Sintering studies of magnesia-chromite refractory composites

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.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat