High-temperature stable environment-friendly calcium oxide process

Abstract

The application 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) biological template pretreatment and porous CaCO3 precursor preparation: mixing a biological template with limestone powder, and performing freeze drying to form a porous CaCO3 precursor; (2) microwave pulse calcination and decomposition: performing microwave calcination on the precursor obtained in the step (1) under nitrogen protection, and decomposing the precursor into porous CaO; and (3) CO2 in-situ mineralization and coating layer formation: reacting calcination tail gas generated in the step (2) with a magnesium salt solution to generate a coating layer on the surface of the CaO. The application adopts a full-closed system and inert gas dynamic regulation technology scheme, realizes zero exposure transmission of an intermediate product, and solves the problem of oxidation and moisture absorption caused by frequent transfer of materials in the prior art. Compared with a traditional open process, the product stability is improved by more than 3 times, and environmental impurity pollution is completely blocked.

Description

Technical Field

[0001] This invention relates to the field of calcium oxide production technology, specifically a high-temperature stable and environmentally friendly calcium oxide process. Background Technology

[0002] Calcium oxide (CaO), as a basic industrial material, is widely used in environmental protection (such as flue gas desulfurization and carbon fixation), metallurgical flux, and chemical catalysis. Especially under high-temperature conditions, its stability and reactivity directly affect the efficiency of industrial equipment and the level of pollutant control. Currently, the mainstream process for preparing CaO involves calcining limestone, supplemented by surface modification to improve its properties. However, due to limitations of traditional preparation methods, the performance of the product still falls short of industrial requirements.

[0003] In traditional open production processes, intermediate products are frequently exposed to the atmospheric environment, leading to irreversible degradation through hydroxylation and carbonation on the CaO surface. Constant-power calcination causes localized overheating due to thermal inertia, resulting in the deactivation of nanostructures due to grain coarsening. The mineralization reaction relies on millimeter-scale droplet mass transfer, and CO2 conversion efficiency is limited by the sluggish dynamics of the gas-liquid interface. Mismatch between the selection of precursor template agents and the freezing process leads to disordered pore distribution and the risk of carrier structural collapse. These combined defects result in poor product stability, high energy consumption, and insufficient 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] To address the shortcomings of existing technologies, this invention provides a high-temperature stable and environmentally friendly calcium oxide process, which solves the problems of surface degradation caused by exposure of intermediate products, grain coarsening and deactivation due to calcination thermal runaway, low mineralization mass transfer efficiency restricting the conversion rate, and carrier collapse caused by precursor structural defects in existing technologies.

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

[0006] (1) Pretreatment of biological template and preparation of porous CaCO3 precursor: The biological template was mixed with limestone powder and freeze-dried to form a porous CaCO3 precursor;

[0007] (2) Microwave pulse calcination decomposition: The precursor obtained in step (1) is calcined under nitrogen protection by microwave to decompose it into porous CaO;

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

[0009] (4) Post-treatment 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 lies in the directional construction of porous structures induced by a bio-template. Traditional processes rely on high-temperature calcination to directly decompose limestone, while this invention introduces carboxymethylated modified cellulose nanofibers (CM-CNF) as a bio-template. After mixing with limestone powder, they self-assemble to form a three-dimensional interpenetrating network structure. The carboxyl groups on the surface of CM-CNF interact with Ca... 2+ CaCO3 is guided to deposit directionally on the template surface through electrostatic adsorption and chemical bonding. 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 (10-50 nm).

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

[0012] Traditional rotary kiln calcination relies on external heat conduction, requiring temperatures above 900℃ and consuming high energy. This invention, however, employs microwave pulse heating technology, directly acting 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℃, reducing the temperature by 100-300℃ compared to conventional processes. A stepped power mode (5-7kW→9-11kW→5-7kW) combined with intermittent heating stops allows for dynamic control of thermal stress, preventing sintering of porous structures due to localized overheating.

[0013] Microwave bulk heating avoids the thermal hysteresis effect of traditional heat conduction, significantly shortening the decomposition time (8-12 minutes). The synergistic effect of stepped power and dynamic nitrogen flow rate (nitrogen flow rate increased to 12-18 L / min during the shutdown phase) effectively dissipates local heat, inhibits abnormal grain growth, and results in a high specific surface area (>50 m²) for the porous structure. 2 / g) is preserved.

[0014] Traditional processes either directly emit calcination exhaust gas (containing CO2) or rely on costly carbon capture technologies. This invention innovatively reacts the exhaust gas with a magnesium salt solution (MgCl2) in a closed system, generating nanoscale MgCO3 through a gas-liquid micro-interface reaction, which is then deposited in situ onto the CaO surface. CO2 and Mg... 2+ A mineralization reaction (CO2 + Mg) occurs at 50-80℃. 2+ +H₂O→MgCO₃+2H₂ +The generated MgCO3 is uniformly coated on the surface of CaO pores using 5-15μm atomized droplets as a carrier.

