Video Transcript
What is the purpose of adding limestone to a blast furnace? (A) The limestone decomposes and the products of that reaction convert impurities into slag. (B) The limestone decomposes, an exothermic reaction, heating up the furnace. (C) The limestone reduces the iron ore to iron. (D) The limestone contributes to the overall yield as it contains iron metal impurities. Or (E) the limestone provides a source of oxygen needed to react with coke.
A blast furnace can be used to extract iron from its ore. Components other than just the ore are added to the furnace to extract the iron. One of these key components is limestone, which is primarily composed of calcium carbonate, CaCO3. The heat from the blast furnace causes the calcium carbonate to decompose into calcium oxide and carbon dioxide. The calcium oxide can react with sand impurities in the iron ore, which is primarily composed of silicon dioxide. This reaction produces calcium silicate. This makes up the majority of a by-product called slag. So the limestone composed of calcium carbonate decomposes, and the product of the reaction converts impurities, such as sand, into slag.
So option (A), the limestone decomposes and the products of that reaction convert the impurities into slag, appears to be the correct answer to this question. But to confirm, let’s look at options (B) to (E). Option (B) is the limestone decomposes, an exothermic reaction, heating up the furnace. Decomposition tends to be an endothermic reaction, not an exothermic reaction. So option (B) is not the answer to this question. Option (C) is the limestone reduces the iron ore to iron. In a blast furnace, the iron ore is reduced to iron by carbon monoxide, not limestone. So option (C) cannot be the answer to this question either.
Option (D) is the limestone contributes to the overall yield as it contains iron metal impurities. Limestone is a form of calcium carbonate and does not contain iron. Thus, option (D) cannot be the answer to this question. Option (E) is the limestone provides a source of oxygen needed to react with coke. Limestone produces calcium oxide and carbon dioxide when it decomposes. So the oxygen atoms are bound to calcium and carbon atoms. They are not free to react with coke. Thus, option (E) cannot be the answer to this question.
So the answer to the question “What is the purpose of adding limestone to a blast furnace?” is (A). The limestone decomposes and the products of that reaction convert impurities into slag.
