The stirring of powders of different flow behavior, namely one free-flowing (semolina) and one cohesive (lactose) is investigated in a four-blade horizontal convective mixer. The kinetics of homogenization are investigated for each powder separately, thanks to tracer experiments performed with colored semolina and talc, respectively tracers of the free flowing and of the cohesive powder. Different stirring times and rotational speeds of the impeller are studied. Three flow mechanisms are identified: convection, avalanching and diffusion. Convection conveys important amounts of powder near the blades, while the avalanching mechanism takes place on the powder bed surface. Diffusion, which is the slowest mechanism, allows powder transport to the zones of the blender in which blades do not pass through. The final tracer dispersion in the blender's volume is better for the free-flowing powder than for the cohesive powder, and is enhanced by an increase of the rotational speed. A simple Markov chain model based on the identified mechanisms is developed as a basis for discussion of the results. (C) 2017 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.