WO2021261410A1

WO2021261410A1 - Method for fixing carbon dioxide

Info

Publication number: WO2021261410A1
Application number: PCT/JP2021/023286
Authority: WO
Inventors: 隆雄中垣; コーリアダムマイヤズ; 翔藤本; 良彦加藤; 悟平野; 和彦石田; 基頼早水; 恵理鈴木; 慶明三保
Original assignee: 学校法人早稲田大学; 日揮グローバル株式会社; 株式会社ササクラ
Priority date: 2020-06-22
Filing date: 2021-06-21
Publication date: 2021-12-30
Also published as: JPWO2021261410A1; AU2021295775A1

Abstract

This method for fixing carbon dioxide comprises: a first step S1 for passing seawater or saline water through a nanofiltration membrane to produce an NF membrane concentrated liquid that is concentrated without permeating the nanofiltration membrane; a second step S2 for precipitating and collecting, from the NF membrane concentrated liquid produced in the first step S1, at least one crystal, which is selected from among calcium sulfate, sodium chloride, potassium chloride, and sodium sulfate; a third step S3 for obtaining an alkaline earth metal oxide from the NF membrane concentrated liquid that has undergone the second step S2; and a fourth step S4 for reacting carbon dioxide with the alkaline earth metal oxide obtained in the third step S3, and thereby fixing the carbon dioxide as carbonate.

Description

Carbon dioxide immobilization method

The present invention relates to a method for immobilizing carbon dioxide on an alkaline earth metal.

As global warming becomes more serious, it is required to suppress the temperature rise, and the goal is to reduce _{anthropogenic carbon dioxide (CO 2) emissions to zero as an evaluation model.} As a means for achieving the above-mentioned goal, a CO ₂ immobilization method can be mentioned.
As _{an effective means of the CO 2} immobilization method, there is a method of using the alkaline earth metals Mg and Ca to bond and immobilize _{these alkaline earth metals and CO 2.} However, the conventional method using ore containing alkaline earth metal requires a treatment linked to _{CO 2} emission such as high temperature and high pressure and addition of chemicals, _{so that CO 2} emission is often generated in the entire process.
In addition, Mg and Ca are also contained in seawater and waste liquid irrigation from seawater desalination plants, and for example, a CO ₂ immobilization method using seawater has been proposed (see, for example, Patent Documents 1 and 2). ..

Japanese Unexamined Patent Publication No. 2005-21870 Japanese Unexamined Patent Publication No. 2010-12354

As a CO ₂ immobilization method using seawater or brine, _{many methods have been studied by injecting CO 2} into seawater or brine, but it is formed around ^{Mg 2+} ions, which are divalent cations with a small ion diameter. Due to the strong hydrated shell and ^{the presence of cations (Na +} , K ⁺ ) competing for carbon dioxide chloride, there is a problem that the efficiency _{of CO 2} immobilization in the liquid phase is reduced. As a solution to this problem, it has been the mainstream to use a means for raising the pH by adding an alkali, which is difficult to recycle, such _{as Ca (OH) 2.} However, these means for promoting the reaction between CO _2, considering CO ₂ emission caused by the additive production of the energy consumption and life cycle assessment, CO ₂ emissions becomes positive in the entire process ..
Even with the techniques of

Patent Documents

1 and 2, it is difficult to make the _{CO 2} emission amount negative as a whole process because pH adjustment, wastewater treatment, etc. are required.
_{As described above, a problem of the CO 2} immobilization method using seawater or brine is the inhibition of the reaction with _{CO 2} due to the presence of molecules and ions other than Mg and Ca. Therefore, in the process of separating Mg and Ca from seawater or brackish water, the CO ₂ reduction capacity must be evaluated in _{consideration of the CO 2 emissions derived from each unit operation.}

Therefore, the present invention provides a method for immobilizing carbon dioxide on an alkaline earth metal, which enhances the carbon dioxide reduction capacity while considering the amount of carbon dioxide emissions.

The object of the present invention is a first step of passing seawater or alkaline water through a nanofiltration membrane to generate a concentrated NF membrane concentrate without permeating the nanofiltration membrane, and the first step. From the second step of precipitating and recovering at least one crystal selected from calcium sulfate, sodium chloride, potassium chloride and sodium sulfate from the generated NF membrane concentrate, and from the NF membrane concentrate that has undergone the second step. Dioxide including a third step of obtaining an alkaline earth metal oxide and a fourth step of reacting the alkaline earth metal oxide obtained in the third step with carbon dioxide and fixing the carbon dioxide as a carbonate. Achieved by carbon immobilization methods.

