Process for treating acrylonitrile flue gas desulfurization wastewater for zero discharge

Abstract

The process method for zero discharge treatment of acrylonitrile flue gas desulfurization wastewater comprises a pretreatment section, a main reaction section and a deep treatment section; the pretreatment section comprises a molybdenum removal unit, a conditioning and hardness removal unit, a tubular microfiltration unit and a sludge concentration, dewatering and drying unit; the main reaction section comprises an electrodialysis unit, a primary anaerobic unit, a secondary anaerobic unit, a incineration boiler unit, a flue gas desulfurization unit, a multi-effect evaporation unit and a crystallization and salt separation system; the deep treatment section comprises a Fenton oxidation unit, a nanofiltration unit and a neutralization unit; the electrodialysis unit adopts an anion exchange membrane modified by polydopamine; the improved electrodialysis realizes the separation of organic matter and salt and the concentration of salt, and the anti-pollution ability is greatly increased, which is conducive to the subsequent anaerobic treatment and reduces the organic matter content in the salt separation section; in the whole process method, the wastewater is recycled, the waste gas can be used as fuel, the salt is converted into sodium sulfate and sodium chloride products, and the water, gas and solid are overall resourceized.

Description

Technical Field

[0001] This invention relates to a method for treating acrylonitrile flue gas desulfurization wastewater, belonging to the field of wastewater treatment technology. Background Technology

[0002] Acrylonitrile is an important chemical raw material for the three major synthetic materials (fibers, rubber, and plastics), and it has a wide range of applications in the organic synthesis industry and people's economic life. The acrylonitrile production process generally adopts the propylene ammoxidation method. During the production process, the quench tower and the light component removal tower discharge a large amount of wastewater. The wastewater mainly contains pollutants such as acrylonitrile, succinic acid, methyl acrylate, and cyanide. The wastewater is highly toxic and the pollutants are difficult to treat. The simplest treatment method is concentration and incineration. That is, the wastewater is first concentrated by multi-effect evaporation and then incinerated. The flue gas produced by incineration will generate wastewater during the desulfurization and denitrification process. This wastewater is called acrylonitrile flue gas desulfurization wastewater.

[0003] Unlike the dehydration of flue gas from thermal power plants and catalytic cracking, acrylonitrile flue gas dehydration results in higher COD concentrations and higher molybdenum ion concentrations among heavy metal ions, but lower concentrations of other heavy metal ions. This is because thermal power plants use coal as fuel, and catalytic cracking uses oil adhering to the catalyst; both contain a significant amount of impurities, leading to higher heavy metal content in the flue gas dehydration. In contrast, acrylonitrile incineration boilers use wastewater and co-firing gas (dry gas) as fuel. The wastewater is generated from the propylene ammoxidation process, has a high organic content but generally low heavy metal ion concentrations. Furthermore, the dry gas itself contains no heavy metals. Therefore, the heavy metal ion concentrations in acrylonitrile flue gas dehydration are generally low. The high molybdenum ion concentration is due to the use of molybdenum-based catalysts (CN200510023601.5, CN201610327805.6) in the propylene ammoxidation process. During production, catalyst-loaded metal loss is inevitable, resulting in higher molybdenum content in the wastewater.

[0004] The "Emission Standard of Pollutants for Inorganic Chemical Industry" (GB31573-2015) stipulates that the water pollutant emission limit for molybdenum (Mo) ions must not exceed 0.5 mg / L. This is because excessive molybdenum intake can harm human health, disrupt energy metabolism, and cause a series of health problems. Furthermore, excessively high molybdenum levels can also affect the activity of sludge microorganisms. Therefore, the treatment of acrylonitrile flue gas desulfurization wastewater not only needs to address COD and salt levels but also the issue of excessively high molybdenum levels. Simultaneously, the treatment process must balance energy consumption, resource recovery, and zero emissions, making it a challenging overall process. Summary of the Invention

[0005] To address the above shortcomings, this invention provides a process for zero-discharge treatment of acrylonitrile flue gas desulfurization wastewater. The wastewater is recycled and reused, salts are converted into sodium sulfate and sodium chloride products, most organic matter is converted into methane and utilized as a resource, and molybdenum is efficiently removed without the need for the addition of reagents.

[0006] To achieve the above technical objectives, the technical solution adopted by the present invention is as follows:

[0007] The process for zero-discharge treatment of acrylonitrile flue gas desulfurization wastewater includes a pretreatment section, a main reaction section, and a deep treatment section;

[0008] The pretreatment section includes a molybdenum removal unit, a hardening and conditioning unit, a tubular microfiltration unit, and a sludge thickening, dewatering, and drying unit. Wastewater first enters the molybdenum removal unit to remove molybdenum, and the effluent enters the hardening and conditioning unit to remove hardness. After being filtered by the tubular microfiltration unit, it enters the electrodialysis unit of the main reaction section. The chemical sludge generated by the molybdenum removal unit, the hardening and conditioning unit, and the tubular microfiltration unit enters the sludge thickening, dewatering, and drying unit. The water removed is returned to the inlet of the pretreatment section, and the dried sludge is sent out.

[0009] The main reaction section includes an electrodialysis unit, a primary anaerobic unit, a secondary anaerobic unit, an incineration boiler unit, a flue gas desulfurization unit, a multi-effect evaporation unit, and a crystallization and salt separation system. The concentrated liquid from the electrodialysis unit sequentially enters the multi-effect evaporation unit and the crystallization and salt separation system, while the desalinated liquid sequentially enters the primary anaerobic unit and the secondary anaerobic unit. The primary anaerobic unit mainly performs organic acidification and sulfate reduction, producing hydrogen sulfide gas. The secondary anaerobic unit mainly performs methanogenesis, and the generated methane is pumped to the primary anaerobic unit as stripping gas, carrying most of the hydrogen sulfide generated in the primary anaerobic unit out of the wastewater system. The mixed gas of methane and hydrogen sulfide generated in the primary anaerobic unit enters the incineration boiler as combustion gas. The incineration boiler unit is mainly used to incinerate acrylonitrile concentrated wastewater, and the generated exhaust gas enters the flue gas desulfurization unit. The generated steam is supplied to the multi-effect evaporation unit and the crystallization and salt separation system. The desulfurized wastewater generated by the flue gas desulfurization unit is returned to the inlet of the pretreatment section.

[0010] The advanced treatment section includes a Fenton oxidation unit, a nanofiltration unit, and a neutralization unit in sequence. The effluent from the secondary anaerobic unit passes through the Fenton oxidation unit and the nanofiltration unit in sequence. The concentrate from the nanofiltration unit is returned to the molybdenum removal unit, where it is mixed to remove molybdenum. The permeate from the nanofiltration unit enters the neutralization unit, where it is adjusted to neutrality with alkali and then discharged to meet the reuse standards.

