Method and device for realizing carbon sequestration of red mud under supercritical conditions

Abstract

This invention discloses a method and apparatus for achieving red mud carbon fixation under supercritical conditions, relating to the field of red mud carbon fixation technology. The method includes a red mud pretreatment step, an ultrasonic cavitation crushing step, and a microchannel mixing reaction step. The apparatus includes a pretreatment unit, an ultrasonic crushing module, a microchannel mixing module, a gradient reaction tower, a temperature and pressure control system, a multi-stage reciprocating compressor, and an external circulation pump. The invention features an ultrasonic crushing zone at the top, a honeycomb-shaped microchannel mixing module integrated in the middle, and a gradient temperature reaction tower at the bottom, equipped with baffles to enhance mass transfer. After the carbon fixation reaction is completed, the slurry is returned to the reaction zone via a circulation pump. Through the synergistic effect of ultrasound and microchannels, the red mud particle size is reduced to 1-5 μm, achieving three-phase mixing with supercritical CO2 within 0.5 seconds. Simultaneously, a porous supported calcium-magnesium composite catalyst is used, utilizing the confinement effect and hydrophobicity of the support to increase the carbon fixation efficiency to over 95%, and solving the problem of silicate side reactions caused by excessive alkalinity.

Description

Technical Field

[0001] This invention relates to the field of red mud carbon fixation technology, specifically to a method and apparatus for achieving red mud carbon fixation under supercritical conditions. Background Technology

[0002] Red mud is an industrial solid waste generated during alumina production. It is characterized by its strong alkalinity, heavy metal content, high viscosity, and fine particle size. Long-term stockpiling not only occupies large amounts of land but also pollutes water and soil. Therefore, large quantities of red mud are a type of solid waste urgently needing treatment and resource utilization. As a massive industrial solid waste, traditional landfilling, stockpiling, or neutralization methods for red mud present problems such as high carbon emissions and waste of its potential resources. Supercritical CO2 technology reacts supercritical CO2 with the alkaline components in red mud to generate stable carbonates, achieving the goal of "carbon sequestration from waste" and combining carbon sequestration with the harmless utilization of red mud. Summary of the Invention

[0003] The main objective of this invention is to provide a method and apparatus for carbon fixation of red mud under supercritical conditions, in order to solve the problem of red mud resource utilization.

[0004] To achieve the above objectives, the present invention provides a method for carbon fixation of red mud under supercritical conditions, comprising the following steps: S1. After washing and drying the red mud to remove free water, the dried red mud is crushed to form dried red mud powder, which is then mixed with deionized water in a certain proportion. S2. Pressurize CO2 to supercritical pressure and supercritical temperature; S3. The slurry obtained in step S1 is ultrasonically crushed and cavitation crushed by an ultrasonic array to reduce the particle size of red mud. S4. The crushed slurry, the pressurized supercritical CO2 from step S2, and the porous supported catalyst are mixed in parallel in a honeycomb microchannel to complete three-phase mixing, with a mixing time ≤0.5 seconds. S5. The mixture enters the gradient reaction tower, where it undergoes a carbonization reaction under segmented temperature control before being cooled.

[0005] As a further improvement of the present invention, the drying temperature in step S1 is T1, 100℃≤T1<120℃, the dried red mud is pulverized to 150~200 mesh, and the pulverized red mud is mixed with deionized water at a liquid-solid ratio of 8~10:1.

[0006] As a further improvement of the present invention, the supercritical CO2 reaction pressure in step S2 is P1, 10MPa < P1 ≤ 25MPa, and the supercritical CO2 reaction temperature in step S2 is T2, 35℃ < T2 ≤ 80℃.

[0007] As a further improvement of the present invention, in step S3, the ultrasonic crushing uses a piezoelectric ceramic transducer with a frequency of 40 kHz and a dynamic amplitude adjustment range of 10-100%, so that the red mud particle size is reduced to 1-5 μm.

[0008] As a further improvement of the present invention, the amount of porous supported catalyst added in step S4 is 2 to 10 wt% of the mass of the dried red mud powder.

