Bubble size and rise velocity, gas holdup, and frother concentration in flotation

Abstract

It was revealed from the literature that although Sauter mean bubble diameter, d₃₂, remained virtually unchanged after critical coalescence concentration (CCC), gas holdup kept rising without reaching a maximum plateau. In addition, for a given d₃₂, different gas holdups were obtained for different types of frothers, contrary to conventional belief regarding bubbly columns. A brief analysis was given in this Note, by considering the effect of different hydrodynamic environment surrounding a bubble in a swarm from a single bubble alone on dynamic frother adsorption. Based on the balance between hydrodynamic drag and surface tension gradient force acting on a bubble, it was speculated that frother adsorption density or compactness in the dynamic adsorption layer on a bubble would be lower in swarms than in single bubble systems of the same size. This could allow further frother adsorption after CCC and CMV (concentration at minimum/terminal velocity for single bubbles). Consequently gas holdup might continue to rise. Moreover, the overlap of adsorption layers between adjacent bubbles caused stronger molecular interactions among frothers of longer hydrocarbon chains, or larger molecular weights with different head groups, and interfered bulk free water flow through the channels of rising bubbles, leading to their stronger drag to slow bubbles thereby higher gas holdups for a given bubble size. It appeared that bubble surfaces in real flotation systems may not be totally immobilized.