Unit 4: Metallurgical Extraction

Ores and Minerals

Definition and Types

  • Minerals: Naturally occurring chemical substances (elements or compounds) in the Earth's crust, obtained by mining.
  • Ores: Minerals from which one or more metals can be extracted economically and conveniently.
Key Concept: All ores are minerals, but not all minerals are ores.
(e.g., Clay (Al2O3 · 2SiO2 · 2H2O) is a mineral of aluminum, but it is not an ore because extracting Al from it is too difficult and expensive. Bauxite (Al2O3 · 2H2O) is the ore of aluminum).

Types of Ores

  • Oxide Ores: Hematite (Fe2O3), Bauxite (Al2O3 · 2H2O), Zincite (ZnO).
  • Sulfide Ores: Argentite (Ag2S), Copper Pyrites (CuFeS2), Zinc Blende (ZnS), Galena (PbS).
  • Carbonate Ores: Siderite (FeCO3), Calamine (ZnCO3).
  • Halide Ores: Rock Salt (NaCl), Cryolite (Na3AlF6).
The general steps of metallurgy are:
  1. Concentration of Ore: Removing the "gangue" (earthy impurities). Methods include froth flotation, magnetic separation, leaching.
  2. Isolation (Reduction): Converting the ore to the metal (e.g., smelting, electrolysis).
  3. Purification (Refining): Purifying the impure metal (e.g., electrorefining, zone refining).

Extraction and purification process of Iron from hematite

  • Ore: Hematite (Fe2O3)
  • 1. Concentration: Done by gravity separation (washing in a stream of water). The dense ore particles settle, while lighter gangue is washed away.
  • 2. Calcination: Ore is heated in air to remove moisture, CO2, and oxidize impurities like S and As.
  • 3. Extraction (Smelting): Done in a Blast Furnace. The charge (ore + coke + limestone) is added from the top.
    • Coke (C): Acts as the fuel and the main reducing agent.
      C + O2 → CO2 (heat)
      CO2 + C → 2CO (reducing agent)
    • Limestone (CaCO3): Acts as a flux to remove the acidic gangue (SiO2).
      CaCO3 → CaO + CO2
      CaO (flux) + SiO2 (gangue) → CaSiO3 (slag)
    • Reduction: Iron oxide is reduced to molten iron by CO.
      Fe2O3 + 3CO → 2Fe(l) + 3CO2
  • Product: Molten "Pig Iron" (93-95% Fe, with C, Mn, Si) and molten slag.
  • Purification: Pig iron is purified into steel by oxidizing the impurities in a converter (e.g., Bessemer converter or L-D process) by blowing hot air or O2 through it.

Extraction and purification process of Ag from argentite

  • Ore: Argentite (Ag2S)
  • 1. Concentration (Leaching): This is a hydrometallurgical process (MacArthur-Forrest Cyanide Process).
    • The ore is crushed and treated with a dilute Sodium Cyanide (NaCN) solution, with air bubbled through.
    • The Ag2S is converted to a soluble complex, Na[Ag(CN)2].
    • Ag2S + 4NaCN leftharpoons 2Na[Ag(CN)2] + Na2S
    • (The reaction is reversible, so air (O2) is blown to oxidize the Na2S to Na2SO4, driving the reaction forward).
  • 2. Extraction (Precipitation): The Ag is recovered from the complex by adding a more electropositive metal, like Zinc (Zn).
    Zn + 2Na[Ag(CN)2] → Na2[Zn(CN)4] + 2Ag(s)
  • 3. Purification: The impure silver (called "Spelter") is purified by electrolytic refining or cupellation.

Extraction and purification process of Copper from copper pyrites

  • Ore: Copper Pyrites (CuFeS2)
  • 1. Concentration: Done by Froth Flotation. The sulfide ore is preferentially "wetted" by pine oil and carried to the top in a froth, while the gangue is wetted by water and sinks.
  • 2. Roasting: The ore is heated in air. CuFeS2 is converted to Cu2S and FeS.
    2CuFeS2 + O2 → Cu2S + 2FeS + SO2
  • 3. Smelting: The roasted ore is mixed with sand (SiO2, a flux) and heated in a furnace. FeS is oxidized to FeO, which then reacts with the flux to form slag.
    FeS + O2 → FeO + SO2
    FeO (impurity) + SiO2 (flux) → FeSiO3 (slag)
    The Cu2S melts and collects at the bottom as "copper matte".
  • 4. Bessemerization: The molten matte is put in a Bessemer converter. Air is blown through it.
    Cu2S + O2 → 2Cu + SO2 (This is self-reduction).
  • Product: "Blister Copper" (98-99% pure), so-named because SO2 gas bubbles out as it cools.
  • 5. Purification: Done by electrolytic refining.
    • Anode: Impure Blister Copper
    • Cathode: Thin sheet of Pure Copper
    • Electrolyte: CuSO4 solution

Extraction and purification process of Zinc from zinc blend

  • Ore: Zinc Blende (ZnS)
  • 1. Concentration: Done by Froth Flotation (same as copper).
  • 2. Roasting: ZnS is converted to ZnO by heating strongly in air.
    2ZnS + 3O2 → 2ZnO + 2SO2
  • 3. Extraction (Reduction): ZnO is reduced using crushed coke (Carbon) at high temperature (~1400°C).
    ZnO + C → Zn + CO
  • Product: Zinc metal is volatile at this temperature, so it distills off as a vapor, which is then condensed. This is "Spelter Zinc".
  • 4. Purification: Done by electrolytic refining or fractional distillation (since Zn is volatile).

Extraction and purification process of Aluminum from bauxite

  • Ore: Bauxite (Al2O3 · 2H2O), with impurities of Fe2O3 (red bauxite) and SiO2 (white bauxite).
  • 1. Concentration (Leaching): The Hall-Héroult Process or Baeyer's Process is used to separate Al2O3 from the impurities.
    • The ore is digested with hot, concentrated NaOH solution under pressure.
    • Al2O3 is amphoteric and dissolves:
      Al2O3 + 2NaOH → 2NaAlO2 (Sodium aluminate) + H2O
    • Fe2O3 (basic) and SiO2 (as sodium silicate) do not dissolve and are filtered off as "red mud".
    • The NaAlO2 solution is cooled, diluted, and "seeded" with Al(OH)3 crystals, causing Al(OH)3 to precipitate.
      NaAlO2 + 2H2O → Al(OH)3(s) + NaOH
    • The Al(OH)3 is filtered, washed, and heated strongly (calcination) to get pure Al2O3 (alumina).
      2Al(OH)3 → Al2O3 + 3H2O
  • 2. Extraction (Electrolysis): The Hall-Héroult Process.
    • Pure Al2O3 is dissolved in molten Cryolite (Na3AlF6) and Fluorspar (CaF2). This (a) lowers the melting point from 2000°C to 1000°C and (b) increases conductivity.
    • The electrolysis is done in a steel tank (cathode) with carbon rods (anode).
    • Cathode (Reduction): Al3+ + 3e- → Al(l) (Molten Al collects at the bottom).
    • Anode (Oxidation): 2O2- + C(s) → CO2(g) + 4e- (The carbon anodes are consumed and must be replaced).
  • 3. Purification: The 99.8% pure Al from electrolysis can be further purified to 99.99% by Hoope's electrolytic process.