Seawater carbon dioxide fixation system and method for producing carbonate minerals

Abstract

To provide a system for fixation of carbon dioxide which can recover CO2 with excellent recovery efficiency and a carbonate mineral production method which can efficiently produce carbonate mineral from seawater.SOLUTION: A system for fixation of carbon dioxide in seawater includes: a bipolar membrane electrodialyzer 1 which produces acidified seawater and basified seawater from the seawater; a carbon dioxide separation unit 2 into which the acidified seawater is introduced from the bipolar membrane electrodialyzer 1 to separate gaseous carbon dioxide; and a precipitation tank 3 into which the basified seawater is introduced from the bipolar membrane electrodialyzer 1 and the gaseous carbon dioxide is introduced from the carbon dioxide separation unit 2, where a carbonate mineral is recovered from the precipitation tank 3. With the system, a dialysis step, a carbon dioxide separation step, and a precipitation step are performed to fix the carbon dioxide so as to form the carbonate mineral.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a carbon dioxide fixation system in seawater and a method for producing carbonate minerals. [Background technology]

[0002] To combat global warming, there is a worldwide demand for reducing carbon dioxide (CO2) emissions and for CO2 capture and storage technologies (CCS). While previous CCS efforts have focused on CO2 from factory exhaust gases and the atmosphere, recently, in conjunction with addressing the problem of ocean acidification, CO2 capture from seawater (DOC; Direct Ocean Capture or Ocean-based Carbon Dioxide Removal) has been attracting attention.

[0003] Approximately 30% of anthropogenic CO2 is dissolved in the ocean, and the concentration of CO2 per unit volume in seawater is about 120 times that of air. When CO2 is recovered from seawater, the seawater absorbs CO2 from the atmosphere to maintain equilibrium with the atmosphere, resulting in a reduction in the amount of CO2 in the atmosphere.

[0004] While methods using gas permeable membranes, ion exchange resins, and electrochemical acidification have been investigated as CO2 recovery technologies from seawater, their recovery efficiency has been low. On the other hand, a method has been proposed to recover gaseous CO2 by making seawater acidic using bipolar membrane electrodialysis (Patent Document 1, Non-Patent Document 1).

[0005] Furthermore, Patent Document 2 proposes a technology for recovering CO2 in seawater as calcium carbonate by basicizing seawater, and utilizing this as a building material. Patent Document 2 also proposes using basicized seawater obtained by treating a portion of the seawater with bipolar membrane electrodialysis as the base for basicizing the seawater. Furthermore, Non-Patent Document 2 proposes treating seawater with bipolar membrane electrodialysis and precipitating the resulting basicized seawater as carbonate minerals in a base chamber. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2013-13889 [Patent Document 2] U.S. Patent Application Publication No. 2018 / 0072626 [Non-patent literature]

[0007] [Non-Patent Document 1] Matthew D. Eisaman, Keshav Parajuly, Alexander Tuganov, Craig Eldershaw, Norine Chang and Karl A. Littau, “CO2 extraction from seawater using bipolar membrane electrodialysis”, Energy & Environmental Science, 2012, Internet<https: / / doi.org / 10.1039 / C2EE03393C> [Non-Patent Document 2] R. Sharifian, L. Boer, RM Wagterveld, DA Vermaas, “Oceanic carbon capture through electrochemically induced in situ carbonate mineralization using bipolar membrane”, Chemical Engineering Journal, Volume 438, 2022, 135326, Internet<https: / / doi.org / 10.1016 / j.cej.2022.135326> [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] However, the CO2 recovery efficiency in all of the patent documents 1 and 2, and non-patent documents 1 and 2, was not considered sufficient. Furthermore, Non-Patent Document 2 describes a problem where mineralization progresses inside the bipolar membrane electrodialysis apparatus, causing carbonate minerals to precipitate on the bipolar membrane, which reduces the CO2 recovery efficiency. In view of the above circumstances, the present invention aims to provide a carbon dioxide fixation system that can recover CO2 with excellent recovery efficiency, and a method for producing carbonate minerals that can efficiently produce carbonate minerals from seawater. [Means for solving the problem]

