Aluminium Extraction Notes

Aluminium extraction

Extraction method:

Aluminium is  a very reactive metal. It cannot be extracted using carbon because it is above carbon in the reactivity series ie. it is  more reactive than carbon. So it is extracted through  electrolysis.

Aluminium ore:
The aluminium ore is bauxite( impure aluminium oxide.)

Major impurities in bauxite:

•  The major impurities include  Silicon dioxide, Iorn Oxide  and Titanium dioxide.(SIT)

 Purifying the aluminium oxide:

• Bauxite ore is heated in a pressure vessel along with a sodium hydroxide solution at a temperature of 150 to 200 °C. 
• At these temperatures, the aluminium is dissolved to form  sodium aluminate ( NaAlO₂ )
•  The insoluble impurities are removed by filtration.
• The sodium aluminate undergoes further treatment to form pure aluminium oxide.

Nature of the electrolyte:
The aluminium oxide must be melted so that electricity can pass through it.

Reducing the temperature of the electrolyte:

Pure aluminium oxide has a melting point of 2070°C  Instead, molten cryolite (Na₃AIF₆) is used to reduce the temperature  to  and  thus reduce energy costs.This reduces the  melting point to 915° 

Sometimes  calcium fluoride is also added which further   reduces the temperature further to 800°

Electrolytic Cell:

The steel container is coated with carbon (graphite) and this is the negative electrode (cathode).The anodes are also carbon electrodes

The reactions are as follows:

Al₂O₃(s) + 2 NaOH(aq) → 2 NaAlO_2(aq) + H₂O(l)
Sodium aluminate undergoes further treatment to form pure aluminium oxide.

Reactions at the electrodes:

                         
                   At the cathode ( Reduction)

Al³⁺  +  3e^-    --->   Al (s)


At the anode.( Oxidation)

2O^2-  -  4e^-   --->   O₂ (g)

• Aluminium metal forms at the cathode  and sinks to the bottom of the tank, where it is tapped off.
• Oxygen forms at the anode. This oxygen reacts with the carbon of the anode , forming carbon dioxide, and they gradually burn away. Consequently, the anodes have to be replaced frequently.

The overall reaction is

2Al³ (l)^{+       +} 2O^3+-_(l)      --->      6Al_(l)   +     3O_2(g)

Gases given out: 
Oxygen is discharged at the positive carbon (graphite) anode.Oxygen reacts with the carbon anode to form carbon dioxide gas.

carbon + oxygen    carbon dioxide
C_(s)  +    O_2(g)             CO_2(g)

Why carbon anode needs to be replaced?
The carbon anode slowly reacts  with oxygen to form carbon dioxide and  hence needs to be replaced regularly.