Method and apparatus for producing calcium carbonate particles

Abstract

To provide a method for producing calcium carbonate particles capable of easily determining an optimal introduction time for fine bubbles when using fine bubbles to produce calcium carbonate.SOLUTION: The present invention relates to a method for producing calcium carbonate particles, the method including the steps of: introducing fine bubbles containing carbon dioxide into a material solution containing calcium; and measuring an electrical conductivity of the material solution over time, wherein the introduction of the fine bubbles is stopped after the measured electrical conductivity reaches a minimum value.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for producing calcium carbonate particles and an apparatus for producing calcium carbonate particles. [Background technology]

[0002] In order to reduce the emission of carbon dioxide, a greenhouse gas, studies have been conducted on the reaction of carbon dioxide with calcium to immobilize it as calcium carbonate. As described in Patent Document 1 and Patent Document 2, calcium carbonate can be produced by reacting calcium extracted into water from calcium-containing waste such as concrete sludge and steel slag with carbon dioxide introduced into the water. These methods are expected to reduce the amount of calcium-containing waste that is disposed of, and therefore to suppress environmental deterioration caused by waste.

[0003] Calcium carbonate particles are 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 their 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. Paper substitute products are used in clear files, business cards, booklets (pamphlets, photo books, etc.), promotional POP, menus, packages, posters, illuminated sheets, stickers and labels, maps, mask cases, and tags. Plastic substitute products are used in applications where plastics were previously used (various molded products, such as plastic shopping bags, food containers, drink cups, hangers, stationery, fans, cosmetic containers, bottles, building materials, etc.).

[0004] When producing calcium carbonate by introducing carbon dioxide, a method of accelerating the reaction rate by reducing the size of carbon dioxide bubbles that react with calcium has also been studied. For example, Patent Document 3 and Patent Document 4 state that calcium carbonate particles with an average particle size of 100 nm or 200 nm or less can be produced in a short time by using bubbles of 100 microns or less (cavitation bubbles) generated by cavitation. In addition, Patent Document 5 and Patent Document 6 state that calcium carbonate particles can be produced in a short time by using ultra-fine bubbles with an average particle size of 1000 nm or less. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2009-279552 A [Patent Document 2] JP 2014-148432 A [Patent Document 3] JP 2015-199654 A [Patent Document 4] JP 2015-199659 A [Patent Document 5] JP 2018-039696 A [Patent Document 6] JP 2023-028788 A Summary of the Invention [Problem to be solved by the invention]

[0006] As described in Patent Documents 3 to 6, it is known that calcium carbonate particles can be produced in a short time by using so-called fine bubbles, which are small carbon dioxide bubbles. However, according to the knowledge of the present inventor, the crystallization rate of calcium carbonate particles is not determined only by the particle size of the bubbles, but can change depending on various conditions. Therefore, even if fine bubbles are used, it is difficult to specify the optimal generation time. And, if the introduction time of fine bubbles is shortened more than necessary, the generation will be stopped before the calcium carbonate particles are sufficiently crystallized, and the yield of calcium carbonate particles will decrease. Also, if the introduction time of fine bubbles is longer than necessary, the reaction rate will not increase as much as expected.

[0007] The present invention has been made in consideration of the problems with the conventional technology described above, and has an object to provide a method for producing calcium carbonate particles that can easily determine the optimal introduction time of fine bubbles when producing calcium carbonate using fine bubbles, and a calcium carbonate particle production apparatus that can carry out the method. [Means for solving the problem]

[0008] One aspect of the present invention for solving the above problems relates to a method for producing calcium carbonate particles as set forth in the following items [1] to [6]. [1] A step of introducing fine bubbles containing carbon dioxide into a material solution containing calcium; and measuring the electrical conductivity of the material solution over time; After the measured electrical conductivity reaches a minimum value, the introduction of the fine bubbles is stopped. A method for producing calcium carbonate particles. [2] The fine bubbles have an average particle diameter of 1 μm or more and 100 μm or less. A method for producing calcium carbonate particles according to [1]. [3] The electrical conductivity of the material solution when the introduction of the fine bubbles is started is EC0, The minimum value of the electrical conductivity is EC min , The electrical conductivity of the material solution when the introduction of the fine bubbles is stopped is EC fin , then After the electrical conductivity reaches a minimum, the formula EC fin / (EC0-EC min The introduction of the fine bubbles is stopped before the ratio (ratio) of the fine bubbles to the total amount of the fine bubbles reaches 0.05. A method for producing calcium carbonate particles according to [1] or [2]. [4] In the step of introducing fine bubbles, The average particle size of the fine bubbles is changed according to the measured electrical conductivity. The method for producing calcium carbonate particles according to any one of [1] to [3]. [5] In the step of introducing fine bubbles, The introduction speed of the fine bubbles is changed according to the measured electrical conductivity. The method for producing calcium carbonate particles according to any one of [1] to [4]. [6] The step of introducing fine bubbles is carried out by setting the pH of the material solution at 7.0 or more. The method for producing calcium carbonate particles according to any one of [1] to [5].

