Question

The Parkes Process is a refining technique used to remove the following impurities from crude lead

• A. Nickel and Zinc
• B. Copper and Iron
• C. Silver and Gold
• D. Bismuth and Antimony

Hint:

The Parkes process is an excellent industrial application of partition law, where precious metals act as solutes distributing themselves preferentially into the solvent zinc.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to identify the specific impurity elements removed from crude lead using the Parkes process during pyrometallurgical refining.

Step 2: Key Formula or Approach:
The process relies on the principle of liquid-liquid extraction.
It utilizes the selective solubility and partition coefficient of precious metals (silver and gold) between liquid zinc and liquid lead, along with the differences in their melting temperatures.

Step 3: Detailed Explanation:

• Operational Principle: Zinc is added to molten lead at temperatures around \( 450^\circ\text{C} \text{ to } 480^\circ\text{C} \).
Silver and gold are much more soluble in liquid zinc than in liquid lead.
Furthermore, zinc and lead are nearly immiscible in this temperature range, forming two distinct liquid layers with the zinc-rich layer floating on top.

• Precipitation and Skimming: On cooling the mixture slightly, a solid crust containing intermetallic compounds of zinc and precious metals (\( \text{Ag}_2\text{Zn}_3 \) and \( \text{Au}_2\text{Zn}_3 \)) forms. This crust has a lower density and higher melting point than the lead bath, causing it to float to the surface where it is skimmed off.

• Precious Metal Recovery: The skimmed zinc crust is then processed via retorting (distillation) to vaporize and recover the zinc, leaving behind a concentrated alloy of silver and gold (dore metal), which is further refined by cupellation.


Step 4: Final Answer:
Thus, the Parkes Process is used to remove silver and gold from crude lead, matching Option (C).