One of the requirements for reducing iron ore into molten iron in
a steel plant is coke, which is a non-volatile residue resulting
from the destructive distillation of coals. Destructive distillation
is the process of decomposing a substance (wood or coal) by heating
it in the absence of air. Air is kept out of the oven so that the
wood or coal will not fire. Coke, along with many useful volatile
by-products, is produced when carefully selected bituminous coals
are heated in a closed atmosphere. Coke used in a blast furnace
must have the following characteristics:
1. It must have enough strength to resist shattering by handling
or crushing under the load of the blast furnace charge.
2. It must be free of dust and particles under 3/4 inches in diameter
for speed of combustion.
3. It must contain as little sulphur and ash as possible as determined
by the coals used in its manufacture.
Coke is a hard, greyish substance obtained when soft coal (bituminous)
is heated in an airtight oven. Coke contains 87 to 89 per/cent carbon,
is hard and porous, produces intense, smokeless heat when it burns
and is valuable in smelting (melting) iron ore. Coke used in smelting
is called metallurgical coke.
Coke was first produced in beehive ovens but is now produced in
the more efficient by-product ovens. The advantages gained in these
by-product ovens are; increased coke yield, through exclusion of
air; shorter coking period through the narrower chambers in use
and the more efficient means of utilising gases; saving in the heat
of the oven through external quenching; saving of time and labour
through the use of mechanical equipment.
Originally, charcoal was produced by stacking hard or soft wood
in piles or long rows that were covered with earth; a small opening
was left in the top through which a fire was lighted, and the wood
was allowed to smoulder for about 10 days. The hole was then closed,
the fire smothered, and the charcoal collected. This early wood
charcoal was used in the production of pig iron because of its low
sulphur content but was replaced by the more economical and efficient
coke produced from bituminous coal.