Chemistry

Conversion of the enriched ore into the oxide of metal
It is easier to obtain metals from their oxides than from their carbonates or sulphides. Hence, the concentrated ore is converted into the oxide of metal which is then reduced to metal. This conversion to oxide is done by the process of calcination or roasting. In this process the ore is heated very strongly in the absence of air, keeping the temperature below its melting point so that volatile impurities are driven off.
EXAMPLES :
(i) Oxide ores are calcined to remove moisture and other volatile impurities.
Al₂O₃ . 2H₂O → Al₂O₃ + 2H₂O
(ii) Carbonate ores are calcined to expel carbon dioxide.
CaCO₃ → CaO + CO₂
CaCO₃ . MgCO₃ → CaO + MgO + 2CO₂

CuCO₃ . Cu(OH) → 2CuO + H₂O + CO₂
Sulphide ores are usually converted to oxides by roasting. The process involves heating the ore at a temperature below its fusion point, but always in the presence of air so that it may be oxidized.
Arsenic and similar other elements present in free state or combined state are also oxidized to volatile oxides.
(i) Zinc blende (ZnS) is roasted in air to convert it into zinc oxide.
2ZnS + 3O₂ → 2ZnO + 2SO₂
(ii) Galena (PbS) is converted into litharge (PbO) by roasting.
2PbS + 3O₂ → 2PbO + 2SO₂
(iii) Cinnabar (HgS) is roasted to convert it directly into mercury (Hg).
HgS + O₂ → Hg + SO₂
(iv) Iron pyrite (FeS₂) is converted into ferric oxide (Fe₂O₃) by roasting.
4FeS₂ + 11O₂ → 2Fe₂O₃ + 8SO₂
Thus, both calcination and roasting produce oxide of the metal. However, there are a few points of difference between the two processes.

Chloride ores remains unchanged by calcination or roasting.