Segregation of particles can occur with many bulk solids as they are being handled and the results can be quite costly!  For example the pharmaceutical industry is susceptible to problems with particle segregation.  If fine and coarse particles segregate, the final tablet or capsule quality can be altered such that valuable drugs have to be discarded.

There are several mechanisms which cause segregation.   Among them are sifting, particles sliding and air entrainment.

Sifting

Sifting occurs when small particles trickle down through a body of larger particles.

This is the most common means for particles to separate.   In order for sifting to occur, the particles must be free flowing, different sizes, fairly large (+100 mesh) and have some means of inter-particle motion (such as forming a pile).  The particles segregate in a horizontal or "side-to-side" pattern.

Sliding Particles

Particles sliding on a surface can segregate because fine particles tend to be more frictional than coarse ones.

If a chute is used, the fine particles in motion settle to the bottom of the chute due to sifting.  The increased friction of the finer particles causes drag and velocity differences between particles as they are sliding on the chute surface.  When the particles discharge from the chute, the fine particles concentrate at the end, while the coarse ones have a trajectory which carries them further away.

Air Entrainment

Air entrainment. affects fine particles as they tend to remain airborne longer than coarse or heavier particles.

As a bin is filled, the fine, light particles tend to settle on top, while the coarse particles fall rapidly, creating a vertical or "top-to-bottom" segregation pattern.

Solutions You Can Use

Here are some ways to correct these problems above,

• Minimize sifting by ensuring a mass flow pattern (first-in-first-out).  Even though the material can segregate side-to-side, the coarse and fine particles will be reunited at the outlet because of mass flow.
• Keep a minimum head of material above the hopper section in a mass flow bin to reduce the velocity gradient that occurs as the material reaches the hopper section in a mass-flow bin.
• Make the material more cohesive by adding water or oil.  One of the prerequisites for segregation is inter-particle motion, increasing cohesiveness causes the particles to stick together and reduces this motion.   Warning:  do not make the material too cohesive or it will not flow.
• Use a tangential entry into bins when handling fine solids that segregate by air entrainment.  Instead of filling the bin from the center of the top of the bin, introduce the material in at the top of the but but on the side, tangential to the circumference of the bin.  This will actually minimize a vertically segregating material by causing side-to-side segregation, the particles will be reunited when they are discharged at the outlet of your mass-flow bin.

Help others by posting your comments, suggestions and experiences with bulk solids feeding or any other materials handling concerns you may have on our On-Line Help Forum.  For past Ask Joe ! Articles, visit the Ask Joe! Archived Articles.

Guest articles for the Ask Joe! Column are always welcome, for more information please contact Joe Marinelli directly at his email address:  joe@solidshandlingtech.com.