A method for the graded separation and recovery of halogens in combustion exhaust gas

Abstract

This invention discloses a method for the cascade separation and recovery of halogens from combustion exhaust gas, belonging to the field of environmental protection technology. This invention utilizes a spray liquid containing alkaline earth metal ions to treat the combustion exhaust gas fed into a spray tower. Calcium ions in the spray liquid react with hydrogen fluoride to form alkaline earth metal fluoride precipitates. The alkaline earth metal fluoride precipitates and exhaust gas absorbent are separated by filtration. The exhaust gas absorbent is then transported to an electrochemical treatment tank for electrochemical treatment, oxidizing bromide ions in the absorbent to bromate ions. This allows for subsequent separation of chlorides and bromates in the reaction water using a nanofiltration membrane. This invention, through an integrated process of "spraying + electrochemical treatment + membrane separation," separates and recovers halogens from exhaust gas, achieving integrated pollution reduction and reuse, simplifying the waste gas treatment process, reducing waste gas treatment costs, and innovatively transforming halogens from a pollution resource.

Description

Technical Field

[0001] This invention relates to the field of environmental protection technology, specifically to a method for the cascade separation and recovery of halogens in combustion exhaust gas. Background Technology

[0002] Liquid crystal displays (LCDs) offer advantages such as rich colors, small size, light weight, and fast response time, and are currently widely used in electronic display devices such as color televisions. As these devices exceed their lifespan, a large number of discarded LCDs are generated. The liquid crystal material in these discarded LCDs is both hazardous and resource-rich. On the one hand, improper recycling methods can pollute the atmosphere and soil; on the other hand, the organic matter, glass, and metal materials in the liquid crystal material can be ultimately utilized through processing. Currently, discarded LCDs are mainly disassembled manually, separating them into several components such as the LCD panel, fluorescent tubes, metal frames, and waste plastics. Discarded LCD panels need to be sorted and recycled first, and then valuable materials are processed. The liquid crystal material, polarizer, and glass in the discarded LCD panels are separated by mechanical grinding. After mechanical grinding, the liquid crystal material, in powder form, is transported to a pyrolysis furnace for pyrolysis. Pyrolysis, as a commonly used method for treating organic solid waste, can reduce the amount of organic matter in discarded liquid crystals and convert large organic molecules into smaller chemical products, thus realizing their resource utilization. However, because liquid crystal materials contain halogens, halogen-containing pyrolysis oil and gas are generated during the pyrolysis process, posing a threat to the environment. In-situ combustion can effectively remove halogen-containing organic matter from waste liquid crystal pyrolysis oil and gas, but the halogen-containing acidic gases in the combustion exhaust are highly corrosive and irritating, requiring further removal and absorption.

[0003] The problem of halogen-containing acidic gases such as hydrogen fluoride, hydrogen bromide, and hydrogen chloride in the exhaust gas produced after the in-situ combustion of liquid crystal material pyrolysis oil and gas is currently lacking a mature and feasible treatment and recovery technology that can achieve the cascade separation of halogens and the resource utilization of halogens after the exhaust gas is generated.

[0004] Therefore, this invention is proposed. Summary of the Invention

[0005] Based on the deficiencies of existing technologies, the purpose of this invention is to provide a method for the graded separation and recovery of halogens in combustion exhaust gas.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] In a first aspect, the present invention provides a method for the graded separation and recovery of halogens in combustion exhaust gas, comprising the following steps:

[0008] S1. The combustion exhaust gas is introduced into a spray tower, and a spray liquid containing alkaline earth metal ions is introduced into the spray tower. The combustion exhaust gas is sprayed in the spray tower using the spray liquid containing alkaline earth metal ions. The solution obtained after spraying is filtered to separate alkaline earth metal fluoride precipitate and exhaust gas absorption liquid. The combustion exhaust gas contains hydrogen fluoride, hydrogen chloride and hydrogen bromide.

[0009] S2. The tail gas absorption liquid obtained in step S1 is fed into an electrochemical treatment cell for electrochemical treatment to obtain reaction effluent.

[0010] S3. The reaction effluent obtained in step S2 is subjected to membrane separation treatment using a nanofiltration membrane to obtain a bromate solution and a chlorine-containing solution.

[0011] It is understood that the halogen cascade separation and recovery method in combustion exhaust gas described in this invention may further include the following steps:

[0012] Alkaline earth metal fluoride precipitates are collected from the spray tower in step S1 and dried to obtain alkaline earth metal fluoride products.

[0013] The chlorine-containing solution obtained in step S3 is evaporated and concentrated to obtain a chloride crystal product.

[0014] The bromate solution obtained in step S3 is concentrated, enriched, and recovered to obtain the bromate product.

[0015] In theory, the drying and recovery of alkaline earth metal fluoride products can be carried out after step S1, before or after steps S2 and S3, or simultaneously with steps S2 and S3. The steps for recovering chloride crystals and bromate products can be carried out after step S3, and the order of the two steps is not required.

[0016] Waste liquid crystal materials possess both resource value and hazardous properties. Halogens in the in-situ combustion exhaust gas of waste liquid crystal pyrolysis oil and gas mainly exist in the form of acidic gases, such as hydrogen fluoride, hydrogen chloride, and hydrogen bromide. The tiered separation and recovery method for halogens in combustion exhaust gas provided by this invention is suitable for recovering and separating halogens from the in-situ combustion exhaust gas of waste liquid crystal pyrolysis oil and gas. Specifically, this invention achieves reduced emissions and reuse of halogens in combustion exhaust gas through an integrated process of "spraying + electrochemical treatment + membrane separation," simplifying the waste gas treatment process and reducing waste gas treatment costs. The recovered alkaline earth metal fluorides, bromates, and chlorides can be directly used in metallurgy, chemical industry, and other fields. In summary, the method described in this invention innovatively transforms the pollution attribute of halogens into a resource attribute by separating and recovering halogens from exhaust gas.

