Method for producing tubular calcium carbonate

Abstract

To provide a method for efficiently producing tubular calcium carbonate without using agents such as an organic acid and a surfactant.SOLUTION: The present invention provides a method for producing tubular calcium carbonate with a through hole along its longitudinal direction. The method includes the step for introducing carbon dioxide fine bubbles with an average particle size of 100 μm or less into a calcium-containing solution, to crystallize tubular calcium carbonate. In the step for crystallizing the tubular calcium carbonate, the temperature of the calcium-containing solution is controlled.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing tubular calcium carbonate. [Background technology]

[0002] Calcium carbonate is used in a wide range of fields, including fillers for plastics, paper, and paints, soil conditioners such as pesticides and fertilizers, adhesives, sealants, erasers and toothpastes that utilize its abrasive properties, and food additives and cosmetics raw materials. Recently, sheet and film-like paper substitute products and plastic substitute products that use calcium carbonate particles as the main raw material have been developed and are widely used. Examples of paper substitute products include clear files, business cards, booklets (pamphlets, photo books, etc.), promotional POP, menus, packages, posters, illuminated sheets, stickers and labels, maps, mask cases, tags, etc. Examples of plastic substitute products include applications where plastic was previously used (various molded products, such as plastic shopping bags, food containers, drink cups, hangers, stationery, fans, cosmetic containers, bottles, building materials, etc.).

[0003] Calcium carbonate is generally synthesized by blowing carbon dioxide into an aqueous solution of calcium hydroxide or by mixing an aqueous solution containing calcium ions such as calcium chloride with an aqueous solution of sodium carbonate. In recent years, in order to reduce the emission of carbon dioxide, a greenhouse gas, it has been considered to react carbon dioxide with calcium and fix the carbon dioxide as calcium carbonate. For example, Patent Document 1 describes the synthesis of calcium carbonate by introducing carbon dioxide as fine bubbles into a calcium-containing solution containing calcium.

[0004] It is known that synthetic calcium carbonate can take various shapes, for example, calcium carbonate in an approximately spherical shape, a prismatic shape, a needle shape, or a spindle shape. Synthetic calcium carbonate in a unique shape such as a hollow shape is also known. For example, Patent Document 2 describes a method for obtaining hollow calcium carbonate microparticles with high circularity and compressive strength by spraying and pyrolyzing an organic acid calcium salt solution. Patent Document 3 describes a method for synthesizing tubular calcium carbonate by adding a surfactant to a water-soluble calcium salt solution in advance and dropping the water-soluble carbonate solution into water. Patent Document 4 describes a method for synthesizing tubular calcium carbonate using a surfactant. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2023-028788 A [Patent Document 2] Patent No. 6386949 [Patent Document 3] Patent No. 5321937 [Patent Document 4] JP 2009-073686 A Summary of the Invention [Problem to be solved by the invention]

[0006] The shape of calcium carbonate is appropriately selected depending on the application, but it has been difficult to stably produce tubular calcium carbonate among the above-mentioned shapes. In addition, as in Patent Documents 2 to 4, the use of chemicals such as organic acids and surfactants tends to complicate the process and make wastewater treatment difficult, which have been major obstacles to industrial mass production.

[0007] The present invention has been made in consideration of the above-mentioned problems of the conventional techniques. Specifically, an object of the present invention is to provide a method for efficiently producing tubular calcium carbonate without using chemicals such as organic acids and surfactants. [Means for solving the problem]

[0008] As a result of intensive research by the present inventors, it has been revealed that in a method for producing calcium carbonate in which calcium carbonate is crystallized by introducing carbon dioxide into a calcium-containing solution, by supplying carbon dioxide in the form of fine bubbles having an average particle size of 100 μm or less and further controlling the temperature of the calcium-containing solution during crystallization, tubular (macaroni-shaped) calcium carbonate having through holes along the longitudinal direction can be produced with extremely high efficiency.

