A ceramic composite membrane for degreasing wastewater and its preparation method

Abstract

This invention relates to a separation membrane for degreasing wastewater and its preparation method. The method involves modifying the surface of a hollow fiber ceramic substrate membrane prepared by a non-solvent phase inversion combined with high-temperature sintering using a TiO2 sol coating method. Specifically, tetrabutyl titanate is used as a precursor, anhydrous ethanol as a solvent, acetic acid as a hydrolysis inhibitor, and hydrochloric acid as a stabilizer to adjust the pH of the mixed solution. Film-forming aids are used to adjust the solution viscosity, and nano-hollow microspheres are added to provide molecular sieve channels. After the solution is thoroughly mixed, it is aged at room temperature. Then, TiO2 is immobilized onto the substrate membrane surface through impregnation coating combined with gradient sintering. This invention constructs a multi-scale porous ceramic composite membrane, further adjusting the average pore size and pore size distribution, surface charge, and hydrophilicity of the membrane surface, aiming to improve the membrane's antifouling performance and achieve the separation and concentration of high-concentration degreasing wastewater.

Description

Technical Field

[0001] This invention relates to a method for preparing modified ceramic membrane materials in the field of industrial wastewater treatment, and particularly to a ceramic composite membrane for degreasing wastewater and its preparation method. Background Technology

[0002] Degreasing solution refers to the liquid used to remove grease and mineral oil from metal surfaces before surface treatments such as painting, electroplating, electroless plating, and phosphating, as well as during cleaning and maintenance cleaning in parts production. However, during the degreasing process, the degreasing capacity of the solution decreases or becomes ineffective as contaminants increase, necessitating regular discharge and replacement. The discharged wastewater still contains a large amount of degreasing agent; direct discharge would waste the degreasing solution and increase the burden on wastewater treatment. Recycling and reusing this wastewater would generate significant economic benefits and make a substantial contribution to environmental protection. Currently, many treatment methods are used domestically and internationally, such as gravity separation, centrifugation, flotation, salting out, and membrane separation. Membrane separation technology, in particular, has developed rapidly in recent years. Its advantages include no phase change during separation, low energy consumption, simple equipment, easy operation, and good separation effect. Ceramic membranes have a long service life, relatively high mechanical strength, and excellent stability, and can withstand acid and alkali corrosion. Ceramic membranes have a hydrophilic surface and a well-defined pore size distribution, offering more controllable pore structure compared to polymer separation membranes. TiO2, as a novel semiconductor material, is environmentally friendly, chemically stable, highly photocatalytically active, non-toxic, low-cost, and readily available. To further improve the membrane's retention rate of degreasing agents and better realize their recycling, it is necessary to adjust the membrane's pore size, hydrophilicity, and charge properties.

