Electrolytic Reduction of Alumina

Aluminium will be produced by the process-electrolysis of fused salts, the use of cryolite as an electrolyte( flux) to dissolve alumina ,and the use of carbon electrodes.

Raw materials

• Carbon: In the electrolysis of aluminium production part of the energy for reducing alumina is supplied as electricity and parts comes from consumption of the carbon anode. About 0.4-0.5 kg of anode is consumed for one kg Aluminium production. Carbon is used as the cathode lining too.
• Aluminium Oxide: About 1.9 -1.95 kg of Al₂O₃ (alumina) are used for production one kg aluminium.
• Electrolyte Materials: The electrolyte is a solution of aluminium oxide in cryolite. Cryolite comprises more than 75% of the electrolyte which contains calcium fluoride (4-8%) ,aluminium fluoride (5- 15%) alumina (1- 6%) and sometimes lithium fluoride (0-5%) and magnesium fluoride (0-5%).These additives lower operating temperature and increase current efficiency.

Theoretical Principles of Fusion Electrolysis

Electrolytes

The mineral cryolite is a component of 3NaF.AlF₃ (Na₃AlF₆) and has a melting point of about 1010 ^oC. Synthetic cryolite can be produced by the following reaction:

6HF + 2NaOH = Na₃AlF₆ + 4H₂O

Cyrolite is produced directly in reduction cell by reaction of the soda impurity in the feed alumina with added aluminium fluoride as following:

3Na₂O + 4ALF₃ = 2Na₃AlF₆ + Al₂O₃

There are a compound with 40% AlF₃ and 60% NaF and called Cyrolite (Na₃AlF₆).It melted at about 1000 ^oC.Al₂O₃ has a melt point about 2150^oC.Figure1shows that about 10-11 %Al₂O₃ is soluble at about 962 ^oC in Cyrolite but this solubility will be reduced by the additive such as AlF₃,CaF₂ and Li₃AlF₆.

During electrolysis of aluminium several additives will be used, they have different influence of the physical properties of the process.