Method for fixing carbon dioxide, method for producing calcium carbonate, and method for utilizing waste gypsum board

Abstract

To effectively reuse a waste gypsum board as it is without discarding it, by efficiently fixing carbon dioxide and efficiently manufacturing calcium carbonate that is a valuable material from the carbon dioxide, and further by using the waste gypsum board for fixing carbon dioxide.SOLUTION: A method includes: a first step in which a first solution containing alkali metal hydroxide is brought into contact with a gaseous matter containing carbon dioxide, and a second solution containing at least one alkali metal salt out of alkali metal carbonate salt and alkali metal bicarbonate is produced; and a second step in which the second solution is brought into contact with a gypsum inclusion to produce calcium carbonate.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for fixing carbon dioxide, a method for producing calcium carbonate, and a method for utilizing waste gypsum board. [Background technology]

[0002] In recent years, interest in global warming has grown, and there is a demand for reducing the amount of carbon dioxide released into the atmosphere. At various facilities, such as power plants, incinerators, cement plants, steel mills, and industrial facilities, methods for capturing exhaust gases containing carbon dioxide generated during operations without releasing them into the atmosphere have been considered. One known method for capturing carbon dioxide involves reacting it with Group 2 elements, such as calcium and magnesium, to produce carbonates.

[0003] For example, Patent Document 1 discloses a method of immobilizing carbon dioxide by reacting carbon dioxide with alkaline earth metal oxides produced from brine. Patent Document 2 discloses a method of immobilizing carbon dioxide as carbonate by dissolving waste concrete, waste materials such as steel slag, and rocks containing Group 2 elements such as calcium in a nitric acid solution to produce a nitrate solution of the Group 2 elements, and separately reacting the nitrate solution with a sodium carbonate solution obtained by reacting carbon dioxide in combustion exhaust gas with a sodium hydroxide solution.

[0004] Furthermore, carbon dioxide contained in exhaust gases emitted from various facilities is sometimes used to produce calcium carbonate. Patent Document 3 discloses a method for producing calcium carbonate by reacting a sodium carbonate solution obtained by absorbing carbon dioxide in exhaust gases such as flue gases with caustic soda by a gas-liquid contact method with milk of lime obtained by hydrating quicklime. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2020-175344 [Patent Document 2] Japanese Patent Application Laid-Open No. 2012-96975 [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-293537 Summary of the Invention [Problem to be solved by the invention]

[0006] The method of Patent Document 1 requires multiple steps, such as removing alkali metals and sulfuric acid from the brine, to obtain alkaline earth metal oxides, making the process complicated and inefficient. Furthermore, some of these steps, such as the evaporation and condensation of the brine and the thermal decomposition of alkaline earth metal chlorides, require a great deal of energy, and as a result, does not lead to an effective reduction of carbon dioxide emissions. In the method of Patent Document 2, it was difficult to efficiently fix carbon dioxide in waste materials such as waste concrete and steel slag, as well as rocks, because they contain a small amount of alkaline earth metals such as calcium and magnesium.

[0007] In the method of Patent Document 3, the quicklime used to produce milk of lime is usually obtained by burning limestone at a high temperature of 900 to 1000°C, but the carbon dioxide contained in the limestone is released during this process. Therefore, although the method of Patent Document 3 absorbs carbon dioxide in exhaust gas, it has the problem of not leading to a reduction in carbon dioxide.

[0008] The present invention has been made in view of the above-mentioned problems, and aims to provide a method for efficiently fixing carbon dioxide contained in exhaust gases and the like generated from various facilities such as power plants, incinerators, cement plants, steelworks, factory facilities, etc., and a method for efficiently producing calcium carbonate using the same. Another aim of the present invention is to provide a new method for utilizing waste gypsum boards. [Means for solving the problem]

[0009] The present inventors focused on gypsum (calcium sulfate) as a calcium source for use in carbon dioxide fixation. Gypsum is not only obtained from natural ores, but also exists in a variety of forms as waste, such as waste gypsum board and materials recovered from flue gas desulfurization processes, and therefore has the advantage of being easily available. Therefore, the present inventors came up with the idea of ​​using gypsum-containing materials, such as waste gypsum board and other gypsum-containing waste, for carbon dioxide fixation as a new method of utilizing such materials, and arrived at the present invention.

[0010] In order to solve the above problems, the present invention provides the following method for fixing carbon dioxide and method for producing calcium carbonate. 1. A first step of contacting a first solution containing an alkali metal hydroxide with a gas containing carbon dioxide to produce a second solution containing at least one alkali metal salt of an alkali metal carbonate or an alkali metal bicarbonate; a second step of contacting the second solution with a gypsum-containing material to produce calcium carbonate. 2. A first step of contacting a first solution containing an alkali metal hydroxide with a gas containing carbon dioxide to produce a second solution containing at least one alkali metal salt selected from the group consisting of an alkali metal carbonate and an alkali metal bicarbonate; and a second step of contacting the second solution with a gypsum-containing material to produce calcium carbonate.

[0011] Furthermore, the present inventors have focused on waste gypsum board, which is a waste material among gypsum-containing materials. Waste gypsum board is waste generated in large quantities when buildings are rebuilt. Waste gypsum board is used as a ground improvement material and a secondary raw material for cement, but due to problems such as the generation of hydrogen sulfide, recycling has not progressed to date. Since waste gypsum board contains a high concentration of calcium, the present inventors have investigated whether it can be reused and have found a new method for effectively reusing waste gypsum board. That is, the present invention provides the following method for utilizing waste gypsum board.

