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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).