Fixation method for carbon dioxide

Abstract

Provided is a fixation method for carbon dioxide, the method comprising: a first step (S1) for allowing seawater or brine to pass through a nano-filtration membrane, thereby generating a first concentrated solution that has been concentrated without permeating through the nano-filtration membrane; a second step (S2) for bringing carbon dioxide into contact with an alkaline solution, thereby generating a treated solution containing carbonate ions; and a third step (S3) for bringing the first concentrated solution and the treated solution into contact, thereby causing the precipitation of carbonated product crystals of an alkaline earth metal contained in the first concentrated solution.

Description

Carbon dioxide fixation method

[0001] The present invention relates to a method for fixing carbon dioxide on an alkaline earth metal.

[0002] As global warming becomes more serious, there is a need to curb the rise in temperature. ２ ) emissions to zero. ２ Immobilization methods include: ２ As an effective immobilization method, alkaline earth metals such as Mg and Ca are used to immobilize CO. ２ However, conventional methods using ores containing alkaline earth metals require high temperature and pressure, and the addition of chemicals. ２ The entire process requires treatment that involves emissions. ２ In many cases, Mg and Ca are also contained in seawater and waste brine from seawater desalination plants, and are used in CO ２ Immobilization methods have been proposed in the past (for example, Patent Document 1).

[0003] WO 2022 / 30529

[0004] Patent Document 1 discloses a method for immobilizing carbon dioxide, comprising: a first step of passing seawater or brine through a nanofiltration membrane to produce a first concentrated liquid that is concentrated without passing through the nanofiltration membrane; a second step of adding an alkali to the produced first concentrated liquid to react carbon dioxide with the alkaline earth metal contained in the first concentrated liquid to immobilize it, thereby precipitating alkaline earth metal carbonate crystals; and a third step of recovering the precipitated alkaline earth metal carbonate crystals from the first concentrated liquid by solid-liquid separation. The first step comprises a salt production step of concentrating the NF membrane permeate that has permeated the nanofiltration membrane to recover precipitated sodium chloride crystals, and an electrodialysis step of electrodialyzing a solution of the sodium chloride crystals recovered in the salt production step to separate an acid solution from an alkaline solution, and the second step comprises adding the alkaline solution obtained in the electrodialysis step to the NF membrane concentrated liquid.

[0005] The invention of Patent Document 1 is characterized in that the first step is performed to remove Na, which may hinder the fixation of carbon dioxide. ＋ , K. ＋ The concentration of carbon dioxide can be easily and efficiently fixed to alkaline earth metals contained in seawater or brine because the concentration of the alkaline earth metals is reduced. Furthermore, in the second step, the alkaline solution obtained in the electrodialysis step of the first step is added to the NF membrane concentrate, which also makes it possible to efficiently fix carbon dioxide to alkaline earth metals.

[0006] The invention of Patent Document 1 is to effectively use energy to reduce CO2 in the entire process. ２ Emission reduction measures have been considered, but there is room for further improvement in this regard.

[0007] Therefore, the present invention provides a method for immobilizing carbon dioxide on alkaline earth metals, which enhances the carbon dioxide reduction capacity while taking into account the amount of carbon dioxide emitted.

[0008] The above-described object of the present invention is achieved by a method for fixation of carbon dioxide, comprising: a first step of passing seawater or brackish water through a nanofiltration membrane to produce an NF membrane concentrate that is concentrated without passing through the nanofiltration membrane and an NF membrane permeate that has permeated the nanofiltration membrane; a second step of producing an alkaline solution and hydrogen by electrolyzing the NF membrane permeate; and a third step of reacting the NF membrane concentrate with carbon dioxide and the alkaline solution to precipitate carbonate crystals of the alkaline earth metal contained in the NF membrane concentrate.

[0009] In this carbon dioxide fixation method, the third step can include a step of bringing the alkaline solution, which has been previously contacted with carbon dioxide, into contact with the NF membrane concentrate.

[0010] The second step includes a concentration step of concentrating the NF membrane permeate and an alkaline earth metal removal step of removing alkaline earth metals contained in the concentrated NF membrane permeate, thereby enabling the NF membrane permeate that has undergone the alkaline earth metal removal step to be electrolyzed. In this case, the alkaline earth metal removal step may include a carbonate separation step of reacting the concentrated NF membrane permeate with carbon dioxide and the alkaline solution to precipitate alkaline earth metal carbonates and perform solid-liquid separation. The carbonate separation step may include a step of contacting the alkaline solution, which has been previously contacted with carbon dioxide, with the concentrated NF membrane permeate.

