Characterization and solidification of arsenic-rich cyanided tailings

LICENTIATE T H E S I S

Department of Civil, Environmental and Natural Resources Engineering Division of Geosciences and Environmental Engineering

Characterization and Solidification of Arsenic-rich Cyanided Tailings

Roger Hamberg

ISSN 1402-1757 ISBN 978-91-7583-071-1 (print)

ISBN 978-91-7583-072-8 (pdf) Luleå University of Technology 2014

ISSN: 1402-1757 ISBN

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Characterization and solidification of arsenic-rich cyanided tailings

Roger Hamberg

Luleå University of Technology

Department of Civil, Environmental and Natural Resources Engineering Division of Geosciences and Environmental Engineering

November 2014

Printed by Luleå University of Technology, Graphic Production 2014 ISSN 1402-1757

ISBN 978-91-7583-071-1 (print) ISBN 978-91-7583-072-8 (pdf) Luleå 2014

Picture: Open pit at Svartliden Gold Mine, Sweden

Abstract

Information on the occurrence of As species and iron sulphide minerals in tailings is essential for predicting the release of As over extended period of time. Tailings originating from a gold mine in northern Sweden with low content of trace elements except for As were used for this purpose. The dominating sulphides were pyrrhotite and arsenopyrite. The samples used in the study were post-cyanided, tailings slurries treated with Fe₂(SO₄)₃ and H₂O₂ to form arsenates and Fe-hydrates for effective As-immobilization.

Speciation of the As in ore and tailings samples revealed that mining processes have dissolved the majority of the arsenopyrite in the ore, causing secondary As phases to co-precipitate with newly formed Fe-hydrates. A minor part of the As retained in the tailings was assumed to be As (III)-species. Weathering cell tests (WCT) involving 32 weekly cycles of wetting and air exposure were conducted to assess the effect of weathering on the stability of As in the tailings. As-bearing Fe-hydrates remained intact during the early stages of the WCT; the low release of As during this period was probably due to the dissolution of soluble As(III)-phases.

During the later stages of the WCT, the release of As, Fe and S increased due to pyrrhotite oxidation and the destabilization of As-bearing Fe-hydrates. The majority of the originally present As was still associated with the tailings by the end of the test, but additional pyrrhotite oxidation with the pH falling to >3 could further destabilize these As-bearing Fe-hydrates.

In the second part of the study, cyanided tailings were converted into a monolith by using a method FDOOHGFHPHQWHGSDVWHEDFN¿OO&3%7ZRPL[WXUHVRI&3%ZHUHWHVWHG&(ZLWKZWRIFHPHQWDQG

&()$FRQVLVWLQJZHLJKWZWRIFHPHQWWRJHWKHUZLWKZWRIELRIXHOÀ\DVK. The stability of As LQ&3%PDVVHVDQGXQDPHQGHGWDLOLQJVZHUHHYDOXDWHGXVLQJWDQNOHDFKLQJWHVWV7/7DQG:&7V7KH

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mixtures were treated with acid in order to consume the buffering minerals and simulate the formation of DFLGPLQHGUDLQDJH$0':KHQDFLGZDVDGGHGWRFUXVKHG&3%PDWHULDOV$VUHOHDVHLQFUHDVHGGXHWR

the dissolution of Fe-hydrates. The addition of binders into tailings could pose more resistance to sulphide oxidation, which in turn means that the stability of As-bearing Fe-hydrates could be prolonged on long term.

Results from the WCT suggested that the addition of low proportions of binders could have a positive effect on As-leaching in a long term perspective. A relatively new method called “Surface paste disposal”

(SPD), where mixtures of low proportions of binders and tailings is placed as a cover on the un-amended tailings has shown promising results in terms of decreasing As-leaching and the generation of AMD. Future research will, therefore, focus on the stability of As in SPD-applications.

List of articles

Papers included in the thesis referred to by roman numbers:

,5+DPEHUJ&0DXULFH/Alakangas.

The release of arsenic from cyanided tailings.

Submitted manuscript

,,5+DPEHUJ&0DXULFH/Alakangas.

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Manuscript

Additional related papers not included in the thesis:

5+DPEHUJ&0DXULFH/Alakangas. 8VHRIFHPHQWHGSDVWHEDFN¿OOEDVHGRQDUVHQLF±ULFKWDLOLQJV

from cyanidation. Proceedings of the 10^th international conference on Acid rock drainage and IMWA annual conference. Santiago, Chile, April 21-2, 2015, 10 pp.

Table of contents

1. Introduction 1

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Cyanidation 1

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2. Scope of the thesis 5

3. Materials 6

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4. Methods 7

5. Findings and discussion 7

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6. Conclusions 11

7. Future research 11

Acknowledgements 11

8. References 12

1. Introduction

Environmental concerns regarding gold extraction in arsenic-rich sulphide mine ores

Mines are often located in ecologically sensitive areas containing many important habitats. Mining gen- erates enormous amounts of waste (waste rock and tailings) that must be handled and disposed of with care.

