Metallurgy and Industrial Extraction of Iron (Fe)

Natural Occurrence & Major Ores

Iron is the second most abundant metal in the Earth’s crust and the fourth most abundant element by mass. It rarely occurs in its native metallic state on Earth due to its reactivity with oxygen and water.

Major Ores of Iron:

• Hematite (Red Iron Ore): Fe₂O₃ (the most important ore for extraction)
• Magnetite (Black Iron Ore): Fe₃O₄ (magnetic, very rich in iron)
• Limonite (Brown Iron Ore): Fe₂O₃·nH₂O (hydrated iron oxide)
• Siderite (Iron Carbonate): FeCO₃
• Iron Pyrites (Fool’s Gold): FeS₂ (sulfide ore, generally not used for iron extraction due to sulfur impurities)

Concentration of the Ore

The chosen method depends on the nature of the ore and impurities (gangue). Iron ores are primarily concentrated using physical methods.

1. Crushing and Grinding

• The mined ore is first crushed into smaller pieces using jaw crushers and then finely ground in ball mills.

2. Gravity Separation (Hydraulic Washing)

• Principle: Based on the difference in specific gravities of the ore particles and the gangue particles.
• Process: The finely ground ore is washed with a current of water. Lighter gangue particles are washed away, leaving the heavier ore particles behind.
• Application: Suitable for heavy oxide ores like Hematite.

3. Magnetic Separation

• Principle: Applicable when either the ore or the gangue is magnetic.
• Process: The finely ground ore is passed over a magnetic roller. Magnetic particles are attracted to the roller and fall in one heap, while non-magnetic particles fall separately.
• Application: Highly effective for Magnetite (Fe₃O₄), which is magnetic, and can also be used for Hematite (Fe₂O₃) after roasting converts it partially to magnetite.

Reduction to Crude Metal (Extraction in Blast Furnace)

The industrial extraction of iron primarily involves the reduction of iron oxides in a Blast Furnace. The raw materials charged into the blast furnace are:

1. Concentrated Iron Ore (Hematite, Magnetite)
2. Coke (C) - acts as a reducing agent and fuel.
3. Limestone (CaCO₃) - acts as a flux to remove acidic impurities (silica, SiO₂).

Blast Furnace Process:

A blast furnace is a tall, cylindrical steel vessel lined with refractory bricks, designed to operate continuously. Hot air is blown into the bottom of the furnace.

1. Preheating and Drying Zone (200-700°C, Top of Furnace)

• Moisture is removed, and the ore starts to get preheated by rising hot gases.
• Some reduction by CO begins: 3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂

2. Reduction Zone (700-1200°C, Middle of Furnace)

• Indirect Reduction: Carbon monoxide (CO), formed from the combustion of coke, acts as the primary reducing agent. Fe₃O₄ + 4CO → 3Fe + 4CO₂ Fe₂O₃ + 3CO → 2Fe + 3CO₂
• Decomposition of Limestone: CaCO₃ → CaO + CO₂

3. Slag Formation Zone (1200-1500°C, Lower Middle)

• Flux Action: Calcium oxide (CaO), formed from the decomposition of limestone, reacts with acidic impurities (gangue like silica) present in the ore. CaO (flux) + SiO₂ (gangue) → CaSiO₃ (slag)
• This molten slag is lighter than molten iron and floats on its surface, protecting the iron from re-oxidation. It is tapped off separately.

4. Combustion and Fusion Zone (1500-1900°C, Bottom of Furnace)

• Combustion of Coke: Hot air (blast) reacts with coke to produce carbon monoxide and generate immense heat. 2C (coke) + O₂ (hot air) → 2CO (exothermic)
• Direct Reduction (at higher temperatures): Some iron oxides are reduced directly by carbon. Fe₂O₃ + 3C → 2Fe + 3CO
• Molten iron collects at the bottom of the furnace. It contains about 4% carbon and small amounts of S, P, Si, and Mn, which dissolve in it. This crude form of iron is called Pig Iron.

Output:

• Pig Iron: Molten iron tapped from the bottom of the furnace. It is brittle due to high carbon content.
• Slag: Molten calcium silicate, tapped from above the molten iron. Used in road construction and cement manufacturing.
• Blast Furnace Gas: Exhaust gases (CO, CO₂, N₂) escaping from the top, which are often used to preheat the incoming air blast.

Refining and Purification

Pig iron, directly produced from the blast furnace, is brittle and contains impurities like carbon (3-5%), silicon, manganese, phosphorus, and sulfur. It is further processed to produce various forms of iron with desirable properties.

1. Cast Iron

• Production: Pig iron is remelted with scrap iron and coke, then cast into molds.
• Properties: Contains about 2.5-4% carbon. It is hard and brittle, non-malleable, and cannot be welded.
• Uses: Manufacture of pipes, stove parts, engine blocks.

2. Wrought Iron

• Production: The purest form of commercial iron. It is prepared from cast iron by oxidizing most of the impurities in a reverberatory furnace lined with hematite (Fe₂O₃). The hematite oxidizes carbon to CO, and other impurities (Si, Mn, P) react with the lining and flux to form slag. Fe₂O₃ (lining) + 3C → 2Fe + 3CO Si + O₂ → SiO₂ Mn + O₂ → MnO 2P + 5O → P₂O₅
• Properties: Contains less than 0.25% carbon. It is soft, malleable, ductile, tough, and can be welded.
• Uses: Anchor chains, rivets, agricultural implements.

3. Steel

• Steel is an alloy of iron with 0.02% to 2% carbon, along with other alloying elements. It is the most widely used form of iron due to its strength and versatility.

• Modern Steel Manufacturing Processes:

  a) Basic Oxygen Furnace (BOF) Process

  • Principle: Pig iron, along with scrap steel, is charged into a large, pear-shaped furnace lined with basic refractories (e.g., magnesia or dolomite). Pure oxygen is blown at high velocity through a water-cooled lance onto the surface of the molten metal.
  • Reactions: Oxygen oxidizes impurities rapidly:
    • Carbon to CO or CO₂ (C + O₂ → CO₂)
    • Silicon to SiO₂ (Si + O₂ → SiO₂)
    • Manganese to MnO (Mn + O₂ → MnO)
    • Phosphorus to P₂O₅ (2P + 5O₂ → P₂O₅)
    • Sulfur to SO₂ (S + O₂ → SO₂)
  • Fluxes (limestone or dolomite) are added to remove acidic oxides (SiO₂, P₂O₅) as slag.
  • Advantages: Fast (20-30 minutes), economical.
  • Output: Molten steel, which is then cast into various forms or used for further alloying.

  b) Electric Arc Furnace (EAF) Process

  • Principle: Scrap metal (steel, cast iron) is melted by heat generated from an electric arc passing between large graphite electrodes and the metal charge. Oxygen is blown in to remove impurities.
  • Advantages: Can use 100% scrap, precise control over temperature and alloying elements, produces high-quality specialty steels.
  • Output: High-quality steel.

After steelmaking, the molten steel is typically cast into semi-finished products (slabs, blooms, billets) and then further processed by rolling, forging, or other methods into final products.