In this explainer, we will learn how to describe the extraction of aluminum from its ore using electrolysis.
Aluminum is one of the most important materials in the world; it is a strong, flexible, and light metal and its alloys have a vast range of applications. The production of aluminum happens at sites all across the world with aluminum smelting plants using electrolysis in a method known as the Hall-Héroult process to produce around 200 000 tonnes of aluminum each year.
Definition: Electrolysis
Electrolysis is a chemical process where electricity is passed through an electrolyte, causing it to decompose into its constituent elements.
The picture below shows an opencut mine in Kazakhstan where bauxite, the most abundant of all the aluminum ores, is mined. Bauxite can be thought of as impure aluminum oxide that gets its red-brown color, clearly seen in the photograph, from impurities of iron (III) oxide.
Aluminum smelting plants are enormous and use a tremendous amount of space. (The picture below shows one such plant in Russia.) They consist of multiple banks of cells and casting facilities as well as anode production areas. But before the extraction process can begin, the bauxite ore must first be purified into pure alumina, . The purification of alumina from bauxite is known as the Bayer process and revolves around a reaction between the aluminum oxide and sodium hydroxide.
Definition: Anode
The anode is the positive electrode in an electrolytic cell.
The alumina will be decomposed using electrolysis, a costly industrial process with the plants using vast amounts of electricity. An aluminum smelting plant uses around 200 MW of power each year, so let us remind ourselves why it is necessary to use electrolysis for the extraction of aluminum.
Aluminum is a very active metal that is more reactive than reducing agents such as carbon and hydrogen. For example, it is not possible to reduce alumina using carbon as we can with other metal ores such as hematite. Hematite is an iron ore containing iron (III) oxide, which can be reduced using carbon in the blast furnace to form iron metal. So for aluminum and other reactive metals such as sodium, we must use the process of electrolysis.
For electrolysis to happen on such a large industrial scale, multiple electrolytic cells are required. Over 300 individual cells are arranged into “pot lines” that need electrical currents of over 150 000 A at around 4 V per cell. Each cell consists of a steel shell lined with heat resistant bricks followed by a layer of carbon that acts as the negative cathode for the electrolysis process.
Definition: Cathode
The cathode is the negative electrode in an electrolytic cell.
Molten cryolite, , fills the cells and serves multiple purposes, which include helping the alumina conduct electricity and effectively lowering the melting point of the alumina. The melting point of alumina is ; however, when mixed with cryolite, the mixture melts at a lower temperature near . The alumina dissolving into the cryolite creates the electrolyte mixture for this reaction.
Definition: Electrolyte
An electrolyte is a substance with freely moving ions that can conduct electricity.
Example 1: Why Cryolite Is Used in the Extraction of Aluminum
Which of the following is not a reason why cryolite is part of a molten electrolyte in the extraction of aluminum?
- Cryolite reduces the temperature at which the alumina melts.
- Cryolite increases the conductivity of the electrolyte.
- Cryolite reduces the working temperature of the electrolytic cell.
- Cryolite increases the amount of electricity used.
- Cryolite provides sodium ions that help to carry the electric current.
Answer
The question is asking us which statement is not a reason why cryolite is added to each of the cells during the electrolysis process.
Answer A suggests that cryolite reduces the temperature at which the alumina melts. However, the temperature inside the electrolytic cell is around , so cryolite must indeed reduce the temperature at which alumina melts. This reduction would have cost implications as less energy is used so this is likely a good reason for having cryolite as part of the electrolyte mixture, meaning that A is not the correct answer. We know that cryolite increases the conductivity of the electrolyte as stated in answer B and so answer B is not the correct answer either. The reasons behind this are quite complex but essentially cryolite increases the mobility of the ions in the electrolyte. Answer C suggests that cryolite reduces the working temperature of the electrolytic cell, which it does, as without the cryolite the cell would have to be heated to around to melt the alumina, and so C is also incorrect. Answer D suggests that cryolite increases the amount of electricity used; however, the amount of electricity would not be affected by a substance that is not involved in either of the reactions at the two electrodes, so this is likely the correct answer, but we will also check answer E. In answer E, the suggestion is that the cryolite provides sodium ions and that they help to carry the electric current. Cryolite does contain the element sodium, and we know that cryolite improves the conductivity. So answer E is incorrect, meaning the previous answer D is the correct answer for this question.
