High-temperature stable environment-friendly calcium oxide process

Abstract

The invention relates to the field of calcium oxide production, and discloses a high-temperature stable environment-friendly calcium oxide process which comprises the following steps: (1) pretreatment of a biological template and preparation of a porous CaCO3 precursor: mixing the biological template with limestone powder, and freeze-drying to form the porous CaCO3 precursor; (2) microwave pulse calcination decomposition: performing microwave calcination on the precursor obtained in the step (1) under the protection of nitrogen, and decomposing the precursor into porous CaO; and (3) CO2 in-situ mineralization and coating layer formation: reacting the calcined tail gas generated in the step (2) with a magnesium salt solution to generate a coating layer on the surface of CaO. According to the invention, a totally-enclosed system and an inert gas dynamic regulation and control technical scheme are adopted, so that zero-exposure transfer of intermediate products is realized, and the problem of oxidation moisture absorption caused by frequent transfer of materials in the prior art is solved. And compared with the traditional open process, the product stability is improved by more than 3 times, and the environmental impurity pollution is thoroughly blocked.

Description

Technical Field

[0001] The invention relates to the technical field of calcium oxide production, in particular to a high-temperature stable and environmentally friendly calcium oxide process. Background Art

[0002] Calcium oxide (CaO), a fundamental industrial material, is widely used in environmental protection (such as flue gas desulfurization and carbon sequestration), metallurgical flux, and chemical catalysis. Its stability and reactivity, particularly under high-temperature conditions, directly impact the efficiency of industrial equipment and the level of pollutant control. The current mainstream process produces CaO by calcining limestone, supplemented by surface modification to enhance performance. However, due to limitations in traditional preparation methods, the product's performance still lags behind industrial requirements.

[0003] In traditional open production processes, intermediates are frequently exposed to the atmosphere, causing irreversible degradation of the CaO surface through hydroxylation and carbonation. Constant-power calcination results in localized overheating due to thermal inertia, deactivating the nanostructure due to grain coarsening. Mineralization reactions rely on millimeter-scale droplet mass transfer, limiting CO2 conversion efficiency due to kinetic hysteresis at the gas-liquid interface. A mismatch between the choice of precursor template and the freezing process can disrupt pore distribution and lead to the risk of carrier structure collapse. These combined shortcomings lead to poor product stability, high energy consumption, and limited feasibility for large-scale continuous production, severely restricting the engineering application of environmentally friendly calcium oxide in high-temperature environments. Summary of the Invention

[0004] In response to the shortcomings of the existing technology, the present invention provides a high-temperature stable and environmentally friendly calcium oxide process, which solves the problems in the existing technology such as surface degradation caused by exposure of intermediate products, grain coarsening and inactivation caused by thermal runaway during calcination, low mineralization mass transfer efficiency restricting the conversion rate, and carrier collapse caused by precursor structural defects.

[0005] To achieve the above objectives, the present invention is implemented through the following technical solutions: a high-temperature stable and environmentally friendly calcium oxide process, comprising the following steps:

[0006] (1) Biotemplate pretreatment and porous CaCO3 precursor preparation: The biotemplate was mixed with limestone powder and freeze-dried to form a porous CaCO3 precursor;

[0007] (2) Microwave pulse calcination and decomposition: the precursor obtained in step (1) is subjected to microwave calcination under nitrogen protection to decompose into porous CaO;

[0008] (3) CO2 in-situ mineralization and coating formation: the calcination tail gas produced in step (2) reacts with a magnesium salt solution to form a coating layer on the CaO surface;

[0009] (4) Post-processing and product activation: The coating layer is heat treated to obtain a high-temperature stable calcium oxide product.

[0010] One of the core innovations of this invention is the directional construction of porous structures induced by biological templates. The traditional process relies on high-temperature calcination to directly decompose limestone, while this invention introduces carboxymethylated cellulose nanofibers (CM-CNF) as biological templates. After mixing with limestone powder, it forms a three-dimensional interpenetrating network structure through self-assembly. The carboxyl groups on the surface of CM-CNF interact with Ca 2+ Through electrostatic adsorption and chemical bonding, CaCO3 is guided to deposit in a directional manner on the template surface. During the subsequent gradient freeze-drying process, ice crystal growth is restricted by the template network, forming a porous CaCO3 precursor with a controllable pore size distribution (10-50nm).

[0011] The biological template not only provides a physical framework for pore formation, but its chemical modification (carboxymethylation) enhances the binding of Ca 2+ The interaction between the two solves the problem of pore collapse caused by weak interfacial bonding in traditional pore-forming agents (polymer microspheres). Gradient freeze-drying controls ice crystal size in stages, avoiding the uneven pore size caused by single low-temperature freezing and laying the structural foundation for subsequent low-temperature efficient decomposition.

[0012] Traditional rotary kiln calcination relies on external heat conduction, requiring maintenance of high temperatures above 900°C and high energy consumption. The present invention, however, uses microwave pulse heating technology, which directly acts on CaCO3 molecules through dielectric loss to achieve rapid bulk heating. Microwave energy selectively excites the polar bonds (CO bonds) of CaCO3, causing it to decompose into CaO at 800-900°C, which is 100-300°C lower than conventional processes. The stepped power mode (5-7kW→9-11kW→5-7kW) combined with intermittent heating stops can dynamically regulate thermal stress and prevent sintering of the porous structure due to local overheating.

[0013] The bulk heating characteristics of microwaves avoid the thermal hysteresis effect of traditional heat conduction and significantly shorten the decomposition time (8-12 minutes). The synergistic effect of step power and dynamic nitrogen flow (nitrogen flow increased to 12-18L / min during the stop phase) effectively diffuses local heat, inhibits abnormal grain growth, and makes the high specific surface area (>50m 2 / g) is retained.

[0014] In traditional processes, calcination tail gas (containing CO2) is directly discharged or relies on high-cost carbon capture technology. However, the present invention innovatively reacts the tail gas with magnesium salt solution (MgCl2) in a closed system, generates nano-sized MgCO3 through gas-liquid micro-interface reaction and deposits it in situ on the CaO surface. 2+ Mineralization reaction (CO2+Mg 2+ +H2O→MgCO3+2H +), the generated MgCO3 is evenly coated on the surface of CaO pores with 5-15μm atomized droplets as the carrier.

[0015] The nano-MgCO3 coating decomposes into MgO during subsequent low-temperature heat treatment (250-350°C), forming a CaO@MgO core-shell structure. MgO acts as a physical barrier to inhibit CaO grain boundary migration, while its high melting point (2852°C) allows it to exist stably at high temperatures. This dual effect significantly enhances CaO's sintering resistance (surface area retention >80% at 1200°C). Furthermore, this step converts CO2 into usable MgCO3, achieving a synergistic effect of carbon emission reduction and improved product performance.

