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Higher and foundation tiers

Metal extraction

Metals are very valuable elements with desirable properties and 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's crust; most metals react with whatever elements are around them to form compounds. Metal ores are rocks which contain a high enough proportion of a metal in them to make it economically viable to extract the metal from them. The images below show two common metal ores, haematite (iron oxide) is a common ore of iron while chalcocite (copper sulphide) is an ore containing copper metal.
Haematite is a common ore of iron which can contain up to about 70% iron. 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 is an alkali metal and it 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 does when it reacts with oxygen. This means that in order to reverse the above reactions and extract the metals from their compounds 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 order to extract the metal from its ore.
lithium oxide(s) + ENERGY lithium (s)
iron oxide(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/ore by heating with hydrogen, the hydrogen will essentially displace the less reactive metal from its compound. Copper for example can be extracted from copper oxide as shown below. Here a stream of hydrogen gas is passed over hot powdered copper oxide sitting 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 copper oxide to copper metal. This is shown by the equations below:

Diagram to show the reduction of copper oxide using hydrogen gas.

The word and symbolic equation for this reaction are shown 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 then 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.  Apparatus set-up with instrctions on how to carry out the experiment.

For example lead oxide, copper oxide and iron oxide can all be reduced by heating with carbon as shown in the diagrams above. Equations for these reactions are shwon below:

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 while the metal oxide is reduced to a metal.

These reactions are therefore an example of a redox reaction and they can also be thought of as a type of displacement reaction.

Extracting metals above carbon in the reactivity series

Metals above carbon in the reactivity series cannot be extracted by heating them 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. Aluminium for example is extracted from its ore, bauxite by electrolysis.

Biological methods for extracting metals- Higher tier only

Image of a copper mine in Sweden.  Large amounts of environmental damage is done when mining for metals. Extracting metals from metal mines is not what you would call an environmentally friendly option! The diagram opposite shows a typical open cast copper mine. 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 more environmentally friendly options to extract copper from its ore. 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 potentially 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. However there are other ways to extract metals from their ores that do not require large amounts of energy or cause large amounts of environmental damage.

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 have be simply been 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. Certain plants when grown on contaminated land will absorb the polluting heavy metals into their roots and leaves, this will concentrate the metal in the plant cells and tissues. When the plants are mature they can then be harvested, dried and then burned. The ash produced will contain the heavy metal compounds which can then be processed and the metals extracted. Using plants to extract metals like this is called phytomining or phytoextraction.

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

Copper extraction using electrolysis.  The copper solution is obtained by a process called phytomining.

Bioleaching- Higher tier only

Bioleaching is another method that can be used to extract metals economically from low grade metal ores. Certain types of bacteria for example thiobacillus ferrooxidans are used in bioleaching. These bacteria feed on sulfide minerals present in low-grade copper ores, the bacteria produce a watery solution called a leachate which contains copper ions (Cu2+) through their metabolic processes.

Bioleaching is a very simple process; 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 metal ore is placed in the liner. The copper ore is sprayed with a watery solution containing the bacteria necessary to extract the metal from the ore. Many metals can be extracted using this method including copper, nickel, zinc and uranium as well as many others. This process is very inexpensive and compared to traditional smelting methods where the copper ore is heated and reduced using charcoal or carbon and is much more environmentally friendly. The downside is that it is very slow; it can take many months or even years years to collect large amounts of metal. The diagram below shows just how simple and easy the process is to set-up with no specialist equipment needed. Although if you decide to set-up your own bioleaching pit in the garden it is probably best to consult with your parents first!

The watery solution or leachate that collects at the bottom of the pit is simply collected and the copper extracted by electrolysis or displacement reactions; similar to the methods used to extract metals using phytomining.

bioleaching set up to obtain metals from metal ores.

Key Points

Practice questions

Check your understanding - Questions on metal extraction

Check your understanding - Additional questions on metal extraction

Check your understanding - Quick Quiz on metal extraction