In principle, there are several motion mechanisms of particulates which contribute to the effect of size segregation, including sliding, surging, slumping, rolling, cascading, cataracting and centrifuging. The performance of particulate motion varies as the rotational speed of the cylinder increases. In this study, the mechanisms of slumping, cataracting and centrifuging are considered. When the rotational speed of the rotating cylinder is low, the motion mechanism of slumping occurs. In this case, the flow of the particulates in the bottom of the cylinder lifts up slightly then avalanches down the free surface of the cylinder. The phenomenon of slumping is dependent on the rotational speed, particulate size and cylinder diameter. When the rotational speed of the rotating cylinder became faster, the motion mechanism transforms from slumping to cataracting. As rotational speed increases, strong cataracting motion is generated and individual particulates detach from the rotating cylinder. The motion of particulates has now reached the condition of turbulent flow. Screening efficiency of particulates is higher during cataracting motion than slumping motion. As the rotational speed increases, a flowing transition to centrifuging takes place. Screening efficiency decreases under the centrifuging condition, because the particulates begin to adhere to the wall of the rotating cylinder.
