Membrane casting solution, composite membrane, preparation method and application

Abstract

The invention relates to a membrane casting solution, a composite membrane, a preparation method and application. The composite membrane comprises a base membrane and a polymer membrane laminated on the base membrane, wherein the polymer membrane comprises a polymer and hydrophobic micron particles distributed in the polymer. The preparation method of the composite membrane comprises the following steps: coating the surface of a base membrane with a membrane casting solution, and drying; the membrane casting solution comprises a polymer precursor and hydrophobic micron particles. The composite membrane is applied to treatment of coating waste liquid. According to the invention, the problems of non-uniform dispersion of particles in a membrane liquid and poor membrane forming stability are solved, and a large number of hydrophobic groups are grafted on the particles, so that the hydrophobicity of a separation layer is further improved, and the separation performance of the membrane on organic matters is enhanced.

Description

Technical Field

[0001] The present invention relates to a casting solution, a composite film, a preparation method and an application. Background Art

[0002] Automobile painting processes are generally divided into two major parts: pretreatment processes and painting construction processes. A large amount of wastewater is generated in multiple processes such as the vehicle body pretreatment process, cathodic electrophoresis process, intermediate coating, and topcoat spraying process. The painting wastewater is mostly discharged intermittently and concentrated. Since the wastewater components and types discharged from each production line are different, the finally collected painting wastewater has the characteristics of complex composition and high organic matter content. At the same time, with the increasing environmental protection requirements, the treatment of painting wastewater has also brought a significant cost increase to relevant enterprises. Therefore, developing an efficient, stable, and environmentally friendly painting wastewater treatment process and method has become a difficult point in the environmental protection water treatment industry.

[0003] The application number CN201810560702.3 discloses a treatment system and method for painting wastewater. The painting wastewater passes through a regulating tank, a coagulation reaction sedimentation tank, a flocculation air flotation device, a neutralization reaction tank, a collection tank, a hydroxyl solution generation system, a filtration adsorber, and a clean water reuse tank that are sequentially connected to each other, and is subjected to primary treatment and advanced treatment by combining conventional physical and chemical methods with advanced oxidation methods. The painting wastewater treatment method of this invention has high treatment efficiency, short treatment time, no secondary pollution, and the effluent quality meets the requirements of wastewater reuse and discharge standards. Compared with the traditional conventional process, it improves the effluent quality and provides a more efficient painting wastewater treatment method.

[0004] The application number CN202322505512.0 discloses a treatment all-in-one machine for painting wastewater purification. This device is composed of a wastewater precipitation device, a solid-liquid separation device, and a collection tank connected in sequence through pipelines. The painting wastewater is precipitated and purified in the wastewater precipitation device, and then sent to the solid-liquid separation device through pipelines for solid-liquid separation. The separated solids are compressed and dehydrated, and finally the filtered wastewater enters the collection tank. This wastewater treatment all-in-one machine has a high degree of automation, simple operation, time-saving and labor-saving. The multi-stage dosing design of the liquid medicine greatly improves the treatment capacity of the painting wastewater, and the overall design of the device also reduces the subsequent treatment cost compared with the traditional painting wastewater treatment method.

[0005] For some existing treatment methods for painting wastewater, such as advanced oxidation method, biochemical method, low-temperature evaporation, reverse osmosis membrane method, etc., there are problems such as high organic load, high operating cost, and difficult separation of azeotropic systems. The pervaporation membrane technology is a cutting-edge and emerging method for wastewater treatment. Polydimethylsiloxane (PDMS) is often studied as a hydrophobic organic-permeable membrane material due to its excellent stability and hydrophobicity. Before the PDMS coating step, cross-linking treatment is required, and the reaction is extremely restricted by objective conditions such as temperature, which affects the rheology of the prepared casting solution and further affects the subsequent membrane preparation steps. In order to improve the separation performance of the PDMS membrane during the pervaporation process, it is usually treated by chemical grafting, modification, physical blending, etc. to achieve the effects of increasing the selectivity of the separation membrane, improving the membrane flux, increasing the mechanical strength of the membrane, and enhancing the chemical stability of the membrane.

