Editing Bayer process

{{Short description|Industrial means of refining bauxite to produce alumina}}
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]] (Al<sub>2</sub>O<sub>3</sub>), the rest being a mixture of [[silica]], various [[iron oxide]]s, and [[titanium dioxide]].<ref>{{cite book |author=Harris, Chris |author2=McLachlan, R. (Rosalie) |author3=Clark, Colin |title=Micro reform – impacts on firms: aluminium case study |publisher=Industry Commission |location=Melbourne |year=1998 |isbn=978-0-646-33550-6 }}</ref> The aluminium oxide must be further purified before it can be refined into aluminium.

The Bayer process is also the main source of [[gallium]] as a byproduct despite low extraction yields.

== Process == 
[[File:Bayer-process-en.svg|thumb|right|500 px|The Bayer [[process flow diagram]].]]
Bauxite ore is a mixture of hydrated aluminium oxides and compounds of other elements such as iron. The aluminium compounds in the bauxite may be present as [[gibbsite]] (Al(OH)<sub>3</sub>), [[boehmite|böhmite]] (γ-AlO(OH)) or [[diaspore]] (α-AlO(OH)); the different forms of the aluminium component and the impurities dictate the extraction conditions. Aluminium oxides and hydroxides are [[amphoteric]], meaning that they are both acidic and basic.  The solubility of Al(III) in water is very low but increases substantially at either high or low pH.  In the Bayer process, bauxite ore is heated in a [[pressure vessel]] along with a [[sodium hydroxide]] solution (caustic soda) at a temperature of {{convert|150|to|200|°C}}. At these temperatures, the [[aluminium]] is dissolved as [[sodium aluminate]] (primarily [Al(OH)<sub>4</sub>]<sup>−</sup>) in an extraction process. After separation of the residue by filtering, gibbsite is precipitated when the liquid is cooled and then [[Seed crystal|seeded]] with fine-grained aluminium hydroxide crystals from previous extractions.  The precipitation may take several days without addition of seed crystals.<ref name="Grocott99">{{cite journal |last1=Hind |first1=Andrew R. |last2=Bhargava |first2=Suresh K. |last3=Grocott |first3=Stephen C. |title=The surface chemistry of Bayer process solids: a review |journal=Colloids and Surfaces A: Physicochemical and Engineering Aspects |date=January 1999 |volume=146 |issue=1–3 |pages=359–374 |doi=10.1016/S0927-7757(98)00798-5}}</ref>

The extraction process (''digestion'') converts the aluminium oxide in the ore to soluble sodium aluminate, NaAlO<sub>2</sub>, according to the [[chemical equation]]:

:Al(OH)<sub>3</sub>+  NaOH   →   NaAlO<sub>2</sub>  +  2 H<sub>2</sub>O

This treatment also dissolves silica, forming sodium silicate :

:2 NaOH + SiO<sub>2</sub>   →   Na<sub>2</sub>SiO<sub>3</sub> + H<sub>2</sub>O 

The other components of Bauxite, however, do not dissolve. Sometimes{{when|date=October 2019}} [[lime (material)|lime]] is added at this stage to precipitate the silica as [[calcium silicate]]. The solution is clarified by filtering off the solid impurities, commonly with a rotary sand trap and with the aid of a [[flocculant]] such as [[starch]], to remove the fine particles. The undissolved waste after the aluminium compounds are extracted, [[bauxite tailings]], contains [[iron oxides]], [[silica]], [[calcia]], [[Titanium dioxide|titania]] and some unreacted alumina. The original process was that the [[alkali]]ne solution was cooled and treated by bubbling carbon dioxide through it, a method by which aluminium hydroxide [[precipitation (chemistry)|precipitates]]:

:2 NaAlO<sub>2</sub> + 3 H<sub>2</sub>O + CO<sub>2</sub> → 2 Al(OH)<sub>3</sub> + [[sodium carbonate|Na<sub>2</sub>CO<sub>3</sub>]]

But later, this gave way to seeding the supersaturated solution with high-purity aluminium hydroxide (Al(OH)<sub>3</sub>) crystal, which eliminated the need for cooling the liquid and was more economically feasible:

:2 H<sub>2</sub>O + NaAlO<sub>2</sub> → Al(OH)<sub>3</sub> + NaOH

Some of the aluminium hydroxide produced is used in the manufacture of water treatment chemicals such as [[aluminium sulfate]], PAC ([[Aluminium chlorohydrate|Polyaluminium chloride]]) or sodium aluminate; a significant amount is also used as a filler in rubber and plastics as a fire retardant.  Some 90% of the gibbsite produced is converted into [[aluminium oxide]], Al<sub>2</sub>O<sub>3</sub>, by heating in [[rotary kiln]]s or fluid flash [[calciner]]s to a temperature of about {{convert|1470|K}}.