Waste rock is dominated by coarse material that is removed to reach the ore, which is then crushed, ground, DQGUH¿QHGE\ÀRWDWLRQPDJQHWLFRUJUDYLPHWULFPHWKRGVWRH[WUDFWWKHGHVLUHGPHWDO7DLOLQJVDUHWKHUH- VLGXDOPDWHULDODULVLQJIURPWKLVSURFHVVRIPHWDOHQULFKPHQWDVOXUU\VOXGJHRI¿QHSDUWLFOHVZLWKDKLJK

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Arsenic (As) is a major contaminant in many mining areas, especially in those housing gold, copper, and XUDQLXPPLQHV,WLVDQDWXUDOFRQVWLWXHQWRIWKHEHGURFNDQGLVSULPDULO\IRXQGLQVXO¿GHVVXFKDVDUVHQR- pyrite (FeAsS), orpiment (As₂S₃), and loellingite (FeAs₂) (Smedley and Kinniburgh, 2002). Under reducing alkaline conditions, these minerals are relatively stable and their natural decomposition is quite slow. How- ever, once arsenic-containing ores have been ground, processed, and deposited in tailings facilities where WKH\DUHH[SRVHGWRDLUDQGZDWHUWKHLUGHJUDGDWLRQLVVLJQL¿FDQWO\HQKDQFHG8QGHUDFLGLFR[LGL]LQJFRQGL- WLRQVRIS+!DUVHQRS\ULWHR[LGDWLRQRFFXUVDFFRUGLQJWRUHDFWLRQ

FeAsS(s) + 4H₂O + 3O₂ DTĺ)H2+₃(s) + H₃AsO₃(aq) + SO₄^2- + 2H⁺ (1)

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FeAsS(s) + 7H₂O + 11Fe³⁺ĺ)H²⁺ + H₃AsO₃(aq) + SO₄^2- + 11H⁺ (2)

Sulphide minerals such as pyrrhotite and pyrite are often co-occurring with arsenopyrite and loellingite LQVXOSKLGHJROGRUHV,IJROGLVSUHVHQWDVLQFOXVLRQVLQWKHVHPLQHUDOVWKHVHQHHGWREHGLVVROYHGWRH[WUDFW

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the generation of acid mine drainage (AMD) (INAP, 2009). Waste rock and tailings from sulphide ore mines RIWHQFRQWDLQVS\UUKRWLWHWKDWR[LGL]HGLQFRQWDFWZLWKDWPRVSKHULFR[\JHQDQGZDWHU)URPWKHVHUHDFWLRQV

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Fe_[6±[2₂[+₂2ĺ±[)H²⁺ + SO₄^2-[+⁺ (3)

2[\JHQLVWKHSULPDU\R[LGDQWRIS\UUKRWLWHDWS+YDOXHVDERYH8QGHUVXFKFRQGLWLRQV)H²⁺ released by WKHLQLWLDOR[LGDWLRQUHDFWLRQLVR[LGL]HGWR)H³⁺ and precipitates as Fe(OH)₃. At pH < 4, pyrrhotite can be R[LGL]HGE\)H³⁺ (reaction 2), which leads to a stronger release of H⁺.

Fe_[6±[)H³⁺ + 4H₂2ĺ±[)H²⁺ + SO₄^2- +8H⁺ (4)

Hydrolysis of generated Fe³⁺LQUHDFWLRQPD\RFFXUDQGIHUULFK\GUR[LGHLVIRUPHGUHDFWLRQ7KH)H³⁺

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Cyanidation

Cyanide (CN^-) is a naturally occurring ion found in low concentrations in fruits, nuts, plants, and insects.

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environment. Precious metal (gold and silver) mines are some of the world’s largest consumers of cyanide, which has been used to separate gold and silver particles from ore by the mining industry for over a decade.

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4AuV + 8NaCN + O₂ + 2H₂2ĺ1D$X&1₂ + 4NaOH (5)

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the free species HCN and CNGXH WR WKHLU KLJK PHWDEROLF LQKLELWLRQ SRWHQWLDO 0HWDOF\DQLGH FRPSOH[HV

such as Fe(CN)₆^3- and Fe(CN) ₆^4-DUHFRQVLGHUHGWREHOHVVWR[LF6KLIULQHWDO7KHWR[LFLW\RIPHW- DOF\DQLGHFRPSOH[HVGHSHQGVRQWKHDELOLW\RIWKHF\DQLGHLRQVWRGLVVRFLDWHIURPWKHFRPSOH[ZHDNHU

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and Ca(CN)₂UHODWLYHO\LQVROXEOHF\DQLGHVVXFKDV=Q&1₂, Cd(CN)₂DQG&X&1ZHDNDFLGGLVVRFLDEOH

cyanides (CN_WADDQGVWURQJDFLGGLVVRFLDEOHF\DQLGHV&1_SAD7KHF\DQLGHLRQVRI&1_WAD species only dissociate under neutral or mildly acidic conditions while those of CN_SAD species are only released under VWURQJO\DFLGLFFRQGLWLRQV,QLURQULFKHQYLURQPHQWVF\DQLGHVJHQHUDOO\IRUPLURQF\DQLGHFRPSOH[HV&1-

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conditions (Meeussen et al., 1992).

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Cyanide and related ions such as cyanate and thiocyanate can be treated and removed by various process- es such as the INCO SO₂DLUSURFHVVFRSSHUFDWDO\]HGK\GURJHQSHUR[LGHWUHDWPHQWWUHDWPHQWZLWK&DUR¶V

acid, alkaline breakpoint chlorination, natural attenuation, cyanide recovery, activated carbon treatment, DQGELRORJLFDOWUHDWPHQWV7KHPRVWZLGHO\XVHGSURFHVVHVIRUF\DQLGHWUHDWPHQWDQGGHVWUXFWLRQLQJROG

PLQHVDUHWKH,1&2SURFHVVDQGWKHDGGLWLRQRIK\GURJHQSHUR[LGH+₂O₂). In the INCO process, cyanide LVR[LGL]HGWROHVVWR[LFF\DQDWH2&1^-VSHFLHVDVVKRZQLQHTXDWLRQ%RWKIUHHDQGFRPSOH[HG&1^- are R[LGL]HGOHDYLQJRQO\IHUURXVF\DQLGHVDQG6&1^- LRQVXQDIIHFWHG7KHPHWDOLRQVUHOHDVHGIURPWKHF\DQLGH

FRPSOH[HVDUHSUHFLSLWDWHGDVK\GUR[LGHV7KHSURFHVVLQYROYHVVSDUJLQJVXOSKXUGLR[LGH62₂ZW into the cyanidation slurries to which Cu²⁺KDVEHHQDGGHGDVDFDWDO\VW7KHF\DQLGHFRQWHQWRIWKHWUHDWHG

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slower at pH values of 5-6 and at lower temperatures.

