Animated Solution for Chemistry - Metallurgy: Which among the following statement(s) is(are) true for the extraction of aluminium from bauxite?

The extraction of aluminium from its primary ore, bauxite, is a beautiful symphony of chemical engineering and electrochemistry. This question takes us on a comprehensive tour of the entire process, testing our knowledge from the initial purification of the ore to the final electrolytic reduction. Let's break down this fascinating journey step by step.

Baeyer's Process

Before we can extract aluminium, we must first purify the bauxite ore, which is heavily contaminated with impurities like iron oxides and silica. This is achieved through Baeyer's Process.

The bauxite is first treated with a hot, concentrated solution of sodium hydroxide ($\text{NaOH}$). The amphoteric nature of aluminium oxide allows it to dissolve, forming a soluble complex called sodium aluminate ($\text{Na}[\text{Al}(\text{OH}{)}_4]$), while the impurities are left behind as 'red mud'.

Once we filter out the impurities, we need to recover the aluminium from the solution. How do we do this? We bubble carbon dioxide (${\text{CO}}_2$) gas through the sodium aluminate solution. Carbon dioxide acts as a weak acid and neutralizes the highly alkaline solution. This shift in pH forces the aluminium to precipitate out as hydrated alumina (${\text{Al}}_2{\text{O}}_3\cdot x{\text{H}}_2\text{O}$ or $\text{Al}(\text{OH}{)}_3$).

This confirms that Statement (A) is absolutely correct.

A Masterpiece of Engineering

Now that we have pure alumina, we need to extract the aluminium metal. Because aluminium is highly reactive, standard chemical reduction (like using carbon in a blast furnace) won't work. We must use electrolysis. This brings us to the Hall-Heroult Process.

However, we face a massive physical hurdle. Pure alumina has an incredibly high melting point of around $2050^{\circ }\text{C}$. Maintaining such a high temperature industrially is economically unviable. Furthermore, pure molten alumina is a very poor conductor of electricity.

Enter the magic ingredient: Cryolite (${\text{Na}}_3{\text{AlF}}_6$). By mixing alumina with molten cryolite (and a bit of fluorspar, ${\text{CaF}}_2$), the melting point of the mixture plummets to a much more manageable $900^{\circ }\text{C}$. Additionally, the cryolite drastically increases the electrical conductivity of the melt.

This brilliant chemical workaround confirms that Statement (B) is completely true.

What Happens at the Electrodes?

Let's look at the architecture of the electrolytic cell. The cell itself is a large steel vessel. To protect the steel and provide a conductive surface, the inside of the vessel is lined with a thick layer of carbon. This carbon lining acts as the cathode (the negative electrode).

This structural fact confirms that Statement (D) is correct.

Suspended from the top and dipping into the molten electrolyte are massive blocks of carbon, which act as the anodes (the positive electrodes).

When the massive electric current is turned on, the magic happens. At the cathode, aluminium ions (${\text{Al}}^{3+}$) gain electrons and are reduced to molten aluminium metal. Because molten aluminium is denser than the electrolyte, it sinks and collects at the bottom of the steel vessel, where it is periodically tapped off.

Meanwhile, at the anodes, oxygen ions (${\text{O}}^{2-}$) lose electrons and are oxidized to oxygen gas. But there's a catch! The temperature is around $900^{\circ }\text{C}$, and the anodes are made of carbon. The freshly liberated oxygen immediately reacts with the carbon anodes, burning them away to produce carbon monoxide ($\text{CO}$) and carbon dioxide (${\text{CO}}_2$) gases.

The reactions at the anode look like this: ${\text{C}}_{(s)}+{\text{O}}^{2-}\to {\text{CO}}_{(g)}+2{\text{e}}^{-}$ ${\text{C}}_{(s)}+2{\text{O}}^{2-}\to {\text{CO}}_{2(g)}+4{\text{e}}^{-}$

Because the carbon anodes are continuously consumed in this process, they must be replaced periodically. The evolution of ${\text{CO}}_2$ at the anode confirms that Statement (C) is also true.

By carefully analyzing the chemistry and engineering behind the extraction of aluminium, we find that every single statement provided in the question is a factual representation of the industrial process. Therefore, the correct options are (A), (B), (C), and (D). This problem serves as a phenomenal review of the entire metallurgy of aluminium!