A process system for preparing pseudo-boehmite by subcritical hydrothermal method of secondary aluminum ash

Abstract

The present application belongs to the technical field of solid waste resource utilization and catalyst carrier preparation, and particularly relates to a process system for preparing pseudoboehmite by a subcritical hydrothermal method of secondary aluminum ash, which comprises a raw material pretreatment unit, a homogenization and preparation unit, a first subcritical hydrothermal unit, a gas-solid-liquid separation unit, a liquid phase impurity removal and purification unit, a precipitation unit, and a second subcritical hydrothermal unit; the first subcritical hydrothermal unit is used for efficiently dissolving aluminum components in the secondary aluminum ash, and the generated tail gas is treated in the gas phase to recover ammonium sulfate products and combustible gas; the dissolved solution is purified in the precipitation unit to form a pseudoboehmite precursor, and the pseudoboehmite product is obtained through the second subcritical hydrothermal crystallization; and the industrial CO2 capture and purification unit provides a carbon dioxide precipitant for the precipitation process, so that the carbon resources are used in a coordinated manner. The present application realizes the high-value utilization of aluminum resources in the secondary aluminum ash, and solves the problems of high energy consumption, environmental pollution, and difficult crystal phase regulation in the existing pseudoboehmite preparation.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization and catalyst carrier preparation technology, specifically involving a process system for preparing pseudoboehmite by a secondary alumina ash subcritical hydrothermal method. Background Technology

[0002] Secondary aluminum ash is a solid waste generated during the recycling of primary aluminum ash and aluminum refining processes. It contains metallic aluminum, alumina, and aluminum nitride, and has significant resource recovery potential. However, untreated secondary aluminum ash readily reacts with moisture in the air to release ammonia, causing air pollution; fluorides and soluble salts easily seep into soil and groundwater, seriously endangering the ecological environment and human health. Given its environmental risks, secondary aluminum ash has been explicitly classified as hazardous waste, and its harmless disposal has become a major environmental issue that the aluminum industry urgently needs to address. Pseudo-boehmite (PB) is a transitional aluminum oxide with high specific surface area and large pore volume. After calcination, it generates γ-Al₂O₃ and is widely used as a catalyst support for heavy oil hydrogenation and methanation. With the continuous growth in demand for high-performance catalyst supports from the refining, chemical, and coal chemical industries, the market gap for high-quality pseudo-boehmite is widening. If secondary aluminum ash can be used as an aluminum source to prepare boehmite, it can not only realize the resource utilization of hazardous waste, but also open up a new way for the green and low-cost production of boehmite, which has both environmental and economic value.

[0003] Previous studies have attempted to prepare boehmite from secondary alumina ash to achieve the resource utilization of secondary alumina ash. However, the current process system for preparing boehmite from secondary alumina ash includes a high-temperature roasting unit, an alkaline leaching unit, and an atmospheric pressure hydrothermal reaction unit. The high-temperature roasting unit is used to remove aluminum nitride and fix fluorides, the alkaline leaching unit is used to convert aluminum components into sodium aluminate and introduce it into the liquid phase, and the atmospheric pressure hydrothermal reaction unit is used to prepare the precursor into boehmite. Among these, the high-temperature roasting unit has high energy consumption, and secondary alumina ash usually contains 10% to 40% aluminum nitride (AlN). Roasting may also be accompanied by the emission of ammonia and dust, causing environmental pollution problems. The alkaline leaching unit has poor aluminum leaching effect. In order to improve the leaching effect, excessive alkali is often required, which not only increases the raw material cost but also increases the burden of subsequent waste liquid treatment, making the current process system unsuitable for large-scale industrial production applications. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a process system for preparing boehmite using a subcritical hydrothermal method from secondary alumina ash. The system employs subcritical water as the primary reaction medium and comprises a primary subcritical hydrothermal unit and a secondary subcritical hydrothermal unit. The primary subcritical hydrothermal unit achieves efficient dissolution of aluminum components. The generated tail gas is treated in the gas phase to recover ammonium sulfate and combustible gases, eliminating the need for traditional high-temperature calcination, thus reducing energy consumption and avoiding environmental pollution caused by ammonia and other gas emissions. After purification, the dissolved solution forms a boehmite precursor in the precipitation unit. The secondary subcritical hydrothermal unit crystallizes the precursor under subcritical hydrothermal conditions to obtain a boehmite product with high crystallinity and controllable crystal phase. An industrial CO2 capture and purification unit provides carbon dioxide precipitant for the precipitation process, achieving synergistic utilization of carbon resources. The process system provided by this invention not only achieves high-value utilization of aluminum resources from secondary alumina ash but also avoids the high energy consumption, environmental pollution, and large alkali consumption problems associated with traditional boehmite preparation, making it suitable for large-scale industrial production applications.

