1. Occurrence of metals: minerals and ores
Metals occur native (free: gold, platinum) or combined as oxides, sulphides, carbonates and other compounds.
- Mineral: a naturally occurring compound of a metal in the earth’s crust.
- Ore: a mineral from which the metal can be extracted conveniently and profitably. Every ore is a mineral; every mineral is not an ore (clay is a mineral of aluminium, not an ore).
- Gangue (matrix): the earthy impurities mixed with the ore.
| Metal | Ores (formula) |
|---|---|
| Iron | Haematite Fe₂O₃; magnetite Fe₃O₄ |
| Aluminium | Bauxite Al₂O₃·2H₂O; cryolite Na₃AlF₆; corundum Al₂O₃ |
| Zinc | Zinc blende ZnS; calamine ZnCO₃ |
Key terms
- Ore
A mineral from which a metal can be extracted conveniently and profitably.
- Gangue
The unwanted earthy or rocky material mixed with an ore.
2. Concentration (dressing) of the ore
Concentration removes the gangue, using a property in which ore and gangue differ.
- Hydraulic washing (gravity separation): a stream of water washes away the light gangue; heavy ore particles remain (haematite).
- Magnetic separation: a magnetic roller separates magnetic ore from non-magnetic gangue (magnetite).
- Froth flotation: for sulphide ores (zinc blende, galena). Powdered ore + water + pine oil; air is blown in. Ore particles are wetted by oil and rise in the froth; gangue is wetted by water and sinks.
- Chemical method: bauxite is purified by Baeyer’s process (section 5).
| Ore | Method | Reason |
|---|---|---|
| Magnetite | Magnetic separation | Ore is magnetic |
| Zinc blende | Froth flotation | Sulphide, wetted by oil |
| Haematite | Hydraulic washing | Ore heavier than gangue |
Key terms
- Froth flotation
Concentration of a sulphide ore in which oil-wetted ore particles are carried up in a froth while water-wetted gangue sinks.
- Magnetic separation
Concentration that uses a magnet to separate a magnetic ore from non-magnetic gangue, or the reverse.
3. Roasting and calcination
The concentrated ore is changed into an oxide, which is easier to reduce.
| Roasting | Calcination |
|---|---|
| Strong heating in excess air, below the melting point | Strong heating in limited air or none |
| For sulphide ores | For carbonate and hydrated ores |
| Gives SO₂ | Gives CO₂ or H₂O |
Both also remove volatile impurities and leave the ore porous.
| Ore | Process | Gas given off |
|---|---|---|
| Zinc blende | Roasting | Sulphur dioxide |
| Calamine | Calcination | Carbon dioxide |
| Bauxite | Calcination | Water vapour |
Formulas
- Roasting of zinc blende
- Roasting of galena
- Calcination of calamine
- Calcination of bauxite
Key terms
- Roasting
Heating a sulphide ore strongly in excess air, below its melting point, to convert it into the oxide.
- Calcination
Heating a carbonate or hydrated ore strongly in a limited supply of air or in its absence to convert it into the oxide.
4. Reduction and the activity series
The method of reduction depends on the position of the metal in the activity series.
- K, Na, Ca, Mg, Al (most reactive): electrolysis of fused compounds. Their oxides are too stable for carbon, and in aqueous solution hydrogen would be discharged instead.
- Zn, Fe, Pb, Cu: oxide reduced by carbon (coke), carbon monoxide or hydrogen on heating.
- Hg, Ag (least reactive): heating the oxide alone, e.g. 2HgO → 2Hg + O₂.
Al₂O₃ and MgO are not reduced by carbon because aluminium and magnesium have a greater affinity for oxygen than carbon has.
| Metals | Method |
|---|---|
| K, Na, Ca, Mg, Al | Electrolysis of fused compound |
| Zn, Fe, Pb, Cu | Reduction by C, CO or H₂ |
| Hg, Ag | Heat alone |
Formulas
- Reduction by hydrogen (black to reddish-brown)
- Reduction by coke
- Reduction by coke
- Reduction by carbon monoxide
Key terms
- Reducing agent
A substance that removes oxygen from (or adds electrons to) another substance, itself being oxidised.
- Electrolytic reduction
Extraction of a reactive metal by electrolysing its fused compound; the metal ions gain electrons at the cathode.
