Calcination and Roasting

Last Updated : 16 Jan, 2024

Calcination and Roasting are processes used to convert ore into oxide. The difference between the two is the presence or absence of air. Roasting involves heating ores with oxygen below their melting point in the presence of air. Calcination involves heating ore to a high temperature in the absence of air.

In this article, we will understand what is Calcination and Roasting, their examples, and processes, along with a comparison between calcination and roasting.

Table of Content

What is Roasting in Chemistry?
What is Calcination?
Difference between Calcination and roasting
Applications of Roasting and Calcination

What is Roasting in Chemistry?

Roasting in metallurgy constitutes a pivotal stage in metal extraction. This process brings about crucial chemical changes in ores by subjecting them to controlled heating in the presence of oxygen. This facilitates the transformation of complex compounds into simpler and more extractable forms, playing a fundamental role in numerous metallurgical operations.

Roasting is very important in the extraction of metals. It serves as a critical precursor to several extraction methods by preparing ores for subsequent processes. Roasting significantly influences the efficiency and feasibility of metal extraction, directly impacting the overall economics of the process.

Examples of Roasting

The detailed explanation of a few examples of roasting is given below:

Copper Extraction

Copper ores, notably sulfide ores like chalcopyrite (CuFeS₂), undergo roasting to eliminate sulfur and convert the ores into oxides or other easily reducible forms, preparing them for subsequent processes like smelting. The chemical equation for roasting chalcopyrite is:

2CuFeS₂(s) + 4O₂(g) →2Cu₂S(s) + 2FeO(s) + 3SO₂(g)

Iron Ore Preparation

In the extraction of iron from its ores, roasting is applied to prepare the iron ore by removing volatile impurities like sulfur, arsenic, and other elements, enhancing the ore’s purity before subsequent refining processes such as smelting or direct reduction.

Mechanism and Process of Roasting

Roasting is a fundamental process in metallurgy where ores undergo transformation through carefully controlled heating in the presence of oxygen. This controlled heating triggers a series of chemical reactions, essential for converting complex compounds within the ores into more desirable forms. The most common process of roasting an ore is oxidization. Let’s take an example of a sulfide ore, for instance, iron pyrite (FeS₂), when subjected to roasting, here’s a simplified explanation of the chemical reactions that might occur.

Oxidation of Sulfide Ore

The iron pyrite ore reacts with oxygen (from the air or supplied externally) during roasting:

4FeS₂ (Iron pyrite) + 11O₂ → 2Fe₂O₃ (Ferric oxide) + 8SO₂ (Sulfur dioxide)

When we heat iron pyrite (FeS₂) in the presence of oxygen during roasting, the heat provides the energy needed to break the bonds between iron and sulfur in the pyrite.

As a result of this reaction with oxygen:

Sulfur atoms join with oxygen atoms and become Sulfur Dioxide Gas (SO₂).

At the same time, the iron atoms stick to oxygen atoms and make Ferric Oxide (Fe₂O₃), which is like rust.

This whole process is really important in making ores easier to work with in metallurgy. It changes the sulfur in the ore into a gas (sulfur dioxide), and the iron turns into a solid (ferric oxide). These changed forms make it easier to do more processes to get metals like iron from the ore.

What is Calcination?

Calcination refers to a thermal treatment process applied to ores and other solid materials in the absence of oxygen or air. The process involves heating a substance to high temperatures, typically between 700 ℃ to 1200 ℃, although the specific temperature can vary based on the melting point of the material and the desired outcome.

During calcination, volatile compounds such as water and carbon dioxide are driven off, resulting in chemical and physical changes within the material. This process can lead to various transformations, such as decomposition, oxidation, and reduction of the original substance, ultimately altering its physical and chemical properties.

Examples of Calcination

Few of the examples of extraction of minerals from their ores through the process of calcination is given below:

Limestone (calcium carbonate – CaCO₃)

When heated, limestone undergoes calcination to produce quicklime (calcium oxide – CaO) and carbon dioxide (CO₂).

Gypsum (calcium sulfate dihydrate – CaSO₄·2H₂O)

Calcination of gypsum removes the water molecules, resulting in the formation of calcium sulfate hemihydrate, commonly known as plaster of Paris.

Bauxite (aluminum ore – Al₂O₃·nH₂O)

Calcination of bauxite removes water and impurities, converting it into alumina (aluminum oxide – Al₂O₃), a precursor for aluminum production.

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Mechanism and Process of Calcination

The detailed process of calcination involves several crucial steps:

Preparation of Material: The raw material is selected and prepared for calcination, ensuring it is free from impurities and suitable for the intended transformation.
Heating: The material is subjected to controlled high temperatures in a furnace. The temperature and duration of heating are critical and vary depending on the specific material and desired outcome.
Chemical Reactions: During heating, chemical reactions occur within the material. These reactions lead to the expulsion of volatile compounds like water, carbon dioxide, sulfur dioxide, etc., resulting in the alteration of the material’s composition.
Cooling and Handling: After the calcination process, the material is cooled down gradually to avoid sudden changes that could affect its properties. Proper handling post-calcination is essential to maintain the integrity of the transformed material.
Utilization: The calcined material, with its altered properties, is used in various industries, such as construction, metallurgy, ceramics, and pharmaceuticals, based on its specific characteristics post-calcination.

Difference Between Calcination and Roasting

The key difference between Calcination and Roasting is given below:

Calcination vs Roasting
Characteristics	Calcination	Roasting
Definition	In this process, an ore is heated in the absence of air.	In this process, an ore is heated in the presence of air
Temperature	Moderate	Relatively higher temperature
Air or inert gases	Absence of oxygen	Presence of oxygen
End Products	Oxides or carbonates	Oxides or sulfides
Examples	Conversion of Limestone (CaCO₃) to lime (CaO)	Conversion of Copper sulfide (CuFeS₂) to copper oxide (Cu₂O)

Applications of Roasting and Calcination

Roasting and calcination are processes that have a vast range of applications, few of them are mentioned below:

Metallurgy: Roasting and calcination plays a vital role in the extraction of metals from their respective ores.

Mineral Processing: In mineral processing, they are utilized to modify the chemical composition of minerals, making them more amenable to subsequent extraction methods.

Precious Metal Recovery: Calcination and Roasting are employed in the extraction of precious metals like gold and silver. Ores containing these metals undergo these processes to eliminate impurities and so enhancing the overall efficiency of precious metal recovery.

Environmental Remediation: They are also used in environmental cleanup efforts, especially for hazardous materials like contaminated soils or solid wastes.

Pharmaceuticals and Chemicals: Roasting is also used in the pharmaceutical and chemical industres for the synthesis of certain chemicals or to modify the properties of materials through controlled thermal treatment.

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