Surface Oxidation and Selectivity in Complex Sulphide Ore Flotation

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DOCTORAL T H E SIS Alireza Javadi Sulphide Minerals: Oxidation and Selectivity in Complex Sulphide Ore Surface Flotation

Department of Civil, Environmental and Natural Resources Engineering Division of Minerals and Metallurgical Engineering

ISSN 1402-1544 Sulphide Minerals: Surface Oxidation and ISBN 978-91-7583-411-5 (print) ISBN 978-91-7583-412-2 (pdf) Selectivity in Complex Sulphide Luleå University of Technology 2015 Ore Flotation

Alireza Javadi

LULEÅ TEKNISKA UNIVERSITET

Sulphide Minerals: Surface Oxidation and Selectivity in Complex Sulphide Ore Flotation

Doctoral thesis

Alireza Javadi Nooshabadi

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

October 2015 Printed by Luleå University of Technology, Graphic Production 2015

ISSN 1402-1544 ISBN 978-91-7583-411-5 (print) ISBN 978-91-7583-412-2 (pdf) Luleå 2015 www.ltu.se

Dedicated to My wife and daughter

III

IV Synopsis

Metal and energy extractive industries play a strategic role in the economic development of Sweden. At the same time these industries present a major threat to the environment due to multidimensional environmental pollution produced in the course of ageing of ore processing tailings and waste rocks. In the context of valuable sulphide mineral recovery from sulphide ore, the complex chemistry of the sulphide surface reactions in a pulp, coupled with surface oxidation and instability of the adsorbed species, makes the adsorption processes and selective flotation of a given sulphide mineral from other sulphides have always been problematic and scientifically a great challenge. Invariably, the problems associated with acid mine drainage and selectivity in flotation are explained to be associated with the oxidation of metal sulphides. Although metal sulphides oxidation and galvanic effects were well known in flotation and leaching of sulphides, recent studies reveal the formation of reactive, oxidizing oxygen species and H2O2 by sulphides due to the catalytic activity of sulphide surfaces.

The inherent formation of H2O2 by single and mixture of sulphide minerals during wet and dry grinding systems and in open and closed environments have been investigated. It was found that pyrite (FeS2), chalcopyrite (CuFeS2), sphalerite ((Zn,Fe)S), and galena (PbS) generated H2O2 in pulp liquid during wet grinding in the presence and absence of dissolved oxygen in water and also when the freshly dry ground solids are placed in water immediately after 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 and capable of breaking down the water molecule leading to hydroxyl free radicals. Type of grinding medium on formation of hydrogen peroxide by pyrite and galena revealed that the mild steel produced 2+ 3+ more H2O2 than stainless steel grinding medium, where Fe and/or Fe ions played a key role in producing higher amounts of H2O2. In addition, 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 the content of a nobler mineral or higher rest potential mineral in a mixed composition. The results of H2O2 formation in pulp liquid of sulphide minerals and mixed minerals at different

V 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. Studies have also been carried out to build correlation between percentage of pyrite in • the concentrate, grinding conditions and concentration of OH /H2O2 in the pulp and as well of controlling the formation of these species through known chemical means for depressing the generation of the oxidant. Flotation tests using a complex sulphide ore with the same reagent scheme that is being used at Boliden concentrator but with the addition of collector and depressant during grinding stage have been performed to judge the beneficial or detrimental role of H2O2 on the selective flotation of sulphides. The results demonstrate that the selectivity of metal sulphides against pyrite increases with increasing generation of H2O2 in the pulp liquid. 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, leaching and environmental degradation in the context of inevitable H2O2 existence in the pulp liquid.

VI 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, Professor Pertti lamberg. I am sincerely grateful to my country Iran- Iranian Ministry of Sciences, Researches & Technologies (MSRT) for awarding to me the scholarship for four years. The financial support for the project by Boliden Company is gratefully acknowledged. I thank all my colleagues at the Department of Civil, Environmental and Natural Resources Engineering. 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. I fully wish to thank my wife Fahimehsadat 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, Oct 2015 Alireza Javadi

VII

VIII Publications

Papers included in the thesis:

I. Alireza Javadi Nooshabadi, Anna-Carin Larsson, Kota Hanumantha Rao, Formation of hydrogen peroxide by pyrite and its influence on flotation– Minerals Engineering, 2013, 49, 128-134. II. Alireza Javadi Nooshabadi, Kota Hanumantha Rao, Formation of hydrogen peroxide by chalcopyrite and its influence on flotation– Minerals and Metallurgical processing, 2013, 30 (4), 212-219. III. Alireza Javadi Nooshabadi, Kota Hanumantha Rao, Formation of hydrogen peroxide by sphalerite– International Journal of Mineral Processing, 2013, 125, 78-85. IV. Alireza Javadi Nooshabadi, Kota Hanumantha Rao, Formation of hydrogen peroxide by galena and its influence on flotation– Advanced Powder Technology, 2014, 25, 832-839. V. Alireza Javadi Nooshabadi, Kota Hanumantha Rao, Formation of hydrogen peroxide by sulphide minerals– Hydrometallurgy, 2014, 141, 82-88. VI. Kota Hanumantha Rao, Alireza Javadi Nooshabadi, Tommy Karlkvist, Anuttam Patra, Annamaria Vilinska, Irina Chernyshova. Revisiting sulphide mineral (bio)processing: A Few Priorities and Directions– Powder Metallurgy & Mining 2: 116. doi: 10.4172/2168-9806.1000116. VII. Alireza Javadi Nooshabadi, Kota Hanumantha Rao, Effect of grinding environment on galena flotation–The Open Mineral Processing Journal, 2015- In press. VIII. Alireza Javadi Nooshabadi, Kota Hanumantha Rao, Complex sulphide ore

flotation: Effect of depressants addition during grinding on H2O2 formation and its influence on flotation – to be submitted to International Journal of Mineral Processing, 2015.

Published papers presented at international conferences (not appended): • Alireza Javadi Nooshabadi, Kota Hanumantha Rao. Sulphide Mineral Flotation – A New Insight into Oxidation Mechanisms

IX • Alireza Javadi Nooshabadi, Kota Hanumantha Rao. A New Insight into Oxidation of Sulphide Minerals XXVIIth International Mineral Processing Congress (IMPC 2014), 20-24 October 2014, Santiago, Chile.

Contrbution of co-authors to papers 1-8:

Experimental work, calculations, results interpretation, and writing of articles 1-5 and 7-8 were carried out by Alireza Javadi Nooshabadi. Hanumantha Rao has contributed in supervisory capacity and assisted with revision of the manuscripts. Article 6, Experimental work, calculations, results interpretation, and writing of part Complex Sulphide ore flotation were carried out by Alireza Javadi Nooshabadi.

X List of symbols and abbreviations

Fe2+ Ferrous ion Fe3+ Ferric ion HO• Hydroxyl radical • − O2 Superoxide radical anion − HO2 Hydroperoxide

H2O2 Hydrogen peroxide Py Pyrite Cp Chalcopyrite Sp Sphalerite Ga Galena

XII Contents Synopsis ...... V Acknowledgements ...... VII Publications ...... IX 1 Introduction ...... 1 1.1 Aim and scope of the thesis ...... 2 1.2 Outline of the thesis ...... 3 2 Literature Review ...... 5 2.1 Oxidation of pyrite ...... 6 2.1.1 Crystal structure of pyrite ...... 6

2.1.2 Effect of atmospheric O2 and water on pyrite oxidation ...... 7 2.1.3 Effect of Fe3+ ions on pyrite oxidation ...... 8 2.1.4 Effects of the grinding environment on pyrite oxidation...... 8 2.1.5 Effect of microorganisms on pyrite oxidation ...... 8 2.2 Oxidation of chalcopyrite ...... 8 2.2.1 Crystal structure of chalcopyrite ...... 8 2.2.2 Effect of atmospheric moisture or water on chalcopyrite oxidation ...... 9 2.2.3 Effect of iron(III) on chalcopyrite oxidation ...... 9 2.2.4 Effect of microrganism on chalcopyrite oxidation ...... 9 2.2.5 Effect of galvanic interactions on chalcopyrite oxidation ...... 10

2.2.6 Oxidation by H2O2 in aqueous system...... 10 2.3 Oxidation of sphalerite ...... 10 2.3.1 Crystal structure of sphalerite ...... 11

2.3.2 Effect of O2 on sphalerite oxidation ...... 11 2.3.3 Effect of Fe3+ on sphalerite oxidation ...... 11 2.3.4 Oxidation by microorganisms ...... 12 2.3.5 Effect of pyrite and chalcopyrite on sphalerite oxidation ...... 12 2.3.6 Effect of galena on sphalerite oxidation ...... 13 2.4 Oxidation of galena ...... 13 2.4.1 Crystal structure of galena ...... 13 3+ 2.4.2 Effect of Fe on galena oxidation...... 14 2.4.3 Effect of grinding environment on galena oxidation ...... 14 2.4.4 Effect of process water on galena oxidation ...... 14 3 Experimental ...... 15 3.1 Materials and reagents ...... 15 3.2 Wet grinding ...... 16 3.3 Dry grinding ...... 17 3.4 Hydrogen peroxide estimation ...... 17 3.5 Pulp chemistry measurement ...... 18 3.6 Gas purging ...... 19 3.7 Flotation tests ...... 19 3.8 Bench-scale flotation tests ...... 20 3.9 Diffuse reflectance FTIR studies ...... 21 3.10 Analysis of flotation products ...... 21 4 Results and discussion ...... 23

4.1 Formation of hydrogen peroxide (H2O2) after dry grinding and mixing with water ...... 23

4.2 Formation of hydrogen peroxide (H2O2) by metal sulphides in wet grinding ...... 28

XIII 4.2.1 The effect of grinding medium type on formation of hydrogen peroxide ...... 29

4.3 Effect of minerals mixture on formation of H2O2 ...... 30 4.4 Effect of gaseous atmosphere ...... 32 4.5 Flotation studies ...... 33 4.5.1 Pyrite flotation ...... 33 4.5.2 Chalcopyrite flotation ...... 34 4.5.3 Galena flotation ...... 35 4.5.4 Sphalerite flotation ...... 36 4.5.5 Bench-scale flotation studies ...... 36 4.6 Diffuse reflectance FTIR studies ...... 46 5 Conclusion ...... 47 6 References ...... 48

XIV

1 Introduction The sulphide minerals are the most important, most diverse, and richest in terms of physical, chemical, and structural properties (Tossell & Vaughan, 1992). Froth flotation is the main separation process used for collecting selectively valuable minerals from a mined ore, and is based on differences in hydrophobicity of particles (del Villar et al., 2010). The process of selective separation of valuable sulphide minerals from gangue (e.g., other non-valuable sulphide mineral like pyrite and metal oxides, silicates, etc.) in flotation is heavily dependent on processing steps involving control of surface oxidation products on the mineral surfaces. Several researchers showed that pyrite (Buckley & Woods, 1987; Buckley AN, 1985), pyrrhotite (Buckley & Woods 1985a; Buckley & Woods 1985b; Jones et al., 1992; Pratt et al., 1994), chalcopyrite (Buckley and Woods, 1984a; Smart, 1991), galena (Buckley and Woods, 1984b; Fornasiero et al., 1994), pentlandite (Richardson and Vaughan, 1989; Buckley and Woods, 1991a), cobaltite (Buckley, 1987) and sphalerite (Buckley et al., 1989a, b, c; Prestidge et al., 1994) exhibit oxide and hydroxide species on their surface after exposure to air or aqueous solution. The mechanisms of surface oxidation are dependent on conditioning time, the gas atmosphere above the sample, pH and Eh. The presence of oxygen in sulphide flotation could play a positive or negative role. Sulpho-oxygen species are produced by the oxidation of sulphide surface (Gaudin, 1957). The galena surface changes from n-type to p-type in the presence of oxygen, which resulted in the adsorption of xanthate ion due to the presence of hole on p-type galena semiconductor (Plaksin et al., 1957; 1963). The presence of oxygen in sulphide flotation leads to hydrophilic products on mineral surfaces because of oxidation and thereby decreases natural floatability of sulphide minerals (Fuerstenau and Sabacky, 1981). The difference of cathodic reduction of oxygen in pyrite and galena affected the adsorption of collector on mineral surfaces (Neeraj, 2000). Although flotation of sulphide ores has been practiced for almost 75 years and several authors studied the oxidation mechansim of metal sulphides, it is not known very well on the kinetics and extent of their oxidation.

1.1 Aim and scope of the thesis

The main aim of this work is to investigate the formation of H2O2 by sulphide minerals in flotation and its influence on flotation recovery and grade of minerals. In case of detrimental effect on flotation results, how to control the formation of these oxidizing species through known chemical means is the second objective. The results were mainly presented in eight papers, appended at end of this thesis as Appendix I to VIII. Experimental studies were performed to investigate the formation of hydrogen peroxide by pyrite (Appendix I), chalcopyrite (Appendix II), sphalerite (Appendix III) and galena (Appendix IV) and its influence on flotation. The Appendix V is a comparison between sulphide minerals in the extent hydrogen peroxide formation by their surface interaction with water. The Appendix VI is about revisiting sulphide mineral processing by flotation. The results on the effect of grinding environment on H2O2 formation by galena and its flotation response were presented in Appendix VII. The results on the studies to control the formation of hydrogen peroxide by the addition of reagents during grinding, i.e., prior to mineral surfaces interaction with water and/or collector, were presented in Appendix VIII. The following research questions are posed on:

1. Can sulphide minerals generate H2O2 during grinding in the presence and absence of dissolved oxygen?

o Pyrite o Chalcopyrite o Sphalerite o Galena

2. What is the effect of grinding environment on formation of H2O2?

3. What is the effect of mixed sulphide minerals on formation of H2O2?

4. What is the contribution of H2O2 in sulphide oxidation relative to galvanic interactions?

5. What is the effect of H2O2 on flotation and how we can control effect of H2O2 in flotation?

1.2 Outline of the thesis The summary of this thesis is based on studies reported in 8 appended papers (papers I to VIII). The first Chapter is the introduction and outline. Literature review on the oxidation of sulphide mineral is provided in Chapter 2. The experimental methods used in the study are explained in Chapter 3. The main findings from the investigations are summarized in Chapter 4. Finally, the conclusions are given in Chapter 5. After presenting a brief summary of the work in the beginning of the thesis in Chapters 1 to 5, the details of the entire work in the form of published papers are included at the end.

2 Literature Review Recently, several investigations have focused on the kinetics of sulphide minerals oxidation and their oxidation products. It has been reported that the oxidation rate of sulphide minerals is in the order of pyrite > chalcopyrite > sphalerite > galena (Feng, 1989; Qin, 1998; Kelsall and Yin, 1999). Rand (1977) investigated the activity of several sulphide minerals (e.g. arsenopyrite, bornite, chalcocite, chalcopyrite, covellite, galena, pentlandite, pyrite and pyrrhotite) for the oxygen reduction in acidic and alkaline solutions and the results are shown in Fig. 1. It can be seen that pyrite is the most active among all the sulphide minerals and galena has the lowest activity. Accordingly, pyrite is a better catalyst for oxygen reduction and galena the poorest. The differences in the surface activity between sulphide minerals for oxygen reduction is explained due to the differences in the hydrophobic nature among the sulphide mineral surfaces. The rest potentials of the sulphide minerals determined by Qing et al. (2007) are shown in Table 1 and they are in good agreement with the activity of the minerals for oxygen reduction determined by Rand (1977). Commonly, more noble is the mineral, lower is the activation energy required for oxygen reduction on its surface.

Fig. 1. Activation controlled current for oxygen reduction on sulphides (Rand, 1977)

Table 1. The rest potential of Sulphide minerals at pH 4 (Qing et al., 2007). Mineral Chemical Formula Rest Potential (V, SHE)

Pyrite FeS2 0.66

Marcasite FeS2 0.63

Chalcopyrite CuFeS2 0.56 Sphalerite (Zn,Fe)S 0.46 Covellite CuS 0.45

Bornite Cu5FeS4 0.42 Galena PbS 0.40

Argentite Ag2S 0.28

Stibnite Sb2S3 0.12

2.1 Oxidation of pyrite

Pyrite (FeS2) is the most abundant metal sulphide in nature that has a major influence on the biogeochemical cycles of iron, sulfur, and oxygen. Pyrite oxidation is economically important in mineral flotation since oxidation changes its hydrophobicity or hydrophilicity character (Buckley et al., 1987). Inadvertent flotation of pyrite reduces concentrate grades and increases smelting costs (Hiskey et al., 1982, Wiersma et al., 1984). Pyrite oxidation processes also play a significant role in acid rock drainage (Rimstidt et al., 2003). Mining industry treatment costs for acid rock drainage, in the US alone, are over $1 million per day (Evangelou, 1995).

2.1.1 Crystal structure of pyrite 2+ 2− Pyrite is composed of a ferrous (Fe ) cation and an S2 anion with an ideal Fe:S ratio of 1:2 as shown in Fig. 2 (Bayliss, P., 1977).

Fig. 2. Crystal structure of pyrite (Bayliss, P., 1977).

2.1.2 Effect of atmospheric O2 and water on pyrite oxidation

Oxidation of pyrite surfaces may occur when mineral is in contact with atmospheric O2 and water (Buckley and Woods, 1987; Todd et al. 2003). Pyrite oxidation is an electrochemical process with three steps, as shown in Fig. 3: (1) cathodic reaction, (2) electron transport, and (3) anodic reaction (Lefticariu et al., 2006a, Rimstidt and Vaughan, 2003).

Fig. 3. The reaction of pyrite oxidation consists of three steps (Rimstidt and Vaughan, 2003).

2.1.3 Effect of Fe3+ ions on pyrite oxidation Ferric iron (Fe3+) is a powerful oxidant of pyrite in highly acidic conditions (Nordstrom and Alpers, 1999), and reacts with pyrite according to the following reaction (1):

3+ 2+ 2- + FeS2+ 14Fe +8 H2O→15Fe +2SO4 +16H (1)

2.1.4 Effects of the grinding environment on pyrite oxidation Sulphide minerals are nobler electrochemically than steel grinding media (balls, rods, and liners) and increase rest potentials under most conditions (Yelloji Rao and Natarajan, 1989a). The contact between media and sulphide minerals during milling takes place through galvanic interactions. Steel grinding media are more active than the sulphide minerals and act as anodes while the sulphides act as cathodes. Galvanic interactions can change the surface properties and pulp chemistry and affect subsequent flotation (Bruckard, et al., 2011).

2.1.5 Effect of microorganisms on pyrite oxidation The iron-oxidizing bacteria accelerates the oxidation of pyrite in acid mine water (Singer and Stumm 1970). Acidophilic sulfur and iron-oxidizing bacteria of the species Acidithiobacillus ferrooxidans, formerly Thiobacillus ferrooxidans (Kelly and Wood, 2000) play an important role in oxidation of sulphide ore. They obtain energy from the oxidation of ferrous iron (Colmer and Hinkle, 1947; Keller and Murr, 1982; Southam and Beveridge, 1992; Ledin and Pedersen, 1996).

2.2 Oxidation of chalcopyrite

Chalcopyrite (CuFeS2) is one of the most abundant and widespread copper-bearing minerals (Habashi, 1978), accounting for approximately 70% of the Earth's copper (Córdoba et al., 2008). Chalcopyrite oxidizes to copper, ferric sulfates and iron oxide (Steger & Desjardins 1978). The rate of chalcopyrite dissolution is slightly dependent on hydrogen ion activity and it increases with decreasing pH (Acero et al., 2009).

2.2.1 Crystal structure of chalcopyrite

Burdick and Ellis (1917) determined the crystal structure of CuFeS2. Chalcopyrite is a covalent copper sulphide as shown in Fig. 4 (Hall, S.R. and Stewart, J.M.,1973).

Fig. 4. The unit cell of chalcopyrite (Hall, S.R. and Stewart, J.M.,1973).

2.2.2 Effect of atmospheric moisture or water on chalcopyrite oxidation

The atmospheric O2 and natural atmospheric moisture or water oxidize chalcopyrite. The interactions between chalcopyrite and the atmosphere is complex and variable and many aspects of the oxidation fundamental of chalcopyrite are still not understood completely (Hackl et al, 1995; Córdoba et al, 2008; Yin et al, 1995; Hiroyoshi et al, 2004; Parker et al, 1981).

2.2.3 Effect of iron(III) on chalcopyrite oxidation The iron(III) is used for the oxidation of chalcopyrite (Dutrizac, 1981; Majima et al., 1985; Lu et al., 2000b; Klauber et al., 2001; Antonijević and Bogdanović, 2002). Equation (2) shows the oxidation of chalcopyrite by acid Fe(III) solution (Dutrizac and MacDonald, 1973; Munoz et al., 1979; Dutrizac, 1981; Mateos et al., 1987; Hackl et al., 1995; Lu et al., 2000a; Lu et al., 2000b; Hiroyoshi et al., 2002). 3+ 2+ 2+ CuFeS2+4Fe →Cu +5Fe +2S (2)

2.2.4 Effect of microrganism on chalcopyrite oxidation The microbes can oxidise or assist in the oxidation of chalcopyrite (Pradhan et al., 2008; Chen et al., 2008) and they are used in bioleaching. Such microbes can oxidise Fe and/or S (Eqs. 3- 6). The above equations indicate that microbes assist in leaching chalcopyrite through generation of Fe3 + and oxidation of S (Zeng et al., 2010).

+ 2+ 2+ 0 CuFeS2(s)+4H +O2(aq)→Cu +Fe +2S (s)+2H2O (3)

2+ + 3+ 4Fe +4H +O2(aq) 4Fe +2H2O (4) 퐹푒−표푥푖푑푖푠푖푛푔 푏푎푐푡푒푟푖푎

0 2- + 2S (s)+3O2(aq)+2H�2⎯O⎯ ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯� 2SO4 (s)+2H (5) 푆−표푥푖푑푖푠푖푛푔 푏푎푐푡푒푟푖푎 3+ 2+ 2+ 0 CuFeS2(s)+4Fe →Cu �+⎯⎯5⎯Fe⎯⎯⎯⎯+⎯⎯2⎯S⎯⎯(⎯s⎯)⎯ � (6)

2.2.5 Effect of galvanic interactions on chalcopyrite oxidation Galvanic interactions take place due to differing rest potentials of semi-conductive minerals (Ekmekci and Demirel, 1997; Qing et al., 2007) and/or grinding media during leaching, bioleaching, flotation (Ekmekci and Demirel, 1997; Sui et al., 1995) and causes acid mine drainage (Dixon et al., 2008; Chmielewski and Kaleta, 2011). When two types of sulphide minerals are in contact with each other a galvanic cell can form (Eqs. 7-8). Galvanic interaction of pyrite has a positive effect on the leaching rate of chalcopyrite and accelerate its leaching rate (Berry et al., 1978; Mehta and Murr, 1983; Tshilombo, 2004; Dixon et al., 2008; Majuste et al., 2012)

+ − O 2 +4H +4e → 2H2O (7) MS(s)→M2++S0(s)+2e− (8)

2.2.6 Oxidation41B by H2O2 in aqueous system

H2O2 increases the oxidation rate of chalcopyrite in sulfuric acid media (Antonijevic and Bogdanovic, 2004). Adebayo et al. (2003) illustrated that there is a linear relationship between Cu leaching rate and H2O2 concentration between 10 and 15% in sulfuric acid solution.

2.3 Oxidation13B of sphalerite 2- The oxidation sphalerite (ZnS) releases Zn and SO4 ions, which can pollute rivers and oceans (Martin et al., 1994). Sphalerite can be oxidized by dissolved molecular O2 or Fe(III) (Eqs. 9-10) (e.g., Seal and Hammarstrom, 2003; Malmström and Collin, 2004; Schippers and Sand, 1999) or under acid pH conditions (Eqs. 11-13) (e.g., Seal and Hammarstrom, 2003; Malmström and Collin, 2004): 2+ 2− ZnS+2O2→Zn +SO4 (9)

3+ 2+ 2+ 2− + ZnS+8Fe +4H2O→Zn +8Fe +SO 4+8H (10) + 2+ ZnS+2H →Zn + H2S(aq) (11)

H2S(aq)→H2S(g) (12) + 2- H2S(aq)+ 2O2→2H +SO4 (13)

2.3.1 Crystal structure of sphalerite The sphalerite crystallizes in the cubic crystal system and the zinc and sulfur atoms are tetrahedrally coordinated (Fig. 5). The structure is closely related to the structure of diamond. The hexagonal analog is known as the wurtzite structure (Skinner, B.J.,1961).

Fig. 5. The crystal structure of sphalerite (Skinner, B.J.,1961).

2.3.2 Effect of O2 on sphalerite oxidation

The sphalerite is oxidized by dissolved molecular O2 as shown in Eq. 14 (Seal and Hammarstrom, 2003; Malmström and Collin, 2004). Ziping et al. (2012) showed that oxidation mechanism of sphalerite by O2 varies with pH value.

2+ 2- ZnS + 2O2 →Zn + SO4 (14)

2.3.3 Effect of Fe3+ on sphalerite oxidation Dissolved Fe(III) ions oxidize sphalerite as shown in equations 15 and 16 (Fig. 6) (Seal and Hammarstrom, 2003; Malmström and Collin, 2004; Aydogan et al., 2005, Dutrizac et al., 2003; Santos et al., 2010). The oxidation rate of sphalerite increases with increasing ferric ion concentration, temperature, Fe content and decreasing pH value (Ziping et al., 2012; Crundwell, 1988). Holmes (2000) showed that sphalerite oxidation depends on its iron content, i.e., the more iron in the solution, the easier the mineral lattice opens up.

3+ 2+ 2+ 2- + ZnS + 8Fe + 4H2O →Zn + 8Fe + SO4 + 8H (15) ZnS + 2Fe3 + → Zn2 + + 2Fe2 + + S0 (16)

Fig. 6. Proposed mechanisms for oxidation of sphalerite by ferric ions.

2.3.4 Oxidation by microorganisms The microorganisms can accelerate sphalerite oxidation (Eqs. 17-19) (Boon et al., 1998; Fowler and Crundwell, 1998; Boon and Heijnen, 1993).

3+ 2+ 0 2+ ZnS +2Fe →Zn +S (ppt)+2Fe (17)

2+ + 3+ 2Fe +2H +0.5O2 2Fe +H2O (18) 퐵푎푐푡푒푟푖푎 0 2- + S +1.5O2+H2O �⎯⎯⎯⎯ ⎯S�O4 +2H (19) 퐵푎푐푡푒푟푖푎 �⎯⎯⎯⎯⎯�

2.3.5 Effect of pyrite and chalcopyrite on sphalerite oxidation Buehler and Gottschalk (1910) reported that when pyrite was mixed with a second sulphide mineral, the second mineral oxidized more rapidly. Pyrite or chalcopyrite addition increases zinc extractions (Harvey and Yen, 1998) due to galvanic interactions that occur between conducting minerals (Rao and Finch, 1988; Kelebek et al., 1996; Zhang et al., 1997; Ekmekçi and Demirel, 1997; Huang and Grano, 2005; Mehta and Murr, 1983; Abraitis et al., 2003; Akcil and Ciftci, 2003).

2.3.6 Effect of galena on sphalerite oxidation The addition of galena to the selective sphalerite leaching system decreased the dissolution of sphalerite (Harvey and Yen, 1998; da Silva et al. 2003) due to electrochemical galvanic interactions (Attia and El-Zeky, 1990b; Berry et al., 1978; Mehta and Murr, 1982).

2.4 Oxidation of galena Galena (PbS) is one of the most commonly observed and important sulphide minerals. This sulphide mineral often occurs with other similar minerals such as pyrite or sphalerite (Mernagh and Trudu, 1993). Table 2 shows the oxidation reactions of galena in different solution conditions.

Table 2. The oxidation of galena in different conditions Eqs. Reference Conditions + _ 1 PbS+2H2O+2xH →Pb1 xS+xPb(OH)2 Ahlberg and Broo, 1991; In alkaline solutions Buckley and Woods, 1984; Buckley and Woods, 1994 + _ 2+ 2 PbS+2xH →Pb1 xS+xPb Kartio et al., 1998, Acidic solution Richardson et al., 1993 + + _ 3 PbS+2H2O+2H →Pb(OH)2+S+2H Schuhmann, 1993; At pH 7 10 Peuporte and Schuhmann,1995 4 PbS→Pb2++S2- Nowak et al., 2000; A congruent Kim et al., 1995 dissolution_oxidation + - 5 2PbS+5H2O →PbS2O3+Pb(OH)2+8H +8e Paul et al., 1978 2- + - 6 2PbS+7H2O→2Pb(OH)2+S2O3 +10H +8e Lam-Thi et al., 1984 2- 2- + - 7 2PbS+2CO3 +3H2O→2PbCO3+S2O3 +6H +8e Gardner and Woods, 1979

2.4.1 Crystal structure of galena Galena belongs to the octahedral sulphide group of minerals with metal ions in octahedral positions, like pyrrhotite. Divalent lead (Pb) cations and sulfur (S) anions form a close packed cubic unit cell much like the mineral halite of the halide mineral group (Fig. 7) (Ramsdell, L.S., 1925).

Fig. 7. The crystal structure of galena (Ramsdell, L.S., 1925).

3+ 2.4.2 Effect of Fe on galena oxidation The oxidation of galena by ferric sulphate has been studied (Dutrizac and Chen, 1995; Vizsolyi et al., 1963) and it proceeds according to equation 20. 3+ 2+ + PbS + 2Fe +1.5O2 + H2O →PbSO4(ppt) + 2Fe + 2H (20)

2.4.3 Effect of grinding environment on galena oxidation Rey and Formanek (1960) showed that galena is more floatable when ground in a ceramic mill than ground in an iron mill. Iron medium lowers rest potential and will act as the anode and iron oxidation takes place (Adam et al., 1986). Peng et al. (2002) showed that iron (III) oxidation species play a dominant role in galena flotation. Iron content of grinding media and the amount of oxygen in the purging gases increased the amount of oxide, hydroxide and sulphate hydrophilic species on galena surface. Grinding with mild steel produces more oxidation products than grinding with 30 wt.% chromium medium (Peng, et al. 2003).

2.4.4 Effect of process water on galena oxidation Ikumapayi et al., (2012) showed that the flotation recovery of galena in process water is lower 2- than in deionised water. It could be due to the presence of a number of species including SO4 2- , SO3 (Houot and Duhamet, 1992), dissolved iron (Kant et al., 1994; Peng et al., 2003b) in the process water.

3 Experimental

3.1 Materials and reagents Crystalline pure sphalerite, galena, pyrite and chalcopyrite samples used in this study were procured from Gregory, Bottley & Lloyd Ltd., UK. Sphalerite sample contains 39.92% Zn, 20.7% S, 4.2% Fe, 1.32% Pb and 0.17% Cu; galena sample contains 73.69% Pb, 13.5% S, 1.38% Fe, 1.26% Zn, 0.2% Cu and some silica (quartz) impurity; pyrite sample contains 44.4% Fe, 50.9% S, and 0.2% Cu, and chalcopyrite sample 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. 8a), chalcopyrite (Fig. 8b), sphalerite (Fig. 8c) and galena (Fig. 8d). 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. The complex sulphide ores from Boliden-Renström mines was used in Bench-scale flotation tests. All chemicals used are technical grade. The flotation reagents being used at Boliden concentrator for treating complex sulphide ores have been abtained. Potassium amyl xanthate (KAX), Isobutyl xanthate (IBX) and Danaflot 871 (dialkyle dithiophoshate mercaptobenzothiazole) were used as collectors. Dowfroth 250 (polypropylene oxide methanol) and MIBC were used as frother. Dextrin, sodium hydrogen sulphite

(NaHSO3), and zinc sulphate (ZnSO4) were used as pyrite and sphalerite depressants respectively in flotation. Copper sulphate (CuSO4) was used as a source of copper ions for activation of sphalerite. Solutions of sodium hydroxide (AR grade) and HCl (1 M) and lime 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 ions (Zn2+, Pb2+, Cu2+, Fe2+ and Fe3+) on the formation of H2O2.

Fig. 8. XRD analysis of the mineral samples: (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).

3.2 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 pH 4.5 with HCl solution and was adjusted before grinding. Experiments were performed at room temperature of approximately 22.5°C.

3.3 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.

3.4 Hydrogen peroxide estimation The most common technique for the quantification of hydrogen peroxide is titration with potassium permanganate (Huckaba & Keyes, 1948; Group, 2003). This method can be used over a broad range of hydrogen peroxide concentrations, but it is time consuming and requires that the peroxide concentration not change significantly with time. Faster analyses can be made by attenuated total reflectance infrared spectroscopy (Voraberger et al. 2003; Yamaguchi et al. 1998), Raman spectroscopy (Vacque et al. 1997; Yamaguchi et al. 1998) and NMR spectroscopy (Stephenson & Bell, 2005). However, these techniques are limited to concentrations above about 30 mM for reflectance infrared and Raman spectroscopy and for NMR spectroscopy from 10-3 to 10 M. Spectrophotometry methods have been used for the measurement of H2O2 in oxidation processes. Table 3 shows a comparison of analytical methods for the quantification 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) 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, will oxidise the mineral surfaces.

Table 3. Comparison of analytical methods

Type Method H2O2 range Reaction mechanism and Performance assess- Reference detection conditions ment/Interference

Permanganate 0.25-70 Wt% Reduce permanganate to Not accurate at low Huckaba & manganese ion. concentration. Keyes, 1948; Group, 2003

Huckaba & Iodimetric 0.1- 6 Wt% Oxidize iodide to iodine, Not accurate at low titrated with thiosulfate concentration; subject Keyes, 1948; and starch. to interference

Titration Group, 2003

Ceric sulfate 1-13 % With ferroin indicator, Not accurate at low Huckaba & titrate to pale blue concentration. Keyes, 1948; Group, 2003

Copper-DMP 2-500 µM Reduction of Cu(II) and Stable color Baga et al., Method formation of copper-DMP 1988 complex; detection at 454 nm

Titanium 40- 12000 Formation of colored Some UV-absorbing Schick, Oxalate µM peroxotitanium complex; species turbidity, Strasser &

Spectrophotometry detection at 400 nm color; combined. Stabel, 1997

Reflectance >30 mM - More rapid analyses Voraberger et infrared al., 2003; spectroscopy Yamaguchi et

al., 1998

Raman >30 mM - More rapid analyses Vacque et al., spectroscopy 1997; Yamaguchi et Spectroscopy al., 1998

1H NMR from 10-3 to - More rapid analyses Stephenson & spectroscopy 10 M Bell, 2005

3.5 Pulp chemistry measurement The pulp chemistry, including pH, Eh (pulp potential), and dissolved oxygen (DO) of pulp were measured in the Renström complex ore grinding circuit. The Eh was measured at room temperature with platinum electrode and reference electrode and was expressed relative to the standard hydrogen electrode, SHE. The pH was measured using a glass electrode. A DO meter (SevenGo (Duo) proTM/OptiOXTM) was used to measuring of DO.

3.6 Gas purging To study the effect of the gaseous atmosphere in pulp liquid, sulphide minerals were 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, sample was added and the laboratory ball mill was again purged with N2 gas for a minimum of 30 min and finally sample was wet-ground for 1 h. The dissolved O2 concentrations in experiments purged for 1 h with N2 gas, did not exceed 0.2 ± 0.05 ppm. The concentration of H2O2 was measured after 60 min of grinding. For air atmosphere the laboratory ball mill was filled with sulphide minerals and 400 ml water and kept open the mill for 5 minutes to have the air and and then sulphide minerals was wet-ground for 1 h.

3.7 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 7.5 or 5 g of sample (100% ‒106 µm) was transferred to a cell of 150 ml capacity (Clausthal flotation equipment as shown in Fig. 9). Also after dry grinding for 60 min, the mill was emptied and the mineral was screened to free from grinding media. In each flotation 5 g of sample (100% ‒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 of 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.

Fig. 9. Image of clausthal flotation equipment.

3.8 Bench-scale flotation tests The flotation procedure is a standard optimized method at Boliden laboratory in the treatment of complex sulphide ores. For flotation tests, 1 kg of ore was wet ground with 0.6 liter of water in a stainless steel mill with 8 kg grinding medium followed by flotation in a WEMCO cell of 2.5 litre capacities (Fig. 10). The sequences of reagent additions were pH regulator, depressants, collectors, frother, and flotation. The dosages of depressants were;

1500 g/t ZnSO4, 300 g/t NaHSO3 and 200 g/t of dextrin. Copper and lead minerals were floated simultaneously followed by zinc mineral flotation. Dosages of collectors in a three stage sequential Cu-Pb mineral flotation were 30+20+10 g/t Danafloat and 10+5+0 g/t KAX. The conditioning times for pH regulator, depressants, and collectors were 5 min, 1 min and 2+1+1 min respectively. Dosages of zinc activator and collectors in a three stage flotation were 400 g/t CuSO4 and 40+20+20 g/t IBX. The conditioning times for zinc activator and collectors were 2 min, 1 min and 1+1+1 min, respectively. The frother dosage was 20 g/t Dowfroth. The pH was regulated to ~10.5 at copper-lead flotation and to ~12 at zinc flotation with powdered calcium oxide. Experiments were performed at room temperature of approximately 22 oC.

Fig. 10. Image of WEMCO flotation equipment.

3.9 Diffuse reflectance FTIR studies The Bruker FTIR spectroscopy model IFS 66 v/s was used for FTIR-DRIFT analysis. Solid fractions of grinding products were subjected to FTIR-DRIFT analysis which was conducted on air-dried samples. Diffuse reflectance infrared Fourier ransform (DRIFT) method was used in measurment with 2.8 wt% concentration of mineral sample in potassium bromide (KBr) matrix. Each spectrum was recorded after 256 scans.

