Sulphide Mineral Flotation

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Department of Civil, Environmental and Natural Resources Engineering Division of Sustainable Process Engineering Alireza Javadi Sulphide Mineral Flotation

ISSN: 1402-1757 Sulphide Mineral Flotation ISBN 978-91-7439-592-1 (tryckt) ISBN 978-91-7439-593-8 (pdf) A New Insight Into Oxidation Mechanisms Luleå University of Technology 2013 A New Insight Into Oxidation Mechanisms

Alireza Javadi

Sulphide Mineral Flotation: A New Insight into Oxidation Mechanisms

Alireza Javadi

Division of Sustainable Process Engineering Department of Civil, Environmental and Natural Resources Engineering Luleå University of Technology, SE-971 87, Sweden

June 2013

Printed by Universitetstryckeriet, Luleå 2013

ISSN: 1402-1757 ISBN 978-91-7439-592-1 (tryckt) ISBN 978-91-7439-593-8 (pdf) Luleå 2013 www.ltu.se “Arrogance is an obstacle of obtaining knowledge and diverts man towards ignorance and humiliation”

Ali al-Hadi (827–868)

3 4 Abstract

Formation of hydrogen peroxide (H2O2), an oxidizing agent stronger than oxygen, by sulphide minerals during grinding was investigated. It was found that pyrite (FeS2), chalcopyrite (CuFeS2), sphalerite ((Zn,Fe)S), and galena (PbS), which are the most abundant sulphide minerals on Earth, generated H2O2 in pulp liquid during wet grinding in the presence or devoid of dissolved oxygen in water and also when the freshly ground solids are placed in water immediately after dry grinding. Pyrite generated more H2O2 than other sulphide minerals and the order of H2O2 production by the minerals found to be pyrite > chalcopyrite > sphalerite > galena. The pH of water influenced the extent of hydrogen peroxide formation where higher amounts of H2O2 are produced at highly acidic pH. The amount of H2O2 formed also increased with increasing sulphide mineral loading and grinding time due to increased surface area and its interaction with water. The sulphide surfaces are highly catalytically active due to surface defect sites and unsaturation because of broken bonds and capable of breaking down the water molecule leading to hydroxyl free radicals. Type of grinding medium on formation of hydrogen peroxide by pyrite revealed that the mild steel produced more H2O2 than stainless steel grinding medium, where Fe2+ and/or Fe3+ ions played a key role in producing higher amounts of H2O2. Furthermore, the effect of mixed sulphide minerals, i.e., pyrite–chalcopyrite, pyrite–galena, chalcopyrite–galena and sphalerite–pyrite, sphalerite–chalcopyrite and sphalerite–galena on the formation of H2O2 showed increasing H2O2 formation with increasing pyrite fraction in chalcopyrite–pyrite composition. In pyrite–sphalerite, chalcopyrite–sphalerite or galena– sphalerite mixed compositions, the increase in pyrite or chalcopyrite proportion, the concentration of H2O2 increased but with increase in galena proportion, the concentration of H2O2 decreased. Increasing pyrite proportion in pyrite–galena mixture, the concentration of H2O2 increased and also in the mixture of chalcopyrite–galena, the concentration of H2O2 increased with increasing chalcopyrite fraction. The results of H2O2 formation in pulp liquid of sulphide minerals and mixed minerals at different experimental conditions have been explained by Eh–pH diagrams of these minerals and the existence of free metal ions that are equally responsible for H2O2 formation besides surfaces catalytic activity. The results also corroborate the amount of H2O2 production with the rest potential of the sulphide minerals; higher is the rest potential more is the formation of H2O2. Most likely H2O2 is answerable for the oxidation of sulphide minerals and dissolution of non-ferrous metal sulphides in the presence of ferrous sulphide besides the galvanic interactions. This study highlights the necessity of revisiting into the electrochemical and/or galvanic interactions between the grinding medium and sulphide minerals, and interaction mechanisms between pyrite and other sulphide minerals in terms of their flotation behaviour in the context of inevitable H2O2 existence in the pulp liquid.

6 Acknowledgements

I would like to appreciate my supervisor, Professor Kota Hanumantha Rao, for his constant guidance, supervision, advices, encouragement, support, valuable discussions and critical evaluation throughout my work; he is the brain behind the success of this project. I gratefully appreciate all practical helps from my Co-supervisor, Dr. Anna-Carin Larsson.

I thank all my colleagues at the Department of Civil, Environmental and Natural Resources Engineering. The following people for helping me with practical issues: Dr. Ulla Grönlund, Assoc. Prof. Ulrika Rova, Prof. Jan Rosenkranz, Assoc. Prof. Bertil Pålsson, Prof. Pertti Lamberg, Dr. Anders Sand, Dr. Anuttam Patra, Tommy Karlkvist, Abdul Mwanga, Mehdi Parian, Mohammad Khoshkhoo Sany and Ulf Nordström. The moral supports of all staff at the mineral processing and process metallurgy groups of our department and mineral processing department at Boliden Mineral AB are greatly appreciated. The financial support in the form of scholarship from the Iranian Ministry of Sciences, Researches & Technologies (MSRT) and Boliden Company is gratefully acknowledged.

I fully wish to thank my wife Fahimeh for her patience, assistance and encouragements, thanks for being supportive finally; I would like to thank everybody who was important to the successful realization of my thesis, as well as expressing my apology that I could not mention personally one by one.

Luleå Sweden, June 2013 ALIREZA JAVADI

List of articles

1. Alireza Javadi Nooshabadi, Anna-Carin Larsson, Kota Hanumantha Rao. Formation hydrogen peroxide by pyrite and its influence on flotation – submitted to Journal of Minerals Engineering.

2. Alireza Javadi Nooshabadi, Kota Hanumantha Rao. Formation hydrogen peroxide by chalcopyrite and its influence on flotation – submitted to Journal of Minerals and Metallurgical processing.

3. Alireza Javadi Nooshabadi, Kota Hanumantha Rao. Formation hydrogen peroxide by sphalerite – submitted to International Journal of Mineral Processing.

4. Alireza Javadi Nooshabadi, Kota Hanumantha Rao. Formation hydrogen peroxide by galena and its influence on flotation – submitted to Journal of Mineral Processing and Extractive Metallurgy (TIMM, Sec. C).

5. Alireza Javadi Nooshabadi, Kota Hanumantha Rao. Formation hydrogen peroxide by sulphide minerals – submitted to Journal of Hydrometallurgy.

6. Kota Hanumantha Rao, Alireza Javadi Nooshabadi, Tommy Karlkvist, Anuttam Patra, Annamaria Vilinska, Irina Chernyshova. Revisiting sulphide mineral (bio)processing: a few priorities and directions – invited key-note paper, XV Balkan Mineral Processing Congress 2013, 12-16 June 2013, Sozopol, Bulgaria.

10 Contents INTRODUCTION ...... 13 Acid Mine Drainage ...... 14 The effects of ferrous ions on oxidation ...... 14 The effects of ferric ions on oxidation ...... 15 Galvanic interactions ...... 15 Effect of Cu2+, Fe2+, and Pb2+ ions on activation of sphalerite ...... 16 EXPERIMENTAL ...... 17 Materials and reagents ...... 17 Wet Grinding ...... 17 Dry Grinding ...... 17 Gas purging ...... 18 Hydrogen peroxide estimation ...... 18 Flotation tests ...... 19 RESULTS AND DISCUSSION ...... 19 Formation of hydrogen peroxide during wet grinding ...... 19 Pyrite–water system ...... 19 Chalcopyrite–water system ...... 20 Sphalerite–water system ...... 21 Galena–water system ...... 21 Formation of hydrogen peroxide after dry grinding mixed with water ...... 22 Pyrite–water system ...... 22 Chalcopyrite–water system ...... 25 Sphalerite–water system ...... 28 Galena–water system ...... 31

Comparison of wet and dry grinding on H2O2 formation ...... 33 Effect of gaseous atmosphere ...... 34 Pyrite ...... 34 Chalcopyrite ...... 35 Galena ...... 35

Mixture of minerals on H2O2 formation ...... 36 Chalcopyrite-pyrite mixture ...... 36 Sphalerite-pyrite mixture...... 38 Galena-pyrite mixture ...... 39 Sphalerite-chalcopyrite mixture ...... 41 Galena-chalcopyrite mixture ...... 42 Sphalerite-galena mixture ...... 44 Flotation studies ...... 45 Pyrite ...... 45 Chalcopyrite ...... 47 Sphalerite ...... 48 Galena ...... 49 CONCLUSIONS ...... 50 FUTURE WORK ...... 52 REFERENCES ...... 53

INTRODUCTION

Oxidation of sulphide minerals takes place when they are exposed to atmosphere and in the grinding process for reducing the particle size suitable for flotation. Numerous studies (Rey and Formanek, 1960, Heyes and Trahar, 1977, Gardner and Woods, 1979, Trahar, 1984, Adam and Iwasaki, 1984a, Adam and Iwasaki, 1984b, Adam and Iwasaki, 1984c, Natarajan and Iwasaki, 1984, Yelloji Rao and Natarajan, 1989, Yelloji Rao and Natarajan, 1990, Ahn and Gebhardt, 1990 and Peng et al., 2003a) have been done on the influence of type of mill and grinding media on the flotation of sulphide ores. An iron mill reduced the natural floatability of sphalerite significantly (Rey and Formanek (1960). Adam and Iwasaki, (1984a, b, and c) reported that the more active the metal (mild steel > austenitic > stainless steel), the larger the decrease in floatability of pyrrhotite. The floatability of chalcopyrite is sensitive to the type of grinding media, grinding atmosphere and even the material of the mill (Heyes and Trahar, 1977, Gardner and Woods, 1979, Trahar, 1984, Yelloji Rao and Natarajan, 1989, Yelloji Rao and Natarajan, 1990, Ahn and Gebhardt, 1990 and Peng et al., 2003a). A glass mill may improve the recovery of chalcopyrite (Heyes and Trahar, 1977), while noble grinding media such as stainless steel (Ahn and Gebhardt, 1990) and 30 wt.% chromium (Peng et al., 2003a) produced a higher recovery of chalcopyrite than active grinding media, such as mild steel or high carbon steel under the same atmosphere. The high chromium medium had a significantly weaker galvanic interaction with arsenopyrite, and produced a very much lower amount of oxidized iron species in the mill discharge than mild steel medium (Huang and Grano, 2006). The use of iron grinding materials slightly depressed the f loatability of galena and pyrite due to iron materials mainly provoking the iron oxy-hydroxide precipitation on the surface of galena and pyrite (Cases et al., 1990). Various mechanisms have been proposed to explain the influence of grinding media on flotation. Many authors have reported (Adam and Iwasaki, 1984b , Natarajan and Iwasaki, 1984, Yelloji Rao and Natarajan, 1989a, Yelloji Rao and Natarajan, 1989b, Iwasaki et al., 1983, Forssberg et al., 1993, Cheng et al.,1993, Yuan et al., 1996, Greet and Steinier, 2004, Greet et al.2004,Wei and Sandenbergh 2007) that galvanic interactions occur in every grinding media-sulphide mineral system, which affects the subsequent flotation properties of the sulphide minerals through unselective surface coatings by iron oxidation products. Recently it was revealed that H2O2 formation take place during wet grinding of complex sulphide ore (Ikumapayi et al., 2012) and that pyrite (FeS2), the most common ferrous sulphide mineral, generates hydrogen peroxide • • (H2O2) (Borda et al., 2001) and hydroxyl radicals ( OH) ( denotes an unpaired electron) (Borda et al., 2003) when placed in water. In the presence of dissolved molecular oxygen, • í ferrous iron associated with pyrite can form superoxide anion (O2 ) (eq. 1), which reacts with ferrous iron to form H2O2 (eq. 2) (Cohn et al., 2006).

II III • ௅ Fe (pyrite) + O2ĺ Fe (pyrite) + (O2 ) (1) II • ௅ + III Fe (pyrite) + (O2 ) + 2H ĺ Fe (pyrite) + H2O2 (2)

Borda et al., (2003) showed that pyrite can also generate H2O2 in the absence of molecular oxygen. He reported that an electron is extracted from water and a hydroxyl radical is formed

(eq. 3). Combining two hydroxyl radicals leads to the formation of H2O2 (eq. 4).

III • + II Fe + H2Oĺ OH + H + Fe (3) • 2 OH ĺ H2O2 (4)

Ahlberg et al. (1996a, b, and c) studied the oxygen reduction on pyrite and galena with a rotating ring disc electrode. Their results suggest that the first electron transfer is the rate- determining step for the oxygen reduction at both galena and pyrite and hydrogen peroxide is found to be an intermediate. While galena is a poor catalyst for oxygen reduction, with minor formation of hydrogen peroxide, pyrite is a relatively good catalyst. This kind of difference may lead to different flotation behaviours. To date, most studies dealing with sulphide mineral-induced ROS (Reactive Oxygen Species) formation have made use of natural or synthetic mineral samples of very high purities suspended in neutral solutions. Pyrite induced ROS formation has been studied most; however other sulphide minerals such as chalcopyrite (CuFeS2), sphalerite (ZnS), pyrrhotite (Fe(1íx)S) and vaesite (NiS2) have also been studied with respect to ROS generation (Borda et al., 2001 and Jones et al., 2011), and reactivities have been found to differ between sulphide minerals.

Acid Mine Drainage

Acid Mine Drainage (AMD) is produced when sulphide-bearing material is exposed to oxygen and water. The production of AMD usually – but not exclusively – occurs in iron sulphide-aggregated rocks. Although this process occurs naturally, mining can promote AMD generation simply through increasing the quantity of sulphides exposed. Naturally-occurring bacteria can accelerate AMD production by assisting in the breakdown of sulphide minerals (Akcil and Koldas, 2006). There are many types of sulphide minerals. Iron sulphides are most common but other metal sulphide minerals may also produce AMD (Akcil and Koldas, 2006, Hao et al., 2006). Pyrite, pyrrhotite, and chalcopyrite are typically the main sources of AMD (Singer and Stumm, 1970). Management of AMD aims to reduce the impact of the effluent to levels that can be tolerated by the environment without significant damage. In order to minimize this pollution, precautions must be taken to ensure that rainwater does not come into contact with pyrite (Akcil and Koldas, 2006).

The effects of ferrous ions on oxidation

In the leaching, chalcopyrite is oxidized by ferric ions and/or by dissolved oxygen whereby copper ions are extracted in sulphuric acid solution. During this process, ferrous ions are also released from chalcopyrite. Ferrous ions found to promote chalcopyrite oxidation with dissolved oxygen (Hiroyoshi, 1997, Hiroyoshi, 1998, Hiroyoshi, 1999a,b), and this phenomenon may contribute to chalcopyrite leaching near the surface of leaching dumps. Also there are several reports of the effects of ferrous ions on chalcopyrite oxidation by ferric ions. Hirato et al. (1987) reported that ferrous ions suppress chalcopyrite oxidation with ferric ions in sulphuric acid solution. However, there is a report implying that ferrous ions promote this reaction. Kametani and Aoki (1985) indicated that the rate of chalcopyrite oxidation with ferric ions is higher in the presence of an adequate amount of ferrous ions than in the absence of ferrous ions. Increasing the ferrous iron to a certain critical concentration (3 g/l) enhanced the oxidation rate while higher ferrous iron concentrations had an inhibitory effect (Barron and Lueking, 1990). Hiroyoshi et al. (1997) and Third et al. (2002) reported that ferrous iron is more effective than ferric iron to dissolve chalcopyrite. The addition of ferrous ions into the leaching medium improves the rate of galena oxidation; giving a recovery over 80% lead (II) (Bang et al., 1995, Baba et al., 2011) and sphalerite oxidation giving a recovery over 80% for zinc (II) (Baba et al., 2011, Hossain et al. 2004). The ferrous ion enhances bioleaching of

14 galena (Baba et al., 2011). In the experiments without Fe (II), zinc extraction and bacterial population are much lower than those where Fe (II) was added (Pina et al., 2005).

The effects of ferric ions on oxidation

Ferric ion is one of the most important oxidants used in leaching processes. Chalcopyrite dissolution in sulphate media with ferric sulphate is an electrochemical reaction, therefore the dissolution process is written as anodic and cathodic half-cell reactions as shown in Eqs. (5) and (6) (Mikhin et al., 2004, Misra and Fuerstenau, 2005,Watling, 2006, Liu et al., 2007).

Anodic half-cell reaction: chalcopyrite oxidation 2+ 2+ - CuFeS2ĺCu +Fe +2S°+4e (5) Cathodic half-cell reaction: reduction of ferric ions 4Fe3++4e-ĺ4Fe2+ (6)

Ferric ion also in either sulphate or chloride media, can be used for pyrite oxidation over the pH range 1-4 (eq. 7) (Mckibben and Barnes, 1986, Mckibben, 1984). 3+ 2+ 2- + FeS2+ 14Fe + 8H2O ĺ 15Fe + 2SO4 + 16H (7)

Jiang et al. (2008) showed that the oxidation capacity of Fe3+ for galena exceeded that of bacterial (Acidithiobacillus ferrooxidans) oxidation of galena; though after Fe3+ was completely consumed, oxidation would cease, but oxidation by At. ferrooxidans would continue. Ferric ion also in either sulphate or chloride media, can be used to leach zinc sulphide, according to the reaction (eq. 8) (Aydogan et al., 2005, Dutrizac et al., 2003, Santos, 2010, Holmes, 2000): ZnS + 2Fe3+ ĺ Zn2+ + 2Fe2+ + S0 (8)

Galvanic interactions

Buehler and Gottschalk (1910) were noted that when pyrite was mixed with a second sulphide mineral, the second mineral oxidized more rapidly. Harvey and Yen (1998) observed that addition of galena to the selective sphalerite leaching system diminished the dissolution of sphalerite. On the other hand, addition of either pyrite or chalcopyrite increased zinc extractions. Many authors have been reported that galvanic interactions are known to occur between conducting minerals and play a significant role in flotation (Rao and Finch, 1988, Kelebek et al., 1996, Zhang et al., 1997, Ekmekçi and Demirel, 1997 and Huang and Grano, 2005), leaching (Mehta and Murr, 1983,Abraitis et al., 2003 and Akcil and Ciftci, 2003), supergene enrichment of sulphide ore deposits (Thornber, 1975 and Sato, 1992), environment governance (Alpers and Blowes, 1994), and geochemical processes (Sikka et al., 1991 and Banfield, 1997). The mineral, or the region, with the highest rest potential will act as the cathode of the galvanic cell and is protected whereas that with the lowest rest potential will serve as anode, and its rate of dissolution will be increased (Nicol, 1975, Mehta and Murr, 1982, Natarajan and Iwasaki, 1983, Liu et al., 2007 and Tshilombo, 2004). Galvanic effects, occurring between conducting and semiconducting minerals in aqueous systems, play an important role in the aqueous processing of ores and minerals, such as in flotation and leaching. For semi-conductive minerals, such as sulphides, direct contact of different minerals with dissimilar rest potentials initiates the galvanic effect. These interactions occur between sulphides, involving the flow of electrons from grains with a higher potential to grains with lower potentials, modifying the Fermi level of both minerals (Cruz et al., 2005 and Holmes

15 and Crundwell, 1995). Mehta and Murr (1983) experimentally studied that the presence of pyrite in the slurry led to a 2 to 15 fold increase in the rate of dissolution of chalcopyrite. Table 1 Shows rest potential values of some common sulphide minerals (Hey et al., 1987).

Table 1. Rest potential of some sulphide minerals in water at pH=4 (Hey et al., 1987).

Mineral Chemical formula Rest potential (V, SHE) Pyrite FeS2 0.66 Chalcopyrite CuFeS2 0.56 Sphalerite ZnS 0.46 Galena PbS 0.40

Effect of Cu2+, Fe2+, and Pb2+ ions on activation of sphalerite

The activation of sphalerite has been studied extensively over several decades (Finkelstein, N.P., 1997, Finkelstein and Poling, 1977, Gerson et al., 1999, Fornasiero and Ralston, 2006). It has been well established that copper activation of sphalerite follows an ion exchange mechanism where the uptake of Cu (II) results in approximately 1:1 release of Zn2+ into the solution (Finkelstein, N.P., 1997, Gerson et al., 1999, Popov, S.R., Vucinic, D.R., 1990). Addition of, or the presence of, lead ions (Pb2+) in the flotation slurry can directly activate sphalerite and promote flotation( Morey et al., 2001), ( Rashchi et al., 2002) and (Sui et al., 1999). Additions of Fe2+ in the presence of oxygen can also activate the sphalerite surfaces (through adsorption of Fe2+ as Fe(OH)+ followed by anodic oxidation to Fe(OH)2+ and aid sphalerite floatation by forming a ferric hydroxy complex with collector molecules at moderately alkaline pH ( Zhang et al. 1992) and ( Finkelstein, 1999).

16 EXPERIMENTAL Materials and reagents

Crystalline pure sphalerite, galena, pyrite and chalcopyrite minerals used in this study were procured from Gregory, Bottley & Lloyd Ltd., UK. Sphalerite contains 39.92% Zn, 20.7% S, 4.2% Fe, 1.32% Pb and 0.17% Cu; galena contains 73.69% Pb, 13.5% S, 1.38% Fe, 1.26% Zn, 0.2% Cu and some silica (quartz) impurity; pyrite contains 44.4% Fe, 50.9% S, and 0.2% Cu, and chalcopyrite contains 29% Fe, 29.5% S, 25.8% Cu, 0.54% Zn, and 0.22% Pb. The XRD analyses of the samples showed that the main mineral phase present in the respective minerals was pyrite (Fig. 1a), chalcopyrite (Fig. 1b), sphalerite (Fig. 1c) and galena (Fig. 1d). All pyrite, chalcopyrite, sphalerite and galena samples used in this study were separately crushed through a jaw crusher and then screened to collect the –3.35 mm particle size fraction. The homogenized sample was then sealed in polyethylene bags. Potassium amyl xanthate (KAX) was used as collector, while MIBC was used as frother in flotation tests. Solutions of sodium hydroxide (AR grade) and HCl (1 M) were added to maintain the pH at the targeted value during flotation. Deionised water was used in the processes of both grinding and flotation. Solutions of 2, 9-dimethyl-1, 10-phenanthroline (DMP) (1%), copper (II) (0.01 M), and phosphate buffer (pH 7.0) were used for estimating H2O2 amount in pulp liquid by UV-Visible spectrophotometer. AR grade zinc sulfate, copper sulphate, lead nitrate, ferrous sulfate and ferric sulfate chemicals were also used to investigate the effect of metal 2+ 2+ 2+ 2+ 3+ ions (Zn , Pb , Cu , Fe and Fe ) on the formation of H2O2.

Wet Grinding

In each grinding test, 100 g of –3.35 mm size fraction of pyrite, chalcopyrite, Galena and sphalerite single mineral was combined with 400 cm3 of water and ground in a laboratory stainless steel ball mill (Model 2VS, CAPCO Test Equipment, Suffolk, UK) with stainless steel or mild steel medium. The slurry samples were collected at pre-determined time intervals and they were filtered (Millipore 0.22 μm) and the liquid (filtrate) was analysed for hydrogen peroxide. The pH in the grinding pulp liquid was regulated to 10.5 and 11.5 with NaOH solution and 4.5 with HCl solution and was adjusted before grinding. Experiments were performed at room temperature of approximately 22.5°C.

Dry Grinding

100 g of –3.35 mm size fraction of sphalerite, pyrite, Galena and chalcopyrite single mineral in each grinding were separately ground in a laboratory stainless steel ball mill with stainless steel medium for 60 min. A 5 g of sphalerite, galena, pyrite or chalcopyrite single mineral and 12.5 g of sphalerite-pyrite, sphalerite-chalcopyrite, sphalerite-galena, galena-pyrite, galena- chalcopyrite or chalcopyrite-pyrite blend mineral (12.5 g in total) that was < 106 μm was mixed with 50 cm3 water using magnetic stirrer. The slurry samples were collected at 0.5, 5 and 11 min intervals and were analysed for hydrogen peroxide. The pH was regulated with HCl and NaOH solutions. Eh (pulp potential) was measured at room temperature in the suspension after the addition of freshly ground solids in water.

Fig. 1. XRD analysis of the sample (a) pyrite (1- pyrite). ((b) chalcopyrite (1- chalcopyrite and 2- pyrrhotite 3- sphalerite) (c) sphalerite (1- sphalerite 2- galena 3- quartz), (d) galena (1- galena 2- sphalerite 3- quartz). Gas purging To study the effect of the atmosphere, pyrite was wet-ground in a laboratory ball mill with stainless steel medium in either air or N2 atmosphere. For N2 atmosphere, first the laboratory ball mill was filled with 400 ml deionized water and purged with N2 gas for a minimum of 30 min. After 30 min, pyrite was added and the laboratory ball mill was again purged with N2 gas for a minimum of 30 min and finally pyrite was wet-ground for 1 h. Though we did not measure dissolved O2 concentrations in our experiments, it has been reported that for 1 L solutions of ultrapure water purged for 1 h with N2 gas, O2 concentrations did not exceed 0.19 ± 0.05 ppm (Butler et al., 1994). The concentration of H2O2 was measured after 60 min of grinding.

Hydrogen peroxide estimation

18 and cobalt (II) ion (Gulyas et al., 1995) that form colour complexes with H2O2, which can then be measured spectrophotometrically. The spectrophotometric method using copper (II) ions and DMP has been found to be reasonably sensitive when applied to advanced oxidation processes (Kosaka et al. 1998). For DMP method (Baga et al., 1988) 1 mL each of 1% DMP in ethanol, 0.01M copper (II) sulphate, and phosphate buffer (pH 7.0) solutions were added to a 10 mL volumetric flask and mixed. A measured volume of liquid (filtrate) sample was added to the volumetric flask, and then the flask was filled up with ultrapure water. After mixing, the absorbance of the sample at 454 nm was measured with DU® Series 700 UV/Vis Scanning Spectrophotometer. The blank solution was prepared in the same manner but without H2O2. The hydrogen peroxide that is measured in the pulp liquid is the rest concentration after its interaction with sulphide minerals. Understandably, H2O2 as soon as it is generated, it will oxidise the mineral surfaces.

Flotation tests

After wet grinding of 100 g of –3.35 size fraction for 60 min, the mill was emptied and the pulp was screened to free from grinding media and it was split into 5 samples for flotation at different pH values. In each flotation almost 7.5 or 5 g of sample that was < 106 μm was transferred to a cell of 150 ml capacity (Clausthal flotation equipment). Also after dry grinding for 60 min, the mill was emptied and the mineral was screened to free from grinding media. In each flotation almost 5 g of sample that was < 106 μm was transferred to Clausthal cell of 150 ml capacity and conditioned with pH modifier, collector and frother. Flotation 3 -1 concentrate was collected after 2.0 min at air or N2 flow rate of 0.5 dm min . The flotation froth was scraped every 10 s. Dosage of collector in flotation was 10-4 M KAX. The conditioning times for adjusting pH and collector were 5 min and 2 min respectively. The frother dosage was one drop MIBC in all cases. Flotation was investigated at different pH and different gas bubbling (air or N2 bubbling). The pH was regulated with NaOH solution and HCl solution. Experiments were performed at room temperature of approximately 22.5°C.

RESULTS AND DISCUSSION

Formation of hydrogen peroxide during wet grinding

Pyrite–water system

The effect of grinding media on formation of hydrogen peroxide during pyrite grinding is shown in Fig. 2. It can be seen that mild steel produced a higher concentration of H2O2 than stainless steel medium. The concentration of H2O2 increased with increasing grinding time, most likely due to increased interactions with water. This is in agreement with other studies where it was observed more active the metal (mild steel > austenitic > stainless steel), larger was the decrease in floatability of pyrrhotite (Adam and Iwasaki, 1984a, b and c). Fig. 3 shows the effect of pH on formation of hydrogen peroxide. It can be seen that with an increase in pH, the concentration of H2O2 decreased up to pH 8 and then increased above this pH.

