Preparation of Coal Agglomerates Using a Water-in-Oil Emulsion

Preparation of Coal Agglomerates Using a
Water-in-Oil Emulsion
K. van Netten 1 , R. Moreno-Atanasio 1 and K.P. Galvin 1
1 Centre for Advanced Particle Processing and Transport
School of Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia
Email: kim.vannetten@uon.edu.au
Abstract— The conventional oil agglomeration process
utilises an immiscible organic liquid, such as diesel, to
agglomerate coal in an aqueous suspension of coal and
mineral particles. With the application of agitation, the
organic liquid selectively wets the surface of the coal and
subsequently binds the coal particles together to form
agglomerates. During this process, the mineral matter
remains finely dispersed and hence the coal agglomerates
can be retrieved by passing the material over a screen.
This process achieves high quality beneficiation of the fine
coal, and very effective dewatering of the product. While
this is a remarkably effective process the technology has
not been adopted because of the very poor economics, due
to the high cost of the oil. In this work an alternative
immiscible binding agent consisting of a high internal
phase (HIP) water-in-oil emulsion was used. This binding
agent provides the interfacial functionality of the oil, with
the space filling functionality provided by the stabilised
water drops of the emulsion. Thus far, the HIP emulsion
has successfully agglomerated fine coal in an aqueous
suspension and, in addition, has done so at a lower oil
requirement than when pure diesel oil was used. A two-tothree
fold reduction in organic liquid consumption was
achieved when the HIP emulsion was used in the place of
pure diesel oil. Further reductions in the organic liquid
consumption are anticipated in the future if improved
methods of preparing the emulsion are realised.
Keywords-oil agglomeration; beneficiation; coal; high
internal phase emulsion
I. INTRODUCTION
The beneficiation of fine coal has been a long standing
problem in the coal processing industry because the size of the
particles means they are difficult and, in most cases,
uneconomical to process. In the past, this fine portion of the
coal feed was simply discarded as waste and held indefinitely
in tailings ponds. More recently, however, a greater focus has
been placed on increasing efficiencies and decreasing waste in
coal processing and utilisation. Spiral concentrators and froth
flotation machines have been used with some success to treat
fine coal particles although both technologies become less
effective when a significant amount of ultra-fine, or slimes,
material (< 75 µm) is present in the feed [1, 2, 3]. Both
technologies are also associated with high dewatering costs as
they require large volumes of water to process coal feeds [4].
The problems encountered in the processing of fine coal are
undeniably due to the small size of the particles and, therefore,
the limited effect gravity has on them. As a direct solution to
this problem, selective agglomeration, or oil agglomeration of
the fine coal particles has been investigated numerous times in
the past. Selective agglomeration is the process by which an
immiscible liquid, usually oil, is added to an aqueous
suspension of coal and mineral particles [5]. Upon the
agitation of the suspension the oil preferentially wets and
agglomerates the hydrophobic coal particles, leaving the
unwanted mineral matter remains finely dispersed in the
suspension. The coal can then be easily retrieved by passing
the agglomerated suspension over a screen. The process of oil
agglomeration, as applied to coal, has been shown to achieve
excellent recovery of combustible material with a reasonable
ash content. The process also has the ability to handle ultrafine
coal feeds. While these attributes make the process sound
highly promising and beneficial, there has never been a fullscale,
long term, implementation of this process. The
dominant reason for this lack of development is the poor
economics of the process due to the high cost of the oil
required.
This study revisited the process of oil agglomeration to test
a novel modification that aimed to reduce the amount of oil
required. The novel modification was the use of a high internal
phase (HIP) water-in-oil emulsion as the immiscible liquid
binder, instead of the pure oil conventionally used. A high
internal phase emulsion is an emulsion in which the internal,
disperse phase has a phase volume of greater than 0.74 [6].
This is an unusual configuration as it is normally the
continuous phase of an emulsion that has the higher phase
volume. The HIP emulsion was considered as an alternative
agglomerating agent as it provided the interfacial functionality
of the oil, with the space filling functionality provided by the
stabilised water drops of the emulsion.
As this is a novel modification, this study was a preliminary
investigation into the functionality of the HIP emulsion as a
binder. Initially, the ability of the HIP emulsion to form coal
agglomerates was tested. As part of this test, the growth of the
agglomerates with time was considered by plotting the size
distribution of a given coal sample for different times during