[0009] In order to solve the above problems, one aspect of the present invention provides the following methods for producing tubular calcium carbonate [1] to [6]. [1] A method for producing tubular calcium carbonate having through holes along the longitudinal axis, comprising the steps of: introducing fine carbon dioxide bubbles having an average particle diameter of 100 μm or less into a calcium-containing solution containing calcium to crystallize tubular calcium carbonate; and controlling the temperature of the calcium-containing solution in the step of crystallizing the tubular calcium carbonate. [2] The method for producing tubular calcium carbonate according to [1], wherein the temperature of the calcium-containing solution is controlled to be higher than 20°C and not higher than 60°C in the step of crystallizing the tubular calcium carbonate. [3] The method for producing tubular calcium carbonate according to [1], wherein the temperature of the calcium-containing solution is controlled to be 25° C. or higher and 40° C. or lower in the step of crystallizing the tubular calcium carbonate. [4] The method for producing tubular calcium carbonate according to any one of [1] to [3], wherein the average particle size of the carbon dioxide fine bubbles introduced into the calcium-containing solution is 1 μm or more and 100 μm or less. [5] The method for producing tubular calcium carbonate according to any one of [1] to [4], wherein the introduction of the fine bubbles of carbon dioxide is carried out while flowing the calcium-containing solution in one direction. [6] The method for producing a tubular calcium carbonate according to any one of [1] to [5], wherein the calcium-containing solution in the step of crystallizing the tubular calcium carbonate has a pH of 7.0 or more. Effect of the Invention

[0010] According to the production method of the present invention, it is possible to efficiently obtain tubular calcium carbonate without using any chemicals such as organic acids or surfactants. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a flow chart of a method for producing tubular calcium carbonate according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic diagram showing the configuration of a production apparatus capable of carrying out the method for producing tubular calcium carbonate shown in FIG. [Diagram 3] FIG. 3A is an SEM image of the tubular calcium carbonate produced in Example 1, and FIG. 3B is an SEM image of the prismatic calcium carbonate produced in Comparative Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, one embodiment of the present invention will be described in detail. However, the present invention is not limited to this embodiment. In addition, in this specification, a numerical range expressed using "to" means a range including the numerical values ​​before and after "to" as the lower and upper limits.

[0013] Fig. 1 is a flow chart of a method for producing tubular calcium carbonate according to one embodiment of the present invention. As shown in Fig. 1, in this embodiment, a calcium-containing solution containing calcium is prepared (preparation step S110). Then, fine bubbles of carbon dioxide having an average particle diameter of 100 μm or less (hereinafter, simply referred to as "fine bubbles") are introduced into the calcium-containing solution to crystallize tubular calcium carbonate (crystallization step S120). After that, the calcium-containing solution after the crystallization of the tubular calcium carbonate is filtered to recover the tubular calcium carbonate (recovery step S130).

[0014] In this embodiment, in the crystallization step S120, the temperature of the calcium-containing solution when fine bubbles are introduced and the temperature of the calcium-containing solution when tubular calcium carbonate is crystallized are controlled. Specifically, the temperature of the calcium-containing solution is measured (temperature measurement step S122), and it is determined whether the temperature is within a predetermined range (temperature determination step S124). If the temperature is not within the predetermined range, the temperature is adjusted (temperature adjustment step S126). Note that the flow chart in FIG. 1 describes that the temperature measurement step S122, the temperature determination step S124, and the temperature adjustment step S126 are performed before the crystallization step S120. However, in this embodiment, these steps may be performed continuously or intermittently not only before the crystallization step S120 but also during the crystallization step S120. On the other hand, when the temperature of the calcium-containing solution during the crystallization step S120 is controlled within a predetermined range, such as by constantly maintaining the temperature of the manufacturing equipment, the temperature measurement step S122 and the temperature determination step S124 may be omitted.

[0015] Conventionally, temperature control in producing calcium carbonate has not been considered important, and in particular, only methods using organic acids or surfactants have been known for tubular calcium carbonate. In contrast, as in the present embodiment, when fine bubbles of carbon dioxide having an average particle size of 100 μm or less are supplied to a calcium-containing solution while controlling the temperature, the shape of the crystallized calcium carbonate can be controlled to be tubular (macaroni-like) without using organic acids or surfactants, and tubular calcium carbonate can be produced more efficiently.

[0016] An example of a manufacturing apparatus 200 capable of carrying out the method for manufacturing tubular calcium carbonate of the present embodiment is shown in Fig. 2. However, the method for manufacturing tubular calcium carbonate of the present embodiment is not limited to the method using the manufacturing apparatus 200.

[0017] The manufacturing apparatus 200 has a circulation section 230 for performing the crystallization process S120, the temperature measurement process S122, the temperature determination process S124, and the temperature adjustment process S126, a separation section 250 for performing the recovery process S130, and a control section (not shown) for controlling these sections.

