The Extraction of Metals.

The Extraction of Metals

Aluminium

Aluminium is the most abundant metal in the Earth's crust. Most aluminium ores do not have a high enough aluminium content to make extraction economical. Bauxite, its main ore, is the one from which aluminium is extracted.

Bauxite is impure aluminium oxide (Al2O3). The first step is to purify it. Aluminium is more reactive than carbon so it cannot be reduced by carbon. Instead, aluminium oxide is electrolysed (see Chapter 11: Uses of Electrolysis). There are, however, a few considerations to take into account for the electrolysis of aluminium oxide:

For electrolysis, as you should know, the ions must be free to move. They are only free when molten or in solution. Aluminium oxide does not dissolve in water and its melting point is over 2000ºC. The energy required to heat it would make the process inefficient. So, aluminium oxide is dissolved in cryolite (Na3AlF6), another ore of aluminium. Cryolite melts at a lower temperature, so the electrolysis can occur at a lower temperature (about 1000ºC).
Graphite electrodes are used. Molten aluminium is produced at the cathode and sinks to the bottom of the cell, from where it is piped off.

Al3+ (l) + 3e- Al(l)

Oxygen is produced at the anode. This means that the graphite electrodes are oxidised to carbon dioxide, so they waste away and must be replaced regularly.

2O2- (l) O2(g) + 4e-

Electrolysis for the extraction of aluminium consumes large amounts of electricity.

Extraction of aluminium by electrolysis

Aluminium is an extremely useful metal. It is a good conductor, is malleable, ductile and has a very low density. Although reactive, it resists corrosion because of the thin oxide layer on its surface, which prevents oxygen from getting to it. Because of its properties, aluminium has many uses.

Uses of aluminium

Iron and steel

Iron is the second most abundant metal in the Earth's crust. Its ores are haematite (Fe2O3) and magnetite (Fe3O4).

It is extracted in a blast ...

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Electrolysis for the extraction of aluminium consumes large amounts of electricity.

Extraction of aluminium by electrolysis

Uses of aluminium

Iron and steel

Iron is the second most abundant metal in the Earth's crust. Its ores are haematite (Fe2O3) and magnetite (Fe3O4).

It is extracted in a blast furnace, where a sequence of reactions takes place:

(Note that the sequence of reactions starts from the bottom upwards. All raw materials are added from the top and the products are collected at the bottom.)

The iron produced is tapped off at the bottom. It contains about 5% of carbon and is called cast iron or pig iron. If cooled in this state, it becomes very hard but also very brittle due to the high carbon content.

To remove impurities such as excess carbon, iron from the blast furnace is mixed with some limestone and burned with oxygen. All the oxides produced are acidic, being non-metal oxides, and most are gases which escape. The rest react with CaO (a basic oxide produced from the limestone). This is called the basic oxygen process. As a result, a molten slag forms on the surface of the molten steel and is run off.

The steel produced is an alloy of iron with some carbon. Its properties depend on the amount of carbon in the steel. The hardness of steel increases the more carbon it contains. However, the strength of steel 'peaks' at around 1% carbon.

Other metals may be added in small quantities to make different alloys with different properties. For example, chromium is added to make stainless steel.

These added metals disrupt the metal lattice structure of the steel, altering its properties.

Rusting
Iron and steel react with oxygen, in the presence of water, forming rust. Rust is brittle, so it weakens the iron or steel. If salt is dissolved in the water, rusting occurs faster.

Demonstrating that iron rusts only in the presence of air and water

Rusting needs both air and water. So if steel is protected from either water or air, it does not rust. This is easily achieved by painting the metal. Greasing it will repel water and plating it in other metals will help. Galvanising is coating with zinc. Since zinc is more reactive than iron, oxygen or water will react with it in preference to the iron, thus protecting the steel.

Titanium

Titanium is a strong transition metal that is resistant to corrosion. Its low density and strength make it ideal for aircraft and space vehicles. It is also used in medicine for artificial hips as it is light and biologically inert.

Its ore is mainly composed of titanium dioxide. You might expect that it can be extracted in the same way as iron, but even small traces of carbon would make it brittle. Instead, a more reactive metal is used. First, it is converted to titanium chloride and then reacted with sodium or magnesium. This is carried out in an inert atmosphere (of argon) since it would react with air or moisture.

TiCl4 + 4Na Ti + 4NaCl

Titanium is made in batches and this is therefore a batch process.

Copper

Copper is a relatively unreactive metal, so it is not difficult to extract from its ores. But when used as electrical cables it must be very pure, so it is purified by the electrolysis of copper sulphate solution (see Chapter 11: Uses of Electrolysis). This results in copper which is about 99.95% pure. At the anode, a sludge containing silver and gold is deposited, which makes the process more economical.

Because copper is quite soft, it is a good conductor of heat and electricity and does not corrode. It is used extensively, especially in alloys as follows: