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Question

Write short notes on the following:
(a) Roasting
(b) Calcination

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Solution

Roasting:
Ores are heated to a high temperature below their melting point in the presence of excess air. The moisture escapes and impurities like sulphur, arsenic, phosphorous etc are oxidized to their volatile oxides. The mass becomes porous.
S+O2SO2
4P+5O22P2O5
4As+3O22As2O3
Sulphide ores are converted into metal oxides.
2ZnS+3O22ZnO+2SO2
2Cu2S+3O22Cu2O+2SO2
Calcination:
The ore is heated to a high temperature below its melting point in the absence of air or in a limited supply of air. The organic matter, moisture, volatile impurities like carbon dioxide and sulphur dioxide are expelled from the ore. It makes the ore porous. For example, zinc carbonate decomposes to give zinc oxide and carbon dioxide.
ZnCO3ΔZnO+CO2

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