Solids suspension in a pilot-scale mechanical flotation cell: A critical impeller speed correlation

This paper develops a correlation for predicting the critical impeller speed, Njs as a function of operating conditions in a mechanical flotation cell. Understanding solids suspension has become increasingly important in recent years due to dramatic increases in flotation cell sizes. The critical impeller speed is commonly used to indicate the effectiveness of solids suspension in stirred tanks. In previous publications, the authors have found the critical impeller speed to be a very useful benchmark for evaluating the effectiveness of solids suspension in a mechanical flotation cell. This study develops a ‘Zwietering-type’ correlation for the critical impeller speed, Njs in terms of particle size, solids density, solids concentration, liquid viscosity and air flow rate. The study was conducted in a 125 ℓ Batequip flotation cell using sized fractions of silica, rutile and zircon at various solids concentrations, liquid viscosities and air flow rates. The critical impeller speed was found to be proportional to the particle size, solids density and solids mass concentration to the exponents of 0.33, 0.70 and 0.17 and to increase linearly with air flow rate. Liquid viscosity was found to have a negligible effect. These findings are similar to those observed in stirred tanks, but indicate that particle size and solids density has more significant influences on the critical impeller speed in mechanical flotation cells. To the authors’ knowledge, this is the first time a ‘Zwietering-type’ correlation, originally developed for stirred tanks, has been presented for solids suspension in a mechanical flotation cell. This correlation can be used to predict how the impeller speed should be adjusted in order to maintain the same level of solids suspension (percentage of critical impeller speed) with changes in variables such as particle size, solids density, solids concentration and air flow rate in mechanical flotation cells. In addition, the power draw at the critical impeller speed, P/Vjs, was found to be proportional to the particle size, solids density and solids concentration to the exponents of 0.88, 1.9 and 0.52 and to increase slightly with air flow rate. The proportionality exponents for power are approximately three times larger than those for impeller speed, as would be predicted by the power number relation. However, the power predictions are not as accurate as those for the critical impeller speed.