Phase Compatibility, Bonding, and Corrosion Resistance of Gahnite and Spinel Refractories

The feasibility of gahnite (zinc aluminate, ZnAl₂O₄) as an alternative to magnesium aluminate spinel (MgAl₂O₄) in high-temperature refractory applications has been explored. ZnAl₂O₄ is being studied due to its high melting point (1950 °C), comparable thermomechanical properties, similar crystal structure, and complete solid solubility with MgAl₂O₄, enabling partial or complete replacement of MgAl₂O₄ in refractories. MgAl₂O₄ is known to exhibit good phase compatibility with calcium aluminate cement (CAC) phases such as hibonite (CaAl₁₂O₁₉, CA₆), which is linked to excellent thermomechanical properties. Phase evolution was studied in the Al₂O₃-rich region of the CaO–Al₂O₃–ZnO ternary system to explore solid-state compatibility between ZnAl₂O₄ and CAC phases. The investigation focused on the formation of ternary phases CAZ-I (Ca₂Zn₂Al₂₈O₄₆), and CAZ-II (CaZn₂Al₁₆O₂₇), drawing parallels to the well-established phase relations in the CaO–Al₂O₃–MgO system. The CaO–Al₂O₃–ZnO ternary phase diagram is found to be analogous to the CaO–Al₂O₃–MgO system on the Al₂O₃-rich side. Formation of CAZ phases using CAC in ZnAl₂O₄-Al₂O₃ castable formulation was investigated using various industry-grade aggregates. Specifically, microstructural development in CAC-bonded ZnAl₂O₄–Al₂O₃ castables was studied, with focus on spatial distribution of acicular phases in matrix and aggregate–matrix interfaces.

Corrosion resistance was evaluated using static cup tests against industry-representative steelmaking slags. A comparative study between ZnAl₂O₄ and MgAl₂O₄ was conducted to examine the influence of slag chemistry, with focus on iron oxide content and basicity—two key factors affecting refractory corrosion. In addition to slag corrosion, resistance to alkali vapor attack was assessed using ASTM C987, simulating vapor-phase corrosion in cement kilns, glass furnaces, and gasifiers. Results reveal distinct differences in corrosion response, highlighting the chemical stability of ZnAl₂O₄ under various slag chemistries.

To complement the experimental findings, computational fluid dynamics (CFD) simulations were used to model convective slag flow in the static cup setup, offering a mechanistic understanding of how thermal and fluid dynamics contribute to refractory wear.

The results support ZnAl₂O₄ as a viable alternative to MgAl₂O₄, either as a partial or complete replacement in refractory castables, with demonstrated compatibility with cement, and superior corrosion resistance to molten slags and alkali vapor.