[0012] 3. A first step of contacting a first solution containing an alkali metal hydroxide with a gas containing carbon dioxide to produce a second solution containing at least one alkali metal salt selected from the group consisting of an alkali metal carbonate and an alkali metal bicarbonate; and a second step of contacting the second solution with the waste gypsum board to produce calcium carbonate. [Effects of the Invention]

[0013] According to the present invention, carbon dioxide can be efficiently fixed and calcium carbonate, a valuable material, can be efficiently produced from carbon dioxide. Furthermore, according to the present invention, by using waste gypsum boards to fix carbon dioxide, the waste gypsum boards can be effectively reused without being discarded as they are. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will be described in detail below. The present invention is not limited to the following embodiments and can be implemented with any modifications within the scope that does not impair the effects of the invention. In this specification, the expression "AA to BB" for a numerical range means "AA or more and BB or less." In addition, in this specification, the numerical values ​​associated with "above," "below," and "to" in describing a numerical range can be arbitrarily combined. For example, when a certain numerical range is described as "CC to DD" and "EE to FF," the numerical ranges "CC to FF" and "EE to DD" are also included.

[0015] [Method of carbon dioxide fixation] One aspect of the present invention is a first step of contacting a first solution containing an alkali metal hydroxide with a gas containing carbon dioxide to produce a second solution containing at least one alkali metal salt of an alkali metal carbonate or an alkali metal bicarbonate; a second step of contacting the second solution with a gypsum-containing material to produce calcium carbonate.

[0016] The reactions for producing the second solution in the first step and for producing calcium carbonate in the second step using a gypsum-containing material both require low energy and tend to proceed stably. Furthermore, examples of the gypsum-containing material used in the second step, which will be described in detail later, include gypsum, waste gypsum board, recovered material from flue gas desulfurization treatment, hydrofluoric gypsum, phosphogypsum, and gypsum-containing sludge obtained in various wastewater treatment processes. Thus, the gypsum-containing material used in the carbon dioxide fixation method can be obtained from a wide range of sources, including waste materials such as waste gypsum board and by-products produced in the manufacturing process of chemical products.

[0017] The carbon dioxide to be fixed can be carbon dioxide contained in exhaust gases released (discarded) into the atmosphere from various facilities such as power plants, incinerators, and factory facilities. As described above, the carbon dioxide fixation method according to one embodiment of the present invention can utilize waste materials, by-products, etc., and employs a reaction that proceeds with low energy, thereby enabling extremely efficient fixation of carbon dioxide. Furthermore, the amount of carbon dioxide released into the atmosphere can be reduced, which can greatly contribute to reducing the environmental load. Hereinafter, the carbon dioxide fixation method according to one embodiment of the present invention will be described in detail, starting from the first step.

[0018] [First step] The first step is a step of contacting a first solution containing an alkali metal hydroxide with a gas containing carbon dioxide to produce a second solution containing at least one alkali metal salt of an alkali metal carbonate or an alkali metal bicarbonate. In the first step, at least one alkali metal salt of an alkali metal carbonate and an alkali metal bicarbonate is produced by the progress of the reactions shown in the following reaction formulas (1) and (2). Therefore, the second solution obtained by contacting the first solution with a gas containing carbon dioxide contains at least one alkali metal salt of an alkali metal carbonate (Ma2CO3) and an alkali metal bicarbonate (MaHCO3).

[0019] 2MaOH+CO2→Ma2CO3+H2O …(1) MaOH+CO2→MaHCO3…(2) In reaction formulas (1) and (2), Ma is an alkali metal.

[0020] (gas containing carbon dioxide) The carbon dioxide (CO2)-containing gas to be brought into contact with the first solution in the first step can be any gas containing carbon dioxide, and can be, for example, exhaust gas emitted from various facilities such as power plants, incinerators, cement factories, steel mills, and factory facilities.

[0021] It is difficult to generalize as this can vary depending on the operating conditions of various facilities and the processes used, but for example, the concentration of carbon dioxide (CO2) in exhaust gas from a power plant is usually 8 to 15% by volume, the concentration of carbon dioxide (CO2) in exhaust gas from an incinerator is usually 5 to 15% by volume, the concentration of carbon dioxide (CO2) in exhaust gas from a cement factory is usually 15 to 25% by volume, and the concentration of carbon dioxide (CO2) in exhaust gas from a steel mill is usually 20 to 30% by volume. Among the above exhaust gases, exhaust gases from cement factories and steelworks contain high concentrations of carbon dioxide, which allows the reactions of the above reaction formulas (1) and (2) to proceed efficiently and also allows the overall equipment, including the size of the piping, to be made compact. Therefore, among the above various exhaust gases, exhaust gases from cement factories and steelworks are preferably used in the method for fixation of carbon dioxide, which is one embodiment of the present invention.

[0022] The various exhaust gases mentioned above that can be used as the carbon dioxide-containing gas used in the first step usually contain impurities such as hydrogen chloride and dust (also called "dust," "combustion fly ash," "soot dust," etc.). Therefore, the various facilities mentioned above are usually equipped with a desalination facility that removes hydrogen chloride, and a dust collector such as an electrostatic precipitator or a bag filter that removes the desalted dust obtained in the desalination facility and the dust. From the viewpoint of improving the efficiency of the reaction between the carbon dioxide in the gas and the alkali metal hydroxide in the first solution, it is preferable to use a gas containing carbon dioxide from which impurities such as hydrogen chloride and dust have been removed.

[0023] For example, when the gas containing carbon dioxide is exhaust gas discharged from a cement factory, it is preferable that the exhaust gas is at least one selected from exhaust gas after passing through a dust collector and exhaust gas discharged from a chlorine bypass facility in which combustion gas is extracted from a kiln end duct and desalted dust is recovered.

[0024] In this way, when the gas containing carbon dioxide is exhaust gas that has passed through a dust collector, the dust collector has removed dust from the gas, so that the carbon dioxide in the exhaust gas can react efficiently with the alkali metal hydroxide in the first solution. Furthermore, if the gas containing carbon dioxide is exhaust gas discharged from a chlorine bypass facility that extracts combustion gas from the kiln end duct and recovers desalted dust, the carbon dioxide will contain a concentration of carbon dioxide similar to that of the exhaust gas that has passed through the dust collector, allowing it to react efficiently with the alkali metal hydroxide in the first solution.

