Method for regenerating caustic soda desulfurization solution

Abstract

The application provides a caustic soda method desulfurization liquid regeneration method, comprising: A) adding the flue gas desulfurization liquid containing sodium sulfite after liquid reduction desulfurization into an oxidation reactor, introducing oxygen, and performing an oxidation reaction in the presence of a catalyst to obtain a solution containing sodium sulfate; B) using bipolar membrane electrodialysis electrolysis to obtain a sodium hydroxide solution and a sulfuric acid solution. The application first oxidizes sodium sulfite and sodium bisulfite in the desulfurization liquid into sodium sulfate, and then uses bipolar membrane electrodialysis electrolysis to obtain sulfuric acid and sodium hydroxide. The obtained sodium hydroxide solution can be used as an absorption liquid for a flue gas desulfurization tower (alkali absorption tower), and sulfuric acid is produced as a byproduct. The purpose of comprehensive utilization of resources and cost reduction is achieved.

Description

Technical Field

[0001] This invention relates to the field of flue gas desulfurization technology, and in particular to a method for regenerating desulfurization liquid using caustic soda. Background Technology

[0002] With increasingly stringent environmental protection requirements, flue gas desulfurization (FGD) technologies in coal-fired power plants and steel mills are facing higher standards and challenges. While the traditional limestone-gypsum FGD process is technically mature, it suffers from problems such as equipment scaling, difficulties in gypsum treatment, and wastewater discharge. In contrast, the caustic soda FGD process offers advantages such as rapid absorption rate, low liquid-to-gas ratio, and the ability to maintain all compounds in solution, avoiding scaling and clogging issues in equipment and pipelines. However, the caustic soda process also has significant drawbacks: caustic soda is expensive, leading to high operating costs, and it generates high-concentration saline wastewater, the discharge of which also impacts the environment.

[0003] Currently, the dual-alkali desulfurization process absorbs sulfur dioxide from flue gas using sodium carbonate or sodium hydroxide solution, generating a solution containing sodium sulfite and sodium bisulfite. Sodium hydroxide is then regenerated using lime or limestone, enabling the recycling of caustic soda. This process reduces operating costs to some extent, but because the regenerated alkali solution is a saturated calcium sulfite solution, it easily forms scale in the transport pipelines and absorption tower, affecting the long-term operation of the system. Furthermore, the sodium-calcium dual-alkali process has a long flow path and numerous control points during actual operation, which can easily lead to unstable control and affect operational efficiency.

[0004] Hundreds of flue gas desulfurization technologies have been researched and developed worldwide, but fewer than 20 are commercially applied. Based on whether water is added during the desulfurization process or the dry / wet state of the desulfurization products, flue gas desulfurization can be divided into three categories: wet, semi-dry, and dry processes. Wet desulfurization technology is mature, efficient, reliable, and simple to operate, but the treatment of desulfurization products is relatively complicated, requiring a large area and significant investment. Dry and semi-dry methods produce dry powder desulfurization products, which are easy to process, and the processes are simpler. Investment is generally lower than traditional wet methods, but dry and semi-dry methods using lime (limestone) as a desulfurizing agent have a high Ca / S ratio, resulting in low desulfurization efficiency and low utilization rate of the desulfurizing agent.

[0005] In recent years, although semi-dry and dry desulfurization technologies and their applications have made significant progress, wet desulfurization remains the most widely used desulfurization technology in the world. Among wet processes, the limestone (lime)-gypsum process is the most mature and widely used, but it requires a high liquid-to-gas ratio, consumes a lot of power, and suffers from scaling and clogging problems in equipment and pipelines.

[0006] Caustic soda absorbs sulfur oxides faster than calcium alkali, requires a much lower liquid-to-gas ratio, thus saving energy. Furthermore, all compounds within the system remain in solution, preventing scaling and clogging of equipment and pipelines. Therefore, the caustic soda absorption method has gained increasing attention. However, the high price of caustic soda leads to high operating costs, and the generation of highly concentrated saline wastewater also impacts the environment.

