Oil-coated bubbles in complex colloidal systems: Fundamentals and applications
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Froth flotation is an efficient method to selectively separate particulate species from aqueous dispersions or slurries. It has been widely utilized in many industries, such as mineral operation, oil recovery, waste paper recycling and waste water treatment, to name but a few. It relies on the principle that hydrophobic particles can attach to gas bubbles while hydrophilic ones cannot. In many cases, however, the particles that we want to separate are too hydrophilic to attach to gas bubbles. To overcome this limitation, our strategy is to coat the gas bubbles with a thin layer of oil, making the bubble surface more polar compared to uncoated bubbles; as a result, less hydrophobic or even mildly hydrophilic particles can attach as well. In this study, we have systematically examined the fundamental aspects of oil-coated bubble flotation and have shown that it has thermodynamic, kinetic and hydrodynamic advantages over the traditional method. Inspired by our previous studies on capillary foams, we also developed a simple and convenient assay: a foaming test to evaluate the ability of particles to be collected and separated by gas bubbles. Finally, we focused on an industrially-relevant separation system (de-inking) to examine the performance of oil-coated bubble flotation. We found that oil-coated bubble flotation often dramatically outperforms the standard technique with uncoated bubbles; the highest ink removal efficiency was observed in systems containing multivalent salts at low pH when using bubbles coated with 1-octanol. This work sheds light on the mechanism of oil-coated bubble flotation in the separation of less hydrophobic or even mildly hydrophilic particles, and is expected to trigger broader interests in many other separation systems.