ELECTRON BEAM MELTING

Refining. KroU-process hafnium sponge and electrowon hafnium do not meet the performance requirements for the two principal uses of hafnium metal. Eurther purification is accompHshed by the van Arkel-de Boer, ie, iodide bar, process (18) and by electron beam melting. 

Hafnium Oxide. Two oxides of hafnium, hafnium monoxide [12029-22-0], HfO, and Hf02, are known to exist but only the dioxide is stable under ordinary conditions. Gaseous hafnium monoxide can be present at >2000° C, especially when the partial pressure of oxygen is low. Hafnium monoxide is probably the compound form in which oxygen is evolved when hafnium metal is melted in an electron-beam melting furnace. HfO(g) is the species observed mass spectrometricaHy when hafnium dioxide vaporizes. 

The product chunks are hydrided, cmshed, and dehydrided. The resultant powder is blended and pressed into bars which are purified by high temperature sintering. The sintering removes all of the carbon and most of the oxygen and is followed by consoHdation by either arc or electron-beam melting. 

Niobium pentoxide also is reduced to metal commercially by the aluminothermic process. The finely ground powder is mixed with atomized aluminum and an accelerator compound which gives extra heat during reaction, then is ignited. The reaction is completed quickly and, after cooling, the slag is broken loose to free the metal derby which is purified by electron-beam melting. 

Consolidation. Because of its high melting point, tungsten is usually processed by powder metallurgy techniques (see Powder metallurgy). Small quantities of rod are produced by arc or electron-beam melting. 

Vanadium metal can be prepared either by the reduction of vanadium chloride with hydrogen or magnesium or by the reduction of vanadium oxide with calcium, aluminum, or carbon. The oldest and most commonly used method for producing vanadium metal on a commercial scale is the reduction of V20 with calcium. Recently, a two-step process involving the alurninotherniic reduction of vanadium oxide combined with electron-beam melting has been developed. This method makes possible the production of a purer grade of vanadium metal, ie, of the quaUty required for nuclear reactors (qv). 

The vanadium alloy is purified and consoHdated by one of two procedures, as shown in the flow diagram of the entire aluminothermic reduction process presented in Figure 1. In one procedure, the brittle alloy is cmshed and heated in a vacuum at 1790°C to sublime most of the aluminum, oxygen, and other impurities. The aluminum faciHtates removal of the oxygen, which is the feature that makes this process superior to the calcium process. Further purification and consoHdation of the metal is accompHshed by electron-beam melting of pressed compacts of the vanadium sponge. 

Impurity Sponge Electron-beam melted First melt Second... 

Compared to that prepared by cirect electron-beam melting of vanadium alloy containing 15 wt % aluminum. 

Electron-beam melting of zirconium has been used to remove the more volatile impurities such as iron, but the relatively high volatiUty of zirconium precludes effective purification. Electrorefining is fused-salt baths (77,78) and purification by d-c electrotransport (79) have been demonstrated but are not in commercial use. 

Rolling and swaging Vacuum-sintered bar can be cold rolled, and reductions up to 90% between anneals are possible. Arc-cast and electron-beam-melted material is generally forged at room temperature prior to rolling and swaging. 

Despite the above disadvantages, some investigations show possible directions for further improvement and development of the process for the production of tantalum powder suitable for the manufacture of capacitors with no additional electron-beam melting and special crushing. 

Finally, it may be pointed out that none of the rare metals can be smelted directly from the ore. The concentrate must first be converted to a pure chemical compound which is utilized as the raw material for the production of the metal. The refractory rare metals are often obtained in the form of a powder or sponge. They are consolidated and refined by powder metallurgy techniques or by arc melting or by electron beam melting. In fact, the current refractory rare metals technology has been crucially dependent on the development of vacuum metallurgical techniques and processes. 

Magnesium chloride and excess magnesium are removed by distillation at reduced pressure. Pure zirconium may be prepared by several methods that include iodide decomposition process, zone refining, and electron beam melting. Also, Zr metal may be electrorefined in a molten salt bath of potassium zirconium fluoride, K2ZrFe... 

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