Batch Crystallization of KCl: the Influence of the Cooling and Mixing Rate on the Granulometric Properties of Obtained Crystals J. Prliæ Kardum, A. Sander, and A. Glasnoviæ Faculty of Chemical Engineering and Technology, University of Zagreb, Marulicev trg 19, 10 000 Zagreb, Croatia; tel., fax: +385 1 4597 259, email: jprlic@fkit.hr, asander@fkit.hr aglasnov@ fkit.hr, The influence of the intensity of mixing (100–500 rpm) and three different cooling rates on crystallization kinetics of KCl and granulometric properties of produced crystals has been investigated on a laboratory scale batch crystallizer. Different process condi- tions result with different crystal size distribution and crystal shapes. Obtained results show that cooling rate influences the supersaturation rate more intensively, the kinetics curves, and the average crystal size. On the other hand, crystal habit changes more with the intensity of agitation. Crystal size distribution can be expressed by log-normal distri- bution, i.e. its characteristic parameters, d 50 and s. Population density of crystal nuclei and overall linear growth rate are evaluated from crystal size distribution. The influence of cooling rate on both quantities (n 0 , & L ) is more intense than the influence of mixing rate. Keywords: Crystallization, crystal habit, crystal size distribution, KCl, kinetics Introduction Crystallization is one of the basic processes in the final treatment of products in the chemical in- dustry. Batch crystallization is an important unit operation in the chemical, photographic, and many other industries as manufacturing process to prepare a wide variety of crystalline products. Of the vari- ous types of batch crystallizer, cooling crystalliza- tion is one of the most common modes used in in- dustry. Optimal operation is important in ensuring the efficiency of the overall process. Control of crystal quality (crystal size distribution, shape and crystal purity) is of special interest in crystallization processes and it is challenging due to the complex- ity and non-linearity of the process. Many different variables (process conditions) influence the kinetics of crystallization and product properties, so that relations should be investigated experimentally. Consequently, experimental data doesn’t always follow the theoretical principles. The crystallization process consists essentially of two stages which generally proceed simulta- neously but which can, to some extent, be inde- pendently controlled. The first stage is the forma- tion of small particles or nuclei, and second stage is the growth of the nuclei. 1 The driving force for both, nucleation and crystal growth is supersatu- ration. Generally speaking, the nucleation rate and the crystal growth rate increases with increasing de- gree of supersaturation. Process parameters, such as cooling rate and intensity of agitation affect the supersaturation (width of metastable zone) and therefore the nucleation and growth rate. The influ- ence of agitation on nucleation process is very com- plex. Mechanical disturbances can enhance nucle- ation, but that is not always true. 2 If the nucleation rate can be controlled, the size of the crystals can be adjusted, and this is the most important feature of the crystallization process. The dimension and the shape of particulate product are important for several reasons. The effi- ciency of any process for production of a particu- late material relies on the size, shape, and size dis- tribution. 3 This is understandable, as the diameter of the crystals and their distribution is one of the criteria that determine the ease of further treatment (separation, drying and, frequently, also applica- tions). It has been found that the crystal size distri- bution can be used for evaluation of a number of fundamental kinetic parameters useful, both, in the description of crystallization processes and in a de- sign of crystallization equipment. 4 The connection between the kinetics of nucle- ation and crystal size distribution is very compli- cated. Bransom et al. were the first to suggest that the kinetic data could be obtained from the distribu- tion of crystal sizes, and they expressed the distri- bution of particle size in terms of quantity taken from mathematical statistics – the population den- sity. 5 Use of this quantity replaced distribution of the sizes of discrete species by coherent function. Presently this method is used regularly as a very J. PRLIÆ KARDUM et al., Batch Crystallization of KCl: the Influence of the Cooling …, Chem. Biochem. Eng. Q. 19 (1) 39–47 (2005) 39 Original scientific paper Received: July 7, 2004 Accepted: November 1, 2004