Extraction of metals

Cambridge IGCSE Chemistry 0620 · Core and Extended

Syllabus points

• State that most metals are extracted from ores found in the Earth's crust, and that unreactive metals can be found as the uncombined element.
• Describe the ease of obtaining a metal from its ore in terms of the position of the metal in the reactivity series.
• Describe the extraction of iron from hematite in the blast furnace, including the burning of coke, the reduction of carbon dioxide to carbon monoxide, the reduction of iron(III) oxide, the thermal decomposition of limestone and the formation of slag.
• Extended only: state the symbol equations for the extraction of iron from hematite.
• Extended only: describe the extraction of aluminium from purified bauxite by electrolysis, including the role of cryolite and the reason the carbon anodes are replaced.

Ores and the choice of method

An ore is a rock containing enough of a metal compound to make extracting the metal worthwhile. Most metals occur as oxides or as sulfides, because they are reactive enough to have combined with other elements long ago.

Gold and silver are found as the uncombined element, sometimes called native metal, precisely because they are so unreactive that they never combined in the first place.

The reactivity series decides how a metal must be won from its ore.

The rule underneath is that carbon can only reduce the oxide of a metal less reactive than itself. A more reactive metal holds its oxygen too tightly for carbon to take it, so electricity must be used instead, which is why aluminium is far more expensive to produce than iron even though it is more abundant in the crust.

The higher a metal is in the series, the more energy its extraction takes and the later in history it was first obtained. Copper and iron have been smelted for thousands of years; aluminium only since electricity became available.

The blast furnace

Raw materials: hematite, which is iron(III) oxide Fe₂O₃; coke, which is carbon; limestone, which is calcium carbonate; and hot air.

The four reactions, in the order they happen:

1. The coke burns in the hot air blast. This is strongly exothermic and supplies the heat, taking the furnace to about 1500 °C.

C + O₂ → CO₂

2. The carbon dioxide is reduced by more coke higher up the furnace, where the temperature is lower.

CO₂ + C → 2CO

3. The carbon monoxide reduces the iron(III) oxide. This is the reaction that makes the iron.

Fe₂O₃ + 3CO → 2Fe + 3CO₂

4. The limestone decomposes and removes the sandy impurity.

CaCO₃ → CaO + CO₂

CaO + SiO₂ → CaSiO₃

The calcium silicate is slag. The molten iron runs to the bottom and is tapped off; the slag is less dense and floats on the iron, so it is tapped off separately and used in road building and cement.

Three points that carry marks:

• Reaction 4 is a thermal decomposition, and calcium oxide is behaving as a base neutralising the acidic oxide silicon dioxide, which ties back to 7.2.
• The reducing agent is carbon monoxide, not carbon. Carbon's job is to make the carbon monoxide and to supply the heat.
• The waste gases leaving the top are mostly nitrogen from the air, with carbon dioxide and some carbon monoxide.

Worked example. How much iron is in 1000 tonnes of pure hematite? The relative atomic masses are Fe 56 and O 16.

2 × 56 + 3 × 16 = 160

so iron is 112 parts in 160, and the percentage of iron by mass is

112 / 160 × 100 = 70%

mass of iron = 1000 × 70 / 100 = 700 tonnes

Extended only: extracting aluminium

The ore is bauxite, which is purified to aluminium oxide, Al₂O₃. Aluminium is above carbon, so it must be extracted by electrolysis, and the oxide must be molten so the ions are free to move.

The role of cryolite. Pure aluminium oxide melts at about 2054 °C, which would be ruinously expensive to maintain. Dissolving it in molten cryolite lets the cell run at about 950 °C instead, saving a great deal of energy and cost. Cryolite is a solvent here, not a reactant, and it is not used up.

The electrodes are made of carbon, which conducts and is cheap.

At the cathode, the negative electrode, aluminium ions gain electrons and molten aluminium collects at the bottom of the cell, to be tapped off:

Al³⁺ + 3e⁻ → Al

At the anode, the positive electrode, oxide ions lose electrons and oxygen gas is produced:

2O²⁻ → O₂ + 4e⁻

Why the anodes must be replaced. The oxygen is released onto carbon electrodes at about 950 °C, so it burns them away:

C + O₂ → CO₂

The anodes are eaten away steadily and have to be replaced at regular intervals, which is one of the running costs of the process. The other, much larger, is the electricity, which is why aluminium smelters are built where power is cheap and why recycling aluminium uses only a small fraction of the energy of extracting it.

Note which electrode gets which name. Reduction always happens at the cathode and oxidation at the anode, so the metal always appears at the cathode.

Common mistakes

• Saying carbon is the reducing agent in the blast furnace, when the equation shows carbon monoxide reducing the ore.
• Writing the ore as iron(II) oxide rather than iron(III) oxide.
• Saying limestone is added to provide heat, rather than to remove the silicon dioxide impurity as slag.
• Saying slag sinks below the iron, when it floats.
• Saying cryolite is a catalyst, when it is a solvent that lowers the operating temperature.
• Saying the carbon anodes wear away by rubbing, rather than by burning in the oxygen released there.
• Extracting aluminium with carbon, which cannot reduce the oxide of a metal above it in the series.
• Producing the aluminium at the anode instead of the cathode.