WO2024080132A1

WO2024080132A1 - Method for fixing carbon dioxide

Info

Publication number: WO2024080132A1
Application number: PCT/JP2023/034902
Authority: WO
Inventors: 隆雄中垣; 悟平野; 慶明三保; 統行島田; 升夫湯淺; 光希有本
Original assignee: 学校法人早稲田大学; 株式会社ササクラ
Priority date: 2022-10-12
Filing date: 2023-09-26
Publication date: 2024-04-18

Abstract

[Problem] To provide a method for fixing carbon dioxide onto an alkaline earth metal, whereby it becomes possible to increase a carbon dioxide emission reduction capability while taking the amount of discharged carbon dioxide into consideration. [Solution] Provided is a method for fixing carbon dioxide, the method comprising: a first step S1 for allowing sea water or brine water to pass through a nano-filtration membrane to generate a first concentrated solution that is concentrated without the penetration through the nano-filtration membrane; a second step S2 for bringing an alkaline solution into contact with carbon dioxide to produce a solution to be treated which contains carbonate ions; and a third step S3 for bringing the first concentrated solution into contact with the solution to be treated, thereby causing the precipitation of a crystal of a carbonated product of the alkaline earth metal contained in the first concentrated solution.

Description

Carbon dioxide fixation method

The present invention relates to a method for fixing carbon dioxide to alkaline earth metals.

As global warming becomes more serious, there is a need to suppress the rise in temperature, and as an evaluation model for this, the goal is to reduce anthropogenic carbon dioxide ( _CO2 ) emissions to zero. One of the means for achieving this goal is the _CO2 fixation method.
One effective method for fixing _CO2 is to use alkaline earth metals such as Mg and Ca to bind and fix _CO2 . However, conventional methods that use ores containing alkaline earth metals require treatments that result in _CO2 emissions, such as high temperature and pressure and the addition of chemicals, and in many cases the entire process results in _CO2 emissions.
Furthermore, Mg and Ca are also contained in seawater and wastewater brine from seawater desalination plants, and a method for fixing _CO2 using seawater has been proposed (see, for example, Patent Documents 1 to 3).

JP 2005-21870 A JP 2010-125354 A International Publication No. 2022/30529

In the _CO2 fixation method using seawater or brine, many methods have been considered that involve blowing _CO2 into seawater or brine, but there are problems such as a decrease in the efficiency of CO2 fixation in the liquid phase due to the strong hydration shell formed around ^Mg2+ ions, which are divalent cations with a small _ion diameter, and the presence of cations (Na ⁺ , K ⁺ ) that compete with carbonation. As a solution to this problem, the mainstream method has been to use a means of increasing the pH by adding alkali such as Ca(OH) ₂ , which is difficult to recycle. However, these means for promoting the reaction with _CO2 result in a positive _CO2 emission in the entire process, considering energy consumption and _CO2 emissions due to additive production as part of life cycle assessment.
Even in the techniques of

Patent Documents

1 and 2, pH adjustment, wastewater treatment, etc. are required, making it difficult to reduce _CO2 emissions throughout the entire process.

As described above, a problem with the _CO2 fixation method using seawater or brackish water is the inhibition of reaction with _CO2 due to the presence of molecules or ions other than Mg and Ca. Therefore, in the process of separating Mg and Ca from seawater or brackish water, the _CO2 reduction capacity must be evaluated taking into account the _CO2 emissions from each unit operation.

Patent Document 3 discloses a method for fixation of carbon dioxide, which includes a first step of generating a first concentrated liquid that is concentrated without passing through the nanofiltration membrane by passing seawater or brackish water through a nanofiltration membrane, a second step of adding an alkali to the generated first concentrated liquid to react and fix carbon dioxide with the alkaline earth metal contained in the first concentrated liquid to precipitate 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 technology of Patent Document 3 can easily and efficiently fix carbon dioxide to alkaline earth metals contained in seawater or brackish water because the concentration of Na ⁺ , K ⁺ , etc., which may hinder the fixation of carbon dioxide, is reduced by the first step, but there is room for improvement in terms of further efficient fixation of carbon dioxide.

The present invention provides a method for fixing carbon dioxide to alkaline earth metals, which increases the carbon dioxide reduction capacity while taking into account the amount of carbon dioxide emitted.

The object of the present invention is achieved by a method for immobilizing carbon dioxide, comprising a first step of generating a first concentrated liquid by passing seawater or brackish water through a nanofiltration membrane without passing through the nanofiltration membrane, a second step of generating a liquid to be treated that contains carbonate ions by contacting carbon dioxide with an alkaline solution, and a third step of precipitating carbonate crystals of the alkaline earth metal contained in the first concentrated liquid by contacting the first concentrated liquid with the liquid to be treated.

In this carbon dioxide fixation method, the first step preferably includes a salt production step in which the first permeate that has permeated the nanofiltration membrane is concentrated to recover precipitated sodium chloride crystals, and an electrodialysis step in which the solution of sodium chloride crystals is subjected to electrodialysis to separate an acid solution and an alkaline solution, and the second step preferably uses the alkaline solution obtained in the electrodialysis step to produce the liquid to be treated.

Alternatively, the first step preferably includes a concentration step of concentrating the first permeated liquid that has permeated the nanofiltration membrane to produce a second concentrated liquid, an alkaline earth metal removal step of removing alkaline earth metals contained in the second concentrated liquid, and an electrodialysis step of electrodialyzing the second concentrated liquid that has been through the alkaline earth metal removal step to separate an acid solution and an alkaline solution, and the second step preferably produces the liquid to be treated using the alkaline solution obtained in the electrodialysis step.

