A method for preparing spherical calcium carbonate particles

Abstract

The application relates to the technical field of calcium carbonate preparation, and particularly discloses spherical calcium carbonate, a preparation method and application thereof, wherein different process conditions are regulated, and different crystal form control agents are added to prepare nano CaCO3 products with different morphological characteristics and special functions; when the spherical calcium carbonate prepared by the application is used as a PE resin film filler, the filling amount can reach 50%, and meanwhile, good tensile strength can be maintained.

Description

Technical Field

[0001] This invention relates to a method for preparing nano-calcium carbonate particles, and more specifically, to a method for preparing nano-spherical calcium carbonate particles. Background Technology

[0002] Calcium carbonate is a commonly used inorganic filler, and its size and shape vary depending on the application. Common forms of calcium carbonate include rhombohedral, rod-shaped, needle-shaped, and spindle-shaped particles, with a typical calcite crystal structure. Nano-calcium carbonate is an ultrafine solid powder material that emerged in the 1980s. Due to the ultrafine particle size, its crystal structure and surface electronic structure produce quantum size and surface effects not found in ordinary calcium carbonate, enabling it to play a reinforcing and toughening role in many applications such as rubber, plastics, papermaking, coatings, adhesives, and inks. However, its generally high viscosity limits its processing performance.

[0003] Carbonation is a common method for the chemical preparation of nano-calcium carbonate. The main process involves first calcining limestone to obtain quicklime and carbon dioxide. The quicklime is then digested and reacted with water to obtain a calcium hydroxide slurry. Purified carbon dioxide is then introduced into the calcium hydroxide slurry to obtain a calcium carbonate slurry. After dehydration and drying, the finished nano-calcium carbonate product is obtained. The carbonation reaction is the most critical process step in the preparation of nano-calcium carbonate. Controlling the reaction conditions determines the particle size and distribution of the product. Improper drying processes can easily lead to agglomeration and caking of nano-calcium carbonate. Unfortunately, current nano-calcium carbonate carbonation and drying equipment requires numerous and stringent controllable process parameters, making it difficult to consistently control product quality and severely hindering the application and development of nano-calcium carbonate.

[0004] On the other hand, spherical calcium carbonate is a research focus due to its high filling capacity and low system viscosity; however, it typically has a spheroidal aragonite structure, making it inherently unstable. If the process is designed to produce spheroidal aragonite-type calcium carbonate, there is a high probability that a significant proportion of calcite-type nano-calcium carbonate particles will remain in the final product.

[0005] Generally, calcium carbonate with a spheroidal structure is synthesized using the metathesis method, which involves adding additives to form spherical particles. The resulting structure is predominantly spheroidal, a metastable structure that easily transforms into a calcite structure. However, because the pure calcite crystal form is highly suitable for crystal growth, the crystals quickly grow into larger calcite particles, which is detrimental to the preparation of nano-sized calcium carbonate.

[0006] Therefore, how to prepare nano-CaCO3 products with different morphological characteristics and special functions by adjusting different process conditions and adding different crystal form control agents, so as to improve their application performance in the industry and expand their use value, is the focus of current research.

[0007] For example, Chinese patent document CN202211027646.X discloses a surface-modified nano-calcium carbonate and its preparation method, the steps of which include: (1) preparation of crystal form control agent: the crystal form control agent includes solution A and solution B, solution A is an aqueous solution of zinc gluconate, and solution B is a protein composite solution; (2) preparation of nano-calcium carbonate; (3) modification of nano-calcium carbonate: the nano-calcium carbonate is surface-modified by a composite aqueous solution of KH550 and sodium stearate to obtain modified nano-calcium carbonate. The prepared nano-calcium carbonate has a large specific surface area, and although its mechanical properties are improved to a certain extent after being added to polyethylene film, it still needs further improvement.

