Extraction of Metals Study Notes

Principles of Metal Extraction

  • Metal ores are rocks containing enough metal compounds, such as iron oxide or aluminium oxide, to make extraction worthwhile.

  • Native metals, such as gold and platinum, are chemically stable and found as uncombined elements in the Earth's crust.

  • Extraction methods are determined by a metal's position in the reactivity series:

    • Metals above carbon (Potassium, Sodium, Lithium, Calcium, Magnesium, Aluminium) are extracted via electrolysis of molten chlorides or oxides. This process is expensive due to high electricity requirements.

    • Metals below carbon (Zinc, Iron, Hydrogen, Copper) are extracted by heating with a reducing agent like carbon or carbon monoxide in a blast furnace.

    • Extraction of metals from oxide ores is a reduction process because oxygen is removed.

Extraction of Iron from Hematite

  • Iron is extracted from hematite ore in a blast furnace, producing approximately 10,000 tonnes of iron daily.

  • Raw materials added to the top: iron ore (hematite), coke, and limestone. Hot air is blown into the bottom.

  • Zone 1: Coke burns to produce heat and carbon dioxide.

    • C(s)+O2(g)→CO2(g)C (s) + O_2 (g) \rightarrow CO_2 (g)

  • Zone 2: Carbon dioxide is reduced by coke to form carbon monoxide.

    • CO2(g)+C(s)→2CO(g)CO_2 (g) + C (s) \rightarrow 2CO (g)

  • Zone 3: Carbon monoxide reduces iron(III) oxide to molten iron, which is tapped from the bottom.

    • Fe2O3(s)+3CO(g)→2Fe(l)+3CO2(g)Fe_2O_3 (s) + 3CO (g) \rightarrow 2Fe (l) + 3CO_2 (g)

  • Impurity Removal: Limestone (calcium carbonate) thermally decomposes to calcium oxide, which reacts with silicon dioxide (impurity) to form molten slag (calcium silicate).

    • CaCO3(s)→CaO(s)+CO2(g)CaCO_3 (s) \rightarrow CaO (s) + CO_2 (g)

    • CaO(s)+SiO2(s)→CaSiO3(l)CaO (s) + SiO_2 (s) \rightarrow CaSiO_3 (l)

Extraction of Aluminium from Bauxite

  • Aluminium is extracted from bauxite ore (aluminium oxide) via electrolysis because it is more reactive than carbon.

  • Aluminium oxide (Al2O3Al_2O_3) is dissolved in molten cryolite to lower its melting point from over 2000∘C2000^\circ\text{C}, reducing energy costs.

  • The electrolysis cell uses a steel container lined with graphite (cathode) and several graphite blocks as anodes.

  • Cathode (reduction): Aluminium ions gain electrons to form molten aluminium.

    • Al3++3e−→AlAl^{3+} + 3e^- \rightarrow Al

  • Anode (oxidation): Oxide ions lose electrons to produce oxygen gas.

    • 2O2−→O2+4e−2O^{2-} \rightarrow O_2 + 4e^-

  • Overall Reaction: 2Al2O3→4Al+3O22Al_2O_3 \rightarrow 4Al + 3O_2

  • Anode Replacement: Oxygen reacts with the graphite (carbon) anodes to form carbon dioxide, causing them to wear away.

    • C(s)+O2(g)→CO2(g)C (s) + O_2 (g) \rightarrow CO_2 (g)