2 leaching mechanisms

7/26/2019 2 Leaching Mechanisms

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HYDROMETALLURGYHYDROMETALLURGY2 Leaching mechanisms2 Leaching mechanisms

Fathi HabashiFathi Habashi

Department of Mining, Metallurgical, and Materials EngineeringDepartment of Mining, Metallurgical, and Materials Engineering

Laval University, Quebec City,,CanadaLaval University, Quebec City,,CanadaFathi.abashi!arul.ulaval.caFathi.abashi!arul.ulaval.ca


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HydrometallurgyHydrometallurgy

Solid-LiquidSeparation

ConcentrationPurifcation

Precipitation

Oxidant Leaching agentre

Solidtowast

Solution

Pure Metalscompounds

Precipitantor electriccurrent

Leaching


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LEAH!"G #LEAH!"G #

Leaching Agents

Solubility$ Large and ra%id solu&ility o' the material to &e leached inthe leaching agent$

Cost$ An e(%ensi)e reagent is undesira&le &ecause any traces lostduring handling *ill re%resent a large economic loss$

Materials of construction$ !' the leaching agent is corrosi)e and hasto &e handled in tan+s made o' stainless steel, titanium, orHastalloy, the ca%ital cost *ill &e high, and there'ore its use *ill &eless desira&le$

Selectivity$ An ideal reagent *ill e(tract only the desired com%onent$

Regeneration$ A&ility o' regenerating the reagent 'or recycle is alsoan im%ortant criteria


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LEAH!"G 2LEAH!"G 2


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LEAH!"G -LEAH!"G -

.ases such as "aOH or "H/OH are more

e(%ensi)e than H20O/&ut they are used

&ecause o' the 'ollo*ing ad)antages1

"egligi&le corrosion %ro&lems$ More selecti)e, i$e$, suita&le 'or ores containing much

gangue *hich reacts *ith acids &ut not *ith the

&ases$ E(am%les o' such gangue minerals are

limestone, dolomite, iron o(ide, iron car&onate, etc$


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LEACH!" #


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LEAH!"G /LEAH!"G /

An o(idiing or a reducing agent is sometimesneeded during leaching to solu&ilie certainminerals *hich do not dissol)e other*ise$ 3or

e(am%le, during leaching o' gold, sul'ideminerals, uranium ores, etc$

ommonly used o(idiing agents are1 o(ygen4or air5, oone, hydrogen %ero(ide, 'erric ion,

manganese dio(ide, sodium nitrate, and sodiumchlorate

ommonly used reducing agents are 'errous ion,and sul'ur dio(ide$


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LEAH!"G 6LEAH!"G 6


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H$drometallurgicalH$drometallurgical

%eactions%eactions

Ph$sical processesPh$sical processes Chemical processesChemical processes

Electrochemical processesElectrochemical processes

Electrol$tic processesElectrol$tic processes


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Ph$sical ProcessesPh$sical Processes

Hydration

Na+Cl(s)+ (n+m)H2O Na(H2O)+n(aq)+ Cl(H2O)m(aq)

Speed o& agitation

'emperature


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Chemical ProcessesChemical Processes

!onic7co)alent solids!onic7co)alent solids

o)alent solidso)alent solids


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Chemic l Processes

hemic l Processes

Ionic-cov lentonic-cov lent

solids

olids


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Chemic l Processes

hemic l Processes

Ionic-cov lentonic-cov lent

solids

olids

"eutraliation"eutraliation om%le( 'ormationom%le( 'ormation

Dis%alcementDis%alcement

O(idationO(idation

8rotonation8rotonation


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Chemic l Processes. Ionic-cov lent solids

hemic l Processes. Ionic-cov lent solids

Neutr liz tion

eutr liz tion


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Neutr liz tion

eutr liz tion


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Complex form tion

Dissolution o' lead sul'ate in sodium chloride solution$ Lead

sul'ate has a lo* solu&ility in *ater1

PbSO4(s) Pb2+(aq)+ SO42 (aq)

!n %resence o' chloride ions, the chloro%lum&ate ion is 'ormed1

Pb2++ 4Cl [PbCl4]2

thus decreasing considera&ly the concentration o' 8&29ion$ This disru%ts

the e:uili&rium o' 8&29

in contact *ith the solid %hase, 8&0O/, thus moresolid goes into solution$ O)erall reaction 1

PbSO4+ 4Cl [PbCl4]2+ SO42


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Complex form tion

The dissolution o' Hg0 in a solution o'

sodium sul'ide can &e re%resented

similarly1 Hg04s5Hg29 9 024a:5

Hg29 9 202;Hg02


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Displ cement 1

The dissolution o' a sul'ide in a solution

containing a metal ion that 'orms a less

solu&le sul'ide is a dis%lacementreaction$ 3or e(am%le, the dissolution o'

=n0 in u0O/solution


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Displ cement


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!xid tion

xid tion

The dissolution of sulfides in oxygenated

water. For exa!le" the aqueous oxidation

of Fe# to ferrous sulfate$Fe#(s)Fe2+(aq)+ #2(aq)

#2+ 2O2#O%2

O&erall rea'tion$Fe#(s)+ 2O2(aq)Fe2++ #O%2

Oxygen shares ele'trons with the sulfide ion


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Proton tion 1

roton tion 1

rotonation rea'tions in&ol&e the dissolution

of 'o!ounds of wea a'ids in strong a'ids"

or foration of &olatile a'ids.


