Coking Coal Washing

Chapter 7

Coking Coal Washing 7.1 COKING COAL WASHING 7.1.1 History of Coal Washing The technological progress in coal preparation from a global perspective is indicated in Fig. 7.1. The history of establishment of coal washeries in India is illustrated in Fig. 7.2, in which the period is divided into landmarks of 10 years, starting with 1950. Prior to 1950, coals were selectively mined and directly used after sizing for blast furnaces without beneficiation. The sequence of installation of washeries is as follows. 1950
59 The first Indian coal washery, known as West Bokaro washery 1, was set up in 1951 regardless of widespread doubts about the washability of Indian coals, followed by the second one installed at Jamadoba in 1952
both by M/s Tata Iron and Steel Co. Ltd (TISCO). Two more washeries were added by 1959. The third washery was at the Lodna colliery set up by M/s Turner Morrison in 1955. The fourth washery in the public sector was commissioned at Kargali by NCDC (now CCL) in 1958. 1960
69 In 1960, SAIL constructed a washery at Durgapur. Central Coal Washeries Organization (CCWO) set up four central washeries from 1961 to 1968. The Durgapur washery of DPL and Chasnalla washery of Indian Iron and Steel Co. (IISCO) were installed in 1967 and 1968, respectively. In 1969, Kathara washery (CCL) was built with a rated input capacity of 3 Mt/y to treat medium coking coal. 1970
79 NCDC (now CCL) installed two washeries
Swang and Gidi
both in 1970 for the upgradation of medium coking coal. 1980
89 Six more washeries came into being. West Bokaro washery 2 of raw coal throughput (1.8 Mt/y) was commissioned in 1982 by TISCO. It had the then latest technology. Two pithead washeries in the Jharia coalfield were installed at Sudamdih and Moonidih to treat prime coking coal, each with a throughput capacity of 2 Mt/y. The Rajrappa washery (CCL) and Nandan washery (WCL) were commissioned in 1984 and 1989, Sustainable Management of Coal Preparation. DOI: https://doi.org/10.1016/B978-0-12-812632-5.00007-0 © 2018 Elsevier Inc. All rights reserved.

133

FIGURE 7.1 Progression of global technological development in coal preparation.

FIGURE 7.2 History of Indian coal washing.

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respectively, to beneficiate medium coking coals with the latest equipment and instruments. A demonstration plant of 100-t/h throughput capacity was set up in 1982 at Barora (BCCL) to beneficiate difficult-towash prime coking coal. 1990
2000 Five more washeries came into existence, with one at Mahuda (BCCL) for medium coking coal in 1990. In the mid-1990s, two washeries were built by TISCO
West Bokar washery 3 and Bhelata washery. Kedla washery (CCL) was installed in 1997 to treat medium coking coal. In 1998, Madhuban washery (BCCL) was installed to treat prime coking coal. The West Bokaro washery 1 had become obsolete and was closed down. The West Bokaro washery 3 was a state-of-the-art plant befitting Tata Steel’s tradition. It could serve as an illustrious benchmark for the coal preparation industry of India.

7.2 THE OBJECTIVE OF WASHING As the reserves of moderately easy washing coking coals are depleting continuously, the future requirement for coking coals will need to be met from seams with inferior or more difficult cleaning characteristics. As the coking coal reserves are limited, every effort should be made to develop new techniques for washing as well as blending of different types of coal properly. The advancements in the field of washing are one of the most important aspects by which the coking coal reserves can be conserved. The basic purpose of washing is to reduce the ash-forming impurities from raw coal so that better coal with less ash content can be produced. If these ash-forming constituents are not removed, and coal is directly charged to the coke-ovens, the ash content in coke will increase and subsequently more slag will be produced in the blast furnace, thereby lowering the efficiency of the blast furnace.

7.3 NATURE OF IMPURITIES The impurities associated with coals depend on two conditions, i.e., the nature of formation and the method of exploitation. The Indian coals are of ‘drift’ origin, so mineral dirt and mineral matters are intimately mixed with most coals. These impurities are mainly of an inherent type and have made the Indian coals very difficult to wash (Kumar, 1981). To meet the increasing demand for coal, mechanisation has been introduced both in mining and loading. As a result, selective mining is not possible and extraneous impurities get mixed up with raw coal produced from the mine. These impurities usually come from the floor, roof and intermediate dirt bands associated with coal seams.

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137

The impurities in coal generally consist of shale, sandstone, pyrites, siderites, calcites, etc., which form the noncombustible or ashy matter of coal. These impurities, when mixed as extraneous matter with coal, can be separated partly or wholly, but if these are present as intergrown consisting of inherent mineral matter then it becomes more difficult to wash.

7.4 EFFECT OF IMPURITIES IN COAL It is found in steel plants that a 1% increase in ash in coking coal results in the following deteriorating effects (Kumar, 1982): G G G G

G

Increases the slag volume involving more expenses for slag dumping; Decreases production of blast furnaces by about 3%
6%; Increases coke consumption by about 4%
5%; Decreases the yield of carbon in coke, resulting in more coke ovens and higher expenditure; Increases limestone consumption by about 5%. Yield of tar and gas is reduced.

7.5 ‘EASE’ OF WASHING The ease of separation depends on the amount of near density material 6 0.01 relative density present at the desired density of separation. It has been observed that the increasing amounts of near density materials have an adverse effect on washing efficiencies in general. In Indian coals, it is found that the percentage of near gravity material is very high, about 25% at the gravity of separation
which has been termed as formidable by Bird’s classification. As the proportion of NGM in coking coal is high, Bird’s classification does not hold good in the Indian context. The Indian coking coals can be broadly classified considering the primary cut for the recovery of clean coal as shown in Table 7.1. Thus, the separation process required in Indian conditions should be exceptionally efficient for maximum recovery of coking coal. However, the amount of near gravity material decreases considerably, TABLE 7.1 Classification of Washability Behaviour According to NGM NGM (%)

Ease/Difficulty in Washing

Up to 20

Easy

21
40

Moderately difficult

41
50

Difficult

Greater than 50

Extremely difficult

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Sustainable Management of Coal Preparation

80

70

60

NGM (%)

50

40

30

20

10

1.4

1.6

1.5

1.7

1.8

1.9

Specific gravity FIGURE 7.3 Specific gravity versus NGM.

as shown in Fig. 7.3, as the density of separation increases, so for deshaling operation at a high density, there will be little increase in recovery and, therefore, revenue for the extra capital cost of an efficient separation process.

7.6 DEGREE OF WASHABILITY The densimetric, characteristics ash and the 6 relative density curves can only compare ash/yield or relative density/yield relationships depending on the quality of raw coals. They do not give a proper measure for comparison between coals. The concept of ‘washability number’ was developed by Sarkar and Das (1974), taking into consideration the raw coal ash, the relative density of separation, and consequently, the inherent ash of coal. Washability number The degree of cleaning may be expressed as, ða 2 bÞ4a where, a 5 raw coal ash; b 5 clean coal ash.

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139

The degree of washability may be expressed as, N 5 wða 2 bÞ4b where, w 5 yield of clean coal. Nopt is a cut-off point where N is maximum. The degree of washability number (N) calculated for a range of cut-off points are plotted and the maximum value (Nopt) with the corresponding cutoff point is determined from the curves. The clean coal ash associated with the cut-off point at Nopt has some added significance and can be expressed as bopt. An expression called washability number (WN) can be developed as, WN 5 Nopt 4bopt where, bopt 5 ash content at Nopt. To magnify the scale for convenience; the above expression can be multiplied by a factor of 10. The final formula becomes
WN 5 Nopt 4bopt 10 The ‘washability number’ and the results (yield and ash) which correspond to the optimum cut-off points for some coal samples from a number of countries around the world have been discussed (Sarkar, 1986; Sanders and Brookes, 1986; Holuszko, 1994). Carboniferous coals of Europe and North America have the highest washability numbers, varying from 96 to 157, with the lowest ash at the optimum degree of washing (between 3% and 6%), while Mesozoic coals have lower washability numbers (ranging from 25 to 95) with optimum clean coal ash from 4% to 12%. The lowest washability numbers are found in Gondwana coals of Permian age, with washability numbers of 19
28 at optimum clean coal ash fluctuating between 8% and 16%, indicating their inherent difficulty in washing. The washability number significantly points to the differences in the case of washing far more clearly than a comparison of yields would do. Gondwana coals are very difficult to clean as compared to Northern Carboniferous coals. This number is a very useful tool for comparative measures of different types of coals. It can highlight the washing difficulties and establish a relative rating order for various coals. Using the knowledge of variability in the washability numbers, it may be possible for a coal process engineer to make decisions on the size of crushing, method of blending and even on designing specific cleaning operations for a particular coal (Holuszko, 1994).

