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Aluminium Production
Smelting
Aluminium smelting is conducted chiefly through the Hall-Héroult electrolysis process. This process involves passing a direct current through a solution of alumina dissolved in molten cryolite; adding cryolite allows the electrolysis to occur at a lower temperature. The electrolyte is placed in an iron vat, which serves as the cathode, lined with graphite. In addition to this, carbon anodes are immersed in the electrolyte. Upon passage of electrical current through the electrolyte, the molten aluminium metal is deposited at the bottom of the cathode while carbon is oxidized to form carbon dioxide at the anode.

The reaction at the cathode is:

Oxygen is formed at the anode, which gets oxidized to form carbon dioxide:

There are two main types of smelting technologies; Soderberg and Pre-Bake. The principal difference between the two technologies is the type of anode used.
  • Soderberg technology
    Söderberg technology uses a continuous anode which is delivered to the cell in the form of a paste and which bakes in the cell itself.
  • Pre-Bake
    Pre-bake technology uses multiple anodes in each cell, which are pre-baked in a separate facility and attached to rods that suspend the anodes in the cell. The newest primary aluminium production facilities use a variant of pre-bake technology called Centre Worked Pre-bake Technology (CWPB).

    Plants using CPWB technology demonstrate very low fugitive emissions, which amounts to less than 2% of total emissions. The balance of the emissions is collected inside the cell and moved to scrubbing systems, which remove particulate matter and gases.
 
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