting in large dosages, high chemical costs, and complex operation.

[0004] Meanwhile, ultrafiltration membrane materials mostly use organic polymers such as polysulfone and polytetrafluoroethylene. Polytetrafluoroethylene is considered an ideal ultrafiltration membrane substrate due to its excellent chemical inertness, high temperature resistance and hydrophobic properties. However, polytetrafluoroethylene has low surface energy, is difficult to form a membrane and has poor hydrophilicity, which leads to problems such as high filtration resistance, serious pollution and low flux when it is directly applied to water treatment systems.

[0005] To improve the hydrophilicity and antifouling properties of polytetrafluoroethylene (PTFE) membranes, methods such as surface oxidation, plasma treatment, or physical blending are often employed. However, these methods suffer from drawbacks such as poor adhesion of the modified layer, insufficient chemical stability, or complex processes and poor repeatability, making it difficult to simultaneously achieve high flux, antifouling properties, and long-term operational stability of the membrane. Therefore, there is an urgent need to develop a PTFE ultrafiltration membrane with stable structure, high hydrophilicity, and strong antifouling capabilities, and to apply it to the wastewater softening and silica removal process to achieve efficient removal of hard ions and silica from wastewater, resulting in stable operation and easy maintenance of the water treatment effect. Summary of the Invention

[0006] The purpose of this invention is to provide a wastewater softening and silicon removal method based on a polytetrafluoroethylene ultrafiltration membrane, which solves the technical problem that the effect of wastewater softening, hardening, silicon removal, and anti-fouling performance needs to be further improved in the prior art.

[0007] The objective of this invention can be achieved through the following technical solution: a wastewater softening and silica removal method based on a polytetrafluoroethylene ultrafiltration membrane, comprising the following steps:

[0008] S1. The ultrafiltration membrane base membrane is completely immersed in the modification solution and ultrasonically dispersed for 40-60 min. The ultrafiltration membrane base membrane is then removed and drained, and then transferred to glutaraldehyde aqueous solution and immersed at room temperature for 3-4 h. After post-treatment, polytetrafluoroethylene ultrafiltration membrane is obtained.

[0009] S2. Add raw water and softening agent to the reaction tank, adjust the pH of the raw water to alkaline, and then transport it to the sedimentation tank. The sludge settles in the sedimentation tank, and the sediment is discharged from the bottom of the sedimentation tank by a pump. The supernatant overflows from the top of the sedimentation tank to obtain pretreated water.

[0010] S3. The pretreated water is transported to a filter tank containing a polytetrafluoroethylene ultrafiltration membrane, and softened water is obtained by negative pressure suction.

[0011] Further, in step S1, the modification solution is composed of hydroxymethyl cellulose, polyethyleneimine, and deionized water in a ratio of 3-5g:2-4g:50mL, and the mass fraction of the glutaraldehyde aqueous solution is 25%. The post-treatment includes: after the reaction is complete, removing the ultrafiltration membrane base membrane from the solution, washing it three times with purified water, transferring it to a drying oven at a temperature of 70-80℃, and drying it to constant weight to obtain a polytetrafluoroethylene ultrafiltration membrane; in step S2, the pH is 9-10; in step S3, the negative pressure suction pressure is 8-15kPa.

[0012] Furthermore, the preparation method of the ultrafiltration membrane base membrane is as follows: several PTFE spun membranes are cut, stacked layer by layer, and hot-pressed to form a composite spun membrane with a thickness of 2-3 mm; the composite spun membrane is placed in an impregnation solution at a temperature of 90-95℃, ultrasonically impregnated for 4-5 hours, and then post-treated to obtain the ultrafiltration membrane base membrane.

[0013] Furthermore, the hot pressing temperature is 160-180℃, the pressure is 0.6-0.8MPa, and the hot pressing time is 8-10min; the impregnation solution is composed of deionized water and dimethyl sulfoxide in a volume ratio of 8:3; the post-treatment includes: after the reaction is completed, the temperature of the impregnation solution is reduced to room temperature, the composite spun membrane is taken out of the impregnation solution, washed three times with purified water, and dried to obtain the ultrafiltration membrane base membrane.

[0014] Furthermore, the preparation method of PTFE spun membrane is as follows: polytetrafluoroethylene emulsion and polyolefin modified liquid are mixed and stirred, pore-forming liquid is added to the reaction system, and the mixture is stirred for 40-60 min. Deionized water is added to the reaction system to adjust the viscosity of the system to 6-9 Pa·s. After post-treatment, a spinning solution is obtained. The spinning solution is electrospun by electrospinning to obtain a PTFE spun membrane with a thickness of 50-60 μm.

[0015] Furthermore, the ratio of the polytetrafluoroethylene emulsion, the polyolefin modified liquid, and the pore-forming liquid is 20-30 mL:15 mL:10-13 mL. The pore-forming liquid is composed of polyvinylpyrrolidone and deionized water at a ratio of 3 g:10 mL. The post-treatment includes: after stirring, passing the solution through a 200-mesh sieve to obtain the spinning solution. The electrospinning voltage is 22 kV, the distance is 12-13 cm, the spinning solution flow rate is 1-1.2 mL / h, the spinning temperature is 27℃, the humidity is 45-50%, the roller diameter is 70-80 mm, and the roller speed is 500 r / min.

[0016] Furthermore, the preparation method of the polyolefin modified liquid is as follows: perfluorobutylethylene, sulfonic acid modified polysiloxane, hydroxyethyl methacrylate and emulsion are mixed and stirred, the temperature of the reaction system is raised to 80-90℃, an initiator solution is added to the reaction system, the reaction is kept at the temperature for 4-5 hours, and then post-treatment is performed to obtain the polyolefin modified liquid.

