Extracting metals

​​In a nutshell

Metals can be extracted from their metal compounds by reduction with carbon or electrolysis. Unreactive metals are found as elements.

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Reduction

Ores are rocks which contain useful minerals, such as metals. Metals less reactive than carbon are extracted from their metal oxide ores by reduction. Zinc, iron and copper are all less reactive than carbon. Carbon displaces these metals out of their metal oxide compounds.

Example

The reduction of iron oxide with carbon:

​$$iron ~oxide + carbon \rightarrow iron + carbon ~dioxide \\ \$$

$$2Fe_2O_3 + 3C \rightarrow 4Fe + 3CO_2$$​​

Note: Reduction is gain of electrons but can also be defined as loss of oxygen. In this example, you can see that iron oxide is reduced because it loses oxygen.

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Electrolysis

Electrolysis is a process where electricity is used to generate a chemical reaction.

• Metals more reactive than carbon are extracted by electrolysis
• Potassium, sodium, lithium, calcium, magnesium and aluminium are more reactive than carbon

Electrolysis requires the free movement of ions, which is not possible in a solid ionic compound due to the strong electrostatic forces of attraction between the oppositely charged ions. The compound is therefore melted or dissolved in water to break these forces, which allows the ions to move freely.

Redox reactions

• Electrolysis involves redox (oxidation and reduction) reactions
• Oxidation is loss of electrons and gain of oxygen
• Reduction is gain of electrons and loss of oxygen

Migration of ions 

• Positively-charged ions migrate towards the negative electrode, where they are reduced
• Negatively-charged ions migrate towards the positive electrode, where they are oxidised

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Extraction of aluminium

Aluminium is extracted from aluminium oxide by electrolysis.

• Aluminium oxide ($$Al_2O_3$$​​) is obtained from purified bauxite (an aluminium ore)
• Aluminium oxide is melted so that the aluminium ions ($$Al^{3+}​$$) and oxide ions ($$O^{2-}​$$) are free to move
  
• The melting point of aluminium oxide is very high so it is mixed with cryolite
• Cryolite is sodium aluminium fluoride and it lowers the melting point of aluminium oxide
• Less energy is required to melt the aluminium oxide when cryolite is added, making the process less expensive
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At the positive electrode, oxide ions are oxidised (lose electrons):

​$$oxide\rightarrow oxygen~ molecules + ~electrons$$

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​$$2O^{2-}(l) \rightarrow O_2(g) + 4e^-$$​​

At the negative electrode, aluminium ions are reduced (gain electrons):

​$$aluminium + electrons \rightarrow aluminium ~atom$$

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​$$Al^{3+}(l) + 3e^-\rightarrow Al(l)$$

Note: Anions are negatively-charged ions and cations are positively-charged ions.

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Unreactive metals  

Unreactive metals, such as gold and silver, are found as their pure metal elements because they do not react with other elements to form compounds.