[0003] The inventor's earlier Chinese invention patent application (CN108854589 A) disclosed a method for preparing mullite whisker ceramic membranes, which has low preparation cost and a high rejection rate of up to 95% for oil-water emulsions, but the flux decreases rapidly, and there are still some difficulties in treating high-concentration degreasing solutions. Chinese invention patent application (CN101941747 A) disclosed a process for treating and reusing degreasing wastewater based on submerged continuous membrane filtration technology. This method uses a self-priming pump external pressure suction filtration method with an operating pressure of -0.03 to -0.08 MPa and an operating temperature of 10 to 80 ℃. It employs an online program-controlled backwashing process with a backwashing interval of 5 to 60 minutes and a backwashing time of 10 to 30 seconds. The membrane used in the membrane filtration unit is a flat sheet membrane or a hollow fiber membrane, and the membrane material is selected from polyvinylidene fluoride, polysulfone, polyethersulfone, or polyvinyl chloride. The filtration accuracy is 0.001 to 0.45 μm. The cleaning rate is over 95% when rinsed at 45 ℃ for 3 min, and there is no secondary pollution. However, the disadvantage is that it involves many steps and the operating equipment is relatively complex. Ren Zhengyan et al. (Ren Zhengyan, Zhang Yuzhong, Li Hong et al., Treatment of degreasing wastewater with polyvinylidene fluoride hollow fiber membrane [J], Journal of Tianjin University of Technology, 2010, 29: 12-15) used PVDF hollow fiber membrane with an average pore size of 0.1 μm to treat degreasing wastewater (oil droplet size mainly concentrated in 0.15~0.60 μm). Under an operating pressure of 0.04 MPa, after the internal feed liquid was concentrated 2.5 times in 25 min, the pure water flux of the membrane module decreased from 460 L / (m2·h) to 150 L / (m2·h), and the oil removal rate could reach over 90%. Moreover, the oil removal effect gradually weakened with the increase of the concentration factor. Wen Huichao et al. (Wen Huichao, Shu Li, Xing Weihong, Xu Nanping. Application of inorganic ceramic membrane in degreasing wastewater treatment [J]. Water Treatment Technology, 2007(03):42-44+88) used a zirconia membrane with a pore size of 200 nm. Under the operating conditions of 0.1 MPa, membrane surface flow velocity of 5-7 m / s, and operating temperature of 40 ℃, the permeate flux of 5 g / L degreasing wastewater can reach 390 L / m2·h, the permeation rate of degreasing agent is more than 85%, and the oil rejection rate is greater than 99.4%. However, this membrane cannot filter degreasing wastewater with higher concentrations.Wang Xuening et al. (Wang Xuening, Yang Jingjing, Zhou Xiaoji, Bai Renbi. Performance study of modified PVDF hollow fiber membrane for treating automobile degreasing wastewater [J]. Plastics Industry, 2020, 48(07):158-163) used a polyvinylidene fluoride ultrafiltration membrane modified with segment P(VDF-co-CTFE)-g-PMAA-g-fPEG with hydrophilic and oleophobic properties to separate oil and water from automobile degreasing wastewater. The pH value of the degreasing wastewater was 10.65 and the average mass concentration of total oil was 246 mg / L. Within the pressure difference range of 0.04 to 0.10 MPa, the removal rate of organic matter by the membrane could reach 92%, but the COD mass concentration in the filtrate still exceeded 1200 mg / L. It has a good retention effect on organic matter with a particle size larger than the membrane pore size, but it cannot remove dissolved organic matter.

[0004] This invention employs a sol-gel method to modify the surface of a ceramic membrane by coating it with TiO2 sol. By adjusting the membrane pore size and pore size distribution, surface charge, and hydrophilicity, the aim is to improve the membrane's antifouling performance and achieve the separation and concentration of high-concentration degreasing wastewater. Summary of the Invention

[0005] This invention uses α-Al2O3 ceramic membranes or mullite phase ceramic membranes obtained by non-solvent phase inversion method (NIPS) combined with high-temperature sintering as the base membrane, and modifies the membrane with a TiO2 functional layer by means of a sol-gel method. The preparation of the above modified membrane mainly includes the following steps:

[0006] (1) Preparation of ceramic substrate film: Using ceramic powder as the main solid raw material, NN dimethylpyrrolidone (NMP) as solvent, polyethersulfone (PES) as binder, and polyvinylpyrrolidone (PVP) as pore-forming agent, the casting solution was prepared by stirring at a certain temperature for a certain time; the substrate film required for the experiment was prepared by combining NIPS and high-temperature sintering, and then vacuum dried for later use.

[0007] (2) Preparation of TiO2 sol: Tetrabutyl titanate was used as a precursor, anhydrous ethanol as a solvent, and acetic acid as a hydrolysis inhibitor. Tetrabutyl titanate was slowly added dropwise to anhydrous ethanol and stirred thoroughly to form an ethanol solution of tetrabutyl titanate, denoted as solution A. Deionized water and glacial acetic acid were added to anhydrous ethanol in sequence and stirred until homogeneous, denoted as solution B. After stirring solutions A and B separately for a certain period of time, solution A was added dropwise to solution B through a dropping funnel. Hydrochloric acid was used as a stabilizer to adjust the pH of the mixed solution, and then it was aged at room temperature for a period of time until it became a sol. Then, film-forming aids and nano-hollow microspheres were added to the mixed solution, stirred evenly at room temperature, and finally aged at room temperature for a period of time.

[0008] (3) Coating modification of TiO2 sol: Soak the base film in distilled water, wipe off the surface moisture with filter paper, immerse the base film in a sol solution at a certain temperature for a period of time, take it out, and then dry it at room temperature.

[0009] (4) Low-temperature calcination: The dried composite film is placed in a muffle furnace for low-temperature calcination to obtain anatase TiO2 / nanosphere functional layer.