[0017] The implementation principle of this invention, which combines "spraying + electrochemical treatment + membrane separation", is as follows:

[0018] In step S1 of this invention, a spray liquid containing alkaline earth metal ions is used to spray the combustion exhaust gas introduced into the spray tower. During the spray treatment, the spray liquid can fully absorb components such as hydrogen fluoride, hydrogen chloride and bromine chloride in the combustion exhaust gas. Calcium ions in the spray liquid react with hydrogen fluoride to generate alkaline earth metal fluoride precipitates. The alkaline earth metal fluoride precipitates and the exhaust gas absorption liquid containing chloride ions and bromide ions are separated by filtration.

[0019] In step S2 of this invention, the exhaust gas absorbent after spraying and filtration is transported to an electrochemical treatment tank for electrochemical treatment, thereby removing bromide ions (Br₂) with an ionic radius of 1.96 Å from the exhaust gas absorbent. - ) is oxidized to bromate ions (BrO3) with an ionic radius of 3.5 Å. - ), while chloride ions (Cl) in the exhaust gas absorbent - The chemical properties of the two elements are stable and they do not undergo oxidation reactions, thus achieving a difference in ionic radii. This allows subsequent steps to utilize nanofiltration membranes to filter out chloride ions (Cl-) from the reacted water. - Chlorides containing bromate ions (BrO3) - The separation of bromate from bromine ions in liquids solves the problem of bromide ions (Br) in liquids. - ) and chloride ions (Cl - The problem of difficulty in separation due to similar ionic radii.

[0020] Preferably, in step S1, the flow rate ratio of the combustion exhaust gas to the spray liquid containing alkaline earth metal ions is (5~12):1.

[0021] For example, in step S1, the flow ratio of the combustion exhaust gas to the spray liquid containing alkaline earth metal ions can be 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, or any range of two such flow ratios.

[0022] Preferably, in step S1, the spray solution containing alkaline earth metal ions includes a first spray solution and a second spray solution, wherein the pH value of the first spray solution is 8-10 and the pH value of the second spray solution is 7-8; the flow rate ratio of the first spray solution and the second spray solution is (0.5-2):1.

[0023] The first spray solution is composed of a soluble alkaline earth metal salt, a pH adjuster, and water. The soluble alkaline earth metal salt includes at least one of calcium chloride, barium chloride, strontium chloride, and magnesium chloride. The pH adjuster includes at least one of potassium hydroxide and sodium hydroxide. The mass concentration of the soluble alkaline earth metal salt in the first spray solution is 13~18 mg / L. The solute in the second spray solution includes at least one of calcium bicarbonate and sodium bicarbonate.

[0024] For example, in step S1, the pH value of the first spray liquid can be 8, 8.5, 9, 9.5, 10, or any two of these values; the pH value of the second spray liquid can be 7, 7.3, 7.5, 7.7, 8, or any two of these values; and the flow rate ratio of the first spray liquid to the second spray liquid is 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.85:1, 0.9:1, 0.95:1, 1:1, 1.2:1, 1.5:1, 1.8:1, 2:1, or any two of these values.

[0025] It is understood that in step S1, the first spray liquid and the second spray liquid can be combined and then fed into the spray tower, or the first spray liquid and the second spray liquid can be fed into the spray tower separately and simultaneously.

[0026] More preferably, in step S1, the soluble alkaline earth metal salt is calcium chloride.

[0027] The first spray solution is composed of a soluble alkaline earth metal salt, a pH adjuster, and water. The soluble alkaline earth metal salt includes at least one of calcium chloride, barium chloride, strontium chloride, and magnesium chloride. The pH adjuster includes at least one of potassium hydroxide and sodium hydroxide. The mass concentration of the soluble alkaline earth metal salt in the first spray solution is 13~18 mg / L. The solute in the second spray solution includes at least one of calcium bicarbonate and sodium bicarbonate.

[0028] The inventors discovered that in step S1, adjusting the pH of the first spray liquid to 8.5-9.5, adjusting the pH of the second spray liquid to 7.3-7.7, setting the flow rate ratio of the first and second spray liquids to (0.8-1):1, and adjusting the flow rate ratio of the combustion exhaust gas to the spray liquid containing alkaline earth metal ions to (7-9):1 can promote the spray liquid to fully absorb fluorides, chlorides, and bromides in the combustion exhaust gas. At the same time, it allows the calcium ions in the spray liquid containing alkaline earth metal ions to preferentially react with fluorides to form alkaline earth metal fluoride precipitates, thereby improving the recovery rate of fluorine, chlorine, and bromine in the combustion exhaust gas.

[0029] Preferably, step S2 specifically includes:

[0030] The tail gas absorption liquid obtained in step S1 is fed into an electrochemical treatment tank for staged electrochemical treatment to obtain reaction effluent.

[0031] More preferably, the phased electrochemical treatment is specifically divided into two stages:

[0032] First electrochemical treatment stage: current density 15~30A / m 2The voltage is 1~3V, the ratio between the air flow rate and the volume of liquid in the electrochemical treatment cell is 0.5~5L / (L·min), and the reaction time is 10~30min;

[0033] Second electrochemical treatment stage: current density of 500~1500 A / m 2 The voltage is 2~5V, the ratio between the air flow rate and the liquid volume in the electrochemical treatment cell is 0.5~5L / (L·min), and the reaction time is 30~90min.

