Method for preparing aluminum oxide by roasting aluminum hydroxide through electric heating and used device

Abstract

The invention discloses a method for preparing aluminum oxide by roasting aluminum hydroxide through electric heating and a used device, and belongs to the technical field of preparation of aluminum oxide through roasting. According to the method, an electric heating furnace is additionally arranged in an aluminum oxide plant or a roasting system of the aluminum oxide plant, an aluminum hydroxide raw material enters a heated fluidized bed staying tank after being preheated and dried, aluminum hydroxide is roasted into an alpha-Al2O3 crystal form in the staying tank, then the aluminum hydroxide is gradually cooled through a four-stage cyclone cooler, meanwhile, the aluminum hydroxide exchanges heat with cold air which is sent reversely, and the aluminum hydroxide is obtained. Further cooling through a fluidization cooler to obtain an aluminum oxide product; and the fluidized bed staying tank is heated by hot air generated by the electric heating furnace. According to the invention, a huge desulfurization and denitrification treatment system is not needed, and a complex combustion furnace system and a fuel gas security system are omitted; aluminum hydroxide is in contact with air without any additional medium, tail gas can be recycled after deep dust removal and water vapor removal, energy consumption is reduced, and green and clean production is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of preparing aluminum oxide by roasting, and specifically relates to a method for preparing aluminum oxide by roasting aluminum hydroxide by electric heating and a device used therein. Background Art

[0002] The roasting process of aluminum hydroxide is the last fuel-consuming step in the alumina production process. Its energy consumption accounts for about 10% of the total energy consumption of the alumina production process. It is also the main heat source loss process and an important link in determining the output, quality, energy consumption and pollutant emissions of alumina.

[0003] At present, my country's aluminum hydroxide roasting technology widely adopts the gas suspension roasting technology (GSC) of Denmark's Smith Company, and some adopts the circulating fluidized bed roasting furnace technology process (CFB) technology. However, the heat source of both roasting uses natural gas or coal gas to burn with air preheated to 700-1000℃ to produce high-temperature flue gas. The high-temperature flue gas heats the aluminum hydroxide material to complete the process of dehydration and partial crystal phase transformation to generate alumina. The alumina is finally cooled in multiple stages to become a qualified alumina product.

[0004] However, the current aluminum hydroxide roasting technology requires a roasting furnace. The gas combustion process produces high-temperature flue gas. The combustion furnace becomes the core equipment of the system and the main source of carbon emissions in the process. The production of one ton of alumina will emit more than 1.5 tons of carbon dioxide. In addition, when high-temperature roasting is achieved, it is inevitable that nitrogen oxide emissions will be high. Therefore, the emission of nitrogen oxides in the process is an important issue that needs to be solved urgently. Tail gas treatment requires a heat exchange system and a selective catalytic reduction (SCR) treatment system, which requires a large amount of additional capital investment.

[0005] In addition, the combustion furnace produces high-decibel noise, which is labor-intensive to maintain and replace, and there are safety hazards in gas combustion; the thermal efficiency of gas is generally only 30% to 40%. In order to improve efficiency, reduce noise, and ensure safety, equipment investment will be increased. SCR and Selective Non-Catalytic Reduction (SNCR) are passive treatment measures that cannot effectively and fundamentally solve the problem of nitrogen oxide generation in the roasting furnace. Therefore, the low-nitrogen, high-efficiency, and green technology of the aluminum hydroxide roasting process still needs further breakthroughs.

[0006] The information disclosed in this background technology section is only intended to enhance the understanding of the overall background of the invention and should not be regarded as an acknowledgment or any form of suggestion that the information constitutes the prior art already known to a person skilled in the art. Summary of the invention

[0007] Aiming at the problems existing in the prior art, one technical problem to be solved by the present invention is to provide a method for preparing alumina by electroheating and roasting aluminum hydroxide. An electroheating system is adopted to replace the original gas furnace calcination system, and the energy heating method is changed to realize a more flexible roasting process of aluminum hydroxide, avoiding the reduction or oxidation of alumina, while eliminating the generation of nitrogen oxides, reducing carbon dioxide emissions, and improving the reliability and safety of operation. If green electricity is used, no harmful emissions and zero carbon emissions can be achieved, and noise pollution can be reduced; there are no pollutants mixed in the roasting process, and the product quality is not polluted. Compared with the thermal efficiency of 30% - 40% of gas roasting, electrothermal can increase the thermal efficiency to more than 95%, or even 100%; in addition, there is no large amount of tail gas emissions, and risks such as leakage and other common safety problems of combustion gas heating systems can be avoided, thus reducing maintenance work.

