Calcination and Roasting - Definition, Example, Principle, FAQs

Roasting

Roasting is the heat treatment of ores in excess air to convert metals into their respective oxides and water-insoluble sulphides into water-soluble sulphates. It may, otherwise, be defined as a process for the deelectronation of ores, in which the process is centered on the removal of volatile impurities and the conversion of sulphide ores into the corresponding oxides. It essentially takes place in reverberatory furnaces.

Examples
Iron Sulfide In excess air, pyrite, $\mathrm{FeS}_2$, oxidizes to give iron oxide, $\mathrm{Fe}_2 \mathrm{O}_3$, and sulfur dioxide,

$\mathrm{SO}_2: \mathrm{FeS}_2 \rightarrow \mathrm{Fe}_2 \mathrm{O}_3+\mathrm{SO}_2$

The above process chain may merely be used as a way of changing the sulfide ore to a more reactive oxide form—the method also removes sulfur impurities.
Cinnabar Mercury ore- Roasting of cinnabar HgS, produces mercury oxide {HgO} and sulfur dioxide, $\mathrm{SO}_2$. The ore becomes an oxide; this makes the extraction of mercury more comfortable.
Zinc Sulfide Zinc sulfide ZnS is roasted to get zinc oxide {ZnO} and sulfur dioxide {$\mathrm{SO}_2$}. This method turns the sulfide ore into an oxide and makes it fit for further reduction.
Lead Sulphide {PbS} is roasted to yield lead oxide {PbO} and sulphur dioxide {$\mathrm{SO}_2$}. The processing of the ore into the oxide form makes it more reactive and thus easier for reduction to metallic lead.

Related Topics

• Distillation
• Filtration
• Difference between Metallic and Non Metallic Minerals
• Processes of Metallurgy

Significance and Uses of Calcination and Roasting

Why are calcination and roasting considered essential steps in metallurgy? Their importance lies in the chemical and physical transformations they bring about in ores before metal extraction.

Significance:

• Both processes help in converting ores into more reactive forms suitable for reduction.

• They remove volatile impurities such as moisture, carbon dioxide, sulphur, and organic matter.

• Calcination and roasting increase the porosity of ores, making subsequent extraction easier.

• Roasting helps in oxidizing sulphide ores into oxides, which are easier to reduce.

Uses:

• Calcination is used mainly for carbonate and hydrated ores like calamine (ZnCO₃) and bauxite.

• Roasting is used for sulphide ores such as zinc blende (ZnS) and galena (PbS).

• These processes are widely used in the extraction of metals like zinc, lead, iron, and copper.

• They are also employed in industries such as cement manufacturing, lime production, and metallurgy.

Significance in Learning

Processes that are hugely fundamental to the courses for learners in Chemistry and Metallurgy, as in Material Science, Environmental Engineering, or Industrial Chemistry, since they assist in having an idea of the applications and the environmental impact caused by the extraction of the metals from their respective ores.

Also read :

Some Solved Examples

Example 1:Which of the following factors is of no significance for roasting sulphide ores to the oxides and not subjecting the sulphide ores to carbon reduction directly?

1)CO₂ is more volatile than CS₂

2)Metal sulphides are thermodynamically more stable than CS₂

3)CO₂ is thermodynamically more stable than CS₂

4) Metal sulphides are less stable than the corresponding oxides

Solution
The reduction of metal sulphides by carbon reduction is positive whereas, ΔG is negative for the same process with metal oxides. So, metal sulphides are more stable than oxides.

Hence, the statement given in Option 4 is incorrect:

"Metal sulphides are less stable than the corresponding oxides"

Hence, the answer is the option (4).

Example 2: The purest form of commercial iron is:

1) wrought iron

2)pig iron

3)scrap iron and pig iron

4)cast iron

Solution
The purest form of iron is wrought iron.
Hence, the answer is the option (1).

Example 3: The process that involves the removal of sulphur from the ores is :

1) Roasting

2)Leaching

3)Smelting

4)Refining

Solution
In the roasting process, metal sulphide (MS) ore is converted into metal oxide, and sulphur is removed in the form of SO₂ gas

$2 \mathrm{MS}+3 \mathrm{O}_2\xrightarrow{\Delta}2 \mathrm{MO}+2 \mathrm{SO}_2 \uparrow$

Hence, the answer is the option (1).

Example 4: The carbon content in wrought iron is usually:
1) High, around 3-4%
2) Medium, around 2-3%
3) (correct) Low, less than 0.1%
4) High, around 8-10%

Solution:

Wrought iron has a low carbon content, typically less than $0.1 \%$, contributing to its malleability and ductility.
Hence, the answer is the option (3).

Practice more questions with the link given