[0016] Traditional post-processing relies heavily on high-temperature sintering or mechanical mixing of additives. However, the present invention uses low-temperature heat treatment (250-350°C) to decompose MgCO3 into a MgO coating while retaining the active sites of porous CaO. During the heat treatment, the decomposition of MgCO3 releases trace amounts of CO2, further cleaning the CaO surface of adsorbed impurities and enhancing its chemical activity. The final product is CaO@MgO core-shell particles, and the thickness of the MgO coating can be controlled by the mineralization reaction time (10-30 minutes corresponds to 5-20nm).

[0017] Low-temperature heat treatment prevents CaO sintering caused by high temperatures. Furthermore, the interfacial compatibility between the MgO coating and CaO is superior to that achieved through mechanical mixing, resulting in a continuous and stable protective layer. In high-temperature applications (such as steel desulfurization), this core-shell structure preferentially reacts with acidic gases (SO2), protecting the active inner CaO core and extending the material's service life.

[0018] Preferably, in step (1):

[0019] The biological template is carboxymethyl-modified cellulose nanofibers (CM-CNF);

[0020] The freeze drying is a gradient freeze drying, comprising:

[0021] The pre-freezing temperature is 30-10°C and the time is 1-3 hours;

[0022] The deep freezing temperature is 90-70°C and the time is 3-5 hours.

[0023] Traditional bio-template (natural cellulose) has insufficient surface active groups and is not suitable for Ca 2+ The binding force of the precursor is weak, resulting in a loose and easily collapsed pore structure. The present invention introduces carboxylic acid groups (-COOH) on the surface of cellulose nanofibers (CNF) through carboxymethylation modification, which significantly enhances its binding to Ca 2+ During the mixing process, Ca 2+It preferentially adsorbs on the carboxylic acid sites of CM-CNF, inducing the directional growth of CaCO3 crystals along the template surface to form a three-dimensional interpenetrating network structure.

[0024] Carboxymethylation not only improves the interaction between template and Ca 2+ The chemical bonding strength is enhanced, and electrostatic repulsion prevents CaCO3 particle agglomeration, solving the problem of uneven pore formation in traditional templates. This directional growth mechanism enables controllable precursor porosity (>80%) and pore size distribution (10-50nm), laying the structural foundation for subsequent low-temperature decomposition and the formation of highly active CaO.

[0025] Traditional freeze-drying uses a single low temperature (-80°C) for direct freezing, and the rapid growth of ice crystals leads to an overly wide pore size distribution (1-100μm). The present invention uses phased gradient freezing, first pre-freezing at -30 to -10°C to form the initial ice crystal skeleton, and then deep freezing at -90 to -70°C to refine the ice crystal size. The larger ice crystals (micrometer-sized) formed in the pre-freezing stage provide the main framework for the pores, while the ultra-low temperature in the deep freezing stage inhibits the secondary growth of ice crystals, ultimately forming a uniform porous structure dominated by nanoscale pores.

[0026] Gradient freezing achieves a gradient distribution of pore size from micrometers to nanometers by regulating ice crystal dynamics in stages. This structure not only enhances the mechanical strength of the precursor (preventing collapse during drying) but also provides a rapid heat transfer path for microwave calcination, significantly reducing the energy required for decomposition. Furthermore, the ultra-low temperature during the deep freeze stage stabilizes the interface structure between the template and CaCO3, preventing pore wall fracture caused by phase transition stress during thawing.

[0027] Preferably, the carboxymethylation modification includes:

[0028] The cellulose nanofibers are dispersed in a 0.3-0.7 mol / L NaOH solution and reacted for 3-5 hours at a molar ratio of CNF to chloroacetic acid of 1:1.5-1:2.5.

[0029] In traditional carboxymethylation processes, excessively high NaOH concentrations (>1 mol / L) can lead to excessive swelling and even hydrolysis of cellulose chains, while too low a concentration (<0.3 mol / L) fails to fully activate hydroxyl groups (-OH) to promote etherification. The present invention limits the NaOH concentration to 0.3-0.7 mol / L. Within this range, NaOH can dissociate the cellulose hydrogen bond network to expose more hydroxyl sites while avoiding excessive damage to the fiber's nanostructure.

[0030] The moderate concentration of NaOH solution balances the reactivity and structural integrity of cellulose through the "limited swelling" strategy. Experiments show that 0.5 mol / L NaOH can increase the carboxymethyl substitution degree (DS) of CNF to 0.4-0.6, which is about 50% higher than the traditional process (DS < 0.3), while maintaining the fiber length (1-2 μm) and aspect ratio (> 50), which is conducive to the subsequent Ca 2+ Self-assembly provides active sites with high specific surface area.

[0031] Chloroacetic acid, an etherification agent, has a direct impact on the amount of carboxylic acid groups introduced. In conventional processes, an excess of chloroacetic acid (molar ratio of 1:3) leads to side reactions (chloroacetic acid self-polymerization), while an insufficient amount (less than 1:1.5) results in a low degree of substitution. The present invention limits the molar ratio of CNF to chloroacetic acid to 1:1.5 to 1:2.5, ensuring that each mole of CNF hydroxyl group reacts with 1.5 to 2.5 moles of chloroacetic acid.

[0032] At this ratio, chloroacetic acid preferentially reacts with the hydroxyl groups in the amorphous regions of cellulose, preserving the mechanical strength of the crystalline regions and achieving a balance between functionality and structural stability. Nuclear magnetic resonance (NMR) analysis shows that this ratio results in a uniform distribution of carboxylic acid groups, avoiding fiber breakage caused by localized oversubstitution and increasing the stability of CM-CNF in aqueous dispersions by more than three times (Zeta potential <-30mV).

[0033] Too short a reaction time (less than 3 hours) will result in incomplete etherification, while too long a reaction time (more than 5 hours) will trigger side reactions (cellulose oxidative degradation). The present invention controls the reaction time to 3 to 5 hours to match the kinetics of cellulose swelling, hydroxyl activation, and chloroacetic acid diffusion.

[0034] Online conductivity monitoring revealed that after 3 hours, the free chloroacetic acid concentration in the reaction system dropped to less than 10% of its initial value, indicating near-complete etherification. No significant fiber degradation products (gluconic acid) were detected within 5 hours. This time window ensures efficient and controllable carboxymethylation. The resulting CM-CNF has a carboxylic acid group density of 1.2-1.8 mmol / g, significantly superior to the 0.8-1.0 mmol / g achieved by conventional processes.

[0035] Preferably, in step (1):

[0036] The mass ratio of limestone powder to biological template is 8:1 to 10:1.

[0037] In traditional processes, the ratio of bio-template to inorganic material is typically low (1:1 to 3:1), resulting in the template network being unable to fully guide the orderly deposition of the inorganic phase, resulting in a loose pore structure that easily collapses. Experimental verification by the present invention has found that when the mass ratio of limestone powder to CM-CNF is less than 8:1 (i.e., the CM-CNF ratio is too high), the excess template fibers entangle with each other, hindering the uniform deposition of CaCO3 and forming a localized closed-pore structure. When the ratio is higher than 10:1, the template fibers' guiding effect is insufficient, CaCO3 particles randomly accumulate, and the porosity decreases significantly (<60%).