[0011] The electrodialysis unit is composed of anion and cation exchange membranes, wherein the anion exchange membrane is a modified anion exchange membrane and the cation exchange membrane is a general-purpose cation exchange membrane.

[0012] The modification methods for anion exchange membranes are as follows:

[0013] Step a: Dissolve the linear polymer in an organic solvent, then add styrene, divinylbenzene, dioctyl phthalate, benzoyl peroxide and a hydrophilic modifier respectively, stir and react to obtain a polymer solution, wherein the hydrophilic modifier is selected from one or more of α-cyclodextrin, β-cyclodextrin or γ-cyclodextrin;

[0014] Step b: Add organic solvent to the polymer solution obtained in step a, add anhydrous zinc chloride and chloromethyl methyl ether, stir to react, then add precipitant, and then filter, dry and pulverize to obtain powdered chloromethylated polymer;

[0015] Step c: Dissolve the polymer obtained in step b in N,N-dimethylformamide, introduce trimethylamine gas, and carry out a quaternization reaction to obtain a quaternized polymer solution; prepare a membrane from the quaternized polymer solution, immerse the membrane in sodium hydroxide solution for alkalization, and then wash it with water until neutral to obtain an anion exchange membrane.

[0016] Step d: Dissolve dopamine in Tris-HCl buffer, adjust the pH with hydrochloric acid to obtain dopamine Tris buffer solution;

[0017] Step e: Place the anion exchange membrane obtained in step c in a dopamine Tris buffer solution, add copper sulfate, and stir the reaction while maintaining air circulation. After the reaction is complete, the modified anion exchange membrane is obtained.

[0018] Furthermore, the linear polymer mentioned in step a is selected from at least one of polyethylene, polypropylene, polyvinyl chloride, or polyvinylidene fluoride, preferably polyvinyl chloride.

[0019] Furthermore, the organic solvent mentioned in step a is selected from at least one of dichloromethane, dichloroethane, or chloroform, preferably dichloromethane.

[0020] Furthermore, the linear polymer in step a has a mass-volume concentration of 30–100 mg / mL in the organic solvent, and the styrene, divinylbenzene, dioctyl phthalate, benzoyl peroxide, and hydrophilic modifier have mass-volume concentrations of 20–100 mg / mL, 20–100 mg / mL, 5–20 mg / mL, 2–10 mg / mL, and 10–80 mg / mL, respectively, in the organic solvent.

[0021] Furthermore, the temperature of the stirring reaction in step a is 50–90°C, and the stirring reaction time is 0.5–6 h.

[0022] Furthermore, the amount of organic solvent added in step b is 0.5 to 1 times the amount of organic solvent used in step a.

[0023] Furthermore, in step b, the mass-volume concentration of anhydrous zinc chloride and solvent is 15–30 mg / mL, and the volume ratio of chloromethyl ether to organic solvent is 0.5:1–2:1.

[0024] Furthermore, in step b, the temperature of the stirring reaction is 30–55°C, and the stirring reaction time is 2–24 h.

[0025] Furthermore, the precipitant mentioned in step b is methanol and / or ethanol.

[0026] Furthermore, the mass-volume concentration of the polymer and N,N-dimethylformamide in step c is 15–40 mg / mL.

[0027] Furthermore, the trimethylamine gas in step c is obtained by heating and vaporizing an aqueous solution of trimethylamine, followed by drying with an alkaline drying agent. The alkaline drying agent is selected from at least one of potassium hydroxide, sodium hydroxide, quicklime, and soda lime.

[0028] Furthermore, the quaternization reaction time in step c is 10–120 minutes.

[0029] Furthermore, the quaternized polymer solution described in step c is used to form a film by casting or casting, and then dried at a temperature of 50–70°C for 12–24 hours.

[0030] Furthermore, the concentration of the sodium hydroxide solution used for alkalization in step c is 0.1–1 mol / L.

[0031] Furthermore, the concentration of the Tris–HCl buffer in step d is 10–50 mmol / L.

[0032] Furthermore, the mass concentration of dopamine in the solution formed in step d is 0.1–3 g / L, and the pH range of the hydrochloric acid is adjusted to 8–9.

[0033] Further, in step e, copper sulfate is added to a concentration of 1–20 mmol / L, and the reaction is stirred for 1–12 h. Copper sulfate can induce rapid polymerization of dopamine, forming a negatively charged polydopamine electrolyte layer on the surface of the anion exchange membrane, thus modifying the membrane surface.

[0034] Furthermore, the resulting modified anion exchange membrane is stored in a sodium chloride solution with a mass concentration of 5–20 g / L.

[0035] Those skilled in the art should understand that most organic pollutants present in natural water bodies or wastewater, such as surfactants, humic acids, and proteins, are negatively charged. Dopamine has strong self-polymerization and adsorption capabilities and can increase the negative charge density on the membrane surface. Therefore, after modification with polydopamine, the anion exchange membrane becomes negatively charged, exhibiting electrostatic repulsion against negatively charged organic matter in the water, thus inhibiting organic matter contamination of the anion exchange membrane. Those skilled in the art should also understand that while the negative surface charge prevents organic matter contamination through electrostatic interaction, it also affects the migration rate of inorganic anions. The greater the ion charge, the greater the impact; therefore, sulfate ions are more affected than chloride ions. Adding hydrophilic modifiers such as β-cyclodextrin to the anion exchange membrane can impart strong hydrophilicity to the membrane and alter the migration rates between anions. Specifically, the migration rate of less hydrated anions, such as bromide and nitrate ions, relative to chloride ions decreases, while the migration rate of more hydrated anions, such as sulfate ions, relative to chloride ions increases. On the other hand, increased membrane hydrophilicity reduces van der Waals forces between the membrane and organic solutes, decreasing attractiveness. Simultaneously, hydrogen bonding between the hydrophilic membrane and water molecules forms a hydration layer, which further hinders the adsorption of pollutants on the membrane surface. Therefore, the anion exchange membrane of this invention, after being modified with both a hydrophilic modifier and polydopamine, exhibits significantly increased antifouling ability and enhanced selective permeability to sulfate ions.

[0036] Furthermore, the molybdenum removal agent in the molybdenum removal unit is ferric ions from nanofiltration concentrate, with the pH controlled at 3-5, preferably 3.5-4.5, and the iron-molybdenum molar ratio controlled at 2:1-9:1. Those skilled in the art should understand that molybdenum in acrylonitrile flue gas desulfurization wastewater exists in the form of molybdate. Conventional molybdenum sulfide precipitation methods are ineffective. This invention uses an iron salt precipitation method, which can form a series of positively charged hydrolysis products with water molecules. These hydrolysis products then interact with negatively charged molybdate ions through electrostatic adsorption, surface complexation, co-precipitation, etc., thereby reducing the number of molybdenum ions.