[0010] (5) Repeat steps (3) and (4) above to obtain a suitable coating thickness and surface pore size, and optimize it by the separation effect of degreasing liquid.

[0011] In step (1) above, the ceramic powder includes one of pure Al2O3 powder or a blend of Al2O3 powder and fly ash, with the particle size range of both powders being 500 nm to 3 mm; in the blend of Al2O3 powder and fly ash, the fly ash content is 50% to 60%; calculated by mass fraction ratio, ceramic powder: NMP : PES : PVP = 100 : 40~70 : 8~15 : 0.5~2; the stirring temperature for preparing the casting solution is 50~70 ℃, stirring for 12~24 h, and the vacuum degassing temperature is 60~80 ℃; hollow fiber membranes are spun using a dry / wet method, with the spinning solution extruded under N2 drive, the spinning solution flow rate being 15~25 mL / min, the air gap being 8~15 cm, and then immersed in a coagulation bath at a temperature of 20~50 ℃ to undergo phase transformation into a hollow fiber membrane green body, the coagulation bath being distilled water; every 6 The distilled water is changed once every hour, and the green body is taken out and dried after 24 hours. The dried green body is then subjected to gradient heating sintering in a muffle furnace: first, the temperature is raised to 900-1000 ℃ at a heating rate of 5-15 ℃ / min and held for 0.5-2 hours, and then the temperature is raised to 1100-1350 ℃ at a heating rate of 2-7 ℃ / min and held for 2-6 hours to obtain the flat plate or hollow fiber ceramic membrane as the base membrane.

[0012] In step (2) above, the volume of anhydrous ethanol added to solution B is approximately twice that of solution A; the dropping rate is 2-5 drops / 5 s; the pH value is 2-4; the film-forming aid can be either polyvinylpyrrolidone (PVP) or polyethylene glycol (PEG); the nano-hollow microspheres can be either nano-SiO2 or fly ash, with a particle size range of 600-800 mm, each accounting for 1-2 wt% of the mixed solution; the aging time is approximately 12 h.

[0013] The base film in step (3) above is the a-Al2O3 ceramic film or mullite phase ceramic film prepared in step (1), and its morphology is a hollow fiber film (outer diameter is 3~5 mm, inner diameter is 2~4 mm); the dip coating temperature is 30~45 ℃, the time is 5~20 s, and the drying time is 1~2 h;

[0014] The gradient heating sintering procedure in step (4) above is as follows: from room temperature to 75 ℃ after 100 min, hold for 30 min; heat up to 280 ℃ after 410 min, hold for 30 min; heat up to 420 ℃ after 280 min, hold for 30 min; heat up to 750 ℃ ​​after 180 min, hold for 180 min.

[0015] The present invention has the following beneficial technical effects: low material cost, simple preparation method, large membrane flux, good antifouling performance, and high separation efficiency. Attached Figure Description

[0016] Figure 1 These are SEM comparison images of the TiO2 sol-coated modified membrane and the hollow fiber ceramic substrate membrane prepared in Implementation Case 1.

[0017] Figure 2 This is a comparison of the water contact angles of the TiO2 sol-coated modified membrane and the hollow fiber ceramic substrate membrane prepared in Implementation Case 2.

[0018] Figure 3 This is a comparison chart showing the antifouling performance of the TiO2 sol-coated modified membrane and the hollow fiber ceramic substrate membrane used in Case Study 1 for separating degreasing liquid wastewater. Detailed Implementation

[0019] Example 1, (1) A mixture of Al2O3 powder and fly ash with a fly ash content of 60%: NMP: PES: PVP = 100: 60: 12: 1. Fly ash and Al2O3 nanoparticles were dried and ground in a mortar. Weighed fly ash, Al2O3, and NMP were sequentially placed into a three-necked flask and mechanically stirred at room temperature. PES was then added to the flask, and the mixture was mechanically stirred for 12 hours in a 60°C oil bath to form a dispersed and uniform casting solution. After vacuum degassing at 80°C, the solution was poured into a clean spinning kettle for dry / wet spinning. The spinneret extruded the spinning solution under a pressure of 0.2 MPa N2, with an air gap of 10 cm, a spinning solution flow rate of 25 mL / min, and a coagulation bath temperature of 30°C. The resulting green body was soaked in distilled water for 24 hours, with the distilled water changed every 6 hours. It was then dried in a vacuum drying oven at 80°C for later use. The dried hollow fibers were cut into 10 cm long segments and sintered using a gradient heating program: heating at a rate of 10°C / min to 900°C. The temperature was increased to 1200℃ and held for 1 hour; then the temperature was increased to 1200℃ at 5℃ / min and held for 3 hours; finally, the temperature was mechanically cooled to 1000℃ and allowed to cool naturally to room temperature to obtain a mullite phase hollow fiber ceramic substrate membrane.