[0034] For example, in the first electrochemical treatment stage, the current density could be 15 A / m. 2 20A / m 2 22A / m 2 24A / m 2 25A / m 2 27A / m 2 30A / m 2 Or a range of any two sets of values; the voltage can be 1V, 1.5V, 1.8V, 2V, 2.3V, 2.5V, 3V or a range of any two sets of values; the ratio between the flow rate of the introduced air and the volume of the liquid in the electrochemical treatment cell can be 0.5L / (L·min), 1L / (L·min), 1.5L / (L·min), 2L / (L·min), 2.5L / (L·min), 3L / (L·min) or a range of any two sets of values; the reaction time can be 10min, 15min, 20min, 25min, 30min or a range of any two sets of values.

[0035] In the second electrochemical treatment stage, the current density can be 500 A / m. 2 600A / m 2 700A / m 2 800A / m 2 900A / m 2 1000A / m 2 1100A / m 2 1200A / m 2 1300A / m 2 1400A / m 2 1500A / m 2Or a range of any two sets of values; the voltage can be 2V, 2.5V, 3V, 3.5V, 4V, 4.5V, 5V or a range of any two sets of values; the ratio between the flow rate of the introduced air and the volume of the liquid in the electrochemical treatment cell can be 0.5L / (L·min), 1L / (L·min), 1.5L / (L·min), 2L / (L·min), 2.5L / (L·min), 3L / (L·min) or a range of any two sets of values; the reaction time can be 30min, 40min, 50min, 60min, 70min, 80min, 90min or a range of any two sets of values.

[0036] In step S2 of this invention, the tail gas absorbent obtained in step S1 is input into an electrochemical treatment tank for staged electrochemical treatment. In the first electrochemical treatment stage, this invention effectively promotes electrocoagulation and / or electroflotation of the tail gas absorbent by adjusting the current density and voltage to an appropriately low range. This causes suspended impurities such as charged colloidal particles in the tail gas absorbent to become unstable and aggregate to form flocs. At the same time, aeration causes these flocs to float to the surface of the liquid to form scum. Scraping off the scum completes the initial purification of the tail gas absorbent.

[0037] In the second electrochemical treatment stage: This invention enhances the electrode polarization effect by increasing the current density and voltage to suitable ranges, thereby promoting the following electrolytic oxidation reaction: Br⁻ + 3H₂O → BrO₃⁻ + 6H⁺ + 6e⁻, oxidizing Br⁻ to BrO₃⁻; furthermore, due to the oxidation of Br⁻, the generated bromate ions (BrO₃)... - The significant difference in ionic radii between Cl⁻ and Cl⁻ facilitates subsequent membrane separation to remove chloride ions (Cl⁻) from the water. - Chlorides containing bromate ions (BrO3) - The bromate of ) is separated.

[0038] This invention employs a phased electrochemical treatment process. First, colloidal impurities are removed, thus eliminating a large amount of suspended matter and some organic matter in advance, reducing the burden on subsequent membrane separation processes. Then, through an electrolytic oxidation reaction, the ionic radii of chlorine and bromine are differentiated, enabling subsequent separation processes to proceed more efficiently and smoothly.

[0039] The inventors discovered through research that the optimal reaction conditions for the first electrochemical treatment stage are: a current density of 20~25 A / m. 2The voltage is 1.5~2.5V, the ratio of the air flow rate to the liquid volume in the electrochemical treatment cell is 1~3L / (L·min), and the reaction time is 15~25min; the preferred conditions for the second electrochemical treatment stage are: current density of 800~1200A / m 2 The voltage is 2.5~3.5V, the ratio of the air flow rate to the liquid volume in the electrochemical treatment tank is 1~3L / (L·min), and the reaction time is 50~70min. When the reaction conditions of the first electrolytic oxidation stage and the second electrolytic oxidation stage are respectively within the above-mentioned preferred ranges, it is possible to more effectively remove colloidal and other suspended impurities in the tail gas absorbent and increase the bromide ion concentration (Br₂). - ) is converted into bromate ions (BrO3) - This improves the conversion rate of bromine and chlorine, thereby enhancing the separation effect of bromine and increasing the purity of bromate solution and chlorine-containing solution.

[0040] Preferably, in step S2, the electrochemical treatment cell is provided with a cathode plate and an anode plate, and the effective reaction area of ​​the cathode plate and the effective reaction area of ​​the anode plate are each independently 80~100 cm². 2 The cathode plate is made of nickel, and the anode plate is made of iron.

[0041] For example, the effective reaction area of ​​the cathode plate can be 80 cm². 2 85cm 2 90cm 2 95cm 2 100cm 2 Or a range consisting of any two sets of values; the effective reaction area of ​​the anode plate can be 80 cm². 2 85cm 2 90cm 2 95cm 2 100cm 2 Or a range consisting of any two sets of values.

[0042] The inventors discovered through research that using an iron anode plate allows the generation of iron ions (Fe) during the electrochemical treatment process. 3+ ), producing iron ions (Fe 3+ It can react with OH in the exhaust gas absorbent liquid. - By combining and generating highly active flocculant groups, the adsorption bridging and netting / sweeping effects of the highly active flocculant groups are used to coagulate colloidal and suspended impurities in the exhaust gas absorption liquid.

[0043] Preferably, in step S3, the membrane separation process is performed using ultrasound at a frequency of 10-60 kHz. For example, the frequency of the ultrasound can be 10 kHz, 20 kHz, 30 kHz, 40 kHz, 50 kHz, 60 kHz, or any two of these values. This invention effectively improves solution mass transfer efficiency and reduces membrane fouling by applying an external ultrasonic field during the membrane separation process.

[0044] Preferably, in step S3, the pore size of the nanofiltration membrane is 0.1~0.5 nm.

[0045] For example, the pore size of the nanofiltration membrane can be 0.1 nm, 0.15 nm, 0.19 nm, 0.2 nm, 0.23 nm, 0.26 nm, 0.30 nm, 0.34 nm, 0.4 nm, 0.5 nm, or a range consisting of any two sets of values.

