Magnesium recovery system from seawater and method for producing magnesium hydroxide

Abstract

To provide a system for recovering magnesium from seawater and a method for producing magnesium hydroxide, by which high purity magnesium hydroxide can be obtained with excellent energy efficiency.SOLUTION: A system for recovering magnesium from seawater includes: a bipolar membrane electrodialysis apparatus 1 for producing acidified concentrated seawater and alkaline concentrated seawater from neutral seawater; an acidification tank 2 for introducing the acidified concentrated seawater into seawater to produce acidified seawater; a carbon dioxide separation unit 3 for producing acidified seawater having a reduced carbon dioxide concentration from the acidified seawater; a neutralization tank 4 for neutralizing the acidified seawater with a basic liquid to produce neutral seawater having a reduced carbon dioxide concentration; and a magnesium hydroxide precipitation tank 5 which receives the basic concentrated seawater from the bipolar membrane electrodialysis apparatus 1 to produce precipitate of magnesium hydroxide, wherein the neutral seawater that has a reduced carbon dioxide concentration and produced in the neutralization tank 4 is introduced into the bipolar membrane electrodialysis apparatus 1.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a magnesium recovery system from seawater and a method for producing magnesium hydroxide. [Background technology]

[0002] Seawater contains metallic resources such as sodium (1.1% by mass), magnesium (0.13% by mass), calcium (0.04% by mass), potassium (0.04% by mass), strontium (7.8 ppm by mass), and lithium (0.17 ppm by mass). Various methods have been used to concentrate and separate these metallic resources.

[0003] In particular, various technologies have been developed to recover magnesium from seawater. For example, methods for obtaining magnesium hydroxide include adding basic minerals to seawater to obtain a precipitate of magnesium hydroxide (Patent Document 1), obtaining a magnesium-concentrated fraction from concentrated seawater by electrodialysis, and then adding a basic solution to obtain a precipitate of magnesium hydroxide (Patent Document 2), and electrolyzing concentrated seawater to precipitate magnesium ions in the resulting basicized seawater as magnesium hydroxide (Patent Documents 3 and 4).

[0004] These methods all involve recovering magnesium hydroxide precipitate by basicizing seawater, but a problem arises when calcium carbonate is simultaneously formed from dissolved carbon dioxide and calcium during the precipitation of magnesium hydroxide, which reduces the purity of the magnesium compound. Therefore, prior to basicizing seawater, Patent Document 4 proposes removing dissolved carbon dioxide by heating and boiling or electrolyzing seawater. Furthermore, Patent Document 5 proposes removing dissolved carbon dioxide by blowing acid and a foaming agent into seawater. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 64-37415 [Patent Document 2] Japanese Patent Publication No. 2012-213767 [Patent Document 3] Special Publication No. 2015-513899 [Patent Document 4] Japanese Patent Publication No. 2021-70861 [Patent Document 5] Japanese Patent Application Publication No. 55-75782 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, in Patent Documents 4 and 5, a large amount of energy was required to remove dissolved carbon dioxide. In view of the above circumstances, the object of the present invention is to provide a magnesium recovery system from seawater and a method for producing magnesium hydroxide that can obtain high-purity magnesium hydroxide with excellent energy efficiency. [Means for solving the problem]

