Proceedingsofihe 1SthSymposiumofMalaysian Chenical Elgineers
SELECTIVE LEACHING OF SILVER FROM SELECTED
SEMICONDUCTOR WASTE
L. L Meng, A. N. Sadikitr",N. Othman,L S.N. KimarudiL H. B. Mat
AdvancedProcessEngineeing ResearchGroup (AIEN), Faculiy of Chemical& Nat1]ial
Engneering,UnivelsitiTeknologiMalaysia,81310Skudai,Johor,Malaysia.
Resources
'Conesponding
Fa{r +607-5581463,
author.Phone:+607-5535526,
Email: aziahrlniza@fkkksa.utm.my
ABSTRACT
Depleiion ofnatural resourcesis becominga seriousissuein rec€ntdecades.This siiuation
gives an arlsing interest to r€cover recyclable rcsouces from indlrstrial waste
Semiconduciorwaste thai containshigh concentrationofprecious m€ials may be treated
as potenlial sources of precious metals if economic extraction is availableCharacterizaiionol the wast€ sampieir xhis sludy has shoqarlucrative silver contenl of
of aluminium,copper,calciumandferum.Thepotentisl
5.35%by masswith co-presence
of ihiosulfate solution in selective leaching of silver from this selected waste was
invesxigat€d.Thiosulfat€ leaching of silver folows a combination of reaction paihs that
include dircct l€aching, indircct leaching and catalytic lea,rhing. At selectedleaching
condilions,nearly 90% of silv€r is leachedand high selectivitvis achieved.Silver
Ieacljng efficiency is irfluence by concentrationofthiosulfate, Li S ratio, A: T ration and
temperanrle.Varying initiai pH ofthe leachitrgagen!did not result in a markeddifferent
in silver leaching efnciency. However, an alkaline medium is preferred to avoid or
minimize decompositionof thiosulfate, loss of anmonia, formation of sulfide ions that
enhanceFecipitation, and dissolulion of undesiredmetals Thiosulfate solution exhibited
a high selectiviryof silver over othermetals.
K€ytordsr Silver,Leaching,S€miconductor'
1 INTRODUCTION
The grcwth of globai economics and human population has triggered the fear of
exhaustionin nahrml resourcesincluding metals due to the increasiry consumptionand
the deplethg sofices of natura] ore. Silver the m€tal of interest in this siudy has been
usedas a monetaiymetal,jewelry and omamentsinceancienttime. In recentyeals,many
technologiesthat utilized silver's propefiy ofhigh electrical andthemal conductance,and
also the light sensitivebehavior of its compoundhave been developed. Nowadays,silver
is conrnonly found in batteriesand cells, medical appamtus,catalvsts,electrical contacts,
coatiry for elecfonic conducton, platirg of printed elech-ical circuit, phorographic
productsandrecordablemediasuchas compacldisks.
By the elld oflhe 19ih century, naiive or high-gade silver ores throughoutthe world
had been largely depleted. Nowadays,most of the primary silver is exaactedas a byproduct in the processingof compiex copp€r, lead and zinc ofes (Masseyet dt, 1973)The de?letionof natural silver sourcesand its increasingconsumptionhas resultedin an
arising interest to recover and recycle silver ftom photogaphic and industrial waste
bLt
conservalior'
Recorer)ard reLlclmgot si.verfrom\ arlesaoronl) ersurerecource
also at the sametime minimize the detrim€ntaleffectsof ore mining to ihe environment
ln this study, potential of seleclive silver recovery from lhe sel€ctedsemiconductor
waste is investigated using hydrometallugical process of solidliquid extaction,
commonlyktrowr as leaching. Comparedto th€ conventionalhydrometallurgicalprocess
thar rcquies complicaiedfumace operationand high consumplionof energyandtime, and
also biological processwhich is complicated,sensitiveand canonly operateat a very low
metal conc€ntration,hy&ometallurgical processcan be a simpler, less energy intensrve,
more effective and economicmethodto recoverysilver ftom semiconductorwasle
5- 168
P.oceedilgs
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fii
ln hydrometaliurgicalprocess,leaching is usually the first step in recovering metal
from solid waste. Whendealingwith multimelal lvast€,which recoveryof only onemetal
is desimbl€,it is importantthat the selecredleachingagentis selectivetowardsthe desired
metal so that the resultant leach solution can be Durified wiih less effon DesDite
seleflrrir).tne .elecleole,icr.ng ageil soord proridea good leaching
petomance
including fast kinetics and high efficiency. Besidethar, the leachingagentshouldhave a
low toxicity. Otherwise,its operationandthe speniieachingsotution wjll posedangerio
both humanand environment.
