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Coal Into Coke
Name: Don
Status: other
Grade: 9-12
Location: N/A
Country: N/A
Date: 5/9/2005
Question:
Why is it necessary to convert coal into coke for use in
smelting steel?
Replies:
First a little background.
Iron ore that is mined contains iron oxide and some impurities. To obtain
iron, it is necessary to get rid of the oxygen and impurities. The easiest
way to do this is to heat the ore along with a carbonaceous material like
charcoal, coal, or coke, in a blast furnace. The carbon serves as the fuel
and burns when air is blown into the blast furnace, thus heating it to high
temperatures. The carbon chemically "grabs" the oxygen away from the iron,
and becomes the gas carbon dioxide which goes away. (Actually, it is carbon
monoxide that does the reacting with the iron.) If limestone (calcium
carbonate) is added, the calcium reacts with the silica impurities to form
slag, which separates from the iron. Presto - lots of pig iron with a
fairly high carbon content and still some sulfur.
You may ask why air is blown into a blast furnace when the idea is to remove
the oxygen, not add more? Well, high temperatures are needed, and there is
ample amount of coke present to react with the oxygen and the iron ore, so
too much oxygen is not a problem.
In fact, when the iron comes out, it is loaded with carbon and still
contains some sulfur. The next step is to remove some of the carbon and
sulfur to make steel. So steel is made by blowing oxygen into molten iron.
The oxygen reacts with the carbon to form carbon dioxide which escapes as a
gas. The sulfur is removed by adding "fluxes" that react with the sulfur
and other impurities to form a slag that floats on the top.
Yes, it seems strange to first add carbon, and then take away carbon, but
attempts at making one-step steel have not worked well.
Up until about 1750, charcoal was used in iron production. Charcoal works
well, but it comes from wood, and wood is too expensive to use making iron.
It takes 100 kg of charcoal to make 1 kg of steel. The forests were being
wiped out.
Coal was tried, but it did not work well because impurities in the coal are
transferred to the metal. This is especially bad for sulfur. Sulfur in
iron makes it very weak.
It was discovered that when the coal was converted into coke (by heating in
the absence of air) that many of the impurities would come out of the coal,
either as gases or liquids. Even some sulfur comes out. The coke is then
acceptable for smelting iron ore into iron.
Bob Erck
Don,
Coke is the carbonaceous re\sidue of the destructive distillation of
bituminous coal, petroleum, or coal tar pitch. In the iron making
process, coke is used for the reduction of iron ore in blast furnaces. I
hope that this helps.
Sincerely,
Bob Trach
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