[0017] The chemical reaction equations involved in the synthesis of polyolefin modified liquids are as follows:

[0018]

[0019] In the formula: .

[0020] Furthermore, the ratio of the amount of perfluorobutylethylene, sulfonic acid modified polysiloxane, hydroxyethyl methacrylate, emulsion, and initiator solution is 6-7g:2-3g:1.6-1.8g:50mL:5mL. The initiator solution is composed of potassium persulfate and deionized water at a ratio of 1g:20mL. The emulsion is composed of potassium perfluorooctane sulfonate, AEO-9, sodium dodecyl sulfate, and deionized water at a ratio of 1g:5-6g:1.5-1.8g:80mL. The post-treatment includes: after the reaction is completed, the reaction system is evacuated to a negative pressure of -0.1MPa, and low-boiling substances are removed by vacuum distillation to obtain a polyolefin modified solution with a solid content of 50-60wt%.

[0021] Furthermore, the preparation method of sulfonic acid modified polysiloxane is as follows: 3-glycidyl etheroxypropylmethyldiethoxysilane, octamethylcyclotetrasiloxane and catalyst are mixed and stirred, the temperature of the reaction system is raised to 75-85℃, and the reaction is maintained at this temperature for 60-80 min. Diallyltetramethyldisiloxane and 3-aminopropanesulfonic acid are added to the reaction system, and the reaction is maintained at this temperature for 3-5 h. After post-treatment, sulfonic acid modified polysiloxane is obtained.

[0022] The reaction equation for the synthesis of sulfonic acid-modified polysiloxanes is as follows:

[0023]

[0024] In the formula: .

[0025] Furthermore, the molar ratio of 3-glycidyl etheroxypropylmethyldiethoxysilane, octamethylcyclotetrasiloxane, diallyltetramethyldisiloxane, and 3-aminopropanesulfonic acid is 2:5-7:1:2, the molar ratio of 3-glycidyl etheroxypropylmethyldiethoxysilane to catalyst is 2g:1mL, the catalyst is 40-60wt% sulfuric acid, and the post-treatment includes: after the reaction is complete, the temperature of the reaction system is lowered to room temperature, 2wt% sodium carbonate solution is added to the reaction system to adjust the pH of the system to 10, the system is allowed to stand and separate, the upper organic matter is washed with purified water until neutral, and then transferred to a drying oven at a water bath temperature of 80-90℃ and dried to constant weight to obtain sulfonic acid modified polysiloxane.

[0026] The present invention has the following beneficial effects:

[0027] 1. This invention combines pretreatment and ultrafiltration membrane filtration. The pretreatment step uses a chemical reaction to precipitate and separate hard ions, while the ultrafiltration membrane filtration further removes residual pollutants. This method can efficiently reduce the total hardness and silica content in wastewater, ensuring that the softened water meets relevant standards and satisfies the water quality requirements of different fields.

[0028] 2. This invention also modifies polytetrafluoroethylene (PTFE) emulsions using a polyolefin-modified liquid. The polyolefin-modified liquid is a copolymer of perfluorobutylene, sulfonic acid-modified polysiloxane, and hydroxyethyl methacrylate. This polyolefin-modified liquid contains polar groups such as hydroxyl, sulfonic acid, and ester groups. These polar segments enhance the interfacial compatibility between PTFE molecules and with polyvinylpyrrolidone (PVP) molecules, thereby forming a uniformly dispersed composite solution during mixing and stirring. After washing and drying, a uniform microporous structure is formed, improving the structural integrity and filtration stability of the membrane. The modified polyolefin solution maintains the chemical stability of the polyolefin chain while introducing polar groups, improving the compatibility between the modified polyolefin solution and the polytetrafluoroethylene emulsion. This results in a tighter molecular chain bond and a more uniform pore size distribution in the membrane material. The sulfonic acid modified polysiloxane molecules contain sulfonic acid groups that form a negatively charged hydrophilic interface layer on the membrane surface, improving the surface energy and wettability of polytetrafluoroethylene. This significantly enhances the compatibility between the membrane and the water system, reduces the adsorption and deposition of hard ions and colloidal silica on the membrane surface, thereby reducing the total hardness and silica content of the effluent.

[0029] 3. This invention achieves localized fusion between fibers and forms a dense and stable support layer through multi-layered interleaved hot pressing. Subsequently, it undergoes ultrasonic impregnation with a dimethyl sulfoxide / water mixed solvent to remove residues within the pores, resulting in a more open pore structure, uniform overall membrane thickness, and minimal flux attenuation during continuous operation. Through hydroxymethyl cellulose-polyethyleneimine crosslinking modification, hydroxymethyl cellulose provides a multi-hydroxyl network, and polyethyleneimine provides multi-amine sites. Under the crosslinking action of glutaraldehyde, a stable three-dimensional network layer rich in hydroxyl and amino groups is formed, further endowing the membrane surface with excellent hydrophilicity and antifouling properties, reducing the adsorption of organic matter, colloids, and metal ions on the membrane surface, and achieving reversible membrane regeneration. Detailed Implementation

[0030] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] In this application, the total hardness of the raw water is 950 mg / L and the silica content is 800 μg / L;

[0032] In this application, AEO-9 is a fatty alcohol polyoxyethylene ether, selected from Shandong Weilin Chemical Co., Ltd., with an effective ingredient content of 99%.

[0033] In this application, the polyvinylpyrrolidone is selected from Jinan Zhengkang Chemical Co., Ltd., model K60, with an active ingredient content of 99%;

[0034] In this application, the polytetrafluoroethylene emulsion is selected from Shenzhen Yubang Plastic Raw Materials Co., Ltd., with a solid content of 60% and a grade of FE-110.

