# Pidgeon process

> Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Pidgeon_process
> Markdown URL: https://mediated.wiki/source/Pidgeon_process.md
> Source: https://en.wikipedia.org/wiki/Pidgeon_process
> Source revision: 1346726098
> License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/)

{{Short description|Method of producing magnesium metal}}
[[Image:Magnesium crystals.jpg|thumb|Vapor-deposited [magnesium](/source/magnesium) crystals from the Pidgeon process]]
The '''Pidgeon process''' is a practical method for smelting [magnesium](/source/magnesium). The most common method involves the raw material, [dolomite](/source/Dolomite_(mineral)), being fed into an externally heated reduction tank and then thermally reduced to metallic magnesium using 75% [ferrosilicon](/source/ferrosilicon) as a [reducing agent](/source/reducing_agent) in a [vacuum](/source/vacuum).<ref name=":0">{{Citation |last1=Wu |first1=Lan'er |title=Magnesium Smelting via the Pidgeon Process |date=2021 |work=Comprehensive Utilization of Magnesium Slag by Pidgeon Process |pages=45–68 |place=Singapore |publisher=Springer Singapore |isbn=978-981-16-2173-4 |last2=Han |first2=Fenglan |last3=Liu |first3=Guiqun|series=SpringerBriefs in Materials |doi=10.1007/978-981-16-2171-0_2 |s2cid=235872413 |doi-access=free }}</ref> Overall the processes in magnesium smelting via the Pidgeon process involve dolomite [calcination](/source/calcination), grinding and pelleting, and vacuum thermal reduction.<ref name=":0" />

Besides the Pidgeon process, [electrolysis](/source/electrolysis) of [magnesium chloride](/source/magnesium_chloride) for commercial production of magnesium is also used, especially for [magnesite](/source/magnesite) ores,<ref name="ayres00">{{cite news |first=John |last=Ayres |publisher=Environment Canada |url=https://www.epa.gov/sites/default/files/2016-02/documents/conf00_ayres_paper.pdf |title=Canadian Perspective on SF6 Management from Magnesium Industry |date=2000}}</ref> which at one point in time accounted for 75% of the world's magnesium production.<ref>{{Cite book |last=Wu |first=Lan'er |title=Comprehensive utilization of magnesium slag by pidgeon process |date=2021 |others=Fenglan Han, Guiqun Liu |isbn=978-981-16-2171-0 |location=Singapore |oclc=1249509843}}</ref>

By 2000, it took between 17 and 20 kilowatt-hours per kilogram of magnesium produced by the Pidgeon process.<ref name="ayres00"/> The Pidgeon processes in Canada in the year 2000 all used [sulfur hexafluoride (SF<sub>6</sub>)](/source/sulfur_hexafluoride) to cover the reaction so as not to introduce stray oxygen to it. Research to replace SF<sub>6</sub> with [boron trifluoride](/source/boron_trifluoride) was underway in 2000.<ref name=ayres00/> By 2011, magnesium production had departed under the [Kyoto Protocol](/source/Kyoto_Protocol) from Canada.<ref name="cim11">{{cite book |editor-last1=Kapusta |editor-first1=Joël |editor-last2=Mackey |editor-first2=Phillip |editor-last3=Stubina |editor-first3=Nathan |title=The Canadian Metallurgical & Materials Landscape 1960 - 2011 |url=https://store.cim.org/en/commemorative-book-the-canadian-materials-landscape-1960-2011 |publisher=Canadian Institute of Metallurgy |date=2011 |chapter=Magnesium Metal Production in Canada |first1=D. |last1=Creber |first2=B. |last2=Davis |first3=S. |last3=Kashani-Nejad}}</ref> Wu, Han and Liu claimed that "China is the world's largest producer of primary magnesium and has a magnesium smelting industry that is mainly based on the Pidgeon process" in an era in which China had obtained an 80% market share of production of magnesium metal.<ref name=:0/>

