Method for producing calcium carbonate, calcium carbonate, method for producing ready-mixed mortar or ready-mixed concrete, method for improving ground, method for filling underground cavity, and method for immobilizing carbon dioxide

Abstract

The purpose of the present invention is to provide a new method for producing, from carbide slag which hitherto has been mostly discarded, calcium carbonate that, when mixed with ready-mixed concrete, does not deteriorate liquidity of the ready-mixed concrete or deteriorate strength of a cured product.　The present invention pertains to a method for producing calcium carbonate, the method including: a classification step for classifying carbide slag containing calcium hydroxide and hydrophobic impurities and removing the hydrophobic impurities; an adjustment step for adjusting the solid content concentration of the carbide slag having the hydrophobic impurities removed in the classification step; and a carbon dioxide introduction step for introducing carbon dioxide into the carbide slag having the hydrophobic impurities removed in the classification step and having the solid content concentration adjusted in the adjustment step. The present invention further pertains to: calcium carbonate produced by said method for producing calcium carbonate; and a method for using the calcium carbonate produced by said method for producing calcium carbonate.

Description

Method for producing calcium carbonate, method for producing calcium carbonate, ready-mixed mortar or ready-mixed concrete, method for improving ground, method for filling underground cavities, and method for fixing carbon dioxide

[0001] The present invention relates to a method for producing calcium carbonate and calcium carbonate.Furthermore, the present invention relates to a method for producing ready-mixed mortar or ready-mixed concrete using calcium carbonate.The present invention relates to a method for improving ground and a method for filling underground cavities using calcium carbonate.Furthermore, the present invention relates to a method for immobilizing carbon dioxide.

[0002] Calcium carbide (CaC) used in the industrial production of acetylene ２ ) is obtained by heating a mixture of calcium oxide and coke to approximately 2000°C. Calcium carbide is known to contain impurities derived from the raw materials, such as calcium phosphide and sulfur. Calcium carbide can be reacted with water to produce acetylene. The by-product of this reaction, carbide slag (also called carbide lime, carbide sludge, carbide slurry, etc., is mainly composed of calcium hydroxide and Ca(OH) ２ ) is an aqueous slurry containing a variety of impurities. Carbide slag is regulated as a type of industrial waste sludge (Article 2, Paragraph 4 of the Industrial Waste Law, Article 2 of the Cabinet Order), and must be properly treated before disposal. Carbide slag is sometimes used as a neutralizer for acidic soil as a substitute for slaked lime (calcium hydroxide), but most of it is not effectively utilized.

[0003] Patent Document 1 discloses a method for producing precipitated calcium carbonate by granulating carbide slag, charging the granulated slag into a rotary kiln, blowing carbon monoxide and oxygen into the rotary kiln, and carbonating the granulated slag at a temperature of 500 to 900°C. Patent Document 2 discloses a method for producing calcium carbonate from lime such as carbide lime, comprising: step (i) of providing an aqueous solution containing 10% to 35% by weight of a dissolved polyhydroxy compound and 1% to 5% by weight of dissolved calcium hydroxide and having a pH of at least 11.5; step (ii) of treating the solution prepared in step (i) to remove solids including suspended matter; step (iii) of dispersing carbon dioxide in the solution to form calcium carbonate with a resulting decrease in the pH of the reaction mixture; step (iv) of adding an alkaline reagent to terminate the dispersion of carbon dioxide and maintain the pH of the product mixture at at least 9.5 during a period beginning at the start of a short pH spike and ending during the subsequent decrease in pH but before the pH reaches 9.5; and step (v) of recovering the precipitated calcium carbonate. Patent Document 2 discloses that the polyhydroxy compound in step (i) is glycerol, a sugar alcohol, a sugar, or the like, and that step (ii) involves removing suspended matter using a flocculant such as a cationic polymer.

[0004] Meanwhile, attempts have been made to increase the strength of concrete by adding calcium carbonate to ready-mixed concrete, ready-mixed mortar, etc. Patent Document 3 discloses an admixture for high-strength concrete containing light calcium carbonate powder having an average particle size of 0.19 μm to 0.33 μm and a pH of 8.5 to 11. Patent Document 4 discloses an admixture slurry for high-strength concrete containing water, a water-reducing agent (polycarboxylic acid type), and light calcium carbonate powder having an average particle size of 0.1 μm to 0.4 μm and a pH of 8.5 to 11, with the light calcium carbonate powder comprising 60% by mass to 75% by mass.

[0005] It is known that it is difficult to achieve both the fluidity of the fresh concrete and the strength of the hardened product when dispersing a mineral admixture in fresh concrete, but Patent Documents 3 and 4 describe that an admixture containing calcium carbonate powder or its slurry can be dispersed in fresh concrete at a relatively high concentration and that the decrease in the fluidity of the fresh concrete can be somewhat suppressed. However, there is a constant demand for the development of ready-mix concrete with further improved workability (the overall construction characteristics of concrete, such as mixing, transportation, pouring, compaction, and finishing).

[0006] JP-A-54-43897 Publication Special Publication No. 2013-506613 Publication Patent No. 6011926 Publication Patent No. 6011927

[0007] Therefore, an object of the present invention is to provide a novel method for producing calcium carbonate from carbide slag, most of which has been discarded in the past, which, when mixed with ready-mixed concrete, does not reduce the fluidity of the concrete or the strength of the hardened product. Another object of the present invention is to provide a method for producing calcium carbonate-blended ready-mixed mortar or ready-mixed concrete using the produced calcium carbonate, a method for improving ground by injecting a ground improvement material containing the produced calcium carbonate into the ground, and a method for filling underground cavities with a filler containing the produced calcium carbonate. Another object of the present invention is to provide a method for producing industrially usable calcium carbonate from carbide slag, which has been discarded in the past, while also immobilizing carbon dioxide contained in exhaust gases, etc., as calcium carbonate.

[0008] One aspect of the present invention is a method for producing calcium carbonate, comprising the following steps: a classification step of classifying carbide slag containing calcium hydroxide and hydrophobic impurities to remove the hydrophobic impurities; an adjustment step of adjusting the solids concentration of the carbide slag from which the hydrophobic impurities have been removed in the classification step; and a carbon dioxide introduction step of introducing carbon dioxide into the carbide slag from which the hydrophobic impurities have been removed in the classification step and whose solids concentration has been adjusted in the adjustment step.

