Question

Giving examples, differentiate between ‘roasting’ and ‘calcination’.

Answer 1

Calcination: The concentrated ore is heated below its melting point in a little amount of air throughout this process. The following changes occur during calcination.

1. The moisture is removed, and the ore becomes dry.

2. Volatile impurities get removed.

3. The ore becomes porous.

4. If carbonates are present in the ore, they break down into oxides and release CO₂. For example,

\[\ce{\underset{Lime stone}{CaCO3} -> CaO + CO2}\]

\[\ce{\underset{Magnesite}{MgCO3} -> 2CuO + H2O + CO2}\]

\[\ce{\underset{Malachite}{CuCO3 * Cu(OH)2} -> 2CuO + H2O + CO2}\]

5. Hydrated oxides get dehydrated and lose their water of crystallisation. For example:

\[\ce{\underset{Haematite}{Fe2O3 * 3H2O} -> Fe2O3 + 3H2O}\]

\[\ce{\underset{Haematite}{Al2O3 * 2H2O} -> Al2O3 + 2H2O}\]

Roasting: The method of roasting involves intensely heating the ore while there is an abundance of air present. Typically, the procedure is used for sulphide ores. The following changes occur during roasting.

1. The moisture is removed.

2. The volatile impurities are removed.

3. The volatile oxides of contaminants such as sulphur, arsenic, phosphorus, etc., are eliminated.

\[\ce{S + O2 -> SO \uparrow}\]

\[\ce{4As + 3O2 -> 2As2O3 \uparrow}\]

\[\ce{P4 + 5O2 -> 2P2O5 \uparrow}\]

4. The sulphide ores get oxidised into metal oxides. For example,

\[\ce{\underset{Copper pyrites}{CuFeS2 -> Cu2S + 2FeS + SO2}\]

\[\ce{2FeS + 3O2 -> 2FeO + 2SO2}\]

\[\ce{2CuS + 3O2 -> 2Cu2O + 2SO2}\]

\[\ce{\underset{Galena}{2PbS} + 3O2 -> 2PbO + 2SO2}\]

\[\ce{2ZnS + 3O2 -> 2ZnO + 2SO2}\]

A special type of furnace known as a reverberatory furnace is used for both roasting and calcination. Metal in the form of its oxide is present in the ore that is produced by roasting or calcining. It can still contain some contaminants.