Extracting metals

Metals are found in the earth's crust. Any naturally occurring substance is a mineral. A mineral from which a metal can be extracted profitably is an ore: think about money, an ore is a mineral that pays to mine. Ores come mixed with soil and sand, called gangue, which is removed first.

How a metal is extracted depends on where it sits in the reactivity series. The more reactive the metal, the harder it holds on to other elements, and the more energy it takes to free it.

Low in the series (gold, silver, platinum, copper): often found free, as the metal itself. Mercury's ore just needs heating:

2HgS + 3O₂ → 2HgO + 2SO₂, then 2HgO → 2Hg + O₂

Middle of the series (zinc, iron, lead): ores are usually sulphides or carbonates. First turn them into oxides:

• Sulphide → roaSting: heated strongly in plenty of air. 2ZnS + 3O₂ → 2ZnO + 2SO₂
• Carbonate → Calcination: heated strongly in limited air. ZnCO₃ → ZnO + CO₂

Then reduce the oxide with carbon: ZnO + C → Zn + CO

A reactive metal can also do the reducing. Aluminium takes oxygen from iron oxide with so much heat that the iron comes out molten: Fe₂O₃ + 2Al → 2Fe + Al₂O₃. This thermit reaction is used to join broken railway tracks.

Top of the series (potassium, sodium, calcium, magnesium, aluminium): carbon can't pull them away from oxygen. They are extracted by electrolysis of their molten chlorides or oxides. The metal collects at the cathode (the negative electrode).

Refining makes the metal pure. In electrolytic refining, a thick block of impure metal is the anode, a thin strip of pure metal is the cathode, and a solution of the metal's salt is the electrolyte. Pure metal builds up on the cathode, and the impurities fall to the bottom as anode mud.