5. Purification of bauxite: Baeyer’s process
Bauxite contains iron(III) oxide (and other impurities) that would spoil the aluminium, so it is purified first.
- Digestion: finely ground bauxite is heated with concentrated NaOH under pressure (about 150–200 °C). Alumina dissolves as sodium aluminate; Fe₂O₃ does not dissolve and is filtered off.
- Precipitation: the solution is diluted and seeded with fresh Al(OH)₃ and stirred; Al(OH)₃ precipitates.
- Ignition: Al(OH)₃ is washed, dried and heated to about 1000 °C to give pure alumina.
| Stage | Aluminium is present as | Impurity |
|---|---|---|
| Digestion | Sodium aluminate (in solution) | Fe₂O₃ filtered off |
| Precipitation | Aluminium hydroxide (solid) | — |
| Ignition | Pure alumina | — |
Formulas
- Digestion
- Precipitation
- Ignition at about 1000 °C
Key terms
- Baeyer’s process
Purification of bauxite by dissolving alumina in hot concentrated sodium hydroxide, filtering off impurities, precipitating aluminium hydroxide and igniting it to alumina.
- Seeding
Adding a few crystals of a substance to a solution to start its precipitation.
6. Extraction of aluminium: Hall–Héroult process
- Electrolyte: pure alumina dissolved in fused cryolite (Na₃AlF₆) with a little fluorspar (CaF₂); about 950 °C.
- Cathode: carbon lining of the steel tank. Anodes: thick carbon rods dipping into the electrolyte.
- Cryolite: lowers the fusion temperature (from about 2050 °C) and increases conductivity. Fluorspar: increases fluidity. Powdered coke on top: reduces heat loss and protects the anodes from air.
- Molten aluminium collects at the bottom and is tapped off; fresh alumina is added, so the process is continuous.
- Oxygen formed at the anodes burns them (C + O₂ → CO₂; 2C + O₂ → 2CO), so the anodes are replaced periodically.
Cathode: Al³⁺ + 3e⁻ → Al. Anode: 2O²⁻ → O₂ + 4e⁻.
| Part of cell | Material or substance |
|---|---|
| Cathode | Carbon lining of steel tank |
| Anode | Thick carbon rods |
| Electrolyte | Alumina + cryolite + fluorspar |
| Product at cathode | Molten aluminium |
| Product at anode | Oxygen (burns anodes to CO, CO₂) |
Formulas
- Overall change
- Anodes burnt by oxygen
Key terms
- Cryolite
Na₃AlF₆; the solvent for alumina in the Hall–Héroult cell, which lowers the fusion temperature and raises conductivity.
- Fluorspar
CaF₂; added to the electrolyte to increase its fluidity.
7. Alloys and amalgams
An alloy is a homogeneous mixture of two or more metals, or of a metal and a non-metal. An amalgam is an alloy containing mercury.
Alloys are preferred because they can be harder and stronger, resist corrosion, have a lower melting point, or look better than the pure metal.
| Alloy | Constituents | Use |
|---|---|---|
| Stainless steel | Fe, Cr, Ni, C | Cutlery, surgical instruments |
| Magnalium | Al, Mg | Balance beams, light instruments |
| Duralumin | Al, Cu, Mg, Mn | Aircraft bodies, pressure cookers |
| Brass | Cu, Zn | Utensils, door fittings |
| Bronze | Cu, Sn | Statues, medals, bells |
| Solder | Pb, Sn | Joining electrical wires |
Key terms
- Alloy
A homogeneous mixture of two or more metals, or of a metal and a non-metal.
- Amalgam
An alloy in which one of the metals is mercury.
Exam tips
Common mistakes
- Writing calamine as ZnS (it is ZnCO₃; zinc blende is ZnS).
- Saying calcination is done in excess air.
- Saying aluminium forms at the anode, or that cryolite is an electrode.
- Forgetting that the anodes are burnt away by oxygen.
- Swapping brass (Cu, Zn) and bronze (Cu, Sn).
How marks are usually awarded
- Equations (1 mark): correct formulae and balancing; add the condition (heat, excess air) where asked.
- Reasons (1 mark each): link to the activity series, affinity for oxygen or a property difference.
- Hall–Héroult questions: electrolyte, electrodes, electrode reactions and the roles of cryolite, fluorspar and coke.