[0007] To achieve the above objectives, the present invention employs the following configuration. [1] A bipolar membrane electrodialysis apparatus that introduces neutral seawater and generates acidified concentrated seawater and alkaline concentrated seawater, An acidification tank that generates acidified seawater by introducing seawater and the acidified concentrated seawater from the bipolar membrane electrodialysis apparatus, A carbon dioxide separation unit that receives the acidified seawater from the acidification tank and separates gaseous carbon dioxide to produce acidified seawater with reduced carbon dioxide concentration, A neutralization tank is provided into which acidified seawater with reduced carbon dioxide concentration is introduced from the carbon dioxide separation unit, and a basic solution is also introduced to produce neutral seawater with reduced carbon dioxide concentration. The system includes a bipolar membrane electrodialysis apparatus into which the basicized concentrated seawater is introduced, and a magnesium hydroxide precipitation tank that generates a magnesium hydroxide precipitate. A magnesium recovery system from seawater, wherein the neutral seawater introduced into the bipolar membrane electrodialysis apparatus is neutral seawater with reduced carbon dioxide concentration produced in the neutralization tank. [2] The magnesium recovery system in seawater according to [1], wherein the basic liquid introduced into the neutralization tank is the basicized concentrated seawater after magnesium hydroxide has been recovered in the magnesium hydroxide precipitation tank. [3] Furthermore, the system is equipped with a carbonate mineral precipitation tank into which the basicized concentrated seawater after the magnesium hydroxide has been recovered in the magnesium hydroxide precipitation tank and the gaseous carbon dioxide separated in the carbon dioxide separation unit are introduced to produce a carbonate mineral precipitate. The magnesium recovery system in seawater according to [1], wherein the basic liquid introduced into the neutralization tank is the basicized concentrated seawater after carbonate minerals have been recovered in the carbonate mineral precipitation tank. [4] A dialysis process that generates acidified concentrated seawater and basicized concentrated seawater from neutral seawater by bipolar membrane electrodialysis, An acidification step to produce acidified seawater by adding the acidified concentrated seawater to seawater, A carbon dioxide separation step is performed to separate gaseous carbon dioxide from the acidified seawater obtained in the acidification step, A neutralization step is performed to neutralize the acidified seawater, whose carbon dioxide concentration has been reduced in the carbon dioxide separation step, with a basic solution to produce neutral seawater with a reduced carbon dioxide concentration. The process includes a magnesium hydroxide precipitation step in which the basicized concentrated seawater obtained in the dialysis step is introduced into a magnesium hydroxide precipitation tank to precipitate magnesium hydroxide, A method for producing magnesium hydroxide, wherein the neutral seawater used in the dialysis step is neutral seawater with a reduced carbon dioxide concentration produced in the neutralization step. [5] The method for producing magnesium hydroxide according to [4], wherein the basic liquid used in the neutralization step is the basified concentrated seawater after magnesium hydroxide is recovered in the magnesium hydroxide precipitation step. [6] The method further comprises a carbonate mineral precipitation step of introducing the basified concentrated seawater after magnesium hydroxide is recovered in the magnesium hydroxide precipitation step and gaseous carbon dioxide separated in the carbon dioxide separation step into a carbonate mineral precipitation tank to precipitate carbonate minerals, The method for producing magnesium hydroxide according to [4], wherein the basic liquid used in the neutralization step is the basified concentrated seawater after carbonate minerals are recovered in the carbonate mineral precipitation step.

Effects of the Invention

[0008] According to the system for recovering magnesium from seawater and the method for producing magnesium hydroxide of the present invention, high-purity magnesium hydroxide can be obtained with excellent energy efficiency.

Brief Description of Drawings

[0009] [Figure 1] It is a schematic configuration diagram of the system for recovering magnesium from seawater according to the first embodiment of the present invention. [Figure 2] It is a schematic diagram of a bipolar membrane electrodialysis apparatus used in the system for recovering magnesium from seawater according to an embodiment of the present invention. [Figure 3] It is a schematic configuration diagram of the system for recovering magnesium from seawater according to the second embodiment of the present invention.

Mode for Carrying Out the Invention

[0010] <First Embodiment> [System for Recovering Magnesium from Seawater] The system for recovering magnesium from seawater according to the first embodiment of the present invention will be described with reference to FIG. 1. In FIG. 1, wavy arrows indicate gas paths, single-line arrows indicate liquid paths, and double-line arrows indicate solid paths. As shown in Figure 1, the magnesium recovery system in seawater according to this embodiment is generally composed of a bipolar membrane electrodialysis apparatus 1, an acidification tank 2, a carbon dioxide separation unit 3 (indicated as "CO2 separation unit" in the figure), a neutralization tank 4, a magnesium hydroxide precipitation tank 5, and a vacuum pump 7.

[0011] The bipolar membrane electrodialysis apparatus 1 is designed to receive neutral seawater from a neutralization tank 4. Electrode solution is also circulated and supplied from an electrode solution tank 9. From the neutral seawater, the bipolar membrane electrodialysis apparatus 1 generates acidified concentrated seawater, basicized concentrated seawater, and desalinated seawater. Details of the bipolar membrane electrodialysis apparatus 1 will be described later.

[0012] In the acidification tank 2, seawater is introduced from the seawater tank 8, and acidified concentrated seawater produced by the bipolar membrane electrodialysis device 1 is also introduced and mixed to obtain acidified seawater. The resulting acidified seawater is introduced into carbon dioxide separation unit 3, where gaseous carbon dioxide is separated from the acidified seawater.

