Fuel Processing Technology 101 (2012) 94–100

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Fuel Processing Technology

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Desulphurization and deashing of oxidized fine coal by Knelson concentrator

Tuncay Uslu ⁎, Ercan Sahinoglu, Mehmet Yavuz
Karadeniz Technical University, Department of Mining Engineering, 61080, Trabzon, Turkey

a r t i c l e i n f o a b s t r a c t

Article history: In order to efficiently clean fine coals, enhanced gravity separators, such as Knelson concentrator, have been
Received 29 February 2012 developed. Although the Knelson concentrator has not been applied yet in industrial scale for fine coal cleaning,
Received in revised form 26 March 2012 different investigations have been conducted in recent years. In this study, an oxidized coal with high sulphur
Accepted 3 April 2012
was subjected to cleaning process by Knelson concentrator after being classified into the size fractions of
Available online 5 May 2012
−106 μm, −300 + 106 μm and −500 + 300 μm. Experimental variables were coal particle size, bowl speed and
Keywords:
fluidizing water pressure. Maximum combustible matter recovery, pyritic sulphur rejection, and ash rejection
Fine coal were achieved to be 99.13%, 91.60%, and 60.94%, respectively. The highest separation efficiency for the pyritic
Oxidized coal sulphur and ash was determined to be 67.91% and 39.53%, respectively. These corresponded to the removal of
Desulphurization 70.45% of pyritic sulphur with combustible matter recovery of 97.46% and removal of 45.96% of the ash with
Deashing combustible matter recovery of 93.57%. Coal particle size, bowl speed and fluidizing water pressure were
Knelson concentrator determined to affect the separation performance with a close interaction between them.
© 2012 Elsevier B.V. All rights reserved.

1. Introduction is thrown outwards. Heavy particles are trapped in the retention zone
between the ribs while the light ones are carried upward into the
Gravity concentration techniques are extensively utilized in coal concentrate stream of coal by the rising of current water. Injection of
cleaning due to their high efficiency and low cost. However, conven- water through ports located in the ribs provides formation of tailings bed
tional gravity processes are not applicable for cleaning fine coal [1]. To consisting of heavier particles [9]. Although the Knelson concentrator has
clean fine coals, the conventional techniques such as jigs, heavy media not been applied yet in industrial scale for fine coal cleaning, different
systems, and concentrating tables are less efficient and more expensive. studies have been conducted in recent years [5,6,10–17]. However, more
Hence, flotation, flocculation and agglomeration are often used for fine detailed works are required to provide data especially for pyritic sulphur
coal cleaning [2]. Although flocculation and agglomeration methods are removal which was disregarded in most of the previous works [5,11–17].
considered to be important in fine coal processing in the future, these The present study aims to determine the cleaning possibility of fine coal
processes have not yet been commercialized by the coal industry due to by using Knelson concentrator and effects of some variables on the
cost of used reagents [3]. Flotation is the most commonly used cleaning cleaning process, and to fill the gap in the literature in this field. This
technique for fine coal in today's coal preparation plants [4,5]. However, study is the one of the limited number of detailed works on the coal
its performance is low in the cleaning of oxidized coals. In addition, it cleaning by using a Knelson concentrator. In addition, Muzret coal
does not produce high quality concentrates [6]. Although flotation sample differs from many coals used in previous studies in that it shows
columns have been introduced to deal with this problem, their ability to typical characteristics of an oxidized coal with its brittle nature, high
treat middling and weakly hydrophobic pyrite particles is not satisfac- specific gravity, high sulphate sulphur content, and poor response to
tory [4,7]. flotation. A total of 48 tests were undertaken in the study in which the
The ability of gravity separating equipments to treat fine coal has effects of the variables including coal particle size, bowl speed and
been much improved with the development and application of enhanced fluidizing water pressure on the rejections of ash and pyritic sulphur,
centrifugal gravity separators [8]. The Knelson concentrator, one of the and combustible matter recovery were investigated. Positive results
enhanced gravity separators, is essentially a hindered settling device, obtained from this study can be a guide for evaluating the low quality
related to a hydrosizer, with centrifugal force substituting for the force of fine coal as energy sources instead of useless and environmentally
gravity. It consists of a rotating ribbed cone (bowl) with perforations pollutant material.
between the ribs [3]. Feed slurry descends on the bottom of the cone and
2. Materials and methods

