A method for mineralization sequestration of carbon dioxide by using red mud and calcium source additives

Abstract

The application belongs to the technical field of industrial solid waste resource utilization and CO2 emission reduction, and provides a method for mineralizing and sealing carbon dioxide by using red mud and calcium source additives, wherein a solution containing red mud and calcium source additives is placed in a reaction device, and CO2 gas is introduced to carry out reaction; wherein the solid-liquid ratio of the red mud suspension is 20-200 g / L; the CO2 gas concentration is 10-100%; the reaction system pressure is 0.1-0.4 MPa; and the calcium source is one or more of CaSO4, Ca(OH)2 or CaCl2. The application breaks through the limitation of single factor regulation by multi-factor synergistic regulation, maximizes the carbon sequestration potential of red mud, accurately matches the calcium source to solve the problem of blindness in existing calcium source selection, and through calcium source supplement and reaction parameter optimization, inhibits the pH rebound after red mud carbonation, realizes long-term stable sealing of CO2, and overcomes the defect of poor carbon sequestration stability of the prior art.

Description

Technical Field

[0001] This application belongs to the field of industrial solid waste resource utilization and CO2 emission reduction technology, specifically involving a method for mineralizing and sequestering carbon dioxide using red mud and calcium source additives. Background Technology

[0002] Under the demand for sustainable development in the alumina industry, red mud, as the largest source of highly alkaline solid waste emitted during alumina production (with annual emissions exceeding 150 million tons in my country and cumulative stockpiles exceeding 1.3 billion tons), not only occupies land resources but also easily leads to environmental problems such as soil salinization and groundwater pollution. Meanwhile, CO2 capture, utilization, and storage (CCUS) technology has become a research hotspot. Mineral carbonation sequestration technology, due to its ability to permanently and stably store CO2 as carbonates, is considered one of the most promising carbon reduction methods. Red mud, rich in alkaline minerals such as Ca and Mg, possesses the potential to sequester CO2 through carbonation reactions.

[0003] In existing technologies, red mud mineralization and carbon sequestration are considered a technological approach that combines environmental protection and resource value. Many scholars both domestically and internationally have conducted in-depth research on the resource utilization of red mud and the carbonation of alkaline solid waste minerals, achieving fruitful results. CO2 mineralization and sequestration, as an emerging carbon emission reduction method, depends on the engineering scale-up feasibility of CO2 mineralization and sequestration technology, particularly the control of the carbonation process environment. However, few scholars have paid attention to the complex factors affecting practical applications, and current technologies still have the following shortcomings: (1) Existing studies mostly focus on the effects of single variables on carbon fixation efficiency, such as temperature, pressure, and solid-liquid ratio, and lack research on the synergistic mechanism of multiple parameters. In the process of red mud carbonation, the parameters are interrelated and mutually influential. Studies of single variables cannot fully reflect the complex situation in actual applications and it is difficult to find the optimal carbon fixation conditions. Therefore, this invention systematically studies the interaction of multiple factors such as CO2 concentration, reaction system pressure, and liquid-solid ratio on carbon fixation in red mud carbonation. Through a combination of macroscopic control and microscopic characterization, the mechanism of red mud carbonation reaction is fully revealed.

[0004] (2) Currently, there are many studies on the carbonation and CO2 sequestration of pure red mud, but few studies on the pH rebound phenomenon of pure red mud after carbonation and exposure to air. After carbonation, the alkaline minerals inside the red mud may continue to undergo chemical reactions in the air, affecting the long-term stability of the carbon fixation effect.

[0005] (3) Existing technologies are unclear about the selection and dosage of calcium sources and lack comparative analysis of the effects of different calcium sources on the carbon fixation enhancement of red mud. (4) The existing carbon fixation efficiency is low, and the actual carbon fixation amount is far lower than the theoretical value, which limits its industrial application. Therefore, it is urgent to develop a multi-factor synergistic optimization and precise calcium source matching method to enhance red mud carbon fixation and solve the above-mentioned technical bottlenecks. Summary of the Invention

[0006] To address the aforementioned technical problems, this application provides a method for mineralizing and sealing carbon dioxide using red mud and calcium source additives. The method involves placing a solution containing red mud and calcium source additives in a reaction apparatus and introducing CO2 gas for reaction. The solid-liquid ratio of the red mud suspension is 20-200 g / L; the CO2 gas concentration is 10-100% (volume fraction); the reaction system pressure is 0.1-0.4 MPa; and the calcium source is one or more of CaSO4, Ca(OH)2, or CaCl2.

[0007] The CO2 gas concentrations mentioned in this application all refer to volume fractions.

[0008] Furthermore, the solid-liquid ratio of the red mud suspension is 20-100 g / L.

[0009] Furthermore, the CO2 gas concentration is 30-100%.

[0010] Furthermore, the reaction time is ≥30 minutes.

[0011] Furthermore, the solid-liquid ratio of the red mud suspension is 50 g / L, the CO2 gas concentration is 100%, and the reaction system pressure is 0.3 MPa.

[0012] Furthermore, the calcium source is CaSO4, the added mass of CaSO4 is 7% of the dry mass of the red mud, and the reaction time for adding CaSO4 is 60 min.

[0013] Furthermore, the calcium source is Ca(OH)2, the added mass of Ca(OH)2 is 3% of the dry mass of red mud, and the reaction time for adding Ca(OH)2 is 30 min.

[0014] Furthermore, the calcium source is CaCl2, and the added mass of CaCl2 is 10% of the dry mass of the red mud, and the reaction time for adding CaCl2 is 180 min.

[0015] Furthermore, the calcium source is one or more of desulfurized gypsum, quicklime, calcium chloride, carbide slag, and limestone powder.

[0016] Furthermore, the carbon fixation products obtained after the reaction are applied in the fields of building materials and roadbed fillers.

[0017] The beneficial effects of this application are as follows: 1. This application adopts a multi-factor synergistic regulation strategy: it identifies the optimal combination of CO2 concentration, partial pressure, and solid-liquid ratio, breaks through the limitations of single-factor regulation, and maximizes the carbon sequestration potential of red mud.

[0018] 2. This application involves precise matching of calcium sources: a comprehensive study of CaSO4 and Ca(OH)2. 2、 The study aimed to enhance the carbon fixation of red mud by using three calcium sources, CaCl2, and determined the optimal dosage and calcium source. This approach not only improves carbon fixation efficiency but also reduces costs, addressing the problem of blind selection of calcium sources in existing technologies. Furthermore, by supplementing calcium sources and optimizing reaction parameters, the study aimed to suppress pH rebound after carbonation of red mud, achieving long-term stable CO2 sequestration and overcoming the poor carbon fixation stability of existing technologies.

