Higher and foundation tier
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.
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.
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:
The word and symbolic equation for this reaction are shwon below:
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:
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:
Extracting 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.
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
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.
Certain types of bacteria and fungi can feed on nutrients in metal ores and as a by-product the metal
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
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.
|metal||method used to extract the metal from its ore|
|lithium||electrolysis of the molten compound|
|iron||heat the metal ore with carbon|
|copper||heat the metal ore with hydrogen or carbon|
|gold||found as an element in the earth|
|platinum||found as an element in the earth|