Liquidus Curves for the Cryolite — AlF₃ — CaF₂ — Al₂O₃ System in Aluminum Cell Electrolytes

Abstract

Measurements of cryolitic electrolyte freezing points in the quaternary system Na₃AlF₆ - AlF₃ - CaF₂ - A1₂O₃ were conducted in order to resolve some of the apparent discrepancies reported by previous investigators. Rather than investigating just the boundaries of the quaternary system, a large number of tests were conducted in the “interior” of the defined system. This was done so that the results could be used to evaluate the freezing point of traditional bath chemistries. The freezing points of synthetic electrolyte samples were combined with accepted literature values for several of the binary boundaries to create a total data set covering the experimental range of 0 – 22% excess AlF₃ (CR = NaF/AlF₃ weight ratio = 1.50 to 0.90), 0 – 12% CaF₂, and 0 – 5% A1₂O₃. Five planes (one component held constant) in the experimental space were examined and fitted with isopleths for the electrolyte liquidus temperature. From the shape of the isopleths and using the complete data set, a single model for predicting freezing point temperatures was developed to cover the entire experimental range for the Na₃AlF₆ - AlF₃ - CaF₂ - Al₂O₃ electrolyte chemistry system.