Process for the preparation of a fine active calcium carbonate

Abstract

The present application relates to the technical field of inorganic non-metallic powder material, and discloses a preparation method of micro-fine active calcium carbonate, which comprises the following steps: adding a calcium source and a surface modifier into a modified deep eutectic solvent containing choline chloride, urea and ammonium chloride, heating to form a homogeneous precursor solution; then, the solution is heated to make urea pyrolyze in-situ to generate calcium carbonate, and the surface modifier is coated synchronously; finally, the product is obtained through separation, washing and drying; by introducing ammonium chloride into the deep eutectic solvent to kinetically control urea pyrolysis, and combining with the construction of the homogeneous precursor, the present application realizes the synergistic control of calcium carbonate nucleation, growth and surface modification; the micro-fine active calcium carbonate prepared by the method has the beneficial effects of small particle size, uniform distribution, complete surface coating and excellent dispersion stability in organic medium.

Description

Technical Field

[0001] This invention relates to the field of inorganic non-metallic powder materials technology, and in particular to a method for preparing finely active calcium carbonate. Background Technology

[0002] Calcium carbonate, as an important inorganic chemical product, is widely used as a functional filler in polymer composites such as plastics, rubber, coatings, and inks. With the continuous improvement of the performance requirements of polymer materials, higher demands are being placed on the performance of calcium carbonate fillers. Developing micro-active calcium carbonate with nanoscale particle size, excellent dispersibility, and hydrophobic and oleophilic surface properties has become a key research focus in this field.

[0003] Currently, the main method for preparing calcium carbonate is precipitation. Traditional precipitation processes, such as carbonation in an aqueous system, suffer from high surface tension. The newly formed nanoscale primary calcium carbonate particles possess extremely high surface energy, making them prone to irreversible hard agglomeration. This results in a final product with particle sizes typically in the micrometer range and a wide particle size distribution, failing to meet the demands of high-performance applications. Surface modification often employs a post-coating process, adding surface modifiers after the calcium carbonate particles have formed. However, this method treats already agglomerated particle aggregates. Modifier molecules struggle to penetrate the aggregates, only providing incomplete coating to the outer surface, leading to low modification efficiency, insufficient activation of the resulting product, and poor dispersion stability in organic matrices.

[0004] To overcome the shortcomings of aqueous systems, some studies have shifted towards preparation in non-aqueous media such as eutectic solvents. Eutectic solvents, as a novel type of green solvent, provide a milder liquid environment for the controllable growth of particles. However, existing non-aqueous methods still face challenges. If carbon dioxide gas is introduced into the reaction, the heterogeneous reaction interface between the gas and liquid phases leads to uneven reactant concentration distribution, which is also detrimental to uniform particle nucleation and growth. While in-situ generation of reactants through the thermal decomposition of components within the system (such as urea) avoids the phase interface problem, the reaction rate is often difficult to control, and the violent decomposition process often leads to explosive nucleation, still resulting in large, unevenly distributed particles.

[0005] Therefore, existing technologies generally lack an effective means to synergistically regulate particle micromorphology control and surface functionalization modification at the reaction source. How to precisely control nucleation kinetics to obtain nanoscale primary particles in a homogeneous reaction system, while ensuring complete and uniform surface coating of the particles at the moment of generation, thereby fundamentally solving the problems of particle agglomeration and insufficient modification, is a pressing technical challenge in this field. Summary of the Invention

[0006] The technical problem solved by this invention is that existing methods for preparing calcium carbonate, such as traditional aqueous phase carbonation or methods involving external gas reaction in conventional solvents, generally suffer from technical defects such as large product particle size, wide particle size distribution, easy hard agglomeration, insufficient surface modification, and poor dispersion stability of the obtained product in organic media.

[0007] To address the aforementioned problems, this invention provides a method for preparing finely activated calcium carbonate, which can produce activated calcium carbonate products with nano-sized particles, uniform distribution, sufficient surface modification, and high stability in organic media.

[0008] The present invention adopts the following technical solution:

[0009] A method for preparing finely activated calcium carbonate includes the following steps:

[0010] (a) A calcium source and a surface modifier are added to a modified eutectic solvent containing choline chloride, urea and ammonium chloride, and a homogeneous precursor solution is formed under heating and stirring conditions;

[0011] (b) The homogeneous precursor solution is heated to a predetermined reaction temperature to pyrolyze the urea to generate calcium carbonate in situ, and the surface modifier is simultaneously coated onto the calcium carbonate.

[0012] (c) The reaction products are separated, washed and dried to obtain the fine active calcium carbonate.

