A method for resource-based treatment of desulfurization wastewater

Abstract

The present invention provides a method for resource-based treatment of desulfurization wastewater, firstly pre-treating the desulfurization wastewater, then selectively concentrating the chloride salt by selective electrodialysis, and finally concentrating the concentrated water of the electrodialysis twice and crystallizing it to prepare a chloride salt de-icing agent, so that the fresh water can be reused. The present invention adopts a higher operating voltage in the early stage of electrodialysis and reduces the voltage in the later stage. Through the variable voltage operation mode, the speed and selectivity of dechlorination can be taken into account. While removing chloride ions faster, the selectivity of the dechlorination process is maintained at a higher level, thereby reducing the investment cost of the equipment and achieving a better separation effect.

Description

Technical Field

[0001] The invention belongs to the field of chemical industry and relates to a method for resource-based treatment of desulfurization wastewater, and specifically relates to a method for resource-based treatment of desulfurization wastewater. Background Art

[0002] In the limestone-gypsum wet desulfurization process, a large amount of desulfurization wastewater is produced. In the desulfurization tower, various soluble substances, in addition to SO2, also contain a small amount of Cl - 、F - As well as heavy metal ions, these are absorbed by the desulfurization slurry in the absorption tower. As the desulfurization liquid circulates continuously, these ions are also continuously absorbed and concentrated by the lime slurry in the desulfurization tower, and the Cl in the slurry - Increased concentration will bring many problems. First, it will lead to a decline in the quality of desulfurized gypsum; second, Cl - When the concentration increases to a certain level, the desulfurization efficiency will decrease significantly. - The maximum concentration limit varies, but most are between 5000-20000 mg / L. - The concentration method is to partially discharge the circulating slurry and add fresh limestone slurry at the same time. The discharged wastewater is the desulfurization wastewater.

[0003] Desulfurization wastewater has a high salt content and large fluctuations in water quality, making it very difficult to treat. Conventional treatment processes are mainly chemical precipitation methods, typically three-tank processes, which mainly include four main processes: neutralization, reaction, flocculation, and clarification. However, conventional processes cannot remove salts from wastewater, and discharge will increase the salt content of the water, which will have an adverse impact on the environment.

[0004] In response to the above problems, in recent years, the reuse and zero-emission technology of desulfurization wastewater has developed rapidly. At present, the domestic zero-emission technology and process of desulfurization wastewater can generally be divided into three parts: pretreatment, concentration and reduction, and solidification treatment. Among them, pretreatment technology includes chemical precipitation standard discharge, softening clarification and tubular microfiltration technology. Concentration and reduction technology is the core of zero-emission of desulfurization wastewater, which is divided into two categories: evaporation concentration and membrane concentration. Evaporation concentration includes multiple-effect evaporation, steam recompression and low-temperature evaporation concentration. Evaporation concentration has a large investment and high operating costs. Membrane concentration mainly includes nanofiltration and reverse osmosis technologies. The main function of nanofiltration is to separate monovalent salts and divalent salts, while reverse osmosis increases the concentration of brine through seawater desalination reverse osmosis membrane. During the membrane concentration process, due to the high enrichment of various ion concentrations, the pretreatment requirements are relatively high. Solidification treatment includes evaporation ponds, evaporation crystallization, flue spray evaporation and bypass flue gas evaporation. In general, although there are many choices for the three parts of the zero-emission process, the processes of these processes are relatively long, and the investment and operating costs are relatively high.

[0005] Therefore, it is necessary to develop a desulfurization wastewater treatment method with a shorter process and higher treatment efficiency, and a technical solution to fully utilize the desulfurization wastewater as a resource is an urgent problem to be solved. Summary of the invention

[0006] In order to solve the above problems, the purpose of the present invention is to provide a method for resource-based treatment of desulfurization wastewater, which can effectively realize the resource utilization of desulfurization wastewater.

