Zone Refining

Zone refining is based on the principle of FRACTIONAL CRYSTALLISATION -- specifically, on the fact that impurities are generally MORE SOLUBLE IN THE MOLTEN (liquid) form of a metal than in its solid, crystalline form. When an impure molten metal is allowed to solidify (crystallise) slowly, the impurities preferentially remain concentrated in whatever portion is still molten, rather than being incorporated into the newly forming solid crystal.

Procedure. The impure metal is first cast in the form of a rod. A MOBILE induction heater is used to melt just one end of the rod, creating a small molten zone. The heater is then moved slowly along the length of the rod towards the other end. As the heater moves forward, the metal just behind it (which the heater has already passed) COOLS AND RE-CRYSTALLISES in a purer form, because the impurities are rejected from the newly forming solid and instead migrate ahead, into the adjacent still-molten zone that is travelling along with the heater. As the heater continues to advance, this impurity-enriched molten zone moves along with it, effectively sweeping the impurities from one end of the rod towards the other. Repeating this heater pass several times, always in the same direction, progressively concentrates the impurities at one end of the rod (which is eventually cut off and discarded) while the bulk of the rod reaches very high purity. The whole process is carried out in an INERT GAS atmosphere specifically to prevent oxidation of the metal during the repeated melting. …