In this method for immobilizing carbon dioxide, the second step is the first concentrated crystallization in which calcium sulfate crystals are precipitated and recovered by adding calcium sulfate as seed crystals to the NF membrane concentrate and evaporating and concentrating the crystals. It is preferable to have a process. In the first concentrated crystallization step, it is preferable to use the recovered calcium sulfate crystals as seed crystals. It is preferable that the second step includes a second concentrated crystallization step of precipitating and recovering sodium chloride crystals by further evaporating and concentrating the NF membrane concentrate after recovering the calcium sulfate crystals.

It is preferable that the second step includes a cooling crystallization step of recovering the precipitated crystals by cooling crystallization of the NF membrane concentrate. In the cooling crystallization step, a first cooling crystallization step for recovering the potassium chloride crystals precipitated by cooling crystallization of the NF film concentrate and an NF film concentrate that has undergone the first cooling crystallization step are used. It is preferable to include a second cooling crystallization step of recovering the precipitated sodium sulfate crystals by cooling crystallization at a temperature lower than the cooling crystallization temperature of the first cooling crystallization step.

It is preferable that the first step includes a pH adjusting step of adding an acid to the produced NF membrane concentrate to adjust the pH.

It is preferable that the first step includes a salt-making step of concentrating the NF membrane permeate that has permeated the nanofiltration membrane and recovering the precipitated sodium chloride crystals. It is preferable that the first step includes a merging step of merging the produced NF membrane concentrate with the blow liquid after the sodium chloride crystals are recovered in the salt-making step. The first step is a pH adjusting step of adding an acid solution obtained by electrodialysis of a solution of sodium chloride crystals produced in the salt-making step to the produced NF membrane concentrate to adjust the pH. It is preferable to prepare.

The alkaline earth metal oxide obtained in the third step preferably contains magnesium oxide.

According to the present invention, it is possible to provide a method for immobilizing carbon dioxide on an alkaline earth metal, which enhances the carbon dioxide reduction capacity while considering the amount of carbon dioxide emissions.

It is a process flow diagram for demonstrating the method of immobilizing carbon dioxide which concerns on one Embodiment of this invention. It is a figure which shows an example of the change of various ion amounts in the processing flow shown in FIG. It is a figure which shows the modification of some steps in the processing flow shown in FIG.

The method for immobilizing carbon dioxide of the present invention provides a method for immobilizing carbon dioxide with respect to alkaline earth metals contained in seawater or brine. In the present embodiment, "alkaline earth metal" means 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 to contain at least Mg as an alkaline earth metal from the viewpoint of ease of reaction with _{CO 2 and the expectation that the carbonate obtained by the reaction can be used for various purposes.}

Further, "seawater or brine" is an aqueous solution containing ions of an alkaline earth metal such as ^{magnesium ion (Mg 2+} ) and calcium ion (Ca ^2+). Seawater or brine contains ions that make up at least one crystal selected from calcium sulfate, sodium chloride, potassium chloride and sodium sulfate, in addition to the ions of alkaline earth metals. Specifically, sea water or brine is chloride ion (Cl ^-), sulfate ion _(SO ^{4 2-),} sodium ion ^(Na +), at least one ion selected from potassium ^{(K +)} There is.

As the above-mentioned seawater or brackish water, those obtained from at least one selected from seawater, salt lakes, and industrial wastewater can be used. In addition to seawater, salt lakes and industrial wastewater, river water, rainwater, treated sewage water, and accompanying water from oil and gas fields can also be used as long as alkaline earth metals are contained. More specifically, as irrigation, irrigation using salt lakes, waste liquid irrigation discharged by desalination and salt formation processes, recovery of valuable resources using seawater and salt lakes, collection of valuable resources from chemical factories, etc. Examples include industrial wastewater.

From the viewpoint of containing a large amount of Mg as described above, reducing the environmental load _{and easing to reduce CO 2} emissions, brackish water is obtained from brackish water obtained from a desalination device using seawater and a process of producing salt from seawater. It is preferably at least one selected from brackish water and brackish water obtained from the process of recovering lithium from a salt lake.

