Effect of solid particles on gas holdup in flotation columns—II. Investigation of mechanisms of gas holdup reduction in presence of solids

Abstract

Part I showed that the presence of solids decreased gas holdup under conditions typical of flotation in columns. Four mechanisms which might explain this effect are evaluated here: bubble coalescence, slurry density and viscosity changes, changes in radial gas holdup and flow profiles, and bubble wake effects. The first two cannot explain the holdup decrease but the second two can. It was shown experimentally that solids do not cause bubbles to coalesce in flotation columns. Using the drift flux model, changes in density and viscosity of the slurry due to the presence of solids could not account for the observed reduction in gas holdup. A drift flux analysis for the gas-slurry system suggested both that the holdup and flow were not distributed uniformly and that the bubbles rose more rapidly in the presence of solids. Two gas holdup and flow profiles were postulated which could account for the lower gas holdup. These profiles had an upward flow in the center of the collumn and an annular downward flow near the wall. The bubble wake mechanism postulates that wake stability is increased in the presence of solids due to increased viscosity. This, in turn, increases the probability of in line bubble-bubble interaction where the wake velocity of the leading bubble increases the rise velocity of the trailing bubble. The resulting higher rise velocity reduces the gas holdup. It is proposed that the effect of solids on reducing gas holdup is a combination of an increase in the rise velocity of bubbles due to stabilization of the bubble wake and a change in the holdup and flow profiles from flat to non-uniform.