Rocks that contain useful metals at a high enough percentage to be extracted economically are called ores.
Aluminium and iron are the most abundant ores in the earths crust.
Most compounds are found as oxides or sulphides; sulphides are usually converted to oxides by roasting in air.
E.g. Zinc sulphide
2ZnS + 3O2 ---> 2ZnO + 2SO2
The oxide is then reduced by carbon.
SO2 can be released into the environment but that produces ACID RAIN; a more useful way of utilising the SO2 is in the contact process to make H2SO4.
The method of extraction depends on;
- Energy requirements
- Purity of the end result
Method 1: Reduction with Carbon
Iron, Manganese and Copper are extracted using this method.
Fe2O3 + 3CO ---> 2Fe + 3CO2
Carbon on its own instead of carbon monoxide can be used, the other product produced is CO.
MnO2 + C ---> Mn + CO2
2CuO + C ---> 2Cu + CO2 ( The CuO comes from the thermal decomposition of malachite)
Carbon and Carbon dioxide are both harmful.
Temp for these reactions is very high.
This method can't be used to extract some metals like tungsten because the metal carbide is formed rather than the metal by itelf.
WC instead of just W.
Method 2: Reduction by Hydrogen
WO3 + 3H2 ---> W + H2O
The WO3 is heated with hydrogen at 900 degrees celcius.
There is a high risk of explosion when handling hydrogen and problems with storage.
BUUUT there are some positives;
- No pollution given off in this reaction.
- Readily available reagent.
- Gives a higher purity than C does.
Method 3: Reaction w/ a more reactive metal
This metal is useful because it is abundant, has low density and is corrosion resistant.
TiO2 + 2Cl2 + 2C ---> TiCL4 + 2CO
TiCl4 + 4Na ---> Ti + 4NaCl ( or Mg can be used instead of NA)
Steps of this process;
- Solid titanium oxide is converted to titanium chloride at 900 degress.
- The titanium chloride is purified by fractional distillation in an argon atomsphere.
- The titanium is then extracted by Mg or Na in an argon atmosphere at 500 degress.
Titanium is costly because;
- Na and Mg is expensive.
- Batch process so it is slow and requires manual labour.
- Argon and the need to remove moisture (hydrolysis) is expensive.
- High temperatures are required.
Method 4: Extraction by electrolysis
If a metal is more reactive than carbon, electrolysis is used.
The aluminium must be molten to conduct electricity.
Has to be dissolved in molten cryolite to lower the melting pt of AL2O3.
It would be uneconomic without cryolite.
Cathode: Al3+ + 3e- ---> Al
Anode: 2O2- ---> 02 + 4e-
- saves resources like ores
- creates less waste
- saves energy
- utilises scrap metal
- reduces air pollution
However there are costs involved in sorting, collecting, trabsporting.
Copper can be extracted by displacement of scrap iron with an aqueos solution of Cu.
The more reactive iron displaces the Cu by reducing the Cu ions.
This is sustainable because even low grade ores can be used.