The aggregation and flotation behavior of hematite and wolframite have been investigated through particle size analysis, flotation tests and measurement of energy input based on hydrodynamic interactions. In has been shown that when fine and coarse particles are mixed and conditioned in surfactant solution, the floes with particle size greater than that of the coarse particle size and the floes with particles size between that of the fine and coarse particle sizes are formed. The bigger size floc is attributed to the adhesion of fine particles onto coarse particles. The medium size floc may be the aggregation among fine particles themselves and detached floes from the coarse substrate due to the shearing forces. Both cases depend on the state of turbulent flow of pulp and hydrodynamic conditions. The studies on the effect of particle size, agitating speed and time, and the structure of agitation tank on aggregation and flotation show the importance of hydrodynamic interactions in aggregation and flotation. (C) 2003 Elsevier Science B.V. All rights reserved.