Method for sequestration of carbon by using strong brine wastewater of steel mill and used device

Abstract

The invention discloses a method for sequestration of carbon by using strong brine wastewater of a steel plant and a used device, and belongs to the technical field of tempering co-production process production. The method comprises a sodium carbonate production process I and an ammonium chloride production process II which form a cycle; raw materials NH3, salt NaCl and gas carbon dioxide are continuously added into the circulating system, and meanwhile sodium carbonate and ammonium chloride products are continuously produced; the raw material carbon dioxide comes from steel mill blast furnace top gas, steel rolling heating furnace tail gas or lime kiln tail gas; the raw material salt NaCl comes from concentrated salt wastewater of a steel mill; the raw material NH3 is from a byproduct coke oven gas generated in the production process of a steel enterprise and a coking plant. According to the method, sodium carbonate is prepared by fully utilizing three wastes, the situation that soxhlet and Hou's alkali production is adopted in the traditional alkali production industry to damage natural ecology and generate a large amount of waste gas, waste water and waste residues is broken, and double protection of resource recycling and environment is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of the production technology of integrated tempered production, and particularly relates to a method for carbon sequestration using concentrated brine wastewater from steel mills and a device used therefor. Background Art

[0002] The iron and steel industry is an important basic industry of the national economy, and at the same time, it is also one of the industrial sectors with high energy consumption and serious environmental pollution. With the construction of green chemical plants, the treatment of high-salt wastewater in the iron and steel industry has become a difficult problem. High-salt wastewater mainly comes from processes such as desulfurization, washing, wastewater reuse, and solid waste pretreatment. Common treatment methods include the slag flushing method, the flue gas waste heat evaporation method, and the evaporation crystallization and salt separation method. The evaporation crystallization and salt separation method can realize the resource utilization of chlorine elements, but it is necessary to reduce the operating cost and solve the blockage problem. Water is indispensable in all processes of steel production. Wet dust removal, furnace wall cooling, pouring cooling, etc. all require water, and it is softened water after treatment. However, a large amount of high-salt dust removal ash will be generated in the sintering, blast furnace and other processes of steel production, which contains relatively high alkali metals (>15%) and chlorine elements (>20%). Directly returning it to sintering will cause problems such as equipment corrosion or scaling due to the enrichment of potassium, sodium, and chlorine, the caking of dust removal ash due to moisture absorption, and the decline in the purification efficiency of the flue gas desulfurization and denitrification system. In traditional methods, chlorine elements usually enter the slag through the slag flushing process or directly return to the production process without being properly disposed of. However, the enrichment of a large amount of chlorine elements will not only cause corrosion of production equipment, but also increase the difficulty of wastewater treatment. In addition, a large amount of chlorine elements may be converted into hydrogen chloride, and hydrogen chloride will not only occupy the effective sites of the flue gas purification catalyst, but also form ammonium chloride with the sprayed ammonia water. Then, ammonium chloride escapes into the flue gas to form a colored plume, which directly threatens the ecological environment quality and the ultra-low emission effect. Therefore, under the environmental protection requirements, the problem of chlorine element disposal begins to emerge.

[0003] In addition, China's steel production accounts for 51% of the global production capacity, mainly using the long process of blast furnace-converter. The CO2 emissions per ton of steel are 1.8 - 2.1 tons. A large amount of carbon dioxide emissions are also important issues under the background of carbon peaking and carbon neutrality. How to achieve green steel and reduce the carbon emissions per ton of steel has required a great deal of energy and financial resources from the government and research institutions. In the short term, an effective method is carbon capture and carbon sequestration. If the waste gas and wastewater generated by the steel mill can be coupled together, not only carbon can be sequestered, but also chemical products that can be applied can be produced, which will be an ideal way with multiple benefits.

[0004] The information disclosed in this background art section is only intended to enhance the overall understanding of the present invention and should not be regarded as an admission or any form of suggestion that this information constitutes prior art already known to those of ordinary skill in the art. Summary of the Invention

[0005] With the dual support of environmental protection and carbon emission reduction in integrated iron and steel enterprises, reducing exhaust gas, waste, and wastewater emissions is the only way for green development. This invention relies on capturing carbon dioxide produced by steel mills during heating, such as blast furnace top gas, reheating furnace tail gas of rolling mills, and lime kiln tail gas; water is indispensable in steel mills, and the concentrated brine wastewater generated is treated. After membrane separation by existing technologies and meeting the discharge standards, it is discharged. Generally, the NaCl content in concentrated brine wastewater exceeds 10%. To further reduce NaCl, evaporation and separation are required; integrated steel enterprises also have coking plants. The by-product of producing metallurgical coke is coke oven gas, which is a raw material for ammonia production. Therefore, by coupling technologies, the substances originally discharged by steel mills are made into valuable soda ash and ammonium chloride, which not only opens up a way for carbon capture but also effectively reduces carbon dioxide emissions. The use of industrial recycled salt with a content of 99.5% reduces manufacturing costs, and the mother liquor is recycled, achieving no solid-liquid waste discharge; improving mature and reliable soda ash technology ensures continuous and stable product production, reduces comprehensive energy consumption, and enhances the market competitiveness of products.

