JP7699796B2 - How to remove carbon dioxide from the air - Google Patents

Description

Application of Article 30, paragraph 2 of the Patent Act. Website of "Environmental Research Letters, Volume 15, Number 12 (Environ. Res. Lett. 15 124018)" disclosed on IOP SCIENCE, an online service of academic literature issued by IOP Publishing, the publishing division of the Institute of Physics (IOP), published on November 27, 2020 https://iopscience.iop.org/article/10.1088/1748-9326/abc217 https://iopscience.iop. org/article/10.1088/1748-9326/abc217/pdf https://iopscience. iop. org/article/10.1088/1748-9326/abc217/epub

The present invention relates to a method for removing carbon dioxide from the air. More specifically, the present invention relates to a method for removing carbon dioxide from the air, which enables carbon dioxide in the air to be converted into a useful and harmless state by mineralizing it as carbonate.

It is self-evident that carbon dioxide emitted into the atmosphere is the cause of global warming, which has become a problem in recent years. Reducing carbon dioxide emissions is a major issue in protecting the global environment.

Although carbon dioxide is naturally captured from the atmosphere through biological processes such as photosynthesis by terrestrial and marine plant life, the concentration of carbon dioxide in the atmosphere continues to rise. Carbon dioxide is also said to be a factor in the acidification of the world's oceans, which poses a threat to marine ecosystems. In response to this, various systems have been proposed for fixing and removing carbon dioxide (CO ₂ ) gas from factory exhaust gases and the atmosphere (see, for example, Patent Document 1, etc.).

Removing or reducing carbon dioxide from the atmosphere by reducing the net concentration of carbon dioxide (CO ₂ ) in the world is called negative emission technology (NET) or carbon dioxide removal (CDR), and various technologies have been provided (see, for example, non-patent literature 1 to non-patent literature 3, etc.).

The released carbon dioxide needs to be removed from the atmosphere to stabilize the Earth's climate within ranges compatible with modern civilization. Generally, the techniques described in the above documents rely on biological systems (e.g., storing carbon dioxide in soil, plants, or oceans) or highly industrialized machinery to filter the carbon dioxide from the air, liquefy it, and store it underground. Industrial processes that rely on such machinery tend to require chemicals, high temperatures, and specialized reactors.

Special Publication No. 2013-504424

Nduagu, E. , Bergerson, J. and Zevenhoven, R. , 2012. Life cycle assessment of CO2 sequencing in magnesium silicate rock A comparative study. Energy Conversion and Management, 55, pp. 116-126. Nkongwane, M. s. , Broadhurst, J. L. and Petersen, J. , 2018. Assessment of the potential carbon footprint of engineered processes fwor the mineral carbonation of PGM tailings. International Journal of Greenhouse Gas Control, 77, pp. 70-81. Keller, d. P. , Lenton, A. , Littleton, E. w. , Oschlies, A. , Scott, v. and Vaughan, N. E. , 2018. The effects of carbon dioxide removal on the carbon cycle. Current climate change reports, 4(3), pp. 250-265.

The mineralization of carbon dioxide into rocks occurs naturally over hundreds of thousands of years, but conventional methods for promoting the reaction include high temperature and pressure, or the addition of chemicals. However, these methods require a large amount of energy and result in large amounts of carbon dioxide emissions, and have not reduced greenhouse gas emissions.

Specifically, the technology disclosed in the above-mentioned non-patent document 1 has the problem that the reaction with carbon dioxide is carried out under high temperature and high pressure conditions, which reduces the actual amount of carbon dioxide reduction. In addition, the technology disclosed in non-patent document 2 requires the addition of chemicals, and due to background emissions during production (e.g., carbon dioxide emissions related to the production, transportation, final processing, and disposal of chemicals and additives), the actual amount of carbon dioxide reduction is small, just like the technology disclosed in non-patent document 1.

Furthermore, the method disclosed in Non-Patent Document 3 involves simply dispersing crushed rocks on the soil surface, but this is not desirable because it is difficult to estimate the amount of carbon dioxide mineralization and there is a possibility that it may have a negative impact on the environment.

However, the direct removal of carbon dioxide from the atmosphere is only a part of the total amount of carbon dioxide removal (CDR) of released carbon dioxide that is consistent with climate change reduction goals. The removed carbon dioxide needs to be transported to a suitable location for disposal and stored deep underground in specific geological structures. Such transportation and storage systems are too large and costly for individual companies to manufacture and operate, and there are also problems with achieving CDR that relies on underground carbon dioxide storage systems, such as various political, economic, and social risks.

The present invention has been made in view of the above, and aims to provide a method for removing carbon dioxide from the air that can be implemented at low cost, can achieve substantial carbon dioxide reduction, does not pose the risk of carbon dioxide accumulation, and is environmentally friendly.

In order to solve the above problems, the method for removing carbon dioxide from air according to the present invention comprises the steps of:
1. A method for removing carbon dioxide from air, comprising the steps of:
a microparticulation step of crushing a rock containing a compound (excluding carbonates ) containing at least one of Ca (calcium) and Mg (magnesium) to obtain fine particles of the rock;
a carbonate formation step of contacting the obtained rock particles with air in a space having a temperature of 15 to 50° C. and a relative humidity of 50 to 100% to form carbonates in the rock particles;
Including,
The amorphized portion of the rock is 30 mass% or less of the total,
The particle size of the rock particles is 1 to 100 μm ;
The rock particles are placed in a tray in a layer spread across the entire bottom surface of the tray, and the tray is placed in the space to contact the air;
The thickness of the rock particles spread in layers is 1 to 50 mm .

The method for removing carbon dioxide from the air according to the present invention is characterized in that the rock contains at least one compound selected from the group consisting of calcium oxide, magnesium oxide, calcium-iron oxide, calcium aluminate, and calcium silicate.

The method for removing carbon dioxide from the air according to the present invention is characterized in that the rock contains at least one compound selected from the group consisting of calcium oxide, magnesium oxide, calcium-iron oxide, calcium aluminate, and calcium silicate in a total amount of 5% by mass or more of the entire rock.