[0007] In order to achieve the above object, the present invention provides a method for resource-based treatment of desulfurization wastewater, comprising the following steps:

[0008] (1) Pretreatment

[0009] Calcium hydroxide is added to the desulfurization wastewater generated in the limestone-gypsum wet desulfurization section to adjust the pH to 8.5-9.5; then sodium carbonate is added in an amount of 5-10g per liter of desulfurization wastewater; then a clarifier is added for flocculation and clarification, and finally a 0.2-1 micron microfiltration membrane is used to further remove suspended matter in the clarified liquid to obtain filtered wastewater;

[0010] (2) Selective electrodialysis

[0011] The pH of the filtered wastewater is adjusted to 5-6.5 with dilute sulfuric acid or hydrochloric acid, and then passed into the first electrodialyzer for anion selective electrodialysis dechlorination; when the chloride ion concentration in the dilute chamber of the first electrodialyzer is ≥3g / L during electrodialysis dechlorination, the voltage difference between each pair of anion exchange membranes and cation exchange membranes in the first electrodialyzer is adjusted to 1-2V; when the chloride ion concentration in the dilute chamber of the first electrodialyzer is <3g / L during electrodialysis dechlorination, the voltage difference between each pair of anion exchange membranes and cation exchange membranes in the first electrodialyzer is adjusted to 0.5-1V; electrodialysis is performed until 90-98% of the chloride ions in the dilute chamber of the first electrodialyzer are removed;

[0012] (3) Primary concentration

[0013] The primary fresh water from the dilute chamber of the first electrodialyzer is returned to the limestone-gypsum wet desulfurization section, and the primary concentrated water from the concentrated chamber of the first electrodialyzer is respectively introduced into the dilute chamber and the concentrated chamber of the second electrodialyzer for electrodialysis in proportion;

[0014] (4) Secondary concentration

[0015] Part of the secondary concentrated water from the concentration chamber of the second electrodialyzer enters the dilution chamber of the third electrodialyzer, and the other part of the secondary concentrated water enters the concentration chamber of the third electrodialyzer for electrodialysis to concentrate the secondary concentrated water; the secondary fresh water from the dilution chamber of the second electrodialyzer returns to the concentration chamber of the first electrodialyzer as the inlet water for the concentration chamber, and if the amount of the secondary fresh water is insufficient, it is supplemented with tap water;

[0016] (5) Removal of heavy metals and crystallization

[0017] The tertiary concentrated water from the concentration chamber of the third electrodialyzer is added with a heavy metal scavenger to remove heavy metals, and then evaporated and crystallized to prepare a chloride salt de-icing agent. The mother liquor remaining from the crystallization is returned to the selective electrodialysis as the influent water for the dilute chamber; the tertiary fresh water from the dilute chamber of the third electrodialyzer is returned to the concentrate chamber of the second electrodialyzer as the influent water for the concentrate chamber.

[0018] As described above, the flocculation and clarification method described in step (1) is: adding 50-200 mg / L of a clarifier into the desulfurization wastewater for flocculation and precipitation until the desulfurization wastewater is clarified.

[0019] As mentioned above, the clarifier is polyferric chloride or polyferric sulfate.

[0020] As described above, the anion exchange membrane of the first electrodialyzer is a monovalent anion selective exchange membrane, and the cation exchange membrane of the first electrodialyzer is a common non-selective cation exchange membrane; the cation exchange membranes of the second electrodialyzer and the third electrodialyzer are both CMX type cation exchange membranes; and the anion exchange membranes of the second electrodialyzer and the third electrodialyzer are both AMX type anion exchange membranes.

[0021] As described above, the monovalent anion selective exchange membrane is an ASA or ASV type selective anion exchange membrane; and the cation exchange membrane is a CMV type cation exchange membrane.

[0022] As described above, in step (3), the volume ratio of the primary concentrated water entering the dilute chamber and the concentrated chamber of the second electrodialyzer respectively is (5-25):1.

[0023] As described above, the method for removing heavy metals in step (5) is: adding a heavy metal scavenger to water, so that the heavy metal scavenger and the heavy metal form a precipitate to remove the heavy metal, and the added amount of the heavy metal scavenger is 10-100 mg / L.

[0024] As described above, the heavy metal scavenger is one or more of dithiocarbamate, trisodium thiocyanate, dithiocarbamate heavy metal ion scavenger, and methyl-substituted sodium dithiocarbamate.

[0025] As described above, the electrode solutions in the first electrodialyzer, the second electrodialyzer and the third electrodialyzer are all sodium sulfate solutions with a mass percent concentration of 1%.

