How to obtain aluminum from alumina?

Despite the fact that aluminum is the most common metal on our planet, it cannot be found in its pure form on Earth. Due to their high chemical activity, aluminum atoms easily form compounds with other substances. However, “winged metal” cannot be obtained by smelting ore in a furnace, as happens, for example, with iron. The process of producing aluminum is much more complicated and is based on the use of enormous power electricity. Therefore, aluminum smelters are always built near large sources of electricity – most often hydroelectric power plants that do not pollute the environment. But first things first. “In nature, nothing arises instantly and nothing appears in the world in a completely ready-made form.” Alexander Herzen
Russian publicist, writer Bauxite mining Metal production is divided into three main stages: the extraction of bauxite – an aluminum-containing ore, its processing into alumina – aluminum oxide, and, finally, the production of pure metal using the process of electrolysis – the breakdown of aluminum oxide into its component parts under the influence of an electric current. From 4-5 tons of bauxite, 2 tons of alumina are obtained, from which 1 ton of aluminum is produced. There are several types of aluminum ores in the world, but bauxite is the main raw material for the production of this metal. It is a rock consisting mainly of aluminum oxide mixed with other minerals. Bauxite is considered high quality if it contains more than 50% aluminum oxide. Bauxite reserves
The world’s total proven reserves of bauxite are estimated at 18,6 billion tons. At the current level of production, this provides the need for aluminum for more than a hundred years. Bauxite can vary greatly from one another. Their structure is either hard and dense or loose and crumbly. In color – as a rule, brick-red, reddish or brown due to the admixture of iron oxide. With a low iron content, bauxite is white or gray in color. But sometimes there are ores of yellow, dark green and even variegated ones – with blue, red-violet or black veins. About 90% of the world’s bauxite reserves are concentrated in countries of the tropical and subtropical zones – of which 73% are in five countries: Guinea, Brazil, Jamaica, Australia and India. Guinea has the most bauxite – 5,3 billion tons (28,4%), while it is of high quality, contains a minimum amount of impurities and lies almost on the surface. Largest bauxite mining countries, 2014 Most often, bauxite is mined using open-pit mining – using special equipment, the ore is “cut off” layer by layer from the surface of the earth and transported for further processing. However, there are places in the world where aluminum ore lies very deep, and mines have to be built to extract it – one of the deepest mines in the world, “Cheremukhovskaya-Glubokaya”, is located in Russia, in the Urals, its depth is 1550 meters. Alumina production The next stage in the production chain is the processing of bauxite into alumina – this is aluminum oxide Al₂O₃, which is a white friable powder. The main method of producing alumina in the world is the Bayer method, discovered more than a hundred years ago, but still relevant today – about 90% of alumina in the world is produced this way. This method is very economical, but it can only be used when processing high-quality bauxite with a relatively low content of impurities – primarily silica. Bayer’s method is based on the following: crystalline aluminum hydroxide, which is part of bauxite, dissolves well at high temperatures in a solution of caustic soda (caustic alkali, NaOH) of high concentration, and when the temperature and concentration of the solution decrease, it crystallizes again. The foreign substances included in the composition of bauxite (the so-called ballast) do not pass into a soluble form or recrystallize and precipitate before crystallization of aluminum hydroxide occurs. Therefore, after dissolving aluminum hydroxide, the ballast can be easily separated – it is called red mud. Red mud This is a thick mass of red-brown color, consisting of compounds of silicon, iron, titanium and other elements. It is stored in carefully isolated areas – sludge storage facilities. They are equipped in such a way that the alkalis contained in the waste do not penetrate into groundwater. Once the storage facility has reached its potential, the area can be returned to its original state by covering it with sand, ash or turf and planting certain types of trees and grasses. Full restoration may take years, but eventually the area returns to its original state. Many experts do not consider red mud a waste, since it can serve as a raw material for processing. For example, scandium is extracted from it for the further production of aluminum-scandium alloys. Scandium gives such an alloy special strength; areas of use include automobile and rocket science, sports equipment, and the production of electrical wires. Red mud can also be used for the production of cast iron, concrete, and rare earth metals. Large particles of aluminum hydroxide are easily separated from the solution by filtration, they are washed with water, dried and calcined – that is, heated to remove water. This is how alumina is obtained. Нефелин
