Carbon dioxide capture methods

Abstract

The present invention provides a method for recovering carbon dioxide and a method for producing carbonates. [Solution] An alkaline mixture A containing alkali metal ions (e.g., sodium ions) and water is brought into contact with a gas B containing carbon dioxide.

Description

Technical Field

[0001] The present invention relates to a method for recovering carbon dioxide and the like.

Background Art

[0002] In recent years, there has been a strong demand for reducing carbon dioxide emissions. However, it is impossible to completely eliminate carbon dioxide emissions. While aiming to reduce emissions, it is also important to consider methods for recovering (utilizing) the emitted carbon dioxide.

[0003] As such a method, for example, a method of recovering (immobilizing) carbon dioxide as magnesium carbonate by utilizing the reaction between carbon dioxide and a magnesium source can be considered (for example, Non-Patent Document 1, etc.).

Prior Art Documents

Non-Patent Documents

[0004]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] An object of the present invention is to provide a method for recovering (immobilizing) carbon dioxide, a method for producing carbonate, and the like.

Means for Solving the Problems

[0006] As described above, carbon dioxide can be recovered as magnesium carbonate. However, according to the study by the present inventor, simply reacting with a magnesium source (such as magnesium hydroxide) may not be able to efficiently recover carbon dioxide in some cases.

[0007] Under these circumstances, the inventors, after diligent research, discovered that carbon dioxide can be efficiently recovered by a specific method, and that carbon dioxide can be efficiently recovered as carbonate (carbonate can be produced). Further research led to the completion of the present invention.

[0008] In other words, the present invention relates to the following inventions, etc. [1] A method for recovering carbon dioxide [carbon dioxide recovery (fixation) method], comprising at least a carbon dioxide recovery step (carbon dioxide fixation step) of contacting an alkaline mixture A containing alkali metal ions (particularly sodium ions) and water with a gas B containing carbon dioxide to recover carbon dioxide [to absorb (fix) the carbon dioxide in gas B into mixture A]. [2] A method for producing a carbonate (particularly a carbonate containing at least an alkali metal carbonate such as sodium carbonate), comprising at least a carbonate production step of contacting an alkaline mixture A containing alkali metal ions (particularly sodium ions) and water with a gas B containing carbon dioxide to produce a carbonate (particularly a carbonate containing at least an alkali metal carbonate such as sodium carbonate). [3] The method according to [1] or [2], wherein the pH of mixture A is 10.6 or higher, and the proportion of alkali metal ions in mixture A is 0.0005 mol / L or higher. [4] The method according to any one of [1] to [3], wherein the pH of mixture A is 11.2 or higher, and the proportion of alkali metal ions in mixture A is 0.0015 mol / L or higher. [5] The method according to any one of [1] to [4], wherein the proportion of alkali metal ions in mixture A (as far as mixture A is concerned) is 30 mol / L or less. [6] The method according to any one of [1] to [5], wherein the proportion of alkali metal ions in mixture A (as far as mixture A is concerned) is 3.5 mol / L or less. [7] The method according to any one of [1] to [6], wherein the proportion of alkali metal ions in mixture A is 1.2 mol / L or less. [8] Alkali metal ions contain sodium ions, The pH of mixture A is 10.6 or higher. The method according to any one of [1] to [7], wherein the proportion of alkali metal ions in mixture A is 0.0005 to 30 mol / L. [9] Alkali metal ions contain sodium ions, The pH of mixture A is 11.2 or higher. The method according to any one of [1] to [8], wherein the proportion of alkali metal ions in mixture A is 0.0015 to 1.2 mol / L.

[0009]

[10] The method according to any one of [1] to [9], wherein mixture A does not contain magnesium ions, or contains magnesium ions (magnesium ion source) in a proportion of 99.99 mol% or less relative to the total amount of alkali metal ions and magnesium ions.

[11] The method according to any one of [1] to

[10] , wherein mixture A does not contain magnesium ions, or contains magnesium ions (magnesium ion source) in a proportion of 95 mol% or less relative to the total amount of alkali metal ions and magnesium ions.

[12] The method according to any one of [1] to

[11] , wherein mixture A does not contain calcium ions, or contains calcium ions (calcium ion source) in a proportion of 99.99 mol% or less relative to the total amount of alkali metal ions and calcium ions in mixture A.

[13] The method according to any of [1] to

[12] , wherein the concentration of carbon dioxide in gas B (the proportion of carbon dioxide in gas B) is 0.05 volume% or more.

[14] The method according to any one of [1] to

[13] , wherein the concentration of carbon dioxide in gas B is 0.3 volume% or more.

[15] The method according to any one of [1] to

[14] , wherein the concentration of carbon dioxide in gas B is 20% by volume or less.

[16] The method according to any one of [1] to

[15] , wherein the concentration of carbon dioxide in gas B is less than 3.5% by volume.

[17] The method according to any one of [1] to

[16] , wherein the concentration of carbon dioxide in gas B is 0.1 to 3.3% by volume.

[18] The method according to any one of [1] to

[17] , wherein the rate of gas B with respect to 1 L of mixture A (introduction rate, rate of gas B to be contacted) is 50 L / min or less.

[19] The method according to any one of [1] to

[18] , wherein the rate of gas B with respect to 1 L of mixture A (introduction rate, rate of gas B to be contacted) is 15 L / min or less.

[0010]

[20] The alkali metal ions contain sodium ions, the pH of mixture A is 10.6 or more, the proportion of alkali metal ions in mixture A is 0.0005 to 30 mol / L, the concentration of carbon dioxide in gas B is 0.05% by volume or more, The method according to any one of [1] to

[19] , wherein the rate of gas B with respect to 1 L of mixture A (introduction rate, rate of gas B to be contacted) is 50 L / min or less.

[21] The alkali metal ions contain sodium ions, the pH of mixture A is 10.8 or more, the proportion of alkali metal ions in mixture A is 0.0007 to 20 mol / L, the concentration of carbon dioxide in gas B is 0.1% by volume or more, The method according to any one of [1] to

[20] , wherein the rate of gas B with respect to 1 L of mixture A (introduction rate, rate of gas B to be contacted) is 15 L / min or less.

