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Extraction of Metals

Extraction of Iron.

Chemistry of the blast furnace (continued).

Limestone is calcium carbonate (CaCO₃) and it is added
to the blast furnace to remove the impurities in the iron ore.
Calcium carbonate is decomposed by heat in the furnace
to give calcium oxide (quicklime) and carbon dioxide.
This is called thermal decomposition
(see examples of other carbonates).

calcium carbonate  calcium oxide  + carbon dioxide.
CaCO_3(s)                   CaO_(s)       +        CO_2(g)

The main impurity is silica (sand or rock) which is silicon dioxide.
Silicon dioxide is solid at the furnace temperature
and the furnace would become blocked if it was not removed.
Silicon dioxide reacts with calcium oxide
to form calcium silicate (called slag) which is liquid in the furnace.
Slag flows to the bottom of the furnace
where it floats on the liquid iron and is easily removed.

calcium oxide  + silicon dioxide  calcium silicate.
 CaO_(s)        +         SiO_2(s)             CaSiO_3(l)

The slag (CaSiO₃) is allowed to cool until it becomes a solid
and is used for road construction.

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