Method for removing carbon dioxide in air

Abstract

To provide a method which can be executed at low cost, can substantially reduce carbon dioxide, and removes carbon dioxide in the air friendly to an environment with no risk of storing the carbon dioxide.SOLUTION: The present invention atomizes a rock containing an alkaline earth metal, immobilizes carbon dioxide to the rock by bringing acquired fine particles into contact with the air in a space as prescribed environment to mineralize carbon dioxide in the air as carbonate, and removes the carbon dioxide in the air. The invention can easily remove the carbon dioxide in the air at low cost without using chemical, and can remove qualitative carbon dioxide with little production amount of accompanying carbon dioxide. In addition, the immobilized carbon dioxide becomes carbonate harmless to environment and friendly to the environment with no risk of storing the carbon dioxide.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for removing carbon dioxide in the air. More specifically, it relates to a method for removing carbon dioxide in the air that enables conversion into a useful and harmless state by mineralizing carbon dioxide in the atmosphere as a carbonate.

Background Art

[0002] It is obvious that carbon dioxide emitted into the atmosphere is the cause of global warming, which has been a problem in recent years. Reducing the emissions of such carbon dioxide has become a major issue in protecting the global environment.

[0003] 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. Also, carbon dioxide is said to be a factor in the acidification of the world's oceans, which poses a threat to the marine ecosystem. In contrast, various systems have been proposed for immobilizing and removing flue gas from factories and carbon dioxide (CO2) gas in the atmosphere (see, for example, Patent Document 1, etc.).

[0004] Net reduction of the world's carbon dioxide (CO2) concentration to remove or reduce carbon dioxide in the atmosphere is called negative emission technology (NET) or carbon dioxide removal (CDR), and various technologies are provided (see, for example, Non-Patent Documents 1 to 3, etc.).

[0005] The carbon dioxide released needs to be removed from the atmosphere in order to stabilize the Earth's climate within the scope compatible with modern civilization. Also, generally, the technologies described in the above-mentioned literature rely on biological systems (such as carbon dioxide storage in soil, plants, or the ocean, etc.) or highly industrialized machinery that filters carbon dioxide from the air, liquefies it, and stores it underground. And in industrial processes that rely on such machinery, etc., chemicals, high temperatures, and special reactors tend to be required.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Non-Patent Documents

[0007]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Disclosure of the Invention

Problems to be Solved by the Invention

[0008] The mineralization of carbon dioxide in rocks occurs naturally over hundreds of thousands of years. Conventional methods for accelerating the reaction include those involving high temperature and high pressure, or the addition of chemicals. However, such methods have a large energy load and associated carbon dioxide emissions, and thus do not contribute to the reduction of greenhouse gases.

[0009] Specifically, in the technology disclosed in Non-Patent Document 1 mentioned above, since the reaction is carried out under high temperature and high pressure conditions, there is a problem that the substantial reduction amount of carbon dioxide is reduced. In addition, the technology disclosed in Non-Patent Document 2 requires the addition of chemicals, and due to its background emissions during production (for example, carbon dioxide emissions related to the preparation, transportation, end-of-life treatment, and disposal of chemicals and additives), similar to the technology disclosed in Non-Patent Document 1, the substantial reduction amount of carbon dioxide is small.

[0010] Furthermore, the method disclosed in Non-Patent Document 3 is a method of simply dispersing crushed rocks on the soil surface. However, it is difficult to estimate the amount of carbon dioxide mineralization, and it may have an adverse impact on the environment, so it is not preferable.

[0011] Note that direct removal of carbon dioxide from the atmosphere accounts for only a part of the total amount of removed carbon dioxide (CDR) that meets the climate change mitigation target. The removed carbon dioxide needs to be transported to an appropriate location for disposal and stored deep underground in specific geological structures. Such a transportation and storage system is too large-scale and costly for individual companies to manufacture and operate, and there are also problems such as various risks in politics, economy, society, etc. arising in achieving CDR that depends on a system for storing carbon dioxide underground.

[0012] The present invention has been made in view of the above, and aims to provide a method for removing carbon dioxide in the air that can be implemented at low cost, can achieve substantial reduction of carbon dioxide, has no risk of carbon dioxide storage, and is environmentally friendly.

Means for Solving the Problems

[0013] In order to solve the above problems, a method for removing carbon dioxide in the air according to the present invention is a method for removing carbon dioxide in the air, comprising At least one of Ca (calcium) or Mg (magnesium) a particle formation step of pulverizing a rock containing a compound (excluding carbonate) to obtain fine particles of the rock; a carbonate formation step of bringing the obtained fine particles of the rock into contact with air in a space where the temperature is 15 to 50 °C and the relative humidity is 50 to 100% to form a carbonate on the fine particles of the rock; including the non-crystallized portion of the rock is 30% by mass or less of the whole; the particle size of the fine particles of the rock is 1 to 100 μm and the fine particles of the rock are placed in the tray in a state of being spread in layers over the entire bottom surface of the tray, and the tray is placed in the space to be brought into contact with the air; the thickness of the fine particles of the rock spread in layers is 1 to 50 mm characterized in that.

[0014] The method for removing carbon dioxide in the air according to the present invention is, in the present invention described above, characterized in that the rock contains at least one compound selected from the group consisting of calcium oxide, magnesium oxide, calcium ferrite, calcium aluminate and calcium silicate.

[0015] The method for removing carbon dioxide in the air according to the present invention is, in the present invention described above, characterized in that the rock contains at least one compound selected from the group consisting of calcium oxide, magnesium oxide, calcium ferrite, calcium aluminate and calcium silicate in a total amount of 5% by mass or more of the whole rock.

[0017] The method for removing carbon dioxide in the air according to the present invention is, in the present invention described above, characterized in that the rock contains at least one of ultrabasic rock, basic rock, slag and waste concrete.

Advantages of the Invention

[0020] The present invention atomizes a rock containing an alkaline earth metal, and brings the obtained fine particles into contact with air in a space with a predetermined environment, thereby mineralizing carbon dioxide in the air as a carbonate, fixing the carbon dioxide to the rock, and removing carbon dioxide in the air.

