CN111170517A

CN111170517A - Treatment process and treatment system for desulfurization wastewater

Info

Publication number: CN111170517A
Application number: CN201811340935.9A
Authority: CN
Inventors: 钟振成; 熊日华; 程子洪; 陈权; 佟振伟; 段亚威; 于双恩; 卫昶
Original assignee: China Energy Investment Corp Ltd; National Institute of Clean and Low Carbon Energy
Current assignee: China Energy Investment Corp Ltd; National Institute of Clean and Low Carbon Energy
Priority date: 2018-11-12
Filing date: 2018-11-12
Publication date: 2020-05-19

Abstract

The invention relates to the field of desulfurization wastewater treatment, and discloses a desulfurization wastewater treatment method and a desulfurization wastewater treatment system, wherein the treatment method comprises the following steps: (1) in the presence of sodium hydroxide, carrying out softening and clarifying treatment on the desulfurization wastewater to obtain softened and clarified effluent; (2) adding sodium sulfate into the neutral softened clear effluent and crystallizing at normal temperature to obtain normal temperature crystallized effluent; (3) adding sodium carbonate into the normal-temperature crystallization effluent to obtain hardness-controlled effluent and calcium carbonate; (4) carrying out nanofiltration separation treatment on the hardness-regulated effluent; (5) performing electrodialysis-reverse osmosis coupling concentration treatment on nanofiltration produced water; (6) and (4) carrying out diaphragm electrolysis on the membrane concentrated water. The treatment method provided by the invention can fully protect the operation of the membrane system, can avoid the addition of extra alkaline agents, and realizes the effective coupling of self-sufficiency in the system and the whole treatment process.

Description

Treatment process and treatment system for desulfurization wastewater

Technical Field

The invention relates to the field of desulfurization wastewater treatment, in particular to a desulfurization wastewater treatment method and a desulfurization wastewater treatment system.

Background

A plurality of flue gas desulfurization systems adopted at home and abroad are limestone-gypsum wet flue gas desulfurization processes, which are large-scale commercialized desulfurization methods in the world, and have the advantages of mature technology, relatively reliable operation, high desulfurization efficiency and good adaptability to coal types. In the desulfurization process, a certain amount of wastewater must be discharged regularly, so as to maintain the balance of the materials of a slurry circulating system of a desulfurization device, prevent the concentration of chlorine in flue gas from exceeding a specified value and ensure the quality of gypsum. The wastewater mainly comes from a flushing water system, gypsum dehydration and the like, and the desulfurization wastewater is generally acidic and has the characteristics of high salt content, high suspended matter content, heavy metal content and large water quality fluctuation. Such as direct discharge, will severely affect the surrounding environment.

The zero discharge of the waste water of the power plant is a water using mode with high water saving level of the power plant, and has good social and environmental benefits. With the rapid development of economy and electric power in China, in areas with more coal and less water in the north of China, the available amount of water resources is reduced day by day, and the water price and the pollution discharge cost are increased continuously, so that zero discharge of waste water of a power plant is necessary and inevitable.

CN205653287U discloses a device of desulfurization waste water zero release processing includes: a remove magnesium and remove the heavy pond group, the intercommunication remove magnesium and remove a calcium sedimentation pond group of heavy pond group, the intercommunication remove a nanofiltration system (it includes a sulfate dense water export and a chlorine salt fresh water export, the sulfate dense water export through a dense water return line with remove calcium sedimentation pond group intercommunication), with the chlorine salt fresh water export through the multistage reverse osmosis system of a concentrated conveying pipeline intercommunication, with an evaporative crystallizer of multistage reverse osmosis system's a dense water export intercommunication. The device can carry out the preliminary treatment to desulfurization waste water, makes desulfurization waste water accord with membrane separation technical requirement to reduce operation and treatment cost by a wide margin.

CN104355473A discloses a method for carrying out desalination zero-emission treatment on power plant desulfurization wastewater by using an electrodialysis technology, wherein the power plant desulfurization wastewater is subjected to pretreatment such as neutralization, precipitation, coagulation, filtration and the like to remove COD (chemical oxygen demand), heavy metals, fluoride ions and the like in the wastewater; then, separating monovalent salt and divalent salt in the wastewater by using nanofiltration; and then desalting and concentrating nanofiltration produced water by utilizing multi-stage countercurrent reverse-flow electrodialysis, and evaporating and concentrating the electrodialysis concentrated water to obtain NaCl salt.

CN103979729A discloses a system and a method for recycling and zero discharge of desulfurization waste water, wherein the desulfurization waste water enters a nanofiltration system after being filtered, concentrated water of the nanofiltration system returns to a desulfurization tower, nanofiltration fresh water is concentrated by a brine concentration device and then is evaporated and crystallized, the obtained fresh water can be recycled, salt is separated out and dried into a crystal salt product, and therefore the zero discharge of the desulfurization waste water is realized, the quality of the recovered fresh water is high, and the whole process saves chemical reagents and operating cost.

CN104478141A discloses a power plant flue gas desulfurization wastewater treatment process, wherein desulfurization wastewater is firstly filtered by a plate-and-frame filter and is filtered by a micropore to obtain clear filtrate without suspended matters; secondly, concentrating the clear filtrate by using an electrodialysis membrane module with pH adjustment, and recycling the concentrated fresh water; and thirdly, performing microporous filtration on the mixture in the concentration chamber, recovering filter residues, and allowing filtrate to enter a calcium sulfate crystallization device for crystallization to separate out calcium sulfate crystals.

CN105174580A discloses a desulfurization waste water zero release processing system, and waste water gets into full-automatic softening filter, ultrafiltration, one-level RO and second grade RO system after neutralization equalizing basin, coagulating sedimentation tank in proper order, and the product water is as the clean water retrieval and utilization, and dense water gets into the salt manufacturing in the evaporation crystallizer. And the zero emission treatment of the desulfurization wastewater is realized through the combination of the membrane system.

CN105110538A discloses a desulfurization wastewater zero-discharge treatment method, wherein desulfurization wastewater is pretreated and then directly treated by an electrodialysis system, concentrated water is directly subjected to furnace-spraying incineration or evaporation, fresh water is treated by a reverse osmosis system, reverse osmosis produced water is directly recycled, and concentrated water is returned to the electrodialysis system for treatment. The invention adopts the 'pretreatment + membrane integration technology' to treat the desulfurization wastewater, so that most of water resources are recycled, and the environmental pollution is reduced.

CN105254104A discloses a low-cost power plant desulfurization wastewater zero-discharge treatment process, which mainly comprises a pretreatment process and an evaporative crystallization process. In the pretreatment process, lime and sodium sulfate are used for reaction in the first-stage reaction, sodium carbonate is used for complete softening in the second-stage reaction, the obtained wastewater enters a plate heat exchanger for temperature rise after pH adjustment, then enters an evaporator for evaporation and crystallization, and crystal slurry is subjected to crystallization and separation.

