Aluminium production


Even though aluminium is the most common metal on the planet, pure aluminium does not occur naturally. Aluminium atoms easily bind with other metals, forming compounds. At the same time it's impossible to isolate aluminium by simply melting down the compounds in a furnace, as is the case with iron, for example. The aluminium production process is much more complex and requires huge amounts of electricity. For this reason, aluminium smelters are always built in the vicinity of power energy sources, usually hydroelectric power plants that don't contaminate the environment. But let's start at the beginning.
«Nothing in nature comes out of nothing and
nothing comes into existence ready for use

Alexander Gertsen
Russian publicist and writer
Bauxite Mining
The aluminium production process can be broken down into three stages; first bauxites, which contain aluminium, are extracted from the ground. Second, bauxites are processed into alumina or aluminium oxide, and finally in stage three, pure aluminium is produced using electrolytic reduction, a process in which aluminium oxide is broken down into its components using electric current. About 4-5 tonnes of bauxites get processed into 2 tonnes of alumina from which about 1 tonne of aluminium can be made.

There are several minerals available in the world from which aluminium can be obtained, but the most common raw material is bauxite. Bauxite is a mineral made up primarily of aluminium oxide mixed with some other minerals. Bauxite is regarded as high quality if it contains more than 50% of aluminium oxide.
Bauxite supplies
Confirmed global bauxite supplies are estimated at 18.6 billion tonnes. At the current level of extraction, this should last for more than a hundred years.
There is a lot of variation in bauxites. Structurally they can be solid and dense or crumbly. The usual colour is brick red, flaming red or brown because of iron oxide. If iron content is low, bauxite can be grey or white. But yellow, dark green and even multi-coloured bauxites with bluish, purple, red and black strains occur too.

About 90% of global bauxite supplies are found in tropical and subtropical areas, with 73% found in just five countries: Guinea, Brazil, Jamaica, Australia and India. Guinea has the largest supply of bauxites, 5.3 billion tonnes (28.4% of the global supply) and the Guinean bauxites are very high quality, containing minimal amounts of admixtures. They're also found very near to the surface, which makes mining them very easy.
The world's largest bauxite producers, 2014
The most common way to mine for bauxites is by using open pit mines. Special equipment is used to cut one layer after another off the surface, with the rock then being transported elsewhere for further processing. However, there are places where aluminium ore has to be mined from deep underground which require underground mines to be built to get at it. One of the deepest mines is the Cheremkhovskaya-Deep mine in the Urals in Russia, its shafts run 1550 metres deep.
Alumina Production
The next stage in the production chain is the processing of bauxite into alumina, or aluminium oxide - Al2O3, - a white powder. The most common process for making alumina from bauxite is the Bayer process, which was first discovered over 100 years ago but is still in wide use today. About 90% of alumina refineries in the world use the Bayer process. It's very efficient but it can only be used on high quality bauxite with fairly low content of admixtures, especially silicon.

The principle of the Bayer process is as follows: the crystallised aluminium hydrate found in bauxite easily dissolves in concentrated caustic soda (NaOH) at high temperatures and when the temperature is lowered and the concentration of the solution increases again, aluminium hydrate crystallises but the other elements contained in the bauxite (the so called ballast) either don't dissolve or recrystallise and settle to the bottom well before aluminium hydrate crystallises. This means that after aluminium hydrate gets dissolved in caustic soda the ballast can be easily isolated and removed. This ballast is known as red mud.

Red Mud

Red mud is a thick red-brown paste consisting of silicon, iron, titanium and other compounds. It's disposed of in special isolated areas, called mud disposal areas. Mud disposal areas are designed to prevent the seepage of alkali contained in the mud into ground water. Once a mud disposal area has been filled up, the territory can be reclaimed by burying it in sand, ash or dirt and planting certain types of trees and plants there. While full reclamation can take years, in the end the territory will return to its original state.

Many experts don't regard red mud as a waste because it can be used as a raw material. For example, scandium can be made from it and then used in aluminium scandium alloys. Scandium makes aluminium alloys extra strong and such alloys can be used in motor vehicles, rockets, sports equipment and in the production of electric wires.

Red mud can also be used in the production of cast iron, concrete and rare earth metals.

