Extraction of Aluminum (Al)

This ppt include all physical , chemical process of extraction of Aluminum.

1. Aluminium Production
Extraction of Non-Ferrous Metals

2. Details about of Al
AtomicNumber:13
DetailsaboutofAl:
AtomicWeight:27amu
Density:2.702gm/cm
3
MeltingPoint:660.37℃
BoilingPoint:2467℃

3. Common Minerals
Bauxite(Al
2O
3.xH
2O)
Kaolinite(Al
2O
3.2SiO
2.2H
2O)
Cyanite(Al
2O
3.SiO
2)
Sillimanite(Al
2O
3.SiO
2)
Corundum(Al
2O
3)
Cryolite(Na
3AlF
3)
Aluminite(Al
2O
3.SO
3.9H
2O)

4. Problems of conventional pyro-metallurgy processing
Carbothermicreductionofaluminiumoxideisnotcommerciallyfeasiblebecauseof:
Hightemperaturerequirement(~2000℃).
Formationofundesirablealuminiumcarbideatthistemperature.
Refractoryrequiredforsuchhightemperatureareexpensiveandalsonotreadily
available.

5. Extractionofaluminiumbyelectrolysisofaluminiumsaltinaqueousmediumisnot
feasible.Thisisbecause:
Hydrogenevolution,duetothedecompositionofaqueoussolutionoccursatamuch
lowervoltagethanthatneededfortheelectrodepositionofaluminiumions.
Thehydrogenovervoltageisinsufficienttoovercomethe1.67-voltdifferenceinpotential
betweenaluminiumdepositionandhydrogenevolution.
Problems of electrolysis of aqueous salt

6. Primary AluminumProduction
Theprimaryoreofaluminumisbauxite,amixtureofhydratedaluminumoxides:
•Gibbsite-Al(OH)
3(mostextractableform)
•Boehmite–γ-AlO•OH(lessextractablethanGibbsite)
•Diaspore–α-AlO•OH(difficulttoextract)
Itisformedbyweatheringofaluminum-bearingrocksbyrainwater,andsobauxitedepositstendtobefound
inareasthatarenow,orwereinthepast,tropicalhigh-rainfallareas.
Aluminumiscontainedinmanyminerals,butbauxiteisthepreferredorebecauseithasthehighest
aluminumoxidecontentandisthereforethecheapesttoprocess.
Althoughaluminumisanextremelycommonelementonearth,itwasnotpracticaltoproducealuminum
metalatareasonablecostuntiltwobreakthroughshadbeenmade:amethodforproducingpurified
aluminumoxidefrombauxite(theBayerprocess),andamethodforconvertingaluminumoxidetometallic
aluminum(theHall-Heroultprocess).

7. General method of extraction:
Step1:ProductionofAlumina(Al
2O
3)fromBauxite(Al
2O
3,xH
2O)BayerProcess.
Step2:ElectrolyticaldecompositionofAlumina(Al
2O
3)dissolvedincryolite(AlF
3.3NaF)
bathHall-HeroultProcess.

8. The Bayer process
StepsinvolvedinBayerProcessare:
Step1.Digestion:Dissolutionofthealuminaatelevatedtemperatures.
Gibbsite:????????????
2�
3.3??????
2�+2�??????�??????
135−145℃
2�??????????????????�
2+4??????
2�
(??????−????????????(�??????)
3)
Boehmite:????????????
2�
3.??????
2�+2�??????�??????
205−245℃
2�??????????????????�
2+2??????
2�
(??????−????????????��??????)
Diaspore:????????????
2�
3.??????
2�+2�??????�??????
??????????????????ℎ??????????????????????????????????????????��????????????
2�??????????????????�
2+2??????
2�
(α−????????????��??????)
Step2.Clarification:Additionofflocculantsthenseparationandwashingoftheinsoluble
impuritiesofbauxite(redmud)torecoverthesolublealuminateandcausticsoda.
High Pressure

9. The Bayer process (continue..)
Step3.Precipitation:PrecipitationofpureGibbsitebyseedingthecooled,clarified
solutionwithpreviouslyprecipitatedcrystals.
NaAlO
2+2H
2O==>Al(OH)
3+NaOH
Step4.Regeneration:Regenerationofthesolutionsforrecyclingtostep1byevaporation
ofthewaterintroducedbythewashings.
Step5.Calcination:HeatingtheprecipitatedGibbsite,to1100°C(calcination),toremove
thechemicallycombinedwater,henceproducingalumina.
2Al(OH)
3==>Al
2O
3+3H
2O

