Aluminum deposition from a molten salt

The classical process for winning aluminum is one developed by Hall and by Herault in 1886 and 1888. Aluminum oxide (AI2O3) of high purity is mixed with cryolite (NajAlFg) in the ratio of 1 10. The mixture melts at approximately 950 °C. The preparation of the pure aluminum oxide, which is usually from bauxite, is the first step in winning aluminum. This process was developed by K.J. Bayer in 1892. The bauxite is treated with concentrated sodium hydroxide and reacted with the aluminum minerals to get sodium aluminate. After aluminum hydroxide precipitates from the aluminate solution, it is then calcinated at 1200-1300 °C. 

The aluminum oxide and the ions in the cryolite melt form oxy-fluoride complexes, e.g., 

From kinetic experiments, the following mechanism of aluminum deposition at the liquid aluminum electrode was derived  

In a technical deposition process one also has to look for the counter-electrode, which in aluminum electrolysis is a graphite electrode. The processes at the carbon electrode are described by the equations 

The electrolysis needs a large amount of energy. Increasing energy prices may open the way to alternative processes, e.g., to win aluminum from chloride melts. 

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