CO2 mineralization accelerator

Abstract

To provide a mineralization accelerator that contributes to promoting the mineralization of CO2. [Solution] A mineralization accelerator for CO2, comprising a silica sol containing silica particles (a) having an average secondary particle diameter of 5 to 200 nm as determined by dynamic light scattering and an aqueous medium (b).

Description

[Technical Field] 【0001】 This invention relates to a CO2 mineralization accelerator. [Background technology] 【0002】 Carbon dioxide capture and storage (CCS) technology is a technique for separating and capturing CO2 emitted from emission sources such as power plants and steel mills, and storing it by injecting it into the ground or sea. It prevents the release of CO2 into the atmosphere and is positioned as an important option for combating global warming. As a storage location for CO2 separated and recovered by CCS, deep underground aquifers (such as sandstone layers containing many voids) are considered promising, and for example, a subsurface storage system utilizing deep aquifers has been proposed, as shown in Patent Document 1. Furthermore, Patent Document 2 proposes a subsurface CO2 storage facility that aims to realize CO2 storage in unstructured storage layers that do not have a sealed structure as the target layer for underground storage. 【0003】 CO2 injected into deep underground saline aquifers (reservoirs) spreads out, displacing the formation water within the reservoir, and is trapped within the reservoir by four mechanisms: structural traps, residual gas traps, dissolution traps, and mineral traps. Among these mechanisms, the formation water containing dissolved CO2 undergoes a chemical reaction with rock minerals, and cations (Ca) are released from these minerals. 2+ Mg 2+Mineral traps, where CO2 reacts with minerals (such as ions) (carbonation) and is fixed underground as secondary minerals such as calcium carbonate and magnesium carbonate, are known to be the most stable form of CO2 storage. However, as described in Non-Patent Literature 1, a drawback is that it is estimated to take 100-1000 years for injected CO2 to reach mineral traps. Non-Patent Literature 2 shows that if the geological formation contains many rocks with basic minerals such as basalt, the number of years until mineral traps are reached can be significantly shortened. On the other hand, Non-Patent Literature 3 shows that the dissolution of basalt is promoted in the presence of supercritical CO2, but not at pH 5-6. Furthermore, Non-Patent Literature 4 shows that the dissolution reaction of minerals is affected by metal ion diffusion. [Prior art documents] [Patent Documents] 【0004】 [Patent Document 1] Japanese Patent Publication No. 2008-307483 [Patent Document 2] Japanese Patent Publication No. 2011-147869 [Non-patent literature] 【0005】 [Non-Patent Document 1] Nature Reviews Earth & Environment volume 1 (2020) 90-102 [Non-Patent Document 2] Science 352 (6291) 1312-1314 [Non-Patent Document 3] Journal of MMIJ 128 (2012) 94-102 [Non-Patent Document 4] International Journal of Coal Science&Technology volume 12, article number 15 (2025) [Overview of the Initiative] 【Problems to be Solved by the Invention】 【0006】 As shown in Non-Patent Document 2 and the like, in a stratum where rocks containing basic minerals are widely distributed, it can be expected that the number of years until mineral trapping by mineralization of CO2 can be significantly shortened. On the other hand, as the mineralization of CO2 progresses, cations (metal ions) dissolved in the formation water in the storage layer are consumed, and it is expected that the progress of mineralization will slow down. In addition, cations (metal ions) are supplied by the reaction between the acid generated by the dissolution of CO2 in the formation water and the basic minerals. However, since this reaction is slow in the pH neutral region or higher, when the reaction progresses beyond a certain level and the pH in the system rises, the reaction slows down, resulting in the supply of no cations (metal ions) and the delay of mineralization. Furthermore, when the diffusion of cations (metal ions) in the formation water is slow, as shown in Non-Patent Document 4, the dissolution reaction of minerals is delayed, and therefore (although not proven), it is also considered that it may cause the delay of the mineralization of CO2 itself. 【Means for Solving the Problems】 【0007】 In view of the above circumstances, the present inventors have conducted intensive studies and, as a result, have focused on the interaction between the silanol groups on the surface of silica particles and positive charges (cations), and have found that utilizing this interaction contributes to the promotion of the mineralization of CO2. 【0008】 That is, as a first aspect, the present invention relates to a promoter for promoting the mineralization of CO2, wherein the silica sol contained in the promoter for promoting mineralization comprises silica particles (a) having an average secondary particle diameter of 5 to 200 nm by the dynamic light scattering method and an aqueous medium (b). As a second aspect, in Mineral Adsorption Test-1, a mixture obtained by adding the promoter for promoting mineralization to a brine having a pH of 3.0 containing 8% by mass of CaCl2 and 2% by mass of NaCl at a concentration such that the silica particle concentration becomes 1% by mass is stored at 60°C for 1 hour, and the mineral adsorption rate is evaluated. This relates to the mineralization accelerator described in the first aspect, wherein the mineral adsorption rate (%) calculated using the following formula (Equation 1) from the Ca concentration in the mixture before storage and the Ca concentration in the filtrate after ultrafiltration of the mixture after storage is 0.1 to 30%. [Mathematics 1] Mineral adsorption rate (%) = {[(Ca concentration of mixture before storage) - (Ca concentration of filtrate of mixture after storage)] / (Ca concentration of mixture before storage)} × 100 As a third point of view, in a mineral adsorption test-2 in which a mixture prepared by adding the mineralization accelerator to a pH 3.0 saline solution containing 8% by mass of CaCl2 and 2% by mass of NaCl at a concentration of 1% by mass of silica particles, is stored at 25°C for 1 hour, and the silica particle size in the mixture after storage is evaluated, The ratio (DLS average particle diameter / BET particle diameter) between the average secondary particle diameter (DLS average particle diameter) of silica particles contained in the mixture after storage, measured by dynamic light scattering, and the specific surface area diameter (BET particle diameter) measured by nitrogen adsorption of silica particles contained in the mineralization accelerator, is 2.0 or more and 5.0 or less. This relates to the mineralization accelerator described in the first point. From a fourth perspective, the silica particles have a specific surface area (S) due to nitrogen adsorption. N2 ) is 25-550m 2 This relates to the mineralization accelerator described in the first aspect, which is / g. As a fifth point of view, regarding silica particle powder obtained by removing unbound components from silica particles by ultrafiltration, and then heating and drying the mineralization accelerator to remove the aqueous medium, This relates to the mineralization accelerator described in the first aspect, wherein the amount of silanol groups calculated using the following two equations based on the mass loss when heated from 25°C to 600°C in thermogravimetric analysis and the molecular weight of water molecules is 3.5 to 20 mmol / g. [Math 2] Silanol group content (mmol / g) = 2 × (M2 - M1) ÷ M H2O ÷(M0)×1000 (However, M0 is the mass of silica particle powder subjected to thermogravimetric analysis at 25°C or below before heating, M1 is the mass decrease of the silica particle powder when it reaches 200°C, M2 is the mass decrease of the silica particle powder when it reaches 600°C, MH2O (These represent the molecular weights of water molecules.) From the sixth perspective, 29 Si-NMR spectrum The peak area from -136 ppm to -82 ppm is defined as PA. The peak area from -116 ppm to -106 ppm is PA4. The peak area from -105.5 ppm to -96 ppm is PA3. When the peak area from -95 ppm to -85 ppm is defined as PA2, The aforementioned silica particles relate to the mineralization accelerator described in the first aspect, wherein the amount of silanol groups (%) calculated by the following three equations is 30 or less. [Math 3] Silanol group content (%) = ((PA2 / PA) × 100) × 2 + ((PA3 / PA) × 100) × 1 + ((PA4 / PA) × 100) × 0 The seventh aspect relates to the mineralization accelerator described in the first aspect, wherein at least a portion of the silica particles are coated with a silane compound (c) having a hydrophilic organic group, and the hydrophilic organic group is an epoxy group-containing organic group, an amino group-containing organic group, a hydroxyl group-containing organic group, or a carboxyl group-containing organic group. The eighth aspect relates to the mineralization accelerator described in the first aspect, wherein in a salt resistance test in which a mixture prepared by adding the mineralization accelerator to a pH 3.0 saline solution containing 8% by mass of CaCl2 and 2% by mass of NaCl at a concentration such that the silica particle concentration is 1% by mass, is stored at 60°C for 7 days, the ratio of the average DLS particle diameter of the mixture after storage to the average DLS particle diameter of the mixture before storage (average DLS particle diameter of the mixture after storage / average DLS particle diameter of the mixture before storage) is 0.9 to 3.0. The ninth aspect relates to the mineralization accelerator described in the first aspect, which, in a high-pressure salt resistance test in which a mixture prepared by adding the mineralization accelerator to a pH 3.0 saline solution containing 8% by mass of CaCl2 and 2% by mass of NaCl at a concentration such that the silica particle concentration is 1% by mass, is subjected to CO2 injection at 8 MPa and stored at 60°C and 8 MPa for 7 days, has a ratio of the average DLS particle diameter of the mixture after storage to the average DLS particle diameter of the mixture before storage (average DLS particle diameter of the mixture after storage / average DLS particle diameter of the mixture before storage) of 2.4 or less. As a tenth aspect, there is provided a method for promoting the mineralization of CO2, which includes a step of injecting, into an underground layer before or after the underground injection of CO2, a mineralization promoter described in the first aspect or a diluent obtained by adjusting the mineralization promoter to a solid content concentration of 0.001 to 30% by mass in an aqueous medium. The present invention relates to a method for promoting mineralization. 