[0009] Here, it is preferable that the particle diameter of the hydrophobic impurities removed in the classification step is 45 μm or more.

[0010] In the adjusting step, it is preferable to adjust the solid content concentration of the carbide slag from which hydrophobic impurities have been removed to 1 to 30 mass %.

[0011] In the carbon dioxide introduction step, it is preferable to introduce carbon dioxide into the carbide slag from which hydrophobic impurities have been removed and the solid content concentration has been adjusted so that the carbonation rate of calcium ions is 1 to 40 cation % / hour.

[0012] Preferably, a calcite-type calcium carbonate having a spindle shape is produced.

[0013] A second aspect of the present invention is calcium carbonate produced by the above-mentioned production method.A third aspect of the present invention is a method for producing calcium carbonate-blended ready-mixed mortar or ready-mixed concrete by classifying carbide slag containing calcium hydroxide and hydrophobic impurities to remove the hydrophobic impurities; adjusting the solids concentration of the carbide slag from which the hydrophobic impurities have been removed; introducing carbon dioxide into the carbide slag from which the hydrophobic impurities have been removed and whose solids concentration has been adjusted to obtain calcium carbonate; and mixing the obtained calcium carbonate with ready-mixed mortar or ready-mixed concrete.

[0014] A fourth aspect of the present invention is a method for improving ground by classifying carbide slag containing calcium hydroxide and hydrophobic impurities to remove the hydrophobic impurities; adjusting the solids concentration of the carbide slag from which the hydrophobic impurities have been removed; introducing carbon dioxide into the carbide slag from which the hydrophobic impurities have been removed and whose solids concentration has been adjusted to obtain calcium carbonate; mixing the obtained calcium carbonate with a ground improvement material; and mixing or injecting the obtained calcium carbonate-mixed ground improvement material into the ground.

[0015] A fifth aspect of the present invention is a method comprising: classifying carbide slag containing calcium hydroxide and hydrophobic impurities to remove the hydrophobic impurities; adjusting the solids concentration of the carbide slag from which the hydrophobic impurities have been removed; introducing carbon dioxide into the carbide slag from which the hydrophobic impurities have been removed and the solids concentration has been adjusted to obtain calcium carbonate; mixing the obtained calcium carbonate with a filler; and filling the obtained calcium carbonate-blended filler into an underground cavity.

[0016] A sixth aspect of the present invention is a method comprising: classifying carbide slag containing calcium hydroxide and hydrophobic impurities to remove the hydrophobic impurities; adjusting the solids concentration of the carbide slag from which the hydrophobic impurities have been removed; and introducing an exhaust gas containing carbon dioxide into the carbide slag from which the hydrophobic impurities have been removed and the solids concentration has been adjusted, thereby immobilizing the carbon dioxide contained in the exhaust gas.

[0017] Calcium carbonate, which does not reduce the fluidity of ready-mixed concrete when mixed with it and does not reduce the strength of the hardened concrete, can be produced from carbide slag, most of which has traditionally been discarded. The calcium carbonate produced in this way can be mixed with ready-mixed mortar or ready-mixed concrete to build structures, or mixed with soil improvement materials or underground cavity fillers to improve the ground or backfill underground cavities. Furthermore, industrially usable calcium carbonate can be produced from waste carbide slag and waste exhaust gas, and carbon dioxide gas in the exhaust gas can be immobilized.

[0018] 1 is a scanning electron microscope photograph (magnification: 10,000 times) of calcium carbonate obtained in Example 2. FIG. 2 is a scanning electron microscope photograph (magnification: 20,000 times) of calcium carbonate obtained in Example 2.

[0019] The embodiments of the present invention will be described in more detail below, but the present invention is not limited to the following embodiments.

[0020] One embodiment is a method for producing calcium carbonate, comprising the following steps: a classification step of classifying carbide slag containing calcium hydroxide and hydrophobic impurities to remove the hydrophobic impurities; an adjustment step of adjusting the solid content concentration of the carbide slag from which the hydrophobic impurities have been removed in the classification step; and a carbon dioxide introduction step of introducing carbon dioxide into the carbide slag from which the hydrophobic impurities have been removed in the classification step and whose solid content concentration has been adjusted in the adjustment step.

[0021] In one embodiment, the carbide slag is mainly calcium carbide (calcium carbide, CaC ２ ) is the residue obtained after the industrial production of acetylene, and is an aqueous slurry. Carbide slag is a mixture of calcium hydroxide (Ca(OH) ２) and hydrophobic impurities. The hydrophobic impurities contained in the carbide slag are mainly coke residues, and may also contain sulfur, silicon carbide, etc. The hydrophobic impurities refer to all impurities other than calcium hydroxide that are present in the aqueous carbide slag without dissolving. One embodiment includes a step of classifying the carbide slag. This classification step is a step of removing the hydrophobic impurities contained in the carbide slag by a classification operation. The particle sizes of calcium hydroxide particles, which are the main component of the carbide slag, and the hydrophobic impurity particles are different, and the particle size of the hydrophobic impurities is generally larger. Therefore, the hydrophobic impurities can be removed from the carbide slag by a known classification operation (such as filtration using filter paper, filter cloth, or a filter, or classification using a classification device). The particle size of the hydrophobic impurities removed in the classification step is preferably 45 μm or more. Increasing the lower limit of the particle size of the hydrophobic impurities removed from the carbide slag (e.g., setting the lower limit to 200 μm) increases the amount of hydrophobic impurities remaining in the carbide slag, and therefore the hydrophobic impurities are more likely to remain in the calcium carbonate finally produced in one embodiment. Furthermore, decreasing the lower limit of the particle size of the hydrophobic impurities removed from the carbide slag (e.g., setting the lower limit to 10 μm) can almost completely remove the hydrophobic impurities from the carbide slag, but calcium hydroxide particles are also removed, resulting in a lower yield of the calcium carbonate finally produced in one embodiment. The particle sizes of the calcium hydroxide and hydrophobic impurities are values ​​obtained by either image analysis using an electron microscope or an optical microscope or by measuring the wet sieve residue, and the values ​​are the same regardless of the measurement method. The separation process of removing hydrophobic impurities from the carbide slag is a very important process in producing calcium carbonate to be mixed with ready-mixed mortar or ready-mixed concrete. If the carbon dioxide introduction step described below is carried out without removing the hydrophobic impurities from the carbide slag, the hydrophobic impurities will remain in the finally obtained calcium carbonate. In this case, the surfaces of the calcium carbonate particles will be water-repellent, making it difficult for the calcium carbonate to be mixed into raw mortar or raw concrete.

