- Title
- Computer simulation of particle-bubble interactions using discrete element method
- Creator
- Gao, Ya
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2017
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Froth flotation, as one of the most important and widely used separation techniques in recovering valuable minerals from a slurry, has been widely studied. The key issue, plaguing the improvement of collection efficiency, lies in the limited understanding of the interaction between mineral particles and air bubbles. Indeed, the particle-bubble interaction is driven by the combined effect of numerous forces, for example gravitational, buoyancy, hydrodynamic, and surface forces, and thus is of great complexity. In this thesis, a joint approach of both numerical simulations and experimental tests was made to investigate the particle-bubble interactions in froth flotation, specifically the capture of hydrophobic particles by a bubble, with eventual goals to provide guidelines to improve the collection efficiency of the froth flotation. A numerical model within the framework of the three-dimensional Discrete Element Method (DEM) was proposed to simulate a representative sample system of the froth flotation, which consists of a stationary bubble located at the centre of the working space and hydrophobic particles nearby. The developed DEM model provides a microscopic description of the particle-bubble interaction in this sample system, and thus is able to clarify the mechanism involved. To verify the appropriateness of the developed three-dimensional DEM model, the simulation results were compared with theoretical predictions based on the Schulze theory under quiescent conditions, and fairly good agreement was obtained. Validations of the proposed model were also conducted against available experimental results regarding the motion of a particle sliding over the bubble surface. In this regard, the simulation results in respect of particle-bubble separation distance versus time also showed great agreement with experimental observations. Furthermore, the simulations could correctly predicted the time when the particle contacted the bubble. To highlight the effect of the interactions between attached particles on the particle-bubble aggregate stability, additional simulations were performed in which particle-particle repetitive collision and rebounding phenomena were observed but without the occurrence of the particle-bubble detachment due to the strong capillary force. The 3D DEM model was then employed to study the capture of hydrophobic particles by a single bubble. The hydrophobic force was calculated on the basis of a single exponential decay law that depends on two parameters, namely, a pre-exponential parameter Κ and a decay length λ. Numerical results indicated that when λ was less than 10 nm, the number of the particles collected was independent of the strength of the hydrophobic force, whereas, for values of λ within the range of 10-500 nm, the capture efficiency increased significantly with the strength of the hydrophobic force and λ. Additionally, simulation results demonstrated how the particle trajectory around the bubble was affected when the strength and activation range of the hydrophobic force were varied, and thus different particle collection was produced. The developed DEM model was further extended to consider the surface forces based on the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. A preliminary theoretical analysis of the surface forces were carried out in order to determine the possibility of particle capture. This analysis included the determination of the total XDLVO potential energy vs. separation distance curves. Five different behaviours with respect to the interaction potential were found: (1) monotonically increasing, (2) with a primary minimum, (3) with an energy barrier and a primary minimum, (4) with an energy barrier and second minimum, and (5) monotonically decreasing. It was found that the capture of the particle on the bubble surface was strongly dependent on characteristics of the interaction potential curve. For example, a curve with a primary minimum (2) can result in contactless “adhesion”. In the case of total XDLVO energy curve with an energy barrier, the approaching particle requires sufficient kinetic energy or gravitational force to overcome the energy barrier. Last but not least, a newly developed experimental device along with the developed DEM approach was utilised to investigate the impact of particle properties in terms of size and wettability on the attachment behaviours to liquid interfaces (e.g., air/water or oil/water). In the experimental tests, an individual stationary aniline droplet was made to be neutrally buoyant in salt solution. The interaction between particles and the aniline droplet were directly observed using a high speed digital camera. The interaction was controlled through variations in particle size and surface hydrophobicity. The results showed that the hydrophobicity of the solid particles is the overriding factor affecting the particle-droplet adhesion, while the particle size mainly determined the collision mechanism. Moreover, an apparent increase of attachment time was observed with increasing polar angle of the initial impact. The DEM simulation results again showed fairly good agreement with the experimental results. Although the findings are for a particle-oil droplet system, it could be readily extended to a particle-bubble system by the fact that the oil droplet and the bubble have many features in common. It is expected that the developed experimental setup has the potential to uncover changes in flotation behaviour or, in conjunction with other factors, such as reagent (collector) and pH, to predict flotation performance.
- Subject
- flotation; discrete element method; extended DLVO theory; particle-bubble interaction; oil/water interface; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1353396
- Identifier
- uon:31091
- Rights
- Copyright 2017 Ya Gao
- Language
- eng
- Full Text
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