Method for fixing carbon dioxide

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

DESCRIPTION METHOD FOR FIXING CARBON DIOXIDE Technical Field

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

[0002] As global warming becomes more serious, an increase in temperature is required to be suppressed, and as an evaluation model thereof, the goal is to reduce an anthropogenic carbon dioxide (CO) emission amount to zero. Examples of means for achieving the above goal include a CO; fixing method. Examples of effective means of the CO: fixing method include a method in which CO: is fixed by bonding Mg or Ca which is an alkaline earth metal to the CO. However, the conventional method using an ore containing the alkaline earth metal requires a treatment associated with CO; emission such as high temperature and high pressure or addition of chemicals, and thus, in many cases, the CO; is emitted in the entire process. Mg and Ca are also contained in seawater and waste brackish water from a desalination plant of the seawater. For example, a CO; fixing method using seawater has been proposed (see, for example, Patent Literatures 1 and 2). Citation List Patent Literature

[0003] Patent Literature 1: JP 2005-21870 A Patent Literature 2: JP 2010-125354 A Summary of Invention Technical Problem

[0004] In the CO fixing method using seawater or brackish water, many methods in which CO; is injected into seawater or brackish water have been studied. However, the methods have a problem that the efficiency of CO; fixation in a liquid phase decreases due to the presence of a strong hydration shell formed around Mg?” ions which are divalent cations having a small ion diameter, and cations (Na*, K*) which compete with the Mg?™ ions in carbonation. As a solution therefor, means for increasing a pH by the addition of an alkali such as Ca(OH), which is difficult to recycle, have been mainly used. However, these means for promoting the reaction with CO; have increased CO: emission in the entire process in consideration of energy consumption and CO: emission due to additive production as life cycle assessment. Also in the techniques of Patent Literatures 1 and 2, pH adjustment and a wastewater treatment and the like are required, which make it difficult to reduce the CO: emission amount as the entire process. As described above, the problem point of the CO: fixing method using seawater or brackish water includes the inhibition of the reaction with the CO> due to the presence of molecules or ions other than Mg and Ca. Therefore, in a process of separating Mg and Ca from seawater or brackish water, CO> reduction capability must be evaluated in consideration of CO; emission derived per unit operation.

[0005] Therefore, the present invention provides a method for fixing carbon dioxide to an alkaline earth metal, which enhances carbon dioxide reduction capability while considering the carbon dioxide emission amount. Solution to Problem

[0006] The object of the present invention is achieved by a method for fixing carbon dioxide including: a first step of causing seawater or brackish water to pass through a nanofiltration membrane to produce an NF membrane-concentrated liquid concentrated without permeating the nanofiltration membrane; a second step of precipitating and collecting at least one crystal selected from calcium sulfate, sodium chloride, potassium chloride, and sodium sulfate from the NF membrane-concentrated liquid produced in the first step; a third step of obtaining an alkaline earth metal oxide from the NF membrane-concentrated liquid subjected to the second step; and a fourth step of reacting the alkaline earth metal oxide obtained in the third step with carbon dioxide to fix the carbon dioxide as a carbonate.

[0007] In the method for fixing carbon dioxide, the second step preferably includes a first concentration crystallization step of adding calcium sulfate as a seed crystal to the NF membrane-concentrated liquid, followed by evaporating and concentrating to precipitate and collect the calcium sulfate crystal. In the first concentration crystallization step, the collected calcium sulfate crystal is preferably used as the seed crystal. The second step preferably includes a second concentration crystallization step of further evaporating and concentrating the NF membrane-concentrated liquid after collecting the calcium sulfate crystal to precipitate and collect the sodium chloride crystal.

[0008] The second step preferably includes a cooling crystallization step of collecting a crystal precipitated by cooling and crystallizing the NF membrane-concentrated liquid. The cooling crystallization step preferably includes: a first cooling crystallization step of collecting the potassium chloride crystal precipitated by cooling and crystallizing the NF membrane-concentrated liquid; and a second cooling crystallization step of collecting the sodium sulfate crystal precipitated by cooling and crystallizing the NF membrane-concentrated liquid subjected to the first cooling crystallization step at a temperature lower than a cooling crystallization temperature of the first cooling crystallization step.