[0008] To solve the above problems, the technical solutions adopted by the present invention are as follows:

[0009] A method for preparing alumina by electroheating and roasting aluminum hydroxide is realized by adding an electroheating roasting furnace process technology system to the roasting system of an existing alumina plant or a newly built alumina plant; specifically, an electroheating system is adopted to replace the original gas furnace calcination system, and the electroheating furnace contains an electric heating element.

[0010] The aluminum hydroxide raw material enters the heated fluidized bed residence tank after preheating and drying, and the aluminum hydroxide is roasted into α - Al2O3 crystal form in the residence tank, and then gradually cooled by a four - stage cyclone cooler, while exchanging heat with the cold air sent in the reverse direction, and further cooled by the cold air in the fluidized bed cooler to obtain the alumina product; the fluidized bed residence tank is heated by the hot air generated by the electroheating furnace.

[0011] The carbon emissions during the roasting of aluminum hydroxide are mainly determined by parameters such as the composition, calorific value, carbon content, and consumption of the selected gas. The lower the calorific value of the gas, the larger the consumption, and the higher the carbon content, the result is an increase in carbon dioxide emissions during the calcination period. Now most enterprises adopt methods such as oxygen enrichment, changing the burner structure, adding catalysts, etc. to promote the further complete combustion of gas in order to improve the combustion efficiency of gas. As a result, a large amount of nitrogen oxides are generated. If the environmental protection emission standards need to be met, the amount of tail gas treatment increases, resulting in the construction of a huge denitration system and an increase in costs. The structure of the combustion furnace is more complex, and the risks of safe and stable operation increase accordingly, and the maintenance cost also increases accordingly. Compared with the one - time cost of constructing an electroheating furnace system and a gas furnace equipment system with the same power, 30% can be saved, and compared with the later maintenance, denitration, gas supply system, etc., the cost can be saved by more than 50%. If green electricity is used, calculated according to the energy consumption of 2.7 GJ per ton of alumina, 750 kWh is required per ton of alumina, and the cost per kWh is 0.17 yuan, then 127.5 yuan is required per ton of aluminum hydroxide, which is both clean and environmentally friendly and cost - saving compared with 0.5 tons of coal fuel required per ton.

[0012] In the method for preparing alumina by electroheating and roasting aluminum hydroxide, the temperature of the fluidized bed holding tank is 850 - 1050 °C.

[0013] In the method for preparing alumina by electroheating and roasting aluminum hydroxide, during operation, the pressure in the entrained flow bed holding tank is 0.04 - 0.165 MPa, or in a slightly negative pressure state of -20 Pa.

[0014] In the method for preparing alumina by electroheating and roasting aluminum hydroxide, the cold air is outside air.

[0015] For the device used in the above method, the electroheating furnace is composed of a furnace body, an electric heating element group, alloy tubes, a protective sleeve, and a fixed tube sheet;

[0016] The furnace body contains a number of combined alloy tubes, and the alloy tubes are fixed to the furnace body through a sealed fixed cover; each alloy tube contains a number of electric heating element groups; the sealed fixed cover is provided with through holes through which the electric heating element groups can pass; a number of electric heating element groups are fixed by a fixed tube sheet; the outer side of the alloy tube is provided with a protective sleeve; the electric heating element group is composed of electric heating elements.

[0017] The device also includes an entrained flow bed, a fluidized product cooling system, a first-stage cyclone preheater, a second-stage cyclone preheater, primary cooling, secondary cooling, an aluminum hydroxide raw material bin, and a dust removal system;

[0018] The first-stage cyclone preheater is provided with an aluminum hydroxide raw material inlet and an aluminum hydroxide outlet on the side, a first hot air inlet at the bottom, and a first waste gas outlet at the top; the second-stage cyclone preheater is provided with an aluminum hydroxide inlet on the side, a hot air outlet and a second waste gas outlet at the top, and is connected to the entrained flow bed through a flange of the equipment pipe orifice at the bottom; the bottom of the entrained flow bed is connected to the electroheating furnace; the bottom of the entrained flow bed is provided with a discharge port;