[0009] As a further improvement of the present invention, the porous supported catalyst in step S4 is prepared by the following steps: (1) The carrier was synthesized by sol-gel method using zinc acetate dihydrate, 2-methylimidazole and methanol; 20 g of zinc acetate dihydrate was weighed and dissolved in 400 mL of methanol to form solution A, and 30 g of 2-methylimidazole was weighed and dissolved in 400 mL of methanol to obtain solution B. Solution A and solution B were then rapidly mixed and stirred at 25℃ for 12 h. After standing for 24 h, the white precipitate was collected by centrifugation. The white precipitate was washed three times with methanol and dried in an oven at 80℃ for 12 h to obtain carrier powder. (2) Prepare an impregnation solution with a total molar mass of 1.5 mol / L by mixing calcium nitrate and magnesium nitrate at a molar ratio of Ca:Mg=2:1. Impregnate the carrier in the impregnation solution to obtain the impregnated carrier. (3) The impregnated support is placed under N2 atmosphere for gradient calcination. Calcination at 100-150℃ for 1-2 h dehydrates it. Calcination at 300-400℃ for 2-3 h decomposes nitrate ions to generate MgO / CaO nanoparticles. Calcination at 550-650℃ for 3-4 h forms a stable oxide in the impregnated support. Then, it is modified by hexamethyldisilazane gas-phase hydrophobic modification to obtain a porous supported catalyst with a water contact angle >130°, which means it has strong hydrophobicity.

[0010] As a further improvement of the present invention, the honeycomb microchannel in step S4 is a multi-layer microchannel hybrid module, which is composed of multiple titanium alloy microchannel plates stacked together, with each plate containing 800 to 4000 regular hexagonal channels with a side-to-side distance of 50 to 300 μm.

[0011] As a further improvement of the present invention, the mixture enters the gradient reaction tower from top to bottom. In step S5, the temperature of the upper low-temperature zone of the gradient reaction tower is 35-50°C, which generates carbonate crystal nuclei; the temperature of the middle medium-temperature zone is 50-65°C, which promotes crystal growth; and the temperature of the lower high-temperature zone is 65-80°C, which deeply carbonizes the mixture.

[0012] An apparatus for a method of achieving carbon fixation of red mud under supercritical conditions includes a red mud drying tower, a ball mill connected to the red mud drying tower, and a mixing outer cylinder located at the outlet of the ball mill. Inside the mixing outer cylinder, from top to bottom, are a first red mud annular distributor, a second red mud annular distributor, and a CO2 annular distributor. An ultrasonic transducer and a titanium alloy amplitude transformer are vertically arranged between the first and second red mud annular distributors. A multi-layer microchannel mixing module is located between the second red mud annular distributor and the CO2 annular distributor. A gradient reaction tower is located below the CO2 annular distributor, and a multi-stage reciprocating compressor is connected to the outside of the CO2 annular distributor. Multiple sets of baffles are spaced apart inside the gradient reaction tower, and a pressure relief valve is located on the outer wall of the gradient reaction tower. A catalyst feed pipe is located at the top of the multi-layer microchannel mixing module. A water inlet pipe and a red mud feed pipe connected to the ball mill are located on the first red mud annular distributor. A temperature and pressure control system is located outside the gradient reaction tower. A conical collector is located at the bottom of the gradient reaction tower, and a circulation pump is connected to the outside of the conical collector. The circulation pump is connected to the top of the gradient reaction tower.

[0013] As a further improvement of the present invention, the CO2 annular distributor is provided with nozzles arranged in a ring shape, the nozzle diameter being 0.1 to 0.5 mm, and the nozzles are positioned toward the second red mud annular distributor so that the CO2 flowing in the CO2 annular distributor flows from the bottom end of the multilayer microchannel mixing module toward the ultrasonic transducer.

[0014] The beneficial effects of this invention are reflected in: 1. This invention, under supercritical CO2 conditions, enables CO2 to maintain high gas diffusivity while achieving strong liquid dissolving ability. Furthermore, it possesses chemical inertness and mildness, allowing for full diffusion and contact with red mud. There, it reacts with pore water or alkaline components in the red mud to generate carbonate ions, dissolving alkaline metal oxides (such as CaO) in the red mud to form corresponding hydroxides and releasing soluble divalent metal ions (such as Mg²⁺). 2+ Ca 2+ Divalent metal ions react with carbonate ions to form carbonate precipitates. This process not only achieves the goal of carbon fixation in red mud but also reduces its alkalinity, preparing it for subsequent comprehensive utilization.

[0015] 2. This invention uses only a supercritical CO2 system, without using a combination of supercritical water and supercritical CO2. Instead, it employs a porous supported calcium-magnesium composite catalyst. Abandoning supercritical water reduces the overall energy consumption of the reaction. Furthermore, achieving a supercritical state for water requires temperatures >374℃ and pressures >22.1 MPa; therefore, using only supercritical CO2 allows the reaction to proceed under relatively mild conditions. Titanates and silicates present in red mud compete with carbonate ions for soluble divalent cations (such as Ca2+) that can be used to precipitate carbonates in the red mud.2+ Mg 2+ Therefore, this invention incorporates a porous supported calcium-magnesium composite catalyst during the reaction to reduce competition between titanates and silicates in red mud, thereby facilitating carbonate precipitation and increasing carbon fixation efficiency. Furthermore, compared to existing technologies, the use of a single calcium-based catalyst results in excessively high local pH due to its strong alkalinity, triggering silicate side reactions (silicates react with alkaline substances to form silicate complexes, consuming alkaline substances and reducing carbonate production, thus affecting carbon fixation efficiency). The composite catalyst used in this invention, with MgO having a slightly lower alkalinity (pH≈10.3) than CaO (pH≈12.5), allows for pH regulation through the addition of a magnesium-based catalyst, reducing byproduct formation and increasing carbon fixation efficiency by over 20%. Moreover, MgO is more stable in a supercritical CO2 environment, and Mg... 2+ With CO3 2+ It has a stronger binding ability and can generate more stable MgCO3 precipitate, resulting in better carbon fixation effect.