[0008] However, there is an urgent need in the existing technology for a technical solution to achieve a stable calcite crystal structure in nano-calcium carbonate particles before they grow into calcite particles. Summary of the Invention

[0009] To address the problem of existing technologies failing to produce stable spherical calcium carbonate particles with calcite structure, thus reducing the proportion of calcite particles, this invention provides a technical solution for the preparation of spherical calcium carbonate particles.

[0010] Specifically, according to a first aspect of the present invention, a method for preparing spherical calcium carbonate particles using a spray drying process is provided. After preparing and synthesizing nano-sized calcium carbonate particles using a lime milk carbonation method, no surface treatment or surface coating is performed. Instead, the particles are directly spray-dried using a spray drying method. By utilizing the high surface tension of nanoparticles and their tendency to generate hard agglomerates, spherical nano-calcium carbonate particles are generated.

[0011] According to the above-described method for preparing spherical calcium carbonate particles using spray drying, the following equipment is used: a lime slurry temperature-controlled carbonation reactor, a Venturi tube treatment device, and a spray drying device. The lime slurry temperature-controlled carbonation reactor includes a carbonation reactor, a cooling component on the outer layer of the reactor, and a cooling coil on the inner layer. The cooling component and the cooling coil are connected to a coolant storage tank via liquid cooling pipes, and the coolant storage tank is connected to a refrigeration device. The bottom of the carbonation reactor is connected to the Venturi tube treatment device via a pipeline. The spray drying device and the calcium carbonate clinker that has been processed and agglomerated by the Venturi tube treatment device are connected via a clinker slurry pump through a pipeline.

[0012] Furthermore, the top of the carbonization reactor is equipped with a slurry inlet, a stirring motor, multiple temperature sensors, and a pH test box; a hollow stirring rod with blades is installed at the center of the carbonization reactor, and carbon dioxide gas is circulated inside the stirring rod.

[0013] Furthermore, a rotating ultrasonic atomizing nozzle is provided at the bottom of the stirring rod.

[0014] Furthermore, the Venturi tube processing device includes a gas-liquid two-phase, two-channel Venturi tube, a heating device, and a cooling device. The cooled carbon dioxide gas is discharged from the gas outlet or collected for other uses. The cooled slurry is fed into a spray drying device, which includes a spray drying growth tower, spray nozzles, an air intake fan, an air heating device, a separator, a fan, and a dust collector. Spray nozzles are installed inside the spray drying growth tower and are connected to the slurry pump. The spray drying reaction tower is also connected to the separator, which is connected to the dust collector via a fan and pipeline.

[0015] Furthermore, in the temperature-controlled carbonization reactor, the raw material slurry is pumped into the carbonization reactor through the inlet. Under the stirring of the stirring rod and blade driven by the stirring motor, carbon dioxide enters the carbonization reactor through the rotating ultrasonic atomizing nozzle and reacts with the raw material slurry. The heat generated by the carbonization reaction is cooled by the coolant through heat exchange between the cooling coil and the cooling component. After absorbing the heat of the carbonization reaction system, the coolant enters the coolant storage tank, is cooled by the refrigeration unit, and returns to the coolant storage tank. It is then pumped back into the cooling coil and cooling component by the refrigerant pump for heat exchange with the carbonization reaction system. The temperature and pH value of the slurry in the carbonization tank are monitored by the temperature sensor and pH test box. When the pH value drops to a certain value, the reaction is stopped. After stirring for a period of time, the slurry is connected to the Venturi tube treatment device. In the spray drying device, the clinker slurry is pumped out from the cooling device in the venturi tube treatment device by the clinker slurry pump and enters the spray drying tower through the spray nozzle. Air is drawn into the spray drying tower by the air intake fan and the air heating device. The dried nano-calcium carbonate material is discharged from the material outlet at the bottom of the spray drying tower. The carbon dioxide gas is separated from the material by the separator and then discharged into the atmosphere by the fan and the dust collector.