[22] Alkali metal ions contain sodium ions, The pH of mixture A is 11.2 or higher. The proportion of alkali metal ions in mixture A is 0.0015 to 12 mol / L. The concentration of carbon dioxide in gas B is 0.1 to 20% by volume. The method according to any one of [1] to

[21] , wherein the rate of gas B relative to 1 L of mixture A (introduction rate, rate of gas B in contact) is 10 L / min or less. [twenty three] Alkali metal ions contain sodium ions, The pH of mixture A is 11.2 or higher. The proportion of alkali metal ions in mixture A is 0.015 to 3 mol / L. The concentration of carbon dioxide in gas B is 0.1 to 20% by volume. The method according to any one of [1] to

[22] , wherein the rate of gas B relative to 1 L of mixture A (introduction rate, rate of gas B in contact) is 8 L / min or less. [twenty four] Alkali metal ions contain sodium ions, The pH of mixture A is 13 or higher. The proportion of alkali metal ions in mixture A is 0.015 to 3 mol / L. Mixture A either does not contain magnesium ions, or contains a magnesium ion source in a proportion of 80 mol% or less relative to the total amount of alkali metal ions and magnesium ions. The carbon dioxide concentration in gas B is 0.3 to 15% by volume. The method according to any one of [1] to

[23] , wherein the rate of gas B relative to 1 L of mixture A (introduction rate, rate of gas B in contact) is 6.7 L / min or less. [twenty five] The method according to any one of [2] to

[24] , wherein the carbonate comprises at least an alkali metal carbonate (in particular, sodium carbonate).

[26] A carbonate production step described in any of [2] to

[25] , A method for producing alkaline earth metal carbonate, comprising an alkaline earth metal carbonate production step, in which the carbonate obtained in the carbonate production step is brought into contact with a mixture B containing at least alkaline earth metal ions (particularly calcium ions) and water to produce an alkaline earth metal carbonate (particularly calcium carbonate).

[27] The method according to

[26] , wherein mixture B further comprises chloride ions (e.g., sodium chloride). [Effects of the Invention]

[0011] This invention provides a method for capturing carbon dioxide. This method allows for the efficient recovery (absorption) of carbon dioxide from gases containing it.

[0012] In another aspect of the present invention, a method for producing carbonates can be provided. This method allows for the efficient recovery (absorption) of carbon dioxide from gases containing carbon dioxide, using carbonates (such as sodium carbonate).

[0013] The carbonate obtained by the present invention can also be used as a raw material for the production of various compounds.

[0014] For example, the carbonate can produce an alkaline earth metal carbonate [for example, calcium carbonate (for example, calcium carbonate with a calcite crystal structure)] upon contact with an alkaline earth metal ion (for example, calcium ion).

[0015] Therefore, the present invention can also provide a method for producing various compounds (for example, calcium carbonate) while efficiently recovering (absorbing) carbon dioxide. [Modes for carrying out the invention]

[0016] <Methods for capturing carbon dioxide and producing carbonates> In the method of the present invention, carbon dioxide is recovered through a process (carbon dioxide recovery step, carbon dioxide fixation step) in which an alkaline mixture A containing alkali metal ions and water is brought into contact with a gas B containing carbon dioxide. Since carbon dioxide is recovered in such a process [by absorbing (fixing) the carbon dioxide in gas B into mixture A], this method can be called a carbon dioxide recovery (fixation, separation, removal) method.

[0017] In such a process, carbon dioxide can also be recovered (immobilized, separated, and removed) as carbonate. Since such a process produces carbonate (particularly carbonate containing at least an alkali metal carbonate such as sodium carbonate), it can also be called a carbonate production process. Therefore, the present invention can also provide a method for producing carbonate via such a process (carbonate production process).

[0018] The following describes the process in detail (carbon dioxide process, carbonate production process).

[0019] [Mixture A] Mixture A is alkaline (basic).

[0020] The pH of such mixture A may be selected from, for example, a range of 8 or higher (or 8 to 14) (e.g., 8.5 or higher, 8.5 to 14, 9 or higher, 9.5 or higher), 10 or higher (e.g., 10.5 or higher, 10.6 or higher), preferably 11 or higher (e.g., 11.2 or higher, 11.5 or higher), more preferably 12 or higher (e.g., 12.2 or higher, 12.5 or higher, 12.7 or higher), and especially 13 or higher (e.g., 13.2 or higher, 13.5 or higher, 13.8 or higher, 14). Note that the pH value may be the value at 25°C.

[0021] Examples of alkali metal ions in mixture A include sodium ions, potassium ions, rubidium ions, cesium ions, and the like.

[0022] Mixture A may contain one or more alkali metal ions.

[0023] Alkali metal ions are usually at least sodium ions. Therefore, alkali metal ions may contain sodium ions.

[0024] In such cases, the alkali metal ions may consist solely of sodium ions, or they may include other alkali metal ions (alkali metal ions other than sodium ions).

[0025] If other alkali metal ions (such as potassium ions) are present, the ratio of other alkali metal ions to the total alkali metal ions may be 20 mol% or less, 15 mol% or less, 10 mol% or less, 8 mol% or less, 5 mol% or less, 3 mol% or less, 2 mol% or less, 1 mol% or less, 0.5 mol% or less, 0.1 mol% or less, 0.05 mol% or less, 0.03 mol% or less, 0.01 mol% or less, 0.005 mol% or less, 0.001 mol% or less, etc.

[0026] Furthermore, if other alkali metal ions (such as potassium ions) are present, the lower limit of the ratio of other alkali metal ions to the total alkali metal ions may be, for example, 0.00001 mol% or more, 0.0001 mol% or more, 0.0005 mol% or more, 0.001 mol% or more, 0.005 mol% or more, 0.01 mol% or more, or 0.01 mol% or more.

[0027] Furthermore, the range can also be defined by appropriately combining the lower and upper limits of the above range (the same applies to the description of the range below).

[0028] Alkali metal ions may originate from alkali metal compounds (compounds composed of alkali metal ions, mixture A or compounds that generate alkali metal ions in water) [they may also be included in mixture A as alkali metal ions formed by the dissociation (ionization) of alkali metal compounds].

[0029] Examples of such compounds include hydroxides (e.g., sodium hydroxide, potassium hydroxide, etc.), non-hydroxides {e.g., halides (e.g., sodium chloride, potassium chloride, sodium bromide, sodium iodide, etc.), sulfates (e.g., sodium sulfate), phosphates (e.g., sodium phosphate), nitrates (e.g., sodium nitrate), etc. salts [e.g., inorganic salts (salts with inorganic acids, etc.)]}.

[0030] Furthermore, alkali metal ions and alkali metal compounds (e.g., sodium chloride) may be derived from seawater.

[0031] Alkali metal compounds may be used individually or in combination of two or more.

[0032] Typical alkali metal compounds include hydroxides (e.g., sodium hydroxide) and halides (e.g., sodium chloride, potassium chloride).

[0033] In particular, hydroxides (for example, sodium hydroxide) may be suitably used from the viewpoint of efficient carbon dioxide recovery. In particular, depending on the pH and the proportion of alkali metal ions in mixture A, using hydroxides among alkali metal compounds can easily lead to efficient carbon dioxide recovery.

[0034] Therefore, the alkali metal compound (mixture A) may contain hydroxides (for example, sodium hydroxide) (mixture A may contain alkali metal ions obtained by the dissociation (ionization) of hydroxides).

