Mechanisms of Solid Blending: Diffusion, Convection, and Shear
Diffusion blending is characterized by the small scale random motion of solid particles. Blender movements increase the mobility of the individual particles and thus promote diffusive blending. Diffusion blending occurs where the particles are distributed over a freshly developed interface. In the absence of segregating effects, the diffusive blending will in time lead to a high degree of homogeneity.
Tumbler blenders like the double cone blenders and v-blenders function by diffusion mixing.
For rapid blending, in addition to the fine-scale diffusion blending there should be a means by which large quantities of particles can be intermixed. This is accomplished by either convection or shear mechanisms.
Convection blending is characterized by the large scale random motion of solid particles. In convection blending groups of particles are rapidly moved from one position to another due to the action of a rotating agitator or to cascading of material within a tumbler blender.
The blending of solids in ribbon blenders, paddle blenders, and plow mixers is mainly a result of convection mixing.
Some texts define shear blending as the development of slip planes or shearing strains within a bed of material. Others define the blending mechanism of shear as high intensity impact or splitting of the bed of material to disintegrate agglomerates or overcome cohesion. For the purpose of this discussion we shall use the latter definition. Shear blending is very effective at producing small-scale uniformity generally on a localized basis.
Blenders with high speed chopper blades and intensifiers are an examples of shear blending.