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Finishing process for the preparation of highly pure metal. This method was invented for refining germanium by American inventor W. G. Pfann. Earlier high-purity metals had been produced by expensive chemical means, unsuitable for mass production. In contrast, the zone melting process as a physical refining method is not only inexpensive, but is also suitable for refining intermetallic and other compounds.
The principle is based on the observation that during the production of metal ingots from a melt, impurities will always accumulate at the place cast last. Therefore, zone melting makes use of a single or a double loop heating coil which is placed in the furnace and the metal slowly moves along it. However, as the metal can move through the coil as well, the use of a ceramic dish is necessary for horizontal movements, which in turn can be a source of impurities. To avoid this, a bare metal rod is hung vertically and gradually lowered. In the process of refining silicon semiconductors, this vertical method is used while the silicon is heated with a high frequency induction coil. By multiple repetition of zone melting, the purity of the metal can be gradually increased.