Modeling of Equilibria in Complex Cryolite Melts

Summary.

In the fully ionized sodium sulfate melt, the solubility of an oxide (at a given melt basicity) is well described by considering a single equilibrium to form a specific simple acidic or basic solute. In this case, the dominant solutes are identified by a simple log–log interpretation describing the dependence of the solute concentration on an acid-base parameter ($\log a_{\rm Na_2 O}$). However, in fused cryolite (Na₃AlF₆) solutions, the solvent itself and the solutes of oxides involve anionic complexes (alumino-fluorides and oxy-fluorides). Therefore, a single simple equilibrium does not suffice to model the complex solution. Rather, as for a high temperature multi-component gas phase containing many complex volatile species, every possible equilibrium must be individually satisfied and coupled to a mass balance. Examples will be given for each limiting type of solution behavior.