Metal extraction

Key Definitions

• Metal ore – Rock containing enough of a metal to make extraction profitable.

• Metal extraction – Removal of a metal from a rock using physical and/or chemical processes.

• Alloy – Mixture of a metal with one or more other elements (metal or non-metal).

• Mineral – Naturally occurring compound or mixture of compounds, or the native metal.

• Rock – Mass of minerals found in the Earth’s crust.

• Ore – Rock with a high enough concentration of minerals (metal compounds or rarely metals) to extract economically.

Extraction of Metals

• Haematite (iron ore) contains iron(III) oxide, Fe₂O₃

• Bauxite (aluminium ore) contains aluminium oxide, Al₂O₃

Steps to extract a metal from its ore:

1. Mining and collecting the ore from the ground

   • Mining is removing ores from the Earth’s crust

   • Environmental problems: habitat destruction, soil erosion, water pollution, dust and noise

2. Extraction of the metal from the ore

   • Extracting a metal usually involves chemical or thermal decomposition to release the metal

   • Environmental problems: high energy use, greenhouse gas emissions, chemical waste

Advantages of mining and mineral extraction:

• Provides raw materials for industry and construction

• Creates jobs and economic growth

• Allows production of metals for technology and infrastructure

Disadvantages of mining and mineral extraction:

• Causes environmental damage (land and water pollution)

• Can displace communities and wildlife

• High energy consumption and CO₂ emissions

Extracting Metals from the Ores

• Most metals naturally occur in rocks as compounds of oxygen or sulfur.

• The more reactive the metal, the more tightly it is bonded to other elements in the ore and the harder it is to separate.

• Extraction usually involves reduction of metal oxides.

• Unreactive metals are found as native elements and do not require chemical separation (e.g., gold).

• The method used to extract a metal depends on its position in the reactivity series.

Questions:

• Could carbon be used to extract iron from haematite?

  • Yes, carbon can reduce iron(III) oxide.

  • Reaction: Fe₂O₃ + 3C → 2Fe + 3CO

• Could carbon be used to extract aluminium from bauxite?

  • No, aluminium is too reactive to be reduced by carbon.

  • Alternative: electrolysis of molten aluminium oxide using cryolite.

Linking Reactivity of the Metal with Method of Extraction

• Metals above carbon in the reactivity series are extracted using electrolysis.

• Metals below carbon are extracted by heating with carbon.

Advantages and Disadvantages:

Electrolysis

• Energy requirements: Very high, requires a lot of electricity

• Purity of product: High, produces very pure metal

• Impact on environment: High energy use → CO₂ emissions if electricity is from fossil fuels

Heating with Carbon

• Energy requirements: Lower than electrolysis

• Purity of product: Lower, may contain impurities

• Impact on environment: Produces CO/CO₂ → contributes to air pollution and greenhouse gases

Extraction of Metals and Recycling

• Metals higher than carbon in the reactivity series (e.g., Groups 1 & 2 and aluminium) are extracted by electrolysis. This requires a lot of energy and is expensive.

• Less reactive metals such as copper, lead, iron, and zinc occur as oxides or sulfides. Since these metals are less reactive than carbon, their ores are heated with charcoal. Carbon reduces the metal oxide to carbon dioxide and leaves the pure metal:

  • Metal oxide + carbon → metal + carbon dioxide

• These metals have been known for thousands of years.

• The cost of a metal depends on the abundance of the ore and the cost of extraction (including purification if needed).

  • Copper is costly due to purification by electrolysis.

  • Gold is expensive because it is rare, though it requires little processing as it occurs natively.

Recycling of Metals – Benefits:

• Saves raw materials and reduces mining.

• Reduces energy consumption compared with extracting from ores.

• Minimizes environmental impact (less habitat destruction and pollution).

• Reduces waste in landfills.

Problems with Recycling:

• Metals can be mixed or contaminated, requiring separation.

• Collection and transport can be expensive and energy-consuming.

• Not all metals can be recycled indefinitely without losing quality.

• Some metals are difficult to recover from complex products.

Displacement of Metals from Their Oxides

1. Displacement of metals from hot oxides by a more reactive metal

• Heating zinc powder with metal oxides.

• Safety: Wear goggles and lab coat, tie hair back. Follow teacher’s instructions.

• Metals can be extracted by reacting the compound with a more reactive metal; less used industrially due to demand for reactive metals.

Method:

• Place half a spatula of zinc powder on scrap paper.