Aiming at the water quality characteristics of the desulfurization wastewater, the zero-emission treatment technology generally adopts two or more technologies of pretreatment, salt separation, membrane concentration, evaporative crystallization and the like to be integrated and combined, and the patent documents relate to the technologies. Through comparison, the subsequent treatment by utilizing the membrane technology is involved, the hardness in the wastewater is completely removed by adopting softening modes such as sodium carbonate, carbon dioxide flue gas or ion resin and the like in the pretreatment process, the running cost of sodium hydroxide, sodium carbonate, resin and the like used in the softening process is very high, and the development of the zero-emission technology is limited. The concentration degree and the reduction degree are different in the subsequent membrane treatment process, and the zero emission treatment is limited to be popularized.

The desulfurization wastewater not only has the characteristics of high suspended matters and heavy metals and is acidic, but also contains high-concentration chloride ions, calcium ions and sulfate ions. Therefore, after the conventional triple-box process is only utilized to adjust the pH value and remove suspended matters and heavy metals, the high-concentration salt-containing wastewater cannot meet the discharge requirement, and a zero-discharge process for recycling and reducing the wastewater is realized. As mentioned above, the recycling and reduction processes are mostly performed by using membrane technology, and in the using process of the membrane technology, the concentration and supersaturation of scaling factors such as calcium ions, magnesium ions and silicon which are easy to cause pollution to membrane elements are rapidly increased after membrane concentration, so that scaling is easily caused on the surface of a membrane concentration system to block the membrane elements, and further, the operation and maintenance costs of the process system are increased. Therefore, the removal of pollution factors such as calcium, magnesium, silicon and the like is important in the reduction and recycling process by using the membrane method. And the power plant desulfurization wastewater contains high-concentration calcium ions and magnesium ions, and scaling influence exists on a membrane system, a water path system and the like in the treatment process. In the conventional treatment process, calcium ions, magnesium ions, silicon and the like are mainly treated by the technologies of chemical precipitation, flue gas precipitation, electrochemical adsorption, resin softening and the like, so that the influence of the existence of the pollution factors on the system is reduced. However, the process flow is long, the operation is complicated, and the medicament cost is high in the operation process.

Therefore, the development of a low-cost and high-resource-recycling desulfurization wastewater treatment method and system has important practical significance and market application value.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention aims to provide a method and a system for treating desulfurization wastewater.

According to a first aspect of the present invention, there is provided a method for treating desulfurization waste water, comprising:

(1) in the presence of sodium hydroxide, carrying out softening and clarifying treatment on the desulfurization wastewater to obtain softened and clarified effluent, and adding acid into the softened and clarified effluent to obtain neutral softened and clarified effluent;

(2) adding sodium sulfate into the neutral softened clarified effluent and crystallizing at normal temperature to obtain normal-temperature crystallized effluent and calcium sulfate product salt;

(3) adding sodium carbonate into the normal-temperature crystallization effluent to obtain hardness-controlled effluent and calcium carbonate, wherein the addition amount of the sodium carbonate enables the concentration of calcium ions in the hardness-controlled effluent to be not higher than 8 mmol/L;

(4) performing nanofiltration separation treatment on the hardness-regulated effluent to obtain nanofiltration product water and nanofiltration concentrated water, and returning the nanofiltration concentrated water to the step (2);

(5) performing electrodialysis-reverse osmosis coupling concentration treatment on the nanofiltration produced water to obtain electrodialysis concentrated water and reverse osmosis produced water;

wherein when the salt content of the nanofiltration produced water is more than or equal to 10g/L, the step (5) comprises the following steps:

5-1: performing electrodialysis treatment on the nanofiltration water to obtain electrodialysis water and electrodialysis concentrated water;

5-2: carrying out reverse osmosis treatment on the obtained electrodialysis produced water to obtain reverse osmosis concentrated water and reverse osmosis produced water, and returning the reverse osmosis concentrated water to carry out electrodialysis treatment; or

When the salt content of the nanofiltration produced water is less than 10g/L, the step (5) comprises the following steps:

5-1: carrying out reverse osmosis treatment on the nanofiltration produced water to obtain reverse osmosis produced water and reverse osmosis concentrated water;

5-2: carrying out electrodialysis treatment on the obtained reverse osmosis concentrated water to obtain electrodialysis concentrated water and electrodialysis produced water, and returning the electrodialysis produced water to carry out reverse osmosis treatment;

(6) diaphragm electrolysis is carried out on the membrane concentrated water to obtain chlorine, a sodium hydroxide aqueous solution and diaphragm electrolysis fresh water, and the sodium hydroxide aqueous solution is returned to the step (1);

optionally, step (5) further comprises: before the electrodialysis-reverse osmosis coupling concentration treatment, the nanofiltration water production is subjected to ion exchange treatment, so that the concentration of calcium ions in the water production is not higher than 0.1 mmol/L.

According to a second aspect of the present invention, there is provided a system for treating desulfurization waste water, comprising: softening and clarifying treatment unit, normal temperature crystallization unit, sodium carbonate reaction and clarification unit, nanofiltration separation unit and membrane concentration unit and diaphragm electrolysis unit;

the softening and clarifying treatment unit is used for softening and clarifying the desulfurization wastewater to obtain softened and clarified effluent;

the normal-temperature crystallization unit is used for adding sodium sulfate into neutral softened and clarified effluent obtained after the softened and clarified effluent is subjected to acid pH adjustment and carrying out normal-temperature crystallization treatment to obtain normal-temperature crystallized effluent and calcium sulfate product salt;

the sodium carbonate reaction clarification unit is used for adding sodium carbonate into the normal-temperature crystallization effluent for reaction to obtain hardness-regulated effluent and calcium carbonate product salt;

the nanofiltration separation unit is used for carrying out nanofiltration separation treatment on the hardness-regulated effluent to obtain nanofiltration product water and nanofiltration concentrated water, and returning the nanofiltration concentrated water to the normal-temperature crystallization unit;

the membrane concentration unit comprises an electrodialysis-reverse osmosis coupling concentration unit and an optional ion exchange unit; the electrodialysis-reverse osmosis coupling concentration unit is used for treating the nanofiltration produced water to obtain electrodialysis concentrated water and reverse osmosis produced water; the electrodialysis-reverse osmosis coupling concentration unit comprises an electrodialysis unit and a reverse osmosis unit;

wherein when the salt content of the nanofiltration water is more than or equal to 10g/L,

the electrodialysis unit is used for carrying out electrodialysis treatment on the nanofiltration water to obtain electrodialysis water and electrodialysis concentrated water; the reverse osmosis unit is used for performing reverse osmosis treatment on the obtained electrodialysis produced water to obtain reverse osmosis concentrated water and reverse osmosis produced water, and returning the obtained reverse osmosis concentrated water to the electrodialysis unit; or

