6. Conclusion
Reductions of segregation rates by orders of magnitude by
tailoring the particle size distribution do not seem to have been
reported in the granular research literature. Yet, mixing of grains and
powders of different sizes and densities is ubiquitous in industrial
operations. In the pharmaceutical industry, small drug crystals are
often blended with larger excipient (
fi
ller) particles and the resulting
mixture is transported and then compacted into tablets. Poor mixing
or segregation can lead to out-of-spec tablets, with too little or too
much drug, resulting in whole batch failures. Our results suggest that
introducing additional intermediate particle sizes in between the
smallest and the largest in the original mixture may reduce the
tendency of the particulate mixture to segregate. There is evidence
that during
fl
uidized bed processing, which is known to lead to
separation of
fi
nes and coarse particles, powders with a broad,continuous particle size distribution do not segregate
[48]
. Our results
have focused on non-cohesive particle mixtures of equal density in
order to avoid complications associated with the Reverse Brazil-nut
effect
[7,21,23]
and agglomeration of particles. It remains to be seen if
these results can be applied to reduce segregation in granular and
powder systems of differing density and cohesion where a range of
other modes of segregation, beyond the Brazil-nut effect appear. Prior
research has shown that cohesion can assist or hinder mixing of
granular materials
[49]
, so further work is required to determine the
effect of cohesion on segregation in a vertically vibrated bed. In
addition, our results have focused on a vertically vibrated cylinder but
there are certainly other scenarios where one can separate particles
completely by size and obtain all the Brazil nuts (large particles) on
the top of a mixed nut (particle) system. With implications in
situations ranging from the production of cereal
[2]
to pharmaceutical
manufacture
[24]
, understanding and reducing granular segregation
has the potential to drastically reduce costs associated with low yield
and out-of-spec products.