[0022] One embodiment includes an adjusting step of adjusting the solid content concentration of the carbide slag from which hydrophobic impurities have been removed in the classification step. The adjusting step is a step of adjusting the solid content concentration of the carbide slag. Here, the solid content refers to all components other than water contained in the carbide slag after the hydrophobic impurities have been removed. The carbide slag from which hydrophobic impurities have been removed in the classification step mainly contains calcium hydroxide particles having a particle size of less than 45 μm as solid content, and a trace amount (5 mass% or less) of hydrophobic impurities may remain. The solid content concentration refers to the mass ratio of all components other than water to the mass of the carbide slag from which hydrophobic impurities have been removed. In the adjusting step, the solid content concentration of the carbide slag from which hydrophobic impurities have been removed can be adjusted to 1-30 mass%, preferably 5-20 mass%, and more preferably 7-10 mass%. If the solids concentration of the carbide slag from which hydrophobic impurities have been removed is higher than the desired range, water can be added to the carbide slag from which hydrophobic impurities have been removed. If the solids concentration of the carbide slag from which hydrophobic impurities have been removed is lower than the desired range, calcium hydroxide can be added to the carbide slag from which hydrophobic impurities have been removed, or the solids of the carbide slag from which hydrophobic impurities have been removed can be allowed to settle over time, and then a portion of the supernatant liquid can be removed. By adjusting the solids concentration to an appropriate range in the adjustment step, the fluidity of the carbide slag (slurry) from which hydrophobic impurities have been removed can be maintained, thereby improving the reaction efficiency in the carbon dioxide introduction step described below. If the solids concentration of the carbide slag from which hydrophobic impurities have been removed is too high or too low, the properties of the calcium carbonate produced in one embodiment may be unsuitable for mixing with ready-mixed mortar or ready-mixed concrete. The desired properties and characteristics of the calcium carbonate produced in one embodiment will be described in detail later.

[0023] One embodiment includes a carbon dioxide introduction step of introducing carbon dioxide into the carbide slag from which hydrophobic impurities have been removed in the classification step and whose solid content has been adjusted in the adjustment step. This step is a step of synthesizing calcium carbonate by reacting carbon dioxide with calcium hydroxide contained in the carbide slag. In the carbon dioxide introduction step, when carbon dioxide is introduced into the carbide slag from which hydrophobic impurities have been removed in the classification step and whose solid content has been adjusted in the adjustment step, it is preferable to introduce carbon dioxide while adjusting the carbon dioxide so that the carbonation rate of calcium ions is 1 to 40 cation % / hour. A calcium ion carbonation rate of 1 to 40 cation atom % / hour means that 1 to 40% of the total calcium ions (cations) of calcium hydroxide contained in the carbide slag from which hydrophobic impurities have been removed in the classification step and whose solid content has been adjusted in the adjustment step reacts and converts to calcium carbonate per hour. In other words, a carbonation rate of 1 cation % / hour means that it takes 100 hours for calcium hydroxide to react and convert to calcium carbonate. A carbonation rate of 40 cation % / hour means that the conversion reaction from calcium hydroxide to calcium carbonate is completed in 2.5 hours. Carbon dioxide can be introduced into the carbide slag from which hydrophobic impurities have been removed in the classification step and whose solid content has been adjusted in the adjustment step, so that the carbonation rate becomes preferably 5 to 30 cation % / hour, more preferably 10 to 20 cation % / hour.

[0024] The temperature in the carbon dioxide introduction step, particularly the reaction initiation temperature of the carbide slag from which hydrophobic impurities have been removed in the classification step and whose solid content has been adjusted in the adjustment step, before carbon dioxide is introduced, is important. The reaction initiation temperature can be set to about 8 to 60°C, preferably about 15 to 50°C. If the reaction initiation temperature is too low or too high, the properties of the calcium carbonate produced in one embodiment will be deteriorated. The carbon dioxide introduction step is preferably carried out so as to constantly maintain the temperature within the above range.

[0025] In one embodiment, calcium carbonate is produced by the formula CaCO ３and is a calcium carbonate represented by the formula (I) and is a main component of seashells, eggshells, limestone, chalk, etc. Calcium carbonate is classified into heavy calcium carbonate (natural calcium carbonate) obtained by crushing and classifying limestone, and light calcium carbonate (synthetic calcium carbonate) obtained by chemical reaction. The calcium carbonate produced in one embodiment is light calcium carbonate. Calcium carbonate exists in crystal polymorphs such as calcite crystals (trigonal rhombohedral crystals), aragonite crystals (orthorhombic crystals), and vaterite crystals (hexagonal crystals). The calcium carbonate produced in one embodiment is a calcite-type crystal, and preferably has a spindle shape.

[0026] The calcium carbonate produced in the first embodiment can be used in a wide range of applications, similar to conventional calcium carbonate. The calcium carbonate produced by the manufacturing method of the first embodiment has various impurity ratios and crystal forms, depending on the properties of the carbide slag used as a raw material. Therefore, it is difficult to clearly define the calcium carbonate of the second embodiment of the present invention, which is produced by the manufacturing method of the first embodiment, as a substance. Calcium carbonate can be used, for example, for neutralizing wastewater and flue gas desulfurization, as well as for use as a filler in building materials, concrete, fertilizer, paint, etc. The calcium carbonate obtained in the first embodiment can be effectively used, particularly as an additive to ready-mixed mortar and ready-mixed concrete, and is intended to be used for ground improvement and filling underground cavities. Ready-mixed concrete is a composite material obtained by mixing and kneading granular aggregates such as sand and gravel, hydraulic cement, binders including finely divided slag powder, fly ash, etc., water, and various admixtures. The recently demanded ready-mixed concrete for high-strength concrete has a low water-binder ratio (weight ratio of water to binder), which can result in low fluidity and poor workability during construction. Furthermore, ready-mixed concrete for high-strength concrete may require long mixing times to obtain ready-mixed concrete with good workability, resulting in a significant amount of time required for concrete construction. Therefore, attempts have been made to add calcium carbonate to ready-mixed concrete to improve the fluidity of the ready-mixed concrete and shorten the mixing time.

