Modelling and measurement of multi-phase hydrodynamics in the Outotec flotation cell

The flow field, gas dispersion and solids concentration in a flotation cell fitted with an Outotec (Outokumpu) flotation mechanism have been studied using both experimental and multi-phase Computational Fluid Dynamics (CFD) modelling. The velocity field of the liquid phase was measured using Laser Doppler Velocimetry (LDV). The bubble size was measured using an imaging window, while void fraction was measured with a conductivity probe and solids concentration was measured by manual sampling. Both the LDV measurements and CFD models revealed that the rotor generated a jet stream towards the wall, which induced two vortexes, one below the impeller level and the other extending from the rotor region to near the free surface. The Sauter mean bubble diameter was measured to be about 0.7-0.9 mm. The population balance model incorporated into the CFD model gave good general agreement in terms of bubble size and distribution. When solids were added to the system, the void fraction profiles changed significantly: the void fraction showed less variation in the vertical direction. The solids concentration was practically uniform across the tank except for the region near the surface, and this was reproduced in the multiphase CFD model. The complex multiphase CFD model thus can be used to predict the major characteristics determining flotation kinetics.