Influence of mixing on the quality of iron precipitates in zinc-rich solutions

Reaction crystallization processes, also known as precipitation, are often characterized by high supersaturation levels and rapid nucleation. In such processes the mixing rate is generally a few orders of magnitude slower than the rates of the chemical reaction and nucleation. Any change in the rate-limiting step is therefore expected to influence supersaturation, which in turn impacts on the nucleation and growth rates. Since the nucleation and growth rates affect the final product quality, an effort needs to be made to reduce supersaturation. In this study iron precipitate quality was improved by changing the mixing environment. It was shown that the final product quality of iron precipitates formed through ferric iron hydrolysis was significantly improved by changes in the micro- and macromixing environments in a continuous crystallizer. Product quality parameters that were studied included the dry solids density, particulate impurity content, particle size and the filterability of the precipitates.