[0013] By employing the above technical solution, this invention utilizes a synergistic system to control the nucleation, growth, and surface modification processes of calcium carbonate. Its mechanism of action is as follows:

[0014] 1. Reaction Medium and Kinetic Regulation: This invention uses a specific eutectic solvent containing ammonium chloride as the reaction medium. Ammonium chloride is not an inert component in this system, but rather acts as a core rate regulator. During the heating phase of the reaction, the presence of ammonium chloride effectively suppresses the initial rate of urea pyrolysis at high temperatures to produce ammonia and isocyanate (which then hydrolyzes into carbon dioxide) through mechanisms such as the common ion effect. This avoids explosive nucleation caused by instantaneous supersaturation of local reactant concentrations, transforming the nucleation process into a gradual and controllable process.

[0015] 2. Construction of the homogeneous precursor: Before the reaction, the calcium source and surface modifier, under the specific process conditions in step (a), have achieved a uniform dispersion at the molecular or micromicelle level in the modified eutectic solvent, forming a macroscopically clear and transparent homogeneous precursor solution. This step constructs a globally uniform reaction environment, ensuring the uniform distribution of reactants and modifiers at the microscale, laying the foundation for subsequent simultaneous reactions.

[0016] 3. Controlled Nucleation and Simultaneous In-situ Coating: In step (b), as the system temperature increases, urea slowly pyrolyzes, releasing reactants in situ and homogeneously. During the controlled nucleation stage, newly formed calcium carbonate nuclei with extremely high surface energy are immediately captured and coated by surface modifier molecules uniformly distributed throughout the system. This simultaneous in-situ coating process fundamentally blocks direct contact between nuclei and the hard agglomeration process caused by van der Waals forces or chemical bonding, while also inhibiting grain coarsening caused by the Oswald ripening effect.

[0017] Therefore, through the above-mentioned multiple synergistic mechanisms, the method of the present invention can suppress particle agglomeration at the source, obtain primary particles with small size and concentrated distribution, and ensure that the surface of each particle is coated with a complete and uniform organic modification layer, thereby obtaining a final product with both fine particle size and high surface activity.

[0018] Preferably, in the modified eutectic solvent, the molar ratio of choline chloride, urea, and ammonium chloride is 1:(1.8–2.2):(0.1–0.3). This ratio range ensures that the solvent system has a suitable melting point and viscosity, and provides sufficient and controllable reactants and reaction rate regulators.

[0019] Preferably, the modified eutectic solvent is prepared by heating and stirring a solid mixture of choline chloride, urea, and ammonium chloride at 80–95°C. Preparation within this temperature range ensures complete melting of all components to form a homogeneous liquid, while preventing premature decomposition of urea.

[0020] Preferably, the calcium source is anhydrous calcium chloride, and the surface modifier is stearic acid.

[0021] Preferably, the molar ratio of anhydrous calcium chloride to urea is 1:(3.0-5.0); the mass ratio of stearic acid to theoretically generated calcium carbonate is (1.5-4.0):100. This material ratio ensures that urea is in excess relative to the calcium source to drive the reaction to proceed completely, while the amount of surface modifier is matched with the specific surface area of ​​the generated calcium carbonate to achieve efficient and complete coating.

[0022] More preferably, the heating temperature in step (a) is 90–110°C; the heating rate in step (b) is 0.8–1.5°C / min; the predetermined reaction temperature in step (b) is 135–155°C, and the reaction is carried out at this temperature for 2.5–4.5 hours. By precisely controlling the above-mentioned dissolution temperature, heating rate, reaction temperature, and time, the stable formation of the homogeneous precursor can be ensured, and the nucleation and growth process of calcium carbonate can be kept within the optimal kinetic control range, thereby stably obtaining the target product.

[0023] In summary, the present invention has at least one of the following beneficial technical effects:

[0024] 1. This invention enables the preparation of fine calcium carbonate products with small particle size and highly uniform distribution. It utilizes an ammonium chloride-modified eutectic solvent and avoids explosive nucleation by kinetically controlling the pyrolysis rate of urea through ammonium chloride. Combined with the homogeneous nucleation environment provided by the homogeneous reaction system and the effective suppression of primary particle aggregation by simultaneous in-situ coating, precise control over the particle size and distribution of the final product is achieved.

[0025] 2. This invention achieves efficient and complete organic coating of calcium carbonate particles. The key lies in dissolving the surface modifier in the reaction medium before the reaction begins, constructing a homogeneous precursor solution. When calcium carbonate crystal nuclei are generated in situ, the modifier molecules dispersed in the system can be adsorbed onto the high-energy newly formed surface immediately and without obstruction, maximizing the coating efficiency and resulting in a product with an extremely high activation index.