【Effects of the Invention】 【0009】 The mineralization promoter of the present invention can adsorb minerals (cations) such as Ca in the system by the cation adsorption action of the silanol groups (anionic) on the surface of silica particles, thereby promoting the formation of carbonate on the surface of silica particles, that is, promoting the mineralization of CO2. In addition, the mineralization promoter of the present invention uses silica particles having an average secondary particle diameter of 5 to 200 nm by the dynamic light scattering method. That is, since it does not contain coarse particles, it can penetrate without blocking the gaps even when injected into the underground rock mass, and adsorb minerals (cations) such as Ca dissolved and released from basic minerals contained in the rock constituting the gaps, and can be expected to contribute to the mineralization of CO2. 【Modes for Carrying Out the Invention】 【0010】 In response to the problem of promoting mineral trapping (mineralization) in CCS technology, the present inventors have proposed a three-step mineralization mechanism of CO2, that is, 1) carbonation by dissolution of CO2 in water, 2) dissolution of rock and minerals by the acid generated by the carbonation to release minerals (Ca 2+ , Mg 2+ , Fe 2+We focused on a three-stage mechanism: 1) the elution and diffusion of minerals, 2) the formation of carbonates (minerals) through the bonding of eluted and diffused minerals (cations) with carbonate ions. We noted that in the 2) mineral diffusion stage, particularly in geological formations with low permeability and small pore sizes, mineral diffusion becomes the rate-limiting factor in carbonate formation; and in the 3) carbonate (mineral) formation stage, the formation of nucleating carbonate particles becomes the rate-limiting factor in carbonate growth (mineralization); on the other hand, the formation (precipitation) and dissociation (dissolution) of carbonates are reversible reactions, and the easily dissolvable carbonates hinder carbonate formation and mineralization. The inventors then considered using pseudo-nuclear particles and concluded that if minerals (cations) could be adsorbed on these pseudo-nuclear particles and the minerals (cations) could be brought to a saturated state, then the diffusion of minerals (cations) could be promoted, the formation of carbonates could be promoted, and further, mineralization could be promoted. Based on this, they proceeded to adopt silica particles with mineral adsorption capacity, leading to the completion of the present invention. 【0011】 In other words, the present invention relates to a CO2 mineralization accelerator, specifically a mineralization accelerator comprising a silica sol containing (a) silica particles having an average secondary particle diameter of 5 to 200 nm as measured by dynamic light scattering, and (b) an aqueous medium. As described above, the silica particles of the present invention are particles that adsorb minerals (cations). The (a) silica particles may be coated with at least a portion of the (c) silane compound having hydrophilic organic groups, as described later. 【0012】 The mineralization accelerator of the present invention can be evaluated for its mineral adsorption performance in the following <Mineral Adsorption Evaluation Test 1> and <Mineral Adsorption Test Evaluation 2>. As mentioned above, since adsorbed minerals form carbonates and can contribute to the mineralization of carbonates, those with high mineral adsorption performance can be considered to have high CO2 mineralization promoting ability. Furthermore, the dispersion stability of the silica particles constituting the mineralization accelerator of the present invention can be evaluated by the <salt resistance test (stability test in a geological environment 1)> and the <high ​​pressure salt resistance test (stability test in a geological environment 2)>. These evaluation tests simulate the conditions of geological water (pH 3-4) after CO2 has dissolved. 【0013】 <Mineral adsorption evaluation test 1> Mineral adsorption Evaluation Test 1 involves storing the target mineralization accelerator in a saltwater environment containing minerals and evaluating the amount of minerals adsorbed onto the silica particles that make up the mineralization accelerator. In detail, a mixture is prepared by adding the mineralization accelerator to a pH 3.0 brine containing 8% by mass of CaCl2 and 2% by mass of NaCl, at a concentration of 1% by mass of silica particles, and storing the mixture at 60°C for 1 hour. To adjust the pH of the brine to 3.0, an inorganic acid that does not easily form precipitates with the mineral components contained in the brine, such as hydrochloric acid, can be used. 2+ In the case where minerals are adsorbed, the amount of minerals adsorbed onto the silica particles can be evaluated by comparing the amount of minerals in the mixture before storage with the amount of minerals in the filtrate after filtering the silica particles (on which minerals have been adsorbed) and comparing the amount of minerals in the filtrate. The filtration of silica particles is not particularly limited as long as it can filter out the silica particles, but for example, an ultrafiltration filter with a molecular weight cutoff of 100,000 can be used. In other words, the mineral adsorption capacity, and consequently the mineralization promoting capacity, of the mineralization accelerator according to the present invention can be evaluated by the mineral adsorption rate (%) calculated using the following formula based on the Ca concentration in the mixture before storage and the Ca concentration in the filtrate after ultrafiltration of the mixture after storage. [Mathematics 1] Mineral adsorption rate (%) = {[(Ca concentration of mixture before storage) - (Ca concentration of filtrate of mixture after storage)] / (Ca concentration of mixture before storage)} × 100 In the mineralization accelerator of the present invention, the mineral adsorption rate (%) is preferably 0.1 to 30%, 0.1 to 20%, 0.1 to 15%, 0.1 to 10%, or 1 to 10%. Although a higher mineral adsorption rate can be expected to promote mineralization, if it is too high, i.e., if the amount of adsorption is too large, it may induce aggregation of silica particles and impair dispersibility, so it is desirable to keep it at around 30% or less. 【0014】 〈Mineral Adsorption Test Evaluation 2〉 The mineralization promoter according to the present invention can also evaluate the presence or absence of mineral adsorption by silica particles as a change in particle size in a salt water environment containing minerals similar to the primary particle diameter of silica particles in the target mineralization promoter and the secondary particle diameter of silica particles constituting the mineralization promoter. Specifically, a mixture in which the mineralization promoter is added to a pH 3.0 salt water containing 8% by mass of CaCl2 and 2% by mass of NaCl at a concentration such that the silica particle concentration is 1% by mass is stored at 25°C for 1 hour, and the dynamic light scattering method of the silica particles contained in the mixture after storage is used. The mineral adsorption ability of the mineralization promoter according to the present invention, and thus the mineralization promotion ability, can be evaluated by the ratio <DLS average particle diameter / BET particle diameter> of the average secondary particle diameter (DLS average particle diameter) and the specific surface area diameter (BET particle diameter) measured by nitrogen adsorption of the silica particles contained in the mineralization promoter. In the mineralization promoter of the present invention, it is preferable that the above ratio is 2.0 or more and 5.0 or less, or 2.0 or more and 3.0 or less. When the above ratio exceeds 5.0, it is not preferable because it reflects the aggregation of silica particles rather than mineral adsorption. 【0015】 〈Salt Resistance Test (Stability Evaluation Test 1 in Formation Environment)〉 In addition, the mineralization promoter according to the present invention can evaluate whether the particle size is maintained by evaluating the particle size of the target mineralization promoter before and after storage at 60°C for 7 days in a salt water environment containing minerals. Specifically, a mixture in which the mineralization promoter is added to a pH 3.0 salt water containing 8% by mass of CaCl2 and 2% by mass of NaCl at a concentration such that the silica particle concentration is 1% by mass is stored at 60°C for 7 days, and the average secondary particle diameter of the silica particles in the mixture before and after storage is measured by the dynamic light scattering method. Then, the particle size maintenance (stability in the formation environment) of the mineralization promoter is evaluated from the ratio <DLS average particle diameter of the mixture after storage / DLS average particle diameter of the mixture before storage> of the average secondary particle diameter of the silica particles after storage and the average secondary particle diameter of the silica particles before storage. In the mineralization accelerator of the present invention, the ratio expressed as <average DLS particle size of the mixture after storage / average DLS particle size of the mixture before storage> is preferably 0.9 to 3.0, or 0.9 to 2.0, or 0.9 to 1.5. If the ratio of <average DLS particle size of the mixture after storage / average DLS particle size of the mixture before storage> is too large, it means that aggregation of silica particles will occur, and in this case, when the mineralization accelerator is injected into an underground reservoir, it may cause blockage of the pores in the underground layers and the pores of the rock. By having the above ratio of 0.9 to 3.0, even when the mineralization accelerator of the present invention is injected into underground bedrock, it can be expected to penetrate without blocking the pores, and consequently, the mineralization accelerator can be expected to exhibit its ability to promote the mineralization of CO2. 