A green electricity coupled closed steam cascade cycle aluminum hydroxide calcination system and method

Abstract

The application provides a green electricity coupling closed steam cascade cycle aluminum hydroxide calcination system and method, and relates to the technical field of alumina metallurgy. The system comprises a raw material pretreatment unit, a first-stage low-temperature pure electricity calcination unit, a second-stage medium-temperature electricity-steam coupling calcination unit, a third-stage high-temperature steam main heating calcination unit, a closed steam compression circulation unit and an alumina cooling waste heat recovery unit. The system can match different heat supply modes according to the decomposition characteristics of aluminum hydroxide in different temperature ranges, avoid the problem of heat load concentration in the medium-high temperature section caused by single electricity heating, and improve the problem that pure steam heat exchange is difficult to adapt to the multi-stage calcination process. Meanwhile, the system can reduce the latent heat loss caused by direct water vapor discharge, improve the recycling degree of the decomposition steam, reduce the dependence on external fuel or external steam supply in the calcination process, improve the internal heat recycling efficiency of the system, and improve the heat coupling degree between the calcination process and the raw material preheating and Bayer process dissolution process.

Description

Technical Field

[0001] This invention relates to the field of alumina metallurgical production technology, and in particular to a green electro-coupled closed-loop steam-cascaded circulating aluminum hydroxide roasting system and method. Background Technology

[0002] Aluminum hydroxide roasting is a crucial step in alumina production. Its main function is to remove free water, water of crystallization, and residual hydroxyl groups from aluminum hydroxide through heating, and to promote corresponding crystal transformations in the material to obtain alumina products that meet subsequent application requirements. Aluminum hydroxide exhibits significant staged thermal decomposition characteristics during roasting. The degree of dehydration, heat demand, and steam production vary across different temperature ranges. Therefore, the configuration of the roasting heat source, the atmosphere organization, and the waste heat recovery method directly affect system energy consumption, steam utilization efficiency, and product stability.

[0003] Existing alumina roasting processes mostly use high-temperature flue gas generated from the combustion of fuels such as natural gas as the main heat source, and utilize this high-temperature flue gas for fluidization and heat exchange of aluminum hydroxide. Some processes also use electric heating to replace combustion heating, or use steam heat exchange to heat the material. These methods can achieve roasting and dehydration of aluminum hydroxide, but overall they still rely mainly on a single heat source or a non-closed heat medium. The water vapor generated during the decomposition of aluminum hydroxide is usually discharged with the airflow, and the sensible heat released during the material cooling process is mostly dissipated through conventional cooling methods. There is a lack of effective steam and heat coupling between the roasting process and the upstream raw material preheating and the downstream Bayer leaching processes.

[0004] Therefore, existing technologies have at least the following shortcomings: Traditional gas-fired roasting relies on fossil fuels for heating, and the combustion flue gas participates in fluidization and heat exchange, making it difficult to form a high-purity, closed-loop steam roasting environment. Furthermore, the water vapor generated by the decomposition of aluminum hydroxide is discharged with the flue gas, and the decomposition steam and its latent heat are not fully recovered and utilized. Single electric heating roasting has a concentrated heat load in the medium and high temperature sections, resulting in high overall power consumption, and it is difficult to meet the different heat requirements of each stage of low-temperature dehydration, medium-temperature deep dehydration, and high-temperature crystal transformation. Simple steam heat exchange roasting also suffers from insufficient steam recycling and insufficient heat exchange adaptability in the medium and high temperature sections, making it difficult to match with the segmented thermal decomposition process of aluminum hydroxide. At the same time, existing roasting systems do not adequately utilize the cascade recovery of the waste heat from the cooling of high-temperature alumina, making it difficult to convert the waste heat into reusable steam for further use in raw material preheating or Bayer leaching processes, resulting in room for improvement in the overall thermal efficiency of the roasting system. Summary of the Invention

[0005] To address the shortcomings of the existing technology, this invention provides a green electric coupling closed steam cascade circulation aluminum hydroxide roasting system, comprising a raw material pretreatment unit, a primary low-temperature pure electric roasting unit, a secondary medium-temperature electric-steam coupling roasting unit, a tertiary high-temperature steam main heating roasting unit, a closed steam compression circulation unit, and an alumina cooling waste heat recovery unit. The solid discharge end of the raw material pretreatment unit is connected to the feed end of the first-stage low-temperature pure electric roasting unit, and the water vapor outlet is connected to the steam inlet of the closed steam compression circulation unit. This is used to preheat the aluminum hydroxide raw material to obtain preheated aluminum hydroxide and separate free water vapor. The solid discharge end of the first-stage low-temperature pure electric roasting unit is connected to the feed end of the second-stage medium-temperature electric-steam coupled roasting unit, and the water vapor outlet is connected to the steam inlet of the closed steam compression circulation unit. This is used to perform low-temperature pure electric roasting of preheated aluminum hydroxide to obtain the first-stage roasted material. The solid discharge end of the secondary medium-temperature electric-steam coupled roasting unit is connected to the feed end of the tertiary high-temperature steam main heating roasting unit, and the water vapor outlet is connected to the steam inlet of the closed steam compression circulation unit. It is used to perform electric-steam coupled roasting on the primary roasting material to obtain the secondary roasting material. The solid phase discharge end of the three-stage high-temperature steam main heating roasting unit is connected to the alumina cooling waste heat recovery unit, and the water vapor outlet is connected to the steam inlet of the closed steam compression circulation unit. It is used to perform high-temperature fine roasting of the secondary roasting material to obtain high-temperature alumina. The circulating steam outlet of the closed steam compression circulation unit is connected to the secondary medium-temperature electric-steam coupled roasting unit and the tertiary high-temperature steam main heating roasting unit, respectively, for collecting, pressurizing and heating water vapor from the raw material pretreatment unit, the primary low-temperature pure electric roasting unit, the secondary medium-temperature electric-steam coupled roasting unit and the tertiary high-temperature steam main heating roasting unit; The waste heat outlet pipeline of the alumina cooling waste heat recovery unit is connected to the raw material pretreatment unit, which is used to cool high-temperature alumina and output finished alumina, while recovering the waste heat of high-temperature alumina.

