[TheForge] Coal clumping

[Bruce Freeman]

I don't know the answer.  I do know a little about graphitic carbon, however, and I can speculate from that.

Graphitic carbon (graphite, charcoal, coke) consists of large sheets of carbon atoms bound together in a hexagonal grid, rather like the cells of a bee hive.  Above and below the plane of the sheet is a dense electron cloud.  Other chemicals like to attach to this cloud of electrons, as it is very polarizable (i.e., takes an induced electric charge easily).  This property makes graphite a good material for adsorbing chemicals, like the nasties out of your tap water.  When water gets on the surface of these sheets, the sheets will slip over each other very easily - making graphite suitable as a lubricant.  (Doesn't work in space because the water evaporates out.)  

Normally, such carbon will have stuff stuck to the surface, be it only water.  If you take a fairly pure form of carbon (charcoal) and heat it in the oven to a moderate degree, you'll drive off this stuff and "activate" the charcoal, making it ready to pick up more stuff.

Now, with that in mind, imagine what happens when you heat bituminous coal or coal dust to high temperatures.  First the volatiles (water and much much more) boil out.  Sometimes the coal puffs like popcorn during this process.  This heating does a good first approximation of activating the carbon.  Hence, the pieces of carbon sheets are "sticky" and will stick to each other.  This apparently happens not only within one chunk of coal, but between chunks or particles.

So, the follow-up question is why DOESN'T this happen for charcoal or anthracite.  Well, I don't know.  It may be because these don't swell when heated and hence don't expose new surface.  

Hope this helps.

Bruce
NJ

>>> trollkeep at gmail.com 4/23/2007 1:51 PM >>>
I was asked at a 4-H Blacksdmithing class why the coal clumps up when
it converts to coke.  As odd as it may seem I did not have the
slightest idea.  Does anyone know in terms that I can at least convert
for a ten year old?

David Childress
Rocky Forge Blacksmith Guild
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