137.

Describe mainly with balanced chemical equations.
(a) Copper from copper pyrites by self reduction.
(b) Zinc from zinc blende by carbon reduction.
(c) Silver from silver glance by cyanide process.

a) copper pyrites is first converted into cuprous sulphide.
2CuFeS₂ + O₂ → Cu₂S + 2FeS + SO₂ 

Cuprous sulphide and ferrous sulphide are further oxidized into their oxides.

2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂ 
2FeS + 3O₂ → 2FeO + 2SO₂

Copper is extracted from the molten matte through bessmerization.  The matte is introduced in to Bessemer. The air is blown through the molten matte. Blast of air convert Cu₂s partly into Cu₂O which react with remaining Cu₂S to give molten copper.

2Cu₂ S + 3O₂ → 2Cu₂ O + 2SO₂ 
2Cu₂O + Cu₂ S → 6Cu + SO₂ 

 This is self-reduction 

b)  Zinc blende is concentrated by floath floatation process. The concentrated ore is heated in excess supply of air above 900^o C on the hearth furnace. During roasting. Zinc sulphide is converted to zinc oxide

 2ZnS + 3O₂ →  2ZnO + 2SO₂

The reduction of zinc oxide is done using coke. The temperature in this case is higher than that in case of copper. For the purpose of heating, the oxide is made into brickettes with coke and clay.

ZnO + C $\stackrel{\mathrm{coke},1673\mathrm{K}}{\to }$ Zn + CO 

The metal is distilled off and collected by rapid chilling.

c) Extraction of silver by cyanide process: silver is extracted from argentite ore. The powdered ore is agitated with a dilute solution of NaCN while a current of O₂ is constantly passed. The metallic sliver from the ore dissolves and forms sodium argentocynide.

 2Ag₂S +8NaCN +O₂ +H₂O → 4Na[Ag(CN)₂] +2NaOH +2S

 This complex is treated with zinc dust when silver gets precipitated as a dark amorphous mass.

4Na[Ag(CN)₂] +Zn → Na[Zn(CN)₄] + 2Ag

Thus, impure Ag is obtained which is purified by electrolysis of a solution of AgNO₃ containing dil HNO₃ using impure Ag as the anode and a strip  of pure Ag as the cathode.