dewatering of fine coal using hyperbaric centrifugation
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Dewatering of Fine Coal UsingHyperbaric CentrifugationRamazan Asmatulu a , Gerald H. Luttrell a & Roe-Hoan Yoon aa Center for Advanced Separation Technologies ,Virginia Polytechnic Institute and State University ,Blacksburg, Virginia, USAPublished online: 25 Feb 2010.
To cite this article: Ramazan Asmatulu , Gerald H. Luttrell & Roe-Hoan Yoon (2005)Dewatering of Fine Coal Using Hyperbaric Centrifugation, Coal Preparation, 25:3,117-127, DOI: 10.1080/07349340590962766
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DEWATERING OF FINE COAL USING HYPERBARIC
CENTRIFUGATION
RAMAZAN ASMATULUGERALD H. LUTTRELLROE-HOAN YOON
Center for Advanced Separation Technologies,Virginia Polytechnic Institute and State University,Blacksburg, Virginia, USA
Many coal preparation plants are forced to discard their fine coal
because of the inability of existing technologies to reduce the moist-
ure content of this product to an acceptable level. In an effort to
overcome this problem, a new mechanical dewatering method has
been developed that combines centrifugation with pressure filtration.
The process, which may be referred to as hyperbaric centrifugation,
is capable of producing a drier product than can be achieved using
either filtration or centrifugation alone. The test data obtained from
batch experiments show that the new method can reduce cake moist-
ures to 10% or below for many fine coal product streams.
Keywords: Dewatering; Filtration; Centrifugation; Hyperbaric
INTRODUCTION
The dewatering of fine particles is widely considered to be the most dif-
ficult operation in coal preparation plants. The difficulty is due to the
very high specific surface area of fine coal. The lack of an effective
Received October 12, 2004; accepted February 2, 2005.
Address correspondence to Roe-Hoan Yoon, Center for Advanced Separation
Technologies, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061,
USA. E-Mail: [email protected]
Coal Preparation, 25: 117–127, 2005
Copyright Q Taylor & Francis Inc.
ISSN: 0734-9343 print
DOI: 10.1080/07349340590962766
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technology for moisture removal has forced many coal producers to
discard their fine coal streams [1–4]. In the United States alone, this
problem has resulted in the discharge of approximately 2–3 billion tons
of fine coal into abandoned ponds and 500–800 million tons in active
ponds over the years. On a yearly basis, U.S. coal producers continue
to discard approximately 30–40 million tons of fresh fine coal to the
active ponds. This represents a loss of valuable natural resources, loss
of profit for coal producers, and creation of significant environmental
concerns [5–10].
Centrifugal dewatering devices are currently the most widely used
processes for fine coal dewatering in the United States. Basket-type cen-
trifuges are typically used to dewater particles larger than approximately
1mm, while finer particles are more commonly dewatered by screen-
bowl centrifuges [11]. The latter is capable of providing considerably
lower moistures than vacuum filters. However, the use of screen-bowl
centrifuges may entail substantial losses of fine coal compared to fil-
tration processes. Recent field studies suggest that the weight recovery
of particles finer than about 0.045mm (325 mesh) is typically only about
50% for screen-bowl centrifuges. Therefore, new technologies are
needed that can achieve high recoveries of fine coal at a low moisture
content.
In light of the difficulties associated with the dewatering of fine coal,
a new mechanical dewatering process known as hyperbaric centrifuga-
tion has been developed. The novel feature of this device is the use of
air pressure to increase the rate of dewatering during centrifugation. In
the present study, batch tests were conducted using a laboratory proto-
type of this new process. The objective of these tests was (i) to demon-
strate the enhanced dewatering performance of this new technology
using a variety of different coal samples and (ii) to evaluate the effects
of operating parameters on moisture removal.
The dewatering method described in this article differs from those
developed by Veal et al. [12–13], in which a gas stream such as air is
injected into the bed of particles during centrifugation. In the latter,
the turbulent flow created by the gas flow strips the water from the sur-
face of the particles. This technique may be useful for the particles in the
range of 0.5 to 30mm that are dewatered in basket centrifuges, but not
for fine coal products. Furthermore, the stream of gas is injected into
an open space; therefore, it cannot significantly increase the pressure
drop across the bed of particles.
118 R. ASMATULU ET AL.
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EXPERIMENTAL
Apparatus
Figure 1 shows a schematic of the laboratory-scale hyperbaric centrifugal
filter used in the present work. The centrifuge vessel was constructed
from stainless steel with a diameter of 90mm and a height of 75mm.
The vessel was lined with filter cloth designed to fit the inside contour
of the vessel. The side walls of the vessel were perforated with equally
spaced 3.0, 2.4, and 1.6mm diameter holes to allow for cake drainage.