The alkaline earth metal removal step preferably includes an adsorption removal step in which the alkaline earth metals contained in the second concentrated liquid are removed by a chelating resin or an ion exchange resin. The first step preferably further includes a resin regeneration step in which the chelating resin or the ion exchange resin is regenerated using the acid solution obtained in the electrodialysis step. In the resin regeneration step, the acid solution in which the chelating resin or the ion exchange resin has been regenerated is preferably merged with the first concentrated liquid.

The alkaline earth metal removal step preferably includes a step of passing the second concentrated liquid through a second nanofiltration membrane different from the nanofiltration membrane, and more preferably includes a step of merging the non-permeated liquid of the second nanofiltration membrane with the first concentrated liquid. In addition, the alkaline earth metal removal step preferably includes a step of contacting the second concentrated liquid with the liquid to be treated, thereby precipitating carbonate crystals of the alkaline earth metal contained in the second concentrated liquid and removing them by solid-liquid separation.

The third step preferably includes a first precipitation step of mixing the first concentrated liquid with the liquid to be treated to produce a first reaction liquid in which calcium carbonate crystals are precipitated, a first solid-liquid separation step of subjecting the first reaction liquid to solid-liquid separation to recover calcium carbonate, a second precipitation step of further mixing the liquid to be treated with the first reaction liquid that has been subjected to the first solid-liquid separation step to produce a second reaction liquid in which magnesium carbonate crystals are precipitated, and a second solid-liquid separation step of subjecting the second reaction liquid to solid-liquid separation to recover magnesium carbonate.

The present invention provides a method for fixing carbon dioxide to alkaline earth metals that increases the carbon dioxide reduction capacity while taking into account the amount of carbon dioxide emitted.

FIG. 1 is a process flow diagram for explaining a carbon dioxide fixation method according to a first embodiment of the present invention. 2 is a diagram showing an example of changes in the amounts of various ions in the process flow shown in FIG. 1 . FIG. 2 is a diagram showing a modification of some of the steps in the processing flow shown in FIG. 1 . FIG. 4 is a diagram showing an example of a component change in the processing flow shown in FIG. 3 . FIG. 5 is a process flow diagram for explaining a carbon dioxide fixation method according to a second embodiment of the present invention. FIG. 6 is a diagram showing a modification of some of the steps in the processing flow shown in FIG. 5 . FIG. 11 is a process flow diagram for explaining a carbon dioxide fixation method according to a third embodiment of the present invention. FIG. 11 is a process flow diagram for explaining a carbon dioxide fixation method according to a fourth embodiment of the present invention. FIG. 11 is a process flow diagram for explaining a carbon dioxide fixation method according to a fifth embodiment of the present invention. FIG. 13 is a process flow diagram for explaining a carbon dioxide fixation method according to a sixth embodiment of the present invention.

The method for fixing carbon dioxide of the present invention provides a method for fixing carbon dioxide to alkaline earth metals contained in seawater or brine. In the present invention, "alkaline earth metal" refers to a broad range including Mg and Be, which are elements of Group 2 of the periodic table, in addition to Ca, Sr, Ba, and Ra. In particular, it is preferable that the alkaline earth metal contains at least Mg, from the viewpoint of ease of reaction with _CO2 and the fact that carbonates obtained by the reaction can be expected to be used for various purposes.

Furthermore, "seawater or brine" is an aqueous solution containing alkaline earth metal ions such as magnesium ions (Mg ²⁺ ) and calcium ions (Ca ²⁺ ). Seawater or brine usually contains, in addition to alkaline earth metal ions, at least one type of ion that forms a crystal selected from calcium sulfate, sodium chloride, potassium chloride, and sodium sulfate. Specifically, seawater or brine usually contains at least one type of ion selected from chloride ions (Cl ^- ), sulfate ions (SO ₄ ^2- ), sodium ions (Na ⁺ ), and potassium (K ⁺ ).

The seawater or brine mentioned above 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 associated water from oil fields and gas fields can also be used so long as they contain alkaline earth metals. More specific examples of brine include wastewater brine discharged from water production processes using salt lakes, desalination and salt production processes, recovery of valuable materials using seawater and salt lakes, and industrial wastewater from chemical plants, etc.

From the viewpoints of containing a large amount of Mg as described above, reducing the environmental load, and facilitating the reduction of _CO2 emissions, 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.

First Embodiment
Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a process flow diagram for explaining a method for immobilizing carbon dioxide according to a first embodiment of the present invention. In the first embodiment, the treatment target is seawater, but the same treatment can be performed in the case of brackish water. As shown in FIG. 1, the method for immobilizing carbon dioxide according to the first embodiment includes a first step S1 of passing seawater through a nanofiltration membrane (NF membrane) to generate a first concentrated liquid that is concentrated without passing through the NF membrane, a second step S2 of contacting carbon dioxide with an alkaline solution to generate a liquid to be treated that contains carbonate ions, and a third step of contacting the generated first concentrated liquid with the liquid to be treated to precipitate carbonate crystals of an alkaline earth metal contained in the first concentrated liquid.

<S1: First step>
In the first step S1, the taken seawater is appropriately pretreated by filtration, coagulation, precipitation, etc. to remove impurities such as fine particles and microorganisms, and then the seawater is supplied to an NF membrane unit by a medium pressure pump or the like and passed through the NF membrane to produce a first permeate that has permeated the NF membrane and a first concentrated liquid that has not permeated the NF membrane and is concentrated.

