Abstract
This investigation is concerned primarily with the volume and structural changes which arose from the CO-CO[2] reduction of relatively pure hematite pellets to magnetite in the temperature range 350°C-1100°C. Hematites of natural and chemically prepared origins were involved in both single and sintered polycrystalline form. The apparent volume expansion of a micaceous hematite as measured by mercury displacement, camera dilatometer and by micrometer was related to the observed reduction zone width, compact porosity, grain size and magnetite morphology. A macroscopic model of swelling was proposed which related the emergence of a cooling tower sample shape to the decrease in reduction zone width with rise in temperature. The variation in volume expansion with pellet porosity was attributed to the effects of a transition in the sintered pore structure. The lower apparent volume expansion experienced for H[2]-H[2]O reduction compared with CO-CO[2] was related to the difference in reduction zone width and magnetite microstructure. A detailed microstructural examination of fully and partially reduced micaceous samples revealed a spectrum of magnetite morphologies. The morphologies included a low temperature porous multi-crystallite growth, an intermediate and high temperature lamellar growth and a high temperature dense nodular growth. The surface relief, interphase boundary appearance and the growth characteristics of the various morphologies were shown to be in broad agreement with the precipitation behaviour in solid state metallic systems in particular the separation of ?-ferrite from ?-austenite in plain carbon steels. The mechanisms of expansion and cracking were based upon the observed surface relief and the anticipated ionic movement associated with diffusional-shear and purely reconstructive transformations. The formation of porosity and fissures in magnetite was accounted for in terms of the cation vacancy diffusion gradient, the relative magnitude of the gas-phase boundary reaction rate, the cation vacancy diffusion rate and oxide plasticity. Calculations of the relative volumes of hematite and magnetite taking into account the differences in thermal expansion and published deviations from stoichiometry of magnetite at temperature suggested that the normally accepted theoretical volume contraction on reduction to magnetite may turn into a small expansion. This was particularly so for decomposition of hematite above 1300°C. A single crystal X-ray diffraction study of natural and synthetic crystals confirmed the presence of the porous crystallite and lamellar morphologies for reduction at 550°C and 1000°C respectively and also the surface relief effects and associated cracking. In addition the conventional (0001)[H] // (111)[M] orientation relationships and habit was confiimed for the lamellar structure. No unique orientation was found for the low temperature porous crystallite morphology. An X-ray line broadening investigation of the micaceous hematite reduced to magnetite was undertaken. The trends in particle size and strain broadening with reduction temperature was in agreement with the variation in magnetite microstructure as evaluated by conventional optical microscopy and S. E. M. Similar growth behaviour was experienced for reduction of a spectroscopically pure hematite and a laboratory grade hematite to magnetite. The distinguishing feature was the appearance of a columnar grain magnetite structure above 850°C which was related to the high solid solution purity and the associated stress relieving capability of the magnetites.