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Performance Optimization

of the FGX Dry Separator for

Cleaning High-Sulfur CoalB. Zhang

a, H. Akbari

a, F. Yang

a, M. K. Mohanty

a

& J. Hirschib

aDepartment of Mining and Mineral Resources

Engineering, Southern Illinois University at

Carbondale, Carbondale, IL, USAbIllinois Clean Coal Institute, Carterville, IL, USA

Published online: 14 Jun 2011.

To cite this article: B. Zhang , H. Akbari , F. Yang , M. K. Mohanty & J. Hirschi (2011):

Performance Optimization of the FGX Dry Separator for Cleaning High-Sulfur Coal,

International Journal of Coal Preparation and Utilization, 31:3-4, 161-186

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PERFORMANCE OPTIMIZATION OF THE FGX

DRY SEPARATOR FOR CLEANING

HIGH-SULFUR COAL

B. ZHANG1, H. AKBARI1, F. YANG1, M. K. MOHANTY1,AND J. HIRSCHI2

1Department of Mining and Mineral Resources

Engineering, Southern Illinois University at

Carbondale, Carbondale, IL, USA2Illinois Clean Coal Institute, Carterville, IL, USA

The main goal of the present study was not only to deshale (removepure rock) raw coal extracted from Illinois mines but also to assess

the maximum ash separation efficiency and sulfur rejection achiev-

able using the FGX Dry Separator for cleaning raw coals of varying

cleaning characteristics. A Model FGX-1 Dry Separator with feed

throughput capacity of 10 tph was extensively tested at the Illinois

Coal Development Park using multiple coal samples having dis-

tinctly different cleaning characteristics. Statistically designed

experimental programs were conducted to indentify critical process

variables and to optimize FGX Dry Separator performance by sys-

tematic adjustments of critical process variable parameters.The coal-cleaning performance of the FGX Dry Separator

was evaluated for the particle size range of 4.7563.5 mm in most

Received 1 June 2010; accepted 9 August 2010.

Research funding received for this project from the Illinois Clean Coal Institute

(ICCI) through the Office of Coal Development and the Department of Commerce and

Economic Opportunity is sincerely appreciated. The authors greatly appreciate the support

and guidance received from FGX SepTech, LLC, during the course of this investigation.

Special appreciation is extended to Mr. Chao Zhao of FGX SepTech, LLC for his constant

guidance during the test program.

Address correspondence to M. K. Mohanty, Department of Mining and Mineral

Resources Engineering, Southern Illinois University at Carbondale, Carbondale, IL,

USA. E-mail: [email protected]

International Journal of Coal Preparation and Utilization, 31: 161186, 2011

Copyright # Taylor & Francis Group, LLC

ISSN: 1939-2699 print=1939-2702 online

DOI: 10.1080/19392699.2011.574943


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cases, although FGX Dry Separator feed consisted of nominal

7minus;63.5 mm run-of-mine coals. Deck vibration frequency,

longitudinal deck angle, feeder frequency, and baffle plate heightwere identified as critical process variables for the FGX Dry Sep-

arator. The best cleaning performance obtained from the FGX Dry

Separator was described by specific gravity of separation (SG50)

and probable error (Ep) values of 1.98 and 0.17, respectively.

For a relatively easy-to-clean coal (having a Cleaning Index of

0.72), only about 0.42% of the clean coal (i.e., 1.6 float fraction)

present in the feed was lost to the tailings stream. For a relatively

difficult-to-clean coal (having a Cleaning Index of 0.53), about

0.98% of the clean coal present in the feed was lost to the tailings

stream. The positive impact of having fine materials in the FGXfeed stream was also noted in this study. A modified log-logistic

partition model was developed using experimental data reported

in literature and validated using new experimental data generated

in this study. The results showed that this model could be

effectively used to predict the FGX Dry Separator coal-cleaning

performance.

Keywords: Dry coal beneficiation; FGX dry separator; Partition

model; Performance optimization

INTRODUCTION

Dry coal beneficiation is typically less restrictive, simpler in process, and

easier to operate in comparison to wet coal-cleaning processes. Several

dry coal beneficiation technologies have been developed and investigated

by researchers worldwide, including Accelerator [1], Rotary Breaker [2],

Allair Jig [3, 4], FGX Compound Dry Separator [5], Air Dense Medium

Fluidized Bed Separator [6], AKAFLOW Separator [7], Tribo-Electrostatic Separator [8], MagMill [9], and so on.

Accelerator, like rotary breaker, utilizes selective breakage principle

to remove large, unbroken pure rock pieces while providing better coal

liberation for subsequent cleaning processes. The rotary breaker repeat-

edly lifts the feed material, whose size is larger than the screen aperture,

and drops it against the strong perforated screen plates. Two products

are generated: one being large unbroken high-ash tailings and the other

being broken product having better liberation characteristics. Rotarybreaker has proven to be a robust machine having very low operating

costs, typically ranging from $ 0.01=ton to $0.04=ton, and a high capacity

up to 2000 tph [2]. The Accelerator could achieve better product yield in

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comparison to the rotary breaker and also improved liberation perform-

ance [1].

