Wednesday, 25 May 11:00am – 12:00pm

This seminar will be delivered in Online Zoom Please email chemistry.researchsupport@sydney.edu.au for zoom link and password.

Speaker: Professor Anh V. Nguyen; School of Chemical Engineering, The University of Queensland

Host: Professor Chiara Neto

Title: About the “love affair” between hydrophobic particles and air bubbles in the flotation separation of minerals

Abstract: Flotation is the separation of hydrophobic particles from a suspension using air bubbles and has been the workhorse of the mining industry for a century. The hydrophobic attraction between hydrophobic surfaces is critical to flotation. However, our understanding of this “love affair” in flotation has been limited quantitatively. In this seminar, the experimental and theoretical research outcomes over the last fifty years are reviewed and reconciled. The early measurements by SFA and AFM produced two types of empirical correlations of hydrophobic force vs separation distance: the single or double exponential dependence and the cubic-power dependence. Most of these early results have been attributed to the bridging of interfacial nanobubbles. The hydrophobic force in the absence of nanobubbles was also measured by AFM colloid probe technique on the first approach. If dissolved gases are removed from the solutions, no hydrophobic force can be detected experimentally but the certainty of the water degassing has been an issue. Recently, we measured the hydrophobic force using the first approach technique and different salt solutions with different gas saturation levels. The hydrophobic force observed in the absence of interfacial nanobubbles is attributed to the interfacial enrichment (IGE) in the form of a dense gas layer (DGL) at hydrophobic surfaces. This force can increase with increasing gas saturation and decrease with decreasing gas solubility. Both IGE and DGL have been confirmed and analyzed by molecular dynamics simulations. The multilayer theory of van der Waals dispersion forces with the inclusion of IGE can now be applied to predict the measured hydrophobic force in the presence or absence of nanobubbles. The asymptotic analysis also shows that the van der Waals dispersion forces can be simplified to obtain the empirical correlations of the exponential or power dependences of hydrophobic force on the separation distance. Significant progress in our understanding of hydrophobic force between solid surfaces has been made but direct evidence of the hydrophobic force between a solid surface and an air bubble being critical to flotation is still awaited.

Biography: Anh Nguyen is a professor in the School of Chemical Engineering, The University of Queensland where he held the BMA (BHP Billiton Mitsubishi Alliance) Chair from 2007 to 2017. He was an ARC QEII Research Fellow and then an associate professor at the University of Newcastle and has held many adjunct and visiting positions overseas, including those at the University of Utah (USA), Central South University (China), and Kyushu University (Japan). His research interests embrace various aspects of colloid and interfacial science and engineering that include coal and minerals processing, saline water, foliar fertilizers, and hydrophobic hydration and hydrate formation of gases. His relevant publications include a research book on the colloidal science of flotation and 340 papers in peer-reviewed journals. He has supervised over 50 PhD projects and has an editorial role in Advances in Colloid and Interface Science, and the International Journal of Mineral Processing. Currently, he is a Chief Investigator at the ARC Centre of Excellence for Enabling Eco-efficient Beneficiation of Minerals.