A method for deep removal of chloride ions from desulfurization wastewater

Abstract

The application provides a method for deeply removing chlorine ions from desulfurization wastewater, which comprises the following steps: firstly, pretreating the desulfurization wastewater; then, selectively concentrating chlorine salts by using a first anion selective electrodialyzer; then, continuously selectively concentrating the chlorine salts by using a second anion selective electrodialyzer; and finally, returning the effluent after desulfurization twice to a desulfurization system to realize reuse. The application realizes efficient selective removal of chlorine ions by twice selective electrodialysis, and effectively inhibits the leakage of sulfate.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of chemical industry, and relates to a desulfurization wastewater treatment method, in particular to a method for deeply removing chlorine ions from desulfurization wastewater. BACKGROUND

[0002] Due to the stability of coal-fired power generation, although new energy is developing rapidly, traditional thermal power generation dominated by coal is still the main form of power generation in China, accounting for more than 60% in China's power supply. Most of the fuel coal in China has a high sulfur content, and a large amount of sulfur dioxide will be generated in the process of burning and generating electricity. In order to avoid the pollution of sulfur dioxide to the atmosphere, desulfurization must be carried out. At present, the limestone-gypsum wet desulfurization technology is mainly used for flue gas desulfurization in power plants.

[0003] In the process of lime-gypsum wet desulfurization, in addition to sulfur dioxide, chlorine ions, sodium ions and other anions and heavy metal ions in the flue gas will also enter the desulfurization slurry. In order to maintain the material balance in the desulfurization slurry, prevent the high concentration of chlorine ions from affecting the desulfurization efficiency and affecting the quality of desulfurization gypsum, and at the same time supplement fresh lime slurry, part of the slurry must be discharged. Due to the difference in water quality, the concentration limit of chlorine ions in the circulating slurry is different in each power plant, but it is usually between 5000-20000 mg / L.

[0004] Desulfurization wastewater is a typical high-salt wastewater, which has high salt content, complex composition, large water quality fluctuation and a certain amount of heavy metal ions. The conventional process represented by the traditional triple tank discharges the desulfurization wastewater after neutralization, reaction, flocculation and clarification. This process was once the mainstream process for desulfurization wastewater in China, but due to the high concentration of salts in the wastewater, the discharge will affect the environment, so in recent years it has been replaced by zero discharge process.

[0005] Zero discharge process mainly includes softening-membrane concentration-crystallization process, pretreatment-evaporation concentration-crystallization process and pretreatment-flue spray evaporation process. Among them, although the softening-membrane concentration-crystallization process can obtain crystalline salt, the consumption of softening reagent is large, and the operation cost is very high; the evaporation concentration cost of the pretreatment-evaporation concentration-crystallization process is very high; the pretreatment-flue spray evaporation process is simple, but the salts in the wastewater are evaporated in the flue to form solid waste salt, and the post-treatment cost is high. Although there are many zero discharge processes to choose from, they all have their own shortcomings, and more effective treatment processes need to be developed.

[0006] The patent CN117720242A provides a method for removing chloride salt from desulfurization wastewater by selective electrodialysis. The wastewater from which the chloride salt is removed can be reused in the desulfurization system. The concentrated water from the electrodialysis is concentrated to obtain a mixed salt of magnesium chloride, sodium chloride and calcium chloride, which can be used as a snow melting agent. By running at a relatively low voltage in the later stage of desalination, the rapid increase of the sulfate permeation flux in the later stage of desalination can be avoided, and the leakage of sulfate can be inhibited. However, when the concentration of chloride ions is low, even if the voltage is low, the running current will still exceed the limit current, and a large amount of sulfate will still be leaked. The leaked sulfate and the calcium ions entering the concentration chamber will form calcium sulfate crystals during subsequent concentration, affecting the stable operation of the system, and also introducing a certain amount of impurity calcium sulfate into the final product.

[0007] Therefore, it is an urgent problem to develop a technical solution that can more efficiently remove chloride ions and effectively inhibit the leakage of sulfate. SUMMARY

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

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

[0010] (1) Pretreatment

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

[0012] (2) First-stage anion selective electrodialysis

[0013] Send the clarified desulfurization wastewater to the feed chamber of the first-stage anion selective electrodialysis device, 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 of the concentration chamber of the first-stage anion selective electrodialysis device, remove the chloride salt in the desulfurization wastewater by first-stage anion selective electrodialysis to reduce the chloride ion concentration in the desulfurization wastewater to 3-5 g / L, send the effluent of the feed chamber of the first-stage anion selective electrodialysis device to the second-stage anion selective electrodialysis device for further removal of chloride salt, and after removing heavy metals from the effluent of the concentration chamber of the first-stage anion selective electrodialysis device with a heavy metal capturing agent, evaporate and crystallize to prepare chloride salt crystals;

[0014] The primary anion selective electrodialyzer is initially fed with 0.3%-1% sodium chloride solution as the concentrated chamber feed water, so that the concentrated water can circulate between the electrodialyzer and the concentrated chamber water tank, and the concentrated chamber is not fed with water thereafter.