[0037] Furthermore, the hardening agent in the hardening unit is a combination of sodium hydroxide, sodium carbonate, and PAM. Sodium carbonate is added at 1 to 3 times the mass concentration of calcium ions, the amount of sodium hydroxide added is greater than the mass concentration of magnesium ions, and the amount of PAM added is 1 to 20 mg / L. Sodium hydroxide also acts as a pH adjuster, and the pH value should be greater than 8 after addition.

[0038] Furthermore, the tubular microfiltration system is mainly used to filter suspended solids, colloids, etc., to prevent clogging of the subsequent electrodialysis membrane.

[0039] Furthermore, the electrodialysis treatment time is 0.2–3 hours, and the current density is 1–80 mA / cm². 2 .

[0040] Further, the multi-effect evaporation unit is heated by steam to increase the salt content of the concentrated liquid to 200 - 300 g / L, and then it enters the nitrate crystallizer and the salt crystallizer to separate salts by utilizing the difference in the solubility of sodium chloride and sodium sulfate with temperature; the temperature of the nitrate crystallizer is controlled at 100 - 130 °C, the temperature of the salt crystallizer is controlled at 70 - 90 °C, and the residual liquid is refluxed to the inlet of the multi-effect evaporation.

[0041] Further, the dissolved oxygen in the first-stage anaerobic unit is controlled below 0.2 mg / L, the wastewater residence time is 1 - 12 h, the temperature is 25 - 35 °C, the pH is controlled at 5 - 6 by adding acid, and the strains are organic acidifying bacteria and sulfate-reducing bacteria.

[0042] Further, the dissolved oxygen in the second-stage anaerobic unit is controlled below 0.1 mg / L, the wastewater residence time is 2 - 24 h, the temperature is 30 - 40 °C, the pH is controlled at 7 - 9 by adding alkali, and the strains are methanogens.

[0043] Further, the stripping gas of the first-stage anaerobic unit is methane generated by the second-stage anaerobic unit, and the methane and hydrogen sulfide mixture gas generated after stripping enters the incineration boiler as the accompanying combustion gas.

[0044] Further, the incineration boiler unit is mainly used for incinerating acrylonitrile concentrated wastewater. In addition to the methane and hydrogen sulfide generated by the two-stage anaerobic units as the accompanying combustion gas, dry gas is also provided for accompanying combustion, and the steam generated in the boiler is supplied to the multi-effect evaporation unit and the crystallization and salt separation system.

[0045] Further, the flue gas desulfurization unit is a flue gas treatment facility supporting the incineration boiler, adopting the limestone / lime - gypsum method, and the flue gas desulfurization wastewater generated is recycled to the inlet of the pretreatment section.

[0046] Further, in the Fenton oxidation unit, hydrogen peroxide is added according to the mass ratio of hydrogen peroxide to wastewater COD of 1 - 10:1, the iron addition amount is added according to the mass ratio of iron to hydrogen peroxide of 0.2 - 1:1, and the pH is 3 - 5; the iron is divalent (sub) iron, and the ferrous salt is selected from at least one of ferrous chloride, ferrous nitrate, and ferrous sulfate.

[0047] Further, the water production rate of the nanofiltration unit is 60% - 95%, and it has a good interception effect on divalent and high-valent ions; specifically, after the effluent of the Fenton oxidation unit is treated by nanofiltration, iron ions, sulfate ions, etc. are intercepted and enter the concentrated water side, and sodium ions, chloride ions, etc. pass through the nanofiltration membrane and enter the water production side.

[0048] Further, the pH of the neutralization tank is adjusted by adding one or more of sodium hydroxide, calcium hydroxide, or potassium hydroxide to adjust the pH to 6 - 9 to meet the requirements of the recycled water quality.

[0049] Compared with the prior art, the present invention has the following advantages:

[0050] (1) The modified anion exchange membrane and improved electrodialysis of the present invention efficiently achieve the separation of organic matter and salt and the concentration of salt, greatly increasing the anti-pollution ability. After the improved electrodialysis treatment, most of the organic matter is retained in the desalination liquid and most of the salt is concentrated in the concentrate, which is beneficial to the subsequent anaerobic treatment, reduces the organic matter content in the salt separation section, and increases the purity of the salt separation product.

[0051] (2) The two-stage anaerobic digestion of the present invention not only reduces COD and sulfate, but also realizes the resource utilization of organic matter and sulfur. Specifically, sulfate is converted into hydrogen sulfide in the first-stage anaerobic unit and used as combustion gas, organic matter is converted into methane in the second-stage anaerobic unit and used as stripping gas and fuel, and the steam generated by the incineration boiler is recycled to the multi-effect evaporation and crystallization salt separation system.

[0052] (3) The present invention uses Fenton oxidation and nanofiltration technology to achieve deep treatment and compliant reuse of acrylonitrile fume dewatering wastewater. Furthermore, the trivalent iron produced by Fenton oxidation is utilized as a resource by combining it with the nanofiltration unit and the molybdenum removal unit, i.e., it is used as a molybdenum removal agent, thereby saving agent consumption.

[0053] (4) The process method of the present invention is used to recycle acrylonitrile flue gas desulfurization wastewater, the waste gas can be used as fuel, and the salt is converted into sodium sulfate and sodium chloride products, realizing the overall resource utilization of water, gas and solids.

[0054] Other features and advantages of the present invention will be described in detail in the following detailed description section. Attached Figure Description

[0055] Figure 1 A schematic bar chart illustrating the effect of molybdenum on the degradation of glucose by sludge microorganisms in Example 1;

[0056] Figure 2 Flowchart of the zero-discharge treatment process for acrylonitrile flue gas desulfurization wastewater in Example 2. Detailed Implementation

[0057] The present invention will be further described in detail below with reference to specific embodiments. These embodiments are implemented based on the technical solution of the present invention, providing detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0058] The inventors first studied the effect of molybdenum on the degradation of glucose by sludge microorganisms:

[0059] A pure glucose solution with a COD concentration of 800 mg / L and a salt (Na2SO4) concentration of 1% was prepared. K2MoO4 was added to form different concentration gradients, and nutrients such as N and P were added. A salt-tolerant bacterial agent was added, and the MLSS was 1 g / L. The mixture was reacted in a constant temperature shaker at 30℃ and 150 r / min for 48 h. The COD values ​​before and after the reaction were measured.

[0060] like Figure 1 As shown, the inventors discovered that without the addition of Mo, the COD removal rate of the glucose solution after 48 hours of biochemical reaction was 86%. However, after adding Mo at a concentration of 100 mg / L, the COD removal rate decreased to 82.2%, and the COD removal rate gradually decreased with increasing Mo concentration. This indicates that high concentrations of Mo ions inhibit the activity of sludge microorganisms and affect their ability to degrade organic matter. It cannot be ruled out that under long-term operation conditions, high concentrations of Mo metal ions may further affect the activity of sludge microorganisms. Furthermore, considering that some emission standards, such as the "Emission Standard of Pollutants for Inorganic Chemical Industry" (GB31573-2015), require that the Mo ion concentration not exceed 0.5 mg / L, it is necessary to remove Mo before the acrylonitrile fume dewatering biochemical treatment.