[0027] The characteristic required for ready-mixed concrete containing calcium carbonate is generally called workability. Workability refers to the workability of concrete in construction, taking into account both the resistance of the ready-mixed concrete to plastic deformation and flow (consistency) and the resistance of the components contained in the ready-mixed concrete to separation, and is the most important indicator of the properties of ready-mixed concrete. Specifically, workability is a general indicator of the workability of concrete related to the series of operations from mixing, transportation, pouring, and compaction of the ready-mixed concrete, to finishing the concrete. The evaluation criteria vary depending on the type of structure, construction location, and construction method. In one embodiment, calcium carbonate as an additive that does not impair the ease of mixing of ready-mixed concrete, which is one of the factors that determine the workability of ready-mixed concrete, can be produced from waste carbide slag as a raw material.

[0028] A third embodiment of the present invention is a method for producing calcium carbonate-blended ready-mixed mortar or ready-mixed concrete by classifying carbide slag containing calcium hydroxide and hydrophobic impurities to remove the hydrophobic impurities; adjusting the solids concentration of the carbide slag from which the hydrophobic impurities have been removed; introducing carbon dioxide into the carbide slag from which the hydrophobic impurities have been removed and whose solids concentration has been adjusted to obtain calcium carbonate; and mixing the obtained calcium carbonate with ready-mixed mortar or ready-mixed concrete.

[0029] The third embodiment includes all steps of the manufacturing method of the first embodiment. The calcium carbonate produced in the first embodiment can be mixed with ready-mixed mortar or ready-mixed concrete to produce calcium carbonate-blended ready-mixed mortar or ready-mixed concrete. Ready-mixed mortar is a mixture of cement, water, and fine aggregate such as sand, while ready-mixed concrete is mortar mixed with coarse aggregate such as gravel. In addition to these components, ready-mixed mortar or ready-mixed concrete can also contain admixtures such as a water-reducing agent to increase the fluidity of the ready-mixed mortar or ready-mixed concrete, and a retarder to delay the setting of the ready-mixed mortar or ready-mixed concrete for a predetermined period of time. Examples of water-reducing agents include so-called anionic surfactants such as carboxyl group-containing polyethers, lignin sulfonates, and hydroxycarboxylates. Examples of retarders include agents containing hydroxycarboxylates such as sodium gluconate as a main component. Various admixtures, such as water-reducing agents and retarders for ready-mixed mortar or ready-mixed concrete, are commercially available to suit various conditions, such as the shape of the structure, the construction location, and the construction period, and can be selected from these.

[0030] The calcium carbonate-blended ready-mixed mortar or ready-mixed concrete produced by the third embodiment is easy to mix and has high workability. Furthermore, the hardened product obtained by hardening the calcium carbonate-blended ready-mixed mortar or ready-mixed concrete produced by the second embodiment has greater strength than the hardened product obtained by hardening ready-mixed mortar or ready-mixed concrete that does not contain calcium carbonate, thereby enabling the production of high-strength mortar structures or high-strength concrete structures.

[0031] A fourth embodiment is a method for improving ground by classifying carbide slag containing calcium hydroxide and hydrophobic impurities to remove the hydrophobic impurities; adjusting the solid content concentration of the carbide slag from which the hydrophobic impurities have been removed; introducing carbon dioxide into the carbide slag from which the hydrophobic impurities have been removed and the solid content concentration has been adjusted to obtain calcium carbonate; mixing the obtained calcium carbonate with a ground improvement material; and mixing or injecting the obtained calcium carbonate-mixed ground improvement material into the ground.

[0032] The fourth embodiment includes all steps of the manufacturing method of the first embodiment. The calcium carbonate manufactured in the first embodiment can be mixed with a soil improvement material, and the calcium carbonate-mixed soil improvement material can be mixed or injected into the ground to improve the ground. Soil improvement materials are materials used to improve the bearing capacity of land that cannot be used as a foundation for a building or structure in its current state when constructing a building or structure. Soil improvement materials are used to increase the strength of the ground to prevent deformation, subsidence, liquefaction, etc. of soft ground. Known ground improvement methods include spread foundations (a surface improvement method in which soil improvement materials are mixed or injected into the ground until the ground is stable) and pile foundations (a columnar improvement method in which soil improvement materials are mixed or injected into the ground until the ground is stable and columnar columns are built, and a steel pipe pile method in which steel pipe piles are injected into the ground until the ground is stable). The method of the third embodiment is particularly applicable to surface improvement methods and columnar improvement methods. Known ground improvement materials include cement, cement-based solidification materials, lime, lime-based solidification materials, cement / lime composite solidification materials, and polymer-based solidification materials. Various types of ground improvement materials are commercially available to suit various conditions such as the topography and environment of the construction site, and it is possible to obtain one from these as appropriate. The third embodiment can be applied to any type of ground improvement material. Ground improvement materials are made of almost the same materials as ready-mixed mortar or ready-mixed concrete. Although it depends on the characteristics of the ground to be improved, the amount of water mixed into ground improvement materials is generally greater than that of ready-mixed mortar or ready-mixed concrete.

[0033] By implementing the fourth embodiment, a strong ground suitable for buildings and structures can be obtained, thereby making it possible to build a stable foundation for the buildings and structures. The calcium carbonate-mixed ground improvement material used in the third embodiment is easy to mix and has high workability.

[0034] A fifth embodiment is a method including: classifying carbide slag containing calcium hydroxide and hydrophobic impurities to remove the hydrophobic impurities; adjusting the solids concentration of the carbide slag from which the hydrophobic impurities have been removed; introducing carbon dioxide into the carbide slag from which the hydrophobic impurities have been removed and the solids concentration has been adjusted to obtain calcium carbonate; mixing the obtained calcium carbonate with a filler; and filling the obtained calcium carbonate-blended filler into an underground cavity.