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

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

[0017] Low-temperature heat treatment avoids CaO sintering caused by high temperatures. Simultaneously, the interfacial compatibility between the MgO coating layer and CaO is superior to that of mechanical mixing doping methods, forming a continuous and stable protective layer. In high-temperature applications (steel desulfurization), this core-shell structure allows the outer MgO layer to preferentially react with acidic gases (SO2), protecting the active core of the inner CaO layer and extending the material's service life.

[0018] Preferably, in step (1):

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

[0020] The freeze-drying is a gradient freeze-drying, including:

[0021] Pre-freezing temperature is ~30~10℃, time is 1~3 hours;

[0022] Deep freezing temperature is ~90~70℃, time is 3~5 hours.

[0023] Traditional biological templates (natural cellulose) lack sufficient surface-active groups, and therefore cannot effectively utilize Ca. 2+ The weak bonding force results in a loose and easily collapsed pore structure in the precursor. This invention addresses this by introducing carboxylic acid groups (-COOH) onto the surface of cellulose nanofibers (CNF) through carboxymethylation modification, significantly enhancing their bonding with Ca. 2+ The chelating ability of Ca. During the mixing process, Ca... 2+The carboxylic acid sites preferentially adsorb to CM-CNF, inducing CaCO3 crystals to grow directionally along the template surface, forming a three-dimensional interpenetrating network structure.

[0024] Carboxymethylation not only enhances the interaction between the 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 allows for controllable precursor porosity (>80%) and pore size distribution (10-50 nm), laying the structural foundation for subsequent low-temperature decomposition and the generation of highly active CaO.

[0025] Traditional freeze-drying uses a single low temperature (-80℃) for direct freezing, resulting in rapid ice crystal growth and an excessively wide pore size distribution (1-100μm). This invention employs a staged gradient freezing process: first, pre-freezing at -30 to -10℃ forms an initial ice crystal framework, followed by deep freezing at -90 to -70℃ refines the ice crystal size. The larger ice crystals (micrometer-sized) formed during the pre-freezing stage provide the main framework for the pores, while the ultra-low temperature of the deep freezing stage inhibits secondary ice crystal growth, ultimately forming a uniform porous structure dominated by nanometer-sized pores.

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

[0027] Preferably, carboxymethylation modification includes:

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

[0029] In traditional carboxymethylation processes, excessively high NaOH concentrations (>1 mol / L) can lead to excessive swelling or even hydrolysis of cellulose chains, while excessively low concentrations (<0.3 mol / L) cannot adequately activate hydroxyl groups (-OH) to promote the etherification reaction. This invention limits the NaOH concentration to 0.3–0.7 mol / L. Within this range, NaOH can both dissociate the hydrogen bond network of cellulose to expose more hydroxyl sites and avoid excessively damaging the nanostructure of the fiber.

[0030] A moderately concentrated NaOH solution, employing a "limited swelling" strategy, balances the reactivity and structural integrity of cellulose. Experiments show that 0.5 mol / L NaOH can increase the degree of carboxymethyl substitution (DS) of CNF to 0.4–0.6, approximately 50% higher than the traditional process (DS < 0.3), while maintaining fiber length (1–2 μm) and aspect ratio (> 50), thus facilitating subsequent Ca2+ dissolution. 2+ Self-assembly provides active sites with high specific surface area.

[0031] As an etherifying agent, the amount of chloroacetic acid used directly affects the amount of carboxylic acid groups introduced. In traditional processes, excess chloroacetic acid (molar ratio 1:3) leads to a side reaction (chloroacetic acid self-polymerization), while insufficient amount (<1:1.5) results in a low degree of substitution. This invention limits the molar ratio of CNF to chloroacetic acid to 1:1.5 to 1:2.5, ensuring that each mole of hydroxyl group in CNF 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 region of cellulose, preserving the mechanical strength of the crystalline region and achieving a balance between "functionalization and structural stability". Nuclear magnetic resonance (NMR) analysis shows that the carboxylic acid groups at this ratio are evenly distributed, avoiding fiber breakage caused by local oversubstitution, resulting in a >3-fold increase in the stability of CM-CNF in aqueous dispersion (Zeta potential < -30mV).

[0033] Too short a reaction time (<3 hours) will lead to incomplete etherification, while too long a time (>5 hours) will trigger side reactions (cellulose oxidative degradation). This invention controls the reaction time to 3-5 hours to match the kinetics of cellulose swelling, hydroxyl activation, and chloroacetic acid diffusion.

[0034] Online conductivity monitoring revealed that after 3 hours, the concentration of free chloroacetic acid in the reaction system dropped to below 10% of its initial value, indicating that the etherification reaction was nearly complete; and no obvious fiber degradation products (gluconic acid) were detected within 5 hours. This time window ensured the high efficiency and controllability of the carboxymethylation reaction, and the resulting CM-CNF had a carboxylic acid group density of 1.2–1.8 mmol / g, which was significantly better than the 0.8–1.0 mmol / g of 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 biological template to inorganic materials is usually low (1:1 to 3:1), resulting in the template network failing to adequately guide the orderly deposition of the inorganic phase, leading to a loose and easily collapsing pore structure. This invention, through experimental verification, found that when the mass ratio of limestone powder to CM-CNF is less than 8:1 (i.e., the CM-CNF proportion is too high), excessive template fibers become entangled, hindering the uniform deposition of CaCO3 and forming localized closed-pore structures. Conversely, when the ratio is higher than 10:1, the guiding effect of the template fibers is insufficient, CaCO3 particles randomly accumulate, and the porosity significantly decreases (<60%).

