Direct carbonation offers simplicity and does not require additional chemicals Bobicki et al. The direct gas—solid carbonation route was first investigated by Lackner et al. The slow kinetics of the direct gas—solid carbonation in Ca- or Mg-bearing silicate minerals is the major challenge to be resolved Lackner et al.
Huijgen et al. They suggested a two-step reaction: i Ca leaching from the silicate mineral and ii crystallization of CaCO 3. However, further efforts are required to enhance the reaction conversion, particularly considering the pretreatment step. The pretreatment process is aimed at promoting the carbonation reaction kinetics by providing larger surface area of the raw materials Sanna et al. The process can be conducted through two major processes: i mechanical and ii thermal pretreatments or hybrid processes.
The purpose of mechanical grinding is to destroy the mineral lattice and reduce the particle size, resulting in an increase of the surface area. Nevertheless, the requirement of high energy input is recognized as a critical drawback that must be mitigated.
Thermal pretreatment was also investigated by many researchers, particularly focusing on serpentine to remove hydroxyl groups, resulting in a chemical transformation to pseudoforsterite Sanna et al. It was revealed that thermotreatment can be a more effective option for Mg-bearing silicate minerals to enhance the carbonation efficiency than mechanical pretreatment, although the energy requirement during the process must be further addressed Fabian et al.
Since the indirect carbonation process separates the precipitation step from dissolution of the raw materials, mineral carbonates with higher purity can be expected compared with the direct carbonation route.
In the dissolution step, various additives including strong acids i. Park et al. Although the extraction efficiencies are promising, the use of such strong acids may provoke significant energy penalties associated with their recovery Teir et al.
Furthermore, some acids such as succinic acid and disodium oxalate often chelate the alkaline metals too strongly, and this does not result in the production of carbonates, but rather the precipitation of succinates or oxalates Bonfils et al.
The precipitated mineral carbonates and their derivatives present versatile applications in industrial uses depending on their purity, polymorphism, shape, size and distribution, color, brightness, density, and other many physicochemical properties. Therefore, such a transformation of CO 2 into value-added solid carbonates through ex situ mineral carbonation can partially offset the total cost of the carbon capture and storage process, thus making the mineral carbonation process more viable.
In addition, the particular use of industrial waste materials i. The final products produced by the ex situ mineral carbonation route can be graded into two categories— i low-end high-volume and ii high-end low-volume mineral carbonates, regarding market needs, as well as their properties i.
If necessary, synthesis variables such as temperature, pressure, pH, and concentration of the ingredients in the precipitation step should be manipulated to acquire specific mineral carbonates with targeted properties. It was revealed that high purity calcium or magnesium carbonate can be obtained by the indirect mineral carbonation process separating silica and iron oxide via a pH swing processes Park and Fan, ; Wang and Maroto-Valer, a , b ; Sanna et al.
Recently, a precipitated calcium carbonate PCC production technology utilizing steel converter basic oxygen furnace slag as a calcium source, referred to as Slag2PCC, has been developed and demonstrated successfully by researchers at Aalto university together with their collaborators Said et al.
The process includes ammonium salt e. This may be advantageous because the quality of the final product can be controlled by the operation conditions i. Precipitated calcium carbonate exhibits various polymorphs with tunable physicochemical properties, which play a critical role in determining potential markets. Accordingly, more versatile applications of the products of mineral carbonation are anticipated particularly for calcium carbonate or its derivatives. In general, calcium carbonates exist either in the form of amorphous calcium carbonate ACC or one of the three polymorphs, namely, calcite, aragonite, and vaterite Figure 1.
Recently, it was revealed that ACC normally exists as a monohydrated calcium carbonate Goodwin et al. Table 2 briefly presents the properties of the anhydrous crystalline forms of calcium carbonates. Figure 1. Crystal structures of A calcite, B aragonite, and C vaterite. Ca atoms are displayed as large yellow balls, and carbonate groups are illustrated with gray carbon and red oxygen balls.
