The performance of bioleaching stirred tank reactors (STR) is related to the homogeneity of biomass, substrates and dissolved gases. This work was focused on the characterization of the impeller design on bioreactor hydrodynamics and, more specifically on power, mixing efficiency and particle stress. Few studies addressed the issue of the impact of the impeller design on these, especially for multi-stage bioreactors which are the most commonly used at the industrial scale. To fill this lack, a two-stage solid-liquid computational fluid dynamics (CFD) model was simulated on more than 50 conditions to assess power consumption, dissipated power, suspension quality and particle stress. A dual impeller configuration was chosen using Rushton turbines, R600, Hydrofoil, Elephant Ear and HTPG impellers. Grinded pyrite-rich materials (average particles size: 80 μm) were considered as the solid phase at 3 different solid concentrations (10, 18 and 26% w/w). Considering the impeller power number (Np), the configuration with an axial impeller consumed less energy than a radial impeller in concordance with literature data. The results show that the impeller design had few to no effect on mixing efficiency considering a given power dissipation per unit volume. Independently on the impeller used, unique relationships were found between particle stress and mixing efficiency. This study gives new insights for reactor design and scaling of bioleaching stirred tank reactor and more specifically on the reduction of shear stress for the attached bacterial communities