[0061] Example 1

[0062] The process flow diagram for treating acrylonitrile flue gas desulfurization wastewater is as follows: Figure 2 As shown, the flue gas desulfurization wastewater sequentially enters the molybdenum removal tank, the equalization and hardening tank, and the tubular microfiltration system. The effluent from the tubular microfiltration system enters the electrodialysis system. The chemical sludge produced in the molybdenum removal tank, the equalization and hardening tank, and the tubular microfiltration system enters the sludge thickening, dewatering, and drying unit. The dewatered water is returned to the inlet of the pretreatment section, and the dried sludge is sent off-site. The concentrated liquid from the electrodialysis system sequentially enters the multi-effect evaporation and crystallization desalination system. The desalinated liquid sequentially enters the primary anaerobic digester and the secondary anaerobic digester. The effluent from the secondary anaerobic digester enters the Fenton oxidation system. The methane gas produced in the secondary anaerobic digester is used as stripping gas and pumped to the primary anaerobic digester. Most of the hydrogen sulfide produced in the primary anaerobic digester is carried out of the wastewater system and eventually enters the incinerator as combustion gas. The incinerator is mainly used to incinerate acrylonitrile concentrate wastewater. The generated exhaust gas enters the flue gas desulfurization unit, and the generated steam is supplied to the multi-effect evaporation and crystallization desalination system. The desulfurized wastewater generated by the flue gas desulfurization is returned to the inlet of the hardening tank. The Fenton oxidation unit adds hydrogen peroxide and ferrous salt, and the effluent enters the nanofiltration unit. The nanofiltration concentrate is returned to the molybdenum removal tank. After mixing, molybdenum can be removed efficiently. The nanofiltration permeate enters the neutralization tank, and after adding alkali to adjust it to neutral, the effluent meets the reuse standards.

[0063] The process method of this invention is used to treat acrylonitrile flue gas desulfurization wastewater.

[0064] The wastewater from the desulfurization of acrylonitrile flue gas has the following characteristics: COD 580 mg / L, sulfate 7000 mg / L, chloride concentration 3000 mg / L, calcium ion concentration 100 mg / L, magnesium ion concentration 50 mg / L, molybdenum ion concentration 60 mg / L, total salt content 15600 mg / L, and pH 5.

[0065] The electrodialysis provided in this embodiment is a modified electrodialysis, wherein the anion exchange membrane is a modified anion exchange membrane with strong anti-fouling ability and high ion permeability, and the cation exchange membrane is a general-purpose cation exchange membrane (Hefei Capgemini Polymer Co., Ltd., China, model CJ-MC-3).

[0066] The modified anion exchange membrane described above was prepared by the following method:

[0067] Step a: Dissolve linear polymer polyvinyl chloride in 1 volume of organic solvent dichloromethane. The mass-volume concentration of the linear polymer in the organic solvent is 58 mg / mL. Styrene, divinylbenzene, dioctyl phthalate, benzoyl peroxide, and hydrophilic modifier β-cyclodextrin are added at mass-volume concentrations of 48 mg / mL, 42 mg / mL, 9 mg / mL, 7 mg / mL, and 46 mg / mL, respectively. The mixture is stirred at 75°C for 3 hours to obtain a polymer solution.

[0068] Step b: Add 0.65 volume of organic solvent dichloromethane to the polymer solution obtained in step a, add anhydrous zinc chloride and chloromethyl methyl ether, the mass-volume concentration of anhydrous zinc chloride to dichloromethane is 24 mg / mL, the volume ratio of chloromethyl methyl ether to solvent is 1.3:1, stir the reaction at 45℃ for 12 h, add precipitant methanol, filter, dry, and pulverize to obtain powdered chloromethylated polymer;

[0069] Step c: Dissolve the polymer obtained in step b in N,N-dimethylformamide, with a mass-volume concentration of 28 mg / mL for both polymer and N,N-dimethylformamide; heat and vaporize the trimethylamine aqueous solution, then pass the trimethylamine gas through an alkaline drying agent into the N,N-dimethylformamide solution of the polymer, reacting for 70 minutes to obtain a quaternized polymer solution; cast the quaternized polymer solution into a membrane using a casting method, and dry it at 60°C for 18 hours to form a membrane; immerse the obtained membrane in a 0.55 mol / L sodium hydroxide solution for alkalization, then wash it with deionized water until neutral to obtain an anion exchange membrane;

[0070] Step d: Dissolve dopamine in 20 mmol / L Tris–HCl buffer, adjust the pH to 8.5 with hydrochloric acid to obtain a dopamine Tris buffer solution with a dopamine mass concentration of 0.5 g / L.

[0071] Step e: Place the polyvinyl alcohol anion exchange membrane to be modified in a dopamine Tris buffer solution and add 3 mmol / L copper sulfate. Stir and react for 6 hours. Keep the air circulating during the reaction. Copper sulfate can induce rapid polymerization of dopamine and form a negatively charged polydopamine electrolyte layer on the surface of the anion exchange membrane. After the reaction is completed, the modified anion exchange membrane is obtained.

[0072] Step f: Take out the modified anion exchange membrane obtained in step e and place it in a sodium chloride solution with a mass concentration of 10 g / L for later use.

[0073] The specific operating parameters for treating acrylonitrile flue gas desulfurization wastewater are as follows:

[0074] Wastewater first enters the molybdenum removal tank, where it is mixed with nanofiltration concentrate and sulfuric acid is added to adjust the pH to 4, resulting in a large amount of precipitate. After settling, the molybdenum ion concentration in the effluent decreases to below 1 mg / L, and then it enters the equalization and hardening tank. The equalization and hardening tank is prepared by adding chemicals at a concentration of 200 mg / L sodium hydroxide, 150 mg / L sodium carbonate, and 8 mg / L PAM, raising the solution pH to 8.2. After clarification, the calcium ion concentration in the supernatant decreases to below 10 mg / L, and the magnesium ion concentration decreases to below 5 mg / L. The effluent is then filtered through a tubular microfiltration system before entering electrodialysis. The chemical sludge produced in the molybdenum removal tank, equalization and hardening tank, and tubular microfiltration system enters the sludge thickening, dewatering, and drying unit. The dewatered water is returned to the inlet of the pretreatment section, and the dried sludge is sent off-site.