[0026] 3. The finely activated calcium carbonate prepared by this invention exhibits excellent long-term dispersion stability in non-polar organic media. This performance stems from the complete and homogeneous organic long-chain layer constructed on the surface of each primary particle using the simultaneous in-situ coating technology. This coating layer provides effective steric hindrance in the organic medium, sufficient to overcome the attractive forces between particles, thereby inhibiting secondary agglomeration of the particles and enabling them to maintain a stable dispersion state. This solves the technical problem of easy agglomeration of traditional products in applications. Detailed Implementation

[0027] To further clarify the purpose, technical solution, and advantages of this invention, specific embodiments are described in detail below. It should be understood that the embodiments described herein are only part of, and not all of, this invention, and are for illustrative purposes only, and should not constitute any limitation on the scope of protection of this invention. Any modifications, equivalent substitutions, improvements, etc., made in accordance with the spirit and principles of this invention should be included within the scope of protection of this invention.

[0028] Preparation Example 1:

[0029] This preparation example provides a method for preparing an ammonium chloride-modified deep eutectic solvent (DES-A1), comprising the following steps:

[0030] Weigh 139.6 g (1 mol) of choline chloride, 120.1 g (2 mol) of urea and 8.0 g (0.15 mol) of ammonium chloride. Place the above solid mixture in a reactor equipped with a mechanical stirrer. Under nitrogen atmosphere protection, heat to 85°C and keep stirring for 2 hours until all solids are completely melted, resulting in a clear and transparent viscous liquid, which is the modified deep eutectic solvent DES-A1. Seal and store for later use.

[0031] Preparation Example 2:

[0032] This preparation example provides a method for preparing an ammonium chloride-modified deep eutectic solvent (DES-A2), comprising the following steps:

[0033] Weigh 139.6 g (1 mol) of choline chloride, 108.1 g (1.8 mol) of urea and 5.4 g (0.1 mol) of ammonium chloride. Place the above solid mixture in a reactor equipped with a mechanical stirrer. Under nitrogen atmosphere protection, heat to 80°C and keep stirring for 2 hours until all solids are completely melted, resulting in a clear and transparent viscous liquid, which is the modified deep eutectic solvent DES-A2. Seal and store for later use.

[0034] Preparation Example 3:

[0035] This preparation example provides a method for preparing an ammonium chloride-modified deep eutectic solvent (DES-A3), comprising the following steps:

[0036] Weigh out 139.6 g (1 mol) of choline chloride, 132.1 g (2.2 mol) of urea and 16.0 g (0.3 mol) of ammonium chloride. Place the above solid mixture in a reactor equipped with a mechanical stirrer. Under nitrogen atmosphere protection, heat to 95°C and keep stirring for 1.5 hours until all solids are completely melted, resulting in a clear and transparent viscous liquid, which is the modified deep eutectic solvent DES-A3. Seal and store for later use.

[0037] Example 1:

[0038] This embodiment provides a method for preparing finely activated calcium carbonate, including the following steps:

[0039] The modified deep eutectic solvent DES-A1 obtained in Preparation Example 1 was transferred to a reactor equipped with a reflux condenser and a mechanical stirrer, and heated to 100°C with continuous stirring. 55.5 g (0.5 mol) of anhydrous calcium chloride and 1.5 g of stearic acid were slowly added, and the mixture was stirred continuously at 100°C for 2 hours until all solids were completely dissolved and the system became clear and transparent, yielding a homogeneous precursor solution. This solution was heated to 145°C at a heating rate of 1.0°C / min and reacted at this temperature for 3.5 hours. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. The reaction product was slowly added to 20 times its volume of deionized water with stirring to dilute it, and then the solid product was collected by vacuum filtration. The resulting filter cake was repeatedly washed with deionized water until no chloride ions were detected in the filtrate using silver nitrate solution. Finally, the filter cake was dried in a vacuum drying oven at 80°C for 12 hours to obtain a white powder product, denoted as S1.

[0040] Example 2:

[0041] This embodiment provides a method for preparing finely activated calcium carbonate, including the following steps:

[0042] The modified eutectic solvent DES-A2 obtained in Preparation Example 2 was transferred to a reactor equipped with a reflux condenser and a mechanical stirrer, and heated to 90°C with continuous stirring. 45.4 g (approximately 0.41 mol) of anhydrous calcium chloride and 0.6 g of stearic acid were slowly added, and the mixture was stirred continuously at 90°C for 2.5 hours until all solids were completely dissolved and the system became clear and transparent, yielding a homogeneous precursor solution. This solution was heated to 135°C at a heating rate of 0.8°C / min and reacted at this temperature for 4.5 hours. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. Subsequent separation, purification, and drying steps were performed as in Example 1, yielding a white powder product, denoted as S2.

[0043] Example 3:

[0044] This embodiment provides a method for preparing finely activated calcium carbonate, including the following steps:

[0045] The modified eutectic solvent DES-A3 obtained in Preparation Example 3 was transferred to a reactor equipped with a reflux condenser and a mechanical stirrer, and heated to 110°C with continuous stirring. 50.0 g (approximately 0.45 mol) of anhydrous calcium chloride and 2.0 g of stearic acid were slowly added, and the mixture was stirred continuously at 110°C for 1.5 hours until all solids were completely dissolved and the system became clear and transparent, yielding a homogeneous precursor solution. This solution was heated to 155°C at a heating rate of 1.5°C / min and reacted at this temperature for 2.5 hours. After the reaction was completed, the mixture was allowed to cool naturally to room temperature. Subsequent separation, purification, and drying steps were performed as in Example 1, yielding a white powder product, denoted as S3.

