Inert Anode Technology

As we have seen, electric current passes through the molten bath carried by carbon anodes immersed in it. Carbon anodes not only are used to carry the current, but also participate in the alumina reduction chemical reaction:

aluminum reduction reaction

On a practical side, this implies the anodes are consumed as the time goes by and need to be replaced on a regular basis (28 – 32 days on average).

Anode change operations are one of the most critical tasks to be performed in a smelter, requiring upstream an entire separated plant for manufacturing new anodes (usually called “Carbon Plant”) and recycling of the spent ones.

Since anodes are made of carbon, they easily react with oxygen in the air, leading to extra consumption of carbon material, exposition of stubs and/or cast iron, increased levels of Fe in the metal, lower current efficiency, burn-offs, etc…

To avoid these problems, anodes need to be properly covered in the top surface and sides with a mixture of crushed bath and alumina, which requires a dedicated bath handling/crushing/mixing facility in the smelter.

Due to all the aforementioned problems, the aluminum industry has extensively researched different materials for the anode manufacturing, in particular anodes that are not consumed during the electrolysis process, called “inert anodes”.

In this case, the alumina reduction chemical reaction can be written as:

As a result, no more CO2 evolves from the anodes, but instead oxygen is produced.
An electrolysis cell equipped with inert anodes has potentially several advantages over a carbon anode equipped cell:

  • No need for frequent anode changes
  • No need of a carbon plant
  • No need of a bath handling facility
  • More stable pot operation (the anode change, in fact, is a quite disturbing process for the electrolysis process)
  • Possible to retrofit with inert anodes an existing cell
  • No more production of greenhouse gases (CO2)

At the same time, inert anode technology brings several disadvantages. The carbon anodes participate in the alumina reduction reaction producing heat right inside the bath (through the reaction C + O2 → CO2). If an inert anode is used, there is no participation in the chemical reaction and no heat generated into the bath. To keep the same pot thermal balance, the missing heat has to be provided through external voltage. As a rule of thumb, the heat generated by the burning of the anodes is equal to 1 Volt in electrical terms. This means that using inert anodes we need to increase pot voltage by 1 Volt to have the same pot thermal balance.
The increase of pot voltage is partially offset by the fact that the anode overvoltage (term ηaa in the equation below):

Voltage Components

Further reductions in voltage could be achieved using drained cathodes technology which allow for reduced ACD (anode-to-cathode distance).
Materials utilized to manufacture inert anode are the following:

  • Ceramics
    • NiFe2O4 based anodes
    • SnO2 based anodes
    • NiO-Li2O based anodes
  • Metals
    • Aluminium bronze
    • Cu-Ni-Fe based anodes
  • Cermets
    • Fe-(NiFe2O4 + NiO) based anodes
    • Cu-Ni-NiFe2O4-NiO based anodes
    • NiFe2O4-Cu based anodes
    • Cu2O-Cu based anodes
    • Ni(NiFe2O4-10NiO) based anodes

In may 2018 Alcoa and Rio Tinto announced the development of a inert anode technology and the forming of a joint venture called Elysis, backed also by founds coming from Apple, to further develop the new technology and made it available for retrofitting existing smelters starting from 2024.