Particle segregation
In particle segregation, particulate solids and also quasi solids, such as foams tend to segregate by virtue of differences in the size,and also physical properties such as volume, density, shape and other properties of particles of which they are composed. The process of segregation occurs during as well as during subsequent handling of completed mix and it is pronounced with free-flowing powders. Powders that are not free flowing or that exhibit high forces of cohesion or adhesion between particles of similar or dissimilar composition are often difficult to mix owing to agglomeration. The clumps of particles can be broken down in such cases by the use of mixtures that generate high shear forces or that subject the powder to impact. When these powders have been mixed, however, they are less susceptible to segregation because of the relatively high interparticulates forces that resist interparticulate motion, leading to unmixing.
Segregation mechanisms
The five major segregation mechanisms[1] are
- Sifting
- Angle of repose
- Fines fluidization,
- Air entrainment
- Chute trajectory
Sifting
Sifting occurs when there is a significant variation of particle diameter in a mix. Interparticle motion causes the finer particles to sift through the coarser ones.
Angle of repose
Disparity of the angles of repose among different particles causes segregation. Particles deposited with greater angles of repose form a steep pile under the deposition point while the ones with lower angle of repose roll away from that point.
Fines fluidization
In this mechanism, the lighter or fluffier particles form a 'fluidized' layer. Only coarser particles can penetrate the fluidized fines and the finer particles remain in the top layer.
Air currents
The finer particles in a mix are susceptible to be airborne in the presence of airflow. They move away from the deposition point whereas the coarser particles tend to remain close to the deposition point.
Chute trajectory
The difference of friction coefficients of different particles in a mix results in different discharge trajectories as the mix slides down a chute. Particles with high friction coefficients show lower discharge angles the end close to the chute, while the trajectory of particles with low friction coefficients deviate further away from the chute.