[0026] As described above, in the first electrodialyzer, the second electrodialyzer and the third electrodialyzer, the anion exchange membranes and the cation exchange membranes are arranged alternately and both ends of the electrodialyzer are cation exchange membranes.

[0027] The advantages of the present invention are:

[0028] 1. The present invention does not soften the desulfurization wastewater, which can save a lot of softener and corresponding equipment costs. The conventional electrodialysis scheme in the prior art also contains the situation that the desulfurization wastewater is not softened, but it will cause the problem of scaling on the surface of the electrodialysis membrane (anion exchange membrane and cation exchange membrane). In the present invention, by controlling the pH in a slightly acidic environment, there is no carbonate in the water that can form scale with calcium and magnesium ions, so the formation of scale can be inhibited, and the long-term stable operation of the entire system can be guaranteed without softening.

[0029] 2. The present invention adopts the method of selective electrodialysis. During the electrodialysis process, due to the selectivity of the anion exchange membrane for monovalent anions, the monovalent anions, mainly chloride ions, preferentially pass through the membrane into the concentrated water, while the polyvalent cation sulfate ions cannot pass through the membrane, thereby selectively removing chloride ions from the desulfurization wastewater; as for the cation exchange membrane, a common type can be used. During the electrodialysis process, there is no selectivity for the removed cations, so the calcium ions, magnesium ions and chloride ions in the desulfurization wastewater migrate into the concentrated water simultaneously, and salts mainly composed of calcium chloride, magnesium chloride and sodium chloride are obtained in the concentrated water. The primary fresh water after most of the chloride ions are removed from the desulfurization wastewater will no longer have the problem of high concentration of chloride ions affecting the desulfurization efficiency, so it can be recycled and used in the desulfurization section. For anion selective electrodialysis, when the voltage is high, the desalination speed is faster, but due to the large voltage driving force, the leakage of sulfate will also increase accordingly, especially in the late stage of desalination, when the chloride ion content is low and the sulfate content is relatively high, the leakage is more; when the voltage is low, the desalination speed is slow, but the leakage of sulfate is less. When the concentration of leaked sulfate radical is too high, calcium sulfate scale may be formed in the subsequent concentration and crystallization process, which affects the operation. Therefore, it is of great significance to control the leakage of sulfate radical. However, if a lower voltage is used, the desalination speed is slower, and more electrodialysis membranes are required, which results in excessive investment costs. In view of this contradiction, the present invention adopts a relatively high voltage in the early stage of electrodialysis to remove most of the chloride ions at a relatively fast speed, and adopts a relatively low voltage to remove the chloride ions in the later stage when the chloride ion concentration is relatively low, so as to control the leakage of sulfate radical, thereby taking into account both the desalination speed and the leakage of sulfate radical.

[0030] 3. Since the salt concentration in the desulfurization wastewater generally does not exceed 5%, the salt content of the concentrated water obtained by selective electrodialysis generally does not exceed 10%, and the energy consumption of direct evaporation and crystallization is relatively high. Therefore, the present invention further uses ordinary electrodialysis to further concentrate the concentrated water of selective electrodialysis, increase the salt content to 20%, and then perform evaporation and crystallization, which can greatly reduce the amount of evaporated water and save a lot of energy consumption.

[0031] 4. The present invention uses the electrodialysis dilute chamber solution and ordinary tap water as the inlet water of the concentrate chamber, and the obtained concentrate components have less harmful substances. After crystallization, the mixed salt of calcium chloride, magnesium chloride and sodium chloride can be used as a deicing agent to achieve resource utilization. Since the concentrate contains not only chloride salts, but also a small amount of sulfates that cannot be avoided from entering the concentrate through the ion exchange membrane, the mother liquor remaining after crystallization will be enriched with sulfates. If all the mother liquors are evaporated and crystallized, sulfates will be brought into the mixed salts. At the same time, calcium sulfate in the sulfates is easy to scale and affect the evaporation and crystallization equipment. Therefore, the small amount of mother liquor remaining after crystallization is returned to the system of the desulfurization section for utilization, which also avoids the influence of sulfates on the crystallization equipment and the crystallization products.

