Polymorphic Change of Calcium Carbonate during Reaction

Apr 23, 2004 - In the reaction crystallization of calcium carbonate using a batch reactor system with aqueous. CaCl2 and Na2CO3 at room temperature, t...
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Ind. Eng. Chem. Res. 2004, 43, 2650-2657

Polymorphic Change of Calcium Carbonate during Reaction Crystallization in a Batch Reactor Woon-Soo Kim,† Izumi Hirasawa,† and Woo-Sik Kim*,‡ Department of Applied Chemistry, Waseda University, 3-4-1 Okubo Shinjuku-ku, Tokyo 169-8555, Japan, and School of Applied Chemistry, MRCID, Kyunghee University, 1 Seochon Kiheung Yongin, Kyungki-Do 449-701, Korea

In the reaction crystallization of calcium carbonate using a batch reactor system with aqueous CaCl2 and Na2CO3 at room temperature, the selective polymorphic control of crystalline calcium carbonate into calcite was achieved by adjusting the operating conditions. The polymorphic ratio of calcite and vaterite nucleated from amorphous calcium carbonate (ACC) was significantly influenced by the initial supersaturation and solution pH, thereby resulting in a different phase transformation rate for the product powder from unstable vaterite to stable calcite. With a high supersaturation and low solution pH, the phase fraction of calcite decreased owing to the favorable transformation of ACC into vaterite during the early stage of the crystallization. Thereafter, the phase fraction of calcite increased as a result of the transformation of vaterite into calcite as the aging progressed. However, with a low supersaturation and high solution pH, calcite was exclusively observed throughout the reaction time without any recrystallization of vaterite. The decrease in the polymorphic ratio of vaterite in the product powders with the reaction time indicated the disappearance of vaterite due to its dissolution at a low initial supersaturation, which matched well with the morphological change in the polymorphs obtained by a scanning electron microscopy analysis. Meanwhile, at a high initial supersaturation, the polymorphic changes from vaterite to calcite did not produce any morphological transformation. Introduction Calcium carbonate, CaCO3, is one of the most plentiful mineral resources occurring in nature.1,2 The preparation of crystalline calcium carbonate by crystallization has recently received a lot of attention because of its potential application as a raw material in the paper, paint, plastic, adhesive, and rubber industries.3-5 However, despite the plethora of research on calcium carbonate, the complex behavior of its crystallization process, structural isomerism, phase transformation, and morphological modification continues to make calcium carbonate a very attractive material to study.6-8 The polymorphism of crystalline calcium carbonate, structurally sorted as calcite, aragonite, and vaterite, has already been the focus of various reports. As such, it has already been observed that the nucleation of the structural isomers of calcium carbonate is dependent on the operating conditions of the crystallization, including the supersaturation, solution composition, pH, temperature, and presence of additives.9-12 For example, calcite is usually the dominant polymorph at a low temperature and high solution pH, while vaterite and aragonite are abundantly produced at a low pH and high temperature.1,9,10 Furthermore, because vaterite and aragonite are thermodynamically less stable than calcite, these two metastable phases eventually transform into calcite in an aqueous solution via a recrystallization mechanism13-17 or in a solid phase at a high temperature of around 400 °C.8 The transformation of vaterite or aragonite into calcite has been explained by * To whom correspondence should be addressed. Tel.: 8231-201-2576. Fax: 82-31-202-1946. E-mail: [email protected]. † Waseda University. ‡ Kyunghee University.

a two-step recrystallization process: the dissolution of an unstable polymorph and growth of a stable polymorph in the product suspension.9,17 The typical morphologies of calcium carbonate polymorphs are generally classified as rhombohedral calcite, spherical vaterite, and needlelike aragonite.3 In many studies on the reaction crystallization of calcium carbonate, amorphous calcium carbonate (ACC) in the form of hydrated calcium carbonate (CaCO3‚ nH2O) has been found in an early stage of crystallization because of rapid precipitation at high supersaturation.9,10,13,17-19 Then, ACC is easily transformed into the crystalline calcium carbonate of calcite, aragonite, and vaterite via dissolution and recrystallization of ACC. Also, the polymorphic fraction in the product powder varies with the crystallization conditions, such as the temperature and supersaturation. The crystal morphology of calcium carbonate is also an important concern for crystallization because it is involved in determining the final usage of calcium carbonate as an advanced material, along with the purity, mean particle size, and particle size distribution.20 Although numerous previous studies have focused on the polymorphic phenomena of crystalline calcium carbonate, the ability to control the structure and morphology of calcium carbonate during crystallization has still not been achieved. Accordingly, the current work investigated the reaction crystallization of calcium carbonate in a batch reactor using an aqueous reacting system of CaCl2 and Na2CO3, especially at room temperature, to determine the effects of the crystallization conditions on the polymorphism of calcium carbonate. Particular interest was paid to the phase transformation and corresponding modification of the morphology during the aging of the product powders.