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

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

[0040] Precise control of the mass ratio enables Ca 2+ The adsorption rate and CaCO3 nucleation rate on the CM-CNF surface reach a dynamic equilibrium. When the mass ratio is higher than 10:1, Ca... 2+ Insufficient supply resulted in some template areas remaining unmineralized, leading to structural defects. When the mass ratio was below 8:1, excessively rapid CaCO3 deposition caused stress concentration in the mineralized layer, making the pore walls prone to fracture during drying. X-ray diffraction (XRD) analysis showed that the CaCO3 crystal orientation degree (Lotgering factor > 0.8) at ratios of 8:1 to 10:1 was significantly higher than other ratios, demonstrating its highly ordered mineralized structure.

[0041] The mass ratio must be matched with the subsequent freeze-drying process. When the mass ratio is 8:1 to 10:1, the mineralized CM-CNF network has moderate rigidity and can resist the mechanical stress generated by ice crystal growth during gradient freezing. If the mass ratio is too high (12:1), the mineralized network strength is insufficient, and the low-temperature brittleness during the deep freezing stage (-90 to -70℃) will lead to pore wall cracking; if the mass ratio is too low (5:1), the excessive fiber content will increase the 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), enabling it to withstand the pressure of ice crystal growth during the pre-freezing stage (-30 to -10℃) and maintain stable pore morphology during the deep-freezing stage (-90 to -70℃). Atomic force microscopy (AFM) mechanical tests show that the compression resilience of the mineralized network corresponding to this ratio is >90%, far exceeding the 50% to 70% of traditional ratios.

[0043] Preferably, in step (2):

[0044] The microwave calcination adopts a stepped power mode, including power changes of 5-7kW → 9-11kW → 5-7kW, with each segment heating for 1.5-2.5 minutes and stopping for 0.5-1.5 minutes at intervals;

[0045] The calcination temperature is 750–900℃.

[0046] Traditional microwave calcination uses constant power heating, which is prone to local overheating and pore structure collapse due to uneven energy input. The stepped power mode of this invention uses a three-stage dynamic control of "preheating-enhanced decomposition-stabilized cooling" to match the endothermic kinetics of CaCO3 decomposition. The first stage (5-7 kW) uniformly heats the precursor to 500-600°C, avoiding thermal shock caused by instantaneous high temperatures; the second stage (9-11 kW) provides high energy density, promoting rapid decomposition of CaCO3 at 800-900°C (reducing the reaction activation energy by approximately 30%); the third stage (5-7 kW) gradually reduces power, alleviating thermal stress and promoting the orderly arrangement of CaO grains.

[0047] The synergistic effect of stepped power and intermittent shutdown (with nitrogen flow rate increased to 12–18 L / min during shutdown) can rapidly dissipate residual heat and suppress abnormal grain growth. Experiments show that the grain size of CaO in this mode is 20–50 nm (compared to >100 nm in conventional processes), and the standard deviation of pore size distribution is reduced to 15% (compared to >40% in conventional processes), significantly improving structural stability.

[0048] Traditional rotary kiln calcination requires maintaining temperatures above 1000℃ to ensure complete decomposition, but the high temperature causes CaO to sinter and densify (specific surface area <10m²).2 / g). This invention utilizes the selective heating characteristics of microwaves to achieve rapid decomposition of CaCO3 (decomposition rate > 98%) at 750–900℃. Within this temperature range, the nucleation rate and grain growth rate of CaO reach equilibrium: below 750℃, the decomposition reaction kinetics are slow (reaction time extended to > 20 minutes); above 900℃, grain boundary migration intensifies, and the risk of pore closure increases significantly.

[0049] Microwave energy acts directly on CaCO3 molecules through dielectric loss, exciting the vibration of polar bonds (CO bonds) and reducing 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 <5 °C / mm (compared to >50 °C / mm in conventional heating), avoiding the "cold nucleus" effect caused by thermal conduction hysteresis and ensuring uniform decomposition.

[0050] The synergistic effect of the stepped power mode and the 750–900℃ temperature range is reflected in two breakthroughs:

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

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

[0053] Innovation Verification: X-ray photoelectron spectroscopy (XPS) analysis showed that the oxygen vacancy concentration on the surface of CaO after stepped power calcination was 3.2 × 10⁻⁶. 15 cm -2 Compared to constant power calcination (1.8×10), 15 cm -2 The increase of nearly 80% indicates its higher surface activity.

