CN108117206B

CN108117206B - Zero-discharge treatment process method for salt-containing wastewater

Info

Publication number: CN108117206B
Application number: CN201611066568.9A
Authority: CN
Inventors: 陈明翔; 高会杰; 孙丹凤; 郭志华; 赵胜楠; 李宝忠
Original assignee: China Petroleum and Chemical Corp; Sinopec Fushun Research Institute of Petroleum and Petrochemicals
Current assignee: China Petroleum and Chemical Corp; Sinopec Fushun Research Institute of Petroleum and Petrochemicals
Priority date: 2016-11-28
Filing date: 2016-11-28
Publication date: 2020-05-19
Anticipated expiration: 2036-11-28
Also published as: CN108117206A

Abstract

The invention relates to a zero-discharge treatment process method for salt-containing wastewater, which comprises the following steps: (1) pre-treating; (2) reverse osmosis treatment; (3) electrodialysis concentration; (4) and (5) circulating and crystallizing. Compared with the prior art, the zero-discharge salt separation treatment process method for the salt-containing wastewater provided by the invention improves the recovery rate of salt while realizing zero discharge or near zero discharge of the wastewater in the coal chemical industry, can recover high-quality sodium sulfate, mirabilite and sodium chloride, realizes reclamation of crystallized salt, and has the advantages of stable membrane treatment unit process, long operation period, low cost and good economical efficiency of the whole process.

Description

Zero-discharge treatment process method for salt-containing wastewater

Technical Field

The invention belongs to the technical field of environmental protection, and particularly relates to a zero-discharge treatment technology for salt-containing wastewater, in particular to a zero-discharge treatment process for high-salt-containing wastewater in the coal chemical industry.

Background

Coal chemical industry is a process of converting coal, which is used as a raw material, into gas, liquid, solid fuel and chemicals through chemical processing. The coal chemical industry mainly comprises coal gasification, liquefaction, dry distillation, tar processing, calcium carbide acetylene chemical industry and the like. The coal chemical engineering project has huge water consumption and high water consumption of the produced wastewater. Most coal chemical engineering projects in China are distributed in northwest areas where water resources are short and nano sewage bodies are short. With the continuous adjustment of the national environmental protection policy, the wastewater of the coal chemical industry enterprises needs to be recycled to the maximum extent, so that zero emission is achieved.

Coal chemical wastewater can be generally divided into organic wastewater and saline wastewater. The salt-containing wastewater mainly comprises circulating sewage, chemical water station drainage and the like, and in order to recycle wastewater to the maximum extent, in general, organic wastewater enters a salt-containing wastewater treatment system after primary treatment, secondary treatment and advanced treatment, and is subjected to further desalting treatment, so that the salt-containing wastewater meets the requirement of quality of circulating water make-up water.

The recycling treatment of the salt-containing wastewater mainly comprises two technologies, one is a membrane separation technology, and the other is a thermal evaporation technology. The method for directly obtaining the reuse water from the salt-containing wastewater by adopting the thermal evaporation technology has huge energy consumption, so that the membrane separation technology is the mainstream technology in the field. The main membrane separation unit technologies include reverse osmosis, forward osmosis, bipolar membrane, dialysis, electrodialysis, microwave membrane, ultrafiltration membrane, nanofiltration membrane, biological membrane, etc., and one or a combination of membrane technologies can be generally adopted to treat wastewater from different sources. The key technical problem of the membrane technology in application is the problem of membrane pollution, and the components of the wastewater are complex, so that the membrane has great influence on the use efficiency and the service life. The prior common method is to carry out advanced pretreatment on the treated wastewater, combine different types of membrane technologies and the like, but the cost of the advanced pretreatment is greatly increased, the prior combined technology does not obtain expected effects, and particularly in the saline wastewater treatment technology in the field of coal chemical industry, the prior combined technology can obtain higher reuse water yield, but does not solve the problems of membrane pollution and membrane service life.

The aim of the salt-containing wastewater treatment technology is zero discharge of wastewater, which needs to recover salt in the wastewater in a solid form, and salt meeting the selling quality standard is difficult to obtain due to complex salt components in the wastewater. The existing method obtains more mixed salt, is difficult to sell and use, and forms solid waste which is difficult to treat. Although theoretically, the salts in the wastewater can be respectively obtained into the monosalts with higher purity by various separation and purification methods, the process is complicated, the cost is increased and the method is difficult to bear economically due to more components.