[0007] Chinese patent CN1302920A discloses an electrolysis method for sodium sulfate and sodium bisulfate used in flue gas desulfurization and suitable for anion exchange membrane electrolyzers, which regenerates sodium hydroxide through anion exchange membrane electrolysis. This invention can effectively reduce electrolysis energy consumption; however, the regenerated solution still contains a large amount of sodium sulfate, while the content of sodium hydroxide, which has the actual absorption capacity for sulfur oxides, is very low. Reusing this regenerated solution in the absorption tower would increase the power consumption of the flue gas desulfurization system. Summary of the Invention

[0008] In view of this, the technical problem to be solved by the present invention is to provide a caustic soda desulfurization and regeneration method, which has a high conversion rate. This invention provides a method for regenerating caustic soda desulfurization liquid, comprising: A) The flue gas desulfurization liquid containing sodium sulfite after liquid subtraction desulfurization is added to an oxidation reactor, oxygen is introduced, and an oxidation reaction is carried out in the presence of a catalyst to obtain a solution containing sodium sulfate. B) The sodium sulfate solution was electrolyzed using bipolar membrane electrodialysis to obtain sodium hydroxide solution and sulfuric acid solution.

[0009] In some specific embodiments, the catalyst is a transition metal salt; the transition metal salt includes one or more of cobalt sulfate or manganese sulfate.

[0010] In some specific embodiments, the oxidation reaction in step A) is carried out at a temperature of 100~130℃, a pressure of 0.3~0.8 MPa, and a reaction time of 1~3 h.

[0011] In some specific embodiments, the concentration of metal ions in the catalyst is 10 to 100 ppm.

[0012] In some specific embodiments, the oxygen flow rate is 0.05~0.2 m / s.

[0013] In some specific embodiments, the pretreatment of the flue gas desulfurization liquid containing sodium sulfite involves adjusting the pH value to 7-9.

[0014] In some specific embodiments, the parameters of the bipolar membrane electrodialysis in step B) include: bipolar membrane current density: 500~1500 A / m²; Operating temperature: 40 - 60 °C; Sodium sulfate concentration in feed: 150 - 250 g / L.

[0015] Sulfuric acid concentration produced: 9% 23%, producing caustic soda with a concentration of 4-10%.

[0016] In some specific embodiments, the residence time of the sodium sulfate-containing solution in step B) in the electrolytic cell of the bipolar membrane electrodialysis is 2 to 15 hours.

[0017] In some specific embodiments, the sodium hydroxide in step B) is returned as an absorbent to the liquid-subtractive desulfurization step to absorb acidic sulfur dioxide gas, resulting in flue gas desulfurization liquid containing sodium sulfite.

[0018] In some specific embodiments, the bipolar membrane is composed of a cation exchange layer (N-type membrane), an intermediate hydrophilic layer (catalytic layer), and an anion exchange layer (P-type membrane).

[0019] Compared with existing technologies, this invention provides a method for regenerating flue gas desulfurization liquid using the caustic soda method, comprising: A) adding the flue gas desulfurization liquid containing sodium sulfite after liquid-subtractive desulfurization to an oxidation reactor, introducing oxygen, and carrying out an oxidation reaction in the presence of a catalyst to obtain a solution containing sodium sulfate; B) electrolyzing the sodium sulfate solution using bipolar membrane electrodialysis to obtain a sodium hydroxide solution and a sulfuric acid solution. This invention first oxidizes the sodium sulfite and sodium bisulfite in the desulfurization liquid to sodium sulfate, and then uses bipolar membrane electrodialysis to electrolyze it into sulfuric acid and sodium hydroxide. The resulting sodium hydroxide solution can be reused as an absorbent in the flue gas desulfurization tower (alkali absorption tower), while simultaneously producing sulfuric acid as a byproduct. This achieves the goals of comprehensive resource utilization and cost reduction. Attached Figure Description

[0020] Figure 1 This is a process flow diagram of a caustic soda desulfurization liquid regeneration method; Figure 2 This is a schematic diagram of bipolar membrane electrolysis of sodium sulfate aqueous solution. Detailed Implementation

[0021] This invention provides a method for regenerating desulfurization liquid using the caustic soda process. Those skilled in the art can refer to this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and fall within the scope of this invention. The method and application of this invention have been described through preferred embodiments. Those skilled in the art can clearly modify or appropriately change and combine the method and application described herein without departing from the content, spirit, and scope of this invention to implement and apply the technology of this invention.

