The electrolyte used in the Hall process for producing aluminum metal consists basically of a fused mixture of alumina (A1^0^) and cryolite (Na^AlF^). Other ingredients including AFF^, LiF, and CaF^ are added to modify properties such as bath density, electrical conductivity, and freezing temperature. Historically, a variety of experimental techniques has been employed to determine the phase diagrams of systems important to the Hall process. Many of the early experiments were conducted using

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... n containers. This practice resulted in the publication of some inaccurate phase diagrams because of the decomposi tion of many of the fluoride compounds at high temperatures. In more recent years, it has become standard procedure to encapsulate all samples in sealed platinum containers. Quenching and differential thermal analysis (DTA) experiments using samples sealed in the platinum containers have been used to revise some of the early phase diagrams and to investiage new systems. In this paper, the phase diagrams that are known in the Na2AlF^-CaF2"AlF^-Al20^ system are reviewed in a systematic manner, with special emphasis on the Hall cell electrolyte composition region.