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CHAPTER VI

CHEMISTRY OF TAILINGS AND COAL WASHERY EFFLUENTS

6.1 INTRODUCTION

Coal is inherently a “dirty” source of energy. Row coal contains non-coal minerals

that will be released as polluting discharge during washing. Coal washeries have been

implicated as one of the major sources of surface and groundwater pollution (Gurdeep

Singh, 1986, Bandopadhyay, 1987; Gupta Ravi and Gurdeep Singh, 1993).

One of the main objectives of coal preparation is to reduce the quantity of

pollutants in coal when it is burned. Washing principal of coal is mainly based on the

differences in specific gravity between coal and its impurities, and the different unit

processes depend on the washibility characteristics of particular coal. Preparation of coal

by physico-chemical methods is known as washing/beneficiation. The wastes

characteristics of coal washing plant are highly dependent on the raw coal utilized and the

final product. With the continuing increasing demand of coking coal in conjunction with

the exhaustion of good quality coking coal Coal washing is a generic term that is used to

designate various operations performed on run-of-mine (ROM) coal to prepare it for

specific end use.

Water is the most common medium for transporting crushed material in coal

washing plant and hence most coal separations take place within this medium. Perhaps the

greatest and the most long-standing problem in coal washery is the disposal of effluents

which contains a suspension of fine solids (Bandopadhyay, 1995). The effluents from coal

washing processes contain large amounts of suspended and dissolved solids, dirty materials

and impuries associated with raw coal and they cause deterioration of water quality of

groundwater into which they are discharged (Ghose 1999). Effluents from coal mine

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contain high load of TDS, calcium carbonate and heavy metals contaminate the aquatic

regime (Dhar et al., 1986).

Heavy metals are one of the most detrimental fractions of mining effluent, which

accumulate in water, soil, sediment and living organism (Miretzky et al., 2004).

Occurrence of toxic metals in plants and water bodies adversely affects the lives of local

people since they utilize this water for daily requirements. The heavy metals can be

incorporated into food chain and their levels can increase through biological magnification

(Cardwell et al., 2002). The wastewater pollutants likely to be generated while washing

coal due to the above activities are : total suspended solids (TSS); (b) chemical oxygen

demand (COD); total dissolved solids (TDS), acidity or alkanity (pH), and heavy metal

contaminants, etc (Arora, V et al., 2006). Regulations of government of Iran severely

restrict the methods of disposal of effluents loaded with fine material, usually produced in

the form of slurry and normally referred to as tailings.

In the present study, we discuss the geochemical characteristics of effluents from

coal washery and accumulated tailings (solid waste). Nature and quantum of pollution

caused by the coal washery can be assessed from the chemical analyses data of raw coal,

clean coal and middlings, initial fresh water used for coal washing, fine coal Jig water and

tailings pond water.

6.2 METHODOLOGY

During August 2009 three samples each of raw coal, clean coal and middlings were

collected from Zarand coal washery. Likewise, fine coal Jig water and water from washery

tailings pond were collected in pre-cleaned bottles (1 Lit capacity). Prior to sampling the

water was filtered through 0.45 µm Supor-450 membrane filters. The bottles were

completely filled and capped air tight to avoid contamination from atmospheric CO2. The

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filtered water was acidified to pH<2 with HNO3- and stored in a dark room until it was

analysed.

During laboratory studies the concentrations of Ca, Mg, Na, K, Fe, Mn, Cu, Ni, Zn,

Pb, Cr, Cd, As, Se, Hg, S, P and Cl (in mg/kg) in raw coal feed, fine clean coal and

middlings were determined. Likewise, concentrations of Ca, Mg, Na, K, As, Hg, Pb, Cd,

Cr, Cu, Zn, Se, Ni, Mn and Fe in raw water (water from bore wells W1 to W5 used for

washing coal), fine coal Jig water and tailings pond water were determined. Concentrations

nitrate nitrogen, sulphate, chloride, COD, TDS, TSS and oil and grease in all water

samples were also determined. The results of the laboratory investigations are presented in

table 6.1 and 6.2.

Fig. 6.1 Satellite imagery of Zarand coal washing plant and tailing pond.

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6.3 CHEMICAL CHARACTERISATION OF TAILINGS WATER

Effluents from coal washery are brownish black in colour. The appearance of these

waters during discharge from the outlet of washery premises is, in general, of brownish-

black in colour.

