Indigo dye filter membrane, manufacturing method thereof and online recovery method of indigo dye

Abstract

The invention discloses an indigo dye filter membrane, a manufacturing method thereof and an indigo dye online recovery method, and belongs to the field of textile wastewater treatment.The recovery method comprises the steps that wastewater containing indigo dye is primarily filtered to remove fiber hairiness, the primarily filtered wastewater is filtered through a special indigo dye filter membrane to obtain a concentrated indigo solution and filtered water, and the concentrated indigo solution and the filtered water are recycled; and oxidizing the indigo dye in the concentrated indigo solution into oxidized indigo by using an oxygenation device, concentrating and crystallizing to obtain a secondary dye, recycling filtered water for cleaning yarns, and recycling the secondary dye for preparing a dyeing mother solution. By preparing the durable indigo dye filter membrane with a good separation effect, the indigo dye and water in the wastewater can be separated, repeated use of indigo and water can be realized in combination with an online recovery method, sewage discharge is greatly reduced, the treatment difficulty of cleaning wastewater containing the indigo dye is reduced, the indigo dye is repeatedly utilized, and environmental pollution is reduced. The production cost is reduced, and the indigo dye dyeing is more environment-friendly.

Description

TECHNICAL FIELD

[0001] The present application relates to an indigo dye filter membrane and a manufacturing method thereof, and an indigo dye online recovery method, and belongs to the field of textile wastewater treatment. BACKGROUND

[0002] Traditional denim fabric is made of pure cotton yarn dyed with indigo dye as warp and pure cotton weft in a twill structure. Based on the characteristics of indigo dye itself, the clothes made can present various styles after washing process, so that denim clothes have always stood on the stage of fashion clothes and occupied an irreplaceable position in the clothing market.

[0003] Indigo dye is mainly adsorbed on the surface of the fiber by van der Waals force, so the warp needs to be immersed for multiple times to achieve the preset dyeing effect. Most indigo dyes are not firmly attached to the warp, in order to avoid color bleeding and dyeing the cloth during finishing, the warp needs to be washed with water after dyeing, generally through three washing tanks for treatment, to remove the excess indigo dye on the surface of the yarn and make the indigo dye that is not firmly attached fall off. The water capacity of each washing tank is about 1000L-1500L, and the wastewater is discharged to the sewage treatment tank after washing treatment, and is discharged externally only after meeting the discharge requirements.

[0004] At present, the means for centralized treatment of the above-mentioned washing wastewater in the denim industry is usually through physical adsorption, bacterial treatment, etc. to form sludge. Some technicians use advanced oxidation process to degrade indigo, but the treatment process is complex and the multi-stage treatment consumes high cost.

[0005] These means are difficult to reuse indigo dye and water, resulting in waste of indigo dye and a large amount of water, which is not conducive to the sustainable production of denim fabric production. In the prior art, there are studies on using new adsorbents to adsorb and treat indigo dye in wastewater, and the indigo can be recovered after desorption, but the regeneration rate of the adsorbent after desorption is low, the effect decreases quickly during repeated use, and secondary pollution is easy to occur. SUMMARY

[0006] In order to overcome the shortcomings of the prior art, the present application provides an indigo dye filter membrane and a manufacturing method thereof, which can intercept indigo dye and facilitate the separation of water and indigo dye in washing wastewater, and also provides an indigo dye online recovery method based on the filter membrane.

[0007] The technical scheme adopted by the present application to solve its technical problems is: In a first aspect, the present application provides a manufacturing method of an indigo dye filter membrane, comprising the following steps: mixing a red clay dispersion liquid and a polyvinyl alcohol solution to obtain a coating liquid; coating the coating liquid to the surface of a ceramic support; drying the surface of the ceramic support to form a film, and then heating the ceramic support to degrade the polyvinyl alcohol, and sintering the film on the ceramic support to obtain the indigo dye filter membrane; The red clay used to prepare the red clay dispersion liquid is subjected to decarbonate treatment, deironing treatment and de-organic matter treatment in sequence.

[0008] The method for manufacturing the indigo dye filter membrane provided in the application reduces impurities in natural red clay by decarbonate treatment, deironing treatment and de-organic matter treatment, avoids problems such as film layer cracking and uneven pore size in subsequent film forming, and controls the pore size of the clay film layer after purification, which is conducive to improving the mechanical strength and separation precision of the film. The purified natural red clay is used as a raw material together with polyvinyl alcohol (PVA) to form a stable porous structure by heat treatment to degrade PVA and sintering, thereby enhancing the anti-pollution and chemical resistance of the film. The filter membrane prepared can efficiently separate indigo dye from water with high yield.

