coal washing

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Coal Washing
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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

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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

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Coal Washing Processes

• Some Types of Coal washing/cleaning plants
• Dense Medium Separators
• Jigs
• Cyclone Washers
• Froth Floatation

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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

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Dense Medium Washers

• Many designs have been employed so not possible
  to generalize
• Mainly there are two processes under this heading
• Chance Process
• (ii) Tromp Process

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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

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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

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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

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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

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Advantages of Jig Washers

• Better separation efficiency for finer sizes of
  coal (25-200mm)

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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

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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
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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

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Froth Floatation process

• 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