[0006] The patent application No. CN201110375074.X discloses a method for preparing an anti-fouling pervaporation membrane. A uniform casting solution is obtained by stirring and reacting a polyorganosiloxane, a fluorosilane containing a CF2 or CF3 segment, a catalyst, and an organic solvent. The casting solution is coated on the surface of the pervaporation membrane, and after heat treatment, an anti-fouling membrane layer is formed on the surface. Compared with the unmodified pervaporation membrane, the anti-fouling pervaporation membrane prepared has greatly improved anti-fouling performance under long-term continuous operation, and the attenuation rate of the membrane performance is reduced from the original 50-60% to less than 10%.

[0007] The patent application No. CN202111232816.3 discloses a phenyl-modified PDMS separation membrane for the separation of aromatic compounds. By replacing the p-dimethyl on the PDMS chain with p-diphenyl, a rigid spacer is constructed between adjacent polymer segments. The resulting increase in the chain spacing reduces the steric hindrance, making the remaining methyl groups on the chain have higher mobility. At the same time, the presence of phenyl inhibits the cooperative action between the polymer main chains and reduces the entanglement between the chains. The lower mass transfer resistance promotes the mass transfer efficiency of aromatic substances with a free volume close to that between the polymer chains in the membrane layer.

[0008] On the one hand, the painting wastewater finally collected in the automotive painting workshop has the characteristics of complex composition and high organic content. According to the feedback from major painting workshops, the painting wastewater is generally outsourced to a company with relevant treatment qualifications as hazardous waste for incineration treatment, and the treatment cost per ton is as high as 3800-6000 yuan, bringing a significant cost increase to automotive enterprises. On the other hand, PDMS is prone to uneven dispersion and unstable film-forming performance during the preparation process. The methods of chemical modification and grafting have problems such as complex processes and difficulty in engineering scale-up in the membrane preparation process. Summary of the Invention

[0009] To solve the defects existing in the prior art, the present invention provides a casting solution, a composite membrane, a preparation method and an application. The present invention solves the problems of uneven dispersion of particles in the membrane solution and poor film-forming stability. Moreover, a large number of hydrophobic groups are grafted onto the particles, further increasing the hydrophobicity of the separation layer and enhancing the separation performance of the membrane for organic substances.

[0010] To achieve the above object, the present invention adopts the following technical solutions:

[0011] In a first aspect, the present invention provides a composite membrane, which comprises a base membrane and a polymer membrane laminated on the base membrane. The polymer membrane comprises a polymer and hydrophobic micron particles distributed in the polymer; the average particle size of the hydrophobic micron particles is 0.1 - 100 μm.

[0012] In the present invention, the base membrane is a conventional choice in the art and may include polyvinylidene fluoride.

[0013] In the present invention, the filtration accuracy of the base membrane can be 0.04 - 0.22 μm.

[0014] In the present invention, the rejection rate of the base membrane can be ≥90%.

[0015] In the present invention, the polymer is a conventional choice in the art. The structure of the polymer may include a siloxane group and a C1 - C4 alkyl group, and the polymer is preferably polydimethylsiloxane.

[0016] In the present invention, the viscosity of the polymer at 25°C can be 900 - 86000 mPa·s, preferably 74000 - 86000 mPa·s.

[0017] In the present invention, the material of the hydrophobic micron particles is a conventional choice in the art, such as hydrophobic silica; preferably silica modified with dimethyldichlorosilane.

[0018] Among them, the silica modified with dimethyldichlorosilane can be prepared by the following method: heating a mixture; the mixture includes silica, dimethyldichlorosilane and a solvent.

[0019] Further, the mass ratio of the silica to the dimethyldichlorosilane is (1 - 4):(0.15 - 0.6).

[0020] Further, the solvent is ethanol and / or water.

[0021] Further, the heating temperature is the boiling point of the mixture, such as 130°C.

[0022] Further, the heating time is 50 - 120 min.

[0023] In the present invention, the thickness ratio of the polymer film to the base film may be (10 - 50):120, preferably is (25 - 30):120.

[0024] In the present invention, the mass ratio of the hydrophobic microparticles to the polymer may be (1 - 30):100, preferably (2.5 - 10):100.

[0025] In the present invention, the average particle size of the hydrophobic microparticles is preferably 0.1 - 20 μm, such as 6.5 μm.

[0026] In a second aspect, the present invention provides a casting solution, which comprises a polymer precursor and hydrophobic microparticles; the average particle size of the hydrophobic microparticles is 0.1 - 100 μm.

[0027] In the present invention, the polymer precursor may comprise polydimethylsiloxane, a crosslinking agent and a catalyst.