[0009] Further, the red clay used to prepare the red clay dispersion liquid is subjected to pre-preparation and sedimentation screening before the decarbonate treatment. The pre-preparation step includes crushing the natural red clay, passing it through a 40 μm-50 μm sieve, adding it to a 2M-4M sodium chloride solution, stirring for 30 min-60 min, and then centrifuging at a speed of 2200 rpm-3000 rpm for 20 min-30 min, and repeatedly washing by centrifugation. The sedimentation screening step includes preparing the pre-prepared red clay into a 12wt%-15wt% water-based suspension, stirring for 25 min-35 min, and then standing for 30 min-60 min.

[0010] Among them, the screening can remove large particle impurities, the sodium chloride solution centrifugal washing can remove soluble salts and loose attachments, and the specific standing time can screen out red clay particles with a particle size of ≤5 μm, so as to ensure that the film layer structure is dense and uniform, and the film layer defects are reduced.

[0011] Further, the decarbonate treatment step includes: The red clay particles with a particle size of not more than 5 μm are dispersed in water, acetic acid solution is added to make the pH reach 4.4-4.6, hydrochloric acid is added dropwise under stirring until no bubbles are generated, and after repeated centrifugal washing, the insoluble matter is retained to obtain the red clay treated by decarbonate.

[0012] Under acidic conditions, carbonates decompose into CO2 and escape, and controlling the pH range is conducive to preventing excessive acidification from damaging the crystal structure of the red clay minerals and maintaining their inherent adsorption and filtration performance.

[0013] Further, the deironing treatment step includes: The red clay is put into a chelating solution, heated to 70-80℃ and stirred for 32-40 minutes, and after washing the residual solid with water, centrifugal separation is performed, and the insoluble substance is retained to obtain the red clay treated by removing iron; The chelating solution contains 1-2 g / L sodium citrate and 0.3-0.8 g / L sodium bicarbonate, and the pH is maintained at 7.5-8; the mass ratio of the red clay to the chelating solution is 1:2.

[0014] The sodium citrate prevents Fe²⁺ from precipitating, and the sodium bicarbonate maintains a weak alkaline environment, ensuring the activity of the sodium hydrosulfite and the stability of the iron-citric acid complex, which is conducive to improving the iron removal efficiency.

[0015] Further, the step of removing organic matter includes: 1 part by mass of the red clay is added to 1 part by mass of a 30% hydrogen peroxide solution, heated to 70-80℃ and stirred for 1.5-2 hours, and after washing and centrifugal separation, the insoluble substance is retained to obtain the red clay treated by removing organic matter.

[0016] The oxidation reaction can be accelerated at 70-80℃, and the strong oxidizing agent H2O2 decomposes part of the residual organic matter in the clay into CO2 and H2O, avoiding excessive organic matter reducing the membrane flux during the use of the membrane.

[0017] Further, the concentration of the red clay dispersion is 0.75-2 wt%, the concentration of the polyvinyl alcohol solution is 0.5-1.5 wt%, and the coating liquid is mixed by mixing the red clay dispersion and the polyvinyl alcohol in a volume ratio of 1:1.

[0018] Optimizing the amount of red clay and PVA can ensure that the viscosity of the coating liquid is moderate, avoiding excessive thickness or thinness of the membrane layer and reducing the generation of cracks. PVA provides temporary bonding strength, and after sintering, a porous red clay skeleton is formed, which takes into account the strength and permeability of the membrane layer.

[0019] Further, the step of drying the surface of the ceramic support to form a film, then heating the ceramic support to degrade the polyvinyl alcohol, and then sintering the film on the ceramic support includes: The ceramic support is naturally dried at 20-30℃ for 20 hours, then the ceramic support is heated at 310℃ for 2.5 hours, and then sintered at 520℃ for 2 hours; the heating and sintering processes have a temperature rise rate of less than 3℃ / min.

[0020] 310℃ can completely decompose PVA, avoiding organic residues to contaminate the film layer; sintering at 520℃ can enhance the binding force between clay particles, improving the film pressure resistance. Slow heating during the heating and sintering process can reduce thermal stress, preventing the film layer from cracking due to sudden temperature change.