[0019] 3. Under ideal conditions, the technology described in this application can sequester 0.051 tons of CO2 per ton of red mud. If 10% of the red mud (approximately 15 million tons) is utilized annually, the annual carbon sequestration will reach 765,000 tons, which is equivalent to saving 63,750 m³ of carbon. 2 Green spaces provide carbon sequestration opportunities while reducing red mud alkalinity, thereby mitigating the risk of soil and groundwater pollution and addressing the pollution problem caused by red mud stockpiles.

[0020] 4. The reaction conditions of this application are mild (room temperature, medium and low pressure), requiring no complex equipment, and the products can be directly used in construction, roadbed and other fields, realizing a closed loop of resource utilization. Attached Figure Description

[0021] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a schematic diagram of the carbonation reaction mechanism in this application; Figure 2 The pH and conductivity of the sample vary with different solid-liquid ratios; Figure 3 The images show the XRD and SEM-EDS spectra of RMS-5 before and after carbonation, where (a) is the XRD pattern of RMS-5. (a) Image; (b) Elemental mass percentage of RMS-5; (c) SEM image of RMS-5 before carbonation; (d) SEM image of RMS-5 after carbonation; Figure 4 The graph shows the pH changes of the samples with different concentrations of CO2; where (a) pH changes of RMS-5; (b) pH changes of RMS-10; (c) pH changes of RMS-15; (d) pH changes of RMS-20; and (e) pH rebound of RMS-5. Figure 5The following figures show the changes in the conductivity of the samples with different concentrations of CO2: (a) conductivity change of RMS-5; (b) conductivity change of RMS-10; (c) conductivity change of RMS-15; and (d) conductivity change of RMS-20. Figure 6 The pH of the samples changes with different reaction system pressures, including (a) pH change of RMS-5; (b) pH change of RMS-10; (c) pH change of RMS-15; (d) pH change of RMS-20; and (e) pH rebound of RMS-5. Figure 7 The following figures show the changes in the conductivity of the samples with different reaction system pressures: (a) conductivity change of RMS-5; (b) conductivity change of RMS-10; (c) conductivity change of RMS-15; and (d) conductivity change of RMS-20. Figure 8 The changes in pH and conductivity of samples with different CaSO4 dosages over carbonation time are shown in (a) pH change of the sample; (b) pH rebound of the sample after aeration equilibrium; and (c) conductivity change of the sample. Figure 9 The images show SEM-EDS images of RMS-D7 before and after the carbonation reaction, where (a) shows the XRD pattern of RMS-D7. (a) Image; (b) Elemental mass percentage of RMS-D7; (c) SEM image of RMS-D7 before carbonation; (d) SEM image of RMS-D7 after carbonation; Figure 10 The changes in pH and conductivity of samples with different Ca(OH)2 dosages as a function of carbonation time; where (a) pH change of the sample; (b) pH rebound of the sample after aeration equilibrium; (c) change in conductivity of the sample. Figure 11 XRD and SEM-EDS images of RMS-Ch3 before and after carbonation, where (a) XRD image of RMS-Ch3; (b) elemental mass percentage of RMS-Ch3; (c) SEM image of RMS-Ch3 before carbonation; (d) SEM image of RMS-Ch3 after 30 min of carbonation; (e) SEM image of RMS-Ch3 after 180 min of carbonation. Figure 12 The changes in pH and conductivity of samples with different CaCl2 dosages with carbonation time, including (a) pH change of the sample; (b) pH rebound of the sample after aeration equilibrium; and (c) change in conductivity of the sample. Figure 13XRD and SEM-EDS images of RMS-Cc10 before and after carbonation, where (a) XRD image of RMS-Cc10; (b) elemental mass percentage of RMS-Cc10; (c) SEM image of RMS-Cc10 before carbonation; (d) SEM image of RMS-Cc10 after carbonation. Figure 14 TGA and DTG curves of the sample at different carbonation times; Figure 15 Comparison of theoretical carbon sequestration capacity and actual carbon sequestration capacity under different conditions in red mud. Detailed Implementation

[0022] Unless otherwise specified, the reagents and instruments used in the following examples are all commercially available products.

[0023] The specific performance test items and methods involved in the implementation are as follows: (1) pH measurement The pH reduction efficiency reflects the carbon fixation effect of red mud. Red mud suspensions after carbonation typically contain a significant amount of solid particles, which may affect the accuracy of pH measurements. Therefore, the suspension is centrifuged before measurement to separate the solid particles. The carbonation-treated red mud suspension is placed in a centrifuge tube and centrifuged at 4000 rpm for 15 minutes. The supernatant is then collected, and the pH value of the suspension is measured and recorded.

[0024] (2) Conductivity measurement Changes in conductivity reflect changes in the ion concentration of the red mud suspension during carbonation. Combined with XRD experiments, the carbonation reaction mechanism of red mud can be revealed. After centrifuging the carbonation-treated red mud suspension, the supernatant was collected to measure the conductivity value, and the value was recorded after the reading stabilized.

[0025] (3) X-ray diffraction (XRD) Before conducting X-ray diffraction tests on the red mud samples, the red mud suspension was dried to constant weight in an oven at 105℃. After the samples were removed, they were placed in a drying tower to cool to room temperature. The red mud samples were then ground using a mortar and pestle and passed through a 0.075 mm sieve. After sieving, the samples were mixed evenly and placed on a 2 mm deep sample holder. The mineral composition was determined by scanning at a speed of 1 ° / min between 5° and 80°.

[0026] (4) Scanning electron microscopy (SEM) Before the experiment, the red mud suspension was dried to constant weight in a 105℃ oven. After the sample was removed, it was placed in a drying tower to cool to room temperature. Carbon conductive adhesive was first attached to the sample stage, and then the sample was fixed onto the conductive adhesive, ensuring that the sample was firmly attached to the sample stage. After fixing, the sample was sputter-coated with gold, and then the sample was magnified at an appropriate magnification to further analyze the sample's surface morphology, crystal formation, and internal hydration at the microscopic level.

[0027] (5) Thermogravimetric analysis (TGA) Before conducting thermogravimetric analysis (TGA) on the red mud samples, the red mud suspension was dried to constant weight in a 105℃ oven. After removal, the samples were placed in a drying tower to cool to room temperature. The red mud samples were then ground using a mortar and pestle, and passed through a 0.075 mm sieve for collection. 5–10 mg of sample was placed in a crucible, which was then placed on a crucible rack and the instrument turned off. High-purity nitrogen was used as the protective gas for the balance. The test temperature range was set to 30–1000℃, with a heating rate of 10℃ / min. After the test, the amount of carbonate decomposition in the red mud was calculated based on the mass loss of the samples, thus assessing the carbon sequestration potential of the red mud. The main change in the carbonized red mud is the reaction of calcium-containing mineral phases to form CaCO3. The mass loss between 400℃ and 800℃ is due to the decomposition of various CaCO3 components. Therefore, after conducting TGA on the carbonized red mud samples, the mass loss caused by phase decomposition at different temperatures was analyzed to understand the various stages of red mud decomposition and to quantitatively determine the proportion of CaCO3 in the samples.