Hereinafter, an embodiment of the present invention will be described with reference to the attached drawings. FIG. 1 is a processing flow chart for explaining a method for immobilizing carbon dioxide according to an embodiment of the present invention. In the present embodiment, the treatment target is seawater, but the same treatment can be performed in the case of brackish water. As shown in FIG. 1, in the method for immobilizing carbon dioxide of the present embodiment, seawater is passed through a nanofiltration membrane (NF membrane) to obtain a concentrated NF membrane concentrate without permeating the NF membrane. A second step of precipitating and recovering at least one crystal selected from calcium sulfate, sodium chloride, potassium chloride and sodium sulfate from the first step S1 to be produced and the NF membrane concentrate produced in the first step S1. Carbon dioxide is reacted with the alkaline earth metal oxide obtained in the third step S3 and the third step S3 to obtain the alkaline earth metal oxide from the S2 and the NF film concentrate obtained through the second step S2. It is provided with a fourth step S4 for fixing carbon dioxide as a carbonate.

<S1: First step>
In the first step S1, seawater was supplied to the NF membrane unit by a medium pressure pump or the like and passed through the NF membrane to concentrate the NF membrane permeable liquid that had permeated the NF membrane and the NF membrane without permeating the NF membrane. Produces an NF membrane concentrate.

Since the NF membrane suppresses the permeation of divalent or higher ions, the monovalent ion has the property of being easily permeated. Therefore, the NF membrane concentrate contains alkaline earth metals that are the targets of carbon dioxide immobilization. As much remains, the concentrations of ^{Na +} , K ^{+, etc. that may interfere with this immobilization are reduced.} Therefore, it is possible to easily and efficiently immobilize carbon dioxide on alkaline earth metals contained in seawater (or brackish water), and it is possible to suppress the generation of carbon dioxide in the entire process including the post-process. .. FIG. 2 shows an example of various ion amounts (mg / h) contained in seawater, NF membrane permeate and NF membrane concentrate when seawater is supplied at a flow rate of ^{100 m 3 / h.}

<S11: Salt making process>
On the other hand, since the NF membrane permeate ^{contains a large amount of monovalent ions Na +} and Cl ⁻ , the first step S1 of the present embodiment is the sodium chloride precipitated by concentrating the NF membrane permeate. (NaCl) The salt making step S11 for recovering crystals is provided. In the salt-making step S11 of the present embodiment, the NF membrane permeate is supplied to the reverse osmosis membrane (RO membrane) unit by a high-pressure pump or the like and water is passed through the RO membrane, so that the membrane is concentrated without permeating the RO membrane. The membrane treatment step S111 for producing a treatment concentrate and the crystallization step S112 for supplying the produced membrane treatment concentrate to a crystal can and heating and evaporating to precipitate NaCl crystals are provided. The vapor discharged from the crystal can is condensed by a condenser or the like to become distilled water, which is merged with the membrane-treated permeate liquid that has permeated the RO membrane and used as production water or the like. The crystal can concentrate concentrated in the crystal can is partially discharged from the crystal can as a slurry liquid containing NaCl crystals, and is dehydrated by a centrifuge or the like to recover the NaCl crystals. NF membrane _permeate since it does not contain ^{SO 4 2-most,} by membrane treatment step S111 is a low energy, can be concentrated at a high concentration.

The membrane treatment step S111 is not limited to the treatment using the RO membrane, and may be another treatment using a semipermeable membrane, or may be a combination of a plurality of membrane treatments. For example, as shown in FIG. 3, the membrane treatment step S111 concentrates the NF membrane permeate with the RO membrane to generate the RO membrane concentrate, and the RO membrane concentrate step S113 and the RO membrane concentrate are separated by a semipermeable membrane. It is possible to provide a composite membrane treatment step S114 that supplies the semipermeable membrane unit to the high pressure chamber and further concentrates the RO membrane concentrate by utilizing the pressure difference from the recovered liquid passing through the low pressure chamber. As the recovery liquid supplied to the low pressure chamber, a part of the RO membrane concentrate that has passed through the high pressure chamber can be used, and the recovery liquid that has passed through the low pressure chamber is used as the NF membrane permeate before the RO membrane concentration step S113. Can be merged. Further, as shown in FIG. 3, the evaporation treatment step S115 in which the membrane treatment concentrate produced in the membrane treatment step S111 is evaporated and concentrated by a horizontal tube type evaporator or the like between the membrane treatment step S111 and the crystallization step S112. May be provided.

