PARTICLE SIZE DISTRIBUTION ANALYSIS FOR CALCIUM CARBONATE PRECIPITATION Raluca Isopescu 1 , Carmencita Mateescu 2 , Mihaela Mihai 1 , Ion Udrea 3 1.University POLITEHNICA of Bucharest, Chemical Engineering Department 1-7 Polizu Street, Bucharest 011061, Romania 2. National Institute for Material Physics, 1 Atomistilor Street, Magurele, Romania 3.University of Bucharest, Faculty of Chemistry, Bucharest, Romania Abstract: Calcium carbonate precipitation is studied experimentally using gas-liquid and liquid-liquid systems, aiming to obtain small particle sizes and narrow particle size distributions. The gas-liquid reaction is carried on by bubbling pure carbon dioxide into a clear solution of calcium hydroxide containing 2 g/L CaO, in a continuous crystallizer. The liquid-liquid reaction is performed in a semi-continuous mode, by controlled double jet precipitation adding simultaneously, at equal flowrates, 1 M and 0.1 M calcium nitrate tetrahydrate and potassium carbonate solutions. The influence of a non-ionic additive, Tween 20 was also studied. The solid phase composition and particle size distribution are analyzed for different operating conditions. A mathematical model based on the population balance equation is developed assuming as main mechanisms nucleation, size-dependent crystal growth and agglomeration. The corresponding kinetic parameters are estimated by correlating the experimental and calculated particle size distribution data. Keywords: Calcium Carbonate, Particle size distribution, Kinetic parameters, Population balance equation 1. INTRODUCTION Precipitated calcium carbonate is a product with various applications in the paint, paper, adhesive and rubber industries, in cosmetics and pharmaceutics. Calcium carbonate exhibits three polymorphic modifications: vaterite, aragonite and calcite. The purity of the solid phase obtained by precipitation is an important concern when certain destinations of the product are considered. Apart from the solid phase purity and morphology, the particle shape and particle size distribution (PSD) are key aspects that affect the end use properties of the product. Special working conditions in terms of temperature, pH, additives, ultrasound field are experimented in order to obtain a certain calcium carbonate polymorph (Shivkumara et al., 2006, Westin and Rasmunson, 2005, Han et al., 2006, Mateescu et al., 2008) or controlled PSD (Dalas et al., 2004, Isopescu et al, 2007). Precipitated calcium carbonate is mainly obtained by the chemical reaction between two soluble salts or in a gas-liquid system by bubbling CO 2 into a calcium hydroxide solution. The synthesis route is of great importance for the final product properties meaning solid phase purity and PSD. In the present work, both synthesis methods were experimentally studied aiming to obtain small particle sizes and narrow PSD. In precipitation processes, the calculation of PSD time evolution needs to assume the mechanisms for particle nucleation, growth and agglomeration. A large number of particles nucleate due to the significant increase of supersaturation created by the chemical reaction. In reactive crystallization crystal growth process often is size dependent (Alexopoulos, 2004). Agglomeration, which is an undesired process, cannot be totally avoided because of the large mass of particles that exist in the precipitation medium. The overlapping of these mechanisms produces specific shapes of PSD. The time evolution of the PSD is obtained from the solution of the population balance equation (PBE) and corresponding kinetic parameters have to be calculated. Therefore, experimental investigations are a must. The present work aims to solve the mathematical model of precipitation, using the population balance technique, considering homogenous nucleation, size dependent growth and agglomeration and estimate the