Since the NF membrane has the property of suppressing the permeation of divalent or higher ions while easily allowing monovalent ions to permeate, most of the alkaline earth metals to be immobilized with carbon dioxide remain in the first concentrated liquid, and the concentrations of Na ⁺ , K ⁺ , etc., which may hinder the immobilization, are reduced. Therefore, carbon dioxide can be easily and efficiently immobilized in the alkaline earth metals contained in the seawater (or brackish water), and the generation of carbon dioxide in the entire process including the subsequent steps can be suppressed. Figure 2 shows an example of the amounts (mg/h) of various ions contained in the seawater, the first permeated liquid, and the first concentrated liquid when seawater is supplied at a flow rate of 100 m ³ /h.

<S11: Salt production process>
On the other hand, since the first permeate contains a large amount of monovalent ions Na ⁺ and Cl ^- , the first step S1 includes a salt production step S11 in which the first permeate is concentrated to recover precipitated sodium chloride (NaCl) crystals. The salt production step S11 of this embodiment includes a first membrane treatment step S111 in which the first permeate is supplied to a reverse osmosis membrane (RO membrane) unit by a high-pressure pump or the like and passed through the RO membrane to produce a second concentrated liquid that is concentrated without passing through the RO membrane, a second membrane treatment step S112 in which the second concentrated liquid produced in the first membrane treatment step S111 is supplied to a high-pressure chamber of a semipermeable membrane unit separated by a semipermeable membrane, and the second concentrated liquid is further concentrated by utilizing the pressure difference with the recovered liquid passing through the low-pressure chamber, and a crystallization step S113 in which the second concentrated liquid further concentrated in the second membrane treatment step S112 is supplied to a crystallizer and heated and evaporated to precipitate NaCl crystals.

The recovery liquid supplied to the low pressure chamber in the second membrane treatment step S112 can utilize a part of the second concentrated liquid that has passed through the high pressure chamber, and the recovery liquid that has passed through the low pressure chamber can be merged with the first permeated liquid before the first membrane treatment step S111. In addition, the steam discharged from the crystallizer in the crystallization step S113 is condensed by a condenser or the like to become distilled water, and is merged with the second permeated liquid that has permeated the RO membrane in the first membrane treatment step S111 to be used as produced water or the like. The crystallizer concentrated liquid concentrated in the crystallizer is discharged from the crystallizer as a slurry liquid containing NaCl crystals, and is dehydrated by a centrifuge or the like to recover NaCl crystals. Since the first permeated liquid contains almost no SO ₄ ^2- , it can be concentrated at a high concentration by the first membrane treatment step S111, which is low energy. The second concentrated liquid may be produced only by the first membrane treatment step S111 without providing the second membrane treatment step S112. In addition, the second concentrated liquid may be evaporated and concentrated using a horizontal tube evaporator or the like before the crystallization step S113 is carried out, or the first permeated liquid may be evaporated and concentrated without being concentrated through a membrane.

<S12: Joining process>
As shown in Fig. 1, the first step S1 includes a confluence step S12 in which the filtrate obtained after recovering NaCl crystals in the salt production step S11 is merged with the first concentrated liquid, thereby suppressing discharge of waste liquid outside the system and reducing the environmental load. The alkaline earth metals to be recovered, such as magnesium, are contained not only in the first concentrated liquid but also in the first permeated liquid, so that the confluence step S12 can increase the recovery rate of the alkaline earth metals required for the fixation of carbon dioxide in the second step S2. From the viewpoint of increasing the purity of the NaCl crystals obtained in the crystallization step S112, it is preferable to increase the amount of filtrate merged with the first concentrated liquid in the confluence step S12 as much as possible.

<S13: Electrodialysis step>
The first step S1 includes an electrodialysis step S13 in which the NaCl crystals produced in the salt production step S11 are electrodialyzed. For example, a bipolar membrane electrodialysis device can be used in the electrodialysis step S13, and the solution in which the NaCl crystals obtained in the crystallization step S113 are dissolved in water is separated into an HCl solution and a NaOH solution. It is preferable that the electrodialysis utilizes renewable energy such as solar energy, and _CO2 emissions in the entire process can be suppressed. In order to increase the purity of the NaCl solution to be electrodialyzed, it is preferable to adsorb impurities such as magnesium and calcium contained in the NaCl solution to a chelating resin to sufficiently reduce their concentrations (for example, to 1 ppm or less).

The NaOH solution obtained in the electrodialysis step S13 can be suitably used as the alkaline solution used in the second step S2. The flow path of the first permeate produced in the first step S1 may be branched so that the alkaline solution produced in the electrodialysis step S13 is only the amount required for the second step S2, and only a part of the first permeate may be subjected to the salt production step S11. This suppresses the energy consumption required for producing the alkali, and reduces _CO2 emissions in the entire process. It is preferable to use renewable energy for producing the alkali, as described above. The remainder of the first permeate not used in the salt production step S11 can be used in another step such as a desalination process, and the first permeate not used in the other step may be discharged into the ocean, etc.

In order to produce only the amount of alkaline solution required in the second step S2 in the electrodialysis step S13, the first step S1 may bypass part of the seawater or brine without passing it through the NF membrane, and the salt production step S11 may be carried out on part or all of the first permeate with the production amount suppressed. The seawater or brine that bypasses the NF membrane can be merged with the first concentrated liquid and then used for the fixation of carbon dioxide in the third step S3.

<S2: Second step>
In the second step S2, an alkaline solution is stored in a storage tank, and a gas containing carbon dioxide is blown into the alkaline solution to bring the solution into gas-liquid contact by bubbling, thereby producing a liquid to be treated that contains carbonate ions (CO ₃ ^2- ). The gas containing carbon dioxide may be the atmosphere or exhaust gas from various combustion devices. There is no limit to the concentration of carbon dioxide contained in the gas, but it may be, for example, from the atmosphere to about 100% by volume.