The method for removing carbon dioxide from the air according to the present invention is characterized in that, in the present invention described above, the rocks include at least one of ultrabasic rock, basic rock, slag, and waste concrete.

The present invention works by microparticulating rocks containing alkaline earth metals, and then bringing the resulting microparticulates into contact with air in a space with a specified environment, thereby mineralizing the carbon dioxide in the air as carbonate, thereby fixing the carbon dioxide in the rocks and removing it from the air.

Therefore, the present invention can easily remove carbon dioxide from the air at low cost without using chemicals or large-scale equipment, and since the amount of carbon dioxide generated is small, it is possible to substantially reduce carbon dioxide. In addition, the present invention is environmentally friendly because it does not require the storage of carbon dioxide and there are no risks associated with such storage, and the fixed carbon dioxide becomes carbonate, which is harmless to the environment, and can be reused together with rocks.

This is a graph (Graph 1) showing the results of measuring the change in mineralization (mineralization rate (%)) of atmospheric carbon dioxide over a 14-day period. This is a graph (Graph 2) showing the results of measuring the change in the mineralization of atmospheric carbon dioxide (mineralization rate (%)) over a period of 12 months (one year).

One embodiment of the present invention will be described below. The method of removing carbon dioxide from the air according to the present invention involves microparticulating rocks containing alkaline earth metals ((I) Microparticulation step), and then bringing the microparticulates into contact with air in a space with a specific environment, thereby converting the carbon dioxide in the air into carbonates, thereby mineralizing and fixing the carbon dioxide ((II) Carbonate formation step), thereby removing the carbon dioxide from the air.

(A) Micronization step:
The microparticulation process is a process in which rocks are crushed to obtain fine particles.

(A-1) About rocks:
First, we will explain the rocks that can be used. Both natural and artificial rocks can be used as long as they are made of the compounds listed below. In the present invention, the term "compound" is a concept that includes "minerals."

For a rock to be usable, it must be thermodynamically possible for it to react with gas-phase carbon dioxide to form carbonates under conditions of, for example, a temperature of 0-100°C, appropriate humidity (e.g., relative humidity of the reaction environment of 50-100%), and atmospheric pressure. In addition, "thermodynamically" is understood to take into account zero-dimensional calculation conditions and not meso-scale structures.

Rocks that can be used in the present invention include rocks containing compounds containing alkaline earth metals, such as so-called slag (steel slag, iron and steel slag, etc.), which is made of a substance containing mineral components of the ore parent rock separated by melting from the metal to be metallurgically produced when smelting metal from ore, and concrete that was originally used in buildings, but which is expected to be disposed of in the future, such as dismantled concrete generated by dismantling buildings that were used for buildings, and surplus concrete generated when buildings are constructed. It is preferable to use these slags and waste concrete. Carbonates do not react with carbon dioxide in the air to become further carbonates, so they are excluded from compounds containing alkaline earth metals.

Rocks contain compounds that contain alkaline earth metals. That is, compounds of alkaline earth metals such as Ca (calcium), Mg (magnesium), Be (beryllium), Sr (strontium), Ba (barium), and Ra (radium) are examples, but it is preferable to use compounds of Ca (calcium) and Mg (magnesium), which are abundant in natural rocks.

Below are examples of compounds containing alkaline earth metals. First, there are oxides of a single alkaline earth metal. For example, calcium oxide (CaO), which is an oxide of calcium, and magnesium oxide (MgO), which is an oxide of magnesium, can be used.

Other examples include barium oxide (BaO), which is an oxide of barium, and strontium oxide (SrO), which is an oxide of strontium, but it is preferable to use calcium oxide (CaO) and magnesium oxide (MgO), which is an oxide of magnesium.

Other oxides that can be _used include magnesium-iron oxide ( _MgFe2O4 , _etc. ), calcium-iron oxide ( _Ca2Fe2O5 , _etc. ), magnesium aluminate ( _magnesium alumina oxide) ( _MgAl2O4 , etc.), calcium aluminate, calcium iron _aluminate , _etc. For example, _calcium aluminates such as _{Ca3Al2O6 and Ca12Al14O33} , _{and calcium} iron aluminates such _{as Ca4Al2Fe2O10} can be used.

As the compound, for _example , calcium silicate can be used, such as _{Ca2SiO4 , Ca3Si2O7 , Ca3SiO5} , CaSiO3 _{, etc.}

As the compound, for example, magnesium silicate or magnesium aluminosilicate can be used. For example, magnesium silicate such as MgSiO ₃ or Mg ₂ SiO ₄ , or magnesium aluminosilicate such as Mg ₃ Al ₂ Si ₃ O ₁₂ can be used.

As the compound, for _example , calcium magnesium silicate can be used, _{such as CaMgSiO4 , Ca3MgSi2O8} , _{Ca7MgSi4O16 , CaMgSi2O6 , Ca2MgSi2O7} , _etc.

As the compound, _{for example} , _{calcium aluminosilicate} can be used, such _{as CaAl2Si2O8} , _Ca2Al2SiO7 , _{Ca3Al2Si3O12 , etc.}

As the compound, for example, calcium iron silicate can be used. For example, calcium iron _silicate , calcium magnesium iron _{silicate ,} magnesium iron _silicate , etc., such as CaFeSiO4, (Ca/ _Fe ) _SiO3 , (Ca/Fe _{) 2SiO4 , Ca3Fe2Si3O12} , _CaFeSi2O6 , _{Ca0.82Fe0.18SiO2} , _etc. , Ca(Fe/Mg) _Si2O6 , _etc. , can be used.

For example, calcium iron silicate can be Ca _x Fe _2-x SiO ₄ (0<x<2), and also a compound in which the calcium magnesium silicate portion is replaced with iron, or a compound in which the iron portion of calcium iron silicate is replaced with magnesium, can be used.

When applying the present invention to compounds containing alkaline earth metals, including the compounds exemplified above, there are differences in the intrinsic reactivity of the compounds (how easily or difficultly they can mineralize carbon dioxide when in contact with carbon dioxide) and in their reactivity when pulverized into fine particles.