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SO₂ + O₂ + H₂O + CN^- ĺ2&1^- + SO₄^2- + 2H⁺ (Cu²⁺, added as a catalyst) (6)

Under the INCO conditions, the Fe³⁺VSHFLHVIRUPHGYLDUHDFWLRQPD\EHK\GURO\]HGSURGXFLQJIHUULF

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However, if the pH is below 4, the Fe³⁺PD\DOVRR[LGL]HS\ULWHUHDFWLRQ,URQF\DQLGHVDUHUHPRYHGE\

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H₂O₂ + CN^-ĺ2&1^- + H₂O (Cu²⁺, added as a catalyst) (7)

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pyrrhotite and arsenopyrite. During the enrichment of gold, these minerals are dissolved in order to release WKHDVVRFLDWHGJROG*ROGH[WUDFWLRQSURFHVVHVPRVWFRPPRQO\LQFOXGHVWKHH[SRVXUHRIWKHFUXVKHGRUHWR

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Table 1: Summary of arsenic removal processes (Twidwell et al., 1994).

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the presence of Fe²⁺&RQYHUVHO\DGVRUEHG$VVKRXOGVWDELOL]HWKHVWUXFWXUHRI)HK\GUDWHVDQGUHWDUGLWV

transformation into more crystalline phases (Rancourt et al. 2001).

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suppression of AMD formation, usually by treatment with lime or CaCO₃. Water or soil coverage (dry or wet FRYHULVZLGHO\XVHGIRUUHPHGLDWLRQEHFDXVHR[\JHQLVPXFKOHVVVROXEOHDQGGLIIXVHVPXFKPRUHVORZO\LQ

water than in air. Soil coverage often contains a liner with low hydraulic conductivity that becomes heavily waterlogged. Unfortunately, these approaches are not suitable for arsenic-containing mine waste because DUVHQLFLVPRUHVROXEOHLQWKHUHGXFHG$V,,,VWDWHWKDQLQWKHR[LGL]HG$V9VWDWH&RQVHTXHQWO\RWKHU

remediation options must be used.

One method that has been suggested for the management of As-rich tailings is their conversion into a PRQROLWKXVLQJVRFDOOHG³&HPHQWHG3DVWH%DFN¿OO´&3%$SDVWHLVGH¿QHGDVVOXUU\WKDWKDVEHHQGH- ZDWHUHGWRWKHSRLQWWKDWQRZDWHURUVROXWLRQZLOOÀRZIURPWKHPDWHULDO3DVWHIRUPDWLRQLVDPHWKRGIRU

the thickening and dewatering of tailings to reduce the risk of dam failures and the consumption of water LQUHPHGLDWLRQDSSOLFDWLRQV$SDVWHFDQEHSXPSHGLILWFRQWDLQVPRUHWKDQVROLGVZLWKDSDUWLFOHVL]H

OHVVWKDQ—P&3%JHQHUDOO\FRQVLVWLQJRIPLQHWDLOLQJVZDWHUDQG3RUWODQGFHPHQW%HQ]DD]RXDHWDO

1999). It is normally prepared from dilute slurries of tailings by dewatering it with conventional thickening RU¿OWHULQJ

2QFHIRUPHG&3%FDQEHEDFN¿OOHGLQWRH[FDYDWHGXQGHUJURXQGDUHDV7KHDLPRIFRQYHUWLQJWDLOLQJV

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underground mine cavities, which increases its operational advantages in the mining (Coussy et al., 2011).

7KHPRVWFRPPRQELQGHUPDWHULDOXVHGLQ&3%LVFHPHQWDQGWKHW\SLFDOZDWHUFRQWHQWRI&3%PDWHULDOV

is 20 - 25%, which is required to enable its transport through pipes. Because mines can easily produce more WKDQRIWRQQHVRIWDLOLQJVWKHFRVWRIWKHFHPHQWUHTXLUHGWRIRUP&3%FDQEHFRPHYHU\KLJK7R

reduce this cost, attempts have been made to replace some of the cement with cheaper materials that have VLPLODUO\SR]]RODQLFDQGDONDOLQHSURSHUWLHVVXFKDVJUDQXODWHGEODVWIXUQDFHVODJ*%)6ELRIXHOÀ\DVK

%)$DQGFHPHQWNLOQGXVW&.'7DULTDQG<DQIXO7KHVHDONDOLQHPDWHULDOVKHOSWRQHXWUDOL]HWKH

acidity arising from the sulphide-rich tailings and prevent the spread of trace elements. However, increasing the alkalinity of the material may increase the risk of leaching for anionic trace elements such as arsenic.

&HPHQW%)$DQG&.'DUHW\SLFDOSR]]RODQLFPDWHULDOV3X]]RODQVDUHVLOLFHRXVRUVLOLFHRXVDQGDOXPL- QRXVPDWHULDOVZKLFKUHDFWZLWKFDOFLXPK\GUR[LGHLQWKHSUHVHQFHRIZDWHU,QWKLVUHDFWLRQFDOFLXPVLOLFDWH

hydrate (C-S-H) and calcium aluminate hydrate compounds are formed possessing cementitious properties.