[0009] To achieve the above objectives, the present invention employs the following configuration. [1] A bipolar membrane electrodialysis apparatus that generates acidified seawater and basicized seawater from seawater, The acidified seawater is introduced from the bipolar membrane electrodialysis apparatus, and a carbon dioxide separation unit separates gaseous carbon dioxide. The apparatus comprises a sedimentation tank into which the basicized seawater is introduced from the bipolar membrane electrodialysis apparatus and gaseous carbon dioxide is introduced from the carbon dioxide separation unit, A carbon dioxide fixation system in seawater in which carbonate minerals are recovered from the aforementioned sedimentation tank. [2] The carbon dioxide fixation system according to [1], further comprising a vacuum pump for promoting the separation of gaseous carbon dioxide in the carbon dioxide separation unit. [3] The carbon dioxide fixation system according to [1] or [2], further comprising a neutralization tank, wherein the acidified seawater after gaseous carbon dioxide has been separated in the carbon dioxide separation unit and the basicized seawater after carbonate minerals have been recovered in the sedimentation tank are mixed in the neutralization tank. [4] A dialysis process that generates acidified seawater and basicized seawater from seawater by bipolar membrane electrodialysis, A carbon dioxide separation step of separating gaseous carbon dioxide from the acidified seawater obtained in the dialysis step, and a precipitation step of introducing the alkalized seawater obtained in the dialysis step and the gaseous carbon dioxide obtained in the carbon dioxide separation step into a precipitation tank to precipitate carbonate minerals. A method for producing carbonate minerals. [5] The method for producing carbonate minerals according to [4], wherein in the carbon dioxide separation step, a vacuum pump is used to promote the separation of gaseous carbon dioxide. [6] A neutralization step of mixing and neutralizing the acidified seawater after separating gaseous carbon dioxide in the carbon dioxide separation step and the alkalized seawater after removing the carbonate minerals precipitated in the precipitation step. The method for producing carbonate minerals according to [4] or [5]. [Advantages of the Invention]

[0010] According to the carbon dioxide immobilization system of the present invention, CO2 can be recovered with excellent recovery efficiency. Also, according to the method for producing carbonate minerals of the present invention, carbonate minerals can be efficiently produced from seawater. [Brief Description of the Drawings]

[0011] [Figure 1] It is a schematic configuration diagram of a carbon dioxide immobilization system according to a first embodiment of the present invention. [Figure 2] It is a schematic diagram of a bipolar membrane electrodialysis device used in the carbon dioxide immobilization system according to the first embodiment of the present invention. [Figure 3] It is a schematic configuration diagram of a carbon dioxide immobilization system according to a second embodiment of the present invention. [Figure 4] It is a schematic diagram of a bipolar membrane electrodialysis device used in the carbon dioxide immobilization system according to the second embodiment of the present invention. [Modes for Carrying Out the Invention]

[0012] [First Embodiment] [Carbon Dioxide Immobilization System] A carbon dioxide fixation system according to the first embodiment of the present invention will be described with reference to Figure 1. In Figure 1, wavy arrows indicate the gaseous path, single arrows indicate the liquid path, and double arrows indicate the solid path. As shown in Figure 1, the carbon dioxide fixation system of this embodiment is generally composed of a bipolar membrane electrodialysis apparatus 1, a carbon dioxide separation unit 2 (indicated as "CO2 separation unit" in the figure), a sedimentation tank 3, a vacuum pump 4, and a neutralization tank 5.

[0013] The bipolar membrane electrodialysis apparatus 1 is designed to receive seawater from a seawater tank 6. Electrode solution is also circulated and supplied from an electrode solution tank 7. The bipolar membrane electrodialysis apparatus 1 then generates acidified seawater and basicized seawater from the seawater. Details of the bipolar membrane electrodialysis apparatus 1 will be described later.

[0014] The acidified seawater generated by the bipolar membrane electrodialysis machine 1 is introduced into the carbon dioxide separation unit 2, where gaseous carbon dioxide is separated from the acidified seawater. The carbon dioxide separation unit 2 is not particularly limited as long as it can separate and extract gaseous carbon dioxide from acidified seawater. For example, a membrane contactor (e.g., Liqui-Cel manufactured by Snowpure) that brings acidified seawater and the gas phase into contact by dissolving a membrane can be used. Alternatively, a tank that has at least one of a showerhead system for spraying acidified seawater and a means for agitating the acidified seawater, and can collect gas from the gas phase, may be used.

[0015] In this embodiment, a vacuum pump 4 is provided in the gas path from the carbon dioxide separation unit 2 to the sedimentation tank 3, allowing the gas phase of the carbon dioxide separation unit 2 to be depressurized. This promotes the separation of gaseous carbon dioxide in the carbon dioxide separation unit 2. The vacuum pump 4 can be omitted.