==Chemistry==
[[File:Enormous asbestos mittens must be worn by men handling the thousands of hot magnesium ingots 8b08228v.jpg|thumb|right|Enormous asbestos mittens must be worn by men handling the thousands of hot magnesium ingots produced daily at [Basic Magnesium](/source/Basic_Magnesium)'s giant plant in the southern Nevada desert in [Gabbs, Nevada](/source/Gabbs%2C_Nevada) near [Las Vegas](/source/Las_Vegas). Full operation commenced in the summer of 1943.]]
The general reaction that occurs in the Pidgeon process is:
:{{chem2|2 MgO*CaO + Si -> 2 Mg + Ca2SiO4}}

For industrial use, ferrosilicon is used in place of pure silicon because its cheaper and more readily available. The iron from the alloy is a spectator in the reaction. CaC<sub>2</sub> may also be used as an even cheaper alternative for silicon and ferrosilicon, but is disadvantageous because it decreases the magnesium yield slightly.<ref name=":1">{{Cite book |title=Magnesium and its alloys: technology and applications |date=2020 |others=Menachem Bamberger, Leszek A. Dobrzański, George E. Totten |isbn=978-1-351-04547-6 |edition=First |location=Boca Raton, FL |oclc=1111577710}}</ref>

The magnesium raw material of this type of reaction is [magnesium oxide](/source/magnesium_oxide), which is obtained in many ways. In all cases, the raw materials must be calcined to remove both water and carbon dioxide.  Magnesium oxide can also be obtained from sea or lake water magnesium chloride hydrolyzed to hydroxide. The Mg(OH)<sub>2</sub> is thermally dehydrated. Another option is to use mined [magnesite](/source/magnesite) (MgCO<sub>3</sub>) calcined to magnesium oxide.

The most used raw material is mined dolomite, a mixed (Ca,Mg)CO<sub>3</sub>, where the calcium oxide present in the reaction zone scavenges the silica formed, releasing heat and consuming one of the products, ultimately helping push the equilibrium to the right.
c(1) Dolomite calcination
:{{chem2|CaCO3*MgCO3 -> MgO*CaO  + 2 CO2}}

(2) Reduction
:{{chem2|MgO*CaO +Si -> 2 Mg  + Ca2SiO4}}

The Pidgeon process is an [endothermic reaction](/source/Endothermic_process) (<math>\bigtriangleup</math>H° ~183.0 kJ/mol<sub>Si</sub>). Thermodynamically speaking, the temperatures decrease when the vacuum is used for both MgO and calcined dolomite.<ref name=":1" />

== Summary of Pidgeon process using dolomite ==
thumb|Flow chart showing steps taken during the Pidgeon process
===Chinese variant===
The Chinese Pidgeon process is described here by Wu, Han and Liu. Being an endothermic reaction, heat is applied to initiate and sustain the reaction. This heat requirement may be very high. To keep reaction temperatures low, the processes are operated under pressure. The [rotary kiln](/source/rotary_kiln) is typically used in dolomite calcination. In the rotary kiln, the raw material, calcinated dolomite, is mixed with the finely ground reducing agent, ferrosilicon and the catalyst, [fluorite](/source/fluorite). The materials are mixed together and pressed into sphere shaped pellets and the mixed materials are charged into cylindrical nickel chromium steel [retort](/source/retort)s. A number of retorts are placed in a furnace in sealed paper bags to avoid moisture absorption so that calcined dolomite activity doesn't reduce magnesium yield. The pellets are then placed into a reduction tank and heated to 1200&nbsp;°C. The inside of the furnace is vacuumed with a 13.3 Pa or higher, to produce magnesium vapour. Magnesium crystals are removed from the condensers, [slag](/source/slag) is removed as a solid, and the retort is recharged. The crude magnesium is refined via [flux](/source/flux), and commercial magnesium [ingot](/source/ingot) is produced. The authors nowhere identify the name or the characteristics of the flux.<ref name=":0" />

Typical flux composition is 49 wt% anhydrous [magnesium chloride](/source/magnesium_chloride), 27 wt% [potassium chloride](/source/potassium_chloride), 20 wt% [barium chloride](/source/barium_chloride) and 4 wt% [calcium fluoride](/source/calcium_fluoride).<ref name="bell06">{{cite news |url=https://ressources-naturelles.canada.ca/sites/www.nrcan.gc.ca/files/mineralsmetals/pdf/mms-smm/busi-indu/rad-rad/pdf/2003-19(cf)cc-eng.pdf}}</ref><ref name=proffitt89>H. Proffitt, "Magnesium and Magnesium Alloys", Metals Handbook, 9, [2], (1989), pp. 801-802.</ref>

