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1997 TMS Annual Meeting: Thursday Session


Sponsored by: LMD Aluminum Committee
Program Organizer: Harald A. Øye, Institute of Inorganic Chemistry, Norwegian University of Science and Technology, N-7034 Trondheim, Norway
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Room: 230A

Session Chairperson: Halvor Kvande, Hydro Aluminium, Hydro Aluminium a.s, P.O. Box 80, 1321 Stabekk, Norway

8:30 am

APPLICATIONS OF NEW STABILITY CRITERIA TO INDUSTRIAL CELL DESIGN: R.I. Lindsay, P.A. Davidson, Department of Engineering, University of Cambridge, Trumpington Street, Cambridge, CB2 1PZ, UK

Previous models of the behaviour of interfacial waves in aluminium reduction cells lead to distinct descriptions of the physical mechanisms involved and different stability criteria. We consider the implications of these criteria for cell design. A new wave equation has recently been developed from shallow-water theory, in which the Lorentz force is expressed explicitly in terms of the fluid motion. The simplicity of this new equation leads to a general energy criterion to establish which types of motion may be unstable. By expressing the new equation in matrix form, we obtain some unexpected results. We discuss the implications of some of these results for cell design. Finally, we introduce a new sufficient condition for the stability of standing waves in a finite domain, which does not require solving the governing equations. The use of Gershgorin's theorem allows us to place a lower bound on the critical value of the background magnetic field at which an instability first appears.

8:55 am

MAGNETOHYDRODYNAMIC EFFECT OF ANODE SET PATTERN ON CELL PERFORMANCE: M. Segatz, Ch. Droste, D. Vogelsang, VAW Aluminium-Technologie, G.-v.-Boeselager-Str. 25, D-53117 Bonn, Germany

Numerical simulation of coupled bath/metal magnetohydrodynamics (MIID) and MIID stability analysis allows the optimization of anode set pattern with respect to minimal cell disturbance. The contribution of anode gas induced forces and impact on the flow field are discussed. During a complete anode set cycle the anode current distribution changes significantly due to varying anode resistances, frozen bath and different metal pad heights. Typically the largest disturbance to cell stability occurs during a short time span after the anode change. With steady-state MIID simulations immediately before and after each anode change - in sequence of the underlying set pattern - the relevant cell current and ACD distribution are determined. The impact of these parameters on cell stability is predicted with a linear MIID stability analysis for a complete anode change cycle.

9:20 am

A NEW MODEL OF INTERFACIAL WAVES IN ALUMINIUM REDUCTION CELLS: P.A. Davidson, R.I. Lindsay, Department of Engineering, University of Cambridge, Trumpington Street, Cambridge, CB2 1PZ, U.K.

We develop a new wave equation for the liquid interface in aluminium reduction cells. It differs from previous models in that the Lorentz force is expressed explicitly in terms of the fluid motion. The equation is valid both for open domains and confined domains of arbitrary shape, and its simplicity makes the instability mechanism explicit. Our new equation predicts that both travelling waves and standing waves may become unstable by the same mecha nism and that the travelling wave instabilities are distinct to those uncovered in previous investigations. We also derive a simple yet general energy criterion which shows which types of motion may extract energy from the background magnetic field. This indicates that a rotating tilted interface is particularly prone to instability, and indeed such motions are often seen in practice.

9:45 am

SIMULATION OF THE DYNAMIC RESPONSE OF ALUMINIUM REDUCTION CELLS: Imad Tabsh, COMPUSIM Inc., 1003D 55 Avenue N.E., Calgary, Alberta, Canada T2E 6W1; Marc Dupuis, GéniSim, 3111 Alger, Jonquière. Québec, Canada G7S 2M9

A comprehensive program (ARC/Dynamic) was developed to simulate the dynamic behavior of aluminium reduction cells during operation. The program uses the mass and energy balance equations to determine the transient evolution of more than 60 process variables. In addition, it simulates various operational and control policies in use during cell operation. This paper describes the use of ARC/Dynamic to study the sensitivity of the cell response to variations in the input parameters. It also looks at the cell performance under various amperage curtailment conditions such as sudden shutdown of the line or scheduled reduction in line amperage. A detailed description of the amperage fluctuation model implemented in the program is presented.