[0006] The raw material pretreatment unit includes an aluminum hydroxide raw material silo, a dryer, and a primary gas-solid separator. The aluminum hydroxide raw material silo is equipped with a variable frequency screw feeder on the discharge side, which is used to feed the aluminum hydroxide raw material in the aluminum hydroxide raw material silo into the dryer. The dryer has a shell-and-tube structure. The shell side of the dryer is connected to the waste heat outlet pipeline of the alumina cooling waste heat recovery unit, which is used to preheat the aluminum hydroxide raw material using the waste heat recovered by the alumina cooling waste heat recovery unit. The discharge end of the dryer is connected to the first-stage gas-solid separator, the solid phase outlet of the first-stage gas-solid separator is connected to the feed end of the first-stage low-temperature pure electric roasting unit, and the water vapor outlet of the first-stage gas-solid separator is connected to the steam inlet of the closed steam compression circulation unit, which is used to perform gas-solid separation on the preheated aluminum hydroxide.

[0007] The primary low-temperature pure electric calcination unit is a primary low-temperature electric heating fluidized bed, which is equipped with modular electric heating tube bundles and air distribution devices. The modular electric heating tube bundle is used to electrically heat the aluminum hydroxide entering the first-stage low-temperature pure electric roasting unit, and the air distribution device is used to make low-temperature saturated steam enter the first-stage low-temperature pure electric roasting unit as a fluidizing medium. The operating temperature of the primary low-temperature pure electric roasting unit is 280℃~400℃, and the furnace is maintained at a slight positive pressure.

[0008] The secondary medium-temperature electric-steam coupled calcination unit includes a secondary coupled fluidized bed and a secondary gas-solid separator; The secondary coupled fluidized bed is equipped with an electric heating tube bundle and a steam heat exchange coil. The electric heating tube bundle is used to provide basic heat to the secondary coupled fluidized bed, and the steam heat exchange coil is used to receive the circulating steam output from the closed steam compression circulation unit and exchange heat with the material in the secondary coupled fluidized bed. The discharge end of the secondary coupled fluidized bed is connected to the secondary gas-solid separator, and the operating temperature is 500℃~760℃; The solid phase outlet of the secondary gas-solid separator is connected to the feed end of the tertiary high-temperature steam main heating and roasting unit, and the water vapor outlet of the secondary gas-solid separator is connected to the steam inlet of the closed steam compression and circulation unit, which is used to perform gas-solid separation on the material output from the secondary coupled fluidized bed.

[0009] The three-stage high-temperature steam main heating and roasting unit is a three-stage high-temperature roasting furnace, which is equipped with a steam heat exchange coil and a fine-tuning electric heating component. The steam heat exchange coil is used to receive the circulating steam output from the closed steam compression circulation unit and serve as the main heating source for the three-stage high-temperature roasting furnace. The fine-tuning electric heating component is used to fine-tune the operating temperature of the three-stage high-temperature roasting furnace. The operating temperature of the three-stage high-temperature steam main heating roasting unit is 760℃~1000℃, which is used to enable the secondary roasting material to complete the crystal transformation and remove residual hydroxyl groups.

[0010] The closed-loop steam compression cycle unit includes a steam main pipe, a steam compressor, and a steam heater; The steam collection main pipe is connected to the steam outlets of the raw material pretreatment unit, the first-stage low-temperature pure electric roasting unit, the second-stage medium-temperature electric-steam coupled roasting unit, and the third-stage high-temperature steam main heating roasting unit, respectively, for collecting free water vapor, decomposing steam, and entraining steam. The steam compressor's air inlet is connected to the steam manifold to pressurize the incoming steam, and the steam compressor is equipped with a steam-water separator. The steam compressor's outlet is divided into two paths: one path connects to the secondary medium-temperature electric-steam coupled roasting unit and the tertiary high-temperature steam main heating roasting unit via a steam heater, and the other path connects to the Bayer process leaching process.

[0011] The alumina cooling waste heat recovery unit includes a solid-solid waste heat boiler, a water supply system, and a finished alumina discharge unit. The feed end of the solid-solid waste heat boiler is connected to the solid discharge end of the three-stage high-temperature steam main heating and roasting unit to receive the high-temperature alumina output from the three-stage high-temperature steam main heating and roasting unit. The waste heat outlet pipeline of the solid-solid waste heat boiler is connected to the raw material pretreatment unit. The water supply system is connected to the solid-solid waste heat boiler and is used to introduce softened water into the solid-solid waste heat boiler, so that the softened water is vaporized into high-temperature and high-pressure steam by the waste heat released by high-temperature alumina. The finished alumina discharge unit is connected to the solid phase discharge end of the solid-solid waste heat boiler.