[0028] The actual CO2 absorption of the carbonated red mud sample was calculated according to formula (1).

[0029] (1) In the above formula: Mass of the red mud sample after calcination at 400℃; Mass of the red mud sample after calcination at 800℃.

[0030] The minerals involved in the carbonization reaction in red mud are mainly CaO and MgO, followed by Na2O and K2O, as shown in chemical equation (2-5). According to the reaction principle, every 1 mol of minerals participating in the carbonization reaction can absorb 1 mol of CO2.

[0031] (2) (3) (4) (5) The maximum theoretical CO2 absorption of the red mud sample was calculated using formula (6-8).

[0032] (6) (7) (8) In the above formula: Red mud quality; The amount of minerals involved in the carbonization reaction in red mud; : The mass fraction of minerals involved in the carbonization reaction in red mud; Molar mass of minerals involved in carbonization reaction in red mud.

[0033] The carbon fixation efficiency of the red mud sample is calculated according to formula (9).

[0034] (9) Example 1 The red mud was pretreated by air-drying and crushing the raw material, then drying it at 105℃ to constant weight, and grinding it through a 0.075 mm square hole sieve for later use. The basic physical properties of the pretreated red mud are shown in Table 1.

[0035] Table 1 Basic physical properties of red mud

[0036] The chemical composition of the red mud was analyzed, and the results are shown in Table 2: Table 2 Chemical composition and content of red mud

[0037] Using water as a solvent, the pretreated red mud was prepared into pure red mud suspensions with different solid-liquid ratios. The red mud suspensions were then placed in a shaking chamber and shaken at 250 rpm for 10 min to ensure uniform mixing. After the carbonation test, the red mud suspensions were dried in a 105℃ oven for later use. The pure red mud suspension samples are shown in Table 3 below. Table 3 Solid-liquid ratio of red mud suspension samples

[0038] The principle of red mud carbonation reaction is as follows: Figure 1 When CO2 enters the red mud suspension, it first dissolves in the pore water and then reacts with water molecules to form H2CO3. H2CO3 is a weak acid, which helps maintain CO32- in the solution. 2- and HCO 3- Dynamic chemical equilibrium, CO32- It is a precursor to carbonate precipitation, and storing carbon in the form of carbonate is the core approach to CO2 sequestration in red mud.

[0039] The solid-liquid ratio is an important parameter for studying the carbonation reaction effect of red mud. An excessively high solid-liquid ratio hinders CO2 diffusion in the liquid phase, while an excessively low ratio leads to insufficient ion concentration for the reaction, thus reducing the carbon fixation rate. The effect of different solid-liquid ratios on the sample pH under the conditions of 100% CO2 concentration and a reaction system pressure of 0.3 MPa is shown in the figure. Figure 2 As shown in (a), under the same solid-liquid ratio, with the extension of reaction time, the pH of the sample first decreased rapidly, and then the rate of decrease gradually slowed down and tended to plateau. Within 0-60 min of CO2 introduction, the pH of each sample basically reached equilibrium. Among them, when the solid-liquid ratio was 50 (RMS-5), the pH decrease rate was the fastest, and the pH equilibrium value was the lowest at 5.5. When the solid-liquid ratio increased to 100 (RMS-10), the pH decrease rate slowed down, and further increasing the liquid-solid ratio did not significantly change the pH decrease rate. When the solid-liquid ratio was small, the H2CO3 formed by the dissolution of CO2 in the suspension could react with OH-. - A complete reaction results in high acid-base neutralization efficiency and a rapid decrease in pH. As the solid-liquid ratio increases, the viscosity of the suspension increases. According to the two-mode theory, increased viscosity leads to a decrease in the liquid film enhancement factor and a reduction in the CO2 diffusion rate. Simultaneously, the limited solubility of CO2 in the liquid phase means that H2CO3 cannot completely neutralize the OH- in the solution. - This slows down the rate of pH decrease and the rate of carbonation reaction.

[0040] The effect of different solid-liquid ratios on the conductivity of red mud suspension is as follows: Figure 2 As shown in (b), the conductivity exhibits a three-stage dynamic change pattern of first increasing, then decreasing, and then increasing again, consistent with the conductivity evolution trend under the control of CO2 concentration and partial pressure. With the increase of the solid-liquid ratio, the conductivity of the suspension increases significantly. When the solid-liquid ratio increases from 50 g / L (RMS-5) to 200 g / L (RMS-20), the initial conductivity of the sample increases by 785 μs / cm, and after 360 min of reaction, the maximum difference in conductivity between samples has increased to 1250 μs / cm. This is the synergistic effect of solid-phase particle concentration and liquid-phase ion equilibrium. The higher the solid-liquid ratio, the more red mud particles are in the suspension, and the more ions are released from particle dissociation. Simultaneously, CaO and Na2O in the red mud dissolve in the water, further increasing the liquid-phase ion concentration, leading to an increase in conductivity, which conforms to the basic law of a positive correlation between conductivity and ion concentration.

[0041] XRD patterns of RMS-5 at different carbonation times are as follows: Figure 3As shown in (a), the main phases of the untreated red mud are Ca2SiO4 and CaTiO3, with a small amount of CaCO3, which is the result of natural carbonation caused by long-term exposure to the atmosphere. With prolonged carbonation time, the phase composition of the sample changed significantly. The diffraction peak intensity of CaCO3 increased, while the characteristic peaks of CaTiO3 and Ca2SiO4 gradually weakened, and the diffraction peak intensities of minerals such as hematite and nepheline showed no significant change. This phenomenon indicates that Ca2SiO4 and CaTiO3 are the main calcium-containing minerals participating in the CO2 reaction during the carbonation process of red mud. Both react with CO2 to form CaCO3 crystals, and the main crystal morphologies detected were calcite and aragonite. The CaCO3 diffraction peak was strongest after 30 min of carbonation, indicating that the carbonation effect of the red mud was best at this time, and the degree of mineral carbonation was highest.

[0042] EDS surface scan results of C, O, and Ca elements before and after RMS-5 carbonation are as follows: Figure 3 As shown in (b), the mass percentages of the three elements in the sample increased to some extent after carbonation compared to before carbonation. Specifically, the C content increased by 57%, and the Ca content increased by 78%. This result confirms that red mud generates CaCO3 through the carbonation reaction. This result is consistent with the enhancement of the CaCO3 characteristic peak in the XRD analysis, indicating that red mud has carbon sequestration capabilities.