[0016] Meanwhile, the basicized seawater produced by the bipolar membrane electrodialysis machine 1 is introduced into the sedimentation tank 3. Furthermore, the gaseous carbon dioxide separated by the carbon dioxide separation unit 2 is also introduced into the sedimentation tank 3.

[0017] In sedimentation tank 3, gaseous carbon dioxide can be blown into the introduced basicized seawater. As a result, carbonate minerals precipitate, and the precipitated carbonate minerals can be removed and recovered. Furthermore, a device for concentrating gaseous carbon dioxide may be installed between the vacuum pump 4 and the sedimentation tank 3. As a concentration device, for example, a UBE CO2 SEPARATOR manufactured by UBE Corporation can be used.

[0018] The neutralization tank 5 receives acidified seawater, which is the result of separating gaseous carbon dioxide in the carbon dioxide separation unit 2, and basicized seawater, which is the result of recovering carbonate minerals in the sedimentation tank 3. The acidified seawater and basicized seawater are then mixed in the neutralization tank 5 to obtain treated seawater that is neutralized and has a reduced carbon dioxide concentration.

[0019] As shown in Figure 2, the bipolar membrane electrodialysis apparatus 1 is configured by alternately arranging multiple bipolar membranes 11 and multiple anion exchange membranes 12 between the anode 21 and the cathode 22. The bipolar membrane 11 is a composite ion exchange membrane with a structure in which an anion exchange layer and a cation exchange layer are bonded together. Specifically, a substrate can be used in which the anion exchange layer is laminated or impregnated on one side and the cation exchange layer is laminated or impregnated on the other side. The bipolar film 11 is arranged such that the anode 21 side is an anion exchange layer and the cathode 22 side is a cation exchange layer.

[0020] As the anion exchange film 12, one can be used in which an anion exchange layer is laminated or impregnated onto a substrate. For the bipolar membrane 11, for example, NeoSepta BP-1 manufactured by Astom Co., Ltd. can be used. For the anion exchange membrane 12, for example, NeoSepta ASE manufactured by Astom Co., Ltd. can be used.

[0021] An acid chamber 16 is formed between the bipolar membrane 11 and the adjacent anion exchange membrane 12 on the cathode 22 side. A base chamber 17 is formed between the bipolar membrane 11 and the adjacent anion exchange membrane 12 on the anode 21 side. Furthermore, an anode chamber 23 is formed between the bipolar film 11 located closest to the anode 21 and the anode 21. A cathode chamber 24 is formed between the bipolar film 11 located closest to the cathode 22 and the cathode 22.

[0022] Seawater flows into the acid chamber 16 and the base chamber 17 from the seawater tank 6. In the acid chamber 16, acidified seawater is generated and supplied to the carbon dioxide separation unit 2. In the base chamber 17, basicized seawater is generated and supplied to the sedimentation tank 3. Electrode solution flows into the anode chamber 23 and the cathode chamber 24 from the electrode solution tank 7, respectively, and also returns to the electrode solution tank 7.

[0023] [Method for producing carbonate minerals] In the carbon dioxide fixation system of this embodiment, a method for producing carbonate minerals can be carried out by the following steps (i) to (iii). (i) A dialysis process that produces acidified seawater and basicized seawater from seawater using bipolar membrane electrodialysis. (ii) A carbon dioxide separation step for separating gaseous carbon dioxide from the acidified seawater obtained in the dialysis step. (iii) A precipitation step in which the basicized seawater obtained in the dialysis step and the gaseous carbon dioxide obtained in the carbon dioxide separation step are introduced into a sedimentation tank to precipitate carbonate minerals.

[0024] The method for producing carbonate minerals using the carbon dioxide fixation system of this embodiment further comprises the following step (iv). (iv) A neutralization step in which the acidified seawater obtained after separating gaseous carbon dioxide in the carbon dioxide separation step is mixed with the basicized seawater obtained after removing the carbonate minerals precipitated in the precipitation step to neutralize it.

[0025] The dialysis process is performed using the bipolar membrane electrodialysis apparatus 1 shown in Figure 2. It is preferable that the seawater supplied from the seawater tank 6 to the bipolar membrane electrodialysis apparatus 1 undergoes pretreatment to remove turbidity. For turbidity removal, filtration devices using, for example, ultrafiltration membranes, microfiltration membranes, or nanofiltration membranes can be used. As the electrode solution supplied from the electrode solution tank 7 to the anode chamber 23 and cathode chamber 24, for example, an aqueous sulfuric acid solution, an aqueous sodium sulfate solution, or an aqueous sodium carbonate solution can be used.

