In conventional flotation machine, flotation performance is always high for intermediate particle sizes (80-100 microns) and decreases for finer and coarser particles. The low flotation recovery and rate constant of fine and coarse hydrophobic particles have been attributed to their low collision efficiency with bubbles and to their detachment from bubbles in the high turbulence regions of the flotation cell, respectively. We have investigated the effect of mineral density and turbulence on particle-bubble collision and detachment in the flotation of hydrophobic quartz, chalcopyrite and galena (10-500 microns) in a Rushton turbine cell as a function of agitation rate and particle size. For these hydrophobic particles, high maximum recoveries were obtained independently of particle size, mineral density and degree of agitation. However, their flotation rates were not constant and increased with particle size up to a maximum value before decreasing for larger particles. It was also found that the particle size for maximum flotation rate increased with decreasing mineral density. The flotation rates of coarse particles were more affected than fine particles by a change in agitation rate. In general, flotation rates increased with agitation rate before decreasing at the higher agitation rates. The Wark flotation model was used to explain these results.