L Journal of Alloys and Compounds 265 (1998) 190–195 Refining of tantalum by silicon deoxidation a, a a a b a * Alok Awasthi , N. Krishnamurthy ,Y.J. Bhatt , R. Venkataramani ,Y. Ueda , S.P. Garg a Materials Processing Division, Bhabha Atomic Research Centre, Mumbai-400 085, India b Department of Energy Science and Engineering, Kyoto University, Kyoto 606, Japan Received 20 May 1997; received in revised form 10 July 1997 Abstract The removal of residual oxygen from tantalum by pyrovacuum treatment using silicon deoxidation (as SiO ) was studied. The (v) possibility of eliminating oxygen from a Ta–Si–O solid solution by preferential evaporation of SiO was indicated by calculations based (v) on the experimental or estimated values for the activities of silicon and oxygen in the solid solution as well as for the standard free energies of formation of tantalum suboxide (TaO ) and silicon monoxide (SiO ). The experimental results obtained by treating (v) (v) 23 Ta–Si–O alloys of known silicon and oxygen contents at temperatures ranging from 2073 to 2273 K under 1310 Pa pressure for 2 h indicated the occurrence of silicon deoxidation. In this temperature range, oxygen removal has occurred faster and to a lower residual level in Ta–O alloys containing silicon as compared to Ta–O alloys containing no silicon. When both silicon and oxygen concentrations have reached relatively lower levels, viz., ¯1 at % O and 0.3 at % Si, oxygen removal by SiO vapourisation decreases with an (v) increasing contribution of sacrificial deoxidation (as TaO ).  1998 Elsevier Science S.A. (v) Keywords: Tantalum; Deoxidation; Refining; SiO ; TaO (v) (v) 1. Introduction alternative to carbon. Silicon deoxidation works essentially in the same way as carbon deoxidation with the possibility As reduced tantalum metal, obtained by sodium reduc- of distillation of the remaining residual silicon at the end tion of potassium tantalum fluoride [1] or by oxide of deoxidation as an added advantage. Silicon deoxidation reduction methods [2] may contain up to about 7.5 at % of tantalum has been investigated in this work. ( |7000 ppm) of residual oxygen. During subsequent pyrovacuum treatments, the residual oxygen is removed from tantalum either by sacrificial deoxidation involving 2. Theoretical considerations the vapourisation of the suboxide TaO [3] or by carbon (v) deoxidation involving the volatilization of carbon monox- A complete assessment of the silicon deoxidation of ide, CO [4]. Sacrificial deoxidation involves tantalum loss. tantalum involves consideration of thermodynamics of For example, the decrease of oxygen content from 7.5 at % each of the major vapourisation processes expected from to 0.5 at % O by sacrificial deoxidation involves .7% loss the Ta–Si–O alloy, viz., vapourisation of TaO , va- (v) of tantalum. This was carried out by treating the Ta–O pourisation of SiO and vapourisation of Si and their 23 (v) (v) alloy at 2320 K for 2 h under 7310 Pa. Under the same relative extents. conditions, if tantalum mixed with carbon corresponding to The reference states throughout this thermodynamic a C / O mole ratio of 0.91 was treated, oxygen content analysis have been taken as pure liquid / solid Si and Ta, decreased to about 0.1 at % with essentially no tantalum pure vapour of Si , SiO and TaO and pure O at (v) (v) (v) 2(g) loss. The metal, however, contains approximately 0.15 5 the given temperature and at 1 atm (1.0133310 Pa) at % residual carbon. This carbon could not be removed pressure. from tantalum by pyrovacuum methods except by intro- ducing more oxygen to result in the evaporation of CO. However, introduction of oxygen in such Ta–C alloy is 2.1. Sacrificial deoxidation extremely difficult. Silicon appears to be an attractive There can be only two phases in equilibrium during * Corresponding author. sacrificial deoxidation of tantalum (below the saturation 0925-8388 / 98 / $19.00  1998 Elsevier Science S.A. All rights reserved. PII S0925-8388(97)00433-7