[0016] To address the problem of exothermic carbonation leading to increased temperature and accelerated reaction rate, which makes it difficult to control the particle size of calcium carbonate, a cooling component and cooling coil are added to the carbonation reactor to remove the heat generated by the carbonation reaction. This controls the temperature of the carbonation system, inhibits the growth rate of calcium carbonate particles, and controls the particle size of the generated calcium carbonate particles to the nanoscale range. To utilize the high surface tension and easy agglomeration of nanoparticles, they are heated in a Venturi tube after carbonation and then cooled to fully agglomerate them. To address the problem that conventional drying processes for light calcium carbonate, such as drum drying and air cannon dispersion, easily cause particle caking and difficulty in dispersion, spray drying is adopted to directly dry the nano-calcium carbonate particle slurry. This utilizes the agglomeration characteristic while ensuring particle dispersibility.

[0017] According to a second aspect of the present invention, the present invention provides a method for preparing spherical calcium carbonate, comprising the following steps:

[0018] S1. Preparation of calcium hydroxide slurry

[0019] Quicklime powder and water are mixed and subjected to a digestion reaction. After the reaction is complete, the mixture is allowed to stand and age, then filtered to remove impurities, resulting in a calcium hydroxide slurry.

[0020] In this step, quicklime is ground and passed through a 1000-mesh sieve. The sieved powder is collected to obtain quicklime powder.

[0021] In this step, the mass ratio of quicklime powder to water is 1:4-8. In some embodiments of the present invention, for example, 1:4, 1:5, 1:6, 1:7, or 1:8 can be selected (but not limited to the listed values, other unlisted values ​​within the range are also applicable).

[0022] In this step, the temperature of the digestion reaction is 60-80℃, for example, 60℃, 65℃, 70℃, 75℃, 80℃ can be selected; the time of the digestion reaction is 90-180min, for example, 90min, 100min, 110min, 120min, 130min, 140min, 150min, 160min, 170min, 180min can be selected (but not limited to the listed values, other unlisted values ​​within the range are also applicable).

[0023] In this step, the settling and aging time is 12-24 hours, for example, you can choose 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h (but not limited to the listed values, other unlisted values ​​within the range are also applicable).

[0024] In this step, impurities are removed by filtration using a 200-mesh standard sieve.

[0025] S2. Preparation of calcium carbonate slurry

[0026] Carbon dioxide gas is introduced into the calcium hydroxide slurry. When the conductivity of the reaction system drops to 3-5 mS / cm, the introduction of carbon dioxide gas is stopped. A crystal form control agent is added to the slurry, and the introduction of carbon dioxide gas is continued until the pH of the reaction system is ≤7.0, thus obtaining a calcium carbonate slurry.

[0027] In this step, the flow rate of carbon dioxide gas is 20-30 m³ / h. 3 / h, for example, you can choose 20m 3 / h、21m 3 / h、22m 3 / h、23m 3 / h、24m 3 / h, 25m 3 / h、26m 3 / h、27m 3 / h、28m 3 / h、29m 3 / h, 30m 3 / h (but not limited to the listed values; other unlisted values ​​within the range also apply).

[0028] In this step, the volume ratio of calcium hydroxide slurry to crystal form control agent is 1L:10-20mL. For example, you can choose 1L:10mL, 1L:12mL, 1L:14mL, 1L:16mL, 1L:18mL, or 1L:20mL, but it is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0029] Specifically, the preparation method of the crystal form control agent is as follows:

[0030] (1) Chitosan was dissolved in acetic acid solution, sodium periodate was added to it, the reaction was stirred under nitrogen atmosphere, and then ethylene glycol was added to stop the reaction. The product was concentrated by rotary evaporation and then dialyzed, and then freeze-dried to obtain aldehyde-modified chitosan.

[0031] (2) Dissolve aldehyde-modified chitosan in an organic solvent, then add sodium hypochlorite as an oxidant, stir and react. After the reaction is complete, concentrate the product by rotary evaporation and then dialyze it, and then freeze dry it to obtain carboxylated chitosan.

[0032] (3) Dissolve carboxylated chitosan in water to prepare an aqueous solution, and the crystal form control agent is obtained.