7.7 MEASURES OF EFFICIENCY As no commercial coal beneficiation is perfectly efficient, some indices are required to measure the efficiency of the process. The best way of indicating

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Sustainable Management of Coal Preparation 100

Distribution numbers (Probability of particle floating)

Theoretical separation 75 EP =

d 25 – d 75 2

Partition density = d 50

50

Actual separation

25

d 75

d 25 d 50

Increasing specific gravity FIGURE 7.4 Tromp curve.

the efficiency of a density separation device is the distribution of the partition curve (Fig. 7.4), which was first proposed by Tromp (1937). This curve depends on the equipment used, the relative density or cut-off point and the size range of the feed coal. Various simpler measures of efficiency have been defined but none are as accurate in predicting the performance of a density separation device as the Tromp curve. It denotes the probability of a particle reporting with the floats to its specific gravity. The distribution numbers are marked on the vertical axis against the various specific gravity fractions shown on the horizontal axis. Thus, if the vertical axis has value x, then the corresponding value of the horizontal axis is denoted as dx. The partition density is denoted by d50
the distribution number is 50, in this condition the particle will have an equal chance of floating or sinking. The Tromp curve is nothing but an error curve, the steeper the curve, the most efficient is the separation. To measure the inclination of the curve, Terra introduced ‘Ecart Probable’ (Ep) which is defined as, Ep 5 ðd25 2 d75 Þ42 When, Ep 5 0, the curve becomes a straight vertical line at the specific gravity of separation
the efficiency of separation will be 100%. The Ep value does not consider the tails of the Tromp curve
that are above the distribution number 75 and below the distribution number 25. The larger tails in the Tromp curve result in lower yield at the desired ash. The shaded area in Fig. 7.4 between the partition curve and the theoretically perfect partition curve is defined as the error area. The imperfection I is defined as,

Coking Coal Washing Chapter | 7

I5

141

Ep ðd50 2 1Þ

The ash error is the difference between the percentage ash in the clear coal and the percentage ash of the floats in a washability test giving the same yield. The yield error is the difference between the actual yield and the theoretical (washability test) yield giving the same percentage ash (or other measure of quality) in the clean coal. The efficiency at any relative density is defined as (Sarkar and Das, 1978) the recovery % of clean coal (ash % of raw coal
ash % of clean coal) divided by the recovery % of float coal (ash % of raw coal
ash % of float coal). It may be noted that the most widely accepted measure of the efficiency with which a cleaning device separates coal from impurities is referred to as probable error (Ep) and Ep/dp which is sometimes called the generalised probable error (Gottfried and Jacobsen, 1977).

7.8 EFFICIENCIES OF VARIOUS PROCESSES The US Bureau of Mines has published detailed data on the generalised partition curves for five processes: dense medium cyclone, water-only cyclone, concentrating table, dense medium vessel, and Baum jig. Fig. 7.5 gives the partition curve for three of these processes for the same density of separation (1.5 RD) and the same size range (1.17
0.59 mm). Table 7.2 gives the generalised probable error (imperfection) for different feed sizes and it also details how the ratio of separation increases as the size of the feed coal decreases. 100

90 80

Partition coefficient

70 60 50 40 Concentrating table

30

Water-only cyclone

20 Dense medium cyclone 10 0.4

0.6

0.8

1.0

1.2

1.4

Specific gravity

FIGURE 7.5 Partition curves for different equipment.

1.6

1.8

2.0

2.2

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TABLE 7.2 Numerical Values of Generalised Probable Error and Density of Separation Ratios Type of Cleaning Equipment

Feed Size (Fraction and Composite) (mm)

GEpa

Feed Size (Fraction) (mm)

Ratiob

Dense-medium cyclone

19
12.7

0.014

19
12.7

0.995

12.7
9.5

0.015

12.7
9.5

0.989

Water-only cyclone

Concentrating table

Dense-medium vessel

9.5
6.3

0.016

9.5
6.3

0.991

6.35
2.38

0.019

6.35
2.38

0.999

2.38
1.19

0.025

2.38
1.19

1.019

1.19
0.59

0.034

1.19
0.59

1.042

19
0.59

0.019

6.35
4.76

0.084

6.35
4.76

0.810

4.76
2.38

0.100

4.76
2.38

0.842

2.38
1.19

0.118

2.38
1.19

0.861

1.19
0.59

0.130

1.19
0.59

0.956

0.595
0.297

0.121

0.595
0.297

1.074

0.297
0.149

0.090

0.297
0.149

1.199

0.149
0.074

0.110

0.149
0.074

1.362

6.35
0.074

0.189

9.5
6.3

0.042

9.5
6.3

0.988

6.35
2.38

0.044

6.35
2.38

0.994

2.38
1.19

0.051

2.38
1.19

0.974

1.19
0.59

0.068

1.19
0.59

0.970

0.595
0.297

0.089

0.595
0.297

1.028

0.297
0.149

0.101

0.297
0.149

1.128

0.149
0.074

0.191

0.149
0.074

1.212

9.5
0.074

0.058

152
102

0.013

152
102

0.969

102
51

0.016

102
51

0.981

51
25.4

0.019

51
25.4

0.993

25.4
12.7

0.025

25.4
12.7

1.010

12.7
6.3

0.030

12.7
6.3

1.031

152
6.3

0.020 (Continued )

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143

TABLE 7.2 (Continued) Type of Cleaning Equipment

Feed Size (Fraction and Composite) (mm)

GEpa

Feed Size (Fraction) (mm)

Ratiob

Baum jig

152
76

0.034

152
76

0.946

76
41

0.030

76
41

0.959

41
12.7

0.061

41
12.7

1.014

12.7
6.3

0.092

12.7
6.3

1.088

6.35
2.38

0.109

6.35
2.38

1.047

2.38
1.19

0.126

2.38
1.19

1.088

1.19
0.3

0.272

1.19
0.3

1.311

152
0.3

0.082

a

GE is the generalised probable error which is equal to the conventional probable error (Ep) divided by the relative density of separation (dp). b The density of separation ratio is the density of separation of the size fraction divided by the density of separation of the composite feed. Source: Modified from Gottfried, B.S., Jacobsen, P.S., 1977. Generalised distribution curve for characterising the performance of coal cleaning equipment, US Department of Interior, Bureau of Mines report of investigations 8238, Washington, DC.

The probable error generally increases with decreasing size of the coal feed. This is further illustrated in Fig. 7.6, which gives the partition curves for three different size ranges in a heavy-medium cyclone. Fig. 7.7 illustrates the distribution curves for various coal-cleaning devices and indicates that the dense-medium devices are the most efficient of all coal-cleaning processes. Fig. 7.8 indicates the relative efficiencies of five coal treatment systems. Fig. 7.9 demonstrates how dirty process water literally sabotages the plant efficiency. Dirty process water contains too many solids, which create their own medium, thus making the specific gravity higher than required and increasing the viscosity. The higher specific gravity and viscosity cause the misplacement of refuse to the clean coal. As a result, the quality of clean coal deteriorates and the overall separating efficiency of the plant goes down. Clean water is vital to the efficiency of the plant and one way to achieve this is through a thickener or settling pond.

7.9 LIBERATION CHARACTERISTIC The liberation size is a fundamental characteristic of the coal type (Vince, 2013). Estimating the required liberation size by physical cleaning requires the ash-forming components of raw coal to be sufficiently liberated from the

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100 90 80

Partition coefficient

70 60 50

Size fraction:

40

6.4 mm x 1.2 mm

30

1.2 mm x 0.6 mm 0.6 mm x 0.2 mm

20

10 1.2

1.4

1.8

1.6 Specific gravity

2.0

FIGURE 7.6 Partition curves for a heavy-medium cyclone for three size fractions.

100 90 80

Partition coefficient

70 60 Dense medium separators Jigs Air tables Water-only clones Concentrating tables

50 40 30 20 10 13

1.4

1.5 Specific gravity

1.6

1.8

2.0

FIGURE 7.7 Partition curves for various separation processes.

coal. The liberation size is conventionally derived by quantifying the size reduction necessary to reduce the gangue
coal particle conglomerates such that the individual low ash value grains prevail. Tree flotation is a technique, particularly applicable to hydrophobic coals, which can be used to estimate the ash value of most liberated coal particles present and this is related to particle-size distribution in a given sample (Nicol, 2001).

Coking Coal Washing Chapter | 7

145

es

Flota

har t

Reic

c .cy

M D.

Efficiency

tion

lon

Spirals

s

ne Fi

jig

W.W. Cyclones

75

500 Size in microns

1000

3000

FIGURE 7.8 Relative efficiencies of fine coal treatment systems.

FIGURE 7.9 Plant efficiency depends on the solid content in process water.