[0018] The circulation section 230 has a crystallization tank 210 for crystallizing tubular calcium carbonate, and a fine bubble introduction section 220 for introducing fine bubbles into the calcium-containing solution 310. The circulation section 230 further has a flow path connecting the crystallization tank 210 and the fine bubble introduction section 220, a valve for controlling the opening and closing of the flow path, and a pump (not shown) for controlling the flow rate and flow of the calcium-containing solution 310 in the flow path. The circulation section 230 also has temperature measurement sections 242 and 244 for measuring the temperature of the calcium-containing solution 310 circulating therein, temperature adjustment sections 246 and 248 for adjusting the temperature of the calcium-containing solution 310, and a gas tank 222 for supplying carbon dioxide, which is connected to the fine bubble introduction section 220. On the other hand, the separation section 250 has a filtration section 252 for separating the tubular calcium carbonate crystallized in the circulation section 230, a flow path connecting the filtration section 252 and the circulation section 230, and the like.

[0019] The control unit may be configured to be able to transmit and receive data to and from the circulation unit 230 and the separation unit 250. The control unit usually has a computer such as a CPU, and a storage unit such as a ROM and a RAM. The control unit controls the operation of each component of the circulation unit 230 and the separation unit 250, the opening and closing of valves, and the operation of a pump, thereby operating the manufacturing apparatus 200. The control unit does not necessarily need to be provided inside the manufacturing apparatus 200, and may be disposed outside the manufacturing apparatus 200.

[0020] The manufacturing apparatus 200 shown in FIG. 2 does not include a configuration for performing the preparation step S110, but may include a configuration for preparing a calcium-containing liquid, if necessary. Hereinafter, a method for producing tubular calcium carbonate using the production apparatus 200 will be described in detail.

[0021] (Preparation step S110) In the preparation step S110, a calcium-containing solution 310 containing a calcium compound is prepared. The calcium-containing solution 310 may contain components (e.g., impurities, etc.) other than the calcium compound and the solvent as long as the purpose of this embodiment is not impaired.

[0022] The calcium compound contained in the calcium-containing solution 310 is not particularly limited as long as it is a compound that can dissolve in a solvent and generate calcium ions, and examples thereof include calcium hydroxide, calcium oxide, calcium sulfate, calcium chloride, and the like.

[0023] On the other hand, the solvent contained in the calcium-containing solution 310 is not particularly limited as long as it can dissolve calcium compounds, and an example thereof is water. The water may be any of ion-exchanged water, tap water, industrial water, etc. Note that, as long as the calcium compounds are well dissolved in the solvent, a solvent other than water may be used, or a mixture of water and another solvent may be used.

[0024] The calcium ion concentration in the calcium-containing solution 310 is preferably 800 mg / L or more and 1900 mg / L or less, and more preferably 1000 mg / L or more and 1700 mg / L or less. When the calcium ion concentration in the calcium-containing solution 310 is in this range, the tubular calcium carbonate is crystallized more efficiently by the crystallization step described below. The method for measuring the calcium ion concentration is not particularly limited, and a known method can be applied. For example, the concentration can be measured by inductively coupled plasma optical emission spectroscopy (ICP-OES) or inductively coupled plasma mass spectrometry (ICP-MS).

[0025] Here, the method for preparing the calcium-containing solution 310 is not particularly limited, and the calcium-containing solution 310 may be prepared by mixing a raw material containing a calcium compound (hereinafter also referred to as a "calcium raw material") with a solvent. In addition, when the calcium raw material contains a solvent together with the calcium compound, this may be used as it is as the calcium-containing solution.

[0026] The calcium raw material may be in a solid state, a liquid state (solution state), or a slurry state. The type of calcium raw material is not particularly limited, and may be a refined product such as limestone or marble, or a recovered product such as waste. Examples of the recovered products include concrete sludge, steel slag, waste concrete, coal ash, biomass ash, incineration ash, waste gypsum, alkaline wastewater, and the like. These calcium raw materials may or may not have been subjected to pretreatment such as neutralization treatment, crushing treatment, or grinding treatment.

[0027] In addition, when preparing the calcium-containing solution 310, a solid-liquid separation operation may be performed. By performing the solid-liquid separation operation, it is possible to obtain a calcium-containing solution with fewer impurities (fine particles, etc.), which makes it easier to crystallize tubular calcium carbonate more efficiently in the crystallization step described below. The solid-liquid separation method is not particularly limited, and separation can be performed using a known solid-liquid separation device such as a filter press, a vacuum belt filter, a vacuum rotary filter, a screw decanter, or a centrifugal dehydrator. After the solid-liquid separation, a flocculant containing sodium, potassium, magnesium, etc. may be added to flocculate fine particles such as colloidal particles and then remove them.

