Abstract
The angle of repose of some fine powders as a means to evaluate their flowability is measured in a rectangularr test box under various experimental parameters, such as shape, particle size, and relative humidity., At any constant relative humidity, the angle of repose of the finely particulated solids increases with decrease in the particle size from 120 to 60μm, but it remains nearly constant for the size range of less than 60 through 70μm. It is concluded that the powder flowability reveals its lowest behavior at 120μm. The powder flowability decreases continuously with increase in the relative humidity above 50%. By using the rectangular test box, it is possible to evaluate the powder flowability more conveniently than by other methods.
The above experimental parameters are examined for the bottom pressure exerted by the fine powders in a test silo. The bottom pressure is directly proportional to the average particle size of the powder, as expected in the Janssen equation which was theoretically developed. The relative humidity more complicatedly affects the bottom pressure exerted by finely particulated solid powders than by coarsely particulated masses, such as harvest grains and the glass beads. At 70% relative humidity, the bottom pressure reaches the highest value. It is not appropriate to predict the bottom pressure of the fine powder with the Janssen equation, so it is recommendable to include the cohesion term into the equation estimating the bottom pressure of the fine powders.