When the salt content of the nanofiltration produced water is less than 10g/L,

the reverse osmosis unit is used for performing reverse osmosis treatment on the nanofiltration produced water to obtain reverse osmosis produced water and reverse osmosis concentrated water; the electrodialysis unit is used for carrying out electrodialysis treatment on the obtained reverse osmosis concentrated water to obtain electrodialysis concentrated water and electrodialysis water, and returning the electrodialysis water to the reverse dialysis unit;

the ion exchange unit is used for enabling the nanofiltration produced water to enter the electrodialysis-reverse osmosis coupling concentration unit for ion exchange so as to further reduce the hardness of the nanofiltration produced water;

and the diaphragm electrolysis unit is used for carrying out diaphragm electrolysis on the electrodialysis concentrated water to obtain chlorine and a sodium hydroxide aqueous solution, and returning the obtained sodium hydroxide aqueous solution to the softening and clarifying treatment unit.

The treatment method provided by the invention adopts a normal temperature crystallization-nanofiltration technology, and firstly adopts low-cost sodium sulfate to replace sodium carbonate to preliminarily regulate and control the hardness and crystallize at normal temperature; the balance of sulfate radicals and calcium ions in water can be destroyed by adding a small amount of sodium carbonate, so that the supersaturation degree of calcium sulfate in wastewater is rapidly reduced, the operation of a subsequent membrane system is fully protected, and the problem of membrane scaling is avoided. The water produced by nanofiltration is further subjected to reduction concentration by electrodialysis-reverse osmosis coupling concentration treatment, the concentration of the concentrated salt can be increased to over 180g/L, high-purity chlorine and sodium hydroxide aqueous solution can be recovered by diaphragm electrolysis, the concentration of the sodium hydroxide aqueous solution is higher, the sodium hydroxide aqueous solution returns to pretreatment for cyclic utilization, so that the addition of extra alkaline agents (such as calcium hydroxide) can be avoided, the self-sufficiency in the system can be realized, the dosage of sodium carbonate for softening corresponding to the sodium hydroxide is greatly reduced, and the effective coupling of the whole treatment process is realized. Moreover, the treatment method of the invention can simultaneously obtain a plurality of byproducts (chlorine, calcium sulfate, magnesium hydroxide, calcium carbonate and the like) with high added values.

In a preferred embodiment, the nanofiltration water production is completely softened by ion exchange before the electrodialysis-reverse osmosis coupling concentration treatment, so that the operation stability of the membrane can be improved; and compared with the method of directly using sodium carbonate to completely soften, the method is more favorable for reducing the operation cost. The regeneration of the resin can be completed in the system, the low-concentration sodium chloride solution after the diaphragm electrolysis is used for regeneration, the water containing calcium, magnesium and other ions after regeneration is returned to the pretreatment working section for continuous treatment, and the process flow is greatly simplified.

In particular, the advantages of the present invention compared to existing methods are:

(1) in view of the operation cost, the sodium sulfate with lower cost is utilized to replace high-cost sodium carbonate to regulate and control calcium ions during normal-temperature crystallization treatment instead of completely removing the calcium ions, so that the operation cost can be greatly reduced; a small amount of sodium carbonate is added to regulate the supersaturation degree of calcium sulfate, so that the pollution of a membrane system is avoided, the service life of the membrane is prolonged, the membrane consumption cost is reduced, the final high-salt water content is reduced to be below 10%, and the energy consumption required by the electrolysis of the membrane is greatly saved;

(2) in consideration of the whole process flow, the incomplete softening is carried out through hardness regulation and control treatment, and further, the treatment is combined with nanofiltration system treatment and membrane concentration treatment, so that the concentration of sodium chloride can be finally concentrated to be more than 180g/L, and the scale, investment and energy consumption of diaphragm electrolysis are saved; the sufficient reduction means that the amount of reusable water is increased and the amount of water in the high energy consumption treatment process is reduced;

in addition, the inventors of the present invention have also unexpectedly found that when the electrodialysis feed water is subjected to electrodialysis treatment at a salt concentration (TDS) of 10g/L or more, the energy consumption in the membrane concentration process can be significantly reduced. Therefore, in the treatment method, the nanofiltration water production with the salt concentration of more than 10g/L is firstly subjected to electrodialysis treatment, or the nanofiltration water production with the salt concentration of less than 10g/L is firstly subjected to reverse osmosis concentration, and the electrodialysis-reverse osmosis coupling concentration treatment of performing the electrodialysis treatment on the reverse osmosis concentrated water with high salt concentration is favorable for further reducing the operation cost of the device;

(3) in consideration of equipment investment, the sodium carbonate reaction tank is combined with the normal-temperature crystallization unit in the treatment system provided by the invention, and compared with the conventional reaction tank and a clarification tank, the reaction time and the retention time are greatly shortened, so that the floor area of the system is saved, and the operation is simple and convenient;

the electrodialysis-reverse osmosis coupling concentration unit is used for concentration and reduction, so that the terminal treatment water quantity is greatly reduced, and the scale and the investment of equipment in the subsequent evaporation concentration section are greatly reduced;

in the preferred embodiment, the ion exchange can reduce the scaling factors such as calcium, magnesium and the like, and improve the stability of the system operation; the stability of a subsequent ionic membrane electrolysis system is particularly facilitated, and the problems of polar plate damage, current efficiency reduction and running cost increase caused by system scaling are avoided;

(4) in view of the treatment of the strong brine, the treatment is carried out by utilizing a diaphragm electrolysis method, and the obtained sodium hydroxide can be returned to a pretreatment system to realize sodium circulation, so that the addition amount of an additional medicament is greatly reduced; on the other hand, the chlorine byproduct obtained by electrolysis has higher added value and can be used as a medicament of a circulating water sterilization system of a power plant.

Additional features and advantages of the invention will be set forth in the detailed description which follows.

Drawings

FIG. 1 is a schematic diagram of one embodiment of a treatment process according to the present invention.

FIG. 2 is a schematic view of another embodiment of the treatment process of the present invention.

Detailed Description

The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value, and such ranges or values should be understood to encompass values close to those ranges or values. For ranges of values, between the endpoints of each of the ranges and the individual points, and between the individual points may be combined with each other to give one or more new ranges of values, and these ranges of values should be considered as specifically disclosed herein.

optionally, step (5) further comprises: before the electrodialysis-reverse osmosis coupling concentration treatment, the nanofiltration water production is subjected to ion exchange treatment, so that the concentration of calcium ions in the water production after ion exchange is not higher than 0.1 mmol/L.