[0013] The carbon dioxide separation unit 3 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.

[0014] In this embodiment, the vacuum pump 7 is provided in a gas path leading to the gas phase of the carbon dioxide separation unit 3, allowing the gas phase of the carbon dioxide separation unit 3 to be depressurized. This promotes the separation of gaseous carbon dioxide in the carbon dioxide separation unit 3. The vacuum pump 7 may be omitted.

[0015] The neutralization tank 4 is supplied with acidified seawater, which remains after gaseous carbon dioxide has been separated in the carbon dioxide separation unit 3, and a basic solution. The acidified seawater and basic solution are then mixed in the neutralization tank 4, resulting in neutralized seawater with reduced carbon dioxide concentration. This neutralized seawater from the neutralization tank 4 is then supplied to the bipolar membrane electrodialysis apparatus 1.

[0016] Meanwhile, the basicized concentrated seawater produced by the bipolar membrane electrodialysis machine 1 is introduced into the magnesium hydroxide precipitation tank 5. In magnesium hydroxide precipitation tank 5, magnesium hydroxide precipitates from the introduced basicized seawater, and the precipitated magnesium hydroxide can be removed and recovered outside. After magnesium hydroxide is recovered in the magnesium hydroxide precipitation tank 5, the concentrated, basicized seawater is introduced into the neutralization tank 4 as a basic solution.

[0017] As shown in Figure 2, the bipolar membrane electrodialysis apparatus 1 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. 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.

[0018] 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.

[0019] As the anion exchange membrane 12, a substrate with an anion exchange layer laminated or impregnated into it can be used. As the cation exchange membrane 13, a substrate with a cation exchange layer laminated or impregnated into it can be used. For example, the NeoSepta BP-1 manufactured by Astom Co., Ltd. can be used as the bipolar membrane 11. For example, the NeoSepta ASE manufactured by Astom Co., Ltd. can be used as the anion exchange membrane 12. For example, the NeoSepta CSE manufactured by Astom Co., Ltd. can be used as the cation exchange membrane 13.

[0020] 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.

[0021] Neutral seawater flows into the acid chamber 16, base chamber 17, and desalination chamber 18 from the neutralization tank 4. In the acid chamber 16, acidified concentrated seawater is generated and supplied to the acidification tank 2. In the base chamber 17, basicized concentrated seawater is generated and supplied to the magnesium hydroxide precipitation tank 5. In the desalination chamber 18, desalination seawater is generated and released into the ocean. Electrode solution flows into the anode chamber 23 and the cathode chamber 24 from the electrode solution tank 9, and also returns to the electrode solution tank 9.

[0022] [Method for producing magnesium hydroxide] In the magnesium recovery system for seawater of this embodiment, a method for producing magnesium hydroxide can be carried out by the following steps (i) to (v). (i) A dialysis process that produces acidified concentrated seawater and basicized concentrated seawater from neutral seawater using bipolar membrane electrodialysis. (ii) An acidification step in which the acidified concentrated seawater is added to seawater to produce acidified seawater. (iii) A carbon dioxide separation step for separating gaseous carbon dioxide from the acidified seawater obtained in the acidification step. (iv) A neutralization step in which the acidified seawater, whose carbon dioxide concentration has been reduced in the carbon dioxide separation step, is neutralized with a basic solution to produce neutral seawater with a reduced carbon dioxide concentration. (v) A magnesium hydroxide precipitation step in which the basicized concentrated seawater obtained in the dialysis step is introduced into a magnesium hydroxide precipitation tank to precipitate magnesium hydroxide.

[0023] (i) is a dialysis process that produces acidified concentrated seawater and basicized concentrated seawater from neutral seawater using bipolar membrane electrodialysis. (i) The dialysis process is performed using the bipolar membrane electrodialysis apparatus 1 shown in Figure 2. In the magnesium hydroxide production method using the magnesium recovery system in seawater of this embodiment, neutral seawater with reduced carbon dioxide concentration produced in the neutralization step (iv) is used as the neutral seawater in the dialysis step (i). For example, an aqueous sulfuric acid solution, an aqueous sodium sulfate solution, or an aqueous sodium carbonate solution can be used as the electrode solution supplied from the electrode solution tank 9 to the anode chamber 23 and cathode chamber 24.