Figures 1 and 2 show the results of measuring the change in mineralization (mineralization rate (%)) of atmospheric carbon dioxide (carbon dioxide concentration: 415 ppm) for the target compounds at a temperature of 30°C and a relative humidity of 90% after 14 days (compounds listed in Figure 1) and 12 months (one year) (compounds listed in Figure 2) when the compounds listed in Figure 1 and Figure 2 were selected from the above-mentioned compounds and crushed to a particle size of less than 3 μm. Figures 1 and 2 show the results of measuring the change in mineralization (mineralization rate (%)) of atmospheric carbon dioxide (carbon dioxide concentration: 415 ppm) after 14 days (compounds listed in Figure 1) and 12 months (one year) (compounds listed in Figure 2). Figure 1 is referred to as "Graph 1" and Figure 2 is referred to as "Graph 2".

Here, the mineralization rate (%) indicates the actual amount of mineralization relative to the _CO2 (carbon dioxide) mineralization potential of each compound, as expressed by the following formula (I) (see formula (II) below).

The _CO2 mineralization potential indicates, so to speak, the amount of mineralization that can be ideally achieved, and the mineralization rate (%), which is the ratio of the actual amount of mineralization to this potential, is an index that indicates the degree of ease of mineralization (mineralization reaction).

1 and 2, it can be confirmed that the compounds listed in FIG. 1 (CaO, MgO, Ca ₂ SiO ₄ , CaSiO ₃ ) have excellent reactivity and are mineralized in a short period of time (note that in FIG. 2, the result lines for Mg ₂ SiO ₄ and MgSiO ₃ almost overlap). In addition to FIG. 1 and FIG. 2, Table 1 shows, for selected compounds listed in Table 1 from the above-mentioned compounds, the CO ₂ mineralization potential (kg-CO ₂ /kg) and the mineralization rate (%) after one year when the particle size of the crushed compound is 1-3 μm and 5-10 μm (although it is abbreviated to "mineralization rate" in Table 1, it is the same as the mineralization rate (similar to others)).

The above-mentioned compounds (minerals) are examples, but among these, it has been confirmed that the mineralization rates of alkaline earth metal oxides calcium oxide (CaO) and magnesium oxide (MgO), calcium iron oxide, calcium aluminate, and calcium silicate are high.

Based on these results, it is preferable that the rocks used in the present invention contain these compounds, which are designated as "Group 1" in Table 1 (which also correspond to "Group 1" described below).

The results of Figures 1, 2 and Table 1 show that these compounds have a high mineralization rate (the mineralization reaction is fast), and when they are contained in rocks, they are thought to efficiently form carbonates when they come into contact with carbon dioxide in the air.

Based on the results in Table 1, the above compounds can be divided into Groups 1 to 3 as shown below. Each group is considered to have the following characteristics regarding mineralization.

Natural rocks, slag, waste concrete, etc. often contain compounds other than the above-mentioned compounds, i.e., compounds that do not contain alkaline earth metals. For example, oxides such as silicon oxide ( _SiO2 ), aluminum oxide (alumina) ( _Al2O3 ), aluminosilicates, iron _oxides ( _{FeO, Fe2O3 , Fe3O4} ), manganese oxide (MnO), iron silicates _{( FeSiO3} , _Fe2SiO4 ), carbonates, and alkali minerals (minerals). All _of these become gangue, and are essentially unnecessary compounds (ores).

Compounds that do not contain alkaline earth metals such as calcium and magnesium, such as these compounds, do not form carbonates even when crushed and contacted with carbon dioxide under specified conditions (classified as Group 4). Compounds that cannot form carbonates under specified conditions are also included in Group 4.

Based on the results in Table 1, Groups 1 to 4 are defined as follows based on the degree of mineralization reaction.

Group 1: Refers to compounds (minerals) (highly reactive compounds) that are considered to be capable of almost completing the mineralization of carbon dioxide within one year (more than 95% if crushed to a particle size of 1 to 3 μm; 60% or more if crushed to a particle size of 5 to 10 μm). For example, compounds listed in Group 1 of _Table 1 that contain at least one of the following compounds are included: calcium oxide (CaO), magnesium oxide (MgO ₎ , calcium-iron oxide ( _{Ca 2} Fe ₂ O ₅ , etc.), calcium aluminate such as Ca ₃ Al ₂ O ₆ and Ca ₁₂ Al ₁₄ O ₃₃ , calcium silicate such as Ca ₂ SiO ₄ , Ca 3 Si ₂ O ₇ , Ca 3 SiO ₅ , and CaSiO 3. Note that compounds containing alkaline earth metals as defined in the present invention, such as these compounds and compounds in the second group, include compounds that are understood to belong to these compounds in which the cation is partially replaced by another element. Specifically, compounds such as calcium silicate, magnesium silicate, and calcium magnesium silicate in which some of the calcium ions or magnesium ions are replaced with iron ions may be used.

Group 2: This refers to compounds (compounds with normal reactivity) that are thought to have a carbon dioxide mineralization rate of 10-95% within one year even when crushed to a particle size of 1-3 μm. For example, calcium iron aluminate such as Ca ₄ Al ₂ Fe ₂ O ₁₀ , magnesium silicate such as MgSiO ₃ and Mg ₂ SiO ₄ , calcium magnesium silicate such as CaMgSiO ₄ , Ca ₃ MgSi ₂ O ₈ , Ca ₂ MgSi ₂ O ₇ , Ca ₇ MgSi ₄ O ₁₆ , CaMgSi ₂ O ₆ , calcium aluminosilicate such as Ca ₂ Al ₂ SiO ₇ and CaAl ₂ Si ₂ O ₈ , oxide (magnesium aluminate) such as MgAl ₂ O ₄ , magnesium-iron oxide such as MgFe ₂ O ₄ , A-CaMgSiO ₄ , A-Ca ₂ MgSi ₂ O ₇ , A-Ca ₃ Si ₂ O ₇ and A-CaSiO ₃ ("A-" refers to a compound in an amorphous state (non-crystallized). The same applies below). Examples of the compounds include those listed in Group 2 (including 2*) in Table 1.

