Characterizing powders in order to determine their flow behavior in a mixer: From small scale observations to macroscopic in-mixer rheology for powders of various flowabilities

Abstract : In this study we examine the relationships between particle's characteristics and in-process powder behavior for single powders and their mixes. Flowing properties of twenty-three powders have been investigated with different techniques at three scales of measurement and three different states of consolidation in order to link their basic characteristics to their behavior during agitation in a mixer. The macroscopic scale concerns powder stirring experiments in a 48 L planetary pilot mixer. The mesoscopic scale properties have been measured with Erweka® volumenometer and FT4 Freeman rheometer under three different consolidation states. Their flow ability ranges from free-flowing to cohesive. Microscopic properties, at the scale of particles, have been obtained from LASER granulometry, SEM observations and He-pycnometry. The comparison between macroscopic experiments and mesoscopic characterizations through specific power shows that the consolidation state of the powder is a key parameter to consider, especially for cohesive powders. The macroscopic scale in-mixer rheological measurements show two main patterns: a free-flowing like pattern and a cohesive like pattern, the ranking of the powders between these two being not intuitive. It is found that the power registered when stirring a powder constituted by a free-flowing media filled with a cohesive one reaches a minimum for a specific ratio of the two powders that has been linked to the microscopic structure of the particle's packing.