Calcium carbonate production method

Abstract

The present invention addresses the problem of providing a novel calcium carbonate production method that is excellent in production efficiency. A calcium carbonate production method according to the present invention comprises: a preparation step in which a raw material containing calcium hydroxide and having a water content of 0.3-40% is prepared; and a reaction step in which a gas containing carbon dioxide is supplied to the raw material while maintaining the water content of the raw material at 0.3-40% and while stirring the raw material to thereby cause the reaction of the calcium hydroxide with the carbon dioxide. In this calcium carbonate production method, the amount of calcium hydroxide in the raw material is preferably 30% or more.

Description

Calcium carbonate manufacturing method

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

[0002] In recent years, in order to achieve carbon neutrality, ２ (carbon dioxide) as a resource and make effective use of it. ２ Efforts are underway to promote "carbon recycling" to reduce carbon dioxide emissions. As disclosed in Patent Document 1, the applicant of the present application has proposed a hydraulic composition containing a predetermined amount of calcium carbonate. This hydraulic composition reduces CO2 emissions that would otherwise be associated with a reduction in the amount of cement used, while maintaining material properties such as strength. ２ Not only can it reduce CO emissions, but it also produces calcium carbonate. ２ It can also be immobilized inside concrete.

[0003] Various techniques have been proposed for producing calcium carbonate used in Patent Document 1. For example, Patent Document 2 proposes a method of producing light calcium carbonate by mixing seed crystals with one or both of an aqueous suspension of calcium hydroxide and a partially carbonated aqueous suspension of calcium hydroxide, and then blowing carbon dioxide or a carbon dioxide-containing gas into the resulting aqueous suspension to carbonate it, wherein the mixing is carried out by shear stirring at a peripheral speed of 7 m / s or more. Patent Document 3 also proposes a method of adding an alkaline agent to waste seawater obtained after producing magnesium hydroxide from seawater to adjust the pH to greater than 11 and equal to or less than 13, and reacting a carbon dioxide-containing gas with the calcium component of the waste seawater for a predetermined time period that increases with the pH of the waste seawater to which the alkaline agent has been added.

[0004] International Publication No. 2021 / 256484 Publication JP 11-11941 Publication Patent No. 7138256

[0005] The calcium carbonate production methods of Patent Documents 2 and 3 are both methods of blowing a gas containing carbon dioxide into a solution containing calcium ions (so-called liquid phase methods). Therefore, the calcium carbonate production methods of Patent Documents 2 and 3 require a large amount of water to disperse the raw materials, and may also require 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 calcium carbonate production method that differs from the liquid phase methods of Patent Documents 2 and 3. However, the present inventors believed that even if a new calcium carbonate production method was proposed, it would not be adopted in practice unless it had 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 (carbon dioxide emission) 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.

[0007] The above-mentioned 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%, thereby reacting the calcium hydroxide with the carbon dioxide. According to the present invention, by controlling the moisture contents of the raw material to be within predetermined ranges in the preparation step and the reaction step, respectively, calcium hydroxide and carbon dioxide react appropriately, thereby efficiently producing calcium carbonate. Furthermore, in the method for producing calcium carbonate according to the present invention, it is preferable that the moisture content of the raw material in the reaction step is lower than the moisture content of the raw material in the preparation step. Furthermore, in the method for producing calcium carbonate according to the present invention, it is preferable that the calcium hydroxide content of the solid content of the raw material is 30% or more. Furthermore, in the method for producing calcium carbonate according to the present invention, it is preferable that the carbon dioxide content of the gas is 5% or more. Furthermore, in the method for producing calcium carbonate according to the present invention, it is preferable that the atmospheric temperature in the reaction step is 10 to 75°C. In addition, in the method for producing calcium carbonate according to the present invention, the raw material is preferably carbide slag, waste concrete, sludge of ready-mixed concrete, cement, or a mixture thereof. According to the present invention, it is possible to more reliably achieve excellent production efficiency of calcium carbonate.

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

[0009] 1 is a graph showing the change over time in the solid phase constitution and moisture content of sample 1 of Example 1. FIG. 2 is a graph showing the change over time in the solid phase constitution and moisture content of sample 2 of Example 1. FIG. 3 is a graph showing the change over time in the solid phase constitution and moisture content of sample 3 of Example 2. FIG. 4 is a graph showing the change over time in the solid phase constitution of samples 4, 5, and 6 of Example 3. FIG. 5 is a graph showing the change over time in the solid phase constitution of samples 7, 8, and 9 of Example 4. FIG. 6 is a graph showing the change over time in the solid phase constitution of samples 10, 11, 12, and 13 of Example 5. FIG. 7 is a graph showing the change over time in the solid phase constitution of sample 14 of Example 6.