[0006] To solve the above problems, the technical solutions adopted in this invention are as follows:

[0007] A method for carbon sequestration using concentrated brine wastewater from steel mills. Specifically, the combined soda process (abbreviated as the combined soda method) is used to produce soda ash and ammonium chloride; the combined soda method is divided into two processes. The first process is the soda ash production process (including the process of calcining heavy soda ash into soda ash), and the second process is the ammonium chloride production process. The two processes form a cycle; raw materials ammonia, salt, and carbon dioxide are continuously added to the circulation system, and at the same time, soda ash and ammonium chloride products are continuously produced; the raw material carbon dioxide comes from blast furnace top gas, reheating furnace tail gas of rolling mills, or lime kiln tail gas of steel mills; the raw material salt NaCl comes from the concentrated brine wastewater of steel mills, where the NaCl content exceeds 10%; the raw material NH3 comes from the by-product coke oven gas generated during the production processes of steel enterprises and coking plants.

[0008] In the method for carbon sequestration using concentrated brine wastewater from steel mills, in the soda ash production process I, the mother liquor MⅡ filtered out after salting out ammonium chloride with salt is preheated, absorbs ammonia to become ammonia mother liquor AⅡ, and undergoes a carbonization reaction with continuously added gaseous carbon dioxide. After the carbonization reaction, it is filtered. The filtrate of the alkali solution is treated to obtain mother liquor MI, and the filter cake is calcined to obtain soda ash; the carbonization tail gas is discharged after passing the washing test.

[0009] In the ammonium chloride production process II, mother liquor MI absorbs ammonia to become ammonia mother liquor AI, undergoes cold crystallization to obtain liquid semi-mother liquor MⅡ and solid ammonium chloride, and salt NaCl is added to the semi-mother liquor MⅡ for salting out to obtain mother liquor MⅡ and solid ammonium chloride.

[0010] For the method of carbon sequestration using concentrated brine wastewater from steel plants, the alkali discharge liquid from the carbonation tower first enters the alkali discharge tank, flows into the alkali liquid buffer tank through the alkali discharge tank, and the alkali discharge liquid flows into the belt filter from the bottom of the alkali liquid buffer tank, and vacuum filtration is carried out by the suction force from the vacuum pump; the filter cake is washed with water from the high-level water tank for washing during the filtration process, and part of the heavy soda ash discharged from the belt filter is transported to the calcination section by the heavy soda ash belt conveyor; the gas-liquid mixture extracted by the vacuum machine enters the mother liquor separation tank for gas-liquid separation, and the liquid at the bottom of the separation tank is called the filtered mother liquor, which flows into the carbonation mother liquor machine by itself, and the precipitate of the carbonation mother liquor machine is sent back to the alkali discharge tank through the cyclone by the heavy soda ash crystal slurry pump, and the clarified mother liquor (abbreviation: MⅠ) flows into the mother liquor MI barrel by itself.

[0011] For the method of carbon sequestration using concentrated brine wastewater from steel plants, the cold ammonia mother liquor MI sent by the filter is heat-exchanged with gaseous ammonia in the metering and gaseous ammonia heat exchanger and then sent into the central circulation pipe of the cold crystallization crystallizer, and together with the circulating mother liquor AI from and the salting-out crystal slurry at the bottom of the hydrocyclone, it reaches the bottom of the cold crystallization crystallizer and rises distributively; the mother liquor in the upper part of the liquid ammonia evaporation external cooler of the crystallizer is sent into the tubes of the external cooler by the cold crystallization axial flow pump, exchanges heat with the liquid ammonia between the tubes and cools down to generate supersaturation, and then returns to the bottom of the central pipe of the cold crystallization crystallizer through the collecting tank and the central circulation pipe, and its supersaturation disappears through the crystal slurry layer, promoting the formation and growth of ammonium chloride crystals; the crystal slurry is in a suspended state, and the overflow semi-mother liquor MII of the cold crystallization crystallizer flows into the central downcomer of the salting-out crystallizer.

[0012] For the method of carbon sequestration using concentrated brine wastewater from steel plants, the raw salt NaCl sent by the belt conveyor is added into the central circulation pipe of the salting-out crystallizer, and together with the overflow semi-mother liquor MII of the cold crystallization crystallizer and the filtrate sent by the filtrate pump, it is sent into the bottom of the crystallizer by the salting-out axial flow pump, rises distributively, and is gradually dissolved, and ammonium chloride crystals are precipitated under the action of the common ion effect; the overflow mother liquor MII of the salting-out crystallizer flows into the mother liquor MII barrel and is sent to the mother liquor exchange section by the mother liquor MII pump; the precipitate at the bottom of the mother liquor II barrel is sent to the salting-out crystallizer by the mother liquor MII precipitation pump.