[0011] The alkaline earth metal removal step can include a step of further concentrating the concentrated NF membrane permeate to recover precipitated sodium chloride crystals and redissolve them in water, thereby regenerating the NF membrane permeate.

[0012] The alkaline earth metal removal step can include a second NF membrane treatment step of passing the concentrated NF membrane permeate through a second nanofiltration membrane different from the nanofiltration membrane.

[0013] The alkaline earth metal removal step can include an adsorption removal step of removing alkaline earth metals contained in the concentrated NF membrane permeate using a chelating resin or an ion exchange resin.

[0014] When the third step comprises a step of contacting the alkaline solution that has been previously contacted with carbon dioxide with the NF membrane concentrate, the second step comprises a concentration step of concentrating the NF membrane permeate and an alkaline earth metal removal step of removing alkaline earth metals contained in the concentrated NF membrane permeate, whereby the alkaline earth metal removal step can comprise a step of contacting a portion of the alkaline solution that has been previously contacted with carbon dioxide, which is used in the third step, with the concentrated NF membrane permeate to precipitate carbonates of the alkaline earth metals contained in the NF membrane permeate and perform solid-liquid separation.

[0015] The first step can include an evaporation and concentration step of evaporating and concentrating the NF membrane concentrate to precipitate calcium sulfate crystals, and a solid-liquid separation step of recovering the precipitated calcium sulfate crystals by solid-liquid separation.

[0016] According to the present invention, it is possible to provide a method for immobilizing carbon dioxide on alkaline earth metals, which enhances the carbon dioxide reduction capacity while taking into account the amount of carbon dioxide emitted.

[0017] FIG. 1 is a process flow diagram illustrating a method for immobilizing carbon dioxide according to a first embodiment of the present invention. FIG. 2 is a process flow diagram illustrating a method for immobilizing carbon dioxide according to a second embodiment of the present invention. FIG. 3 is a diagram illustrating a modified example of some of the steps in the process flow shown in FIG. 2. FIG. 4 is a process flow diagram illustrating a method for immobilizing carbon dioxide according to a third embodiment of the present invention. FIG. 5 is a process flow diagram illustrating a method for immobilizing carbon dioxide according to a fourth embodiment of the present invention. FIG. 6 is a process flow diagram illustrating a method for immobilizing carbon dioxide according to a fifth embodiment of the present invention. FIG. 7 is a process flow diagram illustrating a method for immobilizing carbon dioxide according to a sixth embodiment of the present invention. FIG. 8 is a process flow diagram illustrating a method for immobilizing carbon dioxide according to a seventh embodiment of the present invention. FIG. 9 is a diagram illustrating a modified example of some of the steps in each of the above process flows.

[0018] The carbon dioxide fixation method of the present invention provides a method for fixating carbon dioxide to alkaline earth metals contained in seawater or brine. In the present invention, the term "alkaline earth metal" refers to a broad range of elements including Ca, Sr, Ba, Ra, and also Mg and Be, which are elements of Group 2 of the periodic table. In particular, CO ２ From the viewpoint of ease of reaction with HCl and the fact that the carbonate obtained by the reaction can be expected to be used in various applications, it is preferable that the alkaline earth metal contains at least Mg.

[0019] Also, "seawater or brine" contains magnesium ions (Mg ２＋ ), calcium ions (Ca ２＋Seawater or brine is an aqueous solution containing ions of alkaline earth metals such as calcium sulfate, sodium chloride, potassium chloride, and sodium sulfate. In addition to alkaline earth metal ions, seawater or brine usually contains at least one ion that forms a crystal selected from calcium sulfate, sodium chloride, potassium chloride, and sodium sulfate. Specifically, seawater or brine contains chloride ions (Cl － ), sulfate ions (SO ４ ２－ ), sodium ions (Na ＋ ), potassium (K ＋ ) and usually contains at least one ion selected from the group consisting of:

[0020] The seawater or brine can be obtained from at least one selected from seawater, salt lakes, and industrial wastewater. In addition to seawater, salt lakes, and industrial wastewater, river water, rainwater, treated sewage water, and produced water from oil or gas fields can also be used, as long as they contain alkaline earth metals. More specific examples of brine include waste brine discharged from water production processes using salt lakes, desalination processes, and salt production processes; recovery of valuable materials using seawater and salt lakes; and industrial wastewater from chemical plants.

