Metallothermic reduction as an electronically mediated reaction
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Metallothermic reduction as an electronically mediated reaction Toru H. Okabea) and Donald R. Sadoway Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307 (Received 9 December 1997; accepted 15 April 1998)
The commonly held view that metallothermic reduction is strictly a chemical reaction and that the process is rate limited by mass transfer has been found to be incomplete. In a study of the production of tantalum powder by the reaction of K2 TaF7 with sodium, it has been shown that there are two dominant kinetic pathways, both involving electron transfer. Furthermore, the overall rate of reaction is limited by electron transport between the reactants. This indicates that metallothermic reduction is an “electronically mediated reaction” (EMR). Experiments found that the location of the tantalum deposit and its morphology are governed by the reaction pathway.
I. INTRODUCTION
Tantalum is produced by metallothermic reduction of one of its salts.1,2 At approximately 800 ±C, solid potassium heptafluorotantalate (K2 TaF7 ) and liquid sodium are added to a halide melt (known as a “diluent”) where they react to produce solid tantalum in the form of powder. The central reaction for this process is K2 TaF 7 s,d 1 5 Nas,d Tassd 1 5 NaF s,d 1 2 KFs,d , (1) where the underline denotes that the species is dissolved in the diluent. Figure 1 shows a sketch of the industrial reactor. The stirrer is necessiated by the fact that while K2 TaF7 dissolves in the diluent, sodium is effectively insoluble. This process is representative of a set of primary extraction technologies that produce metal by reaction of one of its compounds with a metallic reducing agent. Magnesium, titanium, zirconium, beryllium, neodymium, and tantalum are all examples of metals produced by this technology. The commonly held view is that metallothermic reduction is strictly a chemical reaction and that the process is rate limited by mass transfer. In the present study it is shown that the reaction mechanism of Eq. (1) includes electron transfer steps, and furthermore, that it is the rate at which electrons produced by the anodic steps are transported to the cathodic steps that controls the overall rate of reaction. For this reason, metallothermic reduction is deemed to be electronically mediated. In addition, there is evidence that the mode of electron transport between reactants influences the morphology a)
Present address: Institute for Advanced Materials Processing, Tohoku University, 2-1-1 Katahira, Aobaku, Sendai, 980 Japan.
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http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 12, Dec 1998
Downloaded: 13 Mar 2015
of the metal product. Recognition of the role of electron transport has important consequences for process design and optimization. II. METALLOTHERMIC REDUCTION AS AN ELECTRONICALLY MEDIATED REACTION
Conventional metallothermic reduction is based on direct physical contact between feed and reductant, schematic
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