Cementation of Cyanidation Tailings – Effects on the Release of As, Cu, Ni and Zn

DOCTORAL T H E SIS Roger Hamberg Cementation of Cyanidation Tailings – Effects on The Release of As, Cu, Ni and Zn

Department of Civil, Environmental and Natural Resources Engineering Division of Geosciences and Environmental Engineering Cementation of Cyanidation Tailings

ISSN 1402-1544 ISBN 978-91-7790-124-2 (print) – Effects on The Release of As, Cu, Ni and Zn ISBN 978-91-7790-125-9 (pdf)

Luleå University of Technology 

Roger Hamberg

Applied Geochemistry

Cementation of Cyanidation Tailings – Effects on The Release of As, Cu, Ni and Zn

Roger Hamberg

Luleå University of Technology Department of Civil, Environmental and Natural Resources Engineering Division of Geosciences and Environmental Engineering Picture: Open pit at Svartliden gold mine, Sweden

Printed by Luleå University of Technology, Graphic Production 2018

ISSN 1402-1544 ISBN 978-91-7790-124-2 (print) ISBN 978-91-7790-125-9 (pdf) Luleå 2018 www.ltu.se Abstract

Knowledge about mineralogy and chemical composition in sulfidic tailings is essential to predict how tailings management may affect the future leachate quality. At a gold mine in the north of Sweden, gold was extracted from inclusions in arsenopyrite and pyrrhotite by the use of cyanide. Sulfides in the ore dissolved to a large extent during the cyanide leaching process causing sulfide-related elements such as As, Cu, Ni and Zn to be mobilized to a various extent. In a subsequent water treatment process, a significant proportion of As and Cu was captured in secondary formed Fe-precipitates. Large proportions of water- soluble Ni- and Zn-species in tailings suggested that this treatment was insufficient to reduce the mobility of Ni and Zn. Maintaining oxidized, neutral conditions is of major importance for the immobility of As, Cu, Ni and Zn during further management of the cyanidation tailings (CT). Part of the CT were planned to be managed in underground cavities by the use of a cemented paste backfill (CPB) -application. In CPB, a monolithic mass is formed as tailings are mixed with small proportions (4-7 weight %) of pozzolanic materials and backfilled into underground excavated areas. Using a CPB-application may decrease the sulphide oxidation rate, reducing exposure of mineral surfaces to oxygen and increasing water saturation levels within the material. In this study, CT was mixed with binders (1-3 wt. %) for the formation of a low-strength (0.2 Mpa) CT-CPB-mass. These mixtures were stored at moisturized conditions and subsequently subjected to oxidized and flooded conditions in a laboratory-based study. During short-term storing, high water saturation levels were preserved in the CT-CPB-mixtures, but, sulfide oxidation still progressed, and the release of Zn, Cu, and Ni was still lower compared to that in CT. The opposite was true for As, probably due to a desorption from Fe-precipitates. The desorbed As was subsequently incorporated into less acid-tolerant species (i.e. Ca-arsenates and As bonded to cementitious phases) in the CT-CPB:s, that readily dissolved and released more As compared to that in CT. A complete flooding of CPB-filled workings may take a long time to be reached. During this transition period, zones with low levels of water saturation forms in the CPB-monoliths, which could increase the sulphide oxidation rate, lower pH and dissolve the cementitious binders. In this study, strength decreased along with the water saturation levels in the CPB-mixtures, due to a more extensive pyrrhotite oxidation. A minimal proportion (1 wt. %) of binders did not suppress Cu and As leaching during flooding, but Ni and Zn-leaching were still lower than from CT. In the CT-CPB:s, proportions of As, Cu, Ni and Zn associated with cementitious phases increased in tandem with the fraction of binders. Using higher binder proportions in the CPB, as water saturation levels were lowered, substantially increased the Zn-release while there was an insignificant change in the As-release, and substantially lower Cu- and Ni-release. Pyrrhotite oxidation proceeded in the CT-CPB-mixtures independent of water saturation level. So, increasing binder proportion in a CPB does not necessary mean that trace metals are more stabilized, due to the formation of acid-intolerant fractions. Results from this study, pinpoints the importance of having knowledge about trace element distribution and mineral assemblage in tailings before management methods are chosen and implemented.

List of articles

I. Hamberg R, Alakangas L, Maurice C, The use of low binder proportions in cemented paste backﬁll: Effects on As-leaching (2015). Minerals Engineering (78, pp 74-82). http://dx.doi.org/10.1016/j.mineng.2015.04.017

II. Hamberg R, Bark G., Alakangas L, Maurice C, Release of arsenic from cyanidation tailings (2016). Minerals Engineering (93, pp 57-64). http://dx.doi. org/10.1016/j.mineng.2016.04.013

III. Hamberg, R., Maurice, C., Alakangas, L. (2017). Lowering the water saturation level in cemented paste backﬁll mixtures – effect on the release of arsenic. Minerals Engineering, 112, 84-91. https://doi.org/10.1016/j.mineng.2017.05.005

IV. Hamberg, R., Maurice, C., Alakangas, L. (2018). The formation of unsaturated zones within cemented paste backﬁll mixtures - Effects on the release of copper, nickel, and zinc, http://dx.doi.org/10.1007/s11356-018-2222-9. Accepted for publication in Environmental Science and Pollution Research.

Additional related papers not included in the thesis:

Hamberg, R., Maurice C., Characterization of green liquor dredges for the remediation of mine waste. (2013), Project report, Cooperation, Luleå University of Technology and Processum, Örnsköldsvik (Only available in Swedish).

Hamberg, R., Characterization and solidiﬁcation of arsenic-rich cyanided tailings (2014), Licentiate Thesis, Luleå University of Technology. 55p.

Hamberg, R., Alakangas, L., Maurice, C., Use of cemented paste backﬁll based on As-rich cyanidation tailings. Proceedings of the 10th on Acid rock drainage and IMWA annual conference. Santiago, Chile, April 21-24, 2015, 10pp.

Kumpiene, J., Nordmark, D., Hamberg, R., Carabante, I., Simanavi ien , R., Aksamitauskas, V. . (2016). Leaching of arsenic, copper and chromium from thermally treated soil. Journal of Environmental Management, 183, 460-466. doi:10.1016/ j.jenvman.2016.08.080

Hamberg, R., Alakangas, L., Maurice, C., The release of Arsenic from cyanidation tailings (2016). Proceedings of the 6th International Congress on Arsenic in the Environment (As2016), June19-23, Stockholm, Sweden. 2pp.

Hamberg, R., Alakangas, L., Maurice, C (2017). Delaying ﬂooding of cemented paste backﬁll mixtures – Effect on the mobility of trace metals. Proceedings of the IMWA 2017 – Mine Water & Circular Economy conference. Lappeenranta, Finland, June 25-30, 2017, 6pp (Vol II) 808 – 814.

Acknowledgements I would like to express my gratitude to Lena Alakangas and Christian Maurice, my supervisors, for guiding me through this research work. I would also like to thank my research colleagues at the division of applied geochemistry for their advice and help on numerous occasions. My grateful thanks are also extended to Mr. Milan Vnuk for his help with designing my articles and the thesis. Financial support from Ramböll Sverige AB, Ramböll Foundation, Norrbottens forskninsgråd NoFo, SUSMIN – Tools for sustainable gold mining in EU, and the Center of Advanced Mining and Metallurgy (CAMM) at Luleå University of Technology are gratefully acknowledged. The project has been carried out in cooperation with the Luleå Technical University and the mining industry. The mining industry provided information about strength requirements and the cyanide leaching process, as well as supplied material (fresh tailings). Mr. Mattias Koot, a process engineer, is greatly acknowledged for taking time to answer questions about the gold extraction process. Finally, I wish to thank my family, Johanna, Astrid, Ebbe and Hedda for their support and encouragement throughout my study.

Appended Papers

Paper I The use of low binder proportions in cemented paste backﬁll: Effects on As-leaching

Paper II Release of arsenic from cyanidation tailings

Paper III Lowering the water saturation level in cemented paste backﬁll mixtures – effect on the release of arsenic.

Paper IV The formation of unsaturated zones within cemented paste backﬁll mixtures - Effects on the release of copper, nickel, and zinc

Table of Contents

1. Introduction and background 1 Environmental concerns regarding base metal extraction in sulphidic mine ores 1 Environmental concerns regarding gold extraction in sulphidic mine ores 2 Methods for containment of Cu, Ni, As and Zn in tailings from a cyanide leaching process 3 Cemented paste backﬁll 4 Background 6 2. Scope and aims 8 3. Materials and methods 8 Preparation of CT-CPB mixtures 10 Methods 10 4. Results 11 Mineralogical studies 12 Sequential leaching test of Ore and CT 13 Sequential leaching test of CE446 and CE-FA446 14 Weathering Cell Test (WCT) 15 pH-dependent leaching test 17 Tank leaching test 18 5. Discussion 20 The cyanide leaching process and the associated water treatment - effects on the mobility of As, Cu, Ni and Zn in CT 20 Managing CT by the use of a CPB-method with low proportions of binders-effects on the leaching behavior of Cu, As, Ni and Zn 22 How does the establishment of unsaturated zones within CT-CPB- mixtures affect the leaching behavior of As, Cu, Ni and Zn? 26 Overall – could a low strength CPB (0.2 Mpa) be an environmentally accepted option for managing tailings from a cyanide leaching process? 28 6. Conclusions 28 7. Future Research 29 8. References 30

1. Introduction and background

Mines can be located in settings of relatively unspoiled nature, which may contain ecologically sensitive areas and many important habitats. Mining operations can generate enormous amounts of waste (i.e. waste rock and tailings) that have to be managed and handled in a way that avoids negative environmental impact. Waste rock is a heterogeneous material that has to been removed to reach the ore. The ore is then removed, crushed and reﬁned using various enrichment methods that extract the desired metal and/or mineral. Tailings is the residual material from the enrichment process and consist of a ﬁne grained, silty slurry with a high water content.

Environmental concerns regarding base metal extraction in sulphidic mine ores

Base metals such as Copper (Cu), Nickel (Ni) and Zinc (Zn) are natural constituents of the bedrock and is can occur in sulfides such as chalcopyrite (CuFeS2), pentlandite (Ni, Fe)9S8 and Sphalerite (ZnS). It is common that Cu, Ni and Zn also occur as impurities in other sulfides such as pyrite and/or pyrrhotite (Janzen et al., 2000). A major environmental concern associated with the mining of sulfide ores is where these metals are associated with sulfide minerals that can oxidize and form acid mine drainage (AMD). AMD is formed when the sulfide mineral oxidizes in contact with air and water (reaction 1). The formation of AMD can pose a severe pollution problem, especially due to acidic conditions, high concentrations of potentially toxic dissolved metal(oid)s, and sulfates. The extent of sulfide oxidation is dependent on the abundance and type of the sulfide mineral but also on the surface area of the exposed mineral, which increases with smaller grainsize. Under reducing alkaline conditions, these minerals are relatively stable and their natural dissolution is quite slow. However, once the ores have been ground, processed, and deposited in tailings facilities where they are exposed to air and water, their dissolution is + significantly enhanced. Pyrite (FeS2) is a common sulfide mineral that generates acid (H ) upon oxidation (net reaction 1):

2− + FeS2 + 3.75O2 + 3.5H2O A Fe(OH)3 + 2SO4 + 4H (1)

Pyrrhotite oxidizes 20-100 times more rapidly than pyrite (Janzen et al., 2000) when it comes into contact with atmospheric oxygen and water (net reaction 2).

2- + Fe(0.9) S + 2.175O2 + 2.35H2O A 0.9Fe(OH)3 + SO4 + 2H (2)

In alkaline oxidizing conditions, oxidation of these sulfides is obstructed by the formation of a surface coating that usually contains different forms of iron oxides (hydroxides, oxyhydroxides) and sulfates (Asta et al., 2010; Pérez-López et al., 2007; Yin et al., 2000; Buckley and Woods, 1989; Malysiak et al., 2004; Belzile et al., 2004). In these cases, the oxygen supply is restricted but not blocked, resulting in a reduction of the sulfide oxidation rate. Sulfide oxidation can continue despite this. Under oxidizing conditions of pH > 5,

1 oxidation of chalcopyrite and sphalerite occurs according to net reactions 3 and 4:

2+ 2− + CuFeS2 + 4O2 + 3H2O A Cu + Fe(OH)3 + 2SO4 + 2H (3)

2+ 2− ZnS + 2O2 A Zn + SO4 (4)

Under acidic (pH <4) conditions, the dominant oxidant is Fe3+ rather than molecular oxygen. In such cases, sulﬁde oxidation produces more acidity (Blowes et al., 1998).

Environmental concerns regarding gold extraction in sulphidic mine ores

Gold can occur in its native form or as inclusions in sulphide minerals such as arsenopyrite, pyrrhotite and pyrite. Cyanide-leaching is a method used to extract gold occurring as inclusions in sulphide minerals that also might contain arsenic, copper, nickel and zinc. Cyanide is used exclusively to dissolve gold, but to increase the extraction efficiency; an oxidation step is often used to dissolve the gold-enclosing sulphide minerals. At this stage, gold is liberated from the sulfide minerals in order to be dissolved in a cyanide solution. A cyanide detoxification step is added to remove toxic cyanide species, before the tailings slurry is to be deposited. The most commonly used cyanide detoxification process is the INCO-process whereas cyanide is oxidized to less toxic cyanate (OCN-) species as shown in reaction (5). The INCO-process utilizes SO2 and O2 in the presence of a soluble copper catalyst to oxidize cyanide to the less toxic compound cyanate (OCN-).

- - 2- + 2+ SO2 + O2 + H2O + CN A OCN + SO4 + 2H (Cu , added as a catalyst) (5)

The INCO process may leave some small quantities of toxic cyanide in the treated material. It is therefore, sometimes followed up with copper-catalyzed hydrogen peroxide treatment to destroy these remaining cyanides oxidatively, as shown in reaction (6):

- - 2+ H2O2 + CN A OCN + H2O (Cu , added as a catalyst) (6)

However, the oxidation step and the addition of hydrogen peroxide (H2O2) may also increase the oxidation rate of copper-, nickel- and zinc-sulﬁdes. The slurries produced must therefore be treated before being discharged into tailings facilities. This is commonly addressed with a supplementary water treatment process where lime (Ca(OH)2) and iron hydroxides (in the form of Fe2(SO4)3) are added to the cyanide leaching slurries to raise pH and form stable metal hydroxides (reaction 7 and 12) and metal-bearing Fe-precipitates. In these cases, metals could adsorb onto the Fe-precipitate-(Fe(OH)3) -surfaces or exchange for Fe in the Fe(OH)3 structure (reaction 8).

2+ 2+ Cu, Ni, Zn + Ca(OH)2 A Cu, Ni , Zn (OH)2 + Ca 7)

2+ Cu, Ni, Zn + Fe2(SO4)3 A Cu, Ni, Zn>Fe(OH)3 (adsorption) and/or Fe(Cu, Ni, Zn)(OH)3 (co-precipitation) (8)

2 The oxidation of arsenopyrite in the ore generates As (III) and As(V) species. As (III) species less attached onto mineral surfaces. Oxidation of As(III) to As(V) is therefore a prerequisite for enhanced stability of As and must precede the co-precipitation step (net reaction 12).

2- + FeAsS(s) + 4H2O + 3O2 (aq) A Fe(OH)3(s) + H3AsO3(aq) + SO4 + 2H (9)

The unloaded arsenite specie formed in reaction (9) can be oxidized as shown in reactions 10 and 11:

2- + H3AsO3(aq) + 0.5O2 A HAsO4 + 2H (10)

- + H3AsO3(aq) + 0.5O2 A H2AsO4 + H (11)

2H3AsO4 + Fe2(SO4)3 + 3Ca(OH)2 A 2FeAsO4(s) + 3CaSO4(s) + 6H2O (12)

To be able to form a stable Fe-arsenate, As (V) must be the dominant As-specie; molar ratio Fe/As in the slurry must be more than 4/1 if pH is 4-7. If pH of the slurry is close to 10, more Fe must be added to get a stable Fe-arsenate; a Fe/As-molar ratio of 8/1 is then preferred (Riveros et al., 2001). If insufﬁcient amounts of Fe are available, As could remain in solution and/or precipitate as a Ca-arsenate, which are more soluble than the Fe- arsenate. In a study of Paktunc et al., (2015), arseniosiderite and yukonite were suggested as probable As-minerals in CT. Yukonite is a highly soluble As-mineral while the pH- stability ﬁeld of arseniosiderite overlaps that of As-bearing Fe precipitates (4-8) (Pactunc et al., 2015; Riveros et al., 2001). Metal(oid)-precipitates formed this way are generally stable in an oxidizing environment at a pH of 4-8. Therefore, reducing environments and acidic or alkaline conditions must be avoided to ensure the stability of Cu, As, Zn and Ni.

Methods for containment of Cu, Ni, As and Zn in tailings from a cyanide leaching process

In the remediation of mine waste there are generally three strategies that can be applied: (1) limiting the source for AMD (sulfide oxidation), (2) the collection and treatment of generated effluents or (3) reducing the amount of sulfides present in the tailings by mechanical removal (i.e. flotation). If AMD is generated in the mining process, treatment of the effluents is usually conducted with lime or CaCO3. Most metals can be precipitated as hydroxides during neutralization (pH adjustment), commonly with addition of lime

(Ca(OH)2). Upon neutralization, aeration could transform reduced metals to oxidized forms to improve the recovery of metals as stable hydroxides. This process is often combined with the precipitation of Fe-precipitates that have a high afﬁnity for metals in semi-neutral pH conditions. A most frequently-used method for metal immobilization is the oxidation of iron in conjunction with pH adjustment and metal precipitation (Bowell and Craw, 2014).

3 To reduce the amount of sulfides in the tailings, a method called desulphurization is often used, in which sulfides are removed by froth flotation (Benzaazoua et al., 2017). One way to limit the source of AMD in surficial tailings impoundments is by the use of a water or soil coverage (dry or wet cover).The satisfactory function of these methods are based on the fact that the solubility and diffusion of oxygen is much lower in water than in air. A soil coverage, therefore, often contains a liner with low hydraulic conductivity reaching high waterlogging (Lottermoser, 2007). However, using these methods is not appropriate if the tailings are containing secondary metal-bearing Fe-precipitates. In that case, severe water logging can generate a reducing environment where these Fe-precipitates become unstable and releasing large amounts of metals. Consequently, other remediation options must be considered.

If tailings are to be managed and stored in underground workings, the use of a method called cemented paste backfill (CPB) is sometimes suggested. A paste is defined as dewatered tailings whereas no water could migrate from the material. In surficial tailings management facilities (TMF), the paste method is commonly used to reduce the hydraulic pressure in dam constructions, but also to reduce water consumption in mining processes. A CPB- application generally includes dewatered mine tailings mixed with a binder material. A common binder material used in CPB is cement. Typical water content of 20-25%; is used within a CPB-application, this water is required for paste transportation through a pipe networking. A CPB-material could be pumped if it contains more than 20wt. % solids with a particle size less than 20 μm (Landriault et al. 1998). In CPB-applications, a monolithic mass is formed as tailings are thickened (water content reduced), mixed with low proportions (3-7 weight %) of cement or other pozzolanic materials and backfilled into an underground excavated area (Fig. 1). A CPB is primarily used to enhance the geotechnical properties/mechanical strength of surrounding rock, increasing the amount of ore that could be excavated. The C-S-H phase is considered to be a main contributor of the mechanical strength in a CPB-material (Peyronnard and Benzaazoua, 2012). C-S-H is formed as Ca-silicates are 2+ 2− − dissolved, and leads to the formation of Ca , H 2SiO4 and OH (reaction 13). A C-S-H- 2+ 2− phase is formed as Ca-silicates dissolved, and leads to the formation of Ca , H 2SiO4 and OH− (reaction 13). In the second stage, Ca, Si is reacts with water and hydroxide ions to form a C-S-H-phase (reaction 14):

2+ - 2− 2Ca3SiO5 + 6H2O A 6Ca + 8OH + 2H2SiO4 (13)

2+ 2− − 3Ca + 2H2SiO4 + 2OH + 2H2O => Ca3H2Si2O7(OH)2·32H2O (14)

The environmental beneﬁt of using a CPB application has been of little concern for the mining industry. However, in this case, such a method could prevent air intrusion into the tailings and thus lower the sulﬁde oxidation rate. The prevention of air intrusion is governed by the water saturation levels within the CPB-mixture and, at levels of

4 Fig. 1. CPB-application >85 %, oxygen migration is largely prevented (Yanful, 1993). In a CPB, trace metals such as Cu, Ni, As and Zn can be immobilized due to physical encapsulation and chemical stabilization (Chen et al., 2009; Paria and Yuet, 2006; Benzaazoua et al., 2004). A physical encapsulation is largely dependent on the amount and durability of the C-S-H that determines the inherent strength of the monolith. A chemical stabilization may occur as the hydroxyl anions from this process react with trace metals, causing the formation of metal-hydroxides. The stability of C-S-H in a CPB is governed by the sulfide and sulfate content, the curing time as well as the type and proportion of binder material (Ercikdi et al., 2009; Kesimal et al., 2005; Benzaazoua et al., 2004). The efficiency of a CPB to limit sulfide oxidation is therefore largely dependent on the C-S-Hs. The dissolution of portlandite contributes to the hardening of the CPB by the formation of C-S-H as Ca(OH)2 reacts with silicic acid (H2SiO4) (CH + SH A C-S-H). But portlandite are also sensitive to a sulfate attack where Ca(OH)2 reacts with sulfates, forming expansive phases such as gypsum and ettringite (reaction 15).

2- − Ca(OH)2 + SO4 + 2H2O => CaSO4·2H2O + 2OH (15)

This may cause cracks to form within the CPB material that increase oxygen migration into the monolith. Sulfates in a CPB may originate from sulfide oxidation, gypsum dissolution, the cyanide leaching process, and/or the binders. The effects of a sulfate attack can be reduced by the addition of fly ash replacing a part of the cement. An addition of fly ash can decrease the amount of Ca(OH)2 formed in the CPB-material. Fly ash contains lower proportions of tri-calcium silicates, compared to that in cement, but higher amounts of di-calcium silicates (C2S) phases. This exchange leads to the generation of less

5 amounts of calcium-hydrate that is sensitive to a sulfate attack (Ercikdi et al., 2009). In CPB materials, dissolution of C-S-H contributes to neutralization of the acid formed by the oxidation of sulfides (reactions 1 and 2). Alkaline cementitious phases neutralize the acidity from sulfide oxidation until exhaustion, at a pH < 9, all cementitious phases are consumed and the matrix consists solely of gypsum, ettringite, Fe or Al oxides (Coussy et al., 2011). As large mines may produce millions of tonnes of tailings, the cost of the cement needed for CPB can be substantial, and many studies have therefore explored the scope for using alternative binders or otherwise minimizing the amount of cement used in CPB formation. Granulated blast furnace slag (GBFS) and biofuel fly ash (BFA) have successfully been used to partially replace cement in CPB due to their pozzolanic properties (Peyronnard and Benzaazoua, 2012). For a CPB to function as a remediation method, the water saturation level within the monolith must be maintained at high levels (more than 85 %) to obstruct oxygen ingress and thereby reduce the sulfide oxidation rate. This is conducted by a reaction rind containing gypsum and oxide-precipitates that are filling the pores within the CPB-monolith. The reaction rind is formed as the binders dissolves and functions as a diffusion barrier that maintains high water saturation levels within the CPB-monolith (Bowell and Craw, 2014). This decreases the sulfide oxidation rate, and prolongs the mechanical stability of the C-S-H.

Background

In Svartliden, gold is extracted from inclusions in arsenopyrite and pyrrhotite using a cyanide leaching process. Occasionally, in outlet process water, levels of As, Ni and Zn have exceeded threshold values. In the gold extraction process, prior to cyanide leaching, an oxidation step is added to liberate the gold occurring as inclusions in arsenopyrite/ pyrrhotite. In the oxidation process, pH is raised to 10-11 while oxygen is added. In the subsequent cyanide leaching process, pH is kept at 10-11, to avoid the formation of hydrogen cyanide that easily volatilizes (Mosher and Figueroa, 1996). This in turn, could cause a loss of free cyanide ions (CN-) from the leaching solution and decreases the gold extraction efﬁciency (reaction 16).

- - - 4Au + 8CN + O2 + 2 H2O A 4Au(CN)2 + 4OH (16)

After the gold has been dissolved and removed, the tailings slurries were treated with

H2O2, CuSO4 and lime (CaO) in order to immobilize heavy metals and detoxify residual cyanides. At this point, pH of the tailings slurry was set to 10-11. To immobilize the metal(oid)s, Fe2(SO4)3 is then added (reaction 12 and 17) until the pH of the tailings slurries has been reduced to 7-8.5 (reaction 17) before deposition in the tailings dams, in accordance with legal guidelines (personal communication, Mattias Koot) .

3+ + Fe + 3H2O A Fe(OH)3(s) + 3H 17)

6 Cyanide Detoxification Preoxidation Co-precipitation Deposition leaching step: Step: H2O2+ step: O2 and step: Fe2(SO4)3, in tailings O2 addition, pH CuSO4, pH: 10- H2O2 10-11 11 pH: 7-8.5 dam

Fig. 2. Process schedule, Svartliden

Overall, the cyanide leaching process and the supplementary water treatment process could have a large impact on the mobility of arsenic, copper, nickel and zinc. Alkaline and/ or acidic conditions may cause desorption from the metal(oid)-bearing Fe precipitates or their dissolution (Moon and Peacock, 2012; Cornell and Schwertmann, 2003). Therefore, it is advantageous if the sulphide minerals are dissolved or otherwise removed during the cyanide-leaching process. However, a complete removal of sulfide minerals seldom happens, because it is a common fact that mining processes are never 100 % effective and remnants of gold and sulfide minerals are always present in the end product (tailings). Where acid-generating sulfide minerals still exist after cyanide leaching, the effects of the sulphide oxidation or the oxidation itself must be prevented to ensure the stability of metal-bearing phases formed during the water treatment process. Before managing CT by the use of a CPB-application, it is critical to understand how the cyanide leaching process and a supplementary water treatment process could affect the mobility of Cu, Ni, As and Zn. Previous studies by Ercikdi et al. (2009) and Peyronnard et al. (2010) have shown that using a CPB application could reduce AMD generation and contaminants release in sulfide- rich tailings. However, in these studies, binder proportions of < 3 wt.% were used in the CPB-applications. Highly alkaline conditions (resulting from C-S-H-dissolution and the subsequent release of OH-) could also increase the mobility of some trace metal(oid)s (i.e. As) (Kumpiene et al., 2008). As the cementitious phases hydrates, pores within the CPBs are filled, high water saturation levels can be maintained, decreasing the oxygen ingress and a the sulfide oxidation rate. Maintaining the C-S-H-structures intact is therefore of major importance for the immobility of trace metals in a CPB application. Before a CPB- application is chosen as a management method for sulfide rich tailings, it is vital to know how this could affect the mobility of trace metals. As remediation methods are expected to withhold a good function for a long time, it is important to address the potential long-term performance of a CPB application. Underground mines become flooded by groundwater after mining processes have ceased, which is seen as a positive thing in respect of sulfide oxidation as the solubility of oxygen (which is the primary oxidation agent of sulphide minerals in a pH of > 4) is much lower in water than in air. However, in field conditions, the groundwater raising is a slow process and a complete flooding of the CPB-materials takes long time to be reached. During this transition period, zones with low levels of water saturation may form in

7 the CPB monoliths, which in turn can increase the sulﬁde oxidation rate, lower the pH and destabilize the metal-bearing phases. A previous study by Kesimal et al. (2005) has shown that the strength within a sulﬁde-rich CPB material could decrease by more than 50% during prolonged (>1 year) periods of curing. The stability of the C-S-Hs must be guaranteed over a long time. It is, therefore, important to determine the effect of unsaturated conditions in CT-CPB mixtures on the leaching behavior of As, Ni, Zn, and Cu.

2. Scope and aims

The overall aim for this study was to gain knowledge about the mobility of As, Cu, Ni and Zn in tailings from a cyanide leaching process, and then address the following questions Could a low strength CPB (0.2 Mpa) be an environmentally acceptable option for managing tailings with elevated concentrations of Cu, As, Ni and Zn from a cyanide leaching process? These speciﬁc questions provide the focus of the thesis: - How could a cyanide leaching process and the associated water treatment affect the mobility of Cu, As, Ni and Zn in tailings?

- How does the use of a low-strength CPB application affect leaching of As, Cu, Ni and Zn?

- How does the establishment of unsaturated zones within low-strength CPB mixtures affect leaching of As, Cu, Ni and Zn?

The results may contribute to knowledge about the preparation/management of CPB mixtures for use in underground mine workings.

3. Materials and methods Tailings from a cyanide leaching process (CT) and grinded ore originating from the Svartliden Gold mine, Sweden were provided by the mining company Dragon Mining AB. Materials The ore was considered to be refractory because the gold occurred as inclusion in sulfide minerals. Fresh tailings were sampled at depths of 0-30 cm from ten different locations on the tailings dam and mixed to form a bulk sample of 15 kg. The weight of the crushed ore samples were approximately 3kg. The materials used for the CT-CPB mixtures were biofuel fly ash (FA) and ordinary cement (CE). CE was provided by Allmix AB in Trollhättan, Sweden. FA (ash from wood products), was provided by a fluidized bed combustion plant in Lycksele, Sweden. The ash was dry, having been collected directly after incineration.

8 Fig. 3. Svartliden mine in Sweden

Fig. 4. Tailings dam, Svartliden, Sweden

9 Preparation of CT-CPB mixtures

CT was managed by the use of a laboratory-based CPB application, where the required strength was set to 0.2 MPa by the mine operator. CT-CPB-mixtures were designed with minimal additions of binders and involved 1-3 wt % of binders (CE and FA) (Paper II). The preparation of the CT-CPB mixtures is described in detail in Paper (II and III). A slump test was not conducted while only 15 kg of CT was available for analyses and experiments. Water contents in the CT-CPB-mixtures were set to ~ 25 wt. % in order to make them pumpable in a pipe system. CT-CPB mixtures of CE and CE-FA were cured for 31 days or 446 days (named CE31, CE446, CE-FA31, and CE-FA446 hereafter). In this study, CT-CPB-mixtures were stored at ~80% humidity until the 31st day (concerning CE31 and CE-FA31) and subsequently at ¾60% humidity until the 446th day (concerning CE446 and CE-FA446). Until the 31th of curing, CT-CPB-mixtures were stored in a container (with a perforated lid) containing water that maintained the humidity at high levels. During day 31 to 446, the container storing the CT-CPB-mixtures was uncovered and drained. A curing period of 31 days was chosen based on the study of (Ercikdi et al., 2009) which suggested that the mechanical strength in CPBs with small proportions of binders develops along with the hydration of cement, which is nearly complete after approximately 30 days. Ouellet et al .,(2006), showed that unsaturated zones could appear on CPB surfaces during underground storage. The reduction of the humidity levels and increase in the curing period in CE446 and CE-FA446 were carried out to enhance the formation of unsaturated zones. The curing period was terminated on the 446th day when brighter areas on the CPB surfaces indicated the formation of unsaturated zones. Mixtures of CE31, CE446, CE-FA31 and CE-FA446 underwent a tank leaching test (TLT) to evaluate the effects of a slow recovery of natural groundwater levels on the mobility of Cu, Ni, Zn and As.

Methods

Most of the methods used in this study are standardized, well-established and are described in detail in Papers I–IV. However, evaluating the release of As, Cu, Ni and Zn from a granular material (CT and CE446) using a tank leaching test (TLT) is not a standardized procedure. CT and CE446 were placed in paper filter bags (1μm) recessed in nylon filters before the TLT was carried out (Papers II–IV). The filter bags were replaced upon each leachate renewal. A TLT using granular CT was carried out to evaluate the effects of cementation on the mobility of Cu, Ni, Zn and As and to examine their behavior under simulated flooded conditions. The sequential leaching test was modified to include 2 grams of material instead of 1 gram. The purposes of these methods are briefly described in Table 1. The weathering cell test (WCT) was carried out over a period of 32 cycles (217 days). During cycles 13–18, acid was added to consume the buffering minerals within the CT-CPB materials and stimulate the formation of acid mine drainage (AMD). This was done to assess element leaching in response to the dissolution of the binders (Table 1).