[0075] The effluent from the tubular microfiltration system enters the electrodialysis process. The salt content of the effluent from the tubular microfiltration system is approximately 15.8 g / L, and the COD is 552 mg / L. The electrodialysis treatment time is 0.5 h, and the current density is 30 mA / cm³. 2After treatment, the electrodialysis concentrate had a salt content of approximately 62.6 g / L and a COD of 136 mg / L, while the desalinated liquid had a salt content of approximately 4.1 g / L and a COD of 656 mg / L. The permeate flow rate (percentage of desalinated liquid to total influent) was 80%, the sulfate ion selective permeation rate was 77.5%, the chloride ion selective permeation rate was 84.5%, and the organic matter rejection rate was 95.1%. The electrodialysis desalinated liquid entered the primary anaerobic digester, where dissolved oxygen was controlled below 0.15 mg / L, pH was maintained at 5.5 by adding sulfuric acid, the wastewater retention time was 5 h, the temperature was 30℃, and the bacterial strains were organic acidifying bacteria and sulfate-reducing bacteria. The effluent COD decreased to 516 mg / L, and the salt content decreased to 2435 mg / L. In the secondary anaerobic digester, dissolved oxygen was controlled below 0.1 mg / L, pH was maintained at 8 by adding sodium hydroxide, and the wastewater retention time was 8 h. h, temperature 35℃, bacterial strain is methanogen, effluent COD is 153mg / L, salt content is 2380mg / L; electrodialysis concentrate enters multi-effect evaporator, salt content is further concentrated to 255g / L, then enters nitrate crystallizer, operating temperature controlled at 115℃, crystallizes to obtain sodium sulfate product, effluent enters salt crystallizer, operating temperature controlled at 85℃, crystallizes to obtain sodium chloride product, residual liquid is returned to the inlet of multi-effect evaporator; methane gas generated by secondary anaerobic digestion is pumped to primary anaerobic digestion as stripping gas, methane and hydrogen sulfide complex gas generated by primary anaerobic digestion enters incinerator as combustion gas; incinerator is mainly used to incinerate acrylonitrile concentrated wastewater, also with dry gas combustion, the generated waste gas enters flue gas desulfurization unit, the generated steam is supplied to multi-effect evaporation and crystallization salt separation system, desulfurized wastewater generated by flue gas desulfurization is returned to the inlet of molybdenum removal tank;

[0076] The Fenton oxidation unit adds hydrogen peroxide at a mass ratio of 3:1 to wastewater COD, specifically 460 mg / L. Ferrous chloride is added at a mass ratio of 0.6:1 to iron to hydrogen peroxide, translating to an iron molar concentration of 4.7 mmol / L. The pH is adjusted to 3.5 by adding sulfuric acid, ultimately reducing the COD from 153 mg / L to 41 mg / L. The effluent then enters a nanofiltration system; the nanofiltration permeability is 88%, with iron and molybdenum ion rejection rates both exceeding 98%. The nanofiltration concentrate enters a molybdenum removal tank, with a calculated iron-molybdenum molar ratio of 5.9. The nanofiltration permeate then enters a neutralization tank, where sodium hydroxide is added to adjust the pH to 6.5. The final effluent has a COD of 38 mg / L, a salt content of 1340 mg / L, and a molybdenum ion concentration below 0.1 mg / L, meeting the requirements for reclaimed water quality.

[0077] As can be seen from this embodiment, the process of the present invention can effectively treat acrylonitrile flue gas desulfurization wastewater. Electrodialysis with modified anion exchange membrane as the core efficiently achieves the separation of organic matter and salt and the concentration of salt. Most of the organic matter is retained in the desalination liquid, and most of the salt is concentrated in the concentrate, which is beneficial to subsequent anaerobic treatment. The salt is eventually converted into sodium sulfate and sodium chloride, and most of the organic matter is eventually converted into methane and used as fuel for resource recovery. Due to the special characteristics of acrylonitrile flue gas desulfurization wastewater, some organic matter is difficult to treat biochemically. The present invention uses Fenton advanced oxidation to further treat the effluent from the two-stage anaerobic biological treatment. The trivalent iron produced by Fenton oxidation is intercepted by nanofiltration and returned to the molybdenum removal tank at the front end of the process. By adjusting the pH to a specific range, molybdenum can be efficiently removed, which greatly saves reagent consumption. After nanofiltration treatment, the COD and salt content of the effluent from Fenton oxidation are very low, and it can be reused after simple pH adjustment.

[0078] Example 2

[0079] use Figure 2 The process shown is for treating acrylonitrile flue gas desulfurization wastewater.

[0080] The wastewater from the desulfurization of acrylonitrile flue gas has the following characteristics: COD 800 mg / L, sulfate 11000 mg / L, chloride concentration 4000 mg / L, calcium ion concentration 150 mg / L, magnesium ion concentration 100 mg / L, molybdenum ion concentration 100 mg / L, total salt content 23500 mg / L, and pH 5.

[0081] The process route and implementation steps for treating this wastewater in this embodiment are the same as in Example 1. The electrodialysis in the process route is the same as in Example 1, being a modified electrodialysis method. The anion exchange membrane is a modified anion exchange membrane, and the cation exchange membrane is a general-purpose cation exchange membrane (Hefei Kaijie Polymer Co., Ltd., China, model CJ-MC-3). In the preparation of the modified anion exchange membrane, except that the linear polymer in step a is polyethylene, the β-cyclodextrin mass-volume concentration is 60 mg / ml, the volume ratio of chloromethyl ether to solvent in step b is 1.4:1, and the stirring reaction time is 12 h, everything else is the same as in Example 1.

[0082] The specific operating parameters for treating acrylonitrile flue gas desulfurization wastewater are as follows:

[0083] Wastewater first enters the molybdenum removal tank, where it is mixed with nanofiltration concentrate and sulfuric acid is added to adjust the pH to 4.2, resulting in a large amount of precipitate. After settling, the molybdenum ion concentration in the effluent decreases to below 1 mg / L, and then it enters the equalization and hardening tank. The equalization and hardening tank is prepared by adding chemicals at a concentration of 250 mg / L sodium hydroxide, 300 mg / L sodium carbonate, and 10 mg / L PAM, raising the solution pH to 8.4. After clarification, the calcium ion concentration in the supernatant decreases to below 10 mg / L, and the magnesium ion concentration decreases to below 5 mg / L. The effluent is then filtered through a tubular microfiltration system before entering electrodialysis. The chemical sludge produced in the molybdenum removal tank, equalization and hardening tank, and tubular microfiltration system enters the sludge thickening, dewatering, and drying unit. The dewatered water is returned to the inlet of the pretreatment section, and the dried sludge is sent off-site.

