Zone Refining

Zone refining is a metal-purification technique in which a narrow molten zone is passed slowly along the length of a solid bar of material, exploiting the fact that most impurities are more soluble in the liquid phase than in the solid, so repeated passes sweep the impurities progressively toward one end of the bar, which is then cut off and discarded. The American physicist William G. Pfann invented the technique in 1952 while working at Bell Telephone Laboratories, developing it specifically to purify the germanium needed for early transistors far beyond what earlier chemical purification methods could achieve. Zone refining subsequently became a standard method for producing the ultra-high-purity germanium and silicon that semiconductor manufacturing depends on, and the same principle has since been applied to purifying various other metals and compounds.