Method for producing calcium carbonate and calcium carbonate

Abstract

The purpose of the present invention is to provide: calcium carbonate having a novel hierarchical structure, and preferably calcite type calcium carbonate; and a novel method for providing calcium carbonate having a novel hierarchical structure, and preferably calcite type calcium carbonate.　The present invention provides: calcium carbonate having a hierarchical structure, which comprises calcium carbonate having a cylindrical, needle-like or columnar elongated structure at the surface of calcium carbonate having a compact structure; and a method for producing calcium carbonate having a hierarchical structure. The method includes the following steps.　A step for mixing calcite type calcium carbonate particles, gelatin and water, and heating and stirring to obtain a gelatin / calcite type calcium carbonate particle mixed slurry A.　A step for dissolving a surfactant in an oil and stirring to obtain a solution B.　A step for adding the gelatin / calcite type calcium carbonate particle mixed slurry A to the solution B, and stirring to obtain an emulsified liquid C.　A step for adding a crosslinking agent to the emulsified liquid C, and stirring to obtain an emulsified liquid D containing gelatin / calcite type calcium carbonate composite particles.　A step for subjecting the emulsified liquid D containing gelatin / calcite type calcium carbonate composite particles to centrifugal separation to obtain gelatin / calcite type calcium carbonate composite particles.　A step for firing the gelatin / calcite type calcium carbonate composite particles to obtain calcite type calcium carbonate compact particles, and a step for placing the calcite type calcium carbonate compact particles in an atmosphere containing at least carbon dioxide to obtain calcium carbonate having a hierarchical structure, which has calcium carbonate having a cylindrical, needle-like or columnar elongated structure at the surface of the calcium carbonate compact particles.

Description

Calcium carbonate and method for producing calcium carbonate

[0001] The present invention relates to calcium carbonate and a method for producing calcium carbonate.

[0002] Calcium carbonate is widely used as a reinforcing filler in rubber, plastics, paper, sealants, paints, inks, etc., a calcium fortifier in foods, feeds, etc., and an alkaline agent in fertilizers, etc. The calcium carbonate used for these purposes is roughly divided into light calcium carbonate, which is obtained by injecting carbon dioxide gas into an aqueous suspension of calcium hydroxide to cause a carbonation reaction, and heavy calcium carbonate, which is obtained by mechanically crushing and classifying high-quality limestone.

[0003] Calcium carbonate is known to have three crystalline phases: calcite, aragonite, and vaterite. Of these, calcite is the most stable crystalline phase, and vaterite, for example, transforms into calcite over time. Calcite-type calcium carbonate is generally a rhombohedral or cubic crystal with a dense structure. In other words, calcium carbonate with a higher structure obtained by conventional crystal growth of calcium carbonate almost always becomes stable calcite crystals with a rhombohedral or cubic shape. Calcium carbonate is inexpensive, easy to prepare, has high whiteness, and is biocompatible and degradable. Therefore, many attempts have been made to produce calcium carbonate in various forms, with the aim of actively utilizing it in fields such as pharmaceuticals and cosmetics.

[0004] The production method disclosed in Patent Document 1 involves blowing carbon dioxide into milk of lime to produce aragonite, adding a dispersant to the dehydrated wet cake, re-slurrying the resulting mixture, wet-pulverizing it, mixing it with another milk of lime, adding a soluble phosphate compound to the mixture, and again introducing carbon dioxide. This method is characterized by the fact that by having aragonite crystalline calcium carbonate present in the raw material water slurry, the calcium carbonate obtained by the carbonation reaction of calcium hydroxide is made to be of aragonite crystalline form.

[0005] On the other hand, in the production method disclosed in Patent Document 2, carbon dioxide is introduced into milk of lime to synthesize calcium carbonate, the introduction of carbon dioxide is temporarily stopped when the carbonation rate is in the range of 10 to 90%, new milk of lime is then added to the suspension to proceed with carbonation, and the introduction of carbon dioxide is stopped again when the carbonation rate reaches 10 to 90%. This process is repeated at least once, and then new milk of lime is added again, and the carbonation reaction is completed while maintaining the temperature of the suspension at 40°C or higher, thereby obtaining calcium carbonate aggregates in which aragonite calcium carbonate having a needle-like or columnar shape is aggregated.

[0006] The calcium carbonates produced by the methods of Patent Documents 1 and 2 are both in the aragonite crystalline form, which is less stable and easily transforms into the calcite crystalline form. Therefore, there has been a demand for the production of calcite crystalline calcium carbonate, which is more stable and has a more complex structure that has not been previously possible to synthesize.

[0007] Japanese Patent Laid-Open No. 4-224110 Japanese Patent Laid-Open No. 2008-273761

[0008] Therefore, an object of the present invention is to provide a calcium carbonate having a novel hierarchical structure, preferably a calcite-type calcium carbonate. Another object of the present invention is to provide a novel method for providing a calcium carbonate having a novel hierarchical structure, preferably a calcite-type calcium carbonate.

[0009] An embodiment of the present invention is calcium carbonate having a hierarchical structure, in which calcium carbonate having an elongated cylindrical, needle-like or columnar structure is provided on the surface of calcium carbonate having a dense structure.

[0010] Here, it is preferred that the calcium carbonate having a dense structure is in the form of particles having a diameter of 10 to 200 μm, the calcium carbonate having an elongated structure has a cross-sectional diameter in the range of 0.1 to 1.0 μm, and the length of the calcium carbonate having an elongated structure in the range of 0.1 to 10 μm.