the agitation of the suspension. The change in the volume
weighted mean diameter with time was also measured.
Finally, a number of agglomeration experiments were carried
out in which the performance of three different binders was
compared: pure diesel oil, a HIP water-in-oil emulsion and a
mixture of diesel and emulsifier. It was in these tests that the
benefit of using the HIP emulsion as the binder, as opposed to
pure oil, could be elucidated.
II.
EXPERIMENTALS
A. Preparation of the High Internal Phase Emulsion
The high internal phase emulsion was prepared prior to
carrying out the agglomeration experiments. In all
experiments the emulsion was prepared such that the internal
dispersed water phase made up 0.85 of the total mass of the
emulsion. The continuous, oil, phase made up the remainder
of the emulsion and consisted of automotive diesel oil and
50% by weight emulsifier. The emulsifier was an
ethanolamine derivative of polyisobutylene. To make the HIP
emulsion, the water was added to the oil and emulsifier during
continuous mixing supplied by a Russell Hobbs Hand Mixer
(350 watt motor) set a maximum speed. To ensure phase
inversion did not occur during the creation of the emulsion the
water was added to the mixture in increments. Each increment
of water was smaller than the original volume of oil in the
emulsion and the previous increment was fully dispersed
before the next was added. The mixture began as a transparent
orange liquid and progressively turned lighter as more water
was added, finally forming an opaque, white and viscous
substance. To further refine (disperse the internal water into
smaller and more uniform droplets) the emulsion was placed
under a Heidolph RZR 2102 Control agitator for 40 min at
2000 rpm.
agglomerated product remained on top while the finely
dispersed mineral matter remained in the flow of the water and
formed the reject under the screen. The agglomerated product
was submitted to a thorough wash to ensure any mineral
matter remaining on the surface of the agglomerates was
removed. The product and reject samples were then dried in an
oven (set at 110 °C )and weighed as to calculate the yield
achieved. The yield was defined as the fraction of the feed
sample that was retained as coal agglomerates on top of the
screen. It should be noted that when the growth of the
agglomerates was under investigation the agglomerated
suspension was not screened. Instead the entire suspension
was dried and the size distribution of the agglomerated sample
was then measured by laser diffraction. The dispersant used
was a 50 wt% ethanol-water mixture.
III.
A. Growth of Agglomerates
RESULTS AND DISCUSSIONS
In the first experiments, the ability of the HIP emulsion to
agglomerate a fine coal feed was investigated. A number of
agglomeration experiments were carried and allowed to run
for a number of time intervals up to and beyond the time at
which inversion of the suspension occurred. In each
experiment a constant and sufficient amount of HIP emulsion
was used such that the feed could be completely agglomerated
if there was sufficient time. As a first measure of the
agglomeration taking place the change in the volume weighted
mean diameter of the sample was considered. Fig. 1 presents
the change in the volume weighted mean diameter of the
sample as a function of agitation time in the agglomeration
experiments.
B. Selective Agglomeration
A lower hunter valley coking coal with a low ash content
was chosen for the experiments. The feed was wet screened
such that the top size of the sample was 260 µm. In the
agglomeration experiments a glass jug held 500 ml of the
prepared coal sample with a solids concentration of 7.5 wt%.
A Waring variable speed 1 L blender set at 12 000 rpm was
then used to disperse the suspension. The immiscible binding
liquid (either the diesel or the HIP emulsion) was then added
in a single dose and the agitation was continued at 12 000 rpm
until inversion of the suspension was achieved. Inversion of
the suspension was easily visible and was designated to be the
point at which the slurry changes from being black to a sandy
yellow colour. Inversion of the suspension occurred when
there was sufficient binder present to fully agglomerate all of
the carbonaceous material, and effectively separate it from the
finely dispersed mineral matter.
At the completion of the experiment the agglomerated
suspension was transferred to a sieve (mesh size = 425 µm)
for sizing. The mesh size was selected such that the aperture
size was about two times larger than the largest particle in the
feed originally. By selecting a screen of this size the
Figure 1: The change in the volume weighted mean diameter with time
measured by laser diffraction in 50 wt% ethanol. The liquid binder used
was the HIP emulsion.