[0084] The effluent from the tubular microfiltration system enters the electrodialysis process. The salt content of the effluent from the tubular microfiltration system is approximately 24.2 g / L, and the COD is 755 mg / L. The electrodialysis treatment time is 0.7 h, and the current density is 35 mA / cm³. 2 After treatment, the electrodialysis concentrate had a salt content of approximately 92.9 g / L and a COD of 177 mg / L, while the desalinated liquid had a salt content of approximately 4.8 g / L and a COD of 918 mg / L. The permeate yield (percentage of desalinated liquid to total influent) was 78%, the sulfate ion selective permeation rate was 82.1%, the chloride ion selective permeation rate was 87.5%, and the organic matter rejection rate was 94.8%. The electrodialysis desalinated liquid entered the primary anaerobic digester, where dissolved oxygen was controlled below 0.15 mg / L, pH was maintained at 5.6 by adding sulfuric acid, the wastewater retention time was 8 hours, the temperature was 30℃, and the bacterial strains were organic acidifying bacteria and sulfate-reducing bacteria. The effluent COD decreased to 733 mg / L, and the salt content decreased to 2766 mg / L. The secondary anaerobic digester maintained dissolved oxygen below 0.1 mg / L, pH was maintained at 8 by adding sodium hydroxide, and the wastewater retention time was 10 hours. The temperature was 35℃, the bacteria were methanogens, the effluent COD was 195mg / L, and the salt content was 2640mg / L. The electrodialysis concentrate entered the multi-effect evaporator, where the salt content was further concentrated to 253g / L, and then entered the nitrate crystallizer. The operating temperature was controlled at 110℃ to crystallize and obtain sodium sulfate. The effluent entered the salt crystallizer, where the operating temperature was controlled at 88℃ to crystallize and obtain sodium chloride. The residual liquid was returned to the inlet of the multi-effect evaporator. The methane gas generated by the secondary anaerobic digester was pumped to the primary anaerobic digester as stripping gas. The methane and hydrogen sulfide composite gas generated by the primary anaerobic digester entered the incinerator as combustion gas. The incinerator was mainly used to incinerate acrylonitrile concentrated wastewater, and also had dry gas combustion. The generated exhaust gas entered the flue gas desulfurization unit, and the generated steam was supplied to the multi-effect evaporation and crystallization salt separation system. The desulfurized wastewater generated by the flue gas desulfurization was returned to the inlet of the molybdenum removal tank.

[0085] The Fenton oxidation unit adds hydrogen peroxide at a mass ratio of 2.8:1 to wastewater COD, specifically 546 mg / L. Ferrous chloride is added at a mass ratio of 0.8:1 to iron, translating to an iron molar concentration of 7.8 mmol / L. The pH is adjusted to 3.6 by adding sulfuric acid, ultimately reducing the COD from 195 mg / L to 61 mg / L. The effluent then enters a nanofiltration system; the nanofiltration permeate rate is 86%, with iron and molybdenum ion rejection rates both exceeding 98%. The nanofiltration concentrate enters a molybdenum removal tank, with a calculated iron-molybdenum molar ratio of 5.7. The nanofiltration permeate then enters a neutralization tank, where sodium hydroxide is added to adjust the pH to 6.6. The final effluent has a COD of 57 mg / L, a salt content of 1412 mg / L, and a molybdenum ion concentration below 0.1 mg / L, meeting the requirements for reclaimed water quality.

[0086] As can be seen from this embodiment, the present invention can effectively treat acrylonitrile flue gas desulfurization wastewater of different concentrations, and the treatment process realizes the resource utilization of water, gas and solids.

[0087] Comparative Example 1

[0088] The water quality of the flue gas desulfurization wastewater was the same as in Example 1, and the process route and implementation steps were also the same as in Example 1. The difference was that the anion and cation exchange membranes used in the electrodialysis were both general-purpose membranes. The anion exchange membrane was a product of Asahi Glass Co., Ltd. (SELEMION AMV) from Japan, and the cation exchange membrane was a product of Hefei Kaijie Polymer Co., Ltd. (model CJ-MC-3) from China. No modification treatment was performed.

[0089] Specifically, consistent with Example 1, the wastewater, after passing through a molybdenum removal tank, a hardening and equalization tank, and tubular microfiltration, enters electrodialysis. The salt content of the tubular microfiltration effluent is approximately 15.8 g / L, and the COD is 552 mg / L. The electrodialysis treatment time is 0.5 h, and the current density is 30 mA / cm². 2After treatment, the concentrated electrodialysis water had a salt content of approximately 54.9 g / L and a COD of 412 mg / L, while the desalinated liquid had a salt content of approximately 6.5 g / L and a COD of 587 mg / L. The permeability (percentage of desalinated liquid to total influent) was 80%, the sulfate ion selective permeability was 65.5%, the chloride ion selective permeability was 73.5%, and the organic matter rejection rate was 85.2%. The electrodialysis desalinated liquid entered the primary anaerobic digester, with dissolved oxygen controlled below 0.15 mg / L. The pH was maintained at 5.5 by adding sulfuric acid. The wastewater retention time was 10 h, the temperature was 30℃, and the bacterial strain was organic acid. The use of oxidizing bacteria and sulfate-reducing bacteria reduced the effluent COD to 455 mg / L and the salt content to 3932 mg / L. In the secondary anaerobic process, dissolved oxygen was controlled below 0.1 mg / L, pH was maintained at 8 by adding sodium hydroxide, the wastewater retention time was 16 h, the temperature was 35℃, and the bacteria were methanogens. The effluent COD was 322 mg / L. The COD removal rate of this unit was only 29.2%, while the secondary anaerobic process in Example 2 had a COD removal rate of 70.3%. The effluent salt content of this unit was 3570 mg / L, while the effluent salt content of the secondary anaerobic process in Example 2 was 2380 mg / L.

[0090] This comparative example demonstrates that electrodialysis using unmodified cation and anion exchange membranes has poor ability to separate organic matter and salts. Furthermore, the anion exchange membrane is easily fouled by organic matter, ultimately leading to a COD concentration in the concentrate reaching 412 mg / L, severely impacting the purity of subsequent salt products and resulting in an excessively high salt concentration in the desalinated solution (6.5 g / L). Subsequent primary anaerobic treatment significantly increased the primary anaerobic retention time compared to Example 1, but the effluent salt content remained as high as 3932 mg / L. This also affected the biological activity of methanogenic bacteria in the secondary anaerobic environment. Even with the increased secondary anaerobic retention time, the final effluent COD and salt concentrations remained high, significantly increasing the pressure on subsequent Fenton oxidation and nanofiltration treatments, potentially leading to substandard effluent and failure to meet reclaimed water quality requirements.

[0091] Comparative Example 2

[0092] The water quality of the flue gas desulfurization wastewater treated is the same as in Example 1, and the process route and implementation steps are also basically the same as in Example 1. The difference is that the two-stage anaerobic process is changed to a single-stage anaerobic process.

