How Aluminium Is Extracted？

No one can deny the importance of aluminium. Have you ever wondered how it came from the earth’s crest to your hand? There are multiple processes involved during its extraction, and that’s why we see aluminium everywhere around us. This article will give you a complete insight into the metal, its importance, and an ultimate guide to its extraction process.

Introduction to Aluminium

Aluminium (Al) is a silvery-white metal with atomic number 13 in the boron group in the periodic table. It is a non-magnetic and ductile metal with only one stable isotope. This metal was first discovered in 1825 by Danish physicist Hans Christian Oersted. Here are some physical and atomic properties of Aluminium:

• Metallic point: 660.32 °C
• Boiling point: 2470 °C
• Density: 2.7 g/cm³
• The heat of vaporization: 284 KJ/Mol
• Young’s modulus: 70 GPa

When it comes to the earth’s crest, aluminium is the third most abundant element after oxygen and silicon. It means the earth’s crust has more aluminium than iron. This metal has some truly amazing properties, which are as follows:

1. Aluminium was a precious metal in the 19th century.
2. This metal has been in commercial use for 150 years.
3. The Aluminium Association was established in 1933.
4. 8% of the earth’s crust consists of aluminium.
5. It is a shiny metal like silver and reflects 92% of the visible light.
6. It can mix with 270 different minerals.
7. The most common ore of aluminium is bauxite.
8. China is the biggest producer of aluminium.
9. It is toxic to gilled animals while non-toxic to humans.
10. Powdered aluminium is in explosives and fireworks.
11. It is a highly reactive metal, and it never rusts.
12. Aluminium is the 12^thmost abundant metal in space.

Top Uses of Aluminium

The uses of aluminium are myriad. It is because of its properties, such as strong, lightweight, durable, ductile, odourless, conductive, and resistant to corrosion. The list of the uses of aluminium is long, but here are some of the most notable uses of aluminium in various industries.

• Transportation: Name any mode of transportation, and you’ll find aluminium there. Fuel efficiency increases with the use of lightweight aluminium in vehicles, high-speed rail systems, and aeroplanes. Additionally, there are no risks of corrosion. The space shuttles also make use of 50% to 90% of aluminium alloys.
• Packaging industry: Aluminium plays a vital role in the packaging industry in making rolls, foils, coils, and other packaging stuff.
• Smartphones and Gadgets: Laptops, TV, smartphones, and all other tech gadgets use aluminium for sleek design and attractive finish. It has largely reduced the use of plastic in gadgets.
• The corrosion resistance nature makes it an ideal metal for gas and oil pipelines and refrigeration equipment.
• Construction industry: Due to its thermal properties, pleasing finish, and corrosion-free nature, engineers make buildings with aluminium.
• House Items: Interior decoration, lamps, chairs, fans, cupboards, tables, doors, frames, and food containers are all made of aluminium. The list of aluminium products in the kitchen is long.
• Aluminium is an excellent catalyst in numerous reactions. It means Al doesn’t directly involve in the reaction but speeds up the reaction. For example, it acts as a deoxidizer during the steelmaking process.

Raw Materials of Aluminium

Aluminium is a plentiful element found in the earth’s crust. Aluminium is extracted in the form of aluminium ores. This metal can conveniently and economically be produced using some raw materials. While aluminium compounds are present in most clays, the most useful for producing this metal is bauxite. Here is a list of the raw materials used to make this metal.

1. Bauxite

Bauxite has around 45 to 60% aluminium oxide in addition to other impurities like iron, sand, etc. While some bauxite deposits are solid and hard, others are usually soft dirt. Usually, 4lb of bauxite can make around 1 lb of aluminium metal.

2. Caustic Soda

Caustic soda or sodium hydroxide is a product used to dissolve the aluminium compounds present in bauxite. This element separates the aluminium from impurities. Based on the bauxite ore composition, other chemicals like lime, sodium sulfide, and starch may also be used in small amounts.

