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To aid development of cost-effective sintered spinel as a refractory raw material, this paper presents an extensive analysis of microstructure and complex phase evolution of Al-rich, Mg-rich, and stoichiometric spinel aggregates derived from Indian magnesite and calcined alumina. Pore morphology in Al-rich spinel was transformed upon sintering at 1650°C and corundum laths embedded in porous Al-rich spinel matrix was formed. Stoichiometric spinel sintered at 1600°C consisted of mostly direct bonded angular equiaxed spinel grains which incorporated the impurities in solid solution. Mg-rich spinel was composed of spinel grains with reduced angularity along with intergranular amorphous phase, small round monticellite grains, and periclase. EDS line scan revealed impurity-free joins existed between direct bonded spinel grains. Mg-rich spinel containing 0.65 wt% ZrO 2 formed cubic ZrO 2 -CaO-MgO solid solution located along spinel boundaries, which reduced both intergranular amorphous phase and monticellite. This increased SiO 2 and MgO content in spinel solid solution triggering exsolution of metastable cubic forsterite manifested as split spinel peaks in XRD pattern. A 14.7% reduction in slag penetration was exhibited upon doping Mg-rich spinel with 0.21% ZrO 2 . Stoichiometric and Mg-rich spinels attained 0.35% and 0.54% apparent porosity at 1600°C, which is better than most commercial sintered refractory spinels. K E Y W O R D S

The Ruhrstahl Heraeus (RH) degasser (Figure 1) is the globally preferred equipment to manufacture high quality vacuum treated steel purified to ppm level of C, H, and N. 1-3 Examples of such steel are ultra-low carbon steel (<30 ppm C) for lightweight automobile applications, high carbon manganese steel (<1.5 ppm H 2 , <40 ppm N 2) for crack resistant railway tracks and electrical steel for transformer cores. Faster operation and the ability to perform gas removal, decarburization, desulphurization, and alloy addition with precise composition control makes RH degassers superior to other metallurgical equipment like the vacuum arc degasser. This paper explores refractories for use in a RH degasser which is used to manufacture steel consumed primarily by the automotive industry, one of the world's largest industries in terms of revenue. The automotive steel market is worth US$