[0035] In such cases, the alkali metal compound may consist solely of hydroxides, or it may contain other alkali metal compounds (alkali metal compounds that are not hydroxides).

[0036] If the alkali metal compound (mixture A) contains hydroxide, the proportion of hydroxide to the total alkali metal compound may be, for example, 1 mol% or more (e.g., 5 mol% or more), preferably 10 mol% or more (e.g., 15 mol% or more), more preferably 20 mol% or more (e.g., 25 mol or more), and may also be 30 mol% or more, 40 mol% or more, 50 mol% or more, 60 mol% or more, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, 100 mol%, etc.

[0037] When alkali metal compounds (mixture A) contain hydroxides and other alkali metal compounds (halides, etc.), the ratio of other alkali metal compounds (e.g., sodium chloride) to the total amount of hydroxides (e.g., sodium hydroxide) and other alkali metal compounds (e.g., sodium chloride) (or the entire alkali metal compound) may be selected from a range of approximately 99.9999 mol% or less (e.g., 99.9995 mol% or less). The following are examples of the following: preferably 99.99 mol% or less (for example, 99.95 mol% or less), more preferably 99.9 mol% or less (for example, 99.85 mol% or less), and it may also be 99.5 mol% or less, 99 mol% or less, 97 mol% or less, 96 mol% or less, 95 mol% or less, 93 mol%, 90 mol% or less, 88 mol% or less, 85 mol% or less, 80 mol% or less, 75 mol% or less, 70 mol% or less, 65 mol% or less, 60 mol% or less, 55 mol% or less, 50 mol% or less, etc.

[0038] When an alkali metal compound (mixture A) contains hydroxides and other alkali metal compounds (halides, etc.), the lower limit of the ratio of the other alkali metal compound (e.g., sodium chloride) to the total amount of hydroxides (e.g., sodium hydroxide) and other alkali metal compounds (e.g., sodium chloride) (or the entire alkali metal compound) may be, for example, 0.0001 mol% or more, 0.001 mol% or more, 0.005 mol% or more, 0.01 mol% or more, 0.05 mol% or more, 0.1 mol% or more, 0.15 mol% or more, 0.5 mol% or more, 1 mol% or more, 5 mol% or more, 10 mol% or more, 15 mol% or more, 20 mol% or more, 25 mol% or more, 30 mol% or more, 35 mol% or more, 40 mol% or more, 45 mol% or more, 50 mol% or more, etc.

[0039] In mixture A, the proportion (concentration, the same applies hereinafter) of alkali metal ions (or alkali metal compounds) [lower limit of proportion (concentration)] may be selected from a range of approximately 0.0001 mol / L or more, preferably 0.0005 mol / L or more (for example, 0.0008 mol / L or more), preferably 0.001 mol / L or more (for example, 0.0012 mol / L or more), and more preferably 0.0015 mol / L or more (for example, 0.002 mol / L or more). It may be 0.005 mol / L or more, especially 0.01 mol / L or more (for example, 0.01 mol / L or more), or 0.05 mol / L or more (for example, 0.08 mol / L or more, 0.1 mol / L or more, 0.2 mol / L or more, 0.3 mol / L or more, 0.4 mol / L or more, 0.5 mol / L or more, 0.6 mol / L or more, 0.7 mol / L or more, 0.8 mol / L or more, 0.9 mol / L or more, 1 mol / L or more).

[0040] In mixture A, the proportion (upper limit of the proportion) of alkali metal ions (or alkali metal compounds) may be selected from a range of approximately 100 mol / L or less (for example, 80 mol / L or less, 50 mol / L or less, 40 mol / L or less, 30 mol / L or less), for example, 25 mol / L or less (for example, 20 mol / L or less), preferably 15 mol / L or less (for example, 12 mol / L or less), and even more preferably 10 mol / L or less (for example, 8 mol / L or less, 7 mol / L or less, 6 mol / L or less, 5 mol / L or less, 4.4 mol / L or less), and especially 3 mol / L or less (for example, 2.8 mol / L or less, 2.5 mol / L or less, 2.2 mol / L or less, 2 mol / L or less), and may be less than 2 mol / L (for example, 1.8 mol / L or less, 1.5 mol / L or less, 1.2 mol / L or less, 1 mol / L or less).

[0041] By setting the alkali metal ion ratio as described above (upper and / or lower limits, in particular, at least the upper limit, preferably both the upper and lower limits), the efficiency of carbon dioxide recovery can be easily increased. Furthermore, this effect can become even more pronounced when combined with other conditions (for example, one or more conditions selected from pH, the ratio of alkaline earth metal ions if present, the ratio of carbon dioxide in gas B, the rate of gas B relative to 1 L of mixture A, etc.).

[0042] Mixture A may or may not contain components that are not alkali metal ions (alkali metal compounds), as long as efficient carbon dioxide recovery is achieved.

[0043] Examples of such components include non-alkali metal ions (metal ions that are not alkali metal ions, such as alkaline earth metal ions).

[0044] Mixture A may contain one or more non-alkali metal ions.

[0045] Among them, magnesium ions and calcium ions are representative examples. Therefore, mixture A may contain at least one selected from magnesium ions and calcium ions.

[0046] In this invention, even with a mixture A containing magnesium ions, calcium ions, etc. (where magnesium ions, calcium ions, etc. coexist with alkali metal ions), carbon dioxide can be efficiently recovered.

[0047] Therefore, it is possible to use a mixture A as a raw material (carbon dioxide to be recovered) that does not separate these components or does not separate them excessively (strictly), and consequently, the carbon dioxide recovery process as a whole can be carried out efficiently.

[0048] Furthermore, non-alkali metal ions may originate from non-alkali metal compounds (compounds composed of non-alkali metal ions, or compounds that can generate non-alkali metal ions in mixture A or water) (they may also be included in mixture A as non-alkali metal compounds).

[0049] Examples of such non-alkali metal compounds (magnesium compounds, calcium compounds) include hydroxides (e.g., magnesium hydroxide, calcium hydroxide, etc.), non-hydroxides {e.g., halides (e.g., magnesium chloride, calcium chloride, magnesium bromide, calcium iodide, etc.), sulfates (e.g., magnesium sulfate, calcium sulfate), phosphates (e.g., magnesium phosphate, calcium phosphate), nitrates (e.g., magnesium nitrate, calcium nitrate), etc. salts [e.g., inorganic salts (salts with inorganic acids, etc.)]}.

[0050] Furthermore, non-alkali metal ions and non-alkali metal compounds may originate from seawater.