[0013] For the method of carbon sequestration using concentrated brine wastewater from steel plants, the crystal slurry taken out from the salting-out crystallizer is sent to the hydrocyclone by the salting-out circulation pump, the top flow of the hydrocyclone flows into the salting-out crystallizer, and the bottom flow enters the cold crystallization crystallizer; the crystal slurry of the cold crystallization crystallizer is sent to the cold crystallization thickener by the cold crystallization extraction pump, thickened, and then enters the centrifugal filter for centrifugal separation; the wet ammonium chloride separated by the centrifugal filter is transported to the dry ammonium section by the scraper conveyor; the overflow liquid of the cold crystallization thickener and the filtrate semi-mother liquor II of the centrifugal filter both flow into the filtrate barrel and are sent into the salting-out crystallizer by the filtrate pump. The semi-mother liquor II of the liquid ammonia evaporation external cooler is emptied into the buffer tank and then sent into the salting-out crystallizer by the external cooler drain pump.

[0014] Part of the wet ammonium chloride from the centrifugal filter in the crystallization separation section enters the ammonium chloride drying bed. After drying, the product is sent to the ammonium chloride powder flow cooler by a bucket elevator, and softened water is used for cooling. Finally, the temperature of the cooled product is lower than 60 °C, and the moisture content of the product is lower than 0.5 wt%. After heat exchange, the softened water enters the soft water bucket, is sent to the soft water cooler by a soft water pump for cooling, and then enters the powder flow cooler. The cooled dry ammonium product is sent to the dry ammonium packaging warehouse by a dry ammonium belt conveyor.

[0015] The NaHCO₃ crystal slurry taken out from the bottom of the carbonation tower enters the cold crystallization thickener. In the cold crystallization thickener, the upper clear alkali liquid flows into the filtrate bucket through the overflow pipe, and the thick crystal slurry at the bottom enters the centrifuge. The wet baking soda material obtained after centrifugal dehydration is sent to the dryer through a wet material belt conveyor and a wet material auger. The filtrate after centrifugation enters the filtrate bucket.

[0016] The wet material sent by the feeding screw conveyor in the dryer is fully heated and dried, and the dried material is sent to the packaging process.

[0017] The calcination reaction formula is:

[0018] 2NaHCO₃ → Na₂CO₃ + CO₂↑ + H₂O↑ (light ash calcination).

[0019] The carbonation and calcination of ammoniated brine involve the following chemical reactions:

[0020] Carbonation:

[0021]

[0022] Calcination:

[0023]

[0024] Total reaction:

[0025]

[0026] Beneficial effects: Compared with the existing technology, the advantages of the present invention include:

[0027] (1) Different from the traditional double soda process, this invention does not require impurity removal of crude salt in raw materials. The existing wastewater from steel mills can meet the process requirements. Carbon dioxide is not obtained by roasting limestone, but by using the tail gas from steel mills, and ammonia comes from the coking plant of steel mills.

[0028] (2) The equipment used in this invention is also different from traditional equipment. Traditional soda production mostly uses ductile iron as the equipment material for carbonation towers and ammonia absorption towers, and it is easy to introduce Fe +When ions enter the product, they need to be removed through a process. In the present invention, 2205 duplex stainless steel is used as the main material for the carbonation tower and ammonia absorption tower, and 316L stainless steel is used for the pipelines and liquid circulation pumps, fundamentally avoiding the retention and erosion of Cl - and Fe + ions.

[0029] (3) In the present invention, flue gas is directly introduced into the middle of the carbonation tower. Without using the captured CO2, the production of soda ash and carbon capture are directly coupled, opening up a new way for direct CO2 capture.

[0030] (4) The present invention makes full use of the three wastes to prepare soda ash, breaking the situation in the traditional soda-making industry where the "Solvay process" and "Hou's process" damage the natural ecology and generate a large amount of waste gas, waste water, and waste residue, achieving the recycling of resources and double environmental protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a schematic structural diagram of a device for carbon sequestration and soda-making with concentrated brine wastewater;

[0032] Figure 2 It is a flow chart of NH4Cl crystallization taken out in countercurrent;

[0033] 1 - Cold crystallization tank; 2 - Salting-out crystallization tank; 3 - Main cold crystallization circulation pump; 4 - Salting-out circulation pump; 5 - External cooler; 6 - Air-lift crystal slurry extraction pipe; 7 - Crystal slurry thickener; 8 - Cold crystallization thickener; 9 - Centrifugal filter; 10 - Mother liquor I tank; 11 - Filtrate tank; 12 - Filtrate pump; 13 - Carbonation tower; 14 - First ammonia absorption tower; 15 - Second ammonia absorption tower; 16 - Filter press; 17 - Cold salting-out crystal kettle; 18 - Concentrated ammonia buffer tank; 19 - Carbonation mother liquor machine; 20 - Automatic discharging centrifuge; 21 - Centrifugal mother liquor tank. DETAILED DESCRIPTION OF THE INVENTION

[0034] In order to make the above objects, features, and advantages of the present invention more obvious and understandable, the following detailed description of the specific embodiments of the present invention will be given in conjunction with specific embodiments.

