Solid State Reduction of Preoxidized Chromite-iron Ore Pellets by Coal

Abstract

A comparative study on coal-based solid state reduction of preoxidized chromite pellets and chromite-iron ore pellets was conducted to investigate the reduction behavior of these two types of pellets. And the enhancement mechanism of chromite reduction reactions due to the pre-oxidation process of chromite with magnetite was revealed by means of optical microscopy, SEM with EDS. The results show the presence of iron ore and pre-oxidation process both are advantageous to the reduction of chromite. Pre-oxidation of chromite and iron ore mixture at 1323 K for 15 min contributes to more formation of sesquioxide solid solution (Fe, Cr, Al)₂O₃ than pure chromite oxidation. Massive cations exchange and diffusion occur between chromite and iron ore, which normally cause the decrease of magnesium content and aluminum content and the increase of iron content in spinel solid solution. Accordingly, the chromite reduction is enhanced with the metallization rate of chromite raised from 20.83% in prereduced chromite pellets to 47.98% in prereduced composite pellets at temperature of 1473 K for 90 min and C/O ratio=1.06, but no further improvement is observed above 1473 K. The mechanism study based on carbothemic reduction thermodynamics and the microstructure of prereduced pellets reveals the reduction of sesquioxide solid solution (Fe, Cr, Al)₂O₃ is more thermodynamically supported and the high iron, low magnesium and aluminum spinel solid solution formed in pre-oxidation process is also favorable for chromite reduction because of lower outward diffusion resistance of Fe²⁺ to the interface with CO gas.