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.