Several studies have been reported for coarse coal cleaning usingpneumatic jigs. Air jigs use pulsed air to achieve particle stratification

and separation through the hindered settling and consolidated trickling

mechanisms. Allminerals air jig technology, known as an Allair jig,

has been commercialized in the United States with the first 50 tph unit

installed in an Ohio surface coal mine in 2001. The system provides

high-density cutpoints as required in rock-removal operation; however

the top particle size that it can treat is about 50 mm [4]. Air-Dense

Medium Fluidized Bed (ADMFB) technology has been an effectivemethod to clean run-of-mine (ROM) coal [10]. Large coal (50

300 mm) was successfully cleaned by ADMFB with Ep values about

0.02. However, a much deeper bed was required to provide sufficient sep-

aration space [11]. The ROM coal of 650 mm size fraction was also

effectively cleaned by ADMFB and the overall Ep value was 0.03.

Detailed results showed that the finer the feed coal particles, the lower

the clean coal recovery as well as the separation efficiency [12]. Vibrated

ADMFB and Magnetically Stabilized ADMFB technologies were

developed to provide dry separation of coal in 60 mm particle size range

[12, 13]. Both technologies could provide low (0.065) Ep values. Mag-

netically Stabilized ADMFB could also provide low (1.52) specific grav-

ity of separation due to better fluidization characteristics of the bed.

Dual-density ADMFB technology was developed and tested to achieve

three-product separation [14]. The results showed that an Ep value of

0.060.09 for the upper layer with an SG50 of 1.501.54 and an Ep value

of 0.090.11 for the lower layer with an SG50 of 1.841.90 were achiev-

able by this process. A pilot ADMFB plant having a feed capacity of 50tph was established to clean the 506 mm size coal. Although good

coal-cleaning performance, indicated by 0.050.07 Ep, was achieved,

the long-term operation was difficult due to the difficulties of medium

recovery and stability control. Full-scale units need to be developed

and demonstrated for successful commercialization of the ADMFB

technologies.

The AKAFLOW Separator [7] has been developed in Germany

using the principle of an air table for cleaning fine size (3 mm) parti-cles. Preliminary testing on fine coal (3 mm) having an ash content

of 29.5% produced a clean coal product having an ash content of

18.3% at 73.5% product yield. The densest reject was 57.7% in ash.

OPTIMIZATION OF THE FGX DRY SEPARATOR 163


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Another air table separator, namely the Cimbria Heid Gravity Table, was

evaluated recently by Patil (2008) and Parekh et al. [15] to clean

6 mm 1 mm size coal. Excellent separation efficiency values, describedby ash rejection values of about 77%80% with a combustible recovery of

about 95% were reported in this study. Pyritic sulfur content was reduced

from 2.65% in feed to about 1.5% in product stream.

Electrostatic separators utilize high electric field to achieve the sep-

aration of charged particles. Three major mechanismsthat is, conduc-

tive induction, contact or tribo-electrification, and ion or corona

bombardmenthave been used to endow the charges. Separators using

one or more of the above-mentioned mechanisms have been developedand evaluated to beneficiate coal [8, 1632].

Dry magnetic separation has also been used to achieve coal cleaning

based on the fact that clean coal is a weakly diamagnetic material and

some of the minerals associated with coal (particularly those containing

iron) are paramagnetic. Various approaches, including irradiation,

High-Gradient Magnetic Separation, and Open-Gradient Magnetic Sep-

aration, have been studied to achieve dry cleaning of ash and sulfur con-

tents [3339]. MagmillTM, a commercialized dry coal magnetic

separation technique, showed that additional an 25%, 30%, and 35%

reduction of sulfur, mercury, and ash, respectively, could be achieved

at 95% BTU recovery [9]. Other dry coal beneficiation techniques,

including optical sorting, radioactivity sorting, scanning and so on, have

been studied for their effectiveness for coal-cleaning applications [40,

41]. Test work showed potentials of using a sensor-based sorting system

to preconcentrate ROM coal. However, a more accurate sensor and

more compact design will be necessary for possible commercialization

in the coal-cleaning industry [41].The FGX Dry Separator is a special type of air table that consists of

a perforated separating deck, three air chambers, a vibrating mechanism,

and a hanging support mechanism. The separating deck, having riffles on

its surface, is suspended inclined both in longitudinal and transverse

directions, as shown in Figure 1. Airflow supplied from a blower fluidizes

feed material on the deck and the vibration mechanism imparts a helical

turning motion to particles as they slide in the longitudinal direction. In

a relatively broad particle size range (top-to-bottom size ratio of$10:1),a density-based particle stratification takes place on the separating deck

under the action of the vibration force, the upward fluidizing force of the

air flow, and the downward gravity force of the solid particles. Past

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results obtained on Chinese coal [5], and a recent study conducted by

Honaker et al. [42] on several U.S. coal samples, indicate the effective

density-based separation achievable from the FGX Dry Separator. Highseparation efficiency along with low cleaning costs have resulted in the

widespread application of the FGX Dry Separator in China. The first

commercial installation of this technology in the United States took

place in the year 2009.