[0009] One aspect of the present invention for solving the above-mentioned problems relates to an apparatus for producing calcium carbonate particles as set forth in the following items [7] to

[11] . [7] A measuring unit that measures the electrical conductivity of a material solution containing calcium over time; An introduction unit that introduces fine bubbles containing carbon dioxide into the material solution; A control unit controls the introduction of the fine bubbles by the introduction unit to stop the introduction of the fine bubbles after the measured electrical conductivity has reached a minimum value. Manufacturing equipment for calcium carbonate particles. [8] The electrical conductivity of the material solution when the introduction of the fine bubbles is started is EC0, The minimum value of the electrical conductivity is EC min , The electrical conductivity of the material solution when the introduction of the fine bubbles is stopped is EC fin , then The control unit is configured to control the electric conductivity of the EC fin / (EC0-EC min The introduction of the fine bubbles is stopped before the ratio (ratio) of the fine bubbles to the total amount of the fine bubbles reaches 0.05. [7] An apparatus for producing calcium carbonate particles. [9] The introduction section comprises a tubular porous ceramic through which the material solution flows; and an injection section for injecting carbon dioxide into the porous ceramic. [7] or [8]. A manufacturing apparatus for calcium carbonate particles.

[10] The control unit controls the introduction of the fine bubbles by the introduction unit to change the average particle diameter of the fine bubbles according to the measured electrical conductivity. The apparatus for producing calcium carbonate particles according to any one of [7] to [9].

[11] The control unit controls the introduction of the fine bubbles by the introduction unit to change the introduction speed of the fine bubbles according to the measured electrical conductivity. The apparatus for producing calcium carbonate particles according to any one of [7] to

[10] . Effect of the Invention

[0010] According to the present invention, there are provided a method for producing calcium carbonate particles that can easily determine the optimal introduction time of fine bubbles when producing calcium carbonate using fine bubbles, and an apparatus for producing calcium carbonate particles that can carry out the method. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a flow chart of a method for producing calcium carbonate particles according to one embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic diagram showing the configuration of a calcium carbonate particle production apparatus capable of carrying out the method for producing calcium carbonate particles shown in FIG. [Diagram 3] FIG. 3A is a graph showing the change in electrical conductivity over time after the start of the introduction of fine bubbles, FIG. 3B is a graph showing the change in the crystallization rate of calcium carbonate particles (CaCO3 crystallization rate) over time after the start of the introduction of fine bubbles, FIG. 3C is a graph showing the change in the crystallization rate of calcium carbonate particles over time after the start of the introduction of fine bubbles, and FIG. 3D is a graph showing the change in carbon dioxide utilization efficiency (CO2 utilization efficiency) over time after the start of the introduction of fine bubbles. [Figure 4] 4A to 4D are graphs showing the change over time in electrical conductivity (FIG. 4A), the change over time in the crystallization rate of calcium carbonate particles (CaCO3 crystallization rate) (FIG. 4B), the change over time in the crystallization rate of calcium carbonate particles (FIG. 4C), and the change over time in carbon dioxide utilization efficiency (CO2 utilization efficiency) (FIG. 4D), respectively, for Example 1. [Diagram 5] 5A to 5D are graphs showing the change over time in electrical conductivity (FIG. 5A), the change over time in the crystallization rate of calcium carbonate particles (CaCO3 crystallization rate) (FIG. 5B), the change over time in the crystallization rate of calcium carbonate particles (FIG. 5C), and the change over time in carbon dioxide utilization efficiency (CO2 utilization efficiency) (FIG. 5D), respectively, for Example 2. [Figure 6] 6A to 6D are graphs showing the change over time in electrical conductivity (FIG. 6A), the change over time in the crystallization rate of calcium carbonate particles (CaCO3 crystallization rate) (FIG. 6B), the change over time in the crystallization rate of calcium carbonate particles (FIG. 6C), and the change over time in carbon dioxide utilization efficiency (CO2 utilization efficiency) (FIG. 6D), respectively, for Example 3. [Figure 7] 7A to 7D are graphs showing the change over time in electrical conductivity (FIG. 7A), the change over time in the crystallization rate of calcium carbonate particles (CaCO3 crystallization rate) (FIG. 7B), the change over time in the crystallization rate of calcium carbonate particles (FIG. 7C), and the change over time in carbon dioxide utilization efficiency (CO2 utilization efficiency) (FIG. 7D), respectively, for Comparative Example 1. [Figure 8]FIG. 8A is a graph showing the relationship between measured electrical conductivity and calcium concentration, and FIG. 8B is a graph showing the relationship between measured pH and calcium concentration. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] [First embodiment] FIG. 1 is a flowchart of a method for producing calcium carbonate particles according to one embodiment of the present invention, and FIG. 2 is a schematic configuration diagram of a calcium carbonate particle production apparatus capable of carrying out the method for producing calcium carbonate particles shown in FIG.

