Journal of Crystal Growth 271 m tic Concepcio´n Domingo�, Jesu´s Garcı´a-Carmona, Eva Loste, Alejandra Fanovich, r 2004 Elsevier B.V. All rights reserved. 3 morphology, specific surface area, polymorphism or chemical purity [1,2]. The most used industrial ARTICLE IN PRESS �Corresponding author. Tel.: 34-935801853; fax: 34- process of obtaining CaCO3 involves the following steps: (a) calcination of limestone (natural CaCO3 0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.07.060 935805729. E-mail address:
[email protected] (C. Domingo). PACS: 81.20.F; 89.20; 64.60.Q Keywords: A1. Crystal morphology; A2. Industrial crystallization; A2. Supercritical crystallization; B1. Calcium compounds; B1. Rhombohedral calcite 1. Introduction The demand for precipitated calcite, the most thermodynamically stable polymorph of calcium carbonate (CaCO ), has been rapidly growing in recent years in various fields of industry: paper, rubber, plastics, paint, food, etc. Industrial appli- cations of CaCO3 are determined by the strict control of a great number of parameters, such as average particle size, particle size distribution, Instituto de Ciencia de Materiales de Barcelona (CSIC), ICMAB, Campus UAB sn 08193 Bellaterra, Barcelona, Spain Received 1 July 2004; received in revised form 19 July 2004; accepted 22 July 2004 Communicated by D.T.J. Hurle Available online 11 September 2004 Abstract This paper reports a new method to produce calcite crystals with scalenohedral and rhombohedral morphologies in the Ca(OH)2–H2O–CO2 system, without the use of any tailor-made additive. Compressed CO2 vapour (25 1C, 50 bar), liquid CO2 (25 1C, 200 bar) and supercritical CO2 (45 1C, 200 bar) were used to precipitate CaCO3. De-aggregated calcite crystals of rhombohedral habit and narrow crystal size distribution were prepared using a novel route that consists in the reaction of an aqueous suspension of Ca(OH)2 with compressed carbon dioxide. In the new method, morphological control, overall production rate and conversion efficiency are enhanced with respect to the established industrial carbonation process. Julio Fraile, Jaime Go´mez-Morales Control of calcium carbonate compressed and supercri (2004) 268–273 orphology by precipitation in al carbon dioxide media www.elsevier.com/locate/jcrysgro at a flow rate of 7.5 lmin through 1.5 l of slaked �1 ARTICLE IN PRESS Cryst rocks) to produce quicklime (CaO) and carbon dioxide; (b) the slaking process, in which the quicklime is transformed to slaked lime slurry (a Ca(OH)2 suspension) by controlled addition of water; and, finally (c) the carbonation reaction (Eq. (1)), in which the CO2 is bubbled through an aqueous slurry of slaked lime. CaðOHÞ2 þ CO2 ! CaCO3 þH2O (1) The carbonation reaction is the crucial step determining the morphology of the obtained product. The morphology of the so precipitated calcite, at typical temperatures of industrial process (between 30 and 70 1C), is normally the scalenohedral one bounded by the f2 1 �1g form. Synthetic scalenohedral calcite is, therefore, pro- duced through a batch carbonation method [3]. The rhombohedral morphology, bounded by the {1 0 4} form, is usually precipitated by using solution routes, but rarely by the mentioned industrial process. An analysis of the literature reveals the lack of methods that allow the morphological control of calcite precipitated by carbonation of slaked lime without the addition of additives [4–6]. Therefore, the development of new industrial carbonation routes for the production of calcite with different morphologies in the absence of expensive additives is of great interest. Recently, Jung et al. [4] and Garcı´a-Carmona et al. [6] have demonstrated that it is possible to produce rhombohedral and scalenohedral shapes by mixing filtered solutions of Ca(OH)2 and pure CO2 in ‘‘stoichiometric’’ and ‘‘non-stoichiometric’’ conditions, respectively. According to these authors, the change in particle morphology was mainly caused by the excess species of the reactants in the solution rather than by super- saturation. An excess of calcium species over the stoichiometry is normally found in the medium by using the described industrial method for the production of calcite. Stoichiometric conditions could be reached by increasing the total concen- tration of carbonate species in the media. Hence, the hypothesis of this work has been that by using compressed CO2 instead of gas, the ratio between the dissolved calcium and carbonate species can be adjusted, allowing the morphological control of C. Domingo et al. / Journal of precipitated CaCO3. lime suspension of