Polymorphs of pure calcium carbonate prepared by the mineral carbonation of flue gas desulfurization gypsum
Graphical abstract
Introduction
Mineral carbonation is one of the methods for mitigating atmospheric CO2 gas. This method mimics global silicate-weathering processes (Eq. (1)), wherein CO2 is converted into inorganic carbonates using Ca/Mg-bearing minerals or industrial waste [1]. This method has attracted both industrial and academic interest because it enables the permanent storage of CO2 without leakage. Nonetheless, some major issues such as high energy consumption and/or the disposal of carbonated products require resolution [2].
The reaction routes for mineral carbonation are divided into two major categories: direct and indirect processes [3]. Direct carbonation is carried out using a single process step and is usually employed to immobilize toxic elements in industrial materials. In contrast, in indirect carbonation the extraction and carbonation steps are performed separately. The indirect method has received more attention because it facilitates an increase in the carbonation rate or in the purity of the Ca/Mg carbonates produced. Recent research has focused on the feasibility of producing pure calcium carbonate (CaCO3) during mineral carbonation using industrial wastes [4].
Flue gas desulfurization (FGD) gypsum is produced by the FGD process, which is the removal of sulfur oxides from flue gas in coal-fired power plants. Mineral carbonation of FGD gypsum is one of the methods that have been investigated for CO2 sequestration [5]. Previously [6], we demonstrated the feasibility of producing high-purity CaCO3 through the direct carbonation of FGD gypsum under ambient conditions, which can be described by the following reaction:
Pure CaCO3 was successfully obtained during an induction period, in which CaCO3 exists in the dissolved form wherein impurities were easily separated before precipitation [7].
CaCO3 is one of the most important materials in polymer industry [8], [9]. Various techniques have been developed to manipulate CaCO3 using environmentally undesirable by-products [10], [11]. It exists as three anhydrous crystalline polymorphs (calcite, aragonite, and vaterite), two hydrated metastable forms (monohydrocalcite and calcium carbonate hexahydrate), and one unstable amorphous phase. To the best of our knowledge, this is the first attempt to control the phase and/or morphology of pure CaCO3 synthesized by using industrial by-products and greenhouse gas CO2. Herein, we demonstrate the possibility of controlling which polymorph of pure CaCO3 is obtained during the direct aqueous carbonation of FGD gypsum.
Section snippets
Materials
FGD gypsum was supplied by Yeongheung Thermal Power Plants, Incheon, Korea and was the same material as was used in our previous work [6], [7]. It was used without performing a pulverizing or grinding process, because the particle size fraction of the sample was in the 1–100 μm range, which corresponds to the fraction generally employed in mineral carbonation [12]. The FGD gypsum was primarily composed of calcium sulfate dihydrate (CaSO4·2H2O) with a purity of approximately 95%; Si, Al, and Fe
Effect of ammonia on the polymorphs of pure CaCO3 precipitated from solution
In a slow precipitation process, CaCO3 exists as a solvated pair (Ca2+ and CO32−) before crystallization is induced [14]. The sequential formation of CaCO3 during the direct aqueous carbonation of FGD gypsum has been described in detail in our previous study [6], [7]. The amount of CaCO3 dissolved in the solution was found to increase with increasing ammonia concentration. Fig. 1 shows the maximum amount of pure CaCO3 (dissolved) at various ammonia concentrations (1.5, 4, 8, 12, and 25 vol.%).
Conclusions
Pure CaCO3 crystals were precipitated by the direct aqueous carbonation of FGD gypsum by exploiting an induction period. The phase and morphology of CaCO3 were controlled by adding ethanol to the solution extracted during the induction period under conditions of stoichiometric and excess ammonia. The amount of dissolved CaCO3 increased with ammonia concentration.
When using a stoichiometric amount of ammonia only calcite crystals with rhombohedral morphology were precipitated. However, the
Acknowledgements
This research was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (KIGAM) funded by the Ministry of Science, ICT and Future Planning of Korea.
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