Aluminium Smelter
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Aluminium smelting is the process of extracting
aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
from its oxide,
alumina Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula . It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. It is commonly ...
, generally by the Hall-Héroult process. Alumina is extracted from the ore
bauxite Bauxite () is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite (), boehmite (γ-AlO(OH)), and diaspore (α-AlO(OH) ...
by means of the
Bayer process The Bayer process is the principal industrial means of refining bauxite to produce alumina (aluminium oxide) and was developed by Carl Josef Bayer. Bauxite, the most important ore of aluminium, contains only 30–60% aluminium oxide (Al2O3), the ...
at an alumina refinery. This is an
electrolytic An electrolyte is a substance that conducts electricity through the movement of ions, but not through the movement of electrons. This includes most soluble salts, acids, and bases, dissolved in a polar solvent like water. Upon dissolving, the ...
process, so an aluminium
smelter Smelting is a process of applying heat and a chemical reducing agent to an ore to extract a desired base metal product. It is a form of extractive metallurgy that is used to obtain many metals such as iron, copper, silver, tin, lead and zin ...
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 A carbon footprint (or greenhouse gas footprint) is a calculated value or index that makes it possible to compare the total amount of greenhouse gases that an activity, product, company or country Greenhouse gas emissions, adds to the atmospher ...
. Smelters are often located near ports, since many smelters use imported alumina.


Layout of an aluminium smelter

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 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

Aluminium is produced by electrolytic reduction of aluminium oxide dissolved in molten
cryolite Cryolite ( Na3 Al F6, sodium hexafluoroaluminate) is a rare mineral identified with the once-large deposit at Ivittuut on the west coast of Greenland, mined commercially until 1987. It is used in the reduction ("smelting") of aluminium, in pest ...
. :Al^3+ + 3e- -> Al At the same time the carbon electrode is oxidised, initially to carbon monoxide :C + 1/2O2 -> CO Although the formation of
carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...
(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 is produced. Thus the idealised overall reactions may be written as : \begin \ce &: \Delta G\circ = (264460 + 3.75 T\log T - 92.52 T)\ \ce \\ \ce &: \Delta G^\circ = (325660 + 3.75 T\log T - 155.07 T)\ \ce \end By increasing the current density up to 1 A/cm2, the proportion of increases and carbon consumption decreases. 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 Hydroelectricity, or hydroelectric power, is Electricity generation, electricity generated from hydropower (water power). Hydropower supplies 15% of the world's electricity, almost 4,210 TWh in 2023, which is more than all other Renewable energ ...
.


Cell components

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.F. Habashi, "Extractive metallurgy of aluminum," in Handbook of Aluminum: Volume 2: Alloy production and materials manufacturing. vol. 2, G. E. Totten and D. S. MacKenzie, Eds., First ed: Marcel Dekker, 2003, pp. 1–45 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 and slower creep with lower amplitude 5than graphitic and graphitized petroleum coke cathodes. Instead, dense cathodes with more graphitic order have higher electrical conductivity, lower energy consumption 4 and lower swelling due to sodium penetration. 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 Energy efficiency may refer to: * Energy efficiency (physics), the ratio between the useful output and input of an energy conversion process ** Electrical efficiency, useful power output per electrical power consumed ** Mechanical efficiency, a rat ...
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). 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 Sintering or frittage is the process of compacting and forming a solid mass of material by pressure or heat without melting it to the point of liquefaction. Sintering happens as part of a manufacturing process used with metals, ceramics, pla ...
at 1100–1200 °C for 300–400 hours, without
graphitization Graphitization is a process of transforming a carbonaceous material, such as coal or the carbon in certain forms of iron alloys, into graphite. Process The graphitization process involves a restructuring of the molecular structure of the carbon ...
, 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. 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. 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

