In the extraction of aluminium from \(Al_2O_3\) we generally add \(Na_3AlF_6\) or \(CaF_2\):

To lower the melting point of the mix and bring conductivity

The correct answer is option 4. To lower the melting point of the mix and bring conductivity.

In the extraction of aluminum from bauxite ore (which contains \( \text{Al}_2\text{O}_3 \), or alumina), the primary method used is the electrolytic reduction process, known as the Hall-Héroult process. Here’s a detailed explanation of why \( \text{Na}_3\text{AlF}_6 \) (sodium aluminate) and \( \text{CaF}_2 \) (calcium fluoride) are added:

Purpose of Adding Fluxing Agents

a. Lowering the Melting Point:

Alumina Melting Point: Alumina (\( \text{Al}_2\text{O}_3 \)) has a very high melting point of about 2050°C. Electrolyzing alumina directly at this temperature would be impractical and energy-intensive.

Fluxing Agents: \( \text{Na}_3\text{AlF}_6 \) and \( \text{CaF}_2 \) act as fluxing agents, meaning they lower the melting point of the alumina mixture. This is important because the electrolytic reduction of alumina requires a molten electrolyte, and lowering the melting point makes the process more feasible.

b. Increasing Conductivity:

Electrical Conductivity: For the electrolysis process to work efficiently, the electrolyte must be electrically conductive. Pure alumina is a poor conductor, so adding \( \text{Na}_3\text{AlF}_6 \) and \( \text{CaF}_2 \) improves the electrical conductivity of the molten mixture.

Effect on Conductivity: \( \text{Na}_3\text{AlF}_6 \), also known as cryolite, dissolves alumina in the electrolyte and improves the overall conductivity of the molten mixture, making the electrolysis process more efficient.

Mechanism

a. Cryolite Addition (\( \text{Na}_3\text{AlF}_6 \)):

Cryolite is added to the electrolyte to dissolve alumina. It forms a complex with \( \text{Al}_2\text{O}_3 \), which helps to lower the melting point of the mixture. The melting point of the cryolite-alumina mixture is significantly lower than that of pure alumina. This mixture melts at around 900-1000°C, which is much more manageable for the electrolytic process.

b. Calcium Fluoride Addition (\( \text{CaF}_2 \)):

Calcium fluoride is often used in combination with cryolite to further reduce the melting point of the mixture and to maintain the desired fluidity and conductivity of the molten electrolyte.

Overall Impact on the Process

Efficient Electrolysis: By lowering the melting point and increasing the conductivity, these fluxing agents make the electrolytic reduction of alumina more energy-efficient and practical.

Process Optimization: This optimized process allows for the extraction of aluminum metal from alumina using electrolysis, which is essential for the industrial production of aluminum.

In summary, \( \text{Na}_3\text{AlF}_6 \) and \( \text{CaF}_2 \) are added to the alumina electrolyte to lower its melting point and increase its conductivity, both of which are crucial for the efficient electrolysis process used in aluminum extraction.