Why smelters are power hungry
‘‘HE inherent chemical attributes of aluminium aren’t completely to blame for the energy intensity of aluminium production, according to a group of researchers from Swinburne University of Technology and CSIRO. Although there are several factors that contribute to the relatively high energy consumption and high capital cost per tonne, most of them can be traced back to materials,’’ they said in a recent review of challenges in light metals production.
Aluminium is one of the more abundant elements in the earth’s surface rocks because aluminium is stable when combined with oxygen. Aluminium’s knack for staying bound to oxygen means the Hall-Heroult process uses about 6 kilowatt hours of energy to strip oxygen away to create 1kg of aluminium.
But to create that kilogram, another six or seven kilowatt hours have been used as well, they said, lost to the surroundings as heat. The reason for the heat loss is that the molten material used to dissolve and host the aluminium oxide — called cryolite — is corrosive at the necessary temperature of about 960C.
Cryolite will attack the inside walls of the HallHeroult reaction cell unless prevented. The only currently feasible way to do this is to freeze the cryolite sitting next to the walls. Keeping the peripheral cryolite frozen while the central portion is molten means a lot of heat must be continually drawn off. The result is high energy consumption.
The researchers said that among CSIRO’s HallHeroult projects was work on developing new wall materials to help address the problem. Other project work included trying to find materials that can dissolve aluminium oxide at lower temperatures.