An ionic diffusion mechanism of chromite reduction
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ODUCTION
IN the Bushveld Complex
of the Transvaal, chromite occurs in 13 layers, m as shown in Table I. These layers differ chiefly in their Cr to Fe ratio, which generally increases with depth. Ferrochromium is currently produced from the LG6 layer; however, the UG2 layer, which is rich in platinum group metals, has been considered as an additional source of ferrochromium ore.[Z1 The behavior of LG6 and UG2 chromites in the presence of a reductant at temperatures above 1000 ~ has been the subject of a number of investigations. Yet, little success has been obtained in elucidating mechanisms to account for the phases which are formed during reduction. Rankin t31 found Cr20 3 to be a phase produced when LG6 chromite was reduced with carbon at temperatures up to 1200 ~ At higher temperatures, the Cr203 phase was no longer present but the spinel, MgA1204, was the predominant phase. Subsequently, several investigators t4'5'6] reported concentration profiles, obtained by electron microprobe analyses, within chromite particles subjected to various degrees of reduction. All these studies had one feature in common: during the initial stages of reduction the iron concentration profile has a characteristic shape, low and flat near the surface and rising sharply in the center to an inner core of nearly constant composition. Such an iron profile is shown in Figure 1 from Searle, tSl who studied the reduction of UG2 chromite at 1300 ~ The extent to which iron had been removed from the outer zone was noted to be equivalent to the removal of Fe30 4. Kinloch, t61 who studied the reduction of LG6 chromite at ~ 1250 ~ explains the shape of the FeO profile as being caused by the rapid diffusion of iron toward the surface of the chromite particle. However, diffusion would result in a smooth FeO profile rather than the nearK. P. D. PERRY is Engineer, Council of Mineral Technology,Randburg, South Africa. C. W. P. FINN, formerlyAssociate Professor, University of the Witwatersrand, is Senior Research Scientist, Vacuum Industries, Inc., Somerville, MA 02143. R.P. KING is Professor and Head of Department, University of the Witwatersrand, Johannesburg, South Africa. Manuscript submittedOctober2, 1987.
METALLURGICALTRANSACTIONSB
step change observed. As reduction progresses, tSj the UG2 chromite particles become increasingly depleted in iron, resulting in the particles separating into two phases, namely a sesquioxide phase consisting of a solid solution of Cr203AI20 3 and a Mg-A1 spinel phase. However, this phenomenon was not observed during the reduction of LG6 chromite at temperatures above 1200 ~ Using thermogravimetric data and chemical analyses of the resulting alloys, Algie and Finn tTl proposed three stages in the reduction of LG6 chromite at 1300 ~ (1) the reduction of ferric to ferrous ions, (2) the reduction of ferrous ions to metallic iron, and (3) the reduction of trivalent chromium to metallic chromium. They presented a reduction model based on the diffusion of iron and chromium in the chromite particles but did not suggest a me
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