[0005] This invention is specifically achieved through the following technical solutions: This invention provides a process system for preparing pseudoboehmite by a secondary subcritical hydrothermal method of aluminum ash, characterized in that it includes a raw material pretreatment unit, a homogenization and mixing unit, a primary subcritical hydrothermal unit, a gas-solid-liquid separation unit, a liquid phase impurity removal and purification unit, a precipitation unit, a secondary subcritical hydrothermal unit, and a post-treatment unit connected in sequence. The raw material pretreatment unit is used to crush, screen and remove iron from the secondary aluminum ash. The homogenization mixing unit is used to mix the pretreated aluminum ash with alkaline solution to form a slurry. The first-stage subcritical hydrothermal unit is used to convert the aluminum component in the slurry into sodium aluminate under subcritical hydrothermal conditions, forming an alkaline leaching mixture. The gas-solid-liquid separation unit is used to separate the gas phase, liquid phase and solid phase in the alkaline leaching mixture system. The liquid phase outlet of the gas-solid-liquid separation unit is connected to the liquid phase impurity removal and purification unit. The solid phase outlet is connected to the residue collection unit. The gas phase outlet is connected to the gas phase treatment unit. The gas phase treatment unit is used to absorb ammonia and produce ammonium sulfate products and recover combustible gases. The liquid phase purification unit is used to remove silicon and heavy metal impurities from sodium aluminate solution; The precipitation unit is used to convert the purified sodium aluminate into amorphous aluminum hydroxide gel under the action of a precipitant to form a precursor slurry. The precipitant includes industrial CO2 that has been captured and purified. The secondary subcritical hydrothermal unit is used to crystallize the precursor in the precursor slurry into pseudoboehmite. The post-processing unit is used to filter, wash and dry the pseudoboehmite crude product system. The filtered liquid phase is then transported to the homogenization and blending unit to form a slurry.

[0006] Preferably, the gas phase processing unit includes a condensation unit and an ammonia absorption unit connected in sequence, and the inlet of the condensation unit is connected to the gas phase outlet of the gas-solid-liquid separation unit. The condensation unit is used to remove water vapor from the exhaust gas. The ammonia absorption unit is filled with dilute sulfuric acid absorbent to absorb ammonia and generate ammonium sulfate. The liquid phase outlet of the ammonia absorption unit is connected to the ammonium sulfate recovery unit, and the gas phase outlet is connected to the combustible gas recovery unit. The ammonium sulfate recovery unit includes an evaporator crystallizer. The combustible gas recovery unit is used to dry the collected hydrogen.

[0007] Preferably, the first-stage subcritical hydrothermal unit is equipped with a first heat exchange component and a first pressurization component, which are used to adjust the reaction temperature to 120℃~160℃ and the pressure to 0.2MPa~0.7MPa.

[0008] Preferably, the secondary subcritical hydrothermal unit is equipped with a second heat exchange component and a second pressurization component to adjust the crystallization temperature to 120℃~160℃; the seed inlet of the secondary subcritical hydrothermal unit is connected to a seed supply unit for adding seed crystals or structure guiding agents.

[0009] Preferably, the precipitation unit is equipped with a stirring component and a pH online monitoring component. The precipitant inlet of the precipitation unit is connected to a precipitant supply unit, and the precipitant provided by the precipitant supply unit is one or more of sodium bicarbonate or carbon dioxide. The precipitant supply unit is also connected to an industrial CO2 capture and purification unit, which is used to capture and purify carbon dioxide from industrial waste gas.

[0010] Preferably, the liquid phase purification unit includes a first filtration unit, a desilication unit, and a heavy metal removal unit connected in sequence. The inlet of the first filtration unit is connected to the liquid phase outlet of the gas-solid-liquid separation unit. The desilication unit has a desilication agent inlet connected to a desilication agent supply unit, which is used to add desilication agent calcium oxide. The heavy metal removal unit is used to adjust the pH of the system to 8-10, so that heavy metal ions precipitate out in the form of hydroxides.