[0022] This invention provides a method for regenerating caustic soda desulfurization liquid, comprising: A) The flue gas desulfurization liquid containing sodium sulfite after liquid subtraction desulfurization is added to an oxidation reactor, oxygen is introduced, and an oxidation reaction is carried out in the presence of a catalyst to obtain a solution containing sodium sulfate. B) The sodium sulfate solution was electrolyzed using bipolar membrane electrodialysis to obtain sodium hydroxide solution and sulfuric acid solution.

[0023] This method uses only caustic soda for desulfurization. The core of caustic soda desulfurization is the absorption of acidic SO2 gas by an alkaline solution. When only caustic soda (NaOH) is used as the absorbent, the following reactions mainly occur: 2NaOH + SO2 → Na2SO3 + H2O (when the alkalinity is relatively strong) NaOH + SO2 → NaHSO3 (when the alkalinity is weak or SO2 is in excess) Typically, a buffer solution system of Na₂SO₃ and NaHSO₃ is formed. However, by adjusting the amount of NaOH added into the desulfurization tower, the main component of the outlet solution can be controlled to be sodium sulfite.

[0024] The caustic soda desulfurization liquid regeneration device of the present invention, such as Figure 1 The device comprises, in sequence, a liquid alkali desulfurization unit, a liquid alkali desulfurization liquid storage tank connected to the liquid alkali desulfurization unit, a clarification tank at the inlet end connected to the outlet of the desulfurization liquid storage tank, an equalization tank at the outlet end of the clarification tank, an oxidation reactor at the equalization tank, a bipolar membrane electrodialysis unit at the outlet end of the oxidation reactor, an alkali outlet at the bipolar membrane electrodialysis unit connected to the alkali inlet of the liquid alkali desulfurization unit, and a sulfuric acid collection unit at the by-product acid outlet.

[0025] The flue gas desulfurization liquid containing sodium sulfite after liquid-subtractive desulfurization is added to an oxidation reactor, oxygen is introduced, and an oxidation reaction occurs under the action of a transition metal salt catalyst, whereby sodium sulfite and sodium bisulfite are completely oxidized to sodium sulfate. The transition metal salt catalyst of this invention is preferably one or more of cobalt sulfate or manganese sulfate.

[0026] In some specific embodiments, the concentration of metal ions in the catalyst is 10~100 ppm; specifically, it can be 10ppm, 20ppm, 30ppm, 40ppm, 50ppm, 60ppm, 70ppm, 80ppm, 90ppm or 100ppm. Within this concentration range, the catalyst can fully exert its catalytic oxidation effect at a low dosage, without introducing too many impurities, and without adding extra burden to subsequent separation and processing.

[0027] In some specific embodiments, the oxidation reaction temperature is 100~130℃, specifically 100℃, 110℃, 120℃, or 130℃; the pressure is 0.3~0.8 MPa, specifically 0.3 MPa, 0.4 MPa, 0.5 MPa, 0.6 MPa, 0.7 MPa, or 0.8 MPa; and the reaction time is 1~3 hours, specifically 1 hour, 1.5 hours, 2 hours, 2.5 hours, or 3 hours. Under these reaction conditions, the oxidation conversion rate of sodium sulfite can reach over 99%, ensuring the purity of the feed for subsequent bipolar membrane electrodialysis treatment.

[0028] During the oxidation reaction, the oxygen flow rate is controlled at 0.05~0.2 m / s, specifically...

[0029] 0.05m / s, 0.1m / s, 0.15m / s, 0.2m / s; this flow rate range can ensure that oxygen and desulfurization liquid are in full contact, providing sufficient oxygen source for oxidation reaction, without wasting oxygen due to excessive flow rate and increasing system operating energy consumption.

[0030] Before the oxidation reaction, the present invention can pretreat the flue gas desulfurization liquid containing sodium sulfite to adjust its pH value to 7-9. This pH range is suitable for the requirements of catalytic oxidation reaction, and can avoid the catalyst activity being affected by excessive acidity or alkalinity, thus ensuring that the oxidation reaction proceeds smoothly.

[0031] After the oxidation reaction is completed to obtain a sodium sulfate-containing solution, it is sent to a bipolar membrane electrodialysis device for electrolysis. Bipolar membrane electrodialysis utilizes the dissociation effect of the bipolar membrane on water under the action of a DC electric field to dissociate sodium sulfate into sodium hydroxide and sulfuric acid, thus directly obtaining sodium hydroxide solution and sulfuric acid solution.