10. The Bayer process (continue..)

11. The Bayer process (continue..)

12. FactorsaffectingBayerProcess:
Finerbauxitegivesbetterdigestionofaluminainliquidliquor.
Wetgrindingismoreefficientthandrygrinding.
Digestionacceleratesathightemperature.Topreventevaporationofwaterathigh
temp.(say220℃)ahighpressure(say25atm.)isapplied.
Thereisalowerlimitoftemp.atwhichdigestionanditssubsequentprocessingcanbe
carriedout.Belowthistemp.aluminumhydroxidemayprecipitateoutduring
clarificationand/orfiltration.Thismayleadtoaluminalossanddeteriorationoffilter
clothduetoclogging.
Sensibleheatofhotpregnantliquorafterfiltrationisrecoveredbyheatexchangers.
Inprecipitationstage,entireprecipitatesofaluminanotallowbecauseitmaycause
precipitationofsilicaalongwithalumina.
Forefficientcalcinationsofaluminainrotarykiln,requiredtemp.is1400℃.
DecompositionofAl(OH)
3producesAl
2O
3.H
2OandAl
2O
3.3H
2O.Required
dehydrationtemp.is1200℃and800℃respectively.Theadditionofsmallamountof
CaF
2sometimeshelpsindehydration.

13. Bayer Process(continue..)
Bauxite
Raw material preparation (crushing, grinding etc.)
Leaching with NaOH
Al
2O
3+ 2NaOHNaAlO
2 + H
2O
T up to 300
0
C
P up to 40 atm.
Solid/Liquid separation
Leach residue (Red Mud)
Fe
2O
3, SiO
2, TiO
2etc.
Precipitation of Al(OH)
3
Separation of Al(OH)
3
Calcination of Al(OH)
3
Al
2O
3(99.6%)
T ~ 1100
0
C
2Al(OH)
3Al
2O
3+ 3H
2O
Seeds of Al(OH)
3
NaOH Regeneration
Hydrometallurgy

14. In1886,Hall(intheU.S.)andHeroult(inFrance)independentlydevelopedthis.processesusingcryolite
(Na
3AlF
6)asamoltensaltelectrolyteforproducingaluminum.Cryoliteiselectricallyconductive,and
dissolvesalumina.
Boththeanodesandthecathodearemadeofcarbon.Theanodesaregraduallyconsumedbytheoxygenthat
migratestotheanodes,andsotheoverallelectrolysisreactionis
2Al
2O
3+ 3C ==> 4Al + 3CO
2
Thetheoreticalvoltageforthisreactiontooccuris1.15volts,butduetoanodeovervoltagesthepotentialin
actualpracticeis1.5-1.7volts.Inordertoovercometheelectricalresistanceoftheelectrodes,conductors,
andcontainers,thetypicaloperatingvoltageis4-5volts.
Operatingconditionsforanaluminumelectrolysiscellare:
•Temperature:935-975°C
•Aluminacontentofelectrolyte:2-6%
•Cellvoltage:4-5volts
•Faradayefficiency:85-90%
Hall-HeroultProcess

15. Additivestocryolitebath:
•AlF
3(improvesFaradayefficiency)
•CaF
2,LiF,MgF
2(reducefreezingpointofelectrolyte)
RawmaterialandpowerusagepertonofAlproduced:
•Al
2O
3:1.90-1.95
•Electrolyte:0.04-0.06
•Anodecarbon:0.43-0.50
•Power,Kw-hr:13,000-16,000
Wastesproducedinaluminumelectrolysis:
•HF,CF
4,andotherfluorocarbons(fromanodereactions)
•CO
2
•“Saltcake”(spentelectrolyte,metaloxides,andentrainedmetallicaluminumdroplets)
Hall-HeroultProcess(continue..)