[0019] The aluminum hydroxide raw material inlet is connected to the aluminum hydroxide raw material bin; the first waste gas outlet and the second waste gas outlet are connected to the dust removal system through a gas pipeline; the aluminum hydroxide outlet is connected to the aluminum hydroxide inlet; the hot air outlet is connected to the first hot air inlet;

[0020] The primary cooling is provided with a first alumina inlet at the top, a first alumina outlet and a first external cold air inlet at the bottom, and a second external cold air inlet on the side; the secondary cooling is provided with a second alumina inlet on the side, a first external cold air outlet at the top, and a second alumina outlet at the bottom; the fluidized product cooling system is provided with a product alumina inlet, a third external cold air inlet, a third waste gas outlet, and a product alumina outlet;

[0021] The first alumina inlet is connected to the outlet, the first alumina outlet is connected to the second alumina inlet, and the second alumina outlet is connected to the product alumina inlet; the first external cold air outlet is connected to the second external cold air inlet; the third exhaust gas outlet is connected to the dust removal system.

[0022] For the device, a four-stage cyclone cooler is adopted for the primary cooling and the secondary cooling; a fluidized bed cooler is adopted for the fluidized product cooling system.

[0023] For the device, the material of the heating element is ferritic chrome-aluminum alloy.

[0024] For the device, in the ferritic chrome-aluminum alloy, the content of metal Cr is 22 wt.%, the content of metal Al is 5.8 wt.%, and the remaining components are metal Fe.

[0025] Advantages: Compared with the existing technologies, the advantages of the present invention include:

[0026] (1) An electric heating system is adopted to replace the original gas furnace calcination system, changing the energy heating method to achieve a more flexible aluminum hydroxide roasting process, avoiding the reduction or oxidation of alumina, simultaneously eliminating the generation of nitrogen oxides, reducing carbon dioxide emissions, and improving the reliability and safety of operation. If green electricity is used, no harmful emissions and zero carbon emissions can be achieved, and noise pollution can be reduced; no pollutants are mixed into the roasting process, and the product quality is not contaminated. Compared with the thermal efficiency of 30% - 40% of gas roasting, the electrothermal method can increase the thermal efficiency to over 95%, or even 100%.

[0027] (2) The entire process flow of the present invention does not have a huge desulfurization and denitrification treatment system compared with the traditional gas alumina roasting, omitting the complex combustion furnace system and the gas security system. The aluminum hydroxide contacts air without any additional medium, the material is not contaminated, and it is a pure heat exchange. Moreover, the tail gas does not need to undergo deep dust removal. After removing the water vapor therein, it can be recycled back to the original heating system for reuse, thereby reducing energy consumption, reducing carbon emissions, and achieving green and clean production.

[0028] (3) The heating element of the electric heating furnace is made of ferritic chrome-aluminum alloy that can withstand a high temperature of 1250 °C to ensure stable operation at this temperature; compared with ordinary NiCr or FeNiCr tubes, alumina (Al2O3) scale will form on the ferritic chrome-aluminum alloy during heating, which can protect the alloy from further corrosion and extend its service life. When the furnace temperature is 1000 °C, the load capacity can be more than twice that of nickel-chromium (Ni-Cr) and iron-nickel-chromium (Fe-Ni-Cr) tubes. This makes the furnace design more flexible and converts the existing heating system to a higher heating furnace power. The same rated power requires fewer tubes. According to the required power, the surface area of the heating element needed is calculated, and then the corresponding quantity is matched. The size and shape of the furnace body can be freely designed according to the space. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a process flow diagram for preparing alumina by electro - heating roasting of aluminum hydroxide;

[0030] Figure 2 is a relationship diagram between temperature and enthalpy during physicochemical reaction;

[0031] Figure 3 is a schematic structural diagram of an electro - heating furnace;

[0032] Figure 4 is a schematic structural diagram of a device for preparing alumina by electro - heating roasting of aluminum hydroxide. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] In order to make the above - mentioned 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.

