Role of oxide additives in the synthesis and sintering of magnesium aluminate spinel*

Magnesia-alumina spinel bricks have already been successfully used as a replacement for magnesia-chrome bricks in iron and steel furnaces as well as in cement and rotary lime kilns. In this regard, magnesium aluminate spinel may be a potential candidate even in non-ferrous industries that need highly basic refractories, albeit with some concerns that need to be addressed. The major concern is the increased energy demands for the formation and sintering of magnesium aluminate spinel as it requires at least two firings. The formation of MgAl₂O₄ from the oxides magnesia and alumina is through a diffusion-controlled reaction accompanied by volume expansion (∼8 %) rendering it difficult to synthesize and sinter in a single firing. The need for firing at high temperature for the synthesis of magnesia-spinel before sintering results in grain growth and grinding steps slowing down the production process, whereas multiple low temperature calcinations result in higher energy demands. The use of sub-micron powders, additives and precursors for nano-particles may result in lower sintering temperature and uniform microstructure. In order to achieve better bonding of the phases, novel methods of synthesis and sintering of spinel need to be developed in order to obtain microstructures similar to the precipitated magnesium chromite spinel in direct bonded, re-bonded and fused grain magnesia-chrome bricks. These precipitated spinels have a partial inversion in the spinel structure making them more stable at high temperatures than either normal or inverse spinel. Additives like ZnO improve densificaron, TiO₂ decreases the synthesis temperature and SnO₂ increases the thermodynamic stability of magnesium aluminate spinel. Fine zirconia particles are also known to retard the grain growth of spinel, but they remain as a separate phase. The F*A*C*T Sage software and thermodynamic database was used to understand the role these additives would have in the formation of spinel. In the present study, magnesia and alumina powders along with bivalent and tetravalent oxide additives were analyzed for their role in activating spinel synthesis and sintering at relatively low temperatures, in order to achieve a better binding system for magnesia-based refractories. These solid solution formers result In improvement of the bonding mainly as sub-micron or nano-size particles, retard the grain growth during sintering, decrease the diffusion mean-free path for spinel formation and increase the solid solubility at elevated temperatures, followed by precipitation of secondary spinel phases, similar to the complex spinel in magnesia-chrome refractories. The diffusion of activating cations through the magnesia-spinel and spinel-spinel interfaces was studied using the peak shift data of the X-ray diffraction patterns and correlated with the changes in density in a single step firing process.