1997 TMS Annual Meeting: Tuesday Session

ALUMINIUM REDUCTION TECHNOLOGY: Session IV: Lining and Cathodes

Session Chairperson: Dave Belitskus, Alcoa Technical Center, Alcoa Center, PA 15069

2:00 pm

IMPROVEMENT OF THE ALUMINIUM CELL LINING BEHAVIOUR BY USING SiC: M.M. Ali, A.A. Mostafa, R&D Management, The Aluminium Company of Egypt, Naga Hamadi, Egypt

Now, the finite element analysis techniques (FEA) are widely used in the aluminium reduction cells. For example, to select new lining materials or to better understanding of the aluminium cell behaviour, through the use of modelling concepts. One of the important studies is the thermal model of the aluminium reduction cells. A 2-D FEA model has been established, tested and verified to study the thermal balance of the cell. This paper presents a study for using a new lining material. The effect of this material on the ledge profile and thermal distribution were discussed.

2:25 pm

PROPERTIES OF A COLLOIDAL ALUMINA-BONDED TiB2-COATING ON CATHODE CARBON MATERIALS: H.A. Øye, Institute of Inorganic Chemistry, The Norwegian University of Science and Technology, 7034 Trondheim, Norway; V. de Nora, J.-J. Duruz, G. Johnston, MOLTECH S.A., 9 Route de Troinex, 1227 Carouge, Geneva, Switzerland

Laboratory studies of colloidal alumina-bonded TiB2 have been carried out. The following properties have been demonstrated: Thermal expansion somewhat higher than carbon materials, electrical conductivity of the same order as graphite, porosity of coating 30%, resistance to sodium is high if the coating is protected by infiltrated aluminium, strong adherence of the coating to carbon materials even after thermal cycling, high abrasion resistance of the coating, wettability of the coating by liquid aluminium, the coating's action as a barrier to sodium penetration. The barrier action to sodium is the ability to form a stable liquid aluminium layer in the pores of the coating which will slow down the reaction: Al(l)+3 NaF(in electrolyte)=3 Na(in C cathode)+ AlF3 (in electrolyte) as the sodium stabilizing C is no longer in close contact with the bath.

2:50 pm

PENETRATION FORCE OF BATH COMPONENTS INTO POT LININGS DESCRIBED BY NONEQUILlBRIUM THERMODYNAMICS: Stig F. Johansson, Skamol a/s, Østergade 58-60, DK-7900 Nykøbing Mors, Denmark; Signe Kjelstrup, The Norwegian University of Science and Technology, Department of Physical Chemistry, N-7034 Trondheim, Norway

From principles of irreversible thermodynamics, a simple expression is developed for the driving force for transport of bath constituents across a non-isothermal matrix into the refractory lining in aluminium electrolysis furnaces. The expression shows that the transport outwards of aggressive substances is favoured by the temperature gradient. Possible applications are demonstrated for 3 penetrating gases with a variety of applied and proposed refractory materials. Common to all results is a possible reduction of the rate of penetration into barriers by a smaller temperature gradient due to the entropies of reaction. This can be accomplished by proper heat insulation below. A set of conclusions regarding the resistance of chemical barriers to penetration by bath components completes the paper.

3:15 pm BREAK

3:35 pm

FLUORIDE ATTACK ON ALUMINO-SILICATE REFRACTORIES IN ALUMINIUM ELECTROLYSIS CELLS: Jørn Rutlin, Tor Grande, Department of Inorganic Chemistry, Norwegian University of Science and Technology, N-7034 Trondheim, Norway

The phase relations in the system sodium fluoride - mullite are relevant to chemical reactions that take place in fireclay based refractory materials used in the bottom lining of aluminium electrolysis cells. The solid-liquid phase relations have been investigated by means of differential thermal analysis, powder X-ray diffraction analysis and microscopy. At sub solidus temperatures the four compounds sodium fluoride, cryolite, nepheline and -alumina have been found at excess amount of sodium fluoride. At lower concentration of sodium fluoride, corundum is formed at the expense of -alumina. The solidus temperature was observed at 857±5°C which corresponds well with results in systems with higher aluminium fluoride content. An apparent eutectic was observed at 12±0.5 mole% mullite. Nepheline and cryolite are the two main components of the melt crystallizing at the solidus temperature. The liquidus temperature at the solubility limit was observed at 943°C and at 7.5±0.5 mole% mullite. The present results are discussed in relation to the deterioration mechanism of alumino-silicate refractories.

