A2.1 Metal Reduction and Reactivity

The reactivity of the metal will have an effect on the method used to extract the metal from the ore. Carbon (coke) is a good reducing agent because it is cheap and when oxidised releases a gas which escapes easily. Some equations:

ZnO + CZn + CO 2Fe2O + 3C4Fe + 3CO2*\textsf{ZnO + C} \rightarrow \textsf{Zn + CO } \\ \\ \\ \textsf{2Fe}_\textsf2\textsf O\textsf{ + 3C} \rightarrow \textsf{4Fe + 3CO}_\textsf2\textsf*

*more correctly the Fe2O3\textsf{Fe}_\textsf2\textsf {O}_\textsf3is reduced by CO in the Blast Furnace

Fe2O3 + 3CO2Fe + 3CO2\textsf{Fe}_\textsf2 \textsf {O}_\textsf3 \textsf{ + 3CO} \rightarrow \textsf{2Fe + 3CO}_\textsf2

Key information 🗝️

1. Some metals are found as metals, these are the most unreactive. For example silver and gold (and also copper).

2. Some metals are reduced by heating with carbon (coke) or carbon monoxide (CO\textsf{CO}) these have medium reactivity. For example iron, zinc, lead and copper.

3. The most reactive metals can only be extracted effectively using electrolysis. Metals involved include: magnesium, aluminium, all alkali metals, and calcium.

Key information 🗝️

Important details about the aluminium extraction process:

  1. Al2O3\textsf{Al}_\textsf2\textsf O_\textsf3 has a very high melting point, so cryolite, Na3AIF6\textsf{Na}_\textsf3\textsf{AIF}_\textsf6, is added which lowers the melting point to just over 1000°C to conserve energy.

  2. An enormous electrical current is used – maybe 100 000 Amps. So there needs to be a good (and preferably cheap) electrical supply (HEP or Geothermal power). This enormous current helps keep the the Al2O3\textsf{Al}_\textsf2\textsf O_\textsf3 / cryolite mixture molten (in liquid form).

  3. The process is carried out in a steel tank, lined with graphite - the tank is the cathode. The reduction of aluminium ion occurs here : Al3+ + 3eAI\textsf{Al}^\textsf{3+} \textsf{ + 3e} \rightarrow \textsf {AI}

  4. The anodes are carbon (graphite) blocks.

  5. The anode reaction is 2O2-O2+ 4e\textsf{2O}^\textsf{2-} \rightarrow \textsf{O}_\textsf2\textsf{+ 4e}^{-} and at 1000°C the oxygen causes the anodes to burn away, so they need to be replaced regularly. The heat produced also helps to keep the electrolyte molten.

  6. Graphite is used because it conducts electricity well and does not react with, or dissolve in the liquid aluminium.

  7. There is an environmental impact: Carbon dioxide and some fluorine is released into the atmosphere and large amounts of waste material is left after the initial purification step (see point 2 above).

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