Equilibrium slag losses in ferrovanadium production
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I. INTRODUCTION
FERROVANADIUM can be produced from the trivalent oxide (V2O3) by reduction with aluminum in an electric arc furnace, adding iron in the form of scrap, and lime (CaO) to flux the alumina (Al2O3), which is produced by reduction of the vanadium oxide. Because of the high value of the ferrovanadium product and the large slag mass, losses of vanadium to the slag are of concern, and the work presented here aimed to quantify the effect of slag composition on the equilibrium amount of unreduced vanadium oxide that remains in the slag. To serve as background to this, a brief process overview is presented first. Typical compositions of the metallic product and the slag are given in Table I, based on figures for a South African producer. A corresponding mass balance is given in Table II; the mass balance shows a mismatch of some 140 kg per tonne of ferrovanadium. This is largely the result of the substantial wear of the magnesia refractory lining of the furnace for which data could be obtained; the entire MgO content of the slag (Table I) is the result of refractory wear in this furnace. The tapping temperature of this furnace (as measured by means of an infrared pyrometer) is in the vicinity of 1840 8C. A high tapping temperature is required because of the high melting point of ferrovanadium and also favors settling of entrained metal droplets from the slag after tapping. Such entrainment of metal droplets has been identified as one of the mechanisms of vanadium loss to the slag and is illustrated in Figure 1, which shows a backscattered electron image of solidified slag from the industrial furnace. The metal droplets show up as bright discs in this image, and the figure suggests that a substantial part of vanadium losses to the slag may be in this form. Quantification of these metallic losses is the topic of work that is to be presented elsewhere; the work presented here is concerned with the M.K.G. VERMAAK, Graduate Student, and P.C. PISTORIUS, Professor, are with the Department of Materials Science and Metallurgical Engineering, University of Pretoria, Pretoria 0002, South Africa. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS. METALLURGICAL AND MATERIALS TRANSACTIONS B
vanadium contained in the oxide phases. The vanadium oxide content of the slag (expressed as the mass percentage of V2O3) was found to be as high as 5 pct (based on energy dispersive X-ray (EDX) analyses in a scanning electron microscope). This high percentage means that dissolved (oxidic) vanadium losses are also of serious concern. Preliminary analyses of the phases present in the solidified slag indicated that, as might be expected, the slag basicity affects oxidic losses strongly. (The basicity here is defined as the molar ratio of the sum of the CaO and MgO contents of the slag to the Al2O3 content.) The three oxide phases that are visible in Figure 1 were identified—based on Xray diffraction and EDX analy
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