[0038] A mass ratio of 8:1 to 10:1 achieves a balance between the bio-template's "guided pore formation" and the inorganic phase's "structural support." At this ratio, the CM-CNF fiber surface is fully covered with CaCO3 particles, forming a continuous mineralized network while retaining sufficient template voids to serve as pore channels. Scanning electron microscopy (SEM) analysis shows that the precursor porosity corresponding to this ratio is 82% to 85%, with an average pore size of 20 to 40 nm and a uniform pore wall thickness (10 to 15 nm), significantly superior to the structural performance of conventional ratios.

[0039] The carboxylic acid groups (-COOH) on the surface of CM-CNFs bind to Ca via electrostatic adsorption. 2+ The mass ratio directly determines the density of active sites per unit volume. At a mass ratio of 8:1, each gram of CM-CNF corresponds to 8 grams of limestone powder (CaCO₃), equivalent to a CaCO₃ layer approximately 50-70 nm thick on the surface of each CM-CNF fiber. This loading ensures continuous CaCO₃ growth along the fiber axis while preventing cracking of the mineralized layer due to excessive thickness.

[0040] Precise control of the mass ratio enables Ca 2+ The adsorption rate on the CM-CNF surface reaches a dynamic equilibrium with the CaCO3 nucleation rate. When the mass ratio is higher than 10:1, Ca 2+ Insufficient supply leaves some template areas unmineralized, forming structural defects. When the mass ratio is lower than 8:1, rapid CaCO3 deposition leads to stress concentration in the mineralized layer, making pore wall fracture more likely during drying. X-ray diffraction (XRD) analysis shows that the degree of crystal orientation (Lotgering factor > 0.8) of CaCO3 in ratios of 8:1 to 10:1 is significantly higher than that of other ratios, demonstrating a highly ordered mineralized structure.

[0041] The choice of mass ratio must be compatible with the subsequent freeze-drying process. When the mass ratio is 8:1 to 10:1, the mineralized CM-CNF network has moderate rigidity, capable of resisting the mechanical stress generated by ice crystal growth during the gradient freezing process. If the mass ratio is too high (12:1), the mineralized network is insufficiently strong, and low-temperature brittleness during the deep freezing stage (-90 to -70°C) can cause pore wall fractures. If the mass ratio is too low (5:1), the excessive fiber content increases shrinkage stress during freeze-drying, increasing the risk of pore structure collapse.

[0042] A mass ratio of 8:1 to 10:1 gives the mineralized network an optimal elastic modulus (approximately 1.5 to 2.0 GPa), allowing it to withstand ice crystal growth pressure during the pre-freeze phase (-30 to -10°C) while maintaining stable pore morphology during the deep freeze phase (-90 to -70°C). Atomic force microscopy (AFM) mechanical testing shows that the mineralized network's compression rebound rate exceeds 90%, significantly higher than the 50% to 70% achieved with conventional ratios.

[0043] Preferably, in step (2):

[0044] The microwave calcination adopts a step power mode, including a power change of 5-7 kW → 9-11 kW → 5-7 kW, with each heating period of 1.5-2.5 minutes and an interval of 0.5-1.5 minutes;

[0045] The calcination temperature is 750-900°C.

[0046] Conventional microwave calcination uses constant power heating, which can easily lead to local overheating and pore structure collapse due to uneven energy input. The stepped power mode of the present invention matches the endothermic kinetic process of CaCO3 decomposition through three-stage dynamic regulation of "preheating-enhanced decomposition-stable cooling". In the first stage (5-7kW), the precursor is evenly heated to 500-600°C to avoid thermal shock caused by instantaneous high temperature; the second stage (9-11kW) provides high energy density to promote the rapid decomposition of CaCO3 at 800-900°C (the reaction activation energy is reduced by about 30%); the third stage (5-7kW) gradually reduces the power to relieve thermal stress and promote the orderly arrangement of CaO grains.

[0047] The synergistic effect of stepped power and intermittent pauses (nitrogen flow increased to 12-18 L / min during pauses) rapidly dissipates residual heat and inhibits abnormal grain growth. Experiments show that this mode achieves a CaO grain size of 20-50 nm (compared to >100 nm in conventional processes), and reduces the standard deviation of pore size distribution to 15% (compared to >40% in conventional processes), significantly improving structural stability.

[0048] Traditional rotary kiln calcination needs to maintain a high temperature of more than 1000℃ to ensure complete decomposition, but high temperature causes CaO sintering and densification (specific surface area <10m2 / g). The present invention leverages the selective heating properties of microwaves to achieve rapid decomposition of CaCO3 (decomposition rate >98%) at temperatures between 750°C and 900°C. Within this temperature range, the nucleation rate of CaO and the grain growth rate are balanced. Below 750°C, the decomposition reaction kinetics are slow (reaction time extended to >20 minutes); above 900°C, grain boundary migration intensifies, significantly increasing the risk of pore closure.

[0049] Microwave energy acts directly on CaCO₃ molecules through dielectric loss, stimulating polar bond (CO) vibrations and lowering the apparent temperature required for the decomposition reaction by 150-200°C. Infrared thermal imaging shows that the internal temperature gradient of CaCO₃ particles in the microwave field is less than 5°C / mm (compared to >50°C / mm with conventional heating), avoiding the "cold core" effect caused by delayed heat conduction and ensuring uniform decomposition.

[0050] The synergy between the stepped power mode and the 750-900°C temperature range results in two breakthroughs:

[0051] Energy consumption optimization: The total energy consumption is 8-12kW·h / kgCaO (traditional process>20kW·h / kg), and the energy saving efficiency is improved by more than 40%.

[0052] Structural protection: The power reduction (5-7kW) in the third stage is combined with nitrogen protection to form a thin amorphous oxide layer (thickness 1-2nm) on the CaO surface, which inhibits excessive penetration of CO2 in the subsequent mineralization step.

[0053] Innovation verification: X-ray photoelectron spectroscopy (XPS) analysis shows that the oxygen vacancy concentration on the CaO surface after step power calcination is 3.2×10 15 cm -2 , compared with constant power calcination (1.8×10 15 cm -2 ) increased by nearly 80%, indicating its higher surface activity.

[0054] Preferably, the nitrogen flow rate during the calcination process in step (2) is dynamically adjusted as follows:

[0055] Heating stage: 8-12 L / min;

[0056] Stop phase: 12-18 L / min.

[0057] In traditional calcination processes, the nitrogen flow rate is typically constant (10 L / min), resulting in the dissipation of microwave energy by excess gas convection, reducing heating efficiency. The present invention controls the nitrogen flow rate to 8-12 L / min during the heating phase. Experimental verification has shown that this flow rate range maintains an inert atmosphere in the reaction chamber (oxygen content <50 ppm) while avoiding microwave field distortion caused by excessive airflow.

[0058] Nitrogen plays a dual role during the heating phase:

[0059] Inhibit local oxidation: When the flow rate is greater than 8L / min, it can effectively isolate air from penetrating and prevent CaO surface oxidation to generate Ca(OH)2 or CaCO3 (XPS test shows that the thickness of the oxide layer is less than 2nm);

[0060] Stable microwave field: When the flow rate is less than 12L / min, the interference of gas turbulence on microwave distribution is weak (field intensity fluctuation is less than 5%), ensuring that the CaCO3 particles are heated evenly.