[0046] Preferably, the combustion exhaust gas is the in-situ combustion exhaust gas of waste liquid crystal pyrolysis oil and gas, wherein the mass concentration of hydrogen fluoride in the combustion exhaust gas is not greater than 15 mg / L, the mass concentration of hydrogen chloride is not greater than 0.5 mg / L, and the mass concentration of hydrogen bromide is not greater than 0.3 mg / L.

[0047] The waste liquid crystal is liquid crystal material removed from at least one of the liquid crystal panels of televisions, computers, mobile phones, and tablets, and the liquid crystal material contains elements such as F, Cl, and Br.

[0048] Preferably, the method for cascade separation and recovery of halogens in combustion exhaust gas is implemented through a cascade separation and recovery system for halogens in combustion exhaust gas, which includes a spray device, an electrochemical treatment device, and a membrane separation device.

[0049] The spraying device includes a spraying tower and a spraying liquid storage tank. The lower part of the spraying tower is provided with an air inlet and a water outlet, and the top of the spraying tower is provided with an air outlet. At least one set of spraying components is provided inside the spraying tower. The spraying components are located between the air inlet and the air outlet. The water outlet of the spraying tower is provided with a filter screen. The spraying components and the spraying liquid storage tank are connected through a spraying water pump and a pipeline. The spraying liquid storage tank containing alkaline earth metal ions is connected to the spraying water pump through a pipeline.

[0050] The electrochemical treatment device includes an electrochemical treatment tank, a cathode mechanism, an anode mechanism, and a pulse power supply. The cathode plate of the cathode mechanism and the anode plate of the anode mechanism are disposed in the electrochemical treatment tank. An aeration device is provided at the bottom of the electrochemical treatment tank. The outlet of the spray tower is connected to the inlet of the electrochemical treatment tank through a pipe.

[0051] The electrochemical treatment cell is connected to the membrane separation device via a pipeline.

[0052] Preferably, control valves and delivery pumps are installed on the pipeline between the outlet of the spray tower and the electrochemical treatment tank, and on the pipeline between the electrochemical treatment tank and the membrane separation device.

[0053] Preferably, the spray liquid storage tank containing alkaline earth metal ions is provided with a partition, which divides the internal space of the spray liquid storage tank containing alkaline earth metal ions into a first spray liquid storage chamber and a second spray liquid storage chamber. The first spray liquid storage chamber is connected to the spray water pump through a pipe, and the second spray liquid storage chamber is connected to the spray water pump through a pipe. The first spray liquid storage chamber is used to store the first spray liquid, and the second spray liquid storage chamber is used to store the second spray liquid.

[0054] More preferably, flow regulating valves are respectively installed on the pipeline between the first spray liquid storage chamber and the spray water pump, and on the pipeline between the second spray liquid storage chamber and the spray water pump.

[0055] Preferably, the spraying device further includes a bubble generator, and the spraying tower is connected to the bubble generator via a bubble delivery pipe, which extends into the interior of the spraying tower and is located below the air inlet.

[0056] Preferably, the number of spray components is not less than two sets, the spray components include a spray pipe and a plurality of atomizing nozzles arranged at equal intervals on the spray pipe, the first spray liquid storage tank is connected to the spray pipe of at least one set of spray components through a spray water pump and a pipeline, and the second spray liquid storage tank is connected to the spray pipe of at least one set of spray components through a spray water pump and a pipeline.

[0057] Preferably, the spray tower is further provided with a packing layer, which is located between the atomizing nozzle and the air outlet.

[0058] Preferably, the cathode mechanism includes a cathode plate and a cathode connector connected to one end of the cathode plate, and the anode mechanism includes an anode plate and an anode connector connected to one end of the anode plate. The cathode plate and the anode plate are respectively disposed in the electrochemical treatment tank, and the cathode connector and the anode connector are respectively installed on the side wall of the electrochemical treatment tank. The cathode connector and the anode connector are respectively used for electrical connection with the pulse power supply.

[0059] Preferably, the cathode plate and the anode plate are arranged parallel to each other and opposite to each other, and the effective reaction area of ​​the cathode plate and the effective reaction area of ​​the anode plate are each independently 80~100cm². 2 The cathode plate is made of nickel, and the anode plate is made of iron.

[0060] Preferably, the electrochemical treatment device further includes a scraper mechanism, which includes a scraper body and a drive motor. The drive motor is disposed on the electrochemical treatment tank, and the output end of the drive motor is connected to one end of the scraper body. The other end of the scraper body is rotatably connected to one side of the electrochemical treatment tank, and the scraper body is located inside the electrochemical treatment tank on the side near the tube.

[0061] Preferably, the membrane separation device includes a membrane separation tank, in which a plurality of membrane separators are arranged. The inlet end of each membrane separator is connected to the electrochemical treatment tank via a pipe. A bromate solution discharge pipe is provided at the bottom of the membrane separation tank. A nanofiltration membrane is installed inside each membrane separator. A chlorine-containing solution discharge pipe is connected to the permeate-side outlet of each membrane separator. A discharge pump is installed on the chlorine-containing solution discharge pipe. A leaching discharge pipe is connected to the residual outlet of each membrane separator. A leaching discharge valve is installed on the residual discharge pipe. The leaching discharge pipe is located inside the membrane separation tank. A bromate solution discharge pipe is provided at the bottom of the membrane separation tank.

[0062] More preferably, the membrane separation tank is provided with ultrasonic rods that correspond one-to-one with the membrane separators, and the ultrasonic rods are perpendicular to the height direction of the membrane separators.

[0063] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0064] (1) The present invention utilizes a spray liquid containing alkaline earth metal ions to spray the combustion exhaust gas introduced into the spray tower. During the spray treatment process, the spray liquid can fully absorb components such as hydrogen fluoride, hydrogen chloride and bromine chloride in the combustion exhaust gas. Calcium ions in the spray liquid react with hydrogen fluoride to generate alkaline earth metal fluoride precipitates. The alkaline earth metal fluoride precipitates and the exhaust gas absorption liquid are separated by filtration.