Coal grain analysis is an optical microscopic imaging method that is used for routine coal petrography assessments (maceral composition and coal rank) and for obtaining compositional information on individual coal grains. This method determines the number of mineral inclusions within the particles and is a good complement to tree flotation and washability by size information techniques (Ofori et al., 2004; O’Brien et al., 2011). In any mineral beneficiation process, the usual practice is to first crush the material to liberate the different minerals from attachment to one another so that they can be separated in the subsequent methods of beneficiation. The separation of different minerals is possible because of differences in their physical or chemical properties. The efficiency of separation depends on the degree of liberation, i.e., freeing of gangue and valuable mineral

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particles. The extent of comminution needs to be studied so as to liberate the valuable minerals from the rock matrix. This depends upon the intergrowth in size and the structure of particles. The dimensions of the particles determine the extent of comminution required. The particle can be termed as a ‘grain’ as it can be seen under a microscope. If all the valuable minerals are made free, then the liberation of the minerals is complete. As comminution is a high-cost affair, an economic approach is to be taken. Here, a study has been undertaken to determine the liberation characteristics of Indian coking coals on the basis of float and sink tests on coal crushed to different top sizes.

7.9.1 Petrological Analysis Quantitative evaluation of minerals by scanning electron microscope and mineral liberation analysis are automated analytical systems provided by FEI Natural Resources to estimate detailed mineralogical information. Although they are specifically designed for metalliferous minerals, they can be used to determine the size of minerals present in a coal matrix. To plan the washing system of any coal, the petrological composition should be studied under a microscope. Under a microscope, various constituents, such as organic macerals of combustible matter and inorganic minerals of ashy matter, can be seen. The petrological texture of coal indicates the possibility of liberation of different constituents by crushing and grinding. The first step of washing is to crush or grind the coal to liberate the coaly matter from ashy matter, which can theoretically yield complete liberation. In the case of intergrown texture, finer grinding, up to a few microns, is necessary for complete liberation, whereas for free dirt ordinary crushing is sufficient (Kumar, 1982).

7.9.2 Degree of Liberation The degree of liberation of a mineral can be defined as the percentage of that mineral occurring as free particles with respect to the total of that mineral present. This can be expressed as per Eq. (7.1).   ð7:1Þ L 5 100 Nf =ðNf 1 Nl Þ % where L 5 degree of liberation; Nf 5 number of free particles; Nl 5 number of locked particles; The liberation information can be obtained by release analysis and float and sink tests. For a binary mineral consisting of concentrate (1) and refuse (2), the composition can be expressed in terms of weight percentages. It is

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147

assumed that the feed or raw mineral is a true physical mixture of two components. The degree of liberation can be defined as the percentage of free concentrate out of the total content of the ore body. L5

Va ρa 100 Va ρa 1 Vb ρb

where, L 5 degree of liberation, V 5 volume, ρ 5 density, and Va 1 Vb 5 1.

7.9.2.1 Evaluation of Liberation Microscopic examination of the polished section of a sample cannot give the correct information as to the optimum point of liberation but it can serve as a guide. Using this method, the degree of grinding required for maximum liberation can be roughly estimated. The evaluation of liberation involves comminution, study under microscope and/or beneficiation tests. From different float and sink data of coal crushed to different sizes, the degree of liberation can be calculated. For coal, the degree of liberation can be expressed as per Eq. (7.2). L5

Yð100 2 ZÞ ð100 2 XÞ

ð7:2Þ

where, Y 5 weight of free particles, i.e., yield % of clean coal; Z 5 ash % of clean coal; X 5 ash % of raw coal.

7.9.2.2 Optimum Level of Crushing It is believed that finer crushing of coal will yield more coal in the cleaning process. But the increase in yield by finer crushing is offset by many factors. Thus, the degree of crushing has to be judiciously selected so that the total system becomes economical. In commercial coal washing practice, crushing the coal will bring many disadvantages. The efficiency of separation in conventional gravity washers is low in the case of fine coal. Further, a lot of slurry will be produced and it is difficult to dewater this slurry below 10% moisture content without resorting to thermal drying. The quantity of slurry or coal fines determines the total moisture content of washed coal. For the same throughput, the size of units treating small-size coal has to be much larger than those for coarse coal. The capital and operating costs will proportionately increase along with the added cost of slurry treatment. 7.9.2.3 Cumulative Float Curves (Yield/Ash Curves) The most important implication of determining the liberation size is the impact of grind size on the yield/ash curve. For the general case of a poor-

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quality coal, it may be necessary to reduce particle sizes to smaller levels to achieve an adequate degree of liberation. From the float and sink data, cumulative float curves can be drawn for the calculation of liberation. These curves show the ash content of clean coal against the yield of clean coal. These are drawn by plotting the cumulative yield against cumulative ash of fraction. The starting point of the curves shows the ash content of the lightest coal particle present in coal, whereas the end of the curve shows the overall ash of coal. From the cumulative float curves, the yields of clean coal at 15%, 16%, 17%, 18%, and 19% ash for each sample crushed to different top sizes are noted. With these data, the degree of liberation can be calculated with the help of Eq. (7.2). The calculated data are presented in Table 7.3 and liberation at 17% ash has been plotted in Fig. 7.10. It is clear that the degree of liberation increases along with finer crushing.

7.10 SPECIAL CHARACTERISTICS The density distribution in Indian coals is characterised by a low proportion or complete lack of low relative density fractions below 1.3 or 1.4, which is evident from the density distribution curves in Fig. 7.11. Lack of density fraction accounts for the lack of low ash fractions. As a consequence, Indian coals, as shown in Fig. 7.12 are incapable of producing clean coal at low ash levels common in Northern Carboniferous coalfields. Most Indian coal seams contain a large proportion of true middlings. Even though the clean coal has a relatively high ash content, the rejects (discards) contain a substantially higher proportion of combustibles than what would normally be acceptable in Europe and North America. Some of the typical washability characteristics of Indian coals are: (1) low recovery of clean coal, (2) more yield of middlings, (3) poor elimination of dirt, and (4) higher content of neat-gravity materials. Due to these factors, it is rather impossible to bring down the ash content of clean coal below 10%, which is the limit of ash in coking coal as supplied to steel plants in other countries. Thus, Indian steel plants have been forced to adopt the technology of steel-making with coking coal of higher ash content (17%). It is observed that the quality of washed coal from different washeries is deteriorating because of an increase in the ash content of raw coal feed to washeries. So, the present system of washing will not hold good any more for coal beneficiation in the future.

7.11 EXISTING SYSTEM OF BENEFICIATION Until the Second World War, it was the common conviction that Indian coals were not amenable to washing. This belief led to selective mining of goodquality coking coals for different uses, even other than steel-making. It

TABLE 7.3 Yield and Degree of Liberation at 17% Ash of Clean Coal Raw Coal Sample

Crushed to 75 mm

Crushed to 25 mm

Crushed to 13 mm

Yield (%)

Liberation (%)

Yield (%)

Liberation (%)

Yield (%)

Liberation (%)

Raw coal ash 34.2%

33.7

42.5

40.2

50.71

43.2

54.49

Raw coal ash 30.4%

48.34

57.65

57.0

67.97

59.0

70.36

Raw coal ash 25.6%

66.82

74.54

74.2

82.78

76.5

85.34

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Raw coal ash 34.2%

Raw coal ash 25.6%

Raw coal ash 30.4%

90

Degree of liberation (%)

80

70 60 50 40 30

75

13

25 Crushing (mm)

FIGURE 7.10 Degree of liberation at 17% ash of clean coal. 50

50

Sample 1

40

40

Weight (%)

Sample 2

Weight (%)

30

20

20

10

10

0 1.3

30

1.4 1.5

1.6 1.7 1.8

Specific gravity

1.9 2.0

0 1.3

1.4 1.5

1.6

1.7

1.8

1.9

2.0

Specific gravity

FIGURE 7.11 Density distribution.

resulted in depletion of better-quality coking coals and now we are left with poor-quality coals. As the ash content of the available coking coal reserves in the country is seldom below 17%, washing of coals has become a must. The average ash content in raw coal usually varies from 30% to 40%.

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Coking Coal Washing Chapter | 7 100 90

Sample 2

Sample 1

80 70

Yield (%)

60 50 40 30 20 10 0

5

10

15 Ash (%)

20

25

30

35

FIGURE 7.12 Cumulative clean coal curves.

It is usually found in Indian coals that the specific gravity varies from 1.45 to 1.50 when the ash contents are 16%
18% and for specific gravity of 1.6
1.7, the ash contents are between 32% and 35%. This difference of property is utilised in gravity separation of coal. In coal washing, artificial suspension of the above-mentioned densities is produced and coals are subjected to float and sink. The coals which float at 1.5 specific gravity are called clean coal and the sinks are put into another suspension of specific gravity of 1.7. Floats of the second suspension are called middlings and the final sinks are rejects. Thus, three products are produced by this process. Clean coal is supplied to steel plants, the middlings to thermal power stations, and the rejects are usually dumped near the washery or utilised for power generation. There are at present 16 coal washeries in the coking coal sector with total raw coal input capacity of 29.7 Mt/annum. The names, capacities and principal beneficiation schemes of the 16 operating plants have been indicated in Tables 7.4 and 7.5. The individual capacities of these plants vary from 70 to 800 t/h, the average lying within the range of 500
600 t/h. The beneficiation and dewatering processes are complex. The different combination of equipment comprises of jigs, heavy-medium separators, heavy-medium cyclones, flotation/water-only cyclones, etc. Jigs are sometimes used for prewashing of raw coals for eliminating free dirt before further treatment in the heavymedium baths or cyclones. Yet, small coals with easy cleaning characteristics are treated in Baum or feldspar jigs. In a composite washing system,

TABLE 7.4 Existing Prime Coking Coal Washeries in India Sl. No.

