The physical chemistry of thermal decomposition of South African chromite minerals
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I. INTRODUCTION
CHROMITE is almost exclusively the only mineral used for the production of metallic chromium, chromium chemicals, and refractories. Extensive investigations on chromite mineral phase equibria were undertaken in the past;[1,2,3] however, the main focus of the previous research works was in the hightemperature region (1573 K) related to the production of ferrochrome alloy, which is related to the phase transformation, chemical bonding in the crystals in the temperature range of 1273 to 2073 K reported by Hino et al.[4] By contrast, the literature on the phase transformation for oxidation conditions below 1473 K used for the extraction of sodium chromate from chromite spinels is rather limited.[5] The alkali roasting process is used worldwide for the extraction of sodium chromate from chromite ores. The roasting temperature varies from 873 to 1473 K and the oxygen partial pressure ranges from 8 to 15 vol pct, depending on the chromite ore and charge compositions used for the extraction of sodium chromate. The structure of natural chromites is described by the cubic spinel having a formula unit (Fe2, Mg) [Cr, Al, Fe3]2O4, which shows that the natural mineral is a solid solution of FeCr2O4, Fe3O4, FeAl2O4, MgCr2O4, MgFe2O4, and MgAl2O4 pure spinel end members. These constituent members form a wide range of continuous solid solutions at high temperatures during the rock-forming process, which may or may not decompose under the geological conditions. In chromite minerals, iron occurs in two different oxidation states (Fe2 and VILAS D. TATHAVADKAR, Scientist, formerly with IMR, University of Leeds, is with the Ferro Alloys and New Business Group, R&D Department, Jamshedpur, JH 831 001, India. M.P. ANTONY, Scientist G, Fuel Chemistry Division, IGCAR, Kalpakkam, TN, 600 102 India. ANIMESH JHA, Professor of Materials Science, is with IMR, University of Leeds, Leeds LS2 9JT, United Kingdom. Contact e-mail: [email protected] Manuscript submitted March 3, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
Fe3) in significant amounts, and therefore, the oxygen potential is an important parameter in the consideration of the phase equilibria. In addition to the oxygen potential, the spinel structure readily permits permutation of divalent and trivalent cations between the tetrahedral and octahedral sites, the distribution of which is a function of temperature and pressure, and therefore makes the equilibrium studies more complex. Muan[6] reviewed the phase relationships in chromium-oxide–based systems at elevated temperatures, and reported that at high temperatures (1473 to 1573 K) and low oxygen partial pressures (PO2 107 atm), the spinel solid solution phase with composition range between Fe- and Mg-based spinel members is very stable. Cremer[7] investigated the Fe3O4-FeCr2O4-FeAl2O4 system and reported extensive immiscibility gaps at the low temperatures, which are consequences of the decomposition of chromite solid-solution under reduced pressure, temperature, and oxygen potential. The phase decompositio
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