1. Solid Fuels

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

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

13. 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

14. 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

15. 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

16. 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

17. 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