Bauxite is the most common, but not the only raw material for alumina production. It can also be obtained from nepheline. In nature, it occurs in the form of apatite-nepheline rocks (apatite is a material from the group of calcium phosphate salts). In the process of producing alumina from nepheline, soda, potash (used in the construction sector, the production of household chemicals, the confectionery industry, etc.), and the rare metal gallium are also obtained. And from production waste – white sludge – high-quality cement. To obtain 1 ton of alumina on average, 4 tons of nepheline and 7,5 tons of limestone are required. Alumina has no shelf life, but it is not easy to store, since at the slightest opportunity it actively absorbs moisture – so manufacturers prefer to send it to aluminum production as quickly as possible. First, the alumina is stacked in stacks weighing up to 30 thousand tons – a kind of layer cake up to 10-12 meters high is obtained. Then the pie is “sliced” and loaded to be sent to railway cars – on average, one car weighs from 60 to 75 tons (depending on the type of car itself). There is another, much less common way to obtain alumina – the sintering method. Its essence is to obtain solid materials from powdered ones at elevated temperatures. Bauxite is sintered with soda and limestone – they bind silica into water-insoluble silicates, which are easily separated from alumina. This method is more expensive than the Bayer method, but at the same time it makes it possible to process bauxite with a high content of harmful silica impurities. Ivittuut
One of the only deposits of natural cryolite on Earth. Located in Greenland and was discovered in 1799. Mining of cryolite ceased there in 1987, when a method of artificially producing this rare mineral was invented. Later, cryolite was found in the Ilmen Mountains in the Southern Urals (Miass) and in the state of Colorado (USA). Alumina acts as a direct source of metal in the aluminum production process. But to create an environment in which this process will occur, one more component is needed – cryolite. This is a rare mineral from the group of natural fluorides with the composition Na3AlF6. It usually forms colorless, white or smoky-gray crystalline clusters with a glassy luster, sometimes almost black or reddish-brown. Cryolite is brittle and melts easily. There are very few natural deposits of this mineral, so artificial cryolite is used in industry. In modern metallurgy, it is obtained by reacting hydrofluoric acid with aluminum hydroxide and soda. Aluminum production So, we mined bauxite, obtained alumina from it, and stocked up on cryolite. Everything is ready for the last stage – aluminum electrolysis. The electrolysis shop is the heart of an aluminum smelter and is not like the shops of other metallurgical enterprises that produce, for example, cast iron or steel. It consists of several rectangular buildings, the length of which often exceeds 1 km. Inside, hundreds of electrolysis baths are installed in rows, connected in series to electricity by massive wires. The constant voltage at the electrodes of each bath is in the range of only 4-6 volts, while the current is 300 kA, 400 kA or more. It is electric current that is the main production force here – there are very few people in this workshop, all processes are mechanized. Current for aluminum production To start the engine, the car battery must provide an electric current of 300-350 A for 30 seconds. That is, 1000 times less than what one electrolyzer needs for constant operation. The process of aluminum electrolysis occurs in each bath. The bath container is filled with molten cryolite, which creates an electrolytic (conducting) environment at a temperature of 950°C. The role of the cathode is played by the bottom of the bath, and the anode is played by coal blocks about 1,5 meters long and 0,5 meters wide, immersed in cryolite; from the outside they look like an impressively sized hammer. Every half hour, using an automatic alumina supply system, a new portion of raw material is loaded into the bath. Under the influence of an electric current, the bond between aluminum and oxygen is broken – aluminum is deposited at the bottom of the bath, forming a layer of 10-15 cm, and oxygen combines with carbon, which is part of the anode blocks, and forms carbon dioxide. Approximately once every 2-4 days, aluminum is removed from the bath using vacuum ladles. A hole is punched into the crust of electrolyte frozen on the surface of the bath, into which the pipe is lowered. Liquid aluminum is sucked through it into a ladle, from which the air is previously pumped out. On average, about 1 ton of metal is pumped out from one bath, and about 4 tons of molten aluminum are placed in one ladle. Next, this ladle is sent to the foundry. Each ton of aluminum produces 280 m000 of gases. Therefore, each electrolyser, regardless of its design, is equipped with a gas collection system that captures the gases released during electrolysis and directs them to the gas purification system. Modern “dry” gas cleaning systems use alumina to capture harmful fluoride compounds. Therefore, before being used to produce aluminum, alumina actually first participates in the purification of gases that were formed earlier in the metal production process. This is such a vicious cycle. The aluminum electrolysis process requires a huge amount of electricity, so it is important to use renewable and non-polluting sources of this energy. Most often, hydroelectric power plants are used for this – they have sufficient power and do not emit emissions into the atmosphere. For example, in Russia, 95% of aluminum production capacity is provided by hydroelectric generation. However, there are places in the world where coal generation still dominates – in particular, in China it accounts for 93% of aluminum production. As a result, to produce 1 ton of aluminum using hydrogen generation, just over 4 tons of carbon dioxide are released into the atmosphere, and when using coal generation, it is five times more – 21,6 tons. Carbon dioxide For comparison, in one sunny day, 1 hectare of forest absorbs 120-280 kg of carbon dioxide from the air and releases 180-200 kg of oxygen. Aluminum is one of the most common chemical elements on Earth. But it is better known as a light metal, which is widely used in industrial production, construction and other areas of life. Aluminum and its alloys are valued for their strength, lightness, good thermal conductivity, corrosion resistance, and ease of processing. However, the silver-white metal is almost impossible to find in a “finished” form. In this article we will talk about the production of aluminum – from origin to remelting.