[0026] When a voltage is applied between the anode 21 and the cathode 22, water molecules from the seawater introduced into the acid chamber 16 and the base chamber 17 penetrate into the bipolar membrane 11 and H + and OH - It is dissociated into H. + It is attracted to the cathode 22 side, passes through the cation exchange layer of the bipolar film 11, and moves to the acid chamber 16. Meanwhile, OH - It is attracted to the anode 21, passes through the anion exchange layer of the bipolar film 11, and moves to the base chamber 17. Also, Cl in the base chamber 17 - These anions are attracted to the anode 21 and move through the anion exchange membrane 12 to the acid chamber 16. As a result, the seawater introduced into the acid chamber 16 becomes acidified seawater, and the seawater introduced into the base chamber 17 becomes basicized seawater.

[0027] The pH of acidified seawater is preferably 3.0 to 5.0, more preferably 4.0 to 5.0, and particularly preferably around 4.5. If the pH of the acidified seawater is below the preferred upper limit, most of the HCO3 dissolved in the seawater will be removed. - Ions and CO3 2-Since ions are converted to CO2, carbon dioxide can be efficiently separated in the carbon dioxide separation process. If the pH of the acidified seawater is above a desirable lower limit, energy consumption in electrodialysis can be reduced.

[0028] The pH of the basicized seawater is preferably 9.0 to 10.0, more preferably 9.0 to 9.5, and particularly preferably around 9.3. If the pH of the basicized seawater is above the preferred lower limit, carbonate minerals can be efficiently precipitated in the precipitation process. If the pH of the basicized seawater is below the preferred upper limit, it is easier to avoid precipitation occurring in the base chamber 17.

[0029] It is preferable not to allow the basicized seawater to remain in the base chamber 17 for longer than necessary. The basicized seawater contains CO3 2- The ions become basic enough to form precipitates with calcium ions, magnesium ions, etc., so if they remain in the base chamber 17 for longer than necessary, precipitates will form. Once precipitates form in the base chamber 17, these precipitates can act as seed crystals, causing further precipitation, which is undesirable.

[0030] The pH of the acidified seawater and the pH of the basicized seawater can be adjusted, for example, by the applied voltage and current, the flow rate of seawater passing through the acid chamber 16 and the base chamber 17, etc. Alternatively, the pH may be adjusted by recirculating the seawater through the bipolar membrane electrodialysis apparatus 1. The electrode solution introduced into the anode chamber 23 becomes basic, and the electrode solution introduced into the cathode chamber 24 becomes acidic, but they are returned to the electrode solution tank 7 and mixed, returning to their original composition.

[0031] The carbon dioxide separation process is carried out in the carbon dioxide separation unit 2 shown in Figure 1. Acidified seawater is introduced into the carbon dioxide separation unit 2 from the bipolar membrane electrodialysis apparatus 1. The dissolved CO2 contained in the acidified seawater is then separated as gaseous CO2. By creating a reduced pressure state using the vacuum pump 4, the conversion from dissolved CO2 to gaseous CO2 is promoted, and gaseous CO2 can be efficiently separated from the acidified seawater. The separated gaseous CO2 may be concentrated if necessary.

[0032] The precipitation process is carried out in precipitation tank 3. Basic seawater is introduced into precipitation tank 3 from bipolar membrane electrodialysis device 1. Also, gaseous CO2 is introduced from carbon dioxide separation unit 2. In precipitation tank 3, gaseous CO2 is blown into the basic seawater, so that calcium ions and magnesium ions in the basic seawater and gaseous CO2 form salts and precipitate as carbonate minerals such as calcium carbonate and magnesium carbonate. Also, CO3 2- ions in the basic seawater also form salts with calcium ions and magnesium ions in the basic seawater by taking sufficient residence time in precipitation tank 3 and precipitate as carbonate minerals such as calcium carbonate and magnesium carbonate.

[0033] The precipitated carbonate minerals can be recovered from precipitation tank 3 by solid-liquid separation. Thereby, carbon dioxide in seawater can be immobilized and carbonate minerals useful as metal resources can be obtained. The obtained carbonate minerals can be utilized, for example, as building materials, paper-making materials, plastic additives, etc.

[0034] In order to promote the precipitation of carbonate minerals in precipitation tank 3, it is preferable to previously introduce seed crystals of carbonate minerals into precipitation tank 3. Also, a part of the carbonate minerals recovered from precipitation tank 3 by solid-liquid separation is preferably returned to precipitation tank 3 as seed crystals.

