Method and system for recovering aluminum hydroxide calcination fumes

Abstract

The application discloses a kind of recovery method and recovery system of aluminium hydroxide calcination dust, it is sent to calcination furnace calcination using venturi dryer drying aluminium hydroxide, and the alumina obtained by calcination is sent to fluidized bed cooler and is cooled to obtain alumina product, and after being treated by primary cyclone separator, secondary cyclone separator, the flue gas is sent to venturi dryer for heating and drying, the waste gas in venturi dryer is treated by tertiary cyclone separator and dust is recovered, and then sent to tail gas treatment system after being treated by bag filter, and then discharged after denitration and washing, wherein, according to the particle size monitoring data of dust in cyclone separator and bag filter, different recovery methods are intelligently controlled and used for processing, so as to effectively reduce resource consumption, improve the yield of alumina product, on the other hand, the waste heat in the fluidized bed cooler is used to pretreat the denitration tail gas to promote the denitration reaction, which is beneficial to the harmlessness of harmful pollutants in flue gas and prevents dust pollution.

Description

Technical Field

[0001] This invention belongs to the field of aluminum hydroxide roasting flue dust recovery and treatment technology, specifically relating to a method and system for recovering aluminum hydroxide roasting flue dust. Background Technology

[0002] The roasting of aluminum hydroxide to produce alumina is a crucial step in the Bayer process for alumina production. The roasting process controls the particle size distribution, hardness, and strength of the final alumina, directly impacting the quality of the alumina product. The flue gas generated during roasting contains pollutants such as nitrogen oxides, alumina dust, and alkali metal salt dust. Therefore, it is necessary to treat the flue gas to render it harmless and to recover as much alumina dust as possible back into the roasting raw materials to improve product yield and prevent environmental pollution. For example, Chinese patent application CN109432934A discloses an integrated flue gas purification system and an alumina roasting process system. This integrated flue gas system includes at least a gravity dust removal unit, an electrostatic precipitator unit, a bag filter unit, and a denitrification unit. This achieves a dust removal efficiency of over 99.95%, maximizing and efficiently separating gas and solids. This ensures that almost all aluminum hydroxide material is converted into alumina in the main roasting furnace, significantly reducing material waste and improving product quality. However, in actual production, there are also drawbacks to centrally returning all the recovered dust to the main roasting furnace. For example, some of the recovered alumina dust particles are too fine (less than 30μm), which have strong adsorption properties and are prone to moisture absorption and clumping. They are easy to clump and block during the return transportation process. Moreover, if the fine particles are directly mixed with the aluminum hydroxide raw material for roasting, it can easily lead to the pulverization of the alumina product. Summary of the Invention

[0003] To address the aforementioned shortcomings, this invention discloses a method and system for recovering calcination dust from aluminum hydroxide. This method not only effectively recovers the calcination dust generated during calcination into the calcination system, improving product yield, but also treats pollutants such as nitrogen oxides in the flue gas, preventing environmental pollution.

[0004] This invention is achieved using the following technical solution: A method for recovering aluminum hydroxide roasting flue dust includes the following steps: S1. The aluminum hydroxide filter cake obtained by drying in a Venturi dryer is sent to a calcining furnace for calcination at 900-1100℃. The calcined alumina is processed by a fluidized bed cooler to obtain alumina product. The flue gas generated during calcination is sent to a primary cyclone separator for treatment. S2. The primary recovered dust collected in the primary cyclone separator is mixed with the aluminum hydroxide filter cake obtained by drying in the Venturi dryer and then sent to the calcination furnace for calcination; the tail gas from the outlet of the primary cyclone separator is sent to the secondary cyclone separator for treatment. S3. The tail gas from the secondary cyclone separator is sent to a Venturi dryer for heating and drying the aluminum hydroxide filter cake. The temperature of the tail gas at the outlet of the secondary cyclone separator is controlled at 300-400℃. The secondary recovered dust collected in the secondary cyclone separator is mixed with water to make a dust slurry with a solid content of 20-25%. Then, calcium hydroxide solution is added to the dust slurry, stirred and mixed, and filtered to obtain a purified slurry. The purified slurry is then sent to a bauxite wet grinding system for recovery. The calcium hydroxide solution is prepared according to a mass ratio of calcium oxide to secondary recovered dust of (1-5):100. S4. The exhaust gas in the Venturi dryer is sent to a three-stage cyclone separator for treatment. The three-stage recovered dust collected in the three-stage cyclone separator is mixed with the aluminum hydroxide filter cake at the inlet of the Venturi dryer, and the exhaust gas at the outlet of the three-stage cyclone separator is sent to a bag filter for treatment. S5. Mix the dust collected in the bag filter with the secondary recovered dust for treatment; send the exhaust gas from the bag filter to the exhaust gas preheater for treatment. S6. The fluidized bed cooler is air-cooled, and the generated hot air is sent to the calcining furnace for combustion. Part of the hot air is sent to a heat exchanger to exchange heat with cold air and then returned to the hot air duct to mix and be sent to the calcining furnace. The cold air is heated to 450-500°C after heat exchange and then split into two paths. One path is sent to the exhaust gas preheater to mix with the exhaust gas from the bag filter, and the other path is injected with urea solution for mixing. The two paths are then mixed to obtain a reaction gas at a temperature of 250-300°C. The reaction gas is then sent to a reaction vessel filled with a vanadium-based catalyst for reaction. The exhaust gas in the reaction vessel is sent to a waste heat boiler to recover waste heat and then sent to an alkaline scrubbing tower for treatment. The exhaust gas from the alkaline scrubbing tower is sent to the chimney for discharge.