concentration 170 g l . The temperature of the experiment was maintained at 45 1C, in spite of the exothermic character of the reaction, by manipulating the temperature of the thermostat. The homogenization of the system was attained at stirring rates of 800 rpm. (b) Addition of compressed CO2: the experimen- tal set-up consisted of a high-pressure stainless steel autoclave (Thar Designs, of 70ml capacity) that has two opposite sapphire windows. The autoclave was also provided with a vertically mounted impeller stirrer (DynaMag, 2500 rpm) and four resistances for heating at the working temperature (T between 25 and 45 1C). A syringe pump (Thar Design, SP-240) manipulated through a computer was used to compress and deliver the CO2 into the autoclave. The reactor was first Supercritical CO2 (SCCO2) is used as a solvent in many commercial processes, including the extraction of caffeine from coffee, and essential oils and spices from plants for use in perfumes and foods [7]. The critical conditions for CO2 are 31.1 1C and 73.8 bar. The advantages of SCCO2 is the combination of gas-like properties of low viscosity, high diffusivity and absence of surface tension with liquid-like properties of high density. Note that, in this study, SCCO2 was not used as a solvent but as a reagent for calcite precipitation. The aim of the present work, therefore, has been to investigate if the controlled processing of slaked lime with vapour, liquid and SCCO2 could be a useful tool to improve the calcite morphological control in the carbonation process. 2. Experimental procedure Calcium carbonate crystals were induced to precipitate either via the diffusion of gaseous CO2 into an atmospheric reactor or via the addition of compressed CO2 into a high-pressure autoclave. (a) Diffusion of gaseous CO2: the experimental set-up consisted of a thermostated double-wall Pyrex reactor of 2.2 l capacity provided with a vertical shaker. The experiments were performed by bubbling a mixture of 20% CO2:80% N2 (v/v) �1 al Growth 271 (2004) 268–273 269 charged with 10ml slaked lime suspension of CO2–H2O, both the supersaturation and the ratio 3.1. Calcite precipitation by using atmospheric CO2 gas During a conventional batch carbonation pro- cess, carried out in an open vessel (P ¼ 1 bar and T ¼ 45 1C), the amount of dissolved CO2, and hence available CO3 2� for carbonation, was re- stricted by low solubility of CO2 (0.036mol% or 3.48 g l�1, Fig. 1(b)) and low miscibility in the alkaline water. Under these experimental condi- tions, the ratio [Ca2+]/[CO3 2�] in the reaction medium was higher than one and aggregates of scalenohedral calcite crystals were precipitated (Fig. 2(a)). Based on the previous studies [9,11], it was deduced that the rhombohedral morphology will be most easily precipitated by enhancing the ARTICLE IN PRESS Cryst between the concentration of calcium and carbo- nate species ([Ca2+]/[CO3 2�]) play an important role in determining the growth morphology of calcite crystals [9]. The ratio [Ca2+]/[CO3 2�] affects the growth rate of {1 0 4} and f2 1 �1g surfaces in a different manner. Indeed, the stoichiometric {1 0 4} faces of rhombohedral calcite tend to dis- appear from the growth morphology when pre- cipitated from solutions with [Ca2+]/[CO3 2�] 41.1–1.2. In contrast, the growth f2 1 �1g face of scalenohedral calcite is inhibited for solutions under identical conditions of supersaturation but with more stoichiometric [Ca2+]/[CO3 2�] ratios. This behaviour has been ascribed to the lower Ca2+ Langmurian adsorption coefficient as com- pared to that of the CO3 2� ion, which entails a higher amount of calcium in relation to carbonate for building a stoichiometric face [9]. Primary concentration 170 g l�1. Experiments were per- formed by adding pure CO2 into the reactor up to working pressure (P between 50 and 200 bar). Stirring efficiency was 400 rpm and 2 h runs were performed. At the end of the experiments an aliquot of the final suspension was analyzed by the light scatter- ing method, using a Coulter LS 130 instrument, to obtain the crystal size distribution. The rest of each suspension was filtered and washed with deionized water. The final white precipitate was dried in an oven with circulating air at 100 1C. Then, the powder was analyzed by X-ray diffrac- tion (XRD) with a Rigaku Rotaflex RU200 B instrument, using CuKa1 radiation and scanning electron microscopy (SEM) using a Hitachi S570 microscope. 