Group 3: This refers to compounds that are considered to have a carbon dioxide mineralization rate of less than 10% within one year even when crushed to a particle size of 1 to 3 μm (compounds with somewhat low reactivity). For example, the compounds listed in Group 3 (including 3*) in Table 1 _, including A-Ca ₃ SiO _{5 ,} A- _{CaAl 2 Si 2 O 8 ,} A-Ca _{2 SiO 4} , A-MgSiO ₃ , A-Ca ₂ Al 2 SiO 7 , A-Ca ₃ MgSi 2 O 8 , A-Ca 7 MgSi ₄ O ₁₆ , A-Ca 2 Fe ₂ O ₅ , and A-CaMgSi ₂ O ₆ , are listed.

Group 4: This refers to compounds (including gangue) that do not contain alkaline earth metals (calcium, magnesium, etc.) or are not considered to be able to form carbonates under ambient conditions (compounds that are not expected to react). Examples include silicon oxide (SiO ₂ ), aluminum oxide (alumina) (Al ₂ O ₃ ), iron oxide (FeO, Fe ₃ O ₄ , Fe ₂ O ₅ , etc.), manganese oxide (MnO), iron silicate (FeSiO ₃ , Fe ₂ SiO ₄ ), carbonates, alkali minerals, etc.

For rocks to which the method of the present invention is applicable, the ratio of the above-mentioned Groups 1 to 4 is considered to be important. Among the above-mentioned groups, Group 1 is the most reactive when considered as a single compound, and Group 4 is the least reactive when considered as a single compound (reactivity is Group 1 > Group 2 > Group 3 > Group 4).

It goes without saying that the composition of the rock used should have a high content of compounds belonging to highly reactive groups. Below, we list the range of content for each group, but as will be described later, the reactivity depends on other components of the rock (including the presence of gangue, etc., in the case of natural rocks), the size (particle size) of the fine particles obtained in the fine particle process, the carbon dioxide concentration of the air with which it comes into contact, etc., so the ranges below are merely a guideline.

Compared to compounds in other groups, we would like to expect that compounds belonging to Group 1 are present in rocks as much as possible, but in the case of natural rocks, it is often not possible to know for sure until they are mined, so it is not possible to make a generalized determination.

Taking such circumstances into consideration, the compounds belonging to Group 1 (at least one of the compounds such as calcium oxide, magnesium oxide, calcium-iron oxide, calcium aluminate, and calcium silicate) are preferably contained in a total of 5% by mass or more, more preferably 10% by mass or more, and particularly preferably 15% by mass or more, based on the entire rock. The more compounds belonging to Group 1 are present, for example, all of the rock (100% by mass) or 90% by mass or more of the entire rock, the better the mineralization performance of the rock will be, but the actual expected content is probably at most about 25% by mass, so it is considered that a content of 5 to 20% by mass would be sufficient. In the case of natural rocks, when Group 1 compounds are present, the compounds belonging to Group 1 are generally contained in a total of 5 to 20% by mass based on the entire rock. In the case of slag, waste concrete, etc., the compounds are often contained in a total of 0 to 90% by mass.

In the case of rocks containing compounds belonging to Group 1, it is desirable to avoid compounds belonging to Groups 3 and 4 (described below) as much as possible, so it is preferable for compounds belonging to Group 2 (excluding the amorphous compounds in Table 1) to contain as much of the remainder as possible (roughly 80 to 100% by mass of the remainder) excluding compounds belonging to Group 1.

On the other hand, if the rock does not contain any compounds belonging to Group 1, the compounds belonging to Group 2 preferably account for 50% or more by mass in total, more preferably 70% or more by mass, and even more preferably 70-100% by mass. In the case of natural rocks, the compounds belonging to Group 2 often account for 70-100% by mass in total (when no compounds belonging to Group 1 are present, and when they are present, the amount is roughly 0-25% by mass), while in the case of slag, waste concrete, etc., the amount is often 0-30% by mass.

It is preferable that compounds belonging to Groups 3 and 4 are as absent as possible, if possible.

When the rock is a natural rock, in the present invention, for example, magnesium oxide (MgO), calcium oxide (CaO), magnesium silicate, calcium magnesium silicate, and magnesium iron silicate can be mainly used.

Natural rocks include, for example, igneous rocks, sedimentary rocks, metamorphic rocks, etc., but it is preferable to use igneous rocks. Igneous rocks include, for example, plutonic rocks, volcanic rocks, etc., but it is preferable to use plutonic rocks.

Plutonic rocks are classified into acidic rocks (generally containing more than 66% by mass of _SiO2 ), neutral rocks (containing 52-66% by mass of _SiO2 ), basic rocks (containing 45-52% by mass of _SiO2 ), and ultrabasic rocks (containing 45% or less by mass). In the present invention, it is preferable to use ultrabasic rocks or basic rocks, and it is particularly preferable to use ultrabasic rocks.

Ultramafic rocks are also called ultramafic rocks, and generally consist of peridotite, dunite, serpentine, orthopyroxene peridotite, clinopyroxene peridotite, dipyroxene peridotite, komatiite, etc. Basic rocks are also called mafic rocks, and generally consist of gabbro, basalt, dolerite, etc.

Ultramafic rocks mainly contain magnesium silicates (magnesium silicates) such as _MgSiO3 (olivine), _Mg2SiO4 (olivine) and _CaMgSi2O6 ( _pyroxene ), and may also contain silicate minerals such as _CaAl2Si2O8 . Such magnesium silicates belong to the _above group ₂ , and the _{mineralization} reaction is not very fast.

On the other hand, ultramafic rocks contain a suitable amount (generally thought to be about 0-10% by mass, but not limited to this range) of magnesium oxide (MgO), which belongs to Group 1 and undergoes very rapid mineralization reactions, and are thought to contain a total of about 5-20% by mass of Group 1 compounds. Furthermore, ultramafic rocks contain almost no silica (SiO ₂ ), alumina silicate, etc., making them suitable natural rocks in terms of mineralization. In addition, ultramafic rocks are natural rocks that are widely available in large quantities.