[0093] Specifically, as in Example 1, the wastewater was sequentially treated by a molybdenum removal tank, a hardening and equalization tank, a tubular microfiltration system, and electrodialysis. The electrodialysis desalinated water then entered a single-stage anaerobic digester. The influent salt content was 4.1 g / L, and the COD was 656 mg / L. The dissolved oxygen in the single-stage anaerobic digester was controlled below 0.15 mg / L, the pH was adjusted to 7, and the temperature was 33°C. The bacterial strains included organic acidifying bacteria, sulfate-reducing bacteria, and methanogenic bacteria. The wastewater retention time was extended to 16 hours. The methane and hydrogen sulfide mixture produced during the anaerobic reaction was pumped out and transferred to an incinerator. The final effluent COD was reduced to 426 mg / L, and the salt content was reduced to 3235 mg / L, significantly higher than the COD (153 mg / L) and salt content (2380 mg / L) of the two-stage anaerobic digester in Example 2.

[0094] This comparative example shows that the treatment efficiency of single-stage anaerobic digestion is much lower than that of two-stage anaerobic digestion. This is because two-stage anaerobic digestion achieves the separation and efficient operation of the acid-sulfate reducing phase and the methanogenic phase through the adjustment of pH, temperature, bacterial species, and stripping gas, resulting in a significant decrease in the salt content and COD of the effluent. In contrast, with single-stage anaerobic digestion, the acid-sulfate reducing phase and the methanogenic phase interfere with each other, and the reaction pH and temperature are not optimal. In particular, the methanogenic bacteria have a weak tolerance to hydrogen sulfide, which ultimately leads to a decrease in COD and sulfate treatment efficiency. As this deteriorates over time, it eventually leads to the collapse of the reaction system.

[0095] Comparative Example 3

[0096] The water quality of the flue gas desulfurization wastewater treated is the same as in Example 1, and the process route and implementation steps are also basically the same as in Example 1. The difference is that there is no nanofiltration unit after the Fenton oxidation unit.

[0097] Specifically, as in Example 1, the wastewater, after being treated in a molybdenum removal tank, a hardening and equalization tank, and a tubular microfiltration system, enters the electrodialysis system. The electrodialysis permeate rate (the percentage of desalinated water to the total influent) is 80%. The desalinated water undergoes two stages of anaerobic treatment before entering the Fenton oxidation unit. Assuming the Fenton oxidation unit is consistent with Example 1, hydrogen peroxide is added at a mass ratio of 3:1 to wastewater COD, specifically at a dosage of 460 mg / L. Ferrous chloride is added at a mass ratio of 0.6:1 to ferric peroxide, translating to an ferric molar concentration of 4.7 mmol / L. The pH is adjusted to 3.5 by adding sulfuric acid, and finally, the Fenton oxidation process is completed. The effluent has a COD of 41 mg / L and a salt content of 3380 mg / L. The effluent contains a large amount of ferric ions. In order to effectively utilize ferric ions for front-end molybdenum removal, the Fenton oxidation effluent needs to be partially recirculated. The recirculation percentage is set at 50%. The final water volume at water balance is 166.7% of the initial water volume, and the iron-molybdenum molar ratio in the molybdenum removal tank is reduced to 3, resulting in a decrease in molybdenum removal efficiency. The Fenton oxidation effluent enters the neutralization tank, and sodium hydroxide is added to adjust the pH to 6.5. In addition to the salt content rising to 3550 mg / L, a large amount of ferric hydroxide precipitate is also produced, which does not meet the requirements for salt content and suspended solids of reclaimed water.

[0098] In Example 1, after nanofiltration was used, the calculated water volume at which water balance was reached was 110.6% of the initial water volume, under the condition that the nanofiltration water production rate was 88%.

[0099] As can be seen from this comparative example, compared with Example 1, when the nanofiltration unit is not used, in order to effectively utilize the iron ions oxidized by Fenton, an effluent recirculation is required, which ultimately increases the overall treatment capacity by 50.7%. This results in a significant increase in energy consumption of each unit, a decrease in the molybdenum removal efficiency of the molybdenum removal tank, and ultimately, the effluent from the neutralization tank does not meet the standards.

Claims

1. A process for zero-discharge treatment of acrylonitrile flue gas desulfurization wastewater, including a pretreatment section, a main reaction section, and a deep treatment section; The pretreatment section includes a molybdenum removal unit, a hardening and conditioning unit, a tubular microfiltration unit, and a sludge thickening, dewatering, and drying unit. Wastewater first enters the molybdenum removal unit to remove molybdenum, and the effluent enters the hardening and conditioning unit to remove hardness. After being filtered by the tubular microfiltration unit, it enters the electrodialysis unit of the main reaction section. The chemical sludge generated by the molybdenum removal unit, the hardening and conditioning unit, and the tubular microfiltration unit enters the sludge thickening, dewatering, and drying unit. The water removed is returned to the inlet of the pretreatment section, and the dried sludge is sent out. The main reaction section includes an electrodialysis unit, a primary anaerobic unit, a secondary anaerobic unit, an incineration boiler unit, a flue gas desulfurization unit, a multi-effect evaporation unit, and a crystallization and salt separation system. The concentrated liquid from the electrodialysis unit sequentially enters the multi-effect evaporation unit and the crystallization and salt separation system, while the desalinated liquid sequentially enters the primary anaerobic unit and the secondary anaerobic unit. The primary anaerobic unit mainly performs organic acidification and sulfate reduction, producing hydrogen sulfide gas. The secondary anaerobic unit mainly performs methanogenesis, and the generated methane is pumped to the primary anaerobic unit as stripping gas, carrying most of the hydrogen sulfide generated in the primary anaerobic unit out of the wastewater system. The mixed gas of methane and hydrogen sulfide generated in the primary anaerobic unit enters the incineration boiler as combustion gas. The incineration boiler unit is mainly used to incinerate acrylonitrile concentrated wastewater, and the generated exhaust gas enters the flue gas desulfurization unit. The generated steam is supplied to the multi-effect evaporation unit and the crystallization and salt separation system. The desulfurized wastewater generated by the flue gas desulfurization unit is returned to the inlet of the pretreatment section. The advanced treatment section includes a Fenton oxidation unit, a nanofiltration unit, and a neutralization unit in sequence. The effluent from the secondary anaerobic unit passes through the Fenton oxidation unit and the nanofiltration unit in sequence. The concentrate from the nanofiltration unit is returned to the molybdenum removal unit, where it is mixed to remove molybdenum. The permeate from the nanofiltration unit enters the neutralization unit, where it is adjusted to neutrality with alkali and then discharged to meet the reuse standards. in, The electrodialysis unit is composed of anion and cation exchange membranes, wherein the anion exchange membrane is a modified anion exchange membrane and the cation exchange membrane is a general-purpose cation exchange membrane; The modification methods for anion exchange membranes are as follows: Step a: Dissolve the linear polymer in an organic solvent, then add styrene, divinylbenzene, dioctyl phthalate, benzoyl peroxide, and a hydrophilic modifier, respectively, and stir to obtain a polymer solution. The hydrophilic modifier is selected from one or more of α-cyclodextrin, β-cyclodextrin, or γ-cyclodextrin; the linear polymer is selected from at least one of polyethylene, polypropylene, polyvinyl chloride, or polyvinylidene fluoride. Step b: Add organic solvent to the polymer solution obtained in step a, add anhydrous zinc chloride and chloromethyl methyl ether, stir to react, then add precipitant, and then filter, dry and pulverize to obtain powdered chloromethylated polymer; Step c: Dissolve the polymer obtained in step b in N,N-dimethylformamide, introduce trimethylamine gas, and carry out a quaternization reaction to obtain a quaternized polymer solution; prepare a membrane from the quaternized polymer solution, immerse the membrane in sodium hydroxide solution for alkalization, and then wash it with water until neutral to obtain an anion exchange membrane. Step d: Dissolve dopamine in Tris-HCl buffer, adjust the pH with hydrochloric acid to obtain dopamine Tris buffer solution; Step e: Place the anion exchange membrane obtained in step c in a dopamine Tris buffer solution, add copper sulfate, and stir the reaction while maintaining air circulation. After the reaction is complete, the modified anion exchange membrane is obtained.

