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Polymorphs of pure calcium carbonate prepared by the mineral carbonation of flue gas desulfurization gypsum
Materials & Design 83 (2015) 308–313
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Polymorphs of pure calcium carbonate prepared by the mineral carbonation of flue gas desulfurization gypsum Kyungsun Song, Wonbaek Kim ⇑, Jun-Hwan Bang, Sangwon Park, Chi Wan Jeon Korea Institute of Geoscience & Mineral Resources (KIGAM), Gwahang-no 124, Yuseong-gu, Daejeon 305-350, Republic of Korea
a r t i c l e
i n f o
Article history: Received 16 March 2015 Revised 19 May 2015 Accepted 7 June 2015 Available online 19 June 2015 Keywords: CaCO3 polymorphism Flue gas desulfurization gypsum Mineral carbonation Ammonia Ethanol
a b s t r a c t We previously developed a process for the precipitation of pure CaCO3 by exploiting the induction period of one-step mineral carbonation of flue gas desulfurization gypsum. Herein, the process was further investigated to elucidate CaCO3 polymorphism as a function of the addition of ammonia and ethanol using quantitative X-ray diffraction and field-emission scanning electron microscopy. Calcite, which was the dominant phase when using a stoichiometric amount of ammonia, was replaced by vaterite upon the addition of excess ammonia. Ethanol tends to induce vaterite and aragonite phases under stoichiometric and excess ammonia conditions, respectively. Thus, when using excess ammonia, single-phase aragonite was crystallized when the ethanol concentration exceeded 30 vol.%. Ethanol stabilized the vaterite phase, which otherwise transformed into superstructure calcite upon contact with water. This process offers a simple method for manipulating the phase and morphology of clean CaCO3 produced using industrial by-products by mineral carbonation. Ó 2015 Elsevier Ltd. All rights reserved.
1. 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].
ðCa; MgÞx Siy Oxþ2yþz H2zðsÞ þ xCO2ðgÞ ! xðCa; MgÞCO3ðsÞ þ ySiO2ðsÞ þ zH2 O
ð1Þ
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]. ⇑ Corresponding author. E-mail address: [email protected] (W. Kim). http://dx.doi.org/10.1016/j.matdes.2015.06.051 0264-1275/Ó 2015 Elsevier Ltd. All rights reserved.
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:
CaSO4 2H2 OðsÞ þ CO2ðgÞ þ 2NH4 OHðaqÞ ! CaCO3ðsÞ þ ðNH4 Þ2 SO4ðaqÞ
ð2Þ
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.
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2. Experimental
Analytical) and field-emission scanning electron microscopy (FE-SEM; S-4700, Hitachi).
2.1. Materials 2.3. Determination of phase composition by X-ray diffraction 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 lm range, which corresponds to the fraction generally employed in mineral carbonation [12]. The FGD gypsum was primarily composed of calcium sulfate dihydrate (CaSO42H2O) with a purity of approximately 95%; Si, Al, and Fe were present as minor impurities. Anhydrous ethanol and nitric acid were purchased from Sigma– Aldrich, and the deionized water used was purified using a Milli-Q 18 MX cm system (Millipore).
2.2. Preparation of pure CaCO3 using ammonia and ethanol Carbonation was carried out by injecting CO2 gas (99.9 vol.% purity) into an aqueous ammonia solution containing solid particles of FGD gypsum. The amount of dissolved CaCO3 was measured under the following conditions: CO2 flow rate = 1 L/min, [NH3] = 0.5–25 vol.%, and solid to liquid (s/l) ratio = 200 g/L. The suspension was filtered in approximately 20 mL portions at 5 min intervals during the carbonation. The solution containing dissolved CaCO3 was initially transparent, but later turned into a milky-white suspension before the precipitation of CaCO3 particles. The amount of CaCO3 precipitated was estimated by measuring the Ca concentration in the solution before and after the precipitation. The Ca concentration in solution after the precipitation was <1.0 mM in all cases. To investigate the effect of ammonia on CaCO3 precipitation, carbonation was carried out under both stoichiometric and excess ammonia conditions. Under stoichiometric conditions (stoichiometric ratio of CO2/NH3 = 2), the sampling was performed after carbonation for 5 min under the following conditions: CO2 flow rate = 0.3 L/min, total injected CO2 volume = 1.5 L (approximately 0.06 mol), s/l ratio = 20 g/L (total Ca = approximately 0.1 mol/L), and [NH3] = 0.5 vol.% (0.13 mol). Under excess ammonia conditions, the sampling was performed after carbonation for 20 min under the following conditions: CO2 flow rate = 1 L/min, s/l ratio = 20 g/L, and [NH3] = 12 vol.% (8.48 mol). For each condition, the sampling time was chosen to be when the dissolved CaCO3 was maximum. To evaluate the effect of ethanol on the CaCO3 polymorphs formed, ethanol (10, 30, 50, 70, and 90 vol.%) was mixed with the solution extracted during the induction period in Nalgene bottles. After mixing, the solutions were gently shaken at 70 rpm using a digital reciprocating shaker (SHR-2D, DAIHAN Scientific). The stability of vaterite in the presence of water was assessed by soaking and shaking it in deionized water for two days. All of the carbonation experiments were conducted under ambient conditions (room temperature and atmospheric pressure) while monitoring the temperature and pH (Orion 410A, Thermo Scientific). The solutions were filtered using a 0.2 lm membrane filter (Nylon, Sartorius). The calcium ion concentrations were determined using inductively coupled plasma-optical emission spectrometry (ICP-OES; Optima 5300DV, PerkinElmer) after the acidification of the samples to pH 2 with instrumental-grade HNO3. All the precipitated CaCO3 particles were washed with anhydrous ethanol or deionized water, and then dried either under vacuum or in air overnight at 30 °C. The precipitated CaCO3 particles were examined by X-ray diffraction (XRD; X’pert MPD, Philips