[0016] 3. The porous supported calcium-magnesium composite catalyst used in this invention is prepared by the sol-gel method and uses zinc acetate dihydrate, 2-methylimidazole and methanol to synthesize the support. 20 g of zinc acetate dihydrate was dissolved in 400 mL of methanol to form solution A. 30 g of 2-methylimidazole was dissolved in 400 mL of methanol to obtain solution B. Solution A and solution B were then rapidly mixed and stirred at 25 °C for 12 h. After standing for 24 h, the white precipitate was collected by centrifugation. The white precipitate was washed three times with methanol and dried in an oven at 80 °C for 12 h to obtain the carrier powder. Calcium nitrate and magnesium nitrate were prepared into a 1.5 mol / L impregnation solution at a Ca:Mg = 2:1 molar ratio, and an equal volume was loaded into the carrier channels. The carrier was calcined at a gradient of 120 °C, 350 °C, and 600 °C under a N2 atmosphere. Calcination at 120 °C for 2 h dehydrated the carrier, and calcination at 350 °C for 3 h decomposed the nitrate ions to generate MgO / CaO nanoparticles. Calcination at 600 °C for 4 h further decomposed the carrier. h forms a stable oxide; finally, it is modified by hexamethyldisilazane in the gas phase to make its water contact angle >130°, that is, it has strong hydrophobicity, which blocks the water erosion in the red mud slurry and prevents the catalyst from deactivating.

[0017] 4. The porous supported calcium-magnesium composite catalyst used in this invention has a support pore size of only 1.2 nm, confining the CaO / MgO active components to the nanoscale and forming a high-density catalytic microreactor. This shortens the CO2 diffusion distance to the nanoscale, significantly increasing the carbon fixation rate. Furthermore, since silicate crystals typically have a diameter greater than 1.5 nm, they are blocked by the support pores, completely eliminating their competing side reactions.

[0018] 5. The reactor of this invention combines ultrasonic crushing, microchannel mixing, and a gradient reaction tower. Traditional methods require more than 30 minutes for the reaction due to uneven stirring, and the carbon fixation rate is only 89.5%. However, this device uses 40 kHz ultrasonic cavitation to crush the red mud to the 1-5 μm level. Combined with supercritical CO2 turbulence in the honeycomb microchannel, the mass transfer efficiency is increased by 60 times, and the reaction time is shortened to less than 10 minutes. At the same time, the three-stage gradient temperature-controlled reaction tower (precisely zoned at 35-80℃) synergistically enhances the confinement effect of the catalyst, which not only increases the carbon fixation rate to 95.5%, but also consumes only 120 kWh of energy per ton of red mud, while the traditional device consumes as much as 200 kWh and has serious side reactions. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the apparatus used in the present invention for a method of achieving carbon fixation of red mud under supercritical conditions; Explanation of reference numerals in the attached figures: 1. Red mud drying tower; 2. Ball mill; 3. Mixing outer cylinder; 4. First red mud annular distributor; 5. Second red mud annular distributor; 6. CO2 annular distributor; 7. Ultrasonic transducer; 8. Titanium alloy amplitude transformer; 9. Multi-layer microchannel mixing module; 10. Gradient reaction tower; 11. Multi-stage reciprocating compressor; 12. Baffle plate; 13. Pressure relief valve; 14. Catalyst feed pipe; 15. Water inlet pipe; 16. Red mud feed pipe; 17. Temperature and pressure control system; 18. Conical collector; 19. Circulating pump; 20. Cooling water jacket. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the described embodiments are merely some, not all, of the embodiments of this invention. Unless otherwise specified, the embodiments and features described in this application can be combined with each other. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0021] The present invention provides a method for achieving carbon fixation of red mud under supercritical conditions, comprising the following steps: S1. After washing the red mud, it is sent to the drying tower to remove free water. The dried red mud is crushed by ball mill 2 to form dried red mud powder, and then mixed with deionized water in a certain proportion. S2. In the compressor, CO2 is pressurized to supercritical pressure and supercritical temperature; S3. The slurry obtained in S1 is fed into the ultrasonic crushing zone and crushed by cavitation through an ultrasonic array to reduce the particle size of the red mud. S4. The crushed slurry, the pressurized supercritical CO2 from step S2, and the porous supported catalyst are introduced into the microchannel mixing module in parallel, and the three-phase mixing is completed in the honeycomb microchannel with a mixing time of ≤0.5 seconds. S5. The mixture enters the gradient reaction tower 10 and undergoes a carbon fixation reaction under segmented temperature control. The temperature is then lowered after 10 minutes of reaction.

