(388b) Selective Collection of Fine Particles by Water Drops

Froth flotation is normally used
to separate the valuable particles from the worthless gangue by taking
advantage of the differences in physical properties such as hydrophobicity and
hydrophilicity. Although it is a remarkably effective technology for recovering
and concentrating hydrophobic particles, there remains a considerable scope for
significant improvement. The recovery of fine hydrophobic particles in
conventional flotation is very low compared to coarser particle recovery,
though due to gravitational forces recovery decreases when the particles become
too large. The exceedingly fine hydrophobic particles do not have sufficient
momentum to collide with the air bubbles. These particles tend to follow the streamlines,
avoiding collision (Figure 1(a)). Even when these particles deviate from the
streamlines and collide, they may still have insufficient kinetic energy to
rupture the air-liquid interface around the air bubble. These problems arise
because of the viscous drag of water. Furthermore, fine hydrophilic particles,
which contaminate the product, are difficult to remove because the liquid
drains slowly from the froth. The hydrophobic product also retains significant
moisture even after filtration.

Recently two research groups examined the potential of an
alternative method of fine particle beneficiation. Tran et al. (2009) initiated
the preliminary experimental studies on selective separation of ultra-fine
particles less than 10 microns using film flotation in a planar trough. Galvin
et al. (2010) separated fine coal less than 90 microns using individual drops
of water fixed in position under a gas dispersion of fine coal and glass
ballotini.

This paper, which builds upon the
previous work, is concerned with a new approach for selectively separating
particles on the basis of their wetting properties. Here, a
dry particle dispersion interacts with falling water drops (Figure
1(b)). The paper also outlines the basic laboratory systems used to test this
concept. Hydrophilic particles pass directly through the gas-liquid interface
of the falling drops, while hydrophobic particles choose to remain with the gas
phase or adhere to the outer surface of the water drops. The underflow
collection efficiency achieved for a range of particles interacting with a
given flux of falling drops was determined experimentally.

The central process of selective
collection is the wetting and particle adhesion. Thomas Young (1804) and Gibbs
(1873) provided the foundation used today to describe the interfacial phenomena
involved in wetting and adhesion. The basic thermodynamic criterion for the drop-particle
attachment involves a finite contact angle at the line formed by the
intersection of three phases. The contact angle is a quantitative
measure of the wetting of a solid by a liquid (Chau,
2009) and indicates the affinity between the water and the solid (Nicol, 1992). If the contact angle has a higher value
(measured through the water), the solid is described as hydrophobic and if a
relatively small angle, is described as hydrophilic.

Figure 1: Comparison
between particles interacting with bubbles in water and drops in air (a)
Conventional Flotation (b) Proposed Selective Collection.

.

Two
experimental approaches have been used to test this concept. Preliminary tests
have been conducted using single water drops to test the validity of the
phenomena at the drop level. Secondly, a laboratory scale experimental system
has been designed and developed for detailed investigations of the concept at
the multiple drop level.

Dry particles
of glass ballotini and coal (75 to 90 microns size range) were used in single
drop experiments. The first set of single drop experiments was conducted using
a vibratory table under a hanging water drop to energise the system of
particles. For glass ballotini (Figure 2(a)), most of the particles exhibited
engulfment into the water drop.  The
second set of single drop experiments was conducted by sprinkling the particles
above the hanging water drops. Video microscopy was used to monitor the
phenomena. When a mixture of ballotini and coal was used, a considerable amount
of ballotini particles was observed (Figure 2(b)) to again penetrate the drop,
while there was some evidence of fine coal at the drop surface. These single
drop experiments clearly demonstrate selectivity in the process; that is, the
ballotini is engulfed into the bulk of the drop while the few coal particles
that do interact with the drop remain at the interface.