[0033] More specifically, in step (1), the mass ratio of chitosan to sodium periodate is 5-10:4-8, for example, 5:4, 5:5, 5:6, 5:7, 5:8, 8:4, 8:5, 8:6, 10:4, 10:5, 10:6, 10:7, 10:8 (but not limited to the listed values, other unlisted values ​​within the range are also applicable).

[0034] More specifically, in step (1), the temperature of the stirring reaction is 4-8℃, for example, 4℃, 5℃, 6℃, 7℃, 8℃ can be selected, and the stirring reaction time is 12-24h, for example, 12h, 15h, 18h, 21h, 24h can be selected (but not limited to the listed values, other unlisted values ​​within the range are also applicable).

[0035] More specifically, in step (1), the specific operation process of dialysis is as follows: first, dialysis is performed in a saturated saline solution for 6 hours, and then dialysis is performed in deionized water for 6 hours to remove impurities.

[0036] More specifically, in step (2), the mass ratio of aldehyde-modified chitosan to sodium hypochlorite is 5-10:8-12, for example, 5:8, 5:10, 5:12, 8:8, 8:10, 8:12, 10:12, 10:8 (but not limited to the listed values, other unlisted values ​​within the range are also applicable).

[0037] More specifically, in step (2), the stirring reaction time is 12-18h, for example, 12h, 13h, 14h, 15h, 16h, 17h, 18h can be selected (but not limited to the listed values, other unlisted values ​​within the range are also applicable).

[0038] More specifically, in step (2), the specific operation process of dialysis is as follows: first, dialysis is performed in a saturated saline solution for 6 hours, and then dialysis is performed in deionized water for 6 hours to remove impurities.

[0039] More specifically, in step (3), the mass fraction of carboxylated chitosan in the aqueous solution is 10-20%, for example, 10%, 12%, 14%, 15%, 18%, 20% can be selected (but not limited to the listed values, other unlisted values ​​within the range are also applicable).

[0040] S3. Preparation of spherical calcium carbonate

[0041] Maleic anhydride and pyridine, an esterification catalyst, are added to calcium carbonate slurry. The esterification reaction is carried out at 50-60℃ for 2-3 hours. After the reaction is completed, the product is dehydrated by pressure filtration, dried, and pulverized to obtain spherical calcium carbonate.

[0042] In this step, based on the mass of carboxylated chitosan, the amount of maleic anhydride added is 20-40%, for example, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40% can be selected; the amount of pyridine added is 1-2%, for example, 1%, 1.2%, 1.5%, 1.8%, 2% can be selected (but not limited to the listed values, other unlisted values ​​within the range are also applicable).

[0043] According to a third aspect of the present invention, the present invention provides spherical calcium carbonate prepared by the above-described preparation method.

[0044] According to a fourth aspect of the invention, the invention also provides the use of the above-described spherical calcium carbonate in the preparation of polyethylene films.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] (1) By adjusting different process conditions, nano-CaCO3 products with different morphological characteristics and special functions can be prepared, thereby improving their application performance in the industry and obtaining stable spherical calcium carbonate particles with calcite structure to reduce the proportion of calcite particles. No surface treatment or surface coating is required. Instead, spray drying is carried out directly. Taking advantage of the high surface tension of nanoparticles and their tendency to generate hard agglomerates, spherical nano-calcium carbonate particles are generated.

[0047] (2) In this invention, chitosan is first modified by aldehyde and then by carboxylation to improve its water solubility. Then, carboxylated chitosan is used as a crystal form control agent. Carboxylated chitosan has a high molecular chain structure, and the carboxyl group has a stronger adsorption capacity for calcium ions than the hydroxyl group. Calcium ions are easy to form a covalent structure with the carboxyl group on the molecular chain of the crystal form control agent, so that a large number of calcium ions are enriched near the molecular chain. At the same time, the positively charged calcium ions will attract nearby carbonate ions, so that calcium carbonate forms a large number of calcium carbonate crystal nuclei in the microenvironment of the high molecular chain. The crystal form control agent with a large number of calcium carbonate crystal nuclei can be further adsorbed onto the surface of calcium carbonate particles, inhibiting the growth of particles along the linear direction and preventing calcium carbonate from forming large agglomerates, thereby obtaining regular spherical calcium carbonate. When used as a filler for PE resin film, it can significantly improve the mechanical properties of the film.