[0034] Embodiment 1

[0035] The schematic structural diagram of the device for preparing alumina by electro - heating roasting of aluminum hydroxide is as shown in Figure 4 The device includes a entrained - flow bed 7, an electro - heating furnace 8, a fluidized product cooling system 10, a first - stage cyclone pre - heater 12, a second - stage cyclone pre - heater 13, a first - stage cooling 14, a second - stage cooling 15, an aluminum hydroxide raw material bin 16 and a dust removal system 17;

[0036] The first - stage cyclone pre - heater 12 is provided with an aluminum hydroxide raw material inlet and an aluminum hydroxide outlet on the side, a first hot air inlet at the bottom, and a first waste gas outlet at the top; the second - stage cyclone pre - heater 13 is provided with an aluminum hydroxide inlet on the side, a hot air outlet and a second waste gas outlet at the top, and is connected to the entrained - flow bed 7 through a flange of the equipment pipe orifice at the bottom, and is lined with wear - resistant material; the bottom of the entrained - flow bed 7 is connected to the electro - heating furnace 8; the bottom of the entrained - flow bed 7 is provided with a discharge port;

[0037] The aluminum hydroxide raw material inlet is connected to the aluminum hydroxide raw material bin 16; the first waste gas outlet and the second waste gas outlet are connected to the dust removal system 17 through a gas pipeline; the aluminum hydroxide outlet is connected to the aluminum hydroxide inlet; the hot air outlet is connected to the first hot air inlet;

[0038] The first - stage cooling 14 is provided with a first alumina inlet at the top, a first alumina outlet and a first external cold air inlet at the bottom, and a second external cold air inlet on the side; the second - stage cooling 15 is provided with a second alumina inlet on the side, a first external cold air outlet at the top, and a second alumina outlet at the bottom; the fluidized product cooling system 10 is provided with a product alumina inlet, a third external cold air inlet, a third waste gas outlet and a product alumina outlet;

[0039] The first alumina inlet is connected to the outlet, the first alumina outlet is connected to the second alumina inlet, and the second alumina outlet is connected to the product alumina inlet; the first external cold air outlet is connected to the second external cold air inlet; the third waste gas outlet is connected to the dust removal system 17.

[0040] The electric heating furnace 8 is composed of a furnace body 6, an electric heating element group 2, alloy tubes 3, protective sleeves 4, and fixed tube plates 5. The structural schematic diagram is as Figure 3 shown. As Figure 3 can be seen, the furnace body 6 contains a number of combined alloy tubes 3, and the alloy tubes 3 are fixed to the furnace body through a sealing fixed cover 1; each alloy tube 3 contains a number of electric heating element groups 2, and the sealing fixed cover 1 is provided with through holes through which the electric heating element groups 2 can pass; a number of electric heating element groups 2 are fixed by fixed tube plates 5, and a number of fixed tube plates 5 are evenly provided; a protective sleeve 4 is arranged outside the alloy tube 3; the electric heating element group 2 is composed of electric heating elements; the heating element is made of an iron-chromium-aluminum alloy resistant to a high temperature of 1425 °C (the content of metal Cr in the alloy is 22 wt.%, the content of metal Al is 5.8 wt.%, and the remaining components are metal Fe), ensuring stable operation at this temperature; compared with ordinary NiCr or FeNiCr tubes, the iron-chromium-aluminum alloy will form alumina (Al2O3) scale during heating, which can protect the alloy from further corrosion and extend its service life. At a furnace temperature of 1000 °C, the load capacity can be more than twice that of nickel-chromium (Ni-Cr) and iron-nickel-chromium (Fe-Ni-Cr) tubes. This makes the furnace design more flexible and converts the existing heating system into a higher heating furnace power. The same rated power requires fewer tubes. According to the required power, the surface area of the heating elements needed is calculated, so as to match the corresponding quantity, and the size and shape of the furnace body are freely designed according to the space.

[0041] The process flow diagram and device structural schematic diagram for preparing alumina by electrically heating and roasting aluminum hydroxide are as Figure 1 and Figure 4 shown. As Figure 1 and Figure 4 can be seen, the calcination of aluminum hydroxide has two material flow directions, one is the flow direction of the electrically heated air ( Figure 1 shown by the dotted line in), and the other is the flow direction of the aluminum hydroxide becoming finished alumina after calcination ( Figure 1 shown by the solid line in).

[0042] The aluminum hydroxide after processing and purification of bauxite is placed in the silo. According to the designed production capacity flow, it enters the feeder through the weighing scale and is sent into the preheater for preheating treatment. The preheated material and the material collected by high-temperature dust removal together enter the high-temperature Venturi swirling fluidized bed residence tank heated by electricity, where roasting dehydration and crystal form transformation physical and chemical reactions are carried out, as shown in the following formula:

[0043] 2Al(OH)3 = Al2O3 + 3H2O + ΔH kJ (calcination).

