Extraction of Copper from its Ores Chemistry Notes
Extraction of Copper from its Ores :
→ The atomic number of copper is 29. It belongs to d-block of periodic table. It exists in native as well as in combined state. Its percentage abundance in earth’s crust in about 0.0001%. The principal ores of copper are as follow :
- Bornite – Cu3 Fes3
- Cuprite (red) – Cu2O
- Cholcocite or copper glance – Cu2S
- Azorite (blue) – 2CuCO3 . Cu(OH)2
- Malachite (green) CuCO3 . Cu(OH)3
Copper is extracted from copper pyrites (CuFeS). The extraction involves the following steps:
Pulverisation : In this process, the ore is converted into powdered form by ball mills or jaw crushers.
→ Concentration : First of all to remove silicious impurities it is concentrated by gravity separation method then it is concentrated by froth floatation process. Pulverised ore with pine oil and with some potassium xanthate is transferred into water tank.
→ A strong stream of air is passed through whole mass in order to produce a lot of froth. The ore particles with froth is carried out at the surface of liquid while impurities settle down to the bottom of the tank. The froth is skimmed off and collected into another tank where concentrated ore can be collected.
→ Roasting : Concentrated ore is now heated strongly in the presence of air in reverberatory furnace. During roasting, the following changes may occur :
Impurities are converted into volatile oxides and removed.
- 4As + 3O2 → 2As2O3
- 4Sb + 3O2 → Sb2O3
- S + O2 → SO3
Copper pyrite ore is oxidised into cuprous sulphide and ferrous sulphide.
The sulphides of Cu and Fe are partially oxidised.
2Cu2S + 3O2 → 2Cu2O + 2SO2
2FeS + 3O2 → 2FeO + 2O2.
- The whole mass becomes porous.
- The moisture is removed.
→ Smelting : Roasted ore is mixed with coke and silica (a flux) and transferred into a small blast furnace. The mixture is heated with the help of blast of hot air. The following reactions occur in blast furnace :
Copper oxide reacts with ferrous sulphide to produce ferrous oxide and copper sulphide.
Cu2O + FeS → FeO + Cu2S
Iron sulphide is oxidised into iron oxide
2FeS + 3O2 → 2FeO + 2SO2 ↑
The gangue of ferrous oxide combines with flux silica and converted into slag.
→ Being lighter this slag floats over the surface of molten Cu2S which can be removed easily from the lower part of furnace. The lower part of furnace consists of a shallow hearth which has molten mixture of Cu2S and little sulphide. This mixture of sulphides (Cu2S + FeS) is known as ‘matte’.
→ Bessmerisation : Molten matte, a mixture of Cu. S and Fes, obtained from smelting is transferred to a “Bessemer Converter”. Some silica (flux) is also added with it and a blast of hot air in blown through the molten mass. In Bessemer converter, following reactions occur:
Remaining FeS is oxidised into Feo.
FeS + 3O2 → 2FeO + 2SO2
Ferrous oxide obtained, now reacts with silica (flux) and converted into slag
This slag is drained out at intervals by tilting the vessel.
Remaining Sio, reacts with CaO present an inner lining in the vessel of Bessemer converter.
SiO2 + CaO → CASiO3
In this way, excess silica is absorbed by basic lining of converter.
Cu2S is oxidised which combines with remaining cuprous sulphide to form free copper metal.
→ The molten copper is poured off and allowed to cool. During cooling, the dissolved SO2 gas comes out and large blisters are formed on the surface of solid copper. This copper with blister of SO2 is known as Blister copper. It consists of about 98% Cu and 2% impurities of Ag, Au, Zn, Pb, Sn etc.
→ Refining of Copper: Blister copper can be refined by poling and electrolytic refining. But electrolytic refining produce about 99.9% pure copper so it is most efficient. During electrolytic refining, a thin sheet of pure copper metal is made cathode while the block of crude metal is made anode. Both electrodes are placed in acidified copper sulphate solution.
→ When electric current is passed through the solution, then impure copper from anode goes into the solution and pure copper from the solution gets deposited on the cathode. Here oxidation process takes place at anode while reduction takes place at cathode. During electrolysis, anode dissolves while cathode is thickened out. The reactions taking place at different electrodes are as follow:
At Anode : Cu → Cu2+ + 2e– (oxidation)
At Cathode : Cu2+ + 2– → Cu (reduction)
→ The impurities present in anode is collected below anode as anode mud. The anode mud has the impurities of Zn, Ni, Fe ete.