Basic rocks mainly contain silicate minerals such as CaAl ₂ Si ₂ O ₈ and CaMgSi ₂ O ₆ (pyroxene), and also contain magnesium silicates (magnesium silicates) such as MgSiO ₃ (olivine) and Mg ₂ SiO ₄ (olivine).

Rocks that contain a lot of amorphous (non-crystallized) compounds (rocks with a lot of amorphous phase) are less reactive and often hinder mineralization. Rocks with a lot of amorphous phase tend to encase other crystals and entrap them in the amorphous phase, preventing the crystals from connecting with each other (which is important for the mineralization reaction to proceed).

In rocks that contain a large amount of amorphous compounds (rocks with a large amount of amorphous phase) before crushing, it is preferable that the amorphous (amorphous) portion is generally 30% or less by mass. If the ratio of the amorphous (amorphous) portion is 30% or less by mass, it is thought that even if the amorphous phase (amorphous portion) occupies the space between crystalline minerals, it is prevented from completely enveloping the crystals. Note that the above range depends on other components of the rock (including the presence of gangue, etc., in the case of natural rocks), the size (particle size) of the fine particles obtained in the fine particle process, the carbon dioxide concentration of the air in contact, etc., so the above range is merely a guideline.

(A-2) Grinding:
In the case of natural rocks, after they are mined, they are crushed into fine particles. In the fine particle process, it is important to maintain the crystalline state of each compound that makes up the rock (to prevent it from becoming non-crystallized (amorphous)).

When the compounds that make up rocks are crushed into fine particles, the smaller the particle size, the greater the surface area of the fine particles, and the greater the surface area that comes into contact with the air (carbon dioxide), making this preferable. If the fine particles of crushed rock are in a crystallized state, it is expected that the mineralization reaction will proceed, but if the particles are too fine (particle size is too small), the rock will become amorphous and its reactivity to mineralization will decrease.

Among the compounds listed above, the compounds listed in Table 2 are selected, and when they are pulverized into fine particles, the particle size (amorphous diameter) at which they become amorphous (non-crystallized) is shown in Table 2 as a guide. Table 2 also shows the "grinding efficiency" and "effective rate of mineralization" for each compound. Also, the amorphous diameters listed in Table 2 are merely a guide, and particles below this diameter will not necessarily become amorphous.

The time t required for a microparticle of radius R (m) to react and reach a depth δ is a function of the density ρ of calcium (Ca) and magnesium (Mg) in the solid, the gas phase concentration _Cg of _CO2 , and the diffusion coefficient D ( ^m2 /sec) of ions passing through the product layer, as shown in the following formula (III).

t: time (seconds)
ρ: The sum of the molar densities of magnesium (Mg) and calcium (Ca) in rock (moles/m ³ -rock)
R: radius of the particle (m)
D: _CO2 diffusion coefficient in the compound ( ^m2 /sec)
C _g : CO ₂ molar concentration in the gas phase (moles/m ³ -gas)
δ: Depth of reaction (m)

On the other hand, if the radius (particle diameter) of the fine particles is too large, it takes a very long time for all of the fine particles to react, which is also undesirable. According to the unreacted nucleus model, the reaction rate is considered to be inversely proportional to the cube of the particle diameter. For example, if a particle of 3 μm is crushed to 6 μm, the reaction is considered to be 8 (2 ³ ) times slower.

Note that the diffusion coefficient D represents the rate of _CO2 diffusion through the product layer in the unreacted core model ( ^m2 /sec) and does not directly indicate the rate of carbon dioxide mineralization due to other factors. On the other hand, since diffusion through the product layer is the rate-limiting step in gas-solid _CO2 mineralization, a large diffusion coefficient D actually indicates a mineral that rapidly mineralizes carbon dioxide.

If the outer diameter (particle size) of the microparticles is such that there are many compounds belonging to Group 1 as described above relative to the total amount of the obtained rock microparticles, it is believed that the reaction will proceed without the rock being crushed into smaller pieces than necessary, which will suppress energy loss or the generation of carbon dioxide due to crushing, and will also suppress the amorphization of the rock (compounds).

On the other hand, the reactivity of the entire rock is influenced by the type of other components of the rock (including the presence of gangue, etc., in the case of natural rocks), the presence and content of amorphous phases, the formation position and relationship of crystalline layers and amorphous phases in the rock composition, and other factors such as the amount of carbonation (hereinafter sometimes referred to as "rock elements"). The particle size range of the fine particles obtained by crushing such rocks is also influenced by these factors.

In view of these, when microparticulating rocks, as a guideline, the particle size of the microparticles is preferably approximately 100 μm or less (more preferably 50 μm or less, even more preferably 25 μm or less, and especially preferably 10 μm or less), and the particle size may be appropriately selected, for example, within the range of 1-25 μm, 1-20 μm, 1-10 μm, 1-5 μm, 1-3 μm, etc., taking into consideration the rock factors. For example, when the highly reactive first group compound is contained, the particle size should be 1-25 μm (preferably 1-10 μm).

If the particle size is 1-3 μm, the surface area is large and good reactivity can be expected, but as shown in Table 2, if the particle size is too small, amorphization progresses and reactivity may worsen, so it is preferable to aim for 1-3 μm and keep the particle size below about 1-10 μm. Regarding the particle size, for example, crushed rock particles can be spread out so that they do not overlap, and images taken with an optical microscope or the like can be analyzed by a computer to determine the particle diameter (particle size) and shape of the complete distribution of the particles (it is preferable to use the maximum particle size). It is also desirable to analyze at least 5,000 (or 10,000) particles for each compound sample.

The rocks can be crushed to produce fine particles using conventional crushing means, but it is preferable to use a vertical roller mill or high pressure grinding rolls. Ball mills are generally considered to have low energy efficiency and a tendency to cause amorphous formation.

In a simplified process of atomization, the rock can be placed in a mortar and crushed to the desired particle size using a mortar and pestle. When crushing rock using a mortar and pestle, for example, it is possible to achieve a very low degree of amorphization by separating very small particles that are expected to be amorphized when they are generated and performing a minimal amount of crushing to achieve the desired particle size.

