Granular Segregation: Why You Can’t Mix Materials in a Silo

Brazil Nut Problem - Granular Segregation in Biomass

From time to time, we meet clients who attempt to store mixed materials in a silo. Either the material is premixed or the client fills the silo with separate materials and assumes the materials will mix as they descend through the system. What they discover, however, is that the materials, whether they be wood chips or grains, do not remain uniformly mixed.

The reason why this occurs has to do with something called granular separation, commonly referred to as the “Brazil Nut Effect” or the “Brazil Nut Problem.” It’s the name for the process in which particles of differing densities and sizes separate in a mixture: in general, larger and denser particles rise while smaller, lighter particles descend. It’s the reason big Brazil nuts end up on top in a can of mixed nuts (hence the common name of the effect).

While percolation (the process of smaller particles settling between larger particles) may seem to explain this process, it is only one of multiple forces at work. The physics behind granular separation are actually so complex that scientists cannot fully explain it. Other factors affecting granular separation include buoyancy (material densities), air pressure, temperature, inertia, friction, particle shape, the shape of the container, and convection (granular materials act as a liquid when together in motion; e.g. one “pours” sand out of a bucket).

To ensure materials arrive properly mixed at the processing step in which the ratio and uniformity of the mixture is crucial, clients have two options, both of which involve storing materials separately. The first option is twofold: blend the materials in a mixer and reduce processing steps thereafter (thus reducing opportunities for the mix to separate). Some processes require an evenly distributed and well-mixed conglomerate. In such cases, a mixer is necessary.

For processes that require a controlled amount of each material in the mix but that do not need the materials mixed, it may be easier to blend the materials upon discharge from storage into a single conveyor. Within any given section of the conveyor, the material will segregate vertically and horizontally but will by and large retain the ratio it had upon input.

(For a detailed explanation of granular separation and suggestions for minimizing or eliminating the effect in powders, read this article in Powder & Bulk Solids.)

A Solution for Bulk Material Mixing

Horizontal silos stacked in modular configuration for automated material handling.

While we do not sell mixers, our SMART Containers allow customers to accomplish the second option. We can arrange these horizontal silos side by side so they discharge simultaneously into a single conveyor (read about our SMART Conveyors™ here). Due to their robust nature, we can stack them up to three units high to increase capacity without taking up more ground space.

SMART Containers are constructed of intermodal shipping containers, which we fit with a push-pull, wedge floor (our SMART Floor system). We prefer this design over slat-based moving-floor systems like Keith’s Walking Floor® or Hallco™ Industry’s Live Floor® because the push-pull system sheers material off the bottom of the material pile for true first in-first out recovery. The design also allows us to meter the output and avoid surges, such as occur with a Keith’s or a Hallco’s™ floor system. Our SMART Containers thus more accurately feed processes. (Centering feeder bins are available that even more accurately meter material from our SMART Containers.)

While granular segregation disallows manufacturers to mix material prior to storage, it is possible to integrate materials without a separate mixing process by mixing upon discharge from the silos. Our containers are a smart solution to accomplish this. Contact us today for more information!