19 2 1.8 1.6 1.4 1.2

(mM) 1

2 Mild steel

O 0.8 2

H 0.6 Stainless Steel 0.4 0.2 0 1 10204560 grinding time (min)

Fig. 2. H2O2 concentration as a function of time during wet grinding of pyrite.

1.2

0.8 (mM) 2

O 0.6 2 H 0.4

0.2

0 3.67 4.3 6 7 8 9.06 10.5 pH

Fig. 3. Effect of pH on H2O2 concentration by pyrite during wet grinding with stainless steel medium after 60 min.

Chalcopyrite–water system

Fig. 4 shows the effect of pH on formation of hydrogen peroxide. It can be seen that with an increase in pH, the concentration of H2O2 decreased up to pH 8 and then increased above this pH.

20 0.45 0.4 0.35 0.3 0.25 (mM)

2 0.2 O 2 0.15 H 0.1 0.05 0 3.5 5.7 8 10.5 11.3 pH

Fig. 4. Effect of pH on H2O2 concentration by chalcopyrite during wet grinding with stainless steel medium after 60 min.

Sphalerite–water system

The effect of pH during grinding on formation of hydrogen peroxide is shown in Fig. 5. It can be seen that pH 2.7 generated more H2O2 than at pHs 3.7 and 6.8.

0.6

0.5

0.4

(mM) 0.3 2 O 2

H 0.2

0.1

0 pH=2.7 pH=3.7 pH= 6.8

Fig. 5. Effect of pH in wet grinding with stainless steel medium on formation of H2O2 by sphalerite.

Galena–water system

The effect of pH during galena grinding on formation of hydrogen peroxide is shown in Fig. 6. It can be seen that at pH 4 and 7.8 hardly any H2O2 is generated but at pH 2.7 not only H2O2 formed but also its concentration increased with increasing grinding time due to increased surface area of solids and its reactions with water.

0.45 0.4 0.35 0.3 0.25 (mM) pH= 7.8 2 0.2 O 2 pH=4

H 0.15 0.1 pH=2.7 0.05 0 0 10203040506070 grinding time

Fig. 6. H2O2 concentration during wet grinding with stainless steel medium as function of pH.

Formation of hydrogen peroxide after dry grinding mixed with water

Pyrite–water system

The effect of water pH, in which dry ground solids are added, on formation of hydrogen peroxide is shown in Fig. 7. It can be seen that with an increase in pH, the concentration of H2O2 decreased to about pH 8 and then increased above this pH. Fig. 7 also shows that Eh increased with decreasing pH. The Eh-pH diagram of pyrite shows that oxidation of pyrite 2+ + yields S°, Fe , Fe (OH) 2 species in the solution for pH < 6 (Fig. 8) (Kocabag et al., 1990). In pH < 6, Fe2+ ions are increased with decreasing pH and therefore, in the presence of dissolved molecular oxygen, ferrous iron associated with pyrite can form superoxide anion • í (O2 ) (eq. 1), which reacts with ferrous iron to form H2O2 (eq. 2) as shown in Fig. 9 (Cohn et al., 2006).

1.8 500 1.6 400 1.4 H2O2 1.2 300 Eh

(mM) 1 2 200 O

2 0.8 H 0.6 100 Eh (mV, SHE) Eh (mV, 0.4 0 0.2 0 -100 2.5 4.5 6 8 11.6 pH

Fig. 7: Effect of water pH on H2O2 formation when 5 g dry-ground solids are mixed in water for 5 min.

22 0 -5 Fig. 8. Eh–pH diagram for FeS2–H2O system at 25 C and for 10 M dissolved species (Kocabag et al., 1990).

Fig. 9. Proposed mechanisms for formation of H2O2 by chalcopyrite. The numericals in the figure represent the equation numbers.

The effect pH on formation of hydrogen peroxide with two grinding media was investigated. Fig. 10 shows that mild steel produced a higher concentration of H2O2 than stainless steel medium at pH 4.5 in agreement with the results from wet grinding (see Fig. 2). Fig. 11 shows the proposed mechanisms for formation of H2O2 where mild steel generated 2+ Fe ions but not stainless steel explaining a higher concentration of H2O2 in mild steel than 2+ stainless steel medium. This is in accordance with our results that Fe ions generate H2O2 as shown in Table 2. These results agree with other studies where the level of dissolved ferrous iron found to be an important secondary factor contributing towards H2O2 generation (Zhao et al. 2001, Jones et al., 2012). The formation of higher amounts of H2O2 when pyrite is ground in mild steel medium than stainless medium may explain the effect of grinding media on flotation in addition to widely published galvanic interaction of electron transfer between the grinding media and pyrite (Adam and Iwasaki, 1984b , Natarajan and Iwasaki, 1984, Yelloji Rao and Natarajan, 1989a, Yelloji Rao and Natarajan, 1989b, Iwasaki et al., 1983, Forssberg et al., 1993, Cheng et al.,1993, Yuan et al., 1996, Greet and Steinier, 2004, Greet et al., 2004,

23 Wei and Sandenbergh 2007). It is obvious that the presence of H2O2 in pulp liquid needs due consideration in controlling the surface properties in flotation besides galvanic interactions.

2.5

1.5 (mM) 2 O 2 1 H

0.5

0 Stainless Steel Mild Steel

Fig. 10. H2O2 concentration after dry grinding in different media at pH 4.5.

a) b)

Fig. 11. Illustration of H2O2 formation by pyrite in contact with a) mild steel and b) stainless steel by the incomplete reduction of oxygen (eqs. 1 and 2) and also from two reacting •OH (eqs. 3 and 4) radicals.

To investigate the effect of pyrite loading or solids concentration on H2O2 formation, pyrite was mixed with 50 ml water at pH 11.6. The results at three different solids concentrations of pyrite on hydrogen peroxide production are shown in Fig. 12. It can be seen that the concentration of H2O2 increased with increasing pyrite loading.

24 0.16

0.14

0.12

0.1 (mM)

2 0.08 O 2

H 0.06

0.04

0.02

0 50 100 250 pyrite loading(g/l)

Fig. 12. Effect of pyrite loading dry-ground with stainless steel medium on H2O2 concentration after 5 min mixing with water at pH 11.6.

Chalcopyrite–water system

Formation of hydrogen peroxide was observed when the freshly dry-ground solids are mixed in water. The effect of water pH in which the solids are mixed, on the formation of hydrogen peroxide is shown in Fig. 13. It can be seen that with an increase in pH, the concentration of H2O2 decreased up to pH 8 and then increased above this pH. These results are in agreement with wet grinding data shown in Fig. 4. Fig. 13 also shows that the concentration of H2O2 increased with increasing mixing time, most likely due to increased chalcopyrite interaction with water. Increasing Eh with decreasing pH also corroborates the formation of high amounts of H2O2 at lower pH values. Fairthorne at al. (1997) using the Eh–pH stability diagram of chalcopyrite (Fig. 14) showed that the formation of insoluble ferric oxide/hydroxide at neutral and basic pH values but also in acidic conditions at high Eh values. Notably the divalent Fe and Cu ions exist at low pH values from negative to high Eh values. These divalent ions are reported to aid the formation of H2O2 from water and our present 2+ results shown in Table 2 demonstrate that Fe ions generates substantial H2O2 followed by Fe3+ and Cu2+ ions. Fig. 14 displays that the concentration of ferrous ions decreases at the expense of ferric oxide/hydroxides at higher pH and Eh values (oxygen conditioning). The schematic diagram of H2O2 formation in the presence of dissolved molecular oxygen is shown • í in Fig. 15, where in acidic pH (pH < 5) forms superoxide anion (O2 ) (eq. 9), which reacts with ferrous iron to form H2O2 (eqs. 10-12). This is in agreement with other studies where it was observed that metal ions-induced formation of free radicals have significantly been evidenced that ferrous ion generates the superoxide and hydroxyl radical (Valko et al., 2005, Jones et al., 2011).

í • í O2 + e ĺ (O2 ) (9) • í + 2(O2 ) + 2H ĺ H2O2+ O2 (10) 2+ 3+ • í Fe + H2O2ĺ Fe + OH + OH (11) • 2 OHĺH2O2 (12)

25 Above pH 5, the stability diagram shows the existence of Fe2O3 solid phase which can also generate hydrogen peroxide as reported by Cohn et al. (2006) where hematite and magnetite solids in water were shown to induce H2O2 formation (Cohn et al., 2006). Fig. 15 exhibits that copper ions also generate hydrogen peroxide. The metal ions induced formation of hydroxyl free radical has been significantly evidenced where copper ions generate the superoxide and hydroxyl radical (eqs. 13-16) (Valko et al., 2005).

í •– O2 + e ĺ O2 (13) •– + 2O2 + 2H ĺ H2O2 + O2 (14) + 2+ • – Cu + H2O2 ĺCu + OH + OH (15) • 2 OHĺ H2O2 (16)

In acidic pH, the higher amount of hydrogen peroxide formation than alkaline pH is due to the presence of copper and ferrous ions in acidic pH which ions are capable to generate hydrogen peroxide (Table 2).

Table 2. H2O2 generation in the presence of metal ions at natural pH and at 22 °C.

H2O2 (mM) Concentration of ions 1 mM 10 mM water 0 0 Fe 2+ 0.552 4.656 Fe 3+ 0.004 0.059 Cu 2+ 0 0.015 Pb 2+ 0.013 0.096 Zn 2+ 0.004 0.088

0.35 450

400 0.3 350 0.25 5 300 0.5 0.2 250

(mM) Eh 2

O 200

2 0.15 H

150 SHE ) Eh (mV, 0.1 100 0.05 50

0 0 4.5 8 10.5 11.6 pH

Fig. 13. Effect of pH on H2O2 concentration with 5 g of chalcopyrite solids after 0.5 and 5 min mixing time with water.

Fig. 14. Eh–pH stability diagram for the Cu–Fe–S–H2O system with the preponderant copper and iron species shown in each domain. Empty and filled circles represent the experimental Eh and pH values for chalcopyrite conditioned in nitrogen and oxygen, respectively (Fairthorne, 1997 ).

Fig. 15. Proposed mechanisms for formation of H2O2 by chalcopyrite. The numericals in this figure represent the equation numbers.

The effect of chalcopyrite loading on hydrogen peroxide was investigated with 50 ml total volume of water at pH 11.6 and the results are shown in Fig. 16. It can be seen that the concentration of H2O2 increased with increasing chalcopyrite loading.

27 0.25

0.2

(mM) 0.15 2 O 2 H 0.1

0.05

0 10 50 100 250 Chalcopyrite or pyrite loading (g/l)

Fig. 16. Effect of chalcopyrite loading on H2O2 concentration after 5 min mixing with water at pH 11.6.

Sphalerite–water system

The effect of pH on formation of hydrogen peroxide by freshly ground dry solids is shown Fig. 17. It can be seen that for sphalerite the concentration of H2O2 increased with decreasing pH. Fig. 17 also shows that Eh increased with decreasing pH. Huai Su (1981) showed by Eh- pH stability diagram of sphalerite species that in pH < 6, Zn2+ and Fe2+ species exist (Fig. 18). 2+ 2+ The results in Table 2 show that Zn and Fe ions generated H2O2 and it is clear that dissolved molecular oxygen reacts with dissolved Fe2+ via the Haber-Wiess reaction • – mechanism and forms H2O2 (eqs. 17 and 18) with superoxide anion (O2 ) as an intermediate • species. The H2O2 can then react with ferrous iron disolved to form OH (eq. 19) (Cohn et al., 2006) and also with combining two hydroxyl radicals leads to formation of H2O2 (eq. 20) (Borda et al., 2003) as shown in Fig. 19.

2+ 3+ • í Fe + O2ĺ Fe + (O2 ) (17) 2+ • ௅ + 3+ Fe + (O2 ) + 2H ĺ Fe + H2O2 (18) 2+ • ௅ 3+ Fe + H2O2ĺ+ OH+ OH + Fe (19) • • OH+ OHĺ H2O2 (20)

2+ • í Also Zn ions can form superoxide anion (O2 ) to form H2O2 (eq. 21):

• ௅ + 2(O2 ) + 2H ĺH2O2+ O2 (21)

28 0.7 450

400 0.6 H2O2 350 0.5 Eh 300

0.4 250

(mM) (mM) 0.3 200 2 O 2

H 150 0.2 SHE) Eh (mV, 100 0.1 50

0 0 2.5 4.5 8 11.6 pH

Fig. 17: Effect of pH on H2O2 concentration.

Fig. 18. Eh–pH stability diagram for the ZnS–H2O–FeS system (Huai Su, 1981)

2+ 2+ Fig. 19. Proposed mechanisms for formation of H2O2 by sphalerite to Zn or Fe react with • water to generate OH and H2O2. The numericals in this figure represent the equation numbers.

To investigate the effect of sphalerite loading, sphalerite was mixed with 50 ml water at pH 2.5. The concentration of H2O2 increased with increasing sphalerite loading as was shown in Fig. 20. Fig. 20 also shows that the concentration of H2O2 increases with increasing mixing time, most likely due to increased sphalerite reaction with water.

2 1.8 1.6 1.4 1.2 0.5 min (mM)

2 1 O 2 5 min

H 0.8 0.6 11 min 0.4 0.2 0 50 100 250 sphaleite loading (g/l)

Fig. 20. Effect of sphalerite loading on H2O2 concentration after 5 min mixing with water at pH 2.5.

30 Galena–water system

The effect of pH on formation of hydrogen peroxide is shown in Fig. 21. It can be seen that for galena the concentration of H2O2 increased with decreasing pH. Fig. 21 also shows that Eh increased with decreasing pH. Woods et al. (1981) showed by the Eh-pH diagram (Fig. 22) of galena that at pH < 4, Pb2+ ions are stable. These divalent metal ions also found to generate H2O2 as shown in Table 2 (Blokhina et al., 2001, Ni et al., 2004, Valko et al., 2005, Jones et al., 2012). Fig. 23 shows in the presence of dissolved molecular oxygen and at acidic pH (pH • í 2+ < 4), lead ions can form superoxide anion (O2 ) which reacts with Pb to form H2O2 (Eqs. 22-25). This is in agreement with other studies where metal-ions induced formation of free radical has been significantly evidenced for Pb2+ ions that generate the superoxide and hydroxyl radical (Ahlberg and Broo, 1996a, b, and c).

í • í O2 + e ĺ (O2 ) (22) • í + 2(O2 ) + 2H ĺ H2O2+2O2 (23) + 2+ • í Pb + H2O2ĺ Pb + OH + OH (24) • 2 OHĺ H2O2 (25)

450 0.16

400 0.14 Eh 350 0.12 galena 300 0.1 250 (mM)

0.08 2 O

200 2 H

Eh (mV, SHE ) Eh (mV, 0.06 150

0.04 100

50 0.02

0 0 2.5 4.5 8 10.44 11.6 pH

Fig. 21. Effect of pH on H2O2 concentration.

Fig. 22. Eh-pH diagram for the Pb-S-H2O system showing the region of stability (solid lines) and of metastability (dashed lines) for the mineral. Equilibrium lines correspond to dissolved species where [Pb] =10-3 M (Woods et al.,1981).

2+ Fig. 23. Proposed mechanism for formation H2O2 by galena that Pb reacts with water to • generate OH and H2O2.

The production of H2O2 increased with increasing galena loading as shown in Fig. 24. This figure also shows that the production of H2O2 increases with increasing time and didn’t reach equilibrium level within 11 min kinetics of galena and water interactions studied.

32 0.25

0.2

0.15

(mM) 0.5 2 O 2 0.1 5 H 11

0.05

0 100 250 galena loading (g/l)

Fig. 24. Effect of galena loading on H2O2 concentration after 0.5 and 5 and 11 min mixing with water at pH 2.5.

Comparison of wet and dry grinding on H2O2 formation A comparison has been made between wet and dry grinding on H2O2 formation. For this study, 100 g of pyrite was ground dry for 50 min and wet with 400 ml water for 60 min at natural pH so that the fineness of ground product was the same in both types of grinding. After dry grinding, the mill was immediately emptied and pyrite was mixed with 400 ml water for 60 min then slurry samples were collected at pre-determined time intervals. Also after wet grinding, slurry samples were collected and analysed for hydrogen peroxide. The medium in both cases was stainless steel. Fig. 25 shows that more H2O2 is produced after dry grinding due to a difference in Eh and pH as shown in Fig. 26.

1.8 1.6 1.4 1.2 1

(mM) 0.8 2 dry O

2 0.6 wet H 0.4 0.2 0 0 10203040506070 Time (min)

Fig. 25. H2O2 concentration after wet grinding and mixing with water for dry-ground pyrite at natural pH.

6 600

5 500

4 400

3 300 pH

pH of wet SHE) Eh (mV, 2 200 pH of dry

1 Eh of dry 100 Eh of wet 0 0 0 10203040506070 Time (min)

Fig. 26. Eh and pH during grinding for wet grinding and during mixing with water for dry grinding at natural pH.

Effect of gaseous atmosphere

Pyrite

To study the effect of gaseous atmosphere in pulp liquid, pyrite was wet-ground in a laboratory ball mill with stainless steel media in either air or nitrogen atmosphere. For N2 atmosphere the laboratory ball mill was filled with 400 ml water and purged with N2 gas for a minimum of 1 h and then pyrite was wet-ground for 1 h. For air atmosphere the laboratory ball mill was filled with pyrite and 400 ml water and kept open the mill for 5 minutes to have the air and then pyrite was wet-ground for 1 h. Pyrite in N2 atmosphere also generated H2O2 as Borda et al. (2003) showed that pyrite can also generate H2O2 in the absence of molecular oxygen. Fig. 27 shows that more H2O2 was generated in a N2 atmosphere than in air. This must be due to a change in pH and Eh of the pulp liquid that in N2 atmosphere pH and Eh after 60 min grinding were 6.3 and 152 mV respectively and in air atmosphere they were 5.4 2+ and 287 mV. The Eh-pH diagram (Fig. 8) shows N2 atmosphere generated more Fe which is responsible for H2O2 generation.

34 2 1.8 1.6 1.4

mM 1.2 2

O 1 2

H 0.8 0.6 0.4 0.2 0 N2 air

Fig. 27. Concentration of H2O2 formed in different grinding atmospheres at natural pH (3.7).

Chalcopyrite

Fig. 28 shows that H2O2 was generated in a N2 atmosphere too. N2 atmosphere generated lower H2O2 than air atmosphere. One can hypothesis that an electron is extracted from water and a hydroxyl radical is formed (eq. 26) and reaction between two hydroxyl radicals leads to the formation of H2O2 (eq. 27): 2+ • + + Cu + H2Oĺ OH + H + Cu (26) • 2 OH ĺ H2O2 (27)

0.07

0.06

0.05

0.04 (mM) 2

O 0.03 2 H 0.02

0.01

0 N2 Air

Fig. 28. The effect of gaseous atmosphere on formation of H2O2 by chalcopyrite at natural pH.

Galena

Fig. 29 shows that H2O2 was generated in a N2 atmosphere too. However, N2 atmosphere generated lower H2O2 that in air atmosphere. This is in agreement with other studies where it

35 was observed that the dissolution of PbS in the absence of dissolved oxygen is lower than those obtained in the presence of oxygen (Hsieh and Huang, 1989). It is not very clear how the divalent metal ions are generating H2O2 from water but one can hypothesise that an electron is extracted by the Pb2+ from water and a hydroxyl radical is formed (eq. 28). Combining two hydroxyl radicals leads to the formation of H2O2 (eq. 29). 2+ • + + Pb + H2Oĺ OH + H + Pb (28) • 2 OH ĺ H2O2 (29)

0.4

0.35

0.3

0.25

0.2 (mM) 2 O

2 0.15 H 0.1

0.05

0 N2 air

Fig. 29. The effect of the atmosphere galena on formation of H2O2 in pH 2.5.

Mixture of minerals on H2O2 formation

Chalcopyrite-pyrite mixture

Samples of chalcopyrite and pyrite are mixed (12.5 g in total) and conditioned with 50 cm3 of water for 5 min. Figs. 30a and 30b show the effect of pyrite proportion in chalcopyrite-pyrite mixture on the formation of hydrogen peroxide at pH 4.5, 9.5 and 10.5. It can be seen that with an increase in pyrite fraction, the concentration of H2O2 increased. This is in agreement with other studies where it was observed when pyrite was mixed with a second sulphide mineral, the second mineral oxidized more rapidly (Buehler and Gottschalk, 1910). This result that an increase in pyrite proportion, the concentration of H2O2 increases could be the explanation for the following reported observations: 1- The floatability of chalcopyrite decreases when it is in contact with pyrite. An increase in pyrite content from 0% to 80% resulted in a decrease in chalcopyrite recovery from 98% to 80% and a concomitant decrease in the flotation rate constant from 0.94 to 0.42 min-1. In contrast to chalcopyrite, the recovery of pyrite gradually increased as the amount of chalcopyrite present in the mixture increased (Owusu et al., 2011). 2- The selective flotation of chalcopyrite from pyrite is possible at pH 10 and is impossible at pH 4 (Mitchell et al., 2005). 3- The amount of pyrite in contact with chalcopyrite increases, the leaching rate of chalcopyrite increases (Koleini et al., 2010, Koleini et al., 2011, Dixon and Tshilombo, 2005, Mehta, and Murr 1983, Holmes and Crundwell, 1995). This is in agreement with other studies where it was observed that increasing hydrogen peroxide

36 concentration accelerates considerably chalcopyrite oxidation (eq. 30) (Antonijevic, 2004).

+ 2+ 3+ 2- 2CuFeS2 + 17H2O2 + 2H ĺ 2Cu + 2Fe +SO4 + 18H2O (30)

4- The generation of H2O2 by pyrite and chalcopyrite particles in solution has been proposed to explain the decreased microbial performance in the presence of pyrite and chalcopyrite under different solids loading operating conditions (Jones et al., 2011, Jones et al., 2012).

Figs. 30a and 30b also show the effect of chalcopyrite proportion in chalcopyrite-pyrite mixture on formation of hydrogen peroxide at pH 4.5, 9.5 and 10.5. It can be seen that with an increase in chalcopyrite proportion, the concentration of H2O2 decreased. This effect may be the explanation for the behaviour of increasing pyrite floatability in the presence of chalcopyrite as reported by Peng et al. (2003). The formation of higher amounts of H2O2 when chalcopyrite is in contact with pyrite may explain the effect of interaction between pyrite and chalcopyrite on flotation, leaching, environment governance and geochemical processes as shown in Fig. 31b and 31c while the entire literature describes so far of galvanic interaction between two contacting sulphide minerals and electron transfer from one to the other as shown in Fig. 31a (Rao and Finch, 1988, Kelebek et al., 1996, Zhang et al., 1997, Ekmekçi and Demirel, 1997 and Huang and Grano, 2005, Mehta and Murr, 1983, Abraitis et al., 2003, Akcil and Ciftci, 2003, Thornber, 1975, Sato, 1992, Alpers and Blowes, 1994, Sikka et al., 1991 and Banfield, 1997).

1.6 0.25 (b)

(a) 1.4

0.2 1.2

Py in 1 10.5 0.15 ( mM) 2 ( mM) Py in 9 O 2 2 0.8 H O 2 Cp in H 0.1 10.5 0.6 Py in 4.5 Cp in 9 0.4 Cp in 4.5 0.05 0.2

0 0 0 20406080100 0 20406080100 Pyrite or chalcopyrite proportion % Pyrite or chalcopyrite proportion %

Fig. 30. Effect of pyrite and chalcopyrite proportion in their mixture on H2O2 formation with total 12.5 g sample after 5 min conditioning in water at (a) pH 9 and 10.5 (b) pH 4.5.

37 a) b)

(c)

Fig. 31. Proposed mechanisms for Oxidation of chalcopyrite (Cp) by a) Galvanic interaction between chalcopyrite and pyrite (Py). Anodic dissolution reactions occur on the surface of Cp and cathodic reactions occur on the surface of Py (Adapted from Koleini et al. (2011)). b)

Formation of H2O2 by Py that are proposed to form via the incomplete reduction of oxygen and be formed from two reacting •OH (eqs. 1-4), c) Both Galvanic interactions between Cp and Py and Formation of H2O2 by Py.

Sphalerite-pyrite mixture

Fig. 32 (left) shows the effect of pyrite proportion in sphalerite–pyrite mixture on formation of hydrogen peroxide at different pH. It can be seen that with an increase in pyrite proportion, the concentration of H2O2 increased. This is in agreement with other studies where pyrite was mixed with a second sulfide mineral, the second mineral oxidized more rapidly (Buehler and Gottschalk, 1910). This mechanism of non-ferrous metal sulphide oxidation with increasing pyrite fraction could be due to the increased H2O2 generation and also that an increase in the rate of sphalerite leaching with increasing chalcopyrite proportion (Harvey and Yen, 1998) is due to increasing H2O2 formation with an increase in chalcopyrite proportion as shown in Fig. 33. The oxidation of sphalerite increases with an increase in hydrogen peroxide concentration (Adebayo et al., 2006 and Aydogan, S., 2006). Fig. 32 (right) shows the effect of sphalerite proportion in sphalerite–pyrite mixture on formation of hydrogen peroxide. It can be seen that with an increase in sphalerite proportion, the concentration of H2O2 decreased.

38 3 3

2.5 2.5

2 2 2.5 2.5 (mM) (mM)

1.5 2 1.5 2 O O 4.5 4.5 2 2 H H 1 11.5 1 11.5 9 9 0.5 0.5

0 0 0 20406080100 0 20406080100 Pyrite proportion % sphaleriteproportion%

Fig. 32. Effect of pyrite (left) and sphalerite (right) proportion in sphalerite-pyrite mixture on concentration of H2O2 at different pH.

Fig. 33. Proposed mechanisms for oxidation of sphalerite by formation of H2O2 intermediates are proposed to form via pyrite or sphalerite.

Galena-pyrite mixture

Blend samples of galena–pyrite (12.5 g in total) were mixed in 50 cm3 of water for 5 min. Figs. 34a and 34b show the effect of pyrite proportion in galena–pyrite mixture on formation of hydrogen peroxide at different pH. It can be seen that increasing pyrite proportion, the production of H2O2 increased. This is in agreement with other studies where pyrite was mixed with a second sulfide mineral, the second mineral oxidized more rapidly (Buehler and Gottschalk, 1910). This result of increasing pyrite proportion increases the production of H2O2 could be the explanation for the following observations:

1. The galena recovery decreased with an increase in pyrite proportion in galena-pyrite mixture (Peng et al., 2003, Peng and Grano, 2010). Also, the floatability of galena decreases in the presence of pyrite in the entire pH range studied (Pecina-Trevino et al., 2003). 2. The amount of pyrite increases in contact with another sulphide mineral, the leaching rate of this contacting mineral increase (Koleini et al., 2010, Koleini et al., 2011, Dixon and Tshilombo, 2005, Mehta, and Murr 1983, Holmes and Crundwell, 1995).

39 This is in agreement with other studies where it was observed that increasing hydrogen peroxide concentration accelerates galena oxidation considerably (Baba and Adekola, 2011).

Figs. 34a and 34b also show that also show that the effect of galena proportion in galena- pyrite mixture on formation of hydrogen peroxide. It can be seen that with an increase in galena proportion, the production of H2O2 decreased. This mechanism may be the explanation for a behavior that increasing pyrite floatability in a mixture of galena-pyrite (Peng et al., 2003, Pecina-Trevino et al., 2003). The formation of higher amounts of H2O2 when galena is in contact with pyrite may explain the effect of interaction between two sulphide minerals on flotation as shown in Figs. 35b and 35c while old literature reported when two sulphide minerals are in contact, electron transfer from one to the other, i.e. galvanic interaction, occurs as shown in Fig. 35a (Rao and Finch, 1988, Holmes and Crundwell, 1995).

