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alumina concentration (magenta line) also rises by about 1% due to this separation effect, but
drops back down again when the electrolysis process is restarted.

During the trial, the bath level in Figure 5c (green line) was similarly affected with a 350mV
addition for preheating, increased ACD as well as the ledge formation process. On cooling,
the electrical resistance of the bath increases by about 20% - see Figure 5c (black line and
right-hand axis) - so that the pot control system lowers the ACD to maintain the pot voltage
after power restart. This points to some measures for avoiding an anode effect during restart.
With a well-validated model, different measures can therefore be evaluated to stabilize pot
operation during power modulation.

Conclusions

This very condensed model validation exercise highlights both the strengths and weaknesses
of mathematical modelling: a well calibrated and, hence, well validated model is able to
represent the complex dynamic behaviour of a Hall-Héroult aluminium electrolysis cell
accurately: Such a model can help to plan and execute strategies for power modulation,
reduce specific power costs and ensure stable pot operation and extended pot life.

References

[1] N.E. Richards and H.O. Forberg, Electrical Power Contracts and Other Factors Affecting
Smelter Economics, Light Metals 1997, 253-258.

[2] A.C. Brant and al, The Operation of a Smelter with Power Modulation, Light Metals1992,
357-362.

[3] L.J. Leal Nunes and al, Power Modulation on Valesul P-19 Pots, Light Metals 1998, 1267-
1271.

[4] M. Dupuis, Modeling Power Modulation, Light Metals 2002, 489-493

[5] I. Tabsh, M. Dupuis and A. Gomes, Process Simulation of Aluminum Reduction Cells,
Light Metals 1996, 451-457.

[6] M. Dupuis, Computation of Aluminum reduction Cell Energy Balance Using ANSYS

Finite Element Models, Light Metals 1998 , 409-417.

[7] M. Dupuis and R. Lacroix, Development of a 2D+ Dynamic Model of an Aluminum
Reduction Cell, Proceeding of the 38

Conference on Light Metal, CIM, 1999, 41-55.

[8] M. Dupuis, Thermo-Electric Design of a 400 kA Cell using Mathematical Models: A
Tutorial, Light Metals 2000, 297-302.

[9] M. Dupuis, Process Simulation, TMS Course on Industrial Aluminum Electrolysis, 1997.

[10] F.J. Stevens McFadden, Energy Balance and Cell Dynamics: Considerations for Cell
Design, Operations and Process Control, Proc. 6

Aust. Al. Smelting Workshop, 1998, 289-

320.

[11] J.N. Bruggeman, Pot Heat Balance Measurements An Overview, Proc. 7

Aust. Al.

Smelting Workshop, 2001, on CD.