[0015] (3) Secondary anion selective electrodialysis

[0016] The feed water of the primary anion selective electrodialyzer is used as the feed water of the secondary anion selective electrodialyzer, and the clarified desulfurization waste water is used as the concentrated chamber feed water of the secondary anion selective electrodialyzer, so that the secondary anion selective electrodialyzer further removes chlorides, the concentrated water of the secondary anion selective electrodialyzer is mixed with the clarified desulfurization waste water and returned as the feed water of the primary chamber, and the feed water of the primary chamber is returned to the desulfurization system for recycling.

[0017] As described above, the flocculation clarification method in step (1) is that the flocculation clarification agent is added to the desulfurization waste water in an amount of 50-200 mg / L for flocculation clarification.

[0018] As described above, the flocculation clarification method in step (1) is that the flocculation clarification agent is added to the desulfurization waste water in an amount of 50-200 mg / L for flocculation clarification.

[0019] As described above, the heavy metal removal method in step (2) is that the heavy metal capturing agent is added to the water, so that the heavy metal capturing agent and the heavy metal form a precipitate to remove the heavy metal, and the addition amount of the heavy metal capturing agent is 10-200 mg / L.

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

[0021] As described above, the feed water amount of the clarified desulfurization waste water as the concentrated chamber feed water of the secondary anion selective electrodialyzer in step (3) is 20-100%.

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

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

[0024] As described above, the primary anion selective electrodialysis is operated in a constant current or constant voltage mode.

[0025] As described above, the secondary anion selective electrodialysis is operated in a constant voltage mode.

[0026] As described above, the primary and secondary anion selective electrodialysis cell uses intermittent method, i.e. after the cell solution is desalted, the fresh water is discharged and then water is continuously added for operation.

[0027] The advantages of the present application are:

[0028] 1. The desulfurization wastewater does not need to be completely softened, only a small amount of sodium carbonate is added to eliminate the supersaturation state of calcium sulfate, which can greatly save the softening cost. At the same time, by controlling the pH of the wastewater to be acidic, the carbonate in the water which can form scale with calcium and magnesium ions is eliminated, thus effectively inhibiting the generation of scale and ensuring the long-term stable operation of the whole system.

[0029] 2. The use of two-stage anion selective electrodialysis can more effectively control the leakage of sulfate, resulting in a low proportion of sulfate in the concentrated cell solution. In subsequent concentration, the risk of forming calcium sulfate scale is greatly reduced, which is conducive 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 high, and the leakage of divalent ions is less. The current efficiency, desalination speed and selectivity coefficient are reduced, and the leakage of divalent ions increases. Because the price of anion selective electrodialysis membrane is high, in addition to the large current and high selectivity coefficient, therefore, within the allowable range of the membrane, a relatively high current is usually used. However, when the concentration of chloride ions decreases, the limiting current will decrease accordingly. If the limiting current is lower than the operating current, 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, causing the rapid increase of the permeation flux of sulfate. The lower the concentration of chloride ions, the smaller the limiting current, and the more the permeation flux of sulfate increases. In the previous research of the inventors, by reducing the voltage and current at the later stage of desalination, the leakage of sulfate ions can be greatly inhibited. However, this process also has some disadvantages. When the removal rate of chloride ions is high and the residual chloride ion concentration is low, the limiting current is small. Even if a relatively small voltage is used, the current will still exceed the limiting current, and the leakage of sulfate will be large. If a smaller voltage is used to make the operating current less than the limiting current, but the current is too small, the desalination speed is too slow, and for the same desalination amount, more electrodialysis membranes are needed, which increases the investment cost. For example, if the current is reduced from 100 A to 20 A, the membrane area needs to be increased by 5 times under the condition that the current efficiency does not change much. In addition, even if the current does not exceed the limiting current, a smaller current also corresponds to a lower selectivity coefficient, and the permeation amount of sulfate will also increase. Therefore, although the leakage of sulfate was effectively inhibited by using a lower voltage at the later stage in the previous research of the inventors, the selectivity coefficient was still reduced to 16-21, and the leakage of sulfate reached about 5%. This will result in the formation of more calcium sulfate during the subsequent evaporation crystallization, the risk of scaling is high, and the impurities in the final crystallization product are also more. The present application divides the dechlorination of wastewater into two stages. The first-stage anion selective electrodialysis is operated in the high-efficiency interval, and a small amount of sulfate is leaked into the concentrated chamber while most of the chloride ions are removed. For the remaining chloride ions, the second-stage anion selective electrodialysis is used to deeply dechlorinate the desulfurization wastewater. At this time, a relatively high voltage can be selected to speed up the desalination speed. Although there is a lot of leakage of sulfate, the concentrated water returned as the feed water of the first-stage electrodialysis does not increase the sulfate in the concentrated water of the first-stage electrodialysis. Through the use of two-stage anion selective electrodialysis, the concentrated water discharged from the system is the concentrated water of the first-stage anion selective electrodialysis, the selectivity coefficient is above 100, the leakage of sulfate is reduced to less than 1%, and the content of sulfate is very small, which has little effect on the subsequent concentration. At the same time, the content of impurities such as calcium sulfate in the product is also greatly reduced.