[0011] Preferably, the homogenization and mixing unit is equipped with a stirring assembly, and the alkali source inlet of the homogenization and mixing unit is connected to an alkali supply unit, which is used to provide sodium hydroxide solution.

[0012] Preferably, the post-processing unit includes a second filtration unit, a washing unit, and a drying unit connected in sequence. The inlet of the second filtration unit is connected to the outlet of the secondary subcritical hydrothermal unit, the liquid phase outlet of the filtration unit is connected to the mother liquor recovery unit, and the outlet of the mother liquor recovery unit is connected to the alkali supply unit.

[0013] The second filtration unit is used to separate the crystallization mother liquor, the washing unit is used to wash with deionized water, and the outlet of the drying unit is connected to the storage silo.

[0014] Preferably, the raw material pretreatment unit includes a crushing and screening unit and an iron removal unit; the outlet of the crushing and screening unit is connected to the inlet of the iron removal unit to control the particle size of the secondary aluminum ash and remove metal impurities; the discharge port of the iron removal unit is connected to the feed port of the homogenization and blending unit.

[0015] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention uses subcritical water as the main reaction medium. The process system includes a primary subcritical hydrothermal unit and a secondary subcritical hydrothermal unit. A homogenization and mixing unit premixes secondary aluminum ash with alkali solution to form a slurry. The primary subcritical hydrothermal unit uses subcritical water as the medium to achieve efficient dissolution of aluminum components and hydrolytic conversion of aluminum nitride under subcritical hydrothermal conditions. This avoids the problem of relying on excessive alkali to increase aluminum dissolution rate in traditional processes. Furthermore, it eliminates the need for high-temperature calcination pretreatment, and the generated gases such as ammonia are recovered and reused through a gas-phase treatment unit, reducing energy consumption and avoiding environmental pollution. The alkali-leached mixture after the primary hydrothermal reaction undergoes separation in a gas-solid-liquid separation unit, impurity removal in a liquid-phase purification unit, and precipitation to prepare a precursor slurry. After hydrothermal crystallization in the secondary subcritical hydrothermal unit, a pseudoboehmite product can be obtained.

[0016] 2. This invention employs a two-stage subcritical hydrothermal process system. The first-stage subcritical hydrothermal unit completes the efficient dissolution of aluminum components, which helps reduce the energy consumption of the alkaline leaching process. The second-stage subcritical hydrothermal unit achieves effective control of the pseudoboehmite crystal phase, crystal morphology, and pore structure by independently adjusting the crystallization temperature and crystallization time, and optionally adding seed crystals or structure guiding agents, thus meeting the requirements of the catalyst support for material performance.

[0017] 3. This invention removes silicon and heavy metals through a liquid-phase purification unit, effectively removing silicon and heavy metal impurities from sodium aluminate solution and reducing the impurity content of the product. The precipitation unit uses sodium bicarbonate or carbon dioxide as a precipitant, avoiding the introduction of new impurity ions and facilitating the acquisition of high-purity boehmite precursors. Simultaneously, the precipitant supply unit is connected to an industrial CO2 capture and purification unit, realizing the resource utilization of carbon dioxide and improving the greenness of the process.

[0018] 4. This invention collects the alkaline leaching residue to the residue collection unit through the gas-solid-liquid separation unit, realizing the centralized disposal and resource utilization of the residue; the gas phase treatment unit absorbs ammonia in the tail gas and converts it into ammonium sulfate product, and the combustible gas is recovered and dried for subsequent resource utilization; the mother liquor recovery unit reuses the crystallized mother liquor in the homogenization and blending unit for alkaline solution preparation, realizing the recycling of alkaline solution.

[0019] The system of this invention forms a complete process system integrating aluminum resource recovery, impurity removal, tail gas resource utilization and mother liquor recycling. It realizes the coordinated recovery and high-value utilization of aluminum, nitrogen and alkaline resources in secondary aluminum ash. The process is controllable and stable in operation, and has good prospects for industrial application. Attached Figure Description

[0020] Figure 1 The flowchart of the process system for preparing pseudoboehmite by the secondary alumina subcritical hydrothermal method provided by the present invention is shown.