[0009] The first step preferably includes a pH adjusting step of adjusting a pH of the produced NF membrane-concentrated liquid by adding an acid to the NF membrane- concentrated liquid.

[0010] The first step preferably includes a salt making step of collecting a sodium chloride crystal precipitated by concentrating an NF membrane-permeated liquid permeating the nanofiltration membrane. The first step preferably includes a merging step of merging a blow liquid after the sodium chloride crystal is collected in the salt making step with the produced NF membrane-concentrated liquid. The first step preferably includes a pH adjusting step of adjusting a pH of the produced NF membrane-concentrated liquid by adding an acid solution to the NF membrane- concentrated liquid, the acid solution obtained by electrodialyzing a solution of the sodium chloride crystal produced in the salt making step.

[0011] The alkaline earth metal oxide obtained in the third step preferably contains magnesium oxide, Advantageous Effects of Invention

[0012] The present invention can provide a method for fixing carbon dioxide to an alkaline earth metal, which enhances carbon dioxide reduction capability while considering the carbon dioxide emission amount. Brief Description of Drawings

[0013] FIG. 1 is a process flow diagram for illustrating a method for fixing carbon dioxide according to an embodiment of the present invention. FIG. 2 is a diagram showing examples of changes in amounts of various ions in a process flow shown in FIG. 1. FIG. 3 is a diagram showing a modification of some steps in the process flow shown in FIG. 1. Description of Embodiments

[0014] A method for fixing carbon dioxide of the present invention provides a method for fixing carbon dioxide to an alkaline earth metal contained in seawater or brackish water. In the present embodiment, the "alkaline earth metal" means a broad range including Mg and Be which are elements of the second group of the periodic table in addition to Ca, Sr, Ba, and Ra. In particular, at least Mg is preferably contained as the alkaline earth metal from the viewpoint of the easiness of a reaction with CO and the viewpoint that a carbonate obtained by the reaction can be expected to be used for various applications.

[0015] The "seawater or brackish water" is an aqueous solution containing ions of alkaline earth metals such as magnesium ions (Mg?") and calcium ions (Ca®”). The seawater or brackish water contains ions constituting 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, the seawater or brackish water contains at least one kinds of ions selected from chloride ions (CI), sulfate ions (SO4%), sodium ions (Na™), and potassium (K*).

[0016] As the seawater or brackish water, those obtained from at least one selected from seawater, a salt lake, and industrial wastewater can be used. If the alkaline earth metal is contained, in addition to the seawater, the salt lake, and the industrial wastewater, river water, rainwater, sewage water treatment water, and associated water of oil fields and gas fields, and the like can also be used. More specific examples of the brackish water include water produced from a salt lake or the like, waste brackish water discharged by a desalination or salt production process, collection of a valuable substance from seawater and a salt lake or the like, and industrial wastewater from a chemical factory or the like.

[0017] The brackish water is preferably at least one selected from brackish water obtained from a water production device using seawater, brackish water obtained from a process for making a salt from seawater, and brackish water obtained from a process for collecting lithium from a salt lake from the viewpoint of containing a large amount of Meg, easily reducing an environmental load, and easily reducing a CO emission amount.

[0018] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a process flow diagram for illustrating a method for fixing carbon dioxide according to an embodiment of the present invention. In the present embodiment, the treatment target is seawater, but a similar treatment can be performed in the case of brackish water. As shown in FIG. 1, a method for fixing carbon dioxide of the present embodiment includes: a first step S1 of causing seawater to pass through a nanofiltration membrane (NF membrane) to produce an NF membrane-concentrated liquid concentrated without permeating the NF membrane; a second step S2 of precipitating and collecting at least one crystal selected from calcium sulfate, sodium chloride, potassium chloride, and sodium sulfate from the NF membrane-concentrated liquid produced in the first step S1; a third step S3 of obtaining an alkaline earth metal oxide from the NF membrane-concentrated liquid subjected to the second step S2; and a fourth step S4 of reacting the alkaline earth metal oxide obtained in the third step S3 with carbon dioxide to fix the carbon dioxide as a carbonate.