[0005] Furthermore, in step S2, if the particle size of the primary recovered dust collected after treatment by the primary cyclone separator is less than 30μm, the primary recovered dust is mixed with the secondary recovered dust for further processing. This is to prevent excessively fine dust from being introduced into the calcination system, which would result in inconsistent particle size of the calcined material, affecting the quality of the alumina product and causing product pulverization.

[0006] Furthermore, in step S3, if the particle size of the tertiary recovered dust collected after treatment by the tertiary cyclone separator is less than 10 μm, the tertiary recovered dust is mixed with the secondary recovered dust for further treatment. This is to avoid introducing excessively fine dust into the drying system, which would affect the drying effect.

[0007] Furthermore, in step S6, the concentration of the urea solution is 40-50%.

[0008] Furthermore, in step S6, the nitrogen oxide (NOx) content in the tail gas of the reaction vessel is less than 100 mg / m³. 3 .

[0009] A system for recovering aluminum hydroxide roasting dust, which can be used to implement the above-mentioned method for recovering aluminum hydroxide roasting dust, includes a Venturi dryer, a roasting furnace, a fluidized bed cooler, a primary cyclone separator, a secondary cyclone separator, a tertiary cyclone separator, a slurry mixing tank, a settling tank, a bag filter, a heat exchanger, a tail gas preheater, a urea solution storage tank, a urea mixer, a reaction gas mixer, a reactor, a waste heat boiler, an alkaline scrubbing tower, a fan, and a central control system. The aluminum hydroxide filter cake dried in the Venturi dryer is sent to the roasting furnace for roasting, and the alumina obtained from roasting in the furnace is sent to the fluidized bed cooler. After being cooled by the bed cooler, the material is sent to the storage silo. The flue gas generated in the roasting furnace is processed by a primary cyclone separator and a secondary cyclone separator before being sent to a Venturi dryer for heating the material. The primary dust collected by the primary cyclone separator is sent to the roasting furnace inlet and mixed with the material from the Venturi dryer before being fed into the roasting furnace. The secondary dust collected by the secondary cyclone separator is sent to the slurry mixing tank for processing. The slurry in the slurry mixing tank is sent to the settling tank for processing. The settling tank is equipped with a reagent inlet for adding chemicals. The slurry in the settling tank is filtered and then sent to the bauxite wet grinding system for recovery. The exhaust gas from the Venturi dryer is sequentially treated by a three-stage cyclone separator and a bag filter before being sent to the exhaust gas emission system. The three-stage recovered dust collected by the cyclone separator is sent to the inlet of the calcining furnace and mixed with the material from the Venturi dryer before being fed into the calcining furnace. The dust collected by the bag filter is sent to a slurry mixing tank for treatment, and the exhaust gas from the bag filter is sent to an exhaust gas preheater. Part of the hot air generated by the fluidized bed cooler is sent to a heat exchanger for heat exchange and then returned to the hot air duct. A fan sends cold air to a heat exchanger for heat exchange, and part of the cold air is sent to the exhaust gas preheater, while the other part is sent to the urea mixer. The urea solution in the urea solution storage tank is pumped to the urea mixer and sprayed in through atomizing nozzles; the mixed gas in the urea mixer and the tail gas preheater is sent to the reaction gas mixer; the gas in the reaction gas mixer is sent to the reactor for reaction, and the tail gas in the reactor is sent to the alkaline scrubbing tower for treatment after the waste heat is recovered by the preheating boiler and then discharged through the chimney; the dust outlet of the first-stage cyclone separator is equipped with an online particle size monitor A, and the dust outlet of the third-stage cyclone separator is equipped with an online particle size monitor B; the central controller is connected to the online particle size monitor A and the online particle size monitor B respectively.

[0010] Furthermore, the dust outlet of the primary cyclone separator is connected to the inlet of the calcining furnace and the inlet of the slurry mixing tank via a three-way feeding valve; the dust outlet of the tertiary cyclone separator is connected to the inlet of the calcining furnace and the inlet of the slurry mixing tank via a three-way feeding valve; the central control system controls the switching of the three-way feeding valves at the primary and tertiary cyclone separators based on the detection results of the online particle size monitor A and the online particle size monitor B.

[0011] Furthermore, the exhaust gas outlet of the bag filter is equipped with a first gas flow meter and an online nitrogen oxide concentration detector, the outlet of the urea solution storage tank is equipped with a metering pump, the inlet of the reactor is equipped with a first thermometer, and the pipes connecting the heat exchanger to the exhaust gas preheater and the urea mixer are respectively equipped with a second flow meter and a second thermometer.

[0012] Compared with existing technologies, this technical solution has the following advantages: 1. This invention utilizes a primary cyclone separator to recover large particulate dust from the roasting flue gas and return it to the roasting furnace for processing. A secondary cyclone separator recovers fine dust, which is then slurryed, purified, and returned to the bauxite wet grinding system for reuse. Simultaneously, a tertiary cyclone separator recovers waste gas dust from the Venturi dryer and returns it to the drying system. A bag filter recovers ultrafine dust, processes it, and returns it to the bauxite wet grinding system, thereby minimizing resource loss and increasing the yield of alumina products.

