Slurries are known to exhibit stiff non-Newtonian properties, in particular yield stress and, from an engineering standpoint, their rheology can be well described by the Bingham model. In industry, the design of the slurry make-down process is a difficult and costly task. The flow of such materials in a twin-blade vertical planetary mixer is studied using 3D computer simulation. The flow structures and the power consumption are investigated for various values of the Bingham number. It is found that this type of kneader can induce adequate radial and axial pumping, making it quite efficient for the preparation of viscoplastic materials. Dimensional analysis is carried out and used to derive power correlations with respect to the Bingham number. Finally, a value of the Metzner-Otto constant K-s is obtained and leads to a generalized power curve.