Role of electrolyte in the extraction of aluminium

An electrolyte is the compound decomposed during electrolysis (electrolysis is the breaking down of a molten or aqueous compound by electricity). An electrolyte must have ions that are able to move freely towards the electrodes (anode and cathode)/ it must allow electricity to pass through it during the electrolysis. In the extraction of Aluminium by electrolysis, the electrolyte used is molten Al₂O₃ (alumina)dissolved in a molten solvent called cryolite (Na₃AlF₆). It’s dissolved in cryolite to reduce the energy costs as cryolite lowers the melting point of the Al₂O₃, thus allowing the ions in aluminium oxide (the electrolyte) to move freely at a lower temperature enabling the aluminium to be extracted from Al₂O₃. 

Aluminium is the most abundant metal on Earth, but it is expensive, largely because of the amount of electricity used in the extraction process.

Aluminium ore is called bauxite (Al₂O₃). The bauxite is purified to yield a white powder – aluminium oxide (also known as alumina) – from which aluminium can be extracted.

The extraction is done by electrolysis, but first the aluminium oxide must be melted so that electricity can pass through it. However, aluminium oxide has a very high melting point (over 2,000°C) so it would be expensive to melt it.

Instead, it is dissolved in molten cryolite – an aluminium compound with a lower melting point than aluminium oxide. The use of molten cryolite as a solvent reduces some of the energy costs involved in extracting aluminium by allowing the ions in aluminium oxide to move freely at a lower temperature.

Both the negative electrode (cathode) and positive electrode (anode) are made of graphite, which is a form of carbon.

Aluminium ions receive electrons at the negative electrode and are reduced to aluminium atoms:

Al³⁺ + 3e^– → Al (reduction – gain electrons)

The molten aluminium sinks to the bottom of the cell, where it is tapped off.

Oxide ions lose electrons at the positive electrodes and are oxidised to oxygen gas:

2O^2– → O₂ + 4e^– (oxidation – lose electrons)

This oxygen reacts with the carbon of the positive electrodes, forming carbon dioxide, so they gradually burn away. As a result, the positive electrodes have to be replaced frequently. This adds to the cost of the process.

It is used to extract the aluminium from its ore called bauxite.

Hi Firdaus,

Aluminium is extracted from it's ore, bauxite. Bauxite is actually aluminium oxide - Al2O3 - and it has to be melted so that the aluminium can be extracted - this is the electrolyte. The melting point of bauxite is really high (around 2000 'C), so it is mixed with cryolite, which means that the electrolyte can be melted at around 1200'C which is cheaper.

Once the electrolyte (bauxite / cryolite mixture) is molten, electricity is passed through it using carbon electrodes.

At the negative electrode (called the cathode), the positive aluminium ions gain electrons and form aluminium atoms.

At the positive electrode (the anode), oxygen ions give up their additional electrons and form oxygen gas.

The oxygen reacts with the carbon electrodes to form carbon dioxide, so the electrodes have to be replaced on a regular basis.

I hope this helps!

Catherine

Molten Cryolite is used in extraction of pure Aluminium. The melting point of aluminium oxide is very high (more than 2000∘C) and so melting it would be a very expensive process. Hence we use cryolite to decrease the melting point of aluminium oxide from 2000 - 2500∘C to 900 - 1000∘C

Hi Firdaus! For aluminium to be extracted through electrolysis, its ions need to be free to move towards the electrodes (cation and anion). This is only possible when it is molten or when it is dissolved in an aqueous solution- the electrolyte itself. Hope this helps!

The role of the electrolyte, molten cryolite, is to dissolve the aluminium oxide from which the aluminium is to be obtained, leaving the aluminium ions free to move within the solution. These positively charged ions gain electrons from the cathode, thereby forming molten aluminium.

The electrolyte is the medium by which electricity flows through the circuit so that dissolved aluminium can move towards the cathode and become molten non-aqueous aluminium. Molten Cryolite is used instead of water as water does not dissolve aluminium oxide (the powder purified from bauxite). Hope this helps

Aluminium is typically found as a compound (e.g. aluminium oxide Al2O3) The electrolyte is essentially a "medium of ions" that allows aluminium to be separated from the other ion in the compound. Aluminium oxide is composed of positive aluminium ions and negative oxide ions. If we want to extract aluminium, aluminium can be melted, forming an "electrolyte". This molten electrolyte allows the two oppositely charged ions to move to the oppositely charged electrodes. Pure aluminium will build up on the negative electrode. It can then be used for industrial applications (e.g. manufacturing soda cans).