The process produces a quantity of
fluoride Fluoride (). According to this source, is a possible pronunciation in British English. is an Inorganic chemistry, inorganic, Monatomic ion, monatomic Ion#Anions and cations, anion of fluorine, with the chemical formula (also written ), whose ...
waste:
perfluorocarbons Fluorocarbons are chemical compounds with carbon-fluorine bonds. Compounds that contain many C-F bonds often have distinctive properties, e.g., enhanced stability, volatility, and hydrophobicity. Several fluorocarbons and their derivatives are ...
and
hydrogen fluoride Hydrogen fluoride (fluorane) is an Inorganic chemistry, inorganic compound with chemical formula . It is a very poisonous, colorless gas or liquid that dissolves in water to yield hydrofluoric acid. It is the principal industrial source of fluori ...
as gases, and
sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...
and
aluminium fluoride Aluminium fluoride is an inorganic compound with the formula . It forms hydrates . Anhydrous and its hydrates are all colorless solids. Anhydrous is used in the production of aluminium. Several occur as minerals. Occurrence and production Aside ...
s and unused
cryolite Cryolite ( Na3 Al F6, sodium hexafluoroaluminate) is a rare mineral identified with the once-large deposit at Ivittuut on the west coast of Greenland, mined commercially until 1987. It is used in the reduction ("smelting") of aluminium, in pest ...
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 hydrocarbon A Polycyclic aromatic hydrocarbon (PAH) is any member of a class of organic compounds that is composed of multiple fused aromatic rings. Most are produced by the incomplete combustion of organic matter— by engine exhaust fumes, tobacco, incine ...
s as the pitch is consumed in the smelter. The linings of the pots end up contaminated with cyanide-forming materials;
Alcoa Alcoa Corporation (an acronym for "Aluminum Company of America") is an American industrial corporation. It is the world's eighth-largest producer of aluminum. Alcoa conducts operations in 10 countries. Alcoa is a major producer of primary alu ...
has a process for converting spent linings into aluminium fluoride for reuse and synthetic sand usable for building purposes and inert waste.


Inert anodes

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. 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.


Types of anodes

Ceramic anode materials include Ni-Fe, Sn, and Ni-Li based oxides. 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. 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. 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 conductivity and toughness of the metal and stability of the ceramic. These anodes often consist of a combination of the above metal and ceramic materials. In industry,
Alcoa Alcoa Corporation (an acronym for "Aluminum Company of America") is an American industrial corporation. It is the world's eighth-largest producer of aluminum. Alcoa conducts operations in 10 countries. Alcoa is a major producer of primary alu ...
and Rio Tinto have formed a joint venture, Elysis, to commercialize inert anode technology developed by Alcoa. The inert anode is a
cermet A cermet is a composite material composed of ceramic and metal materials. A cermet can combine attractive properties of both a ceramic, such as high temperature resistance and hardness, and those of a metal, such as the ability to undergo pla ...
material, a metallic dispersion of copper alloy in a ceramic matrix of nickel ferrite. Unfortunately, as the number of anode components increases , the structure of the anode becomes more unstable. As a result. cermet anodes also suffer from corrosion issues during reduction.


Energy use

Aluminium smelting is highly energy intensive, and in some countries is economical only if there are inexpensive sources of electricity. In some countries, smelters are given exemptions to energy policy like renewable energy targets. To reduce the energy cost of the smelting process, alternative electrolytes such as Na3AlF6 are being investigated that can operate at a lower temperature. 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. 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. Applying these two technologies at the same times allows the anode-cathode distance to be minimized which decreases resistive losses.


