- Title
- Behaviour of bubble clusters in a turbulent flotation cell
- Creator
- Chen, Zhihao
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2014
- Description
- Research Doctorate - Doctor of Philosophy (PhD) (Chemical Engineering)
- Description
- In the flotation process for the separation of valuable minerals from their ores, it is known that the rate of capture of particles decreases as the particle size increases. It has been postulated that the range of particles that can be recovered in conventional machines could be extended by the use of bubble clusters (Ata and Jameson, 2005). However, little is known about the effect of the turbulent shear in the flotation cell, as reflected in the power input or the impeller rotational speed, on the structure of the clusters, or even if clusters can form in existing equipment where the power input is in the range 0.5 to 5 kW/m3. In this thesis, the behaviour of bubble clusters in turbulent conditions in flotation cells has been studied both experimentally and theoretically. The breakup and re-formation of clusters and the effect of bubble size and impeller speed on the behaviour of clusters has been investigated. The apparatus used was essentially a laboratory flotation cell, agitated by a Rushton turbine. The cell was modified to allow pre-formed clusters to rise out of a fluidized bed and into the path of the rotating impeller. The events were captured using a digital camera, and the images were analysed to give the sizes of the bubbles and clusters.In the first part of the investigation, a collector was used but no frother. In these conditions, the bubble diameter was effectively controlled by the collector concentration, and it varied considerably. It was found that the sizes of clusters decreases with increasing shear rate at low impeller speeds, and at higher speeds the clusters are broken up into bubbles and particles and then re-form into clusters through collision. At even higher speeds the bubbles themselves are broken up into smaller bubbles that then re-form into small clusters. In the second part, frother was used at a concentration above the critical coalescence concentration, to control the bubble size, which remained essentially constant at this concentration. The bubbles were too small to be broken by the action of the impeller, so they always remained at the same size. In this case it was found that when the impeller speed was increased, two stages of formation were observed, the fragmentation and equilibrium stages. In the fragmentation stage, at low impeller speeds, the clusters were loose and filamentous, and as the energy input increases, they rupture and re-form. In the second stage, above a critical impeller speed, dense clusters formed whose size was relatively insensitive to the energy input. The cluster behaviour was modelled, based on concepts drawn from the related fields of floc breakup and bubble breakup. The clusters were modelled on the assumption that the bubbles were bridged by particles, so the cohesive strength was determined by the capillary force between the bubbles and the particles. Two different theories were investigated for the disruptive force from the turbulent liquid: the shear rate hypothesis of Camp and Stein (1943), and the turbulent fluctuation model arising from Kolmogorov’s theory of isotropic turbulence (Kolmogorov, 1941). It was found that neither method is applicable in the fragmentation stage. However, in the equilibrium stage, an equation derived from Camp and Stein’s theory was more realistic than that obtained using Kolmogrov’s equation. The results showed that in industrial flotation cells, where the power input is typically in the range 0.5 to 5 kW/m3, clusters that form will be in the equilibrium stage, and their sizes will range from 500 μm to 1 mm.
- Subject
- bubble clusters; minerals; turbulent shear
- Identifier
- http://hdl.handle.net/1959.13/1040877
- Identifier
- uon:13834
- Rights
- Copyright 2014 Zhihao Chen
- Language
- eng
- Full Text
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