10 Table 1. Methods used Method Material Purpose Paper Mineralogical charac- X-ray Diffraction (XRD) Ore, CT I terization Scanning electron microscopy (SEM) com- Ore, CT, CE446 and Mineralogical charac- bined with energy dispersive spectroscopy I, IV CE-FA446 terization (SEM-EDS) Chemical composi- ICP-AES1, ICP-MS2 Ore, CT, CE and FA I – IV tion Particle size distribu- Laser diffraction analyses (CILAS) CT II tion Unconfined Compressive Strength (UCS)-test CE31 and CE-FA31 Mechanical strength II Acidifying and/or Acid Base-Accounting (ABA)- test CT neutralization poten- II tial Ore, CT, CE446 and Prediction of the mo- Sequential leaching test I – IV CE-FA446 bility of trace metals. Maximum element Batch leaching test (Static) CT leaching on a short- I term basis Determine the kinet- Weathering Cell Test (WCT) (Kinetic leach- CT, CE31 and CE- ic release of specific I – IV ing test) (acid addition, AMD-simulation) FA31 elements Leaching of elements pH-dependent leaching test CE446 and CE-FA446 III in response to pH Simulate element CT, CE31, CE-FA31 Tank leaching test (TLT) leaching during un- II – IV CE446 and CE-FA446 derground storage Estimate the extent Leachates from WCT of dissolution and PHREEQC I – IV and TLT precipitation of significant minerals. 1: Inductively coupled plasma-atomic emission spectroscopy; 2: Inductively coupled plasma-mass spectrometry

4. Results

Results from an Acid-Base-Accounting (ABA) test and analyses of the particle size distribution of CT previously conducted in Hamberg (2014) are shown in table 2. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) and inductively coupled plasma-mass spectrometry (ICP-MS) were carried out on ore, CT, ﬂy ash and cement. CT and ore had low contents of sulphide-associated elements such as Cu, Ni, Pb and Zn. The As-content in the tailings was considerably higher in ore compared to that in the CT (Table 2). The opposite concerned for Cu and Zn. According to previous analysis (personal communication, Lars-Åke Lindahl, Dragon Mining AB), the As-content in the ore is highly variable (100-10 000 mg/kg). Results from the ABA-test suggested that CT could be considered acid-generating (-60.8 kg CaCO3/t). Compared with CT,

11 cement and fly ash contained higher total concentrations of Ni and Zn. An addition of CE increased the fraction of Ni by 1.8 %, in CE-mixtures compared to that in CT. The corresponding increase in CE-FA was 21.7 (Zn) and 3.0 % (Ni). CT could be pumped while the particle size distribution showed that close to 20wt. % solids had a particle size less than 20 μm. The fly ash can be classified as a class C fly ash according to (ASTM C618-05) (Table 2).

Table 2. Results from ICP-AES-, ICP-MS-analyses of Ore, CT, Fly ash and Cement (n = 3, ± SD). Results from the ABA test, particle size distribution, (n = 3, ± SD) of CT

Unit Ore CT Fly ash Cement Total Solids (TS) % 99.97 ± 0.06 89.0 ± 0.4 95.2 ± 0.7 99.4 ± 0.0

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

Al2O3 “ “ 1.74 ± 0.07 4.69 ± 0.04 10.7 ± 0.6 5.61 ± 0.45 CaO “ “ 4.26 ± 0.09 4.83 ± 0.25 14.1 ± 1.0 50.3 ± 1.8

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

K2O “ “ 0.32 ± 0.00 0.92 ± 0.03 2.89 ± 0.08 0.83 ± 0.05 MgO “ “ 2.38 ± 0.07 3.24 ± 0.01 2.54 ± 0.08 4.00 ± 0.17 As mg/kg TS 4703 ± 781 1070 ± 30 124 ± 5 10.2 ± 0.2 Cu “ “ 82.0 ± 12.1 147 ± 7 136 ± 10 86.2 ± 2.9 Ni “ “ 126 ± 12 63.8 ± 2.1 114 ± 9 63.8 ± 1.3 S “ “ 25367 ± 2363 20933 ± 493 13700 ± 200 9960 ± 219 Zn “ “ 10.6 ± 1.5 25.0 ± 0.42 374 ± 10 149 ± 3 AP N.D 65.3 N.D N.D

NP kg CaCO3/t “ “ 4.48 ± 1.31 “ “ “ “ NNP “ “ “ “ -60.8 ± 1.32 “ “ “ “

D20 μm “ “ 20. 0 ± 0.3 “ “ “ “

D50 “ “ “ “ 91.0 ± 2.7 “ “ “ “

D90 “ “ “ “ 195 ± 8 “ “ “ “ N.D: Not determined

Mineralogical studies

According to the X-ray diffraction (XRD) analyses (Paper I), the tailings mineral composition in Svartliden consisted of quartz (80 wt. %), tremolite (<5 wt.% ) , albite

( <5 wt.%), microcline (<5 wt.%) and Jarosite (KFe3(SO4)2(OH)6) (<5 wt.%). Analyses of scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (SEM-EDS) revealed that pyrrhotite (< 1 wt %) and arsenopyrite (< 0.1 wt.%) were the main Fe- and As-sulﬁde minerals in the CT (Paper I). No Cu-, Ni- or Zn-carrying minerals were found (Paper IV). Rims of hydrous ferric oxides (HFO) that could sequester arsenates were detected on arsenopyrite grains (Paper I).

12 Sequential leaching test of Ore and CT

Sequential extractions were carried out on tailings and ore samples in triplicate (Paper I). The fractionation of Ca, Fe, As, S, Ni, Zn and Cu in CT was compared to that in the ore. The results are summarized as relative abundances of Ca, Fe, As, S, Ni, Zn and Cu in the fractions:

- Water-soluble fraction: Ni > S > Zn > Cu > Ca > Fe = As. In comparison to those in the ore, large proportions of Ni (80 wt.%), S (35), Zn (20), Cu (10) and Ca (8) were evident. No change in As (0 wt.%) and Fe (0 wt.%) fractionation.

- Acid exchangeable (AEC) fraction: S > Zn > Ni > Cu = Fe = As = Ca. In comparison to the ore, minor/no changes (< 5 wt.%) of Zn, Ni, Cu, As, Fe and Ca. A small change in S (10) was evident.

- Fe (III) oxyhydroxide fraction: As > Fe = Cu > Zn > Ni = S > Ca. In comparison to the ore, there were large proportions of As (95), Fe (30) and Cu (30), a small proportion of S (10) and no changes in Zn, Ni and Ca.

- Fe(III)oxide-fraction: Zn = S = Fe > As = Ni > Cu = Ni. In comparison to the ore, overall small changes in fractionation.

- Sulﬁde/residual fraction: Ca > Fe > Cu > Zn > S > Ni > As. In comparison to the ore, minor proportions of As (3), Ni (10), S (40), Zn (55), Cu (60), Fe (65) were evident. Proportions of Ca were higher in CT compared to that in ore (95).

The most significant difference in fractionation is the higher proportion of water-soluble S, Ni and Zn in the CT compared to that in ore. The sulfide/residual fractions of these elements in the ore have been redistributed to less recalcitrant phases. Major proportions of sulfide-associated As and Cu have been redistributed to the Fe(III) oxyhydroxide phases (Fig. 5).

13 Ore Cu Zn Ni S As Fe Ca

0 102030405060708090100 Extracted (%) Tailings Cu Zn Ni S As Fe Ca

0 102030405060708090100 Extracted (%) Water soluble Acid Exchangeable (AEC) Fe(III)-oxy-hydroxides Fe(III)-oxides Sulphides/Residual Fig. 5. Distributions of elements across different phases in tailings and unprocessed ore based on the results of sequential extraction tests (n =3, averaged results shown)

Sequential leaching test of CE446 and CE-FA446

Sequential extractions were conducted on CE446 and CE-FA446 samples in triplicate (Paper III and IV). Results are summarized as relative abundances of Ca, Fe, As, S, Ni, Zn and Cu in the fractions: - Water-soluble fraction: In CE446, S > Ca > Zn > Ni > As = Fe = Cu. In CE-FA446, S > Ca > Zn > Ni = As = Fe = Cu.

- Acid exchangeable (AEC) fraction: In CE446, Zn > Ni > S > Ni > As > Ca = Fe = Cu. In CE-FA446, Ca = Zn > Cu = Ni > S > As > Fe.

- Fe (III) oxyhydroxide fraction: In CE446: As > Ni = Zn > Cu > Fe > S > Ca. In CE- FA446, As > Ni = Zn > Cu > Fe > S > Ca.

- Fe(III)oxide-fraction: In CE446: S > Zn > As > Ni = Fe > Cu = Ca. In CE-FA446: Zn = S > Fe > As = Ni > Cu = Ca

- Sulﬁde/Residual fraction: In CE446: Ca > Fe > Cu > S > Ni > Zn > As. In CE-FA446: Fe > Ca > Cu = S > Ni > Zn > As

14 The proportions of Ca, Fe, As, S, Ni, Zn and Cu in CE446 and CE-FA446 were compared to those in CT (Table 3). Overall, the most signiﬁcant changes in the fractionation of the studied elements in the CT-CPB mixtures were the proportions of Ni, Cu and Zn associated with the water-soluble phase, being lower in the CT-CPBs compared to those in CT. These water-soluble elements were redistributed into less mobile phases, and most frequently to the AEC and Fe(III) oxyhydroxide phase. A small amount of As in CE- FA446 was redistributed from the Fe(III) oxyhydroxide phase to the AEC and Fe(III) oxide phase (Table 3).

Table 3. Changes in distribution of elements across different phases in crushed CT-CPB-mixtures (CE446 and CE-FA446) (CE-FA446-valuation within brackets) compared to those in CT based on the results of sequential extraction tests.

Element Water- Acid Exchangeable Fe(III) oxy Fe(III)- Sulﬁde/ soluble (AEC) hydroxides oxides Residual Ca + (-) sim (+ +) sim (sim) sim (sim) - (-) Fe sim (sim) sim (+) - (-) sim (sim) + (+) As sim (sim) sim (+) - (-) + (+) sim (sim) S sim (- -) - (+) sim (sim) sim (sim) + (+ +) Ni - - - (- - -) + (+ + +) + + (+ +) sim (sim) + + (+ +) Zn - - (- -) + + (+ + +) + + (+ +) sim (sim) - - (- -) Cu - (-) + (+ + +) - (-) sim (sim) + (sim) sim: similar; +: < 100 % increase and/or < 5 % in total; + +: 100-500 % increase; + + +: > 500% increase- : < 100 % decrease and/or < 5 % in total; - -: 100-500 % decrease; - - - : > 500% decrease

Weathering Cell Test (WCT)

The WCT on CT, CE31 and CE-FA31 exhibited good reproducibility with only small differences between runs; average values are therefore given. The cumulative release of Fe, As, Cu, Ni and Zn from CT was 1393, 1.5, 5.4, 23.3 and 8.7 mg/kg, respectively (Fig. 6). The endpoint pH for CT was approx. 3.5. The Cu and As releases evolved in a similar way in CT and increased most extensively towards the end of the WCT, as the Fe/S molar ratio increased to 5 0.8 and the pH decreased below 4. The release of Ni and Zn in CT were less pH-dependent. Ni release was most abundant during the ﬁrst few cycles but was almost zero thereafter whilst Zn release increased progressively throughout the WCT. In CT, the release of As, Cu, Fe and S increased alongside a pH decrease and a higher solubility of ferrihydrite. The release of Ni from CT showed water-soluble behavior while the Zn release increased gradually throughout the WCT (Fig. 6). Initially in the WCT, the release of S, Fe, Ni, Cu and Zn from CE31 and CE-FA31 showed a pH-dependent behaviour and increased as pH decreased steeply during the ﬁrst days of leaching. The As and Si release evolved in similar ways during this period. The cumulative release of Fe, Cu, Ni, S and Zn from the CT-CPB samples was lower compared to that from CT. Arsenic leaching from CE31 and CE-FA31 was more abundant compared

15 to that from CT in alkaline conditions. In alkaline conditions, the release of Ni, Zn, Si, Cu and Fe from the CT-CPBs was pH-dependent, but overall much lower compared to that from CT. The As release was more abundant than that from CT and increased gradually in alkaline conditions.

S As 2500 3500 2 3000 Acid addition 2000 2500 1.5 1500 2000 Acid addition 1 1500

mg/kg 1000 mg/kg 1000 CT(mg/kg) 0.5 500 500 0 0 0 0 20 40 60 80 100 120 140 160 180 200 220 0 20 40 60 80 100 120 140 160 180 200 220 Days Days Ni pH 2 25 12

20 10 1.5 Acid addition 8 Acid addition 15 1 6 10 4 0.5 CT(mg/kg) 5 2

0 0 0 0 20 40 60 80 100 120 140 160 180 200 220 0 20 40 60 80 100 120 140 160 180 200 220 Days Days Zn Si 4 10 40 Acid addition Acid addition 8 3 30 6 2 20

mg/kg mg/kg 4 mg/kg

1 CT(mg/kg) 10 2

0 0 0 0 20 40 60 80 100 120 140 160 180 200 220 0 20 40 60 80 100 120 140 160 180 200 220 Days Days Cu Fe 0.8 6 80 1500

5 1200 0.6 60 4 Acid addition Acid addition 900 0.4 3 40

mg/kg mg/kg 600 2 0.2 CT(mg/kg) 20 CT(mg/kg) 1 300

0 0 0 0 0 20 40 60 80 100 120 140 160 180 200 220 0 20 40 60 80 100 120 140 160 180 200 220 DaysCE31 CE-FA31 CT Days Fig. 6. Evolution of pH and the cumulative release of Cu, Ni, Zn, Fe, S, Si and As from CE31, CE-FA31 and CT over time during the WCT (Average values shown, n = 2). Acid added on day 70-147.

Acid was added on days 70–147 to consume the buffering minerals in the crushed CT- CPB mixtures. Different amounts of acid were added to each CPB material (CE: 0.47 M H+/kg TS and CE-FA: 0.69 M H+/kg TS). The addition of acid lowered the pH to 4.5 in leachates from CE31 and CE-FA31. At this stage, leaching of Cu, As, Fe, Si and Zn from CE31 increased in conjunction with the Fe/S-molar ratio, the solubility of Ferrihydrite and a pH decrease (Figs. 6 and 7).

16 6 5 4 Acid addition 3 2 1 0 -1 -2 Ferrihydrite (SI) -3 -4 -5 0 20 40 60 80 100 120 140 160 180 200 220 CT CE-FA31 CE31 Days 0.9 0.8 Acid addition 0.7 0.6 0.5 0.4 0.3 0.2 Molar ratio Fe/S 0.1 0 0 20 40 60 80 100 120 140 160 180 200 220 Days

Fig. 7. Evolution of Saturation Index (SI) of ferrihydrite and molar ratios of Fe/S in CE31, CE- FA31and CT over time during the WCT (Average values shown, n = 2). Acid added on day 70- 147. No acid was added to CT.

In CE-FA31, this pattern was only evident for the release of Zn and Si, as it was for the Ni release in CE31. The S-release in the CT-CPB:s was unaffected by the acid addition. During the acid addition, the Cu release was more extensive from CE-FA31 compared to that CE31, while the opposite was true for Fe. Overall, following the acid addition, the leaching of Cu, Ni, S, Si and Zn were lower in the CT-CPB:s compared to that in CT. This was especially evident for Ni that leached more than a 10-fold less from CE-FA31, compared to CT. Arsenic leaching from CE31 was more abundant compared to that in CT, during acidic conditions. The opposite was true for the As release from CE-FA31 (Fig. 6). During the acid addition, the release of As, Cu, Ni, Si, Fe and Zn increased while the S release was not signiﬁcantly affected (Fig. 6). pH-dependent leaching test

The pH-dependent leaching test of CE446 and CE-FA446 exhibited good reproducibility with only small differences between runs; average values are therefore given. The release of Ni and Zn increased in conjunction with a decrease in pH. Cu release reduced as pH decreased from 8 to 6, especially from CE446, as did the release of As from CE- FA446 (Fig. 8). The release of Ni and Zn increased along with a decrease in pH. This was not evident for the release of As and Cu, which was greater in semi-alkaline conditions compared to a pH of 6 (Fig. 8).

17 Ni Zn 1E-5 1E-5

1E-6 1E-6 1E-7 1E-7 1E-8 1E-8 1E-9

1E-9 1E-10 2345678 2345678 pH pH As Cu 1E-6 1E-5 CE-FA446 CE 446 CE-FA446 CE 446 1E-6 1E-7

1E-7 Mole Mole 1E-8 1E-8

1E-9 1E-9 2345678 2345678 pH pH Fig. 8. Release of Ni, Zn, As and Cu (in Mole) during pH-dependent leaching of CE-FA446 and CE446. Averaged values shown, n= 2.

Tank leaching test

The As release was greatest from CE31 and increased along with Si due to diffusion- like behavior throughout the TLT. In CE-FA31 and CE-FA446, the As release was less but evolved in a similar way. The As release in CE446 and CT was lower compared to that from the other CT-CPBs and occurred through a surface wash-off effect. The initial release of As from CT and CE446 occurred along with the dissolution of ferrihydrite. Towards the end of the TLT, As release was almost zero in CT, but increased slightly in CE446. As release increased along with Si as pH increased in CE446 leachates towards the end of the TLT (Fig. 9). Molar ratios of Fe/S were higher in CT compared to those in CE-FA31, CE31, CE-FA446 and CE446 (Table 4). In CT, the molar ratio of Fe/S was highest initially during the TLT, reaching a value of ~0.1.

Table 4: Molar ratios of Fe/S and cumulative amounts of As, Cu, Ni and Zn (in mg/m2) released from CT, CE31, CE-FA31, 46 and CE-FA446 through the dominant leaching mechanism during TLT (Averaged values shown, n=2). Mixture Cu Ni Zn As Molar ratio Fe/S CT 8.0 DF 162 SW 199 SW 5.5 SW 0.01 – 0.1 CE 446 35 SW 8.6 SW 8.9 SW 12 SW 0.002 – 0.03 CE 31 2.7 DF 3.3 DF 31 DF 98 DF 0.001 – 0.003 CE-FA 446 7.7 DF 1.0 DF 124DS 62 DF 9E-5 – 0.003 CE-FA 31 2.5 DF 1.5 DF 68 DF 53 DF 5E-5 – 0.0001 SW: Surface wash-off, DF: Diffusion, DS: Dissolution

18 Cu release from CT occurred initially through a surface wash-off effect where surface- attached Cu was rinsed off the surface. The Cu release decreased steeply during the ﬁrst days of extraction. However, it increased towards the end of the TLT in CT leachates, from which the dissolution of ferrihydrite increased as pH dropped below 4. The Cu release from CE31, CE-FA31 and CE-FA446, was stable throughout the TLT, not pH-dependent and lower compared to that from CT. The Cu release from CE446 was signiﬁcantly higher compared to that from CT. The Cu release from CE446 was greatest initially during the TLT, and occurred along with the dissolution of ferrihydrite, when pH was ~4.5 (Fig. 9).

CT CT 100 10000 2 7 1 0 10 1000 ) 6 2 - 2 1 -2 1 100 5 -3 pH mg/m -4 0.1 10 Si (mg/m -5 4 -

Ferrihydrite (SI) 6 0.01 1 -7 3 0102030405060 0102030405060 Days Days As Cu Ni Zn Si Ferrihydrite pH CE-FA31 CE-FA31 100 300 4 11 250

) 3 10 10 200 2 2 150 2 9 pH

mg/m 1 100

Si (mg/m 1 8 50 Ferrihydrite (SI) 0.1 0 0 7 0102030405060 0 102030405060 Days Days As Cu Ni Zn Si Ferrihydrite pH CE-FA446 CE-FA446- 100 600 3 9 500

) 2

10 2 8

2 400 1 1 300 7 0 pH mg/m 200 0.1 Si (mg/m 6 100 -1 Ferrihydrite (SI) 0.01 0 -2 5 0102030405060 0 102030405060 Days Days As Cu Ni Zn Si Ferrihydrite pH CE31 CE31 100 400 4 11 350 )

10 300 2 3 10

2 250 1 200 2 9 pH

mg/m 150

0.1 100 Si (mg/m 1 8

50 Ferrihydrite (SI) 0.01 0 0 7 0102030405060 0 102030405060 Days Days As Cu Ni Zn Si Ferrihydrite pH CE446 CE446 4 100 900 7 800 3

700 ) 2 10 600 2 1

2 6 500 0 400 - 1 pH

mg/m 1 300 - 2 5 Si (mg/m 200 - 3 - 100 Ferrihydrite (SI) 4 0.1 0 - 5 4 0102030405060 0 102030405060 Days Days As Cu Ni Zn Si Ferrihydrite pH Fig. 9. Evolution of pH, Saturation Index (SI) of Ferrihydrite and release of As, Cu, Ni, Zn, and Si (in mg/m2) during the TLT. Averaged values shown, n= 2.

19 From CT, the Ni release occurred solely as a result of a surface wash-off effect, with the majority of Ni being rinsed off the surface of CT. The Ni release decreased steeply during the ﬁrst days of extraction. From CE-FA31, CE31 and CE446, Ni-release was stable throughout the TLT, not pH-dependent and signiﬁcantly lower compared to that from CT. The cumulative Zn release was higher from CT compared to that from the CT- CPB mixtures. The Zn release was more extensive from the CE-FA mixtures compared to that from the CE mixtures (Table 4). The release of Zn from CE-FA446 increased steeply between days 16 and 32, along with Si but decreased during the last extraction (Fig. 9). The release of Ni and Cu evolved in a similar way in the CT-CPB mixtures (Fig. 9). The amounts of Ni and Cu released from CE-FA446 and CE-FA31 were similar, while there was a small increase in the Ni released from CE446 compared to that from CE31. In CE446 and CT, the release of Ni, Cu, and As was greatest initially at pH < 4.5, with a negative SI of Ferrihydrite suggesting dissolution. This pattern was not seen for CE- FA446, where the release of Ni, Cu and As increased initially and then stabilized. The Zn release from CE-FA31, CE31 and CE446 showed a similar pattern, that is, it increased initially and then stabilized. Again, this pattern was not seen in CE-FA446, where the Zn release increased steeply as pH exceeded 8 (Fig. 9).

5. Discussion

The cyanide leaching process and the associated water treatment - effects on the mobility of As, Cu, Ni and Zn in CT

According to Sciuba (2013), the major sulﬁde minerals in Svartliden consists of pyrrhotite

(Fe7S8), arsenopyrite (FeAsS), traces of chalcopyrite (CuFeS2), pyrite (FeS2), and sphalerite (ZnS). In the detox-, pre-oxidation- and cyanide leaching process these sulfides are likely to have been oxidized, releasing various amounts of As, Cu, Ni and Zn into solution. Whether the stability of As, Cu, Ni and Zn have increased or not after the cyanidation process, and the associated water treatment process, is depending on the environment at the final storage. If a neutral pH and oxidized conditions could be maintained in the tailings storage; As-, Cu-, Ni- and Zn-bearing Ca (and/or Fe) precipitates are stabilized more efficiently compared to the sulphide species. If reducing conditions prevails, the opposite is true.

For As-removal, and Fe-arsenate-precipitation, Fe2(SO4)3 was added to the slurry before discharge onto the tailings dam. A highly variable As-content in the ore suggest that As- species with differed molar Fe/As-ratios could have formed during the water treatment process. Results from the sequential leaching tests suggested that a majority of the As- sulphides in the ore has been oxidized during the cyanide leaching process. Dissolved As in cyanide leaching process was then resented as Arseniosiderite or in As-bearing Fe- precipitates in the water treatment process. These phases could be associated with the Fe(III)-oxy-hydroxide-fraction (Parviainen et al., 2012). In Paper I, arsenates within HFO- rims onto arsenopyrite grains and was found in the tailings. The solubility of these phases increases at acidic, oxidized conditions. In reducing conditions, As in arsenopyrite is largely immobilized while the opposite prevails for As in As-bearing Fe-precipitates. However, a

20 majority (95 wt. %) of the total As in CT was associated with the Fe(III)-oxy-hydroxide- fraction (Fig. 5). So, the stability of As in tailings could therefore be controlled by the solubility of As-bearing Fe-precipitates and arseniosiderite. This suggests that reducing, acidic conditions should be avoided in the further management of CT, otherwise, large amounts of As could be released Results from sequential leaching tests suggested that a large proportion of Cu associated with the sulfide/residual fraction in the ore has been redirected to the water soluble- and Fe(III)-oxy-hydroxide-fraction. However, the water-soluble fraction of Cu could also originate from the addition of CuSO4, in the cyanide detoxification process (reactions 5 and 6). Similar to As, Cu-sulphides in the ore have probably been oxidized during the cyanide leaching process and dissolved Cu was incorporated in newly-formed Fe precipitates in the tailings (Paper IV). This is likely since it has been known for a long time that Cu2+ bonds strongly to iron (hydr)oxides, such as ferrihydrite, at pH 4–8 (Benjamin and Leckie, 1981; Karapinar, 2016). Under conditions prevailing in the cyanide leaching process and the associated water treatment, Ni and Zn can also be incorporated into the structure of Fe(III) oxyhydroxide minerals (Gunsinger et al., 2006). Of Cu, Zn and Ni, Cu is most extensively removed from solution by adsorption onto ferrihydrite under pH-neutral, oxidizing conditions (Karapinar, 2016; Smith and Macalady, 1991). In CT, this would explain why a larger proportion of Cu, compared to Zn and Ni, is associated with the Fe(III) oxyhydroxide fraction (Fig. 5). The mobility of Cu has increased because of the cyanidation process and the associated water treatment process, in tandem with the proportion of Cu associated with the water-soluble phase. Maintaining neutral, oxidized conditions, in the further management of CT is therefore of major importance considering the immobility of Cu. In these conditions, the oxidation rate of chalcopyrite and solubility of Cu-bearing Fe-precipitates is low (Kimball et al., 2010). The proportion of Ni and Zn associated with the water-soluble fraction was ~80 and ~20 wt.%, respectively (Fig. 5) and probably originated from the oxidation of sulfides during the cyanide leaching process (Paper IV). At best, the water-soluble fraction of these elements is small, otherwise large amounts of Cu, Ni and Zn would be released. In the water treatment process, there must be sufficient amounts of Fe precipitates for the adsorption of Ni, Cu and Zn (Lee et al., 2002). While Cu has a higher affinity for Fe- precipitate-surfaces, a large proportion of Ni and Zn could have remained in solution or precipitated as hydroxides (when lime (Ca(OH)2) is added) during the water treatment phase (reaction 7). These species are probably represented by Ni and Zn hydroxides associated with the water-soluble phase and are soluble when pH<7 (Karapinar, 2016). As for Cu, the mobility of Ni and Zn increased because of the cyanidation process and the associated water treatment process. Large proportions of water soluble species must be detained in neutral, oxidized conditions, in the further management of CT.

21 Managing CT by the use of a CPB-method with low proportions of binders-effects on the leaching behavior of Cu, As, Ni and Zn

Leaching behavior - Flooded conditions

In CT, the Zn, Ni and Cu release was mostly due to water-soluble species that were rinsed off the surface in a wash-off process. The mobility of Cu, Zn and Ni is strongly related to pH and decreases in alkaline conditions, but could increase in extreme alkaline conditions due to the formation of negatively charged complexes that have a low affinity for Fe (oxy)hydroxides (Kumpiene et al., 2008). In CT, the pH of the leachates was in the range 3.5–5, and the release of Cu was less than that of Ni and Zn because Cu binds more strongly to Fe precipitates, compared to Ni and Zn. Low (~0.1, Table 4) Fe/S ratios in the CT leachates could be an indicator of a low Cu and As sulfide oxidation rate, which prevents the oxidation-produced As and Cu sulfides becoming the main contributors for Cu and As release. Instead, modelling has suggested that the majority of the As release could be due to leaching of unloaded As(III) species (Table 5) that are less attached to mineral surfaces (Jain et al., 1999). The unloaded arsenite (As(III)) species form during arsenopyrite oxidation in reaction (9) and may be oxidized over days or months to As(V) (Paper I). Ferrihydrite (a Cu and As bearing Fe (oxy)hydroxide) in CT dissolved throughout the whole TLT, but Cu and As release was still low, even though pH decreased below 3.5 at the end of the TLT (Fig. 9). Diffusion in CT could have been hindered by the appearance of a Fe crust. Such iron crusts are common in mine waste dumps cementing the pores and reducing metal release (Valente et al., 2012). This suggested that sulfide oxidation is not to be considered as a source for the As, Cu, Ni and Zn release. Instead, a release of Cu, Ni, and Zn in CT was mainly governed by water soluble phases while an As-release was less abundant and governed by As(III)-species. Maintaining high water saturation levels is of major importance to reduce the sulfide oxidation rate within CT-CPB-mixtures. But the evolution of pH and Si in the leachates from CE31 and CE-FA31, suggested that a sulfide oxidation has occurred to some extent, dissolving the cementitious phases. Trace metal immobilization in CPB has been attributed to a combination of physical encapsulation and chemical stabilization (Chen et al., 2009; Paria and Yuet, 2006; Cornelis et al., 2008). In that way, there is a apperent connection between the stability of C-S-H and the release of metals. In a CPB, porosity decreases by the formation of gypsum and a hydration of cementitious phases filling the pores (Belem et al., 2001). This in turn, reduces permeability and could lead to the formation of a diffusion barrier. If the CPB remains intact, maintaining high water saturation levels, a metal release would then be controlled by a wash-off effect whereas metals are rinsed from the surface (Bowell, 2014) followed by a diminished metal-release. Otherwise, metal release could be governed by diffusion and dissolution processes, increase with time or even continue until depletion. In CE31 and CE-FA31, a C-S-H formation (reactions 13 and 14) increased the pore water pH to alkaline levels where water-soluble species of Cu and Ni to precipitated as hydroxides, and/or adsorbed onto Fe precipitates and C-S-H surfaces (Li et al., 2001). These phases were stable throughout the TLT when very small amounts of Ni and Cu were released. In CE31 and CE-FA31, the release of Cu and Ni evolved in a similar way and was largely immobilized compared to CT (Fig. 9).

22 A Zn release from CE31 and CE-FA31 was greatest when pH<10 and about ten-fold higher compared to the release of Ni and Cu (but still lower than from CT). During the TLT, the pH in leachates from CE31 and CE-FA31 was 8–11, and Zn2+ and HZnO2- were the dominant species, as predicted by modeling (Table 5). A Zn release from CE31 and CE-FA31 could therefore have an amphoteric leaching pattern (Kumpiene et al., 2008), governed by dissolution of Zn hydroxides and/or desorption of HZnO2-. A release of Zn could also originate from the binders themselves, as they are more abundant in Zn compared to CT (Table 2). According to Lou et al., (2011) the Zn release from a class C ﬂy ash (as in CE-FA31) could be governed by the dissolution of calcium aluminosilicate glass that is more soluble under alkaline conditions than acidic. In that case, desorption is 2− not expected to govern the release of Zn from CE-FA31. In cement, Zincate (Zn(OH)4 ) ions are the dominant Zn species, according to Tsuyumoto and Uchikawa (2004). These ions could be incorporated in the C-S-H-structure, and released due to desorption and the dissolution of the cementitious phases. Another reason for the evolution of the release of Zn could be water migration through the CT-CPB mixtures. At the beginning of the TLT, a Zn release from CE-FA31 and CE31 occurred due to a wash-off effect but, as a reaction rind formed, water migration was obstructed and Zn release decreased. This reaction rind within the CE31 and CE-FA31 probably did not hinder water migration through the mixtures efﬁciently, as indicated by an increasing Si release towards the end of the TLT. This caused the Zn release to remain the same or even increase towards the end of the TLT (as in CE31) (Fig. 7). The Zn release from CE-FA31 was more than ten-fold higher than from CE31 (Table 5), but still lower compared to CT. So, in CE31 and CE- FA31, the Zn release could be governed by desorption of negatively charged Zn ions and an abundance of calcium aluminosilicate glass, respectively. The release of As was up to 18 times higher (Fig. 9) from CE31 and CE-FA31 compared to CT and greatest as the pH decreased from 10 to 8. The oxidation rate of arsenopyrite is higher in alkaline conditions than in neutral ones (Yu et al., 2007) but, as Benzaazoua et al. (2004) stipulated, arsenopyrite quickly becomes passivated in a cementitious environment. Instead, alkaline conditions in CT-CPB leachates caused desorption of As(V) from the As-bearing Fe precipitates, while adsorption of As(V) is largely ineffective at pH~10 (Roddick-Lanzilotta et al., 2002). As(V) released in alkaline conditions could then adsorb onto C-S-H-surfaces or form Ca arsenates, as previously suggested by Cornelis et al. (2008) and Benzaazoua et al. (2004). However, these phases are sensitive to a drop in pH, when the surfaces of C-S-H become negatively charged (Cornelis et al., 2008) and Ca arsenates dissolve readily as the pH drops below 10 (Benzaazoua et al., 2004). From the evolution of As release from the CT-CPB, it seems that these phases could govern the mobility of As. In the CT-CPB mixtures, almost twice as much As was released from CE31 compared to CE-FA31. These As-phases are probably associated with the AEC-fraction (Pantuzzo and Ciminelli, 2010). Coussy et al., (2012) suggested a relation Si-As-release from CPB-mixtures, meaning that As are released from being trapped in the cementitious matrix. This is also seen in Paper (II) whereas the evolution of As- and Si-release was similar from CE-FA31 and CE31 during the TLT. Fly ash-based (Class C) CPB is not suitable for the stabilization of As, but increasing amounts of lime could have a stabilizing effect on As in CPB mixtures. Another reason for this to happen could be that, in cement- stabilized materials, leachability is reduced along with permeability and porosity, both

23 of which are connected with mechanical strength (Belie et al., 1996). It is well known that ﬂy ash increases the sulfate resistance and hence the strength-durability of a CPB (Chindaprasirt et al., 2004). The CE mixtures only contain low (1 wt.%) proportions of cement, and therefore have a small sulfate resistance compared to CE-FA mixtures. The mechanical strength can be reduced at a higher grade in CE31, and hence, the As release can be greater. Results from the TLT suggested that the function of the CT-CPBs was not satisfactory when the As release was governed by diffusion and seemed to increase, even after 64 days (Fig. 9 and Table 4). The As release from CE31 and CE-FA31 increased, compared to that from CT, due to the dissolution of less acid-tolerant As species and instability of the cementitious matrix.

