- Title
- Influence of turbulence on bubble-particle detachment in flotation
- Creator
- Wang, Guichao
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2016
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- It is widely known that the rate of capture of particles in the froth flotation process is a function of particle size. Mineral particles that are in the size range 50 to 150 µm in diameter float relatively quickly, while recovery decreases with particles that are smaller and larger. It has been hypothesised that the reason for the decrease observed with coarse particles, is the relative magnitudes of the forces of attachment and detachment. Particles are held on the surfaces of bubbles by a capillary force exerted at the three-phase contact line, which is linearly related to the length of the line and hence to the diameter of the particle. Detachment takes place by a body force acting on a particle, whether by gravity or through a centrifugal or inertial force arising from a change in direction of the particles, that is related to the particle’s mass and hence to the cube of its diameter. As the diameter increases, the magnitude of the detaching force increases much more rapidly relative to the force of attraction, and when the former exceeds the latter, the particle will detach. For bubble-particle detachment in the turbulent field, it is hypothesised that the aggregate becomes trapped in the centre of an eddy, and the particle moves with the liquid as it rotates about the centre of the eddy. If the centrifugal force is higher than the force due to surface tension at the bubble’s surface, detachment occurs. Due to the difficulty in carrying out experiments where the motion of bubble-particle aggregates in in a turbulent field can be observed, classical bubble-particle detachment theory still remains hypothetical. The complex behaviour of the rotating eddy and the bubble-particle aggregates make it difficult to analyse the hydrodynamic forces involved in the bubble-particle detachment process. This appears to be the main obstacle to understanding the mechanisms of bubble-particle detachment in a turbulent field. This thesis explores the mechanisms of bubble-particle detachment in a turbulent field. It starts with the simulation of a particle sliding down the surface of a stationary bubble in a quiescent liquid. Instantaneous forces were calculated to analyse the value of Bond number in relation to bubble-particle detachment. The mechanism of bubble-particle detachment is similar to the scenario where a bubble detaches from a solid surface. Preliminary experiments were undertaken to characterize the influence of turbulent liquid motion on the detachment of a bubble in the presence of oscillating grid turbulence. It was found from the instantaneous velocity field that localised conditions (energy dissipation rate) were crucial to bubble detachment. The model was used to predict the bubble size at detachment, and the best agreement with experiment was found when the localised energy dissipation rate was included. Bubble-particle detachment, where the particle was positioned, was studied with a high-speed particle image velocimetry (PIV) together with laser induced fluorescence (LIF) technology. Flow structures in the instantaneous velocity field around the detaching bubble were analysed and eddy development leading to bubble detachment was identified. Kinetic energy and energy dissipation rates were high in the vicinity of the detaching bubble. Work was extended to study freely moving bubble-particle aggregates in a turbulent field. A new experimental setup was constructed to generate a flow field where a rotating eddy was confined in a square cavity and bubble-particle aggregates were directed into the eddy. For the first time, Schulze’s theory of bubble-particle detachment due to centrifugal movement in the rotating eddy was investigated and verified. Other bubble-particle detachment modes were also identified. In conclusion, for the first time, the centrifugal movement of particles on the surfaces of rotating bubbles inside an eddy is described. The motion of a particle attached to a bubble surface has been observed and quantified experimentally. Rotational speeds of attached particles as high as 200 cycles per second were observed in the rotating eddy. It was also observed that bubble-particle detachment in the turbulent field is a stochastic process, and there is no single dominant mechanism. A number of different detachment mechanisms were identified, including: The classical model in which a bubble rotates about its axis in an eddy, giving rise to a centrifugal force on the attached particle. ; Irregular trajectories of the bubbles, so that particles having sufficient inertia in a given direction, detach from the bubble when it rapidly changes direction. ; Oscillations of the surface of the bubble, often violent and unsymmetrical, which attached particles cannot follow due to their inertia
- Subject
- turbulence; flotation; detachment
- Identifier
- http://hdl.handle.net/1959.13/1322502
- Identifier
- uon:24592
- Rights
- Copyright 2016 Guichao Wang
- Language
- eng
- Full Text
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | ATTACHMENT01 | Thesis | 10 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 195 KB | Adobe Acrobat PDF | View Details Download |