===Canadian variant===
The Canadian variant is described here with reference to the Chinese variant. In 2000, Canada had three magnesium smelters. All three used SF<sub>6</sub> as cover gas to prevent oxidation and combustion of exposed surfaces of magnesium, which is at [STP](/source/Standard_temperature_and_pressure) highly [combustible](/source/combustible). The SF<sub>6</sub> cover gas had been in use at that point for over 20 years by all industries which dealt with raw magnesium.<ref name=ayres00/> Canadian industry was tasked to discover a suitable alternative cover gas in order not to be sacrificed to ''Action Plan 2000 on Climate Change''.<ref name="ap2000">{{cite news |url=https://publications.gc.ca/collections/Collection/M22-135-2000E.pdf |title=Information archivée dans le Web }}</ref><ref name="ayres02">{{cite news |url=https://19january2017snapshot.epa.gov/sites/production/files/2016-02/documents/conf02_fasoyinu_paper.pdf}}</ref> SF<sub>6</sub> had been deemed to have a Global Warming Potential (GWP) factor of 23,900 times that of CO<sub>2</sub>.<ref name=ayres02/> By 2011, magnesium production had departed from Canada because of the [Kyoto Protocol](/source/Kyoto_Protocol).<ref name=cim11/>

== Other routes for magnesium processing ==
{{main|Magnesium#Production}}
Many technologies have been developed for producing magnesium metal. These approaches can be broadly classified as electrolytic and thermic.<ref>{{cite book |doi=10.1002/14356007.a15_559 |chapter=Magnesium |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2003 |last1=Amundsen |first1=Ketil |last2=Aune |first2=Terje Kr. |last3=Bakke |first3=Per |last4=Eklund |first4=Hans R. |last5=Haagensen |first5=Johanna Ö. |last6=Nicolas |first6=Carlos |last7=Rosenkilde |first7=Christian |last8=Van Den Bremt |first8=Sia |last9=Wallevik |first9=Oddmund |isbn=978-3-527-30385-4 }}</ref>   The main manifestation of the electrolytic is the Dow process.  The main application of thermic routes is the Pidgeon process. The [Bolzano process](/source/Bolzano_process) merits mention because it is very similar to the Pidgeon process except that the heating is achieved through electric heating conductors and [retort](/source/retort)s are placed vertically into large blocks in the Bolzano process.<ref name=":1" /><ref name=":2">{{Cite web |title=Magnesium processing {{!}} Techniques & Methods {{!}} Britannica |url=https://www.britannica.com/technology/magnesium-processing |access-date=2023-04-16 |website=www.britannica.com |language=en}}</ref> The Pidgeon process is less technologically complex and because of distillation/vapour deposition conditions, a high purity product is  easily achievable.<ref name=":1" />