3. Results and discussion By bubbling CO2 into solutions of low Ca(OH)2 concentration, rhombohedral calcite is commonly formed [8] in a homogeneous process. The habit is mainly controlled by the supersaturation degree. On the contrary, for high concentrated slurries, solid Ca(OH)2 is suspended in the media. In the three phases system, solid–gas–liquid Ca(OH)2–- C. Domingo et al. / Journal of270 factors affecting the [Ca2+]/[CO3 2�] ratio in the crystallizing media are: (i) the concentration of soluble calcium ion which is controlled by the rate of dissolution of calcium hydroxide, and (ii) the rate of CO2 dissolution and mass transfer into water to form an available carbonate ion. In this work, experiments were performed at a fixed Ca(OH)2 concentration in the aqueous slurry (170 gCaðOHÞ2Þ l �1). In Fig. 1(a) experimental condi- tions are superposed in the CO2 phase diagram. In Fig. 1(b), the huge rise of CO2 solubility in water with the increase in pressure is represented [10]. XRD analysis identified calcite as the polymorph precipitated under the different working conditions. 0 75 150 225 10 25 40 55 T [°C] P [b ar] Liquid Vapor Supercritical 0 1 2 3 0 100 200 P [bar] so lu b CO 2 in H 2O [m ol% ] 45 ºC 25 ºC Fig. 1. (a) CO2 Pressure–Temperature phase diagram, and (b) CO2 solubility in water as a function of pressure. Data extracted from Ref. [10]. Working experimental conditions are included in the graphics: B P ¼ 1 bar; T ¼ 45 1C; m P ¼ 50bar; T ¼ 25 1C; ’ P ¼ 200bar; T ¼ 25 1C; � P ¼ 200bar; T ¼ 45 1C: al Growth 271 (2004) 268–273 concentration of CO3 2� species in the mother ARTICLE IN PRESS Cryst (a) (b) (c) (d) C. Domingo et al. / Journal of solution. In order to increase the dissolution of CO2 in water, experiments were performed in a high-pressure closed vessel. The CO2 pressure in the reactor was varied from 50 to 200 bar. 3.2. Calcite precipitation by using compressed CO2 vapour At 25 1C and partial pressure of CO2 of 50 bar (vapour phase, Fig. 1(a)) the microscopic exam- ination of the precipitated crystals revealed inter- grown stepped rhombohedral aggregates of di- verse sizes, and a large population of very fine (0.1–0.3 mm) crystals (Fig. 2(b)). Previous work performed by the authors on the morphological control of calcite precipitated by carbonation of slaked lime [6,9] has shown that the apparition of submicrometric particles (particle size between 0.1 and 1.0 mm) points towards calcite precipitation by the Ca(OH)2 surface route. In this route, nano- Fig. 2. (a) SEM images of precipitated calcite at: (a) P ¼ 1bar; T ¼ 45 1C; (b) P ¼ 50 bar; T ¼ 25 1C; (c) P ¼ 200bar; T ¼ 25 1C; (d) P ¼ 200bar; T ¼ 45 1C: Bar: 3 mm. metric particles of amorphous calcium carbonate are first precipitated in the lime surface. Dehydra- tion and reorganization of this precursor is a slow process and requires a passage in solution to nucleate dehydrated calcite. The interaction of the new nucleated tiny calcite particles with the substrate surface makes their interfacial free energy to decrease, thus diminishing their trend to grow and stabilizes the submicrometric crystals. Hence, the large population of fine particles observed at 25 1C and 50 bar was likely to be precipitated via this mechanism. 3.3. Calcite precipitation by using liquid CO2 A further pressure increase to 200 bar (CO2 liquid phase, Fig. 1a) produced intergrown crystals which displayed either rhombohedral or scaleno- hedral habits. The population of submicrometric particles was very small (Fig. 2(c)). Therefore, at 25 1C, different crystal growth mechanisms were probably operating either at 50 or 200 bar. At 200 bar, CO2 solubility in water was even higher than at 50 bar (Fig. 1(b)). The main difference between the two systems was the higher viscosity of compressed liquid CO2 (m(25 1C,200 bar)=100 10�6 Pas) with respect to compressed vapour (m(25 1C,50 bar)=17 10�6 Pas). Under similar stir- ring conditions, the rise in viscosity results in the likely difficulties in mixing between the two liquids, shifting the reaction to a diffusion control instead of a chemical control. The reaction rate and the mass transfer rate in the particle growth are critically determined by the mixing of solutions [12]. 