Theory of solids mixing
It is not possible to achieve a completely uniform mixture of dry powders
or particulate solids. The degree of mixing achieved depends on:
- the relative particle size, shape, and density
- the efficiency of the particular mixer for the components being mixed
- the tendency of the materials to aggregate
- the moisture content, surface characteristics, and flow characteristics
of each component
Generally, materials similar in size, shape, and density are able to form
the most uniform mixtures. Differences in these properties can also cause unmixing
or segregation during mixing or mechanical jiggling of the mixture.
The Experimental Nonlinear Physics Group at the Univ. of Toronto describes experiments
of unmixing sand and provides illustrations at
http://mobydick.physics.utoronto.ca/sand.html.
Julio Ottino (Dept. of Chem. Engg., Northwestern Univ.) provides links to many
of his papers on granular mixing. See
http://pg.chem-eng.nwu.edu/mixing/.
Experience shows that materials with a size greater than 75
m will segregate readily during mechanical jiggling of the mixture,
but those below 10 m will not segregate appreciably.
Means of overcoming segregation and poor mixing include:
-- comminution to smaller sizes
-- use of powders with a narrow size distribution
-- use of the same volume-average diameter for all components
-- granulation
-- coating processes
-- controlled continuous mixing