The vessel was mounted vertically atop a rotor shaft and held in place
by means of a locking bolt. The rotational speed of the rotor was
controlled using a variable speed motor. In order to pressurize the rotat-
ing vessel, a bearing=seal connector was used to attach an external
compressed air line to the center of the cover lid. The compressed air
line was equipped with an on-off valve, an airflow meter, and a pressure
gauge so that the air flow could be easily monitored and controlled.
Samples
Four coal samples were collected from operating coal preparation plants
for use in the centrifugal hyperbaric filter tests. The first two samples,
which were taken from a plant processing Pittsburgh seam coal, included
Figure 1. Simplified schematic of the centrifugal hyperbaric filter.
HYPERBARIC CENTRIFUGATION 119
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a minus 0.6mm (28 mesh) conventional flotation product and a minus
1mm disc filter feed. The third sample consisted of a minus 1mm
screen-bowl feed from a processing plant. The fourth and final sample
was obtained as a minus 0.15mm (100 mesh) flotation concentrate from
a column flotation circuit at a pond reclaim facility. All of the samples
were shipped to the laboratory in slurry form for immediate testing in
the centrifugal hyperbaric filter.
Procedure
All tests were performed by operating the centrifuge in a batch mode.
Each coal sample was prepared for testing by thickening the as-received
slurry to 60–70% solids by gravity filtration with a large filter funnel. The
thickened sample was pasted against the filter cloth prior to rotating the
vessel at a desired rotational speed for a preset period of time. After each
run, the cake was removed from the vessel, weighed, dried overnight at
105�C in a convection oven and then weighed again to establish the
percent moisture by weight of the filter cake. Test variables examined
in this study included rotational speed (G-force), air pressure, solids
loading (cake thickness), and spin (centrifugation) time. The rotational
speed of the rotor was monitored in each test run using a hand-held
optical tachometer. The centrifugal force (G) was calculated from the
rotational speed using the following relationship:
G ¼ rx2
g½1�
in which r is the inside radius of the vessel and g is the standard gravi-
tational acceleration. The test unit was typically operated at centrifugal
forces up to 2,500 times that of gravity and at air pressures of up to
400 kPa. The cake thickness, which varied in the 10–20mm range, was
carefully measured after each test run using a caliper scale.
RESULTS AND DISCUSSION
Comparison of Dewatering Methods
In order to demonstrate the combined impact of air pressure and centrifu-
gation, test runs were conducted (i) with air pressure only, (ii) with centri-
fugation only, and (iii) with air pressure and centrifugation. Table 1 shows
the results obtained from these tests for the minus 0.6mm (28 mesh) froth
120 R. ASMATULU ET AL.
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flotation product from the Pittsburgh seam. Each series of tests were
conducted as a function of drying cycle (or spin) time using a solids load-
ing that provided a 15mm cake thickness. The test runs conducted at
100 kPa air pressure in the absence of centrifugal force produced cake
moistures in the range of 23.8 to 27.5% by weight, depending on the
drying cycle times employed. Likewise, the tests conducted at 2,000G
without air pressure produced cake moistures that dropped from 24.4 to
21.0% as the spin time increased from 30 to 120 sec. However, the cake
moistures obtained by applying both the air pressure (100 kPa) and cen-
trifugal force (2,000G) were much lower and ranged from 14.2 to
10.6%. At a common spin time of 60 sec., the three sets of tests produced
filter cakes with total moisture contents of 25.8, 22.6, and 12.9%, respect-
ively. Thus, these results show that the combined use of compressed air
and centrifugation greatly increased the total moisture reduction. The
improvement can be attributed to the increased pressure drop across the
filter cake provided by the air pressure.
Table 2 shows a similar set of test data comparing the various dewater-
ing methods for the minus 1.18mm (14 mesh) sample of screen-bowl feed
from the Eagle seam. The tests conducted using air pressure alone were
able to reduce the total cake moisture from 28.9 to 24.3% by increasing
the drying cycle time from 15 to 120 sec. The tests performed over the
same range of spin times using centrifugal force alone were able to reduce
the moisture from 25.2 to 22.3%. Thus, the use of air pressure alone or
centrifugation alone was unable to reduce the moisture to below 20%.
On the other hand, the combined use of air pressure and centrifugal force
Table 1. Comparison of dewatering tests conducted on a minus 0.6mm (28 mesh)
Pittsburgh seam coal
Cake moisture (%)
Drying cycle
or spin time (sec.)