<S12: Confluence process>
The liquid after recovering the NaCl crystals in the salt-making step S11 is discharged as a blow liquid. The first step S1 of the present embodiment includes a merging step S12 for merging the blow liquid with the above-mentioned NF membrane concentrate, thereby suppressing the discharge of the waste liquid to the outside of the system and reducing the environmental load. Can be planned. Alkaline earth metals such as magnesium to be recovered are contained not only in the NF film concentrate but also in the blow liquid which is the NF film permeation liquid. It is possible to increase the recovery rate of alkaline earth metals required for immobilization of carbon dioxide.

<S13: pH adjustment process>
The first step S1 of the present embodiment includes a pH adjusting step S13 for adjusting the pH of the NF membrane concentrate to which the blow liquid is merged in the merging step S12. The pH adjusting step S13 is a step of adding a pH adjusting agent such as hydrochloric acid (HCl) to the NF membrane concentrate to bring the pH value of the NF membrane concentrate to the acidic side, whereby the second step S2 described later is performed. In, the formation of soft scales such as magnesium hydroxide (Mg (OH) ₂ ) and calcium carbonate (CaCO _{3) can be suppressed.} The pH value of the NF membrane concentrate after pH adjustment is preferably 3.5 to 6.5.

The pH adjusting step S13 includes an electrodialysis step S131 for electrodialyzing the NaCl crystals produced in the salt making step S11, so that the HCl produced in the electrodialysis step S131 can be used as a pH adjusting agent. In the electrodialysis step S131, for example, a bipolar membrane electrodialysis apparatus can be used, and the NaCl solution is separated into the HCl solution and the NaOH solution. As described above, by producing the pH adjuster by electrodialysis using the crystals obtained in the salt making step S11, _{there is no possibility that CO 2} is separately generated in the process of producing the pH adjuster, and the electrodialysis is a renewable energy. Since it can be carried out by using the above, it is possible to suppress _{CO 2 emissions in the entire process.}

<S2: Second step>
In the second step S2, impurities that hinder the immobilization of carbon dioxide on the alkaline earth metal described later are removed from the NF film concentrate produced in the first step S1, and the alkali in the third step S3 is removed. This is a step that facilitates the isolation of earth metal oxides. The second step S2 is the first concentrated crystallization step S21 for precipitating and recovering calcium sulfate crystals by adding calcium sulfate as a seed crystal to the NF membrane concentrate and evaporating and concentrating, and recovering the calcium sulfate crystals. The second concentrated crystallization step S22, in which sodium chloride crystals are precipitated and recovered by further evaporating and concentrating the NF film concentrate, and the crystals precipitated by cooling crystallization of the NF film concentrate are recovered. The cooling crystallization step S23 is provided.

<S21: First concentrated crystallization step>
First concentration crystallization step S21 is concentrated by evaporation by heating by supplying a NF membrane concentrate to a first concentration can, after precipitating the calcium sulfate crystals _{_{(CaSO 4 · 2H 2 O)}} , Slurry It is discharged as a liquid, and calcium sulfate crystals are separated by a solid-liquid separator such as a centrifuge. The NF membrane concentrate contains Ca ²⁺ , Na ⁺ , K ^+, etc., but calcium sulfate crystals have back-solubility, which decreases in solubility as the temperature rises, so that Ca ²⁺ is deposited. The operating temperature of evaporation concentration is maintained so that Na ⁺ and K ^{+ do not precipitate.} This operating temperature is preferably 70 to 90 ° C, for example set to 80 ° C.

In the first concentrating crystallization step S21, in order to prevent scale formation of calcium sulfate, by adding seed crystals of the first evaporator to the CaSO 4 · _2H 2 _O, the crystal growth of the seed crystals as nuclei It is preferable to encourage. This is seed, can be preferably used first of CaSO ₄ · 2H ₂ O produced in the concentration crystallization step S21. Since the NF membrane concentrate is on the acidic side by the above pH adjusting step S13, the scale generation of calcium sulfate can also be suppressed by this.

<S22: Second concentrated crystallization step>
In the second concentrated crystallization step S22, the NF membrane concentrate that has undergone the first concentrated crystallization step S21 is supplied to the second concentrated can and heated to further evaporate and concentrate, mainly containing sodium chloride (NaCl). After precipitating the crystals as components, the sodium chloride crystals are separated by a solid-liquid separator. The operating temperature for evaporation concentration is preferably 60 to 80 ° C, for example set to 70 ° C. The concentration ratio in the second concentration crystallization step S22 is within the range in which _{MgCl 2} does not precipitate so that the recovery rate of the alkaline earth metal oxide mainly composed of magnesium oxide can be increased in the third step S3 described later. It is preferable to set to.