The relative amounts (%) of the CaCO3 crystalline phases, i.e., calcite, aragonite, and vaterite, were estimated using Eqs. (3) and (4), as proposed by Kontoyannis and Vagenas [13]. For a mixture of aragonite and calcite:
XA ¼
3:157I221A 100; I104C þ 3:157I221A
X C ¼ 100 X A
ð3Þ
and for a mixture of vaterite and calcite:
XV ¼
7:691I110V 100; I104C þ 7:691I110V
X C ¼ 100 X V
ð4Þ
where XA, XV, and XC are the relative amount (%) of aragonite, vaterite, and calcite, respectively. I represents the integrated areas of the XRD peaks, and subscripts A, V, and C represent the aragonite, vaterite, and calcite phases, respectively, i.e., I221A, I104C, and I110V are the areas of the (2 2 1), (1 0 4), and (1 1 0) peaks of aragonite, calcite, and vaterite, respectively. 3. Results and discussion 3.1. 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 CO2 3 ) 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.%). The pH was higher than 9.0 in each case. The ammonia tended to slow the carbonation rate, which prolonged the induction period, and accordingly increased the amount of pure CaCO3 obtained. We attributed this effect to the formation of carbamate (NH2CO 2 ) [7], which is the dominant species formed in the reaction between CO2 and excess ammonia. The related reactions can be expressed as follows [15]:
2NH3 þ CO2 ! NH2 CO2 þ NHþ4 ;
K eqð273Þ ¼ 2:35 104
þ 2NH3 þ CO2 þ H2 O ! CO2 3 þ 2NH4 ;
K eqð273Þ ¼ 8:89 102
ð5Þ ð6Þ
The polymorphs of the precipitated CaCO3 were examined after washing with anhydrous ethanol several times and drying under vacuum at 30 °C to prevent a possible transformation of
Fig. 1. Dissolved CaCO3 (mM) as a function of ammonia concentration (vol.%).
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Fig. 2. Field-emission scanning electron microscopy (FE-SEM) images of CaCO3 obtained under (a) excess and (b) stoichiometric ammonia conditions.
It is difficult to anticipate the influence of external factors during the precipitation of CaCO3, including those of the solvent components. However, the use of an aqueous solution of ethanol is considered a typical method for synthesizing or stabilizing metastable vaterite [21]. In the following sections, we evaluate the likelihood of phase changes, control of morphology, and stabilization of vaterite by an ethanol–water binary mixed solvent, from the solution extracted during the induction period.
3.2. Effect of ethanol on the polymorphs of pure CaCO3 precipitated using stoichiometric ammonia
Fig. 3. X-ray diffraction (XRD) patterns of pure CaCO3 precipitated from mixed solvents of (a) 0, (b) 10, (c) 30, and (d) 50 vol.% of ethanol in water under stoichiometric ammonia conditions. c = calcite and v = vaterite.
metastable vaterite into calcite under water-mediated conditions [16]. We found that the phase and morphology of the CaCO3 precipitate were strongly dependent on the amount of ammonia in the solution. For example, spherical vaterite was the only phase observed under conditions of excess ammonia (Fig. 2a), whereas calcite with rhombohedral morphology was formed under stoichiometric conditions (Fig. 2b). Calcite is the most stable polymorph of CaCO3. Although vaterite is the least thermodynamically stable among the anhydrous crystalline CaCO3 phases, it is often observed at ambient temperature in the presence of ammonia/ammonium [17–19]. This is because NH+4 interacts with the CO2 present on the surface of 3 the negatively charged crystal planes. Here, vaterite had a spherical morphology, which is its most frequently observed shape [17]. Spherical vaterite particles were formed in gelatinous amorphous CaCO3, resulting in spherulitic growth with a radiating pattern (see the magnified inset in Fig. 2a) [20].
The effect of ethanol on the polymorphs of pure CaCO3 was diverse, depending on the amount of ammonia present in the solution. In the presence of a stoichiometric amount of ammonia, the amount of vaterite phase increased with increasing ethanol content. Although no vaterite crystals were detected when 10 vol.% ethanol was added, the amount of vaterite increased to 25% and 73% in the presence of 30 and 50 vol.% of ethanol, respectively (Fig. 3). However, in the presence of 70 or 90 vol.% ethanol, the precipitated products were not CaCO3, but rather were compounds composed of calcium, ammonium, and sulfate ions, i.e., (NH4)2SO4 and (NH4)2Ca(SO4)2H2O. This is probably because CO2 3 production was limited under low H2O conditions, as is seen in Eq. (6). The polymorphs of the CaCO3 crystals precipitated from ethanol–water binary mixed solvents are shown in Fig. 4. The CaCO3 crystals prepared from the 10 vol.% ethanol solvent consisted of only the calcite phase (Fig. 3a) with some imperfect rhombohedral shapes (Fig. 4a). With the 30 vol.% ethanol solvent, the amount of imperfect rhombohedral calcite crystals increased, which appeared to be formed through the agglomeration of particles (Fig. 4b). Ethanol is known to damage calcite surfaces by binding more strongly at the calcite surface than with water [22]. This might block the surface and inhibit the perfect growth of calcite. Accordingly, in the presence of 50 vol.% ethanol, spherical vaterite
Fig. 4. Morphology of pure CaCO3 precipitated from a mixed solvent of (a) 10, (b) 30, and (c) 50 vol.% ethanol in water under stoichiometric ammonia conditions.