[0011] It is also preferred that the calcium carbonate having a dense structure is in the form of hollow particles having an outer diameter of 10 to 200 μm, the calcium carbonate having an elongated structure has a cross-sectional diameter in the range of 0.1 to 1.0 μm, and the length of the calcium carbonate having an elongated structure in the range of 0.1 to 10 μm.

[0012] Another aspect of the present invention is a method for producing a calcium carbonate composite particle dispersion comprising the following steps: mixing calcite-type calcium carbonate particles, gelatin, and water, and heating and stirring to obtain a gelatin / calcite-type calcium carbonate particle mixed slurry A; dissolving a surfactant in oil and stirring to obtain a solution B; adding the gelatin / calcite-type calcium carbonate particle mixed slurry A to solution B and stirring to obtain an emulsion C; adding a crosslinking agent to the emulsion C and stirring to obtain an emulsion D containing gelatin / calcite-type calcium carbonate composite particles; centrifuging the emulsion D containing gelatin / calcite-type calcium carbonate composite particles to obtain gelatin / calcite-type calcium carbonate composite particles; calcining the gelatin / calcite-type calcium carbonate composite particles to obtain dense calcite-type calcium carbonate particles; and The method for producing calcium carbonate having a hierarchical structure includes a step of placing calcite-type dense calcium carbonate particles in an atmosphere containing at least carbon dioxide to obtain calcium carbonate having a hierarchical structure, in which calcium carbonate has an elongated cylindrical, needle-like or columnar structure on the surface of the dense calcium carbonate particles.

[0013] In the gelatin / calcite-type calcium carbonate particle mixed slurry A, the mass ratio of gelatin to calcite-type calcium carbonate particles is preferably in the range of 5:10 to 25:10.

[0014] In addition, in solution B, the volume ratio of surfactant to oil is preferably in the range of 1:100 to 5:100.

[0015] The volume ratio of the gelatin / calcite-type calcium carbonate particle mixed slurry A to the solution B is preferably in the range of 1:10 to 10:10.

[0016] It is preferable to add a crosslinking agent to emulsion C so that the mass ratio of gelatin to crosslinking agent is in the range of 1:1 to 1:25.

[0017] Furthermore, it is preferable to bake the gelatin / calcite-type calcium carbonate composite particles at a temperature of 570°C or higher and lower than 650°C.

[0018] In this embodiment, it is preferred that the calcium carbonate having a dense structure is in the form of particles having a diameter of 10 to 200 μm, the cross-sectional diameter of the calcium carbonate having an elongated structure is in the range of 0.1 to 1.0 μm, and the length of the calcium carbonate having an elongated structure is in the range of 0.1 to 10 μm.

[0019] In this embodiment, it is preferable that the calcium carbonate having a dense structure is a hollow particle having an outer diameter of 10 to 200 μm, the cross-sectional diameter of the calcium carbonate having an elongated structure is in the range of 0.1 to 1.0 μm, and the length of the calcium carbonate having an elongated structure is in the range of 0.1 to 10 μm.

[0020] The production method of the present invention can produce calcite-type calcium carbonate granules in a high yield using a relatively simple process. By loading various drugs into the pores of the produced calcite-type calcium carbonate, the granules can be used in drug delivery systems, cosmetic additives, food additives, health foods, catalyst supports, etc.

[0021] FIG. 1 is a schematic diagram of calcium carbonate having a hierarchical structure according to the present invention. FIG. 2 is a scanning electron microscope photograph (magnification: 5,000x) of calcium carbonate having a hierarchical structure obtained by the method of Example 1. FIG. 3 is a scanning electron microscope photograph (magnification: 20,000x) of calcium carbonate having a hierarchical structure obtained by the method of Example 1. FIG. 4 is an X-ray diffraction pattern, obtained by X-ray diffraction, of calcium carbonate granules having a hierarchical structure obtained by the method of Example 1. FIG. 5 is a scanning electron microscope photograph (magnification: 500x) of calcium carbonate having a hierarchical structure obtained by the method of Example 2. FIG. 6 is a scanning electron microscope photograph (magnification: 1,000x) of calcium carbonate having a hierarchical structure obtained by the method of Example 2. FIG. 7 is an X-ray diffraction pattern, obtained by X-ray diffraction, of calcium carbonate granules obtained by the method of Example 2.

[0022] The embodiments of the present invention will be described in more detail below, but the present invention is not limited to the following embodiments.

[0023] One embodiment of the present invention is calcium carbonate having a hierarchical structure, in which calcium carbonate having an elongated cylindrical, needle-like or columnar structure is provided on the surface of calcium carbonate having a dense structure.

[0024] Calcium carbonate has the formula CaCO ３ and is a calcium carbonate represented by the formula (I) and is the main component of seashells, eggshells, limestone, chalk, etc. Calcium carbonate is classified into heavy calcium carbonate (natural calcium carbonate) obtained by crushing and classifying limestone, and light calcium carbonate (synthetic calcium carbonate) obtained by chemical reaction, but the calcium carbonate produced in this embodiment is a type of light calcium carbonate. In this specification, when simply referring to calcium carbonate, it refers to light calcium carbonate (synthetic calcium carbonate) unless otherwise specified. Calcium carbonate exists in crystal polymorphs such as calcite crystals (trigonal rhombohedral crystals), aragonite crystals (orthorhombic crystals), and vaterite crystals (hexagonal crystals), and the calcium carbonate particles of this embodiment may be any of these crystals, but calcite-type ones are particularly preferred.