[0035] The fifth embodiment includes all steps of the manufacturing method of the first embodiment. The calcium carbonate produced in the first embodiment can be mixed with a filler, and the calcium carbonate-containing filler can be filled into an underground cavity. When constructing a building or structure on the ground, if an underground cavity that is not visible from the surface is discovered by a boring survey or the like, it may be necessary to backfill the underground cavity. Leaving the underground cavity unattended can lead to surface subsidence or land subsidence, so filling the underground cavity to prevent these events may be necessary. Here, underground cavities refer to all underground cavities, including natural cavities such as limestone caves, drains, lava tunnels, and weathered caves; artificial cavities such as mining sites for metal mines, coal mines, stone mines, and underground bunkers, as well as underground shopping malls, tunnels, and piping works. The filler used to backfill underground cavities is required to have fluidity that allows it to reach every corner of the cavity when filling the cavity and to become a solidified product with the required strength after solidification. Typical filling materials for such underground cavities include solidifying materials containing gypsum and lime, cement-based solidifying materials, and slag-based fillers containing molten slag or blast furnace slag and aggregate. Various types of filling materials for underground cavities are commercially available to suit the topography, environment, and other conditions of the construction site, and it is possible to obtain one from these. The fourth embodiment can be applied to any type of filling material.

[0036] By carrying out the fifth embodiment, a strong solidified material can be formed all over the underground cavity, making it possible to safely backfill the underground cavity. The calcium carbonate-blended filler used in the fourth embodiment has high fluidity, is easy to mix, and is highly workable.

[0037] A sixth embodiment is a method including: classifying carbide slag containing calcium hydroxide and hydrophobic impurities to remove the hydrophobic impurities; adjusting the solids concentration of the carbide slag from which the hydrophobic impurities have been removed; and introducing an exhaust gas containing carbon dioxide into the carbide slag from which the hydrophobic impurities have been removed and the solids concentration has been adjusted, thereby immobilizing the carbon dioxide contained in the exhaust gas.

[0038] The sixth embodiment includes, in principle, all of the steps of the manufacturing method of the first embodiment. In the first embodiment, exhaust gas is used as a carbon dioxide source for producing calcium carbonate. Exhaust gas generally refers to gases emitted from gasoline engines, diesel engines, internal combustion engines, and factories. Untreated exhaust gas typically contains large amounts of carbon dioxide, a greenhouse gas, in addition to water vapor and harmful gases such as carbon monoxide. Releasing such exhaust gas into the atmosphere without treatment is undesirable, as it leads to environmental pollution and global warming. The fifth embodiment is a method for obtaining calcium carbonate that can be used in the methods of the second, third, or fourth embodiments described above, by reacting carbon dioxide in exhaust gas using carbide slag, which was previously discarded as industrial waste. By practicing the fifth embodiment, carbon dioxide in exhaust gas is immobilized as calcium carbonate, and the obtained calcium carbonate can be used for blending in ready-mixed mortar and ready-mixed concrete, as well as for soil improvement materials, underground cavity fillers, and the like.

[0039] The following describes in detail the embodiments of the present invention, but the present invention is not limited to the following examples.

[0040] Example 1 Production of Calcium Carbonate Carbide slag obtained from Koatsu Gas Kogyo Co., Ltd. was passed through a sieve with a mesh diameter of 100 μm (Japanese Industrial Standard JIS Z 8801, company name: Iida Seisakusho Co., Ltd.) to remove particles with a diameter of 100 μm or more from the carbide slag. The solid content concentration of the obtained carbide slag was adjusted to 10%. Next, the temperature of the obtained carbide slag was raised to 28°C, and the carbide slag was stirred while introducing carbon dioxide gas so that the carbonation rate of calcium ions became 12 cation % / hour (so that the time until completion of the reaction was 8.1 hours). The obtained slurry was filtered and dried in an oven at 105°C for 1 hour, yielding spindle-shaped calcite-type calcium carbonate (BET specific surface area measured according to the method according to JIS Z 8830: 5.9 m). ２ / g).

[0041] <Production of Ready-Mixed Concrete> Ordinary Portland cement (Japanese Industrial Standard JIS R 5210, density: 3.14 to 3.17 g / cm ３ ), blast furnace slag ground powder 4000 (Japanese Industrial Standard JIS A 6206, density: 2.8 g / cm ３ or more), crushed sand with a particle size of 5 mm or less as fine aggregate (Japanese Industrial Standards JIS A 5005, produced in Joyo City, Kyoto Prefecture, density: 2.5 g / cm ３ or more), and crushed stone with a particle size of 5 to 15 mm (Japanese Industrial Standards JIS A 5005, produced in Kameoka City, Kyoto Prefecture, density: 2.5 g / cm ３ or more), and as admixture A, a high-performance water-reducing agent (Type I) containing a carboxyl group-containing polyether compound as the main component (Japanese Industrial Standard JIS A 6204, density: 1.04 to 1.08 g / cm ３ ), Admixture B is a water-reducing agent retardation type (Type I) whose main component is hydroxycarboxylic acid salt (Japanese Industrial Standard JIS A 6206, density: 1.17 to 1.21 g / cm ３), and water were prepared. The calcium carbonate obtained above was blended into the ready-mixed concrete, and the mixture was mixed using an inverter biaxial forced mixing concrete mixer (model: MIC-109-0-31, manufactured by Marui Co., Ltd.). The detailed blending ratio of the ready-mixed calcium carbonate concrete was as shown in Table 1. Concrete specimens for the compressive strength test described below were prepared from this ready-mixed concrete.

[0042] <Compressive Strength Test> The compressive strength test was conducted in accordance with Japanese Industrial Standard JIS A 1108:2018 "Test method for compressive strength of concrete," and the compressive strength of the concrete after 24 hours and after 14 days was measured.

[0043] [Example 2] Calcium carbonate was produced under the same conditions as in Example 1, except that the carbide slag was passed through a sieve with a mesh diameter of 150 μm (Japanese Industrial Standard JIS Z 8801, company name: Iida Seisakusho Co., Ltd.) and carbon dioxide gas was introduced so that the carbonation rate of calcium ions was 17 cation % / hour (so that the time until completion of the reaction was 5.9 hours). A spindle-shaped calcite-type calcium carbonate was obtained (BET specific surface area measured by the method according to JIS Z 8830: 8.4 m ２ / g). The obtained calcium carbonate was mixed with ready-mixed concrete containing the same ingredients as in Example 1, and a compressive strength test was carried out. The detailed mix ratio of the ready-mixed concrete and the mix ratio of calcium carbonate are shown in Table 1. Scanning electron microscope photographs of the spindle-shaped calcite-type calcium carbonate obtained in Example 2 are shown in Figure 1 (magnification: 10,000 times) and Figure 2 (magnification: 20,000 times).

