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

Question

The cyanide process of gold extraction involves leaching out gold from its ore with \(\mathrm{CN}^{-}\)in the presence of \(Q\) in water to form \(R\). Subsequently, \(R\) is treated with \(T\) to obtain \(\mathrm{Au}\) and \(Z\). Choose the correct option(s) (a) \(Q\) is \(\mathrm{O}_{2}\) (b) \(T\) is \(\mathrm{Zn}\) (c) \(Z\) is \(\left[\mathrm{Zn}(\mathrm{CN})_{4}\right]^{2-}\) (d) \(R\) is \(\left[\mathrm{Au}(\mathrm{CN})_{4}\right]^{-}\)

Step-by-Step Solution

Answer

Correct options are (a), (b), and (c).

Step 1: Identify Q in the Reaction

In the cyanide process, also known as the MacArthur-Forrest process, gold is extracted from its ores using cyanide ions (\(\mathrm{CN}^{-}\)). Oxygen \(\mathrm{O}_{2}\) is used in the presence of \(\mathrm{CN}^{-}\) ions for this purpose. Therefore, \(Q\) is \(\mathrm{O}_{2}\).

Step 2: Understand the Formation of R

When gold is extracted, it forms a complex in solution. The typical complex formed by the reaction of gold with cyanide ions is \(\left[\mathrm{Au}(\mathrm{CN})_{2}\right]^{-}\). Therefore, \(R\) should be the gold cyanide complex. Options point to \(\left[\mathrm{Au}(\mathrm{CN})_{4}\right]^{-}\), but the standard formula is \(\left[\mathrm{Au}(\mathrm{CN})_{2}\right]^{-}\), indicating a potential typo or confusion in the problem. Proceed with the intended answer based on typical oxidation state and coordination number.

Step 3: Identify T in the Reduction Process

To recover gold from \(\left[\mathrm{Au}(\mathrm{CN})_{2}\right]^{-}\), zinc \(\mathrm{Zn}\) is commonly used. Zinc displaces gold from the complex due to its position in the electrochemical series, where it is more reactive than gold. Thus, \(T\) is \(\mathrm{Zn}\).

Step 4: Determine the Formation of Z

When zinc displaces gold from the cyanide complex, it forms a new complex. Zinc reacts with cyanide to form \(\left[\mathrm{Zn}(\mathrm{CN})_{4}\right]^{2-}\). Therefore, \(Z\) is \(\left[\mathrm{Zn}(\mathrm{CN})_{4}\right]^{2-}\).

Key Concepts

Gold Extraction

Gold extraction using the cyanide process is a fascinating chemical method. It's primarily used due to its efficiency in extracting gold from low-grade ores. The process begins with the treatment of crushed ore with a cyanide solution. Here, the cyanide ions (\(\text{CN}^-\)) play an essential role in the reaction. They bind with gold present in the ore to form a soluble complex.
This method revolutionized gold mining because it enabled miners to retrieve gold even from poor-quality ore. The use of cyanide ions greatly increases the yield, making it economically viable to extract smaller particles of gold.
Gold extracted through the cyanide process accounts for a significant portion of gold production worldwide, showcasing the importance of chemical advancements in mining.

Complex Formation

In the cyanide process, complex formation is a crucial step. When gold reacts with cyanide in the presence of oxygen, it forms a complex ion. The typical complex ion formed is the \([\text{Au}(\text{CN})_2]^-\).
The role of oxygen (\(\text{O}_2\)) is important; it acts as an oxidizing agent that facilitates the dissolution. The reaction can be summarized as:
\[4\text{Au} + 8\text{CN}^- + \text{O}_2 + 2\text{H}_2\text{O} \rightarrow 4[\text{Au}(\text{CN})_2]^- + 4\text{OH}^-\]
This complex formation is crucial because it converts gold into a water-soluble complex, allowing further processing to separate gold from the rest of the material.

Zinc Reduction

After forming the gold-cyanide complex, the next step is to recover gold. This is done through a process known as zinc reduction or displacement. Zinc (\(\text{Zn}\)) is added to the solution containing the gold-cyanide complex.
The principle here is based on the reactivity series. Zinc is more reactive than gold, allowing it to displace gold from the complex. The relevant reaction is:
\[2[\text{Au}(\text{CN})_2]^- + \text{Zn} \rightarrow 2\text{Au} + [\text{Zn}(\text{CN})_4]^{2-}\]
This step not only recuperates gold but also forms a new zinc-cyanide complex, \([\text{Zn}(\text{CN})_4]^{2-}\), from which zinc can sometimes be recovered in subsequent steps.

Cyanide Leaching

Cyanide leaching is the process where cyanide solution percolates through ore, dissolving gold in the process. It is a form of hydrometallurgy and is widely adopted due to its efficiency in separating gold from dirt and rock.
This method's success lies in its ability to handle large amounts of ore. It is applied in both heap leaching and vat leaching, depending on the scale and nature of the ore being treated. Cyanide leaching leverages cyanide's ability to selectively dissolve gold, minimizing waste.
This approach is highly effective but requires careful environmental and safety management due to the toxicity of cyanide. Regulations ensure the safe containment and detoxification of cyanide solutions after use to prevent environmental contamination.