The detachment of particles from bubbles at various locations in a turbulent flotation cell

Abstract

In a mechanical flotation cell, turbulence is required to suspend particles, to disperse the air flow into small bubbles, and enhance the attachment of bubbles and particles. However, turbulent eddies may also cause particles to detach from bubbles. This paper describes experiments in which particle-laden bubbles were observed at various locations in a turbulent flotation cell. Two different types of particles were used for the flotation experiments: irregular silica particles, and smoothly spherical glass particles, both of Sauter mean diameter 180 µm. A specially designed flotation cell was constructed in which particles were attached to bubbles in an external fluidized bed. Bubble-particle aggregates were carried into the vessel by a stream of liquid rising through its base. Agitation was provided by a Rushton turbine impeller of standard design. The cell was configured so that bubble-particle aggregates could be introduced at a range of radial distances from the axis of the impeller shaft. The data were analysed with reference to the theory of Schulze (1982) for detachment of particles in a turbulent field, using the data of Lee and Yianneskis (1998) for the local energy dissipation rate. The detachment of particles at different locations was observed experimentally and compared with the Bond number calculated using local energy dissipation rates. No single critical Bond number was observed, the fractional detachment increasing gradually with increasing Bond number. Detachment was highest nearest the impeller as expected. The results generally followed the theory over a fourfold reduction of the dissipation rate. As the bubble size increased, particles were observed to detach more easily. For a given Bond number, the fractional detachment of particles was higher for the irregular silica particles, relative to that of the spherical glass particles under the same conditions. The relevance of existing detachment models in expressions for flotation kinetics is questioned.