Zone refining is a purification technique used primarily for ultra pure metals. It works on the principle of fractional crystallization. A narrow molten zone is passed slowly along a metal rod. Impurities, which are more soluble in the melt than in the solid, concentrate in the moving liquid zone. This process is repeated multiple times, progressively pushing impurities to one end of the rod, which is then cut off, leaving behind a highly purified metal.

The final answer is: ultra pure metals.