Aluminum: Approaching the New Millennium

Abstract

The amount of energy currently used to reduce alumina to aluminum in electrolysis cells is staggering, around 10¹¹ kW·h per annum. Yet much of this energy (almost one half) is lost in the form of I ² R heating of the highly resistive electrolyte. Strenuous efforts have been made to minimize these losses by reducing the volume of electrolyte in the cells. However, the aluminum 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, fueled by the intense currents that pass through the liquids. At present, cells operate just above the critical electrolyte depth, but if this depth were reduced from, say, 4.5 cm to 4.0 cm, then the annual savings would exceed $100 million. After a number of false starts, we now have a clear understanding of the physical mechanisms that underpin the instability, and it turns out that these are remarkably simple.