(19b) Relationships Between Particle Properties of Fine Powders and Flow Properties Under Low Compaction Stresses

In the food and pharmaceutical
industries, significant amounts of process material are in the form of fine
powder. Fine powders in general do not flow well because of their cohesive
nature, and various characterization methods have been used to predict powder
flowability. We present here results for thirteen model lactose powders, in
which Hausner ratio and powder flow function data, i.e. compaction
stress/powder strength data, have been correlated with size descriptors, as
well as with each other. Particle size distribution was measured by the laser
diffraction method (Mastersizer 2000, Malvern Instruments Ltd, UK); the key
size descriptors used were surface-volume diameter, d₃₂, and
span, (d₉₀?d₁₀)/d₅₀.
Hausner ratio, ρ_tap/ρ_B, was
determined with tapped density, ρ_tap, measured according
to a GEA Niro standard for dairy powders at 1250 taps in accordance with the
European Pharmacopoeia; loose bulk density, ρ_B, was
based on New Zealand Standard 3111. An annular shear cell (Brookfield
Engineering Laboratories, USA) operated at low compaction stresses, in the
range 0.31?4.85 kPa, was used to obtain powder flow functions, and hence
estimates of particle cohesion, C, and Jenike flow index, σ_c/σ_y;
σ_c is the major consolidating stress and σ_y
is the unconfined yield stress. Hausner ratio and C increased with
decreasing d₃₂ and increasing span, while σ_c/σ_y
increased with increasing d₃₂ and decreasing span. A plot of
Hausner ratio versus σ_c/σ_y at 4?6
kPa suggested a power correlation, and the relationship between Hausner ratio
and C appeared linear.