Sand Handling & Segregation

All the care taken in producing carefully controlled grades can be destroyed by handling systems that cause segregation. Almost any re-handling can cause segregation but perhaps those most troublesome result from poor flow patterns through storage silos. Unless the silo is fitted with an anti-segregation device continual refilling of partially emptied silos will inevitably show segregation. A brief examination of what happens to silo contents will illustrate the point.

During silo filling the contents generally form some sort of cone or peak directly under the filling point. A closer examination will show that some segregation takes place during the filling, more of the larger grains of material rolling further down the slope of the cone toward the walls of the silo. When emptying through a single central outlet, the cone inverts and a hole develops down the center above the outlet. The initial flow of material will show a shift toward fine and when the hole reaches the diameter of the silo it will shift back, becoming coarser. When further material is put into the partially emptied silo it will segregate to form a coarse layer (immediately above the level of the partial load) as the cone builds up to the full diameter of the silo. This will show up as a coarse 'slug' next time material is taken from this level. Repeated cycling can build up layers of segregated material.

Segregation can be reduced by obtaining mass flow within the silo, basically the old 'first in first out' rule. With mass flow the shape of the cone under the filling point should not change very much. A peak under the filling point should be visible whether the silo is being filled or emptied. There are many ideas and theories on how segregation is prevented or mass flow achieved. In our experience this pattern of flow can be promoted, reasonably economically, by installing a horizontal plate near the bottom of the silo, blocking off flow except for a narrow area (6 to 12 inches) around its edge against the walls. Many silos have coned bottoms and the plate should be located where the cone joins the silo. A good rough guide for the plate height in a flat bottomed silo is where a line drawn from the outlet at 45 degrees to the bottom meets the wall. The crucial point is worth repeating, the space for the material to flow through should be around the outer edge of the plate with absolutely no holes anywhere else in the plate. If you have a sand segregation problem and would like more detailed advice on modification of your storage silo, please contact us today.