[0061] Computational fluid dynamics (CFD) simulation shows that the standard deviation of the temperature distribution in the microwave cavity at a flow rate of 10L / min is only ±8°C (±25°C under traditional constant flow rate), significantly improving the decomposition uniformity.

[0062] During the stop phase (0.5-1.5 minutes) of the stepped power mode, the nitrogen flow rate is increased to 12-18 L / min to rapidly dissipate residual heat through forced convection. Conventional processes neglect heat dissipation control during this phase, resulting in microcracks in the calcined product due to residual heat accumulation (cooling rates >200°C / min).

[0063] Thermal stress release: When the flow rate increases to 12-18 L / min, the Reynolds number (Re) transitions from laminar flow (Re < 2000) to turbulent flow (Re > 4000), and the heat transfer coefficient increases by 3-5 times, causing the surface temperature of the CaO particles to drop from 900°C to 300°C within 10 seconds, thereby suppressing structural deformation caused by grain boundary migration.

[0064] Pore ​​structure protection: Rapid cooling "freezes" the porous structure at high temperatures, preventing pore wall shrinkage caused by slow cooling (shrinkage rate drops from >15% to <5%). Scanning electron microscopy (SEM) shows that the CaO pore edges are clear in this mode, with no signs of melt sintering.

[0065] Dynamic regulation of nitrogen flow and stepped power mode form a spatiotemporal synergy:

[0066] The low flow rate (8-12L / min) in the heating stage matches the heating requirements of the stepped power, reducing energy loss;

[0067] The high flow rate (12-18 L / min) during the stop phase corresponds to the power interruption period, enhancing heat dissipation efficiency.

[0068] Innovation verification: Through thermogravimetric-mass spectrometry (TG-MS) analysis, dynamic flow regulation reduces the activation energy (Ea) of the CaCO3 decomposition reaction to 120kJ / mol (traditional process Ea>150kJ / mol), indicating its higher energy utilization efficiency. At the same time, the specific surface area of ​​CaO reaches 60-70m2 / g (BET test), which is more than 40% higher than the traditional nitrogen control process.

[0069] Preferably, in step (3):

[0070] The magnesium salt solution is a water-soluble magnesium salt solution, which is injected into the tail gas with a droplet size of 5 to 15 μm through ultrasonic atomization.

[0071] In traditional processes, magnesium salt (MgO powder) is difficult to achieve uniform coating at the gas-solid interface due to its poor dispersibility and low reaction activity. The present invention uses a water-soluble magnesium salt solution (MgCl2, MgSO4), which generates MgO by ionization. 2+ It can quickly migrate to the CO2 gas-liquid interface through liquid phase diffusion. When the solution concentration is 0.3~0.7mol / L, Mg 2+ The diffusion rate and CO2 dissolution rate reach a dynamic balance to avoid crystal coarsening caused by local oversaturation.

[0072] Water-soluble magnesium salts form free-moving Mg in the liquid phase 2+ , and HCO3 generated by dissolving CO2 - Mineralization reaction (Mg 2+ +2HCO3 - →MgCO3↓+CO2↑+H2O), the reaction rate is 3 to 5 times higher than that of solid phase reaction. According to Raman spectroscopy, the density of MgCO3 nucleation sites in the liquid phase is as high as 10 10 / cm 3 , ensuring the uniform generation of nano-sized MgCO3 particles (20-50nm).

[0073] The droplet size produced by traditional mechanical atomization (pressure nozzle) is large (>50μm), resulting in limited gas-liquid contact area and insufficient reaction time. The present invention uses ultrasonic atomization technology to break the magnesium salt solution into micro droplets of 5-15μm, with a specific surface area (>300m 2 / m 3 ) is more than 10 times higher than the traditional atomization method.

[0074] Optimized mass transfer efficiency: The Stokes number (Stk < 1) of 5-15 μm droplets indicates their strong ability to follow the airflow, forming stable aerosols in the exhaust gas and extending the gas-liquid contact time to 3-5 seconds (compared to < 1 second in traditional processes).

[0075] Reaction kinetics breakthrough: Convection-diffusion effect inside microdroplets enables Mg 2+ The mass transfer rate of CO2 was increased to 1.2×10 -4 mol / (m 2·s), the mineralization reaction completion rate is >95% (conventional process <70%). High-speed camera observations show that 5-15μm droplets exhibit a spiral motion trajectory in the airflow, further enhancing mixing efficiency.

[0076] The optimized design of droplet size must match the reaction temperature (50-80°C) and pressure (0.08-0.12MPa). When the droplet size is less than 5μm, the evaporation rate is too fast (less than 0.1 seconds), resulting in Mg 2+ If the reaction is not complete, the product will be deactivated. When the particle size is greater than 15 μm, the droplets will settle significantly due to gravity and will be difficult to disperse effectively in the exhaust gas.

[0077] The evaporation time of 5-15 μm droplets at 50-80°C (1-3 seconds) is highly consistent with the time required for the mineralization reaction (2-4 seconds), ensuring that MgCO3 completes nucleation and deposition before the droplets completely evaporate. Transmission electron microscopy (TEM) shows that the MgCO3 coating layer corresponding to this particle size range has a thickness of 10-20 nm and forms a tight chemical bond with the CaO substrate (interface energy <0.5 J / m 2 ).

[0078] Preferably, the water-soluble magnesium salt is MgCl2 or MgSO4.

[0079] In traditional processes, magnesium salts (MgO or Mg(OH)2) have low solubility (MgO solubility is about 0.01g / 100mL), so strong acid or high temperature conditions are required to release Mg. 2+ , resulting in low reaction efficiency. The present invention uses MgCl2 (solubility 54.3g / 100mL) or MgSO4 (solubility 35.1g / 100mL), which can quickly dissociate into free Mg in aqueous solution. 2+ The ion concentration can reach 1.2-1.8 mol / L (pH 5-6), significantly improving the mineralization reaction rate with CO2.

[0080] The Cl released during the dissociation of MgCl2 and MgSO4 - or SO4 2- As a counterion, it reduces Mg by the charge shielding effect 2+ The hydration energy of MgCl2 promotes its migration to the gas-liquid interface. 2+ The diffusion coefficient is 1.2×10 -9 m 2 / s, compared with MgSO4 (0.9×10 -9 m 2 / s) is higher, but the crystal form controllability of its mineralized product (MgCO3) is slightly inferior to that of the MgSO4 system. The two can be flexibly selected according to process requirements, taking into account both reaction rate and product uniformity.

[0081] The anion types of MgCl2 and MgSO4 directly affect the crystallization behavior of the mineralization products. - Due to the weak polarization effect, MgCO3 is allowed to grow preferentially along the (104) crystal plane to form flake nanoparticles (thickness 10-20nm); while SO4 2- The strong charge effect can be adsorbed on the surface of MgCO3 crystal nuclei, inhibiting anisotropic growth and forming spherical particles (diameter 20-50nm).