[0021] Therefore, the present invention can easily remove carbon dioxide in the air at low cost without using chemicals or large-scale equipment, etc., and the amount of carbon dioxide generated is also small, so substantial reduction of carbon dioxide can be achieved. In addition, the present invention does not require storage of carbon dioxide, there is no risk in such storage, and the fixed carbon dioxide becomes a carbonate harmless to the environment and can be reused together with the rock, so it is also environmentally friendly.

Brief Description of the Drawings

[0022]

Figure 1

Figure 2

Mode for Carrying Out the Invention

[0023] Hereinafter, one aspect of the present invention will be described. The method for removing carbon dioxide in the air according to the present invention involves pulverizing a rock containing an alkaline earth metal ((I) pulverization step), and bringing such fine particles into contact with air in a space with a specific environment, thereby mineralizing and fixing carbon dioxide in the air as a carbonate ((II) carbonate formation step), so as to remove such carbon dioxide from the air.

[0024] (A) Pulverization step: The pulverization step is a step of pulverizing a rock to obtain fine particles.

[0025] (A-1) Regarding the rock: First, the usable rock will be described. Any natural rock or artificial rock can be used as long as the rock is formed from the compounds etc. exemplified below. In the present invention, the concept of "compound" includes "mineral".

[0026] The usable rock needs to be thermodynamically capable of reacting with gaseous carbon dioxide to form a carbonate under an environment of a temperature of 0 to 100°C, appropriate moisture (for example, the relative humidity of the reaction environment is 50 to 100%, etc.), and atmospheric pressure. Also, "thermodynamically" is understood to not consider the meso-scale structure but consider the 0-dimensional calculation conditions.

[0027] As rocks that can be used in the present invention, as rocks containing a compound containing an alkaline earth metal, substances containing mineral components of ore parent rock separated by melting from the metal to be metallurgically treated, such as so-called slag (steel slag, iron-making slag, etc.) when smelting metal from ore, or concrete that was originally used for buildings, etc., and this building itself becomes unnecessary, or demolition concrete generated by demolishing the building where the building was used, or surplus concrete itself generated when building a building, etc., and waste concrete (concrete waste) such as concrete that is assumed to be discarded in the future is also included, and it is preferable to use these slags and waste concrete. Note that since carbonates do not react with carbon dioxide in the air to form further carbonates, they are excluded from compounds containing alkaline earth metals.

[0028] The rock contains a compound containing an alkaline earth metal. That is, compounds of alkaline earth metals such as Ca (calcium), Mg (magnesium), Be (beryllium), Sr (strontium), Ba (barium), Ra (radium), etc. can be mentioned, but it is preferable to adopt compounds of Ca (calcium) and Mg (magnesium) that are abundantly contained in natural rocks.

[0029] Hereinafter, examples of compounds containing alkaline earth metals will be given. First, oxides of single alkaline earth metals can be mentioned. For example, calcium oxide (CaO) which is an oxide of calcium, magnesium oxide (MgO) which is an oxide of magnesium, etc. can be used.

[0030] In addition, barium oxide (BaO) which is an oxide with barium, strontium oxide (SrO) which is an oxide with strontium, etc. can be mentioned, but it is preferable to use calcium oxide (CaO), magnesium oxide (MgO) which is an oxide of magnesium, etc.

[0031] As oxides, other oxides such as magnesium iron oxide (MgFe2O4, etc.), calcium iron oxide (Ca2Fe2O5, etc.), magnesium aluminate (aluminum oxide magnesium) (MgAl2O4, etc.), and oxides such as calcium aluminate and calcium ferroaluminate can also be used. For example, calcium aluminates such as Ca3Al2O6, Ca 12 Al 14 O 33 and calcium ferroaluminates such as Ca4Al2Fe2O 10 etc. can be used.

[0032] As compounds, for example, calcium silicate can be used. For example, calcium silicates such as Ca2SiO4, Ca3Si2O7, Ca3SiO5, and CaSiO3 can be used.

[0033] As compounds, for example, magnesium silicate and magnesium aluminosilicate can be used. For example, magnesium silicates such as MgSiO3 and Mg2SiO4, and magnesium aluminosilicates such as Mg3Al2Si3O 12 etc. can be used.

[0034] As compounds, for example, calcium magnesium silicate can be used. For example, calcium magnesium silicates such as CaMgSiO4, Ca3MgSi2O8, Ca7MgSi4O 16 , CaMgSi2O6, and Ca2MgSi2O7 can be used.

[0035] As compounds, for example, calcium aluminosilicate can be used. For example, calcium aluminosilicates such as CaAl2Si2O8, Ca2Al2SiO7, and Ca3Al2Si3O 12 etc. can be used.

[0036] As compounds, for example, calcium iron silicate can be used. For example, calcium iron silicates such as CaFeSiO4, (Ca / Fe)SiO3, (Ca / Fe)2SiO4, and Ca3Fe2Si3O12 , CaFeSi2O6, Ca 0.82 Fe 0.18 etc., such as SiO2, calcium iron silicate such as Ca(Fe / Mg)Si2O6, calcium magnesium iron silicate, and magnesium iron silicate can be used.

[0037] Note that, for example, calcium iron silicate can be Ca X Fe 2-X SiO4 (0 < X < 2), and also compounds in which the above-mentioned calcium magnesium silicate part is replaced by iron, or compounds in which the iron part of the above-mentioned calcium iron silicate is replaced by magnesium, etc. can also be applied.

[0038] Regarding compounds containing alkaline earth metals, including the compounds exemplified above, when applying to the present invention, it is considered that there are differences in the original reactivity of the compounds (the ease or difficulty of mineralizing carbon dioxide by contacting with carbon dioxide), and differences in reactivity when pulverized into fine particles.

[0039] Figures 1 and 2 show, among the above-mentioned compounds, for the target compounds when the compounds shown in Figures 1 and 2 are selected, pulverized so that the particle size is less than 3 μm, the temperature is 30 °C, and the relative humidity is 90%, the change in the mineralization effect (mineralization rate (%)) of carbon dioxide in the atmosphere (carbon dioxide concentration: 415 ppm) is shown with the number of elapsed days being 14 days (compounds listed in Figure 1) and the number of elapsed months being 12 months (1 year) (compounds listed in Figure 2). Note that Figure 1 is referred to as "Graph 1" and Figure 2 is referred to as "Graph 2".