[0035] The neutralization process is carried out in neutralization tank 5. Acidified seawater after removing carbon dioxide from carbon dioxide separation unit 2 is introduced into neutralization tank 5. Also, basic seawater after removing carbonate minerals from precipitation tank 3 is introduced. The introduced acidified seawater and basic seawater are mixed in neutralization tank 5 to be neutralized, and treated seawater with reduced carbon dioxide concentration is obtained. The treated seawater can be discharged into the ocean as it is. Note that the basic seawater after removing carbonate minerals may be brought into contact with the atmosphere before being introduced into neutralization tank 5 to absorb carbon dioxide in the atmosphere.

[0036] According to the method for producing carbonate minerals of this embodiment, dissolved CO2 from acidified seawater and CO3 from basicized seawater are produced in the bipolar membrane electrodialysis apparatus 1. 2- From both ions, carbonate minerals can be obtained, and carbon dioxide can be fixed. Also, the CO3 in basicized seawater 2- Since the ions are precipitated in the precipitation tank 3 and not in the base chamber 17, problems such as clogging of the bipolar membrane 11 and the anion exchange membrane 12 are less likely to occur.

[0037] <Second Embodiment> [Carbon dioxide sequestration system] A carbon dioxide fixation system according to a second embodiment of the present invention will be described with reference to Figure 3. In Figure 3, components similar to those in Figure 1 are denoted by the same reference numerals as in Figure 1, and their detailed descriptions are omitted. As in Figure 1, dashed arrows indicate the gaseous path, single arrows indicate the liquid path, and double arrows indicate the solid path.

[0038] As shown in Figure 3, the carbon dioxide fixation system of this embodiment is generally composed of a bipolar membrane electrodialysis apparatus 10, a carbon dioxide separation unit 2 (indicated as "CO2 separation unit" in the figure), a sedimentation tank 3, a vacuum pump 4, and a neutralization tank 5.

[0039] The bipolar membrane electrodialysis apparatus 1 is designed to receive seawater from a seawater tank 6. Electrode solution is also circulated and supplied from an electrode solution tank 7. The bipolar membrane electrodialysis apparatus 1 then generates concentrated acidified seawater (acidified concentrated seawater), concentrated basicized seawater (basic concentrated seawater), and desalinated seawater (desalinated seawater) from the seawater. Details of the bipolar membrane electrodialysis apparatus 10 will be described later.

[0040] The acidified and concentrated seawater produced by the bipolar membrane electrodialysis machine 10 is introduced into the carbon dioxide separation unit 2, where gaseous carbon dioxide is separated from the acidified and concentrated seawater. As the carbon dioxide separation unit 2, the same type as the carbon dioxide separation unit 2 in the first embodiment can be used.

[0041] In this embodiment as well, the vacuum pump 4 is provided in the gas path from the carbon dioxide separation unit 2 to the sedimentation tank 3, allowing the gas phase of the carbon dioxide separation unit 2 to be depressurized. This promotes the separation of gaseous carbon dioxide in the carbon dioxide separation unit 2. The vacuum pump 4 can be omitted.

[0042] Meanwhile, the basicized concentrated seawater produced by the bipolar membrane electrodialysis machine 10 is introduced into the sedimentation tank 3. Furthermore, the gaseous carbon dioxide separated by the carbon dioxide separation unit 2 is also introduced into the sedimentation tank 3. In the sedimentation tank 3, gaseous carbon dioxide can be blown into the introduced basicized concentrated seawater. Carbonate minerals then precipitate, and the precipitated carbonate minerals can be removed and recovered. A device for concentrating gaseous carbon dioxide may be installed between the vacuum pump 4 and the sedimentation tank 3.

[0043] The neutralization tank 5 receives both the acidified concentrated seawater, which is the result of separating gaseous carbon dioxide in the carbon dioxide separation unit 2, and the basicized concentrated seawater, which is the result of recovering carbonate minerals in the sedimentation tank 3. The acidified concentrated seawater and the basicized concentrated seawater are then mixed in the neutralization tank 5 to obtain treated seawater that is neutralized and has a reduced carbon dioxide concentration.

[0044] Figure 4 shows the configuration of the bipolar membrane electrodialysis apparatus 10. In Figure 4, components similar to those in Figure 2 are given the same reference numerals as in Figure 3, and their detailed descriptions are omitted. As shown in Figure 4, the bipolar membrane electrodialysis apparatus 10 is configured with multiple bipolar membranes 11, multiple anion exchange membranes 12, and multiple cation exchange membranes 13 arranged between the anode 21 and the cathode 22.