[0025] The carbon dioxide concentration in the exhaust gas that has passed through the dust collector is usually 15 to 25% by volume, which is the same as that in the exhaust gas that has not passed through the dust collector. The carbon dioxide concentration is preferably 20 to 25% by volume. The carbon dioxide concentration in the exhaust gas discharged from the chlorine bypass facility is also the same.

[0026] Before contacting the carbon dioxide-containing gas used in the first step with the first solution containing an alkali metal hydroxide, the carbon dioxide in the gas may be recovered in advance by a method such as chemical absorption, physical absorption, membrane separation, cryogenic separation, or oxygen combustion to increase the carbon dioxide concentration, and then the gas may be contacted with the first solution containing an alkali metal hydroxide.

[0027] Chemical absorption is a method of chemically absorbing carbon dioxide (CO2) from exhaust gas using a solvent such as an amine to separate it as highly concentrated carbon dioxide (CO2) gas. Physical absorption is a method of separating it as highly concentrated carbon dioxide (CO2) gas by absorbing carbon dioxide (CO2) into a physical absorption solution under high pressure. Membrane separation is a method of separating it as highly concentrated carbon dioxide (CO2) gas using a membrane that selectively allows carbon dioxide (CO2) to pass through. Cryogenic separation is a method of liquefying carbon dioxide at extremely low temperatures and separating it using the difference in boiling points. Additionally, oxyfuel combustion is a method of burning oxygen produced in an air separation unit to separate carbon dioxide (CO2) at a high concentration. By using the above method, the carbon dioxide (CO2) concentration in the gas is increased before the gas is brought into contact with the first solution, which allows the gas to react efficiently with the alkali metal oxide, thereby more efficiently obtaining the alkali metal carbonate.

[0028] (First solution) The alkali metal hydroxide contained in the first solution is not particularly limited, but considering the ease of progress of the reactions of the above reaction formulas (1) and (2) and the ease of availability of the alkali metal hydroxide, the alkali metal is preferably lithium (Li), potassium (K), or sodium (Na), more preferably potassium (K) or sodium (Na), and even more preferably sodium (Na). That is, sodium hydroxide (NaOH), potassium hydroxide (KOH), or lithium hydroxide (LiOH) is preferred, sodium hydroxide (NaOH) or potassium hydroxide (KOH) is more preferred, and sodium hydroxide (NaOH) is even more preferred. In the first step, the alkali metal hydroxide may be any of the above hydroxides, or a combination of two or more of them may be used.

[0029] The first solution can be obtained by electrolysis of an aqueous alkali metal chloride solution, a metathetical reaction between calcium hydroxide and an alkali metal carbonate, or hydration of a solid alkali metal hydroxide. The first solution can also be produced by electrolysis using an ion exchange membrane. Bipolar membrane electrodialysis is also preferred because it can produce alkali metal hydroxide with low energy consumption.

[0030] Considering the above-mentioned production method, the first solution is preferably an aqueous solution. Furthermore, an aqueous solution does not require special consideration for the specifications of the apparatus used, and is also advantageous in terms of environmental impact. When an aqueous solution is used, the water medium is not particularly limited, and various types of water can be used, such as tap water, distilled water, ion-exchanged water, and industrial water.

[0031] The concentration of alkali metal hydroxide contained in the first solution is preferably 0.1 mol / L or more, more preferably 0.5 mol / L or more. By setting the concentration within the above range, the amount of carbon dioxide absorbed by the first solution can be increased, allowing for efficient fixation of carbon dioxide. Furthermore, the alkali metal concentration and carbonate ion concentration in the second solution can be set to concentrations suitable for the second step described below.

[0032] The upper limit of the alkali hydroxide concentration in the first solution is not particularly limited, for example, from the viewpoint of efficient fixation of carbon dioxide, and the higher the concentration, the better. On the other hand, for example, if the concentration exceeds 8 mol / L (corresponding to about 25% by mass), there is a risk of freezing in the piping in winter, and equipment such as insulation and a jacket heater may be required. Furthermore, if the concentration exceeds 1.25 mol / L (corresponding to about 5% by mass), it is treated as a deleterious substance under the Poisonous and Deleterious Substances Control Act, etc., and corresponding equipment is required. In consideration of ease of handling the first solution and suppression of increases in equipment costs due to device specifications, etc., a concentration of 1.1 mol / L or less is preferable, and 1.0 mol / L or less is more preferable. The concentration of the alkali hydroxide contained in the first solution is a concentration measured in accordance with JIS K0102:2019 (48.2 Flame atomic absorption spectrometry) when the alkali metal is sodium, or in accordance with JIS K0102:2019 (49.2 Flame atomic absorption spectrometry) when the alkali metal is potassium.

[0033] The produced first solution may be stored in a tank or the like until it is brought into contact with exhaust gas containing carbon dioxide, or the produced first solution may be directly supplied to equipment (such as a scrubber described below) for contacting the first solution with exhaust gas containing carbon dioxide.

[0034] In the present invention, the facility for producing the second solution by contacting the first solution with a gas containing carbon dioxide is preferably provided adjacent to the facility for discharging the exhaust gas containing carbon dioxide, which is industrially advantageous because it allows the length of piping for transporting the exhaust gas to be shortened.

[0035] (contact) The first step involves contacting a first solution containing an alkali metal hydroxide with a gas containing carbon dioxide, which causes a reaction between the alkali metal hydroxide and carbon dioxide, i.e., reactions (1) and (2) shown above, to produce at least one alkali metal salt of an alkali metal carbonate or an alkali metal bicarbonate.

[0036] The contact in the first step is contact between the first solution and a gas containing carbon dioxide, i.e., gas-liquid contact. The equipment used for the contact is not particularly limited as long as it is equipment used for gas-liquid contact, and preferred examples include equipment such as a packed tower filled with various packing materials such as Rahisch rings or Pall rings, and a plate tower equipped with various trays such as sieve trays or bubble cap trays (collectively referred to as "scrubbers"). The use of such a scrubber facilitates the progress of the reactions of the above reaction formulas (1) and (2), allowing for more efficient fixation of carbon dioxide. When a scrubber is used, the contact in the first step is basically a flow-through type.