[0035] In this application, the hydroxymethyl cellulose is selected from Jinan Mingjiang Chemical Co., Ltd., with a content of 99% and a product number of 684954;

[0036] In this application, the polyethyleneimine is selected from Wuhan Kemike Biomedical Technology Co., Ltd., with a molecular weight of 600 and a content of 99%.

[0037] Example 1

[0038] This embodiment provides a method for preparing a polytetrafluoroethylene ultrafiltration membrane, including the following steps:

[0039] Step 1: Preparation of sulfonic acid modified polysiloxane

[0040] Weigh out 49.7 g of 3-glycidyl etheroxypropylmethyldiethoxysilane, 148.3 g of octamethylcyclotetrasiloxane, and 24.9 mL of 40 wt% sulfuric acid and add them to a reaction flask. Stir the mixture and raise the temperature of the reaction flask to 75 °C. Keep the reaction at this temperature for 60 min. Add 21.4 g of diallyltetramethyldisiloxane and 49.7 g of 3-aminopropanesulfonic acid to the reaction flask and keep the reaction at this temperature for 3 h. Lower the temperature of the reaction flask to room temperature and add 2 wt% sodium carbonate solution to the reaction flask to adjust the pH of the system to 10. Allow the mixture to stand and separate the layers. Wash the upper organic matter with purified water until neutral and then transfer it to a drying oven at 80 °C in a water bath. Dry the mixture to constant weight to obtain sulfonic acid modified polysiloxane.

[0041] Under acid catalysis, octamethylcyclotetrasiloxane undergoes ring-opening polymerization. The 3-glycidyl etheroxypropylmethyldiethoxysilane added to the reaction system has hydrolyzable ethoxy groups and active epoxy groups. Under acidic conditions, the ethoxy groups are partially hydrolyzed to generate silanol structures. Then, the ring-opened octamethylcyclotetrasiloxane copolymerizes with the hydrolyzed and activated silanol groups through a dehydration condensation reaction, forming epoxy group modifications on the siloxane backbone. Diallyltetramethyldisiloxane acts as a capping agent, undergoing hydrolysis followed by condensation at high temperature to form alkenyl groups on the polysiloxane chain. Upon addition of 3-aminopropanesulfonic acid, the epoxy groups undergo ring-opening reactions under the nucleophilic action of the amino groups, forming β-hydroxy-amine bonds. Simultaneously, sulfonic acid groups are introduced onto the side chains, forming sulfonic acid group modifications on the polysiloxane molecular chain. Finally, the reaction is terminated by adding sodium carbonate solution to neutralize excess acid and adjust the pH of the system to alkaline, simultaneously generating a sodium sulfonate salt structure.

[0042] Step 2: Preparation of polyolefin modified liquid

[0043] Potassium perfluorooctane sulfonate, AEO-9, sodium dodecyl sulfate, and deionized water were mixed evenly at a ratio of 1g:5g:1.5g:80mL to obtain the emulsion.

[0044] Potassium persulfate and deionized iron were mixed at a ratio of 1 g: 20 mL to obtain an initiator solution;

[0045] Weigh out 600g of perfluorobutylethylene, 200g of sulfonic acid modified polysiloxane, 160g of hydroxyethyl methacrylate and 5000mL of emulsion and add them to a reaction flask. Stir the mixture and raise the temperature of the reaction flask to 80℃. Add 500mL of initiator solution to the reaction flask and keep it at this temperature for 4 hours. Then, apply a negative pressure to the reaction flask to -0.1MPa and remove low-boiling substances by vacuum distillation to obtain a polyolefin modified liquid with a solid content of 50wt%.

[0046] In the presence of an emulsifier system, a stable oil / water emulsion is formed in the reaction system, in which perfluorobutylethylene (PFME) is the main monomer, and sulfonic acid-modified polysiloxane and hydroxyethyl methacrylate are co-dispersed in the aqueous phase. Under heating conditions, potassium persulfate is added to initiate the free radical polymerization of PFME and hydroxyethyl methacrylate molecules, introducing polar hydroxy ester groups into the polyfluoropolymer molecules, increasing the hydrophilicity and crosslinkability of the system. The sulfonic acid-modified polysiloxane molecular chain contains unsaturated alkenyl groups that participate in the free radical reaction copolymerization, and the polysiloxane segments are chemically grafted onto the copolymer backbone of PFME / hydroxyethyl methacrylate. As the reaction proceeds, the flexible segments of polysiloxane coexist with the polyfluoroalkyl segments, thus preparing a polyolefin-modified liquid.

[0047] Step 3: Preparation of PTFE spinning membrane

[0048] Polyvinylpyrrolidone and deionized water were mixed at a ratio of 3g:10mL to obtain a pore-forming solution;

[0049] Weigh out 100 mL of polytetrafluoroethylene emulsion and 75 mL of polyolefin modified solution and add them to the reaction flask. Stir for 20 min. Add 50 mL of pore-forming solution to the reaction flask and stir for 40 min. Then add deionized water to the reaction flask and adjust the viscosity of the system to 6 Pa·s. After stirring, pass the solution through a 200-mesh sieve to obtain the spinning solution.

[0050] The spinning solution was added to the spinning injector. A 70 mm diameter roller was used as the receiver. The roller speed was set to 500 r / min. The distance between the spinning nozzle and the roller was set to 12 cm. The electrospinning voltage was set to 22 kV. The spinning solution flow rate was 1 mL / h. Electrospinning was carried out in an environment with a temperature of 27℃ and a humidity of 45%, and a PTFE spinning film with a thickness of 50 μm was formed on the receiver.