K. Song et al. / Materials & Design 83 (2015) 308–313
Fig. 5. X-ray diffraction (XRD) patterns of pure CaCO3 precipitated from mixed solvents of (a) 0, (b) 10, (c) 30, and (d) 50 vol.% ethanol in water under excess ammonia conditions. a = aragonite, c = calcite, and v = vaterite.
311
Fig. 7. X-ray diffraction (XRD) patterns of vaterite from (a) 0 and (b) 10 vol.% ethanol in water after soaking for two days. c = calcite and v = vaterite.
3.3. Effect of ethanol on the polymorphs of pure CaCO3 precipitated using excess ammonia particles occur together with calcite crystals, which have rough faces (Fig. 4c). The effect of ethanol on the formation of metastable vaterite has primarily been explained by two mechanisms. One postulates that ethanol lowers the solubility of CaCO3, which eventually increases its supersaturation [23]. This promotes the production of the kinetically favored vaterite phase, rather than thermodynamically favored calcite. The other mechanism is related to the interaction between Ca2+ and CO2 3 ions and the solvents (water and ethanol). 2+ Ca2+ and CO2 ions, 3 ions are highly solvated in H2O, but only Ca which have a higher electric charge density, are solvated in ethanol. In addition, the weak interaction of Ca2+ with ethanol relative to water requires a lower solvation energy, which facilitates the formation of metastable vaterite. Zhang et al. used a molecular dynamics simulation that demonstrated that the morphology and phase of CaCO3 could be controlled by varying the amount of ethanol, which resulted in the control of the polarity difference between Ca2+ and CO2 3 in the mixed solvent [24].
In the absence of ethanol, only vaterite was obtained under the conditions of excess ammonia. However, a complete phase change from vaterite to aragonite occurred upon the addition of either 30 or 50 vol.% ethanol (Fig. 5). This is an interesting result because we could produce pure aragonite using this method. However, solutions containing 70 or 90 vol.% ethanol did not precipitate CaCO3; instead, compounds composed of calcium, ammonium, and sulfate, i.e., (NH4)2SO4 and (NH4)2Ca(SO4)2H2O were formed regardless of the ammonia content. This was consistent with a previous report 2 that the formation of HCO 3 or CO3 is unstable and reversible during CO2 sorption in ethanol solution [25]. Aragonite is the stable phase of CaCO3 at temperatures above 40 °C [26]. Conventionally, pure aragonite is synthesized through two major routes. The first is to use aqueous alcohol solution and varying the solvent properties [23]. The other is the direct synthesis from amorphous CaCO3 in an ethanol mixed solvent containing a large quantity of Mg2+ [27,28]. In the latter route in particular, the
Fig. 6. Peanut-like crystals of aragonite with dandelion-like heads of pure CaCO3 precipitated from ethanol–water binary mixed solvents under excess ammonia conditions: (a and b) 30 and (c and d) 50 vol.% ethanol, respectively.
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Fig. 8. Morphology of transformed vaterite under excess ammonia conditions after soaking in water for two days: (a) superstructure calcite, (b) intermediate multilayered calcite, and (c) calcite crystals on the vaterite surface.
Fig. 9. Morphology of transformed vaterite under excess ammonia conditions after soaking in 10 vol.% ethanol in water for two days. Arrows highlight calcite crystals. (b) is the magnified image of the area marked in (a), and (c) is another magnified region.
