Microstructure and phase evolution of Indian magnesite‐derived MgAl₂O₄ as a function of stoichiometry and ZrO₂ doping

Abstract

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

Designing environment‐friendly chromium‐free Spinel‐Periclase‐Zirconia refractories for Ruhrstahl Heraeus degasser

“…The EDS analysis of the interparticle regions in SP (Figure 11) shows presence of high Si and Ca in the interparticle regions. From the perspective of the erosion‐prone environment of the RH degasser operating up to 1630°C, presence of Ca and Si at interparticle regions can prove dangerous because of formation of eutectic glass and crystalline phases including monticellite (CaMgSiO 4 , T m is 1490°C), 35 merwinite (Ca 3 MgSi 2 O 8 , T m is 1575°C), anorthite (CaAl 2 Si 2 O 8 , T m is 1553°C), and gehlenite (Ca 2 Al 2 SiO 7 , T m is 1593°C) 26 . Liquid formation at interparticle regions would cause particles to slide under load at high temperature, reducing HMOR, and could also cause the refractory to be eroded faster under the force of rapidly moving molten steel.…”

“…Point EDS data in Table shows that the cubic ZrO 2 particles in Z15 contain 6.4 ± 0.5 at% Ca. ZrO 2 is reported as a “lime sponge” in spinel‐ and MgO‐containing refractories 31,35 . This can prevent formation of a low‐melting interparticle film of glass or crystalline phase, which both reduce HMOR, and is a critical factor for RH degasser snorkel and lower vessel performance.…”

“…The MgO‐FeO phase diagram 58 shows complete solid solubility of FeO in MgO, and the melting point decreases with such substitution. Both FeO and Fe 2 O 3 have a significant solubility in spinel 35,59,60 . If Fe is taken up in the MgO solid solution, the viscosity of the infiltrating slag might increase, reducing its penetration into the refractory.…”

“…Like sintered magnesia, sintered spinel (20 µm grain size) was also added in various particle sizes to optimize the thermal shock resistance. The in situ spinel and 22 µm ( d 50 ) sintered spinel powder, because of its smaller size, is expected to enhance corrosion resistance by readily reacting with slag thereby increasing its viscosity locally ( T m of spinel is 2105°C) 26,35 …”

“…The in situ spinel and 22 µm (d 50 ) sintered spinel powder, because of its smaller size, is expected to enhance corrosion resistance by readily reacting with slag thereby increasing its viscosity locally (T m of spinel is 2105°C). 26,35 To enhance HMOR, 27 thermal shock resistance, 28,29 and corrosion resistance, 30,31 2.5, 5, 7.5, 10, and 15 wt% (Z2.5, Z5, Z7.5, Z10, and Z15 in Table 2) fused monoclinic ZrO 2 (CUMI SHARP; d 50 is 6.1 µm) was added into SP while maintaining the 1:1 weight proportion of spinel and periclase. The added monoclinic ZrO 2 may get stabilized into the cubic structure by the MgO raw material or CaO impurity during sintering [36][37][38][39] which may enhance the thermal shock resistance due to thermal expansion mismatch.…”