Slag and tapping
Iron ore is not iron alone. It carries impurities too — and removing them is precisely why limestone is added.
Two impurities
Silica, or silicon dioxide (SiO2)
Alumina, or aluminium oxide (Al2O3)
These react with the calcium oxide (CaO) produced by the decomposition of the limestone and form calcium silicate and calcium aluminate, which together are known as slag.
The slag-forming reactions
SiO2 + CaO → CaSiO3
Al2O3 + CaO → CaAl2O4
These two products together are called the slag.
How they are separated
The slag floats
The slag floats on the surface of the molten iron, being the less dense of the two. So the molten iron and the slag are drawn off separately.
As figure 16.1 shows, there are two outlets low down in the furnace — one for the slag and, below it, one for the molten iron.
Is the slag just waste?
No. Lying as a layer on the molten iron, it also keeps the iron from reacting with the oxygen of the air again. Slag is used in cement and in road building as well.