Collection of hydrophobic particles in the froth phase

doi:10.1016/S0301-7516(01)00066-7

International Journal of Mineral Processing

Volume 64, Issues 2–3, March 2002, Pages 101-122

https://doi.org/10.1016/S0301-7516(01)00066-7 Get rights and content

Abstract

The collection of hydrophobic particles in the froth phase in flotation has been studied. A continuous flotation cell was developed, in which the froth phase could be isolated from the pulp zone. Hematite particles, d₈₀=87 μm, were floated in the pulp, using a conventional oleate collector, while glass particles of similar diameter, d₈₀=82 μm, were introduced in wash water added to the froth. Four samples of glass particles, with contact angles between 0° and 82°, were used. The results indicate that the efficiency of collection of the hydrophobic particles in the froth can be very high. The process is strongly influenced by the hydrophobicity of the particles and the surface area available for attachment in the froth.

Recommendations for the flotation of particles in the froth phase are made, especially regarding the desirable hydrophobicity as reflected in the contact angle, the froth depth, the solids concentration in the feed to the froth, the air rate and wash water rate.

Introduction

The behaviour of mineral particles in the froth phase, and especially the measurement of the collection efficiency in the froth, has not been fully explored. There are indications, however, that the collection of particles in the froth may even be more efficient than the liquid phase. Schultz et al. (1991) for example, obtained an improved concentrate grade and recovery in a column by introducing the feed directly into the froth phase (above the pulp–froth interface). The ore used in the testwork was Alabama shale with a particle size of d₉₀=23.1 μm. Experiments were carried out at a fixed column height, so they could investigate the effect of froth depth on the collection of particles fed into the froth. The column was also operated in the conventional mode, for comparison. The concentrate grade as well as the recovery showed continuous increase with froth height when the slurry was fed into the froth. More interestingly, it was found that operating the column in this mode gave better concentrate grade as well as recovery than when the column was operated conventionally.

Froth-phase separation was well-known in the late Soviet Union since first being introduced in the early 1970s Malinovskii et al., 1973, Young, 1982. The principle is to discharge the conditioned feed on the top of a froth 150 to 1000 mm deep. Hydrophobic particles are retained in the upper layers of froth, while hydrophilic particles are washed away with the feed water. Since the pulp is filtered through the froth, extremely favorable conditions are created in the froth for interaction of particles and bubbles, especially for the recovery of the coarse particles. According to Young (1982), the advantages arise because: a large air–water interface exists in the froth; the time for the feed to pass through the froth is long in relation to the time available for the adhesion to occur in pulp body machines; turbulence is low, and the tendency for particles to break away from bubbles is low; particles adhere to several bubbles, rather that just one as in pulp-body machines, and the wetted perimeter is consequently much larger.

The maximum size of particle that can be effectively separated by froth separation has found to be 5 to 10 times the upper limiting size for mechanical machines. Particles of up to 3000 μm was reported to be effectively recovered (Young, 1982). The main drawback of froth flotation with direct entry of feed to the froth, is poor selectivity in the fines fraction <75 μm.

A careful examination of froth phase studies reported in the literature provide some insight into the collection of particles in the froth zone. For instance, Cutting et al. (1986) found that a significant upgrading occurred in the froth near the centre of the surface of a mechanical cell. Yianatos et al. (1988) studied the selectivity in the froth zone of two industrial flotation columns in a molybdenum cleaning circuit. It was shown that in froths deeper than 1 m, molybdenite upgrading of up to 15% occurs with increasing height in the froth. This was attributed to the selective re-collection of molybdenite. On the other hand, with froth depths less than 50 cm there was no selectivity. This difference in results between deep and shallow froths was ascribed to mixing arising from wash water.

The general conclusion from previous work is that the collection of particles in the froth is not only possible, but that high collection rates may be achieved. However, there has been little systematic study on this point.

In this work, we have conducted experiments in a continuous flotation cell of special design, in which the froth zone is separated from the pulp zone. Hematite is floated in the pulp in the normal way, so that the flotable particles transfer to the froth and travel upwards to discharge at the overflow lip into a launder. The cell also has facilities for introducing wash water and a froth-feed which may contain hydrophilic and hydrophobic glass particles of a similar size to the hematite.

The feed to the froth contains particles of different hydrophobicities as measured by the contact angle. Particles of well-defined hydrophobicity were used to evaluate the effect of hydrophobicity on the recovery of particles as well as froth phase behavior. The interaction of hydrophobic froth-fed particles with particles which are carried out of the flotation pulp and are already attached to the surface of bubbles in the froth, was also studied. Froth-fed particles that attach to the froth will pass out in the product launder. The design of the cell is such that particles that have been fed into the froth and have passed through it, or hematite particles that have dropped out of the froth, can be collected and measured.

Section snippets

The flotation apparatus

A special cell in which the froth phase is isolated from the pulp zone was employed. The main objective was to study the collection of particles in the froth phase alone, so it was necessary to be able to capture all particles that were dropping out of the froth zone. This approach was first used by Falutsu and Dobby (1989) to measure froth flotation recoveries and rates in a laboratory column cell.

Effect of the concentration of glass particles in feed to froth

The recovery of the glass particles is shown in Fig. 2. Three levels of hydrophobicity were investigated: weak, intermediate and strong, corresponding to contact angles of 50°, 66°, 82°, respectively. The results for hydrophilic glass particles are also shown. All the tests were conducted at a superficial gas velocity of 1 cm/s, an applied wash water superficial velocity of 0.13 cm/s (to which should be added a further 0.19 cm/s from the water in the feed to the froth), a hematite feed flowrate

Summary and conclusions

The aim of this work was to investigate the recoveries of particles distributed in the froth phase in a flotation cell, and compare them with those of particles of a similar size that were contacted with bubbles in the pulp zone of the flotation cell.

The work shows that collection in the froth phase can be very rapid, and give high recoveries. For froth depths as low as 100 mm below the entry point of the feed to the froth, recoveries as high as 87% were achieved. However, the work also shows

Acknowledgements

One of us (SA) is grateful to the University of Newcastle, and the Australian Research Council Special Research Centre for Multiphase Processes, for financial support.

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Collection of hydrophobic particles in the froth phase

Abstract

Introduction

Section snippets

The flotation apparatus

Effect of the concentration of glass particles in feed to froth

Summary and conclusions

Acknowledgements

Int. J. Miner. Process.

Int. J. Miner. Process.

Miner. Eng.

Miner. Eng.

Int. J. Miner. Process.

J. Colloid Interface Sci.

Int. J. Miner. Process.

Int. J. Miner. Process.

Int. J. Miner. Process.