[0051] Step 4: Preparation of the ultrafiltration membrane base membrane

[0052] Several PTFE spun membranes are cut into regular rectangles and then laid flat on a hot press to form a continuous membrane material. The membrane material is then laid in an alternating manner, the hot press is closed, a pressure of 0.6 MPa is applied, the temperature of the hot press is raised to 160℃, and the membrane is hot-pressed for 8 minutes. The membrane is then allowed to cool naturally to room temperature to obtain a composite spun membrane with a thickness of 2 mm.

[0053] Deionized water and dimethyl sulfoxide were mixed at a volume ratio of 8:3 to obtain the impregnation solution;

[0054] The composite spun membrane was completely immersed in the impregnation solution, which was heated to 90°C and ultrasonically impregnated for 4 hours. The temperature of the impregnation solution was then reduced to room temperature. The composite spun membrane was then removed from the impregnation solution, washed three times with purified water, and dried to obtain the ultrafiltration membrane base membrane.

[0055] Polyvinylpyrrolidone (PVP) is mixed with deionized water to form a pore-forming liquid. PPVP is a hydrophilic polymer that does not chemically react with the polymer backbone during subsequent spinning and heat treatment; instead, it is physically dispersed in the system, regulating the phase separation rate and forming the pore structure. During subsequent washing, some PPVP is dissolved, leaving micropores and achieving membrane porosity. The polytetrafluoroethylene (PTFE) emulsion and the polyolefin-modified liquid are mixed to form the main film-forming system. PTFE particles exhibit excellent chemical stability in the emulsion state, while the polyolefin-modified liquid contains grafted segments with polar groups (such as hydroxyl, sulfonic acid, and ester groups). These polar segments enhance the interfacial compatibility between PTFE particles and with PPVP molecules, thus forming a uniformly dispersed composite solution during mixing. In the electrospinning stage, the PTFE particles, modified polyolefin segments, and PPVP molecules in the system are stretched into continuous filaments under a high electric field. During electrospinning, the solvent evaporates rapidly, forming a composite nanofiber network with polytetrafluoroethylene (PTFE) as the main framework, polyolefin segments as the interfacial bonding layer, and polyvinylpyrrolidone (PVP) as the dispersing and pore-forming agent. No chemical bonds are formed in this process; instead, a stable fiber structure is primarily constructed through physical entanglement, hydrogen bonding, and van der Waals forces. Subsequently, hot pressing is used to form welding nodes at fiber intersections, achieving a dense bond between the fiber membranes. In the subsequent impregnation and post-treatment stages, the composite membrane is subjected to high-temperature ultrasonic impregnation in a water-dimethyl sulfoxide (DMSO) mixed solution. This process causes residual PDM and unreacted small-molecule additives to dissolve from the membrane, while DDM molecules penetrate into the membrane interior, resulting in microphase separation, further expanding the pore size and connecting the channels to form a uniform microporous structure. After washing and drying, a composite ultrafiltration membrane base membrane with PTFE as the main body, embedded with modified polyolefin flexible segments, and possessing a porous structure is obtained.

[0056] Step 5: Preparation of polytetrafluoroethylene ultrafiltration membrane

[0057] Hydroxymethyl cellulose, polyethyleneimine, and deionized water were mixed at a ratio of 3g:2g:50mL and stirred until the system was dissolved to obtain the modified solution.

[0058] The ultrafiltration membrane base was completely immersed in the modification solution and ultrasonically dispersed for 40 min. The ultrafiltration membrane base was then removed and drained, and then transferred to a 25 wt% glutaraldehyde aqueous solution. It was completely immersed at room temperature for 3 h. The ultrafiltration membrane base was then removed from the solution, washed with purified water for 3 hours, and then transferred to a drying oven at 70 °C and dried to constant weight to obtain a polytetrafluoroethylene ultrafiltration membrane.

[0059] Hydroxymethyl cellulose and polyethyleneimine are dissolved together in water to form a modification solution containing a large number of hydroxyl and primary and secondary amine groups. When the ultrafiltration membrane base is immersed in the modification solution, hydroxymethyl cellulose and polyethyleneimine form a uniform adsorption layer on the membrane surface. When the treated membrane is transferred to a glutaraldehyde solution, the dialdehyde groups on the glutaraldehyde molecule undergo a condensation reaction with the primary / secondary amines on the polyethyleneimine chain or the hydroxyl groups in the hydroxymethyl cellulose molecule, forming a cross-linked network between hydroxymethyl cellulose and polyethyleneimine. During the final drying process, the cross-linked network is further densified and stabilized on the membrane surface, resulting in a polytetrafluoroethylene ultrafiltration membrane.

[0060] Example 2

[0061] This embodiment provides a method for preparing a polytetrafluoroethylene ultrafiltration membrane, including the following steps:

[0062] Step 1: Preparation of sulfonic acid modified polysiloxane

[0063] Weigh out 49.7 g of 3-glycidyl etheroxypropylmethyldiethoxysilane, 178.0 g of octamethylcyclotetrasiloxane, and 24.9 mL of 50 wt% sulfuric acid and add them to a reaction flask. Stir the mixture and raise the temperature of the reaction flask to 80 °C. Keep the reaction at this temperature for 70 min. Add 21.4 g of diallyltetramethyldisiloxane and 49.7 g of 3-aminopropanesulfonic acid to the reaction flask and keep the reaction at this temperature for 4 h. Lower the temperature of the reaction flask to room temperature and add 2 wt% sodium carbonate solution to the reaction flask to adjust the pH of the system to 10. Allow the mixture to stand and separate the layers. Wash the upper organic matter with purified water until neutral and then transfer it to a drying oven at 85 °C in a water bath. Dry the mixture to constant weight to obtain sulfonic acid modified polysiloxane.