[0043] SEM images of RMS-5 before and after carbonation at 10000 magnification are as follows: Figure 3 As shown in Figure 1 (cd), the red mud before carbonation consists of loose, fine particles with uneven particle size and random distribution. In contrast, the microstructure of the carbonated red mud is significantly denser, with the original pores filled by carbonation products, resulting in a slight increase in true density. Combining XRD test results with the principle of red mud carbonation reaction, it can be inferred that the component formed after the carbonation reaction of red mud is CaCO3. The formed CaCO3 mineral forms are mainly of two types: one is calcite existing alone or in aggregates, and the other is aragonite with spindle-shaped individual particles interspersed or clustered. During the carbonation process, Ca... 2+ The carbonated red mud particles dissolve, forming new nanoscale pores. Simultaneously, the resulting carbonates fill the original slit pores, leading to a reduction in macropore volume and an increase in the number of nanopores. As shown in the figure, the surface of the carbonated red mud particles is coated with fine CaCO3 particles, and the pore structure exhibits an irregular distribution.

[0044] Example 2 This embodiment investigates the effect of different CO2 concentrations on the carbonation of red mud. Under normal temperature and pressure conditions and a reaction time of 360 min, the carbonation of RMS-5, RMS-10, RMS-15, and RMS-20 samples was conducted using a CO2 and N2 gas equilibration method. The pH evolution during the carbonation process of each sample is shown in the figure below. Figure 4 As shown in Figure (ad), CO2 concentration has a significant regulatory effect on pH evolution. With increasing CO2 concentration, the rate of pH decrease in the samples accelerates, the time to reach pH equilibrium is shortened, and the pH equilibrium value is lower, indicating that increasing CO2 concentration can effectively promote the carbonation reaction rate of red mud. When the CO2 concentration is below 30%, the pH decrease with increasing CO2 concentration is significant; when the CO2 concentration is above 30%, the pH decrease is significantly reduced. The first 30 minutes of carbonation is the main stage of the reaction; samples with CO2 concentrations above 30% experience a rapid pH decrease, with RMS-5 showing the largest decrease, ranging from 30.9% to 40.4%. This is because CO2 reacts with the alkaline components in the red mud to form carbonates. After 180 minutes of carbonation, the pH of all samples basically reaches equilibrium.

[0045] like Figure 4 As shown in (e), after the RMS-5 carbonation reaction reached pH equilibrium and was exposed to the atmosphere, the pH rebounded significantly. After 360 min of full contact with the atmosphere, the pH of RMS-5 rapidly rebounded to between 7.4 and 7.6, and the rebound magnitude increased with increasing initial CO2 concentration. Specifically, the pH rebounded by 0.96 in the 10% CO2 group, 1.38 in the 30% CO2 group, 1.62 in the 70% CO2 group, and 1.8 in the 100% CO2 group. This indicates that... Figure 4 The pH decrease observed in the middle (ad) is due to CO2 dissolving in the aqueous phase as H2CO3 or free CO2. The pH rebound after exposure to the atmosphere is due to the gradual degassing of dissolved carbonates.

[0046] The evolution of conductivity during the carbonation process of each sample under different CO2 concentrations is as follows: Figure 5 As shown, the conductivity generally exhibits an "increase-decrease-increase" trend. This trend reflects the dynamic changes in ion concentration during the carbonation reaction and is a macroscopic manifestation of the material transformation at each stage of the reaction. In the initial stage of carbonation, the conductivity increases rapidly, attributed to the dissolution of CO2 to generate HCO3. - CO3 2- The plasma is released into the solution, and the changes in this stage reflect the dissolution rate of CO2 and the ion formation rate. During the middle stage of carbonation, the conductivity decreases, attributed to the reaction of basic cations in the sample with CO3. 2-The reaction produces carbonate precipitate, reducing the concentration of freely moving ions in the solution. This indicates that the carbonation reaction is transitioning from a gas-dissolution-dominated stage to a solid-precipitation-dominated stage, which is the core stage of the carbonation reaction. The continued formation of precipitate in the middle stage disrupts the dissolution equilibrium of the red mud suspension, causing further dissolution of ions and a renewed increase in conductivity. As the carbonation reaction proceeds, the number of basic cations that can combine with CO2 gradually decreases, and the concentration of soluble Na+ in the solution also decreases. + Continuous leaching further accelerates the rate of increase in conductivity, at which point the sample has reached a relatively high degree of carbonation. The conductivity change at this stage reflects the redistribution of ions and the dynamic adjustment of the dissolution equilibrium during carbonation. Notably, the sample introduced with 100% CO2 concentration maintained the highest conductivity, indicating that increasing CO2 concentration promotes the carbonation reaction. High CO2 concentrations can provide more CO32-. 2- This accelerates the reaction of alkaline cations with CO3. 2- The reaction drives the overall rate of the carbonation reaction to increase.

[0047] Example 3 This embodiment investigates the effect of different reaction system pressures on the carbonation of red mud. Under the condition of 100% CO2 concentration, the effect of different reaction system pressures on the pH evolution during the carbonation process of RMS-5, RMS-10, RMS-15, and RMS-20 is as follows: Figure 6 As shown in (ad). With increasing pressure, the rate of pH decrease in the sample accelerates, the time to reach pH equilibrium shortens, and the pH equilibrium value is lower. When the pressure is less than 0.3 MPa, the rate of pH decrease with increasing reaction time gradually increases, and the reaction process is controlled by the CO2 dissolution and ionization equilibrium. When the pressure is greater than 0.3 MPa, the rate of pH decrease decreases, and the reaction process is controlled by the diffusion rate of the reactants. With increasing pressure, the carbonation process accelerates. The first 30 minutes of carbonation is the stage of rapid pH decrease, with RMS-5 showing the largest pH decrease, ranging from 40.4% to 46.4%. When the pressure is greater than 0.1 MPa, the sample basically reaches equilibrium after 60 minutes of carbonation, a reduction of 60-120 minutes compared to the sample with a pressure less than 0.1 MPa. The carbonation reaction of red mud particles is essentially a liquid-liquid reaction. High pressure can accelerate the entry of gaseous CO2 into the liquid phase through dissolution and hydration. Calcium-containing minerals in red mud release Ca through dissolution. 2+ To the liquid phase. Ca 2+ and CO3 2-In the liquid phase, CO2 molecules combine to form CaCO3 precipitate, completing the carbonation process. The entry of gaseous CO2 molecules into the liquid phase is a complex process involving physical dissolution and chemical absorption; the concentration of CO2 in the liquid phase can be described by Henry's Law. At a given temperature, the solubility of a slightly soluble gas in solution is directly proportional to its partial pressure.