[0024] When a voltage is applied between the anode 21 and the cathode 22, water molecules from the neutral 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.

[0025] Also, Cl in the desalination chamber 18 and cathode chamber 24- Anions such as the aforesaid are attracted to the anode 21 side, pass through the anion exchange membrane 12, and move to the acid chamber 16 (anions in the desalination chamber 18 closest to the anode 21 move to the anode chamber 23). Further, Na in the desalination chamber 18 + , K + , Li + and other cations are attracted to the cathode 22 side, pass through the cation exchange membrane 13, and move to the base chamber 17. As a result, the neutral seawater introduced into the acid chamber 16 becomes acidified concentrated seawater, the neutral seawater introduced into the base chamber 17 becomes basified concentrated seawater, and the neutral seawater introduced into the desalination chamber 18 becomes desalinated seawater.

[0026] The pH of the acidified concentrated seawater flowing out from the acid chamber 16 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 flowing out from the acid chamber 16 is not higher than the preferred upper limit, the introduced seawater can be sufficiently acidified in the acidification step of (ii), so carbon dioxide can be efficiently separated in the carbon dioxide separation step of (iii). If the pH of the acidified concentrated seawater flowing out from the acid chamber 16 is not lower than the preferred lower limit, energy consumption in electrodialysis can be suppressed.

[0027] The pH of the basified concentrated seawater flowing out from the base chamber 17 is preferably 9.5 to 11.0, more preferably 10.0 to 10.5, and particularly preferably around 10.2. If the pH of the basified concentrated seawater flowing out from the base chamber 17 is not lower than the preferred lower limit, magnesium hydroxide can be efficiently precipitated in the precipitation step of (v). If the pH of the basified concentrated seawater flowing out from the base chamber 17 is not higher than the preferred upper limit, it is easy to avoid precipitation occurring within the base chamber 17.

[0028] It is preferable not to allow the basified concentrated seawater to retain excessively in the base chamber 17. The basified concentrated seawater contains CO3 in the basified concentrated seawater 2-This is because if the mixture is allowed to remain in the base chamber 17 for longer than necessary in a state where ions and calcium ions, magnesium ions, etc., have been basicized to the extent that a precipitate can form, a precipitate will form within the base chamber 17. Once a precipitate forms within the base chamber 17, that precipitate can act as a seed crystal, causing further precipitation, which is undesirable.

[0029] The pH of the acidified concentrated seawater flowing out of the acid chamber 16 and the pH of the basicized concentrated seawater flowing out of the base chamber 17 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 to the bipolar membrane electrodialysis apparatus 1.

[0030] (ii) is an acidification step in which the acidified concentrated seawater is added to seawater to produce acidified seawater. (ii) The acidification process is carried out in the acidification tank 2. It is preferable that the seawater supplied from the seawater tank 8 to the acidification tank 2 is pre-treated to remove turbidity. For turbidity removal, for example, a filtration device using an ultrafiltration membrane, a microfiltration membrane, or a nanofiltration membrane can be used.

[0031] Acidified concentrated seawater is supplied from the acid chamber 16 of the bipolar membrane electrodialysis apparatus 1. The pH of the seawater (acidified seawater) after mixing with the acidified concentrated seawater in the acidification tank 2 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 the ions are converted to CO2, carbon dioxide can be efficiently separated in the carbon dioxide separation process of (iii). If simply mixing the acidified concentrated seawater from the bipolar membrane electrodialysis machine 1 does not sufficiently lower the pH of the acidified seawater, an additional acidic solution prepared separately may be introduced into the acidification tank 2. In this case, hydrochloric acid, sulfuric acid, etc., can be used as the acidic solution.

[0032] (iii) is a carbon dioxide separation process that separates gaseous carbon dioxide from the acidified seawater obtained in the acidification process of (ii). The carbon dioxide separation process is carried out in the carbon dioxide separation unit 3 shown in Figure 1. Acidified seawater is introduced into the carbon dioxide separation unit 3 from the acidification tank 2. The dissolved CO2 contained in the acidified seawater is then separated as gaseous CO2. By creating a reduced pressure state using the vacuum pump 7, the conversion from dissolved CO2 to gaseous CO2 is promoted, and gaseous CO2 can be efficiently separated from the acidified seawater.