[0021] The aforementioned Mg content, reduction of environmental load and CO ２ From the viewpoint of ease of reducing discharge, the brine is preferably at least one selected from the group consisting of brine obtained from a freshwater production system using seawater, brine obtained from a process for producing salt from seawater, and brine obtained from a process for recovering lithium from a salt lake.

[0022] First Embodiment An embodiment of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a process flow diagram illustrating a carbon dioxide fixation method according to a first embodiment of the present invention. In the first embodiment, seawater is treated, but a similar process can be performed with brine. As shown in FIG. 1, the carbon dioxide fixation method according to the first embodiment includes a first step S1 in which seawater is passed through a nanofiltration membrane (NF membrane) to produce an NF membrane concentrate and an NF membrane permeate, a second step S2 in which the NF membrane permeate is electrolyzed to produce an alkaline solution and hydrogen, and a third step S3 in which the NF membrane concentrate is reacted with carbon dioxide and an alkaline solution to precipitate carbonate crystals of an alkaline earth metal contained in the NF membrane concentrate.

[0023] <S1: First Step> In the first step S1, the seawater taken in is subjected to appropriate pretreatment such as filtration, coagulation, and precipitation to remove impurities such as fine particles and microorganisms, and then the seawater is supplied to an NF membrane unit using a medium-pressure pump or the like and passed through the NF membrane, thereby producing an NF membrane concentrate that is concentrated without passing through the NF membrane, and an NF membrane permeate that has passed through the NF membrane.

[0024] NF membranes have the property of suppressing the permeation of divalent and higher ions, while allowing monovalent ions to easily permeate. Therefore, in the NF membrane concentrate, many of the alkaline earth metals that are the target of carbon dioxide fixation remain, and Na, which may hinder this fixation, ＋ , K. ＋ Therefore, carbon dioxide can be easily and efficiently fixed to alkaline earth metals contained in seawater (or brine), and carbon dioxide generation can be suppressed throughout the entire process, including subsequent steps.

[0025] The first step S1 of this embodiment includes the following evaporation concentration step S11 and solid-liquid separation step S12.

[0026] <S11: Evaporation and Concentration Step> In the evaporation and concentration step S11, calcium sulfate crystals are precipitated by evaporating and concentrating the NF membrane concentrate in an evaporator. To prevent scale formation in the evaporator, calcium sulfate crystals are preferably added to the NF membrane concentrate as seed crystals. For example, calcium sulfate crystals recovered in the solid-liquid separation step S12 described below can be used as these seed crystals. As a method for preventing scale formation in the evaporator, it is also preferable to add an acid to the NF membrane concentrate to adjust the pH.

[0027] <S12: Solid-Liquid Separation Step> In the solid-liquid separation step S12, the calcium sulfate crystals precipitated in the evaporation concentration step S11 are recovered from the NF membrane concentrated liquid by solid-liquid separation using a solid-liquid separator. The recovered calcium sulfate crystals can be used, for example, as gypsum. The NF membrane concentrated liquid after solid-liquid separation is subjected to the third step S3, which will be described later.

[0028] By providing the above-mentioned evaporation concentration step S11 and solid-liquid separation step S12, it is possible to reduce the amount of the NF membrane concentrate to be subjected to the third step S3 described later, and therefore the amount of alkali (NaOH) added in the third step S3 can be reduced. Furthermore, since the concentration of alkaline earth metals in the NF membrane concentrate increases, the reaction efficiency of carbon dioxide in the third step S3 increases, and therefore the amount of carbon dioxide bubbling can be reduced, and the power consumption required for bubbling can be suppressed. Furthermore, since calcium is recovered as calcium sulfate crystals from the NF membrane concentrate before the third step S3, the amount of CaCO 3 added in the third step S3 can be reduced. ３ The precipitation of MgCO is suppressed and the recovered ３ The purity of the product can be increased.

[0029] <S2: Second Step> The second step S2 is to dissolve monovalent ions, such as Na ＋ and Cl － The NF membrane permeate, which contains a large amount of chlorine, is electrolyzed in an electrolytic cell. The electrolytic cell is separated between the anode and the cathode by an ion exchange membrane, and when electricity is applied while the NF membrane permeate is stored in the electrolytic cell, chlorine (Cl) is generated on the anode side. ２ ) is generated on the cathode side, and hydrogen (H ２ ) is generated. Sodium ions (Na ＋) and hydroxide ions (OH － ) bonded to each other and sodium hydroxide solution (NaOH （ａｑ） It is preferable to use renewable energy such as solar energy to power the electrolytic cell, and CO ２ The alkaline solution produced in the second step S2 is mostly a sodium hydroxide solution.