[0013] 2. This invention utilizes the waste heat in the fluidized bed cooler to preheat the tail gas containing nitrogen oxides. It is also used to decompose urea to produce ammonia and obtain the reaction raw material gas. Then, the preheated tail gas containing nitrogen oxides is mixed with the reaction raw material gas to carry out a denitrification reaction to remove nitrogen oxides from the tail gas. Finally, after being washed with alkaline solution, the tail gas is ensured to meet the safe emission standards, effectively preventing smoke and dust pollution of the environment.

[0014] 3. This invention uses online monitoring instruments such as flow rate, temperature, and particle size to monitor the operation of key process nodes that affect product quality in real time. The central controller makes intelligent decisions and adjustments to the operating conditions based on real-time data. According to the dust particle size collected by different cyclone separators, the three-way valve is controlled to realize intelligent dust diversion, avoiding the direct return of excessively fine dust to the process system, which would affect the calcination and drying effect and lead to a downgrade in the quality of the obtained alumina product. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the connection of the aluminum hydroxide roasting flue dust recovery system described in Example 4.

[0016] Figure labels: 1-Roasting furnace, 2-Venturi dryer, 3-Fluidized bed cooler, 4-Storage silo, 5-Heat exchanger, 6-Tail gas preheater, 7-Urea solution storage tank, 8-First-stage cyclone separator, 9-Second-stage cyclone separator, 10-Third-stage cyclone separator, 11-Slurry mixing tank, 12-Settling tank, 13-Bag filter, 14-Urea mixer, 15-Reaction gas mixer, 16-Reactor, 17-Waste heat boiler, 18-Alkali scrubbing tower, 19-Fan, 20-First gas flow meter, 21-Nitrogen oxide concentration online detector, 22-Metering pump, 23-Particle size online monitor A, 24-Particle size online monitor B, 25-First thermometer, 26-Second flow meter, 27-Second thermometer, 28-Chimney. Detailed Implementation

[0017] The present invention is further illustrated by the following examples, but these are not intended to limit the invention. Specific experimental conditions and methods not specified in the following examples are generally conventional methods well known to those skilled in the art.

[0018] Example 1: A method for recovering aluminum hydroxide roasting flue dust, comprising the following steps: S1. The aluminum hydroxide filter cake obtained by drying in Venturi dryer 2 is sent to calcination furnace 1 for calcination at 1000℃. The aluminum oxide obtained by calcination is processed by fluidized bed cooler 3 to obtain aluminum oxide product. The flue gas generated by calcination is sent to primary cyclone separator 8 for treatment. S2. The primary recovered dust collected in the primary cyclone separator 8 is mixed with the aluminum hydroxide filter cake obtained by drying in the Venturi dryer 2 and then sent to the calcining furnace 1 for calcination; the tail gas from the outlet of the primary cyclone separator 8 is sent to the secondary cyclone separator 9 for treatment; if the particle size of the primary recovered dust collected after treatment in the primary cyclone separator 8 is less than 30μm, the primary recovered dust is mixed with the secondary recovered dust for further treatment. S3. The tail gas from the secondary cyclone separator 9 is sent to the Venturi dryer 2 for heating and drying the aluminum hydroxide filter cake. The temperature of the tail gas at the outlet of the secondary cyclone separator 9 is controlled at 350℃. The secondary recovered dust collected in the secondary cyclone separator 9 is mixed with water to make a dust slurry with a solid content of 22%. Then, calcium hydroxide solution is added to the dust slurry, stirred and mixed, and filtered to obtain a purified slurry. The purified slurry is then sent to the bauxite wet grinding system for recovery. The calcium hydroxide solution is prepared according to a mass ratio of calcium oxide to secondary recovered dust of 3:100. If the particle size of the tertiary recovered dust collected after treatment by the tertiary cyclone separator 10 is less than 10μm, the tertiary recovered dust is mixed with the secondary recovered dust for further treatment. S4. The exhaust gas in the Venturi dryer 2 is sent to the three-stage cyclone separator 10 for treatment. The three-stage recovered dust collected in the three-stage cyclone separator 10 is mixed with the aluminum hydroxide filter cake at the inlet of the Venturi dryer 2, and the tail gas at the outlet of the three-stage cyclone separator 10 is sent to the bag filter 13 for treatment. S5. The dust collected in the bag filter 13 is mixed with the secondary recovered dust for treatment; the exhaust gas from the bag filter 13 is sent to the exhaust gas preheater 6 for treatment. S6. The fluidized bed cooler 3 is air-cooled, and the generated hot air is sent to the calcining furnace 1 for combustion support. Part of the hot air is sent to the heat exchanger 5 to exchange heat with cold air before returning to the hot air duct and mixing with it before being sent back to the calcining furnace 1. The cold air is heated to 480°C after heat exchange and then split into two streams. One stream is sent to the exhaust gas preheater 6 to mix with the exhaust gas from the bag filter 13, and the other stream is injected with urea solution for mixing. The two streams are then mixed to obtain a reaction gas at 380°C, which is then sent to a reaction vessel filled with a vanadium-based catalyst. The exhaust gas from the reaction vessel is sent to the waste heat boiler 17 to recover waste heat and then to the alkaline scrubbing tower 18 for treatment. The exhaust gas from the alkaline scrubbing tower 18 is then discharged through the chimney 28. The concentration of the urea solution is 45%. The NOx content in the exhaust gas from the reaction vessel is 55 mg / m³. 3 .