[0010] Hereinafter, an embodiment for carrying out the method for producing calcium carbonate according to the present invention (a method for producing calcium carbonate according to the present embodiment) will be described. The method for producing calcium carbonate according to the present 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 Step] In the preparation step, raw materials to be used in the reaction step described below are prepared. (Preparation Step: Raw Materials) The raw materials prepared in the preparation step contain calcium hydroxide and have a moisture content within a predetermined range. The calcium hydroxide content in the solid content of the raw materials (solid content when the raw materials are 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. By having a calcium hydroxide content equal to or greater than a predetermined value, the calcium carbonate production efficiency 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% (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 materials are not particularly limited as long as they satisfy 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 step: 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-mentioned range. Examples of dehydration treatment include thickeners, filter presses, centrifugation, gravity settling, heat drying, and natural drying. Dehydration may be performed using one or more of these treatments, but from the viewpoints of energy efficiency and work efficiency, thickeners and filter presses are preferred. When 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-mentioned 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 solutions separated by the above-mentioned dehydration treatment.

[0013] [Reaction Step] 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 Moisture Content) The moisture content of the raw material 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 adjusting the moisture content of the raw material in the reaction step within a predetermined range, the carbonation reaction of calcium hydroxide proceeds appropriately, and the amount of calcium carbonate produced within a predetermined time increases. In the reaction step, "Ca(OH) ２ +CO ２ →CaCO ３ +H ２ As the reaction of "O" progresses, heat of reaction is generated, causing the temperature of the raw materials to rise. As a result, water evaporates from the raw materials, gradually decreasing the moisture content of the raw materials. Therefore, the raw materials may 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 (e.g., 5 to 40 minute intervals, 15 to 30 minute intervals), using TG-DTA. Alternatively, a predetermined amount of the raw materials may be sampled during the reaction step and calculated based on the change in mass before and after a moisture meter or heating and drying (e.g., heating at 105°C or higher for 1 hour or more). Then, based on the calculated moisture content of the raw materials, hydration may be performed as appropriate. Alternatively, hydration may be performed as appropriate based on the rate of decrease in the moisture content of the raw materials obtained in advance. The water used in the hydration treatment is as described above for the preparation step.

[0014] (Reaction Step: Gas) The gas used in the reaction step contains carbon dioxide. As the gas, exhaust gas from a boiler or the like (having a carbon dioxide concentration of about 10%), carbon dioxide gas highly concentrated by a concentrator, etc. can also be used. From the viewpoint of the production rate of calcium carbonate, the carbon dioxide content in the gas is sufficient as long as it is 5% (specifically, v / v %) or more. If the carbon dioxide content in the gas is 5% or more, the carbon dioxide 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 production rate of calcium carbonate, a higher carbon dioxide content in the gas is preferable, but based on the results of Figure 6 and the like 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, the exhaust gas may be used as it is, or it may be possible to efficiently transport the exhaust gas and / or to produce high-concentration, high-purity CO 2 . ２ Concentrated CO for use in industries requiring ２ Gas may also be used. The gas supply rate may be appropriately set 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 is used in an amount of 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 when carried out under the above conditions, it is, for example, 10 to 130 minutes (preferably 60 to 120 minutes).

[0015] (Reaction Step: 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 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 with 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 (e.g., 20 to 50°C) without treatment such as heating is preferred.

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

[0017] [Example 1] In Example 1, the influence of replenishing water evaporated during the reaction process was confirmed. (Test details) The raw materials for Samples 1 and 2 were 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 calcium carbide is reacted with water to produce acetylene. First, slurry filtration was performed on each raw material, and the water content of the raw material was adjusted to 44.3% (WB). Then, the samples were placed on a tray in the incubator of a carbonation acceleration tester (MIT-639-3-05 manufactured by Marui Co., Ltd.) at a temperature of 50°C, RH of 60%, and CO ２Each raw material was allowed to stand for 5 hours under the condition of a concentration of 5%. Then, every hour from the start of the standing period, a measurement sample was taken from each sample, and the taken sample was 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 in an amount equivalent to the amount of water evaporated during TG-DTA sample collection (i.e., water was supplied so that the moisture content of the raw material was 44.3%) and mixed into the entire 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 based on the change in mass before and after drying after placing the sample taken from each sample in a dryer (drying temperature 105°C) and drying for 24 hours or more.

