Fabrication of spinel composites for sidewall of Al Smelter application

A frozen layer of electrolyte is needed to protect the sidewall materials of Al Smelter in the Hall-Heroult process from corrosion. About 35 % of the heat loss is required to form the ledge and therefore, ledge-free sidewall is preferred as it potentially reduces the energy requirement of aluminium production. Potential materials for the ledge-free sidewall, namely NiFe 2O4, NiAl2O4 and MgAl 2O4 and their combinations have been identified in the previous work. The present work focused on the thermodynamic assessment of the materials in molten aluminium; and fabrication and joining of the materials. Thermodynamic analysis of the NixMg(1-x)Al 2O4 spinels in molten aluminium were discussed. The equilibrium calculation of the NixMg(1-x)Al 2O4 spinels in molten aluminium at 960 °C predict that the concentration of dissolved Mg in the aluminium melts are lower compared to Ni concentration for all the NixMg(1-x)Al 2O4 being studied. In addition, the concentration of Ni in the aluminium melts in contact with the Ni0.5Mg0.5Al 2O4 was about two times lower than the melts in contact with pure NiAl2O4, while the concentration of Mg was slightly decreased from 0.076 wt. % in the molten Al - pure MgAl 2O4 system to about 0.071 wt. % in the molten Al - Ni 0.5Mg0.5Al2O4 system. Experiments on joining the samples using reaction sintering method were also conducted. X-Ray Diffraction, Scanning Electron Microscopy and Energy Dispersive Spectroscopy methods were used to characterize the samples. It was found that crack occurred on the interface of the joined NiFe2O4-MgAl 2O4 and NiAl2O4-MgAl 2O4 specimen. It has been shown that the crack can be minimised by the implementing intermediates in between the two materials. These intermediates were solid solutions of the spinels produced from mixing (with different compositions) the two materials being joined.