Air pressure alone
(100 kPa)
Centrifugal force alone
(2,000G)
Centrifugal force &
air pressure
0 82.21 38.72 38.72
30 27.5 24.4 14.2
60 25.8 22.6 12.9
120 23.8 21.0 10.6
1Amount of water in slurry.2Solids content of samples fed to the basket after thickening.
HYPERBARIC CENTRIFUGATION 121
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dropped the moisture content down to 12.3% after just 15 sec. of spin
time. The moisture was further reduced to single digit values (i.e., 9.6%
moisture) after a spin time of 60 sec. A longer spin time of 120 sec. did
not substantially improve dewatering performance, providing a cake moist-
ure just 0.3 percentage points lower (i.e., 9.3% moisture).
Effect of Air Pressure
Table 3 shows the results of the centrifugal hyperbaric filtration tests
conducted as a function of air pressure using the minus 0.6mm (28 mesh)
flotation product from the Pittsburgh seam. The air pressure was varied
in the range of 50–200 kPa, while the centrifugal force was kept constant
Table 2. Comparison of dewatering tests conducted on a 14 mesh� 0 Eagle seam coal
Cake moisture (%)
Drying cycle
or spin time (sec.)
Air pressure
alone (200 kPa)
Centrifugal force
alone (2,000G)
Centrifugal force &
air pressure
0 86.31 36.52 35.62
15 28.9 25.2 12.3
30 25.7 23.1 10.2
60 24.6 22.5 9.6
120 24.3 22.3 9.3
1Amount of water in slurry.2Solids content of samples fed to the basket after thickening.
Table 3. Centrifugal hyperbaric filtration tests conducted on a minus 0.6mm (28 mesh)
Pittsburgh seam coal 2,000G
Cake moisture (%)
Air pressure (kPa)
Spin time (sec.) Without air pressure 50 100 200
0 36.51 36.51 36.51 36.51
30 23.1 18.3 14.2 13.2
60 22.5 16.3 12.9 10.6
120 22.3 15.1 10.6 9.1
1Solids content of samples fed to the basket after thickening.
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at 2,000G. A constant solids loading was used to maintain a cake thick-
ness of 11.5mm for all tests. As expected, the cake moisture decreased
with increasing spin time and air pressure. At the shortest spin time of
30 sec., an increase in air pressure from 50 kPa to 200 kPa reduced the
moisture content from 18.3 to 13.2%. At the longest spin time of
120 sec., the cake moisture was reduced from 15.1 to 9.1% by increasing
the air pressure over the same range. These results further illustrate the
dramatic impact of air pressure on the dewatering performance of a
centrifugal filter.
Effect of Centrifugal Force
Table 4 shows the results obtained from a series of centrifugal hyperbaric
filtration tests conducted by varying the centrifugal force from 1,500G to
2,500G. The tests were conducted using the 1.18mm� 0 filter feed sam-
ple from the Pittsburgh seam. Two series of tests were performed, i.e.,
one without applying air pressure and the other with 300 kPa of air press-
ure. The spin time was varied from 30 sec. to 60 sec. and the cake thick-
ness was kept constant at 11.5mm. The test results again indicate that
centrifugal dewatering was not effective without using compressed air.
A total moisture content of 20% or less could not be achieved even after
120 sec. of spin time at 2,500G. In contrast, very low moisture contents
of 11–12% could be obtained after just 30 sec. of spin time when using
300 kPa of air pressure together with the centrifugal force. Dewatering
performance did improve slightly in both cases by increasing the
Table 4. Centrifugal hyperbaric filtration tests conducted on a minus 1.18mm (14 mesh)
Pittsburgh seam coal
Cake moisture (% wt)
No air pressure With 300 kPa air pressure
Spin time (sec.) 1,500G 2,000G 2,500G 1,500G 2,000G 2,500G
0 42.31 42.31 42.31 42.31 42.31 42.31
30 24.3 23.0 22.2 12.1 11.4 11.2
60 23.9 22.1 21.1 10.3 9.8 10.0
120 23.2 21.1 20.9 8.4 8.2 8.1
1Solids content of samples fed to the basket after thickening.
HYPERBARIC CENTRIFUGATION 123
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centrifugal force. However, the improvement was relatively small com-
pared to the enhanced dewatering performance obtained by adding air
pressure during centrifugal filtration. These results suggest that an
increase in air pressure has a much larger impact on dewatering perform-
ance than an increase in centrifugal force under higher G forces.
Effect of Particle Size
Several series of laboratory tests were conducted to establish the effec-
tiveness of the centrifugal hyperbaric filter for dewatering very fine coal
samples. To evaluate this capability, the minus 0.6mm (28 mesh) flo-
tation product from the Pittsburgh seam was screened at 0.074mm
(200 mesh), and the screen underflow was used in the laboratory tests.
