Aluminium smelting is the process of extracting aluminium from its oxide, alumina, generally by the Hall-Héroult process. Alumina is extracted from the ore bauxite by means of the Bayer process at an alumina refinery.
This is an electrolytic process, so an aluminium smelter uses huge amounts of electric power; smelters tend to be located close to large power stations, often hydro-electric ones, in order to hold down costs and reduce the overall carbon footprint. Smelters are often located near ports, since many smelters use imported alumina.
Layout of an aluminium smelter
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The Hall-Héroult electrolysis process is the major production route for primary aluminium. An electrolytic cell is made of a steel shell with a series of insulating linings of refractory materials. The cell consists of a brick-lined outer steel shell as a container and support. Inside the shell, cathode blocks are cemented together by ramming paste. The top lining is in contact with the molten metal and acts as the cathode. The molten electrolyte is maintained at high temperature inside the cell. The prebaked anode is also made of carbon in the form of large sintered blocks suspended in the electrolyte. A single Soderberg electrode or a number of prebaked carbon blocks are used as anode, while the principal formulation and the fundamental reactions occurring on their surface are the same.
An aluminium smelter consists of a large number of cells (pots) in which the electrolysis takes place. A typical smelter contains anywhere from 300 to 720 pots, each of which produces about a ton of aluminium a day, though the largest proposed smelters are up to five times that capacity. Smelting is run as a batch process, with the aluminium metal deposited at the bottom of the pots and periodically siphoned off. Particularly in Australia these smelters are used to control electrical network demand, and as a result power is supplied to the smelter at a very low price. However power must not be interrupted for more than 4–5 hours, since the pots have to be repaired at significant cost if the liquid metal solidifies.
Principle
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Aluminium is produced by electrolytic reduction of aluminium oxide dissolved in molten cryolite.
Al 3 + + 3 e − ⟶ Al {\displaystyle {\ce {Al^3+ + 3e- -> Al}}}
At the same time the carbon electrode is oxidised, initially to carbon monoxide
C + 1 2 O 2 ⟶ CO {\displaystyle {\ce {C + 1/2O2 -> CO}}}
Although the formation of carbon monoxide (CO) is thermodynamically favoured at the reaction temperature, the presence of considerable overvoltage (difference between reversible and polarization potentials) changes the thermodynamic equilibrium and a mixture of CO and CO2 is produced.[1][2] Thus the idealised overall reactions may be written as
{ Al 2 O 3 + 3 2 C ↽ − − ⇀ 2 Al + 3 2 CO 2 : Δ G ∘ = ( 264460 + 3.75 T log T − 92.52 T ) cal Al 2 O 3 + 3 C ↽ − − ⇀ 2 Al + 3 CO : Δ G ∘ = ( 325660 + 3.75 T log T − 155.07 T ) cal {\displaystyle {\begin{cases}{\ce {Al2O3 + 3/2C <=> 2Al + 3/2CO2}}&:\Delta G\circ =(264460+3.75T\log T-92.52T)\ {\ce {cal}}\\{\ce {Al2O3 + 3C <=> 2Al + 3CO}}&:\Delta G^{\circ }=(325660+3.75T\log T-155.07T)\ {\ce {cal}}\end{cases}}}
By increasing the current density up to 1 A/cm2, the proportion of CO2 increases and carbon consumption decreases.[3][4]
As three electrons are needed to produce each atom of aluminium, the process consumes a large amount of electricity. For this reason aluminium smelters are sited close to sources of inexpensive electricity, such as hydroelectric.
Cell components
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Electrolyte: The electrolyte is a molten bath of cryolite (Na3AlF6) and dissolved alumina. Cryolite is a good solvent for alumina with low melting point, satisfactory viscosity, and low vapour pressure. Its density is also lower than that of liquid aluminium (2 vs 2.3 g/cm3), which allows natural separation of the product from the salt at the bottom of the cell. The cryolite ratio (NaF/AlF3) in pure cryolite is 3, with a melting temperature of 1010 °C, and it forms a eutectic with 11% alumina at 960 °C. In industrial cells the cryolite ratio is kept between 2 and 3 to decrease its melting temperature to 940–980 °C.[5][6]
Cathode: Carbon cathodes are essentially made of anthracite, graphite and petroleum coke, which are calcined at around 1200 °C and crushed and sieved prior to being used in cathode manufacturing. Aggregates are mixed with coal-tar pitch, formed, and baked. Carbon purity is not as stringent as for anode, because metal contamination from cathode is not significant. Carbon cathode must have adequate strength, good electrical conductivity and high resistance to wear and sodium penetration. Anthracite cathodes have higher wear resistance[7] and slower creep with lower amplitude [15] than graphitic and graphitized petroleum coke cathodes. Instead, dense cathodes with more graphitic order have higher electrical conductivity, lower energy consumption [14], and lower swelling due to sodium penetration.[8] Swelling results in early and non-uniform deterioration of cathode blocks.
