Abstract
The reaction studied in this work is the synthesis of nanometric size calcium carbonate particles by carbonation of a suspension of lime, which represents the most common industrial route. It consists in bubbling carbon dioxide in a suspension of lime to obtain precipitated calcium carbonate (PCC). PCC is a mineral filler with various applications: sealants, paints, paper, ink, pharmacy, cosmetics, food etc. However, there is a challenge related to the synthesis and the use of this precipitate: the agglomeration of the monoparticles. The aim of this work is then to understand the mechanisms of this phenomenon and to study its kinetics to improve the run of the process and the control of its impact on the final product. Experiments realized with a high concentration in sodium chloride (2 M) showed that the modification of the electrostatic environment did not change the particle size distribution and the morphology of the agglomerates. This indicates that the electrostatic interactions are not responsible for the agglomeration but the formation of crystalline bridges induced by the crystal growth. Thus, thanks to an agglomeration model including the crystal growth rate, the agglomeration kernel β and the agglomeration constant β0 can be determined using a mathematical treatment of the experimental particle size distributions. Finally, by varying the experimental conditions, it appears that the agglomeration constant increases with the temperature whereas there is an optimal value regarding the shear rate.
Highlights
The reaction of carbonation is the most common industrial route to synthetize precipitated calcium carbonate (PCC)
Experiments performed with a high concentration in sodium chloride (2 M) show that the modifications of the electrostatic environment do not change the particle size distribution or the morphology of the agglomerates, so the electrostatic interactions are not responsible for the agglomeration
The principal mechanism of agglomeration is the building of crystalline bridges between the elementary particles of calcium carbonate induced by the crystal growth
Summary
The reaction of carbonation is the most common industrial route to synthetize precipitated calcium carbonate (PCC). They are agglomerated in micrometric “loose” structures This agglomeration has consequences on the run of the process and the properties of the final product so the aim of this work is to understand the mechanisms of this phenomenon and to study its kinetics under complex industrial conditions characterized by a high concentration in solid phase and intensive mechanical stirring and gas bubbling. The nucleation and crystal growth kinetics are presented elsewhere [1], while the aim of this work is to clarify the mechanism and to quantify the kernel expression during the agglomeration of calcite particles This phenomenon sensibly influences the end use properties of the PCC. The reactive gas is composed of 30% CO2 and the mass flow rate is controlled by two mass flowmeters
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