Anodes are large carbon blocks which are used to conduct electricity during the aluminium reduction process.
As they do so they are consumed, at a rate of around 450 kg per tonne of aluminium produced. The two families of smelting technology in operation today – Søderberg and prebake – are characterised by the types of anode employed.
Søderberg smelters use a continuously created anode, made by the addition of pitch to the top of the electrolytic cell or “pot”. The heat generated by the reduction process is used to bake the pitch into the carbon form required for reaction with alumina, thus recycling the waste energy in the pot.
Prebake technology utilises anodes which are baked in very large gas-fired ovens at high temperature before being lowered into the pot. These are then replaced once consumed. The efficiency of this technology compared to Søderberg, combined with its lower environmental impact, means that prebake smelters predominate (representing over 90% of worldwide aluminium production), with all new facilities built today utilising this technology.
Prebake anode production
Prebake anodes are produced in an area of the smelter called a “green mill” and are made from petroleum coke, pitch and recycled anode butts (the ends of the consumed anodes remaining at the end of their life) returned from the smelting process. These materials are mixed together in heated containers and poured into moulds. Once formed, the anodes are transferred to a “bakehouse”.
Here, the anodes are placed in a furnace at a temperature of 1120°C for a period of up to two weeks. This bakes the pitch in the mix, forming a solid block of carbon, able to withstand the extreme conditions inside the smelting pots but of a homogenous consistency that allows for efficient conduction of current and an even consumption of the anode.
The last stage of the anode production process takes place in the “rodding room”. Here the carbon blocks are fused to a steel rod (the means by which they will be lowered into the pot and through which the electrical current is passed) with molten cast iron. The rodded anodes are then transported to the smelter potroom to be placed in the reduction cells.
“Without carbon, without anodes, we wouldn’t be able to realise the potential of aluminium metal.”