[0028] (Crystallization process S120) In the crystallization step S120, fine bubbles of carbon dioxide having an average particle diameter of 100 μm or less are introduced into the calcium-containing solution 310 prepared in the preparation step S110 to crystallize tubular calcium carbonate. More specifically, the calcium-containing liquid 310 is introduced into the circulation section 230 (crystallization tank 210) of the manufacturing apparatus 200 shown in FIG. 2, and the calcium-containing liquid 310 is circulated in the circulation section 230. In the circulation section 230, fine bubbles of carbon dioxide having an average particle diameter of 100 μm or less are introduced into the calcium-containing liquid 310 by the fine bubble introduction section 220. Then, the calcium-containing liquid 310 into which the fine bubbles have been introduced returns to the crystallization tank 210. In the crystallization tank 210, the fine bubbles are dissolved in the calcium-containing solution 310, and calcium ions (Ca 2+ ) and carbonate ions (CO3 2- ) to produce calcium carbonate, and then tubular calcium carbonate crystallizes out.

[0029] In general, the dissolution rate of fine bubbles in a calcium-containing solution 310 increases with the concentration of calcium ions (Ca 2+ ) and carbonate ions (CO3 2-) and the precipitation rate of calcium carbonate. Therefore, by circulating the calcium-containing solution 310 in the circulation section 230 and by ensuring sufficient time for the fine bubbles to dissolve in the calcium-containing solution 310 in the crystallization tank 210, tubular calcium carbonate can be easily crystallized. In addition to controlling the temperature by the temperature control step S126 described later, the fine bubbles can be introduced in the fine bubble introduction section 220 while the calcium-containing solution 310 is flowing in one direction, which further facilitates the crystallization of calcium carbonate into a tubular shape.

[0030] Here, the size and the like of the crystallization tank 210 are appropriately selected as long as it can accommodate a desired amount of the calcium-containing solution 310. In the present embodiment, the crystallization tank 210 is provided with an agitator 214 driven by a motor 212, but the present invention is not limited to this configuration.

[0031] On the other hand, the fine bubble introduction part 220 is connected to a gas tank 222 that stores a gas containing carbon dioxide. The structure of the fine bubble introduction part 220 is not particularly limited as long as the gas containing carbon dioxide supplied from the gas tank 222 can be introduced into the calcium-containing solution 310 as fine bubbles having an average particle diameter of 100 μm or less. The fine bubble introduction part 220 of this embodiment has a tubular porous ceramic (not shown) for flowing the calcium-containing liquid 310 inside, and a gas injection part (not shown) arranged on the outside of the porous ceramic. The gas injection part has a gas reservoir for temporarily storing the gas containing carbon dioxide supplied from the gas tank 222. When gas is injected into the gas reservoir of the gas injection part, the gas penetrates into the porous ceramic and passes through it. As a result, the gas becomes fine bubbles and is introduced into the calcium-containing solution 310 flowing in the porous ceramic. According to the fine bubble introduction part 220, fine bubbles can be generated in-line, so that processing delays are unlikely to occur. In addition, the fine bubble introduction section 220 has the advantage that it is difficult for foreign matter to get mixed in and that fine bubbles can be introduced into the calcium-containing solution 310, which has a high viscosity.

[0032] However, the method of introducing fine bubbles and the configuration of the fine bubble introduction section 220 are not limited to these methods and configurations. For example, the fine bubble introduction section 220 may have a configuration using a gas-liquid mixed shear type, a static mixer type, a Venturi type, a cavitation type, a steam condensation type, an ultrasonic type, a swirling jet type, a pressurized dissolution type, or the like.

[0033] In this embodiment, fine bubbles with an average particle size of 100 μm or less are used, so the reaction efficiency between carbonate ions and calcium ions is high. In other words, tubular calcium carbonate can be efficiently crystallized. From the viewpoint of controlling the introduction of fine bubbles, the average particle size of the fine bubbles introduced in the crystallization step S120 is preferably 1 μm or more and 100 μm or less, more preferably 5 μm or more and 50 μm or less. In this specification, the average particle size of the fine bubbles is determined by the configuration of the fine bubble introduction section 220, but may be a value measured by a laser diffraction / scattering type particle size distribution measurement device.

