M.C. Harris

The effect of froth residence time on the kinetics of flotation

Abstract Previous studies by the authors have shown that there is a strong correlation between the overall flotation rate constant k and the bubble surface area flux, S b , in a flotation cell, and that this relationship is independent of impeller type. More recently it was found that results obtained from a 250 litre cell and a 3 m 3 cell treating the same feed could be represented by the same k-S b relationship, as long as the ratio of froth height to superficial gas rate in the cells was the same. This ratio is termed the froth residence time, τ fg . This paper examines the effect of τ fg on the kinetics of flotation, by re-evaluating and re-interpreting these results. It is found that k decreases exponentially with τ fg but the relationship is dependent on cell size and therefore not directly useful for scale-up. When τ fg is divided by a typical cell dimension to take into account the effect of froth transportation distance in cells of different sizes, the relationship between k and the “specific froth residence time” fits the data better and and is found to be independent of cell size. Finally, when the results are evaluated in terms of τ fg , the froth residence time calculated on the basis of the concentrate slurry flow rate, the fitting od the data is found to be very much better again. The implications of these relationships for flotation modelling and scale-up are discussed.

Frother characterisation using dynamic surface tension measurements

There is as yet no accepted method of predicting flotation frother performance in plant processes based on laboratory scale characterisation techniques. This paper investigates the use of dynamic surface tension, using the maximum bubble pressure method, as a means of evaluating frother performance. The method has been refined to facilitate measurement at high bubble rates, an important criterion with respect to the analysis of small, fast adsorbing molecules at low concentration such as flotation frothers. An adsorption model is presented, which has been used to elucidate the role of frothers with respect to bubble size in a flotation cell, based on the correlation of model parameters to physical frother characteristics. The potential for the application of dynamic surface tension to the performance of the froth phase in a flotation system is also discussed.

An evaluation of a direct method of bubble size distribution measurement in a laboratory batch flotation cell

Abstract In this paper a novel bubble sampler is described which allows the measurement of bubble size distributions in flotation cells containing slurries of high solids concentration. The device separates bubbles fromo a slurry into a water solution where they may be measured using a bubble size analyser. An experimental programme of measuring bubble sizes in two and three-phases is reported in which the bubble sampler was evaluated and the effect of different frothers on the bubble size distribution in two and three-phases was investigated. In two-phase operation (liquid/gas), the bubble sampler was proved to supply accurate and reproducible measurements when compared with measurements made by the bubble size analyser without the sampler fitted. The frother type was found to have the greatest influence on bubble size. With the addition of solids (gold plant tailings) to form a three phase (solid/liquid/gas) system, the mean bubble size was found to increase, and only the further addition of frother could reduce it to its former value. Different frothers were found to have very different effects on the size of bubbles produced in slurries compared with those produced in two-phase systems. Of the frothers tested MIBC was found to be the best all around performer in terms of bubble size reduction with increasing solids concentration.

Review of froth modelling in steady state flotation systems

Abstract Froth models proposed in the literature are reviewed with the aim of identifying their significance and usefulness in the modelling and scale-up of the froth phase in steady state flotation systems. Literature indicates that froth phase performance is better understood in terms of froth recovery, the fraction of material presented at the pulp-froth interface that reports to the concentrate. This review suggests that froth recovery is a strong function of drainage rate of particles from the froth phase to the slurry phase. Drainage rate, in turn, is determined by physical factors, such as froth removal technique, geometry of the flotation cell, flux and distribution of air at the pulp-froth interface, the water content, particle size and solids content, and chemical factors, such as froth stability and froth loading. These factors influence the froth residence time, which has been identified as a key froth parameter. Finally, it is proposed that future work should focus largely on the development of a methodology to investigate froth performance based on the froth recovery in different flotation systems. This will enable generic relationships between the froth recovery and froth sub-processes and key froth parameters to be established, and make it possible to relate froth performance in different flotation systems.

A review of methods to model the froth phase in non-steady state flotation systems

Abstract The current of batch flotation froth modelling is critically reviewed in order to identify its significance and usefulness, particularly in the scale-up of batch data to a continuous flotation process. This review suggests that the concept of the froth recovery factor, R f , may provide the most useful way of establishing the performance of the froth phase in a batch flotation process. The froth recovery factor refers to the fraction of material reporting to the pulp-froth interface which is ultimately recovered in the concentrate. It is also proposed that a froth recovery model based on froth retention time can be used for relating batch froth performance to continuous flotation systems. However, a quantitative model which relates the froth recovery factor and froth sub-processes has yet to be developed.