The desulfurization waste water treated by the present invention is not particularly limited, and it is well known in the art that the desulfurization waste water may be waste water from a limestone-gypsum wet desulfurization system in which suspended substances, heavy metal ions, chloride ions, calcium ions, magnesium ions, sulfate ions, silicon, and other impurities are main components, and the waste water is acidic.

According to one embodiment, the desulfurization waste water has a pH of 4-6.5, a TDS value of 20000-40000mg/L, a conductivity of 20-35mS/cm, a calcium ion content of 400-6000mg/L, a magnesium ion content of 500-5000mg/L, a sodium ion content of 200-5000mg/L, a chloride ion content of 5000-20000mg/L, a sulfate ion content of 500-15000mg/L, a turbidity of 4000-15000NTU, a basicity of 0.2-50mg/L, and an ammonia nitrogen content of 10-200 mg/L.

In the treatment method of the present invention, in the step (1), preferably, the acid is sulfuric acid, and the pH of the neutral softened clear effluent is 6 to 8, more preferably 7 to 8. In the step, sulfuric acid is added into the softened and clarified effluent to adjust the pH value to be neutral, and the added sulfate ions react with calcium ions in water to generate a small amount of calcium sulfate crystals, so that the reaction in a subsequent normal-temperature crystallization reactor is facilitated.

In order to obtain a high-purity magnesium hydroxide product, preferably, in step (1), the softening and clarifying treatment is performed in two stages, specifically including:

1-1: adjusting the pH value of the desulfurization wastewater to 7-8 by using a sodium hydroxide aqueous solution, reacting for 45-80min, and then settling for 80-150min to obtain first-stage clarified effluent;

1-2: and adjusting the pH value of the first-stage clarified effluent to 11-12 by using an aqueous solution of sodium hydroxide, reacting for 45-80min, and then settling for 80-150min to obtain magnesium hydroxide and the softened clarified effluent.

More preferably, step 1-1 further comprises: while adding sodium hydroxide to the desulfurized wastewater, an organic sulfur, a coagulant aid and optionally a flocculant are added thereto. The dosage of the organic sulfur can be 5-150mg/L, preferably 5-80 mg/L; the dosage of the flocculating agent can be 0-30mg/L, and the dosage of the coagulant aid can be 3-10 mg/L.

In the step 1-1, pollutants such as heavy metal ions, silicon and the like in water can be removed by firstly adjusting the pH to 7-8 and carrying out reaction and sedimentation.

Optionally, step 1-2 further comprises: while adding sodium hydroxide to the desulfurization waste water, a coagulant aid was added thereto. The coagulant aid can be used in an amount of 0-10 mg/L.

In the step 1-2, the pH is adjusted to 11-12, reaction and sedimentation are carried out, and high-purity magnesium hydroxide (the purity is more than or equal to 98%) can be obtained through separation, so that the concentration of magnesium ions can be reduced to below 10mg/L through the step.

The organic sulfur, the flocculant and the coagulant aid of the present invention are not particularly limited, and may be various organic sulfur, flocculant and coagulant aid, respectively, which are commonly used in the art. Preferably, the organic sulfur is at least one of TMT-15, TMT-55 and DTC; the flocculating agent is at least one of polyaluminium sulfate, polyferric chloride, ferric chloride and aluminium sulfate; the coagulant aid is polyacrylamide.

In the treatment method, in the step (2), sodium sulfate is added into the softened and clarified effluent and the returned nanofiltration concentrated water, and normal-temperature crystallization treatment is carried out, the added sulfate radicals react with calcium ions to generate calcium sulfate, the solubility of the calcium sulfate in the wastewater is low, and the calcium sulfate can be crystallized and separated out under the normal-temperature condition to obtain calcium sulfate product salt.

In the step (2), the addition amount of the sodium sulfate ensures that the concentration of calcium ions in the normal-temperature crystallization water is preferably 9-15 mmol/L. Through the treatment, the supersaturation degree of the calcium sulfate in the obtained normal-temperature crystallized effluent water is 100-120%.

In the treatment method, in the step (3), the normal-temperature crystallized effluent reacts with sodium carbonate, carbonate in the normal-temperature crystallized effluent reacts with calcium ions quickly to generate calcium carbonate precipitate, so that the balance of sulfate radicals and calcium ions in water is broken quickly, and the supersaturation degree of calcium sulfate is reduced. In the process, calcium ions in the wastewater do not need to be completely removed. Wherein the addition amount of the sodium carbonate enables the concentration of calcium ions in the hardness-regulated water to be not higher than 8mmol/L, and preferably 3-6 mmol/L.

In addition, in the step (3), sodium carbonate is added, so that the supersaturation degree of calcium sulfate in the obtained hardness-regulated effluent can be reduced to be below 100%. Preferably, the step may further include: returning the obtained calcium carbonate to a desulfurizing tower for desulfurization reaction.

In the treatment method of the present invention, in the step (4), preferably, the nanofiltration separation treatment is performed in the presence of a scale inhibitor, and the operating pressure of the nanofiltration separation treatment is 0.5 to 2 MPa. The pressures mentioned in the present invention are gauge pressures.

In the treatment method, the flow of nanofiltration water is preferably controlled to be 60-85% of the flow of nanofiltration inlet water. The low flow of nanofiltration concentrated water can cause high membrane operation pressure, easy scaling and high concentration of divalent ions in produced water, thus affecting the quality of the produced water; too high results in low system efficiency, increased equipment size and investment. In the nanofiltration concentrated water, the supersaturation degree of calcium sulfate is more than 200%.

In the treatment method, the step (2) of returning the nanofiltration concentrated water is to mix the nanofiltration concentrated water with neutral softened clarified effluent, add sodium sulfate and crystallize at normal temperature; or sodium sulfate is added into the two streams respectively to carry out normal temperature crystallization, and the former is preferably adopted.

In the treatment method of the present invention, the nanofiltration membrane element used in the nanofiltration separation treatment is required to have a lower rejection rate of monovalent salt and a higher rejection rate of divalent salt so as to better realize the high-efficiency separation of monovalent salt and divalent salt and obtain a higher water recovery rate, preferably, the nanofiltration membrane element used in the nanofiltration separation treatment is a nanofiltration membrane element having a rejection rate of more than 98% for sulfate ions in the nanofiltration influent water and a rejection rate of more than 95% for calcium ions in the nanofiltration influent water, and may be, for example, a GE DL series nanofiltration membrane element, a GE SWSR series nanofiltration membrane, a GE DK series nanofiltration membrane element or a NE8040-40 nanofiltration membrane element of the korean TCK company.

