Location: Outside U.S.
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.
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Update: June 2012