4:00 pm

PYROHYDROLYSIS OF SPENT POTLINING: Vladimir Blinov, Tor Grande, Harald A. Øye, Department of Inorganic Chemistry, Norwegian University of Science and Technology, N-7034 Trondheim, Norway

Due to the toxic nature of spent potlining (SPL) this is becoming one of the major environmental concerns for the aluminium industry today. SPL also represents a major recovery potential because of its fluoride and energy content. The Elkem SPL recycling process is one of several technically feasible alternatives for treatment and recycling of SPL. In this process the fluorides can potentially be recovered by a pyrohydrolysis of the oxyfluoride silicate slag from the Elkem process. The thermodynamic aspects of this pyrohydrolysis are discussed in the present work. Chemical activity of fluorides in actual oxyfluoride silicate melts has been determined by means of high temperature mass spectroscopy and the Knudsen effusion method. Only sodium fluoride was observed as volatile species for dry melts. The chemical activity of sodium fluoride in several melts could therefore be determined by the Knudsen effusion method. Together with data on the activity of sodium oxide, the present activity data of sodium fluoride enable a calculation of the equilibrium pressure of HF during pyrohydrolysis of SPL.

4:25 pm

IDENTIFICATION OF NONLINEAR SWELLING PRESSURE DISTRIBUTION OF ALUMINUM REDUCTION CELL: H.S. Sayed, Structural Eng. Dept., Cairo University, Egypt; M.M. Megahed, Mechanical Eng. and Prod. Dept., Cairo University, Egypt; F. Dawi, S. Abdella, R&E Dept., EgyptAlum Co, Nagi Hammadi, Egypt

The swelling pressure has the most significant effect on determining the cathode life among other loads applied on the steel casing of the aluminium reduction cell. In previous research, an identification technique is applied to determine the average swelling pressure value that is exerted on the side wall of the cell. The identification technique made use of the measured deformation of the cathode steel casing at different elapsed times (after 88 and 615 days) on an existing cathode that belongs to EgyptAlum. A new set of measurements has been conducted at different elapsed time (after 1055 days). This new set of measurements has been conducted at different elapsed times (after 1055 days). This new set of measurements gives a unique opportunity to reassess the identified swelling parameters. On the other hand, the distribution of the swelling pressure along the cathode interface with the vertical steel walls of the cell, has a significant effect on the performance of the steel casing. In this paper, the distribution of the swelling pressure, the interface of the carbon blocks and the steel casing, are identified. A nonlinear elasto-plastic model for the steel casing using combination of shell and beam elements is developed. The carbon blocks are modeled using solid and truss elements. The model utilizes Dewing model that describes the swelling pressure as a function of the elapsed time and confined swelling strain rate to determine the swelling pressure distribution. The swelling pressure is initiated using a predetermined free and confined swelling rate. As a nonlinear analysis progress with item, the confined swelling rate of the previous time step is used to assess the swelling pressure at the interface for the subsequent steps. The swelling pressure distribution is then determined at any elapsed time.

4:50 pm

MODELLING OF DYNAMIC LEDGE HEAT TRANSFER: Chuck C. Wei, John J.J. Chen, Barry J. Welch, The University of Auckland, Chemical & Materials Engineering Dept., Auckland, New Zealand; Vaughan R. Voller, University of Minnesota, Dept. of Civil & Mineral Engineering, Minneapolis, USA; M.P. Taylor, New Zealand Aluminium Smelters Company Ltd., Invercargill, New Zealand

Cell disturbances such as anode effect and process operations such as feeding, anode changing and metal-tapping often cause variations in the heat balance of a reduction cell, resulting in deviations from the optimum operating conditions. A dynamic ledge heat transfer model built on an earlier one-dimensional dynamic simulation by the authors and based on the finite difference method was used to solve the transient heat conduction with phase change in the sidewall/ledge region. Fixed-grid and deforming-grid spacings were respectively superimposed on the sidewall and ledge region in order to track the moving front. Various aspects of the process dynamics with respect to the variation of ledge thickness and sidewall shell temperature were considered.