Wrought iron is the purest form of iron. Write a reation used for the preparation of wrought iron from cast iron. How can the impurities of sulphur, silicon and phosphorus be removed from cast iron?

Options

A. carbon which reduce the impurities
B. water which dissolves the impurities
C. limestone which changes impurities into oxides and pass into slag
D. iron oxide which reacts with impurities by forming slag.

Answer

To prepare wrought iron from cast iron and to remove impurities such as sulfur, silicon, and phosphorus, we can follow these steps:

### Step 1: Understanding the Reaction
Wrought iron is produced from cast iron by oxidizing the impurities present in cast iron. The primary reaction involves the reduction of iron oxide (Fe2O3) using carbon.

### Step 2: Writing the Reaction
The balanced chemical reaction for the preparation of wrought iron from cast iron can be represented as follows:

\[ \text{Fe}_2\text{O}_3 + 3\text{C} \rightarrow 2\text{Fe} + 3\text{CO} \]

In this reaction:
- Iron(III) oxide (Fe2O3) is reduced to pure iron (Fe).
- Carbon (C) is oxidized to carbon monoxide (CO).

### Step 3: Removing Impurities
To remove impurities such as sulfur, silicon, and phosphorus from cast iron, we can use limestone (CaCO3) in the process. The steps are as follows:

1. **Addition of Limestone**: Limestone is added to the furnace during the process.

2. **Decomposition of Limestone**: Upon heating, limestone decomposes to form calcium oxide (CaO):
\[ \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \]

3. **Removal of Sulfur**: Calcium oxide reacts with sulfur dioxide (SO2) produced from the impurities to form calcium sulfate (CaSO4), which is removed as slag:
\[ \text{CaO} + \text{SO}_2 \rightarrow \text{CaSO}_4 \]

4. **Removal of Silicon**: Silicon is converted to silicon dioxide (SiO2), which then reacts with calcium oxide to form calcium silicate (CaSiO3), another form of slag:
\[ \text{Si} + \text{O}_2 \rightarrow \text{SiO}_2 \]
\[ \text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3 \]

5. **Removal of Phosphorus**: Phosphorus is also removed by oxidizing it to phosphorus pentoxide (P2O5), which can react with calcium oxide to form calcium phosphate (Ca3(PO4)2), another slag:
\[ \text{P} + \text{O}_2 \rightarrow \text{P}_2\text{O}_5 \]
\[ \text{CaO} + \text{P}_2\text{O}_5 \rightarrow \text{Ca}_3(\text{PO}_4)_2 \]

### Conclusion
In summary, wrought iron is prepared from cast iron by oxidizing impurities using a reaction with iron oxide and carbon. Impurities like sulfur, silicon, and phosphorus are removed by adding limestone, which converts these impurities into slag.