[0021] Explanation of reference numerals in the attached figures: 1. Raw material pretreatment unit; 1a. Crushing and screening unit; 1b. Iron removal unit; 2. Homogenization and blending unit; 2a. Alkali supply unit; 3. First-stage subcritical hydrothermal unit; 4. Gas-solid-liquid separation unit; 4a. Residue collection unit; 5. Liquid phase impurity removal and purification unit; 5a. First filtration unit; 5b. Desilication unit; 5c. Heavy metal removal unit; 5d. Desilication agent supply unit; 6. Precipitation unit; 6a. Precipitant supply unit; 7. Second-stage subcritical hydrothermal unit; 7a. Seed crystal supply unit; 8. Post-treatment unit; 8a. Second filtration unit; 8b. Washing unit; 8c. Drying unit; 9. Storage silo; 10. Gas phase treatment unit; 10a. Condensation unit; 10b. Ammonia absorption unit; 11. Ammonium sulfate recovery unit; 12. Combustible gas recovery unit; 13. Mother liquor recovery unit; 14. Industrial CO2 capture and purification unit. Detailed Implementation

[0022] To enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below with reference to specific embodiments and accompanying drawings. However, the embodiments described are not intended to limit the present invention. Unless otherwise specified, the experimental methods and detection methods described in the following embodiments are conventional methods; unless otherwise specified, the reagents and materials described are commercially available.

[0023] Currently, the main methods for recycling secondary aluminum ash fall into two categories. One is to use secondary aluminum ash as a raw material for producing basic materials such as refractory materials. The other is to use alkaline or acidic methods to separate aluminum elements from other impurities to generate aluminum compounds. However, these methods generally suffer from low added value and small production scale, making it difficult to achieve high-value recycling of aluminum resources. Traditional processes for preparing boehmite from aluminum ash typically include a high-temperature roasting unit, an alkaline leaching unit, and an atmospheric pressure hydrothermal reaction unit. The high-temperature roasting unit is used to remove aluminum nitride and fix fluorides, the alkaline leaching unit is used to convert aluminum components into sodium aluminate which enters the liquid phase, and the atmospheric pressure hydrothermal reaction unit is used to prepare boehmite from the precursor. Secondary aluminum ash typically contains 10%–40% aluminum nitride (AlN). Conventional roasting treatment is not only energy-intensive but may also result in ammonia and dust emissions, increasing the burden of exhaust gas treatment and potentially causing environmental pollution. The alkaline leaching unit has poor aluminum leaching performance; to improve leaching efficiency, excessive alkali is often required, increasing raw material costs and the burden of subsequent wastewater treatment. Existing technologies employ pretreatment methods such as water washing desalination and acid leaching, but these methods struggle to simultaneously achieve efficient aluminum leaching and complete removal of nitrides at low temperatures, limiting the purity of subsequent products. Therefore, the current process system is not suitable for large-scale industrial production.

[0024] In this invention, relying on a subcritical hydrothermal system, the efficient leaching of aluminum components and the hydrolytic conversion of nitrides are simultaneously achieved under alkaline and subcritical hydrothermal conditions through a first-stage subcritical hydrothermal unit. This eliminates the need for high-temperature calcination pretreatment, reducing energy consumption. Subcritical water refers to water heated above its boiling point but below its critical point, while maintaining it in a liquid state. Aluminum components can be efficiently converted into sodium aluminate and enter the liquid phase at a relatively low temperature (120°C to 160°C) and under autogenous pressure. This reduces the alkaline leaching temperature compared to traditional processes and avoids the problem of relying on excessive alkali to increase aluminum leaching rates.

[0025] Furthermore, the liquid-phase purification unit incorporates desilication and heavy metal removal steps to deeply remove silicon impurities from the sodium aluminate solution, effectively reducing the impurity content in the final product. The desilication unit, after adding calcium oxide, reacts at 80℃~85℃ to efficiently remove silicon; the heavy metal removal unit deeply removes silicon and heavy metal impurities from the sodium aluminate solution by adjusting the pH.

