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Useful Products from Metal Ores

Introduction | Extraction of Iron | Extraction of Aluminium | Purification of Copper

Introduction

The Earth's crust contains metals and metal compounds, but these are always mixed with other substances.

An ore is a rock that contains enough of the metal or metal compound to make it economic to extract the metal.

Most metals exist as metal compounds in the Earth, but gold is very unreactive so is found as the free metal. It just has to be physically separated from the other substances.

Some metals react more vigorously with air, water and acids than others. They can be listed in a reactivity series.

A more reactive metal can displace a less reactive one from its compounds.

The non-metals, carbon and hydrogen, will also displace a less reactive metal from its compound.

For example, copper will displace silver from its compounds, because it is higher up in the reactivity series.

copper + silver nitrate —› silver + copper nitrate

Iron cannot displace magnesium from its compounds because it is less reactive than magnesium.

The position of a metal in the reactivity series can be used to predict how a metal could be extracted from a compound.

Many ores contain a metal oxide, or a substance that can easily be changed into a metal oxide.

To extract the metal, the oxygen must be removed from the metal oxide. This is called reduction.

Extraction of Iron

Iron is less reactive than carbon so can be extracted from its ore using carbon in the blast furnace.

The raw materials are: iron ore (haematite), coke (a form of carbon) and limestone.

Hot air is blown into the furnace, causing the coke to burn, forming carbon dioxide and releasing heat energy.

The high temperature causes the carbon dioxide to react with coke, forming carbon monoxide.

This reduces the iron oxide in the ore to molten iron, which flows to the bottom of the furnace.

The carbon monoxide is oxidised to carbon dioxide, as it gains oxygen from the iron oxide.

The limestone removes acidic impurities from the ore, forming a molten slag that floats on the surface of the molten iron.

Waste carbon dioxide and carbon monoxide escape at the top.

Iron (or steel) is widely used because it is strong and relatively cheap. However, it corrodes (rusts) quickly.

This corrosion can be prevented by connecting iron to a more reactive metal, such as zinc or magnesium.

The more reactive metal corrodes instead of the iron. This is called sacrificial protection. It is used on fence posts and hulls of ships.

Extraction of Aluminium

Aluminium cannot be extracted by heating with carbon, because it is too reactive. Instead, it is extracted by electrolysis.

When ionic compounds are dissolved in water or melted, the ions are free to move about.

If an electric current is passed through the liquid, the positive ions, like metal ions, move to the negative electrode, where they gain electrons. This is called reduction.

The negative ions move to the positive electrode where they lose electrons. This is oxidation.

You can remember this by: Oxidation is electron loss.

In a chemical reaction, whenever something is oxidised, something else is reduced. These reactions are called redox reactions.

During electrolysis, gases may be given off or metals deposited at the electrodes.

The raw material for producing aluminium is aluminium oxide, purified from an aluminium ore called bauxite.

Aluminium oxide has a very high melting point, so it is dissolved in a molten aluminium compound called cryolite. This does not use so much heat energy, so is cheaper because cryolite has a lower melting point.

The electrodes are made of carbon.

Aluminium ions move to the negative electrode where they gain electrons to form aluminium atoms.

The aluminium collects at the bottom of the cell and is tapped off.

At the negative electrode:

Al³⁺	+ 3 electrons	Al
aluminium ion	—›	aluminium atom
	reduction

Oxide ions move to the positive electrode and lose electrons to form atoms.

At the positive electrode:

O^2-	– 2 electrons	O
oxide ion	—›	oxygen atom
	oxidation

The atoms form oxygen molecules (O₂) at the positive electrodes. This makes the positive electrodes burn away, so they frequently have to be replaced.

carbon

oxygen

—›

carbon dioxide

Although aluminium is a reactive metal, it does not corrode very quickly.

This is because it reacts with oxygen in the air to form a layer of aluminium oxide on the surface.

This acts as a barrier to water and oxygen, preventing further corrosion.

Aluminium is a useful structural metal because it is lightweight.

It is made harder, stronger and stiffer by mixing it with small amounts of other metals, such as magnesium, to make alloys.

Purification of Copper

Copper has a low reactivity, so it can be extracted from its ore by heating with carbon.

However, copper extracted this way contains many impurities and is unsuitable to be used for electrical wiring.

The impure copper is purified using electrolysis.

The positive electrode is made of impure copper and the negative electrode is made of a thin piece of pure copper.

When an electric current is passed through the copper sulphate solution the copper atoms in the impure sample lose electrons and form copper ions, which move to the pure copper negative electrode where they gain electrons to form atoms again.

This electrode gradually gets bigger, whilst the positive electrode gets smaller.

The impurities fall to the bottom.

At the positive electrode:

	– 2 electrons	Cu²⁺
copper atom	—›	copper ion
	oxidation

At the negative electrode:

Cu²⁺	+ 2 electrons	Cu
copper ion	—›	copper atom
	reduction