[0019] Example 2: A method for recovering aluminum hydroxide roasting flue dust, comprising the following steps: S1. The aluminum hydroxide filter cake obtained by drying in Venturi dryer 2 is sent to calcination furnace 1 for calcination at 900°C. The aluminum oxide obtained by calcination is processed by fluidized bed cooler 3 to obtain aluminum oxide product. The flue gas generated by calcination is sent to primary cyclone separator 8 for treatment. S2. The primary recovered dust collected in the primary cyclone separator 8 is mixed with the aluminum hydroxide filter cake obtained by drying in the Venturi dryer 2 and then sent to the calcining furnace 1 for calcination; the tail gas from the outlet of the primary cyclone separator 8 is sent to the secondary cyclone separator 9 for treatment; if the particle size of the primary recovered dust collected after treatment in the primary cyclone separator 8 is less than 30μm, the primary recovered dust is mixed with the secondary recovered dust for further treatment. S3. The tail gas from the secondary cyclone separator 9 is sent to the Venturi dryer 2 for heating and drying the aluminum hydroxide filter cake. The temperature of the tail gas at the outlet of the secondary cyclone separator 9 is controlled at 300℃. The secondary recovered dust collected in the secondary cyclone separator 9 is mixed with water to make a dust slurry with a solid content of 20%. Then, calcium hydroxide solution is added to the dust slurry, stirred and mixed, and filtered to obtain a purified slurry. The purified slurry is then sent to the bauxite wet grinding system for recovery. The calcium hydroxide solution is prepared according to a mass ratio of calcium oxide to secondary recovered dust of 1:100. If the particle size of the tertiary recovered dust collected after treatment by the tertiary cyclone separator 10 is less than 10μm, the tertiary recovered dust is mixed with the secondary recovered dust for further treatment. S4. The exhaust gas in the Venturi dryer 2 is sent to the three-stage cyclone separator 10 for treatment. The three-stage recovered dust collected in the three-stage cyclone separator 10 is mixed with the aluminum hydroxide filter cake at the inlet of the Venturi dryer 2, and the tail gas at the outlet of the three-stage cyclone separator 10 is sent to the bag filter 13 for treatment. S5. The dust collected in the bag filter 13 is mixed with the secondary recovered dust for treatment; the exhaust gas from the bag filter 13 is sent to the exhaust gas preheater 6 for treatment. S6. The fluidized bed cooler 3 is air-cooled, and the generated hot air is sent to the calcining furnace 1 for combustion support. Part of the hot air is sent to the heat exchanger 5 to exchange heat with cold air before returning to the hot air duct to mix and be sent to the calcining furnace 1. The cold air is heated to 450°C after heat exchange and then split into two streams. One stream is sent to the exhaust gas preheater 6 to mix with the exhaust gas from the bag filter 13, and the other stream is injected with urea solution for mixing. The two streams are then mixed to obtain a reaction gas at a temperature of 250°C. The reaction gas is then sent to a reaction vessel filled with a vanadium-based catalyst for reaction. The exhaust gas from the reaction vessel is sent to the waste heat boiler 17 to recover waste heat and then sent to the alkaline scrubbing tower 18 for treatment. The exhaust gas from the alkaline scrubbing tower 18 is sent to the chimney 28 for discharge. The concentration of the urea solution is 40%. The content of nitrogen oxides (NOx) in the exhaust gas from the reaction vessel is 93 mg / m³. 3 .