[0022] To ensure sufficient contact between supercritical CO2, porous supported catalyst, and the crushed slurry, see [reference needed]. Figure 1 It also includes a red mud drying tower 1, a ball mill 2 connected to the red mud drying tower 1, and a mixing outer cylinder 3 located at the outlet of the ball mill 2. Inside the mixing outer cylinder 3, from top to bottom, are a first red mud annular distributor 4, a second red mud annular distributor 5, and a CO2 annular distributor 6. An ultrasonic transducer 7 and a titanium alloy amplitude transformer 8 are vertically arranged between the first red mud annular distributor 4 and the second red mud annular distributor 5. A multi-layer microchannel mixing module 9 is provided between the second red mud annular distributor 5 and the CO2 annular distributor 6. A gradient reaction tower 10 is located below the CO2 annular distributor 6. The distributor 6 is externally connected to a multi-stage reciprocating compressor 11; multiple sets of baffles 12 are spaced apart inside the gradient reaction tower 10, and a pressure relief valve 13 is provided on the outer wall of the gradient reaction tower 10; a catalyst feed pipe 14 is provided at the top of the multi-layer microchannel mixing module 9; a water inlet pipe 15 and a red mud feed pipe 16 connected to the ball mill 2 are provided on the first red mud annular distributor 4; a temperature and pressure control system 17 is provided outside the gradient reaction tower 10, and a conical collector 18 is provided at the bottom of the gradient reaction tower 10. A circulation pump 19 is externally connected to the conical collector 18, and the circulation pump 19 is connected to the top of the gradient reaction tower 10.

[0023] As attached Figure 1The apparatus of the present invention shown includes a red mud drying tower 1, a ball mill 2, a red mud feed pipe 16, a water inlet pipe 15, a first red mud annular distributor 4, a second red mud annular distributor 5, an ultrasonic transducer 7, a titanium alloy amplitude transformer 8, a catalyst feed pipe 14, a multi-layer microchannel mixing module 9, a CO2 annular distributor 6, a gradient reaction tower 10, a baffle plate 12, a multi-stage reciprocating compressor 11, a pressure relief valve 13, a temperature and pressure control system 17, a circulating pump 19, a cooling water jacket 20, and a conical collector 18. The ultrasonic transducer 7 and the titanium alloy amplitude transformer 8 are placed in a closed tubular structure and are vertically positioned. The upper end is equipped with a red mud feed pipe 16 and a water inlet pipe 15. The two pipes are respectively filled with dried and crushed red mud and deionized water. After mixing, the mixture enters the first red mud annular distributor 4, ensuring that the red mud slurry enters the ultrasonic crushing stage uniformly. After ultrasonic crushing, the red mud slurry passes through a second red mud annular distributor 5 located at the lower end and then uniformly enters a multi-layer microchannel mixing module 9. The multi-layer microchannel mixing module 9 is situated within a closed tubular structure and is connected to the ultrasonic crushing module via the second red mud annular distributor 5. A catalyst feed pipe 14 is located at the upper end, and a CO2 annular distributor 6 is located at the lower end. The catalyst and supercritical CO2 enter the multi-layer microchannel mixing module 9 through these pipes, respectively. The three phases are instantly mixed via the microchannels. The mixed slurry then enters a gradient reaction tower 10 connected below the multi-layer microchannel mixing module 9. In the upper low-temperature zone (35–50°C) of the gradient reaction tower 10, carbonate crystal nuclei are generated; in the middle medium-temperature zone (50–65°C), crystal growth is promoted; and in the lower high-temperature zone (65–80°C), deep carbonization is completed. The temperature is controlled by a temperature and pressure control system 17. Baffles 12, numbering 8–10 from top to bottom, are placed inside the gradient reaction tower 10. The pressure relief valve 13 is used to release pressure to ensure safety when the reaction pressure is exceeded. The multi-stage reciprocating compressor 11 pressurizes CO2 to the supercritical CO2 reaction pressure and then uniformly inputs it into the multi-layer microchannel mixing module 9 through the CO2 annular distributor 6.

