CHAPTER 5 General methods of refining.pdf

Introduction
Basic approaches to refining
Purification of crude metal produced in bulk
Preparation of pure metals

1. DEFENCE ENGINEERING COLLEGEMetallurgical And Materials Engineering
Present By:
Weldebrhan Hadush (Maj.)
14 Nov., 2019

2. Content
Introduction
Basic approaches to refining
Purification of crude metal produced in bulk
Preparation of pure metals
General methods of refining

3. INTRODUCTION
Everymetaloralloy-evaluated-intermsofsomeusefulproperties,forexample,
i.Mechanicalstrength;
ii.Hardness;
iii.Ductility;
iv.Specificgravity;
General methods of refining
iv.Specificgravity;
v.Meltingandboilingpoints;
vi.Thermalandelectricalproperties;
vii.ResistancetocorrosionandChemicalreactivity
.
Allthesepropertiesareinfluencedbythepropertiesoftheimpurityelements.Insomecases,
eventracesofanimpuritymaysignificantlychangeaspecifiedproperty.Necessaryto
removeimpuritiesandrefinethemetalsoastoimproveitsproperties.
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4. Cont’d
Example:Effectofatraceimpurity
•Tracesofoxygenwouldhardlychangethespecificgravityofcopper.Ifweight
werethecriterionofanyparticularuse,thepresenceofoxygencouldbeignored.
•Thedissolvedoxygencould,however,drasticallyaffecttheelectrical
conductivityofcopper.Purecopperfromthepointofviewofuseasaconductor
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must,therefore,necessarilyhaveaverylowoxygencontent.Thepresenceof
oxygeninfluencestheeffectproducedbythemetalimpuritythroughitsoxidizing
power.Forinstant0.01percentoxygencausesironandtintoprecipitateas
oxidesandtherebyeliminatesthedeleteriouseffectoftheseimpurities,namely
ironandtin,onelectricalconductivitywhentheyarepresentasasolidsolutionin
copper.
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5. BASIC APPROACHES TO REFINING
Metalobtainedfromtheprimaryextractionprocessoftencontainimpuritieswhich
originatedfromtheore,thefluxes,orthefuel.Forfurtheruseofthemetalsitis
usuallynecessarytosubjectthemtooneorseveralprocess.Therefiningmayhaveas
itspurposetoproducethemetalaspureaspossible:insomecasesextremepurityis
desired.Inothercasesas,
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Example:Insteelmakingtherefiningistocontrolledamountsofdesiredimpurities.
Finally,somerefiningprocessesaredoneinordertorecoverimpuritieswhichin
themselvesarenotharmful,butwhichhaveintrinsicvalue,
Example:Therecoveryofsiliverfromlead
Refiningprocessesarealwaysbasedontheprinciplethatdifferentelementsdistrubite
themselvesdifferentlyindifferentphases,andthatthesephasesmaybeseparated
physically.
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6. PURIFICATION OF CRUDE METAL PRODUCED IN BULK
Exceptintheelectrolyticreductionmethod,metalsproducedbyanyothermethodare
generallyimpure.Theimpuritiesmaybeintheformof:othermetals,unreduced
metaloxides,non-metalslikecarbon,silicon,phosphorus,Sulpheretc.andfluxorslag.
Severaltechniquesrefiningcrudemetalareuseddependinguponthedifferencesin
propertiesofthemetalandtheimpurity.Someofthemarelistedbelow.:
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propertiesofthemetalandtheimpurity.Someofthemarelistedbelow.:
i.Distillation
ii.Liquation
iii.Poling
iv.Electrolysis
v.Transport reaction/ Vapourphase refining
vi.Chromatographic methods
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7. Distillation:
Volatilemetals like zincand mercuryare purified by distillation. Distillation is a
metal-gas refining process that, impuritiesseparated from the desired metal by
distillation at a suitable temperature.
It is customary to apply this method when theimpuritiesare morevolatilethan the
metalso that the power required for heating purposes is low.
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Example-destillationrefining
a.Purezinc(B.P=910°C)canbepreferentiallydistilledfrombrass,since
copperhasamuchhigherB.Pthanzinc,i.e.2575°C.
b.Magnesium(B.P=1107
°
C)canbepreferentiallydistilledfromaluminum,
sincealuminiumismuchhigherB.Pthanmagnesiumi.e2467°C.
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8. Liquation:
Liquationimpliestheselectivemelting(differentmeltingpoint)ofacomponentinan
alloy.Easilyfusiblemetalsliketin,leadetc.arerefinedbythisprocess.Inthis
method,theimpuremetalispouredontheslopinghearthofareverberatoryfurnacein
figurebelowandheatedslowlytoatemperaturelittleabovethemeltingpointofthe
metal.Thepuremetaldrainsoutleavingbehindinfusibleimpurities.
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Example:Crudezinccontainingabout5%leadcanberefinedinareverberatoryfurnace
byslowlycoolingthemoltenmetal,i.e.overaperiodoftwoorthreedays.
Itisalsopossibletoremoveironbyliquationfromcrudetin,containingabout
10%iron,.