amorphous CaCO3 is strongly stabilized by Mg2+, showing that the amount of Mg2+ ions present has a great influence on morphology. Tang et al. synthesized peanut-like aragonite using anhydrous ethanol with the Mg2+ to Ca2+ molar ratio of 8:1 [28]. They ascribed this unique morphology to the prolonged stability of amorphous CaCO3 in the presence of both ethanol and Mg2+. The aragonite prepared herein has a unique peanut-like morphology with dandelion-like heads (Fig. 6). In our case, in the absence of Mg2+, ammonia might have stabilized the amorphous CaCO3 in the same way [29]. The preferential nucleation of aragonite crystals could result from the synergetic effect of ammonia and ethanol in disrupting calcite formation. 3.4. Effect of ethanol on the stability of vaterite Metastable vaterite transforms into stable calcite upon exposure to water [30]. However, both ammonia and ethanol stabilize vaterite in aqueous systems. We evaluated the effect of ethanol on the stability of vaterite under conditions of excess ammonia. Fig. 7 shows the XRD patterns of vaterite particles after soaking in water for two days. In the absence of ethanol, vaterite transformed completely into calcite (Fig. 7a). In contrast, in the presence of 10 vol.% ethanol, only 15% of vaterite was transformed into calcite (Fig. 7b). This, it is clear that the vaterite phase was stabilized significantly by the addition of a small amount of ethanol. The morphological transformation of the vaterite crystals under the conditions of excess ammonia is illustrated in Fig. 8. After soaking in water for two days, the particles changed completely into superstructure calcite (Fig. 8a). In contrast, after a brief washing with water, rhombohedral calcite crystals were formed on the surface of vaterite (Fig. 8c), and the intermediate multilayered calcite crystals resembled those of the mother vaterite (Fig. 8b). This showed the templating action of vaterite, which was also observed when using polymer additives [31]. This templating action during the transformation was also detected by Shen et al. [32]. However, upon the addition of 10 vol.% ethanol, the morphology of the
transformed vaterite was different. Fig. 9 shows the transformed vaterite when isolated from its mother crystals. As observed in Fig. 9c, calcite was likely formed by a typical dissolution–crystallization process, rather than by the templating action of vaterite that was observed in the absence of ethanol. The vaterite produced herein was very unstable and readily transformed into the stable calcite phase, especially in the absence of ethanol. Ammonium ions facilitated the formation of vaterite, but their action is more reversible than those of polymers [33], and vaterite crystals formed in the presence of ammonium ions are easily transformed into stable calcite [17,19]. Based upon our previous results, it is obvious that compared to ammonia and water, ethanol binds more strongly to vaterite. This result is consistent with a previous study that showed that ethanol is chemisorbed onto CaCO3 and binds more strongly than water [30].
4. 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 amount of vaterite precipitated increased with the addition of 30 and 50 vol.% ethanol. Under excess-ammonia condition, spherical vaterite crystals were formed. However, peanut-like aragonite crystals with dandelion-like heads were formed when 30 and 50 vol.% ethanol was used under excess-ammonia conditions. However, in solutions containing 70 or 90 vol.% ethanol, the reaction products were not CaCO3, but were rather compounds composed of calcium, ammonium, and sulfate, i.e., (NH4)2SO4 and
K. Song et al. / Materials & Design 83 (2015) 308–313
(NH4)2Ca(SO4)2H2O, regardless of the amount of ammonia in solution. When spherical vaterite, which was precipitated in the absence of ethanol, was immersed in water, it transformed completely into superstructure calcite through the templating action of vaterite. However, with the addition of 10 vol.% ethanol, the amount of vaterite transformed was significantly reduced. Our results show the synergistic effect of ammonia and ethanol the polymorphic selectivity of aragonite in the absence of Mg2+. Thus, the method developed herein can be used to control the phase of morphology of pure CaCO3 produced using industrial by-products as FGD gypsum. 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. References [1] W. Seifritz, CO2 disposal by means of silicates, Nature 345 (1990) 486. [2] A. Kirchofer, A. Becker, A. Brandt, J. Wilcox, CO2 mitigation potential of mineral carbonation with industrial alkalinity sources in the United States, Environ. Sci. Technol. 47 (2013) 7548–7554. [3] E.R. Bobicki, Q. Liu, Z. Xu, H. Zeng, Carbon capture and storage using alkaline industrial wastes, Energy Combust. Sci. 38 (2012) 302–320. [4] M.M.M.G.P.G. Mantilaka, R.M.G. Rajapakse, D.G.G.P. Karunaratne, H.M.T.G.A. Pitawala, Preparation of amorphous calcium carbonate nanoparticles from impure dolomitic marble with the aid of poly(acrylic acid) as a stabilizer, Adv. Powder Technol. 25 (2014) 591–598. [5] M.g. Lee, Y.N. Jang, K.w. Ryu, W. Kim, J.-H. Bang, Mineral carbonation of flue gas desulfurization gypsum for CO2 sequestration, Energy 47 (2012) 370–377. [6] K. Song, Y.-N. Jang, W. Kim, M.G. Lee, D. Shin, J.-H. Bang, C.W. Jeon, S.C. Chae, Precipitation of calcium carbonate during direct aqueous carbonation of flue gas desulfurization gypsum, Chem. Eng. J. 213 (2012) 251–258. [7] K. Song, Y.-N. Jang, W. Kim, M.G. Lee, D. Shin, J.-H. Bang, C.W. Jeon, S.C. Chae, Factors affecting the precipitation of pure calcium carbonate during the direct aqueous carbonation of flue gas desulfurization gypsum, Energy 65 (2014) 527–532. [8] A. Afshar, I. Massoumi, R.L. Khosh, R. Bagheri, Fracture behavior dependence on load-bearing capacity of filler in nano- and microcomposites of polypropylene containing calcium carbonate, Mater. Des. 31 (2010) 802–807. [9] H. He, K. Li, J. Wang, G. Sun, Y. Li, J. Wang, Study on thermal and mechanical properties of nano-calcium carbonate/epoxy composites, Mater. Des. 32 (2011) 4521–4527. [10] N. Ramdani, J. Wang, X.-Y. He, T.-T. Feng, X.-D. Xu, W.-B. Liu, X.-S. Zheng, Effect of crab shell particles on the thermomechanical and thermal properties of polybenzoxazine matrix, Mater. Des. 61 (2014) 1–7. [11] T. Boronat, V. Fombuena, D. Garcia-Sanoguera, L. Sanchez-Nacher, R. Balart, Development of a biocomposite based on green polyethylene biopolymer and eggshell, Mater. Des. 68 (2015) 177–185.
