Week 4 · Chapter 1 (D): iron in nature, the iron ores, ore dressing and roasting, reduction in the blast furnace and the gas-reduction furnace, cast iron and steel, the three types of alloys (conceptual: industrial processes are described, never instructed) · 3 lessons
2
From ore to iron and steel
Chemistry Sec 3 Term 1, Chapter 1: pp. 16–20
GoalWrite the reduction equations for the blast furnace and the gas-reduction furnace, calculate masses on paper, and tell cast iron from steel.
New words
reduction · removal of oxygen from an oxide to leave the metal; the reducing agent (CO or H₂) is oxidisedcoke · an almost pure form of carbon made from coal, the fuel and source of CO in the blast furnacecast iron · iron straight from the blast furnace, high in carbon, hard but brittle
Explanation
After dressing, the oxide is reduced. In the blast furnace the fuel is coke. Carbon burns to carbon dioxide, the carbon dioxide meets more hot carbon and becomes carbon monoxide, and carbon monoxide is the real reducing agent: C + O₂ → CO₂; CO₂ + C → 2CO; Fe₂O₃ + 3CO → 2Fe + 3CO₂. In the gas-reduction furnace the reducing gases are made from natural gas (mostly methane): 2CH₄ + CO₂ + H₂O → 3CO + 5H₂, and then 2Fe₂O₃ + 3CO + 3H₂ → 4Fe + 3CO₂ + 3H₂O. Both routes are equations to study on paper; real furnaces run at very high temperatures under trained operators. The first product, cast iron, holds a lot of carbon and some impurities, so it is hard but brittle. Steel is iron with a small controlled amount of carbon, and often other metals, after the impurities have been removed; it is strong and can be shaped. The oxygen converter, open-hearth and electric furnaces that make steel are only named here.
Route
Reducing gas comes from
Reduction equation
blast furnace
coke, as CO
Fe₂O₃ + 3CO → 2Fe + 3CO₂
gas-reduction furnace
natural gas, as CO and H₂
2Fe₂O₃ + 3CO + 3H₂ → 4Fe + 3CO₂ + 3H₂O
Worked examples
ƒEquation
Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g)
ratio 1 : 3 : 2 : 3
Masses in the blast furnace (Fe = 56, C = 12, O = 16). Take 80 g of Fe₂O₃ in Fe₂O₃ + 3CO → 2Fe + 3CO₂. M(Fe₂O₃) = 160 g/mol, so n = 0.5 mol. The ratio 1 : 3 : 2 : 3 gives 1.5 mol CO = 42 g, 1.0 mol Fe = 56 g and 1.5 mol CO₂ = 66 g. Check: 80 + 42 = 122 and 56 + 66 = 122.
Step
Equation
Masses
1
C + O₂ → CO₂
12 g + 32 g → 44 g
2
CO₂ + C → 2CO
44 g + 12 g → 56 g
Where the carbon monoxide comes from. 12 g of carbon (1 mol) burns in 32 g of oxygen to make 44 g of CO₂. If that 44 g of CO₂ meets another 12 g of carbon, 2 mol of CO form: 2 × 28 = 56 g, and 44 + 12 = 56 g.
Cast iron
Steel
carbon (approximate)
about 3 to 4.5 %
less than about 2 %
impurities
some left
removed
behaviour
hard, brittle
strong, can be shaped
Cast iron and steel compared. The difference is mostly the carbon content and the impurities.
Class activity
A card order game, on paper. Write these on separate cards: crushing, sintering, concentrating, roasting, reduction, cast iron, steel. Shuffle them and put them in the order of the process. Then write the two coke equations and the reduction equation on three more cards and match each to the sentence that describes it. Nothing is heated and no furnace is built: it is a study aid.
Practice
1
Fe₂O₃ + 3CO → 2Fe + 3CO₂. For 32 g of Fe₂O₃ (Fe = 56, C = 12, O = 16) find the mass of CO needed and of Fe and CO₂ formed.