[0026] Furthermore, this invention employs a two-stage subcritical hydrothermal process. The first-stage subcritical hydrothermal unit efficiently dissolves the aluminum component, while the second-stage subcritical hydrothermal unit, through independent control of crystallization temperature and time, and the optional addition of seed crystals, effectively controls the crystalline phase and pore structure of boehmite, resulting in a product with high crystallinity and a single crystalline phase. Simultaneously, the precipitation unit uses sodium bicarbonate or carbon dioxide as a precipitant to avoid introducing new impurity ions, which is beneficial for obtaining a high-purity boehmite precursor. After filtration, washing, and drying in the post-treatment unit, the resulting product meets the material performance requirements of the catalyst support.

[0027] In summary, the process system of this invention not only solves the defects of high-temperature calcination, gas pollution, low product purity, and poor controllability of crystal phase in the prior art, but also achieves high-value utilization of secondary aluminum ash and efficient preparation of pseudoboehmite through the synergistic effect of two-stage subcritical hydrothermal treatment and impurity removal purification, as well as the optimization of precipitant selection, and has good prospects for industrial application.

[0028] The process system of the present invention will be described in detail below.

[0029] See Figure 1 A process system for preparing pseudoboehmite using a secondary subcritical hydrothermal method for aluminum ash includes a raw material pretreatment unit 1, a homogenization and blending unit 2, a primary subcritical hydrothermal unit 3, a gas-solid-liquid separation unit 4, a liquid phase impurity removal and purification unit 5, a precipitation unit 6, a secondary subcritical hydrothermal unit 7, a post-treatment unit 8, a storage silo 9, a gas phase treatment unit 10, an ammonium sulfate recovery unit 11, a combustible gas recovery unit 12, a mother liquor recovery unit 13, and an industrial CO2 capture and purification unit 14. Each unit is connected sequentially through pipelines to form a complete process link.

[0030] The raw material pretreatment unit 1 includes a crushing and screening unit 1a and an iron removal unit 1b, used for crushing, screening, and iron removal of raw materials; the homogenization and blending unit 2 is equipped with an alkaline solution supply unit 2a, used to mix the pretreated aluminum ash with alkaline solution to form a slurry; the primary subcritical hydrothermal unit 3 is used for subcritical hydrothermal alkaline leaching reaction; the gas-solid-liquid separation unit 4 is equipped with a residue collection unit 4a, used to collect inert residue; the liquid phase impurity removal and purification unit 5 includes a first filtration unit 5a, a desilication unit 5b, and a heavy metal removal unit 5c connected in sequence, with the desilication unit 5b connected to a desilication agent supply unit 5d; the precipitation unit 6 is equipped with... The unit includes a precipitant supply unit 6a; a secondary subcritical hydrothermal unit 7 equipped with a seed crystal supply unit 7a; a post-treatment unit 8 comprising a second filtration unit 8a, a washing unit 8b, and a drying unit 8c connected in sequence; a gas phase treatment unit 10 comprising a condensation unit 10a and an ammonia absorption unit 10b connected in sequence; an ammonium sulfate recovery unit 11 connected to the liquid phase outlet of the ammonia absorption unit 10b; a combustible gas recovery unit 12 connected to the gas phase outlet of the ammonia absorption unit 10b; a mother liquor recovery unit 13 connected to the liquid phase outlet of the second filtration unit 8a; and an industrial CO2 capture and purification unit 14 connected to the precipitant supply unit 6a.

[0031] The feed inlet of raw material pretreatment unit 1 is connected to the secondary aluminum ash silo for receiving secondary aluminum ash raw materials. The raw material pretreatment unit 1 includes a crushing and screening unit 1a and an iron removal unit 1b. The outlet of crushing and screening unit 1a is connected to the inlet of iron removal unit 1b. The mechanical action of crushing and screening unit 1a refines the particle size of the secondary aluminum ash to 80-120 mesh, and then the iron removal unit 1b removes magnetic metal impurities through magnetic separation. The crushing and screening unit 1a refines the particle size of the raw material, significantly increasing its specific surface area and reactive sites, promoting sufficient contact between the aluminum components and the alkaline solution during subsequent homogenization and alkaline leaching. However, it is also necessary to avoid problems such as a surge in crushing energy consumption and particle agglomeration due to excessively small particle size, or insufficient reaction and decreased aluminum leaching rate due to excessively large particle size.