[0028] Among them, the crosslinking agent is a conventional selection in the art, such as tetraethyl orthosilicate.

[0029] Among them, the catalyst is a conventional selection in the art, such as dibutyltin dilaurate.

[0030] Among them, the mass ratio of the polydimethylsiloxane to the crosslinking agent may be (4 - 10):(0.5 - 1.5).

[0031] Among them, the mass ratio of the polydimethylsiloxane to the catalyst may be (4 - 10):(0.1 - 0.5).

[0032] Among them, the mass ratio of the hydrophobic microparticles to the polydimethylsiloxane may be (1 - 30):100, preferably (2.5 - 10):100.

[0033] In the present invention, the hydrophobic microparticles may be as described above.

[0034] In a third aspect, the present invention provides a method for preparing a composite membrane, which comprises the following steps: coating the casting solution as described above on the surface of a base film and drying; the casting solution

[0035] In the present invention, the base film may be as described above.

[0036] In the present invention, the thickness of the coating is a conventional selection in the art, such as 100 - 200 μm.

[0037] In the present invention, the drying temperature is a conventional selection in the art, such as 100 - 120 °C;

[0038] In the present invention, the drying time is a conventional selection in the art, such as 12 - 24 h.

[0039] Fourthly, the present invention provides a composite membrane prepared by the preparation method of the composite membrane as described above.

[0040] Fifthly, the present invention provides an application of the composite membrane as described above in treating coating waste liquid.

[0041] In the present invention, the COD value of the coating waste liquid can be 150000 - 200000 mg / L, for example, 160000 mg / L.

[0042] In the present invention, the coating waste liquid may include one or more of ethylene glycol monobutyl ether, ethylene glycol hexyl ether, isopropanol, 1-butoxy-2-propanol, and n-butanol.

[0043] On the basis of not violating the common knowledge in the art, the above preferred conditions can be combined arbitrarily to obtain various preferred examples of the present invention.

[0044] The reagents and raw materials used in the present invention are all commercially available.

[0045] The positive and progressive effects of the present invention are as follows:

[0046] (1) For the composite membrane of the present invention, micron particles after hydrophobic modification are added, which improves the separation performance and has better stability, and has more application prospects for coating wastewater with complex components;

[0047] (2) For the composite membrane of the present invention, due to the better dispersibility of the micron particles after hydrophobic modification, the problem of uneven dispersion of general nano-filler particles in the casting solution is effectively solved, greatly reducing agglomeration, improving the film quality after film formation, and enhancing the organic matter permeation performance of the membrane;

[0048] (3) For the composite membrane of the present invention, the COD of the treated feed liquid is lower and the COD of the permeate is higher, that is, more organic matter passes through and less water passes through. Description of the Drawings

[0049] Figure 1 It is a photograph of the appearance of the casting solution prepared in Example 1;

[0050] Figure 2 It is a surface scanning electron micrograph of the pervaporation separation membrane prepared in Example 1;

[0051] Figure 3 It is a side cross-section scanning electron micrograph of the pervaporation separation membrane prepared in Example 1;

[0052] Figure 4 It is a photograph of the appearance of the casting solution prepared in Comparative Example 1;

[0053] Figure 5 It is a surface scanning electron micrograph of the pervaporation separation membrane prepared in Comparative Example 1;

[0054] Figure 6 It is a scanning electron microscope cross-sectional view of the pervaporation separation membrane prepared in Comparative Example 1. Detailed implementation manners

[0055] The present invention will be further described below by way of examples, but the present invention is not limited to the scope of the described examples. For the experimental methods without specific conditions noted in the following examples, they are carried out according to conventional methods and conditions, or selected according to the product specifications.

[0056] PDMS, purchased from Yuyao Jinchao Polymer, model RTV-107-8, viscosity (25 °C) is 80000 ± 6000 mPa·s, turbidity ≤ 3.0 NTU, volatile matter (150 °C, 3 h) ≤ 3.0%, surface vulcanization time ≤ 1.0 h.

[0057] PVDF base film, purchased from Shenzhen Jiaquan Membrane Filtration Equipment Co., Ltd., with a filtration accuracy of 0.04 - 0.22 μm, Rejection rate 90%, 10 7 CFU / cm 2 , maximum working temperature 90 °C, maximum tolerated temperature 140 °C, water flow rate 0.38 t / m 2 ·h.

