Process for treating flue gas desulfurization wastewater zero discharge

Abstract

A process method for flue gas desulfurization wastewater zero discharge treatment, comprising a pretreatment section, a concentration section and a salt separation section; the pretreatment section comprises a hardness removal adjusting tank, a heavy metal removal tank, a coagulation sedimentation tank, an aeration biochemical tank, an ozone catalytic oxidation device, a sludge concentration and dehydration unit and a sludge drying unit; the concentration section comprises a tubular microfiltration, a medium-high pressure reverse osmosis, a low pressure reverse osmosis, an electrodialysis and a biological reactor; the salt separation section comprises a nanofiltration I, a nanofiltration II, a nitrate crystallizer and a salt crystallizer; wherein the electrodialysis adopts a modified anion exchange membrane, which is modified by a polyanion modifier and beta-cyclodextrin together, so that the anti-pollution ability of the membrane is greatly increased, and the selective permeability of sulfate ions is increased; in the process method, the wastewater is not only zero discharged, but also high-purity sodium chloride and sodium sulfate are obtained, and the process method has the advantages of mild operation condition, simple operation, low energy consumption and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to a treatment method of flue gas desulfurization wastewater, and belongs to the technical field of wastewater treatment. BACKGROUND

[0002] The wastewater produced in the process of boiler flue gas wet desulfurization (limestone / lime-gypsum method) is discharged from the absorption tower. In order to maintain the material balance of the slurry circulating system of the desulfurization device, prevent the soluble part in the flue gas, i.e. the concentration of chloride ions, from exceeding the specified value, and ensure the quality of gypsum, a certain amount of wastewater must be discharged from the system. The impurities contained in the desulfurization wastewater mainly include solid suspended matter, supersaturated sulfite, sulfate, chloride, calcium and magnesium ions, a small amount of organic matter, and trace heavy metals. The desulfurization wastewater must be treated before being discharged.

[0003] Wastewater "zero discharge" means that after the industrial wastewater is reused, the salts and pollutants in the water are concentrated and crystallized in solid form to be discharged from the plant, and the wastewater is recycled without any waste liquid being discharged from the plant. Due to the high salt content of desulfurization wastewater, in the context of increasingly strict wastewater discharge, zero discharge is the future direction of development of flue gas desulfurization wastewater.

[0004] The normal zero discharge approach is concentrated water pretreatment and concentration crystallization. Membrane separation technology is a new technology that uses separation membranes as the core for separation, concentration and purification. The commonly used membrane separation technology is reverse osmosis, which separates the solute from the solvent in the solution under the action of external pressure by means of the selective interception of a semi-permeable membrane that only allows water to pass through but not other substances. By utilizing the separation characteristics of reverse osmosis membranes, dissolved salts, colloids, organic matter, bacteria, microorganisms and other impurities in water can be effectively removed. However, due to the small pore size of reverse osmosis membranes, as the wastewater continues to concentrate, more and more impurities will cause membrane clogging. Therefore, the reverse osmosis technology can only concentrate the salt to 50-100 g / L at most. Deep concentration currently uses multi-effect evaporation or MVR technology, and can further use thermal method to separate salt to produce sodium sulfate and sodium chloride products. Thermal method of salt separation involves high-temperature evaporation crystallization, medium-temperature evaporation crystallization and cooling system, which not only consumes a lot of energy, but also easily causes pipe and equipment crystallization blockage during the transformation of materials between high and low temperatures. At the same time, the purity of the products is greatly affected by the temperature parameters and discharge parameters of each crystallizer, and the overall operation is difficult.

[0005] Electrodialysis is a new technology formed by combining membrane separation with electrochemistry, and the semi-permeable membrane used in the electrodialysis is an ion exchange membrane, which realizes the separation of anions and cations under the driving of an external direct current electric field. The electrodialysis has been widely used in seawater desalination, brackish water desalination and the like due to its simple operation and long service life, and the system desalination rate can be selected in the range of 30% to 99% according to the requirement, and compared with multi-effect evaporation, the energy consumption of the electrodialysis is much lower. However, for industrial wastewater, since the water contains not only soluble inorganic salts, but also organic matters and hardness, the ion exchange membrane will be polluted and scaled in the separation and concentration process, and the service life of the membrane is affected. SUMMARY

[0006] In view of the above problems, the present application provides a flue gas desulfurization wastewater zero discharge treatment system and method, which not only realizes zero discharge of wastewater, but also obtains high-purity sodium chloride and sodium sulfate, and has the advantages of mild operating conditions, simple operation and low energy consumption.

[0007] In order to achieve the above technical purposes, the technical scheme adopted by the present application is as follows:

[0008] The present application provides a process method for flue gas desulfurization wastewater zero discharge treatment, wherein the total salt mass concentration of the flue gas desulfurization wastewater is greater than 10000 mg / L, and the COD is less than 400 mg / L.

[0009] The process method comprises a pretreatment section, a concentration section and a salt separation section.

[0010] The pretreatment section comprises a hardness removal and adjustment tank, a heavy metal removal tank, a coagulation and sedimentation tank, an aeration and biochemical tank, an ozone catalytic oxidation unit, a sludge concentration and dewatering unit and a sludge drying unit; the wastewater is first subjected to hardness removal and adjustment, heavy metal removal and coagulation and sedimentation, the effluent enters the ozone catalytic oxidation unit, the chemical sludge generated is subjected to sludge concentration and dewatering, the water discharged therefrom is returned to the inlet of the pretreatment section, and the remaining sludge is subjected to treatment by the sludge drying unit and then externally sent; the effluent of the ozone catalytic oxidation unit enters the tubular microfiltration of the concentration section.

[0011] The concentration section comprises a tubular microfiltration, a medium-high pressure reverse osmosis, a low-pressure reverse osmosis, an electrodialysis and a biological reactor; the effluent of the tubular microfiltration enters the medium-high pressure reverse osmosis, the concentrated water of the medium-high pressure reverse osmosis enters the electrodialysis, and the produced water enters the low-pressure reverse osmosis; the concentrated liquid of the electrodialysis enters the nanofiltration I of the salt separation section, and the dilution liquid enters the biological reactor; the effluent of the biological reactor enters the low-pressure reverse osmosis, the concentrated water of the low-pressure reverse osmosis enters the ozone catalytic oxidation unit of the pretreatment section, and the produced water of the low-pressure reverse osmosis enters the reuse water tank of the pretreatment section.

