Steps of extraction

Step 1 — Ore dressing (preparation). It aims to:

1. Improve the physical/mechanical properties:
   • Crushing — gives lumps of suitable size for easy reduction.
   • Sintering — the crushing process produces large quantities of fine particles that cannot be fed directly to the furnace (they would be blown out and block the flow of gases), so they are fused together into larger, uniform lumps.
   • Concentration — raises the percentage of iron by removing the impurities mixed with or combined in the ore, using surface tension (flotation), magnetic or electric separation. (Magnetic separation suits magnetite because it is attracted by a magnet while the gangue is not.)
2. Improve the chemical properties by roasting (strong heating in air) to: a. dry the ore and expel moisture / water of hydration / CO₂, which raises the iron percentage:
   • FeCO₃ → FeO + CO₂, then 4FeO + O₂ → 2Fe₂O₃ (overall 4FeCO₃ + O₂ → 2Fe₂O₃ + 4CO₂)
   • 2Fe₂O₃·3H₂O → 2Fe₂O₃ + 3H₂O b. oxidize impurities such as sulphur and phosphorus to gases/oxides, which can be removed: S + O₂ → SO₂; 4P + 5O₂ → 2P₂O₅.

Step 2 — Reduction of the oxide to iron. Done by one of two methods, depending on the reducing agent.

(a) Blast furnace (reducing agent: carbon monoxide from coke).

• The furnace is a tall steel tower lined with refractory (fire) bricks, charged from the top with a mixture of roasted ore, coke and limestone, while a blast of hot air is forced in near the bottom.
• Coke burns: C + O₂ → CO₂ (exothermic, supplies the heat); the CO₂ reacts with hot coke higher up: CO₂ + C → 2CO.
• Reduction (about 700 °C and above): Fe₂O₃ + 3CO → 2Fe + 3CO₂ (taking place stepwise: Fe₂O₃ → Fe₃O₄ → FeO → Fe, at progressively lower levels of the furnace as the temperature rises).
• Limestone is added as a flux: CaCO₃ → CaO + CO₂; CaO + SiO₂ → CaSiO₃ (slag). The silica (SiO₂) of the gangue would otherwise form a refractory solid; as calcium silicate it melts, is less dense than molten iron, floats on it and is run off separately. (Slag is used in cement making and road building.)
• The molten iron collects at the bottom of the furnace and is run into moulds. The product is pig (cast) iron — hard and brittle, with about 4 % carbon plus S, P and Si.

Worked example 5 (blast furnace). A hematite ore contains 80 % Fe₂O₃. For 1000 kg of ore (Fe = 56, O = 16, C = 12, Ca = 40, Si = 28):

• Fe₂O₃ present = 800 kg. Fe₂O₃ + 3CO → 2Fe + 3CO₂: 160 kg gives 112 kg Fe, so the iron obtained = 800 × 112/160 = 560 kg (theoretical).
• CO needed = 800 × (3 × 28)/160 = 420 kg = 420/28 = 15 kmol, which occupies 15 × 22.4 = 336 m³ at STP.
• If the gangue contains 60 kg of SiO₂: CaO + SiO₂ → CaSiO₃ needs 60 × 56/60 = 56 kg CaO, which comes from 56 × 100/56 = 100 kg of CaCO₃; the slag formed is 60 + 56 = 116 kg CaSiO₃.

(b) Midrex process (reducing agent: a mixture of CO and H₂ made from natural gas, about 93 % methane).

• Reforming: 2CH₄ + CO₂ + H₂O → 3CO + 5H₂
• Reduction: Fe₂O₃ + 3CO → 2Fe + 3CO₂ and Fe₂O₃ + 3H₂ → 2Fe + 3H₂O
• The product is a solid porous "sponge iron" (direct-reduced iron), cleaner than blast-furnace iron because the iron never melts and no coke is used.

Worked example 6 (Midrex). To reduce 80 kg of Fe₂O₃ (0.5 kmol) with hydrogen: Fe₂O₃ + 3H₂ → 2Fe + 3H₂O needs 1.5 kmol H₂ = 1.5 × 22.4 = 33.6 m³ at STP, and gives 1 kmol = 56 kg Fe.

Step 3 — Production of cast iron and steel.

• Steel making depends on (1) removing impurities from the furnace iron and (2) adding elements that give the steel the required properties.
• It is carried out in: the oxygen converter, the open-hearth furnace or the electric furnace.
• In the oxygen converter a jet of pure oxygen is blown into the molten iron; carbon is oxidized to CO and silicon, phosphorus and sulphur to oxides that join the slag, and the required alloying elements are then added.