Contact angles, minerals, flotation

Contact angle measurements are of fundamental importance in a range of industrial and everyday processes such as flotation, painting (i.e. the paint must wet the substrate) and weather-proofing. In the flotation process a solid block of the powdered mineral to be floated is often studied using a wide range of collector (i.e. surfactant) solutions to determine optimum flotation conditions. 

The wetting properties of the particles play a crucial role in flotation. We have already discussed the equilibrium position of a particle in the water-air interface (Section 7.2.2). The higher the contact angle the more stably a particle is attached to the bubble (Eq. 7.19) and the more likely it will be incorporated into the froth. Some minerals naturally have a hydrophobic surface and thus a high flotation efficiency. For other minerals surfactants are used to improve the separation. These are called collectors, which adsorb selectively on the mineral and render its surface hydrophobic. Activators support the collectors. Depressants reduce the collector s effect. Frothing agents increase the stability of the foam. 

For example, in mineral flotation, surfactant can be added to adsorb on metal ore particles, increasing the contact angle, so they attach to gas bubbles, but the surfactant does not adsorb much on silicates, so these do not attach to gas bubbles. The surfactant may also stabilize a foam containing the desired particles facilitating their recovery as a particle-rich froth that can be skimmed. Flotation processes thus involve careful modification of surface tension and wettability. 

The numerous papers on contact angles can only be given (in selection) in references.3 The subject is important in the theory of flotation used in the separation of minerals, which has a large literature. ... 

Flotation performance of homologous collectors reported by Wark et al. [19], it was found that contact angle of minerals treated with collector solutions depends only on the structure of the non-polar groups of the reagent, and not on any other portion. In other words, for various structures of non-polar groups, contact angle on sulfide minerals in-... 

The effects of dithiocarbamatic acids with various nonpolar groups on the contact angles of minerals are listed in Table 1.4. And the flotation critical curves of several dithiocarbamatic acids are displayed in Fig. 1.1. 

It is weU known that the selective adsorption of surfactants at the solid-water interface imparts hydrophobicity to the surface of the solid. The relative hydrophobicity of the solid surface is responsible for various macroscopic properties observed experimentally. For example, in mineral separation, the hydrophobicity of the solid surface leads to selective bubble-particle attachment, which accounts for the selective flotation of minerals in large scale industrial plants. The relative measure of mineral surface hydrophobicity is usually quantified in terms of contact angle measurements and flotation experiments (Fuerstenau 1957, 1970, 2000 Fuerstenau and Herrera-Urbina 1989 Fuerstenau and Pradip 2005 Pradip 1988). Molecular-modeling tools can be successfully employed to compute the interaction energies and contact angle on both virgin and surfactant-covered mineral surfaces. The relative flotation efficacy of different surfactants can thus be related to their molecular structure and properties. 

FIGURE 2.12 A comparison of computed contact angles with theoretically computed interaction energies and experimental flotation response of oleate with different calcium minerals. (Data from Pugh, R. and Stenius, P., Int. J. Min. Process. 15 193, 1985.)... 

Flotation leads to separation of ores from the mixtures (add). Especially, in modem mineral industry where rare metals are processed, this techiuque is being widely applied. It has been suggested that among other surface forces, the contact angle plays an important role. The gas (air or other gas) bubble as attached to the solid particle should have a large contact angle for separation. 

Figure 18 Illustration of configurations of three phases involved in detergency (solid-oil-wash-ing bath) and mineral flotation (solid-air-fiotation bath), both pertaining to wetting phenomena in the presence of surfactants, (a) Oily droplet and first 6i contact angle prior to surfactant adsorption and subsequent rollback when 9i > 90°. (b) Air bubble and relevant contact angle 62 that wUl be made greater than 90° upon surfactant adsorption, with resulting good adhesion of the air bubble to the surface.

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