[0044]

[0045] Comparative Example 1 Calcium carbonate was produced under the same conditions as in Example 1, except that the carbide slag was not passed through any sieve and the reaction initiation temperature in the carbon dioxide introduction step was set to 30°C. A spindle-shaped calcite-type calcium carbonate was obtained (BET specific surface area measured according to the method in accordance with JIS Z 8830: 8.5 m). ２The obtained calcium carbonate was mixed with ready-mixed concrete having the same ingredients and mixing ratio as in Example 1, and a compressive strength test was carried out.

[0046] Comparative Example 2 Calcium carbonate was produced under the same conditions as in Example 1, except that the carbide slag was passed through a sieve with a mesh diameter of 75 μm (Japanese Industrial Standard JIS Z 8801, company name: Iida Seisakusho Co., Ltd.) and carbon dioxide gas was introduced so that the carbonation rate of calcium ions was 50 cation % / hour (so that the time until completion of the reaction was 2.0 hours). Cubic calcite-type calcium carbonate was obtained (BET specific surface area measured by the method according to JIS Z 8830: 27.0 m ２ The obtained calcium carbonate was mixed with ready-mixed concrete having the same ingredients and mixing ratio as in Example 1, and a compressive strength test was carried out.

[0047] Comparative Example 3 Calcium carbonate was produced under the same conditions as in Example 1, except that the carbide slag was passed through a sieve with a mesh diameter of 75 μm (Japanese Industrial Standard JIS Z 8801, company name: Iida Seisakusho Co., Ltd.) and carbon dioxide gas was introduced so that the carbonation rate of calcium ions was 0.5 cation % / hour (so that the time until completion of the reaction was 200 hours). A spindle-shaped calcite-type calcium carbonate was obtained (BET specific surface area measured by the method according to JIS Z 8830: 4.0 m). ２ The obtained calcium carbonate was mixed with ready-mixed concrete having the same ingredients and mixing ratio as in Example 1, and a compressive strength test was carried out.

[0048] Comparative Example 4 Calcium carbonate was produced under the same conditions as in Example 1, except that the carbide slag was not passed through any sieve, the reaction initiation temperature in the carbon dioxide introduction step was set to 8°C, and carbon dioxide gas was introduced so that the carbonation rate of calcium ions was 8.9 cation % / hour (so that the time until the reaction was completed was 11.3 hours). Cubic calcite-type calcium carbonate was obtained (BET specific surface area measured by the method according to JIS Z 8830: 42.0 m). ２The calcium carbonate thus obtained was mixed with ready-mixed concrete having the same components as those in Example 1 at the mixing ratio shown in Table 2, and a compressive strength test was carried out.

[0049] Comparative Example 5 Calcium carbonate was produced under the same conditions as in Example 1, except that the carbide slag was passed through a sieve with a mesh diameter of 75 μm (Japanese Industrial Standard JIS Z 8801, company name: Iida Seisakusho Co., Ltd.), the reaction initiation temperature in the carbon dioxide introduction step was set to 65° C., and carbon dioxide gas was introduced so that the carbonation rate of calcium ions became 16.7 cation % / hour (so that the time until the reaction was completed was 6 hours). A spindle-shaped calcite-type calcium carbonate was obtained (BET specific surface area measured by the method according to JIS Z 8830: 4.0 m ２ The calcium carbonate thus obtained was mixed with ready-mixed concrete having the same components as those in Example 1 at the mixing ratio shown in Table 2, and a compressive strength test was carried out.

[0050] Comparative Example 6 Calcium carbonate was produced under the same conditions as in Example 1, except that the carbide slag was passed through a sieve with a mesh diameter of 75 μm (Japanese Industrial Standard JIS Z 8801, company name: Iida Seisakusho Co., Ltd.), the solid content concentration of the carbide slag was adjusted to 0.5% in the adjustment step, and carbon dioxide gas was introduced so that the carbonation rate of calcium ions became 200 cation % / hour (so that the time until the completion of the reaction was 0.5 hours). Cubic calcite-type calcium carbonate was obtained (BET specific surface area measured by the method according to JIS Z 8830: 3.0 m ２ The calcium carbonate thus obtained was mixed with ready-mixed concrete having the same components as those in Example 1 at the mixing ratio shown in Table 2, and a compressive strength test was carried out.

[0051] Comparative Example 7 Calcium carbonate was produced under the same conditions as in Example 1, except that the carbide slag was passed through a sieve with a mesh diameter of 75 μm (Japanese Industrial Standard JIS Z 8801, company name: Iida Seisakusho Co., Ltd.), the reaction initiation temperature in the carbon dioxide introduction step was set to 14° C., the solid content concentration of the carbide slag was adjusted to 32% in the adjustment step, and carbon dioxide gas was introduced so that the carbonation rate of calcium ions was 13 cation % / hour (so that the time until the reaction was completed was 7.6 hours). Calcite-type calcium carbonate aggregates were obtained (BET specific surface area measured by the method according to JIS Z 8830: 16.9 m ２ The calcium carbonate thus obtained was mixed with ready-mixed concrete having the same components as those in Example 1 at the mixing ratio shown in Table 2, and a compressive strength test was carried out.

[0052] The experimental conditions and the mix ratio of the ready-mixed concrete for the comparative example are shown in Table 2.

[0053]

[0054]

[0055] [Reference Example 1] Commercially available precipitated calcium carbonate (Brilliant 1500, Shiraishi Kogyo Co., Ltd., BET specific surface area: 13 m ２ Calcium carbonate was mixed in the mixing ratio shown in Table 3 to ready-mixed concrete of the same composition as in Example 1 except that admixture B was not included, and a compressive strength test was carried out.