[0037] Furthermore, as equipment other than a scrubber, for example, a reactor equipped with a container capable of storing a liquid and a supply port capable of supplying a gas to the container can be used. In this case, a flow type in which the liquid is continuously supplied and discharged may be adopted, or a batch type in which the liquid is held in a container and a gas is supplied and discharged for a certain period of time may be adopted.

[0038] The type of supply port for supplying gas to the container is not particularly limited, but it is preferable to provide a jig such as a dispersion nozzle or bubble generating nozzle that can supply gas in bubbles from the bottom of the container. Since gas-liquid contact is facilitated, the above reactions (1) and (2) proceed more easily, allowing carbon dioxide to be fixed efficiently. For the same reason, the container may also be equipped with a stirrer.

[0039] In the contact, it is preferable to use a larger amount of alkali metal carbonate (Ma2CO3), that is, to promote the reaction of the above reaction formula (1). As shown in reaction formulas (3) and (4) described below, the use of alkali metal carbonate (Ma2CO3) allows calcium carbonate to be obtained with a smaller amount, thereby enabling more efficient fixation of carbon dioxide.

[0040] The reaction of the above reaction formula (1) can be made to proceed preferentially over the reaction of the above reaction formula (2) by adjusting the pH, reaction temperature, reaction time, etc., and adjusting the pH is the easiest and most reliable method.

[0041] The pH is preferably 8.5 or higher, more preferably 10 or higher, even more preferably 11 or higher, and even more preferably 12 or higher. By carrying out the reaction while raising the pH, the reaction of the above reaction formula (1) tends to proceed preferentially compared to the reaction of the above reaction formula (2), and therefore a larger amount of alkali metal carbonate (Ma2CO3) can be obtained. From the viewpoint of preferentially proceeding the above reaction formula (1), there is no particular upper limit. However, in order to more efficiently preferentially proceed the above reaction formula (1), the pH is preferably 13.5 or less, more preferably 13 or less.

[0042] The pH can be adjusted using an inorganic alkali agent such as sodium hydroxide, potassium hydroxide, etc. If necessary, a mineral acid such as hydrochloric acid or sulfuric acid, or an organic acid such as acetic acid may also be used.

[0043] The reaction temperature is not particularly limited and may be, for example, between 10 and 80°C, and is preferably between 15 and 60°C, more preferably between 20 and 45°C, from the viewpoint of more efficient reaction progress. The reaction time is not particularly limited and may be, for example, 10 minutes to 1 hour, and from the viewpoint of more efficient reaction progression, it is preferably 15 to 50 minutes, more preferably 20 to 45 minutes.

[0044] The molar ratio of carbon dioxide contained in the flue gas brought into contact with the first solution to the alkali metal hydroxide contained in the first solution (carbon dioxide / alkali metal hydroxide) is preferably 0.1 to 1.0, more preferably 0.2 to 0.9, and even more preferably 0.3 to 0.8. When the molar ratio is within the above range, the carbon dioxide in the flue gas can be efficiently absorbed into the first solution. Furthermore, since the concentration can be made sufficient for reaction with the gypsum-containing material described below, carbon dioxide can be efficiently fixed.

[0045] [Second step] The second step is a step of bringing the gypsum-containing material into contact with a second solution containing at least one of an alkali metal carbonate and an alkali metal bicarbonate to produce calcium carbonate. In the second step, Depending on the type of alkali metal salt obtained in the first step, namely, alkali metal carbonate or alkali metal bicarbonate, the reactions shown in the following reaction formulas (3) and (4) proceed. Ma2CO3+CaSO4→CaCO3+Ma2SO4…(3) 2MaHCO3+CaSO4→CaCO3+Ma2SO4+CO2+H2O …(4) In reaction formulas (3) and (4), Ma is an alkali metal, and is the same as Ma in the above reaction formulas (1) and (2).

[0046] (Gypsum-containing material) The gypsum-containing material used in the carbon dioxide fixation method according to one embodiment of the present invention is not particularly limited as long as it contains gypsum, which is at least one of calcium sulfate and calcium sulfate hydrate.

[0047] Calcium sulfate, i.e., anhydrous calcium sulfate, is called anhydrous gypsum, and calcium sulfate hydrates include a dihydrate called gypsum dihydrate and a hemihydrate called gypsum hemihydrate. Either the anhydrous or hydrated forms can be used in the fixing method according to one embodiment of the present invention. In addition, for anhydrides, for example, by calcining the above-mentioned hydrate, soluble anhydrous gypsum (also referred to as "type II anhydrous gypsum"), insoluble anhydrous gypsum (also referred to as "type II anhydrous gypsum"), and further, depending on the calcination method, calcined gypsum such as α-type and β-type can be obtained, and any of these anhydrides can be used in the fixing method that is one embodiment of the present invention.

[0048] Examples of the gypsum-containing material include, in addition to gypsum itself, products using gypsum, such as gypsum board and waste gypsum board, or waste products thereof; recovered products from flue gas desulfurization treatment (also called "flue gas desulfurization gypsum" and, for example, recovered products from flue gas desulfurization treatment in copper refining, etc.); and by-products produced in the production process of chemical products, such as hydrofluoric gypsum (a by-product in the process of producing hydrogen fluoride), phosphogypsum (a by-product in the process of producing wet phosphoric acid), titanic gypsum (a by-product in the process of producing titanium oxide), and activated silicate gypsum (a by-product in the process of producing activated silicic acid).

[0049] In addition, for example, incineration ash generated during the incineration process of municipal waste contains anhydrous gypsum. Therefore, waste such as incineration ash can also be used as a gypsum-containing material. As another waste, gypsum-containing sludge obtained in various wastewater treatment processes can also be used as a gypsum-containing material. As described above, in the method for fixation of carbon dioxide according to one embodiment of the present invention, the gypsum-containing material can be widely selected from various waste materials and various by-products in addition to gypsum itself.