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

[0019] (Calculation method for each index) Using the measurement results of TG-DTA, the indexes shown in the table were calculated by the following method. ２ (%: anhydrous equivalent) = "CH loss (%)" / 18 × 74 × 100 / (100 - "amount of water below 300°C (%)") · "CaCO ３ (%: anhydride equivalent)” = “CaCO ３ Weight loss (%) / 44 x 100 x 100 / (100 - Amount of water below 300°C (%)) Other (%: anhydrous equivalent) = 100 - Ca(OH) ２ (%: anhydride equivalent)” - “CaCO ３ (%: Anhydrous equivalent)" ・"CaO meter (%: Anhydrous equivalent)" = "Ca(OH) ２ (%: anhydride equivalent)” / 74×56+”CaCO ３ (%: anhydride equivalent)” / 100×56 ・“Ca(OH) ２ (%: initial CaO equivalent value) = "Ca(OH) ２(%: anhydrous equivalent)) / (CaO total (%: anhydrous equivalent)) x (CaO total of raw materials before reaction (%: anhydrous equivalent)) ３ (%: initial CaO amount equivalent) = "CaCO ３ (%: anhydrous equivalent)) / (CaO total (%: anhydrous equivalent)) x (CaO total of raw materials before reaction (%: anhydrous equivalent)) - "Other (%: initial CaO equivalent)" = "Other of raw materials before reaction (%: anhydrous equivalent)" - "Total (%: initial CaO equivalent)" = "Ca(OH) ２ (%: initial CaO amount equivalent) + CaCO ３ "(%: initial CaO equivalent)" + "Other (%: initial CaO equivalent)" Here, "anhydrous equivalent" refers to the content (%) of each component when the solid content after removing water (such as adhered water) removed by heating up to 300°C is taken as 100%. Also, "initial CaO equivalent" refers to the content (%) of each component based on the initial (before reaction) CaO amount, and is the amount of Ca(OH) ２ How much CaO was present as CaCO after the reaction? ３ The "amount of water below 300°C" in the table is the amount of water removed by heating up to 300°C. The indices used in each figure are those of the above-mentioned indices, such as "Ca(OH) ２ (%: initial CaO equivalent value)," "CaCO ３ (%: value converted into the initial CaO amount)," "Other (%: value converted into the initial CaO amount)," and moisture content.

[0020]

[0021] (Consideration of the results of Example 1) FIG. 1 is a graph showing the change over time in the solid phase composition and moisture content of Sample 1 of Example 1. FIG. 2 is a graph showing the change over time in the solid phase composition and moisture content of Sample 2 of Example 1. From the results of FIG. 1 and Table 1, Sample 1, to which water was supplied in the reaction step, had a consistently high moisture content of the raw materials (over 40%), so the reaction of calcium hydroxide did not occur and the proportion of calcium carbonate in the solid phase composition was very small. On the other hand, from the results of FIG. 2 and Table 1, for Sample 2, to which water was not supplied in the reaction step, the proportion of calcium carbonate in the solid phase composition increased significantly after 2 hours had passed, when the moisture content of the raw materials began to decrease. These results confirmed that the moisture content of the raw materials in the reaction step has a significant impact on the amount of calcium carbonate produced. Note that after removing adherent water in anhydrous form, the initial CaO equivalent value is based on the amount of CaO before the start of the reaction, and is compared by converting to this amount at each reaction time. Therefore, it is possible to determine whether CO was produced by the reaction. ２ The content of each component at each reaction time exceeds 100% due to the amount of absorbed water.

[0022] [Example 2] In Example 2, the influence of carrying out a drying treatment in the reaction process was confirmed. (Test details) The raw material for Sample 3 was calcium hydroxide (commercially available product) shown in Table 2. First, water (tap water) was added to the raw material to adjust the water content of the raw material to 40.0% (WB). Then, CO was added to the raw material while stirring approximately 6 kg of the raw material using a paddle mixer (NARA MACHINERY MFG. CO., LTD., NPD-1.6W-12L-G). ２A 60-minute reaction was carried out while supplying a gas containing 20% ​​of the compound at 250 L / min. During the reaction in Example 2, drying was carried out using a 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, manufactured by Kett Electric Laboratory, drying temperature 105°C), and various indices such as the amount of calcium hydroxide and the amount of calcium carbonate were measured using TG-DTA (the same apparatus as in Example 1). For Sample 3, no water was supplied to the raw materials during the reaction period. The measurement method using TG-DTA and the calculation method for each indices were the same as in Example 1.

[0023]

[0024] (Consideration of the results of Example 2) Figure 3 is a graph showing the change over time in the solid phase composition and moisture content of Sample 3 of Example 2. From the results of 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 in the reaction step. Furthermore, when the results of Sample 3 shown in Figure 3 and Sample 2 shown in Figure 2 are compared, 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 thought that there was room for further improvement in order to further increase the proportion of calcium carbonate in the solid phase composition of the product.

