Method for producing calcium carbonate

Abstract

To provide a novel method for producing calcium carbonate with excellent production efficiency.SOLUTION: The method for producing calcium carbonate according to the present invention comprises a preparation step of preparing a raw material containing calcium hydroxide and having a moisture content of 0.3 to 40%, and a reaction step of, with the moisture content of the raw material maintained at 0.3 to 40%, supplying a gas containing carbon dioxide to the raw material while stirring the raw material to react the calcium hydroxide with the carbon dioxide. Furthermore, in the method for producing calcium carbonate according to the present invention, the content of the calcium hydroxide in the raw material is preferably 30% or more.SELECTED DRAWING: Figure 4

Description

[Technical Field]

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

[0002] In recent years, efforts have been made to achieve carbon neutrality through "carbon recycling," which involves recovering and effectively utilizing CO2 (carbon dioxide) as a resource to reduce CO2 emissions. The applicant of the present application has proposed a hydraulic composition containing a predetermined amount of calcium carbonate, as disclosed in Patent Document 1. This hydraulic composition not only reduces CO2 emissions by reducing the amount of cement used while maintaining material properties such as strength, but also immobilizes CO2 inside the concrete as calcium carbonate.

[0003] Regarding the method for producing calcium carbonate used in Patent Document 1, various techniques have been proposed up to now. For example, Patent Document 2 proposes a method for producing precipitated calcium carbonate by mixing seed crystals with one or both of an aqueous suspension of calcium hydroxide and an aqueous suspension of partially carbonated calcium hydroxide, and then blowing carbon dioxide or a gas containing carbon dioxide into the resulting aqueous suspension to carbonate it, in which the mixing is carried out by a shear stirring treatment at a stirring peripheral speed of 7 m / s or more. Furthermore, Patent Document 3 proposes a method in which an alkaline agent is added to waste seawater after producing magnesium hydroxide from seawater to adjust the pH to greater than 11 and equal to or less than 13, and a gas containing carbon dioxide is reacted with the calcium component of the waste seawater for a preset time that increases as the pH of the waste seawater to which the alkaline agent has been added increases. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2021 / 256484 [Patent Document 2] Japanese Patent Application Publication No. 11-11941 [Patent Document 3] Patent No. 7138256 Summary of the Invention [Problem to be solved by the invention]

[0005] The calcium carbonate production methods of Patent Documents 2 and 3 are both methods in which a gas containing carbon dioxide is blown into a solution containing calcium ions (so-called liquid phase method). Therefore, the methods for producing calcium carbonate in Patent Documents 2 and 3 require a large amount of water to disperse the raw materials, and may also require a prior treatment of the raw materials (shear stirring treatment in Patent Document 2) or pH control in the liquid phase (adjustment treatment in Patent Document 3). Therefore, the present inventors wanted to propose a completely new method for producing calcium carbonate that differs from the liquid phase methods of Patent Documents 2 and 3. However, the present inventors thought that even if a new method for producing calcium carbonate is proposed, it will not be adopted in practice unless it has excellent calcium carbonate production efficiency (balance between the amount of calcium carbonate produced (amount of carbon dioxide absorbed) within a predetermined time and the energy required for production (amount of carbon dioxide emitted) such as operating the equipment and facilities).

[0006] From this viewpoint, an object of the present invention is to provide a new method for producing calcium carbonate with excellent production efficiency. [Means for solving the problem]

[0007] The above problems can be solved by the following means. The method for producing calcium carbonate according to the present invention includes: a preparation step of preparing a raw material containing calcium hydroxide and having a moisture content of 0.3 to 40%; and a reaction step of supplying a gas containing carbon dioxide to the raw material while stirring the raw material and maintaining the moisture content of the raw material at 0.3 to 40%, to react the calcium hydroxide with the carbon dioxide. According to the present invention, by controlling the moisture content of the raw materials to be within a predetermined range in the preparation step and the reaction step, calcium hydroxide and carbon dioxide react appropriately, and calcium carbonate is efficiently produced. In the method for producing calcium carbonate according to the present invention, the moisture content of the raw materials in the reaction step is preferably lower than the moisture content of the raw materials in the preparation step. In the method for producing calcium carbonate according to the present invention, the calcium hydroxide content in the solid content of the raw materials is preferably 30% or more. In the method for producing calcium carbonate according to the present invention, the carbon dioxide content in the gas is preferably 5% or more. In the method for producing calcium carbonate according to the present invention, the atmospheric temperature in the reaction step is preferably 10 to 75°C. In the method for producing calcium carbonate according to the present invention, the raw materials are preferably carbide slag, waste concrete, ready-mixed concrete sludge, cement, or a mixture thereof. According to the present invention, it is possible to more reliably improve the efficiency of calcium carbonate production. [Effects of the Invention]