[0025] Here, in the embodiment, the hierarchical structure generally refers to a structure in which layers are stacked in order from the bottom to the top. Calcium carbonate having a hierarchical structure means that layers of calcium carbonate having different shapes or structures are stacked in order. In the embodiment, it is particularly preferable that the calcium carbonate has at least two layers: a layer of calcium carbonate having a dense structure and a layer consisting of aggregates of calcium carbonate having a tubular, needle-like, or columnar elongated structure. Calcium carbonate having a dense structure refers to calcium carbonate having a dense structure with almost no pores on the surface. Calcium carbonate having a dense structure is preferably composed of calcite-type calcium carbonate. Calcium carbonate having a dense structure is a higher-order structure, such as a dense secondary structure of calcium carbonate, formed by the aggregation of primary particles of calcium carbonate. On the other hand, calcium carbonate having a tubular, needle-like, or columnar elongated structure is preferably composed of calcite-type calcium carbonate, and calcium carbonate having these shapes and elongated structures aggregate to form layers. Calcium carbonate having a cylindrical, needle-like, or columnar elongated structure is also a higher-order structure such as a secondary structure of calcium carbonate formed by the aggregation of primary particles of calcium carbonate. Calcium carbonate having a cylindrical elongated structure is calcium carbonate having a so-called tube-like shape (having a cavity inside). Calcium carbonate having a needle-like elongated structure is calcium carbonate having a tapered tube-like shape. Calcium carbonate having a columnar elongated structure is calcium carbonate having a solid column-like shape rather than a hollow tube-like structure. The term "elongated structure" generally refers to a structure with an aspect ratio such as a cylinder, needle, or column, but also includes a structure with an aspect ratio of 1 (i.e., the length of the long part and the short part are equal).

[0026] Here, the calcium carbonate having a dense structure is preferably particles having a diameter of 10-200 μm. As described above, the calcium carbonate having a dense structure is preferably a high-order structure formed by the aggregation of primary particles of calcite-type calcium carbonate, and is preferably a particle having a high-order structure. The particle diameter is 10-200 μm, preferably 20-150 μm, and more preferably 30-100 μm. Furthermore, the cross-sectional diameter of the calcium carbonate having an elongated structure is in the range of 0.1-1.0 μm, preferably 0.2-0.8 μm, and more preferably 0.3-0.6 μm. The length of the calcium carbonate having an elongated structure is in the range of 0.1-10 μm, preferably 0.2-8 μm, and more preferably 0.3-7 μm, but there is no limit to the length, and it may be possible to increase the length to, for example, 30 μm, in some cases.

[0027] Furthermore, as another embodiment of the present invention, the calcium carbonate having a dense structure may be hollow particles having a diameter of 10-200 μm. As described above, the calcium carbonate having a dense structure is preferably a high-order structure formed by the aggregation of primary particles of calcite-type calcium carbonate, and is preferably a high-order hollow particle. In this case, the calcium carbonate having a hierarchical structure of the present invention has at least three layers: a hollow portion inside the hollow particle, a layer of calcium carbonate having a dense structure, and a layer consisting of aggregates of calcium carbonate having an elongated structure such as a cylindrical, needle-like, or columnar shape. Here, the diameter of the hollow particle is 10-200 μm, preferably 20-150 μm, and more preferably 30-100 μm. Furthermore, the cross-sectional diameter of the calcium carbonate having an elongated structure is in the range of 0.1-1.0 μm, preferably 0.2-0.8 μm, and more preferably 0.3-0.6 μm. The length of calcium carbonate having an elongated structure is in the range of 0.1 to 10 μm, preferably in the range of 0.2 to 8 μm, and more preferably in the range of 0.3 to 7 μm, but there is no limit to the length, and it may be possible to make it as long as 30 μm, for example, in some cases.

[0028] The calcium carbonate having a hierarchical structure of at least two layers according to the embodiment has a unique shape resembling a purple sea urchin or a chestnut, since multiple calcium carbonate particles having elongated cylindrical, needle-like, or columnar structures extend radially from the surface of a calcium carbonate particle having a dense structure in the center. FIG. 1 is a schematic diagram illustrating an example of calcium carbonate having a hierarchical structure according to the embodiment. In FIG. 1, 1 represents calcium carbonate having a dense structure, and 2 represents calcium carbonate having an elongated cylindrical, needle-like, or columnar structure (shown as a needle in FIG. 1 for convenience). Although not illustrated in FIG. 1, the calcium carbonate having a dense structure according to the embodiment (1) may be hollow. The calcium carbonate having a hierarchical structure according to the embodiment can support a drug in the layer of calcium carbonate having an elongated cylindrical, needle-like, or columnar structure, or in the hollow particle having a dense structure in the center, and therefore can be used in a wide range of fields, such as pharmaceuticals, cosmetics, fragrances, food, and daily commodities.

[0029] A second embodiment of the present invention comprises the following steps: a step of mixing calcite-type calcium carbonate particles, gelatin, and water, and heating and stirring to obtain a gelatin / calcite-type calcium carbonate particle mixed slurry A; a step of dissolving a surfactant in oil and stirring to obtain a solution B; a step of adding the gelatin / calcite-type calcium carbonate particle mixed slurry A to solution B and stirring to obtain an emulsion C; a step of adding a crosslinking agent to the emulsion C and stirring to obtain an emulsion D containing gelatin / calcite-type calcium carbonate composite particles; a step of centrifuging the emulsion D containing gelatin / calcite-type calcium carbonate composite particles to obtain gelatin / calcite-type calcium carbonate composite particles; a step of firing the gelatin / calcite-type calcium carbonate composite particles to obtain dense calcite-type calcium carbonate particles; and The method for producing calcium carbonate having a hierarchical structure includes a step of placing calcite-type dense calcium carbonate particles in an atmosphere containing at least carbon dioxide to obtain calcium carbonate having a hierarchical structure, in which calcium carbonate has an elongated cylindrical, needle-like or columnar structure on the surface of the dense calcium carbonate particles.

