- Title
- The effect of turbulence on bubble-particle interaction in flotation
- Creator
- Wang, Ai
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2022
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Mineral separation based on flotation is a complex multiphase multi-step physico-chemical process. In a given flotation system, efficient physical contact between the bubbles and particles is the first step to successful recovery of valuable minerals. Several models are reported in the literature to estimate the efficiency of this physical contact process based on an idealised flow field to simply the problem. Nevertheless, presence of turbulence in any practical flotation system inevitably renders such phase interactions as a stochastic process introducing a distribution of collision efficiency as a function of local flow conditions. This research study aimed to contribute to this less explored area of flotation by investigating the mechanistic effect of turbulence on bubble-particle interactions with specific objectives to quantify collision efficiency distribution; change in bubble rise velocity due to hydrophobic particle attachment to bubble surface and their effect on the overall recovery. Firstly, the effect of turbulence on collision efficiency was numerically studied for a single-bubble-multiple particles system under typical flotation conditions comprising bubble diameter dB~ 1 mm; bubble Reynolds number ReB ~ 230; particle diameter dP ~ 30 to 100 µm; and turbulence intensity Ti ~ 4% - 20%. A theoretical model was developed to estimate bubble-particle collision efficiency incorporating the turbulent dispersion behaviour assuming homogeneous turbulence condition. Also developed was an Eulerian-Lagrangian CFD modelling framework incorporating a large eddy simulation (LES) turbulence model and two-way coupling between the particles and fluid through drag and buoyancy force exchange and the random walk model for momentum exchange between particles and surrounding eddies. The CFD model showed chaotic trajectories of particles in the turbulent flow field and produced a distribution of the collision efficiency parameter depending on the local flow conditions. It was noted that particles injected along the bubble centre line had the highest probability for collision. Both the theoretical model and CFD model had comparable prediction for lower turbulence intensity cases (<7%), however a deviation was apparent in the higher turbulence intensity cases. Secondly, the developed CFD model was applied to simulate the interaction behaviour between a bubble and a particle swarm for a range of solid concentrations (~ 0.39 – 6.16 vol %) with the same range of turbulence intensity value. The maximum collision efficiency along the injecting radius decreased with the increasing solid concentration as more particles were dispersed away from the bubble centre at high solid concentrations. In general, the overall collision efficiency was noted to decrease with increasing solid concentration due to the greater lateral dispersion of particles. An optimal turbulence intensity of ~ 7% was obtained corresponding to maximum overall collision efficiency. Turbulence intensity however was found to have no significant effect on averaged local solid concentration. Thirdly, the decrease in bubble rise velocity due to collection of hydrophobic particles at the interface was quantified experimentally. Highspeed imaging was performed to determine the rise velocity of millimetric size particle-laden bubbles (dB ~ 2.76 to 3.34 mm) with bubble surface loading (BSL) varying from 0 to 0.6 both in the absence and presence of surfactant. An image processing methodology was developed to quantify the bubble surface loading as bubble rises. Bubble rise velocity was observed to decrease with bubble surface loading, but this declining trend was less steep in the presence of surfactant. In absence of surfactant, bubble surface loading contributed significantly to surface immobility. A correction factor to Schiller-Naumann drag coefficient model was proposed accounting for the bubble surface loading. To obtain a more realistic estimation of collision efficiency, earlier developed CFD model was extended to multi-bubble-particles system. The predicted collision efficiency was higher than that in single-bubble domain and the optimal collision efficiency occurred at turbulence intensity ~ 20% compared to ~7% found earlier in single-bubble case. A novel flotation recovery model based on first-order kinetics was finally proposed which included the CFD model predicted collision efficiency and the developed drag correction factor model for particle-laden bubbles. In this recovery model, a maximum bubble surface loading of 0.142 was determined by fitting the model-predicted bubble velocity with available experimental data. With this maximum bubble surface loading constraint, the recovery model predicted two distinct regimes - a loading regime in the early flotation period and a saturated regime wherein the bubble loading capability was entirely exhausted. The model predicted recovery value was compared to a laboratory scale coal flotation test and reasonable agreement was obtained.
- Subject
- turbulence dispersion; bubble-particle collision efficiency; flotation; recovery; CFD modelling; bubble rise velocity; bubble surface loading; drag coefficient
- Identifier
- http://hdl.handle.net/1959.13/1504640
- Identifier
- uon:55560
- Rights
- Copyright 2022 Ai Wang
- Language
- eng
- Full Text
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| View Details Download | ATTACHMENT01 | Thesis | 7 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 541 KB | Adobe Acrobat PDF | View Details Download |