[0024] Preferably, the outer layer of the ultrasonic breaking module is equipped with a cooling water jacket 20 to prevent the temperature from getting too high during ultrasonication.

[0025] Preferably, the ultrasonic crushing module is 1.5 m high and 1 m in diameter, the ultrasonic transducer 7 is 300 mm in diameter and 200 mm in length (including the amplitude transformer), and the cooling water jacket 20 has a wall thickness of 2 mm and an inner diameter of 8 mm.

[0026] Preferably, the multilayer microchannel mixing module 9 is 1.8 m high and 1 m in diameter, with each single-layer microchannel plate being 100 mm thick and 0.98 m in diameter.

[0027] Preferably, the gradient reaction tower 10 is 4 m high and 1 m in diameter, with the upper layer being 1.5 m high, the middle layer being 1 m high, and the lower layer being 1.5 m high, and the bottom conical structure having a cone angle of 45° to 60°.

[0028] Preferably, the CO2 annular distributor 6 has nozzles arranged in a ring shape, with nozzle diameters of 0.1 to 0.5 mm. The nozzles are positioned towards the second red mud annular distributor 5 so that the CO2 flowing in the CO2 annular distributor 6 flows from the bottom end of the multilayer microchannel mixing module 9 toward the ultrasonic transducer 7.

[0029] Preferably, the pH of the feed solution obtained after step S1 is greater than 10.

[0030] Preferably, the temperature of the drying tower in S1 is T1, 100℃≤T1<120℃, and the dried red mud is pulverized to 150~200 mesh by ball mill 2 and mixed with deionized water at a liquid-solid ratio of 8~10:1.

[0031] The supercritical CO2 reaction pressure in step S2 is P1, and the adjustment pressure is the absolute pressure. The supercritical CO2 reaction temperature in step S3 is T2, where 10MPa < P1 ≤ 25MPa and 35℃ < T2 ≤ 80℃.

[0032] Preferably, the ultrasonic crushing described in S3 uses a piezoelectric ceramic transducer with a frequency of 40 kHz and a dynamic amplitude adjustment range of 10-100%, which reduces the red mud particle size to 1-5 μm.

[0033] Preferably, the porous supported catalyst of S4 is prepared by the following steps: (1) The support is synthesized by using the sol-gel method with zinc acetate dihydrate, 2-methylimidazole and methanol. 20 g of zinc acetate dihydrate is weighed and dissolved in 400 mL of methanol to form solution A. 30 g of 2-methylimidazole is weighed and dissolved in 400 mL of methanol to obtain solution B. Solution A and solution B are then rapidly mixed and stirred at 25 °C for 12 h. After standing for 24 h, the white precipitate is collected by centrifugation. The white precipitate is washed three times with methanol and dried in an oven at 80 °C for 12 h to obtain the support powder. (2) Prepare an impregnation solution with a total molar mass of 1.5 mol / L by mixing calcium nitrate and magnesium nitrate at a molar ratio of Ca:Mg=2:1. Impregnate the carrier in the impregnation solution to obtain the impregnated carrier. (3) The impregnated support is placed under N2 atmosphere for gradient calcination. Calcination at 100-150℃ for 1-2 h dehydrates it. Calcination at 300-400℃ for 2-3 h decomposes nitrate ions to generate MgO / CaO nanoparticles. Calcination at 550-650℃ for 3-4 h forms a stable oxide in the impregnated support. Then, it is modified by hexamethyldisilazane gas-phase hydrophobic modification to obtain a porous supported catalyst with a water contact angle >130°, which means it has strong hydrophobicity.

[0034] Preferably, in step S4, the multilayer microchannel mixing module 9 is formed by stacking multiple layers of titanium alloy microchannel plates, with each plate containing 800 to 4000 regular hexagonal channels with a side-to-side distance of 50 to 300 μm.

[0035] Preferably, the amount of porous supported catalyst added in step S4 is 2 to 10 wt% of the dry red mud powder (calculated by the mass of red mud).

[0036] The gradient reaction tower 10 described in S5 is equipped with baffles 12, which extends the material residence time to 8-10 minutes and achieves a carbon fixation rate of ≥95%.

