During smelting an additional substance is added which combines with impurities to form of fusible product which is known as SLAG.
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Introduction:
During the process of smelting, an additional substance is often added to the raw materials. This substance serves the purpose of combining with impurities present in the raw materials to form a fusible product. This product is then separated from the desired metal or mineral, resulting in a purer end product. The addition of this substance helps in enhancing the efficiency of the smelting process and obtaining a higher quality final product.

Key Points:
1. Purpose of adding an additional substance:
- Combining with impurities: The additional substance is specifically chosen to react with impurities present in the raw materials. It forms a chemical reaction with these impurities, facilitating their removal during the smelting process.
- Formation of fusible product: The combination of the additional substance with impurities results in the formation of a fusible product. This product has a lower melting point compared to the desired metal or mineral, allowing for easier separation.

2. Role of the additional substance:
- Fluxing agent: The additional substance is commonly referred to as a fluxing agent. It acts as a medium for facilitating the removal of impurities by promoting chemical reactions.
- Reduction of melting point: The fluxing agent lowers the melting point of impurities and forms a slag, which is a molten mixture of the fluxing agent and impurities. This enables the separation of the impurities from the desired metal or mineral.

3. Selection of appropriate fluxing agent:
- Chemical compatibility: The fluxing agent should be chemically compatible with the impurities present in the raw materials. This ensures effective reaction and formation of the fusible product.
- Melting point considerations: The fluxing agent should have a lower melting point than the desired metal or mineral. This allows for the separation of the fusible product from the desired material.

4. Examples of fluxing agents:
- Limestone (calcium carbonate): Commonly used in smelting iron ore. It reacts with impurities such as silica, forming a slag that can be easily separated.
- Soda ash (sodium carbonate): Used in the extraction of copper. It reacts with impurities like iron oxide, creating a slag that can be removed.
- Borax (sodium borate): Utilized in gold smelting. It helps in removing impurities like metal oxides, forming a slag that can be separated.

Conclusion:
During the smelting process, the addition of an appropriate fluxing agent plays a crucial role in removing impurities from the raw materials. This results in the formation of a fusible product, which can be easily separated from the desired metal or mineral. The selection of the fluxing agent depends on its chemical compatibility with impurities and its lower melting point compared to the desired material. Examples of fluxing agents include limestone, soda ash, and borax.