[0030] 3. The present application reuses the desalted dilute chamber liquid for desulfurization, and through deep dechlorination, the reused water in the desulfurization system can absorb more chloride ions, thereby reducing the amount of desulfurization wastewater and reducing the operation cost.

[0031] 4. In the first electrodialysis concentration chamber, 0.3-1.0wt% sodium chloride solution is added as the inlet water at the beginning of operation, and the addition amount only needs to be circulated between the electrodialysis membrane and the concentrated water tank. Since no concentrated water is added thereafter, the electrodialysis can obtain concentrated water with higher salt concentration. During the electrodialysis process, chloride ions and sodium ions, calcium ions and magnesium ions migrate from the material chamber to the concentrated chamber through the electrodialysis membrane, carrying water into the concentrated chamber. At the same time, when the salt content in the concentrated chamber is high, the salt concentration in the material chamber is low, and the chemical potential of water is high. Under the driving of the chemical potential, water will permeate from the dilute chamber to the concentrated chamber. Due to the migration of these two parts of water to the concentrated chamber, the salt concentration in the concentrated chamber will not continue to rise, but will basically remain unchanged after rising to a certain extent. Due to the dilution of the concentrated water, the concentrated water in the concentrated chamber is provided by the migration of the material chamber, which can make the salt content in the concentrated chamber higher, and is beneficial to reduce the cost of subsequent evaporation crystallization.

[0032] The beneficial effects of the present application are:

[0033] The present application provides a method for deep removal of chloride ions from desulfurization wastewater. The method effectively removes chloride ions from wastewater while effectively inhibiting the leakage of sulfate. Using this process, the selectivity coefficient can be maintained above 100 when the chloride ion removal rate reaches 95%-99%, and the sulfate leakage rate is less than 1%. Not only can the chloride ion capacity of the reused water be improved, but also the risk of calcium sulfate scale formation during the concentration process of the concentrated water is greatly reduced, making the system run more stably and reducing the content of impurities such as calcium sulfate in the final crystalline salt. In addition, since the concentrated chamber does not need to be additionally watered after operation, the salt concentration in the concentrated chamber can be greatly improved, and the cost of subsequent evaporation crystallization is greatly reduced. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 The process flow diagram for resource utilization of desulfurization wastewater in the embodiment of the present application.

[0035] Figure 2 The principle diagram of anion selective electrodialysis in the embodiment of the present application, wherein C represents a cation exchange membrane, and A represents an anion selective exchange membrane. DETAILED DESCRIPTION

[0036] The embodiments of the present application will be described in detail below to make the advantages and features of the present application more easily understood by those skilled in the art, so as to define the protection scope of the present application more clearly and explicitly.

[0037] As Figure 1A process flow chart of the treatment method of the resourceful desulfurization wastewater provided by the application is shown, including the following steps:

[0038] (1) Pretreatment

[0039] In the desulfurization wastewater produced in the limestone-gypsum wet desulfurization process, sodium hydroxide or calcium hydroxide is added 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 desulfurization wastewater, followed by adding a flocculating agent for flocculation and clarification, and finally using an ultrafiltration membrane with a pore size of 0.05-0.1 microns to filter to further remove suspended solids and colloids in the clarified liquid to obtain clarified desulfurization wastewater;

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

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

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

[0043] The method for removing heavy metals is to add a heavy metal capturing agent to the water, so that the heavy metal capturing agent forms a precipitate with the heavy metals to remove the heavy metals. The addition amount of the heavy metal capturing agent is 10-200 mg / L.