The method of bringing the alkaline solution and carbon dioxide into gas-liquid contact may be a method of blowing _CO2 gas (for example, fine bubbles such as microbubbles or ultrafine bubbles) into the alkaline solution, or a method of spraying the alkaline solution into the _CO2 gas with a spray nozzle or tray in a single-stage or multi-stage desulfurization tower or degassing tower, etc., and various known gas-liquid contact devices can be used taking into consideration the reaction rate, reaction amount, _CO2 concentration in gases such as exhaust gas, and the like.

The alkaline solution used in the second step S2 is preferably the NaOH solution obtained in the electrodialysis step S13 as described above, and this makes it possible to suppress an increase in _CO2 emissions associated with the separate production of alkali. However, in the second step S2, an alkaline solution different from the alkali obtained in the electrodialysis step S13 may be used, or the alkaline solution obtained in the electrodialysis step S13 may be used in combination with another alkaline solution.

The pH value of the alkaline solution is maintained at a high value, and by contacting it with carbon dioxide, sodium carbonate (Na ₂ CO ₃ ) is generated by the reaction between sodium hydroxide and carbon dioxide shown in reaction formula (1) below.

2NaOH + _CO2 → _Na2CO3 ₊ _H2O ... (1)

On the other hand, if the pH value of the alkaline solution is too low, sodium bicarbonate (NaHCO ₃ ) may be produced by the reaction between sodium hydroxide and carbon dioxide shown in reaction formula (2) below.

NaOH + _CO2 → _NaHCO3 ... (2)

If sodium bicarbonate is generated, as described below, a problem occurs in that carbon dioxide is generated when alkaline earth metal carbonate crystals are generated in the third step S3. Therefore, it is preferable to adjust the pH value of the alkaline solution to 9 or more, which ensures that a carbonate salt containing carbonate ions (sodium carbonate in this embodiment) is generated. There is no particular upper limit to the pH of the alkaline solution, but if the pH value of the alkaline solution is too high, the amount of carbon dioxide that can be fixed decreases, making it difficult to efficiently fix carbon dioxide in the third step S3. Therefore, it is preferable to adjust the pH value of the alkaline solution to 9 or more and less than 12 using a pH adjuster or the like, and more preferably to 9 or more and less than 10.

<S3: Third process>
In the third step S3, the first concentrated liquid obtained in the first step S1 and the liquid to be treated obtained in the second step S2 are brought into liquid-liquid contact in a reaction tank, whereby the alkaline earth metal contained in the first concentrated liquid reacts with carbonate ions to produce a reaction liquid consisting of a slurry liquid in which alkaline earth metal carbonate crystals such as _MgCO3 and _CaCO3 have precipitated. The first concentrated liquid obtained in the first step S1 may be subjected to evaporation crystallization to precipitate calcium sulfate crystals, followed by solid-liquid separation to remove the calcium sulfate crystals, before the third step S3 is carried out.

<S31: Solid-liquid separation step>
The third step S3 includes a solid-liquid separation step in which the alkaline earth metal carbonate crystals contained in the produced reaction liquid are separated into solid and liquid by a solid-liquid separation device such as a centrifuge or precipitation in a precipitation tank, and then recovered.

<S32: Neutralization step>
The third step S3 includes a neutralization step S32 for neutralizing the filtrate obtained after the alkaline earth metal carbonate crystals are recovered from the reaction solution by the solid-liquid separation step S31. This filtrate usually has a pH of 9 or higher, so by adding an acid to neutralize it (for example, pH 7 to 8), it becomes possible to discharge it directly outside the system, such as the ocean. The acid added to the filtrate is preferably the HCl solution obtained in the electrodialysis step S13, and this makes it possible to suppress an increase in _CO2 emissions associated with the separate generation of acid.

<S33: Water washing step>
The third step S3 includes a water washing step S33 in which the alkaline earth metal carbonate crystals recovered in the solid-liquid separation step S31 are washed with wash water to dissolve and remove Na ⁺ , K ⁺ , and the like adhering to the alkaline earth metal carbonate crystals. The wash water used in the water washing step S33 preferably contains the distilled water obtained in the crystallization step S113, which makes it possible to suppress an increase in CO ₂ emissions associated with the separate production of wash water. In this embodiment, a portion of the produced water obtained by combining the second permeate obtained in the salt production step S11 and the distilled water is used as the wash water.

The alkaline earth metal carbonate crystals such as _MgCO3 and _CaCO3 after washing can be suitably used as building materials such as concrete, cement, etc. Therefore, the present invention can also provide a method for producing alkaline earth metal carbonate salts using the carbon dioxide fixation method.

In the carbon dioxide fixation method of this embodiment, carbon dioxide is brought into contact with an alkaline solution in the second step S2 to produce a liquid to be treated that contains carbonate ions, and then the liquid to be treated is brought into liquid-liquid contact with the first concentrated liquid in the third step S3, which makes it possible to easily produce carbonate crystals of the alkaline earth metal contained in the first concentrated liquid. Therefore, carbon dioxide can be easily and efficiently fixed to the alkaline earth metal contained in seawater or brine, and carbon dioxide fixation can be completed within the system using the alkali, acid, distilled water, etc. produced in each step, so that the carbon dioxide reduction capacity can be increased while taking into account the amount of carbon dioxide emitted throughout the entire process.