When crushing rocks, three things must be considered: preventing amorphization, narrowing the particle distribution (the distribution of particle sizes), and minimizing energy consumption (energy consumption generates carbon dioxide, which reduces the actual amount of carbon dioxide reduction). These goals can be maintained, for example, by using high-pressure crushing and separating the particles that have become too small while grinding (which are considered to be amorphized. Separation can also reduce the load on the crusher (improve energy efficiency)). On the other hand, low-pressure crushing (low-pressure grinding) is preferable for crushing with a vertical roller mill, which is considered to be one form of high-pressure crushing means, because the supplied energy leads to rearrangement of the crystal structure (amorphization) and the generation of heat and sound (energy loss).

It is also preferable to narrow the particle distribution as much as possible when crushing rocks. By narrowing the particle distribution, the air that comes into contact with the rocks to mineralize them is diffused into the fine particle layer.

Such diffusion is an important factor in the mineralization of carbon dioxide; as the particle distribution becomes wider, the porosity between the particles decreases, and only the upper part of the crushed layer comes into contact with the carbon dioxide, which is considered to be inefficient. Less diffusion means that the rock needs to be spread out into a thinner layer (thinner rock layer). Thinner rock layers are undesirable because they reduce the amount of rock that can be processed per unit volume of equipment and at the same time increase the number of sheets, etc. per unit rock, which increases costs.

Classification may be performed using a commercially available air classifier or a sieve, or a pulverizer equipped with a classification function. Particle size distribution may be determined using conventional techniques (e.g., measuring the fine particles after classification using an optical microscope equipped with computer vision operated by commercially available software, etc.).

For example, as shown in the CaMgSiO4 in Table ₁ , a compound that has become amorphous as a result of being pulverized may mineralize _CO2 faster than a non-amorphous compound (crystalline compound). In this case, the structure of the amorphous compound changes depending on the ratio of cations and silica, so that in a specific composition such as _CaMgSiO4 , even if it has an amorphous structure, it reacts well with carbon dioxide and mineralization proceeds.

On the other hand, such cases are the exception and, from an engineering point of view, one never finds pure minerals, there is always some degree of mixing and compositional variation. Thus, the significant reduction in carbon dioxide mineralization associated with most amorphous compounds is outweighed by the slight performance improvement of some compounds. In other words, it is preferable to avoid amorphous compounds, as their reaction rates with carbon dioxide, and therefore mineralization rates, are likely to be very low.

In addition, with regard to the rock microparticles obtained by carrying out the microparticulation process, it is preferable that the above-mentioned Group 4 (compounds that are not expected to react) that are originally contained in the rock microparticles due to the amorphization of the rock, and those that become amorphous as a result of the microparticulation, are not present in the rock microparticles as much as possible, since they are not expected to undergo a mineralization reaction. It is preferable that these compounds are present in total at approximately 80 mass% or less, and it is particularly preferable that they are present at 50 mass% or less.

(B) Carbonate Formation Step:
The resulting rock particles come into contact with air in a specified space, forming carbonates in the particles. That is, carbon dioxide in the air reacts with the rock particles of the above composition, forming carbonates (mainly carbonates of calcium, magnesium, etc.) on the surface or inside of the particles, which are then fixed through mineralization, and as a result, carbon dioxide in the air is removed.

If air can be constantly brought into contact with the rock particles in a stable state (such as by providing a steady supply of air containing carbon dioxide and maintaining the conditions described below for the duration of the experiment), there is no problem if the space is outdoors, but it is preferable to have the space indoors, as this stable state can be easily and reliably created. In addition, it is preferable for the space to be an enclosed space in which air can be supplied at a certain frequency by ventilation, etc.

The air to be introduced refers to air that contains carbon dioxide in general, and can be any air (gas) that is thought to contain carbon dioxide, such as the atmosphere (which is thought to have a carbon dioxide content of approximately 400 to 450 ppm), exhaust gas, etc. There are no particular restrictions on the carbon dioxide concentration of the target air.

The space is set to a temperature of 15-50°C and a relative humidity of 50-100%. By setting both the temperature and humidity conditions within the above ranges, the mineralization of carbon dioxide will be carried out efficiently. The temperature in the space is preferably 20-40°C, and more preferably 30-35°C. The relative humidity is preferably 60-95%.

The flow rate of the air containing carbon dioxide in the space can be appropriately determined depending on the concentration of carbon dioxide, the type and composition of the rock, etc., but for example, if natural ultramafic rock (ultramafic rock) is used as the rock and the air from which carbon dioxide is to be removed is atmospheric air (with the carbon dioxide content being about the level described above), it is preferable to set the flow rate to about 1 to 100 mm/sec, and 1 to 10 mm/sec is particularly preferable. Since the fine rock particles have a small particle size and are pulverized, it is preferable to set the flow rate to the level described above in order to prevent the fine particles from scattering, etc.

There are no particular restrictions on the contact between the rock particles and the air in the space, but for example, the particles can be placed in a shallow container such as a tray of any area, spread in a layer over the entire bottom surface of the tray, and then the tray or the like can be placed in the space.

When rock particles are layered in this way, it is necessary to take into account the diffusion of carbon dioxide into the deepest part of the layer. When rocks are crushed to the micrometer level, it is thought that the diffusion of carbon dioxide in the gaps between the particles is due to molecular diffusion. Therefore, if fine-grained rocks are piled up thickly, there is a concern that the concentration of carbon dioxide will decrease in the lower part (deep part) of the layered rock particles, slowing down the mineralization reaction. On the other hand, the extent of this deep part depends on the type of rock (compound), the size of the space, the particle size of the particles, the air flow speed, etc.

In view of the above, the thickness of the rock particles (particle layer) when spread in layers is, for example, preferably 5 cm (50 mm) or less, more preferably 1 to 50 mm, more preferably 1 to 20 mm, even more preferably 1 to 10 mm, and particularly preferably 1 to 5 mm.