6.3.1 Turbidity (NTU)

Turbidity is a measure of the degree to which the water loses its transparency due

to the presence of suspended particulates. Higher concentration of suspended solids in the

water, imparts higher turbidity. The suspended particles absorb heat from the sunlight, and

make the turbid waters become warmer, and thereby reducing the concentration of oxygen

in the water. Turbidity Units (NTU) are determined for assessing proper ecosystem

functioning. The suspended particles also help the attachment of heavy metals and many

other toxic organic compounds and pesticides. Turbidity appearance of the tailings pond

water varies from 100 to 260 NTU which is very high and exceed permissible limit IS:

2490 (1981), (table 6.1).

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Table 6.1: Chemical Characterisation of tailings water in Zarand coal washing plant.

Sample

No. pH T°(c)

Turbidity TSS TDS EC Oil &

grease COD Ca2+ Mg2+ Na+ K+ SO4

2- HCO3- Cl - NO3

- PO4-

(NTU) (mg/l) (mg/l) (µs/cm) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l)

T-1 7.92 27.7 250 211

1526.13 5130 15.50 335.00 602 242.50 425.62 20.52 146.0 56.40 61.20 0.66 850

T-2 7.94 26.6 110 240 1458.79 5120 13.00 298.00 496 241.78 413.83 19.10 182.5 56.43 77.21 0.75 1110

T-3 7.91 26.6 140 230 1599.37 5160 13.30 310.00 651 244.14 415.59 20.93 155.0 54.90 85.20 0.63 902

T-4 7.95 29 220 251 1651.70 2780 12.80 225.00 689.6 241.01 413.22 19.40 172.0 65.60 83.43 0.92 650

T-5 8.35 25.6 100 185 1112.55 5150 18.50 265.00 414 159.78 289.03 18.72 128.8 65.60 69.23 0.86 770

T-6 8.36 26.8 260 160 1114.34 3790 16.00 373.00 405.6 160.63 281.58 16.88 153.0 50.33 71.00 1.00 840

T-7 8.63 25.5 180 175 1031.92 3800 17.00 350.00 390 159.65 281.84 17.00 103.2 36.60 61.24 1.07 665

T-8 7.88 25.8 120 170 1094.08 3800 21.00 262.00 426 158.36 282.14 17.91 89.4 67.10 86.98 0.43 710

Average 8.12 26.7 172.5 203 1324 4341.25 15.89 302.25 509.28 200.98 350.35 18.81 141.24 56.62 74.44 0.79 812

Permissible Discharge Limit IS : 2490 (1981)

Standard 5.5-9 < 40 50 100 2100 ⁻ 10 250 ⁻ 100 ⁻ ⁻ 1000 1000 10 500

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6.3.2 Total Suspended Solids (TSS)

Total suspended solids is a water quality assessment parameter usually abbreviated

as TSS. It is listed as a conventional pollutant in the U.S. Clean Water Act. TSS constitutes

solid materials, including organic and inorganic, that are suspended in the water. TSS

would include silt, plankton and industrial wastes. High concentrations of suspended solids

can lower water quality by absorbing light. Waters then become warmer and lessen the

ability of the water to hold oxygen necessary for aquatic life. Because of thin aquatic plants

receive less light and as a consequence photosynthesis decreases and less oxygen is

produced.

The concentration of TSS in the waters of tailings pond was found in the range of

111 to 185 mg/l which is higher than the permissible limit of 100 mg/l as per IS: 2490.

This reflects practically non-functional of the clarification system with hardly any recovery

of coal fines/suspended solids. It was observed during this investigation that in Zarand coal

washing plant recovery system is not in good working condition and there is clear over

flow of discharge water containing suspended solids/coal fines. Besides, this gives rise to

enormous economic loss due to escaping of coal fines through the discharge effluents. Coal

washing plant and tailings pond are situated in very close proximity of Zarand city and

agricultural land. There has been continued and uncontrolled discharge of effluents from

coal washery into the groundwater. There is a problem of land availability and as such

proper time for natural settling of suspended solids/coal fines is not provided. The problem

intensifies due to non-provision of application of coagulants/flocculants for clarification of

effluents for proper coal fines recovery system.

