Bubble Detachment from a Steel Ball in Turbulent Field: Application to Mineral Flotation Systems

Abstract

Coarse particle (typically more than 100 micrometers in diameter) flotation is adversely influenced by liquid motion resulting from energy input associated with mixing of the gas and solid phases. In particular, the collected particles can become detached from the bubble as the particle-bubble aggregate passes through regions of different turbulent levels. The dynamics of particle-bubble-turbulence interaction is almost impossible to visualize within a real flotation environment as the phases are in constant motion which changes with time and position. To study the phenomenon of the particle bubble detachment process the problem was mimicked in such a way as to have a bubble detaching from a stationary 3mm diameter steel particle in the turbulent field. A bubble of known volume was firstly introduced onto the submerged particle surface via a syringe needle. Image analysis was used to determine the bubble-particle contact angle and radial position of the three phase contact line under quiescent conditions. An oscillating grid device was then used to generate turbulent liquid motion around the particle-bubble aggregate. Particle image velocimetry (PIV) was used to quantify the instantaneous velocity field around the disturbed bubble. Laser induced-fluorescence (LIF) was applied to filter out the (green) internally reflected light from the bubble so that only the (orange) light from the fluorescing seeding particles was collected. The PIV-LIF images were then analysed by firstly utilising a masking technique to eliminate spurious velocity vectors inside the bubble. The velocity data in an envelope surrounding the bubble was extracted to calculate local, instantaneous values of liquid velocity, turbulent kinetic energy and energy dissipation rate. It was found that the flow structures generated by the oscillating grids resulted in a lateral inclination of the gas-liquid interface at the three phase contact line. The subsequent change in the dynamic contact angle resulted in a reduction in the capillary (attachment) force, and at a high enough turbulence level it became less than the buoyancy (lift-off) force and detachment of the bubble from the particle surface took place. The detachment events observed in this study is analogous to what actually takes place in mineral flotation cells where the bubble-particle aggregate is in motion.