Method for deeply removing chloride ions from desulfurization wastewater

Abstract

The invention provides a method for deeply removing chloride ions from desulfurization wastewater, which comprises the following steps: pretreating the desulfurization wastewater, selectively concentrating chlorine salt by using a first-stage anion selective electrodialyzer, and continuously and selectively concentrating the chlorine salt by using a second-stage anion selective electrodialyzer. And returning the effluent after two times of desulfurization to a desulfurization system to realize reuse. According to the method disclosed by the invention, efficient selective removal of chloride ions is realized through two times of selective electrodialysis, and leakage of sulfate radicals is effectively inhibited.

Description

Technical Field

[0001] The present invention belongs to the field of chemical engineering, and relates to a method for treating desulfurized wastewater, in particular to a method for deeply removing chloride ions from desulfurized wastewater. Background Art

[0002] Due to the stability of power generation in coal-fired power plants, although new energy has developed rapidly, traditional thermal power generation mainly based on coal is still the main form of power generation in China, accounting for more than 60% of the power supply in China. Most of the fuel coal in China has a high sulfur content, and a large amount of sulfur dioxide will be generated during the combustion power generation process. To avoid air pollution caused by sulfur dioxide, desulfurization must be carried out. Currently, the wet limestone-gypsum desulfurization technology is mainly used for flue gas desulfurization in power plants.

[0003] During the wet limestone-gypsum desulfurization process, in addition to sulfur dioxide, chloride ions, sodium ions, and other anions and heavy metal ions in the flue gas will also enter the desulfurized slurry. To maintain the material balance in the desulfurized slurry and prevent the chloride ion concentration from being too high, which affects the desulfurization efficiency and the quality of desulfurized gypsum, part of the slurry must be discharged, and fresh lime slurry must be supplemented. Due to different water qualities, the chloride ion concentration limits in the circulating slurry of each power plant are different, but usually between 5000 and 20000 mg / L.

[0004] Desulfurized wastewater is a typical high-salt wastewater with a high salt content, complex composition, large water quality fluctuations, and also contains a certain amount of heavy metal ions. The conventional process is represented by the traditional triple tank. The desulfurized wastewater is neutralized, reacted, flocculated, and clarified before being discharged. This process used to be the mainstream process for desulfurized wastewater in China, but due to the high concentration of salts in the wastewater, discharging it will affect the environment, so it has been replaced by the zero-discharge process in recent years.

[0005] The zero-discharge process mainly includes the softening-membrane concentration-crystallization process, the pretreatment-evaporation concentration-crystallization process, and the pretreatment-flue duct spray evaporation process, etc. Among them, although the softening-membrane concentration-crystallization process can obtain crystalline salts, due to the large consumption of softening reagents, the operating cost is very high; the evaporation concentration cost of the pretreatment-evaporation concentration-crystallization process is very high; although the pretreatment-flue duct spray evaporation process is simple, the salts in the wastewater are all evaporated in the flue duct to form solid waste salts, and the post-treatment cost is relatively high. Although there are currently multiple zero-discharge processes to choose from, they all have their own deficiencies, and there is an urgent need to develop a more effective treatment process.

[0006] Patent CN117720242A provides a method for removing chlorides from desulfurization wastewater by selective electrodialysis. The wastewater from which chlorides are removed can be recycled to the desulfurization system, and the concentrated water from electrodialysis yields a mixed salt of magnesium chloride, sodium chloride, and calcium chloride after concentration, which can be used as a snowmelt agent. By changing to a relatively low voltage for constant voltage operation in the later stage of desalination, this patent can avoid the rapid increase in the permeation flux of sulfate ions in the later stage of desalination and inhibit the leakage amount of sulfate ions. However, when the chloride ion concentration is low, even though the voltage is low, due to the operating current exceeding the limiting current, there will still be a relatively large amount of sulfate ions leaking. The leaked sulfate ions and the calcium ions entering the concentrated chamber will form calcium sulfate crystals during subsequent concentration, thereby affecting the stable operation of the system and also incorporating a certain amount of impurity calcium sulfate into the final product.

[0007] Therefore, developing a technical solution that can more efficiently and selectively remove chloride ions and effectively inhibit the leakage of sulfate ions has become an urgent problem to be solved. Summary of the Invention

[0008] In order to solve the above problems, the purpose of the present invention is to provide a method for deeply removing chloride ions from desulfurization wastewater, which can further reduce the leakage rate of sulfate ions.