[0035] Example 1

[0036] A method for carbon sequestration using the concentrated brine wastewater from steel plants. The specific approach is to produce soda ash and ammonium chloride using the combined soda process (abbreviated as the joint soda process). The joint soda process is divided into two processes. The first process is the soda ash production process (including the process of calcining heavy soda ash into soda ash), and the second process is the ammonium chloride production process. These two processes form a cycle. Raw materials such as ammonia, salt, and carbon dioxide are continuously added to the circulation system, while soda ash and ammonium chloride products are continuously produced. Carbon dioxide generated from steel plant heating, such as blast furnace top gas, reheating furnace tail gas of rolling mills, and lime kiln tail gas, is captured. Steel plants rely on water and treat the generated concentrated brine wastewater. After membrane separation by existing technologies and reaching the discharge standard, it is discharged. Generally, the NaCl content in the concentrated brine wastewater exceeds 10%, and in order to further reduce NaCl, evaporation salt separation is also required. Associated steel enterprises also have coking plants. The by-product of producing metallurgical coke is coke oven gas, which is a raw material for ammonia production. Therefore, by coupling technologies, the substances originally discharged from the steel plant are made into valuable soda ash and ammonium chloride, which not only opens up a way for carbon capture but also effectively reduces carbon dioxide emissions. The use of industrially recycled salt with a content of 99.5% reduces manufacturing costs, and the mother liquor is recycled to achieve no solid-liquid waste discharge. The improved, mature, and reliable soda ash technology ensures continuous and stable product production, reduces comprehensive energy consumption, and enhances the market competitiveness of products.

[0037] The soda ash production process I includes preheating the mother liquor MⅡ filtered out after salting out ammonium chloride with salt, absorbing ammonia to become ammonia mother liquor AⅡ, carrying out a carbonization reaction with continuously added gaseous carbon dioxide, filtering after the carbonization reaction, treating the filtrate to obtain mother liquor MI, and calcining the filter cake to obtain soda ash; the carbonization tail gas is discharged after passing the washing test.

[0038] The ammonium chloride production process II includes absorbing ammonia by mother liquor MI to become ammonia mother liquor AI, obtaining liquid semi-mother liquor MⅡ and solid ammonium chloride through cold crystallization, and adding salt NaCl to the semi-mother liquor MⅡ for salting out to obtain mother liquor MⅡ and solid ammonium chloride.

[0039] The device used in the above method, as Figure 1 shown, the device includes a carbonization tower 13, a first ammonia absorption tower 14, a second ammonia absorption tower 15, a soda ash filter 16, and a cold salt crystallization kettle 17; a concentrated ammonia buffer tank 18 is connected between the carbonization tower 13 and the second ammonia absorption tower 15; a carbonization mother liquor barrel 19 is connected between the soda ash filter 16 and the cold salt crystallization kettle 17; an automatic discharging centrifuge 20 and a centrifugal mother liquor tank 21 are provided at the bottom of the cold salt crystallization kettle 17.

[0040] The middle position of the carbonization tower 13 is provided with a flue gas inlet, the top is provided with a carbonization tail gas discharge port and a carbonation liquid inlet; a gaseous carbon dioxide inlet is provided below the flue gas inlet; the bottom of the carbonization tower 13 is provided with a liquid discharge port and is connected to the carbonization mother liquor tank 19; the carbonation liquid inlet is connected to the bottom of the concentrated ammonia buffer tank 18. Flue gas is directly blown into the middle part of the carbonization tower, without collecting CO2, directly coupling soda production and carbon capture, and opening up a new way of directly capturing CO2.

[0041] The middle position of the second ammonia absorption tower 15 is provided with a second steel enterprise ammonia inlet, the lower part is provided with a concentrated ammonia brine outlet, and the upper part is provided with a second brine inlet; the concentrated ammonia brine outlet is connected to the concentrated ammonia buffer tank 18, and a concentrated ammonia brine pump is provided on the connecting pipeline between the concentrated ammonia brine outlet and the concentrated ammonia buffer tank 18.

[0042] The middle position of the first ammonia absorption tower 14 is provided with a first steel enterprise ammonia inlet, the lower part is provided with a brine outlet, and the brine outlet is connected to the second brine inlet; the upper part of the first ammonia absorption tower 14 is provided with a saturated brine inlet; the liquid outlet of the centrifugal mother liquor tank 21 is connected to the saturated brine inlet.

[0043] 2205 duplex stainless steel is used as the main material equipment of the carbonization tower and the ammonia absorption tower, and 316L stainless steel is used for the pipelines and the liquid circulation pump, fundamentally avoiding the retention and erosion of Cl - and Fe + ions.

[0044] Its total reaction formula is:

[0045] Carbonization:

[0046] NH3 + NaCl + CO2 + H2O → NaHCO3↓ + NH4Cl (heavy soda carbonization reaction);

[0047] Calcination:

[0048] 2NaHCO3 → Na2CO3 + CO2↑ + H2O↑ (light ash calcination);

[0049] Total reaction:

[0050] 2NH3 + 2NaCl + H2O + CO2 = Na2CO3 + 2NH4Cl.

[0051] Chemical reaction for generating scaling substances during the brine ammoniation process:

[0052] Ca 2+ +(NH4)2CO3 = CaCO3↓ + 2NH 4+ ;

[0053] Mg 2+ +(NH4)2CO3 = MgCO3↓ + 2NH4+ ;

[0054] MgCO3 + (NH4)2CO3 + 4H2O = MgCO3·(NH4)2CO3·4H2O↓;

[0055] MgCO3 + NaCl + Na2CO3 + 4H2O = MgCO3·NaCl·Na2CO3↓;

[0056] This part is a harmful reaction and needs to be avoided. Therefore, when selecting brine, the contents of Ca 2+ and Mg 2+ are strictly controlled. Generally, the total amount of Ca 2+ and Mg 2+ should not exceed 30×10 -6 .