[0020] Example 3: A method for recovering aluminum hydroxide roasting flue dust, comprising the following steps: S1. The aluminum hydroxide filter cake obtained by drying in Venturi dryer 2 is sent to calcination furnace 1 for calcination at 1050℃. The aluminum oxide obtained by calcination is processed by fluidized bed cooler 3 to obtain aluminum oxide product. The flue gas generated by calcination is sent to primary cyclone separator 8 for treatment. S2. The primary recovered dust collected in the primary cyclone separator 8 is mixed with the aluminum hydroxide filter cake obtained by drying in the Venturi dryer 2 and then sent to the calcining furnace 1 for calcination; the tail gas from the outlet of the primary cyclone separator 8 is sent to the secondary cyclone separator 9 for treatment; if the particle size of the primary recovered dust collected after treatment in the primary cyclone separator 8 is less than 30μm, the primary recovered dust is mixed with the secondary recovered dust for further treatment. S3. The tail gas from the secondary cyclone separator 9 is sent to the Venturi dryer 2 for heating and drying the aluminum hydroxide filter cake. The temperature of the tail gas at the outlet of the secondary cyclone separator 9 is controlled at 360℃. The secondary recovered dust collected in the secondary cyclone separator 9 is mixed with water to make a dust slurry with a solid content of 22%. Then, calcium hydroxide solution is added to the dust slurry, stirred and mixed, and filtered to obtain a purified slurry. The purified slurry is then sent to the bauxite wet grinding system for recovery. The calcium hydroxide solution is prepared according to a mass ratio of calcium oxide to secondary recovered dust of 4:100. If the particle size of the tertiary recovered dust collected after treatment by the tertiary cyclone separator 10 is less than 10μm, the tertiary recovered dust is mixed with the secondary recovered dust for further treatment. S4. The exhaust gas in the Venturi dryer 2 is sent to the three-stage cyclone separator 10 for treatment. The three-stage recovered dust collected in the three-stage cyclone separator 10 is mixed with the aluminum hydroxide filter cake at the inlet of the Venturi dryer 2, and the tail gas at the outlet of the three-stage cyclone separator 10 is sent to the bag filter 13 for treatment. S5. The dust collected in the bag filter 13 is mixed with the secondary recovered dust for treatment; the exhaust gas from the bag filter 13 is sent to the exhaust gas preheater 6 for treatment. S6. The fluidized bed cooler 3 is air-cooled, and the generated hot air is sent to the calcining furnace 1 for combustion support. Part of the hot air is sent to the heat exchanger 5 to exchange heat with cold air before returning to the hot air duct and mixing with it before being sent back to the calcining furnace 1. The cold air is heated to 470°C after heat exchange and then split into two streams. One stream is sent to the exhaust gas preheater 6 to mix with the exhaust gas from the bag filter 13, and the other stream is injected with urea solution for mixing. The two streams are then mixed to obtain a reaction gas at 260°C, which is then sent to a reaction vessel filled with a vanadium-based catalyst. The exhaust gas from the reaction vessel is sent to the waste heat boiler 17 to recover waste heat and then to the alkaline scrubbing tower 18 for treatment. The exhaust gas from the alkaline scrubbing tower 18 is then discharged through the chimney 28. The concentration of the urea solution is 48%. The NOx content in the exhaust gas from the reaction vessel is 65 mg / m³. 3 .

[0021] Example 4: A method for recovering aluminum hydroxide roasting flue dust, comprising the following steps: S1. The aluminum hydroxide filter cake obtained by drying in Venturi dryer 2 is sent to calcination furnace 1 for calcination at 1100℃. The aluminum oxide obtained by calcination is processed by fluidized bed cooler 3 to obtain aluminum oxide product. The flue gas generated by calcination is sent to primary cyclone separator 8 for treatment. S2. The primary recovered dust collected in the primary cyclone separator 8 is mixed with the aluminum hydroxide filter cake obtained by drying in the Venturi dryer 2 and then sent to the calcining furnace 1 for calcination; the tail gas from the outlet of the primary cyclone separator 8 is sent to the secondary cyclone separator 9 for treatment; if the particle size of the primary recovered dust collected after treatment in the primary cyclone separator 8 is less than 30μm, the primary recovered dust is mixed with the secondary recovered dust for further treatment. S3. The tail gas from the secondary cyclone separator 9 is sent to the Venturi dryer 2 for heating and drying the aluminum hydroxide filter cake. The temperature of the tail gas at the outlet of the secondary cyclone separator 9 is controlled at 400℃. The secondary recovered dust collected in the secondary cyclone separator 9 is mixed with water to make a dust slurry with a solid content of 25%. Then, calcium hydroxide solution is added to the dust slurry, stirred and mixed, and filtered to obtain a purified slurry. The purified slurry is then sent to the bauxite wet grinding system for recovery. The calcium hydroxide solution is prepared according to a mass ratio of calcium oxide to secondary recovered dust of 5:100. If the particle size of the tertiary recovered dust collected after treatment by the tertiary cyclone separator 10 is less than 10μm, the tertiary recovered dust is mixed with the secondary recovered dust for further treatment. S4. The exhaust gas in the Venturi dryer 2 is sent to the three-stage cyclone separator 10 for treatment. The three-stage recovered dust collected in the three-stage cyclone separator 10 is mixed with the aluminum hydroxide filter cake at the inlet of the Venturi dryer 2, and the tail gas at the outlet of the three-stage cyclone separator 10 is sent to the bag filter 13 for treatment. S5. The dust collected in the bag filter 13 is mixed with the secondary recovered dust for treatment; the exhaust gas from the bag filter 13 is sent to the exhaust gas preheater 6 for treatment. S6. The fluidized bed cooler 3 is air-cooled, and the generated hot air is sent to the calcining furnace 1 for combustion support. Part of the hot air is sent to the heat exchanger 5 to exchange heat with cold air before returning to the hot air duct to mix and be sent to the calcining furnace 1. The cold air is heated to 500°C after heat exchange and then split into two streams. One stream is sent to the exhaust gas preheater 6 to mix with the exhaust gas from the bag filter 13, and the other stream is injected with urea solution for mixing. The two streams are then mixed to obtain a reaction gas at a temperature of 300°C. The reaction gas is then sent to a reaction vessel filled with a vanadium-based catalyst for reaction. The exhaust gas from the reaction vessel is sent to the waste heat boiler 17 to recover waste heat and then sent to the alkaline scrubbing tower 18 for treatment. The exhaust gas from the alkaline scrubbing tower 18 is sent to the chimney 28 for discharge. The concentration of the urea solution is 50%. The content of nitrogen oxides (NOx) in the exhaust gas from the reaction vessel is 72 mg / m³. 3 .