Name of Washery

Company

Year of Commissioning

Annual Rated Input Capacity (Mt)

Main Washing Equipment/Process

Fine Coal Washing (,0.5 mm)

1

Jamadoba

TISCO

1952 (Expn. 1973)

1.72

Two-stage HM bath (75
13 mm), twostage HM cyclone (13
0.5 mm)

Flotation

2

Bhojudih

BCCL

1962 (Expn. 1964)

2.00

HM Leebar bath (75
25 mm), Jig (25
0.5 mm)

Flotation

3

Chasnalla

SAIL

1968

2.00

Two-stage HM Leebar bath (75
20 mm), jig (20
0.5 mm)

Flotation

4

Dugda-II

BCCL

1968

2.40

Deshaling jig (75
13 mm), HM cyclone (13-0.5), Komag jig (cyclone sinks)

Flotation

5

Sudamdih

BCCL

1981

2.00

Two-stage HM cyclone (37
0.5 mm)

Flotation

6

Moonidih

BCCL

1983

2.00

Two-stage HM cyclone (30
0.5 mm)

Water-only cyclone

7

Bhelatand

TISCO

1995

1.00

Two-stage HM cyclone (25
0.5 mm)

Flotation

8

Madhuban

BCCL

1998

1.50

Batac jig (13
0.5 mm)

Flotation

Total 5 14.62

TABLE 7.5 Existing Medium Coking Coal Washeries in India Sl. No.

Name of Washery

Company

Year of Commissioning

Annual Rated Input Capacity (Mt)

Main Washing Equipment/Process

Fine Coal Washing (,0.5 mm)

1

Kathara

CCL

1969

3.00

Deshaling HM Drewboy bath (75
13 mm), HM cyclone (13
0.5 mm)

Flotation

2

Swang

CCL

1970

0.75

Deshaling jig (80
20 mm), HM cyclone (20
0.5 mm)

Water-only cyclone

3

West Bokaro II

TISCO

1982

1.80

Two-stage HM cyclone (13
0.5 mm)

Flotation

4

West Bokaro III

TISCO

Mid-1990s

2.10

Two-stage HM cyclone (13
0.5 mm)

Flotation

5

Nandan

WCL

1984

1.20

Komag jig (75
10 mm), Komag jig (10
0.5 mm)

Flotation

6

Rajrappa

CCL

1989

3.00

Batac jig (80
10 mm), Batac jig (10
0.5 mm)

Flotation

7

Mohuda

BCCL

1990

0.63

HM Cyclone (25
0.5 mm), jig (cyclone sinks)

Flotation

8

Kedla

CCL

1997

2.60

Batac jig (75
13 mm), Batac jig (13
0.5 mm)

Flotation

Total 5 15.08

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Sustainable Management of Coal Preparation

heavy medium baths are used for coarse coal washing and jigs or heavymedium cyclones for small coal washing. Summing up, raw coal is usually crushed to 75 mm and then screened into three fractions 75
25 mm, 25
0.5 mm and ,0.5 mm. The 75
25 mm fraction is washed in heavy medium washer and the 25
0.5 mm fraction either in jigs or heavy-medium cyclones. The ,0.5 fraction is usually upgraded by flotation as shown in Fig. 7.13.

7.11.1 Level of Washing Although three levels of washing are considered in relevance to both earlier and present generations of washeries, the system of washing required for presently available coal resources falls under the category of level III. In the past, fine coal was not properly treated. The level of washing has climbed along with the deterioration of raw coal. As in Fig. 7.14, washeries can be put under different levels but some have ceased operation.

7.11.2 A Classical Flotation Unit The plant is designed with a throughput capacity of 100 t/h with two parallel streams of 50 t/h each. The two streams are interchangeable and meant for fresh slurry. One can be used for dumped coal fines. The fresh thickened slurry from adjusting thickener is pumped to a flotation plant on sieve bends and high-frequency screens for elimination of .0.5 mm coal. The ,0.5 mm coal is pumped to a conditioner for conditioning along with reagents like kerosene oil, pine oil and sodium silicate. After conditioning, the slurry is R.O.M.COAL

Crusher (75–0 mm)

Screening

(75–25 mm) H.M.Washer

Cleans

Cleans storage

Middlings

(25–0.5 mm)

(–0.5 mm)

JIG OR H.M. cyclone

Flotation

Rejects

Cleans

Middlings storage

FIGURE 7.13 Simplified flowsheet for an existing system of washing.

Middlings

Rejects

Rejects storage

Coking Coal Washing Chapter | 7

155

Coking coal Level lll • Coarse, intermediates and fines are all cleaned

Level ll • Coarse coal and intermediates are washed

Level l • Coarse coal treatment only

FIGURE 7.14 Level of washing for coking coal.

adjusted to a pulp density of 150 g/L before being fed to a bank of six cells of 8 m3 each. The froth from the flotation cells is sent to three disc filters through a three-way pulp distributer, with one of the filters working as a standby. The filter cakes are discharged into the existing clean coal conveyor. The tailings of the flotation cells are thickened in two thickeners of 20 m diameter, the underflow of the thickeners is pumped to existing settling ponds and underflow pumped back to flotation unit for reuse. The process flow scheme is shown in Fig. 7.15.

7.11.2.1 Problems and Prospects of Flotation Units It has been observed that the flotation units are not performing well in India, clean concentrates of desired quality and quantity are not produced. Thus, it is necessary to impose proper operational and control measures to achieve the desired results. For that matter, all the parameters controlling the floatability of coal should be known, actually there are many but it is not possible to control all the parameters. As a minimum, the important parameters as enumerated below are to be taken care of assuming the other parameters are constant (Kumar, 1984): G

The coarser size fraction ( . 0.5 mm) should not be allowed to go along with the feed to the flotation cells as it creates trouble in the flotation circuit. The optimum size range for feed coal to flotation is 0.3 mm (48 mesh) to 0.075 mm (200 mesh). Thus, it is preferable to deslime the coal

156

Sustainable Management of Coal Preparation

To washery circuit Reclaimed water Feed from washery

Thickener

Thickener

Sieve bend

(–0.5 mm)

(–0.5 mm)

(+0.5 mm)

Pulp density adjusting tank

Pulp density adjusting tank

Bin for dump

Flotation cells

Pulp distributer

Flotation cells

Tailings thickener

Sump for reclaimed water

Tailings thickener

Reclaimed water

Conditioner

Conditioner

Settling pond

Vibrating screen

Screen undersize collecting sump

Disc filter

Rejects

Clean coal bunker

(+0.5 mm) Screen oversize bin

FIGURE 7.15 Flotation flow scheme of a washery.

G

G

by hydrocyclone and the ,0.075 mm fraction may be discarded or upgraded depending on the quality. As the pulp density is one of the determining factors, this should be adjusted properly, and generally varies from 10% to 12%. Unless sufficient retention time is provided, the performance cannot be effective.

Coking Coal Washing Chapter | 7 G

G

G

G

157

The reagents should be added at the right places, all the reagents should not be added at one point, and they can be distributed throughout the bank of cells. Conditioners ahead of pulp-adjusting tanks should be there so that the coal particles are properly coated with collecting reagents. It is always advisable to keep watch on the proper performance of flotation cells. The pulp should flow freely from cell to cell. The aeration should be sufficient and froths should leave the cells as soon as they are formed. Oxidised coals are difficult to float, and in that case new reagents can be tried as well as the usual MIBC, diesel oil and kerosene oils, and along with other frothers. If these reagents are not properly dispersed, then some coal particles will be coated and others will starve and if more reagents are used to counteract this, a noncoal substance will be floated. If this oil can be broken into fine droplets, then the chances of mass transfer of oils onto coal surfaces increase and coal floatability will improve. Sufficient dispersion of oils is possible by emulsification of these oils.

As beneficiation of coal fines containing enriched vitrinite is essential to maintain the quality of metallurgical coke, due attention needs to be paid to revamp/renovate the fine coal circuits. At least 60% recovery of clean coal at 18% ash from washery tailings is possible. Such a beneficiation project can be set up through an outsourcing mode by adopting a BOM/BOO model (Sengupta and Senapati, 2014).

