Penetration and chemical reactions in carbon cathodes during aluminum electrolysis: part i. laboratory experiments

Abstract

Penetration of metallic sodium and salt melt into cathodic carbon materials was studied in a laboratory aluminum electrolysis cell. Variables were cryolite ratio, current density, N2 or Ar atmosphere, degree of graphitization, and strain on the carbon samples. The carbon sample was analyzed by optical and electron microscopy, ash analysis, sodium detection by phenolphtalein, density, and X-ray diffraction (XRD). The developed XRD technique proved especially useful and allowed quantitative determination of concentration profiles for the crystallized compounds. Sodium was found to be the primary penetration agent and its velocity and saturation concentration increased with increased cryolite ratio of the melt and decreased graphitization of the carbon material. Sodium was found to be important as a wetting agent facilitating melt penetration. Al4C3 was not present within the pores of the carbon material. Melt penetration was enhanced by polarization and formation of nitrogen compounds. NaCN was formed directly from the elements and was destabilized by the advancing melt. In nitrogen atmosphere, A1N was the major nitrogen component and was formed within the melt phase.