16. Synthesise of Cryolite

17. Notethetremendouspowerconsumptionofaluminumproduction.
Theelectrolysismustsupplythreeelectronsforeveryatomofmetallicaluminum,andsoveryhigh
currentsareneededtoproducealuminumatareasonablerate.
Becauseofthis,electricpoweristhesinglelargestcostinaluminumproduction,andsoaluminum
smeltersaretypicallylocatedinareaswhereelectricpowerisinexpensive,generallynearmajor
hydropowersites.
Thehighpowerconsumptionofprimaryaluminumproductionalsoencouragesaluminum
recycling,whichrequiresmuchlesselectricpower
At1000℃densityofmoltencryoliteis2.1gm/cm
3
andAl
2O
3is3.96gm/cm
3
.Alliberated
heavierthanbathandsinksatbottom.HighertheamountofAl
2O
3inbath,thehigherits
densityandconsequentlythesinkingofAlisrenderedmoredifficult.
Hall-HeroultProcess(continue..)

18. ForAlcathodeandoxygenanodethedecompositionreactiongivenas:
�.�????????????
��
���??????��??????��??????��??????
�??????????????????
�=??????????????????+�.���
�??????……….(1)
Inpracticeconsumablegraphiteelectrodeservesasanode.Therefore,decompositionpotentialfor
????????????(??????)cathodeandcarbon(c)anodehastobeincorporatedi.e.Eq.(1)hastobecombinedwith
carbonoxidationreactions:
�.���
�+�.��????????????=�.��??????�
�(??????)………..(2)
�.���
�??????+�.�????????????=�.�??????�(??????)………… (3)
CombiningEq.(1)andEq.(2):
�.�????????????
��
���??????��??????��??????��??????
�??????????????????
�+�.��????????????=??????????????????+�.��??????�
�??????….(4)E=1.19V
CombiningEq.(1)andEq.(3):
�.�????????????
��
���??????��??????��??????��??????
�??????????????????
�+�.�??????(??????)=??????????????????+�.��??????�??????…….(5)E=1.05V
Where??????isthedecompositionpotential.ItistobenotedthatEq.(4)andEq.(5)areofequal
importance.Therefore,thetheoreticaldecompositionpotentialcanbetakenas{(1.19+1.05)/2}or
1.12V.
Hall-HeroultProcess: Decomposition Potential of Al
2O
3in dissolved in Cryolite

19. Carbonreducesdecompositionvoltageofaluminaincryolitebathfrom2.2Vto1.2V.
EffectofH
2addition:
�
�
????????????
��
�+
�
�
??????
�→????????????+
�
�
??????
�O[Decompositionpotentialabout1.3V]
Effectof????????????
4addition:
�
�
????????????
��
�+
�
�
????????????
�→????????????+
�
�
??????
�+
�
�
??????�
�[Decompositionpotentialabout1.06V]
Injectionofhydrogenormethaneisnotmucheffectiveinreducingdecompositionpotential
ofAl
2O
3.Suchinjectionmayeffectiveincutdownthegraphiteconsumption.
Hall-HeroultProcess: Influence of H
2or CH
4injection at Anode

20. Whenthealuminaconcentrationoftheelectrolytefallstoolow(between1and2%),a
phenomenoncalledanodeeffectoccurs.
Asaluminaisdepletedfromtheelectrolyteanodicovervoltageincreases.Theincreasein
surfacetensionofthebathwithdecreasingaluminaconcentrationcoupledwiththe
electrocapillaryeffectresultingfromthehigheranodeovervoltagereduceswettingofthe
anodeandthebubblecontactangledecreases,forminglargebubbles.Gasbubbleshielding
causesthecurrentdensityonunshieldedareastoincrease.At1.2Vanodeovervoltage,
sufficientfluorineactivityisproducedtocausefluorinetobondtothe
carbon.Fluorocarboncompoundshavelowsurfaceenergyandfurtherdecreasewettingof
theanodebyelectrolyte.Soonacontinuousfilmofgasliesbetweenthebathandtheanode.
Sincethecellsareoperatedatessentiallyconstantcurrent,thevoltagerisesandcurrent
flowsacrossthegasfilmbysparkingorarcing.Thisphenomenoniscalledtheanode
effect.DuringanodeeffectthegasfromtheanodechangesfromCO
2toamixtureof2–
10%CO
2,10–20%CF
4,1–3%C
2F
6andthebalanceCO.
Hall-HeroultProcess: Anode Effect

