(Invited) Electrowinning of Aluminium and Carbon Footprint

Abstract

Primary aluminium is produced by the Hall-Heroult process which is based on electrolysis in molten fluoride electrolyte, Na3AlF6-AlF3, at ~960 oC in which the raw material alumina is dissolved and decomposed into pure aluminium and CO2 gas due to the use of carbon anodes. The current efficiency with respect to aluminium can be as high as 96 % in Hall-Heroult cells. The total cell reaction is: Al2O3 (diss) + 3/2 C (s) = 2 Al (l) + 3/2 CO2 (g) The electrical energy consumption is around 12 - 15 kWh/kg Al, which corresponds to an energy efficiency of about 50 - 60 %. The main losses are due to a large ohmic voltage drop in the electrolyte and high anodic overvoltage. There are considerable emissions of CO2 from generation of electricity and the anode reaction. In addition there are emissions of CF4 and C2F6 (PFC gases) during so-called anode effect. Several efforts are under way to reduce the carbon footprint of the electrolysis process. One approach is to develop inert anodes to produce oxygen to eliminate the direct emissions of CO2 and PFC. The most promising inert anode candidates are based on alloys (Fe, Ni, Cu) in an electrolyte rich in KF with a lower liquidus temperature. Another idea is to produce aluminium based alloys directly during electrolysis by adding metal oxides in order to co-deposit alloying elements such as silicon, titanium and manganese. Such a method may give significant savings also in terms of carbon footprint since the current method for producing alloys is to add pure metals to liquid aluminium before casting. These alloying metals are associated with high emissions of CO2 durning their primary production processes.