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Polymorphs of pure calcium carbonate prepared by the mineral carbonation of flue gas desulfurization gypsum Kyungsun Song, Wonbaek Kim ⇑, Jun-Hwan Bang, Sangwon Park, Chi Wan Jeon Korea Institute of Geoscience & Mineral Resources (KIGAM), Gwahang-no 124, Yuseong-gu, Daejeon 305-350, Republic of Korea
a r t i c l e
i n f o
Article history: Received 16 March 2015 Revised 19 May 2015 Accepted 7 June 2015 Available online 19 June 2015 Keywords: CaCO3 polymorphism Flue gas desulfurization gypsum Mineral carbonation Ammonia Ethanol
a b s t r a c t We previously developed a process for the precipitation of pure CaCO3 by exploiting the induction period of one-step mineral carbonation of flue gas desulfurization gypsum. Herein, the process was further investigated to elucidate CaCO3 polymorphism as a function of the addition of ammonia and ethanol using quantitative X-ray diffraction and field-emission scanning electron microscopy. Calcite, which was the dominant phase when using a stoichiometric amount of ammonia, was replaced by vaterite upon the addition of excess ammonia. Ethanol tends to induce vaterite and aragonite phases under stoichiometric and excess ammonia conditions, respectively. Thus, when using excess ammonia, single-phase aragonite was crystallized when the ethanol concentration exceeded 30 vol.%. Ethanol stabilized the vaterite phase, which otherwise transformed into superstructure calcite upon contact with water. This process offers a simple method for manipulating the phase and morphology of clean CaCO3 produced using industrial by-products by mineral carbonation. Ó 2015 Elsevier Ltd. All rights reserved.
1. 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].
ðCa; MgÞx Siy Oxþ2yþz H2zðsÞ þ xCO2ðgÞ ! xðCa; MgÞCO3ðsÞ þ ySiO2ðsÞ þ zH2 O
ð1Þ
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]. ⇑ Corresponding author. E-mail address: [email protected] (W. Kim). http://dx.doi.org/10.1016/j.matdes.2015.06.051 0264-1275/Ó 2015 Elsevier Ltd. All rights reserved.
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:
CaSO4 2H2 OðsÞ þ CO2ðgÞ þ 2NH4 OHðaqÞ ! CaCO3ðsÞ þ ðNH4 Þ2 SO4ðaqÞ
ð2Þ
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.
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2. Experimental
Analytical) and field-emission scanning electron microscopy (FE-SEM; S-4700, Hitachi).
2.1. Materials 2.3. Determination of phase composition by X-ray diffraction 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 lm range, which corresponds to the fraction generally employed in mineral carbonation [12]. The FGD gypsum was primarily composed of calcium sulfate dihydrate (CaSO42H2O) with a purity of approximately 95%; Si, Al, and Fe were present as minor impurities. Anhydrous ethanol and nitric acid were purchased from Sigma– Aldrich, and the deionized water used was purified using a Milli-Q 18 MX cm system (Millipore).
2.2. Preparation of pure CaCO3 using ammonia and ethanol Carbonation was carried out by injecting CO2 gas (99.9 vol.% purity) into an aqueous ammonia solution containing solid particles of FGD gypsum. The amount of dissolved CaCO3 was measured under the following conditions: CO2 flow rate = 1 L/min, [NH3] = 0.5–25 vol.%, and solid to liquid (s/l) ratio = 200 g/L. The suspension was filtered in approximately 20 mL portions at 5 min intervals during the carbonation. The solution containing dissolved CaCO3 was initially transparent, but later turned into a milky-white suspension before the precipitation of CaCO3 particles. The amount of CaCO3 precipitated was estimated by measuring the Ca concentration in the solution before and after the precipitation. The Ca concentration in solution after the precipitation was <1.0 mM in all cases. To investigate the effect of ammonia on CaCO3 precipitation, carbonation was carried out under both stoichiometric and excess ammonia conditions. Under stoichiometric conditions (stoichiometric ratio of CO2/NH3 = 2), the sampling was performed after carbonation for 5 min under the following conditions: CO2 flow rate = 0.3 L/min, total injected CO2 volume = 1.5 L (approximately 0.06 mol), s/l ratio = 20 g/L (total Ca = approximately 0.1 mol/L), and [NH3] = 0.5 vol.% (0.13 mol). Under excess ammonia conditions, the sampling was performed after carbonation for 20 min under the following conditions: CO2 flow rate = 1 L/min, s/l ratio = 20 g/L, and [NH3] = 12 vol.% (8.48 mol). For each condition, the sampling time was chosen to be when the dissolved CaCO3 was maximum. To evaluate the effect of ethanol on the CaCO3 polymorphs formed, ethanol (10, 30, 50, 70, and 90 vol.%) was mixed with the solution extracted during the induction period in Nalgene bottles. After mixing, the solutions were gently shaken at 70 rpm using a digital reciprocating shaker (SHR-2D, DAIHAN Scientific). The stability of vaterite in the presence of water was assessed by soaking and shaking it in deionized water for two days. All of the carbonation experiments were conducted under ambient conditions (room temperature and atmospheric pressure) while monitoring the temperature and pH (Orion 410A, Thermo Scientific). The solutions were filtered using a 0.2 lm membrane filter (Nylon, Sartorius). The calcium ion concentrations were determined using inductively coupled plasma-optical emission spectrometry (ICP-OES; Optima 5300DV, PerkinElmer) after the acidification of the samples to pH 2 with instrumental-grade HNO3. All the precipitated CaCO3 particles were washed with anhydrous ethanol or deionized water, and then dried either under vacuum or in air overnight at 30 °C. The precipitated CaCO3 particles were examined by X-ray diffraction (XRD; X’pert MPD, Philips
