I do not have a ton of experience at it - lead melts at 327.5 C / 621.5 F. Zinc melts at 419.5 C / 787 F - so idea to separate pure Zinc would work - but I do not think it works if the two metals are already alloyed - often creates a "eutectic" alloy that has lower melting point than either parent metal. But, if more or less "pure" lead weights, and more or less "pure" zinc weights - a melt temp circa 650 F to 700 F will melt the lead ones, but not the zinc ones. But, is NOT the same if the lead/zinc is already alloyed - I think.
There was a write-up one time as one of the ways to check an unknown batch of alloy - melt it - insert thermometer and shut off the heat - at what temp does it turn solid? Then turn heat back on - at what temp does it go liquid - is also a process to go through to record the temp every 10 seconds or so - at the "phase change" - is going to be giving off or taking up heat energy, but the temp is not changing as it goes through the phase change from liquid to solid or vice versa.
I believe from my mining days that lead and zinc often found in the same ore - so is likely that most "lead" contains some zinc, and most "zinc" contains some lead. Probably depends how pure the refinery process had to make it. I do not know if wheelweights marked with "ZN" are pure zinc, mostly zinc or have a certain minimum amount of zinc. I have seen lab test reports for "lead" wire - their testing could find all sorts of stuff in trace amounts - zinc, copper and so on - I have no clue if .001% makes a difference when casting bullets or not.