6.3.3 Chemical Oxygen Demand (COD)

Chemical Oxygen Demand (COD) is defined as the quantity of a specified oxidant

that reacts with a sample under controlled conditions. The quantity of oxidant consumed is

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expressed in terms of its oxygen equivalence. COD is often used as a measure of pollutants

in natural and waste waters and to assess the strength of waste such as sewage and

industrial effluent waters. In the present study, the COD levels in washery effluent samples

range from 225 to 373 mg/l. Hence, the COD levels in general, except of T4 sample,

exceed the permissible limit of 250 mg/l, which is mainly due to the reducing nature of

coal fines and other suspended solids in the tailings water.

6.3.4 Oil and Grease

The concentration of dispersed oil and grease is an important parameter for water

quality and safety. Oil and grease in water can cause surface films and shoreline deposits

leading to environmental degradation and can induce human health risks when discharged

to surface or ground waters. In the study area, another water pollution problem identified is

that of oil and grease content which is found in significant quantities in the tailings water.

Oil and grease content in tailings water varies from 12.8 to 21 mg/l. These values exceed

the permissible limit of 10 mg/l. Excessive content of oil and grease in water may interfere

with aerobic and anaerobic biological processes and lead to decreased wastewater

treatment efficiency.

6.3.5 Heavy metals

Heavy metals are a major concern in the treatment of water due to the toxic and

other detrimental effects these materials can produce. In the tailings pond water, the

concentrations of Fe vary from 0.125 to 1.136 mg/l with an average of 0.47mg/l. The Cr

concentration varies from 0.0064 to 0.009 mg/l with an average of 0.007 mg/l. The

concentration of Cu varies from 0.0083 to 0.023 mg/l with an average of 0.012 mg/l. The

concentration of Mn varies from 0.38 to 0.623 mg/l with an average of 0.505 mg/l. The

concentration of Zn varies from 0.011 to 0.474 mg/l with an average of 0.080 mg/l. The

concentration of Pb varies from 0.0013 to 0.017 mg/l with an average of 0.005 mg/l. The

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concentration of Cd varies from below detection limit (BDL) to 0.001 mg/l with an

average of 0.0007 mg/l. The concentration of Hg varies from below detection limit (BDL)

to 0.003 mg/l with an average of 0.0003 mg/l. The concentration of As varies from below

detection limit (BDL) to 0.01 mg/l with an average of 0.004 mg/l. These heavy metals

were observed at significant concentration levels but do not exceed the permissible limits

as per IS : 2490 and as such do not seem to pose any serious pollution problem (Table 6.2).

Table 6.2 Heavy metals chemistry of tailings water in Zarand coal washing plant.

Sample

No.

Fe Cr Cu Mn Pb Zn Cd Hg As

(mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l)

T-1 1.136 0.009 0.023 0.623 0.017 0.474 0.001 BDL 0.01

T-2 0.425 0.008 0.011 0.55 0.006 0.03 BDL BDL 0.005

T-3 0.735 0.008 0.014 0.602 0.005 0.03 0.0008 BDL BDL

T-4 0.481 0.008 0.017 0.528 0.007 0.041 BDL BDL BDL

T-5 0.281 0.0067 0.009 0.444 0.0022 0.022 0.0007 0.0003 0.004

T-6 0.426 0.0067 0.0084 0.485 0.0018 0.02 0.0006 0.0002 0.004

T-7 0.148 0.0066 0.0088 0.425 0.0013 0.011 0.0008 0.0003 0.003

T-8 0.125 0.0064 0.0083 0.38 0.0013 0.012 0.0007 0.0003 0.003

Average 0.470 0.007 0.012 0.505 0.005 0.080 0.0007 0.0003 0.004

Permissible Discharge Limit IS : 2490 (1981)

Standard 3 2 3 2 0.1 15 2 0.01 0.2

BDL = below detection limit

6.4 RELATIONSHIP BETWEEN RAW COAL AND EFFLUENT QUALITY

Processing of coal and minerals involves transfer of potential pollutants from one

sector of the environment into other parts. This leads to degradation of water quality.

Processing results essentially in production of huge quantities of suspended material,

beside other pollutants in the effluents generated. Hence studies on the relationship

between raw coal and effluents from washery are essential.

