Bloomery Iron Smelting

What is modelled

In a bloomery, charcoal burns in the air from the bellows and makes carbon monoxide. That gas, rising through the ore, takes the oxygen away and leaves iron, still below its melting point, as a spongy bloom. Two equilibria set the rules: hot charcoal turns carbon dioxide back into monoxide (Boudouard), and the gas must stay rich enough in monoxide to reduce the iron oxide. Silica in the ore ties up iron in a slag, so poor ore gives a small bloom.

Equations

• Boudouard C + CO2 ⇒ 2CO, ΔG = 170700 − 174.5 T J/mol, with x2 / (1 − x) = K for the CO fraction x at one atmosphere.
• Wustite reduction FeO + CO ⇒ Fe + CO2, ΔG = −17150 + 21.0 T J/mol, so reduction needs x > 1 / (1 + K2).
• Charcoal burned nC = 2 nO2 with the oxygen in the blast, heat 110.5 kJ per mole of carbon.
• Fayalite slag binds two iron atoms per silica, and the hearth settles where heat released equals the endothermic reduction plus the wall and gas losses.

Checks run in the page

• The Boudouard fraction by bisection against the closed form, and its free-energy crossover.
• The reduction threshold found by bisection against a fine scan.
• The heat and oxygen balance at the settled temperature.
• Iron bookkeeping between ore, bloom, slag and unreduced oxide.

Honest simplifications

The furnace is treated as one well-mixed zone at one temperature, the gas utilisation is a fixed fraction of the equilibrium limit, heat losses are a single wall coefficient plus a share of the gas enthalpy, and carburisation, wall wear, charcoal quality and the time-dependent bloom growth are ignored. Real smelting depended on skill and varied from heat to heat, so this explains the principles rather than predicting a smelt.

• Press Weak blast, cold furnace and see the gas become too weak to reduce the ore.
• Press Strong blast, too hot to push the hearth past the welding window.
• Press Poor siliceous ore to watch more iron lock into slag.