[0040] Here, the mineralization rate (%) indicates the actual mineralization amount with respect to the CO2 (carbon dioxide) mineralization amount potential represented by the following formula (I) possessed by each compound (see the following formula (II)).

[0041] The CO2 mineralization potential indicates the mineralization amount that can ideally be achieved. The mineralization rate (%), which is the ratio of the actual mineralization amount to such potential, is an index indicating the degree of ease of mineralization (mineralization reaction).

[0042]

Number

[0043]

Number

[0044] From FIGS. 1 and 2, it can be confirmed that the compounds (CaO, MgO, Ca2SiO4, CaSiO3) listed in FIG. 1 have excellent reactivity and can be mineralized in a short period (note that in FIG. 2, the lines of the results of Mg2SiO4 and MgSiO3 almost overlap). In addition to FIGS. 1 and 2, Table 1 shows the CO2 mineralization potential (kg-CO2 / kg) and the mineralization rate (%) after one year when the particle size of the pulverized compound is 1 to 3 μm and 5 to 10 μm, respectively, for the compounds selected from the above-mentioned compounds and listed in Table 1 (abbreviated as "mineralization rate" in Table 1, which is the same as the mineralization rate (the same applies to others)).

[0045]

Table 1

[0046] The above-mentioned compounds (minerals) are examples. Among them, it can be confirmed that calcium oxide (CaO), magnesium oxide (MgO), which are oxides of alkaline earth metals, and calcium ferrite, calcium aluminate, and calcium silicate have high mineralization rates.

[0047] From this result, it is preferable that the rock used in the present invention contains these compounds, which are classified as "Group 1" in Table 1 (also corresponding to "Group 1" described later).

[0048] From the results in Fig. 1, Fig. 2 and Table 1, it is considered that these compounds have a high mineralization rate (the reaction of mineralization is fast), and when contained in rocks, they can efficiently form carbonates by contacting with carbon dioxide in the air.

[0049] And from the results such as Table 1, the above-mentioned compounds can be divided into the following Group 1 to Group 3. Each group is considered to have the following characteristics regarding mineralization.

[0050] Natural rocks, slag, waste concrete, etc. often contain compounds other than the above-mentioned compounds, that is, compounds not containing alkaline earth metals. For example, compounds (minerals) such as silicon oxide (SiO2), aluminum oxide (alumina) (Al2O3), aluminosilicate, iron oxide (FeO, Fe2O3, Fe3O4), manganese oxide (MnO), iron silicate (FeSiO3, Fe2SiO4), carbonate, and alkali minerals. All of these become gangue and are originally unnecessary compounds (ores).

[0051] Compounds that do not contain alkaline earth metals such as calcium and magnesium, like these compounds, do not form carbonates even when pulverized and contacted with carbon dioxide under predetermined conditions (designated as Group 4). Also, compounds that cannot form carbonates under predetermined conditions are included in this Group 4.

[0052] From the results in Table 1, etc., Group 1 to Group 4 are defined as follows in terms of the degree of mineralization reaction.

[0053] Group 1: Refers to compounds (minerals) (compounds with high reactivity) that are considered to be able to generally complete the mineralization of carbon dioxide within 1 year (exceeding 95%) when pulverized to a particle size of 1 - 3 μm. When pulverized to a particle size of 5 - 10 μm, it is 60% or more. For example, oxides such as calcium oxide (CaO), magnesium oxide (MgO), calcium - iron oxide (Ca2Fe2O5, etc.), Ca3Al2O6, Ca 12 Al 14 O33 Compounds such as calcium aluminate, calcium silicates such as Ca2SiO4, Ca3Si2O7, Ca3SiO5, CaSiO3, etc., and compounds listed in Group 1 of Table 1 containing at least one of these compounds are included. Note that these compounds, compounds of Group 2, etc., compounds containing alkaline earth metals as referred to in the present invention include compounds that are understood to belong to these compounds in which cations are partially substituted by other elements. Specifically, calcium silicate, magnesium silicate, calcium magnesium silicate, etc., in which part of calcium ions or magnesium ions are replaced by iron ions may also be used as compounds.

[0054] Group 2: Refers to compounds (compounds with ordinary reactivity) in which, even when pulverized to a particle size of 1 to 3 μm, the carbon dioxide mineralization rate is considered to be 10 to 95% within 1 year. For example, calcium iron aluminate such as Ca4Al2Fe2O 10 Calcium iron aluminates such as etc., magnesium silicates such as MgSiO3, Mg2SiO4, calcium magnesium silicates such as CaMgSiO4, Ca3MgSi2O8, Ca2MgSi2O7, Ca7MgSi4O 16 , calcium magnesium silicates such as CaMgSi2O6, calcium aluminosilicates such as Ca2Al2SiO7, CaAl2Si2O8, oxides (magnesium aluminate) such as MgAl2O4, magnesium-iron oxides such as MgFe2O4, A-CaMgSiO4, A-Ca2MgSi2O7, A-Ca3Si2O7, A-CaSiO3 (「A-」 refers to an amorphous (non-crystallized) compound. The same applies hereinafter.), etc., and compounds listed in Group 2 (including 2*) of Table 1 are included.

[0055] Group 3: Refers to compounds (compounds with slightly low reactivity) in which, even when pulverized to a particle size of 1 to 3 μm, the carbon dioxide mineralization rate is considered to be less than 10% within 1 year. For example, A-Ca3SiO5, A-CaAl2Si2O8, A-Ca2SiO4, A-MgSiO3, A-Ca2Al2SiO7, A-Ca3MgSi2O8, A-Ca7MgSi4O 16, compounds such as those listed in Group 3 (including 3*) of Table 1, including A-Ca2Fe2O5, A-CaMgSi2O6, etc.