[0008] According to the method for producing calcium carbonate of the present invention, calcium carbonate can be produced with excellent production efficiency. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a graph showing the change over time in the solid phase structure and moisture content of Sample 1 in Example 1. [Figure 2] 1 is a graph showing the change over time in the solid phase structure and moisture content of Sample 2 in Example 1. [Figure 3] 1 is a graph showing the change over time in the solid phase structure and moisture content of Sample 3 in Example 2. [Figure 4] 1 is a graph showing the change over time in the solid phase structure of Samples 4, 5, and 6 of Example 3. [Figure 5] 1 is a graph showing the change over time in the solid phase structure of Samples 7, 8, and 9 in Example 4. [Figure 6] 1 is a graph showing the change over time in the solid phase structure of Samples 10, 11, 12, and 13 of Example 5. [Figure 7] 10 is a graph showing the change over time in the solid phase structure of Sample 14 in Example 6. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, an embodiment for carrying out the method for producing calcium carbonate according to the present invention (the method for producing calcium carbonate according to the present embodiment) will be described. The method for producing calcium carbonate according to this embodiment includes a preparation step and a reaction step, and may include a drying step after the reaction step. Each step will be described in detail below.

[0011] [Preparation process] In the preparation step, raw materials to be used in the reaction step described below are prepared. (Preparation process: raw materials) The raw material prepared in the preparation step contains calcium hydroxide and has a moisture content within a predetermined range. The calcium hydroxide content in the solid content of the raw material (solid content when the raw material is dried at 105°C) is preferably 30% (specifically w / w%) or more, more preferably 50% or more, 70% or more, 80% or more, or 90% or more. When the calcium hydroxide content is a predetermined value or more, the production efficiency of calcium carbonate in the product obtained in the reaction step described below can be improved. The moisture content of the raw materials in the preparation step (in other words, the moisture content of the raw materials at the start of the reaction step) is preferably 0.3 to 40% (more specifically, w / w%) on a wet basis (WB), more preferably 1 to 40%, 5 to 30%, or 10 to 25%. By keeping the moisture content of the raw materials within a predetermined range in the preparation step, the carbonation reaction of calcium hydroxide proceeds quickly in the reaction step described below, and a large amount of calcium carbonate is produced within a predetermined time. The raw material is not particularly limited as long as it satisfies the above two requirements, but examples include sludge (carbide slag) produced as a by-product in the process of producing acetylene gas from calcium carbide, waste concrete, sludge from ready-mixed concrete, cement, etc., and one or more of these can be suitably crushed and used. Among these, it is particularly preferable to use carbide slag, which is a by-product (residue) and has a high calcium hydroxide content.

[0012] (Preparation process: Adjusting moisture content) When the moisture content of the raw material is high, the raw material may be subjected to a dehydration treatment so that the moisture content falls within the above range. Examples of dehydration treatments include thickeners, filter presses, centrifugation, gravity settling, heat drying, and natural drying. One or more of these treatments may be used, but thickeners and filter presses are preferred from the viewpoints of energy efficiency and work efficiency. If the moisture content of the raw material is low, the raw material may be subjected to a hydration treatment so that the moisture content falls within the above range. The water used in the hydration treatment is not particularly limited, and examples thereof include tap water, distilled water, ion-exchanged water, RO water, and the solution separated in the above-mentioned dehydration treatment.

