Occurrence and principles of extraction of zinc
Atomic mass : 65.37 Valence : 2
Atomic number : 30 Symbol : Zn
Position in the periodic table : Period Number -4, Group Number -12.
The ancient used an alloy of Zn and Cu not very different from brass without knowing its actual composition. Zinc has been known in our country for a long time and has been mentioned in Ayurvedic treatises as yashda. It is commonly called jast.
Zinc does not occur in the native form since it is a reactive metal. However, in the combined state, zinc is widely distributed.
The important ores of zinc are
i) Zinc blende, ZnS
ii) Calamine, ZnCO3
iii) Zincite, ZnO
The chief ore of Zinc is Zinc blende. In India large deposits of zinc blende occur in Zawar mines near Udaipur in Rajasthan.
Extraction of Zinc from Zinc blende
The extraction of Zinc from Zinc blende involves the following steps.
The ore is crushed and then concentrated by froth-floatation process.
The concentrated ore is then roasted in the presence of excess of air at about 1200 K.
2 ZnS + 3O2 ® 2ZnO + 2SO 2
Zinc oxide is mixed with powdered coke and heated to 1673 K in a fire clay retort, in which ZnO is reduced to zinc metal.
ZnO + C ----1673K --- > Zn + CO
Zinc is purified by electrolytic refining. In this process, Impure Zinc is anode and cathode is of pure thin sheet of Zinc. The electrolyte is ZnSO4 solution containing a little of dil.H2SO4. On passing electric current, pure zinc get deposited at the cathode.
i) Zinc is a bluish white metal
ii) It is good conductor of heat and electricity. I
ii) It is malleable and ductile.
i) Action of air
When heated in air at 773 K, it burns to form a white cloud of Zinc oxide which settles to form a wooly flock called philosopher's wool.
2Zn + O2 ----773K --- > 2ZnO
ii) Action of water
Pure zinc does not react with water but impure zinc (Zn-Cu couple) decomposes steam quite readily evolving H2 gas.
Zn + H2O (steam) ® ZnO + H2
iii) Action of dilute acids
Pure zinc is not attacked by dilute acids. However, impure zinc reacts with dilute acids with the liberation of H2.
Zn + 2HCl ® ZnCl2 + H2
Zn + H2SO4 ® ZnSO4 + H2
iv) Action of con.H2SO4
Zinc reacts with hot con.H2SO4 forming ZnSO4.
Zn + 2H2SO4 ® ZnSO4 + SO2 + 2H2O
v) Action of HNO3
Zinc reacts with HNO3 at various concentrations and it gives different
4 Zn + 10 HNO3 (dil) ® 4 Zn (NO3)2 + N2O + 5 H2O
4 Zn + 10 HNO3 (very dilute) ® 4 Zn (NO3)2 + NH4NO3 + 3H2O
vi) Action of alkalies
Zinc dissolves in hot NaOH solution forming soluble zincate ion.
Zn + 2NaOH + 2H2O ® Na2ZnO2 + H2O
Uses of zinc
1. It is widely used for galvanizing iron sheets.
2. It is used in the extraction of gold and silver by the cyanide process.
3. Zinc plates and rods are used in batteries and dry cells.
4. Zinc dust and granulated zinc are used in laboratory as reducing agents.