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Why can tungsten be purified by float zoning?
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zone melting
Figure 1: Schematic representation of (A) normal freezing, (B) zone refining
Tungsten can be purified by float zoning because it possesses a high melting temperature, which is associated with high surface tension. This property allows a vertical rod of the metal to maintain a stable molten zone held in place by surface tension during the refining process.1
Float zoning is particularly advantageous for high-melting-point metals like tungsten because it eliminates the risk of contamination from a container. In standard zone refining, the material is typically held in a boat or tube, which can introduce impurities. By using float zoning, the molten zone is suspended between solid sections of the rod, ensuring the material remains free from contact with container walls.1