The type and amount of the scale inhibitor are not particularly limited, and can be selected by referring to the prior art. Preferably, the scale inhibitor is selected from an organic phosphine type scale inhibitor, an organic phosphonate type scale inhibitor, a polycarboxylic acid type scale inhibitor and a composite type scale inhibitor. The composite scale inhibitor is a scale inhibitor containing more than two effective components, for example, two or three of organic phosphine, organic phosphonate and polycarboxylic acid can be combined to be used as the scale inhibitor. In the composite scale inhibitor, the content of each effective component can be selected according to the type of the effective component, and is not particularly limited. The addition amount of the scale inhibitor can be 2-20mg/L, and preferably 3-10 mg/L.

In addition, in order to further remove suspended matters in the desulfurization wastewater, the method of the present invention may further include subjecting the hardness-controlled effluent to sand filtration and ultrafiltration before the nanofiltration separation, and the methods of sand filtration and ultrafiltration are well known in the art and will not be described herein again.

In the treatment method of the present invention, the step (5) can be divided into two embodiments depending on whether the step includes the ion exchange treatment. In particular, the amount of the solvent to be used,

according to a first embodiment, as shown in fig. 1, step (5) comprises: and directly carrying out electrodialysis-reverse osmosis coupling concentration treatment on the nanofiltration produced water to obtain electrodialysis concentrated water and reverse osmosis produced water.

According to a second embodiment, as shown in fig. 2, step (5) comprises: and carrying out ion exchange on the nanofiltration produced water to reduce the calcium ion concentration of the produced water to be not higher than 0.1mmol/L, and then carrying out electrodialysis-reverse osmosis coupling concentration treatment.

In the treatment method, the salt in the nanofiltration produced water is mainly sodium chloride. As will be understood by those skilled in the art, the nanofiltration effluent is mainly chloride ions and sodium ions, so that the salt content in the nanofiltration effluent is related to the quality of the desulfurization wastewater, the desulfurization wastewater is discharged by controlling the chloride ion index in the desulfurization tower, and when the desulfurization wastewater with the above composition is treated, the treatment method is generally such that the salt content of the nanofiltration effluent is more than or equal to 10 g/L. Although the salt content of the nanofiltration produced water is usually more than or equal to 10g/L, the invention is not limited to the method, when the salt content of the nanofiltration produced water is less than 10g/L by the desulfurization wastewater treated by the invention, the salt content of the produced water is firstly concentrated to more than 10g/L by the reverse osmosis, and then the electrodialysis treatment is carried out, so that the compatibility of the water quality fluctuation of the whole process system can be improved.

In the step (5), the operating pressure of the reverse osmosis treatment is preferably 2-5 MPa. Further preferably, the reverse osmosis treatment is carried out so that the recovery rate of the obtained reverse osmosis produced water is 50-75%, and the salt content of the obtained reverse osmosis concentrated water is 50-70 g/L.

In the step (5), it is preferable that the conditions of the electrodialysis treatment include: the current density is 30-35mA/cm²The current is 120-150A, and the voltage is 100-120V.

In the treatment method, in the step (5), the water produced by nanofiltration can be concentrated to more than 180g/L by electrodialysis, and the water produced by electrodialysis is desalted to a certain concentration (less than 10 g/L); the reverse osmosis concentrates the brine with certain concentration (less than 10g/L) and controls certain recovery rate, so that the brine and the brine both operate under optimized conditions, and the operating pressure and the energy consumption are optimized. Wherein, the outlet water of the electrodialysis fresh water chamber (namely electrodialysis produced water) is used as reverse osmosis inlet water, the reverse osmosis concentrated water can be concentrated to 50-70g/L through reverse osmosis treatment, and the concentrated water is continuously subjected to electrodialysis treatment; and the reverse osmosis produced water can reach the standard of recycled water (the salt concentration is less than or equal to 1g/L), and is preferably returned to a factory for recycling. The water produced by nanofiltration is deeply concentrated by electrodialysis-reverse osmosis coupling, so that the problem of high energy consumption caused by separate electrodialysis or reverse osmosis deep concentration can be effectively solved.

In the treatment method of the present invention, in the step (6), the operation conditions of the membrane electrolysis include: 2-4kA/m²The voltage is 30-100V, preferably 30-50V.

In the treatment method of the invention, the diaphragm electrolysis fresh water (also called low-concentration brine, which refers to sodium chloride brine) refers to a solution obtained after dechlorination of an anode region in the diaphragm electrolysis process, and the salt content in the solution is preferably controlled to be 100-150 g/L. Preferably, step (6) further comprises: and returning at least part of the membrane electrolyzed fresh water to the electrodialysis system to continue the concentration treatment.

In the second embodiment shown in fig. 2, the preferable step (6) further includes: returning part of the diaphragm electrolysis fresh water to the ion exchange for resin regeneration to obtain ion exchange regenerated water, and returning the rest of the diaphragm electrolysis fresh water to perform electrodialysis treatment; returning the ion exchange regenerant water to step (1); preferably, the flow ratio of the membrane electrolytic fresh water returning to the electrodialysis treatment and the membrane electrolytic fresh water returning to the ion exchange is 5-15: 1.

according to a second aspect of the present invention, there is provided a system for treating desulfurization waste water, comprising: the device comprises a softening and clarifying treatment unit, a normal-temperature crystallization unit, a sodium carbonate reaction and clarification unit, a nanofiltration separation unit, a membrane concentration unit and a diaphragm electrolysis unit;

When the salt content of the nanofiltration produced water is less than 10g/L,

the diaphragm electrolysis unit is used for carrying out diaphragm electrolysis on the electrodialysis concentrated water to obtain chlorine, a sodium hydroxide water solution and diaphragm electrolysis fresh water, and returning the obtained sodium hydroxide water solution to the softening and clarifying treatment unit.

The treatment method according to the first aspect of the present invention may be performed by using the treatment system of the present invention.

In the treatment system of the present invention, according to a preferred embodiment, the softening and clarifying treatment unit includes a primary neutralization reaction tank, a primary clarifying tank, a secondary neutralization reaction tank, and a secondary clarifying tank;

the primary neutralization reaction tank is used for adding sodium hydroxide into the desulfurization wastewater for reaction to obtain a primary reaction product; the primary clarifying tank is used for settling a primary reaction product to obtain primary clarified effluent;

the second-stage neutralization reaction tank is used for continuously adding sodium hydroxide into the first-stage clarified water for reaction to obtain a second-stage reaction product; and the secondary clarifying tank is used for settling a secondary reaction product to obtain magnesium hydroxide and softened and clarified effluent.

In the treatment system, the sodium carbonate reaction clarification unit comprises a sodium carbonate reaction tank and a clarification tank, wherein the sodium carbonate reaction tank is used for adding a small amount of sodium carbonate into the normal-temperature crystallized effluent for reaction, and the clarification tank is used for settling the obtained reaction product to obtain hardness-regulated effluent and calcium carbonate product salt. The sodium carbonate reaction tank is usually provided with stirring equipment.

