Solid Fuels

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

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

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

Typical washability data for Coal with 15.3% ash Floating material Sinking Material S.G. of Liquid %yield Ash % 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

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

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

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

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

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

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

Advantages of Jig Washers Better separation efficiency for finer sizes of coal (25-200mm)

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

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

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

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

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