[0020] (2) Slowly add 18 mL of tetrabutyl titanate to 40 mL of anhydrous ethanol and stir thoroughly to form an ethanol solution of tetrabutyl titanate, denoted as solution A; add 4 mL of deionized water and 24 mL of glacial acetic acid to 80 mL of anhydrous ethanol and stir until homogeneous, denoted as solution B. After stirring solutions A and B separately for 1 h, add solution A dropwise to solution B through a dropping funnel at a rate of 2 drops / 5 s. Adjust the pH of the mixed solution to 2.5 using hydrochloric acid as a stabilizer, and then age at room temperature for 12 h. Add 1 wt% PVP and 700 nm SiO2 hollow microspheres to the mixed solution to make the solid content of each 2 wt%, stir magnetically until homogeneous, and finally age at room temperature for 12 h;

[0021] (3) The film immersion coating temperature is 30 ℃, the immersion coating time is 15 s, and the drying time is 1 h;

[0022] (4) Then it was placed in a muffle furnace and heated from room temperature to 75 ℃ in 100 min and held for 30 min; heated to 280 ℃ in 410 min and held for 30 min; heated to 420 ℃ in 280 min and held for 30 min; heated to 750 ℃ ​​in 180 min and held for 180 min.

[0023] (5) Repeat steps (3) and (4) a total of 5 times, then remove and dry for later use.

[0024] Example 2, (1) is the same as step (1) in Implementation Case 1;

[0025] (2) Slowly add 18 mL of tetrabutyl titanate to 40 mL of anhydrous ethanol, and stir thoroughly to form an ethanol solution of tetrabutyl titanate, denoted as solution A; add 4 mL of deionized water and 24 mL of glacial acetic acid to 80 mL of anhydrous ethanol, and stir to mix evenly, denoted as solution B. After 1 h of mixing solutions A and B, add solution A dropwise to solution B through a dropping funnel at a rate of 3 drops / 5 s. Adjust the pH of the mixed solution to 2.5 with hydrochloric acid as a stabilizer, and then age at room temperature for 12 h. Add PVP and fly ash with a particle size of 800 nm to the mixed solution, each accounting for 2 wt% of the mixed solution, stir magnetically until evenly mixed, and finally age at room temperature for 12 h;

[0026] (3) Under a constant temperature water bath of 35 ℃, the film immersion time is 10 s and drying time is 30 min;

[0027] (4) The heating procedure is the same as step (4) in Example 1;

[0028] (5) Repeat steps (3) and (4) a total of 4 times, then remove and dry for later use.

[0029] Example 3, (1) is the same as step (1) in Implementation Case 1;

[0030] (2) Slowly add 18 mL of tetrabutyl titanate to 40 mL of anhydrous ethanol and stir under a constant temperature water bath to form an ethanol solution of tetrabutyl titanate, denoted as solution A; add 4 mL of deionized water and 24 mL of glacial acetic acid to 80 mL of anhydrous ethanol and stir until homogeneous, denoted as solution B. After stirring solutions A and B separately for 1 h, add solution A dropwise to solution B through a dropping funnel at a dropping rate of 1 drop / s. Adjust the pH of the mixed solution to 3 with hydrochloric acid as a stabilizer, and then age at room temperature for 12 h. Add PVP and 700 nm SiO2 hollow microspheres to the mixed solution, each accounting for 1.5 wt% of the mixed solution, stir magnetically until homogeneous, and finally age at room temperature for 12 h;

[0031] (3) The film was dipped in a constant temperature water bath at 35 ℃ for 5 s and then removed and dried for 1 h.

[0032] (4) The heating procedure is the same as step (4) in Example 1;

[0033] (5) Repeat steps (3) and (4) a total of 4 times, then remove and dry for later use.