[0032] The solid discharge port of the raw material pretreatment unit 1 is connected to the homogenization and mixing unit 2, which is used to transport the treated secondary aluminum ash to the homogenization and mixing unit 2. The alkali source inlet of the homogenization and mixing unit 2 is connected to the alkali supply unit 2a, which is used to provide sodium hydroxide solution. The homogenization and mixing unit 2 receives the pretreated secondary aluminum ash and simultaneously adds sodium hydroxide solution through the alkali supply unit 2a, and the mixture is mixed by the built-in stirring assembly to form a slurry.

[0033] The outlet of the homogenization and mixing unit 2 is connected to the primary subcritical hydrothermal unit 3. The primary subcritical hydrothermal unit 3 is equipped with a first heat exchange component and a first pressurization component. It receives the slurry from the homogenization and mixing unit 2 and, using subcritical water as the medium, converts the aluminum components in the secondary aluminum ash into sodium aluminate under subcritical hydrothermal conditions (reaction temperature 120℃~160℃, autogenous pressure 0.2MPa~0.7MPa), forming an alkaline leaching mixture. Below 120℃, the subcritical strengthening effect is not significant, the aluminum dissolution rate is slow, and the aluminum nitride hydrolysis is incomplete. Above 160℃, the equipment pressure resistance needs to be increased, increasing equipment costs, and the hydrogen and ammonia generation rates are too fast, increasing the tail gas treatment load.

[0034] The outlet of the primary subcritical hydrothermal unit 3 is connected to the gas-solid-liquid separation unit 4. The gas-solid-liquid separation unit 4 is used to separate the gas phase, liquid phase, and solid phase in the alkaline leaching mixture. The liquid phase outlet of the gas-solid-liquid separation unit 4 is connected to the liquid phase purification unit 5, and the solid phase outlet is connected to the residue collection unit 4a, which is used to collect inert residues (mainly composed of SiO2, Fe2O3, and calcium fluoride, etc.), which can be subsequently used in the building materials field or for safe landfill; the gas phase outlet is connected to the gas phase treatment unit 10.

[0035] The gas phase treatment unit 10 includes a condensation unit 10a and an ammonia absorption unit 10b connected in sequence. The condensation unit 10a is used to remove water vapor from the exhaust gas; the ammonia absorption unit 10b contains dilute sulfuric acid absorbent liquid, which is used to absorb ammonia to generate ammonium sulfate solution. The liquid phase outlet of the ammonia absorption unit 10b is connected to the ammonium sulfate recovery unit 11, which includes an evaporator crystallizer for evaporating and crystallizing the ammonium sulfate solution to obtain ammonium sulfate product; the gas phase outlet of the ammonia absorption unit 10b is connected to the combustible gas recovery unit 12, which is used to collect hydrogen and dry it.

[0036] The liquid-phase purification unit 5 includes a first filtration unit 5a, a desilication unit 5b, and a heavy metal removal unit 5c connected in sequence. The first filtration unit 5a removes suspended solids from the alkaline leaching mixture; the desilication unit 5b is connected to a desilication agent supply unit 5d, used to add calcium oxide to remove silicon impurities from the sodium aluminate solution by generating hydrated calcium silicate precipitate; the heavy metal removal unit 5c adjusts the pH to 8-10 to remove iron ions and heavy metal ions (Cu). 2+ Pb 2+ Zn 2+ (etc.) are converted into hydroxide precipitates, which are then removed by filtration.

[0037] The purified liquid outlet of the heavy metal removal unit 5c is connected to the precipitation unit 6. The precipitant inlet of the precipitation unit 6 is connected to a precipitant supply unit 6a, which provides sodium bicarbonate solution or carbon dioxide gas. The precipitant supply unit 6a is also connected to an industrial CO2 capture and purification unit 14, which provides carbon dioxide as a precipitant or auxiliary precipitant. The industrial CO2 capture and purification unit 14 uses the alkanolamine method for absorption. The precipitation unit 6 receives the purified sodium aluminate solution, adds precipitant through the precipitant supply unit 6a, and adjusts the pH to 8.5-10.5, converting the sodium aluminate into amorphous aluminum hydroxide gel to form a precursor slurry. The precipitation unit 6 is equipped with a stirring assembly and an online pH monitoring assembly.