3.10 Analysis of flotation products

For the analysis of flotation products, X-ray fluorescence (XRF) equipment of Boliden AB in Skelfteå was used. The samples were analyzed by PANalytical AXIOS XRF Spectrometer (Fig. 11). The samples were ground in a laboratory mill kind Herzog for 45 seconds Steel rings that shaped briquettes were filled with ground material and compressed using brikett hydraulic press with a strength of between 11 and 12 tonnes. Finished pellets were placed in the spectrometer for chemical analysis. The device was programmed by the computer with chosen references for the investigation of copper and zinc concentrates. Samples for analysis were compared with calibrated materials and each survey took approximately four minutes. For mineral grade calculation, the values of weight percentage of copper, zinc, lead, sulfur and iron were used.

Fig. 11. Image of XRF in Boliden Company.

4 Results and discussion

4.1 Formation of hydrogen peroxide (H2O2) after dry grinding and mixing with water Formation of hydrogen peroxide was observed when the freshly dry-ground solids were mixed in water. The effect of water pH in which the solids are mixed, on the formation of hydrogen peroxide is shown in Fig. 12. It can be seen that with an increase in pH, the minerals pyrite, chalcopyrite and sphalerite decreased the formation of H2O2 concentration up to pH 8 and then increased slightly above this pH, but in the case of galena, H2O2 was generated at pH

< 4.5 only. Also, pyrite generated more H2O2 than the other sulphide minerals. Kocabag et al. (1990) using the Eh-pH diagram (Fig. 13a) of pyrite showed that oxidation of pyrite yields S°, 2+ 2+ Fe , Fe(OH) species in the solution for pH < 6 and, Fe(OH)2 and Fe(OH)3 species in the solution for pH > 6. 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 (O2 ) (eq. 1), which reacts with ferrous iron to form H2O2 (eq. 2) as shown in Fig. 14a (Cohn et al., 2006). II III • ‒ Fe (pyrite) + O2 → Fe (pyrite) + (O2 ) (1) II • ‒ + III Fe (pyrite) + (O2 ) + 2H → Fe (pyrite) + H2O2 (2)

Fe(III) can also generate H2O2 by extracting an electron from water and thereby 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 + H2O → OH + H + Fe (3) • 2 OH → H2O2 (4) Fairthorne at al. (1997) using the Eh–pH stability diagram of chalcopyrite (Fig. 13b) 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 and 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. Our 2+ present results shown in Table 3 also demonstrate that Fe ions generate substantial H2O2 followed by Fe3+ and Cu2+ ions. Fig. 13b indicates 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. 14b, where at acidic pH (pH < 5) superoxide anion (O2 )

(eq. 5) forms, which reacts with ferrous iron to form H2O2 (eqs. 6-8). This is in agreement with other studies where it was observed that ferrous ion generates the superoxide and hydroxyl radical (Valko et al., 2005; Jones et al., 2011). 2+ 3+ • − O2 +Fe → Fe + (O2 ) (5) • − 2+ 3+ (O2 ) + Fe + 2H+→ Fe +H2O2 (6) 2+ 3+ • − Fe + H2O2→ Fe + OH + OH (7) • 2 OH → H2O2 (8)

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. 14b indicates that copper ions also generate hydrogen peroxide. The metal ion induced formation of hydroxyl free radical has been significantly evidenced where copper ions generate the superoxide and hydroxyl radical (eqs. 9-12) (Valko et al., 2005). − •– O2 + e → O2 (9) •– + 2O2 + 2H → H2O2 + O2 (10) + 2+ • – Cu + H2O2 → Cu + OH + OH (11) • 2 OH → H2O2 (12) The results at acidic pH show that the greater amounts of hydrogen peroxide formation than at alkaline pH due to the presence of copper and ferrous ions in acidic pH which are capable of generating hydrogen peroxide (Table 3). Huai Su (1981) presented the Eh-pH diagram of the sphalerite that showed that at pH < 2+ 2+ 2+ 2+ 6, Zn and Fe are formed (Fig. 13c). Table 3 shows that Zn and Fe ions generate H2O2. • − In the presence of dissolved molecular oxygen, ferrous iron can form superoxide anion (O2 ) 2+ (eq. 5), which 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. 14c. • ‒ + 2(O2 ) +2H → H2O2+ O2 (13) Woods et al. (1981), using the Eh-pH diagram (Fig. 13d) of the galena, showed that at pH < 4, Pb2+ ions exist. Fig. 14d shows that in the presence of dissolved molecular oxygen • − + and at acidic pH (pH < 4), lead ions can form superoxide anion (O2 ) which reacts with H to 2+ form H2O2 (Eqs. 14-17). It can be seen from Table 4 that Pb ions generate H2O2 (Blokhina et al., 2001; Ni et al. 2004). This is in agreement with other studies where significant evidence for Pb2+ ions generating superoxide and hydroxyl radical has been presented (Ahlberg and Broo, 1996a; 1996b; 1996c). − • − O2 + e → (O2 ) (14)

• − + 2(O2 ) + 2H → H2O2+O2 (15) 2+ 0 − • PbS + H2O2 → Pb +S + 2e + 2 OH (16) • 2 OH → H2O2 (17) For investigating the effect of mineral loading, metal ions (Zn2+, Cu2+, Pb2+, Fe2+ and 3+ Fe ) were mixed with 50 ml water at pH 4.5. The concentration of H2O2 increased with increased loading (Fig. 15). Fig. 16 shows that the concentration of H2O2 increased with increasing mixing time, most likely due to increased interaction of minerals with water.

Table 4. 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 0.000 0.000 Fe 2+ 0.552 4.656 Fe 3+ 0.004 0.059 Cu 2+ 0.000 0.015 Pb 2+ 0.013 0.096 Zn 2+ 0.004 0.088

pyrite 1.6 chalcopyrite sphalerite 1.2 galena (mM)

2 O 2

H 0.8

0.4

0 2.5 4.5 8 10.5 11.5 pH

Fig. 12. Effect of pH on production of H2O2 after 5 min mixing of dry ground solids with water.

a) b)

d) c)

Fig. 13. a) Eh–pH stability diagram for the Cu–Fe–S–H2O system with the preponderant copper and iron 0 species shown in each domain (Fairthorne, 1997) b) Eh–pH diagram for FeS2–H2O system at 25 C and for -5 10 M dissolved species (Kocabag et al., 1990). c) Eh–pH stability diagram for the sphalerite–H2O system -3 (Huai Su, 1981) d) Eh–pH diagram for the Pb–S–H2O system where [Pb] =10 M (Woods et al.,1981).

Fig. 14. Proposed mechanisms for formation of H2O2 by a) chalcopyrite b) pyrite c) sphalerite d) galena. The numbers in this figure represent the equation numbers in the text.

1.5

1.2 (mM)

Ga 2 0.9 O 2 Sp H 0.6 Cp Py 0.3

0 50 100 250 Mineral loading (g/l)

Fig. 15. Effect of solids loading on H2O2 production after 5 min mixing with water at pH 4.5.

1.2 1 0.8 Ga (mM)

2 Sp 0.6 O

2 Cp

H 0.4 Py 0.2 0 0.5 5 11 Time of mixing (min)

Fig. 16. Effect of mixing time on H2O2 concentration at pH 4.5.

4.2 Formation of hydrogen peroxide (H2O2) by metal sulphides in wet grinding Formation of hydrogen peroxide was observed in pulp liquid when sulphide minerals were wet-ground as shown in Fig. 17. The effect of pH on the formation of hydrogen peroxide is shown in Fig. 17. In pyrite and chalcopyrite, 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. In sphalerite, it can be seen that pH 2.7 generated more H2O2 than at pHs 3.7 and 6.8. In galena, it can be seen that at pH 4 and 7.8 hardly any H2O2 is generated but at pH 2.7, H2O2 formed.

1.2 pyrite chalcopyrite 0.4 0.8 (mM) (mM)

2 2 O O 2 2 0.2 H 0.4 H

0 0 3.67 4.3 6 7 8 9.06 10.5 3.5 5.7 8 10.5 11.3 pH pH 0.6 0.4 sphalerite galena

0.4 (mM (mM)

2 0.2 O O 2 2 H H 0.2

0 0 2.7 3.7 6.8 2.7 4 7.8 pH pH

Fig. 17. Effect of pH on production of H2O2 after 60 min wet grinding of sulphide minerals.

4.2.1 The effect of grinding medium type on formation of hydrogen peroxide The effect of grinding medium type on formation of hydrogen peroxide with two grinding media was investigated. Fig. 18a and b show that mild steel produced a higher concentration of H2O2 than stainless steel medium in pyrite at pH 4.5 and galena at pH 2.5. Fig. 19 shows 2+ 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 stainless steel 2+ medium. This is in accordance with our results that Fe ions generate H2O2 as shown in Table 3. 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).

2.5 a) 0.2 b) 2

0.15 1.5 (mM) (mM)

2 0.1 2 O O 2 2 1 H H 0.05 0.5

0 0 Stainless Steel Mild Steel Stainless Steel mild Steel

Fig. 18. H2O2 concentration after dry grinding in different media by a) pyrite at pH 4.5 b) galena at pH 2.5.

Fig. 19. Illustration of H2O2 formation by pyrite (up) and galena (down) in contant with mild steel and stainless steel.

4.3 Effect of minerals mixture on formation of H2O2 Fig. 20 shows the effect of pyrite–chalcopyrite, pyrite–sphalerite, pyrite–galena, chalcopyrite– sphalerite, chalcopyrite–galena and sphalerite–galena mixture on formation of hydrogen peroxide at pH 4.5. Fig. 20a 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. 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 the increasing sphalerite percent in the mixture, the concentration of H2O2 decreased as shown in Fig. 20b. Fig. 20c shows the effect of galena or sphalerite proportion in sphalerite–galena 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. Fig. 20d illustrates the effect of pyrite or galena percent in galena–pyrite mixture on formation of hydrogen peroxide. It can be seen that with an increase in pyrite percent, the concentration of H2O2 increases but on increasing galena percent, the concentration of H2O2 decreases. shows the effect of chalcopyrite or galena percent in galena–chalcopyrite mixture on formation of hydrogen peroxide that with increasing chalcopyrite percent, the concentration of H2O2 increases. However, increasing galena percent in the mixture, the production of H2O2 decreases as shown in Fig. 20e. Fig. 20f demonstrates the effect of pyrite or chalcopyrite percent in a 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.

1.6 1 a) b) 1.2 0.8 0.6 (mM) (mM)

2 0.8 2 O O 2 2 0.4 H H 0.4 0.2

0 0 0 20 40 60 80 100 0 20 40 60 80 100 percent of Py percent of Cp 0.3 1.6 c) d) 1.2 0.2 (mM)

(mM)

2 0.8

2 O 2 O 2 H H 0.1 0.4

0 0 0 20 40 60 80 100 0 20 40 60 80 100 percent of Sp percent of Py 1 1.6 e) f ) 0.8 1.2 0.6 (mM) (mM)

2 0.8 2 O O 2 2 0.4 H H 0.4 0.2

0 0 0 20 40 60 80 100 0 20 40 60 80 100 percent of Cp percent of Py

Fig. 20. Effect of (a) Py percent in Py-Sp mixture (b) Cp percent in Cp-Sp mixture (c) Sp percent in Ga-Sp mixture (d) Py percent in Py-Ga mixture (e) Cp percent in Cp-Ga mixture (f ) Py percent in Cp-Py mixture on H2O2 formation with total 12.5 g sample after 5 min conditioning in water at pH 4.5

Fig. 21 shows the effect of single or mixtures of minerals on the 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 greater amounts of H2O2 when pyrite was mixed with a second sulphide mineral may explain the effect of interaction between pyrite and the 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; 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; Banfield, 1997).

1.8

1.5

1.2 (mM)

0.9 2 O 2 H 0.6

0.3

0 Py Cp -Py Sp-Py Ga -Py Cp Sp-Cp Ga -Cp Sp Ga-Sp Ga

Single or Mixture of minerals

Fig. 21. Effect of single minerals and their mixtures on H2O2 formation after 5 min conditioning in water at pH 4.5.

4.4 Effect of gaseous atmosphere To study the effect of gaseous atmosphere in pulp liquid, sulphide minerals were wet-ground in a laboratory ball mill with stainless steel medium in either air or nitrogen atmosphere.

Sulphide minerals in N2 atmosphere also generated H2O2 as Borda et al. (2003) showed that pyrite generates H2O2 in the absence of molecular oxygen. Fig. 22a shows that pyrite generated more H2O2 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 diagram (Fig. 2+ 13) shows N2 atmosphere generated more Fe which is responsible for H2O2 generation. Figs.

22b and 22c show that chalcopyrite and galena generated more H2O2 in a air atmosphere than in N2.

2 a)

1.6

1.2 mM

2 O 2 0.8 H

0.4

0 air purging N2 purging without purging

0.07 b) 0.4 c) 0.06 0.35 0.3 0.05

0.25 0.04 (mM)

(mM) 2 0.2

2 O

2 0.03 O 2 H 0.15 0.02 H 0.1 0.01 0.05 0 0 N2 Air N2 air

Fig. 22. (a) The effect of gaseous atmosphere on formation of H2O2 by pyrite at natural pH 3.7 (b) by chalcopyrite at natural pH (c) by galena in pH 2.5.

4.5 Flotation studies

4.5.1 Pyrite flotation The effect of grinding media on pyrite flotation is shown in Fig. 23. It can be seen that for all pH values studied the stainless steel medium produced a higher pyrite recovery than mild steel medium. 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).

70 Stainless Steel 60 Mild steel 50 40 30

Recovery of pyrite (%) pyrite of Recovery 20 10 0 4.5 10.5 11.5 pH

Fig. 23. Effect of pH on flotation recovery of pyrite with air atmosphere during the flotation.

4.5.2 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. 24. 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. 25 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. 12).

100

20 Recovry of chalcopyrite % chalcopyrite of Recovry 0 0 0.03 0.3 3 30 H2O2 (mM)

Fig. 24. Effect of H2O2 on recovery of chalcopyrite with dry-ground solids at pH 10.5.

100

20 Recovery of chalcopyrite % chalcopyrite of Recovery 0 3.5 5.7 7 9 10 11.3 Initial pH for grinding

Fig. 25. Effect initial pH of grinding on recovery of chalcopyrite at flotation pH 10.5.

4.5.3 Galena flotation

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. 26. 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. 18).

– [Pb (EX) 2] ads + H2O2→Pb (OH) 2 + (EX) 2+ 2e (18)

100 pH of grinding 7.8 pH of grinding 2.5

40 Recovery galena% of Recovery 20

0 3 4 6.3 9 pH

Fig. 26. Effect of pH on flotation recovery of galena with a pulp ground at pH 2.5 and 7.8.

4.5.4 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. 27. It can be seen that H2O2 increased the recovery of sphalerite when its concentration is < 1 mM. 80

40 Recovery(%) 20

0 0 0.01 0.1 1 10 Concentration of H2O2 (mM)

Fig. 27. Effect of H2O2 on recovery of chalcopyrite with dry-ground solids at pH 10.5.

4.5.5 Bench-scale flotation studies

4.5.5.1 Effect of depressant addition during grinding Fig. 28 and Table 5 show the reagent conditions of the flotation tests. Fig. 29 shows the effect of depressant addition during grinding, independently and in combination, on the amount of

H2O2 formation and the quantity of dissolved oxygen in the pulp liquid before and after grinding. Dissolved oxygen in the pulp liquid decreased after grinding in all the cases of depressant addition, either dextrin (test F7) or ZnSO4 (test F4) independently or in combination of dextrin and ZnSO4 (test F18) or all the three depressants including NaHSO3 together. In general, the decrease in dissolved oxygen after grinding is related to its consumption in the formation of H2O2. The decrease in the formation of H2O2 with the addition of depressants during grinding could also be correlated to a decrease in pulp pH (Fig. 3b) since a decrease in pulp pH value, the formation of hydrogen peroxide is shown to decrease (Ikumapayi, et al., 2012). The amount of H2O2 generated is seen to be at the same level in the presence of dextrin whether it is added in the grinding stage (test F7) or in the pulp after grinding (test F1'). The variation in H2O2 amount is seen mostly due to ZnSO4 and

NaHSO3 addition during the grinding.

Fig. 28. Schematic diagram of flotation test procedure.

Table 5. Addition of collector and depressants to mill. Test no. Reagents used in the grinding F1' Reference (normal flotation test where the reagents are added to the ground sulphide pulp) F2 1500 g/t (ZnSO4) + 300 g/t (NaHSO3) + 200 g/t (Dextrin)

F18 1500 g/t (ZnSO4) + 200 g/t (Dextrin) + pH 6.0 (H2SO4)

F4 1500 g/t (ZnSO4) F7 200 g/t (Dextrin) F3 60 g/t ( Danafloat) + 10 g/t ( KAX)

F6 60 g/t ( Danafloat) + 15 g/t ( KAX) + 1500 g/t (ZnSO4) + 300 g/t (NaHSO3) + 200 g/t (Dextrin)

10 0.8

8 0.6

(ppm)

6 2

0.4 (mM) 2 O 4 2 H

Disolved O O2 (before grinding) 0.2 2 O2 (after grinding) H2O2 (mM) 0 0 F7 F4 F18 F2 F1' Test no. as shown in Table 1

Fig. 29. Effect of addition of depressants on H2O2 concentration and dissolved oxygen.

Fig. 30a shows the effect of depressant addition during grinding on Eh of the pulp liquid before and after grinding. The tests clearly show that the addition of depressants during grinding increased the Eh value since the pH is seen to decrease (Fig. 30b). The increase in Eh value is relative to the reference value when the depressants are added during flotation after grinding. However, the difference between Eh before and after grinding when all depressants added to mill (test F2) is more than when all depressants added after grinding (test F1'). This is due to more generation of H2O2 in F1' than F2 (Fig. 29). The effect of depressant addition during grinding on pH of the pulp liquid before and after grinding exhibiting an increase in pH value after grinding as shown in Fig. 30b. Since the measurements of the pulp liquid before grinding is recorded immediately after water addition, hardly any H2O2 formation is expected and the changes in Eh and pH values suggest that the H2O2 generated during grinding interact with surface species.

500 12 a) b) 10 400

8 300

6 pH 200

Eh (mV, SHE) 4 Eh (before grinding) pH (before grinding) 100 Eh (after grinding) 2 pH (after grinding)

0 0 F7 F4 F18 F2 F1' F7 F4 F18 F2 F1' Test no. as shown in Table 1 Test no. as shown in Table 1

Fig. 30. Effect of Addition of depressants on a) pH and b) Eh before and after grinding.

Table 6 shows the chemical analysis of all flotation tests products that are discussed in this paper. Fig. 31(a,b) displays that the Cu-Pb concentrate with addition of depressant independently in test F4 (ZnSO4) or in combination in tests F18 (ZnSO4+Dextrin) and F2

(ZnSO4+Dextrin+NaHSO3) during grinding, the grade of Cu and Pb decreased due to a decrease in H2O2 formation in the pulp liquid as shown Fig. 29. The H2O2 is a strong depressant for pyrite (Hu, et al., 2009) and a decrease in its amount may decrease selectivity between metal sulphides and ferrous sulphide in flotation. Fig. 31a shows that the addition of dextrin (test F7) during grinding has produced high Cu and Pb grade and recovery than when it added after grinding (test F1') since F7 test conditions generated more H2O2 than test F1'

(Fig. 29). Fig. 31c shows that addition of depressant in tests F2, F4 and F18 during grinding the recovery of Fe (pyrite) increased due to a decrease in H2O2 formation in the pulp liquid as shown Fig. 29 but in test F7, the recovery of Fe (pyrite) decreased due to an increase in H2O2 formation in the pulp liquid. It is thus clear that the generated H2O2 is functioning as a strong depressant for pyrite.

a) 16 b)

12 12 F4 F7

F7 F2 8 8 F1' F1'

F2 F18 Concentrategrade (%)

4 Concentrategrade (%) 4 F18 F4

0 0 20 40 60 80 100 20 40 60 80 100 Cumulative recovery (%) Cumulative recovery (%)

100% c) 80%

60%

40%

wt % 20% 0% F18 F7 F4 F2 F1' Feed Gangue 58.71 52.77 60.18 54.46 56.04 80.79 FeAsS 0.13 0.15 0.13 0.16 0.14 0.11 FeS2 9.78 8.33 8.70 9.81 8.27 9.78 PbS 2.17 2.52 2.09 2.50 2.44 0.60 (Zn,Fe)S 15.72 18.92 15.46 17.40 17.48 5.28 CuFeS2 13.50 17.31 13.43 15.67 15.62 3.43

Fig. 31. Effect of depressant addition during grinding on recovery and grade of (a) Cu in Cu-Pb conc. (b) Pb in Cu-Pb conc. (c) mineral recovery in Cu-Pb concentrate.

Table 6. Analysis of flotation product.

Seq. Sample Cu Zn Pb MgO Fe S SiO2 As Mn Na2O Al2O3 K2O CaO name Cu Zn Pb Mg Fe S Si As Mn Na Al K Ca (g/t) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) (%) Zn3 Concentrate 18 0.1 0.33 0.14 7.43 23.09 16.85 31.43 0.08 0.09 0.23 5.19 0.52 2.94 Zn2 Concentrate 0.12 0.89 0.15 6.59 29.08 21.7 24.54 0.12 0.07 0.21 4.18 0.37 2.21 Zn1 Concentrate 0.22 20.1 0.37 4.51 23.76 22.35 13.78 0.19 0.06 0.17 2.66 0.21 1.59 CuPb3 Concentrate 0.62 6.93 0.49 8.56 13.33 10.34 35.34 0.06 0.11 0.16 5.72 0.62 3.34 CuPb2 Concentrate 3.36 13.6 1.69 6.66 16.76 16.79 22.91 0.07 0.08 0.09 4.01 0.37 2.25 CuPb1 Concentrate 10.4 8.87 3.55 4.58 19.67 19.59 18.04 0.05 0.06 0.08 2.8 0.27 1.66 tailing 0.03 0.13 0.04 7.65 4.707 0.844 51.65 0.02 0.13 0.26 7.6 0.98 5.64 Zn3 Concentrate 7 0.09 0.3 0.12 6.79 28.65 21.82 25.51 0.1 0.08 0.22 4.58 0.41 2.44 Zn2 Concentrate 0.13 1.38 0.18 6.88 30.69 21.94 22.72 0.17 0.07 0.22 4.23 0.36 2.23 Zn1 Concentrate 0.27 22.6 0.47 5.07 21.26 19.83 14.15 0.2 0.07 0.12 2.96 0.24 1.95 CuPb3 Concentrate 0.59 8.29 0.51 9.42 13.99 11.18 33.36 0.07 0.1 0.16 5.66 0.61 3.36 CuPb2 Concentrate 4.35 17.5 2.26 6.47 17.59 18.93 17.28 0.08 0.07 0.03 2.92 0.28 1.75 CuPb1 Concentrate 16.4 9.29 4.64 3.06 22.68 23.72 9.09 0.05 0.04 0.04 1.33 0.13 0.92 tailing 0.03 0.14 0.04 7.71 5.51 1.651 51.29 0.02 0.13 0.27 7.69 0.96 5.54 Zn3 Concentrate 6 0.14 0.66 0.22 6.87 33.33 23.75 19.94 0.13 0.07 0.19 3.81 0.31 2.02 Zn2 Concentrate 0.25 5.97 0.87 5.84 35.52 23.82 13.45 0.27 0.05 0.19 2.58 0.19 1.52 Zn1 Concentrate 0.3 42.2 2.75 1.9 13.7 24.63 4.373 0.13 0.05 0.16 0.98 0.09 0.84 CuPb3 Concentrate 1.13 8.8 1.57 9.58 14.17 12.01 31.12 0.08 0.1 0.11 5.14 0.54 2.93 CuPb2 Concentrate 5.87 14.3 3.42 6.92 17.86 18.58 17.57 0.08 0.06 0 2.59 0.26 1.58 CuPb1 Concentrate 19.1 10.5 2.52 2.05 24.31 26.52 6.059 0.04 0.03 0.01 0.86 0.08 0.59 tailing 0.04 0.15 0.05 7.81 5.925 2.003 49.27 0.02 0.13 0.28 7.59 0.95 5.86 Zn3 Concentrate 4 0.09 0.32 0.15 6.93 27.14 20.57 26.56 0.09 0.08 0.21 4.51 0.44 2.52 Zn2 Concentrate 0.12 1.01 0.19 6.5 32.58 23.16 20.29 0.16 0.06 0.2 3.65 0.3 1.86 Zn1 Concentrate 0.26 15 0.51 6.53 22.93 19.21 18.72 0.23 0.07 0.16 3.56 0.31 2.08 CuPb3 Concentrate 0.51 6.15 0.56 9.19 12.69 9.518 37.23 0.06 0.11 0.18 5.99 0.65 3.5 CuPb2 Concentrate 3.06 12.9 1.73 6.54 15.77 15.38 24.83 0.07 0.09 0.11 4.34 0.41 2.54 CuPb1 Concentrate 12.7 9.06 3.64 3.8 20.73 20.82 14.68 0.05 0.05 0.08 2.5 0.23 1.53 tailing 0.03 0.14 0.04 7.7 5.043 1.178 50.92 0.02 0.13 0.26 7.62 0.98 5.63 Zn3 Concentrate 3 0.1 0.4 0.17 7.15 28.95 21.47 24.75 0.11 0.08 0.19 4.24 0.4 2.38 Zn2 Concentrate 0.14 1.93 0.29 7.25 30.15 21.22 22.47 0.19 0.07 0.2 3.86 0.33 2.07 Zn1 Concentrate 0.31 30.1 1.05 4.43 18.43 21.33 10.33 0.18 0.06 0.07 1.97 0.17 1.37 CuPb3 Concentrate 0.6 8.99 1.21 8.43 14.4 11.69 31.41 0.08 0.1 0.13 5.24 0.53 3.16 CuPb2 Concentrate 3.31 16.7 5.35 5.21 17.61 18.46 16.47 0.09 0.07 0.01 2.91 0.26 1.89 CuPb1 Concentrate 16.7 9.43 1.82 3.03 22.87 23.73 10.4 0.03 0.04 0.04 1.66 0.15 1.07 tailing 0.03 0.15 0.05 7.6 5.716 1.946 50.05 0.02 0.13 0.25 7.36 0.96 5.91 Zn3 Concentrate 2 0.11 0.44 0.15 6.71 29.95 21.77 23.33 0.1 0.09 0.21 4.2 0.38 2.38 Zn2 Concentrate 0.15 2.22 0.18 5.71 35.2 24.28 16.55 0.17 0.06 0.19 3.33 0.25 1.65 Zn1 Concentrate 0.29 26.3 0.46 3.74 23 22.8 8.94 0.23 0.06 0.16 2.08 0.16 1.36 CuPb3 Concentrate 0.97 9.72 0.7 9.06 15.16 12.37 29.55 0.08 0.1 0.1 5.13 0.55 2.81 CuPb2 Concentrate 4.04 12.5 1.98 6.96 17.84 17.06 21.74 0.08 0.07 0.07 3.63 0.36 2.02 CuPb1 Concentrate 12.8 9.75 4.22 3.51 21.74 22.01 12.76 0.06 0.05 0.05 1.98 0.19 1.23 tailing 0.04 0.15 0.04 7.66 4.931 1.101 51.3 0.02 0.13 0.27 7.6 0.99 5.9 Zn3 Concentrate 1` 0.09 0.32 0.14 6.8 26.95 20.29 26.72 0.09 0.08 0.21 4.51 0.42 2.74 Zn2 Concentrate 0.15 1.49 0.22 6.68 32.51 23.3 20.16 0.18 0.07 0.19 3.68 0.3 2.02 Zn1 Concentrate 0.26 21.5 0.52 5.36 20.89 19.63 15.14 0.21 0.07 0.09 2.9 0.25 1.98 CuPb3 Concentrate 0.66 9.21 0.71 8.75 13.71 11.35 32.67 0.07 0.1 0.13 5.48 0.57 3.24 CuPb2 Concentrate 3.59 13.5 2.03 6.32 16.6 15.86 21.92 0.07 0.08 0.05 3.85 0.37 2.33 CuPb1 Concentrate 14.8 8.77 4.18 3.29 22.37 22.58 11.81 0.05 0.05 0.06 1.88 0.17 1.24 tailing 0.04 0.14 0.04 7.77 5.191 1.34 50.61 0.02 0.13 0.27 7.66 0.96 6.17

Fig. 32a shows the results of Zn concentrate with depressant addition during grinding. It can be seen that addition of ZnSO4 (test F4) during grinding decreased the Zn grade and recovery relative to when all the depressants are added after grinding (test F1') due to decreasing H2O2 formation in pulp liquid as shown in Fig. 29. Fig. 32a also shows that the addition of all the depressants (test F2) during grinding increased the Zn grade and recovery relative to when all the depressants added after grinding. Fig. 32b shows addition of depressant during grinding

increased the pyrite grade in Zn concentrate due to decreasing H2O2 formation as shown in

Fig. 29 and it is clear that H2O2 is a strong depressant for pyrite.

100% 30 a) b) 80% F2 F7 60% F1' 20 F18 wt % 40% F4 20%

0% F18 F7 F4 F2 F1' Feed Concentrategrade (%) 10

Gangue 59.56 56.31 57.34 52.17 56.78 80.79 FeAsS 0.26 0.31 0.32 0.34 0.32 0.11 FeS2 32.91 35.03 35.08 36.45 34.38 9.78 PbS 0.21 0.24 0.29 0.26 0.28 0.60 0 0 10 20 30 40 (Zn,Fe)S 6.68 7.72 6.56 10.31 7.84 5.28 Cumulative recovery (%) CuFeS2 0.38 0.39 0.40 0.48 0.41 3.43

Fig. 32. Effect of depressant addition on Zn concentrate: (a) Zn grade vs. recovery (b) mineral recovery in Zn concentrate.

The Cu-Pb concentrate results in Fig. 33a reveal that the addition of depressants during grinding (tests F2, F4 and F18 except test F7), flotation selectivity of Cu-Pb minerals against

Fe (pyrite) decreased due to decreasing of H2O2 formation in these tests conditions while the

F7 test conditions increases the generation of H2O2 as shown in Fig. 29. This suggests that the

H2O2 depresses pyrite (Hu, et al., 2009). Fig. 33b demonstrates the same test results of Fig. 33a but the selectivity of Cu-Pb concentrate against Zn. The results demonstrate that the addition of depressants during grinding have a better depression of Zn mineral compared to the reference test where the depressants are added only during the flotation stage. Adding depressant during grinding where freshly broken mineral surfaces are produced, the depressant effect is seen to be significant. Among all the depressants, dextrin is more effective when added during grinding. Yelloji Rao and Natarajan (1988) observed that adding reagents to the mill minimized or often eliminated the effects of the galvanic interaction.

F2 a) b) 80 F18 80

F4 F1' 60 60 F7

40 40

20 20 Cumulative Fe recovery(%) Cumulative Zn recovery(%)

0 0 50 60 70 80 90 100 50 60 70 80 90 100 Cumulative Cu recovery (%) Cumulative Cu recovery (%)

Fig. 33. Effect of depressant addition during grinding on Cu-Pb concentrate: (a) selectivity against pyrite, (b) selectivity against Zn.

The Zn concentrate results in Fig. 34 demonstrates that when all the three depressants (F2) added together during grinding produced high Zn recovery with lower Fe content compared to reference test where the depressants were added during the flotation stage. The effect of ZnSO4 depressant addition during grinding is found to be less effective relative to the normal reference flotation test.

F7 50 F2 F1' 40 F18 F4

10 Cumulative Zn recovery(%)

0 0 20 40 60 Cumulative Fe recovery (%)

Fig. 34. Effect of depressant addition during grinding on Zn concentrate and its selectivity against pyrite.

4.5.5.2 Effect of collector addition during grinding

The effect of both collector and depressant addition during grinding on formation of H2O2 and dissolved oxygen in the pulp liquid as shown in Fig. 35. Test F6 (collector and depressant) results show that more production of H2O2 and lower dissolved oxygen content compared to

the reference flotation test F1'. The lower dissolved oxygen could be due to the oxygen consumption in the generation of hydrogen peroxide as described above in Part 3.1.1.

10 1

8 0.8

(ppm) 6 0.6

2 (mM) 2 O

4 0.4 2 H disolved O disolved

2 O2 (before grinding) 0.2 O2 (after grinding) H2O2 (mM) 0 0 F3 F6 F1' Test no. as shown in Table 1

Fig. 35. Effect of collector and depressant addition on H2O2 concentration and dissolved oxygen in the pulp liquid.

The effect of collector and depressant addition during grinding on Eh of pulp liquid before and after grinding as shown in Fig. 36a. The Eh value of the pulp liquid in test F6 is higher than the reference test (F1') corresponding to higher H2O2 formation (Fig. 35). The lower pH value of pulp liquid in test F6 (Fig. 36b) correlates to the test conditions of higher

H2O2 amount.

300 12 a) b) 250 10

200 8

150 pH 6

Eh (mV, EhSHE) (mV, 100 Eh (before grinding) 4 pH (before grinding) Eh (after grinding) pH (after grinding) 50 2

0 0 F3 F6 F1' F3 F6 F1' Test no. as shown in Table 1 Test no. as shown in Table 1

Fig. 36. Effect of collector and depressant addition on a) Eh and b) pH in pulp liquid before and after grinding.

Fig. 37a shows that Cu grade and recovery in Cu-Pb concentrate improved significantly with the collector (test F3) and in combination with depressant addition (test F6) during grinding. Higher Cu grade and recovery was experienced when depressant and collector were added together (test F6) compared to only collector addition (test F3) correspond to higher

H2O2 formation in pulp liquid (Fig. 35). Fig. 37b shows that the Pb grade and recovery in Cu-

Pb concentrate decreased in tests F3 and F6 due to increasing H2O2 amounts in pulp liquid

(Fig. 35) and that H2O2 depresses galena flotation (Hu, et al., 2009). Fig. 37c shows that the addition of collector and depressant during grinding on pyrite depression is considerable.

20 20 a) b) F1'

F3 15 15 F6

10 10

5 5 Concentrategrade (%)

Concentrategrade (%)

0 0 10 20 30 40 50 60 70 80 90 100 10 30 50 70 90 110 cumulative recovery (%) cumulative recovery (%) 100% C) 80% 60%

wt % 40% 20% 0% F6 F3 F1' Feed Gangue 46.94 51.42 56.04 80.79 FeAsS 0.14 0.15 0.14 0.11 FeS2 7.19 8.23 8.27 9.78 PbS 2.73 2.91 2.44 0.60 (Zn,Fe)S 17.39 18.05 17.48 5.28 CuFeS2 25.60 19.24 15.62 3.43

Fig. 37. Effect of collector and depressant addition during grinding on recovery and grade of (a) Cu in Cu- Pb conc. (b) Pb in Cu-Pb conc. (c) minerals in Cu-Pb concentrate.

The results of Zn concentrate that addition of collector (test F3) alone and collector and depressant (test F6) together significantly increased Zn grade and recovery relative to when collector added in flotation stage as shown in Fig. 38a. Better results in test F6 than test F3 correlate to higher H2O2 formation (Fig. 35) in pulp liquid. H2O2 increases Zn recovery by effectively depressing pyrite. This has been reported by Javadi & Hanumantha Rao (2013;

2013a). Fig. 38b shows that addition of collector and depressant (test F6) decreased flotation recovery of Fe due to increasing H2O2 amount (Fig. 35).

100% 50 a) b) 80% F6 40 F3 60% F1' 40% 30 wt % 20%

Fig. 38. Effect of collector and depressant addition on Zn concentrate: (a) grade and recovery of Zn (b) Fe recovery.

Fig. 39 shows the results of Cu-Pb concentrate where the collector (test F3) and collector and depressant together (test F6) are added during grinding. It can be seen that the Fe content (Fig. 39a) and Zn content (Fig. 39b) decreased in the Cu-Pb concentrate illustrating a better depression of pyrite and sphalerite. The results of Zn concentrate and the addition of reagents during grinding increased the Zn recovery while maintaining pyrite depression at the same level as shown in Fig. 40.

a) F1' b) 80 F3 80

F6 60 60

40 40

20 20 cumulative Fe recovery(%) cumulative Zn recovery(%)

0 0 50 60 70 80 90 100 50 60 70 80 90 100 cumulative Cu recovery (%) cumulative Cu recovery (%)

Fig. 39. Effect of collector (test F3) and collector and depressant (test F6) addition during grinding on Cu- Pb concentrate: (a) selectivity against pyrite (b) selectivity against Zn.

100

60 F6

F3 40 F1'

20 cumulative Zn recovery(%) 0 0 20 40 60 cumulative Fe recovery (%)

Fig. 40. Effect of collector (test F3) and depressant and collector (test F6) addition during grinding on Zn concentrate and its selectivity against pyrite.

4.6 Diffuse reflectance FTIR studies

Solid fractions of grinding products were subjected to FTIR-DRIFT analysis which were conducted on dried samples. Fig. 41 shows that the spectrum with the addition of depressant (test F2) during grinding has the lowest intensity of absorption peaks while the addition of depressant and collector (test F6) together has the highest intensity peaks. Increasing of intensities in test F6 samples corresponds to increasing level of H2O2 formation in the pulp liquid. The bands at 666, 1160, 1453 cm-1 are S–O vibrations in sulphate, and the band at 985 cm-1 can be assigned to S=O stretching in sulphite and sulphate (Smith, 1999; Socrates, 2001).

985.00 778.01 665.69 3567.92 1452.80 1160.26 1.2 1.0 0.8 0.6 Kubelka Munk 0.4 0.2

3500 3000 2500 2000 1500 1000 Wavenumber cm-1

Fig. 41. DRIFT spectra of corresponding ore solid residue separated from solution after grinding for 60 minutes in tap water (blue line : F6 ; green line: F1'; pink line: F2).