[0051] If mixture A contains magnesium ions (magnesium compounds), the ratio of magnesium ions to the total amount of alkali metal ions and magnesium ions may be selected from a range of, for example, 99.9999 mol% or less (for example, 99.9995 mol% or less), preferably 99.99 mol% or less (for example, 99.995 mol% or less), more preferably 99.99 mol% or less (for example, 99.95 mol% or less), and even more preferably 99.9 mol% or less (for example, 99.5 mol% or less, 99 mol% or less, 97 mol% or less). In particular, it may be 95 mol% or less (for example, 93 mol%, 90 mol% or less, 88 mol% or less, 85 mol% or less, 80 mol% or less, 75 mol% or less, 70 mol% or less, 65 mol% or less, 60 mol% or less, 55 mol% or less, 50 mol% or less), etc.

[0052] Furthermore, if mixture A contains magnesium ions (magnesium compounds), the lower limit of the ratio of magnesium ions to the total amount of alkali metal ions and magnesium ions may be, for example, 0.0001 mol% or more, 0.001 mol% or more, 0.005 mol% or more, 0.01 mol% or more, 0.05 mol% or more, 0.1 mol% or more, 0.15 mol% or more, 0.5 mol% or more, 1 mol% or more, 5 mol% or more, 10 mol% or more, 15 mol% or more, 20 mol% or more, 25 mol% or more, 30 mol% or more, 35 mol% or more, 40 mol% or more, 45 mol% or more, 50 mol% or more, etc.

[0053] If mixture A contains calcium ions (calcium compounds), the ratio of calcium ions to the total amount of alkali metal ions and calcium ions may be selected from a range of, for example, 99.9999 mol% or less (for example, 99.9995 mol% or less), preferably 99.99 mol% or less (for example, 99.995 mol% or less), more preferably 99.99 mol% or less (for example, 99.95 mol% or less), and even more preferably 99.9 mol% or less (for example, 99.5 mol% or less, 99 mol% or less, 97 mol% or less), and in particular it may be 95 mol% or less (for example, 93 mol% or less, 90 mol% or less, 88 mol% or less, 85 mol% or less, 80 mol% or less, 75 mol% or less, 70 mol% or less, 65 mol% or less, 60 mol% or less, 55 mol% or less, 50 mol% or less), etc.

[0054] Furthermore, if mixture A contains calcium ions (calcium compounds), the lower limit of the ratio of calcium ions to the total amount of alkali metal ions and calcium ions may be, for example, 0.0001 mol% or more, 0.001 mol% or more, 0.005 mol% or more, 0.01 mol% or more, 0.05 mol% or more, 0.1 mol% or more, 0.15 mol% or more, 0.5 mol% or more, 1 mol% or more, 5 mol% or more, 10 mol% or more, 15 mol% or more, 20 mol% or more, 25 mol% or more, 30 mol% or more, 35 mol% or more, 40 mol% or more, 45 mol% or more, 50 mol% or more, etc.

[0055] By using the above proportions, it is easier to further improve the efficiency of carbon dioxide recovery, even when magnesium ions, calcium ions, etc. are included. Furthermore, this effect can become even more pronounced when combined with other conditions (for example, one or more conditions selected from pH, the proportion of alkali metal ions, the proportion of carbon dioxide in gas B, the rate of gas B relative to 1 L of mixture A, etc.).

[0056] Furthermore, if mixture A contains non-alkali metal ions, the ratio of alkali metal ions (especially sodium ions) to the total amount of alkali metal ions (especially sodium ions) and non-alkali metal ions may be selected from a range of, for example, 99.9999 mol% or less (especially 99.9995 mol% or less), preferably 99.99 mol% or less (especially 99.995 mol% or less), more preferably 99.99 mol% or less (especially 99.95 mol% or less), and even more preferably 99.9 mol% or less (especially 99.5 mol% or less, 99 mol% or less, 97 mol% or less), and especially 95 mol% or less (especially 93 mol%, 90 mol% or less, 88 mol% or less, 85 mol% or less, 80 mol% or less, 75 mol% or less, 70 mol% or less, 65 mol% or less, 60 mol% or less, 55 mol% or less, 50 mol% or less), etc.

[0057] If mixture A contains non-alkali metal ions, the ratio of alkali metal ions (especially sodium ions) to the total amount of alkali metal ions (especially sodium ions) and non-alkali metal ions may be 0.0001 mol% or more, 0.001 mol% or more, 0.005 mol% or more, 0.01 mol% or more, 0.05 mol% or more, 0.1 mol% or more, 0.15 mol% or more, 0.5 mol% or more, 1 mol% or more, 5 mol% or more, 10 mol% or more, 15 mol% or more, 20 mol% or more, 25 mol% or more, 30 mol% or more, 35 mol% or more, 40 mol% or more, 45 mol% or more, 50 mol% or more, 60 mol% or more, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more, etc.

[0058] [Gas B] Gas B, which is brought into contact with mixture A, contains carbon dioxide (CO2).

[0059] The proportion (concentration) of carbon dioxide in gas B may be selected from a range of, for example, 0.001 volume% (vol%) or more (e.g., 0.005 volume%), preferably 0.01 volume% or more (e.g., 0.05 volume%), more preferably 0.1 volume% or more (e.g., 0.15 volume%), even more preferably 0.2 volume% or more (e.g., 0.25 volume%), and particularly preferably 0.3 volume% or more (e.g., 0.35 volume%). It can also be 0.4 volume% or more (for example, 0.45 volume% or more, 0.5 volume% or more, 0.6 volume% or more, 0.7 volume% or more, 0.8 volume% or more, 0.9 volume% or more, 1 volume% or more, 1.2 volume% or more, 1.5 volume% or more, 1.8 volume% or more, 2 volume% or more, 2.2 volume% or more, 2.5 volume% or more, 2.8 volume% or more, 3 volume% or more, 5 volume% or more, 8 volume% or more, 10 volume% or more, 12 volume% or more, 15 volume% or more), etc.

[0060] The proportion (concentration) of carbon dioxide in gas B may be 100% by volume, or it may be less than 100% by volume.

[0061] In cases where the concentration is less than 100% by volume, the proportion (concentration) of carbon dioxide in gas B may be selected from a range such as 99.9% by volume (vol%) or less (for example, 99% by volume or less), and may also be 95% by volume or less, 90% by volume or less, 85% by volume or less, 80% by volume or less, 75% by volume or less, 70% by volume or less, 65% by volume or less, 60% by volume or less, 55% by volume or less, 50% by volume or less, 45% by volume or less, 40% by volume or less, 35% by volume or less, 30% by volume or less, etc.