10.1021/ie034161y CCC: $27.50 © 2004 American Chemical Society Published on Web 04/23/2004

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Figure 1. Typical XRD patterns of product powders, crystallized at Sin ) 393.1 and pH ) 11.7, relative to the reaction time.

Experimental Procedures The reaction crystallization of calcium carbonate was induced by the instant addition of 50 mL of a calcium chloride (ACS-grade) solution to 1200 mL of a sodium carbonate (ACS-grade) solution initially loaded in the reactor equipped with a thermostated water jacket. The reactor had a 1.25 L working volume and was made of Pyrex glass. All of the reagent solutions were prepared with highly purified water and their concentrations adjusted on the basis of the stoichiometry of a chemical reaction with calcium carbonate. That is, on the basis of the concentration after feeding of the calcium chloride solution, the concentrations of calcium chloride and sodium carbonate in the solutions were maintained as equal to each other and controlled within a range of 0.005-0.1 mol/L. The impeller speed and reaction temperature in the reactor were fixed at 300 rpm and 25 °C, respectively, for a reaction time of 24 h after addition of the calcium chloride solution. The remaining experimental apparatus and procedures are described in detail in a previous paper by Hirasawa.12 Samples of the product suspension were taken intermittently and filtered immediately using a micromembrane filter with a 0.45 µm pore diameter (Toyo Roshi, Japan). After being dried at 50 °C for 24 h, the solid residues were characterized by scanning electron microscopy (SEM; Hitachi S2500CX) for their crystal morphology and by powder X-ray diffraction (XRD; Rigaku RAD-IC using Cu KR radiation and a Ni filter) for their polymorphic composition, which was expressed as the mass fraction of calcite. In addition, a thermogravimetric and differential thermal analysis (TG-DTA; Rigaku TG8120 with a heating rate of 20 °C/min in a flow of helium) and FT-IR spectroscopy (FT-IR; JEOL JIR-WINSPEC50 using KBr pellets) were also applied to identify the polymorphs in the resulting crystalline powders. Results and Discussion Polymorphic Ratio and Phase Transformation. Figure 1 shows the typical change in the XRD patterns of the product crystals relative to the reaction time when precipitated at a high initial supersaturation with no adjustment of the solution pH. While calcite was the most stable phase among the structural isomers of

Figure 2. Effect of the initial supersaturation on the polymorphic ratio of calcite and vaterite relative to the reaction time.

Figure 3. Effect of the solution pH on the polymorphic ratio of calcite and vaterite at the same initial supersaturation of 218.1 relative to the reaction time.

calcium carbonate, vaterite was also observed along with calcite. However, no XRD peak for aragonite was detected in the product powders for the range of initial supersaturations and solution pHs used for crystallization in the current study, indicating a preferential

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Figure 4. Morphological changes in product powders, crystallized at Sin ) 218.1 and pH ) 11.6, relative to the reaction time: (a) t ) 2 min; (b) t ) 10 min; (c) t ) 2 h; (d) t ) 24 h.

nucleation of vaterite and calcite from ACC rather than aragonite, even though vaterite is thermodynamically slightly less stable than aragonite. From the XRD patterns relative to the reaction time (Figure 1), it was found that the product powders were polymorphic mixtures of calcite and vaterite and their characteristic peak heights gradually changed with the reaction time because of the simultaneous generation of calcite and vaterite and spontaneous transformation of vaterite into calcite during the crystallization process. Therefore, according to the phase transformation mechanism of calcium carbonate,9,17 the unstable vaterite with a higher solubility (log KSP ) -7.91) than that of calcite (log KSP ) -8.48) was dissolved in the solution, then recrystallized, and grown as stable calcite, thereby resulting in a gradual shift of the XRD pattern from vaterite to calcite during the reaction time. The polymorphic ratio ()phase fraction) of calcite to vaterite in the product powders was evaluated based on the relationship between the peak intensities (I) at the characteristic faces and the abundance of the two polymorphs, as suggested by Rao,14,21 and then expressed as a percentage fraction of calcite, XC, as described in eq 1.

