This work addresses the steady fluidization of small particles (diamet
ers of 0.15, 0.2, 0.3 and 0.4 mm) within a randomly arranged fixed bed
of bigger particles (diameters of 2, 3 and 4 mm). The research is rel
evant in the field of comminution, where crushing processes and materi
al transport phenomena are integrated features. Here measurements are
made of fine particle bed height, solid volume fraction of both specie
s, and liquid flow rate. The results show that existing correlations f
or mono-disperse fluidization in vertical pipes (Richardson and Zaki [
1]) provide a qualitative description of fluid velocity in this new co
nfiguration, when using the effective diameter of the coarse pore spac
e. The effective diameter is defined as the ratio of pore volume to we
tted surface. The present results are correlated within a new proposed
equation. The correlation maintains the form of the Richardson and Za
ki correlation, but one of the empirical coefficients is modified to o
btain a better fit. A simple model is proposed to explain the underlyi
ng physical principles. In this model the hindering effects of the coa
rse bed on the fluidization of the fines are explained in terms of a s
ize exclusion mechanism.