[0034] The concentration of carbon dioxide in the fine bubbles introduced in the fine bubble introduction section 220 is not particularly limited, and can be, for example, 10% by volume or more and 100% by volume or less of the fine bubbles introduced. By setting the carbon dioxide concentration to 10% by volume or more, tubular calcium carbonate can be crystallized in a shorter time.

[0035] The gas containing carbon dioxide that is the material for fine bubbles may be factory exhaust gas, combustion exhaust gas from petroleum and natural gas, waste incineration exhaust gas, etc. Using these exhaust gases can reduce the amount of carbon dioxide emitted into the atmosphere, thereby contributing to reducing the environmental load.

[0036] The conditions for introducing the fine bubbles are not particularly limited. For example, the introduction speed of the fine bubbles can be 0.01 L / min or more and 10 L / min or less. The introduction time of the fine bubbles (the circulation time of the calcium-containing solution 310 in the circulation section 230) can be 20 minutes or more and 240 minutes or less.

[0037] The pH of the calcium-containing solution 310 when the fine bubbles are introduced is preferably 7.0 or more. By setting the pH at 7.0 or more, it is possible to prevent re-dissolution of calcium crystallized as calcium carbonate. The pH of the calcium-containing solution 310 is preferably 7.0 or more and 13.0 or less, and more preferably 8.0 or more and 13.0 or less.

[0038] In this step, seed crystals may be added to the calcium-containing solution 310 to promote crystallization of tubular calcium carbonate. However, it is preferable not to add seed crystals from the viewpoint of suppressing variation in shape due to crystal growth on the surface of the seed crystals and promotion of the generation of new crystal nuclei.

[0039] (Temperature measurement process S122) In the temperature measurement step S122, the temperature of the calcium-containing liquid 310 in the circulation section 230 is measured. In the manufacturing apparatus 200 of this embodiment, the temperature is measured by a first temperature measurement section 242 that measures the temperature of the calcium-containing liquid 310 before being introduced into the fine bubble introduction section 220, and a second temperature measurement section 244 that measures the temperature of the calcium-containing liquid 310 in the crystallization tank 210. The temperature measurement by these temperature measurement sections 242 and 244 may be performed continuously, or may be performed periodically or intermittently at intervals, such as every minute, every 5 minutes, or every 10 minutes. The temperatures measured by the temperature measurement sections 242 and 244 are transmitted to the control section.

[0040] The type of the temperature measuring units 242 and 244 is not particularly limited as long as it can specify the temperature of the calcium-containing solution 310, and may be, for example, a contact thermometer or a non-contact thermometer. The liquid temperature of the calcium-containing solution 310 may be measured directly, or the temperature of the flow pipe or the crystallization tank 210 through which the calcium-containing solution 310 flows may be measured to indirectly specify the temperature of the calcium-containing solution 310.

[0041] The locations where the temperature measuring units 242, 244 are arranged are not limited to the above positions, and they may be arranged at positions other than those described above. The number of temperature measuring units arranged in the circulation unit 230 is also not limited to two, and may be only one, or may be three or more.

[0042] (Temperature determination step S124) The temperature determination step S124 is a step of determining whether the temperature measured in the temperature measurement step S122 is within a predetermined temperature range. In the manufacturing apparatus 200 of this embodiment, the control unit receives the temperature acquired by the temperature measurement units 242 and 244, and determines whether the temperature is within the predetermined temperature range. If the temperature of the calcium-containing solution 310 is within the predetermined range, the crystallization step S120 is continued as is. On the other hand, if the temperature is outside the predetermined range, the temperature adjustment units 246 and 248 are driven to perform the temperature adjustment step S126 described later.

[0043] Here, the temperature threshold value in the temperature determination step S124 is appropriately selected depending on the flow rate of the calcium-containing solution 310, the concentration of the calcium compound, the desired shape of the tubular calcium carbonate, and the like. In this embodiment, the temperature is preferably higher than 20°C and lower than 60°C, and more preferably higher than 25°C and lower than 40°C. If the temperature of the calcium-containing solution 310 is higher than 20°C, the crystallized calcium carbonate is likely to become tubular. In addition, by setting the temperature of the calcium-containing solution 310 to 60°C or lower, the amount of energy required to maintain the temperature of the calcium-containing solution 310 can be reduced.