[0030] The second step S2 of this embodiment includes the following concentration step S21 and alkaline earth metal removal step S22.

[0031] <S21: Concentration Step> In the concentration step S21, a portion of the NF membrane permeate produced in the first step S1 is supplied to a reverse osmosis membrane (RO membrane) unit using a high-pressure pump or the like before electrolysis in the second step S2, and the water is passed through the RO membrane, and the RO membrane permeate is recovered, thereby concentrating the NF membrane permeate. The RO membrane permeate can be used as produced water, etc., together with the distilled water produced in the above-mentioned evaporation and concentration step S11. The remainder of the NF membrane permeate that is not used in the concentration step S21 can be used in another step such as a desalination process, and the NF membrane permeate that is not used in another step may be discharged into the ocean, etc.

[0032] <S22: Alkaline Earth Metal Removal Step> The alkaline earth metal removal step S22 includes a carbonate separation step S221 and an adsorption removal step S222. Alkaline earth metals such as magnesium and calcium are contained to a certain extent in the NF membrane permeate that has passed through the first step S1 and may cause scale in the second step S2. Therefore, the carbonate separation step S221 and the alkaline earth metal removal step S22 enable the second step S2 to be carried out efficiently over a long period of time. In particular, in the electrolysis using an ion exchange membrane in the second step S2, the concentration of alkaline earth metals such as magnesium and calcium must be extremely low, so it is effective to carry out the alkaline earth metal removal step S22.

[0033] <S221: Carbonate Separation Step> In the carbonate separation step S221, the alkaline solution produced in the second step S2 is added to the NF membrane permeate concentrated in the concentration step S21 to adjust the pH of the NF membrane permeate to the alkaline side (for example, pH 9 to 10), and the NF membrane permeate is stored in a storage tank. A gas containing carbon dioxide is blown into the NF membrane permeate to bring the two into gas-liquid contact by bubbling, causing carbon dioxide to react with the alkaline earth metal contained in the NF membrane permeate and immobilize it. As a result, the NF membrane permeate is converted into MgCO ３ and CaCO ３ In the carbonate separation step S221, the alkaline earth metal carbonate crystals contained in the slurry NF membrane permeate are recovered by solid-liquid separation using a solid-liquid separator such as a centrifuge.

[0034] The carbon dioxide-containing gas may be atmospheric air or exhaust gas from various combustion devices. There are no limitations on the carbon dioxide concentration contained in the gas, but for example, the carbon dioxide concentration contained in the gas may be atmospheric to approximately 100% by volume. To maintain the pH during the reaction between the alkaline earth metal and carbon dioxide, the alkaline solution may be added to the NF membrane permeate not only before bubbling the carbon dioxide-containing gas, but also during bubbling. During bubbling, the reaction efficiency between the alkaline earth metal and carbon dioxide can be improved by blowing fine carbon dioxide bubbles (fine bubbles such as microbubbles or ultrafine bubbles).

[0035] The method of bringing the NF membrane permeate into gas-liquid contact with carbon dioxide is to add CO ２ In addition to the method of blowing in gas, CO ２ The method may be a method of spraying the NF membrane permeate into the gas using a spray nozzle or tray, and the reaction rate, reaction amount, CO in the gas such as exhaust gas, etc. ２ Taking into consideration the concentration and the like, various known gas-liquid contactors can be used.

[0036] <S222: Adsorption and Removal Step> The adsorption and removal step S222 is a step in which alkaline earth metals contained in the NF membrane permeate that has been through the carbonate separation step S221 are removed using a chelating resin or an ion exchange resin, and can be performed, for example, by passing the NF membrane permeate through a column packed with a chelating resin. As the chelating resin, it is preferable to use one that can selectively capture alkaline earth metal ions such as magnesium ions and calcium ions, and examples of such chelating resins include iminodiacetic acid type and aminophosphate type.

[0037] <S3: Third Step> In the third step S3, the alkaline solution produced in the second step S2 is added to the NF membrane concentrate produced in the first step S1 to adjust the pH of the NF membrane concentrate to the alkaline side (for example, pH 9 to 10), and the NF membrane concentrate is stored in a storage tank. A gas containing carbon dioxide is blown into the NF membrane concentrate to bring the NF membrane concentrate into gas-liquid contact by bubbling, thereby causing the carbon dioxide to react with the alkaline earth metal contained in the NF membrane concentrate and immobilize it. Examples of the method for bringing the NF membrane concentrate and carbon dioxide into gas-liquid contact include the same method as in the carbonate separation step S221 described above.