[0033] (iv) is a neutralization step in which the acidified seawater, whose carbon dioxide concentration has been reduced in the carbon dioxide separation step of (iii), is neutralized with a basic solution to produce neutral seawater. The neutralization process is carried out in the neutralization tank 4 shown in Figure 1. In this embodiment, the basicized concentrated seawater obtained after the magnesium hydroxide has been recovered in the magnesium hydroxide precipitation process (v) is used as the basic solution.

[0034] If simply mixing the basicized concentrated seawater after the recovery of magnesium hydroxide is insufficient to neutralize the acidified seawater, a separately prepared basic solution may be added to the acidification tank 2. In this case, for example, a sodium hydroxide solution or a potassium hydroxide solution can be used as the additional basic solution.

[0035] (v) is a magnesium hydroxide precipitation process in which the basicized concentrated seawater obtained in the dialysis process of (i) is introduced into the magnesium hydroxide precipitation tank 5 to precipitate magnesium hydroxide. The magnesium hydroxide precipitation process is carried out in the magnesium hydroxide precipitation tank 5. Basicized concentrated seawater is introduced into the magnesium hydroxide precipitation tank 5 from the base chamber 17 of the bipolar membrane electrodialysis apparatus 1.

[0036] Magnesium ions in the basicized concentrated seawater precipitate as magnesium hydroxide in the magnesium hydroxide precipitation tank 5. At this time, the basicized concentrated seawater contains CO3 2-Because the ion concentration is reduced, the simultaneous formation of calcium carbonate is suppressed. As a result, a precipitate of high-purity magnesium hydroxide is obtained.

[0037] The pH of the basicized concentrated seawater in the magnesium hydroxide precipitation tank 5 is preferably 9.5 or higher, more preferably 9.5 to 11.0, and even more preferably 10.0 to 10.5. If the pH of the basicized concentrated seawater in the magnesium hydroxide precipitation tank 5 is above the preferred lower limit, magnesium hydroxide can be efficiently precipitated and recovered. If the pH of the basicized concentrated seawater in the magnesium hydroxide precipitation tank 5 is below the preferred upper limit, the precipitation of other hydroxide minerals (e.g., basic magnesium chloride and calcium hydroxide) can be prevented.

[0038] If the pH of the basicized concentrated seawater supplied from the bipolar membrane electrodialysis machine 1 is not sufficiently high, a separately prepared basic solution may be added to the magnesium hydroxide precipitation tank 5. In this case, for example, a sodium hydroxide solution or a potassium hydroxide solution can be used as the additional basic solution. In the magnesium hydroxide precipitation tank 5, it is preferable to allow a sufficient residence time so that the magnesium hydroxide can settle completely.

[0039] The precipitated magnesium hydroxide can be recovered from the magnesium hydroxide precipitation tank 5 by solid-liquid separation. This allows for the acquisition of magnesium hydroxide, which is useful as a metal resource. The obtained magnesium hydroxide can be used, for example, as a refractory material, an adsorbent, a compounding agent for rubber and plastics, a heavy metal treatment agent, and a water quality improvement agent.

[0040] In order to promote the precipitation of magnesium hydroxide in the magnesium hydroxide precipitation tank 5, it is preferable to introduce magnesium hydroxide seed crystals into the magnesium hydroxide precipitation tank 5 in advance. Furthermore, it is preferable to return a portion of the carbonate minerals recovered from the magnesium hydroxide precipitation tank 5 by solid-liquid separation to the magnesium hydroxide precipitation tank 5 as seed crystals.

[0041] According to the magnesium hydroxide production method of this embodiment, since the precipitate is obtained from basicized concentrated seawater with reduced carbon dioxide concentration, high-purity magnesium hydroxide can be obtained. Also, the CO3 in basicized concentrated seawater 2- Since the ions are precipitated in the magnesium hydroxide precipitation tank 5 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.

[0042] <Second Embodiment> [Magnesium recovery system from seawater] A magnesium recovery system in seawater 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 gas pathways, single arrows indicate liquid pathways, and double arrows indicate solid pathways.

[0043] As shown in Figure 3, the magnesium recovery system in seawater according to this embodiment is generally composed of a bipolar membrane electrodialysis apparatus 1, an acidification tank 2, a carbon dioxide separation unit 3 (indicated as "CO2 separation unit" in the figure), a neutralization tank 4, a magnesium hydroxide precipitation tank 5, a carbonate mineral precipitation tank 6, and a vacuum pump 7.