[0044] (Temperature adjustment process S126) In the temperature adjustment step S126, when it is determined in the temperature determination step S124 that the temperature of the calcium-containing solution 310 is outside the predetermined temperature range, the temperature of the calcium-containing solution 310 is adjusted. In this embodiment, the control unit drives the first temperature adjustment unit 246 arranged in the flow path of the circulation unit 230 and the second temperature adjustment unit 248 arranged near the crystallization tank 210 to control the temperature of the calcium-containing solution 310. However, the locations where the temperature adjustment units 246 and 248 are arranged are not limited to these locations, and they may be arranged in other locations. Furthermore, the number of temperature adjustment units is not limited to two, and may be only one, or may be three or more. In addition, when multiple temperature adjustment units 246 and 248 are arranged in the circulation unit 230 as in this embodiment, only some of them may be driven to adjust the temperature, or all of them may be driven to adjust the temperature.

[0045] The temperature adjustment units 246, 248 may be configured in any manner that can adjust the temperature of the calcium-containing solution 310, and may be configured as known heating means or cooling means.

[0046] (Recovery process S130) In the recovery step S130, the calcium-containing solution 310 after the above-mentioned crystallization step S120 is recovered, and the tubular calcium carbonate is separated from other components. In this embodiment, the calcium-containing solution 310 in which the tubular calcium carbonate has been crystallized is filtered by the filtration section 252 of the separation section 250, and the tubular calcium carbonate is recovered. In the filtration section 252, filtration is performed by a known method such as suction filtration. The recovered tubular calcium carbonate may be dried or pulverized as necessary. In addition, the calcium carbonate may be subjected to a surface treatment with fatty acid or the like depending on the application.

[0047] On the other hand, the liquid component (treated liquid) separated by filtration may be disposed of after waste liquid treatment, or may be used for other purposes or may be reused as a calcium-containing solution.

[0048] [Tubular calcium carbonate and its uses] The tubular calcium carbonate obtained by the above method may have a tubular shape having through holes along the long axis direction, and the size and the like are not particularly limited. In this specification, the long axis may be linear or curved. The average major axis of the tubular calcium carbonate may be about 0.1 to 3 μm. The average minor axis may be 0.05 to 0.2 μm. The average film thickness (wall thickness) of the tubular calcium carbonate is not particularly limited, but is preferably 0.05 to 0.2 μm, and more preferably 0.1 to 0.2 μm. When the average film thickness of the tubular calcium carbonate is 0.05 μm or more, the strength of the tubular calcium carbonate is high, and the tubular shape is easily maintained when used for various purposes. The average film thickness can be increased by extending the crystallization time, but from the viewpoint of production efficiency, it is preferably 0.2 μm or less. Each dimension of the tubular calcium carbonate is specified by image analysis using a scanning electron microscope (SEM) or the like, and each value is an average value of each value when the images of 100 pieces of tubular calcium carbonate are analyzed.

[0049] The crystal phase of the tubular calcium carbonate is not particularly limited, but is preferably mainly calcite. However, a part of the tubular calcium carbonate may contain a different crystal phase. Calcite is known as a stable phase of calcium carbonate, and has a more stable crystal form than other crystal phases (aragonite, vaterite, etc.). Therefore, if the tubular calcium carbonate is mainly calcite, it becomes easier to use the tubular calcium carbonate for various purposes. The crystal phase of the tubular calcium carbonate is identified by X-ray diffraction (XRD).

[0050] The BET specific surface area of ​​the tubular calcium carbonate is not particularly limited, but is preferably 5 to 30 m. 2 / g. When the BET specific surface area is within the above range, good physical properties are likely to be obtained when the tubular calcium carbonate is mixed with a resin or the like as described below. In this specification, the "BET specific surface area" refers to a specific surface area determined by the BET adsorption method (nitrogen gas adsorption method). As a measuring device for the BET specific surface area, "BELSORP-mini" (manufactured by Microtrack-Bell) can be preferably used.

[0051] The method of using the above-mentioned tubular calcium carbonate is not particularly limited, and it may be used as it is or mixed with other components. Examples of applications include fillers for plastics, paper, paints, etc., soil conditioners such as pesticides and fertilizers, adhesives, sealants, erasers and toothpastes that utilize the abrasive effect, sheet / film-like paper substitute products and plastic substitute products that use calcium carbonate as the main raw material, as well as food additives and cosmetic raw materials.

