Aluminium smelting is the process of extracting
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
from its oxide,
alumina, 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 (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO ...
by means of the
Bayer process at an
alumina refinery.
This is an
electrolytic
An electrolyte is a medium containing ions that is electrically conducting through the movement of those ions, but not conducting electrons. This includes most soluble salts, acids, and bases dissolved in a polar solvent, such as water. Upon di ...
process, so an aluminium
smelter uses huge amounts of electric power; smelters tend to be located close to large power stations, often
hydro-electric
Hydroelectricity, or hydroelectric power, is electricity generated from hydropower (water power). Hydropower supplies one sixth of the world's electricity, almost 4500 TWh in 2020, which is more than all other renewable sources combined an ...
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
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
Aluminium is produced by
electrolytic reduction of aluminium oxide dissolved in molten
cryolite
Cryolite ( Na3 Al F6, sodium hexafluoroaluminate) is an uncommon mineral identified with the once-large deposit at Ivittuut on the west coast of Greenland, mined commercially until 1987.
History
Cryolite was first described in 1798 by Danish vete ...
.
:
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 colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
(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
:
By increasing the current density up to 1 A/cm
2, 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.
Cell components
Electrolyte: The electrolyte is a molten bath of cryolite (Na
3AlF
6) 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/cm
3), which allows natural separation of the product from the salt at the bottom of the cell. The cryolite ratio (NaF/AlF
3) 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 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 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 CO
2 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 waste:
perfluorocarbons
Fluorocarbons are chemical compounds with carbon-fluorine bonds. Compounds that contain many C-F bonds often has distinctive properties, e.g., enhanced stability, volatility, and hydrophobicity. Fluorocarbons and their derivatives are commerci ...
and
hydrogen fluoride as gases, and
sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
and
aluminium fluoride
Aluminium fluoride refers to inorganic compounds with the formula AlF3·''x''H2O. They are all colorless solids. Anhydrous AlF3 is used in the production of aluminium metal. Several occur as minerals.
Occurrence and production
Aside from anhydr ...
s and unused
cryolite
Cryolite ( Na3 Al F6, sodium hexafluoroaluminate) is an uncommon mineral identified with the once-large deposit at Ivittuut on the west coast of Greenland, mined commercially until 1987.
History
Cryolite was first described in 1798 by Danish vete ...
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 perfluorocarbons gases are strong greenhouse gases with a long lifetime.
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 a class of organic compounds that is composed of multiple aromatic rings. The simplest representative is naphthalene, having two aromatic rings and the three-ring compounds anthracene and phenanthrene. ...
s 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.
There is ongoing research efforts to develop an inert anode to reduce the environmental impacts of aluminium smelting.
Alcoa and
Rio Tinto Rio Tinto, meaning "red river", may refer to:
Businesses
* Rio Tinto (corporation), an Anglo-Australian multinational mining and resources corporation
** Rio Tinto Alcan, based in Canada
** Rio Tinto Borax in America
*** Rio Tinto Borax Mine, ...
have formed a joint venture, Elysis, to commercialize inert anode technology developed by Alcoa. The inert anode is a cermet material, a metallic dispersion of copper alloy in a ceramic matrix of nickel ferrite.
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.
Example aluminium smelters
*
Alcan Lynemouth Aluminium Smelter
The Alcan Lynemouth Aluminium Smelter is a mothballed industrial facility near Ashington, Northumberland, on the coast of North East England, south of the village of Lynemouth. The smelter is owned by the Canadian aluminium company Alcan, whic ...
, powered by the coal-fired
Lynemouth Power Station in
North East England
*
Anglesey Aluminium
Anglesey Aluminium 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 aluminiu ...
, powered by
Wylfa
Wylfa nuclear power station ( cy, Atomfa'r Wylfa) is a Magnox nuclear power station undergoing decommissioning. Wylfa is situated west of Cemaes Bay on the island of Anglesey, off the northwestern coast of Wales. Construction of the two 490MW n ...
nuclear power station in north-west
Wales
Wales ( cy, Cymru ) is a country that is part of the United Kingdom. It is bordered by England to the east, the Irish Sea to the north and west, the Celtic Sea to the south west and the Bristol Channel to the south. It had a population in ...
(shut down as of 30/09/09)
* The Valco aluminium smelter in
Ghana
Ghana (; tw, Gaana, ee, Gana), officially the Republic of Ghana, is a country in West Africa. It abuts the Gulf of Guinea and the Atlantic Ocean to the south, sharing borders with Ivory Coast in the west, Burkina Faso in the north, and To ...
, powered by the
Akosombo Hydroelectric Project
*
Fjarðaál in
Iceland
Iceland ( is, Ísland; ) is a Nordic island country in the North Atlantic Ocean and in the Arctic Ocean. Iceland is the most sparsely populated country in Europe. Iceland's capital and largest city is Reykjavík, which (along with its s ...
, powered by the
Kárahnjúkar Hydropower Plant
*
Jharsuguda
Jharsuguda is a city and district headquarters of Jharsuguda district of Odisha, India. It is an industrial hub, consisting mainly of metallurgical industries. It is well connected to major cities of India through the rail network, and a rece ...
in
Orissa, India
Odisha (English: , ), formerly Orissa ( the official name until 2011), is an Indian state located in Eastern India. It is the 8th largest state by area, and the 11th largest by population. The state has the third largest population of ...
, 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.
With the successful start-up of a major expansion in 2005, the Alouette Aluminum Smelter, at 550,000 ...
in
Sept-Îles, Québec.
*
Alba Smelter in
Bahrain
Bahrain ( ; ; ar, البحرين, al-Bahrayn, locally ), officially the Kingdom of Bahrain, ' is an island country in Western Asia. It is situated on the Persian Gulf, and comprises a small archipelago made up of 50 natural islands and an ...
, 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 metal. Secondary production is the process of recycling aluminum scrap into aluminum that can be used again. ...
*
List of alumina refineries
This is a list of alumina refineries
A refinery is a production facility composed of a group of chemical engineering unit processes and unit operations refining certain materials or converting raw material into products of value.
Types of ...
*
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, ...
*
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 ...
*
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 to ore, to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. Smelting uses heat and a ...
Electrolysis
de:Aluminium#Gewinnung