【0016】 <High-pressure salt resistance test (stability evaluation test in geological environment 2)> Furthermore, considering the promotion of mineralization in a supercritical CO2 environment, it is desirable that the particle size of the silica particles constituting the mineralization accelerator according to the present invention be maintained even under such conditions. Specifically, a mixture is prepared by adding the mineralization accelerator to a pH 3.0 brine containing 8% by mass of CaCl2 and 2% by mass of NaCl at a concentration of 1% by mass of silica particles. CO2 is then injected under pressure at 8 MPa, and the mixture is stored at 60°C and 8 MPa for 7 days. The average secondary particle diameter of the silica particles in the mixture before and after storage is measured by dynamic light scattering. The ratio of the average secondary particle diameter of the silica particles before storage to the average secondary particle diameter of the silica particles before storage (<DLS average particle diameter of the mixture after storage / DLS average particle diameter of the mixture before storage>) is used to evaluate the particle size retention of the mineralization accelerator under high pressure (stability in the geological environment). In the mineralization accelerator of the present invention, it is preferable that the ratio expressed as <average DLS particle diameter of the mixture after storage / average DLS particle diameter of the mixture before storage> is 2.4 or less (rate of change in average particle diameter is 140% or less) or 1.5 or less (rate of change in average particle diameter is 50% or less) before and after storage under the above-mentioned high-pressure conditions. The lower limit of the above ratio can be, for example, 0.9 or more. By having the above ratio of 2.4 or less, when the mineralization accelerator of the present invention is injected into underground rock mass, the silica particles remain dispersed even when in contact with supercritical CO2, so the mineral adsorption effect can be maintained, and consequently, the mineralization accelerator can be expected to exhibit its ability to promote the mineralization of CO2. 【0017】 As the silica sol containing (a) silica particles and (b) aqueous medium described above, for example, an aqueous silica sol can be used. An aqueous silica sol is a colloidal dispersion system in which colloidal silica particles (i.e., (a) silica particles) are dispersed in an aqueous solvent (i.e., (b) aqueous medium) as the dispersion medium, and can be manufactured by known methods using water glass (aqueous solution of sodium silicate) as a raw material. In this invention, (b) the aqueous medium is usually water, and examples include ordinary industrial water, deionized water, and distilled water. 【0018】 In the silica sol comprising (a) silica particles and (b) an aqueous medium, which is a mineralization accelerator of the present invention, (a) the silica particles have an average secondary particle diameter (DLS average particle diameter (also referred to as DLS particle diameter): Z average particle diameter, harmonic mean particle diameter) in the silica sol determined by dynamic light scattering (DLS method) in the range of 5 to 200 nm, for example, in the range of 5 to 100 nm, or 10 to 100 nm, or 15 to 80 nm. The average particle diameter obtained by dynamic light scattering (DLS) represents the average value of the secondary particle diameter (dispersed particle diameter). It is said that the DLS average particle diameter of particles that are completely dispersed in a medium is about twice the average primary particle diameter (specific surface area diameter obtained by the nitrogen adsorption method (BET method) or Sears method, which represents the average value of the primary particle diameter), as described later. In a medium, particles are usually dispersed in the form of clumps of several particles (secondary particles). If the DLS average particle diameter is about twice the average primary particle diameter, it can be judged that the particles are dispersed without aggregation. The larger the DLS average particle diameter, the more likely it is that the silica particles in the medium are in an aggregated state. By using DLS particles with an average particle diameter greater than 5 nm, the particles will not aggregate in the silica sol and will be more stable. Conversely, by using particles with an average particle diameter smaller than 200 nm, it is expected that penetration into the gaps between subsurface layers and into the pores of rocks will be easier. 【0019】 The above DLS average particle size values ​​are (initial) values ​​before conducting the aforementioned <Mineral Adsorption Evaluation Test 1>, etc. However, the silica particles in the mineralization accelerator according to the present invention are particles with excellent dispersion stability, and even after the above-mentioned salt resistance test (stability evaluation test in geological environment 1) and high-pressure salt resistance test (stability evaluation test in geological environment 2), the ratio of particle size before and after the test can be kept at a maximum of about 5, that is, the DLS average particle size value can be kept within approximately the above numerical range (5 to 200 nm). 【0020】 As described above, particles with a large DLS average particle diameter are undesirable because they can clog voids in subsurface layers. Therefore, it is preferable to use a silica sol in the mineralization accelerator of the present invention that does not contain coarse particles, for example, a silica sol in which the cumulative particle size distribution D90 of the average particle diameter (DLS average particle diameter) measured by dynamic scattering of silica particles is 5 to 200 nm, 5 to 150 nm, 5 to 100 nm, or 5 to 70 nm. Furthermore, it is preferable that the silica particles contained in the silica sol have salt resistance so that when they come into contact with saltwater contained in subsurface layers, the silica particles do not aggregate to an extent that exceeds the range of the DLS average particle diameter or D90. The aforementioned D90 is the particle size that represents 90% of the cumulative particle size distribution from the fine particle side. The cumulative particle size distribution can be obtained, for example, by dynamic light scattering or image analysis. As an example, the value of the cumulative particle size distribution can be measured by the particle size distribution using a dynamic light scattering particle size analyzer. There are two methods for analyzing the D value: the number distribution method and the volume distribution method. In the number distribution method, particles are treated as perfect circles with the same area as the particle itself, and the percentage of particles with a specific particle size is measured. In the volume distribution method, assuming that volume and weight are proportional if the particle density is constant, the percentage of mass of particles with a specific particle size is measured in a given amount of sample. As an example, it is preferable to determine the D value (D90) using the volume distribution method. 【0021】 In the silica sol used in the present invention, which comprises (a) silica particles and (b) an aqueous medium, the average primary particle diameter of the silica particles can be, for example, 5 to 100 nm, or for example, 5 to 70 nm, 5 to 60 nm, 5 to 50 nm, or 10 to 50 nm. Unless otherwise specified, the average primary particle diameter of the silica particles described above may be the specific surface area diameter or Sears particle diameter obtained by the nitrogen adsorption method (BET method). The specific surface area diameter (average particle diameter (specific surface area diameter) D (nm)) obtained by measurement using the nitrogen adsorption method (BET method) is the specific surface area S (m²) measured by the nitrogen adsorption method. 2 From ( / g), it is given by the formula D(nm) = 2720 / S. Sears particle size refers to the average particle size measured based on the method described in GWSears, Anal. Chem. 28(12) p. 1981, 1956, as "A rapid method for measuring colloidal silica particle size." Specifically, it is the equivalent diameter (specific surface area diameter) calculated from the specific surface area of ​​colloidal silica, which is obtained by determining the amount of 0.1N-NaOH required to titrate colloidal silica equivalent to 1.5g of SiO2 from pH 4 to pH 9. 【0022】 <Specific surface area due to nitrogen adsorption (S N2 )> The silica particles according to the present invention have a specific surface area (S) due to nitrogen adsorption, for example.N2 ) for example, 25-550m 2 / g, or 25-300m 2 / g, or 25-250m 2 / g, or 40-550m 2 / g, or 40-250m 2 You can use values ​​within the range of / g. Specific surface area of ​​silica particles (S N2 ) 25m 2 By setting the amount to 1 / g or more, mineral components can be adsorbed onto the surface of the silica particles, thereby promoting mineralization. Also, the specific surface area (S) of the silica particles N2 ) to 550m 2 By keeping the amount below / g, the aggregation of silica particles can be reduced. 