Fluorine-chlorine-containing flue gas treatment method

Abstract

The invention discloses a fluorine and chlorine containing flue gas treatment method, and relates to the technical field of resource recycling. Flue gas containing fluorine and chlorine is sequentially subjected to semi-dry method defluorination, washing tower washing dechlorination and desulfurization tower desulfurization, and fluorine, chlorine and sulfur are removed step by step. The treatment process of washing tower wastewater generated by washing tower water-washing dechlorination is optimized, the procedures of neutralization and impurity removal, calcium precipitation with sodium sulfate, hardness and softening with sodium carbonate, deamination, evaporative crystallization and mother liquor drying are sequentially performed, and required chemicals such as lime and sodium sulfate can be added in a solid manner, so that a large amount of clear water is prevented from being brought in, and the evaporation load and the wastewater treatment cost are reduced. By optimizing pretreatment, designing a functional separation material and constructing a multi-section separation system, fluorine and chlorine are accurately separated and are respectively converted into industrial-grade products, and meanwhile it is ensured that purified flue gas reaches the standard to be discharged.

Description

Technical Field

[0001] This invention relates to the field of resource recycling technology, and more specifically, to a method for treating fluorinated and chlorine-containing flue gas. Background Technology

[0002] In the harmless and resource-based treatment of hazardous waste, high-temperature treatment processes (high-temperature incineration and high-temperature smelting) are the mainstream technologies. High-temperature incineration is mainly for hazardous waste with high organic matter and high calorific value (such as waste plastics and organic residues), achieving harmlessness through high temperature; high-temperature smelting is mainly for hazardous waste containing valuable metals (such as electronic waste and electroplating sludge), recovering the metals and realizing resource reuse.

[0003] However, during high-temperature treatment, the fluorine and chlorine elements in hazardous waste decompose to generate acidic gases such as HF and HCl, and some may even produce highly toxic pollutants such as dioxins, which not only corrode equipment but also threaten the environment and human health. Currently, the "Standard for Pollution Control of Hazardous Waste Incineration" (GB 18484-2020) imposes stringent requirements on flue gas emissions (e.g., HF ≤ 3 mg / m³). 3 HCl ≤ 50 mg / m³ 3 The purification of flue gas from high-fluoride and chlorine hazardous waste has become a major challenge for the industry.

[0004] Existing technologies mostly employ a "wet + dry / semi-dry + adsorption" process to simultaneously remove fluoride and chlorine. While this can meet standards, it has significant limitations: First, the fluoride and chlorine removal process forms mixed salts, which are difficult to recover and can only be disposed of as hazardous waste in landfills, resulting in resource waste and solid waste pressure. Second, high-concentration flue gas is prone to "over-removal" or "uneven removal," increasing treatment costs. Currently, there is almost no technology available for the selective separation and recovery of fluoride and chlorine from high-fluoride and chlorine flue gas.

[0005] Therefore, there is an urgent need to develop technologies for the selective separation and recovery of fluorine and chlorine from high-fluorine flue gas.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] The purpose of this invention is to provide a method for treating flue gas containing fluorine and chlorine, which aims to achieve precise separation of fluorine and chlorine and convert them into industrial-grade products respectively, while ensuring that the purified flue gas meets emission standards.

[0008] This invention is implemented as follows: In a first aspect, the present invention provides a method for treating fluorinated and chlorine-containing flue gas, comprising: The flue gas containing fluorine and chlorine is sequentially subjected to semi-dry defluorination, water washing dechlorination in a scrubbing tower, and desulfurization in a desulfurization tower. The steps for treating the wastewater generated from the dechlorination process in the scrubbing tower are as follows: Neutralization and impurity removal: Lime is added to the wastewater in the washing tower to adjust the pH to 11.3-11.7 and react. Then, iron salt is added for deep impurity removal. After solid-liquid separation, neutralized residue and first-treatment liquid are obtained. Sodium sulfate precipitation of calcium: The first treatment solution and sodium sulfate are mixed and reacted, and after solid-liquid separation, calcium sulfate dihydrate and the second treatment solution are obtained. Sodium carbonate dehardening and softening: The second treatment liquid is mixed with sodium carbonate and reacted. After solid-liquid separation, softened residue and softened liquid are obtained. Deammoniation: The softened liquid is stripped to remove ammonia, resulting in a deammonised liquid; Evaporation crystallization: The deammoniation liquid is evaporated and crystallized to separate and precipitate salt substances, and the mother liquor of evaporation crystallization is obtained; Mother liquor drying: The evaporated crystallization mother liquor is sent to the mother liquor drying system for treatment to achieve complete evaporation and salt separation of the mother liquor.