In the treatment system, the nanofiltration separation unit comprises at least one nanofiltration membrane element, and the nanofiltration membrane element has a retention rate of more than 98% for sulfate ions in the hardness-regulated outlet water and a retention rate of more than 95% for calcium ions in the nanofiltration inlet water, and can be, for example, a GE DL series nanofiltration membrane element, a GE SWSR series nanofiltration membrane element, a GEDK series nanofiltration membrane element or a NE8040-40 nanofiltration membrane element of Korea TCK company. Further preferably, the nanofiltration separation unit comprises at least two nanofiltration membrane elements used in series.

In the treatment system of the present invention, the ion exchange unit includes: a water inlet pump, ion exchange resin, a regenerative pump and the like.

The reverse osmosis unit and the electrodialysis unit are not particularly limited in the present invention, and may be selected from those commonly used in the art. In general, in the treatment system of the present invention, the reverse osmosis unit may include at least one reverse osmosis membrane element, preferably at least two reverse osmosis membrane elements used in series, and the electrodialysis unit may include a three-stage hydraulic series electrodialysis membrane stack and corresponding auxiliary systems, which are well known to those skilled in the art and will not be described herein.

In the treatment system of the present invention, the membrane electrolysis unit may be selected from membrane electrolysis cells commonly used in the salt membrane electrolysis soda production technology, and the membrane electrolysis cell generally comprises: a direct current stabilized voltage supply, a cation selective ion membrane, a chlorine gas recovery device and the like. Wherein the cation selective membrane can be cation selective ion membrane such as Asahi glass, Asahi chemical, and Home-made Dongye. And electrolyzing to obtain a chlorine product at the anode of the diaphragm electrolysis unit and obtain a sodium hydroxide solution at the cathode.

According to a preferred embodiment, the membrane concentration unit comprises the ion exchange unit, in which case the membrane electrolysis unit is also adapted to: returning part of the diaphragm electrolysis fresh water to the ion exchange unit for regeneration of resin, and returning the rest of the diaphragm electrolysis fresh water to the electrodialysis unit.

The present invention will be described in detail below by way of examples, but the scope of the present invention is not limited thereby.

The following examples will explain the method of treating desulfurization waste water according to the present invention with reference to FIGS. 1 and 2.

The softening and clarifying unit comprises a volume of 20m³Primary and secondary neutralization reaction tanks and phasesShould have a volume of 40m³The first-stage clarification tank and the second-stage clarification tank;

the sodium carbonate reaction clarification unit comprises a volume of 20m³Sodium carbonate reaction tank and volume of 60m³The reaction tank is internally provided with a stirrer;

the nanofiltration separation unit comprises a first-stage two-stage nanofiltration system consisting of 6 membrane shells and 4 series-connected GE DK 8040F30 nanofiltration membrane elements arranged in the nanofiltration separation unit;

the normal temperature crystallization unit comprises a stirring device with a volume of 40m³The stainless steel container of (1);

the electrodialysis unit comprises a total membrane area of 1500m²The three-stage hydraulic series electrodialysis membrane stack and the corresponding auxiliary system;

the reverse osmosis unit is a first-stage two-stage reverse osmosis system consisting of 7 pressure containers and 4 series-connected seawater desalination reverse osmosis membrane elements assembled in the pressure containers;

the diaphragm electrolysis unit is a diaphragm electrolysis cell, wherein the cation selective ion membrane adopts an Asahi glass ion exchange membrane;

the flocculant is polyaluminium sulfate, which is purchased from Chengsheng water purification material factory in Chengsheng, Chengshen 05-11;

the coagulant aid is polyacrylamide, purchased from Nalcidae under the trademark 8103 PLUS;

the effective component of the scale inhibitor is organic phosphonate which is purchased from Nalco company and has the trade name of OSMOTREAT OSM 1035;

the power plant desulfurization wastewater comprises the following components: the pH value is 6.15, the TDS value is 27643mg/L, the conductivity is 29.6mS/cm, the calcium ion content is 1013.81mg/L, the magnesium ion content is 4722.03mg/L, the sodium ion content is 200.19mg/L, the chloride ion content is 6880.71mg/L, the sulfate ion content is 13093.41mg/L, the turbidity is 7269NTU, the alkalinity is 18mg/L, and the ammonia nitrogen content is 17.3 mg/L.

Example 1

This example will explain the method of treating desulfurization waste water with reference to FIG. 1.

(1) Adding the sodium hydroxide aqueous solution obtained in the step (6) into the 20t/h power plant desulfurization wastewater in a primary neutralization reaction tank for reaction, adjusting the pH of the wastewater to 7.8, adding 20mg/L organic sulfur TMT-15, 10mg/L flocculating agent and 5mg/L coagulant aid, reacting for 60min, conveying the obtained reaction product to a primary clarifying tank, and standing for 120min for settling to obtain primary clarified effluent;

sending the first-stage clarified effluent to a second-stage neutralization reaction tank, continuously adding the sodium hydroxide aqueous solution obtained in the step (6), adjusting the pH of the effluent to 11.5, adding 5mg/L coagulant aid, reacting for 70min, conveying the obtained reaction product to the second-stage clarification tank, staying for 120min, and settling to obtain a magnesium hydroxide product (with the purity of 98.9%) and softened clarified effluent, wherein the concentration of magnesium ions in the effluent is reduced to 8.5 mg/L;

adding sulfuric acid into the softened and clarified effluent of 20t/h, and adjusting the pH to 7.0 to obtain neutral softened and clarified effluent;

(2) mixing 20t/h neutral softened clarified effluent with nanofiltration concentrated water, feeding the mixture into a normal-temperature crystallization reactor at a flow rate of 40t/h, adding a sodium sulfate solution with a mass concentration of 20%, adding a coagulant aid of 5mg/L, reacting for 60min under a stirring condition, feeding the obtained reaction product into a clarification tank, and settling for 90min to obtain normal-temperature crystallized effluent (the concentration of calcium ions is 9.6mmol/L) and a calcium sulfate product;

(3) enabling 40t/h of normal-temperature crystallized effluent to enter a sodium carbonate reaction tank, adding 20% by mass of sodium carbonate, reacting for 30min under a stirring condition, then enabling a reaction product to enter a clarification tank for settling for 90min, separating to obtain hardness-regulated effluent (the concentration of calcium ions is 5.2mmol/L) and calcium carbonate, and returning the calcium carbonate to a desulfurization tower;

