Surface chemistry aspects of coal flotation in bore water

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Abstract

Several theories have been proposed to explain enhancement of coal flotation in salt solutions. In this paper, surface chemistry aspects of coal flotation in bore (hypersaline) water were examined using bubble-particle attachment time experiments, zeta potential measurements, cyclic measurements of contact angle and Atomic Force Microscopy (AFM). The attachment time experiments showed that the bubble-particle attachment in deionized water was instantaneous and independent of the particle size. The attachment in bore water required longer time, which increased with increasing particle size. The cyclic measurements of contact angle on a flat coal surface showed that the coal hydrophobicity as measured by the advancing (maximum) and receding (minimum) contact angle did not change in the presence of salt ions. The zeta potential measurements show that both the coal particles and air bubbles were negatively charged in bore water. The AFM studies showed that bore water reduced repulsive surface forces between the coal particles and air bubbles but had little effect on the force of adhesion. The overall results suggest that enhancement of coal flotation in hypersaline water is not entirely attributed to the surface chemistry aspects as previously proposed.

Introduction

In the 1930s, researchers in the former USSR found that naturally hydrophobic minerals such as coal could be floated in electrolyte solutions without use of collectors and frothers (Klassen and Mokrousov, 1963). Since then, particularly for coal, there have been many studies investigating this mechanism in seawater and saline waters (Yoon, 1982, Yoon and Sabey, 1989, Li and Somasundaran, 1993, Laskowski, 2001). These studies have brought several theories to explain the coal flotation in inorganic electrolyte solutions. For example, one theory proposes that the inorganic electrolytes prevent bubble coalescence resulting in a reduction of the bubble size and increased population, which in turn increases the encounter efficiency for the bubble-particle attachment and the froth stability, hence flotation efficiency (Yoon, 1982, Paulson and Pugh, 1996). However, this theory cannot explain the enhancement of coal flotation in a Hallimond tube, where a froth phase is essentially not present and coal flotation is primarily determined by the attachment interaction between air bubbles and coal particles (Harvey et al., 2002). Another theory proposes that the presence of electrolytes compresses the electrical double-layer (EDL) between bubbles and particles which corresponds to the reduction of zeta potential of both bubble and particle. Although there are some studies showing that the flotation recovery shows a maximum at minimum zeta potentials (Reay and Ratcliff, 1975, Fuerstenau et al., 1983, Yoon and Sabey, 1989), other studies showed that the flotation recovery has a maximum at pH values both above and below the isoelectric point (Celik and Somasundaran, 1980, Li and Somasundaran, 1993). A final theory proposes that the inorganic electrolytes destabilize the hydrated layers surrounding the coal and reduce the surface hydration of the coal (Klassen and Mokrousov, 1963). The destabilization makes the coal more hydrophobic and enhances the bubble-particle attachment. However, this hypothesis does not support experimental evidence that inorganic salts do not cause flotation of minerals which are not naturally hydrophobic.

The hypotheses discussed above have only partly explained the enhancement of coal flotation performance in salt solution. The aim of this paper is conduct a comprehensive study of the role of salt ions in enhancing coal flotation. In particular, the paper focuses on surface chemistry aspects of coal flotation in bore (ground) water which is frequently used as process water in several flotation plants in Australia (George, 1996).

Section snippets

Materials

The coal samples used in this study were obtained from Peak Down and Saraji, BHP Billiton Mitsubishi Alliance (BMA), Australia. Peak Down coal is more hydrophobic than Saraji coal. The volatile matter for Peak Down and Saraji coal is less than 22% (BMA, 2007). Based on this data, these coals can be specified as low-volatile bituminous coal.

The bore water used for the flotation experiments was received from the MKO plant (BHP Billiton, Australia). The chemical analysis of bore water is presented

3.1.1. Flotation experiments

Flotation of coal was carried out in bore water as a function of particle size without a frother or collector. The results for Peak Down and Saraji coal are presented in Table 2, Table 3, respectively. As seen from the tables, the flotation of coal in bore water without any frother or collector is possible. These results also indicate that it takes only 1 min to recover 90% of the coal particles in bore water.

Fig. 3 shows the combustible recovery and ash content of the flotation products with

Conclusions

Results show that coal particles can float in bore water, which contains mostly Na+ and Mg2+, without using any frother or collector and produce a coal concentrate of 85%. The flotation results showed that 90% of the coal particles float within 1 min which suggests rapid flotation kinetics in the presence of salt ions. These results also indicated that a poor flotation response for particles coarser than 0.106 mm.

Coal flotation in bore water was also investigated to understand the surface

Acknowledgements

The Australian Research Council is gratefully acknowledged for financial support through a Discovery grant (AVN). BHP Billiton Mitsubishi Alliance (BMA) is gratefully acknowledged for funding the BMA Chair of Minerals Processing at the University of Queensland (AVN) and the coal samples (Mr Ian Brake and Mr Ben Cronin). BHP Billiton Nickel West (Perth, Australia) is acknowledged for providing the bore water samples. We are indebted to Mr Maung Aung Min from the JKMRC, the University of

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