Method for recovering tail gas generated in production of lithium iron phosphate

Abstract

The invention provides a method for recovering tail gas generated in lithium iron phosphate production, which comprises the following steps: absorption: introducing tail gas containing ammonia gas and carbon dioxide into an absorption tower, spraying a saturated sodium chloride solution into the absorption tower, and reacting with ammonia gas and carbon dioxide to obtain a mixed solution; crystallization: introducing the mixed solution in the absorption tower into a crystallizer, so that sodium bicarbonate in the mixed solution is crystallized and separated out; and separation: filtering the mixed solution passing through the crystallizer, and separating sodium bicarbonate from the mixed solution to obtain solid sodium bicarbonate and a solution containing ammonium chloride. The saturated sodium chloride solution is used for absorbing ammonia gas and carbon dioxide gas in the tail gas, and environmental pollution caused by direct emission of the ammonia gas is avoided.

Description

Technical Field

[0001] This application relates to the field of waste gas treatment technology in the production of lithium iron phosphate, and more particularly to a method for recovering tail gas generated during the production of lithium iron phosphate. Background Technology

[0002] The production of lithium iron phosphate uses iron oxide, ammonium dihydrogen phosphate, and lithium carbonate as raw materials. During the high-temperature solid-phase reaction stage, a large amount of exhaust gas is generated, mainly composed of ammonia and carbon dioxide. These gaseous components have significant industrial value, but currently, they are commonly treated by being directly fed into incinerators for combustion, resulting in substantial resource waste and environmental pollution. Summary of the Invention

[0003] The purpose of this application is to provide a method for recovering tail gas generated during the production of lithium iron phosphate. By using a non-toxic and harmless saturated sodium chloride solution as an absorbent, the method achieves maximum recovery and utilization of useful resources such as ammonia and carbon dioxide in the tail gas. To achieve one of the above-mentioned objectives, one embodiment of this application provides a method for recovering tail gas generated during the production of lithium iron phosphate, comprising the following steps: Absorption: The tail gas containing ammonia and carbon dioxide is passed into the absorption tower, where a saturated sodium chloride solution is sprayed to react with the ammonia and carbon dioxide to obtain a mixed solution. Crystallization: The mixed solution in the absorption tower is passed into a crystallizer, causing sodium bicarbonate in the mixed solution to crystallize and precipitate out; Separation: The mixed solution after passing through the crystallizer is filtered to separate sodium bicarbonate from the mixed solution, resulting in solid sodium bicarbonate and a solution containing ammonium chloride.

[0004] As a further improvement of one embodiment of this application, in the absorption step, the temperature of the absorption tower is controlled to be 40~50°C.

[0005] As a further improvement of one embodiment of this application, in the crystallization step, the crystallization temperature of the mixed solution in the crystallizer is ≤10°C.

[0006] As a further improvement of one embodiment of this application, in the separation step, a filter press is used to pressurize and filter the mixed solution, and the pressure of the filter press is 0.5~0.6 MPa.

[0007] As a further improvement of one embodiment of this application, in the separation step, the solid sodium bicarbonate obtained after pressure filtration is dried at 80~120 °C for 20~40 min.

[0008] As a further improvement of one embodiment of this application, the exhaust gas comprises, by mass percentage, 10-20% ammonia, 20-35% carbon dioxide, 40-70% nitrogen, with the remainder being impurities.

[0009] As a further improvement of one embodiment of this application, a pretreatment step is included before the absorption step, in which the exhaust gas is passed through a dust collector and coke remover to remove solid impurities.

[0010] As a further improvement of one embodiment of this application, the temperature of the exhaust gas generated during the production of lithium iron phosphate is 200~300 ℃, and the exhaust gas is cooled by a cooler to ≤100 ℃.

[0011] As a further improvement of one embodiment of this application, after the exhaust gas is cooled, the exhaust gas is pressurized to increase the pressure by 20~40kPa.

[0012] As a further improvement of one embodiment of this application, the temperature of the exhaust gas entering the absorption tower after being cooled, pressurized, and treated by a dust and coke remover is 20~40 ℃.