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

[0033] The present invention provides a method for resource-based treatment of desulfurization wastewater. The method can effectively realize resource utilization of desulfurization wastewater. At the same time, a variable voltage operation mode is adopted in the electrodialysis process, which can take into account the speed and selectivity of dechlorination. While removing chloride ions faster, the selectivity of the selective dechlorination process is maintained at a high level, thereby reducing the investment cost of equipment and achieving better separation effect. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0035] Figure 2 Schematic diagram of the principle of selective electrodialysis in the first electrodialyzer in an embodiment of the present invention. DETAILED DESCRIPTION

[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 a clearer and more definite definition of the protection scope of the present invention.

[0037] like Figure 1 The process flow chart of the method for treating resource-based desulfurization wastewater provided by the present invention is shown, which includes the following steps:

[0038] (1) Pretreatment

[0039] Calcium hydroxide is added to the desulfurization wastewater generated in the limestone-gypsum wet desulfurization section to adjust the pH to 8.5-9.5; then sodium carbonate is added in an amount of 5-10g per liter of desulfurization wastewater; then a clarifier is added for flocculation and clarification, and finally a 0.2-1 micron microfiltration membrane is used to further remove suspended matter in the clarified liquid to obtain filtered wastewater;

[0040] The flocculation and clarification method is as follows: 50-200 mg / L of a clarifier is added into the desulfurization wastewater for flocculation and precipitation until the desulfurization wastewater is clarified, and the selected clarifier is polyferric chloride or polyferric sulfate.

[0041] (2) Selective electrodialysis

[0042] The pH of the filtered wastewater is adjusted to 5-6.5 with dilute sulfuric acid or hydrochloric acid, and then passed into the first electrodialyzer for selective electrodialysis dechlorination; when the chloride ion concentration in the dilute chamber of the first electrodialyzer is ≥3g / L during electrodialysis dechlorination, the voltage difference between each pair of anion exchange membranes and cation exchange membranes in the first electrodialyzer is adjusted to 1-2V; when the chloride ion concentration in the dilute chamber of the first electrodialyzer is <3g / L during electrodialysis dechlorination, the voltage difference between each pair of anion exchange membranes and cation exchange membranes in the first electrodialyzer is adjusted to 0.5-1V; the electrodialysis is performed until 90-98% of the chloride ions in the dilute chamber of the first electrodialyzer are removed.

[0043] The anion exchange membrane of the first electrodialyzer is a monovalent anion selective exchange membrane, specifically ASA or ASV, and the cation exchange membrane of the first electrodialyzer is CMV.

[0044] (3) Primary concentration

[0045] The primary fresh water from the dilute chamber of the first electrodialyzer is returned to the limestone-gypsum wet desulfurization section, and the primary concentrated water from the concentrate chamber of the first electrodialyzer enters the dilute chamber and the concentrate chamber of the second electrodialyzer respectively in a volume ratio of (5-25):1.

[0046] The ratio of the primary concentrate water entering the dilute chamber and the concentrate chamber of the second electrodialyzer is related to the chloride content in the original desulfurization wastewater and the goal of the first concentration. The chloride concentration of the secondary concentrate water after the first concentration should be increased to more than 12%. At the same time, the salt content of the dilute chamber after the first concentration should not be less than 0.3% to avoid the problem of too low salt concentration in the dilute chamber and serious water seepage into the concentrate chamber, which affects the concentration effect.

[0047] (4) Secondary concentration

[0048] Part of the secondary concentrate from the concentration chamber of the second electrodialyzer enters the dilution chamber of the third electrodialyzer, and the other part of the secondary concentrate enters the concentration chamber of the third electrodialyzer for electrodialysis to concentrate the secondary concentrate; the secondary fresh water from the dilution chamber of the second electrodialyzer returns to the concentration chamber of the first electrodialyzer as the inlet water for the concentration chamber. If the amount of secondary fresh water is insufficient, it is supplemented with tap water.

[0049] The cation exchange membranes of the second electrodialyzer and the third electrodialyzer are both CMX type cation exchange membranes; and the anion exchange membranes are both AMX type anion exchange membranes.