[0009] To achieve the above purpose, the present invention provides a method for deeply removing chloride ions from desulfurization wastewater, including the following steps:

[0010] (1) Pretreatment

[0011] Add sodium hydroxide or calcium hydroxide to the desulfurization wastewater generated by the limestone-gypsum wet desulfurization process to adjust the pH to 8.5 - 9.5, and then add sodium carbonate at a rate of 1 - 5 g per liter of desulfurization wastewater; subsequently, add a flocculant for flocculation and clarification, and finally use an ultrafiltration membrane with a pore size of 0.005 - 0.1 microns to filter to further remove suspended solids and colloids in the clarified liquid to obtain clarified desulfurization wastewater;

[0012] (2) Primary anion selective electrodialysis

[0013] Feed the clarified desulfurization wastewater into the feed chamber of the primary anion selective electrodialyzer, and control the pH of the feed chamber at 3.0 - 6.5 with dilute sulfuric acid or hydrochloric acid; use a sodium chloride solution with a mass percentage concentration of 0.3% - 1% as the initial feed water for the concentrated chamber of the primary anion selective electrodialyzer, remove the chlorides from the desulfurization wastewater through primary anion selective electrodialysis, reduce the chloride ion concentration in the desulfurization wastewater to 3 - 5 g / L, the effluent from the feed chamber of the primary anion selective electrodialyzer is sent to the secondary anion selective electrodialyzer for further removal of chlorides, and the effluent from the concentrated chamber of the primary anion selective electrodialyzer is treated with a heavy metal scavenger to remove heavy metals and then subjected to evaporation crystallization to prepare chloride salt crystals;

[0014] The above-mentioned primary anion selective electrodialyzer initially uses a 0.3%-1% sodium chloride solution as the concentrated chamber influent to enable the concentrated water to circulate between the electrodialyzer and the concentrated chamber water tank, and thereafter no additional influent is supplied to the concentrated chamber.

[0015] (3) Secondary anion selective electrodialysis

[0016] The effluent from the feed chamber of the primary anion selective electrodialyzer is used as the influent of the secondary anion selective electrodialyzer, and clarified desulfurized wastewater is used as the concentrated chamber influent of the secondary anion selective electrodialyzer. Chloride salts are further removed through the secondary anion selective electrodialyzer. The concentrated water of the secondary anion selective electrodialyzer is mixed with the clarified desulfurized wastewater and returned as the influent of the primary feed chamber, and the effluent from the feed chamber is returned to the desulfurization system for recycling.

[0017] As described above, the flocculation clarification method in step (1) is: adding a clarifying agent in an amount of 50-200 mg / L to the desulfurized wastewater for flocculation precipitation until the desulfurized wastewater is clarified.

[0018] As described above, the flocculation clarification method in step (1) is: adding a clarifying agent in an amount of 50-200 mg / L to the desulfurized wastewater for flocculation clarification.

[0019] As described above, the method for removing heavy metals in step (2) is: adding a heavy metal scavenger to the water to form a precipitate with the heavy metals to remove the heavy metals, and the addition amount of the heavy metal scavenger is 10-200 mg / L.

[0020] As described above, the heavy metal scavenger is one or more of dithiocarbamate, trisodium trithiocyanate, and sodium methyl-substituted dithiocarbamate.

[0021] As described above, the influent flow rate of using clarified desulfurized wastewater as the concentrated chamber influent of the secondary anion selective electrodialyzer in step (3) is 20-100%.

[0022] As described above, the anion exchange membranes of the primary anion selective electrodialyzer and the secondary anion selective electrodialyzer are all monovalent anion selective exchange membranes, and the cation exchange membranes are all non-selective cation exchange membranes.

[0023] As described above, the monovalent anion selective exchange membrane is ASV, ACS, PC-MVA, or A-102; the cation exchange membrane is CMV.

[0024] As described above, the primary anion selective electrodialysis operates in a constant current or constant pressure mode.

[0025] As described above, the secondary anion selective electrodialysis operates in a constant pressure mode.