:2 [[Aluminium hydroxide|Al(OH)<sub>3</sub>]] → [[alumina|Al<sub>2</sub>O<sub>3</sub>]] + 3 [[water|H<sub>2</sub>O]]

The left-over, 'spent' sodium aluminate solution is then recycled. Apart from improving the economy of the process, recycling accumulates [[gallium]] and [[vanadium]] impurities in the liquors, so that they can be extracted profitably.

Organic impurities that accumulate during the precipitation of gibbsite may cause various problems, for example high levels of undesirable materials in the gibbsite, discoloration of the liquor and of the gibbsite, losses of the caustic material, and increased viscosity and density of the working fluid.

For bauxites having more than 10% silica, the Bayer process becomes uneconomic because of the formation of insoluble [[sodium aluminium silicate]], which reduces yield, so another process must be chosen.

{{convert|1.9|-|3.6|ST|t lb|order=out}} of bauxite (corresponding to about 90% of the alumina content of the bauxite) is required to produce {{convert|1|ST|t lb|order=out}} of aluminium oxide. This is due to a majority of the aluminium in the ore being dissolved in the process.<ref name="Grocott99" /> Energy consumption is between {{convert|7|to|21|GJ/t|kWh/lb}} (depending on process), of which most is thermal energy.<ref>{{cite book |doi=10.1007/978-3-319-48248-4_24 |author1=Alessio Angelo Scarsella, Sonia Noack, Edgar Gasafi, Cornelis Klett, Andreas Koschnick |title=Light Metals 2015 |chapter=Energy in Alumina Refining: Setting New Limits|year=2015 |pages=131–136 |isbn=978-3-319-48610-9 }}</ref><ref>{{cite web |title=Energy efficiency |url=http://bauxite.world-aluminium.org/refining/energy-efficiency/ |quote=energy required by the Bayer Process is very much dependent on the quality of the raw material .  average specific energy consumption is around 14.5 GJ per tonne of alumina, including electrical energy of around 150 kWh/t Al2O3}}</ref> Over 90% (95-96%) of the aluminium oxide produced is used in the [[Hall–Héroult process]] to produce aluminium.<ref>{{cite web|title=The Aluminum Smelting Process|url=http://www.aluminum-production.com/important_figures.html |website=Aluminum Production|publisher=aluminumproduction.com |access-date=12 April 2018}}</ref>

== Waste ==
[[Red mud]] is the waste product that is produced in the digestion of bauxite with sodium hydroxide. It has high calcium and sodium hydroxide content with a complex chemical composition, and accordingly is very caustic and a potential source of pollution. The amount of red mud produced is considerable, and this has led scientists and refiners to seek uses for it. It has received attention as a possible source of [[vanadium]]. Due to the low extraction yield much of the gallium ends up in the aluminium oxide as an impurity and in the red mud.

One use of red mud is in ceramic production. Red mud dries into a fine powder that contains iron, aluminium, calcium and sodium. It becomes a health risk when some plants use the waste to produce aluminium oxides.<ref>{{cite journal|title=The Surface Chemistry of Bayer Process Solids: A Review|journal=Colloids and Surfaces A: Physicochemical and Engineering Aspects|volume=146|issue=1–3|pages=359–374|doi=10.1016/S0927-7757(98)00798-5|year=1999|last1=Hind|first1=Andrew R.|last2=Bhargava|first2=Suresh K.|last3=Grocott|first3=Stephen C.}}</ref>

In the United States, the waste is disposed in large [[Reservoir|impoundments]], a sort of reservoir created by a dam. The impoundments are typically lined with clay or synthetic liners. The US does not approve of the use of the waste due to the danger it poses to the environment. The EPA identified high levels of arsenic and chromium in some red mud samples.<ref>{{cite web|title=TENORM: Bauxite and Alumina Production Wastes|url=https://www.epa.gov/radiation/tenorm-bauxite-and-alumina-production-wastes|website=www.epa.gov|publisher=United States Environmental Protection Agency|access-date=12 April 2018|date=2015-04-22}}</ref>