[0062] In particular, the proportion (concentration) of carbon dioxide in gas B may be relatively low, for example, less than 30 volume% (for example, 28 volume% or less), preferably 25 volume% or less (for example, 22 volume% or less, and even more preferably 20 volume% or less (for example, 18 volume% or less)), and 15 volume% or less (for example, 12 volume% or less, 10 volume% or less, 9 volume% or less, 8 volume% or less, 7 volume% or less, 6.5 volume% or less, 6 volume% or less, 5.5 volume% or less, 5 volume% or less). It may also be less than 5 volume%, 4.5 volume% or less, 4 volume% or less, less than 4 volume%, 3.8 volume% or less, 3.5 volume% or less, less than 3.5 volume%, 3.4 volume% or less, 3.3 volume% or less, 3.2 volume% or less, 3.1 volume% or less, 3 volume% or less, 2.9 volume% or less, 2.8 volume% or less, 2.5 volume% or less, 2 volume% or less, 1.8 volume% or less, 1.5 volume% or less, 1.2 volume% or less, 1 volume% or less, 0.8 volume% or less, 0.5 volume% or less, 0.3 volume% or less), etc.

[0063] Using gases containing carbon dioxide in the proportions described above makes it easier to further improve the efficiency of carbon dioxide capture. Furthermore, such effects can become even more pronounced when combined with other conditions (for example, one or more conditions selected from pH, the proportion of alkali metal ions, the rate of gas B in 1 L of mixture A, etc.). Furthermore, surprisingly, this invention can achieve such efficient carbon dioxide recovery even for gases with low carbon dioxide concentrations.

[0064] If gas B contains gases other than carbon dioxide (other gases, gases), the other gases are not particularly limited and include inorganic gases {e.g., nitrogen, oxygen, noble gases (e.g., helium, neon, argon, krypton, etc.), halogens (e.g., iodine), oxides [e.g., nitrogen oxides (e.g., nitrous oxide, nitrogen dioxide), sulfur oxides (e.g., sulfur dioxide)], etc.}, organic gases [e.g., hydrocarbons (e.g., methane)], etc.

[0065] Other gases may be included in gas B, either individually or in combination of two or more.

[0066] Gas B may be air (or derived from air).

[0067] Furthermore, gas B may be exhaust gas (exhaust gas, exhaust fumes) (it may also originate from exhaust gas).

[0068] Examples of exhaust gases include gases emitted from combustion (calcination) equipment or combustion (calcination) devices [for example, automobiles (e.g., gasoline cars, diesel cars), ships, aircraft, boilers, incinerators, dryers, calcination furnaces (e.g., limestone calcination furnaces)].

[0069] Among these, gases discharged from boilers, dryers, etc., often contain carbon dioxide at very low concentrations, but the present invention makes it possible to efficiently recover carbon dioxide from such gases.

[0070] Furthermore, the exhaust gas may be gas emitted from a factory.

[0071] By utilizing such exhaust gas sources or locations as gas B, carbon dioxide can be recovered at the source or location (or its surroundings, for example, within a factory), and consequently, the generation and recovery of carbon dioxide can be completed at the source or location, making this method preferable.

[0072] Furthermore, carbon dioxide (gas B, exhaust gas) may originate from at least fossil fuels (or fossil resources, such as petroleum, coal, and natural gas) (it may include at least combustion gases from fossil fuels). The use of such carbon dioxide (gas B, exhaust gas) contributes to reducing carbon dioxide in the atmosphere.

[0073] [Contact method (carbon dioxide capture process, carbonate production process)] The contact between mixture A and gas B is not particularly limited, but it can usually be brought into contact by passing gas B through mixture A (blowing or introducing it).

[0074] Such contact (transaction) can take place in either a closed or open system. Furthermore, contact (distribution) may be carried out in either a batch or continuous manner.

[0075] Typically, gas B may be continuously circulated (passed through) (blown into) mixture A (within mixture A).

[0076] The contact may also be carried out under stirring of mixture A.

[0077] When the process is carried out under stirring, the stirring method is not particularly limited, and conventional methods can be used [for example, stirring with a stirring blade in a container equipped with baffles as needed].

[0078] In addition, during stirring, the rotation speed of the stirring blade may be, for example, 10 rpm or more (for example, 50 rpm or more, preferably 100 rpm or more, and even more preferably 200 rpm or more), or 1000 rpm or less (for example, 800 rpm or less, preferably 600 rpm or less, and even more preferably 400 rpm or less).

[0079] In contact (flow), the velocity of gas B (flow rate, introduction rate) may be selected according to the amount of mixture A, etc.

[0080] For example, the rate of gas B (per 1 liter of mixture A) relative to 1 liter of mixture A may be selected from a range of approximately 0.0001 L / min [or L / (min·L)] or more (e.g., 0.0005 L / min or more), preferably 0.001 L / min or more (e.g., 0.005 L / min or more), more preferably 0.01 L / min or more (e.g., 0.05 L / min or more), and even more preferably 0.1 L / min or more (e.g., 0.2 L / min or more), and may also be 0.3 L / min or more (e.g., 0.4 L / min or more, 0.5 L / min or more, 0.6 L / min or more, 0.65 L / min or more, 0.67 L / min or more), etc.

[0081] The upper limit of the rate of gas B (per 1L of mixture A) relative to 1L of mixture A may be selected from a range of, for example, 100 L / min [or L / (min·L)] or less (for example, 90 L / min or less), and may be 80 L / min or less (for example, 70 L / min or less), preferably 60 L / min or less (for example, 50 L / min or less), and even more preferably 40 L / min or less (for example, 35 L / min or less, 33.3 L / min or less), and particularly 30 L / min or less (for example, 28 L / min or less). Bottom, 25L / min or less, 22L / min or less, 20L / min or less, 18L / min or less, 15L / min or less, 12L / min or less, 10L / min or less, 9L / min or less, 8L / min or less, 7L / min or less, 6.7L / min or less, 6L / min or less, 5L / min or less , 4L / min or less, 3L / min or less, 2L / min or less, 1.8L / min or less, 1.5L / min or less, 1.2L / min or less, 1L / min or less, 0.9L / min or less, 0.8L / min or less, 0.7L / min or less, 0.67L / min or less).

[0082] By bringing mixture A and gas B into contact at the above ratio (rate of gas B relative to 1 L of mixture A), the efficiency of carbon dioxide recovery can be further increased. Furthermore, these effects can become even more pronounced when combined with other conditions (for example, one or more conditions selected from pH, the proportion of alkali metal ions, the proportion of carbon dioxide in gas B, etc.).