Adapted with permission from Chang et al. Copyright American Chemical Society. Among the anhydrous polymorphs of CaCO 3 , calcite is thermodynamically the most stable at ambient conditions. The order of thermodynamic stability is, from most to least, calcite, aragonite, and vaterite Declet et al. Despite lower stabilities from a thermodynamic point of view, aragonite and vaterite can be formed at ambient conditions owing to the kinetic constraints induced by synthesis factors such as temperature and impurities e.
A number of mechanistic studies have been conducted thus far to reveal the transformation mechanisms among the CaCO 3 polymorphs Kralj et al. Although it is necessary to heat to temperature exceeding K for irreversible transformation of vaterite to calcite Chang et al.
Figure 2. Polymorph formation mechanism of the precipitated calcium carbonate, adapted from Wei et al. It is being reproduced with permission from the copyright holder. Zhang et al. Rodriguez-Blanco et al.
They revealed that the second step of transforming metastable vaterite into stable calcite is the rate-determining step, which is controlled by the surface area of calcite via Ostwald ripening Ostwald, , whereas the first dissolution step of ACC followed by transformation into vaterite occurs rapidly.
According to Ostwald ripening, a redeposition of the dissolved small particles e. It was found that the solubility of each polymorph e. The effect of impurities on the transformation of CaCO 3 polymorphs was investigated in the presence of Mg ions by Zhang et al.
The anhydrous crystalline polymorphs of CaCO 3 strongly depend on the synthesis variables such as temperature, pressure, pH of the solution, reaction time, degree of supersaturation, ion concentration and ratio, ionic strength, stirring, type and concentration of additives, and feeding order Tai and Chen, ; Jung et al.
Various synthesis factors and their effects on the formation of CaCO 3 polymorphs have been investigated thus far using simple model chemicals e.
While the synthesis factors affect the formation of polymorphs in multiple and interacting ways, temperature is considered the most critical factor affecting the formation of the polymorphs of CaCO 3. Ogino et al. During the precipitations, pH was monitored and the values ranged from 7.
Chu et al. However, the exact crystal structure of vaterite is still under debate due to the difficulties in obtaining large, pure, single crystals of vaterite Kamhi, ; Mugnaioli et al. Recently, Chang et al.
Figure 3 illustrates the morphological structures of CaCO 3. Figure 3. D Image of structural transformation of vaterite small spherical particles into calcite planar arrays. The effect of temperature on CaCO 3 polymorphs was also investigated by Zhao et al. They synthesized PCC by using a calcium-bearing silicate mineral, wollastonite, via two steps of Ca source extraction and carbonation. The Ca source was extracted by using 1 M acetic acid, and the leachate was reacted with 0.
Said et al. Although the temperature is the major synthesis factor determining the polymorphs of CaCO 3 , other factors such as pH, concentration of ions, impurities, and CO 2 flow rate also compositively interact in the precipitation and crystallization processes.
For instance, aragonite, which is generally formed at elevated temperature, can be obtained at lower temperature in the presence of impurities such as magnesium Park et al. Instead, the CO 2 flow rate was optimized to obtain aragonite rather than rhombohedral calcite. Solution pH is one of the important factors determining polymorphs of CaCO 3 , affecting not only the equilibrium concentration of carbonate species e. In particular, the pH conditions were adjusted from 7.
In their experiments, vaterite was observed under pH lower than 9. However, when the pH exceeded 9. Therefore, calcite was expected to form at higher pH. These results partially agree with the work performed by Tai and Chen They investigated the formation of CaCO 3 polymorphs in a pH range of 8.
Their results also indicate that the calcite was produced as the pH increased. However, pure calcite formed in a different pH range depending on the reaction temperature.
They also investigated the effect of ionic strength. It was observed that as the ionic strength was increased, the yield of calcite decreased, whereas the yield of aragonite increased even at conditions where calcite favorably formed.
Recently, Ramakrishna et al. They reported that pH 10 is the most suitable condition for obtaining pure aragonite, whereas calcite became the dominant polymorph when pH exceeded Overall, calcite is favorably formed at high pH conditions, in general, greater than pH However, aragonite is preferentially obtainable in a pH range of 9— At low pH conditions, typically below than pH 8, vaterite is favored Han et al.