[0044] During the process of calcining aluminum hydroxide into alumina with a high content of α-Al2O3 crystal form, 750 °C is the optimal temperature for this physicochemical reaction to reach the required state ( Figure 2 ). To ensure product quality and the reaction efficiency for complete conversion to α-Al2O3 type alumina, industrial production will control the temperature between 850 - 1050 °C. After high-temperature calcination, the α-Al2O3 metallurgical-grade material transformed through crystal form will be gradually cooled by a four-stage cyclone cooler, and at the same time, heat exchange will occur with the cold air sent in the reverse direction. The α-Al2O3 content after cooling (below 200 °C) will be further cooled by cold air in a fluidized bed cooler and then packaged as a finished product and sent to the finished product warehouse for processing.

[0045] The specific process flow is as follows: The prepared aluminum hydroxide raw material enters the aluminum hydroxide raw material bin 16 through the feeding system. According to the size of the designed production capacity, the corresponding feeding capacity is matched. If the annual production capacity is 300,000 tons, then 30 tons of aluminum hydroxide are fed per hour. (The material level in the feeding box is interlocked with the belt metering feeder under the bin to control the feed rate of the calciner.) The aluminum hydroxide with a water content of 3% - 5% is transported by a belt conveyor and, after passing through a feeding screw conveyor, is carried into the first-stage cyclone preheater 6 by air flow through the aluminum hydroxide raw material inlet. At this time, the water attached to the aluminum hydroxide will be evaporated and dried, and the dried aluminum hydroxide and hot air will be separated. The temperature of the aluminum hydroxide coming out of the first-stage cyclone preheater 6 reaches 320 - 360 °C, and the attached water is basically removed. Along with the hot air, it enters the dust removal system 11 through the first waste gas outlet. The aluminum hydroxide dried by removing the attached water in the first-stage cyclone preheater 6 enters the second-stage cyclone preheater 7 from the aluminum hydroxide outlet and the aluminum hydroxide inlet. Here, the aluminum hydroxide is mixed with hot air at a temperature of 600 - 850 °C coming from the entrained flow bed 1 for heat exchange. At this time, the aluminum hydroxide Al(OH)3 is completely converted into γ-Al2O3 alumina, and the transformation from γ-Al2O3 to α-Al2O3 alumina begins. After passing through the second-stage cyclone preheater 7, at this time, it is already γ and α alumina, and the temperature of the material reaches 400 - 600 °C. According to the production logistics process, the alumina enters the entrained flow bed 7.

[0046] The γ-Al2O3 that is not converted here will all be converted into α-Al2O3 alumina, and at this time, aluminum hydroxide is converted into the required terminal finished product α-Al2O3 alumina. The hot air used by the fluidized bed 1 comes from the electric heating furnace 8, which is heated to 850-1050°C and enters from the bottom of the fluidized bed 7. At this time, the γ-Al2O3, α-Al2O3 alumina and hot air are fully mixed in the fluidized bed 7, and the roasting of alumina is completed instantly in the fluidized bed 7 (within 3 seconds). The roasted alumina and hot air are separated in the separator in the fluidized bed 7: the hot air passes through the above-mentioned second-stage cyclone preheater 13 and the first-stage cyclone preheater 12, and after heat exchange with aluminum hydroxide, the temperature drops to 165°C, and enters the dust removal system 17 through the second exhaust gas outlet and the first exhaust gas outlet. The purified exhaust gas 18 is sent into the chimney by an exhaust fan and discharged into the atmosphere.

[0047] The high-temperature α-Al2O3 alumina needs to be cooled before it can be packaged and stored. The temperature drops to 80°C after the first cooling 14 and the second cooling 15. The first cooling adopts a four-stage cyclone cooler. During the first four-stage cyclone cooling process, the temperature of the alumina drops from 1050°C to 260°C. The air temperature from the second cooling is preheated to above 400°C. The second cooling adopts a fluidized bed cooler. The external cold air 9 introduced through the third external cold air inlet in the fluidized product cooling system 10 performs the first heat exchange with the high-temperature 260°C alumina. The external water in the fluidized product cooling system 10 is indirectly cooled to reduce the temperature of the alumina from 260°C to 80°C. The alumina coming out of the fluidized product cooling system 10 is sent to the alumina warehouse by a pneumatic flow trough and packaged for storage.

