A. Dutra, F. T. da Silva, A. Espínola
Aug 1, 2001
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Mineral Processing and Extractive Metallurgy
Abstract
high melting-point and corrosion resistance—in which it is matched only by glass, graphite and PTFE1,2,3—other applications, such as in superalloys for the aircraft industry and in the chemical processing industry, are also important. Other well-established applications include tantalum carbide for metal cutting tools, along with prosthetic devices, optical glass, laboratory ware, electroplating devices, etc.4,5 (Fig. 1). Tantalum metal is obtained from the processing of tantalite–columbite concentrates or tin slags; Fig. 2 summarizes the process. Tantalum-bearing materials are concentrated and leached with hydrofluoric acid, in some cases mixed with sulphuric acid to decrease the vapour pressure of the solution.6–8 Tantalum is then removed from the aqueous solution and separated from niobium by liquid–liquid extraction with methyl isobutyl ketone (MIBK). The alternatives for the next step depend on the final product intended: (1) precipitation with ammonia to produce tantalum hydroxide; or (2) crystallization with a potassium-bearing salt to produce potassium fluorotantalate (K2TaF7), known as K-salt. To produce pure tantalum metal as the final product the K-salt can either be electrolysed or be reduced by liquid sodium, the latter being the usual route. Several authors9–11 have advocated fused salt electrolysis for the production of refractory metals, such as tantalum, because purer metals are produced than by thermal processes.10 Nevertheless, since the 1970s the vast majority of the tantalum industry has adopted sodium reduction because the very fine, porous, flaked powder obtained by this route is more suitable for high-performance capacitors than the dendritic powder produced by molten salt electrolysis.6,12 However, the electrolytic process consumes much less energy than sodium reduction13 and is the method of choice for end-uses in the chemistry industry, direct electroplating on steel, working in very aggressive environments or even electroforming of small parts. For these purposes the electrolytic deposit must be smooth, coherent and free of defects—characteristics that can only be obtained under a narrow range of electrolysis conditions owing to the nature of the diffusion-controlled process