Extraction of metals 

Many of our useful metals come from metal ores. A metal ore is a rock that contains enough of the metal to make it worthwhile extracting the metal. Since the earliest times, metals have been extracted from the ground and provided us with useful materials to make tools, weapons, buildings, bridges and aeroplanes.

Although the Earth’s crust contains native metals such as gold and silver, many of the useful metals such as iron and aluminium are locked up in ores, in the form of oxides such as iron(III) oxide and aluminium oxide, so a metal extraction method must be used to obtain the metal. In every case, extraction of metals by reduction takes place. Reduction is the removal of oxygen from an oxide.

Unreactive metals such as gold and platinum do not react with oxygen. Some reactive metals such as sodium react easily with oxygen. Aluminium and iron can also react with oxygen although much more slowly.

metal + oxygen → metal oxide 

When metals react with oxygen a metal oxide is formed, for example:

iron + oxygen → iron(III) oxide 

4Fe (s) + 3O₂ (g) → 2Fe₂O₃ (s)

Methods of extraction of metals from ores

The methods of metal extraction from ores depend on the position of the metal in the reactivity series. The reactivity series is a list of common metals in order from most to least reactive. The general methods of extraction of metals involve electrolysis or reduction with carbon.

Those metals placed higher up on the series (above carbon) have to be extracted using electrolysis. Metals lower down on the series can be extracted by heating with carbon. It seems strange to have carbon in a table of metal reactivity, but it is useful to have it there, because many everyday metals are extracted by carbon, and we can use its position to predict whether reduction by carbon can be used or not for a named metal.

More reactive metals are extracted by electrolysis. The electrolytic extraction of metals consumes large amounts of electricity so this is reflected in the cost of the metal.

Extraction of iron from hematite

The extraction of iron metal from iron ore uses a blast furnace, a huge tower used to extract iron from its ore, hematite. A modern blast furnace operates 24 hours a day and can make approximately 10,000 tonnes of iron per day. 

Diagram showing the carbon extraction of iron

The process can be broken down into three steps:

• Making carbon dioxide by burning carbon (in the form of coke)
• Turning the carbon dioxide into carbon monoxide
• Using carbon monoxide to reduce iron(III)oxide to iron

The word equations for these reactions are:

• carbon + oxygen → carbon dioxide

• carbon + carbon dioxide → carbon monoxide

• iron(III) oxide + carbon monoxide  →  iron + carbon dioxide

For more details about the chemical equations for the extraction of iron, which are required for Extended Level: “Extraction of iron from hematite”

Extraction of aluminium from bauxite

You can see from the reactivity series that aluminium is above carbon in the reactivity series.

Bauxite is the main ore from which aluminium is extracted. Aluminium is higher in the reactivity series than carbon, so it cannot be extracted by reduction using carbon.

Instead, the extraction of aluminium metal is by electrolysis.

Diagram showing the extraction of aluminium by electrolysis

Before the aluminium can be extracted, the bauxite is first purified to make aluminium oxide, Al₂O₃. Aluminium oxide is then dissolved in molten cryolite, a mineral that lowers its melting point.

This is because aluminium oxide has a melting point of over 2000 °C which would use a lot of energy and be very expensive. The resulting mixture has a lower melting point without interfering with the reaction.

The mixture is placed in an electrolysis cell, made from steel, lined with graphite

The graphite lining acts as the negative electrode, with several large graphite blocks as the positive electrodes. Molten aluminium collects at the bottom of the cell

The molten aluminium is syphoned off periodically and fresh aluminium oxide is added to the cell.

For more details about the chemical equations for the extraction of aluminium, which are required for Extended Level: “Extraction of aluminium from bauxite”

What keyword definitions do I need to know for the extraction of metals? 

Some keyword definitions you need to know are:

• Ore - A metal ore is a rock that contains enough of the metal to make it worthwhile to extract
• Hematite - the main ore of iron, in the form of impure iron(III)oxide

• Bauxite - the main ore of aluminium, in the form of impure aluminium oxide

• Reduction - the removal of oxide from an oxide

• Electrolysis - splitting with electricity

This is a quick summary of some key concepts on the extraction of metals - remember to go through the full set of revision notes, which are tailored to your specification, to make sure you know everything you need for your exams!