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Aluminiul

Aluminiul este un element chimic, notat cu simbolul Al. Numrul atomic al aluminiului are valoarea13, iar masa atomic este 26.97. Este un element chimic comun, ocupnd poiia a treia, dup o!i"eni siliciu, ca rspndire terestr, e!istnd #n procent de 7.$%. &ompuii aluminiului constituie '.13%din scoara terestr, (iind #ntlnii #n substanele minerale, precum i #n lumea ve"etal i animal. )n

stare natural este #ntlnit sub (orma mineralelor, dintre care amintim silicaii, silicoaluminaii*(eldspat, mic, ar"ile+, criolitul *(luoaluminat de sodiu+, bau!ita, corindonul.up (ier, acesta a devenit metalul cu cea mai lar" #ntrebuinare. Aluminiul a (ost remarcat pentru(aptul c este un metal uor, cu o densitate de 2.7 "-cm3. Aceasta calitate #l (ace s (ie utiliat #ncantiti mari #n industria naval i aeronautic.&apacitatea mare de re(le!ie este (olosit #n construireao"linilor metalice.Este un bun conductor electric i termic, (iind (olosit #n industria electrochimic sub (orm de srm,#nlocuind conductoarele electrice din cupru, care sunt mai scumpe.Este un metal ductil i maleabil, (iind posibil obinerea unei (oie subiri de ./ mm "rosime.0otodat, aceast proprietate este utiliat #n industria alimentar, aluminiul (iind (olosit la ambalareaproduselor alimentare sau #n industria (armaceutic.

alt proprietate important a acestui metal este reistena la coroiune, care se datorea (ormriiunui strat protector de o!id. eist la aciunea chimic a acidului aotic diluat sau concentrat, iar acestlucru se re(lect #n (abricarea canistrelor transportoare de acid aotic din aluminiu.reint o a(initate mare pentru o!i"en, (iind utiliat #n obinerea altor metale precum &r, 4n, &o, 5din o!ii.

Stare natural

&el mai abundent element metalic din scoara terestr i al treilea element chimic ca rspndire. Nu se"sete #n stare nativ, (iind #ntlnit doar #n combinaii sub (orm de minereuri, dintre care cei maiimportani suntsilicaii i silicoaluminaii *i28Al238e238:2 ar"ila, ;

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ndustrial, aluminiul poate (i ob?inut prin prelucrarea bau!itei, care poate avea loc #n 2 (aerepararea aluminei *Al23+, proces prin care se separ alumina de celelalte impurit?i, precume23, i2, 0i2, etc. Aceast preparare are loc prin procedeul >aBer, bau!ita (iind calcinata latemperatura de 7 C& #ntr-un cuptor rotativ, #n scopul deshidratrii Di distru"erii materiilor or"anice.Are loc un proces de mcinare (in Di apoi o #nclire sub a"itare cu o solu?ie de Na: concentrat #n

autoclave a(late sub presiune de / atmos(ere Di temperaturi de 16-1' C&, timp de cteva ore. !idulde aluminiu trece #n solu?ie sub (orma aluminatului de sodiu, Naucureti, 1999.Frecu, &himie anor"anic, Editura didactic i peda"o"ic, >ucureti, 196$

Aluminium

Aluminium *or aluminum+ is a chemical element in the boron "roup Gith sBmbol Al and atomicnumber 13. t is silverB Ghite, and it is not soluble in Gater under normal circumstances.

Aluminium is the third most abundant element *a(ter o!B"en and silicon+, and the most abundant metal,

in the EarthHs crust. t maIes up about '% bB Gei"ht o( the EarthHs solid sur(ace. Aluminium metal is sochemicallB reactive that native specimens are rare and limited to e!treme reducin" environments.nstead, it is (ound combined in over 27 di((erent minerals. 0he chie( ore o( aluminium is bau!ite.