[0013] As shown in Fig. 1, in this embodiment, calcium is extracted from a calcium-containing material (hereinafter, simply referred to as "calcium raw material") into water (step S110), a calcium-containing solution (hereinafter, simply referred to as "material solution") containing calcium is separated by solid-liquid separation (step S120), and then fine bubbles containing carbon dioxide are introduced into the material solution to crystallize calcium carbonate particles (step S130). At this time, the electrical conductivity (EC) is measured over time (step S140), and when it is confirmed that the electrical conductivity of the material solution has reached a minimum value (step S150), the introduction of fine bubbles is stopped (step S160). After that, the material solution is filtered to recover the crystallized calcium carbonate particles (step S170).

[0014] As shown in FIG. 2, the calcium carbonate particle manufacturing apparatus 200 includes an extraction section 210 for extracting calcium, a separation section 220 for separating the material solution by solid-liquid separation, an introduction section 230 for preparing fine bubbles and introducing them into the material solution, a measurement section 240 for measuring the electrical conductivity of the material solution, a crystallization section 260 for crystallizing calcium carbonate particles from the material solution, and a recovery section 270 for recovering the calcium carbonate particles by filtration.

[0015] Each component of the manufacturing apparatus 200 is configured to be able to transmit and receive data to and from the control unit 250. The control unit 250 has a computer such as a CPU, and a storage unit such as a ROM and a RAM. The control unit 250 controls the operation of these components, as well as the opening and closing of valves and the operation of a pump (not shown) provided in a flow path connecting these components. The control unit 250 does not need to be provided inside the manufacturing apparatus 200, and may be disposed outside the manufacturing apparatus 200 and control these operations and opening and closing through communication by a communication device.

[0016] Hereinafter, each step of the method for producing calcium carbonate particles using the production apparatus 200 will be described.

[0017] (Calcium extraction: step S110) First, the calcium raw material and water are placed in the extraction tank 212 of the extraction section 210, and are stirred to extract calcium from the calcium raw material.

[0018] The calcium raw material fed into the extraction tank 212 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 may be a recovered product such as waste. Examples of the recovered products include waste concrete, ready-mixed concrete sludge, steel slag, waste gypsum, coal ash, biomass ash, incineration ash, alkaline wastewater, and shells. These calcium raw materials may be pretreated, such as neutralization, crushing, or grinding, or may not be pretreated. From the viewpoint of increasing the efficiency of calcium extraction, it is preferable that these calcium raw materials are subjected to crushing or grinding.

[0019] As the water, ion-exchanged water, tap water, industrial water, etc. can be used. Note that, as long as calcium is well dissolved, a solvent other than water may be used, or a mixture of water and another solvent may be used. Note that, when a liquid or slurry calcium raw material is introduced into the extraction tank 212, water does not need to be introduced if it is not necessary.

[0020] The extraction tank 212 has an agitator 216 driven by a motor 214. The extraction unit 210 rotates the agitator 216 under the control of the control unit 250 to agitate the calcium-containing material and water, and extract calcium into the water. This produces a suspension in which the calcium raw material is suspended in the calcium-containing material solution. The agitation conditions, such as the agitation speed and time, are not particularly limited.

[0021] The suspension thus obtained is sent to the separation section 220 under the control of the control section 250.

[0022] (Solid-liquid separation: Step S120) The separation unit 220 separates the material solution by removing the solid calcium raw material and other impurities (such as fine particles) from the suspension by solid-liquid separation. The above-mentioned solid-liquid separation can improve the measurement accuracy of the electrical conductivity by the measurement unit 240. Furthermore, the above-mentioned solid-liquid separation can suppress the mixing of impurities into the calcium carbonate particles crystallized in the crystallization unit 260 to obtain calcium carbonate with high purity, suppress the aggregation of calcium carbonate particles through the surfaces of impurities, and suppress clogging of the flow path by the calcium raw material.

[0023] The type of separation section 220 is not particularly limited, and any 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 can be used.

[0024] The suspension that is subjected to solid-liquid separation in the separation section 220 may be added with a flocculant containing sodium, potassium, magnesium, etc. to flocculate fine particles such as colloidal particles. This allows these fine particles to be removed more efficiently in the separation section 220.

[0025] The material solution obtained by solid-liquid separation is sent to the crystallization section 260 under the control of the control section 250. The solid content removed by solid-liquid separation may be collected and discarded, or may be used for other purposes.

[0026] (Start of introduction of fine bubbles: step S130) In the crystallization section 260, fine bubbles are introduced into the material solution by the introduction section 230.

