Can Particle Size Predict Powder Flow Behavior?

By Robert McGregor, General Manager, Brookfield Engineering Laboratories, Inc.

The general rule of thumb in gravity discharge from bins and silos is that as powder particles become smaller, the flow behavior become more difficult. "Increasingly cohesive" is the term used to describe what happens. For powder mixtures that have particle sizes of variable diameter, the general observation is that the large particles go along for the ride. The smaller size particles determine the flow behavior.

So what is the correct method to measure flow behavior? Can a particle size analyzer get the job done all by itself? Not necessarily. It can certainly tell you the makeup of your powder mixture in terms of size distribution from below 1 micron up to more than 1 millimeter. And from prior processing experience, you may be able to make an educated guess on how well the powder will work in manufacturing. But the actual flow behavior is more directly related to interparticle friction within the powder and sliding friction of the powder against the wall of the hopper.

Types Of Flow Behavior

Mass flow and core flow (also known as "funnel" flow) are the two ways in which a powder discharges from a bin. The desired preference for flow behavior is mass flow in which the particles flow uniformly relative to one another. (See Figure 1) "First in, first out" characterizes this type of flow. More common, however, in most powder processing operations is core flow in which the particles at the top of the bin cascade into the middle and flow down through a central core (somewhat like a "funnel"). Figure 2 shows that this type of behavior is described by "Last in, first out".