[0058] Micron silica particles, with an average particle size of 6.5 μm.

[0059] Nanometer silica particles, with an average particle size of 16 nm.

[0060] Example 1

[0061] A PDMS mixed matrix pervaporation separation membrane doped with hydrophobically modified micron silica particles and its preparation method, including the following steps:

[0062] S1. Preparation of hydrophobically modified micron silica particles:

[0063] Place 1 g of micron silica particles in a flask, heat while stirring and keep the temperature constant at 70 °C, then add 20 mL of ethanol to the flask and continue stirring for 50 minutes. After the micron silica is completely dispersed evenly, add 0.15 g of dimethyldichlorosilane modifier at one time, and at the same time slowly drop 6 mL of deionized water into the reaction system.

[0064] Then raise the temperature to 130 °C and keep refluxing for 60 minutes. After the reaction is completed, wash the suspension with ethanol 3 times, and after drying to constant weight, immediately obtain hydrophobically modified silica micron particles.

[0065] S2. Preparation of a PDMS mixed matrix pervaporation separation membrane doped with hydrophobically modified micron silica particles:

[0066] Dissolve 4 g of PDMS in a n - heptane solution. Add 0.1 g of hydrophobically modified silica microparticles and 0.5 g of cross - linker tetraethyl orthosilicate (TEOS), and stir and mix evenly at 25 °C. Subsequently, add 0.1 g of catalyst dibutyltin dilaurate (DBTDL) and continue stirring. After standing for about 1.5 h, measure its viscosity. When the viscosity is stable, the casting solution is obtained.

[0067] Pour the casting solution evenly onto the surface of a polyvinylidene fluoride (PVDF) substrate membrane, and use a 150 - μm - high scraper to coat the casting solution on the surface of the PVDF substrate membrane. Place the coated composite membrane in an oven and dry it at 120 °C for 12 hours to obtain a PDMS mixed - matrix pervaporation separation membrane doped with hydrophobically modified silica microparticles.

[0068] Comparative Example 1

[0069] A PDMS mixed - matrix pervaporation separation membrane and its preparation method include the following steps:

[0070] Dissolve 4 g of PDMS in a n - heptane solution. Add 0.5 g of cross - linker tetraethyl orthosilicate (TEOS), and stir and mix evenly at 25 °C. Subsequently, add 0.1 g of catalyst dibutyltin dilaurate (DBTDL) and continue stirring. After standing for about 1.5 h, measure its viscosity. When the viscosity is stable, the casting solution is obtained.

[0071] Pour the casting solution evenly onto the surface of a polyvinylidene fluoride (PVDF) substrate membrane, and use a 150 - μm - high scraper to coat the casting solution on the surface of the PVDF membrane. Place the coated composite membrane in an oven and dry it at 120 °C for 12 hours to obtain a PDMS mixed - matrix pervaporation separation membrane.

[0072] Comparative Example 2

[0073] A PDMS mixed - matrix pervaporation separation membrane doped with hydrophobically modified nano - silica particles includes the following steps:

[0074] S1. Preparation of hydrophobically modified nano - silica particles:

[0075] Place 1 g of nano - silica particles in a flask, heat while stirring and keep the temperature constant at 70 °C. Subsequently, add 20 mL of ethanol to the flask and continue stirring for 50 minutes. After the nano - silica is completely dispersed evenly, add 0.15 mL of dimethyldichlorosilane modifier at one time, and at the same time, slowly drip 6 mL of deionized water into the reaction system.

[0076] Subsequently, raise the temperature to 130 °C and keep refluxing for 60 minutes. After the reaction is completed, wash the suspension 3 times with ethanol, and after drying to constant weight, immediately obtain hydrophobically modified silica nanoparticles.

[0077] Preparation of a PDMS mixed matrix pervaporation separation membrane doped with hydrophobically modified nano-silica particles:

[0078] Dissolve 4 g of PDMS in a n-heptane solution, add 0.1 g of hydrophobically modified silica nanoparticles and 0.5 g of crosslinking agent TEOS, stir and mix evenly at 25 °C, then add 0.1 g of DBTDL and continue stirring. After standing for about 1.5 h, measure its viscosity. When the viscosity is stable, the casting solution is obtained.

