Extraction of magnesium (mg)

It's include all pyrometallurgical process of extraction of Magnesium

1. Extraction of Mg
Department of MME
Extraction of Non-Ferrous Metals

2. Details about of Mg
Atomic Number: 12
Atomic Weight: 24 amu
Density: 1.738 gm/cm
3
Melting Point: 650
0
C
Boiling Point: 1107
0
C

3. Common Minerals of Mg
Dolomite–MgCO
3.CaCO
3
Magnesite–MgCO
3
MgCl
2andMgSO
4presentinseawater
Brucite-[Mg(OH)
2]
Carnallite-(MgCl
2.KCl.6H
2O)
Olivine-(Mg
2SiO
4)
Serpentine-(Mg
3Si
2O
7)
Asbestos–(CaSiO
3.3MgSiO
3)
Kainite-(MgSO
4.KCl.3H
2O)
Dolomiteandmagnesitearecommercialminerals.
MagnesiumhydroxidederivedfromseawaterhavesufficientlyhighMgcontentsanddonot
requireupgradationbeforesubsequentextraction.

4. Problems encountered during the extraction of Mg by
Pyrometallurgical process
MgOisverystableoxideandrequiresT>1800℃for
carbothermicreduction.
Suchhightemp.isnotpracticablewithexternal
heating.
Reductiontemp.canbereducebyusingvacuumto
distilofftheMgandtodrivethereductionreactionin
forwarddirection.
????????????????????????+????????????↔??????????????????+????????????(??????)
However,itisverydifficulttopreventthebackward
reaction,andhencecarbothermicreductionhasnot
commerciallyaccepted.
ReductionofMgObyotherreductantslikeCaC
2and
Alat1200℃undervacuumisfeasible,butnot
commerciallyattractive.
????????????????????????+??????????????????
2??????→????????????????????????+??????????????????+2??????
3????????????????????????+2??????????????????→????????????
2??????
3??????+3????????????(??????)

5. Problems encountered in Electrometallurgy viz. direct
electrolysis from aqueous medium
Mgrequires2.3VmorethanH
2fordepositionandthehydrogenovervoltageisnot
sufficientlyhightoovercomethisvoltagehydrogenisevolvedbeforemagnesiumcan
bedeposited.
Togeneratesufficientlyhighhydrogenovervoltage,amalgamcathodecanbeemployed.
Itcanservetwo(2)purposes:
1.Becauseofitsextremelysmoothsurface,hydrogenovervoltageisrendereddifficult.
2.ThelowactivityofMginamalgamcathodelowerstheMgdepositionpotential.
However,amalgamcathodehasnotfoundcommerciallyacceptable.

6. Extraction of Mg by Pyrometallurgical process

7. PIDGEON Process
ThermodynamicallypureSicannotreduceMgOwhenbothreactantandproductsareintheirstandardstate.
Therefore,thePidgeonprocessreducespressuretoremovetheMgvaporanddrivethereductionprocess.In
thisprocess,calsineddolomiteisbriquettedwithpoweredferrosilicon(75%Si),andthereductioniscarried
outundervacuumof0.1mmHgat1100-1200℃inanexternallyheatedretort.
�??????????????????.????????????????????????+�????????????????????????→�??????????????????+�??????????????????.??????????????????
�??????+�??????(??????)……….(1)
SinceMgistheonly,itcanbeeasilyremovedfromthereactionchambertoavoidpossibilityofbackward
reaction.
PresenceofCaOlowersthefreeenergyofthereductionreaction.Thefreeenergiesofthereactionsat
1200℃:
�??????????????????+??????????????????→�??????????????????+??????????????????
�?????? ∆�
�
=���????????????�,∆�
�
=����????????????�……..(2)
�??????????????????+�????????????????????????+??????????????????→�??????????????????+�??????????????????.??????????????????
�?????? �
�
=�??????�????????????�,∆�
�
=����????????????�…(3)
At1200℃theequilibriumpressureofMgforreaction(3)is15mmHg,andtoattain1atm.ofMgabout
1700℃isrequired.Suchhightemp.isnotpracticablewithexternalheating,therefore,Pidgeonprocess
incorporatesvacuumtodrivethereaction(3)inforwarddirection.
Reaction(6)ishighlyendothermic.AdditionofCaF
2andMgF
2actsascatalystandspeedsuptheevolution
ofMgvapor.

8. PIDGEON Process (continue..)

9. PIDGEON Process (continue..)
ReactionMechanism:
Thereactionoccursthroughtransientliquidphaseatsomeintermediatestages.DistinctstagesofPidgeon
processare:
(1)TheinitialreactionbetweenCaOandFe-SiproduceliquidCa-Si-Fealloy,whichpermeatesthebriquette
andformsametallicnetwork.Thereactionoccurataround1000℃andmildlyexothermic.Theternary
alloyservesasthemainreducingagent.
(2)MgvaporsproducebyreductionofMgObyCa-Si-Fealloy.Atthisstage,pressureincreasesrapidlyand
slowsdowntherateofreaction.TesubsequentreactionrateisgovernedbytherateatwhichMgcanescape
fromthebriquettes.
KineticsofthereactioncontrolledbythepresenceofSiOvaporinPidgeonprocessas:
??????????????????(c)+????????????(c)→????????????(g)+??????????????????(g)
2??????????????????(g)+2??????????????????(c)→2??????????????????.??????????????????
2(c)+????????????(c)

10. MAGNETOTHERM Process
Itisessentiallyaferrosiliconreductionprocesscarriedoutat1500℃andthebathis
maintainedinmoltenstatebytheadditionofaluminumtoformamoltenslag.