[0065] (2) In this invention, the tail gas absorbent after spraying and filtration is transported to an electrochemical treatment tank for electrochemical treatment, thereby removing bromide ions (Br₂) with an ionic radius of 1.96 Å from the tail gas absorbent. - ) is oxidized to bromate ions (BrO3) with an ionic radius of 3.5 Å. - ), while chloride ions (Cl) in the exhaust gas absorbent - The chemical properties of the two elements are stable and they do not undergo oxidation reactions, thus achieving a difference in ionic radii. This allows subsequent steps to utilize nanofiltration membranes to filter out chloride ions (Cl-) from the reacted water. - Chlorides containing bromate ions (BrO3) - The separation of bromate from bromine ions in liquids solves the problem of bromide ions (Br) in liquids. - ) and chloride ions (Cl- The problem of difficulty in separation due to similar ionic radii;

[0066] (3) This invention achieves integrated pollution reduction and reuse through the integrated process of "spraying + electrochemical treatment + membrane separation", simplifies the waste gas treatment process, helps to reduce the cost of waste gas treatment, and the recovered alkaline earth metal fluorides, bromates and chlorides can be directly used in metallurgy, chemical industry and other fields.

[0067] In summary, the method described in this invention separates and recovers halogens from exhaust gas, innovatively realizing the transformation of halogens from pollution attributes to resource attributes. Attached Figure Description

[0068] Figure 1 This is a schematic diagram of the halogen cascade separation and recovery system in combustion exhaust gas provided by the present invention.

[0069] In the diagram, 1-spraying device, 11-spraying tower, 111-air inlet, 112-air outlet, 113-water outlet, 12-atomizing nozzle, 13-packing layer, 14-filter screen, 15-bubble generator, 16-spray liquid storage tank, 161-partition, 162-first spray liquid storage chamber, 163-second spray liquid storage chamber, 17-bubble delivery pipe, 2-electrochemical treatment device, 21-electrochemical treatment tank, 22-cathode mechanism, 23-anode mechanism, 24-pulse power supply, 25-scraper mechanism, 26-aeration device, 3-membrane separation device, 31-membrane separation tank, 32-membrane separator, 33-ultrasonic rod, 34-discharge pump, 35-bromate solution discharge pipe, 36-chlorine-containing solution discharge pipe. Detailed Implementation

[0070] To better illustrate the purpose, technical solution, and advantages of this invention, the invention will be further described below with reference to specific embodiments and comparative examples. The purpose of this description is to provide a detailed understanding of the invention, not to limit its scope. All other embodiments obtained by those skilled in the art without inventive effort are within the protection scope of this invention.

[0071] Unless otherwise specified, all components and raw materials used in the embodiments and comparative examples of this invention are commercially available, and the same type of components and raw materials are used in each parallel experiment.

[0072] Examples 1-22

[0073] This invention provides a method for the cascade separation and recovery of halogens in combustion exhaust gas, which is implemented through a cascade separation and recovery system for halogens in combustion exhaust gas. The structure of the cascade separation and recovery system for halogens in combustion exhaust gas is as follows: Figure 1As shown, the halogen cascade separation and recovery system in the combustion exhaust gas includes a spray device 1, an electrochemical treatment device 2, and a membrane separation device 3.

[0074] The spraying device 1 includes a spraying tower 11, a bubble generator 15, and a spray liquid storage tank 16. The lower part of the spraying tower 11 has an air inlet 111 and a water outlet 113. The bottom of the spraying tower 11 has a sedimentation outlet with a movable cover (not shown in the figure). The top of the spraying tower 11 has an air outlet 112. A filter screen 14 is installed at the water outlet 113. Three sets of spraying components 12 are installed inside the spraying tower 11, located at the air inlet 11. Between the air inlet 111 and the air outlet 112, three sets of spray components 12 are arranged at equal intervals along the height direction of the spray tower 11. Each spray component 12 includes a spray pipe and several atomizing nozzles arranged at equal intervals on the spray pipe. A packing layer 13 is also provided inside the spray tower 11, and the packing layer 13 is located between the spray components 12 and the air outlet 112. The spray tower 11 is connected to the bubble generator 15 through a bubble delivery pipe 17. The bubble delivery pipe 18 extends into the interior of the spray tower 11 and is located below the air inlet 111.

[0075] The spray liquid storage tank 16 is equipped with a partition 161, which divides the internal space of the spray liquid storage tank 16 into a first spray liquid storage chamber 162 and a second spray liquid storage chamber 163. The first spray liquid storage chamber 162 is used to store the first spray liquid, and the second spray liquid storage chamber 163 is used to store the second spray liquid. The first spray liquid storage chamber 162 is connected to the spray pipe of the bottommost set of spray components 12 through a spray water pump (not shown in the figure), a flow regulating valve (not shown in the figure), and a pipe. The second spray liquid storage chamber 163 is connected to the spray pipes of the other two sets of spray components 12 through a spray water pump (not shown in the figure), a flow regulating valve (not shown in the figure), and a pipe, respectively.

