Flowability of Bulk Solids
Definition – Flowability of bulk solids is defined as the ease with which a bulk material flows by the influence of gravity.
- Solid blending mechanisms are affected by the properties that affect flow of materials.
- Properties that affect flow of material include
- Bulk density
- Cohesive strength
- Electrostatic attraction
- Mechanical interlocking
- Moisture content
- Plastic welding
- Particle shape
- Particle size and distribution
- Surface attraction
- Temperature conditions
- Wall friction
Quantifying Flowability – The coefficient of friction of a powder is the tangent of the angle of repose and is the measure for its flowability.
Mixing Expert Tips on Determining Flowability of Powders
- Lower angle of repose.
- Angle of repose between 25 degrees and 35 degrees.
- Particle size greater than 250 microns.
- Loosely packed powder (low bulk density).
- Non-cohesive particles.
- Higher angle of repose.
- Angle of repose of greater than 60 degrees
- Particle size less than 75 microns.
- Densely packed powder (high bulk density).
- Cohesive powders.
- Agglomeration of particles due to “van der Walls” forces and electrostatic forces.
Mixing Expert Tips on Flowability and Blender Selection
- Knowledge of flowability of bulk solids is essential for blender selection.
- The primary mechanisms of blending of solids are shear, convection and diffusion. The flow properties of the powder and the choice of blender determine the dominant blending mechanism.
- Shorter blend times are achieved when the major component of the blend is relatively free flowing.
Related Terms – Angle of repose, Coefficient of friction
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