Criolyte Ratio, Mass Ratio and % Excess AlF3
Usually, in a modern point feed pot the bath composition is the following:
- AlF3: 10% ÷ 13%
- Al2O3: 2.5% ÷ 3.5%
- CaF2: 4% ÷ 7%
- MgF2: 0% ÷ 1%
- The rest is cryolite
Calcium fluoride and magnesium fluoride typically build up into the bath because they arrive as alumina impurities.
Al2O3 is the raw material from where the aluminum is produced, while AlF3 is intentionally added to increase the current efficiency.
Cryolite can be also thought as composed by three moles of sodium fluoride (NaF) and one mole of aluminum fluoride (AlF3):
Consequently, in the bath is always present some AlF3, coming from the cryolite. In the industrial process in addition to this AlF3 it is added some other AlF3. There are several ways to measure the bath composition respect to the NaF and AlF3 content:
- Cryolite ratio is the ratio between:
For the pure cryolite the cryolite ratio is equal to 3
- The weight ratio is equal to:
- The AlF3 added can be expressed also as % excess AlF3, where the excess is respect the AlF3 coming from the cryolite. In other terms:
So, for the pure cryolite we have:
1 NaF mole weights: 1 Na mole (22.9897 g) + 1 F mole (18.9984 g) = 41.9881 gAlF3 mole weights: 1 Al mole (26.9815 g) + 3 F mole (3 ∙ 18.9984 g) = 83.9767 g
Electrolyte Ionic Structure
Molten cryolite is completely ionized according to the following reaction:
The AlF3 added in excess reacts with the F- ions in order to form AlF4- ions according to the following reaction:
The AlF3 added acts as a Lewis acid accepting a lone pair (). This is the reason why the percent of excess aluminum fluoride is also referred as “acidity of the bath”.
When we add alumina to the cryolite bath this dissolves forming the following species:
The reactions that guide the aluminum production are the following:
- At the cathode:
Where the electrons come out from the metal pad. The formed aluminum is then deposited into the molten aluminum pad.
- At the anode thermodynamically should be produced CO, but actually is CO2 what evolves, because the reaction producing CO2 is kinetically favoured compared to the reaction producing CO: