Residence time distributions and mass transport in the froth phase of the flotation process

Abstract

A comprehensive description of some aspects of the physical behaviour of the froth phase of the flotation process has been provided by two mathematical models. Both are formulated in terms of familiar design and control variables (e.g. cell dimensions, froth depth, gas rate) and the froth stability, α, defined as the ratio between the volume flowrate of air in the concentrate stream and the volume flowrate of air in bubbles crossing the froth/slurry interface which have a finite probability of entering the concentrate stream. The first model provides a description of the two-dimensional streamline behaviour of the froth as it moves towards the concentrate weir. The second model is a two-stage approximation of this behaviour and provides a simple and tractable model of froth behaviour which can easily be incorporated in existing models of the flotation process. This model involves two more parameters: the residence time ratio δ, which provides a means for estimating the minimum froth phase residence time (it can be given the value 0.5 unless it is possible and practical to measure it more accurately); and the froth removal efficiency, ε, which is a measure of the efficiency with which the available froth chamber volume is used. RTD measurements using two-phase froths have shown that these models constitute a good description of physical reality, and that ε and α can be obtained as functions of control variables such as gas rate, froth height and frother concentration. Insights obtained from this work have led to the development of a method for froth removal which produces an unambiguous improvement in the performance of the flotation cell. The second model can be used to investigate the effect of control actions and scale-up on cell performance.