(a) Extraction of metals
-The most unreactive metals like copper and gold exist in their ores as elements. All that needs to be done is remove sand or soil and other impurities. They can be washed off, and no chemical reaction is needed.
-More reactive metals exist in compounds and have to be reduced, for example how iron oxide is reduced in the blast furnace by carbon monoxide to form iron. Carbon monoxide is a reducing agent.
-The more reactive metals have very stable compounds and must be extracted by electrolysis. Aluminium’s main ore is bauxite, it is purified to get alumina; it is Aluminium (III) oxide (extraction of aluminium).
Extraction of iron from haematite in the blast furnace:
- Coke burns with air: carbon + oxygen → carbon dioxide
(exothermic) - Carbon dioxide reacts with coke: carbon dioxide + carbon →
carbon monoxide (endothermic)
- Iron (III) oxide + carbon monoxide → iron + carbon dioxide
- Limestone reacts with impurities: calcium carbonate (limestone)
+ silicon dioxide (silica) → calcium silicate + carbon dioxide
Iron to steel: Molten iron from the blast furnace is poured into an oxygen furnace. Basic calcium oxide is added, and a jet of oxygen is turned on. The calcium oxide neutralises the acidic impurities, forming slag that is skimmed off. The oxygen burns the other
impurities away. The carbon content is checked continually until it is just right –if there is too much it is brittle, too little and it is weak – then the oxygen is turned off.
Extraction of zinc from zinc blende: Zinc blende is zinc sulphide (ZnS). It is roasted in air, giving zinc oxide and sulphur dioxide.
Then either:
- Zinc oxide is reduced by carbon monoxide in the Then the mixture of zinc and slag undergoes fractional distillation.
- Zinc oxide is insoluble in water and has a melting point just under 2000°C. So it is dissolved in sulphuric This is a neutralisation reaction: zinc sulphate solution is produced. In electrolysis, zinc is deposited at the cathode, then scraped off and melted into bars.