0.3 (a) 0.25 Py in pH 7 0.2 Py in pH 10.5 (mM) 0.15 2

O Py in pH 9 2

H 0.1 Ga in pH 7 0.05 Ga in pH 10.5 0 0 20406080100Ga in pH 9 Pyrite or galena proportion %

3 (b) 2.5 Py in pH 2.5 2 Py in pH 4.5

(mM) 1.5 2

O Ga in pH 2.5 2

H 1 Ga in pH 4.5 0.5

0 0 20406080100 Pyrite or galena proportion %

Fig. 34. Effect of pyrite or galena proportion in galena-pyrite mixture on concentration of H2O2 at (a) alkaline pH and (b) acidic pH.

40 (b) (a)

(c)

Fig. 35. Proposed mechanisms for Oxidation of galena by a) Galvanic interaction between galena (Ga) and pyrite (Py). Anodic dissolution reactions occur on the surface of Ga and cathodic reactions occur on the surface of Py (Adapted from Holmes and Crundwell, 1995). b) Formation of H2O2 by Py that are proposed to form via the incomplete reduction of oxygen and be formed from two reacting •OH (eqs.1-4). c) Simultaneous Galvanic interaction and formation H2O2 by Py.

Sphalerite-chalcopyrite mixture

Fig. 36 (left) shows the effect of chalcopyrite proportion in sphalerite–chalcopyrite mixture on formation of hydrogen peroxide at different pH values. It can be seen that with an increase in chalcopyrite proportion, the concentration of H2O2 increased. This result of increasing H2O2 concentration with increasing chalcopyrite fraction explains the following observations:

1. significant higher recoveries of sphalerite were obtained even in the absence of any added activator but with the addition of chalcopyrite (Yelloji Rao and Natarajan, 1989), which can be explained with increasing H2O2 concentration by increasing chalcopyrite fraction. 2. the rate of sphalerite leaching increased with an increase in chalcopyrite proportion (Harvey and Yen, 1998) as shown Fig. 37. This is in agreement with other studies where it was observed that the oxidation of sphalerite increased with an increase in hydrogen peroxide concentration (Adebayo et al., 2006 and Aydogan, S., 2006)

Fig. 36 (right) shows the effect of Sphalerite proportion in sphalerite–chalcopyrite mixture on formation of hydrogen peroxide. It can be seen that with an increase in sphalerite proportion, the concentration of H2O2 decreased.

41 0.9 0.9 0.8 0.8 4.5 0.7 0.7 11.5 0.6 0.6 4.5 9 0.5 0.5

(mM) 11.5 (mM) 2 2

O 0.4 O 0.4

2 9 2 H 0.3 H 0.3 0.2 0.2 0.1 0.1 0 0 0 20406080100 0 20406080100 chalcopyrite proportion % sphalerite proportion %

Fig. 36. Effect of (a) chalcopyrite and (b) sphalerite proportion in sphalerite-chalcopyrite mixture on concentration of H2O2 at different pH.

Fig. 37. Proposed mechanisms for oxidation of sphalerite by formation of H2O2 intermediates are proposed to form via chalcopyrite or sphalerite.

Galena-chalcopyrite mixture

Blend samples of galena–chalcopyrite (12.5 g in total) were mixed in 50 cm3 of water for 5 min and the amount of H2O2 formed was determined. Fig. 38 shows the effect of chalcopyrite proportion in galena–chalcopyrite mixture on formation of hydrogen peroxide at different pH. It can be seen that with an increase in chalcopyrite proportion, the concentration of H2O2 increased. This mechanism that with an increase in chalcopyrite proportion, the production of H2O2 increased could explain the result that the amount of chalcopyrite in contact with galena increases, the leaching rate of galena increases as shown in Fig. 39. This is in agreement with other studies where it was observed that increasing hydrogen peroxide concentration accelerates considerably galena oxidation (Baba and Adekola, 2011). Fig. 38 also shows the effect of galena proportion in galena–chalcopyrite mixture on formation of hydrogen peroxide. It can be seen that increasing galena proportion, the production of H2O2 decreased. This result could perhaps explain that galena has no effect on the kinetics of chalcopyrite leaching (Nazari et al., 2012).

0.8

0.7 Cp in pH 9 0.6 Cp in pH 0.5 11.5

( mM) Cp in pH

2 0.4

O 11.5 2 H 0.3 Cp in pH 9

0.2 Cp in pH 4.5 0.1 Ga in pH 4.5 0 0 20406080100 chalcopyrite(Cp) or galena(Ga) proportion %

Fig. 38. Effect of chalcopyrite or galena proportion in galena-chalcopyrite mixture on concentration of H2O2 at different pH.

Fig. 39. Proposed mechanisms for Oxidation of galena with formation of H2O2 that is proposed to form via the chalcopyrite.

Sphalerite-galena mixture

Fig. 40 (left) shows the effect of galena proportion in sphalerite-galena mixture on formation of hydrogen peroxide at different pH. It can be seen that with increasing galena proportion, the production of H2O2 decreased. This result could explain the decrease in the rate of sphalerite leaching with increasing galena proportion (Harvey and Yen 1998) due to a decrease in H2O2 formation. Fig. 40 (right) shows the effect of sphalerite proportion in sphalerite–galena mixture on formation of hydrogen peroxide. It can be seen that with an increase in sphalerite proportion, the production of H2O2 increased.

1.2 1.2 2.5

1 4.5 1

0.8 11.5 0.8 2.5 9 (mM) (mM) 2 2 0.6 0.6 4.5 O O 2 2 H H 11.5 0.4 0.4 9

0.2 0.2

0 0 0 20406080100 0 20406080100 galena proportion % sphalerite proportion %

Fig. 40. Effect of (a) galena and (b) sphalerite proportion in galena-sphalerite mixture on concentration of H2O2 at different pH.

In order to assess whether the hydrogen peroxide production was a result of the sulphide suspension or strictly the pH change, the hydrogen peroxide concentrations of deionized water at the same pH values as the sulphide suspensions were analysed. The pH of the deionized water was adjusted to the specific values using HCl and NaOH, just as what was done with the suspensions of sulphide minerals. After about 10 min of light exposure, the deionized water samples were analysed and the hydrogen peroxide concentrations can be observed in Fig. 41. Due to the low concentrations of hydrogen peroxide observed in Fig. 41 for deionized water at pH values of 2.5, 7, and 10.5, the effect of the pH adjustment on the hydrogen peroxide production can be assumed as negligible.

44 0.05

0.04

0.03 (mM)

2 2.5 O 2 0.02 7 H 10.5

0.01

0 2.5 7 10.5 pH

Fig. 41. The initial hydrogen peroxide concentration for various pH values of deionized water

Flotation studies

Pyrite

The effect of grinding media on pyrite flotation is shown in Figs. 42-44. In Fig. 42 it can be seen that for all pH values studied the stainless steel medium produced a higher pyrite recovery than mild steel medium. Our results for pyrite flotation show that mild steel has lower recovery than stainless steel. It can also be seen that the more active metal (mild steel) produces a larger amount of H2O2. This is in agreement with other studies where it was observed that hydrogen peroxide has been shown to greatly reduce the hydrophobicity of pyrite even in the presence of amyl xanthate (Monte et al. 1997). Also this is in agreement with the observations where the use of iron materials mainly provokes the iron oxy-hydroxide precipitation on the surface of galena and pyrite (Cases et al. 1990). Pyrite was depressed at pH 10.5 and 11.5 due to the formation of Fe (OH) 2 and/or Fe (OH) 3 on the pyrite surface (Janetski et al., 1977, Kocabag et al., 1990). Fig. 43 shows that flotation with collector produced a higher pyrite recovery than without collector. In both cases stainless steel medium produced a higher pyrite recovery than mild steel medium. Fig. 44 shows the effect of different atmosphere in flotation. Formation of bubbles with O2 (air) produced insignificantly higher pyrite recovery than N2. In both atmospheres stainless steel medium produced a slightly higher pyrite recovery than mild steel medium. This can possibly be because stainless steel medium produced a lower concentration of H2O2 than mild steel (see Fig. 2).

45 80 70 60 50 40 Stainless Steel 30 Mild steel 20 10 Recovery of pyrite (%) 0 4.5 10.5 11.5 pH

Fig. 42. Effect of pH on flotation recovery of pyrite with air atmosphere during the flotation.

80 70 60 50 40 Stainless Steel

30 Mild steel 20

Recovery of pyrite (%) 10 0 0,0001 without collector Collector concentration (M)

Fig. 43. Flotation recovery of pyrite in the presence and absence of collector at pH 4.5 and air gas bubbling.

100 90 80 70 60

50 Stainless Steel 40 Mild steel 30 20

Pyrite recovery (%) 10 0 air N2

Fig. 44. Flotation recovery of pyrite in different gas bubbling with pH 4.5, KAX 0.1 mM.

46 Chalcopyrite

Shi and Fornasiero (2010) observed that conditioning of chalcopyrite in the presence of hydrogen peroxide induces oxidation of its surface with formation of the hydrophilic species of iron oxide/hydroxide and iron sulphate causing its depression in flotation. After dry- grinding for 60 min, the effect of H2O2 formation in water by the freshly ground solids on chalcopyrite flotation recovery was investigated. The concentration of H2O2 in the pulp liquid vis-à-vis chalcopyrite flotation is shown in Fig. 45. H2O2 decreased the recovery of chalcopyrite when its concentration exceeds 0.3 mM and this result was the same as Castro et al. (2003), who reported that the floatability of chalcopyrite was decreased after pre-treatment with hydrogen peroxide in alkaline condition. Fig. 46 shows the effect of initial pH grinding on recovery of chalcopyrite at pH 10.5. It can be seen that the recovery of chalcopyrite remained almost constant at the pH > 5 but decreases at pH 3.5 due to higher H2O2 concentration (Fig. 4). Fig. 47 shows the effect of pH on recovery of chalcopyrite after wet and dry grinding. It can be seen that the recovery of chalcopyrite after wet-ground at natural pH (5.7) or dry-ground decreased with increasing pH. Ackerman et al. (1987) and Chandraprabha et al. (2006) reported that the recovery of chalcopyrite decreases above pH 8. This decrease in flotation recovery at pH > 8 can be due to increasing H2O2 formation.

100

Recovry of chalcopyrite % Recovry of chalcopyrite 20

0 0 0.03 0.3 3 30

H2O2 (mM)

Fig. 45. Effect of H2O2 on recovery of chalcopyrite with dry-ground solids at pH 10.5.

47 100 90 80 70 60 50 40 30 20

Recovry of chalcopyrite % Recovry of chalcopyrite 10 0 3.5 5.7 7 9 10 11.3 initial pH for grinding

Fig. 46. Effect initial pH of grinding on recovery of chalcopyrite at flotation pH 10.5.

100 90 80 70 60 50 wet 40 dry 30 20 10 0

Recovry of chalcopyrite % Recovry of chalcopyrite 8 10.5 11.5 pH

Fig. 47. Recovery of chalcopyrite at different pH with wet and dry ground solids.

Sphalerite

After dry-grinding for 60 min, the effect of H2O2 formation in water by the freshly ground solids on sphalerite flotation recovery was investigated. The concentration of H2O2 in the pulp liquid vis-à-vis sphalerite flotation is shown in Fig. 48. It can be seen that H2O2 increased the recovery of sphalerite when its concentration is < 1 mM. This result explains better the results reported in the literature as below: 1. sphalerite can be floated with xanthate in the pH range 8-11 in the presence of ferrous (but not ferric) ions and oxygen (Zhang et al., 1992) due to ferrous ions generating H2O2 as shown in Table 2. 2. recovery of sphalerite decreased when pH increases from pH 3 to pH 5 (Ikumapayi et al., 2012) due to H2O2 decreases with increasing pH as shown in Fig. 5.

48 100 90 80 70 60 50 40 30 Recovery(%) 20 10 0 0 0.01 0.1 1 10 concentration of H2O2 (mM)

Fig. 48. Effect of H2O2 on recovery of chalcopyrite with dry-ground solids at pH 10.5.

Galena

After 60 min grinding, the effect of H2O2 formation on flotation recovery of galena was investigated. The effect of grinding pH as a function of flotation pH on galena recovery is shown in Fig. 49. It can be seen that for all pH values studied the galena wet-ground at pH 2.5 produced a lower galena recovery than galena wet-ground at pH 7.8. Since wet-ground galena at pH 2.5 produces higher amount of H2O2, a little decrease in galena recovery could be due to surface oxidation caused by H2O2 oxidant. It was also reported that the addition of H2O2, -3 galena flotation decreases and completely depresses if the concentration of H2O2 exceeds 10 M (Wang, 1992). This strong depressing action of H2O2 on galena is attributed to its strong oxidizing action on lead xanthate in galena surface giving rise to the oxidation and decomposition of lead xanthate (eq. 31).

– [Pb (EX) 2] ads + H2O2ĺPb (OH) 2 + (EX) 2+ 2e (31)

100 90 80 70 pH of 60 grinding 7.8 50 pH of grinding 2.5 40 30

Recovery of galena% 20 10 0 346.39 pH

Fig. 49. Effect of pH on flotation recovery of galena with a pulp ground at pH 2.5 and 7.8.

49 CONCLUSIONS

Following preliminary conclusions were drawn from the experiments carried out in the scope of this thesis. Formation of hydrogen peroxide was detected in the filtrate of pyrite for the first time in mineral processing applications. Mild steel produced a higher concentration of H2O2 than stainless steel medium. The concentration of H2O2 increased with increasing grinding time. Stainless steel produced a higher pyrite recovery than mild steel medium. At all three pH values studied (4.5, 10.5 and 11.5), stainless steel media produced a higher pyrite recovery than mild steel grinding medium due to lower concentration of H2O2 in stainless steel than mild steel medium. The mild steel medium increases the concentration of H2O2 and thereby increases pyrite oxidation and decreases its flotation recovery. Pyrite solids after dry grinding produced more H2O2 when placed in water than wet grinding. The pH of water influenced the formation of hydrogen peroxide where high amounts of H2O2 is produced at highly acidic pH and decreased with increasing pH up to 8 and increased again above this pH. The amount of H2O2 formed also increased with increase in pyrite loading due to increased surface area to react with water. Mild steel produced higher concentration of H2O2 than stainless steel medium due to higher amounts of ferrous ions release in mild steel grinding medium. In addition, dry grinding generated more H2O2 than wet grinding. More H2O2 was generated in N2 gas atmosphere compared to air atmosphere suggesting the breakdown of water molecule and giving raise to hydroxide free radical due to the catalytic activity of reactive pyrite surfaces. Pyrite flotation at pH 4.5 after stainless steel grinding in different atmospheres became lower in N2 purging due to higher H2O2 formation compared to air atmosphere.

Wet grinding of chalcopyrite produced H2O2. At lower acidic pH, formation of higher amounts of H2O2 is more evident and increasing pH decreased its concentration up to pH 8 and then increase above this pH value. Dry grinding also produced hydrogen peroxide when the dry ground solids were placed in water and H2O2 concentration increases with increasing conditioning time and solids loading. In a mixed composition of chalcopyrite-pyrite, an increase in pyrite fraction increases H2O2 formation correlating to a decrease in chalcopyrite recovery with increasing pyrite fraction in mixed mineral composition. This clearly suggests that a decrease in chalcopyrite recovery with increasing pyrite proportion was due to the increase of hydrogen peroxide formation. H2O2 was also generated in the N2 atmosphere and devoid of oxygen illustrating that the oxygen in H2O2 is derived from the water molecules.

Wet grinding of sphalerite produced H2O2 and it increases with decreasing pH. Dry grinding also produced hydrogen peroxide when sphalerite was mixed with water and the effect of pH found to be the same as that of wet grinding. Also the concentration of H2O2

50 increased with increasing mixing time and sphalerite loading.With an increase in pyrite and chalcopyrite proportion in pyrite-sphalerite and chalcopyrite–sphalerite mixtures respectively, the concentration of H2O2 increases. However, with an increase in galena proportion in galena–sphalerite mixture, the concentration of H2O2 decreases.

Wet grinding of galena produced H2O2 and it increases with decreasing pH. However, H2O2 formed only at pH values less than 4. Dry grinding also produced hydrogen peroxide when freshly dry ground galena solids were mixed with water, and the effect of pH on H2O2 formation was similar to wet grinding, i.e., hydrogen peroxide generates at pH < 4. Also the concentration of H2O2 increased with increasing mixing time and galena loading. Hydrogen peroxide was also generated in the absence of oxygen (N2 atmosphere), the amount was slightly lower compared to air atmosphere. With an increase in pyrite proportion in pyrite- galena mixture, the concentration of H2O2 increased. Then one can conclude that a decrease in galena recovery in flotation with an increase in pyrite proportion is due to the increase of hydrogen peroxide formation. Also with increasing chalcopyrite proportion in a mixture of chalcopyrite-galena, the concentration of H2O2 increased.

FUTURE WORK

x Using of Raman Spectroscopy for identifying and measuring H2O2.

x Effect of grinding media on formation of H2O2 by chalcopyrite, galena, and sphalerite x Attempt to build correlation between percentages of pyrite in concentrate, grinding condition and concentration hydrogen peroxide in pulp liquid on flotation response of metal sulphides x Investigate on the issues as addressed briefly in paper 6.

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Paper 1

Alireza Javadi Nooshabadi, Anna–Carin Larsson, Kota Hanumantha Rao, Formation hydrogen peroxide by pyrite and its influence on flotation– submitted to Journal of Mineral Engineering.

1 2 Formation of hydrogen peroxide by pyrite and its influence on flotation

Alireza Javadi Nooshabadi1, Anna-Carin Larsson2, Hanumantha Rao Kota1* 1Mineral Processing Group,2Chemistry of Interfaces Group, Division of Sustainable Process Engineering Department of Civil, Environmental and Natural Resources Engineering Luleå University of Technology, SE-971 87 Luleå, Sweden *Corresponding author:Tel.: +46 920 491705; Fax: +46 920 491199; e-mail: [email protected]

Abstract

Formation of hydrogen peroxide (H2O2), an oxidizing agent stronger than oxygen, by pyrite (FeS2), the most abundant metal sulphide on Earth, during grinding was investigated. It was found that pyrite generated H2O2 in pulp liquid during wet grinding and also the solids when placed in water immediately after dry grinding. Type of grinding medium on formation of hydrogen peroxide revealed that the mild steel produced more H2O2 than stainless steel grinding medium, where Fe2+ and/or Fe3+ ions played a key role in producing higher amounts of H2O2. The effect of grinding atmosphere of air and N2 gas showed that nitrogen environment free from oxygen generated more H2O2 than air atmosphere suggesting that the oxygen in hydrogen peroxide is derived from water molecules. In addition, the solids after dry grinding producing more H2O2 than wet grinding indicate the role of pyrite surface or its catalytic activity in producing H2O2 from water. This study highlights the necessity of relooking into the electrochemical and/or galvanic interaction mechanisms between the grinding medium and pyrite in terms of its flotation behaviour. Keywords: Pyrite; Wet and Dry Grinding; Stainless Steel and Mild Steel grinding media; Hydrogen Peroxide; N2 and Air atmosphere; Flotation

1. Introduction Oxidation of sulphide minerals takes place when they are exposed to atmosphere and in the grinding process for reducing the particle size for flotation. Numerous studies (Rey and Formanek, 1960, Heyes and Trahar, 1977, Gardner and Woods, 1979, Trahar, 1984, Adam and Iwasaki, 1984a, Adam and Iwasaki, 1984b, Adam and Iwasaki, 1984c, Natarajan and Iwasaki, 1984, Yelloji Rao and Natarajan, 1989, Yelloji Rao and Natarajan, 1990, Ahn and Gebhardt, 1990 and Peng et al., 2003a) have been done on the influence of type of mill and grinding media on the flotation of sulphide ores. An iron mill reduced the natural floatability of sphalerite significantly (Rey and Formanek (1960). Adam and Iwasaki, (1984a, b, and c) reported that the more active the metal (mild steel > austenitic > stainless steel), the larger the decrease in floatability of pyrrhotite. The floatability of chalcopyrite is sensitive to the type of grinding media, grinding atmosphere and even the material of the mill (Heyes and Trahar, 1977, Gardner and Woods, 1979, Trahar, 1984, Yelloji Rao and Natarajan, 1989, Yelloji Rao and Natarajan, 1990, Ahn and Gebhardt, 1990 and Peng et al., 2003a). A glass mill may improve the recovery of chalcopyrite (Heyes and Trahar, 1977), while noble grinding media such as stainless steel (Ahn and Gebhardt, 1990) and 30 wt. % chromium (Peng et al., 2003a) produced a larger recovery of chalcopyrite than active grinding media, such as mild steel or high carbon steel under the same atmosphere. The high chromium media had a significantly weaker galvanic interaction with arsenopyrite, and produced a very much lower amount of oxidized iron species in the mill discharge than mild steel medium (Huang and Grano, 2006).

3 The use of iron grinding materials slightly depressed the floatability of galena and pyrite due to iron materials mainly provoking the iron oxy-hydroxide precipitation on the surface of galena and pyrite (Cases et al., 1990). Various mechanisms have been proposed to explain the influence of grinding media on flotation. many authors has been reported (Adam and Iwasaki, 1984b , Natarajan and Iwasaki, 1984, Yelloji Rao and Natarajan, 1989a, Yelloji Rao and Natarajan, 1989b, Iwasaki et al., 1983,Forssberg et al., 1993,Cheng et al.,1993,Yuan et al., 1996,Greet and Steinier, 2004, Greet et al.2004,Wei and Sandenbergh 2007) that galvanic interactions occur in every grinding media-sulphide mineral system, which affects the subsequent flotation properties of the sulphide minerals through unselective surface coatings by iron oxidation products. Recently it was revealed that H2O2 formation take place during wet grinding of complex sulphide ore (Ikumapayi et al., 2012) and that pyrite (FeS2), the most common metal sulphide • mineral, generates hydrogen peroxide (H2O2) (Borda et al., 2001) and hydroxyl radicals ( OH) (• denotes an unpaired electron) (Borda et al., 2003) when placed in water. In the presence of dissolved molecular oxygen, ferrous iron associated with pyrite can form superoxide anion • í (O2 ) (eq. 1), which reacts with ferrous iron to form H2O2 (eq. 2) (Cohn et al., 2006). II III • ௅ Fe (pyrite) + O2ĺ Fe (pyrite) + (O2 ) (1) II • ௅ + III Fe (pyrite) + (O2 ) + 2H ĺ Fe (pyrite) + H2O2 (2) Borda et al., (2003) showed that pyrite can also generate H2O2 in the absence of molecular oxygen. He reported that an electron is extracted from water and a hydroxyl radical is formed

(eq. 3). Combining two hydroxyl radicals leads to the formation of H2O2 (eq. 4): • + Fe (III) + H2Oĺ OH+ H + Fe (II) (3) • 2 OHĺ H2O2 (4) Oxidation of pyrite by H2O2 can be described by the following generalized reaction (Lefticariu et al., 2007): 2+ 2- FeS2+H2O2 +H2Oinitial ĺFe +SO4 + O2 + H2Ofinal + residual FeS2 (5) Oxidation of pyrite by H2O2 increases the level of complexity of the overall system due to the formation of reactive oxygen species during decomposition of H2O2, such as hydroxyl • ௅• • radicals OH, superoxide ion radicals O2 , and hydroperoxy radicals HO2 . (McKibben and Barnes, 1986, Lefticariu et al., 2006). • However, participation of H2O2 and OH, if any, in non-selective oxidation of the sulphide ore pulp components and hence in deteriorating of the concentrate grade and recovery of metal-sulphides has not yet been explored. In an attempt to fill the gap, we have estimated the concentration of H2O2 in pulp liquid during different time of grinding and in different grinding environments. The effect of two types of grinding media (mild steel and stainless steel) on formation of hydrogen peroxide and pyrite flotation was investigated.

2. Experimental 2.1. Materials and reagents. Crystalline pure pyrite mineral sample was procured from Gregory, Bottley & Lloyd Ltd., United Kingdom. The XRD analysis of the sample showed that this pyrite sample was very pure (Fig. 1) and contains 44.4% Fe, 50.9% S and 0.2% Cu. All the pyrite used in this study was simultaneously crushed through a jaw crusher and then screened to collect the ௅3.35 mm particle size fraction. The homogenized sample was then sealed in polyethylene bags. Potassium amyl xanthate (KAX) was used as collector and MIBC was used as frother. Solutions of sodium hydroxide (AR grade) and HCl (1 M) were used to maintain the pH at the targeted value during flotation. Deionised water was used in the processes of both grinding and flotation. Solutions of 2, 9-dimethyl-1, 10-phenanthroline (DMP), copper(II) sulphate (0.01 M), and phosphate buffer (pH 7.0) were used in the estimation of H2O2.

Fig. 1. XRD analysis of the pyrite sample.1) pyrite.

2.2. Wet Grinding and flotation tests One hundred grams of pyrite for each grinding test was combined with 400 cm3 of deionised water and ground in a new laboratory stainless steel ball mill (Model 2VS, CAPCO Test Equipment, Suffolk, UK) with either of two types of grinding media (mild steel or stainless steel) at natural pH (i.e. pH 3–4). The slurry samples were collected at pre-determined time intervals of grinding and they were filtered (Millipore 0.22 μm) and liquid (filtrate) was analysed for hydrogen peroxide. After grinding for 60 min (minutes), the mill was emptied and the pulp was screened to free from grinding media and it was split into 5 samples for flotation at different pH values. In each flotation almost 7.5 g of sample that was < 106 μm was transferred to a cell of 150 ml capacity (Clausthal flotation equipment), conditioned with pH modifier, collector and frother. 3 -1 Flotation concentrate was collected after 2.0 min at air or N2 flow rate of 0.5 dm min . The flotation froth was scraped every 10 s. Dosages of collector in flotation was 10-4 M KAX. The conditioning times for adjusting pH and collector were 5 min and 2 min respectively. The frother dosage was one drop MIBC in all cases. Pyrite flotation was investigated at different pH (pH 10.5, 11.5 and 4.5 with 10-4 M KAX), then it was investigated with collector (10-4 M KAX) and without collector at pH 4.5, and finally it was investigated in different gas -4 bubbling (air or N2 bubbling at pH 4.5 and 10 M KAX). The pH was regulated to 10.5 and 11.5 with NaOH solution and 4.5 with HCl solution. Experiments were performed at room temperature of approximately 22.5°C.

2.3. Dry Grinding One hundred grams of pyrite was ground in a laboratory stainless steel ball mill with two types of grinding media (mild steel and stainless steel) for 60 min. After grinding, the mill was emptied and the pyrite was screened from grinding media. A 5 g of sample that was < 106 μm was mixed with 50 cm3 of water in a magnetic stirrer for 0.5 and 5 min. The slurry sample was then collected and analysed for hydrogen peroxide. The pH was regulated with HCl or NaOH solution.

5 2.4. Gas purging To study the effect of the atmosphere, pyrite was wet-ground in a laboratory ball mill with stainless steel medium in either air or N2 atmosphere. For N2 atmosphere, first the laboratory ball mill was filled with 400 ml deionized water and purged with N2 gas for a minimum of 30 min. After 30 min, pyrite was added and the laboratory ball mill was again purged with N2 gas for a minimum of 30 min and finally pyrite was wet-ground for 1 h. Though we did not measure dissolved O2 concentrations in our experiments, it has been reported that for 1 L solutions of ultrapure water purged for 1 h with N2 gas, O2 concentrations did not exceed 0.19 ± 0.05 ppm (Butler et al., 1994). The concentration of H2O2 was measured after 60 min of grinding.