[0048] The pH rebound of RMS-5 after ventilation equilibration upon exposure to the atmosphere is as follows: Figure 6 As shown in (e). Consistent with the CO2 concentration test results, the pH decreased rapidly during the aeration phase, but rebounded rapidly to between 7.4 and 8 after aeration was stopped and the sample was allowed to fully contact the atmosphere for 360 min. The rebound magnitude was positively correlated with the pressure during carbonation, consistent with the pH rebound pattern under different CO2 concentrations.

[0049] The effect of different reaction system pressures on the conductivity of each sample during carbonation is as follows: Figure 7 As shown, the conductivity exhibits a three-stage dynamic characteristic of "increasing-decreasing-increasing" with the progress of the carbonation reaction. This evolution directly reflects the differences in the carbonation reaction mechanism under different partial pressures. In the initial stage of carbonation, the conductivity increases rapidly, with the most significant increase observed at a partial pressure of 0.2 MPa, attributed to CO2 dissolution and protonation reactions. According to Henry's Law, under higher partial pressures, more CO2 dissolves in the liquid phase, generating HCO3-. - With CO3 2- During the middle stage of carbonation, conductivity decreases; this stage is the core of the carbonation reaction. Ca dissolved from red mud... 2+ With CO3 2- The initial reaction produces a carbonate precipitate, reducing the ion concentration in the solution. During the intermediate stage, precipitation continues, and the ongoing CO2 acidification causes partial dissolution of inert minerals in the solution, leading to a further increase in conductivity. Notably, the higher the reaction system pressure, the higher the sample conductivity, indicating that increasing the reaction system pressure can promote the carbonation reaction process.

[0050] Example 4 This embodiment investigates the effect of adding calcium sources on the carbonation of red mud. CaSO4, Ca(OH)2, and CaCl2 solutions (0.5 mol / L) were selected as calcium sources. A red mud suspension with a concentration of 50 g / L was prepared, and 0%, 3%, 5%, 7%, and 10% of the dry weight of red mud (CaSO4, Ca(OH)2, or CaCl2) were added to it, respectively. The samples with different calcium sources and dosages were numbered as shown in Table 4. The mixed test solution was then placed in a shaking chamber and shaken at 250 rpm for 10 min to ensure thorough mixing of the red mud, calcium source, and pure water, ensuring a homogeneous mixed test solution. A red mud carbonation and carbon fixation test was then conducted, with a CO2 gas concentration of 100% and a reaction system pressure of 0.3 MPa. After the carbonation test, the mixed test solution was dried in a 105℃ oven for later use.

[0051] Table 4. Mixing ratio and number of each sample

[0052] The pH changes of the five groups of samples over time under the condition of CaSO4 as the calcium source are as follows: Figure 8 As shown in (a), the initial pH of RMS-D was significantly lower than that of RMS, and the pH of all five samples decreased as the carbonation reaction proceeded, eventually eliminating most of the alkali in the red mud pores. However, RMS-D showed a faster pH decrease rate and a lower pH equilibrium value. With the addition of CaSO4 to 3%, the reaction rate of the samples increased significantly. Further increases in the addition rate continued to raise the reaction rate, but the increase was significantly reduced. In the first 30 minutes of carbonation, the pH of each sample decreased rapidly; within 30–60 minutes of carbonation, the rate of pH decrease slowed; and by 180 minutes, each sample had essentially reached pH equilibrium. The pH of RMS-D was 0.5–0.7 lower than that of RMS. This indicates that the addition of CaSO4 can provide Ca... 2+ This promotes the absorption of CO2 by red mud, generating CaCO3 and improving the carbonation efficiency of red mud. The reaction principle is shown in chemical equation (10). The pH rebound of the sample after being exposed to the atmosphere after gas equilibration is as follows: Figure 8 As shown in (b), the addition of CaSO4 can suppress pH rebound.

[0053] (10) The test results of the conductivity change of the five groups of samples over time are as follows: Figure 8 As shown in (c), during the early stages of carbonation, the conductivity of the samples increased significantly with increasing CaSO4 content. The initial conductivity of RMS was 566 μS / cm, while that of RMS-D increased by 81–190%. This is attributed to the release of Ca from the dissociation of CaSO4. 2+ and SO4 2-This causes a sharp increase in the ion concentration of the solution. During the middle stage of carbonation, Ca... 2+ As the ions are consumed, the concentration of ions in the solution decreases, and the conductivity begins to decline. This decrease in conductivity during this stage coincides with the pH change, indicating that the acidity or alkalinity of the reaction system directly affects the ion concentration. In the later stages of the reaction, the sample is acidic, and minerals such as calcium aluminum silicates in the red mud undergo acidolysis, releasing Ca2+. 2+ And Al 3+ The presence of plasma leads to a further increase in the ion concentration of the solution, resulting in a corresponding increase in conductivity. Overall, the addition of an appropriate amount of CaSO4 can significantly improve the carbon fixation efficiency during the carbonation process of red mud.

[0054] XRD patterns of RMS-D7 at different carbonation times are as follows: Figure 9 As shown in (a), the evolution of minerals in red mud during the carbonation process is revealed. With prolonged carbonation time, the intensity of the CaCO3 characteristic peak significantly increases, indicating that CO2 is effectively fixed in the form of CaCO3, confirming the carbonation reaction. At 60 min of carbonation, the number of CaCO3 characteristic peaks is highest, indicating that RMS-D7 reaches its maximum degree of carbonation at this point. CaCO3 is a key product of the red mud carbonation reaction, and its intensity change directly reflects the degree of carbonation. The diffraction peaks of minerals such as hematite and quartz do not change significantly throughout the carbonation process. This is because hematite has extremely low solubility at pH values ​​greater than 4, while quartz has a high amorphization barrier; these mineral phases exhibit high stability and do not participate in the carbonation reaction. This finding reveals the characteristics of inert mineral phases in red mud, which do not significantly affect the carbonation process. In the early stages of carbonation, exogenous Ca... 2+ And free Ca in red mud 2+ It reacts rapidly with CO2 to form CaCO3, exhibiting a relatively fast carbonation rate. In the later stages of carbonation, the mineral surface is covered with CaCO3 crystals, hindering the reaction of CO2 with Ca. 2+ Further contact leads to a slower reaction rate.

[0055] EDS surface scan results of C, O, and Ca elements before and after RMS-D7 carbonation are as follows: Figure 9 As shown in (b), the mass percentages of the three elements in the sample increased significantly after carbonation, with the C content increasing by 77% and the Ca content by 96%, indicating that the addition of CaSO4 can provide more Ca for the carbonation reaction of red mud. 2+ This promotes the formation of CaCO3 precipitate and CO2 sequestration. Combined with XRD and SEM analysis results, it is shown that the addition of CaSO4 can improve the carbonation efficiency and carbon sequestration capacity of red mud.

