New directions in flotation machine design

The theoretical background for flotation kinetics of ultrafine and coarse particles are explored. Recent advances in flotation technology for these difficult areas are reviewed, with a focus on improving the flotation rate of ultrafines, and extending the upper limit for coarse particle flotation.For ultra-fine particles, the theory suggests that the rate of flotation can be improved by increasing the rate of shear in the suspension of particles and bubbles. A new cell has been developed, the Concorde Cell, in which the pre-aerated feed is raised to supersonic velocities before passing into a high-shear zone in the flotation cell. The local dissipation rate is of the order of 100 kW/m3, one to two orders of magnitude higher than is available in conventional mechanical cells. The Concorde Cell has been trialed on a finely ground PGM feed in South Africa, with excellent results. By recycling the tailings, and using the mass pull or solids recovery as the control variable, the Cell is capable of producing a high-grade concentrate at high recoveries, over a wide range of particle sizes.Theory for the upper limit of coarse particle flotation suggests that a quiescent flow field is necessary to prevent the particles from becoming detached from the bubbles. A liquid-fluidized bed provides a suitable environment. The flotation feed is introduced into the fluidized bed, and air bubbles are dispersed in the fluidizing water. Coarse particles attach to the bubbles rising through the bed and are lifted into the froth layer that is maintained on top of the cell in the usual way. Particles of galena up to 1 mm in diameter have been recovered in such a bed, while for particles of lower density such as quartz and coal, the upper limit for flotation has been extended to at least 2 mm and 5.6 mm respectively. The fluidized bed technology provides major advantages beyond the ability to recover coarse particles currently lost in existing operations. Thus, if the upper flotation limit can be extended, the top size for grinding can be raised, with significant reductions in energy costs. Liberation of the values is the key limitation. Also, a fluidized bed flotation cell can accept a feed with much higher percent solids, leading to significant reductions in water requirements.