Nitriding of ferromolybdenum powder

Nitriding of ferromolybdenum powder - JOURNAL OF MATERIALS...

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JOURNAL OF MATERIALS SCIENCE LETTERS 12 (1993) 633-636 Nitriding of ferromolybdenum powder A. W. KIRBY*, D. J. FRAYS Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, UK Ferroalloys are generally cheaper to prepare than the extraction of the non-ferrous component but, until recently, there has not been a suitable route for the conversion of the ferroalloy to the pure compon- ents. Selective dissolution of the iron from the ferroalloy is not possible, as the elements usually form a continuous series of solid solutions. If, however, the elements can be segregated before dissolution, this route may offer a possibility. In 1989 Kirby and F*ay [1-4] described a route for upgrading ferrochromium to chromium metal by nitriding to form chromium nitride and iron nitride which, unlike the carbides, do not form a continuous series of solid solutions. Iron nitride is far more reactive with dilute acids and can be readily ex- tracted, leaving chromium nitride which can be dissociated to form chromium and nitrogen at elevated temperatures and low pressures. In 1991 Sommers et al. [5] reported extending this approach to ferroniobium, where ferroniobium was reacted with nitrogen gas at elevated temperatures, render- ing its iron content soluble under mildly acidic conditions. The product was then denitrided by arc melting to yield an economical feedstock for elec- tron-beam refinement. As well as obtaining niobium metal, niobium carbide and aluminide were also prepared by the reaction of the ferroalloy with methane or aluminium powder and subsequent leaching of the iron in an acidic solution [6]. Very recently Suri [7] have also reported a nitriding process for the recovery of niobium from ferro- niobium. This letter reports the nitriding of ferromolyb- denum to form molybdenum nitride and iron which is leached in dilute hydrochloric acid. H~igg [8] was the first to perform an extensive study of molyb- denum nitridation, and found that molybdenum is capable of forming three nitrides MoN (6), MozN (7) and MoxN (fi) where x = 3. Both ammonia and nitrogen were used for nitriding and it was noted there was little reaction below 1273 K, a result which was confirmed by Sieverts and Zapf [9]. The preparation of molybdenum nitride was also dis- cussed by Caillet [10], who studied the reaction of molybdenum with nitrogen and ammonia, and again it was reported that there was negligible interaction with nitrogen below 1273K. Von Hejedt~s [11] considered the effect of oxygen on the Mo-N phase diagram with respect to nitrida- *Now with: Reuters plc. ~Now with: Department of Mining and Mineral Engineering, University of Leeds, Leeds LS2 9JT, UK. tion during the ammonia reduction of molybdenum oxides. The Mo-N phase diagram was modified by Evans and Jack [12], who suggested that the H~igg cubic v-phase was a variant (caused by traces of oxygen) on the pure Mo2N phase (e), as pure Mo2N was found to consist of an h c p structure similar to other M2X nitrides. Jack and Jack [13] provided a detailed version of the Mo-N-O system and reported that
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Nitriding of ferromolybdenum powder - JOURNAL OF MATERIALS...

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