3. Cryolite

Cryolite is a chemical compound consisting of aluminium, sodium, and fluorine. This compound serves as the electrolyte in the process of smelting. While it originally came from Greenland, cryolite is now synthetically produced to be used in aluminium manufacturing. Additionally, aluminium fluoride is added to reduce the melting point of this electrolyte solution.

4. Carbon

Carbon is another raw material used in aluminium manufacturing. These electrodes are responsible for transmitting electric current through this electrolyte solution. In the smelting process, some carbon forms carbon dioxide by reacting with oxygen. 0.2kg of carbon is utilized for every 2.2kg of aluminium made. While most of the carbon used in aluminium smelting is the byproduct of oil refining, some of it is taken from coal.

5. Electrical Energy

Finally, aluminium production requires massive amounts of electrical energy because the smelting process requires the passing of electric current through the molten electrolyte. On average, 15 kWh of electrical energy is required to produce two lb. of aluminium.

The raw materials mentioned above are essential for aluminium production. Once you have these, you can head toward the manufacturing process.

How Aluminium is Extracted?

While aluminium is one of the most abundant metals found on the surface of the earth, it doesn’t exist in a pure form. That’s why it needs to be extracted from the ore, bauxite. The aluminium extraction process is very complex and resource-intensive, making aluminium an expensive metal.

The extraction process is done through electrolysis. The process is further divided into several stages. The final product is aluminium, as you know it. Let’s have a look at the complete process of obtaining aluminium.

Bauxite Mixing

Aluminium can be extracted using different minerals. The raw material most commonly used is bauxite which consists of aluminium oxide combined with various other minerals. During the mining process, bauxite is classified based on its aluminium oxide percentage. High-quality bauxite consists of more than 50% of this oxide. Based on the material combination, it can come in different colours ranging from brick red, white, and grey to black, bluish, purple, etc. Structurally, bauxite may be crumbly, solid, or dense. This also depends on the composition of minerals.

Alumina Extraction from Bauxite- PDF

Alumina Production

Once mined, the bauxite is processed further to make aluminium oxide or alumina, a white powder. Alumina can be obtained using different processes, but the most common method is the Bayer Process.  

Discovered almost 100 years ago, this method has remained popular for making alumina even today. While the process is very effective, it is only used for high-quality bauxite, which has a low silicon composition.

The Bayer Process

This process includes a series of steps, as mentioned below in detail.

1. At the very start, bauxite ore is crushed mechanically and mixed with caustic soda in a large grinding mill. It results in a slurry, a watery suspension with fine ore particles.
2. Next, the slurry is poured into a digester. This tank works like a pressure cooker, heating the slurry to 230 to 530F at 50 lb/in2 pressure. This condition is maintained for half to several hours. At this stage, caustic soda may also be added to ensure that the aluminium compounds dissolve completely.
3. The hot sodium aluminate solution is then passed through several flash tanks to reduce pressure. Heat is also recovered that can be reused later in the refining process.
4. Now, the slurry is poured into the settling tank where the slurry rests. The impurities that remain undissolved in caustic soda will rest at the vessel’s bottom end. Red mud is the residue of the solution that sinks at the bottom. This consists of iron oxide, fine sand, and trace element oxides like titanium. This red mud is disposed of in various areas.
5. Once the impurities settle at the end, the top liquid (much like coffee) goes through multiple cloth filters. Here, any fine impurity particles that remain are trapped. The material is then washed to get caustic soda and alumina that can be reused later.
6. Now, the filtered liquid moves through several six-story precipitation tanks. At the top of each tank, alumina hydrate and seed crystals are added. As these crystals grow, they settle in the liquid. The dissolved alumina then attaches to these crystals.
7. As the crystals precipitate, they settle at the bottom of the tank, from where they are then removed. Once washed, the crystals are taken for calcining in a kiln (removing water from alumina molecules through heating). The crystals are moved through a screw conveyor, where they rotate. The tilted cylindrical kiln allows materials to move through. When the 2,000F temperature is reached, the water molecules drive off. Anhydrous alumina crystals leave the kiln and then go through a cooler.