The relative amounts (%) of the CaCO3 crystalline phases, i.e., calcite, aragonite, and vaterite, were estimated using Eqs. (3) and (4), as proposed by Kontoyannis and Vagenas [13]. For a mixture of aragonite and calcite:
XA ¼
3:157I221A 100; I104C þ 3:157I221A
X C ¼ 100 X A
ð3Þ
and for a mixture of vaterite and calcite:
XV ¼
7:691I110V 100; I104C þ 7:691I110V
X C ¼ 100 X V
ð4Þ
where XA, XV, and XC are the relative amount (%) of aragonite, vaterite, and calcite, respectively. I represents the integrated areas of the XRD peaks, and subscripts A, V, and C represent the aragonite, vaterite, and calcite phases, respectively, i.e., I221A, I104C, and I110V are the areas of the (2 2 1), (1 0 4), and (1 1 0) peaks of aragonite, calcite, and vaterite, respectively. 3. Results and discussion 3.1. 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 CO2 3 ) 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.%). The pH was higher than 9.0 in each case. The ammonia tended to slow the carbonation rate, which prolonged the induction period, and accordingly increased the amount of pure CaCO3 obtained. We attributed this effect to the formation of carbamate (NH2CO 2 ) [7], which is the dominant species formed in the reaction between CO2 and excess ammonia. The related reactions can be expressed as follows [15]:
2NH3 þ CO2 ! NH2 CO2 þ NHþ4 ;
K eqð273Þ ¼ 2:35 104
þ 2NH3 þ CO2 þ H2 O ! CO2 3 þ 2NH4 ;
K eqð273Þ ¼ 8:89 102
ð5Þ ð6Þ
The polymorphs of the precipitated CaCO3 were examined after washing with anhydrous ethanol several times and drying under vacuum at 30 °C to prevent a possible transformation of
Fig. 1. Dissolved CaCO3 (mM) as a function of ammonia concentration (vol.%).
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Fig. 2. Field-emission scanning electron microscopy (FE-SEM) images of CaCO3 obtained under (a) excess and (b) stoichiometric ammonia conditions.
It is difficult to anticipate the influence of external factors during the precipitation of CaCO3, including those of the solvent components. However, the use of an aqueous solution of ethanol is considered a typical method for synthesizing or stabilizing metastable vaterite [21]. In the following sections, we evaluate the likelihood of phase changes, control of morphology, and stabilization of vaterite by an ethanol–water binary mixed solvent, from the solution extracted during the induction period.
3.2. Effect of ethanol on the polymorphs of pure CaCO3 precipitated using stoichiometric ammonia
Fig. 3. X-ray diffraction (XRD) patterns of pure CaCO3 precipitated from mixed solvents of (a) 0, (b) 10, (c) 30, and (d) 50 vol.% of ethanol in water under stoichiometric ammonia conditions. c = calcite and v = vaterite.
metastable vaterite into calcite under water-mediated conditions [16]. We found that the phase and morphology of the CaCO3 precipitate were strongly dependent on the amount of ammonia in the solution. For example, spherical vaterite was the only phase observed under conditions of excess ammonia (Fig. 2a), whereas calcite with rhombohedral morphology was formed under stoichiometric conditions (Fig. 2b). Calcite is the most stable polymorph of CaCO3. Although vaterite is the least thermodynamically stable among the anhydrous crystalline CaCO3 phases, it is often observed at ambient temperature in the presence of ammonia/ammonium [17–19]. This is because NH+4 interacts with the CO2 present on the surface of 3 the negatively charged crystal planes. Here, vaterite had a spherical morphology, which is its most frequently observed shape [17]. Spherical vaterite particles were formed in gelatinous amorphous CaCO3, resulting in spherulitic growth with a radiating pattern (see the magnified inset in Fig. 2a) [20].
The effect of ethanol on the polymorphs of pure CaCO3 was diverse, depending on the amount of ammonia present in the solution. In the presence of a stoichiometric amount of ammonia, the amount of vaterite phase increased with increasing ethanol content. Although no vaterite crystals were detected when 10 vol.% ethanol was added, the amount of vaterite increased to 25% and 73% in the presence of 30 and 50 vol.% of ethanol, respectively (Fig. 3). However, in the presence of 70 or 90 vol.% ethanol, the precipitated products were not CaCO3, but rather were compounds composed of calcium, ammonium, and sulfate ions, i.e., (NH4)2SO4 and (NH4)2Ca(SO4)2H2O. This is probably because CO2 3 production was limited under low H2O conditions, as is seen in Eq. (6). The polymorphs of the CaCO3 crystals precipitated from ethanol–water binary mixed solvents are shown in Fig. 4. The CaCO3 crystals prepared from the 10 vol.% ethanol solvent consisted of only the calcite phase (Fig. 3a) with some imperfect rhombohedral shapes (Fig. 4a). With the 30 vol.% ethanol solvent, the amount of imperfect rhombohedral calcite crystals increased, which appeared to be formed through the agglomeration of particles (Fig. 4b). Ethanol is known to damage calcite surfaces by binding more strongly at the calcite surface than with water [22]. This might block the surface and inhibit the perfect growth of calcite. Accordingly, in the presence of 50 vol.% ethanol, spherical vaterite
Fig. 4. Morphology of pure CaCO3 precipitated from a mixed solvent of (a) 10, (b) 30, and (c) 50 vol.% ethanol in water under stoichiometric ammonia conditions.