7.12 OPERATIONAL PROBLEMS A coal preparation plant, even though very well designed, cannot give good results unless proper care is taken in running it with a well-planned operation and management schedule. The operational problems can be divided into two categories: those due to designed defects and those due to defective operation. The problems faced while operating the washeries and their remedies have been analysed. The objective is to achieve the rated capacity of the plant, thus, it is necessary to know how this can be achieved. The rated capacity of a washery is rarely attained due to nonavailability of essential spares and replacements, as well as lack of proper supervision and/or coordination. The quality of raw coal has changed since the washeries were installed. This has adversely affected the quality of washed products as well as the capacity utilisation of the plants. The scenario of coal preparation in India is quite different from that of many other countries. There are many constraints, which are briefly outlined here:

158 G

G

G

G

G

G

Sustainable Management of Coal Preparation

The quality of raw coal to be treated is deteriorating gradually due to the depletion of better-quality reserves. A greater quantity of fines is generated because of greater mechanisation in the method of mining and handling. Because of low output from individual seams, generally a central washery is built. The size consist and washing characteristics of coal feed are divergent. It is difficult to wash mixed coals of different sources. Thus, proper blending facilities are required to be provided. For a central type of washery, raw coal is transported to the plant across long distances, meaning more cost is involved and size degradation takes place due to repeated handling. Because of continuous degradation of raw coal in terms of quality and size, an adequate facility becomes necessary in the capacity of individual circuits and units so that the rated throughput of the plant is achieved. Indian coals yield a large quantity of middlings and rejects, subsequently, disposal of these products poses problems because of their lower heat value.

7.13 SUGGESTIONS FOR IMPROVEMENT Corrective measures both during the design and operation of coal washeries need to be speedily implemented. Built-in capacity of the different sections of a plant should have extra allowance to take care of anticipated variation of size and washability characteristics of raw coal. All the equipment should be easily accessible for maintenance and overhaul to reduce downtime. Design deficiencies of existing washeries should be rectified as soon as possible. Appropriate modifications should also be done in existing washeries to accommodate the deteriorating quality of raw coal feed. The latest techniques of coal preparation should be adopted in future washeries and also in existing washeries in the form of modification schemes. In addition to this, the following major areas need special consideration.

7.13.1 Performance Study The performance of a washery is evaluated by an examination of the extent of fulfilment of different duties of the plant such as (Kumar, 1995): G G G G G

G

Handling the required hourly throughput capacity of the plant; Effective working hours over a shift or day; Efficiency parameter of individual processing and handling units; Yield and quality of washed products; Consumption of magnetite, reagents, water, power, spares for machinery, etc.; Cost of washing.

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159

While evaluating the performance study, the characteristics of raw coal feed to the washery has to be maintained within a reasonable degree of consistency. The actual quantity and quality of clean coal should be compared with those expected from theoretical assessment. In other words, the organic efficiency of the plant as an integrated unit should be determined. Periodic performance study and evaluation are essential for taking corrective steps.

7.13.1.1 Analysis of Idle Period The performance of four washeries is presented in Table 7.6 and the idle time in given Figs 7.16 and 7.17. It is observed that the downtime in the washeries is due to: (1) start/stop, (2) operational difficulties, (3) breakdowns, (4) power failure, (5) raw coal shortage, and (6) other reasons. Except for raw coal shortage, the total downtime is generally attributed to inadequate maintenance. These breakdowns take place not only due to inadequacy of maintenance but also design defects. Breakdowns can be avoided by rectifying the designed defects as well as replacing the spares of affected items in time. Power failures cause spillage, jamming, system contamination and long start up times. Power failure can be avoided by some relatively simple low-cost methods in a short time basis. This may be TABLE 7.6 Analysis of Idle Period Sl. No.

Particulars

Washery A

Washery B

Washery C

Washery D

1

Break-up of idle time (%)

1a:

Start/stop

2.96

4.33

3.82

4.23

1b:

Operational difficulties

6.55

5.31

20.31

4.35

1c:

Break down

1.7

15.79

12.75

8.13

1d:

Power failure

4.67

3.73

4.86

4.83

1e:

Raw coal shortage

0.5

4.01

3.08

3

1 f:

Other reasons

0.52

7.03

5.68

4.16

Total lost period (%) (subtotal of (a) to (f))

16.9

40.2

50.5

28.7

2

Effective period of operation (%)

83.1

59.8

49.5

71.3

3

Total available period (%)

100

100

100

100

FIGURE 7.16 Analysis of idle-time of four washeries (A, B, C and D).

Coking Coal Washing Chapter | 7

Total lost period (%)

161

Effective period of operaion (%)

100% 80% 60%

59.8

49.5 71.3

83.1

40% 20%

40.2

50.5 28.7

16.9

0% Washery A

Washery B

Washery C

Washery D

FIGURE 7.17 Effective period vis-a`-vis lost period of four washeries (A, B, C and D).

possible through on/off compressed air-powered valves, a retrofit programmable logic controller (PLC) system and other modifications. In addition, there are other types of reasons for idle time, such as nonavailability of wagons for off-take of washed products, and subsequently, no adequate place for stocking the products. In the analysis of four washeries, it is observed that washeries B and C suffer greatly from lost periods. Linkage of raw coal to the washeries is a prerequisite for proper utilisation of the washeries. Similarly, for supply of wagons, proper coordination is necessary with the railway authority.

7.13.1.2 Efficiency of Washing Units The efficiency of washing units as expressed by imperfection value remains the same under normal duty ranges of size and quality of coal. The efficiency may go down in the event of faulty operating conditions. Thus, it is necessary to check the efficiency of all the washing units from time to time as it may be difficult to execute this on a daily routine basis. If any anomalies are found, necessary rectifications are to be taken by examination of root causes. Any deterioration in performance of the units will result in loss of clean coal. Thus, it is necessary to know the exact efficiency range of different washing units. This is not only required at the time of selection of the equipment but also during operation. The study of the efficiency of individual washing units plays an important role in the analysis of the performance of the washery. 7.13.1.3 Cost Analysis The cost of clean coal is a function of the expenditure on raw coal, wages, stores, power, interest on depreciation and capital. In addition, the cost is

162

Sustainable Management of Coal Preparation

largely dependent on the percentage recovery of clean coal as well as utilisation of the plant capacity. The cost of clean coal increases with the decrease in the recovery of the clean coal as well as the utilisation of the plant capacity. It is found that the cost of clean coal is more sensitive to recovery than the utilisation of the plant capacity. At present in the existing washeries, the recovery of clean coal varies from 40% to 60% and the plant utilisation capacity 30%
50%. Thus, there is ample scope for improvement in the aforesaid two items.

7.13.2 Control of Operations The performance of coal preparation plants can be properly controlled by taking recourse to the following measures: G G G G G

G

G G G G

G G

G G G G G

Control on the quality of raw coal feed; Blending of raw coal feed from different sources; Efficient operation of individual equipment and the whole system Proper maintenances and supply of spares and replaceable parts; Regular supply of power, water, magnetite and other consumable materials; Regular receipt of the required quantity of raw coal and despatch of washed products; Plant management, supervisory control and training of personnel Quality control of washed products by statistical control charts; Distributed control system of automation by PLC; Design of optimum flow sheet, selection of the right type of equipment and standardisation; Use of proper type of liners against abrasion; Provision for upgrading of raw magnetite in the plant and design of proper type of magnetite circuit; Protective measures against power tripping; Economic design of water circuit; Pollution control against dusts, slurry and noise; Proper disposal of rejects and their possible utilisation; Modification of existing washeries on feedback from research and development.

7.14 CRUSHING OF MIDDLINGS IN THE EXISTING WASHERIES In the composite washing system of coking coal in India, three types of products are usually obtained: clean coal, middlings and rejects. It is observed that the proportion of middlings is rising day by day on one hand, and on the other hand the yield of clean coal is decreasing in the washeries because

Coking Coal Washing Chapter | 7

163

superior-grade raw coal is depleting gradually. This phenomenon results in short supply of coking coal to the steel industry. The middlings which are used presently for power generation contain a good amount of coking coal. Satisfactory separation of coal from mineral matter is not possible by conventional methods of separation as Indian coals are of high intergrown nature. The ash content of middlings is usually 30%
38%, while production varies from 20% to 30%. This substantial amount of middlings presents an excellent potential source of coking coal in view of the conservation of this scarce national raw material. The coking coal resources can be augmented by recovering extra low ash coking coal from washery middlings by suitable beneficiation processes (Jha, 2012). This possibility has been established from tests. The middlings were crushed to 13 and 3 mm and thereafter subjected to float and sink tests. The results are presented in Fig. 7.18. The recoverable weight of clean coal varies from 3.1% to 29.5% for middlings crushed to 13 mm and varies from 3.9% to 35% for middlings crushed to 3 mm, depending upon the nature of the coal. In pilot plant tests, the middlings samples were crushed to 3 mm and the size (3
0.5) fraction was treated in an HM cyclone and (,0.5 mm) fraction in flotation. The results proved to be quite encouraging. Thus, it can be inferred that the cyclonecum-flotation scheme could be a feasible solution to this problem. This can be incorporated in the existing washing system as shown in Fig. 7.19. As per

40 35 30

Weight percent (On total raw coal basis)

25 20 Middlings crushed to >3 mm 15 10 Middlings crushed to >13 mm

5 0

1

2

3

4

5 6 7 Sample no.