[0057] Other main chemical reactions of industrial recycled salt absorbing ammonia and carbon dioxide:

[0058] Ammonia dissolves in water:

[0059] NH3(g) + H2O(l) = NH4OH(aq) + 8313 kcal / mol.

[0060] Carbon dioxide dissolves in water:

[0061] CO2(g) + H2O(l) = H2CO3(aq) + 4821 kcal / mol.

[0062] Reaction of NH4OH and H2CO3:

[0063] 2NH4OH(aq) + H2CO3(aq) = (NH4)2CO3(aq) + 2H2O.

[0064] The outgoing liquor from the carbonating tower first enters the outgoing liquor tank and then flows into the liquor buffer tank through the outgoing liquor tank. The outgoing liquor flows into the belt filter 16 from the bottom of the liquor buffer tank and is vacuum filtered by the suction force from the vacuum pump; the filter cake is washed with water from the wash water elevated tank during the filtration process. The heavy soda ash discharged from the filter 16 is transported to the calcination section by a part of the heavy soda ash belt conveyor and is used to adjust the pH value in the concentrated brine wastewater concentrator and crystallizer. The gas-liquid mixture extracted by the vacuum machine enters the mother liquor separation tank for gas-liquid separation. The liquid at the bottom of the separation tank is called the filtered mother liquor and flows into the carbonated mother liquor machine 19 by itself. The precipitate in the carbonated mother liquor machine 19 is sent back to the outgoing liquor tank through a hydrocyclone by the heavy soda ash crystal slurry pump, and the clarified mother liquor (abbreviated as MⅠ) flows into the mother liquor MI barrel by itself.

[0065] The cold ammonia mother liquor MI sent by the filter press 16 is metered and exchanges heat with the gaseous ammonia heat exchanger, and then enters the central circulation pipe of the cold crystallization crystallizer 1, where it rises together with the circulating mother liquor AI from the and the salting-out crystal slurry at the bottom of the hydrocyclone to the bottom of the cold crystallization crystallizer 1 and is distributed upward; the mother liquor in the upper part of the liquid ammonia evaporation external cooler 5 of the crystallizer is sent into the tubes of the external cooler by the cold crystallization axial flow pump, exchanges heat with the liquid ammonia in the tube space to cool down and generates supersaturation, and then returns to the bottom of the central pipe of the cold crystallization crystallizer 1 through the collecting tank and the central circulation pipe, and its supersaturation disappears through the crystal slurry layer, promoting the formation and growth of ammonium chloride crystals; the crystal slurry is in a suspended state, and the overflow liquid semi-mother liquor I of the cold crystallization crystallizer 1 (MI in the soda ash production process and semi-mother liquor I in the ammonium chloride production process) flows into the central downcomer of the salting-out crystallizer 2.

[0066] As Figure 2 shown, the NH4Cl crystallization process of countercurrent extraction is as follows: the crystal slurry of the salting-out crystallizer 2 is taken out and sent to the hydrocyclone by the salting-out circulation pump 4. The top flow of the hydrocyclone flows into the salting-out crystallizer 2, and the bottom flow enters the cold crystallization crystallizer 1; the crystal slurry of the cold crystallization crystallizer 1 is sent to the cold thickener 8 by the cold crystallization main circulation pump 3 to be thickened, and then enters the centrifuge 9 for centrifugal separation; the wet ammonium chloride separated by the centrifuge 9 is transported to the dry ammonium section by the scraper conveyor; the overflow liquid of the cold thickener 8 and the filtrate semi-mother liquor II of the centrifuge 9 both flow into the filtrate tank 11 and are sent into the cold crystallization crystallizer 1 by the filtrate pump 12; the semi-mother liquor MII of the liquid ammonia evaporation external cooler 5 is emptied into the buffer tank and then sent into the salting-out crystallizer 1 by the external cooler drain pump.

[0067] According to the flow direction of ammonium chloride crystals, the flow direction of the crystal slurry is as follows: in the salting-out reactor 2, due to the addition and dissolution of NaCl, after the raw salt is added, due to the difference in supersaturation, NH4Cl precipitates, and the grown NH4Cl crystals are enriched into a slurry. The crystal slurry enriched on the top of the salting-out reactor 2 is lifted to the crystal slurry thickener 7 on the upper part of the crystal slurry cold crystallization reactor through the air-lift crystal slurry extraction pipe 6, runs back and forth, continuously crystallizes, reaches the saturation state, forms an ammonium chloride enriched crystal slurry, and ammonium chloride NH4Cl is taken out after passing through the carbonated mother liquor machine 9.