[0019] <Sl1: First Step> In the first step S1, the seawater is supplied to an NF membrane unit by an intermediate-pressure pump or the like to pass through the NF membrane, thereby producing an NF membrane-permeated liquid permeating the NF membrane and an NF membrane-concentrated liquid concentrated without permeating the NF membrane.

[0020] Since the NF membrane has a property of suppressing the permeation of divalent or higher ions and causing monovalent ions to easily permeate, most of an alkaline earth metal for fixing the carbon dioxide remains in the NF membrane- concentrated liquid, and the concentrations of Na* and K* and the like which may hinder the fixation are reduced. This makes it possible to easily and efficiently fix the carbon dioxide to the alkaline earth metal contained in the seawater (or brackish water), whereby the generation of the carbon dioxide in the entire process including post-steps can be suppressed. FIG. 2 shows examples of amounts of various ions (mg / h) contained in the seawater, the NF membrane-permeated liquid, and the NF membrane- concentrated liquid when the seawater is supplied at a flow rate of 100 m*h.

[0021] <S11: Salt Making Step> Meanwhile, since the NF membrane-permeated liquid contains a large amount of monovalent ions Na™ and CI, the first step S1 of the present embodiment includes a salt making step S11 of collecting a sodium chloride (NaCl) crystal precipitated by concentrating the NF membrane-permeated liquid. The salt making step S11 of the present embodiment includes a membrane treatment step S111 of supplying the NF membrane-permeated liquid to a reverse osmosis membrane (RO membrane) unit by a high pressure pump or the like and causing the NF membrane-permeated liquid to pass through the RO membrane to produce a membrane treatment-concentrated liquid concentrated without permeating the RO membrane, and a crystallization step S112 of supplying the produced membrane treatment-concentrated liquid to a crystallizer and heating and evaporating the liquid to precipitate a NaCl crystal. Steam discharged from the crystallizer is condensed in a condenser or the like to become distilled water. The distilled water is merged with the membrane treatment permeated liquid permeating the RO membrane, and is used as produced water or the like. A part of a crystallizer- concentrated liquid concentrated in the crystallizer is discharged from the crystallizer as a slurry liquid containing the NaCl crystal, and dehydrated by a centrifugal separator or the like to collect the NaCl crystal. Since the NF membrane-permeated liquid hardly contains SO4%, the NF membrane-permeated liquid can be concentrated at a high concentration by the low-energy membrane treatment step S111.

[0022] The membrane treatment step S111 is not limited to the treatment using the RO membrane. The membrane treatment step S111 may be another treatment using a semipermeable membrane, or a combination of a plurality of membrane treatments. For example, as shown in FIG. 3, the membrane treatment step S111 can include an RO membrane concentration step S113 of concentrating the NF membrane-permeated liquid with the RO membrane to produce an RO membrane-concentrated liquid, and a composite membrane treatment step S114 of supplying the RO membrane-concentrated liquid to a high-pressure chamber of a semipermeable membrane unit separated by a semipermeable membrane, and further concentrating the RO membrane-concentrated liquid using a pressure difference from a collected liquid passing through a low-pressure chamber. As the collected liquid supplied to the low-pressure chamber, a part of the RO membrane-concentrated liquid passing through the high-pressure chamber can be used, and the collected liquid passing through the low-pressure chamber can be merged with the NF membrane-permeated liquid before the RO membrane concentration step S113. As shown in FIG. 3, an evaporation treatment step S115 of evaporating and concentrating the membrane treatment-concentrated liquid produced in the membrane treatment step S111 by a horizontal tube evaporator or the like may be provided between the membrane treatment step S111 and the crystallization step S112.

[0023] <S12: Merging Step> A liquid after the NaCl crystal is collected 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 of merging the blow liquid with the NF membrane-concentrated liquid, whereby the discharge of a waste liquid to the outside of the system can be suppressed to reduce the environmental load. Since the alkaline earth metal to be collected such as magnesium is contained not only in the NF membrane-concentrated liquid but also in the blow solution which is the NF membrane-permeated liquid, the collection rate of the alkaline earth metal required for fixing the carbon dioxide in post- steps can be increased by providing the above-described merging step S12.