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6.4.1 Physico chemical characterization of washery effluent

During dry operation of washing process huge amount of coal fines and noncoal

minerals matters is generated which are mixed with process of wet washing and constitute

the effluents. Most washeries use wet washing process, Wet process in particular and other

unit process in general are responsible for effluents generation. Hence, to evaluate the

nature and intensity of pollutants in the coal washery effluents, the washery samples of the

washery effluents were collected from three points : (i) raw water (fresh water), (ii) fine

coal jig water, (iii) tailing pond water (table 6.3). Raw water consists of 87 mg/L total

suspended solids. The washery effluents contain high quantities of total suspended solids

ranging from 202 to 8410 mg/l.

The concentration of suspended solids/fines generated depends upon the washing

operation to which the coal is subjected. In fine coal jigs, the coal is subjected to abrasive

forces that generate maximum fines. The concentration of suspended solids in fine coal jig

water is 8410 mg/l (Table 6.3). The tailings from the coal washery are pumped to

settling/slurry ponds, in which all the fine solids gradually settle and reasonably clear

overflow water is discharged into a natural water course. The concentration of suspended

solids in the slurry pond varies depending upon the settling rate. In the present study, it was

found to be 202 mg/l. The over flow from settling ponds at times contains huge amount of

fines. This may be either due to inadequate retrieval or due to retrieval before complete

sedimentation takes place in the tailing pond. Thus, along with the drainage from the slurry

ponds also at times contributes to the pollution of the natural water course. Hence, an

attempt has to be made in the washeries to maximize water recycle in order to reduce the

quantity of effluent discharged outside.

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Table 6.3: Water quality parameters (mg/L) of Zarand coal washing plant.

Sl.

No. Parameter

Sampling stations Below

detection

limit

MOEF-Schedule

VI Class standards Raw water Fine coal jig water

Tailing pond water

1 Colour and odour Acceptable Acceptable Acceptable - Acceptable

2 Total suspended solids 87 8410 202 < 5.0 100

3 pH 7.2 7.7 8.12 < 0.01 5.5-9

4 Temperature ( C ) 25.5 26 26.7 - Shall not exceed 5 C above the receiving temp.

5 Oil and grease BDL 8 15.89 < 1.0 10

6 COD 65 849 302 < 25.0 250

7 Arsenic BDL BDL 0.004 < 0.005 0.2

8 Mercury BDL BDL BDL < 0.001 -

9 Lead 0.004 0.004 0.005 < 0.0005 0.1

10 Cadmium BDL BDL BDL < 0.0005 2

11 Chromium 0.005 0.006 0.007 < 0.0005 2

12 Copper 0.01 0.01 0.012 < 0.001 3

13 Zinc 0.05 0.07 0.08 < 0.001 5

14 Selenium BDL BDL BDL < 0.001 0.05

15 Nickel BDL BDL BDL < 0.001 3

16 Fluoride 0.36 1.05 1.25 < 0.05 2

17 Manganese BDL 0.6 0.5 < 0.05 2

18 Iron 0.89 0.89 0.97 < 0.001 3

19 Nitrate nitrogen 0.43 0.66 0.79 < 0.01 10

20 Sulphate 57 155.3 141.25 < 0.5 1000

21 Chloride 41 99 74.44 < 0.2 -

22 Calcium 126 418 509 < 0.05 -

23 Magnesium 174 393 200 < 0.01 -

24 Sodium 190 345 350 < 0.01 -

25 Potassium 12.5 15.66 18.82 < 0.05 -

All parameter in mg/l unless specified, BDL = below detection limit MOEF = Ministry of Environment and Forest

The other technology used to remove suspended material is mechanical dewatering

and sedimentation. For this, thickners are used. Thickners play the dual role of clarifying

process water and thickening of fines as described earlier. The underflow from the

thickener is pumped to vacuum or pressure filters, centrifuge and finally to the setting

ponds.

Dissolution of minerals or salts can significantly affect the properties of water.

Some minerals and salts, such as chlorides and sulphates of the alkali and alkaline earth

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metals, readily dissolve in water and thus can significantly alter the pH (Osborne, 1988).

