The transport of gangue through flotation froth has been described by solving the convection-diffusion equation. Gangue recovery is predicted to be proportional to liquid recovery, which is consistent with experimental observation. In addition, it is seen that the dependency of gangue recovery upon particle size is due to processes within the pulp phase rather than the froth, insofar as the transport of particles in a given froth is approximately independent of size. The importance of maintenance of positive bias in column flotation, previously stressed by other workers, is reinforced. This model utilises a simplified representation of the froth and, as a consequence, it does not necessarily give accurate gangue recovery estimates for practical flotation processes. However, the convective-diffusive model does illuminate the physical processes behind gangue recovery in the concentrate which will aid the development of automatic control strategies. (C) 2007 Elsevier Ltd. All rights reserved.