Aluminium

Industrial Aluminium Smelting - Hall Heroult Process Overivew

Aluminum is the most widely used nonferrous metal in the world today.

As of, world aluminum production was at 44,100 kilotons (about 41 percent of which was smelted in China alone).

Its popularity is unsurprising actually considering that the metal (both in its pure and alloyed forms) is renowned for its resilience despite its light weight, its high conductivity and its ability to withstand corrosion. And of course, we should not discount that it is, after all, the most abundant metal in the Earth’s crust.

In spite of this, aluminum production does present one main challenge: finding it in pure, elemental form is extremely and impractically difficult—this fact owing much to aluminum’s very high chemical reactivity. That is why, as with most metals, it is first mined in ore form before undergoing purification.

The Bayer process is the first of which, and is concerned with extracting aluminum oxide from bauxite ore. Aluminum oxide, also known as alumina, produced from this process is further refined into pure aluminum using the Hall–Héroult process.

 

 Charles Martin Hall

The Hall–Héroult process starts with the dissolution of alumina in molten cryolite (Na3AlF6; sodium hexafluoride) vat, which is also known as a “cell.” A small amount of aluminum fluoride is added into the alumina–cryolite mixture to reduce the melting point of cryolite from 1,012 °C (1,854 °F) to about less than 1,000 °C (1,830 °F). Take note though that alumina has a melting point of more than 2,000 °C (3,630 °F).

Thus only a small amount of alumina actually dissolves in the cryolite vat, which is then passed with an electric current.

The current itself is used to heat up the cells, and the electrolysis resulting therefrom leads to deposition of pure aluminum at the cathode as precipitate, while the anode, on the other hand, serves to produce carbon dioxide. Liquid elemental aluminum is siphon-transferred either in batches or by continuous flow to casts where they solidify into ingots or final-cast products.

 Paul Heroult

The electrolysis of the alumina–cryolite mixture is mainly driven by the amount of current passed through the vat. Voltage, on the other hand, is a no brainer: electrolysis can be performed efficiently in electric potentials of five Volts to as low as three Volts. The voltage requirement is further reduced as the carbon dioxide anode is oxidized. This means that anodes would have to be constantly replaced in order to increase electrical efficiency and thus minimize associated power costs.

There are two methods that can be employed in manufacturing anodes: the Söderberg and the prebake processes.

  • The Söderberg uses the continuous addition of pitch to the top of the anode. The pitch is readily baked by the excess heat created by the electrolysis of the alumina–cryolite mixture into carbon form, which is used to react with the mixture.
  • The prebake process involves, as its name suggests, the prebaking of pitch in large gas ovens prior to dipping into the mixture—this is as opposed to baking them onsite as with the previous method. The latter process is preferred because it is slightly more efficient and produces less greenhouse emmissions.

The rate of production being proportional to the amount of current, industrial smelting cells usually consume hundreds of thousands of Amperes at any given point in time during operation.

Meanwhile, this enormous amount of current can create a significantly strong magnetic field within the vats—one that is large enough to cause alumina–cryolite mixtures to swirl (and thus aid the further dissolution of alumina in cryolite) on their own even without mechanical assistance. This is a phenomenon often exploited by cell engineers to minimize cost and maximize efficiency.

Power is supplied by in-site transformers which convert grid-sourced alternating current to direct current. The large demand of aluminum smelters for power naturally makes sites where cheap and constant supply of power is readily available an undeniably popular option.

To be precise, hydroelectric power plants are the power source of choice in the developed world, where aluminum smelting factories are usually built a few kilometers from these plants in order to minimize transmission costs as well as the possibility of intermittent power outages due to transmission-side problems.

The Hall–Héroult process in itself was a very large improvement from previous methods, which involved heating bauxite with pure sodium or potassium inside a vacuum. These methods were far more complicated and resource intensive.

The high prices of sodium and potassium then contributed all the more to the historical prices of aluminum, so much so to the extent that aluminum during the early 19th century were far more costlier than gold, silver or platinum. (Interestingly, Napoleon III of France has been fabled to have kept his aluminum silverware only for use of his most important guests.)

With the advance of smelting technology, aluminum prices did go down, but it was only through the high efficiency of the being the Hall–Héroult process that we enjoy cheap aluminum prices today.

Various alternatives have been explored to replace the Hall–Héroult, many of them seeking to minimize the carbon footprint that it produces. There have been no convincing contenders as of yet, however, and the Hall–Héroult process remains exclusively to be the method for aluminum production.