Anode: Carbon anodes have a specific situation in aluminium smelting and depending on the type of anode, aluminium smelting is divided in two different technologies; “Soderberg” and “prebaked” anodes. Anodes are also made of petroleum coke, mixed with coal-tar-pitch, followed by forming and baking at elevated temperatures. The quality of anode affects technological, economical and environmental aspects of aluminium production. Energy efficiency is related to the nature of anode materials, as well as the porosity of baked anodes. Around 10% of cell power is consumed to overcome the electrical resistance of prebaked anode (50–60 μΩm).[5] Carbon is consumed more than theoretical value due to a low current efficiency and non-electrolytic consumption. Inhomogeneous anode quality due to the variation in raw materials and production parameters also affects its performance and the cell stability.
Prebaked consumable carbon anodes are divided into graphitized and coke types. For manufacturing of the graphitized anodes, anthracite and petroleum coke are calcined and classified. They are then mixed with coal-tar pitch and pressed. The pressed green anode is then baked at 1200 °C and graphitized. Coke anodes are made of calcined petroleum coke, recycled anode butts, and coal-tar pitch (binder). The anodes are manufactured by mixing aggregates with coal tar pitch to form a paste with a doughy consistency. This material is most often vibro-compacted but in some plants pressed. The green anode is then sintered at 1100–1200 °C for 300–400 hours, without graphitization, to increase its strength through decomposition and carbonization of the binder. Higher baking temperatures increase the mechanical properties and thermal conductivity, and decrease the air and CO2 reactivity.[9] The specific electrical resistance of the coke-type anodes is higher than that of the graphitized ones, but they have higher compressive strength and lower porosity.[10]
Soderberg electrodes (in-situ baking), used for the first time in 1923 in Norway, are composed of a steel shell and a carbonaceous mass which is baked by the heat being escaped from the electrolysis cell. Soderberg Carbon-based materials such as coke and anthracite are crushed, heat-treated, and classified. These aggregates are mixed with pitch or oil as binder, briquetted and loaded into the shell. Temperature increases bottom to the top of the column and in-situ baking takes place as the anode is lowered into the bath. Significant amount of hydrocarbons are emitted during baking which is a disadvantage of this type of electrodes. Most of the modern smelters use prebaked anodes since the process control is easier and a slightly better energy efficiency is achieved, compared to Soderberg anodes.
Environmental issues of aluminium smelters
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The process produces a quantity of fluoride waste: perfluorocarbons and hydrogen fluoride as gases, and sodium and aluminium fluorides and unused cryolite as particulates. This can be as small as 0.5 kg per tonne of aluminium in the best plants in 2007, up to 4 kg per tonne of aluminium in older designs in 1974. Unless carefully controlled, hydrogen fluorides tend to be very toxic to vegetation around the plants.
The Soderberg process which bakes the Anthracite/pitch mix as the anode is consumed, produces significant emissions of polycyclic aromatic hydrocarbons as the pitch is consumed in the smelter.
The linings of the pots end up contaminated with cyanide-forming materials; Alcoa has a process for converting spent linings into aluminium fluoride for reuse and synthetic sand usable for building purposes and inert waste.