21. BathTemperature:Increaseintemperature,currentefficiencydecrease.Forevery4
0
Criseinbath
temp.(abovethenormal970℃)currentefficiencydecreasesby1%duetheincreaseinNo.ofside
reactionsandgreaterdissolutionofmetalinthebath.Morenumberofsidereactionandhence
dissociationofmetalinbath.
CurrentDensity:Currentefficiencyincreasewithincreaseincurrentdensity,therefore,thecurrent
densityismaintainedataveryhighvalue.
InterpolarDistance(distancefromthebottomoftheanodetothetopthetopofthebath):As
theinter-polardistance,currentefficiencyincreasestoamaximum(~90%)whenthedistanceis~65
cm.
AdditionofAl
2O
3:AtAl
2O
3contentofbathis4%,thecurrentefficiencyisminimum.Atvalue,
higherorlowerthan4%currentefficiencyincreases.Tomaintainhighcurrentefficiencycellshould
beoperatedwithhighAl
2O
3content.
BathDensity:Lowerbathdensityyieldshigherrateofseparationofliberatedmetal.Bathdensity
loweredbyhighAlF
3/NaFratio.But,forlargeAlF
3%lowersbathconductivity,solubilityofalumina
andlargevolumeoffluorineproduced.AhighNaFcontentdecreasescurrentefficiencyduetoan
increaseinconcentrationofNaionsandNadepositiononthecathode.AnNaF/AlF
3ratioslightly
lessthan3isoptimum.
Hall-HeroultProcess: Factors Influencing Electrolysis

22. ElectricalapplicationsandcanningrequiresAlof
morethan99.5%purity,whichcannobeobtained
byHall-Heroultprocess.
Three-layerprocessisanelectrolyticmethod,
employedtoattainsuchhighdegreeofpurity.
Adenseelectrolyteofapproximatecomposition:
aluminumfluoride36%,cryolite30%,barium
fluoride18%andcalciumfluoride16%formsthe
middlelayer.
DensityoftheelectrolyteenablestheAlpurified
duringelectrolysistofloatupwardandformthetop
layer.
TheimpuremetalisalloyedwithCu,sandthis
heavyalloyformsthebottomlayer.
Temp.ofoperationabout950℃.
Electrolytic Refining of Aluminum: Three-Layer Process

23. Averagepurityofthemetal99.99%.
Productivity:about100kgpercellperday.
Consumptionofelectrolytesandgraphiteeach
about6%ofthemetalproduced.
Theprocessisineffectiveiftheimpurealuminum
containsmagnesiumsayAl-Mgscrap,becauseMg
reactswithelectrolytesandrendersitunsuitable.
Electrolytic Refining of Aluminum: Three-Layer Process (continue..)

24. Themainenvironmentalconcernoftheindustryisrelatedtoprimaryaluminumproduction,where
GHGsincludingCO
2,fluorinatedcompounds,SO
2,etc.,aregeneratedasaresultoftheanode
effectsduringtheprocessofaluminumproduction.
CO
2:AmajorGHGintheenvironment,whichmayleadtovarioushealthimpactswithinthe
contextofglobalwarming.
SO
2:Conifers,lichens,andmossesareamongtheplants,whicharemostsensitivetoSO
2,a
principalpollutantcontributingtoacidrain.Thus,SO
2emissioninatmosphereistheoneofthe
majorcausesofatmosphericacidification,therebyaffectinglivingbeingsontheearth.
Fluorinatedcompounds:99%oftheseemissionsareproducedbyelectrolyticcells.Fluorineisa
highlyreactiveandtoxichalogenreadilyabsorbedbyplants(particularlyconifers).Animals,
includinglivestockthatfeedoncontaminatedplantscandeveloposteoporosisoreven
osteosclerosis.Also,itmaycausedentalfluorosisduringthecriticaltoothdevelopmentperiodin
childrenlessthan6yearsofage.Thepermissiblelimitforfluorineinforageisusuallyintheorder
of40ppm.Continuousexposuretofluorinecompoundsfor8h/daymayleadtoeyeandskin
irritations(>900μg/m
3
),oftherespiratorytractirritations(>2100μg/m
3
),dentalfluorosis,and
osteoporosis(>1780μg/m
3
).Aluminumplantworkersarethemostexposed,becauseambient
concentrationsinpotlinescansometimesreach1500μg/m
3
.Chlorofluorocarboncompounds
emissionintoatmospheremaycauseozonelayerdepletion.
Hall-HeroultProcess: Environmental Problems