[0068] Based on the on-site working conditions and the requirements of the gas-based direct reduction process, taking the capture of 1000 tons of carbon dioxide by the MEA method from the tail gas of the steel pipe rolling workshop as the benchmark, the CO2 purity ≥ 98.5%, and the composition of the raw gas is as follows in the table:

[0069] Table 1 Composition and content of raw gas

[0070]

[0071] Flue gas temperature: average 238 °C, minimum: 100 °C, maximum 405 °C;

[0072] Average flue gas pressure: -0.44 KPa;

[0073] Flue gas volume: 1000 Nm 3 / h

[0074] The NH3 used is liquid ammonia prepared by the steel plant's coking plant, and the NaCl used is industrial salt purified from rolling mill wastewater. In the wastewater composition: the sodium ion content is 38 g / L, chloride ion: 92 mg / mL. After adding sodium carbonate, the pH value is controlled at 7. After concentration, the sodium chloride content is 95%, and the rest is crystal water.

[0075] The process of the soda ash production unit is as follows:

[0076] The saturated ammonia salt mother liquor coming out of the ammonia absorption tower is fed into the upper part of the carbonation tower 13; the concentrated CO2 gas captured by the CO2 capture system enters from the bottom of the carbonation tower 13, and the diluted CO2 gas pressurized by the flue gas compression system is introduced into the middle part of the carbonation tower 13. The saturated ammonia salt mother liquor and CO2 conduct a chemical reaction in reverse contact in the carbonation tower 13 to generate sodium bicarbonate crystal slurry, which is pressed from the bottom of the tower to the filtration process by means of the tower pressure.

[0077] The sodium bicarbonate crystal slurry is fed into the filter press 16 (heavy soda filter). In the heavy soda filter, the carbonated mother liquor is evenly distributed in the width direction of the filter cloth, and vacuum filtration is carried out by means of the suction force from the vacuum pump to separate the carbonated mother liquor and solid particles; the solid particles are fed into the heavy soda calciner, and the heavy soda is heated and decomposed into light soda ash by steam at 1.4 MPa in the calciner, generating furnace gas containing carbon dioxide, ammonia and water vapor. The light soda ash is discharged from the bottom by an alkali screw conveyor and enters the automatic quantitative packaging system. A part of the packaged finished product is transported to the concentrated brine wastewater concentration crystallizer (the sewage treatment plant needs sodium carbonate to adjust the pH value, precipitate treatment, reduce the hardness of water and remove pollutants in water) to adjust the pH value, and the rest is sent to other processes such as flue gas desulfurization in the steel plant for recycling; the furnace gas containing carbon dioxide, ammonia, water vapor and a small amount of sodium carbonate dust decomposed enters the furnace gas scrubber to recover ammonia and sodium carbonate dust, and the washing liquid is sent back to the heavy soda filter for recycling; the gas after being washed by the scrubber is clean gas containing carbon dioxide and water vapor, and the gas enters the CO2 dehydrator for dehydration. The dehydrated concentrated CO2 gas is mixed with the concentrated CO2 gas captured by the CO2 capture system after being pressurized and then enters the carbonation tower from the bottom to achieve CO2 recycling; in some embodiments, the temperature of the bottom cooling section of the carbonation tower is controlled at 30 °C, the temperature of the upper absorption section in the middle of the tower is 60 °C, the reaction pressure is 0.3 MPa, and the design pressure is 0.6 MPa. It can be a bubble-cap tower, a sieve-tray tower or a packed tower, and an unequal-diameter tower structure is adopted.

[0078] In some embodiments, the soda ash production unit is equipped with a furnace gas scrubber and a CO2 recovery device. The furnace gas scrubber is mainly used to wash the top gas of the carbonating tower and the gas containing CO2, NH3, H2O and a small amount of sodium carbonate dust generated by the heavy soda calciner, purify the top gas, recover and utilize the effective components, and discharge the gas up to the standard; the CO2 recovery device mainly includes a CO2 dehydrator and a compressor, which are used to recover the CO2 generated by calcination and realize recycling.

[0079] The process of the ammonium chloride production unit is as follows: The NaCl obtained from the industrial brine concentrator crystallizer and the mother liquor MⅠ filtered out by the crude sodium bicarbonate filter are made into saturated brine and enter from the top of the brine ammoniation tower. Ammonia gas is introduced into the lower part of the brine ammoniation tower, and reverse reaction occurs in the ammoniation tower to generate saturated ammonium salt mother liquor and send it to the carbonating tower; the filtrate after filtering the heavy soda in the soda ash production unit enters the ammonium chloride crystallizer kettle, and NaCl and ammonia gas obtained from the concentrator crystallizer are added into the ammonium chloride crystallizer kettle. Cooling crystallization and salting out are carried out in the ammonium chloride crystallizer kettle to precipitate ammonium chloride crystals; the suspension slurry containing ammonium chloride crystals taken out from the ammonium chloride crystallizer kettle enters the ammonium chloride filter for thickening and centrifugal filtration, and the separated solid wet ammonium chloride is sent to the ammonium chloride dryer to produce dry ammonium chloride. In some embodiments, the reaction temperature of the brine ammoniation tower is controlled at 30 °C. In the prepared ammonium mother liquor Ⅱ: the ratio of the free ammonia concentration to the NaCl concentration is 1.1.

[0080] In some embodiments, the ammonium chloride crystallizer kettle can be a set of equipment or composed of two sets of equipment, which has both cooling crystallization function and salting out function, and the temperature is controlled at 10 °C. In the ammonium mother liquor Ⅰ entering the ammonium chloride crystallizer kettle: the ratio of the free ammonia concentration to the CO2 concentration converted by HCO - 3 is 2.15.

