Durable Materials in Primary Aluminium Production

The cathode wear is one of the main factors limiting the service life of aluminium electrolysis cells. During the last decades, anthracitic carbon has been gradually replaced by the now state-of-the-art graphitized cathode blocks, which allow for higher productivity through amperage increase and energy savings due to increased electrical conductivity. However, the benefits of increasing the amperage and productivity need to be weighted against higher materials costs and an experienced lower wear resistance. In addition, graphite cathode blocks have shown to wear unevenly and up to several cm/year leaving a "W"-shaped or even "WW"-shaped surface pattern due to a higher material wear along the periphery of the cell. DuraMat has determined materials reactions taking place at the carbon cathode in laboratory cells and developed a fundamental understanding of the wear mechanisms based on studying reaction kinetics, conditions that catalyse the reactions, and transport mechanisms that are important for the durability of the materials. In DuraMat, computer simulations comprising macro-scale modelling of the cathode, meso-scale modelling of transport mechanisms and atomistic-scale modelling of the cathode structure and sodium diffusion have been utilized in understanding the wear mechanisms. Although a conceptual understanding of the chemical reactions taking place has been developed, there is still some uncertainties related to the observed and large differences in wear across the carbon cathode surface in industrial cells. The Duramat project is a co-operation between NTNU and SINTEF with close involvement of the industry partners Hydro Aluminium, Elkem Carbon and Søral. Results from the project work have been presented at seminars twice each year for the industry and research partners. The results are also presented at international conferences and published in peer reviewed scientific journals.

Today, the service lifetime for aluminium electrolysis cells is limited by the materials used in the bottom of the cell, particularly the carbon cathode. The primary objective of the project is to significantly improve the physical understanding and build a strong competence concerning the thermodynamics, the kinetics, and the transport phenomena that are relevant for materials intended for use in aluminium cells. By gaining such knowledge, it will be possible to design better test methods as well as bett er models concerning chemical reactions and general behaviour of the materials. This will again enable more precise predictions concerning performance, better advice concerning choice of materials, and guidelines for improved cell design. The more challe nging task is probably the achievement of a correct description of the conditions at the top of the cathode carbon. The rate of cathode wear appears to be strongly dependent on the local current density, and the formation of a carbide layer in this region may be critically important. Furthermore, it is necessary to obtain a better description of the transport and behaviour of sodium in the cathode materials. A number of experimental methods will be used in the accomplishment of the project, comprising el ectrochemical studies in laboratory cells mimicking the conditions at the cathode in industrial cells and sodium expansion measurements in atmospheres with different sodium vapour pressures. Advanced characterization of materials and deterioration product s will be carried out. Multilevel mathematical modelling will be important. Micro-scale models will be important tools in the screening of hypotheses, as well as in the interpretation of experimental data. The micro-scale models will be integrated into a macro-scale model intended to be used for industrial cells.