The main goal of the present study was not only to study the deshal-

ing (removal of pure rock by producing a high-ash reject) performance

but also to assess the maximum ash separation efficiency and sulfur

rejection achievable using the FGX Dry Separator for cleaning raw coalsof varying cleaning characteristics. A Model FGX-1 Dry Separator, hav-

ing a maximum feed handling capacity of 10 tph, was extensively tested

at the Illinois Coal Development Park using coal samples of different

cleaning characteristics.

EXPERIMENTAL APPROACH

A schematic diagram of the experimental layout is shown in Figure 2.Coal was introduced to the feed hopper using a conveyor belt and a

front-end loader. Several series of exploratory experiments were conduc-

ted to get a better understanding of various process parameters and the

Figure 1. A schematic diagram of the FGX Dry Separator showing the different product

streams [5].



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nature of their effects on important process responses, such as combust-

ible recovery, ash rejection, and sulfur rejection. After obtaining a good

estimate of parameter ranges, a Plackett and Burman experimental pro-

gram was utilized to indentify the most critical process variables among

the eight listed in Table 1. These eight process variables are (in the order

listed in Table 1): feeder frequency (that controls the feed rate to the

FGX deck); bed vibration frequency; clean coal splitter (that dividesthe clean coal from the middling product); refuse splitter (that divides

middling from refuse product); clean coal air rate (the fluidizing air

reporting to the air chamber underneath the clean coal side of the deck);

baffle plate height (the height of the vertical plate at the clean coal dis-

charge side of the deck); lateral and longitudinal angles of the deck. The

positive values for the deck angle indicate downward inclination,

whereas the negative value indicates an angle in the upward direction.

Figure 2. A schematic diagram of the FGX-1 Dry Separator test circuit layout.

Table 1. List of operating parameters used for the Plackett and Burman experimental

design conducted using Sample 1

Factor Name Units Type Low actual High actual

1 Feeder Frequency Hz Numeric 60 90

2 Bed Vibration Frequency Hz Numeric 80 100

3 Clean Coal Splitter not applicable Categoric Low High

4 Refuse Splitter not applicable Categoric Low High

5 Clean Coal Air Rate not applicable Categoric Half Open Fully Open6 Baffle Plate Height cm Numeric 0 1.9

7 Lateral Deck Angle degree Numeric 5 8.5

8 Longitudinal Deck Angle degree Numeric 1 1

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As indicated the longitudinal deck angle was varied over a range of1 to

1 degree. Based on the findings of the Plackett and Burman test pro-gram, a Central Composite Design consisting of 28 tests was pursued

by varying the four most critical process variables (listed in Table 2) to

optimize ash- and sulfur-cleaning performance achievable from the

FGX Dry Separator.

RESULTS AND DISCUSSIONS

Coal Sample Characterizations

Two different bituminous coal samples were utilized in this study to

evaluate the cleaning efficiency achievable using the FGX Dry Separator.

Approximately 15 tons of Sample 1 was used to do an extensive study

with the FGX Dry Separator. Upon completion of a thorough evaluation

of the optimum ash- and sulfur-cleaning performance achievable from

the FGX Dry Separator, more coal (in a smaller quantity) was collected

from Sample 2 for additional testing and comparison. Representative

bucket samples were collected for the size-by-size characterization oftotal mass, ash content, and sulfur content distributions for both coal

samples. As is shown in Table 3, Sample 2 contained a very small pro-

portion of fine coal (4 mesh or 4.76 mm size fraction), whereas Sample

1 contained as much as 42.71% fines.

Overall ash content for both coal samples were 30.26% and 40.23%,

respectively. The high total sulfur contents of 3.67% and 4.44%, for both

coals are common characteristics of Midwestern U.S. coals. Coal sam-

ples were dry screened to prepare the recommended 63.5 3.0 mm par-ticle size fractions to be fed to the FGX Dry Separator. This significantly

reduced the amount of 4.76 mm size coal in the actual feed stream

reporting to the FGX Dry Separator. Ash and sulfur rejection achievable

Table 2. List of operating parameters used for the Central Composite experimental design

conducted using Sample 1

Factor Name Units Type Low Medium High

1 Feeder Frequency Hz Numeric 60 70 80

2 Longitudinal Deck Angle degree Numeric 1 0 1

3 Bed Vibration Frequency Hz Numeric 80 90 100

4 Baffle Plate Height cm Numeric 0 1.6 3.2



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from the FGX Dry Separator were evaluated only for the 63.5 4.76 mm

size fraction.

A simple analysis of float=sink data for Sample 1 and Sample 2,

listed in Table 4, indicates that coal-cleaning indices (the ratio of 1.3

float and 1.6 float) for both coals are 0.53 and 0.72, respectively. The

lower Cleaning Index for Sample 1 indicates a relatively more difficultcleaning characteristic, which was also evidenced from the cleaning per-

formance obtained by the FGX Dry Separator for Sample 1. Sample 1

was utilized for the majority of the experimental program, including

the exploratory and optimization tests, whereas Sample 2 was used for

additional testing.

