9.6 BETA Verified

Extraction of metals

5 learning objectives • 3 core • 2 extended

Topic 9.6: Extraction of Metals Revision Notes

1. Overview

Most metals are found in the Earth's crust as ores (chemically combined with other elements like oxygen or sulfur). This topic explores how we extract these metals using their reactivity to determine the most efficient method, focusing specifically on the industrial processes for Iron and Aluminium.

Key Definitions

  • Ore: A rock that contains a high enough concentration of a metal or metal compound to make it economically worth extracting.
  • Reduction: The process of removing oxygen from a substance (in this context, removing oxygen from a metal oxide to obtain the pure metal).
  • Electrolysis: The decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current.
  • Hematite: The main ore of iron, containing iron(III) oxide (Fe₂O₃).
  • Bauxite: The main ore of aluminium, containing aluminium oxide (Al₂O₃).
  • Slag: A waste product formed in the blast furnace (calcium silicate) used for road building.

Core Content

Reactivity and Extraction Method

The ease of obtaining a metal depends on its position in the reactivity series:

  • Unreactive metals (e.g., Gold, Platinum): Often found as pure "native" metals.
  • Metals below Carbon (e.g., Zinc, Iron, Copper): Extracted by heating the ore with carbon (reduction). Carbon is more reactive than these metals and "steals" the oxygen.
  • Metals above Carbon (e.g., Aluminium, Magnesium, Sodium): Very stable compounds. They require electrolysis, which uses a lot of electricity and is more expensive.

Extraction of Iron (The Blast Furnace)

Iron is extracted from hematite in a large tower called a blast furnace. Raw Materials: Hematite (Fe₂O₃), Coke (carbon), Limestone (CaCO₃), and Hot Air.

The Process Steps:

  1. Burning of Coke: Coke reacts with oxygen in the air to produce heat and carbon dioxide.
    • Word Equation: carbon + oxygen → carbon dioxide
  2. Production of Carbon Monoxide: The carbon dioxide reacts with more coke to form carbon monoxide (the reducing agent).
    • Word Equation: carbon + carbon dioxide → carbon monoxide
  3. Reduction of Iron(III) Oxide: Carbon monoxide reacts with the iron ore to remove the oxygen, leaving molten iron.
    • Word Equation: iron(III) oxide + carbon monoxide → iron + carbon dioxide
  4. Limestone Decomposition: Limestone is added to remove impurities like sand (silica). It breaks down into calcium oxide.
    • Word Equation: calcium carbonate → calcium oxide + carbon dioxide
  5. Formation of Slag: Calcium oxide reacts with silica (sand) to form liquid slag.
    • Word Equation: calcium oxide + silicon(IV) oxide → calcium silicate (slag)
📊A cross-section of a Blast Furnace showing raw materials entering at the top, hot air entering near the bottom, and molten iron and slag being tapped off at the base.

Extraction of Aluminium

  • Ore: Bauxite (contains aluminium oxide).
  • Method: Because aluminium is more reactive than carbon, it must be extracted by electrolysis.

Extended Content (Extended Curriculum Only)

Symbol Equations for the Blast Furnace

  1. C (s) + O₂ (g) → CO₂ (g) (Exothermic reaction provides heat)
  2. C (s) + CO₂ (g) → 2CO (g) (Reduction of CO₂)
  3. Fe₂O₃ (s) + 3CO (g) → 2Fe (l) + 3CO₂ (g) (Reduction of ore)
  4. CaCO₃ (s) → CaO (s) + CO₂ (g) (Thermal decomposition)
  5. CaO (s) + SiO₂ (s) → CaSiO₃ (l) (Neutralisation reaction to form slag)

Extraction of Aluminium from Purified Bauxite

Aluminium oxide has a very high melting point (over 2000°C).

  1. Role of Cryolite: The aluminium oxide is dissolved in molten cryolite. This lowers the melting point to about 950°C, saving energy and costs. It also improves the conductivity of the electrolyte.
  2. Anode Replacement: The oxygen produced at the positive carbon anodes reacts with the carbon at high temperatures.
    • C (s) + O₂ (g) → CO₂ (g)
    • Because the carbon anodes "burn away" into CO₂ gas, they must be regularly replaced.

Electrode Reactions (Half-Equations):

  • At the Cathode (Negative electrode): Aluminium ions gain electrons (reduction).
    • Al³⁺ (l) + 3e⁻ → Al (l)
  • At the Anode (Positive electrode): Oxide ions lose electrons (oxidation).
    • 2O²⁻ (l) → O₂ (g) + 4e⁻

Key Equations

Reaction Balanced Symbol Equation
Reduction of Iron Ore Fe₂O₃ (s) + 3CO (g) → 2Fe (l) + 3CO₂ (g)
Slag Formation CaO (s) + SiO₂ (s) → CaSiO₃ (l)
Aluminium at Cathode Al³⁺ (l) + 3e⁻ → Al (l)
Aluminium at Anode 2O²⁻ (l) → O₂ (g) + 4e⁻
Anode Wearing Away C (s) + O₂ (g) → CO₂ (g)

Common Mistakes to Avoid

  • ❌ Wrong: Thinking carbon reduces iron ore directly in the main reaction.
    • ✓ Right: In the blast furnace, carbon monoxide (CO) is the main reducing agent that reacts with the iron(III) oxide.
  • ❌ Wrong: Stating cryolite is used to clean the aluminium.
    • ✓ Right: Cryolite is used as a solvent to lower the operating temperature and increase conductivity.
  • ❌ Wrong: Forgetting state symbols in the blast furnace equations.
    • ✓ Right: Remember iron and slag are (l) because the furnace is hot enough to melt them.

Exam Tips

  • Command Words:
    • "State": Give a brief answer (e.g., "State the ore of aluminium" → Bauxite).
    • "Explain": Give reasons (e.g., "Explain why anodes are replaced" → They react with oxygen to form CO₂ and wear away).
  • Frequency: This topic is high-yield (appeared in 29 past papers). Ensure you can draw or label the Blast Furnace and the Aluminium electrolysis cell.
  • Contexts: Expect questions on the environmental impact of CO₂ emissions from these processes or the high energy cost of electrolysis.
  • Numerical Values: Know that iron melts at roughly 1538°C (furnace runs at ~1500°C) and aluminium electrolysis occurs at ~950°C due to cryolite.