[0064] Step 2: Preparation of polyolefin modified liquid

[0065] Potassium perfluorooctane sulfonate, AEO-9, sodium dodecyl sulfate, and deionized water were mixed evenly at a ratio of 1g:5.5g:1.65g:80mL to obtain the emulsion.

[0066] Potassium persulfate and deionized iron were mixed at a ratio of 1 g: 20 mL to obtain an initiator solution;

[0067] Weigh out 650g of perfluorobutylethylene, 250g of sulfonic acid modified polysiloxane, 170g of hydroxyethyl methacrylate, and 5000mL of emulsion and add them to a reaction flask. Stir the mixture and raise the temperature of the reaction flask to 85℃. Add 500mL of initiator solution to the reaction flask and keep it at this temperature for 4.5h. Then, apply a negative pressure to the reaction flask to -0.1MPa and remove low-boiling-point substances by vacuum distillation to obtain a polyolefin modified liquid with a solid content of 55wt%.

[0068] Step 3: Preparation of PTFE spinning membrane

[0069] Polyvinylpyrrolidone and deionized water were mixed at a ratio of 3g:10mL to obtain a pore-forming solution;

[0070] Weigh out 125 mL of polytetrafluoroethylene emulsion and 75 mL of polyolefin modified solution and add them to the reaction flask. Stir for 25 min. Add 57.5 mL of pore-forming solution to the reaction flask and stir for 50 min. Then add deionized water to the reaction flask and adjust the viscosity of the system to 7.5 Pa·s. After stirring, pass the solution through a 200-mesh sieve to obtain the spinning solution.

[0071] The spinning solution was added to the spinning injector. A 75 mm diameter roller was used as the receiver. The roller speed was set to 500 r / min. The distance between the spinning nozzle and the roller was set to 12.5 cm. The electrospinning voltage was set to 22 kV. The spinning solution flow rate was 1.1 mL / h. Electrospinning was carried out in an environment with a temperature of 27℃ and a humidity of 47%, forming a PTFE spinning film with a thickness of 55 μm on the receiver.

[0072] Step 4: Preparation of the ultrafiltration membrane base membrane

[0073] Several PTFE spun membranes are cut into regular rectangles and then laid flat on a hot press to form a continuous membrane material. The membrane material is then laid in an alternating manner, the hot press is closed, a pressure of 0.7 MPa is applied, the temperature of the hot press is raised to 170°C, and the membrane is hot-pressed for 9 minutes. The membrane is then allowed to cool naturally to room temperature to obtain a composite spun membrane with a thickness of 2.5 mm.

[0074] Deionized water and dimethyl sulfoxide were mixed at a volume ratio of 8:3 to obtain the impregnation solution;

[0075] The composite spun membrane was completely immersed in the impregnation solution, which was heated to 93°C and ultrasonically impregnated for 4.5 hours. The temperature of the impregnation solution was then reduced to room temperature. The composite spun membrane was then removed from the impregnation solution, washed three times with purified water, and dried to obtain the ultrafiltration membrane base membrane.

[0076] Step 5: Preparation of polytetrafluoroethylene ultrafiltration membrane

[0077] Hydroxymethyl cellulose, polyethyleneimine, and deionized water were mixed at a ratio of 4g:3g:50mL and stirred until the system was dissolved to obtain the modified solution.

[0078] The ultrafiltration membrane base was completely immersed in the modification solution and ultrasonically dispersed for 50 min. The ultrafiltration membrane base was then removed and drained, and then transferred to a 25 wt% glutaraldehyde aqueous solution. It was completely immersed at room temperature for 3.5 h. The ultrafiltration membrane base was then removed from the solution, washed with purified water for 3 hours, and then transferred to a drying oven at 75 °C and dried to constant weight to obtain a polytetrafluoroethylene ultrafiltration membrane.

[0079] Example 3

[0080] This embodiment provides a method for preparing a polytetrafluoroethylene ultrafiltration membrane, including the following steps:

[0081] Step 1: Preparation of sulfonic acid modified polysiloxane

[0082] Weigh out 49.7 g of 3-glycidyl etheroxypropylmethyldiethoxysilane, 207.6 g of octamethylcyclotetrasiloxane, and 24.9 mL of 60 wt% sulfuric acid and add them to a reaction flask. Stir the mixture and raise the temperature of the reaction flask to 85 °C. Keep the reaction at this temperature for 80 min. Add 21.4 g of diallyltetramethyldisiloxane and 49.7 g of 3-aminopropanesulfonic acid to the reaction flask and keep the reaction at this temperature for 5 h. Lower the temperature of the reaction flask to room temperature and add 2 wt% sodium carbonate solution to the reaction flask to adjust the pH of the system to 10. Allow the mixture to stand and separate the layers. Wash the upper organic matter with purified water until neutral and then transfer it to a drying oven at a water bath temperature of 90 °C. Dry the mixture to constant weight to obtain sulfonic acid modified polysiloxane.

[0083] Step 2: Preparation of polyolefin modified liquid

[0084] Potassium perfluorooctane sulfonate, AEO-9, sodium dodecyl sulfate, and deionized water were mixed evenly at a ratio of 1g:6g:1.8g:80mL to obtain the emulsion.

