All about the material alumina

1. ALUMINA
Irsalan Mohamed Asif - 1333309
[DATE]
HEWLETT-PACKARD
[Company address]

2. Background
Aluminaisone of the mostcost effective andwidelyusedmaterial outof the engineeringceramics.
The raw materialsare readilymaterialsare readilyavailable andreasonablypriced,fromwhichitis
made up. Ittakesform innature as a white ornearly colourlesssubstance andcanserve as the raw
material fora broad range of advancedceramicproducts.
Composition
Aluminaiscomposedof Aluminiumandoxygenwiththe chemical formulaAl2O3.Itisthe most
commonoccurring of several aluminiumoxidesandspecificallyidentifiedasAluminium(III) oxide.Its
mostcommon name isAluminabutisalsoreferredtoas aloxide,aloxite oralundumdependingon
particularformsor applications.InNature itiscommonlyfoundinitspolymorphiccrystalline state
labelledα- Al2O3.Polymorphismreferstothe abilityof aluminatoexistinmore thanone formor
crystal structure.In itsα- Al2O3 structure itcompromisesof the mineral corundumwhichis
transparentbutvarietiesof whichcanformruby and sapphire.Corundumisthe mostnaturally
occurringform of crystalline Aluminabutrubiesare sapphiresare gemforms,whichowe their
colourto impurities.However,itisfromthe more abundantoresof bauxite,cryolite andclaysfrom
whichthe material iscommerciallyextractedandpurified.
Crystal structure and bonding
In the mostcommon andstable crystalline formof aluminaithasa trigonal Bravaiscrystal lattice
structure (1).The oxygenionsformingahexagonal close–packedstructure and Aluminiumions
fillingabouttwothirdsof the octahedral spaces. Hexagonal sitesare the corneratomsin the cell
while the octahedral sitesare presentbetweenthe twolayersof vertical stacking(Figure1).
Aluminapossessstrongionicinteratomicbondingformingafixedlattice,givingrise toitsdesirable
material characteristics (1).Thisisformedbecause the oxygenatomsoneitherendof Al2O3 each
have a double covalentbondwithanaluminiumatom.The central Oxygenatomhasone covalent
bondwitheach aluminiumneighbouragainhavingtwocovalentbonds (1,2).EachAluminiumhas
three covalentbonds,twowithone oxygenatomandone withthe otheroxygenatom. There are
some disagreementsonwhetherAluminaisionicorcovalentasmostionicelementshave an
electronegativedifference of 2.0or above however, aluminahasanelectronegative difference of
1.83. However,recentmeasurementsintothe bondingof aluminahave concludedthat ithas 57%
ionicbondsand43% covalentbondsthus,still classifyingitasan ionicmaterial
Key Properties(2)
 Hard, wearand corrosion resistant– The hardnessof corundumthe most occurring
crystalline formof Aluminais suchthat itmakesit suitable foruse asa componentincutting
tools.Itis alsoresponsibleforresistance of metallicAluminiumtoweathering.Metallic
Figure 1 : illustrating
the crystal structure
of Alumina.

3. Aluminiumisveryreactive withatmosphericoxygen,anda thinpassive layerof Aluminais
formedonany expose Aluminiumsurface. Thislayerprotectsthe metal fromfurther
oxidation.The thicknessandpropertiesof thisoxide layercanbe enhancedusingaprocess
calledanodising.
 Resistsstrongacidand alkali attack – Aluminaisamphotericwhichmeansthatitdisplays
some of the propertiesof bothacidsandbases.Itwill reactwithsome basesandsome
acids.
 Good thermal conductivity –Aluminaisanelectrical insulatorbuthasa highthermal
conductivityforaceramic material.
 Good electrical insulation –The highvolume resistivityanddielectricstrengthmake alumina
an excellentelectrical insulator.
 Highstrengthand stiffness
 Highcompressionstrength
 Resistanttoabrasion
 Available inpurityrangesfrom94%,an easymetalisable composition,to99.8% forthe most
demandinghightemperature applications.
Manufacturing route (2)
Aluminaismainlyfoundinbauxite,the principal ore of aluminium.A mixture of the minerals
comprise bauxite ore,includinggibbsite,boehmiteanddiaspore alongwithimpurities.However,
Bauxite onlycontains30 -54 % aluminaandtwo to three tonnesof bauxite are requiredtoproduce
one tonne of Alumina(2).PurificationoccursviaBayerprocess.
1.Milling:The Bauxite isfirstwashedandcrushed,reducingthe particle size andincreasingthe
available surface areaforthe digestionstage.Lime isaddedtomake itintoa slurry.
2.Desilication:Bauxitesthathave a highlevel of Silica(SiO2) gothroughaprocessto remove this
impurity,because Silicacancause problemswiththe qualityof the final product.
3.Digestion:A NaOH solutionisusedtodissolvethe aluminium-bearingmineralsinthe bauxiteto
forma sodiumaluminatesupersaturatedsolution(2).Conditionswithinthe digesterare set
accordingto the propertiesof the bauxiteore.Oreswithahighgibbsite content canbe processedat
140°C, while böhmiticbauxitesrequire temperaturesbetween200 and 280°C. The pressure isnot
importantforthe processbut isat 3.5 MPa. The slurryis thencooledina seriesof flashtanksto
around106°C at atmosphericpressure.Althoughhighertemperaturesare oftenadvantageous,a
majordisadvantage isthe possibilityof otheroxidesotherdissolvingintothe NaOH.
4.Clarification/Settling:The firststage of clarificationisto separate the solids(bauxite residue) from
the pregnantliquor(sodiumaluminate remainsinsolution) viasedimentation.Flocculantsare added
to assistthe sedimentationprocess(2).Furtherclarificationisdone usingsecurityfilterstoensure
that the final productisnot contaminatedwithimpurities.
5.Precipitation:In thisstage,the aluminaisrecoveredbycrystallisationfromthe pregnantliquor,
whichissupersaturatedinsodiumaluminate(2).Crystallisationisdrivenbyprogressivecoolingof
Al(OH)3 +Na+
+ OH-
→ Al(OH)4
-
+Na+
AlO(OH) +Na+
+ OH-
+ H2O → Al(OH)4
-
+Na+
sodiumaluminate supersaturatedsolutionreactions

