The anode effect is a particular working state of the cell characterized by:
- High voltage
- Interruption of the aluminum production
- Production of green house gases CF4 and C2F6
and usually is triggered by a decrease in the alumina concentration in the bath below 2%.
In the ’60 Piontelli found out that the anode effect starts whenever in one of the anodes of a cell the current density exceeds a critical current density whom value depends upon the bath temperature and the alumina concentration.
During an anode effect the normal reactions that produce aluminum are interrupted and other electrochemical reactions take place with the formations of gases CO, CF4 and C2F6. These gases forms bubbles that adhere to the anode bottom, creating an electrical insulating layer which is the reason why the voltage of the cell increases up to 10 – 50 volts and even more. The maximum voltage reached during the anode effects depends also by the bath level. With high bath levels the maximum voltage reached is lower because some fraction of the line current passes through the sides of the anodes, free from the adhering bubbles.
During an anode effect it is possible to observe on the anode surface small sparks that make visible how the current passes from the anodes to the bath through the gas bubble adhering to the anodes.
In order to turn off an anode effect:
- The alumina concentration must be restored to normal values as quick as possible
- The layer of gas adhering to the anode bottom must be removed
In a modern pot with automated control system and point feeders, the computer detects an anode effect when the voltage raises above 8 V. After the detection the control systems starts an overfeeding of the pot, which means to activate at a very fast rate for some time the pot feeders, and, if enabled, try to remove the layer of adhering gas through particular up and down movements of the anodes.
To remove the layer of adhering gas it can be used also green poles which are inserted into the molten aluminum pad. The green poles provoke a violent stirring of the bath and metal and this stirring remove the adhering layer of bubbles.
In the last decades the aluminum industry has greatly reduced the anode effect frequency because of the CF4 and C2F6 green house gases generation, which have a global warming potential of 6500 and 9200 respectively. The ratio of the mass of produced gas CF4/C2F6 during an anode effect is equal to 10.