Reactivity of carbon cathode materials with electrolyte based on plant and laboratory data

Abstract

During the aluminium electrolysis process, pot linings determine the life span of the cell. As a result there has been considerable and ongoing research internationally on the penetrating chemical species and phases that form in the carbon cathode lining of the cell. The objective of the study is to identify phases that penetrate, react and cause expansion of carbon cathode during the electrolytic production of aluminium. The study relied upon post-mortem carbon cathode samples from South African aluminium smelting plant and the development of a laboratory scale aluminium smelting cell. Samples were analysed using qualitative and quantitative x-ray powder diffraction and scanning electron microscopy, with energy dispersive spectroscopy. A sodium expansion test was done according to a test developed at the Council of Scientific and Industrial Research (CSIR), South Africa, based on the Samoilenko Rapoport method. The following phases were identified in the post mortem carbon cathodes: Cryolite (Na3AlF<sub6), Villiaumite (NaF), Fluorite (CaF2), Alumina (Al2O3), Chiolite (Na5Al3F14), Al-Nitride (AlN), Sodium-Cyanide (NaCN) and beta Diaoyudaoite (NaAl11O17). Cryolite (Na3AlF6), Villiaumite (NaF), Fluorite (CaF2) and beta Diaoyudaoite(NaAl11O17) could be distinguished in the 30% and 100% graphitised carbon cathode samples after the laboratory scale electrolysis process. Chiolite (Na5Al3F14) could only be distinguished in the 100% graphitised carbon cathode sample after the laboratory scale electrolysis test. Wear due to reaction between sodium, the cathode, cryolite and nitrogen could not be confirmed in the laboratory scale samples, but could be inferred in the post mortem samples, according to the following reactions: 3/2N2(g) + 3C(s) + 3Na(in C) = 3NaCN(l) 1/2N2(g) + Na3AlF6(l) + 3Na(in C) = AlN(s) + 6NaF(l) Sodium in the cathode was presumably present in the pores of the cathode, as the presence of intercalated sodium could not be confirmed by XRD analysis on the laboratory scale samples. The electrolyte penetration depends on the degree of graphitisation (heat treatment temperature) of the carbon cathode. Carbon cathode that are heat treated at higher temperature (3000°C) has more graphitised and low porous structure as compared to the carbon cathode heat treated al low temperature (1200°). Therefore the 30% graphitised carbon cathode is more vulnerable to electrolyte penetration than the 100% graphitised carbon cathode. The laboratory scale sodium expansion test confirmed that the degree of expansion of the carbon cathode decreases as the degree of graphitisation (30% graphitised > 100% graphitised) of the carbon cathode increases. Copyright