[0050] The amount of gypsum contained in a gypsum-containing material cannot be generalized because it can vary depending on whether the gypsum is present in the form of anhydrous gypsum or a hydrate (gypsum dihydrate or gypsum hemihydrate), and also depending on whether the gypsum-containing material is the above-mentioned gypsum, a by-product, or a waste material. In the method for fixation of carbon dioxide, which is one aspect of the present invention, even if a small amount of gypsum is contained in a gypsum-containing material, carbon dioxide can be fixed, but from the viewpoint of suppressing the amount of gypsum-containing material used and efficiently fixating carbon dioxide, the content as calcium sulfate (anhydrous) is preferably 1% by mass or more, more preferably 3% by mass or more, and even more preferably 5% by mass or more. Furthermore, from the viewpoint of efficiently fixating carbon dioxide, there is no particular upper limit, and in consideration of ease of availability, it is 97% by mass or less.

[0051] The gypsum-containing material used in the method for fixation of carbon dioxide, which is one embodiment of the present invention, may be appropriately selected from the gypsum-containing materials described above. In consideration of the industrial advantage of reusing gypsum, it is preferable to use various waste materials such as waste gypsum board and incineration ash; and various by-products such as flue gas desulfurization gypsum, hydrofluoric gypsum, phosphate gypsum, titanic gypsum, and activated silicate gypsum. From the viewpoint of more efficient fixation of carbon dioxide, waste gypsum board and the various by-products described above are preferred, and from the viewpoint of effective utilization of waste, waste gypsum board is more preferred.

[0052] The gypsum-containing material may be brought into contact with the second solution containing at least one of an alkali metal carbonate and an alkali metal bicarbonate in the form of a solid or a mixed liquid (slurry).

[0053] When the gypsum-containing material is brought into contact with the second solution as a solid, the average particle size of the gypsum-containing material is preferably 10 mm or less, 1.0 mm or less, or 0.5 mm or less, in consideration of ease of handling. Furthermore, when the gypsum-containing material is brought into contact with the second solution as a mixed liquid (slurry), the particle size of the gypsum-containing material is preferably about 500 μm or less, in consideration of obtaining a stable mixed liquid (slurry). The average particle size of gypsum-containing materials is measured by sieving. A rotary automatic sieve using standard sieves conforming to the provisions of JIS Z 8801:2019 is used, and the samples are stacked in order of smallest opening, and the samples remaining on each sieve are weighed. The particle size at which the cumulative 50% of the samples are removed is taken as the average particle size.

[0054] The gypsum-containing material may be crushed in advance using a crusher such as a roller mill to obtain the above particle size. When using waste gypsum board, it is preferable to remove wallpaper or the like from the surface before crushing.

[0055] When using a gypsum-containing mixed solution, the medium is preferably water. Because calcium sulfate has low solubility in water, the mixed solution becomes a gypsum-containing slurry. The mixing ratio of the gypsum-containing material to the medium (water) (gypsum-containing material / medium) is preferably within a range of 1:3 to 1:50, and more preferably within a range of 1:5 to 1:20. By setting the mixing ratio as described above, it becomes easier to bring the gypsum-containing material into contact with the second solution, and the reactions of the above reaction formulas (3) and (4) can proceed more efficiently.

[0056] (Second solution) The second solution used in the reaction in the second step is a solution containing at least one alkali metal salt of an alkali metal carbonate and an alkali metal bicarbonate obtained in the first step. Considering that the first solution is preferably an aqueous solution and that the second solution is obtained by contacting the first solution with a gas containing carbon dioxide, the second solution is preferably an aqueous solution, just like the first solution. An aqueous solution is advantageous in that it does not require special consideration regarding the specifications of the apparatus used and is also advantageous in terms of environmental impact.

[0057] The second solution may be any of a solution containing at least one alkali metal salt of an alkali metal carbonate and an alkali metal bicarbonate, i.e., a solution containing only an alkali metal carbonate, a solution containing only an alkali metal bicarbonate, and a solution containing an alkali metal carbonate and an alkali metal bicarbonate. However, as described above, from the viewpoint of more efficiently fixing carbon dioxide, a solution in which the reaction of the above reaction formula (1) proceeds preferentially over the reaction of (2), i.e., a solution containing a larger amount of alkali metal carbonate, is preferred.

[0058] As described above, when attempting to obtain a solution containing an alkali metal carbonate as the second solution, the reaction of the above reaction formula (1) can be preferentially carried out by adjusting the pH or the like, but the reaction of the above reaction formula (2) may also proceed, resulting in the inclusion of an alkali metal bicarbonate. In other words, even if an attempt is made to preferentially proceed the reaction of the above reaction formula (1), it is substantially impossible to stop the progression of the above reaction formula (2). Therefore, it can be said that the second solution obtained in the first step is a solution that substantially contains an alkali metal carbonate and an alkali metal bicarbonate.

[0059] In the method for fixating carbon dioxide which is one embodiment of the present invention, the content of alkali metal carbonate in the second solution is not particularly limited because carbon dioxide can be fixed regardless of the content. However, from the viewpoint of more efficient fixation of carbon dioxide, the proportion of the alkali metal that becomes the alkali metal carbonate relative to the total amount of alkali metal contained in the second solution is preferably 90 mol % or more, more preferably 95 mol % or more, and even more preferably 99 mol % or more. The upper limit is preferably as high as possible from the viewpoint of more efficient fixation of carbon dioxide, and may be set at 100 mol % because it can be adjusted relatively easily by means of, for example, adjusting the pH as described above.

[0060] The concentration of the alkali metal contained in the second solution is preferably 0.1 mol / L or more, more preferably 0.25 mol / L or more, and even more preferably 0.4 mol / L or more. By setting the alkali metal concentration and the carbonate ion concentration within the above ranges, the reaction efficiency of the formulas (3) and (4) can be increased, and the amount of calcium carbonate produced can be increased. Furthermore, when waste gypsum board is used as the gypsum-containing material, the treatment efficiency of the waste gypsum board can be increased. For the same reason, the concentration of carbonate ions contained in the second solution is preferably 0.05 mol / L or more, more preferably 0.10 mol / L or more, and even more preferably 0.25 mol / L or more.