CN104016529A discloses a coal chemical industry salt-containing wastewater treatment method based on a multistage countercurrent reverse-electrode electrodialyzer, which can concentrate concentrated water by more than 10 times and improve the fresh water yield to more than 85%. The technology improves the fresh water recovery rate through ozone oxidation, multistage membrane filtration and multistage countercurrent reverse-electrode electrodialysis technology, and relieves the problem of electrodialyzer membrane pollution to a certain extent, but the pretreatment cost is high, the stability of a pretreatment membrane unit has no good solution, the electrodialyzer still contains a small amount of salt after obtaining the reuse water, the overall effect is general, and the strong brine has no further treatment scheme, and zero emission cannot be realized.

CN104230124A discloses a process and special equipment for classifying, collecting and quality-dividing treatment of coal chemical wastewater, which can improve the recovery rate of water and obtain industrial salt. However, in the process, three independently used reverse osmosis units are adopted, and pretreatment is carried out by adopting an ion exchange and other deep pretreatment modes; the membrane separation unit for separating and purifying water and organic concentrated solution is difficult to obtain ideal effect at present; the industrial salt is mixed salt and is difficult to sell or use.

CN103508602A discloses a membrane and evaporative crystallization integrated process for zero discharge of high salinity industrial wastewater, which comprises the steps of carrying out ultrafiltration pretreatment on industrial wastewater, conveying the industrial wastewater to a reverse osmosis unit through a high-pressure pump, recycling effluent at the permeation side, carrying out electrodialysis treatment on a concentrated solution after filtration for multiple times, evaporating and crystallizing a material after electrodialysis concentration to obtain salt mud and condensate water, carrying out aftertreatment on the salt mud, recycling the condensate water and recycling electrodialysis fresh water. The method simply combines reverse osmosis and electrodialysis for use, and plays a role independently, the electrodialysis fresh water needs to meet the recycling requirement, the treatment difficulty is high, the energy consumption is high, the membrane pollution is serious, the stability of the whole device is insufficient, and in addition, the obtained solid salt is still mixed salt.

CN105565569A discloses a high-salt industrial wastewater enhanced deep concentration system and a process thereof, wherein the high-salt industrial wastewater is adjusted by an adjusting tank, precipitated by a softening sedimentation tank, filtered by a V-shaped filter tank, enhanced filtered by an ultrafiltration device, concentrated by a first-stage reverse osmosis device, concentrated by an ion exchange resin device, removed by hardness, and separated by a nanofiltration device; wherein: the nanofiltration device concentrates water, the frequent electrode-reversing electrodialysis device concentrates water, the advanced oxidation device oxidizes water, the total production water tank crystallizes in a concentrated water-freezing crystallization system, and sodium sulfate crystallizes; the method comprises the steps of nanofiltration device water production, two-section reverse osmosis device concentration, frequent reversed-pole electrodialysis device reconcentration, water production, advanced oxidation device oxidation, total production water tank, concentrated water, MVR evaporative crystallization device crystallization and sodium chloride crystallization. The method combines membrane technologies such as nanofiltration, frequent reverse electrodialysis, reverse osmosis and the like, but the functions of different membrane technologies are not effectively integrated, advanced treatment such as ion exchange is needed for pretreatment, and the cost is high; although sodium chloride crystals and sodium sulfate crystals are obtained respectively, analysis and experiments prove that if the purity of the obtained crystallized salt needs to be ensured, a certain amount of high-concentration salt-containing wastewater needs to be discharged, zero emission cannot be realized, and the salt recovery rate needs to be further improved.

In summary, the treatment process of salt-containing wastewater, especially salt-containing wastewater in coal chemical industry, needs further optimization in terms of comprehensive technical effects such as wastewater recovery rate, industrial salt recycling, salt recovery rate, stable operation of membrane units, reduction of production cost, realization of zero discharge, and the like.