XC )

I104(C) I104(C) + I110(V) + I112(V) + I114(V)

(1)

Figure 2 delineates the effect of the initial supersaturation on the phase fraction between calcite and vaterite as a function of the reaction time. The initial supersaturation, Sin, of the solution (eq 2) was calculated based on the reactant concentration, as suggested by Nielsen and Toft.22 IP and KSP in eq 2 indicate the ionic and solubility products of calcium carbonate, respectively. In the calculation of the initial supersaturation, the activity coefficient of each species was considered via the modified Debye-Hu¨ckel equation, and the solubility product of calcite was adopted as KSP.1,23

Sin ) (IP/KSP)1/2 - 1

(2)

The crystallization started instantaneously after the addition of calcium chloride to sodium carbonate because of the fast reaction of calcium carbonate, thereby producing a high supersaturation of calcium carbonate at an early stage of the crystallization in the batch

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Figure 5. Morphological changes in product powders, crystallized at Sin ) 218.1 and pH ) 8.5, relative to the reaction time: (a) t ) 5 min; (b) t ) 30 min; (c) t ) 90 min; (d) t ) 24 h.

reactor. As a result, ACC was first generated through the initial crystallization and then the polymorphic powder of calcite and vaterite was nucleated based on the dissolution of ACC. In addition, the phase fraction of calcite in the polymorphic powder decreased as a result of an increase in the vaterite nucleated from ACC and then gradually increased as a result of the recrystallization of vaterite to calcite during aging. For example, with an initial supersaturation of 218.1, only calcite was detected during the initial aging (up to 2 min), meaning no vaterite emerged from ACC. However, the formation of vaterite from ACC appeared after 2 min of reaction time, at which point the phase fraction of calcite in the polymorphic mixtures decreased until the reaction time reached 1 h and the vaterite nucleation was finished. Thereafter, the polymorphic ratio of vaterite decreased as a result of the phase transformation into calcite and eventually disappeared after 18 h of reaction time. This same phase fraction trend relative to the reaction time was also evident when the initial supersaturation is increased, where the phase fraction of vaterite (XV) emerging from ACC at an initial supersaturation of 393.1 was enhanced to 80% after 10 min of reaction. This increase in the phase fraction of vaterite with time was certainly unexpected because the

phase transformation from vaterite to calcite is irreversible from a thermodynamic perspective. According to previous studies of calcium carbonate crystallization,9,13,17,18 ACC was found first to form as hydrated structures (CaCO3‚nH2O) within a short period of crystallization, like a precursor, and then to turn into crystalline calcium carbonate. Therefore, on the basis of such previous studies, the calcium carbonate crystallization in the present batch reaction system clearly revealed that the amount of calcite and vaterite nucleated from ACC was influenced by the initial supersaturation. That is, with a high initial supersaturation, the transformation of ACC into vaterite rather than calcite was much more favored, resulting in a decrease in the phase fraction of calcite over time. Conversely, the transformation into vaterite was significantly suppressed with a low initial supersaturation (Sin ) 43.7), and calcite was preferentially formed without any recrystallization of vaterite. Moreover, at the end of the transformation of ACC into the crystalline phases, the phase fractions were primarily controlled by the phase transformation of vaterite into calcite, resulting in an increased stable phase of calcite. When the solution pH of the reactant is varied during the crystallization, the behavior of the phase fractions

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Figure 6. Morphological changes in product powders, crystallized at Sin ) 393.1 and pH ) 11.7, relative to the reaction time: (a) t ) 2 min; (b) t ) 10 min; (c) t ) 2 h; (d) t ) 24 h.

Figure 7. FT-IR spectra of product powders, crystallized at Sin ) 393.1 and pH ) 11.7, relative to the reaction time.

