I had an interesting chat with a retired employee of our local smelter this last week. He worked in the melting room for a while where they do the final casting into pigs before shipping out the metals.
An interesting point he made, was that when they were going for five 9's pure lead, they would dump a 45 gallon drum of caustic soda into the melt pot as the final purification. Actually, they would keep doing it until they got the analysed purity they were after.
This got me wondering as to the caustic soda being a simple fluxing process as we are used to, or if this is more of a chemical reaction to remove the trace metals left. In other words, more like the copper sulfate / zinc displacement reaction.
Searching for an answer, I found that caustic soda (sodium hydroxide) should bind Tin, zinc and cadmium but no mention of antimony, copper, silver, calcium or the many other trace metals that are common in the lead refining process.
Any one with more metalurgical or chemistry knowledge want to chime in on this?