[0044] In other words, the magnesium recovery system of this embodiment differs from the magnesium recovery system of the first embodiment in that it is provided with a carbonate mineral precipitation tank 6. The carbonate mineral precipitation tank 6 is supplied with basicized concentrated seawater from the magnesium hydroxide precipitation tank 5, after the magnesium hydroxide has been recovered. Additionally, gaseous carbon dioxide separated in the carbon dioxide separation unit 3 is also supplied via the vacuum pump 7.

[0045] In the carbonate mineral sedimentation tank 6, gaseous carbon dioxide can be blown into the introduced basicized concentrated 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 7 and the carbonate mineral precipitation tank 6. As a concentration device, for example, a UBE CO2 SEPARATOR manufactured by UBE can be used. The concentrated, basicized seawater remaining after the carbonate minerals have been recovered in the carbonate mineral sedimentation tank 6 is introduced into the neutralization tank 4 as a basic solution. Other aspects are the same as in the first embodiment.

[0046] [Method for producing magnesium hydroxide] In the magnesium recovery system for seawater of this embodiment, a method for producing magnesium hydroxide can be implemented in which magnesium hydroxide and carbonate minerals are produced through the following steps (i) to (vi). (i) A dialysis process that produces acidified concentrated seawater and basicized concentrated seawater from neutral seawater using bipolar membrane electrodialysis. (ii) An acidification step in which the acidified concentrated seawater is added to seawater to produce acidified seawater. (iii) A carbon dioxide separation step for separating gaseous carbon dioxide from the acidified seawater obtained in the acidification step. (iv) A neutralization step in which the acidified seawater, whose carbon dioxide concentration has been reduced in the carbon dioxide separation step, is neutralized with a basic solution to produce neutral seawater with a reduced carbon dioxide concentration. (v) A magnesium hydroxide precipitation step in which the basicized concentrated seawater obtained in the dialysis step is introduced into a magnesium hydroxide precipitation tank to precipitate magnesium hydroxide. (vi) A carbonate mineral precipitation step in which the basicized concentrated seawater after the magnesium hydroxide has been recovered in the magnesium hydroxide precipitation step and the gaseous carbon dioxide separated in the carbon dioxide separation step are introduced into a carbonate mineral precipitation tank to precipitate carbonate minerals.

[0047] Steps (i) to (iii) and (v) are the same as in the first embodiment. The neutralization step (iv) differs from the first embodiment in that, as the basic solution, the basicized concentrated seawater is used after magnesium hydroxide has been recovered in the carbonate mineral precipitation step (vi) and further carbonate minerals have been recovered. In this embodiment as well, if simply mixing the basicized concentrated seawater after the recovery of magnesium hydroxide and carbonate minerals is insufficient to adequately neutralize the acidified seawater, a separately prepared basic solution may be additionally introduced into the acidification tank 2.

[0048] (vi) is a carbonate mineral precipitation step in which the basicized concentrated seawater after the magnesium hydroxide has been recovered in the magnesium hydroxide precipitation step and the gaseous carbon dioxide separated in the carbon dioxide separation step are introduced into a carbonate mineral precipitation tank to precipitate carbonate minerals. The precipitation process is carried out in carbonate mineral precipitation tank 6.

[0049] In the carbonate mineral precipitation tank 6, the basicized concentrated seawater, after the recovery of magnesium hydroxide from the magnesium hydroxide precipitation tank 5, is introduced. In addition, gaseous CO2 is introduced from the carbon dioxide separation unit 3 via the vacuum pump 7. In the carbonate mineral precipitation tank 6, 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 carbonate mineral precipitation tank 6, 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.

[0050] The precipitated carbonate minerals can be recovered from the carbonate mineral sedimentation tank 6 by solid-liquid separation. This allows for the fixation of carbon dioxide in seawater and the acquisition of carbonate minerals useful as metal resources. The obtained carbonate minerals can be used, for example, as building materials, papermaking materials, and plastic additives.

[0051] In order to promote the precipitation of carbonate minerals in the carbonate mineral precipitation tank 6, it is preferable to introduce seed crystals of carbonate minerals into the carbonate mineral precipitation tank 6 in advance. Furthermore, it is preferable to return a portion of the carbonate minerals recovered from the carbonate mineral precipitation tank 6 by solid-liquid separation to the carbonate mineral precipitation tank 6 as seed crystals.