[0056] SEM images of RMS-D7 before and after carbonation are as follows: Figure 9 As shown in the middle (cd). From Figure 9 As shown in (c), the sample particles before carbonation are irregularly shaped lumps with uneven particle size distribution and a certain number of pores. These pores provide mass transfer channels for CO2 diffusion, which is beneficial for the contact and reaction between CO2 and red mud particles. In addition, CaSO4 particles adhere to the surface of the red mud through physical adsorption, providing exogenous Ca for the subsequent carbonation reaction. 2+ reserve. Figure 9 Image (d) shows the microstructure of the sample after carbonation. At this stage, the pores in the sample are filled with carbonation products, improving density and stability. Furthermore, the formation of CaCO3 indicates that CO2 is effectively fixed, which is consistent with the enhancement of the CaCO3 characteristic peak in the XRD analysis.

[0057] The pH changes of the five groups of samples over time under the condition of Ca(OH)2 as the calcium source are as follows: Figure 10 As shown in (a), the initial pH value of RMS was 9.97, while the initial pH value of RMS-Ch rose to 11.6-12, consistent with the strongly alkaline characteristics of Ca(OH)2 dissolution. In the first 30 min of carbonation, the pH of all samples decreased rapidly, attributed to the rapid dissolution of CO2 and the formation of carbonates. After 60 min of carbonation, all samples reached a near-equilibrium pH, with RMS-Ch3 exhibiting the lowest equilibrium value. The pH rebound of the samples after atmospheric exposure after equilibrium is shown in (b). Figure 10 As shown in (b), the addition of Ca(OH)₂ can suppress pH rebound. The results of conductivity changes over time are shown in Figure [image missing]. Figure 10 As shown in (c), the higher the Ca(OH)₂ content, the higher the initial conductivity of the sample. The initial conductivity of RMS-Ch was increased by 2254~5034 μs / cm compared to RMS. This is because the dissolution of Ca(OH)₂ provides a large amount of free OH⁻. - and Ca 2+ This significantly improved the ionic strength of the solution. In the first 10 minutes of carbonation, the RMS-Ch conductivity dropped sharply, decreasing by 75-82%. During this stage, CO2 dissolves to generate H₂. + , with OH in the solution - A neutralization reaction occurs, OH - It is being consumed rapidly, while Ca 2+ With CO3 2- The formation of CaCO3 precipitate reduces the amount of free Ca in the solution. 2+ The concentration of [amount] causes a decrease in conductivity. The decrease in conductivity slows significantly after 10–30 min. After 30 min, some minerals in the red mud undergo acidolysis under acidic conditions, offsetting some of the Ca [reactivity]. 2+As the carbon dioxide is consumed, the conductivity slowly recovers, but remains below its initial value. Combining this with the pH change pattern, it was found that the conductivity change trend during carbonation is consistent with the pH change, both reflecting the dynamic synergistic changes in ion concentration and acidity / alkalinity of the system. This phenomenon indicates that adding an appropriate amount of Ca(OH)₂ can optimize the carbonation reaction conditions of red mud and improve carbon fixation efficiency.

[0058] XRD patterns of RMS-Ch3 at different carbonation times are as follows: Figure 11 As shown in (a), the addition of Ca(OH)₂ to the sample mainly serves to provide a calcium source and regulate pH. With increasing carbonation time, the characteristic peak intensity of CaCO₃ significantly increases. In the first 30 min of carbonation, Ca(OH)₂ dissolves rapidly, providing additional Ca to the system. 2+ and OH - This significantly increases the supersaturation of the solution, promoting rapid nucleation and growth of CaCO3. After 30 minutes of carbonation, the characteristic peak intensity of CaCO3 gradually decreases, which is due to the local H+ in the red mud. + The accumulation of [acids] leads to the partial dissolution of CaCO3, as shown in chemical equation (11). This phenomenon is consistent with the results of pH and conductivity tests, indicating that the acidity or alkalinity of the reaction system has a significant impact on the stability of CaCO3. The EDS surface scan results of C, O, and Ca elements before and after RMS-Ch3 carbonation are shown in [the table / image]. Figure 11 As shown in (b), quantitative analysis results indicate that the mass percentages of the three elements in the sample after carbonation significantly increased compared to before carbonation, with the C content increasing by 61% and the Ca content increasing by 713%. This is consistent with the enhanced characteristic peak of CaCO3 in the XRD analysis, demonstrating that the addition of Ca(OH)2 can significantly improve the carbon sequestration capacity of red mud.

[0059] (11) SEM images of RMS-Ch3 before and after carbonation are as follows: Figure 11 As shown in the middle (ce). Figure 11 As shown in (c), the red mud particles before carbonation are relatively dispersed, unevenly distributed, and have many pores, providing mass transfer channels for CO2 diffusion. Figure 11 Image (d) shows the microstructure of RMS-Ch3 after 30 min of carbonation. Compared with before carbonation, the surface morphology of the particles changed significantly. A large number of fine, blocky CaCO3 particles adhered to the surface of RMS-Ch3, increasing the sample density and drastically reducing the porosity. This is because Ca(OH)2 provided sufficient Ca for the carbonation of red mud. 2+ A strongly alkaline environment promotes efficient CO2 absorption and inhibits Fe. 3+ Hydrolysis and precipitation promote the directional formation of carbonates, and the resulting CaCO3 crystals act as a cementing phase, filling the pores. For example... Figure 11 As shown in (e), after 180 min of carbonation, the CaCO3 grain size increases, encapsulating the red mud particles, at which point the carbon fixation capacity of the red mud tends to stabilize. This is because the red mud particles are covered by a dense carbonate layer formed during the carbonation process, which restricts the ion diffusion rate, thereby reducing the reactivity of the remaining non-carbonate minerals, inhibiting further mineral carbonation reactions, and decreasing the overall mineral carbonation potential of the red mud.

