Copper pyrites is an ore of copper.    

(i) Describe the process by which copper pyrites is concentrated.

(ii) Describe all the steps, with equations, to convert the concentrated ore to blister copper.

(iii) Describe the process of conversion of blister copper to pure copper.

Copper pyrites are concentrated by froth flotation process. In this process slurry of finely divided ore is taken in a large tank. Pine oil or eucalyptus oil is used as the frothing agent. Ethyl xanthate is added which is used as a collector (it increases the binding force between ore particles and forth). A mixture of all these things is agitated by current of air and froth is formed. The froth being lighter than water floats over it. Particles of ore stick to it with the help of collector. The froth is separated and washed with water to get the concentrated ore.

(ii) Concentrated ore is converted into blister copper after roasting, smelting and bessemerisation.

Roasting : During roasting, the ore is heated in a reverberatory furnace in excess of air. The volatile impurities are removed during this process.

S + O₂ → SO₂↑

As + O₂ → AsO₂↑

The ore is converted into a mixture of sulphides.

2CuFeS₂ + O₂ →Cu₂S + 2FeS + SO₂

The sulphides are partially oxidised.

2Cu₂S + 3O₂ → 2Cu₂O+ 2SO₂

2FeS + 3O₂ → 2FeO+ 2SO₂

Smelting : In smelting, roasted ore is mixed with coke and sand. It is done in a blast furnace. During smelting ore is heated in excess of air (0₂). As a result, the sulphides of copper and iron are converted into their oxides.

2Cu₂S + 3O₂→ 2Cu₂O+ 2SO₂

2FeS + 3O₂ → 2FeO+ 2SO₂

FeO reacts with sand (silica-SiO₂) to form slag.

FeO + SiO₂ → FeSiO₃ Slag

Cu₂O reacts with unoxidised FeS.

Cu₂O + FeS → Cu₂S + FeO

FeO forms slag. Slag floats over molten mass and is removed. The molten mass is called malte and it is a mixture of Cu₂S, small quantity of FeS and FeO. Bessemerisation: The malte is taken out from the reverberatory furnace and is transferred to Bessemer converter. A blast of air and sand is passed into Bessemer converter and as a result following reaction takes place.

2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂

2FeS + 3O₂ → 2FeO + 2SO₂

Cu₂O reacts with unreacted FeS and converts it into FeO which is removed in form of slag.
2Cu₂S +FeS --> Cu₂S +FeO
FeO +SiO₂--> FeSiO₃

Cu₂S and Cu₂O react together to form blister copper which has about 98% Cu and has blister appearance due to the removal of SO₂ from its surface in form of bubbles.
2Cu₂O +Cu₂S --> 6Cu +SO₂

(iii) Blister copper is converted into pure copper by electrolytic refining. Pure copper strips act as a cathode and impure copper acts as an anode. Acidic CuSO₄ is used as an electrolyte when current is passed through this cell, pure copper deposits at the cathode and an equivalent amount of impure copper goes into solution from the anode. The impurities of Ag, Au and Sn deposit below anode as anode mud. The pure copper deposited at the cathode is scrapped off.