[0046] Comparative Example 1:

[0047] This comparative example provides a traditional aqueous phase carbonation method for preparing activated calcium carbonate. Quicklime is slaked to prepare a lime slurry with a mass fraction of 8%, filtered, and placed in a reaction vessel. The mixture is heated to 40°C and a mixed gas containing 30% CO2 (volume fraction) is introduced for carbonation. When the pH of the reaction system drops to 8.5, a stearic acid ethanol solution equivalent to 3% of the theoretical product mass is added, and carbonation continues until the pH reaches 7.0. After aging, filtration, washing, and drying, product D1 is obtained.

[0048] Comparative Example 2:

[0049] This comparative example provides a method for preparing a product by externally introducing CO2 gas into a deep eutectic solvent. Specifically, a conventional deep eutectic solvent (choline chloride:urea = 1:2) without ammonium chloride is prepared, with the remaining raw material ratios the same as in Example 1. CO2 gas is introduced into this system at 145°C to initiate the reaction, instead of generating the product in situ through urea pyrolysis, and the reaction time is 3.5 hours. The remaining steps are the same as in Example 1, yielding product D2.

[0050] Comparative Example 3:

[0051] The difference from Example 1 is that the eutectic solvent used is an unmodified conventional eutectic solvent, prepared from choline chloride and urea in a 1:2 molar ratio, and does not contain ammonium chloride. All other aspects are the same, resulting in product D3.

[0052] Comparative Example 4:

[0053] Compared to Example 1, the difference is that stearic acid was not added when preparing the homogeneous precursor solution. Instead, after the reaction was completed at a constant temperature of 145°C, the system was cooled to 100°C, and then an equal amount of stearic acid as in Example 1 was added. The mixture was stirred for 1 hour for post-coating modification. All other steps were the same, resulting in product D4.

[0054] Test Example 1:

[0055] This test verifies whether stearic acid can form a macroscopically uniform and stable dispersion system in an ammonium chloride-modified eutectic solvent. The test object is the hot liquid, i.e., the homogeneous precursor solution, obtained by adding anhydrous calcium chloride and stearic acid and stirring at 100°C for 2 hours according to the method in Example 1. The testing equipment is a UV-Vis spectrophotometer.

[0056] The experimental steps include:

[0057] 1. Preheat and calibrate the instrument. Place the modified eutectic solvent DES-A1 without anhydrous calcium chloride and stearic acid in a quartz cuvette. Under constant temperature of 100℃, use this as a blank reference and set the transmittance at 700nm wavelength to 100%.

[0058] 2. Take a portion of the DES-A1 solution (excluding stearic acid) prepared according to the method of Example 1, containing only anhydrous calcium chloride, maintain at 100°C, and quickly transfer it to a preheated cuvette to measure its transmittance at a wavelength of 700 nm.

[0059] 3. Take a homogeneous precursor solution prepared according to the method of Example 1, which has completely dissolved anhydrous calcium chloride and stearic acid, maintain it at 100°C, and quickly transfer it to a preheated cuvette to measure its transmittance at a wavelength of 700 nm.

[0060] 4. As a control, another sample was prepared by simply physically mixing stearic acid powder with DES-A1 at room temperature. In this sample, stearic acid was in an insoluble suspension. Its transmittance was also measured at 100°C.

[0061] The experimental results are shown in Table 1.

[0062] Table 1. Transmittance test data of different systems at 700 nm wavelength:

[0063] sample Light transmittance (%) <![CDATA[DES-A1 + CaCl2 (without stearic acid)]]> 99.6 Homogeneous precursor solution (Example 1) 97.2 physical mixture of stearic acid and DES-A1 13.7 <![CDATA[Final CaCO3 suspension (after reaction)]]> 0.8

[0064] Test data shows that at a wavelength of 700 nm, the transmittance of the eutectic solvent system containing only dissolved calcium chloride is close to 100%, indicating that it is a true solution. After adding a specified amount of stearic acid to this system and undergoing the heat treatment and stirring process described in this invention, the resulting homogeneous precursor solution has a transmittance as high as 97.2%. In the visible and near-infrared bands, the high transmittance directly reflects the absence of particles (typically submicron or larger) in the system that are large enough to cause significant light scattering. This result proves that stearic acid does not exist in the form of conventional, insoluble solid particles, but rather achieves a macroscopically uniform dispersion under these specific solvent system and process conditions. This state may be due to the interaction between the carboxyl terminus of stearic acid and ions or polar groups in the solvent, forming a molecularly dissolved or stable micromictal structure.

