Transport phenomena at the pulp–froth interface in a flotation column: I. Recovery profiles

Abstract

A calculation procedure is proposed whereby the bubble load and interstitial concentration of different species can be estimated at different levels in the pulp phase of a pilot-scale flotation column. The model requires experimentally measured concentrations of species in the pulp (overall concentrations), the flow rates of species in the concentrate and tailings, air hold-up, the rate of aeration, the wash water rate, as well as mineralogical liberation data as input parameters. Various transport mechanisms, such as the preferential rejection of particles from bubble wakes, settling of particles, entrainment of particles, the flow of the bulk slurry and bubble transport, are incorporated in the calculation procedure. Calculated results can be used to investigate transport phenomena at the pulp–froth interface, to estimate pulp recoveries and froth recoveries. The flotation of chromite in a 15-cm diameter column serves as a case study to demonstrate the application of the calculation procedure. The ore originates from the Western Bushveld Complex in South Africa, and contains chromite, enstatite and feldspar as major minerals. Conclusions regarding the froth zone recovery are made in conjunction with a proper quantification of the bubble load and interstitial concentration of species directly below the pulp–froth interface. If the upward transport by entrainment directly below the pulp–froth interface is negligible relative to the corresponding bubble transport, then the froth recovery is largely determined by particle–bubble detachment at and above the pulp–froth interface. Conversely, if entrainment contributes significantly to the upward transport below the pulp–froth interface, then the rejection of these entrained species from the froth could strongly reduce froth recovery. The froth recovery in the chromite case study is insensitive to froth depth, and decreases with a decrease in particle size. Furthermore, the pulp–froth interface plays a dominant role in the upgrading process during flotation.