[0030] A second embodiment is a method for producing calcium carbonate having the hierarchical structure of the first embodiment. The production method of the second embodiment includes a step of mixing calcite-type calcium carbonate particles, gelatin, and water, heating and stirring, and obtaining a gelatin / calcite-type calcium carbonate particle mixed slurry A. The calcium carbonate used as a raw material in this step is calcite-type, and its primary particle size is 10-300 nm, preferably 30-150 nm, more preferably 50-120 nm, and most preferably 80-100 nm. The gelatin used in this step is a protein obtained by extracting collagen fibers contained in animal bones and skin with hot water. Gelatin may be derived from bovine, porcine, or fish. Calcite-type calcium carbonate particles, gelatin, and water are mixed and stirred at 50-60°C, preferably 50-55°C, to obtain a water slurry in which the gelatin and calcite-type calcium carbonate particles are dispersed. In this case, gelatin and calcite-type calcium carbonate are mixed so that the mass ratio of gelatin to calcite-type calcium carbonate particles in the gelatin / calcite-type calcium carbonate particle mixed slurry A is in the range of 5:10-30:10, more preferably 10:10-20:10. If the mass of gelatin relative to the mass of calcite-type calcium carbonate particles is reduced, the calcite-type calcium carbonate granules finally obtained in this embodiment tend to have a dense structure rather than a hollow structure. If the mass of gelatin relative to the mass of calcite-type calcium carbonate particles is increased, the calcite-type calcium carbonate granules finally obtained in this embodiment tend to have a hollow structure. It is also preferable to use water in an amount 2.5-6 times, preferably 3-4.5 times the total mass of gelatin and calcite-type calcium carbonate.

[0031] The manufacturing method of the second embodiment includes, apart from the step of obtaining the gelatin / calcite-type calcium carbonate particle mixed slurry A, a step of dissolving a surfactant in oil and stirring to obtain a solution B. The surfactant used in this step may be any surfactant that dissolves in the oil described below, and can be selected from, for example, anionic surfactants such as monoalkyl sulfates, alkyl polyoxyethylene sulfates, alkyl benzene sulfonates, and monoalkyl phosphates; cationic surfactants such as alkyl trimethyl ammonium salts, dialkyl dimethyl ammonium salts, and alkyl benzyl dimethyl ammonium salts; amphoteric surfactants such as alkyl carboxybetaine; and nonionic surfactants such as polyoxyethylene alkyl ethers, fatty acid sorbitan esters, alkyl polyglycosides, fatty acid diethanolamides, and alkyl monoglyceryl ethers. In particular, water-soluble nonionic surfactants such as polyoxyethylene sorbitan monooleate, polyoxyethylene sorbitan monostearate, polyoxyethylene sorbitan monopalmitate, and polyoxyethylene sorbitan monolaurate, known as Tween or polysorbates, and sorbitan monooleate, sorbitan monostearate, sorbitan monopalmitate, and sorbitan monolaurate, known as SPAN, are preferred. Furthermore, oils that are liquid at room temperature, such as paraffinic base oils, naphthenic base oils, vegetable or animal edible oils, and industrial oils and fats, can be used as oils to dissolve the surfactants. Paraffinic base oils or edible oils are particularly preferred. The volume ratio of surfactant to oil can be in the range of 1:100-5:100, preferably 1:100-2:100.

[0032] After obtaining the gelatin / calcite-type calcium carbonate particle mixed slurry A and solution B, a step of mixing and stirring the gelatin / calcite-type calcium carbonate particle mixed slurry A with solution B is carried out to obtain an emulsion C. The gelatin / calcite-type calcium carbonate particle mixed slurry A is added to solution B and mixed so that the volume ratio of the gelatin / calcite-type calcium carbonate particle mixed slurry A to solution B is in the range of 1:10 to 10:10, preferably in the range of 1:10 to 2:10. When mixing the two, the gelatin / calcite-type calcium carbonate particle mixed slurry A may be added to solution B all at once, or solution B may be added dropwise while stirring solution B. Alternatively, solution B may be added all at once to the gelatin / calcite-type calcium carbonate particle mixed slurry A, or solution B may be added dropwise while stirring the gelatin / calcite-type calcium carbonate particle mixed slurry A. The mixture of the two can be stirred using an existing stirring device such as a magnetic stirrer, a stirrer equipped with various stirring blades, or an ultrasonic stirrer. The stirring speed is, for example, 300-1500 rpm, preferably 700-1200 rpm. In this manner, emulsion C is obtained. Emulsion C is a liquid in which aggregates of gelatin and calcite-type calcium carbonate particles are emulsified (dispersed) in oil. By appropriately changing the stirring speed in this step, the aggregation morphology of the gelatin and calcite-type calcium carbonate particles can be changed. In general, slowing the stirring speed tends to result in a structure in which gelatin (hydrophilic portion) and calcite-type calcium carbonate (hydrophobic portion) are randomly aggregated around large aggregates of gelatin (hydrophilic portion), and the diameter of the aggregates tends to be relatively large. Furthermore, if the stirring speed is increased, the gelatin (hydrophilic portion) and the calcite-type calcium carbonate (hydrophobic portion) tend to form a structure in which they are randomly aggregated, and the diameter of the aggregates tends to become relatively small.

