Growth process of ammonium sulfate crystals produced by secondary nucleation in stirred-tank crystallizer

Abstract

Growth behavior of ammonium sulfate secondary nuclei originated from a seed crystal in a stirred-tank crystallizer was investigated using a Coulter Counter. From the time dependence of the cumulative curves it appeared that the particles can be divided according to growth behavior into two groups. Particles larger than 40 pm grow fast, independently of size (group I), and particles less than 20–30 pm grow at a rate 1–2 orders lower than that of group I (group II). It was also suggested that the secondary nucleation mechanisms of groups I and II are different, and that of group I may be initial breeding. It was observed that for a portion of the group II particles the growth rate increases abruptly during the crystallization. Dissolution rates of group I particles obtained from the cumulative curves were consistent with the values calculated by the mass transfer rates equation, but those of group II were largely lower than the calculated values. The difference in growth behavior between groups I and II was attributed neither to Gibbs-Thomson effect nor to mass transfer rate changes with particle size, but to the difference in the contribution of surface kinetic process. It was found that the growth process of large ammonium sulfate crystals (group I) is much more controlled by volume diffusion than are those of other salts such as potassium sulfate. © 1991, The Society of Chemical Engineers, Japan. All rights reserved.