Table 5. Dominating species of As, Cu, Ni, Zn and Fe in leachates from CT, CE31, CE446, CE-FA31 and CE-FA446 during the TLT. As Cu Ni Zn Fe CT u, u, -, -, -, -, -, - + overall + overall + overall + overall CE31 - overall -, -, +, -, -, +, +, + + overall +,+, -, +, -, +, +, + - overall CE446 u, u, -, -, -, -, -, - +overall + overall + overall + overall CE-FA31 - overall -, -, -, -, -, -, +, + + overall +, -, -, -, -, -, +, + - overall CE-FA446 - overall + overall + overall + overall +,+,+,+,-,- ,+,+ -: anionic species; +: cationic species, u: unloaded As(III) species, overall: during all extractions

In CE31 and CE-FA31, the release of As and Zn were governed by a diffusion-like behavior. High saturation levels within the CT-CPBs did not block pyrrhotite oxidation to occur and this caused a strength loss as the C-S-H-phases dissolved. As the cementitious phases dissolves during the TLT, As (V)-species, FA-related Zn, Zn-hydroxides surface- attached or encapsulated onto C-S-H could migrate into solution (Malviya and Chaudhary, 2006). Leaching behaviour – reactivity of CT and CT-CPB in oxidized conditions

During the WCT, wetting and drying cycles accelerated the sulﬁde oxidation rate in the CT. A Ni-release in the WCT exhibited a typical water-soluble-behaviour while most abundant initially and thereafter steeply decreased and diminished towards the end (Fig. 6). Towards the end of the WCT, the molar ratios of Fe/S in CT leachates increased to ~0.8, indicative of pyrrhotite oxidation occurring at a higher rate (Fig. 7). Small amounts of As were released from CT in the WCT as the pH was higher than 4. Arseniosiderite and As-bearing Fe precipitates are stable in gypsum-saturated solutions at a pH of 3.5–7.5 (Paktunc et al., 2015). Therefore, these phases and remnants of arsenopyrite are probably the main As-controlling media in the CT. At a pH less than 4, the oxidation rate of arsenopyrite and chalcopyrite increases while both the adsorption capacity and the stability of ferrihydrite decrease (Dzombak and Morel, 1990; Riveros et al., 2001). This is also evident at the end of the WCT where the release of Cu and As increased along with the solubility of ferrihydrite, while the pH decreased to < 4, due to more extensive pyrrhotite

24 oxidation (Fig. 6), as suggested in Papers I and IV. At this stage, Zn release in the WCT was not related to pH, Fe release or the dissolution of ferrihydrite (Fig.5). Instead, Zn release probably originated either from the oxidation of Fe-poor sphalerite, which liberate small amounts of Fe over Zn, or Zn impurities in pyrrhotite (Moncur et al., 2005, Janzen et al., 2001). From CT, the Ni release was largely governed by water-soluble phases. With respect to Zn, As and Cu, acidic conditions can liberate large proportions of these elements due to desorption from Fe precipitates and oxidation of Zn, As, and Cu-bearing sulﬁdes. The WCT was carried out using crushed CT-CPB-mixtures in an oxidized environment. Therefore, cementitious phases dissolved at a higher rate (a higher Si-release) in the WCT compared to that in TLT (Fig. 10), followed by a rapid decrease in pH (pH of 11 or 10 to a pH of 8). In the CT-CPB-mixtures, the Ni-, Cu- and Zn-release were substantially lowered compared to that in CT. The opposite was true for the As-release, which was greater (2-4 times) compared to that from CT (Fig. 7). Initially, the release of As, Cu, Ni and Zn from CE31 and CE-FA31 increased alongside with a pH-decrease from alkaline conditions to semi-neutral. At this stage, a lower As-release at semi-neutral conditions could be due to a less comprehensive desorption from Fe-precipitates (Roddick-Lanzilotta et al., 2002). In the CT-CPB-leachates from the TLT, pH could be maintained at alkaline levels for a prolonged period compared to that in WCT due to a lower reactivity at water saturated conditions. This probably caused more As, Cu, Ni and Zn to desorb from Fe precipitates in the CT-CPB:s during the TLT compared to that in WCT (Fig. 10).

100

TLT 10 Weathering Accelerated Weathering

0.1

0.01

Ratio elements CT-CPB/CT released elements Ratio 0.001 CE CE-FA CE CE-FA CE CE-FA CE CE-FA CE CE-FA Si As Cu Ni Zn

Fig. 10. The release of Si, As, Cu, Ni and Zn in CT-CPB-mixtures during TLT, WCT and Accel- erated weathering (acid addition) compared to CT.

As acid was added to stimulate dissolution of the buffering capacity in CT-CPB mixtures, the release of As, Ni, Cu and Zn increased in conjunction with Fe, and was more abundant from CE31 compared to CE-FA31 (Fig. 6). Large proportions of these elements were associated with amorphous Fe precipitates (Fig.7) phases that are more soluble in acidic conditions (Dzombak and Morel, 1990; Riveros et al., 2001). This might explain a greater release of Ni, Zn, As and Cu from CE31 compared to CE-FA31, while the pH was slightly lower in CE31 leachates, leading to greater leaching of Fe (Fig. 6). In the CT-CPB

25 mixtures, Cu release seemed less affected by accelerated weathering, compared to Ni and Zn. This might be explained by Cu being readily adsorbed onto Fe precipitates at a pH of 4, whereas other metals (i.e. Ni and Zn) remained in solution (Dzombak and Morel, 1990). In CE-FA, less Zn was released under acidic conditions compared to alkaline ones (Fig. 10). This might due to the presence of Zn-bearing calcium aluminosilicate glass in FA, which is more soluble in alkaline conditions (Luo et al., 2011).

How does the establishment of unsaturated zones within CT-CPB- mixtures affect the leaching behavior of As, Cu, Ni and Zn?

An establishment of unsaturated zones within the CT-CPB mixtures promoted sulﬁde oxidation and hence, acidic conditions in the TLT leachates. This facilitated oxygen migration through the CT-CPB-mixtures, while the solubility of oxygen is hampered in water (Yanful, 1993). In the outer areas of the CT-CPB mixtures, unsaturated zones occurred and these can have a lower permeability than the bulk CPB (Ouellet et al., 2006). In CE446 and CE-FA446, this will initially promote wash-off behaviour where water rinses highly soluble oxidation products from the surface, causing acidic conditions in the leachate (Fig. 9). In leachates from CE446 and CE-FA446, the pH increased throughout the TLT. As the oxidation products were rinsed off from the surfaces of CE446 and CE-FA446, permeability increased, and water gradually penetrated deeper into the unoxidized CPB mass, dissolving the cementitious phases and causing a pH increase. This scenario has a negative effect on the strength within the CT-CPB mixtures, something demonstrated by CE446 which fell apart into a granular material during the test period. As the cementitious phases dissolve, gypsum becomes more abundant in the CT-CPBs, suggesting a loss of strength. Low strength within CE446 is also indicated by a larger amount of gypsum, suggested by the higher fraction of water-soluble Ca and S (Dold, 2003) compared to that of CE-FA446 (Table 3). The evolution of pH in the CT-CPB leachates during TLT suggested that a loss of strength has occurred, as the cementitious phases are highly soluble at pHs lower than 9 (Benzaazoua et al., 1999). Belie et al. (1996) suggested that a loss of strength within a cement-stabilized material could cause the leachability of metals to increase. In this study, this was most signiﬁcant for the Zn release from CE-FA446 and the Cu release from CE446. A majority of the Cu increase occurred initially in the TLT, when the pH was <4, and as Cu-bearing Fe precipitates dissolved (Fig. 7). Cu release was greater from CE446 than from CT, even though the proportion of Cu associated with the water-soluble fraction was less for CE446 (Table 3 and Fig. 9). This might be because Cu has a sorption edge onto Ferrihydrite at a lower pH compared to that of Ni and Zn (Munk et al., 2002). The fraction of Cu released under neutral conditions could therefore be higher. This is also shown in the pH-dependent test, where a higher fraction of Cu compared to Zn and Ni was released under neutral conditions (Fig. 8). As pH is lowered, this fraction will re-adsorb onto Fe-precipitates. These newly-formed Cu-Fe precipitates are, however, less stable in acidic conditions than those originally formed (Munk et al., 2002). This, in turn, probably resulted in a more abundant release of Fe-associated Cu from CE446 during the TLT than from CT. In the CT-CPB mixtures, a large proportion of Cu is associated with the AEC fraction. Based on these results, Cu release seemed to be pH-dependent as more

26 Cu was released as the pH decreased. Therefore, a key issue in reducing the release of Cu is maintaining the strength within the CT-CPB mixtures. Otherwise, water migration increases along with pyrrhotite oxidation, promoting more acidic conditions. The establishment of unsaturated conditions in CE-FA446 increased Zn leaching but, in CE446, the effect was the opposite. The amount of Zn released from CE-FA446 was twice that from CE-FA31, but more than 10-fold higher compared to CE446 (Fig. 9). The majority of the Zn release from CE-FA446 occurred during the ﬁnal extractions of the TLT (Fig. 9), and was greatest as pH increased above 8. The evolution of pH and Zn could be explained with the variance of Zn species, and the water migration in the CT- CPB mixtures. Water migration is largely governed by the water saturation level within the CT-CPBs. At high water saturation levels, oxygen and water migration through the CT-CPBs is largely obstructed. Increasing the binder proportion by adding FA (such as in CE-FA mixtures) may thicken the microstructure and thus decrease the oxygen and water migration through the CT-CPBs (Aldhafeeri and Fall, 2017). The extent of pyrrhotite oxidation was therefore lower in CE-FA mixtures, causing the start and end-points of pH to be higher (6-8) in CE-FA446 compared to CE446 (4-6). The evolution of Zn release is somewhat similar for all the CT-CPBs, being greatest at the highest pH. However, less Zn was released from CE-FA31 compared to CE-FA446, despite the higher pH of CE-FA31 leachates (Fig. 9). This might be due to increased water transport through the unsaturated CE-FA446 mixture. At this stage, Zn release is not only due to desorption from Fe precipitate surfaces, but also to the dissolution of a Zn-bearing calcium aluminosilicate glass in FA, which is more soluble in alkaline conditions than acidic ones (Luo et al., 2011). This could have caused lower Zn release from CE446 compared to CE-FA446. Ni release was low, irrespective of the water saturation level in the CT-CPBs. As the water saturation levels decreased within the CT-CPB mixtures, pyrrhotite oxidation could proceed more extensively, and this seems to have increased As release from CE-FA mixtures while the opposite was true for CE mixtures. An As-Si relationship still exists for CE-FA446 as shown in Paper III, while in CE446, the As release seems to have been governed mostly by the occurrence of As(III) species that are less well attached to oxide surfaces (Table 5). This could mean that a larger proportion of cementitious As phases still persisted in CE-FA446, while more acidic conditions, as in CE446, dissolved these phases to a greater extent. The As release is less from CE446, regardless of the fact that more Si is released, compared to that from CE-FA446. This could be explained by the binding process of As(V) onto positively charged C-S-H surfaces. As C-S-Hs dissolve buffering acidity from pyrrhotite oxidation, Ca is released before Si (Coussy et al., 2012). The release of positively charged Ca2+-ions causes the C-S-Hs to have a net negative surface charge, while negative SiO moieties dominate (Phenrat et al., 2005). At this stage, negatively-loaded As species, that dominate in the leachates of CE446 and CE-FA446 (Table 3), will desorb from the C-S-Hs. In a pH of 4–7, As anions can then quickly re- adsorb onto Fe-precipitate surfaces. With pH above 7, the ability for Fe –precipitates to adsorb decreases (Roddick-Lanzilotta et al., 2002), and more As is maintained in solution. However, as stated in Paper II, upon re-adsorption of As, a release of Ca2+ (due to binder dissolution) may function as a multi-absorbent for As, increasing the adsorption ability for Fe (oxy)hydroxides, especially at pH values of 4–9 (Jia and Demopoulos, 2005; Wilkie and Hering, 1996). Considering the stability of As, it seems that As is stable under acidic

27 conditions (as in CT), releasing small amounts of As into solution. Adding binders to the CT is therefore unnecessary to maintain the stability of As, at least as long as a cementing oxide surface within the CT is present.

Overall – could a low strength CPB (0.2 Mpa) be an environmentally accepted option for managing tailings from a cyanide leaching process?

As the water saturation levels decreased within the CT-CPB mixtures, sulﬁde oxidation progressed more extensively and the release of Cu, Ni, and Zn increased from all of the CT-CPB mixtures. However, the Zn and Ni release was still lower compared to CT. In CE mixtures, as water saturation levels decreased, Cu release increased and became greater compared to CT, probably due to the formation of a substantial proportion of acid-intolerant phases that are susceptible to remobilization in acidic conditions. Arsenic leaching increased compared to that from CT, regardless of binder proportion and water saturation level. In water-saturated, alkaline conditions, pyrrhotite oxidation is retarded. The pH is maintained at alkaline levels for prolonged periods, as As has a higher mobility. This causes more As to be released from the CT-CPBs. The results from this study indicated that using a low strength CPB is not an environmentally-friendly option for underground storage of CT. If used, larger amounts of Cu and As compared to that from unmodiﬁed tailings (CT) might be released.

6. Conclusions In this study, CT used in the CPBs originates from an oxidizing cyanide leaching process used to extract gold from inclusions in arsenopyrite and loellingite. In a subsequent water treatment process, Fe-oxides were added to form metal(oid)-bearing Fe-precipitates that could ensure the immobility of Cu, Ni, Zn and As. In the ore, pyrrhotite and arsenopyrite were the main sulfide minerals, but remnants of Cu- and Zn-sulfides were also present. A significant proportion of sulfides oxidized during cyanidation, an addition of Fe-oxides seemed to have been insufficient to adsorb all the dissolved Cu, Zn and Ni. A significant proportion of Cu, Zn and Ni in the tailings were, therefore, water-soluble and could dissolve in mildly acidic conditions. Pyrrhotite and arsenopyrite still occurred in the tailings to a small extent, governing the acid generation and some of the As release. Overall, the cyanide leaching process increased the mobility of Cu, Ni and Zn due to the formation of water soluble species. A major proportion of As were associated with amorphous Fe-precipitates. Consequently, the management of CT should focus on avoiding the establishment of, acidic and reducing conditions. Managing the CT by using a laboratory-based low strength CT-CPB-application decreased leaching of Cu, Ni and Zn. In water saturated conditions, the release of As was low from granular CT, due to the stability of As in acidic conditions and to the formation of a enclosing Fe-oxide crust that hindered metal(oid)-diffusion. In CT-CPB- mixtures stored to maintain high levels of water saturation, low amounts of Cu, Ni and Zn were released, due to desorption and dissolution of secondary formed cementitious species. At this stage, alkaline conditions caused As-desorption and the dissolved As to be attached onto cementitious phases. These As-phases were highly acid-intolerant and their dissolution caused the As-release to be a 10-18-fold higher in CT-CPB:s compared to

28 that from CT. In oxidized conditions, the cementitious phases dissolved at a higher rate in the CT-CPB:s. This occurred alongside with a lower As-, Ni-, Cu- and Zn-release. The As-release was still higher compared to that in CT, especially in alkaline conditions. This was not true regarding Cu, Ni and Zn, which showed an enhanced stability in acidic and alkaline conditions, compared to that in CT. To address the long-term performance of a CPB application, it is important to understand that unsaturated zones could form within CPBs, as complete flooding of CPB-filled workings could take several years to happen. The establishment of unsaturated zones in CT-CPB mixtures caused the strength to decrease due to a more extensive pyrrhotite oxidation. Using a small proportion of binders did not suppress Cu and As leaching, but Ni and Zn leaching were still less than from unmodified tailings. Proportions of Zn, Ni, As and Cu associated with acid-soluble phases increased in conjunction with the binder fraction. In CT-CPB mixtures with 3 wt.% of binders, Zn release increased substantially while the opposite was true for Cu, As and Ni. This was due to the occurrence of a binder- related Zn phase soluble in alkaline conditions. Increasing the binder proportion in a CPB, does not necessary mean that trace metals are more stabilized, due to the formation of acid-soluble fractions. This increases the importance of minimizing the effects of a sulfide oxidation in a low-strength CPB application. Overall, the use of a low strength CPB presents no major environmental gains, because of a more abundant release of Cu and As compared to that from unmodified tailings.

7. Future Research Gold is most often extracted in an oxidized, alkaline cyanide solution. Generally, in the associated water treatment, heavy metals and metal(loid)s originating from gold- bearing sulfide minerals are stabilized through co-precipitation and the formation of metal hydroxides. Metals such as arsenic and copper, nickel and zinc are mobilized in different conditions. Arsenic is mobilized in neutral to alkaline conditions, while the majority of heavy metals are less mobile in these conditions. In the slurries from a cyanide leaching process, it is common that remnants of sulfides and secondary forms of arsenic and heavy metals co-exist. In that case, the water treatment process must be adapted to ensure the stability of these co-occurring constituents. Previous research has often focused on the stability of the end product (the tailings), whilst process-like studies of whether an efficient gold extraction method could work alongside the long-term immobility of the metal(loid) s have had less attention. Focus for future research could therefore be on obtaining knowledge about the mobility of metal(loid)s in common gold extraction processes. This knowledge could be used to adapt the processes for an efficient gold extraction that works with the long-term immobility of metal(loid)s. In this study, a method called Cemented Paste Backfill (CPB) was used to manage tailings from a cyanide leaching process. In the CT-CPBs large proportions of acid intolerant metal (oid) species could have formed. Little is known about the speciation of arsenic, copper, nickel and zinc within a low-strength CPB-mixture. Using low-strength CPBs could be an option for storing tailings in surficial facilities. It is therefore of interest to investigate the speciation of As, Cu, Ni and Zn in various CPB-mixtures in order to identify compositions suitable for the stabilization and management of CT.

29 8. References

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Paper I

Release of Arsenic from cyanidation tailings

Roger Hamberg, Glenn Bark, Christian Maurice and Lena Alakangas

Published in:

Minerals Engineering, 93, 57-64. (2016)

Minerals Engineering 93 (2016) 57–64

Contents lists available at ScienceDirect

Minerals Engineering

journal homepage: www.elsevier.com/locate/mineng

Release of arsenic from cyanidation tailings ⇑ Roger Hamberg , Glenn Bark, Christian Maurice, Lena Alakangas

Department of Civil, Environmental and Natural Resources Engineering, Division of Geosciences and Environmental Engineering, Luleå University of Technology, Luleå, Sweden article info abstract

Article history: At a gold mine in northern Sweden, gold occurring as inclusions in pyrrhotite and arsenopyrite is leached Received 25 January 2016 by cyanidation of the ore. The main sulphide minerals in the ore are pyrrhotite and arsenopyrite. Effluents Revised 20 April 2016 from the cyanidation process are treated with Fe2(SO4)3 to form Fe-precipitates suitable for the co- Accepted 21 April 2016 precipitation of As. The aim of this study was to perform static and kinetic leaching tests on the ore Available online 30 April 2016 and tailings to define geochemical processes governing As mobility. Sequential leaching tests suggested that the majority of dissolved As deriving from the sulphide fraction in the ore was incorporated in newly Keywords: formed Fe-precipitates in the tailings. The mobility of As in the tailings was therefore mainly dependent Cyanidation tailings on the stability of these As-bearing Fe-precipitates. Weathering cell tests (WCT) involving 31 weekly Sulphide ore Arsenic cycles of wetting and air exposure were conducted to assess the stability of the As in the tailings under As (V)-immobilization accelerated weathering conditions. The first stage of the WCT was characterized by a pH 5 and low As leaching, probably driven by the dissolution of amorphous Fe-As species. In the second stage of the WCT, leaching of Fe, S and As increased and the pH decreased to <3.5. An increase of the leachate’s molar Fe/S- ratio suggested that pyrrhotite oxidation was occurring. The falling pH destabilized As-bearing Fe- precipitates, causing further As release. The total As release during the WCT corresponded to only a small proportion of the tailings’ total As content. The accelerated As-leaching observed towards the end of the WCT could thus indicate that its release could increase progressively over time. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction management of As-rich tailings generated by gold extraction processes generally focuses on the stability of secondary As-phases Gold can occur in its native form (free gold) or may be because As-sulphides are assumed to be oxidized during gold encapsulated in other minerals (e.g. pyrite, quartz, arsenopyrite, extraction (Paktunc et al., 2004). The main secondary As minerals loellingite, etc.). Gold particles are usually separated by gravity present in tailings after cyanide leaching are believed to be arsen- or recovered by leaching the ore using a cyanide solution. During ates (Craw et al., 2003). Under oxidizing conditions, arsenates form the cyanidation process, the ore is subjected to a cyanide solution complexes with metals such as Fe, Al and Ca. The stability of these under alkaline conditions (pH 9.5–10) for up to 36 h. Many gold metal arsenates depends on several factors, including the pH, ores contain arsenic (As) in the form of arsenopyrite, which usually metal/As ratio, redox conditions, availability of O2 and CO2 and 2 co-exists with other sulphide minerals, such as pyrrhotite and pyr- the presence of anions, such as SO4 , that compete with arsenates ite. Tailings from sulphide ore mines are often chemically reactive for sorption surfaces. Iron and Al-arsenates are dominant phases in due to their content of Fe-sulphide minerals, such as pyrite and acidic environments and are less soluble than Ca-arsenates, which pyrrhotite. The weathering of Fe-sulphide minerals can be a predominate in calcareous tailings (Williams et al., 1996). Arsenate strongly acidifying process and is considered to be the main cause may precipitate as scorodite, FeAsO42H2O(Paktunc et al., 2008), or of acid mine drainage (AMD) (INAP, 2009). Arsenic does not form poorly-crystalline Fe(III)-arsenates (Le Berre et al., 2007; stable complexes with cyanide like Au, but it is very soluble under Parviainen et al., 2012). Alternatively, it may co-precipitate as the alkaline conditions used during cyanidation. Therefore, the Ca-Fe-arsenates (Pantuzzo et al., 2008), sorb to amorphous Fe- water used in this process must be treated to remove dissolved oxy-hydroxide phases (Korte and Fernando, 1991) or substitute arsenic before it can be discharged into tailings facilities. The for sulphate in jarosite (Paktunc and Dutrizac, 2003). Gold mine tailings slurries from As-enriched sulphide ores are

⇑ Corresponding author at: Department of Civil, Environmental and Natural often treated with lime and Fe2(SO4)3 to promote the formation Resources Engineering, Division of Geosciences and Environmental Engineering. of stable Fe-As-precipitates and increase the pH to prevent AMD. Luleå University of Technology, 971 87 Luleå, Sweden. The tailings slurries are also often pre-treated with H2O2 and O2 E-mail address: [email protected] (R. Hamberg). http://dx.doi.org/10.1016/j.mineng.2016.04.013 0892-6875/Ó 2016 Elsevier Ltd. All rights reserved. 58 R. Hamberg et al. / Minerals Engineering 93 (2016) 57–64 in order to promote the oxidation of arsenite complexes into less arsenates, which is the preferred species when considering the sta- mobile arsenate species. Gaining knowledge about the distribution bility of As. and speciation of As in such tailings is essential for their future þ ð Þ þ ð Þ ! ð Þþ ð Þ management. For this reason, we studied cyanided tailings from 2H3AsO4 Fe2 SO4 3 3Ca OH 2 2FeAsO4 s 3CaSO4 2H2O s a mine in the north of Sweden where gold has been extracted from ð1Þ inclusions in arsenopyrite. The original ore and tailings (which had The pH of the treated efﬂuent is reduced to 8–8.5 before depo- been pre-treated with Fe (SO ) ) were studied in detail by static 2 4 3 sition in the tailings dams, in accordance with legal guidelines. and kinetic leaching experiments, analysis of their chemical composition and geochemical modelling. The speciﬁc objectives of this investigation were as follows: 3.1. Tailings and ore characteristics

– Chemically characterize the ore and tailings at the mine and The acid potential (AP), neutralization potential (NP) and net speciation of As phases in each case. neutralization potential (NNP) of the tailings samples were deter- – Determine the mobility of As under oxidizing conditions by sta- mined in triplicate by the acid-base accounting (ABA) test accord- tic and kinetic leaching tests. ing to Swedish standard (SS-EN 15875:2011). Tailings were dried at room temperature and sieved to a particle size of <0.125 mm It was anticipated that these investigations would yield new before testing. Two grams of dried, sieved tailings were mixed in insights into the geochemical processes governing As mobility at distilled water for 22 h with a magnetic stirrer, after which the the studied mine and, in general, allow the development of more pH of the suspension was determined. The suspension was then effective ways of managing As-bearing tailings that prevent the treated with 1 M HCI until its pH reached 2–2.5 and the mixtures occurrence of high As concentrations in mine drainage. were back-titrated to a pH of 8.3. Total sulphur content was determined by ICP-AES analysis. Powder X-ray traces were conducted on a Siemens D5000 2. Site description diffractometer using a Cu Ka radiation source (40 kV and 40 mA) over the range 20 = 5–90° using a step size of 0.02° and scan rate ° The Svartliden gold ore deposit is situated along the so called of 1 per minute. Crystalline minerals present at proportions of Gold Line (Fig. 1), southwest of the Skellefte ore district in northern <2 wt% were qualitatively determined. Sweden. The operational time was 2005–2014 and until 2012, From the primary ore and tailings, fifteen samples were pre- approx. 2.4 Mt of ore at a grade of 4.4 ppm Au was generated pared as polished sections and examined by optical and scanning (Schlöglova et al., 2013). The Svartliden gold deposit is considered electron microscopy for phase identification and textural analysis to be a structurally controlled epigenetic lode gold deposit (Hart at the Luleå University of Technology. The electron microscope et al., 1999; Bark and Weihed, 2003, 2012) with key features used was a Zeiss Merlin FEG-SEM equipped with energy- and resembling orogenic gold style mineralization. Gold is the only wavelength-dispersive spectrometers. An accelerating voltage of economic metal at Svartliden. The mineralization is spatially asso- 20 kV and beam current of 200–1000 pA were used for the ciated with a banded iron-formation (Schlöglova et al., 2013; analysis. Sciuba, 2013) situated at the contact zone between metasedimentary rocks and amphibolites. The metasedimentary host rocks 3.2. Leaching tests exhibit interbedded carbonaceous shale horizons with up to 30% pyrrhotite (Hart et al., 1999). Environmental assessment leaching tests (batch and weather- The mineralogy of the ore zone (including the banded iron- ing cell) were conducted to evaluate the behaviour of As in the tail- formation) constitutes mainly quartz, magnetite, diopside, heden- ings. Batch leaching tests (BLT) were performed to assess the bergite, tremolite, grunerite, fayalite, pyrrhotite, arsenopyrite and leaching of selected elements from the tailings over the short term. löllingite (Sciuba, 2013). Gold occurs as electrum (Au/Ag alloy). Weathering cell tests (WCT) were conducted in duplicate to simu- The gold grain size ranges between 5 and +100 lm with an average late weathering of the tailings over extended periods of time and to grain size of ca. 45 lm, and the gold occurs as inclusions in determine the kinetic release of specific elements. arsenopyrite-hosted löllingite, commonly at the grain boundary Batch leaching tests (BLT) were conducted on tailings in trip- to arsenopyrite and within arsenopyrite. Gold grains are also found licate according to the SS-EN 12457-4 standard (SIS, 2003). Tail- in the silicate matrix, commonly spatially related to arsenopyrite ings (100 g, dry weight) were placed in acid-washed bottles

(Bark et al., 2013). (1500 ml) together with 1000 ml of milliQ H2O to generate a liquid/solid ratio of 10 (L/S = 10). The samples were agitated for 24 h in a rotary shaker (10 rpm). The leachates were then filtered 3. Materials and methods through a 0.45-mm Filtropur S non-pyrogenic Sterile-R syringe filter (cellulose acetate membrane) into acid-washed bottles Tailings and crushed ore originating from the Svartliden Gold and stored cold (4 °C) in darkness until analysis. The pH, redox mine, Sweden were provided by the mining company Dragon Min- potential (Eh) and electrical conductivity (EC) of the tailings were ing AB. Tailings have been deposited in a tailings impoundment at determined using unfiltered samples. The chemical compositions the mine for 8 years (2005–2013) at a rate of approx 0.3 million of the water samples were analyzed as described in the ‘‘Sample tonnes/year. Tailings were sampled at depths of 0–30 cm from analysis” section. approx. ten different locations on the impoundment and mixed In the weathering cell test (WCT), a modified version devel- to form a bulk sample of approx. 15 kg. The masses of the crushed oped by Cruz et al. (2001) was used to simulate accelerated ore samples were approx. 3 kg. The tailings slurries at the mine are weathering of the tailings and assess the reactivity of their min- routinely treated by aeration and hydrogen peroxide (H2O2) eral content over time. This protocol was selected because it together with Fe2(SO4)3 and lime in order to generate As-bearing enhances evaporation and reduces the degree of saturation of Fe-precipitates (reaction (1)). Raising the pH in the solution helps the tested sample, avoiding reactivity inhibition. Approximately to avoid the formation of AMD and toxic cyanide gas. Hydrogen 70 g of tailings was placed on a paper filter in a Büchner-type peroxide is used to increase the oxidation rate of arsenites into funnel. The samples were exposed to weekly cycles, involving R. Hamberg et al. / Minerals Engineering 93 (2016) 57–64 59

7225000 Fäbodliden Vargbäcken Storuman A-C Middagsberget Skellefte District 1575000 Barsele 1-2

Stortjärnhobben

Sjöliden Mejvankilen Paubäcken

Svartliden Gold Line

Fäboliden Lycksele Bothnian Basin

7150000 1650000 Knaften

10km

Dolerite, 1.27–1.00 Ga Granitoids; Skellefte and Revsund type, 1.86–1.75 Ga

Svartliden Mafic metavolcanic rocks, 1.88–1.86 Ga Metavolcanic and metaigneous rocks, 1.91–1.88 Ga Metagreywackes, 1.95–1.87 Ga Metavolcanic and metaigneous rocks, 1.96–1.91 Ga Village Au deposit Au mine Tectonic lineament Occurrence of banded ion-formation 250 km

Fig. 1. The gold ore deposit of Svartliden, mineralization of the ‘‘Gold Line” situated in northern Sweden.

one day of leaching, three days of ambient air exposure, a second After each extraction, the suspensions were subjected to centrifu- day of leaching and finally two days of air exposure. The samples gation at 1200 rpm for 15 min. The leachate was then filtered using were leached by covering them with 50 ml of milliQ H2O for a 0.45 lm syringe filter with an acetate cellulose membrane into approximately 2 h and then recovering the leachates by applying acid-washed (60 ml) plastic bottles. The samples were stored cold a vacuum to the funnel. The pH, Eh and EC of the leachates were (4 °C) in darkness until analysis. The solid residues were washed measured after filtration; the chemical composition of the water with milliQ H2O to remove all traces of the previous extraction samples was analyzed as described in the section entitled ‘‘Sam- solution before each step in the sequence after the first. Finally, ple analysis”. Duplicate tailings samples (Tailings 1 and 2) were the filtrates from each extraction step were submitted for total ele- leached. The experiment was conducted over a period of 31 ment analysis. Their chemical compositions were determined as cycles (217 days). described in the ‘‘Sample analysis” section. Arsenic speciation was assessed using the modified sequential extraction scheme described by Dold (2003). In each extraction 3.3. Modelling – PHREEQC sequence, 2 g of tailings was placed in an acid-washed round- bottomed centrifuge tube with a capacity of 60 ml and extracted Speciation-solubility calculations were performed with the geo- with seven different solutions in succession as listed in Table 1. chemical code PHREEQC (Parkhurst and Appelo, 1999) using the 60 R. Hamberg et al. / Minerals Engineering 93 (2016) 57–64

Table 1 Modiﬁed sequential extraction procedure for sulphide mine waste (Dold, 2003).

