Flowability of Bulk Solids

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

Good flowability

  • 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.

Poor flowability

  • 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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