[0056] Group 4: Refers to compounds (including gangue) that do not contain alkaline earth metals (such as calcium, magnesium, etc.) or are considered unable to form carbonates under ambient conditions (compounds for which no reaction can be expected). For example, silicon dioxide (SiO2), aluminum oxide (alumina) (Al2O3), iron oxides (FeO, Fe3O4, Fe2O5, etc.), manganese oxide (MnO), iron silicates (FeSiO3, Fe2SiO4), carbonates, alkali minerals, etc.

[0057] Regarding the rock applied to the method of the present invention, the ratios of the above-mentioned Groups 1 to 4 are considered important. In the above-mentioned groups, Group 1 has the highest reactivity when considered as a single compound, and Group 4 has the lowest reactivity when considered as a single compound (the reactivity is Group 1 > Group 2 > Group 3 > Group 4).

[0058] It goes without saying that the composition of the rock used should preferably have a higher content of compounds belonging to the group with high reactivity. Below, the range of the content of each group is listed. However, as described later, the reactivity depends on other components of the rock (including the presence of gangue, etc. in the case of natural rock), the size (particle size) of the fine particles obtained in the fine particle process, the carbon dioxide concentration of the air in contact, etc. Therefore, the following ranges are only a rough guide.

[0059] Compounds belonging to Group 1 are expected to exist in the rock as much as possible compared to compounds of other groups. However, in the case of natural rock, it is often impossible to know without mining, and it cannot be determined unconditionally.

[0060] Taking such circumstances into consideration, the compound belonging to Group 1 (at least one of compounds such as calcium oxide, magnesium oxide, calcium ferrite, calcium aluminate, and calcium silicate) preferably contains 5% by mass or more, more preferably 10% by mass or more, and particularly preferably 15% by mass or more, in total, with respect to the whole rock. The compound belonging to Group 1, for example, the more the amount is, such as all of the rock (100% by mass) or 90% by mass or more with respect to the whole rock, the more the mineralization performance of the rock is improved. However, the actually expectable content limit would be about 25% by mass, and it is considered that it may be sufficient to contain 5 to 20% by mass. In the case of natural rock, when the compound belonging to Group 1 exists, generally, it often contains 5 to 20% by mass in total with respect to the whole rock. In the case of slag, waste concrete, etc., it often contains 0 to 90% by mass.

[0061] The compound belonging to Group 2 (excluding the amorphized ones in Table 1) preferably contains as much as possible (generally 80 to 100% by mass of the remainder) among the remainder excluding the compound belonging to Group 1, because in the case of a rock in which the compound belonging to Group 1 exists, it does not want to contain the compounds belonging to Group 3 and Group 4 described later as much as possible.

[0062] On the other hand, when the compound belonging to Group 1 is not contained, the compound belonging to Group 2 preferably contains 50% by mass or more, more preferably 70% by mass or more, and particularly preferably 70 to 100% by mass, in total, with respect to the whole rock. In the case of natural rock, the compound belonging to Group 2 generally contains 70 to 100% by mass in total with respect to the whole rock (when the compound belonging to Group 1 does not exist, and when it exists, generally 0 to 25% by mass). In the case of slag, waste concrete, etc., it often contains 0 to 30% by mass.

[0063] The compounds belonging to Group 3 and Group 4 preferably do not exist as much as possible if possible.

[0064] In the case where the rock is a natural rock, in the present invention, for example, magnesium oxide (MgO), calcium oxide (CaO), magnesium silicate, calcium magnesium silicate, magnesium iron silicate can be mainly used.

[0065] Natural rocks include, for example, igneous rocks, sedimentary rocks, metamorphic rocks, etc. However, it is preferable to use igneous rocks. Also, igneous rocks include plutonic rocks, volcanic rocks, etc. However, it is preferable to use plutonic rocks.

[0066] Plutonic rocks can be classified from the content of SiO2 into acidic rocks (generally, the content of SiO2 exceeds 66% by mass of the whole), intermediate rocks (the content of SiO2 is 52 - 66% by mass of the whole), basic rocks (the same, 45 - 52%), and ultrabasic rocks (the same, 45% or less). In the present invention, it is preferable to use ultrabasic rocks or basic rocks, and it is particularly preferable to use ultrabasic rocks.

[0067] Ultrabasic rocks are also called ultramafic rocks and generally consist of peridotite, dunite, serpentine, orthopyroxene peridotite, clinopyroxene peridotite, websterite, komatiite, etc. Also, basic rocks are also called mafic rocks and generally consist of porphyry, basalt, diorite, etc.

[0068] Ultrabasic rocks mainly contain magnesium silicate (magnesium silicate) such as MgSiO3 (olivine), Mg2SiO4 (olivine), etc., and may also contain CaMgSi2O6 (pyroxene), and may also contain silicate minerals such as CaAl2Si2O8. Such magnesium silicate belongs to Group 2 above, and the mineralization reaction is not very fast.

[0069] On the one hand, ultrabasic rocks belong to Group 1 and appropriately contain magnesium oxide (MgO) which belongs to Group 1 and has a very fast mineralization reaction (generally considered to be about 0 to 10% by mass, but not limited to this range). It is considered that the compounds of Group 1 are contained in a total amount of about 5 to 20% by mass. Furthermore, ultrabasic rocks contain almost no silica (SiO2), aluminosilicate, etc., and are natural rocks suitable in terms of mineralization. In addition, ultramafic rocks are natural rocks that are widely and abundantly available.

[0070] Basic rocks mainly contain silicate minerals such as CaAl2Si2O8 and CaMgSi2O6 (pyroxene), and also contain magnesium silicates (magnesium silicate salts) such as MgSiO3 (olivine) and Mg2SiO4 (olivine).

[0071] Note that rocks containing many amorphized (non-crystalline) compounds (rocks with many amorphous phases) have low reactivity and often hinder mineralization. Rocks with many amorphous phases wrap around other existing crystals and entangle them in the amorphous phase, thus preventing the connection of crystals (which is important for promoting the mineralization reaction).

[0072] In the state of the rock before pulverization, it is generally preferable that the proportion of the amorphized (non-crystalline) part in a rock containing many amorphized (non-crystalline) compounds (rock with many amorphous phases) is 30% by mass or less of the whole. If the proportion of the amorphized (non-crystalline) part is 30% by mass or less, even if the amorphous phase (amorphized part) occupies the space between crystalline minerals, it is considered that the complete encapsulation of the crystals is suppressed. 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 such a range is only a guideline after all.

