Problem 79 - A Complete Resource Book in Chemistry for JEE Main

Question

During the extraction of zinc, \(\mathrm{ZnO}\) is obtained from zinc blende by (a) poling (b) roasting (c) cupellation (d) electrolytic reduction

Step-by-Step Solution

Answer

The correct process is (b) roasting.

Step 1: Understanding the Question

We need to identify the correct process used to convert zinc blende into zinc oxide (
4ZnO) during the extraction of zinc. The given options are poling, roasting, cupellation, and electrolytic reduction.

Step 2: Identifying Zinc Blende Composition

Zinc blende is the mineral form of zinc sulfide (
4ZnS). To convert it into zinc oxide, the sulfur needs to be removed.

Step 3: Evaluating Option (a): Poling

Poling is a process used mainly in the purification of metals like copper. It does not apply to the conversion of ZnS to
4ZnO.

Step 4: Evaluating Option (b): Roasting

Roasting is a process where sulfide ores are heated in the presence of air, which converts the sulfide into oxide. Thus,
4ZnS is roasted in air to produce
4ZnO and
4SO_2 gas: \[2\mathrm{ZnS} + 3\mathrm{O}_2 \rightarrow 2\mathrm{ZnO} + 2\mathrm{SO_2}\]

Step 5: Evaluating Option (c): Cupellation

Cupellation is a refining process for separating precious metals from base metals, not typically used in zinc extraction from zinc blende.

Step 6: Evaluating Option (d): Electrolytic Reduction

Electrolytic reduction is used for reducing metal oxides to metals, not for converting zinc sulfide to zinc oxide.

Step 7: Conclusion

The appropriate process for converting zinc blende (
4ZnS) to zinc oxide (
4ZnO) is roasting, which involves heating the sulfide in the presence of oxygen to form the oxide and sulfur dioxide.

Key Concepts

Zinc Blende

Zinc blende is the common mineral form of zinc sulfide and is scientifically denoted as \( ext{ZnS} \). It is an important ore in the production of zinc. This mineral appears shiny and metallic, and it is often found in conjunction with other minerals such as galena. To obtain zinc from zinc blende, the extraction process involves transforming zinc sulfide into zinc oxide. This transformation is a vital step as it sets the stage for zinc metal to be derived in its metallic form.

• Zinc blende is also known as sphalerite.
• It often contains impurities like iron and is found in association with lead, cadmium, and silver ores.

Thus, understanding its composition helps in identifying the necessary steps to extract zinc effectively.

Roasting Process

The roasting process is crucial in the conversion of zinc sulfide into zinc oxide. During roasting, zinc sulfide (ZnS) is heated in the presence of oxygen, leading to a chemical reaction that produces zinc oxide (ZnO) and sulfur dioxide (SO₂) gas. This step is significant because it prepares zinc for the next stage in its extraction.
Roasting involves the following reaction:\[2 ext{ZnS} + 3 ext{O}_2 ightarrow 2 ext{ZnO} + 2 ext{SO}_2\]

• The process requires high temperatures and the presence of air or oxygen.
• Producing sulfur dioxide as a byproduct has environmental considerations and is often captured to prevent air pollution.

Roasting serves as an efficient way to oxidize zinc sulfide, making it easier to extract zinc in subsequent processes.

Sulfur Removal

In the context of zinc extraction, sulfur removal is a pivotal step. It is achieved during the roasting process. When zinc sulfide is heated, sulfur is released in the form of sulfur dioxide gas. This removal of sulfur effectively changes the chemical form of the zinc compound from sulfide to oxide, which is a crucial transformation allowing for further refinement.

• Removing sulfur is essential for avoiding impurities in the final metal product.
• The reaction with oxygen not only changes the chemical composition but also contributes to cleaning the ore.

Efficient sulfur removal facilitates a purer form of zinc metal later in the extraction process.

Metal Oxides

Metal oxides, like zinc oxide formed during roasting, are essential intermediates in metal extraction. When zinc sulfide converts to zinc oxide, it becomes more amenable to reduction processes that extract pure metal. This intermediary oxide form is easier to handle and reduce, compared to treating the original sulfide directly.

• Metal oxides have high melting points, making them stable for various refining steps.
• They act as a bridge between mineral ores and pure metal forms.

In zinc extraction, converting sulfides to oxides is an essential step that precedes further refining procedures.

Refining Metals

The process of refining metals follows the conversion of metal ores into their oxides. Once zinc oxide has been obtained, the next step is to refine it into metallic zinc. This involves reduction techniques, such as heating with carbon or electrolysis, which removes oxygen and leaves zinc metal behind. Refining ensures the purity and usability of zinc for various applications.

• Electrolytic reduction is a common method used in refining high-purity metals.
• The choice of refining method depends on the desired zinc purity and industrial application.

The ultimate aim of refining is to produce zinc that has a high level of purity, suitable for uses in corrosion-resistant coatings or as alloying material.