Zinc
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ZINC
Silver is a bluish white, metallic element. It is characterised by
- Atomic number 30
- Relative atomic mass 65.38
- Relative density 7.1
- Melting point 419.57OC
- Boiling point 907OC
- Is highly malleable and ductile.
OCCURRENCE
Zinc does not occur free in nature. The important zinc minerals are sphalerite (zinc blende, ZnS), zincite (ZnO), calamine (zinc carbonate, ZnCO3), franklinite (ZnFe2O4).
Metallurgy of Zinc:
The sulphide ore is roasted in air to form the oxide :
2ZnS + 3O2 è 2ZnO + 2SO2
This zinc oxide is mixed with coke and heated in a blast furnace ;
ZnO + C è Zn + CO
The zinc vapours leaving the furnace meet a stream of molten lead which condenses about the zinc. On further cooling, the zinc separates as crystals from the lead melt (zinc is insoluble in molten lead at 450OC). Many zinc ores also contain lead, so that any lead formed as a by-product during this process accumulates with the lead used in the cooling process.
Zinc is also refined by the electrolysis of purified zinc sulphate solution, using a high current density and an aluminium cathode. The zinc sulphate used for this process is obtained by roasting low-grade sulphide ore :
ZnS + 2O2 è ZnSO4
NOTE: The electrolytic method produces very pure zinc, while zinc from the blast furnace process is purified by distillation.