[0012] A green electrocoupling closed-loop steam-cascaded aluminum hydroxide calcination method includes the following steps: S1. The aluminum hydroxide raw material is fed into the dryer for preheating and then separated into gas and solid by a first-stage gas-solid separator to obtain preheated aluminum hydroxide. The separated free water vapor is then collected into the steam collection manifold. S2. The preheated aluminum hydroxide is fed into the first-stage low-temperature pure electric roasting unit for low-temperature pure electric roasting to obtain the first-stage roasting material. The decomposition steam generated is then collected into the steam collection manifold. S3. The primary roasted material is fed into the secondary medium-temperature electric-steam coupled roasting unit. The primary roasted material is deeply dehydrated by electric heating and circulating steam to obtain the secondary roasted material. The generated water vapor and entrained steam are collected into the steam collection manifold. S4. The secondary roasted material is fed into the tertiary high-temperature steam main heating roasting unit, where it is mainly heated by circulating steam and assisted by electric heating, so that the secondary roasted material can complete the crystal transformation and remove residual hydroxyl groups to obtain high-temperature alumina. S5. High-temperature alumina is fed into the alumina cooling waste heat recovery unit and softened water is introduced to cool the high-temperature alumina into finished alumina after releasing heat. At the same time, the softened water is vaporized to form high-temperature and high-pressure steam. At least part of the high-temperature and high-pressure steam is returned to the dryer. S6. After the water vapor collected in the steam collection main pipe is pressurized, purified and heated, a portion is returned to the secondary medium-temperature electric-steam coupled roasting unit and the tertiary high-temperature steam main heating roasting unit for circulating heating.

[0013] In S5, a portion of the high-temperature, high-pressure steam is returned to the dryer, while the other portion is output to the Bayer process leaching step. In S6, the water vapor collected in the steam collection manifold is pressurized by the steam compressor and then divided into two paths. One path is heated by the steam heater and returned to the secondary medium-temperature electric-steam coupled roasting unit and the tertiary high-temperature steam main heating roasting unit. The other path is used as surplus high-pressure steam and transported to the Bayer process leaching process.

[0014] In step S1, the aluminum hydroxide raw material is preheated to 100°C to 280°C; In S2, the operating temperature of the primary low-temperature pure electric calcination unit is 280℃~400℃, and the furnace pressure is maintained at a slight positive pressure of 100Pa~300Pa. In S3, the operating temperature of the secondary medium-temperature electric-steam coupled calcination unit is 500℃~760℃; In S4, the operating temperature of the three-stage high-temperature steam main heating calcination unit is 760℃~1000℃; In step S5, the high-temperature alumina is cooled to below 80°C and then output as finished alumina, with a loss on ignition (LOI) of less than 0.8%. In step S6, the water vapor collected in the steam collection manifold is pressurized to 0.8 MPa to 1.0 MPa, and the volume ratio of water vapor in the gas phase inside the system is not less than 95%.

[0015] The beneficial effects of this invention are: 1. This invention sets up a first-stage low-temperature pure electric roasting unit, a second-stage medium-temperature electric-steam coupled roasting unit, and a third-stage high-temperature steam main heating roasting unit, so that aluminum hydroxide passes through low-temperature pure electric roasting, medium-temperature electric-steam coupled roasting, and high-temperature steam main heating roasting in sequence. It can match different heating methods according to the decomposition characteristics of aluminum hydroxide in different temperature ranges, avoid the problem of concentrated heat load in the medium and high temperature range by single electric heating, and also improve the problem that simple steam heat exchange is difficult to adapt to multi-stage roasting process.

[0016] 2. This invention uses a closed steam compression and circulation unit to collect, pressurize, purify, and heat the free water vapor, decomposition steam, and entrained steam generated by the raw material pretreatment unit, the first-stage low-temperature pure electric roasting unit, the second-stage medium-temperature electric-steam coupled roasting unit, and the third-stage high-temperature steam main heating roasting unit, and then recycle them. This can reduce the latent heat loss caused by the direct discharge of water vapor and improve the degree of recovery and utilization of decomposition steam.

[0017] 3. This invention connects the circulating steam outlet of the closed steam compression cycle unit to the secondary medium-temperature electric-steam coupled roasting unit and the tertiary high-temperature steam main heating roasting unit, respectively, so that the pressurized and heated circulating steam can be returned to the medium- and high-temperature roasting stage as a heat source, thereby reducing the dependence of the roasting process on external fuel or external steam supply and improving the efficiency of internal heat recycling.

[0018] 4. This invention uses an alumina cooling waste heat recovery unit to cool the high-temperature alumina output from the three-stage high-temperature steam main heating roasting unit, and uses the waste heat released by the high-temperature alumina to vaporize softened water to form high-temperature and high-pressure steam. At least a portion of the high-temperature and high-pressure steam is returned to the dryer and can also be incorporated into the Bayer process leaching process. This allows the high-temperature alumina cooling waste heat to be converted into usable steam, thereby improving the heat coupling between the roasting process and the raw material preheating and Bayer process leaching processes. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of a green electrical coupling closed steam cascade circulation aluminum hydroxide calcination system according to the present invention. Detailed Implementation

[0020] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] like Figure 1As shown in the embodiment of the present invention, a green electric coupled closed-loop steam-cascaded alumina calcination system includes a raw material pretreatment unit, a primary low-temperature pure electric calcination unit, a secondary medium-temperature electric-steam coupled calcination unit, a tertiary high-temperature steam main heating calcination unit, a closed steam compression circulation unit, and an alumina cooling waste heat recovery unit. The raw material pretreatment unit includes an alumina raw material silo, a dryer 1, and a primary gas-solid separator 2; the primary low-temperature pure electric calcination unit includes a primary low-temperature electric heating fluidized bed 3; the secondary medium-temperature electric-steam coupled calcination unit includes a secondary coupled fluidized bed 4 and a secondary gas-solid separator 5; the tertiary high-temperature steam main heating calcination unit includes a tertiary high-temperature calcination furnace 6; the alumina cooling waste heat recovery unit includes a solid-solid waste heat boiler 7 and a finished alumina discharge unit 8; and the closed steam compression circulation unit includes a steam main pipe 9, a steam compressor 10, and a steam heater 12. The excess steam generated after steam compression circulation can be transported to the Bayer process leaching process 11.

