True, zinc melts at 787 degrees Fahrenheit, and lead melts at 671 degrees Fahrenheit. Wheel weight alloys (mostly lead) melts somewhat lower than pure lead. That is why the initial signs of zinc contamination in a lead melt is a mushy, oatmeal like layer floating on top of the lead. You can try skimming off and discarding most of it, but once the melt temperature is cranked up, some of it gets into the alloy.
According to the NRA advisor, once a bullet melt is contaminated with molten zinc, you can never get all of it out, and it would be best to discard the melt, clean the pot, and start over. That has been my experience too. Once a batch pours poorly in clean, hot molds, if it fails to fill out even after increasing tin content, I pour it out and start over.