Chemical agent added to the solid sample Duration Target phase

50 ml milliQ H2O 1 h shaking Water-extractable As

50 ml 1 M NH4 - acetate, pH 4.5 2 h shaking Acid-Exchangeable (AEC) As

50 ml 0.2 M NH4 – oxalate, pH 3.0 1 h shaking in darkness As bound to Fe (III) oxy-hydroxides

50 ml 1 M NH4 – oxalate, pH 3.0 2 h heating in water bath (80 °C) As bound to Fe (III) oxides

50 ml 35% hydrogen peroxide (H2O2) 1 h heating in water bath (85 °C) Secondary sulphides

Combination of KClO3 and HCl, followed by 4 M HNO3 boiling 20 min heating in water bath (90 °C) Primary sulphides

Aqua regia (HCl:HNO3 = 3:1) – Residual fraction

Wateq4f database. Element concentrations, redox-potential (Eh) and pH in leachates from the WCT were used for this purpose. Table 2 Chemical composition of the tailings and ore (n = 3, ±SD) for selected elements.

Element Unit Tailings Ore 3.4. Sample analysis Total Solids (TS) % 89.0 ± 0.4 99.97 ± 0.06 SiO % TS 55.0 ± 4.9 30.3 ± 0.9 Water samples from the leaching tests were analyzed to deter- 2 Al2O3 % TS 4.69 ± 0.04 1.74 ± 0.07 mine their elemental composition. The pH, Eh and Ec of the water CaO % TS 4.83 ± 0.25 4.26 ± 0.09 samples was measured on unfiltered samples. Element composi- Fe2O3 % TS 16.7 ± 0.6 12.6 ± 0.5 tions were analyzed by an laboratory accredited by the Swedish K2O % TS 0.92 ± 0.03 0.32 ± 0.00 board of accreditation and conformity assessment (SWEDAC). Total MgO % TS 3.24 ± 0.01 2.38 ± 0.07 MnO % TS 0.14 ± 0.01 0.15 ± 0.03 contents of As, Cd, Cr, Cu, Mn, Ni, P, Pb and Zn were determined by Na2O % TS 0.82 ± 0.03 0.18 ± 0.01 high resolution inductively coupled plasma–mass spectrometry P2O5 % TS 0.24 ± 0.01 0.10 ± 0.00 (HR-ICP-MS), whereas Al, Ca, Fe, K, Mg, Na and S were determined TiO2 % TS 0.18 ± 0.00 0.09 ± 0.01 by inductively coupled plasma atomic emission spectroscopy (ICP- LOI (1000°C) % TS 1.83 ± 0.06 1.13 ± 0.11 As mg/kg TS 1070 ± 30 4703 ± 781 AES). Before analysis, samples were acidified with super-pure Cr mg/kg TS 166 ± 3 247 ± 45 nitric acid using 1 ml of acid per 100 ml of sample. Analyses of Cu mg/kg TS 147 ± 7 82.0 ± 12.1 HR-ICP-MS and ICP-AES were performed using modified versions Ni mg/kg TS 63.8 ± 2.1 126 ± 12 of US Environmental Protection Agency (US EPA) methods 200.8 Pb mg/kg TS 4.56 ± 0.46 7.71 ± 2.58 and 200.7, respectively. S mg/kg TS 20933 ± 493 25367 ± 2363 Zn mg/kg TS 25.0 ± 0.4 10.6 ± 1.5 The solid samples were analyzed for major and minor elements by the accredited laboratory. For the determination of As, Cd, Co, Cinorganic % 0.19 ± 0.03 N.D Cu, Ni, Pb, S and Zn, samples were dried at 50 °C and digested with 7 M nitric acid in closed Teflon vessels in a microwave oven. Other elements were determined after fusion with lithium metaborate, 4.1.1. Scanning electron microscopy followed by dissolution in dilute nitric acid. The solutions were Results from SEM showed the occurrence of gold in ore samples subjected to centrifugation and diluted before analysis. For data and grains of arsenopyrite in tailings samples (Fig. 2). No arsenic- quality control, two in-house reference materials were analyzed bearing Fe-precipitates were detected, due to low content of As in parallel with the solid samples. The dry matter content of the in tailings (less than 1 wt%). Indications of a hydrous ferric oxide tailings samples were determined in triplicate by drying the sam- (HFO)-rim enclosing As-sulphide grains could be detected (Fig. 2d). ples in an oven at 105 °C for 24 h according to Swedish standard SS 028113-1 (SIS, 1981). 4.2. Leaching tests

4. Results 4.2.1. Sequential extraction tests Sequential extractions were conducted on tailings and ore sam- 4.1. Tailings and ore characteristics ples in triplicate. In ore samples, analyzed contents of As, Fe and Ca in each fraction deviated by less than 10% from the average. How- According to the XRD-analyses, the major tailings mineral ever, the standard deviation of S in each fraction was 12.6–33.8% assemblage in Svartliden consisted of quartz (SiO2), tremolite (Fig 3), with the largest variation measured in the secondary sul- (Ca2(Mg5-4.5,Fe0-2.5)[Si8O22](OH2)), albite (NaAlSi3O8) and micro- phide fraction. Results (Fig. 2) showed that most (98 wt%) of the cline (KAlSi3O8). Jarosite (KFe3(SO4)2(OH)6) was present in small As in the ore was associated with the primary sulphide fraction amounts and no crystalline phases of As were found. The relative and approx. 1 wt% with the residual fraction (silicates). The major- proportions of the minerals were calculated from the peak heights ity (75 wt%) of the Fe was associated with the residual fraction but of the XRD spectra. The dominant elements in the tailings and ore lesser (16.7 wt%) quantities were associated with primary sul- were Al, As, Ca, Si, Fe and S (Table 2). Both the tailings and ore had phides, Fe-oxides (5.6 wt%) and the adsorbed-exchangeable- low contents of sulphide-associated elements such as Cu, Pb and carbonate (AEC) fraction (1.8 wt%). Most of the S (71 wt%) was Zn. The total sulphur contents of the tailings and ore were associated with the primary sulphide fraction, with smaller 2.1 ± 0.05 and 2.5 ± 0.23 wt%, respectively. Low proportions of (15 wt%) quantities associated with secondary sulphides and the inorganic carbon (0.19 ± 0.03 wt%, Table 2) and a NNP of less than residual fraction (12 wt%). A small proportion (3.1 wt%) of the Ca

20 kg CaCO3/ton (60.8 ± 1.32), showed that tailings could be in the ore was associated with the AEC fraction and primary sul- considered acid-generating. The As contents of the tailings and phides (2.8 wt%), but the majority (93 wt%) was associated with ore were 1070 ± 30 and 4703 ± 781 mg/kg, respectively (Table 2). the residual fraction. Total contents of As and S differed (2654 According to a previous analysis (personal communication, and 10,344 mg/kg) in these samples, indicating a large difference Lars-Åke Lindahl, Dragon Mining AB), the As-content in the ore is from the chemical composition analyses of the ore (Table 2). The highly variable (100–10,000 mg/kg). non-existence of S-silicates and Ca-sulphides in natural environ- R. Hamberg et al. / Minerals Engineering 93 (2016) 57–64 61

Fig. 2. Mineralogical relationships of primary gold ore and tailings from the Svartliden gold deposit (backscattered images from scanning electron microscopy). (a) Common mineral association in the primary ore, showing gold as inclusions at the boundaries between arsenopyrite and löllingite, which are hosted by silicate minerals. (b) Gold together with Bi-tellurides scattered in the silicate matrix in the primary ore. (c and d) Non-dissolved arsenopyrite grains in the tailings with indications of HFO-rims. Apy – arsenopyrite, Loll – löllingite, Au – gold.

Ore Residual S Primary sulphides As Secondary sulphides Fe(III) oxides

Element Fe Fe(III) oxyhydroxides Exchangeable Ca Water soluble

020406080100 Extracted (%)

Fig. 3. Distributions of elements across different phases in unprocessed ores from the Svartliden gold mine based on the results of sequential extraction tests (Error bars, n = 3, ±SD).

Fig. 4. Distributions of elements across different phases in tailings from the Svartliden gold mine based on the results of sequential extraction tests (Error bars, n = 3, ±SD). ments implies that the sulphide fraction of S and silicate fraction of mary sulphides. A minor proportion of As (0.8 wt%) was associated Ca were underestimated. with the AEC fraction. The majority of Fe was associated with the Results from the sequential extractions of tailings (Fig. 4) sug- residual fraction (61 wt%) and Fe (III) oxy-hydroxides (30 wt%), gested that the majority of As (94.6 wt%) was co-precipitated with but a smaller proportion was associated with primary sulphides secondary Fe (III) minerals, whereas 2.5 ± 1.3 wt% occurred as pri- (3.6 wt%). Most of the S was associated with the primary sulphide 62 R. Hamberg et al. / Minerals Engineering 93 (2016) 57–64

Table 3 Fe (650 mg/l), Mg (34.7 mg/l) and Al (60.3 mg/l) (Table 3). It was Batch leaching test results (L/S 10) (n = 3, ±SD) for the Svartliden tailings. also quite highly oxidized (Eh: 347 ± 3 mV). The tailings leachate Element Unit had a pH of approx. 3 (Table 3) and could be classiﬁed as acid mine Ca mg/l 360 ± 12 drainage (AMD) according to INAP (2009). Fe mg/l 65.0 ± 4.1 K mg/l 1.61 ± 0.11 4.2.3. Weathering cell tests Mg mg/l 34.7 ± 0.9 Na mg/l 2.35 ± 0.16 The WCT of the tailings exhibited good reproducibility with Si mg/l 2.78 ± 0.28 only small differences between runs. The cumulative release of Al mg/l 60.3 ± 11.2 As from the tailings was 1.52 ± 0.06 mg/kg and the concentrations Ni mg/l 4.78 ± 0.10 of As ranged from 22 to 554 lg/l. Similar trends were initially S mg/l 532 ± 26 observed for the release of As, Fe, Ca and S: the concentrations of Mn mg/l 4.59 ± 0.11 Zn mg/l 0.97 ± 0.03 all four elements decreased gradually during the ﬁrst few weeks Sr mg/l 0.43 ± 0.02 of leaching. The concentrations of As, Fe and S subsequently As lg/l 447 ± 13 increased but leaching of Ca remained low throughout the remain- l Ba g/l 20.1 ± 1.6 der of the leaching period. After 80 days, the release of As increased Cd lg/l 16.7 ± 0.5 and concentrations of S and Fe increased in tandem once the pH Co lg/l 1004 ± 28 Cr lg/l 38.1 ± 7.0 fell below 4.5 (Fig. 5). Cu lg/l 838 ± 12 pH 3.07 ± 0.02 4.2.4. Modelling – PHREEQC Eh mV 347 ± 3 The dissolution of ferrihydrite was most extensive at the end of Ec mS/cm 2.86 ± 0.11 the WCT and coincided with increased leaching of As. Gypsum was the most dominant Ca-mineral and was under saturated during the WCT (Fig. 6). fraction (37 wt%) and water-soluble fraction (35 wt%); smaller quantities were associated with the AEC (11 wt%) and Fe (III) 5. Discussion oxy-hydroxides (7.0 wt%). The proportion of Ca in the residual fraction was (88 wt%), whereas the rest was associated with the water- Based on results from the sequential extraction tests (Figs. 3 soluble fraction (7.4 wt%) and primary sulphides (3.8 wt%). The and 4), the cyanidation process appeared to have redistributed non-existence of Ca-sulphides in natural environments suggests the majority of As originating from the dissolution of arsenopyrite that the silicate fraction of Ca was underestimated. into secondary phases. Our studies suggested that multiple mechanisms of As attenuation could occur within oxidized tailings at the 4.2.2. Batch leaching tests studied site. Thus, the release of As from these cyanidation tailings The leachate from the BLT had an electrical conductivity (Ec) of can be attributed to various processes, including (1) dissolution of 2.86 mS/cm due to its high contents of S (532 mg/l), Ca (360 mg/l), water-soluble and exchangeable As-species; (2) oxidation and

Fig. 5. Evolution of pH and release of Ca, Fe, S and As from Svartliden gold mine tailings over time during the WCT.

Fig. 6. Evolution of As-release and Saturation Index (SI) of gypsum, goethite and ferrihydrite during the WCT. R. Hamberg et al. / Minerals Engineering 93 (2016) 57–64 63

Fig. 7. Evolution of the pH and molar Fe/S-ratios in Svartliden gold mine tailings during the WCT.

ð Þþ þ 3þ ! 2þ þ ð Þþ 2 þ þ dissolution of arsenopyrite; and (3) precipitation and dissolution of FeAsS s 7H2O 11Fe 12Fe H3AsO3 aq SO4 11H As-bearing Fe-hydrates. ð6Þ

3+ 5.1. Presence of water-soluble and exchangeable species of As in At pH < 4, Fe becomes the dominant oxidant of arsenopyrite, + tailings accelerating its oxidation and the generation of H according to reaction (6) (Blowes et al., 1998). The oxidation of arsenopyrite The initial period of the WCT (approx. until the 80th day) was could therefore be a significant contributor of released As in the characterized by low As release, a pH of 4.5–5.5. At this stage, later stages of the WCT when pH < 4. As-release could derive from the most soluble phases of As (water-soluble and AEC) which constitute of a small proportion 5.3. Stability of As-bearing Fe-hydrates in tailings (0.8%) or the total content of As. Modelling suggested however, that the release of As at this stage could be due to the dissolution Results from the sequential extraction tests suggested that the of poorly crystalline As-bearing Fe-hydrates (Fig. 6). majority (95%) of As was co-precipitated with secondary Fe(III) minerals. Using the modified extraction scheme of Dold (2003), 5.2. Stability of As-sulphides in tailings not adapted specifically for As, may have overestimated the amount of As in the Fe(III)-oxyhydroxide fraction. However, use

The sequential extraction tests showed that a small quantity of NH4–oxalate has been shown to be effective for the dissolution (2.5 ± 1.3 wt%) of As was associated with primary sulphides. of As-bearing Fe-hydrates (Kumpulainen et al., 2007; Keon et al., Observations in SEM confirmed the presence of As-sulphides 2001; Parviainen et al., 2012). The addition of ferric sulphate (reac- (Fig. 7c and d). This means that a small proportion of the As tion (1)) to the tailings slurry prior to disposal aimed to resent As in and acid release from the tailings may be controlled by the dis- newly formed Fe-precipitates. These As-bearing Fe-precipitates solution of As-sulphides. As-sulphides is generally stable under could not be detected by SEM-EDS, due to low total content of As reducing conditions at pH values of 2–12 but is oxidized under (approx. 1 wt%). The diversity of the As-content in the ore suggests near-surface oxidizing conditions at pH > 5 (Blowes et al., 1998) that As-bearing precipitates with differing Fe/As-ratios are pres- (reaction (2)): ence in the tailings. As-bearing Fe-precipitates with a Fe/As < 4 are assumed to be stable under oxidized conditions when pH 4– FeAsSðsÞþ1:5H O þ 2:75O ðaqÞ 2 2 7, but the presence of cations such as Zn2+ and Ca2+ increases the ! 2þ þ ð Þþ 2 ð Þ Fe H3AsO3 aq SO4 2 stable pH range to 4–9 (Riveros et al., 2001). Leaching of As increased in the WCT most extensively when the pH decreased However, at pH values > 5, Fe2+ rapidly oxidizes to Fe3+, under- below 4 at the end of the test (Fig. 5). The evolution of As, Fe and goes hydrolysis and precipitates as Fe(OH) (s) (HFO:s) (reaction 3 pH in the WCT suggests that As-bearing Fe-precipitates with Fe/ (3)). Thus, HFO:s may form rims on the surfaces of the As- As-ratios > 4 and/or poorly crystalline phases of Fe(III)arsenates sulphide grains. Such rims form rapidly under alkaline conditions could be responsible for the As-release during the later stages of prevailed in the cyanidation process, creating a thin permeable the WCT. coating that retards the minerals’ further oxidation (Belzile et al., 2004; Asta et al., 2013). Indications of HFO-rims on these grains were detected in SEM (Fig. 2d). Such HFO-rims could also be a 5.4. Potential long-term risks of cyanidation tailings source for As-release by sequestering secondary forms of As (par- ticularly arsenates) (Walker et al., 2009). The evolution of pH and redox conditions in tailings has important implications for the long-term stability of As-bearing phases. ð Þþ þ ð Þ! ð Þ ð Þþ ð Þþ 2 þ þ FeAsS s 4H2O 3O2 aq Fe OH 3 s H3AsO3 aq SO4 2H The evolution of the pH in tailings is mainly governed by the pres- ð3Þ ence of pyrrhotite and calcite. The sequential extraction tests and analysis of the chemical composition of the tailings suggested The unloaded arsenite species formed in reaction (3) may be low contents of calcite. Pyrrhotite is the main sulphide mineral oxidized over days or months as shown in reactions (4) and (5): in the Svartliden ore (Sciuba, 2013). The sequential extraction tests 2 þ suggested that the content of Fe-sulphides in the tailings was H3AsO3ðaqÞþ0:5O2 ! HAsO þ 2H ð4Þ 4 approx. 5.8 wt% and the sulphide and sulphate contents of the tail- þ ings accounted for 44% and 56% of the total S, respectively. A minor H AsO ðaqÞþ0:5O ! H AsO þ H ð5Þ 3 3 2 2 4 part of the total S and As in the tailings was released during the The aeration in reaction (4) and (5) would make arsenate the WCT (22.8% and 0.1% respectively). Until the 110th day of the dominant As species. Moreover, the above reactions are acid- WCT, Ca/S-molar ratios 1 in the leachates and modelling generating and the solubility of Fe(OH)3 increases as the pH (Fig. 6) suggesting the dissolution of gypsum formed during treat- decreases. ment of the tailings slurry (reaction (1)). Towards the end of the 64 R. Hamberg et al. / Minerals Engineering 93 (2016) 57–64 test period (217 days), the pH of the leachate fell to <3.5 and con- Blowes, D.W., Jambor, J.L., Hanton-Fong, C.J., Lortie, L., Gould, W.D., 1998. centrations of As, Fe and S increased (Fig. 5). The Fe/S molar ratio in Geochemical, mineralogical and microbiological characterization of a sulphide-bearing carbonate-rich gold-mine tailings impoundment, Joutel, the leachate increased to 0.8 by the end of the WCT, suggesting Quebec. Appl. Geochem. 13 (6), 687–705. that pyrrhotite had been oxidized over the course of the experi- Craw, D., Falconer, D., Youngson, J.H., 2003. Environmental arsenopyrite stability ment (Fig. 7). and dissolution: theory, experiment and field observations. Chem. Geol. 199, 71–82. Modelling in PHREEQC suggested that the dissolution of Fe- Cruz, R., Bertrand, V., Monroy, M., González, I., 2001. Effect of sulfide impurities on precipitates occurred predominantly at the end of the WCT when the reactivity of pyrite and pyritic concentrates: a multi-tool approach. Appl. more As was released (Fig. 6). Geochem. 16 (7–8), 803–819. Dold, B., 2003. Speciation of the most soluble phases in a sequential extraction procedure adapted for geochemical studies of copper sulfide mine waste. J. 6. Conclusions Geochem. Explor. 80 (1), 55–68. Hart, I., Marsh, S., Laurent, I., 1999. Svartliden – a new style of mineralisation in the Skellefteå district. In: Cook, N.J., Sundblad, K. (Eds.), Nordic Mineral Resources A speciation of As-phases in ore and tailings from a gold mine Symposium Gold ’99 Trondheim. Geological Survey of Norway, Trondheim, pp. suggested that As occurred dominantly as arsenopyrite in the 87–88. ore, while a majority of As in the tailings was associated with As- INAP (International Network for Acid Prevention), 2009. Global Acid Rock Drainage Guide (GARD Guide). The Acid Rock Drainage Process (Chapter 2). bearing Fe-hydrates. Results from the speciation suggested that a Keon, N.E., Swartz, C.H., Brabander, D.J., Harvey, C., Hemond, H.F., 2001. Validation minor proportion of the total As retained in the tailings were As- of an arsenic sequential extraction method for evaluating mobility in sulphides. Mineralogical analyses conducted on tailings showed sediments. Environ. Sci. Technol. 35 (13), 2778–2784. Korte, N.E., Fernando, Q., 1991. A review of arsenic (III) in groundwater. Crit. Rev. grains of arsenopyrite with indications of HFO-rims. These HFO- Environ. Contr. 21 (1), 1–39. rims may sequester arsenates, which also could be a source for Kumpulainen, S., Carlson, L., Räisänen, M., 2007. Seasonal variations of ochreous As-release. precipitates in mine effluents in Finland. Appl. Geochem. 22 (4), 760–777. Le Berre, J.F., Gauvin, R., Demopoulos, G.P., 2007. Characterization of poorly- The stability of As in tailings under accelerated weathering con- crystalline ferric arsenate precipitated from equimolar Fe(III)-As(V) solutions in ditions was intact as pH < 4.5. As pH was lowered to >3.5 due to an the pH range 2 to 8. Metall. Mater. Trans. B Sci. 38 (5), 751–762. oxidation of pyrrhotite, As-release increased while As-bearing Fe- Paktunc, D., Foster, A., Heald, S., Laflamme, G., 2004. Speciation and characterization hydrates were destabilized. As-release in the tailings had an of arsenic in gold ores and cyanidation tailings using X-ray absorption spectroscopy. Geochim. Cosmochim. Acta 68 (5), 969–983. increasing trend while pH was lowered and the proceeding of pyr- Paktunc, D., Dutrizac, J.E., 2003. Characterization of arsenate-for-sulfate rhotite oxidation could destabilize As-bearing Fe-hydrates further. substitution in synthetic jarosite using X-ray diffraction and X-ray absorption spectroscopy. Can. Mineral. 41 (4), 905–919. Paktunc, D., Dutrizac, J., Gertsman, V., 2008. Synthesis and phase transformations Acknowledgements involving scorodite, ferric arsenate and arsenical ferrihydrite: implications for arsenic mobility. Geochim. Cosmochim. Acta 72 (11), 2649–2672. Financial support from Ramböll Sverige AB, Ramböll Founda- Pantuzzo, F.L., Ciminelli, V.S.T., De Brito, W., 2008. New evidences for the role of precipitation and adsorption during Fe(III)-As(V) co-precipitation. In: Paper tion, SUSMIN – Tools for sustainable gold mining in EU, and the Presented at the Hydrometallurgy 2008: Proceedings of the 6th International Center of Advanced Mining and Metallurgy (CAMM) at Luleå Symposium, pp. 130–139. University of Technology are gratefully acknowledged. Personnel Parkhurst, D.L., Appelo, C.A.J., 1999. User’s guide to PHREEQC (version 2). US Geol. Surv. Water Resour. Inv. Rep. 99-4259, 312p. at Dragon Mining AB are also gratefully acknowledged for valuable Parviainen, A., Lindsay, M.B.J., Pérez-López, R., Gibson, B.D., Ptacek, C.J., Blowes, D. information about the mining processes and for providing the W., Loukola-Ruskeeniemi, K., 2012. Arsenic attenuation in tailings at a former materials needed for this study. Cu-W-As mine, SW Finland. Appl. Geochem. 27 (12), 2289–2299. Riveros, P.A., Dutrizac, J.E., Spencer, P., 2001. Arsenic disposal practices in the metallurgical industry. Can. Metall. Quart. 40 (4), 395–420. References Schlöglova, K., Gordon, C., Hanes, R., Ask, H., Broman, C., 2013. Svartliden gold mine: shear zone and BIF-hosted orogenic gold deposit, Gold Line, northern Sweden. Asta, M.P., Pérez-López, R., Román-Ross, G., Illera, V., Cama, J., Cotte, M., Tucoulou, In: Jonsson, Erik et al. (Eds.), Mineral Deposit Research for a High-Tech World, R., 2013. Analysis of the iron coatings formed during marcasite and arsenopyrite 12th Biennial SGA Meeting, Uppsala, Sweden. Proceedings, vol. 3, pp. 1193– oxidation at neutral-alkaline conditions. Geol. Acta 11 (4), 465–481. 1196. Bark, G., Weihed, P., 2003. The new Lycksele-Storuman gold ore province, northern Sciuba, M., 2013. Mineralogy and Geochemistry of the Banded Iron-Formation in Sweden; with emphasis on the early Proterozoic Fäboliden orogenic gold the Svartliden Gold Deposit, Northern Sweden Master Thesis. Department of deposit. In: Eliopoulos, D.G. et al. (Eds.), Mineral Exploration and Sustainable Civil, Environmental and Natural Resources Engineering, Division of Development, 7th Biennial SGA Meeting, Athens, Greece. Millpress, Rotterdam, Geosciences and Environmental Engineering. Luleå University of Technology, pp. 1061–1064. Luleå, Sweden. Bark, G., Weihed, P., 2012. Geodynamic settings for Paleoproterozoic gold Walker, S.R., Parsons, M.B., Jamieson, H.E., Lanzirotti, A., 2009. Arsenic mineralogy of mineralization in the Svecofennian domain: a tectonic model for the near-surface tailings and soils: influences on arsenic mobility and Fäboliden orogenic gold deposit, northern Sweden. Ore Geol. Rev. 48, 403–412. bioaccessibility in the nova scotia gold mining districts. Can. Mineral. 47 (3), Bark, G., Wanhainen, C., Pålsson, B., 2013. Textural setting of gold and its 533–556. http://dx.doi.org/10.3749/canmin.47.3.533. implications on mineral processing: preliminary results from three gold Williams, M., Fordyce, F., Paijitprapapon, A., Charoenchaisri, P., 1996. Arsenic deposits in northern Sweden. In: Jonsson, Erik et al. (Eds.), Mineral Deposit contamination in surface drainage and groundwater in part of the southeast Research for a High-Tech World, 12th Biennial SGA Meeting, Uppsala, Sweden. Asian tin belt, Nakhon Si Thammarat province, southern Thailand. Environ. Proceedings, vol. 1, pp. 302–305. Geol. 27 (1), 16–33. Belzile, N., Chen, Y., Cai, M., Li, Y., 2004. A review on pyrrhotite oxidation. J. Geochem. Explor. 84 (2), 65–76. Paper II

The use of low binder proportions in cemented paste backfill – effects on As-leaching

Roger Hamberg, Christian Maurice and Lena Alakangas

Published in:

Minerals Engineering, 78, 74-82. (2015)

Minerals Engineering 78 (2015) 74–82

Contents lists available at ScienceDirect

Minerals Engineering

journal homepage: www.elsevier.com/locate/mineng

The use of low binder proportions in cemented paste backﬁll – Effects on As-leaching ⇑ Roger Hamberg , Christian Maurice, Lena Alakangas

Department of Civil, Environmental and Natural Resources Engineering, Division of Geosciences and Environmental Engineering, Luleå University of Technology, Luleå, Sweden article info abstract

Article history: Gold is extracted by cyanide leaching from inclusions in arsenopyrite at a mine in the north of Sweden. Received 15 December 2014 The major ore mineral assemblage consists of pyrrhotite and arsenopyrite–loellingite. Arsenopyrite is Revised 17 April 2015 assumed to be oxidized during cyanidation and the stability of secondary As-phases needs to be assessed. Accepted 20 April 2015 One way of managing such tailings is to convert them into a monolithic mass by using a method called Available online 16 May 2015 cemented paste backfill (CPB). In CPB, tailings are traditionally mixed with water (typically 25% by weight) and small amounts (3–7%) of binders, and backfilled into excavated underground areas. To inves- Keywords: tigate the release of arsenic (As) from CPB prepared from As-rich tailings, tailings containing approx. Cyanidation tailings 1000 ppm of As, mainly in the form of As-bearing iron (Fe)-precipitates (FEP), were mixed with small Cemented paste Arsenic quantities (1–3%) of biofuel fly ash (BFA), ordinary cement, and water to produce monolithic CPB masses. As-bearing FEPs CPB-recipes were designed to meet the strength demand of 200 kPa, stated by the mine operators. Tank leaching tests (TLT) and the weathering cell test (WCT) were used to compare the leaching behavior of As in unmodified tailings and CPB-materials. Results from the leaching tests (TLT and WCT) showed that the inclusion of As-rich tailings into a cementitious matrix increased leaching of As. This behavior could partially be explained by an increase of pH where As sorbed to FEPs becomes unstable. In the CPB mixtures, small (>1%) proportions of the total As in the solid material was released from less acid-tolerant species (i.e. Ca-arsenates and As bonded to cementitious phases). Unmodified tailings generated an acidic environment in flooded conditions at which As-bearing FEPs were stable. Acid was added to the crushed CPB materials during later stages of the WCTs to mimic the effects of weathering. This increased the leaching of Fe and had minor effects on that of As but did not affect S-leaching. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction problem is commonly addressed by the addition of a iron sulfate

salt (Fe2(SO4)3) to the cyanidation slurries aiming to form stable In sulfide gold ores, fine gold particles mainly occur as inclu- As-bearing Fe-precipitates (FEP) (Wilkie and Hering, 1996) (reac- sions in sulfide minerals such as arsenopyrite. Cyanidation has tion (1)). been the dominant gold extraction technology since the 1970s. Prior to cyanidation, the ore is milled and pH is raised in the result- 2H AsO þ Fe ðSO Þ þ 3CaðOHÞ ing slurry to avoid conversion of cyanide ions into the toxic cyanide 3 4 2 4 3 2 ! ð Þþ ð Þþ ð Þ gas of HCN (Hinton et al., 2003). While arsenopyrite is assumed to 2FeAsO4 s 3CaSO4 2H2O s 6H2O 1 be oxidized during gold enrichment, mobility of arsenic in cyanida- Arsenopyrite usually co-exists with other sulfide minerals such tion slurries is governed by the relative abundance of secondary as pyrrhotite and pyrite (Genkin et al., 1998). These minerals can As-phases, namely arsenates (Craw et al., 2003). Arsenates are sol- be oxidized on contact with atmospheric oxygen and water, pro- uble in alkaline conditions (Craw et al., 2003) that prevail during ducing an acid leachate called acid mine drainage (AMD) (INAP, cyanidation, so slurries produced in this way must be treated to 2009). In general, arsenic bearing FEP with a Fe/As-molar ratio of immobilize As before being discharged into tailings facilities. This >3 are stable in the pH range 4–8 (Riveros et al., 2001). The co-occurrence of As-bearing FEPs and sulfide minerals could therefore pose difficulties for tailings remediation. Cemented paste ⇑ Corresponding author at: Department of Civil, Environmental and Natural Resources Engineering, Division of Geosciences and Environmental Engineering, backfill (CPB) is a method where tailings are converted into a Luleå University of Technology, 971 87 Luleå, Sweden. Tel.: +46 705978952. monolithic mass by the addition of low proportions (3–7%) of bin- E-mail address: [email protected] (R. Hamberg). ders. CPB focuses on being a geotechnical support to underground http://dx.doi.org/10.1016/j.mineng.2015.04.017 0892-6875/Ó 2015 Elsevier Ltd. All rights reserved. R. Hamberg et al. / Minerals Engineering 78 (2015) 74–82 75

mine cavities increasing operational benefits for the mining indus- with H2O2 and O2 to promote the oxidation of As (III) complexes try. CPB offers moreover on the minimization and/or enhancement into less mobile As(V) species. The pH of the outlet water is of the quality of percolating water (Coussy et al., 2011; Benzaazoua restricted to 8–8.5. Fe2(SO4)3 is added to the effluent in order to et al., 2004b). CPB is typically formed using cement as the binder form Fe-precipitates (FEP) that facilitate As-immobilization. material and usually has a water content of 20–25%; this level of Tailings were provided by the mining company Dragon Mining water is required to enable the transport of the initially-formed AB and was sampled from approximately ten different sites on paste through pipe networks (Landriault, 2001). Because large the mine’s tailings dam and mixed to form a bulk sample. mines may produce millions of tonnes of tailings, the cost of the Results from previously conducted (Hamberg, 2014) sequential cement used when forming CPB can be substantial, and many stud- leaching tests on unmodified tailings, analyses of its particle size ies have therefore explored the scope for using alternative binders distribution, Acid–Base-Accounting (ABA) test, oxide analyses, or otherwise minimizing the amount of cement used in CPB and inductively coupled plasma atomic emission spectroscopy (Benzaazoua et al., 2004b; Peyronnard and Benzaazoua, 2012). (ICP-AES) determinations of S and As in the tailings are presented Granulated blast furnace slag (GBFS) and biofuel fly ash (BFA) have in Tables 1 and 2. successfully been used to partially replace cement in CPB due to Speciation of As and S in tailings was assessed using the modi- their pozzolanic and alkaline properties (Coussy et al., 2011; fied sequential extraction scheme described by Dold (2003) and Benzaazoua et al., 1999, 2002). It is generally believed that alkaline suggested that a majority (approx. 95 wt.%) of As in the tailings materials (fly ashes and slags) increase the mobility of As, but some of Svartliden was associated with the Fe (III) results indicate that they may decrease its mobility by promoting oxy-hydroxides-fraction (Table 2). The majority of S was associated the formation of stable Ca–As complexes (Hartley et al., 2004; with the water-soluble fraction and the primary sulfides fraction. Porter et al., 2004; Bothe and Brown, 1999). The sequential extraction tests suggested that the sulfide and sul- Studies of Benzaazoua et al. (2004a), Coussy et al. (2011, 2012), fate contents of the tailings accounted for 42% and 46% of their Randall (2012) has successfully used CPB to immobilize arsenopy- total S, respectively. rite, scorodite, and spiked As in natural and synthetic CPB. Water samples from the leaching tests were analyzed by an Proportions of binders used in these studies were 4–7% with a accredited laboratory (ALS Scandinavia, Luleå, Sweden) to deter- strength demand of 0.5–1 MPa. The mechanisms suggested for mine their elemental composition. The samples’ contents of As, the stability of As in these cases were: formation of Ca-arsenates, Cd, Cr, Cu, Mn, Ni, P, Pb and Zn were determined using inductively the entrapment of As in the cementitious matrix by sorption onto coupled plasma sector field mass spectroscopy (ICP-SFMS) while calcium-silicate-hydrates (C-S-H), or substitution of As into the their contents of Al, Ca, Fe, K, Mg, Na and S were determined by crystal lattice of secondary cementitious minerals. However, CPB ICP-AES. Before analysis, samples were acidified with 1 ml of super has also been evaluated as backfill of non-load bearing construc- pure nitric per 100 ml of sample. The ICP-SFMS and ICP-AES anal- tions or tailings dams (Yilmaz et al., 2014; Deschamps et al., yses were conducted according to the modified EPA methods 200.8 2011, 2008). In such cases the strength of the CPB-material is of and 200.7, respectively (USEPA, 1991). less concern but aims to prevent the release of metal ions and min- Solid samples of the binders used to form the CPB materials imize AMD still remains. So, when less strength is needed in the were analyzed for major and minor elements by the accredited CPB-material, how does it affect the leaching of arsenic? A former ALS Analytical laboratory. Their contents of As, Cd, Co, Cu, Ni, Pb, study of Cruz et al. (2001) has shown that low proportions of bin- S and Zn were determined after drying samples at 50 °C and then ders could be insufficient to suppress the generation of acid mine digesting them with 7 M nitric acid in closed Teflon vessels in a drainage. It is therefore important to investigate the chemical sta- microwave oven. Other elements were determined after fusion bility of As-bearing FEP in CPB materials where iron sulfides are with lithium metaborate followed by dissolution in diluted nitric occurring. At lower strength demands, proportions of binders used acid. The solutions were centrifuged and diluted before analysis. in CPB materials could be reduced. The aim of this study was, For quality control of the data, two in-house reference materials therefore, to evaluate the behavior of As in CPB-materials with were analyzed in parallel with the solid samples. The ICP-AES anal- low proportions of binders and its response to weathering and yses were conducted according to the modified EPA method 200.7 flooded conditions. In this study, mine operators considered that a compressive strength of 200 kPa was sufficient for the CPB-material. Recipes for CPB-materials are listed (in Table 3) Table 1 and were also chosen by considering economic criteria minimizing Results from ABA test, ICP-AES analyses of S and As, particle size distribution, (n =3, the binder proportions. The effect of cementation on As-leaching ±SD) in the Svartliden tailings. was evaluated by comparing CPB-materials and unmodified Element Units Tailings tailings. Total Solids (TS) % 89.0 ± 0.4