[0082] MgCl2 system: Cl - The weak coordination ability of MgCO3 makes the crystallization process dominated by the interface energy, and the flake structure is more likely to form a close contact with the CaO substrate (interface binding energy>1.5J / m 2 );

[0083] MgSO4 system: SO4 2- By suppressing the difference in crystal growth through adsorption, the packing density of spherical particles is higher (>80%) and the porosity of the coating layer is <5%).

[0084] Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis showed that the (104) crystal plane diffraction peak intensity of the MgCl2 mineralization product was 30% higher than that of the MgSO4 system, verifying its oriented growth characteristics.

[0085] Some magnesium salts (Mg(NO3)2) release harmful by-products (NO3 - ), while the anions of MgCl2 and MgSO4 (Cl - 、SO4 2- ) is chemically inert under mineralization conditions and will not participate in redox reactions. At a mineralization temperature of 50-80°C, Cl - and SO4 2- It exists in the liquid phase in a free state and is finally removed through a water washing step to avoid impurity residues.

[0086] Cl - and SO4 2- The low reactivity ensures the purity of the mineralized product (MgCO3 content> 99%), while its high solubility in water (Cl - Solubility 355g / L, SO4 2- Solubility 33g / L) is convenient for subsequent wastewater treatment. Ion chromatography (IC) detection shows that Cl in the wastewater - or SO4 2- The concentration can be reduced to <10ppm, meeting environmental emission standards.

[0087] Preferably, in step (3):

[0088] The mineralization reaction temperature is 50-80° C., the pressure is 0.08-0.12 MPa, and the reaction time is 10-30 minutes.

[0089] Conventional mineralization processes are usually carried out at room temperature (25°C) or high temperature (>100°C). The former has a low reaction rate (CO2 conversion rate <50%), while the latter has high energy consumption and is prone to side reactions (MgCO3 dehydration to produce MgO). The present invention controls the temperature to 50-80°C. In this temperature range, the Henry constant (solubility) of CO2 is 0.8-1.2 mol / (L·atm), which is 2-3 times higher than that at room temperature. At the same time, the nucleation rate of MgCO3 (10 10 ~10 12 nuclei / (m 3 ·s)) reaches a dynamic equilibrium with the crystal growth rate (1~3nm / s).

[0090] Mild heating conditions of 50-80°C offer dual advantages:

[0091] Promote CO2 dissolution: Increased temperature reduces liquid viscosity and accelerates CO2 mass transfer from gas phase to liquid phase (mass transfer coefficient increases to 1.5×10 -4 m / s);

[0092] Inhibition of side reactions: When the temperature is less than 80°C, the activation energy barrier (>150kJ / mol) of the dehydration reaction of MgCO3 (MgCO3→MgO+CO2) is not broken, ensuring the purity of the product is greater than 99%. Infrared spectroscopy (FTIR) analysis shows that no MgO characteristic peak (470cm -1 ), verifying the uniqueness of the reaction pathway.

[0093] The gas-liquid mass transfer efficiency at atmospheric pressure (0.1 MPa) is limited by the low partial pressure of CO2, while high pressure (>0.15 MPa) requires complex sealing equipment. The present invention fine-tunes the pressure to 0.08-0.12 MPa, and achieves a match between the gas-liquid interface renewal rate and the reaction rate by slightly increasing (or reducing) the pressure. When the pressure is 0.1 MPa, the mass transfer flux of CO2 in the liquid phase reaches 2.5×10 -5 mol / (m 2 ·s), which is 20% higher than that at normal pressure. When the pressure is greater than 0.12MPa, the bubble merging effect is aggravated and the effective contact area is reduced.

[0094] The pressure window of 0.08-0.12 MPa optimizes gas-liquid mixing through a “micro-perturbation” strategy:

[0095] 0.08~0.1MPa (slightly lower than normal pressure): micron-sized bubbles (10~50μm in diameter) are formed to increase the gas-liquid contact area;

[0096] 0.1-0.12 MPa (slightly higher than atmospheric pressure): Inhibits bubble merging and prolongs gas-liquid contact time. Computational fluid dynamics (CFD) simulations show that at 0.1 MPa, the gas-liquid interface update frequency is 120-150 Hz, a 50% increase compared to atmospheric pressure (80-100 Hz), significantly enhancing mass transfer efficiency.

[0097] Traditional processes often extend reaction times to several hours in pursuit of high conversion rates, resulting in increased energy consumption and excessive product growth (coating thickness > 50nm). Experiments conducted by the present invention have confirmed that the mineralization reaction can achieve 80% CO2 conversion within 10 minutes, with conversion rates reaching >95% at 30 minutes. Further extensions in reaction time contribute less than 2% to the conversion rate increase.

[0098] Nucleation stage (0-10 minutes): MgCO3 nucleus density increases rapidly to 10 15 / m 3 , covering >90% of the CaO surface;

[0099] Growth stage (10 to 30 minutes): The grains grow epitaxially along the CaO pores, and the thickness increases from 5 nm to 20 nm, forming a continuous coating layer.

[0100] Real-time monitoring using a quartz crystal microbalance (QCM) revealed that the mass deposition rate of the coating layer approached zero at 20 minutes, indicating that the reaction was nearing its endpoint. X-ray photoelectron spectroscopy (XPS) revealed no significant difference in the composition of the coating layer after 10 and 30 minutes (Mg / Ca atomic ratios were both between 0.25 and 0.3), confirming the feasibility of a short, efficient reaction.

[0101] Preferably, steps (1) to (3) are all completed in a closed system, and the exposure time of the intermediate product is less than 5 minutes.

[0102] In traditional processes, materials must be transferred multiple times between steps, exposing intermediate products to the environment (humidity, oxygen), leading to moisture absorption and oxidation. The closed system of the present invention connects modular reaction units (precursor preparation - microwave calcination - mineralization coating) in series, using a nitrogen circulation pipeline to maintain internal oxygen content below 50ppm and humidity below 5%RH. Material transfer between units is achieved through pneumatic valves and screw feeders, without manual intervention.

[0103] Inert atmosphere maintenance: Nitrogen positive pressure (0.05-0.1MPa) runs through the entire system to block air infiltration and prevent CaO from absorbing moisture to generate Ca(OH)2 (moisture absorption weight gain rate <0.1%, traditional process >3%);

[0104] Pollution prevention and control: The closed system isolates external dust and impurities. X-ray fluorescence spectroscopy (XRF) shows that the content of impurity elements (Fe, Al) in the product is less than 50ppm, which is 90% lower than that of the open system.

[0105] The exposure time of the intermediate products (porous CaCO3 precursor, nano-CaO) during the transport process must be strictly limited. By optimizing the feed rate (0.5-1.5 kg / min) and the path length (<3 m), the present invention ensures that the interval from the end of step (1) freeze-drying to the start of step (2) microwave calcination is less than 3 minutes, and the transition time from step (2) to step (3) is less than 2 minutes.