[0038] <S31: Solid-liquid separation step> The NF membrane concentrated liquid that has undergone the third step S3 is subjected to a reaction between an alkaline earth metal and carbon dioxide to form MgCO ３ and CaCO ３ In the solid-liquid separation step S31, the alkaline earth metal carbonate crystals contained in the slurry are separated into solid and liquid using a solid-liquid separator such as a centrifugal separator, and are recovered. The recovered alkaline earth metal carbonate crystals are washed with wash water to remove any adhering Na ＋ , K. ＋ By dissolving the alkali metal carbonate in washing water and removing the alkali metal carbonate, the alkali metal carbonate can be suitably used as a building material such as concrete or cement. Therefore, the present invention can also provide a method for producing alkaline earth metal carbonate using the carbon dioxide fixation method.

[0039] In the carbon dioxide fixation method of this embodiment, the NF membrane permeate produced in the first step S1 is electrolyzed in the second step S2 to produce an alkaline solution and hydrogen, and the produced alkaline solution is efficiently used for carbon dioxide fixation in the third step S3 to fix CO ２ This can suppress an increase in emissions and utilize the generated hydrogen gas as clean energy. Therefore, it is possible to enhance the carbon dioxide reduction capability while taking into consideration the carbon dioxide emissions throughout the process. Furthermore, the generated hydrogen gas can be utilized for methanation, co-firing, etc. Chlorine gas generated together with hydrogen gas can also be recovered as a valuable resource. The recovered chlorine gas can be utilized for the production of vinyl chloride monomer, hydrochloric acid, sodium hypochlorite, and isocyanate, for example.

[0040] Furthermore, since carbon dioxide is fixed not only in the third step S3 but also in the carbonate separation step S221, the amount of carbon dioxide that can be fixed can be increased, thereby increasing the production amount of alkaline earth metal carbonate salts.

[0041] <Second embodiment> Fig. 2 is a process flow diagram for explaining a carbon dioxide fixation method according to a second embodiment of the present invention. The carbonate separation step S221 of the first embodiment is configured to blow a gas containing carbon dioxide into the alkaline solution produced in the second step S2 to cause a gas-liquid contact reaction. However, as shown in Fig. 2, in the second embodiment, a CO2 fixation step S221 is configured to bring carbon dioxide into contact with the alkaline solution produced in the second step S2. ２ By providing the absorption step S224, the precipitation of alkaline earth metal carbonates in the carbonate separation step S223 is carried out by liquid-liquid contact between the NF membrane permeate and an alkaline solution that has been previously contacted with carbon dioxide. In Figure 2, the same steps as in Figure 1 are designated by the same reference numerals, and repeated explanations will be omitted (the same applies to each of the following embodiments).

[0042] <S224:CO ２ Absorption process> CO ２In the absorption step S224, the alkaline solution produced in the second step S2 is stored in a storage tank, and a gas containing carbon dioxide is blown into the alkaline solution to bring the gas into contact with the liquid by bubbling, thereby absorbing carbonate ions (CO ２－ The method for bringing the alkaline solution and carbon dioxide into gas-liquid contact can be exemplified by the same method as in the carbonate separation step S221 described above.

[0043] The pH value of the alkaline solution is maintained at a high value, and by contacting it with carbon dioxide, sodium carbonate (Na2CO3) is generated by the reaction between sodium hydroxide and carbon dioxide shown in the following reaction formula (1).

[0044] 2NaOH + CO2 → Na2CO 3 + H2O...(1)

[0045] On the other hand, if the pH value of the alkaline solution is too low, sodium bicarbonate (NaHCO3) may be generated by the reaction between sodium hydroxide and carbon dioxide shown in the following reaction formula (2).

[0046] NaOH + CO2 → NaHCO3...(2)

[0047] If sodium bicarbonate is generated, carbon dioxide is generated during the production of alkaline earth metal carbonate crystals. Therefore, it is preferable to adjust the pH value of the alkaline solution to 9 or higher, which ensures the reliable production of carbonate salts containing carbonate ions (sodium carbonate in this embodiment). The pH of the alkaline solution can be adjusted by changing the amount of carbon dioxide brought into contact with the alkaline solution. While there is no particular upper limit on the pH of the alkaline solution, maintaining a high pH value of the alkaline solution requires reducing the amount of carbon dioxide brought into gas-liquid contact, making it difficult to efficiently immobilize carbon dioxide. While increasing the amount of alkaline solution can also maintain a high pH value of the alkaline solution, this increases the energy required for electrolysis to produce the alkaline solution. Therefore, a too high pH value of the alkaline solution is also undesirable, and therefore the pH value of the alkaline solution is preferably adjusted to between 9 and 12, and more preferably between 9 and 10.