5 Conclusion In the scope of this thesis, 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 minerals 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 the 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 mineral surfaces. 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. The following conclusions were drawn from the results of our initial investigations. The sulphide minerals in contact with water during grinding produced hydrogen peroxide (H2O2) where the amount of H2O2 production by the minerals found to be in the order of pyrite > chalcopyrite > sphalerite > galena. The H2O2 effect on the oxidation of pulp components and its influence on the concentrate grade and recovery in flotation have also been studied. Studies have also been carried out to build correlation between percentage of pyrite in • the concentrate, grinding conditions and concentration of OH /H2O2 in the pulp and as well of controlling the formation of these species through known chemical means for depressing the generation of the oxidant. Flotation tests using a complex sulphide ore with the same reagent scheme that is being used at Boliden concentrator in Sweden but with the addition of collector and depressant during grinding stage have been performed to judge the beneficial or detrimental role of H2O2 on the selective flotation of sulphides. The results demonstrate that the selectivity of metal sulphides against pyrite increases with increasing generation of H2O2 in the pulp liquid.

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Appended papers

Paper 1

Formation of hydrogen peroxide by pyrite and its influence on flotation

Alireza Javadi Nooshabadi, Anna- Carin Larsson, Kota Hanumantha Rao

Minerals Engineering, 2013, 49, 128-134

Minerals Engineering 49 (2013) 128–134

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Minerals Engineering

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Formation of hydrogen peroxide by pyrite and its inﬂuence on ﬂotation ⇑ Alireza Javadi Nooshabadi a, Anna-Carin Larsson b, Hanumantha Rao Kota a, a 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 b Chemistry 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 article info abstract

Article history: Formation of hydrogen peroxide (H2O2), an oxidising agent stronger than oxygen, by pyrite (FeS2), the Received 21 February 2013 most abundant metal sulphide on Earth, during grinding was investigated. It was found that pyrite gen- Accepted 19 May 2013 erated H2O2 in pulp liquid during wet grinding and also the solids when placed in water immediately Available online 12 June 2013 after dry grinding. Type of grinding medium on formation of hydrogen peroxide revealed that the mild 2+ 3+ steel produced more H2O2 than stainless steel grinding medium, where Fe and/or Fe ions played a Keywords: key role in producing higher amounts of H2O2. The effect of grinding atmosphere of air and N2 gas showed Pyrite that nitrogen environment free from oxygen generated more H O than air atmosphere suggesting that Wet and dry grinding 2 2 the oxygen in hydrogen peroxide is derived from water molecules. In addition, the solids after dry grind- Stainless steel and mild steel grinding media Hydrogen peroxide ing producing more H2O2 than wet grinding indicate the role of pyrite surface or its catalytic activity in

N2 and air atmosphere producing H2O2 from water. This study highlights the necessity of relooking into the electrochemical and/ Flotation or galvanic interaction mechanisms between the grinding medium and pyrite in terms of its ﬂotation behaviour. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction steel or high carbon steel under the same atmosphere. The high chromium media had a significantly weaker galvanic interaction Oxidation of sulphide minerals takes place when they are ex- with arsenopyrite, and produced a very much lower amount of oxi- posed to atmosphere and in the grinding process for reducing the dised iron species in the mill discharge than mild steel medium particle size for flotation. Numerous studies (Rey and Formanek, (Huang and Grano, 2006). The use of iron grinding materials 1960; Heyes and Trahar, 1977; Gardner and Woods, 1979; Trahar, slightly depressed the floatability of galena and pyrite due to iron 1984; Adam et al., 1984; Adam and Iwasaki, 1984a,b,c; Natarajan materials mainly provoking the iron oxy-hydroxide precipitation and Iwasaki, 1984; Yelloji Rao and Natarajan, 1989a,b,1990; Ahn on the surface of galena and pyrite (Cases et al., 1990). and Gebhardt, 1990 and Peng et al., 2003a) have been done on Various mechanisms have been proposed to explain the influ- the influence of type of mill and grinding media on the flotation ence of grinding media on flotation. many authors has been re- of sulphide ores. An iron mill reduced the natural floatability of ported (Adam and Iwasaki, 1984b; Natarajan and Iwasaki, 1984; sphalerite significantly (Rey and Formanek (1960). Adam and Iwa- Yelloji Rao and Natarajan, 1989a,b; Iwasaki et al., 1983; Forssberg saki (1984a,b, c) reported that the more active the metal (mild et al., 1993; Cheng et al.,1993; Yuan et al., 1996; Greet and Steinier, steel > austenitic > stainless steel), the larger the decrease in float- 2004; Greet et al., 2005; Wei and Sandenbergh, 2007) that galvanic ability of pyrrhotite. The floatability of chalcopyrite is sensitive to interactions occur in every grinding media-sulphide mineral sys- the type of grinding media, grinding atmosphere and even the tem, which affects the subsequent flotation properties of the sul- material of the mill (Heyes and Trahar, 1977; Gardner and Woods, phide minerals through unselective surface coatings by iron

1979; Trahar, 1984; Yelloji Rao and Natarajan, 1989a,b,1990; Ahn oxidation products. Recently it was revealed that H2O2 formation and Gebhardt, 1990 and Peng et al., 2003a). A glass mill may im- take place during wet grinding of complex sulphide ore (Ikumapayi prove the recovery of chalcopyrite (Heyes and Trahar, 1977), while et al., 2012) and that pyrite (FeS2), the most common metal sul- noble grinding media such as stainless steel (Ahn and Gebhardt, phide mineral, generates hydrogen peroxide (H2O2)(Borda et al., 1990) and 30 wt.% chromium (Peng et al., 2003a) produced a larger 2001) and hydroxyl radicals, HOÅ (Å denotes an unpaired electron) recovery of chalcopyrite than active grinding media, such as mild (Borda et al., 2003) when placed in water. In the presence of dissolved molecular oxygen, ferrous iron associated with pyrite can Å form superoxide anion ðO2Þ (Eq. (1)), which reacts with ferrous ⇑ Corresponding author. Tel.: +46 920 491705; fax: +46 920 97364. iron to form H2O2 (Eq. (2))(Cohn et al., 2006). E-mail address: [email protected] (H.R. Kota).

II III Å Fe ðpyriteÞþO2 ! Fe ðpyriteÞþðO2Þ ð1Þ value during flotation. Deionised water was used in the processes of both grinding and flotation. Solutions of 2, 9-dimethyl-1, 10- II Å þ III phenanthroline (DMP), copper (II) (0.01 M), and phosphate buffer Fe ðpyriteÞþðO2Þ þ 2H ! Fe ðpyriteÞþH2O2 ð2Þ (pH 7.0) were used in the estimation of H2O2. Borda et al. (2003) showed that pyrite can also generate H2O2 in the absence of molecular oxygen. He reported that an electron is 2.2. Wet grinding and flotation tests extracted from water and a hydroxyl radical is formed (Eq. (3)). Combining two hydroxyl radicals leads to the formation of H2O2 One hundred grams of 3.35 mm size fraction of pyrite for each (Eq. (4)): grinding test was combined with 400 cm3 of deionised water and Å ground in a new laboratory stainless steel ball mill (Model 2VS, FeIIIðpyriteÞþH O ! HO þ Hþ þ FeIIðpyriteÞð3Þ 2 CAPCO Test Equipment, Suffolk, UK) with either of two types of Å grinding media (mild steel or stainless steel) at natural pH (i.e. 2HO ! H O ð4Þ 2 2 pH 3–4). The slurry samples were collected at pre-determined time

Oxidation of pyrite by H2O2 can be described by the following intervals of grinding and they were filtered (Millipore 0.22 lm) generalised reaction (Lefticariu et al., 2007): and liquid (filtrate) was analysed for hydrogen peroxide. After grinding for 60 min, the mill was emptied and the pulp 2þ 2 FeS2 þ H2O2 þ H2Oinitial ! Fe þ SO4 þ O2 þ H2Ofinal was screened to free from grinding media and it was split into five

þ residual FeS2 ð5Þ samples for flotation at different pH values. In each flotation almost 7.5 g of sample that was <106 lm was transferred to a cell Oxidation of pyrite by H2O2 increases the level of complexity of of 150 ml capacity (Clausthal flotation equipment), conditioned the overall system due to the formation of reactive oxygen species with pH modifier, collector and frother. Flotation concentrate Å during decomposition of H O , such as hydroxyl radicals OH, 3 1 2 2 was collected after 2.0 min at air or N2 flow rate of 0.5 dm min . Å Å superoxide ion radicals O2 , and hydroperoxy radicals HO2.(McKib- The flotation froth was scraped every 10 s. Dosages of collector in ben and Barnes, 1986; Lefticariu et al., 2006). flotation was 104 M KAX. The conditioning times for adjusting Å However, participation of H2O2 and HO , if any, in non-selective pH and collector were 5 min and 2 min respectively. The frother oxidation of the sulphide ore pulp components and hence in dete- dosage was one drop MIBC in all cases. Pyrite flotation was inves- riorating of the concentrate grade and recovery of metal-sulphides tigated at different pH (pH 10.5, 11.5 and 4.5 with 104 M KAX), has not yet been explored. In an attempt to fill the gap, we have then it was investigated with collector (104 M KAX) and without estimated the concentration of H2O2 in pulp liquid during different collector at pH 4.5, and finally it was investigated in different gas time of grinding and in different grinding environments. The effect 4 bubbling (air or N2 bubbling at pH 4.5 and 10 M KAX). The pH of two types of grinding media (mild steel and stainless steel) on was regulated to 10.5 and 11.5 with NaOH solution and 4.5 with formation of hydrogen peroxide and pyrite flotation was HCl solution. Experiments were performed at room temperature investigated. of approximately 22.5 °C.

2. Experimental 2.3. Dry grinding

2.1. Materials and reagents One hundred grams of 3.35 mm size fraction pyrite was ground in a laboratory stainless steel ball mill with two types of Crystalline pure pyrite mineral sample was procured from grinding media (mild steel and stainless steel) for 60 min. After Gregory, Bottley & Lloyd Ltd., United Kingdom. The XRD analysis grinding, the mill was emptied and the pyrite was screened from of the sample showed that this pyrite sample was very pure grinding media. A 5 g of sample that was <106 lm was mixed with (Fig. 1) and contains 44.4% Fe, 50.9% S and 0.2% Cu. All the pyrite 50 cm3 of water in a magnetic stirrer for 0.5 and 5 min. The slurry used in this study was simultaneously crushed through a jaw sample was then collected and analysed for hydrogen peroxide. crusher and then screened to collect the 3.35 mm particle size The pH was regulated with HCl or NaOH solution. fraction. The homogenised sample was then sealed in polyethylene bags. Potassium amyl xanthate (KAX) was used as collector and 2.4. Gas purging MIBC was used as frother. Solutions of sodium hydroxide (AR grade) and HCl (1 M) were used to maintain the pH at the targeted 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 labo-

ratory 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. Hydrogen peroxide analysis

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 Fig. 1. XRD analysis of the pyrite sample. (1) pyrite. et al., 1993; Roche and Prados, 1995; Sunder and Hempel, 1997; 130 A. Javadi Nooshabadi et al. / Minerals Engineering 49 (2013) 128–134

Leitner and Dore´, 1997) and cobalt (II) ion (Gulyas et al., 1995) that H2O2 was consumed more in mild steel grinding medium since form colour complexes with H2O2, which can then be measured the rest potential is lower in this case compared to stainless steel spectrophotometrically. The spectrophotometric method using medium (Peng and Grano, 2010). Distinguishing H2O2 formation copper (II) and DMP has been found to be reasonably sensitive in pulp liquid, the effect of this strong oxidising agent on solid sur- when applied to advanced oxidation processes (Kosaka et al., faces and its consequence on ﬂotation have been addressed.

1998). For DMP method (Baga et al., 1988) one millilitre of each After grinding for 60 min, the effect of H2O2 formation on flo- of 1% DMP in ethanol, copper(II) (0.01 M), and phosphate buffer tation recovery of pyrite was investigated. The effect of grinding (pH 7.0) solutions were added to a 10 mL volumetric flask and media on pyrite flotation is shown in Figs. 4–6.InFig. 4 it can mixed. A measured volume of liquid (filtrate) sample was added be seen that for all pH values studied the stainless steel medium to the volumetric flask, and then the flask was filled up with ultra- produced a higher pyrite recovery than mild steel medium. Our pure water. After mixing, the absorbance of the sample at 454 nm results for pyrite flotation show that mild steel has lower recovery was measured with DUÒ Series 700 UV/Visible Scanning Spectro- than stainless steel. It can also be seen that the more active metal photometer. The blank solution was prepared in the same manner (mild steel) produces a larger amount of H2O2. This is in agree- but without H2O2. ment 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., 3. Results and discussion 1997). Many oxidising agents such as hydrogen peroxide (H2O2), sodium hypochlorite, potassium permanganate, potassium chro- 3.1. Formation of hydrogen peroxide (H2O2) during wet grinding and mate have been used as depressants for pyrite, arsenopyrite and its implications on flotation galena (Beattie and Poling, 1987; Shimoiizaka et al., 1976; Nagaraj et al., 1986). The dissolution of pyrite by hydrogen peroxide in Initially the extent of H2O2 formation during wet grinding of acidic solutions is characterised by the reaction (Eary, 1985; pyrite was investigated. For these studies, pyrite was wet-ground McKibben, 1984): in a laboratory stainless steel ball mill with two kinds of grinding media at natural pH and slurry samples were collected at pre- FeS 7:5HO Fe3þ 2SO2 Hþ 7H O 6 determined time intervals. The slurry samples were filtrated (Mil- 2 þ 2 2 ! þ 4 þ þ 2 ð Þ lipore 0.22 lm) and the liquid (filtrate) was analysed for hydrogen Castro and Baltierra (2003) reported that the floatability of pyr- peroxide. Formation of hydrogen peroxide was detected in the fil- ite was decreased after pre-treatment with hydrogen peroxide in trate for the first time in mineral processing applications. The pro- alkaline condition. Jones and Woodcock (1978) showed that hydro- posed mechanism, under anoxic condition hypothesizes that the gen peroxide reacts with xanthates to form perxanthates which do Å + dissociation of H2O, to form HO and H , occurs on non-stoichiom- not have collector properties. Also this is in agreement with the Å etric Fe(III) sites on pyrite. The combination of two HO radicals, observations where Shi and Fornasiero (2010) observed that condi- then produces H2O2 (Borda et al., 2003). tioning of chalcopyrite in the presence of hydrogen peroxide in- The effect of grinding media on formation of hydrogen peroxide duces oxidation of its surface with formation of the hydrophilic is shown in Fig. 2. It can be seen that mild steel produced a higher species of iron oxide/hydroxide and iron sulphate causing its concentration of H2O2 than stainless steel medium. The concentra- depression in flotation. Pyrite was depressed at pH 10.5 and 11.5 tion of H2O2 increased with increasing grinding time due to in- due to the formation of Fe(OH)2 and/or Fe(OH)3 on the pyrite sur- creased surface area of solids interacting with water. This is in face (Janetski et al., 1977; Kocabag et al., 1990). Fig. 5 shows that agreement with other studies where it was observed that more ac- flotation with collector produced a higher pyrite recovery than tive the metal in grinding medium (mild steel > austenitic > stain- without collector. In both cases stainless steel medium produced less steel), larger was the decrease in floatability of pyrrhotite a higher pyrite recovery than mild steel medium. (Adam and Iwasaki, 1984a, b,c), identifying the depressing effect Fig. 6 shows the effect of different atmosphere in flotation. For- of H2O2 in flotation. When Eh was measured immediately after mation of bubbles with O2 (air) produced insignificantly higher adding pyrite in jars of grinding mill, Eh in both mild and stainless pyrite recovery than N2. In both atmospheres stainless steel med- steel was the same as shown in Fig. 3, but Eh in mild steel de- ium produced a slightly higher pyrite recovery than mild steel creases more during grinding due to formation of hydrophilic spe- medium. This can possibly be because stainless steel medium pro- cies such as Fe(OH)2, Fe(OH)3, FeOOH and oxidation of sulphur to duced a lower concentration of H2O2 than mild steel (see Fig. 2). various reduced sulfo-oxy species in mild steel medium. The Table 1 shows the Eh in flotation after 5 min contitioning time.

2 Stainless Steel Mild steel 3.2. Formation of hydrogen peroxide (H O ) after dry grinding in 1.8 2 2 contact with water 1.6 1.4 Pyrite mineral (100 g) was dry-ground in a laboratory ball mill. 1.2 Formation of hydrogen peroxide was detected after the solids are

(mM) 1 2 mixed with water. The effect of water pH on formation of hydrogen O 2 0.8 peroxide is shown in Fig. 7. It can be seen that with an increase in H 0.6 pH, the concentration of H2O2 decreased to about pH 8 and then in- 0.4 creased above this pH. Fig. 7 also shows that Eh increased with

0.2 decreasing pH. This is agreement with the amount of H2O2 forma- 0 tion, which is one of the most powerful oxidisers known. The Eh- 110204560 pH diagram of pyrite shows that oxidation of pyrite yields S, Grinding time (min) 2+ þ Fe ,FeðOHÞ2 species in the solution for pH < 6 (Fig. 8)(Kocabag et al., 1990). In pH < 6, Fe2+ ions are increased with decreasing Fig. 2. Production of H2O2 in pulp liquid as a function of grinding time during wet grinding. (For interpretation of the references to colour in this ﬁgure legend, the pH and therefore, in the presence of dissolved molecular oxygen, reader is referred to the web version of this article.) ferrous iron associated with pyrite can form superoxide anion A. Javadi Nooshabadi et al. / Minerals Engineering 49 (2013) 128–134 131

6.5 700 6 5.5 600 5 Mild steel pH 4.5 500 Stainless Steel pH 4 Stainless Steel Eh 400 3.5 Mild steel Eh pH 3 300 2.5 2 Eh (mV, SHE) 200 1.5

1 100 0.5 0 0 0 10203040506070 Grinding time (min)

Fig. 3. Eh and pH during wet grinding.

80 Stainless Steel Mild steel Table 1 70 Eh of pulp liquid during ﬂotation. 60 Collector Gas pH Eh (mild Eh (stainless 50 bubbling steel) steel) 40 1 104 M Air 10.5 153 161 30 1 104 M Air 11.5 112 112 4 20 1 10 M Air 4.5 397 427

Pyrite recovery (%) Without Air 4.5 389 432 10 collector 4 0 1 10 MN2 4.5 387 387 4.5 10.5 11.5 pH

Fig. 4. Effect of pH on flotation recovery of pyrite with air atmosphere during the 1.8 500 flotation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 1.6 400 1.4 H2O2 80 1.2 300 Stainless Steel Mild steel Eh 70 1

(mM) 200 60 2 0.8 O 2

50 H 0.6 100 Eh (mV, SHE) 40 0.4 0 30 0.2 20 0 -100 Pyrite recovery (%) 2.5 4.5 6 8 11.6 10 pH 0 0,0001 withoutcollector Fig. 7. Effect of water pH on H2O2 formation when 5 g dry-ground solids are mixed Collector concentration (M) in water for 5 min.

Fig. 5. Flotation recovery of pyrite in the presence and absence of collector at pH 4.5 and air gas bubbling. (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.) Å ðO2Þ (Eq. (1)), which reacts with ferrous iron to form H2O2 (Eq. (2)) (Cohn et al., 2006). 100 The effect pH on formation of hydrogen peroxide with two Stainless Steel Mild steel 90 grinding media was investigated. Fig. 9 shows that mild steel pro- 80 duced a higher concentration of H2O2 than stainless steel medium 70 at pH 4.5 in agreement with the results from wet grinding (see 60 Fig. 2). Fig. 10a shows the proposed mechanisms for formation of 50 2+ H2O2 where mild steel generated Fe ions but not stainless steel 40 explaining a higher concentration of H2O2 in mild steel than stain- 30 2+ 20 less steel medium since Fe ions can generate H2O2 as shown in Pyrite recovery (%) 10 Table 2. This is in agreement with other studies where the dis- 0 solved ferrous iron concentration was found to be an important air N2 secondary factor contributing towards Reactive Oxygen Species Å (ROS) generation such as O ,HO and HOÅ (Jones et al., 2012). Fig. 6. Flotation recovery of pyrite in different gas bubbling at pH 4.5 and KAX 2 2 2 0.1 mM. (For interpretation of the references to colour in this ﬁgure legend, the The formation of higher amounts of H2O2 when pyrite is ground reader is referred to the web version of this article.) in mild steel medium than stainless medium may explain the 132 A. Javadi Nooshabadi et al. / Minerals Engineering 49 (2013) 128–134

Table 2

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

Concentration of ions H2O2 (mM) 1mM 10mM Water 0 0 Fe2+ 0.552 4.656 Fe3+ 0.004 0.059

3.3. Effect of pyrite loading on formation of hydrogen peroxide

To investigate the effect of pyrite loading or solids concentra-

tion 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 is shown in Fig. 11. It can be

seen that the concentration of H2O2 increased with increasing pyr- 5 Fig. 8. Eh–pH diagram for FeS2–H2O system at 25 °C and for 10 M dissolved ite loading. species (Kocabag et al., 1990).

3.4. A comparison between dry and wet grinding for formation of

2.5 hydrogen peroxide (H2O2)

2 A comparison was made between wet and dry grinding on H2O2 formation. For this comparison 100 g of pyrite was ground dry for 50 min and wet with 400 ml water for 60 min at natural pH so that 1.5 the ﬁneness of ground product was equated. After dry grinding, the (mM)

2 mill was immediately emptied and pyrite was mixed with 400 ml

2 1 water for 60 min then slurry samples were collected at pre-deter- HO mined time intervals. Also after wet grinding, slurry samples were collected and analysed for hydrogen peroxide. The medium in both 0.5 cases was stainless steel. Fig. 12 shows that more H2O2 is produced by the solids after dry grinding. 0 Stainless Steel Mild Steel 3.5. Gas purging

Fig. 9. H2O2 concentration after dry grinding in different media at pH 4.5. To study the effect of gaseous 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 effect of grinding media on flotation in addition to the widely pub- mill was filled with 400 ml water and purged with N2 gas for a lished galvanic interaction of electron transfer between the grind- minimum of 1 h and then pyrite was wet-ground for 1 h. For air ing media and pyrite (Adam and Iwasaki, 1984b; Natarajan and atmosphere the laboratory ball mill was filled with pyrite and Iwasaki, 1984; Yelloji Rao and Natarajan, 1989a,b; Iwasaki et al., 400 ml water and kept open the mill for 5 min to have the air 1983; Forssberg et al., 1993; Cheng et al.,1993; Yuan et al., 1996; and then pyrite was wet-ground for 1 h. Pyrite in N2 atmosphere Greet and Steinier, 2004; Greet et al., 2005; Wei and Sandenbergh, also generated H2O2 as Borda et al. (2003) showed that pyrite 2007). It is obvious that the presence of H O in pulp liquid needs 2 2 can also generate H2O2 in the absence of molecular oxygen. due consideration in controlling the surface properties in flotation Fig. 13 shows that more H2O2 was generated in a N2 atmosphere besides galvanic interactions. than in air. This must be due to a change in pH and Eh of the pulp

Fig. 10. 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 OHÅ (Eqs. (3) and (4)) radicals. A. Javadi Nooshabadi et al. / Minerals Engineering 49 (2013) 128–134 133

160 The above results may be summarised with the occurrence of 140 the following events: (a) mild steel grinding medium increases 120 concentration of H2O2 in the pulp liquid, (b) increasing the concen- 100

(µM) tration of H2O2 increases the oxidation of pyrite and decreases its

2 80

O ﬂotation, and (c) increasing concentration of H O increases pyrite 2 60 2 2

H 40 leaching leading to unwarranted enhanced acid mine drainage. 20 0 50 100 250 4. Conclusions Pyrite loading (g/l) In the scope of this paper, some studies to ascertain the inher- Fig. 11. Effect of stainless steel medium dry-ground pyrite loading on H2O2 ent formation of H2O2 in sulphide ore pulp liquid due to their sur- concentration after 5 min mixing time with water at pH 11.6. face reactivity were carried out to resolve or reveal one of the main problems (i.e., oxidation) in sulphide ore flotation. The initial 1.8 grinding stage was evaluated and the oxidation of the ores was 1.6 investigated at this stage. For the first time in mineral processing 1.4 applications, it was established that the formation of H2O2 takes dry 1.2 place in pulp during grinding (especially fine grinding). Hydrogen wet 1 peroxide is a strong oxidising agent and can easily oxidise sulphide (mM) 2 0.8 minerals and even cause dissolution of some metal ions from min- O 2 0.6 H eral surfaces. That’s why H2O2, rather than oxygen, could be the 0.4 main reason for oxidation, inadvertent activation and non-selectiv- 0.2 ity in the sulphide ore flotation. Prevention or reduction of H2O2 0 0 10 20 30 40 50 60 70 formation may result in improved flotation results and we are Time (min) working on these aspects. Following preliminary conclusions were drawn from the exper-

Fig. 12. H2O2 concentration after dry and wet milling at natural pH. iments carried out in the scope of this paper. Mild steel produced a

higher concentration of H2O2 than stainless steel medium. The con- 2 centration of H2O2 increased with increasing grinding time. Stain- less steel produced a higher pyrite recovery than mild steel 1.5 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

mM grinding medium that could be due to lower concentration of H2O2 2 1 O

2 in stainless steel than mild steel medium. The mild steel medium H increases the concentration of H O and thereby increases pyrite 0.5 2 2 oxidation and decreases its ﬂotation recovery.

Pyrite solids after dry grinding produced more H2O2 when 0 placed in water than wet grinding. The pH of water inﬂuenced N2 air the formation of hydrogen peroxide where high amounts of H2O2

Fig. 13. Concentration of H2O2 formed in different grinding atmospheres at natural is produced at highly acidic pH and decreased with increasing pH pH (3.7). up to 8 and increased again above this pH. The amount of H2O2 formed also increased with increase in pyrite loading due to in- liquid that in N2 atmosphere pH and Eh after 60 min grinding were creased surface area to react with water. Mild steel produced high- 6.3 and 152 mV respectively and in air atmosphere they were 5.4 er concentration of H O than stainless steel medium due to higher and 287 mV. The Eh-pH diagram (Fig. 8) shows the presence of 2 2 2+ amounts of ferrous ions release in mild steel grinding medium. In Fe species at lower pH, which species are responsible for H2O2 addition, dry grinding generated more H2O2 than wet grinding. generation in N2 atmosphere. More H O was generated in N gas atmosphere compared to air The effect on pyrite flotation in the presence and absence of 2 2 2 atmosphere suggesting the breakdown of water molecule and giv- oxygen during grinding is shown in Fig. 14. It can be seen that ing raise to hydroxyl free radical due to the catalytic activity of N2 purging decreased pyrite recovery compared to air atmosphere. reactive pyrite surfaces. Pyrite flotation at pH 4.5 after stainless This is in agreement with other studies where it was observed that steel grinding in different atmospheres became lower in N purg- hydrogen peroxide greatly reduce the hydrophobicity of pyrite and 2 ing due to higher H O formation compared to air atmosphere. flotation response in the presence of amyl xanthate (Monte et al., 2 2 1997). Acknowledgements 100 90 Financial support from the Centre for Advanced Mining and 80 Metallurgy (CAMM), Luleå University of Technology, Sweden, 70 established under Swedish Strategic Research Initiative pro- 60 50 gramme and Boliden Mineral AB, Boliden, Sweden, is gratefully 40 acknowledged. 30 20 Pyrite recovery (%) 10 References 0 N2 air Adam, K., Iwasaki, I., 1984a. Effect of polarization on the surface properties of pyrrhotite. Miner. Metall. Process., 246–253. Fig. 14. Flotation recovery of pyrite after wet-ground in N2 and air atmospheres Adam, K., Iwasaki, I., 1984b. Grinding media wear and its effect on the floatation of using 0.1 mM KAX collector at pH 4.5. sulphide minerals. Int. J. Miner. Process. 12, 39–54. 134 A. Javadi Nooshabadi et al. / Minerals Engineering 49 (2013) 128–134

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Eng. 15, 405–418. Huang, G., Grano, S., 2006. Galvanic interaction between grinding media and Wei, Y., Sandenbergh, R.F., 2007. Effects of grinding environment on the flotation of arsenopyrite and its effect on flotation, Part I. Quantifying galvanic interaction Rosh Pinah complex Pb/Zn ore. Miner. Eng. 20, 264–272. during grinding. Int. J. Miner. Process. 78, 182–197. Yelloji Rao, M.K., Natarajan, K.A., 1989a. Effect of electrochemical interactions Ikumapayi, F., Sis, H., Johansson, B., Hanumantha, Rao.K., 2012. Recycling process among sulfide minerals and grinding medium in chalcopyrite flotation. Miner. water in sulfide flotation, Part B: Effect of H2O2 and process water components Metall. Process. 7, 146–151. on sphalerite flotation from complex sulfide. J. Miner. Metall. Process. 29, 192– Yelloji Rao, M.K., Natarajan, K.A., 1989b. Electrochemical effects of mineralmineral 198. interactions on the flotation of chalcopyrite and sphalerite. Int. J. Miner. Process. 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Paper 2

Formation of hydrogen peroxide by chalcopyrite and its influence on flotation

Alireza Javadi Nooshabadi, Kota Hanumantha Rao

Minerals and Metallurgical processing, 2013, 30 (4), 212-219

Formation of hydrogen peroxide by chalcopyrite and its influence on flotation

A. Javadi Nooshabadi and K. Hanumantha Rao* PhD student and professor, respectively, Mineral Processing Group, Division of Sustainable Process Engineering Department of Civil, Environmental and Natural Resources Engineering Luleå University of Technology, Luleå, Sweden *Corresponding author email: [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 in the solids when placed in water immediately after dry grinding. The generation of H2O2 during 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. The effect of pH in the grinding medium or in the water in which solids are added immediately after dry grinding showed that the lower the pH value, the higher 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 behavior.

Minerals & Metallurgical Processing, 2013, Vol. 30, No. 4, pp. 212-219. An official publication of the Society for Mining, Metallurgy, and Exploration, Inc. Key words: Chalcopyrite, Pyrite, Wet and dry grinding, Hydrogen peroxide, Flotation

Introduction its surface, with the formation of the hydrophilic species of iron Hydrogen peroxide causes nonselective oxidation oxide/hydroxide and iron sulfate on the surface, which causes of sulfide minerals if present in pulp liquid. Oxidation chalcopyrite depression in flotation (Shi and Fornasiero, 2010). of sulfide minerals takes place when they are exposed Various mechanisms have been proposed to explain the to atmosphere and during the grinding process when influence of pyrite on the flotation of chalcopyrite. It has been the particle size is reduced for flotation. The oxida- reported by many authors that galvanic interactions occur tion of chalcopyrite leads to iron dissolution from the between conducting minerals and play a significant role in particle surface and from a metal-deficient, sulfur-rich flotation (Rao and Finch, 1988; Kelebek et al., 1996; Zhang surface (Fairthorne et al., 1997). Other studies report et al., 1997; Ekmekçi and Demirel, 1997; Huang and Grano, the formation of iron hydroxide on the surface with 2005), leaching (Mehta and Murr, 1983; Abraitis et al., 2003; the metal ion dislodged on the surface (Buckley and Akcil and Ciftci, 2003), supergene enrichment of sulfide ore Woods, 1984; Chander, 1991). The chemical reactions deposits (Thornber, 1975; Sato, 1992), environment gover- for the oxidation of chalcopyrite in alkaline and in nance (Alpers and Blowes, 1994) and geochemical processes acidic solutions with the addition of oxidants, such (Sikka et al., 1991; Banfield and Nealson, 1997). Recently, it as oxygen or hydrogen peroxide, are shown in Eqs. was revealed that formation of H2O2 takes place during wet (1) and (2), respectively (Shi and Fornasiero, 2010). grinding of complex sulfide ore (Ikumapayi et al., 2012). Pyrite (FeS2) has been shown to generate hydrogen peroxide (H2O2)  CuFeS2 + (3/2) xO2 + (3/2) xH2O (Cohn et al., 2006a; Javadi et al., 2013) when placed in water. CuFe S + xFe (OH) (1) In the presence of dissolved molecular oxygen, ferrous iron 1-x 2 3 • - associated with pyrite can form superoxide anion (O2 ) (Eq.  2+ − (3)), which reacts with ferrous iron to form H2O2 (Eq. (4)). CuFeS2 CuFe1-x S2 + xFe + 2xe (2) II  III • - Buehler et al. (1910) noted that when pyrite was Fe (pyrite) + O2 Fe (pyrite) + (O2 ) (3) mixed with a second sulfide mineral, the second FeII (pyrite) + (O •)- + 2H+  mineral oxidized more rapidly. Owusu et al. (2011) 2 FeIII (pyrite) + H O (4) showed that with a proportional increase in pyrite in 2 2 the solids, chalcopyrite recovery decreased. It was also Borda et al. (2003) showed that pyrite can also generate reported that the conditioning of chalcopyrite in the H2O2 in the absence of molecular oxygen. He reported that presence of hydrogen peroxide induces oxidation of an electron is extracted from water and a hydroxyl radical is

Paper number MMP-13-010. Original manuscript submitted February 2013. Revised manuscript accepted for publication August 2013. Discussion of this peer-reviewed and approved paper is invited and must be submitted to SME Publications Dept. prior to May 31, 2014. Copyright 2013, Society for Mining, Metallurgy, and Exploration, Inc.

November 2013 • Vol. 30 No. 4 212 MINERALS & METALLURGICAL PROCESSING Figure 1 — XRD analysis of (a) the pyrite sample (1-pyrite) and (b) chalcopyrite sample (1- chalcopyrite, 2- pyrrhotite and 3- sphalerite). formed (Eq. (5)) and combining two hydroxyl radicals leads of water and ground in a laboratory stainless steel ball mill to the formation of H2O2 (Eq. (6)): (Model 2VS, CAPCO Test Equipment, Suffolk, UK) with stainless steel medium for 60 min. The slurry samples were Fe III + H O  •OH + H+ + Fe II (5) 2 collected at predetermined time intervals and they were immediately filtered (Millipore 0.22 µm) and the liquid (filtrate) 2 •OH  H O (6) 2 2 was analyzed for hydrogen peroxide. The pH was regulated Antonijevic et al. (2004) also showed that the accelerated with HCl and NaOH solution. chalcopyrite oxidation was due to increasing hydrogen per- After grinding for 60 min, 7.5 g of the sample that was oxide concentration. < 106 µm was subjected to flotation using a cell of 150 mL • However, participation of H2O2 and OH, if any, in non- capacity (Clausthal flotation equipment), where the pulp is selective oxidation of the sulfide ore pulp components and, conditioned with pH modifier, collector and frother. Flotation hence, in deteriorating the concentrate grade and recovery concentrate was collected after 2.0 min at an air flow rate of of metal-sulfides in flotation has not yet been explored. In an 0.5 dm3/min-1. The flotation froth was scraped every 10 s. The attempt to fill the gap, we have estimated the concentration dosage of collector in flotation was 10-4 M KAX. The con- of H2O2 in pulp liquid during different times of grinding and ditioning times at specified pH and collector were 5 min and in different grinding environments. The effect of pH, type of 2 min, respectively. The frother dosage was one drop MIBC grinding (wet or dry), grinding atmosphere and pyrite propor- in all cases. Chalcopyrite flotation was conducted at pH 8, tion in mixed solids on the formation of hydrogen peroxide 10.5 and 11.5. The pH was regulated with HCl and/or NaOH and chalcopyrite flotation were investigated. The results are solutions. Experiments were performed at room temperature presented and discussed in this paper. of approximately 22.5°C.

Experimental Dry grinding and flotation test. Chalcopyrite and pyrite Materials and reagents. Crystalline pure chalcopyrite single minerals of 100 g each were separately ground in a labo- and pyrite minerals used in this study were procured from ratory stainless steel ball mill with stainless steel medium for Gregory, Bottley & Lloyd Ltd., UK. Chalcopyrite contains 60 min. After grinding, 5 or 12.5 g of chalcopyrite and pyrite 29% Fe, 29.5% S, 25.8% Cu, 0.54% Zn, 0.22% Pb and pyrite minerals or chalcopyrite-pyrite mixture that were < 106 µm contains 44.4% Fe, 50.9% S. X-ray diffraction (XRD) analyses were mixed with 50 cm3 of water using a magnetic stirrer, and of the samples showed that the main mineral phases present the slurry sample was collected at 0.5-, 5- and 11-min inter- were the pyrite (Fig. 1a) and chalcopyrite (Fig. 1b). All pyrite vals, filtered and analyzed for hydrogen peroxide. The pH was and chalcopyrite samples used in this study were separately regulated with HCl and NaOH solutions. Eh (pulp potential) crushed through a jaw crusher and then screened to collect the was also measured at room temperature during mixing using -3.35 mm particle size fraction. The homogenized sample was a platinum electrode and expressed relative to the standard then sealed in polyethylene bags. Potassium amyl xanthate hydrogen electrode, SHE. After dry grinding for 60 min, 5 g of (KAX) was used as collector and MIBC as frother in flotation sample that was < 106 µm was used in flotation with the same tests. Solutions of sodium hydroxide (AR grade) and HCl (1 flotation conditions as were used after wet grinding the material. M) were used to maintain the pH at the targeted value during flotation. Deionized water was used in the processes of both Gas purging. To study the effect of an oxygen-free atmo- grinding and flotation. Solutions of 2, 9-dimethyl-1, 10-phen- sphere, chalcopyrite was wet-ground in a laboratory ball mill anthroline (DMP), copper (II) sulfate (0.01 M) and phosphate with stainless steel medium in an N2 atmosphere at natural pH. buffer (pH 7.0) were used for measuring H2O2 following the For this study, the laboratory ball mill was first filled with 400 method described by Baga et al. (1988). A 30% H2O2 solution mL of deionized water and purged with N2 gas for a minimum was used to investigate the effect of H2O2 concentration on of 30 min. Chalcopyrite was added and the laboratory ball mill flotation of chalcopyrite. jar was purged with N2 gas for a minimum of 30 min. Then, the chalcopyrite sample was wet-ground for 1 h. Though dissolved Wet grinding. Chalcopyrite mineral (100 g) of -3.35 mm O2 concentrations were not measured in the experiments, it has size fraction in each grinding test was combined with 400 cm3 been reported that for 1 L ultrapure water purged for 1 h with

MINERALS & METALLURGICAL PROCESSING 213 Vol. 30 No. 4 • November 2013 (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. Results and discussion 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 Figure 2 — Formation of H2O2 in wet grinding with increasing grinding time samples were collected at predetermined time at natural pH (5.7). 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 first decreased 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 H2O2’s reaction with water.