Previously,Tang (1999) has repodedihe potenlialof using acid and chlondejn
ieaching of silver fiom semiconductorwaste. The best leaching efficiency is achieved
with a solution of hydrochloic acid and sodium chloride. Though only about 55% of
silver l€achingjs rcportedandthe selectivity on silver is very low, suggesringthat a berier
leachingagen! shall be used. Cyanideis currently the major leachingagent of precious
metal in most ofthe mining industryrhroughoutthe world (Han e, at, 1994). Howeverirs
exfeme toxicity eliminat€s-it $om consid€rationin this study. The leaching agent
select€d
is thiosulfaie(S,O3'J,whichis regardedby manyas theporentialsubsrilution
of
cyanid€ in the leaching of precious metal due io irs 1ow toxicity and effectivenessin
leachingrefractoryorc (Berezowsky
er dt, 1978;Kerley, 1981;Abbruzzese
er dt, 1995;
BrionesandLapidus,1998;Tlomase1al, 1998jBreuerandJefftey,2000;Jetrr€y,2001j
Aylnore,200I; AybnoreandMuir, 200l).
Tbiosulfate(SzOi) is an inexpensiveand non{oxic chemical.Reagenrsin ihe folrn of
sodium thiosulfate and ammonium thiosulfate are commonly used in leaching, the
removai of excesschlodne ill paperand textiie bleaching,ihe manufacturingofmatches,
as a photographic fixing agent and the whitening of cotton fabric, bone and ivory.
Medically, thiosulfateis kno*n asthe antidoleto cyanideandarsenicpoisoning(Aylmore
and Muir, 2001). In addition. thiosulfate in the form of ammoniumthiosulfatehas been
usedasa fertilizer for soil deficientin sulfur for decades.
Theimportance
ofthiosulfat€hpr€ciousmetalrecoverywasftst discovered
in 1900s.
This processis known as the Von Pateraprocess(Aylmore and Mui, 2001) and it bas
b€enusedfor silver recoveryin SouthAmerica for many years. In this process,silver is
exira.red by sodium rhiosulfate ieaching of dre product ftom a chlorodizing roasr of a
silver-rich su1fideore (Flett s/ al, 1983). The chemisrry of rhe ammoniacalrhiosulfare
l€achingsystemis complicated
presence
dueto the simultaneous
of complexingligands
such as ammonia and L\iosulfate, catal'tic cupdc'cuprous(Cu(II)-Cu(I)) redox couple,
decompositionproductsof thiosulfateincluding tetrathionateand other sultur compound
(Kerley andBemafd, 1981). Thiosulfateis a meta-stableanionthat tendsro decomposein
aqueoussolurion,especial]yln acidic mediun. The factoa ihat influencethe stability of
thiosuliat€areconcentration,
pH, prcsence
ofmetal ionsandsulfurmetabolizing
bacteria
(Aylnore andMuir, 2001).
Th€ investigation of thiosulfate performance in the leaching of silv€r from
semlconductorwaste,lvhich includesits ieachingeffici€ncy andparametersthar influence
its leachingperformance,is the subjectof this study.
2 trXPERIMENTAI2.I MATERIALS
Sampl€swere collected from the filter cake dryer of a wastewatertreatmentplan! of a
semiconductorindustry. The sampieis grayish and is in the folnl of damp solid. Ior the
initial pH adjustneni of leachingagenis,sodium hydroxide and sulturic acid were used.
Leachingagentswere preparedjlom analyticalgade sodiumthiosulfai€penrahydrateand
distilled water.
