Reducing CO2 emissions in the aluminum industry

BY JOSE PALU-AY DACUDAO

IN THE previous article, we talked about decreasing anthropomorphic CO₂ emission from the iron industry. By far in terms of quantity, the most important elemental metal is iron. Most of the iron goes into steel-making, and the world produces a whopping 1.2 billion tons or more of steel each year. (China is the leading producer with more than 500 million tons of steel annually.)

A far second to iron is aluminum, with around 60 million tons produced annually. (Fifty five percent comes from China.) Almost all structural metal that we use that isn’t iron is aluminum. Because of the ease of its fabrication; its lightness, strength, and durability; its non-toxicity; its resistance to corrosion, splintering, rust, and fire; and high conductivity; it is widely used in:

* transports (road vehicles, trains, aircraft, boats, spacecraft, etc.)

* construction (doors, windows, roofs, sheathing, etc.)

* packaging (aluminum cans, foils, frames)

* household items (cooking utensils, furniture)

* miscellaneous machines, tools, pipes of all kinds

* electricity-related equipment (household and industrial electrical appliances, motors, generators, transformers, capacitors, cables and wires from electricity-generating plants to our homes and industries).

Our elemental aluminum comes from the mineral bauxite. After a complicated process (Bayer process), bauxite is transformed to aluminum oxide (Al₂O₃ alumina). Unlike iron oxide, it takes highly energy-consuming electrolysis in order to separate elemental aluminum from its oxide because of the strong affiliation of aluminum for oxygen. Thus, production of one kilogram of aluminum requires 7 kilograms of oil energy equivalent, as compared to 1.5 kilograms for that of iron. Thus, most aluminum smelters (electrolysis plants) are built near sources of electricity, such as hydroelectric dams.

The electrolysis of alumina to elemental aluminum is achieved by the Hall–Héroult process. A solution of alumina in a molten mixture of cryolite (Na3AlF6) with calcium fluoride is electrolyzed to produce the elemental metal.

2 Al₂O₃ (alumina) + 3 C → 4 Al + 3 CO₂ (carbon dioxide)

In the anode made of elemental carbon, as seen from above, carbon dioxide is produced.

However, if the anode is made of an inert substance, not elemental carbon, then no carbon dioxide is produced.

2 Al₂O₃ (alumina) → 4 Al + 3 O₂

The only “waste” is just oxygen.

This has already been done with various experimental anodes and processes, but so far, the Hall–Héroult process has always turned out to be cheaper. So we’re stuck with it.

Note that in all present-day aluminum smelters, the carbon anodes come from coal. Thus, there is a net carbon dioxide output into the atmosphere.

But there is a way for the Hall–Héroult process to be carbon-neutral. Just use charcoal as the carbon source for the anode. Recall that the carbon from charcoal comes directly from atmospheric CO₂ that has just recently been fixed by plants in photosynthesis.

6 CO₂ (taken from the atmosphere) + 6 H₂O → C₆H₁₂O₆ (organic substance) + 6 O₂

Equation for charring/ carbonization (charcoal production):

C₆H₁₂O₆ (organic substance) → 6 H₂O (water) + 6 C (charcoal carbon)

This is then followed, as explained above in the Hall–Héroult electrolytic process, by:

2 Al₂O₃ (alumina) + 3 C → 4 Al + 3 CO₂ (carbon dioxide)

In brief, by using charcoal instead of coal, the CO₂ that the Hall–Héroult electrolytic process emits in making elemental aluminum comes from atmospheric CO₂ that plants just took from the atmosphere a few years earlier, and thus there is no net CO₂ output into the atmosphere.

This is the same reason why using charcoal to smelt iron oxides (as done in Brazil) is carbon neutral./PN