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Session Chairperson: Boris M. Triko, Aluminum Company of America, 1000 Riverview Tower, 900 South Gay Street, Knoxville, TN 37902-1848
GREEN PASTE POROSITY AS AN INDICATOR OF MIXING EFFICIENCY: Per Stokka, Norsk Hydro Research Centre, N-3901 Porsgrunn, Norway
Variations in green paste porosity were studied as a function of pitch content and mixing parameters. Porosity in paste from paste plants operating with different mixing systems, was measured and used to indicate the efficiency of the mixing. Changes in paste porosity during remixing in the laboratory were also studied.
POTENTIALITIES IN THE PASTE PLANT: S. Wilkening, VAW Aluminium-Technologie GmbH, P.O. Box 2468, 53114 Bonn, Germany
Over the last decade much progress in paste plant technology originated from computerized process control, improved process philosophy and P & ID's as well as full level 2 operation. This paper will focus on materials aspects to get out more from the intrinsic properties of coke and pitch. Proposals will be made how to utilize in a better way the structural properties of petroleum coke and the fluid and carbonization properties of binder pitch. Potential changes in equipment and lay-out will also be discussed.
VERTICAL ANODE CRACKING - THE VALCO EXPERIENCE: Norbert A. Ambenne, Volta Alumium Company Ltd (VALCO), P.O. Box 625, Tema, Ghana, West Africa
During the period 1990 - 1995 the VALCO aluminum smelter experienced a serious anode cracking problem. Approximately 12 % of the anodes split mainly along a particular set of anode stubs. About half of these splits resulted in carbon pieces falling into the reduction cell with attendant operating difficulties. This cracking phenomenon started when the plant was converted from 41.5 to 48 inches longer anodes with the extra length added to one end of the anode. Results of the investigations revealed that several operating variables contributed to anode cracking in varying degrees. These factors include mix profile in anode former mold, anode stub-to-gap ratio, anode setting practices, pot condition, anode symmetry, and moisture content in anode aggregate. This paper discusses these factors and how they affected anode cracking at Valco.
AN APPROACH FOR A COMPLETE EVALUATION OF RESISTANCE OF A MATERIAL TO THERMAL SHOCK (PART 1): APPLYING TO THE CASES OF ANODES AND CATHODES: Christian Dreyer, Bernard Samanos, Aluminium Pechiney, LRF BP 114, 73303 Saint Jean de Maurienne, Cedex, France
The aim of this article is to describe a new method for characterizing a material in terms of resistance to thermal shock. Such characterization takes into account the criteria of thermal shock resistance to initiation (Kingery criterion) and propagation (Hasselman criteria). It can be applied equally well to hard or soft thermal shock. This new approach has the following main advantages : 1) characterization of the material in terms of thermal shock is complete, 2) thermal shock tests or empirical formulae, the validity of which are often hard to establish, become unnecessary. Examples of the use of this new approach are presented for anodes and cathodes.
10:10 am BREAK
AN APPROACH FOR A COMPLETE EVALUATION OF RESISTANCE OF A MATERIAL TO THERMAL SHOCK (PART 2): APPLYING TO AN INDUSTRIAL PROBLEM AT ALBA: Bernard Samanos, Christian Dreyer, Aluminium Pechiney, LRF BP 114, 73303 Saint Jean de Maurienne Cedex, France; A. Hameed, G. Abbas, Jaffar G. Ameeri, Aluminium Bahrain, 570 Manama, Bahrain
This article presents in detail the steps taken to successfully resolve the problem of anode breakage due to thermal shock et ALBA. These steps can be essentially characterized as follows: 1) establishment of a method of accounting for anode breakages, 2) parametric studies on benchscale anodes, 3) effects on the process. Characterizing anodes in terms of resistance to thermal shock, following the method described in Part 1, allowed us to appreciate the importance of each of the adjustable parameters of the process. The applied solution draws on a new grain size distribution formulation.
ANODE PROPERTY DEVELOPMENT DURING HEAT TREATMENT: Mona Jacobsen, Department of Thermal Energy and Hydro Power, the Norwegian Institute of Science and Technology, N-7034 Trondheim, Norway; Oyvind Gundersen, Department of Engineering Cybernetics, The Norwegian Institute of Science and Technology, N-7034 Trondheim, Norway
Change of weight, density, permeability and porosity of anode specimens were measured after being baked to temperatures between 300 and 1250°C. Test specimens were cut from a green anode and analysed prior to the baking to study green property variations. Density variations in the green anode were significant and influenced the permeability measured in the green specimens. In mathematical modeling of the baking process, anode property models for description of heat and mass transport phenomena in the anodes are required. As basis for the derivation of the property models, the anode was considered to be a composite medium consisting of pitch coke, filler coke and pores. The filler coke was divided into a coarse and fine size fraction. The fine fraction mix with the binder pitch to constitute the binder matrix. It was assumed that the binder matrix was uniformly distributed on the surface of the remaining coarse filler particles. The models were compared and verified against the data.
ANODE IMPREGNATION SYSTEM FOR ALUMINIUM REDUCTION CELLS: Georges Berclaz, Avenue St-François, 3968 Veyras/Sierre, Switzerland; Vittorio de Nora, Jean-Jacques Duruz, Gaynor Johnston, MOLTECH, 9 Route de Troinex, CH 1227 Carouge-Geneva, Switzerland
The minimization of anode carbon consumption in aluminium reduction cells has an important technical, environmental and economical impact. The basic anode elements, such as coke and pitch, do not always have the desired properties and there is an increasing need to protect the anodes against oxidation by air and CO2. A solution based on boric acid impregnation of the upper part of the anode has been tested at the industrial scale. Anodes have been impregnated using specially designed equipment to place the protection in the appropriate part of the anode and to avoid metal contamination. Results are shown for different levels of impregnation. Comparison to metal spraying protection is also analyzed.
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