[0050] (5) Removal of heavy metals and crystallization

[0051] The tertiary concentrated water from the concentration chamber of the third electrodialyzer is added with a heavy metal scavenger to remove heavy metals, and then evaporated and crystallized to prepare a chloride salt de-icing agent. The mother liquor remaining from the crystallization is returned to the selective electrodialysis as the influent water for the dilute chamber; the tertiary fresh water from the dilute chamber of the third electrodialyzer is returned to the concentrate chamber of the second electrodialyzer as the influent water for the concentrate chamber.

[0052] The mother liquor is sent to the dilute chamber of selective electrodialysis, where it is returned to the desulfurization section after some chloride salts are removed.

[0053] Wherein, the method for removing heavy metals in step (5) is: adding a heavy metal scavenger to water, so that the heavy metal scavenger and the heavy metal form a precipitate to remove the heavy metal, and the addition amount of the heavy metal scavenger is 10-100 mg / L. The heavy metal scavenger is one or more of dithiocarbamate, trisodium thiocyanate, dithiocarbamate heavy metal ion scavenger, and methyl-substituted sodium dithiocarbamate.

[0054] The electrode liquid in the first electrodialyzer, the second electrodialyzer and the third electrodialyzer is a sodium sulfate solution with a mass percentage concentration of 1%. The anion exchange membrane and the cation exchange membrane in the first electrodialyzer, the second electrodialyzer and the third electrodialyzer are arranged alternately and both ends are cation exchange membranes.

[0055] The "concentrated water" and "fresh water" mentioned in the present invention are relative concepts, referring to the effluents with different chloride contents after anion and cation exchange membrane treatment in the same electrodialyzer, wherein the effluent with high chloride content is concentrated water, and the effluent with low chloride content is fresh water. Since the effluents from different electrodialyzers are all called concentrated water and fresh water, in order to distinguish them, they are distinguished and named as "primary", "secondary", "tertiary" ...

[0056] The present invention has no special restrictions on the cation exchange membrane in the first electrodialyzer for selective electrodialysis, and any cation exchange membrane known in the art can be used; the anion exchange membrane can be a monovalent anion exchange membrane. In the present invention, the anion exchange membrane and the cation exchange membrane respectively use 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 as substitutes.

[0057] The present invention has no special restrictions on the cation and anion exchange membranes in the second electrodialyzer and the third electrodialyzer used for concentrated water. Ion exchange membranes well known to those skilled in the art can be used. However, in order to obtain brine with a higher concentration, an ion exchange membrane with less water permeation is more preferably used. The cation exchange membrane and anion exchange membrane used for concentrated water in the present invention are respectively the CMX cation exchange membrane and the AMX anion exchange membrane of Astom Co., Ltd., which have less water permeability and are conducive to obtaining concentrated water with a high concentration. Other cation exchange membranes or anion exchange membranes with the same effect can also be used instead.

[0058] The first electrodialyzer, the second electrodialyzer and the third electrodialyzer used in the present invention are all conventional electrodialyzers, and only the type and arrangement of the ion exchange membranes used in the electrodialyzers are adjusted, wherein both ends are cation exchange membranes, and the middle is anion exchange membrane and cation exchange membrane arranged alternately, and the involved concentration chamber, dilution chamber, polar chamber, electrodes, etc. are all conventional structures. Figure 2 The schematic diagram of the principle of selective electrodialysis in the first electrodialyzer is shown, in which the anion exchange membrane uses ASV and the cation exchange membrane uses CMV, wherein the anion exchange membrane carries a positive charge (+) so as to combine with anions and allow anions to enter the membrane, and the cation exchange membrane carries a negative charge (-) so as to combine with cations and allow cations to enter the membrane. The second and third electrodialyzers only need to replace the anion exchange membrane and the cation exchange membrane.

[0059] One of the main indicators for evaluating the performance of selective electrodialysis separation is the permselectivity coefficient P (Permselectivity), which is the ratio of the number of chloride ions and sulfate ions that migrate to the concentrated water during the electrodialysis process to the concentration of chloride ions and sulfate ions in the initial feed solution:

[0060]

[0061] In the formula, C 0,Cl —Chloride ion concentration in the material chamber solution at the initial state, mol / L

[0062] C 0,S —Sulfate concentration in the chamber solution at the initial state, mol / L

[0063] V0—volume of solution in the material chamber at the initial state, L

[0064] C Nt,Cl —Chloride ion concentration in the solution in the concentration chamber at time t, mol / L

[0065] C N0,Cl —Chloride ion concentration in the concentration chamber solution at the initial state, mol / L

[0066] V Nt—The volume of the solution in the concentration chamber at time t, L

[0067] V N0 —The volume of the solution in the concentration chamber at the initial state, L

[0068] C Nt,S —Sulfate concentration in the solution in the concentration chamber at time t, mol / L

[0069] C N0,S —Sulfate concentration in the concentration chamber solution at the initial state, mol / L

[0070] The selective permeability coefficient P is large, the number of sulfate ions migrating to the concentrated water is small, and the separation performance is good.

