Barrier layer against fluoride diffusion in linings of aluminum

Abstract

A barrier layer for prevention of solid fluorides penetration, e.g. NaF, from the interior of an aluminum production cell holding a fluoride-containing melt into a refractory layer intended to provide heat and electrical insulation for the cell. The barrier layer contains CaO, which may be derived from a CaO precursor such as calcite (CaCO.sub.2) by decomposition during cell start-up. Since CaO does not appear to react with sodium fluoride (NaF), and indeed according to thermodynamic calculations seems incapable of doing so at cell operating temperatures, it is effective for preventing the penetration of that compound when the latter is in the solid state. The barrier layer makes it possible to use insulating low cost refractories which are attacked by solid fluorides in the cell wall lining with improvement of the life of such materials. The barrier layer also keeps fluorides out of the refractory materials so that these materials can be disposed of in conventional land-fill sites without risk of causing pollution due to leaching of fluoride salts.