:KHQFDOFLXPVLOLFDWHFRPSRXQGVLQSX]]RODQVLVK\GUDWHGSRUWODQGLWH&D2+₂), hydrates such as C-S-H DUHIRUPHG7D\ORUUHDFWLRQ,QWKH¿UVWVWDJHRIK\GUDWLRQ&DVLOLFDWHVDUHGLVVROYHGDQGOHDGVWR

the formation of Ca²⁺, H₂SiO₄^í and OH^í (reaction 9).

2Ca₃SiO₅ + 6H₂2ĺ&D²⁺ + 8OH^- + 2H₂SiO₄^í (9)

$SR]]RODQLFSURSHUW\LVGH¿QHGDVWKHDELOLW\RI6LR[LGHVWRUHDFWZLWKFDOFLXPK\GUR[LGHVIRUPLQJD

FHPHQWLWLRXV&6+JHO7KH&6+JHOLVWKHPDLQFRQWULEXWRUWRWKHSK\VLFDOVWUHQJWKLQWKHPDWHULDO3H\- URQQDUGDQG%HQ]DD]RXD5HDFWLRQVZLWKDQHQGSURGXFWRI&6+DUHFDOOHGSR]]RODQLFUHDFWLRQV

3Ca²⁺ + 2H₂SiO₄^í+ 2OH^í + 2H₂O => Ca₃H₂Si₂O₇(OH)₂·32H₂O (10)

Portlandite Ca(OH)₂ is a cementitious mineral which dissolves into secondary phases such as ettringite DQGJ\SVXPZKHQS+LVORZHUHG7KHR[LGDWLRQRIDUVHQRS\ULWHLVREVWUXFWHGE\WKHDGGLWLRQRIFH- PHQWLWLRXVPDWHULDOVEXWS\UUKRWLWHUHPDLQVDFWLYH%HQ]DD]RXDHWDO2[LGDWLRQRIS\UUKRWLWHDWWKLV

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Ca(OH)₂ + SO₄^2- + 2H₂O => CaSO₄·2H₂O + 2OH^í (11)

7KHSUHVHQFHRIHWWULQJLWHDQGJ\SVXPFRXOGLQFUHDVHWKHSHUPHDELOLW\LQWKHPDWUL[$ONDOLQHFHPHQ- WLWLRXVSKDVHVQHXWUDOL]HWKHDFLGLW\IURPVXO¿GHR[LGDWLRQXQWLOH[KDXVWLRQDWDS+DOOFHPHQWLWLRXV

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$SDUWRIWKHFHPHQWFRXOGEHUHSODFHGZLWKÀ\DVKWRGHFUHDVHWKHHIIHFWRIDVXOIDWHDWWDFN7KH

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hydrate and calcium hydrate in CPB, hence decreasing the amount of Ca (OH)₂ formed. Fly ash contains OHVVDPRXQWVRIWULFDOFLXPVLOLFDWHVWKDQFHPHQW7KLVZLOOUHGXFHWKHJHQHUDWLRQRIH[SDQVLYHHWWULQJLWHDQG

KHQFHWKHHIIHFWVRIDVXOIDWHDWWDFNUHDFWLRQ7DULTDQG<DQIXO7KHVHHIIHFWVRULJLQDWHVIURPWKH

H[FKDQJHRIWULFDOFLXPVLOLFDWHV&₃S) into di-calcium silicates (C₂S) which produces less calcium-hydrates DQGKHQFHOHVVH[SDQVLYHSKDVHVDUHIRUPHG(UFLNGLHWDO

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or substitution within the crystal lattice of secondary cementitious minerals (Coussy et al., 2011). Stable calcium (Ca)-arsenates are formed in pH>10 but in slightly alkaline conditions (at pH 8), Ca-arsenates and FHPHQWLWLRXVSKDVHVDUHYHU\VROXEOH7KHLPPRELOL]DWLRQRI$VDWVHPLQHXWUDOFRQGLWLRQVLQF\DQLGHGFH- mented tailings is, therefore, primarily controlled by the physical stability, secondary cementitious phases in WKHPDWUL[DQGWKH$VEHDULQJ)HK\GUDWHV

2. Scope of the thesis

Gold occurs as inclusions in arsenopyrite in a gold mine situated in the north of Sweden. Arsenopyrite is KLJKO\VROXEOHGXULQJF\DQLGDWLRQDQGGLVVROYHVLQWRDUVHQDWHVDQGDUVHQLWHV7KHSURFHVVHVLQWKHJROGPLQH

LQFOXGHGF\DQLGDWLRQDQGVXEVHTXHQWWUHDWPHQWRIWKHHIÀXHQWVZLWK)H₂(SO₄)₃, H₂O₂DQGOLPH7KHVHSUR- FHVVHVPD\FKDQJHWKHGLVWULEXWLRQDQGVSHFLDWLRQRI$V7KHWUHDWPHQWRIWKHVH$VULFKHIÀXHQWVDLPVWRLP- PRELOL]H$VE\FRSUHFLSLWDWLRQZLWKLQWKHVWUXFWXUHRI)HK\GUDWHV7KHVWDELOLW\RI$VLQWDLOLQJVRULJLQDWLQJ

IURPWKHVHSURFHVVHVLVHYDOXDWHGE\SHUIRUPLQJVWDWLFDQGG\QDPLFOHDFKLQJWHVWV7KHJHQHUDODLPIRUWKHVH

evaluations is to make it possible to develop more effective ways of managing As-bearing tailings that will SUHYHQWWKHRFFXUUHQFHRIKLJK$VFRQFHQWUDWLRQVLQWKHPLQHGUDLQDJH0RUHVSHFL¿FDLPVLQFOXGHGWKHVXS- SUHVVLRQRIWKHVHWDLOLQJVLQWRVLPXODWHGÀRRGHGFRQGLWLRQVDFFHOHUDWHGZHDWKHULQJDQGHYDOXDWHWKHHIIHFWV

on As-leaching. Speciation of As in ore and tailings were conducted to get knowledge about the origin of As.

