Let us say we are making carrot juice. The first thing that we need to do is source the carrot. Carrots are grown underground, so we need to extract and dig them. Now, we have our primary ingredient, and we need to process it to make carrot juice. What we do next, is either put it through a juicer or grate it and put it in a blender, and this makes our carrot juice. But how is this related to aluminium you may ask? Well, similar to other metals we do not just find them randomly on the ground. Rather we extract and process them and this allows us to give the aluminium that we use in so many everyday products. Today we are going to explore, how we can do this.

• This article is about the extraction of aluminium.

• We will go over what aluminium is, this will include what types of things we use it for and where we source aluminium from.

• After, will explore how we extract aluminium using electrolysis.

• We will then go over the equation for this reaction.

• Finally, we will explore what an electrolysis diagram looks like.

Extraction of aluminium: definition

Aluminium is a type of metal. It has common metal properties such as malleable, and soft and can conduct electricity. Due to its many properties, we can use aluminium to produce a variety of products, such as; pans, cables, foil, some types of power cables and many more things that we use in our everyday life.

Aluminium is also quite reactive. If we look at our reactivity series table, you will notice it is more reactive than carbon. This means the carbon is not reactive enough to be used to extract aluminium, so the alternative is to use the process of electrolysis. In electrolysis, we do not directly use aluminium. Instead, we use aluminium oxide Al₂O₃ and we source this from something that we call an ore.

Fig. 2: Bauxite ore, Vaia Originals.

Ores are natural rocks that contain different types of minerals, one type being aluminium oxide Al₂O₃. If we go back to the beginning of our article, remember that to first get our carrots we had to dig and extract them from the soil and this is similar to what we do with ores. In order to source ores, we have to dig them up from the surface of the earth. For aluminium in particular we need to find ores that are called bauxite, these mainly contain aluminium oxide Al₂O₃.

Like carrots, our ores can have impurities, but instead of cleaning, we separate what we need and any other products. For bauxite, we mainly fine iron (III) oxide, and when we separate this from our desired product, rusty brown water waste is produced which needs to be stored in large lagoons.

Now we know how we source the primary materials to make aluminium, we can move on to how we produce aluminium using electrolysis.

Extraction of aluminium by electrolysis

So far, we have gone through where we can find aluminium oxide and that we need to purify it, in order for it to be used for electrolysis. But before we go on to the electrolysis process, there is one more step we need to carry out.

Aluminium oxide needs to be melted and molten before electrolysis is carried out, this is because it facilitates ions to move to the electrodes. However, in order to melt the aluminium oxide, we require a lot of energy, which can be expensive and harmful to the environment. This is because the melting point of aluminium oxide is quite high at 2050℃. Fortunately, scientists have discovered a way to save some energy. The way that they do this is by mixing aluminium oxide with molten cryolite, which is another type of ionic compound. This mixture has a much lower melting point of 850 ℃. We then also require a large amount of energy to keep this molten so electrolysis can take place.

Extraction of aluminium diagram

To carry out electrolysis we use a cell to extract aluminium. Looking at the diagram on the right we see several things. At the bottom, we have molten aluminium, which when produced is taken from the cell. We also have a carbon positive electrode and the lining cell is a carbon negative electrode. We also have a power source and carbon dioxide gas being produced.

We extract aluminium this way, it is because it allows aluminium oxide to be separated into aluminium (our desired product) and oxygen. The aluminium is formed at the negative electrode (cathode), which in this case is in the lining and stays molten so we can extract it from the cell. The oxygen is formed at the positive electrode (anode), which then reacts with our carbon dioxide anode to produce carbon dioxide, as can be seen in the bubbles of our diagram.

Extraction of aluminium equation

For the electrolysis of aluminium, we use aluminium oxide (Al₂O₃), which produces aluminium and oxygen. Try to write a balanced equation yourself, before scrolling down the article.

The full equation of this reaction is:

$$2Al_2O_3(l) \rightarrow 4Al(l) + 3O_2(g)$$

This equation shows that two moles of aluminium oxide (Al₂O₃) which is in the liquid state, produce 4 moles of aluminium, also in a liquid state and 3 moles of oxygen which is in a gas state.

This is not where our reaction stops. Once oxygen gas is produced, it reacts with the carbon positive electrode and produces carbon dioxide, this is presented in our equation below.

$$C(s) + O_2(g) \rightarrow CO_2(g)$$

Typically, we use inert materials as our cathodes, this means that they do not react. However, we know that our carbon electrode does react. As a result, eventually, all of the carbon reacts and must be replaced to continue the process.

To explore our reaction further, instead of using regular equations with reactants and products, we use half equations. Half equations are quite different as we explore how the ionic state of an element is changed within our reaction. They allow us to explore what has been oxidised and what has been reduced:

Let us look at what is happening at our anode first, this is the positive electrode. At the anode in this reaction, oxygen is produced. The oxygen in our reactants has an ionic charge of -2. Therefore, to turn it into oxygen atoms we need to lose two electrons, as electrons are negative. This is presented in our half equation below.

$$2O^{2-}(l) \rightarrow O_2(g) + 4e^-$$

Here we can see in order to produce our oxygen atom, we need two moles of the oxide ion. We take 4 electrons away and this produced our oxygen atom in a gas state.

Moving on to our cathode, which is our negative electrode. Aluminium is produced. Here our aluminium ion has a 3+ charge. Therefore, to make aluminium atoms we need to add 3 electrons. This can be explored in our equation below:

$$Al^{3+}(l) + 3e^- \rightarrow Al(l)$$

As we can see, our aluminium ion has had 3 electrons added to it, forming aluminium atoms. These are in a liquid state and are extracted from the cell to be used.