[0013] [Reaction process] In the reaction step, a gas is supplied to the raw material, and calcium hydroxide contained in the raw material reacts with carbon dioxide contained in the gas to produce calcium carbonate. (Reaction step: Adjustment of water content) The moisture content of the raw materials in the reaction step is preferably within a range of 0.3 to 40%, more preferably 1 to 40%, 5 to 30%, or 10 to 20%. By keeping the moisture content of the raw materials within a predetermined range in the reaction step, the carbonation reaction of calcium hydroxide proceeds appropriately, and a large amount of calcium carbonate is produced within a predetermined time. In the reaction step, the reaction "Ca(OH)2 + CO2 → CaCO3 + H2O" generates heat of reaction, causing the temperature of the raw materials to rise. As a result, water evaporates from the raw materials, gradually reducing the moisture content of the raw materials. Therefore, the raw materials can be appropriately hydrated so that the moisture content of the raw materials falls within the above-mentioned range. Note that, taking into account the evaporation of water during the above reaction, unless extreme hydration is performed, the moisture content of the raw materials in the reaction step will be lower than that of the raw materials in the preparation step. The moisture content of the raw materials in the reaction step may be measured at predetermined time intervals (for example, every 5 to 40 minutes, or every 15 to 30 minutes). TG-DTA may be used, or a predetermined amount may be extracted from the raw materials during the reaction step and the moisture content may be calculated based on the change in mass before and after drying with a moisture meter or by heating (for example, heating at 105°C or higher for 1 hour or more). Then, based on the calculated moisture content of the raw material, the hydration treatment can be carried out as appropriate. Alternatively, the hydration treatment can be carried out as appropriate based on the rate of decrease in the moisture content of the raw material obtained in advance. The water used in the hydration treatment is as described in the preparation step above.

[0014] (Reaction process: gas) The gas used in the reaction process contains carbon dioxide. Examples of gas that can be used include exhaust gas from a boiler (with a carbon dioxide concentration of about 10%) and highly concentrated carbon dioxide gas obtained using a concentrator. From the viewpoint of the rate of calcium carbonate production, the carbon dioxide content in the gas should be 5% (specifically, v / v%) or more. If the carbon dioxide content in the gas is 5% or more, the carbonation reaction can be rapidly advanced and calcium carbonate can be produced in a short time, thereby reducing the energy required for production. On the other hand, from the viewpoint of the rate of calcium carbonate production, it is better for the gas to have a higher carbon dioxide content, but based on the results shown in Figure 6, which will be described later, even if the carbon dioxide content is too high, the final amount of calcium carbonate produced does not change much. Therefore, from an economic viewpoint, it is possible to use the exhaust gas as it is, or to use concentrated CO2 gas for efficient transportation of the exhaust gas or for use in industries that require high-concentration and high-purity CO2. The gas supply rate can be set appropriately depending on the amount of raw material used, the carbon dioxide content of the gas, etc. For example, when the raw material is about 6 kg, it is 20 to 100 L / min (preferably 50 to 75 L / min). Therefore, when the raw material used is Z kg, the gas supply rate is [3.33×Z] to [16.6×Z] L / min (preferably [8.33×Z] to [12.5×Z] L / min). The time for supplying the gas (time for the reaction step) depends on the progress of the reaction, but is, for example, 10 to 130 minutes (preferably 60 to 120 minutes) if the reaction is carried out under the above conditions.

[0015] (Reaction process: other conditions) In the reaction step, the gas is supplied to the raw materials while stirring the raw materials to bring them into contact with each other. The strength and method of stirring are not particularly limited as long as the raw materials and the gas are properly brought into contact with each other, and the stirring can be carried out using a known stirrer. Any device having a stirring function can be used, and therefore a sealed stirrer or a dryer having a stirring function can also be used. The ambient temperature in the reaction step is not particularly limited as long as it is in the range of 10 to 75°C, and from the viewpoint of energy efficiency, room temperature (for example, 20 to 50°C) without treatment such as heating is preferred.