The slag2PCC process includes two stages of Ca extraction and carbonation, based on the recirculation of an aqueous ammonium salt solution Mattila and Zevenhoven, Interestingly, however, the final PCC product was found to be rhombohedral calcite, rather than vaterite or aragonite, which often have been found at similar pH ranges in the previous model chemical studies. In conclusion, pH condition affects the formation of polymorphs of calcium carbonate by shifting the equilibrium concentration of coronate species e.
Other synthesis factors such as reaction time, stirring rate, impurities, and ultrasound treatment are also known to influence the formation of CaCO 3 polymorphs. Reaction aging time is another important factor determining the size and shape as well as the polymorphs of CaCO 3 , by influencing the dissolution and recrystallization of crystals. They were differentiated into two stages, ACC to vaterite and vaterite to calcite transformations.
The composition of the polymorphs was analyzed by using PXRD. They observed the formation of vaterite at the initial reaction; however, when the reaction time was increased to 4 h, calcite was found, although vaterite was still the dominant phase. They concluded that the reaction time more strongly affects the size and shape of the CaCO 3.
The stirring rate in the precipitation of CaCO 3 process affects the particle size and morphologies of the polymorphs. Yan et al. At a lower stirring speed rpm , calcite phase was dominantly observed, and a further increase of stirring speed led to the formation of vaterite. Although the stirring rate has relatively minor effect on the formation of CaCO 3 polymorphs compared to other factors such as temperature, it often influences the hydrodynamics on the particle dynamics Han et al.
Han et al. This could be ascribed to the total surface free energy: because the initially formed fine particles were unstable due to their high surface free energy, the fine crystals tended to aggregate to achieve a minimum total surface free energy. The impurities are also known to significantly influence the formation of CaCO 3 particles. Therefore, providing an efficient impurity e.
De Crom et al. Morandeau and White investigated the effect of MgO content in the mineral carbonation using blast furnace slag. An ultrasound irradiation technique has been applied to the formation of CaCO 3 polymorphs Price et al. The use of ultrasound can help control the crystallization process, and this is referred to as sonocrystallization de Castro and Priego-Capote, Price et al.
Aragonite only formed at the high-intensity ultrasonic irradiation conditions. Wagterveld et al. The results revealed that applying ultrasonic treatment helped increase the available surface area for polymorph growth, resulting in a higher volumetric precipitation rate. It was also reported that a more uniform size distribution of the precipitated polymorphs could be anticipated. The particle size is compositively affected by various factors such as temperature, pH, additive types impurities , concentration of calcium and CO 2 , solvent ratio, CO 2 flow rate, stirring rate, and reaction time Boyjoo et al.
Bang et al. They found that the Ca OH 2 concentration and the CO 2 flow rate significantly influenced the specific surface area, as well as the size of primary and secondary CaCO 2 particles.
Specifically, as the Ca OH 2 concentration was increased from 0. They also revealed that the CO 2 bubble size also affected the particle size and specific surface area Bang et al. Because ex situ mineral carbonation entails a series of chemical processes above ground via reactions between CO 2 and alkaline earth metals such as calcium or magnesium that are extracted not only from naturally occurring silicate minerals, i. First, this technique allows the utilization of alkaline-metal feedstock extracted from industrial wastes.
Therefore, it could provide an appropriate method for proper disposal or for recycling of the industrial wastes. Second, the final products, i. The precipitate calcium carbonate quality is of utmost importance for practical implementation of an ex situ mineral carbonation because it could partly offset the total cost of CCS. In this review, we addressed the key factors affecting the formation of CaCO 3 polymorphs, including temperature, pH, concentration, reaction time, stirring, impurities, and ultrasound, particularly focusing on model chemical studies.
Therefore, a careful review of how each factor affects the polymorphs of CaCO 3 could offer insight into the creation of highly value-added CaCO 3 and its emerging applications for economically viable deployment.