Agitator used in the leaching exp€riment is mechanical shaker manufacturedby
Labline (Mode14626). For leachingexperimentwith varied temperature,the mecharical
shakeris r€placedby a temperature-controlled
shaker(New Brunswick Scientific Irmova
4080).pH is measuredby
EcometP25pH meter.Afier leachingis conpleted,laboratory
c€ntituge (Hettich Zeniritugen EBA12R) is used to separatesolid and supematant.The
concentation of silver and other select€dmetalsin the supematantis anallzed by aiomic
absorylion
spectrophotometer,
AAS (GBCAvanta2000)
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Prcceedlngs
of theI sdrSynrposiunol Mataysian
ChemicatEngineeB
2.2 SAMPLE CHARACTERJZATTON
Physrcaiard chernicai €haracle zations of sample were perfomed ro obi.tD pilysicat
properties and ch€flical composition of ahesample. Physicai characierizatiois rn rhis
siud, irclude waier and orgrnic content, apparent densiry, particle size with screen
aid purosry wrh gdsadsorprior.
14h,lerbrcnemicat
charrcrelTdrion.
Lrera\
-cl].i!
-D,spe|li\e
X-ra)analysisrl
D{XrandX-ra)Did.ac,ion
anat}<srXRn/ \\ereJpptied
2.3METHODS
20 ml oi ieachingagentwasdispensedwith bufetteinto a I 00 ml Erlenmeyerflask. Theq
solid sa1r}pleaccording 1o the specified liquid: solid miio was caretully weighted and
addedto the le6chingagent. The mixrurewas broughtinro conracib/ nechanical shaking
at 200 qrm for a period oi 3 hours- Throughout the coune oi shaknrg,ihe flask was
cavered by aluminium foil ro flinimize evaporaiion of leach soludor and lojs of
arlrmonia.The leachingp€riodof 3 houls x'as selecredfrom preliminary srudy.which was
rdEquatefor the systemto reach equilibrilun. Aiier 3 hours of sh.rijtg. sample ot lhe
rnixh.fe wastaken andcentrituged(2000rpm lor 3 min res).
In theinvesligaiion
ofthe eflectofl€achingagents'pHon leachingequiiibrium,initiat
pH of leaching ag€ntswere first adjustedby concedr.ared(2.0 M) sodirnit hyiiroxid€ of
sulfuricac d. Uhen lnesolllrorpH wasclose'oLhedesignfled
\alue.adllLswenr
wa!
follow€d by th€ samereagentswilh more diluted concentation (0.2 M).
In the investigaiionof the effect of temperatureon leaching equilibrium, exp€rimenrs
were canied out in temperature-contolledshaker ll,irh a similar rotational speed and
leachingtim€ wirh oiher expedments. Leaching agentsin the flasks were heaxedto the
desigmtedtenperatu€ beforesolid sampteswere added.
Experimentalprocedur€for selectiviry of leaching agent on sihc. qus similar to the
study of leaching €quilibridn but ar the end of the experiment,th€ ieach solurior was
analyzedfor concm1mtionof Ag, Cu, Al, Fe and Ca by AAS. Two seis of experiments
rnd Letuperurure
wc e cafriedoL ro ;\esrigare
"ith dillerenr!h.o:u farecon.enndr.oo
whether differencesin these parameterswill inlll,rence selectivify oi siiver. Leaching
conditionsfor thesetesrswere: (i) I NI sodiumthicsuifate,A:T=1, L:S=200.50"C.
leachingtime of3 hours,and (ii) 0.1 M sodiun thiosulfate,AiT=i. L:S=200.30.C.
leachingtime of 3 hours.
3 RESULTSANI} DISCUSSION
3.1 SAMPLE CIIARACTERIZATION
The semiconductolwaste sampleobtainedfrom dried filter cake consisrsof solids wirh
varying sizes. The vaiue ofmass meandiametercalcuiatedis 490.0 !lm, which is taken as
the averagediamet€ror padcle sie ofthe sample. Apparenrdensityafsanple was found
to be 2.270g/cm' by muliipycnometer.