[0048] (3) The present invention esterifies the remaining hydroxyl groups on the chitosan molecular chain and reduces the surface energy of calcium carbonate by grafting maleic anhydride hydrolysate. At the same time, it improves the compatibility between calcium carbonate powder and PE resin, and further improves the mechanical properties of PE resin film. When the spherical calcium carbonate prepared by the present invention is used as a filler for PE resin film, the filling amount can reach 50%, while maintaining good tensile strength.

[0049] (4) One aspect of this invention utilizes the agglomeration property by introducing a Venturi tube and controlling conditions such as temperature and pH during spray drying to produce spherical calcium carbonate particles with a reduced proportion of calcite crystals. Another aspect involves controlling the crystal form to produce spherical calcium carbonate particles with a reduced proportion of calcite crystals. Regardless of the method used, whether utilizing the agglomeration property or preventing large agglomerations, spherical calcium carbonate particles can be perfectly formed. Attached Figure Description

[0050] Figure 1 This is an electron microscope image at a scale of 100 micrometers, prepared by the method for preparing spherical calcium carbonate particles in Example 1 of the present invention.

[0051] Figure 2 This is an electron microscope image at a scale of 80 micrometers, prepared by the method for preparing spherical calcium carbonate particles in Example 2 of the present invention. Detailed Implementation

[0052] The method for preparing spherical calcium carbonate particles according to the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. Those skilled in the art will understand that this description is exemplary and the present invention is not limited to the specific embodiments described herein.

[0053] Example 1

[0054] A method for preparing spherical calcium carbonate particles using spray drying technology: Raw slurry is pumped into a carbonization reactor through the inlet. Under the stirring of a stirring motor driving a stirring rod with blades, carbon dioxide enters the carbonization reactor through a rotating ultrasonic atomizing nozzle and reacts with the raw slurry. The heat generated by the carbonization reaction is cooled by the coolant through a cooling coil and cooling components. The coolant absorbs the heat from the carbonization reaction system and enters a coolant storage tank. After being cooled by a chiller, it returns to the coolant storage tank and is pumped back into the cooling coil and cooling components by a chiller pump for further heat exchange. The temperature and pH value of the slurry in the carbonization tank are monitored by a temperature sensor and a pH test box. When the pH value drops to a certain value, the reaction is stopped. After stirring for a period of time, the slurry is connected to a Venturi tube treatment device. In the spray drying apparatus, the clinker slurry is pumped from the cooling device in the venturi tube treatment unit by a clinker slurry pump and enters the spray drying tower through spray nozzles. Air is drawn into the spray drying tower by an intake fan and an air heating device. The dried nano-calcium carbonate material is discharged from the material outlet at the bottom of the spray drying tower. Carbon dioxide gas is separated from the material by a separator and then discharged into the atmosphere by a fan and a dust collector. Spray drying is performed directly, utilizing the high surface tension of nanoparticles and their tendency to form hard agglomerates, to produce... Figure 1 The image shows spherical nano-calcium carbonate particles.

[0055] Example 2

[0056] A method for preparing spherical calcium carbonate includes the following steps:

[0057] S1. Quicklime powder is passed through a 1000-mesh sieve. Then, quicklime powder and water are mixed at a mass ratio of 1:5. The mixture is then subjected to a digestion reaction at 60°C for 180 minutes. After the reaction is completed, the mixture is allowed to stand for 12 hours. The mixture is then filtered through a 200-mesh standard sieve to remove impurities, resulting in calcium hydroxide slurry.