<Modification of the first embodiment>
In the third step S3 of the first embodiment, the first concentrated liquid and the liquid to be treated are brought into liquid-liquid contact in a reaction tank to generate a slurry liquid in which alkaline earth metal carbonate crystals such as _MgCO3 and _CaCO3 are precipitated. Since calcium ions contained in the first concentrated liquid are more likely to bind to carbonate ions than magnesium ions, _MgCO3 and _CaCO3 can be separated and recovered as described below.

Fig. 3 is a diagram showing a modified example of the third step S3 in the treatment flow shown in Fig. 1. In the third step S3 shown in Fig. 3, the precipitation of alkaline earth metal carbonate crystals by contact between the first concentrated liquid and the liquid to be treated in the third step S3 shown in Fig. 1 is divided into a first precipitation step S301 for precipitating calcium carbonate (CaCO ₃ ) crystals and a second precipitation step S302 for precipitating magnesium carbonate (MgCO ₃ ) crystals. In addition, the solid-liquid separation step S31 included in the third step S3 shown in Fig. 3 includes a first solid-liquid separation step S311 for separating and recovering the calcium carbonate crystals precipitated in the first precipitation step S301, and a second solid-liquid separation step S312 for separating and recovering the magnesium carbonate crystals precipitated in the second precipitation step S302.

In the first precipitation step S301, the first concentrated liquid produced in the first step S1 and the liquid to be treated produced in the second step S2 shown in FIG. 1 are stirred and mixed in a reaction tank, mainly causing calcium ions to react with carbonate ions, and calcium carbonate crystals are precipitated in the first reaction liquid, which is a mixed liquid. If the amount of liquid to be treated is too large, most of the calcium ions will crystallize and the carbonate ions will be more likely to react with magnesium ions, so it is preferable to set the amount appropriately according to the amount of calcium contained in the first concentrated liquid. In the first solid-liquid separation step S311, the first reaction liquid produced in the first precipitation step S301 is transferred to a precipitation tank, and the first precipitate, mainly consisting of calcium carbonate crystals, is precipitated and separated and recovered.

The second precipitation step S302 involves stirring and mixing the first reaction liquid that has undergone the first solid-liquid separation step S311 and the liquid to be treated produced in the second step S2 in a reaction tank, thereby causing magnesium ions to react with carbonate ions and precipitating magnesium carbonate crystals in the second reaction liquid, which is the mixed liquid. The second solid-liquid separation step S312 involves transferring the second reaction liquid produced in the second precipitation step S302 to a precipitation tank, where a second precipitate, mainly consisting of magnesium carbonate crystals, is precipitated and separated and recovered.

When the pH value of the liquid to be treated is maintained high (e.g., pH 9 or higher), calcium carbonate is mainly produced in the first precipitation step S301, and magnesium carbonate is mainly produced in the second precipitation step S302, according to the reactions shown in the following reaction equations (3) and (4).

_Na2CO3 ₊ _CaCl2 → _CaCO3 +2NaCl...(3)

_Na2CO3 ₊ _MgCl2 → _MgCO3 +2NaCl...(4)

On the other hand, when the pH value of the treated liquid is low and bicarbonate ions (HCO ₃ ⁻ ) are present in the treated liquid, carbon dioxide is produced in addition to calcium carbonate and magnesium carbonate in the first precipitation step S301 and the second precipitation step S302, respectively, through the reactions shown in the following reaction equations (5) and (6).

_2NaHCO3 + _CaCl2 → _CaCO3 + 2NaCl + _H2O
+ _CO2 ... (5)

_2NaHCO3 + _MgCl2 → _MgCO3 + 2NaCl + _H2O
+ _CO2 ... (6)

In the treatment flow shown in Figure 3, the results of measuring the change in components when the pH value of the treated liquid is maintained at 9 or higher are shown in Figure 4. In Figure 4, the "NaOH equivalent to be added to dissolved Ca" and the "NaOH equivalent to be added to dissolved Mg" are values when the amount of NaOH added to dissolved Ca and dissolved Mg is 1 equivalent, respectively, when all of the calcium and magnesium dissolved in the first concentrated liquid react according to the above reaction formulas (3) and (4), respectively. As shown in Figure 4, the Ca reduction rate shows a large value in the first reaction liquid, whereas the Mg reduction rate shows a small value in the first reaction liquid and a large value in the second reaction liquid, indicating that calcium carbonate and magnesium carbonate are separated and recovered.

Second Embodiment
Fig. 5 is a process flow diagram for explaining a method for immobilizing carbon dioxide according to a second embodiment of the present invention. The method for immobilizing carbon dioxide according to the second embodiment shown in Fig. 5 includes, as in the first embodiment, a first step S1 of generating a first concentrated liquid that is concentrated without passing through the NF membrane by passing seawater through the NF membrane, a second step S2 of generating a liquid to be treated that contains carbonate ions by contacting carbon dioxide with an alkaline solution, and a third step of precipitating carbonate crystals of an alkaline earth metal contained in the first concentrated liquid by contacting the generated first concentrated liquid with the liquid to be treated. In the second embodiment, the treatment target is seawater, but the same treatment can be performed in the case of brine. In Fig. 5, the same steps as those in Fig. 1 are assigned the same reference numerals, and repeated explanations will be omitted.