[0022] Example 5: A system for recovering aluminum hydroxide roasting dust, applicable to the aluminum hydroxide roasting dust recovery method described in any one of Examples 1-4, comprising a Venturi dryer 2, a roasting furnace 1, a fluidized bed cooler 3, a primary cyclone separator 8, a secondary cyclone separator 9, a tertiary cyclone separator 10, a slurry mixing tank 11, a settling tank 12, a bag filter 13, a heat exchanger 5, a tail gas preheater 6, a urea solution storage tank 7, a urea mixer 14, a reaction gas mixer 15, a reactor 16, a waste heat boiler 17, an alkaline scrubbing tower 18, a fan 19, and a central control system; the aluminum hydroxide filter cake dried by the Venturi dryer 2 is sent to the roasting furnace 1 for roasting, and the alumina obtained from roasting in the roasting furnace 1 is sent to the fluidized bed cooler 3. After cooling, the material is sent to storage silo 4 for storage. The flue gas generated in the roasting furnace 1 is processed sequentially by a primary cyclone separator 8 and a secondary cyclone separator 9 before being sent to the Venturi dryer 2 for heating the material. The primary recovered dust collected by the primary cyclone separator 8 is sent to the inlet of the roasting furnace 1 and mixed with the material from the Venturi dryer 2 before being fed into the roasting furnace 1. The secondary recovered dust collected by the secondary cyclone separator 9 is sent to the slurry mixing tank 11 for processing. The slurry in the slurry mixing tank 11 is sent to the settling tank 12 for processing. The settling tank 12 is equipped with a reagent inlet for adding chemicals. The slurry in the settling tank 12 is filtered and then sent to the bauxite wet grinding system for recovery. The tail gas in the Venturi dryer 2 is processed sequentially by a tertiary cyclone separator 10 and a bag filter dust collector 13 before being sent to the Venturi dryer 2 for further processing. The exhaust gas is fed into the exhaust gas system; the three-stage cyclone separator 10 collects the three-stage recovered dust, which is then mixed with the material from the Venturi dryer 2 at the inlet of the calcining furnace 1 and fed into the calcining furnace 1; the bag filter dust collected by the bag filter 13 is sent to the slurry mixing tank 11 for treatment, and the exhaust gas in the bag filter 13 is sent to the exhaust gas preheater 6; part of the hot air generated by the fluidized bed cooler 3 is sent to the heat exchanger 5 for heat exchange and then returned to the hot air duct; the fan 19 sends cold air to the heat exchanger 5 for heat exchange, and part of it is sent to the exhaust gas preheater 6, and the other part is sent to the urea mixer 14; the urea solution in the urea solution storage tank 7 is sent to the urea mixer 14 by a pump and sprayed in through an atomizing nozzle; the mixed gas in the urea mixer 14 and the exhaust gas preheater 6 are both sent to the reaction Gas mixer 15; the gas in the gas mixer 15 is sent to reactor 16 for reaction, and the tail gas in reactor 16 is sent to alkaline scrubbing tower 18 for treatment after waste heat recovery in preheating boiler and then discharged through chimney 28; the dust outlet of the first-stage cyclone separator 8 is equipped with an online particle size monitor A23, and the dust outlet of the third-stage cyclone separator 10 is equipped with an online particle size monitor B24; the central controller is connected to the online particle size monitor A23 and the online particle size monitor B24 respectively; the dust outlet of the first-stage cyclone separator 8 is connected to the inlet of the roasting furnace 1 and the inlet of the slurry tank 11 respectively through a three-way feeding valve; the dust outlet of the third-stage cyclone separator 10 is connected to the inlet of the roasting furnace 1 and the inlet of the slurry tank 11 respectively through a three-way feeding valve.The central control system switches the three-way feed valves at the primary cyclone separator 8 and the tertiary cyclone separator 10 based on the detection results of the online particle size monitors A23 and B24. The exhaust outlet of the bag filter 13 is equipped with a first gas flow meter 20 and an online nitrogen oxide concentration detector 21. The outlet of the urea solution storage tank 7 is equipped with a metering pump 22. The inlet of the reactor 16 is equipped with a first thermometer 25. The pipes connecting the heat exchanger 5 to the exhaust gas preheater 6 and the urea mixer 14 are respectively equipped with a second flow meter 26 and a second thermometer 27.

[0023] In this embodiment, the particle size online monitoring instruments A23 and B24 in the aluminum hydroxide roasting dust recovery system both employ laser diffraction online particle size analyzers. These instruments can monitor the particle size of the dust collected by the primary cyclone separator 8 and the tertiary cyclone separator 10. During system operation, the central controller collects data from the particle size online monitoring instruments A23 and B24 in real time, monitoring the dust particle size of the primary cyclone separator 8 and the tertiary cyclone separator 10. Monitoring the particle size of the primary cyclone separator 8 ensures that the material returned to the roasting furnace 1 system has uniform fineness and does not introduce excessive ultrafine dust, thus providing the roasting furnace 1 with uniform particle size and fine powder. To ensure the quality of the final alumina product and reduce the occurrence of product pulverization, the central controller should control the three-way feed valve at the primary cyclone separator 8 to switch when the particle size of the primary recovered dust is less than 30μm, sending the primary recovered dust to the slurry mixing tank 11 to mix with the secondary recovered dust for processing. Similarly, monitoring the particle size of the tertiary cyclone separator 10 is also to avoid the dust in the alumina filter cake being too fine and affecting the drying effect. When the particle size of the tertiary recovered dust is less than 10μm, the central controller should control the three-way feed valve at the tertiary cyclone separator 10 to switch when the tertiary recovered dust is sent to the slurry mixing tank 11 to mix with the secondary recovered dust for processing. Meanwhile, during the drying process in the Venturi dryer 2, the wet fine particles collide in the high-speed airflow and, with the help of residual moisture, generate force, which can promote the particles to agglomerate into larger, loose aggregates, thus having a self-de-fineness effect. Therefore, the particle size requirements for the dust in the three-stage cyclone separator 10 are more lenient, while the particle size of the dust in the one-stage cyclone separator 8 must be strictly controlled to be above 30μm.

