2. Coal preparation, which is also called washing,
cleaning, processing, and beneficiation, is the
method by which mined coal is upgraded in order to
satisfy size and purity specifications dictated by a
given market. The upgrading, which occurs after
mining and before transport of the cleaned product
to market , is achieved using low-cost, solid-solid
and solid-liquid separation processes that remove
waste rock and water from the mined coal. The
processing is driven by a desire to reduce freight
costs, improve utilization properties, and minimize
environmental impacts.
Coal Washing
3. Washing of Coal
• Most of the coals when mined contain impurities
associated with it. Impurities are removed by
washing.
• Fixed impurities:
-Originate from the coal forming plants
- Can not be removed by washing
• Free impurities:
-Adhering to the surface
-Can be removed by washing of coal
4. Objectives of Coal Washing
• To reduce its ash content
• To reduce transportation and storage costs
• To increase its heating value
• To increase the fusion point of its ash by
removing alkali chlorides
• To reduce its clinkering tendency
• To increase its efficiency in use
• To reduce its sulphur and phosphorous contents
5. Coal Washing Processes
• Most coal washing processes depend upon differences in specific
gravity between coal and its impurities
• Approx. Sp. Gr. Coal: 1.3, shale: 2.5 pyrite: 4.0
• Liquids of different densities for floating off the coal fractions may be
used.
• Such liquids may be mixtures of Carbon tetrachloride or bromoform
with toluene or solutions of zinc chloride
• Some Types of Coal washing/cleaning plants
• Dense Medium Separators
• Jigs
• Cyclone Washers
• Concentrating tables
• Froth Floatation
6. Typical washability data for Coal with 15.3% ash
Floating material Sinking Material
S.G. of
Liquid
%yield Ash % %yield Ash%
1.25 5 0.8 95 16
1.3 65 2.4 35 39.3
1.35 75 3.1 25 51.8
1.4 79 3.8 21 58.5
1.5 82 4.5 18 64.1
1.6 85 5.6 15 70.0
1.8 87 6.8 13 72.7
7. Dense Medium Washers
• Properties of the Medium
• The medium used to float the coal from its impurities
must be of low viscosity
• The medium must have also some stability so that it has
effective uniform density
• High specific gravity of the medium solid (the grains
suspended in water) is also desirable to give effective
medium densities upto or over 2.0
• Mixture of sand and water can be used as washing
medium
8. Dense Medium Washers
• Many designs have been employed so not
possible to generalize
• Mainly there are two processes under this
heading
(i) Chance Process
(ii) Tromp Process
9. Chance Process
• Washing medium is a mixture of sand and
water
• A deep bath of sand and water having
intermediate specific gravity between clean
coal and ash is used
• Clean coal floats and dirt sinks and removed
10. Tromp Process
• A shallow bath instead of deep bath of sand
and water is used
• Clean coal floats and dirt sinks which is
removed by scraper conveyor
• Requires much lesser volume of fluid
• Requires lesser building height
11. Jig Washers
• A coal bed is maintained on a perforated
plate
• Coal bed is subjected to action of upward
and downward current of water
• Clean coal being the lighter is
concentrated at the top while the dirt sinks
and removed from the bottom of the
washer
12. Baum Jig
• This is the most common type of Jig Washers
• Baum Jig is U-shaped vessel divided into washing
and air compartments
• Compressed air is used to create the pulsating
movement of water
13. Advantages of Jig Washers
• Better separation efficiency for finer sizes
of coal (25-200mm)
14. Cyclone Washer
• The cyclone washer employs the same principle as
cyclone dust collector except that the coal is fed in a
STREAM OF WATER (or dense medium) instead in air
• It is a settling chamber in which settling force is replaced
by centrifugal force
• The clean coal is obtained as an overflow from the
cyclone
• The underflow from the cyclone contains the heavier
material or impurities
15. Advantages
• Efficient for small sized coal
• High flexibility of operation
with respect to near gravity
material, coal size and feed
size
• Disadvantages :
• Higher power consumption
• Higher maintenance cost
Cyclone Washer
16. Froth Floatation process
• Froth is made in a floatation cell by bubbling air
through water in presence of frothing agents
• Frothing agents used are normally cresol, pine
oil or alcohols
• When dirty coal powder is added to cell, the pure
coal particles adhere to the bubbles of the froth
• Dirt particles sink in the cell
• The clean coal is recovered by filtration
17. • Advantages:
• It can produce ultra pure coal
• It is suitable for fine coals
• Disadvantages:
• High capital and running cost
• Coal must be grinded to less than 0.5 mm
Froth Floatation process
18. Concentrator tables
• The table is fitted with wood riffles and is
oscillated lengthwise.
• Coal is fed to the top and water flows
transversely
• The heavy refuse is trapped in the riffles
and transported to the end
• The coal passes over the riffle and travels
to the other corner
20. • The mechanism is attached to the table, and it moves the table along the
long axis a distance adjustable between 1/2" and 1" and then back to the
starting position between 200 and 300 times per minute. This reciprocal
movement is faster on the reverse stroke than it is on the forward stroke.
This shaking movement helps transport the concentrates or heavy material
to the concentrate end of the table. A very important operating variable of a
shaking table is the tilt adjustment. Normally, the feed side is lower, and the
concentrate end is higher on the table, which creates an upward slope
where the heavy material will ascend, while the light density material will
not, and consequently, flow over the riffles. The tailing (low density) side is
usually near level to lower than the feed side.
Feed is introduced to the feed box in a slurry at 25% solids, and is
distributed along the feed side of the table by the feed trough. Additional
wash water is added ad the end of the feed trough, and distributed along
the remainder of the length of the table by a wash water launder,
21. Dry Coal Sorting
• Coal is fed onto a conveyor belt as a thin layer. The bed
of material passes through a proprietary X-ray analyzer
that subjects the particles to hundreds of sequential X-
ray scans. This phenomenon makes it possible to
distinguish coal (organic matter composed mostly of
carbon with a low atomic number) from rock (inorganic
mineral matter composed of various elements such as
silicon and aluminum with higher atomic numbers). The
resolution and speed of the scanner and associated
electronics is of sufficient quality so that a compositional
profile of each particle can be reconstructed in fractions
of a second. Once identified, controlled microbursts of
compressed air from a horizontal array of pneumatically
actuated jets divert unwanted particles of rock into the
reject stream, while coal particles follow their normal
trajectory into the clean coal product stream.
