dewatering of fine coal using hyperbaric centrifugation

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This article was downloaded by: [University of Utah] On: 09 October 2014, At: 19:03 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Coal Preparation Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/gcop19 Dewatering of Fine Coal Using Hyperbaric Centrifugation Ramazan Asmatulu a , Gerald H. Luttrell a & Roe- Hoan Yoon a a Center for Advanced Separation Technologies , Virginia Polytechnic Institute and State University , Blacksburg, Virginia, USA Published 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 To link to this article: http://dx.doi.org/10.1080/07349340590962766 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content.

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Page 1: Dewatering of Fine Coal Using Hyperbaric Centrifugation

This article was downloaded by: [University of Utah]On: 09 October 2014, At: 19:03Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,UK

Coal PreparationPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/gcop19

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

To link to this article: http://dx.doi.org/10.1080/07349340590962766

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all theinformation (the “Content”) contained in the publications on our platform.However, Taylor & Francis, our agents, and our licensors make norepresentations or warranties whatsoever as to the accuracy, completeness,or suitability for any purpose of the Content. Any opinions and viewsexpressed in this publication are the opinions and views of the authors, andare not the views of or endorsed by Taylor & Francis. The accuracy of theContent should not be relied upon and should be independently verified withprimary sources of information. Taylor and Francis shall not be liable for anylosses, actions, claims, proceedings, demands, costs, expenses, damages,and other liabilities whatsoever or howsoever caused arising directly orindirectly in connection with, in relation to or arising out of the use of theContent.

Page 2: Dewatering of Fine Coal Using Hyperbaric Centrifugation

This article may be used for research, teaching, and private study purposes.Any substantial or systematic reproduction, redistribution, reselling, loan,sub-licensing, systematic supply, or distribution in any form to anyone isexpressly forbidden. Terms & Conditions of access and use can be found athttp://www.tandfonline.com/page/terms-and-conditions

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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.

122 R. ASMATULU ET AL.

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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.

124 R. ASMATULU ET AL.

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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.

REFERENCES

1. G. R. Couch, Advanced Coal Cleaning Technology, IEA Coal Research,

Report IEACR=44, Gemini House, United Kingdom, Dec. 1991.

2. R.-H. Yoon and C. I. Basilio, Chemical-Mechanical Dewatering, U. S. Patent

No. 5,670,056, 1997.

3. R.-H. Yoon, R. Asmatulu, and G. H. Luttrell, Development of Novel Fine

Dewatering Aids, Proceedings International Coal Preparation Conference,

Lexington, Kentucky, 113–122, May 1–4, 2001.

4. R.-H. Yoon, R. Asmatulu, and G. H. Luttrell, Technical and Economical

Benefits of Using Advanced Cleaning Technology, Proceedings United

States-China Clean Energy Technology Forum, Beijing, China, Aug. 28,

2001–Sept. 1, 2001.

5. N. C. Lockhart and C. J. Veal, Coal Dewatering: Australian R&D Trends,

Coal Preparation, Vol. 17, pp. 5–24 (1996).

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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6. B. P. Singh, The Influence of Surface Phenomena on the Dewatering of Fine

Clean Coal, Filtration and Separation, Vol. 34, pp. 159–163 (1997).

7. A. Rushton and M. Spear, Centrifugal Filtration and Separation, Filtration

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8. B. Basim and R.-H. Yoon, Dewatering Fine Coal using Novel Methods, SME

Annual Meeting, Orlando, Florida, March 9–11, 1998.

9. L. B. Svarovsky, Solid-Liquid Separation, 3rd ed., Butterworths Publishing,

London, 1990.

10. H. B. Gala, R. Kakwani, S. H. Chiang, J. W. Tierney, and G. E. Klinzing,

Filtration and Dewatering of Fine Coal, Separation Science & Technology,

Vol. 16, pp. 1611–1632 (1981).

11. D. G. Osborne, Solid-Liquid Separation, Coal Preparation Technology,

Graham & Trotman Publishers, London, 1988.

12. C. J. Veal and S. K. Nicol, Process for the Dewatering of Coal and Mineral

Slurries, U. S. Patent No. 5,771,601, 1998.

13. C. J. Veal and S. K. Nicol, Apparatus for the Dewatering of Coal and

Mineral Slurries, U. S. Patent No. 5,956,858, 1999.

HYPERBARIC CENTRIFUGATION 127

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