[0052] In addition, the tubular calcium carbonate may be mixed with a thermoplastic resin to form a resin composition for producing various molded products. Since the tubular calcium carbonate has the through holes, the thermoplastic resin can easily enter the through holes. Therefore, by mixing with various thermoplastic resins, the elastic modulus of the resin composition containing the thermoplastic resin can be increased. The type of thermoplastic resin to be mixed with the tubular calcium carbonate is not particularly limited, but from the viewpoint of versatility, etc., polypropylene resin and polyethylene resin (low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, etc.) are preferably used.

[0053] The content of the tubular calcium carbonate in the resin composition is preferably 90.0% by mass or less, more preferably 80.0% by mass or less. On the other hand, the content of the tubular calcium carbonate in the resin composition is preferably 50.0% by mass or more, more preferably 60.0% by mass or more. The content of the thermoplastic resin in the resin composition is preferably 50.0% by mass or less, more preferably 40.0% by mass or less. On the other hand, the content of the thermoplastic resin in the resin composition is preferably 10.0% by mass or more, more preferably 20.0% by mass or more.

[0054] The mass ratio of the tubular calcium carbonate to the thermoplastic resin in the resin composition (calcium carbonate:thermoplastic resin) is preferably 50:50 to 90:10, more preferably 60:40 to 80:20, and further preferably 60:40 to 70:30. When the amount of the tubular calcium carbonate is within this range, the elastic modulus of the resin composition tends to be high.

[0055] The resin composition may contain components other than the tubular calcium carbonate and the thermoplastic resin, such as additives such as a plasticizer, a resin other than the thermoplastic resin, a filler, a coloring agent, a lubricant, an antioxidant, a flame retardant, and a foaming agent.

[0056] The method for preparing the resin composition is not particularly limited, and the resin composition can be obtained by mixing tubular calcium carbonate, a thermoplastic resin, and, if necessary, other components, and kneading the mixture by a known method (e.g., kneading with a twin-screw kneader).

[0057] The resin composition may be formed into pellets of any size and shape. The shape of the pellets may be, for example, cylindrical, spherical, elliptical, etc. The size of the pellets is not particularly limited, but for example, in the case of spherical pellets, the diameter may be 1 to 10 mm. In the case of elliptical pellets, the aspect ratio may be 0.1 to 1.0, and the length and width may be 1 to 10 mm. In the case of cylindrical pellets, the diameter may be 1 to 10 mm, and the length may be 1 to 10 mm.

[0058] The molding method of the resin composition is not particularly limited, and examples thereof include inflation molding, extrusion molding, injection molding, foam injection molding, injection compression molding, blow molding, press molding, calendar molding, vacuum molding, etc. Molded products obtained from such resin compositions include films, sheets, containers (food containers, etc.), daily necessities, automobile parts, electric and electronic parts, various consumables, etc. EXAMPLES

[0059] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0060] [Example 1] Commercially available Portland cement was prepared as a calcium raw material. 800 g of the calcium raw material was mixed with 3200 mL of ion-exchanged water and stirred to obtain a suspension in which the calcium raw material was suspended. The suspension was then filtered using a solid-liquid separator to obtain a calcium-containing solution. The amount of calcium in the calcium-containing solution was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES).

[0061] The pH of the calcium-containing solution was adjusted to 7.0 or more. The calcium-containing solution was then put into the crystallization tank 210 of the circulation section 230 shown in FIG. 2 and circulated in the circulation section 230. At this time, a mixed gas of carbon dioxide (CO2) and nitrogen (N2) was introduced into the fine bubble introduction section 220 to generate fine bubbles. The CO2 concentration in the mixed gas was set to 10% by volume. As the fine bubble introduction section 220, an in-line FB test device manufactured by Noritake Co., Ltd., which has a tubular porous ceramic and a gas injection section disposed outside it, was used. The average particle diameter of the fine bubbles to be introduced was adjusted to 50 μm. The flow rate of the mixed gas during introduction was set to 50 mL / min. In this Example 1, the temperature of the calcium-containing solution 310 circulating in the circulation section 230 was controlled to be more than 20°C and less than 60°C from the start to the end of the introduction of the fine bubbles.

[0062] The introduction of fine bubbles was stopped 30 minutes after the start of the introduction, and the calcium-containing solution was subjected to suction filtration to obtain tubular calcium carbonate. The shape of the obtained tubular calcium carbonate was identified using a scanning electron microscope (SEM). In addition, the shapes of 100 tubular calcium carbonates were analyzed by image analysis to determine the 50% particle size (d50) and 90% particle size (d90) in the volume distribution. Furthermore, the obtained tubular calcium carbonate was analyzed by X-ray diffraction to determine the crystal phase.