<S114: Concentration step>
The first step S1 shown in FIG. 5 includes a concentration step S114 in which the first permeate is supplied to a reverse osmosis membrane (RO membrane) unit by a high-pressure pump or the like and passed through the RO membrane to produce a second concentrated liquid that is membrane concentrated without permeating the RO membrane. Since the first permeate contains almost no SO ₄ ^2- , the first permeate can be concentrated at a high concentration by low-energy membrane treatment using the RO membrane. The concentration step S114 may include the first membrane treatment step S111 and the second membrane treatment step S112 shown in FIG. 1. In addition, the concentration step S114 may be a step in which the first permeate is evaporated and concentrated instead of membrane concentration using the RO membrane, or a step in which membrane concentration using the RO membrane and evaporation concentration are used in combination. The second permeate that has permeated the RO membrane in the concentration step S114 can be recovered, for example, as produced water.

<S115: Alkaline earth metal removal step (adsorption removal step)>
The first step S1 includes an alkaline earth metal removal step S115 for removing alkaline earth metals contained in the second concentrated liquid generated in the concentration step S114. The alkaline earth metal removal step S115 is an adsorption removal step for removing alkaline earth metals contained in the second concentrated liquid using a chelating resin or an ion exchange resin, and can be performed, for example, by passing the second concentrated liquid through a column filled 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 amino phosphoric acid type. When the alkaline earth metal ion concentration of the second concentrated liquid generated in the concentration step S114 is high (for example, about several hundred ppm), a step of adding sodium hydroxide or sodium carbonate to the second concentrated liquid before the alkaline earth metal removal step S115 to crystallize and remove alkaline earth metal ions such as magnesium ions and calcium ions may be included. On the other hand, when the alkaline earth metal ion concentration of the second concentrated liquid produced in the concentrating step S114 is low, the alkaline earth metal removing step S115 may not be included. Since alkaline earth metals such as magnesium and calcium are likely to cause scale in the electrodialysis step S13, the alkaline earth metal removing step S115 allows the electrodialysis step S13 to be carried out efficiently for a long period of time.

<S13: Electrodialysis step>
The first step S1 includes an electrodialysis step S13 in which the second concentrated liquid that has been through the alkaline earth metal removal step S115 is subjected to electrodialysis to separate the acid solution and the alkaline solution. The second concentrated liquid that has been through the alkaline earth metal removal step S115 is a high-purity NaCl solution in which the concentration of impurities such as magnesium and calcium has been sufficiently reduced (for example, 1 ppm or less), and is separated into an HCl solution and a NaOH solution by the electrodialysis step S13. The electrodialysis step S13 of the second embodiment can be performed in the same manner as the first electrodialysis step S13 shown in FIG.

<S116: Resin regeneration process>
The first step S1 includes a resin regeneration step S116 in which a regeneration liquid is passed through the chelating resin or ion exchange resin that has captured the alkaline earth metal in the alkaline earth metal removal step S115 to desorb the alkaline earth metal, thereby regenerating the chelating resin or ion exchange resin. As the regeneration liquid, an acid solution is preferably used, and for example, the HCl solution obtained in the electrodialysis step S13 can be suitably used. Since the alkaline earth metals to be recovered, such as magnesium, are contained not only in the first concentrated liquid but also in the first permeated liquid, it is preferable to include a confluence step S121 in which the acid solution that has regenerated the chelating resin or ion exchange resin in the resin regeneration step S116 is confluent with the first concentrated liquid before the third step S3 is performed. This makes it possible to increase the recovery rate of the alkaline earth metals required for the fixation of carbon dioxide in the third step S3.

Note that a portion of the HCl solution obtained in the electrodialysis step S13 may be merged with the seawater before the first step S1 is performed. This can suppress fouling of the membrane surfaces of the NF membrane and the RO membrane in the first step S1. The seawater into which the above-mentioned HCl solution is merged may be seawater before pretreatment, or may be seawater after pretreatment. By merging the HCl solution with the seawater before pretreatment, the effect of flocculating impurities can be improved when a flocculant is added in pretreatment.

<Modification of the second embodiment>
The alkaline earth metal removal step S115 of the second embodiment includes an adsorption removal step of removing alkaline earth metals such as calcium and magnesium contained in the second concentrated liquid obtained in the concentration step S114 using a chelating resin or an ion exchange resin, but the alkaline earth metal removal step can be modified in various ways as long as the alkaline earth metals such as calcium and magnesium contained in the second concentrated liquid can be reduced.

FIG. 6 is a diagram showing a modified example of the alkaline earth metal removal step S115 in the process flow shown in FIG. 5. The alkaline earth metal removal step S1151 shown in FIG. 6 includes an adsorption removal step S115 similar to the alkaline earth metal removal step S115 shown in FIG. 5, as well as an adsorption removal pretreatment step S117 in which the second concentrated liquid before the adsorption removal step S115 is passed through a second NF membrane different from the NF membrane through which seawater is passed in the first step S1, and the adsorption removal step S115 is performed on the permeated liquid of the second NF membrane in the adsorption removal pretreatment step S117. The non-permeated liquid that does not pass through the second NF membrane in the adsorption removal pretreatment step S117 contains alkaline earth metals such as calcium and magnesium, so it is preferable to merge it with the first concentrated liquid generated in the first step S1, which can increase the recovery rate of alkaline earth metals required for carbon dioxide fixation in the third step S3. The alkaline earth metal removal step S1151 shown in FIG. 6 can also be performed by only passing the second concentrated liquid through the second NF membrane, without including the adsorption removal step S115.

Third Embodiment
Fig. 7 is a process flow diagram for explaining a method for immobilizing carbon dioxide according to a third embodiment of the present invention. The alkaline earth metal removal step S1152 in the method for immobilizing carbon dioxide according to the third embodiment shown in Fig. 7 includes an adsorption removal pretreatment step S118 for reducing the alkaline earth metal contained in the second concentrated liquid before the adsorption removal step S115 is performed, in addition to the adsorption removal step S115 similar to the alkaline earth metal removal step S115 shown in Fig. 5. In Fig. 7, steps similar to those in Fig. 5 are denoted by the same reference numerals, and repeated explanations will be omitted.

