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9.6.2 Blast furnace equations and aluminium extraction

Blast furnace equations

  1. Each stage in the extraction of iron from hematite can be written as a balanced symbol equation.
  2. Burning the coke
    1. Carbon burns in the hot air to form carbon dioxide and release heat: C(s)+O2(g)→CO2(g)\mathrm{C(s) + O_2(g) \rightarrow CO_2(g)}C(s)+O2​(g)→CO2​(g).
  3. Making carbon monoxide
    1. Carbon dioxide reacts with more hot carbon to form carbon monoxide: C(s)+CO2(g)→2CO(g)\mathrm{C(s) + CO_2(g) \rightarrow 2CO(g)}C(s)+CO2​(g)→2CO(g).
  4. Reducing the iron(III) oxide
    1. Carbon monoxide removes oxygen from iron(III) oxide to leave molten iron: Fe2O3(s)+3CO(g)→2Fe(l)+3CO2(g)\mathrm{Fe_2O_3(s) + 3CO(g) \rightarrow 2Fe(l) + 3CO_2(g)}Fe2​O3​(s)+3CO(g)→2Fe(l)+3CO2​(g).
    2. Each formula unit of iron(III) oxide loses three oxygen atoms, and each carbon monoxide molecule gains one, so three molecules of carbon monoxide are needed.
  5. Decomposing the limestone
    1. Calcium carbonate breaks down on heating: CaCO3(s)→CaO(s)+CO2(g)\mathrm{CaCO_3(s) \rightarrow CaO(s) + CO_2(g)}CaCO3​(s)→CaO(s)+CO2​(g).
  6. Forming the slag
    1. Calcium oxide reacts with silicon(IV) oxide to form calcium silicate: CaO(s)+SiO2(s)→CaSiO3(l)\mathrm{CaO(s) + SiO_2(s) \rightarrow CaSiO_3(l)}CaO(s)+SiO2​(s)→CaSiO3​(l).
    2. Calcium oxide is a basic oxide and silicon(IV) oxide is an acidic oxide, so this reaction is a neutralisation.

Redox in the blast furnace

Definition

Reducing agent

A substance that reduces another substance and is itself oxidised.

  1. In the main reaction, iron(III) oxide loses oxygen, so it is reduced, while carbon monoxide gains oxygen, so it is oxidised.
  2. Carbon monoxide is therefore the reducing agent for iron(III) oxide.
  3. The oxidation numbers confirm this, because iron falls from +3+3+3 in Fe2O3\mathrm{Fe_2O_3}Fe2​O3​ to 000 in Fe\mathrm{Fe}Fe, while carbon rises from +2+2+2 in CO\mathrm{CO}CO to +4+4+4 in CO2\mathrm{CO_2}CO2​.
  4. In the reaction C+CO2→2CO\mathrm{C + CO_2 \rightarrow 2CO}C+CO2​→2CO, carbon is the reducing agent, because it removes oxygen from carbon dioxide and is itself oxidised.
Example

Checking that the main equation balances

  • The equation to check is the reduction of iron(III) oxide by carbon monoxide.
Fe2O3+3CO→2Fe+3CO2 \mathrm{Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2} Fe2​O3​+3CO→2Fe+3CO2​
  • On the left there are 222 iron atoms, 333 carbon atoms, and 3+3=63 + 3 = 63+3=6 oxygen atoms.
  • On the right there are 222 iron atoms in 2Fe\mathrm{2Fe}2Fe, and 333 carbon atoms and 666 oxygen atoms in 3CO2\mathrm{3CO_2}3CO2​.
  • Every element has the same number of atoms on both sides, so the equation is balanced.

Electrolysis of aluminium oxide

Definition

Cryolite

An ionic compound of sodium, aluminium and fluorine that is melted and used as the solvent for aluminium oxide in the extraction of aluminium by electrolysis.

  1. Bauxite is first purified to give aluminium oxide, Al2O3\mathrm{Al_2O_3}Al2​O3​, which is used as the electrolyte.
  2. Aluminium oxide is an ionic compound made of Al3+\mathrm{Al^{3+}}Al3+ and O2−\mathrm{O^{2-}}O2− ions, and these ions must be free to move for electrolysis to work.
  3. Pure aluminium oxide has a melting point of over 2000 ∘C2000\ ^\circ\text{C}2000 ∘C, so melting it would need an enormous amount of energy.
  4. Instead, the aluminium oxide is dissolved in molten cryolite.
  5. The role of cryolite is to lower the operating temperature to about 950 ∘C950\ ^\circ\text{C}950 ∘C, which saves a large amount of energy and money.
  6. The molten mixture also conducts electricity better than molten aluminium oxide on its own.
  7. The cell is a steel tank lined with carbon, which acts as the cathode, while large blocks of carbon dip into the molten mixture as the anodes.
  8. Molten aluminium is denser than the electrolyte, so it collects at the bottom of the cell and is siphoned off at intervals.