[0009] In an optional embodiment, during the neutralization and impurity removal process, lime is first added to adjust the pH value to 11.3-11.7 and react for 30-120 minutes. Then, iron salt is added and reacted for 10-20 minutes to ensure that the fluoride ion concentration in the first treatment solution is ≤10 mg / L and the arsenic ion concentration is ≤0.1 mg / L. And / or, the lime is selected from at least one of quicklime and hydrated lime; the amount of lime added is related to the acidity, fluoride ions, and sulfate ions in the wastewater of the washing tower, and the amount of lime added is adjusted according to the pH of the reaction endpoint; And / or, the iron salt is selected from at least one of ferric sulfate and ferric chloride, and the amount of Fe in the iron salt corresponds to the volume of wastewater per liter of the washing tower. 3+ The amount is 0.2g~0.3g; And / or, the neutralized slag is returned to the pyrometallurgical system for further processing; And / or, a portion of the first-treatment liquid is returned to the scrubbing tower to balance acidity and increase chloride ion concentration; And / or, the solid-liquid separation method in the neutralization and impurity removal stage adopts vacuum belt filtration.

[0010] In an optional embodiment, during the sodium sulfate precipitation process, the amount of sodium sulfate added is 1.1 to 1.5 times the theoretical amount, and the total concentration of dissolved sulfate ions in the solution after the reaction is ≥5 g / L, and the reaction time is 20 min to 60 min; wherein the theoretical amount of sodium sulfate is calculated based on 1 mol of calcium ions equaling 1 mol of sodium sulfate. And / or, the solid-liquid separation method in the sodium sulfate precipitation stage is vacuum belt filtration, and the dry basis CaSO4·2H2O content in the obtained calcium sulfate dihydrate product is ≥95%.

[0011] In an optional embodiment, during the sodium carbonate dehardening and softening process, the amount of sodium carbonate added is 1.1 to 1.2 times the theoretical amount, wherein the theoretical amount of sodium carbonate is calculated based on 1 mol of calcium ions equaling 1 mol of sodium carbonate, and the reaction time is 30 min to 60 min, so that the concentration of calcium ions in the softened solution is ≤10 mg / L, magnesium ions are ≤5 mg / L, and the total hardness is ≤50 mg / L. And / or, softened slag is used in the neutralization and impurity removal stage; And / or, the solid-liquid separation method in the sodium carbonate dehardening and softening stage is to use a plate and frame filter press.

[0012] In an optional implementation, during the ammonia removal process, the concentration of the recovered ammonia water is controlled to be ≥15wt%, so that the recovered ammonia water can be used in the plant's denitrification system.

[0013] In an optional implementation, a triple-effect evaporation crystallization system is used during the evaporation crystallization process; And / or, the crystalline salts produced in the evaporation and crystallization process are classified and processed according to their quality, and the resulting evaporation condensate and steam condensate are used in the production system.

[0014] In an optional implementation, the evaporation crystallization process requires periodically opening the mother liquor and controlling the color of the mother liquor solution to be ≤500. And / or, the whiteness of the crystalline salt produced by the evaporation and crystallization process is required to be ≥80%.

[0015] In an optional implementation, during the drying process of the mother liquor, the moisture content of the final residue is controlled to be ≤15wt%.

[0016] In an optional embodiment, during the semi-dry defluorination process, the defluorinating agent used is selected from at least one of calcium oxide, calcium hydroxide, sodium carbonate, and sodium bicarbonate, and the amount of water added is controlled to be 4%-6% of the mass of the defluorinating agent; Preferably, the removing agent is sodium carbonate, and the sodium fluoride is purified by rinsing or leaching with water at a temperature of 30℃-60℃, or a mixture of sodium chloride and sodium fluoride is directly used as one of the raw materials for preparing aluminum alloy refining agents; Preferably, the defluorinating agent is controlled to circulate in the semi-dry tower. Fluorine-containing chlorine flue gas at a temperature of 160℃-200℃ is introduced into the semi-dry tower for treatment. Intermittent quantitative discharge and quantitative replenishment are adopted. The discharge is based on the fluorine content of the semi-dry defluorination ash on a dry basis being ≥30wt%.

[0017] In an optional implementation, during the desulfurization process in the desulfurization tower, limestone slurry is used as a medium to absorb sulfur dioxide in the flue gas, while air is simultaneously blown in for oxidation, producing desulfurized gypsum slag.

