We learned yesterday that a joint venture between Alcoa, Rio Tinto and Apple was created in order to produce primary aluminum using a zero-CO₂-emission process. Here are some simple explanations about the new primary metal production strategy.

The conventional smelting strategy to produce aluminum from alumina (Al₂O₃) is based on the Hall-Héroult electrochemical process:

or (referring the reaction to pure alumina-corundum)

  with

which leads to the following voltage E^o:

In the Hall-Héroult process, alumina is dissolved in the cryolite (Na₃AlF₆, a molten salt), which will lower the activity of Al₂O₃ (typical values: 3 wt.%; a_Al2O3=0.09). Assuming a partial pressure of CO₂(g) of about 0.9 atm in the exhaust gas, a theoretical value of E=-1.23V is found.

It is interesting to note that CO₂(g) production (-1.2V) is more energetically demanding than the production of CO(g) (-1.1V). Because of the kinetics associated to the use of high current density in Hall-Héroult cells, CO₂(g) is produced under industrial operating conditions (Haupin, J. Chem. Educ., 1983, 60 (4), p 279).

The newly envisaged strategy is to evolve O₂(g) rather than CO₂(g) at the anode by replacing the carbon electrode with an inert one:

or (referring the reaction to pure alumina-corundum)

  with

leading to a new voltage E^o:

This new process, although more energy-consuming from a theoretical point of view, leads to zero CO₂(g) emissions.

Of course, Canada, and especially Quebec, with nearly 100% electricity production coming from renewable energy (hydro) is the perfect playground to develop this technology (0kg CO₂ per kWh for hydro vs 0.9kg CO₂ per kWh for coal).

Let’s hope that all the scientific challenges associated to inert anode material selection, aggressive O₂(g) hot atmosphere and heat flux problems can be resolved by our scientific community.

Time to work!