Using electrolysis to extract metals

What you'll learn

Why some metals cannot be extracted using carbon.
How aluminium is extracted industrially using electrolysis.
Why we mix aluminium oxide with cryolite.
Why the positive electrodes (anodes) constantly burn away and need replacing.

When do we use electrolysis?

Earlier in your chemistry course, you will have seen that many metals are extracted from their ores by heating them with carbon. Carbon is cheap and abundant, so this is the preferred method for industry.

However, carbon extraction only works if the metal is less reactive than carbon. If a metal is more reactive than carbon (like potassium, sodium, calcium, magnesium, or aluminium), carbon isn't "strong" enough to displace the metal from its compounds.

For these highly reactive metals, we have to use a much more powerful (and expensive) method: electrolysis.

Definition

Electrolysis

The splitting up of an ionic compound using electricity. For this to work, the compound must be molten (melted as a liquid) or dissolved in water so that its ions are free to move and carry the charge.

We also use electrolysis if the metal reacts directly with carbon to form a metal carbide instead of the pure metal.

The high cost of electrolysis

Using electrolysis to extract metals is extremely expensive for two main reasons:

Heat energy: The ionic compounds we extract metals from (like aluminium oxide) usually have giant ionic lattices with incredibly high melting points. It takes huge amounts of energy to melt them.
Electrical energy: Passing a massive, continuous electric current through the molten compound requires a lot of electricity.

Because of these costs, chemical engineers are always looking for clever tricks to save energy. The extraction of aluminium is the perfect example of this.

Extracting Aluminium: The classic example

Aluminium is the most abundant metal in the Earth's crust. We mine it as an ore called bauxite, which is purified into a white powder called aluminium oxide ( $Al2O3\text{Al}_2\text{O}_3$ ).

To extract the pure aluminium, we must melt the aluminium oxide so the $Al3+\text{Al}^{3+}$ and $O2−\text{O}^{2-}$ ions can move. The problem is that pure aluminium oxide has a melting point of over $\text{ } ^\circ\text{C}$ . Heating large industrial tanks to this temperature would cost an absolute fortune.

The Cryolite Trick

To solve this, the aluminium oxide is dissolved in a substance called cryolite.

Key Idea

The role of cryolite

Mixing aluminium oxide with cryolite lowers the melting point of the mixture to around $\text{ } ^\circ\text{C}$ . This massive drop in temperature saves a huge amount of heat energy, making the process much cheaper.

Common Mistake

Cryolite is not a catalyst

A very common mistake in exams is to call cryolite a "catalyst". It is not a catalyst. It does not speed up the reaction; it acts as a solvent that lowers the melting point of the mixture.

Inside the electrolysis cell

Once the mixture is molten, the electrolysis can begin. The setup uses huge electrodes made of carbon (specifically graphite, because it conducts electricity and has a very high melting point).

Aluminium electrolysis cell

At the cathode (negative electrode): The steel tank is lined with carbon, acting as the negative electrode. The positive aluminium ions ( $Al3+\text{Al}^{3+}$ ) are attracted to it. They gain electrons to become pure aluminium atoms. Because liquid aluminium is denser than the molten electrolyte, it sinks and pools at the bottom of the tank, where it is easily tapped off.
At the anode (positive electrode): Thick blocks of carbon are suspended into the liquid. The negative oxide ions ( $O2−\text{O}^{2-}$ ) are attracted to these positive electrodes. They lose electrons to form oxygen gas ( $O2\text{O}_2$ ).

Tip

Remembering the electrodes

If you ever mix up your anodes and cathodes, use the acronym PANIC: Positive is Anode, Negative Is Cathode.

The disappearing anodes

If you run an aluminium extraction plant, you will quickly notice a major problem: the thick carbon anodes at the top of the tank gradually shrink and disappear. You have to keep shutting down the cell to replace them, which adds to the cost of the process.

Why does this happen? It comes down to a secondary chemical reaction between the products and the apparatus.

Example

Deducing why the anode must be replaced

Let's walk through the chemistry happening right at the surface of the positive electrode to understand why it degrades.

First, identify the material of the anode. In this setup, the positive electrodes are made of solid carbon.
Next, identify the product forming at the anode. The $O2−\text{O}^{2-}$ ions migrate to the anode and form hot oxygen gas ( $O2\text{O}_2$ ).
Consider the conditions. The electrolysis cell is operating at around $\text{ } ^\circ\text{C}$ .
Put it together: We have highly reactive, hot oxygen gas bubbling against solid carbon. They react together to form carbon dioxide gas: $C(s)+O2(g)→CO2(g)\begin{aligned} \text{C(s)} + \text{O}_2\text{(g)} \to \text{CO}_2\text{(g)} \end{aligned}$
Because the carbon is turning into a gas ( $CO2\text{CO}_2$ ) and floating away, the solid electrode literally burns away into the atmosphere over time.

Exam technique

In the exam

If asked why a metal is extracted by electrolysis instead of carbon, state clearly: "The metal is more reactive than carbon, so carbon cannot displace it."
Always mention both heat energy (to melt the solid) and electrical energy (to run the current) when discussing the high costs of electrolysis.
Be exact with your wording for cryolite: "It lowers the melting point of the mixture, which saves energy."
If a question asks why the positive electrode needs replacing, you must mention that the electrode is made of carbon, it reacts with oxygen, and produces carbon dioxide.