[0032] Bipolar membranes are a novel type of ion-exchange composite membrane, typically composed of a cation exchange layer (N-type membrane), an intermediate hydrophilic layer (catalytic layer), and an anion exchange layer (P-type membrane), making them true reactive membranes. The thickness of the intermediate interface layer is on the nanometer scale (10⁻⁶). -9 Within m), under the action of a DC electric field, the water in the intermediate interface layer dissociates, and hydrogen ions and hydroxide ions are obtained on both sides of the membrane.

[0033] This invention relates to the continuous feeding and discharging of brine and acid / alkali solutions within a bipolar membrane structure. All existing bipolar membranes disclosed in the technology can be used in this invention. The bipolar membrane described in this invention can be a commercially available bipolar membrane, including but not limited to the following: Hangzhou Lanran Technology Co., Ltd.'s BL3T-50 bipolar membrane electrodialysis, Shandong Tianwei Membrane Technology Co., Ltd.'s bipolar membrane, Dongyue Group's bipolar membrane, and Zhejiang Baichen Low Carbon Technology Co., Ltd.'s CLM-SJM type bipolar membrane, etc.

[0034] The theoretical voltage for water electrolysis is 2.057V, of which 1.229V is consumed in the evolution of H2 and O2, and 0.828V is consumed in the water dissociation reaction. The energy consumption for water electrolysis is 198.5kJ. mol⁻¹. In contrast to the electrode electrolysis of water, the bipolar membrane dissociation of water requires only 0.828 V to cause water molecules to dissociate, since no H₂ or O₂ is generated, and the energy consumption is only 79.9 kJ. Therefore, the energy consumption of the bipolar membrane water dissociation process is relatively low, as the energy consumption is mol⁻¹.

[0035] The electrolytic cell is divided into an alkali chamber, a salt chamber, and an acid chamber by an assembly method of bipolar membrane-anion exchange membrane-cation exchange membrane-bipolar membrane.

[0036] The operating parameters for bipolar membrane electrodialysis in this invention are as follows: The bipolar film current density is 500 ~ 1500 A / m², specifically 500 A / m², 600 A / m², 700 A / m², 800 A / m², 900 A / m², 1000 A / m², 1100 A / m², 1200 A / m², 1300 A / m², 1400 A / m², or 1500 A / m².

[0037] The operating temperature is controlled between 40 and 60°C, specifically 40°C, 45°C, 50°C, 55°C, and 60°C. This temperature range can reduce the solution resistance, reduce energy consumption during electrodialysis, and avoid performance damage to the bipolar membrane and ion exchange membrane caused by excessively high temperatures. The concentration of sodium sulfate in the feed is controlled at 150~250 g / L, specifically 150 g / L, 170 g / L, 190 g / L, 210 g / L, 230 g / L, and 250 g / L.

[0038] By controlling the above parameters, a sulfuric acid concentration of 9% can be produced. 23%, producing caustic soda with a concentration of 4-10%. Preferably, the sulfuric acid content is about 10-20%, and the caustic soda content is about 4-10%. This meets the requirements for subsequent reuse and product collection.

[0039] In this invention, the residence time of the sodium sulfate solution in the bipolar membrane electrodialysis electrolysis cell is controlled to be 2-15 hours, specifically 2 hours, 5 hours, 8 hours, 10 hours, 12 hours, or 15 hours, which ensures that the sodium sulfate is fully dissociated and improves the raw material conversion rate.

[0040] The sodium hydroxide solution obtained by electrolysis has a concentration that meets the requirements for desulfurization absorbent and can be directly returned to the caustic soda desulfurization step to absorb acidic sulfur dioxide gas, thereby obtaining flue gas desulfurization liquid containing sodium sulfite again, realizing the recycling of caustic soda. The by-product sulfuric acid solution can be directly collected and sold as a product, realizing the comprehensive utilization of resources.

[0041] The bipolar membrane is composed of an N-type cation exchange layer, an intermediate hydrophilic catalytic layer, and a P-type anion exchange layer. Under the action of an electric field, the water in the bipolar membrane interface layer will dissociate to generate hydrogen ions and hydroxide ions, which will combine with sulfate ions and sodium ions to obtain sulfuric acid and sodium hydroxide, respectively.

[0042] The by-product sulfuric acid, ranging from 12% to 96%, can be applied to various scenarios, including acid leaching (12%–15%), acid washing (40%), wet phosphate production (60%–65%), and drying (92%–96%). The method of this invention can achieve zero discharge of concentrated brine, comprehensive resource utilization, and cost reduction.