8

9

FIGURE 7.18 Recovery of clean coal (17% ash) from crushed middlings.

10

11

164

Sustainable Management of Coal Preparation

Run-of-mine coal Crushing (75-0 mm)

75–25 mm

Middlings

25–0.5 mm

–0.5 mm

H.M.Washer

Clean coal

Screening

JIG or H.M. cyclone

Rejects

Crushing (3–0 mm)

Screening

(3–0.5 mm)

H.M.Cyclone

–0.5 mm Flotation

Clean coal

Rejects

Middlings

Dotted block represents modification scheme

FIGURE 7.19 Modification scheme for recovery of coking coal from existing washeries (simplified).

this scheme, the middlings of the existing washeries will be crushed to 3 mm and then screened at 0.5 mm. The coarser size will be treated in a heavymedium cyclone and the fines by a flotation process. The fines of the existing system can also be put into the new flotation scheme. The cleans produced from the modified scheme can be mixed with the clean coal of the existing washeries. It is anticipated that the proportion of middlings may further rise to as high as 35%
40% of total raw coal feed to the washery. This will mean less availability of washed metallurgical coal. This situation calls for immediate remedial measures. It is, however, believed that this scheme should hold good more or less for coals to be washed in the existing Indian washeries. This integrated approach will go a long way in solving the problem of the acute shortage of coking coal in the country.

Coking Coal Washing Chapter | 7

165

7.15 YIELD OPTIMISATION IN THE WASHERIES The main objective of coal washing is to recover the maximum amount of clean coal without exceeding the specified ash content. In the first instance, the optimum level of crushing for economic liberation of coal particles from dirt is established so that the crushed materials can be beneficiated efficiently in available washers. Then comes the question of optimisation of the yield of clean coal. The crushed coals are usually screened into different size fractions and then washed in different vessels. The different size fractions may have their own washability characteristics and these may again vary in the day to day operation of the plant. In most washeries, the different coal-washing equipment operates in parallel, the feed to each one of them being of different size. This type of system can be termed as composite washing (Cierpisz and Gottfried, 1977). In the Indian washeries also, the same system is followed. It is most essential that the performance of the washeries be controlled so as to maximise the yield, which ultimately helps in conservation. It is known that the yield of clean coal will be maximised if each stream is washed to the same level of direct elementary ash content. Now, if the variation of elementary ash with specific gravity of separation is the same for each stream, then the yield of clean coal is at its maximum if each stream is washed at the same specific gravity of separation. This phenomenon can also be proved graphically by plotting the washability curves of each stream. The blending of two clean coal streams of different size fractions are considered here, as in Fig. 7.20. Fc1 and Fc2 are the products of separation from the respective feed streams F1 and F2 at the corresponding specific gravity of separation ρs1 and ρs2. Fr1 and Fr2 are the respective rejects.

F1

Ps1

Fc1

Fc

Fr1

F2

Ps2 Fc2

Fr2

FIGURE 7.20 Two washers in parallel.

166

Sustainable Management of Coal Preparation

7.15.1 Application in Indian Coal From the float and sink data of the two size fractions, 75
25 mm and 25
0.5 mm of a raw coal sample, washability curves are drawn as in Fig. 7.21. From the curves, the results are read and recorded in Table 7.7 where it was found that the yield of clean coal was maximum when the specific gravity cuts of the two fractions are same, and also the direct ash of both is the same. The direct ash at the particular specific gravity was read from characteristic curves and yield gravity curves. The combined yield of clean coal at 17% ash was 65.5% when the two fractions are washed at sp. gr. cuts of 1.43 and 1.52 and the direct ash are 22.5% and 31%, respectively. Yet at the same specific gravity (sp.gr.) of cut (1.48), the corresponding yield becomes 70% with the direct ash of 27% for both the fractions.

FIGURE 7.21 Washability curves for two washers.

Coking Coal Washing Chapter | 7

167

TABLE 7.7 Graphical Optimisation of Clean Coal Blend in the Ratio of 50:50 Particulars 1.

Separation densities

2.

Yield from each washery (%)

3.

Yield of blended clean coal (%)

4.

Ash content of clean coal (%)

5.

Ash content of blended clean coal (%)

6.

Direct ash content (%)

1.

Separation densities

2.

Yield form each washery (%)

3.

Yield of blended clean coal (%)

4.

Ash content of clean coal (%)

5.

Ash content of blended clean coal (%)

6.

Direct ash content (%)

Washer no. 1

Washer no. 2

1.43

1.52

56

Blended Clean Coal From Two Washers

75 65.5

17

17 17

22.5 1.48 70

31 1.48 70 70

18

16 17

27

27

In the Indian washeries, composite washing of the coal is done at different sp.gr. of cut. Since this practice cannot produce the maximum yield of clean coal at a particular ash content, the coals should be washed by the same sp.gr. of cut for proper optimisation of the plant, which in turn will reduce losses in the beneficiation process.

7.16 NEW PRIVATE COKING COAL WASHERIES Two new washeries in the private sector with details are indicated in Table 7.8.

168

Sustainable Management of Coal Preparation

TABLE 7.8 New Private Coking Coal Washeries Sl. No.

Name of Washery

Company

Annual Rated Input Capacity (Mt)

Main Washing Equipment/ Process

Fine Coal Washing (,0.5 mm)

1

Near Bokaro

Electro Steal

NA

HM Cyclone (13
0.5 mm)

Flotation

2

Bhelatand

TISCO & SAIL

1.8

HM Cyclone (13
1 mm), Teeter Bed Separator (TBS) (,1 .0.25 mm)

Flotation (,0.25 mm)

7.17 STANDARDISATION Since the installation of the first washery in India in 1951, a number of washeries have been constructed and many are under construction. Standardisation of the process and equipment has however been lacking, largely due to the fact that all the washeries have been built by agencies of different foreign countries. This has resulted in an acute shortage of spares, especially for process equipment. This situation has been further aggravated by the restriction of foreign exchange and the delays in issuing of import licences. Thus, the idle hours in the washery due to mechanical breakdowns are increasing. The indigenous capability to manufacture washery equipment with foreign collaboration is slowly building up in the country. If the equipment as well as the circuits of coal preparation plants are standardised, the problems of spares can be solved by indigenous manufacturing. The modular concept of design should be adopted, by which standard circuits of equal capacity can be incorporated. In a modular system, if any breakdown takes place in a stream, the other stream can run effectively without affecting the production of the whole plant. The individual stream should be designed so that larger-capacity equipment can be accommodated to gain advantage of reduced floor space, capital cost and operating cost with lower power consumption as well as lower cost of spares.

7.17.1 Benefits of Standardisation The adoption of standard flow sheets in comparison to diverse systems of washing as they prevail in India can offer many benefits (Kumar, 1988):

Coking Coal Washing Chapter | 7 G

G

G

G

G

G G

169

Standardisation will help in the development of the manufacture of equipment and spares indigenously. Costs will reduce in the design, manufacture and installation of washeries. Stores inventory of spares will reduce as the demand will be limited to a few standard designs. Training of personnel for operation and maintenance of washery will be easier as they can be interchanged. Operation and maintenance of washeries will be easier due to ready availability of spares and equipment. Interchangeability of equipment and spares becomes possible. Time schedule for the construction of new washeries will be less.

Unitised or modular construction is a current trend in advanced countries. Plants are preengineered and fabricated in the manufacturing shop and then transported to the site by trucks or rail as is convenient. Plants up to 400 t/h capacity can be built within a period of 6 months. A module of 500 t/h raw coal capacity has already been built very successfully and it is expected that with modular systems the total plant capacity can be more than 2000 t/h of raw coal input. With the modular plant concepts the following advantages accrue: G G

G

G

G

G

G

Design cost of the modular plant is lowered; Modular plants can be built much faster than conventional plants once the site is ready; Capacity of the preparation plants can be increased by adding modules in parallel to an existing circuit; Modular plants can be easily dismantled and transported to a new place for reuse; Maintenance of circuits can be programmed alternatively without interruption of the whole plant; Modular units are flexible by providing matching capacity to the production programme of the mines; Higher utilisation of investment.