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

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

[0046] The "concentrated water" and "diluted water" mentioned in the present application are relative concepts, referring to the effluent with different amounts of chloride salt after the treatment of cation exchange membrane and anion exchange membrane in the same electrodialyzer, wherein the effluent with high amount of chloride salt is the concentrated water, and the effluent with low amount of chloride salt is the diluted water. The effluent in different electrodialyzers is also called the concentrated water and the diluted water.

[0047] The cation exchange membrane in the electrodialyzer for selective electrodialysis is not particularly limited in the present application, and the cation exchange membrane known in the art can be used; the anion exchange membrane uses a monovalent anion exchange membrane. In the examples, the anion exchange membrane and the cation exchange membrane use the monovalent anion selective exchange membrane ASV and the conventional cation exchange membrane CMV of Japan Asahi Glass Co., Ltd. respectively, and other monovalent anion exchange membranes and cation exchange membranes meeting the requirements can also be selected to replace.

[0048] The primary anion selective electrodialyzer and the primary anion selective electrodialyzer used in the present application are conventional electrodialyzers, and only the types and arrangement modes of the ion exchange membranes used in the electrodialyzers are adjusted, wherein both ends are cation exchange membranes, and the middle is anion exchange membranes and cation exchange membranes arranged alternately. The involved concentrated chamber, feed chamber and the like are of conventional structures. For example, Figure 2 The selective electrodialysis principle of the anion selective electrodialyzer is shown in the schematic diagram, wherein the anion exchange membrane uses ASV, and the cation exchange membrane uses CMV. The anion exchange membrane has positive charge (+) to combine with anions so that the anions enter the membrane, and the cation exchange membrane has 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 permselectivity P, which is the ratio of the number of chloride ions and sulfate ions migrated to the concentrated water to the concentration value ratio of chloride ions and sulfate ions in the initial feed solution in the electrodialysis process:

[0050]

[0051] In the formula, c i is the concentration of ions; V i is the volume of the solution; the subscripts d and c represent the diluted chamber and the concentrated chamber respectively; t and 0 are the time t and the initial time respectively.

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

[0053]

[0054] The smaller the leakage rate of sulfate ions, the less the sulfate in the concentrated chamber, and the higher the purity of chloride salt.

[0055] Example 1

[0056] Desulfurization wastewater produced in the limestone-gypsum wet desulfurization process of a factory was treated by anion selective electrodialysis to remove chloride salt. The chloride ion content in the wastewater was 17500 mg / L, the sulfate ion content was 3100 mg / L, the calcium ion content was 3150 mg / L, and the magnesium ion content was 1630 mg / L. The specific steps were as follows:

[0057] Calcium hydroxide was added to adjust the pH to about 9.0;

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

[0059] 100 mg / L of polymeric ferric sulfate was added;

[0060] 0.05 micron hollow fiber ultrafiltration membrane was used for filtration

[0061] The desulfurization wastewater after ultrafiltration was sent to the feed chamber of the first anion selective electrodialysis, and the pH automatic detection and dosing system was used to control the pH of the feed chamber between 3 and 5;

[0062] 0.3 wt% sodium chloride solution was added to the concentrated chamber as the influent of the concentrated chamber, and the amount of addition was enough to make the concentrated chamber liquid circulate between the electrodialysis membrane and the concentrated chamber water tank, and no more water was added subsequently.

[0063] The feed chambers of the first and second anion selective electrodialysis were used in intermittent method, the feed chamber was first filled with influent, then desalination was carried out, after desalination, the dilute water was discharged, and then the water was continuously added and desalination was carried out.

[0064] The anion exchange membrane of the selective electrodialysis was ASV, and the cation exchange membrane was CMV. The current density of the first selective electrodialysis was controlled at 250 A / m 2 , and the chloride ion was removed to about 6000 mg / L; then the current density was reduced to 180 A / m 2 , when the chloride ion was removed to 3500 mg / L, the feed liquid of the feed chamber was sent to the second selective electrodialysis for further removal of chloride salt, 100 mg / L of heavy metal capture agent trisodium trithiocyanate was added to the concentrated water to remove heavy metal ions, and then the chloride salt crystals were prepared by evaporation crystallization. According to the national standard GB / T23851-2017, the concentrations of toxic and harmful 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 content in the crystals was detected, and the concentrations of Hg, Cd, Cr, Pb and As were 0.8993 mg / L, 0.1831 mg / L, 0.0009 mg / L, 3.7186 mg / L and 0.0017 mg / L respectively, which were all lower than the maximum concentrations of heavy metal elements in the national standard.