[0043] This method does not introduce calcium-based compounds, thus avoiding scaling problems at the source. The entire process operates in a closed loop, without generating high-salt wastewater discharge. At the same time, caustic soda can be recycled, significantly reducing the operating cost of caustic soda desulfurization and solving the problems of high operating costs and large pollution emissions of traditional caustic soda methods.

[0044] This invention relates to a method for regenerating caustic soda desulfurization liquid. A typical caustic soda wet desulfurization system includes an absorption tower, a circulating pump, an alkali preparation and delivery system, a demister, and a flue system. SO2-containing flue gas enters from the bottom of the absorption tower and comes into countercurrent contact with NaOH solution sprayed from the top of the tower, undergoing an absorption reaction. The washed and purified flue gas is discharged after the demister removes droplets. The sodium bisulfite and sodium sulfite solutions (desulfurization liquid) generated in the reaction are collected in a circulating pool at the bottom of the tower. A portion is pumped back to the top of the tower for continued spraying, while the other portion is drawn off as a byproduct and sent to a subsequent treatment unit. By adjusting the amount of NaOH added to the desulfurization tower, the main component of the outlet solution can be controlled to be sodium sulfite. The method includes oxidizing the sodium sulfite-containing flue gas desulfurization liquid into a sodium sulfate-containing solution using catalytic wet air oxidation, and then electrolyzing the sodium sulfate-containing solution into a sodium hydroxide solution and a sulfuric acid solution using bipolar membrane electrodialysis. The electrolytic conversion rate of sodium sulfate in this invention can reach over 95%, and the resulting sodium hydroxide solution can be reused as an absorbent in the flue gas desulfurization tower, while simultaneously producing sulfuric acid as a byproduct. Using the method of this invention, the goals of zero discharge of desulfurization liquid, comprehensive resource utilization, and cost reduction can be achieved.

[0045] It should be understood that the expression “one or more of…” individually includes each of the objects described after the expression, as well as various different combinations of two or more of the described objects, unless otherwise understood from the context and usage. The expression “and / or” combined with three or more described objects should be understood to have the same meaning, unless otherwise understood from the context.

[0046] The terms “including,” “having,” or “containing,” including the use of their grammatical synonyms, should generally be understood as open-ended and non-restrictive, for example, not excluding other unstated elements or steps, unless otherwise specifically stated or understood from the context.

[0047] In this application, the term "and / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural.

[0048] In this application, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of single or multiple items.

[0049] It should be understood that the order of the steps or the order in which certain actions are performed is not important as long as the invention remains operational. Furthermore, two or more steps or actions can be performed simultaneously.

[0050] The use of any and all instances or exemplary language such as “e.g.” or “including” in this document is merely intended to better illustrate the invention and is not intended to limit the scope of the invention unless the claims are made. No language in this specification should be construed as indicating that any unclaimed element is essential to the practice of the invention.

[0051] Furthermore, the numerical ranges and parameters used to define the present invention are approximate values, and the relevant values ​​in the specific embodiments have been presented as precisely as possible. However, any value inevitably contains standard deviations due to individual test methods. Therefore, unless explicitly stated otherwise, it should be understood that all ranges, quantities, values, and percentages used in this disclosure are modified with the word "approximately". Here, "approximately" generally means that the actual value is within plus or minus 10%, 5%, 1%, or 0.5% of a specific value or range.

[0052] It should be understood that in the various embodiments of this application, the order of the above processes does not imply the order of execution. Some or all steps may be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0053] The embodiments and comparative examples of this invention describe some examples, in which the embodiments illustrate certain implementations of the invention. However, this does not mean that the effects of the invention can only be achieved in these examples.

[0054] To further illustrate the present invention, the following describes in detail a method for regenerating caustic soda desulfurization liquid provided by the present invention with reference to embodiments.