[0056] [Reference Example 2] Commercially available precipitated calcium carbonate (Silver W, Shiraishi Kogyo Co., Ltd., BET specific surface area: 6 m ２ Calcium carbonate was mixed with ready-mixed concrete having the same components as those in Example 1 in the mixing ratio shown in Table 3, and a compressive strength test was carried out.

[0057] [Reference Example 3] Commercially available large particle heavy calcium carbonate (Whiten H, Toyo Fine Chemical Co., Ltd., BET specific surface area: 1.5 m ２ Calcium carbonate was mixed with ready-mixed concrete having the same components as those in Example 1 in the mixing ratio shown in Table 3, and a compressive strength test was carried out.

[0058] Reference Example 4 Calcium carbonate (BET specific surface area: 0.26 m) produced by the method disclosed in JP 2014-148432 A (a method for producing calcium carbonate from concrete waste (concrete sludge) containing unsolidified cement fine particles) was used. ２ Calcium carbonate was mixed with ready-mixed concrete having the same components as those in Example 1 in the mixing ratio shown in Table 3, and a compressive strength test was carried out.

[0059]

[0060] [Example 3] <Production of calcium carbonate> A spindle-shaped calcite-type calcium carbonate was obtained under the same conditions as in the calcium carbonate production method of Example 1 (BET specific surface area measured according to the method in accordance with JIS Z 8830: 5.9 m ２ / g).

[0061] <Production of soil improvement material> Ground granulated blast furnace slag 4000 (Japanese Industrial Standard JIS A 6206, density: 2.8 g / cm ３ or more), crushed stone powder with a particle size of 2.5 mm or less as aggregate (Japanese Industrial Standards JIS A 5041, density: 2.5 g / cm ３ or more), sodium silicate (SiO ２ / Na ２ O molar ratio 2.0, Na ２ Mass percent concentration of O: 14.1 to 14.5 wt%, density 1.48 to 1.53 g / cm ３ A ground improvement material containing the above) and water was prepared. The calcium carbonate obtained above was mixed with the ground improvement material and mixed using a kneading machine (name: Mortar Mixer MIC-362-1-01, company name: Marui Co., Ltd.). The detailed mix ratio of the calcium carbonate-mixed ground improvement material was as shown in Table 4. Test specimens for the uniaxial compression test described below were prepared from this ground improvement material.

[0062] <Unconfined compression test> The unconfined compression test was conducted in accordance with Japanese Industrial Standard JIS A 1216:2020 "Unconfined compression test method for soil," and the unconfined compressive strength of the ground improvement material specimen after 28 days and after 91 days was measured.

[0063] [Example 4] A spindle-shaped calcite-type calcium carbonate was obtained under the same conditions as in the calcium carbonate production method of Example 2 (BET specific surface area measured according to the method in accordance with JIS Z 8830: 8.4 m ２ / g). The obtained calcium carbonate was mixed with a soil improvement material containing the same components as in Example 3, and a uniaxial compression test was performed. The detailed mixing ratio of the soil improvement material and the mixing ratio of calcium carbonate are shown in Table 4.

[0064]

[0065] Comparative Example 8 Calcium carbonate was produced under the same conditions as in Example 1, except that the carbide slag was not passed through any sieve. A spindle-shaped calcite-type calcium carbonate was obtained (BET specific surface area measured according to JIS Z 8830: 6.6 m). ２ The obtained calcium carbonate was mixed with a soil improvement material having the same components and mixing ratio as in Example 3, at the same mixing ratio as in Example 3, and a uniaxial compression test was carried out.

[0066] Comparative Example 9 Calcium carbonate was produced under the same conditions as in Example 1, except that the carbide slag was passed through a sieve with a mesh diameter of 75 μm (Japanese Industrial Standard JIS Z 8801, company name: Iida Seisakusho Co., Ltd.) and carbon dioxide gas was introduced so that the carbonation rate of calcium ions was 50 cation % / hour (so that the time until the reaction was completed was 2.0 hours). A spindle-shaped calcite-type calcium carbonate was obtained (BET specific surface area measured by the method according to JIS Z 8830: 27.0 m ２ The obtained calcium carbonate was mixed with a soil improvement material having the same components and mixing ratio as in Example 3, at the same mixing ratio as in Example 3, and a uniaxial compression test was carried out.

[0067] Comparative Example 10 Calcium carbonate was produced under the same conditions as in Example 1, except that the carbide slag was passed through a sieve with a mesh diameter of 75 μm (Japanese Industrial Standard JIS Z 8801, company name: Iida Seisakusho Co., Ltd.) and carbon dioxide gas was introduced so that the carbonation rate of calcium ions was 0.5 cation % / hour (so that the time until completion of the reaction was 200 hours). A spindle-shaped calcite-type calcium carbonate was obtained (BET specific surface area measured by the method according to JIS Z 8830: 4.0 m). ２ The obtained calcium carbonate was mixed with a soil improvement material having the same components and mixing ratio as in Example 3, at the same mixing ratio as in Example 3, and a uniaxial compression test was carried out.

[0068]

[0069] [Reference Example 5] Commercially available light calcium carbonate (Shiraenka CC, Shiraishi Kogyo Co., Ltd., BET specific surface area: 26 m ２ Calcium carbonate was mixed with a soil improvement material having the same components and mixing ratio as in Example 3 at the same mixing ratio as in Example 3, and an unconfined compression test was carried out.

[0070] [Reference Example 6] Commercially available precipitated calcium carbonate (PC, Shiraishi Kogyo Co., Ltd., BET specific surface area: 6 m ２ Calcium carbonate was mixed with the soil improvement material having the same components and mixing ratio as in Example 3 at the same mixing ratio as in Example 3, and an unconfined compression test was carried out.

[0071] [Reference Example 7] Commercially available large particle heavy calcium carbonate (Whiten P50, Toyo Fine Chemical Co., Ltd., BET specific surface area: 1.5 m ２ Calcium carbonate was mixed with a soil improvement material having the same components and mixing ratio as in Example 3 at the same mixing ratio as in Example 3, and an unconfined compression test was carried out.

[0072] [Reference Example 8] Commercially available precipitated calcium carbonate (Silver W, Shiraishi Kogyo Co., Ltd., BET specific surface area: 6 m ２ Calcium carbonate was mixed in the same mixing ratio as in Example 3 with a soil improvement material having the same components as in Example 3, and a uniaxial compression test was carried out.