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Fig. 5. X-ray diffraction (XRD) patterns of pure CaCO3 precipitated from mixed solvents of (a) 0, (b) 10, (c) 30, and (d) 50 vol.% ethanol in water under excess ammonia conditions. a = aragonite, c = calcite, and v = vaterite.
311
Fig. 7. X-ray diffraction (XRD) patterns of vaterite from (a) 0 and (b) 10 vol.% ethanol in water after soaking for two days. c = calcite and v = vaterite.
3.3. Effect of ethanol on the polymorphs of pure CaCO3 precipitated using excess ammonia particles occur together with calcite crystals, which have rough faces (Fig. 4c). The effect of ethanol on the formation of metastable vaterite has primarily been explained by two mechanisms. One postulates that ethanol lowers the solubility of CaCO3, which eventually increases its supersaturation [23]. This promotes the production of the kinetically favored vaterite phase, rather than thermodynamically favored calcite. The other mechanism is related to the interaction between Ca2+ and CO2 3 ions and the solvents (water and ethanol). 2+ Ca2+ and CO2 ions, 3 ions are highly solvated in H2O, but only Ca which have a higher electric charge density, are solvated in ethanol. In addition, the weak interaction of Ca2+ with ethanol relative to water requires a lower solvation energy, which facilitates the formation of metastable vaterite. Zhang et al. used a molecular dynamics simulation that demonstrated that the morphology and phase of CaCO3 could be controlled by varying the amount of ethanol, which resulted in the control of the polarity difference between Ca2+ and CO2 3 in the mixed solvent [24].
In the absence of ethanol, only vaterite was obtained under the conditions of excess ammonia. However, a complete phase change from vaterite to aragonite occurred upon the addition of either 30 or 50 vol.% ethanol (Fig. 5). This is an interesting result because we could produce pure aragonite using this method. However, solutions containing 70 or 90 vol.% ethanol did not precipitate CaCO3; instead, compounds composed of calcium, ammonium, and sulfate, i.e., (NH4)2SO4 and (NH4)2Ca(SO4)2H2O were formed regardless of the ammonia content. This was consistent with a previous report 2 that the formation of HCO 3 or CO3 is unstable and reversible during CO2 sorption in ethanol solution [25]. Aragonite is the stable phase of CaCO3 at temperatures above 40 °C [26]. Conventionally, pure aragonite is synthesized through two major routes. The first is to use aqueous alcohol solution and varying the solvent properties [23]. The other is the direct synthesis from amorphous CaCO3 in an ethanol mixed solvent containing a large quantity of Mg2+ [27,28]. In the latter route in particular, the
Fig. 6. Peanut-like crystals of aragonite with dandelion-like heads of pure CaCO3 precipitated from ethanol–water binary mixed solvents under excess ammonia conditions: (a and b) 30 and (c and d) 50 vol.% ethanol, respectively.
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Fig. 8. Morphology of transformed vaterite under excess ammonia conditions after soaking in water for two days: (a) superstructure calcite, (b) intermediate multilayered calcite, and (c) calcite crystals on the vaterite surface.
Fig. 9. Morphology of transformed vaterite under excess ammonia conditions after soaking in 10 vol.% ethanol in water for two days. Arrows highlight calcite crystals. (b) is the magnified image of the area marked in (a), and (c) is another magnified region.
