The effect of physical parameters on the rupture of bubble films in two-phase foams
Date
1999-11
Authors
Journal Title
Journal ISSN
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Publisher
Stellenbosch : Stellenbosch University
Abstract
ENGLISH ABSTRACT: The effects of various physical parameters on the rupture of bubble films in two-phase
foams were investigated in order to develop a better understanding of the
behaviour of coarse particles in the froth phase of a novel flotation cell. This novel
flotation technique is based on the fact that coarse particles, if they are selectively
rendered hydrophobic by conditioning, would act as bubble film breakers. If the
feed was introduced onto the surface of the froth, such particles would settle
through the froth under gravity to be recovered as an underflow (concentrate)
product, while the gangue would be supported by the bubble films and be recovered
as a float (tailings) product.
The efficiency of this technique - reverse froth flotation - depends on the interaction
between various characteristics of particles and the froth. In order to simulate the
experimentally observed trends, and hence investigate the various mechanisms
qualitatively, a fundamental model of these interactions was developed. Various
particle properties were taken into account, including surface properties, shape, size
and density. To account for the changing nature of the froth at different positions in
the cell, the model predicts the trajectory of a particle over discrete time events.
This was accomplished by calculating bubble flow streamlines and modelling the
bubble size, thickness of bubble films, air hold-up and bubble velocity at any point
on the streamline.
The experimental results showed that the behaviour of particles (within the size
range tested) in the froth phase of the cell is primarily dependent on the mass of a
particle. In general, the higher the mass, the steeper the trajectory of the particle in
the froth, i.e. an increase in particle mass results in an increased recovery to the
concentrate. The contact angle on the particle surface has only a secondary
influence on the overall particle trajectory, in that an increase in the equilibrium
contact angle will result in an increased recovery. However, the particle contact
angle has very little influence on the behaviour of large, high-density particles, as
well as small, low-density particles.
Particles will therefore only separate on the basis of contact angle as long as their
mass is between an upper and lower critical value. Any particle with a mass greater
that the critical value will fall through the froth irrespective of the contact angle.
Similarly, the upward force component acting on a particle with mass less than the
lower critical value will dominate the force balance. The particle will therefore
remain supported by the froth, irrespective of the particle contact angle and bubble
film rupture time. For particles within these mass limits, the effect of the contact
angle increases with a decreased mass. It was further concluded that these mass limits are dependent on the operating
conditions of the cell as well as the particle shape. The particle shape influences
the mass to cross-sectional surface area ratio (M/Ao). Where particles therefore
. have the same mass, the M/Ao ratio would govern the particle trajectory. The higher
the M/Ao ratio, the more particles would be recovered to the concentrate, while a
decrease in the M/Ao ratio would result in flatter particle trajectories in the froth,
thereby increasing the probability of a particle reporting to the tailings. The model
provided a unique understanding of the interrelationship between the various
parameters which would not have been gained to the same extent by alternative
modelling methods.
Two potential industrial applications for the reverse froth flotation process were
evaluated. As a coarse particle flotation technique for sulphide bearing ores, it was
found that Xanthate pre-conditioning of the ore results in concentrating the sulphur
bearing particles to the concentrate. By increasing the Xanthate addition, the
relative % mass recovery as well as % sulphur recovery increases. This increase in
mass and sulphur recovery, however, is not linear. The greatest concentration
effect was obtained by the initial Xanthate addition. An economically optimum
Xanthate addition point therefore exists. These results were very promising in terms
of finding a coarse particle flotation technique for the pre-concentration of sulphide
bearing minerals. HOl(Vever, it is recognised that several potential problems might
exist with the technique.
The results further indicated that the reverse froth flotation process is not suitable
for the replacement of grease belts for fine (-3 mm) diamond recovery. The major
concern is the high potential losses of valuable material to the tailings. These
losses are mainly due to the fact that the major separation process in the reverse
froth flotation cell is based on particle mass. The surface properties of the particle
account only for a secondary separation process.
Description
Thesis (Ph.D. (Ing.)) -- University of Stellenbosch, 1999.
Keywords
Separation (Technology), Flotation, Dissertations -- Metallurgical engineering, Reverse froth flotation