<S23: Cooling crystallization step>
In the cooling crystallization step S23, the NF film concentrate that has undergone the second concentration crystallization step S22 is supplied to the cooling crystallization can and cooled to a predetermined cooling crystallization temperature while stirring to obtain the desired impurities. After precipitating the crystals of, the crystals are separated by a solid-liquid separator. The cooling crystallization step S23 of the present embodiment has undergone a first cooling crystallization step S231 for recovering the potassium chloride crystals precipitated by cooling crystallization of the NF film concentrate and a first cooling crystallization step S231. It is provided with a second cooling crystallization step S232 for recovering the sodium sulfate crystals precipitated by cooling crystallization of the NF film concentrate at a temperature lower than the cooling crystallization temperature of the first cooling crystallization step S231. .. Cooling crystallization temperature of the first cooling crystallization step S231, while KCl crystals of entities are _deposited, a temperature not crystals precipitated in _{_{Na 2 SO 4 · 10H 2 O}} , to be 33 ~ 40 ° C. Preferably, it is set to, for example, 36 ° C. The cooling crystallization temperature of the second cooling crystallization step S232 _{is a temperature at which crystals of Na 2} SO ₄・ 10H ₂ O precipitate, and is set to, for example, 0 to 10 ° C.

In this way, the NF membrane concentrate that has undergone the first concentrated crystallization step S21, the second concentrated crystallization step S22, and the cooling crystallization step S23 is a brine containing an alkaline earth metal chloride such as _{MgCl 2 as a main component.} be. In the second step S2, it is not necessary to perform all of the first concentrated crystallization step S21, the second concentrated crystallization step S22 and the cooling crystallization step S23, and calcium sulfate, sodium chloride, potassium chloride and the like from the NF membrane concentrate are used. Only necessary steps may be appropriately selected depending on the components of seawater or brackish water to be treated so that at least one kind of crystal selected from sodium sulfate can be precipitated and recovered.

<S3: Third step>
The third step S3 is a step of obtaining an alkaline earth metal oxide from the brine containing an alkaline earth metal chloride such as _{MgCl 2 obtained in the second step S2 as a main component, and is a drying step S31 and thermal decomposition.} The process S32 is provided.

<S31: Drying process>
The brine obtained in the second step S2 of the present embodiment _{is a slurry containing magnesium chloride (MgCl 2} ) hydrate as a main component, and is dried in the drying step S31 to obtain magnesium chloride dihydrate (Mgnesium chloride dihydrate (MgCl 2) hydrate. MgCl ₂ · _2H ₂ O) to become. Drying temperature in the drying step S31, to the generation of MgCl _₂ · 2H ₂ O, preferably at atmospheric pressure is at 130 ° C. or less, more preferably at 100 ~ 129 ° C.

<S32: Pyrolysis process>
Pyrolysis step step S32, at least a portion of the MgCl _₂ · 2H ₂ O produced in the drying step S31 the thermally decomposed to produce the hydroxy magnesium chloride (MgOHCl), further dehydrochlorination by thermally decomposing MgOHCl Generates magnesium oxide (MgO). The thermal decomposition temperature of the thermal decomposition step S32 is preferably 235 ° C. or lower, more preferably 160 to 235 ° C., under atmospheric pressure with respect to the formation of MgOHCl. The temperature can be further lowered by lowering the pressure below the atmospheric pressure. Further, for the formation of MgO, the temperature is preferably 300 to 500 ° C, more preferably 350 to 450 ° C under atmospheric pressure. The temperature can be further lowered by lowering the pressure below the atmospheric pressure.

In the thermal decomposition step S32, since HCl gas is discharged when MgO is generated, the HCl gas may be recovered and reused. This HCl gas can be used, for example, as a pH adjuster in the above pH adjusting step S13, thereby avoiding _{CO 2 emissions due to the separate production of HCl.}

<S4: Fourth step>
The fourth step S4 is a step of reacting carbon dioxide with an alkaline earth metal oxide such as MgO obtained in the third step S3 to fix carbon dioxide as a carbonate. Carbon dioxide is immobilized on the alkaline earth metal oxide by a solid-gas reaction between the alkaline earth metal oxide and a gas containing carbon dioxide. The gas containing carbon dioxide may be the atmosphere, or may be the exhaust gas of various combustion devices. The concentration of carbon dioxide contained in the gas is not limited, but the concentration of carbon dioxide contained in the gas is about 100% by volume from the atmosphere from the viewpoint of the ease of progress of the solid-gas reaction. When the CO ₂ is reacted to MgO, is a magnesium carbonate trihydrate _{_{(MgCO 3 · 3H 2 O)}} , carbon dioxide is immobilized. The method for fixing carbon dioxide as a carbonate in the fourth step S4 is not limited to the above method, and for example, the alkaline earth metal oxide such as MgO obtained in the third step S3 is dissolved in water. It is also possible to generate an aqueous solution and bring carbon dioxide into gas-liquid contact with the aqueous solution by bubbling or the like to immobilize carbon dioxide on the alkaline earth metal oxide.

