Nanobubbles, hydrophobic effect, heterocoagulation and hydrodynamics in flotation

It has been shown in recent years that the long-range attractive interaction forces between hydrophobic surfaces in aqueous systems are caused by the capillary forces of gas bridges which form at the coalescence of nanobubbles adhering on the surfaces. The coalescence of nanobubbles on selectively hydrophobized particles with coarser bubbles initiates the jump into the three-phase contact at the attachment events in flotation. Therefore, one should no longer speak of hydrophobic forces in these events but of hydrophobic effects because they are caused by capillary forces. However, in the selective hydrophobization of particles by long-chain collectors (surfactants), there is another situation. Here, the association of nonpolar groups in the adsorption layers plays an important role in the energy balance of adsorption. This association is caused by “truly” hydrophobic interactions, which support the spotty distribution of the adsorbed collector ions on the particle surfaces and promote the formation of nanobubbles. This paper is intended to show to what extent the results obtained by basic research on the nanobubble formation as well as the force-distance dependence of collision events can be applied to flotation processes. This particularly requires the consideration of the highly turbulent flow conditions in the impeller stream of the flotation machines, in which the attachments almost exclusively occur. Various phenomena which occur in these machines and affect the reagent regime and the hydrodynamics point to the fact that nanobubbles can form, exist and even grow into microbubbles in that region. Therefore, so-called combined attachment events should predominate as already imagined several decades ago. The highly turbulent pressure fluctuations in the impeller stream in addition to the dispersion of the bubbles effect also their oscillations in size and shape, so that adsorption equilibria in the interfaces liquid/gas cannot be supposed. However, the pressure fluctuations present the possibility to overcome potential barriers in the wetting films at attachment events.