Inert anodes
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Inert anodes are non-carbon based alternatives to traditional anodes used during aluminum reduction. These anodes do not chemically react with the electrolyte, and are therefore not consumed during the reduction process. Because the anode does not contain carbon, carbon dioxide is not produced.[11] Through a review of literature, Haradlsson et al. found that inert anodes reduced the green house gas emissions of the aluminum smelting process by approximately 2 tonnes CO2eq/ tonne Al.[12]
Types of anodes
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Ceramic anode materials include Ni-Fe, Sn, and Ni-Li based oxides.[13] These anodes show promise as they are extremely stable during the reduction process at normal operating temperatures (~1000 °C), ensuring that the Al is not contaminated. The stability of these anodes also allows them to be used with a range of electrolytes. However, ceramic anodes suffer from poor electrical conductivity and low mechanical strength.[13]
Alternatively metal anodes boast high mechanical strength and conductivity but tend to corrode easily during the reduction process. Some material systems that are used in inert metal anodes include Al-Cu, Ni-Cu, and Fe-Ni-Cu systems.[13] Additional additives such as Sn, Ag, V, Nb, Ir, Ru can be included in these systems to form non reactive oxides on the anode surface, but this significantly increases the cost and embodied energy of the anode.
Cermet anodes are the combination of a metal and ceramic anode, and aim to take advantage of the desirable properties of both; the electrical cpnductivity and toughness of the metal and stability of the ceramic.[13] These anodes often consist of a combination of the above metal and ceramic materials. In industry, Alcoa and Rio Tinto have formed a joint venture, Elysis, to commercialize inert anode technology developed by Alcoa.[14] The inert anode is a cermet material, a metallic dispersion of copper alloy in a ceramic matrix of nickel ferrite.[15] Unfortunately, as the number of anode components increases , the structure of the anode becomes more unstable. As a result. cermet anodes also suffer from corosion issues during reduction.[16]
Energy use
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Aluminium smelting is highly energy intensive, and in some countries is economical only if there are inexpensive sources of electricity.[17][18] In some countries, smelters are given exemptions to energy policy like renewable energy targets.[19][20]
To reduce the energy cost of the smelting process, alternative electrolytes such as Na3AlF6 are being investigated that can operate at a lower temperature.[21] However, changing the electrolyte changes the kinetics of the liberated oxygen from the Al2O3 ore. This change in bubble formation can alter the rate the anode reacts with Oxygen or the electrolyte and effectively change the efficiency of the reduction process.[22]
Inert anodes, used in tandem with vertical electrode cells, can also reduce the energy cost of aluminum reduction up to 30% by lowering the voltage needed for reduction to occur.[13] Applying these two technologies at the same times allows the anode-cathode distance to be minimized which decreases restive losses.
Example aluminium smelters
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See also
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References
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The steady growth of the global aluminium industry continues to be fuelled by the world’s top 10 largest aluminium companies and the top 10 leading aluminium producing countries in 2020. The rising demand of aluminium is mainly posed by the boom of the downstream industries such as automotive, packaging, electrical applications, construction, and consumer products etc. As the consumption of aluminium in various of industries around the world still remains high, those world’s largest aluminium manufacturers are expected to increase their revenues, while the business opportunities in the global aluminium market are projected to flourish further in the near future.
This industry fact sheet is to assist industry professionals, investors, and people in general that are interested in gaining insights into the global aluminium industry including:
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Due to its advantages of light weight and resistance to corrosion, aluminium is one of the world’s most important and widely used non-ferrous metal materials in various of industrial, commercial, and domestic applications. Thus, the production and consumption of aluminium around the world also reflects the economy performance.
According to the latest data from International Aluminium Institute, the global primary aluminium production amounted 63.7 million metric tonnes in 2019, slightly reduced from 64.3 million metric tonnes in 2018. Another recent market report from Allied Market Research also shows that the global aluminium market size was valued at $147.2 billion in 2018, and is expected to reach $189.8 billion by 2026, registering a strong CAGR of 3.2% during the forecast period.
The growth of the global aluminium market is mainly driven by the rapid development of downstream industries including transportation, construction, electrical engineering, packaging and consumer goods sectors etc. Meanwhile, technological advancements in aluminium manufacturing as well as aluminium recycling also play an important role in propelling the growth of the global aluminium market in the near future.
Geographically, Asia-Pacific, led by China and India, is the largest aluminium markets in terms of production and consumption, followed by followed by Europe and North America. Asia-Pacific’s dominant position in the global aluminium industry mainly attributed to its massive urbanization, increasing income, and rapid industrial development in the region.
Source: USGS
China remains on the No.1 spot on the chart of the world’s top 10 largest aluminium producing countries in 2020. China’s production of aluminium has surged from around 16 million metric tonnes in 2010 to a whopping 36 million metric tonnes in 2019, mainly driven by its booming downstream sectors, particularly the automotive industry and construction industry. In recent years, the Chinese government has been targeting a capacity reduction of up to 30% of the country’s full aluminium output capacity since 2017 as part of nation’s “supply-side reform” to tackle chronic overproduction and environment pollution. Despite the significantly cut on its aluminium production capacity, China is still the world’s largest aluminium producer in 2020, accounting for over 55.6% of the world’s total aluminium production.
