Reducing atmosphere
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Areducing atmosphereis anatmosphericcondition in whichoxidationis prevented by absence ofoxygenand other oxidizing gases or vapours, and which may contain activelyreductantgases such ashydrogen,carbon monoxide,methaneandhydrogen sulfidethat would be readily oxidized to remove any free oxygen. AlthoughEarly Earthhad had a reducingprebiotic atmosphereprior to theProterozoiceon,starting at about 2.5 billion years ago in the lateNeoarchaeanperiod,theEarth's atmosphereexperienceda significant rise in oxygenand transitioned to an oxidizing atmosphere with a surplus of molecularoxygen(dioxygen,O2) as the primaryoxidizing agent.
Foundry operations
[edit]The principal mission of aniron foundryis the conversion of iron oxides (purified iron ores) to iron metal. This reduction is usually effected using a reducing atmosphere consisting of some mixture ofnatural gas,hydrogen(H2), andcarbon monoxide.The byproduct iscarbon dioxide.[1]
Metal processing
[edit]In metal processing, a reducing atmosphere is used inannealingovens for relaxation of metalstresseswithout corroding the metal. A non-oxidizing gas, usuallynitrogenorargon,is typically used as a carrier gas so that diluted amounts of reducing gases may be used. Typically, this is achieved through using the combustion products of fuels and tailoring the ratio of CO:CO2.However, other common reducing atmospheres in the metal processing industries consist of dissociated ammonia, vacuum, and/or direct mixing of appropriately pure gases of N2,Ar, and H2.[2]
A reducing atmosphere is also used to produce specific effects onceramicwares being fired. Areductionatmosphere is produced in a fuel fired kiln by reducing the draft and depriving the kiln of oxygen. This diminished level of oxygen causes incomplete combustion of the fuel and raises the level of carbon inside the kiln. At high temperatures the carbon will bond with and remove the oxygen in the metal oxides used as colorants in the glazes. This loss of oxygen results in a change in the color of the glazes because it allows the metals in the glaze to be seen in an unoxidized form. A reduction atmosphere can also affect the color of the clay body. If iron is present in the clay body, as it is in moststoneware,then it will be affected by the reduction atmosphere as well.
In most commercialincinerators,exactly the same conditions are created to encourage the release of carbon bearing fumes. These fumes are then oxidized in reburn tunnels where oxygen is injected progressively. The exothermic oxidation reaction maintains the temperature of the reburn tunnels. This system allows lower temperatures to be employed in the incinerator section, where the solids are volumetrically reduced.
Origin of life
[edit]The atmosphere of Early Earth is widely speculated to have been reducing. TheMiller–Urey experiment,related to some hypotheses for the origin of life, entailed reactions in a reducing atmosphere composed of a mixed atmosphere ofmethane,ammoniaandhydrogen sulfide.[3][4]Some hypotheses for the origin of life invoke a reducing atmosphere consisting ofhydrogen cyanide(HCN). Experiments show that HCN can polymerize in the presence of ammonia to give a variety of products includingamino acids.[5]The same principle applies toMars,VenusandTitan.
Cyanobacteriaare suspected to be the firstphotoautotrophsthat evolvedoxygenic photosynthesis,which over the latter half of theArchaeneon eventually depleted all reductants in the Earth's oceans, terrestrial surface and atmosphere, gradually increasing the oxygen concentration in the atmosphere, changing it to what is known as an oxidizing atmosphere. This rising oxygen initially led toa 300 million-year-long ice agethat devastated the then-mostlyanaerobe-dominatedbiosphere,forcing the surviving anaerobic colonies to evolve intosymbioticmicrobial matswith the newly evolvedaerobes.Some aerobicbacteriaeventually becameendosymbiontwithin other anaerobes (likelyarchaea), and the resultantsymbiogenesisled to the evolution of a completely new lineage of life — theeukaryotes,who took advantage ofmitochondrialaerobic respirationto power their cellular activities, allowing life to thrive and evolve into ever more complex forms.[6]The increased oxygen in the atmosphere also eventually created theozone layer,which shielded away harmfulionizingultraviolet radiationthat otherwise would havephotodissociatedawaysurface waterand rendered life impossible on land and the ocean surface.
In contrast to the hypothesized early reducing atmosphere, evidence exists thatHadeanatmospheric oxygen levels were similar to those of today.[7]These results suggests prebiotic building blocks were delivered from elsewhere in the galaxy. The results however do not run contrary to existing theories on life's journey from anaerobic to aerobic organisms. The results quantify the nature of gas molecules containing carbon, hydrogen, and sulphur in the earliest atmosphere, but they shed no light on the much later rise of free oxygen in the air.[8]
See also
[edit]- Atmosphere of Earth
- Great Oxidation Event– Paleoproterozoic surge in atmospheric oxygen
- Paleoclimatology– Study of changes in ancient climate
- Paleoatmosphere– Ancient atmosphere, particularly of Earth, in the geological past
- Redox– Chemical reaction in which oxidation states of atoms are changed
Notes
[edit]- ^Formanek, Lothar; Lüngen, Hans Bodo; Prölss, Julian; Rose, Fritz; Stellmacher, Ulrike (2019-07-30),"Iron, 3. Direct Reduction Processes",Ullmann's Encyclopedia of Industrial Chemistry,Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA, pp. 1–21,doi:10.1002/14356007.o14_o02.pub3,ISBN9783527306732,retrieved2022-02-28
- ^Koria, S. C."Fuels Refractory and Furnaces"(PDF).Indian Institute of Technology Kanpur.Retrieved28 December2018– via National Programme on Technology Enhanced Learning.
- ^McGrath, John W.; Chin, Jason P.; Quinn, John P. (2013). "Organophosphonates Revealed: New Insights into the Microbial Metabolism of Ancient Molecules".Nature Reviews Microbiology.11(6): 412–419.doi:10.1038/nrmicro3011.PMID23624813.S2CID32515430.
- ^Orgel, Leslie E. (1998). "The Origin of Life—a Review of Facts and Speculations".Trends in Biochemical Sciences.23(12): 491–495.doi:10.1016/S0968-0004(98)01300-0.PMID9868373.
- ^Ruiz-Bermejo, Marta; Zorzano, María-Paz; Osuna-Esteban, Susana (2013)."Simple Organics and Biomonomers Identified in HCN Polymers: An Overview".Life.3(3): 421–448.doi:10.3390/life3030421.PMC4187177.PMID25369814.
- ^Gribbin, J. (1995-12-09)."Structure of the Earth's atmosphere".New Scientist, 2007. p. 1.
- ^Trail, Dustin; Watson, E. Bruce; Tailby, Nicholas D. (2011). "The oxidation state of Hadean magmas and implications for early Earth's atmosphere".Nature.480(7375): 79–82.Bibcode:2011Natur.480...79T.doi:10.1038/nature10655.PMID22129728.S2CID4338830.
- ^"Earth's Early Atmosphere: An Update".NASA Astrobiology Institute.