,QWKHVHFRQGSDUWRIWKHVWXG\WKHXVHRIDPHWKRGFDOOHGFHPHQWHGSDVWHEDFN¿OO&3%ZDVHYDOXDWHG

IRUPDQDJHPHQWRI$VULFKF\DQLGHGWDLOLQJV7KHXVHRI&3%PL[WXUHVFRQWDLQLQJFHPHQWÀ\DVKVODJV

DQGWDLOLQJVFRXOGGHFUHDVHOHDFKLQJRIDUVHQLFDFFRUGLQJWRSUHYLRXVO\FRQGXFWHGVWXGLHVRI%HQ]DD]RXD

HWDO&RXVV\HWDO5DQGDOODQG&RXVV\HWDO+RZHYHULQWKHVHVWXGLHVWKHSUR- SRUWLRQVRIFHPHQWYDULRXVVODJVDQGDVKHVUDQJHGIURP'HPDQGVRIVWUHQJWKLQWKHVHPL[WXUHVZHUH

GHWHUPLQHGE\WKHPLQHSURFHVVRUWRPD[LPL]HWKHDPRXQWRIH[WUDFWHGRUH:KHQOHVVVWUHQJWKLVQHHGHG

LQWDLOLQJVPDQDJHPHQWORZHUSURSRUWLRQVRIELQGHUVFRXOGEHDGGHG7KLVFRXOGKRZHYHUUHGXFHWKHOHYHO

RIVDWXUDWLRQZLWKLQWKH&3%PDWHULDOVLQFUHDVLQJÀXLGWUDQVIHUDQGZHDWKHULQJ7KHSULPDU\REMHFWLYHIRU

this part of the study was to evaluate the effects of cementation on arsenic leaching. For cost-reduction, low SURSRUWLRQVRIFHPHQWDQGELRIXHOÀ\DVKZWZHUHXVHGDVDPHQGPHQWV7KHHIIHFWRIFHPHQWDWLRQ

RQ$VOHDFKLQJZDVHYDOXDWHGE\FRPSDULQJ&3%PDWHULDOVDQGXQDPHQGHGWDLOLQJV7KHVSHFL¿FDLPVRI

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- 7KHOHDFKLQJEHKDYLRURI$VLQ&3%PDWHULDOVDQGWDLOLQJV

- 7KHLQÀXHQFHRIZHDWKHULQJRQWKHEHKDYLRURI$VLQ&3%PDWHULDOV - 7KHORQJWHUPVWDELOLW\RI$VLQ&3%

- 7KHHIIHFWRIFHPHQWDWLRQRQWKHVWDELOLW\RI$V

7KHGHOLPLWDWLRQVRIWKHVWXG\ZHUH

- 7KHSURSRUWLRQVRIWDLOLQJVDQGELQGHUVLQ&3%PL[WXUHVZHUHDGMXVWHGWRDFKLHYHWKHGHVLUHGKDUG- HQHGVWUHQJWKRIN3DZKLOHPLQLPL]LQJWKHELQGHUFRQWHQWLQWKH&3%

- 7KHGXUDWLRQRIFRQGXFWHGOHDFKLQJWHVWVFRXOGEHSURORQJHGWRKDYHUHVXOWVUHÀHFWLQJWKHORQJ±WHUP

stability of As.

- Arsenic was the only element evaluated.

3. Materials

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Cyanided tailings and grinded ore originating from the Svartliden Gold mine, Sweden were provided by WKHPLQLQJFRPSDQ\'UDJRQ0LQLQJ$%7DLOLQJVKDYHEHHQGHSRVLWHGLQWDLOLQJVGDPVDWWKHPLQHIRU

\HDUV±DWDUDWHRIDSSUR[PLOOLRQWRQQHV\HDU(IÀXHQWVIURPWKHF\DQLGHOHDFKLQJSURFHVV

were treated with Fe₂(SO₄)₃, H₂O₂, CuSO₄DQGOLPH&D2LQRUGHUWRLPPRELOL]H$VDQGGHWR[LI\UHVLGXDO

F\DQLGHV7KHS+RIWKHWUHDWHGHIÀXHQWLVWKHQUHGXFHGWREHIRUHGHSRVLWLRQLQWKHWDLOLQJVGDPVLQ

DFFRUGDQFHZLWKOHJDOJXLGHOLQHV7DLOLQJVZHUHVDPSOHGDWGHSWKVRI±FPIURPDSSUR[WHQGLIIHUHQW

ORFDWLRQVRQWKHWDLOLQJVGDPDQGPL[HGWRIRUPDEXONVDPSOHRIDSSUR[NJ7KHZHLJKWRIWKHFUXVKHG

RUHVDPSOHVZHUHDSSUR[NJ7KHGRPLQDWLQJVXOSKLGHPLQHUDOLQWKHRUHZDVS\UURWLWKHDQGDUVHQRS\ULWH

was the most common As-sulphide. In tailings, total concentrations of trace elements such as Cu, Pb and Cr ZDVUHODWLYHO\ORZEXWWKH$VFRQWHQWZDVPJNJ