[0071] Example 1

[0072] The desulfurization wastewater generated during the limestone-gypsum wet desulfurization process of a certain factory has a chloride ion content of 17500 mg / L, a sulfate ion content of 3100 mg / L, a calcium ion content of 3150 mg / L, and a magnesium ion content of 1630 mg / L.

[0073] Specific process flow Figure 1 As shown, the specific steps are as follows:

[0074] 1) Calcium hydroxide is added to the desulfurization wastewater to adjust the pH to 9.0, and then 5-10 g of sodium carbonate is added per liter of desulfurization wastewater, followed by adding polyferric chloride. After flocculation and clarification, a 0.5 micron microfiltration membrane is used to further remove suspended matter in the clarified liquid to obtain filtered wastewater.

[0075] 2) The membrane stack of the electrodialyzer is composed of anion selective ion exchange ASA and cation exchange membrane CMV, such as Figure 2 As shown, the filtered wastewater is subjected to anion selective electrodialysis to selectively remove chloride ions in the wastewater.

[0076] In the early stage of selective electrodialysis to remove chloride salts, the voltage difference of each pair of anion and cation membranes is controlled at 2.0V, and the operation is carried out until the chloride ion removal in the first electrodialyzer is 83% and the concentration is reduced to below 3g / L; then the operating voltage is reduced and the voltage difference of each pair of anion and cation membranes is controlled at 0.8V, and the operation is carried out until the chloride ion concentration is reduced to about 0.8g / L, and 95.3% is removed. At this time, the sulfate ion is removed by 5.9%, the calcium ion concentration is 2800mg / L, the magnesium ion concentration is 1360mg / L, and the selective permeability coefficient P is 16.1.

[0077] 3) The chloride ion concentration of the primary fresh water obtained after selective electrodialysis is greatly reduced and reused in the desulfurization section; the salt content of the primary concentrated water is 3.4%, and the CMX cation exchange membrane and AMX anion exchange membrane with less water permeability are used for general electrodialysis concentration. The second electrodialyzer increases the salt content of the concentrated water to about 12%, and the third electrodialyzer further increases the salt content to more than 20%. Subsequently, the heavy metal scavenger trisodium thiocyanate is added to remove heavy metals, and a mixed salt of calcium chloride, magnesium chloride and sodium chloride is obtained by evaporation and crystallization, which is used as a chloride salt de-icing agent. No obvious scaling phenomenon was observed during the evaporation process. A small amount of leaked sulfate was sent to the dilute chamber of the selective electrodialysis with the remaining small amount of crystallization mother liquor after evaporation and crystallization, and returned to the desulfurization section with the primary fresh water after most of the chloride salts were removed.

[0078] Example 2

[0079] The same wastewater and pretreatment as in Example 1 are used, and an anion selective ion exchange membrane ASA and a cation exchange membrane CMV are used to form an electrodialyzer. The pretreated wastewater is subjected to anion selective electrodialysis to selectively remove chloride ions in the wastewater.

[0080] In the early stage of selective electrodialysis to remove chloride salts, the voltage difference of each pair of anion and cation membranes was controlled at 1.3V, and the operation was performed until the chloride ion concentration dropped to 3g / L; then the operating voltage was reduced and the voltage difference of each pair of anion and cation membranes was controlled at 0.5V, and the operation was performed until the chloride ion concentration dropped to about 0.9g / L, the chloride ion removal rate was 94.9%, the sulfate removal rate was 4.6%, the calcium ion concentration was 2760mg / L, the magnesium ion concentration was 1340mg / L, and the selective permeability coefficient P was 20.1.