[0033] Next, a crosslinking agent is added to the emulsion C, followed by stirring to obtain an emulsion D containing gelatin / calcite-type calcium carbonate composite particles. Any compound can be used as the crosslinking agent as long as it can crosslink the gelatin contained in the gelatin / calcite-type calcium carbonate particles emulsified and dispersed in the emulsion D. Examples of crosslinking agents include 3-ethylcarbodiimide hydrochloride (1-(3-dimethylaminopropyl-3-ethylcarbodiimide hydrochloride, etc.), 3-ethylcarbodiimide sulfate (1-(3-dimethylaminopropyl-3-ethylcarbodiimide sulfate, etc.), 3-ethylcarbodiimide nitrate (1-(3-dimethylaminopropyl-3-ethylcarbodiimide nitrate, etc.), transglutaminase, olefin-maleic anhydride copolymer, ammonia reaction products with ethylenically unsaturated anhydride-maleic anhydride copolymer, polyamide epoxy resin, Glutaraldehyde or genipin can be used. In this case, the crosslinking agent can be added to emulsion C so that the mass ratio of gelatin to crosslinking agent is in the range of 1:1 to 1:25, preferably 1:1 to 1:10. When the crosslinking agent is added to emulsion C and stirred, the gelatin in the gelatin / calcite-type calcium carbonate particles is crosslinked to form gelatin / calcite-type calcium carbonate composite particles. In other words, emulsion D is a liquid in which gelatin / calcite-type calcium carbonate composite particles are emulsified and dispersed in oil.

[0034] Next, emulsion D containing the gelatin / calcite-type calcium carbonate composite particles is centrifuged to obtain gelatin / calcite-type calcium carbonate composite particles. The obtained gelatin / calcite-type calcium carbonate composite particles are granular solids formed by the combination of crosslinked gelatin and calcite-type calcium carbonate. For example, they may be composite particles in which gelatin is used as a template and calcite-type calcium carbonate surrounds it. Alternatively, they may be composite particles having a sea-island structure, in which islands of crosslinked gelatin are contained within a sea structure of calcite-type calcium carbonate. Furthermore, gelatin / calcite-type calcium carbonate composite particles may have a structure in which specifically crosslinked gelatin aggregates in the center, and the crosslinked gelatin and calcite-type calcium carbonate form a sea-island structure around it. The gelatin / calcite-type calcium carbonate composite particles obtained by centrifugation can be washed with a cleaning solvent such as water or alcohol and appropriately dried. The particle size of the gelatin / calcite type calcium carbonate particles depends on the stirring speed and the amount of crosslinking agent in the step of obtaining emulsion C, and is usually in the range of 1 to 500 μm, preferably 5 to 200 μm.

[0035] The gelatin / calcite-type calcium carbonate composite particles thus obtained are then calcined to obtain dense calcite-type calcium carbonate particles. The gelatin / calcite-type calcium carbonate composite particles are calcined in air, carbon dioxide, or an inert gas atmosphere such as nitrogen or argon at a temperature of 570°C or higher but lower than 650°C, preferably 580°C or higher but lower than 600°C. When calcined in air, the temperature can be 570°C or higher but lower than 650°C; when calcined in carbon dioxide, the temperature can be 580°C or higher but lower than 630°C; and when calcined in an inert gas, the temperature can be 590°C or higher but lower than 650°C. The gelatin / calcite-type calcium carbonate composite particles can be calcined using an existing ceramics calcination furnace. When the gelatin / calcite-type calcium carbonate composite particles are calcined, the gelatin portion is first incinerated (combusted), leaving only the calcite-type calcium carbonate portion. That is, only the island structure portion (gelatin) of the gelatin / calcite-type calcium carbonate particles is removed by firing (combustion), leaving only the sea structure portion (calcite-type calcium carbonate), i.e., particles (granules) with a generally dense structure with holes in places. If firing is further continued at a temperature within the above range, the holes that existed in the granules are closed, and part of the calcite-type calcium carbonate (particularly the calcium carbonate on the surface of the granules) is converted to calcium oxide. In this way, dense calcite-type calcium carbonate particles, some of which have been converted to calcium oxide, are obtained. In this specification, the term "dense calcite-type calcium carbonate particles" is intended to include particles in which part of the calcite-type calcium carbonate has been converted to calcium oxide. The diameter of the calcite-type calcium carbonate dense particles is almost the same as that of the gelatin / calcite-type calcium carbonate particles obtained in the previous step, and is usually in the range of 1-500 μm, preferably 5-200 μm, or the particle diameter may be slightly smaller due to blocking of the pores of the gelatin / calcite-type calcium carbonate particles obtained in the previous step. The obtained calcite-type calcium carbonate dense particles are almost spherical.

