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Name: TanPhat
Status: student
Grade: 9-12
Location: Outside U.S.
Country: Vietnam
Date: Summer 2009

In the "zone refining" (or "zone melting"), why and how do most impurities tend to remain in the molten region rather than re-solidify, when the process is complete?

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.

Richard Barrans, Ph.D., M.Ed.
University of Wyoming

The basic principle of "zone refining" is that most impurities cannot "fit" into the crystal lattice of the substance being purified. So the majority of impurities remain in the liquid as the substance is frozen. This does not have to be 100% true, because a "zone refining" apparatus typically consists of multiple hot/cold zones. So a cylinder of the substance is passed slowly through the zones. The substance melts, solidifies, melts, solidifies, .... as the cylinder passes through the apparatus. The net effect, unless the impurity does fit into the crystal lattice, is that the impurities keep being concentrated in the liquid phase. If the impurities do "fit" into the crystal lattice they will tend to collect at the "bottom" of the cylinder and be depleted, while liquid soluble impurities collect at the "top" of the cylinder. So typically, the cylinder of refined substance is cut into 1/3's, and the middle third is (roughly speaking) taken as the "pure" substance. In principle, this third can be further refined by repeating the process on this first stage.

Vince Calder

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