9. Poling:
Polinginvolvesstirringtheimpuremoltenmetalwithgreenlogsorbamboo.The
hydrocarbonscontainedinthepolereduceanymetaloxidepresentasimpurity.
Copperandtinarerefinedbythismethod
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10. Electrolytic Refining:
A large number of metals like copper, silver, zinc, tin etc. are refined by electrolysis.A
block of impure metal is made the anode and a thin sheet of pure metal forms the
cathode of the electrolytic cell containing suitable metal salt solution which acts as an
electrolyte. On passing current, pure metal deposits at the cathode sheet while more
electropositive impurities are left in solution. Less electropositive metals do not dissolve
General methods of Refining
and fall away from the anode to settle below it as anode mud.
The reactions are:
Anode:M → M
n+
+ n??????
-
Cathode:M
n+
+ n??????
-
→ M
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11. Cont’d
Example: In the electrolytic refining of crude copper (blister copper), a large piece of
impure copper is made anodeand a thin piece of pure copper is made the cathode.
An acidified solution of copper sulphateis used as an electrolyte. On passing an
electric current of low voltage through the solution copper (II) ions obtained from
copper sulphatesolution go to the cathode where they are reduced to the free copper
General methods of Refining
metal and get deposited.
Reaction at Cathode: Cu
+2
+2
-2
→ (Reduction)
An equivalent amount of the metal from the anode dissolves into the electrolyte as
Cu
+2
ions.
Reaction at Anode: → Cu
+2
+ 2
-2
(Oxidation)
As the process goes on, anode becomes thinner while the cathode becomes thicker.
The impurities like silver, gold settle down at the bottom of the cell as 'anode mud'.
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12. Transport reaction/ Catalyticdistillations/ vapourphase refining:
In this method, the metal is converted into its volatile compound. It is then
decomposed to give pure metal. So, the two requirements are:
i.The metal should form a volatile compound with an available reagent,
ii.The volatile compound should be easily decomposable, so that the recovery is easy.Examples:MondProcess for Refining Nickel:
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Examples:MondProcess for Refining Nickel:
In this process, nickel is heated in a stream of carbon monoxide forming a volatile
complex, nickeltetra carbonyl.
Ni
(s)
+ 4CO → Ni(CO)
4
(g)
at 50 °C Impuritymetaldo not formcarbonyl
The nickel carbonyl gas is passed on another reactor which is kept at about 230 °C
which contain metallic nickel seeds. Carbonyl dissociates, solid nickel precipitated on
the seeds. Ni(CO)
4
→ Ni + 4CO NB: @ 1atm
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13. Cont’d
Examples:Sub-chlorideprocessofAluminumrefining:
Inthisprocess,theimpurityalloyisreactedwithAlCl
3
(g)atabout1200°Cwhereby
3AlCl(g)isformed.Oncoolingofthesubchloridetoabout700 °Cthereaction
reverses,purealuminumcondenses,andvolatileAlCl
3
isreformedandreturntothe
process
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2Al(l) +AlCl
3
(g) = 3AlCl(g)
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14. Chromatographic methods:
Thismethodisbasedontheprinciplethatdifferentcomponentsofamixtureare
differentlyadsorbedonanadsorbent.
Themixtureisputinaliquidmediumwhichis movedthroughtheadsorbent.
Differentcomponentsareadsorbedatdifferentlevelsonthecolumn.Laterthe
adsorbedcomponentsareremovedbyusingsuitablesolvents.
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adsorbedcomponentsareremovedbyusingsuitablesolvents.
Example: Al
2
O
3
is prepared in a glass tube and the moving medium containing a
solutionof the components is in liquid form. This is very useful for purification of
the elements which are available in minute quantities and the impurities are not very
different in chemical properties from the element to be purified.
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15. PREPARATION OF PURE COMPOUNDS
•Itisalmostasdifficultastodefine«pure»asitistodefine«rare»metals.Some
metalsaresufficientlypurebynormalproductionmethodes.Forthemorereactive
metalssmallamountofimpuritiessuchascarbon,nitrogenandoxygen may
affectthepropertiesandusefulnessofthemetal.
•Foralackofbetterdefinitionwecansaythatapuremetalisonewheretheharmful
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•Foralackofbetterdefinitionwecansaythatapuremetalisonewheretheharmful
effectsoftheimpuritieshavebeenvertuallyeliminated.
•Themorenoblemetalssuchasiron,copper,etc.maybeobtainedrelativelypureby
refiningofcrudemetals,thiscannotbedoneforthemorereactivemetals,the
impurityuseuallybeinglessreactivethanthemetalitself.
•Thismaybeobtainedbyvarioushydrometallurgicalmethodes.Theoreisleached,
thesolutionispurifiedandapuremetalcompoundisprecipitated.
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16. Cont’d
As alreadymentionedpure oxidemaybereducedwithcarbonor siliconin vacuum,
or withaluminium, magnesium, or calcium atatmosphericpressure. Oxidesof the
more noble metalssuchas molybdenumand tungstonmayalsobereducedwith
hydrogenin a tube furnace.
General methods of Refining
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17. “The future can not be predicted,
but it can be made !”