MODELLING THE INCORPORATION OF IMPURITIES INTO SUGAR CRYSTALS

This paper describes work whose object was to quantify and model the partitioning of impurity species into sugar crystals. Impurities are trapped (layered in) within the crystalline structure of sucrose by co-precipitating with sucrose into the crystal lattice during crystal growth. As well, some impurities are present in gross molasses inclusions within the crystal, and in the molasses or syrup film around the crystals. Expressions were derived to quantify the various mechanisms for species inclusion. Data obtained at five raw sugar factories were processed to estimate partitioning coefficients for a number of impurity species. Two methods, one involving a simple calculation and the other a more complex pan stage mathematical model, were used to process the data. They gave similar results for the coefficients in the partitioning expressions. Using best estimates of partitioning coefficients, predictions were made to indicate where beneficial reductions in crystal and sugar impurity species could be achieved through changes to the flow scheme for crystallisation.