Table 2: Total element composition of materials used in the study

Element Unit Tailings Ore FA CE

TS % 89.0 ± 0.4 99.97 ± 0.06 95.2 ± 0.7 99.4 ± 0.0

SiO₂ % TS 55.0 ± 4.9 30.3 ± 0.9 34.6 ± 1.3 20.6 ± 0.8

Al₂O₃ ” 4.69 ± 0.04 1.74 ± 0.07 10.7 ± 0.6 5.61 ± 0.45

CaO ” 4.83 ± 0.25 4.26 ± 0.09 14.1 ± 1.0 50.3 ± 1.8

Fe₂O₃ ” 16.7 ± 0.6 12.6 ± 0.5 13.9 ± 1.0 2.81 ± 0.05

K₂O ” 0.92 ± 0.03 0.32 ± 0.00 2.89 ± 0.08 0.83 ± 0.05

MgO ” 3.24 ± 0.01 2.38 ± 0.07 2.54 ± 0.08 4.00 ± 0.17

MnO ” 0.14 ± 0.01 0.15 ± 0.03 0.92 ± 0.03 0.10 ± 0.01

Na₂O ” 0.82 ± 0.03 0.18 ± 0.01 1.24 ± 0.08 0.65 ± 0.07

P₂O₅ ” 0.24 ± 0.01 0.10 ± 0.00 2.25 ± 0.05 0.06 ± 0.00

TiO₂ ” 0.18 ± 0.00 0.09 ± 0.01 0.34 ± 0.01 0.42 ± 0.02

As mg/kg TS 1070 ± 30 4703 ± 781 124 ± 5 10.2 ± 0.2

Cr ” 166 ± 3 247 ± 45 132 ± 4 98.1 ± 4.4

Cu ” 147 ± 7 82.0 ± 12.1 136 ± 10 86.2 ± 2.9

Ni ” 63.8 ± 2.1 126 ± 12 114 ± 9 63.8 ± 1.3

Pb ” 4.56 ± 0.46 7.71 ± 2.58 78.6 ± 1.1 8.46 ± 0.11

S ” 20933 ± 493 25367 ± 2363 13700 ± 200 9960 ± 219

Zn ” 25.0 ± 0.4 10.6 ± 1.5 374 ± 10 149 ± 3

C _inorganic % 0.19 ±0.03 N.D N.D N.D

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4. Methods

,QWKH¿UVWSDUWRIWKHVWXG\3DSHU,DFKDUDFWHUL]DWLRQRIWKHF\DQLGHGWDLOLQJVZDVFRQGXFWHG/HDFK- LQJWHVWVEDWFKDQGZHDWKHULQJFHOOZHUHFRQGXFWHGWRHYDOXDWHWKHEHKDYLRXURI$VLQWKHWDLOLQJV7KHVH

leaching tests were performed to determine the potential for As-leaching into ground and surface waters IURPWKHWDLOLQJV:HDWKHULQJFHOOWHVWV:&7DQGDQ$FLG%DVH$FFRXQWLQJ$%$WHVWZHUHFRQGXFWHG

to simulate weathering reactions and predict the tailings’ potential for acid generation and metal release. In :&7JRIWDLOLQJVZHUHSODFHGRQDSDSHU¿OWHULQD%FKQHUW\SHIXQQHO7KHVDPSOHVZHUHOHDFKHGE\

FRYHULQJWKHPZLWKPORIGHLRQL]HG0LOOLSRUHŠTXDOLW\ZDWHUIRUDSSUR[LPDWHO\KDQGWKHQUHFRYHULQJ

WKHOHDFKDWHVE\DSSO\LQJDYDFXXPWRWKHIXQQHO7KHVDPSOHVZHUHH[SRVHGWRZHHNO\F\FOHVLQYROYLQJ

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In the second part of the study (Paper II), leaching tests (tank and weathering cell) were conducted to evaluate the effects of cementation and weathering on As-rich cyanided tailings. Cement (CE) and biofuel À\DVK)$ZHUHPL[HGZLWKWDLOLQJVDQGFXUHGIRUGD\VIRUPLQJD&3%PRQROLWKZLWKWKHFRPSRVL- WLRQVRIWKHVWXGLHGPL[WXUHVEHLQJDGMXVWHGWRDFKLHYHDKDUGHQHGVWUHQJWKRIN3DZKLOHPLQLPL]LQJWKH

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5. Findings and discussion

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associated with the water-soluble fraction, these As-precipitates are the most soluble fraction of As and dissolves readily in contact with water. Conducted sequential leaching tests suggested that the majority of

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was primarily governed by a wash-off effect where a minor amount of As was rinsed from the tailings´ sur- face. Modeling suggested that an As-release at this stage could be related to the presence of As (III) (Paper ,7KHHYROXWLRQRIS+DQGLWVHIIHFWRQ$VOHDFKLQJZHUHGLIIHUHQWZKHQUHVXOWVIURP:&7DQG7/7ZHUH

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—PFRXOGDOVRKDYHEHHQH[XGHGGXULQJWKH¿UVWVWDJHVRI7/7WKLVLQWXUQFRXOGKDYHUHGXFHGWKHUHDFWLY- ity of the tailings. 0LQRUDPRXQWVRI$VZDVUHOHDVHGGXULQJ:&7DQG7/7WKHUHOHDVHRI&D)HDQG6ZDV

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has occurred.