【0023】 <Silanol group content (mmol / g) obtained by thermogravimetric analysis> Furthermore, the silica particles constituting the mineralization accelerator of the present invention may have a predetermined amount of silanol groups (mmol / g). That is, the silica particle powder obtained by ultrafiltration of the mineralization accelerator containing the silica particles to remove components unbound to the silica particles, and then heating and drying to remove the aqueous medium, can have a silanol group amount calculated by the following formula from the mass loss when heated from 25°C to 600°C and the molecular weight of the water molecules, for example, 3.5 to 20 mmol / g, 3.5 to 15 mmol / g, 3.5 to 10 mmol / g, or 3.8 to 10 mmol / g. [Math 2] Silanol group content (mmol / g) = 2 × (M2 - M1) ÷ M H2O ÷(M0)×1000 However, M0 is the mass of the silica particle powder subjected to thermogravimetric analysis at 25°C or below before heating, M1 is the mass loss when 200°C is reached, M2 is the mass loss when 600°C is reached, and M H2O The values ​​of each represent the molecular weight of a water molecule. By setting the silanol content (mmol / g) of the silica particles to a range of 3.5 to 20 mmol / g, it is possible to reduce the condensation and aggregation of silanol groups on the surface of the silica particles between the particles while simultaneously obtaining the mineral adsorption effect. 【0024】 To obtain silica particle powder by the above-mentioned heat drying method, first, the target mineralization accelerator is ultrafiltered, and ultrafiltration is repeated as needed to remove components that are not bound to the silica particle powder. The components after ultrafiltration (filter material: silica sol) are recovered, and by heating and drying this at 120°C to 150°C using, for example, a hot plate or oven, the aqueous dispersion medium (water) is removed, and the desired silica particle powder can be obtained. Furthermore, silanol groups are present on the surface of silica particles, and these silanol groups readily adsorb water through hydrogen bonding. Therefore, to quantify the amount of silanol groups in silica particles from the mass loss due to heating, it is preferable to heat them to approximately 200°C, where the adsorbed water is desorbed, to remove it. 【0025】 < 29 Silanol group content (%) obtained from Si NMR spectrum > In this specification, 29 The silanol group content (%) obtained from the Si-NMR spectrum represents the total amount of silanol groups present in the Q2-Q4 structure of silica particles. 29 The amount of silanol groups obtained from Si-NMR spectroscopy represents the amount of silanol groups present throughout the silica particle (particle surface and interior), meaning it includes not only silanol groups deep within the particle that are not thought to contribute to mineralization, but also isolated silanol groups on the particle surface that cannot be fully analyzed by thermogravimetric analysis (since silanol groups are not adjacent, dehydration condensation does not occur). On the other hand, the amount of silanol groups obtained from the aforementioned thermogravimetric analysis represents the amount of silanol groups located in positions from which water molecules can detach from the silica particle, meaning it primarily evaluates silanol groups that are thought to contribute to mineralization. In silica particles, silicon atoms exist that are not bonded to hydroxyl groups, and silicon atoms that are bonded to one or two hydroxyl groups. In other words, the silicon atoms in silica particles can take on four structures, as shown in the following formula: a silicon atom (Q2) bonded to two oxygen atoms and two hydroxyl groups, a silicon atom (Q3) bonded to three oxygen atoms and one hydroxyl group, and a silicon atom (Q4) bonded to four oxygen atoms. [ka] Then, by determining the ratio of Q2, Q3, and Q4 in the silicon atoms within the silica particles, the amount of silanol (Si-OH) groups in the silica can be estimated. The higher the silanol group content (%), the higher the mineral content (Ca 2+ It can be estimated that the adsorption capacity of ) is high. The amount of silanol groups present in the silicon atoms of the above Q2-Q4 structures was determined, for example, by using a water-dispersed silica sol containing the silica particles under investigation. 29 It can be measured by Si NMR (liquid NMR). Specifically, the silica particles (water-dispersed silica sol) that are the target of measurement 29 In the Si-NMR spectrum, the peak observed between chemical shifts of -136 ppm and -80 ppm is identified as originating from the Q structure (overall) of Si, the peak observed between -116 ppm and -106 ppm as originating from the Q4 structure, the peak observed between -105.5 ppm and -96 ppm as originating from the Q3 structure, and the peak observed between -95 ppm and -85 ppm as originating from the Q2 structure. The ratio of the peak area originating from each of the Q2 to Q4 structures to the peak area originating from the Q structure (overall) represents the content ratio (mol%) of each structure (Q2 to Q4) in the silica particles being measured. Furthermore, since the silanol group has two hydroxyl groups in the Q2 structure, one in the Q3 structure, and zero in the Q4 structure, the amount of silanol group can be calculated by taking into account the number of silanol groups in each structure using the following formula. [Math 3] Silanol group content (%) = ((PA2 / PA) × 100) × 2 + ((PA3 / PA) × 100) × 1 + ((PA4 / PA) × 100) × 0 In the above formula, PA represents the peak area from -136 ppm to -80 ppm, PA4 represents the peak area from -116 ppm to -106 ppm, PA3 represents the peak area from -105.5 ppm to -96 ppm, and PA2 represents the peak area from -95 ppm to -85 ppm. The silica particles constituting the mineralization accelerator of the present invention preferably have a silanol group content of 30% or less. By setting it to 30% or less, the condensation and aggregation of silanol groups on the surface of the silica particles between the silica particles can be reduced. The lower limit can be 5% or 8%, and a mineral adsorption effect can be obtained by setting it to 5% or more. As will be described later, the silica particles constituting the mineralization accelerator of the present invention are as follows: 29 No peaks in the Si-NMR spectrum with a chemical shift of -80 to -85 ppm, i.e., peaks originating from the Q1 structure (Si(OH)3OSi), were observed. 【0026】 The above-mentioned silica sol (aqueous silica sol) can be a commercially available product. Furthermore, aqueous silica sols with a silica concentration of 5-50% by mass are generally commercially available, which is preferable because they are easily obtainable. Furthermore, aqueous silica sols include alkaline aqueous silica sols and acidic aqueous silica sols, and both can be used, but acidic aqueous silica sols are preferably used. Examples of commercially available acidic aqueous silica sols include Nissan Chemical Corporation's Snowtex (product names) ST-OXS, ST-OS, ST-O, ST-O-40, ST-OL, ST-OYL, etc.; and ADEKA Corporation's Adelite (product name) AT-series, etc. Examples of commercially available alkaline aqueous silica sols include Nissan Chemical Corporation's Snowtex (product names) ST-30, ST-XL, etc. The silica solid content concentration in the aqueous silica sol used is preferably 5 to 55% by mass. Here, silica solids concentration is a value determined by the calcination method, specifically, it is the value obtained by dividing the mass of the calcination residue obtained by calcining aqueous silica sol at 1000°C for 30 minutes or more under air by the mass of the aqueous silica sol. The mass of the calcination residue is also called "silica solids". 【0027】 In the mineralization accelerator according to the present invention, (a) the concentration of silica particles (silica solid content concentration) is not particularly limited, but based on the total mass of the mineralization accelerator, it can be, for example, 0.01% to 50.0% by mass, or 0.1% to 50.0% by mass, or 1.0% to 50.0% by mass, or 10.0% to 40.0% by mass, or 15.0% to 35.0% by mass, or 15.0% to 25.0% by mass, or 0.001% to 30.0% by mass, or 0.01% to 30.0% by mass, or 0.1% to 30.0% by mass, or 1.0% to 30.0% by mass. 【0028】 In the mineralization accelerator of the present invention, the (a) silica particles may be coated with (c) a silane compound having a hydrophilic organic group, at least a portion thereof. 【0029】 In this invention, "coating with a silane compound" refers to a configuration in which the surface of silica particles is coated with a silane compound, and also includes a configuration in which the silane compound is bonded to the surface of silica particles. "An embodiment in which the surface of silica particles is coated with a silane compound" means that the silane compound coats at least a portion of the surface of the silica particles, that is, it includes embodiments in which the silane compound covers a portion of the surface of the silica particles and embodiments in which the silane compound covers the entire surface of the silica particles. This embodiment does not depend on whether or not there is bonding between the silane compound and the surface of the silica particles. Furthermore, "a mode in which the silane compound is bonded to the surface of silica particles" means that the silane compound is bonded to at least a part of the surface of the silica particles, that is, a mode in which the silane compound is bonded to a part of the surface of the silica particles, a mode in which the silane compound is bonded to a part of the surface of the silica particles and covers at least a part of the surface, and a mode in which the silane compound is bonded to the entire surface of the silica particles and covers the entire surface. 【0030】 Examples of the above (c) silane compound include silane compounds having an epoxy group-containing organic group, an amino group-containing organic group, a hydroxyl group-containing organic group, or a carboxyl group-containing organic group as a hydrophilic organic group. Furthermore, it is preferable that the (c) silane compound has a hydrolyzable group such as an alkoxy group, an acyloxy group, or a halogen group in addition to the hydrophilic organic group. 