- Title
- Optimising the hydrodynamic performance of the REFLUX Flotation Cell
- Creator
- Cole, Matthew James
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2021
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- This thesis examined the hydrodynamics underpinning the interplay between grade, recovery and feed throughput in the novel REFLUXTM Flotation Cell (RFC). The RFC consists of a series of parallel inclined channels located below a vertical chamber. The inclined channels enhance the rate of segregation of the low density gas bubbles from the liquid phase via the Boycott effect (Boycott, 1920), which permits the use of increased gas and liquid fluxes far beyond those typical of conventional flotation technologies, and thus provides a broader hydrodynamic window of operation. Further, the enhanced bubble-liquid segregation also permits operation of the system beyond the so-called flooding condition, where the interface separating the pulp and froth zones is lost, and is instead replaced by a bubbly mixture having a bubble volume fraction of ~0.5. Operation without a conventional froth zone, instead forming a concentrated bubbly zone of high bubble holdup within the RFC, facilitates the uniform application and permeation of wash water via downwards fluidization, and thus enables the effective rejection of hydraulically entrained slimes from reporting to the overflow. One-dimensional drift flux theory was utilised to provide a theoretical description of two-phase gas-liquid flow, and hence details the hydrodynamic constraints governing flotation. The hydrodynamic interplay between the inlet feed, gas and fluidization (wash) water fluxes was investigated in order to evaluate the performance constraints of a flotation cell operating as a single stage separator. For this thesis, fine, well-liberated coal slurries were utilised as the flotation feeds, sampled from industrial tailings streams. Further, high dosages of diesel collector were added to the feeds to improve the homogeneity of the surface hydrophobicity of the different coal macerals. Thus, these coal slurries provided representative ‘binary’ flotation feeds composed of hydrophobic coal particles and hydrophilic gangue mineral matter. Here, the feeds were considered model binary flotation systems, which was supported by the results of Coal Grain Analysis which demonstrated the high degree of coal maceral liberation, along with the presence of well-liberated mineral matter. Thus, throughout this thesis the product grade and recovery were inferred from the product ash and combustible recovery respectively, and the flotation results were benchmarked against recovery-ash curves generated from the results of Coal Grain Analysis and the tree flotation procedure. The experimental study began with an investigation of the impact of the liquid bias flux (jb) on the relationship between recovery and grade, with the volumetric feed flux limited to the order of 1 cm/s. This volumetric feed flux represents that typical of conventional flotation devices (Fuerstenau et al., 2007), ensuring operation below the flooding constraint. In this study, however, flooding was induced through application of downwards fluidization water. As the liquid bias flux was increased from neutral to positive, representing the transition from zero net motion of liquid to a downwards flow of fluidization water, the product grade (inferred by the product ash value) was demonstrated to improve. This result clearly highlighted the enhanced desliming of ultrafine particles from the overflow concentrate at stronger liquid biases. However, above the bias flux of 1.0 cm/s, no further benefit to the product grade was observed, suggesting that full desliming of the product had been achieved, arguably by jb = 0.6 cm/s. As the liquid bias flux was increased, the combustible recovery was shown to gradually decrease, clearly representing an inverse relationship between recovery and grade. However, the recovery for a given product ash was shown to converge to the maximum theoretical value outlined by Coal Grain Analysis, indicating virtually full recovery for a given grade. These results demonstrated the establishment of liquid bias fluxes and permeability far superior to that attainable using conventional flotation systems. The influence of the overflow gas volume fraction on recovery and grade was also assessed at a volumetric feed flux approximating 1 cm/s. The overflow gas volume fraction is defined as the ratio of gas to the total gas and liquid in the overflow discharging the RFC. This parameter is thus different in magnitude to the internal bubble volume fraction within the RFC (Dickinson & Galvin, 2014). Bubble coalescence was induced at overflow gas volume fractions exceeding 0.90, resulting in hydrophobic particle detachment and thus a decrease in recovery. Combustible recoveries further declined as the gas fraction approached unity. However, selective hydrophobic recovery was observed as the product ash value simultaneously declined with recovery. As the overflow gas volume fraction reduced from ~0.85, and particularly below 0.80, the bubble concentration within the RFC was visually observed to decrease under the imposed 0.6 cm/s positive bias flux, likely hindering the effectiveness of the fluidization wash water in desliming the concentrate. Hence, the product ash increased relatively linearly at a fixed recovery as the overflow gas volume fraction reduced. Further, this study, for the first time, examined the potential process simplification arising from merging two previously distinct operating modes of the RFC into a single stage separation process. Thus, the elevated volumetric feed rates per unit of vessel cross-sectional area characteristic of “Fast Flotation” (Dickinson et al., 2015) were combined with the establishment of a positive liquid bias flux, a critical requirement in “Desliming Flotation” (Dickinson & Galvin, 2014). Here, volumetric feed fluxes effectively covering an order of magnitude increase were delivered to the RFC, whilst establishing a positive bias flux to ensure desliming of the product. The grade (product ash) and recovery were maintained over a comprehensive range of volumetric feed fluxes, up to the order of 7 cm/s, representing up to a 7-fold increase over conventional flotation devices. A clear divergence in the typical grade-recovery relationship as a function of throughput was therefore observed. Importantly, these results were also shown to converge towards the theoretical limits defined by Coal Grain Analysis, and were far superior to the tree flotation curve. Finally, the influence of feed solids concentration on the separation performance was assessed when deploying the RFC as a single stage separator. A strong correlation between the imposed gas flux and combustible recovery was evident as the feed solids concentration was increased. By scaling the gas flux with the inlet flux of hydrophobic particles, restrictions in recovery arising from kinetic limitations could be avoided, and thus the recovery could be preserved. Estimations of the bubble surface coverage with attached hydrophobic particles yielded values ranging 6-18% for the suite of experiments, with an approximate asymptotic surface coverage of 58% for the given flotation feed. Overall, the results indicated an inverse correlation between bubble surface coverage and recovery. Further, the effectiveness of the concentrate desliming was assessed as the feed solids concentration was increased approximately 16-fold, including via the addition of ultrafine (400G) silica to the coal suspension. The results exhibited consistently high grades (low product ashes), showing the strong downwards permeation of fluidization water (jb ≈ 0.7 cm/s) prevented the hydraulic entrainment of ultrafine particles from entering the product.
- Subject
- fine particle flotation; Reflux Flotation Cell (RFC); hydrodynamics; fluidization; grade; recovery; throughput
- Identifier
- http://hdl.handle.net/1959.13/1453238
- Identifier
- uon:44629
- Rights
- Copyright 2021 Matthew James Cole
- Language
- eng
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