- Title
- Selective collection of fine particles by water drops
- Creator
- Liyanaarachchi, Kathika Rashanthi
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2015
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Fine particle processing has long been a challenge in the minerals processing industry. Ores often have to be ground to less than 100 μm in size in order to liberate the valuable components. Even in the coal industry, where there is minimal comminution, a considerable fraction of the run-of-mine coal is less than 100 μm in size. Hence, it can be difficult to efficiently recover these fine particles using existing beneficiation methods, such as flotation. In the past few decades, there has been on-going effort focussed on developing methods to separate finer particles in an efficient way, with an increasing interest in approaches that involve dry particles. This Thesis is concerned with the potential for achieving selective separations through contacting particles with water drops. More specifically, the objective was to achieve selective capture of the hydrophilic particles via the water phase, leaving the hydrophobic particles in a dry state. Two different approaches were investigated through the establishment of novel experimental systems. The first approach involved a system consisting of falling water drops passing through a gas-suspension of particles. Conceptually, this approach is the opposite of conventional flotation. A gaseous dispersion of particles was used instead of a water based dispersion of particles, and a system of falling water drops was used instead of a system of rising air bubbles. Thus the particles travelled horizontally with the airflow, colliding with drops via the gas side of the gas-liquid interface of the falling drops. The hydrophobic particles either adhered at the surface or were deflected, while the hydrophilic particles were engulfed completely, settling through to the base of the drop. Ideally the hydrophobic particles passed through the collection zone, reporting in a dry form. Thus, in the context of the coal industry, a high calorific dry product should be generated. Due to the low viscosity of air compared to water, the collision efficiency of the particles with the water drops should increase significantly compared to conventional flotation. Thus viscous forces become far less significant, allowing the inertial forces to dominate, even for ultrafine particles. Energy consumption should then be much lower. A new laboratory-scale experimental system was then designed and set up to further investigate this approach. The development of this facility represented a major outcome of the study, the goal being to control the steady flow of the particles and the drops to achieve reproducible and meaningful data sets. A number of design modifications were needed to achieve these requirements. The experimental system consisted of a feed column, water distribution chamber mounted over a collision chamber, underflow collection system, and dry particle cyclone stage. The airflow was driven by an air suction pump. Hydrophilic and hydrophobic particles covering a broad size range (8-116 μm) and spherical and irregular shapes were used in the experimental program to investigate the selective collection efficiency of this system. Results, expressed in terms of the recovery to underflow, were found to be highly reproducible, to within 3%. A simple mathematical model was also developed to describe the interception of the particles with the water drops. Particles were assumed to be collected if there was any overlap in the trajectories of the particles and the drops. In one version, the particle velocity was assumed to be much higher than the falling speed of the drops, while in a second version the particle velocity was assumed to be much smaller than the drop speed. These models were then validated using the Discrete Element Method, demonstrating the accuracy of the model in these two extremes. Moreover, the DEM approach allowed intermediate, less extreme, conditions to be examined. Thus the modelling approach provided an appreciation of the principle parameters governing the interception, most notably the water flux, droplet diameter, and active length of the collection zone. Clearly, the model did not consider the subtle effects associated with the particle-drop adhesion, dynamic wetting, disengagement from the fluid streamlines, and particle shape effects. These more subtle effects were manifested through the experimental work. The recovery of the particles in the underflow was calculated relative to the total feed that passed through the experimental system. There was a linear dependence between the particle recovery in the underflow and the water flux, with a slope dependent on the relative selectivity between the different particle species. The analysis was examined using a water flux of up to 0.0167 m3/(m2s). The results agreed well with the analytical and DEM models within the aforementioned water flux range. With the second approach, electrostatic formation of liquid marbles was investigated as an alternative mechanism for collecting hydrophilic glass ballotini particles into a water drop. In these experiments curious electrostatic phenomena were observed, resulting in the formation of particle agglomerates. Fine hydrophilic and hydrophobic particles on a charged substrate were seen to intermittently jump across a gap of several millimetres to penetrate the surface of a hanging water drop. The substrate was wrapped in Teflon (PTFE) tape, which provided an ideal substrate for frictional charging. Within around 300 milliseconds, the drop filled up like a 'sack of marbles'. Charge and volume data analysis of these experiments revealed that there is a tendency of the particles to jump to the drop intermittently in sudden 'cascades' or 'avalanches', rather than through gradual and incremental transfer as the substrate was moved toward the hanging drop. Although the distance between the substrate and the drop was important in affecting particle lift, the substrate speed had relatively little effect on the onset of the particle ‘avalanche’. This interesting phenomenon was also observed using different organic liquids such as glycerine and hexane. Overall, it was apparent that a limited selectivity between hydrophilic and hydrophobic particles can be achieved through direct interaction between a gaseous dispersion of the particles and a system of water drops. However, the level of selectivity was deemed to be insufficient for justifying the development of a successful industrial separation. The data sets generated from this work can be useful in any follow-up work in improving our understanding of the precise particle-drop interception, and final interaction. Electrostatic forces were clearly powerful across significant distances, and offered the potential for developing innovative methods for processing fine, dry particles. This approach may also prove useful in the formation of both hydrophilic and hydrophobic liquid marbles.
- Subject
- fine particles; drops; dry processing; electrostatics; liquid marbles
- Identifier
- http://hdl.handle.net/1959.13/1317961
- Identifier
- uon:23546
- Rights
- Copyright 2015 Kathika Rashanthi Liyanaarachchi
- Language
- eng
- Full Text
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