[0106] Surface active site protection: After CaO is exposed to air for 5 minutes, its specific surface area decreases by more than 30% due to moisture absorption and carbonation (BET test). However, the present invention controls the exposure time to less than 5 minutes, and the specific surface area retention rate is more than 98%;

[0107] Enhanced reaction continuity: The short exposure time matches the surface reaction kinetics of the intermediate product. The oxygen vacancy concentration on the CaO surface decays to 60% of the initial value after 10 minutes of exposure, while it only decays by <5% within 5 minutes (electron paramagnetic resonance EPR analysis).

[0108] The closed system's inert environment and short exposure time work together to overcome performance fluctuations caused by environmental interference in traditional processes:

[0109] Protection of humidity sensitive materials: The porous CaCO3 precursor will partially hydrolyze into Ca(HCO3)2 at humidity > 30% RH. The closed system humidity < 5% RH completely blocks this side reaction (no HCO3 is detected by Raman spectroscopy). - characteristic peaks);

[0110] Maintaining thermodynamic stability: Nano-CaO is prone to sintering in air due to its high surface energy. Low-temperature (<50°C) transmission in a closed system inhibits grain boundary migration, and the grain size growth is <5% (TEM observation).

[0111] The present invention provides a high-temperature stable and environmentally friendly calcium oxide process. It has the following beneficial effects:

[0112] 1. This invention utilizes a fully enclosed system and dynamic inert gas control technology to achieve zero-exposure transfer of intermediate products, resolving the oxidation and moisture absorption issues associated with frequent material transfer in existing technologies. Compared to traditional open processes, product stability is more than three times greater, completely eliminating environmental contamination by impurities.

[0113] 2. This invention overcomes the grain coarsening bottleneck of traditional constant-temperature calcination by combining stepped-power microwave calcination with intermittent heat dissipation. While existing technologies severely sinter due to sustained high temperatures, this invention preserves the active surface of the nano-calcium oxide, increasing adsorption capacity by 2.2 times.

[0114] 3. This invention combines micro-droplet atomization with pressure micro-perturbation technology to improve the gas-liquid mass transfer efficiency of the CO2 mineralization reaction to near saturation levels. Existing mechanical stirring or spraying methods suffer from low mass transfer efficiency and take several hours to complete the reaction. This invention achieves efficient conversion in just 10 to 30 minutes.

[0115] 4. This invention, based on the directional growth mechanism of ice crystals induced by gradient freezing, overcomes the uneven porosity distribution and structural collapse inherent in conventional freeze-drying. Compared to a single-stage deep-freeze process, the present invention increases the compressive strength of the precursor by 40%, providing reliable support for industrial continuous production. BRIEF DESCRIPTION OF THE DRAWINGS

[0116] Figure 1 Schematic diagram of the method of the present invention. DETAILED DESCRIPTION

[0117] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the drawings in the present specification. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0118] Example 1: Preparation of high-porosity calcium oxide

[0119] Step (1) Precursor preparation:

[0120] Raw material ratio: limestone powder to CM-CNF mass ratio 8:1;

[0121] Freeze drying: gradient freezing (-20℃ pre-freezing for 2h → -80℃ deep freezing for 6h);

[0122] Precursor indicators: porosity 85%, average pore diameter 25nm.

[0123] Step (2) microwave calcination:

[0124] Power mode: Step power 5kW (2min) → 9kW (2.5min) → 5kW (2min), with a 1min stop interval;

[0125] Calcination temperature: 750℃, total time: 12min;

[0126] Nitrogen control: 8L / min during heating phase, 12L / min during stopping phase;

[0127] CaO index: specific surface area 70m 2 / g, grain size 25nm.

[0128] Step (3) CO2 mineralization and coating:

[0129] Magnesium salt solution: 0.3 mol / LMgCl2, ultrasonic atomization droplet size 5 μm;

[0130] Reaction conditions: temperature 50°C, pressure 0.08 MPa, time 10 min.

[0131] Example 2: Preparation of high mechanical strength calcium oxide

[0132] Step (1) Precursor preparation:

[0133] Raw material ratio: limestone powder to CM-CNF mass ratio 10:1;

[0134] Freeze drying: gradient freezing (-30℃ pre-freezing for 1.5h→-70℃ deep freezing for 5h);

[0135] Precursor indicators: porosity 80%, average pore diameter 35nm, compressive strength 8MPa.

[0136] Step (2) microwave calcination:

[0137] Power mode: Step power 7kW (1.5min) → 11kW (2min) → 7kW (1.5min), with an interval stop of 0.5min;

[0138] Calcination temperature: 900℃, total time: 9min

[0139] Nitrogen control: 12 L / min during heating phase, 18 L / min during stopping phase;

[0140] CaO index: specific surface area 55m 2 / g, grain size 40nm, compressive strength 7.5MPa.

[0141] Step (3) CO2 mineralization and coating:

[0142] Magnesium salt solution: 0.7 mol / LMgSO4, ultrasonic atomization droplet size 15 μm;

[0143] Reaction conditions: temperature 80°C, pressure 0.12 MPa, time 30 min.

[0144] Example 3: Preparation of Rapidly Mineralized Calcium Oxide

[0145] Step (1) Precursor preparation:

[0146] Raw material ratio: limestone powder to CM-CNF mass ratio 9:1;

[0147] Freeze drying: gradient freezing (-25℃ pre-freezing for 3h → -90℃ deep freezing for 4h);

[0148] Precursor indicators: porosity 82%, average pore diameter 30nm.

[0149] Step (2) microwave calcination:

[0150] Power mode: Step power 6kW (2min) → 10kW (2min) → 6kW (2min), with an interval stop of 1.5min;

[0151] Calcination temperature: 850℃, total time: 10min;

[0152] Nitrogen control: 10L / min during heating phase, 15L / min during stopping phase;

[0153] CaO index: specific surface area 65m 2 / g, grain size 30nm.

[0154] Step (3) CO2 mineralization and coating:

[0155] Magnesium salt solution: 0.5 mol / LMgCl2 / MgSO4 (1:1 mixture), ultrasonic atomization droplet size 10 μm;

[0156] Reaction conditions: temperature 70°C, pressure 0.1 MPa, time 20 min.

[0157] Comparative Example 1 (corresponding to Example 1)

[0158] The difference from Example 1 is that in step (1), the mass ratio of limestone powder to CM-CNF is 5:1 (outside the range of 8:1 to 10:1), and the other conditions are the same.

[0159] Comparative Example 2 (corresponding to Example 1)

[0160] The difference from Example 1 is that in step (2), the microwave calcination adopts a constant power of 9 kW (the step power mode is cancelled), and the other conditions are the same.

[0161] Comparative Example 3 (corresponding to Example 2)

[0162] The difference from Example 2 is that the magnesium salt solution in step (3) is replaced by Mg(NO3)2 solution (not preferably MgCl2 / MgSO4), and the other conditions are the same.

[0163] Comparative Example 4 (corresponding to Example 2)

[0164] The difference from Example 2 is that the particle size of the ultrasonic atomized droplets in step (3) is 50 μm (outside the range of 5 to 15 μm), and the other conditions are the same.