[0085] Potassium persulfate and deionized iron were mixed at a ratio of 1 g: 20 mL to obtain an initiator solution;

[0086] Weigh out 700g of perfluorobutylethylene, 300g of sulfonic acid modified polysiloxane, 180g of hydroxyethyl methacrylate and 5000mL of emulsion and add them to a reaction flask. Stir the mixture and raise the temperature of the reaction flask to 90℃. Add 500mL of initiator solution to the reaction flask and keep it at this temperature for 5h. Then, apply a negative pressure to the reaction flask to -0.1MPa and remove low-boiling substances by vacuum distillation to obtain a polyolefin modified liquid with a solid content of 60wt%.

[0087] Step 3: Preparation of PTFE spinning membrane

[0088] Polyvinylpyrrolidone and deionized water were mixed at a ratio of 3g:10mL to obtain a pore-forming solution;

[0089] Weigh out 150 mL of polytetrafluoroethylene emulsion and 75 mL of polyolefin modified solution and add them to the reaction flask. Stir for 30 min. Add 65 mL of pore-forming solution to the reaction flask and stir for 60 min. Then add deionized water to the reaction flask and adjust the viscosity of the system to 9 Pa·s. After stirring, pass the solution through a 200-mesh sieve to obtain the spinning solution.

[0090] The spinning solution was added to the spinning injector. An 80 mm diameter roller was used as the receiver. The roller speed was set to 500 r / min. The distance between the spinning nozzle and the roller was set to 13 cm. The electrospinning voltage was set to 22 kV. The spinning solution flow rate was 1.2 mL / h. Electrospinning was carried out in an environment with a temperature of 27℃ and a humidity of 50%, forming a 60 μm thick PTFE spun film on the receiver.

[0091] Step 4: Preparation of the ultrafiltration membrane base membrane

[0092] Several PTFE spun membranes are cut into regular rectangles and then laid flat on a hot press to form a continuous membrane material. The membrane material is then laid in an alternating manner, the hot press is closed, a pressure of 0.8 MPa is applied, the temperature of the hot press is raised to 180°C, and the membrane is hot-pressed for 10 minutes. The membrane is then allowed to cool naturally to room temperature to obtain a composite spun membrane with a thickness of 3 mm.

[0093] Deionized water and dimethyl sulfoxide were mixed at a volume ratio of 8:3 to obtain the impregnation solution;

[0094] The composite spun membrane was completely immersed in the impregnation solution, which was heated to 95°C and ultrasonically impregnated for 5 hours. The temperature of the impregnation solution was then reduced to room temperature. The composite spun membrane was then removed from the impregnation solution, washed three times with purified water, and dried to obtain the ultrafiltration membrane base membrane.

[0095] Step 5: Preparation of polytetrafluoroethylene ultrafiltration membrane

[0096] Hydroxymethyl cellulose, polyethyleneimine, and deionized water were mixed at a ratio of 5g:4g:50mL and stirred until the system was dissolved to obtain the modified solution.

[0097] The ultrafiltration membrane base was completely immersed in the modification solution and ultrasonically dispersed for 60 min. The ultrafiltration membrane base was then removed and drained, and then transferred to a 25 wt% glutaraldehyde aqueous solution. It was completely immersed at room temperature for 4 h. The ultrafiltration membrane base was then removed from the solution, washed with purified water for 3 hours, and then transferred to a drying oven at 80 °C and dried to constant weight to obtain a polytetrafluoroethylene ultrafiltration membrane.

[0098] Example 4

[0099] This embodiment provides a wastewater softening and silica removal method based on a polytetrafluoroethylene ultrafiltration membrane, comprising the following steps:

[0100] S1. The polytetrafluoroethylene ultrafiltration membrane prepared in Example 1 is stacked and installed on the filter tank to form a filter layer with a thickness of 2.8 cm on the filter tank.

[0101] S2. Add raw water and 10wt% sodium hydroxide aqueous solution to the reaction tank, adjust the pH of the raw water to 9, and transport it to the sedimentation tank. The sludge settles in the sedimentation tank, and the sludge is discharged from the bottom of the sedimentation tank by a pump. The supernatant overflows from the top of the sedimentation tank to obtain pretreated water.

[0102] S3. The pretreated water is transported to the filter tank in step S1. A negative pressure of -8 kPa is drawn at the bottom of the filter tank, and the water is filtered through a polytetrafluoroethylene ultrafiltration membrane to obtain softened water. The precipitates trapped in the pretreated water remain in the filter tank.

[0103] Example 5

[0104] This embodiment provides a wastewater softening and silica removal method based on a polytetrafluoroethylene ultrafiltration membrane, comprising the following steps:

[0105] S1. The polytetrafluoroethylene ultrafiltration membrane prepared in Example 2 is stacked and installed on the filter tank to form a filter layer with a thickness of 3.0 cm on the filter tank.

[0106] S2. Add raw water and 10wt% sodium hydroxide aqueous solution to the reaction tank, adjust the pH of the raw water to 9.5, and transport it to the sedimentation tank. The sludge settles in the sedimentation tank, and the sludge is discharged from the bottom of the sedimentation tank by a pump. The supernatant overflows from the top of the sedimentation tank to obtain pretreated water.

[0107] S3. The pretreated water is transported to the filter tank in step S1. A negative pressure of -12 kPa is drawn at the bottom of the filter tank. The water is then drawn through a polytetrafluoroethylene ultrafiltration membrane to obtain softened water. The precipitates trapped in the pretreated water remain in the filter tank.

[0108] Example 6

[0109] This embodiment provides a wastewater softening and silica removal method based on a polytetrafluoroethylene ultrafiltration membrane, comprising the following steps:

[0110] S1. The polytetrafluoroethylene ultrafiltration membrane prepared in Example 3 is stacked and installed on the filter tank to form a filter layer with a thickness of 3.2 cm on the filter tank.

