Production method 2 - Electrolysis

Electrolysis is carried out in large steel vessels lined with a graphite cathode, and into which an array of carbon-based anodes is suspended. Other features include:

  • A way of breaking the crust that forms on the electrolyte in order to add aluminium oxide
  • A means of adding more aluminium oxide
  • A point from which molten aluminium can be removed
  • A removable hood to collect waste gases, mostly carbon dioxide and some fluorocarbons
The cell may be up to 8m long, 4m wide, and about 1m in depth, excluding the hood. A typical production facility may contain as many as 400 such cells, each producing between 1 and 2 tonnes of aluminium each day, amounting to over 200,000 tonnes per year.

Cell processes

The cell voltage is in the region of 4v, but the current can be as much as 300,000 amps in each cell. The heating effect of the electric current is responsible for maintaining the temperature of the cell.

At the cathode that lines the cell, aluminium is produced and collects below the less dense electrolyte.

chemical equations

At the anode oxygen is produced, which reacts with carbon in the anode to produce carbon dioxide. This eats away at the anode, which has to be replaced about every 20 days.

chemical equations

The Electrolyte

Electrolyte composition is important, and it is necessary to replace fluorine lost as fluorocarbons and inorganic fluorides. This is achieved by adding aluminium fluoride, made from aluminium hydroxide available from the Bayer process.
Many modern facilities now use many sensors and computer control to ensure optimum cell conditions. This helps to keep emissions lower, as well as maximising output. If conditions are not maintained correctly, fluorocarbons are produced from a reaction with the anodes. Although these are not harmful, they are greenhouse gases.

Anodes

Pre-baked anodes are constructed from graphite or coke, mixed with pitch (made from coal tar) and baked. Used, crushed anode stubs are included in the mix. Each anode lasts about 20 days, and so 20 anodes will typically be used in a cell, with one being changed every day. This allows production to continue more or less without interruption.

Green Anodes

Anode production forms a significant part of an aluminium plant. A 200,000 tonnes per year aluminium facility will consume in the region of 100,000 tonnes of anodes, generating a lot of carbon dioxide in the process. Much research is therefore directed towards designing inert, non-consumable anodes. This would significantly reduce carbon dioxide emissions from the electrolysis, and remove the need for the continual anode preparation and baking.

Click here for additional information about development of alternative anodes

diagram: Aluminium Electrolysis Cell diagram: Aluminium Electrolysis Cell diagram: Aluminium Electrolysis Cell diagram: Aluminium Electrolysis Cell diagram: Aluminium Electrolysis Cell diagram: Electrolysis Cell diagram: Aluminium Electrolysis Cell diagram: Aluminium Electrolysis Cell diagram: Aluminium Electrolysis Cell diagram: Aluminium Electrolysis Cell diagram: Aluminium Electrolysis Cell
diagram: Aluminium Electrolysis Cell
diagram: Aluminum Electrolysis Cell diagram: Aluminium Electrolysis Cell diagram: Aluminium Electroloysis Cell

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