[0027] Specifically, the crystallization section 260 has a crystallization tank 262 for crystallizing calcium carbonate particles, and the crystallization tank 262 has an agitator 266 driven by a motor 264. In addition, the crystallization tank 262 is connected to an extraction flow path 232 for extracting a part of the material solution to the outside of the crystallization tank 262. The extraction flow path 232 is a circulation flow path configured so that the extracted material solution is reintroduced into the crystallization tank 262 from an inlet 236 through a fine bubble generator 234.

[0028] A gas tank 238 that stores a gas containing carbon dioxide is connected to the fine bubble generator 234. The fine bubble generator 234 converts the gas containing carbon dioxide supplied from the gas tank 238 into fine bubbles and introduces the fine bubbles into the material solution flowing through the extraction flow path 232. The introduced fine bubbles are introduced into the crystallization tank 262 from the introduction port 236 together with the material solution. The extraction flow path 232, the fine bubble generator 234, the introduction port 236, and the gas tank 238 constitute the introduction section 230.

[0029] The configuration of the fine bubble generator 234 is not particularly limited, and may be any known configuration for generating fine bubbles. For example, the fine bubble generator 234 may be configured to generate fine bubbles by a method such as 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 a micropore type. Among these, the micropore type is preferred because it can generate fine bubbles in-line, which makes it less likely to cause processing delays and to be less likely to cause foreign matter to be mixed in, and because it can introduce fine bubbles into a highly viscous material solution.

[0030] In this embodiment, the fine bubble generator 234 has a gas reservoir to which gas containing carbon dioxide is supplied under pressure from a gas tank 238, and a tubular porous ceramic disposed inside the gas reservoir through which the material solution flows. The gas reservoir is a pressurizing section that pressurizes the gas containing carbon dioxide into the porous ceramic. The pressed gas passes through the porous ceramic and becomes fine bubbles, which are then introduced into the material solution.

[0031] In this specification, fine bubbles refer to fine bubbles with an average particle size of 100 μm or less. By using fine bubbles with a large surface area for the crystallization of calcium carbonate particles, the reaction efficiency between carbonate ions and calcium ions can be increased, and the crystallization rate of calcium carbonate particles can be accelerated. From the viewpoint of controlling the introduction of fine bubbles, the average particle size of fine bubbles is preferably 1 μm or more and 100 μm or less, and more preferably 5 μm or more and 50 μm or less. In this specification, the average particle size of fine bubbles is determined by the configuration of the fine bubble generator 234, but may be a value measured by a laser diffraction / scattering type particle size distribution measuring device.

[0032] In this manner, the carbon dioxide in the fine bubbles introduced by the introduction section 230 reacts with the calcium in the material solution, and calcium carbonate particles are crystallized inside the crystallization tank 262.

[0033] The concentration of carbon dioxide in the fine bubbles introduced 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, calcium carbonate particles can be crystallized in a short time.

[0034] 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.

[0035] The conditions for introducing the fine bubbles are not particularly limited. For example, the speed of introducing the fine bubbles can be 0.01 L / min or more and 10 L / min or less. The time for introducing the fine bubbles can be 60 minutes or more and 240 minutes or less. The temperature for introducing the fine bubbles can be 10°C or more and 40°C or less.

[0036] The pH of the material solution when 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 material solution is preferably 7.0 or more and 13.0 or less, and more preferably 8.0 or more and 13.0 or less.

[0037] In addition, seed crystals may be added to the material solution to promote the crystallization of calcium carbonate particles, but it is preferable not to add seed crystals from the viewpoint of suppressing the variation in particle size due to the growth of crystals on the surface of the seed crystals and the promotion of the generation of new crystal nuclei.

[0038] (Measurement of electrical conductivity: step S140) The measuring unit 240 measures the electrical conductivity of the material solution inside the crystallization tank 262 over time while the fine bubbles are being introduced.

[0039] The type of the measuring unit 240 is not particularly limited, and may be any measuring device capable of measuring the electrical conductivity of a liquid. For example, the measuring unit 240 may be a device that measures electrical conductivity by an electrode method such as an AC two-electrode method or an AC four-electrode method, or may be a device that measures electrical conductivity by an electromagnetic induction method. The measuring unit 240 may be configured to extract a part of the material solution from the crystallization tank 262 and measure the electrical conductivity of the extracted solution.

[0040] The measuring section 240 measures the electrical conductivity of the material solution in the crystallization section 260 over time under the control of the control section 250. In this specification, "over time" means measuring the electrical conductivity multiple times at different times. In this respect, this embodiment does not necessarily require continuous measurement of the electrical conductivity, and may measure the electrical conductivity at intervals, such as every minute, every 5 minutes, or every 10 minutes. It is preferable that the measuring section 240 measures the electrical conductivity over time before starting the introduction of fine bubbles. Then, the measuring section 240 transmits the measured electrical conductivity to the control section 250.