[0021] Further, the surface of the ceramic support is polished by sandpaper with 600 mesh before being coated, then ultrasonic cleaning is performed to remove residual particles, and then drying is performed at 105℃.

[0022] In the second aspect, the application provides an indigo dye filter film prepared by the method for preparing an indigo dye filter film.

[0023] The filter film has less impurities, uniform pore size, good anti-pollution ability, high indigo dye interception rate, and stable water flux.

[0024] In the third aspect, the application provides an online indigo dye recovery method, comprising the following steps: Primary filtration of wastewater containing indigo dye to remove fiber fluff; The wastewater after primary filtration is filtered by the indigo dye filter film according to the second aspect, and the operating pressure is 2bar-4bar, to obtain an indigo concentrated solution and filtered water; The indigo dye in the indigo concentrated solution is oxidized to an oxidized indigo, concentrated and crystallized to obtain a secondary dye; The filtered water is used for washing yarn, and the secondary dye is used for preparing dyeing mother liquor.

[0025] The method first removes large particle impurities through primary filtration, prolonging the service life of the indigo dye filter film. The indigo dye filter film can work under smaller positive pressure, has low energy consumption, is suitable for continuous production, and the indigo dye after oxidation and concentration can be directly used for preparing dyeing mother liquor. The filtered water is used for washing yarn, reducing water resource waste, reducing sewage discharge and dye consumption, and meeting the sustainable production goal.

[0026] The application has the beneficial effects that: the application prepares a durable and good separation effect indigo dye filter film, can separate indigo dye and water in wastewater, realizes the reuse of indigo and water by combining the online recovery method, greatly reduces sewage discharge, reduces the difficulty of treating washing wastewater containing indigo dye, reuses indigo dye, is conducive to reducing production cost, and makes indigo dye dyeing more environmentally friendly. DETAILED DESCRIPTION

[0027] To make the objects, technical solutions, and advantages of the embodiments of the present disclosure clearer, the technical solutions of the embodiments of the present disclosure are clearly and completely described in the following. Obviously, the described embodiments are some but not all of the embodiments of the present disclosure. Based on the described embodiments of the present disclosure, all other embodiments obtained by a person of ordinary skill in the art without creative effort belong to the scope of the present disclosure.

[0028] It should be understood that any and all embodiments of the present application can be combined with the technical features in any other embodiment or multiple other embodiments to obtain additional embodiments without conflict. The present application includes such combinations to obtain additional embodiments.

[0029] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this application belongs. If there is a conflict between the definitions of terms in different sections of the specification, the definitions provided in this section prevail.

[0030] At present, the wastewater and dyes generated during the dyeing of yarn are not recycled and used online in the prior art, and the dyes and water in the wastewater during the dyeing process of denim yarn can be recycled and used online. Therefore, the prior art needs to be improved.

[0031] The embodiment of the present application provides an indigo dye online recycling method, comprising the following steps: S1: preliminary filtration of wastewater containing indigo dye to remove fiber hair.

[0032] S2: filtering the wastewater filtered in S1 with a specially prepared indigo dye filter membrane, and the operation pressure is 2bar~4bar, to obtain an indigo concentrated solution and filtered water.

[0033] S3: oxidizing the indigo dye in the indigo concentrated solution into an oxidized indigo, concentrating and crystallizing to obtain a secondary dye.

[0034] S4: recycling the filtered water for washing yarn and recycling the secondary dye for preparing a dyeing mother liquor.

[0035] The embodiment of the present application first removes the fiber hair by preliminary filtration to protect the indigo dye filter membrane. The specially prepared indigo dye filter membrane can separate the indigo concentrated solution from water under a lower positive pressure (2bar~4bar), has low energy consumption, and can restore the dyeing performance of the indigo dye by oxidizing and re-concentrating the indigo, thereby realizing closed-loop circulation. The filtered water can be directly recycled for washing, and the secondary dye can be recycled for dyeing, which is beneficial to reducing the difficulty of wastewater treatment and reducing the cost of dye raw materials.

[0036] In step S3, the concentration and crystallization specifically refers to concentrating the volume of the mixed solution (the indigo dye is oxidized into an oxidized state, and insoluble substances have begun to appear) to one-third to one-fifth.

[0037] Preferably, the filtered water is first introduced into the water storage tank for collection, and then is transported to the cleaning tank through the pipeline, so that the dyed yarn can be cleaned, and the filtered water can be transported to the spraying device through another pipeline, the spraying device being used for spraying the yarn just leaving the cleaning tank, so as to remove the floating color on the surface of the yarn.

