Abstract
The investigation was conducted with four iron ores which, after grinding and compaction, were sintered at elevated temperatures to give constant porosity. The rates of reduction were determined at 550, 750 and 1050°C, using hydrogen/argon, hydrogen/steam and hydrogen atmosphere. The results from the last series of tests were treated so as to determine, apparent activation energies. Reduction was also effected isothermally in controlled hydrogen/steam reduction atmospheres, to each of the lower iron oxides and iron. The changes in specimen volume, microstructure and compressive strength at temperature, resulting after each reduction step, were determined over the temperature range 550-1050 °C. The three high grade ores underwent volume increases on reduction to magnetite, the magnitude of which was dependent on the reduction temperature. Associated with the volume change was a large decrease in the compressive strength. Microscopic examination revealed that the magnetite growth pattern changed as the reduction temperature was increased, and that reduction was accompanied by serious cracking, with only a slight tendency for the formation of porous magnetite. Reduction to wustite and iron of Stripa and Kiruna ores was accompanied by a decrease in the volume expansion compared with reduction to magnetite, whilst Conakry ore underwent greatly increased expansion. The compressive strength was decreased because of the inherent weakness of both wustite and iron, which readily deformed plastically at moderate temperatures. The low grade ore, Oxfordshire, differed from the high grade ores in that the iron-bearing materials were calcium ferrites. The specimens after all reduction stages showed a contraction. The reduced specimens suffered a less significant decrease in the compressive strength, due both to negligible cracking and the presence of a large volume of relatively strong slag reinforcing the weak plastic wustite and iron. The probable influence of the structural and strength changes during reduction of iron agglomerates in the blast furnace has been suggested.