Removal of alloying elements from zirconium alloys by hydrogen plasma-arc melting

The removal of alloying elements such as Sn and Fe from zircaloy scrap was investigated using H₂Ar (0% up to 50% H₂) plasma-arc melting under atmospheric pressure and reduced pressure. While the removal rates of Sn and Fe are very low when the plasma gas is only Ar, they become higher with increasing hydrogen content of the plasma gas and with decreasing pressure during the melting process. Zirconium metal with above 99.9% purity is recovered from zircaloy scrap by hydrogen plasma-arc melting for 3 h at a reduced pressure of 9.3 kPa. Furthermore, hydrogen plasma-arc melting is confirmed to be superior to electron beam melting for the removal of alloying elements from the scrap. To clarify the refining effect and its mechanism, the hydrogen plasma-arc melting of Zr-1mass%Sn and Zr-1mass%Fe binary alloys was examined. The reactions of Sn and Fe removal from the Zr alloy melts obey a first-order rate law and their removal rates are found to increase proportionally to P_H2 and P_H2 ^{3 4} for Sn and Fe respectively, where P_H2 is the partial pressure of hydrogen in the plasma gas. This unique and excellent refining effect is supposed to be caused by activated hydrogen atoms formed in the high temperature plasma-arc.