[0034] Experimental comparison example:

[0035] Al2O3 powder and fly ash were mixed, with fly ash accounting for 60%; the ratio of ceramic powder to NMP to PES to PVP was 100 to 60 to 12 to 1. Fly ash and Al2O3 nanoparticles were dried and ground in a mortar. Weighed fly ash, Al2O3, and NMP were sequentially placed into a three-necked flask and mechanically stirred at room temperature. PES was then added to the flask, and the mixture was mechanically stirred for 12 hours in a 60°C oil bath to form a dispersed and uniform casting solution. After vacuum degassing at 80°C, the solution was poured into a clean spinning kettle for dry / wet spinning. The spinneret extruded the spinning solution under a pressure of 0.2 MPa N2, with an air gap of 10 cm, a spinning solution flow rate of 25 mL / min, and a coagulation bath temperature of 30°C. The resulting green body was soaked in distilled water for 24 hours, with the distilled water changed every 6 hours, and then dried in a vacuum drying oven at 80°C. The dried hollow fibers were cut into 10 cm long segments and sintered using a gradient heating program: heating at a rate of 10°C / min to 900°C. The temperature was increased to 1200℃ and held for 1 hour; then the temperature was increased to 1200℃ at 5℃ / min and held for 3 hours; finally, the temperature was mechanically cooled to 1000℃ and allowed to cool naturally to room temperature to obtain a mullite phase hollow fiber ceramic membrane.

[0036] Experimental results:

[0037] This invention designs and prepares a hollow fiber membrane for degreasing wastewater. This membrane exhibits strong hydrophilicity, high flux, and good stability during the separation of degreasing liquid. Comparative tests were conducted using samples prepared in Examples 1 and 2.

[0038] This invention designs and prepares a TiO2-coated modified ceramic membrane with photocatalytic self-cleaning function for the separation of degreasing solutions. The base membrane is prepared from ceramic powder using a combination of solvent-free phase inversion and high-temperature sintering to create a flat ceramic membrane or hollow fiber ceramic membrane. Further TiO2 surface modification enhances the hydrophilicity of the resulting composite membrane. The addition of hollow microspheres to the TiO2 sol thickens the TiO2 sol, reduces the pore size of the membrane surface, and increases additional molecular sieve-like channels. Figure 1 As shown in the figure, the surface morphology of the base membrane and the modified membrane in Implementation Case 1 shows that the hollow fiber membrane has large pores on its surface, with an average pore size of less than 10 μm. After modification, the membrane surface is smoother and denser than the original membrane, and the average pore size is significantly smaller than that of the base membrane. This helps to improve the strength, superhydrophilicity, separation efficiency and antifouling performance of the ceramic membrane.

[0039] like Figure 2 As shown, the water contact angle of the base membrane in the experimental proportion was 36.5°, while the water contact angle of the modified membrane prepared in Case 2 was 10°, which was significantly lower than that of the base membrane.

[0040] like Figure 3 As shown, after pre-pressurization at 1 bar for 1 h, the flux of the modified membrane filtering degreasing wastewater decreased from the initial 233.11 L·m⁻²·h⁻¹ to 61.73 L·m⁻²·h⁻¹, while the flux of the base membrane filtering degreasing wastewater under the same conditions decreased from the initial 172.85 L·m⁻²·h⁻¹ to 47.92 L·m⁻²·h⁻¹. The flux of the modified membrane for degreasing wastewater was consistently higher than that of the base membrane. Therefore, it can be concluded that the hydrophilicity of the base membrane was improved after modification, which is more conducive to the separation of degreasing wastewater.