=== Ajka alumina plant accident ===
{{main|Ajka alumina plant accident}}

On October 4, 2010, the Ajka alumina plant in Hungary had an [[Ajka alumina plant accident|incident]] where the western dam of its red mud reservoir collapsed. The reservoir was filled with {{convert|700,000|m3|e6cuft|abbr=off}} of a mixture of red mud and water with a pH of 12. The mixture was released into the valley of Torna river and flooded parts of the city of Devecser and the villages of Kolontár and Somlóvásárhely. The incident resulted in 10 deaths, more than a hundred injuries, and contamination in lakes and rivers.<ref>{{cite journal|last1=Ruyters|first1=Stefan|last2=Mertens|first2=Jelle|last3=Vassilieva|first3=Elvira|last4=Dehandschutter|first4=Boris|last5=Poffijin|first5=Andre|last6=Smolders|first6=Erik|title=The Red Mud Accident in Ajka (Hungary): Plant Toxicity and Trace Metal Bioavailability in Red Mud Contaminated Soil|journal=Environmental Science & Technology|volume=45|issue=4|pages=1616–1622|doi=10.1021/es104000m|pmid=21204523|year=2011|bibcode=2011EnST...45.1616R|url=https://figshare.com/articles/journal_contribution/2692039/files/4367983.pdf}}</ref>

==History==
In 1859, [[Henri Étienne Sainte-Claire Deville]] in France developed a method for making alumina by heating bauxite in sodium carbonate, {{chem|Na|2|CO|3}}, at {{convert|1200|°C}}, leaching the sodium aluminate formed with water, then precipitating aluminium hydroxide by [[carbon dioxide]], {{CO2}}, which was then filtered and dried. This process is known as the [[Deville–Pechiney process]]. In 1886, the [[Hall–Héroult process|Hall–Héroult electrolytic aluminium process]] was invented, and the [[Gold cyanidation|cyanidation process]] was invented in 1887. 

The Bayer process was invented in 1888 by [[Carl Josef Bayer]].<ref name=":0" /> Working in Saint Petersburg, Russia to develop a method for supplying alumina to the textile industry (it was used as a [[mordant]] in dyeing cotton), Bayer discovered in 1887 that the aluminium hydroxide that precipitated from alkaline solution was crystalline and could be easily filtered and washed, while that precipitated from acid medium by neutralization was gelatinous and difficult to wash.<ref name=":0" /> The industrial success of this process caused it to replace the Deville–Pechiney process,<ref name=":0"/> marking the birth of the modern field of [[hydrometallurgy]].

The engineering aspects of the process were improved upon to decrease the cost starting in 1967 in [[Germany]] and [[Czechoslovakia]].<ref name=":0" /> This was done by increasing the heat recovery and using large [[autoclaves]] and precipitation tanks.<ref name=":0" /> To more effectively use energy, [[heat exchangers]] and flash tanks were used and larger reactors decreased the amount of heat lost.<ref name=":0" /> Efficiency was increased by connecting the autoclaves to make operation more efficient.<ref name=":0">{{cite web | title=Bayer's Process for Alumina Production: A Historical Production | url=http://www.scs.illinois.edu/~mainzv/HIST/bulletin_open_access/num17-18/num17-18%20p15-19.pdf |website=scs.illinois.edu | publisher=Fathi Habashi, [[Laval University]] | access-date=6 April 2018}}</ref>

Today, the process produces nearly all the world's alumina supply as an intermediate step in aluminium production.

==See also==
*[[Ajka alumina plant accident]]
*[[Deville process]]
*[[Hall–Héroult process]]
*[[History of aluminium]]
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==References==
<references/>
* {{cite journal | author = Habashi, F. | title = A short history of hydrometallurgy | journal = [[Hydrometallurgy (journal)|Hydrometallurgy]] | volume = 79 | issue = 1–2 | pages = 15–22 | year = 2005 | doi = 10.1016/j.hydromet.2004.01.008| bibcode = 2005HydMe..79...15H }}

{{Extractive metallurgy}}

[[Category:Chemical processes]]
[[Category:Aluminium industry]]
[[Category:Metallurgical processes]]