A model for aluminium trihydroxide crystallization from pure and impure solutions

Mannitol has been used as a model hydroxy organic compound to simulate the influence of organic compounds on the growth, nucleation and agglomeration mechanisms of aluminium trihydroxide crystallization. As little as 151.6 mg mannitol/m(2) of seed was sufficient to completely suppress the crystalliz ation. A population balance equation including terms for nucleation, growth and agglomeration, coupled with a mass balance equation forms the basis of a mechanistic model for batch crystallization of aluminium trihydroxide. T he presence of mannitol at a level as low as 92.2 mg mannitol/m(2) of seed reduced the growth rate coefficient by an order of magnitude and the nuclea tion rate coefficient by two orders of magnitude. This low mannitol concent ration also increased the agglomeration parameter. Four different kernels o f shear flow, turbulent flow, size independent, and an empirical kernel wer e checked for best fit in the present study. Crystallization in the absence of mannitol was best fitted with no agglomeration term in the model.