Question

Explain the following:
(a) Roasting
(b) Calcination

Answer 1

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.
$S+O_2\to S{O}_2\uparrow$
$4P+5O_2\to 2P_2{O}_5\uparrow$
$4As+3O_2\to 2A{s}_2{O}_3\uparrow$
Sulphide ores are converted into metal oxides.
$2ZnS+3O_2\to 2ZnO+2S{O}_2\uparrow$
$2C{u}_{2}S+3O_2\to 2C{u}_{2}O+2S{O}_2\uparrow$
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.
$ZnC{O}_3\stackrel{\Delta }{\to }ZnO+C{O}_2$