SiO2 % of TS 55.0 ± 4.9

Al2O3 % of TS 4.69 ± 0.04 2. Materials and methods CaO % of TS 4.83 ± 0.25 Fe2O3 % of TS 16.7 ± 0.6

K2O % of TS 0.92 ± 0.03 2.1. Tailings characteristics MgO % of TS 3.24 ± 0.01 MnO % of TS 0.14 ± 0.01 At a mine in Svartliden, Sweden, gold is extracted by cyanida- Na2O % of TS 0.82 ± 0.03 P O % of TS 0.24 ± 0.01 tion from inclusions in arsenopyrite (FeAsS), which is predomi- 2 5 TiO2 % of TS 0.18 ± 0.00 nantly associated with pyrrhotite (Bark and Weihed, 2007). The As mg/kg TS 1070 ± 30 major ore mineral assemblage in Svartliden consists of pyrrhotite, S mg/kg TS 20,933 ± 493 and arsenopyrite–loellingite. Chalcopyrite, pyrite, and localized AP 65.3 traces of sphalerite are also present in small amounts. The NP kg CaCO3/t 4.48 ± 1.31 arsenopyrite and pyrrhotite are primarily associated with silicates NNP kg CaCO3/t 60.8 ± 1.32 and magnetite (Sciuba, 2013). Efﬂuents from the enrichment pro- D10 lm 6.15 ± 0.03 D 91.0 ± 2.7 cess are treated with Fe2(SO4)3, CuSO4 and lime to immobilize As 50 and destroy toxic cyanides. Tailings slurries are also pre-treated D90 195 ± 8 76 R. Hamberg et al. / Minerals Engineering 78 (2015) 74–82

Table 2 Speciation of As and S in tailings and ore from Svartliden, extraction scheme adopted from Dold (2003) (values in % of total content) (n = 3, ±SD).

Fraction Water soluble Exchangeable Fe (III) oxyhydroxide Fe (III)-oxides Secondary sulﬁdes Primary sulﬁdes Residual Total (mg/kg) Ca 7.39 ± 0.35 0.50 ± 0.03 0.00 0.00 0.84 ± 0.10 3.77 ± 0.52 87.5 ± 4.3 45 258 ± 2552 Fe 0.09 ± 0.003 0.29 ± 0.01 30.0 ± 2.8 4.73 ± 0.3 0.23 3.59 ± 0.26 61.1 ± 3.1 121 772 ± 8122 As 0 0.81 ± 0.06 94.6 ± 3.2 1.81 ± 0.09 0.02 2.75 ± 0.38 0.03 ± 0 997 ± 50 S 35.0 ± 2.5 10.8 ± 0.8 6.99 ± 0.55 5.36 ± 0.36 4.41 ± 0.26 37.5 ± 2.7 0 14 259 ± 1378

Table 3 2.3.1. Flooded monoliths – Tank leaching test (TLT) Composition of tested cemented paste backﬁll mixtures (values reported in wt.%). Tests were conducted according to the Dutch standard EA NEN

Mixture FA CE H2O Tailings 7375:2004, which examines leaching from a monolithic block. CPB FA 5 5 25 70 samples were removed from their bottles after 31 days of curing FA 15 15 25 60 and shaped into regular cylinders. The resulting monoliths were CE 3.2 3 2 25 70 then placed inside tanks on 2 cm tall plastic supports, after which CE 1 1 26 73 the tanks were charged with distilled water until there was a 2 cm CE 3 3 26 71 water head surrounding the CPB sample and a liquid/solid (L/S) CE 5 5 25 70 3 2 CE–FA 1.2 1 2 26 71 ratio of 10 cm of solution per cm of exposed solid was estab- TA 11 89 lished. Reference samples (tailings) were placed in paper filter bags with 1 lm pores inside nylon filters, which were hung from the using a Perkin Elmer Optima DV 5300 instrument, and the lids of the tanks such that the tailings were fully immersed in ICP-SFMS analyses were conducted according to the modified the water. Duplicate experiments were conducted for each CPB EPA method 200.8 using a Thermo Scientific Element instrument. material and for the unmodified tailings, so in total six samples were leached. The leachate was removed and the tank was charged 2.2. Paste backfill samples preparation and mechanical strength with the same volume of fresh distilled water after 6 h, 1 day, 2.25 days, 4 days, 9 days, 16 days, 36 days and 64 days. In all cases, Two types of binders were tested in different paste mixtures: the leachate was stirred manually before collection and analysis. Portland cement (Ordinary Portland Cement, or CE) and biofuel The collected leachates were filtered through a 0.45 mm mem- fly ash (FA) from a biofuel incineration plant located in Lycksele, brane filter after which their pH, Eh and conductivity were mea- Sweden. Tailings (TA) were mixed with the binders and distilled sured and their elemental composition was determined by water was then added slowly until a of water content of 25– ICP-AES. 26 wt.%. The proportions of tailings and binders (Table 3) were tar- The mass transfer of As from the tailings and CPB materials was geting to achieve the desired hardened strength of 200 kPa while calculated using Eq. (2), as specified in the EA NEN 7375:2004 minimizing the binder content of the CPB. standard: CE and FA mixed with tailings were poured into plastic bottles ¼ð Þ= ð Þ with a diameter of 5 cm and a height of 10 cm. A small hole was Mti Ci V i A 2 drilled in the bottom of each bottle before filling to allow the drai- 2 Mti (in mg/m ) is the mass of As released during leaching period nage of excess water. After filling, the bottles were sealed and i, Ci (mg/l) is the As concentration for period i, Vi (L) is the leachate cured for 31 days at 22 °C in a moisturized environment. volume for period i, and A is the specimen surface area exposed to the leachate (m2). The apparent diffusion coefficient of As was 2.2.1. Mechanical strength (Unconfined Compressive Strength, UCS) determined by plotting the logarithm of the cumulative Mt until testing the i-th period against the logarithm of the time elapsed since The standard test of a material’s Unconfined Compressive the start of the experiment. Strength, UCS, (SIS, 2005) involves loading a cylindrical test piece at a steady rate until failure occurs. In this work, UCS tests were conducted on solidified samples of the test material that had been 2.3.2. Accelerated weathering – Weathering cell test (WCT) removed from the bottles in which they were prepared. The test A modified version of the WCT developed by Cruz et al. (2001) samples were then cut into pieces with lengths of 100 mm and was used. The CPB samples (hardened for 31 days) were crushed diameters of 50 mm, and weighed to determine their densities, and approximately 70 g of the resulting material was placed on a after which they were coated with a thin layer of cement-based paper filter in a Büchner-type funnel. The samples were then paste to create an even surface for testing. This paste was allowed to harden before the material’s compressive strength was tested. Table 4 The loading rate in the UCS tests was 1 mm/min. ICP-AES analysis of major elements, S, and As in CE and FA (n = 3, ±SD).

Unit FA CE 2.3. Leaching tests Total Solids (TS) % 95.2 ± 0.7 99.4 ± 0.0 SiO % of TS 34.6 ± 1.3 20.6 ± 0.8 For the environmental assessment, leaching tests (tank and 2 Al2O3 % of TS 10.7 ± 0.6 5.61 ± 0.45 weathering cell) were conducted to evaluate the behavior of As CaO % of TS 14.1 ± 1.0 50.3 ± 1.8 in the CPB materials and tailings. Tank leaching tests (TLTs) were Fe2O3 % of TS 13.9 ± 1.0 2.81 ± 0.05 conducted to simulate the behavior of ﬂooded CPB materials and K2O % of TS 2.89 ± 0.08 0.83 ± 0.05 weathering cell tests (WCTs) were conducted to simulate the MgO % of TS 2.54 ± 0.08 4.00 ± 0.17 MnO % of TS 0.92 ± 0.03 0.10 ± 0.01 (accelerated) effect of weathering on CPB materials and tailings. Na2O % of TS 1.24 ± 0.08 0.65 ± 0.07

All TLTs and WCTs were conducted in duplicate, under oxidizing P2O5 % of TS 2.25 ± 0.05 0.06 ± 0.00 conditions. The acidity of the solutions used in the WCT was TiO2 % of TS 0.34 ± 0.01 0.42 ± 0.02 adjusted over time to mimic the gradual depletion of buffering As mg/kg TS 124 ± 5 10.2 ± 0.2 S mg/kg TS 13,700 ± 200 9960 ± 219 minerals. R. Hamberg et al. / Minerals Engineering 78 (2015) 74–82 77 subjected to weekly cycles involving one day of leaching, three 600 FA 5 wt % days of ambient air exposure, another day of leaching and ﬁnally FA 15 wt % 501 500 CE (2 wt %) + FA (3 wt %) two days of air exposure. Leaching was performed with 50 ml of CE (2 wt %) + FA (1 wt %) CE 1 wt % 426 deionized water at a time over approximately 2 h, after which CE 3 wt% the leachate was recovered by applying suction to the funnel. 400 CE 5 wt % 341 The pH, Eh and conductivity of the leachates were measured and 334 300

their elemental composition was analyzed by ICP-AES after ﬁltra- kPa 214 tion. Duplicate weathering tests were performed for each CPB sam- 207 198 200 200 184 179 ple, so 4 cells were set up. The test was conducted over a period of 32 cycles (217 days). During cycles 13–18, 1 M HCl was added to 100 consume the buffering minerals within the CPB-materials and 23 26 26 28 stimulate the formation of acid mine drainage (AMD). 0 28 60 Curing time (days) 3. Results Fig. 1. Results from unconﬁned compressive strength tests on prospective CPB- 3.1. The characteristics of the binder materials used for CPB materials, hardened for 28 and 60 days.

3.1.1. Elemental composition of the binder materials 3.2.2. Weathering cell test See Table 4. In the weathering cell tests, the cumulative As release from CE (0.65 mg/kg) after 52 days was greater than that from CE–FA 3.1.2. Characterization of binder material and CPB mixtures (0.42 mg/kg) or the unmodified tailings (0.29 mg/kg). The trends The composition of the CPB mixtures was adjusted to achieve in pH for CE and CE–FA were similar; in both cases, the pH of the the desired hardened strength of 200 kPa while minimizing the washings declined from 11.4 (CE–FA) or 10.1 (CE) to a stable value amount of binder used. of approx. 8 after 10 days (Fig. 3). Small deviations are observed for the duplicated tests of tailings, CE and CE–FA, average values are 3.1.3. Mechanical strength (Unconfined Compressive Strength, UCS) therefore presented. The mixtures of CE 1 wt.% and CE (2 wt.%)–FA (1 wt.%) gained a The release of As from CPB samples seemed to be linked to the mechanical strength of approx. 200 kPa with a minimal proportion changes in pH, peaking at pH values of 8–10. The amount of As lea- of binders and were chosen for further studies. The CE mixture ched from CE increased progressively over the first 24 days of the achieved the highest compressive strength, exhibiting a value of experiment and was substantially greater than that from CE–FA 214 kPa after 60 days’ curing (see Fig. 1). or tailings during the same period. Similar trends were observed for the release of Ca, S, Fe, As and Si from CE, CE–FA and tailings: 3.2. Leaching tests in all cases, the extent of leaching increased progressively over the first 24 days and then stabilized, increasing only slightly 3.2.1. Tank leaching test (TLT) between days 24 and 52. The cumulative release of S, Fe and Si The highest As concentrations released during a leachate from tailings was greater than that from CE or CE–FA. The leaching renewal cycle for CE, CE–FA, and unmodified tailings were of Ca and S evolved in the same way as that of the other elements, 41.3 mg/m2, 13.7 mg/m2, and 1.6 mg/m2, respectively. The largest with an initial sharp increase to 400–600 mg/kg over the first and smallest cumulative releases of As occurred in the CE 4 days of the experiment followed by a stabilization, with the (101.5 mg/m2) and tailings (5.5 mg/m2). As shown in Fig. 2, the cumulative amount of material released after day 24 being only amount of As that leached from the tailings during the tank leach- 40–70 mg/kg. The cumulative release of Fe was more than ing tests declined over the course of the experiments whereas that 100-fold higher from tailings (363 mg/kg) than from CE for both CPB mixtures increased over time. The trends in the (3.15 mg/kg) or CE–FA (3.30 mg/kg). The release of Fe from the release of As, Ca and S from the CE monolith were quite similar CPB materials mirrored that of As, peaking as the pH of the lea- to those for CE–FA, but the amount of S released from the tailings chate fell from 10 (CE) or 11 (CE–FA) to 8. was comparatively high. The release of S from both CPB monoliths increased progressively toward the end of the test as the solutions 3.2.3. Accelerated weathering in the presence of added acid became more oxidized. In the middle of the test a reddish crust After 70 days, acid was added to the leachate in the weathering appeared on the unmodified tailings surface. Small deviations are experiments to consume the buffering minerals in the observed for the duplicated tests of tailings, CE and CE–FA, average CPB-materials. Different amounts of acid (1 M HCl) were added values are therefore presented. over 77 days (between days 70 and 147) to each CPB material A wash-off process, in which surface-deposited As dissolves or (CE: 0.47 M H+/kg TS and CE–FA: 0.69 M H+/kg TS) reducing the is otherwise released from the surface of the tailings or CPB mate- pH in each case to approximately 4.5, i.e. the initial value deter- rial is expected to give rise to a graph with a slope (rc) of <0.4 when mined for the unmodified tailings. Small deviations are observed the logarithm of the total As mass transfer is plotted against the for the duplicated tests of tailings, CE and CE–FA, average values logarithm of the time elapsed during the experiment. Conversely, are therefore presented. diffusion-controlled As release should produce a straight-line The addition of acid increased the leaching of Ca, Si, As and Fe graph with a slope of 0.4–0.6. The sharp initial increase in the from CE and CE–FA. However, the acid treatment had no significant arsenic concentration of the leachates from CPB samples can be effect on S-leaching from CE or CE–FA. The cumulative As release at attributed to wash-off controlled processes but diffusion becomes 187 days was higher for CE (1.74 mg/kg) than for tailings largely responsible for arsenic release during the later stages of the (1.50 mg/kg) or CE–FA (1.20 mg/kg). The addition of acid also experiment. According to calculations based on the EA NEN increased the leaching of Si, Fe and As from CE more strongly than 7375:2004 standard, the As leaching from the tailings was due to from CE–FA. In addition, the leaching of Fe and As from CE was the wash-off-effect whereas that from the CPB materials was diffu- more progressive than that from CE–FA. The endpoint pH values sion controlled (Table 5). for CE and CE–FA were approximately 4.5 whereas that for the 78 R. Hamberg et al. / Minerals Engineering 78 (2015) 74–82

16000 12000 Ca S 14000 10000 12000 8000

2 10000 2 8000 6000 mg/m 6000 mg/m 4000 4000 2000 2000 0 0 12345678 12345678

10000 45 Fe As 40 1000 35

2 30 2 100 25 mg/m

mg/m 20 10 15 1 10 5 0.1 0 12345678 12345678

12 600 pH Si 10 500

8 400 2 6 300 mg/m 4 200

2 100

0 0 12345678 12345678 Extraction Extraction CE-FACE Tailings

Fig. 2. Evolution of the leachate pH and leaching of S, Ca, Fe, As and Si from tailings, CE and CE–FA (in mg/m2) during the TLT (average values presented).

Table 5 off from the tailings surface (Voglar and Leštan, 2013). Saturated Mass transfer mechanisms of As in tailings and CPB materials during TLT (n = 3, ±SD). conditions during the TLT limited potential for direct sulfide oxida- 2 Mix Cumulative As mg/m rc Mechanism tion by oxygen. So, initial leaching of As could be due to a presence of As (III) in the tailings pore water existing as a consequence of Tailings 5.5 ± 0.7 0.21 ± 0.09 Wash-off CE 98 ± 4 0.62 ± 0.01 Diffusion incomplete pre-oxidation of As (III) to As(V) during the tailings CE–FA 53 ± 0 0.46 ± 0.13 Diffusion treatment process. The As release declined and were almost diminished after the 2nd extraction in the TLT. A reddish crust formed on the surface of the tailings mid-way through the TLT. This was tailings was approx. 3.2. In WCT, conducted with unmodified tail- assumed to be due to the formation of an enclosing ings, leaching of As, Fe and S increased while pH <3.5 (Fig. 4). iron-oxide-precipitate reducing permeability and the rate of weathering enhancing the wash-off effect. In the WCT, leaching of As increased most extensively when pH 4. Discussion decreases below 3.5 in the end of the test (Fig. 4). At this stage, the Fe/S molar ratio in the leachate increased to 0.8, suggesting that 4.1. As-leaching in unmodified tailings pyrrhotite had been oxidized over the course of the experiment. In unsaturated conditions prevailing during WCT leaching of As could As-bearing FEP with Fe/As-ratios <3 are stable at a pH of 4–8 originate from the destabilization of As-bearing FEP and the oxida- (Riveros et al., 2001). The pH-stability range increases with higher tion of As-sulfides that remains in the tailings at small quantities Fe/As-ratios and the presence of Ca2+ and Zn2+ (Paktunc et al., (2.75 ± 0.38 wt.%). 2008). However, as suggested by sequential extraction tests (Table 2), small proportions of As-sulfides were still present in the tailings and could contribute to an As-release. At more acidic 4.2. The use of low binder proportions in CPB – Effects on As-leaching (pH < 4) conditions, oxidation of arsenopyrite and pyrrhotite are accelerated (Blowes et al., 1998). The tailings released relatively In cementitious environments, the reactivity of arsenopyrite small quantities of As during the TLT when pH 3.8–5.7. The becomes quickly passivized but pyrrhotite remains active As-release was not correlated with pH and standard calculations (Benzaazoua et al., 2004a). This would suggest that As-leaching in suggest that the evolution of arsenic leaching in tailings during CPB-materials largely originate from secondary As-phases. The TLT was dominantly a wash-off-effect where arsenic was rinsed pH-variation in the TLT is related to leaching of Ca2+ from the R. Hamberg et al. / Minerals Engineering 78 (2015) 74–82 79

3000 2500 S Ca 2500 2000 2000 1500 1500 mg/kg mg/kg 1000 1000

500 500

0 0 0 102030405060 0102030405060

400 3.5 0.8 Fe As 350 3.0 0.7 300 0.6 2.5 250 0.5 2.0 200 0.4 1.5 mg/kg 150 mg/kg 0.3 1.0 100 0.2 0.5

50 CE and CE-FA (mg/kg) 0.1 0 0.0 0 0 102030405060 0102030405060

14 35 pH Si 12 30

10 25

8 20

6 mg/kg 15 mg/kg

4 10

2 5

0 0 0 102030405060 0102030405060 Days Days CE-FACE Tailings

Fig. 3. Evolution of the leachate pH and cumulative leaching of S, Ca, Fe, As and Si from tailings, CE and CE–FA over the ﬁrst 52 days of the weathering cell tests (average values presented). dissolution of the cementitious binders. The oxidation of Fe-sulﬁdes during the TLT was initially lower than that from the tailings. in cementitious media consumes hydroxyl ions (OH) and could This may have been due to the ability of cementitious oxides contribute to the excessive release of sulfates in leachates as well (especially MnO) to oxidize As (III) to As(V) (Cornelis et al., as the precipitation of Fe-oxides (Benzaazoua et al., 1999) (reaction 2008). Leaching of As from CPB materials was most intensive when (3)). Iron tends to precipitate in the water/sample interface while the pH fell from 10 to 8 at the end of the TLT (Fig. 2). In alkaline sulfates are released into the leachate. conditions during TLT (pH 7.7–10.4), arsenic could have desorbed þ þ ð Þ! ð Þ þ 2 þ ð Þ from FEPs and re-precipitate as Ca-arsenates, sorbed onto 4FeS2 15O2 16 OH 4Fe OH 3 8SO4 2H2O 3 2 calcium-silicate-hydrates (C-S-H), or substituted for SO4 in the At pH <9, all hydrated cement phases are assumed to have crystal lattice of secondary cementitious minerals (i.e. gypsum, been dissolved, resulting in the formation of expansive gypsum ettringite). The mechanism of As leaching from CPB materials (CE and ettringite (Benzaazoua et al., 1999) increasing As, Ca, and Si and CE–FA) was different to that in the tailings. Initially, cementa- contents of the leachate. C-S-H surfaces contain SiO moieties tion hindered water to percolate through the CPB-materials and that can repel the anionic arsenic species formed under alkaline the release of As would have occurred by rinsing the surfaces of conditions (10–13). The mechanism of As bonding to C-S-H has the monoliths. However, as the cementitious phases dissolved been related to the availability of Ca2+, which acts as a (indicated by a Si-release) (Fig. 2), permeability increased and charge-balancing counter ion that increases the positive charge As-leaching became more diffusion-driven where As could spread on surfaces amenable to As-binding, the normally positively through the CPB-matrix. This is corroborated by calculations per- charged C-S-H-surfaces may become negatively charged if the formed according to the TLT standard (Table 5). Consequently, Ca/Si-ratio falls below 1.2 (Phenrat et al., 2005). However, the the release of As from CE and CE–FA indicate that no plateau was availability of Ca2+ may also increase the surface charge on the reached, so the release of As could have continued after the surfaces of FEPs and increase their ability to adsorb As(V), espe- 64-day experimental period. The amount of As released from CE cially at pH <9 (Wilkie and Hering, 1996). The availability of was greater than that from CE–FA during TLT, probably because Ca2+ therefore strongly affects the immobilization of As in cemen- the greater content of binder material in CE–FA allowed less water titious systems. to percolate to the matrix and increased the availability of Ca2+. Cementation reduced the Fe contents of the TLT leachates rela- The release of As from CE–FA was more strongly correlated with tive to those observed for the tailings, but increased leaching of S, that of Si (Fig. 5) than was the case for CE, suggesting that more Ca, As and the leachate pH. As-leaching from the CPB materials arsenic was associated with C-S-H (Coussy et al., 2011). 80 R. Hamberg et al. / Minerals Engineering 78 (2015) 74–82

4000 3000 S 3500 Ca 2500 3000 2000 2500 2000 1500 mg/kg 1500 mg/kg 1000 1000 500 500 0 0

1600 90 2.0 Fe As 1400 80 1.8 1.6 1200 70 60 1.4 1000 1.2 50 800 1.0 40 600 mg/kg 0.8 30

Tailings (mg/kg) 0.6 400 20 0.4 CE-FA, CE (mg/kg) 200 10 0.2 0 0 0.0

9 pH 40 8 Si 7 30 6 5 20 4 mg/kg 3 2 10 1 0 0 50 75 100 125 150 175 200 225 50 75 100 125 150 175 200 225 Days Days

CE-FACE Tailings

Fig. 4. Evolution of the leachate pH and cumulative leaching of S, Ca, Fe, As and Si from tailings, CE and CE–FA between days 52 and 217 of the weathering cell tests (average values presented).

The greater leaching of As from CE compared to CE–FA could phases (Benzaazoua et al., 2004). Weathering during WCT therefore be due to the lower levels of Ca2+ in the former but increased the availability of Ca2+ in CPB-leachates, which may have may also be partially due to the formation of greater quantities increased the positive charge on the FEP surfaces. of Ca-arsenates. These Ca-arsenates are stable at pH >10 but become very soluble at lower pH values (Benzaazoua et al., 2004a). 4.3. The effect of accelerated weathering on the stability of As in CPB As in TLT, the release of As in CE during the WCT was greater materials than that from CE–FA when the pH fell from 10 to 8. However, this pH change was more rapid during WCT, occurring within 10 days The addition of acid to leachate solutions during the later stages of the start of the experiment. The mechanisms of As leaching dur- of the WCT was conducted to simulate accelerated weathering and ing WCT differed from those for TLT because the exposed surface determine its effects on As leaching. The addition of acid increased area in WCT is greater and the sample is more susceptible to oxi- the leaching of As, Fe, Si, and Ca but had no signiﬁcant effect on dation. Surﬁcial conditions could have a negative effect on that of S (Figs. 4 and 6). As-leaching in the CPB-materials while carbonation (which Arsenic released from the cementitious phases could potentially depends on the availability of CO2) is known to have a destabilizing be re-adsorbed onto FEP surfaces, the high sulfate concentrations effect on Ca-arsenates (Krause and Ettel, 1989) and cementitious formed under these conditions would compete with As(V) species

1.2 25 1.8 20 CE-FA CE 1.6 18 1.0 20 1.4 16 14 0.8 1.2 15 12 1 0.6 10 0.8 10 8 As (mg/kg) Si (mg/kg) Si (mg/kg) As (mg/kg) 0.4 0.6 6 5 0.4 4 0.2 0.2 2 0.0 0 0 0 12345678 012345678 Extraction Extraction As Si

Fig. 5. Changes in the Si and As contents of the TLT leachates produced from CE–FA and CE. R. Hamberg et al. / Minerals Engineering 78 (2015) 74–82 81

3000 surfaces, reducing their capacity for As adsorption. Unmodified tailings generated an acidic environment in which As-bearing 2500 Acid addition FEPs were stable. A crust of Fe-oxides enclosing the unmodified 2000 tailings was formed during flooded conditions, which decreased the surface area susceptible for oxidation. In the CPB mixtures, 1500 small (>1%) proportions of the total As in the solid material was Tailings 1000 released. CE 500 CE-FA Acknowledgements 0 Cumulative Ca+Mg+Mn (mg/kg) 0 500 1000 1500 2000 2500 3000 3500 Financial support from Ramböll Sverige AB, Ramböll Cumulative S (mg/kg) Foundation, SUSMIN – Tools for sustainable gold mining in EU,

Fig. 6. The relationship between the levels of major neutralizing elements and S in and the Center of Advanced Mining and Metallurgy (CAMM) at the leachate during WCT. Results obtained following acid treatment are enclosed Luleå University of Technology are gratefully acknowledged. The within the ellipse. personnel at Dragon Mining AB are also gratefully acknowledged for valuable information about the mining processes and for providing the tailings needed for this study. for FEP binding (Frau et al., 2010). This negative effect could be counteracted by the co-adsorption of Ca2+, which may function References as a multi-absorbent for As at pH values of 4–9 (Jia and