[0073] (A-2) Pulverization: In the case of natural rocks, after being mined, the rocks are crushed and micronized. In micronization, it is important to maintain the crystalline state of each compound forming the rock (so as not to cause amorphization).

[0074] When the compounds constituting the rock are crushed into fine particles, the smaller the particle size, the larger the surface area of the fine particles and the larger the contact area with air (carbon dioxide), which is preferable. Here, if the fine particles of the crushed rock are in a crystallized state, a mineralization reaction can be expected to proceed. However, if they are too fine (if the particle size is too small), the rock will become amorphous and the reactivity to mineralization will decrease.

[0075] Among the above-mentioned compounds, the compounds listed in Table 2 were selected, and the criteria for the particle size (amorphization diameter) at which amorphization (non-crystallization) occurs when crushed into fine particles were listed in Table 2. In addition, Table 2 also lists the "grinding efficiency of each compound" and the "efficiency of mineralization". Also, the amorphization diameter listed in Table 2 is only a criterion, and it does not necessarily mean that amorphization will surely occur when the diameter is below this value.

[0076]

Table 2

[0077] Also, the time t until the fine particles with a radius R (m) react and reach a depth δ is, as shown in the following formula (III), a function of the density ρ of calcium (Ca) and magnesium (Mg) in the solid, the gas-phase concentration C of CO2 g , and the diffusion coefficient D (m 2 / s) of ions passing through the product layer, etc.

[0078]

Equation

[0079] t: Time (seconds) ρ: Sum of the molar densities of magnesium (Mg) and calcium (Ca) in the rock (moles / m 3 -rock) R: Radius of the fine particles (m) D: CO2 diffusion coefficient in the compound (m 2 / s) C g : CO2 molar concentration in the gas phase (moles / m 3 -gas) δ: Depth of the reaction (m)

[0080] On the other hand, if the radius (particle size) of the fine particles is too large, it will take a very long time for all of the fine particles to react, which is also not preferable. According to the unreacted core model, the reaction rate is considered to be inversely proportional to the cube of the particle diameter. For example, assuming that the result of pulverization is 6 μm when it was assumed to be 3 μm, the reaction is considered to be 8 (2 3 ) times slower.

[0081] Note that the diffusion coefficient D represents the rate of CO2 diffusion through the product layer in the unreacted core model (m 2 / s), and due to other factors, it does not directly indicate the mineralization rate of carbon dioxide. On the other hand, since diffusion through the product layer is the rate-determining step of gas-solid CO2 mineralization, a large diffusion coefficient D indicates a mineral that can actually rapidly mineralize carbon dioxide.

[0082] If the outer diameter (particle size) of the fine particles is such that there is a large amount of the compound belonging to the above-described Group 1 with respect to all of the obtained rock fine particles, it is considered that the reaction will proceed without pulverizing the rock more finely than necessary, and energy loss or the generation of carbon dioxide due to pulverization can be suppressed. In addition, it is considered that the amorphization of the rock (compound) can also be suppressed.

[0083] On the one hand, the reactivity of the entire rock depends on factors such as the types of other components of the rock (including the presence of gangue, etc. in the case of natural rocks), the presence and content of the amorphous phase, the formation positions and relationships between the crystal layers and the amorphous phase in the rock structure, and other factors such as the amount of carbonation (hereinafter sometimes referred to as "rock factors"). The particle size range of the fine particles obtained by crushing such rocks is also affected by these factors.

[0084] In view of these, when atomizing the rock, as a rough guide, the particle size of the fine particles is generally preferably 100 μm or less (more preferably 50 μm or less, still more preferably 25 μm or less, and particularly preferably 10 μm or less). Considering the rock factors, for example, it may be appropriately selected from ranges such as particle sizes of 1 to 25 μm, 1 to 20 μm, 1 to 10 μm, 1 to 5 μm, 1 to 3 μm, etc. For example, when containing a compound in the first group with high reactivity, it is considered that the particle size may be 1 to 25 μm (preferably 1 to 10 μm).

[0085] If the particle size is 1 to 3 μm, the surface area is large and good reactivity can be expected. However, as shown in Table 2, if the particle size is made too small, amorphization progresses and the reactivity may conversely deteriorate. Therefore, it is preferable to suppress it to about 1 to 10 μm or less with the target of 1 to 3 μm. Regarding the particle size of the fine particles, for example, the fine particles of the crushed rock may be spread out so as not to overlap, and an image obtained by an optical microscope or the like may be analyzed by a computer to determine the particle diameter (particle size) and shape of the complete distribution of the fine particles (it is preferable to adopt the maximum value for the particle size). Also, it is desirable that at least 5000 (or 10000) fine particles are analyzed for each sample of the compound.

[0086] The pulverization operation of the rock during micronization can be carried out using conventionally known pulverization means. For example, it is preferable to carry out the pulverization of the rock using a vertical roller mill or high pressure grinding rolls. Note that a ball mill is generally considered to have low energy efficiency and a tendency to become amorphous.

[0087] In addition, when carrying out micronization simply, the rock may be put into a mortar and pulverized using a pestle and grinding. In the pulverization of the rock using a mortar and pestle, etc., for example, when extremely small fine particles that are expected to be amorphous are generated, they are separated and the minimum amount of pulverization is carried out to achieve the desired particle size, thereby achieving the suppression of amorphization to a very low level.

[0088] The pulverization of the rock needs to consider three aspects: prevention of amorphization, making the particle distribution (distribution of the size of the fine particles) as narrow as possible, and minimizing energy consumption (carbon dioxide is generated by energy consumption, and the substantial reduction amount of carbon dioxide decreases). For example, these can be achieved by using high pressure pulverization and separating the fine particles that have become too small while grinding (which are considered to be amorphous. Also, the load on the pulverizer can be reduced (energy efficiency can be increased) by separation). On the other hand, low pressure pulverization (low pressure grinding) causes the supplied energy to result in dislocation of the crystal structure (amorphization) and generation of heat and sound (energy loss), so it is preferable to pulverize using a vertical roller mill, which is considered to be an aspect of high pressure pulverization means.

