Why is aluminum fluoride added to the electrolyte in aluminum smelting?

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aluminum processing Rolls of processed aluminum.

In the Hall-Héroult process used for aluminum smelting, alumina is dissolved in a molten electrolyte bath composed primarily of cryolite. During the electrolytic process, some fluoride from the cryolite is inevitably lost.12

Aluminum fluoride is added to the electrolyte as needed to restore the chemical composition of this bath. Maintaining the bath with an excess of aluminum fluoride is essential because it provides maximum efficiency for the smelting operation.1