[0018] This invention offers the following advantages: It sequentially removes fluoride, chlorine, and sulfur from fluoride- and chlorine-containing flue gas through semi-dry defluorination, water washing dechlorination in a scrubbing tower, and desulfurization in a desulfurization tower. The wastewater treatment process from the water washing dechlorination in the scrubbing tower is optimized, involving neutralization and impurity removal, sodium sulfate precipitation, sodium carbonate softening and hardening, ammonia removal, evaporation and crystallization, and mother liquor drying. Required reagents such as lime and sodium sulfate can be added as solids, avoiding the introduction of large amounts of clean water and reducing evaporation load and wastewater treatment costs. By optimizing pretreatment, designing functional separation materials, and constructing a multi-stage separation system, precise separation of fluoride and chlorine is achieved, converting them into industrial-grade products while ensuring that the purified flue gas meets emission standards. Attached Figure Description

[0019] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a process flow diagram for flue gas scrubbing wastewater treatment. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0022] High-fluorine and high-chlorine materials will produce hydrogen fluoride and hydrogen chloride after being processed in high-temperature metallurgical furnaces. Especially in the process of using electrostatic precipitators, because the operating temperature of electrostatic precipitators is higher than that of ordinary bag filters, and hydrogen fluoride and hydrogen chloride do not have the opportunity to react with alkaline oxides such as zinc oxide in the bag filter ash, a large amount of free hydrogen chloride and hydrogen fluoride will be present in the flue gas.

[0023] To address the technical problem of the difficulty in resource utilization of high-fluoride and chlorine flue gas in existing technologies, this invention optimizes the separation and recovery process of fluoride and chlorine-containing flue gas. An embodiment of this invention provides a method for treating fluoride and chlorine-containing flue gas, comprising the following steps: The fluoride and chlorine-containing flue gas is sequentially subjected to semi-dry defluorination, water washing dechlorination in a scrubbing tower, and desulfurization in a desulfurization tower. Through the combination of defluorination, dechlorination, and desulfurization processes, fluorine, chlorine, and sulfur are removed stepwise, and fluorine is utilized as a resource.

[0024] Semi-dry defluorination Semi-dry defluorination uses a mixture of a defluorinating agent and a small amount of water as a treatment reagent to treat fluorinated and chlorine-containing flue gas, thereby removing most of the fluorine.

[0025] In some embodiments, during the semi-dry defluorination process, the defluorinating agent used is selected from at least one of calcium oxide, calcium hydroxide, sodium carbonate, and sodium bicarbonate. The defluorinating agent can be any one or more of these. The amount of water added is controlled to be 4%-6% of the defluorinating agent mass; a small amount of water can achieve a good defluorination effect, and the addition amount can be 4%, 5%, 6%, etc.

[0026] In a preferred embodiment, sodium carbonate is used as the removing agent. Sodium fluoride is purified by rinsing or leaching with water at a temperature of 30℃-60℃, or a mixture of sodium chloride and sodium fluoride can be directly used as a raw material for preparing aluminum alloy refining agents. Sodium carbonate has significant advantages in the fluoride resource recovery process: firstly, the solubility of sodium fluoride in water is much lower than that of sodium chloride and sodium sulfate, allowing for purification by water rinsing or leaching to produce the final product; secondly, impurities such as arsenic that may be present in the flue dust can form soluble salts with sodium carbonate, which can then be purified using sodium fluoride. If calcium hydroxide is used as the removing agent, the resource recovery process is quite complex; see patent CN202311826560.8 (application number) for details.

[0027] Furthermore, the main process control points for semi-dry defluorination are: the temperature of the fluorinated chlorine flue gas in the defluorination step is 180±20℃ (i.e., 160℃-200℃), the defluorinating agent circulates in the semi-dry tower, and the method of intermittent quantitative discharge and quantitative replenishment is adopted, and the discharge is carried out first and then the replenishment is carried out. The discharge basis is that the fluorine content of the semi-dry defluorination ash dry basis is ≥30wt%.

[0028] [Scrubber tower water washing and dechlorination] The flue gas after defluorination in the semi-dry tower enters the scrubbing tower for water washing and dechlorination. In the scrubbing tower, hydrogen chloride in the waste gas is effectively removed by absorption of aqueous solution. The scrubbing tower uses water as a medium to wash and dissolve hydrogen fluoride and hydrogen chloride in the flue gas, producing wastewater (highly acidic wastewater).

