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Aluminium smelting

January 26, 2023

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.

Overview of the Point Henry smelter, operated by Alcoa World Alumina and Chemicals in Australia
Straumsvik aluminum smelter, operated by Rio Tinto Alcan in Iceland.

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

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.

 

At the same time the carbon electrode is oxidised, initially to carbon monoxide

 

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

 

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

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

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 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 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.

There is ongoing research efforts to develop an inert anode to reduce the environmental impacts of aluminium smelting.[11] Alcoa and Rio Tinto have formed a joint venture, Elysis, to commercialize inert anode technology developed by Alcoa.[12] The inert anode is a cermet material, a metallic dispersion of copper alloy in a ceramic matrix of nickel ferrite.[11]

Energy use

Aluminium smelting is highly energy intensive, and in some countries is economical only if there are inexpensive sources of electricity.[13][14] In some countries, smelters are given exemptions to energy policy like renewable energy targets.[15][16]

Example aluminium smelters

See also

References

  1. ^ K. Grjotheim and C. Krohn, Aluminium electrolysis: The chemistry of the Hall-Heroult process: Aluminium-Verlag GmbH, 1977.
  2. ^ F. Habashi, Handbook of Extractive Metallurgy vol. 2: Wiley-VCH, 1997.
  3. ^ Kuang, Z.; Thonstad, J.; Rolseth, S.; Sørlie, M. (April 1996). "Effect of baking temperature and anode current density on anode carbon consumption". Metallurgical and Materials Transactions B. 27 (2): 177–183. doi:10.1007/BF02915043. S2CID 97620903.
  4. ^ Farr-Wharton, R.; Welch, B.J.; Hannah, R.C.; Dorin, R.; Gardner, H.J. (February 1980). "Chemical and electrochemical oxidation of heterogeneous carbon anodes". Electrochimica Acta. 25 (2): 217–221. doi:10.1016/0013-4686(80)80046-6.
  5. ^ a b 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
  6. ^ P. A. Foster, "Phase diagram of a portion of system Na3AlF6-AlF3-Al2O3," Journal of the American Ceramic Society, vol. 58, pp. 288–291, 1975
  7. ^ Welch, B. J.; Hyland, M. M.; James, B. J. (February 2001). "Future materials requirements for the high-energy-intensity production of aluminum". JOM. 53 (2): 13–18. Bibcode:2001JOM....53b..13W. doi:10.1007/s11837-001-0114-8. S2CID 136787092.
  8. ^ Brisson, P.-Y.; Darmstadt, H.; Fafard, M.; Adnot, A.; Servant, G.; Soucy, G. (July 2006). "X-ray photoelectron spectroscopy study of sodium reactions in carbon cathode blocks of aluminium oxide reduction cells". Carbon. 44 (8): 1438–1447. doi:10.1016/j.carbon.2005.11.030.
  9. ^ W. K. Fischer, et al., "Baking parameters and the resulting anode quality," in TMS Annual Meeting, Denver, CO, USA, 1993, pp. 683–689
  10. ^ M. M. Gasik and M. L. Gasik, "Smelting of aluminum," in Handbook of Aluminum: Volume 2: Alloy production and materials manufacturing. vol. 2, G. E. Totten and D. S. MacKenzie, Eds., ed: Marcel Dekker, 2003, pp. 47–79
  11. ^ a b Sadoway, Donald (May 2001). "Inert Anodes for the Hall-Héroult Cell: The Ultimate Materials Challenge" (PDF). Retrieved 29 April 2022.
  12. ^ "Rio Tinto and Alcoa announce world's first carbon-free aluminum smelting process; Apple assist; Elysis JV to commercialize". Green Car Congress. Retrieved 2022-04-30.
  13. ^ "World Aluminium — Primary Aluminium Smelting Energy Intensity".
  14. ^ . Geoscience Australia. Archived from the original on 2015-09-23. Retrieved 2015-09-02. A great amount of energy is consumed during the smelting process; from 14 – 16 MWh of electrical energy is needed to produce one tonne of aluminium from about two tonnes of alumina. The availability of cheap electricity is therefore essential for economic production.
  15. ^ (PDF). Department of Industry, Science and Resources – Australian Government. July 2000. Archived from the original (PDF) on 2015-09-24. Retrieved 2015-09-02.
  16. ^ "Australian Aluminium Council – Submission to the Productivity Commission Inquiry into Energy Efficiency" (PDF).

