Turbulent Particle Transport and its Effect on Flotation

Abstract

This thesis studies the hydrodynamics of froth flotation, a process that is used in industry as a means of removing small inclusions from the treated liquid by gas bubble injection. This work is particularly motivated by the removal of aluminium and silicon oxides from liquid steel. Hydrodynamic aspects have a strong influence on the efficiency of froth flotation. The flow around a rising bubble, especially the turbulent flow in the near-wake, is of particular importance as it may cause preferential concentration of particles as well as significantly change the collision and attachment rates of particles with the surface of the bubble. Both effects are studied in this work. Preferential concentration of particles in the near-wake is a result of the time-averaged balance of inertial and pressure forces on a dispersed particle, resulting in a drift towards the wake. It can therefore be observed by an effective time-averaged particle slip velocity over the wake boundary. Likewise, a direct confirmation of preferential concentration is possible by measurements of the average concentration of particles in the wake. These measurements were done in the wake of a solid mock-up of a spherical-cap bubble in the VerMeer vertical water tunnel. The particle slip velocity was obtained from simultaneous two-phase Particle Image Velocimetry measurements and numerically integrated in post-processing. The particle concentration was obtained directly from the average scattered light intensity. Both experimental results are compared to the predictions of a model for the preferential concentration in the wake, derived from a local balance of inertial, gravitational and diffusive fluxes. The influence of turbulence on particle attachment rates was measured in the newly constructed DABuT (Dynamic Air Bubble Trap) facility, using bubbles of different shapes and volumes. The research was again focused on the semi-spherical bubbles, these were also found to have the highest attachment rates. The results of the measurements are compared to a model based on a turbulent attachment flux and the effective shielding of particle attachments by a mono layer of particles building up and finally covering the rear side of the bubble in proximity to the wake. In conclusion, both effects contribute to the overall efficiency of flotation. the results of this work suggest that spherical-cap bubbles — although potentially difficult to generate — have the highest potential for the optimisation of flotation processes in industry.