Effect of pH on H2O2 formation during wet grinding. Figure 4 shows the effect of initial pH on formation of hydrogen peroxide. It can be seen that with an increase in pH, the Figure 3 — pH and Eh of pulp liquid during wet grinding. concentration of H2O2 decreased up to pH 8 and then increased above this pH. After estab- lishing the formation of H2O2, the effect of this strong oxidizing agent on solid surfaces and its consequence on flotation is addressed.

Formation of H2O2 after dry grinding and mixing with water. Chalcopyrite or pyrite mineral of 100 g was dry-ground in a laboratory ball mill and the formation of hydrogen peroxide was studied when the dry-ground solids were mixed in water. The effect of the pH of the water in which the solids are mixed on the formation of hydrogen peroxide is shown in Fig. 5a and Fig. 5b (Javadi et al., 2013). It can be seen that with an increase in pH, the concentration of H2O2 decreased up to pH 8, then increased above this pH. These results are in agreement with wet Figure 4 — Effect of pH on H2O2 concentration by chalcopyrite during wet grinding after 60 min. grinding data shown in Fig. 4. Figure 5a also shows that the concentration of H2O2 increased with increasing mixing time, most likely due N2 gas, the O2 concentrations do not exceed 0.19 ± 0.05 ppm to increased chalcopyrite interaction with water. Increasing Eh (Butler et al., 1994). The concentration of H2O2 was measured with decreasing pH also corroborates the formation of high after 60 min of grinding. amounts of H2O2 at lower pH values. Fairthorne at al. (1997), using the Eh-pH stability diagram of chalcopyrite (Fig. 6a), Hydrogen peroxide quantification.The spectrophotomet- showed the formation of insoluble ferric oxide/hydroxide at ric method using copper (II) ions and DMP has been found to neutral and basic pH values, as well as in acidic conditions at be reasonably sensitive when applied to advanced oxidation high Eh values. Notably, the divalent Fe and Cu ions exist at processes (Kosaka et al., 1998). For the DMP method (Baga low pH values from negative to high Eh values. These divalent et al., 1988), one mL each of DMP, copper (II) sulfate and ions are reported to aid the formation of H2O2 (Valko et al., phosphate buffer (pH 7.0) solutions were added to a 10-mL 2005; Jones et al., 2011) from water, and our present results volumetric flask and mixed. A measured volume of liquid shown in Table 1 demonstrate that the presence of Fe2+ ions

November 2013 • Vol. 30 No. 4 214 MINERALS & METALLURGICAL PROCESSING Figure 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. generate substantial H2O2, followed by the presence of Fe3+ and Cu2+ ions. When the pulp liquid is free from dissolved oxygen, hydrogen peroxide is also seen to form (Fig. 9), with little difference due to low Eh value compared to the system open to the atmosphere (Fig. 6a). Figure 6a shows 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 Figs. 7a and 7b, where in acidic pH (pH<5) − superoxide anion (O2•) (Eq. (7)) is formed and reacts with ferrous ion to form H2O2 (Eqs. (8)-(10)). This is in agreement with other studies, where it was observed that Figure 6 — a) Eh–pH stability diagram for the Cu–Fe–S–H2O system. Empty metals induce formation of free radical and and filled circles represent the experimental Eh and pH values for chalcopyrite that ferrous ion generates the superoxide conditioned in nitrogen and oxygen, respectively (Fairthorne, 1997 ). b) Eh–pH -5 and hydroxyl radical (Valko et al., 2005; diagram for the FeS2 – H2O system at 25° C and for 10 M dissolved species Jones et al., 2011). That the metal ions (Kocabag et al., 1990). induced formation of hydroxyl free radical has been significantly shown where copper ions also generate the superoxide and hydroxyl radical (Valko et al., 2005). In acidic pH, the higher amounts of hydrogen peroxide formation than in alkaline pH is due to the presence of copper and ferrous ions in acidic pH. These ions are capable of generating hydrogen peroxide (Table 1). −  • − O2 + e (O2 ) (7) • − +  2(O2 ) + 2H H2O2 + O2 (8) 2+  3+ Fe + H2O2 Fe Figure 7 — Proposed mechanisms for formation of H O by a) chalcopyrite and • − 2 2 + OH + OH (9) b) pyrite via the incomplete reduction of oxygen and from two reacting •OH. The numbers in parentheses in this figure represent equation numbers. (For •  2 OH H2O2 (10) pyrite, the proposed mechanism is adapted from Borda et al. (2003) and Cohn Above pH 5, the stability diagram et al. (2006a)). shows the Fe2O3 solid phase, the surface of + 2+ which generates hydrogen peroxide. This is in agreement with Fe (OH)2 species in a solution at pH < 6. At pH < 6, Fe ions other studies where hematite and magnetite solids in water were are increased with decreasing pH and, therefore, in the pres- shown to induce H O formation (Cohn et al., 2006b). Figure ence of dissolved molecular oxygen, ferrous iron associated 2 2 • − 7a shows that copper ions also generate hydrogen peroxide. with pyrite (Fig. 7b) can form superoxide anion (O2 ) (Eq. Kocabag et al. (1990), using the Eh-pH diagram (Fig. 6b) (3)), which reacts with ferrous iron to form H2O2 (Eq. (4)) of pyrite, showed that the oxidation of pyrite yields S°, Fe2+, (Cohn et al., 2006a).

MINERALS & METALLURGICAL PROCESSING 215 Vol. 30 No. 4 • November 2013 Table 1 — H2O2 generation in the presence of metal ions min and, then, chalcopyrite was wet-ground for 1 h. Figure 9 at natural pH and at 22° C. shows that H2O2 was generated in an N2 atmosphere, though N2 atmosphere generated lower H2O2 than air atmosphere. H2O2 (mM) One can hypothesize that an electron is extracted from water Concentration of ions 1 mM 10 mM and a hydroxyl radical is formed (Eq. (11)), and that the reac- Water 0 0 tion between two hydroxyl radicals leads to the formation of Fe 2+ 0.552 4.656 H2O2 (Eq. (12)): 2+  • + + Fe 3+ 0.004 0.059 Cu + H2O OH + H + Cu (11) 2+ Cu 0 0.015 •  2 OH H2O2 (12)

Effect of H2O2 on chalcopyrite flotation. Shi and For- nasiero (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 sulfate causing its depression in flotation. After dry-grinding for 60 min, the effect of H2O2 formation in water on chalcopyrite flotation recovery caused by the freshly ground solids was investigated. The concentration of H2O2 in the pulp liquid vis-à-vis chalcopyrite flotation is shown in Fig. 10. H2O2 decreases the recovery of chalcopyrite when its concentration exceeds 0.3 mM, and this result was the same as Castro and Baltierra (2003), who reported that the floatability of chalcopyrite was decreased after pretreatment with hydrogen peroxide in alkaline condition. Figure 11 shows the effect of initial pulp liquid pH of grinding on recovery of chalcopyrite Figure 8 — Effect of chalcopyrite or pyrite loading on H2O2 at pH 10.5. It can be seen that the recovery of chalcopyrite concentration after 5 min mixing with water at pH 11.6. remains almost constant at pH > 5, but decreases at pH 3.5, which could be due to higher H2O2 concentration (Fig. 4). Figure 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-grinding at natural pH (5.7) or after dry-grinding decreased with increasing pH. Ackerman et al. (1987) and Chandraprabha and Natarajan (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 (Figs. 4 and 5a).

Chalcopyrite-pyrite mixture. Samples of chalcopyrite– pyrite (12.5 g in total) were mixed with 50 cm3 of water for 5 min. Figure 13a shows the effect of the pyrite proportion in the 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 Figure 9 — The effect of gaseous atmosphere on the increase in the pyrite fraction, the concentration of H2O2 increased. This is in agreement with other studies, where it was formation of H2O2 by chalcopyrite at natural pH. observed that 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 Effect of chalcopyrite loading on formation of H2O2. in pyrite proportion, the concentration of H2O2 increased, can The effect of chalcopyrite and pyrite loading on hydrogen be the explanation for a behavior that: peroxide formation was investigated with 50 mL total volume of water at pH 11.6, and the results are shown in Fig. 8. It can 1. The floatability of chalcopyrite decreases when in con- be seen that the concentration of H2O2 increased with increas- tact with pyrite. Owusu et al. (2011) observed that an ing chalcopyrite and pyrite loading, due to increased mineral increase in pyrite content from 0% to 80% resulted in surface interaction with water. a decrease in chalcopyrite recovery from 98% to 80% and a concomitant decrease in the flotation rate constant Gas purging. To study the effect of gaseous atmosphere from 0.94 to 0.42 min-1. In contrast to chalcopyrite, the of the presence or absence of oxygen on hydrogen peroxide recovery of pyrite gradually increased as the amount of formation, chalcopyrite was wet-ground in a laboratory ball chalcopyrite present in the mixture increased. Therefore, mill with stainless steel media in either air or an N2 atmosphere. at a certain H2O2 concentration, the complete decompo- For the N2 atmosphere, the laboratory ball mill was filled sition of copper xanthate preadsorbed on chalcopyrite with 400 mL water and purged with N2 gas for a minimum renders the chalcopyrite surface hydrophilic and leads of one hr and, then, chalcopyrite was wet-ground for 1 h. For to the depression of chalcopyrite, as shown in Fig. 10. air atmosphere, the laboratory ball mill was left open for 5 2. The selective flotation of chalcopyrite from pyrite is pos-

November 2013 • Vol. 30 No. 4 216 MINERALS & METALLURGICAL PROCESSING sible at pH 10. Chalcopyrite flotation is impossible at pH 4 because xanthate decomposes (Mitchell et al., 2005). 3. As the amount of pyrite in contact with chalcopyrite increases, the leaching rate of chalcopyrite increases as well (Koleini et al., 2010; Koleini et al., 2011; Dixon and Tshilombo, 2005; Mehta and Murr, 1983; Holmes and Crundwell, 1995). Antonijevic et al. (2004) showed that increasing hydrogen peroxide concentration considerably accelerates chalcopyrite oxidation: +  2+ 3+ 2CuFeS2 + 17H2O2 + 2H 2Cu + 2Fe + 2- SO4 + 18H2O (13)

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 Figure 10 —Effect of H2O2 on recovery of chalcopyrite with conditions of solids loading (Jones et al., 2011; Jones dry-ground solids at pH 10.5. et al., 2012). Figure 13b also shows the effect of chalcopyrite proportion in the 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 chalcopyrite proportion, the concentration of H2O2 decreases. 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) . 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 Figs. 14b and 14c, while the entire literature describes a galvanic interaction between two contacting sulfide 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; Huang and Grano, 2005; Figure 11 — Effect initial pH of grinding on recovery of Mehta and Murr, 1983; Abraitis et al., 2003; Akcil and Ciftci, chalcopyrite at flotation pH 10.5. 2003; Thornber, 1975; Sato, 1992; Alpers and Blowes, 1994; Sikka et al., 1991; Banfield, 1997). Effect of ferric ions on chalcopyrite. It has been reported that chalcopyrite dissolution in sulfate media with ferric sulfate is an electrochemical reaction where the anodic and cathodic half-cell reactions are written as in Eqs. (14) and (15) (Mikhin et al., 2004; Misra and Fuerstenau, 2005; Watling, 2006; Liu et al., 2007). Anodic half-cell reaction: chalcopyrite oxidation  2+ 2+ - CuFeS2 Cu + Fe + 2S° + 4e (14) Cathodic half-cell reaction: reduction of ferric ions Fe3+ + e-  Fe2+ (15)

The ferric ions would generate ferrous ions according to Eq. Figure 12 — Recovery of chalcopyrite at different pHs with (16), and ferrous ions can generate H2O2 (Eqs. (17) and (18)), wet and dry ground solids. as shown in Table 1. H2O2 oxidizes chalcopyrite (Eq. (13)), as displayed in Fig. 15 (Antonijevic et al., 2004). 3+ - 2+ Fe + e  Fe (16) of chalcopyrite and decreases the recovery of chalcopyrite flotation; (c) increasing the concentration ofH O also in- 2+  3+ • - 2 2 Fe + O2 Fe + (O2 ) (17) creases the chalcopyrite leaching recovery; (d) decreasing the 2+ • - +  3+ concentration of H2O2 decreases the oxidation of pyrite and Fe + (O2 ) + 2H Fe + H2O2 (18) increases the recovery of pyrite flotation. The above discussion may be summarized by the following outcomes: (a) increasing the pyrite proportion increases the Conclusions concentration of H2O2 in the pyrite-chalcopyrite mixture; (b) For this paper, fundamental studies were carried out to resolve increasing the concentration of H2O2 increases the oxidation or reveal the source of one of the most common problems of

MINERALS & METALLURGICAL PROCESSING 217 Vol. 30 No. 4 • November 2013 Figure 13 — Effect of a) pyrite or b) chalcopyrite proportion in the chalcopyrite-pyrite mixture on concentration of H2O2 with 12.5 g sample after 5 min with water at different pHs.

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. Increasing pH decreased its concentration up to pH 8, at which point it increases. 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 the pyrite fraction increases H2O2 formation, correlating to a decrease in chalcopyrite recovery with increasing pyrite fraction in mixed mineral composition. This 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, which is devoid of oxygen, illustrating that the oxygen in H2O2 is derived from the water molecules. Figure 14 — Proposed mechanisms for oxidation of chalcopyrite (Cp) by a) galvanic interaction between chalcopyrite and pyrite (Py): Anodic dissolution Acknowledgments reactions occur on the surface of Cp and cathodic reactions occur on the surface Financial support from the Centre for of Py (adapted from Koleini et al., 2011); b) formation of H2O2 by Py via the incomplete reduction of oxygen from two reacting •OH (Eqs. (3)-(6)); c) both Advanced Mineral and Metallurgy (CAMM) galvanic interactions between Cp and Py and the formation of H O by Py. Research Centre, LTU and Boliden Mineral 2 2 AB is gratefully acknowledged. flotation, i.e., oxidation in sulfide ore flotation. For this reason, References the oxidation of sulfide mineral at the grinding stage was evalu- Abraitis, P.K., Pattrick, R.A.D., Kelsall, G.H., and Vaughan, D.J., 2003, “Acid leaching and dissolution of major sulfide ore minerals: process and galvanic ated. For the first time in mineral processing applications, it effects in complex systems,” Electrochemistry in Mineral and Metals Process- was found out that during grinding (especially fine grinding) ing, Vol. VI, F.M. Doyle, G.H. Kelsall, R. Woods, Eds., The Electrochemical formation of H O in pulp liquid takes place. It is known that Society Inc., pp. 143-153. 2 2 Ackerman, P.K., Harris, G.H., Klimpel, R.R., and Aplan, F.F., 1987, “Evaluation of hydrogen peroxide is a strong oxidizing agent and can easily flotation collectors for copper sulfides and pyrite: Effect of xanthate chain oxidize sulfide minerals. It can even cause dissolution of some length and branching,” Int. J. Miner. Process., Vol. 21, pp. 105-127. metal ions from the ore surface. That is why H O , rather than Akcil, A., and Ciftci, H., 2003, “Metals recovery from multimetal sulfide con- 2 2 centrates (CuFeS2–PbS–ZnS): combination of thermal process and pressure oxygen, can be the main reason for oxidation, inadvertent leaching,” Int. J. Miner. Process., Vol. 71, pp. 233-246. activation and nonselectivity in sulfide ore flotation. Preven- Alpers, C.N., and Blowes, D.W., 1994, “Secondary iron-sulfate minerals as sources of sulfate and acidity,” Environmental Geochemistry of Sulfide Oxidation, tion or reduction of H2O2 formation may result in improved C.N. Alpers and D.W. Blowes, Eds., Am. Chem. Soc. Series 550, pp. 345-364. flotation results and the authors are working on these aspects. Antonijevic, M., Jankovic, Z., and Dimitrijevic, M., 2004, “Kinetics of chalcopyrite Preliminary conclusions were drawn from the results of our dissolution by hydrogen peroxide in sulfuric acid,” Hydrometallurgy, Vol. initial investigations. Formation of hydrogen peroxide takes 71, pp. 329-334.

November 2013 • Vol. 30 No. 4 218 MINERALS & METALLURGICAL PROCESSING components on sphalerite flotation from complex sulfide,”J. Miner. Metall. Process., Vol. 29, pp. 192-198. Javadi Nooshabadi, A., Larsson, A-C., Hanumantha Rao, K., 2013, “Formation of hydrogen peroxide by pyrite and its influence on flotation,”Minerals Engineering, Vol. 49, pp. 128-134. Jones, G.C., Corin, K.C., van Hille, R.P., and Harrison, S.T.L., 2011, “The generation of toxic reactive oxygen species (ROS) from mechanically activated sulfide concentrates and its effect on thermophilic bioleaching,” Minerals Engineering, Vol. 24, pp. 1198-1208. Jones, G., van Hille, R.P., and Harrison, S.T.L., 2012, “Reactive oxygen species generated in the presence of fine pyrite particles and its implication in thermophilic mineral bioleaching,” Appl. Micrbiol. Biotechnol., doi: 10.1007/ s00253-012-4116-y. 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Kosaka K., Yamada H., Matsui S., Echigo S., and Shishida K., 1998, “A comparison among the methods for hydrogen peroxide measurements to evaluate advanced oxidation processes: application of a spectrophotometric method Baga, A.N., Johnson G.R.A., Nazhat, N.B., and Saadalla-Nazhat, R.A., 1988, “A using copper(II) ion and 2,9-dimethyl-1,10-phenanthroline,” Environ. Sci. simple spectrophotometric determination of hydrogen peroide at low con- Technol., Vol. 32, pp. 3821-3824. centrations in aqueous solutions,” Anal. Chim. Acta, Vol. 204, pp. 349-353. Liu, Q.Y., Li, H., and Zhou, L., 2007, “Study of galvanic interactions between Banfield, J.F., and Nealson, K.H., Eds., 1997,Geomicrobiology: Interactions pyrite and chalcopyrite in a flowing system: implications for the environment,” Between Microbes and Minerals, Mineralogical Society of America, Wash- Environ. Geol., Vol. 52, pp. 11-18. ington, D.C. p. 448. Mehta, A.P., and Murr, L.E., 1983, “Fundamental studies of the contribution Borda, M.J., Elsetinow, A.R., Strongin, D.R., and Schoonen, M.A., 2003, “A of galvanic interaction to acid-bacterial leaching of mixed metal sulfides,” mechanism for the production of hydroxyl radical at surface defect sites on Hydrometallurgy, Vol. 9, pp. 235-256. pyrite,” Geochimica et Cosmochimica Acta, Vol. 67, pp. 935-939. Mikhin, Y.L., Tomashevich, Y.V., Asanov, I.P., Okotrub, A.V., Varnek, V.A., and Buckley, A., and Woods, R., 1984, “An X-ray photoelectron spectroscopy study Vyalikh, D.V., 2004, “Spectroscopic and electrochemical characterization of the oxidation of chalcopyrite,” Australian Journal of Chemistry, Vol. 37, of the surface layers of chalcopyrite (CuFeS2) reacted in acidic solutions,” No. 12, pp. 2403-2413. Appl. Surf. Sci., Vol. 225, pp. 395-409. Buehler, H.A., and Gottschalk, V.H., 1910, “Oxidation of sulfides,” Econ. 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Advances in Colloid Castro, S.H., and Baltierra, L., 2003, “Redox condition in the selective flota- and Interface Science, Vols. 114-115, pp. 227-237. tion of enargite,” Proceedings of Electrochemistry in Mineral and Metal Owusu, C., Zanin, M., Fornasiero, D., and Addai-Mensah, J., 2011, “Influence of Processing VI, pp. 27-36. pyrite content on the flotation of chalcopyrite after regrinding with Isamaill,” Chander, S., 1991, Electrochemistry of sulfide flotation: growth characteristics of CHEMECA 2011, Engineering a Better World, 1-10, Sydney, Australia. surface coatings and their properties, with special reference to chalcopyrite Peng, Y., Grano, S., Fornasiero, D., and Ralston, J., 2003, “Control of grinding and pyrite,” International J, Mineral Processing, Vol. 33, pp. 121-134. conditions in the flotation of chalcopyrite and its separation from pyrite,” Chandraprabha, M.N., and Natarajan, K.A., 2006, “Surface chemical and flota- Int. J. Miner. 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A chemical model Fairthorne, G., Fornasiero, D., and Ralston, J., 1997, “Effect of oxidation on based on massive nickel sulfide deposits at Kambalda, Western Australia,” the collectorless flotation of chalcopyrite,” International Journal of Mineral Chemical Geology, Vol. 15, pp. 1-14. Processing, Vol. 49, pp. 31-48. Valko, M., Morris, H., and Cronin, M.T.D, 2005, “Metals,toxicity and oxidative Holmes, P.R., and Crundwell, F.K., 1995, “Kinetic aspects of galvanic interactions stress,” Current Medicinal Chemistry, Vol. 12, pp. 1161-1208. between minerals during dissolution,” Hydrometallurgy, Vol. 39, pp. 353-375. Watling, H.R., 2006, “The bioleaching of sulfide minerals with emphasis on cop- Huang, G. and Grano, S., 2005, “Galvanic interaction of grinding media with pyrite per sulfides – a review,” Hydrometallurgy, Vol. 84, pp. 81-108. and its effect on flotation,” Minerals Engineering, Vol. 18, pp. 1152-1163. 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MINERALS & METALLURGICAL PROCESSING 219 Vol. 30 No. 4 • November 2013

Paper 3

Formation of hydrogen peroxide by sphalerite

Alireza Javadi Nooshabadi, Kota Hanumantha Rao

International Journal of Mineral Processing, 2013, 125, 78-85

International Journal of Mineral Processing 125 (2013) 78–85

Contents lists available at ScienceDirect

International Journal of Mineral Processing

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

Formation of hydrogen peroxide by sphalerite

Alireza Javadi Nooshabadi, Hanumantha Rao Kota ⁎

Mineral Processing Group, Division of Sustainable Process Engineering, Luleå University of Technology, SE-971 87 Luleå, Sweden Department of Civil, Environmental and Natural Resources Engineering, Luleå University of Technology, SE-971 87 Luleå, Sweden article info abstract

Article history: Formation of hydrogen peroxide (H2O2), an oxidizing agent stronger than oxygen, by sphalerite ((Zn, Fe) S) was Received 3 April 2013 examined during its grinding process. It was observed that sphalerite generated H2O2 in pulp liquid during wet Received in revised form 22 September 2013 grinding and also when the freshly ground solids placed in water immediately after dry grinding. The generation Accepted 28 September 2013 of H O in either wet or dry grinding was thought to be due to a reaction between sphalerite and water where the Available online 09 October 2013 2 2 mineral surface is catalytically active to produce •OH free radicals by breaking down the water molecule. Effect of pH on the formation of H O by sphalerite showed that acidic pH values generated more H O . Mixtures of pyrite, Keywords: 2 2 2 2 chalcopyrite and galena with sphalerite on the formation of H2O2 were also probed. It was shown that the con- H2O2 – – Oxidation centration of H2O2 increases with increasing pyrite or chalcopyrite fraction in pyrite sphalerite or chalcopyrite Pyrite sphalerite or galena–sphalerite mixtures but with an increase in galena proportion, the concentration of H2O2 de- Chalcopyrite creased. The oxidation or dissolution of one mineral than the other in a mixture can also be accounted with the Galena and sphalerite extent of H2O2 formation in the pulp liquid in addition to galvanic interactions. H2O2 plays a greater role in the oxidation of sulfides or aiding the extensively reported galvanic interactions since the amount of H2O2 generated with a specific mineral followed the rest potential series of sulfide minerals. This study highlights the necessity of

1. Introduction Sphalerite and pyrite frequently occur together in ore deposits along with galena and copper containing ores such as chalcopyrite. Separation Hydrogen peroxide, a stronger oxidizing agent than oxygen, causes of sphalerite from pyrite and silicate gangue minerals is normally carried oxidation of sulfide minerals. The oxidation of sulfide minerals takes out by anionic xanthate flotation with copper ion activation of sphalerite place during the grinding process when the particle size is reduced for at high pH (Shen et al., 1998) although some plants such as Teck

flotation. Recently it was shown that formation of H2O2 takes place dur- Cominco also do this at lower pH (Harmer et al., 2008). Buehler and ing wet grinding of complex sulfide ore (Ikumapayi et al., 2012a,b). Gottschalk (1910) reported that when pyrite was mixed with a second Other works have highlighted the significance of reactive oxygen spe- sulfide mineral, the second mineral gets oxidized more rapidly. Harvey • − • cies (ROS), i.e., superoxide anion (O2 ) , hydroxyl radical ( OH) and and Yen (1998) observed that addition of galena to the selective sphaler- H2O2 generated from milled sulfide concentrates and their potential ef- ite leaching system decreased the dissolution of sphalerite. Alternatively, fect on thermophilic Fe- and S-oxidizing bioleaching microorganisms a pyrite or chalcopyrite addition increased zinc extractions. Many au- through oxidative stress (Jones et al., 2011; Jones et al., 2013a,b). To thors have been reported that galvanic interactions are known to occur date, most studies dealing with sulfide mineral-induced ROS formation between conducting minerals and play a significant role in flotation have made use of natural or synthetic mineral samples, of very high pu- (Rao and Finch, 1988; Kelebek et al., 1996; Zhang et al., 1997; Ekmekçi rities, suspended in neutral solutions. Pyrite induced ROS formation has and Demirel, 1997; Huang and Grano, 2005), leaching (Mehta and been studied most; however other sulfide minerals such as chalcopyrite Murr, 1983; Abraitis et al., 2003; Akcil and Ciftci, 2003), supergene en-

(CuFeS2), sphalerite (ZnS), pyrrhotite (Fe(1 − x)S) and vaesite (NiS2) richment of sulfide ore deposits (Thornber, 1975; Sato, 1992), environ- have also been studied with respect to ROS generation (Borda et al., ment governance (Alpers and Blowes, 1994), and geochemical 2001; Jones et al., 2011; Javadi et al., 2013; Javadi and Hanumantha processes (Sikka et al., 1991; Banfield and Nealson, 1997). • Rao, 2013), and reactivities have been found to differ between sulfide However, participation of H2O2 and OH, if any, in oxidation of the minerals. 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 this gap, we have estimated the concentration of H2O2 ⁎ Corresponding author at: Department of Civil, Environmental and Natural Resources in pulp liquid at different times of grinding and different grinding envi- Engineering, Luleå University of Technology, SE-971 87 Luleå, Sweden. Tel.: +46 920 491705; fax: +46 920 491199. ronments. Thus the effect of pH, type of grinding (wet or dry grinding), E-mail address: [email protected] (H.R. Kota). and different pyrite, chalcopyrite and galena fractions in mixtures with

0301-7516/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.minpro.2013.09.010 A. Javadi Nooshabadi, H.R. Kota / International Journal of Mineral Processing 125 (2013) 78–85 79 sphalerite on formation of hydrogen peroxide was investigated. In this liquid by UV–Visible spectrophotometer. Zinc sulfate, copper sulfate, fer- paper, the results of these investigations were presented and discussed rous sulfate and ferric sulfate chemicals were also used to investigate the 2+ 2+ 2+ 3+ various pathways for the generation of H2O2. effect of metal ions (Zn ,Cu ,Fe and Fe ) on the formation of H2O2.TheH2O2 level in flotation tests was adjusted with H2O2 addition. 2. Experimental 2.2. Wet grinding 2.1. Materials and reagents In each grinding test, a 100 g of −3.35 mm size fraction of sphalerite Crystalline pure sphalerite (Sp), galena (Ga), pyrite (Py) and chalcopy- mineral was mixed with 400 cm3 of water and ground in a new labora- rite (Cp) minerals used in this study were procured from Gregory, Bottley tory stainless steel ball mill (Model 2VS, CAPCO Test Equipment, Suffolk, & Lloyd Ltd., UK. Sphalerite contains 39.92% Zn, 20.7% S, 4.2% Fe, 1.32% Pb UK) with stainless steel medium for 60 min. The pH of the pulp liquid and 0.17% Cu; galena contains 73.69% Pb, 13.5% S, 1.38% Fe, 1.26% Zn, 0.2% was adjusted to a specified value by HCl and NaOH solutions before Cu and some silica (quartz) impurity; pyrite contains 44.4% Fe, 50.9% S, grinding. Aliquot portions of slurry samples were collected at pre- and 0.2% Cu, and chalcopyrite contains 29% Fe, 29.5% S, 25.8% Cu, 0.54% determined grinding time intervals and filtered immediately through Zn, and 0.22% Pb. The XRD analyses of the samples showed that the Millipore filter of 0.22 μmporesizeandthefiltrate was analyzed for hy- main mineral phases present were the pyrite (Fig. 1a), chalcopyrite drogen peroxide. (Fig. 1b), sphalerite (Fig. 1c) and galena (Fig. 1d), in the respective sulfide mineral samples. The pyrite, chalcopyrite, sphalerite and galena samples 2.3. Dry grinding as received and used in this study were separately crushed through a jaw crusher and then screened to collect the −3.35 mm particle size frac- 100 g of −3.35 mm size fraction of sphalerite, pyrite, galena and tion. The homogenized sample was then sealed in polyethylene bags. chalcopyrite minerals in each grinding was independently separately Potassium amyl xanthate (KAX) collector and MIBC frother were used ground in a laboratory stainless steel ball mill with stainless steel medi- in flotation studies. Dilute solutions of AR grade NaOH and HCl were used um for 60 min. 5 g of sphalerite, galena, pyrite or chalcopyrite single to maintain the pH at the targeted value during flotation. Deionised water mineral and 12.5 g in total of mineral mixture either sphalerite–pyrite was used in the processes of both grinding and flotation. 2,9-dimethyl- or sphalerite–chalcopyrite or sphalerite–galena that was b106 μmwas 1,10-phenanthroline (DMP), copper (II) sulfate (0.01 M), and phosphate conditioned with 50 cm3 water using a magnetic stirrer and the slurry buffer (pH 7.0) solutions were used for estimating H2O2 amount in pulp sample was collected successively after 0.5, 5 and 11 min conditioning

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). 80 A. Javadi Nooshabadi, H.R. Kota / International Journal of Mineral Processing 125 (2013) 78–85

0.6

0.5

0.4

(mM) 0.3 2 O 2

H 0.2

0.1

0 02468 pH

Fig. 2. Effect of pH in wet grinding on formation of H2O2 by sphalerite. time and analyzed for hydrogen peroxide. The pH was regulated with HCl and/or NaOH solution. Eh (pulp potential) was measured at room temperature during mixing.

2.4. Estimation of hydrogen peroxide

Fig. 4. Eh–pH stability diagram for the ZnS–H2O–FeS system (Huai Su, 1981). The spectrophotometric method using copper (II) ions and DMP has been found to be reasonably sensitive when applied to advanced oxida- The pH was regulated with dilute NaOH and HCl solutions. Experiments tion processes (Kosaka et al., 1998). For DMP method (Baga et al., 1988), were performed at ambient temperature of approximately 22.5 °C. 1 mL each of 1% DMP in ethanol, 0.01 M copper (II) sulfate, and phosphate buffer (pH 7.0) solutions were added to a 10 mL volumetric 3. Results and discussion flask and mixed. A measured volume of liquid (filtrate) sample was added to the volumetric flask, and the volume was made up to the 3.1. Formation of hydrogen peroxide (H O ) in wet grinding mark with ultrapure water. After mixing, the absorbance of the sample 2 2 at 454 nm was measured with DU® Series 700 UV/Vis Scanning Spec- Initially the extent of H O formation during wet grinding of sphal- trophotometer. The blank solution was prepared in the same manner 2 2 erite was investigated. For these studies, sphalerite single mineral was but without H O . 2 2 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 2.5. Flotation test (filtrate) was analyzed for hydrogen peroxide. Formation of hydrogen peroxide was detected in the filtrate. The effect of pulp pH during grind- After dry grinding for 60 min of 100 g of −3.35 mm feed size, the ing on formation of hydrogen peroxide is shown in Fig. 2.Itcanbeseen mill was emptied and the solids were screened at 106 μmsize.For that at pH 2.7 generated more H O than at pHs 3.7 and 6.8. This is in each flotation 5 g of sample that was b106 μm was transferred to a 2 2 agreement with our earlier studies where pyrite was shown to generate cell of 150 mL capacity (Clausthal flotation equipment), conditioned H O during wet grinding (Javadi et al., 2013). with pH modifier (HCl/NaOH), collector and frother. Flotation concen- 2 2 trate 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 3.2. Formation of hydrogen peroxide (H2O2)afterdrygrindingandmixing were 10−4 M KAX. The conditioning times for adjusting pH and collec- with water tor were 5 min and 2 min respectively. The frother dosage was one drop MIBC in all cases. Sphalerite flotation was investigated at different pH. 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 concentra- 0.7 450 tion of H2O2 increased with decreasing pH. Fig. 3 also shows that Eh in- 400 – 0.6 creased with decreasing pH. Huai Su (1981) showed by Eh pH stability b 2+ 2+ H2O2 350 diagram of sphalerite species that in pH 6, Zn and Fe species exist 0.5 (Fig. 4). The results in Table 1 show that Zn2+ and Fe2+ generated H O . 300 2 2 Eh In the presence of dissolved molecular oxygen, ferrous iron can form 0.4 250 (mM) 2 200 O 0.3

2 Table 1 2+ 3+ 2+ H 150 H2O2 generation measured after 1 hfrominitialdissolvedFe ,Fe and Zn 0.2 Eh (mV, SHE) concentrations, performed at natural pH and at 22 °C. 100 Concentration of ions H O (mM) 0.1 50 2 2 1mM 10mM 0 0 2.5 4.5 8 11.6 Water 0 0 2+ pH Fe 0.552 4.656 Fe3+ 0.004 0.059 Zn2+ 0.004 0.088 Fig. 3. Effect of pH on H2O2 concentration. A. Javadi Nooshabadi, H.R. Kota / International Journal of Mineral Processing 125 (2013) 78–85 81

3.3. 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 increases with increasing sphalerite loading as shown in Fig. 6.Thisﬁgure also

shows that the concentration of H2O2 increases with increasing mixing time, obviously due to increased sphalerite surface reactions with water.

