Precipitation of calcium carbonate particles by gas–liquid reaction: Morphology and size distribution of particles in Couette-Taylor and stirred tank reactors Wang-Mo Jung a , Sung Hoon Kang a , Kyo-Seon Kim b , Woo-Sik Kim c,n , Chang Kyun Choi a a School of Chemical and Biological Engineering, Seoul National University, Kwanak-ku Kwanak-ro 599, Seoul 151-742, Republic of Korea b Department of Chemical Engineering, Kangwon National University, Choonchun Hyoja2-dong 192-1, Kangwon-Do 200-701, Republic of Korea c Department of Chemical Engineering, ILRI, Kyung Hee University, Yoing Kiheung-ku Seochun-dong 1, Kyungki-do 449-701, Republic of Korea article info Article history: Received 16 November 2009 Received in revised form 12 August 2010 Accepted 17 August 2010 Communicated by J.J. Derby Available online 25 August 2010 Keywords: A1. Agglomeration A1. Crystal morphology A1. Crystal size distribution B1. Calcium carbonate B3. Couette-Taylor reactor abstract The morphology and size of CaCO 3 precipitated by CO 2 –Ca(OH) 2 reaction in stirred tank and Couette- Taylor reactors were experimentally investigated. The Taylor vortex in CT reactor encouraged more homogeneous mixing conditions, resulting in the production of smaller particles with a uniform shape throughout the reactor. However, in the stirred tank reactor, the local non-homogeneity of the mixing intensity led to the simultaneous production of cube-like and spindle-like particles at a high reactant concentration. The agglomeration of CaCO 3 resulted in a bimodal size distribution. However, the morphology and size of a single particle were predominantly changed by the excess species in the solution. The largest mean size and cube-like particles were observed under stoichiometric reaction conditions. As the excess species concentration increased, the morphology was transformed to a spindle-like shape and the mean size decreased due to selective adsorption of the excess species on the crystal faces. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Calcium carbonate is one of the most popular materials for precipitation studies, as it is used in numerous areas, including plastics, textiles, rubbers, adhesives, paints, and wastewater treatment. For industrial applications in the above mentioned fields, the product properties are determined by the morphology and size distribution of the precipitates. Therefore, a considerable amount of attention and effort has been devoted to control these characteristics of calcium carbonate. The polymorphs and size distribution of calcium carbonate have already been reported to depend on the precipitation conditions, such as the supersaturation, temperature [1,2], solution pH [3,4], ionic ratio of the reactants [5], and polymeric additives [6–8]. Also, an extensive study on the effect of the operating variables, including the reactant concentration, resi- dence time, specific power input and feed point location, on calcium carbonate precipitation was performed [9]. The charged ions in the solution have also been found to change the morphology and size of the particles. Nielsen [10] qualitatively demonstrated the effect of an ionic additive on the growth and morphology of a particle based on the selective adsorption on the crystal face. Plus, additives such as Mg 2+ , Ni 2+ , Co 2+ and Fe 3+ were found to favor the formation of aragonite while the formation of calcite was favored in the presence of Mn 2+ , Ca 2+ and Pb 2+ [1,11,12]. Jung et al [13] demonstrated that the monovalent ionic additives of Na + ,K + and NH þ 4 were much effective to the modification of crystal morphology of calcite due to the lattice inclusion. Excess species of reactants in the solution, which may be caused by a deviation from the reaction stoichiometry, can also affect the morphology and size of the particles. However, studies on the effect of excess species are rare, as most experiments are conducted under stoichiometric condi- tions due to their simplicity. Besides, most previous studies have adopted liquid–liquid reaction processes, as they are easy to operate and control. However, with this type of system, the simultaneous formation of a by-product from the reaction is inevitable. For example, when using the reaction between calcium chloride and sodium carbonate, sodium chloride is simultaneously produced as a by- product. This by-product salt dissolved in the solution can then influence the precipitation, like an ionic additive, yet this is invariably missed in most studies. When compared with a liquid–liquid system, a gas liquid reaction system using carbon dioxide and calcium hydroxide is advantageous by eliminating the effect of by-products, plus this Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.08.026 n Corresponding author. Tel.: + 82 31 201 2576; fax: + 82 31 273 2971. E-mail address: wskim@khu.ac.kr (W.-S. Kim). Journal of Crystal Growth 312 (2010) 3331–3339