Table 3. Distribution of mass, ash, and total sulfur in coal samples utilized for FGX Dry

Separator tests

Coal samples Sample 1 Sample 2

Weight % 4.76 mm 57.29 91.42

4.76 mm 42.71 8.58

Total 100 100

Ash % 4.76 mm 25.39 39.14

4.76 mm 36.80 51.84

Total 30.26 40.23

Sulfur% 4.76 mm 3.80 4.49

4.76 mm 3.50 3.89

Total 3.67 4.44

Table 4. Washability data obtained from float=sink analyses of the 63.5 mm size fraction

of the two major coal samples utilized in this study

Sample 1 Sample 2

Specific Gravity Weight % Ash % Weight % Ash %

Float 1.3 42.57 5.60 53.86 10.61

1.3 1.4 16.94 10.20 10.57 15.51

1.4 1.5 17.27 15.88 8.51 19.34

1.5 1.6 3.71 22.94 2.25 29.37

1.6 1.8 2.86 35.38 2.33 41.15

1.8 2.0 2.70 57.80 1.62 56.922.0 2.2 2.25 77.55 1.14 72.83

2.2 Sink 11.70 82.05 19.74 92.00

Total 100.0 21.62 100.0 30.53

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FGX Dry Separator Testing Results

Plackett and Burman Experimental Program. A Plackett and

Burman experimental design was utilized to identify the most critical

process parameters for the FGX Dry Separator. Experimental

conditions for 12 tests varying eight process parameters and the

resulting ash-cleaning performance are listed in Table 5. These data

were statistically analyzed to develop corresponding half-normal

probability plots (Figure 3) for four responses. As marked in Figures 3a

and 3b, longitudinal deck angle (Factor H) and feeder frequency (Factor

A) were the most critical process parameters for ash separation

efficiency, which is a function of combustible recovery and ashrejection. Figure 3c indicates the importance of feeder frequency

(Factor A) and baffle plate height (Factor F) for the product ash

response. Figure 3d shows that longitudinal deck angle (Factor H) and

deck vibration frequency (Factor B) are critical for the tailings ash

response. Based on these findings, Factors A, B, F, and H were further

evaluated in a more detailed study for optimizing FGX Dry Separator

performance.

Optimization Test Program. After identifying the four critical process

parameters, a Central Composite Design (CCD) was utilized to optimize

the ash- and sulfur-cleaning performance achieved from the FGX Dry

Separator. Experimental conditions for 28 CCD tests and resulting

ash- and sulfur-cleaning performances are listed in Table 6. Statistical

perturbation plots of these tests, shown in Figure 4, revealed the

relative importance of the four key process parameters on various ash-

and sulfur-cleaning process responses. Two parameters, feederfrequency and baffle plate height, had significant effects on product

ash response, whereas longitudinal angle had the most significant

effect on tailings ash content. Feeder frequency and bed vibration

frequency played the most significant role in affecting ash separation

efficiency, whereas longitudinal deck angle affected the sulfur rejection

response the most. Low feeder frequency allowed a low rate of feed

coal causing less crowding on the separating deck that resulted in

improved selectivity and thus lower product ash content of the cleancoal product. The effect of baffle plate height can be explained by the

fact that due to the combined effect of fluidizing air and deck

vibration, lightest particles form the topmost layer of the coal flowing



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Table5.Op

eratingconditionsandashclea

ningresultsobtainedfromthe

PlackettandBurmanexperimentalprogramusingSample1

Test#

Feeder

frequency

(

Hz)

Deck

vibration

frequency

(Hz)

Clean

coal

splitter

position

Refuse

splitter

position

Clean

coalairvalve

Baffle

plate

height

(cm)

Lateral

deck

a

ngle()

Longitudinaldeck

angle()

Concentrate

ash(%)

Refuse

ash(%)

Combust

ible

recovery

(CM

)

(%)

Ash

rejection

(R)(%)

1

90

85

High

High

Half

1.90

5.0

1

16.49

30.09

87.94

22.75

2

90

101

High

Low

Full

1.90

5.0

1

16.75

57.91

97.94

11.70

3

60

85

Low

Low

Half

0.00

5.0

1

14.71

49.81

75.56

63.43

4

90

85

High

Low

Half

0.00

8.5

1

15.98

61.32

98.83

8.83

5

60

101

High

High

Half

1.90

8.5

1

10.81

28.90

54.74

73.36

6

60

85

Low

High

Full

1.90

5.0

1

14.54

59.04

97.36

16.69

7

60

85

High

High

Full

0.00

8.5

1

16.38

58.19

95.25

25.97

8

90

101

Low

High

Half

0.00

5.0

1

19.41

50.66

98.15

7.49

9

60

101

High

Low

Full

0.00

5.0

1

12.29

23.74

69.20

49.36

10

90

101

Low

High

Full

0.00

8.5

1

17.44

23.45

85.50

20.97

11

90

85

Low

Low

Full

1.90

8.5

1

15.46

33.05

93.50

16.11

12

60

101

Low

Low

Half

1.90

8.5

1

12.12

52.72

96.03

19.47

Note.C,M

,andRrefertoconcentrate,m

iddlings,andrefusestreams,r

espectively.

