The effect of froth residence time on the kinetics of flotation

Previous studies by the authors have shown that there is a strong correlation between the overall flotation rate constant k and the bubble surface area flux, Sb, in a flotation cell, and that this relationship is independent of impeller type. More recently it was found that results obtained from a 250 litre cell and a 3 m3 cell treating the same feed could be represented by the same k-Sb relationship, as long as the ratio of froth height to superficial gas rate in the cells was the same. This ratio is termed the froth residence time, τfg. This paper examines the effect of τfg on the kinetics of flotation, by re-evaluating and re-interpreting these results. It is found that k decreases exponentially with τfg but the relationship is dependent on cell size and therefore not directly useful for scale-up. When τfg is divided by a typical cell dimension to take into account the effect of froth transportation distance in cells of different sizes, the relationship between k and the “specific froth residence time” fits the data better and and is found to be independent of cell size. Finally, when the results are evaluated in terms of τfg, the froth residence time calculated on the basis of the concentrate slurry flow rate, the fitting od the data is found to be very much better again. The implications of these relationships for flotation modelling and scale-up are discussed.