[0026] As described above, the feed chambers of the primary and secondary anion selective electrodialysis adopt the batch method, that is, after the solution in the feed chamber is desalted, the fresh water is discharged and then the water is fed in to continue running.

[0027] The advantages of the present invention are as follows:

[0028] 1. It is not necessary to completely soften the desulfurized wastewater. It is only necessary to add a small amount of sodium carbonate to eliminate the supersaturated state of calcium sulfate, which can save a large amount of softening costs. At the same time, by controlling the pH of the wastewater to be acidic, the carbonate ions that can form scale with calcium and magnesium ions in the water are eliminated, so the formation of scale is effectively inhibited, ensuring the long-term stable operation of the whole system.

[0029] 2. By adopting two-stage anion selective electrodialysis, the leakage of sulfate can be controlled more effectively. The proportion of sulfate in the concentrated chamber solution obtained is low, and in subsequent concentration, the risk of forming calcium sulfate scale is greatly reduced, which is beneficial to maintaining the stable operation of the system. When the selective electrodialysis membrane separates monovalent and divalent ions, the current is large, the desalination speed is fast, and the selectivity coefficient is also relatively high, with less leakage of divalent ions; when the current efficiency decreases, the desalination speed decreases, and the selectivity coefficient also decreases accordingly, with an increase in the leakage of divalent ions. Due to the relatively high price of the anion selective electrodialysis membrane, and in addition, the current is large and the selectivity coefficient is high, therefore, within the range allowed by the membrane, a relatively high current is usually adopted. However, when the chloride ion concentration decreases, the limiting current will also decrease accordingly. If the limiting current is lower than the operating current, relatively serious concentration polarization will occur. At this time, the insufficient chloride ions on the membrane surface will be compensated by the permeation of sulfate ions, resulting in a rapid increase in the permeation flux of sulfate. The lower the chloride ion concentration, the smaller the limiting current, and the more the permeation flux of sulfate increases. In previous studies, the inventors could significantly inhibit the leakage of sulfate ions by reducing the voltage and current in the later stage of desalination. However, this process also has certain deficiencies. Mainly when the chloride ion removal rate is relatively high and the residual chloride ion concentration is relatively low, the limiting current is small. Even if a relatively small voltage is adopted, the current will still exceed the limiting current, resulting in a relatively large leakage of sulfate. And if a smaller voltage is adopted to make the operating current less than the limiting current, but the desalination speed of too small a current is too slow. For the same desalination amount, more electrodialysis membranes are required, and the investment cost increases significantly. For example, when the current is reduced from 100 A to 20 A, without much change in the current efficiency, the membrane area needs to be increased by 5 times. In addition, even if the current does not exceed the limiting current, a smaller current corresponds to a lower selectivity coefficient, and the permeation amount of sulfate will also increase. Therefore, although the inventors effectively inhibited the leakage of sulfate in previous studies by adopting a lower voltage in the later stage, from the examples, its selectivity coefficient still decreased to between 16 and 21, and the sulfate leakage reached about 5%. This will lead to the formation of more calcium sulfate during subsequent evaporation and crystallization, with a relatively high scaling risk, and at the same time, the impurities in the final crystalline product are relatively more. In the present invention, the dechlorination of the wastewater is divided into two stages. Among them, the first-stage anion selective electrodialysis operates in the high-efficiency range. While removing most of the chloride ions, less sulfate leaks into the concentrated chamber; for the remaining chloride ions, the second-stage anion selective electrodialysis is used to deeply dechlorinate the desulfurized wastewater. At this time, a relatively high voltage can be selected to accelerate the desalination speed. Although there is more sulfate leakage, the concentrated water is returned as the fresh chamber inlet water of the first-stage electrodialysis, and it does not increase the sulfate in the concentrated water of the first-stage electrodialysis. Through the setting of two-stage anion selective electrodialysis, the concentrated water discharged from the whole system is the concentrated water of the first-stage anion selective electrodialysis, the selectivity coefficient reaches more than 100, the sulfate leakage is reduced to less than 1%, the contained sulfate is extremely small, and the influence on subsequent concentration is small. At the same time, the content of impurity calcium sulfate in the product is also greatly reduced.

[0030] 3. The fresh chamber feed liquid after desalination in the present invention is recycled for desulfurization. Through deep dechlorination, the recycled water returned to the desulfurization system can absorb more chloride ions, which is conducive to reducing the generation amount of desulfurization wastewater and lowering the operation cost.