In the adsorption removal pretreatment step S118, the second concentrated liquid obtained in the concentration step S114 and a part of the liquid to be treated obtained in the second step S2 are brought into liquid-liquid contact in a reaction tank, so that alkaline earth metals such as calcium and magnesium contained in the second concentrated liquid react with carbonate ions to precipitate alkaline earth metal carbonate crystals such as MgCO ₃ and CaCO _3. In the adsorption removal pretreatment step S118, the second concentrated liquid containing the precipitate is transferred to a precipitation tank, and the precipitate is removed by solid-liquid separation. This makes it possible to remove calcium, magnesium, etc., which cause scale in the electrodialysis step S13, in advance, and also makes it possible to use calcium, magnesium, etc., which permeate the NF membrane in the first step S1 for the fixation of carbon dioxide, so that the fixation of carbon dioxide can be performed efficiently. In the adsorption removal pretreatment step S118, when pH adjustment in the reaction tank is required, NaOH obtained in the electrodialysis step S13 may be used. The alkaline earth metal removal step S1152 shown in FIG. 7 can also be further combined with the adsorption removal pretreatment step S117 shown in FIG. 6. Alternatively, the alkaline earth metal removal step S1152 shown in FIG. 7 may be performed only by a step of bringing the second concentrated liquid into liquid-liquid contact with the liquid to be treated, without including the adsorption removal step S115.

Fourth Embodiment
Fig. 8 is a process flow diagram for explaining a method for immobilizing carbon dioxide according to a fourth embodiment of the present invention. The method for immobilizing carbon dioxide according to the fourth embodiment shown in Fig. 8 further includes an evaporation concentration step S14 and a solid-liquid separation step S15 described below in addition to the method for immobilizing carbon dioxide according to the third embodiment shown in Fig. 7. In Fig. 8, the same steps as those in Fig. 7 are denoted by the same reference numerals, and repeated explanations will be omitted (the same applies to the following figures).

<S14: Evaporation and concentration step>
In the evaporation and concentration step S14, the first concentrated liquid produced in the first step S1 is evaporated and concentrated in an evaporator to precipitate calcium sulfate crystals. In order to prevent scale formation in the evaporator, it is preferable to add calcium sulfate crystals as seed crystals to the first concentrated liquid. As the seed crystals, for example, calcium sulfate crystals recovered in the solid-liquid separation step S15 described later can be used. As a method for preventing scale formation in the evaporator, it is also preferable to add an acid to the first concentrated liquid to adjust the pH.

<S15: Solid-liquid separation step>
In the solid-liquid separation step S15, the calcium sulfate crystals precipitated in the evaporation concentration step S14 are separated from the first concentrated liquid by a solid-liquid separator and recovered. The recovered calcium sulfate crystals can be used as, for example, gypsum. The third step S3 is performed on the filtrate, which is the first concentrated liquid after the solid-liquid separation.

By providing the above-mentioned evaporation concentration step S14 and solid-liquid separation step S15, the amount of the first concentrated liquid to be subjected to the third step S3 can be reduced, so that the reaction tank of the third step S3 can be made smaller. In addition, the increase in the concentration of alkaline earth metal in the first concentrated liquid increases the reaction efficiency of the alkaline earth metal and carbonate ions in the third step S3, so that the production rate of alkaline earth metal carbonate crystals can be increased. Furthermore, since calcium is recovered as calcium sulfate crystals from the first concentrated liquid before the third step S3, the precipitation of CaCO ₃ in the third step S3 is suppressed, and the purity of the recovered MgCO ₃ can be increased.

Fifth embodiment
Fig. 9 is a process flow diagram for explaining a method for immobilizing carbon dioxide according to a fifth embodiment of the present invention. In the third embodiment shown in Fig. 7, a part of the liquid to be treated obtained by contacting carbon dioxide with an alkaline solution in the second step S2 is brought into liquid-liquid contact with a second concentrated liquid in an adsorption removal pretreatment step S118, whereas in the fifth embodiment shown in Fig. 9, an alkaline solution that has been previously contacted with carbon dioxide in a CO ₂ absorption step S119 different from the second step S2 is brought into liquid-liquid contact with the second concentrated liquid in the adsorption removal pretreatment step S118. In the CO ₂ absorption step S119, carbon dioxide can be brought into gas-liquid contact with an alkaline solution in the same manner as in the second step S2.

Sixth embodiment
Fig. 10 is a process flow diagram for explaining a method for immobilizing carbon dioxide according to a sixth embodiment of the present invention. In the third embodiment shown in Fig. 7, a part of the liquid to be treated obtained by contacting carbon dioxide with an alkaline solution in the second step S2 is brought into liquid-liquid contact with a second concentrated liquid in an adsorption/removal pretreatment step S118, whereas in the sixth embodiment shown in Fig. 10, the second concentrated liquid is brought into gas-liquid contact with carbon dioxide to perform an adsorption/removal pretreatment step S118'.

