Ultra-Purification of Electrolytic Iron by Cold-Crucible Induction Melting and Induction-Heating Floating-Zone Melting in Ultra-High Vacuum

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The effect of melting under ultra-high vacuum of 10−7 Pa on the purification of the high-purity electrolytic iron developed was investigated using a new cold-crucible induction melting furnace which was able to melt high-purity iron of 10 kg and a new induction-heating floating-zone melting furnace, which were designed and constructed using ultra-high vacuum technology. Ultra-high vacuum melting was quite useful even for the ultra-purification of iron 10 kg in weight. The high-purity electrolytic iron of 7.5 kg with residual resistivity ratio of 2000 to 2700 was melted using the cold-crucible induction melting furnace, and the pressure during melting after melt-down was kept from 1×10−6 to 8×10−7 Pa for 9 min. The 7.5 kg ultra-pure iron ingot of more than 99.9988 mass% purity after the chemical analysis of 33 elements, most of which were lower than each detection limit, was successfully obtained, even if this ingot contains their all elements of the detection limit value. Also the ultra-pure iron 35 g in weight of more than 99.9988 mass% purity was obtained from electrolytic iron of 99.996 mass% purity by the combination of cold-crucible induction melting in ultra-high vacuum and induction-heating floating-zone melting in UHV of 4×10−7 Pa after the analysis of 35 elements. The concentration of C+N+O+S decreases from 14.4 mass ppm before zone-leveling to 2.4 mass ppm after four zone-leveling passes in ultra-high vacuum. For further ultra-purification of iron it is absolutely necessary to develop and establish the trace analysis techniques of measuring accurately the concentration of non-metallic impurity elements at levels below 0.1 mass ppm and metallic impurity elements at levels below 0.01 or 0.001 mass ppm.

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