Chemical reactions and mineral formation by sodium aluminium fluoride attack on anorthite based refractories

摘要

The present thesis investigates the use of anorthite as a barrier layer for aluminium electrolysis cells. The system NaF-CaF(sub 2)-NaAlSiO(sub 4)-CaAl(sub 2)Si(sub 2)O(sub 8) is relevant to chemical reactions that take place in anorthite based refractory materials used in the bottom lining of aluminium electrolysis cells. The phase relations in this system have been investigated by means of differential thermal analysis, quenching experiments, X-ray diffraction analysis and optical and electron microscopy. A phase diagram of the system has been constructed, based on the experimental results. The different co-ordination chemistry in typical ionic melts and the tetrahedrally co-ordinated silica based network is presented as a plausible explanation for the miscibility gap. A statistical thermodynamic model, the ''cell model'', was applied to the system. The ''cell model'' describes the structure of the melt as a cationic and an anionic sub-lattice with no vacancies. The simple model was not adequate to describe these complex oxy-fluoride melts. Some improvements of the models have been suggested. The refractory bottom linings of two industrial aluminium electrolysis cells have been investigated. The cells were lined with a refractory powder material based on the mineral anorthite (CaAl(sub 2)Si(sub 2)O(sub 8)). The autopsies of these two cells constituted visual observations, X-ray diffraction analysis and chemical analysis of the spent pot linings. The powder material has been evaluated, and the present results show that only a small amount of liquid material was present in the anorthite based bottom linings at the operating temperature (<950(sup o)C). The penetrating material has reacted with the barrier powder as expected. The observations were in accordance with the measured phase diagram. The investigation is to be very important for understanding the degradation mechanism of insulation linings of electrolysis cells. 81 refs., 45 figs., 32 tabs.