Modelling of particle mixing and segregation in the transverse plane of a rotary kiln

Thorough mixing of particles in the transverse plane of a rotary kiln or drier is essential to the uniform heating or cooling of the charge and ultimately, to the generation of a homogeneous product. However, differences in particle size and density result in a de-mixing process whereby smaller or denser particles segregate to form an inner core or kidney of segregated material which may never reach the bed surface to be exposed to freeboard temperatures. A model has been developed to predict the preferential movement of particles in the shearing active layer. This model determines the extent of fine particle segregation and is based on the principle of percolation in the active layer, whereby fines sift through the matrix of the bed to form the segregated core. Incorporating results of a granular flow model developed for this purpose, it has been possible to establish the dimensions of the segregated core as well as fines (jetsam) concentration in the rest of the bed cross-section. This result is necessary in assessing the effect of segregation on bed temperature nonuniformities in rotary kilns. Copyright (C) 1996 Elsevier Science Ltd