When particles are placed in trays or the like in this manner and are exposed to air in a layered state, the trays or the like may be placed in multiple tiers in the height direction inside the space.

The contact time between the air and the rock particles (which is also the reaction time) can be determined appropriately depending on the concentration of carbon dioxide in the air and the type and amount of rock used, but as a guide it can be set to 7 days to 1 year, and can be arbitrarily determined, for example, between 7 days and 2 months. Although it depends on the amount and type of rock, the mineralization reaction is generally considered to be complete after 1 year has passed since the start of contact (the start of the mineralization reaction).

After carbonates are formed in the rock particles and carbon dioxide is mineralized, carbonates are deposited on the surface, forming a mass of rock particles in which many particles have aggregated together. First, the mass is heated at less than 100°C to remove the adsorbed moisture. Note that this operation and the analytical operation described below are not particularly necessary if the carbonates formed are not to be analyzed.

After removing the moisture, the amount of mineralized carbon dioxide can be confirmed by comparing the mass of the rock particles with that before the carbonate formation process as an analytical procedure and confirming the change in mass. In addition, the amount of carbon dioxide present can be analyzed in detail using thermogravimetric differential thermal analysis (TG-DTA) and X-ray fluorescence analysis (XRF) to determine the mineralized carbon dioxide (carbonate).

In addition, the mineralized rocks in which environmentally harmless carbonates (mainly carbonates of calcium, magnesium, etc. (calcium carbonate ( _CaCO3 , _MgCO3 , etc.)) are formed can be used, for example, as roadbed materials and construction materials for landfill. In this way, the mineralized rocks are environmentally friendly.

(C) Effects of the present invention:
The present invention described above works by microparticulating rocks containing alkaline earth metals, and then contacting the resulting microparticulates with air in a space with a specified environment, thereby mineralizing carbon dioxide in the air as carbonate, thereby fixing the carbon dioxide in the rocks and removing carbon dioxide from the air.

Therefore, the present invention can easily remove carbon dioxide from the air at low cost without using chemicals or large-scale equipment, and since the amount of carbon dioxide generated is small, it is possible to substantially reduce carbon dioxide. In addition, the present invention is environmentally friendly because it does not require the storage of carbon dioxide and there are no risks associated with such storage, and the fixed carbon dioxide becomes carbonate, which is harmless to the environment, and can be reused together with rocks.

According to the present invention, CDR can be carried out without relying on other technologies or infrastructure. Previous technologies required a large amount of energy to remove carbon dioxide from the air, and many of these technologies did not result in a substantial reduction in carbon dioxide emissions when the carbon dioxide generated by such energy was taken into consideration. On the other hand, the present invention removes carbon dioxide from the air using a simple method consisting of a rock microparticulation process and a carbonate formation process in which the resulting rock microparticles react with carbon dioxide in the air to form carbonates, so it requires less energy and is considered to be a technology that leads to a substantial reduction in carbon dioxide emissions.

There are currently two main technologies for CDR: Bioenergy with Carbon Capture and Storage ( _BECCS ) and Direct Air Capture with Carbon _Capture (DACS).

Of these, BECCS relies on burning biomass, capturing the carbon dioxide, transporting it to a storage site, and storing it underground. DACCS also uses machinery to filter the carbon dioxide from the air before transport and underground storage. The main points of comparison between these are considered to be energy requirements (GJ/t-CO ₂ ), land requirements (km ² /Mt-CO ₂ ), water requirements (m ³ /t-CO ₂ ), time required for CO ₂ capture, biological limitations (Gt-CO ₂ /year), and cost. In all these cases, t-CO ₂ refers to the net carbon dioxide (CO ₂ ) removed from the environment (taking into account the carbon dioxide emissions from the process and supply chain). A table comparing the present invention and these two technologies is shown in Table 3. From Table 3, it can be seen that the present invention works favorably compared to other technologies.

Furthermore, there is the issue of scale-up and scale-out rates. BECCS and DACCS are usually limited to very large scale (>1 Mt- _CO2 /year) due to the need for storage and transportation infrastructure, and the significant costs involved in developing storage sites. The present invention, on the other hand, can be applied at any scale, be it kg scale or Mt scale. It is therefore conceivable that small, medium and large scale systems can be developed without worrying about transportation, storage etc.

Furthermore, with other methods that use soil or the ocean, it is impossible to accurately quantify the amount of carbon dioxide removed, and in addition, these other methods have the problem that expected carbon dioxide may be lost in the future due to the effects of climate change. The present invention solves these problems by fixing carbon dioxide as carbonates and removing it from the air using the simple means described above.

(D) Modifications of the embodiment:
The above-described aspects show one aspect of the present invention, and the present invention is not limited to the above-described embodiments, and it goes without saying that modifications and improvements that have the configuration of the present invention and can achieve the object and effect are included in the contents of the present invention. Furthermore, the specific structure and shape, etc., when implementing the present invention may be other structures and shapes, etc., as long as the object and effect of the present invention can be achieved. The present invention is not limited to the above-described embodiments, and modifications and improvements that can achieve the object of the present invention are included in the present invention.

For example, the necessary operations for the (A) microparticulation step and (B) carbonate formation step are partially exemplified, but there is no problem if other operations not exemplified are used within the scope of not impeding the purpose and effect of the present invention. Similarly, although examples of compounds capable of constituting rocks are given, they are merely examples, and there is no problem if the rocks used in the present invention contain other compounds such as compounds containing alkaline earth metals.
In addition, the specific structure and shape when implementing the present invention may be other structures and shapes within the scope of the invention in which the object of the present invention can be achieved.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these.

( Reference Example 1, Examples 2 to 6, Reference Example 7, Example 8)
Eight types of slag (Slag 1 to Slag 8 (referred to as Reference Example 1, Examples 2 to 6, Reference Example 7, and Example 8 )) with the compositions shown in Table 4 below (analysis methods for compound compositions, etc. will be described later) were prepared as equivalents to rocks. Note that "%" indicates the content (mass%) relative to the total.