11. DOW Process: Extraction of Mg from Sea-Water

12. DOW Process: Flow Chart

13. DOW Process: Flow Chart Explanation
Developed by the Dow Chemical Company (U.S.A).
It involves the collection of Mg as insoluble hydroxide by precipitation with lime as:
????????????????????????
2+????????????(????????????)
2→????????????(????????????)
2+????????????????????????
2
????????????????????????
4+????????????(????????????)
2=????????????(????????????)
2=????????????????????????
4
Thesuspensionisdistributedtothickeners,inwhichthehydroxidesettleandthecleaneroverflow
pumpedbacktothesea.
Themilkofmagnesia(slurry)underflowispumpedtofiltertank,andsubsequentlyagitatedwith
10%HCl,whichconvertsittoaweaksolutionofchloride.Thisdilutesolutionisconcentratedand
filteredtorecoverthecrystallizedsolids.
Thefiltrateisthensubjectedtoevaporationtorecoverremainingchloride.Evaporationis
carefullycontrolledasthehydrouschlorideisliabletolosehydrogenchloride,whichresultsin
oxidecontaminationinthefinalproduct.Thereactionis:
????????????????????????
2.6??????
2??????=??????????????????+2??????????????????+5??????
2??????

14. DOW Process: Flow Chart Explanation
ElectrolysisofMagnesiumChloride:
Principle:
Mgisreactivemetal,hasahighdecompositionpotential.Therefore,duringaqueouselectrolysis
hydrogenevolutionoccursbeforethedecompositionofMgCl
2Notfeasible
ForelectrolysisofMgCl
2,theelectrolyteisnormallycomposedof25-30%MgCl
2,15%CaCl
2and
50-60%NaCl.Alargeamountoffluxisnecessarytomaintainfluidityandtoincreasethedensityof
thebathsothattheliberatedmetalatcathodefloatsonthesurfaceoftheelectrolyte.Thetheoretical
decompositionvoltageofdissolvedMgCl
2is3.3,buttheresistanceofthebathraisesthevoltageto
6.6.Acurrentof30,000-50,000Aisemployedandthetemperatureoftheelectrolyticcellis
maintainedat670-730℃.
Celldesign:
TheelectrolyticcellforMgextractionmaybedividedintotwo(2)categories:linedandunlined.
Unlinedcellmustbeprovidedwithseparatecathodes,usuallymadeofsteel,whereasincaseof
unlinedcell,thesteelwallofthecellitselfmaybeusedascathode.Graphiteanodeisusedforboth
cell.Theanodesareinert,therefore,notconsumed.Thecelloperateinaninertatmosphere.

15. DOW Process: Flow Chart Explanation
Itisnecessarytopreventchlorinationofmetallic
productbygaseouscholineevolvedattheanode.Thus,
themetalmustbedivertedawayfromtheanodezone
andrecombinationpreventedbyspecialdesignfeatures.
Clgasiscollectedinaspecialhoodandthecollection
ofthemetalisfacilitatedbydivertingtoapool.The
chlorinesetfreeattheanodeisusedtodrythe
incomingMgCl2andalsotochlorinateMg(OH)2.
However,completedryingisbothexpensiveand
difficult.Therefore,forelectrolyzingachloridethathas
beenoneandtwomolesofcombinedwaterhavebeen
evolved.

16. Uses or Applications of Mg
Magnesium
Non-structural
Uses
Alloying
De-oxidation and
De-sulphurization
Modifying
structure of
graphite in cast
iron
Pyrotechnics
and
Photography
Cathodic
protection
structural
Uses

17. Uses or Applications of Mg
Nonstructuraluses:
1.Alloying:Mgiswidelyusedinalloying,especiallyinAl-Basedalloys,toimprovestrength,
hardnessandcorrosionresistance.ItalsoimprovesmechanicalpropertiesofPb-basedalloys
containingCa,SnandIn.
2.Deoxidationanddesulphurization:Mghasstrongaffinityforoxygen,effectivelydeoxidizes
manymoltenmetals.Forproductionofironandsteel,theScontentintheBFcokeisratherhigh,
andthepigironcontainsalargequantityofS.ToreducetheSulphurlevellessthan0.01%,Mag-
Cokeormagnesiumwireisinjectedintothehotmetal.
3.Modifyingstructureofgraphiteincastiron:Mgiseffectivenodulizerofgraphiteparticlesin
castironforproductionofspheroidalgraphite(S.G.iron).S.G.ironisstronger,tougherandmore
ductilethanordinarycastiron,andiswidelyusedinautomotiveindustry.
4.PyrotechnicsandPhotography:Mgemitsabrilliantlightwhenignited.Thispropertyis
extensivelyusedinpyrotechnicsandphotography.FlashlightpowderconsistofMgpowdermixed
withchemicalswhichreadilygiveoffoxygenwhenheated,e.g.potassiumchlorate,potassium
permanganateandmagnesiumdioxide.

18. Uses or Applications of Mg (continue..)
Nonstructuraluses(continue..):
5.Cathodicprotection:ForPipelines,storagetankbasesandlead-sheathedcables,‘sacrificial’Mg
anodeareplacedinthesoilandconnectedbymeansofinsulatedwiretothematerialthatistobe
protectedagainstcorrosion.Insuchanarrangement,thecorrosionattackisdirectedpreferentially
againstthemagnesium,andthematerialisspared.
Structuraluses:
Magnesiumandaluminumalloysareimportantforpropertieslikehighspecificstrength,damping
capacity,andresistancetocorrosionbychemicals.Consequently,Mgalloysfindstructuralusemainly
intheaircraftindustryandalsolandtransportationandmaterialhandling.