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Bioleachingofachalcopyriticoreincolumns

Fernando

Torres,

Adriana

Tua,

Miguel

Mas

and

Olga

Regalado

UniversidadNacionaldeCatamarca,Argentina

BlancaEscobar

UniversidaddeChile

ABSTRACT

Bioleachingoflowgradecopperoresisahydrometallurgicalalternativeofgreatimportanceforthe

future. Many factfinding efforts have focused on recovering sulphur ores from copper. Bio

hydrometallurgyhasbeenappliedintherecoveryofsecondarysulphidecopperoreslikechalcocite

andhasobtainedgoodresultsnotonlyinrecoverybutalsoinkineticsbyapplyingbacteriaknown

as Acidithiobacillusferrooxidans and Acidithiobacillus

thiooxidans. This research introduces a

bioleachingtestinaPVCcolumnof1minheightand0.105mindiameter,loadedwith13.31kgof

copper ore with a grade of 0.41 % of total copper. The applied mineralogy comprises copper

sulphides, chalcopyriteand covellite, copperoxides (principallymalachite)and abundantpyrite,

hematiteandmagnetitecontainingimportantquantitiesoffinelimonite.ThemineralsizegaveaP80

of5,500microns(70% #4,Tyler).Thismineralyieldedlowrecoveriesfromtheflotation,between

40%and50%.The testwascarriedout in two stages.The first stagewas the leachingprocess,

whichtookplaceduringfourdaysusingconcentratedsulphuricacidtoextractsolublecopper.The

secondstageaimedatrecyclingtheacidicsolutionandapplyingabasalculturemediumforan86

day period of time. Additionally, isolation tests with PLS samples were carried out for At.

ferrooxidansandAt.thiooxidans,soastodemonstratetheirpresenceasawild type in thismineral.

Theobtained results suggest thepresenceofbothbacteria,At.ferrooxidansandAt.

thiooxidans,as

wellasacloseextractionofcopperof65%for86daysofleaching.Nearly10%ofthisresultcanbe

assignedtobacterialaction.

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INTRODUCTION

Worldwide copper production is continuously increasing. More than 20 % of the manufactured

copper

is

now

being

treated

hydrometallurgically.

An

indirect

indicator

of

the

outstanding

growth

in thehydrometallurgicalcopperproduction is the increase in thecapabilityofsolventextraction

plants (SXEW) to produce copper cathodes. It increased from 0.8 million tonnes to two million

tonnes between 1993 and 1997 [1]. As the demand of copper grows, the mining industry is

accessing lowgrade minerals and the methods of treatment must have lower operation and

inversioncosts.Millionsoftonnesoflowgradeoresonwastedepositsandtailingsarewaitingfor

the development of an economical and efficient technology to obtain copper of chalcopyrite

(CuFeS2).Bioleachingoflowgradesulphidemetallicoresisthemostimportanthydrometallurgical

road and has a promissory future. Since it is an alternative, many factfinding works and

researchers efforts are intended to enlarge knowledge on this topic. With regards to copper

extraction, biohydrometallurgy has been applied on secondarysulphide ores of copper as

chalcocite(Cu2S)withgoodresultsnotonlyinrecoveriesbutalsoinkinetics,particularlyapplying

bacteria likeAcidithiobacillusferrooxidansandAcidithiobacillus

thiooxidans,both from themesophile

group. These bacteria do not behave in the same way when copper ore is in the form of

chalcopyrite,presentingaveryslowkinetics.

For these ores, studies are being carried out using bacteria from the group of the moderate

termophilessuchasAcidithiobacilluscaldusandSulfobacillus thermosulfidooxidans, from theextreme

thermophilesarchaeasor fromaconsortiaofmoderateandextrememesophilesand termophiles

suchasSulfolobusmetallicus[3,4],AcidianusBrierleyorMetallospherasedula[2].

