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.

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.