[0031] 4. In the first-stage electrodialysis of the present invention, only a sodium chloride solution with a concentration of 0.3 - 1.0 wt% is added as the concentrated chamber feed water at the beginning of operation, and the addition amount only needs to be sufficient for the concentrated water to circulate between the electrodialysis membrane device and the concentrated water tank. Since no additional concentrated water is replenished thereafter, the electrodialysis can obtain concentrated water with a higher salt concentration. During the electrodialysis process, when chloride ions, sodium ions, calcium ions, and magnesium ions migrate from the feed chamber to the concentrated chamber through the electrodialysis membrane, bound water will be carried into the concentrated chamber together; at the same time, when the salt content in the concentrated chamber is relatively high, due to the low salt concentration in the feed chamber and the high chemical potential of the corresponding water, water will permeate from the fresh chamber to the concentrated chamber under the drive of the chemical potential. Due to the migration of these two parts of water to the concentrated chamber, the salt concentration in the concentrated chamber will not continuously increase but will basically remain unchanged after rising to a certain level. Without the dilution of makeup water, all the water in the concentrated chamber is provided by the migration from the feed chamber, which can make the salt content in the concentrated chamber higher and is conducive to reducing the cost of subsequent evaporation and crystallization.

[0032] The beneficial effects of the present invention are as follows:

[0033] The present invention provides a method for deeply removing chloride ions from desulfurization wastewater. While effectively removing chloride ions in the wastewater, it can effectively inhibit the leakage of sulfate ions. Using this process, when the chloride ion removal rate reaches 95% - 99%, the selectivity coefficient can be maintained above 100, and the sulfate ion leakage rate is lower than 1%. It can not only increase the chloride ion accommodation capacity of the recycled water but also greatly reduce the risk of forming calcium sulfate scale during the concentration of the concentrated water, making the operation of the system more stable and reducing the content of impurity calcium sulfate in the final crystalline salt. In addition, since no additional makeup water is added to the concentrated chamber after operation, the salt concentration in the concentrated chamber can be greatly increased, and the subsequent evaporation and crystallization cost is significantly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a process flow chart of the resource-based desulfurization wastewater treatment in the embodiment of the present invention.

[0035] Figure 2 It is a schematic diagram of the principle of anion selective electrodialysis in the embodiment of the present invention, where C represents a cation exchange membrane and A represents an anion selective exchange membrane. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0036] The embodiments of the present invention will be described in detail and comprehensively below, so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby making the protection scope of the present invention more clearly defined.

[0037] Such as Figure 1The process flow diagram of the treatment method for resource-based desulfurized wastewater provided by the present invention is shown as follows, including the following steps:

[0038] (1) Pretreatment

[0039] Sodium hydroxide or calcium hydroxide is added to the desulfurized wastewater generated in the limestone-gypsum wet desulfurization process to adjust the pH to about 8.5 - 9.5. Then, sodium carbonate is added in an amount of 1 - 5 g per liter of the desulfurized wastewater. Subsequently, a flocculant is added for flocculation and clarification. Finally, ultrafiltration membranes with a pore size of 0.05 - 0.1 microns are used for filtration to further remove suspended solids and colloids in the clarified liquid, obtaining clarified desulfurized wastewater;

[0040] (2) First-stage anion selective electrodialysis

[0041] After adjusting the pH of the clarified desulfurized wastewater to 5.0 - 6.5 with dilute sulfuric acid or hydrochloric acid, it is fed into the feed chamber of the first-stage anion selective electrodialyzer. A low-concentration sodium chloride solution is used as the feed water for the concentrated chamber of the first-stage anion selective electrodialyzer. Chloride salts in the desulfurized wastewater are removed through the first-stage anion selective electrodialysis, reducing the chloride ion concentration in the desulfurized wastewater to 3 - 5 g / L. The effluent from the feed chamber of the first-stage anion selective electrodialyzer is sent to the second-stage anion selective electrodialyzer for further removal of chloride salts. The effluent from the concentrated chamber of the first-stage anion selective electrodialyzer is treated with a heavy metal scavenger to remove heavy metals, and then crystallized to prepare chloride salt crystals, which can be used as a snowmelt agent;

[0042] Among them, the anion exchange membranes of the first-stage anion selective electrodialyzer and the second-stage anion selective electrodialyzer are all monovalent anion selective exchange membranes, specifically ASV, ACS, PC-MVA or A-102, and the cation exchange membranes are all non-selective cation exchange membranes, specifically CMV;

[0043] The method for removing heavy metals is: adding a heavy metal scavenger to the water to form a precipitate with the heavy metals to remove the heavy metals. The addition amount of the heavy metal scavenger is 10 - 200 mg / L.