[0045] Specifically, an anion exchange film 12 is placed on the side closest to the anode 21, and thereafter, starting from the anode 21 side, a cation exchange film 13, a bipolar film 11, and an anion exchange film 12 are arranged in a repeating pattern, with each set consisting of three films. In other words, the anion exchange membrane 12 is positioned on the cathode 22 side of the bipolar membrane 11, and the cation exchange membrane 13 is positioned on the anode 21 side of the bipolar membrane 11.

[0046] The configuration of the bipolar membrane 11 and the anion exchange membrane 12 is the same as described in the first embodiment. The cation exchange membrane 13 can be made by laminating or impregnating a substrate with a cation exchange layer. For example, NeoSepta CSE manufactured by Astom Co., Ltd. can be used as the cation exchange membrane 13. The bipolar film 11 is arranged such that the anode 21 side (cation exchange film 13 side) is the anion exchange layer and the cathode 22 side (anion exchange film 12 side) is the cation exchange layer.

[0047] An acid chamber 16 is formed between the bipolar membrane 11 and the adjacent anion exchange membrane 12 on the cathode 22 side. A base chamber 17 is formed between the bipolar membrane 11 and the adjacent cation exchange membrane 13 on the anode 21 side. A desalting chamber 18 is formed between the anion exchange membrane 12 and the cation exchange membrane 13. Furthermore, an anode chamber 23 is formed between the anion exchange membrane 12 located closest to the anode 21 and the anode 21. A cathode chamber 24 is formed between the anion exchange membrane 12 located closest to the cathode 22 and the cathode 22.

[0048] Seawater flows into the acid chamber 16, base chamber 17, and desalination chamber 18 from the seawater tank 6. In the acid chamber 16, acidified concentrated seawater is produced and supplied to the carbon dioxide separation unit 2. In the base chamber 17, basicized concentrated seawater is produced and supplied to the sedimentation tank 3. In the desalination chamber 18, desalination seawater is produced and released into the ocean. Electrode solution flows into the anode chamber 23 and the cathode chamber 24 from the electrode solution tank 7, respectively, and also returns to the electrode solution tank 7.

[0049] [Method for producing carbonate minerals] In the carbon dioxide fixation system of this embodiment, a method for producing carbonate minerals can be carried out by the following steps (i') to (iii'). (i') A dialysis process that produces desalinated seawater in addition to acidified seawater (acidified concentrated seawater) and concentrated basicized seawater (basic concentrated seawater) obtained by bipolar membrane electrodialysis from seawater. (ii') A carbon dioxide separation step for separating gaseous carbon dioxide from the acidified concentrated seawater obtained in the dialysis step. (iii') A precipitation step in which the basicized concentrated seawater obtained in the dialysis step and the gaseous carbon dioxide obtained in the carbon dioxide separation step are introduced into a sedimentation tank to precipitate carbonate minerals.

[0050] The method for producing carbonate minerals using the carbon dioxide fixation system of this embodiment further comprises the following step (iv'). (iv') A neutralization step in which the acidified concentrated seawater obtained after separating gaseous carbon dioxide in the carbon dioxide separation step is mixed with the basicized concentrated seawater obtained after removing the carbonate minerals precipitated in the precipitation step and neutralized.

[0051] The dialysis process is performed in the bipolar membrane electrodialysis apparatus 1 shown in Figure 4. It is preferable that the seawater supplied from the seawater tank 6 to the bipolar membrane electrodialysis apparatus 1 undergoes pretreatment to remove turbidity. A filtration device similar to that described in the first embodiment can be used for turbidity removal. The electrode solution supplied from the electrode solution tank 7 to the anode chamber 23 and cathode chamber 24 can be the same as that described in the first embodiment.

[0052] When a voltage is applied between the anode 21 and the cathode 22, water molecules from the seawater introduced into the acid chamber 16 and the base chamber 17 penetrate into the bipolar membrane 11 and H + and OH - It is dissociated into H. + It is attracted to the cathode 22 side, passes through the cation exchange layer of the bipolar film 11, and moves to the acid chamber 16. Meanwhile, OH - It is attracted to the anode 21, passes through the anion exchange layer of the bipolar membrane 11, and moves to the base chamber 17.