The increasing of pH was observed in Zarand coal washery, the pH is seen to increase

from 7.2 (that of raw water) to 8.12 (that of tailings pond water) (Table 6.3). When NaOH

is added in the flotation cell, the slurry is subjected to a pH increase. Such changes in pH

can cause precipitation of metal species which affects the flotation behavior of the

particles. The concentrations of dissolved Fe, Al, Ca and Mg decrease as the pH is

increased, with the mode of alkali addition being irrelevant. If the pH increases during coal

processing, there will be precipitation of metal ion species whereas if the pH decreases,

there will be dissolution of mineral species. Temperature (°C) varies from 25.5°C in raw or

fresh water to 26.7 °C in tailing pond water. Slight increase in temperature can be due to

intense interaction between raw coal and water during coal washery processing. However,

temperature does not exceeded 5°C above the receiving temperature in three points, raw or

raw (fresh) water, fine coal jig water and tailing pond water during coal washing process.

Oil and grease content varies from below detection limit fresh water to 15.89 mg/l

in tailings water and exceedes the permissible limit of 10 mg/l (Table 6.3). Increasing of

oil and grease can be due to distribution of these material as a collector and co-collector for

hydrophobe the suspended material in flotation process unit of coal washery.

6.4.2 Trace elements

The term ‘trace elements’ refers to chemical elements present in a natural material

at concentration < 0.1 wt% (Zevenhoven and Kilipinen, 2001). A sub-class of trace

elements are the ‘heavy metals’ such as Cd, Pb, Hg, Zn, and Cu, having a density of

approximately 5000 kg/m3 or higher. During coal benefication process the concentrations

of the mineral matter and trace elements in coal, the concentration are reduced due to

decrease through the beneficiation process (Swaine, 1990).

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Trace elements are generally decrease by beneficiation, the extent being variable

for different elements and coals. In general, those elements associated with the mineral

matter are more readily removed than those that are mainly organically bound. In

commonly used coal washing methods many trace elements are concentrated in the heavy

fraction (sink). Thus, although it is advantageous to remove some trace elements, from

some coals, especially those of environmental significance, which can lead to increasing

contaminant materials in the rejects of washing process. Extensive studies have been

carried out in the US (Karr, 1978) and in India (Banerjee at al., 2000) to show the

distribution of trace elements in the various specific gravity fractions of coal.

The problem of release of metals from coal into water has been reported from

several earlier studies. The metals reported in coal are Al, Ca, Co, Cu, Fe, Mg, Mn, Ni, Pb

and Zn. When the metals are listed in order of leaching rate the following series emerged-

Mn>Ca>Mg>Zn>Pb>Fe>Ni>Cu>Co>Al (Vlado, 1983). In the present study, the order of

abundance of metals in the tailings pond water are Ca>Mg>Fe>Mn>Zn>Cu>Cr>Ni.

Amongst the parameters analysed, the maximum value of Iron (0.97 mg/l) was observed in

tailings pond water which is more than the content of the same in raw water and fine jig

water. Similar results were also reported by Arora et al (2006) and Vetrivel et al. (2008).

In coal samples the concentration of iron vary from 6500 mg/kg in fine clean coal

sample to 16750 mg/kg in raw coal feed and the latter has highest concentration of trace

elements (table 6.4). The concentration of calcium varies from 3100 mg/kg in fine clean

coal sample to 8944 mg/kg in middlings. The concentration of magnesium varies from 842

mg/kg in fine clean coal to 3280 mg/kg in raw coal. The content of sodium varies from 618

mg/kg in fine clean coal to 2180 mg/kg in raw coal. The concentration of potassium varies

from 117 mg/kg in fine clean coal to 514 mg/kg in raw coal.

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Table 6.4 : Trace elements in various fractions of Zarand coal washing plant.

Sl.