Resultsfrom EnergyDispersiveX-ray Spectropholometer
(EDAX) reveatedthat silver
content in fie sampleis 5-35%by dry mass. Other metals thai co-pleseri h rhe sample
mciudealuminium, copper,calciumand ferun
3.2 LEACHING EQUILIBRIUM
1.2.| | eaching
Equilibrium:
A Funcrion
of fime
Th€ time dependencyof leaching equilibrium of silver with thiosulfaaesolution is
illwtrated in Figwe 1. It shows that the incremenrof silver leaching levels off after 5
minutes(300s),suggestingthat leachingequilibrium is achievedas early as5 minures.
The resultsindicatethat ieachingtime needto be minimized oncel€achingequilib.ium
is achieved. This is necessaryto avoid the deirimental effect of prolong leachingto the
leaching efficiency as the equilibrium is shift€d toward reprecipirationof silver in rhe
leach soiution. Leaching time of more than l0 houn should be avoided since ir was
reportedthat silver app€aredlo be unsrablethereafler(Aytmor€, 2001).
5 ,170
Procccdilgso ne l3iir Symposium
oli,IaiaysEnCtehiclt Engdr.eL:
FICURE 1. percenl3geof silve. leachedtor rhe firsr I 0 rrirutes
(experimenral
colrditions:
0.1M sodiumthiosujfare,
30.C,L: 5=200,Ar T=1)
3.2-2Effectof Thiosuuate
Concentration
Fig re 2 shows that increasingthe rhiosulfate concmtrarion &om 0.01
M to 0.05 M
drasr,cdllyincfease
rhe percerlage
or sil\er teacb,og
from o2oolo S1",. q l0_f"lj
Increase
or rhroruttdreconcLrnalionfrom0.tM,ot.0Moolyinprove(abou,t00"offie
'eachiogeffcieDcy. Tbr. sligbr increa,edra) due ro rhe t""*.i"" ot **"r
Jf,t*..i
s l\er-trrosullhle
comple\er.
$hichrs,avored
b] ncrea.ing
corceDralon
of!hio.utfare.
FIGURE 2. Efect of thiosulfar€concenhationon silver leaching
(experimental
condirionsr
30"C,L:5=200,A:T=1,Leachingtime=3hours)
3.2.3EffectofL: S Ratio
F;gurcI :llusraLe.rhal a.hary ircrea(ein .rtver leachingefficienc' $as ob.erued
bv
S_m:o
t0 ro 200 rigue I a,soi usiaresttrari"".r* i" r*.tri
l.:re.asine,rhe
!:
lom
ettrcrency
levelsotf afterL:S rafioof200.
l
i
i
i
l
I
)
5-t7l
Proceedingsoftle 181hSymposiumofMalaysian ChemicalEngineers
FIGURE 3. Effect ofI-rs ratio on silver leaching(experimentalconditions:
0.1M sodiumthiosulfate,
30"C,A:T-1, Leachingtime=3houn)
3.2.4Effec!ofA: T Ratio
The effect of A:T ratio on silver leaching €quilibrium for solutions wilh thiosulfate
concentation fixed ai 0.I M arepreseftedin Figure 4 Thereappearsro be a sharpincrease
in leachingefficiencywhenthe A:T ratio is increased
ftom 0.1 io 1.0. Maximumsilver
leachingis achievedat A: T ratio of 1.0 and turther incr€asein A: T raxioslightly lowels
the leachingeffici€ncy.
560
: bo
IIGURE 4. Effect of A:T ratio on silver leaching(experimedalconditions:
0.1 M sodiumthiosulfare,30.C, L:S=200, Leachingtim€=3 hoxrs)
3.2.5EfIectOfpH
Figur€ 5 ilustrates the €ffect of pH.q to leachingequilibriun of silver. It show€dclearly
that varying pH"qdid not fesult in a marked diff€(ence in leaching efiiciency. Leaching
efiiciencyfor pH"qrangeof8.02 to 10.82waswithin 80% to 83%while at pH.qof 12.9,
lbeleach.ng
effcrenc)decrease
sligbrly.o "70..