[0055] Example 1

[0056] Adjust the pH of a 5L desulfurization solution containing 18% sodium sulfite and 2% sodium bisulfite to 7-9 using liquid alkali. Add 5g of cobalt sulfate (CoSO4) and introduce oxygen into the solution through an aerator at a flow rate of 0.1-0.2 m / s. Maintain the aeration oxidation for 3 hours until the oxidation rate reaches 100%, yielding a 20% sodium sulfate solution. (Follow the attached...) Figure 2 The bipolar membrane-anion exchange membrane-cation exchange membrane-bipolar membrane assembly method divides the electrolytic cell into an alkali chamber, a salt chamber, and an acid chamber. A 20% sodium sulfate solution with a volume of 5 L is injected into the salt chamber. After starting the circulation pump, the power is turned on, with a current density of 1000 A / m³. 2 Na + and SO4 2- Under the influence of voltage, water molecules pass through the anion exchange membrane and cation exchange membrane respectively into the alkaline and acidic chambers. Water molecules between the bipolar membrane layers dissociate into H+ and OH- under the influence of voltage. OH- molecules pass through the cation exchange layer of the bipolar membrane into the alkaline chamber and react with Na+. + NaOH solution is formed; H+ ions pass through the anion exchange layer of the bipolar membrane into the acid chamber and react with SO42-. 2- An H2SO4 solution is formed. After 1.5 hours of electrolysis, the conversion rate of sodium sulfate reaches 95%, the sodium hydroxide concentration in the alkali chamber is 8.0%, and the sulfuric acid concentration in the acid chamber is 19.2%.

[0057] Example 2

[0058] The operating steps are the same as in Example 1, but the operating conditions are as follows: Adjust the pH of a 5L desulfurization solution containing 15.8% sodium sulfite and 2% sodium bisulfite to 7-9 using liquid alkali. Add 5g of manganese sulfate (MnSO4) and purge with oxygen at a flow rate of 0.1-0.2 m / s. Maintain the aeration oxidation for 3 hours until the oxidation rate reaches 100%, yielding a 17.8% sodium sulfate solution. Inject the 5L sodium sulfate solution into the salt chamber of the bipolar membrane electrolyzer. After starting the circulation pump, turn on the power supply at a current density of 1000 A / m. 2 After 1.5 hours of electrolysis, the electrolytic conversion rate of sodium sulfate reached 90%, the sodium hydroxide concentration in the alkali chamber was 7.2%, and the sulfuric acid concentration in the acid chamber was 16.8%.

[0059] Example 3

[0060] The operating steps are the same as in Example 1, but the operating conditions are as follows: Adjust the pH of a 5L desulfurization solution containing 18% sodium sulfite and 2% sodium bisulfite using liquid alkali, controlling the pH to 7-9. Add 5g of manganese sulfate (MnSO4), and introduce oxygen into the desulfurization solution through an aerator at a flow rate of 0.05-0.1 m / s. Maintain the aeration oxidation for 3 hours until the oxidation rate reaches 100%, yielding a 20% sodium sulfate solution. Follow the instructions in the appendix... Figure 2 The bipolar membrane-anion exchange membrane-cation exchange membrane-bipolar membrane assembly method divides the electrolytic cell into an alkali chamber, a salt chamber, and an acid chamber. A 20.0% sodium sulfate solution with a volume of 5 L is injected into the salt chamber. After the circulation pump is started, the power is turned on, with a current density of 1000 A / m³. 2 . Na+ and SO4 2- Under the influence of voltage, water molecules pass through the anion exchange membrane and cation exchange membrane respectively into the alkaline and acidic chambers. Water molecules between the bipolar membrane layers dissociate into H+ and OH- under the influence of voltage. OH- molecules pass through the cation exchange layer of the bipolar membrane into the alkaline chamber and react with Na+. + A NaOH solution is formed; H+ ions permeate through the anion exchange layer of the bipolar membrane into the acid chamber, where they react with SO42- to form an H2SO4 solution. After 1 hour of electrolysis, the electrolytic conversion rate of sodium sulfate reaches 96%, the sodium hydroxide concentration in the alkali chamber is 6.8%, and the sulfuric acid concentration in the acid chamber is 16.5%.