[0029] This invention optimizes the steps for treating wastewater from a scrubbing tower. The wastewater, by mass fraction, comprises: fluoride ions 2-5 g / L, chloride ions 30-50 g / L, hydrogen ions 0.5-1.2 g / L, and arsenic ions 0.2-0.8 g / L. For example... Figure 1 As shown, the specific steps are as follows: S1, Neutralization and Impurity Removal Wastewater from the scrubbing tower is collected in a wastewater collection tank and then enters a neutralization reaction tank (such as a four-stage neutralization reaction tank, but not limited to this four-stage). First, lime is added to adjust the pH to 11.3-11.7 for reaction, removing F, As, Mg, and heavy metals. After the reaction is complete, iron salts are added for further removal of As and other impurities. After solid-liquid separation, neutralization sludge and the first-stage treated liquid are obtained. The first-stage treated liquid enters a neutralization sedimentation tank for further treatment. The resulting sludge is returned to the neutralization reaction tank, and the filtrate enters a neutralization filtrate collection tank. A portion of the first-stage treated liquid is returned to the scrubbing tower for reuse, used to balance acidity and increase chloride ion concentration.

[0030] In some embodiments, lime is selected from at least one of quicklime and hydrated lime, and lime can be any one or more of the above. The amount of lime added is related to the acidity, fluoride ions, and sulfate ions in the wastewater of the washing tower, and the amount of lime added is adjusted according to the pH of the reaction endpoint. The iron salt is selected from at least one of ferric sulfate and ferric chloride, and iron salt can be any one or more of the above. Each liter of wastewater volume in the washing tower corresponds to the Fe content in the iron salt. 3+ The amount is 0.2g~0.3g. By adding iron salt, the precipitation reaction or adsorption precipitation of impurities such as arsenic can be enhanced, so as to achieve a deep purification effect.

[0031] Furthermore, during the neutralization and impurity removal process, lime is first added, and the pH is adjusted to 11.3-11.7 by an automatic control system, followed by a reaction of 30-120 minutes. Then, iron salt is added and the reaction is carried out for 10-20 minutes, so that the fluoride ion concentration in the first treatment solution is ≤10 mg / L and the arsenic ion concentration is ≤0.1 mg / L. Specifically, the reaction pH can be 11.3, 11.4, 11.5, 11.6, 11.7, etc., and the reaction time can be 30 minutes, 50 minutes, 80 minutes, 100 minutes, 120 minutes, etc. The reaction time after adding iron salt can be 10 minutes, 13 minutes, 15 minutes, 18 minutes, 20 minutes, etc.

[0032] In some embodiments, a vacuum belt filtration method is used for solid-liquid separation, resulting in a lower water content in the filtered solid material. Specifically, the arsenic- and fluorine-containing waste residue obtained from the filtration of the neutralized slurry is returned to the pyrometallurgical system for further processing, while the filtrate proceeds to the next step. Considering the possibility of returning a portion of the neutralized filtrate to the washing tower to balance acidity and increase chloride ion concentration, a neutralization reaction filtrate storage tank and related valves, pumps, and other equipment are required.

[0033] S2, sodium sulfate precipitation The first treatment solution obtained in step S1 is mixed with sodium sulfate and reacted to precipitate most of the calcium ions in the solution, generating calcium sulfate dihydrate as a byproduct with high whiteness and purity, thereby reducing the cost of dehardening. After the reaction is complete, solid-liquid separation is performed to obtain the byproduct calcium sulfate dihydrate and the second treatment solution, which then proceeds to the next process.

[0034] In an optional embodiment, during the sodium sulfate precipitation process, the amount of sodium sulfate added is 1.1 to 1.5 times the theoretical amount (e.g., 1.1, 1.2, 1.3, 1.4, 1.5 times, etc.), and the total concentration of dissolved sulfate ions in the solution after the reaction is ≥5 g / L. The reaction time is 20 min to 60 min, such as 20 min, 30 min, 40 min, 50 min, 60 min, etc. By adjusting the reaction conditions, calcium ions can be removed more effectively.

[0035] Specifically, there are no restrictions on the form of sodium sulfate addition. For example, self-produced wet sodium sulfate (containing about 5%-10% water) or concentrated solution rich in sodium sulfate can be used.

[0036] Furthermore, the solid-liquid separation method in the sodium sulfate precipitation stage can employ vacuum belt filtration to reduce the water content of the byproduct calcium sulfate dihydrate. The resulting byproduct calcium sulfate dihydrate has a dry basis CaSO4·2H2O content of ≥95%.

[0037] S3, sodium carbonate dehardening and softening The second treatment solution is mixed with sodium carbonate (Na2CO3, purity ≥99%) to further precipitate the remaining calcium ions, achieving deep dehardening and softening of the wastewater. After solid-liquid separation, softened slag and softened liquid are obtained. The softened slag is mainly composed of calcium carbonate and can be reused in the upstream neutralization and impurity removal stage, reducing lime usage and reagent costs. The resulting softened liquid is a qualified pre-treated liquid and is transported to the ammonia removal system.