[0079] Pour the casting solution evenly onto the surface of the PVDF substrate membrane, and coat the casting solution on the PVDF membrane surface with a 150-μm height scraper. Place the coated composite membrane in an oven and dry it at 120 °C for 12 hours to obtain a PDMS mixed matrix pervaporation separation membrane doped with hydrophobically modified nano-silica particles.

[0080] Effect Example 1: Stability of the casting solution

[0081] 1. Test object: The casting solutions in some of Example 1 and Comparative Example 2.

[0082] 2. Test method: Let the casting solution stand for half an hour to one hour, and observe whether there is agglomeration or sedimentation.

[0083] 3. Test results:

[0084] Figure 1 is a photograph of the appearance of the casting solution prepared in Example 1. The hydrophobically modified micron-sized silica particles are evenly dispersed in the casting solution, and there is almost no agglomeration or sedimentation after standing for a long time.

[0085] Figure 4 is a photograph of the appearance of the casting solution prepared in Comparative Example 1. The hydrophobically modified nano-silica particles are initially evenly dispersed in the casting solution, but there are agglomeration and sedimentation phenomena after a little standing. The agglomerates are clearly attached to the cup wall, and the stability is lacking.

[0086] Effect Example 2: Morphology of the pervaporation membrane

[0087] 1. Test object: The pervaporation membranes in Example 1 and Comparative Example 2.

[0088] 2. Test method: Use the commonly used scanning electron microscope SEM in the field for testing.

[0089] 3. Test results:

[0090] Figure 2 is a scanning electron microscope (SEM) image of the surface of the PDMS mixed matrix pervaporation separation membrane doped with hydrophobically modified micron-sized silica particles prepared in Example 1. Figure 3SEM cross-sectional view of the PDMS mixed matrix pervaporation separation membrane doped with hydrophobically modified micron silica particles prepared in Example 1. From Figure 2 - 3 it can be seen that the hydrophobically modified micron silica particles are uniformly dispersed in the PDMS membrane, and the thickness ratio of the polymer membrane to the base membrane is about (25 - 30):120.

[0091] Figure 5 SEM surface view of the PDMS mixed matrix pervaporation separation membrane doped with hydrophobically modified nano silica particles prepared in Comparative Example 1. Figure 6 SEM cross-sectional view of the PDMS mixed matrix pervaporation separation membrane doped with hydrophobically modified nano silica particles prepared in Comparative Example 1. From Figure 5 - 6 it can be seen that the hydrophobically modified nano silica particles are not uniformly dispersed in the PDMS membrane, and the agglomeration phenomenon is relatively obvious. The agglomerated particles are generally above 100 microns, and the thickness uniformity of the membrane layer is poor, showing undulating shapes. A large number of agglomerated particles result in uneven distribution of nano silica particles in the membrane.

[0092] Effect Example 3: Pervaporation experiment of the pervaporation membrane

[0093] 1. Test objects: The pervaporation membranes in Example 1 and Comparative Examples 1 - 2.

[0094] 2. Test methods:

[0095] (1) Treat the feed liquid: The painting wastewater collected from the painting workshop of an automobile factory in Shanghai. Currently, most of the samples are mainly alcohol ethers. The organic substances in the wastewater are mainly ethylene glycol monobutyl ether, ethylene glycol hexyl ether, isopropyl alcohol, 1-butoxy-2-propanol, n-butanol, etc. Among them, the content of ethylene glycol butyl ether is the highest, and in most cases, it accounts for about 50% of the total mass of the organic substances. This painting wastewater has been treated by coagulation and low-temperature distillation. After measurement, the COD value of this wastewater is about 160000 mg / L.

[0096] (2) Test conditions: Adopt a continuous and full-circulation operation mode. That is, the volume of the feed tank is about 4 L; the stable flow rate of the feed liquid is about 8 L / min; the membrane area in the pervaporation system is about 0.1 m 2 , the length of a single membrane is about 28 cm and the width is about 18.5 cm; the operation temperature of the pervaporation system is controlled at about 65 °C; the temperature of the condensate system for collecting the permeate is maintained below 0 °C; the pressure on the permeate side of the pervaporation system by the vacuum pump is maintained at 450 - 550 Pa.

