S6 Chemistry – Unit 2: Extraction of Metals

2.1

Principles of Metal Extraction

What is Metallurgy?

Metallurgy is the science and technology of obtaining metals from their ores and preparing them for use. Most metals occur in nature as compounds (ores) — oxides, sulphides, carbonates, or chlorides — because they have reacted with elements in the environment.

The choice of extraction method depends on the position of the metal in the reactivity series: the more reactive the metal, the harder it is to extract.

Definition — Ore A naturally occurring mineral from which a metal can be extracted economically. Not all minerals containing a metal are ores — the concentration must be high enough to make extraction profitable.

Reactivity	Metal	Extraction Method	Example
Very high	K, Na, Ca, Mg, Al	Electrolysis of molten compound	Al from Al₂O₃
Medium	Zn, Fe, Sn, Pb	Reduction with carbon (coke)	Fe from Fe₂O₃
Low	Cu	Reduction or smelting	Cu from Cu₂S
Very low	Ag, Au, Pt	Found native; simple physical methods	Au panning

General Steps in Extraction

1. Mining: The ore is dug from the ground (open-cast or underground mining).

2. Concentration (ore dressing): Removal of impurities (gangue). Methods include froth flotation, magnetic separation, and gravity separation.

3. Reduction/Smelting: The metal compound is converted to the free metal by reduction (using carbon, CO, hydrogen, or electrolysis).

4. Refining/Purification: The crude metal is purified — by electrolytic refining, zone refining, or distillation.

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Thermite Reduction Some metals (Cr, Mn) are extracted by reduction with aluminium — the thermite reaction. Al is a stronger reducing agent than C at the temperatures involved: Cr₂O₃ + 2Al → Al₂O₃ + 2Cr. This reaction is highly exothermic.

2.2

Extraction of Iron — The Blast Furnace

Raw Materials

The blast furnace uses three raw materials: haematite (Fe₂O₃) or magnetite (Fe₃O₄) as the ore, coke (C) as the reducing agent and fuel, and limestone (CaCO₃) to remove acidic impurities (silica, SiO₂).

Reactions in the Blast Furnace

Zone 1 — Combustion (bottom, ~2000°C): C(s) + O₂(g) → CO₂(g) ΔH = −394 kJ/mol Zone 1 — CO formation: CO₂(g) + C(s) → 2CO(g) ΔH = +173 kJ/mol Zone 2 — Reduction of iron ore (~900°C): Fe₂O₃(s) + 3CO(g) → 2Fe(l) + 3CO₂(g) Fe₃O₄(s) + 4CO(g) → 3Fe(l) + 4CO₂(g) Zone 3 — Limestone decomposition (~800°C): CaCO₃(s) → CaO(s) + CO₂(g) Zone 3 — Slag formation: CaO(s) + SiO₂(s) → CaSiO₃(l) [slag]

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Why limestone? Sand (SiO₂) is an acidic impurity in the ore. Limestone provides CaO (a base) to react with SiO₂ to form calcium silicate (CaSiO₃) — the slag. Slag is less dense than iron so it floats on top and is tapped off separately.

Products and Uses

Pig iron (cast iron): The direct product of the blast furnace — contains about 4% carbon plus other impurities. It is brittle and used directly for decorative castings or further processed.

Steel: Pig iron is converted to steel by reducing the carbon content (to 0.1–1.5%) in a basic oxygen furnace (BOF) or electric arc furnace (EAF). Steel is stronger, more malleable, and widely used in construction, vehicles, and tools.

Slag: Used as a building material and in road construction.

Worked Example 2.1

Blast Furnace Mass Calculation

Calculate the mass of iron produced when 1600 kg of Fe₂O₃ reacts completely with CO. (M_r: Fe = 56, O = 16)

Write the balanced equation:
Fe₂O₃ + 3CO → 2Fe + 3CO₂

Molar mass of Fe₂O₃:
M(Fe₂O₃) = 2(56) + 3(16) = 112 + 48 = 160 g/mol

Moles of Fe₂O₃:
n = 1,600,000 g ÷ 160 g/mol = 10,000 mol

Moles of Fe (ratio 1:2):
n(Fe) = 2 × 10,000 = 20,000 mol

Mass of Fe:
m = 20,000 × 56 = 1,120,000 g = 1120 kg

2.3

Extraction of Tin

Ore and Process

Tin is extracted from cassiterite (SnO₂), the main ore. The process involves:

1. Concentration: The crushed ore is concentrated by gravity separation (shaking tables) because SnO₂ is denser than gangue minerals.