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

[0055] Heating phase: 8–12 L / min;

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

[0057] In traditional calcination processes, the nitrogen flow rate is typically constant (10 L / min), leading to excessive microwave energy dissipation through gas convection and reduced heating efficiency. This invention controls the nitrogen flow rate at 8–12 L / min during the heating stage. Experimental verification shows that this flow rate range maintains an inert atmosphere (oxygen content <50 ppm) within the reaction chamber while avoiding microwave field distortion caused by excessive gas flow.

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

[0059] Suppressing local oxidation: When the flow rate is >8L / min, it can effectively prevent air infiltration and prevent CaO surface oxidation to form Ca(OH)2 or CaCO3 (XPS detection shows that the oxide layer thickness is <2nm);

[0060] Microwave field stability: When the flow rate is <12L / min, the interference of gas turbulence on microwave distribution is weak (field strength fluctuation <5%), ensuring uniform heating of CaCO3 particles.

[0061] Computational fluid dynamics (CFD) simulations show that the standard deviation of temperature distribution within the microwave cavity at a flow rate of 10 L / min is only ±8℃ (compared to ±25℃ under conventional constant flow rate), significantly improving the uniformity of decomposition.

[0062] During the shutdown phase (0.5–1.5 minutes) of the stepped power mode, the nitrogen flow rate is increased to 12–18 L / min, and residual heat is rapidly removed through forced convection. Traditional processes neglect heat dissipation control during this phase, leading to microcracks in the calcined products due to residual heat accumulation (cooling rate >200℃ / min).

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

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

[0065] Dynamic regulation of nitrogen flow rate and spatiotemporal coordination with stepped power mode:

[0066] During the heating phase, a low flow rate (8-12 L / min) is used to match the temperature rise requirements of the stepped power, thereby reducing energy loss.

[0067] During the shutdown phase, the high flow rate (12-18 L / min) corresponds to the power interruption period, which enhances heat dissipation efficiency.

[0068] Innovative Validation: Thermogravimetric-mass spectrometry (TG-MS) analysis showed that dynamic flow rate adjustment reduced the activation energy (Ea) of the CaCO3 decomposition reaction to 120 kJ / mol (compared to Ea > 150 kJ / mol in traditional processes), indicating higher energy utilization efficiency. Simultaneously, the specific surface area of ​​CaO reached 60–70 m² / s.2 / 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 exhaust gas in the form of 5-15 μm droplets through ultrasonic atomization.

[0071] In traditional processes, magnesium salts (MgO powder) are difficult to uniformly coat at the gas-solid interface due to poor dispersibility and low reactivity. This invention uses water-soluble magnesium salt solutions (MgCl2, MgSO4), whose ionization produces Mg... 2+ It can rapidly migrate to the CO2 gas-liquid interface via liquid-phase diffusion. When the solution concentration is 0.3–0.7 mol / L, Mg… 2+ The diffusion rate and CO2 dissolution rate reach a dynamic equilibrium, avoiding crystal coarsening caused by local supersaturation.

[0072] Water-soluble magnesium salts form freely moving Mg in the liquid phase. 2+ HCO3 formed by dissolving with CO2 - Mineralization reaction occurs (Mg) 2+ +2HCO3 - The reaction (MgCO3↓ + CO2↑ + H2O) occurs, with a reaction rate 3–5 times higher than that of the solid-phase reaction. Raman spectroscopy analysis shows that the density of MgCO3 nucleation sites in the liquid phase reaches 10-1. 10 / cm 3 This ensures the uniform generation of nano-sized MgCO3 particles (20-50 nm).

[0073] Traditional mechanical atomization (pressure nozzle) produces droplets with a large diameter (>50 μm), resulting in limited gas-liquid contact area and insufficient reaction time. This invention utilizes ultrasonic atomization technology to break down magnesium salt solutions into microdroplets of 5–15 μm, with a specific surface area (>300 m²). 2 / m 3 It is more than 10 times more efficient than traditional atomization methods.

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

[0075] Breakthrough in reaction kinetics: Convection-diffusion effects within microdroplets enable Mg 2+ The mass transfer rate with CO2 was increased to 1.2 × 10⁻⁶. -4 mol / (m 2•s), the mineralization reaction completion rate is >95% (compared to <70% in traditional processes). High-speed camera observations show that 5–15 μm droplets exhibit a spiral trajectory in the airflow, further enhancing the mixing efficiency.

[0076] The optimal design of the droplet size needs to be matched with the reaction temperature (50–80℃) and pressure (0.08–0.12 MPa). When the droplet size is <5 μm, the evaporation rate is too fast (<0.1 seconds), leading to Mg... 2+ It becomes inactive before it fully reacts; when the particle size is >15μm, the droplets settle significantly due to gravity, making it difficult to disperse them effectively in the exhaust gas.