[0013] One or more technical solutions provided in this application have at least the following technical effects or advantages: The method for recovering tail gas generated during lithium iron phosphate production provided in this application uses a saturated sodium chloride solution to absorb ammonia and carbon dioxide in the tail gas. By using a non-toxic and harmless saturated sodium chloride solution as the absorbent, the maximum recovery and utilization of useful resources such as ammonia and carbon dioxide in the tail gas is achieved. Simultaneously, the byproduct sodium bicarbonate can be used as an industrial product, and ammonium chloride can be used as nitrogen fertilizer. The emitted tail gas meets environmental standards, and no other toxic or flammable chemicals are introduced into the entire recovery and utilization system. The entire production process is safe and reliable, and suitable for existing lithium iron phosphate production enterprises belonging to the industrial and trading sectors. Attached Figure Description

[0014] Figure 1 This is a flowchart of a method for recovering tail gas generated during the production of lithium iron phosphate, as described in this application. Detailed Implementation

[0015] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0016] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "connected" and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. For those skilled in the art, the specific meaning of the above terms in this application can be understood according to the specific circumstances.

[0017] This application provides a method for recovering tail gas generated during the production of lithium iron phosphate, comprising the following steps: Absorption: The tail gas containing ammonia and carbon dioxide is passed into the absorption tower, where a saturated sodium chloride solution is sprayed to react with the ammonia and carbon dioxide to obtain a mixed solution. Crystallization: The mixed solution in the absorption tower is passed into a crystallizer, causing sodium bicarbonate in the mixed solution to crystallize and precipitate out; Separation: The mixed solution after passing through the crystallizer is filtered to separate sodium bicarbonate from the mixed solution, resulting in solid sodium bicarbonate and a solution containing ammonium chloride.

[0018] In some embodiments of this application, the exhaust gas comprises, by mass percentage, 10-20% ammonia, 20-35% carbon dioxide, 40-70% nitrogen, with the remainder being impurities.

[0019] Nitrogen constitutes the majority of the exhaust gas produced during lithium iron phosphate production. While nitrogen does not react with sodium chloride and water, it can be directly emitted without impacting the ecological environment. Ammonia and carbon dioxide, although only a small portion, can easily cause environmental pollution if emitted directly. This application utilizes a saturated sodium chloride solution to absorb ammonia and carbon dioxide from the exhaust gas, producing sodium bicarbonate and ammonium chloride. By using a non-toxic and harmless saturated sodium chloride solution as the absorbent, the maximum recovery and utilization of useful resources such as ammonia and carbon dioxide from the exhaust gas is achieved.

[0020] Meanwhile, the byproduct sodium bicarbonate can be used as industrial products, and ammonium chloride can be used as nitrogen fertilizer; the exhaust gas meets environmental protection standards, and no other toxic or flammable chemicals are introduced into the entire recycling system. The entire production process is safe and reliable, and is suitable for existing lithium iron phosphate production enterprises belonging to the industrial and trading sector.

[0021] A crystallizer is connected to the outlet of the absorption tower. After the reaction in the absorption tower, the mixed liquid is discharged from the absorption tower and enters the crystallizer for crystallization, causing sodium bicarbonate in the mixed solution to precipitate out as a solid. After passing through the crystallizer, the mixed solution containing solid sodium bicarbonate is filtered to obtain sodium bicarbonate residue and the remaining mixed solution, thus separating sodium bicarbonate from the mixed solution to obtain the product.

[0022] In some embodiments of this application, the temperature of the absorption tower is controlled at 40~50°C during the absorption step.

[0023] The exhaust gas is fed into the absorption tower from bottom to top, and the saturated sodium chloride solution is sprayed downward from the top of the absorption tower. The gas phase and the liquid phase react in countercurrent contact, and the reaction products are discharged from the bottom of the absorption tower.

[0024] Ammonia, carbon dioxide, water, and sodium chloride react at room temperature, but sodium bicarbonate has low solubility in water and easily precipitates. Maintaining a higher temperature in the absorption tower allows more of the reaction product, sodium bicarbonate, to dissolve in water rather than precipitate as a solid, thus preventing solid sodium bicarbonate from draining from the bottom and clogging the absorption tower's outlet.