It can be seen in Fig. 1 that as the sample is agitated the mean
size of the particles increases from an initial size of 127 µm up
until a peak which occurs at about 430 µm at about 40 s. After
this time the mean particle size drops and plateaus at about
300 µm. From observations of the experiments it was clear
that during the initial mixing, from 0 – 40 s, all of the finely
dispersed hydrophobic coal particles were being agglomerated
as the suspension gradually turned from black to a sandy
yellow and reached complete suspension inversion by about
40 s. No further changes in the appearance of the suspension
were observed after the point of inversion.
Fig. 1 clearly shows that the HIP emulsion possesses the
capability to agglomerate fine coal in an aqueous suspension
as there is a significant increase in the mean size of the
particles. Further to this, Fig. 2 presents the size distribution of
the fine coal feed sample initially and at five different stages
during the mixing; 10 s, 30 s, 45 s, 90 s and 120 s.
On inspection of Fig. 2, it can be seen that the initial
sample contained a high proportion of fine material (< 100
µm). However as the agglomeration progressed this fine
material was collected and formed into larger agglomerates.
At the time the of inversion, about 40 s, there remains only a
small portion of fine material and the main peak in the size
distribution plot has shifted to the right and narrowed,
indicating that the majority of the agglomerates are much
larger than the particles in the original sample. Once again, it
can be seen that continuation of the agitation beyond the
inversion point (40 s) has led to a decrease in agglomerate size
as the peak has shifted to the left for the samples taken at 90 s
and 120 s. However, it should be noted that it has also
narrowed and the amount of fine particles remaining in the
system has been reduced again.
On consideration of Fig. 1 and 2 it may be suggested that
by agitating the agglomerated suspension beyond the point at
which the suspension inverted the agglomerates were sheared
and reworked to an equilibrium size (as defined by the shear
force imparted by the mixer). This decrease, in the overall
agglomerate size, with further agitation time may also be due
to consolidation of the agglomerates and perhaps the
agglomeration process was not yet complete. A more thorough
investigation into the kinetics of agglomeration in which the
HIP emulsion is used as the liquid binder would reveal with
greater clarity the mechanisms at work in this system.
B. Performance Comparison of Different Binding Liquids
In the previous section it was shown that the HIP emulsion
can indeed agglomerate fine coal if there was sufficient time
and an adequate amount of binding liquid was added.
However, it was the purpose of this study to investigate
whether the HIP emulsion can agglomerate coal with a lower
oil requirement than if pure oil was used. Therefore in the next
stage of this study, a number of experiments were carried out
to compare the ability of the HIP emulsion and pure diesel to
agglomerate coal at a given level of oil addition. In traditional
oil agglomeration 10 – 20 wt% (dry coal feed basis) oil is
usually required to fully agglomerate and process a coal feed
and it was this figure that was to be reduced through the use of
the HIP emulsion. For completeness, a third binding liquid
was also tested; a mixture of diesel and emulsifier.
Figure 2: The change in the size distribution of the coal sample at different stages during the mixing measured by laser
diffraction in 50 wt% ethanol. The liquid binder used was the HIP emulsion. Note logarithmic scale for particle size.

Fig. 3 presents the results from these experiments and it shows
the yield achieved as a function of the organic liquid addition.
As aforementioned, the yield is defined as the fraction of the
feed sample that is retained as coal agglomerates on top of the
screen. The abscissa is presented as organic liquid addition to
allow for a fair comparison between the different binders. In
the case of pure oil, 100% of the liquid used is an organic
liquid, however in the case of the HIP emulsion only a small
fraction of the liquid is organic and the remainder is water.
The organic liquid fraction in the HIP emulsion consists of
emulsifier and the diesel oil.
As depicted in Fig. 3, all three binders exhibit an increase in
yield with an increase in organic liquid addition until a certain
point after which a plateau occurs. It is at this plateau that
sufficient organic liquid has been added to completely
agglomerate the coal and form strong, discrete agglomerates.
The plateau can be seen to occur at an organic liquid addition
of approximately 6 wt% for the HIP emulsion and 15 wt% for
both the pure diesel and the diesel and emulsifier mixture.
This close similarity between the diesel and the diesel and
emulsifier mixture is not surprising as they are both pure
organic liquids. Indeed, [5] described experiments in which
oils of different densities and viscosities achieved similar
recoveries in the oil agglomeration process if an appropriate
agitation time was used for each oil.
Evidence for the enhanced ability of the HIP emulsion to
agglomerate fine coal in suspension can also be seen in Fig. 3.
This evidence is based on the fact that for a given yield, the
HIP emulsion requires 2 - 3 times less organic liquid than
when a pure oil is used. While this result is encouraging, it is
believed that the HIP emulsion has not reached its full
potential in that further reductions in oil consumption are
conceivably possible. It was considered that a HIP emulsion
with a water fraction of 0.85 (as used in these experiments)
could achieve at least a 6 fold reduction if the emulsion was
perfectly dispersed and the oil in the emulsion was used to the
full extent. One explanation put forward for the results
obtained in this study is that the emulsion is being degraded
during the agglomeration process. If this is the case then
further refinement (reduction of the internal droplet diameter)
of the emulsion may enhance its stability and improve the
results observed here.
Figure 3: The yield obtained for the three different binding liquids tested. Diesel and the diesel and emulsifier mixture comprise entirely of
organic liquid, whereas the HIP emulsion has only a small fraction of organic liquid (emulsifier and diesel) and the remainder is water.

IV.
CONCLUSIONS
When considering all the experimental results
presented here it is clear that the HIP emulsion has the
capability to agglomerate fine coal using the traditional oil
agglomeration process. Analysis of the size distribution of the
coal during the agglomeration process showed that the fine
particles (< 100 µm) were successfully collected and formed
into agglomerates that were mostly bigger than the original top
size of the feed. Evidence of the enhanced ability of the HIP
emulsion to agglomerate at a lower oil requirement was also
seen in these results. A two - three fold reduction in oil
consumption was observed when the HIP emulsion was used
as the binder, as compared to the performance of pure diesel
oil. This result was encouraging, however it is believed that
the HIP emulsion is not yet being used to its full potential as
degradation of the emulsion may be occurring during the
agglomeration process. It has been speculated that further
reductions in oil consumption may be achieved in the future if
the emulsion can be made in a more stable and refined way.
ACKNOWLEDGMENT
The financial support from ACARP for this project is greatly
appreciated.
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