Bioleachingof lowgradecopperminerals isbeingappliedatpresent.Copperprimaryrefractory

ores like chalcopyrite is less extended. However, applications in waste deposits of lowgrade

copperarefoundmainlyinChile[1,5].Bacteriaarerecognisedduringthebioleachingprocessby

their

catalytic

action

in

the

natural

processes

of

metal

separation,

such

as

copper,

zinc,

gold

and

uranium.Sincethefirstbacteriumwasisolated,manybacteriahavebeenidentified.Inthisprocess,

Acidithiobacillus stand out and have been largely studied and analysed. Since Acidithiobacillus

ferrooxidanswasisolated,itwasthoughtthattheonlyimportantmicroorganisminbioleachingcame

from minerals. Nonetheless, in the last 30 years, many strains have been discovered.

Acidithiobacillus ferrooxidans is, by now, the more frequently isolated microorganism in the

bioleachingprocesses.

Mechanismsofinteractionmineralbacteria

Bacteria cover its demand of carbon with CO2 as their energetic needs are obtained from the

oxidationof the ferrous ionaswellas from reducedspeciesofsulphur. In thisway,bioleaching

occursin

an

acidic

medium

with

ferric

ion.

Therearetwomechanismsofbacteriainteractionwithsulphideminerals:anindirectmechanism,in

which the bacteria oxidise iron (II) to iron (III) and the sulphide to sulphate, and a direct

mechanism that implies thebacteria adhere directly on the solid crystals. In this case, it is a

sulphideoressentialsulphuractingasbridgetotransferelectrons.

Lowgrade copper ores and those with low recoveriesbefore flotation due to the presence of

primarymineralsofcopper,likechalcopyrite,havegonethroughbacterialleaching.Thisisanother

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possibilityof lowcostextractionthatallowsrecovery;copperwastedeposits,forexample,canbe

processedbythismethod.

METHODOLOGY

Mineral

The mineral sample was named MM1. The total weight of the lot was 147 kg. To prepare the

mineralsample,itwasputthroughjawsgrinderat1setopening.Arepresentativemineralsample

of13.31kgwasextractedandchargedina1mhighcolumnwithadiameterof105mm.

Chemicalcompound

The sample for chemical analysis was prepared according to Richards table. The chemical

compositionofthemineralwas:TotalCu:0.42%;solubleCu:0.13%andtotalFe:5.1%.

Mineralogy

The test was performed using a lowgrade copper ore, containing 0.42 % copper, mostly

chalcopyrite,0.95%,covellite,0.08%,malachite,0.15%,pyrite6.04%,jarosite2.74%bymass,and

somemagnetiteandhematite.Thisoreshowslowrecoveriesinflotationprocessbetween4050%.

Somemineralogicalassociationswiththismineralare:

CovellitereplacingchalcopyriteinthebordersChalcopyriteintergrowthinhematiteHematitereplacingmagnetitePyritereplacedbylimonites.

Sizinganalysisofthesample

ParticlesizedistributionsweremadewithamechanicalRoTapsieve,withacapabilityforseven

sieves.Forthisprocess,Tylersieveswereused.Fortheweight,a2353Sartoriusscalewasused.It

hasamaximumcapacityof3kgandaccuracy0.1g.Inthefollowingtable,thegrainsizeanalysisof

thesampleisshown:

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Table1 Sizinganalysisofmineralsample

Mesh Fractionsize

(microns)

Oversizeweight

(g)

Ov.weight

(%)

Ov.weightacum.

(%)

Passing

(%)

4

4750

3860

29.00

29.00

71.00

6 3350 2220 16.68 45.68 54.32

8 2360 510 3.83 49.51 50.49

10 1700 760 5.71 55.22 44.78

14 1180 1140 8.56 63.79 36.21

20 850 1080 8.11 71.90 28.10

2830 600 1060 7.96 79.86 20.14

6570 212 1620 12.17 92.04 7.96

100 150 620 4.66 96.69 3.31

100

440

3.31

100.00

0.00

TOTAL 13310 100.00

TESTOFBIOLEACHINGINCOLUMN

AtransparentPVCcolumnof0.105mofdiameterand1mhighwasfilledwith13.31kgofmineral,

and watered during 86 days with a solution containing culture medium T&K [7], and pH= 2

regulated with sulphuric acid, with an irrigation rate of 82 cm3/h and an equivalent flow of 11

L/h/m2.Thecolumnhadasystem forcollectingsolutionsenablingcleansolutions in itswayout.