2.5. Estimation of Hydrogen peroxide

So far, various methods have been used for the measurement of H2O2 in oxidation processes. Such methods use metallic compounds such as titanium oxalate, titanium tetrachloride (Volk et al.,1993, Roche and Prados,1995, Sunder and Hempel, 1997, Leitner and Dore´,1997) and cobalt (II) ion (Gulyas et al., 1995) that form colored complexes with H2O2, which can then be measured spectrophotometrically. The spectrophotometric method using copper(II) ions and DMP has been found to be reasonably sensitive when applied to advanced oxidation processes (Kosaka et al. 1998). For DMP method (Baga et al., 1988) one millilitre each of DMP, copper(II) sulphate, and phosphate buffer (pH 7.0) solutions were added to a 10 mL volumetric flask and mixed. A measured volume of liquid (filtrate) sample was added to the volumetric flask, and then the flask was filled up with ultrapure water. After mixing, the absorbance of the sample (at 454 nm) was measured with DU® Series 700 UV/Vis Scanning Spectrophotometer. The blank solution was prepared in the same manner but without H2O2.

3. Results and discussion

3.1. Formation of hydrogen peroxide (H2O2) during wet grinding and its implications on flotation

Initially the extent of H2O2 formation during wet grinding of pyrite was investigated. For these studies, pyrite was wet-ground in a laboratory stainless steel ball mill with two kinds of grinding media at natural pH and slurry samples were collected at pre-determined time intervals. The slurry samples were filtrated (Millipore 0.22 μm) and liquid (filtrate) was analysed for hydrogen peroxide. Formation of hydrogen peroxide was detected in the filtrate for the first time in mineral processing applications. The proposed mechanism, under anoxic • + condition hypothesizes that the dissociation of H2O, to form OH and H , occurs on nonstoichiometric Fe(III) sites on pyrite. The combination of two •OH radicals, then produces H2O2 (Borda et al., 2003). The effect of grinding media on formation of hydrogen peroxide is shown in Fig. 2. It can be seen that mild steel produced a higher concentration of H2O2 than stainless steel medium. The concentration of H2O2 increased with increasing grinding time, most likely due to increased reactions with water. This is in agreement withother studies where it was observed that the more active the metal (mild steel > austenitic > stainless steel), the larger the decrease in floatability of pyrrhotite (Adam and Iwasaki, 1984a, b, and c). After detection of H2O2 formation, the effect of this strong oxidizing agent on solid surfaces and its consequence on flotation should be addressed.

6 After grinding for 60 min, the effect of H2O2 formation on flotation recovery of pyrite was investigated. The effect of grinding media on pyrite flotation is shown in Figs. 3-5. In Fig. 3 it can be seen that for all pH values studied the stainless steel medium produced a higher pyrite recovery than mild steel medium. Our results for pyrite flotation show that mild steel has lower recovery than stainless steel. It can also be seen that the more active metal (mild steel) produces a larger amount of H2O2. This is in agreement with other studies where it was observed that hydrogen peroxide has been shown to greatly reduce the hydrophobicity of pyrite even in the presence of amyl xanthate (Monte et al. 1997). Also this is in agreement with the observations where the use of iron materials mainly provokes the iron oxy-hydroxide precipitation on the surface of galena and pyrite (Cases et al. 1990). Pyrite was depressed at pH 10.5 and 11.5 due to the formation of Fe(OH)2 and/or Fe(OH)3 on the pyrite surface (Janetski et al., 1977, Kocabag et al., 1990). Fig. 4 shows that flotation with collector produced a higher pyrite recovery than without collector. In both cases stainless steel medium produced a higher pyrite recovery than mild steel medium.

Fig. 2. H2O2 concentration as a function of time during wet grinding.

Fig. 5 shows the effect of different atmosphere in flotation. Formation of bubbles with O2 (air) produced insignificantly higher pyrite recovery than N2. In both atmospheres stainless steel medium produced a slightly higher pyrite recovery than mild steel medium. This can possibly be because stainless steel medium produced a lower concentration of H2O2 than mild steel (see Fig. 2).

Fig. 3. Effect of pH on flotation recovery of pyrite with air atmosphere during the flotation.

Fig. 4. Flotation recovery of pyrite in the presence and absence of collector at pH 4.5 and air gas bubbling.

Fig. 5. Flotation recovery of pyrite in different gas bubbling with pH 4.5, KAX 0.1 mM

3.2. Formation of hydrogen peroxide (H2O2) after dry grinding in contact with water Pyrite mineral (100 g) was dry-ground in a laboratory ball mill. Formation of hydrogen peroxide was detected after the solids are mixed with water. The effect of water pH on formation of hydrogen peroxide is shown in Fig. 6. It can be seen that with an increase in pH, the concentration of H2O2 decreased to about pH 8 and then increased above this pH. Fig. 6 also shows that Eh increased with decreasing pH. This is agreement with that H2O2 is one of the most powerful oxidizers known. The Eh-pH diagram of pyrite shows that oxidation of 2+ + pyrite yields S°, Fe , Fe (OH)2 species in the solution for pH < 6 (Fig. 7) (Kocabag et al., 1990). In pH < 6, Fe2+ ions are increased with decreasing pH and therefore, in the presence of

8 dissolved molecular oxygen, ferrous iron associated with pyrite can form superoxide anion • í (O2 ) (eq. 1), which reacts with ferrous iron to form H2O2 (eq. 2) (Cohn et al., 2006).

Fig. 6: Effect of water pH on H2O2 formation when 5 g dry-ground solids are mixed in water for 5 min.

0 -5 Fig. 7. Eh – pH diagram for FeS2 – H2O system at 25 C and for 10 M dissolved species (Kocabag et al., 1990). The effect pH on formation of hydrogen peroxide with two grinding media was investigated. Fig. 8 shows that mild steel produced a higher concentration of H2O2 than stainless steel medium at pH 4.5 in agreement with the results from wet grinding (see Fig. 2). 2+ Fig. 9 shows the proposed mechanisms for formation of H2O2 where mild steel generated Fe ions but not stainless steel explaining a higher concentration of H2O2 in mild steel than 2+ stainless steel medium due to Fe ions can generate H2O2 as shown in Table 1. This is in agreement withother studies where the dissolved ferrous iron concentration was found to be

9 •௅ • an important secondary factor contributing towards ROS (O2 , H2O2 and OH) generation (Jones et al., 2012). The formation of higher amounts of H2O2 when pyrite is ground in mild steel medium than stainless medium may explain the effect of grinding media on flotation better than the widely published galvanic interaction of electron transfer between the grinding media and pyrite (Adam and Iwasaki, 1984b , Natarajan and Iwasaki, 1984, Yelloji Rao and Natarajan, 1989a, Yelloji Rao and Natarajan, 1989b, Iwasaki et al., 1983, Forssberg et al., 1993,Cheng et al.,1993,Yuan et al., 1996,Greet and Steinier, 2004,Greetet al.2004,Wei and Sandenbergh 2007). It is obvious that the presence of H2O2 in pulp liquid needs due consideration in controlling the surface properties in flotation besides galvanic interactions.

Fig. 8. H2O2 concentration after dry grinding in different media at pH 4.5.

a) b)

Fig. 9. Illustration of H2O2 formation by pyrite in contant with a) mild steel and b) stainless steel by the incomplete reduction of oxygen (eqs. 1 and 2) and also from two reacting •OH (eqs. 3 and 4) radicals.

Table 1. Effect of metal ions on H2O2 generation at two initial concentrations (conditioning time 1 h, natural pH, and 22 °C).

H2O2 (mM) Concentration of ions 1 mM 10 mM water 0 0 Fe 2+ 0.552 4.656 Fe 3+ 0.004 0.059

3.3. Effect of pyrite loading on formation of hydrogen peroxide

To investigate the effect of pyrite loading or solids concentration on H2O2 formation, pyrite was mixed with 50 ml water in pH 11.6. The results at three different solids concentrations of pyrite on hydrogen peroxide production are shown in Fig. 10. It can be seen that the concentration of H2O2 increased with increasing pyrite loading.

Fig. 10. Effect of pyrite loading dry-ground with stainless steel medium on H2O2 concentration after 5 min mixing with water at pH 11.6.

3.4. A comparison between dry and wet grinding for formation of hydrogen peroxide (H2O2)

A comparison was made between wet and dry grinding in H2O2 formation. For this comparison 100 gram of pyrite was ground dry for 50 min and wet (with 400 ml water) for 60 min at natural pH so that the fineness of ground product was equated. After dry grinding, the mill was immediately emptied and pyrite was mixed with 400 ml water for 60 min then slurry samples were collected at pre-determined time intervals. Also after wet grinding, slurry samples were collected and analysed for hydrogen peroxide. The medium in both cases was stainless steel. Fig. 11 shows that more H2O2 is produced after dry grinding.

Fig. 11. H2O2 concentration after dry and wet milling at natural pH.

11 3.5. Gas purging To study the effect of atmosphere, pyrite was wet-ground in a laboratory ball mill with stainless steel media in either air or nitrogen atmosphere. For N2 atmosphere the laboratory ball mill was filled with 400 ml water and purged with N2 gas for a minimum of 1 h and then pyrite was wet-ground for 1 h. For air atmosphere the laboratory ball mill was filled with pyrite and 400 ml water and kept open the mill for 5 minutes to have the air and then pyrite was wet-ground for 1 h. Pyrite in N2 atmosphere also generated H2O2 as Borda et al. (2003) showed that pyrite can also generate H2O2 in the absence of molecular oxygen. Fig. 12 shows that more H2O2 was generated in a N2 atmosphere than in air. This must be due to a change in pH and Eh of the pulp liquid that in N2 atmosphere pH and Eh after 60 min grinding were 6.3 and 152 mV respectively and in air atmosphere they were 5.4 and 287 mV. The Eh-pH 2+ diagram (Fig. 7) shows N2 atmosphere generated more Fe which is responsible for H2O2 generation.

Fig. 12. Concentration of H2O2 formed in different grinding atmospheres at natural pH (3.7).

Fig. 13. Flotation recovery of pyrite after wet-ground in N2 and air atmospheres using 0.1 mM KAX collector at pH 4.5.

12 The effect on pyrite flotation in the presence and absence of oxygen during grinding is shown in Fig. 13. It can be seen that N2 purging decreased pyrite recovery compared to air atmosphere. This is in agreement with other studies where it was observed that hydrogen peroxide greatly reduce the hydrophobicity of pyrite and flotation response in the presence of amyl xanthate (Monte et al. 1997). The above results may be summarized with the occurrence of the following events: (a) mild steel grinding medium increases concentration of H2O2 in the pulp liquid, (b) increasing the concentration of H2O2 increases the oxidation of pyrite and decreases its flotation, and (c) increasing concentration of H2O2 increases pyrite leaching leading to unwarranted enhanced acid mine drainage.

4. Conclusions

In the scope of this paper, some studies to ascertain the inherent formation of H2O2 in sulphide ore pulp liquid due to their surface reactivity were carried out to resolve or reveal one of the main problems (i.e., oxidation) in sulphide ore flotation. The initial grinding stage was evaluated and the oxidation of the ores was investigated at this stage. For the first time in mineral processing applications, it was established that the formation of H2O2 takes place in pulp during grinding (especially fine grinding). Hydrogen peroxide is a strong oxidizing agent and can easily oxide sulphide minerals and even cause dissolution of some metal ions from ore surface. That’s why H2O2, rather than oxygen, can be the main reason for oxidation, inadvertent activation and non-selectivity in the sulphide ore flotation. Prevention or reduction of H2O2 formation may result in improved flotation results and we are working on these aspects. Following preliminary conclusions were drawn from the experiments carried out in the scope of this paper. Mild steel produced a higher concentration of H2O2 than stainless steel medium. The concentration of H2O2 increased with increasing grinding time. Stainless steel produced a higher pyrite recovery than mild steel medium. At all three pH values studied (4.5, 10.5 and 11.5), stainless steel media produced a higher pyrite recovery than mild steel grinding medium due to lower concentration of H2O2 in stainless steel than mild steel medium.The mild steel medium increases the concentration of H2O2 and thereby increases pyrite oxidation and decreases its flotation recovery. Pyrite solids after dry grinding produced more H2O2 when placed in water than wet grinding. The pH of water influenced the formation of hydrogen peroxide where high amounts of H2O2 is produced at highly acidic pH and decreased with increasing pH upto 8 and increased again above this pH. The amount of H2O2 formed also increased with increase in pyrite loading due to increased surface area to react with water. Mild steel produced higher concentration of H2O2 than stainless steel medium due to higher amounts of ferrous ions release in mild steel grinding medium. In addition, dry grinding generated more H2O2 than wet grinding. More H2O2 was generated in N2 gas atmosphere compared to air atmosphere suggesting the breakdown of water molecule and giving raise to hydroxyl free radical due to the catalytic activity of reactive pyrite surfaces. Pyrite flotation at pH 4.5 after stainless steel grinding in different atmospheres became lower in N2 purging due to higher H2O2 formation compared to air atmosphere.

Acknowledgement

Financial support from the Centre for Advanced Mining and Metallurgy (CAMM), Luleå University of Technology, Sweden, established under Swedish Strategic Research Initiative programme and Boliden Mineral AB, Boliden, Sweden, is gratefully acknowledged.

13 References

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Paper 2

Alireza Javadi Nooshabadi, Kota Hanumantha Rao, Formation hydrogen peroxide by chalcopyrite and its influence on flotation– submitted to Journal ofMineral and Metallurgical Processing.

18 Formation of hydrogen peroxide by chalcopyrite and its influence on flotation

Alireza Javadi Nooshabadi and Kota Hanumantha Rao* Mineral Processing Group, Division of Sustainable Process Engineering Department of Civil, Environmental and Natural Resources Engineering Luleå University of Technology, SE-971 87 Luleå, Sweden *Corresponding author:Tel.: +46 920 491705; Fax: +46 920 491199; e-mail: [email protected]

Abstract

Formation of hydrogen peroxide (H2O2), an oxidizing agent stronger than oxygen, by chalcopyrite (CuFeS2), which is a copper iron sulfide mineral, during grinding, was investigated. It was observed that chalcopyrite generated H2O2 in pulp liquid during wet grinding and also the solids when placed in water immediately after dry grinding. The generation of H2O2 in either wet or dry grinding was thought to be due to a reaction between chalcopyrite and water where the mineral surface is catalytically active in producing •OH free radicals by breaking down the water molecule. Effect of pH in grinding medium or water pH in which solids are added immediately after dry grinding showed lower the pH value more was the H2O2 generation. When chalcopyrite and pyrite are mixed in different proportions, the formation of H2O2 was seen to increase with increasing pyrite fraction in the mixed composition. The results of H2O2 formation in pulp liquid of chalcopyrite and together with pyrite at different experimental conditions have been explained by Eh-pH diagrams of these minerals. This study highlights the necessity of revisiting the electrochemical and/or galvanic interaction mechanisms between the chalcopyrite and pyrite in terms of their flotation behaviour.

Keywords: Chalcopyrite; Wet and Dry Grinding; Hydrogen peroxide; Pyrite proportion, N2 and Air atmosphere; Flotation

1-Introduction

Hydrogen peroxide causes non-selective oxidation of sulfide minerals, if present in pulp liquid. Oxidation of sulfide minerals takes place when they are exposed to atmosphere and during the grinding process when the particle size is reduced for flotation. The oxidation of chalcopyrite leads to iron dissolution from the particle surface and form a metal deficient sulfur-rich surface (Fairthorne et al. 1997). Other studies report the formation of iron hydroxide on the surface with the metal ion dislodged on the surface (Buckley and Woods, 1984, Chander, 1991). The chemical reactions for the oxidation of chalcopyrite in alkaline and in acidic solutions with addition of oxidants, such as oxygen or hydrogen peroxide, are shown in equations 1 and 2, respectively (Shi and Fornasiero, 2010). CuFeS2 + (3/2) xO2 + (3/2) xH2OĺCuFe1-x S2 +xFe (OH) 3 (1) 2+ о CuFeS2ĺCuFe1-x S2+xFe +2xe (2) Buehler et al. (5) noted that when pyrite was mixed with a second sulfide mineral, the second mineral oxidized more rapidly. Owusu et al. (6) showed that with an increase in pyrite proportion in solids, chalcopyrite recovery decreased. Shi at el. also reported that conditioning of chalcopyrite in the presence of hydrogen peroxide induces oxidation of its surface with the

19 formation of the hydrophilic species of iron oxide/hydroxide and iron sulphate on surface which cause chalcopyrite depression in flotation (Shi and Fornasiero, 2010). Various mechanisms have been proposed to explain the influence of pyrite on flotation of chalcopyrite. Many authors has been reported that galvanic interactions are known to occur between conducting minerals and play a significant role in flotation (Rao and Finch, 1988, Kelebek et al., 1996, Zhang et al., 1997, Ekmekçi and Demirel, 1997 and Huang and Grano, 2005), leaching (Mehta and Murr, 1983, Abraitis et al., 2003 and Akcil and Ciftci, 2003), supergene enrichment of sulfide ore deposits (Thornber, 1975 and Sato, 1992), environment governance (Alpers and Blowes, 1994), and geochemical processes (Sikka et al., 1991 and Banfield, 1997). Recently was revealed that formation of H2O2 take place during wet grinding of complex sulfide ore (Ikumapayi et al., 2012). Pyrite (FeS2) has been shown to generate hydrogen peroxide (H2O2) (Cohn et al., 2006) when placed in water. In the presence of dissolved molecular oxygen, ferrous iron associated with pyrite can form superoxide anion • í (O2 ) (eq. 3), which reacts with ferrous iron to form H2O2 (eq. 4). II III • ௅ Fe (pyrite) + O2ĺ Fe (pyrite) + (O2 ) (3) II • ௅ + III Fe (pyrite) + (O2 ) + 2H ĺ Fe (pyrite) + H2O2 (4) Borda et al., (2003) showed that pyrite can also generate H2O2 in the absence of molecular oxygen. He reported that an electron is extracted from water and a hydroxyl radical is formed

(eq. 5). Combining two hydroxyl radicals leads to the formation of H2O2 (eq. 6): III • + II Fe + H2Oĺ OH + H + Fe (5) • 2 OH ĺ H2O2 (6) Also Antonijevic´ et al., (2004) showed that the accelerated chalcopyrite oxidation was due to increasing hydrogen peroxide concentration. • However, participation of H2O2 and OH, if any, in non-selective oxidation of the sulfide ore pulp components and hence in deteriorating the concentrate grade and recovery of metal- sulfides in flotation has not yet been explored. In an attempt to fill the gap, we have estimated the concentration of H2O2 in pulp liquid during different time of grinding and in different grinding environments. The effect of pH, type of grinding (wet or dry grinding), grinding atmosphere and pyrite proportion in mixed solids on the formation of hydrogen peroxide and chalcopyrite flotation was investigated and the results have been presented and discussed in this paper.

2. Experimental

2.1. Materials and reagents Crystalline pure chalcopyrite and pyrite minerals used in this study were procured from Gregory, Bottley & Lloyd Ltd., UK. Chalcopyrite contains 29% Fe, 29.5% S, 25.8% Cu, 0.54% Zn, 0.22% Pb and pyrite contains 44.4% Fe, 50.9% S. The XRD analyses of the samples showed that the main mineral phase present were the pyrite (Fig. 1a) and chalcopyrite (Fig. 1b). All pyrite and chalcopyrite samples used in this study were separately crushed through a jaw crusher and then screened to collect the –3.35 mm particle size fraction. The homogenized sample was then sealed in polyethylene bags. Potassium amyl xanthate (KAX) was used as collector and MIBC as frother in flotation tests. Solutions of sodium hydroxide (AR grade) and HCl (1 M) were used to maintain the pH at the targeted value during flotation. Deionized water was used in the processes of both grinding and flotation. Solutions of 2, 9- dimethyl-1, 10-phenanthroline (DMP), copper (II) sulfate (0.01 M), and phosphate buffer (pH 7.0) were used for measuring H2O2 following the method described by Baga et al. (1988). A 30% H2O2 solution was used to investigate the effect of H2O2 concentration on flotation of chalcopyrite.

Fig. 1. XRD analysis of (a) the pyrite sample (1-pyrite) and (b) chalcopyrite sample (1- chalcopyrite and 2- pyrrhotite 3- sphalerite). 2.2. Wet Grinding

Chalcopyrite mineral (100 g) of –3.00 mm size fraction in each grinding test was combined with 400 cm3 of water and ground in a new laboratory stainless steel ball mill (Model 2VS, CAPCO Test Equipment, Suffolk, UK) with stainless steel medium for 60 min. The slurry samples were collected at pre-determined time intervals and they were immediately filtered (Millipore 0.22 μm) and liquid (filtrate) was analyzed for hydrogen peroxide. The pH was regulated with HCL and NaOH solution. After grinding for 60 min, a 7.5 g of the sample that was < 106 μm was subjected to flotation using a cell of 150 ml capacity (Clausthal flotation equipment), where the pulp is conditioned with pH modifier, collector and frother. Flotation concentrate was collected after 2.0 min at air flow rate of 0.5 dm3 min-1. The flotation froth was scraped every 10 s. Dosages of collector in flotation was 10-4 M KAX. The conditioning times for adjusting pH and collector were 5 min and 2 min respectively. The frother dosage was one drop MIBC in all cases. Chalcopyrite flotation was conducted at pH 8, 10.5, and 11.5. The pH was regulated with HCl and/or NaOH solution. Experiments were performed at room temperature of approximately 22.5°C.

2.3. Dry Grinding and Flotation test Chalcopyrite and pyrite single minerals of 100 g each is separately ground in a laboratory stainless steel ball mill with stainless steel medium for 60 min. After grinding, 5 or 12.5 grams of chalcopyrite and pyrite minerals or chalcopyrite-pyrite mixture that was < 106 μm was mixed with 50 cm3 of water using magnetic stirrer and the slurry sample was collected at 0.5, 5 and 11 min interval, filtered and was analyzed for hydrogen peroxide. The pH was regulated with HCL and NaOH solution. Eh (pulp potential) was also measured at room temperature during mixing. After dry grinding for 60 min, a 5 g of sample that was < 106 μm was used in flotation with the same flotation conditions as were used after wet grinding the material.

2.4. Gas purging To study the effect of oxygen free atmosphere, chalcopyrite was wet-ground in a laboratory ball mill with stainless steel medium in N2 atmosphere at natural pH. For this study, the laboratory ball mill was first filled with 400 ml of deionized water and purged with N2 gas for a minimum of 30 min. Chalcopyrite was added and the laboratory ball mill jar was purged

21 with N2 gas for a minimum of 30 min again and then chalcopyrite sample was wet-ground for 1 h. Though we did not measure dissolved O2 concentrations in our experiments, it was reported that 1 L ultrapure water purged for 1 h with N2 gas, the O2 concentrations do not exceed 0.19 ± 0.05 ppm (Butler et al., 1994). The concentration of H2O2 was measured after 60 min of grinding. 2.5. Hydrogen peroxide quantification The spectrophotometric method using copper (II) ions and DMP has been found to be reasonably sensitive when applied to advanced oxidation processes (Kosaka et al. 1998). For DMP method (Baga et al., 1988) one milliliter each of DMP, copper (II) sulphate, and phosphate buffer (pH 7.0) solutions were added to a 10 mL volumetric flask and mixed. A measured volume of liquid (filtrate) sample was added to the volumetric flask, and then the flask was filled up with ultrapure water. After mixing, the absorbance of the sample (at 454 nm) was measured with DU® Series 700 UV/Vis Scanning Spectrophotometer. The blank solution was prepared in the same manner but without H2O2.

3. Results and discussion

3.1. Formation of hydrogen peroxide (H2O2) during wet grinding

Initially the extent of H2O2 formation during wet grinding of chalcopyrite was investigated. For these studies, chalcopyrite mineral alone was wet-ground in a laboratory stainless steel ball mill. Slurry samples were collected at pre-determined time intervals. The slurry samples were filtrated (Millipore 0.22 μm) and liquid (filtrate) was analyzed for hydrogen peroxide. The results of hydrogen peroxide formation in pulp liquid by chalcopyrite are shown in Fig. 2. It can be seen that the concentration of H2O2 decreased first with increasing grinding time, most likely due to increased pH in the pulp as shown Fig. 3, but after 10 min the concentration of H2O2 increased with increasing grinding time due to increasing surface area of solids and its reaction with water.

Fig. 2. Formation of H2O2 in wet grinding with increasing grinding time at natural pH (5.7).

Fig. 3. pH and Eh of pulp liquid during wet grinding.

3.2 Effect of pH on hydrogen peroxide (H2O2) formation during wet grinding Fig. 4 shows the effect of pH on formation of hydrogen peroxide. It can be seen that with an increase in pH, the concentration of H2O2 decreased up to pH 8 and then increased above this pH. After establishing the formation of H2O2, the effect of this strong oxidizing agent on solid surfaces and its consequence on flotation is addressed.

Fig. 4. Effect of pH on H2O2 concentration by chalcopyrite during wet grinding after 60 min.

3.3. Formation of hydrogen peroxide (H2O2) after dry grinding and mixing with water

Chalcopyrite or pyrite mineral of 100 g was dry-ground in a laboratory ball mill. Formation of hydrogen peroxide was observed when the dry-ground solids are mixed in water. The effect of water pH in which the solids are mixed, on the formation of hydrogen peroxide is shown in Fig. 5. It can be seen that with an increase in pH, the concentration of H2O2 decreased up to pH 8 and then increased above this pH. These results are in agreement with wet grinding data shown in Fig. 4. Fig. 5 also shows that the concentration of H2O2 increased with increasing mixing time, most likely due to increased chalcopyrite interaction with water. Increasing Eh with decreasing pH also corroborates the formation of high amounts of H2O2 at lower pH

23 values. Fairthorne at al. (1997) using the Eh–pH stability diagram of chalcopyrite (Fig. 6a) showed that the formation of insoluble ferric oxide/hydroxide at neutral and basic pH values but also in acidic conditions at high Eh values. Notably the divalent Fe and Cu ions exist at low pH values from negative to high Eh values. These divalent ions are reported to aid the formation of H2O2 (reference) from water and our present results shown in Table 1 2+ 3+ 2+ demonstrate that Fe ions generates substantial H2O2 followed by Fe and Cu ions. When the pulp liquid is free from dissolved oxygen, hydrogen peroxide is also seen to form (Fig. 9) with a little difference of low Eh value compared to the system open to atmosphere (Fig. 6). Fig. 6 displays that the concentration of ferrous ions decreases at the expense of ferric oxide/hydroxides at higher pH and Eh values (oxygen conditioning). The schematic diagram of H2O2 formation in the presence of dissolved molecular oxygen is shown in Fig. 7, where in • í acidic pH (pH < 5) forms superoxide anion (O2 ) (eq. 7), which reacts with ferrous iron to form H2O2 (eqs. 8-10). This is in agreement with other studies where it was observed that metals-induced formation of free radical has been significantly been evidenced that ferrous ion generates the superoxide and hydroxyl radical (Valko et al., 2005, Jones et al., 2011).

í • í O2 + e ĺ (O2 ) (7) • í + 2(O2 ) + 2H ĺ H2O2 +O2 (8) 2+ 3+ • í Fe + H2O2ĺ Fe + OH + OH (9) • 2 OHo H2O2 (10)

Above pH 5, the stability diagram shows the Fe2O3 solid phase that also generates hydrogen peroxide. This is in agreement with other studies where hematite and magnetite solids in water were shown to induce H2O2 formation (Cohn et al., 2006). Fig. 7a exhibit that copper ions also generate hydrogen peroxide. The metal ions induced formation of hydroxyl free radical has been significantly evidenced where copper ions generate the superoxide and hydroxyl radical (eqs. 11-14) (Valko et al., 2005). о •– O2 + e ĺ O2 (11) •– + 2O2 + 2H ĺ H2O2 + O2 (12) + 2+ • – Cu + H2O2 oCu + OH + OH (13) • 2 OHo H2O2 (14) In acidic pH, the higher amounts of hydrogen peroxide formation than alkaline pH is due to the presence of copper and ferrous ions in acidic pH which ions are capable to generate hydrogen peroxide (Table 1).

Table 1. H2O2 generation in the presence of metal ions at natural pH and at 22 °C.

