Limestone helps purify iron by reacting with impurities, primarily silicon dioxide, to form a separate, easily removable substance called slag.
When iron ore is processed in a furnace to extract the iron, impurities are often present. The most common impurity is silicon dioxide (also known as sand). To remove these impurities, limestone (calcium carbonate) is added to the furnace mixture.
Here's a breakdown of the process:
- Adding the Flux: Limestone acts as a 'flux' - a substance added to reduce the melting point of minerals and to react with impurities so they can be removed.
- The Reaction: Inside the hot furnace, the calcium carbonate in the limestone decomposes and then reacts with the silicon dioxide impurities.
- Calcium carbonate (CaCO₃) + Silicon dioxide (SiO₂) → Calcium silicate (CaSiO₃) + Carbon dioxide (CO₂)
- This reaction forms molten calcium silicate, which is also known as slag.
- Separation: The reference states that molten calcium silicate is less dense than the molten iron. This is crucial.
- Removal: Because the molten slag is less dense, it floats on top of the molten iron. This layer of slag can then be easily skimmed off or tapped out of the furnace separately from the purified molten iron.
Essentially, limestone converts solid impurities into a liquid waste product (slag) that floats and can be physically separated from the liquid iron.
Summary of the Purification Process:
| Step | Action | Result |
|---|---|---|
| 1. Addition | Limestone added to furnace | Acts as a flux |
| 2. Reaction | Limestone reacts with SiO₂ | Forms molten calcium silicate (slag) |
| 3. Separation | Slag density vs. Iron density | Molten calcium silicate is less dense |
| 4. Removal | Slag floats on top of iron | Slag can easily be removed |
This process effectively removes a significant portion of the silicon impurities, leading to purer iron.