[0037] The working principle and process of the device of this invention are as follows: First, the red mud is heated to 100-120°C in the red mud drying tower 1 to dry it. Then, the dried red mud is ground to 150-200 mesh in the ball mill 2. The pretreated red mud enters the ultrasonic crushing module through the red mud feed pipe 16 and the first red mud annular distributor 4, and deionized water is introduced through the water inlet pipe 15. After ultrasonic crushing, the porous loaded catalyst enters the multi-layer microchannel mixing module 9 through the catalyst feed pipe 14. While the red mud is being pretreated, CO2 is pressurized to the pressure required for supercritical CO2 in the multi-stage reciprocating compressor 11 and uniformly distributed into the multi-layer microchannel mixing module 9 through the CO2 annular distributor 6. After the above materials are introduced into the multi-layer microchannel mixing module 9, the supercritical CO2, red mud slurry, and catalyst are instantaneously mixed. The mixed three-phase slurry enters the gradient reaction tower 10 for gradient temperature reaction and is then processed by ultrasonic-microchannel mixing. The synergistic effect reduces the red mud particle size to 1-5 μm, enabling three-phase mixing with supercritical CO2 within 0.5 seconds. Simultaneously, a porous supported calcium-magnesium composite catalyst is employed, utilizing the confinement effect and hydrophobicity of the support to increase carbon fixation efficiency to over 95%, completely resolving the silicate side reaction problem caused by excessive alkalinity. After the three-phase mixture reacts for a period, the circulation pump 19 is activated periodically to extract the red mud accumulated in the conical collector 18 and return it to the upper reaction zone of the gradient reaction tower 10 via the return pipe, forming a material circulation. This not only improves the uniformity of the reaction and the utilization rate of red mud carbon fixation but also reduces corrosion of the equipment and extends its lifespan. After the reaction is complete, heating is stopped and the temperature is lowered. While operating inside the reactor, new red mud and CO2 can also be processed in the red mud drying tower 1, ball mill 2, and multi-stage reciprocating compressor 11, making the entire process continuous.

[0038] The present invention will be further described below with reference to the embodiments.

[0039] Example 1 S1. Take Bayer process red mud from an aluminum plant. The red mud is washed, dried at 105℃ for 24 hours, crushed in a ball mill, and passed through a 150-mesh sieve. S2. In the multi-stage reciprocating compressor 11, CO2 is pressurized to the supercritical CO2 reaction pressure, and the absolute pressure P1 is adjusted to 12MPa. S3. The slurry enters the ultrasonic crushing zone (frequency 40 kHz, amplitude 60%), and the red mud particle size is crushed to 4 μm. S4. The crushed slurry and supercritical CO2 enter the multi-layer microchannel mixing module 9, with a mixing time of 0.4 seconds; S5. The mixture is reacted in gradient reaction tower 10 (upper layer 38℃ / middle layer 55℃ / lower layer 70℃), with 5 wt% porous supported catalyst added, and the reaction time is 8 min. After the reaction was completed, the carbon fixation efficiency was determined by acid hydrolysis-gas chromatography (acid hydrolysis: the red mud sample after the reaction was passed through a 200-mesh sieve and reacted with hydrochloric acid and shaken, the CO2 generated by the reaction was collected and analyzed by gas chromatography and quantitatively calculated), and the carbon fixation rate was 94.7%.

[0040] Example 2 S1. Take Bayer process red mud from an aluminum plant. The red mud is washed, dried at 110℃ for 26 hours, crushed in a ball mill, and passed through a 200-mesh sieve. S2. In the multi-stage reciprocating compressor 11, CO2 is pressurized to the supercritical CO2 reaction pressure, and the absolute pressure P1 is adjusted to 18MPa. S3. The slurry enters the ultrasonic crushing zone (frequency 40 kHz, amplitude 80%), and the red mud particle size is crushed to 2 μm. S4. The crushed slurry and supercritical CO2 enter the multi-layer microchannel mixing module 9, with a mixing time of 0.3 seconds; S5. The mixture is reacted in gradient reaction tower 10 (upper layer 42℃ / middle layer 58℃ / lower layer 75℃), with 8 wt% porous supported catalyst added, and the reaction time is 9 min. After the reaction was completed, the carbon fixation efficiency was determined by acid hydrolysis-gas chromatography, and the carbon fixation rate was 96.2%.

[0041] Example 3 S1. Take Bayer process red mud from an aluminum plant. The red mud is washed, dried at 100℃ for 26 hours, crushed in a ball mill, and passed through a 200-mesh sieve. S2. In the multi-stage reciprocating compressor 11, CO2 is pressurized to the supercritical CO2 reaction pressure, and the absolute pressure P1 is adjusted to 22MPa. S3. The slurry enters the ultrasonic crushing zone (frequency 40 kHz, amplitude 70%), and the red mud particle size is crushed to 1.5 μm; S4. The crushed slurry and supercritical CO2 enter the multi-layer microchannel mixing module 9, with a mixing time of 0.5 seconds; S5. The mixture is reacted in gradient reaction tower 10 (upper layer 45℃ / middle layer 62℃ / lower layer 78℃), with 3 wt% porous supported catalyst added, and the reaction time is 10 min. After the reaction was completed, the carbon fixation efficiency was determined by acid hydrolysis-gas chromatography, and the carbon fixation rate was 95.1%.