[0061] The upper limit of the concentration of the alkali metal contained in the second solution is not particularly limited from the viewpoint of efficient fixation of carbon dioxide, and the higher the concentration, the better. However, taking into consideration the need to suppress increases in equipment costs due to the specifications of the apparatus and the like, the upper limit is preferably 2.0 mol / L or less, more preferably 1.5 mol / L or less, and even more preferably 1.0 mol / L or less. For the same reason, the upper limit of the carbonate ion concentration in the second solution is preferably 1.5 mol / L or less, more preferably 1.0 mol / L or less, and even more preferably 0.5 mol / L or more.

[0062] The concentration of alkali metals in the second solution can be measured by the same method as that for measuring the concentration of alkali metals in the first solution. The concentration of carbonate ions in the second solution is a value calculated from the concentration of carbon measured using a total organic carbon (TOC) measuring device.

[0063] (contact) In the second step, the second solution is contacted with the gypsum-containing material to obtain calcium carbonate. As described above, the gypsum-containing material is a solid or a mixed liquid (slurry), and the second solution is a liquid. Therefore, the contact between the second solution and the gypsum-containing material is solid-liquid contact or liquid-liquid contact. When the gypsum-containing material is a solid, the solid gypsum-containing material is added to the second solution, or the second solution is added to the solid gypsum-containing material and mixed. When the gypsum-containing material is a mixed liquid (slurry), the second solution and the gypsum-containing material are mixed.

[0064] More specifically, preferred methods for contacting the second solution with the gypsum-containing material include supplying the second solution to a container containing the gypsum-containing material. In this case, for example, there are (i) a method in which a gypsum-containing material containing a predetermined concentration of calcium sulfate is placed in a container and a predetermined amount of the second solution is supplied according to the calcium sulfate concentration, (ii) a method in which a predetermined amount of the second solution is placed in a container and the gypsum-containing material containing calcium sulfate is supplied, which is the opposite of (i), or (ii) a method in which the gypsum-containing material and the second solution are continuously or intermittently supplied to a container so that a predetermined ratio between the gypsum-containing material and the second solution is satisfied. The above methods (i), (i), and (ii) are applicable whether the gypsum-containing material is a solid or a mixed liquid (slurry).

[0065] For the contact of the second solution with the gypsum-containing material, for example, a reactor having a container capable of storing the gypsum-containing material and a supply port through which the second solution can be supplied, or a reactor having a container capable of storing the second solution and a supply port through which the gypsum-containing material can be supplied, can be used.

[0066] The supply port capable of supplying the second solution preferably includes a supply means according to the method of supplying the second solution. For example, when the second solution is supplied dropwise, it is preferable to include a drip nozzle. It is also preferable to include a flow rate adjusting means. When the gypsum-containing material is supplied as a mixed liquid (slurry), the same applies as for the second liquid. The reactor used for bringing the second solution into contact with the gypsum-containing material is preferably equipped with a stirrer in order to promote the contact between the second solution and the gypsum-containing material and to more efficiently proceed with the above reaction formulas (3) and (4).

[0067] The contact between the second solution and the gypsum-containing material may be carried out by a flow system or a batch system (batch system). The above (ia) and (ib) are basically batch systems (batch systems), but they can also be carried out like a flow system by supplying a gypsum-containing material every time the gypsum-containing material in the container is consumed due to contact with the second solution. The type of contact in the second step may be determined by taking into consideration the type of contact in the first step, for example, the site area available for carrying out the method for fixation of carbon dioxide according to one embodiment of the present invention, requirements regarding production efficiency, and other various circumstances in total.

[0068] The molar ratio of carbonate ions contained in the second solution to calcium sulfate (CaSO4) contained in the gypsum-containing material (carbonate ions / calcium sulfate) is preferably at least 1.0 or more, based on the above reaction formula (3). The upper limit is preferably 1.2 or less, more preferably 1.1 or less, from the viewpoint of efficiently proceeding the reaction of reaction formula (3) by supplying carbonate ions in excess of calcium sulfate.

[0069] Furthermore, the molar ratio of bicarbonate ions contained in the second solution to calcium sulfate (CaSO4) contained in the gypsum-containing material (bicarbonate ions / calcium sulfate) is preferably at least 2.0 or more, more preferably 2.3 or more, and even more preferably 2.5 or more, based on the above reaction formula (4). The upper limit is preferably 3.5 or less, more preferably 3.2 or less, and even more preferably 3.0 or less, from the viewpoint of supplying carbonate ions in excess of calcium sulfate to efficiently progress the reaction of reaction formula (4).

[0070] The reaction temperature of the reaction caused by the contact in the second step is not particularly limited and may be, for example, between 10 and 80°C. From the viewpoint of more efficient reaction progression, the reaction temperature is preferably 15 to 60°C, more preferably 20 to 45°C. The reaction time is not particularly limited and may be, for example, 10 minutes to 1 hour, and from the viewpoint of more efficient reaction progression, it is preferably 15 to 50 minutes, more preferably 20 to 45 minutes.

[0071] (Calcium carbonate) Calcium carbonate is produced in the second step. The calcium carbonate produced in the second step is poorly soluble in water, so the calcium carbonate forms a precipitate. In the carbon dioxide fixation method according to one embodiment of the present invention, the precipitated calcium carbonate may be recovered. The calcium carbonate may be recovered by, for example, decantation, filtration, or the like, and may further be dried by heating. The recovered calcium carbonate can be used in cement compositions, materials for mortar and concrete, fillers, building materials, etc.