H2O2 (mM) Concentration of ions 1 mM 10 mM water 0 0 Fe 2+ 0.552 4.656 Fe 3+ 0.004 0.059 Cu 2+ 0 0.015

Kocabag et al. (1990) using the Eh-pH diagram (Fig. 6b) of pyrite showed that oxidation of 2+ + 2+ pyrite yields S°, Fe , Fe (OH)2 species in the solution for pH < 6. In pH < 6, Fe ions are increased with decreasing pH and therefore, in the presence of dissolved molecular oxygen, • í ferrous iron associated with pyrite (Fig. 7b) can form superoxide anion (O2 ) (eq. 3), which reacts with ferrous iron to form H2O2 (eq. 4) (Cohn et al., 2006).

Fig. 5. Effect of pH on H2O2 concentration with 5 g of solids: a) chalcopyrite and b) pyrite after 0.5 and 5 min mixing time with water.

a) b)

Fig. 6. a) Eh–pH stability diagram for the Cu–Fe–S–H2O system with the preponderant copper and iron species shown in each domain. Empty and filled circles represent the experimental Eh and pH values for chalcopyrite conditioned in nitrogen and oxygen, respectively (Fairthorne, 0 -5 1997 ) b)Eh – pH diagram for FeS2 – H2O system at 25 C and for 10 M dissolved species (Kocabag et al., 1990).

25 b) a)

Fig. 7. Proposed mechanisms for formation of H2O2 by a) chalcopyrite b) pyrite to form via the incomplete reduction of oxygen and be formed from two reacting OH. The numericals in this figure represent the equation numbers. (For pyrite, proposed mechanism adapted from Borda et al. (2003) and cohn et al. (2006))

3.4. Effect of chalcopyrite loading on formation of hydrogen peroxide The effect of chalcopyrite and pyrite loading on hydrogen peroxide was investigated with 50 ml total volume of water at pH 11.6 and the results are shown in Fig. 8. It can be seen that the concentration of H2O2 increased with increasing chalcopyrite and pyrite loading.

Fig. 8. Effect of chalcopyrite or pyrite loading on H2O2 concentration after 5 min mixing with water at pH 11.6.

3.5. Gas purging To study the effect of gaseous atmosphere or the presence or absence of oxygen on hydrogen peroxide formation, chalcopyrite was wet-ground in a laboratory ball mill with stainless steel media in either air or N2 atmosphere. For N2 atmosphere the laboratory ball mill was filled

26 with 400 ml water and purged with N2 gas for a minimum of 1 h and then chalcopyrite was wet-ground for 1 h. For air atmosphere the laboratory ball mill was allowed open for 5 minutes to have the air and then chalcopyrite was wet-ground for 1 h. Fig. 9 shows that H2O2 was generated in a N2 atmosphere too. N2 atmosphere generated lower H2O2 than air atmosphere. One can hypothesis that an electron is extracted from water and a hydroxyl radical is formed (eq. 15) and reaction between two hydroxyl radicals leads to the formation of H2O2 (eq. 16):

2+ • + + Cu + H2Oĺ OH + H + Cu (15) • 2 OH ĺ H2O2 (16)

Fig. 9. The effect of gaseous atmosphere on formation of H2O2 by chalcopyrite at natural pH.

3.6. Effect of hydrogen peroxide (H2O2) on chalcopyrite flotation

Shi and Fornasiero (2010) observed that conditioning of chalcopyrite in the presence of hydrogen peroxide induces oxidation of its surface with formation of the hydrophilic species of iron oxide/hydroxide and iron sulphate causing its depression in flotation. After dry- grinding for 60 min, the effect of H2O2 formation in water by the freshly ground solids on chalcopyrite flotation recovery was investigated. The concentration of H2O2 in the pulp liquid vis-à-vis chalcopyrite flotation is shown in Fig. 10. H2O2 decreased the recovery of chalcopyrite when its concentration exceeds 0.3 mM and this result was the same as Castro et al. (2003), who reported that the floatability of chalcopyrite was decreased after pre-treatment with hydrogen peroxide in alkaline condition. Fig. 11 shows the effect of initial pH grinding on recovery of chalcopyrite at pH 10.5. It can be seen that the recovery of chalcopyrite remained almost constant at the pH > 5 but decreases at pH 3.5 due to higher H2O2 concentration (Fig. 4). Fig. 12 shows the effect of pH on recovery of chalcopyrite after wet and dry grinding.It can be seen that the recovery of chalcopyrite after wet-ground at natural pH (5.7) or dry-ground decreased with increasing pH. Ackerman et al. (1987) and

27 Chandraprabha et al. (2006) reported that the recovery of chalcopyrite decreases above pH 8. This decrease in flotation recovery at pH > 8 can be due to increasing H2O2 (see Figs. 4 and 5a) formation.

Fig. 10. Effect of H2O2 on recovery of chalcopyrite with dry-ground solids at pH 10.5.

Fig. 11. Effect initial pH of grinding on recovery of chalcopyrite at flotation pH 10.5.

Fig. 12. Recovery of chalcopyrite at different pH with wet and dry ground solids.

28 3.7. Chalcopyrite-pyrite mixture

Samples of chalcopyrite–pyrite mixture (12.5 g in total) mixed with 50 cm3 of water for 5 min. Figs. 13 a, and b show the effect of pyrite proportion in chalcopyrite-pyrite mixture on the formation of hydrogen peroxide at pH 4.5, 9.5 and 10.5. It can be seen that with an increase in pyrite fraction, the concentration of H2O2 increased. This is in agreement with other studies where it was observed when pyrite was mixed with a second sulfide mineral, the second mineral oxidized more rapidly (Buehler and Gottschalk, 1910). This mechanism that with an increase in pyrite proportion, the concentration of H2O2 increased can be the explanation for this behavior that: 1- The floatability of chalcopyrite decreases when in contact with pyrite as Owusu et al. (2011) observed that an increase in pyrite content from 0% to 80% resulted in a decrease in chalcopyrite recovery from 98% to 80% and a concomitant decrease in the flotation rate constant from 0.94 to 0.42 min-1. In contrast to chalcopyrite, the recovery of pyrite gradually increased as the amount of chalcopyrite present in the mixture increased Owusu et al. (2011). Therefore, at certain H2O2 concentration, the complete decomposition of copper xanthate preadsorbed on chalcopyrite renders chalcopyrite surface hydrophilic and depression of chalcopyrite as shown in Fig. 10. 2- The selective flotation of chalcopyrite from pyrite is possible at pH 10 and is impossible at pH 4 (Mitchell et al., 2005). 3- The amount of pyrite in contact with chalcopyrite increases, the leaching rate of this mineral increases (Koleini et al., 2010, Koleini et al., 2011, Dixon and Tshilombo, 2005,Mehta, and Murr 1983,Holmes and Crundwell, 1995). This is in agreement with other studies where it was observed that increasing hydrogen peroxide concentration accelerates considerably chalcopyrite oxidation (eq. 17) (Antonijevic, 2004). + 2+ 3+ 2- 2CuFeS2 + 17H2O2 + 2H ĺ 2Cu + 2Fe +SO4 + 18H2O (17) 4- The generation of H2O2 by pyrite and chalcopyrite particles in solution has been proposed to explain the observed decreased microbial performance in the presence of pyrite and chalcopyrite under different solids loading operating conditions (Jones et al., 2011, Jones et al., 2012).

Figs. 13a and b also show the effect of chalcopyrite proportion in chalcopyrite-pyrite mixture on formation of hydrogen peroxide at pH 4.5, 9.5 and 10.5. It can be seen that with an increase in chalcopyrite proportion, the concentration of H2O2 decreased. This mechanism may be the explanation for the behavior of increasing pyrite floatability in the presence of chalcopyrite as reported by Peng et al. (2003) of increased pyrite flotation after addition of chalcopyrite. The formation of higher amounts of H2O2 when chalcopyrite is in contact with pyrite may explain the effect of the interaction between pyrite and chalcopyrite on flotation, leaching, environment governance and geochemical processes as shown in Fig. 14 B and C while the entire literature describes so far of galvanic interaction between two contacting sulphide minerals and electron transfer from one to the other as shown in Fig. 14A (Rao and Finch, 1988, Kelebek et al., 1996, Zhang et al., 1997, Ekmekçi and Demirel, 1997 and Huang and Grano, 2005, Mehta and Murr, 1983, Abraitis et al., 2003, Akcil and Ciftci, 2003, Thornber, 1975, Sato, 1992, Alpers and Blowes, 1994, Sikka et al., 1991 and Banfield, 1997).

Fig. 13. Effect of pyrite and chalcopyrite proportion in their mixture on H2O2 formation with total 12.5 g sample after 5 min conditioning in water at (a) pH 9 and 10.5 (b) pH 4.5.

A) B)

(C)

Fig. 14. Proposed mechanisms for Oxidation of chalcopyrite (Cp) by A) Galvanic interaction between chalcopyrite and pyrite (Py). Anodic dissolution reactions occur on the surface of Cp and cathodic reactions occur on the surface of Py(Adapted from Koleini et al. (2011)). B) Formation of H2O2 by Py that are proposed to form via the incomplete reduction of oxygen and be formed from two reacting OH (eqs. 3-6), C) Both Galvanic interactions between Cp and Py and Formation of H2O2 by Py.

3.8. Effect of ferric ions on chalcopyrite

It was reported that chalcopyrite dissolution in sulphate media with ferric sulphate was an electrochemical reaction where the anodic and cathodic half-cell reactions are written as in Eqs. (18) and (19) (Mikhin et al., 2004, Misra and Fuerstenau, 2005; Watling, 2006; Liu et al., 2007). Anodic half-cell reaction: chalcopyrite oxidation 2+ 2+ വ CuFeS2 ĺ Cu +Fe +2S°+4e (18)

30 Cathodic half-cell reaction: reduction of ferric ions Fe3++eവ ĺ Fe2+ (19) The ferric ions would generate ferrous ions according to equation 20 and ferrous ions can generate H2O2 (eqs. 21 and 22) shown in Table 1 and H2O2 oxidizes chalcopyrite (eq. 17) as displayed in Fig. 15 (Antonijevic, 2004). Fe3++eവĺ Fe2+ (20) 2+ 3+ • ௅ Fe + O2 ĺ Fe + (O2 ) (21) 2+ • ௅ + 3+ Fe + (O2 ) +2H ĺFe +H2O2 (22)

Fig. 15. Proposed mechanism for oxidation of chalcopyrite (Cp) by H2O2 through ferric and ferrous ions.

The above discussion may be summarized by the flowing events: (a) increasing pyrite proportion increases concentration of H2O2 in mixture of pyrite-chalcopyrite; (b) increasing concentration of H2O2 increases oxidation of chalcopyrite and decreases recovery of chalcopyrite flotation; (c) increasing concentration of H2O2 also increases chalcopyrite leaching recovery; (d) increasing chalcopyrite proportion decreases concentration of H2O2 in mixture of pyrite-chalcopyrite; and (e) decreasing concentration of H2O2 decreases oxidation of pyrite and increases recovery of pyrite flotation.

4. Conclusions

In the scope of this paper, fundamental studies were carried out to resolve or reveal one of the main problems, i.e., oxidation, in sulfide ore flotation. For this reason, the oxidation of sulfide mineral at the grinding stage was evaluated. For the first time in mineral processing applications, it was found out that during grinding (especially fine grinding) formation of H2O2 in pulp liquid takes place. It is known that hydrogen peroxide is a strong oxidizing agent and can easily oxidize sulfide minerals and can even cause dissolution of some metal ions from ore surface. That’s why H2O2, rather than oxygen, can be the main reason for oxidation, inadvertent activation and non-selectivity in the sulfide ore flotation. Prevention or reduction of H2O2 formation may result in improved flotation results and we are working on these aspects. Following preliminary conclusions were drawn from the results of our initial investigations. Formation of hydrogen peroxide takes place in the pulp liquid and its significance in mineral processing applications hitherto not considered in flotation literature has been clear. At lower acidic pH, formation of higher amounts of H2O2 is more evident and increasing pH decreased its concentration up to pH 8 and then increase above this pH value. Dry grinding also produced hydrogen peroxide when the dry ground solids were placed in

31 water and H2O2 concentration increases with increasing conditioning time and solids loading. In a mixed composition of chalcopyrite-pyrite, an increase in pyrite fraction increases H2O2 formation correlating to a decrease in chalcopyrite recovery with increasing pyrite fraction in mixed mineral composition. This clearly suggests that a decrease in chalcopyrite recovery with increasing pyrite proportion was due to the increase of hydrogen peroxide formation. H2O2 was also generated in the N2 atmosphere and devoid of oxygen illustrating that the oxygen in H2O2 is derived from the water molecules.

Acknowledgements

Financial support from the Centre for Advanced Mineral and Metallurgy (CAMM) Research Centre, LTU and Boliden Mineral AB is gratefully acknowledged.

References

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Paper 3

Alireza Javadi Nooshabadi, Kota Hanumantha Rao, Formation hydrogen peroxide by sphalerite and its influence on flotation– submitted to International Journal of Mineral Processing.

36 Formation of hydrogen peroxide by sphalerite

Abstract

Formation of hydrogen peroxide (H2O2), an oxidizing agent stronger than oxygen, by sphalerite ((Zn, Fe) S) was examined during its grinding process. It was observed that sphalerite generated H2O2 in pulp liquid during wet grinding and also when the freshly ground solids placed in water immediately after dry grinding. The generation of H2O2 in either wet or dry grinding was thought to be due to a reaction between sphalerite and water where the mineral surface is catalytically active to produce •OH free radicals by breaking down the water molecule. Effect of pH on the formation of H2O2 by sphalerite was shown that the acidic pH generated more H2O2. Mixtures of pyrite, chalcopyrite and galena with sphalerite on the formation of H2O2 were also probed. It was shown that the concentration of H2O2 increases with increasing pyrite or chalcopyrite fraction in pyrite–sphalerite, chalcopyrite– sphalerite mixtures but with an increase in galena proportion, the concentration of H2O2 decreased in galena–sphalerite mixture. The oxidation or dissolution of one mineral than the other in a mixture can be explained better with the extent of H2O2 formation in the pulp liquid than galvanic interactions. It is clear of the greater role of H2O2 in the oxidation of sulphides or aiding the extensively reported galvanic interactions since the amount of H2O2 generated with a specific mineral followed the rest potential series. This study highlights the necessity of further investigations into the role of H2O2 in electrochemical and/or galvanic interaction mechanisms between pyrite, chalcopyrite and galena with sphalerite.

Keywords: H2O2, sphalerite, flotation, pyrite, chalcopyrite, galena

1. Introduction Hydrogen peroxide, a stronger oxidizing agent than oxygen, causes non-selective oxidation of sulphide minerals. The oxidation of sulphide minerals takes place during the grinding process when the particle size is reduced for flotation. Buehler and Gottschalk (1910) were noted that when pyrite was mixed with a second sulfide mineral, the second mineral oxidized more rapidly.Harvey and Yen (1998) observed that addition of galena to the selective sphalerite leaching system retarded the dissolution of sphalerite. Alternatively, a pyrite or chalcopyrite addition increased zinc extractions. Many authors have been reported that galvanic interactions are known to occur between conducting minerals and play a significant role in flotation (Rao and Finch, 1988, Kelebek et al., 1996, Zhang et al., 1997, Ekmekçi and Demirel, 1997 and Huang and Grano, 2005), leaching (Mehta and Murr, 1983,Abraitis et al., 2003 and Akcil and Ciftci, 2003), supergene enrichment of sulfide ore deposits (Thornber, 1975 and Sato, 1992), environment governance (Alpers and Blowes, 1994), and geochemical processes (Sikka et al., 1991 and Banfield, 1997). Recently it was revealed that H2O2 take

37 place during wet grinding of complex sulphide ore (Ikumapayi et al., 2012). Previous work published by the authors has highlighted the significance of reactive oxygen species (ROS), • H2O2 and hydroxyl radical ( OH), generated from milled sulfide concentrates and their potential effect on thermophilic Fe- and S-oxidising bioleaching microorganisms through oxidative stress (Jones et al., 2011 and Jones et al., 2012). To date, most studies dealing with sulfide mineral-induced ROS formation have made use of natural or synthetic mineral samples, of very high purities, suspended in neutral solutions. Pyrite induced ROS formation has been studied most; however other sulfide minerals such as chalcopyrite (CuFeS2), sphalerite (ZnS), pyrrhotite (Fe(1íx)S) and vaesite (NiS2) have also been studied with respect to ROS generation (Borda et al., 2001 and Jones et al., 2011), and reactivities have been found to differ between sulfide minerals. • However, participation of H2O2 and OH, if any, in non-selective oxidation of the sulfide ore pulp components and hence in deteriorating of the concentrate grade and recovery of metal-sulfides has not yet been explored. In an attempt to fill the gap, we have estimated the concentration of H2O2 in pulp liquid during different time of grinding and in different grinding environments. Thus the effect of pH, type of grinding (wet or dry grinding), and different pyrite, chalcopyrite and galena fractions in mixtures with sphalerite on formation of hydrogen peroxide was investigated. In this paper, the results of these investigations were presented and discussed various path-ways for the generation of H2O2.

2. Experimental

2.1. Materials and reagents Crystalline pure sphalerite, galena, pyrite and chalcopyrite minerals used in this study were procured from Gregory, Bottley & Lloyd Ltd. Sphalerite contains 39.92% Zn, 20.7% S, 4.2% Fe, 1.32% Pb and 0.17% Cu; galena contains 73.69% Pb, 13.5% S, 1.38% Fe, 1.26% Zn, 0.2% Cu and some silica (quartz) impurity; pyrite contains 44.4% Fe, 50.9% S, and 0.2% Cu, and chalcopyrite contains 29% Fe, 29.5% S, 25.8% Cu, 0.54% Zn, and 0.22% Pb. The XRD analyses of the samples showed that the main mineral phase present were the pyrite (Fig. 1a), chalcopyrite (Fig. 1b), sphalerite (Fig. 1c) and galena (Fig. 1d). All pyrite, chalcopyrite, sphalerite and galena samples used in this study were separately crushed through a jaw crusher and then screened to collect the –3.35 mm particle size fraction. The homogenized sample was then sealed in polyethylene bags. Potassium amyl xanthate (KAX) collector and MIBC frother were used in flotation studies. Dilute solutions of AR grade sodium hydroxide and HCl were used to maintain the pH at the targeted value during flotation. Deionised water was used in the processes of both grinding and flotation. 2, 9-dimethyl-1, 10-phenanthroline (DMP), copper (II) sulphate (0.01 M), and phosphate buffer (pH 7.0) solutions were used for estimating H2O2 amount in pulp liquid by UV-Visible spectrophotometer. Zinc sulfate, copper sulphate, ferrous sulfate and ferric sulfate chemicals were also used to investigate the effect of 2+ 2+ 2+ 3+ metal ions (Zn , Cu , Fe and Fe ) on the formation of H2O2. 2.2. Wet grinding In each grinding test, 100 g of –3.35 mm size fraction of sphalerite mineral was mixed with 400 cm3 of water and ground in a new laboratory stainless steel ball mill (Model 2VS, CAPCO Test Equipment, Suffolk, UK) with stainless steel medium for 60 min. The pH of the pulp liquid was adjusted to a specified value by HCl and NaOH solutions before grinding. Aliquot portions of slurry samples were collected at pre-determined grinding time intervals and filtered immediately through millipore filter of 0.22 μm pore size and the filtrate was analyzed for hydrogen peroxide.

2.3. Dry grinding 100 g of –3.35 mm size fraction of sphalerite, pyrite, galena and chalcopyrite single mineral in each grinding was separately ground in a laboratory stainless steel ball mill with stainless steel medium for 60 min. 5 g of sphalerite, galena, pyrite or chalcopyrite single mineral and 12.5 g in total of mineral mixture either sphalerite-pyrite or sphalerite-chalcopyrite or sphalerite-galena that was < 106 μm was conditioned with 50 cm3 water using a magnetic stirrer and the slurry sample was collected successively after 0.5, 5 and 11 min conditioning time and analyzed for hydrogen peroxide. The pH was regulated with HCl and/or NaOH solution. Eh (pulp potential) were measured at room temperature during mixing.

Fig. 1. XRD analysis of the mineral samples: (a) pyrite (1- pyrite); (b) chalcopyrite (1- chalcopyrite, 2- pyrrhotite, 3- sphalerite); (c) sphalerite (1- sphalerite, 2- galena, 3- quartz); (d) galena (1- galena, 2- sphalerite, 3- quartz).

2.4. Estimation of hydrogen peroxide The spectrophotometric method using copper (II) ions and DMP has been found to be reasonably sensitive when applied to advanced oxidation processes (Kosaka et al., 1998). For DMP method (Baga el al., 1988), 1 mL each of 1% DMP in ethanol, 0.01M copper (II) sulphate, and phosphate buffer (pH 7.0) solutions were added to a 10 mL volumetric flask and mixed. A measured volume of liquid (filtrate) sample was added to the volumetric flask, and the volume was made up to the mark with ultrapure water. After mixing, the absorbance of

39 the sample (at 454 nm) was measured with DU® Series 700 UV/Vis Scanning Spectrophotometer. The blank solution was prepared in the same manner but without H2O2.

2.5. Flotation test After dry grinding for 60 min of 100 g of –3.35 mm feed size, the mill was emptied and the solids were screened at 106 μm size. For each flotation 5 g of sample that was < 106 μm was transferred to a cell of 150 ml capacity (Clausthal flotation equipment), conditioned with pH modifier (HCl/NaOH), collector and frother. Flotation concentrate was collected after 2.0 min at air flow rate of 0.5 dm3 min-1. The flotation froth was scraped every 10 s. Dosages of collector in flotation was 10-4 M KAX. The conditioning times for adjusting pH and collector were 5 min and 2 min respectively. The frother dosage was one drop MIBC in all cases. Sphalerite flotation was investigated at different pH. The pH was regulated with NaOH and HCL solution. Experiments were performed at ambient temperature of approximately 22.5°C.

3. Results and discussion

3.1. Formation of hydrogen peroxide (H2O2) in wet grinding

Initially the extent of H2O2 formation during wet grinding of sphalerite was investigated. For these studies, sphalerite single mineral wet-ground in a laboratory stainless steel ball mill with stainless steel medium and slurry samples were collected at pre-determined time intervals. The slurry samples were filtered (Millipore 0.22 μm) and liquid (filtrate) was analyzed for hydrogen peroxide. Formation of hydrogen peroxide was detected in the filtrate for the first time in mineral processing applications. The effect of pulp pH during grinding on formation of hydrogen peroxide is shown in Fig. 2. It can be seen that at pH 2.7 generated more H2O2 than at pHs 3.7 and 6.8.

Fig. 2. Effect of pH in wet grinding on formation of H2O2 by sphalerite.

3.2. Formation of hydrogen peroxide (H2O2) after dry grinding and mixing with water

Sphalerite (100 g) was dry-ground in a laboratory ball mill. The effect of pH on formation of hydrogen peroxide by freshly ground dry solids is shown in Fig. 3. It can be seen that for sphalerite the concentration of H2O2 increased with decreasing pH. Fig. 3 also shows that Eh

40 increased with decreasing pH. Huai Su (1981) showed by Eh-pH stability diagram of sphalerite species that in pH < 6, Zn2+ and Fe2+ species exist (Fig. 4). The results in Table 1 2+ 2+ show that Zn and Fe ions generated H2O2. dissolved molecular oxygen reacts with 2+ disolved Fe via the Haber-Wiess reaction mechanism and forms H2O2 (eqs. 1 and 2) with • – superoxide anion (O2 ) as an intermediate species. The H2O2 can then react with ferrous iron • disolved to form OH (eq. 3)(Cohn et al., 2006) and also with combining two hydroxyl radicals leads to formation of H2O2 (eq. 4)(Borda et al., 2003) as shown in Fig. 5. 2+ 3+ • í Fe + O2ĺ Fe + (O2 ) (1) 2+ • ௅ + 3+ Fe + (O2 ) + 2H ĺ Fe + H2O2 (2) 2+ • ௅ 3+ Fe + H2O2ĺ+ OH+ OH + Fe (3) • • OH+ OHĺ H2O2 (4) 2+ • í Also Zn ions can form superoxide anion (O2 ) to form H2O2 (eq. 5): • ௅ + (O2 ) + 2H ĺH2O2 (5)

Fig. 3. Effect of pH on H2O2 concentration.

Table 1. Effect of metal ions on H2O2 generation at two initial concentrations (conditioning time 1 h, natural pH, and 22 °C).

H2O2 (mM) Concentration of ions 1 mM 10 mM water 0 0 Fe 2+ 0.552 4.656 Fe 3+ 0.004 0.059 Cu 2+ 0.00068 0.015 Zn 2+ 0.004 0.088

Fig. 4. Eh–pH stability diagram for the ZnS–H2O–FeS system (Huai Su, 1981)

2+ 2+ Fig. 5. Proposed mechanisms for formation of H2O2 by sphalerite through Zn or Fe ions • reacting with water to generate OH and H2O2. The numericals in this figure represent the equation numbers.

3.2. Effect of sphalerite loading on formation of hydrogen peroxide

For investigation of the effect of sphalerite loading it was mixed with 50 ml water at pH 2.5. The concentration of H2O2 production increased with increasing sphalerite loading as was shown in Fig. 6. This figure also shows that the concentration of H2O2 increased with increasing mixing time, most likely due to increased sphalerite reactions with water.

Fig. 6. Effect of sphalerite loading on H2O2 production after 5 min mixing with water at pH 2.5.

3.3. Effect of hydrogen peroxide on sphalerite flotation

After dry-grinding for 60 min, the effect of H2O2 formation in water by the freshly ground solids on sphalerite flotation recovery was investigated. The concentration of H2O2 in the pulp liquid vis-à-vis sphalerite flotation is shown in Fig. 7. It can be seen that H2O2 increased the recovery of sphalerite when its concentration is < 1 mM. This result explains better the results reported in the literature as below: 1. sphalerite can be floated with xanthate in the pH range 8-11 in the presence of ferrous (but not ferric) ions and oxygen (Zhang et al., 1992) due to ferrous ions generating H2O2 as shown in Table 1. 2. recovery of sphalerite decreased when pH increases from pH 3 to pH 5 (Ikumapayi et al., 2012) due to H2O2 decreases with increasing pH as shown in Fig. 3.

Fig. 7. Effect of H2O2 on recovery of sphalerite with dry-ground solids at pH 10.5.

43 3.4. Sphalerite-pyrite mixture

Fig. 8 (left) shows the effect of pyrite proportion in sphalerite–pyrite mixture on formation of hydrogen peroxide at different pH. It can be seen that with an increase in pyrite proportion, the concentration of H2O2 increased. This is in agreement with other studies where it was observed when pyrite was mixed with a second sulfide mineral, the second mineral oxidized more rapidly (Buehler and Gottschalk, 1910). This mechanism of non-ferrous metal sulphide oxidation with increasing pyrite fraction could be due to the increased H2O2 generation and also that an increase in the rate of sphalerite leaching with increasing pyrite proportion (Harvey and Yen ,1998) is due to increasing H2O2 formation with an increase in pyrite proportion as shown in Fig. 9. This is in agreement withother studies where it was observed that the oxidation of sphalerite increased with increase in hydrogen peroxide concentration (Adebayo et al., 2006 and Aydogan, S., 2006). Fig. 8 (right) shows the effect of sphalerite proportion in sphalerite–pyrite mixture on formation of hydrogen peroxide. It can be seen that with an increase in sphalerite proportion, the concentration of H2O2 decreased.

Fig. 8. Effect of pyrite (left) and sphalerite (right) proportion in sphalerite-pyrite mixture on concentration of H2O2 at different pH.

Fig. 9. Proposed mechanisms for oxidation of sphalerite with increasing of H2O2 intermediates by pyrite.