[0077] The evaporation time (1–3 seconds) of 5–15 μm droplets at 50–80 °C closely matches the time required for the mineralization reaction (2–4 seconds), ensuring that MgCO3 nucleation and deposition are completed before the droplets completely evaporate. Transmission electron microscopy (TEM) shows that the MgCO3 coating thickness corresponding to this particle size range is 10–20 nm, and it forms a tight chemical bond with the CaO substrate (interfacial 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 0.01 g / 100 mL), requiring strong acid or high temperature conditions to release Mg. 2+ This leads to low reaction efficiency. The present invention uses MgCl2 (solubility 54.3 g / 100 mL) or MgSO4 (solubility 35.1 g / 100 mL), which can rapidly dissociate into free Mg in aqueous solution. 2+ The ion concentration can reach 1.2–1.8 mol / L (pH 5–6), which significantly enhances the mineralization reaction rate with CO2.

[0080] The dissociation processes of MgCl2 and MgSO4 release Cl... - or SO4 2- As a counterion, it reduces Mg through charge shielding effect. 2+ The hydration energy of MgCl2 promotes its migration to the gas-liquid interface. Experiments show that Mg in MgCl2 solution... 2+ The diffusion coefficient is 1.2 × 10⁻⁶. -9 m 2 / s, compared to MgSO4 (0.9×10 -9 m 2 The reaction rate is higher ( / s), but the crystal form controllability of its mineralization product (MgCO3) is slightly inferior to that of the MgSO4 system. Both can be flexibly selected according to process requirements, balancing reaction rate and product uniformity.

[0081] The types of anions in MgCl2 and MgSO4 directly affect the crystallization behavior of mineralization products. - Due to weak polarization, MgCO3 preferentially grows along the (104) crystal plane, forming sheet-like nanoparticles (10–20 nm thick); while SO4… 2- The strong charge effect can adsorb onto the surface of MgCO3 crystal nuclei, inhibiting anisotropic growth and forming spherical particles (20-50 nm in diameter).

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

[0083] MgSO4 system: SO4 2- By adsorption to suppress differences in crystal plane growth, the spherical particles have a higher packing density (>80%) and a coating porosity of <5%.

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

[0085] Some magnesium salts (Mg(NO3)2) release harmful byproducts (NO3) during the reaction. - ), while the anions of MgCl2 and MgSO4 (Cl - SO4 2- Under mineralization conditions, Cl is chemically inert and does not participate in redox reactions. At mineralization temperatures of 50–80°C, Cl... - and SO4 2- It exists in a free state in the liquid phase and is eventually removed by a water washing step to avoid impurity residue.

[0086] Cl - and SO4 2- Its low reactivity ensures the purity of the mineralized products (MgCO3 content > 99%), while its high solubility in water (Cl) - Solubility 355 g / L, SO4 2- (Solubility 33 g / L) facilitates subsequent wastewater treatment. Ion chromatography (IC) detection showed that Cl in the waste liquid... - 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℃, the pressure is 0.08–0.12 MPa, and the reaction time is 10–30 minutes.

[0089] Traditional mineralization processes are typically carried out at room temperature (25℃) or high temperature (>100℃). The former results in a low reaction rate (CO2 conversion rate <50%), while the latter is energy-intensive and prone to side reactions (MgCO3 dehydration to MgO). This invention controls the temperature between 50 and 80℃. Within this temperature range, the Henry's constant (solubility) of CO2 is 0.8–1.2 mol / (L·atm), which is 2–3 times higher than at room temperature. Simultaneously, the nucleation rate of MgCO3 (10⁻⁶ mol / (L·atm)) is significantly increased. 10 ~10 12 nuclei / (m 3 The crystal growth rate (1-3 nm / s) reaches a dynamic equilibrium with the crystal growth rate (1-3 nm / s).

[0090] The mild heating conditions of 50-80℃ offer a dual advantage:

[0091] Promoting CO2 dissolution: Increased temperature reduces liquid phase viscosity, accelerating CO2 mass transfer from the gas phase to the liquid phase (mass transfer coefficient increases to 1.5 × 10⁻⁶). -4 m / s);

[0092] Suppression of side reactions: At temperatures <80℃, the activation energy barrier (>150kJ / mol) for the dehydration reaction of MgCO3 (MgCO3→MgO+CO2) remains intact, ensuring product purity >99%. FTIR spectroscopy analysis shows that no characteristic MgO peak (470cm⁻¹) was detected in the product at this temperature range. -1 This verifies the singularity of the reaction pathway.

[0093] At atmospheric pressure (0.1 MPa), the gas-liquid mass transfer efficiency is limited by the low partial pressure of CO2, while high pressure (>0.15 MPa) requires complex sealing equipment. This invention fine-tunes the pressure to 0.08–0.12 MPa, achieving a match between the gas-liquid interface renewal rate and the reaction rate through slight pressurization (or depressurization). At a pressure of 0.1 MPa, the mass transfer flux of CO2 in the liquid phase reaches 2.5 × 10⁻⁶. -5 mol / (m 2 The pressure (·s) is 20% higher than that under normal pressure; when the pressure is >0.12MPa, the bubble merging effect is aggravated and the effective contact area is reduced.

[0094] The gas-liquid mixing was optimized within a pressure window of 0.08–0.12 MPa using a “micro-perturbation” strategy.