[0060] The pH changes of the five groups of samples over time under the condition of CaCl2 as the calcium source are as follows: Figure 12 As shown in (a), the higher the CaCl2 content, the lower the initial pH value of the sample; the initial pH value of RMS-Cc decreases to 7.3–8.2 compared to RMS. CaCl2 dissociates in solution to release Cl... - With Ca 2+ Cl - The addition of CaCl2 can reduce the alkalinity of the solution, directly leading to a decrease in the initial pH, indicating that the addition of CaCl2 promotes the reduction of red mud alkalinity. In the first 10 minutes of carbonation, the pH of all five samples decreased, but RMS-Cc decreased faster. The final pH of RMS-Cc decreased to 4.4–4.6, which was 1.4–1.6 lower than RMS, indicating that the addition of CaCl2 can significantly accelerate the initial process of the carbonation reaction. Within 10–30 minutes, the rate of pH decrease of the samples slowed down, indicating that the alkaline substances in the reaction system were gradually consumed, and the carbonation reaction entered a stable stage. After 30 minutes, the pH slightly rebounded, possibly due to the passivation phenomenon on the surface of the red mud, and then tended to reach dynamic equilibrium. Among all samples, RMS-Cc10 had the lowest pH equilibrium value, indicating that when the CaCl2 dosage was 10%, the promoting effect on the carbonation reaction of red mud was most significant. The pH rebound of the samples after aeration equilibrium and exposure to the atmosphere is as follows: Figure 12 As shown in (b), without the addition of a calcium source, atmospheric CO2 can only neutralize the alkalinity of the pore water in the red mud, and the pH value will rebound after the CO2 supply is stopped. Adding CaCl2 can improve the red mud's ability to absorb CO2 and provide a more durable CO2 sequestration solution, as shown in chemical equation (12).

[0061] (12) The test results of the conductivity of the five groups of samples changing over time are as follows: Figure 12 As shown in (c), during the initial stage of carbonation, the initial conductivity of the sample significantly increased with the increase of CaCl2 doping. CaCl2 solution, as a strong electrolyte, directly increased the ionic strength of the mixed test solution through dissociation, thereby increasing the conductivity. The RMS-Cc conductivity decreased rapidly in the first 30 minutes of carbonation. This is due to the formation of CaCO3 precipitate, leading to a decrease in Ca2+ concentration in the solution. 2+As concentration decreases, conductivity decreases. Cl - It does not participate in the precipitation reaction, and its concentration remains stable. Therefore, the decrease in conductivity is mainly due to Ca. 2+ Consumption-driven. After carbonation for 30 min, it is affected by Cl... - Due to the complexation effect and the influence of surface passivation, the conductivity tends to level off.

[0062] XRD patterns of RMS-Cc10 at different carbonation times are as follows: Figure 13 As shown in (a), with the extension of carbonation time, the system exhibits significant mineral phase transformation and recombination characteristics. The characteristic peak intensity of CaCO3 significantly increases, directly reflecting the gradual increase in CaCO3 formation. CaCl2 has high solubility in water, therefore no characteristic peak of CaCl2 was detected in the experiment. In the first 10 minutes of carbonation, a characteristic peak of CaCO3 began to appear in the sample, but the peak value was weak, indicating that the amount of CaCO3 formed at this time was small, and the carbonation reaction was in its initial stage. After 10 minutes, the characteristic peak of CaCO3 gradually strengthened, and the amount of CaCO3 formed increased significantly, indicating that the carbonation reaction entered a rapid advancement stage. At the same time, the peak intensity of Ca2Al2SiO7 decreased significantly, indicating that adding CaCl2 to red mud can induce the conversion of Ca2Al2SiO7 to CaCO3, further increasing the amount of CaCO3 formed and strengthening CO2 fixation. As the reaction progressed, a characteristic peak of NaCl was detected in the sample. This is due to the reaction between CaCl2 and Na in the red mud. + An ion exchange reaction occurred, a phenomenon that confirms that CaCl2 can participate in carbonation reactions.

[0063] EDS surface scan results of C, O, and Ca elements before and after RMS-Cc10 carbonation are as follows: Figure 13 As shown in (b), the mass percentages of the three elements in the sample after carbonation increased significantly compared to before carbonation, with C increasing by 14% and Ca by 251%. This indicates that the addition of CaCl2 significantly promotes the formation of CaCO3 and enhances the carbon sequestration capacity of the sample, consistent with the XRD analysis results. SEM images of RMS-Cc10 before and after the carbonation reaction are shown below. Figure 13 Shown in (cd). Figure 13 Image (c) shows the surface morphology of the red mud before carbonation. The sample exhibits an irregular blocky structure with some porosity, a relatively smooth surface, and no obvious deposits. As the carbonation reaction proceeds, the microstructure of the red mud changes, such as... Figure 13As shown in (d), the surface of the red mud is covered with plate-like CaCO3 crystals. These crystals are distributed on the particle surface and fill the original pores, significantly reducing the porosity of the red mud. This indicates that the carbonation reaction not only leads to the formation of CaCO3 precipitate but also effectively alters the surface structure and pore characteristics of the red mud. Further analysis shows that adding CaCl2 can increase the formation rate of CaCO3 and significantly improve the carbonation efficiency of the red mud.

[0064] Figure 14 The TGA and DTG curves of each sample at different carbonation times are shown, and the CaCO3 content in the red mud samples at each carbonation time period is calculated. The results are shown in Table 5. The thermal decomposition of the red mud samples after carbonation can be divided into three stages. The weight loss peak at around 100℃ corresponds to the removal of free water in the red mud. The mass loss in the 200~380℃ range corresponds to the partial hydroxylation of Al(OH)3. At the same time, FeOOH undergoes a dehydroxylation reaction at approximately 270℃ to generate Fe2O3. The mass loss between 400℃ and 800℃ represents the decomposition of various types of CaCO3, which, in descending order of increasing decomposition temperature, are amorphous CaCO3, aragonite, and calcite.

