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Cryoscopy in the System Na3AlF6-Fe2O3

F. Šimko and V. Daněk

Institute of Inorganic Chemistry, Slovak Academy of Sciences, SK-842 36 Bratislava


Abstract: Iron(III) oxide is regarded as impurity introduced into the aluminium reduction cells together with alumina. Reactions taking place in the melt at the dissolution of Fe2O3 in molten cryolite were studied using the cryoscopic method. Three different reaction schemes were taken into account, leading to the formation of Na3FeF6 and Na3Al2OF6, and/or Na2Al2O2F4. The cryoscopic experiments were supplemented by the calculation of equilibrium composition using material balance.From the results of the cryoscopic measurements it follows that the number of new species is close to 5, which indicates that Fe2O3 reacts with cryolite under formation of Na3FeF6 and Na2Al2OF6. Next addition of Fe2O3 enables also the formation of Na2Al2O2F4, the presence of which was confirmed by the IR spectroscopic analysis of quenched melts.The Na3AlF6 liquidus curve in the phase diagram of the system Na3AlF6-Fe2O3 shows an inflex point, which is characteristic of reciprocal systems and indicates that some chemical reaction takes place in the melt. The coordinates of the eutectic point are 1.06 mole % Fe2O3 and 1002.7°C. The chemical nature of the substance crystallizing on the right side of the phase diagram could not be identified due to its very low concentration in the quenched samples. However, according to the result of the IR spectroscopy, it could be probably Na2Al2O2F4.

Full paper in Portable Document Format: 555a269.pdf


Chemical Papers 55 (5) 269–272 (2001)

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