An oxidized coal sample from Muzret mine was used in this study.
⁎ Corresponding author. Tel.: + 90 462 3773530; fax: + 90 462 3257405. Proximate and sulphur analyses, and particle size distribution of coal
E-mail address: tuncay43@ktu.edu.tr (T. Uslu). sample are illustrated in Tables 1 and 2, respectively. Mineralogical

0378-3820/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.fuproc.2012.04.002
 T. Uslu et al. / Fuel Processing Technology 101 (2012) 94–100 95

Table 1 and petrographical examination of the sample (Fig. 1) has shown that
Proximate and sulphur analyses of the coal sample. pyrite is the major mineral matter in the coal. Clays, calcite, gypsum,
Components Air dried Dried quartz, and siderite are present as other mineral matters. Oxidation
products of pyrite such as limonite, hematite and goethite were also
Moisture (%) 4.30 –
Ash (%) 17.06 17.83 observed. Among the macerals, vitrinite is the dominant group. Pyrite
Volatile matter (%) 21.12 22.07 occurs as massive and framboidal forms with particles of 1–500 μm in
Fixed carbon (%) 57.52 60.10 size. Cracks and micro cracks are observed in the coal sample.
Sulphate sulphur (%) 1.18 1.23
Schematic representation of Knelson concentrator and photo of
Pyritic sulphur (%) 2.72 2.84
Total sulphur (%) 6.99 7.30 batch type (KC-MD3) Knelson concentrator used in this study are
Organic sulphur (%) 3.09 3.23 illustrated in Fig. 2a,b. The coal samples were fed to the Knelson
Calorific value (kJ/kg) 23643 24702 concentrator as slurries with a coal content of 10% at a rate of 0.33 l/min.
Overflow (clean coal slurry) was collected in a bucket (Fig. 2c) while
underflow (tailings) remained in the bowl (Fig. 2d). Tailings in the bowl
Table 2 were washed into bakers. After dewatering by using vacuum filter
Particle size distribution of the coal sample. (Fig. 2d), the clean coal (Fig. 2e) was dried, weighed, and analyzed for
Particle size (mm) Weight (%) ash and pyritic sulphur. Test conditions and variables are illustrated in
− 0.5 + 0.3 32.03
Table 3.
− 0.3 + 0.106 42.29 The combustible matter recovery (CMR), ash rejection (AR),
− 0.106 25.68 pyritic sulphur rejection, (PSR), ash separation efficiency (ASE), and
Total 100 pyritic sulphur separation efficiency (PSSE) were determined using

Fig. 1. Mineralogical and petrographical examinations of the coal sample.

96 T. Uslu et al. / Fuel Processing Technology 101 (2012) 94–100

the following equations: CMR (%)=(WP /WF)×100; AR (%)=[(AF − Increasing bowl speed adversely affected the combustible matter
AP)/AF]×100; PSR (%)=[(PSF −PSP)/PSF]×100; ASE (%)=CMR+AR− recovery (Fig. 4). Increasing centrifugal force at high bowl speeds
100; PSSE (%)=CMR+PSR−100. In the above equations, WP: weight of caused also organic coal matter as well as mineral matter to retain
dry ash-free product (g), WF: weight of dry ash-free feed (g), AF: ash in between the ribs despite its lower specific gravity than mineral
dry feed (wt.%), AP: ash in dry product (wt.%), PSF: pyritic sulphur in dry matter, i.e., light particles were also overwhelmed by centrifugal
feed (wt.%), PSP: pyritic sulphur in dry product (wt.%). force. The adverse effect of increasing bowl speed on combustible
matter recovery was more dominant at higher particle size range due
to the higher effect of centrifugal force at coarse particle sizes. The
3. Results and discussion lower the pressures of fluidizing water, lower combustible matters
were recovered at the same bowl speeds. Because, light coal particles
Test results revealed that the Knelson concentrator provided a trapped or misplaced in the retention zone could not be removed due
considerable amount of ash and pyritic sulphur removal from the to insufficient water pressure.
coal. The performance of the cleaning process can be obviously seen In general, a higher amount of ash was rejected with increasing
from the pyrite contents of feed, tailings and clean coal (Fig. 3). particle size in contrary to expectation that higher ash removal would

Fig. 2. Schematic representation of Knelson Concentrator (a) [3], Knelson concentrator used in the tests (b), Clean coal slurry collected in buckets (c), Tailings remained between the
ribs of the bowl (d), Dewatering of clean coal slurry by vacuum filter (e), Final clean coal product (f) [6].
 T. Uslu et al. / Fuel Processing Technology 101 (2012) 94–100 97

Table 3
Test conditions and variables.

