What is zone refining?

Zone refining is simply a sophisticated recrystallization technique. It does not always work; sometimes impurity components form stable "co-crystals" with the main component and are actually enriched by crystallization. Most of the time, however, the crystal lattice of the solid is stabler without the impurity component, so the probability is higher for a main component molecule or atom to be included than for one of the impurities.