The Occurrence and Principles of Extraction of Aluminium

Aluminium is one of the most abundant metals found on Earth and known to be the most commonly used non ferrous metal. As it possesses high chemical activities, pure aluminium cannot be found in nature. However, aluminium compounds can be found in almost all kinds of rocks, vegetation, and even animals. The most significant ore of aluminium is bauxite (Al2O3.2H2O), with varying degrees of purity. Other important aluminium ores are corundum (Al2O3), cryolite (Na3AlF6), etc.

Occurrence of Aluminium 

Since aluminium is a highly reactive metal, it cannot be found free in nature. However, its metal compounds are highly abundant in the Earth’s crust. Examples of such hugely available compounds are aluminosilicates like clay, kaolin, mica, etc. 

Other examples of the oxide compounds of aluminium are corundum (Al2O3) and emery. Among the ores of aluminium, bauxite is known as the most crucial and highly economical raw material for aluminium production. Bauxite is known to contain approximately 60% of aluminium hydroxide [Al (OH)3], 30% iron oxide (Fe2O3), and silica (SiO2).

Characteristics of Aluminium

Due to its characteristics, aluminium is widely used in the aerospace, transportation, and construction industries. Some characteristics of aluminium include:

• Non-corrosive in nature
• Can be cast easily
• Lightweight
• Highly durable
• Non-magnetic in nature
• Good conductor of heat and electricity

Properties of Aluminium 

Principles of Extraction of Aluminium 

The term metallurgy refers to the process of extracting pure metals from their ores via economically favourable methods. One can use various methods for the metallurgy of aluminium. However, aluminium is primarily extracted from its ore, called bauxite. 

As aluminium is highly reactive, its extraction is not favourable using carbon reduction methods. It is not an economical process, as the required temperature for such a reaction is remarkably high. 

Therefore, for the extraction of aluminium, the electrolysis method is used. In this process, the ore is converted into pure aluminium oxide via the Bayer Process. Bauxite contains impurities like ferric oxides, titanium dioxide, silica, etc. Thus, the concentration of this ore is necessary to remove impurities.

The Bayer process is generally the most widely used process for the production of alumina. It is then used to produce aluminium by the methods of the Hall-Héroult electrolytic process.

The Bayer Process

(a) Reaction with Sodium Hydroxide (NaOH) solution

Using this process, the red varieties of bauxite ores that contain mainly iron oxide impurities are purified to extract aluminium. 

First, bauxite ore is crushed and then treated with concentrated sodium hydroxide (NaOH) solution. Here, the reaction temperatures generally vary between 140-240°C, and pressures can rise to about 35 atmospheres.

In this process, high pressure is used so that the water molecule of the sodium hydroxide solution remains in the liquid form at temperatures above 100°C. 

Aluminium oxide reacts with the concentrated NaOH solution and produces sodium tetrahydroxoaluminate solution.

Al2O3 + 2NaOH + 3H2O ⟶ 2NaAl(OH)4 …………………..(1)

In this process, the impurities of the bauxite ore remain solids, as the other metal oxides present in the ore do not react with the NaOH solution and therefore remain unaltered. 

These solid impurities are separated from the sodium tetrahydroxoaluminate solution via filtration. After filtration, a “red mud” is produced that generally gets stored in huge lagoons.

(b) Precipitation of Aluminium Hydroxide [Al(OH)3]

The sodium tetrahydroxoaluminate solution is cooled. The extracted aluminium hydroxide is added into that solution to “seed” it so that a new batch of aluminium hydroxide can precipitate around it.

NaAl(OH)4 ⟶ Al(OH)3 + NaOH             …………………………(2)

(c) Formation of pure Aluminium Oxide(Al2O3)

The aluminium hydroxide [Al(OH)3] is then heated to a temperature of about 1100-1200°C, and aluminium oxide, also known as alumina, is produced.

2Al(OH)3 ⟶ Al2O3 + 3H2O   ……………………………………….(3)

(d) Conversion of the Aluminium Oxide (Al2O3) into Aluminium by electrolysis method

The aluminium oxide (Al2O3) is then electrolysed in a solution containing molten cryolite (Na3AlF6), another type of aluminium ore.

Conclusion

In this article, we have discussed the occurrence and the extraction process of aluminium. Aluminium is a metal of group 13 in the periodic table. Aluminium is extracted from its ores that are naturally found. The most widely used ore of aluminium is bauxite, which can be found in abundance. 

Aluminium is extracted via the process of electrolysis. It involves passing electricity via an ionic substance in a solvent solution in the presence of a cathode and anode. During this process, aluminium gets deposited in one of the electrodes.