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Difficulty: MediumIron: Blast Furnace Extraction, Rusting Prevention, and Chemical Compounds

During the industrial extraction of iron in a blast furnace, limestone (CaCO3\text{CaCO}_3) is added to remove silica impurities (SiO2\text{SiO}_2). Arrange the following stages of slag formation and separation in their correct chronological sequence from first to last.

  1. 1Thermal decomposition of limestone (CaCO3\text{CaCO}_3) to form basic calcium oxide (CaO\text{CaO}) flux and gaseous carbon(IV) oxide (CO2\text{CO}_2).
  2. 2Acid-base reaction between basic calcium oxide (CaO\text{CaO}) flux and acidic silicon(IV) oxide (SiO2\text{SiO}_2) impurity.
  3. 3Formation of molten calcium trioxosilicate(IV) (CaSiO3\text{CaSiO}_3).
  4. 4Collection of molten slag as a protective layer floating above the denser liquid pig iron at the hearth.

Answer

The correct chronological sequence is: Thermal decomposition of limestone (CaCO3\text{CaCO}_3) into basic calcium oxide (CaO\text{CaO}) and carbon(IV) oxide (CO2\text{CO}_2) →\rightarrow Acid-base reaction between basic calcium oxide (CaO\text{CaO}) flux and acidic silicon(IV) oxide (SiO2\text{SiO}_2) impurity →\rightarrow Formation of molten calcium trioxosilicate(IV) (CaSiO3\text{CaSiO}_3) →\rightarrow Collection of molten slag as a protective layer floating above the denser liquid pig iron at the hearth.
Limestone (CaCO3\text{CaCO}_3) decomposes thermally under high temperatures into calcium oxide (CaO\text{CaO}) and carbon(IV) oxide (CO2\text{CO}_2). Next, the basic CaO\text{CaO} flux reacts with acidic silica (SiO2\text{SiO}_2) impurities in an acid-base neutralization to form molten calcium trioxosilicate(IV) (CaSiO3\text{CaSiO}_3, slag). Finally, the molten slag collects at the hearth and floats on top of the denser molten pig iron, forming a protective layer.

Step-by-Step Solution

1
Identify the initial thermal reaction of limestone in the furnace.
Limestone (CaCO3\text{CaCO}_3) decomposes at around 800∘C−1000∘C800^\circ\text{C}-1000^\circ\text{C} to yield calcium oxide (CaO\text{CaO}) and carbon(IV) oxide (CO2\text{CO}_2).
Calcium oxide (CaO\text{CaO}) must first be synthesized to act as a basic flux.
2
Determine the chemical interaction between the flux and raw ore impurities.
Basic calcium oxide (CaO\text{CaO}) reacts with acidic silicon(IV) oxide (SiO2\text{SiO}_2).
Silica is the main acidic impurity in hematite ore, requiring neutralization by the basic flux.
3
Identify the chemical product formed from this reaction.
Molten calcium trioxosilicate(IV) (CaSiO3\text{CaSiO}_3, slag) is formed via the reaction CaO(s)+SiO2(s)→CaSiO3(l)\text{CaO(s)} + \text{SiO}_2\text{(s)} \rightarrow \text{CaSiO}_3\text{(l)}.
Neutralization of silica forms the molten compound known as slag.
4
Describe the physical separation of slag at the hearth.
The molten slag drains down to the hearth and floats above the denser liquid iron layer.
Density differences allow slag to float on molten iron, preventing its re-oxidation by blast air.

Key Concept

Slag Formation and Impurity Removal in the Blast Furnace
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Topics

Iron: Blast Furnace Extraction, Rusting Prevention, and Chemical Compounds

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