Animated Solution for Chemistry - Metallurgy: In the leaching of alumina from bauxite, the ore expected to leach out in the process by reacting with NaOH is

Purifying Bauxite

When we extract aluminum, we don't start with pure metal; we start with its principal ore, Bauxite (${\text{Al}}_2{\text{O}}_3\cdot x{\text{H}}_2\text{O}$). However, nature rarely gives us pure compounds. Bauxite comes mixed with several unwanted guests, primarily iron oxide (${\text{Fe}}_2{\text{O}}_3$), titanium dioxide (${\text{TiO}}_2$), and silica (${\text{SiO}}_2$).

To extract pure aluminum, our first major hurdle is to separate the valuable alumina from these impurities. This is achieved through a chemical concentration process known as Leaching, specifically the Bayer's Process.

The genius of the Bayer's process lies in exploiting the acid-base properties of the ore and its impurities. We treat the crushed bauxite ore with a hot, concentrated solution of sodium hydroxide ($\text{NaOH}$).

Here is the catch: $\text{NaOH}$ is a strong base. It will only react with substances that are either acidic or amphoteric (capable of acting as both an acid and a base).

Let's look at our impurities: 1. ${\text{Fe}}_2{\text{O}}_3$ and ${\text{TiO}}_2$ are basic oxides. Since a base does not react with another base, these impurities remain completely unaffected by the $\text{NaOH}$ and stay as solid, insoluble residues (often called 'red mud'). 2. ${\text{SiO}}_2$ (Silica), on the other hand, is an acidic oxide.

Because alumina (${\text{Al}}_2{\text{O}}_3$) is amphoteric, it happily reacts with the strong base to form a soluble complex called sodium aluminate:

But what about the silica? Since ${\text{SiO}}_2$ is acidic, it also reacts with the basic $\text{NaOH}$ to form soluble sodium silicate:

After the reaction, we filter the mixture. The unreacted basic oxides (${\text{Fe}}_2{\text{O}}_3$ and ${\text{TiO}}_2$) are left behind on the filter paper. However, both the sodium aluminate and the sodium silicate pass through the filter because they have leached out into the solution.

Therefore, the impurity that leaches out along with the alumina is ${\text{SiO}}_2$.