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W. Richard Bowen and Nidal Hilal 4

W. Richard Bowen and Nidal Hilal 4

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82 3. QUANTIFICATION OF PARTICLE–BUBBLE INTERACTIONs<br />

3.1 IntroDuCtIon<br />

The attachment of particles to bubbles in solution is of fundamental<br />

importance to several industrial processes most notably in froth flotation.<br />

Froth flotation is a significant industrial process, used primarily in the<br />

separation of mineral particles <strong>and</strong> also in the treatment of wastewater.<br />

The process of flotation involves the suspension of finely ground mineral<br />

particles in a chamber through which large volumes of air or another<br />

gas are bubbled. Hydrophobic particles will attach more readily to air<br />

bubbles passing through the medium <strong>and</strong> will thus rise to the top of<br />

the chamber where they form froth at the surface <strong>and</strong> become separated<br />

from other materials. Hydrophilic particles, which are less able to attach<br />

to the air bubbles, will eventually sink to the bottom of the chamber. In<br />

addition, a number of additives, including surfactants, termed collectors,<br />

may also be introduced to increase the hydrophobicity of the particles of<br />

interest <strong>and</strong> consequently increase the efficiency or specificity of the flotation<br />

process [1, 2].<br />

It follows that an underst<strong>and</strong>ing of the nature <strong>and</strong> strength of the interactions<br />

between colloidal particles suspended in solution <strong>and</strong> air bubbles<br />

is of fundamental importance to developing new ways of increasing flotation<br />

efficiency <strong>and</strong> modulating specificity. Atomic force microscopy<br />

is one technique which is able to quantitatively measure interactions<br />

between single particles <strong>and</strong> interfacial boundaries. Over the past decade<br />

the atomic force microscope (AFM) has been adapted for use in studying<br />

the forces involved in the attachment of single particles to bubbles in<br />

the laboratory. This allows the measurement of actual Derjaguin, L<strong>and</strong>au,<br />

Vervey <strong>and</strong> Overbeek (DLVO) forces <strong>and</strong> hydrodynamic, hydrophobic<br />

<strong>and</strong> adhesive contacts to be measured under different conditions. In<br />

addition, contact angles may be calculated from features of force versus<br />

distance curves. The purpose of this chapter is to illustrate how the AFM<br />

<strong>and</strong> particularly the colloid probe technique can be used to make measurements<br />

of single particle–bubble interactions <strong>and</strong> to summarise the<br />

current literature describing such experiments.<br />

3.2 PArtICLE–BuBBLE IntErACtIonS<br />

During collisions between mineral particles <strong>and</strong> air bubbles in flotation<br />

cells, attachment is determined by the thinning of a film of water in the<br />

intervening space. This consists of a layer of bulk water <strong>and</strong> a hydration<br />

layer which surround both the particle <strong>and</strong> the bubble (see Figure 3.1). On<br />

initial approach, the bulk fluid layer will become displaced [3]. However,<br />

the time required for this bulk layer to drain from the confined space

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