The modelling of froth zone recovery in batch and continuously operated laboratory flotation cells

This paper proposes an integrated methodology for modelling froth zone performance in batch and continuously operated laboratory flotation cells. The methodology is based on a semi-empirical approach which relates the overall flotation rate constant to the froth depth (FD) in the flotation cell; from this relationship, a froth zone recovery (R,) can be extracted. Froth zone recovery, in turn, may be related to the froth retention time (FRT), defined as the ratio of froth volume to the volumetric flow rate of concentrate from the cell. An expansion of this relationship to account for particles recovered both by true flotation and entrainment provides a simple model that may be used to predict the froth performance in continuous tests from the results of laboratory batch experiments. Crown Copyright (C) 2002 Published by Elsevier Science B.V. All rights reserved.

Relationship between surface froth features and process conditions in the batch flotation of a sulphide ore

Flotation processes occurring in the bulk and froth phases have a characteristic influence on the structural features and dynamics of the flotation froth. In principle the froth features can therefore be used as a useful indicator of the performance of the flotation system. In this study the surface froth features and dynamics are represented by three features extracted from the digitized images of the froths, viz. a statistical feature which is a rough indication of the average bubble size of the froth, a measure of the froth stability, as well as the average grey level of the froth, which is an indication of mineral loading. The effect of high intensity conditioning on the batch flotation of a sulphide ore from the Merensky reef in South Africa was investigated, and the significantly beneficial effect of high intensity conditioning on the performance of the flotation was clearly reflected in the smaller bubble size distributions and greater stability of the flotation froths.

An on-site evaluation of different flotation technologies for fine coal beneficiation

Abstract This paper presents the results of an investigation of the performance of a number of different flotation cell technologies for the beneficiation of fine coal. The work was conducted on-site at the Grootegeluk Colliery in the northern Transvaal province of South Africa, using a pilot-scale conventional column cell, a pilot-scale Jameson-type cell, and an air-sparged hydrocyclone (ASH). In addition, characterisation and conventional batch flotation tests were conducted in the laboratory in the Department of Chemical Engineering at the University of Cape Town. All three units tested on-site demonstrated improved selectivity compared to conventional subaeration flotation. In the column cell, optimum performance could only be achieved at very low throughputs. Substantial losses of coal occured in the coarser size fractions. The Jameson-type cell was able to operate effectively at about double the throughput of the column cell at similar recoveries. Coal recovery in the coarser size fractions was still poor, but better than that of the column cell. The ASH was characterised by a very high throughput, more than 150 times that of the column cell on the basis of solids capacity per unit cross-sectional area. However, the ASH required more than three times the reagent dosage of the other two units to achieve this. The ASH performed particularly well in the recovery of the coarser size fractions, but was less effective than the other cells on the finer size fractions. Overall, the best performance for this application was that of the Jameson cell, owing to its higher capacity in comparison to the column cell. The high reagent requirement of the ASH makes this technology uneconomic in this application.

An evaluation of the role of particle size in the flotation of coal using different cell technologies

Abstract A large variety of new flotation cells has been introduced in the last few years, probably as a result of the successful introduction of column flotation in the minerals processing industry. In common with the column cell, a number of these new cells employ an essentially quiescent separation zone. However, a number of novel cell designs have been introduced that use agitation mechanisms similar to those employed on conventional flotation cells. The aim of this investigation was to evaluate flotation behaviour in both an agitated and non-agitated environment, particularly with respect to particle size. A hybrid ‘agitated’ column cell was designed for the investigation, and the operation of this unit was compared to that of a column cell, and to a batch flotation cell, on a laboratory scale. The testwork was conducted on coal fines, as problems with the flotation of coarse coal particles in a column cell had previously been identified. It was demonstrated that the addition of an agitated stage to a column cell can significantly improve the coarse particle recovery in comparison to the conventional column cell, while maintaining good selectivity in the fine sizes.

Fine pyrite flotation in an agitated column cell

Abstract The flotation of fine pyrite has been studied in a 0.1 m×1.9 m agitated column. The variables studied were agitation, air and slurry feed rates. The recovery and selectivity were determined for various size fractions as a function of the column operating variables. The results obtained indicate that particles less than 25 μm may be selectively recovered provided the agitation rates are kept below 400 rpm. Increasing the airflow rate at a fixed agitation rate of 400 rpm resulted in a decrease in the selectivity. The loss of selectivity at higher agitation and airflow rates is attributed to the increased entrainment of fine gangue particles. The results obtained suggest that maximum external power input of about 0.3 kW/m3 is required for separation of the fine pyrite.