Example aluminium smelters

*
Alcan Lynemouth Aluminium Smelter The Alcan Lynemouth Aluminium Smelter was an industrial facility near Ashington, Northumberland, Ashington, Northumberland, on the coast of North East England, south of the village of Lynemouth. The smelter was owned by the Canadian aluminium c ...
, powered by the coal-fired Lynemouth Power Station in
North East England North East England, commonly referred to simply as the North East within England, is one of nine official regions of England. It consists of County DurhamNorthumberland, , Northumberland, Tyne and Wear and part of northern North Yorkshire. ...
, ceased production in 2012, demolished in 2018. *
Anglesey Aluminium Anglesey Aluminium Metal Ltd. was a joint venture between Rio Tinto and Kaiser Aluminum. Its aluminium smelter, located on the outskirts of Holyhead, was one of the largest employers in North Wales, with 540 staff members, and began to produce ...
, powered by Wylfa nuclear power station in north-west
Wales Wales ( ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by the Irish Sea to the north and west, England to the England–Wales border, east, the Bristol Channel to the south, and the Celtic ...
, closed in 2013, with redevelopment of the site announced in 2022. * The Valco aluminium smelter in
Ghana Ghana, officially the Republic of Ghana, is a country in West Africa. It is situated along the Gulf of Guinea and the Atlantic Ocean to the south, and shares borders with Côte d’Ivoire to the west, Burkina Faso to the north, and Togo to t ...
, powered by the Akosombo Hydroelectric Project * Fjarðaál in
Iceland Iceland is a Nordic countries, Nordic island country between the Atlantic Ocean, North Atlantic and Arctic Oceans, on the Mid-Atlantic Ridge between North America and Europe. It is culturally and politically linked with Europe and is the regi ...
, powered by the
Kárahnjúkar Hydropower Plant Kárahnjúkar Hydropower Plant ( ), officially called Fljótsdalur Power Station ( ) is a hydroelectric power plant in Fljótsdalshérað municipality in eastern Iceland, designed to produce annually for Alcoa's Fjarðaál Aluminium smelting, alu ...
* Jharsuguda in
Orissa, India Odisha (), formerly Orissa ( the official name until 2011), is a state located in Eastern India. It is the eighth-largest state by area, and the eleventh-largest by population, with over 41 million inhabitants. The state also has the thi ...
, to be powered by its own coal-fired power station. *
Aluminerie Alouette Aluminerie Alouette is an aluminum manufacturing company based in Sept-Îles, Quebec, Canada, on the North Shore of the Gulf of St. Lawrence. History In 2005, the Alouette Aluminum Smelter, at 550,000 metric tonnes capacity per year, became the ...
in Sept-Îles, Québec, powered by the Churchill Falls Hydro Electric project. * Alba Smelter in
Bahrain Bahrain, officially the Kingdom of Bahrain, is an island country in West Asia. Situated on the Persian Gulf, it comprises a small archipelago of 50 natural islands and an additional 33 artificial islands, centered on Bahrain Island, which mak ...
, powered by its own four power stations with a total generating capacity of .


See also

*
List of aluminium smelters This is a list of primary aluminium smelters in the world. Primary production is the process by which alumina is smelted to pure aluminum. Secondary production is the process of recycling aluminum scrap into aluminum that can be used again. Capa ...
*
List of alumina refineries This is a list of alumina Refining (metallurgy), refineries in the world. The list is incomplete and missing some data. Smelter-grade alumina refineries Specialty alumina plants See also *List of aluminium smelters References Sources

...
*
Lead smelter Plants for the production of lead are generally referred to as lead smelters. Primary lead production begins with sintering. Concentrated lead ore is fed into a sintering machine with iron, silica, limestone fluxes, coke, soda ash, pyrite, zinc, ...
*
Nuclear power Nuclear power is the use of nuclear reactions to produce electricity. Nuclear power can be obtained from nuclear fission, nuclear decay and nuclear fusion reactions. Presently, the vast majority of electricity from nuclear power is produced by ...
*
Zinc smelting Zinc smelting is the process of converting zinc concentrates ( ores that contain zinc) into pure zinc. Zinc smelting has historically been more difficult than the smelting of other metals, e.g. iron, because in contrast, zinc has a low boiling poin ...
* Solid oxide Hall–Héroult process


References

{{Authority control Metallurgical processes
smelting Smelting is a process of applying heat and a chemical reducing agent to an ore to extract a desired base metal product. It is a form of extractive metallurgy that is used to obtain many metals such as iron-making, iron, copper extraction, copper ...
Electrolysis de:Aluminium#Gewinnung