[0048] According to the flow direction of the gas, the specific method is: according to the alumina production capacity required by the customer, the power consumption of 750kWh per ton of alumina, calculate the power of the required electric heating furnace 8, configure the corresponding heating pipes and furnace size, calculate the required heating body surface area, so as to match the corresponding quantity, and design the corresponding furnace size and shape according to the space. According to the air volume of 810-850m3 / t per ton of Al2O3 (one part comes from the exhaust gas 18, mainly from the first cooling 14, the second cooling 15 two cooling sections and the fluidized product cooling system 10 preheated air) is blown into the electric heating furnace pipe network. After the air is heated to 850-1050℃, the aluminum hydroxide raw material goes up in the reverse way into the fluidized bed 7, and exchanges gas-solid heat energy with aluminum hydroxide fine powder with a particle size of 80-95μm in the fluidized bed 7, so that aluminum hydroxide is converted into α-alumina.

[0049] To ensure the surface quality of the α-aluminum oxide particle balls, the pressure in the entrained flow bed 7 is between 0.04 and 0.165 MPa, or in a slightly negative pressure state of -20 Pa. Since the α-aluminum oxide has a small particle size and is in a hollow spherical shape, when viewed under an electron microscope, the surface of the small balls is smooth, the particle size is uniform, and if there is no damage, it is considered a product with good quality. And its particle size is mainly determined by the particle size of aluminum hydroxide. Controlling the pressure is mainly to ensure that the particle size is not damaged during boiling calcination, which is not conducive to gas-solid separation and is mainly a process operation parameter.

[0050] After heat energy exchange, together with the crystal water vapor decomposed from aluminum hydroxide, it continues to move forward with air at 450 - 600 °C into the second-stage cyclone preheater 13. In the second-stage cyclone preheater 13, natural water and crystal water are removed. Then, it continues to enter the first-stage cyclone preheater 12 within the range of 320 - 360 °C while carrying water vapor, meets the aluminum hydroxide raw material added from the silo, and attaches water is removed. Finally, it enters the dust removal system 17 for dust removal treatment. After dust separation, the dust (aluminum hydroxide and aluminum oxide) is discharged into the second-stage cyclone preheater 13. After the hot air passes through the above two-stage cyclone preheaters and the venturi dryer for heat exchange with aluminum hydroxide, the temperature drops to <180 °C, enters the electrostatic precipitator. After purification, a part of the air is sent into the chimney and discharged into the atmosphere by the exhaust fan, and the other part enters the electric heating furnace for continued use.

[0051] Taking an alumina production line with an annual output of 1.68 million tons as an example, 5000 t / d of alumina is calcined every day, the required calcining furnace is 100 million yuan per unit, and the heat consumption is 2.77 GJ / t.AO.

[0052] Using natural gas for calcination, the consumption per ton is 82.716 m 3 / t.AO (calorific value 8000). Calculated at the natural gas price of 1.7 yuan per ton, the fuel cost per ton of alumina is: 82.716 × 1.7 = 140.69 yuan / t.AO, and the annual fuel cost is 140.69 × 1.68 million = 236.3592 million yuan.

[0053] Using an electric heating calcining furnace, the power consumption per ton of alumina is 750 kWh, and an electric heating furnace of 126 MW needs to be equipped. Calculated at the cost of 0.17 yuan per kWh, each ton of aluminum hydroxide requires 127.5 yuan, directly saving 1.68 × (140.69 - 127.5) = 22.1592 million yuan in fuel costs compared with the gas calcining furnace.

[0054] In addition, the original combustion furnace needs to be equipped with a denitration device. For a production capacity of 1.68 million tons, 10 million yuan of denitration equipment is required for investment, and 1.5 million yuan of denitration agent needs to be consumed every year to maintain normal operation. Therefore, for an alumina production line with an annual output of 1.68 million tons, not only can the one-time equipment investment cost be saved by 30 million yuan, but also the annual operation cost can be saved by 25 million yuan.

[0055] Using an electric heating roasting furnace, the entire technological process does not have a huge desulfurization and denitrification treatment system compared with the traditional gas-fired alumina roasting. The complex combustion furnace system and the gas security system are omitted. The aluminum hydroxide contacts the air without any additional medium, the material is not polluted, and it is a pure heat energy exchange. Moreover, the tail gas does not need to go through deep dust removal. After removing the water vapor in it, it can be recycled back to the original heating system for reuse, thereby reducing energy consumption, reducing carbon emissions, and achieving green and clean production. Roasting alumina with an electric heating furnace is a better option under the dual-carbon background.