[0044] (3) Second-stage anion selective electrodialysis

[0045] The effluent from the feed chamber of the first-stage anion selective electrodialyzer is used as the feed water for the second-stage anion selective electrodialyzer. A part of the clarified desulfurized wastewater is used as the feed water for the concentrated chamber of the second-stage anion selective electrodialyzer. Chloride salts are further removed through the second-stage anion selective electrodialyzer. The concentrated water from the second-stage anion selective electrodialyzer is mixed with the clarified desulfurized wastewater and returned as the feed water for the first-stage feed chamber, and the effluent from the feed chamber is returned to the desulfurization system for desulfurization.

[0046] The "concentrate water" and "fresh water" mentioned in the present invention are relative concepts, referring to the effluents with different chlorine salt contents after treatment by cation and anion exchange membranes in the same electrodialyzer. Among them, the effluent with a high chlorine salt content is the concentrate water, and the effluent with a low chlorine salt content is the fresh water. Since the effluents in different electrodialyzers are all called concentrate water and fresh water.

[0047] The present invention has no special restrictions on the cation exchange membrane in the electrodialyzer for selective electrodialysis, and a cation exchange membrane well-known in the art can be used; a monovalent anion exchange membrane can be used for the anion exchange membrane. In the examples, the anion exchange membrane and the cation exchange membrane are respectively the monovalent anion selective exchange membrane ASV and the conventional cation exchange membrane CMV of Asahi Glass Co., Ltd. of Japan, and other monovalent anion exchange membranes and cation exchange membranes that meet the requirements can also be selected for substitution.

[0048] The first-stage anion selective electrodialyzer and the first-stage anion selective electrodialyzer used in the present invention are both conventional electrodialyzers, and only the types and arrangement modes of the ion exchange membranes used in the electrodialyzer are adjusted. Among them, both ends are cation exchange membranes, and in the middle, anion exchange membranes and cation exchange membranes are arranged at intervals. The involved concentrate chambers, feed chambers, etc. are all conventional structures. As Figure 2 shown is the schematic diagram of the principle of selective electrodialysis in the anion selective electrodialyzer. In the figure, the anion exchange membrane uses ASV, and the cation exchange membrane uses CMV. Among them, the anion exchange membrane has a positive charge (+) to combine with anions so that the anions enter the membrane, and the cation exchange membrane has a negative charge (-) to combine with cations so that the cations enter the membrane.

[0049] A main index for evaluating the separation performance of selective electrodialysis is the selectivity coefficient P (Permselectivity), which is the ratio of the number of chloride ions and sulfate ions migrated into the concentrate water during the electrodialysis process to the concentration ratio of chloride ions and sulfate ions in the initial feed solution:

[0050]

[0051] In the formula, c i is the concentration of the ion; V i is the volume of the solution; the subscripts d and c respectively represent the dilute chamber and the concentrate chamber; t and 0 are the time t and the starting time respectively.

[0052] In addition, the sulfate leakage rate α i can also be used to characterize the proportion of sulfate in the feed chamber solution leaking into the concentrate chamber. The leakage rate calculation formula at time t is:

[0053]

[0054] A small leakage rate of sulfate ions also indicates less sulfate in the concentrate chamber and higher purity of the chlorine salt.