January 26, 2023
aluminium, smelting, this, article, needs, additional, citations, verification, please, help, improve, this, article, adding, citations, reliable, sources, unsourced, material, challenged, removed, find, sources, news, newspapers, books, scholar, jstor, novemb. This article needs additional citations for verification Please help improve this article by adding citations to reliable sources Unsourced material may be challenged and removed Find sources Aluminium smelting news newspapers books scholar JSTOR November 2013 Learn how and when to remove this template message Aluminium smelting is the process of extracting aluminium from its oxide alumina generally by the Hall Heroult process Alumina is extracted from the ore bauxite by means of the Bayer process at an alumina refinery Overview of the Point Henry smelter operated by Alcoa World Alumina and Chemicals in AustraliaStraumsvik aluminum smelter operated by Rio Tinto Alcan in Iceland 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 Contents 1 Layout of an aluminium smelter 2 Principle 3 Cell components 4 Environmental issues of aluminium smelters 4 1 Energy use 5 Example aluminium smelters 6 See also 7 ReferencesLayout of an aluminium smelter EditThe Hall Heroult 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 EditAluminium is produced by electrolytic reduction of aluminium oxide dissolved in molten cryolite Al 3 3 e Al displaystyle ce Al 3 3e gt 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 gt 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 D G 264460 3 75 T log T 92 52 T cal Al 2 O 3 3 C 2 Al 3 CO D G 325660 3 75 T log T 155 07 T cal displaystyle begin cases ce Al2O3 3 2C lt gt 2Al 3 2CO2 amp Delta G circ 264460 3 75T log T 92 52T ce cal ce Al2O3 3C lt gt 2Al 3CO amp 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 EditElectrolyte 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 mWm 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 EditThe 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 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 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 There is ongoing research efforts to develop an inert anode to reduce the environmental impacts of aluminium smelting 11 Alcoa and Rio Tinto have formed a joint venture Elysis to commercialize inert anode technology developed by Alcoa 12 The inert anode is a cermet material a metallic dispersion of copper alloy in a ceramic matrix of nickel ferrite 11 Energy use Edit Aluminium smelting is highly energy intensive and in some countries is economical only if there are inexpensive sources of electricity 13 14 In some countries smelters are given exemptions to energy policy like renewable energy targets 15 16 Example aluminium smelters EditAlcan Lynemouth Aluminium Smelter powered by the coal fired Lynemouth Power Station in North East England Anglesey Aluminium powered by Wylfa nuclear power station in north west Wales shut down as of 30 09 09 The Valco aluminium smelter in Ghana powered by the Akosombo Hydroelectric Project Fjardaal in Iceland powered by the Karahnjukar Hydropower Plant Jharsuguda in Orissa India to be powered by its own 1 215 megawatts 1 629 000 hp coal fired power station Aluminerie Alouette in Sept Iles Quebec Alba Smelter in Bahrain powered by its own four power stations with a total generating capacity of 2 265 megawatts 3 037 000 hp See also EditList of aluminium smelters List of alumina refineries Lead smelter Nuclear power Zinc smelting Solid oxide Hall Heroult processReferences Edit K Grjotheim and C Krohn Aluminium electrolysis The chemistry of the Hall Heroult process Aluminium Verlag GmbH 1977 F Habashi Handbook of Extractive Metallurgy vol 2 Wiley VCH 1997 Kuang Z Thonstad J Rolseth S Sorlie M April 1996 Effect of baking temperature and anode current density on anode carbon consumption Metallurgical and Materials Transactions B 27 2 177 183 doi 10 1007 BF02915043 S2CID 97620903 Farr Wharton R Welch B J Hannah R C Dorin R Gardner H J February 1980 Chemical and electrochemical oxidation of heterogeneous carbon anodes Electrochimica Acta 25 2 217 221 doi 10 1016 0013 4686 80 80046 6 a b 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 P A Foster Phase diagram of a portion of system Na3AlF6 AlF3 Al2O3 Journal of the American Ceramic Society vol 58 pp 288 291 1975 Welch B J Hyland M M James B J February 2001 Future materials requirements for the high energy intensity production of aluminum JOM 53 2 13 18 Bibcode 2001JOM 53b 13W doi 10 1007 s11837 001 0114 8 S2CID 136787092 Brisson P Y Darmstadt H Fafard M Adnot A Servant G Soucy G July 2006 X ray photoelectron spectroscopy study of sodium reactions in carbon cathode blocks of aluminium oxide reduction cells Carbon 44 8 1438 1447 doi 10 1016 j carbon 2005 11 030 W K Fischer et al Baking parameters and the resulting anode quality in TMS Annual Meeting Denver CO USA 1993 pp 683 689 M M Gasik and M L Gasik Smelting of aluminum in Handbook of Aluminum Volume 2 Alloy production and materials manufacturing vol 2 G E Totten and D S MacKenzie Eds ed Marcel Dekker 2003 pp 47 79 a b Sadoway Donald May 2001 Inert Anodes for the Hall Heroult Cell The Ultimate Materials Challenge PDF Retrieved 29 April 2022 Rio Tinto and Alcoa announce world s first carbon free aluminum smelting process Apple assist Elysis JV to commercialize Green Car Congress Retrieved 2022 04 30 World Aluminium Primary Aluminium Smelting Energy Intensity Aluminium Fact Sheet Geoscience Australia Archived from the original on 2015 09 23 Retrieved 2015 09 02 A great amount of energy is consumed during the smelting process from 14 16 MWh of electrical energy is needed to produce one tonne of aluminium from about two tonnes of alumina The availability of cheap electricity is therefore essential for economic production Energy efficiency best practice in the Australian aluminium industry PDF Department of Industry Science and Resources Australian Government July 2000 Archived from the original PDF on 2015 09 24 Retrieved 2015 09 02 Australian Aluminium Council Submission to the Productivity Commission Inquiry into Energy Efficiency PDF Retrieved from https en wikipedia org w index php title Aluminium smelting amp oldid 1115766314, wikipedia, wiki, book, books, library,

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