[0065] Take 50% of the desulfurization clarified wastewater as the influent of the secondary selective electrodialysis concentration chamber, control the voltage to be 0.4V per pair of membranes, and operate until the chloride ion concentration is reduced to 0.5g / 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 desulfurization clarified wastewater to serve as the dilute chamber influent of the primary electrodialysis.

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

[0067] Since the salt concentration in the extracted concentrated brine is already high enough, it does not need to be further concentrated by electrodialysis, but can be directly evaporated and crystallized.

[0068] Example 2

[0069] The same wastewater as in Example 1 is used for treatment to remove chloride ions:

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

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

[0072] Add 100mg / L of polymeric ferric sulfate;

[0073] Use 0.01 micron hollow fiber ultrafiltration membrane for filtration

[0074] Send the desulfurized wastewater after ultrafiltration to the feed chamber of the primary anion selective electrodialysis, and use the pH automatic detection and dosing system to control the pH of the feed chamber to be between 3 and 5;

[0075] Add 0.3wt% sodium chloride solution to the concentrated chamber as the influent of the concentrated chamber. The amount of addition is enough to enable the liquid in the concentrated chamber to circulate between the electrodialysis membrane device and the concentrated chamber water tank. No subsequent water is added.

[0076] The feed chambers of the primary and secondary anion selective electrodialysis use the intermittent method. First, fill the feed chamber with the feed liquid, then desalt, and after desalination is completed, discharge the fresh water and continue to add water and desalt.

[0077] The anion exchange membrane of the selective electrodialysis is ASV, and the cation exchange membrane is CMV. The current density of the primary selective electrodialysis is controlled to be 200A / m 2 , and the chloride ion is removed to about 5000mg / L; then the current density is reduced to 160A / m 2When the chloride ion is removed to 3000 mg / L, the feed liquid in the feed chamber is sent to the secondary selective electrodialyzer for further removal of chloride salt, and the concentrated water is added with 100 mg / L of heavy metal capturing agent dithiocarbamic acid to remove heavy metal ions, followed by evaporation crystallization to prepare chloride salt crystals. According to the national standard GB / T 23851-2017 for snow melting agent, the concentrations of toxic and harmful metal elements Hg, Cd, Cr, Pb and As should be no higher than 1 mg / L, 5 mg / L, 15 mg / L, 25 mg / L and 5 mg / L respectively. The heavy metal content in the crystals is 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 of which are lower than the maximum concentrations of heavy metal elements in the national standard.

[0078] Take 70% of the desulfurization clarified wastewater as the influent of the secondary selective electrodialysis concentration chamber, control the voltage to be 0.3V per pair of membranes, and run until the chloride ion concentration is reduced to 0.2g / 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 desulfurization clarified wastewater to serve as the dilute chamber influent of the primary electrodialysis.

[0079] The salt concentration in the extracted concentrated brine is 16.3wt%, of which the chloride ion is 103.4g / L and the sulfate ion is 0.18g / L, the selectivity coefficient is 104, and the sulfate leakage rate is 0.95%. The chloride ion concentration in the dilute chamber effluent of the secondary electrodialysis is 0.2g / 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 extracted concentrated brine is already high enough, there is no need to further concentrate it by electrodialysis, and direct evaporation crystallization can be used.

[0081] Comparative Example 1

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

[0083] The desulfurization wastewater after ultrafiltration is sent to the feed chamber of the anion selective electrodialysis, and the pH automatic detection and dosing system is used to control the pH of the feed chamber between 3 and 5.

[0084] The anion selective electrodialysis is operated by the intermittent method, 0.3wt% sodium chloride solution is added to the concentration chamber as the influent of the concentration chamber, the ratio of the influent of the feed chamber to the influent of the concentration chamber is controlled to be 10:1, desalination is carried out after the feed of the feed chamber and the concentration chamber, and then the dilute water and the concentrated water are discharged, followed by the next round of desalination.

