Methods of extracting metals

The Earth's crustcrust: The outer layer of the Earth. contains metals and metal compoundscompounds: Substances formed by the chemical union (involving bond formation) of two or more elements. such as gold, iron oxide and aluminium oxide, but when found in the Earth these are often mixed with other substances. To be useful, the metals have to be extracted from whatever they are mixed with. A metal ore is a rock containing a metal, or a metal compound, in a high enough concentration to make it economic to extract the metal.

The method used to extract metals from the ore in which they are found depends on their reactivity. For example, reactive metals such as aluminium are extracted by electrolysiselectrolysis: Electrolysis is the decomposition (separation or break-down) of a compound using an electric current., while a less-reactive metal such as iron may be extracted by reductionreduction: Reduction is a reaction in which oxygen is removed from a substance. Reduction also means a gain in electrons. with carbon or carbon monoxide.

Thus the method of extraction of a metal from its ore depends on the metal's position in the reactivity series:

Reactivity and extraction method

Note that gold, because it is so unreactive, is found as the native metal and not as a compound, so it does not need to be chemically separated. However, chemical reactions may be needed to remove other elements that might contaminate the metal.

Making iron

In the blast furnace

Blast furnace in a modern steel works

Iron is extracted from iron ore in a huge container called a blast furnace. Iron ores such as haematite contain iron oxide. The oxygen must be removed from the iron oxide to leave the iron behind. Reactions in which oxygen is removed are called reduction reactions.

Carbon is more reactive than iron, so it can push out or displace the iron from iron oxide. Here are the equations for the reaction:

iron oxide + carbon    →    iron + carbon dioxide

2Fe₂O₃ + 3C    →    4Fe + 3CO₂

In this reaction, the iron oxide is reduced to iron, and the carbon is oxidised to carbon dioxide.

In the blast furnace, it is so hot that carbon monoxide can be used to reduce the iron oxide in place of carbon:

iron oxide + carbon monoxide    →    iron + carbon dioxide

Fe₂O₃ + 3CO    →    2Fe + 3CO₂

Raw materials for the reaction

The raw materials for extracting iron and their function in the process

Steel

Iron

Layers of atoms slide over each other when metals are bent or stretched

Pure iron is soft and easily shaped. This is because its atoms are arranged in a regular way that lets layers of atoms slide over each other. Pure iron is too soft for many uses.

Iron from the blast furnace is an alloy of about 96 per cent iron with carbon and some other impurities. It is hard, but too brittle for most uses. So, most iron from the blast furnace is converted into steel by removing some of the carbon.

Steel

Carbon is removed by blowing oxygen into the molten metal. It reacts with the carbon producing carbon monoxide and carbon dioxide. These escape from the molten metal. Enough oxygen is used to achieve steel with the desired carbon content. Other metals are often added, such as vanadium and chromium.

There are many different types of steel, depending on the other elements mixed with the iron. The table summarises the properties of some different steels.

A summary of the properties of some different steels

Alloys

The properties of a metal are changed by including other elements, such as carbon. A mixture of two or more elements, where at least one element is a metal, is called an alloy. Alloys contain atoms of different sizes, which distort the regular arrangements of atoms. This makes it more difficult for the layers to slide over each other, so alloys are harder than the pure metal.

It is more difficult for layers of atoms to slide over each other in alloys

Copper, gold and aluminium are too soft for many uses. They are mixed with other metals to make them harder for everyday use. For example:

• Brass, used in electrical fittings, is 70 per cent copper and 30 per cent zinc.
• 18 carat gold, used in jewellery, is 75 per cent gold and 25 per cent copper and other metals.
• Duralumin, used in aircraft manufacture, is 96 per cent aluminium and 4 per cent copper and other metals.

Smart alloys can return to their original shape after being bent. They are useful for spectacle frames and dental braces.

The transition metals

You need to know where to find the transition metals in the periodic table. The transition metals are found in the large block between Groups 2 and 3 in the periodic table. Most metals are placed here, including iron, titanium, copper and nickel.

The transition metals

Common properties

The transition metals have these properties in common:

• They are metals.
• They form coloured compounds.
• They are good conductors of heat and electricity.
• They can be hammered or bent into shape easily.
• They are less reactive than alkali metals such as sodium, they have higher melting points - but mercury is a liquid at room temperature -and they are hard and tough.
• They have high densitiesdensities: Density is the ratio of mass to volume. It is usually measured in grams per cubic centimetre or grams per cubic decimetre..

Copper

Copper is a transition metal. It is soft, easily bent and it is a good conductor of electricity. This makes copper useful for electrical wiring. Copper does not react with water, which makes it useful for plumbing.

Copper is purified by electrolysis. Electricity is passed through solutions containing copper compounds, such as copper sulfate - sometimes spelt sulphate. Pure copper forms on the negative electrode. The animation shows how this works, but note that you do not need to know the details of the extraction process for your examination.

Problems

We are running out of ores rich in copper. Research is being carried out to find new ways to extract copper from the remaining ores, without harming the environment too much. This research is very important, as traditional mining produces huge open-cast mines, and the remaining ores are low-grade, which means that they contain relatively little copper and produce a lot of waste rock.

Aluminium and titanium

Block of aluminium metal - image does not show the transparent oxide layer

Aluminium and titanium are two metals with a low density. This means that they are lightweight for their size. They also have a very thin layer of their oxides on the surface, which stops air and water getting to the metal, so aluminium and titanium resist corrosion. These properties make the two metals very useful.

Aluminium is used for aircraft, trains, overhead power cables, saucepans and cooking foil. Titanium is used for fighter aircraft, artificial hip joints and pipes in nuclear power stations.

Extraction

Unlike iron, aluminium and titanium cannot be extracted from their oxides by reduction with carbon:

• Aluminium is more reactive than carbon, so the reaction does not work.
• Titanium forms titanium carbide with carbon, which makes the metal brittle.

Aluminium extraction is expensive because the process needs a lot of electrical energy. Titanium extraction is expensive because the process involves several stages and a lot of energy. This especially limits the uses of titanium.

Recycling

Aluminium is extensively recycled because less energy is needed to produce recycled aluminium than to extract aluminium from its ore. Recycling preserves limited resources and requires less energy, so it causes less damage to the environment.

Now try a Test Bite