Characterisation of flowability of loosely compacted cohesive powders by indentation

In this paper we report on our investigations of the flowability of bulk powders using indentation and unconfined direct compression testing on a small assembly of powders compacted at low pressure levels. Samples of a-lactose, magnesium carbonate and glass beads (made cohesive by silanization) have been consolidated to different pressure levels (ranging from 3 to 20 kPa) and the indentation hardness and unconfined yield stress of the compacts were then characterised. The latter was obtained by unconfined direct compression. The experimental results show that there is a linear relationship between the yield stress and consolidation pressure for the cases of a-lactose and magnesium carbonate, from which it is possible to deduce the powder flow behaviour. The results of yield stress obtained from indentation and unconfined compression measurements are similar for the cases of a-lactose and magnesium carbonate, whilst the results are different for the case of cohesive glass beads. Furthermore simulations were carried out using the Distinct Element Method to predict the bulk behaviour of the cohesive powders based on single particle mechanical properties. The simulation results qualitatively agree with the experiment, i.e. there is a linear relationship between the unconfined yield stress and the consolidation pressure, from which the role of inter-particle adhesion on the flow behaviour may be deduced.