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

[0086] As can be seen from the above examples, the method for deeply removing chloride ions from desulfurization wastewater provided by the application can achieve a chloride ion removal rate of 95%-99%, i.e., deeply remove chloride ions, while the selectivity coefficient is kept above 100, the sulfate leakage rate is less than 1%, the sulfate ion concentration in the concentrated water is less than 0.2 g / L, which is much lower than the comparative examples, greatly reducing the risk of calcium sulfate scale formation in the concentration process of the concentrated water, making the system run more stably, and also reducing the content of impurities calcium sulfate in the final crystalline salt. In addition, since the concentration chamber no longer needs additional water after operation, the salt concentration of the concentrated water is higher, which is nearly 2 times that of the comparative examples, and the subsequent evaporation and crystallization cost is greatly reduced.

[0087] The above examples only express several embodiments of the application, and the description is more specific and detailed, but it should not be understood as limiting the scope of the patent of the application. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the application, several modifications and improvements can be made, which belong to the protection scope of the application. Therefore, the protection scope of the patent of the application should be subject to the appended claims.

Claims

1. A method for the advanced removal of chloride ions from desulfurization wastewater, characterized by, The method comprises the following steps: (1) Pretreatment The desulfurization wastewater generated in a limestone-gypsum wet desulfurization process is added with sodium hydroxide or calcium hydroxide to adjust the pH to 8.5-9.5, and then added with sodium carbonate at an amount of 1-5 g per liter of the desulfurization wastewater; subsequently, a flocculating agent is added for flocculation and clarification, and finally, a 0.005-0.1 micron ultrafiltration membrane is used for filtration to further remove the suspended solids and colloids in the clarified liquid to obtain clarified desulfurization wastewater; (2) First-stage anion selective electrodialysis The clarified desulfurization wastewater is fed into a feed chamber of a first-stage anion selective electrodialyzer, and dilute sulfuric acid or hydrochloric acid is used to control the pH of the feed chamber at 3.0-6.5; a sodium chloride solution with a mass percentage concentration of 0.3%-1% is used as the initial feed water of a concentrated chamber of the first-stage anion selective electrodialyzer, and the chlorides in the desulfurization wastewater are removed by the first-stage anion selective electrodialysis to reduce the concentration of chloride ions in the desulfurization wastewater to 3-5 g / L; the effluent of the feed chamber of the first-stage anion selective electrodialyzer is fed into a second-stage anion selective electrodialyzer for further removal of chlorides; and the effluent of the concentrated chamber of the first-stage anion selective electrodialyzer is subjected to heavy metal capture to remove heavy metals, and then evaporated and crystallized to prepare chloride crystals; (3) Second-stage anion selective electrodialysis The effluent of the feed chamber of the first-stage anion selective electrodialyzer is used as the feed water of the second-stage anion selective electrodialyzer, and the clarified desulfurization wastewater is used as the feed water of the concentrated chamber of the second-stage anion selective electrodialyzer; the second-stage anion selective electrodialyzer is used for further removal of chlorides; the concentrated water of the second-stage anion selective electrodialyzer is mixed with the clarified desulfurization wastewater and returned as the feed water of the feed chamber of the first-stage anion selective electrodialyzer; and the effluent of the feed chamber is returned to the desulfurization system for recycling.

2. The method of claim 1, wherein, The method for flocculation and clarification in step (1) is that a clarifying agent is added into the desulfurization wastewater at an amount of 50-200 mg / L for flocculation and clarification.

3. The method of claim 1, wherein, The method for removing heavy metals in step (2) is that a heavy metal capture agent is added into the water to form a precipitate with the heavy metals to remove the heavy metals, and the amount of the heavy metal capture agent added is 10-200 mg / L.

4. The method of claim 3, wherein, The heavy metal capture agent is one or more of dithiocarbamate, trisodium trithiocyanate, and sodium methyl-substituted dithiocarbamate.

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

6. The method of claim 1, wherein, The anion exchange membranes of the first-stage anion selective electrodialyzer and the second-stage anion selective electrodialyzer are monovalent anion selective exchange membranes, and the cation exchange membranes are non-selective cation exchange membranes.

7. The method of claim 6, wherein, The monovalent anion selective exchange membrane is ASV, ACS, PC-MVA, or A-102, and the cation exchange membrane is CMV.

8. The method of claim 1, wherein, The first-stage anion selective electrodialysis is operated in a constant current or constant voltage mode.

9. The method of claim 1, wherein, The second-stage anion selective electrodialysis is operated in a constant voltage mode.

10. The method of claim 1, wherein, The feed chambers of the first-stage anion selective electrodialyzer and the second-stage anion selective electrodialyzer are intermittently operated by discharging fresh water after the desalination of the solution in the feed chamber and then feeding water.

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