Demopoulos, 2005; Wilkie and Hering, 1996). The cumulative Asta, M.P., Pérez-López, R., Román-Ross, G., Illera, V., Cama, J., Cotte, M., Tucoulou, release of As and Fe from CE was greater than from CE–FA. R., 2013. Analysis of the iron coatings formed during marcasite and arsenopyrite This suggests further that higher binder content introduces more oxidation at neutral-alkaline conditions. Geol. Acta 11 (4), 465–481. 2+ Bark, G., Weihed, P., 2007. Orogenic gold in the new Lycksele–Storuman ore Ca which could enhance the ability of FEP to adsorb As. province, northern Sweden; the Palaeoproterozoic Fäboliden deposit. Ore Geol. Fe-precipitates originated from sulfide oxidation at alkaline condi- Rev. 32 (1–2), 431–451. tions (reaction (3)) could have formed permeable crusts covering Belzile, N., Chen, Y., Cai, M., Li, Y., 2004. A review on pyrrhotite oxidation. J. the sulfide minerals´ surfaces retarding their further oxidation Geochem. Explor. 84 (2), 65–76. Benzaazoua, M., Quellet, J., Servant, S., Newman, P., Verburg, R., 1999. Cementitious (Belzile et al., 2004; Asta et al., 2013). backfill with high sulfur content physical, chemical, and mineralogical characterization. Cem. Concr. Res. 29 (5), 719–725. 4.4. Future aspects concerning the use of CPB based on As-rich tailings Benzaazoua, M., Belem, T., Bussière, B., 2002. Chemical factors that influence the performance of mine sulphidic paste backfill. Cem. Concr. Res. 32 (7), 1133– from cyanidation 1144. Benzaazoua, M., Marion, P., Picquet, I., Bussière, B., 2004a. The use of pastefill as a Most of the As in the tailings were present as As-bearing FEP: S solidification and stabilization process for the control of acid mine drainage. Miner. Eng. 17 (2), 233–243. which constituted approximately 95% of the total As. Arsenic in Benzaazoua, M., Fall, M., Belem, T., 2004b. A contribution to understanding the these precipitates is sensitive to reducing, extreme acidic or alka- hardening process of cemented pastefill. Miner. Eng. 17 (2), 141–152. line environments. Arsenic dissolutions under reductive environ- Blowes, D.W., Jambor, J.L., Hanton-Fong, C.J., Lortie, L., Gould, W.D., 1998. Geochemical, mineralogical and microbiological characterization of a ment may contribute to the As release into solution when the sulphide-bearing carbonate-rich gold-mine tailings impoundment, Joutel, management of tailings are not adequately designed. Although, Quebec. Appl. Geochem. 13 (6), 687–705. results from the tailings TLT showed that As-bearing FEPs may Bothe Jr., J.V., Brown, P.W., 1999. The stabilities of calcium arsenates at 23 ± 1 °C. J. Hazard. Mater. 69 (2), 197–207. be stable under acidic and oxidizing conditions common to the Cornelis, G., Johnson, C.A., Gerven, T.V., Vandecasteele, C., 2008. Leaching tailings piles. Increased molar Fe/S-ratios (up to 0.8) in the WCT mechanisms of oxyanionic metalloid and metal species in alkaline solid indicated that more extensive pyrrhotite oxidation generating wastes: a review. Appl. Geochem. 23 (5), 955–976. Coussy, S., Benzaazoua, M., Blanc, D., Moszkowicz, P., Bussière, B., 2011. Arsenic acidic conditions may increase As-leaching on a long term basis. stability in arsenopyrite-rich cemented paste backfills: a leaching test-based Results from TLT suggested that the addition of binders may assessment. J. Hazard. Mater. 185 (2–3), 1467–1476. increase As-leaching on a short term basis. However, the addition Coussy, S., Benzaazoua, M., Blanc, D., Moszkowicz, P., Bussière, B., 2012. Assessment of binders increased the tailings’ acid-neutralizing capacity and of arsenic immobilization in synthetically prepared cemented paste backfill specimens. J. Environ. Manage. 93 (1), 10–21. introduced more Fe precipitates into the tailings matrix, both of Craw, D., Falconer, D., Youngson, J.H., 2003. Environmental arsenopyrite stability which may reduce the potential for sulfide oxidation. Pyrrhotite and dissolution: theory, experiment and field observations. Chem. Geol. 199, oxidation is not obstructed, but in alkaline conditions, Fe may pre- 71–82. Cruz, R., Bertrand, V., Monroy, M., González, I., 2001. Effect of sulfide impurities on cipitate on the surfaces of sulfide grains retarding its further oxida- the reactivity of pyrite and pyritic concentrates: a multi-tool approach. Appl. tion (reaction (3)). Hydroxyl anions are consumed faster (due to Geochem. 16 (7–8), 803–819. lower proportions of binder) and more As is released in CE com- Deschamps, T., Benzaazoua, M., Bussière, B., Aubertin, M., Belem, T., 2008. Microstructural and geochemical evolution of paste tailings in surface pared to that in CE–FA, but conduction of more extensive tests disposal conditions. Miner. Eng. 21 (4), 341–353. are needed to access the long-term stability of As. A high degree Deschamps, T., Benzaazoua, M., Bussière, B., Aubertin, M., 2011. Laboratory study of of saturation in the material reduces the intrusion of oxygen and surface paste disposal for sulfidic tailings: physical model testing. Miner. Eng. 24 (8), 794–806. water percolation, this could however lead to reduced conditions Dold, B., 2003. Speciation of the most soluble phases in a sequential extraction in which As-bearing FEP are less stable. procedure adapted for geochemical studies of copper sulfide mine waste. J. Geochem. Explor. 80 (1), 55–68. EA NEN 7375:2004, 2004. Leaching characteristics of moulded or monolithic 5. Conclusions building and waste materials. Determination of leaching of inorganic components with the diffusion test. The tank test. The addition of low proportions of binders to tailings increased Frau, F., Addari, D., Atzei, D., Biddau, R., Cidu, R., Rossi, A., 2010. Influence of major anions on As(V) adsorption by synthetic 2-line ferrihydrite. Kinetic As leaching due to the relocation of desorbed As from FEPs into less investigation and XPS study of the competitive effect of bicarbonate. Water acid-tolerant species such as Ca-arsenates and cementitious Air Soil Pollut. 205 (1–4), 25–41. As-phases. Compared to CE–FA, CE contained lower levels of bin- Genkin, A.D., Bortnikov, N.S., Cabri, L.J., Wagner, F.E., Stanley, C.J., Safonov, Y.G., Gamyanin, G.N., 1998. A multidisciplinary study of invisible gold in ders and Ca-ions in its cementitious matrix. Consequently, it may arsenopyrite from four mesothermal gold deposits in Siberia, Russian have had a lower positive charge density on its FEP and C-S-H Federation. Econ. Geol. 93 (4), 463–487. 82 R. Hamberg et al. / Minerals Engineering 78 (2015) 74–82

Hamberg, R., 2014. Characterization and solidification of arsenic-rich cyanidation Porter, S.K., Scheckel, K.G., Impellitteri, C.A., Ryan, J.A., 2004. Toxic metals in the tailings. Licentiate Thesis. Department of Civil, Environmental and Natural environment: thermodynamic considerations for possible immobilization Resources Engineering, Division of Geosciences and Environmental Engineering. strategies for Pb, Cd, As, and Hg. Crit. Rev. Environ. Sci. Technol. 34 (6), 495–604. Luleå University of Technology, Luleå, Sweden. Randall, P.M., 2012. Arsenic encapsulation using Portland cement with ferrous Hartley, W., Edwards, R., Lepp, N.W., 2004. Arsenic and heavy metal mobility in iron sulfate/lime and terra-bond™ technologies – microcharacterization and oxide-amended contaminated soils as evaluated by short- and long-term leaching studies. Sci. Total Environ. 420, 300–312. leaching tests. Environ. Pollut. 131 (3), 495–504. Riveros, P.A., Dutrizac, J.E., Spencer, P., 2001. Arsenic disposal practices in the Hinton, J.J., Veiga, M.M., Veiga, A.T., 2003. Clean artisanal gold mining: a utopian metallurgical industry. Can. Metall. Quart. 40 (4), 395–420. approach? J. Clean. Prod. 11 (2), 99–115. Sciuba M., 2013. Mineralogy and geochemistry of the banded iron-formation in the INAP (International Network for Acid Prevention), 2009. Global Acid Rock Drainage Svartliden gold deposit, Northern Sweden. Master Thesis. Department of Civil, Guide (GARD Guide). The acid rock drainage process (chapter 2). Environmental and Natural Resources Engineering, Division of Geosciences and Jia, Y., Demopoulos, G.P., 2005. Adsorption of arsenate onto ferrihydrite from Environmental Engineering. Luleå University of Technology, Luleå, Sweden. aqueous solution: influence of media (sulfate vs nitrate), added gypsum, and pH SIS, 2005. Geoteknisk undersökning och provning – Laboratorieundersökning av alteration. Environ. Sci. Technol. 39 (24), 9523–9527. jord – Del 7: Enaxligt tryckförsök på finkorning jord (ISO/TS 17892-7:2004) (in Krause, E., Ettel, V.A., 1989. Solubilities and stabilities of ferric arsenate compounds. Swedish). Hydrometallurgy 22 (3), 311–337. USEPA, 1991. Methods for the determination of metals in environmental samples. Landriault, D., 2001. Backfill in underground mining. In: Hustrulid, W.A., (Ed.), Environmental Protection Agency, Office of research and development, Underground Mining Methods Engineering Fundamentals and International Washington, DC. EPA 600/4-91/010. Case Studies, SME, USA. pp. 608–609. Voglar, G.E., Leštan, D., 2013. Equilibrium leaching of toxic elements from cement Paktunc, D., Dutrizac, J., Gertsman, V., 2008. Synthesis and phase transformations stabilized soil. J. Hazard. Mater. 246–247, 18–25. involving scorodite, ferric arsenate and arsenical ferrihydrite: implications for Wilkie, J.A., Hering, J.G., 1996. Adsorption of arsenic onto hydrous ferric oxide: arsenic mobility. Geochim. Cosmochim. Acta 72 (11), 2649–2672. effects of adsorbate/adsorbent ratios and co-occurring solutes. Colloids Surf. A Peyronnard, O., Benzaazoua, M., 2012. Alternative by-product based binders for 107, 97–110. cemented mine backfill: recipes optimisation using Taguchi method. Miner. Yilmaz, E., Benzaazoua, M., Bussière, B., Pouliot, S., 2014. Influence of disposal Eng. 29, 28–38. configurations on hydrogeological behaviour of sulphidic paste tailings: a field Phenrat, T., Marhaba, T.F., Rachakornkij, M., 2005. A SEM and X-ray study for experimental study. Int. J. Miner. Process. 131, 12–25. investigation of solidified/stabilized arsenic–iron hydroxide sludge. J. Hazard. Mater. 118 (1–3), 185–195.

Paper III

Lowering the water saturation levels in cemented paste backfill mixtures – effect on the release of Arsenic

Roger Hamberg, Christian Maurice and Lena Alakangas

Published in:

Minerals Engineering, 112, 84-91. (2017)

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Contents lists available at ScienceDirect

Minerals Engineering

journal homepage: www.elsevier.com/locate/mineng

Lowering the water saturation level in cemented paste backﬁll mixtures – Eﬀect on the release of arsenic ⁎ R. Hamberg , C. Maurice, L. Alakangas

Department of Civil, Environmental and Natural Resources Engineering, Division of Geosciences and Environmental Engineering, Luleå University of Technology, 971 87 Luleå, Sweden

ARTICLE INFO ABSTRACT

Keywords: The Cemented Paste Backfill (CPB) method allows the mixing of dewatered tailings slurries with cementitious Arsenic binders to backfill excavated underground workings. After mine closure, CPB workings are permanently flooded Cyanidation tailings by rising groundwater. This flooding is considered beneficial for reducing the risk of acid generation associated Flooding with CPB containing sulphide minerals. In general, CPB workings are slowly flooded and the process may lead to Cemented paste backfill regions with a low degree of water saturation to form within the CPB. This in turn, may increase oxygen ingress in the CPB, thereby prolonging oxidation of the minerals. To investigate the environmental impact of this oxidation, tailings containing elevated concentrations of arsenic (As) and pyrrhotite were handled via CPB. In this study, CPB mixtures containing 1–3 wt.% of cementitious binders and tailings was studied. The water saturation level in the CPB-mixtures was lowered as curing time extended. In mimicked flooded conditions, the mobility of As in the CPB mixtures was correlated with As-bearing cementitious phases that are sensitive to a reduction in the pH. In CPB-mixtures with lower proportions of binders, cementitious As-phases dissolved while the water saturation level decreased to form more stable As-phases. Increasing binder fractions, most of the cementitious As-phases persisted in the CPB while water saturation levels were lowered and release of As increased. Regardless of curing conditions, managing these tailings via the CPB method yielded increased mobility of As compared with that in the unmodified tailings; this resulted possibly from the formation of less acid-tolerant As species.

1. Introduction Arsenates) could also form during the cyanide leaching process, due to the interaction of Ca-rich, alkaline solutions (by the addition of Ca

Arsenic (As) minerals (i.e. arsenopyrite and loellingite) are typical (OH)2) containing amorphous ferric arsenates (Paktunc et al., 2015). gold-bearing constituents in gold ore deposits. These minerals often co- The stability of As in these precipitates varies with the pH. For example, occur with other sulphide minerals, such as pyrite and pyrrhotite. Gold under acidic conditions, the Fe-oxides dissolve and As is released, encapsulated in these minerals is typically extracted by using cyanide. whereas As adsorbed on the surfaces of the oxides can be desorbed To increase the gold-extraction eﬃciency, an aeration step (addition of under alkaline conditions (Riveros et al., 2001).

H2O2 and O2) is added with the aim of oxidizing sulphide minerals to Tailings are typically managed by using a cemented paste backfill facilitate cyanide dissolution of gold (Mesa Espitia and Lapidus, 2015). (CPB). In CPB, tailings are dewatered, converted into a monolithic mass The tailings slurry produced in this process must be treated to im- through the addition of low fractions (3–7%) of cementitious binders mobilize As before discharge at the storage facility (Kyle et al., 2012; (i.e. cement and biofuel fly ash), and backfilled into underground mine Deschênes et al., 1998; Paktunc et al., 2004). The As in these slurries is cavities (Coussy et al., 2011; Benzaazoua et al., 1999, 2002). The re- usually immobilized by adding an iron sulphate salt that promotes the sulting monoliths can enhance the geotechnical properties/mechanical formation of iron arsenates (reaction (1)). strength of the surrounding rock, thereby increasing the amount of ore that can be excavated (Kesimal et al., 2005; Fall et al., 2007). In a CPB 2H AsO++→ Fe (SO ) 3Ca(OH) 2FeAsO (s) + 3CaSO ·2H O(s) 34 243 2 4 42 material, the physical strength of the monolith stems mainly from the + 6H2 O (1) formation of Calcium-Silicate-Hydrates (C-S-H) (Peyronnard and However, As-species such as arseniosiderite and yukonite (Ca-Fe- Benzaazoua, 2012). The strength of CPB materials is governed by the sulphide and sulphate content, curing time, as well as the type and

⁎ Corresponding author. E-mail address: [email protected] (R. Hamberg). http://dx.doi.org/10.1016/j.mineng.2017.05.005 Received 1 February 2017; Received in revised form 15 May 2017; Accepted 30 May 2017 $YDLODEOHRQOLQH-XO\ (OVHYLHU/WG$OOULJKWVUHVHUYHG R. Hamberg et al. 0LQHUDOV(QJLQHHULQJ ² fraction of binder material (Ercikdi et al., 2009; Kesimal et al., 2005; materials cured under conditions of low water saturation is therefore Benzaazoua et al., 2004a). Using CPB results in increased saturation essential. In this regard, the objectives of this study were to: levels in the material, and reduced availability of oxygen (Ouellet et al., 2006) and may therefore lead to a decrease in acid mine drainage - Lower the water saturation level in CPB mixtures, and assess the (AMD). mobility of As Arsenopyrite and loellingite often co-occur with other sulphide - Evaluate the mobility of As in such CPB mixtures during an event of minerals, such as pyrite and pyrrhotite. Pyrrhotite is a highly reactive flooding sulphide mineral which generates acid upon contact with water and air (Belzile et al., 2004). If pyrrhotite is left in cyanidation tailings, further The aim of this investigation was to predict the post-mining water actions are needed to avoid acidic conditions and thereby preserve the quality, to prevent acid generation and the occurrence of high As stability of the iron arsenates. The oxidation of pyrrhotite in the CPB concentrations in mine drainage. mixtures may contribute to the excessive release of sulphates in the leachates (reaction (2))(Benzaazoua et al., 1999). In CPB mixtures based on tailings from a cyanide leaching process (CT), sulphates could 2. Material and methodology also origin from the destruction of cyanide (INCO SO2-air process) but also from the binder themselves. These sulphates react with the binders, Tailings were collected from a gold mine in the north of Sweden. thereby forming gypsum and ettringite (a sulphate attack; reactions (3) Effluents from the cyanidation process are treated with the INCO SO2- and (4)) that may generate cracks and increase the availability of air process, H2O2,Fe2(SO4)3 and lime to immobilize As and destroy oxygen in the CPB material (Ercikdi et al., 2009). This phenomenon is toxic cyanides. Hydrogen peroxide (H2O2) and Oxygen gas (O2)is referred to as a sulphate attack (Benzaazoua et al., 1999; Kesimal et al., added aiming to oxidize arsenopyrite and pyrrhotite to facilitate the 2005), where the formation of expansive phases (such as gypsum and cyanide dissolution of gold. The pH of the outlet water is restricted to ettringite) may lead to decreased strength of the CPB material. In sul- 8–8.5. Fe2(SO4)3 is added to the effluent in order to form iron arsenates. phidic CPB materials, the dissolution of these cementitious binders The content of fine material (< 20 μm in size) in tailings from contributes to neutralization of the acidity generated from the oxidation Svartliden is about 19 wt.%. In the ore, gold occurred as inclusions in of sulphides (reactions (2) and (3)). arsenopyrite. Portland cement (CE) and biofuel fly ash (FA) were used

2− + as cementitious binders in the preparation of various cemented paste FeS222++ 15/4O 7/2H O → Fe(OH) 3 ++ 2SO4 4H (2) mixtures. FA was obtained from a biofuel incineration plant located 2− − near the mine. The main elements and the corresponding content of the Ca(OH)2 ++→ 2SO4 2HO242 CaSO·2HO2OH + (3) tailings, CE, and FA are shown in Tables 1 and 2. The content of 2− 2+ Ca42 Al (SO) 4 ·12H 2 O+++ 2SO4 2Ca 20H2624312 O → Ca Al (SO ) (OH) · SiO2 +Al2O3 +Fe2O3 in FA is approx. 60 wt.%. In comparison with ashes from coal combustion, FA could be classified as a class C fly ash 26H2 O (4) according to (ASTM C618-05). SEM-analyses suggested that arseno- Previous studies have shown that CPB can immobilize As pyrite grains have iron-oxide-rims that could sequester arsenates (Benzaazoua et al., 2004a; Coussy et al., 2011, 2012) in tailings. In (Hamberg et al., 2016). Hamberg et al. (2016) assessed the speciation of these studies, cement based CPB:s were used, whereas As occurred as a As in cyanidation tailings, using the modified sequential extraction primary mineral or as spiked As. In addition, 4–7 wt.% of binders scheme described by Dold (2003). The results suggested that most yielded strengths of 0.5–1 MPa for CPB mixtures cured for 28–90 days. (∼95%) of the As in the tailings is associated with Fe-(III)oxy-hydro- These times are considered sufficient for achieving the mechanical re- xides (Hamberg et al., 2016). A small fraction (< 3%) of As is asso- sistance needed in the mining operations (Benzaazoua et al., 2004a). ciated with the primary sulphide fraction corresponding to remnants of Kesimal et al. (2005) showed that sulphate attack may lead to a > 50% arsenopyrite. decrease in the strength of a CPB material (binder fraction: 5 wt.%), XRD, sequential extractions, and SEM-EDS revealed that pyrrhotite subjected to curing times of > 1 year. and arsenopyrite are the main Fe- and As-sulphide minerals in tailings Hamberg et al. (2015) determined the effect of binder fraction (Hamberg et al., 2016). Quartz is the main mineral in the studied (1–3 wt.%) on the mobility of As in cyanidation tailings containing As tailings (approx. 80 wt.%). Other important minerals are Tremolite, mainly as associated with Fe(III)-oxy-hydroxides. These tailings con- Albite and Microcline that comprises for less than 5 wt.% each. Total tained 2 wt.% of pyrrhotite, 1000 ppm of As and was mixed with bin- element contents were determined by high resolution inductively cou- ders (cement and biofuel fly ash) for the formation of a monolithic CPB pled plasma–mass spectrometry (HR-ICP-MS). mass. The CPB-mixtures were then exposed to a tank leaching test (TLT). In a TLT, the mixtures are submerged in a solution of deionized water to simulate an initial underground storage of a monolithic mass. Results from the TLT showed that more As is leached from the CPB Table 1 materials than from the unmodified tailings. A decrease in the pH Oxide composition and total Sulfur-content (Stot) weight (wt.)% in Tailings, Portland during TLT indicates that the sulphides in the CPB monoliths are oxi- Cement (CE), and Biofuel fly ash (FA) (n = 3, ± SD) (DS = Dry solids). dized. TLT results also indicated that As-release from the CPB materials can increase over time (Hamberg et al., 2015). However, in that study, Element Unit Tailings FA CE fl As-behaviour of the materials was evaluated during ooding (by Total solids (TS) % 89.0 ± 0.4 95.2 ± 0.7 99.4 ± 0.0 groundwater) after 31 days of curing under water-saturated conditions. SiO2 wt.% of DS 55.0 ± 4.9 34.6 ± 1.3 20.6 ± 0.8 ” Under complete water-saturated conditions, sulphide oxidation in these Al2O3 4.69 ± 0.04 10.7 ± 0.6 5.61 ± 0.45 materials is obstructed and sulphates affecting the mechanical strength CaO ” 4.83 ± 0.25 14.1 ± 1.0 50.3 ± 1.8 Fe O ” 16.7 ± 0.6 13.9 ± 1.0 2.81 ± 0.05 originate mainly from the initial tailings (pre-oxidized products) 2 3 K O ” 0.92 ± 0.03 2.89 ± 0.08 0.83 ± 0.05 fi 2 (Benzaazoua et al., 2004b). Under eld conditions, groundwater rises MgO ” 3.24 ± 0.01 2.54 ± 0.08 4.00 ± 0.17 very slowly and complete flooding of the CPB materials is achieved MnO ” 0.14 ± 0.01 0.92 ± 0.03 0.10 ± 0.01 ” after long time (INAP, 2009). During the transition period, zones with Na2O 0.82 ± 0.03 1.24 ± 0.08 0.65 ± 0.07 ” low levels of water saturation could form at the CPB monoliths surfaces P2O5 0.24 ± 0.01 2.25 ± 0.05 0.06 ± 0.00 TiO ” 0.18 ± 0.00 0.34 ± 0.01 0.42 ± 0.02 which in turn may increase the sulphide oxidation rate (Ouellet et al., 2 Stot ” 2.10 ± 0.50 1.37 ± 0.02 1.00 ± 0.02 2006, 2003). An investigation of the geochemical stability of As in CPB

R. Hamberg et al. 0LQHUDOV(QJLQHHULQJ ²

3 2 Table 2 10 cm water/cm of exposed solid for all samples to ensure that the Results from ICP-SMS (HR-ICP-MS) - analysis of the duplicated samples were leached under the same conditions (Coussy main elements in tailings. et al., 2011). The leachate was removed and the tanks was charged with

Elements wt.% of DS the same volume of fresh distilled water after 6 h, 1 day, 2.25 days, 4 days, 9 days, 16 days, 36 days and 64 days The overall experimental Al 2.7 set-up is described by Hamberg et al. (2015). The 446-day-cured CE Ca 3.9 monoliths were disintegrated and placed in paper filter bags, with 0.45- Fe 11 Mg 2 μm-sized pores, inside nylon sample holders. The bags were changed on Mn 0.12 a weekly basis and hung from the lid of each tank to ensure that the As 0.1 disintegrated CE mixtures were fully immersed in the water. The mass transfer of As from the tailings and CPB materials was calculated using Eq. (1), as specified in the EA NEN 7375:2004 standard: Table 3 Composition (wt.%) of cemented paste backfill (CPB) mixtures containing Cement (CE) Mti=×(C i V)/ i A (1) and/or Fly ash (FA) and unmodified Cyanidation tailings (CT).

2 Names Portland Biofuel Fly Total Binder/ Water Tailings where Mti (mg/m ): mass of As released during the leaching period i, Ci Cement (wt. ash (wt.%) dry weight content (wt. (wt.%) (mg/L): As concentration during period i, Vi (L): leachate volume as- %) (wt.%) %) sociated with period i, and A: specimen surface area exposed to the leachate (m2). The apparent diﬀusion coeﬃcient of As was determined CE 100 1 26 73 CE-FA 67 33 3 26 71 by plotting the logarithm of the cumulative Mt (for times preceding and CT 0 0 0 11 89 including the i-th period) against the logarithm of the time elapsed since the start of the experiment.

2.1. Preparation of the CPB-materials 2.2.2. pH-dependent leaching test In the study of Hamberg et al. (2015), mine operators considered The protocol of the test was inspired by previous studies of Glass that a compressive strength of 200 kPa was sufficient for the CPB-ma- and Buenfeld (1999) and Peyronnard et al. (2009). Materials from the terial. CPB-materials with a minimal proportion of binders were used outermost areas of CE-FA446 and CE446 were crushed into a pulverized (Table 3 for proportions). Tailings were mixed with the binders (CE and material, divided into 12.5-g sub-samples, and placed in acid-cleaned FA) and distilled water was slowly added to 25% (w/w). The CPB plastic bottles. The aim of the test was to estimate the release of As in mixtures (CE and CE-FA) were prepared as described in Hamberg et al. response to a decrease in the pH of the CPB material. The acid-cleaned (2015). A slump test was not conducted while only 15 kg of tailings was bottles containing CE-FA446 and CE446 were filled with 50 mL of nitric available for analyses and experiments. Mixtures of CE and CE-FA were acid solution with the aim of achieving a liquid/solid ratio (L/S) of four. cured for 31 days or 446 days (hereafter referred to as CE31, CE446, Duplicates were included for three HNO3 concentrations (pH: 6, 4, and CE-FA31, and CE-FA446). CE31 and CE-FA31 were kept at +25 °C at ∼2.7) associated with each CPB recipe. The amount of nitric acid approximately 80% humidity (similar to underground conditions ac- added to each bottle was based on preliminary test results that revealed cording to Benzaazoua et al., 2004b), under dark conditions, until the a correlation between nitric-acid addition and the pH. The pH ranged 31st day of curing. As for CE446 and CE-FA446, these samples were from 8 to ∼2.7 corresponding to the normal pH of the semi-alkaline cured at the same conditions as CE31 and CE-FA31 until the 31th day, cementitious samples of CE446, CE-FA446 and unmodified tailings, while the subsequent curing was conducted at approx. 60% humidity, respectively. The bottles were stirred for 8 days to reach a constant pH, in dark, room-temperature conditions until the 446th day. These con- and then leachates were filtered with 0.45 μm polypropylene mem- ditions were chosen to enhance the formation of an unsaturated area at branes on a Büchner filter. the surface of the CPB-mixtures as found in Ouellet et al. (2003). The curing period was terminated while local desaturation areas appeared at the surfaces of the CPB-mixtures. A curing period of 31 days were 2.2.3. Sequential extraction test chosen based on results from the studies of (Ercikdi et al., 2009; Huang A sequential extraction test, adapted from Dold (2003), was con- et al., 2011). In CPB:s with low proportions of binders (less than 5 wt. ducted on crushed, pulverized, CPB materials (taken from the CPB- %) the hydration of cement is assumed to be nearly completed after surfaces) and the fraction of As associated with certain target phases in 30 days and the strength does not change much afterwards (Ercikdi CE446 and CE-FA446 was compared. The obtained results were then et al., 2009; Huang et al., 2011). Increasing the curing period to more compared with those of a sequential extraction test (Hamberg et al., than a year could mean a strength loss in the CPB-material, especially 2015) conducted on unmodified tailings (see Table 4). when containing sulphide/sulphate rich tailings (Kesimal et al., 2005).

2.2. Leaching tests Table 4 A modified sequential extraction procedure for sulphide mine waste (Dold, 2003). 2.2.1. Tank leaching test (TLT) Chemical agent added to the Duration Target phase A TLT is suitable for monolithic (low permeability) materials, where solid sample the leaching fluid flows around the material, thereby providing the driving force for maintaining leaching. This test provides a mechanism 50 mL milliQ H2O 1 h shaking Water-extractable As 50 mL 1 M NH – acetate, pH 2 h shaking Adsorbed-Exchangeable- for evaluating the surface area-related (i.e. diffusion-controlled) release 4 4.5 Carbonate (AEC) bound As that occurs at the external boundary of the material. Results are 50 mL 0.2 M NH4 – oxalate, pH 1 h shaking in As bound to Fe (III) oxy- 2 therefore expressed as mass released on a surface area (mg/m ) and 3.0 darkness hydroxides – tests on the CPB samples (CE-FA and CE) were conducted in accordance 50 mL 0.2 M NH4 oxalate, pH 2 h heating in As bound to Fe (III) oxides with the Dutch standard EA NEN 7375:2004. CPB duplicate samples 3.0 water bath (80 °C) were removed from the bottles after 31 or 446 days and shaped into Aqua regia (HCl:HNO3 = 3:1) – Sulphides/residual fraction regular cylinders. The liquid/solid (L/S) ratio was fixed at

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Fig. 1. Evolution of the leachate pH and cumulative release (in mg/m2) of elements (Si, As, S, Ca, and Fe) from uncemented low sulphide As-bearing tailings, cemented tailings (by addition of Cement (CE)), and cement and ﬂy ash (CE-FA) cured for 31 (CE31, CE-FA31) and 446 days (CE446, CE-FA446) during the TLT. Averaged values are presented.

3. Results extraction. In the case of stored CPB-materials, the amount of As leached from CE-FA during the TLTs increases with time, whereas the 3.1. Tank leaching test (TLT) amount leached from CE decreases. Similar amounts of Ca and S are released from CE446 and CE-FA446, but a significantly higher amount The release of elements experienced small deviations in the dupli- of Fe is released from CE446, especially while pH is lower than 4.5. A cated samples, averaged values is therefore presented. More As is re- lower amount of Ca, S, and As is released from the unmodified tailings leased in CE31 than in CE446 (Fig. 1), where more Si, Ca, and S are than from CE31, CE446, CE-FA31, and CE-FA446. However, a higher released. As-release in CE31 is lower than that in CE446 at the begin- amount of Fe-leaching occurs in these tailings than in CE31, CE446, CE- ning of the test, but increases thereafter. Maximum As concentrations of FA31, and CE-FA446 (Fig. 1). 32 mg/m2 (0.32 mg/L) for CE31 and 6 mg/m2 (0.06 mg/L) for CE446 The occurrence of higher molar ratios (Ca/S) in the TLT-leachates, occur at the 8th and 1st extraction, respectively, of the leachate renewal compared with those in the unmodified tailings from fresh CE31, in- cycle. In addition, opposite trends are observed for the pH evolution of dicates that Ca is derived from an additional source, besides gypsum. As CE31 and CE446, where a low initial value of 3.2 increases to 6.2 in the case of CE31, molar ratios (Ca/S) of TLT-leachates from CE-FA31 during the last extraction (Fig. 1). The pH of CE31, in contrast, has a are larger than those of the unmodified tailings, again indicative of an high initial value of 9.3 which decreases to 7.7 during the last extrac- additional source of Ca. The occurrence of alkaline conditions suggests tion (Fig. 1). During the leachate renewal cycle of CE-FA, the highest that cementitious phases have dissolved in CE-FA31 (Fig. 2). Moreover, concentrations of As (14 mg/m2 and 12 mg/m2 for CE-FA31 and CE- Ca/S molar ratios corresponding to CE446 and CE-FA446 are lower FA446, respectively) are released at the 8th extraction. As in the case of than those associated with CE31 and CE-FA31 (Fig. 2).

CE, more Si, Ca, and S are released in aged CE-FA-materials (CE-FA446) A plot with a slope (rc) of < 0.4 is expected for the surface wash-oﬀ than in fresh CE-FA (CE-FA31). The pH evolution of CE-FA446 is similar process, where surface-captured As is released from the surface of the to that of CE446, i.e. the initial value of 5.2 increases to ∼8 at the 7th CPB material. The slope is deﬁned as the logarithm of the total mass of

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Fig. 2. Evolution of pH and Ca/S molar ratios (averaged values) associated with leachates from CT, CE-FA31, CE31, CE-FA446, and CE446 during extraction 1–8 in TLT.

