Aluminium is extracted from alumina $(Al_2O_3)$ by electrolysis of a molten mixture of
- $Al_2O_3 + HF + NaAlF_4$
- $Al_2O_3 + CaF_2 + NaAlF_4$
- $Al_2O_3 + Na_3AlF_6 + CaF_2$
- $Al_2O_3 + KF + Na_3AlF_6$
The Correct Option is C
Solution and Explanation
The extraction of aluminium from alumina \((Al_2O_3)\) involves the electrolytic reduction of a molten mixture. The process used is known as the Hall-Héroult process. This method is chosen due to its efficiency in separating aluminium from its ore.
The key components of the molten mixture are:
- Al_2O_3: Alumina, the primary source of aluminium.
- Na_3AlF_6 (cryolite): This serves as a solvent, lowering the melting point of alumina and enhancing conductivity.
- CaF_2: Calcium fluoride acts as a flux to further decrease the melting point and assist in maintaining the molten state of the mixture.
Now, let's examine the given options:
- Al_2O_3 + HF + NaAlF_4: HF (hydrofluoric acid) is not typically used in the electrolytic process; hence, this option is incorrect.
- Al_2O_3 + CaF_2 + NaAlF_4: While CaF2 can be part of the mixture, NaAlF4 is not commonly involved in the Hall-Héroult process, making this option incorrect.
- Al_2O_3 + Na_3AlF_6 + CaF_2: This combination matches the composition used in the Hall-Héroult process, with Na3AlF6 and CaF2 lowering the melting point and aiding electrolysis. This option is correct.
- Al_2O_3 + KF + Na_3AlF_6: Although KF (potassium fluoride) could theoretically serve a similar function, it is not the standard choice, thus incorrect.
Thus, based on the composition used in the Hall-Héroult process, the correct answer is:
Al_2O_3 + Na_3AlF_6 + CaF_2.
This choice effectively reduces alumina efficiently while ensuring economic feasibility by maintaining manageable temperatures and conductivity for the electrolytic reduction process.
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