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.
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

lithium_(s) + oxygen_(g) → lithium oxide_(s) + ENERGY

iron_(s) + oxygen_(g) → iron oxide_(s) + energy

lithium oxide_(s) + ENERGY → lithium _(s)

iron oxide_(s) + energy → iron _(s)

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:

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

copper oxide_(s) + hydrogen_(g) → copper_(s) + hydrogen oxide_(g)

CuO_(s) + H_2(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:

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) + CO_{2 (g)}

lead oxide_(s) + carbon_(s) → lead_(s) + carbon dioxide_(g)

2PbO_(s) + C_(s) → 2Pb_(s) + CO_{2 (g)}

iron oxide_(s) + carbon_(s) → iron_(s) + carbon dioxide_(g)

2Fe₂O_3(s) + 3C_(s) → 4Fe_(s) + 3CO_{2 (g)}

Extracting metals above carbon in the reactivity series

Biological methods for extracting metals- Higher tier only

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.

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.

Key Points

• High grade metal ores are rocks which contain a high percentage of metal in them; low grade ores contain only a small percentage of metal.
• Phytomining (phytoextraction) and bioleaching are two common biological methods which can be used to extract metals from low grade ores. These two methods of extracting metals are more environmentally friendly than traditional mining but they are a much slower way of obtaining the metal from its ore.
• The method used to extract a metal from its ore will depend on its position in the reactivity series. Metals above carbon are extracted by electrolysis, whereas metals less than reactive than carbon are extracted by heating with carbon or charcoal. Hydrogen can be used to extract metals close to the bottom of the reactivity series, the metal needs to be less reactive than hydrogen. This is summarised in the table below:

metal	method used to extract the metal from its ore
potassium
sodium
lithium	electrolysis of the molten compound
magnesium
aluminium
carbon
zinc
iron	heat the metal ore with carbon (smelting)
tin
lead
hydrogen
lead
copper	heat the metal ore with hydrogen or carbon
silver
gold	found as an element in the earth
platinum	found as an element in the earth

• A few metals such as gold and platinum are found as pure elements. This is simply because these metals are chemically unreactive.

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