Metal Mists and Aluminum Losses in the Hall Process

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Abstract
Reaction of aluminum with Hall cell electrolyte forms aluminum monofluoride and sodium which dissolve in some form to give the bath a reducing capacity equivalent to a free aluminum content of about one tenth per cent. The so‐called “metal mist” forms in the presence of moisture and consists of hydrogen bubbles containing a small partial pressure of aluminum monofluoride, sodium, and sodium tetrafluoroaluminate. Increasing the ratio of the electrolyte increases the sodium and decreases the aluminum monofluoride partial pressure, but sodium has a much higher partial pressure; hence, a low ratio is desirable to minimize metal reoxidation. Carbon dioxide is abundant in the anode gas of a smelting cell and is soluble in the bath. It oxidizes the free metal both in and over the bath. The rate of this reoxidation appears to be controlled by the rate of diffusion and convection of dissolved metal away from the metal‐bath interface.

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