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June 2020 / 15 AGGLOMERATION ADVISOR Greg Mehos, president Greg Mehos & Associates Mitigating agglomerate segregation A gglomeration processes often yield a material with an array of particle sizes. When handled, agglomerated materials, such as cat litter and fertilizer, can separate by particle size. An example of this is when a bin is filled from a single inlet. Once a pile is formed in the bin, the larger particles, which are rela- tively free-flowing, will roll down the pile's surface toward the bin's periphery while smaller particles percolate through the material bed and concentrate in the pile's center. The pile will inevitably avalanche, and when it does, the larger parti- cles' momentum will cause them to travel farther than the finer parti- cles. This phenomenon is referred to as sifting segregation or Christ- mas tree segregation 1 , as shown in Figure 1. In the pile shown, the lighter-colored particles are smaller and the darker-colored particles are larger. The particles' tendency to travel as they do is called Christ- mas tree segregation because the resulting pile — when bisected down the middle — looks like a Christmas tree. If the material is handled in a funnel-flow bin, its average particle size may vary during discharge. In funnel flow, material will flow in a flow channel above the outlet. As the flow channel empties, material along the walls, which will have a larger average-particle size, will fall into the channel. As a result, the material's average particle size upon leaving a funnel-flow bin will PBE FIGURE 1 Christmas tree or sifting segregation Fine particles concentrated in the center Large particles concentrated at the edges cycle from small to large and back to small again. To mitigate sifting segregation, agglomerates with a wide particle size distribution should be han- dled in a mass-flow bin in which the hopper walls are steep enough and fabricated from materials low enough in friction to allow the material to flow along the walls. For agglomerates that have very high wall friction, mass flow can be nearly impossible to achieve in a conical hopper and would result in funnel flow. Mass flow can usually be achieved in a wedge-shaped hopper with flat walls, but a spe- cialized feeder is required. A dispersion cone is an alterna- tive device for mitigating sifting segregation. The device is com- prised of an inverted cone above a ring that has equally spaced extensions or "teeth" that redirect approximately half of the particles when they travel past the cone, as shown in Figure 2. The particle redirection results in the particles mixing as they fall inside the bin, forming multiple piles of mixed particle sizes instead of a single pile of particles in which fines have accumulated in the center. You can imagine the particles hitting the cone's tip and surrounding area in looking at the dispersion cone's aerial view in Figure 2. Notice how the ring's diameter is larger than