Studies on synthesis and characterization of Magnesia based aggregates derived from Indian Magnesite

Abstract

Natural magnesite is the primary source for the production of magnesia based refractory materials. India has vast deposit of magnesite in Salem and Almora region. However due to the presence of large amount of impurities which form low melting phases on sintering, high temperature applications of Indian magnesite are restricted. In this background, an attempt has been made to synthesize and characterize magnesite based refractory aggregates and to improve its mechanical & thermo-mechanical properties using different methodologies. Natural magnesite of Salem region was used as the starting material. Raw magnesite was characterized in terms of chemical analysis, differential thermal analysis, thermo gravimetric analysis, phase assemblage and Infrared spectroscopy. The sintering temperature range across methodologies was chosen to be 1550-1700 °C. All sintered samples were characterized in terms of densification, flexural strength, thermal shock resistance, phase assemblage and microstructural evolution. In the first methodology, lime/silica ratio of the system was changed by adding CaCO3 in the raw magnesite. It was observed that when the lime/silica ratio was modified into 1:1, only low melting monticellite phase was formed as a secondary phase. However, when the ratio was changed into 2:1, dicalcium silicate phase was formed which was high melting in nature. This helped in improvement of thermo-mechanical properties. In the second methodology, 1 to 5 wt % zirconia was added to the raw magnesite to reduce the low melting phase formation at high temperatures. It was found that addition of zirconia minimized the formation of low melting monticellite phase as zirconia utilized one of the impurities (lime) to stabilize its tetragonal form at room temperature. As the low melting phase formation was reduced, high temperature mechanical properties were improved. In the third methodology, magnesium aluminate spinel samples were prepared using the raw magnesite and calcined alumina in one stage sintering. Developed spinel aggregates showed good mechanical and thermo-mechanical properties. Quantification of the crystalline phases showed that the spinel content increased with sintering temperature. In the fourth and final methodology, the raw magnesite was beneficiated using reverse froth flotation technique. Pine oil was used as frother, Flotigam EDA as collector and sodium hexametaphosphate as depressant of carbonate groups. Various process parameters were optimized using Response Surface Methodology (RSM). A quadratic model equation was formulated for the experiment. The effect of different process parameters was analyzed using Box-Behnken Design. The model was significant as suggested by statistical analysis. Optimum process conditions were predicted by RSM after analyzing the experimental data. The beneficiated sample for which the highest silica in froth was achieved was characterized in terms of X-ray diffraction analysis. Quantification of the crystalline phases showed that amount of quartz was reduced from 2.3 % in the raw magnesite to 1.8 % in the processed magnesite. A new experiment was carried out using optimized values of process parameters. The results of the new experiment and the software predicted data are in good agreement with each other.