UBC Theses and Dissertations
Sintering studies of magnesia-chromite refractory composites Zargar, Hamidreza
The magnesia-chromite refractory composites are the best candidates for the lining of non-ferrous metal smelting and refining furnaces, due to their high melting temperature, chemical inertness, and excellent thermal shock resistance. However, their high sintering temperatures (>1700°C) increase the processing complexity and costs. In this investigation, the primary goal was to study the sintering of these composites, with the long-term engineering goal to reduce their sintering temperature to <1500°C in air. The materials were synthesized via chemical methods using nitrates to assure chemical homogeneity. Al₂O₃, Fe₂O₃, and SnO₂ affected the kinetics of the MgCr₂O₄ formation and increased the density of MgCr₂O₄ in the order Fe₂O₃>Al₂O₃>SnO₂. The enhanced MgCr₂O₄ densification was attributed to the cation distribution in spinel structure (inversion phenomenon), caused by the inherent affinity of Fe⁺³ and Al⁺³ to tetrahedral sites. Fe₂O₃ and Al₂O₃ showed to form inverted spinel, while SnO₂ resulted in the formation of normal spinel solid solutions. Twelve magnesia-chromite composites were synthesized to study the effects of Al₂O₃, Fe₂O₃ and Cr₂O₃ on their sintering conditions; Cr₂O₃ decreased the density, while Fe₂O₃ and Al₂O₃ enhanced the densification of composites. The microstructural studies revealed that Fe₂O₃ and Al₂O₃ reduced the dihedral angle between MgO and spinel, while Cr₂O₃ increased it. The increased densification by Fe₂O₃ and Al₂O₃ was attributed to the decreased dihedral angle and formation of inverted solid solutions. The optimized composition [MgO6.9Cr₂O₃6.9Al₂O₃2.7Fe₂O₃]mol% (MK) reached nearly full density in air at 1475°C for 70minutes; 1700°C is currently used for magnesia-chromite refractories. In order to study the effects of the particle size on densification, magnesia-chromite composites (NMK) with average particle size of ~20 nm were synthesized via Pechini's method. Reducing the particle size from 1.2 um for MK to 20 nm for NMK reduced the onset sintering temperature by 200°C to 1000°C. The densification results were evaluated using master sintering curve theory for the first time for this kind of composites. The sintering activation energy was 443.7 and 302.6 kJ/mol for MK and NMK respectively. It was hypothesized that the oxygen diffusion through lattice and grain boundaries was rate controlling mechanism for MK and NMK respectively.
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