Claims

1. A ceramic composite membrane for treating degreasing wastewater, characterized by impregnating and coating a titanium dioxide sol onto the surface of a ceramic substrate membrane, followed by gradient sintering, for treating degreasing wastewater and improving the recycling rate of the degreasing liquid. The specific preparation method of the ceramic composite membrane is as follows: (1) Preparation of ceramic substrate film: Using ceramic powder as the main solid raw material, NN dimethylpyrrolidone (NMP) as solvent, polyethersulfone (PES) as binder, and polyvinylpyrrolidone (PVP) as pore-forming agent, the casting solution was prepared by stirring at a certain temperature for a certain time; the substrate film was prepared by combining NIPS and high-temperature sintering, and then vacuum dried for later use. (2) Preparation of TiO2 sol: Using tetrabutyl titanate as a precursor, anhydrous ethanol as a solvent, and acetic acid as a hydrolysis inhibitor, tetrabutyl titanate was slowly added dropwise to anhydrous ethanol and stirred thoroughly to form an ethanol solution of tetrabutyl titanate, denoted as solution A; deionized water and glacial acetic acid were added to anhydrous ethanol in sequence and stirred until homogeneous, denoted as solution B; after stirring solutions A and B separately for a certain period of time, solution A was added dropwise to solution B through a dropping funnel, and hydrochloric acid was used as a stabilizer to adjust the pH of the mixed solution. Then, it was aged at room temperature for a period of time until it became a sol; then, the template agent and SiO2 nano-hollow microspheres or fly ash nano-hollow microspheres were added to the mixed solution, stirred evenly at room temperature, and finally aged at room temperature for a period of time. (3) Coating modification of TiO2 sol: Soak the base film in distilled water, wipe off the surface moisture with filter paper, immerse the base film in a sol solution at a certain temperature for a period of time, take it out, and then dry it at room temperature. (4) Low-temperature calcination: The dried composite film is placed in a muffle furnace for low-temperature calcination to obtain anatase TiO2 / nanosphere functional layer; (5) Repeat steps (3) and (4) above to obtain a suitable coating thickness and surface pore size, and optimize it by the separation effect of degreasing liquid; in, In step (1), the ceramic powder is one of pure Al2O3 powder or a blend of Al2O3 powder and fly ash, with a particle size range of 500 nm to 3 μm; in the blend of Al2O3 powder and fly ash, the fly ash content is 50% to 60%; the ceramic powder ratio is calculated as follows: NMP: PES: PVP = 100: 40~70: 8~15: 0.5~2; the stirring temperature for preparing the casting solution is 50~70 ℃, stirring for 12~24 h, and the vacuum degassing temperature is 60~80 ℃; a flat film is prepared using a film scraper with a thickness of 500~800 μm, a solvent evaporation time of 30~60 s, and then placed in hot water at 60~80 ℃ for phase inversion to form a film; or a hollow fiber membrane is spun using a dry / wet method, with the spinning solution extruded under N2 drive and the spinning solution flow rate being 15~25. The membrane is prepared by immersing it in distilled water at a flow rate of mL / min and an air gap of 8-15 cm. The membrane undergoes phase inversion to form a hollow fiber membrane. The coagulation bath is distilled water at a temperature of 20-50 °C. The dried green body is then subjected to gradient heating sintering in a muffle furnace: first, the temperature is increased to 900-1000 °C at a heating rate of 5-15 °C / min and held for 0.5-2 h, then increased to 1100-1350 °C at a heating rate of 2-7 °C / min and held for 2-6 h, thus obtaining the flat or hollow fiber membrane as the base membrane.

2. The ceramic composite membrane for degreasing wastewater according to claim 1, characterized in that... The base film is either an a-Al2O3 ceramic film or a mullite phase ceramic film obtained by NIPS combined with high-temperature sintering.

3. The ceramic composite membrane for degreasing wastewater according to claim 1, characterized in that... In step (2), the volume of anhydrous ethanol added to solution B is twice that of solution A; the dropping rate is 2-5 drops / 5 s; the pH value is 2-4; the particle size range of the nano-hollow microspheres is 600-800 μm, each accounting for 1-2 wt% of the mixed solution; the constant temperature water bath is 30-45 ℃; and the aging time is 12 h.

4. A ceramic composite membrane for degreasing wastewater according to claim 1, characterized in that... The base film morphology in step (3) is a hollow fiber membrane with an outer diameter of 3~5 mm and an inner diameter of 2~4 mm; the immersion temperature is 30~45 ℃; the immersion time is 5~20s; and the drying time is 1~2 h.

5. A ceramic composite membrane for degreasing wastewater according to claim 1, characterized in that... In step (4), the low-temperature calcination procedure is as follows: from room temperature to 75 ℃ after 100 min, hold for 30 min; after 410 min, raise the temperature to 280 ℃ and hold for 30 min; after 280 min, raise the temperature to 420 ℃ and hold for 30 min; after 180 min, raise the temperature to 750 ℃ ​​and hold for 180 min.

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