Purification of hafnium by hydrogen plasma arc melting

Purification of two kinds of Hf metal, a high-purity Hf (HP-Hf) with 99.7 mass% and a low-purity Hf (LP-Hf) with 98 mass%, by hydrogen plasma arc melting (HPAM) has been carried out and the removal behavior of various impurity elements has been examined. Many metallic impurities (Fe, Al, Cr, Cu, Mn, Sn, Ti etc.) in the melted Hf, with higher vapor pressures than that of Hf, were removed to the very low levels by vaporization during HPAM and their removal rates increased with the hydrogen content of plasma arc. Then, the purity (except for Zr, O, N and C) of LP-Hf and HP-Hf were improved to nearly 99.998% and above 99.999%, respectively, after 60 min of 20%H2+Ar plasma arc melting. For the removal of Fe, Al, Cu and Ti from Hf, the refining effect of HPAM under atmospheric pressure was found to be superior to that of EBM under high vacuum. Therefore, HPAM has been confirmed to be a very useful method for the purification of Hf and the dissociated and activated hydrogen atoms formed in the high temperature plasma arc must play an important role for the unique refining process of HPAM.