According to the present invention, since seawater or brine is an aqueous solution containing a plurality of ions as described above, water (pure water), salt, gypsum, potassium chloride and the like can be obtained as by-products in each of the above steps. Therefore _{, in addition to CO 2} immobilization, various by-products are produced, and it can be expected that these will be used in more environmentally friendly products. Further, since the present invention can use the waste liquid as the brackish water as described above, it can be used as the waste liquid treatment, and it is considered that the present invention also contributes to the reduction of the waste liquid treatment cost. Furthermore, magnesium carbonate on which carbon dioxide is immobilized can also be used as a building material. Therefore, the present invention can also provide a method for producing an alkaline earth metal carbonate using the above-mentioned method for immobilizing carbon dioxide.

S1 1st step S11 Salt making step S12 Confluence step S13 pH adjustment step S2 2nd step S21 1st concentrated crystallization step S22 2nd concentrated crystallization step S23 Cooling crystallization step S3 3rd step S4 4th step

Claims

The first step of producing a concentrated NF membrane concentrate without permeating the nanofiltration membrane by passing seawater or brine through the nanofiltration membrane.
A second step of precipitating and recovering at least one crystal selected from calcium sulfate, sodium chloride, potassium chloride and sodium sulfate from the NF membrane concentrate produced in the first step.
The third step of obtaining an alkaline earth metal oxide from the NF membrane concentrate that has undergone the second step, and
A method for immobilizing carbon dioxide, which comprises a fourth step of reacting carbon dioxide with an alkaline earth metal oxide obtained in the third step and fixing the carbon dioxide as a carbonate.
The second step is according to claim 1, further comprising a first concentrated crystallization step of precipitating and recovering calcium sulfate crystals by adding calcium sulfate as seed crystals to the NF membrane concentrate and evaporating and concentrating the crystals. Calcium immobilization method.
The method for immobilizing carbon dioxide according to claim 2, wherein the first concentrated crystallization step uses the recovered calcium sulfate crystals as seed crystals.
The second step comprises the second concentrated crystallization step of precipitating and recovering sodium chloride crystals by further evaporating and concentrating the NF membrane concentrate after recovering the calcium sulfate crystals, according to claim 2 or 3. The method for immobilizing carbon dioxide as described.
The method for immobilizing carbon dioxide according to any one of claims 1 to 4, wherein the second step comprises a cooling crystallization step of recovering the precipitated crystals by cooling crystallization of the NF membrane concentrate.
In the cooling crystallization step, a first cooling crystallization step of recovering potassium chloride crystals precipitated by cooling crystallization of the NF film concentrate and an NF film concentrate that has undergone the first cooling crystallization step are used. The carbon dioxide according to claim 5, further comprising a second cooling crystallization step of recovering the precipitated sodium sulfate crystals by cooling crystallization at a temperature lower than the cooling crystallization temperature of the first cooling crystallization step. Immobilization method.
The method for immobilizing carbon dioxide according to any one of claims 1 to 6, wherein the first step comprises a pH adjusting step of adding an acid to the produced NF membrane concentrate to adjust the pH.
The method for immobilizing carbon dioxide according to any one of claims 1 to 7, wherein the first step comprises a salt-making step of concentrating the NF membrane permeate that has permeated the nanofiltration membrane and recovering the precipitated sodium chloride crystals. ..
The method for immobilizing carbon dioxide according to claim 8, wherein the first step comprises a merging step of merging the generated NF membrane concentrate with a blow liquid after the sodium chloride crystals have been recovered in the salt-making step.
The first step is a pH adjusting step of adding an acid solution obtained by electrodialysis of a solution of sodium chloride crystals produced in the salt-making step to the produced NF membrane concentrate to adjust the pH. The method for immobilizing carbon dioxide according to any one of claims 8 or 9.
The method for immobilizing carbon dioxide according to any one of claims 1 to 10, wherein the alkaline earth metal oxide obtained in the third step contains magnesium oxide.