The mixing process of powders in an agitation type of solid mixer was described based on a modified comminution model. The mixing procedure for the fine and adhesive powders was expressed by means of the disintegration process of the tracer agglomerates (iron-oxide) in the bulk powder bed (calcium carbonate) in this model. The hypothetical breakage region (maximum stress solid volume) formed around the impeller passing through the powder bed was defined as functions of the scale of the mixer vessel, the number of impeller blades and the rotary speed. Model parameters of the selection function and the breakage distribution function were determined by measuring the changing of size and distribution with time for the agglomerates of the tracer. The effects of the scale of the mixer and several operating conditions on the mixing processes were discussed using the presented model and experimental results.