[0016] [Drying process] In the drying step, the calcium carbonate-containing product obtained in the reaction step is dried as needed, for example, to a moisture content (called "moisture" in JIS) of 1.0% or less, as specified in JIS A 6201 "Fly ash for concrete" and JIS A5041 "Crushed stone powder for concrete." The drying process may be performed by known methods such as heat drying, natural drying, or reduced-pressure drying. The drying time and intensity may be appropriately set according to the needs of the product (e.g., if a product with a low moisture content is required, the drying time may be longer or the drying intensity may be increased). [Example]

[0017] [Example 1] In Example 1, the effect of replenishing water that evaporates during the reaction process was confirmed. (Exam contents) The raw material for samples 1 and 2 was a slurry of carbide slag (solid content in the slurry: approximately 20%, calcium hydroxide content in the solid content: 86.9%), which is a by-product generated when producing acetylene by reacting calcium carbide with water. First, each raw material was subjected to slurry filtration to obtain a moisture content of 44.3% (WB). Then, each raw material was placed on a tray in the incubator of a carbonation acceleration tester (MIT-639-3-05, manufactured by Marui Co., Ltd.) under the following conditions: temperature: 50°C, RH: 60%, and CO2 concentration: 5% for 5 hours. Then, every hour from the start of the incubation, samples for measurement were taken from each sample, and the collected samples were measured using a thermogravimetric and differential thermal analyzer (TG-DTA: Thermo Mass Photo, manufactured by Rigaku Corporation). For Sample 1, water was supplied to the raw material at the time of measurement to replace the evaporated water found when the TG-DTA sample was taken (i.e., water was supplied so that the moisture content of the raw material was 44.3%) and mixed into the raw material. On the other hand, for Sample 2, no water was supplied to the raw material during the standing period. The moisture content of each sample was calculated by placing a sample taken from each sample in a dryer (drying temperature 105°C) and drying for 24 hours or more, and then calculating the moisture content based on the change in mass before and after drying.

[0018] (TG-DTA measurement method) For TG-DTA measurements, approximately 30 mg of the collected sample was placed in a platinum cell, and while nitrogen gas was flowing at 300 mL / min, the sample was heated from 20°C to 1000°C at a temperature increase rate of 20°C / min to obtain TG and DTA curves, and the measurement results shown in the table were then obtained.

[0019] (Calculation method for each indicator) Using the results of each TG-DTA measurement, the indices shown in the table were calculated using the following method. "Ca(OH)2 (%: anhydrous equivalent)" = "CH loss (%)" / 18 x 74 x 100 / (100 - "amount of water below 300°C (%)") "CaCO3 (%: anhydrous equivalent)" = "CaCO3 weight loss (%)" / 44 x 100 x 100 / (100 - "amount of water below 300°C (%)") "Other (%: anhydrous equivalent)" = 100 - "Ca(OH)2 (%: anhydrous equivalent)" - "CaCO3 (%: anhydrous equivalent)" · "CaO meter (%: anhydrous equivalent)" = "Ca(OH)2 (%: anhydrous equivalent)" / 74 x 56 + "CaCO3 (%: anhydrous equivalent)" / 100 x 56 "Ca(OH)2 (%: initial CaO equivalent)" = "Ca(OH)2 (%: anhydrous equivalent)" / "CaO total (%: anhydrous equivalent)" x "CaO total of raw material before reaction (%: anhydrous equivalent)" "CaCO3 (%: initial CaO equivalent)" = "CaCO3 (%: anhydrous equivalent)" / "CaO total (%: anhydrous equivalent)" x "CaO total of raw material before reaction (%: anhydrous equivalent)" "Others (%: initial CaO equivalent)" = "Others in raw materials before reaction (%: anhydrous equivalent)" "Total (%: converted value of initial CaO amount)" = "Ca(OH)2 (%: converted value of initial CaO amount)" + "CaCO3 (%: converted value of initial CaO amount)" + "Other (%: converted value of initial CaO amount)" Here, "anhydrous equivalent" refers to the content (%) of each component when the solid content after removing water (such as adhering water) removed by heating up to 300°C is taken as 100%. Furthermore, "initial CaO equivalent" refers to the content (%) of each component based on the initial (before reaction) amount of CaO, and is an index that can determine how much of the CaO initially present as Ca(OH)2 has been converted to CaCO3 after the reaction. Furthermore, "amount of water below 300°C" in the table refers to the amount of water removed by heating up to 300°C. The indices used in each figure are, among the above-mentioned indices, "Ca(OH)2 (%: converted value of initial CaO amount)," "CaCO3 (%: converted value of initial CaO amount)," "Other (%: converted value of initial CaO amount)," and moisture content.