[0073] [Reference Example 9] Commercially available precipitated calcium carbonate (Brilliant 1500, Shiraishi Kogyo Co., Ltd., BET specific surface area: 13 m ２ Calcium carbonate was mixed in the same mixing ratio as in Example 3 with a soil improvement material having the same components as in Example 3, and an unconfined compression test was carried out.

[0074]

[0075] The meanings of the abbreviations in Tables 1 to 6 are as follows: P: powder amount, the sum of B and F below B: binder amount, the sum of C and BFS below C: cement amount BFS: amount of ground granulated blast furnace slag F: calcium carbonate amount W / P: water-to-powder ratio s / a: fine aggregate rate, s / a = Sv / Av x 100 [%], Sv is the fine aggregate volume, Av is the total aggregate volume (fine aggregate volume + coarse aggregate volume) A: admixture A, addition rate is the percentage of the total weight of fresh concrete, "addition amount P wt%" is the weight of the total volume of powder contained in the fresh concrete B: admixture B, "addition rate" is the percentage of the total weight of fresh concrete, "addition amount P wt%" is the weight of the total volume of powder contained in the fresh concrete S2: aggregate (crushed stone powder) amount WG: sodium silicate amount

[0076] In Tables 1 to 6, the evaluations refer to the workability of the concrete or ground improvement material. As mentioned above, workability is a comprehensive evaluation that takes into account the workability and constructability of the concrete or ground improvement material. In the series of experiments in this specification, the mixing characteristics of the ready-mixed concrete or ground improvement material and the compressive strength of each hardened product were comprehensively evaluated, and the workability was ranked as excellent, good, fair, or poor, starting with the highest. The remarks are comments regarding noteworthy characteristics of the ready-mixed concrete or ground improvement material during mixing and the strength of the hardened product. "Thixotropy immediately after mixing" indicates that thixotropy (the property of decreasing viscosity when shear force is applied continuously) was observed immediately after starting mixing of the ready-mixed concrete or ground improvement material. Thixotropy is a property that cannot be considered good as a general property of ready-mixed concrete.

[0077] Ready-mixed concrete containing calcium carbonate produced by the manufacturing method of the present invention exhibits excellent mixing performance without losing fluidity, and the concrete has high strength after hardening. In the Reference Examples, experimental examples in which commercially available calcium carbonate (limestone) was blended into ready-mixed concrete or ground improvement materials were shown. However, the calcium carbonate produced by the manufacturing method of the present invention exhibited performance equivalent to that of commercially available calcium carbonate (e.g., Reference Examples 2 and 6), and excellent ready-mixed concrete or ground improvement materials could be provided. The manufacturing method of the present invention can produce industrially useful calcium carbonate using carbide slag, which is industrial waste, as a raw material. In the manufacturing method of the present invention, if exhaust gas is used as a carbon dioxide source, carbon dioxide emissions into the atmosphere can be reduced, thereby contributing to halting the progression of global warming and environmental pollution.

Claims

1. The following steps: a classification step of classifying carbide slag containing calcium hydroxide and hydrophobic impurities to remove the hydrophobic impurities; an adjustment step of adjusting the solid content concentration of the carbide slag from which the hydrophobic impurities have been removed in the classification step; and a carbon dioxide introduction step of introducing carbon dioxide into the carbide slag from which the hydrophobic impurities have been removed in the classification step and the solid content concentration of which has been adjusted in the adjustment step, a method for producing calcium carbonate.

2. The method for producing calcium carbonate according to claim 1, wherein the particle size of the hydrophobic impurities removed in the classification step is 45 μm or more.

3. The method for producing calcium carbonate according to claim 1 or 2, wherein in the adjustment step, the solid content concentration of the carbide slag from which the hydrophobic impurities have been removed is adjusted to 1-30% by mass.

4. The method for producing calcium carbonate according to any one of claims 1 to 3, wherein in the carbon dioxide introduction step, carbon dioxide is introduced into the carbide slag from which the hydrophobic impurities have been removed and the solid content concentration of which has been adjusted so that the carbonation rate of calcium ions is 1-40 cation % / hour.

5. The method for producing calcium carbonate according to any one of claims 1 to 4, wherein calcite-type calcium carbonate having a spindle shape is produced.

6. Calcium carbonate produced by the production method according to any one of claims 1 to 5.

7. A method of classifying carbide slag containing calcium hydroxide and hydrophobic impurities to remove the hydrophobic impurities; adjusting the solid content concentration of the carbide slag from which the hydrophobic impurities have been removed; introducing carbon dioxide into the carbide slag from which the hydrophobic impurities have been removed and the solid content concentration of which has been adjusted to obtain calcium carbonate, and mixing the obtained calcium carbonate with raw mortar or raw concrete to produce calcium carbonate-blended raw mortar or calcium carbonate-blended raw concrete.

8. A method of classifying carbide slag containing calcium hydroxide and hydrophobic impurities to remove the hydrophobic impurities; adjusting the solid content concentration of the carbide slag from which the hydrophobic impurities have been removed; introducing carbon dioxide into the carbide slag from which the hydrophobic impurities have been removed and the solid content concentration of which has been adjusted to obtain calcium carbonate, mixing the obtained calcium carbonate with a ground improvement material, and mixing or injecting the obtained calcium carbonate-blended ground improvement material into the ground to improve the ground.

9. A method of classifying carbide slag containing calcium hydroxide and hydrophobic impurities to remove the hydrophobic impurities; adjusting the solid content concentration of the carbide slag from which the hydrophobic impurities have been removed; introducing carbon dioxide into the carbide slag from which the hydrophobic impurities have been removed and the solid content concentration has been adjusted to obtain calcium carbonate; mixing the obtained calcium carbonate with a filler; and filling the obtained calcium carbonate-containing filler into an underground cavity.

10. A method of classifying carbide slag containing calcium hydroxide and hydrophobic impurities to remove the hydrophobic impurities; adjusting the solid content concentration of the carbide slag from which the hydrophobic impurities have been removed; introducing exhaust gas containing carbon dioxide into the carbide slag from which the hydrophobic impurities have been removed and the solid content concentration has been adjusted to immobilize the carbon dioxide contained in the exhaust gas.

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