Preparation of MgO-NiFe2O4-TiO2 materials and their corrosion in Na3AlF6-AlF3-K3AlF6 bath

The development of new electrolysis technology in aluminum industry requires a new generation of refractory materials to replace the Si3N4 bonded SiC refractories, which are currently used widely as sidewall materials in Hall-Héroult cells. In the present paper, the MgO based materials are considered as potential candidates for this type of application and are prepared using fused magnesia, pre-synthesized nickel ferrite and anatase powder as starting materials. The reaction sintering process of the MgO-NiFe2O4-TiO2 materials is investigated by means of X-ray diffraction (XRD), scanning electron microscope (SEM). All the specimens are corroded in a Na3AlF6-AlF3-K3AlF6 bath to assess the electrolyte corrosion resistance and the results are compared with those for nickel ferrite materials. The results show that reaction sintering occurs in the MgO-NiFe2O4-TiO2 system in the range of 1000-1600 °C. Firstly MgO reacts with TiO2 to produce the Mg2TiO4 phase at 1000 °C, which in turn reacts with NiFe2O4 to form the NiχTi1−χFe2χMg2−2χO4 composite spinel at temperatures above 1000 °C. As a result, the mass transfer and densification in the MgO-NiFe2O4-TiO2 specimens are enhanced. All the specimens are composed of the composite spinel and periclase phases. As the amount of NiFe2O4 added increases, more composite spinel phase precipitates inside MgO grains at high temperatures. The MgO-NiFe2O4-TiO2 refractories prepared exhibit good corrosion resistance to the electrolyte melts. And their corrosion resistance increases with the increase in the NiFe2O4 addition. When the amount of NiFe2O4 added is 30 wt%, the refractory shows a corrosion resistance comparable to that of the pure NiFe2O4 refractory; the reasons for this much improved corrosion resistance are investigated and discussed.