Reactions at the electrodes

Definition

Reduction

The gain of electrons by a substance.

Definition

Oxidation

The loss of electrons by a substance.

  1. At the cathode
    1. Positive aluminium ions are attracted to the negative cathode, where each ion gains three electrons to form an aluminium atom: Al3+(l)+3e−→Al(l)\mathrm{Al^{3+}(l) + 3e^- \rightarrow Al(l)}Al3+(l)+3e−→Al(l).
    2. Gaining electrons is reduction, so the aluminium ions are reduced.
  2. At the anode
    1. Negative oxide ions are attracted to the positive anode, where they lose electrons to form oxygen gas: 2O2−(l)→O2(g)+4e−\mathrm{2O^{2-}(l) \rightarrow O_2(g) + 4e^-}2O2−(l)→O2​(g)+4e−.
    2. Losing electrons is oxidation, so the oxide ions are oxidised.
  3. The overall equation for the electrolysis is 2Al2O3(l)→4Al(l)+3O2(g)\mathrm{2Al_2O_3(l) \rightarrow 4Al(l) + 3O_2(g)}2Al2​O3​(l)→4Al(l)+3O2​(g).
Example

Combining the two half-equations

  • The cathode half-equation uses 333 electrons and the anode half-equation releases 444, so both are multiplied to involve 121212 electrons.
4Al3++12e−→4Al \mathrm{4Al^{3+} + 12e^- \rightarrow 4Al} 4Al3++12e−→4Al 6O2−→3O2+12e− \mathrm{6O^{2-} \rightarrow 3O_2 + 12e^-} 6O2−→3O2​+12e−
  • Adding the two half-equations and cancelling the 121212 electrons on each side gives the ionic equation for the whole cell.
4Al3++6O2−→4Al+3O2 \mathrm{4Al^{3+} + 6O^{2-} \rightarrow 4Al + 3O_2} 4Al3++6O2−→4Al+3O2​
  • Grouping the ions back into aluminium oxide gives the overall equation 2Al2O3→4Al+3O2\mathrm{2Al_2O_3 \rightarrow 4Al + 3O_2}2Al2​O3​→4Al+3O2​.

Replacing the carbon anodes

  1. The oxygen produced at the anodes forms at about 950 ∘C950\ ^\circ\text{C}950 ∘C, right next to hot carbon.
  2. The oxygen reacts with the carbon of the anodes to form carbon dioxide: C(s)+O2(g)→CO2(g)\mathrm{C(s) + O_2(g) \rightarrow CO_2(g)}C(s)+O2​(g)→CO2​(g).
  3. The anodes are gradually burned away, so they become smaller and must be replaced regularly.
  4. The carbon lining of the cathode is not used up, because aluminium forms there and no oxygen is produced at it.
  5. Replacing the anodes adds to the cost of the process, and the carbon dioxide released adds to the greenhouse gases in the atmosphere.
Exam technique

Answering aluminium extraction questions

  • For the role of cryolite, state that it lowers the operating temperature and so saves energy, or that it improves the electrical conductivity of the electrolyte.
  • For the anodes, give both steps: oxygen forms at the anode, and the oxygen reacts with the carbon to make carbon dioxide.
  • Balance each half-equation for charge as well as atoms, so 2O2−\mathrm{2O^{2-}}2O2− carries four negative charges and releases four electrons.
  • Name the products at each electrode clearly: aluminium at the cathode and oxygen at the anode.
Common Mistake
  • Do not call cryolite a catalyst, because it acts as a solvent that lowers the operating temperature.
  • Do not write the anode product as single oxygen atoms, because oxygen forms as O2\mathrm{O_2}O2​ molecules.
  • Do not say that the anodes are used up by the electric current, because they are used up by reacting with oxygen.
Self review
  • Write the symbol equation for the reduction of iron(III) oxide by carbon monoxide.
  • Write the symbol equation for the formation of slag.
  • Why is aluminium oxide dissolved in molten cryolite?
  • Write the half-equation for the reaction at the cathode.
  • Why must the carbon anodes be replaced regularly?
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9.6.2 Blast furnace equations and aluminium extraction Revision Guide

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