In a flowchart describing the steps to refine lead obtained after the reduction step, my book1 gives a short note on the process 'Softening of lead' where the crude lead is melted in the presence of air. It's stated that this removes scum of oxides of copper, tin, arsenic, antimony and bismuth which constitute the hard lead. After this soft lead with silver and gold is obtained.
Earlier, under the section which explained about refining of gold, the book described about the process cupellation where crude gold is melted in the presence of air. This method was meant to remove lead and the success behind this method was attributed to the high oxygen affinity of lead. The lead oxide being volatile gets easily separated from the system.
I understood the above two scenarios independently. However, when I thought about the lead refining process with the volatile nature of lead oxide in mind, it seems contradictory on why in the first case lead remains in the molten state whereas in the second it gets oxidised and escapes into the atmosphere.
My question is, why does 'softening' of lead oxidise only elements other than lead? Why doesn't the lead get oxidised and escape into atmosphere as volatile lead oxide? Compared to the impurities, as per the Pauling scale of electronegativity, lead is more electropositive and this seems to suggest it must get oxidised before the impurities which is opposite of what must be inferred in order for the lead softening process to be successful.
- Guha, Sudarsan. Concise Inorganic Chemistry for JEE (Main and Advanced). 4th ed., Wiley India Pvt. Ltd., 2017.