[0038] In some embodiments, during the sodium carbonate dehardening and softening process, the amount of sodium carbonate added is 1.1 to 1.2 times the theoretical amount, and the reaction time is 30 to 60 minutes (e.g., 30 minutes, 40 minutes, 50 minutes, 60 minutes, etc.). By adjusting the reaction conditions, the calcium ion concentration in the softened solution is ≤10 mg / L, the magnesium ion concentration is ≤5 mg / L, and the total hardness (calculated as CaCO3) is ≤50 mg / L.

[0039] Furthermore, the solid-liquid separation method in the sodium carbonate dehardening and softening stage adopts a plate and frame filter press, but is not limited to this.

[0040] S4, deamination The softened liquid is transported to the stripping and ammonia removal system, where stripping technology separates and removes ammonia from the wastewater, yielding ammonia-removed liquid. The recovered ammonia water is reused in the plant's denitrification system, achieving efficient recycling of ammonia resources. The ammonia-removed liquid then proceeds to the next processing stage.

[0041] In some embodiments, during the ammonia removal process, the concentration of the recovered ammonia water is controlled to be ≥15wt%, so that the recovered ammonia water can be used in the plant's denitrification system.

[0042] S5, Evaporation and Crystallization After ammonia removal, the liquid enters the wastewater evaporation system, where water vaporizes through evaporation, and salts crystallize out. The crystals are then separated into solid and liquid components by a centrifuge, yielding evaporated crystallized salt and mother liquor. The evaporated crystallized salt is processed according to its grade and quality. Evaporation condensate and steam condensate are recycled into the production system, while the mother liquor undergoes further treatment in a mother liquor drying process.

[0043] In some embodiments, a triple-effect evaporation crystallization system is used for treatment during the evaporation crystallization process, but it is not limited to this. The mother liquor from the triple-effect evaporation is transferred to the original evaporation system for further concentration. The salt composition of the wastewater in this project is mainly sodium chloride. A triple-effect evaporation system is used, equipped with crystallizers, centrifuges, and ton bag packaging, etc. The evaporation crystallization process requires periodic opening of the mother liquor and control of the mother liquor solution color to ≤500. The whiteness of the crystalline salt produced in the evaporation crystallization process is required to be ≥80%.

[0044] S6, Mother liquor drying During the evaporation process, pollutants in the mother liquor will continue to accumulate. After repeated evaporation and concentration to a certain concentration, the evaporation crystallization mother liquor obtained in step S5 is sent to the mother liquor drying system for treatment to achieve complete evaporation and salt separation of the mother liquor, thereby achieving the goal of zero discharge of wastewater.

[0045] The mother liquor from the original evaporation system is treated using a mother liquor dryer. Vacuum-reduced evaporation is employed to completely evaporate the mother liquor, resulting in a final residue with a moisture content of ≤15%, achieving complete evaporation and salt separation of the mother liquor. Steam condensate is temporarily stored and reused, while the evaporated condensate, after cooling, is directly reused in the flue gas scrubbing tower.

[0046] It should be noted that during the wastewater treatment process in the scrubbing tower, the required reagents, such as lime and sodium sulfate, should preferably be added in solid form, or a filtrate-to-lime slurry preparation facility should be considered to avoid introducing large amounts of clean water during reagent addition, which would increase the evaporation load and wastewater treatment costs. The original wastewater is acidic and may contain some acidic volatile gases; the treated solution is an ammonium-containing alkaline liquid with ammonia volatilization, so the operating environment needs to be considered.

[0047] Desulfurization tower During the desulfurization process in the desulfurization tower, limestone slurry is used as a medium to absorb sulfur dioxide in the flue gas, while air is blown in at the same time for oxidation, producing desulfurized gypsum slag and a small amount of weakly acidic wastewater (generally requiring no treatment).

[0048] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0049] The composition of the fluorinated flue gas to be treated in the following examples and comparative examples is as follows: SO2 15680 mg / Nm³ 3 HF 760mg / Nm 3 HCl 1370 mg / Nm3 .

[0050] Example 1 This embodiment provides a method for treating flue gas containing fluorine and chlorine, the steps of which are as follows: the flue gas containing fluorine and chlorine is subjected to semi-dry defluorination, water washing dechlorination in a scrubbing tower, and desulfurization in a desulfurization tower in sequence.

[0051] (1) Semi-dry defluorination The main process control points for semi-dry defluorination are as follows: Sodium carbonate and water are used as the defluorinating agent, with water accounting for 5% of the sodium carbonate mass. The temperature of the fluorinated flue gas is 180℃ during the defluorination process. The defluorinating agent (sodium carbonate) circulates within the semi-dry tower. The fluorinated flue gas is input from the bottom of the tower and discharged from the top. Intermittent quantitative discharge and quantitative replenishment of the defluorinating agent are adopted, with discharge preceding replenishment. Discharge is based on a fluorine content of ≥30wt% on a dry basis in the semi-dry defluorination ash. Sodium fluoride is purified by leaching with water at 50℃.