• Add half a spatula of metal oxide and mix.

• Transfer mixture to a metal cap.

• Heat strongly with a roaring flame until changes occur.

• Let cool.

• Tip mixture onto scrap paper to look for iron, lead, and copper. Use a magnet for iron detection.

Results:

Conclusion:

• The metals compete for oxygen.

• The more reactive metal removes the less reactive metal from its oxide.

• This is a displacement reaction and also a redox reaction.

Balanced Chemical Equations and Redox:

• Zn + Fe₂O₃ → 3ZnO + 2Fe

  • Zn is oxidised, Fe³⁺ is reduced

• Zn + PbO → ZnO + Pb

  • Zn is oxidised, Pb²⁺ is reduced

• Zn + CuO → ZnO + Cu

  • Zn is oxidised, Cu²⁺ is reduced

• More reactive metal is oxidised.

• Cation of least reactive metal is reduced.

Word Equations:

• Magnesium + zinc oxide → magnesium oxide + zinc

• Copper + iron(III) oxide → no reaction (copper less reactive than iron)

• Lithium + copper(II) oxide → lithium oxide + copper

Thermit(e) Reaction

• The reaction between aluminium and iron(III) oxide is a metal displacement reaction: more reactive aluminium displaces less reactive iron from its oxide.

• Equation: Fe₂O₃ + 2Al → 2Fe + Al₂O₃

• The reaction is ignited using magnesium ribbon.

• The diagram below shoes how this thermit reaction can be carried out.

• This reaction is extremely exothermic.

• Temperature exceeds the melting point of iron, and the iron produced is molten.

• Use: Joining railway tracks – molten iron flows into the gap between rails and solidifies, welding them together.

• Advantage: Can be done in remote areas without electricity.

• The reaction mixture is ignited and molten iron pours into the mould. The mould is removed and molten iron solidifies to create a join between the two rails.

Displacement of Metals from Hot Oxides by Carbon

• Safety: Wear goggles and lab coat, tie hair back, follow teacher’s instructions.

Method:

• Mix 1 spatula of carbon powder with 1 spatula of copper(II) oxide on scrap paper, transfer to a test-tube.

• Heat in a roaring Bunsen flame for 5 minutes using a test-tube holder.

• Let cool, tip mixture onto scrap paper, check for copper.

• Repeat with lead(II) oxide and iron(III) oxide.

Results:

• Removing oxygen from a substance is called reduction.

• Carbon acts as a reducing agent.

• Metals obtained this way: zinc, iron, lead, copper.

• Metals whose oxides are easily reduced by carbon are less reactive than metals whose oxides do not react.

Equations and Redox:

• Fe₂O₃ + 3C → 2Fe + 3CO

  • C is oxidised, Fe³⁺ is reduced

• PbO + C → Pb + CO

  • C is oxidised, Pb²⁺ is reduced

• CuO + C → Cu + CO

  • C is oxidised, Cu²⁺ is reduced

Extraction of Aluminium

• Aluminium makes up 7.5% of the Earth’s crust.

• Ore: bauxite

• Mineral in bauxite: aluminium oxide, Al₂O₃. Oxygen must be removed to release aluminium.

• Aluminium is more reactive than carbon, so it cannot be extracted by carbon reduction.

• Aluminium is extracted by electrolysis.

  • Aluminium oxide is a poor conductor with a high melting point (2015°C).

  • Cryolite (Na₃AlF₆) is added to dissolve Al₂O₃ and lower melting point to ~1000°C.

• Electrolyte: molten mixture of aluminium oxide and cryolite at 1000°C.

• Electrodes: carbon/graphite

  • Cathode (-): aluminium forms

  • Anode (+): oxygen forms

  • Oxygen reacts with carbon anodes → carbon dioxide, so anodes must be replaced regularly

• Major costs: electricity and anode replacement

  
  

Uses of Metals – Alloys

• Pure metals (e.g., iron) are soft because layers of atoms slide easily.

• Alloy: mixture of two or more elements, at least one metal.

  • Atoms of different sizes distort layers → harder and stronger than pure metal.

• Examples:

  • Brass: 70% copper, 30% zinc → harder, used in electrical fittings

  • 18 carat gold: 75% gold, 25% copper/other metals → harder, cheaper, used in jewellery

  • Duralumin: 96% aluminium, 4% copper/other metals → low density, stronger, used in aircraft manufacture

Uses of Metals and Alloys