[0025] Preferably, a gas distribution plate is installed at the bottom of the absorption tower. The exhaust gas enters the absorption tower from below the gas distribution plate and flows upward after passing through it. The gas distribution plate ensures that the upward-flowing gas is evenly distributed. A uniform spray structure is also installed at the top of the absorption tower, where a saturated sodium chloride solution is sprayed evenly from top to bottom. This ensures that the ammonia and carbon dioxide in the exhaust gas come into uniform contact with the saturated sodium chloride solution and react, thereby improving the reaction efficiency.

[0026] In some embodiments of this application, during the crystallization step, the crystallization temperature of the mixed solution in the crystallizer is ≤10°C.

[0027] Sodium bicarbonate has low solubility, and its solubility in water decreases with decreasing temperature. At sufficiently low temperatures, sodium bicarbonate becomes supersaturated in water and crystallizes out, thus enabling separation from the mixed solution in the separation step.

[0028] In some embodiments of this application, in the separation step, a filter press is used to pressurize and filter the mixed solution, and the pressure of the filter press is 0.5~0.6MPa.

[0029] Pressure filtration increases the filtration rate, quickly filtering out some of the dissolved sodium bicarbonate, ammonium chloride, and unreacted sodium chloride. It also reduces the content of mixed solution in the crystallized sodium bicarbonate filter residue, resulting in less ammonium chloride and sodium chloride impurities and higher purity in the filtered sodium bicarbonate.

[0030] The filtrate obtained after pressure filtration still contains sodium bicarbonate dissolved in water. The filtrate is recycled into a crystallizer for recrystallization, which allows sodium bicarbonate in the mixed solution to precipitate out to a greater extent and reduce residue.

[0031] In some embodiments of this application, during the separation step, the solid sodium bicarbonate obtained after pressure filtration is dried at 80~120°C for 20~40 min.

[0032] After pressure filtration, although sodium bicarbonate contains less of the mixed solution, some of the mixed solution still remains on the surface of sodium bicarbonate. The surface moisture needs to be dried to obtain dry sodium bicarbonate, which can be used as an industrial sodium bicarbonate product.

[0033] In some embodiments of this application, a pretreatment step is included before the absorption step, in which the exhaust gas is passed through a dust collector and coke remover to remove solid impurities.

[0034] In addition to gases, the exhaust gas also contains a very small amount of solid particles such as dust and tar with small diameters, which are carried out by the airflow. In order to reduce impurities, a dust collector and tar remover are connected before the air inlet of the absorption tower. The exhaust gas is filtered to remove solids, resulting in exhaust gas containing only gaseous substances. This avoids solid particles from clogging and corroding subsequent absorption towers, crystallizers and other equipment, and extends the service life of the equipment.

[0035] In some embodiments of this application, the temperature of the exhaust gas generated during the production of lithium iron phosphate is 200~300°C, and the exhaust gas is cooled to ≤100°C by a cooler.

[0036] The production of lithium iron phosphate involves high temperatures, resulting in high exhaust gas temperatures. Directly introducing the high-temperature exhaust gas into the dust collector and coking remover can easily damage the equipment. Therefore, the exhaust gas needs to be cooled to below 100°C to avoid damage to downstream equipment.

[0037] In some embodiments of this application, after the exhaust gas is cooled, the exhaust gas is pressurized to increase the pressure by 20~40 kPa.

[0038] After the exhaust gas exits the lithium iron phosphate production equipment, it needs to pass through a dust collector and coke remover to remove solid particulate matter. This requires a certain pressure; therefore, the exhaust gas, cooled to below 100°C, is pressurized by 20-40 kPa to provide sufficient pressure and prevent insufficient pressure from reducing efficiency. A fan is preferred for pressurization.

[0039] In some embodiments of this application, the exhaust gas, after being cooled, pressurized, and processed by a dust and coke remover, enters the absorption tower at a temperature of 20-40°C. After being cooled by the cooler, the exhaust gas temperature drops from 200-300°C to below 100°C. The temperature further decreases after being pressurized by a fan, and is further reduced during the removal of solid particulate matter in the dust and coke remover, resulting in a final exhaust gas temperature of 20-40°C. This relatively low temperature allows for reaction with water and sodium chloride.

