Liquidus Temperature Depression in Cryolitic Melts

The electrolyte in Hall-Héroult cells for the manufacture of primary aluminum nominally contains only cryolite (Na₃AlF₆) with additions of AlF₃, CaF₂, and Al₂O₃. However, impurities are present, entering the process with the feedstock. The effect on the liquidus temperature by the impurities cannot be calculated correctly by the well-known equation for freezing-point depression in binary systems simply because the electrolyte cannot be regarded as a binary system. By extending the equation for freezing-point depression to the ternary system NaF-AlF₃-B, it appeared that the acidity of the impurity B plays a major role. Some calculations were made using an ideal Temkin model, and for most types of impurities, the effect on the liquidus temperature will be larger in an industrial electrolyte than what can be estimated from the equation for freezing-point depression in cryolite. Experimental data on the liquidus temperature in the system Na₃AlF₆-AlF₃-Al₂O₃-CaF₂-MgF₂ show that the effect of MgF₂ on the liquidus temperature increases strongly with decreasing NaF/AlF₃ molar ratio, and it is suggested that MgF₂ forms an anion complex, probably MgF ₄ ²⁻ .