Effect of impeller speed on properties of quiescent zone and entrainment in mechanical flotation cells

Abstract

Flotation process in mechanical cells is carried out in highly turbulent conditions. In this work, the impact of impeller speed on four characteristics of the quiescent zone, i.e. zone height, turbulence, solid percentage, and gas holdup, and their relationship with the entrainment is investigated, and it is shown why at a higher impeller speed, entrainment is not significant. The height of the quiescent zone and its turbulence are measured using a piezoelectric sensor, while an electrical conductivity sensor measures the gas hold-up. A peristaltic pump is applied to take samples from the pulp to measure the solid percentage. The results obtained showed that with increase in the impeller speed from 750 to 1100 rpm, the entrainment value changed from 2.01% to 5.69%. However, the variations in entrainment were not significant at speeds higher than 1100 rpm. It was found that the height of the quiescent zone was independent from the impeller speed, while raising the impeller speed, as long as the solid percentage, turbulence, and gas hold-up are increased, caused a drastic increase in entrainment. Despite the increase in the solid percentage and turbulence, the gas hold-up decreased at impeller speeds higher than 1100 rpm due to the variation in the bubble distribution pattern, so the entrainment raised with a smaller slope. Finally, a model is presented for the entrainment as a function of the three correlated variables using the Ridge regression. The entrainment is then correlated to the impeller speed, explaining the contradictory results from the literature on the effect of impeller speed on the entrainment.