7.17.2 Standard Flowsheet for Coking Coals The conventional method of washing the size range of 75
25 mm in an HM washer and ,25 mm in a Baum jig washer will no longer hold good for coal seams which are of inferior quality. These seams are now being mined by mechanical and sand stowing, which increase the ash content of raw coal. Surface mining adds to the problem. Thus, the conventional processing methods cannot produce clean coal with 17% ash at an economic recovery limit to meet the requirements of steel plants. It has been observed that crushing coals to lower sizes produces more clean coal through washing. The extent

170

Sustainable Management of Coal Preparation

of crushing is limited by the economics of washing as finer crushing generates fines which have to be upgraded by the relatively costly method of flotation. Furthermore, the overall moisture content of clean coal will be greater. On examination of float and sink tests data of different coal samples, it has been established that the maximum yield of clean coal is attained when the level of crushing is brought down to the range of 13
6 mm. For economic reasons, the crushing limit may not be below 13 mm. To improve the raw coal feed to the washery, it can be suggested that raw coal be first deshaled before resorting to further crushing for all future washery installation, and, if possible, in existing washeries also. A flow scheme as given in Fig. 7.22 can be recommended as the optimum standard flowsheet for Indian coking coal washeries. The proposed standard flowsheet is briefly described here: G

G G

G

Crushing ROM coal down to 75 mm and blending it for uniform feed to washery. Deshaling of 75
0 mm size raw coal by Baum jig. Screening and desliming of deshaled coal at 13 and 0.5 mm size aperture, respectively. Crushing of .13 mm size of coal to ,13 mm and desliming of 13
0 mm size.

Raw coal feed (75–0)mm

Deshaling jig

Screening

+13 mm

Screening at 13 mm

Dewatering

Cruching at 13 mm

Primary H.M. cyclones

Desliming at 0.5 mm

Secondary H.M. cyclones

Medium recovery

Dewatering

Medium recovery

Dewatering

Medium recovery

Dewatering

MAG, SEP, circuit

Magnetite circuit

(13–0.5)mm

Middlings

–0.5 mm

Clarified water to plant

Froth flotation

Dewatering

Thickening

Dewatering

FIGURE 7.22 Standard flowsheet for coking coal washeries.

Clean coal

Rejects

Coking Coal Washing Chapter | 7 G G

G

G

G

G G

G

G

171

Dewatering of rejects obtained from deshaler. Washing of 13
0.5 mm size fraction by two-stage HM cyclones to produce clean, middlings and rejects. Rinsing and dewatering of clean coal, middlings and rejects produced from HM cyclones. Treating the dilute media pulp in two-stage magnetic separators for regeneration of magnetite. Upgradation of ,0.5 mm fines by forth flotation to produce clean coal and tailings. Dewatering of flotation concentrates and tailings. Storage of cleans obtained from HM cyclones and flotation cells in clean coal bunker for despatch to steel plants. Storage of middlings obtained from HM cyclones in middlings bunker for despatch to power stations. Storage of rejects obtained from HM cyclones and flotation cells in a reject bin for disposal to dumping yard or other utilisation.

Standardisation is the only rational approach and it should find its place in coal washing too. The standard flowsheet so selected will not only yield optimum recovery of clean coal and middlings but also simplify the layout, design, operation and maintenance of washeries. The benefits of standardisation can be reaped both for new and existing washeries.

7.17.3 A Review The diversity of design and capacity of different washery equipment and plant circuits offer limited scope for standardisation in existing plants vis-a`vis new ones. As a number of coal washeries are being planned in the coking coal sector, there is an urgent need for standardisation. A flowsheet of standard design has been formulated which is most suitable for maximising the recovery of clean coal and middlings at the desired ash levels. The existing washeries can also be modified in line with the proposed circuit. This will also go a long way in solving many problems through import substitution. The design and construction of complete washeries along with the timely supply of spares will be possible from indigenous sources. It is now widely accepted that one large unit should be preferred to two or more smaller capacity units for better process control and for economies of scale. Through large-capacity process units, modular design of plants can be made and in turn a higher degree of standardisation can be achieved. There is enough scope for simplification in the design of modular plants which will be economical to build. The modules will have provision for separate arrangement for raw coal handling and homogenisation, washing section and despatch of washed products. The modules can also be erected at a site according to the local conditions.

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Sustainable Management of Coal Preparation

7.18 LVMC COAL PROCESSING As the reserves of good-quality coking coals of upper seams are almost depleted, low volatile medium coking (LVMC) coals of lower seams have become the alternative choice. These coals, being of lower seams are likely to be more mature (Ro B1.3) than the upper seams and consequently exhibit lower values of volatile matter (Kumar and Saxena, 2014). They constitute about 50% of the total coking coal reserves of India. These coals are characterised by high ash content and difficult cleaning potential. Proper utilisation of these LVMC coals for metallurgical purposes after suitable beneficiation will not only help in reducing the dependence on imported coal but also minimise the present improper utilisation for power generation. These coals generally occur in the lower seams of Jharia Coalfield (combined seam V/VI/VII/VIII and individual seam II, III and IV) and Karo group of seams (seam VI to XI) in the eastern part of East Bokaro Coalfield of the Damodar Valley basin. These coals have at present not found entry into the steel industry. The laboratory and pilot plant investigations on these coals after washing at 10%, 15% and 17% ash levels have firmly demonstrated that the coals show good coking properties and can be blended in a proportion up to 20% to produce good-quality blast furnace coke (Mishra and Chopra, 2013). The yield percentage of the cleans is also encouraging, and hence, these coals can be considered for utilisation in the metallurgical industry for coke making. The typical characteristics of LVMC coals with parametric feature range (Bhattacharya et al., 2013) are as follows: G G G G G G

High ROM coal ash 35%
50%; High maturity Ro B 1.3; Low volatile matter content 15%
16%; High inertinite content 60%
70%; Washability potentially poor to very poor; Crushing requirement fine to very fine (,13/ to ,3 mm).

On the basis of various reports and washability data, the following observations can be made: G

G

Percentage of near gravity materials (NGM) at 1.45
1.65 cut density range are high, and thus the coals become difficult to wash (Sapru, 2013). Generally, washability of LVMC coal can be improved by crushing it to smaller size. The difficult washability characteristics of LVMC coals are due to fine dissemination of mineral particles with macerals. The ash % of marketable product can be further improved by blending with prime coking coal, so that this coal can be used in various steel plants.

Coking Coal Washing Chapter | 7

173

TABLE 7.9 Proposed LVC Coal Washeries Sl. No.

Proposed Washery

Company

Annual Rated Input Capacity (Mt)

1

Madhuban Washery

BCCL

5.0

2

Patherdih Washery 21

BCCL

5.0

3

Dugda Washery

BCCL

2.5

4

Dahibadi Washery

BCCL

1.6

5

Patherdih Washery -II

BCCL

2.5

6

Dhori Washery

CCL

2.5 Total 5 19.1

Source: Modified from http://www.bcclweb.in/?page_id 5 990.

G

G

Crushing at lower size involves increased cost of size reduction, cost of beneficiation and handling cost of materials can be outweighed by an increase in the yield of the beneficiated coal. From the washability analyses, crushing of coal to less than 13 mm is economical.

Coal India Ltd. has taken the initiative to set up six washeries with a total capacity of about 19 Mt/y for washing LVMC coal in BCCL and CCL (Table 7.9). The mode of setting up of five washeries in BCCL is under a BOM concept, i.e., build
operation
maintenance basis, through a global bidding process. Coal India Ltd. will provide the funds and infrastructure facilities like land, power, water, railway sidings. Another washery of 2.5 Mt/y capacity has been envisaged to wash LVMC coals at CCL and is to be set up on a turnkey basis. The real task is to wash these coals with suitable washing techniques and to obtain 25%
30% yield at 17%
18% ash levels. The middlings are to be used in the power sector.

7.18.1 Patherdih Washery The proposed flowsheet developed for the 5 Mt/y washery at Patherdih is briefly described as (Fig. 7.23): G G G

Crushing ROM coal down to 50 mm; Screening of crushed coal at 13 mm; Deshaling of 50
13 mm size raw coal by jig;

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Sustainable Management of Coal Preparation

(–200 mm) From thickener overflow Raw coal (–200 mm) Crushing (–50 mm)

Screening (–13 mm)

Overhead water tank

(50–13 mm) Sinks (rejects)

(–13 mm)

Deshaling jigs

Magnetite medium

Floats

Crushing (–13 mm)

(–0.5 mm)

(13–0.5 mm)

Desliming DM cyclones feed tank

DM cyclones Over flow

Flotation

Tailings

Flotation concentrate Under flow

Clean coal bunker (Despatch to steel plants)

Middlings bunker (Despatch to power houses)

Rejects

FIGURE 7.23 Process flowsheet of Patherdih washery (5 Mt/y). G

G

G

G

G

Crushing of floats (,50 .13 mm) size of coal to ,13 mm and desliming of 13
0 mm size. Washing of 13
0.5 mm size fraction by HM cyclones to produce clean and middlings. Upgradation of ,0.5 mm fines by forth flotation to produce clean coal and tailings. Storage of cleans obtained from HM cyclones and flotation cells in clean coal bunker for despatch to steel plants. Storage of middlings obtained from HM cyclones in middlings bunker for despatch to power stations.

