The yield in the Bayer precipitation circuit is affected primarily by the alumina supersaturation, caustic concentration, temperature profile, seed surface area, holding time and impurities.
Optimisation of the precipitation yield requires a knowledge of the interaction of these various factors, and is best handled by computer simulation using rate data obtained in the laboratory.
Batch experiments were carried out over a range of conditions on Plant and synthetic liquors to derive rate constants and equilibrium relationships. The synthetic liquors were organic free, but contained similar levels of other impurities as Plant liquors.
The rate constants were used in a continuous precipitation model with results matched to actual Plant performance using a seed effectiveness factor. The model was then used to determine the optimum temperature and caustic concentration for the continuous precipitators.
The model is also a useful tool for quantifying the benefit of other strategies such as additional holding time or reduction in the organic level in Plant liquors.
The model assists with the optimisation of precipitation to extract the best yield performance while maintaining or improving product quality. Further project development on particle balances and product soda content is required to determine the limitations these product quality values place on yield in Plant liquors and liquors in which the organics are controlled to a low level.