【0031】 Specific examples of these (c) silane compounds include silane coupling agents having an epoxy group-containing organic group or an amino group-containing organic group. Examples of silane coupling agents having the above-mentioned epoxy group-containing organic group include 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 2-(3,4-epoxycyclohexyl)propyltrimethoxysilane, 2-(3,4-epoxycyclohexyl)propyltriethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyltriethoxysilane, 2-(3,4-epoxycyclohexyl)methyltrimethoxysilane, 2-(3,4-epoxycyclohexyl)methyltriethoxysilane, 1-(3,4-epoxycyclohexyl)methyltrimethoxysilane, and 1-(3,4-epoxycyclohexyl)methyltriethoxysilane. Examples of silane coupling agents having the above-mentioned amino group-containing organic group include 3-(2-(2-aminoethylamino)ethylamino)propyltriethoxysilane, N-2-(aminoethyl)-3-aminopropylmethyldimethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, 3-aminopropyltrichlorosilane, 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-aminopropylmethyldimethoxysilane, 3-aminopropylmethyldiethoxysilane, 3-triethoxysilyl-N-(1,3-dimethylbutylidene)propylamine, N-phenyl-3-aminopropyltrimethoxysilane, and N-phenyl-3-aminopropyltriethoxysilane. 【0032】 Silica particles according to the present invention, which are coated with a silane compound having at least a portion of a hydrophilic organic group (also referred to as "silica particles surface-treated with a silane compound"), can be obtained, for example, by adding a silane compound to an aqueous silica sol and then heat-treating it at 50 to 100°C for about 1 to 20 hours. At this time, the amount of silane compound added to the silica particles (silica solids) in the aqueous silica sol can be, for example, a ratio of silane compound / silica particles = 0.1 to 10.0 by mass ratio. The amount of surface treatment by the above silane compound, that is, the amount of silane compound bonded to the silica particle surface, is the amount of the silica particle surface at 1 nm 2 For example, it is preferable that the number of items per unit be around 0.1 to 12. 【0033】 Furthermore, in the mineralization accelerator of the present invention, when (a) silica particles are used as (c) particles coated with a silane compound having a hydrophilic organic group (also referred to as surface-modified silica particles), unreacted (c) silane compounds may remain in the mineralization accelerator, unmodified silica particles may remain, and it is also possible to use a product from which these unreacted (c) silane compounds and unmodified silica particles have been removed. 【0034】 <Methods to accelerate mineralization> The present invention also covers a method for accelerating the mineralization of CO2 using the mineralization accelerator, and more specifically, a method for accelerating mineralization that includes the step of injecting the mineralization accelerator, or a diluted solution of the mineralization accelerator prepared in an aqueous medium so that the silica particle concentration is 0.001 to 30% by mass, into an underground layer. The step of injecting the above-mentioned mineralization accelerator (diluted solution) into the subsurface layer may be performed either before or after the injection of CO2. The order in which the mineralization accelerator and CO2 are injected is not particularly important, and the effects of the present invention can be obtained regardless of which comes first. [Examples] 【0035】 The present invention will be described in further detail below based on synthesis examples, embodiments, and comparative examples, but the present invention is not limited in any way by these embodiments. 【0036】 (Measuring device) • DLS average particle diameter (dynamic light scattering particle diameter): Dynamic light scattering particle diameter measurement device, product name Zetasizer Nano (manufactured by Spectris Corporation, Malvern Division) was used. • pH: A pH meter (manufactured by Toa DKK Co., Ltd.) was used. • Electrical conductivity: An electrical conductivity meter (manufactured by Toa DKK Co., Ltd.) was used. • ICP: We used Agilent 5110 ICP-OES (Agilent Technologies, Inc.). 【0037】 [Measurement of silica solid content concentration (silica concentration)] The silica sol was placed in a crucible, dried at 150°C, and the resulting gel was calcined at 1000°C under air for 30 minutes. The calcination residue was then weighed and calculated. [pH measurement] The pH of the silica sol was measured at 20°C using a pH meter (manufactured by Toa DKK Co., Ltd., product name: MM-43X). [Electrical conductivity measurement] The electrical conductivity of silica sol was measured at 20°C using an electrical conductivity meter (product name CM-30R, manufactured by Toa DKK Co., Ltd.). [Measurement of average primary particle diameter (particle diameter by nitrogen adsorption method)] Specific surface area value (S) of silica particles by nitrogen adsorption method N2 The silica sol was prepared by removing water-soluble cations using an H-type cation exchange resin (Dow Chemical, product name: Amberlite IR-120B), drying the silica sol at 290°C, and then grinding it in a mortar for 10 minutes to obtain the measurement sample. This sample was then measured using a nitrogen adsorption specific surface area analyzer, product name Monosorb (Quantachrome Instruments Japan LLC), with a mixed gas of 30% N2 (nitrogen) and 70% He (helium) as the carrier gas, using the BET single-point method. The obtained specific surface area value S N2 (m 2 From ( / g), the average primary particle diameter (nm) = 2720 / S N2 It was calculated using [the method described below]. [Measurement of average secondary particle diameter using dynamic light scattering (DLS) method] The average secondary particle diameter (also called DLS average particle diameter) was measured using a dynamic light scattering particle diameter analyzer (Malvern Panalytical, product name: Zetasizer Nano). 0.1 g of the target silica sol was placed in a glass cell with a 10 mm optical path length, and a 0.15 mass% NaCl aqueous solution was added to obtain a silica sol with a silica particle concentration adjusted so that the count rate was 200-400 kcps when the attenuator was set to 7. The prepared silica sol was placed in the cell, adjusted so that the liquid level was approximately 1 cm from the bottom of the cell, and the DLS average particle diameter of the silica sol was measured using the attenuator 7. Although this analyzer can also calculate the number-average particle diameter and the volume-average particle diameter, the Z-average particle diameter was used in this application. 【0038】 (Example 1) 1000 g of aqueous silica sol 1 (Snowtex (product name) ST-O, manufactured by Nissan Chemical Corporation, silica concentration = 20.5% by mass, average particle size by BET method 11.7 nm, average particle size by DLS 18.6 nm) was added to a 2000 ml glass round-bottom flask. Then, a magnetic stir bar was added, and while stirring with a magnetic stirrer, 149.7 g of 3-glycidoxypropyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., product name KBM-403) was added. Subsequently, the round-bottom flask was placed in an oil bath that had been preheated to 60°C. A condenser running tap water was placed on top of the round-bottom flask, and the mixture was maintained at 60°C for 3 hours under reflux, after which it was cooled. After cooling to room temperature, the aqueous sol was removed and filtered using a 460 mesh to obtain 1145 g of aqueous silica sol surface-treated with a silane compound. The pH, electrical conductivity, silica solids content, and DLS average particle size of the aqueous silica sol from Example 1 were evaluated. The mineral adsorption performance of the aqueous silica sol of Example 1 was evaluated according to the [Mineral Adsorption Test-1] and [Mineral Adsorption Test-2] in the <Evaluation Test> below. Furthermore, the stability of the aqueous silica sol of Example 1 in a CCS geological environment was evaluated according to [Stability Test in CCS Geological Environment - 1] and [Stability Test in CCS Geological Environment - 2]. 【0039】 (Example 2) 1352 g of aqueous silica sol was obtained by the same procedure as in Example 1, except that 1000 g of aqueous silica sol 1 (Snowtex (product name) ST-O, manufactured by Nissan Chemical Corporation, silica concentration = 20.5% by mass, average particle size by BET method 11.7 nm, average particle size by DLS 18.6 nm) was added, followed by the addition of 228.2 g of lactic acid (manufactured by Kanto Chemical Co., Ltd., concentration 89% by mass) and stirring with a magnetic stirrer, and 130.7 g of aminoethylaminopropylmethyldimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., product name KBM-602) was added instead of 3-glycidoxypropyltrimethoxysilane. The pH, electrical conductivity, silica solids content, and DLS average particle size of the aqueous silica sol of Example 2 were evaluated. The mineral adsorption performance of the aqueous silica sol of Example 2 was evaluated according to the [Mineral Adsorption Test-1] and [Mineral Adsorption Test-2] in the <Evaluation Test> below. Furthermore, the stability of the aqueous silica sol of Example 2 in a CCS geological environment was evaluated according to [Stability Test in CCS Geological Environment - 1] and [Stability Test in CCS Geological Environment - 2]. 【0040】 (Example 3) Except for adding 1000g of aqueous silica sol 2 (Snowtex (trade name) ST-OL, manufactured by Nissan Chemical Corporation, silica concentration = 20.6% by mass, average particle size by BET method 45.4nm, average particle size by DLS 76.4nm) instead of aqueous silica sol 1, a magnetic stir bar was added, and while stirring with a magnetic stirrer, 38.7g of 3-glycidoxypropyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., trade name KBM-403) was added, and the procedure was the same as in Example 1 to obtain 1038.0g of aqueous silica sol for Example 3. The pH, electrical conductivity, silica solids content, and DLS average particle size of the aqueous silica sol of Example 3 were evaluated. The mineral adsorption performance of the aqueous silica sol of Example 3 was evaluated according to the [Mineral Adsorption Test-1] and [Mineral Adsorption Test-2] in the <Evaluation Test> below. Furthermore, the stability of the aqueous silica sol of Example 3 in a CCS geological environment was evaluated according to [Stability Test in CCS Geological Environment - 1] and [Stability Test in CCS Geological Environment - 2]. 