170


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with a helical motion across the bed. As a result, the higher the baffle

plate (that obstructs the lateral flow of material) height, the lighter the

density and lower the ash content of the coal flowing over the baffle

plate to the clean coal stream. The effect of the longitudinal angle can

be explained by the fact that its high value (10) indicates an upward

angle for the deck, whereas, the low value (10) indicates a downward

angle for the deck along the longitudinal direction. The upward angle

tends to hinder the flow of coal (mostly the high-density fraction thatis left in the bottom-most layer on the deck) along the longitudinal

direction and allows only the densest coal fraction to report to the

tailings port at the farthest end of the deck. Consequently, the ash

Figure 3. Half-normal probability plots generated from the Plackett and Burman

experimental program identifying critical process parameters for the FGX Dry Separator.



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content of the tailings material is the highest with the high longitudinal

angle of the deck. However, due to the hindered flow in the

longitudinal direction caused at high longitudinal deck angle, some of

the heavy-middling type of particles are allowed to flow across the bedand to report to the clean coal product launder, thus lowering ash

separation efficiency and sulfur rejection achieved by the FGX

Separator.

Table 6. Operating conditions and resulting ash- and sulfur-cleaning performance obtained

from the optimization test program where A is feeder frequency (in Hz), B is longitudinal

deck angle (in degrees), C is bed frequency (in Hz), D is baffle plate height (in cm)

CCD

design

ID A B C D

Feed

ash

(%)

Concentrate

ash

(%)

Middling

ash

(%)

Refuse

ash

(%)

Combustible

recovery

(CM)

(%)

Ash

rejection

(R) (%)

Sulfur

rejection

(R) (%)

1 60 1 80 3.2 21.07 11.21 16.49 30.28 68.57 51.13 45.65

2 70 0 90 1.6 19.01 14.42 13.60 37.88 84.21 41.03 28.33

3 70 0 90 1.6 19.73 13.94 16.32 39.47 85.47 38.54 29.74

4 80 1 100 0 14.66 12.38 21.55 59.06 98.78 10.24 4.71

5 70 0 80 1.6 18.58 13.84 16.27 43.12 90.34 32.10 23.76

6 60 1 100 3.2 18.72 11.25 14.84 46.04 91.00 33.32 20.88

7 70 0 90 0 19.56 15.61 17.17 38.85 87.62 32.33 21.34

8 60 1 80 0 19.32 13.33 36.83 66.70 97.11 24.20 9.31

9 60 1 100 0 17.20 13.45 21.19 59.74 97.47 18.07 7.73

10 80 1 80 0 19.31 16.37 17.26 41.98 92.15 23.74 17.77

11 70 0 90 1.6 20.58 13.27 17.98 43.13 86.46 39.64 27.52

12 80 1 100 3.2 16.71 13.17 14.64 61.89 97.38 21.17 12.51

13 60 1 100 3.2 19.21 11.35 11.28 24.25 42.70 77.16 70.81

14 60 1 100 0 27.26 14.00 14.87 46.68 70.63 68.61 56.96

15 70 0 100 1.6 18.97 12.56 15.71 30.38 75.02 46.57 45.14

16 70 1 90 1.6 14.16 11.32 18.44 54.43 99.14 6.20 3.39

17 80 1 80 3.2 15.65 13.53 13.21 63.58 98.06 18.24 8.82

18 60 1 80 3.2 25.74 13.07 27.35 71.85 97.25 20.24 11.62

19 70 0 90 3.2 18.00 12.73 12.46 40.16 85.61 44.00 31.31

20 80 0 90 1.6 17.96 14.14 13.30 42.66 90.00 33.98 24.33

21 80 1 80 3.2 15.44 12.98 13.31 28.72 87.72 27.11 25.04

22 70 1 90 1.6 20.57 12.93 13.32 35.79 73.48 57.09 48.08

23 60 0 90 1.6 24.59 14.87 14.23 42.55 72.55 62.33 50.79

24 60 1 80 0 20.28 13.03 13.65 46.33 85.50 49.20 31.23

25 70 0 90 1.6 16.59 11.91 15.63 29.06 83.21 34.57 28.68

26 80 1 100 0 20.44 15.40 21.59 34.00 82.93 34.23 28.49

27 80 1 80 0 18.02 15.73 35.88 74.76 99.60 5.38 1.8228 80 1 100 3.2 22.71 11.74 16.36 31.64 59.58 63.69 60.51

Note. C, M, and R refer to concentrate, middlings, and refuse streams, respectively.

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Empirical models were developed for three key process responses

using the step-wise regression technique. It may be noted that tailings

ash content response is the most critical performance parameter when

deshaling is the primary purpose of the FGX Dry Separator application.

As exhibited by the empirical model of Equation 1, tailings ash content

was found to be a function of longitudinal deck angle, bed vibration fre-

quency, and baffle plate height. Interaction effects of Bed Vibration Fre-quency x Baffle Plate Height and Longitudinal Deck Angle x Bed

Vibration Frequency were also significant for the tailings ash response.

Separation efficiency response is the most useful performance parameter

Figure 4. Perturbation plots for various process responses: (a) product ash content, (b)

tailings ash content, (c) separation efficiency, and (d) sulfur rejection.