Other world’s top 10 largest aluminium producing countries in 2020 include India, Russia, Canada, UAE, Australia, Bahrain, Norway, the U.S., and Iceland. Together, these top 10 largest aluminium producers account for over 85.6% of the world’s total aluminium production volume every year, and their leading position in the global market is expected to continue in the coming years.
Aluminum Corporation of China (Chalco) is the world’s largest alumina producer and largest aluminium company by production volume, with operations in bauxite and coal mining, alumina refining and primary aluminium smelting. Headquartered in Beijing, China, the company is also involved in producing and selling aluminium alloy products, carbon products, and other electrolytic aluminium products. Chalco also has business operations in other segments such as energy, trading, corporate and others.
China Hongqiao Group Limited is a leading Chinese aluminium producer based in Shangdong. The Group is mainly engaged in the production and sales of molten aluminium alloy, aluminium alloy ingots, aluminium alloy casting-rolling products and aluminium busbars. Today, it is one of the top 10 largest aluminium companies in the world.
Rusal is a leading Russian aluminium producer based in Moscow. The company is engaged in the production and sale of primary aluminium and other related products. The company is involved in mining and refining of bauxite into alumina. The company is currently one of the world’s largest aluminium companies by production, and it has 10 aluminium smelters, of which nine are in Russia and one in Sweden.
Xinfa Group is another world’s largest aluminium manufacturer based in Guangdong, China. The Company mainly produces and processes electrolytic aluminium and other related aluminium products. Xingfa Aluminium is one of the oldest aluminium extrusion producers in Chinese mainland and has now become a large-scale enterprise specializing in the production of construction and industrial aluminium profiles.
Rio Tinto’s aluminium division is a subsidiary of the world’s leading mining company Rio Tinto, based in Montreal. Rio Tinto operates through several mining and metal manufacturing businesses, including iron ore, aluminium, copper and diamonds, energy and minerals, and other operations. The company has 24 production sites across the globe. Products of the company include sheet ingot, extrusion billet, forging stock, and others.
Alcoa is one of the world’s leaders in the production of Bauxite, Alumina and Aluminium products, with corporate headquarters in Pittsburgh, Pennsylvania. Alcoa conducts business across 10 countries and has facilities located in Australia, Guinea, Suriname, Texas, Sao Luis, Brazil, and Spain. Alcoa has the largest bauxite mining operations in the world, a leading alumina refining system, as well as a global network of aluminium smelting plants.
Emirates Global Aluminium is the world’s leading aluminium producer based in the UAE. The company produces high-quality aluminium products in three main categories including primary aluminium, bauxite, and water. Its product portfolio comprises of high-purity and foundry re-melt products, rolling products, and billets for extrusion and forging.
Headquartered in Norway, Hydro is one of the world’s leading company in areas of Bauxite & Alumina, Primary Metal, Rolled Products, Extruded Solutions, and Energy. Hydro is engaged within all market segments for aluminium, from production of the primary metal to recycling and repurposing it. Today, Hydro is a fully integrated aluminium company with 35000 employees with operations in 40 countries.
East Hope Group Co. Ltd is one of the largest investment companies in China. The Company invests in electricity, aluminium, non-ferrous metals, bio-chemicals, and other industries. It owns and operates several largest aluminium smelters in China and abroad, making it one of the largest aluminium companies in the world.
China Power Investment Corporation, known as CPI, is one of largest state-owned electricity producers and investment companies in China. Headquartered in Beijing, China, CPI has huge capacities in electricity, coal production, aluminium smelting, alumina refinery, and bauxite production. It is now one of the world’s top 10 aluminium producers.
The growing demand and usage for recycled aluminium have become one of the major trends in the global aluminium market. Aluminium is also well-known for being a widely recycled metal, with over 75% of all produced aluminium still in circulation and being reused around the world. Meanwhile, the production process of secondary aluminium is more energy-efficient than the primary production process which further increases its significance in end-user industries. Growing demand and increasing use of recycled aluminium products are expected to create more lucrative opportunities in the global market for the near future.
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