Segregation
In the context of bulk solids, segregation means separation by gravity.
Example: a homogeneously mixed bulk solid with particles of different densities is in a glass container. The filling level is, for example, 90 %. The glass container is now transported by road over several kilometres. Depending on the condition of the road, the bulk solid experiences different vibrational excitations. This causes relative movements between the particles. The internal friction of the powder is dissipated by the relative movements of the particles. This can be seen in the glass container: heavy particles sink, while light particles ‘float’ upwards. Gravity causes segregation.
The causes of segregation in bulk solids are
- different particle sizes
- different bulk densities
- different particle shapes
- different friction coefficients (presence of liquids)
- type of powder movement (flow rate)
- type of powder storage/packaging
- ....
Avoiding segregation:
If a homogenous bulk solid mixture is to be protected from segregation, the powder can be agglomerated, for example. Agglomeration is described elsewhere in the glossary.
Another possibility is to compact the powder so that the particles are fixed in their position. In a vacuum package, the particles can no longer move against each other.
The homogeneously mixed state of a powder can also be fixed by mixing a suitable liquid into the powder. The liquid increases the internal friction of the bulk material. This reduces flowability.
Segregation also has useful aspects, for example, when a foreign substance is to be removed from a bulk material. Segregation due to different particle densities occurs, for example, when a bulk material is allowed to trickle down evenly, comparable to a waterfall. If the trickling bulk material is blown from the side with air, the lighter the particles, the greater the deflection of the flow. In this way, separation takes place based on differences in density. This separation process is in contrast to sieving, in which separation takes place based on grain size.
Powder mixer
After a mixing process has been completed, there is usually a high degree of mixing quality in the mixing chamber. Nevertheless, segregation can occur. This is when the mixing goods flow many metres down into a silo in free fall. A material cone then forms in the silo. The coarse particles roll outwards and the small particles collect inside. The drop height under the mixer should therefore be low.
Another measure against segregation:
If mixing goods tend to segregate, the mixer should discharge the mixed batches very quickly. This prevents the formation of an angle of repose. This is the case, for example, when a KoneSlid® mixer is used.