Table 5 higher, (iii) Fe (III)-oxides in CE446 and CE-FA446 is higher and un- Mass-transfer mechanism of As in tailings, CE31, CE446, CE-FA31, and CE-FA446 during altered, respectively, and (iv) low-solubility phases (sulphides, silicates) TLT (averaged values). in the CPB-materials is unaltered. Sequential extraction tests on fresh

2 CPB mixtures were impossible due to unexpected changes in the un- Mix Cumulative mg/m Slope (rc) Mechanism modified tailings. Tailings 5.5 0.21 Wash CE31 98 0.62 Diff CE-FA31 53 0.46 Diff 3.3. pH-dependent leaching test CE446 12 0.23 Wash ff CE-FA446 62 0.68 Di At pH ≠ 6, more As is released from CE-FA446 than from CE446. At pH = 4 and pH = 2.7, leaching of Fe from CE-FA446 is lower than and Wash = wash-off,Diff =Diffusion. similar to, respectively, that from CE446. Conducting a pH-dependent leaching test on fresh CPB mixtures were impossible due to unexpected As transferred plotted against the logarithm of the time elapsed during changes in the unmodified tailings. the experiment. Conversely, diffusion-controlled As release should yield a linear plot with a slope of 0.4–0.6. Calculations performed in accordance with the EA NEN 7375:2004 standard, revealed that As 4. Discussion leaching from CE446 and CE31 results from a wash-off-effect and is diffusion controlled (Table 5), respectively. The sharp initial increase in 4.1. Lowering the water saturation levels in CPB-mixtures – Effect on the the As concentration of leachates from the CE-FA446 samples is at- stability of As tributed to mass transfer-controlled processes, whereas As release during the later stages of the experiment results mainly from diffusion. In Hamberg et al. (2016), results from a sequential leaching test suggested that most (95%) of the As in the unmodified tailings is as- 3.2. Sequential extraction test sociated with Fe (III)-oxy-hydroxides. As-bearing Fe (III)-oxy-hydroxides formed during the tailing treatment process are stable at pH va- Crushing CE446 and CE-FA446 prior to the sequential leaching tests lues of 4–8(Riveros et al., 2001). This As-phase account for 85–90 wt.% revealed dry (bright), superficial regions which could be associated of the total As in CE446 and CE-FA446. Specifically, the fraction of As with a low degree of water saturation. Results from the corresponding associated with crystalline Fe-phases in CE446 and with the AEC frac- sequential extractions (Fig. 3) showed that the addition of cementitious tion is higher than that of the tailings. This is also true of As associated binders leads to slight modification of As fractionation, compared with with the AEC fraction of CE-FA446 (Fig. 3). However, the fraction of As- that of the unmodified tailings. Furthermore, compared with that of the sulphides in the CPB materials is unaltered compared with that of the unmodified tailings, the fraction of As associated with (i) amorphous Fe tailings (Fig. 3), and is indicative of a low oxidation rate of Arseno- (III) oxy-hydroxides in CE446 and CE-FA446 is lower, (ii) the AEC pyrite. The re-distribution of As phases in CE446 could mean that a fraction in the cementitious mixtures (especially in CE-FA446) is larger proportion of crystalline As-Fe precipitates and less acid tolerant

Arsenic Fig. 3. Fractionation of As during sequential extraction 100 Water soluble testing of tailings, CE 446 and CE-FA446, (n = 3, error 90 bars ± SD). 80 Acid Exchangeable 70 (AEC) 60 50 Fe(III)-oxy- hydroxides 40

Extracted (%) 30 Fe(III)-oxides 20 10 Sulphides/Residual 0 Tailings CE446 CE-FA446

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As-species (Fig. 3) has formed compared to that in unmodified tailings. 4.2. Lowering water saturation levels in CPB-mixtures – effect on As However, these results should be interpreted with precaution whilst leaching changes in the distribution of As is lower than 10% (Tessier et al., 1979). In general, tailings material used in CPB-mixtures must contain at The As content of the Svartliden ore varies significantly least 15 wt.% of fine particles (< 20 μm in size) in order to retain (100–10000 ppm), suggesting that As-Fe precipitates with various Fe/ sufficient water to form a paste (Landriault, 2001). In case of CT, this As molar ratios have formed in the tailings (Hamberg et al., 2016). As proportion stands for 19 wt.%. During the TLT, As leaching from the species having an Fe/As molar ratio of 1–1.5 are considered co-pre- CPB mixtures is higher, regardless of curing conditions (time and water cipitated, but adsorbed As species dominate at Fe contents (of the As-Fe saturation), than that of the unmodified tailings. Lowering the water precipitates) corresponding to Fe/As-ratios of > 1.5 (Paktunc et al., saturation level in the CPB-mixtures yields higher and lower As 2003). However, cyanide leaching of gold ores under semi-neutral/al- leaching from CE-FA and CE, respectively (Fig. 1). During the TLT, kaline conditions could also promote the formation of Ca-Fe arsenates ≈100 mg/m2 and ≈10 mg/m2 of As are released from CE31 and from Fe- and As-rich process solutions. Yukonite is a highly soluble Ca- CE446, respectively. As-release from the CE-FA-mixtures increased Fe-arsenate while arseniosiderite is stable at pH 3.5–7.5 in gypsum- while water saturation levels decreased. Standard calculations (EA NEN saturated solutions (Paktunc et al., 2015). In Hamberg et al. (2016), As- 7375:2004) suggest that the evolution of As-release from CE446 (during release from unmodified tailings is low at pH > 3.5 and is therefore TLT) occurs predominantly via a wash-off-effect, where As is rinsed off probably not governed by Yukonite. The pH-stability field of arsenio- the surface. This is manifested in the As-release from CE446 that is most siderite overlaps that of iron-arsenates (Paktunc et al., 2015; Riveros abundant during the first extraction, but decreases immediately after- et al., 2001), that could be associated with the Fe(III)-oxy-hydroxide- wards, and is almost diminished at the end of the TLT. In the case of CE- fraction (Parviainen et al., 2012). So, the stability of As in tailings could FA31, CE-FA446, and CE31, the transport mechanism of As can be therefore be controlled by the solubility of iron-arsenates and arsenio- described as diffusion-like, where As diffuses from the pore water siderite. (Table 5) and increases during the TLT. The evolution of As-release In Hamberg et al. (2016), rims of Fe-oxides that could sequester from CE-FA31, CE-FA446, and CE31 suggests that no plateau was arsenates (As V) were detected on arsenopyrite grains. Several studies reached. So, As release could continue after 64 days. have shown that adsorbed As(V) can transform into a Ca-arsenate in a The mobility of As in CPB mixtures has been correlated with phy- cementitious matrix (Jing et al., 2003; Vandecasteele et al., 2002; sical encapsulation (Bowell, 2014), and the formation of Ca-As-, Ca-Fe- Coussy et al., 2012), as in the case of CE446 and CE-FA446. The in- As-, and As-species sorbed onto cementitious phases (Coussy et al., troduction of alkaline conditions (resulting in desorption of adsorbed 2012). The occurrence of a diffusion-like transport mechanism of As in As) and an increase in the Ca content of pore water (through the ad- CE-FA31, CE-FA446, and CE31 (Table 5) suggests that physical en- dition of a Ca-rich, soluble binder) may increase the solubility of Fe-As capsulation has been ineffective in decreasing As leaching. Ca-arsenates precipitates over a wide range (3–12) of pH values (Swash and and cementitious As-phases formed in a CPB mixture may therefore Monhemius, 1995; Drahota et al., 2009). Ca-arsenates formed under dissolve in the presence of carbon dioxide and/or acidic conditions, alkaline conditions are known to be thermodynamically unstable in the thereby releasing As into solution (Benzaazoua et al., 2004b). As-re- presence of atmospheric carbon dioxide or carbonate ions (Riveros lease from CE31, CE-FA31 and CE-FA446 are largely dependent on the et al., 2001; Krause and Ettel, 1985, Benzaazoua et al., 2004b). The stability of cementitious phases. The stability of cementitious phases in higher amount of As associated with the AEC fraction of CE446 and CE- the CPB mixtures is sensitive to reductions in pH that depends strongly FA446, compared with that of the unmodified tailings, results possibly on the sulphide-oxidation rate. Initial pH (pH: 3–4) in CE446- and CE- from the formation of Ca-arsenates (Pantuzzo and Ciminelli, 2010). The FA446-leachates are lower than those of CE31 and CE-FA31 and in- proportion of As associated with the AEC fraction, is larger in CE-FA446 creased to semi-neutral (pH: 6–8) levels at the end of the TLT; this re- compared to that in CE446 (Fig. 3) and is therefore more susceptible to sults possibly from a more extensive sulphide oxidation that occurred dissolution in an acid medium. This may have resulted in the greater while water saturation levels were lowered. Most of the acid generated stability of As in CE446 than in CE-FA446 during the pH-dependent from the CPB mixtures during TLT should originate from the oxidation leaching test (Figs. 2 and 4); especially in the acidic range. This poses a of pyrrhotite. Benzaazoua et al. (2004b) showed that arsenopyrite risk for destabilization of a large fraction of As, due to sulphide-mineral surfaces can become rapidly passivized in cementitious environments. components in the CPB mixtures. However, the surfaces of pyrrhotite grains remain active in a cementitious environment, leading to further pH-control of the CPB materials (Benzaazoua et al., 2004b). Pyrrhotite undergoes oxidation in the CPB mixtures, regardless of curing conditions (length and level of

Fig. 4. Release of As during pH-dependent leaching of CE-FA446 (1 and 2) and CE446 (1 and 2).

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Fig. 5. The release of As and Si from CE31, CE446, CE-FA31, and CE-FA446 during TLT. water saturation), and seems to increase As-release from CE-FA-mix- leaching of As. Nevertheless, adding a Ca-rich binder may destabilize tures while the opposite concerned for CE-mixtures. initially formed As-Fe precipitates in unmodified tailings and thereby A presence of gypsum in unmodified tailings is suggested by water- decrease the stability of As, especially in the acidic range. Maintaining soluble Ca and S (Dold, 2003). Ca/S molar ratios of > 1 for TLT-lea- high saturation levels is therefore essential for preventing acidic con- chates from CE31 and CE-FA31 (Fig. 2), are indicative of Ca-leaching ditions and extensive As-release in CPB mixtures. from another source, besides gypsum, probably a cementitious phase. Ca/S molar ratios and pH of TLT-leachates from CE446 and CE-FA446 Acknowledgements are lower than those of CE31 and CE-FA446 (Fig. 2) suggesting that some of the cementitious phases have dissolved while water saturation This work was supported by the Ramböll Sverige AB, Ramböll levels decreased as the curing time was prolonged. This is especially Foundation, SUSMIN – Tools for sustainable gold mining in EU, and the evident for CE446 that disintegrated during the curing period. A com- Center of Advanced Mining and Metallurgy (CAMM) at Luleå University paratively low binder proportion (1 wt.%) in CE446 has probably been of Technology. The personnel at Dragon Mining AB are gratefully ac- insufficient to maintain the stability of C-S-H (physical strength), during knowledged for valuable information about the mining processes and a more extensive sulphide oxidation. During the TLT, a low amount of for providing the tailings evaluated in this study. As is released from CE446 and CE-FA446 in the acidic range. This amount increases with pH in CE-FA446, suggesting that As is released References while the cementitious phases dissolves to buffer acidity from pyrrhotite oxidation. The release of cementitious As-phases manifested as a ASTM C618–05, 2005. Standard Specification for Coal Fly Ash and Raw or Calcined simultaneous release of As and Si (Coussy et al., 2011)(Fig. 5) from CE- Natural Pozzolan for Use in Concrete, American Society of Testing and Materials, West Conshohocken, PA. FA31, CE-FA446 and CE31. In CE446, a simultaneous As-Si-release is Belzile, N., Chen, Y., Cai, M., Li, Y., 2004. A review on pyrrhotite oxidation. J. Geochem. less obvious (Fig. 5), instead, As could origin from the dissolution of Fe, Explor. 84 (2), 65–76. which is comparatively more abundant (Fig. 2). During the TLT, a Benzaazoua, M., Quellet, J., Servant, S., Newman, P., Verburg, R., 1999. Cementitious backfill with high sulfur content physical, chemical, and mineralogical character- majority of As released from CE31, CE-FA31 and CE-FA446 is origi- ization. Cem. Concr. Res. 29 (5), 719–725. nated from cementitious As-phases that dissolves when binders are Benzaazoua, M., Marion, P., Picquet, I., Bussière, B., 2004a. The use of pastefill as a buffering acidity generated from pyrrhotite oxidation. solidification and stabilization process for the control of acid mine drainage. Miner. – Coussy et al. (2011), suggested that As is more stable in CPB-mix- Eng. 17 (2), 233 243. Benzaazoua, M., Fall, M., Belem, T., 2004b. A contribution to understanding the hard- tures containing larger amounts of lime, but also that fly-ash based ening process of cemented pastefill. Miner. Eng. 17 (2), 141–152. http://dx.doi.org/ CPB:s are not suitable for the stabilization of As. These suggestions 10.1016/j.mineng.2003.10.022. could to some extent be evident in this study while in the TLT, As- Coussy, S., Benzaazoua, M., Blanc, D., Moszkowicz, P., Bussière, B., 2011. Arsenic stability in arsenopyrite-rich cemented paste backfills: a leaching test-based assessment. release is most abundant from CE-FA446. Lime content, is however, J. Hazard Mater. 185 (2–3), 1467–1476. higher in CE-FA-mixtures (due to higher binder proportions) and As- Coussy, S., Paktunc, D., Rose, J., Benzaazoua, M., 2012. Arsenic speciation in cemented release is less abundant in CE-FA31 compared to that in CE31. How- paste backfills and synthetic calcium-silicate-hydrates. Miner. Eng. 39, 51–61. http:// dx.doi.org/10.1016/j.mineng.2012.05.016. ever, as water saturation levels are lowered As-release in CE-FA446 Deschênes, G., Rousseau, M., Tardif, J., Prud'homme, P.J.H., 1998. Effect of the compo- becomes more abundant than that from CE-FA31 and CE446. As-release sition of some sulphide minerals on cyanidation and use of lead nitrate and oxygen to from CE446 is lower and had a different release mechanism (wash-off) alleviate their impact. Hydrometallurgy 50 (3), 205–221. Drahota, P., Rohovec, J., Filippi, M., Mihaljevič, M., Rychlovský, P., Červený, V., Pertold, compared to that in CE31, CE-FA31 and CE-FA446 (Table 5). This Z., 2009. Mineralogical and geochemical controls of arsenic speciation and mobility might be due to the shape of CE446 which is granular rather than under different redox conditions in soil, sediment and water at the Mokrsko-west gold monolithic, but also due to higher stability of As in the acidic range deposit, Czech Republic. Sci. Tot. Environ. 407 (10), 3372–3384. http://dx.doi.org/ 10.1016/j.scitotenv.2009.01.009. compared to that in CE-FA446 (Fig. 4). Dold, B., 2003. Speciation of the most soluble phases in a sequential extraction procedure adapted for geochemical studies of copper sulfide mine waste. J. Geochem. Explor. 80 (1), 55–68. 5. Conclusions EA NEN 7375:2004, 2004. Leaching Characteristics of Moulded or Monolithic Building and Waste Materials, Determination of Leaching of Inorganic Components with The Diffusion Test, The Tank Test. Management of cyanidation tailings by using CPB with a low frac- Ercikdi, B., Kesimal, A., Cihangir, F., Deveci, H., Alp, I., 2009. Cemented paste backfill of tion of binders results in increased leaching of As, regardless of curing sulphide-rich tailings: importance of binder type and dosage. Cem. Concr. Comp. 31 – conditions. Lowering the water saturation level in the CPB mixtures (4), 268 274. http://dx.doi.org/10.1016/j.cemconcomp.2009.01.008. Fall, M., Belem, T., Samb, S., Benzaazoua, M., 2007. Experimental characterization of the leads to the dissolution of cementitious phases. In CPB-mixtures with stress-strain behaviour of cemented paste backfill in compression. J. Mater. Sci. 42 lower fractions of binders, this yields a reduction in As leaching, where (11), 3914–3922. ff most of the cementitious As phases dissolve to form relatively stable As Glass, G.K., Buenfeld, N.R., 1999. Di erential acid neutralisation analysis. Cem. Concr. Res. 29 (10), 1681–1684. species. Cementitious phases of As persist in CPB mixtures with a higher Hamberg, R., Maurice, C., Alakangas, L., 2015. The use of low binder proportions in proportion of binders, and lowering the water saturation level increased cemented paste backfill - effects on As-leaching. Miner. Eng. 78, 74–82. http://dx.doi.

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org/10.1016/j.mineng.2015.04.017. 37 (10), 2067–2074. http://dx.doi.org/10.1021/es026185m. Hamberg, R., Bark, G., Maurice, C., Alakangas, L., 2016. Release of arsenic from cyani- Paktunc, D., Foster, A., Heald, S., Laflamme, G., 2004. Speciation and characterization of dation tailings. Miner. Eng. 93, 57–64. http://dx.doi.org/10.1016/j.mineng.2016.04. arsenic in gold ores and cyanidation tailings using X-ray absorption spectroscopy. 013. Geochim. Cosmochim. Acta 68 (5), 969–983. http://dx.doi.org/10.1016/j.gca.2003. Huang, S., Xia, K., Qiao, L., 2011. Dynamic tests of cemented paste backfill: effects of 07.013. strain rate, curing time, and cement content on compressive strength. J. Mater. Sci. Paktunc, D., Majzlan, J., Huang, A., Thibault, Y., Johnson, M.B., White, M.A., 2015. 46 (15), 5165–5170. http://dx.doi.org/10.1007/s10853-011-5449-0. Synthesis, characterization, and thermodynamics of arsenates forming in the Ca-Fe

INAP (International Network for Acid Prevention), 2009. Global Acid Rock Drainage (III)-As(V)-NO3 system: implications for the stability of Ca-Fe arsenates. Am. Miner. Guide (GARD Guide). The Acid Rock Drainage Process (Chapter 2). 100 (8–9), 1803–1820. http://dx.doi.org/10.2138/am-2015-5199. Jing, C., Korfiatis, G.P., Meng, X., 2003. Immobilization mechanisms of arsenate in iron Pantuzzo, F.L., Ciminelli, V.S.T., 2010. Arsenic association and stability in long-term hydroxide sludge stabilized with cement. Environ. Sci. Technol. 37 (21), 5050–5056. disposed arsenic residues. Water Res. 44 (19), 5631–5640. http://dx.doi.org/10. http://dx.doi.org/10.1021/es021027g. 1016/j.watres.2010.07.011. Kesimal, A., Yilmaz, E., Ercikdi, B., Alp, I., Deveci, H., 2005. Effect of properties of tailings Parviainen, A., Isosaari, P., Loukola-Ruskeeniemi, K., Nieto, J.M., Gervilla, F., 2012. and binder on the short-and long-term strength and stability of cemented paste Occurrence and mobility of as in the ylöjärvi Cu-W-as mine tailings. J. Geochem. backfill. Mater. Lett. 59 (28), 3703–3709. http://dx.doi.org/10.1016/j.matlet.2005. Explor. 114, 36–45. http://dx.doi.org/10.1016/j.gexplo.2011.12.002. 06.042. Peyronnard, O., Benzaazoua, M., Blanc, D., Moszkowicz, P., 2009. Study of mineralogy Kyle, J.H., Breuer, P.L., Bunney, K.G., Pleysier, R., 2012. Review of trace toxic elements and leaching behavior of stabilized/solidified sludge using differential acid neu- (Pb, Cd, Hg, As, Sb, Bi, Se, Te) and their deportment in gold processing: Part II: tralization analysis. Part I: experimental study. Cem. Concr. Res. 39 (7), 600–609. deportment in gold ore processing by cyanidation. Hydrometallurgy 111–112 (1), http://dx.doi.org/10.1016/j.cemconres.2009.03.01. 10–21. http://dx.doi.org/10.1016/j.hydromet.2011.09.005. Peyronnard, O., Benzaazoua, M., 2012. Alternative by-product based binders for ce- Landriault, D., 2001. Backfill in underground mining. In: Hustrulid, W.A. (Ed.), mented mine backfill: recipes optimisation using Taguchi method. Miner. Eng. 29, Underground Mining Methods Engineering Fundamentals and International Case 28–38. http://dx.doi.org/10.1016/j.mineng.2011.12.010. Studies. SME, USA, pp. 608–609. Riveros, P.A., Dutrizac, J.E., Spencer, P., 2001. Arsenic disposal practices in the me- Mesa Espitia, S.L., Lapidus, G.T., 2015. Pretreatment of a refractory arsenopyritic gold ore tallurgical industry. Can. Metall. Quart. 40 (4), 395–420. using hydroxyl ion. Hydromet 153, 106–113. http://dx.doi.org/10.1016/j.hydromet. Swash, P.M., Monhemius, A.J., 1995. Synthesis, Characterization and Solubility Testing 2015.02.013. of Solids in the Ca-Fe-AsO4 System, Sudbury ’95 - Mining and the Environment. In: Ouellet, S., Bussière, B., Mbonimpa, M., Benzaazoua, M., Aubertin, M., 2006. Reactivity Hynes, T.P., Blanchette, M.C. (Eds.), CANMET, Ottawa, Canada, pp. 17–28. and mineralogical evolution of an underground mine sulphidic cemented paste Tessier, A., Campbell, P.G.C., Bisson, M., 1979. Sequential extraction procedure for the backfill. Miner. Eng. 19 (5), 407–419. http://dx.doi.org/10.1016/j.mineng.2005.10. speciation of particulate trace metals. Anal. Chem. 51 (7), 844–851. 006. Vandecasteele, C., Dutré, V., Geysen, D., Wauters, G., 2002. Solidification/stabilization of Paktunc, D., Foster, A., Laflamme, G., 2003. Speciation and characterization of arsenic in arsenic bearing fly ash from the metallurgical industry, immobilization mechanism of Ketza river mine tailings using x-ray absorption spectroscopy. Environ. Sci. Technol. arsenic. Waste Manage. 22, 143–146.

Paper IV

The formation of unsaturated zones within cemented paste backfill mixtures - Effects on the release of copper, nickel, and zinc

Roger Hamberg, Christian Maurice and Lena Alakangas

Accepted for publication in:

Environmental Science and Pollution Research

Environmental Science and Pollution Research https://doi.org/10.1007/s11356-018-2222-9

RESEARCH ARTICLE

The formation of unsaturated zones within cemented paste backfill mixtures—effects on the release of copper, nickel, and zinc

Roger Hamberg1 & Christian Maurice1 & Lena Alakangas1

Received: 5 February 2018 /Accepted: 3 May 2018 # The Author(s) 2018

Abstract Flooding of cemented paste backfill (CPB) filled mine workings is, commonly, a slow process and could lead to the formation of unsaturated zones within the CPB fillings. This facilitates the oxidation of sulfide minerals and thereby increases the risk of trace metal leaching. Pyrrhotitic tailings from a gold mine (cyanidation tailing (CT)), containing elevated concentrations of nickel (Ni), copper (Cu), and zinc (Zn), were mixed with cement and/or fly ash (1–3 wt%) to form CT-CPB mixtures. Pyrrhotite oxidation progressed more extensively during unsaturated conditions, where acidity resulted in dissolution of the Ni, Cu, and Zn associated with amorphous Fe precipitates and/or cementitious phases. The establishment of acidic, unsaturated conditions in CT-CBP:s with low fractions (1 wt%) of binders increased the Cu release (to be higher than that from CT), owing to the dissolution of Cu- associated amorphous Fe precipitates. In CT-CPB:s with relatively high proportions of binder, acidity from pyrrhotite oxidation was buffered to a greater extent. At this stage, Zn leaching increased due the occurrence of fly ash-specific Zn species soluble in alkaline conditions. Irrespective of binder proportion and water saturation level, the Ni and Zn release were lower, compared to that in CT. Fractions of Ni, Zn, and Cu associated with acid-soluble phases or amorphous Fe precipitates, susceptible to remobilization under acidic conditions, increased in tandem with binder fractions. Pyrrhotite oxidation occurred irrespective of the water saturation level in the CPB mixtures. That, in turn, poses an environmental risk, whereas a substantial proportion of Ni, Cu, and Zn was associated with acid-soluble phases.

Keywords Tailing management . Cement . Trace metal leaching

Introduction 2015). The goal of this step is to increase the availability of cyanide for dissolving the gold. However, this process is sel- Cyanide is typically used to extract gold occurring as inclu- dom 100% effective, and remnants of sulfide minerals often sions in sulfide minerals, such as pyrrhotite and pyrite. In the occur in tailings obtained through a cyanide leaching process cyanide leaching process, lime addition can avoid the forma- (cyanidation tailing (CT)). These sulfides often contain trace tion of toxic cyanide species by reaching a pH of ≈ 10. In metals such as copper (Cu), arsenic (As), and zinc (Zn). refractory gold ores, an oxidation step aimed at oxidizing the Furthermore, sulfide minerals exposed to atmospheric oxygen sulfide minerals enclosing the gold is usually performed prior and water may become oxidized. These reactions (reaction 1) to the cyanide leaching process (Mesa Espitia and Lapidus yield sulfate and an acid leachate, referred to as acid mine drainage (AMD), with a high content of metals (Blowes et Highlights - Pyrrhotite oxidation occurrence within CPB irrespective of al. 2003). Owing to the co-occurrence of these trace metals the water saturation level and the sulfide minerals, CT must be carefully handled to - Unsaturated conditions in CPB increased Cu and Zn leaching - Using CPB increased proportion of Ni, Zn, and Cu associated to acid- avoid pollution of the surrounding environment. intolerant phases þ = þ = → ðÞþ 2− þ þð Þ FeS2 15 4O2 7 2H2O Fe OH 3 2SO4 4H 1 Responsible editor: Philippe Garrigues Tailing management often aims to restrict the interaction of * Roger Hamberg tailings with the environment. A commonly used method is by [email protected] placing the tailings in underground engineered facilities. Managing of CT in underground workings often includes 1 Luleå University of Technology, 971 87 Luleå, Sweden the use of a so-called BCemented paste backfill^ (CPB) Environ Sci Pollut Res method. In CPB, low fractions (3–7 wt%) of cementitious will increase the risk of a significant sulfide oxidation to occur binders are mixed with tailings and backfilled into under- within the CT-CPB. A previous study of Kesimal et al. (2005) ground facilities (Coussy et al. 2011). Using CPB can primary showed that the strength of a CPB material (especially sulfide- lead to the formation of a monolith that can serve as a geotech- rich CT-CPB) could decrease by more than 50% during nical support to underground mine cavities, thereby increasing prolonged (> 1 year) curing. Cruz et al. (2001)haveshown the operational benefits of the mining industry. Increasing the that low fractions of binders may be insufficient to prevent the fraction of binders in a CT-CPB may yield intensified hydra- occurrence of AMD. Knowledge of trace metal and sulfide tion and pozzolanic reactions, thereby enhancing the mechan- mineral mobility in CT-CPB is therefore essential for the ical strength. The use of CPB could also prevent air intrusion prediction/management of contaminant release over an ex- into CT and thereby lower the sulfide-oxidation rate. Trace tended period of time. In some underground workings, the metal immobilization in CPB has been applied to a combina- strength of the CT-CPB material is of less concern, but aims tion of physical encapsulation and chemical stabilization. to minimize water percolation and enhancing trace metal sta- Studies of Chen et al. (2009) and Paria and Yuet (2006) sug- bility still exist. So, when less strength is needed in the CPB gested that nickel (Ni) and chromium (Cr) can be encapsulated material, how does it affect the leaching of trace metals? CT in the calcium-silicate-hydrate (C-S-H) structure. This physical from a gold mine in the north of Sweden was to be managed encapsulation is largely dependent on the inherent strength of by the use of a CPB application, whereas the strength demand the CPB. The CPB strength results primarily from the abun- was set to 0.2 MPa by the mine operator. The overall aim of dance of C-S-H (Peyronnard and Benzaazoua 2012) formed this study was to determine whether a low-strength CPB by the addition of calcium-rich binders that hydrate in solution. (0.2 MPa) could be an environmentally friendly option for In CT-based CPB materials (CT-CPB), dissolution of C-S-H managing tailings (obtained through a cyanide leaching pro- contributes to neutralization of the acid formed by the oxida- cess) with elevated concentrations of Cu, Ni, and Zn. tion of sulfides (reactions 1 and 2). Sulfide oxidation, gypsum As part of the overall aim, primary objectives of the study dissolution, the cyanide leaching process, and/or the binders were to may generate sulfates in the CTand the CT-CPB mixtures. The stability of C-S-H in a CPB is governed by the sulfide and – Evaluate the effect of using low-strength CT-CPB mix- sulfate content, curing time as well as the type and fraction tures on the release of Ni, Cu, and Zn, of binder material (Ercikdi et al. 2009; Kesimal et al. 2005; – Determine the effect of unsaturated conditions in low- Benzaazoua et al. 2004). The cementitious phase reacts with strength CT-CPB mixtures on leaching of these elements. sulfates, thereby forming expansive phases such as gypsum and ettringite (a sulfate attack, reactions 2 and 3)thatmaylead The results of this study may increase knowledge of the to a decrease in the inherent strength, generate cracks, and preparation/management of CPB mixtures for use in mine increase the availability of oxygen in the CPB material. A facilities with slow recovery of natural groundwater levels. chemical stabilization may occur and the hydroxyl anions from this process may react with trace metals (i.e., Pb and Zn), forming hydroxides that can adsorb onto C-S-H. However, Materials and methods highly alkaline conditions (resulting from the release of OH−) could also increase the mobility of some trace metals Tailings and ore were collected from a gold mine in the north (Cr, Pb, Zn, and Cu) (Kumpiene et al. 2008). Maintaining of Sweden. The ore was considered refractory as the gold the stability of C-S-H is therefore essential from the viewpoint occurred as inclusions in co-occurring sulfide minerals. of trace metal mobility in a CT-CPB. According to personnel at the site, hydrogen peroxide (H O ) was added in the 1st step of the cyanide leaching ðÞþ 2‐ þ → ⋅ þ ‐ ð Þ 2 2 Ca OH 2 2SO4 2H2O CaSO4 2H2O 2OH 2 process, with the aim of oxidizing the sulfide minerals. This ðÞ⋅ þ 2− þ 2þ oxidation will increase the availability of cyanide for gold Ca4Al2 SO 4 12H2O 2SO4 2Ca dissolution. Effluents from the H2O2 step were treated with þ → ðÞðÞ⋅ ð Þ 20H2O Ca6Al2 SO4 3 OH 12 26H2O 3 lime (to maintain a pH of 10) to avoid the formation of haz-

ardous cyanide species. Subsequently, Fe(SO4)3 was added Maintaining a high grade of water saturation in a CT-CPB for immobilization of the metal(oid)s. Fe(SO4)3 addition con- will hinder sulfide oxidation and the subsequent sulfate attack tinued until a pH of ~ 8 was reached for the outlet water. on the C-S-H. In field conditions, a CT-CPB is typically Portland cement (CE) and biofuel fly ash (FA) were tested flooded when mine operations are disclosed and the ground as cementitious binders for the preparation of various water has recovered to natural levels. Complete flooding of a cemented paste mixtures. The ash was provided from a biofuel CT-CPB-filled stope may take several years, and unsaturated incineration plant located near to the mine site and could be zones may form within the CT-CPB (Ouellet et al. 2006). This classified as a class C fly ash (SiO2 +Al2O3 +Fe2O3 ≥ 60 Environ Sci Pollut Res weight (wt%)), according to ASTM C618-05 (2005). The contents of Cu, Ni, and Zn were determined by high- main elements and total content of Cu, Ni, S, and Zn in the resolution inductively coupled plasma-mass spectrometry CT, ore, CE, and FA are presented in Table 1. (HR-ICP-MS), whereas Al, Ca, Fe, K, Mg, Na, and S were In Hamberg et al. (2016), X-ray diffraction (XRD) and determined by inductively coupled plasma atomic emission scanning electron microscopy combined with energy disper- spectroscopy (ICP-AES). Prior to analysis, samples were acid- sive spectroscopy (SEM-EDS) revealed that pyrrhotite and ified with super-pure nitric acid using 1 mL of acid/100 mL of arsenopyrite were the main Fe- and As-sulfide minerals in sample. Analyses of HR-ICP-MS and ICP-AES were per- the CT. Quartz constituted the main mineral (~ 80 wt%) in formed using modified versions of the US Environmental the studied tailings. Tremolite, albite, and microcline each Protection Agency (US EPA) methods 200.8 and 200.7, accounted for < 5 wt% of the tailings, and no Cu-, Ni-, or respectively. Zn-carrying minerals were found. The acid-base accounting Solid samples were analyzed for major and minor elements (ABA) test was conducted in Hamberg et al. (2015)and by the accredited laboratory. For the determination of Cu, Ni, showed that the tailings were considered to be acid generating S, and Zn, samples were dried at 50 °C, digested with 7 M with an acid potential (AP) of 65.3, neutralization potential nitric acid in closed Teflon vessels, and treated in a microwave (NP) of ~ 4.5. This rendered a net neutralization potential oven. Concentrations of other elements were determined after (NNP) of − 60.5. Calculation performed in Hamberg et al. fusion with lithium metaborate, followed by dissolution in (2016) suggested that the sulfide and sulfate content in CT dilute nitric acid. The solutions were subjected to centrifuga- accounted for 44 and 56 wt%. These estimations were based tion and diluted before analysis. For quality control, two in- on the suggestions of Dold (2003), which encountered the house reference materials were analyzed in parallel with the sulfate content to be the amount of S released in steps 1–4 solid samples. The dry matter content of CT, CE, and FA during the sequential extraction test. The sulfide content was samples were determined in triplicate by drying the samples equal to total S released in step 5–6. in an oven at 105 °C for 24 h according to Swedish standard SS 028113-1 (SIS 1981). Sample analysis Preparation of CT-CPB mixtures The pH of the water samples was measured on unfiltered samples. The pH of the water samples was measured on un- Mixtures (desired strength 0.2 MPa) were designed with min- filtered samples. The paste-pH was measured by a methods imal fractions of binders (see Table 2). The tailings were suggested by Sobek et al. (1978)andPageetal.(1982). mixed with the binders (CE and FA), and ~ 25% wt% of Twenty grams of solids were mixed with 20 g of deionized distilled water was slowly added (see Table 2 for the compo- water, stirred for 5 s, and then left for 10 min before pH was sition of the CT-CPB mixtures (CE and CE-FA)). The prepa- measured. A pH meter called BMeterlab PHM201^ pH-Eh ration is described in detail by Hamberg et al. (2015). CT-CPB meter calibrated against standard buffers at pH 4.01 and 7.0 mixtures of CE and CE-FA were cured for 31 or 446 days was used. Element compositions in the water samples were (referred to as CE31, CE446, CE-FA31, and CE-FA446 here- analyzed by a laboratory accredited by the Swedish board of after). CE31 and CE-FA31 were kept at room temperature at accreditation and conformity assessment (SWEDAC). Total ~ 80% humidity under dark conditions, until the 31st day. In

Table 1 Total content of elements and pH in the ore, tailings, fly ash, Ore CT Fly ash Cement and cement (a selection from 31 elements is presented) (n =3,± Total solids (TS) 99.97 ± 0.06 89.0 ± 0.4 95.2 ± 0.7 99.4 ± 0.0