[0048] <S223: Carbonate Separation Step> In the carbonate separation step S223 of the second embodiment, CO ２ In the absorption step S224, carbon dioxide is preliminarily contacted with the NF membrane permeate produced in the first step S1, and then the carbon dioxide is brought into liquid-liquid contact in the reaction tank. This causes the alkaline earth metal contained in the NF membrane permeate to react with the carbonate ions to form MgCO ３ and CaCO ３ The resulting mixture is in the form of a slurry in which alkaline earth metal carbonate crystals such as the above are precipitated.

[0049] The calcium ions contained in the NF membrane permeate are more likely to bind to carbonate ions than magnesium ions, so as shown below, ３ and CaCO ３ The above may be separated and recovered.

[0050] <Modification of Second Embodiment> Fig. 3 is a diagram showing a modification of the carbonate separation step S223 in the treatment flow shown in Fig. 2. In the carbonate separation step S223 shown in Fig. 3, the precipitation of alkaline earth metal carbonate crystals is separated from calcium carbonate (CaCO ３ a first precipitation step S2231 of precipitating magnesium carbonate (MgCO ３ ) crystals are precipitated in a second precipitation step S2232.

[0051] In the first precipitation step S2231, the NF membrane permeate and an alkaline solution in contact with carbon dioxide are stirred and mixed in a reaction tank, causing mainly calcium ions to react with carbonate ions and precipitating calcium carbonate crystals in the NF membrane permeate after the first reaction. If the amount of alkaline solution supplied is too large, most of the calcium ions will crystallize, making the carbonate ions more likely to react with magnesium ions. Therefore, it is preferable to set the amount of alkaline solution supplied appropriately according to the amount of calcium contained in the NF membrane permeate. The NF membrane permeate after the first precipitation step S2231 is transferred to a settling tank in a first solid-liquid separation step S2233. In the first solid-liquid separation step S2233, a first precipitate mainly composed of calcium carbonate crystals is precipitated and separated and recovered from the NF membrane permeate after the first reaction.

[0052] In the second precipitation step S2232, the NF membrane permeate that has been through the first solid-liquid separation step S2233 and an alkaline solution that has been contacted with carbon dioxide are stirred and mixed in a reaction tank, causing mainly magnesium ions to react with carbonate ions and precipitating calcium carbonate crystals in the NF membrane permeate after the second reaction. The NF membrane permeate that has been through the second precipitation step S2232 is transferred to a settling tank in a second solid-liquid separation step S2234. In the second solid-liquid separation step S2234, a second precipitate mainly composed of magnesium carbonate crystals is precipitated from the NF membrane permeate after the second reaction, and separated and recovered.

[0053] When the pH value of the alkaline solution brought into contact with carbon dioxide is maintained high (for example, pH 9 or higher), calcium carbonate is mainly produced in the first precipitation step S2231 and magnesium carbonate is mainly produced in the second precipitation step S2232 through the reactions shown in the following reaction formulas (3) and (4).

[0054] Na2CO3 + CaCl2 → CaCO 3 +2NaCl...(3)

[0055] Na2CO3 + MgCl2 → MgCO 3 +2NaCl...(4)

[0056] On the other hand, since the pH value of the alkaline solution contacted with carbon dioxide is low, bicarbonate ions (HCO － ) is present, carbon dioxide is produced in addition to calcium carbonate and magnesium carbonate in the first precipitation step S2231 and the second precipitation step S2232 by the reactions shown in the following reaction formulas (5) and (6), respectively.

[0057] 2NaHCO3 + CaCl2 → CaCO 3 + 2NaCl+ H2O + CO2...(5)

[0058] 2NaHCO3 + MgCl2 → MgCO 3 + 2NaCl+ H2O + CO2...(6)

[0059] Therefore, it is preferable that the alkaline solution contacted with carbon dioxide and used in the first precipitation step S2231 and the second precipitation step S2232 be maintained at a pH of 9 or higher.

