Method of refining metal to high degree of purity and high-frequency vacuum induction melting apparatus

A method of refining metal to be processed to a high degree of purity, said method comprising the steps of:

• providing a vacuum vessel, a metal crucible including an induction heating coil wound therearound, the metal crucible placed in the vacuum vessel, and metal in the metal crucible;
• drying the vacuum vessel, the metal crucible and the metal by preheating them under vacuum;
• controlling a total pressure in the vacuum vessel so as to keep the total pressure at 1.times.10.sup.-8 Torr or lower,
• controlling an attainable degree of vacuum in the refining atmosphere to keep the partial pressure of oxygen (P.sub.O.sbsb.2) at 1.times.10.sup.-10 Torr or lower, to keep the partial pressure of hydrogen (P.sub.H.sbsb.2) at such a level so that the ratio of the partial pressure of hydrogen (P.sub.H.sbsb.2) to the partial pressure of water (P.sub.H.sbsb.2.sup.O), P.sub.H.sbsb.2 /P.sub.H.sbsb.2.sup.O, is 10 or higher, and to keep the partial pressure of carbon monoxide (P.sub.CO) at such a level that the ratio of the partial pressure of carbon monoxide (P.sub.CO) to the partial pressure of carbon dioxide (P.sub.CO.sbsb.2), P.sub.CO /P.sub.CO.sbsb.2, is 10 or higher;
• and
• electrifying the induction heating coil wound around the crucible to melt the metal.