This paper presents a numerical study of the flow and segregation of particles over flat blades in a vertical cylindrical mixer by means of a modified discrete element method. Simulations are conducted using a binary mixture of spheres, where the blade's rotating speed, particle size, volume fraction and particle density are the variables considered. Similar flow features are found for all the cases considered: particles rise to form a heap in front of a blade and then either flow downward on the bed surface over the blade or to the base of the heap to rejoin the flow toward the blade, and there is a recirculating zone in front of the blade. However, different types of mixing behaviour can be observed. Particles with large size or light density are mainly in the top layer of the mixing bed, while small and heavy particles remain in the bottom of the mixing bed and in front of the blades. The segregation pattern resulting from size difference is not the same as that resulting from density difference. The force evolution in these two binary systems also differs, although at the initial stage large/light particles always receive a relatively larger vertical force than small/heavy particles, leading to segregation that can be maintained at the later stage. Decreasing the size or density difference promotes the mixing performance. Changing the blade speed affects the mixing kinetics but not the mixing quality. (C) 2003 Society of Chemical Industry.