Purification of Vanadium by Vacuum Melting

Abstract

The Bureau of Mines compared the purification of two commercially available vanadium samples by three vacuum melting techniques: consumable-electrode arc melting in a conventional deep mold, consumable-electrode arc melting at the top of the mold, and conventional electron-beam melting. Top-of-the-mold (TOM) and electron-beam (EBF) melts were both conducted in the same vessel equipped with a bottom-withdrawal mechanism. Pressures during melting were in the range of 0.10–0.50 mTorr; the pressure in the deep mold could not be measured, but probably exceeded 100–300 mTorr during a typical melt. Arc melting was characterized by melting rates of 500–800 g/min, but electron-beam melts were limited to rates of 20–30 g/min. Forty-nine percent of the oxygen content was removed by double melting vanadium sponge at the top of the mold, and 42% of the oxygen content was removed from electrorefined vanadium treated in the same manner; however, the carbon and nitrogen increased. The purity of samples melted at the TOM approached the purity of samples prepared in the EBF. In some instances, a further decrease in the oxygen and metallic impurity content resulted from remelting selected materials in the electron-beam furnace. Little or no purification resulted from melting vanadium in the conventional deep mold. Purification proceeded faster and more completely on melting vanadium with small additions of yttrium, carbon, or aluminum; aluminum was largely removed on double melting, but carbon and yttrium were retained in the products. Metal or carbon additions tended to stabilize the arc and improve metal recovery from 85 to greater than 90% during a consumable-electrode arc melt. Typical metal hardness ranged from Rhn B-20 to B-30. Mass-spectrometric studies conducted in conjunction with the purification work demonstrated that CO, CO2, Al2O, and AlO were the principal oxides, while Al, Fe, Ti, Mn, Ni, V, Cr, Ca, and Cu were the principal metallics evolved when heating and melting vanadium. The volatilization of VO was slow and incomplete; therefore, sacrificial dissociation was of minor importance in the purification of vanadium.

Thermodynamics of Interstitial Impurities Removal from Refractory Metals

“…Distillation Distillation is an important process by which metallic impurities are removable from vanadium, niobium and tantalum during pyrovacuum treatment. The impurity elements, which typically include aluminium, calcium, chromium, copper, nickel, iron, magnesium, silicon, are usually more volatile than the refractory metals and when the impure metal is heated or melted under high vacuum, the more volatile elements simply distill off [3]. The rate and extent of their removal thus, depend upon the partial pressures of the elements involved.…”