In the adsorption removal pretreatment step S118' shown in Fig. 10, the alkaline solution produced in the electrodialysis step S13 is added to the second concentrated liquid to adjust the pH of the second concentrated liquid to the alkaline side (for example, pH 9 to 10) and store it in a storage tank, and a gas containing carbon dioxide is blown into the second concentrated liquid to bring the liquid into gas-liquid contact by bubbling, thereby reacting and immobilizing the alkaline earth metal contained in the second concentrated liquid. As a result, the second concentrated liquid becomes a slurry in which alkaline earth metal carbonate crystals such as _MgCO3 and _CaCO3 are precipitated. In the adsorption removal pretreatment step S118', the alkaline earth metal carbonate crystals contained in the slurry-like second concentrated liquid are separated into solid and liquid using a solid-liquid separator such as a centrifuge and recovered.

The gas containing carbon dioxide may be the atmosphere or exhaust gas from various combustion devices. There is no limit to the concentration of carbon dioxide contained in the gas, but for example, the concentration of carbon dioxide contained in the gas is from the atmosphere to about 100% by volume. In order to maintain the pH during the reaction between the alkaline earth metal and carbon dioxide, the alkaline solution may be added to the second concentrated liquid not only before bubbling the gas containing carbon dioxide, but also during bubbling. When bubbling, the reaction efficiency between the alkaline earth metal and carbon dioxide can be improved by blowing in fine bubbles of carbon dioxide (fine bubbles such as microbubbles or ultrafine bubbles).

The method of bringing the second concentrated liquid into gas-liquid contact with carbon dioxide may be, in addition to the method of blowing _CO2 gas into the second concentrated liquid, a method of spraying the second concentrated liquid into _CO2 gas with a spray nozzle or tray in a single-stage or multi-stage desulfurization tower or degassing tower, etc., and various known gas-liquid contact devices can be used taking into consideration the reaction rate, reaction amount, _CO2 concentration in gases such as exhaust gas, and the like.

<Other embodiments>
The alkaline earth metal removal step S1152 shown in Figures 7 to 10 may be configured to include only the adsorption removal pre-treatment steps S118, S118' when the scale components can be sufficiently removed by the adsorption removal pre-treatment steps S118, S118'.

S1 First step S11 Salt production step S111 First membrane treatment step S112 Second membrane treatment step S113 Crystallization step S114 Concentration step S115 Alkaline earth metal removal step (adsorption removal step)
S116 Resin regeneration step S1151, S1152 Alkaline earth metal removal step S12 Confluence step S13 Electrodialysis step S2 Second step S3 Third step S301 First precipitation step S302 Second precipitation step S31 Solid-liquid separation step S311 First solid-liquid separation step S312 Second solid-liquid separation step S32 Neutralization step S33 Water washing step

Claims

A first step of passing seawater or brackish water through a nanofiltration membrane to produce a first concentrated liquid that is concentrated without permeating the nanofiltration membrane;
A second step of producing a liquid to be treated containing carbonate ions by contacting the alkaline solution with carbon dioxide;
a third step of precipitating carbonate crystals of an alkaline earth metal contained in the first concentrated liquid by contacting the first concentrated liquid with the liquid to be treated.
The first step includes a salt production step of concentrating the first permeate that has permeated the nanofiltration membrane to recover precipitated sodium chloride crystals, and an electrodialysis step of subjecting the solution of sodium chloride crystals to electrodialysis to separate an acid solution and an alkaline solution,
2. The method for fixation of carbon dioxide according to claim 1, wherein the second step produces the liquid to be treated using the alkaline solution obtained in the electrodialysis step.
The first step includes a concentration step of concentrating the first permeated liquid that has permeated the nanofiltration membrane to produce a second concentrated liquid, an alkaline earth metal removal step of removing alkaline earth metals contained in the second concentrated liquid, and an electrodialysis step of electrodialyzing the second concentrated liquid that has been subjected to the alkaline earth metal removal step to separate an acid solution and an alkaline solution,
2. The method for fixation of carbon dioxide according to claim 1, wherein the second step produces the liquid to be treated using the alkaline solution obtained in the electrodialysis step.
The carbon dioxide fixation method according to claim 3, wherein the alkaline earth metal removal step includes an adsorption removal step in which the alkaline earth metal contained in the second concentrated liquid is removed by a chelating resin or an ion exchange resin.
The carbon dioxide fixation method according to claim 4, wherein the first step further comprises a resin regeneration step of regenerating the chelating resin or ion exchange resin using the acid solution obtained in the electrodialysis step.
The carbon dioxide fixation method according to claim 5, wherein the resin regeneration step comprises merging the acid solution in which the chelating resin or ion exchange resin has been regenerated with the first concentrated liquid.
The carbon dioxide fixation method according to claim 3, wherein the alkaline earth metal removal step includes a step of passing the second concentrated liquid through a second nanofiltration membrane different from the nanofiltration membrane.
The carbon dioxide fixation method according to claim 7, wherein the alkaline earth metal removal step includes a step of merging the non-permeate of the second nanofiltration membrane with the first concentrated liquid.
The method for immobilizing carbon dioxide according to claim 3, wherein the alkaline earth metal removal step includes a step of contacting the second concentrated liquid with the liquid to be treated to cause the alkaline earth metal carbonate crystals contained in the second concentrated liquid to precipitate and be removed by solid-liquid separation.
The carbon dioxide fixation method according to claim 1, wherein the third step includes a first precipitation step of mixing the first concentrated liquid with the liquid to be treated to produce a first reaction liquid in which calcium carbonate crystals are precipitated, a first solid-liquid separation step of performing solid-liquid separation of the first reaction liquid to recover calcium carbonate, a second precipitation step of further mixing the liquid to be treated with the first reaction liquid that has been subjected to the first solid-liquid separation step to produce a second reaction liquid in which magnesium carbonate crystals are precipitated, and a second solid-liquid separation step of performing solid-liquid separation of the second reaction liquid to recover magnesium carbonate.