[0165] Comparative Example 5 (corresponding to Example 3)

[0166] The difference from Example 3 is that in step (1), freeze drying adopts a single-stage -80°C deep freezing (gradient freezing is eliminated), and the other conditions are the same.

[0167] Comparative Example 6 (corresponding to Example 3)

[0168] The difference from Example 3 is that the mineralization reaction temperature in step (3) is 25° C. (lower than the range of 50 to 80° C.), and the other conditions are the same.

[0169] Comparative Example 7 (corresponding to Example 1)

[0170] The difference from Example 1 is that the calcination temperature in step (2) is 1000° C. (outside the range of 750-900° C.), and the other conditions are the same.

[0171] Comparative Example 8 (corresponding to Example 2)

[0172] The difference from Example 2 is that the reaction time in step (3) is 60 minutes (outside the range of 10 to 30 minutes), and the other conditions are the same.

[0173] Comparative Example 9 (corresponding to Example 3)

[0174] The difference from Example 3 is that the nitrogen flow rate in step (2) is fixed at 10 L / min throughout the process (dynamic adjustment is cancelled), and the other conditions are the same.

[0175] Comparative Example 10 (corresponding to Example 1)

[0176] The difference from Example 1 is that the pressure in step (3) is 0.05 MPa (lower than the range of 0.08 to 0.12 MPa), and the other conditions are the same.

[0177] Experiment 1: Precursor preparation and structural regulation verification

[0178] Description of experimental steps:

[0179] Raw material mixing:

[0180] Example 1-3: Limestone powder and CM-CNF were mixed at a mass ratio of 8:1, 10:1, and 9:1, respectively, and deionized water (solid content 10%) was added and stirred for 2 hours.

[0181] Comparative Example 1: mass ratio 5:1, other parameters are the same as Example 1.

[0182] Comparative Example 5: mass ratio 9:1, but gradient freezing was canceled and deep freezing at -80°C was performed directly for 6 hours.

[0183] Freeze drying:

[0184] Example 1-3: Gradient freezing (pre-freezing -20°C / 2h→deep freezing -80°C / 6h);

[0185] Comparative Example 1: Same as Example 1;

[0186] Comparative Example 5: Single-stage deep freezing at -80°C / 6h.

[0187] Precursor characterization:

[0188] SEM test: observe pore structure and pore size distribution;

[0189] BET specific surface area: calculate porosity and average pore size;

[0190] Compressive strength test: measured using a universal testing machine (loading rate 1 mm / min).

[0191] The experimental data are shown in Table 1 below:

[0192] Table 1: Precursor performance comparison

[0193]

[0194] The porous structure of the precursor directly determines the efficiency of subsequent calcination and mineralization reactions. When the mass ratio of limestone to CM-CNF is reduced to 5:1 (Comparative Example 1), the template effect of CM-CNF is overly diluted and cannot effectively guide the directional deposition of CaCO3, resulting in a cliff-like drop in porosity to 59.8%, and a significant coarsening of the pore size distribution (48.5nm). SEM images show that the precursor of Comparative Example 1 presents a locally collapsed structure, while the gradient freezing process of Examples 1-3 suppresses ice crystal growth in stages, making the pore size uniformly distributed in the range of 25 to 35nm, providing a highly active surface for subsequent calcination.

[0195] The single-stage freezing of Comparative Example 5 was directly deep-frozen to -80°C. Due to the excessively fast cooling rate, ice crystals formed large-sized channels (52.1nm) in disordered growth, and the porosity dropped to 74.9%. This is closely related to the microcrystalline nuclei formed in the pre-freezing stage (-20°C) in the gradient freezing. The pre-freezing process guides the orderly migration of water by slowly cooling, avoiding stress concentration in the deep freezing stage, thereby maintaining the integrity of the pore structure. The compressive strength test further confirmed that the mass ratio of 10:1 (Example 2) has a higher compressive strength of 8.1MPa due to the higher cross-linking density of CM-CNF, while the imbalance of mass ratio (Comparative Example 1) or the loss of control of freezing (Comparative Example 5) both lead to deterioration of mechanical properties.

[0196] From a mechanistic perspective, the carboxylic acid groups of CM-CNFs chelate Ca 2+The mineralization sites are regulated, and the phase transition dynamics of gradient freezing determine the path of pore structure formation. The optimal mass ratio range of 8:1 to 10:1 ensures a dynamic balance between the template and the mineralized component, giving the precursor both high porosity and mechanical stability, laying the structural foundation for the subsequent efficient preparation of nano-CaO.

[0197] Experiment 2: Verification of synergy of microwave calcination process

[0198] Description of experimental steps:

[0199] Calcination parameter settings:

[0200] Example 1-3: Calcination was performed in a stepped power mode (Example 1: 5 kW → 9 kW → 5 kW) with a dynamic nitrogen flow rate (8-12 L / min in the heating stage, 12-18 L / min in the stopping stage), and the temperature was controlled at 750-900°C.

[0201] Comparative Example 2: constant power 9 kW, other parameters are the same as Example 1.

[0202] Comparative Example 7: The calcination temperature was raised to 1000° C., and the rest was the same as in Example 1.

[0203] Comparative Example 9: The nitrogen flow rate was fixed at 10 L / min throughout the process, and the rest was the same as in Example 3.

[0204] Product characterization:

[0205] TEM: observe CaO grain size and pore structure;

[0206] BET: Determination of specific surface area and pore volume;

[0207] XPS: analyze the surface oxygen vacancy concentration (O_vacancy).

[0208] The experimental data are shown in Table 2 below:

[0209] Table 2: Comparison of calcined product properties

[0210]

[0211]

[0212] The stepped power mode effectively balances the decomposition kinetics of CaCO3 and the grain growth rate through intermittent heating and cooling cycles. The constant power in Example 2 leads to local overheating, and the grain size increases to 52.8nm, while the specific surface area decreases to 38.6m 2 / g, while the stepped power in Example 1 inhibited grain boundary migration through staged heat dissipation (stop stage), keeping the grain size at 25.3nm. XPS results showed that the oxygen vacancy concentration (12.5%) under the stepped power was significantly higher than that under the constant power (5.3%), indicating that the dynamic thermal field can stabilize surface active sites and enhance the activity of subsequent mineralization reactions.

[0213] The effect of calcination temperature on the product structure has a critical threshold. Example 2 increases the grain size to 40.5 nm at 900 °C, but still maintains 55.2 nm. 2 / g specific surface area, while in comparative example 7, due to the sintering effect at 1000℃, the grains coarsened to 80.2nm, and the specific surface area dropped drastically to 15.3m 2 / g. At high temperatures, the rate of grain boundary migration far exceeds the mass transfer limitation of the decomposition reaction, leading to pore collapse and loss of activity. This phenomenon is directly related to the temperature dependence of CaO surface energy. The optimal temperature window (750-900°C) maintains the stability of the nanostructure by suppressing the Ostwald ripening process.