[0111] S2. Add raw water and 10wt% sodium hydroxide aqueous solution to the reaction tank, adjust the pH of the raw water to 10, and transport it to the sedimentation tank. The sludge settles in the sedimentation tank, and the sludge is discharged from the bottom of the sedimentation tank by a pump. The supernatant overflows from the top of the sedimentation tank to obtain pretreated water.

[0112] S3. The pretreated water is transported to the filter tank in step S1. A negative pressure of -15 kPa is drawn at the bottom of the filter tank, and the water is filtered through a polytetrafluoroethylene ultrafiltration membrane to obtain softened water. The precipitates trapped in the pretreated water remain in the filter tank.

[0113] Comparative Example 1

[0114] The difference between this comparative example and Example 6 is that 3-aminopropanesulfonic acid was not added in step 1 during the preparation of the polytetrafluoroethylene ultrafiltration membrane used.

[0115] Comparative Example 2

[0116] The difference between this comparative example and Example 6 is that, in the preparation of the polytetrafluoroethylene ultrafiltration membrane, sulfonic acid-modified polysiloxane was not added in step 2.

[0117] Comparative Example 3

[0118] The difference between this comparative example and Example 6 is that perfluorobutylethylene was not added in step 2 during the preparation of the polytetrafluoroethylene ultrafiltration membrane used.

[0119] Comparative Example 4

[0120] The difference between this comparative example and Example 6 is that, in the preparation of the polytetrafluoroethylene ultrafiltration membrane, the impregnation solution in step 4 is deionized water.

[0121] Comparative Example 5

[0122] The difference between this comparative example and Example 6 is that step S3 is omitted, and the pretreated water is used as softened water.

[0123] Performance testing:

[0124] The total hardness (calculated as calcium carbonate) of the softened water prepared in Examples 4-6 and Comparative Examples 1-5 was determined according to the standard GB 5749-2022 "Standards for Drinking Water Quality".

[0125] The silica content of the softened water prepared in Examples 4-6 and Comparative Examples 1-5 was determined according to the standard GB / T 12149-2017 "Determination of Silicon in Industrial Circulating Cooling Water and Boiler Water".

[0126] In Examples 4-6 and Comparative Examples 1-5, the polytetrafluoroethylene ultrafiltration membrane was used to prepare softened water for pretreatment of water, which operated continuously for 8 hours, referring to the formula. The clogging rate of the polytetrafluoroethylene ultrafiltration membrane was determined. In the formula, V0 is the effluent flow rate after the softened water prepared from the pretreated water filtered by the polytetrafluoroethylene ultrafiltration membrane has stabilized, and V1 is the effluent flow rate at 8 hours after the softened water prepared from the pretreated water filtered by the polytetrafluoroethylene ultrafiltration membrane has stabilized.

[0127] In Examples 4-6 and Comparative Examples 1-5, the pre-treated water from the polytetrafluoroethylene ultrafiltration membrane was continuously treated for 1 hour, followed by backwashing with 3 mol / L hydrochloric acid for 2 minutes. The wastewater from the backwash was then transferred to the original water source. This cycle was repeated 100 times, referring to the formula. The effluent degradation rate of a polytetrafluoroethylene ultrafiltration membrane after 100 cycles was determined. In the formula, S0 is the effluent volume of the first cycle, and S... 100 The output water volume for the 100th cycle is shown in Table 1 below.

[0128] Table 1 - Performance Test Data of Samples

[0129] Group Project <![CDATA[Total hardness / mg·L -1 > <![CDATA[Silicon dioxide content / μg·L -1 > Clog rate / % Water attenuation rate / % Example 5 215 152 6.3 4.3 Example 6 208 148 6.1 4.1 Example 7 212 151 6.2 4.2 Comparative Example 1 253 196 9.3 7.2 Comparative Example 2 301 235 9.6 7.9 Comparative Example 3 287 218 9.5 7.5 Comparative Example 4 254 196 8.3 6.4 Comparative Example 5 485 500 -- --

[0130] Data Analysis:

[0131] Comparative analysis of the data in Table 1 shows that the total hardness of the softened water purified by this invention is reduced to 208 mg / L, and the silica content is reduced to 148 μg / L. The clogging rate of the polytetrafluoroethylene (PTFE) ultrafiltration membrane after 8 hours of continuous filtration is only 6.1%, and the effluent decay rate of the PTFE ultrafiltration membrane after 100 cycles of filtration is only 4.1%. All performance test data are superior to the comparative example. This indicates that this invention constructs a PTFE-based ultrafiltration membrane with high hydrophilicity, low fouling, and high stability by modifying the PTFE membrane with sulfonated siloxane, polyolefin interfacial blending, electrospinning, hot pressing, and hydrophilic crosslinking. This membrane is used to soften and remove silica from wastewater after alkaline pretreatment, which not only effectively reduces the hardness and silica content of the softened water, but also makes the membrane less prone to clogging and ensures stable operation.

[0132] The above description is merely an example and illustration of the structure of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described, or use similar methods to replace them, as long as they do not deviate from the structure of the invention or exceed the scope defined in the claims, all of which should fall within the protection scope of the present invention.