[0089] Also, it is preferable to narrow (make as narrow as possible) the particle distribution in the rock pulverization. By narrowing the particle distribution, the air that becomes the contact target for mineralizing carbon dioxide diffuses into the fine particle layer.

[0090] Such diffusion is an important factor in the mineralization of carbon dioxide. When the particle distribution becomes wider, the porosity between the particles decreases, and it is considered inefficient because only the upper part of the crushed layer comes into contact with carbon dioxide. Less diffusion means that it is necessary to spread the rock into thinner layers (the rock layer becomes thinner). When the rock layer becomes thinner, the amount of rock that can be processed per unit volume of the equipment decreases, and at the same time, the amount of sheets per unit rock increases, resulting in an increase in cost, which is not preferable.

[0091] Classification may be performed using a commercially available air classifier or a sieve or the like. Further, a crusher equipped with a classification function may be used. The particle size distribution may be determined using a conventionally known technique (for example, by measuring the fine particles after classification using an optical microscope equipped with computer vision operated by commercially available software, etc.).

[0092] In addition, for example, as in the case of CaMgSiO4 in Table 1 described above, there are cases where a compound that has become amorphous as a result of micronization mineralizes CO2 faster than a non-amorphous compound (crystalline compound). In this case, since the structure of the amorphized compound changes depending on the ratio of cations to silica, for a specific composition such as CaMgSiO4, even if it has an amorphous structure, it reacts well with carbon dioxide and mineralization proceeds.

[0093] On the other hand, such cases are exceptions, and from an engineering perspective, it is never possible to find pure minerals, and there is always some degree of mixing and compositional variation. Therefore, the significant decrease in the mineralization of carbon dioxide associated with most amorphous compounds outweighs the slight performance improvement of some compounds. In other words, since the reaction rate with carbon dioxide and the mineralization rate are very likely to be extremely low, it is preferable to avoid amorphous compounds.

[0094] Regarding the fine particles of rock obtained by carrying out the micronization process, for those fine particles of rock that have been amorphized or the like, and for those that have become amorphous during micronization, such as the above-mentioned Group 4 compounds (compounds for which no reaction can be expected) originally contained in the fine particles of rock, since no mineralization reaction can be expected, it is preferable that they do not exist in the fine particles of rock as much as possible. In total, these are preferably at most about 80% by mass, and particularly preferably at most 50% by mass.

[0095] (B) Carbonate formation step: The obtained fine particles of rock come into contact with air in a predetermined space, and carbonates are formed on the fine particles. That is, carbon dioxide in the air reacts with the fine particles of rock having the above composition, and becomes carbonates (mainly carbonates of calcium, magnesium, etc.) on the surface or inside of the fine particles and is immobilized by mineralization. As a result, carbon dioxide in the air is removed.

[0096] When the space can bring air into contact with the fine particles of rock in a constantly stable state (constantly supplying air containing carbon dioxide and maintaining the conditions described below during the implementation period, etc.), there is no problem even outdoors. However, in terms of being able to easily and surely form such a stable state, it is preferable to use an indoor space. Also, the space is preferably a sealed space that can supply air at a certain frequency by ventilation or the like.

[0097] The introduced air refers to general air containing carbon dioxide. For example, air (the carbon dioxide content is considered to be approximately 400 to 450 ppm), exhaust gas, etc., all air (gases) considered to contain carbon dioxide can be used. There is also no particular limitation on the concentration of carbon dioxide in the target air.

[0098] The space is set under the conditions of a temperature of 15 to 50°C and a relative humidity of 50 to 100%. By setting both the temperature condition and the humidity condition within the above-mentioned ranges, the mineralization of carbon dioxide can be efficiently carried out. The temperature in the space is preferably 20 to 40°C, and particularly preferably 30 to 35°C. The relative humidity is preferably 60 to 95%.

[0099] The flow rate of the air containing carbon dioxide in the space can be appropriately determined according to the concentration of carbon dioxide, the type and composition of the rock, etc. For example, when using natural ultrabasic rock (ultramafic rock) as the rock and the air to be treated for carbon dioxide removal is the atmosphere (the carbon dioxide content is as described above), it is preferably about 1 to 100 mm / second, and particularly preferably 1 to 10 mm / second. Since the fine particles of the rock are finely pulverized with a small particle size, it is preferable to set it to the above-mentioned degree in order to prevent the scattering of the fine particles.

[0100] In the space, the contact between the fine particles of the rock and the air is not particularly limited. For example, the fine particles can be placed in a shallow-bottomed container such as a tray with an arbitrary area, and the tray or the like placed in the space can be placed in a state where it is spread in layers over the entire bottom surface of the tray.

[0101] When the fine particles of the rock are layered in this way, it is necessary to consider the diffusion of carbon dioxide to the deepest part of the layer. When the rock is pulverized to the micrometer level, the diffusion of carbon dioxide in the voids between the fine particles is considered to be due to molecular diffusion. Therefore, when the fine particles of the rock are stacked thickly, there is a concern that the concentration of carbon dioxide will decrease in the lower part (deep part) of the layered fine particles of the rock, and the mineralization reaction will be slowed down. On the other hand, the degree of this deep part depends on the type of rock (compound), the size of the space, the particle size of the fine particles, the air flow rate, etc.

[0102] From the above, when spreading in layers, the thickness of the rock fine particles (fine particle layer) is preferably, for example, 5 cm (50 mm) or less, 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.

[0103] In addition, when putting fine particles into a tray or the like in this way and bringing them into contact with air in a state of being spread in layers, such a tray or the like may be placed in a multi-stage manner in the height direction inside the space.

[0104] The contact time between the air and the rock fine particles (which also becomes the reaction time) may be appropriately determined according to the concentration of carbon dioxide in the air, the type and amount of the rock used, etc. As a guide, it can be arbitrarily determined, for example, as 7 days to 1 year, for example, selected between 7 days and 2 months. Although it also depends on the amount and type of the rock, it is considered that the mineralization reaction has generally ended if one year has passed since the start of the contact (the start of the mineralization reaction).

