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Aluminium smelting

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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.

Aluminum smelter near Bellingham, Washington
Straumsvik aluminum smelter in Iceland
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

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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 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.

 

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

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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 conductivity 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 corrosion 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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  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. Bibcode:1996MMTB...27..177K. 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. Bibcode:2006Carbo..44.1438B. 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. ^ Obaidat, Mazin; Al-Ghandoor, Ahmed; Phelan, Patrick; Villalobos, Rene; Alkhalidi, Ammar (2018-04-17). "Energy and Exergy Analyses of Different Aluminum Reduction Technologies". Sustainability. 10 (4): 1216. doi:10.3390/su10041216. ISSN 2071-1050.
  12. ^ Haraldsson, J. (August 26, 2020). "Effects on primary energy use, greenhouse gas emissions and related costs from improving energy end-use efficiency in the electrolysis in primary aluminium production". Energy Efficiency. 13 (7): 1299–1314. Bibcode:2020EnEff..13.1299H. doi:10.1007/s12053-020-09893-1. S2CID 225243592.
  13. ^ a b c d e Siberian Federal University; Sai Krishna, Padamata; Andrey S., Yasinskiy; Siberian Federal University; Peter V., Polyakov; Siberian Federal University (March 2018). "Progress of Inert Anodes in Aluminium Industry: Review". Journal of Siberian Federal University. Chemistry. 11 (1): 18–30. doi:10.17516/1998-2836-0055.
  14. ^ "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.
  15. ^ Sadoway, Donald (May 2001). "Inert Anodes for the Hall-Héroult Cell: The Ultimate Materials Challenge" (PDF). Retrieved 29 April 2022.
  16. ^ Liu, Jian-yuan; Li, Zhi-you; Tao, Yu-qiang; Zhang, Dou; Zhou, Ke-chao (March 2011). "Phase evolution of 17(Cu-10Ni)-(NiFe2O4-10NiO) cermet inert anode during aluminum electrolysis". Transactions of Nonferrous Metals Society of China. 21 (3): 566–572. doi:10.1016/S1003-6326(11)60752-8.
  17. ^ "World Aluminium — Primary Aluminium Smelting Energy Intensity". 18 January 2021.
  18. ^ "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.
  19. ^ "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.
  20. ^ "Australian Aluminium Council – Submission to the Productivity Commission Inquiry into Energy Efficiency" (PDF).
  21. ^ Gupta, Amit; Basu, Biswajit (August 2019). "Sustainable Primary Aluminium Production: Technology Status and Future Opportunities". Transactions of the Indian Institute of Metals. 72 (8): 2135–2150. doi:10.1007/s12666-019-01699-9. ISSN 0972-2815. S2CID 181342550.
  22. ^ Yasinskiy, A. S.; Polyakov, P. V.; Klyuchantsev, A. B. (March 2017). "Motion dynamics of anodic gas in the cryolite melt–alumina high-temperature slurry". Russian Journal of Non-Ferrous Metals. 58 (2): 109–113. doi:10.3103/S1067821217020122. ISSN 1067-8212. S2CID 100529685.
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