[0036] Next, the resulting calcite-type dense calcium carbonate particles are placed in an atmosphere containing at least carbon dioxide to obtain calcium carbonate with a hierarchical structure, in which the surface of the dense calcium carbonate particles has calcium carbonate with elongated structures such as cylindrical, needle-like, or columnar structures. The surface of the calcite-type dense calcium carbonate particles obtained by firing in the previous step becomes unstable, resulting in the formation of numerous cracks and pores. If the calcite-type dense calcium carbonate particles are left at room temperature in this state, two changes may occur on the surface of the dense calcium carbonate particles. First, the surface of the dense calcium carbonate particles absorbs moisture from the air and releases calcium ions and carbonate ions. In an atmosphere containing at least carbon dioxide, the calcium ions released into the cracks and pores react with carbonate ions or carbon dioxide to form calcium carbonate. The calcium carbonate formed by the reaction of the released calcium ions with carbonate ions or carbon dioxide is calcite-type. The reaction between the released calcium ions and carbonate ions or carbon dioxide occurs simultaneously throughout the surface of the calcite-type calcium carbonate dense particles, and calcium carbonate crystal growth also begins simultaneously throughout the surface of the calcite-type calcium carbonate dense particles. The other change is that trace amounts of calcium oxide present on the surface of the aggregates of calcite-type calcium carbonate dense particles obtained by firing in the previous step, due to decomposition, react with carbon dioxide in an atmosphere containing at least carbon dioxide to form calcium carbonate. The calcium carbonate formed by the reaction of calcium oxide and carbon dioxide is calcite-type. The reaction between calcium oxide and carbon dioxide occurs simultaneously throughout the surface of the calcite-type calcium carbonate dense particles, and calcium carbonate crystal growth also begins simultaneously throughout the surface of the calcite-type calcium carbonate dense particles. As a result, the calcite-type calcium carbonate grows to have an elongated cylindrical, needle-like, or columnar structure. In this process, the growth of elongated cylindrical, needle-like, or columnar calcite-type calcium carbonate structures can be controlled by placing the calcite-type calcium carbonate dense particles in an atmosphere with a controlled carbon dioxide concentration.If the carbon dioxide concentration in the atmosphere in which the dense calcite-type calcium carbonate particles are placed is similar to that of the atmosphere (at least 0.03%), calcite-type calcium carbonate with an elongated cylindrical, needle-like, or columnar structure will grow. Furthermore, the temperature of the atmosphere containing at least carbon dioxide in which the dense calcite-type calcium carbonate particles are placed should be in the range of from near room temperature (about 20°C) to 80°C, preferably 20-40°C. In this way, calcium carbonate with a hierarchical structure can be obtained, which has at least a layer of dense calcite-type calcium carbonate particles and a layer of elongated calcite-type calcium carbonate with a cylindrical, needle-like, or columnar structure.

[0037] Among calcium carbonates having a hierarchical structure, calcium carbonates having a dense structure are preferably particles having a diameter of 10-200 μm. Calcium carbonates having a dense structure are preferably formed by aggregation of primary particles of calcite-type calcium carbonate, and are preferably particles having a high-order structure. The diameter of these particles is 10-200 μm, preferably 20-150 μm, and more preferably 30-100 μm. Furthermore, among calcium carbonates having a hierarchical structure, calcium carbonates having an elongated structure have a cross-sectional diameter in the range of 0.1-1.0 μm, preferably 0.2-0.8 μm, and more preferably 0.3-0.6 μm. The length of calcium carbonates having an elongated structure is in the range of 0.1-10 μm, preferably 0.2-8 μm, and more preferably 0.3-7 μm, although there is no limit to the length, and it may be possible to increase the length to, for example, 30 μm, in some cases.

[0038] Furthermore, as another example of the second embodiment, the calcium carbonate having a hierarchical structure may be hollow particles having a diameter of 10-200 μm. The calcium carbonate having a dense structure is preferably a high-order structure formed by the aggregation of primary particles of calcite-type calcium carbonate, and is preferably a high-order hollow particle. In this case, the calcium carbonate having a hierarchical structure produced in the second embodiment has at least three layers: a hollow portion inside the hollow particle, a layer of calcium carbonate having a dense structure, and a layer consisting of aggregates of calcium carbonate having an elongated structure such as a cylindrical, needle-like, or columnar shape. Here, the diameter of the hollow particle is 10-200 μm, preferably 20-150 μm, and more preferably 30-100 μm. Furthermore, the cross-sectional diameter of the calcium carbonate having an elongated structure is in the range of 0.1-1.0 μm, preferably 0.2-0.8 μm, and more preferably 0.3-0.6 μm. Among calcium carbonates having a hierarchical structure, calcium carbonates having an elongated structure have a length in the range of 0.1 to 10 μm, preferably in the range of 0.2 to 8 μm, and more preferably in the range of 0.3 to 7 μm, but there is no limit to the length, and it is possible in some cases to make the length as long as 30 μm, for example.

[0039] The production method of the second embodiment allows for the production of calcite-type calcium carbonate having a hierarchical structure in high yield through a relatively simple process. The calcium carbonate having at least two layers of a hierarchical structure obtained by the second embodiment has a unique shape resembling a purple sea urchin or a chestnut, since multiple calcium carbonate particles having elongated cylindrical, needle-like, or columnar structures extend radially from the surface of a calcium carbonate particle having a dense structure in the center. For example, a drug can be supported in the layer of calcium carbonate having elongated cylindrical, needle-like, or columnar structures, or in the hollow particle portion having a dense structure in the center, making the calcium carbonate useful in a wide range of fields, including pharmaceuticals, cosmetics, fragrances, food, and daily necessities.

[0040] The following describes in detail the embodiments of the present invention, but the present invention is not limited to the following examples.

