- Title
- Flotation of coarse particles in the reflux flotation cell
- Creator
- Sutherland, Joshua Leigh
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Froth flotation is a separation process that has widespread application throughout the coal and minerals industry. However, flotation has historically been limited by the upper size of particles that can be floated due to hydrodynamic issues, such as turbulence. Conventional flotation cells require vigorous mixing to contact particles with bubbles; however the turbulence generated also promotes the detachment of increasingly larger particles. This presents issues in industry, where significant losses can occur due to oversize particles passing through worn classifying screens and into the flotation circuit. Coal particles up to 0.5 mm in diameter are generally able to be recovered using conventional flotation cells, with significant losses for coarser particle sizes. The upper size of floatable mineral particles is typically less than this, due to their higher density, and limited surface liberation. A single process that can float a much wider size range, i.e. particle diameters up to 2 mm, would therefore minimise the current losses due to screen wear, as well as potentially simplify some aspects of processing plant circuit design. This thesis investigates coarse particle flotation with coal particles up to a screened size of 2.8 mm using the Reflux Flotation Cell (RFC), a novel device being developed at the University of Newcastle, Australia. It was originally hypothesized that a fluidized bed of bubbles with a volume fraction of ~0.5, generated with high gas fluxes, would promote coarse particle recovery. It is emphasized that the fluidization involved here is inverted, hence in the opposite direction to that used by other new technologies. Due to the high concentration of nearby bubbles in the fluidized bed, any particles that detached from a bubble would therefore immediately encounter another bubble, and re-attach. Experiments revealed, however, another factor to be more significant. The experiments showed coarse particle recovery was highly effective below a gas flux of 0.5 cm/s, and significantly lower at gas fluxes beyond 0.5 cm/s. It was also observed that coarse particle recovery was relatively unaffected with the introduction of higher volumetric feed fluxes. Nevertheless, it remains plausible that high bubble concentrations generated by high feed fluxes at moderate gas fluxes would allow for higher probabilities of particle re-attachment. Two distinct investigations were undertaken in this thesis. The first investigation involved the flotation of coarse coal tracer particles up to a screened top-size of 2.0 mm. Tracer particles were added to the system individually, with the RFC operating at a specific hydrodynamic condition defined by the feed and gas fluxes. All particles were of a known size and were strongly hydrophobic having relative densities (compared to water) within the range 1.25 – 1.30. Coal of such density is known to be highly hydrophobic. The particles were also saturated by collector prior to each experiment. After adding numerous particles to the system one at a time, the particles eventually partitioned to either the product or reject. The partition number was defined by the ratio of the number of particles reporting to the product to the total number of particles added to the system, effectively the yield. The tracer particle experiments revealed a general trend of decreasing coarse particle yield with increasing gas flux; the highest yields being achieved at gas fluxes below 0.5 cm/s. At the same time, particle yields were observed to be independent of the feed flux. The second investigation applied the findings of the tracer particle study to the continuous steady-state flotation of feed slurries, sourced from industry, at different solids concentrations. While the approach used in the tracer particle study gave valuable insight into the flotation of coarse particles, it bore little resemblance to flotation as performed in industry. Hence, the processing of industrial feeds, containing a full size distribution of coal particles up to 2.8 mm in diameter, provided the opportunity to investigate the effects that particle-particle interactions had on coarse particle recovery. Feed solids concentrations of 1, 5 and 15 wt% were investigated at operating conditions identified in the tracer particle study to be ideal for coarse particle recovery. Results from the experiments were compared with two benchmarks: -0.125 mm particles were compared to the Tree Flotation Curve, while +0.125 mm particles were compared to a Float/Sink separation performed at a relative density of 1.6. High recoveries of all +0.125 mm particles, including the coarsest fractions, were achieved in the RFC at both 1 and 5 wt% solids concentrations. Similarly for -0.125 mm particles, the recoveries were comparable or better than the Tree Curve. It was obvious that particle-particle interactions had little to no effect at these lower feed concentrations. However, increasing the feed concentration to 15 wt% solids resulted in a decrease in coarse particle recovery. An experiment was performed at half the baseline throughput, as well as a pair of two-stage runs, all revealing similar combined recoveries. Further batch flotation tests confirmed that the decrease in coarse particle recovery was due to the natural limit of floatability of the particular feed used, which was considerably higher in head ash content than for previous experiments. However, the recovery of -0.125 mm particles continued to match or better the Tree Curve benchmark. Overall, the Reflux Flotation Cell was demonstrated to be effective in floating coarse particles using gas fluxes at or below 0.5 cm/s, irrespective of the volumetric feed flux. The universal decline in coarse particle recovery in the RFC at gas fluxes higher than 0.5 cm/s was concluded to be a result of bubble plume effects occurring at the exit of the downcomer.
- Subject
- coal; froth flotation; coarse particles; reflux flotation cell; fluidized bed
- Identifier
- http://hdl.handle.net/1959.13/1400485
- Identifier
- uon:34776
- Rights
- Copyright 2019 Joshua Leigh Sutherland
- Language
- eng
- Full Text
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