Extraction of Metals

Lessons

Senior 6 · Chemistry

Extraction of Metals

4 lessons

Lesson 2 of 4

Carbon Reduction: Iron, Copper, Zinc and Tin

25 min

A furnace that never goes out

A modern blast furnace is a steel tower lined with heat-resistant brick, often 30 m or more high. Once lit, it runs day and night for years: raw materials go in at the top, and molten iron is drawn off at the bottom every few hours. Blast furnaces make most of the world's iron, and they rest on one idea from Lesson 1: iron is below carbon in the reactivity series, so carbon can take the oxygen from iron oxide.

Figure 2.1: The blast furnace. The numbers match the equations below; temperatures are approximate.

The reactions, zone by zone

1. Near the tuyeres, hot air meets coke and it burns. The reaction is strongly exothermic and heats the furnace to about 1900 °C:

$\mathrm{C(s) + O_2(g) \rightarrow CO_2(g)} \qquad \Delta H = -394\,\mathrm{kJ\,mol^{-1}}$

2. Just above, carbon dioxide passing over more hot coke is reduced to carbon monoxide, the main reducing agent. This step is endothermic:

$\mathrm{CO_2(g) + C(s) \rightarrow 2CO(g)} \qquad \Delta H = +173\,\mathrm{kJ\,mol^{-1}}$

3. Higher up, the rising carbon monoxide reduces the iron(III) oxide:

$\mathrm{Fe_2O_3(s) + 3CO(g) \rightarrow 2Fe(s) + 3CO_2(g)}$

The iron forms as a spongy solid. As it sinks it absorbs carbon, which lowers its melting point to about 1150 °C, so it melts and trickles into the hearth. In the hottest zone any remaining iron oxide is reduced directly by coke, for example $\mathrm{FeO(s) + C(s) \rightarrow Fe(l) + CO(g)}$ .

4 and 5. Removing the sand. The main impurity in the ore is silica, $\mathrm{SiO_2}$ , which melts only at about 1700 °C. Limestone decomposes, and the basic calcium oxide reacts with the acidic silica to form calcium silicate, a liquid slag:

$\mathrm{CaCO_3(s) \rightarrow CaO(s) + CO_2(g)}$

$\mathrm{CaO(s) + SiO_2(s) \rightarrow CaSiO_3(l)}$

Slag is less dense than molten iron, so it floats on top and is tapped off separately. It is used to make cement and road stone.

Definition

Flux: a substance added in smelting to combine with the gangue and form a liquid slag that can be run off. Example: limestone (as CaO) removes silica in the blast furnace.

Pig iron: the iron from the blast furnace, containing about 4% carbon with silicon, phosphorus and sulfur. It is hard but brittle.

Worked example

Problem. A blast furnace is fed 1.00 tonne of haematite ore that contains 80.0% $\mathrm{Fe_2O_3}$ by mass. What is the maximum mass of iron it can produce? (Fe = 55.85, O = 16.00)

Solution. Mass of $\mathrm{Fe_2O_3}$ = 0.800 × 1000 kg = 800 kg = $8.00 \times 10^{5}$ g.

$M(\mathrm{Fe_2O_3}) = 2(55.85) + 3(16.00) = 159.7\,\mathrm{g\,mol^{-1}} \qquad n = \frac{8.00 \times 10^{5}}{159.7} = 5009\,\mathrm{mol}$

Each mole of $\mathrm{Fe_2O_3}$ gives 2 mol of Fe:

$m(\mathrm{Fe}) = 2 \times 5009 \times 55.85 = 5.595 \times 10^{5}\,\mathrm{g}$

Answer: about 560 kg of iron (a real furnace recovers slightly less).

From pig iron to steel

Pig iron is too brittle for most uses. In the basic oxygen process, pure oxygen is blown through the molten pig iron. It oxidises carbon to carbon monoxide, which escapes, and silicon and phosphorus to acidic oxides, which react with added calcium oxide to form slag. The carbon content is brought down to the value needed: mild steel, with about 0.25% carbon, for roofing sheets, nails and car bodies; high-carbon steel, with up to about 1.5%, for hard tools such as hoes and machetes. Adding chromium and nickel gives stainless steel.

