IGCSE Chemistry 0620: The Blast Furnace and Extraction of Iron

Iron is extracted from iron(III) oxide ore in a blast furnace. The furnace is charged from the top with iron ore, coke and limestone, and a blast of hot air is blown in near the bottom.

(a) Near the bottom of the furnace, coke reacts with the hot air blast, releasing heat. Write a word equation for this reaction. [1]

(b) Further up the furnace, the hot gas formed in (a) reacts with more unburnt coke to form a different gas. Write a word equation for this reaction. [1]

(c) This second gas then reduces the iron(III) oxide to iron. Write a word equation for this reaction, and state the name given to a reaction in which oxygen is removed from a compound. [2]

(d) Explain the purpose of the limestone added to the furnace, including what happens to it on heating and what solid by-product it forms. [2]

Part (a): Coke burns in the hot air blast

Near the bottom of the furnace, the coke (carbon) burns in the blast of hot air, releasing the heat needed to keep the furnace at a high working temperature:

$\text{carbon} + \text{oxygen} \rightarrow \text{carbon dioxide}$

Part (b): Carbon dioxide reacts with more coke

As this hot carbon dioxide rises through the furnace, it meets more unburnt coke and reacts with it to form carbon monoxide:

$\text{carbon dioxide} + \text{carbon} \rightarrow \text{carbon monoxide}$

Part (c): Carbon monoxide reduces the ore

The carbon monoxide formed in (b) is the gas that actually reduces the iron(III) oxide, removing its oxygen and leaving behind molten iron:

$\text{iron(III) oxide} + \text{carbon monoxide} \rightarrow \text{iron} + \text{carbon dioxide}$

Because oxygen is removed from the iron(III) oxide in this reaction, it is called a reduction reaction.

Part (d): The role of limestone

Limestone is calcium carbonate. On heating in the furnace, it thermally decomposes:

$\text{calcium carbonate} \rightarrow \text{calcium oxide} + \text{carbon dioxide}$

The calcium oxide formed then reacts with the silicon dioxide (sand) impurity present in the iron ore, forming calcium silicate:

$\text{calcium oxide} + \text{silicon dioxide} \rightarrow \text{calcium silicate (slag)}$

This slag is molten but less dense than the molten iron, so it floats on top and can be tapped off separately, leaving behind purer iron.

Final answers

(a) carbon + oxygen → carbon dioxide
(b) carbon dioxide + carbon → carbon monoxide
(c) iron(III) oxide + carbon monoxide → iron + carbon dioxide; reduction
(d) Limestone decomposes to calcium oxide (+ carbon dioxide); calcium oxide reacts with silicon dioxide impurity to form calcium silicate (slag), which is removed separately from the molten iron.