[0012] The salt fractionation section comprises nanofiltration I, nanofiltration II, nitrate crystallizer and salt crystallizer; the water produced by the two-stage nanofiltration is mixed and then enters the salt crystallizer, the concentrated water of the nanofiltration I enters the nanofiltration II, the concentrated water of the nanofiltration II enters the nitrate crystallizer, the mirabilite mother liquor produced by the nitrate crystallizer and the salt mother liquor produced by the salt crystallizer are returned to the electrodialysis for secondary treatment; the nitrate crystallizer produces sodium sulfate as a product, and the salt crystallizer produces sodium chloride as a product.

[0013] Further, the electrodialysis is composed of anion exchange membrane and cation exchange membrane, wherein the anion exchange membrane is a modified anion exchange membrane, and the cation exchange membrane is a general cation exchange membrane.

[0014] The modified anion exchange membrane is prepared by the following method:

[0015] Step a: polyvinyl alcohol is added to water to prepare a polyvinyl alcohol aqueous solution, then a positively charged amine compound, β-cyclodextrin and a crosslinking agent are added to the above solution, an inorganic acid is added to adjust the pH, and a casting solution is obtained after mixing and stirring reaction;

[0016] Step b: the casting solution obtained in step a is cast on a horizontal panel and dried to obtain a base film;

[0017] Step c: the base film obtained in step b is washed and then alkalinized in a lye, and then soaked in water to obtain a polyvinyl alcohol anion exchange membrane;

[0018] Step d: dopamine is dissolved in a Tris-HCl buffer solution, and the pH is adjusted to 8-9 with hydrochloric acid to obtain a dopamine Tris buffer solution;

[0019] Step e: the anion exchange membrane obtained in step c is placed in the dopamine Tris buffer solution, and copper sulfate is added, and the reaction is stirred under the condition of air flow to obtain a surface initially modified anion exchange membrane;

[0020] Step f: a polyanion modifier and sodium chloride are dissolved in a Tris-HCl buffer solution, and the pH is adjusted to 8-9 with hydrochloric acid to obtain an electrodeposition mother liquor;

[0021] Step g: the anion exchange membrane obtained in step e is placed in the middle of a direct current electrodeposition device to form two compartments, the electrodeposition mother liquor prepared in step f is placed in the cathode side compartment, and water is placed in the anode side compartment, and an electrodeposition reaction is carried out to obtain a final modified anion exchange membrane.

[0022] Further, the positively charged amine compound is one or more of 2,3-epoxypropyltrimethylammonium chloride, polyethyleneimine, polyepoxy chloropropane amine and quaternary ammonium chitosan, and is preferably 2,3-epoxypropyltrimethylammonium chloride; the mass ratio of the positively charged amine compound to polyvinyl alcohol is 0.1:1-0.6:1.

[0023] Further, the mass ratio of the β-cyclodextrin to the polyvinyl alcohol is 0.05:1-0.4:1.

[0024] Further, the crosslinking agent is selected from aldehyde or acid crosslinking agent, preferably glutaraldehyde, and the mass ratio of the crosslinking agent to the polyvinyl alcohol is 0.01:1-0.1:1.

[0025] Further, the inorganic acid in step a is sulfuric acid, hydrochloric acid or nitric acid, and the pH of the solution is adjusted to 4-6.

[0026] Further, the mass fraction of the polyvinyl alcohol aqueous solution in step a is 5%-15%, and the mixing reaction time of the positively charged amine compound, the β-cyclodextrin and the crosslinking agent is 4-16h.

[0027] Further, the polyvinyl alcohol aqueous solution is dissolved in water by stirring at 60-90℃.

[0028] Further, the drying in step b is first dried at room temperature for 2-6h, and then dried at 40-80℃ under vacuum for 4-10h.

[0029] Further, the washing in step c is washed with water until neutral.

[0030] Further, the alkalization adopts 0.5-3mol / L sodium hydroxide aqueous solution, and the alkalization time is 12-24h.

[0031] Further, the soaking time in step c is 12-24h.

[0032] Further, the concentration of the Tris-HCl buffer in steps d and f is 10-50mmol / L.

[0033] Further, the mass concentration of dopamine in step d is 0.2-3g / L.

[0034] Further, the concentration of copper sulfate in step e is 1-15mmol / L, and the stirring reaction time is 1-10h. The copper sulfate can induce rapid polymerization of dopamine, and form a negatively charged polydopamine electrolyte layer on the surface of the anion exchange membrane, thereby preliminarily modifying the membrane surface.

[0035] Further, the polyanion modifier in step f is one or more of poly(4-styrenesulfonic acid) sodium, p-styrenesulfonic acid sodium, polyvinyl sulfonic acid sodium and polypropylene sulfonic acid sodium, and preferably poly(4-styrenesulfonic acid) sodium.

[0036] Further, the mass concentration of the polyanion modifier in the electrodeposition mother liquor of step f is 0.5-5 g / L, and the mass concentration of sodium chloride is 3-30 g / L.

[0037] Further, the electrodeposition reaction time of step g is 0.2-2 h, and the current density is 1-50 mA / cm 2 The polyanion modifier is secondarily deposited on the surface of the anion exchange membrane by the electrodeposition method, so as to sulfonate and modify the membrane surface.

[0038] Further, after the modified anion exchange membrane is obtained, the modified anion exchange membrane is stored in a sodium chloride solution, and the mass concentration of the sodium chloride solution is 5-20 g / L.