Large aluminium hydrate particles can be filtered out from the solution with relative ease. They're then washed with water, dried and calcined: i.e. heated up to remove water. The output of this process is alumina.
Bauxites are the most common raw materials for making alumina from but they're not the only ones. Alumina can also be made from nepheline. Nepheline occurs in the form of apatite-nepheline rock (apatite is a calcium phosphorous oxide). The production process for making alumina from nepheline also by-produces soda, potash (a material used in construction, production of some chemicals, food industry etc.), and the rare metal gallium. The production waste, white mud, can be used to make high quality cement. It takes 4 tonnes of nepheline and 7.5 tonnes of limestone to make 1 tonne of alumina
Alumina has unlimited shelf life but it has to be stored under the right conditions as it will absorb moisture at the first opportunity, so alumina producers prefer to ship it off to smelters as soon as possible. First alumina is stacked into piles weighing up to 30,000 tonnes. In the end a kind of layered pie 10-12 metres high is built in this manner. The pile is then cut and loaded into railroad cars, 60-75 tonnes per car (depending on the type of car) for dispatch to smelters.

There is another far less common method for making alumina. It's called sintering. The idea is to make solid materials from powders at high temperature. Bauxites are sintered with soda and lime. The latter two elements bind the silica into insoluble silicates that can then be easily separated from alumina. The sintering process is more energy intensive than the Bayer process but it can be used to make alumina from bauxites with a high content of toxic silica admixtures.
One of the few natural deposits of cryolite on Earth is in the town of Ivituuit in Greenland. It was discovered in 1799. Mining for Cryolite stopped there in 1987 when the process for making artificial cryolite was developed. Later cryolite was found in the Ilmen mountains in the Southern Urals (Miass) and in Colorado, US.
Alumina is the direct source of aluminium in the aluminium production process, but in order to create the right environment for electrolysis another component is necessary, and that component is cryolite. It's a rare natural fluoride mineral which due to its scarcity in natural form has been manufactured artificially. In modern metal production, cryolite is made by mixing hydrofluoric acid with aluminium hydroxide and soda.
Aluminium Production
So we've mined bauxite, made alumina from it and stockpiled cryolite, and now everything is ready for the last stage: electrolytic reduction to make aluminium. The reduction area is the heart of an aluminium smelter and it looks very different from the production shops in your typical steel works that make cast iron or steel. The reduction area consists of several rectangular buildings whose length sometimes exceeds 1 kilometre. Inside there are hundreds of reduction cells or pots arranged in rows and hooked up to power sources via massive cables. The constant voltage at the electrodes of each reduction cell varies in the range of between 4 and 6 volts, while the amperage can reach 300, 400 KA and more. It's the electric current that is the main production force in this process. There are only a handful of people in a typical reduction area as all the key processes are automated.
Current for aluminium production
To start a car engine, current of 300-350A is needed for 30 seconds. That's 1000 times less than one reduction cell requires on an ongoing basis.
In each reduction cell, aluminium is produced from alumina via the electrolytic reduction process. The entire cell is filled up with molten cryolite that creates a conductive environment at a temperature of 950oC. The bottom of the cell works as the cathode while the role of the cathode is played by special cryolite-carbon blocks 1.5 metres in length and 0.5 metres in width that are lowered into the cell. These blocks look like massive hammers.

Every thirty minutes an automatic alumina feeding system dumps a new portion of alumina into the cell. The electric current flowing through the cell breaks down the bond between aluminium and oxygen, causing aluminium to settle to the bottom of the cell and form a layer 10-15 cm deep while the oxygen binds with the carbon in the anode blocks to form carbon dioxide.

Two to four times per day, aluminium gets extracted from the cell with special vacuum buckets. A hole is punched in the cryolite crust that forms on the surface of the reduction cell, then a pipe is lowered in through the hole. Through this pipe liquid aluminium is sucked into the bucket, from which all air is pumped out in advance. On average, about 1 tonne of metal is recovered from every reduction cell while a vacuum bucket can hold 4 tonnes of molten aluminium. Once the bucket is full it is taken to the casthouse.
For every tonne of aluminium produced, 280,000 cubic metres of gases are emitted. For this reason, every reduction cell, regardless of its design, is equipped with a gas removal system that catches the gases emitted during the reduction process and directs them into a gas treatment plant. Modern dry gas treatment systems use alumina to filter out toxic fluoride compounds from the gases. So before being used in aluminium production, alumina is first used to treat the gases emitted during the earlier production of aluminium. So it's a closed loop, in a sense.

The aluminium reduction process requires huge amounts of electric power, so it's important to use renewable energy sources that don't contaminate the environment. The most common renewable energy source is a hydroelectric power plant, as they can deliver the required power without contaminating the atmosphere. For example, in Russia 95% of aluminium smelters derive electricity from hydroelectric power plants. However, there are places in the word that are still dominated by coal-fired generation, for example in China, 93% of aluminium production gets its power from coal fired power plants. When hydroelectric power is used just 4 tonnes of carbon dioxide is emitted into the atmosphere per each tonne of aluminium produced, but when coal-fired generation is used, five times as much carbon dioxide is emitted per each tonne of output, or 21.6 tonnes of carbon dioxide.