[0055] Example 1

[0056] Anion selective electrodialysis was used to remove chlorides from the desulfurization wastewater generated in the limestone-gypsum wet desulfurization process of a certain factory. The chloride ion content in the wastewater was 17,500 mg / L, the sulfate ion content was 3,100 mg / L, the calcium ion content was 3,150 mg / L, and the magnesium ion content was 1,630 mg / L. The specific steps were as follows:

[0057] Add calcium hydroxide to adjust the pH to about 9.0;

[0058] Add 1 g of sodium carbonate per liter of wastewater;

[0059] Add 100 mg / L of polyferric sulfate;

[0060] Filter using a 0.05-μm hollow fiber ultrafiltration membrane

[0061] Feed the ultrafiltered desulfurization wastewater into the feed chamber of the first-stage anion selective electrodialysis, and use a pH automatic detection and dosing system to control the pH of the feed chamber between 3 and 5;

[0062] Add a 0.3 wt% sodium chloride solution to the concentrated chamber as the inlet water for the concentrated chamber. The addition amount should enable the liquid in the concentrated chamber to circulate between the electrodialysis membrane device and the concentrated chamber water tank, and no additional water supply is required subsequently.

[0063] The feed chambers of the first-stage and second-stage anion selective electrodialysis adopt the batch method. First, fill the feed chamber with the feed liquid, then desalt. After desalting, discharge the fresh water and then continue to feed water and desalt.

[0064] The anion exchange membrane of the selective electrodialysis is ASV, and the cation exchange membrane is CMV. Control the current density of the first-stage selective electrodialysis at 250 A / m 2 , and remove the chloride ions to about 6,000 mg / L; then reduce the current density to 180 A / m 2 . When the chloride ions are removed to 3,500 mg / L, send the feed liquid in the feed chamber to the second-stage selective electrodialyzer to further remove chlorides. Add 100 mg / L of the heavy metal scavenger trisodium trithiocyanurate to the concentrated water to remove heavy metal ions, and then go to evaporation crystallization to prepare chloride salt crystals. According to the national standard for snow melting agents GB / T 23851-2017, the concentrations of toxic heavy metal elements Hg, Cd, Cr, Pb, and As should not be higher than 1 mg / L, 5 mg / L, 15 mg / L, 25 mg / L, and 5 mg / L, respectively. Detect the heavy metal content in the crystals. The concentrations of Hg, Cd, Cr, Pb, and As are 0.8993 mg / L, 0.1831 mg / L, 0.0009 mg / L, 3.7186 mg / L, and 0.0017 mg / L, respectively, all lower than the maximum concentrations of heavy metal elements in the national standard.

[0065] Take 50% of the desulfurized clarified wastewater as the feed water for the concentrated chamber of the secondary selective electrodialysis, control the voltage at 0.4 V per pair of membranes, and operate until the chloride ion concentration is reduced to 0.5 g / L. Send the diluted chamber solution of the secondary electrodialysis back to the desulfurization system for reuse, and mix the concentrated chamber solution with the desulfurized clarified wastewater as the feed water for the diluted chamber of the primary electrodialysis.

[0066] In the extracted concentrated brine, the salt concentration is 17.2 wt%, including 106.8 g / L of chloride ions, 0.17 g / L of sulfate ions, a selectivity coefficient of 109, and a sulfate ion leakage rate of 0.89%. The chloride ion concentration in the effluent from the diluted chamber of the secondary electrodialysis is 0.5 g / L, and the chloride ion removal rate is 97%.

[0067] Since the salt concentration in the extracted concentrated brine is already high enough, there is no need to further concentrate it by electrodialysis, and direct evaporation and crystallization can be carried out.

[0068] Example 2

[0069] Use the same wastewater as in Example 1 to remove chloride ions:

[0070] Add calcium hydroxide to adjust the pH to about 9.2;

[0071] Add 2 g of sodium carbonate per liter of wastewater;

[0072] Add 100 mg / L of polyferric sulfate;

[0073] Filter using a 0.01 - micron hollow fiber ultrafiltration membrane

[0074] Send the ultrafiltered desulfurized wastewater into the feed chamber of the primary anion - selective electrodialysis, and use a pH automatic detection and dosing system to control the pH in the feed chamber between 3 and 5;

[0075] Add a 0.3 wt% sodium chloride solution to the concentrated chamber as the feed water for the concentrated chamber. The addition amount should enable the concentrated chamber liquid to circulate between the electrodialysis membrane device and the concentrated chamber water tank, and no additional water supply is required subsequently.

[0076] The feed chambers of the primary and secondary anion - selective electrodialysis adopt the batch method. First, fill the feed chamber with the feed liquid, then desalt. After desalting, discharge the diluted water and then continue to feed water and desalt.

