Effect of microturbulence on bubble-particle collision during the bubble rise in a flotation cell

Abstract

Holistically, air bubbles in flotation must rise to the top surface while free solid particles must settle to the cell bottom. Inside the stator-rotor space of the mechanical flotation cells, the turbulence of mixing can surpass the gravity-driven motion of the bubble-particle suspension. Outside the space, the bubbles rise to the surface, but it is not known how the turbulence affects the bubble-particle interaction during the bubble rise. Here we numerically investigate the effect of microturbulence on the bubble-particle collision during the bubble rise. The focus is on the interaction asymmetry caused by the inertial forces and bubbles with fully mobile (MBS) or immobile (IMBS) surfaces at intermediate bubble Reynolds number (Re). The Reynolds-Averaged Navier-Stokes equations with the RNG k-ɛ turbulence model are solved for the mean water flow. Its fluctuating components are predicted using the stochastic formulation that is adopted with the discrete phase formulation for determining the particle grazing trajectories. The results show microturbulence influences the bubble-particle collision interaction. The asymmetry of the collision interaction is also affected by the microturbulence intensity. Its negative effects are found for the CCA (the critical collision angle, which also affects the bubble-particle attachment) and the overall collision efficiency. These negative effects are unexpected because turbulence has been postulated to enhance the collision interaction. Numerical results agree with the MBS model for overall collision efficiency and show some significant deviations for the IMBS model. This paper provides a potential technique to examine the microturbulence effect on attachment efficiency via the CCA.