- Title
- Film flotation of airborne particles impacting a gas-liquid interface
- Creator
- Liu, Dongmei
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2012
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Flotation is a well-known separation process widely used in many industrial processes. The fundamental principle of conventional flotation is that the hydrophobic particles are captured by the rising bubbles and carried into the froth concentrate. Flotation recovery is governed by surface properties, size of both particles and bubbles, and energy dissipation within the cell. Often, high energy input is needed to suspend the solids and to propagate bubble-particle interaction. The detrimental effect of operating under these conditions is that the probability of particle detachment is increased resulting in a lowering of flotation recovery. The focus of this PhD study was to develop a flotation environment whereby energy requirements for solids suspension, bubble generation, and bubble-particle contact are decoupled and optimized independently. A consideration of such a system has led to the notion of flotation of airborne particles, whereby falling particles are impacted upon a free liquid surface and depending upon their surface properties, will either penetrate through or be captured at the gas-liquid interface. Generally, the approach is referred to as film flotation, and has the advantage that energy requirements for bubble generation and interaction of the particles with the gas-liquid interface are no longer required. A mathematical model was developed that described the behaviour of a particle impacting with a gas-liquid interface. The model was based on a system which included the receiving liquid volume and the falling particle that was travelling at a critical velocity, vc. Beyond this value the particle would penetrate entirely through the gas-liquid interface. An energy balance was applied to the system at the critical condition, which could be solved to obtain (1) the critical velocity and corresponding critical fall height of the particle; (2) velocity profile of the particle as it passed through the liquid; and (3) resultant profile of the gas-liquid interface. The model was based on the physical and surface properties of both the liquid and particle and accounted for the presence of a confining solid boundary. Moreover, the model also incorporated the dynamic behaviour of the advancing contact angle. An extensive experimental program was carried out as part of the study. The critical impact velocity for glass beads, polypropylene spheres, non-spherical seeds, and coal and silica particles were determined by dropping them from different heights and recording their interaction with the liquid volume. The liquids included Milli-Q water, NaCl, sucrose and surfactant (CTAB) solutions. The velocities of both the particle and the gas-liquid interface (three phase contact line) moving over the surface of the particle were recorded using high speed video for each experimental system. The high speed imaging also allowed the temporal cavity profile and advancing contact angle to be measured. Experimental studies showed that the critical impact velocity increased with (i) an increase in hydrophobicity and surface tension and (ii) a decrease in particle size and ratio of particle-liquid density. The motions of the particle and three phase contact line (TPCL) were measured from the high speed video images. It was found that the velocities of both the particle and the TPCL could be fitted as a function of particle position relative to the interface by a simple quadratic relationship using only the dimensionless parameter, m and n. The relative velocity difference was then able to be used to estimate the advancing contact angle, by applying a combined molecular-hydrodynamic approach, which changes magnitude as the particle moves through the liquid. Finally, it was found that the critical fall height could be successfully modelled by using m and n only and including the influence of advancing contact angle. It was found that for a particle penetrating into a confined volume of liquid that the critical velocity increased with increasing static contact angle of the container wall. The critical velocity remained relatively unchanged with particle-to-liquid volume fraction less than 0.05. The liquid depth also seemed to have little influence on vc for the different liquids tested. However, for a given particle diameter vc was found to decrease with decreasing container diameter, especially when the particle-to-container diameter ratio increased beyond about 0.2. The experimental system was also modelled using the Young-Laplace equation using both static and advancing contact angle measurements for both the particle and container surfaces. The model predictions were generally in good agreement with the experimental observations, including showing increase in particle penetration depth with increasing diameter and meniscus profiles, both at the particle impact point and the wall of the container. The predictions were improved when the advancing contact angle was used, especially for the smaller diameter containers where there was more liquid motion. A simple model was also presented, based on the Young-Laplace equation, which could provide an estimation of the critical (minimum) diameter of container required so that the cavity profile generated by the impacting particle is unlikely to be influenced by the container walls. The estimated critical diameter was consistent with experimental observations. Float fraction experiments for non-spherical particles (black sesame seeds) followed a similar trend to those of spherical particles, in that at a higher impact velocity the particle was more likely to report to sinks stream. The orientation of particle also had a major effect on the critical velocity, with a minimum value occurring when the particle impacted the interface on its sharpest point. The spherical particle impact model used to determine the critical fall height was still applicable, however, provided the impact orientation of the non-spherical was taken into account. For a two particle system it was found that there was an increase in the normalised critical velocity as the normalised separation distance was decreased. However, this increase is likely to be less than about 10 percent. It seemed that there was no effect of the normalised separation distance of greater than 0.043 on the critical impact velocity of particles. Particle image velocimetry measurements were undertaken in an attempt to quantify the liquid velocities generated by the moving particle. The experimental observations highlighted the fluid motion and provided quantitative evidence that it was reasonable to assume that the liquid possessed negligible kinetic energy due to its very low velocity, at the (critical) point where the impacting particle attained zero downward velocity. It was also observed that at the critical point there was still motion at the TPCL. For this reason, the modelling analysis should include the advancing contact angle. Finally, the model was applied to the design of a continuous film flotation device for used for the separation of a coal-silica mixture. The fall height was designed so that the impact velocities for the coal and silica particles were below and above their critical velocities, respectively, it was found that greater than 95 percent of coal recovery could be achieved. However, both grade and recovery were reduced with increased loading of floating particles on the free surface. In conclusion, the research to date has highlighted the potential of the new approach in providing a low energy alternative to conventional flotation. The successful separation of coal-silica mixtures indicates the possibility for separating ore particles and gangue particles by choosing an appropriate fall height based on the mathematical model developed in the thesis. The experimental rig includes a belt from which the particle mixtures can be projected onto the liquid surface with an ability of adjusting fall height, and the equipment could be modified or redesigned to improve the efficiency so that it could be used in industrial flotation, e.g. widening the belt, adding more belts on the same surface, speeding up the belt and liquid surface velocity.
- Subject
- mineral flotation; film flotation; critical velocity; dynamic contact angle; meniscus profile; particle separation; three-phase contact line
- Identifier
- http://hdl.handle.net/1959.13/936141
- Identifier
- uon:12225
- Rights
- Copyright 2012 Dongmei Liu
- Language
- eng
- Full Text
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