[0020] [Table 1]

[0021] (Consideration of the results of Example 1) Fig. 1 is a graph showing the change over time in the solid phase structure and moisture content of Sample 1 in Example 1. Fig. 2 is a graph showing the change over time in the solid phase structure and moisture content of Sample 2 in Example 1. From the results in Figure 1 and Table 1, in Sample 1, in which water was supplied during the reaction process, the moisture content of the raw material was always high (over 40%), so the reaction of calcium hydroxide did not occur and the proportion of calcium carbonate in the solid phase was very small. On the other hand, from the results of Figure 2 and Table 1, in Sample 2, in which water was not supplied during the reaction process, the proportion of calcium carbonate in the solid phase composition increased significantly after 2 hours had passed, when the moisture content in the raw materials began to decrease. From these results, it was confirmed that the moisture content of the raw materials in the reaction process has a very large effect on the amount of calcium carbonate produced. After removing the adhering water by converting it to anhydrous, the initial CaO amount equivalent value is based on the amount of CaO before the reaction started, and the amount converted to each reaction time is also compared. Therefore, the content of each component at each reaction time exceeds 100% due to the amount of CO2 absorbed by the reaction.

[0022] [Example 2] In Example 2, the effect of carrying out a drying treatment in the reaction step was confirmed. (Exam contents) As the raw material for Sample 3, calcium hydroxide (commercially available product) shown in Table 2 was used. First, water (tap water) was added to the raw materials to adjust the moisture content of the raw materials to 40.0% (WB). Then, a paddle mixer (NARA MACHINERY MFG. CO., LTD., NPD-1.6W-12L-G) was used to stir approximately 6 kg of raw materials while supplying a 20% CO2 gas at 250 L / min for a 60-minute reaction. During the reaction in Example 2, drying was performed using the paddle mixer (110°C steam was circulated on the wall (jacket) of the dryer container). The moisture content of the raw materials sampled before the reaction and 10, 20, 35, and 55 minutes after the start of the reaction was measured using an infrared moisture meter (FD-800, Kett Electric Laboratory, drying temperature 105°C). The calcium hydroxide and calcium carbonate contents were also measured using TG-DTA (the same equipment as in Example 1). For Sample 3, no water was supplied to the raw materials during the reaction period. The measurement method by TG-DTA and the calculation method for each index were the same as in Example 1.

[0023] [Table 2]

[0024] (Discussion of the results of Example 2) FIG. 3 is a graph showing the change over time in the solid phase structure and moisture content of Sample 3 in Example 2. From the results in Figure 3 and Table 2, it was confirmed that the proportion of calcium carbonate in the solid phase composition increases as the moisture content of the raw materials decreases during the reaction process. Furthermore, by comparing the results of Sample 3 shown in Figure 3 with those of Sample 2 shown in Figure 2, it was found that Sample 3, in which the raw materials were reacted while being stirred, was able to significantly shorten the time until calcium carbonate was produced. However, the present inventors believed that there was room for further improvement in order to increase the proportion of calcium carbonate in the solid phase of the product.

[0025] [Example 3] In Example 3, the influence of the moisture content of the raw material was confirmed. (Exam contents) The raw material for Samples 4, 5, and 6 was calcium hydroxide (commercially available product) shown in Table 3. First, water (tap water) was added to each raw material, and the moisture content of the raw material for Sample 4 was 10% (WB), the moisture content of the raw material for Sample 5 was 20% (WB), and the moisture content of the raw material for Sample 6 was 30% (WB). Then, using a paddle mixer (the same apparatus as in Example 2), 6 kg of each raw material was stirred while supplying a gas containing 20% CO at 50 L / min, and a 90-minute reaction treatment was carried out. During the reaction treatment in Example 3, no temperature control (heating or cooling) was performed, but the temperature (ambient temperature) inside the paddle mixer was 20 to 50°C. Then, before the reaction treatment and 30, 60, and 90 minutes after the start of the reaction treatment, the moisture content of each sampled raw material was measured using an infrared moisture meter (the same apparatus as in Example 2), and various indicators were measured using TG-DTA (the same apparatus as in Example 1). For samples 4, 5, and 6, water was supplied to the raw materials in an amount equal to the amount of water that had evaporated during the moisture content measurement (i.e., water was supplied so that the moisture content of the raw materials was 10% for sample 4, 20% for sample 5, and 30% for sample 6). The measurement method by TG-DTA and the calculation method for each index were the same as in Example 1.

