Cohesion as a strategy to control segregation and implications for flowability
Paul Mort (Purdue University, IFPRI-Elsevier Liaison, PrM3 Consulting)
The addition of a small amount of liquids, often called “spray-on”, is a commonly used as a means to mitigate segregation in mixed-granular products. The spray-on creates liquid bridges between particles in a mixture, having the dual effects of reducing the rate of segregation in downstream handling (e.g., packing) and making the product flow more cohesive. In practice, the amount of spray-on is optimized to balance these effects. This presentation considers this practice in light of previous experimental work on liquid-bridge cohesion and more recent work on fundamentals of granular flow , and segregation. ,
The author’s previous work2 investigated the transition from slow frictional to intermediate granular flows, including the effect of small liquid additions on measurable stress fluctuations and yield-locus transitions (incipient/steady-state at the critical state). This previous work was motivated by process applications such as binder agglomeration, powder mixing, precise metering, and pneumatic conveying. Slow frictional (i.e., quasi-static) flows are typically characterized using a Mohr-Coulomb analysis of shear-cell data. Quasi-static analyses (tensile intercept and curvature approaching the critical state) are very sensitive to the analysis of yield-locus curvature. As the dense flow regime transitions to the intermediate (a.k.a., elastic-inertial) regime, the direct measurement of normal stress and variation thereof becomes practical and informative.
This presentation discusses the above characterizations in context of more recent work sponsored by the International Fine Particle Research Institute (IFPRI), specifically in context of flow fluctuations in the transition from non-local to local rheology3-4 and the observed inertial scaling of segregation time in granular flows.5 Taken together, this recent work suggests that stress-field fluctuations correlate with both density and size-driven segregation. Further, this suggests a strategy to manage segregation potential by dampening stress fluctuations, e.g., via liquid-bridge cohesion. We will discuss the mechanistic basis for this strategy across both quasi-static and intermediate flow regimes. In quasi-static flows, liquid-bridge cohesion can increase the curvature of yield loci and reduce the stress differential between critical state incipient and steady-state flows, effectively reducing slip-stick behavior. In the transition to intermediate flows, liquid-bridge cohesion reduces the magnitude of local stress fluctuations.
Paul R Mort III Professor Department of Materials Engineering Center for Particulate Products and Processes (CP3) FLEX Lab 3021B, Gates Rd, West Lafayette, IN 47906
Paul Mort recently joined Materials Engineering faculty in support of Purdue’s Center for Particulate Products and Processes (CP3). He is a 24-year veteran of the Procter & Gamble Company and is globally recognized as an expert in particulate processing and powder technology. He has a demonstrated history of product innovation and driving process efficiency in the consumer goods industry. Paul is an Editor for Powder Technology journal and consultant with the International Fine Particle Research Institute (IFPRI), working to develop a pipeline of perspective articles for the journal. He is active in linking particle technology with adjacent technical communities including pharmaceutical processing, materials science and engineering, and process control.