Higher and foundation tier

Metal extraction

metal mining Metals are very valuable elements with many uses. You only have to take a look around at the many objects that are made from metals and alloys to realise how useful metals are. However not many metals are found as pure elements in the Earth, most metals react with whatever elements are around them to form compounds. Metal ores are rocks which contain a high proportion of metal in them.

If it is economic to extract the metal from these ores then they may be mined out of the ground and the metal extracted from them. If the metal ore contains an expensive metal such as copper it may be economically viable to extract it even if the amount of metal present is low. However if the ore contains a small amount of a less valuable metal such as iron, then it would not be economically viable to extract it.

Ease of extracting metals

The method used to extract the metal from its ore will depend on the metals position in the reactivity series. Some metals such as platinum, gold and silver and those near the bottom of the reactivity series can be found as metallic elements due to their lack of chemical reactivity. However most metals need to be chemically extracted from their ores and in some cases this can require large amounts of energy. Consider the reactions of two highly sought after metals, lithium and iron with oxygen. Lithium, an alkali metal is fairly close to the top of the reactivity series and iron, which is used to make steel, is roughly in the middle of the reactivity series.
lithium(s) + oxygen(g) → lithium oxide(s) + ENERGY
iron(s) + oxygen(g) → iron oxide(s) + energy
lithium is so much more reactive than iron, it releases a lot more energy than iron when it reacts with oxygen. This means that in order to reverse the above reactions if we needed to extract the metals then we have to put back more energy to extract lithium than iron, the same amount of energy that was released when the metal reacted will have to be put back in to extract the metal from its ore.
lithium oxide(s) + ENERGY → lithium (s)
iron(s) + energy → iron (s)
This means that the higher the metal is in the reactivity series the more energy will be needed to extract it from its ore.

Extracting metals using hydrogen gas

A metal lower than hydrogen in the reactivity series can be extracted from its oxide by heating with hydrogen. Copper can be extracted from copper oxide as shown. A stream of hydrogen gas is passed over hot copper oxide powder in a glass tube. The hydrogen being higher in the reactivity series than copper will remove the oxygen from the copper oxide. The hydrogen will reduce the coppper oxide to copper metal. This is shown by the equation:

reduction of copper oxide using hydrogen gas

The word and symbolic equation for this reaction are shwon below:

copper oxide(s) + hydrogen(g) copper(s) + hydrogen oxide(g)
CuO(s) + H2(g) Cu(s) + H20(g)

Extracting metals using carbon

If the metal is above hydrogen in the reactivity series the it cannot be extracted from its ore by heating with hydrogen. Instead the metal ore is heated with carbon. Carbon is a non-metal but it has been long used to extract metals such as lead and iron from their ores. The basic method is shown below:

reduction of metal ores using carbon

For example lead oxide, copper oxide and iron oxide can all be reduced by heating with carbon as shown in the diagrams above. Heating the mixture of metal oxide and powdered charcoal (carbon) tends to work better using the crucible method rather than the boiling tube especially when using iron oxide. Equations for these reactions are:

copper oxide(s) + carbon(s) copper(s) + carbon dioxide(g)
2CuO(s) + C(s) 2Cu(s) + CO2 (g)
lead oxide(s) + carbon(s) → lead(s) + carbon dioxide(g)
2PbO(s) + C(s) → 2Pb(s) + CO2 (g)
iron oxide(s) + carbon(s) → iron(s) + carbon dioxide(g)
2Fe2O3(s) + 3C(s) → 4Fe(s) + 3CO2 (g)
In each of these reactions the carbon is oxidised to carbon dioxide and the metal oxide is reduced to a metal.

Extracting metals above carbon in the reactivity series

Metals above carbon in the reactivity series cannot obviously be extracted by heating with carbon. To extract these metals their ores need to be melted to form molten compounds and an electric current passed through this molten ore. This method of extracting metals is called electrolysis. Electrolysis is expensive and it is only used to extract those metals that cannot be obtained from their ores by heating with carbon. Alumnium for example is extracted from its ore, bauxite by electrolysis.

Biological methods for extracting metals- Higher tier only

copper mineExtracting metals from metal mines is not what you would call environmentally friendly! It uses large amounts of energy and destroys large areas of land. Recycling metals would reduce the need for these mines and would also use less energy and reduce the amount of valuable materials being sent to landfill as well as producing less carbon dioxide and other pollutants.

As an example consider copper. Copper is a very valuable metal which is in high demand. Most of the world's high grade copper ore has been used and so scientists have had to develop methods to extract copper from low grade ores. Using traditional methods to extract copper from low grade ores would not be economically viable due to the large amount of copper ore/rock that would have to be dug up and processed to obtain a fairly small amount of copper. This would also produce a very large amount of waste that would have to be disposed of and ultimately put somewhere and this of course would lead to loss of land and habitat as well as incurring financial costs.

Phytomining- Higher tier only

Plants have been used for many years to clean up land contaminated with heavy metals such as mercury and lead. Traditionally the contaminated soil would be simply scooped up by bulldozers and shipped elsewhere for disposal. This is expensive and polluting. Plants can do a similar job but much more cheaply and in a less polluting and more environmentally friendly and sustainable way. The plants absorb the heavy metals into their roots and leaves and when they plants are mature they are simply chopped down, dried and then burned. The ash produced will contain the heavy metal compounds which can be processed and the metals extracted. Using plants to extract metals like this is called Phytomining.

Phytomining is often used to extract copper from its low grade ores. Here the plants will be grown on the ore and when the plants are fully grown they will be harvested, dried and burned as described above. The ash maybe dissolved in sulfuric acid to form a solution of copper sulfate. The copper can then be extracted from this solution by electrolysis or by a displacement reaction using scrap iron, as shown below.

copper extraction

Bioleaching- Higher tier only

Certain types of bacteria and fungi can feed on nutrients in metal ores and as a by-product the metal is produced. The process is very simple and you can even buy kits to do it yourself on the internet! Basically a large hole is dug in the ground and it is lined with a plastic liner. The ore is placed in the liner. Many of the natural bacteria that are used require acidic conditions, so the ore is sprayed or soaked with dilute sulfuric acid. Many metals can be extracted using this method including copper, nickel and uranium as well as many others. This process is very inexpensive and compared to traditional smelting methods much more environmentally friendly. The downside is that it is very very slow. The diagram below shows just how simple and easy the process is to set-up with no specialist equipment needed.
The acidic solution which collects at the bottom of the pit is simply collected and the copper extracted by electrolysis or displacement reactions, similar to that used to extract metals by phytomining.

bioleaching to obtain metals

Key Points

Practice questions

Check your understanding - Questions on metal extraction