The effect of impeller-stator design on froth stability, flotation performance and flotation hydrodynamics

Abstract

Froth flotation is the most important mineral concentration process. Over the last few decades, throughput has increased due to flotation equipment becoming exponentially larger. Small improvements in performance, therefore, significantly impact the economic revenue and sustainability of the process. The optimisation of flotation has thus been extensively studied, resulting in several improvements, especially in operating conditions, and important advances in our understanding of flotation. Publications on flotation equipment designs and their impact on flotation phenomena, however, are scarce and flotation tank design has, to a large extent, remained in-house know-how of manufacturing companies. Currently, flotation tank design tends to focus mainly on pulp zone parameters without considering its effect on the froth zone, despite the importance of froth stability in flotation performance. Moreover, while most new designs are studied using computational fluid dynamics models, these are typically validated with one- or two-phase systems, which oversimplify flotation phenomena. To solve these problems, large experimental datasets quantifying the effects of design variables on flotation phenomena are required.

This thesis fills these and other gaps identified in the literature, by studying the effects of different impeller-stator designs on flotation phenomena. A continuously overflowing laboratory-scale flotation tank was used to study two different impeller designs, a Rushton turbine and a rotor, both with and without a stator. Bubble size distribution was measured for two- and three-phase systems, froth stability was quantified using air recovery, and flotation performance was assessed. Furthermore, a scaled-down bench-scale flotation cell was used to study the effect of these impeller-stators on the hydrodynamics of the flotation system, using Positron Emission Particle Tracking, which determines the motion of radioactive particle tracers, representative of hydrophobic and hydrophilic particles.

Results show that the effect of the stator is predominant over the effect of impeller design. This thesis quantifies, for the first time in the literature, stator-induced bubble size reduction, caused by changes in bubble size distribution. Similarly, it shows for the first time that stators enhance froth stability and drastically change the hydrodynamics of the system by altering flow patterns and particle speed. All these effects improve flotation performance, markedly increasing metallurgical recovery. The information presented herein allows for a better understanding of physical phenomena within a flotation tank. Moreover, these datasets represent an important contribution to the validation and improvement of three-phase computational fluid dynamics models and will be valuable for the design of pilot- and industrial-scale test work.