[0039] Those skilled in the art should understand that most of the organic pollutants existing in natural water bodies or sewage, such as surfactants, humic acid, proteins, aromatic hydrocarbon derivatives, etc., are negatively charged, dopamine has strong self-polymerization ability and adsorption capacity, and can improve the negative charge density of the membrane surface and the stability of the membrane, so that the surface of the anion exchange membrane modified by the polydopamine is negatively charged, has electrostatic repulsion to the negatively charged organic matters in water, and can inhibit the pollution of the organic matters to the anion exchange membrane, and the surface is sulfonated and modified by the polyanion modifier, so as to reduce the roughness of the membrane surface caused by the adsorption of dopamine and further improve the negative charge density of the membrane surface. Those skilled in the art should also understand that the negative surface charge prevents the pollution of the organic matters by electrostatic action, and also affects the migration rate of inorganic anions, and the more the ion charge number, the greater the influence, so that the sulfate ion is more affected than the chloride ion. The addition of β-cyclodextrin to the anion exchange membrane can endow the membrane with strong hydrophilicity and change the migration rate of anions, in which the migration number of anions with low hydration degree such as bromide and nitrate relative to chloride is reduced, and the migration number of anions with high hydration degree such as sulfate relative to chloride is increased. On the other hand, the hydrophilicity of the membrane is improved, the van der Waals force between the membrane and the organic solute is reduced, the attraction is reduced, and at the same time, the hydrophilic membrane and water molecules form a hydration layer due to hydrogen bonding, which can further hinder the adsorption of pollutants on the membrane surface. Therefore, after the anion exchange membrane of the present application is modified by dopamine, polyanion modifier and β-cyclodextrin, the anti-pollution ability of the membrane is greatly improved, and the selective permeability of sulfate ions is increased.

[0040] Further, the hardness removal agent for adjusting the hardness removal tank is preferably a combination of sodium hydroxide and sodium carbonate, the sodium carbonate is added in an amount of 1-3 times the mass concentration of calcium ions, and the sodium hydroxide is added in an amount greater than the mass concentration of magnesium ions, and at the same time, the sodium hydroxide also serves as a pH adjuster, and after being added, the pH value should be greater than 8.5.

[0041] Further, the heavy metal removal tank adds a reagent of sodium sulfide or sodium hydrosulfide, which utilizes the principle that the solubility of sulfides of all heavy metals is lower than that of corresponding hydroxides, to further treat the heavy metals in the wastewater.

[0042] Further, the coagulation sedimentation tank adds an aluminum salt coagulant or an iron salt coagulant, and additionally adds a coagulant aid PAM; the coagulant addition amount is 10-200 mg / L, and the coagulant aid addition amount is 1-20 mg / L.

[0043] Further, the biological species of the aeration biochemical tank adopts salt-tolerant bacteria, adopts an aerobic process, and adopts one of a BAF, an MBR, a contact oxidation tank, and an MBBR process. The salt-tolerant bacteria are strains that can tolerate a salt mass concentration of 10-50 g / L and have strong tolerance to sulfides; and the salt-tolerant bacteria are preferably the salt-tolerant bacteria GXNYJ-DL-1 disclosed in CN114686391A, with a preservation number of CGMCC No. 20350.

[0044] Further, in the ozone catalytic oxidation, the ozone dosage is 0.1-2 times the required amount of oxidizing agent calculated according to the COD value of the wastewater, the reaction time is 10-120 minutes, and the catalyst is a conventional solid-supported metal catalyst.

[0045] Further, the tubular microfiltration is mainly used for filtering suspended solids, colloids, microorganisms, and the like to prevent the subsequent reverse osmosis membrane from being blocked.

[0046] Further, the medium-high pressure reverse osmosis unit is multi-stage and is composed of multiple medium-high pressure reverse osmosis membrane components, the operating pressure is between 2.0-4.2 Mpa, the salt content of the wastewater can be concentrated to 40-110 g / L, and the overall concentration ratio is 2-10; the low pressure reverse osmosis is single-stage, the operating pressure is 1.0-2.0 Mpa, and the water production rate is 50%-80%.

[0047] Further, the electrodialysis treatment time is 0.5-3 h, the current density is 1-80 mA / cm 2 .

[0048] Further, the biological reactor adopts a BAF or MBR process, which is consistent with the aeration biochemical tank, and the biological species adopts salt-tolerant bacteria.

[0049] Further, the nanofiltration has good selective permeability to chloride ions and good interception effect on sulfate ions, and the sulfate ion interception rate is greater than 90%; the nanofiltration I water production rate is 40%-75%, and the nanofiltration II water production rate is 30%-65%.

[0050] Further, the salt crystallizer operating temperature is controlled at 35-60 DEG C, and concentrated mother liquor is obtained in addition to sodium chloride product; further, when the salt crystallizer has been operated for a period of time, the concentrated mother liquor has a sodium sulfate concentration of up to 300 g / L, and is returned to the second treatment of electrodialysis, according to the solubility of sodium sulfate at 35-60 DEG C being between (40 g-49 g) / 100 g water, when the sodium sulfate concentration is lower than 300 g / L, the sodium sulfate in the salt crystallizer can be kept in a dissolved state, and the purity of the precipitated sodium chloride product is not affected, and the purity of the sodium chloride product is ensured to be greater than 99%.

[0051] Further, the nitrate crystallizer operating temperature is controlled at 40-60 DEG C; further, when the nitrate crystallizer has been operated for a period of time, the concentrated mother liquor has a sodium chloride concentration of up to 250 g / L, and is returned to the second treatment of electrodialysis, according to the solubility of sodium chloride increasing with temperature in the range of 40-60 DEG C, and the solubility of sodium sulfate decreasing with temperature, under medium temperature conditions, with salt concentration, sodium sulfate in the high sodium sulfate solution is precipitated first due to supersaturation, and the time required for sodium chloride saturation is long, in order to keep sodium chloride as an unsaturated component, when the sodium chloride concentration reaches a certain value, the concentrated solution is discharged in whole, and this method can ensure that the purity of the sodium sulfate product is always above 99%.

