The reduction mechanism of chromite in the presence of a silica flux
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INTRODUCTION
F E R R O C H R O M I U M is one of the most important alloying materials used in the production of stainless and high-alloy ferritic steels. At present, high-carbon ferrochromium is produced in submerged-arc furnaces. Disadvantages of this process are the limited use of chrome ore fines and friable chrome ores and the fact that more than two-thirds of the total electrical energy consumption (4000 to 4200 k W h / t o n metal) is used for heating and reducing the chromite. To improve cost effectiveness, solid-state carbothermic prereduction has gained importance. A new process in which unagglomerated particles are reduced in a rotary kiln has been developed by Krupp GmbH.IU The process is characterized by a high operating temperature (up to 1500 ~ the presence of a silica flux, and a large excess of reductant. The solid-state carbothermic reduction mechanism and kinetics of a natural chromite from the LG-6 layer (the sixth layer of the lower group of chromite seams) of the Bushveld Complex of South Africa has been investigated in some detail; the most significant results at temperatures of about 1400 ~ were recently published. 12.31 These studies, which did not involve the use of a silica flux, clearly demonstrated a number of points. Zoning was observed in partially reduced chromites, with degrees of reduction of up to approximately 70 pct. The inner cores were rich in iron, whereas the outer cores were depleted of iron. Using a stoichiometric ionic model for the spinel] 21 EDAX revealed that, within the outer core, Fe 2+ and C r 3+ ions diffused outward, whereas Cr 2+, A13+, and M f + ions diffused inward. Initially, Fe 3+ and Fe 2+ ions at the surface of the chromite
P. WEBER, Postgraduate Student, and R.H. ERIC, Professor and Head, are with the Department of Metallurgy and Materials Engineering, University of the Witwatersrand, Johannesburg, W I T S 2050, South Africa. Manuscript submitted October 5, 1992. METALLURGICAL TRANSACTIONS B
particle were reduced to the metallic state. This was followed immediatel)/ by the reduction of Cr 3+ ions to the divalent state. Cr -+ ions diffusing toward the center of the particle reduced the Fe 3§ ions in the spinel under the surface of the particle to Fe 2§ at the interface between the inner and outer cores. Fe 2§ ions diffused toward the surface, where they were reduced to metallic iron. After the iron was completely reduced, C r 3+ and any Cr 2§ that was present were reduced to the metallic state, leaving an iron- and chromium-free spinel, MgA1204. The metallized iron and chromium carburized during the reduction into (Fe, Cr)7C3. The ionic reduction model proposed and modeled by Soykan et al. t23j involves rather complex reactions among the reductant graphite, altered spinel phases, and various ionic species and also involves site-exchange mechanisms between Fe 2- and C r 3+ ions, the trivalent chromium being placed in octahedral sites due to its very high preference for them. Furthermore, the proposed mechanism 121 involves a swap mechanism between the Cr 2
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