[0061] Comparative Example 1

[0062] A 5L desulfurization solution containing 18.0% sodium sulfite and 2.0% sodium bisulfite was prepared overnight to adjust the pH to 7-9. Air was then introduced at a flow rate of 0.1-0.2 m / s for 3 hours, resulting in an oxidation rate of 80% and a 16.0% sodium sulfate solution. This 5L solution was then injected into the salt chamber. The circulation pump was started, and the power was turned on at a current density of 1000 A / m³. 2 Na + and SO4 2- Under the influence of voltage, water molecules pass through the anion exchange membrane and cation exchange membrane respectively into the alkaline and acidic chambers. Water molecules between the bipolar membrane layers dissociate into H+ and OH- under the influence of voltage. OH- molecules pass through the cation exchange layer of the bipolar membrane into the alkaline chamber and react with Na+. + NaOH solution is formed; H+ ions pass through the anion exchange layer of the bipolar membrane into the acid chamber and react with SO42-. 2- An H2SO4 solution is formed. After 1.5 hours of electrolysis, the conversion rate of sodium sulfate reaches 67%, the sodium hydroxide concentration in the alkali chamber is 4.2%, and the sulfuric acid concentration in the acid chamber is 8.9%.

[0063] Comparative Example 2

[0064] The operating steps are the same as in Comparative Example 1, but the operating conditions are as follows: Adjust the pH of a 5L desulfurization solution containing 18.0% sodium sulfite and 2.0% sodium bisulfite to 7-9 using liquid alkali. Add 5g of ferrous sulfate (FeSO4). Introduce oxygen into the desulfurization solution through an aerator at a flow rate of 0.1-0.2 m / s. Maintain the aeration oxidation for 3 hours, achieving an oxidation rate of 89%, resulting in a 17.8% sodium sulfate solution.

[0065] A 17.8% sodium sulfate solution with a volume of 5 L was injected into the salt chamber of the bipolar membrane electrolyzer. After the circulation pump was started, the power was turned on, and the current density was 1000 A / m. 2 Na + and SO4 2- Under the influence of voltage, water molecules pass through the anion exchange membrane and cation exchange membrane respectively into the alkaline and acidic chambers. Water molecules between the bipolar membrane layers dissociate into H+ and OH- under the influence of voltage. OH- molecules pass through the cation exchange layer of the bipolar membrane into the alkaline chamber and react with Na+. + NaOH solution is formed; H+ ions pass through the anion exchange layer of the bipolar membrane into the acid chamber and react with SO42-. 2- An H2SO4 solution is formed. After 1.5 hours of electrolysis, the conversion rate of sodium sulfate reaches 80%, the sodium hydroxide concentration in the alkali chamber is 6.0%, and the sulfuric acid concentration in the acid chamber is 14.3%.

[0066] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for regenerating caustic soda desulfurization liquid, characterized in that, include: A) The flue gas desulfurization liquid containing sodium sulfite after liquid subtraction desulfurization is added to an oxidation reactor, oxygen is introduced, and an oxidation reaction is carried out in the presence of a catalyst to obtain a solution containing sodium sulfate. B) The sodium sulfate solution was electrolyzed using bipolar membrane electrodialysis to obtain sodium hydroxide solution and sulfuric acid solution.

2. The method according to claim 1, characterized in that, The catalyst is a transition metal salt; the transition metal salt includes one or more of cobalt sulfate or manganese sulfate.

3. The method according to claim 1, characterized in that, The oxidation reaction in step A) is carried out at a temperature of 100-130℃, a pressure of 0.3-0.8MPa, and a reaction time of 1-3h.

4. The method according to claim 1, characterized in that, The concentration of metal ions in the catalyst is 10~100ppm.

5. The method according to claim 1, characterized in that, The oxygen flow rate is 0.05~0.2 m / s.

6. The method according to claim 1, characterized in that, The pretreatment of the flue gas desulfurization liquid containing sodium sulfite involves adjusting the pH value to 7-9.

7. The method according to claim 1, characterized in that, The parameters for the bipolar membrane electrodialysis in step B) include: bipolar membrane current density: 500 ~ 1500 A / m²; Operating temperature: 40 - 60 °C; Sodium sulfate feed concentration: 150 - 250 g / L; Sulfuric acid concentration produced: 9% 23%, producing caustic soda with a concentration of 4-10%.

8. The method according to claim 1, characterized in that, In step B), the sodium sulfate solution is kept in the bipolar membrane electrodialysis cell for 2 to 15 hours.

9. The method according to claim 1, characterized in that, In step B), the sodium hydroxide is returned as an absorbent to the liquid-subtractive desulfurization step to absorb acidic sulfur dioxide gas, resulting in a flue gas desulfurization liquid containing sodium sulfite.

10. The method according to claim 1, characterized in that, The bipolar membrane is composed of a cation exchange layer (N-type membrane), an intermediate hydrophilic layer (catalytic layer), and an anion exchange layer (P-type membrane).

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