4. the pregnantliquor,resultinginsmall crystalsof aluminiumtrihydroxite (Al(OH)3,),whichthengrow
and agglomerate toformlargercrystals
6.Classification:The crystalsformedinprecipitationare classifiedintosize ranges.Thisisnormally
done usingcyclones orgravityclassificationtanks.The coarse size crystalsare destinedfor
calcinationafterbeingseparated. The fine crystals,afterbeingwashedtoremove organic
impurities,are returnedtothe precipitationstage tobe agglomerated.
8.Calcination: The SeparationisfedintoRotarykilns(Calciners) wheretheyare roastedat
temperaturesof upto1100°C to drive off free moisture andchemically-connectedwater,producing
aluminasolids. Calcinationreaction:
2Al(OH)3 → Al2O3 + 3H2O
Alumina,awhite powder,isthe productof thisstepandthe final productof the BayerProcess.
Applicationsof Alumina(3)
As a medical implant,Aluminaisahighlyinert material andresistanttomostcorrosive environment
eventhe dynamicenvironmentwhichisthe humanbody.Itis unreactive thuselicitinglittle response
fromsurroundingtissuesandcanremainunchangedaftermanyyearsinuse. However,the body
doesrecognise Aluminaasa foreignmaterial andwillcoveritwithafibroustissue where possible.
Articulating surfacesin jointreplacements:Due to itsabilitytobe polishedtoa highsurface finish
and itsexcellentwearresistance,Aluminaisoftenusedforwearsurfacesinjointreplacement
prosthesis.Examplesof these are Femoral headsforhipreplacementsandwearplatesinknee
replacemnets.
Dental applications:Dense Aluminahasbeenusedfortoothreplacements specificallyfor
replacementsof teeth. Inmanycasessingle crystal Aluminaisused since polycrystallineAlumina
can be fracturedwhile insertingthe implantintothe dental root.Typically,singlecrystal Alumina is
the core of the implantaroundwhichpolycrystallineAluminaisfused.
Bone Spacers: Aluminawithmore than30% porositycan be usedas bone spacersto replace
sectionsof bone thathave beenremoveddue tocanceror trauma. The porous nature of the implant
allowsforbone cell infiltrationof the implant,eventuallyresultinginnew bone formation. Alumina
can alsobe usedformaxillofacial regioninreplacingthe jaw bone orpartsof it.
Cochlearimplants: Some partsof the cochlearimplantsuchas the antennae have beenmade with
Aluminabecause of the highstiffnessandgoodthermal conductivityitdisplays.
References
Figure 1 : http://sist.fnal.gov/archive/97-topics/papers/JosePerez/paper.html.Accessedon
15/04/2014
1. Paglia,G. (2004). "Determinationof the Structure of γ-AluminausingEmpirical andFirstPrinciples
CalculationsCombined withSupportingExperiments" .CurtinUniversityof Technology,Perth.
2.Gitzen,W.H (2006) Alumina asa Ceramic Material,Westerville:JohnWiley&sons.
3. Ratner,B.D; Hoffman,A.S;Schoen,F.J;Lemons,J.E(2012) BiomaterialsScience: An Introduction to
Materialsin Medicine,3rd edn.,Oxford:Elsevier.