3.4. Effect of hydrogen peroxide on sphalerite ﬂotation

Leroux et al. (1987) in batch flotation tests on two ores found that sphalerite floated at pH 9.5–11.0 without activation by Cu2+ ions. He suggested that the presence of ferrous ions in solution was responsible. Also Zhang et al. (1992) reported that sphalerite floated readily at pH 8–11 in the presence of ferrous ions, but not in the presence of ferric ions. He showed by electrokinetic measurements that the surface charge increased in the presence of Fe2+ ions and oxygen while we

showed in this paper that ferrous ion can generate H2O2 as shown in Table 1. For indication that H2O2 can be responsible for sphalerite flota- 2+ 2+ – 2+ Fig. 5. Proposed mechanisms for formation of H2O2 by sphalerite through Zn or Fe tion at pH 9.5 11.0 without activation by Cu , after dry-grinding for • fi reacting with water to generate OH and H2O2. The numericals in this gure represent 60 min, the effect of H2O2 formation in water by the freshly ground the equation numbers. solids on sphalerite flotation recovery was investigated. The concentra-

tion 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 b1 mM. This result can be the explanation for a be- superoxide anion (O2 ) (Eq. (1)), which reacts with ferrous iron to havior that Hallimond ﬂotation recovery of sphalerite decreased when form H2O2 (Eq. (2))(Cohn et al., 2006)asshowninFig. 5. pH increases from pH 3 to pH 5 (Ikumapayi et al., 2012a,b)mostlycan 2þ 3þ − • be due to H2O2 decreases with increasing pH as shown in Fig. 3. Fe þ O2 →Fe þðO2 Þ ð1Þ

3.5. Sphalerite–pyrite mixture

2þ • − þ 3þ Fe þðO2 Þ þ 2H →Fe þ H2O2: ð2Þ Many authors have reported that galvanic interactions are known to − occur between sulfide minerals and play a significant role in flotation Also ZnS can generate superoxide anion (O •) to form H O 2 2 2 (Rao and Finch, 1988; Kelebek et al., 1996; Zhang et al., 1997; Ekmekçi (Eqs. (3)–(5)): and Demirel, 1997; Huang and Grano, 2005), and we have investigated – ZnS→Zn2þ þ 2e− þ S0 ð3Þ the effect of sphalerite pyrite mixture on formation of hydrogen peroxide. Samples of sphalerite and pyrite are separately dry-ground in a laboratory ball mill. Fig. 8 shows the effect of pyrite proportion in sphalerite–pyrite mixture on formation of hydrogen peroxide at differ- − • − e þ O2 →ðO2 Þ ð4Þ ent pH. It can be seen that with an increase in pyrite proportion, the concentration of H2O2 has increased. This is in agreement with the studies wherepyrite(FeS2) generated hydrogen peroxide (Borda et al., 2001) and hydroxyl radicals, HO• (Borda et al., 2003) when placed in water. 2ðO •Þ− þ 2Hþ →H O þ O ð5Þ 2 2 2 2 In the presence of dissolved molecular oxygen, ferrous iron associated

2 1.8 1.6 Sp loading (g/l) 1.4 1.2 50

(mM) 1 2 O 2 0.8 H 100 0.6 0.4 0.2 250 0 0.5 5 11 Mixing time (min)

Fig. 6. Effect of sphalerite loading on H2O2 production after 5 min mixing with water at pH 2.5. 82 A. Javadi Nooshabadi, H.R. Kota / International Journal of Mineral Processing 125 (2013) 78–85

100 90 80 70 60 50 40 30 20 Recovery of Sphalerite (%) 10 Fig. 9. Proposed mechanisms for oxidation of sphalerite by formation of H2O2 intermedi- 0 ates are proposed to form via the incomplete reduction of oxygen and be formed from 010.01 0.1 10 two reacting •OH.

concentration of H2O2 (mM)

Fig. 7. Effect of H2O2 on recovery of sphalerite with dry-ground solids at pH 10.5. sphalerite increases with increasing hydrogen peroxide concentration (Adebayo et al., 2006; Aydogan, 2006). • − – with pyrite can form superoxide anion (O2 ) (Eq. (6)), which reacts Fig. 8 shows the effect of sphalerite proportion in sphalerite pyrite with ferrous iron to form H2O2 (Eq. (7))(Cohn et al., 2006; Javadi mixture on formation of hydrogen peroxide. It can be seen that with et al., 2013). Borda et al. (2003) showed that pyrite can also generate an increase in sphalerite proportion, the concentration of H2O2 de-

H2O2 in the absence of molecular oxygen (Eqs. (8), (9)). creased. This mechanism that an increase in sphalerite proportion, the concentration of H2O2 decreases could be a better explanation for the II III • − ﬁ ﬂ Fe ðpyriteÞþO2 →Fe ðpyriteÞþðO2 Þ ð6Þ results where a signi cant depression of activated pyrite otation in the presence of sphalerite at all pH values happened with recovery con- tinuously decreasing to nearly 2% with increasing sphalerite content at pH 11 (Zhang et al., 1997; Dichmann and Finch, 2001). II • − þ III Fe ðpyriteÞþðO2 Þ þ 2H →Fe ðpyriteÞþH2O2 ð7Þ

3.6. Sphalerite–chalcopyrite mixture

III • þ II Fe ðpyriteÞþH2O→HO þ H þ Fe ðpyriteÞð8Þ Yelloji Rao and Natarajan (1989) observed that significantly higher flotation recoveries of sphalerite could be obtained even in the absence of Cu-ion activation from a sphalerite–chalcopyite mixture. Harvey and Yen (1998) reported that the rate of sphalerite leaching increased with 2HO•→H O : ð9Þ 2 2 an increase in chalcopyrite proportion in the sphalerite–chalcopyite mixture, and indicated that this behavior is due to galvanic interaction This methodical process where an increase in pyrite proportion, the between sphalerite and chalcopyite. We may suggest that the increase in sphalerite flotation recovery (Fig. 7) without metal-ion activation production of H2O2 increases (Fig. 9) would perhaps explain better for a behavior that the rate of sphalerite leaching increases with increasing could also be due to the formation of H2O2 as it creates oxidizing envi- chalcopyrite proportion (Harvey and Yen, 1998). This is in agreement ronment for the formation dixanthogen, the responsible surface species fl with other studies where it was observed that the oxidation of generally considered for the otation to take place. To ascertain this hy- pothesis, samples of different proportions of sphalerite and chalcopyrite

minerals were dry-ground and the formation of H2O2 at different pH has 3

2.5 0.9 pH 0.8 2 Py 2.5 0.7 pH Py 4.5 Py 11.5 0.6 Cp 4.5 (mM)

2 1.5 Py 9 0.5 Cp 11.5 O 2 Sp 2.5 (mM) 2

H Cp 9

Sp 4.5 O 0.4 1 2 Sp 11.5 H 0.3 Sp 4.5 Sp 11.5 Sp 11.5 0.5 0.2 Sp 9 0.1 0 0 0 20 40 60 80 100 0 20 40 60 80 100 Pyrite or Sphalerite proportion % Chalcopyrite or Sphalerite proportion %

Fig. 8. Effect of pyrite or sphalerite proportion in sphalerite–pyrite mixture on concentra- Fig. 10. Effect of chalcopyrite or sphalerite proportion in sphalerite–chalcopyrite mixture tion of H2O2 at different pH. on concentration of H2O2 at different pH. A. Javadi Nooshabadi, H.R. Kota / International Journal of Mineral Processing 125 (2013) 78–85 83 been evaluated (Fig. 10). It can be seen that with an increase in chalco- 1.2 pyrite proportion, the concentration of H2O2 increased. This is in agreement with other studies where chalcopyrite (CuFeS ) was shown to 1 2 pH generate hydrogen peroxide (Jones et al., 2011; Jones et al., 2013a; Ga 2.5 Javadi and Hanumantha Rao, 2013). This phenomenon of increasing 0.8 Ga 4.5 – chalcopyrite proportion in sphalerite chalcopyrite mixture increases Ga 11.5 (mM) 0.6 H2O2 formation would also explain the results reported by earlier 2 Ga 9 O authors: 2 Sp 2.5 H 0.4 Sp 4.5 1. Signiﬁcantly higher recoveries for sphalerite could be obtained even Sp 11.5 in the absence Cu-metal ion activation from the Sp–Cp mixture 0.2 Sp 9 (Yelloji Rao and Natarajan, 1989). 2. The rate of sphalerite leaching increased with an increase in chalco- 0 pyrite proportion (Harvey and Yen, 1998) as shown in Fig. 11. This 0 20 40 60 80 100 is in agreement with other studies where it was shown that the oxi- Galena or Sphalerite proportion % dation of sphalerite increases with increasing hydrogen peroxide concentration (Adebayo et al., 2006; Aydogan, 2006). Fig. 12. Effect of galena or sphalerite proportion in galena–sphalerite mixture on concen-

tration of H2O2 at different pH. Fig. 10 shows the effect of sphalerite proportion in sphalerite– chalcopyrite mixture on formation of hydrogen peroxide. It can be ﬁ seen that with an increase in sphalerite proportion, the concentra- medium, can be used to leach zinc sul de (Eq. (10))asshowninFig. 14a (Aydogan et al., 2005; Dutrizac et al., 2003; Santos et al., 2010): tion of H2O2 decreased. ZnS þ 2Fe3þ →Zn2þ þ 2Fe2þ þ S0: ð10Þ 3.7. Sphalerite–galena mixture Also Holmes (2000) reported that sphalerite reactivity depends on Galena and sphalerite minerals are the most important raw mate- its iron content, i.e., the more iron in the solution, the easier the sphal- rials to obtain Pb and Zn base metals, respectively, and generally are erite lattice opens up, while the mechanism could be that ferric ions found associated together. Harvey and Yen (1998) showed that the generate ferrous ions (Eq. (11)), which ions in turn generate H2O2 rate of sphalerite leaching decreased with an increase in galena propor- (Eqs. (1) and (2))andH2O2 oxidizes sphalerite (Eq. (12))asshownin tion in sphalerite–galena mixture due to galvanic interactions. We pro- Fig. 14b(Adebayo et al., 2006; Aydogan, 2006). pose that it could be due to formation of H2O2. To elucidate this 3þ − 2þ proposition, samples of sphalerite and galena are separately dry- Fe þ e →Fe ð11Þ ground, mixed and the formation of H2O2 was investigated. Fig. 12 shows the effect of galena proportion in sphalerite–galena mixture on formation of hydrogen peroxide at different pH. It can be seen that þ 2þ 2− 2ZnS þ 4H O þ 2H →2Zn þ SO þ H S þ 4H O: ð12Þ with increasing galena proportion in the mixture, the production of 2 2 4 2 2

H2O2 decreased. This result of decreasing H2O2 generation with an in- The above discussion may be summarized as follows: (i) increasing crease in galena proportion would explain a decrease in sphalerite pyrite proportion increases production of H2O2 in pyrite–sphalerite leaching rate with increasing galena proportion (Harvey and Yen, mixture; (ii) increasing concentration of H2O2 increases oxidation of 1998). Fig. 12 also shows the effect of sphalerite proportion in sphaler- sphalerite and accelerates leaching of sphalerite; (iii) increasing chalco- – ite galena mixture on formation of hydrogen peroxide, where with in- pyrite proportion increases production of H2O2 in chalcopyrite–sphaler- creasing sphalerite proportion, the production of H2O2 increased. A ite mixture; (iv) increasing galena proportion decreases concentration – schematic illustration of H2O2 formation in galena sphalerite mixture of H2O2 in galena–sphalerite mixture; and (v) decreasing concentration is shown in Fig. 13. of H2O2 decreases oxidation of sphalerite and decreases leaching rate of sphalerite. 3.8. Effect of ferric ions on sphalerite

Earlier studies reported that ferric ion, which is one of the most im- 4. Conclusions portant oxidants used in leaching processes in either sulfate or chloride In the scope of the present work, a few fundamental studies were carried out to reveal one of the main problems (i.e., oxidation) in sulﬁde

Fig. 11. Proposed mechanisms for oxidation of sphalerite by formation of H2O2 intermedi- Fig. 13. Proposed mechanisms for oxidation of galena by formation of H2O2 intermediates ates are proposed to form via the incomplete reduction of oxygen and be formed from two are proposed to form via the incomplete reduction of oxygen and be formed from two reacting •OH. reacting •OH. 84 A. Javadi Nooshabadi, H.R. Kota / International Journal of Mineral Processing 125 (2013) 78–85

Fig. 14. Proposed mechanisms for oxidation of sphalerite a) by ferric ions b) both ferric ions and H2O2 that generated by ferrous ions and sphalerite.

ore flotation. For this reason, the grinding stage was initially examined Cohn, C.A., Mueller, S., Wimmer, E., Leifer, N., Greenbaum, S., Strongin, D.R., Schoonen, and the possible oxidation of the ores was investigated at this stage. It M.A.A., 2006. Pyrite-induced hydroxyl radical formation and its effect on nucleic acids. Geochem. Trans. 7, 1. fi was found that during grinding (especially ne grinding) of metal Dichmann, T.K., Finch, J.A., 2001. The role of copper ions in sphalerite–pyrite flotation. – sulfides formation of H2O2 takes place. Hydrogen peroxide was a strong Miner. Eng. 14, 217 225. oxidizing agent and can easily oxidize sulfide minerals and can even Dutrizac, J.E., Pratt, A.R., Chen, T.T., 2003. The mechanism of sphalerite dissolution in ferric – fi sulphate sulphuric acid media. Yazawa International Symposium, Metallurgical and cause dissolution of some metal ions from sul de minerals. Therefore, Materials Processing: Principles and Technologies. Aqueous and Electrochemical Pro- we suggest that H2O2, rather than oxygen, could be the main reason cessing, vol. III, pp. 139–161. for oxidation, inadvertent activation and non-selectivity in the sulfide Ekmekçi, Z., Demirel, H., 1997. Effects of galvanic interaction on collectorless flotation behaviour of chalcopyrite and pyrite. Int. J. Miner. Process. 52, 31–48. ore flotation. Prevention or reduction of H O formation may result in 2 2 Harmer, S.L., Mierczynska-Vasilev, A., Beattie, D.A., Shapter, J.G., 2008. The effect of bulk improved flotation results and we are working on these aspects. iron concentration and heterogeneities on the copper activation of sphalerite. Miner. – The formation of H2O2 occurs during wet grinding only at pH b 6. Eng. 21, 1005 1012. Harvey, T.J., Yen, W.T., 1998. Influence of chalcopyrite, galena and pyrite on the selective Also, when freshly dry-ground sphalerite was mixed with water, H2O2 – b extraction of zinc from base metal sulphide concentrates. Miner. Eng. 11, 1 21. formation took place at pH 6. The concentration of H2O2 increased Holmes, P.R., 2000. The kinetics of the oxidation of pyrite by ferric ions and dissolved oxygen: with increasing mixing time and sphalerite loading. With an increase in an electrochemical study. Geochim. Cosmochim. Acta 64, 263–274. pyrite or chalcopyrite proportion in pyrite–sphalerite or chalcopyrite– Huang, G., Grano, S., 2005. Galvanic interaction of grinding media with pyrite and its effect on flotation. Miner. Eng. 18, 1152–1163. sphalerite mixture respectively, the concentration of H2O2 increased. Ikumapayi, F, Makitalo, M., Johansson, B., Hanumantha Rao, K., 2012a. Recycling of pro- However, with an increase in galena proportion in galena–sphalerite cess water in sulphide flotation, part A: effect of calcium and sulphate on sphalerite recovery. Miner. Metall. Process. 29 (4), 183–191. mixture, the concentration of H2O2 decreased. Ikumapayi, F., Sis, H., Johansson, B., Hanumantha, Rao K., 2012b. Recycling process water

in sulfide flotation, part B: effect of H2O2 and process water components on sphalerite fl fi – Acknowledgments otation from complex sul de. J. Miner. Metall. Process. 29 (4), 192 198. Javadi Nooshabadi, A., Hanumantha Rao, K., 2013. Formation of hydrogen peroxide by chalcopyrite and its influence on flotation. Miner. Metall. Process. 30 (4), 212–219. Financial support from the Centre for Advanced Mineral and Metal- Javadi, A., Larsson, A.C., Hanumantha Rao, K., 2013. Formation of hydrogen peroxide by lurgy (CAMM) Research Centre, LTU and Boliden Mineral AB is grateful- pyrite and its influence on flotation. J. Miner. Eng. 49 (2013), 128–134. Jones, G.C., Corin, K.C., van Hille, R.P., Harrison, S.T.L., 2011. The generation of toxic reac- ly acknowledged. tive oxygen species (ROS) from mechanically activated sulphide concentrates and its effect on thermophilic bioleaching. Miner. Eng. 24, 1198–1208. Jones, G.C., Becker, M., van Hille, R.P., Harrison, S.T.L., 2013a. The effect of sulfide concen- References trate mineralogy and texture on Reactive Oxygen Species (ROS) generation. Appl. Geochem. 29, 199–213. Abraitis, P.K., Pattrick, R.A.D., Kelsall, G.H., Vaughan, D.J., 2003. Acid leaching and dissolu- Jones, G.C., van Hille, R.P., Harrison, S.T.L., 2013b. Reactive oxygen species generated in the tion of major sulfide ore minerals: process and galvanic effects in complex systems. presence of fine pyrite particles and its implication in thermophilic mineral In: Doyle, F.M., Kelsall, G.H., Woods, R. (Eds.), Electrochemistry in Mineral and Metals bioleaching. Appl. Microbiol. Biotechnol. 97, 2735–2742. Processing, vol. VI. The Electrochemical Society Inc., pp. 143–153. Kelebek, S., Wells, P.F., Fekete, S.O., 1996. Differential flotation of chalcopyrite, pentlandite Adebayo, A.O., Ipinmoroti, K.O., Ajayi, O.O., 2006. Leaching of sphalerite with hydrogen and pyrrhotite in Ni–Cu sulfide ores. Can. Metall. Q. 35, 329–336. peroxide and nitric acid solutions. J. Miner. Mater. Charact. Eng. 5, 167–177. Kosaka, K., Yamada, H., Matsui, S., Echigo, S., Shishida, K., 1998. A comparison among the Akcil, A., Ciftci, H., 2003. Metals recovery from multimetal sulfide concentrates methods for hydrogen peroxide measurements To evaluate advanced oxidation pro- (CuFeS2–PbS–ZnS): combination of thermal process and pressure leaching. Int. cesses: application of a spectrophotometric method using copper(ii) ion and 2,9-di- J. Miner. Process. 71, 233–246. methyl-1,10-phenanthroline. Environ. Sci. Technol. 32, 3821–3824. Alpers, C.N., Blowes, D.W., 1994. Secondary iron-sulfate minerals as sources of sulfate and Leroux, M., Rao, S.R., Finch, J.A., 1987. Selective flotation of sphalerite from Pb–Zn ore acidity. In: Alpers, C.N., Blowes, D.W. (Eds.), Environmental Geochemistry of Sulfide without copper activation. CIM Bull. 80, 41–44. Oxidation. Am. Chem. Soc. Series, 550, pp. 345–364. Mehta, A.P, Murr, L.E., 1983. Fundamental studies of the contribution of galvanic interac- Aydogan, S., 2006. Dissolution kinetics of sphalerite with hydrogen peroxide in sulphuric tion to acid-bacterial leaching of mixed metal sulfides. Hydrometallurgy 9, 235–256. acid medium. Chem. Eng. J. 123, 65–70. Rao, S.R., Finch, J.A., 1988. Galvanic interaction studies on sulfide minerals. Can. Metall. Q. Aydogan, S., Aras, A., Canbazoglu, M., 2005. Dissolution kinetics of sphalerite in acidic fer- 27, 253–259. ric chloride leaching. Chem. Eng. J. 114, 67–72. Santos, S.M.C., Machado, R.M., Correia, M.J.N., Reis, M.T.A., Ismael, M.R.C., Carvalho, J.M.R., Baga, A.N., Johnson, G.R.A., Nazhat, N.B., Saadalla-Nazhat, R.A., 1988. Anal. Chim. Acta. 204, 2010. Ferric sulphate/chloride leaching of zinc and minor elements from a sphalerite 349–353. concentrate. Miner. Eng. 23, 606–615. Banfield, J.F., Nealson, K.H. (Eds.), 1997. Geomicrobiology: Interactions between Microbes Sato, M., 1992. Persistency-field Eh–pH diagrams for sulfides and their application to su- and Minerals. Mineralogical Society of America, Washington, DC, p. 448. pergene oxidation and enrichment of sulfide ore bodies. Geochim. Cosmochim. Acta Borda, M., Elsetinow, A., Schoonen, M., Strongin, D., 2001. Pyrite-induced hydrogen per- 56, 3133–3156. oxide formation as a driving force in the evolution of photosynthetic organisms on Shen, W.Z., Fornasiero, D., Ralston, J., 1998. Effect of collectors, conditioning pH and gases an early Earth. Astrobiology 1, 283–288. in the separation of sphalerite from pyrite. Miner. Eng. 11, 145–158. Borda, M.J., Elsetinow, A.R., Strongin, D.R., Schoonen, M.A., 2003. Amechanismforthe Sikka, D.B., Petruk, W., Nehru, C.E., Zhang, Z., 1991. Geochemistry of secondary copper production of hydroxyl radical at surface defect sites on pyrite. Geochim. Cosmochim. minerals from a Proterozoic porphyry copper deposit, Malanjkhand, India. In: Acta 67 (5), 935–939. 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Huai Su, 1981. Dissolution of sphalerite in ferric chloride solution, Open-File Yelloji Rao, M.K., Natarajan, K.A., 1989. Electrochemical effects of mineral–mineral interac- Report 81–609. http://download.egi.utah.edu/geothermal/GL00457/GL00457. tions on the flotation of chalcopyrite and sphalerite. Int. J. Miner. Process. 27, 279–293. pdf. Zhang, Q., Rao, S.R., Finch, J.A., 1992. Flotation of sphalerite in the presence of iron ions. Thornber, M.R., 1975. Supergene alteration of sulfides, I. A chemical model based on Colloids Surf. 66, 81–89. massive nickel sulfide deposits at Kambalda, Western Australia. Chem. Geol. 15, Zhang, Q., Xu, Z., Bozkurt, V., Finch, J.A., 1997. Pyrite flotation in the presence of metal ions 1–14. and sphalerite. Int. J. Miner. Process. 52, 187–201.

Paper 4

Formation of hydrogen peroxide by galena and its influence on flotation

Alireza Javadi Nooshabadi, Kota Hanumantha Rao

Advanced Powder Technology, 2014, 25, 832-839

Advanced Powder Technology 25 (2014) 832–839

Contents lists available at ScienceDirect

Advanced Powder Technology

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

Original Research Paper Formation of hydrogen peroxide by galena and its inﬂuence on ﬂotation ⇑ Alireza Javadi Nooshabadi, 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 article info abstract

Article history: The formation of hydrogen peroxide (H2O2), an oxidising agent stronger than oxygen, during the grinding Received 18 September 2013 of galena (PbS) was examined. It was observed that galena generated H2O2 in the pulp liquid during wet Received in revised form 17 November 2013 grinding and also when the freshly ground solids were placed in water immediately after dry grinding. Accepted 12 December 2013 The generation of H O during either wet or dry grinding was thought to be due to a reaction between Available online 27 December 2013 2 2 galena and water, when the mineral surface is catalytically active, to produce OHÅ free radicals by break-

ing down the water molecule. It was also shown that galena could generate H2O2 in the presence or Keywords: absence of dissolved oxygen in water. Hydrogen peroxide The concentration of H O formed increased with decreasing pH. The effects of using mixtures of pyrite Hydroxyl radical 2 2 Galena or chalcopyrite with galena were also investigated. In pyrite–galena mixture, the formation of H2O2 Pyrite increased with an increase in the proportion of pyrite. This was also the case with an increase in the frac- Chalcopyrite tion of chalcopyrite in chalcopyrite–galena mixtures. The oxidation or dissolution of one speciﬁc mineral

Flotation rather than the other in a mixture can be explained better by considering the extent of H2O2 formation rather than galvanic interactions. It appears that H2O2 plays a greater role in the oxidation of sulphides or in aiding the extensively reported galvanic interactions. This study highlights the necessity of further study of electrochemical and/or galvanic interaction mechanisms between pyrite and galena or chalcopyrite and galena in terms of their ﬂotation behaviour. Ó 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.

1. Introduction Borda et al. showed that pyrite can also generate H2O2 in the absence of molecular oxygen [3]. They reported that an electron is ex- Hydrogen peroxide causes non-selective oxidation of sulphide tracted from water and a hydroxyl radical is formed (Eq. (4)). minerals. The oxidation of sulphide minerals takes place during Combining two hydroxyl radicals leads to the formation of H2O2: the grinding process when the particle size is reduced for flotation. 3þ 2þ Å þ Recently it was shown that formation of H2O2 takes place in pulp Fe þ H2O ! Fe þ OH þ H ð4Þ liquid during wet grinding of complex sulphide ore [1]. Previous works showed that pyrite (FeS ) [2–8], chalcopyrite (CuFeS ) Å 2 2 2 OH ! H2O2 ð5Þ [7,9], sphalerite (ZnS) [10] and galena (PbS) [11–13] too generate Ahlberg et al. studied the oxygen reduction on pyrite and galena hydrogen peroxide (H2O2) when placed in water. Cohn et al. showed that ferrous iron associated with pyrite in the presence with a rotating ring disc electrode [11–13]. Their results suggest Å that the first electron transfer is the rate-determining step for of dissolved molecular oxygen, can form superoxide anion (O2) the oxygen reduction at both galena and pyrite interfaces and (Eq. (1)), which further reacts with ferrous iron to form H2O2 (Eq. (2)) [4]. Thus hydrogen peroxide can then react with ferrous iron hydrogen peroxide is found to be an intermediate. While galena to form hydroxyl radicals through the Fenton reaction (Eq. (3)) [4]. is a poor catalyst for oxygen reduction, with minor formation of hydrogen peroxide, pyrite is a relatively better catalyst. This kind 2þ 3þ Å of difference may lead to different flotation behaviours. Numerous Fe ðpyriteÞþO2 ! Fe ðpyriteÞþðO Þ ð1Þ 2 studies [14–19] on galena have shown that the floatability of gale- Å na can be affected by the grinding environment. The galena ground Fe2þðpyriteÞþðO Þ þ 2Hþ ! Fe3þðpyriteÞþH O ð2Þ 2 2 2 in an iron mill was much less floatable than galena ground in a ceramic mill [18]. Guy and Trahar demonstrated that the oxida- Fe2þ pyrite H O ÅOH OH Fe3þ pyrite 3 ð Þþ 2 2 ! þ þ ð ÞðÞ tion–reduction environment during grinding had a pronounced influence on the subsequent floatability of galena [14]. Peng et al. ⇑ Corresponding author. Tel.: +46 920 491705; fax: +46 920 491199. showed that galena had a lower floatability when ground with E-mail address: [email protected] (K. Hanumantha Rao). mild steel than with chromium grinding media [15]. According to

0921-8831/$ - see front matter Ó 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. http://dx.doi.org/10.1016/j.apt.2013.12.008 A. Javadi Nooshabadi, K. Hanumantha Rao / Advanced Powder Technology 25 (2014) 832–839 833 the Nernst equation, the partial pressure of oxygen in the mill can a laboratory stainless steel ball mill (Model 2VS, CAPCO Test Equip- also influence the grinding environment [16,19]. Oxygen purging ment, Suffolk, UK) with stainless steel media. The amount of med- causes the grinding environment to be more oxidising while nitro- ia, size of media ball, grinding time for the charge are 850 g, gen purging induces the opposite effect. Natarajan and Iwasaki 1.87 cm and 60 min respectively. The slurry samples were col- studied the electrochemical aspects of grinding media and ob- lected at pre-determined time intervals and they were immedi- served that galvanic coupling of mild steel medium with magnetite ately filtered (Millipore 0.22 lm) and the liquid (filtrate) was or pyrrhotite resulted in the formation of iron hydroxide species on analysed for hydrogen peroxide. the mineral surface [17]. Buehler and Gottschalk noted that when After grinding for 60 min, the mill was emptied and the pulp pyrite was mixed with a second sulphide mineral, the second min- was screened and it was sampled to different portions. In each flo- eral oxidised more rapidly [20]. The galena oxidation was acceler- tation test, 7.5 g of sample that was <106 lm was transferred to a ated electrochemically when in contact with pyrite [21,22]. cell of 150 ml capacity (Clausthal flotation equipment), condi- However, Rao et al. demonstrated that in a 50:50 weight percent tioned with pH modifier, collector and frother. The flotation con- mixture of galena and pyrite the flotation response of galena was centrate was collected after 2.0 min at air flow rate of dramatically impaired compared with the same best performance 0.5 dm3 min1. The flotation froth was scraped every 10 s. Dosages with galena only [23]. They proposed that galvanic interactions of collector for flotation was 104 M KAX. The conditioning times may occur between conducting minerals and play a significant role for adjusting pH and collector were 5 min and 2 min respectively. in flotation. The frother dosage was one drop of MIBC in all cases. The flotation Å However, participation of H2O2 and OH, if any, in the non-selec- of galena was investigated at different pH. The Eh (pulp potential) tive oxidation of the sulphide mineral pulp components and hence was measured at room temperature and during the conditions of in the deterioration of the concentrate grade and recovery of me- all our experiments using a platinum electrode and reference elec- tal-sulphides has not yet been explored. In an attempt to fill the trode and was expressed relative to the standard hydrogen elec- gap, we have estimated the concentration of H2O2 in pulp liquid trode, SHE. The pH was measured using a glass electrode. The pH during different times of grinding and in different grinding envi- was regulated with NaOH and HCl solutions. Experiments were ronments. The effects of pH, type of grinding (wet or dry grinding), performed at room temperature of approximately 22.5 °C. grinding atmosphere (nitrogen/air) and the proportion of pyrite/ chalcopyrite mixed with galena on the formation of hydrogen peroxide and on galena flotation were investigated. 2.3. Dry grinding

100 g of galena, pyrite and chalcopyrite single minerals were 2. Experimental separately ground in a laboratory stainless steel ball mill with stainless steel medium for 60 min. 5 g of either galena, pyrite or 2.1. Materials and reagents chalcopyrite mineral and 12.5 g in total of galena–pyrite blend or galena–chalcopyrite blend with particle size < 106 lm was mixed Crystalline pure galena, pyrite and chalcopyrite minerals used in with 50 ml of water and conditioned with a magnetic stirrer. The this study were procured from Gregory, Bottley & Lloyd Ltd., United slurry sample was collected after 0.5, 5 and 11 min conditioning Kingdom. Table 1 shows the chemical composition of the minerals. time and was analysed for hydrogen peroxide. The pH was regu- The XRD analyses of the samples showed that the main mineral lated with HCl and NaOH solutions. The Eh (pulp potential) of phase present was pyrite (Fig. 1a), chalcopyrite (Fig. 1b) and galena the suspension was measured at room temperature during mixing. (Fig. 1c) respectively in these minerals. Pyrite, chalcopyrite and galena samples used in this study were separately crushed through a 2.4. Gas purging jaw crusher and then screened to collect the 3.35 mm particle size fraction. The homogenised sample was then sealed in polyethylene To study the effect of gaseous atmosphere, galena was wet- bags. Potassium amyl xanthate (KAX) and methyl isobutyl carbinol ground in a laboratory ball mill with stainless steel media in either (MIBC) used as collector and frother respectively were obtained air or N atmosphere. For these tests, the jar of the laboratory ball from Boliden Mineral AB, and these samples are being used in the 2 mill was filled with 400 ml deionised water and purged with either flotation plant concentrator. Dilute solutions of AR grade sodium air or N gas for a minimum of 30 min. After 30 min, galena (100 g) hydroxide and HCl were added to maintain the pH at the targeted 2 was added to the jar and purged with either air or N gas for a min- value during flotation. Deionised water was used in both grinding 2 imum of 30 min again and then galena was wet ground for 1 h. and flotation experiments. Solutions of 2,9-dimethyl-1,10-phenan- Though the dissolved O concentration was not measured in these throline (DMP), copper (II) sulphate (0.01 M) and phosphate buffer 2 experiments, Butler et al. have reported that for 1 l solution of (pH 7.0) used in the analytical method for determining H O and 2 2 ultrapure water purged for 1 h with N gas, O concentrations do lead nitrate, ferrous sulphate and ferric sulphate used for investi- 2 2 not exceed 0.19 ± 0.05 ppm [24]. The concentration of H O in gating the effect of these metal ions on the formation of H O were 2 2 2 2 the pulp liquid was measured after 60 min of grinding. purchased from VWR, Sweden.

2.5. Estimation of hydrogen peroxide 2.2. Wet grinding and ﬂotation test So far, various methods have been used for the measurement of Crushed galena sample (100 g) of 3.35 mm size in each H2O2 in oxidation processes. Such methods use metallic com- grinding test was combined with 400 ml of water and ground in pounds such as titanium oxalate, titanium tetrachloride [25–28] and cobalt (II) ions [29] that form coloured complexes with H O , Table 1 2 2 Chemical composition of the minerals used in the studies. which can then be measured spectrophotometrically. The spectrophotometric method using copper (II) ions and 2,9-dimethyl-1, Minerals % Pb % S % Fe % Zn % Cu 10-phenanthroline (DMP) has been found to be reasonably sensi- Galena 73.69 13.5 1.38 1.26 0.2 tive when applied to advanced oxidation processes [30]. The Pyrite – 50.9 44.4 – 0.02 principle of DMP method is reduction of copper(II) with H O . Chalcopyrite 0.22 29.5 29 0.54 25.8 2 2 The stoichiometry is as follows: 834 A. Javadi Nooshabadi, K. Hanumantha Rao / Advanced Powder Technology 25 (2014) 832–839

Fig. 1. XRD analysis of the sample (a) pyrite (1 – pyrite), (b) chalcopyrite (1 – chalcopyrite and 2 – pyrrhotite 3 – sphalerite) and (c) galena (1 – galena 2 – sphalerite 3 – quartz).

2þ þ þ 2Cu þ 4DMP þ H2O2 ! 2CuðDMPÞ2 þ O2 þ 2H ð6Þ wet-ground in a laboratory stainless steel ball mill with stainless steel medium and the slurry samples were collected at pre-deter- Cu DMP þ is a bright yellow complex with maximal absorbance ð Þ2 mined time intervals. Slurry samples were filtered (Millipore at 454 nm and is stable under ordinary light [31]. The blank solu- 0.22 lm) and the liquid (filtrate) was analysed for hydrogen perox- tion without H2O2 has a different colour, and differences of the ide. Formation of hydrogen peroxide was detected in the filtrate for absorbance between the sample and blank solutions are approxi- the first time in mineral processing applications. The effect of pH mately proportional to H2O2 concentration as long as the concen- 2þ during grinding on formation of hydrogen peroxide is shown in trations of the copper(II)DMP complexes [e.g., CuðDMPÞ2 , + 2+ Table 2. It can be seen that at pH 4 and 7.8 hardly any H2O2 is Cu(DMP)2OH , and Cu(DMP) ] are high enough and H2O2 concen- generated but at pH 2.7 not only is H2O2 formed but also its tration is relatively low. For DMP method [31], 1 ml each of 1% concentration is increased with increasing grinding time due to DMP in ethanol, 0.01 M copper (II), and phosphate buffer (pH increased surface area of solids and their interaction with water. 7.0) solutions were added to a 10 ml volumetric flask and mixed. After 60 min grinding, the effect of H2O2 formation on the flota- A measured volume of liquid (filtrate) sample was added to the tion recovery of galena was investigated. The effect of grinding pH volumetric flask, and then the volume was made up to the mark as a function of flotation pH on galena recovery is shown in Table 3. with ultrapure water. After mixing, the absorbance of the sample Ò It can be seen that for all pH values studied the wet-ground at 454 nm was measured with DU Series 700 UV/Vis Scanning galena at pH 2.5 produced a lower galena recovery than that of Spectrophotometer. The blank solution was prepared in the same manner but without H2O2. Table 2

H2O2 concentration during wet grinding as function of grinding time. 3. Results and discussion Time (min) H2O2 (mM)

3.1. Formation of hydrogen peroxide (H2O2) during wet grinding and Initial pH = 2.7 Initial pH = 4 Initial pH = 7.8 its implications on ﬂotation 1 0.25 0.000 0.000 10 0.40 0.000 0.000 30 0.42 0.0042 0.0021 Initially the extent of H2O2 formation during wet grinding of 60 0.38 0.0083 0.0076 galena was investigated. For these studies, galena mineral was A. Javadi Nooshabadi, K. Hanumantha Rao / Advanced Powder Technology 25 (2014) 832–839 835

Table 3 0.16 400 Effect of pH on flotation recovery of galena with a pulp ground at pH 2.5 and 7.8. H2O2 0.14 pH Recovery (%) of galena with a Recovery (%) of galena with a Eh pulp ground at initial pH 7.8 pulp ground at pH 2.5 0.12 300 (final pH after grinding 5.5) (final pH after grinding 6.1) 365 61 0.1 470 66

(mM) 0.08 200

6.3 76 69 2 O

950 43 2

H 0.06 Eh (mV, SHE ) 0.04 100 wet-ground galena at pH 7.8. Since wet-ground galena at pH 2.5 0.02 produces a higher amount of H2O2, a little decrease in galena recovery could be due to surface oxidation caused by H2O2 oxidant. 0 0 02468101214 It was also reported that with the addition of H2O2, galena ﬂotation pH decreases and completely depresses if the concentration of H2O2 3 exceeds 10 M [32]. This strong depressing action of H2O2 on ga- Fig. 2. Effect of pH on H2O2 concentration in pulp liquid. lena is attributed to its strong oxidising action on lead xanthate on galena surface, giving rise to the oxidation and decomposition of lead xanthate.

PbðEXÞ2 þ H2O2 ! PbðOHÞ2 þðEXÞ2 ð7Þ

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 the formation of hydrogen peroxide is shown in Fig. 2. It can be seen that the concentration of

H2O2 is increased with decreasing pH. Fig. 2 also shows that Eh is increased with increasing formation of H2O2 in the pulp liquid. Woods et al. showed by the Eh-pH diagram (Fig. 3) of galena that at pH < 4, Pb2+ ions are stable [33]. These divalent metal ions are also found to generate H2O2 as shown in Table 4 [34–36,6]. Fig. 4 shows that in the presence of dissolved molecular oxygen and at Å acidic pH (pH < 4), lead ions can form superoxide anion (O2) 2+ which reacts with Pb to form H2O2 (Eqs. (8)–(11)). Further, Peng et al. showed Pb2+ ions were produced during galena grinding with Fig. 3. Eh-pH diagram for the Pb–S–H2O system. Equilibrium lines correspond to stainless mill media [15]. This is in agreement with other studies dissolved species where [Pb] = 103 M [32]. where metal-ions induced formation of free radical has been sig- niﬁcantly evidenced for Pb2+ ions that generate the superoxide and hydroxyl radical [11–13]. Table 4 Effect of metal ions on H2O2 generation at two initial concentrations (conditioning Å O2 þ e !ðO2Þ ð8Þ time 1 h, 6–7 pH, and 22 °C). Concentration of ions H O (mM) Å þ 2 2 2ðO Þ þ 2H ! H2O2 þ O2 ð9Þ 2 1mM 10mM

2þ 0 Å Water 0 0 PbS þ H2O2 ! Pb þ S þ 2e þ 2 OH ð10Þ Pb2+ 0.013 0.096 Fe2+ 0.552 4.656 Å 3+ 2 OH ! H2O2 ð11Þ Fe 0.004 0.059 Cu2+ 0.00068 0.015

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 1 h and then galena was wet-ground for 1 h. For air atmosphere the

H2O2 is increased with increasing loading of galena as shown in ball mill jar was ﬁlled with galena and 400 ml water and the lid of Table 5. This table also shows that the production of H2O2 is the jar was kept open for 5 min and then galena was wet-ground increased with increasing time and does not reach equilibrium for 1 h. Table 6 shows that H2O2 was generated in N2 atmosphere level within 11 min of interaction time of galena and water. too. However, N2 atmosphere generated lower H2O2 than that in air atmosphere. This is in agreement with other studies where it 3.4. Gas purging was observed that the dissolution of PbS in the absence of dissolved oxygen is lower than those obtained in the presence of oxy- To study the effect of gaseous atmosphere, galena was wet- gen [37]. It is not very clear how the divalent metal ions are ground in a laboratory ball mill with stainless steel media in either generating H2O2 from water but one can hypothesise that an elec- 2+ air or N2 atmosphere. For N2 atmosphere the jar of the ball mill was tron is extracted by the Pb from water and a hydroxyl radical is ﬁlled with 400 ml water and purged with N2 gas for a minimum of formed (Eq. (12)). Combining two hydroxyl radicals leads to the 836 A. Javadi Nooshabadi, K. Hanumantha Rao / Advanced Powder Technology 25 (2014) 832–839

1. The galena recovery decreases with an increase in pyrite proportion in galena–pyrite mixture [41,15]. Also, the ﬂoatability of galena decreases in the presence of pyrite in the entire pH range studied [42]. 2. With the increase in the amount of pyrite in contact with another sulphide mineral, the leaching rate of this contacting mineral increases [43–47]. This is in agreement with other studies where it was observed that increasing hydrogen peroxide concentration accelerates galena oxidation considerably [48].

