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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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    Chemic l Processes. Ionic-cov lent solids

    hemic l Processes. Ionic-cov lent solids

    Neutr liz tion

    eutr liz tion

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    Chemic l Processes. Ionic-cov lent solids

    hemic l Processes. Ionic-cov lent solids

    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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    Chemic l Processes. Ionic-cov lent solids

    hemic l Processes. Ionic-cov lent solids

    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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    Chemic l Processes. Ionic-cov lent solids

    hemic l Processes. Ionic-cov lent solids

    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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    Chemic l Processes. Ionic-cov lent solids

    hemic l Processes. Ionic-cov lent solids

    !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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    Chemic l Processes. Ionic-cov lent solids

    hemic l Processes. Ionic-cov lent solids

    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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    Electrochemical Processes

    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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    Electrochemical Processes

    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