[0072] In a method for fixation of carbon dioxide according to one embodiment of the present invention, carbon dioxide contained in gas, more specifically, carbon dioxide contained in exhaust gas generated from various facilities such as power plants, incinerators, and factory facilities, is fixed as calcium carbonate. Because calcium carbonate is a valuable material as described above, the method for fixation of carbon dioxide according to one embodiment of the present invention can be said to be a fixation method that not only simply fixes carbon dioxide but also adds value. Furthermore, by using waste materials such as waste gypsum board and by-products such as hydrofluoric gypsum as the gypsum-containing material, there is also the advantage that these waste materials and by-products can be effectively reused without being discarded. The method for fixing carbon dioxide, which is one aspect of the present invention, is a method that can fix carbon dioxide efficiently, and not only provides economic added value, but also has the secondary effect of reducing the environmental load by reducing waste and by-products.

[0073] [Method for producing calcium carbonate] One aspect of the present invention is a first step of contacting a first solution containing an alkali metal hydroxide with a gas containing carbon dioxide to produce a second solution containing at least one alkali metal salt selected from the group consisting of an alkali metal carbonate and an alkali metal bicarbonate; a second step of contacting the second solution with a gypsum-containing material to produce calcium carbonate; Including, A method for producing calcium carbonate.

[0074] In the method for producing calcium carbonate according to one embodiment of the present invention, the first step and the second step are the same as those described in the method for fixing carbon dioxide. As explained in the above-mentioned method for fixation of carbon dioxide, carbon dioxide can be fixed efficiently by the first step and the second step, and calcium carbonate is obtained by fixating carbon dioxide, so efficient fixation of carbon dioxide means efficient production of calcium carbonate. Therefore, according to the method for producing calcium carbonate which is one aspect of the present invention, it is possible to efficiently produce calcium carbonate.

[0075] According to one embodiment of the method for producing calcium carbonate of the present invention, the yield of calcium carbonate can be 90% by mass or more, 95% by mass or more, 98% by mass or more, 99% by mass or more, 99.5% by mass or more, or even 100% by mass. Although a portion of calcium carbonate that is soluble at the temperature during contact in the second step, i.e., the temperature of the second solution, may remain in the second solution, the remainder being precipitated as calcium carbonate and can be recovered. Here, the yield of calcium carbonate is the mass ratio of the amount of calcium (Ca) in the calcium carbonate to the amount of calcium (Ca) supplied by the gypsum-containing material. As described above, it can be said that the method for producing calcium carbonate according to one aspect of the present invention makes it possible to efficiently produce calcium carbonate.

[0076] Furthermore, according to the method for producing calcium carbonate which is one aspect of the present invention, not only can calcium carbonate be produced efficiently, but also carbon dioxide contained in exhaust gases and the like can be fixed, thereby contributing to environmental improvement. Furthermore, by using waste materials such as waste gypsum boards and by-products such as hydrofluoric gypsum as the gypsum-containing material, these waste materials and by-products can be effectively reused without being discarded, and there are also advantages in that they contribute to environmental improvement.

[0077] [How to use waste gypsum board] One aspect of the present invention is contacting a first solution containing an alkali metal hydroxide with a gas containing carbon dioxide to produce a second solution containing at least one alkali metal salt of an alkali metal carbonate or an alkali metal bicarbonate; and a step of contacting the second solution with waste gypsum boards to generate calcium carbonate; Including, This is a way to utilize waste gypsum board.

[0078] In one embodiment of the present invention, the method for utilizing waste gypsum boards includes the same steps as those described in the carbon dioxide fixation method, and in the second step, waste gypsum boards are used as the gypsum-containing material. As explained in the above method for fixating carbon dioxide, carbon dioxide can be fixed efficiently by the first and second steps, and calcium carbonate can be obtained by fixating carbon dioxide even when waste gypsum board, which is a waste material, is used as a gypsum-containing material.

[0079] When waste gypsum board is disposed of, it must be disposed of as industrial waste, which incurs costs just for the disposal itself. However, according to a method for utilizing waste gypsum board, which is one aspect of the present invention, it is possible to reduce the amount of waste gypsum board disposed of and produce calcium carbonate, which is a valuable resource. This method for utilizing waste gypsum board is a new method that is not a conventionally known method. Not only is it a new method of utilization, but it can also add economic value, fix carbon dioxide, and reduce waste, thereby greatly contributing to environmental improvement. [Example]

[0080] [Example 1] A simulated exhaust gas was produced by mixing 80% by volume of air and 20% by volume of CO2 gas, simulating the exhaust gas emitted from a cement factory.

[0081] A sodium hydroxide solution (0.5N) was prepared as a first solution using tap water as a solvent and sodium hydroxide (NaOH, 20% by mass, special grade reagent, manufactured by Kanto Chemical Co., Inc.). 1 L of this sodium hydroxide solution was stored in a container with a supply port at the bottom that could supply gas containing carbon dioxide, and the above-mentioned simulated exhaust gas (80% by volume of N2, 20% by volume of CO2) was aerated through the supply port at a flow rate of 2 L / min for 30 minutes. During the aeration process, the pH of the solution in the container was 11, and a second solution containing sodium carbonate (Na2CO3) was obtained. The sodium concentration in the resulting second solution was 0.5 mol / L, and the carbonate ion concentration was 0.25 mol / L.

[0082] A second solution (4 L) containing an amount equivalent to 1 mol of carbonate ions was added all at once to a container containing 1 mol of gypsum dihydrate powder (average particle size: 0.5 mm), and the mixture was stirred for 30 minutes at 20° C. After stirring, the reaction solution was filtered, and the recovered product was dried.

[0083] To confirm whether the alkali metal salt contained in the second solution was sodium carbonate or sodium bicarbonate, a part of the second solution was heated and the resulting powder was subjected to X-ray diffraction measurement by the following method. In addition, the recovered material obtained by the drying was also subjected to X-ray diffraction measurement in the same manner.

[0084] (X-ray diffraction measurement) The measurement was carried out using a powder X-ray diffractometer ("X'pert Pro" (trade name), manufactured by PANalytical) under the following measurement conditions. Measurement range: 2θ=10~70° Step size: 0.01° Scan speed: 0.05° / s Voltage: 45kV Current: 40mA

[0085] Material identification was performed from the X-ray diffraction profile obtained by X-ray diffraction measurement, and it was confirmed that the majority of the alkali metal salt contained in the second solution was sodium carbonate (approximately 99.5 mol%). Material identification was also performed similarly on the recovered material obtained by drying, and it was confirmed that the entire amount was calcium carbonate. In addition, the yield of calcium carbonate was 100%.

