The Blast Furnace
Ferrous metals are alloys of iron with carbon; these alloys may contain
also some other elements such as silicon (Si), phosphorus (P), etc., but
carbon is the most important of all elements present in ferrous alloys.
Ferrous metals are used in industry in two general forms: cast iron and
steel. These two ferrous alloys are usually produced from pig iron and they
have different carbon content. Steel is iron containing from 0.05 to 1.7 per
cent carbon, while pig iron is an alloy of iron and carbon with the carbon
content more than 2.0 per cent. Pure iron is not used in industry because it
is too soft.
The furnace that is used for separating iron from the other elements
combined with it in the iron ore is called a blast furnace. It is called so
because a blast of hot air is forced into it, while producing the pig iron.
This is a vertical furnace from 50 to 100 feet high and from 10 to 20 feet
in diameter. The walls of this furnace are made of refractory bricks, which
can stand great heat.
The largest parts of the blast furnace are the shaft and the hearth. The
charging mechanism is in the shaft top; through this charging mechanism the
iron ore, coke, and flux are charged into the blast furnace. The blast of
hot air forced into the hearth through tuyers, special holes in the top part
of the hearth. These tuyers are cooled with water while working. The pig
iron flows out of the furnace through the tap which is a special hole in the
wall of the hearth near the hearth bottom. The slag, which is lighter than
the pig iron, flows out through another hole made in the wall of the furnace
higher than the tap. This hole is called the slag hole.
For separating iron from the impurities the iron ore must be melted. The
ore melts at a very high temperature that is why the temperature inside the
blast furnace is held about 2,500 degrees Fahrenheit. This great heat is
produced by the coke burning in the blast of hot air. The air is heated in
special stoves which are called hot-blast stoves. In these stoves the blast
furnace gases are burned to produce heat and to heat the air.
A blast furnace may hold about 1,000 tons of iron ore, coke, and fluxes.