Do you remember the extraction of metal ores in AS/A-level Chemistry?
Metal Extraction
- An ore is a natural substance that a metal can be economically extracted from.
- Metals are often found in ores as sulphides (such as lead sulphide and zinc sulphide) or oxides (like titanium dioxide and iron(III) oxide). The metal element needs to be removed from these compounds.
Sulphide Ores
- The first step to extract a metal from a sulphide ore is to turn it into an oxide by roasting the sulphide in air.
Zinc sulphide + oxygen → zinc oxide + sulphur dioxide
ZnS(s) + 3O2(g) → 2ZnO(s) + SO2(g)
- Sulphur dioxide causes acid rain.
- Acid rain can cause harm to plants and aquatic life and damage limestone buildings, so the sulphur dioxide cannot be released into the atmosphere.
- By converting the sulphur dioxide into sulphuric acid, a pollutant is avoided, and a valuable product is made – sulphuric acid is in demand because it is used in many chemical and manufacturing processes.
Reduction of Oxides
- The method for reducing the oxide depends on the metal that are going to be extracted.
- E.g. Carbon (as coke – a solid fuel made from coal) and carbon monoxide are used as reducing agents for many metals – usually the ones that are less reactive than carbon.
(1) Iron(III) oxide (haematite) is reduced by carbon or carbon monoxide to iron and carbon dioxide.
2Fe2O3 + 3C → 4Fe + 3CO2
Fe2O3 + 3CO → 2Fe + 3CO2
This happens in a blast furnace at temperatures greater than 700°C.
Coke and haematite are added continuously to the top of the blast furnace.
The coke reacts with oxygen at the base of the furnace to produce carbon dioxide, as this is an exothermic reaction, it heats the furnace.
C(s) + O2(g) → CO2(g)
To make the reducing agent, the carbon dioxide rises up the furnace and reacts with more coke to produce carbon monoxide (endothermic).
C(s) + CO2(g) → 2CO(g)
(2) Manganese(IV) oxide is reduced with carbon (as coke) or carbon monoxide in a blast furnace).
MnO2 + C → Mn + CO2
MnO2 + 2CO → Mn + 2CO2
- This needs higher temperatures than iron(III) oxide – about 1200°C.
(3) Copper Carbonate (malachite) is reduced by heating directly with carbon.
2CuCO3 + C → 2Cu + 3CO2
- Another method involves heating the carbonate until it decomposes, then reducing the oxide with carbon.
CuCO3 → CuO + CO2
2CuO + C → 2Cu + CO2
Advantages of Reduction with Carbon
- The raw materials are very cheap.
- The reaction is exothermic, so less energy is needed to keep the furnace hot.
- The purity of the iron is sufficient for most steel-making purposes.
- It is a continuous process which means iron can be made more efficiently.
Disadvantages of Reduction with Carbon
- The use of carbon means carbon dioxide is released – which is a greenhouse gas.
- Metals which are higher than carbon in the reactivity series (e.g. aluminium) cannot be extracted in this way.
- Metals which react with carbon to for carbides (e.g. titanium and tungsten) cannot be extracted this way.
- Metals that need to be very pure cannot be produced in this way, as the metal produced always contains significant quantities of carbon.
Carbon and carbon monoxide are the first choice for extracting metals because they are cheap. But they are not always suitable – some metals have to be extracted by other methods. To be continued...
References:
1. https://slideplayer.com/slide/7751551/
Drafted by Bonnie (Chemistry)