Further efforts to precisely control the morphology, agglomeration, and particle size distribution as well as polymorphs of the PCCs via research on the carbonation kinetics should be made together with designing demonstration processes for commercializing the technology. All appropriate permissions have been obtained from the copyright holders of any work that has been reproduced in the manuscript. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Utilization of carbon dioxide by chemically accelerated mineral carbonation. Bang, J. Specific surface area and particle size of calcium carbonate precipitated by carbon dioxide microbubles. Energies 8, — Beck, R. The onset of spherulitic growth in crystallization of calcium carbonate. Growth , — Bobicki, E. Carbon capture and storage using alkaline industrial wastes. Energy Combust.
Bonfils, B. Comprehensive analysis of direct aqueous mineral carbonation using dissolution enhancing organic additives. Greenhouse Gas Control 9, — Boyjoo, Y. This increased storage security. A team in the United States found a similar mineralization rate on the flood basalts of the Columbia River. The European Union has sponsored future versions of CarbFix, and an international consortium has formed with the goal of using CCS to lower geothermal emissions.
If some of those rock types are feasible to use for this method, we could broaden the applicability even more. The team is also looking into how well offshore injections using seawater might work. Offshore injection would make this method an option in regions with limited freshwater resources or that might be prone to induced seismicity.
Covering Climate Now is a global journalism collaboration committed to strengthening coverage of the climate story. Cartier, K. Published on 20 March Geologic storage is defined as the placement of CO 2 into a subsurface formation so that it will remain safely and permanently stored. The U. Department of Energy DOE is investigating five types of underground formations for geologic carbon storage:.
Myth: Carbon capture and storage is not a feasible way to reduce human CO 2 emissions. Reality: Developing the technologies and know-how to successfully capture and store CO 2 emissions will allow for a viable industry that will reduce the human contribution to atmospheric CO 2 levels. Carbon dioxide CO 2 can be stored underground as a supercritical fluid. Supercritical CO 2 means that the CO 2 is at a temperature in excess of At such high temperatures and pressures, the CO 2 has some properties like a gas and some properties like a liquid.
In particular, it is dense like a liquid but has viscosity like a gas. At depths below about meters about 2, feet , the natural temperature and fluid pressures are in excess of the critical point of CO 2 for most places on Earth. This means that CO 2 injected at this depth or deeper will remain in the supercritical condition given the temperatures and pressures present. Myth: The CO 2 gas behaves the same in the atmosphere as it does when injected deep underground. Reality: The elevated temperatures and pressures that exist at the depths where CO 2 is injected changes its characteristics, allowing for storage of much greater volumes of CO 2 than at the surface.
Trapping refers to the way in which the carbon dioxide CO 2 remains underground in the location where it is injected. There are four main mechanisms that trap the injected CO 2 in the subsurface. Each of these mechanisms plays a role in how the CO 2 remains trapped in the subsurface. The following provides a description of each type of trapping mechanism. Structural Trapping — Structural trapping is the physical trapping of CO 2 in the rock and is the mechanism that traps the greatest amount of CO 2.
The rock layers and faults within and above the storage formation where the CO 2 is injected act as seals, preventing CO 2 from moving out of the storage formation. Once injected, the supercritical CO 2 can be more buoyant than other liquids present in the surrounding pore space. Therefore, the CO 2 will migrate upwards through the porous rocks until it reaches and is trapped by an impermeable layer of seal rock. Diagram depicting two examples of structural trapping.
The top image shows the CO 2 being trapped beneath a dome, preventing it from migrating laterally or vertically. The bottom image shows that CO 2 is prevented from migrating vertically by the overlying seal rock and a fault to the right of the CO 2.
Residual Trapping — Residual trapping refers to the CO 2 that remains trapped in the pore space between the rock grains as the CO 2 plume migrates through the rock. The existing porous rock acts like a rigid sponge. When supercritical CO 2 is injected into the formation, it displaces the existing fluid as it moves through the porous rock. As the CO 2 continues to move, small portions of the CO 2 can be left behind as disconnected, or residual, droplets in the pore spaces which are essentially immobile, just like water in a sponge.
Diagram depicting the pockets of residually trapped CO 2 in the pore space between the rock grains as the CO 2 migrates to the right through the openings in the rock.
Solubility Trapping — In solubility trapping, a portion of the injected CO 2 will dissolve into the brine water that is present in the pore spaces within the rock.
0コメント