[0105] When carbonates are formed on the rock fine particles and carbon dioxide is mineralized, for the mass of the rock fine particles in a state where carbonates are deposited on the surface and a large number of fine particles are aggregated, first, heat or the like is performed at less than 100 °C to remove the adsorbed moisture. Note that this operation and the analysis operation described below are not particularly necessary when the generated carbonates or the like are not analyzed.

[0106] After removing the moisture, as an analysis operation, by comparing with the mass of the rock fine particles before the carbonate formation step and confirming the mass change, the amount of mineralized carbon dioxide can be confirmed. In addition, the amount of carbon dioxide present and other details of the mineralized carbon dioxide (carbonate) can be analyzed in detail by thermogravimetry differential thermal analysis (TG-DTA) and X-ray fluorescence analysis (XRF).

[0107] In addition, the carbonate harmless to the environment (mainly carbonates such as calcium and magnesium (calcium carbonate (CaCO3, MgCO3, etc.).) The mineralized rock formed can be used, for example, as a roadbed material or a landfill construction material. Thus, the mineralized rock is environmentally friendly.

[0108] (C) Effects of the present invention: As described above, the present invention atomizes a rock containing an alkaline earth metal, and brings the obtained fine particles into contact with air in a space having a predetermined environment, thereby mineralizing carbon dioxide in the air as a carbonate, fixing carbon dioxide in the rock, and removing carbon dioxide in the air.

[0109] Therefore, the present invention can easily remove carbon dioxide in the air at low cost without using chemicals or large-scale equipment, and the amount of carbon dioxide generated is also small, so that substantial reduction of carbon dioxide can be achieved. In addition, the present invention does not require storage of carbon dioxide and has no risk in such storage, and the immobilized carbon dioxide becomes a carbonate harmless to the environment and can be reused together with the rock, so it is also environmentally friendly.

[0110] According to the present invention, CDR can be carried out without relying on other technologies and infrastructures. In the conventional technologies, in order to remove carbon dioxide in the air, a large amount of energy is required, and considering the carbon dioxide generated by such energy, many of them do not result in substantial reduction of carbon dioxide. On the other hand, the present invention removes carbon dioxide from the air by a simple method of a rock atomization step and a carbonate formation step of reacting the obtained fine particles of the rock with carbon dioxide in the air to form a carbonate, so it is considered to be a technology that requires less energy and leads to substantial reduction of carbon dioxide.

[0111] There are currently two major technologies as CDR. They are Bioenergy with Carbon Capture and Storage (BECCS) involving carbon dioxide (CO2) recovery and storage, and DIRECT Air CO2 Capture with Storage (DACCS).

[0112] Among these, BECCS depends on biomass combustion, carbon dioxide recovery, transportation to the storage site, and underground storage. Also, DACCS uses machinery to filter carbon dioxide from the air before transportation and underground storage. The main points for comparing these are considered to be energy requirements (GJ / t-CO2), land requirements (km 2 / Mt-CO2), water requirements (m 3 / t-CO2), the period required for CO2 recovery, biological limitations (Gt-CO2 / year), and cost. In all these cases, t-CO2 means the net carbon dioxide (CO2) removed from the environment (considering carbon dioxide emissions from the process and supply chain). Table 3 shows a table comparing the present invention and these two technologies. It can be confirmed from Table 3 that the present invention acts advantageously compared to other technologies.

[0113]

Table 3

[0114] Furthermore, there are issues regarding the scale-up and scale-out rates. For BECCS and DACCS, a storage site and transportation infrastructure are required, and the development of storage sites incurs significant costs, so they are usually limited to a very large scale (>1 Mt-CO2 / year). On the other hand, the present invention can be adapted at any scale, whether in kg scale or Mt scale. Therefore, it is considered that small-scale, medium-scale, and large-scale systems can be developed without worrying about transportation, storage, etc.

[0115] Furthermore, in other methods that utilize soil or the ocean, it is impossible to accurately quantify the amount of carbon dioxide removed. Additionally, such other methods also have the problem that the expected carbon dioxide in the future may be lost due to the influence of climate change. Since the present invention immobilizes carbon dioxide as carbonate by such simple means and removes it from the air, these problems can also be solved.

[0116] (D) Modifications of Embodiments: It should be noted that the aspects described above show one aspect of the present invention, and the present invention is not limited to the above-described embodiments. Needless to say, modifications and improvements within the scope that includes the configuration of the present invention and can achieve the object and effect are included in the content of the present invention. Also, regarding the specific structure, shape, etc. when implementing the present invention, there is no problem even if it is other structures, shapes, etc. within the scope that can achieve the object and effect of the present invention. The present invention is not limited to the above-described embodiments, and modifications and improvements within the scope that can achieve the object of the present invention are included in the present invention.

[0117] For example, although the necessary operations for the (A) atomization step and the (B) carbonate formation step are partially exemplified, there is no problem in implementing them using other operations not exemplified as long as the object and effect of the present invention are not hindered. Similarly, although the compounds that can constitute rocks are exemplified, since it is merely an example, there is no problem even if the rocks used in the present invention contain compounds other than those containing alkaline earth metal compounds. In addition, the specific structure, shape, etc. when implementing the present invention may be other structures, etc. within the scope that can achieve the object of the present invention.

Examples

[0118] Hereinafter, the present invention will be described in more detail based on examples and the like, but the present invention is not limited thereto.

[0119] ( Reference Example 1 Example 2 ~ Example 6, Reference Example 7 Example 8) As materials corresponding to rocks, eight types of slags (slag 1 to slag 8 ( Reference Example 1 Examples 2 ~ Example 6, Reference Example 7 are taken as Example 8.)) with the compositions shown in Table 4 below (the analysis means for the compound compositions, etc. will be described later) were prepared. Note that “%” indicates the content (mass %) relative to the whole.

[0120] (Composition of slag)

Table 4

[0121] Slags 1 to 8 were pulverized using a mortar and pestle, and the pulverized slags were classified using a sieve. The particle size distribution of the fine particles of the classified slags was measured using an optical microscope equipped with computer vision operated by “ImageJ” software, which is one of the image processing methods, and fine particle samples with a particle size of 25 to 90 μm and fine particle samples with a particle size of less than 25 μm were obtained respectively. Furthermore, the elemental compositions of slags 1 to 8 were analyzed by XRF, and the compound compositions of slags 1 to 8 were analyzed by Q-XRD respectively (the compound compositions, etc. are shown in Table 4).