== Disadvantages of the Pidgeon process ==
thumb|right|Schematic cut of a retort: (a) outlet for creating a vacuum, (b) cold water inlet and outlet, (c) retort door, (d) magnesium crown, (e) heat shield, and (f) retort furnace wall.
Although the Pidgeon process has many advantages, there are some environmental disadvantages of the process as well. With the increased demand for magnesium in recent years, production through ore reduction has been emitting larger amounts of [carbon dioxide](/source/carbon_dioxide) and [particulate matter](/source/Particulates).<ref>{{Cite journal |last1=Wada |first1=Yuji |last2=Fujii |first2=Satoshi |last3=Suzuki |first3=Eiichi |last4=Maitani |first4=Masato M. |last5=Tsubaki |first5=Shuntaro |last6=Chonan |first6=Satoshi |last7=Fukui |first7=Miho |last8=Inazu |first8=Naomi |date=2017-04-12 |title=Smelting Magnesium Metal using a Microwave Pidgeon Method |journal=Scientific Reports |language=en |volume=7 |issue=1 |article-number=46512 |doi=10.1038/srep46512 |issn=2045-2322 |pmc=5388895 |pmid=28401910|bibcode=2017NatSR...746512W }}</ref>  There are environmental impacts because to create lightweight materials in the first place, more energy is needed compared to the material being replaced, typically [iron](/source/iron) or [steel](/source/steel). Approximately 10.4&nbsp;kg of coal is burned and 37&nbsp;kg of carbon dioxide is released per 1&nbsp;kg of magnesium obtained, compared with less than 2&nbsp;kg of carbon dioxide to produce 1&nbsp;kg of steel.<ref>{{Cite journal |last1=Johnson |first1=M. C. |last2=Sullivan |first2=J. L. |date=2014-09-01 |title=Lightweight Materials for Automotive Application: An Assessment of Material Production Data for Magnesium and Carbon Fiber |url=http://www.osti.gov/servlets/purl/1172026/ |language=en |pages=ANL/ESD––14/7, 1172026 |doi=10.2172/1172026|osti=1172026 |journal=Argonne National Lab }}</ref><ref>{{Cite journal |last1=Gao |first1=Feng |last2=Nie |first2=Zuo-ren |last3=Wang |first3=Zhi-hong |last4=Gong |first4=Xian-zheng |last5=Zuo |first5=Tie-yong |date=June 2008 |title=Assessing environmental impact of magnesium production using Pidgeon process in China |url=https://linkinghub.elsevier.com/retrieve/pii/S1003632608601296 |journal=Transactions of Nonferrous Metals Society of China |language=en |volume=18 |issue=3 |pages=749–754 |doi=10.1016/S1003-6326(08)60129-6|url-access=subscription }}</ref><ref name=":3">{{Cite journal |last1=Ramakrishnan |first1=S. |last2=Koltun |first2=P. |date=August 2004 |title=Global warming impact of the magnesium produced in China using the Pidgeon process |journal=Resources, Conservation and Recycling |volume=42 |issue=1 |pages=49–64 |doi=10.1016/j.resconrec.2004.02.003 |bibcode=2004RCR....42...49R |issn=0921-3449}}</ref> In China, production of magnesium using the Pidgeon process has a 60% higher global warming impact than aluminum, a competing metal mass-produced in the country as well.<ref name=":3" />

== History ==
thumb|right|Retort storage at modern Iranian production plant
The silicothermic reduction of dolomite was first developed by Amati in 1938 at the [University of Padua](/source/University_of_Padua). Immediately afterward, an industrial production was established in [Bolzano](/source/Bolzano) (Italy), using what is now better known as the [Bolzano process](/source/Bolzano_process).<ref>{{Cite book |url=http://link.springer.com/10.1007/3-540-30812-1 |title=Magnesium Technology |date=2006 |publisher=Springer-Verlag |isbn=978-3-540-20599-9 |location=Berlin/Heidelberg |language=en |doi=10.1007/3-540-30812-1}}</ref>

A few years later in 1939, when Canada and its allies entered [WW2](/source/World_War_II), they were short on supplies that required magnesium such as bombs, other military devices and aluminum alloys needed for aircraft. [Dr. Lloyd Montgomery Pidgeon](/source/Lloyd_Montgomery_Pidgeon) at the [National Research Council](/source/National_Research_Council_Canada) was able to create a method for extracting magnesium from dolomite in a vacuum at high temperature with ferrosilicon as the reducing agent. At this time, the ferrosilicon method was known, however it had yet to be commercialized. By early 1942, a successful pilot test took place.<ref>{{Cite web |date=2005-02-23 |title=Science & Tech Innovations - National Research Council Canada |url=http://www.nrc-cnrc.gc.ca/education/sti-1940s_pidgeon_e.html |access-date=2023-04-16 |archive-url=https://web.archive.org/web/20050223001123/http://www.nrc-cnrc.gc.ca/education/sti-1940s_pidgeon_e.html |archive-date=2005-02-23 }}</ref>

Since then, the Pidgeon process has continually been widely used, especially in China, the world's largest magnesium producer.

== References ==
<references/>

Category:Chemical processes
Category:Magnesium processes
Category:Metallurgical processes
Category:Materials science
Category:1942 introductions
Category:20th-century inventions
Category:Canadian inventions

---
Adapted from the Wikipedia article [Pidgeon process](https://en.wikipedia.org/wiki/Pidgeon_process) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Pidgeon_process?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.