【0041】 (Example 4) 309.0 g of aqueous silica sol for Example 4 was obtained by following the same procedure as in Example 1, except that 195.2 g of aqueous silica sol 3 (Snowtex (product name) ST-30, manufactured by Nissan Chemical Corporation, silica concentration = 30.6% by mass, average particle size by BET method 11.4 nm, average particle size by DLS 19.3 nm) was added to a 500 mL glass round-bottom flask in place of aqueous silica sol 1, 100.6 g of pure water was added, and 13.8 g of 3-glycidoxypropyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., product name KBM-403) was added while stirring with a magnetic stirrer. The pH, electrical conductivity, silica solids content, and DLS average particle size of the aqueous silica sol of Example 4 were evaluated. The mineral adsorption performance of the aqueous silica sol of Example 4 was evaluated according to the [Mineral Adsorption Test-1] and [Mineral Adsorption Test-2] in the <Evaluation Test> below. Furthermore, the stability of the aqueous silica sol of Example 4 in a CCS geological environment was evaluated according to [Stability Test in CCS Geological Environment - 1] and [Stability Test in CCS Geological Environment - 2]. 【0042】 (Example 5) Except for adding 146.4 g of aqueous silica sol (Snowtex (product name) ST-XL, manufactured by Nissan Chemical Corporation, silica concentration = 41.7% by mass, average particle size by BET method 45.4 nm, average particle size by DLS 79.5 nm) instead of aqueous silica sol 1, adding 149.4 g of pure water, and then adding 5.1 g of 3-glycidoxypropyltrimethoxysilane (manufactured by Shin-Etsu Chemical Co., Ltd., product name KBM-403) while stirring with a magnetic stirrer, the procedure was the same as in Example 4 to obtain 300.3 g of aqueous silica sol for Example 5. The pH, electrical conductivity, silica solids content, and DLS average particle size of the aqueous silica sol of Example 5 were evaluated. The mineral adsorption performance of the aqueous silica sol of Example 5 was evaluated according to the [Mineral Adsorption Test-1] and [Mineral Adsorption Test-2] in the <Evaluation Test> below. Furthermore, the stability of the aqueous silica sol of Example 5 in a CCS geological environment was evaluated according to [Stability Test in CCS Geological Environment - 1] and [Stability Test in CCS Geological Environment - 2]. 【0043】 (Example 6) Aqueous silica sol 1 (Snowtex (product name) ST-O, manufactured by Nissan Chemical Corporation, silica concentration = 20.5% by mass, average particle diameter by BET method 11.7 nm, average particle diameter by DLS 18.6 nm) was used as the aqueous silica sol for Example 6, and its pH, electrical conductivity, silica solids content, and average particle diameter by DLS were evaluated. The mineral adsorption performance of the aqueous silica sol of Example 6 was evaluated according to the [Mineral Adsorption Test-1] and [Mineral Adsorption Test-2] in the <Evaluation Test> below. Furthermore, the stability of the aqueous silica sol of Example 6 in a CCS geological environment was evaluated according to [Stability Test in CCS Geological Environment - 1] and [Stability Test in CCS Geological Environment - 2]. 【0044】 (Comparative Example 1) Sol-gel silica sol 1 (manufactured by Fuso Chemical Co., Ltd., PL-2L (trade name), primary particle size 17 nm, secondary particle size 26 nm, pH 7.3) was used as the aqueous silica sol for Comparative Example 1, and its pH, electrical conductivity, silica solids content, and DLS average particle size were evaluated. The mineral adsorption performance of the aqueous silica sol of Comparative Example 1 was evaluated according to the [Mineral Adsorption Test-1] and [Mineral Adsorption Test-2] in the <Evaluation Test> below. The stability of the aqueous silica sol of Comparative Example 1 in a CCS geological environment was evaluated according to [Stability Test in CCS Geological Environment - 1] and [Stability Test in CCS Geological Environment - 2]. 【0045】 <Evaluation Test> [Mineral Adsorption Test - 1] After placing a stirring bar in a 300 mL polystyrene bottle, 133.3 g of brine-1 containing 12% by mass of CaCl2 and 3% by mass of NaCl was added. While stirring, aqueous silica sols (mineralization accelerators) from Examples 1-6 or Comparative Example 1 were added until the silica concentration reached 1.0% by mass when the total volume of the solution was 200 g. Then, the pH was adjusted to 3.0 using 0.1 N hydrochloric acid (manufactured by Kanto Chemical Co., Ltd.). After that, pure water was added to prepare 200 g of mineral adsorption test solution-1 containing 8% by mass of CaCl2, 2% by mass of NaCl, and a silica concentration of 1.0% by mass. The pH, conductivity, and DLS average particle size of the mineral adsorption test solution were evaluated. This mineral adsorption test solution-1 was stored at 60°C for 1 hour. 2g of the above-mentioned stored mineral adsorption test solution-1 was added to a filter-equipped centrifuge cell (Merck, product name Amicon Ultra-15, molecular weight cutoff 100,000, centrifugal ultrafiltration filter unit). The centrifuge cell was then placed in a centrifuge (Tommy Industries, Ltd., product name Suprema 21) and centrifuged at 5000 rpm for 15 minutes to obtain filtrate-1 from which silica particles were removed by the ultrafiltration membrane.2+ When the silica particles are adsorbed, the adsorbed minerals are filtered out along with the silica particles, and the amount of minerals adsorbed onto the silica particles (absolute value) can be defined as the difference between the amount of minerals in the mineral adsorption test solution-1 before storage and the amount of minerals in the filtrate-1 after storage. 【0046】 The Ca concentration in mineral adsorption test solution-1 before storage and in filtrate-1 after storage was quantified by ICP measurement, and the mineral adsorption rate-1 was calculated using the following formula. Ca concentration was based on volume (mg / L). Mineral adsorption rate -1[%] = {[(Ca concentration of mineral adsorption test solution-1 before storage) - (Ca concentration of filtrate-1 after storage)] / (Ca concentration of mineral adsorption test solution-1 before storage)} × 100 The mineral adsorption rate of -1 was evaluated according to the following criteria. <Criteria for determining mineral adsorption rate -1> A: Mineral adsorption rate -1 is between 1% and 10% B: Mineral adsorption rate -1 is 0.1% or more but less than 1%, and more than 10% but 30% or less. C: Mineral adsorption rate -1 is 0.01% or more and less than 0.1%, and more than 30% and less than or equal to 40%. D: Mineral adsorption rate -1 is less than 0.01 and greater than 40% The results for determining the mineral adsorption rate -1 indicate that A is the most preferable result, followed by B, C, and D in that order of preference. However, a C rating is undesirable because it is difficult to obtain the effect of a mineralization accelerator, and a D rating is undesirable because it does not obtain the effect of a mineralization accelerator or may induce aggregation of silica particles. 【0047】 [Mineral Adsorption Test - 2] After placing a stirring bar in a 300 mL polystyrene bottle, 133.3 g of brine-1 containing 12% by mass of CaCl2 and 3% by mass of NaCl was added. While stirring, aqueous silica sols (mineralization accelerators) from Examples 1-6 or Comparative Example 1 were added until the silica concentration reached 1.0% by mass when the total volume of the solution was 200 g. Then, the pH was adjusted to 3.0 using 0.1 N hydrochloric acid (manufactured by Kanto Chemical Co., Ltd.). After that, pure water was added to prepare 200 g of mineral adsorption test solution-2, which contained 8% by mass of CaCl2, 2% by mass of NaCl, and a silica concentration of 1.0% by mass. This mineral adsorption test solution-2 was stored at 25°C for 1 hour. The particle size of silica particles in mineral adsorption test solution-2 after storage was measured using DLS, and the DLS average particle size-1 (nm) was defined as the particle size of the silica particles with minerals (Ca 2+ If adsorbed substances are present, the particle size can be evaluated as containing those adsorbed substances. 【0048】 Next, 2 g of aqueous silica sol (mineralization accelerator) from Examples 1-6 or Comparative Example 1 was placed in a centrifuge cell with a filter (Merck, product name Amicon Ultra-15, molecular weight cutoff 100,000, centrifugal ultrafiltration filter unit), and then 4 g of pure water was added to the centrifuge cell. The centrifuge cell was placed in a centrifuge (Tommy Industries, Ltd., product name Suprema 21) and centrifuged at 3000 rpm for 20 minutes. After collecting and quantifying the filtrate, the same amount of pure water as the filtrate was added back to the centrifuge cell, and centrifuged again at 3000 rpm for 20 minutes. After repeating this process five times, the silica sol remaining in the filter was collected. Using a hot plate, the collected silica sol was dried at 120°C to obtain silica gel, which was then crushed in a mortar and pestle, and then further dried at 150°C for 3 hours to obtain dried silica powder. The specific surface area (m²) of the obtained silica dry powder was determined by nitrogen adsorption method based on the BET theory. 2The specific surface area ( / g) was measured using the BET method, i.e., the nitrogen gas BET method. The value obtained by dividing the specific surface area calculated by the BET method by 2720 was defined as the BET particle diameter - 1 (nm). Note that BET particle diameter - 1 represents the average value of the primary particle diameter of silica particles (only). In the form of aqueous silica sol (in water), silica particles are usually thought to be dispersed in the form of clumps of several particles (secondary particles), and the DLS average particle diameter is thought to be about twice the BET particle diameter. Therefore, if the DLS average particle diameter - 1 is more than twice as large as the BET particle diameter - 1, it can be evaluated that minerals are adsorbed on the particle surface of the clumped silica particles (secondary particles). (However, as will be described later, if this DLS average particle diameter is too large compared to the BET particle diameter, it is considered that the silica particles are in an aggregated state, which is undesirable.) 