Claims

1. A wastewater softening and silica removal method based on a polytetrafluoroethylene ultrafiltration membrane, characterized in that, Includes the following steps: S1. The ultrafiltration membrane base membrane is completely immersed in the modification solution and ultrasonically dispersed for 40-60 min. The ultrafiltration membrane base membrane is then removed and drained, and then transferred to glutaraldehyde aqueous solution and immersed at room temperature for 3-4 h. After post-treatment, polytetrafluoroethylene ultrafiltration membrane is obtained. S2. Add raw water and softening agent to the reaction tank, adjust the pH of the raw water to alkaline, and then transport it to the sedimentation tank. The sludge settles in the sedimentation tank, and the sediment is discharged from the bottom of the sedimentation tank by a pump. The supernatant overflows from the top of the sedimentation tank to obtain pretreated water. S3. The pretreated water is transported to a filter tank containing a polytetrafluoroethylene ultrafiltration membrane, and softened water is obtained by negative pressure suction.

2. The wastewater softening and silicon removal method based on a polytetrafluoroethylene ultrafiltration membrane according to claim 1, characterized in that, In step S1, the modification solution is composed of hydroxymethyl cellulose, polyethyleneimine and deionized water in a ratio of 3-5g:2-4g:50mL, and the mass fraction of the glutaraldehyde aqueous solution is 25%; in step S2, the pH is 9-10; in step S3, the pressure of the negative pressure suction is 8-15kPa.

3. The wastewater softening and silicon removal method based on a polytetrafluoroethylene ultrafiltration membrane according to claim 1, characterized in that, The preparation method of the ultrafiltration membrane base membrane is as follows: several PTFE spun membranes are cut, stacked layer by layer, and hot-pressed to form a composite spun membrane with a thickness of 2-3 mm; the composite spun membrane is placed in an impregnation solution at a temperature of 90-95℃ and ultrasonically impregnated for 4-5 hours, followed by post-treatment to obtain the ultrafiltration membrane base membrane.

4. The wastewater softening and silicon removal method based on a polytetrafluoroethylene ultrafiltration membrane according to claim 3, characterized in that, The hot pressing temperature is 160-180℃, the pressure is 0.6-0.8MPa, and the hot pressing time is 8-10min; the impregnation solution is composed of deionized water and dimethyl sulfoxide in a volume ratio of 8:

3.

5. The wastewater softening and silicon removal method based on a polytetrafluoroethylene ultrafiltration membrane according to claim 3, characterized in that, The preparation method of PTFE spun membrane is as follows: polytetrafluoroethylene emulsion and polyolefin modified liquid are mixed and stirred, pore-forming liquid is added to the reaction system, and the mixture is stirred for 40-60 min. Deionized water is added to the reaction system to adjust the viscosity of the system to 6-9 Pa·s. After post-treatment, a spinning solution is obtained. The spinning solution is electrospun by electrospinning to obtain a PTFE spun membrane with a thickness of 50-60 μm.

6. The wastewater softening and silicon removal method based on a polytetrafluoroethylene ultrafiltration membrane according to claim 5, characterized in that, The ratio of polytetrafluoroethylene emulsion, polyolefin modified liquid, and pore-forming liquid is 20-30 mL:15 mL:10-13 mL. The pore-forming liquid is composed of polyvinylpyrrolidone and deionized water at a ratio of 3 g:10 mL. The electrospinning voltage is 22 kV, the distance is 12-13 cm, the spinning liquid flow rate is 1-1.2 mL / h, the spinning temperature is 27℃, the humidity is 45-50%, the roller diameter is 70-80 mm, and the roller speed is 500 r / min.

7. The wastewater softening and silicon removal method based on a polytetrafluoroethylene ultrafiltration membrane according to claim 5, characterized in that, The preparation method of polyolefin modified liquid is as follows: perfluorobutylethylene, sulfonic acid modified polysiloxane, hydroxyethyl methacrylate and emulsion are mixed and stirred, the temperature of the reaction system is raised to 80-90℃, an initiator solution is added to the reaction system, the reaction is kept at the temperature for 4-5 hours, and then post-treatment is performed to obtain polyolefin modified liquid.

8. The wastewater softening and silicon removal method based on a polytetrafluoroethylene ultrafiltration membrane according to claim 7, characterized in that, The ratio of perfluorobutylethylene, sulfonic acid-modified polysiloxane, hydroxyethyl methacrylate, emulsion, and initiator solution is 6-7g:2-3g:1.6-1.8g:50mL:5mL. The initiator solution is composed of potassium persulfate and deionized water at a ratio of 1g:20mL. The emulsion is composed of potassium perfluorooctane sulfonate, AEO-9, sodium dodecyl sulfate, and deionized water at a ratio of 1g:5-6g:1.5-1.8g:80mL.

9. A wastewater softening and silica removal method based on a polytetrafluoroethylene ultrafiltration membrane according to claim 7, characterized in that, The preparation method of sulfonic acid modified polysiloxane is as follows: 3-glycidyl etheroxypropylmethyldiethoxysilane, octamethylcyclotetrasiloxane and catalyst are mixed and stirred, the temperature of the reaction system is raised to 75-85℃, and the reaction is maintained at this temperature for 60-80 min. Diallyltetramethyldisiloxane and 3-aminopropanesulfonic acid are added to the reaction system, and the reaction is maintained at this temperature for 3-5 h. After post-treatment, sulfonic acid modified polysiloxane is obtained.

10. A wastewater softening and silica removal method based on a polytetrafluoroethylene ultrafiltration membrane according to claim 9, characterized in that, The molar ratio of 3-glycidyl etheroxypropylmethyldiethoxysilane, octamethylcyclotetrasiloxane, diallyltetramethyldisiloxane, and 3-aminopropanesulfonic acid is 2:5-7:1:2, the molar ratio of 3-glycidyl etheroxypropylmethyldiethoxysilane to catalyst is 2g:1mL, and the catalyst is 40-60wt% sulfuric acid.

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