Test no. Particle size (μm) Bowl speed (rpm) Fluidizing water pressure (kPa) Test no. Particle size (μm) Bowl speed (rpm) Fluidizing water pressure (kPa)

1 − 106 500 10 25 − 300 + 106 1500 10

2 − 106 500 20 26 − 300 + 106 1500 20
3 − 106 500 30 27 − 300 + 106 1500 30
4 − 106 500 40 28 − 300 + 106 1500 40
5 − 106 1000 10 29 − 300 + 106 2000 10
6 − 106 1000 20 30 − 300 + 106 2000 20
7 − 106 1000 30 31 − 300 + 106 2000 30
8 − 106 1000 40 32 − 300 + 106 2000 40
9 − 106 1500 10 33 − 500 + 300 500 10
10 − 106 1500 20 34 − 500 + 300 500 20
11 − 106 1500 30 35 − 500 + 300 500 30
12 − 106 1500 40 36 − 500 + 300 500 40
13 − 106 2000 10 37 − 500 + 300 1000 10
14 − 106 2000 20 38 − 500 + 300 1000 20
15 − 106 2000 30 39 − 500 + 300 1000 30
16 − 106 2000 40 40 − 500 + 300 1000 40
17 − 300 + 106 500 10 41 − 500 + 300 1500 10
18 − 300 + 106 500 20 42 − 500 + 300 1500 20
19 − 300 + 106 500 30 43 −500 + 300 1500 30
20 − 300 + 106 500 40 44 − 500 + 300 1500 40
21 − 300 + 106 1000 10 45 − 500 + 300 2000 10
22 − 300 + 106 1000 20 46 − 500 + 300 2000 20
23 − 300 + 106 1000 30 47 − 500 + 300 2000 30
24 − 300 + 106 1000 40 48 − 500 + 300 2000 30

a
100
90
Combustible Matter

80
Recovery (%)

70
60
50 FWP (10 kPA)
40 FWP (20 kPA)
30 FWP (30 kPA)
20 FWP (40 kPA)
10
0
500 1000 1500 2000
Bowl Speed (rpm)

b 100
90
Combustible Matter

80
Recovery (%)

70
60
50
40 FWP (10 kPA)
30 FWP(20 kPA)
20 FWP (30 kPA)
10 FWP (40 kPA)
0
500 1000 1500 2000
Bowl Speed (rpm)

c 100
90
Combustible Matter

80
Recovery (%)

70
60
50
40
FWP(10 kPA)
30 FWP (20 kPA)
20 FWP (30 kPA)
10 FWP (40 kPA)
0
500 1000 1500 2000
Bowl Speed (rpm)

Fig. 4. Effects of bowl speed and fluidizing water pressure (FWP) on combustible
Fig. 3. Microscopic examination of tailings (a), feed (b), and concentrate (c) of the matter recovery for size fractions of − 106 μm (a), − 300 + 106 μm (b), − 500 +
cleaning process. 300 μm (c).
98 T. Uslu et al. / Fuel Processing Technology 101 (2012) 94–100

matter trapped in the retention zone. Therefore, the ash percent of the
concentrate could not be reduced considerably due to loss of organic coal
matter even if some degree of ash was removed from the coal. The
adverse effect of increasing bowl speed on the ash rejection especially at
low fluidizing water pressures was resulted from the reduction of
elutriation effect of water by centrifugal force. Lower water pressure at
high bowl speeds could not remove entrained coal particles from the
retention zone and mineral matters could not find sufficient space in the
retention zone to enter.
Increasing bowl speed created more pronounced effect on pyritic
sulphur rejection than ash rejection (Figs. 5, 6). Therefore, higher
rejections of pyritic sulphur than that of ash were obtained owing to
higher specific gravity of pyrite than other ash forming minerals, i.e.
higher effect of centrifugal force on pyrite. The critical bowl speeds above
which a trend of decline in the pyritic sulphur rejection occurred was
observed. The rejection of pyritic sulphur improved at coarse fractions.
High pyritic sulphur and ash rejections were generally observed at low
fluidizing water pressures at the expense of low combustible matter
losses.