2. Roasting: If sulphide impurities are present, the ore is roasted to convert them to oxides.

3. Reduction with coke: SnO₂ is reduced by carbon in a reverberatory furnace at ~1300°C:

SnO₂(s) + 2C(s) → Sn(l) + 2CO(g)

4. Refining: Molten tin is heated gently on a sloping hearth — tin melts and runs down (liquation), leaving higher-melting impurities behind. Further electrolytic refining gives high-purity tin.

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Uses of Tin Tin is used for tin-plating steel (tin cans for food), making bronze (Cu-Sn alloy), solder (Sn-Pb or Sn-Ag-Cu), and pewter. Rwanda and neighbouring DRC are among the world's leading producers of cassiterite.

2.4

Extraction of Zinc

Ore and Process

The main ore is sphalerite (ZnS), also called zinc blende. Extraction steps:

1. Froth flotation: The crushed ore is mixed with water, pine oil, and air. ZnS particles attach to bubbles and float to the surface while gangue sinks.

2. Roasting (in excess air): ZnS is converted to ZnO:

2ZnS(s) + 3O₂(g) → 2ZnO(s) + 2SO₂(g)

3. Reduction with coke: ZnO is reduced by carbon in a retort at ~1200°C. Because Zn boils at 907°C, it distils as a vapour and is condensed:

ZnO(s) + C(s) → Zn(g) + CO(g)

4. Electrolytic refining: For high-purity zinc, the crude metal is dissolved in H₂SO₄ and electrolytically deposited on aluminium cathodes.

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Uses of Zinc Galvanising steel (protecting against corrosion), die-casting alloys, brass (Cu-Zn), and zinc-carbon batteries. SO₂ produced during roasting is used to make sulphuric acid (contact process).

Worked Example 2.2

Zinc from Roasting ZnS

If 9.7 g of ZnS is roasted completely, what mass of ZnO is produced? (M_r: Zn = 65, S = 32, O = 16)

Equation: 2ZnS + 3O₂ → 2ZnO + 2SO₂

M(ZnS) = 65 + 32 = 97 g/mol; n(ZnS) = 9.7 ÷ 97 = 0.1 mol

Ratio ZnS:ZnO = 2:2 = 1:1, so n(ZnO) = 0.1 mol

M(ZnO) = 65 + 16 = 81 g/mol; m(ZnO) = 0.1 × 81 = 8.1 g

2.5

Extraction of Sodium — The Downs Cell

Why Electrolysis?

Sodium is too reactive to be extracted by chemical reduction — carbon cannot reduce sodium compounds. Electrolysis of molten sodium chloride (NaCl) is used in the Downs cell.

Water is NOT used because sodium reacts violently with water. CaCl₂ is added to lower the melting point of NaCl from 801°C to about 600°C, saving energy.

Downs Cell — Electrode Reactions

Cathode (−): Na⁺(l) + e⁻ → Na(l) [reduction] Anode (+): 2Cl⁻(l) → Cl₂(g) + 2e⁻ [oxidation] Overall: 2NaCl(l) → 2Na(l) + Cl₂(g)

A cylindrical iron cathode surrounds a central graphite anode. A steel gauze diaphragm separates the products — liquid sodium (less dense) rises to a collector ring, while Cl₂ gas exits from a dome above the anode.

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Why keep products separate? If Na and Cl₂ were allowed to mix they would recombine violently: 2Na + Cl₂ → 2NaCl. The steel gauze diaphragm is essential for safety and efficiency.

Uses of Sodium and Chlorine

Sodium: Production of sodium peroxide, tetraethyl lead (anti-knock agent), sodamide (NaNH₂), heat exchange fluid in nuclear reactors, and sodium vapour lamps.

Chlorine: Manufacture of PVC (polyvinyl chloride), pesticides, bleach (NaOCl), hydrochloric acid, and water treatment.

2.6

Extraction of Aluminium — Hall-Héroult Process

Ore and Purification (Bayer Process)

Aluminium is the most abundant metal in the Earth's crust. It is extracted from bauxite (Al₂O₃·2H₂O).

Bayer Process — purification of bauxite:

1. Bauxite is dissolved in hot concentrated NaOH: Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O. Iron oxide impurities do not dissolve and are filtered off as "red mud".

2. The solution is diluted and seeded with Al(OH)₃ crystals — aluminium hydroxide precipitates: NaAlO₂ + 2H₂O → Al(OH)₃ + NaOH

3. Al(OH)₃ is filtered and calcined (heated strongly) to give pure Al₂O₃ (alumina): 2Al(OH)₃ → Al₂O₃ + 3H₂O

Hall-Héroult Electrolysis

Pure Al₂O₃ (m.p. 2072°C) is dissolved in molten cryolite (Na₃AlF₆) which reduces the operating temperature to ~950°C. Calcium fluoride (CaF₂) is added to lower the melting point further.