[0060] <Third embodiment> Fig. 4 is a process flow diagram for explaining a carbon dioxide fixation method according to a third embodiment of the present invention. As shown in Fig. 4, in the third embodiment, the CO fixation method according to the second embodiment shown in Fig. 2 is ２ A part of the alkaline solution that has been brought into contact with carbon dioxide in the absorption step S224 is brought into contact with the NF membrane concentrate in the third step S3. The contact between the alkaline solution that has been brought into contact with carbon dioxide in advance and the NF membrane permeate can be carried out in a reaction tank, as in the carbonate separation step S223 in the second embodiment. As in the modified example of the second embodiment, the precipitation of alkaline earth metal carbonate crystals can be carried out by separating calcium carbonate (CaCO ３ and precipitating magnesium carbonate (MgCO ３ ) crystallization process may be carried out separately.

[0061] <Fourth embodiment> Fig. 5 is a process flow diagram for explaining a carbon dioxide fixation method according to a fourth embodiment of the present invention. As shown in Fig. 5, the fourth embodiment includes a second NF membrane treatment step S225 in which the NF membrane permeate concentrated in the concentration step S21 is passed through a second NF membrane different from the NF membrane through which seawater is passed in the first step S1, instead of the carbonate separation step S221 in the first embodiment shown in Fig. 1.

[0062] <S225: Second NF membrane treatment step> In the second NF membrane treatment step S225, the NF membrane permeate that has permeated the second NF membrane is subjected to the adsorption removal step S222, while the non-permeate that does not permeate the second NF membrane is merged with the NF membrane concentrate produced in the first step S1. The non-permeate that does not permeate the second NF membrane contains alkaline earth metals such as calcium and magnesium, so by merging it with the NF membrane concentrate, the recovery rate of the alkaline earth metals required for carbon dioxide fixation in the third step S3 can be increased.

[0063] <Fifth Embodiment> Fig. 6 is a process flow diagram illustrating a carbon dioxide fixation method according to a fifth embodiment of the present invention. As shown in Fig. 6, the fifth embodiment includes, instead of the carbonate separation step S221 in the first embodiment shown in Fig. 1, a crystallization step S226 in which the NF membrane permeate concentrated in the concentration step S21 is supplied to a crystallizer and re-concentrated by heating and evaporating, thereby precipitating NaCl crystals.

[0064] <S226: Crystallization Step> In the crystallization step S226, a portion of the NF membrane permeate concentrated in the crystallizer is discharged from the crystallizer as a slurry containing NaCl crystals, and the slurry is dehydrated using a centrifuge or the like to recover NaCl crystals. The recovered NaCl crystals are redissolved in water to regenerate the NF membrane permeate, which is then supplied to the adsorption removal step S222.

[0065] In the crystallization step S226, the filtrate from which NaCl crystals have been recovered is merged with the NF membrane concentrate produced in the first step S1. This prevents wastewater from being discharged outside the system, thereby reducing the environmental impact. Furthermore, because the filtrate contains alkaline earth metals, such as magnesium, that are targets for recovery, the recovery rate of alkaline earth metals required for carbon dioxide fixation in the third step S3 can be increased.

[0066] The steam discharged from the crystallizer is condensed in a condenser or the like to become distilled water, which is then combined with the RO membrane permeate produced in the concentration step S21 and the distilled water produced in the evaporation concentration step S11 and can be used as produced water, etc.

[0067] <Sixth embodiment> Fig. 7 is a process flow diagram illustrating a carbon dioxide fixation method according to a sixth embodiment of the present invention. As shown in Fig. 7, in the sixth embodiment, the carbonate separation step S223 of the third embodiment shown in Fig. 4 is replaced with the carbonate separation step S221 of the first embodiment shown in Fig. 1, whereby the NF membrane concentrate and carbon dioxide are brought into gas-liquid contact in the carbonate separation step S221.

[0068] <Modification of Sixth Embodiment> The carbonate separation step S221 of the sixth embodiment shown in FIG. 7 can be replaced with the second NF membrane treatment step S225 or the crystallization step S226 shown in FIG. 5 or FIG.

[0069] Seventh Embodiment Fig. 8 is a process flow diagram for explaining a method for immobilizing carbon dioxide according to a seventh embodiment of the present invention. ２ In the absorption step S224, a part of the alkaline solution that has been previously contacted with carbon dioxide is brought into contact with the NF membrane permeate in the carbonate separation step S223. In the seventh embodiment shown in FIG. ２ CO absorption step S224 ２ The alkaline solution that has been previously contacted with carbon dioxide in the absorption step S227 is brought into contact with the NF membrane permeate in the carbonate separation step S223. ２ The absorption step S227 is ２ This can be performed in the same manner as in the absorption step S224.

