Producing Aluminum without direct CO2 emissions

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:

3C(s)+6O^{2-}+4Al^{3+}\rightarrow 4Al(l) +3CO_{2}(g)

or (referring the reaction to pure alumina-corundum)

3C(s)+2Al_2O_3(s)\xrightarrow{960^oC} 4Al(l) +3CO_{2}(g)  with \Delta G^o(960^oC)=1.37925E+03 kJ

which leads to the following voltage Eo:

E^o= \frac{-\Delta G^o}{nF}=-1.2V

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.%; aAl2O3=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:

6O^{2-}+4Al^{3+}\rightarrow 4Al(l) +3O_{2}(g)

or (referring the reaction to pure alumina-corundum)

2Al_2O_3(s)\xrightarrow{960^oC} 4Al(l) +3O_{2}(g)  with \Delta G^o(960^oC)=2.56806E+03 kJ

leading to a new voltage Eo:

E^o= \frac{-\Delta G^o}{nF}=-2.2V

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!

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