Aluminium Ore: Extraction of Aluminium

We get minerals and ores in abundance in the earth’s crust. Some ores have been proven themselves a great resource for mankind. Such as iron obtained from ore of iron (Hematite) built the foundation of industrial revolution. On the other hand, aluminium was a crucial strategic resource for aviation during World War I and World War II. Still, aluminium metal dominates in the various fields of the market due to its unique properties and easy and cost - effective extraction. Generally, Aluminium ores are found in the form of its oxides. Bauxite is the most common aluminium ore and Hall – Heroult process is the major industrial process for extraction of aluminium from its oxide alumina. In this article we will discuss occurrence and extraction of aluminium in detail. 

Occurrence of Aluminium 

As aluminium is a highly electropositive element so it does not occur in the free state. It is one of the top five elements of the reactivity series. So, it is a highly reactive element or metal and generally reacts with atmospheric oxygen and is found in the form of its oxide. 

A mineral from which metal can be economically extracted in a maximum amount is called an ore. Most common aluminium ores are bauxite, corundum, cryolite and alunite. In India, most of the aluminium metal is extracted from its bauxite ore. India has large deposits of bauxite aluminium ore in Gujarat, Madhya Pradesh, Jharkhand, Uttar Pradesh and Karnataka. Here we have given a list of common aluminium ores with their chemical formulae –

Ores 

Bauxite 

Corundum 

Cryolite 

Alunite 

Chemical Formula 

Al2O3.2H2O

Al2O3

Na3AlF3

K.2SO4.Al2(SO4)3.4Al(OH)3

 

Extraction of Aluminium from Bauxite 

The science of extracting pure metals by economically effective methods from their ores is called metallurgy. Metallurgy of aluminium or extraction process of aluminium from its ore involves various methods. Mostly aluminium is extracted from its ore called bauxite. The extraction of aluminium from its ore involves following steps –

  • Dressing or Concentration of the bauxite ore by Hall’s method 

  • Electrolysis by Hall – Heroult Method

Dressing or concentration of the bauxite ore– Bauxite is generally found as an impure form of aluminium oxide. The process which gives the ore such a physical form so that the gauge can be easily removed from it, is called dressing of the ore. It is done by hand – picking, grinding etc. 

Bauxite ore generally contains ferric oxide and silica as impurities. Dressing of bauxite ore is done by crushing and pulverizing. To remove ferric oxide impurities from the bauxite, magnetic separation method is used. The ore obtained is concentrated by Hall’s method. 

Concentration of bauxite by Hall’s method – The hall method of concentration of bauxite was invented in 1886 by the American chemist Charles Martin Hall. This is the reason the method is named as Hall’s method. It takes place by following three steps –

Step 1. Conversion of impure bauxite into sodium aluminate – The ore is fused to red heat with sodium carbonate and formation of sodium aluminate takes place. Reaction involved is given below –

Al2O3.2H2O + Na2CO3 + heat 🡪 2NaAlO2 +2H2O +CO2

Step 2. Conversion of sodium aluminate into aluminium hydroxide – 

2NaAlO2 +3H2O +CO2 🡪 Na2CO3 + 2Al(OH)3

Step 3. Conversion of aluminium hydroxide into pure alumina – 

2Al(OH)3 1100℃→ Al2O3 + 3H2O

Electrolysis by Hall – Heroult Method – Alumina is highly stable oxide and melts at 2050 that’s why alumina cannot be directly electrolyzed. Its electrolysis is done with cryolite (3 parts by weight) and fluorspar (1 part by weight). In this process for electrolysis an iron tank lined with heat resistant material and has a sloping floor, provided with an outlet for tapping molten aluminium metal is used. Gas carbon or graphite are used as cathode and thick carbon rods are used as anode. Coke powder covering is used to prevent burning of carbon anodes and to prevent heat loss from molten electrolyte. A direct current of 100 A is passed through the electrolyte and the temperature is maintained at 950. In this process sodium, calcium and aluminium ions are formed which migrates towards the cathode. However, only aluminium ions reach to the cathode due to their lower position in the electrochemical series. Thus, pure aluminium get deposited at cathode and melts due to 950 temperature of the electrolyte, as it is heavier than electrolyte, so it gets deposited at the base of the electrolytic tank. While at anode nascent oxygen is formed which reacts with carbon of coke and forms carbon mono oxide which reacts with atmospheric oxygen and forms carbon dioxide. Although nascent oxygen formed at anode reacts with carbon of carbon-anode as well. That’s why carbon – anodes are consumed gradually and need to be replaced time to time. 

Reactions involved in the electrolysis are given below –

Al2O3 2Al+3 + 3O-2

Na3AlF3 3Na+ + Al+3 + 6F-

CaF2 Ca+2 + 2F-

At Cathode –

2Al+3 + 3e− → Al

At Anode –

O-2 - 2e− 🡪 [O]

2[O] 🡪 O2

C(coke) + [O] 🡪 CO

2CO + (air)O2 🡪 2CO2

Thus, pure aluminium metal is obtained from the chief ore of aluminium which is bauxite. 

Uses of Aluminium 

Due to its properties aluminium is used in a number of fields. Main uses of aluminium are as follows –

  • It is widely used in aviation. 

  • We have been using aluminium vessels since ages. Many types of utensils of aluminium are available. As it is a good conductor of heat.

  • It is used in many alloys.

  • It is used in electric wires. As it is a very good conductor of electricity. 

  • It is used as aluminium foils to keep the things warm. 

  • It is used in the packaging industry.

  • Paints are prepared by mixing aluminium powder in linseed oil.

  • It is used in explosives.

  • It is used in the extraction of gold and silver.

  • It works as a reducing agent for many organic reactions. 

  • It is used in thermite welding. 

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