The alumina, which we remember is the common name given to aluminum oxide, is dissolved in the molten cryolite at temperatures of around . Rows of carbon anodes are then lowered into the electrolyte mixture completing the circuit. The electrolytic cell used for extracting aluminum (shown in the diagram below) was developed in 1886 simultaneously by American chemist Charles Martin Hall and French chemist Paul Héroult.
We can describe the reactions that take place during electrolysis in terms of oxidation and reduction and the movement of electrons. Oxidation is considered to be the loss of electrons, whereas reduction involves the gain of electrons. A simple mnemonic can help us to quickly memorize this idea, “OIL RIG”: Oxidation Is Loss of electrons, Reduction Is Gain of electrons.
During the electrolysis, aluminum ions are attracted to the cathode where they are reduced, gain three electrons each, and become aluminum metal atoms that then sink to the bottom of the cell. The oxide ions, which have a negative charge, move to the anode where they are oxidized, losing two electrons each and then pairing up to form oxygen molecules. The half-equations and overall equation for the electrolysis of the alumina are
We need to add these two half-equations to give us our overall equation for the electrolytic cell. We need to balance the number of electrons on each side of the equation:
This gives us an overall equation for the reaction, which can be written as
Example 2: The Movement of Electrons in the Reduction of Aluminum Ions
Fill in the blank: In the extraction of aluminum, the aluminum are because they electrons.
- ions, reduced, lose
- atoms, oxidized, lose
- atoms, reduced, lose
- ions, oxidized, gain
- ions, reduced, gain
Answer
This question is asking us to fill in the blanks in a sentence about aluminum and electrons. Looking down the answers, we can see that we have two choices for the first space, ion or atoms. We know we are trying to form aluminum atoms through this process and that we start with the ionic substance alumina, so the first space is likely to be ions, excluding answers B and C, but let’s continue and check the rest of the sentence. For metal ions to become atoms, they need to gain electrons and from “OIL RIG”. We know that gaining of electrons is reduction, which gives us our final correct sentence: “In the extraction of aluminum, the aluminum ions are reduced because they gain electrons.” So the correct answer is E.
The electrolysis process runs 365 days a year, 24 hours per day, with aluminum oxide decomposing into aluminum metal and oxygen gas. A crust continually forms at the top of the cell, which needs to be broken every 3 to 4 minutes for the cell to be refilled with additional alumina. The high temperature causes a reaction between the carbon anodes and the oxygen produced during electrolysis, creating carbon dioxide: And, consequently, the carbon anodes burn away and have to be replaced every month.
Example 3: Identifying the Waste Gases Produced by a Hall–Héroult Cell
Given the contents of the electrolytic cell used for the extraction of aluminum, which of the following is unlikely to be a waste gas?
Answer
This question is asking us which of four different gases are we unlikely to find being generated by an electrolytic cell during the extraction of aluminum.
Our first option is oxygen, ; we know that alumina is, in fact, aluminum oxide and that those oxide ions form oxygen molecules at the anode, and so it is very likely that oxygen gas will be produced, so answer A is incorrect.
Answer B is hydrogen gas, . There are no hydrogen-containing compounds in the electrolyte or in the cell itself, so it is hard to think of where hydrogen gas may come from. Therefore, answer B is likely correct but we will check our final two options. Answers C and D are both oxides of carbon; we know that the anodes placed into the top of the cells are made from carbon and that oxygen is produced at the anode, so it is highly likely that we will get either carbon dioxide or carbon monoxide, from incomplete oxidation, formed as waste gases.
With A, C, and D discounted, it appears that, as we thought, answer B is correct!
Each cell produces around 50 kg of aluminum per hour, which is then sucked out of the cell and taken off for casting into pure aluminum ingots or combined with other metals such as iron, silicon, manganese, and magnesium in the production of alloys. After casting into the desired shapes and quantities, the aluminum and its alloys are shipped around the world for use in a wide range of items. Some typical applications of this extraordinary metal include aluminum window and door frames, the foil we use daily in our kitchens, and overhead electricity cables, with aluminum alloys also being critical in the production of aircraft.
Key Points
- Aluminum must be extracted from its ore using electrolysis as it is a very reactive metal, more reactive than hydrogen and carbon.
- The electrolysis happens in an electrolytic cell with an alumina and cryolite electrolyte.
- Cryolite helps to reduce the temperature needed for the cell and reduces costs.
- Molten aluminum is produced through reduction in the cell as well as oxygen waste gases that react with the carbon anode.
- Aluminum is an incredibly important metal to society with many different uses.