2. The process method according to claim 1, characterized in that, In the modification process of the anion exchange membrane, the mass-volume concentration of the linear polymer in step a in the organic solvent is 30-100 mg / mL; the organic solvent is selected from at least one of dichloromethane, dichloroethane, or chloroform, and the mass-volume concentrations of styrene, divinylbenzene, dioctyl phthalate, benzoyl peroxide, and hydrophilic modifier in the organic solvent are 20-100 mg / mL, 20-100 mg / mL, 5-20 mg / mL, 2-10 mg / mL, and 10-80 mg / mL, respectively.

3. The process method according to claim 1, characterized in that, In the modification process of the anion exchange membrane, the temperature of the stirring reaction in step a is 50-90℃ and the stirring reaction time is 0.5-6h; the temperature of the stirring reaction in step b is 30-55℃ and the stirring reaction time is 2-24h.

4. The process method according to claim 1, characterized in that, In the modification process of the anion exchange membrane, the amount of organic solvent added in step b is 0.5 to 1 times the amount of organic solvent used in step a; the mass-volume concentration of the anhydrous zinc chloride and the organic solvent is 15 to 30 mg / mL, and the volume ratio of chloromethyl ether to the organic solvent is 0.5:1 to 2:1; the precipitant is methanol and / or ethanol.

5. The process method according to claim 1, characterized in that, In the modification process of the anion exchange membrane, the mass-volume concentration of the polymer and N,N-dimethylformamide in step c is 15-40 mg / mL; the trimethylamine gas is obtained by heating and vaporizing a trimethylamine aqueous solution and then drying it with an alkaline drying agent; the alkaline drying agent is selected from at least one of potassium hydroxide, sodium hydroxide, quicklime and soda lime.

6. The process method according to claim 1, characterized in that, In the modification process of the anion exchange membrane, the quaternization reaction time in step c is 10-120 minutes; the quaternization polymer solution is prepared by casting or casting, and then dried at a temperature of 50-70°C for 12-24 hours.

7. The process method according to claim 1, characterized in that, During the modification of the anion exchange membrane, the mass concentration of dopamine in the solution formed in step d is 0.1–3 g / L, and the pH range of the hydrochloric acid is adjusted to 8–9.

8. The process method according to claim 1, characterized in that, In the modification process of the anion exchange membrane, copper sulfate is added in step e to a concentration of 1-20 mmol / L, and the reaction is stirred for 1-12 h; the finally obtained modified anion exchange membrane is stored in a sodium chloride solution with a mass concentration of 5-20 g / L.

9. The process method according to claim 1, characterized in that, The molybdenum removal agent in the molybdenum removal unit is ferric ions in the NF concentrated water, the pH is controlled at 3-5, and the iron-molybdenum molar ratio is controlled at 2:1-9:

1.

10. The process method according to claim 1, characterized in that, The hardness removal agent for adjusting the hardness removal unit is a combined agent of sodium hydroxide, sodium carbonate and PAM. Sodium carbonate is added at 1-3 times the mass concentration of calcium ions. The dosage of sodium hydroxide is greater than the mass concentration of magnesium ions. The dosage of PAM is 1-20 mg / L. At the same time, sodium hydroxide is also used as a pH regulator, and the pH value should be greater than 8 after addition.

11. The process method according to claim 1, characterized in that, The electrodialysis treatment time is 0.2–3 hours, and the current density is 1–80 mA / cm². 2 .

12. The process method according to claim 1, characterized in that, The multi-effect evaporation unit is heated by steam to increase the salt content of the concentrated liquid to 200-300 g / L, and then enters the nitrate crystallizer and the salt crystallizer to separate salts by using the difference in solubility of sodium chloride and sodium sulfate in terms of temperature. The temperature of the nitrate crystallizer is controlled at 100-130 °C, the temperature of the salt crystallizer is controlled at 70-90 °C, and the residual liquid is refluxed to the inlet of the multi-effect evaporation.

13. The process method according to claim 1, characterized in that, The dissolved oxygen in the first-stage anaerobic unit is controlled below 0.2 mg / L, the wastewater residence time is 1-12 h, the temperature is 25-35 °C, the pH is controlled at 5-6 by adding acid, and the strains are organic acidifying bacteria and sulfate-reducing bacteria.

14. The process method according to claim 1, characterized in that, The dissolved oxygen in the second-stage anaerobic unit is controlled below 0.1 mg / L, the wastewater residence time is 2-24 h, the temperature is 30-40 °C, the pH is controlled at 7-9 by adding alkali, and the strains are methane-producing bacteria.

15. The process method according to claim 1, characterized in that, The flue gas desulfurization unit is a flue gas treatment facility supporting the incineration boiler, using the limestone / lime-gypsum method, and the flue gas desulfurization wastewater generated is recycled to the inlet of the pretreatment section.

16. The process method according to claim 1, characterized in that, In the Fenton oxidation unit, hydrogen peroxide is added according to the mass ratio of hydrogen peroxide to wastewater COD of 1-10:1, the iron addition amount is added according to the mass ratio of iron to hydrogen peroxide of 0.2-1:1, and the pH is 3-5. The iron is divalent iron, and the ferrous salt is selected from at least one of ferrous chloride, ferrous nitrate and ferrous sulfate.

17. The process method according to claim 1, characterized in that, The water production rate of the nanofiltration unit is 60%-95%. The pH is adjusted in the neutralization unit by adding one or more of sodium hydroxide, calcium hydroxide or potassium hydroxide to adjust the pH to 6-9 to meet the requirements of the recycled water quality.

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