The concentrated ore is heated strongly with silicon dioxide (silica) and air or oxygen in a furnace or series of furnaces.
The Full Process
Large amount of copper are obtained from copper pyrite (CuFeS2) by smelting. Ores containing 4% or more copper are used and are treated by smelting process.
Concentration - Froth-Floatation process
The finely crushed ore is concentrated by Froth-Floatation process. The finely crushed ore is suspended in water containing a little amount of pine oil. A blast of air is passed through the suspension. The particles get wetted by the oil and float as a froth which is skimmed. The gangue sinks to the bottom.
The concentrated ore is then roasted in a furnace in the presence of a current of air. Sulphur is oxidized to SO2 and impurities of arsenous and antimony are removed as volatile oxides.
The following reaction takes place.
2CuFeS2 + O2 → Cu2S + 2FeS + SO2
S + O2 è SO2
4As + 3O2 → As2O3
4Sb + 3O2 → 2Sb2O3
Cuprous sulphide and ferrous sulphide are further oxidized into their oxides.
2Cu2S + 3O2 → 2Cu2O + 2SO2
2FeS + 3O2 → 2FeO + 2SO2
The roasted ore is mixed with coke and silica (sand) SiO2 and is introduced in to a blast furnace. The hot air is blasted and FeO is converted in to ferrous silicate (FeSiO3).
FeO + SiO2 → FeSiO3
Cu2 O + FeS → Cu2 S + FeO
FeSiO3 (slag) floats over the molten matte of copper.
Copper metal is extracted from molten matte through bessemerization . The matte is introduced in to Bessemer converter which uphold by tuyers. The air is blown through the molten matte. Blast of air converts Cu2S partly into Cu2O which reacts with remaining Cu2S to give molten copper.
2Cu2 S + 3O2 → 2Cu2 O + 2SO2
2Cu2O + Cu2 S → 6Cu + SO2
The copper so obtained is called "Blister copper" because, as it solidifies, SO2 hidden in it escapes out producing blister on its surface.
Removal of impurities from blister copper
Blister copper is 99% pure. It contains impurities mainly iron but little amount of As, Zn, Pb, Ag and Au may also be present. These impurities adversely affect the electrical as well as mechanical properties of copper. Therefore, they must be removed. Refining is used for this purpose.
Blister copper is refined by electrolysis. Blocks of blister copper are used as anodes and thin sheets of pure copper act as cathodes. The cathode plates are coated with graphite in order to remove depositing copper. The electrolyte is copper sulphate (CuSO4) mixed with a little amount of H2 SO4 to increase the electrical conductivity. Optimum potential difference is 1.3 volt for this electrolytic process. During electrolysis, pure copper is deposited on the cathode plates and impurities which are soluble and fall to the bottom of the cell as anode mud or sludge.
Reactions during electrolysis at electrodes
Cu → Cu+2 + 2e‾ (at the anode)
Cu+2 +2e‾ → Cu (at the cathode)
We get copper is 100% pure copper from electrolytic refining.