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 fittable constant. This model provided a reasonable fit to the apparent viscosity data measured in the original 20 L continuous flotation cell test data, and was validated using data generated from additional flotation tests in which frother dosage was varied and higher feed grades were used. Correlations were observed between the froth rheology and key flotation parameters which are known to affect overall flotation performance. As expected, a positive correlation was observed between the froth height above the lip and the froth viscosity. As the froth viscosity and thus the resistance to froth flow increases, the froth rises up until a new equilibrium is reached within the froth. The froth height above the lip affects froth retention time; an increase in froth retention time is known to decrease froth recovery and increase drainage of water and entrained solids. As the valuable minerals are recovered by attachment to bubble surfaces, air recovery was used as a proxy for froth recovery, owing to the difficulty in measuring froth recovery. The extent of drainage in a flotation system was evaluated by investigating the recovery of non-floating gangue mineral. In the 20 L laboratory flotation cell testwork, froth rheology was found to be correlated to both air recovery and gangue recovery. The correlations, however, were not straight-forward as the parameters that affect froth rheology (i.e. bubble size and surface coverage by particles) also affect the froth stability and the resistance to drainage within the froth. A preliminary industrial study was performed during which froth rheology was measured in a concentrator processing a platinum ore. It was found that the froth exhibited similar rheological characteristics to the froth generated in the laboratory work using the synthetically created copper ore. The industrial data suggests that there is also a correlation between the froth viscosity and the froth height above the lip, as observed in the laboratory work. In summary, this PhD project has delivered the following major outcomes: • A novel method to measure froth rheology • An evaluation of the effect of froth properties on froth rheology • A model to predict froth rheology as a function of froth properties • Observed correlations between froth rheology and key flotation parameters which are known to affect overall flotation performance