2. Washability Characteristics and Economically Washable Coal:
 All the coal being mined and being categorized as Washery Grade coking coal is
not amenable for washing.
 Coal washability depends on intrinsic property of coal and distribution of
impurities in the coal matrix.
 Washery grade coal from two different sources possessing same ash content might
have different washability characteristics, resulting in different yield percent of
Washed Coal (Clean Coal).
 Washing of coal is an economic activity and yield of clean coal (the primary
product) and pricing of products plays a major role in assessing its viability.

3. Fundamentals of Coal Washability
Washability characteristics determine maximum attainable yield of washed coal and other products;
whereas product pricing and cost of washing including cost of raw material determine whether the coal
is suitable for economic washing or not.
Schematic for the Sink-Float Analysis
• Sink-Float Tests also known as Washability Tests are carried
out in the laboratory to determine how coal particles will
behave when subjected to density separation process
• In this test, coal particles are immersed in liquids of varying
densities as shown in the Figure.
Specific
Gravity
Cumulative Float Cumulative Sink NGM
Wt % Ash % Wt % Ash % Wt%
1.3 3.86 4.55 96.14 33.90
1.4 9.01 11.35 90.99 34.88 22.75
1.5 26.61 19.29 73.39 37.65 61.38
1.6 70.39 27.22 29.61 45.95 59.66
1.7 86.27 29.29 13.73 54.56
> 1.7 100.00 32.76
Example: A typical Coal Washability Data

4. Typical Coal Washability Curves
0
10
20
30
40
50
60
70
80
90
100
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30 40 50 60 70 80 90 100
Cumulative
Sink
Wt
%
Cumulative
Float
Wt%
Cumulative Ash %
Specific Gravity
Float Curve Yield Gravity Curve NGM Curve Sink Curve
2.2 2.1 2.0 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2

5. Thank You