[0041] (Determination of electrical conductivity: step S150) When the control unit 250 receives the electrical conductivity from the measurement unit 240, it checks whether the electrical conductivity of the material solution has decreased over time, has not changed over time, or has increased over time, and determines whether the electrical conductivity has reached a minimum rate. The fact that the electrical conductivity has reached a minimum rate can be determined by, for example, the change in electrical conductivity changing from a decrease to an increase.

[0042] According to the inventor's new findings, when the introduction of fine bubbles is started, the electrical conductivity of the material solution decreases. Then, after the electrical conductivity reaches a minimum value, it gradually increases. Meanwhile, the crystallization rate of calcium carbonate particles in the material solution is highest when the electrical conductivity reaches a minimum value or immediately thereafter. Then, while the electrical conductivity gradually increases thereafter, the crystallization rate does not change for a while or decreases gradually. Even if the introduction of fine bubbles is continued while this crystallization rate does not change, the crystallization rate and the utilization efficiency of carbon dioxide (fine bubbles) will only decrease, so it is desirable to stop the introduction of fine bubbles when the electrical conductivity reaches a minimum value or immediately thereafter.

[0043] Although the reason for this is unclear, it is thought that this is because the electrical conductivity reflects the calcium concentration of the material solution relatively accurately. The time when the electrical conductivity reaches a minimum is when the calcium concentration is also at a minimum, and when the calcium in the material solution has crystallized to the maximum extent as calcium carbonate particles. After that, even if the introduction of fine bubbles continues, no further crystallization of calcium carbonate particles can be expected, or calcium carbonate is redissolved in the material solution by the carbon dioxide supplied by the fine bubbles, and the crystallization rate gradually decreases.

[0044] Furthermore, according to further findings by the inventors, if the introduction of fine bubbles is continued while the electrical conductivity is gradually increasing, the crystallization rate of calcium carbonate particles will rapidly decrease at a certain point. This is thought to be because the pH of the material solution rapidly decreases due to the carbon dioxide introduced by the fine bubbles, and the crystallized calcium carbonate is rapidly re-dissolved.

[0045] Fig. 3A is a graph showing the change over time in electrical conductivity after the start of fine bubble introduction, Fig. 3B is a graph showing the change over time in the crystallization rate of calcium carbonate particles (CaCO3 crystallization rate) after the start of fine bubble introduction, Fig. 3C is a graph showing the change over time in the crystallization rate of calcium carbonate particles after the start of fine bubble introduction, and Fig. 3D is a graph showing the change over time in carbon dioxide utilization efficiency (CO2 utilization efficiency) after the start of fine bubble introduction. Note that Figs. 3A to 3D show typical change patterns.

[0046] As shown in Figures 3A to 3D, when the introduction of fine bubbles begins, the crystallization rate of calcium carbonate particles increases over time (Figure 3B), and therefore the crystallization rate (Figure 3C) and carbon dioxide utilization efficiency (Figure 3D) also increase. At the same time, the electrical conductivity of the material solution decreases (Figure 3A).

[0047] After a certain amount of time has passed (time point A), new crystallization of calcium carbonate particles stops, and thereafter the crystallization rate of calcium carbonate particles remains almost unchanged or decreases slightly (Figure 3B). Even if fine bubbles are continued to be introduced after this point, the crystallization rate of calcium carbonate particles does not change even as time passes, so the crystallization rate (Figure 3C) decreases and the carbon dioxide utilization efficiency (Figure 3D) also decreases. At this time, the electrical conductivity of the material solution remains almost unchanged or increases slightly (Figure 3A).

[0048] If the introduction of fine bubbles is continued, at a certain point (time point B), the pH of the material solution drops rapidly due to the carbon dioxide, and calcium carbonate rapidly redissolves, causing a rapid drop in the crystallization rate of calcium carbonate particles (Figure 3B). As a result, both the crystallization rate (Figure 3C) and the utilization efficiency of carbon dioxide (Figure 3D) drop rapidly.

[0049] Therefore, the electrical conductivity decreases from the initial electrical conductivity (EC0) to a minimum value (EC min ), the introduction of fine bubbles is stopped during the period when the temperature gradually increases (between time point A and time point B), thereby making it possible to increase the crystallization rate and crystallization speed of calcium carbonate particles and the efficiency of carbon dioxide utilization, and to efficiently recover calcium carbonate particles.