[0083] The rate of gas B can be selected according to the amount of mixture A to be brought into contact with, as described above, and is not particularly limited. However, it can be selected from a range that is not excessively high, for example, from 2000 L / min or less (for example, 1500 L / min or less), preferably 1200 L / min or less (for example, 1000 L / min or less), preferably 800 L / min or less (for example, 700 L / min or less), and even more preferably 600 L / min or less (for example, 500 L / min or less), and 450 L / min or less. It can also be set to "below" (for example, below 400 L / min, below 380 L / min, below 350 L / min, below 320 L / min, below 300 L / min, below 280 L / min, below 250 L / min, below 220 L / min, below 200 L / min, below 180 L / min, below 150 L / min, below 120 L / min, below 100 L / min, below 80 L / min, below 60 L / min, below 50 L / min, below 40 L / min, below 30 L / min, below 25 L / min, below 20 L / min, below 15 L / min, below 10 L / min), etc.

[0084] Furthermore, the lower limit of the gas B velocity may be selected from a range that is not extremely low, for example, 0.0001 L / min or more (for example, 0.0005 L / min or more), preferably 0.001 L / min or more (for example, 0.005 L / min or more), more preferably 0.01 L / min or more (for example, 0.05 L / min or more), and even more preferably 0.1 L / min or more (for example, 0.5 L / min or more), and may also be 1 L / min or more (for example, 2 L / min or more, 3 L / min or more, 4 L / min or more, 5 L / min or more, 6 L / min or more, 7 L / min or more, 8 L / min or more, 9 L / min or more, 10 L / min or more), etc.

[0085] The velocity of gas B relative to 1 L of mixture A (and furthermore, the velocity of gas B) may be constant or variable. If it is variable, it may be varied within a range that satisfies the above-described velocity of gas B relative to 1 L of mixture A (and furthermore, the velocity of gas B).

[0086] The contact temperature (temperature of mixture A) is not particularly limited, but may be, for example, 0 to 100°C, preferably 5 to 95°C, and more preferably 10 to 90°C (for example, 15 to 85°C, 20 to 80°C, or 25 to 75°C).

[0087] In particular, the contact temperature may be room temperature, ambient temperature, or the surrounding temperature. From the viewpoint of efficiently recovering carbon dioxide, contact may be made without heating or cooling.

[0088] The timing of the termination of contact is not particularly limited and can be chosen as appropriate. For example, when the percentage of carbon dioxide X1 (volume %) in gas B and the percentage of carbon dioxide X2 (volume %) in the gas emitted from mixture A (circulating through mixture A and being emitted outside mixture A) become exactly the same or substantially unchanged (for example, when the value of X2 ÷ X1 is between 0.999 and 1), it can be considered time to terminate the process, as it is no longer possible to recover carbon dioxide even if contact is made (or if possible, it would take an extremely long time and is not practical). Alternatively, even if the gas emitted from mixture A contains carbon dioxide, the contact may be terminated at an appropriate time [for example, when the value of X2 ÷ X1 is less than 0.999 (for example, 0.99 or less, 0.95 or less, 0.9 or less, 0.8 or less)], as long as carbon dioxide recovery is expected (for example, X1 > X2).

[0089] Through this contact process (process, carbon dioxide capture process, carbonate formation process), carbon dioxide is recovered (carbon dioxide is absorbed into mixture A). Here, carbon dioxide is an alkali [hydroxide ion (OH] - )] reacts with (see reaction below) to produce carbonate ions (CO3 2- ) and this carbonate ion can form a carbonate with cations contained in the system. 2OH - +CO2→CO3 2- +H2O

[0090] In other words, carbonates can also be produced through this process. Such carbonates typically contain at least an alkali metal carbonate (a carbonate corresponding to an alkali metal ion, such as sodium carbonate), although this depends on factors such as the non-alkali metal ions (non-alkali metal compounds) contained in mixture A and the timing of the end of contact with gas B. Since this process (method) can separate (recover) carbon dioxide from gas B or fix it as a carbonate, it can also be called a method for carbon dioxide recovery (fixation, separation) or a method for producing carbonate.

[0091] <Uses of carbonates> The carbonate obtained through the method of the present invention (step, carbon dioxide recovery step, carbonate production step) may be separated from the mixture (which may be called mixture A, mixture C, etc. after contact with gas B) by conventional methods, or it may be used as is as a mixture containing carbonate (carbonate that may be ionized).

[0092] Furthermore, the carbonate may be used as is, or it may be modified into another component using the carbonate as a raw material. In this case, if mixture C is used (for example, as is, or after minor operations such as concentration or dilution), the carbonate separation step can be omitted.

[0093] One example of such modification is converting a carbonate (alkali metal carbonate) to an alkaline earth metal carbonate (e.g., calcium carbonate). For example, sodium carbonate becomes calcium carbonate based on the following reaction. Na2CO3 + CaCl2 → 2NaCl + CaCO3

[0094] Specifically, alkaline earth metal carbonates can be produced by an alkaline earth metal carbonate production step, which involves contacting the carbonate obtained by the above method (carbonate production step) with a mixture B containing at least alkaline earth metal ions (particularly calcium ions) and water to produce alkaline earth metal carbonates (particularly calcium carbonate).

[0095] In this alkaline earth metal production process, the carbonate may be used as a separated product as described above, or a mixture C may be used.

[0096] Mixture B may also be produced by conventional methods [for example, by mixing a component containing calcium ions, such as calcium chloride (for example, the compounds exemplified in the aforementioned non-alkali metal compounds), with water].

[0097] Furthermore, mixture B may or may not contain components (trace components) that do not fall under the category of alkaline earth metal ions (alkaline earth metal compounds), as long as it can produce alkaline earth metal carbonates.

[0098] Examples of such components include non-alkali earth metal ions (metal ions that are not alkaline earth metal ions, such as alkali metal ions like sodium ions).

[0099] Such non-alkaline earth metal ions may originate from non-alkaline earth metal compounds (compounds composed of non-alkaline earth metal ions, or compounds that can generate non-alkaline earth metal ions in mixture B or water) (they may also be included in mixture B as non-alkaline earth metal compounds). Examples of such compounds include the compounds exemplified above [for example, non-hydroxides {for example, halides (e.g., sodium chloride, potassium chloride, sodium bromide, sodium iodide, etc.), sulfates (e.g., sodium sulfate), phosphates (e.g., sodium phosphate), nitrates (e.g., sodium nitrate), etc. salts [e.g., inorganic salts (salts with inorganic acids, etc.)]].

[0100] In particular, mixture B may contain halide ions (e.g., chloride ions). Such halide ions may be included in mixture B as non-alkaline earth metal compounds (or their counterions), such as sodium chloride.

[0101] Furthermore, mixture B (alkaline earth metal ions, alkaline earth metal compounds, non-alkaline earth metal ions, non-alkaline earth metal compounds, etc.) may be derived from seawater.

[0102] In mixture B (or used in the alkaline earth metal carbonate production process), the amount (concentration, proportion) of alkaline earth metal ions can be selected according to the amount of alkali metal ions in the carbonate (sodium carbonate, etc.) obtained through the carbonate production process to be mixed. For example, alkaline earth metal ions may be used in an amount of 0.5 moles or more (e.g., 0.5 to 1000 moles, 1 mole or more, 1 to 666 moles, 1 to 500 moles, etc.) per mole of alkali metal ions.