[0022] The aforementioned units form a continuous coordination relationship of material flow, steam flow, and waste heat flow. The material flows sequentially through the raw material pretreatment unit, the primary low-temperature pure electric roasting unit, the secondary medium-temperature electric-steam coupled roasting unit, the tertiary high-temperature steam main heating roasting unit, and the alumina cooling waste heat recovery unit. The water vapor generated by these units enters the closed-loop steam compression circulation unit. The circulating steam output from the closed-loop steam compression circulation unit returns to the secondary medium-temperature electric-steam coupled roasting unit and the tertiary high-temperature steam main heating roasting unit. After recovering the high-temperature alumina waste heat, the alumina cooling waste heat recovery unit returns the waste heat to the raw material pretreatment unit for raw material preheating.

[0023] The raw material pretreatment unit is used to preheat the aluminum hydroxide raw material to obtain preheated aluminum hydroxide and separate free water vapor. The raw material pretreatment unit includes an aluminum hydroxide raw material silo, a dryer 1, and a primary gas-solid separator 2. The aluminum hydroxide raw material silo stores the aluminum hydroxide raw material to be calcined. A variable frequency screw feeder is installed on the discharge side of the aluminum hydroxide raw material silo to feed the aluminum hydroxide raw material into the dryer 1. Continuous feeding via the variable frequency screw feeder ensures that the aluminum hydroxide raw material entering the dryer 1 is continuously and uniformly conveyed, which is beneficial for the stable operation of subsequent preheating and calcination processes.

[0024] Dryer 1 is used to preheat aluminum hydroxide raw materials. Dryer 1 has a shell-and-tube structure, with aluminum hydroxide material conveyed in the tube side and high-temperature waste heat flue gas or steam recovered from the alumina cooling unit introduced into the shell side. The shell side of the dryer is connected to the waste heat outlet pipeline of the alumina cooling waste heat recovery unit, specifically, the shell side of dryer 1 is connected to the waste heat outlet pipeline of the solid-solid waste heat boiler 7, allowing the waste heat recovered by the solid-solid waste heat boiler 7 to enter dryer 1 and serve as a heat source for preheating the aluminum hydroxide raw materials. After being preheated in dryer 1, the free adsorbed water on the surface of the aluminum hydroxide raw materials is removed, forming preheated aluminum hydroxide.

[0025] The discharge end of dryer 1 is connected to primary gas-solid separator 2, which is used for gas-solid separation of preheated aluminum hydroxide. After separation by primary gas-solid separator 2, the solid material enters the primary low-temperature electrically heated fluidized bed 3 of primary low-temperature pure electric roasting unit from the solid outlet of primary gas-solid separator 2, while the separated free water vapor enters the steam collection manifold 9 of closed steam compression circulation unit from the water vapor outlet of primary gas-solid separator 2. Thus, the raw material pretreatment unit not only completes the preheating of raw materials but also incorporates the free water vapor generated during the preheating process into the subsequent closed steam circulation.

[0026] The primary low-temperature pure electric roasting unit is used to perform low-temperature pure electric roasting of preheated aluminum hydroxide to obtain primary roasted material. The primary low-temperature pure electric roasting unit is a primary low-temperature electrically heated fluidized bed 3, which contains modular electric heating tube bundles and an air distribution device. The modular electric heating tube bundles are used to electrically heat the aluminum hydroxide entering the primary low-temperature electrically heated fluidized bed 3, and the air distribution device is used to introduce low-temperature saturated steam as a fluidizing medium into the primary low-temperature electrically heated fluidized bed 3, so that the aluminum hydroxide is heated in a fluidized state. The operating temperature of the primary low-temperature electrically heated fluidized bed 3 is 280℃~400℃. Within this temperature range, the preheated aluminum hydroxide undergoes low-temperature dehydration roasting to form the primary roasted material.

[0027] The furnace of the primary low-temperature electrically heated fluidized bed 3 maintains a slight positive pressure, making it difficult for external air to enter the furnace and helping to maintain a closed steam atmosphere inside the system. The decomposition steam generated during the low-temperature pure electric roasting process enters the steam collection manifold 9 through the water vapor outlet of the primary low-temperature electrically heated fluidized bed 3, and the primary roasting material enters the secondary medium-temperature electric-steam coupled roasting unit through the solid phase discharge end of the primary low-temperature electrically heated fluidized bed 3.

[0028] The secondary medium-temperature electric-steam coupled roasting unit is used to perform electric-steam coupled roasting on the primary roasted material to obtain the secondary roasted material. The secondary medium-temperature electric-steam coupled roasting unit includes a secondary coupled fluidized bed 4 and a secondary gas-solid separator 5. The secondary coupled fluidized bed 4 is equipped with an electric heating tube bundle and a steam heat exchange coil. The electric heating tube bundle provides basic heat to the secondary coupled fluidized bed 4, and the steam heat exchange coil receives the circulating steam output from the closed steam compression circulation unit and facilitates heat exchange between the circulating steam and the material within the secondary coupled fluidized bed 4. The operating temperature of the secondary coupled fluidized bed 4 is 500℃~760℃, within which the primary roasted material undergoes further deep dehydration to form the secondary roasted material.

