Extraction of copper from copper pyrites involves the following steps.
Crushing and concentration
The ore is crushed and then concentrated by froth-floatation process.
Roasting
The concentrated ore is heated strongly in the reverberatory furnace, in excess of air. During roasting,
- Moisture is removed.
- The volatile impurities are removed.
- Sulphur, phosphorus, arsenic and antimony which are present as impurity are removed as volatile oxides.
S + O2 → SO2
P4 + 5O2 → 2P2O5
4As + 3O2 → 2As2O3
- The copper pyrite is partly converted into sulphides of copper and iron.
2CuFeS2 + O2 → Cu2S + 2FeS + SO2
2FeS + 3O2 → 2FeO + 2SO2
Smelting
The roasted ore is mixed with powdered coke and sand and is heated in a blast furnace. It is made of steel plates lined inside with fire clay bricks. Hot air at 800°C is introduced from the tuyers near the base of the furnace. As a result, the following changes occur.
- 2FeS + 3O2 → 2FeO + 2SO2
- FeO + SiO2 → FeSiO3 (fusible slag)
- 2Cu2S + 3O2 → 2Cu2O + 2SO2
- Cu2O + FeS → Cu2S + FeO
- FeO + SiO2 → FeSiO3 (fusible slag)
As a result of smelting, two separate molten layers are formed at the bottom of the furnace. The upper layer consists of slag and is removed as a waste while the lower layer is called matte. It chiefly consists of cuprous sulphide and some unchanged ferrous sulphide.
Refining: Blister copper contains about 2% of impurities and it is purified by electrolytic refining.