(4) supplying the water with the hardness of 40t/h to a nanofiltration separation unit, carrying out nanofiltration separation treatment in the presence of 5mg/L of scale inhibitor, wherein the operating pressure is 1.69MPa, so as to obtain nanofiltration concentrated water and nanofiltration produced water (the salt content is 12.1g/L), and the nanofiltration concentrated water and the neutral softened and clarified water enter a normal-temperature crystallization reactor together for reaction;

(5) performing electrodialysis treatment on nanofiltration water production for 20t/h to obtain electrodialysis water production and electrodialysis concentrated water, performing reverse osmosis treatment on the obtained electrodialysis water production to obtain reverse osmosis concentrated water (with salt content of 62g/L) and reverse osmosis water production, and returning the obtained reverse osmosis concentrated water to continue the electrodialysis treatment;

wherein the current density of the electrodialysis operation is 30mA/cm²Current 120A, voltage 110V;

the reverse osmosis operating pressure is 3.9MPa, and the water recovery rate is 66.7%;

the coupling treatment is carried out to obtain 1.9t/h electrodialysis concentrated water (the salt content is 211g/L) and 18.1t/h reverse osmosis produced water (the salt concentration is 0.4g/L), and the produced water is reused as recycled water;

(6) feeding the electrodialysis concentrated water at 1.9t/h into a diaphragm electrolytic cell, electrolyzing to obtain 110kg of chlorine (with the purity of 99.6%), 0.5t/h of sodium hydroxide aqueous solution (with the mass concentration of 40%) and 1.4t/h of diaphragm electrolysis fresh water (with the salt content of 126g/L), returning the sodium hydroxide aqueous solution to the step (1), and returning the diaphragm electrolysis fresh water to an electrodialysis system for continuous concentration treatment;

wherein, the diaphragm electrolysis conditions are as follows: the current density is controlled to be 3.2kA/m²And a voltage of 30V.

The results prove that the method of the embodiment can realize the reduction treatment of the desulfurization wastewater, simultaneously can recover and obtain magnesium hydroxide, calcium sulfate, calcium carbonate, sodium hydroxide and high-purity chlorine products, the electrodialysis concentrated water entering the diaphragm electrolysis unit is 9.5 percent of the dosage of the desulfurization wastewater, and 90.5 percent of reuse water is recovered. In the embodiment, compared with the case that the pH value of the calcium hydroxide aqueous solution is adjusted to be the same, the recycling of the sodium hydroxide can reduce the adding amount of calcium hydroxide into 14.8 kg/ton of water, and finally reduce the adding amount of sodium carbonate into 10.6 kg/ton of water. The energy consumption of electrodialysis was 6.0kWh/t and the energy consumption of membrane electrolysis was 4.8 kWh/t.

Example 2

This example will explain the method of treating desulfurization waste water with reference to FIG. 2.

(1) Adding the sodium hydroxide aqueous solution obtained in the step (6) into the 20t/h power plant desulfurization wastewater in a primary neutralization reaction tank for reaction, adjusting the pH of the wastewater to 7.2, adding 30mg/L organic sulfur TMT-15, 8mg/L flocculant and 5mg/L coagulant aid, reacting for 60min, conveying the obtained reaction product to a primary clarifying tank, staying for 130min, and settling to obtain primary clarified effluent;

sending the first-stage clarified effluent to a second-stage neutralization reaction tank, continuously adding the sodium hydroxide aqueous solution obtained in the step (6), adjusting the pH of the effluent to 11.6, adding 5mg/L coagulant aid, reacting for 60min, conveying the obtained reaction product to a second clarification tank, staying for 120min, and settling to obtain a magnesium hydroxide product (with the purity of 98.9%) and softened clarified effluent, wherein the concentration of magnesium ions in the effluent is reduced to 7.3 mg/L;

adding sulfuric acid into the softened and clarified effluent of 20t/h, and adjusting the pH to 7.2 to obtain neutral softened and clarified effluent;

(2) mixing 20t/h neutral softened clarified effluent with nanofiltration concentrated water, feeding the mixture into a normal-temperature crystallization reactor at a flow rate of 40t/h, adding a sodium sulfate solution with a mass concentration of 20%, adding a coagulant aid of 4mg/L, reacting for 60min under a stirring condition, feeding the obtained reaction product into a clarification tank, and settling for 90min to obtain normal-temperature crystallized effluent (the calcium ion concentration is 11.4mmol/L) and a calcium sulfate product;

(3) enabling 40t/h of normal-temperature crystallized effluent to enter a sodium carbonate reaction tank, adding 20% by mass of sodium carbonate, reacting for 30min under a stirring condition, then enabling a reaction product to enter a clarification tank for settling for 90min, separating to obtain hardness-regulated effluent (the concentration of calcium ions is 5.4mmol/L) and calcium carbonate, and returning the calcium carbonate to a desulfurization tower;

(4) supplying the water with the hardness of 40t/h to a nanofiltration separation unit, carrying out nanofiltration separation treatment in the presence of 4mg/L of scale inhibitor, wherein the operating pressure is 1.78MPa, so as to obtain nanofiltration concentrated water and nanofiltration produced water (the salt content is 16.2g/L), and the nanofiltration concentrated water and the neutral softened and clarified water enter a normal-temperature crystallization reactor together for reaction;

(5) the nanofiltration water production is supplied to an ion exchange system for treatment at 20t/h, so that the calcium ion concentration hardness of the water production is reduced to 0.03 mol/L;

then, performing electrodialysis treatment on the nanofiltration water product with reduced hardness to obtain electrodialysis water product and electrodialysis concentrated water, performing reverse osmosis treatment on the electrodialysis water product to obtain reverse osmosis concentrated water (with salt content of 68.4g/L) and reverse osmosis water product, and returning the reverse osmosis concentrated water product to continue the electrodialysis treatment;

the reverse osmosis operating pressure is 3.9MPa, and the water recovery rate is 72 percent;

the coupling treatment is carried out to obtain 2.0t/h electrodialysis concentrated water (the salt content is 201g/L) and 18t/h reverse osmosis produced water (the salt concentration is 0.4g/L), and the produced water is reused as recycled water;

(6) feeding the electrodialysis concentrated water at 2.0t/h into a diaphragm electrolytic cell, electrolyzing to obtain 113kg of chlorine (with the purity of 99.5%), 0.6t/h of sodium hydroxide aqueous solution (with the mass concentration of 40%) and 1.4t/h of diaphragm electrolysis fresh water (with the salt content of 131g/L), returning 1.2t/h of diaphragm electrolysis fresh water to an electrodialysis system for continuous concentration treatment, and returning 0.2t/h of diaphragm electrolysis fresh water to an ion exchange system for resin regeneration;

wherein, the diaphragm electrolysis conditions are as follows: the current density is controlled to be 3.0kA/m²And a voltage of 36V.