Figure 8. TG-DTA curves of product powders crystallized at Sin ) 393.1, pH ) 11.7, and t ) 10 min.

relative to the reaction time was also similar, as shown in Figure 3. In the current study, the solution pH, meaning the initial pH of the aqueous sodium carbonate solution before the reaction, was adjusted by adding a

solution of 0.1 mol/L NaOH or 0.1 mol/L HCl. A low solution pH with a fixed initial supersaturation of 218.1 provided favorable conditions for the transformation of ACC into vaterite rather than calcite. As such, the phase

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Figure 9. SEM images implying agglomeration of vaterite on calcite under crystallization conditions of Sin ) 393.1 and pH ) 11.7: (a) t ) 20 min; (b) t ) 1 h.

fraction of vaterite increased up to 65% of the crystalline calcium carbonate with a solution pH of 8.5 after about 2 h of reaction. When the solution pH was increased, calcite was exclusively generated at a solution pH of 13.3. The present experimental results are also consistent with Tai and Chen’s observations,1 where vaterite was the major product at a solution pH below 11.0 and then calcite was the dominant crystalline phase at a solution pH above 12.0. It is also interesting to note that, with a solution pH of 13.3, calcium hydroxide was detected during the initial 10 min of reaction because of the NaOH added to adjust the solution pH, yet this quickly disappeared because the solubility of calcium hydroxide is extremely high.4,24 Relationship between the Polymorphic Ratio and Morphology. The influence of the crystallization conditions, including the initial supersaturation and solution pH, on the morphology of the product powders was microscopically observed relative to the reaction time. As shown in Figure 4a, the representative shape for the crystalline powder obtained with an initial supersaturation of 218.1 and a solution pH of 11.6 after 2 min of reaction was rhombic, the typical morphology of calcite. However, a spherical-shaped powder, the typical morphology of vaterite, became evident after 2 min (Figure 4b,c). Thereafter, the spherical shape disappeared and only a rhombic powder was observed after 24 h of reaction (Figure 4d). Meanwhile, no spherical vaterite was detected for the whole reaction time at an initial supersaturation of 43.7, as anticipated from the XRD determination in Figure 2. These experimental results microscopically demonstrated that the amount of calcite and vaterite nucleated from ACC was dependent on the initial supersaturation, along with the recrystallization of vaterite to calcite. However, it should also be noted that during the initial crystallization, although the XRD analysis indicated calcite as the dominant phase in the powder (Figure 2), a considerable amount of powder with an unidentified shape was mixed in with the rhombic calcite, as observed via microscopic investigation. The amount of this powder was significantly reduced and eventually disappeared as the spherical shape of vaterite was generated. As such, it could be inferred that this

unidentified powder was ACC transformed into a crystalline phase. At a low solution pH of 8.5, the dominant crystalline phase clearly shifted from rhombic calcite to plateshaped vaterite and then back to rhombic calcite with the reaction time, as given in Figure 5. Because of the favorable pH conditions for the transformation of ACC into vaterite, a large fraction of the crystalline phases was occupied by vaterite after 30-90 min of aging and then calcite crystals emerged via a phase transformation from vaterite. This morphological shift also corresponded to the change in the XRD pattern relative to the reaction time. Interestingly, the changes in the phase fractions of calcite and vaterite with a high initial supersaturation of 393.1 were not always reflected by morphological changes in the crystals over the entire reaction period, as shown in Figure 6. Externally, the crystalline powders looked like spherical vaterite with only a few rhombic calcites during the initial crystallization. However, the powders were actually determined to be a crystalline mixture of calcite and vaterite with different polymorphic ratios relative to the reaction time when using the XRD technique (see Figure 2). To reconfirm the polymorphic composition of the spherical crystals, FT-IR spectra and TG-DTA analyses were also applied, as shown in Figures 7 and 8, respectively. Figure 7 shows the typical IR spectra of the spherical crystals with the reaction time. Among the characteristic IR peaks for calcite (714, 848, 876, and 1800 cm-1) and vaterite (745, 848, 873, and 1070 cm-1), the bands at 714 and 745 cm-1 were applied to identify calcite and vaterite, respectively.5,25,26 The IR spectra revealed that the characteristic peak area for each polymorph changed according to the reaction time. Accordingly, the IR spectra confirmed that calcite was included in the spherical crystalline crystals and that the phase fraction of calcite to vaterite varied with the reaction time, despite the observation of only minimal morphological changes. The same analytical results were also acquired from the TG-DTA determination, as shown in Figure 8. From the TG curves, the total weight loss for the powder, attributed to the decomposition of calcium carbonate into CaO and CO2, was around 44% between 400 and 1000 °C, inferring that the product powder was