[0058] S2. Carbon dioxide gas is introduced into the calcium hydroxide slurry at a flow rate of 25 m³ / h. When the conductivity of the reaction system drops to 3 mS / cm, the introduction of carbon dioxide gas is stopped. A crystal form control agent is added, with a volume ratio of calcium hydroxide slurry to crystal form control agent of 1 L: 150 mL. Carbon dioxide gas is then introduced again to continue the reaction at a flow rate of 25 m³ / h. 3 / h, until the pH of the reaction system is ≤7.0, to obtain calcium carbonate slurry;

[0059] The preparation method of the crystal form control agent is as follows:

[0060] (1) Dissolve 5g of chitosan in 100mL of 10wt% acetic acid solution, add 4g of sodium periodate, stir and react for 12h under nitrogen atmosphere and 5℃, then add 5mL of ethylene glycol to stop the reaction, concentrate the product by rotary evaporation and then dialyze it. The specific operation of dialysis is as follows: first dialyze in saturated saline solution for 6h, then dialyze in deionized water for 6h to remove impurities, and then freeze dry to obtain aldehyde-modified chitosan.

[0061] (2) Dissolve 5g of aldehyde-modified chitosan in 100mL of organic solvent tetrahydrofuran, then add 5g of oxidant sodium hypochlorite, stir and react at room temperature for 12h. After the reaction is completed, the product is concentrated by rotary evaporation and then dialyzed. The specific operation of dialysis is as follows: first dialyze in saturated saline solution for 6h, then dialyze in deionized water for 6h to remove impurities, and then freeze dry to obtain carboxylated chitosan.

[0062] (3) Dissolve 10 carboxylated chitosan in 100g of water to prepare an aqueous solution, and the crystal form control agent is obtained.

[0063] S3. Add maleic anhydride and pyridine, the esterification catalyst, to the calcium carbonate slurry. The amount of maleic anhydride added is 30% of the mass of carboxylated chitosan, and the amount of pyridine added is 1% of the mass of carboxylated chitosan. The esterification reaction is carried out at 50°C for 3 hours. After the reaction is complete, the product is dehydrated by pressure filtration, dried, and pulverized to obtain the desired product. Figure 2 The image shows spherical calcium carbonate particles.

[0064] Comparative Example 1

[0065] A method for preparing calcium carbonate includes the following steps:

[0066] S1. Quicklime powder is passed through a 1000-mesh sieve. Then, quicklime powder and water are mixed at a mass ratio of 1:5. The mixture is then subjected to a digestion reaction at 60°C for 180 minutes. After the reaction is completed, the mixture is allowed to stand for 12 hours. The mixture is then filtered through a 200-mesh standard sieve to remove impurities, resulting in calcium hydroxide slurry.

[0067] S2. Carbon dioxide gas is introduced into the calcium hydroxide slurry at a flow rate of 25 m3 / h. When the conductivity of the reaction system drops to 3 mS / cm, the introduction of carbon dioxide gas is stopped. A crystal form control agent is added to the slurry at a volume ratio of 1 L: 150 mL. Carbon dioxide gas is then introduced at a flow rate of 25 m3 / h until the pH of the reaction system is ≤ 7.0, thus obtaining a calcium carbonate slurry.

[0068] The crystal form control agent is prepared by dissolving 10g of soluble corn starch in 100g of water to prepare an aqueous solution, thereby obtaining the crystal form control agent.

[0069] S3. Add maleic anhydride and pyridine, the catalyst for esterification reaction, to the calcium carbonate slurry. The amount of maleic anhydride added is 30% of the mass of carboxylated chitosan, and the amount of pyridine added is 1% of the mass of carboxylated chitosan. The esterification reaction is carried out at 50°C for 3 hours. After the reaction is completed, the product is dehydrated by pressure filtration, dried, and pulverized to obtain calcium carbonate.

[0070] Compared with Comparative Example 1 and Example 2, soluble corn starch was used instead of carboxylated chitosan.