[0025] [Example 3] In Example 3, the influence of the moisture content of the raw materials was confirmed. (Test details) The raw materials for Samples 4, 5, and 6 were 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 set to 10% (WB), the moisture content of the raw material for Sample 5 was set to 20% (WB), and the moisture content of the raw material for Sample 6 was set to 30% (WB). Then, using a paddle mixer (the same device as in Example 2), 6 kg of each raw material was stirred while CO ２A 90-minute reaction was carried out while supplying a gas containing 20% ​​of the hydroxybenzoate at 50 L / min. During the reaction 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. Before the reaction and 30, 60, and 90 minutes after the start of the reaction, the moisture content of each sampled raw material was measured using an infrared moisture meter (the same apparatus as in Example 2), and each index was measured using a 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 equivalent to the amount of water evaporated during the moisture content measurement (i.e., water was supplied so that the moisture content of the raw material was 10% for Sample 4, 20% for Sample 5, and 30% for Sample 6). The measurement method using TG-DTA and the calculation method for each index were the same as in Example 1.

[0026]

[0027] (Consideration of the Results of Example 3) Figure 4 is a graph showing the change over time in the solid-phase composition of Samples 4, 5, and 6 of Example 3. From the results of Figure 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 period of time, such as 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 large, and excellent production efficiency can be achieved. Furthermore, while good results were obtained for Samples 4, 5, and 6, particularly for Samples 4 and 5, very favorable results were obtained. Note that the value in parentheses in the lower row of the moisture content in Table 3 is the moisture content of the raw materials immediately 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 ambient temperature in the reaction process was confirmed. (Test details) The raw materials for Samples 7, 8, and 9 were 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 set to 10% (WB), the moisture content of the raw material for Sample 8 was set to 20% (WB), and the moisture content of the raw material for Sample 9 was set to 30% (WB). Then, using a paddle mixer (the same device as in Example 2), 6 kg of each raw material was stirred while CO ２ A 90-minute reaction was carried out while supplying a gas containing 20% ​​of the compound at 50 L / min. During the reaction in Example 4, the temperature (ambient temperature) inside the paddle mixer was controlled to be approximately 70 to 90°C. Before the reaction and 30, 60, and 90 minutes after the start of the reaction, the moisture content of each sampled raw material was measured using an infrared moisture meter (the same apparatus as in Example 2), and each index was 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 equivalent to the amount of water evaporated during the moisture content measurement (i.e., water was supplied so that the moisture content of the raw material was 10% for Sample 7, 20% for Sample 8, and 30% for Sample 9). The measurement method using TG-DTA and the calculation method for each index were the same as in Example 1.

[0029]

[0030] (Consideration of the results of Example 4) Figure 5 is a graph showing the change over time in the solid-phase composition of Samples 7, 8, and 9 of Example 4. From the results of Figure 5 and Table 4, it was confirmed that even when the ambient temperature of the reaction step was controlled to 70 to 90°C, the proportion of calcium carbonate in the solid-phase composition could be increased in a short period 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, in which the ambient temperature was 20 to 50°C, had a slightly higher proportion of calcium carbonate in the solid-phase composition. Therefore, it was confirmed that, from the viewpoint of energy efficiency, it is not necessary to intentionally control the ambient temperature of the reaction step to a high temperature.

[0031] [Example 5] In Example 5, the influence of the carbon dioxide content of the gas was confirmed. (Test details) The raw materials for Samples 10, 11, 12, and 13 were calcium hydroxide (commercially available product) shown in Table 5. First, water (tap water) was added to each raw material to adjust the water 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 CO ２ For sample 11, the gas contained 10% of CO ２ For sample 12, the gas contained 20% of CO. ２ For sample 13, the gas contained 50% of CO. ２ A 60-minute reaction (90 minutes for Samples 10 and 11 only) was carried out while supplying a gas containing 100% of the ammonium hydroxide at 50 L / min. During the reaction 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. Before the reaction and 15, 30, 60, and 90 minutes after the start of the reaction (Samples 10 and 11 only), the moisture content of each sampled raw material was measured using an infrared moisture meter (the same apparatus as in Example 2), and each index was measured using TG-DTA (the same apparatus 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 evaporated during the moisture content measurement (i.e., water was supplied so that the moisture content of each raw material was 20%). The measurement method using TG-DTA and the calculation method for each index were the same as in Example 1.

[0032]

[0033] (Consideration of the results of Example 5) Figure 6 is a graph showing the change over time in the solid phase composition 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 more rapidly the reaction occurs and the proportion of calcium carbonate in the solid phase composition can be increased in a shorter time. 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 confirmed, and it was found that a carbon dioxide content of about 50% was sufficient to exert the effect of accelerating the reaction.

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

[0035]

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

Claims

1. A method for producing calcium carbonate, comprising: 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.

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. The method for producing calcium carbonate according to claim 1 or 2, characterized in that the content of calcium hydroxide in the solid content of the raw material is 30% or more.

4. The method for producing calcium carbonate according to claim 1 or 2, characterized in that the carbon dioxide content in the gas is 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. A method for producing calcium carbonate according to claim 1 or 2, characterized in that the raw material is carbide slag, waste concrete, sludge of ready-mixed concrete, cement, or a mixture thereof.

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