[0081] The main chemical reactions are as follows:

[0082] Carbonation reaction: NaCl + NH3 + CO2 + H2O → NaHCO3↓ + NH4Cl

[0083] Calcination decomposition reaction: 2NaHCO3 → Na2CO3 + CO2↑ + H2O↑

[0084] Table 2 shows the solubility differences of three salts, namely NaCl, NaHCO3, and NH4Cl, at different temperatures:

[0085] 0℃ 10℃ 20℃ 30℃ 40℃ 50℃ 60℃ 70℃ 80℃ 90℃ 100℃ NaCl 35.7 35.8 36.0 36.3 36.6 37.0 37.3 37.8 38.4 39.0 39.8 <![CDATA[NaHCO3]]> 6.9 8.15 9.6 11.1 12.7 14.45 16.4 — — — — <![CDATA[NH4Cl]]> 29.4 33.3 37.2 41.1 45.8 50.4 55.2 60.2 65.6 71.3 77.3

[0086] Theoretically calculated, 2049 tons of soda ash and 2431 tons of ammonium chloride can be produced. Actually, the result of producing 4 tons of double alkali (soda ash, ammonium chloride) from 1 ton of carbon dioxide can be achieved.

[0087] In this pilot production line, a carbonation and carbonization tower is adopted, and a spare tower is not used. The tower height is 18 m, and the diameter of the absorption section of the tower is 0.5 m. Two ammonia absorption towers of the same height are configured to avoid ammonia escape. All three towers are made of 2205 duplex stainless steel. 1000 t of carbon dioxide comes from the capture of the tail gas of the heating furnace in the steel pipe factory of the steel plant, 773 t of ammonia comes from the coking plant, the brine is taken from the concentrated brine after rolling steel treatment, and the salt comes from the industrial salt after evaporation of the concentrated salt wastewater, with a purity of over 99.5%. The entire project cost is low, and industrial steam, condensate, instrument sealing gas, etc. all come from the steel plant itself. Excluding the depreciation of fixed investment, the production cost of the double alkali is 20% lower than that of the traditional method, with good environmental friendliness and good benefits. The present invention provides a new way for the steel plant to recycle solid waste, while using carbon dioxide capture to treat and consume the brine with a high NaCl content, having a broad application prospect and greatly promoting the development of green chemical industry and circular economy.

Claims

1. A method for carbon fixation using concentrated brine wastewater from a steel plant, characterized in that: The invention comprises a soda ash production process I and an ammonium chloride production process II, and the two processes constitute a cycle; raw materials NH3, salt NaCl and gaseous carbon dioxide are continuously added into the circulation system, and soda ash and ammonium chloride products are continuously produced at the same time; the raw material carbon dioxide comes from the top gas of the blast furnace of the steel plant, the tail gas of the steel rolling heating furnace or the tail gas of the lime kiln; the raw material salt NaCl comes from the concentrated salt wastewater of the steel plant; the raw material NH3 comes from the coke oven gas, a by-product generated in the production process of steel enterprises and coking plants.

2. The method for carbon fixation using steel mill concentrated brine wastewater according to claim 1, characterized in that: The soda ash production process I is to preheat the mother liquor MⅡ filtered out after salting out ammonium chloride, absorb ammonia to become ammonia mother liquor AⅡ, and react with the continuously added gaseous carbon dioxide to carbonize, filter after the carbonization reaction, treat the alkali liquor from the filtrate to obtain the mother liquor MI, and calcine the filter cake to obtain soda ash; The ammonium chloride production process II is as follows: the mother liquor MI absorbs ammonia to become ammonia mother liquor AI, and liquid half-mother liquor MⅡ and solid ammonium chloride are obtained through cold precipitation, and salt NaCl is added to the half-mother liquor MⅡ to obtain mother liquor MⅡ and solid ammonium chloride through salting out.

3. The method for carbon fixation using concentrated brine wastewater from a steel plant according to claim 2, characterized in that: The alkali liquid after carbonization first enters the alkali outlet tank, flows into the alkali liquid buffer tank through the alkali outlet tank, and then flows into the alkali filter (16) through the bottom of the alkali liquid buffer tank for vacuum filtration; the filter cake is washed with water from the washing water high-level tank during the filtration process, and the heavy alkali out of the alkali filter (16) is transported to the calcination section via a part of the heavy alkali belt conveyor; the gas-liquid mixture extracted by the vacuum machine enters the mother liquor separation tank for gas-liquid phase separation, and the liquid at the bottom of the separation tank is called the filtered mother liquor, which flows into the carbonization mother liquor machine (19) by gravity, and the precipitate of the carbonization mother liquor machine (19) is sent back to the alkali outlet tank through the heavy alkali crystal slurry pump and the cyclone, and the clarified mother liquor is the mother liquor MⅠ which flows into the mother liquor MI barrel by gravity.