Self review

Check yourself

Why can't we extract aluminium by heating aluminium oxide with carbon?
Why is aluminium oxide mixed with cryolite before electrolysis?
What material are the electrodes made of in aluminium extraction, and why?
Write a word equation for the reaction that causes the positive electrodes to degrade.

Using electrolysis to extract metals

What you'll learn

Why some metals cannot be extracted using carbon.
How aluminium is extracted industrially using electrolysis.
Why we mix aluminium oxide with cryolite.
Why the positive electrodes (anodes) constantly burn away and need replacing.

When do we use electrolysis?

For these highly reactive metals, we have to use a much more powerful (and expensive) method: electrolysis.

Definition

Electrolysis

We also use electrolysis if the metal reacts directly with carbon to form a metal carbide instead of the pure metal.

The high cost of electrolysis

Using electrolysis to extract metals is extremely expensive for two main reasons:

Heat energy: The ionic compounds we extract metals from (like aluminium oxide) usually have giant ionic lattices with incredibly high melting points. It takes huge amounts of energy to melt them.
Electrical energy: Passing a massive, continuous electric current through the molten compound requires a lot of electricity.

Because of these costs, chemical engineers are always looking for clever tricks to save energy. The extraction of aluminium is the perfect example of this.

Extracting Aluminium: The classic example

Aluminium is the most abundant metal in the Earth's crust. We mine it as an ore called bauxite, which is purified into a white powder called aluminium oxide ( $Al2O3\text{Al}_2\text{O}_3$ ).

The Cryolite Trick

To solve this, the aluminium oxide is dissolved in a substance called cryolite.

Key Idea

The role of cryolite

Common Mistake

Cryolite is not a catalyst

A very common mistake in exams is to call cryolite a "catalyst". It is not a catalyst. It does not speed up the reaction; it acts as a solvent that lowers the melting point of the mixture.

Inside the electrolysis cell

Once the mixture is molten, the electrolysis can begin. The setup uses huge electrodes made of carbon (specifically graphite, because it conducts electricity and has a very high melting point).

Aluminium electrolysis cell

At the cathode (negative electrode): The steel tank is lined with carbon, acting as the negative electrode. The positive aluminium ions ( $Al3+\text{Al}^{3+}$ ) are attracted to it. They gain electrons to become pure aluminium atoms. Because liquid aluminium is denser than the molten electrolyte, it sinks and pools at the bottom of the tank, where it is easily tapped off.
At the anode (positive electrode): Thick blocks of carbon are suspended into the liquid. The negative oxide ions ( $O2−\text{O}^{2-}$ ) are attracted to these positive electrodes. They lose electrons to form oxygen gas ( $O2\text{O}_2$ ).

Tip

Remembering the electrodes

If you ever mix up your anodes and cathodes, use the acronym PANIC: Positive is Anode, Negative Is Cathode.

The disappearing anodes

Why does this happen? It comes down to a secondary chemical reaction between the products and the apparatus.

Example

Deducing why the anode must be replaced

Let's walk through the chemistry happening right at the surface of the positive electrode to understand why it degrades.

First, identify the material of the anode. In this setup, the positive electrodes are made of solid carbon.
Next, identify the product forming at the anode. The $O2−\text{O}^{2-}$ ions migrate to the anode and form hot oxygen gas ( $O2\text{O}_2$ ).
Consider the conditions. The electrolysis cell is operating at around $\text{ } ^\circ\text{C}$ .
Put it together: We have highly reactive, hot oxygen gas bubbling against solid carbon. They react together to form carbon dioxide gas: $C(s)+O2(g)→CO2(g)\begin{aligned} \text{C(s)} + \text{O}_2\text{(g)} \to \text{CO}_2\text{(g)} \end{aligned}$
Because the carbon is turning into a gas ( $CO2\text{CO}_2$ ) and floating away, the solid electrode literally burns away into the atmosphere over time.

Exam technique

In the exam

If asked why a metal is extracted by electrolysis instead of carbon, state clearly: "The metal is more reactive than carbon, so carbon cannot displace it."
Always mention both heat energy (to melt the solid) and electrical energy (to run the current) when discussing the high costs of electrolysis.
Be exact with your wording for cryolite: "It lowers the melting point of the mixture, which saves energy."
If a question asks why the positive electrode needs replacing, you must mention that the electrode is made of carbon, it reacts with oxygen, and produces carbon dioxide.

Self review

Check yourself

Why can't we extract aluminium by heating aluminium oxide with carbon?
Why is aluminium oxide mixed with cryolite before electrolysis?
What material are the electrodes made of in aluminium extraction, and why?
Write a word equation for the reaction that causes the positive electrodes to degrade.

Using electrolysis to extract metals

What you'll learn

When do we use electrolysis?

The high cost of electrolysis

Extracting Aluminium: The classic example

The Cryolite Trick

Inside the electrolysis cell

The disappearing anodes

Using electrolysis to extract metals Revision Guide

Using electrolysis to extract metals

What you'll learn

When do we use electrolysis?

The high cost of electrolysis

Extracting Aluminium: The classic example

The Cryolite Trick

Inside the electrolysis cell

The disappearing anodes

Using electrolysis to extract metals Revision Guide