Cathode (−): Al³⁺ + 3e⁻ → Al(l) [reduction] Anode (+): 2O²⁻ → O₂(g) + 4e⁻ [oxidation] Overall: 2Al₂O₃(l) → 4Al(l) + 3O₂(g)

The carbon anode is consumed by the hot O₂ produced (C + O₂ → CO₂), requiring regular replacement. Molten aluminium (denser than cryolite) sinks to the bottom of the cell and is tapped off.

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Energy Consumption The Hall-Héroult process uses about 15 kWh of electricity per kg of Al produced — very energy-intensive. This is why aluminium smelters are located near cheap hydroelectric power. Recycling Al uses only 5% of this energy.

Worked Example 2.3

Electrolytic Deposition of Aluminium

A current of 5000 A is passed through a Hall-Héroult cell for 2 hours. Calculate the mass of aluminium deposited. (M_r: Al = 27; F = 96500 C/mol)

Charge Q = I × t = 5000 A × (2 × 3600 s) = 36,000,000 C = 3.6 × 10⁷ C

Al³⁺ + 3e⁻ → Al; 3 mol electrons deposit 1 mol Al

Moles of electrons = Q ÷ F = 3.6 × 10⁷ ÷ 96500 = 373.1 mol e⁻

Moles of Al = 373.1 ÷ 3 = 124.4 mol

Mass of Al = 124.4 × 27 = 3358 g ≈ 3.36 kg

2.7

Extraction of Tungsten

Ore and Process

Tungsten is extracted mainly from wolframite [(Fe,Mn)WO₄] and scheelite (CaWO₄). It has the highest melting point of all metals (3422°C), making conventional smelting impossible.

1. Ore treatment: The ore is fused with NaOH to form sodium tungstate: (Fe,Mn)WO₄ + 2NaOH → Na₂WO₄ + Fe(OH)₂ + Mn(OH)₂

2. Acidification: The solution is acidified to precipitate tungstic acid: Na₂WO₄ + 2HCl → H₂WO₄ + 2NaCl

3. Calcination: Tungstic acid is heated to give WO₃: H₂WO₄ → WO₃ + H₂O

4. Hydrogen reduction: WO₃ is reduced by hydrogen gas at ~700–850°C:

WO₃(s) + 3H₂(g) → W(s) + 3H₂O(g)

5. Powder metallurgy: Tungsten powder is pressed into shapes and sintered (heated under pressure) at high temperature to produce solid tungsten parts.

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Uses of Tungsten Tungsten's exceptionally high melting point makes it ideal for light bulb filaments, X-ray tube targets, cutting tools (WC — tungsten carbide), rocket nozzles, and armour-piercing shells.

2.8

Extraction of Tantalum

Ore and Global Significance

Tantalum is extracted from coltan (columbite-tantalite, (Fe,Mn)(Nb,Ta)₂O₆). The DRC and Rwanda together produce over 60% of the world's coltan. Tantalum is a critical element in modern electronics.

Extraction Process

1. Ore concentration: Coltan is concentrated by gravity separation (similar to tin extraction) using shaking tables and sluices.

2. Digestion with HF/H₂SO₄: The concentrated ore is dissolved in a mixture of hydrofluoric acid and sulphuric acid to form fluorotantalate and fluoroniobate complexes:

Ta₂O₅ + 14HF → 2H₂TaF₇ + 5H₂O

3. Solvent extraction: Tantalum (H₂TaF₇) is separated from niobium (H₂NbF₇) by selective extraction into methyl isobutyl ketone (MIBK). Niobium is more soluble in the aqueous phase at higher HF concentrations.

4. Precipitation and calcination: Ammonia is added to the organic phase: H₂TaF₇ + 5NH₄OH → Ta(OH)₅ + 5NH₄F + H₂O. The hydroxide is calcined to Ta₂O₅.

5. Reduction with sodium: Ta₂O₅ is reduced by sodium metal (sodium reduction / metallothermic reduction):

Ta₂O₅ + 10Na → 2Ta + 5Na₂O

Alternatively, K₂TaF₇ can be reduced by sodium in a molten salt bath (SodiumReduction / MSE process).

6. Electron beam melting: The tantalum powder is melted and refined using an electron beam furnace to produce high-purity ingots.