These aluminium hydrate crystals then move on to the Hall-Heroult process, the next step for aluminium extraction.

The Hall-Heroult Process

The process of smelting alumina into the metallic form of aluminium is done in a reduction pot. The pot bottom is set with carbon serving as an electrode for conducting current or electricity within the system. Opposite electrodes are set in the form of carbon rods hanging over the pot. Slowly, the electrodes are lowered in this solution, being held at almost 1.5 inches over the molten aluminium surface accumulating on the pot floor.

Reduction pots are set in rows with around 50 to 200 pots connected in a series form to run electric current. Each of the potlines makes 66,000 to 110,000 tons of aluminium in a year. Two or three potlines are present, usually in a smelting plant. Here is the process covered below.

1. Inside the reduction pot, the alumina crystals dissolve inside molten cryolite. The temperature is kept between 1,760 to 1,780F forming an electrolyte solution. This solution conducts electricity between the carbon rods and the carbon-lined pot bed. A 4 to 6 volts current is passed through this solution. In these conditions, the bonds between oxygen atoms and aluminium are broken down, resulting in alumina molecules. As the oxygen molecules move to the carbon rods and form carbon dioxide. At the bottom of the pot, the aluminium atoms set to free settle down.

This aluminium smelting process is continuous, where more alumina is added to this cryolite solution. It takes the place of the decomposed compound. The electric current is also run constantly and maintained. The electricity flow generates heat that helps to keep the pot contents in liquid form. The crust, however, is broken occasionally, so more alumina can be added. At the bottom, pure aluminium gathers in molten form. It is then siphoned. The pots run continuously throughout, all time, throughout the week.

2. Next, a crucible is used to gather aluminium from the bottom of the pot. In the end, around 9,000 lbs. of 99.8% pure molten aluminium are collected. This metal is then carried into a holding furnace and poured into moulds. One of the most common aluminium moulds is the horizontal, long one. As the hot aluminium moves through, cool water is run at the exterior end to solidify the metal. At the very end, solid shafts of aluminium come out that are then cut to the right ingots of the desired size. The common weight of aluminium ingots is between 6kgs to around 30 kg tons. The aluminium casting process also remains continuous.

After the completion of the entire process above, solid blocks of aluminium of varied sizes are taken off to different shops and markets where they are used for further manufacturing purposes. Aluminium is available in an impressive alloy variety, with over 100 different ones being used in the industry today.

Frequently Asked Questions – FAQs

• Why is the Extraction of Aluminum Difficult?

Pure aluminium oxide melts at 2050 °C only, which results in the loss of a large amount of aluminium during this process.

• What is the Best Extraction Method for Aluminium?

It is commonly extracted by bauxite ore, and then electrolysis takes place for its extraction. The process completes in two steps: The extraction of aluminium from alumina and then the electrolysis process.

• Why Can’t Aluminium be Extracted by Carbon?

There is a reactivity series. It is more reactive than carbon which makes it difficult to be extracted by carbon.

• What Impurities Are Present in Aluminium Ore?

Bauxite contains impurities like iron oxide, silicon dioxide, and titanium oxides as impurities.

• Will Aluminium Rust?

Aluminium doesn’t corrode; it is rust-free because of aluminium oxide coating, which is highly resistant.

• Can Aluminium Be Extracted by Smelting?

Due to its high affinity for oxygen, aluminium can’t be extracted by the smelting process.

Conclusion

Aluminium is one of the most abundant metals in the earth’s crust. It is widely used in countless places, from the kitchen to big industries like chemical, electrical, tech, construction and transportation industries. Aluminium is extracted from its ore called bauxite. It is then separated during the electrolysis process. Bayer’s Process and Hall Heroult Processes are used in this regard. We have articulated everything you need to know in detail.