Fig. 5 also shows the effect of the proportion of galena in galena–pyrite mixture on the formation of hydrogen peroxide. It can be seen that with an increase in galena proportion, the production of

H2O2 is decreased. This mechanism may be the explanation for the observed increase in pyrite ﬂoatability in a mixture of galena–pyr-

ite [15,42]. The formation of higher amounts of H2O2 when galena 2+ Fig. 4. Proposed mechanism for formation of H2O2 by galena showing Pb react is in contact with pyrite may explain the effect of interaction be- with water to generate ÅOH and H O . 2 2 tween these two sulphide minerals on their ﬂotation as shown in Fig. 6, rather than that reported earlier namely that when two sul-

Table 5 phide minerals are in contact, electron transfer from one to the

Effect of galena loading on H2O2 concentration after 0.5 and 5 and 11 min mixing with other, i.e. galvanic interaction, occurs [23,47]. water at pH 2.5.

Time (min) H2O2 (mM) 3.6. Galena–chalcopyrite mixture 100 g/L(0.42 M) galena 250 g/L(1.04 mM) galena 0.5 0.08 0.12 Blend samples of galena–chalcopyrite (12.5 g in total) were 5 0.14 0.18 mixed in 50 ml of water for 5 min and the amount of H2O2 formed 11 0.2 0.23 was determined. Fig. 7 shows the effect of chalcopyrite proportion in galena–chalcopyrite mixture on the formation of hydrogen peroxide at different pH. It can be seen that with an increase in chalcopyrite proportion, the concentration of H O is increased. This is formation of H O (Eq. (13)). As observed by Peng et al., Pb2+ ex- 2 2 2 2 in agreement with other studies, wherein it has been reported that isted in the pulp after grinding of galena with nitrogen gas [15]: chalcopyrite (CuFeS2) generates hydrogen peroxide [5,7,9]. This Å Å 2þ 0 PbS þ H2O ! OH þ H þ Pb þ S þ 2e ð12Þ mechanism, namely that with an increase in chalcopyrite proportion, the production of H2O2 is increased, could explain the result Å that as the amount of chalcopyrite in contact with galena increases 2 OH ! H2O2 ð13Þ the leaching rate of galena increases, as shown in Fig. 8. This is in agreement with other studies where it was observed that by 3.5. Galena–pyrite mixture increasing the hydrogen peroxide concentration, galena oxidation considerably accelerated [48]. Fig. 7 also shows the effect of galena Many authors have reported that galvanic interactions are proportion in galena–chalcopyrite mixture on the formation of known to occur between sulphide minerals and play a signiﬁcant hydrogen peroxide. It can be seen that by increasing galena propor- role in ﬂotation [23,30,38–40]. The effect of galena–pyrite mixture tion, the production of H2O2 is decreased. This result could perhaps on formation of hydrogen peroxide was next investigated. Blend explain that galena has no effect on the kinetics of chalcopyrite samples of galena–pyrite (12.5 g in total) were mixed in 50 ml of leaching [49]. water for 5 min. Fig. 5 shows the effect of pyrite proportion in galena–pyrite mixture on the formation of hydrogen peroxide at different pH. It can be seen that by increasing the pyrite propor- 2.5 tion, the production of H2O2 has increased. This is in agreement with other studies where it was shown that pyrite (FeS2) generates pH 2.5 hydrogen peroxide [2] and hydroxyl radicals, HOÅ [3] when placed 2 in water. In the presence of dissolved molecular oxygen, ferrous pH 4.5 Å iron associated with pyrite can form superoxide anion (O ) (Eq. 2 1.5 (1)), which reacts with ferrous iron to form H2O2 (Eq. (2)) [4,8]. pH 7 (mM)

Borda et al. showed that pyrite can also generate H2O2 in the ab- 2 O sence of molecular oxygen (Eqs. (4) and (5)) [3]. 2 pH 9 H 1 This result of increasing the proportion of pyrite, leading to in- pH 10.5 crease in the production of H2O2 could be the explanation for the following observations: 0.5

Table 6

The effect of N2 and air atmosphere on formation of H2O2 by galena at pH 2.5. 0 0 20406080100 Type of gas H2O2 (mM) Pyrite proportion %

N2 0.36 Air 0.38 Fig. 5. Effect of pyrite (Py) proportion in galena–pyrite mixture on concentration of H2O2 at alkaline and acidic pH. A. Javadi Nooshabadi, K. Hanumantha Rao / Advanced Powder Technology 25 (2014) 832–839 837

Fig. 6. 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 [46]). (B) Formation of H2O2 by Py that are proposed to form via the incomplete Å reduction of oxygen and be formed from two reacting OH (Eqs. (1)–(4)). (C) Simultaneous galvanic interaction and formation H2O2 by Py.

0.8 pH 4.5 0.7 pH 11.5 0.6 pH 9 0.5

(mM) 0.4 2 O 2

H 0.3

0.2

0.1

0 0 20406080100 Fig. 8. Proposed mechanisms for oxidation of galena by H2O2 that increased with chalcopyrite proportion % increasing chalcopyrite proportion in galena–chalcopyrite mixture.

Fig. 7. Effect of chalcopyrite proportion in galena–chalcopyrite mixture on recovery of pyrite ﬂotation, (f) increasing chalcopyrite proportion concentration of H2O2 at different pH. 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

The above discussion may be summarised by the following galena proportion decreases concentration of H2O2 in a mixture observations: (a) increasing pyrite proportion increases production of chalcopyrite–galena. 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 4. Conclusions leaching of galena, (d) increasing galena proportion decreases concentration of H2O2 in pyrite–galena mixture, (e) decreasing Hydrogen peroxide can oxidise galena during grinding and de- concentration of H2O2 decreases oxidation of pyrite and increases press its flotation. In the scope of the present work, fundamental 838 A. Javadi Nooshabadi, K. Hanumantha Rao / Advanced Powder Technology 25 (2014) 832–839 studies were carried out to resolve or reveal one of the main prob- [12] E. Ahlberg, A.E. Broo, Oxygen reduction at sulphide minerals. 2. A rotating ring lems (i.e., oxidation) encountered in sulphide ore flotation. For this disc electrode (RRDE) study at galena and pyrite in the presence of xanthate, Int. J. Miner. Process. 47 (1–2) (1996) 33–47. reason, the grinding stage was evaluated and the oxidation of the [13] E. Ahlberg, A.E. Broo, Oxygen reduction at sulphide minerals. 3. The effect of sulphide minerals was investigated. For the first time in mineral surface pre-treatment on the oxygen reduction at pyrite, Int. J. Miner. Process. processing applications, it was found out that during grinding 47 (1–2) (1996) 49–60. [14] P. Guy, W. Trahar, The influence of grinding and flotation environments on the (especially fine grinding) of galena, formation of H2O2 takes place. laboratory batch flotation of galena, Int. J. Miner. Process. 12 (1984) 15–38. It is known that hydrogen peroxide is a strong oxidising agent and [15] Y. Peng, S. Grano, D. Fornasiero, J. Ralston, Control of grinding conditions in the can easily oxide sulphide minerals and can even cause dissolution flotation of galena and its separation from pyrite, Int. J. Miner. Process. 70 (2003) 67–82. of some metal ions from the mineral surface. That is why H2O2, [16] X. Yuan, B. Palsson, K. Forssberg, Flotation of a complex sulphide ore I. Cu/Zn rather than oxygen, can be the main reason for oxidation, inadver- selectivity control by adjusting pulp potential with different gases, Int. J. tent activation and non-selectivity in the sulphide mineral flota- Miner. Process. 46 (1996) 155–179. [17] K. Natarajan, I. Iwasaki, Electrochemical aspects of grinding media-mineral tion. Prevention or reduction of H2O2 formation may result in interactions in magnetite ore grinding, Int. J. Miner. Process. 13 (1984) 53–71. improved flotation results and we are working on these aspects. [18] M. Rey, V. Formanek, Some factors affecting selectivity in the differential The following preliminary conclusions were drawn from the flotation of lead–zinc ores in the presence of oxidized lead mineral, in: 5th experiments carried out in the scope of this paper. Formation of International Mineral Processing Congress, Institution of Mining and Metallurgy, London, UK, 1960. hydrogen peroxide was detected in the filtrate of galena pulp liquid [19] S. Rao, G. Labonte, J. Finch, Electrochemistry in the plant, in: Innovations in for the first time in mineral processing applications. However, Flotation Technology, Greece, 1992. H O formed only at pH values less than 4. Dry grinding also pro- [20] H. Buehler, V. Gottschalk, Oxidation of sulphides, Econ. Geology. 5 (1910) 28– 2 2 35. duced hydrogen peroxide when freshly dry ground galena solids [21] H. Majima, How oxidation affects selective flotation of complex sulphide ores, were mixed with water, and the effect of pH on H2O2 formation Can. Metall. Q. 8 (1969) 269–273. was similar to wet grinding, i.e., hydrogen peroxide generates at [22] D. Kocabag, M. Smith, Processing of Ores, Concentrates and By-Products, The Metallurgical Society, New York, 1985. pH < 4. Also the concentration of H2O2 was increased with increas- [23] S. Rao, J. Finch, Galvanic interaction studies on sulphide minerals, Can. Metall. ing mixing time and galena loading. Hydrogen peroxide was also Q. 27 (1988) 253–259. [24] I. Butler, M. Schoonen, D. Rickard, Removal of dissolved-oxygen from water – a generated in the absence of oxygen (N2 atmosphere), the amount comparison of 4 common techniques, Talanta 41 (2) (1994) 211–215. being lower compared to air atmosphere. [25] C. Volk, P. Roche, C. Renner, H. Paillard, J.C. Joret, Effects of ozone–hydrogen With an increase in pyrite proportion in pyrite–galena mixture, peroxide combination on the formation of biodegradable dissolved organic carbon, Ozone Sci. Eng. 15 (1993) 405–418. the concentration of H2O2 was increased. Then one can conclude [26] P. Roche, M. Prados, Removal of pesticides by use of ozone or hydrogen that a decrease in galena recovery in flotation with an increase in peroxide, Ozone Sci. Eng. 17 (1995) 657–672. pyrite proportion is due to the increase of hydrogen peroxide for- [27] M. Sunder, D.C. Hempel, Oxidation of tri- and perchloroethene in aqueous mation. Also with increasing chalcopyrite proportion in a mixture solution with ozone and hydrogen peroxide in a tube reactor, Water Res. 31 of chalcopyrite–galena, the concentration of H O was increased. (1997) 33–40. 2 2 [28] K. Leitner, M. Dore, Mechanism of the reaction between hydroxyl radicals and glycolic, glyoxylic, acetic and oxalic acids in aqueous solution: consequence on Acknowledgements hydrogen peroxide consumption, Water Res. 31 (1997) 1383–1397. [29] H. Gulyas, R. von Bismark, L. Hemmerling, Treatment of industrial wastewaters with ozone/hydrogen peroxide, Water Sci. Technol. 32 (1995) Financial support from the Centre for Advanced Mineral and 127–134. Metallurgy (CAMM), Luleå University of Technology, Luleå, [30] K. Kosaka, H. Yamada, S. Matsui, S. Echigo, K. Shishida, A comparison among the methods for hydrogen peroxide measurements to evaluate advanced Sweden, and Boliden Mineral AB, Boliden, Sweden, is gratefully oxidation processes: application of a spectrophotometric method using copper acknowledged. (II) ion and 2, 9-dimethyl-1, 10-phenanthroline, Environ. Sci. Technol. 32 (1998) 3821–3824. [31] A. Baga, G.R.A. Johnson, N. Nazhat, R. Saadalla-Nazhat, A simple References spectrophotometric determination of hydrogen peroxide at low concentrations in aqueous solution, Anal. Chim. Acta 204 (1988) 349–353. [1] F. Ikumapayi, H. Sis, B. Johansson, K. Hanumantha Rao, Recycling process water [32] D. 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Paper 5

Formation of hydrogen peroxide by sulphide minerals

Alireza Javadi Nooshabadi, Kota Hanumantha Rao

Hydrometallurgy, 2014, 141, 82-88

Hydrometallurgy 141 (2014) 82–88

Contents lists available at ScienceDirect

Hydrometallurgy

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

Formation of hydrogen peroxide by sulphide minerals

Alireza Javadi Nooshabadi, Kota Hanumantha Rao ⁎

Article history: Formation of hydrogen peroxide (H2O2), an oxidizing agent stronger than oxygen, by sulphide minerals during Received 4 April 2013 grinding was investigated. It was found that pyrite (FeS2), chalcopyrite (CuFeS2), sphalerite ((Zn,Fe)S), and gale- Received in revised form 7 October 2013 na (PbS), which are the most abundant sulphide minerals on Earth, generated H2O2 in pulp liquid during wet Accepted 21 October 2013 grinding in the presence of dissolved oxygen in water and also when the solids are placed in water immediately Available online 30 October 2013 after dry grinding. Pyrite generated more H2O2 than other minerals and the order of H2O2 production by the minerals found to be pyrite N chalcopyrite N sphalerite N galena. The pH of water inﬂuenced the extent of hydrogen Keywords: Hydrogen peroxide peroxide formation where higher amounts of H2O2 are produced at highly acidic pH. Furthermore, the effect of Pyrite mixed sulphide minerals, i.e., pyrite–chalcopyrite, pyrite–galena, chalcopyrite–galena and sphalerite–pyrite, Chalcopyrite sphalerite–chalcopyrite and sphalerite–galena on the formation of H2O2 showed increasing H2O2 formation Sphalerite with increasing pyrite fraction in chalcopyrite–pyrite, galena–pyrite and sphalerite–pyrite compositions. The re- Galena sults also corroborate the amount of H2O2 production with the rest potential of the sulphide minerals; higher rest

potential of a sulphide mineral results in more formation of H2O2.MostlikelyH2O2 is responsible for the oxidation of sulphide minerals and dissolution of non-ferrous metal sulphides in the presence of ferrous sulphide in addition to galvanic interactions. This study highlights the necessity of revisiting the electrochemical and/or galvanic interactions between pyrite and other sulphide minerals in terms of their ﬂotation and leaching behav-

1. Introduction rapidly. Harvey and Yen (1998) discovered that addition of galena to the sphalerite selective leaching system diminished the dissolution of Hydrogen peroxide, which is a strong oxidizing agent, stronger than sphalerite. Alternatively, a pyrite or chalcopyrite addition increased molecular oxygen, causes non-selective oxidation of sulphide minerals. zinc extractions. Many authors have been reported that galvanic inter- The oxidation of sulphide minerals takes place during the grinding pro- actions are known to occur between conducting minerals and play a sig- cess when the particle size is reduced for flotation. Recently it was nificant role in flotation (Ekmekçi and Demirel, 1997; Huang and Grano, shown that formation of H2O2 takes place in the pulp liquid during 2005; Kelebek et al., 1996; Rao and Finch, 1988; Zhang et al., 1997), wet grinding of complex sulphide ore (Ikumapayi et al., 2012). Previous leaching (Abraitis et al., 2003; Akcil and Ciftci, 2003; Mehta and Murr, work has highlighted the potential effect of reactive oxygen species 1983), supergene enrichment of sulfide ore deposits (Sato, 1992;

(ROS), H2O2 and hydroxyl radical (⋅OH), generated from milled sul- Thornber, 1975), environment governance (Alpers and Blowes, 1994), phide concentrates on thermophilic Fe- and S-oxidizing bioleaching mi- and geochemical processes (Banﬁeld and Nealson, 1997; Sikka et al., croorganisms through oxidative stress (Jones et al., 2011; 2013a; 1991). In addition, other researchers (Dixon and Tshilombo, 2005; 2013b). To date, most studies dealing with sulphide mineral-induced Holmes and Crundwell, 1995; Koleini et al., 2010, 2011; Mehta, and ROS formation have made use of natural or synthetic mineral samples, Murr, 1983) reported when the amount of pyrite in contact with chalco- of very high purity, suspended in solutions. Pyrite induced ROS forma- pyrite increases, the leaching rate of chalcopyrite increases. • tion has been studied most; however other sulphide minerals such as However, participation of H2O2 and OH, if any, in non-selective chalcopyrite (CuFeS2), sphalerite (ZnS), pyrrhotite (Fe(1 − x)S) and oxidation of the sulphide mineral pulp components and hence in dete- vaesite (NiS2) have also been studied with respect to ROS generation riorating of the concentrate grade and recovery of metal-sulphides has (Borda et al., 2001; Javadi Nooshabadi and Hanumantha Rao, 2013a; not yet been explored. In an attempt to ﬁll the gap, we have estimated

2013b; Javadi Nooshabadi et al., 2013; Jones et al., 2011), and reactiv- the concentration of H2O2 in pulp liquid during different grinding time ities have been found to differ between sulphide minerals. intervals and in different grinding environments. The effect of pH, Buehler and Gottschalk (1910) noted that when pyrite was mixed type of grinding (wet or dry grinding), and varying proportion of pyrite, with a second sulphide mineral, the second mineral oxidized more chalcopyrite and galena mixed with sphalerite on the formation of hydrogen peroxide was investigated. The results are presented and ﬂ ⁎ Corresponding author. Tel.: +46 920 491705; fax: +46 920 491199. discussed in the context of otation and leaching phenomena of E-mail address: [email protected] (K. Hanumantha Rao). sulphide minerals.

2. Experimental 2.2. Dry grinding

2.1. Materials and reagents 100 g of −3.35 mm size fraction of sphalerite, pyrite, galena and chalcopyrite minerals in each grinding test was separately ground in a Crystalline pure sphalerite (Sp), galena (Ga), pyrite (Py) and chalco- laboratory stainless steel ball mill (Model 2VS, CAPCO Test Equipment, pyrite (Cp) minerals used in this study were procured from Gregory, Suffolk, UK) with stainless steel medium for 60 min. 5 g of sphalerite, Bottley & Lloyd Ltd., UK. Sphalerite contained 39.92% Zn, 20.7% S, 4.2% galena, pyrite or chalcopyrite single mineral and 12.5 g in total of mineral Fe, 1.32% Pb and 0.17% Cu, galena contained 73.69% Pb, 13.5% S, 1.38% mixture either sphalerite–pyrite or sphalerite–chalcopyrite or sphalerite– Fe, 1.26% Zn, 0.2% Cu and some silica (quartz) impurity; pyrite contained galena or galena–pyrite or galena–chalcopyrite or chalcopyrite–pyrite 44.4% Fe, 50.9% S, and 0.2% Cu, and chalcopyrite contained 29% Fe, 29.5% S, that was b 106 μmwasmixedwith50cm3 water using magnetic stirrer. 25.8% Cu, 0.54% Zn, and 0.22% Pb. The XRD analyses of the samples Then the slurry samples were collected at 0.5, 5 and 11 min, filtered and showed that the main mineral phases were the pyrite (Fig. 1a), chalcopy- they were analyzed for hydrogen peroxide. The pH was regulated with rite (Fig. 1b), sphalerite (Fig. 1c), and galena (Fig. 1d) in the respective HCl and NaOH solutions. mineral samples. 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 homog- 2.3. Analysis of hydrogen peroxide enized sample was then sealed in polyethylene bags. Potassium amyl xanthate (KAX) was used as collector and the frother was methyl isobutyl The spectrophotometric method using copper (II) ions and DMP has carbinol (MIBC) in flotation tests. Solutions of sodium hydroxide (1 M) found to be reasonably sensitive when applied to advanced oxidation and HCl (1 M) were added to maintain the pH at the targeted value dur- processes (Kosaka et al., 1998). For DMP method (Baga et al., 1988) ing flotation. Deionized water was used in the processes of both grinding 1 mL each of 1% DMP in ethanol, 0.01 M copper (II), and phosphate and flotation. Solutions of 2, 9-dimethyl-1, 10-phenanthroline (DMP), buffer (pH 7.0) solutions were added to a 10 mL volumetric flask and copper (II) (0.01 M), and phosphate buffer (pH 7.0) were used for esti- mixed. A measured volume of liquid (filtrate) sample was added to mating H2O2 amount in pulp liquid by UV-Visible spectrophotometer. the volumetric flask, and then the flask was filled up with ultrapure Zinc sulphate, copper sulphate, lead nitrate, ferrous sulphate and ferric water. After mixing, the absorbance of the sample at 454 nm was mea- sulphate chemicals were also used to investigate the effect of metal sured with DU® Series 700 UV/Vis Scanning Spectrophotometer. The 2+ 2+ 2+ 2+ 3+ ions (Zn ,Cu ,Pb ,Fe and Fe ) on the formation of H2O2. blank solution was prepared in the same manner but without H2O2.

Fig. 1. XRD analysis of the mineral samples: (a) pyrite (1 — pyrite), (b) chalcopyrite (1 — chalcopyrite and 2 — pyrrhotite 3 — sphalerite), (c) sphalerite (1 — sphalerite 2 — galena 3 — quartz), and (d) galena (1 — galena 2 — sphalerite 3 — quartz). 84 A. Javadi Nooshabadi, K. Hanumantha Rao / Hydrometallurgy 141 (2014) 82–88

1.2 1.6 1

1.2 0.8 (mM)

2 0.6 O (mM) 2

2 0.8

H 0.4 O 2

H 0.2 0.4 0 0.5 5 11 0 Mixing time (min) 2.5 4.5 8 10.5 11.5 pH Ga Sp Cp Py pyrite chalcopyrite sphalerite galena Fig. 4. Effect of mixing time on H2O2 concentration at pH 4.5.

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

3. Results dissolution with increasing pyrite fraction could be due to the increased

H2O2 generation as Harvey and Yen (1998) showed an increase in the 3.1. Formation of hydrogen peroxide (H2O2) after dry grinding and mixing rate of sphalerite leaching with increasing pyrite proportion. This is in with water agreement with other studies where it was shown that the oxidation of sphalerite increases with increasing hydrogen peroxide concentra- Formation of hydrogen peroxide was observed when the freshly tion (Adebayo et al., 2006; Aydogan, 2006). dry-ground solids were mixed in water. The effect of water pH in Fig. 5b shows the effect of chalcopyrite or sphalerite proportion in which the solids were mixed, on the formation of hydrogen peroxide sphalerite–chalcopyrite mixture on formation of hydrogen peroxide is shown in Fig. 2. It can be seen that the formation of H2O2 decreases where an increase in chalcopyrite proportion increases the concentra- with increasing pH in the cases of pyrite, chalcopyrite and sphalerite tion of H2O2 but with increasing sphalerite proportion in the mixture, up to pH 8, above which it increases again. However, in the case of ga- the concentration of H2O2 decreased. This result of increasing chalcopy- lena, H2O2 generation occurs only at pH b 4.5 and no H2O2 is formed rite proportion increases the formation of H2O2 explains the observation above this point. Also, pyrite generated more H2O2 than other minerals. of increased leaching rate of sphalerite with an increase in chalcopyrite For investigation of the effect of mineral loading, pyrite, chalcopyrite, proportion (Harvey and Yen, 1998). sphalerite, and galena were mixed with 50 ml water at pH 4.5. The con- Fig. 5c shows the effect of galena or sphalerite proportion in sphaler- centration of H2O2 increased with increasing mineral loading as shown in ite–pyrite mixture on formation of hydrogen peroxide. It can be seen Fig. 3. Fig. 4 shows that the concentration of H2O2 increased with increas- that with an increase in galena proportion, the concentration of H2O2 ing mixing time, most likely due to increased minerals interaction with decreased but with an increase in sphalerite proportion, the concentra- water. tion of H2O2 increased. This result could explain the decrease in the rate of sphalerite leaching with increasing galena proportion (Harvey and Yen, 1998). 3.2. Effect of minerals mixture on formation of H O 2 2 Fig. 5d shows the effect of pyrite or galena proportion in galena–pyrite mixture on formation of hydrogen peroxide. It can be seen Fig. 5 shows the effect of pyrite–chalcopyrite, pyrite–sphalerite, pyrite– that an increase in pyrite proportion, the concentration of H O in- galena, chalcopyrite–sphalerite, chalcopyrite–galena or sphalerite– 2 2 creases but increasing galena proportion, the concentration of H O galena mixture on formation of hydrogen peroxide at pH 4.5. Fig. 5a 2 2 decreases. shows the effect of pyrite or sphalerite proportion in sphalerite–pyrite Fig. 5e shows the effect of chalcopyrite or galena proportion in mixture on formation of hydrogen peroxide. It can be seen that with galena–chalcopyrite mixture on formation of hydrogen peroxide that an increase in pyrite proportion, the concentration of H O increased 2 2 increasing chalcopyrite proportion, the concentration of H O increases. but with an increase in sphalerite proportion, the concentration of 2 2 However, with an increase in galena proportion, the production of H O H O decreased. The result of non-ferrous metal sulphide oxidation or 2 2 2 2 decreases. This result could perhaps explain that galena has no effect on the kinetics of chalcopyrite leaching (Nazari et al., 2012). 1.4 Fig. 5f shows the effect of pyrite or chalcopyrite proportion in chalcopyrite–pyrite mixture on the formation of hydrogen peroxide that with 1.2 an increase in pyrite proportion, the concentration of H2O2 increases 1 but with an increase in chalcopyrite proportion, the concentration of H O decreases. As a result, with an increase in pyrite proportion, the 0.8 2 2 (mM) concentration of H2O2 increases. This would explain why increasing 2

O 0.6 the amount of pyrite in contact with chalcopyrite results in an increase 2

H 0.4 of the leaching rate of chalcopyrite increases (Dixon and Tshilombo, 2005; Holmes and Crundwell, 1995; Koleini et al., 2010, 2011; Mehta, 0.2 and Murr, 1983). 0 Fig. 6 shows the effect of the rest potential on the formation of 50 100 250 hydrogen peroxide. It can be seen from this ﬁgure that with an increase

Solids loading (g/l) in rest potential, the concentration of H2O2 increases. The results also Ga Sp Cp Py show that the amount of H2O2 produced correlates with the rest potential series of sulphide minerals; the higher the rest potential of the sulphide

Fig. 3. Effect of solids loading on H2O2 production after 5 min mixing with water at pH 4.5. mineral the more H2O2 is formed. A. Javadi Nooshabadi, K. Hanumantha Rao / Hydrometallurgy 141 (2014) 82–88 85

1.6 0.9 a)a) 0.80.8 b)b) 1.2 0.70.7 ) 0.60.6 M m

(mM) 0.50.5 (mM) (

2 0.8 2 0.40.4 O O 2 2

H 0.30.3 H SSpp % H SpSp % 0.4 0.20.2 PPyy % 0.10.1 CpCp % 0 0 0 2040608010020 40 60 80 100 0 2040608010020 40 60 80 100 SpSp oror PPyy proportionproportion CpCp oror SpSp proportionproportion

0.3 c)c) 11.6.6 d) 11.2.2 0.2 ) M m (mM) ( (mM) 00.8.8 2 2 O O 2 2 0.1 H H SSpp % 00.4.4 GGaa %

GaGa % pypy % 0 0 0 2 204060801000 40 60 80 100 0 2 204060801000 40 60 80 100 SpSp oror GaGa proportionproportion GaGa oror PPyy proportionproportion

1 11.6.6 e)e) f)f) 0.8 11.2.2 )

0.6 M m (mM) (mM) (

2 00.8.8 2 O O 2 0.4 2 H H H CCp%p% GGa%a% 00.4.4 0.2 PyPy % CpCp % 0 0 0 20406080100 0 20406080100 Cp or Ga proportion Cp or Py proportion

Fig. 5. Effect of mineral proportion in a mixture on H2O2 formation: a) Sp or Py b) Sp or Cp c) Sp or Ga d) Ga or Py e) Cp or Ga f) Cp or Py. Total 12.5 g sample after 5 min conditioning in water at pH 4.5.

Fig. 7 shows the effect of single or mixture of minerals on formation agreement with other studies where it was noticed when pyrite was of hydrogen peroxide at pH 4.5. It can be seen that pyrite and a mixture mixed with a second sulphide mineral, the second mineral oxidized of pyrite and other sulphide mineral generated more H2O2.Thisisin more rapidly (Buehler and Gottschalk, 1910). The formation of higher amounts of H2O2 with the presence of pyrite in a mixture of different sulphide minerals may explain the effect of interaction between pyrite 2

1.51.5 1.6 1.4 (mM)

2 1 1.2 O

2 1 (mM) H 2 0.8 O

0.50.5 2 0.6 H 0.4 0 0.2 -30 20 70 120 170 220 270 320 370 420 470 0 Py Cp -Py Sp-Py Ga -Py Cp Sp-Cp Ga -Cp Sp Ga-Sp Ga Eh (mV, SHE) Mixture of minerals Py Cp Sp Ga

Fig. 7. Effect of minerals in their mixture on H2O2 formation with 12.5 g blend sample after

Fig. 6. Effect of rest potential on H2O2 production. 5 min conditioning in water at pH 4.5. 86 A. Javadi Nooshabadi, K. Hanumantha Rao / Hydrometallurgy 141 (2014) 82–88

and other sulphide minerals in the context of ﬂotation, leaching, en- which reacts with ferrous iron to form H2O2 (Eq. (2))(Cohn et al., vironmental control and geochemical processes, which the literature 2006a, 2006b). so far has described as galvanic interactions between two contacting II III • − sulphide minerals and electron transfer from one to the other Fe ðpyriteÞþO2 →Fe ðpyriteÞþðO2Þ ð1Þ (Abraitis et al., 2003; Akcil and Ciftci, 2003; Alpers and Blowes, II • − þ III 1994; Banﬁeld and Nealson, 1997; Ekmekçi and Demirel, 1997; Fe ðpyriteÞþðO2Þ þ 2H →Fe ðpyriteÞþH2O2 ð2Þ Huang and Grano, 2005; Kelebek et al., 1996; Mehta and Murr, 1983; Rao and Finch, 1988; Sato, 1992; Sikka et al., 1991; Thornber, Fe (III) can also generate H2O2 via electron extraction from water 1975; Zhang et al., 1997). and a formation of hydroxyl radical (Eq. (3)). Combining two hydroxyl radicals leads to the formation of H2O2 (Eq. (4))(Borda et al., 2003):

III • þ II 4. Discussion Fe þ H2O→ OH þ H þ Fe ð3Þ

• In the case of pyrite, chalcopyrite and sphalerite, the formation of 2 OH→H2O2 ð4Þ H2O2 decreases with increasing pH up to 8 and above this pH value, it is seen to increase. However, in the case of galena, formation of H2O2 Fairthorne et al. (1997) using the Eh–pH stability diagram of chalco- occurs only at pH values less than 4.5. Also, pyrite generated more pyrite (Fig. 8b) exempliﬁed the formation of insoluble ferric oxide/

H2O2 than other minerals as shown in Fig. 2. Kocabag et al. (1990) hydroxide at neutral and basic pH values but also in acidic conditions using the Eh–pH diagram (Fig. 8a) of pyrite showed that oxidation of at high Eh values. Notably the Fe(II) and Cu(II) ions exist at low pH 2+ + pyrite yields S°, Fe ,Fe(OH)2 species in the solution for pH b 6and values from negative to high Eh values. These divalent ions are reported Fe (OH)2,Fe(OH)3 species in the solution for pH N 6. With decreasing to support the formation of H2O2 (Jones et al., 2011; 2013a; 2013b; pH value less than 6, the concentration of Fe2+ ions increased. Valko et al., 2005) from water and our present results shown in 2+ And therefore, in the presence of dissolved molecular oxygen, ferrous Table 1 demonstrate that Fe ions generate substantial H2O2 and • − 2+ 2+ 3+ 2+ iron associated with pyrite can form superoxide anion (O2) (Eq. (1)), Pb and Zn give higher concentrations than Fe and Cu . Fig. 8b

a) b)

c) d)

Fig. 8. a) Eh–pH stability diagram for the Cu–Fe–S–H2O system with the preponderant copper and iron species shown in each domain (Fairthorne, et al., 1997). b) Eh–pH diagram for FeS2– -5 H2O system at 25 °C and for 10 M dissolved species (Kocabag et al., 1990). c) Eh–pH stability diagram for the sphalerite–H2Osystem(Huai Su, 1981). d) Eh–pH diagram for the Pb–S–H2O system where [Pb] =10-3 M(Woods, 1981). A. Javadi Nooshabadi, K. Hanumantha Rao / Hydrometallurgy 141 (2014) 82–88 87 displays that the concentration of ferrous ions decreases at the expense significantly evidenced for Pb2+ ions that generate superoxide and of ferric oxide/hydroxides at higher pH and Eh values (oxygen condition- hydroxyl radical (Ahlberg and Broo, 1996a; 1996b; 1996c). ing). In the presence of dissolved molecular oxygen, at acidic pH (b5.5) − • − • − O2 þ e →ðO2Þ ð16Þ superoxide (O2) is formed (Eq. (5)), which reacts with ferrous ion to – • − þ form H2O2 (Eqs. (6) (8)). This is in agreement with other studies 2ðO2Þ þ 2H →H2O2 þ O2 ð17Þ where it was observed that metal-induced formation of free radical gen- 2þ 0 − • erates the superoxide and hydroxyl radical (Jones et al., 2011; Valko PbS þ H2O2 →Pb þ S þ 2e þ 2 OH ð18Þ et al., 2005). • 2 OH→H2O2 ð19Þ 2þ 3þ • − O2 þ Fe →Fe þðO2Þ ð5Þ • − 2þ þ 3þ 5. Conclusions ðO2Þ þ Fe þ 2H →Fe þ H2O2 ð6Þ 2þ 3þ • − In the scope of this paper, fundamental studies were carried out to re- Fe þ H2O2 →Fe þ OH þ OH ð7Þ veal oxidation in sulphide mineral flotation. For this reason, the oxidation • 2 OH→H2O2 ð8Þ of sulphide mineral at the grinding stage was evaluated. In mineral processing applications, it was found that (especially fine grinding) for- Above pH 5, the stability diagram shows the Fe O solid phase that 2 3 mation of H2O2 in pulp liquid takes place. It is known that hydrogen per- also generates hydrogen peroxide. This is in agreement with other stud- oxide is a strong oxidizing agent and can easily oxidize sulphide minerals ies where hematite and magnetite solids in water were shown to induce and can even cause dissolution of some metal ions from ore surface. That H O formation (Cohn et al., 2006a, 2006b). Copper ions also generate 2 2 is why H2O2, rather than molecular oxygen, can be the main reason for hydrogen peroxide. This is in agreement with other studies where oxidation, inadvertent activation and non-selectivity in the sulphide copper ions are shown to generate superoxide and hydroxyl radical ore flotation. Prevention or reduction of H2O2 formation may result in – (Eqs. (9) (12))(Javadi Nooshabadi and Hanumantha Rao, in press; improved flotation results and we are working on these aspects. Valko et al., 2005): Following preliminary conclusions were drawn from the results − • – of our initial investigations. It was found that pyrite, chalcopyrite, O2 þ e →ðO2Þ ð9Þ sphalerite, and galena generated H2O2 in pulp liquid during wet • – þ 2ðO2Þ þ 2H →H2O2 þ O2 ð10Þ grinding in the presence of dissolved oxygen in water and also