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Uses of Tantalum About 60% of tantalum goes into capacitors used in mobile phones, laptops, and medical devices. It is also used in surgical implants (biocompatible), jet engine components, and chemical equipment (resistant to most acids).

2.9

Dangers of Metal Extraction & Prevention

Metal/Process	Hazard	Prevention
Blast furnace (Fe)	CO gas (odourless, toxic); molten metal burns; slag explosions	CO detectors; protective clothing; dry slag handling
Downs cell (Na)	Molten Na reacts violently with water; Cl₂ gas (toxic)	Keep cell dry; Cl₂ extraction hoods; gas masks
Hall-Héroult (Al)	Fluoride fumes from cryolite (respiratory damage); electrical hazards	Fume extraction; insulated equipment; PPE
Roasting (Zn, Sn)	SO₂ gas (causes acid rain; toxic); high temperatures	SO₂ scrubbers; recover SO₂ for H₂SO₄ manufacture
Coltan mining (Ta)	Radiation (coltan is mildly radioactive); lung disease from dust; unsafe mining practices	Radiation monitoring; dust masks; regulation of artisanal mining
Tungsten processing	H₂ gas (flammable, explosive); HF (corrosive)	No ignition sources; HF-resistant gloves and face shields; fume cupboards

Environmental Impact

Air pollution: SO₂ from roasting causes acid rain. CO₂ from coke combustion contributes to climate change.

Water pollution: Acid mine drainage (AMD) — water reacts with exposed sulphide ores, producing sulphuric acid that contaminates rivers and groundwater.

Land degradation: Open-cast mining destroys habitats. Tailings ponds (containing toxic chemical waste) can leak.

Mitigation: Recycling metals (especially Al) dramatically reduces energy use and emissions. SO₂ recovery for H₂SO₄ production turns a pollutant into profit. Land reclamation and rehabilitation programmes restore mined areas.

Exercises

State the three raw materials added to a blast furnace and give the role of each.
Haematite (Fe₂O₃) — the iron ore (source of iron). Coke (C) — fuel (burns to provide heat) and reducing agent (produces CO which reduces iron oxide). Limestone (CaCO₃) — decomposes to CaO which reacts with silica impurities to form slag (CaSiO₃).
Write equations for: (a) roasting of ZnS, and (b) reduction of ZnO with carbon.
(a) 2ZnS(s) + 3O₂(g) → 2ZnO(s) + 2SO₂(g)
(b) ZnO(s) + C(s) → Zn(g) + CO(g)
Explain why CaCl₂ is added to NaCl in the Downs cell, and why the products must be kept separate.
CaCl₂ lowers the melting point of NaCl from 801°C to ~600°C, reducing energy costs. The products must be kept separate because sodium metal reacts vigorously with chlorine gas to re-form NaCl, wasting the products and causing a dangerous exothermic reaction.
A current of 2000 A is passed through a Hall-Héroult cell for 3 hours. Calculate the mass of Al deposited. (F = 96500 C/mol, M_r(Al) = 27)
Q = 2000 × (3 × 3600) = 21,600,000 C
mol e⁻ = 21,600,000 ÷ 96500 = 223.8 mol
Al³⁺ + 3e⁻ → Al, so mol Al = 223.8 ÷ 3 = 74.6 mol
Mass = 74.6 × 27 = 2014 g ≈ 2.01 kg
Explain why carbon cannot be used to extract sodium, but can be used to extract iron. What method is used for sodium instead?
Sodium is above carbon in the reactivity series — it is more reactive than carbon, so carbon cannot displace sodium from its compounds. Iron is below carbon in the reactivity series, so carbon (as coke/CO) can reduce iron oxides. Sodium is extracted by electrolysis of molten NaCl in the Downs cell.
Describe the role of cryolite (Na₃AlF₆) in the Hall-Héroult process and explain why the carbon anodes must be replaced regularly.
Cryolite acts as a solvent for Al₂O₃, lowering the operating temperature from 2072°C to ~950°C, which saves energy and makes the process economically feasible. The carbon anodes must be replaced regularly because the O₂ produced at the anode reacts with the hot carbon: C + O₂ → CO₂, gradually burning the anodes away.