Carbon dioxide
In one sunny day one hectare of forest consumes 120-280 kg of carbon dioxide from the atmosphere and emits 180-200kg of oxygen.
Molten aluminium istransported in buckets to the casthouse of the smelter. At this stage the metal still contains a lot of iron, silicon, copper and other elements. However, even the smallest amounts of admixtures can have a drastic impact on the properties of aluminium, so in the casthouse all admixtures are removed by remelting the aluminium in a special furnace at 800 oC. The resultant pure aluminium is cast into special moulds where it is allowed to solidify.
The smallest aluminium ingots, often called pigs, weigh between 6 and 22.5 kg. When customers get aluminium delivered to them in pigs they remelt them, add whatever components they need and then recast them in the shape needed for their purposes.

The largest ingots, 30-tonne slabs 11.5 metres in length, are made in special moulds buried up to 13 metres in the ground. Hot aluminium is poured into a mould like this over a period of two hours, with the slab 'growing' in the mould like an icicle, only from the bottom up. As the slab is cast, it is cooled down with water and as soon as the casting process is complete, the slab is ready for shipment. Slabs are usually then rolled into thin sheets that are then used in the manufacture of foil, beverage cans or automobile body panels.

7 metre long aluminium billets are used to make extrusions - this is when the billet is pushed through a hole of the required shape. Extrusion is the process used for making the vast majority of aluminium products.

In the casthouse, not only is aluminium given the required shape but also the required chemical composition. The thing is that pure aluminium is used far less than aluminium alloys.

Aluminium alloys are made by mixing aluminium with various other metals (the so-called alloying elements). Some increase the strength of aluminium, others make it denser, still others change its heat transmission properties, etc. Common alloying elements include boron, iron, silicon, magnesium, manganese, copper, nickel, lead, titanium, chromium, zinc, zirconium, lithium, scandium, silver and others. In addition, aluminium alloys can include dozens of other alloying elements such as strontium, phosphorous and others, so the total number of possible alloys is very impressive. Today over 100 aluminium alloys are used in industry.

New Technologies
Aluminium producers are constantly refining their production processes to maximise quality while minimising costs and the environmental footprint. Reduction cells have already been designed that operate at 400 and 500 KA amperage, older generation reduction cells are being modernised.

One of the most cutting edge technologies aluminium producers are working on today is the inert anode process. It's a unique revolutionary process that can allow aluminium producers to stop using carbon anodes altogether. The inert anode can potentially be used ad infinitum, but most importantly, what the inert anode based reduction process emits into the atmosphere is not carbon dioxide, but pure oxygen. One inert anode call can produce as much oxygen as 70 hectares of forest. Currently, the inert anode process is being developed in secret and undergoing industrial trials but who knows, maybe in the near future it will turn the aluminium industry into the lungs of our planet.

One important property of aluminium is that it preserves its properties after processing, which means aluminium products can be recycled into new products. This helps preserve the colossal amount of energy that has to be used to produce primary aluminium.

The International Institute of Aluminium estimates that since 1880 almost a billion tonnes of aluminium has been produced around the world with three fourths of this amount still being used today. About 35% is used in buildings and structures, 30% in electric cables and equipment and 30% in transport.


Buildings and structures


Electric cables


Aluminium scrap is collected all over the world. In the home, it mostly consists of aluminium beverage cans. It's been estimated that 1 kg of recycled empty beverage cans save 8 kg of bauxite, 4 kg of various fluorides and 14 KWH of electricity. In addition, recycling aluminium significantly reduces the negative environmental impact of ever-expanding landfills. As the idea of environmental responsibility is gaining more and more traction, separate garbage recycling is becoming more and more popular around the world.

Aluminium cans

Aluminium cans are one of the most recycled products in the world. Approximately 6 weeks after being used, aluminium cans reappear on store shelves
Every year, 220 billion beverage cans are manufactured around the world, 90% of these get recycled in Europe and often these cans are recycled and the aluminium obtained from them is used to make new aluminium cans. That's one reason that the aluminium beverage can is often referred to as an eternal product. But anything can be recycled: car parts, used aluminium cooking foil, bicycle frames, you name it, if it's made from aluminium it can be recycled.

Photo: © Shutterstock and © Rusal.