[0052] According to this embodiment, not only can magnesium hydroxide be produced, but carbonate minerals can also be produced, and furthermore, carbon dioxide in seawater can be fixed, thereby contributing to the reduction of carbon dioxide in the atmosphere.

[0053] The Sustainable Development Goals (SDGs) are 17 international goals adopted at the UN Summit in September 2015. The magnesium recovery system from seawater and the method for producing magnesium hydroxide 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]

[0054] 1. Bipolar membrane electrodialysis machine 2 Acidification Tank 3. Carbon Dioxide Separation Unit 4. Neutralization tank 5. Magnesium hydroxide precipitation tank 6. Carbonate mineral precipitation tank 7. Vacuum pump 8 seawater tanks 9. Electrode solution tank 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 is used to introduce neutral seawater and generate acidified concentrated seawater and alkaline concentrated seawater. An acidification tank that generates acidified seawater by introducing seawater and the acidified concentrated seawater from the bipolar membrane electrodialysis apparatus, A carbon dioxide separation unit that receives the acidified seawater from the acidification tank and separates gaseous carbon dioxide to produce acidified seawater with reduced carbon dioxide concentration, A neutralization tank is provided into which acidified seawater with reduced carbon dioxide concentration is introduced from the carbon dioxide separation unit, and a basic solution is also introduced to produce neutral seawater with reduced carbon dioxide concentration. The system includes a bipolar membrane electrodialysis apparatus into which the basicized concentrated seawater is introduced, and a magnesium hydroxide precipitation tank that generates a magnesium hydroxide precipitate. A magnesium recovery system from seawater, wherein the neutral seawater introduced into the bipolar membrane electrodialysis apparatus is neutral seawater with reduced carbon dioxide concentration produced in the neutralization tank.

2. The magnesium recovery system from seawater according to claim 1, wherein the basic liquid introduced into the neutralization tank is the basicized concentrated seawater after magnesium hydroxide has been recovered in the magnesium hydroxide precipitation tank.

3. Furthermore, the system includes a carbonate mineral precipitation tank into which the basicized concentrated seawater, after the magnesium hydroxide has been recovered in the magnesium hydroxide precipitation tank, and the gaseous carbon dioxide separated in the carbon dioxide separation unit are introduced to generate a carbonate mineral precipitate. The magnesium recovery system from seawater according to claim 1, wherein the basic liquid introduced into the neutralization tank is the basicized concentrated seawater after carbonate minerals have been recovered in the carbonate mineral precipitation tank.

4. A dialysis process that generates acidified concentrated seawater and basicized concentrated seawater from neutral seawater using bipolar membrane electrodialysis, An acidification step to produce acidified seawater by adding the acidified concentrated seawater to seawater, A carbon dioxide separation step is performed to separate gaseous carbon dioxide from the acidified seawater obtained in the acidification step, A neutralization step is performed to neutralize the acidified seawater, whose carbon dioxide concentration has been reduced in the carbon dioxide separation step, with a basic solution to produce neutral seawater with a reduced carbon dioxide concentration. The process includes a magnesium hydroxide precipitation step in which the basicized concentrated seawater obtained in the dialysis step is introduced into a magnesium hydroxide precipitation tank to precipitate magnesium hydroxide, A method for producing magnesium hydroxide, wherein the neutral seawater used in the dialysis step is neutral seawater with a reduced carbon dioxide concentration produced in the neutralization step.

5. The method for producing magnesium hydroxide according to claim 4, wherein the basic solution used in the neutralization step is the basicized concentrated seawater after magnesium hydroxide has been recovered in the magnesium hydroxide precipitation step.

6. Furthermore, the system includes a carbonate mineral precipitation step in which the basicized concentrated seawater after the magnesium hydroxide has been recovered in the magnesium hydroxide precipitation step, and the gaseous carbon dioxide separated in the carbon dioxide separation step are introduced into a carbonate mineral precipitation tank to precipitate carbonate minerals. The method for producing magnesium hydroxide according to claim 4, wherein the basic solution used in the neutralization step is the basicized concentrated seawater after the carbonate minerals have been recovered in the carbonate mineral precipitation step.

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