10:10 am BREAK

10:30 am

MATHEMATICAL MODELLING OF CURRENT DISTRIBUTION AND ANODE SHAPE IN PREBAKE ALUMINIUM CELLS: J. Zoric, I. Rousar, Department of Inorganic Technology, Institute of Chemical Technology, 16628 Prague 6, Czech Republic; J. Thonstad, Department of Electrochemistry, Norwegian University of Science and Technology, N-7034 Trondheim, Norway

Two approaches were used to determine the current distribution in aluminium cells with prebaked anodes, i.e., primary and secondary current distribution, the latter giving far more realistic results. Current densities obtained for secondary current distribution were used to model the changes in anode shape from the time when a new anode has been set until it achieves the typical rounded off steady state profile. Mathematical modelling of the anode consumption using current densities obtained by the solution of the Laplace equation in 2D space, showed that a constant shape was reached after 6 - 8.6 days, depending on the width of the gap to a neighbouring anode or to the sidewall/sideledge. The calculated steady state shapes were similar to the shapes of anodes taken out of a cell. The current density decreases up along the side of the anode from the nominal value at the underside (0.75 A cm-2) to a minimum near the surface of the electrolyte (0.08 - 0.28 A cm-2), depending on the geometry. The percentage of the current which passes through the sides of the anodes is of the order of 15%. The current density on the anode side facing the periferic channel increases with increasing distance to the sideledge, while the exact shape of the ledge makes little difference. The cathodic current density in the periferic channel goes down to 0.07 A cm-2 for a 30 cm wide channel.

10:55 am

MATHEMATICAL MODELLING FOR COKE BED PREHEATING OF ALUMINIUM REDUCTION CELL: Shaher A. Mohammed, R&D Dept., Aluminium Company of Egypt, Nagi-Hammadi, Egypt; Maher M. Abdulwahab, Attia A. Arif, Omar M. Dahab, Power and Energy Dept., Minya University, Egypt

Many methods are available to preheat and bake-out the cathodes of aluminium reduction cells but only few of them are widely used. Resistor coke bed with shunt rheostat is one of the most common methods to preheat the cell. To get optimum design parameters for resistance elements and preheating method, a finite element model was built to simulate the process. The effect of starting current, coke bed thickness, rate of increase of current and finally total time needed for preheating, were studied. The obtained results indicated that the practical technique used has to be subjected to some modifications in order to reach satisfactory conditions.

11:20 am

HIERARCHICAL INTELLIGENT CONTROL SYSTEM FOR ALUMINUM REDUCTION CELLS: Jie Li, Yexiang Liu, Jin Xiao, Department of Metallurgy, Central South University of Technology, Changsha, Hunan 410083, China; Feng Wang, Shengwen Shi, Department of Technology, Liancheng Aluminum Plant, Lanzhou, Gansu 730335, China

A hierarchical intelligent control system for aluminium reduction cells has been developed. There are two intelligent levels in the system. The higher level built on the principle of neural network expert system, aims at analyzing medium and long-term change trends of the state of the process, calculating settings for the lower level, interacting with operators and offering operation proposals. The lower level, composed of several subsystems which are set up on the principle of fuzzy control and expert control, is used to realize short-term analyses and real-time control of the process. As the system has a hierarchical and modular structure, its design, realization and testing are simplified. Test-run results verified that, combined together in one system, the controllers have good properties of robustness and adaptability with respect to changes of operating conditions and severe disturbances.

11:45 am

DETERMINATION OF METAL CURVATURE AND IMPROVED ANODE CONSUMPTION: Jon H. Stefansson, Electrolysis Department, Icelandic Aluminium Co. Ltd., IS-222 Hafnarfjördur, Iceland; René von Kaenel, Jacques Antille, Technology Center Chippis, Alusuisse Technology & Management Ltd., CH-3965 Chippis, Switzerland

The Alusuisse ISAL smelter in Iceland does not have an in house anode plant. This makes it increasingly important to operate with small and evenly thick anode butts. Since the insertion height is calculated according to the carbon burning rate, it is important to know the difference in metal level according to the position in the pots. Two methods have been used to determine the metal upheaval. The first consists in measuring the height of many enough anode butts since the lower anode level adjusts to the metal level with time. The second method is based on mathematical modelling. Measured values have been compared to calculated values with good agreement. Insertion of the anodes according to metal level has resulted in improved gross anode consumption due to more even butt thickness and stable operation.

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