MHD Instabilities in Reduction Cells

Abstract

It is easier to write ten volumes on theoretical principles than to put one into practice. Tolstoy The amount of energy required to reduce alumina to aluminium in electrolysis cells is staggering. In North America, for example, around 2% of all generated electricity is used to produce aluminium. Worldwide, around 2×10 10 kg of aluminium are produced annually, and this requires in excess of 10 11 kWh p.a. The corresponding electricity bill is around £10 10 p.a.! Yet much of this energy (around one half) is wasted in the form of I 2 R heating of the electrolyte used to dissolve the alumina. Needless to say, strenuous efforts have been made to reduce these losses, mostly centred around minimising the volume of electrolyte. However, the aluminium industry is faced with a fundamental problem. When the volume of electrolyte is reduced below some critical threshold, the reduction cell becomes unstable. It is this instability, which is driven by MHD forces, which is the subject of this chapter. Interfacial Waves in Aluminium Reduction Cells Early attempts to produce aluminium by electrolysis It is not an easy matter to produce aluminium from mineral deposits. The first serious attempt to isolate elemental aluminium was that of Humphrey Davy, Faraday's mentor at the Royal Institution. (In fact, Davy's preferred spelling – aluminum – is still used today in North America.) In 1809 he passed an electric current through fused compounds of aluminium and into a substrate of iron.