Difference between Calcination and Roasting

Calcination and Roasting are two processes of refining carbonate and sulfide metal ores respectively into pure metal forms. Minerals are naturally occurring chemical substances in the Earth’s crust that are obtained by mining. Metals that are known to us are derived from some of these minerals and such minerals are known as ores.

The science that deals with extracting metals from ores is known as metallurgy. There are many ways by which metals are extracted from its ore and one such process to extract crude metal from its concentrated ore is conversion to oxide which is further divided into Calcination and Roasting. In this article, we briefly discuss calcination and roasting and the difference between calcination and roasting.

What is Calcination and Roasting?

Calcination and Roasting are the processes of refining pure metals from their ores. These two processes are used for different types of ores and in different conditions. Let’s learn them in detail below:

Calcination Definition

The process of converting ore into oxide by heating it strongly either in absence of air or in limited supply of air is known as Calcination. It is a thermal process that is used to convert ores and other solid materials by thermal decomposition. It is mainly used to convert carbonates and hydroxides to their respective oxides. During this process, the reaction mostly occurs at or above the thermal decomposition temperature. Moreover, moisture and volatile impurities are also removed in this process. Hence, it is sometimes considered as a process of purification. The products derived from calcination are known as calcining.

Roasting Definition

The process of converting ore into oxide by heating it at a temperature below its melting point in presence of air is known as Roasting. It is mainly carried out for sulphide minerals. It consists of solid-gas thermal reaction which includes oxidation, reduction, sulfation, chlorination and pyro hydrolysis. During this process, moisture and non-metallic impurities are released in the form of volatile gases. Roasting is not preferred as it results in the release of toxic gases and compounds that are acidic in nature. All this results in degradation of air quality and contributes to pollution.

Difference between Calcination and Roasting

The difference between calcination and Roasting is tabulated below:

Calcination	Roasting
This is the process of converting ore into oxide by heating it strongly in absence of air.	This is the process of converting ore into oxide by heating it at a temperature below its melting point in presence of air.
It is used to convert carbonates and hydroxides into their oxides.	It is mainly carried out for sulphide ores.
Calcination takes place either in absence or in limited presence of air.	Roasting takes place in presence of air.
The metal oxide assists the production of carbon dioxide.	The metal oxide assists the production of sulphur dioxide.
In Calcination, Moisture is removed.	In Roasting, there is no dehydration of ore.

Applications of Calcination and Roasting

Since, we know the basics of Calcination and Roasting. Let’s learn more about the applications of Calcination and Roasting with the chemical reactions involved in these processes.

Calcination Applications

Calcination is used to convert carbonate and hydroxide into their oxide. It is used to remove volatile impurities. The reactions involved in the process of Calcination is listed below:

• Calcination is widely used in the decomposition of Calcium carbonate(limestone) to Calcium oxide(lime) and Carbon dioxide.

CaCO₃ → CaO + CO₂

• Anhydrous Alumina is left behind when bauxite is calcined, and water is removed.

Al₂O₃.2H₂O → 2Fe₂O₃ + 3H₂O

• Anhydrous Iron is formed during iron calcination.

2Fe₂O₃.3H₂O → 2Fe₂O₃ + 3H₂O

Roasting Application

Roasting is used to convert sulphides into their oxides. Non-metallic impurities are removed basically in the process of Roasting. Let’s learn some of the chemical reactions involved in the process of Roasting.

• Zinc sulphide is converted to zinc oxide with the help of roasting.

2ZnS + 3O₂ → 2ZnO + 2SO₂

• Roasting of Chalcocite to obtain Copper Oxide.

2Cu₂S + 3O₂ → 2Cu₂O + 2SO₂

• Conversion of metal compounds to chlorides through Chloridising roasting.

2NaCl + MS + 2O₂ → Na₂SO₄ + MCl

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

Calcination and Roasting – FAQs

1. Define Calcination and Roasting.

Calcination is the process of converting ore into oxide by heating it at high temperature in the absence of air whereas Roasting is the process of converting ore into oxide by heating it at a temperature below its melting point in presence of air.

2. What is Metallurgy?

The entire scientific and technological process used for isolation of the metal from its ore is known as metallurgy.

3. What is the Purpose of Calcination?

Calcination is used for purification purpose of mainly carbonates ore. Moisture and volatile impurities are removed during calcination.

4. Why is Calcination done in the Absence of Air?

In the metal carbonates, the calcination process is carried out to turn metal carbonates into carbon dioxides and metal oxides. No reaction will occur in the presence of air hence calcination is carried out in the absence of air.

5. Which Process is Used to Convert Hydroxides into Oxides?

Calcination is used to convert hydroxides into oxides.

6. Give an Example of Calcination

Calcination is widely used in the decomposition of Calcium carbonate(limestone) to Calcium oxide(lime) and Carbon dioxide.

CaCO₃ → CaO + CO₂

7. What is Reduction Roasting?

Reduction roasting partially reduces an oxide ore before the actual smelting process.

8. Give an Example of Roasting.

Zinc sulphide is converted to zinc oxide with the help of roasting.

ZnS + 3O₂ → 2ZnO + 2SO₂

9. Why Roasting is Hazardous for Environment?

Roasting releases sulphides which are major contributor of air pollution as they have acidic and toxic metallic compounds.

10. What is Chloridising Roasting?

Chloridising roasting transforms certain metal compounds to chlorides through oxidation or reduction. Some metals such as uranium, titanium, beryllium are processed in their chloride form.

• 2NaCl + MS + 2O₂ → Na₂SO₄ + MCl
• 4NaCl + 2MO + S₂ + 3O₂ → 2Na₂SO₄ + 2MCl₂

11. Differentiate between Calcination and Roasting.

Calcination is the process of heating a substance without air to remove volatiles or change its structure. Roasting, on the other hand, involves heating in the presence of air to remove impurities or alter the material’s composition, typically used in metallurgy.