[0095] 0.08~0.1MPa (slightly below atmospheric pressure): forms micron-sized bubbles (10~50μm in diameter), increasing the gas-liquid contact area;

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

[0097] Traditional processes, in pursuit of high conversion rates, often extend reaction times to several hours, leading to increased energy consumption and excessive product growth (coating thickness > 50 nm). This invention experimentally demonstrates that the mineralization reaction can achieve 80% CO2 conversion within 10 minutes, with a conversion rate > 95% after 30 minutes, and that further extending the reaction time contributes < 2% to the overall conversion rate improvement.

[0098] Nucleation stage (0–10 minutes): The density of MgCO3 crystal nuclei rapidly increases to 10. 15 / m 3 It covers >90% of the CaO surface;

[0099] Growth stage (10-30 minutes): Grains grow epitaxially along the CaO pores, increasing in thickness from 5 nm to 20 nm, forming a continuous coating layer.

[0100] Real-time monitoring using a quartz crystal microbalance (QCM) showed that the deposition rate of the coating layer approached zero after 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 between 10 and 30 minutes (Mg / Ca atomic ratios were both 0.25–0.3), verifying the feasibility of the short-time, high-efficiency reaction.

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

[0102] In traditional processes, materials need to be transferred multiple times between steps, causing intermediate products to be exposed to the environment (humidity, oxygen), leading to moisture absorption and oxidation. The closed system of this invention uses modular reaction units (precursor preparation - microwave calcination - mineralization coating) connected in series, employing nitrogen circulation pipelines to maintain an internal oxygen content of <50ppm and humidity of <5%RH. Material transfer between units is achieved via pneumatic valves and screw feeders, requiring no manual intervention throughout the entire process.

[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 and generating Ca(OH)2 (moisture absorption weight gain rate <0.1%, traditional process >3%).

[0104] Pollution 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 <50ppm, which is 90% lower than that of the open system.

[0105] The exposure time of intermediate products (porous CaCO3 precursor, nano-CaO) during transport must be strictly limited. This invention optimizes the feeding rate (0.5-1.5 kg / min) and path length (<3 m) to ensure that the interval from the end of freeze-drying in step (1) to the start of microwave calcination in step (2) is <3 minutes, and the transition time from step (2) to step (3) is <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), while the present invention controls the exposure time to be less than 5 minutes, and the specific surface area retention rate is greater than 98%.

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

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

[0109] Protection of humidity-sensitive materials: The porous CaCO3 precursor will partially hydrolyze to Ca(HCO3)2 when the humidity is >30%RH. The closed system with humidity <5%RH completely blocks this side reaction (HCO3 was not detected by Raman spectroscopy). - Characteristic peaks);

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

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

[0112] 1. This invention employs a fully enclosed system and inert gas dynamic control technology, achieving zero-exposure transfer of intermediate products and solving the oxidation and moisture absorption problem caused by frequent material transfer in existing technologies. Compared to traditional open processes, product stability is improved by more than 3 times, completely preventing environmental contamination.

[0113] 2. This invention overcomes the grain coarsening bottleneck of traditional isothermal calcination through the synergistic effect of stepped power microwave calcination and intermittent heat dissipation. Existing technologies suffer from severe sintering due to continuous high temperatures, while this invention preserves the active surface of nano-calcium oxide intact, increasing its adsorption capacity by 2.2 times.

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

[0115] 4. This invention, based on the gradient freezing-induced directional ice crystal growth mechanism, solves the defects of uneven pore distribution and easy structural collapse in conventional freeze-drying. Compared with single-stage deep freezing process, the compressive strength of the precursor of this invention is increased by 40%, providing reliable support for industrial continuous production. Attached Figure Description

[0116] Figure 1 This is a schematic diagram of the method flow of the present invention. Detailed Implementation

[0117] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort 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 (pre-freezing at -20℃ for 2 hours → deep freezing at -80℃ for 6 hours);

[0122] Precursor specifications: porosity 85%, average pore size 25nm.

[0123] Step (2) Microwave calcination:

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

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

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

[0127] CaO index: specific surface area 70m² 2 / g, grain size 25nm.

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

[0129] Magnesium salt solution: 0.3 mol / L MgCl2, ultrasonic atomization droplet size 5 μm;

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

[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 (pre-freezing at -30℃ for 1.5 hours → deep freezing at -70℃ for 5 hours);

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

[0136] Step (2) Microwave calcination:

[0137] Power mode: stepped power 7kW (1.5min) → 11kW (2min) → 7kW (1.5min), with a 0.5min pause interval;

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

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

[0140] CaO index: specific surface area 55m³ 2 / g, grain size 40nm, compressive strength 7.5MPa.

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

[0142] Magnesium salt solution: 0.7 mol / L MgSO4, ultrasonic atomization droplet size 15 μm;

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

[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 (pre-freezing at -25℃ for 3 hours → deep freezing at -90℃ for 4 hours);

[0148] Precursor specifications: porosity 82%, average pore size 30nm.

[0149] Step (2) Microwave calcination:

[0150] Power mode: stepped power 6kW (2min) → 10kW (2min) → 6kW (2min), with a 1.5min pause interval;

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

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

[0153] CaO index: specific surface area 65m³ 2 / g, grain size 30nm.

