Solids motion in rolling mode rotating drums operated at low to medium rotational speeds

The industrial-scale rotating drums are usually operated in the rolling or slumping mode. For the rolling mode, the granular material bed can be divid ed into two regions, namely, a 'passive' region where particles are carried up by the drum wall, and an 'active' region where particles cascade down. As solids mixing mainly occurs in the active region, solids motion in this region and solids exchange between the active and passive regions are of pr ime importance for the overall performance of the drum. This paper reports some observations on particle motion in the transverse plane of a three-dim ensional relating drum operated at low and medium rotational speeds. The no n-invasive PEPT (positron emission particle tracking) technique is used to follow particle trajectory and velocity. A mathematical model based on the thin-layer approximation is proposed to describe solids motion in the activ e layer. Reasonable agreement between the model predictions and experiments is obtained. A new parameter termed the 'solids exchange coefficient' is p roposed to characterise particle exchange between the passive and active re gions. A theoretical expression for this parameter is also derived. This ex pression, upon application of the thin-layer approximation, is reduced to g ive an explicit relationship between the solids exchange coefficient and dr um operating parameters such as rotational speed and fill percentage. as we ll as the bed material theological properties. The solids exchange coeffici ent is also shown to give a possible scale-up rule for rotating drums opera ted in a rolling mode. (C) 2001 Elsevier Science Ltd. All rights reserved.