AU2021295775A1

AU2021295775A1 - Method for fixing carbon dioxide

Info

Publication number: AU2021295775A1
Application number: AU2021295775A
Authority: AU
Inventors: Sho FUJIMOTO; Motoyori HAYAMIZU; Satoru Hirano; Kazuhiko Ishida; Yoshihiko Kato; Yoshiaki Miho; Corey Adam Myers; Takao Nakagaki; Eri Suzuki
Original assignee: Waseda University; Sasakura Engineering Co Ltd
Current assignee: Waseda University; Sasakura Engineering Co Ltd
Priority date: 2020-06-22
Filing date: 2021-06-21
Publication date: 2023-02-02
Also published as: WO2021261410A1; JPWO2021261410A1

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 (C02) emission amount to zero. Examples of means for

achieving the above goal include a C02 fixing method.

Examples of effective means of the C02 fixing method include a method in

which C02 is fixed by bonding Mg or Ca which is an alkaline earth metal to the C02.

However, the conventional method using an ore containing the alkaline earth metal

requires a treatment associated with C02 emission such as high temperature and high

pressure or addition of chemicals, and thus, in many cases, the C02 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 C02 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 C02 fixing method using seawater or brackish water, many methods in

which C02 is injected into seawater or brackish water have been studied. However,

the methods have a problem that the efficiency of C02 fixation in a liquid phase

decreases due to the presence of a strong hydration shell formed around Mg2+ ions

which are divalent cations having a small ion diameter, and cations (Na*, K) which

compete with the Mg2+ ions in carbonation. As a solution therefor, means for

increasing a pH by the addition of an alkali such as Ca(OH)2, which is difficult to

recycle, have been mainly used. However, these means for promoting the reaction

with C02 have increased C02 emission in the entire process in consideration of energy

consumption and C02 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

C02 emission amount as the entire process.

As described above, the problem point of the C02 fixing method using

seawater or brackish water includes the inhibition of the reaction with the C02 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, C02 reduction capability must

be evaluated in consideration of C02 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 C02 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 (Mg2+) and calcium ions (Ca2+). 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 (Cl-), sulfate ions (SO42-), 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

Mg, easily reducing an environmental load, and easily reducing a C02 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 SI

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 Sl; 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]

<SI: 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 m3 /h.

[0021]

<S11: Salt Making Step>

Meanwhile, since the NF membrane-permeated liquid contains a large amount

of monovalent ions Na' and Cl-, the first step S of the present embodiment includes a

salt making step SlIof collecting a sodium chloride (NaCl) crystal precipitated by

concentrating the NF membrane-permeated liquid. The salt making step S1 Iof the

present embodiment includes a membrane treatment step S Illof 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. Apart 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 SO42-, the NF membrane-permeated liquid can be concentrated at a high concentration by the low-energy membrane treatment step S11.

[0022]

The membrane treatment step S Illis not limited to the treatment using the

RO membrane. The membrane treatment step Sillmay 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 Sillcan 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 Si llby a horizontal tube evaporator or the like may be provided

between the membrane treatment step Si land the crystallization step S112.

[0023]

<S12: Merging Step>

A liquid after the NaCl crystal is collected in the salt making step Sli is discharged as a blow liquid. The first step Si 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 S Iof 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. ThepH adjusting step S13 is a step of

adding a pH adjusting agent such as hydrochloric acid (HCl) 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)2) or calcium carbonate (CaCO3) 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 Sll, 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 HCl 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 Si1. This has no possibility that C02 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 C02 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 S Ito 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

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 (CaSO 4 2H20) 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 Ca2, 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 Ca2+ 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 CaSO 4 -2H20 to

the first evaporation can to promote crystal growth with the seed crystal as a core. For

this seed crystal, CaSO 4 -2H20 produced in thefirst 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]

<S22: Second Concentration Crystallization Step>

The second concentration crystallization step S22 is performed by supplying

the NF membrane-concentrated liquid subjected to thefirst 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 MgCl2 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]

<S23: 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 H 2 0 is not precipitated. Thecooling

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 S232

is a temperature at which the crystal of Na2SO4-10H 2 0 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 MgCl2 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 MgCl2 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 (MgC2) hydrate as a main component, and is dried in

the drying step S31 to form magnesium chloride dihydrate (MgC2-2H20). 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 MgCl2-2H20.

[0034]

<S32: Thermal Decomposition Step>

In the thermal decomposition step S32, at least a part of MgCl2-2H20 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

MgOHCl. 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 HCl gas may be collected and reused. This HCl gas

can be used, for example, as the pH adjusting agent in the pH adjusting step S13,

whereby C02 emission due to the separate production of HCl 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 C02, magnesium carbonate trihydrate (MgCO3 3H 2 0) 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 C02 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]

Sl first step

Sil salt making step

S12 merging step

S13 pH adjusting step

S2 second step

S21 first concentration crystallization step

S22 second concentration crystallization step

S23 cooling crystallization step

S3 third step

S4 fourth step

Claims

1. A method for fixing carbon dioxide comprising:

a first step of causing seawater or brackish water to pass through a

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.

3. 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 I 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

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.

11. 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.

Fig. 1 NF MEMBRANE- PERMEATED LIQUID MEMBRANE TREATMENT SEAWATER PERMEATED LIQUID PRODUCED (OR BRACKISH 海水 WATER WATER) （又はかん水） MEMBRANE DISTILLED NF TREATMENT WATER MEMBRANE- CONCENTRATED CONCENTRATED LIQUID ELECTRO- CRYSTALLIZE LIQUID DIALYZE

BLOW LIQUID

1/2 ADJUST DRAWINGS

pH S23 MgCl2 S31 S32 S3 S21 S22 S231 S232 (MAIN COMPO- FIRST SECOND FIRST SECOND NENT) CONCENT- CONCENT- COOLING COOLING BRINE THERMALLY DRY RATION RATION CRYSTALLI- CRYSTALLI- DECOMPOSE CRYSTALLI CRYSTALLI- ZATION ZATION -ZATION ZATION MgO CaSO4・2H2O KCI (MAIN COMPONENT) NaCl AND THE LIKE FIX CO2

NaCl (MAIN COMPONENT) Na2SO4・10H2O KCI AND THE LIKE CO2 S4

S2

Fig. 2

NF NF MEMBRANE- MEMBRANE- SEAWATER PERMEATED CONCENTRATED LIQUID LIQUID

Ｍｇ 2+ 140 42 98

Ｃａ 2+ 40 21 19

Ｎａ + 1070 731 339

Ｋ+ 40 27 13 - Ｃｌ 1900 1281 619

ＳＯ 42- 235 1 234

Fig. 3

NF S113 MEMBRANE- PRODUCED PERMEATED WATER LIQUID RO MEMBRANE- CONCENTRATED S115 LIQUID S114 EVAPORATION TREATMENT

TO CRYSTALLIZATION STEP S112 S111

2/2