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almost pure crystalline calcium carbonate. Meanwhile, in the DTA analysis, an exothermic peak was detected between 400 and 450 °C because of the phase transformation of vaterite into calcite,26,27 indicating that the product powder crystallized with a high initial supersaturation (Sin ) 393.1) was actually a polymorphic mixture of calcite and vaterite, although the morphological shape appeared spherical. The reason few morphological changes were observed in the product powders relative to the reaction time with a high initial supersaturation of 393.1 was most likely because the calcite was covered with a predominant fraction of vaterite via agglomeration during the initial stage of crystallization. This hypothesis was supported by SEM images showing vaterite-agglomerated calcite, as presented in Figure 9. Observations similar to those in Figure 9 were also reported by Hostomsky and Jones7 and Franke and Mersmann28 and considered to be agglomerates comprised of calcite and vaterite. Conclusions In the reaction crystallization of calcium carbonate carried out in a batch reactor system using aqueous CaCl2 and Na2CO3 solutions, especially at room temperature, the polymorphic ratio between calcite and vaterite nucleated from ACC was found to be controlled by the crystallization conditions, including the initial supersaturation and solution pH. In the current study, only calcite and vaterite were observed relative to the reaction time and crystallization conditions. With a high supersaturation and low solution pH, the decrease in the phase fraction of calcite during an early stage of crystallization suggested that ACC was simultaneously generated by crystallization and preferentially transformed into the crystalline phase of vaterite rather than calcite. The phase fraction of calcite then recovered with the recrystallization of vaterite to calcite based on the thermodynamic stability. Furthermore, the generation and transformation of amorphous and crystalline calcium carbonate also appeared to depend on the crystallization conditions. For example, when the supersaturation was decreased and the solution pH increased, the formation of calcite was preferred, resulting in the rapid and selective formation of calcite. The above structural transformation was consistent with the morphological shift in the product powder during the crystallization. Rhombic calcite was the major product of calcium carbonate with a low initial supersaturation (Sin ) 43.7) and high solution pH (pH ) 13.3). Yet, when the initial supersaturation was increased and solution pH decreased, spherical and platelike vaterite became dominant. However, as the aging period progressed, rhombic calcite reemerged as vaterite was transformed into calcite. It was interesting to note that the changes in the polymorphic ratio of calcite with a high initial supersaturation of 393.1 were not accompanied by a morphological shift, even though the polymorphic ratio of the powders changed, which was also reconfirmed by FT-IR, TG-DTA, and XRD analyses. Acknowledgment The authors are grateful for financial support from the Korea-Japan co-work program of KOSEF (2003). Nomenclature ACC ) amorphous calcium carbonate Iabc ) XRD peak intensity at the abc face

IP ) ionic product (kmol/m3)2 KSP ) solubility product of calcium carbonate (kmol/m3)2 t ) reaction time (s) Sin ) initial supersaturation XC ) mass fraction of calcite in the product powders XV ) mass fraction of vaterite in the product powders