[0052] Compared with the prior art, the present application has the following advantages:

[0053] (1) In the pretreatment section, the present application first removes hardness and heavy metals to complete the removal of hardness and heavy metals in the flue gas desulfurization wastewater, which not only removes the obstacles for subsequent microbial biochemical reactions, but also provides a guarantee for the purity of the products in the salt separation section.

[0054] (2) In the concentration section, the modified anion exchange membrane and improved electrodialysis of the present application not only efficiently realize the concentration of salt, but also retain most of the organic matter in the dilute solution, thereby reducing the organic matter content in the salt separation section and further increasing the purity of the products in the salt separation section.

[0055] (3) In the salt separation section, since the nanofiltration membrane has a high rejection rate for sulfate, the proportion of sodium chloride in the salt crystallizer is very high, and the purity of the sodium chloride obtained by crystallization is also very high; the separation of sodium chloride by two-stage nanofiltration also greatly increases the proportion of sodium sulfate in the salt in the nitrate crystallizer, so that high-purity sodium sulfate can be crystallized.

[0056] (4) With the improved electrodialysis, reverse osmosis, nanofiltration and other membrane technologies, the separation of salt and organic matter, the separation of sodium sulfate and sodium chloride, and the crystallization of sodium sulfate and sodium chloride are all carried out under normal or medium temperature conditions, the overall operating conditions are mild, the operation is simple and easy to control, and the energy consumption advantage is outstanding.

[0057] Other features and advantages of the present application will be described in detail in the following detailed description section. BRIEF DESCRIPTION OF DRAWINGS

[0058] Figure 1 The flow chart of treating flue gas desulfurization wastewater in Example 1;

[0059] Figure 2 The schematic diagram of anion exchange membrane electrodeposition modification in Example 1. DETAILED DESCRIPTION

[0060] The present application will be further described in detail below in combination with specific examples. The examples are implemented on the premise of the technical solution of the present application, and detailed implementation modes and specific operation processes are given, but the protection scope of the present application is not limited to the following examples.

[0061] Example 1

[0062] The process flow chart of treating flue gas desulfurization wastewater is shown in Figure 1 The flue gas desulfurization wastewater first enters a conditioning and hardness removal pool, a heavy metal removal pool and a coagulation sedimentation pool in sequence, the effluent of the coagulation sedimentation pool enters an aeration biochemical pool, the chemical sludge generated in the conditioning and hardness removal pool, the heavy metal removal pool and the coagulation sedimentation pool enters a sludge concentration and dehydration unit, the discharged water is returned to the water inlet at the starting end, and the remaining sludge is externally sent after being treated by a sludge drying unit; the effluent of the aeration biochemical pool enters ozone catalytic oxidation, the effluent of the ozone catalytic oxidation is filtered by a tubular microfiltration, and then enters a medium-high pressure reverse osmosis, the produced water of the medium-high pressure reverse osmosis is treated by a low-pressure reverse osmosis, and then the produced water is discharged to a recycled water pool, and the concentrated water is returned to the ozone catalytic oxidation for secondary treatment; the concentrated water of the medium-high pressure reverse osmosis flows to an electrodialysis, the dilute liquid of the electrodialysis flows to a biological reactor, and the effluent of the biological reactor enters a low-pressure reverse osmosis; the concentrated liquid of the electrodialysis enters a two-stage nanofiltration, the produced water of the two-stage nanofiltration is mixed and then enters a salt crystallizer, the concentrated water of the nanofiltration I enters the nanofiltration II, the concentrated water of the nanofiltration II enters a sodium sulfate crystallizer, the sodium sulfate crystallizer obtains a product sodium sulfate, the salt crystallizer obtains a product sodium chloride, and the mirabilite mother liquor generated by the sodium sulfate crystallizer and the salt mother liquor generated by the salt crystallizer are returned to the electrodialysis for secondary treatment.

[0063] The flue gas desulfurization wastewater is treated by the process method of the present application.

[0064] A flue gas desulfurization wastewater has the following water quality: COD 285 mg / L, sulfate 2500 mg / L, chloride concentration 8000 mg / L, calcium ion concentration 450 mg / L, magnesium ion concentration 300 mg / L, mercury ion concentration 0.5 mg / L, chromium ion concentration 0.8 mg / L, nickel ion concentration 1.5 mg / L, cadmium ion concentration 2 mg / L, copper ion concentration 10 mg / L, total salt content 18100 mg / L, and pH 5.1, and the wastewater flow rate is 20 t / h.

[0065] The electrodialysis provided in the present embodiment is modified electrodialysis, in which the anion exchange membrane is a modified anion exchange membrane, which has strong anti-pollution ability and high ion permeability, and the cation exchange membrane is a general cation exchange membrane (CJ-MC-3, China Hefei Kaijie Polymer Co., Ltd.).

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

[0067] Step a: polyvinyl alcohol is added to deionized water to prepare a 12% mass fraction aqueous solution, which is dissolved under stirring at 86°C, and then 2,3-epoxypropyltrimethylammonium chloride, β-cyclodextrin and glutaraldehyde are added to the water, the mass ratio of 2,3-epoxypropyltrimethylammonium chloride to polyvinyl alcohol is 0.4:1, the mass ratio of β-cyclodextrin to polyvinyl alcohol is 0.25:1, and the mass ratio of glutaraldehyde to polyvinyl alcohol is 0.05:1, hydrochloric acid is used to adjust the pH to 5, and the reaction is carried out for 10 h to obtain a viscous liquid, which is a casting solution;

[0068] Step b: the casting solution is cast on a horizontal glass plate, dried at room temperature for 4 h, and then the formed membrane is further dried under vacuum at 65°C for 5 h to obtain a base membrane;

[0069] Step c: the base membrane obtained in step b is washed with deionized water until neutral, and then is alkalinized in a 1 mol / L sodium hydroxide aqueous solution for 16 h, and then is soaked in deionized water for 18 h to obtain a polyvinyl alcohol anion exchange membrane;

[0070] Step d: dopamine is dissolved in a 15 mmol / L Tris-HCl buffer solution, and hydrochloric acid is used to adjust the pH to 8.5 to obtain a dopamine Tris buffer solution, and the mass concentration of dopamine in the buffer solution is 0.4 g / L;

