Filling a silo is a common situation where segregation takes place, because of a number of segregation mechanisms, see Figure 1. In the heap, fine, needle like or flaky particles with low mobility will be collected near the top of the heap, whereas large, spherical and light particles with high mobility will move all the way to the edge of the heap. Dispersed in air, fine particles are easily carried by air currents to the walls, whereas coarse ones drop out directly. Aerated by the free fall, the fine particles will flow like a liquid on top of the material accumulating in the silo, ending up at the walls and at the top of the material in the silo.

Fig. 1 Segregation during filling of a silo

In fact, segregation could occur in all stages along the unit operations of solids handling (see Figure 2). Active mechanisms are e.g.: trajectory segregation, percolation of fine particles; rise of coarse particles on vibration and elutriation segregation.

Fig. 2 Mechanisms of segregation

Segregation exerts severe consequences on performance and efficiency of units, for example, even if satisfactory mixing of constituents is achieved in a powder mixing device, unless great care is taken, subsequent processing and handling of the mixture will result in demixing or segregation. (See Figure 3) This can give rise to variations in quality of products, and more seriously, the chemical composition of the product may be off specification (e.g. in blending of constituents for detergents or drugs). It is crucially important to avoid or reduce segregation in most circumstances.

Fig 3. Satisfactory mix and further segregation after mixing for longer than optimal time