ISSN print edition: 0366-6352 ISSN electronic edition: 1336-9075 Registr. No.: MK SR 9/7 Published monthly

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)