[0065] In contrast, the simple physical mixture of stearic acid and eutectic solvent exhibited extremely low transmittance (13.7%), indicating that the unreacted stearic acid was suspended in the system as coarse particles, scattering most of the incident light.

[0066] Because stearic acid is pre-distributed in a highly uniform form throughout the liquid reaction medium before the reaction begins, it creates the necessary preconditions for the simultaneous, in-situ, and homogeneous coating of calcium carbonate crystal nuclei during their formation. If stearic acid exists as insoluble large particles, it would be impossible to achieve immediate coating of each newly formed crystal nucleus during the reaction; instead, incomplete surface adhesion would only occur after particle aggregation, thus failing to effectively inhibit aggregation and achieve comprehensive surface-active modification. Therefore, the method of this invention can successfully construct a homogeneous reaction precursor, laying the foundation for subsequent controllable nucleation and simultaneous surface modification, proving the feasibility of the scheme.

[0067] Test Example 2:

[0068] This test quantitatively evaluates the surface hydrophobic and oleophilic properties of the calcium carbonate products prepared in the examples, and characterizes the integrity and effectiveness of surface modification by measuring their activation index. The test subjects are products S1, S2, and S3 obtained in Examples 1-3.

[0069] The experimental steps include:

[0070] 1. Weigh 1.0g of the sample powder to be tested, in duplicate, and place each portion into a clean, dry 50mL stoppered graduated cylinder.

[0071] 2. Add 30 mL of deionized water to one graduated cylinder and precisely add 30 mL of liquid paraffin to the other graduated cylinder.

[0072] 3. Tighten the stopper of the measuring cylinder and simultaneously vibrate both measuring cylinders vigorously by inverting them 30 times at a constant speed to ensure that the powder is fully dispersed in their respective liquid phases.

[0073] 4. Place the two graduated cylinders side by side on a horizontal and vibration-free experimental platform after shaking, and let them stand for 2 hours.

[0074] 5. After settling, carefully observe and record the scale values ​​corresponding to the upper interfaces of the sedimentation layers formed by the sample powder in deionized water and liquid paraffin, and record them as sedimentation volumes V_water and V_oil, respectively.

[0075] 6. According to Formula I a =(V oil / V water)×100% to calculate the activation index of each sample.

[0076] The experimental results are shown in Table 2.

[0077] Table 2. Activation index test data of the products from the examples:

[0078] Sample number The sedimentation volume V_water (mL) in water. Sedimentation volume Voil (mL) in liquid paraffin <![CDATA[Activation index I a (%)]]> S1 1.8 18.2 1011.1 S2 2.1 17.5 833.3 S3 1.6 19.3 1206.3

[0079] Test data show that the calcium carbonate products prepared in Examples 1-3 all exhibited extremely high activation indices, exceeding 800%. The activation index is a parameter measuring the surface polarity of powder particles. Its physical meaning lies in the fact that particles coated with non-polar groups will strongly aggregate in polar solvents (water) due to hydrophobic effects, forming a dense sedimentation layer, resulting in a small sedimentation volume Vwater. However, in non-polar solvents (liquid paraffin), the particles have good affinity with the solvent, forming a stable dispersion system with slow sedimentation and a loose sedimentation layer, resulting in a larger sedimentation volume Voil. Therefore, an activation index far exceeding 100% proves that the surface of the product particles has changed from the inherent hydrophilicity of calcium carbonate to a significant hydrophobic-oleophilic property.

[0080] In the method of this invention, stearic acid, as a surface modifier, is already dispersed in a highly uniform molecular or micromictal dispersion in a high-temperature eutectic solvent before the reaction occurs. When urea pyrolysis produces reactants and calcium carbonate nuclei are generated in situ in the system, these newly formed nuclei with extremely high surface energy instantly become the most favorable adsorption centers. The dispersed stearic acid molecules can immediately and synchronously adsorb onto the surface of the nuclei through their carboxyl ends, while the nonpolar long carbon chains face the solvent. This synchronous in-situ coating process ensures that each primary particle is covered by a complete and uniform organic layer before aggregation occurs.

[0081] This invention demonstrates that by constructing a homogeneous precursor and utilizing in-situ reaction conditions, the synergistic control of the two processes of inorganic particle generation and organic surface modification is achieved, thereby producing a calcium carbonate product with high surface activity.

[0082] Test Example 3:

[0083] This test accurately characterizes and compares the particle size and uniformity of the calcium carbonate products prepared in each example and comparative example. The test subjects are products S1, S2, and S3 prepared in Examples 1-3, and products D1, D2, D3, and D4 prepared in Comparative Examples 1-4. The testing equipment is a laser particle size analyzer.