5- 172
Proceedilgs
ofthe 18thSymposilmofMataysianChenicatErgircers
r-!-
FIGURE5. EffectofpHeqon silverleaching(ereerimental
condiiionsr
0-1M sodiumrhiosulfate,
30"C,A:T=1,Lr5=200,LeachingtimF3 hours)
It is concludedihat the idtial pH shouldbe mainlainedin th€ alkaline rangesincethe
resultrngleach solurionswere more stable. Fomation ofprecipitate was noricid to occur
faster with leach solution thar iniiial pH was adjustedto acidjc range. This was not a
suryrising observationsince decomposirionof thiosulfate, formation of sulfide ions and
loss of anrmoniawere all accele€tedby tower pH.
3.2.6Effectof Temperatue
Th€ effect of remperatur€on silver leaching equilibrium with thiosulfate solution is
presentedin Figure 6. The leachingefficiency increasesfrom rhe temperarueof3ooc ro
50"C but falls asthe temperatur€reached60"C. The decreaseofteaching efficiency at the
temperatureof60'C is probably dueto loss of ammoniafiom the leachsolution.
i;4
r4'd!c
f c)
FIGURE 6- Effect oftemperatur€on silver leaching(experimedratconditions:
0.1M sodiumrhiosulfate,
ArT=1,L:S=200,Leachirytime=3hours)
3.3 SELECTIVITY OF LEACHING AGENT ON SILVER
Figure 7 pr€sentsmetal conc€ntrationin the leach soluiion for two setsof experiment. In
both cases,silver showed the highest conceni.ation, followed by aluminium, coppef
calcium and ferum. The concentaiion of calcium and ferum in the leachsolution are-1ow,
which are either und€rectedor less rhan 0.3 mg,{. Silver concentrationfor both sers of
experimmtsis quite close.
5- t73
Proceedirysof the I8th Synposiur olMalaysian ChemicalEnginee6
FIGURE7.liletal conc€nlration
in theieachsoiution(Eaperimental
condirions:
Set 1- I M sodiumthiosulfare,
50"C,A:T=1,L:3=200.3 hour!leaching;
Ser2- 0.I M sodirmlthiosufare,30.C,A:T:1, LiS-200,3 houlr leaching)
1 CONCLUSION
Semiconahrctor
wastein this studyis a nrultimetalwastethat conrains5.35%of silverby
mass.Co"existed
netalsin lhe wasteinchrdeallnnnriun(16.94),copper(12.67),calcilrm
(i.977")andfemn (1.30%).
Severalteachingpanmetershave been idenLifiedtu influence silver leaching
efficiency with thiosulfate.Theseparamete$are concentrationof thiosr late, L:S ratio,
A:T ratio and iemperaflre. ln o.der to achievea significant amount of sitver dissolution
with batchleaching,it is impotantthatconcertradon
ofthiosulfareis 0.05M of more,L:S
mtio is 50 or more, A:T ratio around I and temperaturenot higher rhan 50oC.For rhe
cuaent semiconductorwaste sample,leachingtime of 5 lrlin tes io 5. |ours is adequate.
Varying initial pH of the leacling agent did not rcsult in a rnl]lk€d different in silver
leachingefiiciercy. However,an aikalinemediurnis prefenedio avoid or minimize
decompositionof thiosulfate, loss of ammonia,fomation of sullate ions that eniance
precipitation,
anddissolutionof undesired
metals.
Finally, in terns of seleciivity. thiosulfare the leaching agent investigaredin rhis
siudy have showeda geat potentialin selectivilyrecoveryof silver from selected
semicondrctor
waste-At the selected
leachingconditions,
almostcompietedissolution
of
silveris achievedwifi a selecrivityof 368.5overothermerals.This a grcatimprovemeni
over the acid and chiorideleacLingmediumFevious investigaredby Tang( I 999).
ACKNOWLEDGEMENTS
A graieful acknov/iedgemenito the MinisLj, of Science,Technology & Environment of
Malaysia for providing financial suppot through the IRPA gmnr and rhe Universiti
Teknologi Malaysia for the ResearchFellowshipawardedro Mr. L. K. Meng.
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