[0071] Step e: the polyvinyl alcohol anion exchange membrane to be modified is placed in the dopamine Tris buffer solution, and 2 mmol / L copper sulfate is added, and the reaction is carried out for 4 h under stirring and air flow, wherein the copper sulfate can induce rapid polymerization of dopamine to form a negatively charged polydopamine electrolyte layer on the surface of the anion exchange membrane, and the preliminary modified anion exchange membrane is obtained after the reaction;

[0072] Step f: a polyanion modifier and sodium chloride are dissolved in a 20 mmol / L Tris-HCl buffer solution, and hydrochloric acid is used to adjust the pH to 8.5, the mass concentration of the polyanion modifier is 1 g / L, and the mass concentration of sodium chloride is 8 g / L, and the polyanion modifier is selected to be poly 4-styrene sulfonic acid sodium, to obtain an electrodeposition mother liquor;

[0073] Step g: surface sulfonation and modification are carried out by electrodeposition, such asFigure 2 As shown, the anion exchange membrane to be modified is placed in the middle of the direct current electrodeposition device, forming two compartments, the cathode side compartment is placed into the electrodeposition mother liquor, and the anode side compartment is placed into water, the electrodeposition time is 0.5h, and the current density is 20mA / cm 2 , to obtain the final modified anion exchange membrane;

[0074] Step h: the modified anion exchange membrane prepared in step g is taken out and placed in a sodium chloride solution with a mass concentration of 10g / L for use.

[0075] The specific operation parameters for treating the flue gas desulfurization wastewater are as follows:

[0076] The wastewater first enters the conditioning and hardness removal pool, and the hardness removal reagent is added according to the mass concentration of sodium hydroxide of 450mg / L and the mass concentration of sodium carbonate of 500mg / L, the solution pH is raised to 9.1, and after clarification, the supernatant calcium ion concentration is reduced to below 10mg / L, and the magnesium ion concentration is reduced to below 5mg / L, and the effluent enters the heavy metal removal pool; sodium sulfide is added to the heavy metal removal pool, and the concentration is 70mg / L, and after clarification, the supernatant concentration of each heavy metal ion is less than 0.05mg / L; polyaluminum chloride and PAM are added to the coagulation and sedimentation pool, with mass concentrations of 100mg / L and 8mg / L respectively, and the pH is reduced to 8.2, and after clarification, the effluent enters the aeration and biochemical pool, and the chemical sludge in the coagulation and sedimentation pool enters the sludge thickening and dewatering unit together with the chemical sludge produced in the conditioning and hardness removal pool and the heavy metal removal pool, the water discharged is returned to the water inlet of the conditioning and hardness removal pool, and the remaining sludge is treated by the sludge drying unit and then sent out; the aeration and biochemical pool adopts the MBR process, the influent sulfide concentration is 25mg / L, the COD concentration is 255mg / L, and the total salt content is 18800mg / L, salt-tolerant bacteria are added, the salt-tolerant bacteria GXNYJ-DL-1 disclosed in the patent CN114686391A is selected, the preservation number is CGMCC No.20350, the residence time is 6h, the effluent COD concentration is 116mg / L, and the sulfide is 0.3mg / L, the effluent enters the ozone catalytic oxidation unit, the ozone addition concentration is 80mg / L, the reaction time is 30min, the effluent COD is 38mg / L, and the total salt content is 19150mg / L, and after being filtered by the tubular microfiltration, it enters the middle and high pressure reverse osmosis unit.

[0077] The operation pressure of the medium-high pressure reverse osmosis is between 3-4.2 Mpa, the salt content of the concentrated water is close to 100 g / L after concentration, and the concentration multiple is 5.2; the salt content of the water produced by the medium-high pressure reverse osmosis is 3-10 g / L, which enters the low pressure reverse osmosis, the low pressure reverse osmosis is a first stage, the operation pressure is 1.5 Mpa, the water production rate is 72%, the salt content of the concentrated water is greater than 10 g / L, and the COD is higher than 60 mg / L, which is returned to the ozone catalytic oxidation unit for secondary treatment, the COD of the water produced by the low pressure reverse osmosis is lower than 60 mg / L, the salt content is lower than 2500 mg / L, and the requirements for recycling are met; the concentrated water of the medium-high pressure reverse osmosis enters the electrodialysis, the salt content of the concentrated water of the medium-high pressure reverse osmosis is about 100 g / L, the COD is 170 mg / L, the treatment time of the electrodialysis is 1.2 h, and the current density is 40 mA / cm 2 After treatment, the salt content of the concentrated water of the electrodialysis is about 200 g / L, the COD is 34 mg / L, the water volume of the concentrated water accounts for 39% of the total water volume, the salt content of the diluted liquid is about 36 g / L, the COD is 257 mg / L, the water volume of the diluted liquid accounts for 61% of the total water volume, the selective permeation rate of the sulfate ion is 75.2%, the selective permeation rate of the chloride ion is 79.5%, and the interception rate of the organic matter is 92.2%; the diluted liquid of the electrodialysis enters the biological reactor, the BAF process is adopted, the salt-tolerant bacteria GXNYJ-DL-1 is added, the residence time is 6 h, the COD of the effluent is as low as 76 mg / L, the COD is less than 40 mg / L after the low pressure reverse osmosis treatment, and the salt content is lower than 2500 mg / L, which meets the requirements for recycling.

[0078] The concentrated liquid of the electrodialysis enters two-stage nanofiltration, the water production rate of the first-stage nanofiltration is 55%, the water production rate of the second-stage nanofiltration is 45%, the salt content of the mixed water produced by the two-stage nanofiltration is about 204 g / L, the proportion of chloride salt is more than 90%, the salt content of the concentrated water of the nanofiltration II is about 190 g / L, and the proportion of sulfate is more than 80%; the water produced by the two-stage nanofiltration enters the salt crystallizer, the temperature of the salt crystallizer is controlled to be 50 ℃, sodium chloride with a purity of 99.3% is obtained through crystallization, when the residual mother liquor is concentrated to a sodium sulfate mass concentration of 300 g / L, in order not to affect the purity of the product, the concentrated mother liquor is returned to the electrodialysis for secondary treatment; the concentrated water of the nanofiltration II enters the nitrate crystallizer, the temperature is controlled to be 55 ℃, sodium sulfate with a purity of 99.5% is obtained through crystallization, when the residual mother liquor is concentrated to a sodium chloride mass concentration of 250 g / L, in order not to affect the purity of the product, the concentrated mother liquor is returned to the electrodialysis for secondary treatment.