[0084] The experimental steps include:

[0085] 1. Weigh approximately 0.1g of the sample powder to be tested and place it in a 50mL beaker. Add 20mL of anhydrous ethanol as the dispersion medium.

[0086] 2. Place the beaker in an ice-water bath and use a probe-type ultrasonic cell disruptor to process the suspension. Set the power to 200W, the working mode to work for 2 seconds and then pause for 3 seconds, and the total ultrasonic time to 5 minutes to ensure that the aggregates are fully opened.

[0087] 3. Immediately transfer the ultrasonically dispersed suspension to the sample circulation cell of the laser particle size analyzer, and start the circulation pump and stirrer to keep the suspension in a uniform state.

[0088] 4. Set the refractive index of calcium carbonate particles in ethanol to 1.590 and the absorption rate to 0.01.

[0089] 5. Start the test; the instrument will automatically collect data and perform calculations. Record the particle size distribution parameters for each sample, mainly including D10, D50 (median diameter), and D90.

[0090] 6. Calculate the particle size distribution width using the formula Span=(D90-D10) / D50.

[0091] The experimental results are shown in Table 3.

[0092] Table 3. Particle size distribution test data for each sample:

[0093] Sample number D10 (μm) D50 (μm) D90 (μm) Particle size distribution width (Span) S1 0.045 0.082 0.115 0.85 S2 0.051 0.095 0.144 0.98 S3 0.041 0.076 0.103 0.82 D1 2.18 5.31 15.62 2.53 D2 0.39 1.15 2.55 1.88 D3 0.16 0.42 0.85 1.64 D4 0.23 0.68 1.54 1.93

[0094] Test data show that the median diameter D50 of Examples S1 to S3 is all below 100 nm, and the particle size distribution width Span is less than 1.0, indicating that the product particles are small in size and highly concentrated in distribution. In contrast, the D50 and Span values ​​of all comparative samples (D1 to D4) are significantly increased.

[0095] A direct comparison with Comparative Example D3 (without ammonium chloride) reveals the crucial role of ammonium chloride in the system of this invention. In the absence of ammonium chloride, the product particle size (0.42 μm) is significantly larger than that of Example S1 (0.082 μm), and the particle size distribution is wider. This confirms that the presence of ammonium chloride effectively moderates the initial rate of urea pyrolysis, regulates the chemical equilibrium within the system through mechanisms such as the common ion effect, and avoids explosive nucleation caused by instantaneous supersaturation of reactant concentration. The controlled nucleation rate provides more ideal conditions for subsequent crystal growth and simultaneous coating, thereby effectively suppressing the hard aggregation of primary particles.

[0096] Compared with Comparative Example D4 (post-coating modification), the superiority of simultaneous in-situ modification is highlighted. In the post-coating system, calcium carbonate particles have already formed and may have a certain degree of agglomeration. At this point, adding stearic acid can only coat the outer surface of the agglomerates and cannot prevent the aggregation of primary particles. Therefore, the particle size of D4 (0.68 μm) is much larger than that of Example S1. In this invention, stearic acid is pre-uniformly dispersed in the reaction system, so that its high surface energy is saturated by stearic acid molecules at the moment of crystal nucleation, fundamentally blocking Oswald ripening and the agglomeration process caused by van der Waals forces between particles. This is the core of obtaining nanoscale, narrowly distributed particles.

[0097] The results of comparative examples D1 (traditional aqueous phase method) and D2 (external aeration method) demonstrate the advanced nature of the overall process route of this invention. The high surface tension of the aqueous phase system and the heterogeneous gas-liquid reaction interface are inherently unfavorable for preparing well-dispersed fine particles. The homogeneous non-aqueous liquid phase system used in this invention provides a mild environment with low interfacial energy for particle generation, ensuring controllable particle growth from a physical perspective.

[0098] In summary, the particle size distribution test data proves that by constructing a homogeneous precursor solution and synergistically employing two methods—reaction kinetic regulation (introduction of ammonium chloride) and simultaneous surface modification (pre-dispersion of stearic acid)—this invention achieves control over the nucleation, growth, and aggregation processes of calcium carbonate, thereby enabling the stable preparation of fine and uniformly distributed micro-active calcium carbonate products.

[0099] Test Example 4:

[0100] This test determined the mass fraction of calcium carbonate and the coating amount of surface organic modifier in each sample. The test subjects were products S1, S2, and S3 prepared in Examples 1-3, and products D1, D2, D3, and D4 prepared in Comparative Examples 1-4.

[0101] I. Calcium carbonate purity test (acid-base back titration method)

[0102] This test determines the calcium carbonate content in the product using stoichiometry. The steps are as follows:

[0103] 1. Weigh approximately 0.2g of the dried sample to be tested, denoted as m1, and place it in a 250mL Erlenmeyer flask.

[0104] 2. Accurately add 25.00 mL of C1 standard hydrochloric acid solution using a pipette, gently shake and heat slightly to ensure complete reaction and dissolution of the sample.