25. Polycyclicaromatichydrocarbons:Severalpolycyclicaromatichydrocarbons(PAHs)are
consideredpotentialcarcinogens.Workersaremostatrisk,andbladdercancerhasbeenassociated
withPAHexposure.Benzo{a}pyreneisatypicalPAH,anditsambientairconcentrationshouldnot
exceed0.9ng/m
3
.
Spentpotlining:Therefractorymaterialsintheelectrolyticcellsarecommonlyreferredasspent
potlining.Becauseofmainlycarbon,cyanide,andfluoridecontents,spentpotliningisconsidered
hazardouswasteandmustbestoredsafely,whenexposedtowater,acids,oralkalinesubstances,
potliningcanemittoxic,flammable,andpotentiallyexplosivegases(fluorides,ammonia,
hydrogencyanide,sulfuroxides,hydrogen,andacetylene).
Coaltarpitch:Volatilesfromcoaltarpitchintheworkroomatmosphereofaluminumsmelters
containPAHs.Atherosclerosisisacharacteristiclesioninischemicheartdiseaseandperipheral
arteriosclerosis.
Otherhumanhealthproblemsassociatedwithtoxicemissionsfromaluminumproductionare
cancer,senilepsychoses,hypertensivedisease,Ischemicheartdisease,cerebrovasculardisease,
peripheralarteriosclerosis,pneumonia,chronicbronchitis,emphysema,asthma,pepticulcer,liver
cirrhosis,chronicnephritis,andinextremecasesmaycausesuddendeath.
Hall-HeroultProcess: Environmental Problems (continue..)

26. ThealuminaproducedbyBayerprocessischlorinated
underreducingconditioninpresenceofcarbonsat700-
900℃toproduceamixturecontainingAlCl
3,COsand
CO
2.
????????????
��
�??????+�????????????+�????????????
�??????→�????????????????????????
�??????+
??????�??????+??????�
�(??????)
TheAlCl
3vaporisseparatedbycondensationatabout
70℃inafluidizedbedcontainingAlCl
3particles.
ThesolidAlCl
3particlesthusformedarecontinuously
feedintosanelectrolyticcellcontainingafusedchloride
electrolytemadeof5%AlCl
3,50%NaClsand45%LiCl;
maintainedat700℃.
Uponelectrolysisbydirectcurrent,liquidaluminum
formedsatcathodesandgaseouschlorineliberatedat
anode(recycledtochlorinatefreshalumina).
Newer Processes for Aluminium Production: ALCOA Process
Flow Sheet of ALCOA Process
ALCOAProcess:Thechlorideprocesswasdevelopedby
AluminumCompanyofAmerica(ALCOA)is'muchmoreenergy
efficient.

27. TheALCOAbipolarcellconsistsofseveralbipolarelectrodes,
whichbehaveslikeascathodesatthetopsurfacesandlikesan
anodesatthebottomsurface.
Acontinuousflowofelectrolytemustbemaintainedacrossthe
celltopreventformationofmoltenaluminumpoolonthe
electrodes.
Arrangementsaremadetocollectmoltenaluminumatthebottom
andchlorineatthetop.
Onebipolarcellisequivalenttofive(5)conventionalmonopolar
cellsinseries.Theproductivityandcellvoltagearealso5times
thanthosemonopolarcell.(Claimedoperationalcurrentdensity
upto23,000A/m
2
withelectrodegaplessthan1.3cm).
TheconductivityoftheChlorideelectrolyte(4.0mho/cm)ismuch
higherthanHall-Heroultcell(2.8mho/cm).alsothedistance
betweentheelectrodesarelowerthistwo(2)factorslowers
energywastedinformofheat.Further,thechlorideelectrolyte
leadstohighercurrentefficiencysanddecreasesoverallenergy
consumptionbyover30%.
Newer Processes for Aluminium Production: ALCOA Process (continue..)
ALCOA Bipolar Cell