[0079] It can be seen from the embodiment that the wastewater of flue gas desulfurization can be effectively treated by the application, the electrodialysis taking the modified anion exchange membrane as the core not only realizes the concentration of salt, but also realizes the separation of organic matter and salt, and high-purity sodium chloride and sodium sulfate are prepared, and the wastewater is zero discharged. The overall operation condition is mild, the operation is simple, and the control is easy.

[0080] Example 2

[0081] UtilizeFigure 1 The process shown treats certain flue gas desulfurization wastewater.

[0082] The wastewater quality is as follows: COD 350 mg / L, sulfate 5000 mg / L, chloride concentration 12000 mg / L, calcium ion concentration 600 mg / L, magnesium ion concentration 400 mg / L, mercury ion concentration 1 mg / L, chromium ion concentration 1.5 mg / L, nickel ion concentration 2 mg / L, cadmium ion concentration 3 mg / L, copper ion concentration 15 mg / L, total salt content 28500 mg / L, pH 5.5, and wastewater flow rate 20 t / h.

[0083] The process route and implementation steps for treating the flue gas desulfurization wastewater in this example are the same as in Example 1. The electrodialysis I and electrodialysis II in the process route are also the same as in Example 1, and are both modified electrodialysis. The anion exchange membrane is a modified anion exchange membrane, and the cation exchange membrane is a general cation exchange membrane (CJ-MC-3, Hefei Kaitai Polymer Co., Ltd.). In the preparation of the modified anion exchange membrane, in step a, the positively charged amine compound is polyethyleneimine, the mass ratio of polyethyleneimine to polyvinyl alcohol is 0.45:1, the mass ratio of β-cyclodextrin to polyvinyl alcohol is 0.3:1, and the reaction time of step a is 10 h, and the other steps are the same as in Example 1.

[0084] The specific operation parameters for treating the flue gas desulfurization wastewater are as follows:

[0085] The wastewater first enters the conditioning and hardness removal tank, and a hardness removal agent is added according to a mass concentration of 550 mg / L of sodium hydroxide and a mass concentration of 650 mg / L of sodium carbonate, the solution pH is increased to 9.4, the supernatant calcium ion concentration is reduced to less than 10 mg / L, the magnesium ion concentration is reduced to less than 5 mg / L after clarification, and the effluent enters the heavy metal removal tank; sodium hydrosulfide is added to the heavy metal removal tank at a concentration of 100 mg / L, and the supernatant concentration of each heavy metal ion is less than 0.05 mg / L after clarification; polymeric ferric sulfate and PAM are added to the coagulation and sedimentation tank at mass concentrations of 120 mg / L and 10 mg / L respectively, the pH is reduced to 8.5, and the effluent after clarification enters the biological aerated filter tank; the chemical sludge in the coagulation and sedimentation tank, together with the chemical sludge produced in the conditioning and hardness removal tank and the heavy metal removal tank, enters the sludge concentration and dewatering unit, the discharged water is returned to the inlet of the conditioning and hardness removal tank, and the remaining sludge is treated by the sludge drying unit and then sent out; the biological aerated filter tank adopts the BAF process, the influent sulfide concentration is 30 mg / L, the COD concentration is 310 mg / L, the total salt content is 29100 mg / L, salt-tolerant bacteria are added, the salt-tolerant bacteria GXNYJ-DL-1 disclosed in the patent CN114686391A is selected, the preservation number is CGMCC No. 20350, the residence time is 8 h, the effluent COD concentration is 145 mg / L, the sulfide is 0.2 mg / L, the effluent enters the ozone catalytic oxidation unit, the ozone addition concentration is 90 mg / L, the reaction time is 30 min, the effluent COD is 55 mg / L, the total salt content is 29350 mg / L, and after being filtered by the tubular microfiltration, the effluent enters the medium-high pressure reverse osmosis unit.

[0086] The medium-high pressure reverse osmosis operating pressure is between 3 and 4.2 MPa, the salt content of the concentrated water is close to 100 g / L after concentration, and the concentration ratio is 3.4; the medium-high pressure reverse osmosis water salt content is 3-10 g / L, enters the low pressure reverse osmosis, the low pressure reverse osmosis is the first level, the operating pressure is 1.5 MPa, the water production rate is 71%, the concentrated water salt content is greater than 10 g / L, and the COD is higher than 60 mg / L, which is returned to the ozone catalytic oxidation unit for secondary treatment, the low pressure reverse osmosis water COD is lower than 60 mg / L, the salt content is lower than 2500 mg / L, which meets the reuse requirements; the medium-high pressure reverse osmosis concentrated water enters the electrodialysis, the medium-high pressure reverse osmosis concentrated water salt content is about 100 g / L, the COD is 188 mg / L, the electrodialysis treatment time is 1.25 h, the current density is 40 mA / cm 2, the salt content of the concentrated water of the electrodialysis is about 210 g / L, the COD is 38 mg / L, the water volume of the concentrated water accounts for 40% of the total water volume, the salt content of the diluting liquid is about 27 g / L, the COD is 288 mg / L, the water volume of the diluting liquid accounts for 60% of the total water volume, the selectivity of the sulfate ion is 80.2%, the selectivity of the chloride ion is 85.5%, and the interception rate of the organic matter is 91.9%; the diluting liquid of the electrodialysis enters a biological reactor, an MBR process is adopted, the salt-resistant bacteria GXNYJ-DL-1 is added, the residence time is 8 h, the COD of the effluent is as low as 80 mg / L, the COD is less than 40 mg / L after the effluent is treated by low-pressure reverse osmosis, and the salt content is less than 2300 mg / L, which meets the recycling requirement.

