How does Pattinson's process use the Pb–Ag phase diagram to desilverise lead?

In one line

Pattinson's process enriches silver from argentiferous lead (lead with a very small silver content) by cooling the melt so that pure lead crystallises out, leaving a liquid richer in silver up to the eutectic composition.

Explained simply

Imagine juice with very little sugar. If you freeze it slowly, pure ice crystals form first and you scoop them out; the remaining liquid gets sweeter. In the same way, molten lead with a little silver is cooled, pure lead solidifies first and is removed, and the leftover melt becomes richer in silver. It works because the alloy composition lies on the lead side of the eutectic.

How it works

1. Argentiferous lead contains only about 0.1% Ag, so its composition lies on the Pb-rich side of the eutectic (to the right of O, below 2.6% Ag).
2. Heat it above the melting point to get a homogeneous melt (point in the liquid area).
3. Cool slowly. On reaching the freezing curve BO, pure solid lead starts to separate.
4. Remove the lead crystals with perforated ladles. The melt left behind becomes richer in silver.
5. Continue cooling and removing lead. The melt composition moves along BO towards O.
6. At the eutectic point O (303 °C), the melt contains 2.6% Ag; no further enrichment by this method is possible because the whole liquid now solidifies together.
7. The silver-enriched eutectic alloy is then processed (by cupellation, i.e., oxidising Pb to PbO in a current of air) to recover silver.

Key formula

On curve BO: P = 2 (solid Pb + melt), \( F' = 2 - 2 + 1 = 1 \). Choosing a temperature fixes the melt composition, which is why controlled cooling gives a predictable enrichment.

Quick example

Starting with 0.1% Ag, the process raises the silver content of the remaining melt step by step towards 2.6%, about 26 times enrichment, before cupellation. This reduces the amount of lead that has to be oxidised in cupellation.

Common mistakes

• Saying silver crystallises first. For compositions below 2.6% Ag, lead separates first.
• Claiming pure silver is obtained directly by Pattinson's process. It only enriches up to the eutectic composition.
• Not linking the process to curve BO and point O on the diagram.

How MU asks it

"Explain Pattinson's process of desilverisation of lead with reference to the Pb–Ag phase diagram" (3–5 marks) or as the application part of the Pb–Ag question. For full marks: initial composition and its position on the diagram, stepwise cooling and removal of Pb, reason enrichment stops at 2.6% Ag, and the cupellation step.