【0049】 The mineral adsorption amount -2 was calculated from the above DLS average particle size -1 and BET particle size -1 using the following formula. Mineral adsorption amount -2 = (DLS average particle size - 1) / (BET particle size - 1) The mineral adsorption amount of -2 was evaluated according to the following criteria. <Criteria for determining mineral adsorption amount -2> A: 2.0 or higher, less than 3.0 B: 1.5 or higher and less than 2.0, and 3.0 or higher and 5.0 or lower C: 1.1 or higher and less than 1.5, and greater than 5.0 and less than 10.0 D: Less than 1.1, and 10.0 or higher. The results for Mineral Adsorption Amount-2 indicate that A is the most preferable result, followed by B, C, and D in that order. Considering the effect as a mineralization accelerator, a rating of A or B is desirable. However, if Mineral Adsorption Amount-2 exceeds 5.0, it indicates excessive aggregation of silica particles, which is undesirable. 【0050】 [Salt resistance test: Stability test in a CCS geological environment - 1] After placing a stirring bar in a 300 mL polystyrene bottle, 133.3 g of brine-1 containing 12% by mass of CaCl2 and 3% by mass of NaCl was added. While stirring, aqueous silica sols (mineralization accelerators) from Examples 1-6 or Comparative Example 1 were added until the silica concentration reached 1.0% by mass when the total volume of the solution was 200 g. Then, the pH was adjusted to 3.0 using 0.1 N hydrochloric acid (manufactured by Kanto Chemical Co., Ltd.). By adding pure water thereafter, 200 g of stability test solution-1 for CCS geological formations, containing 8% by mass of CaCl2, 2% by mass of NaCl, and a silica concentration of 1.0% by mass, was prepared. This stability test solution-1 was also used as the test solution in the high-pressure salt resistance test described later. 【0051】 100g of Stability Test Solution-1 was placed in a sealed container and stored at 60°C for 7 days. The pH, electrical conductivity, and DLS average particle size of the aqueous silica sol (silica particles) in the sample were evaluated before and after storage of Stability Test Solution-1, and the appearance of Stability Test Solution-1 after storage was visually inspected. The stability in a CCS (Carbon Capture and Storage) geological environment was evaluated according to the following criteria based on the ratio of the average secondary particle diameter (DLS average particle diameter) of silica particles in the storage stability test solution-1 to the average secondary particle diameter (DLS average particle diameter) before storage <average secondary particle diameter of silica particles after storage (DLS average particle diameter after storage) / average secondary particle diameter of silica particles before storage (DLS average particle diameter before storage)> and the appearance of the storage test solution. <Status test in CCS geological environment - 1 result> A: The ratio of the average DLS particle size after storage to the average DLS particle size before storage is between 0.9 and 1.5. B: The ratio of the average DLS particle size after storage to the average DLS particle size before storage is greater than 1.5 and less than or equal to 3.0. C: The ratio of the average DLS particle size after storage to the average DLS particle size before storage is greater than 3.0 and less than or equal to 8.0. D: The ratio of the average DLS particle size after storage to the average DLS particle size before storage is greater than 8.0 and less than or equal to 20.0. E: The ratio of the average DLS particle size after storage to the average DLS particle size before storage is greater than 20.0, or turbidity occurs and solid-liquid separation occurs. The results of the stability test in the CCS geological environment - 1 indicate that A is the most preferable result, followed by B, C, D, and E in that order of preference. Considering the effect as a mineralization accelerator, a rating of A or B is desirable. 【0052】 [High-pressure salt resistance test: Stability test in a CCS geological environment - 2] To evaluate the stability in a supercritical CO2 environment, tests were conducted under high pressure. 100g of the aforementioned stability test solution-1 (hereinafter referred to as stability test solution-2 for the purpose of distinguishing between test types) was placed in a glass inner cylinder (manufactured by Taikatsu Glass Industry Co., Ltd., capacity 200mL), and the glass inner cylinder was placed in a portable reactor (manufactured by Taikatsu Glass Industry Co., Ltd., product name TPR5 (TVS-N2 type portable reactor N2-300 with cross joint, sampling tube, valve, pressure gauge, and pressure gauge connection nozzle installed)). After sealing the portable reactor with a torque wrench to 60 N·m, it was placed in a 60°C oven and held there for 30 minutes. Then, liquefied CO2 was injected into the portable reactor so that the pressure gauge read 8 MPa, and after sealing, the internal pressure was maintained at 8-9 MPa at 60°C. If the internal pressure dropped to 8 MPa, liquefied CO2 was reinjected to 9 MPa, and it was stored for 7 days. After storage, the temperature and pressure were released. The stability in a CCS geological environment simulating a high-pressure environment was evaluated according to the following criteria, based on the ratio of the average secondary particle diameter (DLS average particle diameter) of silica particles in the test solution-2 after high-pressure storage to the average secondary particle diameter (DLS average particle diameter) before high-pressure storage <average secondary particle diameter of silica particles after storage (DLS average particle diameter after storage) / average secondary particle diameter of silica particles before storage (DLS average particle diameter before storage)> and the appearance of the test solution after high-pressure storage. <Status test results for CCS geological environment - 2> A: The ratio of the average DLS particle size after (high pressure) storage to the average DLS particle size before (high pressure) storage is between 0.9 and 1.5. B: The ratio of the average DLS particle size after (high pressure) storage to the average DLS particle size before (high pressure) storage is greater than 1.5 and less than or equal to 2.4. C: The ratio of the average DLS particle size after (high pressure) storage to the average DLS particle size before (high pressure) storage is greater than 2.4 and less than or equal to 8.0. D: The ratio of the average DLS particle size after (high pressure) storage to the average DLS particle size before (high pressure) storage is greater than 8.0 and less than or equal to 20.0. E: The ratio of the average particle size of DLS after (high pressure) storage to the average particle size of DLS before (high pressure) storage is greater than 20.0, or turbidity occurs and solid-liquid separation occurs. The results of the stability test in the CCS geological environment - 2 indicate that A is the most preferable result, followed by B, C, D, and E in that order of preference. Considering the effect as a mineralization accelerator, a rating of A or B is desirable. 【0053】 [Silanol Base Amount Evaluation - 1] The amount of silanol groups (mmol / g) related to mineral adsorption was evaluated using TG-DTA. 2 g of aqueous silica sol (mineralization accelerator) from Examples 1-5 or Comparative Example 1 was placed in a centrifuge cell with a filter (Merck, product name Amicon Ultra-15, molecular weight cutoff 100,000, centrifugal ultrafiltration filter unit), and then 4 g of pure water was added. The centrifuge cell was placed in a centrifuge (Tommy Industries, Ltd., product name Suprema 21) and centrifuged at 3000 rpm for 20 minutes. After collecting and quantifying the filtrate, the same amount of pure water as the filtrate was added to the centrifuge cell, and centrifuged again at 3000 rpm for 20 minutes. This process was repeated 5 times to remove components unbound to the silica particles. The silica sol remaining in the filter (ultrafiltration membrane) was then collected and dried at 120°C using a hot plate. Subsequently, the sample for TG-DTA measurement was prepared by drying in an oven at 150°C. After placing 10 mg of the TG-DTA measurement sample into an aluminum container, it was placed in a TG-DTA (manufactured by NETZSCH Japan Co., Ltd., product name STA 2500 Regulus). Nitrogen gas was introduced at a flow rate of 100 cc / min, and the temperature was increased from 25°C at a rate of 10°C per minute until it reached 600°C. The mass loss M1 at the point when the internal temperature reached 200°C was defined as the amount of water removed adsorbed on the surface of the silica particles contained in the TG-DTA measurement sample. The value obtained by subtracting M1 from the mass loss M2 at the point when the internal temperature reached 600°C was defined as the amount of water removed by dehydration condensation between silanol groups on the surface of the silica particles. The amount of silanol groups (mmol / g) in the silica particles was calculated using the following formula. [Math 2] Silanol group content (mmol / g) = 2 × (M2 - M1) ÷ M H2O ÷(M0)×1000 However, M0 is the mass of the silica particle powder subjected to thermogravimetric analysis at 25°C or below before heating, M1 is the mass loss when 200°C is reached, M2 is the mass loss when 600°C is reached, and M H2O The values ​​of each represent the molecular weight of a water molecule. Table 3 shows the mass of silica particle powder before heating (mass before heating), the mass of silica particle powder at 200°C (mass at 200°C), the mass of silica particle powder at 600°C (mass at 600°C), and the amount of silanol groups (mmol / g) obtained from these measurements. 