[0029] The discharge end of the secondary coupled fluidized bed 4 is connected to the secondary gas-solid separator 5. The secondary gas-solid separator 5 is used to separate the gas and solid phases of the material output from the secondary coupled fluidized bed 4. After separation by the secondary gas-solid separator 5, the solid phase material enters the tertiary high-temperature roasting furnace 6 of the tertiary high-temperature steam main heating roasting unit from the solid phase outlet of the secondary gas-solid separator 5. The water vapor and entrained steam generated during the secondary coupled roasting process enter the steam collection manifold 9 from the water vapor outlet of the secondary gas-solid separator 5.

[0030] The three-stage high-temperature steam main heating roasting unit is used for high-temperature fine roasting of the secondary roasting materials to obtain high-temperature alumina. The three-stage high-temperature steam main heating roasting unit is a three-stage high-temperature roasting furnace 6, which is equipped with a steam heat exchange coil and a fine-tuning electric heating component. The steam heat exchange coil receives the circulating steam output from the closed steam compression circulation unit and serves as the main heating source for the three-stage high-temperature roasting furnace 6; the fine-tuning electric heating component is used to fine-tune the operating temperature of the three-stage high-temperature roasting furnace 6 to compensate for temperature fluctuations during operation.

[0031] The operating temperature of the three-stage high-temperature roasting furnace 6 is 760℃~1000℃. After the secondary roasting material enters the three-stage high-temperature roasting furnace 6, it completes the crystal transformation and removes residual hydroxyl groups under the auxiliary regulation of the main circulating steam heating and fine-tuning electric heating components to form high-temperature alumina. The water vapor generated in the three-stage high-temperature roasting furnace 6 enters the steam collection manifold 9, and the high-temperature alumina enters the solid-solid waste heat boiler 7 of the alumina cooling waste heat recovery unit from the solid phase discharge end of the three-stage high-temperature roasting furnace 6.

[0032] The closed-loop steam compression cycle unit is used to collect, pressurize, and heat the steam from the raw material pretreatment unit, the primary low-temperature pure electric roasting unit, the secondary medium-temperature electric-steam coupled roasting unit, and the tertiary high-temperature steam main heating roasting unit. The closed-loop steam compression cycle unit includes a steam collection manifold 9, a steam compressor 10, and a steam heater 12. The steam collection manifold 9 is connected to the steam outlets of the primary gas-solid separator 2, the primary low-temperature electric-heated fluidized bed 3, the secondary gas-solid separator 5, and the tertiary high-temperature roasting furnace 6, respectively, and is used to collect the free steam separated during the raw material pretreatment process, the decomposition steam generated during the primary low-temperature pure electric roasting process, the steam and entrained steam generated during the secondary medium-temperature electric-steam coupled roasting process, and the steam generated during the tertiary high-temperature steam main heating roasting process.

[0033] The steam compressor 10 has its inlet connected to the steam manifold 9 for pressurizing the incoming steam. The steam compressor 10 is equipped with a steam-water separator for separating steam and water during the steam compression process. The pressurized steam from the steam compressor 10 is output from its outlet. The outlet of the steam compressor 10 is divided into two paths: one path connects to the secondary medium-temperature electric-steam coupled roasting unit and the tertiary high-temperature steam main heating roasting unit via the steam heater 12, allowing the pressurized steam to be heated by the steam heater 12 and then returned to the secondary coupled fluidized bed 4 and the tertiary high-temperature roasting furnace 6 for circulating heating; the other path connects to the Bayer process leaching process 11, allowing excess high-pressure steam to be transported to the Bayer process leaching process 11. Through the above steam collection, pressurization, heating, and reuse processes, the water vapor generated during the aluminum hydroxide roasting process can be recycled and reused within the system in a closed loop.

[0034] The alumina cooling waste heat recovery unit is used to cool high-temperature alumina and output finished alumina, while simultaneously recovering the waste heat from the high-temperature alumina. The alumina cooling waste heat recovery unit includes a solid-solid waste heat boiler 7, a feedwater system, and a finished alumina discharge unit 8. The feed end of the solid-solid waste heat boiler 7 is connected to the solid-phase discharge end of the three-stage high-temperature calcining furnace 6, and is used to receive the high-temperature alumina output from the three-stage high-temperature calcining furnace 6. The feedwater system is connected to the solid-solid waste heat boiler 7 and is used to introduce softened water into the solid-solid waste heat boiler 7. After the high-temperature alumina enters the solid-solid waste heat boiler 7, it releases heat and is gradually cooled. The softened water absorbs the waste heat released by the high-temperature alumina and vaporizes to form high-temperature, high-pressure steam.

[0035] The waste heat outlet pipeline of the solid-solid waste heat boiler 7 is connected to the raw material pretreatment unit, specifically to the dryer 1, so that at least a portion of the high-temperature and high-pressure steam is returned to the dryer 1 for preheating the aluminum hydroxide raw material. The finished alumina discharge unit 8 is connected to the solid phase discharge end of the solid-solid waste heat boiler 7, and the alumina cooled by the solid-solid waste heat boiler 7 enters the finished alumina discharge unit 8.

[0036] Based on the above-mentioned green electrocoupling closed-loop steam-cascaded circulating aluminum hydroxide calcination system, the present invention also provides a green electrocoupling closed-loop steam-cascaded circulating aluminum hydroxide calcination method, which includes the following steps: S1. Aluminum hydroxide raw material is fed from the aluminum hydroxide raw material silo into dryer 1 for preheating, and then undergoes gas-solid separation in primary gas-solid separator 2 to obtain preheated aluminum hydroxide. The separated free water vapor is collected into steam collection manifold 9. Specifically, aluminum hydroxide raw material can be continuously fed from aluminum hydroxide raw material silo into dryer 1 via a variable frequency screw feeder. Dryer 1 uses the waste heat recovered from solid-solid waste heat boiler 7 to preheat the aluminum hydroxide raw material. The preheated material enters primary gas-solid separator 2 to complete gas-solid separation. The solid phase material enters the subsequent low-temperature roasting process, and the free water vapor enters steam collection manifold 9.

