Did a quick 'Google' using:
"caking properties" coal
and got many hits.
The following was taken from one website:
http://eyrie.shef.ac.uk/eee/cpe630/comfun4.html
Caking Properties of Coals
This is a unique property of coals in the bituminous group of coals and is
an essential property for coals which are required for coking. As a caking
coal is heated it passes through a region where it becomes very plastic,
softens, swells and then re solidifies. The residue is a cellular coke
mass. Coals which do not cake are simply form a non coherent or weakly
coherent char.
A number of tests have been devised to classify the caking properties of
coals including the Roga test, Free Swelling Index and Gray - King test.
The free swelling index test entails heating a standard powder of the coal
in a crucible and comparing the resultant "button" with a standard profile,
fig 2, an index is given between 0 and 9. A non coherent sample is given 0.
generally 0 - 3 implies marginal caking behaviour. The Gray King index is
essentially the same except the residue is compared with a number of
previously made standard cakes.
The caking behaviour is critical to coke making. A successful coke must be
strong and not powdery. Prime coking coals have GK indexes of G (and its
subdivisions). Much less and a weak coke unstable coke is made.
Hope this helps.
John
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