Copper, zinc and tin

Table 2.1: Extracting three more metals

Metal and ore	Concentration and roasting	Reduction	Refining
Copper: chalcopyrite, $\mathrm{CuFeS_2}$	froth flotation; partial roasting: $\mathrm{2CuFeS_2 + 4O_2 \rightarrow Cu_2S + 2FeO + 3SO_2}$ ; iron removed as slag, $\mathrm{FeO + SiO_2 \rightarrow FeSiO_3}$	air blown through molten $\mathrm{Cu_2S}$ : $\mathrm{Cu_2S(l) + O_2(g) \rightarrow 2Cu(l) + SO_2(g)}$ , giving "blister" copper, about 98–99% pure	electrolysis: impure copper anode, pure copper cathode, copper(II) sulfate electrolyte; gives 99.99% copper
Zinc: zinc blende, ZnS	froth flotation; roasting: $\mathrm{2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2}$	(a) with coke above about 1000 °C: $\mathrm{ZnO(s) + C(s) \rightarrow Zn(g) + CO(g)}$ ; the zinc vapour is condensed; or (b) ZnO dissolved in sulfuric acid, then electrolysis of the $\mathrm{ZnSO_4}$ solution	distillation, or the electrolytic route gives pure zinc directly; most zinc today is made by route (b)
Tin: cassiterite, $\mathrm{SnO_2}$	gravity separation (cassiterite is dense); magnetic separation removes wolframite; roasting drives off sulfur and arsenic impurities	with coke at about 1200–1300 °C: $\mathrm{SnO_2(s) + 2C(s) \rightarrow Sn(l) + 2CO(g)}$	tin melts at only 232 °C, so it is refined by melting it away from higher-melting impurities, or by electrolysis

In every roasting step, the sulfur dioxide is captured and converted into sulfuric acid by the contact process (Unit 1), instead of being released to cause acid rain.

Refining copper by electrolysis

At the impure anode, copper atoms are oxidised and go into solution; at the pure cathode, copper ions are reduced:

$\text{anode: } \mathrm{Cu(s) \rightarrow Cu^{2+}(aq) + 2e^-} \qquad \text{cathode: } \mathrm{Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)}$

Less reactive impurities such as silver and gold do not dissolve; they fall to the bottom as anode sludge, which is worth recovering. More reactive impurities such as zinc and iron dissolve but stay in solution, because $\mathrm{Cu^{2+}}$ is reduced in preference to them.

Worked example

Problem. A current of 200 A flows through a copper refining cell for 10.0 hours. What mass of pure copper is deposited on the cathode? ( $F = 96\,485\,\mathrm{C\,mol^{-1}}$ , Cu = 63.55)

Solution.

$Q = It = 200 \times 10.0 \times 3600 = 7.20 \times 10^{6}\,\mathrm{C}$

$n(e^-) = \frac{7.20 \times 10^{6}}{96\,485} = 74.62\,\mathrm{mol} \qquad n(\mathrm{Cu}) = \frac{74.62}{2} = 37.31\,\mathrm{mol}$

$m(\mathrm{Cu}) = 37.31 \times 63.55 = 2371\,\mathrm{g}$

Answer: 2.37 kg of copper.

Warning

Carbon monoxide. Blast-furnace and smelter gases contain carbon monoxide, which is colourless, odourless and deadly. Plants burn the waste gas to heat the incoming air, and workers carry CO detectors.

Tip

For blast-furnace questions, write the five equations in order from the bottom up: burn, make CO, reduce, decompose limestone, make slag. Say which is the heat source (1) and which is the main reducing agent (CO).

ChemJungle: The essential reactions in the extraction of iron in the blast furnace (5 min). Watch for where each reaction happens in the furnace, and for the role of limestone in removing silica.