[0081] Fresh water from selective electrodialysis is reused for desulfurization; the salt content of concentrated water is 3.0%, and the less permeable CMX cation exchange membrane and AMX anion exchange membrane are used for two-stage electrodialysis concentration. The first stage increases the salt content of concentrated water to about 12%, and the second stage further increases the salt content to more than 20%. Then, the heavy metal scavenger trisodium thiocyanate is added to remove heavy metals, and a mixed salt of calcium chloride, magnesium chloride and sodium chloride is obtained by evaporation and crystallization, which is used as a chloride salt de-icing agent. No obvious scaling phenomenon is observed during the evaporation process. A small amount of leaked sulfate is sent to the desulfurization chamber of the selective electrodialysis after evaporation and crystallization with the remaining small amount of crystallization mother liquor, and returns to the desulfurization section with the first-stage fresh water after most of the chloride salts are removed.

[0082] Comparative Example 1

[0083] The same wastewater and pretreatment as in Example 1 were used, and an anion selective ion exchange membrane ASA and a common cation exchange membrane CMV were used to form an electrodialyzer. The pretreated wastewater was subjected to anion selective electrodialysis to selectively remove chloride ions from the wastewater.

[0084] When selective electrodialysis is used to remove chloride salts, the voltage difference between each pair of anion and cation membranes is controlled at 2.0V, and the chloride ion is removed by 95.1%, with a residual of 0.86g / L; the sulfate ion is removed by 12.8%. The calcium ion concentration is 2890mg / L, the magnesium ion concentration is 1520mg / L, and the selective permeability coefficient P is 7.4.

[0085] Fresh water from selective electrodialysis is reused for desulfurization; the salt content of concentrated water is 3.5%, and the less permeable CMX cation exchange membrane and AMX anion exchange membrane are used for two-stage electrodialysis concentration. The first stage increases the salt content of concentrated water to about 12%, and the second stage further increases the salt content to more than 20%. Subsequently, through evaporation and crystallization, a mixed salt of calcium chloride, magnesium chloride and sodium chloride is obtained as a chloride salt de-icing agent. Due to the high sulfate concentration, the solution becomes obviously thicker in the later stage of evaporation and crystallization, and there is obvious scaling on the evaporator, which affects the evaporation efficiency. Finally, the amount of residual mother liquor is 3 times that of Example 1, which is sent to the desalination chamber of selective electrodialysis. After most of the chloride salts are removed, it returns to the desulfurization section with the first-stage fresh water.

[0086] Through selective electrodialysis, the chloride ions in the desulfurization wastewater can be removed as much as possible, which can effectively reduce the chlorine content in the desulfurization wastewater, so that more desulfurized flue gas can be treated. If the voltage is kept high, although the chloride ions can be effectively removed, the migration speed of sulfate into the concentrated water in the later stage is greatly accelerated, thereby greatly increasing the sulfate ions in the concentrated water, which is not conducive to subsequent crystallization, and the yield of the obtained crystalline salt is also correspondingly low. Through voltage control, the chloride ions can be removed as much as possible while greatly reducing the removal rate of sulfate, so that the sulfate ion concentration in the concentrated water can be effectively controlled, thereby improving the evaporation crystallization efficiency, reducing the amount of mother liquor generated, and preventing calcium sulfate scaling of ion exchange membranes and evaporation crystallization equipment.

[0087] It can be seen from the above embodiments that the method for resource-based treatment of desulfurization wastewater provided by the present invention can fully utilize the desulfurization wastewater, chloride salt is used to prepare snow-melting agent, concentrated water and fresh water can be recycled between different electrodialyzers or returned to the desulfurization section for reuse; and through variable voltage control during the electrodialysis process, the control of desalination speed and sulfate leakage is taken into account, thereby achieving efficient selective desalination and preventing sulfate scaling.

[0088] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which 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 attached claims.