[0050] On the other hand, since the calcium carbonate particles rapidly re-dissolve after time point B, it is preferable to stop the introduction of fine bubbles at least before time point B. According to the findings of the present inventors, when the introduction of fine bubbles is stopped, the electrical conductivity (EC fin ) for the formula EC fin / (EC0-EC min By stopping the introduction of fine bubbles before the temperature (EC) becomes 0.05, the introduction of fine bubbles can be stopped before time point B. From the above viewpoint, fin / (EC0-EC min It is more preferable to stop the introduction of fine bubbles before the formula EC becomes 0.03, and it is even more preferable to stop the introduction of fine bubbles before the formula EC becomes 0.01.fin / (EC0-EC min ) has a lower limit of 0.

[0051] In this case, by using electrical conductivity to determine the crystallization state of calcium carbonate particles, it is believed that the crystallization state can be accurately determined without being affected by conditions such as the temperature and composition of the material solution, unlike pH, which is easily affected by temperature and the amount of other components. In addition, since electrical conductivity can measure minute changes, it is believed that the crystallization state can be determined more precisely. For these reasons, it is believed that by using electrical conductivity as an indicator, the time point at which the crystallization rate, crystallization speed, carbon dioxide utilization efficiency, etc. become high can be more accurately understood.

[0052] The behavior described above is considered to be characteristic of the use of fine bubbles, which have a large surface area and allow carbon dioxide to dissolve easily in the material solution. When bubbles with a larger particle size are used, it is difficult to accurately determine the crystallization state even by the electrical conductivity.

[0053] (Stopping the introduction of fine bubbles: step S160) Based on the above findings, when it is determined that the electrical conductivity measured by the measuring unit 240 has reached a minimum value (step S150: YES), the introducing unit 230 stops the introduction of fine bubbles under the control of the control unit 250. When it is not determined that the electrical conductivity has reached a minimum value (step S150: NO), the introducing unit 230 does not stop the introduction of fine bubbles. At this time, the measurement of electrical conductivity by the measuring unit 240 and the determination by the control unit 250 are repeated.

[0054] After the introduction of the fine bubbles is stopped, the material solution containing the crystallized calcium carbonate particles is sent to the recovery section 270 under the control of the control section 250.

[0055] (Filtration: Step S170) The recovery section 270 filters the material solution containing the calcium carbonate particles to recover the calcium carbonate particles.

[0056] The filtration by the collection unit 270 may be performed by a known method such as suction filtration.

[0057] The recovered calcium carbonate particles may be dried or pulverized as necessary, and may be surface-treated with a fatty acid or the like depending on the intended use.

[0058] The liquid component (processed liquid) separated by filtration may be disposed of as waste liquid, or may be used for other purposes, or may be reused as a material solution.

[0059] The calcium carbonate particles obtained in this way can be used in a wide range of applications, such as fillers for plastics, paper and paints, soil conditioners such as pesticides and fertilizers, adhesives, sealants, erasers and toothpastes that take advantage of their abrasive properties, sheet and film-like paper substitutes and plastic substitutes made primarily from calcium carbonate particles, as well as food additives and cosmetic ingredients.

[0060] [Other embodiments] It should be noted that the above-described embodiment is merely an example of the present invention, and the present invention is not limited to the above-described embodiment. Needless to say, various other embodiments are possible within the scope of the concept of the present invention.

[0061] For example, the introduction unit 230 may change the fine bubble introduction conditions, such as the average particle size of the fine bubbles, the introduction speed, and the carbon dioxide concentration, depending on the electrical conductivity measured by the measurement unit 240. Specifically, after the electrical conductivity reaches a minimum (between time points a and b in Figs. 3A to 3D), the average particle size of the fine bubbles may be increased, the introduction speed of the fine bubbles may be slowed, or the carbon dioxide content in the fine bubbles may be reduced, thereby slowing down the dissolution rate of carbon dioxide in the material solution and slowing down the rate at which the electrical conductivity increases, thereby improving the EC. fin This allows for a wider range of possible times for the introduction of fine bubbles to be stopped, making it possible to more precisely adjust the timing for stopping the introduction of fine bubbles. EXAMPLES

[0062] The present invention will be described in detail based on examples, but the present invention is not limited to these examples.

[0063] [Experiment 1] [Example 1] Commercially available Portland cement was prepared as the 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 (material solution).

[0064] At this time, the mixing conditions of the Portland cement were changed to obtain two calcium-containing solutions with different compositions (hereinafter simply referred to as "Ca solution 1" and "Ca solution 2"). The calcium content in these calcium-containing solutions was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES).

[0065] The pH of the calcium-containing solution was adjusted to 7.0 or more, and a mixed gas of carbon dioxide (CO2) and nitrogen (N2) was introduced to crystallize calcium carbonate. The mixed gas had a CO2 concentration of 10% by volume. The mixed gas was introduced into the extracted calcium-containing solution by a fine bubble generator installed in an extraction flow path that extracts a part of the calcium-containing solution from the crystallization tank in which calcium carbonate crystallization was performed. The calcium-containing solution into which the fine bubbles were introduced was reintroduced into the crystallization tank, and fine bubbles were also introduced into the calcium-containing solution in the crystallization tank. The fine bubble generator used was an in-line FB test device (product name) manufactured by Noritake Co., Ltd., which has a configuration in which carbon dioxide is pressed from the outside to the inside of a tubular porous ceramic through which the calcium-containing solution flows. The average particle size of the fine bubbles introduced was adjusted to 30 μm. The flow rate of the mixed gas at the time of introduction was 500 mL / min.