[0105] 3. After cooling to room temperature, add 2-3 drops of methyl orange indicator.

[0106] 4. Titrate the remaining hydrochloric acid in the conical flask with a sodium hydroxide standard solution of concentration C2 until the solution color changes from red to orange-yellow, and record the volume of sodium hydroxide standard solution consumed, V2.

[0107] 5. The mass fraction of calcium carbonate ω(CaCO3) is calculated using the following formula: ω(CaCO3) = [(C1 × 0.025 - C2 × V2) × 100.09 × 0.5] / m1 × 100%;

[0108] II. Organic matter coating amount test (Thermogravimetric analysis, TGA):

[0109] This test utilizes the difference in thermal decomposition temperatures between organic and inorganic substances to quantify the content of the organic coating layer. The steps are as follows:

[0110] 1. Weigh approximately 5 mg of the sample to be tested and place it in an alumina crucible.

[0111] 2. Place the crucible on the sample holder of the thermogravimetric analyzer.

[0112] 3. In a flowing high-purity nitrogen atmosphere (flow rate 50 mL / min), heat from room temperature (approximately 30 °C) to 600 °C at a heating rate of 10 °C / min.

[0113] 4. Record and analyze the TGA curve of sample mass change with temperature. The percentage of sample mass loss in the temperature range of 200℃ to 500℃ is determined as the organic matter coating amount.

[0114] The experimental results are shown in Table 4.

[0115] Table 4. Test data on purity and organic matter coating amount of each sample:

[0116] Sample number <![CDATA[CaCO3 purity (%)]]> Organic matter coating amount (%) S1 96.8 2.81 S2 98.2 1.43 S3 95.5 4.09 D1 93.4 2.15 D2 92.8 1.18 D3 96.1 2.47 D4 97.5 0.92

[0117] Test data show that the samples in Examples S1 to S3 not only have high calcium carbonate purity, but their organic coating amount also highly matches the theoretical addition amount. For example, the theoretical addition ratio of stearic acid in Example S1 is about 3.0%, and the measured coating amount is 2.81%, indicating extremely high coating efficiency. Examples S2 and S3 also show this trend. This result confirms that the method of the present invention can achieve efficient surface modification. The fundamental reason is that the present invention places stearic acid in a homogeneous eutectic solvent system before the reaction, so that it exists in a highly dispersed state. When calcium carbonate crystal nuclei are generated in situ, stearic acid molecules can be adsorbed onto the newly formed high-energy surface without hindrance and immediately, thereby maximizing the coating efficiency.

[0118] The results of Comparative Example D4 provide strong evidence for the above mechanism. D4 used the exact same material ratio as S1, only changing the synchronous coating to post-coating. The test results showed that although an equal amount of stearic acid was added as in S1, the final effective coating amount was only 0.92%, far lower than S1's 2.81%. This is because during the post-coating process, the primary calcium carbonate particles had already formed and aggregated. Stearic acid molecules could not effectively enter the aggregates and coat the surface of all the primary particles; most of them only adhered to the outer surface of the aggregates, resulting in low coating efficiency.

[0119] The coating efficiency of Comparative Example D1 (traditional aqueous phase method) was also low, demonstrating the difficulty of achieving uniform and effective coating in heterogeneous systems. The coating amount of Comparative Example D3 (ammonium chloride-free) was slightly lower than that of S1, which is consistent with the conclusion of Test Example 3: uncontrolled rapid nucleation leads to particle coarsening and a decrease in the specific surface area per unit mass of powder. Therefore, with the same amount of stearic acid added, the saturated coating amount it can support is also correspondingly reduced.

[0120] In summary, the purity and coating amount test data, from the perspective of chemical composition, confirm the superiority of the technical solution of this invention. By constructing a homogeneous reaction precursor and achieving simultaneous in-situ coating, this invention can not only prepare high-purity calcium carbonate products, but also achieve efficient and precise control over the amount of surface organic coating, which is difficult to achieve with traditional post-coating methods or other heterogeneous preparation methods.

[0121] Test Example 5:

[0122] This test evaluates the long-term dispersion stability of each sample in a non-polar organic medium. The test subjects are products S1, S2, and S3 prepared in Examples 1-3, and products D1, D2, D3, and D4 prepared in Comparative Examples 1-4.

[0123] The experimental steps include:

[0124] 1. Weigh 1.0g of the sample to be tested and place it in a 100mL beaker. Add 50mL of liquid paraffin.

[0125] 2. Use a high-speed shear homogenizer to shear the mixture at 5000 rpm for 5 minutes to prepare a uniform suspension.

[0126] 3. Quickly and completely transfer the sheared suspension into a 100mL stoppered graduated cylinder, tighten the stopper, and place it vertically on a vibration-free experimental platform.