Aluminium can only be extacted from its ore (aluminium oxide - Al2O3) by a process called electrolysis. This is where free moving Positive Aluminium ions can move towards the negitive electrode. For this to happen the solid aluminium oxide needs to be in a molten liquid form or dissolved in a solution. However, this would mean heating it at high temperatures to make it molton and this is expensive and aluminium oxide does not dissolve in water. However, Al2O3 does dissolve in cryolite to make this liquid electrolyte and so can extract Aluminium by electrolysis (splitting using electricity).

allow change to be created as movement of charged particles

Aluminium is extracted from the ore bauxite by the process of electrolysis. To enable electrolysis to occur, you need an ionic solution or molten ionic compound. Bauxite contains Al2O3 (aluminium oxide) which has a very high boiling point (2072 degrees Celsius). To help lower the energy required to heat the aluminium oxide to its molten state, a substance called cryolite is added, which significantly reduces the melting point by approximately half. In this extraction, the molten aluminium oxide is the electrolyte and consists of Al3+ ions and O2- ions. In this electrolysis reaction, the positive aluminium ions are attracted to the negative electrode, where they gain 3 electrons and become neutral aluminium atoms which sink to the bottom of the tank and can be drained off as liquid aluminium. The negative oxygen ions are attracted to the positive electrode, where they lose two electrons, becoming neutral oxygen atoms, two of which can then combine to become O2 molecules.

Hi Firdaus! That is a really good question.

We can break down this question into 2 parts:

1) What is an electrolyte?

2) What is electrolysis, and how does the electrolyte and electrodes play a part in this process?

Let's dive into the first question and understand what an electrolyte is, and its role.

An electrolyte is a liquid which can conduct electricity. It is usually the molten or dissolved form of the ionic compound which we are separating. For example, if we are trying to separate Aluminium Chloride (or AlCl3) into its atoms or molecules (and so into Aluminium (Al) and Chlorine (Cl2), then the electrolyte would be molten or dissolved Aluminium Chloride. These electrolytes are needed as part of a process called 'electrolysis'.

Now, what is electrolysis?

Electrolysis is a process by which an ionic substance (such as Aluminium Chloride) is broken down into the atoms/molecules which make up this ionic substance (in this case, if AlCl3 was to undergo electrolysis, it would result in the breakdown of AlCl3 into its atoms and molecules, which are: aluminium (Al) and chlorine (Cl2)). Now, it is important to note that this process can only happen if the ionic substance is molten or is dissolved. Okay, we have now understood what electrolysis is, so let's understand the role of an electrolyte within this process.

Electrolysis needs an electrolyte, and two electrodes- an electrode is a solid that conducts electricity and is placed within the electrolyte (the liquid which can conduct electricity). As the electrodes are placed within the electrolyte, ions can now move from one electrode to the other- this allows the conduction of electricity through the circuit.

• the positive ions in the electrolyte will move towards the negative electrode (the cathode) and will gain electrons
• the negative ions in the electrolyte will move towards the positive electrode (the anode) and will lose electrons

as ions gain or lose electrons, they become atoms or molecules and are released. these atoms or molecules are the products of electrolysis. Therefore, we would have now separated the molten/dissolved Aluminium Chloride (which is the electrolyte) into Aluminium (Al) atoms and Chlorine (Cl2) molecules.

I hope this helped! Please feel free to let me know if you have any additional questions you would like to ask.

Generally, the role of electrolytes is to allow the movement of free ions. In the extraction of aluminium, the electrolyte used is molten cryolite which has a lower melting point than aluminium oxide. The role of molten cryolite is to dissolve aluminium oxide where aluminium ions are extracted and free to move.

Electrolysis is the decomposition of a substance using electricity. Electricity is passed through the electrolyte which is a solution of or molten ionic substance. The ions of the substance are free to move. In the case of aluminium extraction from its oxide, the electrolyte is molten aluminium oxide. The positive aluminium ions are attracted to the negative electrode and move to it. At the cathode (negative electrode) each aluminium ion gains three electrons to become the element Aluminium. Gaining electrons is called reduction.