Watering was carried out with a low flow pump and frequency regulation. The collection of

solutions was made in two stages; during the first stage it was watered to the extraction of the

copperoxides inopencircuit.The fourthday,aclosedcircuit irrigationbeganwithrichsolution

return.Sampleswerecollected in flasksata fixedvolumeof30cm3. Ineach sampling,chemical

analyses were made so as to determine total Cu and Fe and measurements of pH and Eh. The

volume of each sampling of rich solution in the phase of open circuit was measured. In the re

circulation phase, a volume of 10 L kept constant, calculating thebalance of copper fines. The

temperature(30C)inthecolumnwasregulatedbyaheatingbelt.Copperandironcontentswere

determinedbymeansofspectrophotometerofatomicabsorption.

Isolationofbacteriafromthemineral

Todetectthepresenceofbacteriainthemineral,5gofmineralsampleweretakenfromthecolumn

andplacedinsterileErlenmeyerwith100cm3ofbasalmediumcontaining3g/LFeSO4.Thebasal

culturemediumhadthefollowingcomposition:0.4gMgSO4*7H2O,0.056gK2HPO4*3H2Oand0.4

g/L (NH4)2SO4, adjusted to pH 1.6 with concentrated H2SO4. The flasks were incubated in an

environmentalshakerat30C.Thecultivationsolutionsweremonitoredperiodicallytodetermine

pHandEh.Whenferrous ironbacteriaweredetectedbythe increaseofEh,10cm3ofthisculture

wereusedtodetectsulphuroxidisingbacteria;thismediumcontainedpotassiumtetrathionate10

mM inabasalmediumspecific to thisbacteria:6g (NH4)2SO4,1gMgSO4*7H2O,0.02gCa(NO3)2,

and0.2g/LKClacidifiedtopH3.5withconcentratedsulphuricacid[8].Theseexperimentswere

monitoredforsomedaystodeterminepH.Allexperimentswerecarriedoutinduplicate.

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RESULTSANDDISCUSSION

Parametersvariationintheexperiment

ThevaluesofpHkeptclosetotwoasshowninFigure2.Duringthelastphaseofthetest,thepH

descendedto1.7soastoallowthedevelopmentofmicroorganismsironoxidisingATandAF.

Figure3showsthatthevariationofpotential(vs.SHE)valuesgoupastimeadvances,obtaininga

close peak of 550 mV, a fact that would show iron oxidisingbacteria in the column filled with

mineral.Itwaspossibletokeepthetemperatureofthesolutionbetween25Cand30C(Figure4),

theoptimalgrowingtemperatureforthesemicroorganisms.

As itcanbededucedbyobservingFigures5and6,copperoxideextraction isquicklyproduced

with the already known kinetic when confronted to the action of sulphuric acid. After this, the

curveofextractionvs.timeagreeswithkineticsofchalcopyriteinferricoxidation,asitisdescribed

inthefollowingequation:

CuFeS2+4Fe3+ Cu2+ +5Fe2++25 (1)

Underthisequation,practically50%isextractedquickly,apercentageagreeingwiththepresented

test,inwhich48%ofextractionofcopperwasobtainedin60hofleaching,(Figure1).

Figure1Extractionofcopperandiron

Figure2VariationofpHduringtest

010

2030405060708090

100

0 10 20 30 40 50 60 70 80 90

%Cuy%Fe

Time (days)

Extraccin Cu (%)

Extraccin Fe (%)

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Figure3 VariationofpotencialredoxintheisolationsolutionsofFeoxidisingbacteria

Afterthreedaysof leaching, itwasconsideredthattheextractedcopperoxidisedandthesample

onlycontained

copper

in

sulphur

form.

