The reduction of small, dense magnetite samples in H2/N2 and H2/H20 gas mixtures has been investigated at temperatures between 450 and 1100°C. In H2/N2 gas mixtures hydrogen partial pressures in the range from 50 to 750 mmHg were utilized, whilst in the H2/H20 system ratios corresponding to reduction potentials ranging from 2 to 40 kJ / mol H2 were used. The reduction reactions have been followed using both direct in situ observations and a new technique involving the rapid quenching of partially reduced samples. Microstructural examinations of fractured and polished cross-sections were made using optical and scanning electron microscopy. Measurements of the surf ace and internal kinetics have been made from these observations.
Examination of partially reduced magnetite samples has shown that one of two initial product microstructures may occur:
(i) porous iron type morphologies - where the oxide is completely reduced to iron metal as the pore tips advance into the bulk; and
(ii) porous oxide/dense iron or 'tunnel' type morphologies where much coarser structures occur but only a thin dense iron layer is formed on the pore walls as the tips advance.
The conditions under which the structures are formed vary considerably, depending upon both the reaction temperature and gas composition. These primary reduction products may be subsequently modified by additional transformations:
(i) at temperatures below 600°C large radial fissures form in the individual iron nuclei;
(ii) under most conditions where porous oxide/dense iron type morphologies are formed large cracks approximately parallel to the main reaction front occur; and
(iii) at high temperatures breakdown of the dense iron layers on the walls of the tunnels results in secondary porous iron growth.
Comparison of the present observations on the reduction of magnetite with earlier work on the reduction of wiistite indicates that many aspects of the reduction of these two oxides are similar. A detailed analysis of these findings has shown that each stage of the reduction is dependent upon the relative rates of the various elementary chemical reaction and mass transport sub-processes. Earlier models of the initial stages of wiistite reduction have been considered in more detail and shown to be also applicable to the reduction of magnetite. This treatment has been extended to include a model for porous iron growth and the transition to porous oxide/dense iron type morphologies. A mechanism for the formation of large cracks where porous oxide/dense iron structures occur is also proposed. The reaction kinetics are discussed with reference to the observed product morphologies.