[0041] <Production of calcite-type calcium carbonate having a hierarchical structure> [Example 1] Gelatin (FUJIFILM Wako Pure Chemical Industries, Ltd.) and calcite-type calcium carbonate with a particle size of 80 nm (Shiraishi Central Research Institute Co., Ltd.) were dispersed in water at a mass ratio of 1:1. The mixture was heated to a temperature of 50-60°C and stirred using a stirrer to obtain a gelatin / calcite-type calcium carbonate particle mixed slurry (gelatin / calcite-type calcium carbonate particle mixed slurry A).

[0042] On the other hand, a surfactant (Span 80, Fujifilm Wako Pure Chemical Industries, Ltd.) and liquid paraffin (Fujifilm Wako Pure Chemical Industries, Ltd.) were mixed at a volume ratio of 1:100 and stirred to obtain a solution (Solution B). Next, the gelatin / calcite-type calcium carbonate particle mixed slurry A was added to Solution B so that the volume ratio of the gelatin / calcite-type calcium carbonate particle mixed slurry A to Solution B was 1:10, and the mixture was stirred at room temperature at a stirring speed of 1200 rpm to obtain an emulsion (Emulsion C).

[0043] To emulsion C, carbodiimide hydrochloride (1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, Fujifilm Wako Pure Chemical Industries, Ltd.) was added so that the mass ratio of gelatin to the crosslinking agent was 1:2. Stirring the mixture yielded an emulsion (emulsion D) containing gelatin / calcite-type calcium carbonate particles. Emulsion D was separated using a centrifuge, and the resulting solid portion was washed five times with acetone and dried at room temperature in the air to obtain gelatin / calcite-type calcium carbonate composite particles. The gelatin / calcite-type calcium carbonate composite particles were calcined in a calcination furnace at 570°C in the air to obtain dense particles of calcite-type calcium carbonate.

[0044] The resulting calcite-type calcium carbonate dense particles were then left to stand at room temperature in the air for 3 days. A hierarchical structure was obtained in which numerous elongated calcium carbonate particles were formed on the surface of the calcite-type calcium carbonate dense particles. The elongated structures had a diameter of approximately 0.3 μm and were needle-like in shape.

[0045] Figure 2-A is a scanning electron microscope photograph (magnification: 5000x) of calcium carbonate having a hierarchical structure obtained by the method of Example 1. Within the observation field, calcium carbonate particles with numerous needle-like elongated structures were observed. Figure 2-B is a scanning electron microscope photograph at a magnification of 20,000x, which is an enlarged view of the elongated structure portion on the surface of the calcium carbonate. Parts of the elongated structure with cracks were observed. This photograph revealed that there was a cavity in the center of the elongated structure. When the hierarchical structure obtained in Example 1 was subjected to crystal structure analysis by X-ray diffraction, it was confirmed that the calcium carbonate was a calcite crystal (Figure 3).

[0046] Example 2 Gelatin (FUJIFILM Wako Pure Chemical Industries, Ltd.) and calcite-type calcium carbonate with a particle size of 80 nm (Shiraishi Central Research Institute Co., Ltd.) were dispersed in water at a mass ratio of 2:1. The mixture was heated to a temperature of 50-60°C and stirred using a stirrer to obtain a gelatin / calcite-type calcium carbonate particle mixed slurry (gelatin / calcite-type calcium carbonate particle mixed slurry A).

[0047] Separately, a surfactant (Span 80, Fujifilm Wako Pure Chemical Industries, Ltd.) and liquid paraffin (Fujifilm Wako Pure Chemical Industries, Ltd.) were mixed at a volume ratio of 1:100 and stirred to obtain a solution (Solution B). Solution B was placed in an ice bath and cooled to 0°C. Next, the gelatin / calcite-type calcium carbonate particle mixed slurry A was added to Solution B so that the volume ratio of the gelatin / calcite-type calcium carbonate particle mixed slurry A to Solution B was 1:10, and the mixture was stirred at 0°C (in the ice bath) at a stirring speed of 1000 rpm to obtain an emulsion (Emulsion C).

[0048] To emulsion C, carbodiimide hydrochloride (1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, Fujifilm Wako Pure Chemical Industries, Ltd.) was added so that the mass ratio of gelatin to the crosslinking agent was 1:1. Stirring the mixture yielded an emulsion (emulsion D) containing gelatin / calcite-type calcium carbonate particles. Emulsion D was separated using a centrifuge, and the resulting solid portion was washed five times with acetone and dried at room temperature in the air to obtain gelatin / calcite-type calcium carbonate composite particles. The gelatin / calcite-type calcium carbonate composite particles were fired at 570°C in the air in a firing furnace to obtain dense calcite-type calcium carbonate particles with a hollow center.

[0049] The resulting calcite-type calcium carbonate dense particles were then left to stand in the air at room temperature for 3 days. A hierarchical structure of calcium carbonate was obtained, in which numerous elongated calcium carbonate particles were formed on the surface of the calcite-type calcium carbonate dense particles. The elongated structures had a diameter of approximately 0.1 to 0.3 μm and were needle-like in shape.

[0050] Figure 4-A is a scanning electron microscope photograph (magnification: 500x) of calcium carbonate (hollow body) having a hierarchical structure obtained by the method of Example 2. Numerous hollow particles of calcium carbonate with needle-like elongated structures were observed within the observation field. Figure 4-B is a scanning electron microscope photograph at a magnification of 1,000x, which shows an enlarged view of a hollow calcium carbonate body having a hierarchical structure. Crystal structure analysis of the calcium carbonate having a hierarchical structure obtained in Example 2 by X-ray diffraction confirmed that the calcium carbonate was a calcite crystal (Figure 5).