The

seventh

day,

basal

culture

medium

9K

without

ferrous

sulphate was added, because it was considered that the sample contained iron in high

concentrations,accordingtomineralogy.

Figures1,5and6showafivepointjumpintherecoveryinonlyfourdaysofoperation.Thisfact

agreeswiththebacterialactioninexponentialgrowth.Afterthisperiod,theextractionphasebegan,

whichcanalsobeseeninthegraphics:9.61%extractionwasgivenina60daysperiodoftime.

Figure4Variationoftemperatureofsolution

Figure5Extractionphasesofcopper.Solublecopperbyacidleachingandcoppersulphurbybacteriaaction

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Copperextractionforbioleachingcanbeestimatedina14%foranapproximate80daysperiodof

time, as the total extraction comes from the 64.68 % for an approximate time of 90 days. It is

consideredthatthisvaluedoesnotcorrespondtothetotalpossiblerecovery,becausethetestwas

stoppedbeforethecurvereachedtheplateau.Consequently,ahigherrecoverycouldbeexpectedin

anextendedperiodoftime.

Thisphaseoflowerextraction,represented inFigure5,wouldbeso,duetotheformationof iron

sulphateprecipitates(likejarosites),producingpassivationofthesurfaceofchalcopyrite,becoming

a barrier for the solution in the process of diffusion, delaying the contact of these with the

chalcopyritecrystalsandthedesorptionoftheproducttothesolution.

Figure6 Copperrecoveryvs.rateofdeleaching

Figure7 VariationofpotentialredoxintheisolationsolutionsofFeoxidisingbacteria

Figure8 pHvariationintheisolationsolutionsofsulphuroxidisingbacteria

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Apossibilitytoincrementthekineticsfordissolvingcopperinthelowgrowthphasewouldbeto

produceremilling,oncethe fastkineticsphase isfinished(60h forthis test)and toreinitiatethe

treatment, which would be in stages that would reduce the times of leaching here exposed.

However, remilling related costs are sometimes prohibitive, taking into account that the new

kineticswillcorresponditselfagaintotheonegiveninthepreviousphase,repeatingtheprocess.

Presenceofbacteriainthecolumn

Figure7showsavariationofredoxpotentialofthesolutionofironoxidisingbacteriaisolatedfrom

the mineral. In approximately 15 days, Eh reached 600 mV (vs. Ag/AgCl), a fact indicating the

presence of iron oxidisingbacteria, very probablyAcidithiobacillusferrooxidans and Leptospirillum

ferrooxidans.ThisisprobablyduetothehighEhreachedattheendofexperiments(about700mV).

Assoonasredoxpotentialreached700mVinthesolutionsofcultureofironoxidisingbacteria,

therewasan inoculumofthesebacteriaputtoaspecificculturemediumforthedevelopment

sulphuroxidisingbacteria.TheresultsobtainedinthistestshowthatthepHdecreasedevento

1.5, which indicates the probable presence ofAcidithiobacillusferrooxidans andA. thiooxidans

(Figure 8). Both bacteria generate sulphuric acid from the oxidation of reduced sulphurcontainedinthetetrationate.

CONCLUSIONS

Theresultsobtainedinthisworkevidencethatthemineralutilisedinthisstudyclearlycontained

ironand sulphuroxidisingbacteriadevelopedwhenpHconditions, temperatureand iron in the

solutionofirrigationenabledtheirdevelopment.

Inbioleachingteststherewasanaverageof45%copperrecovery;33%correspondtotheleaching

oxidesand12%isattributabletothesulphurbioleaching.

Dependingon

the

obtained

results,

it

can

be

concluded

that

it

is

possible

to

apply

the

bioleaching process to these ores, considering the low recoveries obtained for this mineral

withthe flotationroutes.

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