Regimes of drainage instability caused by wash water

Abstract

Wash water is widely used in flotation to increase the gravity-driven drainage rate of water within the froth phase and the efficiency of removing entrained (hydrophilic) particles. At a high rate, it can disturb the froth. Here, we investigate the drainage instability caused by wash water using the forced drainage experiments. We quantitatively confirm that foam suffers from convective instability when a sufficiently high flow rate of wash water is employed. A comparison between the theoretical onset predictions of convective instability and experimental results shows that some available drainage experiments might have suffered from convective instability. Our new experimental results show three drainage regimes and structures associated with increased flow rate of wash water: (1) stable foam structure, (2) relative motion of bubbles and (3) convective roll. Our study presents both theoretical analysis and experimental results for quantifying convective instability, which can be eliminated to improve flotation.