[0127] 4. After standing for 24 hours, observe and record the scale value corresponding to the upper interface of the lower sediment in the graduated cylinder, which is the sedimentation volume.

[0128] The experimental results are shown in Table 5.

[0129] Table 5. Settling volume of each sample in liquid paraffin after 24 hours:

[0130] Sample number 24-hour sedimentation volume (mL) S1 31.2 S2 26.5 S3 35.8 D1 5.8 D2 9.2 D3 11.5 D4 7.1

[0131] Test data show that after samples from Examples S1, S2, and S3 were left to stand in liquid paraffin for 24 hours, they all formed loose sedimentation layers with large volumes and blurred interfaces. Their sedimentation volumes were significantly larger than those of all comparative samples. In colloid science, large sedimentation volumes indicate effective repulsion between particles, preventing them from packing together tightly under gravity, and are direct evidence of the system's excellent dispersion stability.

[0132] This macroscopic performance stems from the unique surface structure of the particles prepared in this invention. Through simultaneous in-situ coating, the surface of each primary calcium carbonate particle is covered with a complete and homogeneous long stearic acid chain. When these particles are dispersed in a nonpolar medium such as liquid paraffin, the nonpolar long carbon chains on their surface have a good affinity with solvent molecules, while simultaneously forming an effective steric hindrance layer between the particles. This steric hindrance layer is sufficient to overcome the van der Waals forces between particles, thereby inhibiting irreversible aggregation of the particles and enabling the particles to be dispersed stably in the medium as independent or small aggregates for a long period of time.

[0133] The significant difference in sedimentation volume between Comparative Example D4 (post-coating) (7.1 mL) and S1 (31.2 mL) reveals the irreplaceable nature of simultaneous coating. The post-coating process treats already aggregated particle clusters, and stearic acid molecules cannot penetrate the clusters to effectively coat all primary particles. Therefore, the particles still exhibit agglomerated behavior in the organic medium, settling rapidly and forming a dense accumulation layer.

[0134] The sedimentation volume of Comparative Example D3 (ammonium chloride-free) (11.5 mL) was also much smaller than that of S1, which is consistent with the particle size analysis results of Test Example 3. The particles of D3 were coarser and more widely distributed, and the surface coating uniformity was not as good as that of S1, resulting in a weakened steric hindrance effect and decreased stability. Due to their inherent inhomogeneity and incomplete surface modification, the products of Comparative Examples D1 (aqueous phase method) and D2 (external aeration method) exhibited the worst dispersion stability in organic media and rapidly formed hard agglomerates.

[0135] Therefore, the results of the dispersion stability test, from the perspective of application performance, verify the effectiveness of the technical solution of this invention. It demonstrates that the complete organic coating layer constructed at the particle generation source using the method of this invention can be successfully transformed into high macroscopic dispersion stability in organic systems. This is of decisive significance for the application of activated calcium carbonate as a functional filler in polymer matrices and other fields.

[0136] 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 method for preparing finely activated calcium carbonate, characterized in that, Includes the following steps: (a) A calcium source and a surface modifier are added to a modified eutectic solvent containing choline chloride, urea and ammonium chloride, and a homogeneous precursor solution is formed under heating and stirring conditions; (b) The homogeneous precursor solution is heated to a predetermined reaction temperature to pyrolyze the urea to generate calcium carbonate in situ, and the surface modifier is simultaneously coated onto the calcium carbonate. (c) The reaction products are separated, washed and dried to obtain the fine active calcium carbonate.

2. The method for preparing finely activated calcium carbonate according to claim 1, characterized in that, In the modified eutectic solvent, the molar ratio of choline chloride, urea, and ammonium chloride is 1:(1.8-2.2):(0.1-0.3).

3. The method for preparing finely activated calcium carbonate according to claim 1 or 2, characterized in that, The modified deep eutectic solvent is prepared by heating and stirring a solid mixture of choline chloride, urea and ammonium chloride at 80-95°C.

4. The method for preparing finely activated calcium carbonate according to claim 1, characterized in that, The calcium source is anhydrous calcium chloride, and the surface modifier is stearic acid.

5. The method for preparing finely activated calcium carbonate according to claim 4, characterized in that, The molar ratio of anhydrous calcium chloride to urea is 1:(3.0-5.0); the mass ratio of stearic acid to theoretically generated calcium carbonate is (1.5-4.0):

100.

6. The method for preparing finely activated calcium carbonate according to claim 1, characterized in that, The heating temperature in step (a) is 90–110°C.

7. The method for preparing finely activated calcium carbonate according to claim 1, characterized in that, The heating rate described in step (b) is 0.8 to 1.5 °C / min.

8. The method for preparing finely activated calcium carbonate according to claim 1, characterized in that, The predetermined reaction temperature in step (b) is 135–155°C, and the reaction is carried out at this temperature for 2.5–4.5 hours.