5.2      describe  and  explain  the  extraction  of  aluminium  from  purified  aluminium  oxide  by electrolysis, including:

i     the use of molten cryolite as a solvent and to decrease the required operating temperature

The aluminium oxide needs to be molten in order to conduct electricity and be electrolysed (see Section 1.50).   Pure aluminium oxide has a very high melting point of around 2000 °C, which is too high to be practical.  Instead it is dissolved in molten cryolite (another aluminium compound). The melting temperature of the mixture is much lower, at around 950 °C.

ii    the need to replace the positive electrodes

Oxygen is produced at the red-hot carbon anodes.  This means they burn away, forming carbon dioxide and so need constant replacement.

iii   the cost of the electricity as a major factor

Aluminium smelters operate using current of around 200,000 A  at about 5 V.  A lot of electrical energy is needed to keep the mixture molten and to electrolyse the aluminium oxide.  This is very expensive.

 (The area labelled cryolite is actually aluminium oxide dissolved in cryolite)