Crystallization of Nano-Calcium Carbonate: The Influence of Process Parameters Steliana Aldea 1 , Mathias Snåre 2 , Kari Era ¨ nen 1 , Henrik Grenman 1 , Anne-Riikka Rautio 3 , Krisztian Korda ´s 3 , Jyri-Pekka Mikkola 1,4 , Tapio Salmi 1 , and Dmitry Y. Murzin 1, * DOI: 10.1002/cite.201600028 Dedicated to Prof. Dr.-Ing. Andreas Seidel-Morgenstern on the occasion of his 60th birthday Precipitated calcium carbonate was synthesized by carbonation of calcium hydroxide in the presence and absence of ultra- sound (conventional stirring) at atmospheric as well as at elevated pressures and different initial concentrations of Ca(OH) 2 . Spherical morphology of the formed calcite was favored at high Ca(OH) 2 concentrations and low CO 2 pres- sures. The presence of ultrasound did not show any influence on the reaction rate in case of efficient mixing. A small increase of the reaction rate was observed at lower CO 2 pressures. Elevated pressures in combination with ultrasound did not lead to notable changes of reaction rate or particle morphology. Keywords: Carbonation, CO 2 pressure, Precipitated calcium carbonate, Process intensification, Ultrasound Received: April 25, 2016; revised: June 28, 2016; accepted: July 01, 2016 1 Introduction Precipitated calcium carbonate (PCC) is a highly desired material finding multiple industrial uses in many applica- tions. PCC is a synthesized calcium carbonate with high chemical purity compared with grinded calcium carbonate typically available in ca. 0.7 mm size. Compared to the latter, PCC is characterized by its smaller particle size, narrower particle size distribution, and applicability in diversified applications. For example, PCC is used as a pigment, filler, or extender in the production of paper, plastics, paints, ad- hesives, textiles, pharmaceuticals, cosmetics, or food [1 – 3]. The various applications are strictly defined by the charac- teristic properties of the prepared material, such as the aver- age particle size, particle size distribution, specific surface area, morphology, or chemical purity. The physicochemical properties of PCC are highly influenced by different synthe- sis methods or synthesis conditions [4 – 7]. Most of the industrial production of PCC is obtained via the carbonation route representing a gas-liquid-solid sys- tem. Starting from limestone (CaCO 3 ), the process itself involves the following steps: calcination with the release of CO 2 and formation of lime (CaO), followed by the slaking process, in which the quicklime (CaO) is transformed to slaked lime (a Ca(OH) 2 suspension) by controlled addition of H 2 O CaO + H 2 O fi Ca(OH) 2 DH = –65 kJ mol –1 (1) and finally the carbonation reaction, in which CO 2 gas is in- troduced into the aqueous slurry, dissolves, and reacts with dissolved Ca(OH) 2 . Ca(OH) 2 + CO 2 fi CaCO 3 +H 2 O DH = –113 kJ mol –1 (2) As already mentioned, the control of crystal size and shape is fundamental from the viewpoint of technical appli- cations [8]. Although extensively studied, due to the com- plexity involved in the industrial carbonation process, only a few methods allow for the morphological control of the precipitated calcite without the help of additives which are increasing the production costs and can compromise the purity needed in certain applications [6, 9, 10]. Beside addi- tives, also the concentration of calcium hydroxide, CO 2 flow rate, Ca 2+ /CO 3 2– ratio, temperature, and stirring quality are among the parameters that influence carbonation reaction. A thorough insight of the processes and kinetics involved in the carbonation step together with the development of new industrial ways to produce PCC with controlled mor- phology are of significant interest. Therefore, several studies have been devoted to explore the influence of synthesis parameters on the morphological properties of thus ob- Chem. Ing. Tech. 2016, 88, No. 11, 1609–1616 ª 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cit-journal.com – 1 Steliana Aldea, Dr. Kari Era ¨nen, Dr. Henrik Grenman, Prof. Dr. Jyri-Pekka Mikkola, Prof. Dr. Tapio Salmi, Prof. Dr. Dmitry Y. Murzin (dmurzin@abo.fi), Laboratory of Industrial Chemistry and Reaction Engineering, Johan Gadolin Process Chemistry Centre, Åbo Akademi University, Biskopsgatan 8, 20500 Åbo- Turku, Finland; 2 Dr. Mathias Snåre, Nordkalk Oy Ab, Parainen- Pargas, Finland; 3 Dr. Anne-Riikka Rautio, Dr. Krisztian Korda ´s, Microelectronics Research Unit, University of Oulu, 90014 Oulu, Finland; 4 Prof. Dr. Jyri-Pekka Mikkola, Technical Chemistry, Department of Chemistry, Chemical-Biological Centre, Umeå Uni- versity, 90187 Umeå, Sweden. Research Article 1609 Chemie Ingenieur Technik