(501g) Avalanching, Bed Collapse and Cohesion: Characteristic Behavior of Geldart A, B and C Powders

The Geldart powder classification diagram is a useful predictor of powder fluidization behavior based on particle density and surface-volume mean particle diameter. It also provides useful identification of and insights to the flow behavior and avalanching of powders in rotating systems in the context of powder handling and process engineering. In this work, avalanching, bed collapse, and cohesion of selected Geldart Groups A, B, and C powders were measured and evaluated. Avalanche behavior was measured with a novel tumbling device equipped with a cylindrical drum, ~200 mm in diameter and ~300 mm long, and a load cell that captured changes in the centre of gravity of the tumbling powder mass; the standard deviation of the load cell signal provided an indication of avalanche activity. Powder de-aeration was measured by fluidization and the double-drainage bed collapse technique. Cohesion at zero consolidation was estimated by shear testing at low consolidation stresses. The standard deviation of the load cell signal is presented as a function of drum rotating speed, 5-30 revolutions per minute (rpm); the standard deviation profile for each powder is different and the data are reproducible. For the Group C powder used, avalanching increases with drum speed; a change of slope occurs at ~15 rpm. In comparison, the avalanche activity of the Group B powder used increased linearly up to 20 rpm before leveling off; the standard deviation values are lower than for the Group C powder. The avalanching profile of a Group A/B powder shows a characteristic similar to that of the Group B powder, but even lower standard deviation values. When two Group A powders of similar surface-volume mean particle diameter, ~40 μm, and span of size distribution, ~1.4-1.6, but different measured values of standardized bed collapse rate and cohesion are compared, the powder with a lower bed collapse rate and higher cohesion shows lower avalanche activity. Another Group A powder of ~60 μm and span of ~2.3 seem to give relatively constant avalanching behavior regardless of drum speed.