✦

Practice

Name the three raw materials fed into the top of a blast furnace, and give the role of each.
Write balanced equations, with state symbols, for the formation of carbon monoxide in the blast furnace and for its reaction with iron(III) oxide.
Explain, with equations, how silica is removed from the furnace. Why does the slag collect above the iron?
Magnetite ore contains $\mathrm{Fe_3O_4}$ . Write a balanced equation for its reduction by carbon monoxide.
Calculate the mass of tin that can be obtained from 500 kg of a concentrate containing 70.0% $\mathrm{SnO_2}$ by mass. (Sn = 118.71, O = 16.00)
Explain why zinc forms as a gas when zinc oxide is reduced with carbon at 1100 °C, and how it is collected.
In copper refining, explain what happens to (a) silver and (b) iron impurities in the anode.
Steelworks sell blast-furnace slag and capture sulfur dioxide from roasting. Evaluate these practices from the point of view of the company and of the environment.

Show answersHide answers

Iron ore (haematite, $\mathrm{Fe_2O_3}$ ): the source of iron. Coke: the fuel, and the source of the reducing agent CO. Limestone: the flux, which removes silica as slag.
$\mathrm{CO_2(g) + C(s) \rightarrow 2CO(g)}$ ; $\mathrm{Fe_2O_3(s) + 3CO(g) \rightarrow 2Fe(s) + 3CO_2(g)}$ .
$\mathrm{CaCO_3(s) \rightarrow CaO(s) + CO_2(g)}$ ; $\mathrm{CaO(s) + SiO_2(s) \rightarrow CaSiO_3(l)}$ . Basic CaO reacts with acidic $\mathrm{SiO_2}$ to form liquid calcium silicate. It is less dense than molten iron, so it floats.
$\mathrm{Fe_3O_4(s) + 4CO(g) \rightarrow 3Fe(s) + 4CO_2(g)}$ .
Mass of $\mathrm{SnO_2}$ = 350 kg; $M = 118.71 + 32.00 = 150.71$ g mol⁻¹; $n = \frac{3.50 \times 10^{5}}{150.71} = 2322$ mol; mass of Sn = $2322 \times 118.71 = 2.757 \times 10^{5}$ g, about 276 kg.
Zinc boils at 907 °C, so at 1100 °C it is formed as a vapour. The vapour leaves the furnace with the carbon monoxide and is condensed to liquid zinc in a cooler condenser.
(a) Silver is less reactive than copper, so it does not dissolve; it falls as anode sludge. (b) Iron is more reactive, so it dissolves as $\mathrm{Fe^{2+}}$ but stays in solution: $\mathrm{Cu^{2+}}$ ions are reduced at the cathode instead.
For the company, both turn waste into products (cement and road stone; sulfuric acid), which earns money and avoids disposal costs. For the environment, less slag is dumped, less quarrying is needed, and capturing $\mathrm{SO_2}$ prevents acid rain. A limitation: both still depend on an energy-hungry process that releases large amounts of $\mathrm{CO_2}$ .

Sources and credits

Image: DSC03739 Blast Furnace 1 at Night, voestalpine Donawitz, 2026-02.jpg by Aciarium, via Wikimedia Commons, licensed under CC BY 4.0.
Image: Cassiterite (GeoDIL number - 37).jpg by Shannon Heinle, via Wikimedia Commons, dedicated to the public domain under CC0 1.0.
Video: The essential reactions in the extraction of iron in the blast furnace, ChemJungle, embedded from YouTube.
The text, blast-furnace diagram, table, worked examples and Practice items were written and drawn by Ganzaa. The statement that cassiterite is one of Rwanda's main minerals follows the Rwanda Mines, Petroleum and Gas Board's Rwanda Mining Investment Pitchbook (May 2025).

1A furnace that never goes out

2The reactions, zone by zone

3From pig iron to steel

4Copper, zinc and tin

5Refining copper by electrolysis

A furnace that never goes out

The reactions, zone by zone

From pig iron to steel

Copper, zinc and tin

Refining copper by electrolysis