Flowability Assessment of Weakly Consolidated Powders

Executive Summary
Measurement of powder flow behaviour is important for many powder handling operations. The most widely established method for measuring powder flow is by shear cell analysis, with procedures developed for designing hoppers based on mass or funnel flow behaviour using shear cell measurements. As the consolidation stress applied to the powder is reduced, the reliability and reproducibility of the measurement decreases. Powder flow measurement at low stresses is assessed here by ball indentation, uniaxial compression and shear cell methods.
Powder flow measurement at low stresses is more challenging due to (i) the required resolution of the force measurement and (ii) the reproducibity of the loose packing state. Shear cells pre-shear the sample in an effort to ensure a reproducible packing state, however the vertical consolidation applied in indentation and uniaxial compression approaches does not achieve this alone. Ball indentation measurements are assessed by pre-shearing and by blade conditioning, wire conditioning and sieve filling prior to vertical consolidation. At low stresses, pre-shearing is found to provide a large coefficient of variation in the bed hardness measurement by indentation, whereas sieve-filling is able to produce a coefficient of variation < 5% at low consolidation stresses. Uniaxial compression measurements correlate with ball indentation measurements at moderate stresses, allowing constraint factor to be determined and ultimately for unconfined yield stress to be inferred from indentation measurements. However, at lower stresses the uniaxial compression test underestimates the unconfined yield stress.
Shear cell measurements using the FT4 shear cell and Schulze RST-XS.s low stress shear cell agree at moderate stresses, however below 2 kPa the FT4 shear cell does not achieve the intended applied stresses for titania DT51, and therefore does not provide a reliable measurement in this range. The Schulze RST-XS.s provides unconfined yield stress measurements < 5% for the very cohesive titania at pre-shear stresses as low as 100 Pa, however the variability increaseses for more free-flowing powders.
The reproducibility of flow measurement at low stresses is similar for the RST-XS.s shear cell and for the ball indentation method by sieve filling using an indentation attachment to the FT4 Powder Rheometer, however the measured values of unconfined yield stress do not agree. A small hopper will be designed based on the flow measurements of both instruments at low stresses. The critical outlet diameter for flow initiation will be experimentally determined and compared to the dimensions predicted by the flow measurements of these two instruments. To minimise the powder quantity, gypsum powder is proposed for this investigation due to its high bulk density and cohesive flow behaviour.