[0214] Dynamic nitrogen flow regulation plays an irreplaceable role in protecting surface oxygen vacancies. While the fixed nitrogen flow rate in Comparative Example 9 resulted in an increase in the local oxygen partial pressure during calcination, reducing the oxygen vacancy concentration to 4.7%, the dynamic regulation (10 → 15 L / min) in Example 3 maintained the oxygen vacancy concentration at 11.2% by intermittently flushing the reaction interface. The synergistic effect of nitrogen flow and stepped power achieved a dynamic equilibrium between grain growth and surface active site exposure, providing a highly active substrate for subsequent mineralization reactions.

[0215] Experiment 3: Verification of mineralization reaction efficiency and coating quality

[0216] Description of experimental steps:

[0217] Mineralization reaction settings:

[0218] Examples 1-3: The calcined CaO was placed in a mineralization reactor, and CO2 mineralization was carried out using different magnesium salts (MgCl2, MgSO4, mixed salts), ultrasonic atomized droplets (5-15 μm), temperature (50-80°C), pressure (0.08-0.12 MPa) and time (10-30 minutes).

[0219] Comparative Example 3: The magnesium salt was replaced by Mg(NO3)2, and the rest was the same as in Example 2.

[0220] Comparative Example 4: The droplet size was increased to 50 μm, and the rest was the same as in Example 2.

[0221] Comparative Example 6: The reaction temperature was lowered to 25°C, and the rest was the same as in Example 3.

[0222] Comparative Example 8: The reaction time was extended to 60 minutes, and the rest was the same as in Example 2.

[0223] Comparative Example 10: The pressure was reduced to 0.05 MPa, and the rest was the same as in Example 1.

[0224] Product characterization:

[0225] Gas chromatography (GC): Determination of CO2 conversion rate;

[0226] SEM-EDS: analysis of coating thickness, coverage and element distribution;

[0227] XRD: Detect by-products (unreacted Mg(OH)2 or CaCO3).

[0228] The experimental data are shown in Table 3 below:

[0229] Table 3: Comparison of mineralization product properties

[0230]

[0231]

[0232]

[0233] The choice of magnesium salt type directly affects the mineralization reaction path and by-product generation. When Mg(NO3)2 is used in Comparative Example 3, nitrate ions undergo a partial reduction reaction in an acidic reaction environment to generate NO2 - By-products (XRD detected 8.6% impurities), while in Example 2, when MgSO4 was used, the stable coordination effect of sulfate inhibited the side reaction and the by-product content was only 1.1%. 2+ With CO3 2- The difference in binding energy results in a thinner coating layer (10.2 nm) in the MgCl2 system (Example 1), while the coating layer in the MgSO4 system (Example 2) thickens to 19.8 nm due to the slower ion migration rate, but still maintains a high coverage (>95%).

[0234] The correlation between droplet size and mass transfer efficiency is particularly significant in the data. When the droplet size of Comparative Example 4 is increased to 50 μm, the gas-liquid contact area is greatly reduced, the CO2 conversion rate drops to 61.2%, and the coating thickness is only 8.3 nm (SEM shows that there is no coverage in some areas). In contrast, the micro-droplets (5-15 μm) of Examples 1-3 are ultrasonically atomized to form an aerosol interface, which makes Mg 2+ The mass transfer flux with CO2 increased by more than 3 times. Dynamic pressure control (0.08-0.12MPa) further optimized the bubble distribution. When the pressure of comparative example 10 was reduced to 0.05MPa, the bubbles merged and caused a mass transfer dead zone, and the CO2 conversion rate dropped to 69.4%.

[0235] The balance between the temperature control window and the reaction time is the key to the mineralization efficiency. The reaction rate of Comparative Example 6 is extremely low at 25°C (conversion rate 42.8%), and the low temperature leads to incomplete nucleation of MgCO3 (coating layer thickness 5.1nm), while Example 3 breaks through the activation energy barrier at 70°C and the conversion rate reaches 92.5%. It is worth noting that although the conversion rate of Comparative Example 8 increases slightly to 95.1% by extending the reaction time to 60 minutes, the excessive growth of the coating layer (31.7nm) causes pore blockage, which is not conducive to subsequent applications. This shows that the preferred time window (10-30 minutes) achieves the best balance between efficiency and quality by suppressing excessive crystal growth.

[0236] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. High temperature stable environmentally friendly calcium oxide process, characterized by: The following steps are involved: (1) Biotemplate pretreatment and porous CaCO3 precursor preparation: The biotemplate was mixed with limestone powder and freeze-dried to form a porous CaCO3 precursor; (2) Microwave pulse calcination and decomposition: the precursor obtained in step (1) is subjected to microwave calcination under nitrogen protection to decompose into porous CaO; (3) CO2 in-situ mineralization and coating formation: the calcination tail gas produced in step (2) reacts with a magnesium salt solution to form a coating layer on the CaO surface; (4) Post-processing and product activation: The coating layer is heat treated to obtain a high-temperature stable calcium oxide product.

2. The high-temperature stable and environmentally friendly calcium oxide process according to claim 1, characterized in that: In step (1): The biological template is carboxymethyl-modified cellulose nanofiber; The freeze drying is a gradient freeze drying, comprising: The pre-freezing temperature is 30-10°C and the time is 1-3 hours; The deep freezing temperature is 90-70°C and the time is 3-5 hours.

3. The high temperature stable and environmentally friendly calcium oxide process according to claim 2, characterized in that: Carboxymethylation modifications include: The cellulose nanofibers are dispersed in a 0.3-0.7 mol / L NaOH solution and reacted for 3-5 hours at a molar ratio of CNF to chloroacetic acid of 1:1.5-1:2.

5.

4. The high-temperature stable and environmentally friendly calcium oxide process according to claim 1, characterized in that: In step (1): The mass ratio of limestone powder to biological template is 8:1 to 10:

1.

5. The high temperature stable and environmentally friendly calcium oxide process according to claim 1, characterized in that: In step (2): The microwave calcination adopts a step power mode, including a power change of 5-7 kW → 9-11 kW → 5-7 kW, with each heating period of 1.5-2.5 minutes and an interval of 0.5-1.5 minutes; The calcination temperature is 750-900°C.

6. The high-temperature stable and environmentally friendly calcium oxide process according to claim 1, characterized in that: The nitrogen flow rate during the calcination process in step (2) is dynamically adjusted to: Heating stage: 8-12 L / min; Stop phase: 12-18 L / min.

7. The high temperature stable and environmentally friendly calcium oxide process according to claim 1, characterized in that: In step (3): The magnesium salt solution is a water-soluble magnesium salt solution, which is injected into the tail gas with a droplet size of 5 to 15 μm through ultrasonic atomization.

8. The high-temperature stable and environmentally friendly calcium oxide process according to claim 7, characterized in that: The water-soluble magnesium salt is MgCl2 or MgSO4.

9. The high-temperature stable and environmentally friendly calcium oxide process according to claim 1, characterized in that: In step (3): The mineralization reaction temperature is 50-80° C., the pressure is 0.08-0.12 MPa, and the reaction time is 10-30 minutes.

10. The high temperature stable and environmentally friendly calcium oxide process according to claim 1, characterized in that: Steps (1) to (3) are all completed in a closed system, and the exposure time of the intermediate product is less than 5 minutes.

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