zone melting

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A purification technique for crystalline substances in which a heating system passes slowly over a bar of the material to be refined, creating a molten region that carries impurities with it across the bar.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics, chemistry A technique used to purify a metal, semiconductor or other solid in which a heating system slowly causes a series of molten zones to traverse the solid, leaving purer material behind it.