3.2.7 Extraction of Metals - Environmental aspects of metal extraction
Students should:
|
Economic factors and recycling
The production of metals is expensive. Old iron (girders, cars, metal cans etc) and aluminium (old planes, trains, milk bottle tops, cans etc) are pure forms of the 'ore'. Without any additional processing costs they can be melted up in routine production process to reduce costs.
The major problem with recycling is the collection and sorting of the material from other waste materials. This has to be done on a large scale to make it economically viable.
The benefits of recycling are that:
- 1 Waste material does not accumulate in the environment and cause visual and actual chemical pollution;
- 2 If the scale is right recycling can cut the cost of production by providing a cheaper ore;
- 3 Less mining need to be done. This leads to less disruption and pollution of the environment.
Copper extraction
Cu is extracted from the ore chalcopyrite, CuFeS2, in a three stage process
In the first stage chalcopyrite is heated with silicon dioxide and oxygen
2CuFeS2 + + 2SiO2 + 4O2 
Cu2S + 2FeSiO3 + 3SO2
The SO2 produced would cause acid rain if released into the atmosphere. However, the SO2 can be captured and used to make sulphuric acid, H2SO4
In stage 2, the copper(I) sulphide is roasted with oxygen at a high temperature
Cu2S + 2O2 2CuO +SO2
In stage 3, the copper(II) oxide is reduced by heating with carbon
CuO + C Cu +CO
Extraction of copper from low grade ores
Aqueous solutions of copper compounds can be made by leaching from low grade ores. The copper can then be extracted by displacement using scrap iron as the reducing agent
Fe + Cu2+ Fe2+ + Cu
This is a low cost method of extracting copper because it uses scrap iron as the reducing agent and it has a low energy requirement