[0037] S2. The preheated aluminum hydroxide is fed into the primary low-temperature electric heating fluidized bed 3 of the primary low-temperature pure electric roasting unit for low-temperature pure electric roasting to obtain primary roasted material. The decomposition steam generated is collected into the steam collection manifold 9. Specifically, after the preheated aluminum hydroxide enters the primary low-temperature electric heating fluidized bed 3, it is purely electric heated by modular electric heating tube bundles and low-temperature roasting is carried out under the condition of low-temperature saturated steam as the fluidizing medium, so that the material removes part of the crystal water and forms primary roasted material. The decomposition steam generated by low-temperature roasting enters the steam collection manifold 9.

[0038] S3. The primary roasted material is fed into the secondary medium-temperature electric-steam coupled roasting unit. Electric heating and circulating steam are coupled for heating to deeply dehydrate the primary roasted material, yielding the secondary roasted material. The generated water vapor and entrained steam are collected in the steam manifold 9. Specifically, after the primary roasted material enters the secondary coupled fluidized bed 4, it receives basic heat from the electric heating tube bundle and receives circulating steam for heat exchange through the steam heat exchange coil, allowing for deep dehydration of the primary roasted material within the secondary coupled fluidized bed 4. The material output from the secondary coupled fluidized bed 4 enters the secondary gas-solid separator 5 for gas-solid separation. The solid phase material enters the tertiary high-temperature roasting furnace 6, while the water vapor and entrained steam enter the steam manifold 9.

[0039] S4. The secondary roasted material is fed into the tertiary high-temperature roasting furnace 6 of the tertiary high-temperature steam main heating roasting unit. It is mainly heated by circulating steam and assisted by electric heating to ensure the secondary roasted material completes its crystal transformation and removes residual hydroxyl groups, yielding high-temperature alumina. Specifically, after the secondary roasted material enters the tertiary high-temperature roasting furnace 6, the circulating steam is received by the steam heat exchange coil and used as the main heating source. The operating temperature of the tertiary high-temperature roasting furnace 6 is finely adjusted by the electric heating components to complete the high-temperature fine roasting of the material. The steam generated in the tertiary high-temperature roasting furnace 6 enters the steam collection manifold 9, and the high-temperature alumina enters the solid-solid waste heat boiler 7.

[0040] S5. High-temperature alumina is fed into the alumina cooling waste heat recovery unit and softened water is introduced. The high-temperature alumina releases heat and is then cooled into finished alumina. Simultaneously, the softened water vaporizes to form high-temperature, high-pressure steam. At least a portion of this high-temperature, high-pressure steam is returned to dryer 1. Specifically, after the high-temperature alumina enters the solid-solid waste heat boiler 7, it releases heat. Softened water is introduced into the solid-solid waste heat boiler 7 through the feedwater system. The softened water absorbs the waste heat released by the high-temperature alumina and vaporizes to form high-temperature, high-pressure steam. At least a portion of this high-temperature, high-pressure steam is returned to dryer 1 through the waste heat outlet pipe of the solid-solid waste heat boiler 7 for preheating the aluminum hydroxide raw material. The cooled alumina then enters the finished alumina discharge unit 8.

[0041] S6. After the water vapor collected by the steam collection manifold 9 is pressurized, purified, and heated, a portion is returned to the secondary medium-temperature electric-steam coupled roasting unit and the tertiary high-temperature steam main heating roasting unit for circulating heating. Specifically, the water vapor collected by the steam collection manifold 9 enters the steam compressor 10, which pressurizes the steam and separates it from the water vapor through a steam-water separator. After the pressurized steam is heated by the steam heater 12, a portion is returned to the secondary coupled fluidized bed 4 and the tertiary high-temperature roasting furnace 6 for circulating heating, while the other portion is transported as surplus high-pressure steam to the Bayer process leaching process 11.

[0042] During the operation of the above method, the aluminum hydroxide raw material can be preheated to 100℃~280℃ in the dryer 1; the operating temperature of the primary low-temperature electric heating fluidized bed 3 is 280℃~400℃, and the furnace pressure is maintained at 100Pa~300Pa slightly positive pressure; the operating temperature of the secondary coupled fluidized bed 4 is 500℃~760℃; the operating temperature of the tertiary high-temperature calcining furnace 6 is 760℃~1000℃; the high-temperature alumina can be cooled to below 80℃ in the solid-solid waste heat boiler 7 and then enter the finished alumina discharge unit 8, and the loss on ignition (LOI) of the finished alumina is less than 0.8%; the water vapor collected by the steam collection manifold 9 can be pressurized to 0.8MPa~1.0MPa by the steam compressor 10, and the volume ratio of water vapor in the gas phase inside the system is not less than 95%.

[0043] Through the above system structure and process flow, the raw material pretreatment unit uses the waste heat recovered by the alumina cooling waste heat recovery unit to preheat the aluminum hydroxide raw material. The first-stage low-temperature pure electric roasting unit performs low-temperature pure electric roasting through the first-stage low-temperature electric heating fluidized bed 3. The second-stage medium-temperature electric-steam coupled roasting unit performs electric-steam coupled roasting through the second-stage coupled fluidized bed 4. The third-stage high-temperature steam main heating roasting unit performs high-temperature fine roasting through the third-stage high-temperature roasting furnace 6. The closed steam compression circulation unit recovers and circulates water vapor at each stage through the steam collection manifold 9, steam compressor 10 and steam heater 12. The alumina cooling waste heat recovery unit recovers high-temperature alumina waste heat through the solid-solid waste heat boiler 7 and prepares high-temperature and high-pressure steam, thereby realizing the cascade heating, closed steam circulation and waste heat recovery and utilization in the aluminum hydroxide roasting process.

