Fluid flow and particle collision intensity in a rotating-drum bioreactor are investigated by numerical simulation and a conventional stirred-tank bioreactor is selected for comparison. Fluid flow is simulated by the computational fluid dynamics (CFD) software package FLUENT (R) whereas particle collision intensity is approached numerically through a hard-sphere model. The dissipation rate of turbulent kinetic energy and the maximum particle collision intensity in the rotating-drum bioreactor are about one order of magnitude smaller than those in the stirred-tank bioreactor. The rotating-drum bioreactor is likely to have a less severe impact on bioleaching microorganisms, and thus is expected to have great potential in the field of bioleaching processes.