[0024] In addition, the central control system collects data in real time from the first gas flow meter 20 and the nitrogen oxide concentration online detector 21 as a basis. The amount of ammonia used in the denitrification reaction is calculated according to the preset rules. For example, the amount of nitrogen oxides is calculated based on the nitrogen oxide concentration and the gas flow rate of the tail gas. Then, the theoretical amount of urea to be used and the amount to be added are calculated. Then, based on the data collected in real time from the first thermometer 25, the second flow meter 26 and the second thermometer 27, the gas flow rate entering the tail gas preheater 6 and the urea mixer 14 and the amount of urea solution injected are adjusted. This controls the gas flow rate, gas temperature and the ratio of nitrogen oxides and ammonia in the gas entering the reactor 16 to achieve the optimal level, thereby promoting the denitrification reaction and effectively removing nitrogen oxides. Meanwhile, the central control system adjusts the ratio of the hot air from the heat exchanger entering the exhaust gas preheater branch (Q1) and the branch air entering the urea mixer branch (Q2) based on the data from the second flow meter 26 and the second thermometer 27. For example, the initial setting of Q1:Q2 is 7:3 or 6:4. Then, the optimal air volume ratio is obtained by controlling the opening and closing of the valves on the regulating pipeline according to the actual air volume.

[0025] Experimental Example 1: Aluminum hydroxide slurry was obtained by processing the same batch of bauxite raw materials using the Bayer process. Then, the slurry was roasted using the system described in Example 4 according to the methods in Examples 1 to 3. The product yield of alumina, the proportion of -45μm particles in the product, and the sodium impurity content were statistically analyzed. The specific results are shown in Table 1.

[0026]

[0027] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and 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.

Claims

1. A method for recovering flue gas from aluminum hydroxide roasting, characterized in that: Includes the following steps: S1. The aluminum hydroxide filter cake obtained by drying in a Venturi dryer is sent to a calcining furnace for calcination at 900-1100℃. The calcined alumina is processed by a fluidized bed cooler to obtain alumina product. The flue gas generated during calcination is sent to a primary cyclone separator for treatment. S2. The primary recovered dust collected in the primary cyclone separator is mixed with the aluminum hydroxide filter cake obtained by drying in the Venturi dryer and then sent to the calcination furnace for calcination; the tail gas from the outlet of the primary cyclone separator is sent to the secondary cyclone separator for treatment. S3. The tail gas from the secondary cyclone separator is sent to a Venturi dryer for heating and drying the aluminum hydroxide filter cake. The temperature of the tail gas at the outlet of the secondary cyclone separator is controlled at 300-400℃. The secondary recovered dust collected in the secondary cyclone separator is mixed with water to make a dust slurry with a solid content of 20-25%. Then, calcium hydroxide solution is added to the dust slurry, stirred and mixed, and filtered to obtain a purified slurry. The purified slurry is then sent to a bauxite wet grinding system for recovery. The calcium hydroxide solution is prepared according to a mass ratio of calcium oxide to secondary recovered dust of (1-5):

100. S4. The exhaust gas in the Venturi dryer is sent to the three-stage cyclone separator for treatment. The three-stage recovered dust collected in the three-stage cyclone separator (10) is mixed with the aluminum hydroxide filter cake at the inlet of the Venturi dryer, and the exhaust gas at the outlet of the three-stage cyclone separator is sent to the bag filter for treatment. S5. Mix the dust collected in the bag filter with the secondary recovered dust for treatment; send the exhaust gas from the bag filter to the exhaust gas preheater for treatment. S6. The fluidized bed cooler is air-cooled, and the generated hot air is sent to the calcining furnace for combustion. Part of the hot air is sent to a heat exchanger to exchange heat with cold air and then returned to the hot air duct to mix and be sent to the calcining furnace. The cold air is heated to 450-500°C after heat exchange and then split into two paths. One path is sent to the exhaust gas preheater to mix with the exhaust gas from the bag filter, and the other path is injected with urea solution for mixing. The two paths are then mixed to obtain a reaction gas at a temperature of 250-300°C. The reaction gas is then sent to a reaction vessel filled with a vanadium-based catalyst for reaction. The exhaust gas in the reaction vessel is sent to a waste heat boiler to recover waste heat and then sent to an alkaline scrubbing tower for treatment. The exhaust gas from the alkaline scrubbing tower is sent to the chimney for discharge.

2. The method for recovering aluminum hydroxide roasting flue dust according to claim 1, characterized in that: In step S2, if the particle size of the primary recovered dust collected after treatment by the primary cyclone separator is less than 30 μm, the primary recovered dust is mixed with the secondary recovered dust for further treatment.