[0103] In particular, when it is desired (intended, planned) to produce a specific alkaline earth metal salt (e.g., calcium carbonate), the proportion of other alkaline earth metal ions (e.g., magnesium ions) in mixture B, other than the specific alkaline earth metal ion (e.g., calcium ion), may be reduced. In such cases, for example, the ratio of the specific alkaline earth metal ion (e.g., calcium ion) to the total amount of the specific alkaline earth metal ion (e.g., calcium ion) and other alkaline earth metal ions (e.g., magnesium ion) may be 10 mol% or more (e.g., 20 mol% or more), preferably 30 mol% or more (e.g., 40 mol% or more), and more preferably 50 mol% or more (e.g., 60 mol% or more, 70 mol% or more, 80 mol% or more, 90 mol% or more, 95 mol% or more), etc.

[0104] The pH of mixture B may be selected from a range of 1 or higher (e.g., 2 or higher, 3 or higher, greater than 3, 3.5 to 14), or it may be 4 or higher (e.g., 5 or higher, 6 or higher, 7 or higher, 8 or higher, 9 or higher, 10 or higher, 11 or higher, 12 or higher, 12.7 or higher), etc. Note that the pH value may be the value at 25°C.

[0105] The method of contact between the carbonate and mixture B is not particularly limited; for example, contact may be brought into contact by mixing them.

[0106] The contact temperature (or the temperature of mixture B) is not particularly limited, but may be, for example, 0 to 100°C, preferably 5 to 95°C, and more preferably 10 to 90°C (for example, 15 to 85°C, 20 to 80°C, or 25 to 75°C). In particular, the contact temperature may be room temperature, ambient temperature, or the surrounding temperature.

[0107] The alkaline earth metal carbonate (such as calcium carbonate) obtained through such an alkaline earth metal carbonate production process may be separated by conventional methods, or it may be used as is as a mixture containing alkaline earth metal carbonate.

[0108] Furthermore, alkaline earth metal carbonates may be used as they are, or they may be modified into other components using alkaline earth metal carbonates as raw materials.

[0109] The resulting alkaline earth metal carbonate crystals are not particularly limited, but for example, calcium carbonate is easily and efficiently obtained as calcite crystals. [Examples]

[0110] Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited in any way by these examples, and many modifications are possible within the technical concept of the present invention by those with ordinary skill in the art.

[0111] The various measurements and evaluations were carried out as follows.

[0112] [pH] The pH was measured at room temperature (25°C) using a pH meter (manufactured by Toa DKK Kaibu Co., Ltd., model: MM-43X, pH electrode: GST5841C).

[0113] [Carbon dioxide sheet] The carbon dioxide (CO2) concentration was measured using a carbon dioxide gas detection tube (Gastec Co., Ltd., NO.2H).

[0114] [Gas speed] The gas velocity was measured using a clamp-on gas flow meter (manufactured by Keyence Corporation, model FD-G).

[0115] [Recovery efficiency] It was calculated using the following formula. Carbon dioxide capture efficiency (%) = Theoretical reaction completion time (minutes) ÷ Actual reaction completion time (minutes) × 100

[0116] The theoretical reaction completion time is the time it takes for the alkaline component (OH) in mixture A to complete. - This refers to the theoretical time it takes for the amount of alkaline components to be completely consumed by carbon dioxide, and can be calculated from the amount of alkaline components (mol) and the rate of introduction of carbon dioxide contained in gas B (mol / min) using the following formula.

[0117] Theoretical reaction completion time (minutes) = Alkaline component (OH) contained in mixture A - Amount (mol) ÷ 2 ÷ Introduction rate of oxygen dioxide contained in gas B (mol / min)

[0118] Furthermore, the actual reaction completion time (in minutes) refers to the time (in minutes) when the carbon dioxide concentration in gas B becomes equal to the carbon dioxide concentration in the emitted gas. As an example, the recovery efficiency in Reference Example 1, described below, can be calculated as follows. In other words, alkaline components (OH - The amount (mol) of ) is 30 (= 15 (L) × 1 (mol / L) × 2 (equivalents)), the introduction rate of oxygen dioxide contained in gas B (mol / min) is 0.03 × 10 (L / min) ÷ 22.4 (L / mol), and the actual reaction completion time is 7467 minutes, so the recovery efficiency is 15% [= 30 (mol) ÷ 2 (equivalents) ÷ (0.03 × 10 (L / min) ÷ 22.4 (L / mol)) ÷ 7467 (min) × 100].

[0119] [Reference example 1] As mixture A, water was prepared by mixing magnesium hydroxide [Mg(OH)2] at a ratio of 1 mole (mol) per liter of water. The pH of mixture A (at 25°C) was 10.6. Since mixture A does not contain alkali metal ions or calcium ions, the ratio of magnesium hydroxide to the total amount of alkali metal ions (or sodium ions) and magnesium hydroxide (or magnesium ions) [the value of Mg / (Na+Mg)×100 in the table] is 100 mol%, and the ratio of alkali metal ions (or sodium ions) to the total amount of alkali metal ions (or sodium ions), magnesium hydroxide (or magnesium ions), and calcium ions (or calcium hydroxide) [the value of Na / (Na+Ca+Mg)×100 in the table], and the ratio of calcium ions to the total amount of alkali metal ions and calcium ions [the value of Ca / (Na+Ca)×100 in the table] are both 0 mol%.

[0120] Then, 15 L of mixture A was placed into a 20 L capacity acrylic reaction vessel with baffles, and at room temperature (25°C), while stirring at a rotation speed of 350 rpm using a stirrer equipped with a single stage turbine blade, gas B with a carbon dioxide concentration of 3 vol% (the remainder having a composition similar to air) was circulated into the reaction vessel (mixture A) at a rate (inlet rate, flow rate) of 10 L / min through the gas inlet in the reaction vessel (circulation rate, bringing mixture A and gas B into contact). In other words, the rate (inlet rate, flow rate) of gas B per 1 L of mixture A was set to 0.67 (=10 / 15) L / min [L / (min·L)].

[0121] When the carbon dioxide concentration was measured hourly using a detector tube at the exhaust outlet of the reaction vessel lid, it reached the same value as the inlet (i.e., 3% by volume) after 7467 minutes. Therefore, it was determined that no further sufficient CO2 recovery was expected (all alkaline components had been consumed), and the flow of gas B was terminated, yielding a mixture (a mixture that had undergone the carbon dioxide recovery process and the carbonate production process).

[0122] Based on these factors, the carbon dioxide capture efficiency was calculated to be 15%.