[0076] The electrochemical treatment device 2 includes an electrochemical treatment cell 21, a cathode mechanism 22, an anode mechanism 23, a pulse power supply 24, and a scraper mechanism 25. The cathode mechanism 22 includes a cathode plate and a cathode connector connected to one end of the cathode plate. The anode mechanism 23 includes an anode plate and an anode connector connected to one end of the anode plate. The cathode plate and anode plate are respectively disposed within the electrochemical treatment cell 21, parallel to each other and facing each other. The effective reaction area of ​​the cathode plate and the effective reaction area of ​​the anode plate are the same, as shown in Table 1. The cathode plate is made of nickel, and the anode plate is made of iron. The cathode connector and... The anode connectors are respectively installed on the side wall of the electrochemical treatment tank 21. The cathode connector and the anode connector are respectively used to electrically connect to the pulse power supply 24. The scraper mechanism 25 includes a scraper body and a drive motor. The drive motor is set on the electrochemical treatment tank 21. The output end of the drive motor is connected to one end of the scraper body. The other end of the scraper body is rotatably connected to one side of the electrochemical treatment tank 21. The scraper body is located in the upper part of the electrochemical treatment tank 21. An aeration device 26 is set at the bottom of the electrochemical treatment tank 21. The outlet 113 of the spray tower 11 is connected to the inlet of the electrochemical treatment tank 21 through a pipe 4.

[0077] The membrane separation device 3 includes a membrane separation tank 31, which contains several membrane separators 32. The inlet of each membrane separator 32 is connected to an electrochemical treatment tank 21 via a pipe. Each membrane separator 31 contains an ultrasonic rod 33 corresponding to one of the membrane separators 32. A bromate solution discharge pipe 35 is located at the bottom of the membrane separation tank 31. Each membrane separator 32 contains a nanofiltration membrane with a pore size of 0.26 nm. The permeate-side outlet of each membrane separator 32 is connected to a chlorine-containing solution discharge pipe 36, which is equipped with a discharge pump 34. The osmotic-side outlet of each membrane separator 32 is connected to an osmotic discharge pipe (not shown in the figure), which is equipped with an osmotic discharge valve (not shown in the figure). The osmotic discharge pipe is located inside the membrane separation tank 31, and the bromate solution discharge pipe 35 is located at the bottom of the membrane separation tank 31.

[0078] Control valves and transfer pumps (not shown in the figure) are installed on the pipes between the outlet of the spray tower 11 and the electrochemical treatment tank 21, the pipes between the inlet of the membrane separator 32 and the electrochemical treatment tank 21, the effluent discharge pipe, the bromate solution discharge pipe 35, and the chlorine-containing solution discharge pipe 36.

[0079] The method for the graded separation and recovery of halogens in combustion exhaust gas includes the following steps:

[0080] S1. The combustion exhaust gas is introduced into the spray tower 11 through the air inlet 111. At the same time, the spray water pump is started to atomize the first spray liquid and the second spray liquid into droplets through the atomizing nozzles of the corresponding spray components and spray them into the spray tower 11 to spray the combustion exhaust gas introduced into the spray tower 11.

[0081] After the solution in the spray tower 11 is filtered through the filter screen 14, the calcium fluoride precipitate and the tail gas absorption liquid are separated. The tail gas after spray treatment is adsorbed by the packing layer 13 and discharged from the outlet 112.

[0082] In step S1, the first spray solution is a mixed solution prepared from NaOH, CaCl2 and water. The pH value of the first spray solution is shown in Table 1. The mass concentration of CaCl2 in the first spray solution is 15 mg / L.

[0083] In all embodiments other than Example 4, the second spray solution is a mixed solution of calcium bicarbonate (Ca(HCO3)2) and water. In Example 4, the second spray solution is water. The pH value of the second spray solution is shown in Table 1.

[0084] In step S1, the ratio of the inflow rate of the first spray liquid to the second spray liquid, the ratio of the flow rate of the combustion exhaust gas to the flow rate of the liquid entering the spray tower 11 are shown in Table 1. The flow rate of the liquid entering the spray tower 11 is the sum of the inflow rate of the first spray liquid and the inflow rate of the second spray liquid.

[0085] S2. Input the tail gas absorption liquid obtained in step S1 into the electrochemical treatment cell 21, connect the cathode and anode mechanisms to an external pulse power supply, and carry out electrochemical treatment in stages to obtain reaction effluent.

[0086] The phased electrochemical treatment is performed as follows: the first electrochemical treatment stage and the second electrochemical treatment stage are performed sequentially, and the electrochemical treatment conditions for each stage are shown in Table 2.

[0087] During the electrochemical reaction, aeration is carried out by aeration device 26. The ratio between the air flow rate of aeration and the volume of tail gas absorption liquid input into electrochemical treatment tank 21 is denoted as gas-liquid ratio. The gas-liquid ratio in each stage is shown in Table 2.

[0088] In the first and second electrochemical treatment stages, scum on the liquid surface is scraped off by the scraper mechanism 25.

[0089] S3. Input the reaction effluent obtained in step S2 into membrane separator 32. Under the conditions of ultrasonic frequency as shown in Table 1, use nanofiltration membrane (the pore size is shown in Table 1) to perform membrane separation treatment on the reaction effluent to separate bromate solution and chlorine-containing solution.

[0090] S4. Collect calcium fluoride precipitate from spray tower 11, and obtain calcium fluoride product after drying; evaporate and concentrate the chlorine-containing solution to obtain chloride crystal product; concentrate and enrich the bromate solution to obtain bromate product.

[0091] Table 1

[0092]

[0093] Table 2

[0094]

[0095] Comparative Example 1

[0096] The difference between this comparative example and Example 1 is that step S2 in this comparative example only involves the first electrochemical stage, without the second electrochemical stage. Step S2 in this comparative example is as follows: the tail gas absorbent obtained in step S1 is input into the electrochemical treatment tank, and the cathode and anode mechanisms are externally connected to a pulse power supply. The tail gas absorbent is then electrochemically treated in the electrochemical treatment tank under the following conditions: current density of 23 A / m. 2 The voltage is 2V, the gas-liquid ratio (the ratio between the air flow rate of aeration and the volume of tail gas absorption liquid input into the electrochemical treatment tank) is 2L / (L·min), and the reaction time is 20min; and the scum is scraped off by a scraper mechanism during the electrochemical treatment process.

