Thermodynamics of Interstitial Impurities Removal from Refractory Metals

Abstract

The applied thermodynamic aspects of removing hydrogen, nitrogen, oxygen, carbon, and silicon from vanadium, niobium and tantalum metals by pyrovacuum treatments are considered in this paper. Two major processes operate in refining by pyrovacuum treatment. One is distillation and the other is degassing. Distillation is mainly used to remove substitutional impurities that are either already present in the metal or have been added for removing any interstitial impurity. The success is determined by the partial pressure of the impurity element as well as by the difference in partial pressures of the impurity and the metal. The maximum rate of vaporization of impurity can then be estimated using the free evaporation equation. Interstitial impurities, particularly the gases hydrogen, nitrogen and oxygen, are removed by degassing. Thermodynamics of classical degassing, which is essentially the reverse of absorption, is described usefully by the pressure-composition isotherms. This is applicable mainly for the r...