Literature Cited (1) Tai, C. Y.; Chen, F.-B. Polymorphism of CaCO3 precipitated in a constant composition environment. AIChE J. 1998, 44, 17901798. (2) Kanakis, J.; Dalas, E. The crystallization of vaterite on fibrin. J. Cryst. Growth 2000, 219, 277-282. (3) Jung, W. M.; Kang, S. H.; Kim, W.-S.; Choi, C. K. Particle morphology of calcium carbonate precipitated by gas-liquid reaction in a Couette-Taylor reactor. Chem. Eng. Sci. 2000, 55, 733-747. (4) Chakraborty, D.; Agarwal, V. K.; Bhatia, S. K.; Bellare, J. Steady-state transitions and polymorph transformation in continuous precipitation of calcium carbonate. Ind. Eng. Chem. Res. 1994, 33, 2187-2197. (5) Xyla, A. G.; Koutsoukos, P. G. Quantitative Analysis of Calcium Carbonate Polymorphs by Infrared Spectroscopy. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3165-3172. (6) Chen, P.-C.; Tai, C. Y.; Lee, K. C. Morphology and growth rate of calcium carbonate crystals in a gas-liquid-solid reactive crystallizer. Chem. Eng. Sci. 1997, 52, 4171-4177. (7) Hostomsky, J.; Jones, A. G. Calcium carbonate crystallization, agglomeration and form during continuous precipitation from solution. J. Phys. D: Appl. Phys. 1991, 24, 165-170. (8) Arai, Y.; Yasue, T. Controls of Crystal Shape and Modification in Preparation of Calcium Carbonate. Gypsum Lime 1990, 228, 291-301. (9) Sawada, K. The mechanisms of crystallization and transformation of calcium carbonates. Pure Appl. Chem. 1997, 69, 921928. (10) Kojima, Y.; Kawanobe, A.; Yasue, T.; Arai, Y. Controls of Polymorphism and Morphology of Calcium Carbonate Compounds Formed by Crystallizing Amorphous Calcium Carbonate Hydrate. J. Ceram. Soc. Jpn. 1994, 102, 1128-1136. (11) Tai, C. Y.; Chen, P.-C. Nucleation, Agglomeration and Crystal Morphology of Calcium Carbonate. AIChE J. 1995, 41, 68-77. (12) Hirasawa, I. Formation of Calcium Carbonate by Reactive Crystallization. J. Soc. Inorg. Mater. Jpn. 2000, 7, 307-312. (13) Kabasci, S.; Althaus, W.; Weinspach, P.-M. Batch-precipitation of Calcium Carbonate from Highly Supersaturated Solutions. Trans Inst. Chem. Eng. 1996, 74, 765-772. (14) Rao, M. S. Kinetics and Mechanism of the Transformation of Vaterite to Calcite. Bull. Chem. Soc. Jpn. 1973, 46, 1414-1417. (15) Manoli, F.; Dalas, E. Spontaneous precipitation of calcium carbonate in the presence of chondroitin sulfate. J. Cryst. Growth 2000, 217, 416-421. (16) Katsifaras, A.; Spanos, N. Effect of inorganic phosphate ions on the spontaneous precipitation of vaterite and on the transformation of vaterite to calcite. J. Cryst. Growth 1999, 204, 183-190. (17) Ogino, T.; Suzuki, T.; Sawada, K. The rate and mechanism of polymorphic transformation of calcium carbonate in water. J. Cryst. Growth 1990, 100, 159-167. (18) Loste, E.; Wilson, R. M.; Seshadri, R.; Meldrum, F. C. The role of magnesium in stabilizing amorphous calcium carbonate and controlling calcite morphologies. J. Cryst. Growth 2003, 254, 206218. (19) Kojima, Y.; Kawanobe, A.; Yasue, T.; Arai, Y. Synthesis of Amorphous Calcium Carbonate and its Crystallization. J. Ceram. Soc. Jpn. 1993, 101, 1145-1152. (20) Vucak, M.; Peric, J.; Krstulovic, R. Precipitation of Calcium Carbonate in a Calcium Nitrate and Monoethanolamine Solution. Powder Technol. 1997, 91, 69-74. (21) Kojima, Y.; Endo, N.; Yasue, T.; Arai, Y. Morphological Control of Sn2+ Doped Calcium Carbonate Phosphors by Crystallizing Amorphous Calcium Carbonate and its Fluorescence Property. J. Ceram. Soc. Jpn. 1997, 105, 395-400. (22) Nielsen, A. E.; Toft, J. M. Electrolyte crystal growth kinetics. J. Cryst. Growth 1984, 67, 278-288.

Ind. Eng. Chem. Res., Vol. 43, No. 11, 2004 2657 (23) Plummer, L. N.; Busenberg, E. The solubilities of calcite, aragonite and vaterite in CO2-H2O solutions between 0 and 90 °C, and an evaluation of the aqueous model for the system CaCO3CO2-H2O. Geochim. Cosmochim. Acta 1982, 46, 1011-1040. (24) Min, K.-S.; Choi, S.-H. Synthsis and morphology control of fine potassium hydroxide powder. Sci. Technol. Ceram. Mater. 1993, 8, 56-62. (25) Wang, L.; Sondi, I.; Matijevic, E. Preparation of Uniform Needle-like Aragonite Particles by Homogeneous Precipitation. J. Colloid Interface Sci. 1999, 218, 545-553. (26) Nassrellah-Aboukais, N.; Boughriet, A.; Laureyns, J.; Aboukais, A.; Fischer, J. C.; Langelin, H. R.; Wartel, M. Transformation of Vaterite into Cubic Calcite in the Presence of Copper(II) Species. Chem. Meter. 1998, 10, 238-243.

(27) Sugihira, H.; Ono, K.; Adachi, K.; Setoguchi, Y.; Ishihara, T.; Takita, Y. Synthesis of Disk-Like Calcium Carbonate (Part I) Effect of Various Organic Compounds on the Carbonation of the Basic Calcium Carbonate. J. Ceram. Soc. Jpn. 1996, 104, 832836. (28) Franke, J.; Mersmann, A. The influence of the operational conditions on the precipitation process. Chem. Eng. Sci. 1995, 50, 1737-1753.

Received for review October 2, 2003 Revised manuscript received February 13, 2004 Accepted February 28, 2004 IE034161Y