[0122] The obtained fine particle samples were respectively placed in circular trays with a diameter of 10 cm and a height of 1 cm and spread so that the thickness became 3 mm.

[0123] The indoor space of a 22 L incubator (3-6815-01 CO2 incubator, manufactured by AS ONE Corporation) was set to a relative humidity of 90% and a temperature of 30°C. Under such humidity and temperature conditions, a tray containing a particulate sample was placed in the indoor space of the incubator, the door was closed to seal the state, and then air with a carbon dioxide concentration of 5% (slag 1 - slag 5) and air with a carbon dioxide concentration of 20% (slag 5 - slag 8) were introduced. Then, while maintaining this state, the particulate sample was brought into contact with air to mineralize carbon dioxide (form carbonate). Separating the samples, the above operations were carried out with the number of days elapsed in the test being 1 day, 3 days, 7 days, 14 days, 21 days, and 28 days (the blanks were not carried out). Each sample was carried out 3 times (n = 3) (the average values were listed in Table 5).

[0124] After the number of days had elapsed, the mass of the particulate sample was measured, and the mineralized carbon dioxide (carbonate) was analyzed by thermogravimetric differential thermal analysis (TG-DTA) and X-ray fluorescence analysis (XRF). The mineralization rate (%) of carbon dioxide corresponding to the number of days elapsed was shown in Table 5 and Table 6 (the experimental values are in Table 5, but the predicted values were listed in Table 6 for reference). The evaluation was carried out at two carbon dioxide concentrations to confirm the unreacted core model.

[0125]

Table 5

[0126]

Table 6

[0127] From the results in Table 5, first, it can be confirmed that the mineralization rates of Slag 1 and Slag 7 are extremely low (for example, 1% or less under the carbon dioxide concentration conditions implemented (in the case of Slag 1 with an elapsed days of 28 days and Slag 7 with an elapsed days of 7 days)). The reason for this is considered to be that both Slag 1 and Slag 7 originally have a high content of amorphous phase (the part that has been amorphized; the same applies hereinafter) (Slag 1 is 34.1% and Slag 7 is 100%). Since all of Slag 7 (100%) is in the amorphous phase, it is considered that the mineralization rate is low.

[0128] On the other hand, in the case of Slag 1, although the amorphous phase is 34.1%, since the proportion of the amorphous phase with respect to the whole is still high, crystals are involved in the amorphous phase, the crystals are not connected to each other, and only the crystals on the surface reacted. Therefore, it is considered that the mineralization rate was low.

[0129] Generally, when molten rock (natural rock or slag) is cooled, crystalline or amorphous compounds are formed. When cooled rapidly, amorphous compounds are likely to be generated, and when cooled slowly, crystals are likely to be generated. The distribution of nucleation crystals in molten rock is randomly distributed, and these crystals will grow until they come into contact with other existing crystals. On the other hand, in the liquid phase between crystals, if the crystals solidify as an amorphous phase before growing with each other, the crystals will be separated from each other and cannot be connected. With such an arrangement, it is considered that the progress of the mineralization action of carbon dioxide will be hindered.

[0130] In addition, slags other than Slag 1 and Slag 7 (Slag 2 - Slag 6, Slag 8) also contained an appropriate amount of amorphous phase, but the above-mentioned behavior was not observed, and from the results in Table 5, it was confirmed that mineralization also proceeded well (for example, under the carbon dioxide concentration conditions implemented, the mineralization rate was 5% or more when the elapsed days were 7 days). This is considered to be because the amorphous phase occupied the space between crystalline minerals but did not completely wrap the crystals.

[0131] In addition, slags (slag 2, slag 4, slag 6, and slag 8) containing compounds such as calcium oxide (oxide) belonging to Group 1 exhibit carbon dioxide mineralization at an initial stage. In particular, the rapid mineralization effect of slag 2 and slag 4 is considered to be due to the calcium content mainly in the form of oxides (CaO and Ca2Fe2O5) and calcium silicate (both belonging to Group 1).

[0132] On the other hand, the relatively low mineralization rates of slag 3 and slag 5 compared to the above-mentioned slag 3 and slag 5 are considered to be because, compared to these, the "mixed cation" silicates belonging to Group 2 (where a certain amount of Ca is replaced by Mg or Fe) are relatively abundant, and compounds belonging to Group 4 are present.

Industrial Applicability

[0133] The present invention can be advantageously used as a means for providing a technology that enables the fixation of carbon dioxide in the air as a carbonate in rock and the reduction of carbon dioxide in the air, and has high industrial applicability.

Claims

1. A method for removing carbon dioxide in air, comprising: a particle size reduction step of pulverizing a rock containing at least one compound (excluding carbonates) of Ca (calcium) or Mg (magnesium) to obtain fine particles of the rock; a carbonate formation step of bringing the obtained fine particles of the rock into contact with air in a space where the temperature is 15 to 50 °C and the relative humidity is 50 to 100% to form carbonates on the fine particles of the rock; wherein the amorphous portion of the rock is 30% by mass or less of the whole; the particle size of the fine particles of the rock is 1 to 100 μm; the fine particles of the rock are placed in the tray in a state of being spread in a layer over the entire bottom surface of the tray, and the tray is placed in the space to be brought into contact with the air; A method for removing carbon dioxide in air, characterized in that the thickness of the fine particles of the rock spread in a layer is 1 to 50 mm.

2. The method for removing carbon dioxide in 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 ferrite, calcium aluminate, and calcium silicate.

3. The method for removing carbon dioxide in air according to claim 1 or claim 2, characterized in that the rock contains at least one compound selected from the group consisting of calcium oxide, magnesium oxide, calcium ferrite, calcium aluminate, and calcium silicate in a total amount of 5% by mass or more of the whole rock.

4. The method for removing carbon dioxide in air according to any one of claims 1 to 3, characterized in that the rock contains at least one of ultrabasic rock, basic rock, slag, and waste concrete.

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