Effect of Nanocrystalline Magnesium Aluminate Precursor and Powder Granulometryon Spinellization and Densification to Develop Magnesium Aluminate Spinel Aggregates

Nanocrystalline spinel forming precursor spinel hydrate (PSH), prepared by co-precipitation technique with controlled concentration of mixed salt, pH, temperature and time, was used as additive in calcined alumina-dead burnt magnesia (DBM) admixture to develop spinel (MgAl2O4) aggregates. Powder granulometry, mixing and dispersion of additive PSH were controlled during milling. Three batches of varying particle size distribution with same proportion of additive were used for body making. Firing was conducted in 1500°-1650°C temperature range. True specific gravity (TSG) of alumina, DBM, spinel and individual fired pellets were used to estimate wt% spinel formation based on the derived mass balance equation and subsequently verified by X-ray diffraction (XRD) analysis. Fired properties, i.e. bulk density, apparent porosity, closed porosity, %linear shrinkage and densification rate were measured. XRD was conducted for phase analysis. SEM and HR-TEM were performed to study the grain/ pore morphology and elemental analysis was done to calculate the spinel stoichiometry. PSH was found to have positive effect both in spinellization and densification; the effect gradually decreased with finer particle size distribution and increasing firing temperatures. The maximum fired density achieved for three batches were 89.21, 92.84 and 96.2% ρth, respectively. Based on the requirements of specific powder granulometry, addition of PSH and firing schedule can be used for bulk production of spinel aggregates.