Aluminium is remarIable (or the metalHs loG densitB and (or its abilitB to resist corrosion due to thephenomenon o( passivation. tructural components made (rom aluminium and its alloBs are vital to theaerospace industrB and are important in other areas o( transportation and structural materials. 0he mostuse(ul compounds o( aluminium, at least on a Gei"ht basis, are the o!ides and sul(ates.

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espite its prevalence in the environment, aluminium salts are not InoGn to be used bB anB (orm o(li(e. n Ieepin" Gith its pervasiveness, aluminium is Gell tolerated bB plants and animals. Gin" totheir prevalence, potential bene(icial *or otherGise+ biolo"ical roles o( aluminium compounds are o(continuin" interest.

Natural occurrence

table aluminium is created Ghen hBdro"en (uses Gith ma"nesium either in lar"e stars or insupernovae.

n the EarthHs crust, aluminium is the most abundant *'.3% bB Gei"ht+ metallic element and the thirdmost abundant o( all elements *a(ter o!B"en and silicon+. >ecause o( its stron" a((initB to o!B"en, it isalmost never (ound in the elemental stateJ instead it is (ound in o!ides or silicates. eldspars, the mostcommon "roup o( minerals in the EarthHs crust, are aluminosilicates. Native aluminium metal can onlBbe (ound as a minor phase in loG o!B"en (u"acitB environments, such as the interiors o( certainvolcanoes. Native aluminium has been reported in cold seeps in the northeastern continental slope o(

the outh &hina ea and &hen et al. *211+ have proposed a theorB o( its ori"in as resultin" bBreduction (rom tetrahBdro!oaluminate Al*:+$K to metallic aluminium bB bacteria.

t also occurs in the minerals berBl, crBolite, "arnet, spinel and turLuoise. mpurities in Al23, such aschromium or iron Bield the "emstones rubB and sapphire, respectivelB.

Althou"h aluminium is an e!tremelB common and Gidespread element, the common aluminiumminerals are not economic sources o( the metal. Almost all metallic aluminium is produced (rom theore bau!ite *Al!*:+3K2!+. >au!ite occurs as a Geatherin" product o( loG iron and silica bedrocI intropical climatic conditions. Mar"e deposits o( bau!ite occur in Australia, >rail, Fuinea and amaicaand the primarB minin" areas (or the ore are in Australia, >rail, &hina, ndia, Fuinea, ndonesia,

amaica, ussia and uriname.

roduction

Aluminium (orms stron" chemical bonds Gith o!B"en. &ompared to most other metals, it is di((icult toe!tract (rom ore, such as bau!ite, due to the hi"h reactivitB o( aluminium and the hi"h meltin" point o(most o( its ores. or e!ample, direct reduction Gith carbon, as is used to produce iron, is not chemicallBpossible because aluminium is a stron"er reducin" a"ent than carbon. ndirect carbothermic reductioncan be carried out usin" carbon and Al23, Ghich (orms an intermediate Al$&3 and this can (urtherBield aluminium metal at a temperature o( 19K2 C&. 0his process is still under developmentJ it

reLuires less ener"B and Bields less &2 than the :all-:Oroult process, the maPor industrial process (oraluminium e!traction. ElectrolBtic smeltin" o( alumina Gas ori"inallB cost-prohibitive in part becauseo( the hi"h meltin" point o( alumina, or aluminium o!ide, *about 2, C& *3,6 C++. 4anB minerals,hoGever, Gill dissolve into a second alreadB molten mineral, even i( the temperature o( the melt issi"ni(icantlB loGer than the meltin" point o( the (irst mineral. 4olten crBolite Gas discovered todissolve alumina at temperatures si"ni(icantlB loGer than the meltin" point o( pure alumina Githoutinter(erin" in the smeltin" process. n the :all-:Oroult process, alumina is (irst dissolved into moltencrBolite Gith calcium (luoride and then electrolBticallB reduced to aluminium at a temperature betGeen9/ and 9' C& *1,7$ to 1,' C+. &rBolite is a chemical compound o( aluminium and sodium

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(luorides *Na3Al6+. Althou"h crBolite is (ound as a mineral in Freenland, its sBnthetic (orm is usedin the industrB. 0he aluminium o!ide itsel( is obtained bB re(inin" bau!ite in the >aBer process.