[0038] The outlet of precipitation unit 6 is connected to secondary subcritical hydrothermal unit 7. The inlet of secondary subcritical hydrothermal unit 7 is connected to the outlet of precipitation unit 6, and the seed inlet is connected to seed supply unit 7a for adding seed crystals or structure guiding agents. The seed crystals in seed supply unit 7a are preferably boehmite seed crystals, and the structure guiding agent is preferably one or more of polyethylene glycol (PEG), P123, and CTAB. Secondary subcritical hydrothermal unit 7 is equipped with a second heat exchange component and a second pressurization component. Using subcritical water as the medium, under subcritical hydrothermal conditions (crystallization temperature 120℃~160℃), the amorphous aluminum hydroxide gel precursor is crystallized into boehmite, forming a boehmite mixture. Amorphous aluminum hydroxide gel is a metastable phase and thermodynamically unstable. Under secondary subcritical hydrothermal conditions, part of the gel dissolves and re-enters the liquid phase. The dissolved aluminum species undergo a directional dissolution-reprecipitation reaction on the seed crystal surface in the system, crystallizing to form structurally ordered boehmite.

[0039] The outlet of the secondary subcritical hydrothermal unit 7 is connected to the post-processing unit 8. The post-processing unit 8 includes a second filtration unit 8a, a washing unit 8b, and a drying unit 8c connected in sequence. The second filtration unit 8a separates the crystallization mother liquor, and its liquid phase outlet is connected to the mother liquor recovery unit 13. The outlet of the mother liquor recovery unit 13 is connected to the alkali supply unit 2a, used to reuse the crystallization mother liquor for alkali preparation. In a preferred embodiment, the mother liquor recovery unit 13 controls the number of mother liquor cycles to 1-3 times. Within 3 cycles, the unreacted alkali source and silicon-aluminum components in the mother liquor can still meet the feed requirements. When the number of cycles reaches a set value, a portion of the mother liquor is discharged for purification. The washing unit 8b is used to wash the product with deionized water; the drying unit 8c is used to dry the product. The outlet of the drying unit 8c is connected to the storage silo 9 for storing the pseudoboehmite product.

[0040] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, it is intended to include any modifications and variations that fall within the scope of the claims and their equivalents.

Claims

1. A process system for preparing pseudoboehmite using a secondary alumina subcritical hydrothermal method, characterized in that, It includes a raw material pretreatment unit (1), a homogenization and blending unit (2), a primary subcritical hydrothermal unit (3), a gas-solid-liquid separation unit (4), a liquid phase impurity removal and purification unit (5), a precipitation unit (6), a secondary subcritical hydrothermal unit (7), and a post-treatment unit (8) connected in sequence. The raw material pretreatment unit (1) is used to crush, screen and remove iron from secondary aluminum ash; The homogenization mixing unit (2) is used to mix the pretreated aluminum ash with alkaline solution to form a slurry; The first-stage subcritical hydrothermal unit (3) is used to convert the aluminum component in the slurry into sodium aluminate under subcritical hydrothermal conditions to form an alkaline leaching mixture. The gas-solid-liquid separation unit (4) is used to separate the gas phase, liquid phase and solid phase in the alkaline leaching mixture system. The liquid phase outlet of the gas-solid-liquid separation unit (4) is connected to the liquid phase impurity removal and purification unit (5). The solid phase outlet is connected to the residue collection unit (4a). The gas phase outlet is connected to the gas phase treatment unit (10). The gas phase treatment unit (10) is used to absorb ammonia and produce ammonium sulfate products and recover combustible gases. The liquid phase purification unit (5) is used to remove silicon and heavy metal impurities from sodium aluminate solution; The precipitation unit (6) is used to convert the purified sodium aluminate into amorphous aluminum hydroxide gel under the action of a precipitant to form a precursor slurry. The precipitant includes industrial CO2 that has been captured and purified. The secondary subcritical hydrothermal unit (7) is used to crystallize the precursor in the precursor slurry into pseudoboehmite; The post-processing unit (8) is used to filter, wash and dry the pseudoboehmite crude product system, and the filtered liquid phase is transported to the homogenization and blending unit (2) to form a slurry.