[0052] The flue gas composition after semi-dry sodium carbonate defluorination was tested and found to be as follows: SO2 15184 mg / Nm³ 3 HF 76mg / Nm 3 HCl 929mg / Nm 3 .

[0053] (2) Scrubber water washing and dechlorination The flue gas after defluorination in the semi-dry tower enters the scrubbing tower for water washing and dechlorination. In the scrubbing tower, hydrogen chloride in the waste gas is effectively absorbed and removed through spray washing. The scrubbing tower uses water as a medium to wash and dissolve hydrogen fluoride and hydrogen chloride in the flue gas, producing wastewater with high acidity.

[0054] The composition of the wastewater from the scrubbing tower is as follows: fluoride ions 2.88 g / L, chloride ions 35.27 g / L, hydrogen ions 0.62 g / L, and arsenic ions 0.42 g / L. For example... Figure 1 As shown, the specific steps for wastewater treatment in the scrubbing tower are as follows: S1, Neutralization and Impurity Removal After the wastewater is transported to the neutralization reaction tank, quicklime is added to adjust the pH to 11.5 and react for 60 minutes. Then, iron salt is added for deep impurity removal. F, As, and Mg are removed through the reaction. The slurry is vacuum belt filtered to obtain arsenic- and fluorine-containing waste residue (neutralization residue), which is returned to the pyrometallurgical system for treatment. The filtrate enters the neutralization sedimentation tank for treatment. The resulting bottom sludge is returned to the neutralization reaction tank. The obtained filtrate enters the neutralization filtrate collection tank. 30% of the filtrate is returned to the washing tower for reuse to balance acidity and increase chloride ion concentration. Most of the filtrate enters step S2.

[0055] The iron salt is ferric sulfate, and the amount of Fe in the iron salt corresponds to the volume of wastewater per liter of the washing tower. 3+ The amount is 0.25g.

[0056] Tests showed that the filtrate entering step S2 contained 7.81 mg / L of fluoride ions and 0.08 mg / L of arsenic ions.

[0057] S2, sodium sulfate precipitation of calcium (byproduct calcium sulfate dihydrate) Sodium sulfate reacts with the filtrate obtained in the previous step, precipitating most of the calcium ions in the solution to form calcium sulfate dihydrate, a byproduct with high whiteness and purity. After vacuum belt filtration, the filtrate proceeds to the next step. During the sodium sulfate precipitation process, the reaction time is controlled at 40 minutes. The amount of sodium sulfate added is 1.3 times the theoretical amount, and the total concentration of dissolved sulfate ions in the solution after the reaction is 14.35 g / L.

[0058] S3, sodium carbonate deep dehardening and softening Sodium carbonate (Na2CO3, purity ≥99%) reacts with the filtrate obtained in the previous step to further precipitate the remaining calcium ions, achieving deep dehardening and softening of the wastewater. The resulting filter residue is mainly calcium carbonate, which is recycled in the upstream neutralization process; the resulting filtrate, as a qualified softened liquid, is transported to the deammoniation system.

[0059] During the dehardening and softening process of sodium carbonate, the reaction time is controlled at 35 minutes, and the amount of sodium carbonate added is 1.1 times the theoretical amount.

[0060] Tests showed that the calcium ion concentration in the softened solution was 3.71 mg / L, the magnesium ion concentration was 0.89 mg / L, and the total hardness (calculated as CaCO3) was 12.98 mg / L.

[0061] S4, Deammonia Removal Process The softened liquid is transported to the stripping ammonia removal system, where stripping technology separates and removes ammonia from the wastewater. The recovered ammonia is reused in the plant's denitrification system. The ammonia-removed liquid then proceeds to the next processing stage. During the ammonia stripping process, the bottom temperature of the stripping tower is controlled at 90℃~98℃, and the upper temperature at 105℃~115℃. The recovered ammonia concentration is 15.4wt%, and the ammonia nitrogen content in the wastewater after ammonia removal is 87mg / L.

[0062] S5, Evaporation and Crystallization Process The wastewater after ammonia removal enters a triple-effect evaporation crystallization system. Evaporation causes the water to vaporize, and the precipitated salts are then separated into solid and liquid components by centrifuge. The evaporated crystallized salts are processed according to their quality and grade. The evaporation condensate and steam condensate are recycled into the production system, while the mother liquor is transferred to a mother liquor drying process for further treatment. After treatment by the triple-effect evaporation crystallization system, the concentration of sodium chloride in the mother liquor is 217.51 ​​g / L, the concentration of sodium sulfate is 87.64 g / L, and the solution color is 480.

