1.5 - Extraction of Metal from Its Ore

Choosing an Extraction Method

• Very unreactive metals may occur naturally as elements.

• Metals more reactive than carbon, such as aluminium, are extracted by electrolysis of molten compounds.

• Metals less reactive than carbon, such as iron, can be extracted by reduction with carbon or carbon monoxide.

• Ores are metal-containing compounds found with rocks and soil, commonly oxides, sulphides or carbonates. Gold and silver can occur as elements.

• The reactivity series guides extraction: K, Na, Mg and Al by electrolysis; Zn, Fe, Sn and Pb by carbon reduction; less reactive Cu and Hg by direct heating in air; Ag and Au occur as elements.

Aluminium by Electrolysis

• Bauxite is purified to aluminium oxide, $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ (alumina).
• Alumina melts near $2000{}^{\circ }\mathrm{C}$, so molten cryolite, $\mathrm{N}{\mathrm{a}}_3\mathrm{Al}{\mathrm{F}}_6$, is added to lower the operating temperature and energy demand.
• Cathode: $\mathrm{A}{\mathrm{l}}^{3+}(\mathrm{l})+3{\mathrm{e}}^{-}\to \mathrm{Al}(\mathrm{l})$; molten aluminium sinks and is tapped off.
• Anode: $2{\mathrm{O}}^{2-}(\mathrm{l})\to {\mathrm{O}}_2(\mathrm{g})+4{\mathrm{e}}^{-}$; oxygen reacts with carbon anodes to form carbon dioxide, so the anodes wear away.
• The process consumes much electricity, produces $\mathrm{C}{\mathrm{O}}_2$ and bauxite purification produces toxic red mud. Recycling aluminium is 95% more efficient than extraction from bauxite and reduces these impacts.

Iron in a Blast Furnace

• Hematite, $\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3$, is heated with coke, $\mathrm{C}$, and limestone, $\mathrm{CaC}{\mathrm{O}}_3$.

• Other iron ores include magnetite, $\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4$; iron pyrite, $\mathrm{Fe}{\mathrm{S}}_2$; and siderite, $\mathrm{FeC}{\mathrm{O}}_3$.

• Zone 1: Coke combustion is exothermic and raises the furnace temperature to about $1600{}^{\circ }\mathrm{C}$.

• Zone 2: Carbon dioxide reacts with coke to form carbon monoxide; this endothermic reaction lowers the temperature.

• Zone 3: Coke and carbon monoxide reduce iron(III) oxide to molten iron. Direct carbon reduction is $2F{\mathrm{e}}_2{\mathrm{O}}_3+3C\to 4Fe+3C{\mathrm{O}}_2$.

• Coke burns and generates the reducing agent carbon monoxide: $\mathrm{C}+{\mathrm{O}}_2\to \mathrm{C}{\mathrm{O}}_2$ $\mathrm{C}{\mathrm{O}}_2+\mathrm{C}\to 2CO$

• Carbon monoxide reduces iron(III) oxide: $\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+3CO\to 2Fe+3C{\mathrm{O}}_2$

• Limestone decomposes: $\mathrm{CaC}{\mathrm{O}}_3\to \mathrm{CaO}+\mathrm{C}{\mathrm{O}}_2$.

• Basic calcium oxide removes acidic silicon(IV) oxide impurity as molten slag: $\mathrm{CaO}+\mathrm{Si}{\mathrm{O}}_2\to \mathrm{CaSi}{\mathrm{O}}_3$

• Denser molten iron collects below the slag and both layers are tapped separately.

Other Extraction Routes

• Less reactive metals can be extracted by heating their ores directly in air, while unreactive metals such as gold occur as elements.
• A metal oxide can also be reduced by a metal higher in the reactivity series. In the thermite reaction, aluminium reduces iron(III) oxide and produces enough heat to form molten iron for welding railway tracks: $\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3(\mathrm{s})+2Al(\mathrm{s})\to \mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3(\mathrm{s})+2Fe(\mathrm{l})$
• Chromium and titanium can similarly be extracted from their oxides using a more reactive metal.