In the metallurgy of Aluminium, what is mixed with purified $Al_{2}O_{3}$ to lower the melting point of the mixture ?

Question

Accepted Answer

In the metallurgy of aluminium, the process involves the extraction of aluminium from its oxide, alumina ($Al_{2}O_{3}$). One of the key steps in this process is the reduction of the melting point of alumina to make the extraction process more energy-efficient. This is achieved by mixing alumina with a substance that lowers its melting point.

1. **Explanation for Option 2 ($Na_{3}AlF_{6}$):**

The correct answer is Option 2, $Na_{3}AlF_{6}$, which is commonly known as cryolite. Cryolite is used in the Hall-Héroult process, which is the primary method for the production of aluminium. The addition of cryolite to alumina serves to lower the melting point of the mixture from about 2050°C to around 950°C. This significant reduction in melting point is crucial because it allows the electrolysis process to occur at a much lower temperature, thereby saving energy and reducing costs. Cryolite also increases the conductivity of the molten mixture, which is beneficial for the electrolysis process.

2. **Explanation for why other options are incorrect:**

- **Option 1 ($FeSiO_{3}$):** Iron silicate ($FeSiO_{3}$) is not used in the aluminium extraction process. It does not have the properties required to lower the melting point of alumina. Instead, it is more commonly associated with slag formation in other metallurgical processes.

- **Option 3 ($Al(OH)_{3}$):** Aluminium hydroxide ($Al(OH)_{3}$) is not used to lower the melting point of alumina. In fact, it is a precursor to alumina itself. When heated, aluminium hydroxide decomposes to form alumina and water vapor, but it does not aid in reducing the melting point of alumina.

- **Option 4 (CaO):** Calcium oxide (CaO), also known as lime, is not used in the process of lowering the melting point of alumina. While it is used in various metallurgical processes as a flux to remove impurities, it does not serve the purpose of reducing the melting point of alumina in the production of aluminium.

In conclusion, $Na_{3}AlF_{6}$ (cryolite) is the correct choice because it effectively lowers the melting point of alumina, making the extraction process more efficient and cost-effective. The other options do not possess the necessary properties to achieve this effect in the context of aluminium metallurgy.