Two modes of transverse solids motion, slumping and rolling, in rotary kiln operation have been experimentally characterized and mathematically modelled in this study. Other modes of bed behaviour encountered in rotary cylinders;​ slipping, cascading, cataracting and centrifuging have been formulated mathematically. The models have been verified using experimental observations reported in the literature.

An experimental study of those conditions under which the bed changed from slumping to rolling was undertaken and the characteristics of these modes of motion quantified using different types of solids in three horizontal rotary cylinders and a small pilot kiln. A Bed-Behaviour Diagram which is a plot of bed depth versus rotational speed was developed to delineate the various areas of dominance of slumping and rolling and it was shown using this Diagram that bed behaviour observations made on batch cylinders were representative of solids motion in a continuous kiln operation, the effects of bed depth, particle size, particle shape and cylinder diameter on the position of the slumping-rolling boundary were also experimentally investigated. The quantitative characterization of slumping and rolling indicated that a new interpretation of the change in bed motion from slumping to rolling was required.

A study of segregation in the bed reveald that while the presence of fines affecte d the siumping-rolling boundary, they had little effect on the slumping frequency, the shear angle, the stati c and dynamic angles of repose and the active layer thickness of the mixtures when compared to those fo r the parent materials. This indicate d that for the bulk solid s tested, segregatio n occurred by the percolation and not by the flow mechanism. Sampling of the bed reveale d two segregation cores whose formation and effect on kiln operations is discussed. A mathematical model is also presented to predict the composition and size of the central segregation zone.

A semi-empirical mathematical model of the slumping - rolling boundary was developed and the effects of operating , material and cylinder variables were illustrated. Scale-up criteri a were found to be the fill ratio, the Froude number, and the minimum shear wedge. For material s having the same shape but different size, this latter criterion may be replaced by the cylinder diameter to particle size ratio. Slipping, cascading and cataractin g were als o modelled and their boundaries illustratedo n the Bed-Behaviour Diagram. Observation s by other workers of these modes of bed behaviour are compared to the model predictions and the appropriate scale-up criteria are presented.