W. Richard Bowen and Nidal Hilal 4
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