2. The process system for preparing pseudoboehmite using a secondary alumina subcritical hydrothermal method according to claim 1, characterized in that, The gas phase processing unit (10) includes a condensation unit (10a) and an ammonia absorption unit (10b) connected in sequence. The inlet of the condensation unit (10a) is connected to the gas phase outlet of the gas-solid-liquid separation unit (4). The condensation unit (10a) is used to remove water vapor from the tail gas. The ammonia absorption unit (10b) is filled with dilute sulfuric acid absorbent liquid to absorb ammonia and generate ammonium sulfate. The liquid phase outlet of the ammonia absorption unit (10b) is connected to the ammonium sulfate recovery unit (11), and the gas phase outlet is connected to the combustible gas recovery unit (12). The ammonium sulfate recovery unit (11) includes an evaporator crystallizer. The combustible gas recovery unit (12) is used to dry the collected hydrogen.

3. The process system for preparing pseudoboehmite using a secondary alumina subcritical hydrothermal method according to claim 1, characterized in that, The first-stage subcritical hydrothermal unit (3) is equipped with a first heat exchange component and a first pressurization component, which are used to adjust the reaction temperature to 120℃~160℃ and the self-generated pressure to 0.2MPa~0.7MPa.

4. The process system for preparing pseudoboehmite by secondary alumina subcritical hydrothermal method according to claim 1, characterized in that, The secondary subcritical hydrothermal unit (7) is equipped with a second heat exchange component and a second pressurization component, which are used to adjust the crystallization temperature to 120℃~160℃; the seed inlet of the secondary subcritical hydrothermal unit (7) is connected to a seed supply unit (7a), which is used to add seed crystals or structure guiding agents.

5. The process system for preparing pseudoboehmite by secondary alumina subcritical hydrothermal method according to claim 1, characterized in that, The precipitation unit (6) is equipped with a stirring assembly and a pH online monitoring assembly. The precipitant inlet of the precipitation unit (6) is connected to a precipitant supply unit (6a). The precipitant provided by the precipitant supply unit (6a) is one or more of sodium bicarbonate or carbon dioxide. The precipitant supply unit (6a) is also connected to an industrial CO2 capture and purification unit (14). The industrial CO2 capture and purification unit (14) is used to capture and purify carbon dioxide from industrial waste gas.

6. The process system for preparing pseudoboehmite by secondary alumina subcritical hydrothermal method according to claim 1, characterized in that, The liquid phase purification unit (5) includes a first filtration unit (5a), a desiliconization unit (5b), and a heavy metal removal unit (5c) connected in sequence. The inlet of the first filtration unit (5a) is connected to the liquid phase outlet of the gas-solid-liquid separation unit (4). The desilication inlet of the desilication unit (5b) is connected to the desilication supply unit (5d), which is used to add the desilication agent calcium oxide. The heavy metal removal unit (5c) is used to adjust the pH of the system to 8-10, so that heavy metal ions are precipitated out in the form of hydroxides.

7. The process system for preparing pseudoboehmite by secondary alumina subcritical hydrothermal method according to claim 1, characterized in that, The homogenization unit (2) is equipped with a stirring assembly; the alkali source inlet of the homogenization unit (2) is connected to an alkali supply unit (2a), which is used to provide sodium hydroxide solution.

8. The process system for preparing pseudoboehmite by secondary alumina subcritical hydrothermal method according to claim 7, characterized in that, The post-processing unit (8) includes a second filtration unit (8a), a washing unit (8b), and a drying unit (8c) connected in sequence. The inlet of the second filtration unit (8a) is connected to the outlet of the secondary subcritical hydrothermal unit (7), the liquid phase outlet of the second filtration unit (8a) is connected to the mother liquor recovery unit (13), and the outlet of the mother liquor recovery unit (13) is connected to the alkali supply unit (2a).

9. The process system for preparing pseudoboehmite by secondary alumina subcritical hydrothermal method according to claim 8, characterized in that, The second filtration unit (8a) is used to separate the crystallization mother liquor, the washing unit (8b) is used to wash with deionized water, and the outlet of the drying unit (8c) is connected to the storage bin (9).

10. The process system for preparing pseudoboehmite by secondary alumina subcritical hydrothermal method according to claim 1, characterized in that, The raw material pretreatment unit (1) includes a crushing and screening unit (1a) and an iron removal unit (1b); the outlet of the crushing and screening unit (1a) is connected to the inlet of the iron removal unit (1b) to control the particle size of the secondary aluminum ash and remove metal impurities; the outlet of the iron removal unit (1b) is connected to the inlet of the homogenization and blending unit (2).

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