Claims

1. A method for resource treatment of desulfurization wastewater, characterized in that: The following steps are involved: (1) Pretreatment Calcium hydroxide is added to the desulfurization wastewater generated in the limestone-gypsum wet desulfurization section to adjust the pH to 8.5-9.5; then sodium carbonate is added in an amount of 5-10g per liter of desulfurization wastewater; then a clarifier is added for flocculation and clarification, and finally a 0.2-1 micron microfiltration membrane is used to further remove suspended matter in the clarified liquid to obtain filtered wastewater; (2) Selective electrodialysis The pH of the filtered wastewater is adjusted to 5-6.5 with dilute sulfuric acid or hydrochloric acid, and then passed into the first electrodialyzer for anion selective electrodialysis dechlorination; when the chloride ion concentration in the dilute chamber of the first electrodialyzer is ≥3g / L during electrodialysis dechlorination, the voltage difference between each pair of anion exchange membranes and cation exchange membranes in the first electrodialyzer is adjusted to 1-2V; when the chloride ion concentration in the dilute chamber of the first electrodialyzer is <3g / L during electrodialysis dechlorination, the voltage difference between each pair of anion exchange membranes and cation exchange membranes in the first electrodialyzer is adjusted to 0.5-1V; electrodialysis is performed until 90-98% of the chloride ions in the dilute chamber of the first electrodialyzer are removed; (3) Primary concentration The primary fresh water from the dilute chamber of the first electrodialyzer is returned to the limestone-gypsum wet desulfurization section, and the primary concentrated water from the concentrated chamber of the first electrodialyzer is respectively introduced into the dilute chamber and the concentrated chamber of the second electrodialyzer for electrodialysis in proportion; (4) Secondary concentration Part of the secondary concentrated water from the concentration chamber of the second electrodialyzer enters the dilution chamber of the third electrodialyzer, and the other part of the secondary concentrated water enters the concentration chamber of the third electrodialyzer for electrodialysis to concentrate the secondary concentrated water; the secondary fresh water from the dilution chamber of the second electrodialyzer returns to the concentration chamber of the first electrodialyzer as the inlet water for the concentration chamber, and if the amount of the secondary fresh water is insufficient, it is supplemented with tap water; (5) Removal of heavy metals and crystallization The tertiary concentrated water from the concentration chamber of the third electrodialyzer is added with a heavy metal scavenger to remove heavy metals, and then evaporated and crystallized to prepare a chloride salt de-icing agent, and the remaining mother liquor of the crystallization is returned to the selective electrodialysis as the influent water of the dilute chamber; the tertiary fresh water from the dilute chamber of the third electrodialyzer is returned to the concentration chamber of the second electrodialyzer as the influent water of the concentration chamber; The flocculation and clarification method in step (1) is as follows: adding 50-200 mg / L of a clarifier into the desulfurization wastewater for flocculation and precipitation until the desulfurization wastewater is clarified; The method for removing heavy metals in step (5) is: adding a heavy metal scavenger to water, so that the heavy metal scavenger and the heavy metal form a precipitate to remove the heavy metal, and the added amount of the heavy metal scavenger is 10-100 mg / L.

2. The method according to claim 1, characterized in that The clarifier is polyferric chloride or polyferric sulfate.

3. The method according to claim 1, characterized in that The anion exchange membrane of the first electrodialyzer is a monovalent anion selective exchange membrane, and the cation exchange membrane of the first electrodialyzer is a common non-selective cation exchange membrane; the cation exchange membranes of the second electrodialyzer and the third electrodialyzer are both CMX type cation exchange membranes; the anion exchange membranes of the second electrodialyzer and the third electrodialyzer are both AMX type anion exchange membranes.

4. The method according to claim 3, characterized in that The monovalent anion selective exchange membrane is an ASA or ASV type selective anion exchange membrane; the cation exchange membrane is a CMV type cation exchange membrane.

5. The method according to claim 1, characterized in that In the step (3), the volume ratio of the primary concentrated water entering the dilute chamber and the concentrated chamber of the second electrodialyzer respectively is (5-25):

1.

6. The method according to claim 1, characterized in that The heavy metal scavenger is one or more of dithiocarbamate, trisodium thiocyanate, dithiocarbamate heavy metal ion scavenger, and methyl-substituted sodium dithiocarbamate.

7. The method according to claim 1, characterized in that The electrode solutions in the first electrodialyzer, the second electrodialyzer and the third electrodialyzer are all sodium sulfate solutions with a mass percent concentration of 1%.

8. The method according to claim 1, characterized in that In the first electrodialyzer, the second electrodialyzer and the third electrodialyzer, anion exchange membranes and cation exchange membranes are arranged alternately and both ends are cation exchange membranes.

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