3. The method for recovering aluminum hydroxide roasting flue dust according to claim 1, characterized in that: In step S3, if the particle size of the tertiary recovered dust collected after treatment by the tertiary cyclone separator is less than 10 μm, the tertiary recovered dust is mixed with the secondary recovered dust for further treatment.

4. The method for recovering aluminum hydroxide roasting flue dust according to claim 1, characterized in that: In step S6, the concentration of the urea solution is 40-50%.

5. The method for recovering aluminum hydroxide roasting flue dust according to claim 1, characterized in that: In step S6, the nitrogen oxide content in the tail gas of the reaction vessel is less than 100 mg / m3.

6. A system for recovering aluminum hydroxide roasting flue dust, used to implement the method for recovering aluminum hydroxide roasting flue dust as described in any one of claims 1 to 5, characterized in that: The system includes a Venturi dryer (2), a calcining furnace (1), a fluidized bed cooler (3), a primary cyclone separator (8), a secondary cyclone separator (9), a tertiary cyclone separator (10), a slurry mixing tank (11), a settling tank (12), a bag filter (13), a heat exchanger (5), a tail gas preheater (6), a urea solution storage tank (7), a urea mixer (14), a reaction gas mixer (15), a reactor (16), a waste heat boiler (17), an alkaline scrubbing tower (18), a fan (19), and a central control system. The aluminum hydroxide filter cake dried by the Venturi dryer (2) is sent to the calcining furnace (1) for calcination. The alumina obtained from calcination in the calcining furnace (1) is sent to the fluidized bed cooler (3) for cooling and then sent to the storage silo. 4) Storage; The flue gas generated in the roasting furnace (1) is processed by the first-stage cyclone separator (8) and the second-stage cyclone separator (9) and then sent to the Venturi dryer (2) for heating materials; The first-stage recovered dust collected by the first-stage cyclone separator (8) is sent to the inlet of the roasting furnace (1) and mixed with the material from the Venturi dryer (2) before being fed into the roasting furnace (1); The second-stage recovered dust collected by the second-stage cyclone separator (9) is sent to the slurry mixing tank (11) for processing, and the slurry in the slurry mixing tank (11) is sent to the settling tank (12) for processing. The settling tank (12) is equipped with a reagent inlet for adding chemicals. The slurry in the settling tank (12) is filtered and then sent to the bauxite wet grinding system for recovery; The tail gas in the Venturi dryer (2) is processed sequentially After being processed by a three-stage cyclone separator (10) and a bag filter (13), the gas is sent to the exhaust gas system. The three-stage recovered dust collected by the three-stage cyclone separator (10) is sent to the inlet of the roasting furnace (1) and mixed with the material from the Venturi dryer (2) before being fed into the roasting furnace (1). The bag filter dust collected by the bag filter (13) is sent to the slurry tank (11) for processing. The exhaust gas in the bag filter (13) is sent to the exhaust gas preheater (6). The hot air generated by the fluidized bed cooler (3) is sent to the heat exchanger (5) and returned to the hot air duct after heat exchange. The fan (19) sends cold air to the heat exchanger (5). After heat exchange, part of the air is sent to the exhaust gas preheater (6), and the other part is sent to the urea mixer (14). The urea solution is pumped to the urea mixer (14). The urea solution in the storage tank (7) is sent to the urea mixer (14) and sprayed in through the atomizing nozzle; the mixed gas in the urea mixer (14) and the tail gas preheater (6) is sent to the reaction gas mixer (15); the gas in the reaction gas mixer (15) is sent to the reactor (16) for reaction, and the tail gas in the reactor (16) is sent to the alkaline scrubbing tower (18) for treatment after the waste heat is recovered by the preheating boiler and then discharged through the chimney (28); the dust outlet of the first-stage cyclone separator (8) is equipped with an online particle size monitor A (23), and the dust outlet of the third-stage cyclone separator (10) is equipped with an online particle size monitor B (24); the central controller is connected to the online particle size monitor A (23) and the online particle size monitor B (24) respectively.

7. The aluminum hydroxide roasting flue dust recovery system according to claim 6, characterized in that: The dust outlet of the first-stage cyclone separator (8) is connected to the inlet of the calcining furnace (1) and the inlet of the slurry mixing tank (11) respectively through a three-way feeding valve; the dust outlet of the third-stage cyclone separator (10) is connected to the inlet of the calcining furnace (1) and the inlet of the slurry mixing tank (11) respectively through a three-way feeding valve; the central control system controls the three-way feeding valves at the first-stage cyclone separator (8) and the third-stage cyclone separator (10) to switch according to the detection results of the online particle size monitor A (23) and the online particle size monitor B (24).

8. The aluminum hydroxide roasting flue dust recovery system according to claim 6, characterized in that: The exhaust outlet of the bag filter (13) is equipped with a first gas flow meter (20) and an online nitrogen oxide concentration detector (21). The outlet of the urea solution storage tank (7) is equipped with a metering pump (22). The inlet of the reactor (16) is equipped with a first thermometer (25). The pipes connecting the heat exchanger (5) to the exhaust gas preheater (6) and the urea mixer (14) are respectively equipped with a second flow meter (26) and a second thermometer (27).

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