1997 TMS Annual Meeting: Thursday Session

CARBON TECHNOLOGY: Session V: Cathode

Session Chairperson: André L. Proulx, Alcan International Limitée, 1955 Boulevard Mellon, C.P. 1250, Jonquière, Québec, Canada G75 4K8

8:30 am

ECA FOR IMPROVED CATHODE PERFORMANCE: Johan A. Johansen, Herman Gran, Elkem Carbon, P.O. Box 8040, Vagsbygd, N-4602 Kristiansand, Norway

ECA (Electrically Calcined Anthracite) is the main raw material for the carbon part of the electrolysis cells. Demand for increased potlife and more efficient cathodes (lower voltage drop) have led to use of more graphite in the carbon materials. Little has been done to improve the main raw material, ECA. The characteristics of ECA can be strongly affected by calcining conditions and selection of raw anthracites. In this study, different ECA qualities were produced in full scale calciners and the effect of improved ECA quality on cathode materials studied. A lab scale test program was established to evaluate the effect on different baked properties. Important properties for improved cathode performance, like sodium resistance, electrical conductivity and mechanical properties, can be effected by use of selected ECA.

8:55 am

HIGH PERFORMANCE TECHNOLOGY FOR THE PREPARATION OF CARBON PASTE FOR CATHODES AND GRAPHITE ELECTRODES: Berthold Hohl, Machinenfabrik Gustav Eirich, D-74732 Hardheim, Federal Republic of Germany

The preparation of carbon paste for manufacture of cathodes and graphite electrodes requires the heating-up of the raw materials as well as the following mixing and cooling of the paste. In the last few years, a newly developed plant type has proved successful in practical operation. It consists mainly of an electric resistance heater and a high-performance mixer. The coke fractions are heated up to temperatures between 150-200°C by the resistance heater and fed into the high-performance mixer along with liquid pitch. After completed homogenization of the components, the temperature required for the moulding is achieved with greatest accuracy by addition of water and evaporation cooling. One single machine of this type replaces 8-12 conventional batch mixers. The special merits of this process are the minimal maintenance compared to conventional techniques, the reduced energy consumption, the shortened cycle time and the considerably improved environmental protection. The plant is a closed system; dusts and steams are purified in an exhaust air decontamination plant and partially reused. The plant flexibility makes it possible to prepare carbon paste for both cathodes and graphite electrodes with exactly the same equipment by only simply changing the recipe. As an example, the latest state of the art is presented as well as the most significant operating results by means of a plant which was commissioned in 1995 at a great international manufacturer of carbon and graphite products.

9:20 am

THE OXIDATION OF HETEROGENEOUS CARBON ELECTRODES AND FURNACE LINERS--PART 1: X-RAY DIFFRACTION APPLICATIONS: F. Hiltmann, SGL Carbon AG, Frankfurt-Griesheim, Germany; B.J. James, B.J. Welch, M.M. Hyland, Department of Chemical and Materials Engineering, The University of Auckland, New Zealand

The primary purpose of this study is to fully understand the role played by oxidation reactions in the degradation of carbon cathodes and furnace liners such as those used in aluminium smelting cells. While we have developed a sensitive technique for detecting oxidation at low rates, full mechanistic understanding is dependent on supplementary characterisation of the residual unreacted materials. This is complicated by the heterogeneous nature of the electrode materials. This paper presents details of the application of X-ray diffraction for studying the various component materials of these composites, namely pitch, graphite and anthracite. Also presented in this paper is supporting compositional analysis of the heterogeneous carbon samples together with element maps to indicate the distribution of typical trace elements.

9:45 am

THE OXIDATION OF HETEROGENEOUS CARBON ELECTRODES AND FURNACE LINERS--PART II: OVERALL REACTIVITY CORRELATIONS: B.J. James, B.J. Welch, M.M. Hyland, Department of Chemical and Materials Engineering, the University of Auckland, New Zealand; F. Hiltmann, SGL Carbon AG, Frankfurt-Griesheim, Germany

Using the method described previously (Light Metals, 1996) the oxidation reactions of carbon cathode materials have been characterised for onset temperature and rate of oxidation at low (less than 550°C) temperatures. Combining the results of composition and structural analysis described in Part I, correlations have been made between the inherent structure of the component materials and their onset temperature and relative rates of oxidation thus explaining the preferential nature observed in the oxidation of composite samples. Using the results of polarised light microscopy and scanning electron microscopy the relationship between bulk structure of the heterogeneous samples and their oxidation reactions has been examined with important implications for the performance of these materials in service.