[0087] The concentrated liquid of the electrodialysis enters two-stage nanofiltration, the water yield of the first-stage nanofiltration is 55%, the water yield of the second-stage nanofiltration is 50%, the salt content of the mixed water of the two-stage nanofiltration is about 195 g / L, the proportion of the chloride salt is more than 90%, the salt content of the concentrated water of the nanofiltration II is about 260 g / L, and the proportion of the sulfate salt is more than 80%; the water of the two-stage nanofiltration enters a salt crystallizer, the temperature of the salt crystallizer is controlled at 55 ℃, sodium chloride with a purity of 99.3% is obtained by crystallization, when the residual mother liquor is concentrated to a sodium sulfate mass concentration of 300 g / L, the concentrated mother liquor is returned to the electrodialysis for secondary treatment so as not to affect the purity of the product; the concentrated water of the nanofiltration II enters a sulfate crystallizer, the temperature is controlled at 52 ℃, sodium sulfate with a purity of 99.6% is obtained by crystallization, when the residual mother liquor is concentrated to a sodium chloride mass concentration of 250 g / L, the concentrated mother liquor is returned to the electrodialysis for secondary treatment so as not to affect the purity of the product.

[0088] It can be seen from the embodiment that the application can effectively treat flue gas desulfurization wastewater with different concentrations, the wastewater finally realizes zero discharge, and high-purity sodium chloride and sodium sulfate are prepared.

[0089] Comparative Example 1

[0090] The wastewater treated in Comparative Example 1 has the same water quality as that in Example 2, the process route and implementation steps are also the same as those in Example 2, and the difference is that the anion exchange membrane and the cation exchange membrane of the electrodialysis are both general membranes, the anion exchange membrane is a product of Asahi Glass Company, Japan (SELEMION AMV), and the cation exchange membrane is a product of Hefei Kaitai Polymer Co., Ltd., China (model CJ-MC-3), and is not modified.

[0091] Specifically, the wastewater is treated by the pretreatment section and the concentration section, the concentrated water of the high-pressure reverse osmosis unit enters the electrodialysis, the salt content of the concentrated water of the medium-high-pressure reverse osmosis is about 100 g / L, the COD is 188 mg / L, the treatment time of the electrodialysis is 1.25 h, the current density is 40 mA / cm 2, the salt content of the concentrated water after treatment is about 178 g / L, the COD is 140 mg / L, the concentrated water accounts for 40% of the total amount of the incoming water, the salt content of the desalination liquid is about 48 g / L, the COD is 220 mg / L, the desalination liquid accounts for 60% of the total amount of the incoming water, the selectivity of sulfate ions is 70.5%, the selectivity of chloride ions is 72.3%, and the interception rate of organic matter is 70.2%; the desalination liquid of the electrodialysis is introduced into a biological reactor, an MBR process is adopted, salt-tolerant bacteria are added, the high-efficiency salt-tolerant bacteria GXNYJ-DL-1 in the patent CN114686391A is selected, the residence time is 8 h, the COD of the effluent is as low as 62 mg / L, and the effluent is introduced into a low-pressure reverse osmosis; the water production rate of the low-pressure reverse osmosis is 71%, the COD of the effluent after the low-pressure reverse osmosis treatment is less than 40 mg / L, and the salt content is 3570 mg / L; general industrial water such as circulating water requires that the salt content be less than 3000 mg / L, so the reverse osmosis water cannot meet the recycling requirements; the salt content of the electrodialysis concentrated liquid is relatively low, the COD is relatively high, and finally the subsequent crystallization efficiency is reduced, the purity of the sodium sulfate decahydrate is reduced to 98.9%, and the purity of the sodium chloride is reduced to 98.8%.

[0092] It can be known from the present comparative example that the electrodialysis adopts unmodified cation and anion exchange membranes, the separation ability of the organic matter and the salt is poor, the anion exchange membrane is easily polluted by the organic matter, the desalination liquid is difficult to treat to meet the standard recycling, and the purity of the sodium sulfate decahydrate and the sodium chloride prepared from the concentrated liquid is reduced.