SD) SiO2 30.3 ± 0.9 55.0 ± 4.9 34.6 ± 1.3 20.6 ± 0.8

Al2O3 1.74 ± 0.07 4.69 ± 0.04 10.7 ± 0.6 5.61 ± 0.45 CaO 4.26 ± 0.09 4.83 ± 0.25 14.1 ± 1.0 50.3 ± 1.8

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

K2O 0.32 ± 0.00 0.92 ± 0.03 2.89 ± 0.08 0.83 ± 0.05 MgO 2.38 ± 0.07 3.24 ± 0.01 2.54 ± 0.08 4.00 ± 0.17 Cu 82.0 ± 12.1 147 ± 7 136 ± 10 86.2 ± 2.9 Ni 126 ± 12 63.8 ± 2.1 114 ± 9 63.8 ± 1.3 S 25,367 ± 2363 20,933 ± 493 13,700 ± 200 9960 ± 219 Zn 10.6 ± 1.5 25.0 ± 0.4 374 ± 10 149 ± 3 Paste-pH N.D 5.03 ± 0.25 13.2 ± 0.6 12.7 ± 0.3

N.D not determined Environ Sci Pollut Res

Table 2 Composition weight % (wt%) of cemented paste backfill Names Portland Biofuel fly Total binder/dry Water Tailings (wt%) (CPB) mixtures containing cement (wt%) ash (wt%) weight (wt%) content (wt%) cement (CE) and/or fly ash (FA) and unmodified cyanidation CE 100 1 26 73 tailings (CT) CE-FA 67 33 3 26 71 CT 0 0 0 11 89 addition, CE446 and CE-FA446 were cured under the same the experiment. Conversely, diffusion-controlled release conditions as CE31 and CE-FA31 until the 31st day, and sub- should yield a linear plot with a slope of 0.4–0.6, and sequently at ~ 60% humidity, in the dark, under room temper- dissolution controls the release if the slope is > 0.6. ature conditions until the 446th day. These conditions were chosen to enhance the formation of an unsaturated region at Sequential extraction test the surface of the CPB mixtures, as described by Ouellet et al. (2006). The curing period was terminated when brighter Fractionation of Ca, Fe, S, Ni, Zn, and Cu was assessed using (drier) regions (compared with those in the bulk) formed on a modified sequential extraction scheme described by Dold thesurfacesoftheCPBmixtures. (2003).Ineachextractionsequence,2gofCT,ore,CE446, and CE-FA446 were used and extracted with five different solutions consecutively. Materials from CE446 and CE- Leaching tests FA446 were taken from the outermost (brighter) regions with a lower water saturation level than the other regions. Overall Flooded monoliths—tank leaching test details about the extraction procedure are presented in Hamberg et al. (2016). Tank leaching tests (TLTs) were conducted, in accordance with Dutch standard EA NEN 7375: 2004,onCT,CE- Weathering cell test FA31, CE-FA446, CE31, and CE446. Duplicate samples were removed from the bottles after 31 or 446 days and shaped into Approximately 70 g of tailings was placed on a paper filter in regular cylinders. The overall experimental setup is described a Büchner-type funnel. Duplicate samples were exposed to 31 in Hamberg et al. (2015). CE 446 fell apart upon contact with weekly cycles (217 days), involving 1 day of leaching, 3 days water and, therefore, placed in paper filter bags (with of ambient air exposure, a second day of leaching, and two μ 0.45- m pores) inside nylon sample holders. The filter bags subsequent days of air exposure. The samples were leached by containing the granular CT and CE mixtures were changed covering each with 50 mL of Milli-Q H2O for ~ 2 h. The upon each leachate renewal and were fully immersed in the leachates were then recovered by applying a vacuum to the water. In addition, the water was exchanged and analyzed funnel. The pH, Eh, and EC of the filtered leachates were after 0.25, 1, 2.25, 4, 9, 16, 36, and 64 days. The mass trans- subsequently measured; the chemical composition of the wa- fer of Cu, Fe, S, Ca, Zn, and Ni from CT and the CT-CPB ter samples was determined as described in Hamberg et al. materials was calculated from (as specified in the EA NEN (2016). 7375: 2004 standard): pH-dependent leaching test M ti ¼ ðÞCi V i =A ð4Þ

2 Where, Mti (mg/m ): mass of the element released dur- The pH-dependent leaching test was inspired by the previous ing leaching period i; Ci (mg/L): element concentration study of Peyronnard et al. (2009). Surficial materials of CE- for period I; Vi (L): leachate volume for period I; and A: FA446 and CE446 were crushed into a pulverized material, specimen surface area (m2) exposed to the leachate. The divided into 12.5-g sub-samples, and placed in acid-cleaned apparent diffusion coefficient of Cu, Fe, S, Ca, Zn, and Ni plastic bottles. The aim of the test was to estimate the release was determined by plotting the logarithm of the cumula- of Cu, Ni, and Zn in response to a decrease in the pH of the tive release Mt until the ith period against the logarithm of CPB material. A more detailed description of the test is de- the time elapsed since the start of the experiment. A plot scribed in Hamberg et al. (2017). The pH ranged from 8 to ~ with a slope (rc)of<0.4isexpectedforthesurfacewash- 2.7 corresponding to the normal pH of CE446, CE-FA446, off process, where surface-captured elements are released and unmodified tailings, respectively. The bottles were stirred from the surface of the CT-CPB. The slope is defined as for 8 days to reach a constant pH, and then leachates were the logarithm of the total mass of the transferred elements filtered with 0.45-μm polypropylene membranes on a plotted against the logarithm of the time elapsed during Büchner filter. Environ Sci Pollut Res

Modeling—PHREEQC analyzed elements in the ore was associated with the sulfide/ residual fraction. Less than 30 wt% of Zn was associated with Speciation-solubility calculations were performed with the more soluble phases. The corresponding proportions of Cu, geochemical code PHREEQC (Parkhurst and Appelo 1999) Ni, S, Ca, Fe, and Si associated with more soluble phases were using the ThermoDem database (Blanc et al. (2007), version 3–12 wt%. In the ore, there were no or very small amounts of V1:10, which includes cementitious phases inferred by analyzed elements associated with the water soluble phase. In Lothenbach et al. (2008). Element concentrations, redox po- CT, proportions of elements associated with the sulfide/ tential (Eh), and pH in leachates from the TLT were used in residual fraction were lower compared to that in the ore, espe- these calculations. cially that of Ni. Instead, significant proportions of Ni (77 wt%), Cu (~ 10 wt%), Zn (~ 20 wt%), Ca (~ 8 wt%), Scanning electron microscopy and S (35 wt%) were associated with the water soluble fraction in CT. Concerning Fe and Si, minor amounts in CT were A JSM-IT 100 scanning electron microscopy (SEM), by associated with the water-soluble phase. The proportions of JEOL, was used for the observation of particle morphology. Cu, S, and Fe associated with AEC, Fe(III)-oxy-hydroxide, Investigated samples were dried, powdered, and placed over a and Fe(III) oxides in CT were higher compared to that in the conductive carbon tape. Secondary electron detector (SED) ore, but unchanged for Zn and Si (Fig. 1). Total extracted mode, with high vacuum and acceleration of 10 kV, is used amount of Cu was significantly higher in CT compared to that to obtain all the micrographs. The morphology is analyzed at in the ore (Fig. 1). Detailed results from the sequential extrac- different magnifications. tion tests are attached in an appendix Table 4. In the CT-CPB mixtures, less proportions of Cu, Ni, and Zn were associated with the water-soluble fraction, compared to Results that in CT. The opposite concerned for S in CE-FA446. In general, higher proportions of Ni, Zn, Cu, Fe, S, Si, and Ca Sequential extraction test in CE-FA446 were associated with the AEC fraction, compared to that in CE446. This difference is most significant Sequential extractions were conducted on triplicate samples of concerning Ca (Fig. 2). Proportions of Ni, Zn, Cu, Fe, S, Si, ore, CT, CE446, and CE-FA446. A majority of all the and Ca associated with Fe(III)-oxy-hydroxide and Fe(III)

Fig. 1 Fractionation of Ni, Zn, Cu, Fe, S, Si, and Ca in the ore and CT (average values presented, n = 3). Total content (n = 3; ± SD) of extracted elements in attached tables Environ Sci Pollut Res

Fig. 2 Fractionation of Ni, Zn, Cu, Fe, S, and Ca in the CE446 and CE-FA446 (average values presented, n = 3). Total content (n = 3; ± SD) of extracted elements in attached tables oxides were similarin CE-FA446 and CE446 (Fig. 2). Detailed during day 9–36 and was significantly lower in the last extrac- results from the sequential extraction tests are attached in an tion. A Ni release from CE446 increased throughout the TLT appendix Table 4. (Fig. 3). The release of Si was most abundant in CT and Compared with CT, CE and FA contained higher total con- increased steeply toward the end of the TLT. At the last ex- centrations of Ni and Zn. Similarly, Ni and Zn occurred with traction, the Si release from CE31, CE-FA446, and CT in- higher fractions in the CT-CPB mixtures (CE31, CE-FA 31, creased, whereas the opposite occurred in CE-FA31. A Si CE-FA446, and CE446) than in the CT. An addition of CE release from CE446 increased throughout the TLT. The pH increased the fraction of Ni by 1.8%, in the CE mixture, com- evolved in a similar way in CE31 and CE-FA31. In each case, pared to that in CT. The corresponding increase in CE-FAwas the pH increased initially, stabilized at ~ 10–10.5, and dropped 21.7 (Zn) and 3.0% (Ni). to 8 during the last extractions. The pH of CE446 and CE- FA446 were lower than those of CE31 and CE-FA31. Tank leaching test Furthermore, the pH of CE-FA446 increased initially and stabilized at a pH of 8. In CE446, pH increased throughout the In the TLT, small deviations are observed in results from the TLT from 3 to ~ 6. duplicated tests of CT, CE 31, CE 446, CE-FA 31, and CE-FA The saturation index of gypsum in CT decreased during the 446 and, therefore, average values are presented. The release TLT. The saturation indexes of ferrihydrite in CE 446 and CE- of Fe, Ni, Zn, and Si from CT was significantly higher than FA 446 were negative (suggesting dissolution) during extrac- that released from the CT-CPB-materials. The opposite con- tions 1–2, but positive during extractions 3–8. Furthermore, cerned for Cu, whereas higher amounts were released from the saturation indexes of ferrihydrite in CT were negative dur- CE446. In the CT-CPB materials, the largest amount of Fe, Ni, ing all extractions, except for extraction 4 (Fig. 4). and Si was released from CE446 (Fig. 3), while the most Results of the calculations based on the EA NEN 7375: 2004 abundant release of Zn was encountered from CE-FA446. In standard indicated that similar Cu and Ni leaching mechanisms all materials, the release of Cu and Fe evolved in a similar way operate in CE31 and CE-FA31 (Table 3). In the CT-CPB mate- and increased considerable during the first days (until the rials, the cumulative release of Zn from CE-FA31 and CE-FA446 ninth day) of the TLT but was lowered toward the end. This was higher than that from CE31 and CE446. Similarly, more Fe was also somewhat true for the evolution of Ni and Zn. and Cu were released from CE-FA446 and CE446 than from However, the Zn release in CE-FA446 increased steeply CE31 and CE-FA31. The release of Cu and Ni from CE31 and Environ Sci Pollut Res

Fig. 3 Evolution of pH and cumulative release of Si, Cu, Zn, Ni, and Fe (in mg/m2) from CT, CE-FA31, CE31, CE-FA446, and CE446 during TLT (averaged values, n =2)

CE-FA31 occurred via diffusion induced by the dispersal of these Weathering cell test elements from pore water. However, the release of Zn from the granular materials (CT and CE446) and the monoliths (CE- After 217 days of the weathering cell tests, the cumulative FA446, CE31, and CE-FA31) was induced by wash-off behavior release of Ni, Zn, Fe, and Cu from the CT-CPBs was lower and diffusion, respectively (Table 3). than that from CT. More Cu was released from CE-FA than

Fig. 4 Saturation indexes of gypsum and two line (2 L) ferrihydrite during TLT Environ Sci Pollut Res

2 Table 3 Cumulative amounts of elements (in mg/m ) released via the experienced a different leaching pattern and was more abun- dominant leaching mechanism during TLT dant at pH 8 compared to that in pH 6 (Fig. 6). Mixture Cu Ni Zn Scanning electron microscopy CT 8.0 DF 162 SW 199 SW CE 446 35 SW 8.6 SW 8.9 SW In Fig. 7a, tabular-shaped particles with approximately length CE 31 2.7 DF 3.3 DF 31 DF of 30–40 μm suggested the presence of gypsum (Myagkaya et CE-FA 446 7.7 DF 1.0 DF 124DS al. 2016). In addition, typical gypsum morphology particles CE-FA 31 2.5 DF 1.5 DF 68 DF are detected in CE-446, Fig. 7b. From Fig. 7c, d, it was visible that tailings treated with CE and FA have developed needle- DF diffusion, SW surface wash off, DS dissolution shaped morphology, typical of the hydrated cement paste phases, suggesting the formation of C-S-H and ettringite from CE-FA, but the opposite was true for Ni and Zn. Similar (Panchal et al. 2018). In addition, typical spherical shaped pH trends were observed for CE and CE-FA. In both cases, the fly ash particles are observed in CE-FA446, Fig. 7d(Grauet pH of the washings decreased from 11.4 (CE-FA) or 10.1 (CE) al. 2015). to a stable value of ~ 8 after 10 days (Fig. 4). Small deviations were observed in the results of the duplicated tests of CT, CE, and CE-FA, and, therefore, average values are presented. The Discussion release of Ni from the CT-CPB samples seemed to be correlated with changes in the pH (i.e., the maximum release oc- Unsaturated conditions in CT-CPB mixtures—effect curred when the pH decreased from 11 to 8). Similar Zn and on the solubility of cementitious binders Cu evolution was observed in the CT-CPB samples, with maximum values occurring on the 10th day of the weathering Immobilization of trace metals in cementitious mixtures has cell test (WCT). The values then decreased until day 24, but been applied to physical encapsulation and chemical stabili- then increased until day 52. The amount of Si decreased until zation (Chen et al. 2009; Paria and Yuet 2006; Benzaazoua et the tenth day of the experiment, but increased thereafter. Prior al. 2004). The physical strength is strongly dependent on the to the addition of acid (after day 70), a low amount of Fe was stability of cementitious C-S-H. Ideally, cementation will re- released from the CT-CPBs. duce the open porosity and obstruct water percolation through After 70 days, acid was added to the leachate with the aim the CT-CPB mixtures. In these mixtures, the addition of ce- of consuming the buffering minerals in the CPB materials. mentitious binders was probably manifested as an increase in Different amounts of acid (1 M HCl) were added over 77 days the amount of Ca and S associated with the AEC fraction (between days 70 and 147) to each CPB material (CE 0.47 M (Dold 2003). Pyrrhotite oxidation generated sulfates that dis- H+/kg TS and CE-FA 0.69 M H+/kg TS). Consequently, the solved the binders, resulting in the formation of gypsum and pH of each material decreased to ~ 4.5, i.e., the initial value ettringite (reaction 4) and, in turn, a reduction in the mechan- determined for the unmodified tailings. Small deviations were ical strength. Dissolution of this kind is suggested by the pres- observed in the results of the duplicated tests of tailings CE ence of C-S-H and ettringite in CE-FA446 (Fig. 7). The solu- and CE-FA, and, therefore, average values are presented. The bility of the cementitious phases increased at pH values of < 9 leaching of Ni, Zn, Si, Cu, and Fe from CE and CE-FA in- (Benzaazoua et al. 1999). The evolution of pH in leachates creased with the addition of acid. At 217 days, the cumulative from all the CT-CPB mixtures during the TLT suggested that release of all the presented elements from the CT-CPBs was the cementitious phases have dissolved to some extent, espe- lower than the release from the CT. With the addition of acid, cially in CE446 that fell apart into a granular material. the leaching of Si, Fe, Zn, and Ni from CE was more strongly Cementitious phases were more abundant in CE-FA446 com- increased than the corresponding leaching from CE-FA. pared to that in CE446 that seems to contain more gypsum Endpoint pH values of (i) ~ 4.5 and (ii) 3.2 were obtained (Fig. 7). The low strength of CE446 was probably also for (i) CE and CE-FA and (ii) the tailings, respectively. Cu reflected in the larger amount of gypsum formed, as indicated and Fe leaching from the unmodified tailings increased during by the higher fraction of water-soluble Ca and S (Dold 2003), WCT, and pH values of < 3.5 were obtained (Fig. 5). compared with that of CE-FA446 (see Fig. 2).

Unsaturated conditions in CT-CPB mixtures—effect pH-dependent leaching test on the release of Ni, Cu, and Zn

The Ni and Zn release from CE446 and CE-FA446 increased According to Sciuba (2013), the major sulfide minerals in in tandem with a pH decrease. The release of Cu and Fe Svartliden consist of pyrrhotite (Fe7S8) and arsenopyrite Environ Sci Pollut Res

Fig. 5 Evolution of pH and release of Ni, Zn, Cu, Si, and Fe from CT, CE31, and CE-FA31 with time during the WCT. Acid was added during days 70– 147; no acid was added to CT

(FeAsS). However, traces of chalcopyrite (CuFeS2), pyrite Zn(OH)2), and Ni(OH)2) (Donahue et al. 2000;Hidmi (FeS2), and sphalerite (ZnFeS) are also found. No chalco- and Edwards 1999). These hydroxides are stable under pyrite, sphalerite, and Ni species was found during min- pH-neutral, oxidized conditions (Karapınar 2016; eralogical analyses, but it is a common fact that mining Kumpiene et al. 2008). In CT, pyrrhotite oxidation prod- processes is never 100% effective, so remnants of sulfides ucts leads to acidic conditions (pH: 3.5–5.0) in TLT- are probably present in CT. Results from the fractionation leachates (Fig. 3) that probably dissolved a majority of (Fig. 1) suggested that significant amounts of sulfides in these hydroxides. A majority of the Zn, Ni, and Cu were the ore oxidized during the cyanide leaching process. A rinsed of the surface via a wash-off-effect in CT (Table 3) substantial fraction of Zn, Cu, and Ni from this process whereas initially high amounts of these elements de- was re-distributed to water-soluble-species and/or co- creased toward the end TLT (Fig. 3). This evolution is precipitated with Fe-(oxy)-hydroxides. The cyanide typical for a wash-off-effect, probably induced by the for- leaching process yielded a pH of ≈ 8fortheoutletwater. mation of a reddish crust on the surface of CT mid-way This value resulted from the addition of lime and conse- through the TLT. The crust was assumed to be an quent formation of metal hydroxides (i.e., Cu(OH)2, enclosing iron-oxide precipitate. As hydroxides of Ni, Environ Sci Pollut Res

Fig. 6 The release of Ni, Cu, Fe, and Zn (expressed in mole) during the pH-dependent leaching test (averaged release, n =2)

Cu, and Zn dissolves, these could be re-adsorbed onto Fe were more than ten times higher than the Cu release in the precipitates, but this process becomes less effective in CT. This may have resulted from the fact that Cu is read- acidic conditions. In addition, Ni release and Zn release ily adsorbed onto Fe precipitates at a pH of 4, whereas

Fig. 7 SEM micrographs of CT (a), CE446 (b), and CE-FA446 (c, d) Environ Sci Pollut Res other metals (i.e., Ni and Zn) remain in the solution governed by the stability of the cementitious phases and/ (Dzombak and Morel 1990). or desorption from Fe precipitates. In CE31 and CE-FA31, the dissolution of the cementi- The establishment of unsaturated conditions in the CT-CPB tious phases increased the pH of the TLT leachates to mixtures resulted in increased pyrrhotite oxidation and pro- alkaline (pH 8–10.5) levels. The diffusion-like behavior moted acidic conditions in the TLT leachates. In the TLT of Cu, Zn, and Ni suggests that these elements were leachates, initial pH was lower in CE446 compared to that in transported from the CT-CPB pore water to the surround- CE-FA446. In CE446, pyrrhotite oxidation may have ing leachate. Heavy metals in a CPB matrix could occur progressed more efficiently compared to that in CE-FA446. as metal hydrated phases or metal hydroxides precipitated Increasing the binder proportion by adding FA (such as in CE- on C-S-H-surfaces (Li et al. 2001). In these cases, a metal FA mixtures) may enhance the microstructure and decrease release occurs as the C-S-H:s are dissolving, causing al- the chemical (oxygen transport) reactivity in the CT-CPB mix- kaline conditions in the CT-CPB-leachates. This phenom- tures, by reducing the diffusion rate of oxygen through the CT- enon could result in the formation of anionic complexes CPB mixtures (Aldhafeeri and Fall 2017). A Cu release in- with a low affinity for Fe precipitates in the pore water of creased, especially in CE446, and was higher than the Cu CE31 and CE-FA31. A Ni and Cu release could then be release from CT. An increase in the Cu leaching was most induced by desorption from Fe precipitate surfaces evident at pH < 4 and was probably correlated with the disso- (Kumpiene et al. 2008). The Zn release from the CT- lution of ferrihydrite (Fig. 4). According to Munk et al. (2002), CPB mixtures was approximately a tenfold higher than Cu has a sorption edge onto ferrihydrite at a lower pH com- the Ni and Cu release and most extensive at pH levels pared to that of Ni and Zn. The fraction of Cu released in of < 8 or > 10 (Fig. 3). A Zn release was probably neutral/alkaline conditions could therefore be higher; this is governed by zinc hydroxides (Zn(OH)2) that are ampho- also seen in the pH-dependent leaching test (Fig. 5). The Cu teric and mobile in both alkaline and acidic conditions fraction desorbed is probably secondarily connected to Fe(III) (Malviya and Chaudhary 2006). Zinc hydroxides could oxy-hydroxides, which are more easily released (especially in gradually be transformed into cationic species (Zn2+, the acidic range) than the originally formed Cu-Fe precipitates Zn(OH)+) as the pH decreased to < 10 and anionic species (Munk et al. 2002). This probably resulted in a greater release at pH > 10 (Degen and Kosec 2000). Anionic Zn species of Fe-associated Cu (especially under acidic conditions) from could then be released due to desorption from Fe precip- CE446, during the TLT, than from CT (Fig. 8). itates. The Zn release may also origin from the fly ash An establishment of unsaturated zones in the CT-CPBs had that, compared to CT, contains substantially higher frac- a negligible effect on the Ni release, which was low during the tions of Zn (Table 1). In the CE-FA mixtures, fly ash TLT. The Ni release from CE446 was more abundant com- could contain Zn attached to calcium alumino-silicate pared to that in CE-FA446. This could be due to a more abun- glass that is more soluble under alkaline than acidic con- dant dissolution of amorphous Ni-bearing Fe phases and a ditions (Luo et al. 2011). That might explain a compara- comparatively lower pH (Figs. 3 and 4). The Zn release from tively higher release of Zn in alkaline conditions. Overall, the CT-CPB was lower than that from CT, irrespective of under saturated conditions, the amounts of Cu, Ni, and Zn binder fraction and the water saturation level. As unsaturated released from the CT-CPBs were lower compared to that zones formed in the CT-CPB mixtures, the Zn release from the in CT. Furthermore, this release (from the CT-CPBs) was CE-FA mixtures increased, while the opposite occurred in the

Fig. 8 Release of Cu and Fe (expressed in mole) from CE446, CE-FA446, and CT during the TLT, linear regression analyses (R2) Environ Sci Pollut Res

CE mixtures. A minor Zn release from CE446, compared with cementitious phases. When acid was added to the CT- that from CE-FA446, could be due to a comparatively lower CPBs, the cumulative release of Ni, Cu, and Zn from total content of Zn (Table 1), owing to the addition of a lower the CT-CPB mixtures was substantially lower than that fraction of binder. Elements associated with the AEC fraction from the CT. This may have resulted from the fact that are often bound to carbonate phases that are sensitive to pH the pH at the endpoint (3.2) of the CT leachates was fluctuations. Carbonate phases formed under alkaline condi- lower than that (pH: 4.5) of the CT-CPB leachates. The tions are known to be thermodynamically unstable in the pres- reaction kinetics of the WCTs differed from that of the ence of atmospheric carbon dioxide (Benzaazoua et al. 2004). TLT. In this case, the solubility of Cu, Ni, and Zn At the surface (with a lower water saturation level than the depended mainly on the pH and, hence, the elements were bulk) of CE446 and CE-FA446, carbonation resulting in dis- probably released via the dissolution/precipitation phases solution of the cementitious phases may have occurred. In in the weathered CPB samples. CE446, this yielded a reduction in Zn leaching, where most of the cementitious Zn phases on the surface dissolved, thereby forming more acid-tolerant species. A Zn decrease may Conclusions have also resulted from the fact that the leachate/CPB contact area during a surface wash-controlled release (as in CE446) is Flooding of cemented paste backfill (CPB)-filled mine fa- lower than the area associated with a dissolution-like release cilities is, commonly, a slow process where unsaturated (as in CE-FA446). In CE-FA446, more Zn is released at neu- zones may form within the CPB mass. Cyanidation tailings tral pH than under acidic conditions (Fig. 3). That substanti- (CT) with elevated concentrations of Cu, Ni, and Zn and a ates a Zn release from FA, which might contain Zn attached to pyrrhotite content of 1 wt% were managed by using CPB calcium alumino-silicate glass that is more soluble under al- (CT-CPB). Two low strength (0.2 MPa) CPB mixtures kaline conditions than acidic, suggested by Lou et al. (Luo et were used, CE (tailings mixed with 1 wt% cement) and al. 2011). By adding binders to CT, a significant fraction of CE-FA (tailings mixed with 2 wt% of cement and 1 wt% Cu, Ni, and Zn might have been re-distributed from the water- of fly ash). Using a low strength CPB resulted in decreased soluble fraction to AEC phases in CE446 and CE-FA446. leaching of Zn and Ni, regardless of water saturation level However, a large part of Zn, Cu, and Ni associated with the and binder proportion compared to that occurring in the AEC fraction in CE446 and CE-FA446 could also originate CT. The establishment of unsaturated zones within the from the binder themselves, as previously suggested in many CT-CPBs leads to significant pyrrhotite oxidation, which studies (Pöykiö et al. 2011; Nurmesniemi et al. 2008; Świetlik resulted in acidity and, in turn, dissolution of the cementi- et al. 2013). In these cases, maintaining high water saturation tious phases. At this stage, the mobility of Zn increased in levels in the CT-CPB mixtures is essential to preventing the tandem with pH in the CT-CPB mixtures. In CE mixtures, effects of ongoing pyrrhotite oxidation. semi-acidic conditions established, Zn-release decreased Mobility of Ni, Cu, and Zn in CT and CT-CPB mixtures whereas relatively stable Zn-species formed. In CE-FA during accelerated weathering mixtures, a dissolution of larger amounts of binders caused alkaline conditions to be established, and Zn release increased due to the solubility of fly ash-specific Zn species. Ni release from CT seemed to be correlated with the A Ni release was low irrespective of binder proportions water-soluble Ni phases. This initially high release de- and unaffected by varying saturation levels. In CE, an es- creased sharply and diminished toward the end of the tablishment of unsaturated zones increased Cu release to be WCT, in contrast to the Zn release that was low during higher than that from CT. This increase resulted possibly the WCT. The Cu release seemed to be correlated with from the formation of Cu phases that were less acid toler- the Fe-release, increased rapidly as the pH dropped below ant than those comprising the CT. The aforementioned re- 4(Fig.5). The pH increased after the binders were added. sults indicated that using a low strength CPB is not an Moreover, the results from the fractionation suggested that environmentally friendly option for underground storage; a large fraction of water-soluble phases was re-distributed otherwise, large amounts of Cu could be released. into AEC phases (Fig. 1), which are sensitive to pH fluctuations (Dold 2003). In the CT-CPBs, the Cu, Ni, and Zn Acknowledgements Authors wish to thank Ms. Ilda Tole at the Luleå release exhibited similar trends and the maximum levels University of Technology for her help in SEM-EDS analyses. This work occurred with a sharp decrease (from 11 to 8) in pH until was supported by the Ramböll Sverige AB, Ramböll Foundation, day 10. The release of Cu, Ni, and Zn decreased when SUSMIN—Tools for sustainable gold mining in EU, and the Center of the pH of the CT-CPB leachates stabilized at 8 (Fig. 5). Advanced Mining and Metallurgy (CAMM) at the Luleå University of Technology. The personnel at Dragon Mining AB are gratefully acknowl- This suggested that these elements are released, owing to edged for providing valuable information about the mining processes and desorption from Fe precipitates and/or the dissolution of the tailings evaluated in this study. Environ Sci Pollut Res

Appendix

Table 4 Results from the sequential extraction test (results expressed in mg/kg, n = 3, ± SD)

Ore Water soluble AEC Fe(III) oxyhydroxides Fe(III) oxides Sulfides/residual Total

Ca mg/kg 160 ± 3 1427 ± 115 ––44,652 ± 2980 46,249 ± 3111 Fe BB 1.58 ± 0.39 1968 ± 23 3014 ± 83 2916 ± 57 97,606 ± 4689 105,506 ± 4693 Si BB 13 ± 1.1 57.5 ± 5.2 346 ± 17 643 ± 26 336,282 ± 1367 344,410 ± 9056 Cu BB – 0.02 ± 0.01 0.64 ± 0.03 – 22 ± 0.7 23 ± 1.0 Ni BB 0.01 ± 0 5.1 ± 0.34 3.05 ± 0.11 0.65 ± 0.06 58 ± 1.4 67 ± 2.2 Zn BB 0.16 ± 0.17 2.37 ± 1.52 3.21 ± 1.55 1.37 ± 1.18 19 ± 0.2 51 ± 22 S BB 86.5 ± 0.5 0 ± 0 – 173 ± 21 10,084 ± 268 10,354 ± 304 CT Ca mg/kg 3346 ± 56 227 ± 29 ––41,684 ± 1691 45,258 ± 1666 Fe BB 41.4 ± 2.56 353 ± 3 36,495 ± 202 5761 ± 303 79,122 ± 2046 121,772 ± 2656 Si BB 21.7 ± 0.7 28.1 ± 0.4 960 ± 4 673 ± 22 342,727 ± 9023 337,341 ± 1370 Cu BB 8.99 ± 0.69 0.82 ± 0.07 31.1 ± 0.8 – 54 ± 7.9 96 ± 8 Ni BB 36 ± 3 1.75 ± 0.42 3.7 ± 0.12 0.8 ± 0.08 4.5 ± 0.8 47 ± 2.7 Zn BB 9.36 ± 0.96 2.92 ± 2.56 6.48 ± 0.18 2.7 ± 1.76 30 ± 16 51 ± 22 S BB 5268 ± 141 1626 ± 44 1053 ± 13 807 ± 136 6312 ± 991 15,066 ± 998 CE-FA446 Ca mg/kg 2443 ± 132 11,915 ± 250 0 ± 0 39 ± 5.4 37,405 ± 2124 51,802 ± 1953 Fe BB 0.3 ± 0 2104 ± 78 15,624 ± 908 5795 ± 556 92,400 ± 3208 115,923 ± 2795 Si BB 46 ± 0.4 1453 ± 212 2206 ± 198 1271 ± 24 278,818 ± 8801 283,793 ± 8717 Cu BB 0.1 ± 0.0 28 ± 0.9 35 ± 2.1 0.3 ± 0.1 101 ± 3.9 165 ± 6.6 Ni BB 0 ± 0 12 ± 1.7 21 ± 1.2 1.9 ± 0.2 33 ± 2.4 67 ± 1.7 Zn BB 1.1 ± 0.6 15 ± 2.4 22 ± 1.7 5.3 ± 0.5 15 ± 0.7 58 ± 2.1 S BB 2082 ± 100 2221 ± 183 200 ± 22 83 ± 17 16,700 ± 755 21,287 ± 805 CE446 Ca mg/kg 7378 ± 112 707 ± 38 0 ± 0 21 ± 0.6 38,119 ± 1327 43,560 ± 3396 Fe BB 0.2 ± 0.1 458 ± 14 19,086 ± 96 3756 ± 482 90,300 ± 700 105,721 ± 13,356 Si BB 25 ± 0.7 212 ± 5.7 2075 ± 30 917 ± 72 263,856 ± 7142 265,977 ± 5412 Cu BB 0.0 ± 0.0 6.0 ± 1.4 35 ± 1.8 0.9 ± 0.2 107 ± 2.0 135 ± 22 Ni BB 0.8 ± 0.1 5.5 ± 0.3 30 ± 0.7 1.8 ± 0.2 30 ± 0.6 55 ± 21 Zn BB 4.8 ± 1.2 9.8 ± 1.2 25 ± 6.1 4.1 ± 1.4 14 ± 1.2 46 ± 29 S BB 6766 ± 91 228 ± 31 73 ± 1.9 36 ± 2.2 14,600 ± 700 19,370 ± 4188

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