Is the in-situ ZnAl2O4 formation an alternative for magnesia-alumina spinel zero-carbon shaped refractories?

The ever-increasing demand for cleaner steels is the driving force for developing zero-carbon bricks. These refractories have some advantages such as lower thermal conductivity and no direct CO2 emissions, reducing thermal losses and environmental impacts related to their use. Aiming at enhancing some properties of these refractories, the addition of spinel-like inducers (MgO or ZnO) was compared. These oxides react with alumina forming MgAl2O4 and ZnAl2O4, respectively. The spinel formation of the former phase is well-known to be a counter-diffusional solid-state reaction, in which the Kirkendall effect leads to residual porosity. This work assessed the diffusion mechanisms in the Al2O3–MgO and Al2O3–ZnO systems, addressing the role played by the Kirkendall effect on the physical and thermomechanical properties of shaped refractory. Supported by XRD analyses, the early gahnite formation, which starts close to 800 °C and is fully completed at 1200 °C, was attested. This fast spinelization, the thermal expansion mismatch between ZnAl2O4 and Al2O3, and the residual porosity due to the Kirkendall effect led to a higher thermal shock damage resistance. Further analysis of the microstructural features highlighted that a homogeneous sintering reaction was carried out between the MgO and Al2O3 oxides. However, for the ZnO-containing composition, substantial voids were present between the matrix and aggregates due to the Kirkendall effect, inhibiting the sample densification. Additionally, a coating was formed at the alumina-coarse boundaries, indicating a high ZnO diffusion from the matrix to the aggregates. This resulted in higher porosity, and thus diminishing the mechanical strength, which could not be suitable for the working linings of steel ladles. However, controlled generation of residual porosity enhanced the thermal shock damage resistance. Moreover, the addition of ZnO can help to anticipate the in-situ spinalization for applications in the aluminum industry or to produce pre-reacted spinel-like ZnAl2O4 raw materials.