[0044] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make modifications, alterations, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A green electrocoupling closed-loop steam-cascaded circulating aluminum hydroxide calcination system, characterized in that, It includes a raw material pretreatment unit, a primary low-temperature pure electric roasting unit, a secondary medium-temperature electric-steam coupled roasting unit, a tertiary high-temperature steam main heating roasting unit, a closed steam compression circulation unit, and an alumina cooling waste heat recovery unit. The solid discharge end of the raw material pretreatment unit is connected to the feed end of the first-stage low-temperature pure electric roasting unit, and the water vapor outlet is connected to the steam inlet of the closed steam compression circulation unit. This is used to preheat the aluminum hydroxide raw material to obtain preheated aluminum hydroxide and separate free water vapor. The solid discharge end of the first-stage low-temperature pure electric roasting unit is connected to the feed end of the second-stage medium-temperature electric-steam coupled roasting unit, and the water vapor outlet is connected to the steam inlet of the closed steam compression circulation unit. This is used to perform low-temperature pure electric roasting of preheated aluminum hydroxide to obtain the first-stage roasted material. The solid discharge end of the secondary medium-temperature electric-steam coupled roasting unit is connected to the feed end of the tertiary high-temperature steam main heating roasting unit, and the water vapor outlet is connected to the steam inlet of the closed steam compression circulation unit. It is used to perform electric-steam coupled roasting on the primary roasting material to obtain the secondary roasting material. The solid phase discharge end of the three-stage high-temperature steam main heating roasting unit is connected to the alumina cooling waste heat recovery unit, and the water vapor outlet is connected to the steam inlet of the closed steam compression circulation unit. It is used to perform high-temperature fine roasting of the secondary roasting material to obtain high-temperature alumina. The circulating steam outlet of the closed steam compression circulation unit is connected to the secondary medium-temperature electric-steam coupled roasting unit and the tertiary high-temperature steam main heating roasting unit, respectively, for collecting, pressurizing and heating water vapor from the raw material pretreatment unit, the primary low-temperature pure electric roasting unit, the secondary medium-temperature electric-steam coupled roasting unit and the tertiary high-temperature steam main heating roasting unit; The waste heat outlet pipeline of the alumina cooling waste heat recovery unit is connected to the raw material pretreatment unit, which is used to cool high-temperature alumina and output finished alumina, while recovering the waste heat of high-temperature alumina.

2. The green electrocoupling closed-loop steam-cascaded circulating aluminum hydroxide calcination system according to claim 1, characterized in that, The raw material pretreatment unit includes an aluminum hydroxide raw material silo, a dryer, and a primary gas-solid separator. The aluminum hydroxide raw material silo is equipped with a variable frequency screw feeder on the discharge side, which is used to feed the aluminum hydroxide raw material in the aluminum hydroxide raw material silo into the dryer. The dryer has a shell-and-tube structure. The shell side of the dryer is connected to the waste heat outlet pipeline of the alumina cooling waste heat recovery unit, which is used to preheat the aluminum hydroxide raw material using the waste heat recovered by the alumina cooling waste heat recovery unit. The discharge end of the dryer is connected to the first-stage gas-solid separator, the solid phase outlet of the first-stage gas-solid separator is connected to the feed end of the first-stage low-temperature pure electric roasting unit, and the water vapor outlet of the first-stage gas-solid separator is connected to the steam inlet of the closed steam compression circulation unit, which is used to perform gas-solid separation on the preheated aluminum hydroxide.

3. The green electrocoupling closed-loop steam-cascaded circulating aluminum hydroxide calcination system according to claim 1, characterized in that, The primary low-temperature pure electric calcination unit is a primary low-temperature electric heating fluidized bed, which is equipped with modular electric heating tube bundles and air distribution devices. The modular electric heating tube bundle is used to electrically heat the aluminum hydroxide entering the first-stage low-temperature pure electric roasting unit, and the air distribution device is used to make low-temperature saturated steam enter the first-stage low-temperature pure electric roasting unit as a fluidizing medium. The operating temperature of the primary low-temperature pure electric roasting unit is 280℃~400℃, and the furnace is maintained at a slight positive pressure.

4. The green electrocoupling closed-loop steam-cascaded circulating aluminum hydroxide calcination system according to claim 1, characterized in that, The secondary medium-temperature electric-steam coupled calcination unit includes a secondary coupled fluidized bed and a secondary gas-solid separator; The secondary coupled fluidized bed is equipped with an electric heating tube bundle and a steam heat exchange coil. The electric heating tube bundle is used to provide basic heat to the secondary coupled fluidized bed, and the steam heat exchange coil is used to receive the circulating steam output from the closed steam compression circulation unit and exchange heat with the material in the secondary coupled fluidized bed. The discharge end of the secondary coupled fluidized bed is connected to the secondary gas-solid separator, and the operating temperature is 500℃~760℃; The solid phase outlet of the secondary gas-solid separator is connected to the feed end of the tertiary high-temperature steam main heating and roasting unit, and the water vapor outlet of the secondary gas-solid separator is connected to the steam inlet of the closed steam compression and circulation unit, which is used to perform gas-solid separation on the material output from the secondary coupled fluidized bed.