[0123] Table 1 summarizes the above (results).

[0124] [Example 1] As mixture A, water was prepared by mixing 1 mole of magnesium hydroxide [Mg(OH)2] and 2 moles of sodium chloride (NaCl) with 1 liter of water. The pH of mixture A (at 25°C) was 10.6.

[0125] Since mixture A does not contain calcium ions, the ratio of magnesium hydroxide to the total amount of sodium chloride (or sodium ions) and magnesium hydroxide (or magnesium ions) [the value of Mg / (Na+Mg)×100 in the table] is 33.3 mol%, the ratio of sodium chloride (or sodium ions) to the total amount of sodium chloride (or sodium ions), magnesium hydroxide (or magnesium ions), and calcium ions (or calcium hydroxide) [the value of Na / (Na+Ca+Mg)×100 in the table] is 66.7 mol%, and the ratio of calcium ions to the total amount of sodium chloride (or sodium ions) and calcium ions [the value of Ca / (Na+Ca)×100 in the table] is 0 mol%.

[0126] Then, 15 L of mixture A was placed into a 20 L capacity acrylic reaction vessel with baffles, and at room temperature (25°C), while stirring at a rotation speed of 350 rpm using a stirrer equipped with a single stage turbine blade, gas B with a carbon dioxide concentration of 3 vol% (the remainder having a composition similar to air) was circulated into the reaction vessel (mixture A) at a rate (inlet rate, flow rate) of 10 L / min through the gas inlet in the reaction vessel (circulation rate, bringing mixture A and gas B into contact). In other words, the rate (inlet rate, flow rate) of gas B per 1 L of mixture A was set to 0.67 (=10 / 15) L / min [L / (min·L)].

[0127] When the carbon dioxide concentration was measured hourly using a detector tube at the exhaust outlet of the reaction vessel lid, the CO2 concentration reached the same value as the inlet (i.e., 3% by volume) after 3394 minutes. Therefore, it was determined that no further sufficient CO2 recovery was expected (all alkaline components had been consumed), and the flow of gas B was terminated, yielding a mixture (a mixture that had undergone the carbon dioxide recovery process and the carbonate production process).

[0128] Based on these factors, the carbon dioxide capture efficiency was calculated to be 33%.

[0129] Table 1 summarizes the above (results).

[0130] [Example 2] As mixture A, water was prepared by mixing 0.0007 moles of NaOH (sodium hydroxide) with 1 liter of water. The pH of mixture A (at 25°C) was 10.8.

[0131] Then, using the same procedure as in Example 1, except for the use of mixture A, a mixture (a mixture obtained through a carbon dioxide recovery process and a carbonate production process) was obtained, and the carbon dioxide recovery efficiency was calculated to be 41%.

[0132] Table 1 summarizes the above (results).

[0133] [Examples 3-18] Under the conditions shown in Tables 1-3 (carbon dioxide concentrations in mixture A and gas B, and the introduction rate of gas B), a mixture (a mixture that has undergone a carbon dioxide recovery process and a carbonate production process) was obtained in the same manner as in Example 1, and the carbon dioxide recovery efficiency was calculated.

[0134] Tables 1-3 summarize these findings (results).

[0135] [Table 1]

[0136] [Table 2]

[0137] [Table 3]

[0138] As is clear from the results in the table above, carbon dioxide could be recovered efficiently according to the present invention. In particular, the efficiency of this recovery could be further improved by selecting the pH, alkali metal ion (sodium ion) concentration, etc., in mixture A. Furthermore, the recovery efficiency could be further improved by selecting the rate of gas B relative to mixture A (and even the carbon dioxide concentration in gas B). Furthermore, carbon dioxide could be recovered with high efficiency even when magnesium ions or calcium ions were present, or when carbon dioxide was present at low concentrations.

[0139] [Example 19] 20 L of seawater (from the vicinity of Kamishima Chemical Industry Co., Ltd.) was mixed with calcium hydroxide at a concentration of 0.053 mol / L. The resulting magnesium hydroxide was separated by vacuum filtration using a Nutche filter to obtain mixture B (seawater from which magnesium hydroxide has been removed).

[0140] Mixture B contained calcium hydroxide (calcium ions) at a concentration of 0.065 mol / L, and also contained sodium chloride (sodium ions, chloride ions) at a concentration of 0.46 mol / L. Furthermore, the pH of mixture B (at 25°C) was 8.9.

[0141] Then, 15.6 L of mixture B and 1 L of the mixture obtained in Example 1 (the mixture after the carbon dioxide recovery step and the carbonate production step) were simultaneously added to a 20 L reaction vessel under stirring, and stirring was continued for approximately 30 minutes (the reaction was continued). After that, the mixture was filtered, washed with approximately 5 times the amount of water relative to the solid content, dried at 110°C for 12 hours, and ground to obtain a powder.

[0142] Analysis of the crystalline structure of the powder (calcium carbonate crystals) using the following method revealed that it was calcite. (Analysis method) After compacting and fixing the powder sample onto a designated sample stage using a spatula, measurements were performed using an XRD instrument (MiniFlex600-C, Rigaku Corporation) to identify and analyze it as a crystalline substance. The peak appearing at approximately 29° at a measurement angle of 2θ is the main peak of calcite.

[0143] [Example 20] The powder was obtained in the same manner as in Example 19, except that sodium hydroxide was used instead of calcium hydroxide. Mixture B contained calcium chloride (calcium ions) at a concentration of 0.010 mol / L, and also contained sodium chloride (sodium ions, chloride ions) at a concentration of 0.56 mol / L. Furthermore, the pH of mixture B (at 25°C) was 9.0.

[0144] Then, using the same procedure as in Example 19, 15.6 L of mixture B and 155 mL of the mixture obtained by the carbonate production step in Example 1 were simultaneously added to a 20 L reaction vessel under stirring, and stirring was continued for approximately 30 minutes (the reaction was allowed to continue). After that, the mixture was filtered, washed with approximately 5 times the amount of water relative to the solid content, dried at 110°C for 12 hours, and ground to obtain a powder. The obtained powder was analyzed for its crystal structure using the method described in Example 19, and it was found to be calcite.

[0145] [Examples 21-37] Powders were obtained in the same manner as in Examples 19 and 20, except that the mixtures obtained in Examples 2 to 18 (mixtures that have undergone a carbon dioxide recovery process and a carbonate production process) were used instead of the mixture obtained in Example 1 (mixture that has undergone a carbon dioxide recovery process and a carbonate production process). Furthermore, when the crystal structure of each powder was analyzed using the method described in Example 19, it was found that all of them were calcite. [Industrial applicability]

[0146] According to the present invention, a method for recovering carbon dioxide and a method for producing carbonates can be provided.

Claims

[Claim 1] The invention described herein.

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