The cryolite ledge
In an aluminum electrolysis cell the electrolyte is found in a molten state with a working temperature only 10 ÷ 20 °C above the freezing point, while the molten aluminum of the metal pad is 300 °C above its freezing point.
Because the electrolyte is so close to its freezing point, we find a ledge of frozen electrolyte attached to the sides of the cell, being these ones the coldest area of a pot.
This frozen electrolyte ledge is made only of cryolite. In fact, looking at the phase diagram of the system NaF – AlF3, we see that when the electrolyte freezes the first phase which is separated is the cryolite.
The ledge plays an important role in the heat balance and bath chemistry control of a pot:
- Heat balance: the thickness of the cryolite ledge changes according to the thermal behavior of the cell. In fact, whenever the temperature of the electrolyte increases with an increase in the temperature difference between the bath temperature and the liquidus temperature, some ledge melts. The melting of ledge happens at constant temperature, thus “absorbing” heat and decreasing the temperature excursions. Similarly, whenever the difference between the bath temperature and the liquidus temperature decreases, some bath freezes increasing the ledge thickness and helping the cell to reduce the temperature excursion
- Because the ledge is made only of cryolite, the melting and freezing of it will change the bath composition, primarily the ratio (or, in other terms, the AlF3 excess) because some liquid cryolite will be added or subtracted from the molten bath.
The cryolite ledge protects also the carbon side of the cell from the chemical attack of the electrolyte. In fact, without this protecting layer, the molten electrolyte in direct contact with the carbon sides would destroy them very quickly, leading to bath tap-out and loss of the cell.