Explain calcination and roasting with example.

Calcination : Ore is heated to a high temperature below its melting point in the absence of dry air or in a limited supply of air. Organic matter, moisture, volatile impurities like carbon dioxide and sulphur dioxide are expelled from the ore. It makes ore porous. Decomposition of carbonate ores gives  metal oxides and carbon dioxide.  
$ZnC{O}_3\stackrel{\Delta }{\to }ZnO(s)+C{O}_2(g)$

Roasting:
Ore is heated to high temperature below melting point in presence of excess air. Moisture escapes and impurities of sulphur, arsenic, phosphorous etc are oxidized to volatile oxides. It makes mass porous. During extraction of magnesium, roasting is used for magnesite.
$S+O_2\to S{O}_2\uparrow$
$2ZnS+3O_2\to 2ZnO+2S{O}_2\uparrow$