Disclosure of Invention

Aiming at the defects of the conventional salt-containing wastewater treatment process, the invention provides a zero-discharge treatment process method for salt-containing wastewater, which improves the recovery rate of salt while realizing zero discharge or near zero discharge of wastewater in the coal chemical industry, can recover high-quality sodium sulfate, mirabilite and sodium chloride, realizes reclamation of crystalline salt, and has the advantages of stable process of a multi-stage membrane treatment unit, long running period, low cost and good economical efficiency of the whole process.

The invention relates to a zero-discharge treatment process method for salt-containing wastewater, which comprises the following steps:

(1) and (4) preprocessing. The pretreatment comprises softening, coagulating sedimentation, rough filtration and ultrafiltration, and the pretreated effluent is subjected to reverse osmosis treatment. The pretreatment can also comprise the processes of oil removal, biochemical treatment, homogeneous and uniform treatment and the like according to the quality of the incoming water.

(2) And (5) reverse osmosis treatment. The membrane pollution resisting combined membrane process combining medium pressure reverse osmosis and high pressure reverse osmosis is adopted, pretreated effluent is firstly subjected to medium pressure reverse osmosis, produced water of the medium pressure reverse osmosis is recycled, concentrated water of the medium pressure reverse osmosis and fresh water of an electrodialysis unit are subjected to high pressure reverse osmosis together, produced water of the high pressure reverse osmosis is recycled, and concentrated water of the high pressure reverse osmosis is subjected to electrodialysis concentration. Recycle refers to the need for reuse in industrial settings or other uses.

(3) And (4) electrodialysis concentration. The high-pressure reverse osmosis concentrated water is firstly subjected to softening, coagulating sedimentation, rough filtration and ultrafiltration pretreatment, then electrodialysis is carried out for further concentration, electrodialysis fresh water is circulated to be used as high-pressure reverse osmosis inlet water, and the generated concentrated water is circulated and crystallized.

(4) And (5) circulating and crystallizing. Carrying out ozone catalytic oxidation on concentrated water generated by electrodialysis, then carrying out primary evaporation crystallization to obtain sodium sulfate and primary mother liquor, carrying out freezing crystallization on primary mother liquor to obtain mirabilite and secondary mother liquor, carrying out secondary evaporation crystallization on secondary mother liquor to obtain sodium chloride and tertiary mother liquor, and circulating crystallization from the tertiary mother liquor to the primary evaporation crystallization step.

Further, in the step (1), the softening, flocculation and precipitation processes are carried out in a high-density clarification tank, the coarse filtration process is carried out in a V-shaped filter tank, and the average pore size of the ultrafiltration membrane is 1-100 nm. The homogenizing and homogenizing process is carried out in a regulating tank.

Further, in the step (1), a dosing device is arranged at the front end of the high-density clarification tank, a softening agent and a flocculating agent are added, a dosing device is also arranged at the outlet of the high-density clarification tank, hydrochloric acid or sulfuric acid is respectively added to adjust the pH value to 6.5-7.5, and a non-oxidizing bactericide is added.

Further, in the step (1), the softener is one or more of calcium hydroxide, sodium hydroxide and sodium carbonate with different concentrations according to different water qualities of the salt-containing wastewater in the coal chemical industry; the coagulant is a composite coagulant of polyferric sulfate (PFS) and Polyacrylamide (PAM), the concentration of PFS is 5-200 mg/L, and the concentration of PAM is 0.5-12 mg/L.

Further, in the step (1), after the pretreatment process, the wastewater index is controlled to be less than 200mg/L, COD and less than 70 mg/L, and the turbidity is less than 3 NTU.

Further, in the step (2), the operating pressure of the medium-pressure reverse osmosis membrane is 1-3 MPa, the operating pressure of the high-pressure reverse osmosis membrane is 3-6 MPa, and the medium-pressure membrane and the high-pressure membrane are both filtered by cross flow. The medium-pressure membrane and the high-pressure membrane can be selected from membranes for reverse osmosis commonly used in the field.

Further, in the step (2), after the two-stage reverse osmosis membrane separation process, the TDS of the generated concentrated water reaches more than 50000 mg/L.

Further, in the step (3), the softening and flocculation precipitation processes are carried out in a high-density clarification tank, the coarse filtration process is carried out in a V-shaped filter tank, and the average pore diameter of the ultrafiltration membrane is 1-100 nm.

