Roasting: Identifying the Metallurgical Process

Roasting is a crucial chemical process used in metallurgy, particularly for extracting metals from their ores. It involves heating solid materials, typically sulfide ores, strongly in the presence of excess air or oxygen. This heating is done below the melting point of the ore. The primary purpose of roasting is to convert the ore into a form that is more easily reduced to the metal. Often, it involves the oxidation of the metal sulfide to its corresponding metal oxide, releasing gases like sulfur dioxide.

Analyzing Options to Find Roasting

Let's examine each chemical reaction provided to determine which one represents roasting:

Option 1 Reaction Analysis

The reaction is:

Fe2O3 + 3C \(\xrightarrow{\Delta }\) 2Fe + 3CO

In this process, iron(III) oxide (Fe2O3) is heated with carbon (C). Carbon acts as a reducing agent, removing oxygen from the iron ore to produce elemental iron (Fe) and carbon monoxide (CO). This method, involving reduction with carbon at high temperatures, is known as smelting, not roasting.

Option 2 Reaction Analysis

The reaction is:

2PbS + 3O2 \(\xrightarrow{\Delta }\) 2PbO + 2SO2

This reaction shows lead(II) sulfide (PbS), a common sulfide ore, being heated in the presence of oxygen (O2). The lead(II) sulfide is converted into lead(II) oxide (PbO) and sulfur dioxide (SO2). This process aligns perfectly with the definition of roasting, as it involves heating a sulfide ore in air to form its oxide.

Option 3 Reaction Analysis

The reaction is:

ZnCO3 \(\xrightarrow{\Delta }\) ZnO + CO2

Here, zinc carbonate (ZnCO3) is heated, causing it to decompose into zinc oxide (ZnO) and carbon dioxide (CO2). This process, where ores are heated in the absence or limited supply of air to remove volatile impurities or decompose carbonates/hydrates, is called calcination. It differs from roasting, which requires excess air.

Option 4 Reaction Analysis

The reaction is:

Al2O3.2H2O \(\xrightarrow{\Delta }\) Al2O3 + 2H2O

This reaction represents the heating of hydrated alumina (Al2O3.2H2O) to remove the water molecules. This is a dehydration process, which is a specific type of calcination. It does not involve reaction with air or oxygen and is used to remove water or carbon dioxide from ores.

Roasting Compared to Other Ore Treatments

It's helpful to distinguish roasting from similar processes:

Conclusion: Identifying the Roasting Process

Based on the analysis, the reaction that correctly represents roasting is the oxidation of a sulfide ore in the presence of air. Option 2, the conversion of PbS to PbO using oxygen, is the process of roasting.