Impurity removal from Zr, Nb and Ta metals by hydrogen plasma arc melting and thermodynamic estimation of hydride formation

Commercial Zr, Nb and Ta have been purified by hydrogen plasma arc melting (HPAM) and the removal degree of impurities was compared with the results of thermodynamic estimations. In the case of Zr and Nb, 40–60% of the total amount of impurities was removed by Ar plasma melting and 60–80% by HPAM. The Ta melting by HPAM allowed to remove 80% of the total amount of impurities and to decrease the concentration of almost impurities down to a few mass ppb levels. For the systems with Ar–H2 plasma gas, experimental and calculated results are closer to each other than that for the system with Ar plasma gas. According to the thermodynamic calculations, none of impurities forms hydrides in the gas phase because of the high melting temperatures of these metals. The effect of H2 addition to the plasma gas on the enhancement of the impurity removal may be attributed to the increase of the surface temperature of molten metals.