[0024] <S13: pH Adjusting Step> The first step S1 of the present embodiment includes a pH adjusting step S13 for adjusting the pH of the NF membrane-concentrated liquid with 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 (HCI) to the NF membrane- concentrated liquid to set the pH value of the NF membrane-concentrated liquid to an acidic side. This makes it possible to suppress the production of a soft scale such as magnesium hydroxide (Mg(OH)z) or calcium carbonate (CaCOs) in the second step S2 to be described later. The pH value of the NF membrane-concentrated liquid after the pH adjustment is preferably 3.5 to 6.5.

[0025] The pH adjusting step S13 includes an electrodialysis step S131 of electrodialyzing the NaCl crystal produced in the salt making step S11, whereby HCl produced in the electrodialysis step S131 can be used as a pH adjusting agent. The electrodialysis step S131 can use, for example, a bipolar membrane electrodialyzer, and separates a NaCl solution into a HCI solution and a NaOH solution. As described above, the pH adjusting agent is produced by electrodialysis using the crystal obtained in the salt making step S11. This has no possibility that CO is separately generated in the production process of the pH adjusting agent, and makes it possible to perform electrodialysis using renewable energy or the like, whereby CO> emission in the entire process can be suppressed.

[0026] <S2: Second Step> The second step S2 is a step of removing impurities which hinder the fixation of the carbon dioxide to the alkaline earth metal to be described later from the NF membrane-concentrated liquid produced in the first step S1 to facilitate the isolation of the alkaline earth metal oxide in the third step S3. The second step S2 includes a first concentration crystallization step S21 of adding calcium sulfate as a seed crystal to the NF membrane-concentrated liquid, followed by evaporating and concentrating to precipitate and collect the calcium sulfate crystal, a second concentration crystallization step S22 of further evaporating and concentrating the NF membrane-concentrated liquid after collecting the calcium sulfate crystal to precipitate and collect the sodium chloride crystal, and a cooling crystallization step S23 of collecting a crystal precipitated by cooling and crystallizing the NF membrane-concentrated liquid.

[0027] <S21: First Concentration Crystallization Step> In the first concentration crystallization step S21, the NF membrane- concentrated liquid is supplied to a first concentration can, and heated to be evaporated and concentrated, whereby the calcium sulfate crystal (CaSO4-2H>0) is precipitated. Then, the NF membrane-concentrated liquid is discharged as a slurry liquid, and the calcium sulfate crystal is separated by a solid-liquid separator such as a centrifugal separator. The NF membrane-concentrated liquid contains Ca®*, Na*, and K™ and the like, but the calcium sulfate crystal has inverse solubility in which solubility decreases with an increase in temperature, and thus an operation temperature for evaporating and concentrating is maintained so that Ca®* is precipitated while Na~ and K™ are not precipitated. The operation temperature is preferably 70 to 90°C, and is set to, for example, 80°C.

[0028] In the first concentration crystallization step S21, in order to prevent the scale generation of the calcium sulfate, it is preferable to add a seed crystal of CaSO4:2H20 to the first evaporation can to promote crystal growth with the seed crystal as a core. For this seed crystal, CaSO4-2H,O produced in the first concentration crystallization step S21 can be preferably used. The NF membrane-concentrated liquid is on an acidic side in the pH adjusting step S13, whereby the scale generation of the calcium sulfate can be suppressed.

[0029] <S822: Second Concentration Crystallization Step> The second concentration crystallization step S22 is performed by supplying the NF membrane-concentrated liquid subjected to the first concentration crystallization step S21 to a second concentration can, heating the NF membrane-concentrated liquid to further evaporate and concentrate the NF membrane-concentrated liquid, thereby precipitating a crystal containing sodium chloride (NaCl) as a main component, and then causing a solid-liquid separator to separate the sodium chloride crystal. An operating temperature for evaporating and concentrating is preferably 60 to 80°C, and is set to, for example, 70°C. A concentration ratio in the second concentration crystallization step S22 is preferably set to a range in which MgCl is not precipitated such that the collection rate of the alkaline earth metal oxide mainly containing magnesium oxide can be increased in the third step S3 to be described later.