0he electrolBtic process replaced the QRhler process, Ghich involved the reduction o( anhBdrousaluminium chloride Gith potassium. >oth o( the electrodes used in the electrolBsis o( aluminium o!ideare carbon. nce the re(ined alumina is dissolved in the electrolBte, it disassociates and its ions are (ree

to move around. 0he reaction at the cathode isAl3@ @ 3 eS T Al

:ere the aluminium ion is bein" reduced. 0he aluminium metal then sinIs to the bottom and is tappedo((, usuallB cast into lar"e blocIs called aluminium billets (or (urther processin".

At the anode, o!B"en is (ormed2 2S T 2 @ $ eS

0o some e!tent, the carbon anode is consumed bB subseLuent reaction Gith o!B"en to (orm carbondio!ide. 0he anodes in a reduction cell must there(ore be replaced re"ularlB, since theB are consumed

in the process. 0he cathodes do erode, mainlB due to electrochemical processes and metal movement.A(ter (ive to ten Bears, dependin" on the current used in the electrolBsis, a cell has to be rebuilt becauseo( cathode Gear.

Aluminium electrolBsis Gith the :all-:Oroult process consumes a lot o( ener"B, but alternativeprocesses Gere alGaBs (ound to be less viable economicallB andUor ecolo"icallB. 0he GorldGideavera"e speci(ic ener"B consumption is appro!imatelB 1/V./ IiloGatt-hours per Iilo"ram o(aluminium produced */2 to /6 4UI"+. 0he most modern smelters achieve appro!imatelB 12.' IQWhUI"*$6.1 4UI"+. *&ompare this to the heat o( reaction, 31 4UI", and the Fibbs (ree ener"B o( reaction,29 4UI".+ eduction line currents (or older technolo"ies are tBpicallB 1 to 2 IiloamperesJ state-

o(-the-art smelters operate at about 3/ IA. 0rials have been reported Gith / IA cells .0he :all-:eroult process produces aluminium Gith a puritB o( above 99%. urther puri(ication can bedone bB the :oope process. 0he process involves the electrolBsis o( molten aluminium Gith a sodium,barium and aluminium (luoride electrolBte. 0he resultin" aluminium has a puritB o( 99.99%.

Electric poGer represents about 2% to $% o( the cost o( producin" aluminium, dependin" on thelocation o( the smelter. Aluminium production consumes rou"hlB /% o( electricitB "enerated in theX.. melters tend to be situated Ghere electric poGer is both plenti(ul and ine!pensive, such as theXnited Arab Emirates Gith e!cess natural "as supplies and celand and NorGaB Gith ener"B "enerated(rom reneGable sources. 0he GorldHs lar"est smelters o( alumina are eopleHs epublic o( &hina,ussia, and Yuebec and >ritish &olumbia in &anada.

e(erences Altenpohl, ietrich. Aluminum 5ieGed (rom Qithin An ntroduction into the 4etallur"B o(Aluminum abrication *En"lish translation+. usseldor( Aluminium-5erla", 19'2.Mide, . . *2+. Z4a"netic susceptibilitB o( the elements and inor"anic compoundsZ. &&:andbooI o( &hemistrB and hBsics *'1st ed.+. && ress. >N '$93$'1$.

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:elmboldt, . *27+. ZAluminum &ompounds, nor"anicZ. XllmannHs EncBclopedia o( ndustrial&hemistrB. QileB-5&:.Elschenbroich, &. *26+. r"anometallics. QileB-5&:. >N 97'-3-/27-2939-2.