Question

The products of roasting a sulphide ore is (are)

• A. oxide only
• B. sulphate only
• C. oxide and sulphate
• D. dependent on temperature and partial pressures of oxygen and sulphur dioxide

Hint:

The stability of compounds in roasting can be visualized on an Ellingham-Richardson Diagram for M-S-O systems. These diagrams plot the stability fields of the metal, sulfide, oxide, and sulfate as a function of temperature, p\(_\text{O2}\), and p\(_\text{SO2}\). This confirms that the product is condition-dependent.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Roasting is a process in extractive metallurgy where a sulfide ore is heated in the presence of an oxidizing agent, typically air. The goal is to convert the metal sulfide into a form that is easier to reduce, usually an oxide or sometimes a sulfate.

Step 2: Detailed Explanation:
The chemical reactions that occur during roasting are governed by thermodynamics. For a generic metal sulfide (MS), the possible reactions are:

• Oxidizing Roast (to oxide): \( 2\text{MS} + 3\text{O}_2 \rightarrow 2\text{MO} + 2\text{SO}_2 \)

• Sulfating Roast (to sulfate): \( \text{MS} + 2\text{O}_2 \rightarrow \text{MSO}_4 \)
Which product is stable and forms depends on the conditions:

• Temperature: At lower temperatures, sulfates are often more stable. At higher temperatures, sulfates decompose to form oxides.

• Partial pressure of Oxygen (p\(_\text{O2}\)): A high oxygen potential favors oxidation.

• Partial pressure of Sulphur Dioxide (p\(_\text{SO2}\)): A high SO\(_2\) pressure can favor the formation of sulfates.

Therefore, one cannot say that the product is always an oxide or always a sulfate. The outcome is a direct function of the thermodynamic conditions. For example, in a fluid bed roaster, the temperature and gas composition can be controlled to produce either an oxide (dead roast) or a sulfate (sulfating roast) as desired. The statement in option (D) is the most scientifically accurate and comprehensive description.

Step 3: Final Answer:
The products of roasting are not fixed but depend entirely on the thermodynamic conditions of the process, specifically the temperature and the partial pressures of the reacting gases.