[0030] <823: Cooling Crystallization Step> The cooling crystallization step S23 is performed by supplying the NF membrane-concentrated liquid subjected to the second concentration crystallization step S22 to a cooling crystallization can, cooling the NF membrane-concentrated liquid to a predetermined cooling crystallization temperature while stirring the NF membrane- concentrated liquid to precipitate a crystal of an intended impurity, and then causing a solid-liquid separator to separate the crystal. The cooling crystallization step S23 of the present embodiment preferably includes: a first cooling crystallization step S231 of collecting a potassium chloride crystal precipitated by cooling and crystallizing the NF membrane-concentrated liquid; and a second cooling crystallization step S232 of collecting a sodium sulfate crystal precipitated by cooling and crystallizing the NF membrane-concentrated liquid subjected to the first cooling crystallization step S231 at a temperature lower than a cooling crystallization temperature of the first cooling crystallization step S231. The cooling crystallization temperature of the first cooling crystallization step S231 is a temperature at which a crystal mainly containing KCl is precipitated but a crystal of Na2SO4-10 HzO is not precipitated. The cooling crystallization temperature is preferably 33 to 40°C, and is set to, for example, 36°C. The cooling crystallization temperature of the second cooling crystallization step $232 is a temperature at which the crystal of Na2SO4-10H20 is precipitated, and is set to, for example, 0 to 10°C,

[0031] Thus, the NF membrane-concentrated liquid subjected to the first concentration crystallization step S21, the second concentration crystallization step S22, and the cooling crystallization step S23 is brine containing an alkaline earth metal chloride such as MgCl as a main component. In the second step S2, it is not necessary to perform all of the first concentration crystallization step S21, the second concentration crystallization step S22, and the cooling crystallization step S23, and only necessary steps may be appropriately selected according to components of seawater or brackish water to be treated so that at least one crystal selected from calcium sulfate, sodium chloride, potassium chloride, and sodium sulfate can be precipitated and collected from the NF membrane-concentrated liquid.

[0032] <S3: Third Step> The third step S3 is a step of obtaining an alkaline earth metal oxide from the brine containing the alkaline earth metal chloride such as MgCl. obtained in the second step S2 as a main component, and includes a drying step S31 and a thermal decomposition step S32.

[0033] <S31: Drying Step> The brine obtained in the second step S2 of the present embodiment is a slurry containing magnesium chloride (MgCl) hydrate as a main component, and is dried in the drying step S31 to form magnesium chloride dihydrate (MgClz2'2H20). A drying temperature in the drying step S31 is preferably 130°C or lower, and more preferably 100 to 129°C under the atmospheric pressure for the production of MgCl, 2H>O.

[0034] <S832: Thermal Decomposition Step> In the thermal decomposition step S32, at least a part of MgCla-2H,O produced in the drying step S31 is thermally decomposed to produce hydroxy magnesium chloride (MgOHCl), and the MgOHCl is further thermally decomposed for dehydrochlorination to produce magnesium oxide (MgO). A thermal decomposition temperature in the thermal decomposition step S32 is preferably 235°C or lower, and more preferably 160 to 235°C under the atmospheric pressure for the production of the MgOHCI. If the pressure is made lower than the atmospheric pressure, the temperature can be further lowered. For the production of the MgO, the temperature is preferably 300 to 500°C, and more preferably 350 to 450°C under the atmospheric pressure. If the pressure is made lower than the atmospheric pressure, the temperature can be further lowered.

[0035] In the thermal decomposition step S32, an HCl gas is discharged when the MgO is produced, whereby the HCI gas may be collected and reused. This HCI gas can be used, for example, as the pH adjusting agent in the pH adjusting step S13, whereby CO; emission due to the separate production of HCI can be avoided.

[0036] <S4: Fourth Step> The fourth step S4 is a step of reacting the alkaline earth metal oxide such as MgO obtained in the third step S3 with carbon dioxide to fix the carbon dioxide as a carbonate. The carbon dioxide is fixed to the alkaline earth metal oxide by a solid-gas reaction between the alkaline earth metal oxide and the carbon dioxide-containing gas. The carbon dioxide-containing gas may be atmospheric air, or an exhaust gas of various combustion apparatuses, or the like. The concentration of the carbon dioxide contained in the gas is not limited, but the concentration of the carbon dioxide contained in the gas is about atmospheric air to 100% by volume from the viewpoint of the easiness of the progression of the solid-gas reaction. When MgO is reacted with CO, magnesium carbonate trihydrate (MgCOs-3H20) is formed, and the carbon dioxide is fixed. The method for fixing carbon dioxide as a carbonate in the fourth step S4 is not limited to the above method, and for example, it is also possible to dissolve the alkaline earth metal oxide such as MgO obtained in the third step S3 in water to produce an aqueous solution, and bring the aqueous solution into gas-liquid contact with carbon dioxide by bubbling or the like to fix the carbon dioxide to the alkaline earth metal oxide.

