16.1 Metal extraction

16.1 Metal Extraction and the Reactivity Series

Overview of Metal Extraction

  • Metals are found in nature as pure metals or as compounds in ores (metal oxides).

  • The extraction process for metals often requires reduction to remove oxygen from their compounds.

    • Example: Metal Ore > reduction > Metal

Reactivity Series

  • Metals differ in reactivity, affecting their extraction methods.

  • Reactive metals form strong compounds and positive ions easily.

  • Hydrogen is included in the reactivity series for comparative purposes:

    • Less reactive than zinc.

    • More reactive than copper.

    • Can reduce copper oxide to copper but not zinc oxide to zinc.

Carbon's Role in Metal Extraction

Use of Carbon in Extraction

  • Carbon serves as a reducing agent in extracting metals.

  • Metals below carbon can be extracted by heating their oxides with carbon.

  • Metals above carbon must be extracted via electrolysis.

Electrolysis for Reactive Metals

  • Metals more reactive than carbon require electrolysis for extraction.

    • Cathodic reduction occurs: metal ions gain electrons to form free metal.

    • Example: Electrolytic extraction of aluminum.

Extraction of Iron

Iron Ores and Properties

  • Iron is the second most common metal, primarily found in hematite (Fe2O3).

  • Hematite contains over 60% iron.

Blast Furnace Production

  • Blast Furnace Structure:

    • A 30-meter high steel tower lined with heat-resistant bricks (magnesium oxide).

  • Raw Materials Used:

    1. Iron ore (hematite)

    2. Coke (almost pure carbon)

    3. Limestone (CaCO3)

Steps in Iron Production

  1. Preparation: Ore is powdered and mixed with coke and limestone.

  2. Combustion: Hot air is blasted into the furnace.

  3. Chemical Reactions:

    • Carbon burns with oxygen to produce CO2.

    • CO2 reacts with carbon to produce CO.

    • Iron ore is reduced by CO to produce iron.

    • The molten iron is collected at the bottom of the furnace.

Key Reactions in the Blast Furnace

  • Combustion of Coke:

    • Carbon + Oxygen → Carbon Dioxide

    • C + O2 → CO2

  • Reduction of Carbon Dioxide:

    • CO2 + C → 2CO

  • Reduction of Iron Ore:

    • Fe2O3 + CO → Fe + CO2

Role of Limestone in Iron Production

  • Limestone (calcium carbonate) decomposes to calcium oxide and carbon dioxide:

    • CaCO3 → CaO + CO2

  • Calcium oxide reacts with silica (SiO2) to form calcium silicate (slag):

    • CaO + SiO2 → CaSiO3

  • Slag forms a separate layer and is tapped off for use in construction.

Benefits of Blast Furnace Extraction

  • Efficient use of waste gases for energy.

  • Continuous extraction process is cost-effective compared to electrolytic methods.

  • Key features include the production methods and the removal of impurities.

Extraction of Aluminum

Aluminum Extraction Process

  • Aluminum is more reactive than carbon; therefore, it cannot be extracted through carbon reduction.

  • Electrolysis: Used for aluminum extraction from bauxite (aluminum oxide).

  • Bauxite is purified to produce aluminum oxide, which is dissolved in molten cryolite to facilitate the process.

Electrolysis Steps

  1. At the cathode (negative electrode):

    • Aluminum ions gain electrons to form molten aluminum:

    • Al3+ + 3e- → Al

  2. At the anode (positive electrode):

    • Oxide ions lose electrons to form oxygen.

    • 2O2- → O2 + 4e-

  3. The complete reaction for aluminum extraction:

    • 2Al2O3 → 4Al + 3O2

  • A significant amount of electricity is required, leading to high operational costs.

Role of Cryolite

  • Cryolite reduces the melting point of aluminum oxide, lowering energy costs during extraction.

Replacement of Carbon Anodes

  • Anodes made of graphite erode due to reaction with oxygen, necessitating regular replacement.