Alumina is reduced to aluminium metal in electrolytic cells known as pots, which are organised into potlines within the smelter.

Potlines

Modern potlines tend to have pots arranged side-to-side and almost all operation and maintenance is carried out using overhead multipurpose cranes - several tasks are carried out regularly including replenishing alumina supplies, changing the anodes and removing the finished product - molten aluminium metal.

The molten metal, which is removed (or tapped) from the cell, is then transferred to a holding furnace prior to processing.

Pots

A pot consists of two main parts:

• a block of carbon that has been formed by baking a mixture of coke and pitch. This block serves as an anode (or positive electrode)
• under the anode is a large rectangular steel box lined with carbon made by baking a mixture of metallurgical coke and pitch. This lining is the cathode (or negative electrode).

Between the anode and the cathode is a space filled by electrolyte. This mixture must be heated to about 980°C, at which point it melts and the refined alumina is added, this then dissolves in the molten electrolyte. This hot molten mixture is electrolyzed at a low voltage of 4-5 volts, but a high current of 50,000-350,000 amperes. This process reduces the aluminium ions to produce molten aluminium metal at the cathode, oxygen is produced at the graphite anode and reacts with the carbon to produce carbon dioxide.

2Al2O3 + 3C ---> 4Al + 3CO2

However some of the metal, instead of being deposited at the bottom of the cell, is dissolved in the electrolyte and reoxidised by the CO2 evolved at the anode:

2Al+ 3CO2 ---> Al2O3 + 3CO

This reaction can reduce the efficiency of the cell and increases the cell's carbon consumption.

The electrolyte

The electrolyte used is cryolite (Na3AlF6), which is the best solvent for alumina. To improve the performance of the cells other compounds are added including aluminium fluoride and calcium fluoride (used to lower the electrolyte's freezing point).

The electrolyte ensures that a physical separation is maintained between the liquid aluminium (at the cathode) and the carbon dioxide/carbon monoxide (at the anode).

The anode

As we have seen, the carbon anodes used in the Hall-Héroult process are consumed during electrolysis. Two designs exist for these anodes; Söderberg and Pre-Bake.

Söderberg anodes are baked by the heat from the electrolytic cell, they do not need changing but are "continuously consumed".

Pre-Bake anodes are made separately, using coke particles bonded with pitch and baked in an oven. Pre-bake anodes are consumed and must then be changed.

The cathode

The cathode consists of a graphite shell embedded with steel bars to minimise current resistance. During operation the liquid aluminium itself begins to operate as the cathode, a feature that can complicate cell design because of the inevitable magnetic effects of such large currents. Typically a cathode will last between 1000 and 2500 days before it needs replacing.

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