Further, in the step (3), a dosing device is arranged at the front end of the high-density clarification tank, a softening agent and a flocculating agent are added, a dosing device is also arranged at the outlet of the high-density clarification tank, a pH regulator is added to regulate the pH value to 6.5-7.5, and a non-oxidizing bactericide is added according to the requirement.

Further, in the step (3), the softener is one or more of calcium hydroxide, sodium hydroxide and sodium carbonate with different concentrations according to different water qualities of the high-pressure reverse osmosis concentrated water; the flocculant is a PFS and PAM composite coagulant, the concentration of PFS is 10-300 mg/L, and the concentration of PAM is 0.5-12 mg/L. The electrodialytic water inlet index is controlled to be less than 50mg/L, COD and less than 400 mg/L, and the turbidity is less than 3 NTU.

Further, in the step (3), after concentration by electrodialysis, TDS in the concentrated water will reach over 200000 mg/L. The electro-dialysis fresh water TDS is less than 25000 mg/L. Generally, the concentration is 10000 mg/L or more, preferably 15000 mg/L or more.

Further, in the step (4), the catalytic oxidation by ozone can adopt the prior art, for example, the catalyst can adopt one or more of the published patents CN105709737A, CN105709743A or CN 105709744A. COD in the ozone catalytic oxygen effluent is controlled below 500 mg/L.

Further, in the step (4), when the primary evaporation crystallization is carried out, the evaporation concentration temperature is 50-150 ℃, and the crystallization temperature is 50-100 ℃; during secondary evaporation crystallization, the temperature of evaporation concentration is 50-150 ℃, and the crystallization temperature is 30-50 ℃; the temperature of freezing crystallization is 0 to-8 ℃.

Further, in the step (4), the device used for evaporative crystallization comprises an evaporator and a crystallizer. The evaporator is any one of a natural circulation evaporator, a forced circulation evaporator, a climbing-film evaporator or a falling-film evaporator; the crystallisers are any of an Oslo crystalliser, a DP crystalliser or a variation of the above types, respectively. The evaporator and crystallizer may be prior art devices or designed according to the prior art.

Further, in the step (4), the generated sodium sulfate, mirabilite and sodium chloride are collected by a centrifugal dehydrator. The purity of the generated sodium sulfate can reach more than 95 percent, and the product quality reaches the quality standard of national standard industrial anhydrous sodium sulfate III qualified products. The concentration of the generated sodium chloride can reach more than 98 percent, and the product quality reaches the secondary quality standard of national standard refined industrial salt. The quality of the mirabilite product is more than 90%. The purity of the product is calculated by mass percent.

The method of the invention obtains the following comprehensive technical effects:

1. the combination of measures ensures stable operation of the membrane unit (reverse osmosis and electrodialysis). The reverse osmosis units with two stages and different pressures are adopted, and the electrodialysis units (still having relatively high salt content in fresh water) with specific operation modes are organically integrated into the reverse osmosis process, and the processes and conditions of the three processes are cooperatively matched, so that the long-period stable operation of the membrane units is realized while the concentration multiple and the water outlet speed are ensured, the membrane pollution problem is effectively controlled, and the membrane service life is reduced due to frequent regeneration of the membrane. When the scheme of the invention is adopted, the membrane unit can stably run for more than 3 years, and tests and simulations show that the membrane unit can only run for 1-1.5 years under the same pretreatment condition if the cooperation of the synergistic combination process and conditions is not adopted.

2. The operation cost of the pretreatment unit is low. The flow and the conditions of the membrane unit are cooperated, so that the severity requirement of the water inlet index of the membrane unit is reduced, and the operation cost of the water inlet pretreatment operation can be greatly reduced. For example, the invention can use the modes of softening, flocculation precipitation, rough filtration, ultrafiltration and the like with lower cost, and the methods have low cost and stable operation and do not need the pretreatment units with high cost such as ion exchange, nanofiltration and the like in the prior art.