[0063] [Examples 2 and 3] Calcium-containing solutions with different calcium contents were prepared by changing the stirring conditions when obtaining the calcium-containing solutions, and tubular calcium carbonate was obtained in the same manner as in Example 1, except that the introduction time of fine bubbles was set as shown in Table 1. Note that in Examples 2 and 3 as well, the temperature of the calcium-containing solution was controlled to be greater than 20°C and less than 60°C from the start to the end of the introduction of the fine bubbles.

[0064] [Comparative Examples 1 and 2] Calcium carbonate was obtained in the same manner as in Example 1, except that the stirring conditions for obtaining the calcium-containing solution were changed to prepare calcium-containing solutions with different calcium contents, and the introduction time of fine bubbles was set as shown in Table 1. In Comparative Examples 1 and 2, the temperature of the calcium-containing solution was not controlled.

[0065] [Comparative Example 3] The calcium-containing solutions having different calcium contents were prepared by changing the stirring conditions for obtaining the calcium-containing solutions, and calcium carbonate was obtained in the same manner as in Example 1, except that carbon dioxide was introduced as millibubbles. In Comparative Example 3, the temperature of the calcium-containing solution was not controlled.

[0066] [Table 1]

[0067] [Consideration] As shown in Table 1 above, when calcium carbonate was produced by introducing fine carbon dioxide bubbles of 100 μm or less into a calcium-containing solution and controlling the temperature, the shape of the calcium carbonate became tubular (Examples 1 to 3). FIG. 3A shows an SEM image of the tubular calcium carbonate obtained in Example 1. The BET specific surface area of ​​the tubular calcium carbonate of Example 1 was measured using a "BELSORP-mini" (manufactured by Microtrack-Bell Co., Ltd.) and was found to be 13.7 m. 2 / g.

[0068] On the other hand, even if fine carbon dioxide bubbles of 100 μm or less were introduced into the calcium-containing solution, the shape became prismatic when the temperature was not controlled (Comparative Examples 1 and 2). Figure 3B shows an SEM image of the prismatic calcium carbonate obtained in Comparative Example 1. The BET specific surface area of ​​the prismatic calcium carbonate was 1.49 m. 2 / g.

[0069] Furthermore, even when carbon dioxide was introduced into the calcium-containing solution, when it was in the form of millibubbles, the shape of the bubbles became prismatic (Comparative Example 3). [Industrial Applicability]

[0070] According to the production method of the present invention, it is possible to efficiently obtain tubular calcium carbonate without using chemicals such as organic acids and surfactants. Therefore, it is expected that the use of calcium carbonate in various applications will be further promoted. [Explanation of symbols]

[0071] 200 Manufacturing equipment 210 Crystallization tank 212 Motor 214 Stirrer 220 Fine bubble introduction section 222 Gas Tank 230 Circulation Department 242, 244 Temperature measurement section 246, 248 Temperature adjustment section 250 Separation section 252 Filtration section

Claims

1. A method for producing tubular calcium carbonate having through holes along a longitudinal axis, comprising the steps of: The method includes a step of introducing fine bubbles of carbon dioxide having an average particle size of 100 μm or less into a calcium-containing solution containing calcium, thereby crystallizing tubular calcium carbonate, In the step of crystallizing the tubular calcium carbonate, the temperature of the calcium-containing solution is controlled. A method for producing tubular calcium carbonate.

2. In the step of crystallizing the tubular calcium carbonate, the temperature of the calcium-containing solution is controlled to be higher than 20° C. and not higher than 60° C. A method for producing tubular calcium carbonate according to claim 1.

3. In the step of crystallizing the tubular calcium carbonate, the temperature of the calcium-containing solution is controlled to be 25° C. or higher and 40° C. or lower. A method for producing tubular calcium carbonate according to claim 1.

4. The average particle diameter of the carbon dioxide fine bubbles introduced into the calcium-containing solution is 1 μm or more and 100 μm or less. A method for producing tubular calcium carbonate according to claim 1.

5. The introduction of the carbon dioxide fine bubbles is performed while the calcium-containing solution is flowing in one direction. A method for producing tubular calcium carbonate according to claim 1.

6. The pH of the calcium-containing solution in the step of crystallizing the tubular calcium carbonate is 7.0 or more. A method for producing tubular calcium carbonate according to claim 1.

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