[0071] Comparative Example 2

[0072] A method for preparing calcium carbonate includes the following steps:

[0073] S1. Quicklime powder is passed through a 1000-mesh sieve. Then, quicklime powder and water are mixed at a mass ratio of 1:5. The mixture is then subjected to a digestion reaction at 60°C for 180 minutes. After the reaction is completed, the mixture is allowed to stand for 12 hours. The mixture is then filtered through a 200-mesh standard sieve to remove impurities, resulting in calcium hydroxide slurry.

[0074] S2. Carbon dioxide gas is introduced into the calcium hydroxide slurry at a flow rate of 25 m3 / h. When the conductivity of the reaction system drops to 3 mS / cm, the introduction of carbon dioxide gas is stopped. A crystal form control agent is added to the slurry at a volume ratio of 1 L: 150 mL. Carbon dioxide gas is then introduced at a flow rate of 25 m3 / h until the pH of the reaction system is ≤ 7.0, thus obtaining a calcium carbonate slurry.

[0075] The crystal form control agent is prepared by dissolving 10 carboxymethyl chitosan in 100g of water to prepare an aqueous solution, thereby obtaining the crystal form control agent.

[0076] S3. Add maleic anhydride and pyridine, the catalyst for esterification reaction, to the calcium carbonate slurry. The amount of maleic anhydride added is 30% of the mass of carboxylated chitosan, and the amount of pyridine added is 1% of the mass of carboxylated chitosan. The esterification reaction is carried out at 50°C for 3 hours. After the reaction is completed, the product is dehydrated by pressure filtration, dried, and pulverized to obtain calcium carbonate.

[0077] Compared with Comparative Example 2 and Example 2, carboxymethyl chitosan was used instead of carboxylated chitosan.

[0078] Comparative Example 3

[0079] A method for preparing calcium carbonate includes the following steps:

[0080] S1. Quicklime powder is passed through a 1000-mesh sieve. Then, quicklime powder and water are mixed at a mass ratio of 1:5. The mixture is then subjected to a digestion reaction at 60°C for 180 minutes. After the reaction is completed, the mixture is allowed to stand for 12 hours. The mixture is then filtered through a 200-mesh standard sieve to remove impurities, resulting in calcium hydroxide slurry.

[0081] S2. Carbon dioxide gas is introduced into the calcium hydroxide slurry at a flow rate of 25 m3 / h. When the conductivity of the reaction system drops to 3 mS / cm, the introduction of carbon dioxide gas is stopped. A crystal form control agent is added to the slurry at a volume ratio of 1 L: 150 mL. Carbon dioxide gas is then introduced at a flow rate of 25 m3 / h until the pH of the reaction system is ≤ 7.0, thus obtaining a calcium carbonate slurry.

[0082] The preparation method of the crystal form control agent is as follows:

[0083] (1) Dissolve 5g of chitosan in 100mL of 10wt% acetic acid solution, add 4g of sodium periodate, stir and react for 12h under nitrogen atmosphere and 5℃, then add 5mL of ethylene glycol to stop the reaction, concentrate the product by rotary evaporation and then dialyze it. The specific operation of dialysis is as follows: first dialyze in saturated saline solution for 6h, then dialyze in deionized water for 6h to remove impurities, and then freeze dry to obtain aldehyde-modified chitosan.

[0084] (2) Dissolve 5g of aldehyde-modified chitosan in 100mL of organic solvent tetrahydrofuran, then add 5g of oxidant sodium hypochlorite, stir and react at room temperature for 12h. After the reaction is completed, the product is concentrated by rotary evaporation and then dialyzed. The specific operation of dialysis is as follows: first dialyze in saturated saline solution for 6h, then dialyze in deionized water for 6h to remove impurities, and then freeze dry to obtain carboxylated chitosan.

[0085] (3) Dissolve 10 carboxylated chitosan in 100g of water to prepare an aqueous solution, and the crystal form control agent is obtained.

[0086] S3. Add maleic anhydride to the calcium carbonate slurry. The amount of maleic anhydride added is 30% of the mass of carboxylated chitosan. Stir the reaction at 50°C for 3 hours. After the reaction is completed, filter the product to remove water, dry it and pulverize it to obtain calcium carbonate.