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

[0155] Magnesium salt solution: 0.5 mol / L MgCl2 / MgSO4 (1:1 mixture), ultrasonic atomization droplet size 10 μm;

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

[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 (exceeding the range of 8:1 to 10:1), while 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 uses a constant power of 9kW (the stepped power mode is canceled), and the other conditions are the same.

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

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

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

[0164] The difference from Example 2 is that the ultrasonic atomization droplet size in step (3) is 50 μm (exceeding the range of 5 to 15 μm), while 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), the freeze-drying adopts a single-stage -80℃ deep freeze (gradient freezing is cancelled), 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 (below the range of 50-80°C), while 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℃ (exceeding the range of 750~900℃), while 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 (exceeding the range of 10 to 30 minutes), while the other conditions are the same.

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

[0174] The difference from Example 3 is that in step (2), the nitrogen flow rate is fixed at 10 L / min throughout the process (dynamic adjustment is canceled), 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 (below the range of 0.08 to 0.12 MPa), while the other conditions are the same.

[0177] Experiment 1: Precursor Preparation and Structure Regulation Verification

[0178] Experimental procedure instructions:

[0179] Raw material mixing:

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

[0181] Comparative Example 1: Mass ratio 5:1, the rest is the same as in Example 1.

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

[0183] Freeze-drying:

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

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

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

[0187] Precursor characterization:

[0188] SEM testing: Observe the pore structure and pore size distribution;

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

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

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

[0192] Table 1: Comparison of Precursor Performance

[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 decreased to 5:1 (Comparative Example 1), the template effect of CM-CNF was excessively diluted, failing to effectively guide the directional deposition of CaCO3, resulting in a sharp drop in porosity to 59.8% and a significantly coarsened pore size distribution (48.5 nm). SEM images showed that the precursor in Comparative Example 1 exhibited a locally collapsed structure, while the gradient freezing process in Examples 1-3, by suppressing ice crystal growth in stages, resulted in a uniform pore size distribution within the range of 25–35 nm, providing a highly active surface for subsequent calcination.

[0195] In Comparative Example 5, the single-stage freezing directly to -80°C resulted in excessively rapid cooling, leading to the formation of large-sized pores (52.1 nm) during disordered ice crystal growth, and a decrease in porosity to 74.9%. This is closely related to the microcrystal nuclei formed during the pre-freezing stage (-20°C) in gradient freezing. The pre-freezing process guides the orderly migration of moisture through slow cooling, avoiding stress concentration during the deep freezing stage and thus maintaining the integrity of the pore structure. Compressive strength tests further confirmed that the 10:1 mass ratio (Example 2) achieved a compressive strength of 8.1 MPa due to the higher crosslinking density of CM-CNF, while mass ratio imbalance (Comparative Example 1) or uncontrolled freezing (Comparative Example 5) both resulted in deterioration of mechanical properties.

[0196] From a mechanistic perspective, the carboxylic acid group of CM-CNF chelates Ca... 2+The formation pathway of the pore structure is determined by the phase transition kinetics of gradient freezing, which regulates the mineralization sites. An optimal mass ratio of 8:1 to 10:1 ensures a dynamic balance between the template agent and the mineralization components, enabling the precursor to possess both high porosity and mechanical stability, thus laying the structural foundation for the efficient preparation of nano-CaO.

[0197] Experiment 2: Verification of the Synergistic Effect of Microwave Calcining Process

[0198] Experimental procedure instructions:

[0199] Calcination parameter settings:

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

[0201] Comparative Example 2: Constant power 9kW, the rest is the same as Example 1.

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

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

[0204] Product characterization:

[0205] TEM: Observe the grain size and pore structure of CaO;

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

[0207] XPS: Analyzes surface oxygen vacancy concentration.

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

[0209] Table 2: Comparison of Properties of Calcined Products

[0210]

[0211]

[0212] The stepped power mode effectively balances the CaCO3 decomposition kinetics and grain growth rate through intermittent heating and cooling cycles. In contrast, the constant power in Comparative Example 2 resulted in localized overheating, increasing the grain size to 52.8 nm and decreasing the specific surface area to 38.6 m². 2 / g, while the stepped power in Example 1 suppressed 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 stepped power was significantly higher than that under 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. In Example 2, the grain size increased to 40.5 nm at 900 °C, but still remained at 55.2 nm. 2 The specific surface area was [value missing] / g, while in Comparative Example 7, due to the sintering effect at 1000℃, the grains coarsened to 80.2nm, and the specific surface area plummeted to 15.3m. 2 / g. At high temperatures, the grain boundary migration rate 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℃) maintains the stability of the nanostructure by suppressing the Ostwald ripening process.

[0214] The protective effect of dynamic nitrogen flow rate regulation on surface oxygen vacancies is irreplaceable. In Comparative Example 9, the fixed nitrogen flow rate led to a local increase in oxygen partial pressure during calcination, reducing the oxygen vacancy concentration to 4.7%. In contrast, 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 rate and step power enabled a dynamic balance between grain growth and exposure of surface active sites, providing a highly active substrate for subsequent mineralization reactions.

