Electrolysis of Aluminium Oxide and Sodium Chloride

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The electrolysis of aluminium oxide and sodium chloride is an important industrial process. It is a way of extracting the element aluminium from its principal commercial source, the bauxite ore.

Aluminium is the most abundant metallic element on Earth, but it occurs in nature only in very complex forms and is hard to isolate in its pure metal form. The Hall-Heroult process of producing aluminum by electrolysis was developed in 1886 by Charles M. Hall, then a 21-year-old student at Oberlin College in Ohio. It was the first large-scale application of electrolysis — a process that uses electrical energy to drive unfavorable chemical reactions to completion. Electrolysis is used to isolate the reactive metals such as sodium, potassium and aluminum from their natural compounds.

Unlike other aluminium refining processes that use water-based solutions, the Hall process electrolyses a molten salt solution. This is because it would be expensive to melt the highly insoluble aluminium oxide (Al2O3), whose melting point is 2072 degC. Instead, it is dissolved in a molten cryolite electrolyte that lowers the melting point to 950 degC.

A direct electric current passes through the ionic solution in an electrolytic cell and chemical oxidation and reduction reactions take place. At the cathode, ions of aluminium are reduced to produce liquid aluminium (Al3+ - Al0). At the anode, chloride ions discharge to form chlorine gas. Because aluminium is denser than the electrolyte, the liquid aluminium sinks to the bottom of the cell and is collected as the pure molten metal. The aluminium is then alloyed with copper (and smaller amounts of magnesium, silicon and iron) to make a stronger, lighter and better corrosion-resistant material called 'Duralumin' that is used in car components, greenhouses and window frames and the steel strands that hold overhead power lines.

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