The Dissolution of FeO and FeF3 in Cryolite

Abstract

Abstract Theoretical derivation of relations valid for cryoscopic measurements in solvents with dystectic mode of melting (solvents, which undergo at melting a more or less extended thermal dissociation), when also chemical reaction between solvent and solute takes place, is presented. When one of the products of this chemical reaction is identical with the product of thermal dissociation of the solvent, arise the problem of defining the number of “foreign” substances, originating in the reaction. The presented approach shows that the partially dissociated solvent cannot distinguish as foreign substance between the own dissociation product and the originating one and made it easier to determine the nature of the probable chemical reaction. The presented approach is applied in the systems Na3AlF6–FeO and Na3AlF6–FeF3, which are of great importance in the fundamentals of aluminium electrolysis. In the system Na3AlF6–FeO for the dependence of the temperature of primary crystallization of cryolite on x(Na3AlF6) the equation T pc/K = 1014.4 + 267.86x(Na3AlF6) was obtained, which yields for the Stortenberker’s correction factor the value k St = 2.09. This means that dissolving FeO in cryolite two new particles are formed. This can be interpreted by the chemical reaction 2Na3AlF6 + FeO ⇔ Na2FeF4 + Na2Al2OF6 + 2NaF. In the system Na3AlF6–FeF3 for the dependence of the temperature of primary crystallization of cryolite on x(Na3AlF6) the equation T pc/K = 1547.2 − 659.88x(Na3AlF6) + 394.99x 2 (Na3AlF6) was obtained. For the Stortenbeker’s correction factor the value k St = 1.02 was then calculated, which means that one new particle is created when FeF3 is dissolved in cryolite. The most probable reaction is 1.5Na3AlF6 + FeF3 ⇔ Na3FeF6 + 1.5NaAlF4.