Zinc boils at a comparatively low temperature next to metals like lead and iron that usually contaminate it, and this large gap in boiling points is the whole trick behind purifying it. By heating crude zinc under reduced pressure, zinc vaporises readily while the heavier, less volatile impurities are simply left behind in the liquid, and the pure zinc vapour is then condensed separately. Electro refining suits copper better because copper does not vaporise conveniently, fire refining is a traditional method for copper and lead rather than zinc, and zone refining is reserved for ultra pure materials like silicon in tiny quantities, not for bulk industrial zinc. So vacuum distillation is the natural choice for refining zinc.