Mixing and gas dispersion in mineral flotation cells

Mineral flotation in mechanically agitated vessels (cells) involves complex interaction between bubbles, particles and the liquid phase. Ideally, just enough power input from the impeller is needed to so that the frequency of particle–bubble collision and attachment is maximised, while at the same time detachment events are minimised. This paper firstly investigated how the slip velocity of 2–10 mm diameter bubbles, a size commonly encountered in flotation devices, was influenced by turbulence intensity. The measurements confirmed the earlier correlation by [Lane, G.L., 2005, Numerical modelling of gas–liquid flow in stirred tanks, Ph.D. Thesis, University of Newcastle, Australia], which was then inputted into a computational fluid dynamic model to describe the gas dispersion in a mechanically agitated tank. The model provided turbulence intensity values that were then coupled with both slip velocity and critical Weber number models to generate both bubble size and gas holdup profiles for the entire vessel. Moreover, a simple equation was introduced to allow prediction of cavity formation behind the rotating impeller blades, which is a common occurrence in most flotation cells they normally operate at high gas loadings. This inclusion allowed the model to predict power reduction resulting from the presence of the cavities. Finally, extension of the computational model to include flotation hydrodynamics, such as probabilities of collision, adhesion and stabilisation of the particles at the bubble surface, is also described. The model is able to compute net attachment rates, and hence the particle flux entering the froth recovery phase, as a function of bubble and particle diameter, gas flowrate and power input.