3. The salt separation scheme does not discharge waste liquid on the basis of ensuring the purity of the crystallized salt, and really realizes zero discharge. The sodium sulfate is crystallized by adopting two crystallization modes, and the sodium sulfate and the mirabilite are crystallized to reasonably distribute other impurities in the wastewater into different crystallized products, so that the problem of unqualified purity of the crystallized salt caused by full circulation of the mother solution is avoided, and zero discharge of the wastewater is realized on the premise of ensuring qualified quality of the crystallized salt. If the sodium sulfate is separated in a crystal salt form, tests show that the two crystal salts (sodium sulfate and sodium chloride) are unqualified in purity; if the purity of the two kinds of crystal salt is qualified, salt-containing waste liquid needs to be discharged. Therefore, the scheme of the invention improves the recovery rate of the salt while realizing zero discharge or near zero discharge of the wastewater in the coal chemical industry, and can recover high-quality sodium sulfate, mirabilite and sodium chloride to realize the recycling of the crystallized salt. Meanwhile, the process of the quality-divided salt is at the tail end, so that the process is simplified, and the construction cost is reduced.

Drawings

FIG. 1 is a schematic diagram of the components and connection relationship of unit devices for implementing the method of the present invention.

Detailed Description

With reference to fig. 1, a coal chemical industry salt-containing wastewater zero-discharge full-flow treatment process is further described in detail, and the scope of the present invention is not limited to the following embodiments. The units are conventional devices or equipment in the field, and referring to the following specific implementation mode of the invention, technicians in the field can perform conventional adjustment according to the quality of wastewater to obtain optimal treatment control conditions of different water qualities, so that each unit meets the requirement of the controlled effluent quality, and the final process effect is realized.

The average concentration of each pollutant of the high-salt industrial wastewater is as follows: COD is 80 mg/L, total hardness is as CaCO₃Calculated as CaCO) of 2000 mg/L and total alkalinity₃Calculated) is 2000 mg/L, the silicon dioxide is 30 mg/L, TDS is 5000mg/L, the chloride ion concentration is 700 mg/L, the sulfate ion concentration is 1000 mg/L, and the pH value is 8.

As shown in fig. 1, the zero discharge treatment process for the salt-containing wastewater in the coal chemical industry comprises the following steps:

(1) the method comprises the steps of enabling salt-containing wastewater to enter a high-density clarification tank, adding 1.8 g/L of calcium hydroxide into a reaction zone of the high-density clarification tank according to the quality of the salt-containing wastewater to maintain the pH value at 11-11.5, then adding 3.2 g/L of sodium carbonate to completely precipitate the residual calcium ions, then adding 50mg/L of PFS for coagulation, adding 4 mg/L of PAM for coagulation assistance, enabling the mixture to enter a clarification zone of the high-density clarification tank for precipitation after the reaction zone is fully coagulated, finally adding hydrochloric acid or sulfuric acid into an outlet of the high-density clarification tank to adjust the pH value to about 7, and adding a non-oxidizing bactericide to prevent membrane pollution. After treatment, the hardness of the effluent of the high-density clarification tank is less than 200 mg/L.

(2) The effluent of the high-density clarification tank is treated by a V-shaped filter to remove suspended matters, colloid particles and the like in the water, and finally the turbidity of the wastewater is controlled to be about 3NTU and the COD is controlled to be about 50mg/L, and the wastewater enters an ultrafiltration device. The operating pressure of the ultrafiltration device is 0.05 MPa, and after further treatment of ultrafiltration, the wastewater index finally reaches the hardness of less than 200mg/L, COD and less than 50mg/L and the turbidity of less than 1 NTU.

(3) The pretreated wastewater enters a reverse osmosis device for concentration, wherein the reverse osmosis device comprises medium-pressure reverse osmosis and high-pressure reverse osmosis, and is provided with a microporous filter of 25 mu m as a guarantee measure for water inlet. The medium-pressure reverse osmosis device adopts an anti-pollution medium-pressure reverse osmosis membrane, the operating pressure is about 2 MPa, and the TDS of the concentrated water reaches about 20000 mg/L; concentrated water of medium-pressure reverse osmosis and fresh water of an electrodialysis unit enter a high-pressure reverse osmosis device, the high-pressure reverse osmosis device is an anti-pollution high-pressure reverse osmosis membrane, the operating pressure is about 3.5 MPa, and the TDS of the concentrated water reaches about 50000 mg/L. The COD of the fresh water obtained by the medium-pressure reverse osmosis and the high-pressure reverse osmosis is less than 30 mg/L, the TDS is less than 300 mg/L, and the hardness is less than 10 mg/L, so that the water quality index requirement of the circulating water replenishing water is met. The service life of the reverse osmosis membrane component can reach more than 3 years.