3.4. Calcite precipitation using supercritical CO2 A rather homogeneous population of multi- nucleated rhombohedral calcite was obtained using SCCO2 (P ¼ 200 bar; T ¼ 45 1C), as shown in Fig. 2(d) and Fig. 3. The combination of the gas-like (low viscosity) and liquid-like (high density) properties makes SCCO2 a unique synth- esis and processing medium [7]. The low surface tension and high diffusivity allow rapid mass transfer across boundaries and enhanced reaction al Growth 271 (2004) 268–273 271 rates. Moreover, by increasing the temperature ARTICLE IN PRESS Cryst C. Domingo et al. / Journal of272 from 25 to 45 1C the solubility of Ca(OH)2 decreases [13]. Hence, the ratio [Ca2+]/[CO3 2�] was probably close to the stoichiometric one and rhombohedral calcite was the main precipitated phase. Finally, at 45 1C the rate of transformation of nanometric amorphous carbonate into calcite increases with respect to 25 1C. Submicrometric particles were not observed. In fact, the obtained morphology is more similar to that obtained when the precipitation is carried out in a homogeneous system rather than in a heterogeneous one with solid lime in suspension [8]. A general trend observed in the carbonation process when working with high supersaturated solutions is considerable agglomeration and ce- mentation of individual crystals. Analysis of crystal size distribution by laser light scattering showed a significant reduction in agglomeration when employing SCCO2 as a precipitant instead of gas, or even dense vapour or liquid phases (Fig. 4). For the last three mentioned conditions, the mean 15 µm Fig. 3. SEM image or precipitated calcite at supercritical conditions: P ¼ 200bar and T ¼ 45 1C: 4. Conclusions In general, currently utilized manufacturing processes, performed in atmospheric reactors, are slow, with low carbonation efficiencies. Thus, manufacturing plants required large equipment, resulting in high capital cost per unit of calcium carbonate production. The new batch method and median particle size values were approxi- mately 4.5 and 3.5, respectively. On the other hand, for the supercritical conditions, the mean and median particle size values were reduced to 2.8 and 2.5, respectively. The low degree of agglom- eration will be beneficial for the use of CaCO3 as a coating pigment or filler. 0 2 4 6 8 0 5 10 15 20 particle size [µm] % v ol um e Fig. 4. Crystal size distribution obtained by laser light scattering: B P ¼ 1bar; T ¼ 45 1C; m P ¼ 50bar; T ¼ 25 1C; ’ P ¼ 200bar; T ¼ 25 1C; � P ¼ 200bar; T ¼ 45 1C: al Growth 271 (2004) 268–273 described in this article represents an alternative with respect to the conventional one, where the overall production rate of CaCO3 is increased. By using compressed CO2, reactor size, necessary for a desired production rate, is also decreased. Optimization of the new route may result into precipitation of de-aggregated rhombohedral crys- tals with high specific surface area and sizes going from a few microns to nanocrystals. The analysis of calcite precipitation under SCCO2 conditions was also performed with the intent to extend the application to the formation of composite materi- als containing fine crystals of CaCO3. Examples are the in situ precipitation of rhombohedral cal- cite between the fibres of cellulose paper [14] or inside the pores of alkaline Portland cement [15]. Acknowledgements We thank Cales de Llierca S.A. for providing the slaked lime used in this work. References [1] C.T. Tai, P.C. Chen, AIChE J. 4 (1995) 68. [2] J. Franke, A. Mersmann, Chem. Eng. Sci. 50 (1995) 1737. [3] P.C. Chen, C.Y. Tai, K.C. Lee, Chem. Eng. Sci. 52 (1997) 4171. [4] W.M. Jung, S.H. Kang, W.-S. Kim, C.K. Choi, Chem. Eng. Sci. 55 (2000) 733. [5] J. Garcı´a Carmona, J. Go´mez Morales, J. Fraile-Sainz, R. Rodrı´guez Clemente, Powder Technol. 130 (2003) 307. [6] J. Garcı´a Carmona, J. Go´mez Morales, R. Rodrı´guez Clemente, J. Crystal Growth 249 (2003) 561. [7] M. McHugh, V. Krukonis, Supercritical Fluid Extraction: Principles and Practices, Butterworth-Heineman Publisher, US, 1994. [8] S.R. Dickinson, G.E. Henderson, K.M. McGrath, J. Crystal Growth 244 (2002) 369. [9] J. Garcı´a Carmona, J. Go´mez Morales, R. Rodrı´guez Clemente, J. Colloid Interf. Sci. 261 (2003) 434. [10] R.J. Bakker, L.W. Diamond, Geochim. Cosmochim. Acta 64 (2000) 1753. [11] V.K. Mathur, US Patent 6 (251) (2001) 356. [12] K. Uebo, R. Yamazaki, K. Yoshida, Adv. Powder Technol. 3 (1992) 71. [13] D.E. Giles, I.M. Ritchie, B.A. Xu, Hydrometallurgy 32 (1993) 119. [14] E. Dalas, P.G. Klepetsanis, P.G. Koutsoukos, J. Coll. Interf. Sci. 224 (2000) 56. [15] N.R. Short, P. Purnell, C.L. Page, J. Mater. Sci. 36 (2001) 35. ARTICLE IN PRESS C. Domingo et al. / Journal of Crystal Growth 271 (2004) 268–273 273 Control of calcium carbonate morphology by precipitation in compressed and supercritical carbon dioxide media Introduction Experimental procedure Results and discussion Calcite precipitation by using atmospheric CO2 gas Calcite precipitation by using compressed CO2 vapour Calcite precipitation by using liquid CO2 Calcite precipitation using supercritical CO2 Conclusions Acknowledgements References