[0070] 9, each of the above embodiments may include a composite membrane treatment step S211 in which the NF membrane permeate concentrated in the concentration step S21 is supplied to a high-pressure chamber of a semipermeable membrane unit separated into a high-pressure chamber and a low-pressure chamber by a semipermeable membrane, and the NF membrane permeate is further concentrated by utilizing the pressure difference with the recovered liquid passing through the low-pressure chamber, and is then supplied to the alkaline earth metal removal step S22. The recovered liquid supplied to the low-pressure chamber may be a portion of the NF membrane permeate that has passed through the high-pressure chamber, and the recovered liquid that has passed through the low-pressure chamber may be merged with the NF membrane permeate before being subjected to the concentration step S21.

[0071] The alkaline earth metal removal step S22 in each of the above embodiments may not include the adsorption and removal step S222 if scale components can be sufficiently removed by the carbonate separation steps S221 and S223, the second NF membrane treatment step S225, or the crystallization step S226. Alternatively, the alkaline earth metal removal step S22 may include only the adsorption and removal step S222.

[0072] S1 1st process S11 Distillation and concentration process S12 Solid-liquid separation process S2 2nd process S21 Concentration process S22 Alkali earth metal removal process S221, S223 Carbonate separation process S222 Adsorption removal process S224 CO ２ Absorption process S225 Secondary NF membrane treatment process S226 Crystallization process S3 Third process S31 Solid-liquid separation process

Claims

1. A method for fixation of carbon dioxide, comprising: a first step of passing seawater or brackish water through a nanofiltration membrane to produce an NF membrane concentrate that is concentrated without passing through the nanofiltration membrane, and an NF membrane permeate that has permeated the nanofiltration membrane; a second step of producing an alkaline solution and hydrogen by electrolyzing the NF membrane permeate; and a third step of reacting the NF membrane concentrate with carbon dioxide and the alkaline solution to precipitate carbonate crystals of the alkaline earth metal contained in the NF membrane concentrate.

2. The method for fixation of carbon dioxide according to claim 1, wherein the third step comprises a step of contacting the alkaline solution, which has been previously contacted with carbon dioxide, with the NF membrane concentrated liquid.

3. A method for fixation of carbon dioxide as described in claim 1 or 2, wherein the second step comprises a concentration step of concentrating the NF membrane permeate and an alkaline earth metal removal step of removing alkaline earth metals contained in the concentrated NF membrane permeate, and the NF membrane permeate that has been subjected to the alkaline earth metal removal step is electrolyzed.

4. A method for fixation of carbon dioxide as described in claim 3, wherein the alkaline earth metal removal process includes a carbonate separation process in which the concentrated NF membrane permeate is reacted with carbon dioxide and the alkaline solution to precipitate alkaline earth metal carbonate and perform solid-liquid separation.

5. A method for fixation of carbon dioxide according to claim 4, wherein the carbonate separation step comprises a step of contacting the alkaline solution, which has been previously contacted with carbon dioxide, with the concentrated NF membrane permeate.

6. The method for fixation of carbon dioxide described in claim 3, wherein the alkaline earth metal removal step comprises a step of further concentrating the concentrated NF membrane permeate to recover precipitated sodium chloride crystals and re-dissolving them in water, thereby regenerating the NF membrane permeate.

7. The carbon dioxide fixation method according to claim 3, wherein the alkaline earth metal removal step comprises a second NF membrane treatment step of passing the concentrated NF membrane permeate through a second nanofiltration membrane different from the nanofiltration membrane.

8. A method for fixation of carbon dioxide as described in claim 3, wherein the alkaline earth metal removal process includes an adsorption removal process for removing alkaline earth metals contained in the concentrated NF membrane permeate using a chelating resin or an ion exchange resin.

9. The method for fixation of carbon dioxide described in claim 2, wherein the second step comprises a concentrating step of concentrating the NF membrane permeate and an alkaline earth metal removal step of removing alkaline earth metals contained in the concentrated NF membrane permeate, and the alkaline earth metal removal step comprises a step of contacting a portion of the alkaline solution used in the third step, which has been previously contacted with carbon dioxide, with the concentrated NF membrane permeate, thereby precipitating carbonates of the alkaline earth metals contained in the NF membrane permeate and performing solid-liquid separation.

10. The method for fixation of carbon dioxide as described in claim 1, wherein the first step comprises an evaporation and concentration step of evaporating and concentrating the NF membrane concentrated liquid to precipitate calcium sulfate crystals, and a solid-liquid separation step of recovering the precipitated calcium sulfate crystals by solid-liquid separation.

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