The making of Aluminum

PHASE ONE:

EXTRACTING ALUMINA, the base material for making aluminium:

1. Bauxite, the mineral that contains alumina, is first ground in giant crushers.

2. Once powdered, bauxite is mixed in autoclaves with a solution of caustic soda, which is used to dissolve aluminium oxide while impurities remain in a solid state.

3. Impurities are separated from the aluminate solution through scrubbing and filtration under pressure with the red mud residue discarded.

4. The resulting solution is pumped into precipitator tanks, which are 25 to 30 meters high, and in which pure alumina trihydrate is added as seed material. As the solution cools down more trihydrate crystals are formed.

5. To be separated from caustic soda the crystals are precipitated and the rest of the solution is filtered with caustic soda before returning to the autoclaves to be reused.

6. Finally, the collected crystals are calcinated in long rotating furnaces where a temperature of approximately 1,000 o C removes all the water from the crystals.

7. What remains of the process is calcined alumina, a kind of white powder not unlike table salt, which will be later transformed in aluminium metal.

PHASE TWO:

CONVERTING ALUMINA INTO ALUMINIUM:

1. Aluminium is extracted from alumina through an electrolytic reduction process (known as the Hall-Héroult process, named after its inventors), which takes place in pots through which flows a high-amperage direct current. Each rectangular steel shelled pot is lined with refractory bricks and carbon blocks acting as the cathode.

2. Anodes, made from petroleum coke and liquid pitch mixed into a paste and then baked for several days at a temperature of approximately 1,100 o C to be hardened, are hung over the pots in which a molten electrolyte dissolves the supplied alumina.

3. In the electrolytic bath, the current passes from the anode to the cathode (i.e. the pot itself) and, at a temperature of approximately 950 o C, reduces (i.e. breaks up) alumina molecules into aluminium and oxygen (it’s true, aluminium smelters actually do make oxygen!).

4. Molten aluminium settles at the bottom of the pot from where, at regular intervals, it is tapped into giant ladles and transferred to holding furnaces.

5. It is in these furnaces containing up to 90 tonnes of molten aluminium that the metal is further refined and optionally mixed with metal additives to produce aluminium alloys.

6. From there, the metal is cast to produce ingots, billets and slabs. Alternatively, the metal may be cast into semi-finished products
.
ELECTRICAL POWER:

The electrolysis process used to produce aluminium requires large quantities of electrical power. When the cost of producing one tonne of primary aluminium is broken down almost one third is devoted to electrical power. This forces the industry to strive to use each kilowatt to its full potential.

In fact, among all industrial activities, it is the aluminium industry that is achieving the best utilization factor in the use of this precious resource. Furthermore, when one considers that each small gain in energy consumption quickly impacts on costs, all producers have implemented energy efficiency programs that yield excellent results.

With this in mind, the quantity of energy required to produce primary aluminium has been cut by 30% over the past 35 years.

At the end of the journey, this process gives us this metal, literally an "energy bar" which, like a giant horn of plenty, provides so many consumer goods of all shapes and sizes.