44 3.4. Sphalerite-Chalcopyrite mixture Fig. 10 (left) shows the effect of chalcopyrite proportion in sphalerite–chalcopyrite mixture on formation of hydrogen peroxide at different pH values. It can be seen that with an increase in chalcopyrite proportion, the concentration of H2O2 increased. This result of increasing H2O2 concentration with increasing chalcopyrite fraction explains the following observations: 1. significant higher recoveries of sphalerite were obtained even in the absence of any added activator but with the addition of chalcopyrite (Yelloji Rao and Natarajan, 1989), which can be explained with increasing H2O2 concentration by increasing chalcopyrite fraction (Fig. 10) which causes positive Eh pulp environment needed for dixanthogen surface species and thereby flotation (Fig. 7). 2. the rate of sphalerite leaching increased with an increase in chalcopyrite proportion (Harvey and Yen, 1998) as shown Fig. 11. This is in agreement withother studies where it was observed that the oxidation of sphalerite increased with an increase in hydrogen peroxide concentration (Adebayo et al., 2006 and Aydogan, S., 2006).

Fig. 10 (right) shows the effect of sphalerite proportion in sphalerite –chalcopyrite mixture on formation of hydrogen peroxide. It can be seen that with an increase in sphalerite fraction, the concentration of H2O2 decreased.

Fig. 10. Effect of chalcopyrite (left) and sphalerite (right) proportion in sphalerite- chalcopyrite mixture on concentration of H2O2 at different pH.

Fig. 11. Proposed mechanisms for oxidation of sphalerite with increasing of H2O2 intermediates by chalcopyrite.

45 3.5. Effect of Cu2+ ion on sphalerite The activation of sphalerite has been studied extensively over several decades (Finkelstein, N.P., 1997, Finkelstein and Poling, 1977, Gerson et al.,1999, Fornasiero and Ralston, 2006). It has been well established that copper activation of sphalerite follows an ion exchange mechanism where the uptake of Cu (II) results in approximately 1:1 release of Zn2+ into the solution (Finkelstein, N.P., 1997, Gerson et al.,1999, Popov, S.R., Vucinic, D.R.,1990) while the mechanism could be copper ions can generate H2O2 as shown in Table 1 and H2O2 increses recovery of sphalerite (Fig. 7).

3.6. Sphalerite-galena mixture

Fig. 12 (left) shows the effect of galena proportion in sphalerite-galena mixture on formation of hydrogen peroxide at different pH. It can be seen that with increasing galena proportion, the production of H2O2 decreased. This result could explain that a decrease in the rate of sphalerite leaching with increasing galena proportion (Harvey and Yen 1998) by the decrease in H2O2 formation. Fig. 12 (right) shows the effect of sphalerite proportion in sphalerite– galena mixture on formation of hydrogen peroxide. It can be seen that with an increase in sphalerite proportion, the production of H2O2 increased.

Fig. 12. Effect of (a) galena and (b) sphalerite proportion in galena-sphalerite mixture on concentration of H2O2 at different pH.

3.7. Effect of ferric ions on sphalerite

In literature, it has been widely reported that ferric ions are one of the most important oxidants used in leaching processes, in either sulphate or chloride media, that were used to leach zinc sulphide (eq. 6) as shown in Fig. 13a (Aydogan et al., 2005 and Dutrizac et al., 2003, Santos, 2010): ZnS + 2Fe3+ ĺ Zn2+ + 2Fe2+ + S0 (6) Also Holmes (2000) reported that sphalerite reactivity depends on its iron content, i.e., the more iron in the solution, the easier the sphalerite can be leached, while ferric ions generates ferrous ions (eq. 7), which in turn responsible to form H2O2 (eqs. 1-2 and 4) and H2O2 oxidizes sphalerite (eq. 8) as shown in Fig. 13b. Fe3++e-ĺFe2+ (7)

46 + 2+ 2í 2ZnS + 4H2O2 +2H ĺ 2Zn +SO4 +H2S + 4H2O (8)

a) b)

Fig. 13. Proposed mechanisms for oxidation of sphalerite (Sp) a) by ferric ions b) both ferric ions and H2O2 that generated by ferrous ions and sphalerite The above discussion may be summarized as follows: (a) increasing pyrite proportion increases production of H2O2 in a mixture of pyrite-sphalerite; (b) increasing concentration of H2O2 increases oxidation of sphalerite and accelerate leaching of sphalerite; (c) increasing chalcopyrite proportion increases production of H2O2 in mixture of chalcopyrite-sphalerite; (d) increasing galena proportion decreases concentration of H2O2 in mixture of galena-sphalerite; (e) decreasing concentration of H2O2 decreases oxidation of sphalerite and retarded leaching of sphalerite.

4. Conclusions

In the scope of this paper, fundamental studies were carried out to reveal one of the main problems (i.e., oxidation) in sulfide ore flotation. For this reason, the grinding stage was initially examined and oxidation of sulphide minerals was investigated at this stage. For the first time in mineral processing applications, it was observed that during grinding (especially fine grinding) formation of H2O2 takes place in the pulp liquid. It is known that hydrogen peroxide is a strong oxidizing agent and can easily oxide sulfide minerals and can even cause dissolution of some metal ions from minerals. That’s why H2O2, rather than oxygen, can be the main reason for oxidation, inadvertent activation and non-selectivity in the sulfide ore flotation. Prevention or reduction of H2O2 formation may result in improved flotation results and we are working on these aspects. Wet grinding of sphalerite produced H2O2 and it increases with decreasing pH. Dry grinding also produced hydrogen peroxide when sphalerite was mixed with water and the effect of pH found to be the same as that of wet grinding. Also the concentration of H2O2 increased with increasing mixing time and sphalerite loading. With an increase in pyrite and chalcopyrite proportion in pyrite-sphalerite and chalcopyrite– sphalerite mixtures respectively, the concentration of H2O2 increases. However, with an increase in galena proportion in galena–sphalerite mixture, the concentration of H2O2 decreases.

Acknowledgements

Financial support from the Centre for Advanced Mineral and Metallurgy (CAMM) Research Centre, LTU and Boliden Mineral AB is gratefully acknowledged

47 References

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Paper 4

Alireza Javadi Nooshabadi, Kota Hanumantha Rao, Formation hydrogen peroxide by galena and its influence on flotation– submitted to Journal of Mineral Processing and Extractive Metallurgy (Trans. IMM C).

52 Formation of hydrogen peroxide by galena and its influence on flotation

Abstract

Formation of hydrogen peroxide (H2O2), an oxidizing agent stronger than oxygen, by galena (PbS) was examined during its grinding process. It was observed that galena generated H2O2 in pulp liquid during wet grinding and also when the freshly ground solids placed in water immediately after dry grinding. The generation of H2O2 in either wet or dry grinding was thought to be due to a reaction between galena and water where the mineral surface is catalytically active to produce •OH free radicals by breaking down the water molecule. In addition, it was shown that galena could generate H2O2 in the presence or absence of dissolved oxygen in water. The concentration of H2O2 formed increases with decreasing pH. Mixtures of pyrite or chalcopyrite with galena on the formation of H2O2 also were investigated. In pyrite-galena mixture, the formation of H2O2 increased with increasing pyrite proportion and was also the case of increasing chalcopyrite fraction in chalcopyrite-galena mixture. The oxidation or dissolution of one specific mineral than the other in a mixture can be explained better with the extent of H2O2 formation in the pulp liquid than galvanic interactions. It appears that H2O2 play a greater role in the oxidation of sulphides or aiding the extensively reported galvanic interactions since the amount of H2O2 generated with a specific mineral followed the rest potential series. This study highlights the necessity of revisiting into electrochemical and/or galvanic interaction mechanisms between pyrite and galena or chalcopyrite and galena in terms of their flotation behavior.

Keywords: Hydrogen peroxide,Hydroxyl radical, Galena, Pyrite, Chalcopyrite, Flotation.

1- Introduction

Hydrogen peroxide causes non-selective oxidation of sulfide minerals. The oxidation of sulfide minerals takes place during the grinding process when the particle size is reduced for flotation. Recently it was shown that formation of H2O2 take place in pulp liquid during wet grinding of complex sulfide ore (Ikumapayi et al., 2012). Previous works showed that pyrite (FeS2) (Borda et al., 2001, Borda et al.,2003,Cohn et al., 2006, Jones et al., 2011, Jones et al., 2012, Jones et al., 2013), chalcopyrite (CuFeS2) (Jones et al., 2013) and galena ( Ahlberg et al. 1996a,b,c) have been shown to generate hydrogen peroxide (H2O2) when placed in water. Cohn et al. (2006) showed that pyrite in the presence of dissolved molecular oxygen, ferrous • í iron associated with pyrite can form superoxide anion (O2 ) (eq. 1), which reacts with ferrous iron to form H2O2 (eq. 2): II III • ௅ Fe (pyrite) + O2ĺ Fe (pyrite) + (O2 ) (1) II • ௅ + III Fe (pyrite) + (O2 ) + 2H ĺ Fe (pyrite) + H2O2 (2)

Borda et al. (2003) showed that pyrite can also generate H2O2 in the absence of molecular oxygen. He reported that an electron is extracted from water and a hydroxyl radical is formed

(eq. 3). Combining two hydroxyl radicals leads to the formation of H2O2 (eq. 4): III • + II Fe + H2Oĺ OH+ H + Fe (3) • 2 OH ĺ H2O2 (4) Ahlberg et al. (1996a, b, and c) studied the oxygen reduction on pyrite and galena with a rotating ring disc electrode. Their results suggest that the first electron transfer is the rate- determining step for the oxygen reduction at both galena and pyrite and hydrogen peroxide is found to be an intermediate. While galena is a poor catalyst for oxygen reduction, with minor formation of hydrogen peroxide, pyrite is a relatively good catalyst. This kind of difference may lead to different flotation behaviors. Numerous studies (Rey and Formanek, 1960, Guy and Trahar, 1984, Peng et al., 2003, Rao et al., 1992, Yuan et al., 1996, Natarajan and Iwasaki, 1984) on galena have shown that the floatability of galena can be affected by the grinding environment.Thegalena ground in an iron mill was much less floatable than galena ground in a ceramic mill (Rey and Formanek, 1960). Guy and Trahar (1984) demonstrated that the oxidation–reduction environment during grinding had a pronounced influence on the subsequent floatability of galena. Peng et al. (2003) showed that galena has a lower floatability when ground with mild steel than with chromium grinding media. According to the Nernst equation, the partial pressure of oxygen in the mill can also influence the grinding environment (Rao et al., 1992, Yuan et al., 1996). Oxygen purging causes the grinding environment to be more oxidizing while nitrogen purging induces the opposite effect. Natarajan and Iwasaki (1984) studied the electrochemical aspects of grinding media and observed that galvanic coupling of mild steel medium with magnetite or pyrrhotite resulted in the formation of iron hydroxide species on the mineral surface. Buehler and Gottschalk (1910) were noted that when pyrite was mixed with a second sulfide mineral, the second mineral oxidized more rapidly. The galena oxidation was accelerated electrochemically when in contact with pyrite (Majima (1969),Kocabag and Smith (1985). However, Rao et al. (1988) demonstrated that in a 50:50 weight percent mixture of galena and pyrite the flotation response of the galena was dramatically impaired compared with the same best performance with galena only. He proposed that galvanic interactions may occur between conducting minerals and play a significant role in flotation. • However, participation of H2O2 and OH, if any, in non-selective oxidation of the sulfide ore pulp components and hence in deteriorating of the concentrate grade and recovery of metal-sulfides has not yet been explored. In an attempt to fill the gap, we have estimated the concentration of H2O2 in pulp liquid during different time of grinding and in different grinding environments. The effect of pH, type of grinding (wet or dry grinding), grinding atmosphere (nitrogen/air) and pyrite/chalcopyrite proportion mixed with galena on formation of hydrogen peroxide and galena flotation was investigated.

2. Experimental

2.1. Materials and reagents Crystalline pure galena, pyrite and chalcopyrite minerals used in this study were procured from Gregory, Bottley & Lloyd Ltd., United Kingdom. Galena contains 73.69% Pb, 13.5% S, 1.38% Fe, 1.26% Zn, 0.2% Cu and some silica (quartz) impurity; pyrite contains 44.4% Fe, 50.9% S, and 0.2% Cu, and chalcopyrite contains 29% Fe, 29.5% S, 25.8% Cu, 0.54% Zn, and 0.22% Pb. The XRD analyses of the samples showed that the main mineral phase present were the pyrite (Fig. 1a), chalcopyrite (Fig. 1b) and galena (Fig. 1c) respectively in these minerals. Pyrite, chalcopyrite and galena samples used in this study were separately crushed

54 through a jaw crusher and then screened to collect the –3.35 mm particle size fraction. The homogenized sample was then sealed in polyethylene bags. Potassium amyl xanthate (KAX) and MIBC used as collector and frother respectively were obtained from Boliden Mineral AB that are being used in the flotation plant concentrator. Dilute solutions of AR grade sodium hydroxide and HCl were added to maintain the pH at the targeted value during flotation. Deionized water was used in both grinding and flotation experiments. Solutions of 2, 9- dimethyl-1, 10-phenanthroline (DMP), copper (II) sulphate (0.01 M) and phosphate buffer (pH 7.0) used in the analytical method for determining H2O2 and lead nitrate, ferrous sulfate and ferric sulfate used for investigating the effect of these metal ions on the formation of H2O2 were purchased from VWR, Sweden.

Fig. 1. XRD analysis of the sample (a) pyrite (1- pyrite) (b) chalcopyrite (1- chalcopyrite and 2- pyrrhotite 3- sphalerite) (c) galena (1- galena 2- sphalerite 3- quartz). 2.2. Wet Grinding and Flotation test Crushed galena sample (100 g) of –3.35 mm size in each grinding test was combined with 400 cm3 of water and ground in a new laboratory stainless steel ball mill (Model 2VS, CAPCO Test Equipment, Suffolk, UK) with stainless steel medium. The slurry samples were collected at pre-determined time intervals and they were immediately filtered (Millipore 0.22 μm) and the liquid (filtrate) was analyzed for hydrogen peroxide. After grinding for 60 min, the mill was emptied and the pulp was screened and it was sampled to different portions. In each flotation test, 7.5 g of sample that was < 106 μm was transferred to a cell of 150 ml capacity (Clausthal flotation equipment), conditioned with pH modifier, collector and frother. Flotation concentrate was collected after 2.0 min at air flow rate of 0.5 dm3 min-1. The flotation froth was scraped every 10 s. Dosages of collector in flotation was 10-4 M KAX. The conditioning times for adjusting pH and collector were 5 min and 2 min respectively. The frother dosage was one drop MIBC in all cases. Galena flotation was investigated at different pH. The pH was regulated with NaOH and HCL solutions. Experiments were performed at room temperature of approximately 22.5°C.

2.3. Dry Grinding 100 g of galena, pyrite and chalcopyrite single minerals were separately ground in a laboratory stainless steel ball mill with stainless steel medium for 60 min. 5 g of galena, pyrite and chalcopyrite single mineral and 12.5 g galena-pyrite blend or galena-chalcopyrite blend (12.5 g in total) with particle size < 106 μm was mixed with 50 cm3 of water and conditioned with a magnetic stirrer. The slurry sample was collected after 0.5, 5 and 11 min conditioning time and was analyzed for hydrogen peroxide. The pH was regulated to with HCL and NaOH

55 solutions. The Eh (pulp potential) of the suspension was measured at room temperature during mixing. 2.4. Gas purging To study the effect of gaseous atmosphere, galena was wet-ground in a laboratory ball mill with stainless steel media in either air or N2 atmosphere. For these tests, the jar of laboratory ball mill was filled with 400 ml deionized water and purged with either air or N2 gas for a minimum of 30 min. After 30 min, galena (100 g) was added to the jar and purged with either air or N2 gas for a minimum of 30 min again and then galena was wet ground for 1 h. Though we did not measure dissolved O2 concentrations in our experiments, Butler et al. (1994) was reported that for 1 L solutions of ultrapure water purged for 1 h with N2 gas, O2 concentrations did not exceed 0.19 ± 0.05 ppm. The concentration of H2O2 in the pulp liquid was measured after 60 min of grinding. 2.5. Estimation of hydrogen peroxide

So far, various methods have been used for the measurement of H2O2 in oxidation processes. Such methods use metallic compounds such as titanium oxalate, titanium tetrachloride (Volk et al., 1993,Roche and Prados, 1995,Sunder and Hempel, 1997, Leitner and Dore´,1997) and cobalt (II) ions (Gulyas et al., 1995) that form colored complexes with H2O2, which can then be measured spectrophotometrically. The spectrophotometric method using copper (II) ions and DMP has been found to be reasonably sensitive when applied to advanced oxidation processes (Kosaka et al., 1998). For DMP method (Baga el al., 1988) one milliliter each of DMP, copper (II) sulphate, and phosphate buffer (pH 7.0) solutions were added to a 10 ml volumetric flask and mixed. A measured volume of liquid (filtrate) sample was added to the volumetric flask, and then the volume was made up to the mark with ultrapure water. After mixing, the absorbance of the sample at 454 nm was measured with DU® Series 700 UV/Vis Scanning Spectrophotometer. The blank solution was prepared in the same manner but without H2O2.

3. Results and discussion

3.1. Formation of Hydrogen peroxide (H2O2) during wet grinding and its implications on flotation

Initially the extent of H2O2 formation during wet grinding of galena was investigated. For these studies, galena mineral was wet-ground in a laboratory stainless steel ball mill with stainless steel medium and the slurry samples were collected at pre-determined time intervals. Slurry samples were filtered (Millipore 0.22 μm) and the liquid (filtrate) was analyzed for hydrogen peroxide. Formation of hydrogen peroxide was detected in the filtrate for the first time in mineral processing applications. The effect of pH during grinding on formation of hydrogen peroxide is shown in Fig. 2. It can be seen that at pH 4 and 7.8 hardly any H2O2 is generated but at pH 2.7 not only H2O2 formed and also its concentration increased with increasing grinding time due to increased surface area of solids and its reactions with water.

Fig. 2. H2O2 concentration during wet grinding as function of grinding time.

After 60 min grinding, the effect of H2O2 formation on flotation recovery of galena was investigated. The effect of grinding pH as a function of flotation pH on galena recovery is shown in Fig. 3. It can be seen that for all pH values studied the galena wet-ground at pH 2.5 produced a lower galena recovery than galena wet-ground at pH 7.8. Since wet-ground galena at pH 2.5 produces higher amount of H2O2, a little decrease in galena recovery could be due to surface oxidation caused by H2O2 oxidant. It was also reported that the addition of H2O2, -3 galena flotation decreases and completely depresses if the concentration of H2O2 exceeds 10 M (Wang, 1992). This strong depressing action of H2O2 on galena is attributed to its strong oxidizing action on lead xanthate in galena surface giving rise to the oxidation and decomposition of lead xanthate (eq. 5).

– [Pb (EX) 2] ads + H2O2ĺPb (OH) 2 + (EX) 2+ 2e (5)

Fig. 3. Effect of pH on flotation recovery of galena with a pulp ground at pH 2.5 and 7.8.

57 3.2. Formation of hydrogen peroxide (H2O2) after dry grinding and mixing with water Galena (100 g) of –3.35 mm size fraction was dry-ground in a laboratory ball mill. The effect of pH on formation of hydrogen peroxide is shown in Fig. 4. It can be seen that the concentration of H2O2 increased with decreasing pH. Fig. 4 also shows that Eh increased with increasing formation of H2O2 in the pulp liquid. Woods et al. (1981) showed by the Eh-pH diagram (Fig. 5) of galena that at pH < 4, Pb2+ ions are stable. These divalent metal ions also found to generate H2O2 as shown in Table 1 (Blokhina et al., 2001, Ni et al. 2004, Valko et al., 2005, Jones et al., 2012). Fig. 6 shows in the presence of dissolved molecular oxygen and • í 2+ at acidic pH (pH < 4), lead ions can form superoxide anion (O2 ) which reacts with Pb to form H2O2 (Eqs. 6-9). This is in agreement with other studies where metal-ions induced formation of free radical has been significantly evidenced for Pb2+ ions that generate the superoxide and hydroxyl radical (Ahlberg and Broo, 1996a, b, and c).

í • í O2 + e ĺ (O2 ) (6) • í + 2(O2 ) + 2H ĺ H2O2+O2 (7) + 2+ • í Pb + H2O2ĺ Pb + OH + OH (8) • 2 OHo H2O2 (9)

Fig. 4: Effect of pH on H2O2 concentration in pulp liquid.

Table 1. Effect of metal ions on H2O2 generation at two initial concentrations (conditioning time 1 h, natural pH, and 22 °C).

H2O2 (mM) Concentration of ions 1 mM 10 mM water 0 0 0.013 0.096 Pb 2+ 0.552 4.656 Fe 2+ 0.004 0.059 Fe 3+ 0.00068 0.015 Cu 2+

Fig. 5. Eh-pH diagram for the Pb-S-H2O system. Equilibrium lines correspond to dissolved species where [Pb] =10-3 M (Woods et al.,1981).

2+ Fig. 6. Proposed mechanism for formation of H2O2 by galena showing Pb react with water to • generate OH and H2O2.

3.3. Effect of galena loading on formation of hydrogen peroxide

The effect of galena loading was investigated with increasing solids concentration in 50 ml water at pH 2.5. The production of H2O2 increased with increasing galena loading as shown in Fig. 7. This figure also shows that the production of H2O2 increases with increasing time and didn’t reach equilibrium level within 11 min kinetics of galena and water interactions studied.

Fig. 7. Effect of galena loading on H2O2 concentration after 0.5 and 5 and 11 min mixing with water at pH 2.5.

3.4. Gas purging To study the effect of gaseous atmosphere, galena was wet-ground in a laboratory ball mill with stainless steel media in either air or N2 atmosphere. For N2 atmosphere the jar of ball mill was filled with 400 ml water and purged with N2 gas for a minimum of 1 h and then galena was wet-ground for 1 h. For air atmosphere the ball mill jar was filled with galena and 400 ml water and kept open the lid of the jar for 5 min and then galena was wet-ground for 1 h. Fig. 8 shows that H2O2 was generated in a N2 atmosphere too. However, N2 atmosphere generated lower H2O2 that in air atmosphere. This is in agreement withother studies where it was observed that the dissolution of PbS in the absence of dissolved oxygen is lower than those obtained in the presence of oxygen (Hsieh and Huang, 1989). It is not very clear how the divalent metal ions are generating H2O2 from water but one can hypothesise that an electron is extracted by the Pb2+ from water and a hydroxyl radical is formed (eq. 10). Combining two hydroxyl radicals leads to the formation of H2O2 (eq. 11):

2+ • + + Pb + H2Oĺ OH+ H + Pb (10) • 2 OHĺ H2O2 (11)

Fig. 8. The effect of N2 and air atmosphere on formation of H2O2 by galena at pH 2.5.

3.5. Galena-pyrite mixture

Blend samples of galena–pyrite (12.5 g in total) were mixed in 50 cm3 of water for 5 min. Figs. 9a and 9b show the effect of pyrite proportion in galena–pyrite mixture on formation of hydrogen peroxide at different pH. It can be seen that increasing pyrite proportion, the production of H2O2 increased. This is in agreement withother studies where it was observed when pyrite was mixed with a second sulfide mineral, the second mineral oxidized more rapidly (Buehler and Gottschalk, 1910). This result of increasing pyrite proportion increases the production of H2O2 could be the explanation for the following observations: 1. The galena recovery decreased with an increase in pyrite proportion in galena-pyrite mixture (Peng et al., 2003, Peng and Grano, 2010). Also, the floatability of galena decreases in the presence of pyrite in the entire pH range studied (Pecina-Trevino et al., 2003). 2. The amount of pyrite increases in contact with another sulphide mineral, the leaching rate of this contacting mineral increase (Koleini et al., 2010, Koleini et al., 2011, Dixon and Tshilombo, 2005, Mehta, and Murr 1983, Holmes and Crundwell, 1995). This is in agreement with other studies where it was observed that increasing hydrogen peroxide concentration accelerates galena oxidation considerably (Baba and Adekola, 2011). Figs. 9a and 9b also show that the effect of galena proportion in galena-pyrite mixture on formation of hydrogen peroxide. It can be seen that with an increase in galena proportion, the production of H2O2 decreased. This mechanism may be the explanation for a behavior that increasing pyrite floatability in a mixture of galena-pyrite (Peng et al., 2003, Pecina-Trevino et al., 2003).The formation of higher amounts of H2O2 when galena is in contact with pyrite may explain the effect of interaction between two sulphide minerals on flotation as shown in Fig. 10, while old literature reported when two sulphide minerals are in contact, electron transfer from one to the other, i.e. galvanic interaction, occurs (Rao and Finch, 1988, Holmes and Crundwell, 1995).

Fig. 9. Effect of pyrite or galena proportion in galena-pyrite mixture on concentration of H2O2 at (a) alkaline pH and (b) acidic pH.

62 (A) (B)

(C)

Fig. 10. Proposed mechanisms for oxidation of galena by A) Galvanic interaction between galena (Ga) and pyrite (Py). Anodic dissolution reactions occur on the surface of Ga and cathodic reactions occur on the surface of Py (adapted from Holmes and Crundwell, 1995). B) Formation of H2O2 by Py that are proposed to form via the incomplete reduction of oxygen and be formed from two reacting •OH (eqs.1-4). C) Simultaneous galvanic interaction and formation H2O2 by Py.

3. 6. Galena-chalcopyrite mixture

Blend samples of galena–chalcopyrite (12.5 g in total) were mixed in 50 cm3 of water for 5 min and the amount of H2O2 formed was determined. Fig. 11 shows the effect of chalcopyrite proportion in galena–chalcopyrite mixture on formation of hydrogen peroxide at different pH. It can be seen that with an increase in chalcopyrite proportion, the concentration of H2O2 increased. This mechanism that with an increase in chalcopyrite proportion, the production of H2O2 increased could explain the result that the amount of chalcopyrite in contact with galena increases the leaching rate of galena increases as shown in Fig. 12. This is in agreement with other studies where it was observed that increasing hydrogen peroxide concentration accelerates considerably galena oxidation (Baba and Adekola, 2011). Fig. 11 also shows the effect of galena proportion in galena–chalcopyrite mixture on formation of hydrogen peroxide. It can be seen that increasing galena proportion, the production of H2O2 decreased. This result could perhaps explain that galena has no effect on the kinetics of chalcopyrite leaching (Nazari et al., 2012).

Fig. 11. Effect of chalcopyrite or galena proportion in galena-chalcopyrite mixture on concentration of H2O2 at different pH.

Fig. 12. Proposed mechanisms for Oxidation of galena by H2O2 that increased with increasing chalcopyrite proportion in galena–chalcopyrite mixture.

The above discussion may be summarized by the flowing events: (a) increasing pyrite proportion increases production of H2O2 in mixture of pyrite-galena, (b) increasing concentration of H2O2 increases oxidation of galena and decreases recovery of galena flotation, (c) also increasing concentration of H2O2 increases recovery of galena leaching, (d) increasing galena proportion decreases concentration of H2O2 in pyrite-galena mixture, (e) decreasing concentration of H2O2 decreases oxidation of pyrite and increases recovery of pyrite flotation, (f) increasing chalcopyrite proportion increases concentration of H2O2 in a mixture of chalcopyrite-galena, (g) increasing concentration of H2O2 increases oxidation of galena and may decrease recovery of galena flotation, and (h) increasing galena proportion decreases concentration of H2O2 in a mixture of chalcopyrite-galena.