Fig. 5. Effects of bowl speed and fluidizing water pressure (FWP) on ash rejections for
size fractions of − 106 μm (a), − 300 + 106 μm (b), − 500 + 300 μm (c).

be obtained at small particle sizes due to better liberation of mineral

matter (Fig. 5). This can be attributed to diminishing effects of density
and centrifugal force on the separation process with the reduction in
size. In other words, the effectiveness of centrifugal force to capture the
heavy mineral matters into retention zone increased with increasing
particle sizes. However, the large coal particles (−500+300 μm) were
also retained in the retention zone at high bowl speeds reducing the
extent of ash rejection. At −500+300 μm size fraction, ash rejection
increased with increasing bowl speed up to 1500 rpm above which a
reverse trend was observed at the fluidizing water pressures of 20, 30,
and 40 kPa. However, at the lowest water pressure of 10 kPa, the bowl
speed above which ash rejection reduced was 1000 rpm. At the size
fractions of −300+106 μm, the reduction in ash rejection at >1500 rpm
occurred only at the fluidizing water pressures of 10 and 20 kPa above
which no negative effect was observed. At the size fraction (−106 μm)
tested, ash rejections generally increased slightly with high bowl speeds.
Increased removal of ash at high bowl speeds can be attributed to greater
effect of centrifugal force on fine pyrite and other ash forming minerals.
However, positive effect of increasing centrifugal force on the ash
removal diminished beyond a certain bowl speed at low fluidizing water Fig. 6. Effects of bowl speed and fluidizing water pressure (FWP) on pyritic sulphur
pressures due to lack of fluidizing water to remove the organic coal rejection for size fractions of − 106 μm (a), − 300 + 106 μm (b), − 500 + 300 μm (c).
 T. Uslu et al. / Fuel Processing Technology 101 (2012) 94–100 99

c 100
90
Pyritic Sulphur Separation

80
10 kPA

70
Efficiency (%)

60
30 kPA

50
20 kPA

40
40 kPA

30
20
10
0
-10
-20
-30
500 1000 1500 2000
Bowl Speed (rpm)
Fig. 7. Effects of bowl speed and fluidizing water pressure (FWP) on ash separation
efficiency for size fractions of − 106 μm (a), − 300 + 106 μm (b), − 500 + 300 μm (c). Fig. 8. Effects of bowl speed and fluidizing water pressure (FWP) on pyritic sulphur
separation efficiency for size fractions of − 106 μm (a), − 300 + 106 μm (b), − 500 +
300 μm (c).

Maximum ash separation efficiency was determined to be 15.29%,

34.62% and 39.53% for the size fractions of −106 μm, −300 + 106 μm pyritic sulphur removals were 36.8% and 75.7%, respectively from
and −500 + 300 μm, respectively (Fig. 7) for which maximum unclassified feed. Paul and Honaker [10] reported 57% ash removal and
separation efficiency for pyritic sulphur was 45.72%, 67.91%, and 85% pyritic sulphur removal from the coal. Butcher and Rowson [11]
67.46% (Fig. 8). These findings suggest that the Knelson concentrator obtained maximum ash rejection of 45.4% and sulphur reduction of
is more successful in removing pyrite than ash. The separation 25.9%. Honaker and Das [14] reported approximately 63% ash reduction
efficiency for ash and pyritic sulphur tended to be inter-dependent on from coal with 70% combustible recovery. Honaker et al. [15] reported
both bowl speed and particle size in that it was high for −106 μm size 55.6% ash reduction from a coal sample while recovering 80% of energy
fraction at high bowl speeds, for −300 + 106 μm at medium speeds, value. Majumder at al. [17] removed 45.38% of ash from the coal with 40%
and for − 500 + 300 μm at low speeds. Centrifugal force surpassing yield. In agreement with the current findings, the previous studies have
the fluidizing water pressure as a result of increase in particle size also shown that low fluidizing water pressures resulted in the coal with
resulted in the passing of coarser and heavier coal particles into low ash content [15,17] and high combustible matter [17]. Reducing of
retention zone and deterioration of selectivity. Reduced efficiency of ash content and combustible matter recovery with increasing bowl speed
ash and pyritic sulphur rejection at high bowl speeds and coarse sizes was also reported [15].
was resulted from decline in combustible matter recovery.
In this study, optimum values of ash rejections and pyritic sulphur 4. Conclusions
rejections have revealed to be 45.96% and 70.45%. However, their
maximum values were determined to be 60.94% and 91.60%, respec- Pyritic sulphur and ash were found to be substantially rejected
tively. In the preliminary study by the authors [6] maximum ash and from oxidized coal by using the Knelson concentrator. The Knelson
100 T. Uslu et al. / Fuel Processing Technology 101 (2012) 94–100

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