[0065] Depend on Figure 14 As shown in (a), RMS-5 has the highest loss rate in the 400~800℃ range when carbonation lasts for 30 min, indicating that the amount of CaCO3 decomposed is the largest during this stage, which means that RMS-5 absorbs the most CO2 when carbonation lasts for 30 min. Figure 14 As shown in (b), the DTG peak shifts towards higher temperatures as the carbonation time increases, indicating that the crystallinity of CaCO3 increases and its thermal stability is enhanced in the later stages of carbonation. Figure 14 Figure (c) shows that the CaCO3 decomposition of RMS-D7 initially increases and then decreases with increasing carbonation time. At 60 min of carbonation, the mass loss rate reaches its highest point at 3.9581%, 0.45% higher than the uncarbonated sample, corresponding to a CaCO3 content of 9 wt.%. This result indicates that the optimal carbonation time for RMS-D7 is 60 min. In the later stages of carbonation, the amount of CaCO3 decomposed decreases due to the influence of the passivation layer. Figure 14 In the middle (d) image, the DTG peak shifts to the right with carbonation time, consistent with the RMS-5 pattern, indicating that the CaCO3 crystallinity is better in the later stage of carbonation. Figure 14As shown in Figure (e), the mass loss rate of RMS-Ch3 reached its maximum of 4.8586% after 30 min of carbonation. In the later stages of carbonation, the passivation layer limited the continued formation of CaCO3, resulting in a slight decrease in the mass loss rate. This phenomenon is reflected in the DTG curve as a slower rate of mass loss. Combined with Table 6, it can be seen that the CaCO3 formation peaked at 11.15 wt.% after 30 min of carbonation, indicating that the optimal carbonation time for RMS-Ch3 is 30 min. Compared to other samples, RMS-Ch3 exhibited significantly higher CaCO3 formation, suggesting that Ca(OH)2, as a strongly alkaline calcium source, is more conducive to the carbonation reaction of red mud. However, the long-term carbon fixation is inhibited due to the limitations imposed by the passivation layer. Figure 14 The results (g) show that the amount of CaCO3 generated by RMS-Cc10 exhibits an increasing-decreasing-increasing trend with reaction time. In the first 30 min of carbonation, the mass loss rate increased from 3.6272% to 4.2503%, and the CaCO3 content increased from 8.24 wt.% to 9.66 wt.%. From 30 to 60 min of carbonation, the reaction of CO2 and H2... + The diffusion of CaCO3 was inhibited, and the CaCO3 content decreased to 9.21 wt.%. After carbonation for 180 min, the long-term acidic environment promoted the dissolution of calcium aluminum silicates in the red mud, releasing Ca. 2+ This pushed the CaCO3 content back up to 9.73 wt.%. However... Figure 14 The DTG curve (h) shows that an acidic environment also reduces the stability of carbonates. Therefore, the optimal carbonation time for RMS-Cc10 is 30 min, at which point the amount of CaCO3 produced is relatively large, and the CaCl2 provides a significant amount of Ca2+. 2+ With Cl - The regulatory effect reached its optimal level.

[0066] Table 5. CaCO3 content of samples at different carbonation times

[0067] The maximum theoretical carbon sequestration capacity of red mud and the actual carbon sequestration capacity under different conditions are as follows: Figure 15 As shown. The theoretical carbon sequestration of red mud is much greater than the actual carbon sequestration calculated based on the mass loss rate. RMS-Ch3 showed a significant increase in carbon sequestration in the early stages of carbonation, reaching a maximum of 51.59 g / kg after 30 min of carbonation, which is 57.2% of the maximum theoretical carbon sequestration and the highest among all conditions. The carbon sequestration of RMS-Cc10 showed a fluctuating upward trend with increasing carbonation time, reaching 45.9347 g / kg after 180 min of carbonation, achieving 50.9% of the maximum theoretical carbon sequestration. -The complexation effect can delay the formation of the passivation layer, demonstrating long-term carbon fixation potential. RMS-D7 reached its peak carbon fixation at 60 min of carbonation, and the subsequent downward trend reflects the inhibitory effect of the passivation layer. The carbon fixation of RMS-5 first increased and then decreased with prolonged carbonation time, with an overall carbon fixation level lower than that of the sample with added external calcium source. CaO and MgO in red mud are the main minerals participating in the carbonation reaction, but due to the limited availability of soluble CaO... 2+ and Mg 2+ The concentration is very low, limiting carbonate formation. Na₂O and K₂O, as minor minerals involved in the carbonation reaction, require temperatures above 600℃ to react, making the reaction conditions extremely demanding. Furthermore, the CaCO₃ products generated in the later stages of carbonation coat the red mud particles, hindering CO₂ diffusion into the red mud, thus resulting in an actual carbon fixation capacity of red mud that is far lower than the theoretical carbon fixation capacity.

[0068] The calculated carbon sequestration efficiency of red mud under different conditions is shown in Table 6. RMS-D7 achieved the highest carbon sequestration efficiency (46.53%) after 60 min of carbonation, a 2.5% improvement compared to RMS-5. RMS-Ch3 achieved the highest carbonation efficiency (57.21%) after 30 min of carbonation, a 26% improvement compared to RMS-5. RMS-Cc10 achieved the highest carbonation efficiency (51.00%) after 180 min of carbonation, a 12.4% improvement compared to RMS-5. In summary, RMS-Ch3 exhibits the best carbon fixation capacity and highest carbon sequestration efficiency under all conditions. Furthermore, if the addition of Ca(OH)2 is to promote carbon fixation in red mud, the reaction time should be controlled within 30 min.

[0069] Table 6. Carbon sequestration efficiency of red mud under different conditions

[0070] The various embodiments in this specification are described in a progressive manner. Similar or identical parts between embodiments can be referred to interchangeably. Each embodiment focuses on describing the differences from other embodiments. In particular, the system embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments.

[0071] The above description is merely an embodiment of this application and is not intended to limit the scope of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of the claims of this application.

Claims

1. A method for mineralizing and sequestering carbon dioxide using red mud and calcium-based additives, characterized in that, A solution containing red mud and calcium source additives is placed in a reaction apparatus and CO2 gas is introduced to carry out the reaction; wherein, the solid-liquid ratio of the red mud suspension is 20-200 g / L; the CO2 gas concentration is 10-100%; the reaction system pressure is 0.1-0.4 MPa; and the calcium source is one or more of CaSO4, Ca(OH)2 or CaCl2.

2. The method for mineralizing and sequestering carbon dioxide using red mud and calcium source additives according to claim 1, characterized in that, The solid-liquid ratio of the red mud suspension is 20-100 g / L.

3. The method for mineralizing and sequestering carbon dioxide using red mud and calcium source additives according to claim 1, characterized in that, The CO2 gas concentration is 30-100%.

4. The method for mineralizing and sequestering carbon dioxide using red mud and calcium source additives according to claim 1, characterized in that, Reaction time ≥ 30 minutes.

5. The method for mineralizing and sequestering carbon dioxide using red mud and calcium source additives according to claim 1, characterized in that, The red mud suspension has a solid-liquid ratio of 50 g / L, a CO2 gas concentration of 100%, and a reaction system pressure of 0.3 MPa.

6. The method for mineralizing and sequestering carbon dioxide using red mud and calcium source additives according to claim 5, characterized in that, The calcium source is CaSO4, and the added mass of CaSO4 is 7% of the dry mass of red mud. The reaction time for adding CaSO4 is 60 min.

7. The method for mineralizing and sequestering carbon dioxide using red mud and calcium source additives according to claim 5, characterized in that, The calcium source is Ca(OH)2, and the added mass of Ca(OH)2 is 3% of the dry mass of red mud. The reaction time for adding Ca(OH)2 is 30 min.

8. The method for mineralizing and sequestering carbon dioxide using red mud and calcium source additives according to claim 5, characterized in that, The calcium source is CaCl2, and the mass of CaCl2 added is 10% of the dry mass of the red mud. The reaction time for adding CaCl2 is 180 min.

9. The method for mineralizing and sequestering carbon dioxide using red mud and calcium source additives according to claim 1, characterized in that, The carbon fixation products obtained after the reaction are used in building materials and roadbed fillers.

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