[0042] Example 4 S1. Take Bayer process red mud from an aluminum plant. The red mud is washed, dried at 115℃ for 18 hours, crushed in a ball mill, and passed through a 160-mesh sieve. S2. In the multi-stage reciprocating compressor 11, CO2 is pressurized to the supercritical CO2 reaction pressure, and the absolute pressure P1 is adjusted to 15MPa. S3. The slurry enters the ultrasonic crushing zone (frequency 40 kHz, amplitude 50%), and the red mud particle size is crushed to 4 μm. S4. The crushed slurry and supercritical CO2 enter the multi-layer microchannel mixing module, with a mixing time of 0.6 seconds; S5. The mixture is reacted in gradient reaction tower 10 (upper layer 35℃ / middle layer 50℃ / lower layer 65℃), with 10 wt% porous supported catalyst added, and the reaction time is 8.5 min. After the reaction was completed, the carbon fixation efficiency was determined by acid hydrolysis-gas chromatography, and the carbon fixation rate was 94.3%.

[0043] Example 5 S1. Take Bayer process red mud from an aluminum plant. The red mud is washed, dried at 108℃ for 22 hours, crushed in a ball mill, and passed through a 170-mesh sieve. S2. In the multi-stage reciprocating compressor 11, CO2 is pressurized to the supercritical CO2 reaction pressure, and the absolute pressure P1 is adjusted to 25MPa. S3. The slurry enters the ultrasonic crushing zone (frequency 40 kHz, amplitude 90%), and the red mud particle size is crushed to 1 μm. S4. The crushed slurry and supercritical CO2 enter the multi-layer microchannel mixing module 9, with a mixing time of 0.2 seconds; S5. The mixture is reacted in gradient reaction tower 10 (upper layer 48℃ / middle layer 65℃ / lower layer 80℃), with 2 wt% porous supported catalyst added, and the reaction time is 9.5 min. After the reaction was completed, the carbon fixation efficiency was determined by acid hydrolysis-gas chromatography, and the carbon fixation rate was 96.8%. Example 6 S1. Take Bayer process red mud from an aluminum plant. The red mud is washed, dried at 112℃ for 20 hours, crushed in a ball mill, and passed through a 190-mesh sieve. S2. In the multi-stage reciprocating compressor 11, CO2 is pressurized to the supercritical CO2 reaction pressure, and the absolute pressure P1 is adjusted to 10MPa. S3. The slurry enters the ultrasonic crushing zone (frequency 40 kHz, amplitude 75%), and the red mud particle size is crushed to 5 μm. S4. The crushed slurry and supercritical CO2 enter the multi-layer microchannel mixing module, with a mixing time of 0.7 seconds; S5. The mixture is reacted in gradient reaction tower 10 (upper layer 40℃ / middle layer 55℃ / lower layer 70℃), with 6 wt% porous supported catalyst added. The reaction time is 10 min, and the return ratio of the circulating pump group 19 is 15%. After the reaction was completed, the carbon fixation efficiency was determined by acid hydrolysis-gas chromatography, and the carbon fixation rate was 95.9%.

[0044] Example 7 S1. Take Bayer red mud, wash it, dry it at 105℃ for 24 hours, crush it in a ball mill, and pass it through a 150-mesh sieve. S2. In the multi-stage reciprocating compressor 11, CO2 is pressurized to the supercritical CO2 reaction pressure, and the absolute pressure P1 is adjusted to 20MPa. S3. The slurry enters the ultrasonic crushing zone (frequency 40 kHz, amplitude 85%), and the red mud particle size is crushed to 2.5 μm; S4. The crushed slurry and supercritical CO2 enter the multi-layer microchannel mixing module 9, with a mixing time of 0.4 seconds; S5. The mixture is reacted in gradient reaction tower 10 (upper layer 50℃ / middle layer 65℃ / lower layer 80℃), with 7 wt% porous supported catalyst added, and the reaction time is 8 min. After the reaction was completed, the carbon fixation efficiency was determined by acid hydrolysis-gas chromatography, and the carbon fixation rate was 95.3%.

