Flow abilities of powders and granular materials evidenced from dynamical tap density measurement - 2013

This paper offers an overview of the flow properties of granular systems, including voids, granular porosity and random packing characteristics. For the purposes of the study, the notion of additional porous volume is introduced. This volume is defined as the additional air volume added to the optimal granular packing. It represents the difference between the volume of the bulk powder bed and that of the same powder but when ideally packed. It appears as the volume of additional air (or voids) trapped/stored between the grains when the powder passes from a dynamical state to a static state (during the filling of a container or the formation of a powder heap, for example). Therefore, if the powder bed traps air, it is then able to restore air partially or completely or not at all, depending on the intergranular cohesion level. This mechanism of the storing and releasing of air can be analysed from the measurement of compressibility curves. If the powder is non-cohesive or free flowing, it traps a small amount of air in its static state. Conversely, if the powder is cohesive, it traps more air. These data can be related to the flow properties of granular materials. Indeed, the compressibility curves obtained for granular materials provide information such as additional porosity, a kinetic parameter which characterizes the compressibility dynamics, a granular relaxation index which predicts how far a powder is from its optimal packing state and an index which gauges the de-areation speed of the powder. Measurement of such properties provides a better understanding of the nature of granular materials. Measurements of dynamical compressibility were carried out on five granular materials (two different lactose powders, hydrated lime Ca(OH)₂, yttrium stabilized zirconia balls and polystyrene balls). The overall results are presented using a radar graph. The use of this tool and its advantages are discussed in relation to the measurement and characterization of powder flow properties.

Abstract