[0053] Also, Cl in the desalination chamber 18 and cathode chamber 24 - These anions are attracted to the anode 21 and move through the anion exchange membrane 12 to the acid chamber 16 (anions in the desalting chamber 18 closest to the anode 21 move to the anode chamber 23). Also, the Na in the desalination chamber 18 + , K + Li + These cations are attracted to the cathode 22 and move through the cation exchange membrane 13 to the base chamber 17. As a result, the seawater introduced into the acid chamber 16 becomes acidified and concentrated seawater, the seawater introduced into the base chamber 17 becomes basicized and concentrated seawater, and the seawater introduced into the desalination chamber 18 becomes desalination seawater.

[0054] The pH of the acidified concentrated seawater is preferably 3.0 to 5.0, more preferably 4.0 to 5.0, and particularly preferably around 4.5. If the pH of the acidified concentrated seawater is below the preferred upper limit, most of the HCO3 dissolved in the seawater will be removed. - Ions and CO3 2-Since ions are converted to CO2, carbon dioxide can be efficiently separated in the carbon dioxide separation process. If the pH of the acidified concentrated seawater is above a desirable lower limit, energy consumption in electrodialysis can be reduced.

[0055] The pH of the basicized concentrated seawater is preferably 9.0 to 10.0, more preferably 9.0 to 9.5, and particularly preferably around 9.3. If the pH of the basicized concentrated seawater is above the preferred lower limit, carbonate minerals can be efficiently precipitated in the precipitation process. If the pH of the basicized concentrated seawater is below the preferred upper limit, it is easier to avoid precipitation in the base chamber 17.

[0056] It is preferable not to allow the basicized concentrated seawater to remain in the base chamber 17 for longer than necessary. The basicized concentrated seawater contains CO3 2- The ions become basic enough to form precipitates with calcium ions, magnesium ions, etc., so if they remain in the base chamber 17 for longer than necessary, precipitates will form. Once precipitates form in the base chamber 17, these precipitates can act as seed crystals, causing further precipitation, which is undesirable.

[0057] The pH of the acidified concentrated seawater and the pH of the basicized concentrated seawater can be adjusted, for example, by the applied voltage and current, the flow rate of seawater passing through the acid chamber 16 and the base chamber 17, etc. Alternatively, it may be adjusted by recirculating the seawater through the bipolar membrane electrodialysis apparatus 1.

[0058] The carbon dioxide separation process is carried out in the carbon dioxide separation unit 2 shown in Figure 3. Acidified concentrated seawater is introduced into the carbon dioxide separation unit 2 from the bipolar membrane electrodialysis apparatus 10. The dissolved CO2 contained in the acidified concentrated seawater is then separated as gaseous CO2. By creating a reduced pressure state using the vacuum pump 4, the conversion from dissolved CO2 to gaseous CO2 is promoted, and gaseous CO2 can be efficiently separated from the acidified concentrated seawater. The separated gaseous CO2 may be concentrated as needed.

[0059] The sedimentation process takes place in sedimentation tank 3. Basicized concentrated seawater is introduced into sedimentation tank 3 from the bipolar membrane electrodialysis unit 10. Gaseous CO2 is also introduced from the carbon dioxide separation unit 2. In sedimentation tank 3, when gaseous CO2 is blown into the basicized concentrated seawater, the calcium ions and magnesium ions in the basicized concentrated seawater and the gaseous CO2 form salts, which precipitate as carbonate minerals such as calcium carbonate and magnesium carbonate. Also, CO3 in basicized concentrated seawater 2- By allowing sufficient residence time in the sedimentation tank 3, the ions form salts with calcium and magnesium ions in the basicized concentrated seawater, and precipitate as carbonate minerals such as calcium carbonate and magnesium carbonate.

[0060] The precipitated carbonate minerals can be recovered from the sedimentation tank 3 by solid-liquid separation. This allows for the fixation of carbon dioxide in seawater and the acquisition of carbonate minerals useful as a metallic resource. The obtained carbonate minerals can be used, for example, as building materials, papermaking materials, and plastic additives.

[0061] To promote the precipitation of carbonate minerals in the sedimentation tank 3, it is preferable to introduce seed crystals of carbonate minerals into the sedimentation tank 3 beforehand. Furthermore, it is preferable to return a portion of the carbonate minerals recovered from the sedimentation tank 3 by solid-liquid separation to the sedimentation tank 3 as seed crystals.