[0038] The steps for manufacturing a special indigo dye filter film are as follows: A. The broken natural red clay sample is sieved with a fine mesh sieve with a pore size of 40-50 μm to obtain a uniform powder.

[0039] B. The obtained uniform powder is added to a 2-4 M sodium chloride solution, stirred for 30-60 min, and then centrifuged at a speed of 2200-3000 rpm for 20-30 min to separate the clay particles from the liquid phase. The centrifugation is repeated 2-3 times, and then the residual salt is removed by washing with clean water and centrifugation.

[0040] C. The clay obtained in step B is prepared into a 12-15 wt% clay water-based suspension, stirred for 25-35 min, and then poured into a glass tube. The particles with a particle size greater than 5 μm are naturally settled from the suspension by controlling the settling time, and finally the fine particles are separated by centrifugation and dried at room temperature.

[0041] D. The fine particles obtained in step C are added to water to disperse them, and acetic acid is added to adjust the pH of the solution to 4.4-4.6. Then hydrochloric acid is slowly added and stirred until the carbon dioxide bubbles completely disappear, so that the carbonate is completely decomposed. Subsequently, the solution is repeatedly washed by centrifugation (3-5 times) until the supernatant is clear and the chloride ion is difficult to detect. This method can effectively remove the carbonate while retaining the structural properties of the clay minerals.

[0042] E. The clay minerals obtained in step D are treated by mixing the clay with a solution containing sodium bicarbonate, sodium hydrosulfite (sodium dithionite) and sodium citrate. The sodium hydrosulfite acts as a strong reducing agent to reduce the iron oxide (Fe³ + ) to a soluble Fe² + state, so that the iron oxide is dissolved out of the clay matrix. The sodium citrate acts as a chelating agent to form a soluble complex with Fe² + , preventing its reprecipitation. The sodium bicarbonate maintains the pH value in the range of 7.5-8, ensuring the stability of the sodium hydrosulfite and the iron-citric acid complex. The mixture is heated to 70-80°C and stirred for 32-40 min. The residual solids are thoroughly washed with water and separated by centrifugation.

[0043] F.The obtained clay is treated with H2O2, which can effectively oxidize and decompose part of the organic matter into CO2 and water as a strong oxidizing agent. Specifically, 30% mass concentration of H2O2 is added to the treated red clay, the mixture is heated to 70-80°C and stirred, the stirring time is 1.5-2h, then the clay particles are separated by centrifugation, and the residual H2O2 is removed by repeated washing with water, finally the obtained clay is dried at 100-110°C for 24h. The obtained clay will be used to prepare indigo dye filter membrane.

[0044] Due to the excellent dye removal capacity and structural stability of ceramic membranes, ceramic membranes are widely used in microfiltration (MF) field, and natural red clay is also widely developed. However, due to the presence of impurities such as organic matter, carbonate and amorphous silicate in natural red clay, it is easy to cause the pore size to be too large or even the membrane layer to crack (the thinner the more obvious), so the purity of red clay is very important to the performance of the membrane.

[0045] The above treatment can effectively purify the red clay, and the purification process can effectively remove organic matter and carbonate impurities while maintaining the structural integrity of the clay, significantly improving its purity, and preparing high-performance nanofiltration membranes, which have good structural stability, filtration capacity and anti-fouling ability, and good treatment effect on indigo-containing wastewater. The natural red clay can be used as raw material, and the preparation cost is relatively low, which is conducive to the sustainable development of wastewater treatment.

[0046] G.Preparation of nanofiltration membrane: first polish one side of the ceramic support with P600 sandpaper, then remove the residual particles by ultrasonic water washing, and dry the cleaned ceramic support in a 105°C oven. The ceramic support can be, for example, silicon carbide ceramic, without special requirements.

[0047] H.Disperse the red clay powder obtained in step F in water to form a suspension, the mass concentration of red clay is 0.75%-2%, then dissolve polyvinyl alcohol in water to form a 0.5wt%-1.5wt% polyvinyl alcohol solution, mix the suspension and polyvinyl alcohol solution in a volume ratio of 1:1 and stir uniformly, then perform ultrasonic treatment to realize homogenization.

