Page 200 - Chemistry ICSE Class X

186 ICSE Chemistry – 10
Calcination of an ore is done to

ƒ convert a carbonate ore into oxide
ƒ remove moisture/water from the wet/hydrated ores
ƒ remove volatile impurities from the ore
During calcination, the ore becomes porous and dry.
For example.
ƒ Calamine (a carbonate ore of zinc) when calcined decomposes to give
zinc oxide.
'
ZnCO o ZnO + CO (g)
2
3
calamine ore zinc oxide
ƒ Calcination of malachite (an ore of copper) gives copper oxide.
Malachite '
CuCO .Cu(OH) o 2CuO + CO (g) + H O(g)
2
3
2
2
malachite copper oxide
What is meant by roasting of an ore
Sulphide ores are converted to oxide by roasting. The process of heating
an ore (generally, a sulphide ore) strongly below its melting point in the
presence of an excess of air is called roasting. Roasting of an ore is done to
ƒ convert a sulphide ore into an oxide ore
ƒ remove volatile impurities and moisture
The chemical reactions during roasting of some common ores are
ƵƌŝŶŐ ƌŽĂƐƟŶŐ͕ Ăůů ƐƵůƉŚŝĚĞ ŽƌĞƐ given below:
ƉƌŽĚƵĐĞ sulphur dioxide (SO ) ŐĂƐ͘
2
heat
2ZnS + 3O o 2ZnO + 2SO (g)
2
2

zinc blende (in air) zinc oxide
(zinc sulphide)
heat
2PbS + 3O o 2PbO + 2SO (g)
2
2
(in air)
galena lead oxide
(lead sulphide)
heat
HgS + O o Hg + SO (g)
2
2
cinnabar (in air) mercury
(mercuric sulphide)
heat
4FeS 2 + 11O o 2Fe O 3 + 8SO (g)
2
2
2

iron pyrites (in air) ferric oxide
How does calcination differ from roasting
Both calcination and roasting involve heating of an ore to obtain oxide of
metal in the ore. The two processes, however, differ from each other in
some aspects (Table 7.6).
Table 7.6 Differences between calcination and roasting
Calcination Roasting
1. During calcination, the ore is heated During roasting, the ore is heated in
in the presence of no or limited the presence of an excess of air.
quantity of air.
2. Calcination is generally used to Roasting is generally used to convert
convert carbonate ores into oxide sulphide ores into oxide ores.
ores.