64 4. Conclusions

In the scope of the present work, fundamental studies were carried out to resolve or reveal one of the main problems (i.e., oxidation) in sulfide ore flotation. For this reason, the grinding stage was evaluated and the oxidation of the sulphide minerals was investigated. For the first time in mineral processing applications, it was found out that during grinding (especially fine grinding) of galena, formation of H2O2 takes place. It is known that hydrogen peroxide is a strong oxidizing agent and can easily oxide sulfide minerals and can even cause dissolution of some metal ions from mineral surface. That’s why H2O2, rather than oxygen, can be the main reason for oxidation, inadvertent activation and non-selectivity in the sulfide mineral flotation. Prevention or reduction of H2O2 formation may result in improved flotation results and we are working on these aspects. Following preliminary conclusions were drawn from the experiments carried out in scope of this paper. Formation of hydrogen peroxide was detected in the filtrate of galena pulp liquid for the first time in mineral processing applications. However, H2O2 formed only at pH values less than 4. Dry grinding also produced hydrogen peroxide when freshly dry ground galena solids were mixed with water, and the effect of pH on H2O2 formation was similar to wet grinding, i.e., hydrogen peroxide generates at pH < 4. Also the concentration of H2O2 increased with increasing mixing time and galena loading. Hydrogen peroxide was also generated in the absence of oxygen (N2 atmosphere), the amount was slightly lower compared to air atmosphere. With an increase in pyrite proportion in pyrite-galena mixture, the concentration of H2O2 increased. Then one can conclude that a decrease in galena recovery in flotation with an increase in pyrite proportion is due to the increase of hydrogen peroxide formation. Also with increasing chalcopyrite proportion in a mixture of chalcopyrite-galena, the concentration of H2O2 increased.

Acknowledgements

Financial support from the Centre for Advanced Mineral and Metallurgy (CAMM) Research Centre, LTU and Boliden Mineral AB is gratefully acknowledged

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Paper 5

Alireza Javadi Nooshabadi, Kota Hanumantha Rao, Formation hydrogen peroxide by sulphide minerals–submitted to Journal ofHydrometallurgy.

70 Formation of Hydrogen peroxide by sulphide minerals

Abstract

Keywords: hydrogen peroxide, oxidation, pyrite, chalcopyrite, sphalerite and galena

1. Introduction Hydrogen peroxide, which is a strong oxidizing agent, stronger than oxygen, causes non- selective oxidation of sulphide minerals. The oxidation of sulphide minerals takes place during the grinding process when the particle size is reduced for flotation. Recently it was shown that formation of H2O2 take place in the pulp liquid during wet grinding of complex sulphide ore (Ikumapayi et al., 2012). Previous work published by the authors has highlighted • the significance of reactive oxygen species (ROS), H2O2 and hydroxyl radical ( OH), generated from milled sulphide concentrates and their potential effect on thermophilic Fe- and S-oxidizing bioleaching microorganisms through oxidative stress (Jones et al., 2011 and Jones et al., 2012). To date, most studies dealing with sulphide mineral-induced ROS formation have made use of natural or synthetic mineral samples, of very high purities, suspended in neutral solutions. Pyrite induced ROS formation has been studied most; however other

71 sulphide minerals such as chalcopyrite (CuFeS2), sphalerite (ZnS), pyrrhotite (Fe(1íx)S) and vaesite (NiS2) have also been studied with respect to ROS generation (Borda et al., 2001 and Jones et al., 2011), and reactivities have been found to differ between sulphide minerals. Buehler and Gottschalk (1910) noted that when pyrite was mixed with a second sulphide mineral, the second mineral oxidized more rapidly.Harvey and Yen (1998) discovered that addition of galena to the sphalerite selective leaching system diminished the dissolution of sphalerite. Alternatively, a pyrite or chalcopyrite addition increased zinc extractions. Many authors has been reported that galvanic interactions are known to occur between conducting minerals and play a significant role in flotation (Rao and Finch, 1988, Kelebek et al., 1996, Zhang et al., 1997, Ekmekçi and Demirel, 1997 and Huang and Grano, 2005), leaching (Mehta and Murr, 1983,Abraitis et al., 2003 and Akcil and Ciftci, 2003), supergene enrichment of sulfide ore deposits (Thornber, 1975 and Sato, 1992), environment governance (Alpers and Blowes, 1994), and geochemical processes (Sikka et al., 1991 and Banfield, 1997). In addition, other researchers (Koleini et al., 2010, Koleini et al., 2011, Dixon and Tshilombo, 2005, Mehta, and Murr 1983, Holmes and Crundwell, 1995) reported when the amount of pyrite in contact with chalcopyrite increases, the leaching rate of chalcopyrite mineral increases. • However, participation of H2O2 and OH, if any, in non-selective oxidation of the sulphide ore pulp components and hence in deteriorating of the concentrate grade and recovery of metal-sulphides has not yet been explored. Similarly, the influence of H2O2 has been recognized lately on bioleaching of metal sulphides (Jones et al., 2012). In an attempt to fill the gap, we have estimated the concentration of H2O2 in pulp liquid during different time of grinding and in different grinding environments. The effect of pH, type of grinding (wet or dry grinding), and varying proportion of pyrite, chalcopyrite and galena mixed with sphalerite on the formation of hydrogen peroxide was investigated and the results have been presented and discussed in this paper in the context of flotation and leaching phenomena of sulphide minerals.

2. Experimental

2.1. Materials and reagents

Crystalline pure sphalerite (Sp), galena (Ga), pyrite (Py) and chalcopyrite(Cp) minerals used in this study were procured from Gregory, Bottley & Lloyd Ltd. Sphalerite contains 39.92% Zn, 20.7% S, 4.2% Fe, 1.32% Pb and 0.17% Cu, galena contains 73.69% Pb, 13.5% S, 1.38% Fe, 1.26% Zn, 0.2% Cu and some silica (quartz) impurity; pyrite contains 44.4% Fe, 50.9% S, and 0.2% Cu, and chalcopyrite contains 29% Fe, 29.5% S, 25.8% Cu, 0.54% Zn, and 0.22% Pb. The XRD analyses of the samples showed that the main mineral phase present were the pyrite (Fig. 1a), chalcopyrite (Fig. 1b), sphalerite (Fig. 1c), and galena (Fig. 1d). All pyrite, chalcopyrite, sphalerite and galena samples used in this study were separately crushed through a jaw crusher and then screened to collect the –3.35 mm particle size fraction. The homogenized sample was then sealed in polyethylene bags. Potassium amyl xanthate (KAX) was used as collector and the frother was MIBC in flotation tests. Solutions of sodium hydroxide (AR grade) and HCl (1 M) were added to maintain the pH at the targeted value during flotation. Deionised water was used in the processes of both grinding and flotation. Solutions of 2, 9-dimethyl-1, 10-phenanthroline (DMP), copper (II) (0.01 M), and phosphate buffer (pH 7.0) were used for estimating H2O2 amount in pulp liquid by UV-Visible spectrophotometer. Zinc sulphate, copper sulphate, lead nitrate, ferrous sulphate and ferric

72 sulphate chemicals were also used to investigate the effect of metal ions (Zn2+, Cu2+, Pb2+, 2+ 3+ Fe and Fe ) on the formation of H2O2.

2.2. Dry Grinding 100 g of –3.35 mm size fraction of sphalerite, pyrite, galena and chalcopyrite minerals in each grinding test was separately ground in a laboratory stainless steel ball mill (Model 2VS, CAPCO Test Equipment, Suffolk, UK) with stainless steel medium for 60 min. 5 g of sphalerite, galena, pyrite or chalcopyrite single mineral and 12.5 g in total of mineral mixture either sphalerite-pyrite or sphalerite-chalcopyrite or sphalerite-galena or galena-pyrite or galena-chalcopyrite or chalcopyrite-pyrite that was < 106 μm was mixed with 50 cm3 water using magnetic stirrer and then the slurry sample was collected at 0.5, 5 and 11 min, filtered and was analysed for hydrogen peroxide. The pH was regulated with HCl and NaOH solutions.

Fig. 1. XRD analysis of the sample (a) pyrite (1- pyrite). (b) chalcopyrite (1- chalcopyrite and 2- pyrrhotite 3- sphalerite) (c) sphalerite (1- sphalerite 2- galena 3- quartz), (d) galena (1- galena 2- sphalerite 3- quartz). 2.3. Estimation of hydrogen peroxide The spectrophotometric method using copper (II) ions and DMP has been found to be reasonably sensitive when applied to advanced oxidation processes (Kosaka et al., 1998). For DMP method (Baga et al., 1988) 1 mL each of 1% DMP in ethanol, 0.01M copper (II), and phosphate buffer (pH 7.0) solutions were added to a 10 mL volumetric flask and mixed. A measured volume of liquid (filtrate) sample was added to the volumetric flask, and then the flask was filled up with ultrapure water. After mixing, the absorbance of the sample at 454 nm

73 was measured with DU® Series 700 UV/Vis Scanning Spectrophotometer. The blank solution was prepared in the same manner but without H2O2.

3. Results and discussion

3.1. Formation of hydrogen peroxide (H2O2) after dry grinding and mixing with water

Formation of hydrogen peroxide was observed when the freshly dry-ground solids are mixed in water. The effect of water pH in which the solids are mixed, on the formation of hydrogen peroxide is shown in Fig. 2. It can be seen that pyrite, chalcopyrite and sphalerite with an increase in pH, the concentration of H2O2 decreased up to pH 8 and then increased above this pH, but in the case of galena, H2O2 generates at pH < 4.5 only. Also, pyrite generated more H2O2 than other materials. Kocabag et al. (1990) using the Eh-pH diagram (Fig. 3a) of pyrite 2+ + showed that oxidation of pyrite yields S°, Fe , Fe (OH)2 species in the solution for pH < 6 2+ and Fe (OH)2 , Fe (OH)3 species in the solution for pH > 6. In pH < 6, Fe ions are increased with decreasing pH and therefore, in the presence of dissolved molecular oxygen, ferrous iron • í associated with pyrite can form superoxide anion (O2 ) (eq. 1), which reacts with ferrous iron to form H2O2 (eq. 2) as shown in Fig. 4a (Cohn et al., 2006). II III • ௅ Fe (pyrite) + O2ĺ Fe (pyrite) + (O2 ) (1) II • ௅ + III Fe (pyrite) + (O2 ) + 2H ĺ Fe (pyrite) + H2O2 (2) Fe (III) can also generate H2O2 that an electron is extracted from water and a hydroxyl radical is formed (eq. 3). Combining two hydroxyl radicals leads to the formation of H2O2 (eq. 4) (Borda et al., 2003): III • + II Fe + H2Oĺ OH + H + Fe (3) • 2 OH ĺ H2O2 (4) Fairthorne at al. (1997) using the Eh–pH stability diagram of chalcopyrite (Fig. 3b) exemplified the formation of insoluble ferric oxide/hydroxide at neutral and basic pH values but also in acidic conditions at high Eh values. Notably the divalent Fe and Cu ions exist at low pH values from negative to high Eh values. These divalent ions are reported to aid the formation of H2O2 (Valko et al., 2005, Jones et al., 2011, Jones et al., 2012) from water and 2+ our present results shown in Table 1 demonstrate that Fe ions generates substantial H2O2 followed by Fe3+ and Cu2+ ions. Fig. 3b displays that the concentration of ferrous ions decreases at the expense of ferric oxide/hydroxides at higher pH and Eh values (oxygen conditioning). The schematic diagram of H2O2 formation in the presence of dissolved molecular oxygen is shown in Fig. 4b, where in acidic pH (pH < 5) forms superoxide anion • í (O2 ) (eq. 5), which reacts with ferrous iron to form H2O2 (eqs. 6-8). This is in agreement with other studies where it was observed that metals-induced formation of free radical has been significantly been evidenced that ferrous ion generates the superoxide and hydroxyl radical (Valko et al., 2005, Jones et al., 2011).

2+ 3+ • í O2 +Fe ĺ Fe + (O2 ) (5) • í 2+ + 3+ (O2 ) + Fe + 2H ĺ Fe +H2O2 (6) 2+ 3+ • í Fe + H2O2ĺ Fe + OH + OH (7) • 2 OH ĺ H2O2 (8)

74 free radical has been significantly evidenced where copper ions generate the superoxide and hydroxyl radical (eqs. 9-12) (Valko et al., 2005). о •– O2 + e ĺ O2 (9) •– + 2O2 + 2H ĺ H2O2 +O2 (10) + 2+ • – Cu + H2O2 oCu + OH + OH (11) • 2 OH ĺ H2O2 (12) In acidic pH, the higher amounts of hydrogen peroxide formation than alkaline pH is due to the presence of copper and ferrous ions in acidic pH which ions are capable to generate hydrogen peroxide (Table 1). Huai Su (1981) by the Eh-pH diagram of the sphalerite displayed that at pH < 6, Zn2+ and 2+ 2+ 2+ Fe formed (Fig. 3c). Table 1 shows that Zn and Fe ions generated H2O2. In the presence • í of dissolved molecular oxygen, ferrous iron can form superoxide anion (O2 ) (eq. 5), which 2+ • í reacts with ferrous iron to form H2O2 (eq. 6). Also Zn ions can form superoxide anion (O2 ) to form H2O2 (eq. 13) as shown in Fig. 4c. • ௅ + 2(O2 ) +2H ĺ H2O2+O2 (13) Woods et al. (1981) by the Eh-pH diagram (Fig. 3d) of the galena showed that at pH < 4, Pb2+ ions exist. Fig. 4d shows that in the presence of dissolved molecular oxygen and in acidic pH • í + (pH < 4), lead ions can form superoxide anion (O2 ) which reacts with H to form H2O2 (Eqs. 2+ 14-17). It can be seen from Table 1 that Pb ions generate H2O2 (Blokhina et al., 2001, Ni et al. 2004). This is in agreement with other studies where metal ions induced formation of free radicals has been significantly evidenced for Pb2+ ions that generate superoxide and hydroxyl radical (Ahlberg and Broo, 1996a, b, and c).

í • í O2 + e ĺ (O2 ) (14) • í + 2(O2 ) + 2H ĺ H2O2+O2 (15) + 2+ • í Pb + H2O2ĺ Pb + OH + OH (16) • 2 OHo H2O2 (17)

Fig. 2. Effect of pH on production of H2O2 after 5 min mixing of dry ground solids with water.

75 Table 1. H2O2 generation in the presence of metal ions at natural pH and at 22 °C.

H2O2 (mM) Concentration of ions 1 mM 10 mM water 0 0 Fe 2+ 0.552 4.656 Fe 3+ 0.004 0.059 Cu 2+ 0 0.015 Pb 2+ 0.013 0.096 Zn 2+ 0.004 0.088

a) b)

c) d)

Fig. 3. a) Eh–pH stability diagram for the Cu–Fe–S–H2O system with the preponderant copper and iron species shown in each domain (Fairthorne, 1997) b)Eh–pH diagram for FeS2–H2O 0 -5 system at 25 C and for 10 M dissolved species (Kocabag et al., 1990). c) Eh–pH stability diagram for the sphalerite–H2O system (Huai Su, 1981) d) Eh–pH diagram for the Pb–S–H2O system where [Pb] =10-3 M (Woods et al.,1981).

76 a) b)

Fig. 4. Proposed mechanisms for formation of H2O2 by a) chalcopyrite b) pyrite c) sphalerite d) galena. The numericals in this figure represent the equation numbers.

For investigation of the effect of mineral loading, they were mixed with 50 ml water at pH 4.5. The concentration of H2O2 increased with increasing loading as was shown in Fig. 5. Fig. 6 shows that the concentration of H2O2 increased with increasing mixing time, most likely due to increased minerals interaction with water.

Fig. 5. Effect of solids loading on H2O2 production after 5 min mixing with water at pH 4.5.

Fig. 6. Effect of mixing time on H2O2 concentration at pH 4.5.

3.2. Effect of minerals mixture on formation of H2O2

Fig. 7 shows the effect of pyrite–chalcopyrite, pyrite–sphalerite, pyrite–galena, chalcopyrite– sphalerite, chalcopyrite–galena or sphalerite–galena mixture on formation of hydrogen peroxide at pH 4.5. Fig. 7a shows the effect of pyrite or sphalerite percent in sphalerite–pyrite mixture on formation of hydrogen peroxide. It can be seen that with an increase in pyrite percent, the concentration of H2O2 increased but with an increase in sphalerite percent, the concentration of H2O2 decreased. This result of non-ferrous metal sulphide oxidation with increasing pyrite fraction could be due to the increased H2O2 generation and that an increase in the rate of sphalerite leaching with increasing pyrite proportion (Harvey and Yen, 1998). This is in agreement withother studies where it was shown that the oxidation of sphalerite increases with increasing hydrogen peroxide concentration (Adebayo et al., 2006 and Aydogan, S., 2006). Fig. 7b shows the effect of chalcopyrite or sphalerite percent in sphalerite–chalcopyrite mixture on formation of hydrogen peroxide where an increase in chalcopyrite percent increases the concentration of H2O2 but with increasing sphalerite percent in the mixture, the concentration of H2O2 decreased. This result of increasing chalcopyrite proportion increases the formation of H2O2 explains the observation of increased leaching rate of sphalerite with an increase in chalcopyrite proportion (Harvey and Yen, 1998). Fig. 7c shows the effect of galena or sphalerite percent in sphalerite–pyrite mixture on formation of hydrogen peroxide. It can be seen that with an increase in galena percent, the concentration of H2O2 decreased but with an increase in sphalerite percent, the concentration of H2O2 increased. This result could explain that a decrease in the rate of sphalerite leaching with increasing galena proportion (Harvey and Yen 1998). Fig. 7d shows the effect of pyrite or galena percent in galena–pyrite mixture on formation of hydrogen peroxide. It can be seen that an increase in pyrite percent, the concentration of H2O2 increases but increasing galena percent, the concentration of H2O2 decreases. Fig. 7e shows the effect of chalcopyrite or galena percent in galena–chalcopyrite mixture on formation of hydrogen peroxide that increasing chalcopyrite percent, the concentration of H2O2 increases. However, an increase in galena percent in the mixture, the production of H2O2 decreases. This result could perhaps explain that galena has no effect on the kinetics of chalcopyrite leaching (Nazari et al., 2012).

78 Fig. 7f shows the effect of pyrite or chalcopyrite percent in chalcopyrite–pyrite mixture on formation of hydrogen peroxide that with an increase in pyrite percent, the concentration of H2O2 increases but with an increase in chalcopyrite percent, the concentration of H2O2 decreases. This result that an increase in pyrite proportion, the concentration of H2O2 increases could explain the behavior that the amount of pyrite in contact with chalcopyrite increases, the leaching rate of chalcopyrite increases (Koleini et al., 2010, Koleini et al., 2011, Dixon and Tshilombo, 2005,Mehta, and Murr 1983,Holmes and Crundwell, 1995). Fig. 8 shows the effect of single or mixture of minerals on formation of hydrogen peroxide at pH 4.5. It can be seen that pyrite and a mixture of pyrite and other sulphide mineral generated more H2O2. This is in agreement with other studies where it was noticed when pyrite was mixed with a second sulphide mineral, the second mineral oxidized more rapidly (Buehler and Gottschalk, 1910). The formation of higher amounts of H2O2 when pyrite was mixed with a second sulphide mineral may explain the effect of interaction between pyrite and second sulphide mineral on flotation, leaching, environment governance and geochemical processes while the entire literature describes so far of galvanic interaction between two contacting sulphide minerals and electron transfer from one to the other (Rao and Finch, 1988, Kelebek et al., 1996, Zhang et al., 1997, Ekmekçi and Demirel, 1997 and Huang and Grano, 2005, Mehta and Murr, 1983, Abraitis et al., 2003, Akcil and Ciftci, 2003, Thornber, 1975, Sato, 1992, Alpers and Blowes, 1994, Sikka et al., 1991 and Banfield, 1997).

Fig. 7. Effect of percent of a) Sp or Py b) Sp or Cp c) Sp or Ga d) Ga or Py e) Cp or Ga f ) Cp or Py in their mixture on H2O2 formation with total 12.5 g sample after 5 min conditioning in water at 4.5

Fig. 8. Effect of minerals in their mixture on H2O2 formation with 12.5 g blend sample after 5 min conditioning in water at pH 4.5.

So far two mechanisms have proposed for the action of microorganisms that increases the leaching rate of metals from ores over that due to purely physico-chemical processes. In direct action, microorganisms will directly oxidize minerals and solubilize metals (eqs. 18-19) (Fig. 9a): 0 MS+H2SO4+ (1/2) O2ĺMSO4+S +H2O (18) 0 S + (1/2) O2+H2OĺH2SO4, (19) Where M is a divalent metal. In indirect action of microorganisms, ferric ion (Fe3+) is the oxidizing agent for minerals and the role of organisms is simply regeneration of Fe3+ from Fe2+ (eqs. 20-21)(Fig. 9b): MS+2Fe3+ĺM2++2Fe2++S0 (20) 2+ + 3+ 2Fe + (1/2) O2+2H ĺ2Fe +H2O (21) In the actual microbial leaching of metals by Acidithiobacillus ferrooxidans (Af) and Leptospirillum ferrooxidans (Lf), both direct and indirect leaching as well as physicochemical reactions can contribute (Suzuki, 2001). Ehrlich (1999) mentions a number of references in support of the direct cell attachment on the surface of minerals and the role of Fe3+bound on the cell surface for sulphide oxidation. Fowler and Crundwell (1998) concluded that the leaching of zinc from sphalerite (ZnS) by T. ferrooxidans was strictly by indirect mechanism and the only bacterial contribution was the regeneration of Fe3+ from Fe2+ (eqs. 22-23): 2+ + 3+ 2Fe + (1/2) O2+2H ĺ2Fe +H2O (22) ZnS+2Fe3+ĺZn2++S0+2Fe2+ (23) Sand et al. (1995) suggested that bacterial leaching of metal sulphides proceeds essentially by indirect mechanism initiated by ferric ion, Fe3+. Sand and colleagues (Gehrke et al., 1998, Sand et al., 1995,Sand et al., 1999, Schippers and Sand, 1999) consider all of these bacterial contributions as ‘indirect.’ It is currently believed that the leaching of sphalerite proceeds according to an indirect mechanism (Boon et al., 1998 and Fowler and Crundwell, 1998), and it has been postulated that this mechanism is applicable to all other sulphide minerals of the form Me2+S2í(Boon and Heijnen, 1993), such as galena. Using these routes, bacteria are taught to accelerate the reaction rate by oxidizing the elemental sulphur product layer (Boon et al., 1998). While H2O2 could also be the oxidizing agent for minerals and the role of 2+ 2+ organisms is simply generation Fe and Fe ions can generate H2O2 as shown in Fig. 9c. The 2+ concentration of H2O2 generated from contacting Fe ion with water for 1 h, is presented as a function of Fe2+ concentration in Table 1. This is in agreement with other studies where the

80 dissolved ferrous iron concentration was found to be an important secondary factor contributing towards ROS generation (Jones et al., 2012).

a) b) c)

Fig. 9. Proposed mechanisms of microorganisms for oxidize minerals by a) direct action b) indirect action c) both indirect action and formation of H2O2.

4. Conclusions In the scope of this paper, fundamental studies were carried out to resolve or reveal one of the main problems, i.e., oxidation, in sulphide ore flotation. For this reason, the oxidation of sulphide mineral at the grinding stage was evaluated. For the first time in mineral processing applications, it was found that during grinding (especially fine grinding) formation of H2O2 in pulp liquid takes place. It is known that hydrogen peroxide is a strong oxidizing agent and can easily oxidize sulphide minerals and can even cause dissolution of some metal ions from ore surface. That’s why H2O2, rather than oxygen, can be the main reason for oxidation, inadvertent activation and non-selectivity in the sulphide ore flotation. Prevention or reduction of H2O2 formation may result in improved flotation results and we are working on these aspects. Following preliminary conclusions were drawn from the results of our initial investigations. Formation of hydrogen peroxide takes place in the pulp liquid and its significance in mineral processing applications hitherto not considered in flotation literature has been clear. It was found that pyrite, chalcopyrite, sphalerite, and galena generated H2O2 in pulp liquid during wet grinding in the presence or devoid of dissolved oxygen in water and also when the solids are placed in water immediately after dry grinding. Pyrite generated more H2O2 than other minerals and the order of H2O2 production by the minerals found to be pyrite > chalcopyrite > sphalerite > galena. The pH of water influenced the extent of hydrogen peroxide formation where higher amounts of H2O2 are produced at highly acidic pH. H2O2 concentration increases with increasing conditioning time and solids loading. H2O2 formation increased with increasing pyrite fraction in chalcopyrite-pyrite, pyrite–galena and sphalerite– pyrite compositions. Also with increasing chalcopyrite fraction in chalcopyrite–sphalerite and chalcopyrite–galena mixtures, the concentration of H2O2 increases but with an increase in galena proportion in galena–sphalerite mixture, the concentration of H2O2 decreases.

Acknowledgements

Financial support from the Centre for Advanced Mineral and Metallurgy (CAMM) Research Centre, LTU and Boliden Mineral AB is gratefully acknowledged.

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Paper 6

Kota Hanumantha Rao, Alireza Javadi Nooshabadi, Tommy Karlkvist, Anuttam Patra, Annamaria Vilinska, Irina Chernyshova, Revisiting sulphide mineral (bio)processing: a few priorities and direction– invited key-note paper, XV Balkan Mineral Processing Congress 2013, 12-16 June 2013, Sozopol, Bulgaria.

86 REVISITING SULPHIDE MINERAL (BIO)PROCESSING: A FEW PRIORITIES AND DIRECTIONS

K. Hanumantha Rao1,*, A. Javadi1, T. Karlkvist1, A. Patra1 , A. Vilinska2 and I.V. Chernyshova2

1Mineral Processing Group, Division of Sustainable Process Engineering, Department of Civil, Environmental and Natural Resources Engineering, Luleå University of Technology, SE-971 87 LULEÅ, Sweden, *E-mail: [email protected]

2Department of Earth and Environmental Engineering, Henry Krumb School of Mines, Columbia University, New York, NY 10027, USA

ABSTRACT.

Large efforts are being made to streamline the conventional (chemical and physical) technological schemes of ore processing, remediation and environmental protection towards reducing overall costs, limiting the use of dangerous substances, decreasing waste streams and improving waste disposal and recycling practice. Hitherto, search for such innovations has been performed mainly empirically and there is an urgent need to shift these technologies to be more innovative and effective. Alternative biotechnological solutions and solutions mimicking natural processes are also being proposed. However, except for bioleaching, practical exploitation of the biotechnological potential in extractive industries and accompanying environmental protection measures remains far from feasibility.

Understanding of the fundamental concepts of aquatic chemistry of minerals–selective adsorption and selective redox reactions at mineral–bacteria–solution interfaces, impact innovating conventional and bio-flotation, as well as (bio)remediation/detoxification of mineral and chemical wastes. Molecular-level knowledge and coherent understanding of minerals contacted with aqueous solutions is required that underlie great opportunities in controlling abiotic and biotic mineral–solution interfaces towards the grand challenge of tomorrow’s science and mineral processing technology.