[0087] Compared with Comparative Example 3 and Example 2, no esterification catalyst pyridine was added, and the maleic anhydride hydrolysis product was attached to the surface of calcium carbonate through hydrogen bonding.

[0088] The calcium carbonate in Examples 2 and Comparative Examples 1-3 was extruded and granulated according to the formulation of 49.5% PE resin (CAS: 9002-88-4) + 50% calcium carbonate + 0.5% polyethylene wax. Then, it was processed by a casting machine to obtain a breathable film and made into 100mm×45mm samples for testing. The tensile properties were tested according to GB / T 1040-2018 method, and the tensile speed was set to 100mm / min. The test results are shown in Table 1.

[0089] Table 1 Mechanical performance test results

[0090]

[0091] As can be seen from Table 1, the calcium carbonate prepared in Comparative Examples 1 and 2 using starch and carboxymethyl chitosan, respectively, as commonly used crystal form control agents, has lower mechanical properties than that of Example 2 of the present invention when used as filler. This may be because carboxylation treatment can effectively increase the polarity of chitosan molecules, thus significantly enhancing their effectiveness in contacting calcium ions and carbonate ions in the aqueous phase. Compared to Comparative Example 3, grafting maleic anhydride hydrolysate through esterification reaction in Example 2 of the present invention is beneficial to improving the mechanical properties of PE resin film.

[0092] The present invention has been described in detail above with reference to specific embodiments. One embodiment, 1, utilizes the agglomeration property by introducing a Venturi tube and controlling conditions such as temperature and pH during spray drying to produce spherical calcium carbonate particles with a reduced proportion of calcite crystals. Another embodiment, 2, achieves the production of spherical calcium carbonate particles with a reduced proportion of calcite crystals by controlling the crystal form. However, it is obvious that the present invention is not limited to these specific embodiments, and those skilled in the art can make various modifications and improvements based on the present invention. Any modifications or improvements made without departing from the spirit and purpose of the present invention should fall within the scope of protection of the present invention, which is defined by the appended claims.

Claims

1. A method for preparing spherical calcium carbonate, characterized in that, Includes the following steps: S1. Mix quicklime powder and water to carry out a digestion reaction. After the reaction is complete, let it stand for aging, filter to remove impurities, and obtain calcium hydroxide slurry. S2. Carbon dioxide gas is introduced into the calcium hydroxide slurry. When the conductivity of the reaction system drops to 3-5 mS / cm, the introduction of carbon dioxide gas is stopped. A crystal form control agent is added to it, and carbon dioxide gas is introduced again to carry out the reaction until the pH of the reaction system is ≤7.0, and calcium carbonate slurry is obtained. S3. Add maleic anhydride and pyridine, the esterification catalyst, to the calcium carbonate slurry. Perform esterification at 50-60℃ for 2-3 hours. After the reaction is complete, filter, dehydrate, dry, and pulverize the product to obtain spherical calcium carbonate. The amount of maleic anhydride added is 20-40% and the amount of pyridine added is 1-2% based on the mass of carboxylated chitosan. In step S2, the preparation method of the crystal form control agent is as follows: (1) Chitosan was dissolved in acetic acid solution, sodium periodate was added to it, the reaction was stirred under nitrogen atmosphere, and then ethylene glycol was added to stop the reaction. The product was concentrated by rotary evaporation and then dialyzed, and then freeze-dried to obtain aldehyde-modified chitosan. (2) Dissolve aldehyde-modified chitosan in an organic solvent, then add sodium hypochlorite as an oxidant, stir and react. After the reaction is complete, concentrate the product by rotary evaporation and then dialyze it, and then freeze dry it to obtain carboxylated chitosan. (3) Dissolve carboxylated chitosan in water to prepare an aqueous solution, and the crystal form control agent is obtained.

2. The spherical calcium carbonate prepared by the preparation method described in claim 1.

3. The application of spherical calcium carbonate as described in claim 2 in the preparation of polyethylene film.

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