7.18.2 A Critique G

G

G

There is no guarantee for supply of such a low raw coal ash (, 40%). In fact, there is every likelihood of an increase in the raw coal ash. It is not known how far the dilution effect has been considered. If the ash content of ROM coal increases by 3%
5%, the whole washing circuit will be imbalanced. The dilution effect has been discussed in Section 1.4.2. A good amount of middlings will be produced; there is enough possibility of recovery of coking coal.

Coking Coal Washing Chapter | 7 G

G

G G

175

Middlings are required to be crushed to 3 mm as described in Section 7.14. Deshaling at the lower top size of coal will lead to an increase in NGM and washing becomes more difficult. All the possibilities of crushing to lower top sizes must be explored. Hopefully, the mistakes and experience gained from this washery will lead to success. Eventually, an optimum design of washery circuit will evolve.

7.19 CLOSING REMARKS Most coal washeries in India are not working properly, resulting in deterioration both in the quality and quantity of washed clean coal. The low performance of coal washeries causes a great deal of damage to the national economy because of increased import of coking coal. It is necessary to analyse the problems very critically so that the present situation can be improved. If the real causes of failures are identified, proper action can be taken, whether of a technical character or management techniques. The latest technology of management science should be introduced in this industry as is done in other industries. The present steel production in India is not very high compared to that of other countries around the world. However, there is a rising trend in steel consumption, accordingly the steel industry is trying to increase its production. Coking coal is used in steel plants in the form of hard coke for two purposes: as a reducing agent and for smelting. Deterioration in the quality of coal in any form, whether in relation to ash, moisture, volatile matter or coking propensities affects the operation of steel plants in one way or other. The high ash content in coal produces less heat value than what is required and at the same time gives rise to a greater quantity of slag. Thus, this situation warrants improvement in the system of coal washing not only from the angle of techniques of beneficiation but also for operational aspects. The smooth operation of the plant requires an efficient management system which should be supported by good organisation, maintenance management, material management, financial control, etc. Most washeries are 40
50 years old, characterised by technological obsolescence and aging equipment, suffer from loss of operational efficiency in terms of yield percentage and capacity utilisation, as shown in Fig. 7.24. The lack of proper beneficiation of fines has added to the problems. The size of the raw coal feed has also changed. The washeries were originally designed for treating raw coal with a top size of 200 mm from underground mines. The top size has gone up to 1200 mm as the supply comes mostly from surface mining. The existing raw coal crushing and handling system of the washeries has become incompatible to handle the present feed of raw coals. As the better-quality upper seam coals become less available, the

176

Sustainable Management of Coal Preparation

Individual washery

6 5 4 3 2 1 0

20

40

60

80

(%) Yield (%)

Capacity utilisation (%)

FIGURE 7.24 Performance of some washeries.

present washeries can be revamped/upgraded to treat the abundant supply of inferior-grade lower-seam coals. This is likely to bring many benefits like better yield, increased capacity utilisation, more availability of coking coal, and improved performance of washeries. As a result, the overall economy of the operating company will improve. It is a fact that the size of coal generally influences its washability characteristics. It is expected that a reduction in the top size of a coal will increase the value of its washability number until the critical top size for the coal sample is reached, below which there is no release of clean coal. Keeping this in perspective, all the pros and cons should be explored to arrive at the optimum top size of crushing for future washeries, especially when dealing with LVMC coals. Liberation of coal substance is not complete unless coal is crushed to a certain level of top size. There is every likelihood of losing some combustibles along with rejects produced from the washery if the coaly matter is not made free before washing. It is suggested that the raw coals should be crushed to 6 mm for liberation of more combustibles (Kumar, 1982). This crushed fraction 6
0.5 mm may be treated in heavy-medium cyclones. Fines (,0.5 mm) should be upgraded by flotation as shown in Fig. 7.25. An elaborate scheme for the treatment of fines needs to be incorporated. As India is importing a sizable amount of coking coal of good quality at a much higher price, it is imperative that coking coal should not be lost in any form and the aim should be for a higher yield by the suggested scheme for washing.

Coking Coal Washing Chapter | 7

177

R.O.M. Coal

Crusher (75–0 mm) Crusher (6–0 mm) Screening (6–0.5 mm) (–0.5 mm)

H.M.cyclone or JIG

Cleans

Middlings

Rejects

Flotation Cleans

Cleans storage

Middlings storage

Reject

Rejects storage

FIGURE 7.25 Simplified flowsheet for future system of washing (suggested).

REFERENCES Bhattacharya, S., Singh, A.K., Choudhury, A., 2013. Coal resources, production and use in India. In: Osborne, D. (Ed.), The Coal Handbook, Towards Cleaner Production Volume 2: Coal utilisation. pp. 169
199. Cierpisz, S., Gottfried, B.S., 1977. Theoretical aspects of coal washer performance. International Journal of Mineral Processing. Elsevier, pp. 261
278. Gottfried, B.S., Jacobsen, P.S., 1977. Generalised Distribution Curve for Characterising the Performance of Coal Cleaning Equipment. US Department of Interior, Bureau of Mines report of investigations 8238, Washington, DC. Holuszko, M.E., 1994. Washability Characteristics of British Columbia Coals. Ministry of Energy, Mines and Petroleum Resources, Geological Survey Branch, Province of British Columbia, Canada, pp. 1994
2002. Jha, N.C., 2012. Sustainable Development of Indian Coal Industry
Daunting Challenges, Proceedings of 4th Asian Mining Congress, 29-31 January, MGMI, Kolkata, India. pp. 1-8. Kumar, V., Saxena, V.K., 2014. Studies on the variation in coal properties of low volatile coking coal after beneficiation. IJCER 04 (1), 39
57. January. Kumar, D., 1984. Flotation von Kohlenschla¨mmen in Indien. Glu¨ckauf 120 (20), 1336
1340. Kumar, D., 1981. Beneficiation of coking coal in India with special reference to conservation. Urja 69
71. Feb. Kumar, D., 1982. Aufbereitung von Kokskohle in Indien. Autbereitungs-Technik (11), 610
614. Kumar, D., 1988. The standard flowsheet for preparation of indian coking coal. Aufbereitungs Technik. (1), 16
21. Kumar, D., 1995. Potential of performance improvement of Indian Coal Washeries. Coal Prep Int. Lexington, Kentucky 2
4, 303
319. May.

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Sustainable Management of Coal Preparation

Mishra, H.K., Chopra, R.K., 2013. Utilization of Indian Non Coking Coals for Blast Furnace Coke Making, Proceedings of the 17th International Coal Preparation Congress 1-6 October, Istanbul/Turkey, pp. 715-721. Nicol, S.K., 2001. Fine coal beneficiation, in Swanson, A.R. (Ed.), Australian Coal Preparation Monograph Series, Volume IV, Part9. Ofori, P., O’Brien, G., Firth, B., Jenkins, B., 2004. Flotation process diagnosis and modelling by coal grain analysis, in Wembrey. In: WB. (Ed.), Proceedings of the Tenth Australian Coal Preparation Conference, C9. O’Brien, G., Firth, B., Adair, B., 2011. Application of the coal grain analysis method to coal liberation studies. Int. J. Coal Prep. Utilization 31, 96
111. Sanders, G.J., Brookes, G.F., 1986. Preparation of the gondwana coals I. Washability characteristics. Coal Preparation III (3), 105
132. Sapru, H.L., 2013. Beneficiation of low volatile medium coking coal (LVMC) in India, Proceedings of the 17th International Coal Preparation Congress 1-6 October, Istanbul/ Turkey, pp. 627-630. Sarkar, G.G., 1986. An Introduction to Coal Preparation Practice. Oxford & IBH Publishing Co, New Delhi. Sarkar, G.G., Das, H.P., 1978. A simple expression to evaluate coal-washing efficiency. World Coal, June 33
35. Sarkar, G.G., Das, H.P., 1974. A World pattern of the optimum ash levels of cleans from the washability data of typical coal seams. Fuel 53, 74
84. Sengupta, A.K., Senapati, G. 2014. Improvement in Washed Coal Yield in BCCL Washeries by Up-gradation of Washeries, Proceedings of 5th Asian Mining Congress, 13-15 February, MGMI, Kolkata, India, pp. 1-7. Tromp, K.F., 1937. New Methods of Computing the Washability of Coals. Glu¨ckauf 37 (125
131), 151
156. February 6 and 13. Vince, A., 2013. Post-treatment of coal. In: Osborne, D (Ed.) The Coal Handbook, Towards cleaner production Volume 1: Coal production, pp. 467-528.

FURTHER READING Sarkar, G.G., Das, H.P., Ghose, S., 1977. Sedimentation patterns: do they offer clues to coal quality? World Coal 10
13. August.