The basic answer is that powder flow depends on many factors that are not addressed by a rheometer as powders are fundamentally different to liquids in flow situations. The main difference is that loose solids can sustain shear loads that changes with the state of compaction of the mateiral.

As conditions of density, porosity, shear strength and wall friction bear on how a bulk material will flow in a given situation it is not possible to represent 'flowability' in a single value.

A graphical approach, based on a 'spider' diagram, has been developed by Eddie McGee, of Ajax Equipment and is in a paper presented at the recent 7th World Congress on Chemical Engineering at Glasgow.

An Ajax publication on 'The Characterisation of Bulk Materials' sets out the various relevant parameters and a pragmatic introduction to the flow related properties of powders. see www.ajax.co.uk.

In practical terms, bulk density and wall friction should be measured in almost every case that bulk material is required to be stored and handled and an assessment made as to the potential strength of the material to establish whether more thorough investigations are warrented as the the shear strength. ■