Calcination/Roasting: Definition, Examples, Difference

Calcination and Roasting are the two widely used processes of metallurgy which help in converting ores to respective sulphides and carbonates either through the process of reduction or oxidation. Generally, metals are obtained from the oxide ores. While it is easy to reduce the oxide ores, things are not the same with sulphates and carbonates. They needed to be converted into oxide ores first. Calcination and Roasting are the processes that help in converting sulphides and carbonates into oxide ores.

Keyterms: Calcination, Roasting, Metallurgy, Sulphates, Carbonates, Oxidation, Reduction, Oxide ores, Heating, Metals, Ores, Thermal decomposition process

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Calcination

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Calcination comes from the Latin word “calculate” which means “to burn lime”. It refers to the process of converting ore into its oxide form by strong heating. The ore is heated below its melting point and in the absence or limited supply of air. Apart from converting ores into their oxide forms, Calcination also helps in removing moisture and volatile impurities. Calcination can also be called a thermal decomposition process as the reactions occur at or above the thermal decomposition temperature. 

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Roasting

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Roasting refers to the process of converting ore into its oxide form by heating the ore below its melting point. Roasting takes place in the presence of an excess amount of air. Apart from converting ores into their oxide forms, Roasting also helps in removing moisture and metallic impurities in the form of volatile gases. Roasting is a solid-gas thermal reaction that includes oxidation, reduction, sulfation, chlorination and pyro hydrolysis.

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Examples 

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Calcination: Calcination is commonly used for converting carbonates and hydroxides into oxides. For example, calcium carbonate (limestone) is decomposed into calcium oxide (lime) through calcination. 

CaCO₃ → CaO + CO₂

Some other examples of Calcination include: 

• Calcination of Bauxite and Gypsum in which hydrated minerals are decomposed to remove the water of crystallization. 
• During the synthesis of zeolites, ammonium ions are removed through the calcination process. 
• Calcination is also used to decompose the volatile matter contained in the raw petroleum coke.

Calcination

Roasting: Roasting is commonly used for converting sulphides into oxides. Because of this, it causes air pollution by releasing a large amount of metallic as well as toxic and acidic compounds in the air. 

For example, roasting includes conversion of zinc sulphide into zinc oxide. 

2ZnS + 3O₂ →  2ZnO + 2SO₂ 

Some other examples of Roasting are: 

• Roasting of Chalcocite (Cu₂S) to obtain Copper Oxide (Cu₂O).

2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂

• Conversion of metal compounds to chlorides through Chloridizing Roasting.

2NaCl + MS + 2O₂ → Na₂SO₄ + MCl,

4NaCl + 2MO + S₂ + 3O₂ → 2Na₂SO₄ + 2MCl₂

• Controlled roasting of a non-magnetic ore like haematite to convert it into magnetite which is a magnetic ore.

Roasting

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Difference between Calcination and Roasting

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Some of the key differences between Calcination and Roasting are as follows:

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Things to Remember 

• Calcination and Roasting are the processes that help in converting sulphides and carbonates into oxide ores.
• Calcination refers to the process of converting ore into its oxide form by strongly heating the ore.
• It takes place in the absence or limited supply of air.
• Roasting refers to the process of converting ore into its oxide form by heating the ore below its melting point.
• It takes place in the excess of air.
• Calcination is commonly used for converting Carbonates and Hydroxides into oxides whereas roasting is commonly used for converting Sulphides into oxides.

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