5. The green electrocoupling closed-loop steam-cascaded circulating aluminum hydroxide calcination system according to claim 1, characterized in that, The three-stage high-temperature steam main heating and roasting unit is a three-stage high-temperature roasting furnace, which is equipped with a steam heat exchange coil and a fine-tuning electric heating component. The steam heat exchange coil is used to receive the circulating steam output from the closed steam compression circulation unit and serve as the main heating source for the three-stage high-temperature roasting furnace. The fine-tuning electric heating component is used to fine-tune the operating temperature of the three-stage high-temperature roasting furnace. The operating temperature of the three-stage high-temperature steam main heating roasting unit is 760℃~1000℃, which is used to enable the secondary roasting material to complete the crystal transformation and remove residual hydroxyl groups.

6. The green electrocoupling closed-loop steam-cascaded circulating aluminum hydroxide calcination system according to claim 1, characterized in that, The closed-loop steam compression cycle unit includes a steam main pipe, a steam compressor, and a steam heater; The steam collection main pipe is connected to the steam outlets of the raw material pretreatment unit, the first-stage low-temperature pure electric roasting unit, the second-stage medium-temperature electric-steam coupled roasting unit, and the third-stage high-temperature steam main heating roasting unit, respectively, for collecting free water vapor, decomposing steam, and entraining steam. The steam compressor's air inlet is connected to the steam manifold to pressurize the incoming steam, and the steam compressor is equipped with a steam-water separator. The steam compressor's outlet is divided into two paths: one path connects to the secondary medium-temperature electric-steam coupled roasting unit and the tertiary high-temperature steam main heating roasting unit via a steam heater, and the other path connects to the Bayer process leaching process.

7. The green electrocoupling closed-loop steam-cascaded circulating aluminum hydroxide calcination system according to claim 1, characterized in that, The alumina cooling waste heat recovery unit includes a solid-solid waste heat boiler, a water supply system, and a finished alumina discharge unit. The feed end of the solid-solid waste heat boiler is connected to the solid discharge end of the three-stage high-temperature steam main heating and roasting unit to receive the high-temperature alumina output from the three-stage high-temperature steam main heating and roasting unit. The waste heat outlet pipeline of the solid-solid waste heat boiler is connected to the raw material pretreatment unit. The water supply system is connected to the solid-solid waste heat boiler and is used to introduce softened water into the solid-solid waste heat boiler, so that the softened water is vaporized into high-temperature and high-pressure steam by the waste heat released by high-temperature alumina. The finished alumina discharge unit is connected to the solid phase discharge end of the solid-solid waste heat boiler.

8. A green electro-coupled closed-loop steam-cascaded circulating aluminum hydroxide calcination method, implemented using the green electro-coupled closed-loop steam-cascaded circulating aluminum hydroxide calcination system as described in any one of claims 1-7, characterized in that, Includes the following steps: S1. The aluminum hydroxide raw material is fed into the dryer for preheating and then separated into gas and solid by a first-stage gas-solid separator to obtain preheated aluminum hydroxide. The separated free water vapor is then collected into the steam collection manifold. S2. The preheated aluminum hydroxide is fed into the first-stage low-temperature pure electric roasting unit for low-temperature pure electric roasting to obtain the first-stage roasting material. The decomposition steam generated is then collected into the steam collection manifold. S3. The primary roasted material is fed into the secondary medium-temperature electric-steam coupled roasting unit. The primary roasted material is deeply dehydrated by electric heating and circulating steam to obtain the secondary roasted material. The generated water vapor and entrained steam are collected into the steam collection manifold. S4. The secondary roasted material is fed into the tertiary high-temperature steam main heating roasting unit, where it is mainly heated by circulating steam and assisted by electric heating, so that the secondary roasted material can complete the crystal transformation and remove residual hydroxyl groups to obtain high-temperature alumina. S5. High-temperature alumina is fed into the alumina cooling waste heat recovery unit and softened water is introduced to cool the high-temperature alumina into finished alumina after releasing heat. At the same time, the softened water is vaporized to form high-temperature and high-pressure steam. At least part of the high-temperature and high-pressure steam is returned to the dryer. S6. After the water vapor collected in the steam collection main pipe is pressurized, purified and heated, a portion is returned to the secondary medium-temperature electric-steam coupled roasting unit and the tertiary high-temperature steam main heating roasting unit for circulating heating.

9. The green electrocoupling closed-loop steam-cascaded aluminum hydroxide roasting method according to claim 8, characterized in that, In S5, a portion of the high-temperature, high-pressure steam is returned to the dryer, while the other portion is sent to the Bayer process leaching step. In S6, the water vapor collected in the steam collection manifold is pressurized by the steam compressor and then divided into two paths. One path is heated by the steam heater and returned to the secondary medium-temperature electric-steam coupled roasting unit and the tertiary high-temperature steam main heating roasting unit. The other path is used as surplus high-pressure steam and transported to the Bayer process leaching process.

10. The green electrocoupling closed-loop steam-cascaded aluminum hydroxide roasting method according to claim 8, characterized in that, In step S1, the aluminum hydroxide raw material is preheated to 100°C to 280°C; In S2, the operating temperature of the primary low-temperature pure electric calcination unit is 280℃~400℃, and the furnace pressure is maintained at a slight positive pressure of 100Pa~300Pa. In S3, the operating temperature of the secondary medium-temperature electric-steam coupled calcination unit is 500℃~760℃; In S4, the operating temperature of the three-stage high-temperature steam main heating calcination unit is 760℃~1000℃; In step S5, the high-temperature alumina is cooled to below 80°C and then output as finished alumina, with a loss on ignition (LOI) of less than 0.8%. In step S6, the water vapor collected in the steam collection manifold is pressurized to 0.8 MPa to 1.0 MPa, and the volume ratio of water vapor in the gas phase inside the system is not less than 95%.

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