An investigation of flotation froth rheology

Abstract

Flotation froth is of significant importance because it ultimately determines overall flotation performance. Froth recovery, the fraction of valuable minerals coming into the froth phase which survive and report to the concentrate, has been widely used to evaluate the efficiency of transporting the valuable minerals from the pulp-froth interface to the concentrate launder.Froth recovery is related to the time that particles spend in the froth and the rate at which bubbles burst.It is believed that froth rheology has an impact on froth recovery in flotation. A more viscous froth should resist motion towards the lip, rising vertically before overflowing into the cell launder, resulting in significantly higher froth residence times and increased probability of froth drop back as well as froth collapse. Very little work has been done to study the rheology of a highly mineralised flotation froth and how it impacts on froth recovery. Therefore, this project aims to gain an insight into the correlations between froth properties, froth rheology and flotation performance.To achieve this goal, a novel method to measure froth rheology properly was developed in this project. The method involves the use of a rotating vane surrounded by a tube. The tube is required to minimise the adverse effects of the horizontal flow on the measurement.Equations have been provided to convert the vane rotational speed and measured torque to shear rate and shear stress, depending on whether the froth within the tube is fully or partially sheared.In order to investigate the correlation between froth rheology and froth properties, flotation tests were performed in a 20 L continuous flotation cell using a synthetic ore which was a mixture of chalcopyrite and silica. The flotation tests, conducted under different flotation conditions (i.e. air rate, froth depth, impeller speed, feed grade and feed P80), resulted in significant changes in the froth properties and the viscosity of the froth. The froth was shown to be shear thinning with minor yield stress. It could be modelled using a power-law model with the consistency index of this function being able to be used to represent the degree of viscosity of the froth phase.The observed change in the froth viscosity was shown to be largely a function of the change in the bubble size and the percentage of the bubble surface covered by particles. Smaller bubbles and a greater coverage of particles on the bubble surface result in a greater resistance to froth motion. Thus changes in cell operation which reduce the froth bubble size (e.g. higher impeller speeds, higher frother dosage rates) will result in a more viscous froth. Higher feed grades and a smaller particle size which result in a greater proportion of bubble surface coverage will also result in higher froth viscosities.A model was proposed which relates the apparent viscosity of the froth to the shear rate, the froth bubble size, the gas volume fraction in the froth, the percentage of the bubble surface covered by particles and a fittabl...

A Brief Review of Pulp and Froth Rheology in Mineral Flotation

“…In flotation, mineral slurries made of good quality ores often display a Newtonian behaviour, whereas complex and low-grade ore slurries normally exhibit a pseudoplastic or Bingham behaviour [4]. Flotation froths, as indicated from the literature, exhibit pseudoplastic characteristics, similar to air-liquid foams [12][13][14].…”