The discrete element method (DEM) offers the possibility of understanding mixing processes at a microscopic, particle level. In connection with previous studies, DEM is used in this work to investigate the mixing of monosized spherical particles with blade speeds in the range 2-100 rpm. Recirculating flows are observed in both horizontal and cylindrical sections of the bed, providing a mixing mechanism. The recirculating flows formed in front of the blade in cylindrical sections seem to disappear, bringing a reduction of mixing rate at high blade speeds. Viewed at the particle scale, a zone of large interparticle forces is present in front of the blade, and this zone moves toward the vessel walls at high speeds. These are the regions where particle breakage or fracture can happen. Interparticle forces in the bed as a whole increase with the blade speed. Moreover, particle mixing has been studied at both macroscopic and particle scales using conventional mixing index and coordination or contact numbers. One conventional mixing index shows a slight reduction in mixing rate in the transition mixing region up to a certain blade speed and a slight improvement thereafter. This improvement is probably caused by the toroidal motion of particles induced at high blade speeds. Coordination number is successful in capturing the mixing process, and indicates a new way of studying particle-scale mixing.