amorphous CaCO3 is strongly stabilized by Mg2+, showing that the amount of Mg2+ ions present has a great influence on morphology. Tang et al. synthesized peanut-like aragonite using anhydrous ethanol with the Mg2+ to Ca2+ molar ratio of 8:1 [28]. They ascribed this unique morphology to the prolonged stability of amorphous CaCO3 in the presence of both ethanol and Mg2+. The aragonite prepared herein has a unique peanut-like morphology with dandelion-like heads (Fig. 6). In our case, in the absence of Mg2+, ammonia might have stabilized the amorphous CaCO3 in the same way [29]. The preferential nucleation of aragonite crystals could result from the synergetic effect of ammonia and ethanol in disrupting calcite formation. 3.4. Effect of ethanol on the stability of vaterite Metastable vaterite transforms into stable calcite upon exposure to water [30]. However, both ammonia and ethanol stabilize vaterite in aqueous systems. We evaluated the effect of ethanol on the stability of vaterite under conditions of excess ammonia. Fig. 7 shows the XRD patterns of vaterite particles after soaking in water for two days. In the absence of ethanol, vaterite transformed completely into calcite (Fig. 7a). In contrast, in the presence of 10 vol.% ethanol, only 15% of vaterite was transformed into calcite (Fig. 7b). This, it is clear that the vaterite phase was stabilized significantly by the addition of a small amount of ethanol. The morphological transformation of the vaterite crystals under the conditions of excess ammonia is illustrated in Fig. 8. After soaking in water for two days, the particles changed completely into superstructure calcite (Fig. 8a). In contrast, after a brief washing with water, rhombohedral calcite crystals were formed on the surface of vaterite (Fig. 8c), and the intermediate multilayered calcite crystals resembled those of the mother vaterite (Fig. 8b). This showed the templating action of vaterite, which was also observed when using polymer additives [31]. This templating action during the transformation was also detected by Shen et al. [32]. However, upon the addition of 10 vol.% ethanol, the morphology of the
transformed vaterite was different. Fig. 9 shows the transformed vaterite when isolated from its mother crystals. As observed in Fig. 9c, calcite was likely formed by a typical dissolution–crystallization process, rather than by the templating action of vaterite that was observed in the absence of ethanol. The vaterite produced herein was very unstable and readily transformed into the stable calcite phase, especially in the absence of ethanol. Ammonium ions facilitated the formation of vaterite, but their action is more reversible than those of polymers [33], and vaterite crystals formed in the presence of ammonium ions are easily transformed into stable calcite [17,19]. Based upon our previous results, it is obvious that compared to ammonia and water, ethanol binds more strongly to vaterite. This result is consistent with a previous study that showed that ethanol is chemisorbed onto CaCO3 and binds more strongly than water [30].
4. 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 amount of vaterite precipitated increased with the addition of 30 and 50 vol.% ethanol. Under excess-ammonia condition, spherical vaterite crystals were formed. However, peanut-like aragonite crystals with dandelion-like heads were formed when 30 and 50 vol.% ethanol was used under excess-ammonia conditions. However, in solutions containing 70 or 90 vol.% ethanol, the reaction products were not CaCO3, but were rather compounds composed of calcium, ammonium, and sulfate, i.e., (NH4)2SO4 and
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(NH4)2Ca(SO4)2H2O, regardless of the amount of ammonia in solution. When spherical vaterite, which was precipitated in the absence of ethanol, was immersed in water, it transformed completely into superstructure calcite through the templating action of vaterite. However, with the addition of 10 vol.% ethanol, the amount of vaterite transformed was significantly reduced. Our results show the synergistic effect of ammonia and ethanol the polymorphic selectivity of aragonite in the absence of Mg2+. Thus, the method developed herein can be used to control the phase of morphology of pure CaCO3 produced using industrial by-products as FGD gypsum. 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. References [1] W. Seifritz, CO2 disposal by means of silicates, Nature 345 (1990) 486. [2] A. Kirchofer, A. Becker, A. Brandt, J. Wilcox, CO2 mitigation potential of mineral carbonation with industrial alkalinity sources in the United States, Environ. Sci. Technol. 47 (2013) 7548–7554. [3] E.R. Bobicki, Q. Liu, Z. Xu, H. Zeng, Carbon capture and storage using alkaline industrial wastes, Energy Combust. Sci. 38 (2012) 302–320. [4] M.M.M.G.P.G. Mantilaka, R.M.G. Rajapakse, D.G.G.P. Karunaratne, H.M.T.G.A. Pitawala, Preparation of amorphous calcium carbonate nanoparticles from impure dolomitic marble with the aid of poly(acrylic acid) as a stabilizer, Adv. Powder Technol. 25 (2014) 591–598. [5] M.g. Lee, Y.N. Jang, K.w. Ryu, W. Kim, J.-H. Bang, Mineral carbonation of flue gas desulfurization gypsum for CO2 sequestration, Energy 47 (2012) 370–377. [6] K. Song, Y.-N. Jang, W. Kim, M.G. Lee, D. Shin, J.-H. Bang, C.W. Jeon, S.C. Chae, Precipitation of calcium carbonate during direct aqueous carbonation of flue gas desulfurization gypsum, Chem. Eng. J. 213 (2012) 251–258. [7] K. Song, Y.-N. Jang, W. Kim, M.G. Lee, D. Shin, J.-H. Bang, C.W. Jeon, S.C. Chae, Factors affecting the precipitation of pure calcium carbonate during the direct aqueous carbonation of flue gas desulfurization gypsum, Energy 65 (2014) 527–532. [8] A. Afshar, I. Massoumi, R.L. Khosh, R. Bagheri, Fracture behavior dependence on load-bearing capacity of filler in nano- and microcomposites of polypropylene containing calcium carbonate, Mater. Des. 31 (2010) 802–807. [9] H. He, K. Li, J. Wang, G. Sun, Y. Li, J. Wang, Study on thermal and mechanical properties of nano-calcium carbonate/epoxy composites, Mater. Des. 32 (2011) 4521–4527. [10] N. Ramdani, J. Wang, X.-Y. He, T.-T. Feng, X.-D. Xu, W.-B. Liu, X.-S. Zheng, Effect of crab shell particles on the thermomechanical and thermal properties of polybenzoxazine matrix, Mater. Des. 61 (2014) 1–7. [11] T. Boronat, V. Fombuena, D. Garcia-Sanoguera, L. Sanchez-Nacher, R. Balart, Development of a biocomposite based on green polyethylene biopolymer and eggshell, Mater. Des. 68 (2015) 177–185.
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