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when using the FGX Dry Separator to produce a final clean coal pro-

duct. As indicated in Equation 2, the separation efficiency response

was a function of all four key process parameters investigated in the opti-mization study. Parameter interaction effects significantly affecting sep-

aration efficiency included Feeder Frequency x Baffle Plate Height and

Longitudinal Angle x Baffle Plate Height. All four key process para-

meters also contributed significantly to the sulfur rejection response, as

indicated in Equation 3. Longitudinal Deck Angle x Feeder Frequency

and Longitudinal Deck Angle x Bed Vibration Frequency were factor

interactions significantly affecting the extent of sulfur rejection achieved

by the FGX Dry Separator. The three empirical models were developedwith R2 values of .89, .90, and .97, respectively, and an overall F-ratio of

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Figures 5c and 5d indicate levels of sulfur rejection achievable for

these two scenarios, that is, for a high separation efficiency target versus

a high-tailings ash-content target. For the high separation efficiency tar-

get, high-sulfur rejection of nearly 56% is achievable as indicated in the

upper left corner of the experimental region. A slightly lower level of sul-

fur rejection (about 47%) is achievable in the same experimental regionthat would result in the highest ash separation efficiency. When the

objective is high-tailings ash content, a lower level of sulfur rejection

(not low-tailings sulfur content) has to be tolerated.

Figure 5. Simulated response surface contours generated with empirical model equations

for: (a) separation efficiency, (b) tailings ash, (c) sulfur rejection with high separation

efficiency as a target, and (d) sulfur rejection with high tailings ash as a target.



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An additional coal sample (Sample 2) with a relatively easier clean-

ing characteristic was utilized to evaluate the cleaning potential achiev-

able from the FGX Separator. Selected ash- and sulfur-cleaningperformances achieved for both coal samples are listed in Table 7.

Due to the presence of a large proportion of out-of-seam dilution mate-

rials in Sample 2, it was easy to produce a tailings stream having an

extremely high-ash content (>80%). The best ash reduction for Samples

1 and 2 achieved were from 22.71% and 34.45% in the feed to 11.74%

and 13.48% in the product streams, respectively. The best sulfur reduc-

tions achieved for both samples were from 4.17% and 5.13% in the feed

to 3.05% and 4.08% in the product streams.It is generally believed that the presence of fines (-1

4-inch size frac-

tion) in the feed aids the separation process by facilitating the formation

of a fluidized bed on the deck that serves as an autogenous dense

medium. It was desired to investigate this hypothesis by conducting four

Table 7. Selected results obtained from the FGX Dry Separator for cleaning Samples 1

and 2

Ash % Total sulfur% Yield %

Feed

Clean

coal Middlings Tailings Feed

Clean

coal Middlings Tailings

Clean

Coal Middlings

Coal Sample 1

19.79 16.13 29.67 64.86 4.81 4.68 5.71 5.66 87.20 7.32

27.26 14.00 14.87 46.68 4.36 3.06 3.29 6.19 40.86 19.07

22.71 11.74 16.36 31.64 4.17 3.05 3.03 5.52 13.78 40.51

19.32 13.33 36.83 66.70 3.89 3.69 4.68 5.17 83.40 9.59

18.02 15.73 35.88 74.76 4.57 4.54 4.59 6.40 91.12 7.59

15.65 13.53 13.21 63.58 3.09 3.04 2.83 6.08 54.63 40.88

25.74 13.07 27.35 71.85 3.79 2.96 3.98 6.07 33.86 58.89

21.36 13.53 16.21 53.85 5.11 4.71 4.06 7.25 70.71 10.59

Coal Sample 2

29.05 16.91 56.03 88.39 3.89 3.77 3.79 4.72 79.53 7.70

42.88 17.65 46.08 89.26 4.52 3.91 4.04 6.12 50.99 22.85

40.06 15.24 51.88 88.16 4.28 4.23 2.78 6.47 52.28 27.50

42.36 16.33 30.55 80.83 5.13 4.08 4.66 6.66 38.70 26.87

30.84 14.39 45.60 82.53 4.33 4.07 3.76 6.21 65.15 19.77

28.23 15.49 47.61 89.90 4.66 4.16 5.41 7.09 74.02 15.5934.45 13.48 39.57 85.09 4.68 3.87 4.62 6.87 58.07 19.90

36.79 27.12 84.00 84.58 4.64 4.58 4.83 6.377 83.00 16.12

33.16 21.56 81.14 92.07 4.00 4.00 4.01 4.138 80.93 16.89

176 B. ZHANG ET AL.


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series of experiments adding varying proportions of fines (4.76 mm size

fraction) to Sample 1. Ash rejection results are shown in Figure 6, first

mention>They indicate that ash separation efficiency gradually

increased as the proportion of fines increased from 0% to 29% at similar

levels of ash rejection ($40%). However, examining combustible recov-ery versus sulfur rejection in Figure 7 suggests that at similar level of

combustible recovery ($85%), the greatest sulfur rejection was achieved

when the feed had only 18% fines. Therefore, further investigation is

needed to establish an optimum proportion of fines in FGX feed.