[0063] S6, Mother liquor drying The evaporation crystallization mother liquor obtained in step S5 is sent to the mother liquor drying system for treatment to achieve complete evaporation and salt separation, thereby achieving the goal of zero discharge of wastewater. The mother liquor from the original evaporation system is treated by the mother liquor dryer to completely evaporate the mother liquor, and the final residue moisture content is ≤15%. The steam condensate is temporarily stored and reused, while the evaporation condensate is directly reused in the flue gas scrubbing tower.

[0064] (3) Desulfurization in desulfurization tower During the desulfurization process in the desulfurization tower, limestone slurry is used as a medium to absorb sulfur dioxide in the flue gas, while air is blown in at the same time for oxidation, producing desulfurized gypsum slag and a small amount of weakly acidic wastewater.

[0065] Tests showed that the dry basis CaSO4·2H2O content of the by-product calcium sulfate dihydrate in this embodiment was 98.2%; the product obtained by evaporation of wastewater after stripping and deammoniation had a salt whiteness of 83.7, a sodium chloride content of 69.1%, and a sodium sulfate content of 28.4%.

[0066] Example 2 The only difference from Example 1 is that in step S1, the pH is adjusted to 11.7, the reaction time is 120 min, 40% of the filtrate is returned to the washing tower for reuse to balance the acidity and increase the chloride ion concentration, and most of the filtrate enters step S2.

[0067] Tests showed that the filtrate entering step S2 contained 6.26 mg / L of fluoride ions and 0.06 mg / L of arsenic ions.

[0068] Tests showed that the dry basis CaSO4·2H2O content in the by-product calcium sulfate dihydrate of this embodiment was 98.4%; the product obtained by evaporation of wastewater after stripping and deammoniation had a salt whiteness of 81.4, a sodium chloride content of 72.5%, and a sodium sulfate content of 25.7%.

[0069] Comparative Example 1 The only difference from Example 1 is that step S2 is skipped and step S3 is performed directly.

[0070] Tests showed that in step S3 of this comparative example, sodium carbonate was used instead of sodium sulfate for decalcification, resulting in a large consumption of sodium carbonate and higher costs, but the calcium removal effect was not affected.

[0071] Tests showed that the calcium ion concentration in the softened solution was 2.58 mg / L, the magnesium ion concentration was 0.65 mg / L, and the total hardness (calculated as CaCO3) was 9.16 mg / L.

[0072] Comparative Example 2 The only difference from Example 1 is that calcium hydroxide is used instead of sodium carbonate in the semi-dry defluorination process.

[0073] Tests showed that the purity of calcium fluoride in the dry ash base of this comparative example was low, containing 65.49 wt% calcium fluoride, 7.63 wt% chlorine, and 2.52 wt% sulfur. The purification process for calcium fluoride is quite complex.

[0074] Comparative Example 3 The only difference from Example 1 is that in step S1 of this comparative example, the pH is adjusted to 10.5.

[0075] Tests showed that the filtrate from this comparative example entering step S2 contained 86.5 mg / L of fluoride ions and 0.37 mg / L of arsenic ions.

[0076] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for treating fluorinated and chlorine-containing flue gas, characterized in that, include: The flue gas containing fluorine and chlorine is sequentially subjected to semi-dry defluorination, water washing dechlorination in a scrubbing tower, and desulfurization in a desulfurization tower. The steps for treating the wastewater generated from the washing tower dechlorination are as follows: Neutralization and impurity removal: Lime is first added to the wastewater in the washing tower to adjust the pH value to 11.3-11.7 for reaction, and then iron salt is added for deep impurity removal. After solid-liquid separation, neutralization residue and first treatment liquid are obtained. Sodium sulfate precipitation of calcium: The first treatment solution and sodium sulfate are mixed and reacted, and after solid-liquid separation, calcium sulfate dihydrate and the second treatment solution are obtained. Sodium carbonate dehardening and softening: The second treatment liquid is mixed with sodium carbonate and reacted. After solid-liquid separation, softened residue and softened liquid are obtained. Deammoniation: The softened liquid is stripped to remove ammonia, resulting in a deammonised liquid; Evaporation and crystallization: The deammoniation liquid is evaporated and crystallized to separate and precipitate salt substances, and the mother liquor of evaporation and crystallization is obtained; Mother liquor drying: The evaporated crystallized mother liquor is sent to the mother liquor drying system for treatment to achieve complete evaporation and salt separation of the mother liquor.