(4) Feeding high-pressure reverse-osmosis concentrated water into a high-density clarification tank, adding 0.55 g/L sodium hydroxide into a reaction zone of the high-density clarification tank according to the water quality of the concentrated water to maintain the pH value at about 11.5, then adding 2.2 g/L sodium carbonate, then adding 100 mg/L PFS for coagulation, adding 6 mg/L PAM for coagulation assistance, fully coagulating in the reaction zone, then feeding into a clarification zone of the high-density tank for precipitation, finally adding hydrochloric acid or sulfuric acid into an outlet of the high-density clarification tank to adjust the pH value to about 7, and adding a non-oxidative bactericide to prevent membrane pollution.

(5) The effluent of the high-density clarification tank is treated by a V-shaped filter to remove suspended matters, colloid particles and the like in the water, and finally the turbidity in the wastewater is controlled to be about 3NTU and enters an ultrafiltration device. The operating pressure of the ultrafiltration device is 0.05 MPa, and after the further treatment of ultrafiltration, the index hardness of the wastewater is finally less than 50mg/L, COD and less than 400 mg/L, and the turbidity is less than 1 NTU.

(6) The wastewater enters an electrodialysis device for further concentration, the operating voltage of the electrodialysis device is 40V, in order to ensure the long-period stable operation of electrodialysis, the TDS of the produced water (fresh water) is set to be about 15000 mg/L, and the produced water enters a high-pressure reverse osmosis membrane for further desalination treatment; the TDS in the concentrated water reaches over 200000 mg/L. The water produced by the electrodialysis device is controlled to have higher salt content and is organically combined with the reverse osmosis unit, so that the long-period stable operation of the reverse osmosis unit is realized, and meanwhile, the long-period stable operation of the electrodialysis unit is ensured.

(7) The electrodialysis concentrated water enters an advanced catalytic oxidation device to reduce the COD which is difficult to degrade in the water to about 500 mg/L, and then enters an evaporation crystallization unit.

(8) In a primary evaporation crystallization device, obtaining sodium sulfate with the product quality reaching the quality standard of national standard industrial anhydrous sodium sulfate III qualified products at an evaporation temperature of 100 ℃ and a crystallization temperature of 50-80 ℃; the mother liquor left after crystallization enters a freezing crystallization device to be further obtained at the crystallization temperature of about minus 5 ℃, and the mirabilite with the purity of more than 90 percent is obtained; and the residual liquid enters a secondary evaporation crystallization device again, sodium chloride with the product quality reaching the secondary quality standard of national standard refined industrial salt is obtained at the evaporation temperature of 100 ℃ and the crystallization temperature of 30-50 ℃, the TDS of the crystallization mother liquor of the secondary evaporation crystallization is controlled to be above 400000 mg/L, the sodium chloride is circulated to the primary evaporation crystallization to evaporate the residual salt in the crystallization mother liquor again, the purity of three crystallization products is qualified through the circulation process and condition control, and the recovery rates of water and salt are improved. The circulating mother liquor with too high COD accumulation (such as about 10000 mg/L) can be treated by circulating to an ozone catalytic oxidation device or other modes.

Claims

1. A zero-emission treatment process method for salt-containing wastewater comprises the following steps:

(1) pre-treating; the pretreatment comprises softening, coagulating sedimentation, rough filtration and ultrafiltration, and the pretreated effluent is subjected to reverse osmosis treatment; after the pretreatment process, the wastewater index is controlled to be that the hardness is less than 200mg/L, COD and less than 70 mg/L and the turbidity is less than 3 NTU; wherein the softener is calcium hydroxide, calcium hydroxide and sodium carbonate, or calcium hydroxide, sodium hydroxide and sodium carbonate; the coagulant is a composite coagulant of polymeric ferric sulfate and polyacrylamide, the concentration of the polymeric ferric sulfate is 5-200 mg/L, and the concentration of the polyacrylamide is 0.5-12 mg/L;