[0062] The neutralization process takes place in neutralization tank 5. Acidified concentrated seawater, obtained after carbon dioxide has been removed from carbon dioxide separation unit 2, is introduced into neutralization tank 5. Basicized concentrated seawater, obtained after carbonate minerals have been removed from sedimentation tank 3, is also introduced into neutralization tank 5. The introduced acidified concentrated seawater and basicized concentrated seawater are mixed in neutralization tank 5 to obtain treated seawater that is neutralized, has a reduced carbon dioxide concentration, and is concentrated. The treated seawater can be discharged directly into the ocean. Furthermore, the concentrated, basicized seawater after the removal of carbonate minerals may be exposed to the atmosphere to absorb carbon dioxide from the atmosphere before being introduced into the neutralization tank 5.

[0063] According to the method for producing carbonate minerals of this embodiment, dissolved CO2 from acidified concentrated seawater and CO3 from basicized concentrated seawater produced in the bipolar membrane electrodialysis apparatus 10 2- From both ions, carbonate minerals can be obtained, and carbon dioxide can be fixed. Also, the CO3 in basicized concentrated seawater 2- Since the ions are precipitated in the precipitation tank 3 and not in the base chamber 17, problems such as clogging of the bipolar membrane 11 and cation exchange membrane 13 are less likely to occur.

[0064] In this embodiment, compared to the first embodiment, the ion concentration of the basicized seawater can be increased by concentrating the seawater, thereby further promoting precipitate formation in the sedimentation tank 3. However, there is a disadvantage in that carbon dioxide contained in the desalinated seawater cannot be recovered. On the other hand, in the first embodiment, the amount of current required to achieve the desired pH change in the acid chamber 16 and the base chamber 17 may be less than in the second embodiment. In other words, the energy consumption is lower.

[0065] The Sustainable Development Goals (SDGs) are 17 international goals adopted at the UN Summit in September 2015. The carbon dioxide sequestration system and carbonate mineral production method according to the above embodiment can contribute to achieving some of the 17 SDGs, such as "Goal 9: Build resilient infrastructure, promote inclusive and sustainable industrialization and foster innovation" and "Goal 13: Take urgent action to combat climate change and its impacts." [Explanation of Symbols]

[0066] 1. Bipolar membrane electrodialysis machine 2. Carbon Dioxide Separation Unit 3. Sedimentation tank 4. Vacuum pump 5. Neutralization tank 6 seawater tanks 7. Electrode solution tank 10 Bipolar Membrane Electrodialysis Machine 11 Bipolar membrane 12 Anion exchange membrane 13 Cation exchange membrane 16 Acid chamber 17 base chambers 18 Desalination room 21 Anode 22 Cathode 23 Anode chamber 24 Cathode Chamber

Claims

1. A bipolar membrane electrodialysis machine that generates acidified seawater and alkaline seawater from seawater, The acidified seawater is introduced from the bipolar membrane electrodialysis apparatus, and a carbon dioxide separation unit separates gaseous carbon dioxide. The apparatus comprises a sedimentation tank into which the basicized seawater is introduced from the bipolar membrane electrodialysis apparatus and gaseous carbon dioxide is introduced from the carbon dioxide separation unit, A carbon dioxide fixation system in seawater in which carbonate minerals are recovered from the aforementioned sedimentation tank.

2. Furthermore, the carbon dioxide fixation system according to claim 1 is further equipped with a vacuum pump for promoting the separation of gaseous carbon dioxide in the carbon dioxide separation unit.

3. Furthermore, the carbon dioxide fixation system according to claim 1 or 2, comprising a neutralization tank, wherein the acidified seawater after gaseous carbon dioxide has been separated in the carbon dioxide separation unit and the basicized seawater after carbonate minerals have been recovered in the sedimentation tank are mixed in the neutralization tank.

4. A dialysis process that generates acidified seawater and alkaline seawater from seawater using bipolar membrane electrodialysis, A carbon dioxide separation step is performed to separate gaseous carbon dioxide from the acidified seawater obtained in the dialysis step, A method for producing carbonate minerals, comprising a precipitation step of introducing basicized seawater obtained in the dialysis step and gaseous carbon dioxide obtained in the carbon dioxide separation step into a sedimentation tank to precipitate carbonate minerals.

5. The method for producing carbonate minerals according to claim 4, wherein the separation of gaseous carbon dioxide is promoted by a vacuum pump in the carbon dioxide separation step.

6. A method for producing carbonate minerals according to claim 4 or 5, comprising a neutralization step of mixing the acidified seawater after separating gaseous carbon dioxide in the carbon dioxide separation step with the basicized seawater after removing the carbonate minerals precipitated in the precipitation step to neutralize the mixture.

Images