[0026] [Table 3]

[0027] (Discussion of the results of Example 3) FIG. 4 is a graph showing the change over time in the solid phase structure of Samples 4, 5, and 6 of Example 3. 4 and Table 3, it was confirmed that by controlling the moisture content of the raw materials within a predetermined range in the reaction step, the proportion of calcium carbonate in the solid phase composition can be significantly increased in a short time of 30 to 90 minutes. In other words, it was confirmed that according to the present invention, the amount of calcium carbonate produced within a predetermined time is increased, and excellent production efficiency can be demonstrated. Samples 4, 5, and 6 all showed good results, but samples 4 and 5 in particular showed very favorable results. In Table 3, the value in parentheses in the lower row of the moisture content is the moisture content of the raw material just before water was replenished, and the value in the upper row of the moisture content is the target value, and the same applies to Tables 4 and 5 described below.

[0028] [Example 4] In Example 4, the influence of the atmospheric temperature in the reaction step was confirmed. (Exam contents) The raw material for Samples 7, 8, and 9 was calcium hydroxide (commercially available product) shown in Table 4. First, water (tap water) was added to each raw material, and the moisture content of the raw material for Sample 7 was adjusted to 10% (WB), that for Sample 8 to 20% (WB), and that for Sample 9 to 30% (WB). Then, using a paddle mixer (the same apparatus as in Example 2), 6 kg of each raw material was stirred while supplying a gas containing 20% CO at 50 L / min, and a 90-minute reaction treatment was carried out. During the reaction treatment in Example 4, the temperature (ambient temperature) inside the paddle mixer was controlled to approximately 70 to 90°C. Then, before the reaction treatment and 30, 60, and 90 minutes after the start of the reaction treatment, the moisture content of each sampled raw material was measured using an infrared moisture meter (the same apparatus as in Example 2), and various indicators were measured using TG-DTA (the same apparatus as in Example 1). For samples 7, 8, and 9, water was supplied to the raw materials in an amount equal to the amount of water that had evaporated during the moisture content measurement (i.e., water was supplied so that the moisture content of the raw materials was 10% for sample 7, 20% for sample 8, and 30% for sample 9). The measurement method by TG-DTA and the calculation method for each index were the same as in Example 1.

[0029] [Table 4]

[0030] (Discussion of the results of Example 4) FIG. 5 is a graph showing the change over time in the solid phase structure of Samples 7, 8, and 9 of Example 4. From the results of FIG. 5 and Table 4, it was confirmed that even when the atmospheric temperature in the reaction process was controlled to 70 to 90°C, the proportion of calcium carbonate in the solid phase composition could be increased in a short time of 30 to 90 minutes. However, when comparing Samples 7, 8, and 9 shown in Figure 5 with Samples 4, 5, and 6 shown in Figure 4, the latter group, where the ambient temperature was 20 to 50°C, had a slightly higher proportion of calcium carbonate in the solid phase. Therefore, it was confirmed that, from the viewpoint of energy efficiency, it is not necessary to intentionally control the ambient temperature of the reaction process to a high temperature.

[0031] [Example 5] In Example 5, the effect of the carbon dioxide content of the gas was confirmed. (Exam contents) The raw material for Samples 10, 11, 12, and 13 was calcium hydroxide (commercially available product) shown in Table 5. First, water (tap water) was added to each raw material to adjust the moisture content of each raw material to 20% (WB). Then, using a paddle mixer (the same device as in Example 2), 6 kg of each raw material was stirred while supplying gas with a CO2 content of 10% for Sample 10, gas with a CO2 content of 20% for Sample 11, gas with a CO2 content of 50% for Sample 12, and gas with a CO2 content of 100% for Sample 13 at 50 L / min, respectively, for a 60-minute reaction treatment (90 minutes for Samples 10 and 11 only). Note that during the reaction treatment in Example 5, no temperature control (heating or cooling) was performed, but the temperature (ambient temperature) inside the paddle mixer was 20 to 50°C. Then, before the reaction treatment, and 15 minutes, 30 minutes, 60 minutes, and 90 minutes (only for samples 10 and 11) after the start of the reaction treatment, the moisture content of each sampled raw material was measured using an infrared moisture meter (the same device as in Example 2), and each indicator was measured using TG-DTA (the same device as in Example 1). For Samples 10, 11, 12, and 13, water was supplied to the raw materials in an amount equivalent to the amount of water that had evaporated during the measurement of the moisture content (that is, water was supplied so that the moisture content of each raw material was 20%). The measurement method by TG-DTA and the calculation method for each index were the same as in Example 1.