4. The method for carbon fixation using steel mill concentrated brine wastewater according to claim 2, characterized in that: After being metered and heat exchanged in the gas-ammonia heat exchanger, the cold ammonia mother liquor MI is sent to the central circulation pipe of the cold precipitation crystallizer (1), and together with the mother liquor AI and the salting-out crystal slurry of the bottom flow of the cyclone separator, it reaches the bottom of the cold precipitation crystallizer (1) and is distributed upward; the upper mother liquor of the crystallizer liquid ammonia evaporation external cooler (5) is sent to the tube array of the external cooler through the cold precipitation axial flow pump, and is cooled by heat exchange with the liquid ammonia between the tubes to produce supersaturation, and then returns to the bottom of the central tube of the cold precipitation crystallizer (1) through the collecting tank and the central circulation pipe, and its supersaturation disappears through the crystal slurry layer, thereby promoting the formation and growth of ammonium chloride crystals; the crystal slurry is in a suspended state, and the overflow liquid half mother liquor I of the cold precipitation crystallizer flows into the central downcomer of the salting-out crystallizer.

5. The method for carbon fixation using steel mill concentrated brine wastewater according to claim 2, characterized in that: The salt NaCl is added into the central circulation pipe of the salting-out crystallizer (2), and is sent to the bottom of the crystallizer together with the overflow semi-mother liquor MII of the cold crystallizer (1) and the filtrate sent by the filtrate pump, where it is distributed upward and gradually dissolved to precipitate ammonium chloride crystals; the mother liquor MII overflowing from the salting-out crystallizer (2) flows into the mother liquor MII barrel and is sent to the mother liquid exchange section by the mother liquor MII pump; the precipitate at the bottom of the mother liquor MII barrel (10) is sent to the salting-out crystallizer by the mother liquor MII precipitation pump.

6. The method for carbon fixation using concentrated brine wastewater from a steel plant according to claim 2, characterized in that: The crystal slurry from the salting-out crystallizer (2) is taken out and sent to the hydrocyclone separator via the salting-out circulation pump (4), the top flow of the hydrocyclone separator flows into the salting-out crystallizer (2), and the bottom flow enters the cold precipitation crystallizer (1); the crystal slurry from the cold precipitation crystallizer (1) is sent to the cold precipitation thickener (8) via the cold precipitation main circulation pump (3) for thickening, and then enters the centrifugal filter (9) for centrifugal separation; the wet ammonium chloride separated by the centrifugal filter (9) is sent to the dry ammonium section via an embedded scraper conveyor; the overflow liquid of the cold precipitation thickener (8) and the filtrate semi-mother liquor II of the centrifugal filter (9) both flow into the filtrate bucket (11), and are sent to the salting-out crystallizer (1) via the filtrate pump (12); the semi-mother liquor II of the liquid ammonia evaporation external cooler (5) is emptied and flows into the buffer tank, and then is sent to the salting-out crystallizer via the external cooler discharge pump.

7. The dedicated device for the method of carbon fixation using concentrated brine wastewater from a steel mill as claimed in any one of claims 1 to 6, characterized in that: It comprises a carbonization tower (13), a first ammonia absorption tower (14), a second ammonia absorption tower (15), an alkali filter (16), and a cold salting out crystallization kettle (17); A concentrated ammonia buffer tank (18) is connected between the carbonization tower (13) and the second ammonia absorption tower (15) via a pipeline; a carbonization mother liquor barrel (19) is connected between the alkali filter (16) and the cold salting-out crystallization kettle (17) via a pipeline; an automatic unloading centrifuge (20) and a centrifugal mother liquor tank (21) are connected at the bottom of the cold salting-out crystallization kettle (17) via a pipeline; the main body material of the carbonization tower (13), the first ammonia absorption tower (14) and the second ammonia absorption tower (15) is 2205 duplex stainless steel.

8. The dedicated device for the method of carbon fixation using concentrated brine wastewater from a steel mill according to claim 6, characterized in that: A flue gas inlet is provided in the middle of the carbonization tower (13), and a carbonization tail gas discharge port and a carbonated liquid inlet are provided at the top; a gaseous carbon dioxide inlet is provided below the flue gas inlet; a liquid discharge port is provided at the bottom of the carbonization tower (13) and is connected to an alkali filter (16); and the carbonated liquid inlet is connected to the bottom of a concentrated ammonia buffer tank (18).

9. The dedicated device for the method of carbon fixation using concentrated brine wastewater from a steel mill according to claim 6, characterized in that: A second steel enterprise ammonia gas inlet is provided in the middle of the second ammonia absorption tower (15), a concentrated ammonia brine outlet is provided at the bottom, and a second brine inlet is provided at the top; the concentrated ammonia brine outlet is connected to a concentrated ammonia buffer tank (18), and a concentrated ammonia brine pump is provided on the connecting pipeline between the concentrated ammonia brine outlet and the concentrated ammonia buffer tank (18).

10. The dedicated device for the method of carbon fixation using concentrated brine wastewater from a steel mill according to claim 8, characterized in that: A first steel enterprise ammonia inlet is provided in the middle of the first ammonia absorption tower (14), and a brine outlet is provided at the bottom, and the brine outlet is connected to a second brine inlet; a saturated brine inlet is provided at the top of the first ammonia absorption tower (14); and the liquid outlet of the centrifugal mother liquid tank (21) is connected to the saturated brine inlet.

Images