The Effects of Powder Cohesion

A substantial problem that remains open is how to account for the effect of cohesion of powder flow and scale-up, in particular for mixing operations. The problem is extensive, and only a brief discussion is provided here. 

In simple terms, a cohesive powder can be defined as a material where the adhesive forces between particles exceed the particle weight by at least an order of magnitude. In such systems, particles no longer flow independently rather, they move in chunks whose characteristic size depends on the intensity of the cohesive stresses. 

The effective magnitude of cohesive effects depends primarily on two factors the intensity and nature of the cohesive forces (e.g., electrostatic, van der Waals, capillary) and the packing density of the material (which determines the number of interparticle contacts per unit area). This dependence on density is the source of great complexity cohesive materials often display highly variable densities that depend strongly on the immediate processing history of the material. 

In spite of this complexity, a few guidelines can be asserted within a fixed operational scale  

slightly cohesive powders mix faster than free flowing materials, 

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Extensions to Mass Discharge Relations Johanson (Tram. Soc. Min. Eng., March 1965) extended the Beverloo relations to include the effect of powder cohesion, with mass discharge rate given by... 

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