Zone Refining
An ultra-purification technique in which a narrow molten zone is passed repeatedly along a solid rod.
Unit
Dimensionless
In depth
Impurities are more soluble in the melt than in the solid, so they are swept along with the travelling molten band and concentrate at one end, which is then cut off. Repeated passes achieve purities of better than one part per billion.
Examples in the real world
Producing semiconductor-grade silicon and germanium for the electronics industry.