[0066] After the introduction of fine bubbles, the introduction of fine bubbles was stopped at a predetermined time point, and the calcium-containing solution was subjected to suction filtration to obtain calcium carbonate particles. The introduction of fine bubbles was stopped 10, 20, 30, 40, 50, and 60 minutes after the start of the introduction of fine bubbles, and multiple experiments were performed. The electrical conductivity of the calcium-containing solution at the time when the introduction of fine bubbles was stopped was measured using a portable electrical conductivity meter WQ-300 (product name) manufactured by Horiba, Ltd.

[0067] For each experiment, the ratio of the amount of calcium (moles) in the calcium carbonate particles, calculated from the mass of the obtained calcium carbonate particles, to the amount of calcium (moles) in the calcium-containing solution was calculated to calculate the crystallization rate (%) of the calcium carbonate particles. In addition, the crystallization rate (g / (hr×L)) of the calcium carbonate particles was calculated from the mass (g) of the obtained calcium carbonate particles, the time (hr) for which the fine bubbles were introduced, and the volume (L) of the calcium-containing solution. In addition, the ratio of the amount of CO2 (moles) in the calcium carbonate particles, calculated from the mass of the obtained calcium carbonate particles, to the amount of carbon dioxide (moles) in the introduced fine bubbles was calculated to calculate the carbon dioxide utilization efficiency (%).

[0068] FIG. 4A is a graph showing the change over time in the electrical conductivity of material solution 1, FIG. 4B is a graph showing the change over time in the crystallization rate of calcium carbonate particles in material solution 1 (CaCO3 crystallization rate), FIG. 4C is a graph showing the change over time in the crystallization rate of calcium carbonate particles in material solution 1, and FIG. 4D is a graph showing the change over time in the carbon dioxide utilization efficiency (CO2 utilization efficiency) of material solution 1.

[0069] The initial electrical conductivity of the calcium-containing solution is EC0, and the electrical conductivity at which it reaches a minimum is EC min The electrical conductivity when the crystallization rate of calcium carbonate particles rapidly decreases is called EC fin Then, EC fin / (EC0-EC min ) was 0.07.

[0070] [Example 2] Crystallization of calcium carbonate particles was carried out in the same manner as in Example 1, except that material solution 1 was changed to material solution 2. Figures 5A to 5D are graphs showing the change over time in electrical conductivity (Figure 5A), the change over time in the crystallization rate of calcium carbonate particles (CaCO3 crystallization rate) (Figure 5B), the change over time in the crystallization rate of calcium carbonate particles (Figure 5C), and the change over time in the efficiency of carbon dioxide utilization (CO2 utilization efficiency) (Figure 5D), respectively, for Example 2.

[0071] The initial electrical conductivity of the calcium-containing solution is EC0, and the electrical conductivity at which it reaches a minimum is EC min The electrical conductivity when the crystallization rate of calcium carbonate particles rapidly decreases is called EC fin Then, EC fin / (EC0-EC min ) was 0.08.

[0072] [Example 3] Except for changing the particle size of the fine bubbles to 70 μm, the crystallization of calcium carbonate particles was carried out in the same manner as in Example 1. Figures 6A to 6D are graphs showing the change over time in electrical conductivity (Figure 6A), the change over time in the crystallization rate of calcium carbonate particles (CaCO3 crystallization rate) (Figure 6B), the change over time in the crystallization rate of calcium carbonate particles (Figure 6C), and the change over time in the efficiency of carbon dioxide utilization (CO2 utilization efficiency) (Figure 6D), respectively, for Example 3.

[0073] The initial electrical conductivity of the calcium-containing solution is EC0, and the electrical conductivity at which it reaches a minimum is EC min The electrical conductivity when the crystallization rate of calcium carbonate particles rapidly decreases is called EC fin Then, EC fin / (EC0-EC min ) was 0.05.

[0074] [Comparative Example 1] Except for changing the average particle size of the fine bubbles to 3 mm, calcium carbonate particles were crystallized in the same manner as in Example 1. Figures 7A to 7D are graphs showing the change over time in electrical conductivity (Figure 7A), the change over time in the crystallization rate of calcium carbonate particles (CaCO3 crystallization rate) (Figure 7B), the change over time in the crystallization rate of calcium carbonate particles (Figure 7C), and the change over time in the efficiency of carbon dioxide utilization (CO2 utilization efficiency) (Figure 7D), respectively, for Comparative Example 1.