Table 2: Comparison of cumulative leaching of Ca, Fe, K, Mg, Si, As, Mn, S and pH in WCT (L/S 10.7*) and TLT (L/S 10), (n = 2, ± SD) in tailings from Svartliden

Tailings

WCT TLT

Ca mg/kg 1966 ± 10 493 ± 127

Fe “ “ 356 ± 20 110 ± 23

K “ “ 178 ± 2 62.4 ± 5.8

Mg “ “ 93.5 ± 6.0 38.3 ± 2.3

Si “ “ 25.6 ± 2.9 3.98 ± 1.46

As “ “ 0.26 ± 0.01 0.18 ± 0.02

Mn “ “ 0.03 ± 0.00 12.7 ± 1.4

S “ “ 2540 ± 37 640 ± 33

pH 4.57 ± 0.07 3.86 ± 0.12

pH min-max* 4.36 – 5.32

* = a period of 15 cycles (59 d)

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are sorption onto C-S-H surfaces, As (V) replacing SO₄^í in ettringite, and the formation of calcium–arsenic FRPSRXQGV3KHQUDWHWDO7KHVHSURFHVVHVDOOGHSHQGRQWKHFKHPLFDODQGSK\VLFDOSURSHUWLHVRIWKH

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the leaching of As from the CE and CE-FA materials, which contain relatively small quantities of binders ZWLQFUHDVHGWKHOHDFKLQJRI$VUHODWLYHWRWKDWVHHQLQXQPRGL¿HGWDLOLQJVHVSHFLDOO\ZKHQWKHS+

was lowered from 10 to 8 (Fig. 2). As-species adsorbed onto C-S-H and Ca-arsenates are only stable under DONDOLQHFRQGLWLRQVS+ZKLFKPD\H[SODLQWKLVLQFUHDVHLQ$VOHDFKLQJ

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JHQHUDOO\RFFXUVDWDS+§$WDONDOLQHFRQGLWLRQVGXULQJOHDFKLQJWHVWV:&7DQG7/7RIWKH&3%PDWH- rials, As adsorbed onto Fe-hydrates could desorb and enter into solution. Alkaline solutions originating from the dissolution of cementitious phases obstructed dissolved As to re-adsorb onto Fe-hydrates. Dissolved As formed less acid-tolerant As species, which subsequently dissolved during leaching. However, it should be QRWHGWKDWRQO\DVPDOOSURSRUWLRQRIWKHWRWDO$VLQWKHWDLOLQJVZDVUHOHDVHGGXULQJWKHOHDFKLQJWHVWVWKH

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Previously conducted study of Jia and Demopoulos (2005) highlighted the importance of Ca ions for WKHVWDELOLW\RI$VLQFHPHQWLWLRXVPDWHULDOV7KHFRQWHQWRIZDWHUVROXEOH&DSKDVHVLQWKHWDLOLQJVZDV

primarily due to the presence of gypsum. Cementitious materials contain highly soluble Ca species whose dissolution may have raised the concentration of Ca ions in the leachates formed during the leaching tests.

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conditions, meaning that dissolved As could more easily re-adsorb onto their surfaces.

Acid (1M HCI) was added to consume the cementitious and buffering minerals in the crushed, cemented WDLOLQJV7KLVLQFUHDVHGOHDFKLQJRI)H&DDQG$VEXWKDGQRVLJQL¿FDQWHIIHFWRQ6OHDFKLQJ¿J,Q

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acid-tolerant. Acid was added to the crushed CPB-samples to reach a pH of 4.5, this represents the average S+LQWDLOLQJVGXULQJWKH¿UVWVWDJHVRIOHDFKLQJ7KHDGGLWLRQRIDFLGLQFUHDVHG$VOHDFKLQJPRVWVLJQL¿FDQW- O\LQ&(WKDWKDGWKHORZHVWSURSRUWLRQRIELQGHUV7KHDGGLWLRQRIELQGHUVLQFUHDVHGWKHDFLGQHXWUDOL]LQJ

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Figure 2: The evolution of As-leaching and pH in tailings, CE, CE-FA during TLT.

0 2 4 6 8 10 12

0 500 1000 1500 2000 2500

1 2 3 4 5 6 7 8

pH g/kg

Extraction

Arsenic

Tailings CE CE-FA

pH Tailings pH CE pH CE-FA

Future aspects concerning the use of CPB in cyanided As-rich tailings

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precipitates to more crystalline phases contribute to the reduction of sites amenable for As-adsorption thus HQKDQFHWKHPRELOL]DWLRQRI$V$UVHQLFGLVVROXWLRQVXQGHUUHGXFWLYHHQYLURQPHQWPD\FRQWULEXWHWRWKH

As release into solution when the management of tailings are not adequately designed. Although the As–Fe SUHFLSLWDWHVPD\EHVWDEOHXQGHUDFLGLFDQGR[LGL]LQJFRQGLWLRQVFRPPRQWRWKHWDLOLQJVSLOHVUHVXOWVIURP

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from the formation of a monolithic mass is that the high degree of saturation in the material reduces the in- WUXVLRQRIR[\JHQDQGZDWHUSHUFRODWLRQ7KHPDMRUFKHPLFDOHIIHFWRIWKHDGGLWLRQRIFHPHQWLWLRXVELQGHUV

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on a long term basis.

Figure 3: Cumulative leaching of Fe, Si, As and S before and after acid addition in CPB materials during the weathering cell test.

286

680

416

1 10 100 1000 10000

Tailings CE CE-FA WCT before acid alteration

1234 1058

738

1 10 100 1000 10000

Tailings CE CE-FA WCT after acid alteration

pH: 5.0-3.2 pH: 4.5 pH: 4.5 pH: 5.3-4.4 pH: 10.2-8.2 pH: 11.4-7.8

Arsenic (μg/kg) Fe (mg/kg) S (mg/kg) Si (mg/kg)

6. Conclusions

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$PDMRULW\RI$VLQWDLOLQJZDVDVVRFLDWHGZLWK)HK\GUDWHVDV)H$VSUHFLSLWDWHV7KHVHSUHFLSLWDWHVZHUH

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DQGGHVFHQGHGGXULQJWKHZKROHWHVWGXUDWLRQHYHQLIS+GHFUHDVHGWR7KLVPD\EHGXHWRWKHIRUPD- WLRQRIDUHGGLVKFUXVWRQWKHWDLOLQJVVXUIDFH7KHFUXVWZDVDVVXPHGWREHDSUHFLSLWDWHRI)HK\GUDWHVWKDW

PLJKWGHFUHDVHWKHSHUPHDELOLW\DQGZHDWKHULQJDELOLW\,Q:&7WKHUDWHRIS\UUKRWLWHR[LGDWLRQLQFUHDVHG

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however, very small proportions of the total content of As.

