Mass Transfer at the Cathode in Alumina Reduction Cells: Behavior of Different Structure Models for NaF-AlF 3 Melts

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THE electrolyte in cells for production of primary aluminum contains sodium fluoride and aluminum fluoride with additions of alumina (2 to 5 wt pct) and calcium fluoride (3 to 7 wt pct). Typically, the NaF/ AlF3 molar ratio (r) is about 2.3 in industrial cells. In the molten state, the system NaF-AlF3 contains Na+ as the only cation, F, and a number of fluoro-aluminate complex anions. The ‘‘classic’’ structure model comprises F, AlF63, and AlF4. In addition, Al2F7 was suggested in order to explain that the system can be in the molten state at r < 1.[1] Although the existence of the mentioned ions appears to be generally accepted, the structure of the melt is still controversial, and a number of other structural entities have been suggested.[2,3] Particularly, the existence and concentration of AlF52 have been much disputed. Knowledge of the melt structure and the concentration of the different species may be important in the understanding and interpretation of physical and chemical data, such as density, conductivity, and heat of mixing. Also, it may be of importance to know which species are involved in electrochemical reactions and in

the different mechanisms for loss in current efficiency in aluminum electrolysis cells.[4] The behavior of one possible structure model in the diffusion layer at the cathode at different NaF/AlF3 molar ratios was reported earlier.[5] The objective of the current study is to expand this treatment to three other structure models. The motivation was to obtain better knowledge about the motion of the different ionic entities in the cathodic diffusion layer, and to reveal any systematic differences giving reason to prefer or avoid any structure model in further studies of the cathode reaction and the loss in current efficiency. The models examined all contain the four anions F, AlF63, AlF4, and Al2F7. In addition, a fifth species was assumed to be present; this was AlF52 (Model I), Al2F104 (Model II), Al2F92 (Model III), or Al3F145 (Model IV).

II.

DERIVATION AND DESCRIPTION OF STRUCTURE MODELS

A. Thermodynamic Modeling Based on formal activity data for NaF and AlF3, the activity (a) of any species NaaAlbF(a+3b) can be calculated by aNaa Alb Fðaþ3bÞ ðlÞ ¼ K  aaNaFðlÞ  abAlF3 ðsÞ ;

ASBJØRN SOLHEIM, Chief Scientist, is with SINTEF Materials and Chemistry, P.O. Box 4760 Sluppen, 7465 Trondheim, Norway. Contact e-mail: [email protected] Manuscript submitted November 13, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B

½1

where K is the equilibrium constant. The standard state is the liquid compound NaaAlbF(a+3b). When building up a structure model, a natural way is to adjust the equilibrium constant for the compound j in such a way that, at the melt composition corresponding to the stoichiometry of compound j, the numerical

values of the activities of all compounds i included in the model satisfy the criterion: n X ai ¼ 1 ½2

where n is the number of ions, and the subscripts ‘‘+’’ and ‘‘’’ represent cations and anions, respectively. The activity of, e.g., Na3AlF6 is then