Claims

1. A method for preparing alumina by electroheating and roasting aluminum hydroxide, characterized in that, Install an electric heating furnace in an alumina plant or the roasting system of an alumina plant. The electric heating furnace contains an electric heating element; The aluminum hydroxide raw material enters the heated fluidized bed holding tank after preheating and drying. In the holding tank, the aluminum hydroxide is roasted into α-Al2O3 crystal form, and then gradually cooled by a four-stage cyclone cooler. At the same time, heat exchange is carried out with the cold air sent in the reverse direction. After further cooling by the fluidized cooler with cold air, alumina products are obtained; The fluidized bed holding tank is heated by the hot air generated by the electric heating furnace.

2. The method for preparing alumina by electroheating and roasting aluminum hydroxide according to claim 1, characterized in that, The temperature of the fluidized bed holding tank is 850 - 1050 °C.

3. The method for preparing alumina by electrically heating and roasting aluminum hydroxide according to claim 1, characterized in that, During operation, the pressure in the entrained flow bed holding tank is 0.04 - 0.165 MPa, or in a slightly negative pressure state of -20 Pa.

4. The method for preparing alumina by electrically heating and roasting aluminum hydroxide according to claim 1, wherein, The cold air is outside air.

5. The apparatus used in the method according to any one of claims 1-4, characterized in that, The electric heating furnace (8) is composed of a furnace body (6), an electric heating element group (2), alloy tubes (3), a protective sleeve (4), and a fixed tube sheet (5); The furnace body (6) contains several alloy tubes (3). The alloy tubes (3) are fixed to the furnace body through a sealed fixed cover (1); each alloy tube (3) contains several electric heating element groups (2); the sealed fixed cover (1) is provided with through holes, and the electric heating element groups (2) can pass through the through holes; several electric heating element groups (2) are fixed by a fixed tube sheet (5); a protective sleeve (4) is provided on the outer side of the alloy tube (3); the electric heating element group (2) is composed of electric heating elements.

6. The device according to claim 5, characterized in that, The device also includes an entrained flow bed (7), a fluidized product cooling system (10), a first-stage cyclone preheater (12), a second-stage cyclone preheater (13), a primary cooler (14), a secondary cooler (15), an aluminum hydroxide raw material bin (16), and a dust removal system (17); The first-stage cyclone preheater (12) is provided with an aluminum hydroxide raw material inlet and an aluminum hydroxide outlet on the side, a first hot air inlet at the bottom, and a first waste gas outlet at the top; the second-stage cyclone preheater (13) is provided with an aluminum hydroxide inlet on the side, a hot air outlet and a second waste gas outlet at the top, and is connected to the entrained flow bed (7) through a device pipe flange at the bottom; the bottom of the entrained flow bed (7) is connected to the electric heating furnace (8); the bottom of the entrained flow bed (7) is provided with a discharge port; The aluminum hydroxide raw material inlet is connected to the aluminum hydroxide raw material bin (16); the first waste gas outlet and the second waste gas outlet are connected to the dust removal system (17) through a gas pipeline; the aluminum hydroxide outlet is connected to the aluminum hydroxide inlet; the hot air outlet is connected to the first hot air inlet; The primary cooler (14) is provided with a first alumina inlet at the top, a first alumina outlet and a first external cold air inlet at the bottom, and a second external cold air inlet on the side; the secondary cooler (15) is provided with a second alumina inlet on the side, a first external cold air outlet at the top, and a second alumina outlet at the bottom; the fluidized product cooling system (10) is provided with a product alumina inlet, a third external cold air inlet, a third waste gas outlet, and a product alumina outlet; The first alumina inlet is connected to the discharge port, the first alumina outlet is connected to the second alumina inlet, and the second alumina outlet is connected to the product alumina inlet; the first external cold air outlet is connected to the second external cold air inlet; the third exhaust gas outlet is connected to the dust removal system (17).

7. The device according to claim 6, wherein The primary cooling (14) and secondary cooling (15) adopt a four-stage cyclone cooler; the fluidized product cooling system (10) adopts a fluidized bed cooler.

8. The device according to claim 5, wherein The material of the heating element is iron-chromium-aluminum alloy.

9. The device according to claim 8, characterized in that, The content of metal Cr in the iron-chromium-aluminum alloy is 22 wt.%, the content of metal Al is 5.8 wt.%, and the remaining components are metal Fe.

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