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conditions in CE and CE-FA during leaching tests caused a small proportion of As to desorb from the Fe-hy- GUDWHV'HVRUEHG$VFRXOGWKHQKDYHEHHQLQFRUSRUDWHGLQWRWKHFHPHQWLWLRXVPDWUL[DQGRUUHSUHFLSLWDWHG

as Ca-arsenates. Leaching of As in CE-FA followed the behavior of Si, which implies that the dissolution of cementitious phases governed the As-release. As-leaching in CE was more strongly than in CE-FA. Leach- ing of As in CE increased more strongly as pH decreased from 10 to 8, suggesting the formation of soluble

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proportion of binders.

7. Future research

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(SPD) is a recently proposed method for tailings management that was developed by adapting underground FHPHQWHGSDVWHEDFN¿OO&3%WHFKQRORJ\,WLQYROYHVWKHXVHRI&3%OLNHSDVWHVDVVXUIDFHFRYHULQJVIRU

tailings dams, and has lower strength requirements than conventional underground applications of CPB.

Consequently, pastes with relatively low contents of binders can be used without issue. Preliminary results indicate that SPD is effective at preventing metal ion release from tailings while suppressing AMD forma- WLRQ$WSUHVHQW63'KDVRQO\EHHQXVHGLQRQHIXOOVFDOHRSHUDWLRQDQGKDVQRWEHHQVWXGLHGH[WHQVLYHO\,W

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As-rich tailings.

Acknowledgements

,ZRXOGOLNHWRH[SUHVVP\GHHSJUDWLWXGHWR/HQD$ODNDQJDVDQG&KULVWLDQ0DXULFHP\UHVHDUFKVX- pervisors, for their patient guidance, enthusiastic encouragement and useful critiques of this research work.

I would also like to thank my research colleagues at the division of applied geology for their advice and assistance in keeping my progress on schedule. A special thanks to Mr. Asif Saleh Qureshi that have assisted PHDWQXPHURXVRFFDVLRQV0\JUDWHIXOWKDQNVDUHDOVRH[WHQGHGWR0U0LODQ9QXNIRUKLVKHOSDQGWHFKQL- FDOVXSSRUW,ZRXOGDOVROLNHWRH[WHQGP\WKDQNVWRWKHWHFKQLFLDQVRIWKHODERUDWRU\RIWKHGHSDUWPHQWRI

&LYLO(QYLURQPHQWDODQG1DWXUDO5HVRXUFHV(QJLQHHULQJIRUWKHLUKHOSLQH[SHULPHQWSODQQLQJDQGH[HFX- tion. Financial support from Ramböll Sverige AB, Ramböll Foundation, Norrbottens forskninsgråd NoFo, 6860,1±7RROVIRUVXVWDLQDEOHJROGPLQLQJLQ(8DQGWKH&HQWHURI$GYDQFHG0LQLQJDQG0HWDOOXUJ\

&$00DW/XOHn8QLYHUVLW\RI7HFKQRORJ\DUHJUDWHIXOO\DFNQRZOHGJHG7KHSHUVRQQHODW'UDJRQ0LQLQJ

AB are also gratefully acknowledged for valuable information about the mining processes and for providing the tailings needed for this study.

Finally, I wish to thank my family, Johanna and Astrid, for their support and encouragement throughout my study.

8. References

%DUN*:HLKHG32URJHQLFJROGLQWKHQHZ/\FNVHOH6WRUXPDQRUHSURYLQFHQRUWKHUQ6ZHGHQ

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VWDELOL]DWLRQSURFHVVIRUWKHFRQWURORIDFLGPLQHGUDLQDJH0LQHUDOV(QJLQHHULQJ

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*XLGH&KDSWHU7KH$FLG5RFN'UDLQDJH3URFHVV

-DQ]HQ031LFKROVRQ596FKDUHU-03\UUKRWLWHUHDFWLRQNLQHWLFV5HDFWLRQUDWHVIRUR[L- GDWLRQE\R[\JHQIHUULFLURQDQGIRUQRQR[LGDWLYHGLVVROXWLRQ*HRFKLPLFD(W&RVPRFKLPLFD$FWD

1511-1522.

-LD<'HPRSRXORV*3$GVRUSWLRQRIDUVHQDWHRQWRIHUULK\GULWHIURPDTXHRXVVROXWLRQ,QÀX- HQFHRIPHGLDVXOIDWHYVQLWUDWHDGGHGJ\SVXPDQGS+DOWHUDWLRQ(QYLURQPHQWDO6FLHQFHDQG7HFKQROR- gy, 39(24), 9523-9527.

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0HHXVVHQ-&/.HL]HU0*9DQ5LHPVGOMN:+'H+DDQ)$0Dissolution behavior RILURQF\DQLGHSUXVVLDQEOXHLQFRQWDPLQDWHGVRLOV(QYLURQPHQWDO6FLHQFH7HFKQRORJ\

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5DQGDOO30$UVHQLFHQFDSVXODWLRQXVLQJSRUWODQGFHPHQWZLWKIHUURXVVXOIDWHOLPHDQGWHU- UDERQGŒWHFKQRORJLHVPLFURFKDUDFWHUL]DWLRQDQGOHDFKLQJVWXGLHV6FLHQFHRIWKH7RWDO(QYLURQPHQW

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