While iron ores are abundant, iron itself is never found free,
but always in combination with other elements. Therefore, the first
step in the production of iron is to separate it from the ore, which
is done in the blast furnace. The method was first used in Germany
in the fourteenth century.
The blast furnace process is a most interesting one. A blast furnace
consists of two divisions:
1. A tall stack or chimney lined with fire brick, into which the
iron ore, fuel, and fluxing material, that is, material which helps
to melt the iron and cause it to flow, are dumped from the top and
in which they are all reduced and melted.
2. A crucible called the "hearth" of the furnace for collecting
the molten products.
The stack may be 60, 80, or even 100 feet high. It is really a
steel shell with a fire-brick lining. It is not a perfect cylinder
like a chimney, but widens out gradually from the top to a point
about two-thirds of the way down and then narrows quickly to the
hearth. The widest part is called the " bosh". The top of the stack
is closed with a funnel arrangement called the " bell and hopper".
The hearth has straight sides and various openings, some of which
are for pipes called "tuyeres" connected with a stove which sends
through them strong blasts of heated air. There are also two small
openings in the front of the hearth, one at the bottom of the furnace
and the other above it at the side. When the furnace is working
these are usually stopped with clay.
On the outside of the furnace is machinery for hoisting the ore
and other materials to the top where they are dumped in.
The fuel is coke, a form of bituminous or soft coal, which has
had the gases and impurities burned out of it. The "flux" is limestone.
As the ore, limestone, and coke fill the stack, blasts of hot air
are forced up from the bottom. Soon the intense heat from the fuel
melts the ore and limestone and the elements separate. The iron
falls to the bottom of the hearth, taking up some of the carbon
from the coke on its way.
The lime, alumina, and ash from the coke are lighter than the molten
iron, and therefore lie on top of it in the form of slag, while
the waste gases pass off through a vertical pipe, or "downcomer".
At periods averaging six hours each, the hole in the hearth-front
on the level of the melted slag is tapped by pushing in the clay
stopper opposite, and the slag is drawn off through sand gutters
or into great pots called ladles, and thence into water pits where
it is granulated.
From twenty to thirty hours after the furnace is lighted the clay
stopper in the lowest hole in the hearth is punctured and a stream
of liquid iron flows out, gradually enlarging with the flow.
The brilliant seething mass runs into a long trench, or gutter,
cut in sand, which has small side trenches crossing it at right
angles. The side trenches are 40 inches long, 4 inches wide and
4 inches deep.
The entrances to the side trenches or molds are all closed with
iron gates until the iron has reached the bottom one, which it fills
first. The gates are then removed from each in turn until the whole
series are filled, sand is sifted over the metal, and water sprayed
upon it so that it is soon hard enough to be broken away from the
channel. The channel itself is then broken up. From some fancied
resemblance the bars have always been called "pigs", and the iron
at this stage is called "pig iron".
Figure 1 shows a blast furnace with the long trench and pig beds.
The iron from many blast furnaces is no longer run into a "pig
bed" but into ladles or pots made of iron lined with firebrick.
These pots are on trucks which may be rolled directly to the steel
mill, and the contents subjected to further treatment.
Pig iron is the most impure form of iron, containing about three
parts carbon and a number of other elements in small quantities.
It is hard and brittle; it cannot be hammered nor drawn out into
wire that is, it is not malleable nor ductile nor can it be welded.
Figure 1. Blast Furnace and Pig Beds.
For cast iron it is remelted and cast into molds without any change
in its composition. For wrought iron or steel it must go through