[0075] [Experiment 2] The introduction of fine bubbles was stopped at each point in Experiment 1, and the calcium-containing solution obtained was subjected to measurement of electrical conductivity, pH, and calcium concentration in the solution measured by ICP-OES.

[0076] FIG. 8A is a graph showing the relationship between calcium concentration and measured electrical conductivity. There is a strong correlation (coefficient of determination R 2 =0.9351). FIG. 8B is a graph showing the relationship between calcium concentration and measured pH. The correlation between calcium concentration and pH is not strong (coefficient of determination R 2 =0.6505).

[0077] As is clear from these experimental results, it is possible to easily grasp the time when the crystallization rate of calcium carbonate particles is highest by using the electrical conductivity as an indicator. Therefore, it is found that the crystallization rate can be increased and calcium carbonate particles can be efficiently produced by stopping the introduction of fine bubbles using the electrical conductivity as an indicator. [Industrial Applicability]

[0078] According to the present invention, the yield of calcium carbonate particles can be increased by a simple method. Therefore, the present invention is expected to increase the efficiency of carbon dioxide fixation by calcium carbonate particles, further reduce the environmental load, and further promote the spread of calcium carbonate particles to various applications. [Explanation of symbols]

[0079] 200 Manufacturing equipment 210 Extraction part 212 Extraction tank 214 Motor 216 Stirrer 220 Separation section 230 Introduction 232 Extraction channel 234 Fine Bubble Generator 236 Introduction 238 Gas Tank 240 Measuring section 250 Control section 260 Crystallization Section 262 Crystallization tank 264 Motor 266 Stirrer 270 Recovery Department

Claims

1. A step of introducing fine bubbles containing carbon dioxide into a material solution containing calcium; and measuring the electrical conductivity of the material solution over time; After the measured electrical conductivity reaches a minimum value, the introduction of the fine bubbles is stopped. A method for producing calcium carbonate particles.

2. The fine bubbles have an average particle diameter of 1 μm or more and 100 μm or less. The method for producing calcium carbonate particles according to claim 1 .

3. The electrical conductivity of the material solution when the introduction of the fine bubbles was started was determined as EC ０ , The minimum value of the electrical conductivity is EC ｍｉｎ , The electrical conductivity of the material solution when the introduction of the fine bubbles is stopped is EC ｆｉｎ , then After the electrical conductivity reaches a minimum value, the formula EC ｆｉｎ / (EC ０ -EC ｍｉｎ The introduction of the fine bubbles is stopped before the ratio (ratio) of the fine bubbles to the total amount of the fine bubbles reaches 0.

05. The method for producing calcium carbonate particles according to claim 1 .

4. In the step of introducing fine bubbles, The average particle size of the fine bubbles is changed according to the measured electrical conductivity. The method for producing calcium carbonate particles according to claim 1 .

5. In the step of introducing fine bubbles, The introduction speed of the fine bubbles is changed according to the measured electrical conductivity. The method for producing calcium carbonate particles according to claim 1 .

6. The step of introducing fine bubbles is carried out with the pH of the material solution being 7.0 or more. The method for producing calcium carbonate particles according to claim 1 .

7. A measuring unit that measures the electrical conductivity of the calcium-containing material solution over time; An introduction unit that introduces fine bubbles containing carbon dioxide into the material solution; A control unit controls the introduction of the fine bubbles by the introduction unit to stop the introduction of the fine bubbles after the measured electrical conductivity has reached a minimum value. Manufacturing equipment for calcium carbonate particles.

8. The electrical conductivity of the material solution when the introduction of the fine bubbles was started was determined as EC ０ , The minimum value of the electrical conductivity is EC ｍｉｎ , The electrical conductivity of the material solution when the introduction of the fine bubbles is stopped is EC ｆｉｎ , then The control unit is configured to control the electric conductivity to be a minimum value and the EC ｆｉｎ / (EC ０ -EC ｍｉｎ The introduction of the fine bubbles is stopped before the ratio (ratio) of the fine bubbles to the total amount of the fine bubbles reaches 0.

05. The apparatus for producing calcium carbonate particles according to claim 7.

9. The introduction section is a tubular porous ceramic through which the material solution flows; and an injection section for injecting carbon dioxide into the porous ceramic. The apparatus for producing calcium carbonate particles according to claim 7.

10. The control unit controls the introduction of the fine bubbles by the introduction unit to change the average particle diameter of the fine bubbles in accordance with the measured electrical conductivity. The apparatus for producing calcium carbonate particles according to claim 7.

11. The control unit controls the introduction of the fine bubbles by the introduction unit to change the introduction speed of the fine bubbles in accordance with the measured electrical conductivity. The apparatus for producing calcium carbonate particles according to claim 7.

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