In an alumina reduction cell the aluminum is produced by the following reaction:
driven by the electrical energy given from the external of the cell.
In parallel with this reaction, another reaction occurs which reoxidizes part of the aluminum produced to alumina, according to the reaction:
This reaction is usually called “back reaction” and is the main factor driving a current efficiency less than 100%.
Let’s see now more in detail how the back reaction works and what can be done in order to minimize it, hence increasing the current efficiency.
The picture above describe in some detail the mechanism of the back reaction:
- Some metal diffuses from the aluminum pad into the molten bath
- At the same time some CO2 diffuses into the bath
- Aluminum and CO2 diffused can than react according to the reaction (2)
To minimize the back reaction we need to work on the factors that reduce the mass transfer of aluminum into the molten bath. In particular:
- Reduce the bath temperature, because this reduces the aluminum solubility in it
- Increase the bath acidity, because this reduces the aluminum solubility in it
- Keep the interface of the metal pad as flat as possible, hence reducing the cell noise
- Optimizing the magnetic design of the pot in order to decrease the metal velocity
- Keep an adeguate ACD, in order to have more space between the zone where the dissolved aluminum is present from the zone where the dissolved CO2 is present
The back reaction is the most important factor reducing current efficiency, but it is not the only ones. Some other factors reduce the current efficiency:
- In presence of a vigorous agitation of the metal pad due to unbalances in the anodic current distribution, metal waves can touch directly one or more anodes. In this case the electrical current passes directly from the anode to the metal pad without producing electrolysis of the alumina and hence reducing the current efficiency
- During an anode effect the alumina reduction reactions are interrupted. The electrical current flows without producing aluminum and hence reducing the current efficiency