Figure 2:  (a) Pendent drop of water saturated with glass
ballotini.  (b) Pendent drop covered with
coal while ballotini filled the inside of the drop

An experimental system was then
designed and fabricated to test this novel concept at the multi-drop level in
the laboratory. A schematic diagram of the initial prototype is shown in Figure
3. The system consists of a feed column, water distribution chamber mounted on
a collision chamber and an air suction pump. Falling drops are surrounded by
gas with a viscosity 100 times lower than the viscosity of water. The capture
efficiency of the hydrophilic particles by the water drops should increase
significantly due to the low viscosity of the air. In other words, inertial
forces should dominate, hence ultrafine particles should engage with the
gas-liquid interface. Hydrophilic particles covering a broad size range should
pass through the interface with little or no resistance. Hydrophobic particles
should fail to pass through the interface, and hence should remain with the gas
phase or attach to the outside of the drops. Thus the proposed approach should
result in improved separation kinetics.

Figure 3: Schematic
illustration of modified experimental system.

The feed was projected into the
collision chamber horizontally while water drops were released from the water
distribution chamber vertically. The particles were fed to the system in
multiple 10g portions within a time period of 30 seconds between each portion,
while maintaining a constant water drop size and flux.

The dispersion then collided with
the water drops, with the hydrophilic particles (silica) engulfed. Water
collected at the base of the chamber, and then drained into the underflow
collection tank. The recovery of the particles in the underflow was calculated
relative to the total feed that passed through the experimental system. In all
of these experiments the velocity of the air stream and hence the velocity of
the particles was kept constant.

The results from these
experiments produced a linear correlation between the recovery of the particles
in the underflow and the water flux. The underflow recovery for two types of
particles provided a measure of the selectivity of the process. The selectivity
of different samples of glass ballotini (glass ballotini as received from the
commercial pack and glass ballotini treated with Extran® to clean the surface) were tested initially. The
results were found to be reproducible, hence the experimental system was found
to be reliable. Interestingly the experimental data indicated some selectivity
between the ballotini samples, as received, versus
those that were cleaned, though the difference was slight (Figure 4). 

Figure 4: Underflow recoveries of two different ballotini samples, as received, and cleaned.

The underflow recovery of
hydrophilic silica particles were observed to be in the same range as that of
cleaned ballotini. For hydrophobic coal particles
relatively low recoveries were obtained. This work proved that there is
selectivity between hydrophilic silica and hydrophobic coal. These results are
supported by the experimental data shown in Figure 5.

Figure 5: Underflow recoveries of silica and coal showing clear,
but limited, selectivity.

One key advantage of the technique
described in this paper is that the hydrophobic product obtained is dry.
Overall, there was clear selectivity measured, dependent on the particle size
and other surface properties of the material. Further work is needed to study
other variables such as the air flow velocity, and water chemistry, as well as
mixtures of hydrophobic and hydrophilic particles. Given the limited
selectivity observed, however, it is unlikely that this concept will develop
into a new technology competitive with conventional froth flotation.

Acknowledgements

The authors thank the Australian Coal Association Research Program
(ACARP) for financial support for this work.

References:

Adamson, A.W., Gast, A.P., 1997. Physical chemistry of surfaces, sixth ed. John Wiley & Sons,
New York.

Tran, D.N.H, Whitby, C.P., Fornasiero, D., Ralston, J. 2010, Selective
separation of very fine particles at a planar air-water interface,
International Journal of  Mineral
Processing 94, 35-42

Galvin, K.P., Webber, G.B., Mason, M.,
Liyanaarachchi, Inverse Flotation-A New Method of Fine Particle Beneficiation,
Chemeca 2010, Adelaide, 2010

Young, T.1805, An essay on the cohesion of fluid, Philosophical
Transactions, Royal Society Publish.

Chau, T.T., 2008. A review of techniques for
measurement of contact angles and their applicability on mineral surfaces,
Minerals Engineering 22 (2009) 213-219

Nicol, S.,1992. The Principles
of Coal
Preparation, Australian
Coal Preparation Society, Newcastle, NSW, p. 232