[0051] The present invention can provide calcium carbonate having a novel and unique hierarchical structure. Using the production method of the present invention, calcite-type calcium carbonate having a hierarchical structure can be produced through a relatively simple process. The produced calcite-type calcium carbonate having a hierarchical structure can be loaded with various drugs and the like on the calcium carbonate layers having an elongated structure. The production method of the present invention can also be used to produce calcite-type calcium carbonate having at least a three-layer structure with a central cavity. The obtained calcite-type calcium carbonate having a three-layer structure is expected to be useful in drug delivery systems, cosmetic additives, food additives, health foods, catalyst supports, and the like.

Claims

1. Calcium carbonate having a hierarchical structure, wherein the surface of the calcium carbonate having a dense structure has calcium carbonate having a cylindrical, needle-shaped or columnar elongated structure.

2. The calcium carbonate having a hierarchical structure according to claim 1, wherein the calcium carbonate having a dense structure is particles with a diameter of 10 - 200 μm, the cross-sectional diameter of the calcium carbonate having an elongated structure is in the range of 0.1 - 1.0 μm, and the length of the calcium carbonate having an elongated structure is in the range of 0.1 - 10 μm.

3. The calcium carbonate having a hierarchical structure according to claim 1, wherein the calcium carbonate having a dense structure is hollow particles with an outer diameter of 10 - 200 μm, the cross-sectional diameter of the calcium carbonate having an elongated structure is in the range of 0.1 - 1.0 μm, and the length of the calcium carbonate having an elongated structure is in the range of 0.1 - 10 μm.

4. The following steps: a step of mixing calcite-type calcium carbonate particles, gelatin, and water, heating and stirring to obtain a gelatin / calcite-type calcium carbonate particle mixed slurry A; a step of dissolving a surfactant in oil and stirring to obtain a solution B; a step of adding the gelatin / calcite-type calcium carbonate particle mixed slurry A to the solution B and stirring to obtain an emulsion emulsion C; a step of adding a cross-linking agent to the emulsion emulsion C and stirring to obtain an emulsion D containing gelatin / calcite-type calcium carbonate composite particles; a step of centrifuging the emulsion D containing gelatin / calcite-type calcium carbonate composite particles to obtain gelatin / calcite-type calcium carbonate composite particles; a step of firing the gelatin / calcite-type calcium carbonate composite particles to obtain calcite-type calcium carbonate dense particles; and a step of placing the calcite-type calcium carbonate dense particles in an atmosphere containing at least carbon dioxide to obtain calcium carbonate having a hierarchical structure, wherein the surface of the calcium carbonate dense particles has calcium carbonate having a cylindrical, needle-shaped or columnar elongated structure. A method for producing calcium carbonate having a hierarchical structure.

5. The method for producing calcium carbonate having a hierarchical structure according to claim 4, wherein in the gelatin / calcite-type calcium carbonate particle mixed slurry A, the mass ratio of gelatin to calcite-type calcium carbonate particles is in the range of 5:10 - 25:

10.

6. The method for producing calcium carbonate having a hierarchical structure according to claim 5, wherein in Solution B, the volume ratio of the surfactant to the oil is in the range of 1:100 to 5:

100.

7. The method for producing calcium carbonate having a hierarchical structure according to claim 5, wherein the volume ratio of the gelatin / calcite-type calcium carbonate particle mixed slurry A to Solution B is in the range of 1:10 to 10:

10.

8. The method for producing calcium carbonate having a hierarchical structure according to claim 6, wherein the volume ratio of the gelatin / calcite-type calcium carbonate particle mixed slurry A to Solution B is in the range of 1:10 to 10:

10.

9. The method for producing calcium carbonate having a hierarchical structure according to claim 5, wherein a crosslinking agent is added to the emulsion C such that the mass ratio of gelatin to the crosslinking agent is in the range of 1:1 to 1:

25.

10. The method for producing calcium carbonate having a hierarchical structure according to claim 6, wherein a crosslinking agent is added to the emulsion C such that the mass ratio of gelatin to the crosslinking agent is in the range of 1:1 to 1:

25.

11. The method for producing calcium carbonate having a hierarchical structure according to claim 7, wherein a crosslinking agent is added to the emulsion C such that the mass ratio of gelatin to the crosslinking agent is in the range of 1:1 to 1:

25.

12. The method for producing calcium carbonate having a hierarchical structure according to claim 8, wherein a crosslinking agent is added to the emulsion C such that the mass ratio of gelatin to the crosslinking agent is in the range of 1:1 to 1:

25.

13. The method for producing calcium carbonate having a hierarchical structure according to any one of claims 4 to 12, wherein the gelatin / calcite-type calcium carbonate composite particles are calcined at a temperature of 570 °C or higher and less than 650 °C.

14. The method for producing calcium carbonate having a hierarchical structure according to claim 13, wherein the calcium carbonate having a dense structure is particles with a diameter of 10 - 200 μm, the cross-sectional diameter of the calcium carbonate having an elongated structure is in the range of 0.1 - 1.0 μm, and the length of the calcium carbonate having an elongated structure is in the range of 0.1 - 10 μm.

15. The method for producing calcium carbonate having a hierarchical structure according to claim 13, wherein the calcium carbonate having a dense structure is hollow particles with an outer diameter of 10 - 200 μm, the cross-sectional diameter of the calcium carbonate having an elongated structure is in the range of 0.1 - 1.0 μm, and the length of the calcium carbonate having an elongated structure is in the range of 0.1 - 10 μm.

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