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Proton tion

roton tion

For exa!le dissolution of CaCO*in a'ids$

CaCO*(s)Ca2+(aq)+ CO*2(aq)

CO*2+ H+HCO*

HCO*+ H+H2CO*

H2CO*H2O + CO2

The o&erall rea'tion$

CaCO*+ 2H+Ca2++ CO2+ H2O


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Chemic l Processes

hemic l Processes

cov lent solidsov lent solids o)alent &ond, e$g$, that &et*een silicon and

o(ygen in silica and the silicates, is a strong

&ond that cannot &e &ro+en easily &y a:ueous

solutions

The strong #iO ond in sili'a is not soluili,ed y

oiling with 'on'entrated a'ids ex'e!t hydrofluori'

e'ause of the foration of #iF%whi'h is a gas thatay &olatili,e or rea't with water to for

fluorosili'i' a'id" H2#iF-.


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Electrochemical Processes

#

"he solid is a conductor, e.g., a metal, or a solid #ithpartially ionic and partially covalent bonds that possessessemiconducting properties, e.g., U$%, &n', and CuFe'%.

"here is a transfer of electrons from the solid phase or fromsolution to the reacting species, i.e., the process involvesan o(idation)reduction couple. "he ions or molecules insolution simply diffuse through the boundary layer to meetthe solid surface to pic* up or give a#ay electrons."he solid dissolves at certain locali+ed points #hile theelectron transfer ta*es place at other points usually muchfurther.

"he o(idation and the reduction reactions proceedsimultaneously.


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2

"he *inetics of these reactions areinfluenced by the presence of latticedefects in the solid andor the presence oftrace impurity #hich may increase or

decrease the electrical conductivity of thesolid, and hence the availability of theelectrons."he rate of dissolution increases #ith

increased reagent concentration and #ithincreased temperature. -hen t#oreagents are present, the rate dependsonly on one reagent at a time.


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Metals in oxygenated water

Metals in dilute acid

Metals by displacement

Metal with complex formation

$(ides

'ulfides


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ELEC!OL"#C P!OCESSESELEC!OL"#C P!OCESSES

An outside %otential 4EM35 is im%osed on

the solid *hich is made as anode in an

electrolytic cell to cause its dissolution$

3or a metal1 MMn99 ne7

3or a semiconducting di)alent sul'ide1 M0

M299 0 9 2e


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S(MMA%)S(MMA%)


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>inetics o'>inetics o'

$%&rometall'rgical !eactions$%&rometall'rgical !eactions

"heory of diffusion introduced by the"heory of diffusion introduced by the

/erman physicist 0dolf Fic* in 1233/erman physicist 0dolf Fic* in 1233 "he concept of the boundary layer"he concept of the boundary layer

introduced by the /erman chemistintroduced by the /erman chemist

-alther 4ernst in 1567-alther 4ernst in 1567


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(i))'sion(i))'sion

Diffusion is the tendency ofDiffusion is the tendency of

any substance to spreadany substance to spreaduniformly throughout theuniformly throughout the

space available to it. 8t is aspace available to it. 8t is a

result of molecular movementresult of molecular movement


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Fick's LawFick's Law

d.

dC/01dt

dn

*


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$E *O,(A!" LA"E!$E *O,(A!" LA"E!


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$E *O,(A!" LA"E!$E *O,(A!" LA"E!

-hen a solid C is agitated in #ater and the solution is-hen a solid C is agitated in #ater and the solution is

analy+ed at intervals, it #ill be found that the rate ofanaly+ed at intervals, it #ill be found that the rate of

increase of the solute concentration follo#s the e9uationincrease of the solute concentration follo#s the e9uation

( )2C32C3)1dt

d3C2s

33

/

1dt

dnCC

s

*


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$E *O,(A!" LA"E!$E *O,(A!" LA"E!

ConcentrationConcentration

4

n123C

= 333

CCV

A

dt

Cds

*


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Rate Determining 0te%Rate Determining 0te%

Diffusion:Diffusion:

controlledcontrolled

processesprocesses

Chemically:Chemically:

controlledcontrolled

processesprocesses


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Characteristics o) $%&rometall'rgicalCharacteristics o) $%&rometall'rgical !eactions!eactions

Effect of fluidEffect of fluid

velocityvelocity

2/315ate C

*


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Characteristics o) $%&rometall'rgicalCharacteristics o) $%&rometall'rgical

!eactions!eactions

Effect of temperatureEffect of temperature

r2

6

N

5T1*

+ , A e-E%'


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Change o) -echanismChange o) -echanism

Effect of concentrationEffect of concentrationDissolution of Cu$ in sulfuric acidDissolution of Cu$ in sulfuric acid


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Change o) -echanismChange o) -echanismEffect of temperatureEffect of temperatureReaction: PdReaction: Pd2+2++ Cu+ Cu Pd + CuPd + Cu2+2+


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0UMMARY0UMMARY

Di''usion o' reacting s%ecies to the

inter'ace

Adsor%tion at the inter'ace

Reaction at the inter'ace

Desor%tion o' the %roducts

Di''usion o' the %roducts 'rom the inter'ace


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0UMMARY0UMMARY

Diffusion-controlled processesCheically controlled processes

!nterediate-control processes


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2 leaching mechanisms

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