The flotation process has been used for more than a century to separate valuable minerals from bulk ores. The separation process is based on utilising the differences in the physico-chemical properties of liberated particles, mainly the particle hydrophobicity which allows the particles to be attached to air bubbles rising from the pulp phase into the froth phase and subsequently collected to the launder. The stability of the froth phase which is be defined as the ability of bubbles to resist coalescing and bursting (Triffet & Cilliers, 2004), has been shown to have a significant effect on the efficiency of the flotation process. An unstable froth will result in poor valuable mineral recovery as these desired hydrophobic particles are detached from air bubbles and drain with the water back into the pulp phase due to bubble coalescence. On the other hand, a very stable froth may result in poor concentrate grade as the unwanted gangue materials are unselectively entrained to the concentrate. As a result, a substantial amount of research has been performed on improving control of froth stability by the manipulation of frother type and dosage. A recent study investigated the manipulation of flotation operating parameters such as air rate, froth height and depressant dosage which resulted in minimal changes in froth stability. The present study then investigated the effect of particle size and solids concentration on the stability of the froth phase using a UG2 ore and an Itabirite ore. Froth stability was determined using Bikerman tests on a laboratory scale noncontinuous stability column. A novel continuously operated agitated hybrid cell was also used to assess froth stability, with water recovery and froth recovery used as proxies for froth stability. The agitated hybrid cell was then included in the experimental design as it allowed for continuous floatation system to be evaluated which resembles more industrial operations as compared to the stability column. The hybrid also incorporated the agitation zone benefits of a lab scale batch flotation cell which allows for better attachment of coarse particles and also allowing for the formation of deeper froths enabling improved froth stability measurements. The viability of using the top froth average bubble size and the side of froth axial bubble coalescence rate as froth stability proxies was also evaluated as the columns were clear glass. An evaluation of the particle size distributions of the feed and the concentrate reporting to the launder showed that the concentrate was consistently finer than the feed. Feed particle sizes for the UG2 ore ranged from 157-78 μm with concentrate sizes ranging from 83-39 μm from the coarsest to the finest grind. Feed particle sizes for the iron ore ranged from 29-62% passing 38 μm with concentrate sizes ranging from 49-82% passing 38 μm from the coarsest to the finest grind. It is hypothesised that this was due to the increased weight of the coarse particles resulting in the particles draining back into the pulp zone at a faster rate. As a result, a smaller fraction of the coarser particles reports to the concentrate resulting in the finer particle size distribution. The effect was shown to be more pronounced for the UG2 ore as compared to the iron ore, as the UG2 ore forms a less stable froth which has a higher rate of bubble coalescence. Changing the feed particle size was also shown to alter the concentrate particle size thereby allowing for the investigation into the effect that the size of particles present in the actual froth has on froth stability. Test results show that froth stability increased with decreasing particle size for both ores. It was hypothesised that a decrease in particle size would result in an increase in the maximum capillary pressure thereby reducing capillary drainage. It was also hypothesised that a decrease in the particle size of the entrained particles would increase the viscosity of the interfilm fluid, thereby reducing drainage rate and increasing stability. Froth stability was shown to follow a decreasing power law relationship with feed particle size. Froth stability was also shown to decrease sharply with increasing particle size over the fine feed size range of less than 100 μm, with the effect becoming less pronounced with increasing particle size over the coarser range. The steep decrease was shown to correspond to concentrate particle sizes approximately less than 50 μm, the range in which particles are expected to report to the froth through entrainment. Froth stability followed an increasing linear relationship with feed specific area for the size range tested (UG2 ore: 78-157 μm and iron ore: 48-118 μm). More importantly, froth stability was assessed as a function of total surface area imparted by the concentrate particles and stability was shown to increase with increased total surface area. Decreasing the feed particle size was shown to result in higher solids recovery and finer concentrate particle size thus higher specific area, therefore total surface area imparted was critical to the assessment of froth stability. Increasing solids concentration was shown to result in an increase in froth stability and the effect was shown to be less pronounced for UG2 ore as it is a sparsely mineralised PGM-bearing ore. The rheology of the interfilm is suggested to play a more significant role in particle stabilisation of the froth as the stability increased steeply in the particle size range where entrainment is expected as shown by the entrainment factor increasing steeply in the same size range. Future work evaluating the rheology of the froth is recommended to further clarify this.

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