Quiz

25-Question Multiple Choice Quiz

Unit 2 — Extraction of Metals

25 Questions

Test

50-Mark Unit Test

Unit 2 Test — Extraction of Metals

Total: 50 marks • Answer all questions • Show all working for calculation questions

[6 marks] Question 1

(a) Name the main ore of iron and give its chemical formula. [2]

(b) Write the equation for the formation of slag in the blast furnace. [2]

(d) Name one other product of the blast furnace and give one use. [1]

(a) Haematite, Fe₂O₃ [2]
(b) CaO(s) + SiO₂(s) → CaSiO₃(l) [2]
(c) Road building / cement / aggregate [1]
(d) Pig iron — used in construction / converted to steel [1]

[8 marks] Question 2

Write equations for the four main reactions that occur in the blast furnace, indicating approximate temperatures. [8]

1. C + O₂ → CO₂ (~2000°C) [2]
2. CO₂ + C → 2CO (~1500°C) [2]
3. Fe₂O₃ + 3CO → 2Fe + 3CO₂ (~900°C) [2]
4. CaCO₃ → CaO + CO₂ (~800°C) [1] and CaO + SiO₂ → CaSiO₃ [1]

[8 marks] Question 3

(a) Draw a labelled diagram of the Downs cell used to extract sodium. [4]

(b) Write electrode equations for both the cathode and anode reactions. [2]

(d) Why must the products be kept separate? [1]

(a) Diagram showing: cylindrical iron cathode surrounding a graphite anode, steel gauze diaphragm between them, outlet for Na(l) at top of cathode, outlet for Cl₂(g) above anode dome, molten NaCl/CaCl₂ electrolyte. [4]
(b) Cathode: Na⁺ + e⁻ → Na(l); Anode: 2Cl⁻ → Cl₂(g) + 2e⁻ [2]
(c) To lower the melting point of NaCl (from 801°C to ~600°C) saving energy [1]
(d) Na reacts with Cl₂ to reform NaCl (2Na + Cl₂ → 2NaCl) — products would be lost [1]

[10 marks] Question 4

(a) Describe the Bayer process for purifying bauxite to give pure Al₂O₃. Include equations. [5]

(b) Write the electrode equations in the Hall-Héroult process. [2]

(c) A current of 10,000 A is passed through a Hall-Héroult cell. How long (in hours) is needed to produce 270 kg of aluminium? (F = 96500 C/mol, M_r(Al) = 27) [3]

(a) Bauxite dissolved in hot NaOH: Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O [1]; iron oxide impurities filtered off [1]; dilute and seed with Al(OH)₃: NaAlO₂ + 2H₂O → Al(OH)₃ + NaOH [1]; filter Al(OH)₃ and calcine: 2Al(OH)₃ → Al₂O₃ + 3H₂O [1]; dissolved in molten cryolite [1] [5]
(b) Cathode: Al³⁺ + 3e⁻ → Al(l); Anode: 2O²⁻ → O₂ + 4e⁻ [2]
(c) mol Al = 270,000 ÷ 27 = 10,000 mol; mol e⁻ = 30,000 mol; Q = 30,000 × 96500 = 2.895 × 10⁹ C; t = Q ÷ I = 2.895 × 10⁹ ÷ 10,000 = 289,500 s = 80.4 hours [3]

[8 marks] Question 5

(a) Name the main ore of zinc and describe how it is concentrated before smelting. [3]

(b) Write the equation for roasting of ZnS. Why is the SO₂ produced important? [3]

(a) Sphalerite / zinc blende (ZnS) [1]; froth flotation — crushed ore mixed with water and pine oil, air blown through, ZnS sticks to bubbles and floats to surface, gangue sinks [2] [3]
(b) 2ZnS + 3O₂ → 2ZnO + 2SO₂ [1]; SO₂ is used to manufacture sulphuric acid (contact process), converting a pollutant into a valuable product [2] [3]
(c) Zinc boils at 907°C which is below the reaction temperature of ~1200°C, so Zn vaporises as it forms and can be collected by condensation [2]

[5 marks] Question 6

Describe the extraction of tungsten from wolframite. Include all relevant equations. [5]

Ore fused with NaOH to form Na₂WO₄ [1]; acidified with HCl to precipitate H₂WO₄ [1]; H₂WO₄ calcined to WO₃ [1]; WO₃ reduced with H₂: WO₃ + 3H₂ → W + 3H₂O [1]; tungsten powder sintered (powder metallurgy) to produce solid metal [1].

[5 marks] Question 7

(a) State two environmental hazards associated with roasting sulphide ores. [2]

(b) State two dangers specific to the Hall-Héroult process and how each is controlled. [3]

(a) Any 2: SO₂ causes acid rain; CO₂ from coke contributes to greenhouse effect / climate change; mining destroys habitats; acid mine drainage contaminates water [2]
(b) Fluoride fumes from cryolite — fume extraction systems [1]; high electrical currents — insulated equipment [1]; molten aluminium at 950°C — heat-resistant PPE / protective clothing [1] [accept any 2 dangers with controls, 3 marks]