Table 5 shows the results obtained as a function of air pressure and spin
time for a constant 2,000G force and 14mm cake thickness. The moist-
ure reductions achieved using the centrifugal force alone (without air
pressure) was relatively poor due to the very fine particle size. After
30 sec. of spin time, the moisture was reduced from 42.3% down to only
37.1%. The moisture reduction did not improve significantly after longer
spin times of up to 120 sec. When air pressure was applied, however, the
cake moisture was substantially reduced. The extent of moisture
reduction achieved by the combined application of compressed air and
centrifugal force increased with increasing air pressure and spin time.
At 400 kPa of air pressure and 150 sec. spin time, the cake moisture
was reduced to as low as 17.8% for the minus 0.074mm (200 mesh)
product.
Table 5. Centrifugal hyperbaric filtration tests conducted on a fine minus 0.074mm
(200 mesh) Pittsburgh seam coal at 2,000G
Cake moisture (% wt)
Air pressure (kPa)
Spin time (sec.) Without air pressure 100 200 300 400
0 42.31 47.91 47.91 47.91 47.91
30 37.1 31.9 27.6 24.5 22.5
60 36.9 31.2 24.6 21.2 19.7
120 36.6 29.7 23.0 19.1 17.8
1Solids content of samples fed to the basket after thickening.
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Another series of dewatering tests was conducted using sized sam-
ples prepared by wet screening the minus 0.15mm (100 mesh) column
flotation product from the pond reclaim facility. In this case, the sample
was sized at 0.038mm (400 mesh) and the oversize and undersize pro-
ducts were separately subjected to laboratory dewatering tests. The data
obtained using the minus 0.038mm (400 mesh) undersize fraction is
shown in Table 6. The tests were conducted at 2,500G and 10mm cake
thickness. When no air was used, the cake moisture was as high as 39.1%
after 120 sec. of spin time and 350 kPa of air pressure. In the presence of
air, however, the moisture was reduced to 21.6% under the same operat-
ing conditions. This moisture content is considered to be very good
considering the extremely fine size of this particular sample.
Table 7 shows the results of the dewatering tests conducted using the
0.15� 0.038mm (100� 400 mesh) oversize fraction of the column
flotation product sized at 400 mesh. These tests were conducted at
2,500G and 15mm cake thickness. As shown, the tests conducted with-
out air pressure provided moisture contents for the 0.15� 0.038mm
(100� 400 mesh) product of just 25.4% even after 120 sec. of spin time.
However, the moisture reduction improved dramatically as the air press-
ure was increased from 50 to 250 kPa. At 120 sec. of spin time and
250 kPa of air pressure, the moisture was decreased to as low as 4.6%
for the deslimed sample. This result demonstrates that very low levels
of moisture can be achieved for deslimed coal samples by combining
air pressure and centrifugal force during dewatering.
Table 6. Centrifugal hyperbaric filtration tests conducted on a fine minus 0.038mm (400
mesh) Pittsburgh seam coal at 2,500G
Cake moisture (% wt)
Air pressure (kPa)
Spin time (sec.) Without air pressure 50 150 250 350
0 47.91 47.91 47.91 47.91 47.91
30 39.6 37.3 35.7 31.9 27.2
60 39.3 36.8 34.4 29.0 24.8
120 39.1 36.2 33.6 26.1 21.6
1Solids content of samples fed to the basket after thickening.
HYPERBARIC CENTRIFUGATION 125
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CONCLUSIONS
A new mechanical dewatering method has been developed that combines
centrifugation with pressure filtration. This innovative process, which
may be referred to as hyperbaric centrifugation, is capable of substan-
tially improving the removal of surface moisture from fine coal products.
Laboratory tests conducted using a batch unit showed that hyperbaric
centrifugation provides substantially better moisture reductions than
with air pressure or centrifugation alone. For the clean coal products
evaluated in this study, moisture contents of 10% or lower have been
obtained by this technique for particle sizes typically treated in industrial
fine coal dewatering circuits.
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Table 7. Centrifugal hyperbaric filtration tests conducted on a deslimed 0.15� 0.038mm
(100� 400 mesh) column flotation product at 2,500G
Cake moisture (%)
Air pressure (kPa)
Spin time (sec.) Without air pressure 50 150 250
0 41.11 41.11 41.11 41.11
30 27.5 12.2 10.0 9.1
60 26.2 10.9 8.0 7.1
120 25.4 8.0 6.3 4.6
1Solids content of samples fed to the basket after thickening.
126 R. ASMATULU ET AL.
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HYPERBARIC CENTRIFUGATION 127
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