Reduction of aluminium oxide by electrolysis
The electrolysis method may be used if the metal oxide is extremely stable. The manufacture of aluminium is carried out by the electrolysis of purified bauxite, Al O , which is dissolved in molten cryolite, Na AlF . The melting point of aluminium oxide is over 2000 C, but by dissolving the oxide in molten cyrolite, the temperature of the melt is reduced to about 970 C. The electrodes are made of carbon and the reactions at the electrodes are:
at the cathode: Al (l) + 3e -> Al(l)
at the anode: 2O (l) -> O (g) + 4e
Some of the oxygen evolved reacts with the carbon anodes at the high temperature:
2C(s) + O (g) -> 2CO (g) and C(s) + O (g) -> CO (g)
The process consumes large amounts of electricity (electricity is needed to melt the cryolite and to decompose Al O ) and is economic only where electricity is relatively inexpensive. The process is continuous, but regular additions of aluminium oxide are needed, and the carbon anodes need replacing as they are consumed. There is a potential environment problem through waste cryolite causing fluoride pollution.
Extraction of titanium
Titanium is obtained in two steps:
Step 1: Conversion of titanium(IV) oxide into titanium(IV) chloride.
The ore rutile, impure titanium(IV) oxide, is converted into titanium(IV) chloride using chlorine and coke at around 900 C:
TiO (s) + 2C(s) + 2Cl (g) -> TiCl (g) + 2CO(g)
At room temperature, titanium(IV) chloride is a colourless liquid which fumes in moist air because of hydrolysis. It is purified from other chlorides by fractional distillation under argon or nitrogen.
Step 2: Reduction of…