[0077] The anion - exchange membrane of the selective electrodialysis is ASV, and the cation - exchange membrane is CMV. Control the current density of the primary selective electrodialysis at 200 A / m 2 , and reduce the chloride ions to about 5000 mg / L; then reduce the current density to 160 A / m 2, after the chloride ions are removed to 3000 mg / L, the liquid in the feed chamber is sent to a secondary selective electrodialysis unit for further removal of chlorides. 100 mg / L of a heavy metal scavenger, dithiocarbamic acid, is added to the concentrated water to remove heavy metal ions, and then it goes to evaporation crystallization to prepare chloride salt crystals. According to the national standard for snow melting agents GB / T 23851-2017, the concentrations of toxic heavy metal elements Hg, Cd, Cr, Pb, and As should not be higher than 1 mg / L, 5 mg / L, 15 mg / L, 25 mg / L, and 5 mg / L respectively. The heavy metal contents in the crystals are detected, and the concentrations of Hg, Cd, Cr, Pb, and As are 0.2923 mg / L, 0.1616 mg / L, 0.3583 mg / L, 1.6343 mg / L, and 4.5464 mg / L respectively, all lower than the maximum concentrations of heavy metal elements in the national standard.

[0078] Take 70% of the desulfurized clarified wastewater as the feed water for the concentrated chamber of the secondary selective electrodialysis unit, control the voltage at 0.3 V per pair of membranes, and operate until the chloride ion concentration is reduced to 0.2 g / L. The dilute chamber solution of the secondary electrodialysis is sent back to the desulfurization system for reuse, and the concentrated chamber solution is mixed with the desulfurized clarified wastewater and used as the feed water for the dilute chamber of the primary electrodialysis unit.

[0079] In the concentrated brine drawn out, the salt concentration is 16.3 wt%, including 103.4 g / L of chloride ions, 0.18 g / L of sulfate ions, a selectivity coefficient of 104, and a sulfate leakage rate of 0.95%. The chloride ion concentration in the dilute chamber effluent of the secondary electrodialysis is 0.2 g / L, and the chloride ion removal rate is 99%. In the case of achieving deep dechlorination, the sulfate ion leakage rate is extremely low.

[0080] Since the salt concentration in the concentrated brine drawn out is already high enough, there is no need to further concentrate it by electrodialysis, and it can be directly evaporated and crystallized.

[0081] Comparative Example 1

[0082] The same wastewater and pretreatment method as in Example 1 are used.

[0083] The desulfurized wastewater after ultrafiltration is sent to the feed chamber of the anion selective electrodialysis unit, and the pH of the feed chamber is controlled between 3 and 5 by a pH automatic detection and dosing system;

[0084] The anion selective electrodialysis operates in an intermittent manner. 0.3 wt% sodium chloride solution is added to the concentrated chamber as the feed water for the concentrated chamber, and the ratio of the feed water for the feed chamber to the feed water for the concentrated chamber is controlled at 10:1. After the feed water for the feed chamber and the concentrated chamber are fed, desalination is carried out. After desalination, the dilute water and the concentrated water are discharged, and then the next round of desalination is carried out.

[0085] The anion exchange membrane of the selective electrodialysis is ASV, and the cation exchange membrane is CMV. Control the current density of the primary selective electrodialysis at 200 A / m 2, the chloride ions were removed to about 5000 mg / L; then it was changed to the constant pressure control mode, and the membrane voltage drop of each pair was controlled at 0.4 V. The operation was stopped after the chloride ions were removed to about 700 mg / L. At this time, the chloride ion removal rate was 96.1%, the sulfate leakage rate was 7.3%, and the selectivity coefficient was 13.2. In the concentrated water, the total salt content was 8.6 wt%, among which the chloride ion concentration was 54.4 g / L and the sulfate ion was 0.73 g / L.

[0086] As can be seen from the above embodiments, by using the method for deeply removing chloride ions from desulfurization wastewater provided by the present invention, while the chloride ion removal rate reaches 95%-99%, that is, deeply removing chloride ions, the selectivity coefficient can be maintained above 100, the sulfate leakage rate is lower than 1%, and the sulfate ion concentration in the concentrated water is less than 0.2 g / L, far lower than that of the comparative example, greatly reducing the risk of calcium sulfate scale formation during the concentration of the concentrated water, making the operation of the system more stable, and also reducing the content of impurity calcium sulfate in the final crystalline salt. In addition, since no additional water is replenished in the concentrated chamber after operation, the salt concentration of the extracted concentrated water is relatively high, which is nearly twice that of the comparative example, greatly reducing the subsequent evaporation and crystallization cost.