No. Parameters

Coal samples

Raw coal feed to the

washery Middlings Fine clean coal

1 Manganese 137.78 101.24 84.30

2 Copper 29.04 12.40 12.20

3 Nickel 17.23 10.13 11.05

4 Zinc 84.5 53 51.72

5 Lead 29.42 24.30 14.40

6 Chromium 33.71 25.25 25.60

7 Cadmium 0.26 0.15 0.08

8 Arsenic 3.22 BDL BDL

9 Selenium 1.05 0.85 0.88

10 Mercury 1.25 0.96 0.96

11 Sodium 2180 1490 618

12 Potassium 514 331 117

13 Calcium 7350 8944 3100

14 Magnesium 3280 2165 842

15 Iron 16750 13490 6500

16 Sulphur 0.88 0.68 0.65

17 Chlorine 0.14 0.1 0.08

18 Phosphorous 0.05 0.05 0.05

All parameter in mg/kg, BDL = below detection limit

The values of sodium, potassium, calcium and magnesium were also in the range of

117 mg/kg to 8944 mg/kg. The concentration of manganese is in the range of 84.30 (fine

clean coal) to 135.78 mg/kg in raw coal feed to the washery (table 6.4). Hence, the order of

abundance of concentrations of metals in coal are Fe>Ca>Mg>Na>

K>Mn>Zn>Cr>Pb>Cu>Ni>As>Hg>Se>Cd. In comparision with raw coal, in coal samples

collected from different points of coal washing plant contain lesser concentration of Na, K

and Mg except of Ca (Fig 6.2). The order of leaching rate of metals in tailing pond water is

as follows:

Ca>Na>Mg>K>Fe>Mn>Zn>Cu>Cr>Pb>As. Other metals in tailing water are

below detection limit. In water samples collected from different points of coal washing

plant show that the Na, K and Mg contents during washing process increased in tailing

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pond water (Fig 6.3). In Iranian coal, generally the concentration of iron is more than other

elements such as calcium, magnesium, sodium and potassium.

The concentration of magnesium was found to be more in the process water than

the concentration found in the raw intake water. Magnesium present in raw water intake in

Zarand washery is < 0.02 mg/l, whereas in process water, i.e. in effluent from the fine coal

jig, it is 393 mg/l, and in effluent from the tailing pond water it is 200 mg/l, which is higher

than the permissible limit of 100 mg/l (Fig. 6.3). The concentration of sodium varies from

190 mg/l in raw water to 350 mg/l in tailing pond water (table 6.3). Calcium content varies

from 126 mg/l (raw water) to 509 mg/l (tailing pond water). The concentration of

potassium varies from 12.5 mg/l (raw or fresh water) to 18.82 mg/l (tailing pond water).

Sodium, calcium and potassium in coal have also been found to dissolve in water (orhan,

1994). Thus, the concentrations of Na, Ca, K and Mg in water during the process of

washing the coal are higher than those present in the raw water.

6.4.3 Fluoride

The concentration of fluoride in case of Zarand washery is higher in the process

water. There is maximum concentration of fluoride in tailing pond water (1.25 mg/l).

Fluorosis may occur when the fluoride level exceeds the recommended limits. However,

the fluoride content is within the tolerance limit stipulated by WHO (1993).

The mode of occurrence of fluorine in coal is questionable. Minerals appear to be

the site of F in coal could present as fluorapatite and fluorine. Clay minerals, viz.,

kaolinite, illite, and montmorillonite may contain varying amounts of fluorine. The

concentration of F in raw coal is higher than that in clean coal (Swaine, 1990).

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Fig 6.2: Major element content of coal samples collected at different stages of coal

washing.

Fig 6.3: Major element content of water samples collected at different stages of coal

washing.

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6.4.4 Sulphate and Chloride

The concentrations of sulphate and chloride are much higher in the process water.

The sulphate content varies from 57 mg/l (raw water) to 155.30 mg/l (fine coal jig water).

The concentration of chloride varies from 41 mg/l (raw water) to 99 mg/l (fine coal jig

water). Chlorine is probably organically and inorganically bound to coal (Swaine, 1990).

The inorganic chlorides are slightly water-soluble and their removal during preparation is

dependent on size of coal, Cl content of the original coal and wash water, and the duration

of washing. Fresh water flushing of coal is generally effective for Cl reduction (Vlado,

1983).

Coal contains varying amounts of sulphar, which can be grouped into three

categories, namely, inorganic, organic and sulphate sulphur. Whether the sulphur is pyritic

or sulphatic it is a part of the mineral matter and washing the coal can lower its content.

Organic sulphur is distributed through the carbonaceous part of the coal and cannot be

washed out by coal washing techniques. The increase of values in the process water may

be due to minerals and salts present as chlorides and sulphates of the alkali and alkaline

earth metals, which readily dissolve in water (Leonard, 1979).