Effect of Coal Addition on Reduction of Iron Oxide Pellets in the Atmosphere of H2 and CO

Abstract

Isothermal reduction of mixture comprising of hematite pellets and anthracite coal was performed at 850 and 1000°C in the atmosphere of H2 and CO. Mass fraction of coal in the mixture was 5, 10 and 15% with respect to total weight of mixture. The off-gas during reduction was measured by Quadruple Mass Spectrometry (QMS), which was used to evaluate the degrees of reduction and gasification in terms of the balance in oxygen and carbon, respectively. The phases in reduced pellets were identified by XRD, and their morphologies were analyzed by SEM-EDS.

Topo-chemical reduction behavior was observed on individual grain of iron oxide. The degrees of reduction and gasification were higher at 1000°C than those at 850°C since both reactions were more activated at higher temperature. Addition of coal improved the reduction by providing additional H2 and CO gas which were produced from gasification reaction.

Thick carbon layer was formed at the surface of reduced pellet at 850°C due to the carbon deposition, and XRD peaks showed the formation of C and Fe3C. Sticking and clustering of pellets were observed at 1000°C, which could be explained by sintering of freshly precipitated iron and formation of low- melting eutectic phase (CaO-SiO2-FeO). Addition of coal cannot eliminate the sticking and clustering. Considerable volume expansion also observed during reduction at 1000°C and the maximum value was obtained at reduction degree between 70 - 80%.

Considering the activation of gasification reaction, gas replacement ratio, utilization efficiency of coal, and time saving for reduction, the optimum parameters from this study was reduction at 1000°C with 5% of coal addition. Swelling and sticking of pellets should be taken into account as it will affect the gas permeability inside the furnace on industrial application.