Keywords: Sulphide minerals; Redox chemistry; FT-IR spectercopy; Adsorption; Fine particle; Flotation

87 INTRODUCTION providing means for bioremediation of environmental problems generated by the Processes on minerals in aquatic media give mineral industries. Many other uses of the world as we know it. Understandings these microorganisms and their derivatives are processes is central to understanding the Earth potentially possible. These include the use of and its sustainability and to developing microorganisms in flocculation and flotation of technologies as diverse as ore processing, minerals [2]. The biomodification of mineral heterogeneous catalysis in solution, (bio)reme- surfaces involves the complex action of diation of contaminants, radioactive waste microorganism on the mineral surface. There storage and disposal, biochemical engineering, are three different mechanisms by means of development of novel surfactants, sorbents, which the biomodification can occur: i) sensors, synthesis of novel materials and attachment of microbial cells to the solid coatings, and especially in emerging substrate, ii) oxidation reactions and iii) nanotechnologies, where stringent control of adsorption and/or chemical reaction with the materials and surfaces is crucial. Despite the metabolite products (EPS). Several types of obvious importance and the long history of autotropic and heterotrophic bacteria, fungi, chemistry of aquatic heterogeneous processes yeasts and algae are implicated in minerals [1], many fundamental issues, especially biobeneficiation. related to biotic interfaces, are still unresolved or poorly understood. AQUEOUS REDOX CHEMISTRY OF Froth flotation is the main separation process SULPHIDES used for collecting selectively valuable minerals from a mined ore, and is based on differences The sulphide minerals are the most common in hydrophobicity of particles. Particles with on the Earth, most important, most diverse, size below 10 microns are a common and richest in terms of physical, chemical, and constituent of a flotation pulp, originating from structural properties. Such diversity originates non-sulphide gangue, grinding in steel mills, from the more complex crystal and electronic and oxidation of sulphides. Iron oxide/sulphide structures compared to other classes of fines are the primary component of acid mine materials [3]. The main reasons are found in drainage that control scavenging of toxic the variety of oxidation states, coordination cations. Such fine particles are produced numbers, symmetry, crystal field stabilization, during grinding of iron ores pose a serious density, stoichiometry, and acid-base surface problem for flotation and degrade the quality of properties, metal sulphides exhibit. The the concentrate. Generally, attempts to remove combination of variety of properties and and discard fines before beneficiation applications of sulphides makes redox operations result in significant economic reactions catalyzed by these minerals in losses. On the other hand, the continual aqueous solutions a very important subject of reduction in the ore grade is forcing miners to research from both fundamental and industrial produce ultrafines in order to liberate valuable standpoints. minerals. Hence, improvements in both Quantifying and predicting redox chemistry of flotation of fines and flotation of coarse sulphides is separately motivated within particles in the presence of fines will increase geochemical, electrochemical, and technolog- the sustainability of the mineral resources, ical communities. These minerals significantly while also reducing the potential environmental impact bioenvironment geochemistry of near impact of the discarded minerals. In addition, surface systems by altering pH and redox there are persistent problem in sulphide conditions, thereby increasing or decreasing flotation of non-ferrous metal sulphides mineral bioavailability [4]. Charge transfer selectivity against ferrous pyrite, inadvertent processes on semiconducting sulphide activation of silicate gangue by metal ions, electrodes are today under intense heterocoagulation between sulphide and investigation towards novel techniques for gangue minerals, and fine particle flotation. conversion of solar energy and environmentally The solutions to these problems are highly clean fuels such as hydrogen [5]. Within a few rewarded: even small technological improve- years well over a thousand publications about ments would provide high economical and this topic appeared (see ref. 5 for a list). ecological return on investments, with regard to Therefore, increased knowledge of electronic the tonnages of material treated and the multi- properties of the mineral surfaces, the detailed dimensional impact of ore processing. mechanisms of charge transfer across their Recent developments in biotechnology have interfaces, and driving forces in the interactions given promise of not only aiding mineral with species from solution will help in improving processing hydrometallurgy operations but also the research in this area of electrochemistry.

88 Despite significant steps have been taken FTIR spectroscopy has become the dominant theoretically and experimentally to obtain a tool for in situ studies of reactions at the molecular-level and coherent understanding of electrode/electolyte or mineral/water interface aquatic reactivity of sulphides [6-8], a general over the last decade due to a large body of theory of redox chemistry of these solids in information provided, simplicity of the waters still does not exist. In the case of application in situ, and a short-time scale of the oxides, the chemical mechanism is commonly measurements, which can be performed considered [9,10], which implies one of the simultaneously with a voltammogram. Three following charge transfer reactions: 1) from the FTIR techniques have so far been applied to solid to the adsorbate, 2) between sorbates, or strongly absorbing IR radiation natural 3) from the adsorbate to the solid. The main sulphides. Based on the comparitive analysis idea behind this mechanism is that catalytic performed [20], the authentic ATR technique effect of the surface is caused by a decrease in that uses a glued mineral plate [21] has more the energy needed to modify the solvent advantages as it is freer from kinetic limitations structure, the standard redox potential of the and technical artifacts (Figure 1). In addition to reductant, and steric hindrance of the oxidant- in-situ FTIR measurements, complimentary reductant interaction [11]. However, the data by XPS for the atomic surface chemical model does not explain different composition and the valence state of the modes and different rates of the oxidation film surface atoms will provide surface structures of growth on different minerals [12-14] as well as adsorbed species. heterogeneous processes on sulphides [15]. A The mechanisms suggested for the oxidation of general disadvantage of this model is in its galena in previous indirect studies at alkaline explanatory character, low ability to predict. pH are: An alternative is the electrochemical a) initial stage: + mechanism, in which the anodic and cathodic PbS + 2H2O + 2xh = Pb1-xS + xPb(OH) (1) + 2+ semi-reactions of the sum redox reaction are PbS + 2xh = Pb1-xS + xPb (2) + + spatially separated. This mechanism has been PbS + 2H2O + 2h = Pb(OH)2 + S + 2H (3) invoked for interpreting abiotic chemistry of PbS = Pb2+ + S2- (4) semiconducting sulphides [15-18] and b) bulk oxidation: + photochemical reactions on both oxides and 2PbS + 5H2O = PbS2O3 + Pb(OH)2 + 8H + 8e (5) 2- + 2PbS + 7H2O = 2Pb(OH)2 + S2O3 + 10H + 8 (6) sulphides [5,10]. Apart from the energy level 2- 2- + redistribution among species in the (inner and 2PbS + 2CO3 + 3H2O = 2PbCO3 + S2O3 + 6H + 8e (7) outer) Helmholtz layer, this model takes into account semiconducting properties of the solid [19]. Provided the position of the Fermi level relative to edges of the conduction and valence bands as well as specification, relative density and position of the surface states are known, the electron-accepting and electron-donating abilities of different solids can be compared and correlated with the direction and rates of a particular heterogeneous redox reaction, including biotic reductive/oxidative degradation of oxides and sulphides.

OXIDATION OF SULPHIDES

Since reactivity of semiconducting sulphides Figure 1. Experimental design of in-situ under flotation conditions is mainly FTIR electrochemical, spectroelectrochemical studies that allows characterizing the sulphide The products of anodic oxidation determined surface (its composition and structure) as a by in-situ FTIR spectra (Figure 2) with function of the mineral potential and the increasing potential in deaerated 0.01 M borate electrolyte composition (including deoxygen- buffer of pH 9.2 at 0.1 V: Pb1-xS; 0.2 V: ation and/or carbonization) are expected to be Pb(OH)2 and PbSO3; 0.3 V: PbS2O3; and 0.4 V: 2- most advanced. Easily manipulating the SnO6 [17,18]. interfacial potential difference by a potentiostat, one can determine all states of the mineral Based on the spectral results (Figure 2), the 0 surface reactivity, which is of substantial first stage produces Pb1-xS and then S : + 2+ importance for modeling the flotation process. PbS + 2xh = Pb1-xS + xPb (8)

89 PbS + 2h+ = Pb2+ + S0 (E0 = 0.354 V) (9) and chemically (by ferric hydroxide) to sulfate Then, Pb(OH)2 is formed by a precipitation at the outer side of the pyriote surface, while mechanism followed by lead sulphite and elemental sulfur can be formed from thiosulfate from the elemental sulfur produced polythionates. in the first stage: 0 + 2- + S + 3H2O + 4h = SO3 + 6H (E0 = +0.59 V) (10) On some surfaces a pseudo-electrochemical 2- 2+ SO3 + Pb = PbSO3 (11) (PE) mechanism is expected, when the redox 2- 0 2- SO3 + S = S2O3 (lg K = 8.14) (12) process is activated at the edge of the surface Oxygen not only provides the cathodic half- heterogeneity domain and proceeds through reaction of the galena oxidation but also react enlarging the initial spot, not starting new ones. with the anodic areas, accelerating the Relative contribution of the different pathways incongruent dissolution. obviously depends on the electronic properties of the mineral bulk and surface. Assuming that Similarly, the anodic oxidation of pyrite by in- bacteria recognize favourable materials for situ FTIR spectra (Figure 3) revealed that the colonization through redox sensing [22], it is surface consist at 0 V: bulk hydrated ferric reasonable to suggest that there can exist a hydroxide gel and bulk FeSO3 in the amount “redox” source of enhanced and selective less than one monoloayer; 0.4 V: less than one reactivity of minerals with redox-active solutes. monolayer of a surfce ferric iron-hydroxyl complex; 0.5 V: bulk oxidation produced ferric hydroxide, ferrous sulphite and polythionates; 2- and at 0.6 V: SO4 [15].

Figure 3. A FTIR study of anodic oxidation Figure 2. Anodic oxidation of galena by in- of pyrite with increasing redox potential. situ FTIR spectra. For redox modification of mineral surfaces as a Anodic reactions on pyrite are: 1) FeS = Fe2+ + tool for providing adsorption selectivity, it is S2-, followed by hydrolysis, oxidation of imperative to investigate and answer the (bi)sulphide ion and hydrolyzed iron(II) either following questions: chemically by dissolved oxygen or x Do the mineral surfaces have intrinsic oxidizing and reducing sites? In situ FTIR- electrochemically, and precipitation of the – 3– products (elemental sulfur, sulphite, and ferric ATR study of the CO, CN , Fe(CN)6 , and + - Fe(CN) 4– adsorption. Discrimination and hydroxide); 2) SSred + h + OH = SSoxOH; and 6 + characterization of mono- and clustered 3) thiosulfate pathway: FeS2 + 3H2O + 6h = 2+ 2- + surface defects based on the effect of Fe + S2O3 + 6H , thiosulfate and ferrous ions are further oxidized both electrochemically dynamic coupling of the adsorbates on the

90 vibrational spectra and, where possible, on Interaction of galena with xanthate by in-situ the STM/AFM data FTIR spectroscopy is shown in Figure 4 x What are adsorption forms of the redox- [16,18]. These spectral results confirmed that i) active anions ubiquitous in geochemical at the first stage xanthate is adsorbed with systems (sulphite/ sulphate, selenate/sele- charge transfer (chemisorbed), ii) dixanthogen nite, arsenate/arsenite, chro-mate, etc.)? is formed at the reversible potential for the 2– – How does introduction of S , CN , O2, and xanthate/dixanthogen pair, independently of H2O2 before/after the anion adsorption the xanthate concentration. The appearance of influence this picture? Complex in situ and dixanthogen makes the surface most ex situ spectroscopic study. hydrophobic, which can be followed from the x How does the partitioning between changes in the water spectrum, and iii) the adsorbed and adsorbed-and-reacted mechanisms of the xanthate adsorption athigh anions change with time? Evaluation of the and low concentrations have differences. At a -3 kinetic parameters of the redox reactions concentration of 10 M, oxidative catalysed by the minerals. Inferring the decomposition of galena and adsorbed reaction mechanisms xanthate is inhibited by the xanthate x What are adsorption forms of redox-active chemisorption, which is followed by the surfactants (xanthates and dithiophosph- formation of lead xanthate, Pb(X)2, and (X)2. At -5 ates) in the absence and presence of the 8.10 M, bulk Pb(X)2 is formed by the pre-adsorbed anions? How this picture is precipitation mechanism. At higher applied 2– – influenced by solvated S , CN , O2, and potentials first Pb(X)2 transforms into Pb(OH)2. H2O2? In situ and ex situ complex Afterward (X)2 decomposes into a dimer of spectroscopic measurements monothiocarbonate, - + + x What is the difference between the anodic (ROCS2)2 + 2OH + 2h = (ROCSO)2 + 2H (13) semireaction paths on (semi)conducting while galena decomposes into lead sulphite oxides and sulphides in the redox process and lead thiosulfate. controlled “chemically” and electrochem- Thus, surface speciation of sulphides in the ically with a potentiostat? FTIR-ATR absence and presence of collectors as a spectroelectrochemical study. function of the sulphide potential controlled x Analysis including surface complexation electrochemically using a potentiostat or modeling of the whole data. Development chemically by changing the redox potential of of a predictive general model of redox the pulp needs to be carried out by in-situ catalytic activity of wide band-gap metal spectroelectrochemistry. The results may lead oxides and sulphides. to surface speciation with significant inference of hydrophobic or hydrophilic nature of the MECHANISMS OF COLLECTOR surface for different redox conditions. ADSORPTION

Molecule-level knowledge of the mechanisms of the interfacial processes will certainly boost technological innovations in ore processing and (bio)remediation of organic and inorganic contaminants. For example, despite a more than one-century history of flotation the major industrial process for mineral separation suitable reagents and reagent regimes are still being developed mainly empirically and scientists are just in the beginning of understanding of basic principles of selective interactions of minerals with hydrophobizing reagents (collectors). One of the possible ways to selectively control adsorption of a collector is conducting redox reactions before or after its adsorption at the mineral surface, as used, e.g., in sulphide flotation with xanthates [22]. It is well known that the toxicity, solubility, Figure 4. Differential ATR spectra for (100) sorption, bioavailability, and transport of PbS and 8 x 10-5 M Na n-butyl xanthate in elements in soil and aquatic systems are deaerated 0.01 M borate buffer at anodic strongly dependent on the oxidation state [9]. potential scan from –0.5 V.

91 COMPLEX SULPHIDE ORE FLOTATION Recently it was revealed [32] that ferric defects on ground pyrite surfaces can generate OH•

The use of NaHSO3 as a flotation depressant radicals upon interaction with water. It may be • [23] is being practiced in selective flotation of the existence and reactivity of OH that plays a sulphide minerals. The depressing effect crucial role in catalytic degradation of organic generally increases from copper sulphides to pollutants by pyrite. However, participation of galena, pyrite and sphalerite. However, there is these species, if any, in non-selective oxidation no common agreement about the depression of the pulp components and hence in mechanisms. In particular, the following three deteriorating the concentrate grade has not still effects have previously been proposed for been explored yet. To fill the gap, it is important 2– depression of pyrite flotation by SO3 : 1) to build correlation between percentage of stripping/decomposition of xanthate [24]; 2) pyrite in the concentrate, grinding conditions • reaction with the pyrite surface to form and concentration of OH /H2O2 in the pulp as hydrophilic iron oxides [25]; and 3) a decrease well as to study possible ways of flexibly of redox potential of the pulp below a level at controlling the formation of these species which binding/oxidation of a collector (electron through known chemical means. One of such donor) becomes energetically unfavorable [26]. ways can be addition of chloride ions, which In the presence of copper, sulphite was shown are known to inhibit the deposition of elemental [27] to promote the oxidation of copper on the sulfur on the pyrite electrode surface by pyrite surface, preventing the adsorption of promoting the oxidation of an adsorbed xanthate and thus leading to the mineral intermediate, believed to be the thiosulfate ion, depression, but has no effect on sphalerite. At to soluble tetrathionate ions. In the absence of the same time, in the case of chalcopyrite, it chloride ions, the thiosulfate intermediate was postulated [28] that sulphite removes both undergoes acid decomposition on the pyrite the sulfur-rich phase and iron oxides from the surface to yield elemental sulfur [33]. surface, leaving a sulfur-rich sulphide layer, which in turn promotes collector adsorption. Also, it was found [29] that the depressing effect of sulphite on chalcopyrite flotation depends on the presence of Fe3+ ions released from grinding media. Apart from the decomposition of xanthate/ dixanthogen and the decrease in xanthate adsorption following a decrease of the redox potential of the pulp, several additional mechanisms have been put forward to explain the depression of the flotation of sphalerite [30]. They include: the formation of a zinc sulphite hydrophilic layer at the mineral surface; the reduction of copper- activation as a result of consumption of copper in solution as copper sulphite; and the Figure 5. Effect of water pH on H2O2 consumption of dissolved oxygen. Sulphite ions formation when 5 g dry-ground solids are are also known to react with polysulphide or mixed in water for 5 min. elemental sulfur and form thiosulfate ions; a decrease in surface hydrophobicity is therefore For the problem of low selectivity between expected from this reaction. Finally, compared ferrous and non-ferrous sulphides, and among to sulfate, this reduced sulfoxyanion has higher sulphides, the effect of production of H2O2 by adsorption affinity due to lower S–O bond order metal sulphides needs to be examined. To [31]. Therefore, we expect that sulfate anions pinpoint the dominant contribution (natural produced upon catalytic oxidation of sulphite hydrophobicity, formation of elemental sulfur on species will much more strongly be bound to the surface under the flotation conditions, the sulphide surface compared with the sulfate and/or activation) to low selectivity of sulphide anions that are directly adsorbed through ion flotation against pyrite, surfaces of pyrite exchange/outer-sphere complexation, thus particles from both concentrate and tailings competing more efficiently with collectors for need spectroscopy characterization for surface the adsorption sites on sulphides, which may speciation. strengthen the depressing effect of sulphite. Our recent results of dry-ground pyrite when This effect, if properly understood, can open a mixed with water leading to the formation of new cost-effective approach to selectively H2O2 are shown in Figure 5. The water pH in regulate surface properties of sulphides. which the ground solids are placed has a

92 significant influence on H2O2 formation of its chemistry of submicron particles and to get a increase with decreasing pH below 8. This possibility to control their reactivity, which is observation found to be valid with other important for flotation, it is necessary to sulphide minerals as well (Figure 6). perform a systematic study of how and why reactivity of sulphides changes with particle sizes.

BIOBENEFICATION OF METAL SULFIDES

The biobeneficiation includes the processes of bio-flotation and bio-flocculation where selective removal of undesirable mineral constituents from an ore is accomplished. Several studies showed that the microorganisms could function similarly as that of traditional reagents in flotation and flocculation processes [2, 35]. Adhesion of Figure 6. Formation of hydrogen peroxide microbes to particle surfaces can alter the by different sulphide minerals as function hydrophobicity of minerals markedly. Such of pH. surface modification to impart hydrophobicity or hydrophilicity is used in flotation of sulphide FINE PARTICLE FLOTATION and non-sulphide minerals. In some systems they also cause excellent flocculation of fine particles. Since the bacteria adhere to mineral The floatability of fine particles has slow surfaces and alter the surface properties that kinetics due to low momentum through the pulp are essential in mineral beneficiation resulting by their small mass, which leads to a techniques, the microorganisms have lower probability of collision with passing air formidable applications in flotation and bubbles. Furthermore, slimes have a significant flocculation processes. The adhesion of depressant effect on polysulphide ores and microorganisms results an alteration of the their depressant effect is three-fold: 1) fines of surface chemistry of minerals due to a non-sulphide gangue report to concentrate, 2) consequence of the formation of a biofilm on valuable sulphides are lost due to low the surface or surface-active chemicals and floatability of the fine particles and 3) non- their adsorption or sorption, accumulation and sulphide gangue slimes covering the originally- precipitation of ions and compounds on the hydrophobic sulphide particles through hetero- surface. coagulation mechanism rendering the particles Microorganisms adhere to mineral surfaces for hydrophilic. various reasons. S. oneidensis utilize minerals

as the terminal electron acceptor in the However, recent data showed that there are respiratory cycle. Bacteria of Thiobacilli group intrinsic differences in the surface and bulk recover energy from minerals by the enzymatic stoichiometry and crystal structure of very fine oxidation. Common to both these microbiologic particles from those of the corresponding large processes is the bacterial need to access, bulk crystals [34]. Atoms exposed on nano- adhere to, and react with the mineral-water sized particles experience an anisotropic interface. Previous studies have shown that environment. To lower free energy per unit under most physiological conditions the surface, the crystal structure of the near bacterial cell surface carries a net negative surface region is distorted, which in real charge, while, along with electrostatic forces, conditions is accompanied by adsorption of hydrophobic, entropic, acid-base, and Van der water/hydroxyls. It is expected that with Waals interactions and H-bonding are decreasing particle size the redox potential of important in the bacterial adhesion [2]. the sulphide decreases, along with sorption The information on the mechanisms of both capacity per nm2. These effects can be bacterium adsorption and reagent (collector) balanced by using the more easily oxidizable adsorption in the presence of the adsorbed homologues of xanthate and carbamate with bacteria is necessary. However, this area is a longer chain lengths and/or employing dark spot at the moment. The tremendously sterically appropriate chelating legands with complex problem of gaining insight into the flexible distance between the reactive groups. mechanisms of adsorption of living organisms Thus to shed light on solution and solid state is met by a not-less-complex surface chemistry

93 in a pulp as far as even in the absence of shown in Fig. 8. Spectra (1) on the top and adsorbed bacteria the sulphide surface species bottom panels show the sulphide–water are instable. We have showed recently that interface after conditioning with a suspension adapted bacteria of the Thiobacilli group of Paenibacius polymixa adapted to FeS2. The (Thiobacillus ferrooxidans, Thibacillus microbe concentration was 10–8 ml–1 (pH 6.5). thiooxidans, and Leptospirilum ferrooxidans), One can see the appearance of weak bands of –1 which are associated of sulphide ores and PbSO4 at 1170 and 985 cm in the case of mine water, and Paenibacillus polymyxa, can galena. The spectral picture is somewhat be selectively attached to sulphides, thereby different in the case of pyrite: First, except for essentially modifying the following adsorption bands of a Fe(III)–SO4 complex at 1105 and of flotation reagents [36-38] (these systems are 980 cm–1, prominent bands at 1420 and 1542 among few that have been extensively reported cm–1 assignable to Fe(III) carbonate are in the literature). This phenomenon can be put observed. Second, intensity of these bands is into the basis of elaboration of principally novel noticeably higher than for galena, implying that flotation schemes providing both ecological and interaction with the microbe results in a higher economical gain. degree of corrosion for pyrite. The presence of Though the mechanisms of adsorption of the the carbonate species implies the formation of microbe’s extracellular polymeric substance amorphous ferric (hydr)oxide, the absorption of (EPS) components onto solid surfaces are which is masked by the water absorption below generally known [39], they are system specific. 800 cm–1. The interaction of the biopolymers produced by The subsequent addition of a 10–5 Ɇ aqueous bacteria grown in the presence of specific solution of ethyl xanthate results in the minerals with surfactants requires investigation formation of both lead xanthate (1215, 1090, focusing on the applicability to bioflotation. and 1009 cm–1) and dixanthogen (a component More specifically, the EPS produced by at 1265 cm–1 marked by an asterisk) on galena. bacteria of Acidithiobacilli group during the This process is accompanied by appearance of growth in 1) culture media, 2) in the presence a strong negative band (įH2O) of water at 1630 of either chalcopyrite, pyrite, galena or cm–1, which indicates that water is pushed off sphalerite, and 3) in the presence of both the interface, consistent with experimentally mineral and flotation reagent (xanthate and observed hydrophobicity of the mineral under dithiophosphate). The interaction of the similar conditions. Though some dixanthogen biopolymers produced by bacteria grown in the is formed upon the xanthate adsorption on presence of specific minerals and minerals with pyrite (bands at 1265, 1090, and 1009), the xanthate and dithiophosphate require special presence of a band at 1035 cm–1 testifies to the attention. concomitant formation of ferric xanthate- hydroxide species. Positive intensity of the –1 0.001 PbS water band at 1630 cm shows that density of 1

) the interfacial water increases, in agreement 0 1170 985 0.000 2 with the observed depression of pyrite.

1265 Inspection with an optical microscope of the

-log(R/R * -0.001 sulphide minerals after the experiments 1090 1215 1009 described revealed that about 10 microbes had -0.002 3000 2500 2000 1500 1000 adhered to the polished pyrite surface of 10 mm2, while none to galena. 0.001

1420 Based on these observations and since no

FeS 1542 2 1105 0.000 typical bands of biopolymers (proteins, )

0 1 polysaccharides, etc.) are present in the in situ 1265 2 1009 1090 -0.001 FTIR spectra, one can make preliminary

-log(R/R conclusion that the main mechanism of 1035 -0.002 bioregulation of the surface properties relates 3000 2500 2000 1500 1000

-1 to biooxidation of the sulphides and/or to Q, cm change in the sulphide potential (in that Figure 7. In situ FTIR at the PbS-aqueous particular experiment we did not measure solution and FeS2-aqueous solution potential of the sulphide electrode but this can interfaces under open atmosphere. 10 min be done) rather than to surface properties of in 108 unit/ml of microbes followed by 10 the adsorbed microbes themselves or min conditioning in 10-5 M ethyl xanthate. expressed biopolymers. To provide a new insight into the phenomena, Our initial in-situ FTIR spectra of Paenibacius the whole problem of the mineral-bacterium polymixa interaction with galena and pyrite are interaction can be conveniently divided into two

94 problems: biochemical and geochemical. The minerals and iii) both mineral and surfactant former is related to the bacterium side of the (flotation collector) need special attention interface (the bacterium envelope) in terms of focusing on the applicability to bioflotation. particular mechanisms of sensing/recognition Another environmentally sound and cost- and response to extracellular minerals, effective biochemical innovation in sulphide molecule-specific pathways of charge transfer, flotation could be replacement of Na2S and and genomic mechanisms of regulation of reduced sulphur compounds, which are widely these processes. The geochemical problem used in flotation practice as regulator concerns the mineral response to the (reductants), by sulphate reducing bacteria. bacterium presence, which is essentially Clearly, to develop a biochemical control of interplay between microorganism and the sulphide flotation, it is necessary first of all to physicochemical properties of the mineral understand difference in floatability of the surface, such as the atomic and electronic sulphide surfaces reduced chemically and structure, the net charge/potential, acid-base biochemically, which require intensified effort. properties, and wettability of the surface. Consequently, the biocatalyzed electron- SUMMARY transfer reactions on metal oxides and sulphides can be controlled through variation of The molecule-level understanding of the these parameters, which require a systematic aquatic solids interfaces is the key to innovate study in order to find answers to the following many society-formative technologies including questions: ore processing, waste recycling and the x Are the redox mechanisms established for environmental protection that are based on abiotic interfaces are valid in the case of stringent control of interfacial processes. There bacterium-mediated reduction/oxidation of are no general concepts of selective the mineral? interactions of minerals with solutes, which x Is direct bacteria-mineral contact imply that fundamental additions to aquatic necessary for oxidative or reductive chemistry of minerals are one of the major dissolution of sulphides, iron oxides, or demands of the day. The interface between manganese oxides? biological and geological materials, as well as a x Do bacteria exploit the energetic means to design and manipulate that interface, perturbations present in the surface defects is currently virtually completely unexplored. to adhere and oxidize/reduce the mineral? The lack of knowledge could be recognized to If yes, defects of which type (point vs. the complexity and technical constraints of the clustered) are more preferable by bacteria? problem. Progress in this area is hampered by x Which type of surface defects bacteria a deficit of direct, not to mention in situ, generate by their own? What is the spectroscopic molecular/atomic-level data mechanism of generation of these defects? about the mineral–bacterium interfaces. What x What are the roles of different kinds of the is exactly happening at the mineral-bacteria- initial and bacterium-induced mineral solution interfaces at the molecular level and in heterogeneity in the overall biogeochemical real time controls mineral (bio)processing interaction? technologies and there is an urgent need to x How size of the mineral particle influences understand the interfacial charge transfer and the microbe adhesion and the rate of the adsorption mechanisms to shift the biotic oxidation/reduction of the mineral? technologies to be more innovative and x How the mineral potential (regulated either effective. by pH or potentiostatically) influences the attachment of the whole bacteria and the ACKNOWLEDGEMENT EPS components and the heterogeneous The financial support from the Centre for charge transfer reaction? Advanced Mining and Metallurgy (CAMM), x How the mineral potential affects forms of Luleå University of Technology, Luleå, collector adsorption and surface Sweden, is gratefully acknowledged. equilibrium of model redox couples in the presence of the bacteria? REFERENCES Though the mechanisms of adsorption of the 1. M.F. Hochella, Jr. and A.F. White, Eds., microbe’s extracellular polymeric substance Mineral-Water Interface Geochemistry, (EPS) components onto solid surfaces are Washington DC, Mineralogical Society of generally known, they are system specific. The America, 1990. mechanisms of the interaction of the 2. K. Hanumantha Rao and S. Subramanian, biopolymers produced by bacteria grown in the In: Microbial processing of Metal Sulphides, E. presence of i) culture media, ii) specific

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