[0032] [Table 5]

[0033] (Discussion of the results of Example 5) FIG. 6 is a graph showing the change over time in the solid phase structure of Samples 10, 11, 12, and 13 of Example 5. From the results of Figure 6 and Table 5, it was confirmed that the higher the carbon dioxide content of the supplied gas, the faster the reaction occurs and the more quickly the proportion of calcium carbonate in the solid phase can be increased. Furthermore, when comparing Sample 12, in which the carbon dioxide content of the supplied gas was 50%, with Sample 13, in which the carbon dioxide content was 100%, no significant difference was observed, indicating that a concentration of around 50% was sufficient to achieve the effect of accelerating the reaction.

[0034] [Example 6] In Example 6, the effect of using carbide slag as a raw material was confirmed. (Exam contents) The raw material for Sample 14 was the carbide slag shown in Table 6 (calcium hydroxide content in solids: approximately 86%). First, the raw material was subjected to slurry filtration, and the moisture content of the raw material was adjusted to 37.7% (WB). Then, using a Pam Apex mixer (WB-20V manufactured by Pacific Machinery Co., Ltd.), 5 kg of raw material was stirred while supplying a gas with a CO2 content of 20% at 50 L / min, and a reaction treatment was carried out for 120 minutes. During the reaction treatment in Example 6, no drying treatment was carried out using a mixer, and the atmospheric temperature was not controlled. Then, before the reaction treatment, and 30 minutes, 60 minutes, 90 minutes, and 120 minutes after the start of the reaction treatment, the moisture content of the sampled raw material was measured using an infrared moisture meter (the same device as in Example 2), and each index was measured using TG-DTA (the same device as in Example 1). For Sample 14, no water was supplied to the raw materials during the reaction period. The measurement method by TG-DTA and the calculation method for each index were the same as in Example 1.

[0035] [Table 6]

[0036] (Discussion of the results of Example 6) FIG. 7 is a graph showing the change over time in the solid phase structure of Sample 14 of Example 6. From the results of Figure 7 and Table 6, it was confirmed that the proportion of calcium carbonate in the solid phase composition can be sufficiently increased even when carbide slag is used as a raw material. 7 and Table 6, it was confirmed that favorable results were obtained even though water was not supplied to the raw materials in the reaction step (even though the moisture content of the raw materials was not controlled to be constant). From these results, it was found that in the present invention, it is not essential to control the moisture content of the raw materials in the reaction step to be constant, but it is important that the moisture content of the raw materials in the reaction step is within a predetermined range.

Claims

1. a preparation step of preparing a raw material containing calcium hydroxide and having a moisture content of 0.3 to 40%; a reaction step of supplying a gas containing carbon dioxide to the raw materials while stirring the raw materials and maintaining a moisture content of the raw materials at 0.3 to 40%, to react the calcium hydroxide with the carbon dioxide.

2. 2. The method for producing calcium carbonate according to claim 1, wherein the moisture content of the raw materials in the reaction step is lower than the moisture content of the raw materials in the preparation step.

3. 3. The method for producing calcium carbonate according to claim 1, wherein the content of calcium hydroxide in the solid content of the raw material is 30% or more.

4. 3. The method for producing calcium carbonate according to claim 1, wherein the gas has a carbon dioxide content of 5% or more.

5. The method for producing calcium carbonate according to claim 1 or 2, characterized in that the atmospheric temperature in the reaction step is 10 to 75°C.

6. 3. The method for producing calcium carbonate according to claim 1, wherein the raw material is carbide slag, waste concrete, sludge of ready-mixed concrete, cement, or a mixture thereof.

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