Roasting is the process of converting sulphide ores to oxides by heating the ores in a regular supply of air at a temperature below the melting point of the metal. For example, sulphide ores of Zn, Pb, and Cu are converted to their respective oxides by this process.

2ZnS + 3O₂ \(\overset{\Delta}\longrightarrow\) 2ZnO + 2SO₂

Zinc blende

2PbS + 3O₂ \(\overset{\Delta}\longrightarrow\) 2PbO + 2SO₂

Galena

2Cu₂S + 3O₂ \(\overset{\Delta}\longrightarrow\) 2Cu₂O + 2SO₂

Copper glance

On the other hand, calcination is the process of converting hydroxide and carbonate ores to oxides by heating the ores either in the absence or in a limited supply of air at a temperature below the melting point of the metal. This process causes the escaping of volatile matter leaving behind the metal oxide. For example, hydroxide of Fe, carbonates of Zn, Ca, Mg are converted to their respective oxides by this process.

Fe₂O₃.3H₂O \(\overset{\Delta}\longrightarrow\) Fe₂O₃ + 3H₂O

Limonite

ZnCO₃(s)\(\overset{\Delta}\longrightarrow\) ZnO(s) + CO₂(g)

Calamine

CaMg(CO₃)₂ \(\overset{\Delta}\longrightarrow\) CaO(s) + MgO + 2CO

Dolomite