An investigation of the critical influence of potassium on the reduction of wustite
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INTRODUCTION
IT is known that potassium, although introduced as traces with the ore and with the coke, concentrates in the blastfurnace as a consequence of recycling processes.l According to thermodynamics,2'3 many potassium compounds, reaching lower parts of the blast-furnace, are partially or totally (depending on the kinetics 4 reduced, giving rise to volatile potassium species which are then carried upward with the gas flow. In the upper part these species oxidize and condense on the descending load thereby increasing its potassium content. The blast-furnace operations are disturbed because potassium increases the ore or pellets' embrittlement, with production of the fine particles which oppose the gas free circulation; on the other hand, potassium is also known to improve the ore reductibility. 5'6 However, in spite of these striking features, little is understood about the mechanism of interactions between potassium and iron ores or oxides. Several interpretations have been provided for the improved rate of reduction and/or the embrittlement of the ore particles: 1. The embrittlement is known to occur mainly during the hematite-magnetite reduction stage 7 as this transformation is anisotropic. 8 When alkalis are present, the increase in the embrittlement results from an increase in relative value of the internal stresses which preexist or develop during the reduction in the materials because (i) potassium entering into the crystal lattice introduces additional stresses; 9 (ii) liquid phases, which form from gangue in the presence of a nonuniform distribution of potassium, exert capillarity force; 6 (iii) potassium increases the anisotropic character of the hematite-magnetite transformation as a consequence of a nonuniform distribution of the additive or as a consequence of a preferential reduction along some particular crysM. GOUGEON, Docteur Ingrnieur, B. DUPRI~, Charg~ de recherche, and C. GLEITZER, Directeur de recherche, are with Universit6 de Nancy I, Laboratoire de Chimie du Solide Minrral, U.A. 158, BP 239, 54506 Vandoeuvre Les Nancy Cedex, France. Manuscript submitted September 17, 1984. METALLURGICALTRANSACTIONS B
tallographic planes; 1~and (iv) the relaxation mechanisms of the stresses generated by the reduction proportionally become less efficient as the reduction process becomes faster, 5'1~ or are less efficient since it was observed that potassium retards sintering of iron oxides. 10 The increased embrittlement would also result from a decrease in the mechanical strength of the materials because (i) the magnetite produced is formed at a lower temperature and therefore is more porous and brittlef (ii) the presence of potassium allows a preferential reduction of the ironoxide bridges; ~~(iii) potassium retards sintering and hence the formation of new oxide bridges during reduction; 1~and (iv) alkalis soften the gangue due to the formation of low melting point components 6'12 and this degrades the mechanical properties, although some authors consider that liquids can furnish oxide bridges whi
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