Figure 6. FGX Dry Separator ash-cleaning performance with varying proportions of coal

fines (4.76 mm) in the feed.

Figure 7. FGX Dry Separator sulfur-cleaning performance with varying proportions of coal

fines (4.76 mm) in the feed.



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FGX Dry Separator Partition Data

The cleaning efficiency of any density-based separator is evaluated by

doing float=sink analyses on collected clean coal product and tailings

samples and using the resulting data to generate partition curves. Par-

tition data were generated for measuring the cleaning efficiency of the

FGX Dry Separator by processing coals of four different size fractions,

that is, 63.5 50.8 mm, 50.8 25.4 mm, 25.4 12.7 mm, and

12.7 4.76 mm. The overall (4.76 mm) apparent partition curve for

Sample 2 and those for individual size fractions are shown in Figure 8a.

Effective specific gravity of separation (SG50) and probable error (Ep)

values obtained from these apparent partition curves are listed inTable 8. Apparent partition curves were corrected for clean coal bypass

to tailings and high-density reject bypass to product to generate corre-

sponding corrected partition curves shown in Figure 8b. Corresponding

SG50c and Epc values obtained from these corrected partition curves are

also listed in Table 8 for comparison purposes.

Moreinsight into the cleaning performance was obtained by examin-

ing clean coal (i.e., 1.6 SG float) lost to middlings and reject streams and

recovery of reject material (i.e., 2.0 SG sink) to clean coal product andmiddlings streams of the FGX Separator. These performance details

are listed in Table 9 for two coal samples having different cleaning char-

acteristics. As indicated previously in Table 7, tailings ash content of

85% was shown to be achievable for Sample 2. This phenomenon is also

evident in the small amount of clean coal lost (i.e., 2.54% of 1.6 float) in

Figure 8. (a) Apparent and (b) corrected partition data for the Sample 2 and for different

size fractions of Sample 2.

178 B. ZHANG ET AL.


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the tailings stream for the overall 4.76 mm size coal, as shown in

Table 9. The amount of high-density reject material in the clean coal pro-

duct was maintained at a reasonably low level of 3.73%. A simple analy-

sis for Sample 2 would indicate that only about 0.42% of the entire clean

coal present in the feed was lost to the tailings stream. Nearly 95.54% of

the clean coal reported to the product stream and the remaining 4.04%

reported to the middlings stream.

Mixing the middlings stream with either the clean coal stream or the

refuse stream may be possible in some cases, based on target product

specifications. However, analysis of the easier-to-clean Sample 2 indi-

cates the middlings stream contained 40.41% clean coal and 53.42%

high-density reject materials. Hence, a more profitable solution may be

to clean the middlings stream a second time. However, it should be noted

that direct recirculation of the middlings stream to the feed may not be

the right option. Cleaning characteristics of the middlings coal will be

significantly more difficult in comparison to the original feed. A simple

analysis of data provided in Table 10 would indicate that the CleaningIndex (CI) for the Sample 2 middlings stream was 0.43 in comparison

to 0.72 for the original feed. Therefore, middlings coal should undergo

a size reduction (i.e., crushing) step prior to being recirculated to the

FGX Dry Separator feed stream. This should improve liberation charac-

teristics of the middlings coal giving it a Cleaning Index similar to that of

the original feed.

FGX Dry Separator performance data for the more difficult-to-

clean Sample 1, listed in Table 9, indicate greater amounts ofhigh-density reject material bypassing to the product (6.85%) and a sig-

nificantly higher clean coal loss to the tailings stream (14.77%). For the

two coarsest size fractions, clean coal losses to the tailings stream were

Table 8. Size-by-size specific gravity of separation and probable error values for the

Sample 2

Test

Size fraction

(mm) SG50 Ep SG50c Epc

Clean coal bypass

to tailings (%)

Reject bypass to

clean coal (%)

Sample 2 50.8 1.90 0.12 1.90 0.11 5.25 4.23

50.8 25.4 1.95 0.18 1.94 0.16 2.47 8.19

25.4 12.7 2.01 0.19 1.98 0.11 2.57 19.1

12.7 4.76 2.03 0.23 1.99 0.08 2.54 24.49

4.76 1.98 0.17 1.94 0.14 2.59 14.73



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Table9.Siz

e-by-sizeseparationefficiencydataobtainedfromtheFGXDrySeparator

Sizefraction

(mm)

FGXfeed

(Weight%)

1.6Floatintailings

(Weight%)

1.6Floatinmiddlings

(Weight%)

2.0Sin

kinclean

coal(Weight%)

2.0Sinkinmiddlings

(Weight%)