[0048] The content of red clay needs to be reasonably controlled, because as the content of red clay increases, the surface roughness of the membrane gradually increases and the thickness of the membrane increases. Although the increase of the content of red clay can increase the thickness and strength performance of the membrane layer, it will also increase the roughness of the membrane surface and the risk of cracking, and the permeability of the membrane will be significantly reduced with the increase of the content of purified red clay, thereby affecting the efficiency of the filter membrane in the filtering application, therefore, the content of purified red clay and its treatment process need to be reasonably controlled to ensure the stable and excellent performance of the membrane.

[0049] I. The mixture obtained in step H is deposited on the surface of the ceramic support by spin coating at a drop rate of 1 drop per second and a spin speed of 420 rpm to 450 rpm, and the rotation is continued until the excess solution is completely removed to ensure uniform film formation, to obtain a 10 μm to 30 μm thick film layer.

[0050] J. The film layer formed in step I is allowed to stand and dry at room temperature for 20 h, and then heat treated, in the first stage, at a constant temperature of 310°C for 2.5 h to remove the PVA, and in the second stage, sintered at 520°C for 2 h, with the whole process being controlled at a heating rate of 3°C / min to avoid cracking of the film layer.

[0051] Implementation Case A silicon carbide ceramic support with a porosity of 40% and a pore size of 50 μm is selected, and the single side of the ceramic support is polished using 600 mesh sandpaper, and then the residual particles are removed by ultrasonic water washing. The cleaned ceramic support is placed in a 105°C oven for drying. The natural red clay (Yuyi Oubait Clay Material Co., Ltd.) purified according to steps A to F is dispersed in deionized water to prepare a 2 wt% red clay dispersion liquid. Polyvinyl alcohol (Shanghai Aldrich Biochemical Technology Co., Ltd., P140398) is dissolved in water to form a 1.5 wt% polyvinyl alcohol solution. The red clay dispersion liquid and the polyvinyl alcohol solution are mixed uniformly at a volume ratio of 1:1. The mixture is spin coated on the ceramic support by spin coating to form a film layer with a thickness of about 25 μm. The formed film layer is allowed to stand and dry at room temperature for 20 h, and then heat treated, in the first stage, at a constant temperature of 310°C for 2.5 h, and in the second stage, sintered at 520°C for 2 h, with the whole process being controlled at a heating rate of 3°C / min, to obtain an indigo dye filter membrane.

[0052] The prepared indigo dye filter membrane is used for online recovery: The wastewater containing indigo dye is subjected to preliminary filtration in a security filter, and the wastewater subjected to preliminary filtration is filtered by the indigo dye filter membrane at an operating pressure of 4 bar to obtain an indigo concentrated solution and filtered water. The indigo dye in the indigo concentrated solution is oxidized to an oxidized indigo dye by using an oxygenation device, concentrated and crystallized to obtain a secondary dye. The filtered water is used for washing yarn, and the secondary dye is used for preparing a dyeing mother liquor. The concentration of the dyeing mother liquor is the same as that of the dyeing mother liquor prepared by using the primary dye (purchased indigo dye) for the first time.

[0053] The fabric made by using the same treatment process is tested, specifically the K / S value of the fabric is tested. The first batch uses the primary dye, and the K / S value is measured to be 16.4; the second batch uses the secondary dye, and the K / S value is measured to be 16.6. This round of production experiment shows that the secondary dye recovered by the present application can achieve the same dyeing effect as the primary dye.

[0054] The interception rate of the indigo dye filter membrane to indigo dye was also measured (interception rate = (wastewater indigo concentration - filtered water indigo solution concentration) / wastewater indigo concentration), and the interception rate was measured to be 92.3%. The indigo dye filter membrane after working was backwashed with water, and the permeation flux was tested, which could reach 95% of the indigo dye filter membrane just after being made, and the effective removal of indigo dye could still be achieved. The dye interception rate of the membrane reached the upper middle level of the interception capacity of dye filter membranes on the market, and the preparation cost of the membrane was low, the raw material source was wide, which provided a feasible treatment scheme for the treatment of wastewater containing indigo, and was conducive to the sustainable development of textile wastewater treatment.

[0055] In the description of the present specification, the description referring to the terms "one embodiment", "certain embodiments", "exemplary embodiment", "example", "specific example", or "some examples" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the exemplary description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0056] The above is the preferred embodiment of the present application, it should be pointed out that for those skilled in the art, without departing from the principles of the present application, can make a number of improvements and refinements, these improvements and refinements also regarded as the protection scope of the present application.

