Animated Solution for Chemistry - Metallurgy: The correct statement(s) related to the metal extraction processes is(are)

Metallurgy is not just about digging rocks out of the ground; it is a highly sophisticated chemical dance. To extract a pure metal from its ore, we must carefully manipulate oxidation states, exploit differences in reactivity, and use clever physical processes to separate the valuable metal from the worthless rock (gangue). In this classic JEE Advanced problem, we are tasked with evaluating four distinct metallurgical statements covering Lead, Copper, and Gold. Let's break down the chemistry behind each one.

The Magic of Self-Reduction in Lead

When we extract lead from its primary sulphide ore, Galena ($PbS$), we don't always need to add an external reducing agent like carbon. Nature provides a more elegant solution known as self-reduction (or auto-reduction).

First, the ore is partially roasted in a limited supply of air. This converts a portion of the lead sulphide into lead oxide:

Once enough lead oxide is formed, the air supply is cut off. The temperature is maintained, and a fascinating reaction occurs. The newly formed lead oxide reacts directly with the remaining unoxidized lead sulphide:

Here, the sulphide ion acts as the reducing agent, reducing the $P{b}^{2+}$ ions to pure metallic lead ($P{b}^0$) while being oxidized to sulphur dioxide gas. Because this process perfectly describes the statement, Option A is correct.

The Role of Silica in Copper Smelting

Copper pyrites ($CuFe{S}_2$) is the most common ore of copper. As the formula suggests, it contains a significant amount of iron, which is a major impurity that must be removed. During the smelting process in a reverberatory furnace, the ore is heated, and the iron is oxidized to iron(II) oxide ($FeO$).

However, $FeO$ is infusible and difficult to remove on its own. To solve this, we add an acidic flux, specifically silica ($Si{O}_2$). The flux reacts with the basic iron oxide impurity to form a fusible liquid called slag:

This iron silicate slag is lighter than the molten copper matte and floats on top, allowing it to be easily skimmed off. The statement claims that silica is added to produce copper silicate. This is a trap! If copper formed a slag, we would lose our precious metal. We want to remove iron, not copper. Therefore, Option B is incorrect.

The Birth of Blister Copper

After smelting, the resulting copper matte (a mixture of $C{u}_{2}S$ and $FeS$) is transferred to a Bessemer converter. Here, a blast of air is blown through the molten mass. Any remaining iron sulphide is oxidized and removed as slag.

Once the iron is gone, the copper sulphide begins to oxidize partially:

Just like with lead, a self-reduction reaction takes over. The copper oxide reacts with the remaining copper sulphide:

As the molten copper cools and solidifies, the dissolved sulphur dioxide gas violently escapes, leaving behind a rough, blistered surface on the metal. This is why the product is famously called blister copper (about 98% pure). Since the statement accurately describes this sequence of partial oxidation followed by self-reduction, Option C is correct.

The Elegance of the Cyanide Process

Gold is a noble metal, meaning it is highly unreactive. To extract it from its native ore, we use a hydrometallurgical technique called the Macarthur-Forrest cyanide process.

First, the crushed ore is leached with a dilute solution of sodium cyanide ($NaCN$) in the presence of atmospheric oxygen. The oxygen oxidizes the gold, allowing it to dissolve and form a stable, soluble complex:

Now that the gold is trapped in an aqueous solution, how do we get it back as a solid metal? We introduce a more reactive metal to take its place. Zinc powder is added to the solution. Because zinc is much higher in the electrochemical series (more electropositive), it acts as a powerful reducing agent. It displaces the gold from the complex:

The gold precipitates out as a fine dark powder, which is then collected and melted down. The statement correctly identifies zinc as the reducing agent used to precipitate gold. Thus, Option D is correct.

By carefully analyzing the chemical principles behind each metallurgical process, we have determined that statements A, C, and D are scientifically accurate, while statement B presents a fundamental misunderstanding of the role of a flux. The correct answer is (A, C, D).