Overview Overcoming instabilities in aluminium reduction cells: A route to cheaper aluminium

Abstract

The amount of energy currently used to reduce alumina to aluminium in electrolysis cells is staggering, around 10¹¹ kWh/year. Yet much of this energy (almost one-half) is lost in the form of I ²R heating of the highly resistive electrolyte. Strenuous efforts have been made to minimise these losses by reducing the volume of electrolyte in the cells. However, the aluminium industry has come up against a fundamental problem: when the depth of the electrolyte is reduced below a critical threshold (around 4–5 cm), the liquids in the cell start to ‘slosh around’ in an uncontrolled fashion. This is an instability, fuelled by the intense currents which pass through the liquids. At present, cells operate just above the critical electrolyte depth, but if this depth were reduced from, say, 4·5 to 4 cm, then the annual savings would exceed £10⁸. After a number of false starts, we now have a clear understanding of the physical mechanisms which underpin the instability, and it turns out that these are remarkably simple. These mechanisms are described here and it is shown that, although the cell geometry is too complex to produce an accurate mathematical model, the underlying mechanisms are so simple that it becomes clear how to suppress the instability. Thus, after two decades of research, we are finally in a position to design inherently stable cells. For example, it is shown that slow, small movements of the anode assembly can lower the critical electrolyte depth to around 2 cm. Such a control system could be retrofitted to most existing cells.