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

[0216] Experimental procedure instructions:

[0217] Mineralization reaction settings:

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

[0219] Comparative Example 3: The magnesium salt was replaced with 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): Determines CO2 conversion rate;

[0226] SEM-EDS: Analysis of coating thickness, coverage, and elemental distribution;

[0227] XRD: Detection of byproducts (unreacted Mg(OH)2 or CaCO3).

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

[0229] Table 3: Comparison of the properties of mineralized products

[0230]

[0231]

[0232]

[0233] The choice of magnesium salt directly affects the mineralization reaction pathway and the formation of byproducts. In Comparative Example 3, when Mg(NO3)2 was used, nitrate ions underwent partial reduction in an acidic reaction environment, producing NO2. - Byproducts (8.6% impurities detected by XRD), while in Example 2, when MgSO4 was used, the stabilizing coordination of sulfate ions suppressed the side reactions, and the byproduct content was only 1.1%. Furthermore, Mg... 2+ With CO3 2- The difference in binding energy resulted in a thinner coating layer (10.2 nm) for the MgCl2 system (Example 1), while the coating layer of the MgSO4 system (Example 2) was thicker to 19.8 nm due to the slower ion migration rate, but still maintained a high coverage (>95%).

[0234] The correlation between droplet size and mass transfer efficiency was particularly significant in the data. In Comparative Example 4, when the droplet size increased to 50 μm, the gas-liquid contact area decreased dramatically, the CO2 conversion rate dropped to 61.2%, and the coating thickness was only 8.3 nm (SEM showed localized uncovering). In contrast, the microdroplets (5-15 μm) in Examples 1-3 formed an aerosol interface through ultrasonic atomization, allowing Mg... 2+ The mass transfer flux with CO2 was increased by more than 3 times. Dynamic pressure control (0.08-0.12 MPa) further optimized the bubble distribution. In Comparative Example 10, when the pressure was reduced to 0.05 MPa, bubble merging caused a mass transfer dead zone, and the CO2 conversion rate decreased to 69.4%.

[0235] The balance between temperature control window and reaction time is crucial for mineralization efficiency. Comparative Example 6 exhibited an extremely low reaction rate (42.8% conversion) at 25°C, and the low temperature resulted in incomplete MgCO3 nucleation (coating thickness of 5.1 nm). In contrast, Example 3 achieved a conversion of 92.5% at 70°C by overcoming the activation energy barrier. Notably, while extending the reaction time to 60 minutes in Comparative Example 8 slightly increased the conversion to 95.1%, excessive coating growth (31.7 nm) led to pore blockage, which was detrimental to subsequent applications. This indicates that the optimal time window (10-30 minutes) achieves the best balance between efficiency and quality by suppressing excessive crystal growth.

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

Claims

1. A high-temperature stable and environmentally friendly calcium oxide process, characterized in that, Includes the following steps: (1) Pretreatment of biological template and preparation of porous CaCO3 precursor: The biological template was mixed with limestone powder and freeze-dried to form a porous CaCO3 precursor; (2) Microwave pulse calcination decomposition: The precursor obtained in step (1) is calcined under nitrogen protection by microwave to decompose it into porous CaO; (3) CO2 in-situ mineralization and coating layer formation: The calcination tail gas generated in step (2) is reacted with magnesium salt solution to form a coating layer on the surface of CaO; (4) Post-treatment and product activation: The coating layer is heat-treated to obtain a high-temperature stable calcium oxide product; In step (1): The biological template is carboxymethylated modified cellulose nanofibers; The freeze-drying is a gradient freeze-drying, including: Pre-freezing temperature is -30 to -10℃, time is 1 to 3 hours; Deep freezing temperature is -90 to -70℃, time is 3 to 5 hours; Carboxymethylation modification includes: Cellulose nanofibers were dispersed in a 0.3–0.7 mol / L NaOH solution and reacted for 3–5 hours at a molar ratio of cellulose nanofibers to chloroacetic acid of 1:1.5–1:2.

5. In step (1): The mass ratio of limestone powder to biological template is 8:1 to 10:1; In step (2): The microwave calcination adopts a stepped power mode, including power changes of 5-7kW → 9-11kW → 5-7kW, with each segment heating for 1.5-2.5 minutes and stopping for 0.5-1.5 minutes at intervals; The calcination temperature is 750–900℃; The nitrogen flow rate during the calcination process in step (2) is dynamically adjusted as follows: Heating phase: 8–12 L / min; Stopping phase: 12-18 L / min; 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 5-15 μm by ultrasonic atomization; The water-soluble magnesium salt is MgCl2 or MgSO4; In step (3): The mineralization reaction temperature is 50–80℃, the pressure is 0.08–0.12 MPa, and the reaction time is 10–30 minutes. Steps (1) to (3) are completed in a closed system, with intermediate product exposure time of less than 5 minutes.

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