【0054】 [Silanol Base Amount Evaluation - 2] The amount of silanol groups (%) related to mineral adsorption in liquid 29 The evaluation was performed using Si-NMR. Two g of aqueous silica sol (mineralization accelerator) from Examples 1-6 or Comparative Example 1 was placed in a centrifuge cell with a filter (Merck, product name Amicon Ultra-15, molecular weight cutoff 100,000, centrifugal ultrafiltration filter unit), and then four g of pure water was added. The centrifuge cell was placed in a centrifuge (Tommy Industries, Ltd., product name Suprema 21) and centrifuged at 3000 rpm for 20 minutes. After collecting and quantifying the filtrate, an amount of pure water equal to the mass of the filtrate was added to the centrifuge cell, and centrifuged again at 3000 rpm for 20 minutes. After repeating this process five times, pure water was added to the silica sol remaining in the filter portion so that its mass was 2 g, thereby removing excess silane compounds and obtaining a sample for NMR measurement. For NMR measurement, 0.1 mL of heavy water was added to 0.5 mL of the sample, then sealed in a 10 mm Teflon tube. NMR (JEOL, product name ECA500) was used to analyze the liquid sample using a 90-degree pulse, a 120-second waiting time, and 1000 integration cycles. 29 Si-NMR was measured. The obtained spectra were analyzed, and the peak area from -136 ppm to -82 ppm was identified as the peak area originating from the Q structure of Si. The peak area from -116 ppm to -106 ppm was identified as the peak area originating from the Q4 structure derived from Si(OSi)4, the peak area from -105.5 ppm to -96 ppm was identified as the peak area originating from the Q3 structure derived from SiOH(OSi)3, and the peak area from -95 ppm to -85 ppm was identified as the peak area originating from the Q2 structure derived from Si(OH)2(OSi)2. The ratio of Q4, Q3, and Q2 structures to the total Q structure was calculated as (peak area originating from Q4, Q3, or Q2 structure) / (peak area originating from Q structure). The amount of silanol groups was evaluated by analyzing the ratio of Q structures from the integral ratio. The silanol group content -2 (%) was calculated as ((percentage of Q2 structure) × 2 + (percentage of Q3 structure)). Furthermore, in all of Examples 1 to 6 and Comparative Example 1, 29 No peaks in the Si-NMR spectrum with a chemical shift of -80 to -85 ppm, i.e., peaks originating from the Q1 structure (Si(OH)3OSi), were observed. 【0055】 [Table 1] 【0056】 [Table 2] 【0057】 [Table 3] 【0058】 [Table 4] 【0059】 As shown in Tables 1 to 2, the aqueous silica sols (mineralization accelerators) of Examples 1 to 6 received an A rating in the [Mineral Adsorption Test-2], and also received A or B ratings in the [Mineral Adsorption Test-1], [Stability Test in CCS Geological Environment-1] (Salt Tolerance Test), and [Stability Test in CCS Geological Environment-1] (High Pressure Salt Tolerance Test), confirming that they possess mineral adsorption capacity and good salt tolerance. Furthermore, the silica particles constituting the aqueous silica sols (mineralization accelerators) of Examples 1 to 6 are particles with a silanol group content (mmol / g) in the range of 3.5 to 10 mmol / g as determined by thermogravimetric analysis as shown in Table 3, as shown in Table 4. 29 The Si-NMR spectral analysis revealed that the particles had a silanol group content (%) in the range of 8-28%. The silanol groups on the silica particle surface did not condense or aggregate between the silica particles, and as confirmed by the aforementioned mineral adsorption test, these particles are expected to exhibit a mineral adsorption effect. On the other hand, while the aqueous silica sol of Comparative Example 1 showed stability in the CCS geological environment, it was evaluated as having almost no mineral adsorption capacity. (See Table 4) 29Although the silanol group content (%) determined by Si-NMR spectral analysis was 31%, the silanol group content (mmol / g) determined by thermogravimetric analysis (as shown in Table 3) was 3.4 mmol / g. This means that even though the total silanol group content (%) of the silica particles is high, the amount of silanol group content (mmol / g) that can contribute to mineral adsorption is small, resulting in particles where mineral adsorption effect is unlikely.

Claims

[Claim 1] CO ２ A mineralization accelerator comprising silica sol contained in the mineralization accelerator, which comprises silica particles (a) having an average secondary particle diameter of 5 to 200 nm as measured by dynamic light scattering and an aqueous medium (b), Regarding silica particle powder obtained by removing unbound components from silica particles by ultrafiltration, and then heating and drying the mineralization accelerator to remove the aqueous medium, In thermogravimetric analysis, the amount of silanol groups calculated from the mass loss when heated from 25°C to 600°C and the molecular weight of the water molecules using the following formula 1 is 3.5 to 20 mmol / g, and / or, 29. In Si-NMR spectra The peak area from -136 ppm to -82 ppm is called PA. The peak area from -116 ppm to -106 ppm is PA4. The peak area from -105.5 ppm to -96 ppm is PA3. When the peak area from -95 ppm to -85 ppm is defined as PA2, The silica particles are a mineralization accelerator in which the amount of silanol groups (%) calculated by the following two equations is 30 or less. [Mathematics 1] Silanol group content (mol / g) = 2 × (M2 - M1) ÷ M H2O ÷ (M0) × 1000 (However, M0 represents the mass of the silica particle powder subjected to thermogravimetric analysis at 25°C or below before heating, M1 represents the mass decrease of the silica particle powder when it reached 200°C, M2 represents the mass decrease of the silica particle powder when it reached 600°C, and MH2O represents the molecular weight of the water molecule.) [Equation 2] Silanol group content (%) = ((PA2 / PA) × 100) × 2 + ((PA3 / PA) × 100) × 1 + ((PA4 / PA) × 100) × 0 [Claim 2] CaCl ２ In Mineral Adsorption Test-1, a mixture prepared by adding the mineralization accelerator to a pH 3.0 saline solution containing 8% by mass of [substance name] and 2% by mass of NaCl at a concentration of 1% by mass of silica particles, and storing the mixture at 60°C for 1 hour, the mineral adsorption rate was evaluated. The mineralization accelerator according to claim 1, wherein the mineral adsorption rate (%) calculated by the following three equations, based on the Ca concentration in the mixture before storage and the Ca concentration in the filtrate after ultrafiltration of the mixture after storage, is 0.1 to 30%. [Mathematics 3] Mineral adsorption rate (%) = {[(Ca concentration of mixture before storage) - (Ca concentration of filtrate of mixture after storage)] / (Ca concentration of mixture before storage)} × 100 [Claim 3] CaCl ２ In a mineral adsorption test-2, a mixture prepared by adding the mineralization accelerator to a pH 3.0 saline solution containing 8% by mass of [substance name] and 2% by mass of NaCl at a concentration of 1% by mass of silica particles, and storing the mixture at 25°C for 1 hour, the silica particle size in the mixture after storage was evaluated. The mineralization accelerator according to claim 1, wherein the ratio (DLS average particle diameter / BET particle diameter) of the average secondary particle diameter (DLS average particle diameter) of silica particles contained in the mixture after storage, measured by dynamic light scattering, to the specific surface area diameter (BET particle diameter) measured by nitrogen adsorption of silica particles contained in the mineralization accelerator is 2.0 or more and 5.0 or less. [Claim 4] The silica particles have a specific surface area (S) due to nitrogen adsorption. Ｎ２ ) 25-550m ２ The mineralization accelerator according to claim 1, wherein the amount is / g. [Claim 5] The mineralization accelerator according to claim 1, wherein at least a portion of the silica particles are coated with a silane compound (c) having a hydrophilic organic group, and the hydrophilic organic group is an epoxy group-containing organic group, an amino group-containing organic group, a hydroxy group-containing organic group, or a carboxy group-containing organic group. [Claim 6] CaCl ２ In a salt resistance test in which a mixture obtained by adding the mineralization accelerator to a pH 3.0 saline solution containing 8% by mass of and 2% by mass of NaCl, at a concentration such that the silica particle concentration is 1% by mass, is stored at 60°C for 7 days, the ratio of the DLS average particle diameter of the mixture after storage to the DLS average particle diameter of the mixture before storage (DLS average particle diameter of the mixture after storage / DLS average particle diameter of the mixture before storage) is 0.9 to 3.0, according to claim 1. [Claim 7] CaCl ２ To a mixture of saline solution containing 8% by mass of and 2% by mass of NaCl at pH 3.0, the mineralization accelerator was added at a concentration that resulted in a silica particle concentration of 1% by mass, and then CO ２ The mineralization accelerator according to claim 1, wherein in a high-pressure salt resistance test in which the mixture is injected under pressure at 8 MPa and stored at 60°C and 8 MPa for 7 days, the ratio of the DLS average particle size of the mixture after storage to the DLS average particle size of the mixture before storage (DLS average particle size of the mixture after storage / DLS average particle size of the mixture before storage) is 2.4 or less. [Claim 8] CO ２ A method for promoting mineralization of CO, comprising injecting into an underground layer before or after underground injection of CO, the mineralization promoter according to claim 1, or a dilution obtained by adjusting the mineralization promoter to a solid content concentration of 0.001 to 30% by mass in an aqueous medium. ２ A method for promoting mineralization, which includes the step of injection.

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