(2) reverse osmosis treatment; adopting a membrane pollution resistant combined membrane process combining medium-pressure reverse osmosis and high-pressure reverse osmosis, firstly carrying out medium-pressure reverse osmosis on pretreated effluent, recycling produced water of the medium-pressure reverse osmosis, carrying out high-pressure reverse osmosis on concentrated water of the medium-pressure reverse osmosis and fresh water of an electrodialysis unit together, recycling produced water of the high-pressure reverse osmosis, and carrying out electrodialysis concentration on the concentrated water of the high-pressure reverse osmosis; after the two-stage reverse osmosis membrane separation process, the TDS of the generated concentrated water reaches more than 50000 mg/L;

(3) electrodialysis concentration; firstly, softening, coagulating sedimentation, rough filtration and ultrafiltration pretreatment are carried out on high-pressure reverse osmosis concentrated water, then electrodialysis is carried out for further concentration, electrodialysis fresh water is circulated to be used as high-pressure reverse osmosis inlet water, and the generated concentrated water is circulated and crystallized; controlling the electrodialysis water inlet index to be less than 50mg/L, COD and less than 400 mg/L, and controlling the turbidity to be less than 3 NTU; after concentration by electrodialysis, the TDS in the concentrated water reaches over 200000 mg/L; electrodialysis fresh water TDS is 10000-25000 mg/L; wherein the softening agent is one or more of calcium hydroxide, sodium hydroxide and sodium carbonate; the flocculating agent is a composite coagulant of polymeric ferric sulfate and polyacrylamide, the concentration of the polymeric ferric sulfate is 10-300 mg/L, and the concentration of the polyacrylamide is 0.5-12 mg/L;

(4) circulating crystallization; carrying out ozone catalytic oxidation on concentrated water generated by electrodialysis, controlling COD (chemical oxygen demand) in ozone catalytic oxidation effluent to be below 500 mg/L, carrying out primary evaporation crystallization to obtain sodium sulfate and primary mother liquor, carrying out freezing crystallization on the primary mother liquor to obtain mirabilite and secondary mother liquor, carrying out secondary evaporation crystallization on the secondary mother liquor to obtain sodium chloride and tertiary mother liquor, and circulating crystallization from the tertiary mother liquor to the primary evaporation crystallization step.

2. The method of claim 1, wherein: in the step (1), softening, flocculation and sedimentation processes are carried out in a high-density clarification tank, a rough filtration process is carried out in a V-shaped filter tank, and the average pore diameter of an ultrafiltration membrane is 1-100 nm.

3. The method of claim 2, wherein: in the step (1), a dosing device is arranged at the front end of the high-density clarification tank, a softening agent and a flocculating agent are added, a dosing device is arranged at the outlet of the high-density clarification tank, hydrochloric acid or sulfuric acid is respectively added to adjust the pH value to 6.5-7.5, and a non-oxidizing bactericide is added.

4. The method of claim 1, wherein: in the step (2), the operating pressure of the medium-pressure reverse osmosis membrane is 1-3 MPa, and the operating pressure of the high-pressure reverse osmosis membrane is 3-6 MPa.

5. The method of claim 1, wherein: in the step (3), softening and flocculating settling processes are carried out in a high-density clarification tank, a rough filtration process is carried out in a V-shaped filter tank, and the average pore diameter of an ultrafiltration membrane is 1-100 nm.

6. The method of claim 5, wherein: in the step (3), a dosing device is arranged at the front end of the high-density clarification tank, a softening agent and a flocculating agent are added, a dosing device is arranged at the outlet of the high-density clarification tank, a pH regulator is added to adjust the pH to 6.5-7.5, and a non-oxidizing bactericide is added according to the requirement.

7. The method of claim 1, wherein: in the step (4), when primary evaporation crystallization is carried out, the temperature of evaporation concentration is 50-150 ℃, and the crystallization temperature is 50-100 ℃, so that sodium sulfate crystals are obtained; during secondary evaporation crystallization, the temperature of evaporation concentration is 50-150 ℃, and the crystallization temperature is 30-50 ℃ to obtain sodium chloride crystals; the temperature of the freezing crystallization is 0 to-8 ℃, and mirabilite crystals are obtained.

8. The method of claim 1, wherein: in the step (4), the purity of the generated sodium sulfate is 95% by mass or more, the purity of the generated sodium chloride is 98% by mass or more, and the purity of the generated mirabilite is 90% by mass or more.