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

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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 are wood chips or grains, don’t 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 why 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 is 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 when the ratio and uniformity of the mixture are 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 retain the ratio it had upon input.

A Solution for Bulk Material Mixing

 

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 our SMART Floor system, a push-pull wedge floor. We prefer this design over slat-based moving-floor systems 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 the surges found with other systems. Our SMART Containers more accurately feed processes. (Centering feeder bins are available that even more accurately meter material from our SMART Containers.)

While granular segregation makes it impossible for manufacturers to mix material before 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!

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