Control of particle size in rapid crystallizing processes, also called precipitation processes, present a challenge for a number of industries such as pharmaceutical, food or water treatment. It is commonly found that milling and agglomeration processes are needed in the latest stages of the synthesis to achieve the desired particle size. These processes are costly and may alter the polymorphic form. It is believed that a better fundamental understanding of the synthesis would allow better control over the process parameter in order to yield the desired product
In the present study the hyrozincite will be firstly characterized in terms of its crystal structure, solubility, induction time and morphology. Then, a continuous precipitation process was chosen to study its synthesis, as it allows fine control of process parameters such flow rate, geometry and residence time which are known to control the final particle size.
The methodologies used in this study aim to elucidate the kinetics of the process of mixing, nucleation, aggregation and breakage. Turbidity measurements in batch reactor allows the determination of the nucleation kinetics as function of reactant concentration. CFD was used to the determination of mixing kinetics well as supersaturation profiles inside the Y-junction as well as within the tubular section. Finally, the the particle size distribution as function of the residence time was determined by in-line static light scattering.
It is been observed that the small surface energy (0.051 J/m^2) of hydrozincite allows nucleation to occur rapidly. However, the rate of crystal generation decrease with decreasing pH as the reactant CO_3 decreases. The study of the relative importance of reaction over mixing (Damkholer number) reveals a general dominance of mixing.
We can conclude that pH is the single most important process parameter for controlling the particle size, followed by residence time.