2. The method for treating fluorinated chlorine-containing flue gas according to claim 1, characterized in that, During the neutralization and impurity removal process, lime is first added to adjust the pH value and react for 30 to 120 minutes. Then, iron salt is added and reacted for 10 to 20 minutes to ensure that the fluoride ion concentration in the first treatment solution is ≤10 mg / L and the arsenic ion concentration is ≤0.1 mg / L. And / or, the lime is selected from at least one of quicklime and hydrated lime; the amount of lime added is related to the acidity, fluoride ions, and sulfate ions in the wastewater of the washing tower, and the amount of lime added is adjusted according to the pH of the reaction endpoint; And / or, the iron salt is selected from at least one of ferric sulfate and ferric chloride, and the amount of Fe in the iron salt corresponds to the volume of wastewater per liter of the washing tower. 3+ The amount is 0.2g~0.3g; And / or, the neutralized slag is returned to the pyrometallurgical system for further processing; And / or, a portion of the first treated liquid is returned to the scrubbing tower to balance acidity and increase chloride ion concentration; And / or, the solid-liquid separation method in the neutralization and impurity removal stage adopts vacuum belt filtration.

3. The method for treating fluorinated chlorine-containing flue gas according to claim 1, characterized in that, During the sodium sulfate precipitation process, the amount of sodium sulfate added is 1.1 to 1.5 times the theoretical amount, and the total concentration of dissolved sulfate ions in the solution after the reaction is ≥5 g / L, and the reaction time is 20 min to 60 min. And / or, the solid-liquid separation method in the sodium sulfate precipitation stage is vacuum belt filtration, and the dry basis CaSO4·2H2O content in the obtained dihydrate calcium sulfate product is ≥95%.

4. The method for treating fluorinated chlorine-containing flue gas according to claim 1, characterized in that, During the sodium carbonate dehardening and softening process, the amount of sodium carbonate added is 1.1 to 1.2 times the theoretical amount, and the reaction time is 30 min to 60 min, so that the calcium ion concentration in the softened solution is ≤10 mg / L, the magnesium ion concentration is ≤5 mg / L, and the total hardness is ≤50 mg / L. And / or, the softened slag is used in the neutralization and impurity removal stage; And / or, the solid-liquid separation method in the sodium carbonate dehardening and softening stage is to use a plate and frame filter press.

5. The method for treating fluorinated chlorine flue gas according to claim 1, characterized in that, During the ammonia removal process, the concentration of the recovered ammonia water is controlled to be ≥15wt%, so that the recovered ammonia water can be used in the plant's denitrification system.

6. The method for treating fluorinated chlorine flue gas according to claim 1, characterized in that, The evaporation and crystallization process is carried out using a triple-effect evaporation and crystallization system; And / or, the crystalline salts produced in the evaporation and crystallization process are classified and processed according to their quality, and the resulting evaporation condensate and steam condensate are used in the production system.

7. The method for treating fluorinated chlorine flue gas according to claim 1, characterized in that, The evaporation and crystallization process requires periodically opening the mother liquor and controlling the color of the mother liquor solution to be ≤500. And / or, the whiteness of the crystalline salt produced by the evaporation and crystallization process is required to be ≥80%.

8. The method for treating fluorinated chlorine flue gas according to claim 1, characterized in that, During the drying process of the mother liquor, the moisture content of the final residue is controlled to be ≤15wt%.

9. The method for treating fluorinated chlorine flue gas according to claim 1, characterized in that, In the semi-dry defluorination process, the defluorinating agent used is selected from at least one of calcium oxide, calcium hydroxide, sodium carbonate, and sodium bicarbonate, and the amount of water added is controlled to be 4%-6% of the mass of the defluorinating agent. Preferably, the removing agent is sodium carbonate, and sodium fluoride is purified by rinsing or leaching with water at a temperature of 30℃-60℃, or a mixture of sodium chloride and sodium fluoride is directly used as a raw material for preparing aluminum alloy refining agent; Preferably, the defluorinating agent is controlled to circulate in the semi-dry tower. Fluorine-containing chlorine flue gas at a temperature of 160℃-200℃ is introduced into the semi-dry tower for treatment. Intermittent quantitative discharge and quantitative replenishment are adopted. The discharge is based on the fluorine content of the semi-dry defluorination ash on a dry basis being ≥30wt%.

10. The method for treating fluorinated chlorine flue gas according to claim 1, characterized in that, During the desulfurization process in the desulfurization tower, limestone slurry is used as a medium to absorb sulfur dioxide in the flue gas, while air is simultaneously blown in for oxidation, producing desulfurized gypsum slag.

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