A PhD studentship sponsored by BP Chemicals. The work investigated the effects of adding different fines cuts on the fluidization and the rheological behaviour of an alumina powder. The fluidization behaviour of the alumina was studied at process temperatures ranging from ambient to 400ºC and by adding to the material previously deprived of fines two fine sub-cuts of nominal size 0-25 and 26-45micron respectively. On the other hand, a new equipment (the msFBR) was designed, built and commissioned at ambient temperature to the determine the rheological behaviour of the powders containing different fines sub cuts. A model was developed, based on the failure properties of the materials, to determine the stress distribution corresponding to any given rate of aeration in the msFBR.