SynopsisThe rate of reduction of hematite and magnetite pellets containing coal char in a nitrogen atmosphere and at the temperatures between 800 °C and 1200 °C was investigated. The reduction rate was found to increase with increasing temperatures, and increasing calcium oxide content of the pellets. The reduction was not stepwise. That is, iron, hematite, magnetite and wustite were detected by X-ray analysis as coexisting phases during early stages of the reduction process. Large temperature differences were found between the core and surface o f the pellets, indicating that the reduction process occurred under non-isothermal conditions.Calculated values for the apparent activation energy ranged between 38 kcal/mol to 51 kcal/mol. These values are lower than those reported by previous workers, but still within the accepted range for chemically controlled processes. The rate of the carbon gasification reaction and the rate of heat transfer from the furnace wall to the reacting pellet were found to exert the strongest influence on the overall rate of the process. Considering the observed non-isothermal conditions, the highly endotermic nature of the carbon gasification reaction and the large increase in the reduction rate with increasing temperature, it appeared that the overall process was heat transfer controlled.

The structural changes that occurred during the reduction of hematite and magnetite pellets containing coal char, in the temperature range between 900 °C and 1200 °C, were studied. The behavior of the pellets was complex. Significant temperature gradients within the pellet were attained during the process, and the reduction was accompanied by catastrophic swelling of the pellets between 900 °C and 1 000 °C, while shrinkage was observed at 1 100 and 1200 °C. Observed microstructural features such as intragranular porosity and cracking of oxide grains resulted from severe lattice disturbances occurring upon transformation of hematite to magnetite. The catastrophic swelling observed for hematite and magnetite pellets 900 and 1 000 °C was due to the filamentary or whisker growth of iron outward from the wustite surfaces. The whisker formation appears to be due to a changing reducing potential of the gas phase within the pellet coupled with the presence of calcium oxide on the wustite surface. The shrinkage reported at 1 100 and 1200 °C is accounted for in terms of sintering of iron filaments. The pellets strength was found to decrease between 900 °C and 1 000 °C, and increased at higher temperatures. The weakness at low temperatures was due to the absence of bonding between iron filaments or whiskers. The high strength obtained at higher temperatures (1100 and 1200 °C) was produced by sintering of iron filaments.

SynopsisThe rate o f dissolution of sponge iron into molten steel has been studied. Reduced pellets from Midrex and HyL processes and pellets containing coal char were used. The dissolution of the pellets is accompanied by continuous gas evolution which is due mainly to the reduction of iron oxides remaining in the pellets by the carbon within the system. It was found that HyL pellets dissolved faster than Midrex and Midrex pellets dissolved faster than Sartenejas. The rate of heat transfer from the bath to the pellet was found to increase with increasing gas evolution. The melting process was simulated by a mathematical model by assuming heat transfer control. This model allowed the calculation of the heat transfer coefficient as a function of the instantaneous radius of the pellet.

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