Problem 65 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Give reasons for the following : (i) Why is chalcocite roasted and not calcinated during recovery of copper?

Step-by-Step Solution

Answer

Chalcocite is roasted to convert sulfides to oxides, which can then be reduced to copper; calcination doesn't provide the necessary oxygen for this transformation.

Step 1: Understanding Calcinating and Roasting

Calcinating is a thermal process that removes volatile substances from ores by heating them in the absence of air or in a restricted supply of air. On the other hand, roasting involves heating in the presence of excess air, converting sulphides to oxides. Both methods are used to prepare an ore for metal extraction.

Step 2: Composition of Chalcocite

Chalcocite (Cu_2S) is a copper(I) sulfide mineral and is an important source of copper. It predominantly consists of copper and sulfur, lacking any significant volatile impurities such as carbon dioxide or water that are typically removed through calcination.

Step 3: Purpose of Roasting in Copper Extraction

The main goal of roasting chalcocite is to convert the copper sulfide (Cu_2S) into copper oxide (CuO), releasing sulfur dioxide gas in the process. This transformation is crucial because copper oxide can be easily reduced to copper metal, whereas copper sulfide cannot be directly reduced to copper using traditional reduction processes.

Step 4: Chemical Reaction during Roasting

During roasting, chalcocite is oxidized according to the following reaction: 2Cu_2S + 3O_2 → 2Cu_2O + 2SO_2. This reaction highlights the conversion of copper sulfide to copper oxide and the release of sulfur dioxide gas, preparing the ore for subsequent reduction steps.

Step 5: Conclusion: Why Roasting is Preferred

Roasting is favored over calcination for chalcocite because it effectively oxidizes sulfide ions, converting them into oxides, which are far easier to reduce to metal. Calcination does not supply the oxygen needed to transform metal sulfides like Cu_2S into oxides, making roasting essential.

Key Concepts

Copper Extraction

Copper extraction is a crucial process in metallurgy, aimed at retrieving copper from its ores. One of the primary steps involved in copper extraction is separating copper from the mineral's impurities. This process is typically carried out using techniques like roasting, smelting, and eventually, refining. The choice of method highly depends on the type of ore and its composition.

• Roasting involves heating the ore in the presence of oxygen.
• This step is essential to convert sulfide ores into their corresponding oxides.

Once the copper ore is effectively converted into copper oxide through roasting, it is ready for reduction. This process involves further heating the oxide with a reducing agent. The outcome is pure copper metal. Therefore, understanding the steps and why certain methods are chosen is vital for efficient copper extraction.

Chalcocite

Chalcocite, or copper(I) sulfide, is one of the most significant copper ores exploited in mining. Represented by the chemical formula Cu\(_2\)S, it holds a substantial amount of copper with minimal impurities.

• Chalcocite consists primarily of copper and sulfur, aligning it as an ideal candidate for roasting.
• This mineral doesn't contain volatile components, thereby diminishing the effectiveness of calcination.

Due to its unique composition, chalcocite undergoes roasting to transform into a more manageable form of copper oxide. This transformation is a crucial step in the extraction process as it directly influences the ease with which pure copper can be obtained. The absence of volatile substances in chalcocite necessitates the use of roasting for copper recovery.

Oxidation of Sulfides

The oxidation of sulfides is a pivotal chemical transformation in the metallurgical processing of ores like chalcocite. This process involves a reaction where sulfides are heated with oxygen, resulting in the formation of oxides.

• For chalcocite, the roasting process converts copper sulfide (Cu\(_2\)S) into copper oxide (Cu\(_2\)O).
• The chemical reaction liberates sulfur dioxide (SO\(_2\)) as a by-product.

During this oxidation process, substances are chemically altered to prepare them for metal extraction. The equation depicting the oxidation of chalcocite is expressed as \[ 2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2 \]. This conversion is fundamental because oxides are easier to reduce into pure metal than sulfides. Hence, the oxidation of sulfides during the roasting phase is indispensable for efficient copper extraction by providing a more amendable form for subsequent reduction processes.