Granular flow and mixing efficiency in powder mixers are important issues for many industrial processes, yet are often not well understood. Here experiments on agitation of free flowing powder were performed in a 650 mm long horizontal mixer of internal diameter 270 mm with six long flat blades located on radial arms fixed to a rotor shaft. These were related to work on a single-bladed agitator. Many features of the two designs had qualitative similarities but for levels of fill exceeding 60% the material was stirred essentially as a solid body by the six-bladed agitator. Velocity fields and axial dispersion coefficients scaled with rotor speed but transaxial effects were more complex. The pulsing regime induced by the single-bladed agitator enhanced radial displacement whereas the continuous stirring of the bed by the six-blades showed an increase of the mean velocity of the particles and of the rate of axial dispersion. The next steps are to extend the range of designs evaluated and to understand the influence of vessel size. The studies provide a method for selecting a blade design for a powder processing system, an aspect which has been empirical to date.