Stability and coalescence of bubbles in salt solutions in a bubble column and thin liquid films

Abstract

Salts are known to inhibit bubble coalescence at the concentration greater than a critical (transition) concentration. The phenomenon of bubble coalescence governs the size distribution of bubbles, which is vital to many natural and industrial processes including mass transfer in the ocean, multiphase reactors and mineral recovery in froth flotation. Despite intensive studies over nearly a century resulting in multiple hypotheses, mechanisms of the coalescence inhibition are still unclear. Researchers have greatly agreed that it is the liquid films between bubbles that control the coalescence and must be firstly understood. Surprisingly, most studies have just focused on the bubble tests, but there was a lack of comprehensive studies about the liquid films in a surfactant-free salt system. This thesis aims to provide experimental evidence and extend the current understanding of the liquid film stability and coalescence in salt solutions. Firstly, the effect of superficial gas velocity (3.5, 10, and 18 mm/s), salt type (NaCl, NaI and CsCl) and concentration (0.001M to 3M) on bubble coalescence and gas holdup in a small bubble column were studied. For the first time, it was found that the transition concentration for coalescence inhibition is not a unique entity of salts as reported but it decreases with increasing gas velocity, which highlights the importance of hydrodynamic conditions. The results also confirmed the ion specific responses of investigated salts to transition concentration and gas holdup. To understand the coalescence, liquid films of the simplest subject, deionised water (DI), were studies using the micro-interferometry method with the Sheludko cell. It was showed that films of DI water drained very fast and ruptured instantly when two bubbles were first brought into contact. However, the film drainage rate and rupture thickness sharply decreased and the film lifetime steeply increased with increasing contact time up to 10 min, but then they levelled off. It was argued that the phenomenon was due to the migration of inherently dissolved gases which act as the long-range hydrophobic attraction. The JC Erickson et al.’ theory of the hydrophobic attraction and the extended Stefan-Reynolds models to include the air/water surface mobility were also employed. The liquid film study was then extended for salts without a presence of surfactants. Drainage and stability of liquid films in different salts (LiCl, NaCl, KCl, CsCl, KNO3, NaClO3, CaCl2, and MgSO4) over a wider range of concentration (0.001M - 3M) were studied in the close (saturation) Sheludko cell with the strictly controlled saturation level and surface contact time. Films of all salts drained slower and lasted longer with increasing salt concentration. Also, the so-called non-inhibiting ii salt, NaClO3, indeed inhibits coalescence at high concentrations (greater than 1M). The effect of the DLVO forces was totally ruled out. Based on the correlation between the experimental results and the data of oxygen solubility and solution viscosity, it was argued that the weaker hydrophobic force with higher salt concentrations was due to a reduction in the concentration of soluble gas. Also, it was suggested that the influence of solution viscosity should not be neglected, and the coalescence was controlled by multiple mechanisms including dissolved gases and solution viscosity. Finally, the drainage of those experimental film results was modelled by using the power law for hydrophobic attraction and the extended Stefan-Reynolds to incorporate the surface mobility. From the model results, it was convincingly demonstrated that the film surfaces were immobilised by salts, which explained the drainage retardation and the increased film stability with increasing salt concentration. The results were attributable to the surface viscosity caused by the ion-water interaction beneath the air/water interfaces. The impact of hydrophobic forces on the film stability was also confirmed but shown not as critical as the surface mobility. The outcomes of this work will provide a better understanding of the underlying mechanisms of bubble coalescence inhibition. As proposed, salts affect the coalescence via multiple mechanisms including dissolved gases and surface mobility. In fact, those mechanisms are not separate but interrelated. It would be a real challenge to correlate them into a single model but is worthwhile to be further investigated.