Origin

A significant amount of the world’s aluminum reserves are contained in the earth’s crust, being part of many compounds – such as corundum, boehmite, kyanite, anorthite, albite, beryl, etc. There are about 25 compounds in total. Metal particles are also present in fresh and sea waters, although in quite a small amount. It is impossible to extract aluminum by smelting the ore, as is usually done with iron – more complex processing methods are required. The countries that produce the most aluminum are China, USA, Russia, Australia, Brazil.

Production technology

Aluminum production is carried out in several stages. Let’s look at each of them in detail.

Bauxite mining

There are different types of aluminum ores, but the metal is extracted from bauxite. Bauxite is a rock containing aluminum oxide (as well as other minerals). Rocks that contain more than half of aluminum oxide are considered to be of high quality. Bauxites differ in structure – they are dense or loose. They may also differ in color – the shade of the rock depends on the amount of iron impurities. The more iron in the composition, the more rusty the bauxite has. But there are breeds that are white or gray, as well as various other colors – dark green, yellow, variegated and even multi-colored. Most of the world’s bauxite is located near the equator. Among the leading countries in rock deposits are Brazil, Jamaica, Guinea, India, and Australia. The largest amount of bauxite is found in Guinea – 28,4% of the world’s reserves. They are taken out of the earth’s crust by cutting off the ore layer by layer, and then these layers are transported for further processing. But bauxite does not always lie close to the surface – sometimes mines have to be set up to extract it. A striking example is the Cheremukhovskaya-Glubokaya mine, which is located in the Sverdlovsk region of the Russian Federation. Its depth is more than 1,5 kilometers.

Alumina production

• crushing – bauxite is split and ground to reveal the grains of aluminum in its composition;
• leaching – ground oxide is dissolved in concentrated alkali, adding lime;
• decomposition – after hydrolysis and crystallization, sodium aluminate is precipitated.

The ballast remaining after processing is called red mud. This is a dense mass consisting of silicon, ferrous and other compounds. Red mud is also actively used in other industries – for example, for the production of concrete or cast iron.

The second, less popular method of producing alumina is sintering. It is more expensive than the Bayer method, but can be used to work with rock containing a high percentage of impurities.

Electrolysis

Next, the composition obtained after processing is placed in a bath with molten cryolite, which creates a conductive environment, and then an electric discharge with a power of over 400 kiloamperes is passed through the solution. Aluminum is freed from oxygen and distributed in a thin layer at the bottom of the container – it is collected in liquid form by vacuum ladles and sent to the foundry for further processing.

Obtaining refined aluminum

Often, metal obtained by electrolysis contains various impurities, such as silicon, zinc, carbon, nitrogen, hydrogen, etc. They negatively affect the properties of aluminum. Therefore, the metal is additionally subjected to refining – purification from impurities. This is done in two ways:

• chlorination (blowing with a chlorine solution);
• electrolytic refining using a three-layer method.

As a result of these processes, the purity of aluminum increases to 99,5-99,9%.

Calculation of raw materials

Not all raw materials obtained from bauxite are converted into aluminum – quite a large part goes into impurities or losses during production. To produce one kilogram of aluminum, about 1,9 kilograms of alumina are required. This is not an exact number – it can vary depending on the type of equipment used, the quality of the alumina and other factors.

Necessary equipment

Aluminum production is a labor-intensive process that requires high technical equipment of enterprises. The most difficult stage is electrolysis – it requires a lot of electricity. For this reason, aluminum smelters are often built near sources of cheap electricity, such as hydroelectric power plants.

Working with aluminum also requires a lot of different equipment, including:

• alumina dispensing machines;
• cathode busbar;
• gas purification plants;
• assembly and linear cranes;
• electrolyser

Not to mention equipping foundries, which also require a large amount of equipment for further metal processing.

Manufacturing of products

Once aluminum is obtained, it is smelted into different types of ingots. Standard rectangular ingots (slabs) are used for the production of sheet aluminum – sheets, foil, cladding materials, food containers. Cylindrical ingots are used for the production of various types of rolled products by extrusion – this is how pipes, profiles, etc. are produced.

During processing, other substances can be added to the metal – this is how alloys are obtained. There are already more than one hundred types of aluminum alloys. The addition of metals such as manganese, zinc, copper, silicon, etc. affects some properties of aluminum, for example, making it more ductile, corrosion-resistant, or increasing electrical conductivity.

According to official statistics, the largest percentage of aluminum consumption occurs in Asian countries – about 78%. Europe is in second place, with the US and Latin America in third.

Remelting

Due to its properties, aluminum is highly recyclable. It is collected, melted down and put back into production. 90% of aluminum beverage containers are recycled and put back on store shelves. Any aluminum product can be recycled – this allows you to significantly save resources spent on its production.