[0097] Comparative Example 2

[0098] The difference between this comparative example and Example 1 is that step S2 in this comparative example is as follows: the tail gas absorbent obtained in step S1 is input into an electrochemical treatment cell, and a pulse power supply is externally connected to the cathode and anode mechanisms to perform electrochemical treatment on the tail gas absorbent in the electrochemical treatment cell. The conditions for the electrochemical treatment are: current density of 1000 A / m 2 The voltage is 3V, the gas-liquid ratio (the ratio between the air flow rate of aeration and the volume of tail gas absorption liquid input into the electrochemical treatment tank) is 2L / (L·min), and the reaction time is 60min; and the scum is scraped off by a scraper mechanism during the electrochemical treatment process.

[0099] Comparative Example 3

[0100] The difference between this comparative example and Example 1 is that step S2 is as follows: the tail gas absorption liquid obtained in step S1 is input into the electrochemical treatment tank, and a pulse power supply is externally connected to the cathode and anode mechanisms to perform electrochemical treatment in stages to obtain reaction effluent; wherein, the staged electrochemical treatment is as follows: a first electrochemical treatment stage and a second electrochemical treatment stage are performed sequentially, and the conditions for the first electrochemical treatment stage are: a current density of 1000 A / m 2 The voltage is 3V, the gas-liquid ratio (the ratio between the air flow rate of aeration and the volume of the tail gas absorption liquid input into the electrochemical treatment tank) is 2L / (L·min), and the reaction time is 60min; the conditions for the first electrochemical treatment stage are: current density of 23A / m 2The voltage is 2V, the gas-liquid ratio (the ratio between the air flow rate of aeration and the volume of tail gas absorption liquid input into the electrochemical treatment tank) is 2L / (L·min), and the reaction time is 20min; during the electrochemical treatment process, the scum is scraped off by a scraper mechanism.

[0101] Comparative Example 4

[0102] The difference between this comparative example and Example 1 is that the first spray liquid was not delivered into the spray tower 11 in step S1 of this comparative example.

[0103] Comparative Example 5

[0104] The difference between this comparative example and Example 1 is that in this comparative example, the second spray liquid was not supplied to the spray tower 11 in step S1.

[0105] Comparative Example 6

[0106] The difference between this comparative example and Example 1 is that there is no ultrasonic condition during the filtering in step S3 of this comparative example.

[0107] Effect test

[0108] The waste liquid crystal pyrolysis oil and gas from Jiangsu Ningda Environmental Protection Co., Ltd. was treated using the methods described in the above embodiments and comparative examples. The amount of combustion gas treated was the same in each embodiment and comparative example, and the fluorine content w1 in the waste liquid crystal pyrolysis oil and gas in-situ combustion tail gas was 12906.36 mg / m³. 3 The bromine content w2 is 278.50 mg / m³. 3 The chlorine content w3 is 104.42 mg / m³. 3 .

[0109] The recovery rates of fluorine, chlorine, and bromine were tested using the following methods:

[0110] (1) Fluorine recovery rate: Weigh the mass m1 (in mg) of the calcium fluoride product obtained in step S4, and calculate the fluorine recovery rate according to the following formula: Fluorine recovery rate (%) = [(m1 × 2 × M] F [ / M1) / (w1×V0)]×100%, where M1 is the relative molecular mass of calcium fluoride, M F is the relative atomic mass of fluorine.

[0111] (2) Bromine recovery rate: The volume of the bromate solution obtained in step S3 is measured and recorded as V1 (in L). The bromate is reduced to bromide ions using thiosulfate. The mass concentration of bromine in the bromate solution obtained in step S3 is detected by ion chromatography and the result is recorded as ρ1 (in g / L). The bromine recovery rate (%) is calculated according to the following formula: [(ρ1×V1) / (w2×V0)]×100%.

[0112] (3) Chlorine recovery rate: The volume of the chlorine-containing solution obtained in step S3 is measured and recorded as V2 (in L). The mass concentration of chlorine in the obtained chlorine-containing solution is detected by the existing silver nitrate spectrophotometric method, and the detection result is recorded as ρ1 (in g / L).

[0113] The chlorine recovery rate is calculated using the following formula: Chlorine recovery rate (%) = [(ρ2×V2) / (w3×V0)]×100%.

[0114] The results are shown in Table 3 below.

[0115] Table 3

[0116]

[0117] Compared with Examples 6-7, the recovery rates of chlorine and bromine in Examples 1-3 are higher. This indicates that by controlling the process parameters of the first and second electrochemical treatment stages within appropriate ranges, the present invention can promote electrocoagulation and electroflotation, avoid excessive residual colloids and other impurities in the reacted water that would affect the passage of chloride ions through the nanofiltration membrane, and also promote the generation of bromate ions, thereby improving the separation effect of chlorine and bromine, increasing the recovery rate of chlorine and bromine, and improving the purity of chloride and bromate products in subsequent steps.

[0118] As can be seen from Examples 1 and 8-12, by adjusting the flow rate ratio of the first spray liquid and the second spray liquid to a suitable range, the present invention can reduce the escape of components such as hydrogen halides in the spray treatment, promote the formation of calcium fluoride, and thus improve the recovery rate of each halogen.

[0119] As can be seen from Examples 1 and 13-15, when the ratio of the flow rate of the combustion exhaust gas to the flow rate of the gas entering the spray tower is (7-9):1, the recovery rate of each halogen is high, and the amount of spray liquid consumed is small, resulting in lower cost.

[0120] As can be seen from Examples 1 and 16-19, selecting a nanofiltration membrane with a suitable pore size can effectively retain bromate ions while ensuring that chloride ions pass smoothly through the nanofiltration membrane, thereby comprehensively improving the recovery rate of chlorine and bromine and increasing the purity of chloride and bromate products in subsequent steps.