[0045] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for achieving carbon fixation of red mud under supercritical conditions, characterized in that, Includes the following steps: S1. After washing and drying the red mud to remove free water, the dried red mud is crushed to form dried red mud powder, which is then mixed with deionized water in a certain proportion. S2. Pressurize CO2 to supercritical pressure and supercritical temperature; S3. The slurry obtained in step S1 is ultrasonically crushed and cavitation crushed by an ultrasonic array to reduce the particle size of red mud. S4. The crushed slurry, the pressurized supercritical CO2 from step S2, and the porous supported catalyst are mixed in parallel in a honeycomb microchannel to complete three-phase mixing, with a mixing time ≤0.5 seconds. S5. The mixture enters the gradient reaction tower, where it undergoes a carbonization reaction under segmented temperature control and is then cooled. The porous supported catalyst in step S4 is prepared through the following steps: (1) The carrier was synthesized by using the sol-gel method with zinc acetate dihydrate, 2-methylimidazole and methanol; (2) Prepare an impregnation solution with a total molar mass of 1.5 mol / L by mixing calcium nitrate and magnesium nitrate at a molar ratio of Ca:Mg=2:

1. Impregnate the carrier in the impregnation solution to obtain the impregnated carrier. (3) The impregnated support is placed under N2 atmosphere for gradient calcination. Calcination at 100-150℃ for 1-2 h dehydrates it. Calcination at 300-400℃ for 2-3 h decomposes nitrate ions to generate MgO / CaO nanoparticles. Calcination at 550-650℃ for 3-4 h forms a stable oxide in the impregnated support. Then, it is modified by hexamethyldisilazane gas-phase hydrophobic modification to obtain a porous supported catalyst. In step S4, the honeycomb microchannel is a multi-layer microchannel hybrid module; The mixture enters the gradient reaction tower from top to bottom. In step S5, the temperature of the upper low-temperature zone of the gradient reaction tower is 35-50°C, the temperature of the middle medium-temperature zone is 50-65°C, and the temperature of the lower high-temperature zone is 65-80°C.

2. The method for achieving carbon fixation of red mud under supercritical conditions according to claim 1, characterized in that: In step S1, the drying temperature is T1, where 100℃≤T1<120℃. The dried red mud is pulverized to 150~200 mesh. After pulverizing, the red mud is mixed with deionized water at a liquid-to-solid ratio of 8~10:1, where the liquid-to-solid ratio is the ratio of the volume of the solution to the mass of the solid, in ml:g.

3. The method for achieving carbon fixation of red mud under supercritical conditions according to claim 1, characterized in that: In step S2, the supercritical CO2 reaction pressure is P1, 10MPa < P1 ≤ 25MPa, and the supercritical CO2 reaction temperature in step S2 is T2, 35℃ < T2 ≤ 80℃.

4. The method for achieving carbon fixation of red mud under supercritical conditions according to claim 1, characterized in that: In step S3, ultrasonic crushing uses a piezoelectric ceramic transducer with a frequency of 40 kHz and a dynamic amplitude adjustment range of 10-100%, which reduces the red mud particle size to 1-5 μm.

5. The method for achieving carbon fixation of red mud under supercritical conditions according to claim 1, characterized in that: The amount of porous supported catalyst added in step S4 is 2 to 10 wt% of the mass of the dried red mud powder.

6. The method for achieving carbon fixation of red mud under supercritical conditions according to claim 1, characterized in that: The microchannel mixing module is composed of stacked multilayer titanium alloy microchannel plates, with each plate containing 800 to 4000 regular hexagonal channels with a side-to-side distance of 50 to 300 μm.

7. The apparatus used in the method for achieving carbon fixation of red mud under supercritical conditions according to claim 1, characterized in that: The system includes a red mud drying tower, a ball mill connected to the red mud drying tower, and a mixing outer cylinder located at the outlet of the ball mill. Inside the mixing outer cylinder, from top to bottom, are a first red mud annular distributor, a second red mud annular distributor, and a CO2 annular distributor. An ultrasonic transducer and a titanium alloy amplitude transformer are vertically installed between the first and second red mud annular distributors. A multi-layer microchannel mixing module is located between the second red mud annular distributor and the CO2 annular distributor. A gradient reaction tower is located below the CO2 annular distributor, and a multi-stage reciprocating compressor is connected to the outside of the CO2 annular distributor. Multiple sets of baffles are spaced apart inside the gradient reaction tower, and a pressure relief valve is installed on the outer wall of the gradient reaction tower. A catalyst feed pipe is located at the top of the multi-layer microchannel mixing module. A water inlet pipe and a red mud feed pipe connected to the ball mill are located on the first red mud annular distributor. A temperature and pressure control system is located outside the gradient reaction tower, and a conical collector is located at the bottom of the gradient reaction tower. A circulation pump is connected to the outside of the conical collector and is connected to the top of the gradient reaction tower.

8. The apparatus used in the method for achieving red mud carbon fixation under supercritical conditions according to claim 7, characterized in that: The CO2 annular distributor has nozzles arranged in a ring, with nozzle diameters of 0.1 to 0.5 mm. The nozzles are positioned towards the second red mud annular distributor so that the CO2 flowing in the CO2 annular distributor flows from the bottom of the multi-layer microchannel mixing module toward the ultrasonic transducer.

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