þ 2þ • – when the solids are placed in water immediately after dry grinding. Cu þ H2O2 →Cu þ OH þ OH ð11Þ Pyrite generated more H2O2 than other minerals and the order of • H2O2 production by the minerals found to be pyrite N chalcopyrite N 2 OH→H2O2 ð12Þ sphalerite N galena. The pH of water inﬂuenced the extent of hydrogen The higher amounts of hydrogen peroxide formation at acidic pH peroxide formation where higher amounts of H2O2 are produced at than at alkaline pH are caused by copper and ferrous ions which are ca- highly acidic pH. H2O2 concentration increases with increasing condi- pable to generate hydrogen peroxide (Table 1). tioning time and solids loading. H2O2 formation increased with increasing pyrite fraction in chalcopyrite-pyrite, pyrite–galena and sphalerite– Huai Su (1981) displayed by the Eh–pH diagram of the sphalerite that at pH b 6, Zn2+ and Fe2+ formed (Fig. 8c) and as Table 1 shows that pyrite compositions. Also with increasing chalcopyrite fraction in 2+ 2+ chalcopyrite–sphalerite and chalcopyrite–galena mixtures, the concen- Zn and Fe ions generated H2O2. In the presence of dissolved molec- • − tration of H2O2 increases but with an increase in galena proportion in ular oxygen, ferrous iron can form superoxide anion (O2) (Eq. (5)), galena–sphalerite mixture, the concentration of H2O2 decreases. which reacts with ferrous iron to form H2O2 (Eq. (6)). Also ZnS can • − generate superoxide anion (O2) to form H2O2 (Eq. (13)–(15))(Javadi Nooshabadi and Hanumantha Rao, 2013): Acknowledgements ZnS→Zn2þ þ 2e þ S0 ð13Þ Financial support from the research Center for Advanced Mineral − • – and Metallurgy (CAMM), Luleå University of Technology, Luleå, and e þ O2 →ðO2Þ ð14Þ Boliden Mineral AB, Boliden, Sweden, is gratefully acknowledged. • – þ 2ðO2Þ þ 2H →H2O2 þ O2 ð15Þ References Woods (1981) by the Eh–pH diagram (Fig. 8d) of the galena showed 2+ that at pH b 4, Pb ions exist. In the presence of dissolved molecular Abraitis, P.K., Pattrick, R.A.D., Kelsall, G.H., Vaughan, D.J., 2003. Acid leaching and dissolu- oxygen and in acidic pH (pH b 4), lead ions can form superoxide tion of major sulphide ore minerals: process and galvanic effects in complex systems. • − + – In: Doyle, F.M., Kelsall, G.H., Woods, R. (Eds.), Electrochemistry in Mineral and Metals anion (O2) which reacts with H to form H2O2 (Eqs. (16) (19)). – 2+ Processing, vol. VI. The Electrochemical Society Inc., pp. 143 153. It can be seen from Table 1 that Pb ions generate H2O2 (Blokhina Adebayo, A.O., Ipinmoroti, K.O., Ajayi, O.O., 2006. Leaching of sphalerite with hydrogen et al., 2001; Ni et al., 2004). This is in agreement with other peroxide and nitric acid solutions. J. Miner. Mater. Charact. Eng. 5, 167–177. studies where metal ions induced formation of free radicals has been Ahlberg, E., Broo, A.E., 1996a. Oxygen reduction at sulphide minerals. 1. A rotating ring disc electrode (RRDE) study at galena and pyrite, Int. J. Miner. Process 46 (1–2), 73–89. Ahlberg, E., Broo, A.E., 1996b. Oxygen reduction at sulphide minerals. 2. A rotating ring Table 1 disc electrode (RRDE) study at galena and pyrite in the presence of xanthate, Int. J. H2O2 generation in the presence of metal ions at pH 6–7 and at 22 °C. Miner. Process 47 (1–2), 33–47. Ahlberg, E., Broo, A.E., 1996c. Oxygen reduction at sulphide minerals. 3. The effect of sur- H2O2 (mM) face pre-treatment on the oxygen reduction at pyrite, Int. J. Miner. Process 47 (1–2), – Concentration of ions 1 mM 10 mM 49 60. Akcil, A., Ciftci, H., 2003. Metals recovery from multimetal sulphide concentrates Water 0 0 (CuFeS2–PbS–ZnS): combination of thermal process and pressure leaching. Int. Fe 2+ 0.552 4.656 J. Miner. Process. 71, 233–246. Fe 3+ 0.004 0.059 Alpers, C.N., Blowes, D.W., 1994. Secondary iron-sulphate minerals as sources of sulphate Cu 2+ 0 0.015 and acidity. In: Alpers, C.N., Blowes, D.W. (Eds.), Environmental geochemistry of sul- – Pb 2+ 0.013 0.096 phide oxidation. Am. Chem. Soc. Series, 550, pp. 345 364. Zn 2+ 0.004 0.088 Aydogan, S., 2006. Dissolutionkineticsofsphaleritewithhydrogenperoxideinsulphuric acid medium. Chemical Engineering Journal 123, 65–70. 88 A. Javadi Nooshabadi, K. Hanumantha Rao / Hydrometallurgy 141 (2014) 82–88

Baga, A.N., Johnson, G.R.A., Nazhat, N.B., Saadalla-Nazhat, R.A., 1988. A simple spectropho- Jones, G.C., Becker, M., Van Hille, R.P., Harrison, S.T.L., 2013a. Theeffectofsulfide concen- tometric determination of hydrogen peroxide at low concentrations in aqueous solu- trate mineralogy and texture on Reactive Oxygen Species (ROS) generation. Appl. tion. Anal. Chim. Acta 204, 349–353. Geochem. 29, 199–213. Banfield, J.F., Nealson, K.H. (Eds.), 1997. Geomicrobiology: interactions between microbes Jones, G.C., Van Hille, R.P., Harrison, S.T.L., 2013b. Reactive oxygen species generated in and minerals, p. 448. Mineralogical Society of America, Washington, DC. the presence of fine pyrite particles and its implication in thermophilic mineral Blokhina, O.B., Chirkova, T.V., Fagerstedt, K.V., 2001. Anoxic stress leads to hydrogen bioleaching. Appl. Microbiol. Biotechnol. 97 (6), 2735–2742. peroxide formation in plant cells. J. Exp. Bot. 52, 1179–1190. Kelebek, S., Wells, P.F., Fekete, S.O., 1996. Differential flotation of chalcopyrite, pentlandite Borda, M., Elsetinow, A., Schoonen, M., Strongin, D., 2001. Pyrite-induced hydrogen and pyrrhotite in Ni-Cu sulfide ores. Can. Metall. Q. 35, 329–336. peroxide formation as a driving force in the evolution of photosynthetic organisms Kocabag, D., Shergold, H.L., Kelsall, G.H., 1990. Natural oleophilicity/hydrophobicity of on an early Earth. Astrobiology 1, 283–288. sulfide minerals, II. Pyrite. Int. J. Miner. Process. 29, 211–219. Borda, M.J., Elsetinow, A.R., Strongin, D.R., Schoonen, M.A., 2003. A mechanism for Koleini, S.M.J., Jafarian, M., Abdollahy, M., Aghazadeh, V., 2010. Galvanic leaching of the production of hydroxyl radical at surface defect sites on pyrite. Geochimica et chalcopyrite in atmospheric pressure and sulphate media: kinetic and surface Cosmochimica Acta 67, 935–939. study. Ind. Eng. Chem. Res. 49, 5997–6002. Buehler, H.A., Gottschalk, V.H., 1910. Oxidation of sulfides. Econ. Geology 5 (28–35), 1. Koleini, S.M.J., Aghazadeh, V., Sandström, Å., 2011. Acidic sulphate leaching of chalcopyrite Cohn, C.A., Laffers, R., Schoonen, M.A.A., 2006a. Using yeast RNA as a probe for generation concentrates in presence of pyrite. J. Miner. Eng. 24, 381. of hydroxyl radicals by earth materials. Environ. Sci. Technol. 40, 2838–2843. Kosaka, K., Yamada, H., Matsui, S., Echigo, S., Shishida, K., 1998. A comparison among Cohn, C.A., Mueller, S., Wimmer, E., Leifer, N., Greenbaum, S., Strongin, D.R., Schoonen, the methods for hydrogen peroxide measurements to evaluate advanced oxidation M.A.A., 2006b. Pyrite-induced hydroxyl radical formation and its effect on nucleic processes: application of a spectrophotometric method using Copper(II) Ion and acids. Geochem. Trans. 7, 1. 2,9-dimethyl-1,10-phenanthroline. Environ. Sci. Technol. 32, 3821–3824. Dixon, D.G., Tshilombo, A.F., 2005. Leaching Process for Copper Concentrates, US Patent, Mehta, A.P., Murr, L.E., 1983. Fundamental studies of the contribution of galvanic interac- Pub No.: US2005/0269208Al. tion to acid-bacterial leaching of mixed metal sulfides. Hydrometallurgy 9, 235–256. Ekmekçi, Z., Demirel, H., 1997. Effects of galvanic interaction on collectorless flotation Nazari, G., Dixon, D., Dreisinger, D., 2012. The role of galena associated with silver- behaviour of chalcopyrite and pyrite. Int. J. Miner. Process. 52, 31–48. enhanced pyrite in the kinetics of chalcopyrite leaching during the Galvanox™ pro- Fairthorne, G., Fornasiero, D., Ralston, J., 1997. Effect of oxidation on the collectorless cess. Hydrometallurgy 111–112, 35–45. flotation of chalcopyrite. Int. J. Miner. Process. 49, 31–48. Ni, Z., Hou, S., Barton, C.H., Vaziri, N.D., 2004. Lead exposure raises superoxide and hydrogen Harvey, T.J., Yen, W.T., 1998. Influence of chalcopyrite, galena and pyrite on the selective peroxide in human endothelial and vascular smooth muscle cells. J. Kidney Int. 66 (6), extraction of zinc from base metal sulphide concentrates. Miner. Eng. 11, 1–21. 2329–2336. Holmes, P.R., Crundwell, F.K., 1995. Kinetic aspects of galvanic interactions between Rao, S.R., Finch, J.A., 1988. Galvanic interaction studies on sulfide minerals. Can. Metall. Q. minerals during dissolution. Hydrometallurgy 39, 353–375. 27, 253–259.

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sulfide flotation, Part B: effect of H2O2 and process water components on sphalerite Vassiliou, A.H., Hausen, D.H. (Eds.), Applied Mineralogy in Exploration. Ore Geology flotation from complex sulfide ore. J. Miner. Metall. Process. 29 (4), 192–198. Reviews, 6, pp. 257–290. Javadi Nooshabadi, A., Hanumantha Rao, K., 2013a. Formation of hydrogen peroxide by Thornber, M.R., 1975. Supergene alteration of sulfides, I. A chemical model based on massive sphalerite. Int. J. Miner. Process. http://dx.doi.org/10.1016/j.minpro.2013.09.010. nickel sulfide deposits at Kambalda, Western Australia. Chemical Geology 15, 1–14. Javadi Nooshabadi, A., Hanumantha Rao, K., 2013b. Formation of hydrogen peroxide by Valko, M., Morris, H., Cronin, M.T.D., 2005. Metals, toxicity and oxidative stress. Current chalcopyrite and its influence on flotation. Miner. Metall. Process. 30 (4), 212–219. Medicinal Chemistry 12, 1161–1208. Javadi Nooshabadi, A., Larsson, A.-C., Hanumantha Rao, K., 2013. Formation of hydrogen Woods, R., 1981. Mineral flotation. In: Bockris, J.O.M., Conway, B.E., Yeager, E., White, R.E. peroxide by pyrite and its influence on flotation. Miner. Eng. 49, 128–134. (Eds.), Comprehensive treatise on electrochemistry.ElectrochemicalProcessing,vol.2. Jones, G.C., Corin, K.C., van Hille, R.P., Harrison, S.T.L., 2011. The generation of toxic reac- Plenum Press, New York, pp. 571–595. tive oxygen species (ROS) from mechanically activated sulphide concentrates and Zhang, Q., Xu, Z., Bozkurt, V., Finch, J.A., 1997. Pyrite flotation in the presence of metal ions its effect on thermophilic bioleaching. Miner. Eng. 24, 1198–1208. and sphalerite. Int. J. Miner. Process. 52, 187–201.

Paper 6

Revisiting sulphide mineral (bio)processing: A Few Priorities and Directions

Kota Hanumantha Rao, Alireza Javadi Nooshabadi, Tommy Karlkvist, Anuttam Patra, Annamaria Vilinska, Irina Chernyshova

Powder Metallurgy & Mining 2: 116. doi: 10.4172/2168-9806.1000116

Hanumantha Rao et al,. J Powder Metall Min 2013, 2:4 http://dx.doi.org/10.4172/2168-9806.1000116 Powder Metallurgy & Mining

Research Article Article OpenOpen Access Access Revisiting Sulphide Mineral (Bio) Processing: A Few Priorities and Directions Hanumantha Rao K*1, Javadi A1, Karlkvist T1, Patra A1, Vilinska A2 and Chernyshova IV2 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 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– EDFWHULD±VROXWLRQLQWHUIDFHVLPSDFWLQQRYDWLQJFRQYHQWLRQDODQGELRÀRWDWLRQDVZHOODV ELR UHPHGLDWLRQGHWR[L¿FDWLRQ of mineral and chemical wastes are necessary. 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: Sulfide mineral; Flotation; Adsorption; Bioprocessing; Recent developments in biotechnology have given promise of Surface reactivity; FTIR Spectroscopy not only aiding mineral processing–hydrometallurgy operations but also providing means for bioremediation of environmental Introduction problems generated by the mineral industries. Many other uses of Processes on minerals in aquatic media give the world as we know microorganisms and their derivatives are potentially possible. These it. Understanding these processes is important for mineral resources include the use of microorganisms in flocculation and flotation of sustainability and to develop technologies as diverse as ore processing, minerals [2]. The biomodification of mineral surfaces involves the heterogeneous catalysis in solution, (bio)remediation of contaminants, complex action of microorganism on the mineral surface. There are radioactive waste storage and disposal, biochemical engineering, three different mechanisms by means of which the biomodification development of novel surfactants, sorbents, sensors, synthesis of novel can occur: i) attachment of microbial cells to the solid substrate, ii) materials and coatings, and especially in emerging nanotechnologies, oxidation reactions and iii) adsorption and/or chemical reaction where stringent control of materials and surfaces is crucial. Despite with the metabolite products (EPS). Several types of autotrophic the obvious importance and the long history of chemistry of aquatic and heterotrophic bacteria, fungi, yeasts and algae are implicated in heterogeneous processes [1], many fundamental issues, especially minerals biobeneficiation. related to biotic interfaces, are still unresolved or poorly understood. Aqueous redox chemistry of sulfides Froth flotation is the main separation process used for collecting selectively valuable minerals from a mined ore, and is based on The sulfide minerals are the most common on the Earth, most differences in hydrophobicity of particles. Particles with size below 10 important, most diverse, and richest in terms of physical, chemical, and microns are a common constituent of a flotation pulp, originating from structural properties. Such diversity originates from the more complex non-sulfide gangue, grinding in steel mills, and oxidation of sulfides. crystal and electronic structures compared to other classes of materials Iron oxide/sulfide fines are the primary component of acid mine [3]. The main reasons are found in the variety of oxidation states, drainage that control scavenging of toxic cations. Such fine particles coordination numbers, symmetry, crystal field stabilization, density, are produced during grinding of iron ores pose a serious problem stoichiometry, and acid-base surface properties that metal sulfides for flotation and degrade the quality of the concentrate. Generally, exhibit. The combination of variety of properties and applications of attempts to remove and discard fines before beneficiation operations sulfides makes redox reactions catalyzed by these minerals in aqueous result in significant economic losses. On the other hand, the continual reduction in the ore grade is forcing miners to produce ultrafines in order to liberate valuable minerals. Hence, improvements in both *Corresponding author: K. Hanumantha Rao, Mineral Processing Group, Division flotation of fines and flotation of coarse particles in the presence of of Sustainable Process Engineering, Department of Civil, Environmental and Natural fines will increase the sustainability of the mineral resources, while also Resources Engineering, Luleå University of Technology, SE-971 87 LULEÅ, Sweden, E-mail: [email protected] reducing the potential environmental impact of the discarded minerals. In addition, there are persistent problem in sulfide flotation of non- Received September 19, 2013; Accepted October 10, 2013; Published October ferrous metal sulfides selectivity against ferrous pyrite, inadvertent 17, 2013 activation of silicate gangue by metal ions, hetero-coagulation between Citation: Hanumantha Rao K, Javadi A, Karlkvist T, Patra A, Vilinska A, et al. (2013) sulfide and gangue minerals, and fine particle flotation. The solutions Revisiting Sulphide Mineral (Bio) Processing: A Few Priorities and Directions. J Powder Metall Min 2: 116. doi:10.4172/2168-9806.1000116 to these problems are highly rewarded: even small technological improvements would provide high economical and ecological return Copyright: © 2013 Hanumantha Rao K, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which on investments, with regard to the tonnages of material treated and the permits unrestricted use, distribution, and reproduction in any medium, provided multi-dimensional impact of ore processing. the original author and source are credited.

J Powder Metall Min ISSN: 2168-9806 JPMM, an open access journal Volume 2 • Issue 4 • 1000116 Citation: Hanumantha Rao K, Javadi A, Karlkvist T, Patra A, Vilinska A, et al. (2013) Revisiting Sulphide Mineral (Bio) Processing: A Few Priorities and Directions. J Powder Metall Min 2: 116. doi:10.4172/2168-9806.1000116

Page 2 of 8 solutions a very important subject of research from both fundamental and position of the surface states are known, the electron-accepting and industrial standpoints. and electron-donating abilities of different solids can be compared and correlated with the direction and rates of a particular heterogeneous Quantifying and predicting redox chemistry of sulfides is separately redox reaction, including biotic reductive/oxidative degradation of motivated within geochemical, electrochemical, and technological oxides and sulfides. communities. These minerals significantly impact bio-environment geochemistry of near surface systems by altering pH and redox Oxidation of sulfides conditions, thereby increasing or decreasing mineral bioavailability [4]. Charge transfer processes on semiconducting sulfide electrodes Since reactivity of semiconducting sulfides under flotation are today under intense investigation towards novel techniques for conditions is mainly electrochemical, spectro-electrochemical studies conversion of solar energy and environmentally clean fuels such as that allows characterizing the sulfide surface (its composition and hydrogen [5]. Within a few years well over a thousand publications structure) as a function of the mineral potential and the electrolyte composition (including deoxygenation and/or carbonization) are about this topic appeared (see ref. 5 for a list). Therefore, increased expected to be most advanced. Easily manipulating the interfacial knowledge of electronic properties of the mineral surfaces, the detailed potential difference by a potentiostat, one can determine all states of mechanisms of charge transfer across their interfaces, and driving forces the mineral surface reactivity, which is of substantial importance for in the interactions with species from solution will help in improving the modeling the flotation process. FTIR spectroscopy has become the research in this area of electrochemistry. dominant tool for in situ studies of reactions at the electrode/electolyte Despite significant steps have been taken theoretically and or mineral/water interface over the last decade due to a large body experimentally to obtain a molecular-level and coherent understanding of information provided, simplicity of the application in situ, and of aquatic reactivity of sulfides [5-8], a general theory of redox a short-time scale of the measurements, which can be performed chemistry of these solids in waters still does not exist. In the case of simultaneously with a voltammogram. Three FTIR techniques have oxides, the chemical mechanism is commonly considered [9,10], which so far been applied to strongly absorbing IR radiation natural sulfides. implies one of the following charge transfer reactions: 1) From the solid Based on the comparitive analysis performed [20], the authentic ATR to the adsorbate, 2) Between sorbates, or 3) From the adsorbate to the technique that uses a glued mineral plate [21] has more advantages as solid. The main idea behind this mechanism is that catalytic effect of it is freer from kinetic limitations and technical artifacts (Figure 1). In in situ the surface is caused by a decrease in the energy needed to modify the addition to FTIR measurements, complimentary data by XPS solvent structure, the standard redox potential of the reductant, and for the atomic surface composition and the valence state of the surface steric hindrance of the oxidant-reductant interaction [11]. However, atoms will provide surface structures of adsorbed species. the chemical model does not explain different modes and different rates The mechanisms suggested for the oxidation of galena in previous of the oxidation film growth on different minerals [12-14] as well as indirect studies at alkaline pH are: heterogeneous processes on sulfides [15]. A general disadvantage of this model is in its explanatory character and low ability to predict. a) Initial stage: PbS + 2H O + 2xh+ = Pb S + xPb(OH) An alternative is the electrochemical mechanism, in which the anodic 2 1-x 2 and cathodic semi-reactions of the sum redox reaction are spatially + 2+ PbS + 2xh = Pb1-xS + xPb separated. This mechanism has been invoked for interpreting abiotic PbS + 2H O + 2h+ = Pb(OH) + S + 2H+ chemistry of semiconducting sulfides [15-18] and photochemical 2 2 reactions on both oxides and sulfides [5,9]. Apart from the energy level PbS = Pb2+ + S2- redistribution among species in the (inner and outer) Helmholtz layer, b) Bulk oxidation: this model takes into account semiconducting properties of the solid + - [19]. Provided the position of the Fermi level relative to edges of the 2PbS + 5H2O = PbS2O3 + Pb(OH)2 + 8H + 8e conduction and valence bands as well as specification, relative density 2- + - 2PbS + 7H2O = 2Pb(OH)2 + S2O3 + 10H + 8e 2- 2- + - 2PbS + 2CO3 + 3H2O = 2PbCO3 + S2O3 + 6H + 8e Where h+ stands for hole [16,17], and has been initially suggested [22,23] to explain the prepeak in voltammograms (at -0.05 V (SHE) in 0.001 M n-butyl xanthate solution [24]). The products of anodic oxidation determined by in situ FTIR spectra [16, 17] (Figure 2) with increasing potential in deaerated 0.01

M borate buffer of pH 9.2 at 0.1 V: Pb1-xS; 0.2 V: Pb(OH)2 and PbSO3; 2- 0.3 V: PbS2O3; and 0.4 V: SnO6 .

Based on the spectral results (Figure 2), the first stage produces Pb1- 0 xS and then S : + 2+ PbS + 2xh = Pb1-xS + xPb PbS + 2h+ = Pb2+ + S0 (E0 = 0.354 V)

Then, Pb(OH)2 is formed by a precipitation mechanism followed Figure 1: Experimental design of in situ FTIR [21]. by lead sulfite and thiosulfate from the elemental sulfur produced in the first stage:

Page 3 of 8

Figure 2: Anodic oxidation of galena by in situ FTIR spectra [17].

Figure 3: A FTIR study of anodic oxidation of pyrite with increasing redox potential [15].

0 + 2- + 0 S + 3H2O + 4h = SO3 + 6H (E = +0.59 V) V: bulk hydrated ferric hydroxide gel and bulk FeSO3 in the amount less than one monoloayer; 0.4 V: less than one monolayer of a surfce SO 2- + Pb2+ = PbSO 3 3 ferric iron-hydroxyl complex; 0.5 V: bulk oxidation produced ferric 2- 0 2- hydroxide, ferrous sulfite and polythionates [15]; and at 0.6 V: SO 2-. SO3 + S = S2O3 (log K = 8.14) 4 Anodic reactions on pyrite are: 1) FeS = Fe2+ + S2-, followed by hydrolysis, Oxygen not only provides the cathodic half-reaction of the oxidation of (bi)sulfide ion (SS) and hydrolyzed iron(II) either galena oxidation but also react with the anodic areas, accelerating the chemically by dissolved oxygen or electrochemically, and precipitation incongruent dissolution. Similarly, the anodic oxidation of pyrite by of the products (elemental sulfur, sulfite, and ferric hydroxide); 2) SSred in situ FTIR spectra (Figure 3) Revealed that the surface consist at 0 + - + + h + OH = SSoxOH; and 3) thiosulfate pathway: FeS2 + 3H2O + 6h

Page 4 of 8

of mono- and clustered surface defects based on the effect of dynamic coupling of the adsorbates on the vibrational spectra and, where possible, on the STM/AFM data • What are adsorption forms of the redox-active anions ubiquitous in geochemical systems (sulfite/sulfate, selenate/ selenite, arsenate/arsenite, chromate, etc.)? How does 2– – introduction of S , CN , O2, and H2O2 before/after the anion adsorption influence this picture? Complex in situ and ex situ spectroscopic study • How does the partitioning between adsorbed and adsorbed- and-reacted anions change with time? Evaluation of the kinetic parameters of the redox reactions catalyzed by the minerals. Inferring the reaction mechanisms • What are adsorption forms of redox-active surfactants (xanthates and dithiophosphates) in the absence and presence of the pre-adsorbed anions? How this picture is influenced by 2– – In situ ex situ solvated S , CN , O2, and H2O2? and complex spectroscopic measurements • What is the difference between the anodic semi-reaction paths on (semi)conducting oxides and sulfides in the redox process controlled “chemically” and electrochemically with a potentiostat? FTIR-ATR spectro-electrochemical study • Analysis including surface complexation modeling of the whole data. Development of a predictive general model of redox catalytic activity of wide band-gap metal oxides and sulfides Mechanisms of collector 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 Figure 4: Differential ATR spectra for (100) PbS and 8 x 10-5 M sodium n-butyl xanthate in deaerated 0.01 M borate buffer at anodic potential scan from –0.5 V [16]. of a collector is conducting redox reactions before or after its adsorption at the mineral surface, as used, e.g., in sulfide flotation with xanthates [25]. It is well known that the toxicity, solubility, sorption, 2+ 2- + = Fe + S2O3 + 6H , thiosulfate and ferrous ions are further oxidized bioavailability, and transport of elements in soil and aquatic systems are both electrochemically and chemically (by ferric hydroxide) to sulfate strongly dependent on the oxidation state [10]. at the outer side of the pyriote surface, while elemental sulfur can be in situ formed from polythionates. On some surfaces a pseudo-electrochemical Interaction of galena with xanthate by FTIR spectroscopy is (PE) mechanism is expected, when the redox process is activated at shown [16,18] in Figure 4. The se spectral results confirmed that i) At the edge of the surface heterogeneity domain and proceeds through the first stage xanthate is adsorbed with charge transfer (chemisorbed), ii) dixanthogen is formed at the reversible potential for the xanthate/ enlarging the initial spot, not starting new ones. Relative contribution dixanthogen pair, independently of the xanthate concentration. The of the different pathways obviously depends on the electronic properties appearance of dixanthogen makes the surface most hydrophobic, of the mineral bulk and surface. Assuming that bacteria recognize which can be followed from the changes in the water spectrum, favorable materials for colonization through redox sensing [25], it is and iii) the mechanisms of the xanthate adsorption at high and low reasonable to suggest that there can exist a “redox” source of enhanced concentrations have differences. At a concentration of 10-3 M, oxidative and selective reactivity of minerals with redox-active solutes. decomposition of galena and adsorbed xanthate is inhibited by the For redox modification of mineral surfaces as a tool for providing xanthate chemisorption, which is followed by the formation of lead -5 adsorption selectivity, it is imperative to investigate and answer the xanthate, Pb(X)2, and (X)2. At 8.10 M, bulk Pb(X)2 is formed by the following questions: precipitation mechanism. At higher applied potentials first Pb(X)2

transforms into Pb(OH)2. Afterward (X)2 decomposes into a dimer of • Do the mineral surfaces have intrinsic oxidizing and reducing monothiocarbonate, In situ – 3– sites? FTIR-ATR study of the CO, CN , Fe(CN)6 , and 4– (ROCS ) + 2OH- + 2h+ = (ROCSO) + 2H+ Fe(CN)6 adsorption. Discrimination and characterization 2 2 2

Page 5 of 8

upon catalytic oxidation of sulfite species will much more strongly be 1.8 500 bound to the sulfide surface compared with the sulfate anions that are directly adsorbed through ion exchange/outer-sphere complexation, 1.6 H2O2 400 thus competing more efficiently with collectors for the adsorption sites 1.4 on sulfides, which may strengthen the depressing effect of sulfite. This Eh 1.2 300 effect, if properly understood, can open a new cost-effective approach 1 to selectively regulate surface properties of sulfides. (mM)

2 200 Recently it was revealed [35] that ferric defects on ground pyrite O 0.8 2 •

H surfaces can generate OH radicals upon interaction with water. It

0.6 100 SHE) Eh (mV, may be the existence and reactivity of OH• that plays a crucial role 0.4 0 in catalytic degradation of organic pollutants by pyrite. However, 0.2 participation of these species, if any, in non-selective oxidation of the 0 -100 pulp components and hence in deteriorating the concentrate grade 2.5 4.5 6 8 11.6 has not still been explored yet. To fill the gap, it is important to build pH correlation between percentage of pyrite in the concentrate, grinding • conditions and concentration of OH /H2O2 in the pulp as well as to Figure 5: Effect of water pH on H O formation after 5 min mixing of dry 2 2 study possible ways of flexibly controlling the formation of these species ground solids with water [37]. through known chemical means. One of such ways can be addition of chloride ions, which are known to inhibit the deposition of elemental while galena decomposes into lead sulfite and lead thiosulfate. sulfur on the pyrite electrode surface by promoting the oxidation of an adsorbed intermediate, believed to be the thiosulfate ion, to soluble Thus, surface speciation of sulfides in the absence and presence tetrathionate ions. In the absence of chloride ions, the thiosulfate of collectors as a function of the sulfide potential controlled intermediate undergoes acid decomposition on the pyrite surface to electrochemically using a potentiostat or chemically by changing the yield elemental sulfur [36]. redox potential of the pulp needs to be carried out by in situ spectro- electrochemistry. The results may lead to surface speciation with For the problem of low selectivity between ferrous and non-ferrous significant inference of hydrophobic or hydrophilic nature of the sulfides, and among sulfides, the effect of production of2 H O2 by metal surface for different redox conditions. sulfides needs to be examined. To pinpoint the dominant contribution (natural hydrophobicity, formation of elemental sulfur on the surface Complex sulfide ore flotation under the flotation conditions, and/or activation) to low selectivity of sulfide flotation against pyrite, surfaces of pyrite particles from both The use of NaHSO3 as a flotation depressant [26] is being practiced in selective flotation of sulfide minerals. The depressing effect generally concentrate and tailings need spectroscopy characterization for surface increases from copper sulfides to galena, pyrite and sphalerite. However, speciation. there is no common agreement about the depression mechanisms. In Our recent results of dry-ground pyrite when mixed with water particular, the following three effects have previously been proposed leading to the formation of H2O2 are shown in Figure 5. The water pH 2– for depression of pyrite flotation by SO3 : 1) stripping/decomposition in which the ground solids are placed has a significant influence [37] of xanthate [27]; 2) reaction with the pyrite surface to form hydrophilic on H2O2 formation of its increase with decreasing pH below 8. Figure iron oxides [28]; and 3) a decrease of redox potential of the pulp below a 5 shows that the formation of H2O2 is lower at pH 8 and above. This level at which binding/oxidation of a collector (electron donor) becomes observation found to be valid with other sulfide minerals [38] as well energetically unfavorable [29]. In the presence of copper, sulfite was (Figure 6). shown [30] to promote the oxidation of copper on the pyrite surface, preventing the adsorption of xanthate and thus leading to the mineral Fine particle flotation depression, but has no effect on sphalerite. At the same time, in the The floatability of fine particles has slow kinetics due to low case of chalcopyrite, it was postulated [31] that sulfite removes both the momentum through the pulp resulting by their small mass, which leads sulfur-rich phase and iron oxides from the surface, leaving a sulfur-rich to a lower probability of collision with passing air bubbles. Furthermore, sulfide layer, which in turn promotes collector adsorption. Also, it was found [32] that the depressing effect of sulfite 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 1.6 xanthate adsorption following a decrease of the redox potential of the 1.2 pulp, several additional mechanisms have been put forward to explain pyrite the depression of the flotation of sphalerite [33]. These include: the chalcopyrite 0.8 formation of a zinc sulfite hydrophilic layer at the mineral surface; the (mM)

2 sphalerite O reduction of copper-activation as a result of consumption of copper in 2 galena solution as copper sulfite; and the consumption of dissolved oxygen. H 0.4 Sulfite ions are also known to react with polysulfide or elemental 0 sulfur and form thiosulfate ions; a decrease in surface hydrophobicity 2.5 4.5 8 10.5 11.5 is therefore expected from this reaction. Finally, compared to sulfate, pH this reduced sulfoxyanion has higher adsorption affinity due to lower Figure 6: Effect of pH on production of H2O2 after 5 min mixing of dry S–O bond order [34]. Therefore, we expect that sulfate anions produced ground solids with water [38].

Page 6 of 8

have formidable applications in flotation and flocculation processes. 0.001 PbS The adhesion of microorganisms results an alteration of the surface 1 chemistry of minerals due to a consequence of the formation of a ) 0 1170 0.000 985 biofilm on the surface or surface-active chemicals and their adsorption, 2 accumulation and precipitation of ions and compounds on the surface. 1265

-log(R/R -0.001 * Microorganisms adhere to mineral surfaces for various reasons. S. oneidensis

1090 utilize minerals as the terminal electron acceptor in 1215 1009 the respiratory cycle. Bacteria of Thiobacilli group recover energy -0.002 3000 2500 2000 1500 1000 from minerals by the enzymatic oxidation. Common to both these microbiologic processes is the bacterial need to access, adhere to, and 0.001 react with the mineral-water interface. Previous studies have shown that

1420 under most physiological conditions the bacterial cell surface carries a FeS2 1542 1105 net negative charge, while, along with electrostatic forces, hydrophobic, 0.000 )

0 1 entropic, acid-base, and Van der Waals interactions and H-bonding are 1265 2 1009 important in the bacterial adhesion [2]. 1090 -0.001 The information on the mechanisms of both bacterium adsorption -log(R/R

1035 and reagent (collector) adsorption in the presence and absence of the -0.002 3000 2500 2000 1500 1000 adsorbed bacteria is necessary. However, this area is a dark spot at the moment. The tremendously complex problem of gaining insight into -1 Q, cm the mechanisms of adsorption of living organisms is met by a not- less-complex surface chemistry in a pulp as far as even in the absence Figure 7: In situ FTIR at the PbS-aqueous solution and FeS2-aqueous solution interfaces under open atmosphere. 10 min in 108 unit/ml of of adsorbed bacteria the sulfide surface species are instable. We have microbes followed by 10 min conditioning in 10-5 M ethyl xanthate. showed that adapted bacteria of the Thiobacilli group (Thiobacillus ferrooxidans, Thibacillus thiooxidans, and Leptospirilum ferrooxidans), Paenibacillus slimes have a significant depressant effect on polysulfide ores and their which are associated of sulfide ores and mine water, and polymyxa depressant effect is three-fold: 1) fines of non-sulfide gangue report to , can be selectively attached to sulfides, thereby essentially concentrate; 2) valuable sulfides are lost due to low floatability of the modifying the following adsorption of flotation reagents [41-43] (these fine particles and 3) non-sulfide gangueslimes covering the originally- systems are among few that have been extensively reported in the hydrophobic sulfide particles through hetero-coagulation mechanism literature). This phenomenon can be put into the basis of elaboration rendering the particles hydrophilic. of principally novel flotation schemes providing both ecological and economical gain. However, recent data showed that there are intrinsic differences in the surface and bulk stoichiometry and crystal structure of very fine Though the mechanisms of adsorption of the microbe’s extracellular particles from those of the corresponding large bulk crystals [39]. polymeric substance (EPS) components onto solid surfaces are Atoms exposed on nano-sized particles experience an anisotropic generally known [44], they are system specific. The interaction of the environment. To lower free energy per unit surface, the crystal structure biopolymers produced by bacteria grown in the presence of specific of the near surface region is distorted, which in real conditions is minerals with surfactants requires investigation focusing on the accompanied by adsorption of water/hydroxyls. It is expected that with applicability to bioflotation. More specifically, the EPS produced by decreasing particle size the redox potential of the sulfide decreases, bacteria of Acidithiobacilli group during the growth in 1) culture media, along with sorption capacity per nm2. These effects can be balanced by 2) in the presence of either chalcopyrite, pyrite, galena or sphalerite, using the more easily oxidizable homologues of xanthate and carbamate and 3) in the presence of both mineral and flotation reagent (xanthate with longer chain lengths and/or employing sterically appropriate and dithiophosphate). The interaction of the biopolymers produced by chelating legands with flexible distance between the reactive groups. bacteria grown in the presence of specific minerals and minerals with Thus to shed light on solution and solid state chemistry of submicron xanthate and dithiophosphate require special attention. particles and to get a possibility to control their reactivity, which is in situ Paenibacius polymixa important for flotation, it is necessary to perform a systematic study of Our initial FTIR spectra of interaction how and why reactivity of sulfides changes with particle sizes. with galena and pyrite are shown in Figure 7. Spectra (1) on the top and bottom panels show the sulfide–water interface after conditioning Paenibacius polymixa Biobeneficiation of metal sulfides with a suspension of adapted to FeS2. The microbe concentration was 10–8 ml–1 (pH 6.5). One can see the appearance of The biobeneficiation includes the processes of bio-flotation and weak bands of PbSO at 1170 and 985 cm–1 in the case of galena. The bio-flocculation where selective removal of undesirable mineral 4 spectral picture is somewhat different in the case of pyrite: First, except constituents from an ore is accomplished. Several studies showed that the for bands of a Fe(III)–SO complex at 1105 and 980 cm–1, prominent microorganisms could function similarly as that of traditional reagents 4 bands at 1420 and 1542 cm–1 assignable to Fe(III) carbonate are in flotation and flocculation processes [2,40]. Adhesion of microbes to observed. Second, intensity of these bands is noticeably higher than for particle surfaces can alter the hydrophobicity of minerals markedly. galena, implying that interaction with the microbe results in a higher Such surface modification to impart hydrophobicity or hydrophilicity degree of corrosion for pyrite. The presence of the carbonate species is used in flotation of sulfide and non-sulfide minerals. In some systems implies the formation of amorphous ferric (hydr)oxide, the absorption they also cause excellent flocculation of fine particles. Since the bacteria of which is masked by the water absorption below 800 cm–1. adhere to mineral surfaces and alter the surface properties that are essential in mineral beneficiation techniques, the microorganisms The subsequent addition of a 10–5 М aqueous solution of ethyl

Page 7 of 8 xanthate results in the formation of both lead xanthate (1215, 1090, bacteria and the EPS components and the heterogeneous and 1009 cm–1) and dixanthogen (a component at 1265 cm–1 marked charge transfer reaction? by an asterisk) on galena. This process is accompanied by appearance • How the mineral potential affects forms of collector adsorption of a strong negative band (δH O) of water at 1630 cm–1, which indicates 2 and surface equilibrium of model redox couples in the presence that water is pushed off the interface, consistent with experimentally of the bacteria? observed hydrophobicity of the mineral under similar conditions. Though some dixanthogen is formed upon the xanthate adsorption on Though the mechanisms of adsorption of the microbe’s extracellular pyrite (bands at 1265, 1090, and 1009), the presence of a band at 1035 polymeric substance (EPS) components onto solid surfaces are generally cm–1 testifies to the concomitant formation of ferric xanthate-hydroxide known, they are system specific. The mechanisms of the interaction species. Positive intensity of the water band at 1630 cm–1 shows that of the biopolymers produced by bacteria grown in the presence of i) density of the interfacial water increases, in agreement with the culture media, ii) specific minerals and iii) both mineral and surfactant observed depression of pyrite. Inspection with an optical microscope of (flotation collector) need special attention focusing on the applicability the sulfide minerals after the experiments described revealed that about to bioflotation. Another environmentally sound and cost-effective 10 microbes had adhered to the polished pyrite surface of 10 mm2, biochemical innovation in sulfide flotation could be replacement of while none to galena. Based on these observations and since no typical Na2S and reduced sulfur compounds, which are widely used in flotation bands of biopolymers (proteins, polysachharides, etc.) are present in practice as regulator (reductants), by sulfate reducing bacteria. Clearly, the in situ FTIR spectra, one can make preliminary conclusion that the to develop a biochemical control of sulfide flotation, it is necessary main mechanism of bioregulation of the surface properties relates to first of all to understand difference in floatability of the sulfide surfaces biooxidation of the sulfides and/or to change in the sulfide potential (in reduced chemically and biochemically, which require intensified effort. that particular experiment we did not measure potential of the sulfide electrode but this can be done) rather than to surface properties of the Summary adsorbed microbes themselves or expressed biopolymers. The molecule-level understanding of the aquatic solids interfaces is To provide a new insight into the phenomena, the whole problem the key to innovate many society-formative technologies including ore of the mineral-bacterium interaction can be conveniently divided into processing, waste recycling and the environmental protection that are two problems: biochemical and geochemical. The former is related to based on stringent control of interfacial processes. There are no general the bacterium side of the interface (the bacterium envelope) in terms concepts of selective interactions of minerals with solutes, which imply of particular mechanisms of sensing/recognition and response to that fundamental additions to aquatic chemistry of minerals are one extracellular minerals, molecule-specific pathways of charge transfer, of the major demands of the day. The interface between biological and genomic mechanisms of regulation of these processes. The and geological materials, as well as a means to design and manipulate geochemical problem concerns the mineral response to the bacterium that interface, is currently virtually completely unexplored. The lack presence, which is essentially interplay between microorganism and the of knowledge could be recognized to the complexity and technical physicochemical properties of the mineral surface, such as the atomic constraints of the problem. Progress in this area is hampered by a deficit and electronic structure, the net charge/potential, acid-base properties, of direct, not to mention in situ, spectroscopic molecular/atomic-level and wettability of the surface. Consequently, the biocatalyzed electron- data about the mineral–bacterium interfaces. What is exactly happening transfer reactions on metal oxides and sulfides can be controlled at the mineral-bacteria-solution interfaces at the molecular level and through variation of these parameters, which require a systematic study in real time controls mineral (bio) processing technologies and there in order to find answers to the following questions: is an urgent need to understand the interfacial charge transfer and adsorption mechanisms to shift the technologies to be more innovative • Are the redox mechanisms established for abiotic interfaces are and effective. valid in the case of bacterium-mediated reduction/oxidation of the mineral? Acknowledgement 7KH ¿QDQFLDO VXSSRUW IURP WKH &HQWUH IRU$GYDQFHG 0LQLQJ DQG 0HWDOOXUJ\ • Is direct bacteria-mineral contact necessary for oxidative or (CAMM), Luleå University of Technology, Luleå, Luossavaara-Kiirunavaara AB reductive dissolution of sulfides, iron oxides, or manganese (LKAB), Kiruna and Boliden Mineral AB, Boliden, all in Sweden, are gratefully oxides? acknowledged. • Do bacteria exploit the energetic perturbations present in the References surface defects to adhere and oxidize/reduce the mineral? If yes, 1. Hochella MFJ, White AF (1990) Mineral-water interface geochemistry. defects of which type (point vs. clustered) are more preferable Washington DC: Mineralogical Society of America. by bacteria? 2. Hanumantha Rao K, Subramanian S (2007) Microbial processing of metal VXO¿GHV6SULQJHU • Which type of surface defects bacteria generate by their own? What is the mechanism of generation of these defects? 3. 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10. Stumm W, Morgan JJ (1996) Aquatic chemistry. New York: Wiley 679. 33. Misra H, Miller D, Song QY (1985) Developments in mineral processing, ÀRWDWLRQRIVXO¿GHPLQHUDOVHG.6()RUVVEHUJ$PVWHUGDP(OVHYLHU 11. Wehrli B, Sulzberger B, Stumm W (1989) Redox processes catalyzed by hydrous oxide surfaces. Chemical Geology 78: 167-179. 34. Arai Y, Sparks DL, Davis JA (2004) Effects of dissolved carbonate on arsenate adsorption and surface speciation at the hematite-water interface. 12. Junta JL, Hochella Jr MF (1994) Manganese (II) oxidation at mineral surfaces: Environmental Science and Technology 38: 817-824. A microscopic and spectroscopic study. Geochimica et Cosmochimica Acta 58: 4985. 35. Borda M, Elsetinow A, Strongin D, Schoonen M (2003) A mechanism for the production of hydroxyl radical at surface defect sites on pyrite. Geochimica et 13. Leland JK, Bard AJ (1987) Photochemistry of colloidal semiconducting iron Cosmochimica Acta 67: 935-939. oxide polymorphs. Journal of Physical Chemistry 91: 5076. 36. Lehmann MN, Stichnoth M, Walton D, Bailey SI (2000) Effect of chloride ions 14. Pehkonen SO, Siefert RL, Hoffmann MR (1995) Photoreduction of iron on the ambient electrochemistry of pyrite oxidation in acid media. Journal of the oxyhydroxides and the photooxidation of halogenated acetic acids. Electrochemical Society 147: 3263-3271. Environmental Science and Technology 29: 1215. 37. Javadi A, Larsson AC, Hanumantha Rao K (2013) Formation of hydrogen 15. Chernyshova IV (2003) An in situ FTIR study of galena and pyrite oxidation in SHUR[LGH E\ S\ULWH DQG LWV LQÀXHQFH RQ ÀRWDWLRQ - 0LQHUDOV (QJLQHHULQJ aqueous solution. Journal of Electroanalytical Chemistry 558: 83-98. 128–134.