[0121] As can be seen from Examples 1, 20-22 and Comparative Example 6, the present invention performs membrane separation in ultrasound at a suitable frequency, which ensures the cleanliness of the nanofiltration membrane while avoiding membrane damage. This ensures the nanofiltration membrane retains bromate ions while accelerating the diffusion and mass transfer of chloride ions, thereby increasing the recovery rate of chlorine and bromine, improving the separation efficiency of chloride and bromate ions, and thus improving the purity of chloride and bromate products in subsequent steps.

[0122] Compared with Example 1, Comparative Example 1 did not undergo staged electrochemical treatment, which significantly reduced the bromine recovery rate. This may be because the bromide ions were not fully oxidized under lower potential conditions, resulting in a poorer chlorine-bromine separation effect.

[0123] Compared with Example 1, the recovery rates of chlorine and bromine in Comparative Examples 2 and 3 were significantly reduced. This may be because the electrochemical treatment was carried out directly under higher potential conditions, which caused other impurities (such as organic matter) in the tail gas absorbent to be electrolytically oxidized, affecting the oxidation efficiency of bromide ions and the flocculation effect of other impurities in the tail gas absorbent. This may have changed the properties of the flocs, resulting in more residual flocs and / or colloidal particles in the reaction water, which in turn affected the subsequent membrane separation effect, leading to a significant decrease in the recovery rates of chlorine and bromine.

[0124] Compared with Example 1, Comparative Examples 4 and 5 used a different spraying liquid to spray the combustion exhaust gas, which resulted in insufficient absorption of halogen gases in the exhaust gas, leading to a significant decrease in the halogen recovery rate.

[0125] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for stepwise separation and recovery of halogens from combustion tail gas, characterized by, The method comprises the following steps: S1, passing the combustion tail gas into a spray tower, and passing a spray liquid containing alkaline earth metal ions into the spray tower, and performing spray treatment on the combustion tail gas in the spray tower by using the spray liquid containing alkaline earth metal ions, and filtering the solution obtained after the spray to separate out alkaline earth metal fluoride precipitate and tail gas absorption liquid; the combustion tail gas contains hydrogen fluoride, hydrogen chloride and hydrogen bromide; S2, inputting the tail gas absorption liquid obtained in step S1 into an electrochemical treatment tank to perform electrochemical treatment, to obtain reaction effluent; S3, performing membrane separation treatment on the reaction effluent obtained in step S2 by using a nanofiltration membrane, to obtain a bromate solution and a chlorine-containing solution; In step S1, the flow rate ratio of the combustion tail gas to the spray liquid containing alkaline earth metal ions is (5-12):1, the spray liquid containing alkaline earth metal ions comprises a first spray liquid and a second spray liquid, the mass concentration of the soluble alkaline earth metal salt in the first spray liquid is 13-18 mg / L, and the solute in the second spray liquid comprises at least one of calcium bicarbonate and sodium bicarbonate; the pH value of the first spray liquid is 8-10, and the pH value of the second spray liquid is 7-8; the flow rate ratio of the first spray liquid to the second spray liquid is (0.5-2):1, the first spray liquid is mixed by a soluble alkaline earth metal salt, a pH adjuster and water, and the soluble alkaline earth metal salt comprises at least one of calcium chloride, barium chloride, strontium chloride and magnesium chloride; Step S2 specifically comprises: inputting the tail gas absorption liquid obtained in step S1 into an electrochemical treatment tank to perform electrochemical treatment in stages, and scraping off the dross in the electrochemical treatment tank during the electrochemical treatment, to obtain reaction effluent; The electrochemical treatment in stages is specifically divided into two stages: First electrochemical treatment stage: current density 15~30A / m 2 The voltage is 1~3V, the ratio between the air flow rate and the volume of liquid in the electrochemical treatment cell is 0.5~5L / (L·min), and the reaction time is 10~30min; Second electrochemical treatment stage: current density of 500~1500 A / m 2 The voltage is 2~5V, the ratio between the air flow rate and the volume of liquid in the electrochemical treatment cell is 0.5~5L / (L·min), and the reaction time is 30~90min; In step S3, the membrane separation treatment is performed in ultrasonic waves with a frequency of 10-60 kHz.

2. The method for stepwise separation and recovery of halogens in combustion exhaust gas according to claim 1, wherein In step S1, the pH adjuster comprises at least one of potassium hydroxide and sodium hydroxide.

3. The method for stepwise separation and recovery of halogens in combustion exhaust gas according to claim 1, wherein In step S2, the electrochemical treatment tank is provided with a cathode plate and an anode plate, the effective reaction area of the cathode plate and the effective reaction area of the anode plate are each independently 80-100m 2 2, the material of the cathode plate is nickel, and the material of the anode plate is iron.

4. The method for stepwise separation and recovery of halogens in combustion exhaust gas according to claim 1, wherein In step S3, the pore size of the nanofiltration membrane is 0.1-0.5 nm.

5. The method for stepwise separation and recovery of halogens in combustion exhaust gas according to claim 1, wherein The method comprises the following steps: collecting the alkaline earth metal fluoride precipitate from the spray tower in step S1, and obtaining an alkaline earth metal fluoride product after drying; performing evaporation concentration on the chlorine-containing solution obtained in step S3, to prepare a chloride crystal product; performing concentration enrichment recovery on the bromate solution obtained in step S3, to obtain a bromate product.

6. The method for stepwise separation and recovery of halogens in combustion exhaust gas according to claim 1, wherein The combustion tail gas is waste liquid crystal pyrolysis oil gas in-situ combustion tail gas, in which the mass concentration of hydrogen fluoride is not more than 15 mg / L, the mass concentration of hydrogen chloride is not more than 0.5 mg / L, and the mass concentration of hydrogen bromide is not more than 0.3 mg / L.

Images