[0040] The technical solution of this application will be further described below with reference to some specific embodiments.

[0041] Example 1 Pretreatment steps: The exhaust gas produced by lithium iron phosphate production is 250°C. After being cooled to 32°C by a cooler, it is pressurized to 30 kPa by a fan and then passes through a dust collector and coke remover to remove solid particulate matter from the exhaust gas. Absorption Steps: The exhaust gas, after pretreatment, enters from the bottom of the absorption tower and flows upwards evenly after passing through the gas distribution plate at the bottom of the tower. Saturated sodium chloride solution is sprayed downwards evenly through the spray structure at the top of the absorption tower. The temperature of the absorption tower is 40℃. After the ammonia and carbon dioxide in the exhaust gas react with the saturated sodium chloride solution, the mixed solution is discharged from the bottom of the absorption tower, while the nitrogen exhaust gas is vented from the top of the absorption tower. Crystallization steps: The mixed solution is crystallized in a crystallizer at 10°C; Separation steps: After solid sodium bicarbonate crystallizes out, the mixed solution is filtered through a filter press at a pressure of 0.5 MPa to obtain sodium bicarbonate filter residue and mixed solution. The sodium bicarbonate filter residue is dried at 100℃ for 30 min to obtain dried sodium bicarbonate product.

[0042] It should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

[0043] The detailed descriptions listed above are merely specific descriptions of feasible implementation methods of this application and are not intended to limit the scope of protection of this application. All equivalent implementation methods or modifications made without departing from the spirit of the art of this application should be included within the scope of protection of this application.

Claims

1. A method for recovering tail gas generated during the production of lithium iron phosphate, characterized in that, Includes the following steps: Absorption: The tail gas containing ammonia and carbon dioxide is passed into the absorption tower, where a saturated sodium chloride solution is sprayed to react with the ammonia and carbon dioxide to obtain a mixed solution. Crystallization: The mixed solution in the absorption tower is passed into a crystallizer, causing sodium bicarbonate in the mixed solution to crystallize and precipitate out; Separation: The mixed solution after passing through the crystallizer is filtered to separate sodium bicarbonate from the mixed solution, resulting in solid sodium bicarbonate and a solution containing ammonium chloride.

2. The method for recovering tail gas generated during the production of lithium iron phosphate according to claim 1, characterized in that, In the absorption step, the temperature of the absorption tower is controlled at 40~50℃.

3. The method for recovering tail gas generated during the production of lithium iron phosphate according to claim 2, characterized in that, In the crystallization step, the crystallization temperature of the mixed solution in the crystallizer is ≤10℃.

4. The method for recovering tail gas generated during the production of lithium iron phosphate according to claim 3, characterized in that, In the separation step, a filter press is used to pressurize and filter the mixed solution at a pressure of 0.5~0.6MPa.

5. The method for recovering tail gas generated during the production of lithium iron phosphate according to claim 4, characterized in that, In the separation step, the solid sodium bicarbonate obtained after pressure filtration is dried at 80~120℃ for 20~40 min.

6. The method for recovering tail gas generated during the production of lithium iron phosphate according to claim 1, characterized in that, By mass percentage, the exhaust gas contains 10-20% ammonia, 20-35% carbon dioxide, 40-70% nitrogen, and the remainder is impurities.

7. The method for recovering tail gas generated during the production of lithium iron phosphate according to claim 6, characterized in that, The absorption step also includes a pretreatment step, in which the exhaust gas is passed through a dust and coke remover to remove solid impurities.

8. The method for recovering tail gas generated during the production of lithium iron phosphate according to claim 7, characterized in that, The temperature of the exhaust gas produced during the production of lithium iron phosphate is 200~300℃. The exhaust gas is cooled to ≤100℃ by a cooler.

9. The method for recovering tail gas generated during the production of lithium iron phosphate according to claim 8, characterized in that, After the exhaust gas is cooled, it is pressurized by 20-40 kPa.

10. The method for recovering tail gas generated during the production of lithium iron phosphate according to claim 9, characterized in that, The exhaust gas, after being cooled, pressurized, and processed by a dust and coke remover, enters the absorption tower at a temperature of 20~40℃.

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