[0086] [Example 2] A second solution was obtained in the same manner as in Example 1, except that simulated exhaust gas (80% by volume of N2, 20% by volume of CO2) was aerated into 1 L of a sodium hydroxide solution (0.5 N) at a flow rate of 2 L / min for 45 minutes and the pH was adjusted to 10. The obtained second solution had a sodium concentration of 0.5 mol / L and a carbonate ion concentration of 0.34 mol / L. A portion of the obtained second solution was sampled, and the dissolved substances in the second solution were identified by the above-mentioned method, confirming that the second solution was composed of half sodium carbonate and half sodium hydrogencarbonate (50 mol% each).

[0087] The second solution (2.94 L) in an amount corresponding to 1 mol of carbonate ion content was added all at once to a container containing 1 mol of gypsum dihydrate powder (particle size: 0.5 mm or less), and the mixture was stirred for 30 minutes at 20° C. After stirring, the reaction solution was filtered, and the recovered product was dried. X-ray diffraction measurement was performed on the recovered material of Example 2 under the same conditions as in Example 1, and an X-ray diffraction profile was obtained. As a result of material identification from the X-ray diffraction profile, it was confirmed that the entire amount of the recovered material obtained was calcium carbonate. In addition, the yield of calcium carbonate was 100%.

[0088] [Example 3] Waste gypsum board was prepared as a gypsum-containing material. Wallpaper was removed from the surface of the waste gypsum board and crushed. The crushed waste gypsum board was classified using a sieve, and powder of 5 mm or less was collected. The calcium sulfate content in the resulting powder of waste gypsum board was 93% by mass. The calcium sulfate content in the waste gypsum board was measured in accordance with JIS R 9101:2018 "Methods for chemical analysis of gypsum."

[0089] Waste gypsum board powder (equivalent to 1 mol of calcium sulfate) was placed in a container. The same second solution as in Example 1 was added to the container all at once in an amount corresponding to a carbonate ion content of 1 mol, and the mixture was stirred at 20°C for 30 minutes. After stirring, the reaction solution was filtered, and the recovered material was dried. X-ray diffraction measurement was performed on the recovered material of Example 3 under the same conditions as in Example 1, and an X-ray diffraction profile was obtained. As a result of material identification from the X-ray diffraction profile, it was confirmed that the entire amount of the recovered material obtained was calcium carbonate. In addition, the yield of calcium carbonate was 100%.

Claims

1. a first step of contacting a first solution containing an alkali metal hydroxide with a gas containing carbon dioxide to produce a second solution containing at least one alkali metal salt of an alkali metal carbonate or an alkali metal bicarbonate; a second step of bringing the second solution into contact with a gypsum-containing material to produce calcium carbonate, wherein the concentration of an alkali metal contained in the second solution is 0.1 mol / L or more and 2.0 mol / L or less, the concentration of carbonate ions contained in the second solution is 0.05 mol / L or more and 1.5 mol / L or less, and the gypsum-containing material is selected from gypsum board, waste gypsum board, a product recovered from a flue gas desulfurization treatment, hydrofluoric gypsum, phosphate gypsum, titanic gypsum, activated silicate gypsum, gypsum-containing sludge obtained in a wastewater treatment process, and incineration ash.

2. 2. The method for fixation of carbon dioxide according to claim 1, wherein the first solution is contacted with the gas containing carbon dioxide at a pH of 8.5 to 13.

5.

3. 3. The method for fixation of carbon dioxide according to claim 1, wherein the gypsum content in the gypsum-containing material is 1% by mass or more in terms of calcium sulfate.

4. The method for fixation of carbon dioxide according to any one of claims 1 to 3, wherein the gypsum-containing material is waste gypsum board.

5. a first step of contacting a first solution containing an alkali metal hydroxide with a gas containing carbon dioxide to produce a second solution containing at least one alkali metal salt of an alkali metal carbonate or an alkali metal bicarbonate; a second step of contacting the second solution with a gypsum-containing material to produce calcium carbonate, wherein the second solution has an alkali metal concentration of 0.1 mol / L or more and 2.0 mol / L or less, and a carbonate ion concentration of 0.05 mol / L or more and 1.5 mol / L or less, and the gypsum-containing material is selected from gypsum board, waste gypsum board, a recovered product from a flue gas desulfurization treatment, hydrofluoric gypsum, phosphogypsum, titanic gypsum, activated silicate gypsum, gypsum-containing sludge obtained in a wastewater treatment process, and incineration ash.

6. The method for producing calcium carbonate according to claim 5, wherein the first solution is contacted with the gas containing carbon dioxide at a pH of 8.5 to 13.

5.

7. 7. The method for producing calcium carbonate according to claim 5, wherein the content of gypsum contained in the gypsum-containing material is 1% by mass or more in terms of calcium sulfate.

8. The method for producing calcium carbonate according to any one of claims 5 to 7, wherein the gypsum-containing material is waste gypsum board.

9. a first step of contacting a first solution containing an alkali metal hydroxide with a gas containing carbon dioxide to produce a second solution containing at least one alkali metal salt of an alkali metal carbonate or an alkali metal bicarbonate; a second step of contacting the second solution with waste gypsum board to produce calcium carbonate, wherein the concentration of alkali metal contained in the second solution is 0.1 mol / L or more and 2.0 mol / L or less, and the concentration of carbonate ions contained in the second solution is 0.05 mol / L or more and 1.5 mol / L or less.

10. The method for utilizing waste gypsum board according to claim 9, wherein the first solution is contacted with the gas containing carbon dioxide at a pH of 8.5 or more and 13.5 or less.

11. The method for utilizing waste gypsum boards according to claim 9 or 10, wherein the content of gypsum contained in the waste gypsum boards is 1% by mass or more in terms of calcium sulfate.