[0037] According to the present invention, the seawater or brackish water is the aqueous solution containing a plurality of ions as described above, whereby water (pure water), salt, gypsum, and potassium chloride and the like can be obtained as by- products in each step described above. Therefore, in addition to CO; fixation, various by-products are produced, and these can be expected to be used for more environmentally friendly products. The present invention can use the waste liquid as the brackish water as described above, whereby the present invention can be used as a waste liquid treatment, and is considered to contribute to the reduction of waste liquid treatment cost. Furthermore, the magnesium carbonate to which the carbon dioxide is fixed 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- described method for fixing carbon dioxide. Reference Signs List

[0038] S1 first step S11 salt making step merging step pH adjusting step second step first concentration crystallization step second concentration crystallization step cooling crystallization step third step fourth step

Claims

CLAIMS 1, A method for fixing carbon dioxide comprising: a first step of causing seawater or brackish water to pass through a nanofiltration membrane to produce an NF membrane-concentrated liquid concentrated without permeating the nanofiltration membrane; a second step of precipitating and collecting at least one crystal selected from calcium sulfate, sodium chloride, potassium chloride, and sodium sulfate from the NF membrane-concentrated liquid produced in the first step; a third step of obtaining an alkaline earth metal oxide from the NF membrane- concentrated liquid subjected to the second step; and a fourth step of reacting the alkaline earth metal oxide obtained in the third step with carbon dioxide to fix the carbon dioxide as a carbonate. 2 The method according to claim 1, wherein the second step includes a first concentration crystallization step of adding calcium sulfate as a seed crystal to the NF membrane-concentrated liquid, followed by evaporating and concentrating to precipitate and collect the calcium sulfate crystal. % The method according to claim 2, wherein in the first concentration crystallization step, the collected calcium sulfate crystal is used as the seed crystal.

4. The method according to claim 2 or 3, wherein the second step includes a second concentration crystallization step of further evaporating and concentrating the NF membrane-concentrated liquid after collecting the calcium sulfate crystal to precipitate and collect the sodium chloride crystal.

5. The method according to any one of claims 1 to 4, wherein the second step includes a cooling crystallization step of collecting a crystal precipitated by cooling and crystallizing the NF membrane-concentrated liquid.

6. The method according to claim 5, wherein the cooling crystallization step includes: a first cooling crystallization step of collecting the potassium chloride crystal precipitated by cooling and crystallizing the NF membrane-concentrated liquid; and a second cooling crystallization step of collecting the sodium sulfate crystal precipitated by cooling and crystallizing the NF membrane-concentrated liquid subjected to the first cooling crystallization step at a temperature lower than a cooling crystallization temperature of the first cooling crystallization step.

7. The method according to any one of claims 1 to 6, wherein the first step includes a pH adjusting step of adjusting a pH of the produced NF membrane- concentrated liquid by adding an acid to the NF membrane-concentrated liquid.

8. The method according to any one of claims 1 to 7, wherein the first step includes a salt making step of collecting the sodium chloride crystal precipitated by concentrating an NF membrane-permeated liquid permeating the nanofiltration membrane.

9. The method according to claim 8, wherein the first step includes a merging step of merging a blow liquid after the sodium chloride crystal is collected in the salt making step with the produced NF membrane-concentrated liquid.

10. The method according to claim 8 or 9, wherein the first step includes a pH adjusting step of adjusting a pH of the produced NF membrane-concentrated liquid by adding an acid solution to the NF membrane-concentrated liquid, the acid solution obtained by electrodialyzing a solution of the sodium chloride crystal produced in the salt making step.

1. The method according to any one of claims 1 to 10, wherein the alkaline earth metal oxide obtained in the third step contains magnesium oxide.

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