This work addresses the steady fluidization of small particles (diameters 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 relevant in the field of comminution, where crushing processes and material transport phenomena are integrated features. Here measurements are made of fine particle bed height, solid volume fraction of both species, and liquid flow rate. The results show that existing correlations for mono-disperse fluidization in vertical pipes (Richardson and Zaki [1]) provide a qualitative description of fluid velocity in this new configuration, when using the effective diameter of the coarse pore space. The effective diameter is defined as the ratio of pore volume to wetted surface. The present results are correlated within a new proposed equation. The correlation maintains the form of the Richardson and Zaki correlation, but one of the empirical coefficients is modified to obtain a better fit. A simple model is proposed to explain the underlying physical principles. In this model the hindering effects of the coarse bed on the fluidization of the fines are explained in terms of a size exclusion mechanism.