Extraction using carbon

Metals such as zinc, iron and copper are present in ores as their oxides. Each of these oxides is heated with carbon to obtain the metal.

The metal oxide loses oxygen, and is therefore reduced. The carbon gains oxygen, and is therefore oxidised.

Using iron as an example:

iron oxide + carbon → iron + carbon dioxide

2Fe₂O₃(s) + 3C(s) → 4Fe(l) + 3CO₂(g)

The source of carbon for this reduction is coke, obtained by heating coal in the absence of oxygen. Note that the iron is liquid when it is formed, due to the very high temperature at which the reaction takes place.

Some metals, such as aluminium, are so reactive that their oxides cannot be reduced by carbon.

Electrolysis

Ionic compounds contain charged particles called ions. For example, copper(II) chloride contains positively charged copper ions and negatively charged chloride ions. Ionic substances can be broken down into the elements they are made from by electricity, in a process called electrolysis.

For electrolysis to work, the ions must be free to move. When an ionic compound is dissolved in water, or melts, the ions break free from the ionic lattice. These ions are then free to move.

For example, if electricity is passed through copper(II) chloride solution, the copper(II) chloride is broken down to form copper metal and chlorine gas.

Electrolysis

There is a similar result if electricity is passed through molten copper(II) chloride.

The solution or molten ionic compound is called an electrolyte. The negative electrode is called the cathode, while the positive electrode is called the anode.

This is what happens during electrolysis:

• Positively charged ions move to the negative electrode. Metal ions are positively charged, so metals are produced at the negative electrode (cathode).

• Negatively charged ions move to the positive electrode. Non-metal ions, such as oxide ions and chloride ions, are negatively charged, so gases such as oxygen or chlorine are produced at the positive electrode (anode).

Extraction of aluminium

Aluminium is the most abundant metal on Earth. Despite this, it is expensive, largely because of the amount of electricity used up in the extraction process.

Aluminium ore is called bauxite. The bauxite is purified to yield a white powder, aluminium oxide, from which aluminium can be extracted.

The extraction is done by electrolysis. But first the aluminium oxide must be made molten so that electricity can pass through it. Aluminium oxide has a very high melting point (over 2,000°C), so it would be expensive to melt it. Instead, it is dissolved in molten cryolite, an aluminium compound with a lower melting point than aluminium oxide. The use of cryolite reduces some of the energy costs involved in extracting aluminium.

Diagram showing cell for aluminium extraction

The diagram shows an aluminium oxide electrolysis tank. Both the negative electrode (cathode) and positive electrode (anode) are made of graphite, a form of carbon.

Aluminium metal forms at the negative electrode and sinks to the bottom of the tank, where it is tapped off.

Oxygen forms at the positive electrodes. This oxygen reacts with the carbon of the positive electrodes, forming carbon dioxide, and they gradually burn away. Consequently, the positive electrodes have to be replaced frequently, which adds to the cost of the process.

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Mass calculations - Higher

Given the equation for the extraction of a metal, it is possible to calculate the mass of metal produced by a particular mass of mineral.

Example calculation

What mass of iron metal can be made from 5 tonnes of iron(III) oxide?

The equation is:

2Fe₂O₃(s) + 3C(s) = 4Fe(l) + 3CO₂(g)

Using the Periodic Table, the relative atomic mass of each element can be found.

Relative atomic mass (A_r) of each element is:

C = 12, Fe = 56, O = 16

These relative atomic masses can be used to find the formula masses of the metal compound.

Fe₂O₃ has the formula mass = (2 x 56) + (3 x 16) = 160

From the equation, 2Fe₂O₃ produces 4Fe. Working this out in tonnes gives:

2Fe₂O₃ = (2 x 160) = 320 tonnes produces 4Fe = (4 x 56) = 224 tonnes of iron metal.

Therefore:

5 tonnes of iron(III) oxide produces 5 x 224/320 = 3.5 tonnes of iron.

Example - simpler method

It is also possible to calculate the maximum mass of a metal that can be obtained from a specific mass of ore by comparing their formulae.

Fe₂O₃ contains 2Fe

So 160 tonnes of Fe₂O₃ contains 2 x 56 = 112 tonnes of Fe

Therefore:

5 tonnes of iron(III) oxide produces 5 x 112/160 = 3.5 tonnes of iron.

Electrolysis equations - Higher

During electrolysis, metal ions, which are positive, gain electrons from the negative electrode (cathode) to form neutral metal atoms.

In the extraction of aluminium, this is the equation for the reaction at the negative electrode:

Al³⁺ + 3e⁻ → Al

At the positive electrode (anode), non-metal ions, which are negative, lose electrons to form neutral atoms. These atoms join to make molecules of the non-metal element, such as a molecules of oxygen gas.

In the extraction of aluminium, this is the equation for the reaction at the positive electrode:

2O²⁻ → O₂ + 4e⁻

The removal of electrons from the cathode and addition of electrons to the anode means that an electrical current is passing through the electrolyte.

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