[0097] (3) Test standard: The removal rate of organic substances in the feed liquid by the separation membrane is measured by respectively measuring the COD values of the feed liquid before pervaporation and after 4 hours using HACH's COD detection reagent. Specifically as follows:

[0098]

[0099] 3. Test results: See the following table

[0100]

[0101] After adding the hydrophobically modified silica particles, both the selectivity and permeability of the pervaporation membrane to organic substances are improved. In comparison, the COD value of the feed liquid treated by the pervaporation membrane with hydrophobically modified micron silica particles added in Example 1 is lower, the organic matter removal rate is higher, and the membrane flux is higher, indicating a stronger permeation and mass transfer ability for organic substances in the coating wastewater; at the same time, the permeate concentration is higher, indicating a stronger water retention ability, which further increases the concentration efficiency of the waste liquid.

Claims

1. A composite film, characterized in that, It includes a base film and a polymer film laminated on the base film. The polymer film includes a polymer and hydrophobic micron particles distributed in the polymer; the average particle size of the hydrophobic micron particles is 0.1 - 100 μm.

2. The composite film according to claim 1, wherein It satisfies at least one of the following conditions: (1) The base film includes polyvinylidene fluoride; (2) The filtration accuracy of the base film is 0.04 - 0.22 μm; (3) The rejection rate of the base film is ≥ 90%; (4) The structure of the polymer includes a siloxane group and a C1 - C4 alkyl group, and the polymer is preferably polydimethylsiloxane; (5) The viscosity of the polymer at 25°C is 900 - 86000 mPa·s, preferably 74000 - 86000 mPa·s; (6) The material of the hydrophobic micron particles is hydrophobic silica; preferably silica modified with dimethyldichlorosilane; (7) The thickness ratio of the polymer film to the base film is (10 - 50):120, preferably (25 - 30):120; (8) The mass ratio of the hydrophobic micron particles to the polymer is (1 - 30):100, preferably (2.5 - 10):100; (9) The average particle size of the hydrophobic micron particles is 0.1 - 20 μm, such as 6.5 μm.

3. The composite film according to claim 2, wherein The silica modified with dimethyldichlorosilane is prepared by the following method: heating a mixture; the mixture includes silica, dimethyldichlorosilane and a solvent.

4. The composite film according to claim 3, characterized in that, It satisfies at least one of the following conditions: (1) The mass ratio of the silica to the dimethyldichlorosilane is (1 - 4):(0.15 - 0.6); (2) The solvent is ethanol and / or water; (3) The heating temperature is the boiling point of the mixture, such as 130°C; (4) The heating time is 50 - 120 min.

5. A casting solution, characterized in that, It includes a casting solution including a polymer precursor and hydrophobic micron particles; the average particle size of the hydrophobic micron particles is 0.1 - 100 μm.

6. The casting solution according to claim 5, characterized in that, It satisfies at least one of the following conditions: (1) The polymer precursor includes polydimethylsiloxane, a crosslinking agent and a catalyst; the crosslinking agent is preferably tetraethyl orthosilicate; the catalyst is preferably dibutyltin dilaurate; the mass ratio of the polydimethylsiloxane to the crosslinking agent is preferably (4 - 10):(0.5 - 1.5); the mass ratio of the polydimethylsiloxane to the catalyst is preferably (4 - 10):(0.1 - 0.5); the mass ratio of the hydrophobic micron particles to the polydimethylsiloxane is preferably (1 - 30):100, preferably (2.5 - 10):100; (2) The average particle size of the hydrophobic micron particles is 0.1 - 20 μm, such as 6.5 μm.

7. A method for preparing a composite film, characterized in that, It includes the following steps: Coating the casting solution as claimed in claim 5 or 6 on the surface of the base film and drying.

8. The method for preparing the composite film according to claim 7, characterized in that, It satisfies at least one of the following conditions: (1) The base film is as defined in claim 2; (4) The coating thickness is 100 - 200 μm; (5) The drying temperature is 100 - 120°C; (6) The drying time is 12 - 24 h.

9. A composite membrane prepared by the method for preparing a composite membrane according to claim 7 or 8.

10. Use of a composite membrane according to any one of claims 1-4 and 9 in treating coating waste liquid; preferably, it satisfies at least one of the following conditions: (1) The COD value of the coating waste liquid is 150,000-200,000 mg / L, such as 160,000 mg / L; (2) The coating waste liquid includes one or more of ethylene glycol monobutyl ether, ethylene glycol hexyl ether, isopropanol, 1-butoxy-2-propanol, and n-butanol.

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