Extracting Metals (Edexcel IGCSE Chemistry (Modular))

Extracting Metals

Extraction of metals and the reactivity series

• The most reactive metals are at the top of the series

• The tendency to become oxidised is thus linked to how reactive a metal is and therefore its position on the reactivity series

• Metals higher up are therefore less resistant to oxidation than the metals placed lower down which are more resistant to oxidation

• The position of the metal on the reactivity series determines the method of extraction

Metals extraction method table

The extraction method depends on the position of a metal in the reactivity series

• Higher placed metals (above carbon) have to be extracted using electrolysis as they are too reactive and cannot be reduced by carbon

• Lower placed metals can be extracted by heating with carbon which reduces them

The extraction method depends on the position of a metal in the reactivity series

Extraction of Iron from Hematite

• Iron is extracted in a large container called a blast furnace from its ore, hematite 

• Modern blast furnaces produce approximately 10,000 tonnes of iron per day 

• This is a continuous process with new raw materials added and products removed all the time due to the time and cost associated with getting the furnace up to temperature

The Blast Furnace

There are three main zones in the blast furnace

• The raw materials: iron ore (hematite), coke (an impure form of carbon), and limestone are added into the top of the blast furnace

• Hot air is blown into the bottom

Table of raw materials and their uses

Zone 1

• Coke burns in the hot air forming carbon dioxide 

• The reaction is exothermic so it gives off heat, heating the furnace

carbon + oxygen → carbon dioxide

C (s)  +  O₂ (g)  →  CO₂ (g)

Zone 2

• At the high temperatures in the furnace, more coke reacts with carbon dioxide forming carbon monoxide

• Carbon dioxide has been reduced to carbon monoxide

carbon + carbon dioxide → carbon monoxide

CO₂ (g)  +  C (s)  →  2CO (g)

Zone 3

• Carbon monoxide reduces the iron(III) oxide in the iron ore to form iron 

• This will melt and collect at the bottom of the furnace, where it is tapped off:

iron(III) oxide + carbon monoxide  →  iron + carbon dioxide

Fe₂O₃ (s)  +  3CO (g)  →  2Fe (I)  +  3CO₂ (g)

Removal of impurities 

• Limestone (calcium carbonate) is added to the furnace to remove acidic impurities in the ore

  • The calcium carbonate in the limestone thermally decomposes to form calcium oxide

calcium carbonate → calcium oxide + carbon dioxide

CaCO₃ (s)  →  CaO (s)  +  CO₂ (g)

• The calcium oxide formed reacts with the silicon dioxide, which is an impurity in the iron ore, to form calcium silicate by neutralisation

calcium oxide + silicon dioxide →  calcium silicate

CaO (s)  +  SiO₂ (s)  →  CaSiO₃ (l)

• This melts and collects as a molten slag floating on top of the molten iron, which is tapped off separately

Extraction of Aluminium

• Aluminium is a reactive metal, above carbon in the reactivity series 

• Its main ore, is bauxite, which contains aluminium oxide

• Aluminium is higher in the reactivity series than carbon, so it cannot be extracted by reduction using carbon

• Instead, aluminium is extracted by electrolysis  

The electrolytic cell for extraction of aluminium

Diagram showing the extraction of aluminium by electrolysis

• Bauxite is first purified to produce aluminium oxide, Al₂O₃

• Aluminium oxide is then dissolved in molten cryolite 

  • This is because aluminium oxide has a melting point of over 2000°C which would use a lot of energy and be very expensive

  • The resulting mixture has a lower melting point without interfering with the reaction

• The mixture is placed in an electrolysis cell, made from steel, lined with graphite

• The graphite lining acts as the negative electrode, with several large graphite blocks as the positive electrodes

• At the cathode (negative electrode): 

  • Aluminium ions gain electrons (reduction) 

  • Molten aluminium forms at the bottom of the cell

  • The molten aluminium is siphoned off from time to time and fresh aluminium oxide is added to the cell

Al³⁺ +  3e^–  → Al 

• At the anode (positive electrode):

  • Oxide ions lose electrons (oxidation)

  • Oxygen is produced at the anode:

2O^2– → O₂ + 4e^–

• The carbon in the graphite anodes reacts with the oxygen produced to produce CO₂

C (s) + O₂ (g)   →   CO₂ (g)

• As a result the anode wears away and has to be replaced regularly

• A lot of electricity is required for this process of extraction, this is a major expense