[0087] The above embodiments only represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the patent of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the appended claims.

Claims

1. A method for deeply removing chloride ions from desulfurized wastewater, characterized in that, The method includes the following steps: (1) Pretreatment Add sodium hydroxide or calcium hydroxide to the desulfurization wastewater generated by the limestone-gypsum wet desulfurization process to adjust the pH to 8.5-9.5, and then add sodium carbonate at a dosage of 1-5 g per liter of desulfurization wastewater; subsequently, add a flocculant for flocculation and clarification, and finally use an ultrafiltration membrane with a pore size of 0.005-0.1 μm to filter to further remove suspended solids and colloids in the clarified liquid to obtain clarified desulfurization wastewater; (2) First-stage anion selective electrodialysis Feed the clarified desulfurization wastewater into the feed chamber of the first-stage anion selective electrodialyzer, and control the pH of the feed chamber at 3.0-6.5 with dilute sulfuric acid or hydrochloric acid; use a sodium chloride solution with a mass percentage concentration of 0.3%-1% as the initial feed water for the concentrated chamber of the first-stage anion selective electrodialyzer, remove the chloride salts in the desulfurization wastewater through the first-stage anion selective electrodialysis, reduce the chloride ion concentration in the desulfurization wastewater to 3-5 g / L, the effluent from the feed chamber of the first-stage anion selective electrodialyzer is sent to the second-stage anion selective electrodialyzer for further removal of chloride salts, and the effluent from the concentrated chamber of the first-stage anion selective electrodialyzer is treated with a heavy metal scavenger to remove heavy metals and then subjected to evaporation and crystallization to prepare chloride salt crystals; (3) Second-stage anion selective electrodialysis Use the effluent from the feed chamber of the first-stage anion selective electrodialyzer as the feed water for the second-stage anion selective electrodialyzer, and use the clarified desulfurization wastewater as the feed water for the concentrated chamber of the second-stage anion selective electrodialyzer. Further remove chloride salts through the second-stage anion selective electrodialyzer. The concentrated water from the second-stage anion selective electrodialyzer is mixed with the clarified desulfurization wastewater and returned as the feed water for the first feed chamber, and the effluent from the feed chamber is returned to the desulfurization system for recycling.

2. The method according to claim 1, wherein The method of flocculation and clarification described in step (1) is: add the clarifying agent at a dosage of 50-200 mg / L to the desulfurization wastewater for flocculation and clarification.

3. The method according to claim 1, wherein The method of removing heavy metals described in step (2) is: add a heavy metal scavenger to the water to form a precipitate with the heavy metals to remove the heavy metals, and the dosage of the heavy metal scavenger is 10-200 mg / L.

4. The method according to claim 3, characterized in that The heavy metal scavenger is one or more of dithiocarbamate, trisodium trithiocyanate, and sodium methyl-substituted dithiocarbamate.

5. The method according to claim 1, wherein In step (3), the feed water volume of using the clarified desulfurization wastewater as the feed water for the concentrated chamber of the second-stage anion selective electrodialyzer is 20-100%.

6. The method according to claim 1, characterized in that, The anion exchange membranes of the first-stage anion selective electrodialyzer and the second-stage anion selective electrodialyzer are all monovalent anion selective exchange membranes, and the cation exchange membranes are all non-selective cation exchange membranes.

7. The method according to claim 6, characterized in that, The monovalent anion selective exchange membrane is ASV, ACS, PC-MVA or A-102; the cation exchange membrane is CMV.

8. The method according to claim 1, wherein The first-stage anion selective electrodialysis operates in a constant current or constant voltage mode.

9. The method according to claim 1, characterized in that, The second-stage anion selective electrodialysis operates in a constant voltage mode.

10. The method according to claim 1, wherein The feed chambers of the first-stage and second-stage anion selective electrodialysis continue to operate in an intermittent method of discharging fresh water after the feed chamber solution is desalted and then re-feeding water.

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