Principles of aluminum electrolysis

Abstract

For years Hall-Heroult cells have been operated successfully using procedures and recipes based largely upon experience. In today's competitive environment it is valuable to supplement experience with scientific principals. This paper attempts to describe some rather complex concepts in a simplified manner for those who never have had or have forgotten their training in electrochemistry, thermodynamics or kinetics. Although sodium ions carry the current through the bath, the cathode reaction extracts aluminum and leaves at the interface a melt that is richer in sodium fluoride and therefore has a higher liquidus temperature (melting point) than the bulk. The anode reaction produces aluminum fluoride at the anode-bath interface. Good convection mixes electrolyte, restoring the bulk composition, but this mixing aggravates the reoxidation of dissolved metal by carbon dioxide thereby lowering current efficiency. Kinetics causes the anode reaction to produce carbon dioxide instead of carbon monoxide, the thermodynamically favored product. These kinetic factors increase the cell voltage by about 500 mv. When the oxide dissolved in the electrolyte is nearly depleted, fluorine is codeposited on the anode surface producing a non-wettable fluorocarbon film. Current then flows by arcing, producing an Anode Effect. This causes emission of carbon tetrafluoride and increases the cell voltage by over 25 volts.