Sample2with0.72

CleaningIndex

50.8

5.61

5.31

39.55

3.23

56.23

50.8

25.4

40.58

2.78

44.29

2.03

46.76

25.4

12.7

30.91

1.17

40.34

4.16

54.16

12.7

4.76

22.90

1.02

34.58

6.14

62.54

4.76

100.00

2.54

40.41

3.73

53.42

Sample1with0.53

CleaningIndex

50.8

5.71

30.64

58.78

0.00

18.26

50.8

25.4

31.95

17.06

69.84

1.16

13.26

25.4

12.7

26.69

5.27

68.87

5.67

22.24

12.7

4.76

35.65

4.81

60.10

1

2.99

32.90

4.76

100.00

14.77

67.13

6.85

19.76

180


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30.64% and 17.06%, which may be considered unacceptably high. More

detailed analysis indicates that about 0.98% of the entire clean coal

present in the feed was lost to the tailings stream. Approximately,

93.1% of the clean coal reported to the product and the remaining

5.92% to the middlings stream. For this type of coal, instead of losing

a considerably high amount of clean coal to tailings, an option worthconsidering is to mix the tailings stream with the middlings stream and

to crush the resulting mixture to improve its liberation characteristics

prior to cleaning it again using a second stage FGX Dry Separator. In

other words, a rougher-scavenger type FGX Dry Separator circuit along

with the above-mentioned crushing operation would be recommended

for relatively difficult-to-clean coal.

FGX Dry Separator Partition Model Development andValidation

Partition models, that is, distribution functions, are commonly used to

approximate partition coefficients for density-based separators. Based

on the FGX Dry Separator data available in literature [42] a modified

log-logistic model, as expressed in Equations 4 and 5, was found to be

the most suitable one.

PN 100 1

1 expalnXb; 4

X Dm=D50 5

Table 10. Float=sink data for feed and middlings coal of the Sample 2

Sample 2 FGX feed FGX middlings

SG Weight % Weight %

Float 1.3 53.86 28.09

1.3 1.4 10.57 13.76

1.4 1.5 8.51 18.50

1.5 1.6 2.25 6.09

1.6 1.8 2.33 0.45

1.8 2.0 1.62 7.33

2.0 2.2 1.14 5.02

2.2 Sink 19.74 20.76

Total 100.0 100.0



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where a and b are fitting constants; Dm is the mean density and D50 is the

separation density.

The results showed in Table 11 indicate that a high coefficient of

determination (R2) value of .997 and a narrow 95% confidence interval

for both constants a and b were achieved. The new partition data gener-

ated in the present study was also used to fit this model and the results

showed that a high R2 of .973 was achieved and the fitted a and b values

were within the 95% confidence interval predicted by using

above-mentioned data available in the literature.

CONCLUSIONS

Some of the key technical findings of this study are listed in the following

section. An economic analysis conducted to determine the capital and

operating cost of cleaning coal using the FGX Dry Separator will be

reported in another publication.

1. Of the eight process variables for the FGX Dry Separator investigatedusing the Plackett and Burman experimental design, four parameters

had the most significant effects on process responses such as com-

bustible recovery, ash rejection, and product ash and tailings ash con-

tent. These four process variables were feeder frequency, deck

vibration frequency, clean coal gate (baffle plate) height, and longi-

tudinal deck angle.

2. The optimization study conducted utilizing a Central Composite

Design revealed the relative importance of these four key process para-meters. Baffle plate height and feeder frequency affected product ash

the most. Longitudinal deck angle affected tailings ash the most. Fee-

der frequency and longitudinal deck angle were the most significant

Table 11. Development and validation of the partition model for the FGX Dry Separator

R2 Parameter 95% Confidence interval

Literature Data

a 0.997 8.965 9.498 8.431

b 1.0053 0.9989 1.0117

New Data

a 0.973 9.278

b 0.9995

182 B. ZHANG ET AL.


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factors contributing to ash separation efficiency. Deck vibration fre-

quency and longitudinal deck angle affected sulfur rejection the most.

3. The best density-based cleaning performance obtained from the FGXDry Separator is described by SG50 and Ep values of 1.98 and 0.17,

respectively, for4.76 mm size coal. SG50 and Ep values for individ-

ual size fractions were as follows: 1.90 and 0.12 for the 50.8 mm size

fraction, 1.95 and 0.18 for the 50.8 25.4 mm size fraction, 2.01 and

0.19 for the 25.4 12.7 mm size fraction, and 2.03 and 0.23 for the

12.7 4.76 mm size fraction.

4. The presence of fines (4.76 mm size coal) in the feed ranging from

0% to 29% improved cleaning performance significantly. However,the highest ash separation efficiency was achieved at 29% fines,

whereas the best sulfur rejection was achieved at a fines content of

18% in the feed. More study may be needed to resolve this anomaly.

5. For a relatively easy-to-clean coal sample (having a CI of 0.72), only

0.42% of clean coal (i.e., 1.6 float fraction) present in the feed was

lost to the tailings stream, whereas 95.54% was recovered to the pro-

duct. For this type of coal, recleaning of the middlings coal following

a size reduction step (to improve liberation) will most likely allow

recovery of almost all clean coal to the product.

6. For a relatively difficult-to-clean coal sample (having a CI of 0.53),

93.1% of clean coal present in the feed was recovered to the pro-

duct, whereas 0.98% of recoverable clean coal was lost to the tail-

ings stream. For coal of such type, a rougher-scavenger type of

FGX Dry Separator circuit along with an intermediate crushing

step for the rougher middlings and tailings streams would be

recommended.

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