The degradation mechanism of iron oxides caused by alumina during reduction from magnetite has been studied. The experiments were carried out by reducing briquettes containing various mixtures of commercial magnetite fines and Al₂O₃ in a CO/CO₂ atmosphere at 950°C in thermo-gravimetric equipment. Following reduction, samples were examined using optical microscopy, SEM-EDS and XRD.
The diffusion of Al cations into the magnetite during reduction induces swelling and cracking of the briquettes. The composition of the magnetite phase approaches the composition of hercynite during reduction. Assuming the spinel structure is that of hercynite, reduction to non-stoichiometric wüstite (Fe_0.88O) causes an expansion of over 16%. This transformation causes high tension forces in the magnetite/hercynite phase in the vicinity of the interface with the wüstite which presumably account for the breakdown of the structure during the reduction.