(Slag Composition)

Slags 1 to 8 were crushed using a mortar and pestle, and the crushed slags were classified using a sieve. The particle size distribution of the fine particles in the classified slags was measured using an optical microscope equipped with computer vision operated by "ImageJ" software, an image processing method, to obtain fine particle samples with particle sizes of 25 to 90 μm and fine particle samples with particle sizes of less than 25 μm. Furthermore, the elemental composition of Slags 1 to 8 was analyzed by XRF, and the compound composition of Slags 1 to 8 was analyzed by Q-XRD (compound compositions, etc. are listed in Table 4).

The resulting microparticle samples were each placed in a circular tray measuring 10 cm in diameter and 1 cm in height, and spread out to a thickness of 3 mm.

The indoor space of a 22L incubator (3-6815-01 _CO2 incubator: manufactured by AS ONE Co., Ltd.) was set to a relative humidity of 90% and a temperature of 30°C. Under such humidity and temperature, a tray containing the fine particle samples was placed in the indoor space of the incubator, the door was closed to create a sealed state, and then air with a carbon dioxide concentration of 5% (slag 1 to slag 5) and 20% (slag 5 to slag 8) were introduced. Then, while maintaining this state, the fine particle samples were brought into contact with air, and carbon dioxide was mineralized (formed carbonate). The above operation was carried out for each sample on the elapsed days of the test of 1 day, 3 days, 7 days, 14 days, 21 days, and 28 days (blanks indicate not carried out). Each sample was carried out three times (n=3) (average values are shown in Table 5).

After the number of days had passed, the mass of the fine particle samples was measured, and the mineralized carbon dioxide (carbonate) was analyzed using thermogravimetric differential thermal analysis (TG-DTA) and X-ray fluorescence analysis (XRF). The mineralization rate (%) of carbon dioxide corresponding to the number of days passed is shown in Tables 5 and 6 (the experimental values are in Table 5, but the predicted values are listed in Table 6 for reference). The evaluation was performed using two carbon dioxide concentrations in order to confirm the unreacted core model.

From the results in Table 5, it can first be confirmed that the mineralization rates of Slag 1 and Slag 7 are very low (for example, less than 1% under the carbon dioxide concentration conditions used (28 days have passed for Slag 1, and 7 days for Slag 7)). The reason for this is believed to be that Slag 1 and Slag 7 both originally had a high content of amorphous phase (non-crystallized portion; same below) (34.1% for Slag 1, 100% for Slag 7). As all of Slag 7 (100%) is amorphous phase, it is believed to have a low mineralization rate.

On the other hand, in the case of Slag 1, the amorphous phase accounts for 34.1%. However, because the amorphous phase still accounts for a high proportion of the total, the crystals were caught up in the amorphous phase, and the crystals were not connected to each other. Only the crystals on the surface reacted, which is thought to be why the mineralization rate was low.

Generally, when molten rock (natural rock or slag) cools, it forms crystalline or amorphous compounds, but rapid cooling favors the formation of amorphous compounds, while slow cooling favors the formation of crystals. The distribution of nucleated crystals in molten rock is random, and these crystals will grow until they come into contact with other existing crystals. On the other hand, if the liquid phase between the crystals solidifies as an amorphous phase before they can grow together, the crystals will be separated from each other and will not be able to connect. This arrangement is thought to hinder the progress of carbon dioxide mineralization.

Slags other than Slag 1 and Slag 7 (Slag 2 to Slag 6, Slag 8) also contained a suitable amount of amorphous phase, but did not exhibit the above-mentioned behavior, and the results in Table 5 confirmed that mineralization had progressed well (for example, the mineralization rate after 7 days under the carbon dioxide concentration conditions used was 5% or more). This is thought to be because the amorphous phase occupied the space between the crystalline minerals, but did not completely encase the crystals.

Additionally, slags containing compounds such as calcium oxide (oxide) belonging to group 1 (Slag 2, Slag 4, Slag 6 and Slag 8) show early mineralization of carbon dioxide. In particular, the rapid mineralization of Slag 2 and Slag 4 is believed to be due to the calcium content being mainly in the form of oxides ( _{CaO and Ca2Fe2O5} ) and calcium silicates (both belonging to group 1).

On the other hand, the mineralization rates of Slag 3 and Slag 5 are relatively low compared to Slag 3 and Slag 5 described above, which is thought to be due to the presence of a relatively large amount of "mixed cation" silicates (where a certain amount of Ca has been replaced by Mg or Fe) belonging to Group 2, as well as the presence of compounds belonging to Group 4.

The present invention can be advantageously used as a means of providing a technology that makes it possible to fix carbon dioxide in the air as carbonate in rocks and reduce carbon dioxide in the air, and has high industrial applicability.

Claims (4)

1. A method for removing carbon dioxide from air, comprising the steps of:
a microparticulation step of crushing a rock containing a compound (excluding carbonates ) containing at least one of Ca (calcium) and Mg (magnesium) to obtain fine particles of the rock;
a carbonate formation step of contacting the obtained rock particles with air in a space having a temperature of 15 to 50° C. and a relative humidity of 50 to 100% to form carbonates in the rock particles;
Including,
The amorphized portion of the rock is 30 mass% or less of the total,
The particle size of the rock particles is 1 to 100 μm ;
The rock particles are placed in a tray in a layer spread across the entire bottom surface of the tray, and the tray is placed in the space to contact the air;
A method for removing carbon dioxide from the air, characterized in that the thickness of the rock particles spread in layers is 1 to 50 mm . The method for removing carbon dioxide from the air according to claim 1, characterized in that the rock contains at least one compound selected from the group consisting of calcium oxide, magnesium oxide, calcium-iron oxide, calcium aluminate, and calcium silicate. The method for removing carbon dioxide from the air according to claim 1 or 2, characterized in that the rock contains at least one compound selected from the group consisting of calcium oxide, magnesium oxide, calcium-iron oxide, calcium aluminate, and calcium silicate in a total amount of 5% by mass or more of the entire rock. The method for removing carbon dioxide from the air according to any one of claims 1 to 3, characterized in that the rocks include at least one of ultrabasic rock, basic rock, slag, and waste concrete.

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