The results prove that the method can realize the reduction treatment of the desulfurization wastewater, and simultaneously can recover and obtain magnesium hydroxide, calcium sulfate, calcium carbonate, sodium hydroxide and high-purity chlorine products; the reverse osmosis concentrated water entering the diaphragm electrolysis unit accounts for 10% of the consumption of the desulfurization wastewater, and 90% of the reuse water is recovered. In addition, the recycling of sodium hydroxide in this example can reduce the addition of 14.6 kg/ton water to calcium hydroxide and finally 10.4 kg/ton water to sodium carbonate, compared with the adjustment of the same pH value of the calcium hydroxide aqueous solution. The energy consumption of electrodialysis was 6.1kWh/t and the energy consumption of membrane electrolysis was 5.2 kWh/t. In addition, compared with the embodiment 1, the embodiment realizes the complete softening of the wastewater under the condition of not additionally adding a medicament, can better ensure the stability of membrane concentration and the operation of the diaphragm electrolysis unit, is not easy to scale, and reduces the cleaning and maintenance of equipment.

Comparative example 1

The desulfurization wastewater was treated by referring to the method of example 1, except that the nanofiltration feed water was not subjected to the electrodialysis-reverse osmosis coupling concentration treatment, but to the two-stage electrodialysis treatment, and the operating conditions of each stage were: the current density of the first stage operation is 30mA/cm²Current 120A, voltage 130V, current density of 33mA/cm for second stage operation²Carrying out current 132A and voltage 210V to obtain 17.0t/h electrodialysis produced water (the salt content is 0.7g/L) and 3.0t/h electrodialysis concentrated water (the salt content is 188 g/L);

3.0t/h of electrodialysis concentrated water is sent into a diaphragm electrolytic cell for electrolysis to obtain 94kg of chlorine (the purity is 99.3%), 1.0t/h of sodium hydroxide aqueous solution (the mass concentration is 36%) and 2.0t/h of low-concentration brine (the salt content is 103g/L), and the sodium hydroxide aqueous solution is returned to the step (1).

In this comparative example, in order to obtain water production and high-concentration electrodialysis concentrate satisfying the water quality requirements, the operating voltage of the secondary electrodialysis was much higher than that of the primary electrodialysis. The operation energy consumption of the electrodialysis part is far higher than that of electrodialysis-reverse osmosis coupling concentration treatment and reaches 30 kWh/t. The reverse osmosis concentrated water entering the diaphragm electrolysis unit accounts for 15% of the consumption of the desulfurization wastewater, 85% of reuse water is recovered, and the energy consumption of diaphragm electrolysis is 8.4 kWh/t. ,

the preferred embodiments of the present invention have been described above in detail, but the present invention is not limited thereto. Within the scope of the technical idea of the invention, many simple modifications can be made to the technical solution of the invention, including combinations of various technical features in any other suitable way, and these simple modifications and combinations should also be regarded as the disclosure of the invention, and all fall within the scope of the invention.

Claims

1. A method for treating desulfurization wastewater is characterized by comprising the following steps:

2. The method according to claim 1, wherein in the step (1), the softening and clarifying treatment method comprises the following steps:

1-2: adjusting the pH value of the first-stage clarified effluent to 11-12 by using an aqueous solution of sodium hydroxide, reacting for 45-80min, and then settling for 80-150min to obtain magnesium hydroxide and the softened clarified effluent;

preferably, the acid is sulfuric acid, and the pH of the neutral softened clear effluent is 6-8.

3. The treatment method according to claim 1, wherein in the step (2), the sodium sulfate is added in an amount such that the calcium ion concentration in the normal temperature crystallization effluent is 9 to 15 mmol/L.

4. The treatment method according to claim 1, wherein in the step (4), the nanofiltration separation treatment is carried out in the presence of a scale inhibitor, and the operating pressure of the nanofiltration separation treatment is 0.5 to 2 MPa;

preferably, the flow of nanofiltration water production is controlled to be 60-85% of the nanofiltration water inlet flow.

5. The treatment method according to claim 1, wherein in the step (5), the reverse osmosis treatment is performed at an operating pressure of 2 to 5 MPa;

preferably, the reverse osmosis treatment ensures that the recovery rate of the obtained reverse osmosis produced water is 50-75 percent, and the salt content of the obtained reverse osmosis concentrated water is 50-70 g/L.

6. The treatment method as claimed in claim 1 or 5, wherein, in step (5), the conditions of the electrodialysis treatment include: the current density is 30-35mA/cm²The current is 120-150A, and the voltage is 100-120V.

7. The treatment method according to claim 1, wherein in step (6), the conditions of the membrane electrolysis include: the current density is 2-4kA/m²The voltage is 30-100V.

8. The processing method according to claim 1, wherein the step (5) comprises: before the electrodialysis-reverse osmosis coupling concentration treatment, carrying out ion exchange treatment on the nanofiltration produced water;

the step (6) further comprises: returning part of the diaphragm electrolysis fresh water to the ion exchange for resin regeneration to obtain ion exchange regenerated water, and returning the rest of the diaphragm electrolysis fresh water to perform electrodialysis treatment; returning the ion exchange regenerant water to step (1);

preferably, the flow ratio of the membrane electrolytic fresh water returning to the electrodialysis treatment and the membrane electrolytic fresh water returning to the ion exchange is 5-15: 1.

9. a system for treating desulfurization waste water, comprising: the device comprises a softening and clarifying treatment unit, a normal-temperature crystallization unit, a sodium carbonate reaction and clarification unit, a nanofiltration separation unit, a membrane concentration unit and a diaphragm electrolysis unit;

When the salt content of the nanofiltration produced water is less than 10g/L,

10. The treatment system of claim 9, wherein the softening and clarifying treatment unit comprises a primary neutralization reaction tank, a primary clarifier, a secondary neutralization reaction tank, and a secondary clarifier;

the secondary neutralization reaction tank is used for adding sodium hydroxide into the primary clarified effluent for reaction to obtain a secondary reaction product; and the secondary clarifying tank is used for settling a secondary reaction product to obtain magnesium hydroxide and softened and clarified effluent.

11. The treatment system according to claim 9, wherein the sodium carbonate reaction and clarification unit comprises a sodium carbonate reaction tank and a clarification tank, the sodium carbonate reaction tank is used for adding sodium carbonate into the normal-temperature crystallization effluent for reaction, and the clarification tank is used for settling the obtained reaction product to obtain hardness-controlled effluent and calcium carbonate product salt.

12. The treatment system of claim 9, wherein the membrane concentration unit comprises the ion exchange unit; the membrane electrolysis unit is also used for returning part of the membrane electrolysis fresh water to the ion exchange unit for regeneration of resin, and the rest of the membrane electrolysis fresh water is returned to the electrodialysis unit.