Upgrading ferrochromium to chromium by nitriding, leaching, and dissociation
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I. I N T R O D U C T I O N C H R O M I U M is produced industrially in three main forms: ferrochromium, chromium made by the aluminothermic reaction, and electrolytic chromium. [q Ferrochromium usually contains about 75 pct chromium, but the price differential between this and the chromium produced by the other routes is much greater than is reflected in the difference by chromium content. Therefore, there is an incentive to develop a method for upgrading ferrochromium to chromium metal. Various routes can be considered, such as the selective dissolution of the iron in aqueous solution, but as the iron and chromium form a solid solution and the resistance to attack depends on the formation of the chromium oxide layer rather than the inertness of the chromium, this does not appear to be a suitable method. A more promising approach might be to form a stable intermetallic compound with the chromium and to leach the iron away from the chromium compound. A search of possible compounds shows that although chromium forms stable carbides, the carbides form a continuous solid solution with iron carbide; therefore, phase segregation is unlikely to occur. Another element which forms stable compounds is nitrogen, and a search of the literature shows that on thermodynamic grounds, the stability of iron nitride is far less than that of chromium
A.W. KIRBY, formerly Research Student in the Department of Materials Science and Metallurgy, University of Cambridge, is with Reuters, London. D.J. FRAY, University Lecturer, is with the Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3Q2, United Kingdom. This paper is based on a presentation made in the T.B. King Memorial Symposium on "Physical Chemistry in Metals Processing" presented at the Annual Meeting of The Metallurgical Society, Denver, CO, February, 1987, under the auspices of the Physical Chemistry Committee and the PTD/ISS. METALLURGICAL TRANSACTIONS B
nitride. Therefore, nitriding seems a possible way to segregate the iron and chromium into separate phases for subsequent chemical treatment. Chromium nitrides can be prepared readily by the reaction of chromium powder with nitrogen or a m m o nia, f2,31 and it has been noted that impurities in the chromium tend to enhance the nitriding reaction.r3~ This observation was confirmed by Duparc et al., [41 who found that the presence of about 1 pct iron in the chromium resulted in consistently higher weight gains and faster reaction kinetics, whereas Valensi ~31found that the presence of oxygen in the gas stream inhibited nitridation through the formation of a surface oxide layer. Two nitildes of chromium exist, CrN and Cr2N, and Mills and Hill ~51 have specified the conditions under which each nitride could be formed. In ferrochromium, the other major element is iron. There is extensive literature describing the nitriding characteristics of iron, because nitriding is an important surface process in the hardening of steels. There have been several studies of the precipitation
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