Claims

1. A method for preparing an indigo dye filter membrane, characterized in that: The following steps are involved: mixing a red clay dispersion and a polyvinyl alcohol solution to obtain a coating solution; Applying the coating liquid to the surface of the ceramic support; drying the surface of the ceramic support to form a film, then heating the ceramic support to degrade the polyvinyl alcohol, and then sintering the film on the ceramic support to obtain the indigo dye filter membrane; The red clay used to prepare the red clay dispersion has been subjected to decarbonation treatment, deiron treatment and deorganic matter treatment in sequence.

2. The method for preparing an indigo dye filter membrane according to claim 1, wherein: The red clay used to prepare the red clay dispersion is also prefabricated and subjected to sedimentation screening before undergoing the decarbonation treatment; The prefabrication steps include: crushing the natural red clay, passing it through a 40 μm to 50 μm sieve, adding it to a 2M to 4M sodium chloride solution, stirring it for 30 minutes to 60 minutes, and then centrifuging it at a speed of 2200 rpm to 3000 rpm for 20 minutes to 30 minutes, and repeatedly centrifuging and washing it; The sedimentation screening step comprises: preparing the prefabricated red clay into a 12 wt% to 15 wt% water-based suspension, stirring for 25 min to 35 min, and then standing for 30 min to 60 min.

3. The method for preparing an indigo dye filter membrane according to claim 1 or 2, wherein: The step of decarbonation treatment comprises: Red clay particles with a particle size of no more than 5 μm are dispersed in water, acetic acid solution is added to make the pH reach 4.4-4.6, hydrochloric acid is added dropwise under stirring until no bubbles are generated, and after repeated centrifugation and washing, insoluble matter is retained to obtain decarbonated red clay.

4. The method for preparing an indigo dye filter membrane according to claim 3, wherein: The step of the iron removal treatment comprises: The red clay is added to the chelating solution, heated to 70°C to 80°C and stirred for 32 min to 40 min, the residual solid is washed with water and then centrifuged to retain the insoluble matter to obtain the iron-removed red clay; The chelating solution contains 1g / L-2g / L of sodium hydrosulfite and 0.3g / L-0.8g / L of sodium citrate, and sodium bicarbonate is used to maintain the pH value at 7.5-8; the mass ratio of red clay to the chelating solution is 1:

2.

5. The method for preparing an indigo dye filter membrane according to claim 4, wherein: The steps of removing organic matter include: 1 part by mass of red clay was added to 1 part by mass of a 30% hydrogen peroxide solution, heated to 70° C. to 80° C. and stirred for 1.5 h to 2 h. After centrifugal washing, insoluble matter was retained to obtain red clay that had been treated with organic matter removal.

6. The method for preparing an indigo dye filter membrane according to claim 1, wherein: The concentration of the red clay dispersion is 0.75wt%~2wt%, the concentration of the polyvinyl alcohol solution is 0.5wt%~1.5wt%, and the coating liquid is prepared by mixing the red clay dispersion and the polyvinyl alcohol in a volume ratio of 1:

1.

7. The method for preparing an indigo dye filter membrane according to claim 6, wherein: The steps of drying the surface of the ceramic support to form a film, then heating the ceramic support to degrade the polyvinyl alcohol, and then sintering the film on the ceramic support include: The ceramic support was placed in a temperature of 20° C. to 30° C. and dried naturally for 20 hours. The ceramic support was then heated at 310° C. for 2.5 hours and then sintered at 520° C. for 2 hours. During the heating and sintering process, the heating rate was below 3° C. / min.

8. The method for preparing an indigo dye filter membrane according to claim 1, wherein: Before coating, the surface of the ceramic support was polished with 600-grit sandpaper, then ultrasonically cleaned to remove residual particles, and then dried at 105°C.

9. An indigo dye filter membrane, characterized in that: The indigo dye filter membrane is prepared by the indigo dye filter membrane preparation method according to any one of claims 1 to 8.

10. An online recovery method for indigo dye, characterized in that: The following steps are involved: Initial filtration of wastewater containing indigo dye to remove fiber hairiness; Filtering the wastewater after the primary filtration through the indigo dye filter membrane according to claim 9 at an operating pressure of 2 bar to 4 bar to obtain an indigo concentrated solution and filtered water; oxidizing the indigo dye in the concentrated indigo solution into oxidized indigo, concentrating and crystallizing the solution to obtain a secondary dye; The filtered water is reused for cleaning yarn, and the secondary dye is reused for preparing dyeing mother liquor.