Claims

1. A process for treating flue gas desulfurization wastewater, wherein the total salt mass concentration of the flue gas desulfurization wastewater is greater than 10000 mg / L, and the COD is less than 400 mg / L; The process comprises a pretreatment section, a concentration section and a salt separation section; The pretreatment section comprises a hardness removal and adjustment tank, a heavy metal removal tank, a coagulation and sedimentation tank, an aeration and biochemical tank, an ozone catalytic oxidation unit, a sludge concentration and dewatering unit and a sludge drying unit; The wastewater is first subjected to hardness removal and adjustment, heavy metal removal and coagulation and sedimentation, and then the effluent is subjected to ozone catalytic oxidation, chemical sludge produced in the process is subjected to sludge concentration and dewatering, the water removed is returned to the inlet of the pretreatment section, and the remaining sludge is subjected to sludge drying and then sent out; The effluent of the ozone catalytic oxidation unit is subjected to tubular microfiltration in the concentration section; The concentration section comprises a tubular microfiltration unit, a medium-high pressure reverse osmosis unit, a low pressure reverse osmosis unit, an electrodialysis unit and a biological reactor; The effluent of the tubular microfiltration unit is subjected to medium-high pressure reverse osmosis, the concentrated water of the medium-high pressure reverse osmosis unit is subjected to electrodialysis, and the produced water is subjected to low pressure reverse osmosis; The concentrated solution of the electrodialysis unit is subjected to nanofiltration I in the salt separation section, and the diluted solution is subjected to the biological reactor; The effluent of the biological reactor is subjected to low pressure reverse osmosis, the concentrated water of the low pressure reverse osmosis unit is subjected to ozone catalytic oxidation in the pretreatment section, and the produced water of the low pressure reverse osmosis unit is subjected to reuse water storage in the pretreatment section; The nitrate crystallizer produces sodium sulfate, and the salt crystallizer produces sodium chloride; The electrodialysis unit comprises anion exchange membranes and cation exchange membranes, wherein the anion exchange membranes are modified anion exchange membranes, and the cation exchange membranes are general cation exchange membranes; The modified anion exchange membranes are prepared by the following method: Step a:Polyvinyl alcohol is added to water to prepare a polyvinyl alcohol aqueous solution, and then a positively charged amine compound, β-cyclodextrin and a crosslinking agent are added to the solution, an inorganic acid is added to adjust the pH, and then the mixture is stirred and reacted to obtain a casting solution; The positively charged amine compound is one or more of 2,3-epoxypropyltrimethylammonium chloride, polyethyleneimine, polyepoxy chloropropane amine and quaternized chitosan; The fractionation section comprises nanofiltration I, nanofiltration II, a nitrate crystallizer and a salt crystallizer; the water produced by the two-stage nanofiltration is mixed and then enters the salt crystallizer, the concentrated water of the nanofiltration I enters the nanofiltration II, the concentrated water of the nanofiltration II enters the nitrate crystallizer, and the mother liquor of mirabilite produced by the nitrate crystallizer and the mother liquor of salt produced by the salt crystallizer are returned together to the electrodialysis for secondary treatment; Step b:The casting solution obtained in step a is cast on a horizontal panel and dried to obtain a base film; Step c:The base film obtained in step b is washed, alkalinized in a lye and then soaked in water to obtain a polyvinyl alcohol anion exchange membrane; Step d:Dopamine is dissolved in a Tris-HCl buffer solution, and the pH is adjusted to 8-9 with hydrochloric acid to obtain a dopamine Tris buffer solution; Step e:The anion exchange membrane obtained in step c is placed in the dopamine Tris buffer solution, copper sulfate is added, and the mixture is stirred under the condition of air flow to obtain a surface-modified anion exchange membrane; Step f:A polyanion modifier and sodium chloride are dissolved in a Tris-HCl buffer solution, and the pH is adjusted to 8-9 with hydrochloric acid to obtain an electrodeposition mother liquor; The polyanion modifier is one or more of poly(4-sodium styrene sulfonate), sodium p-styrene sulfonate, polyvinyl sulfonate and polypropylene sulfonate. ​ ​ ​ ​ Step g: the anion exchange membrane obtained in step e is placed in the middle of a direct current electrodeposition device to form two compartments, the cathode side compartment is put into the electrodeposition mother liquor prepared in step f, and the anode side compartment is put into water to carry out electrodeposition reaction to obtain the final modified anion exchange membrane.

2. The process of claim 1, wherein, The mass ratio of the positively charged amine compound to polyvinyl alcohol is 0.1:1-0.6:1, and the mass ratio of the beta-cyclodextrin to polyvinyl alcohol is 0.05:1-0.4:

1.

3. The process of claim 1, wherein, The crosslinking agent is glutaraldehyde, and the mass ratio of the crosslinking agent to polyvinyl alcohol is 0.01:1-0.1:

1.

4. The process of claim 1, wherein, In step a, the pH of the solution is adjusted to 4-6, the mass fraction of the polyvinyl alcohol aqueous solution is 5%-15%, and the mixing reaction time of the positively charged amine compound, beta-cyclodextrin and crosslinking agent is 4-16 h.

5. The process of claim 1, wherein, The alkalization is carried out by using 0.5-3 mol / L sodium hydroxide aqueous solution, and the alkalization time is 12-24 h.

6. The process of claim 1, wherein, The mass concentration of dopamine in the dopamine Tris buffer solution in step d is 0.2-3 g / L.

7. The process of claim 1, wherein, The concentration of copper sulfate in step e is 1-15 mmol / L, and the stirring reaction time is 1-10 h.

8. The process of claim 1, wherein, The mass concentration of the polyanion modifier in the electrodeposition mother liquor in step f is 0.5-5 g / L, and the mass concentration of sodium chloride is 3-30 g / L; the electrodeposition reaction time is 0.2-2 h, and the current density is 1-50 mA / cm 2 .

9. The process of claim 1, wherein, The hardness removal agent of the hardness removal tank is a combined agent of sodium hydroxide and sodium carbonate, the sodium carbonate is added in an amount of 1-3 times the mass concentration of calcium ions, and the sodium hydroxide is added in an amount greater than the mass concentration of magnesium ions; meanwhile, the sodium hydroxide also serves as a pH adjuster, and after being added, the pH value should be greater than 8.

5.

10. The process of claim 1, wherein, The de-heavy metal tank adds sodium sulfide or sodium hydrosulfide.

11. The process of claim 1, wherein, The coagulation and sedimentation tank adds an aluminum salt coagulant or an iron salt coagulant, and additionally adds a coagulant aid PAM; the aluminum salt coagulant or iron salt coagulant is added in an amount of 10-200 mg / L, and the coagulant aid is added in an amount of 1-20 mg / L.

12. The process of claim 1, wherein, The aeration biochemical tank adopts one of BAF, MBR, contact oxidation tank and MBBR aerobic process.

13. The process of claim 1, wherein, The medium-high pressure reverse osmosis is multi-stage, composed of multiple medium-high pressure reverse osmosis membrane components, and the operating pressure is between 2.0-4.2 Mpa, which concentrates the salt content of the wastewater to 40-110 g / L; the low pressure reverse osmosis is single-stage, and the operating pressure is 1.0-2.0 Mpa, and the water production rate is 50%-80%.

14. The process of claim 1, wherein, The biological reactor adopts BAF or MBR process.

15. The process of claim 1, wherein, The water production rate of nanofiltration I is 40%-75%, and the water production rate of nanofiltration II is 30%-65%.

16. The process of claim 1, wherein, The operation temperature of the salt crystallizer is controlled at 35-60℃, and sodium chloride product and concentrated mother liquor are obtained.

17. The process of claim 1, wherein, The operation temperature of the nitrate crystallizer is controlled at 40-60℃, and sodium sulfate product and concentrated mother liquor are obtained.

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