16. Chernyshova IV (2001) Anodic processes on a galena (PbS) electrode in the 38. Javadi A, Hanumantha Rao K (accepted) Formation of Hydrogen peroxide by presence of n-Butyl xanthate studied FTIR-spectroelectrochemically. Journal of VXO¿GHPLQHUDOV-+\GURPHWDOOXUJ\ Physical Chemistry B 105: 8185-8191. 39. Maira AJ, Yeung KL, Lee CY, Yue PL, Chan CK (2000) Size effects in gas- 17. Chernyshova IV (2001) Anodic oxidation of galena (PbS) studied FTIR- phase photo-oxidation of trichloroethylene using nanometer-sized TiO2 spectroelectrochemically. Journal of Physical Chemistry B 105: 8178-8184. catalysts. Journal of Catalysis 192: 185-196.

18. Chernyshova IV (2002) In situ FTIR-spectroelectrochemical study of the 40. 9LOLQVND $ +DQXPDQWKD 5DR . /HSWRVULULOOXP IHUURR[LGDQVVXO¿GH anodic processes on a galena (PbS) electrode under open-air conditions in the PLQHUDO LQWHUDFWLRQV ZLWK UHIHUHQFH WR ELRÀRWDWLRQ DQG ELRÀRFFXODWLRQ absence and presence of n-butyl xanthate. Langmuir 18: 6962-6968. Transactions of Nonferrous Metals Society of China (English Edition) 18: 1403.

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28. Khmeleva TN, Beattie DA, Georgiev TV, Skinner WM (2003) Surface study of WKHHIIHFWRIVXO¿WHLRQVRQFRSSHUDFWLYDWHGS\ULWHSUHWUHDWHGZLWK[DQWKDWH Minerals Engineering 16: 601-608.

29. )RUVVEHUJ.6(HG )ORWDWLRQRIVXO¿GHPLQHUDOV(OVHYLHU

30. Shen WZ, Fornasiero D, Ralston J (2001) Flotation of sphalerite and pyrite in WKHSUHVHQFHRIVRGLXPVXO¿WH,QWHUQDWLRQDO-RXUQDORI0LQHUDO3URFHVVLQJ Submit your next manuscript and get advantages of OMICS 17-28. Group submissions 31. *UDQR 65 &QRVVHQ + 6NLQQHU : 6XUIDFH PRGL¿FDWLRQV LQ WKH Unique features: FKDOFRS\ULWHVXO¿WHLRQV\VWHP,,'LWKLRSKRVSKDWHFROOHFWRUDGVRUSWLRQVWXG\ International Journal of Mineral Processing 50: 27. • User friendly/feasible website-translation of your paper to 50 world’s leading languages • Audio Version of published paper • Digital articles to share and explore Special features:

• 250 Open Access Journals • 20,000 editorial team • 21 days rapid review process • Quality and quick editorial, review and publication processing • Indexing at PubMed (partial), Scopus, EBSCO, Index Copernicus and Google Scholar etc • Sharing Option: Social Networking Enabled Citation: Hanumantha Rao K, Javadi A, Karlkvist T, Patra A, Vilinska A, et • $XWKRUV5HYLHZHUVDQG(GLWRUVUHZDUGHGZLWKRQOLQH6FLHQWLÀF&UHGLWV al. (2013) Revisiting Sulphide Mineral (Bio) Processing: A Few Priorities and • Better discount for your subsequent articles Directions. J Powder Metall Min 2: 116. doi:10.4172/2168-9806.1000116 Submit your manuscript at: http://omicsgroup.info/editorialtracking/primatology

J Powder Metall Min ISSN: 2168-9806 JPMM, an open access journal Volume 2 • Issue 4 • 1000116

Paper 7

Effect of grinding environment on galena flotation

Alireza Javadi Nooshabadi, Kota Hanumantha Rao

In press, The Open Mineral Processing Journal, 2015

Send Orders for Reprints to [email protected]

The Open Mineral Processing Journal, 2014, 7, 00-00 1 Open Access Effect of Grinding Environment on Galena Flotation

Alireza Javadi Nooshabadi and Kota Hanumantha Rao*

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 2Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology, No. 7491 Trondheim, Norway

Abstract: The generation of H2O2 during the grinding of galena and its effect on the oxidation of galena particles leading to a decrease in flotation recovery has been studied. The influence of two types of grinding media in wet and dry grinding of galena on the formation of hydrogen peroxide and its flotation response was examined. Galena ground in mild steel grinding media generated more hydrogen peroxide compared to stainless steel media. Thus, lower flotation recovery of galena ground in mild steel could also be attributed due to the presence of higher amounts of H2O2 in the pulp liquid, besides widely reported galvanic interactions between grinding medium and mineral. The extent of galena surface oxidation because of either galvanic interactions or H2O2 presence or both, is not very clear. Clearly, both mechanisms operate in galena oxidation and needs further investigation to distinguish the predominant mechanism among the two or the extent of each contributing to surface oxidation.

Keywords: Galena; Wet and Dry Grinding; Stainless Steel and Mild Steel, Grinding Media, Hydrogen Peroxide, Flotation.

INTRODUCTION interaction of a mild steel medium with minerals resulted in Galena is the main mineral of lead and it is commonly the formation of iron hydroxide species on the mineral associated with other sulphide minerals, such as pyrite surface [7]. Recently it was revealed that formation of hydrogen peroxide (H2O2), an oxidizing agent stronger than (FeS2), chalcopyrite (CuFeS2) and sphalerite (ZnS). Guy and Trahar (1984) reported that the floatability of galena is oxygen, takes place in pulp liquid during the wet grinding of dependent upon the grinding environment [1] and galena complex sulphide ore [8]. Previous works showed that pyrite ground in a stainless mill had more recovery than in a steel [9-15] chalcopyrite [14, 16], sphalerite [17] and galena [18- mill. It was found that the oxidation–reduction environment 21] ground particles generated hydroxyl free radicals during grinding is strongly linked to the presence of interacting with water and thus the formation of H2O2 in pulp dissolved iron species from the grinding media [2]. Peng liquid. Pyrite generated more H2O2 than other sulphide et al. (2003) observed the highest amount of iron species minerals and the order of H2O2 production by the minerals coating on galena particles when ground with mild steel [2]. found to be pyrite > chalcopyrite > sphalerite > galena [22]. The dissolved iron ions and their oxidation species played a Javadi et al. (2013) showed that the mild steel generated dominant role in galena flotation. When galena is ground more H2O2 than stainless steel grinding media since the with iron as the grinding medium, galvanic interaction takes dissolved ferrous ions play a key role in generating higher place due to a difference in their rest potentials [3, 4]. The amounts of H2O2 [15]. Clearly, hydrogen peroxide oxidizes steel grinding medium has a lower potential than galena [5]. galena leading to its depression in flotation [23]. The iron medium, which has a lower rest potential will act as the anode and galena with a higher rest potential, will act as In our recent work we found that galena generates H2O2 the cathode. Electrochemical models have been proposed to in pH<4 [21]. In this study, the results on the effect of two explain galvanic interaction between minerals and grinding types of grinding media (mild steel and stainless steel) on media [6]. Natarajan and Iwasaki (1984) used the galena grinding in formation of hydrogen peroxide and its electrochemical models and observed that galvanic influence on surface oxidation and flotation recovery were presented and discussed.

*Address correspondence to this author at the Mineral Processing Group, MATERIALS AND METHODS Division of Sustainable Process Engineering, Department of Civil, Environmental and Natural Resources Engineering, Luleå University of Pure galena sample used in this study was procured from Technology, SE-971 87 Luleå, Sweden; Tel: +47-73594837; Gregory, Bottley & Lloyd Ltd., United Kingdom. Galena Fax: +47-73594814; E-mail: [email protected]

1874-8414/14 2014 Bentham Open 2 The Open Mineral Processing Journal, 2014, Volume 7 Nooshabadi and Rao

Fig. (1). XRD analysis of the galena sample (1- galena 2- sphalerite 3- quartz). contains 73.69% Pb, 13.5% S, 1.38% Fe, 1.26% Zn, 0.2% After grinding for 60 min, the mill was emptied and the Cu and some silica (quartz) impurity. The XRD analyses of pulp was screened and it was sampled to different portions. the sample showed that the main mineral phase present was In each flotation test, 15 g sample that was < 75 µm was the galena (Fig. 1). The sample was initially crushed with a transferred to a cell of 150 ml capacity (Clausthal flotation jaw crusher and screened to collect the –3.35 mm particle equipment), conditioned with pH modifier, collector and size fraction. Homogenised samples of 100 g each were then frother. The flotation time was 2 min at an air flow rate of sealed in polyethylene bags. All flotation reagents that are 0.5 dm3 min-1 in each test. The flotation froth was scraped being used at the Boliden sulphide mineral beneficiation every 10 s to collect the floated particles. The dosage of plant were obtained from Boliden Mineral AB, Boliden, collector in flotation was 5×10-4 M KAX and the frother Sweden. Potassium amyl xanthate (KAX) and MIBC were dosage was one drop of MIBC in all the tests, unless used as collector and frother respectively. Dilute solutions of otherwise specified. The pH and collector conditioning of the AR grade sodium hydroxide and HCl were used to maintain pulp were 5 min and 2 min respectively. All flotation tests the desired pH value during flotation. Deionised water was were performed at room temperature of approximately used in both grinding and flotation experiments. Solutions of 22.5°C. 2, 9-dimethyl-1, 10-phenanthroline (DMP), copper (II) sulphate (0.01 M) and phosphate buffer (pH 7.0) used in the Dry Grinding analytical method for determining H2O2 and lead nitrate, ferrous sulphate and ferric sulphate used for investigating the One hundred grams of galena was ground in a laboratory effect of these metal ions on the formation of H2O2 were stainless steel ball mill with two types of grinding media purchased from VWR, Sweden. (mild steel and stainless steel) for 60 min. After grinding, the mill was emptied and the galena was screened from grinding media. A 10 g of sample that was < 75 µm was mixed with Wet Grinding and Flotation Tests 100 cm3 of water in a magnetic stirrer for 5 min. The slurry A 100 g of crushed galena sample of –3.35 mm size for sample was then collected and analysed for hydrogen each grinding test was combined with 400 ml of water and peroxide. The pH was regulated with HCl or NaOH solution. ground in a new laboratory stainless steel ball mill (Model 2VS, CAPCO Test Equipment, Suffolk, UK) either with Analysis of Hydrogen Peroxide stainless steel or mild steel media. The slurry samples during grinding were collected at pre-determined time intervals and For the amount of hydrogen peroxide formation in pulp they were immediately filtered (Millipore 0.22 µm) and the liquid, the liquid (filtrate) sample was immediately analysed liquid (filtrate) was analysed for hydrogen peroxide. by spectrophotometric method using copper (II) ions and

Effect of Grinding Environment on Galena Flotation The Open Mineral Processing Journal, 2014, Volume 7 3

Fig. (2). Production of H2O2 in pulp liquid as a function of grinding time during wet grinding.

Table 1. Effect of pH on the flotation recovery of galena with air atmosphere during the flotation.

pH Stainless Steel Mild Steel

4 76 55

7 86 67

8 81 62

9 77 57

10 68 52

10.5 61 48

DMP [24]. One millilitre each of DMP, copper (II) sulphate, grinding media on the formation of hydrogen peroxide where and phosphate buffer (pH 7.0) solutions were added to a 10 mild steel produced a higher concentration of H2O2 than mL volumetric flask and mixed. 1 ml of liquid (filtrate) stainless steel medium. Mild steel grinding medium was sample was added to the volumetric flask, and then the flask known to produce dissolved iron ions and thereby several was filled with ultrapure water. After mixing, the absorbance hydrolysed iron species that adsorb on galena particles [2]. of the sample (at 454 nm) was measured with DU® Series Dissolved Fe2+ ions also react with dissolved molecular 700 UV/Vis Scanning Spectrophotometer. The blank oxygen via Haber-Weiss reaction mechanism and forms • − solution was prepared in the same manner but without H2O2. superoxide anion (O2 ) (eq. 1), which reacts with ferrous iron to form H2O2 (eq. 2) [11].

RESULTS 2+ 3+ • ‒ Fe + O2→ Fe + (O2 ) (1)

2+ • ‒ + 3+ Formation of Hydrogen Peroxide (H2O2) During Wet Fe + (O2 ) + 2H → Fe + H2O2 (2) Grinding and its Implications for Flotation This was in agreement with other studies where mild The effect of the nature of grinding media on the steel produced a higher concentration of H2O2 than stainless formation of hydrogen peroxide during wet grinding of steel medium during grinding of pyrite [16]. By realizing the galena was investigated. The galena was wet-ground in a formation of H2O2 in the pulp liquid, the effect of this strong laboratory stainless steel ball mill with two kinds of grinding oxidizing agent on solid surfaces and its consequences for media at natural pH and slurry samples were collected at a flotation have been addressed below. pre-determined time intervals. The slurry samples were filtered (Millipore 0.22 µm), and the liquid (filtrate) was The effect of grinding media type after 60 min grinding analysed for hydrogen peroxide. Fig. (2) shows the effect of on the flotation recovery of galena was investigated. Table 1 4 The Open Mineral Processing Journal, 2014, Volume 7 Nooshabadi and Rao

Fig. (3). Formation of H2O2 in pulp liquid as a function of grinding time during wet grinding. shows the effect of grinding media type on galena recovery. are shown in Fig. (3). It can be seen that at all pH levels mild It can be seen that the galena wet-ground with mild steel steel generated a higher concentration of H2O2 than stainless media has a lower galena recovery than galena wet-ground steel medium. These results correspond to the results of wet with stainless steel media. This is in good agreement with grinding (Fig. 2). other studies where 30 wt% chromium medium produced a much higher galena flotation recovery than the mild steel medium [25]. Since wet-ground galena with mild steel media DISCUSSION produces higher amount of H2O2, a decrease in galena Tao (2004) used Eh-pH diagram to demonstrate that Mild recovery could be due to surface oxidation caused by H2O2 steel medium corrodes under reducing and moderately oxidant. Ikumapayi et al. (2012) showed the presence of oxidizing conditions (Fig. 4a,b) [27]. The oxidation of Mild surface oxidized species such as sulfoxy and hydroxyl steel yields Fe2+ species at acid weak (pH < 6) and with a species on galena. [26]. Wang (2002) reported that the decreasing pH, Fe2+ species increase in solution and these addition of H2O2, galena flotation decreases and completely • – -3 ferrous ions generate superoxide anion (O2 ) in the presence depresses if the concentration of H2O2 exceeds 10 M [23]. of dissolved molecular oxygen (eq. 1), which reacts with He attributes this strong depressing action of H2O2 on galena ferrous iron to form H O (eq. 2) [11]. to its strong oxidising action on lead xanthate in the galena 2 2 surface which gives rise to the oxidation and decomposition Under neutral and mildly alkaline conditions, Mild steel of lead xanthate (eq. 3) [23]. medium does not corrode, because it is either immune under strongly reducing conditions or is in a passive state for more [Pb (EX) ] + H O →Pb (OH) + (EX) (3) 2 ads 2 2 2 2 oxidizing conditions. In strong alkaline environments, Mild steel is free from corrosion except for a small region of Formation of Hydrogen Peroxide (H2O2) after Dry potentials and pHs where soluble, alkaline, corrosion Ground Solids are Placed in Water products form [27]. The effect on the type of grinding media during dry Tao (2004) also showed by Eh-pH diagram that stainless grinding on the formation of hydrogen peroxide when the steel medium does not corrode in strongly reducing or dry ground solids are placed in water was investigated. The oxidizing conditions except when the pH values are below 3 galena was dry-ground in a laboratory stainless steel ball mill [27], in which case, the stainless steel is more stable than with two kinds of grinding media. Afterwards, the dry mild steel.

The proposed mechanisms of H2O2 generation are shown ground solids were mixed with water for 5 min. The liquid in Fig. (5) where Fe2+ ions formed by mild steel are (filtrate) was analysed for hydrogen peroxide and the results responsible for generating H2O2.

Effect of Grinding Environment on Galena Flotation The Open Mineral Processing Journal, 2014, Volume 7 5

b 3+ -6 Fig. (4a). Eh-pH Diagram for Fe-O-H System, Assuming Fe(OH)3 as Stable Fe Phase and Activity of Dissolved Fe = 10 M. b) Eh-pH -2 -3 Diagram for Fe-Cr-H2O System, Assuming Total Concentrations of 10 M Fe and 5×10 M Cr [27].

Fig. (5). Production of H2O2 by galena in contact with a) Mild steel and b) Stainless steel by the incomplete reduction of oxygen (eqs. 1 and 2). 6 The Open Mineral Processing Journal, 2014, Volume 7 Nooshabadi and Rao

CONCLUSION [11] C. Cohn, S. Mueller, E. Wimmer, N. Leifer, S. Greenbaum, D. Strongin and M. Schoonen, “Pyrite-induced hydroxyl radical The formation of H2O2 in pulp liquid during galena formation and its effect on nucleic acids,” Geochem. Trans. vol.7, grinding is more in Mild steel grinding media than stainless p.1, 2006. steel and with increasing grinding time, the concentration of [12] G. Jones, K. Corin, R. van Hille and S. Harrison, “The generation of toxic reactive oxygen species (ROS) from mechanically H2O2 increased. More flotation recovery of galena ground in activated sulphide concentrates and its effect on thermophilic Stainless steel than mild steel medium is obviously due to a bioleaching,” Miner. Eng. vol. 24, pp. 1198-1208, 2011. lower concentration of hydrogen peroxide, which acts as a [13] G. Jones, R. van Hille and S. Harrison, “Reactive oxygen species strong depressant for galena. generated in the presence of fine pyrite particles and its implication in thermophilic mineral bioleaching,” Appl. Microbiol. Biotechnol., vol. 97, pp. 2735-2742, 2013. [14] G. Jones, B. Megan, R. van Hille and S. Harrison, “The effect of CONFLICT OF INTEREST sulphide concentrate mineralogy and texture on Reactive Oxygen The authors declare that there is no conflict of interest Species (ROS) generation,” Appl. Geochem., vol. 29, pp. 199-213, 2013. regarding the publication of this article. [15] A. Javadi, A. Larsson and K. Hanumantha Rao, “Formation of hydrogen peroxide by pyrite and its influence on flotation,” Min. Eng. vol. 49, pp. 128-134, 2013. ACKNOWLEDGEMENTS [16] A. Javadi and K. Hanumantha Rao, “Formation of hydrogen peroxide by chalcopyrite and its influence on flotation,” Miner. Financial support from the research Centre for Advanced Metallurg. Proc., vol. 30, no. 4, pp. 212-219, 2013. Mining and Metallurgy (CAMM), Luleå University of [17] A. Javadi and K. Hanumantha Rao, “Formation of hydrogen Technology, Luleå, Sweden, and Boliden Mineral AB, peroxide by sphalerite,” Int. J. Miner. Process., vol. 125, pp. 78-85, Boliden, Sweden, is gratefully acknowledged. 2013. [18] E. Ahlberg and A. Broo, “Oxygen reduction at sulphide minerals. 1. A rotating ring disc electrode (RRDE) study at galena and pyrite,” Int. J. Miner. Process., vol. 46, pp. 73-89, 1996. REFERENCES [19] E. Ahlberg and A. E. Broo, “Oxygen reduction at sulphide minerals. 2. A rotating ring disc electrode (RRDE) study at galena [1] P. Guy and W. Trahar, “The influence of grinding and flotation environments on the laboratory batch flotation of galena.,” Int. J. and pyrite in the presence of xanthate,” Int. J. Miner. Process, vol. 47, p. 33- 47, 1996. Miner. Process, vol. 12, pp. 15-38, 1984. [2] Y. Peng, S. Grano, D. Fornasiero and J. Ralston, “Control of [20] E. Ahlberg and A. E. Broo, “ Oxygen reduction at sulphide minerals. 3. The effect of surface pre-treatment on the oxygen grinding conditions in the flotation of galena and its separation from pyrite,” Int. J. Miner. Process, vol. 70, pp. 67-82, 2003. reduction at pyrite,” Int. J. Miner. Process, vol. 47, no.1-2, pp. 49- 60, 1996. [3] S. Rao, K. Moon and L. J., “Effect of grinding media on the surface reactions and flotation of heavy metal sulphides,” in Flotation, [21] A. Javadi Nooshabadi and Hanumantha Rao Kota, “Formation of hydrogen peroxide by galena and its influence on flotation,” Adv A.M. Gaudin Memorial Volume, 1976, pp. 509-527. [4] S. Rao, G. Labonte and J. Finch, “Electrochemistry in the plant,” In Powder Technol., vol. 25, no. 3, pp. 832-839, 2014. [22] A. Javadi Nooshabadi and K. Hanumantha Rao, “Formation of Innovations in Flotation Technology, Greece, 1992. [5] Y. Penga and S. Grano, “Effect of iron contamination from hydrogen peroxide by sulphide minerals,” Hydrometallurgy Volume, vol. 141, pp. 82-88 , 2014. grinding media on the flotation of sulphide minerals of different particle size,” Int. J. Miner. Proc., vol. 97, no. 1-4, pp. 1-6, 2010. [23] D. Wang, “Potential Adjustment of Sulphide Mineral and Collectorless Flotation. In: New Development of Flotation [6] K. Adam, K. Natarajan, S. Riemer and I. Iwasakim, “Electrochemical aspects of grinding media-mineral interaction in Theory,” Beijing: Science Press, 1992, pp. 79-143. [24] K. Kosaka, H. Yamada, S. Matsui, S. Echigo and K. Shishida, “A sulphide ore grinding,” Corrosion (Houston), vol. 42, pp. 440-447, 1986. Comparison among the Methods for Hydrogen Peroxide Measurements To Evaluate Advanced Oxidation Processes: [7] K. Natarajan and I. Iwasaki, “Electrochemical aspects of grinding media-mineral interactions in magnetite ore grinding,” Int. J. Application of a Spectrophotometric Method Using Copper (II) Ion and 2, 9-Dimethyl-1, 10-phenanthroline,” Environ. Sci. Technol., Miner. Process., vol. 13, pp. 53-71, 1984. [8] F. Ikumapayi, H. Sis, B. Johansson and K. Hanumantha Rao, vol. 32, pp. 3821-3824, 1998. [25] Y. Peng, S. Grano, J. Ralston and D. Fornasiero, “Towards “Recycling process water in sulphide flotation, Part B: Effect of H2O2 and process water components on sphalerite flotation from prediction of oxidation during grinding I. Galena flotation,” Miner. Eng., vol. 15, pp. 493-498, 2002. complex sulphide,” Miner. Metall. Process, vol. 29, pp. 192-198, 2012. [26] F. Ikumapayi, M. Makitalo, B. Johansson and K. Hanumantha Rao, “Recycling of process water in sulphide flotation: Effect of calcium [9] M. Borda, A. Elsetinow, M. Schoonen and D. Strongin, “Pyrite- induced hydrogen peroxide formation as a driving force in the and sulphate ions on flotation of galena,” Miner. Eng., vol. 39, pp. 77-88, 2012a. evolution of photosynthetic organisms on an early Earth,” Astrobiology, vol. 1, pp. 283-288, 2001. [27] D. Tao, “Corrosion Protection of Grinding Mills in the Phosphate Industry Using Impressed Current Technology,” Florida Institute of [10] M. Borda, A. Elsetinow, D. Strongin and M. Schoonen, “A mechanism for the production of hydroxyl radical at surface defect Phosphate Research, Bartow, FL 33830 USA, 2004.

sites on pyrite,” Geochim. Cosmochim. Acta, vol. 67, no.5, pp. 935 -939, 2003.

Received: March 11, 2015 Revised: June 28, 2015 Accepted: July 03, 2015

This is an open access article licensed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/3.0/) which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.

Paper 8

Complex sulphide ore flotation: Effect of depressants addition during grinding on H2O2 formation and its influence on flotation

Alireza Javadi Nooshabadi, Kota Hanumantha Rao

To be submitted to, International Journal of Mineral Processing, 2015

Complex sulphide ore flotation: Effect of depressants addition during grinding on H2O2 formation and its influence on flotation

Alireza Javadi Nooshabadi1 and Kota Hanumantha Rao2* 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 2Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology, NO-7491 TRONDHEIM, Norway

*Corresponding author: Tel.: +47-73594837; E-mail: [email protected]

Abstract

Recent studies revealed that the ground sulphide minerals in contact with water generate H2O2 but its effect on the oxidation of pulp components and hence in deteriorating the concentrate grade and recovery in flotation has not been explored yet. In this study, the effect of depressant and collector additions during grinding on the formation of H2O2 by metal sulphides and its influence on complex ore flotation was investigated. The results showed a decrease in the generation of H2O2 with the addition of depressants during grinding while the addition of depressant and collector together increased H2O2 formation in the flotation pulp liquid. The flotation results showed that pyrite is effectively depressed when the depressant and collector are added together during grinding, corresponding to a condition of higher amounts of H2O2 in the pulp liquid causing the oxidation of pyrite surface and depression in flotation. The best results of metal sulphides grade and recovery achieved also correspond to the same flotation conditions.

Kewords: flotation, hydrogen peroxide, collector and depressant, oxidation

1 Introduction

The use of NaHSO3 as a flotation depressant (Forssberg, et al., 1993; Gül, 2007) is being practiced in selective flotation of sulphide minerals. The depressing effect of sodium bisulphite generally increases from copper sulphides to galena, pyrite and sphalerite (Peres, 1981). However, there is no common agreement about the depression mechanisms. In particular, the following three effects have previously been proposed for depression of pyrite 2– flotation by SO3 : 1) stripping/decomposition of xanthate (Yamamoto, 1980); 2) reaction with the pyrite surface to form hydrophilic iron oxides (Khmeleva, et al., 2003); and 3) a decrease of redox potential of the pulp below a level at which binding/oxidation of a collector (electron donor) becomes energetically unfavourable (Forssberg, 1990).

Liu and Laskowski (1989a; 1989b) have used dextrin depressant in sulphide flotation and showed that the adsorption of dextrin on sulphide minerals depends on metallic sites. Dextrin depresses galena in the pH range of 10–12 while sphalerite is depressed only around pH 7.5 (Huerta-Cerdán, et al., 2003). The depressant action of dextrin decreases with increase in pH for sphalerite, thereby facilitating its selective separation from galena (Rath & Subramanian, 1999).

Zinc sulphate has been also used as a depressant, mainly to depress sphalerite flotation (Pearse, 2005; El-Shall, et al., 2000). Zinc sulfate is a selective sphalerite depressant that reduces genuine flotation by making the mineral surface hydrophilic and also coagulate the minerals for reducing mechanical entrainment (Cao & Liu, 2006), thereby suggesting that zinc hydroxyl species possessed the dual function of hydrophilizing and coagulation.

Our recent studies have illustrated the formation of hydrogen peroxide (H2O2), an oxidizing agent stronger than oxygen, by metal-sulphides (Javadi Nooshabadi & Hanumantha Rao,

2013a; 2013b; 2013c; 2014). 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 absence 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.

However, participation of these oxidising H2O2 species, if any, in selective oxidation of the pulp components and hence in deteriorating the concentrate grade and recovery has not been explored yet. To fill this gap, it is important to build correlation between percentage of pyrite • in the concentrate, grinding conditions and concentration of OH /H2O2 in the pulp liquid on flotation grade and recovery, and as well as to study possible ways of flexibly controlling the formation of these species through known chemical means for depressing the generation of the oxidant. One of such possible ways for controlling the effect of H2O2 could be the addition of collector and depressant during grinding so that the reagents will be adsorbed on sulphide surfaces before they are affected by the oxidising species. This would be a useful strategy when oily collectors are used, as they require more intense agitation or mixing to adequately disperse in the pulp. Adding collector during grinding where freshly broken mineral surfaces are produced, and therefore interact with surfaces before they are oxidised, is also thought to be beneficial. (Yelloji Rao & Natarajan, 1988) observed that adding collector to the grinding mill minimized or often eliminated the effects of galvanic interactions.

The addition of collector and depressant during grinding is expected to change the pulp chemistry of sulphides but no detailed investigations reported in the literature on the effect of sulphides flotation. In this study, the effect of depressant and collector addition during grinding on the formation of H2O2 and its influence on complex ore flotation was investigated. The results are presented and discussed in terms of H2O2 generation vis-à-vis concentrate grade and recovery in flotation.

2 Experimental

2.1 Materials and reagents

The sulphide ores were from Renström mine of Boliden Mineral AB, Sweden. A bulk ore sample was received in a drum in wet condition and it was dried and crushed in three stages to < 3 mm size and divided into 1 Kg sub-samples using a rotary splitter. These 1 Kg samples were stored in sealed polyethylene bags in a freezer at temperature minus 18 oC. All chemicals used were of technical grade. The flotation reagents that are being used at the Boliden concentrator for treating complex sulphide ores have been obtained. Potassium amyl xanthate (KAX), Isobutyl xanthate (IBX) and Danaflot 871 (dialkyle dithiophoshate mercaptobenzothiazole) were used as collectors. Dowfroth 250 (polypropylene oxide

methanol) was used as frother. Dextrin and sodium hydrogen sulphite (NaHSO3) were used

as pyrite depressants while zinc sulphate (ZnSO4) was used for sphalerite depression in

flotation. Copper sulphate (CuSO4) was used as a source of copper ions for the activation of sphalerite flotation. Deionised water was used in both grinding and flotation. Solutions of 2, 9-dimethyl-1, 10-phenanthroline (DMP) (1%), copper (II) ions (0.01 M), and phosphate

buffer (pH 7.0) were used for estimating H2O2 amount in pulp liquid by UV-Visible spectrophotometer.

2.2 Pulp chemistry measurement

The pH, Eh (pulp potential), and dissolved oxygen (DO) were measured before and after grinding of the complex sulphide ore. The Eh was measured at room temperature in all experiments using a platinum electrode and KCl (3 mol/litre) reference electrode and expressed relative to the standard hydrogen electrode, SHE. The pH was measured using a glass electrode. A DO meter (SevenGo (Duo) proTM/OptiOXTM) was used for measuring the dissolved oxygen.

2.3 Assessment of H2O2 production in pulp liquid

One Kg ore sample is wet-ground with 0.6 litre of water in a laboratory stainless steel rod mill. The slurry samples were collected at a pre-determined time intervals, filtered and the

liquid (filtrate) was immediately analysed for H2O2 amount by spectrophotometric method using copper (II) ion and 2, 9-dimethyl-1, 10-phenanthroline (DMP) (Baga el al., 1988). The solids after grinding are subjected to FTIR-DRIFT spectral analysis to judge the nature of surface species and extent of surface oxidation.

2.4 Bench-scale flotation tests

The procedure followed for flotation tests is a standard optimised method at Boliden laboratory in the treatment of complex sulphide ores. For each flotation test, 1 Kg of ore of

–3 mm is wet ground with 0.6 litre of water for 20 min. The k80 size of the ground material is about 65μm which material is subjected to flotation test in a WEMCO cell of 2.5 litre capacity. The sequence of reagent additions was pH regulator, depressants, collectors, frother, and flotation. The dosages of depressants are; 1500 g/t ZnSO4, 300 g/t NaHSO3 and 200 g/t of dextrin. Copper and lead minerals were floated all together,

4 followed by zinc mineral flotation. Dosages of collectors in a three stage sequential C u - P b flotation are 30+20+10 g/t Danafloat and 10+5+0 g/t KAX. The conditioning times for pH regulator, depressants, and collectors are 5 min, 1 min and 2+1+1 min respectively.

Dosages of zinc activator and collectors in a three stage zinc flotation were 400 g/t CuSO4 and 40+20+20 g/t IBX. The conditioning times for zinc activator and collectors are 2 min, 1 min and 1+1+1 min respectively. The frother dosage was 20 g/t Dowfroth. The pH was regulated to ~10.5 at copper-lead flotation and to ~12 at zinc flotation with powdered calcium oxide.

Experiments were performed at room temperature of approximately 22 oC. Total flotation time in successive three stages was 4.5 min (1+1.5+2) as shown in Fig. 1. The three float products in each test were analysed chemically at Boliden laboratories using PANalytical AXIOS XRF spectrometer. Table 1 shows the reagent conditions of the flotation tests that are discussed in this paper.

Fig. 1. Schematic diagram of flotation test procedure.

Table 1. Addition of collector and depressants that added to mill. Test no. Reagents used in the grinding F1' Reference (normal flotation test where the reagents are added to the ground sulphide pulp) F2 1500 g/t (ZnSO4) + 300 g/t (NaHSO3) + 200 g/t (Dextrin)

F18 1500 g/t (ZnSO4) + 200 g/t (Dextrin) + pH 6.0 (H2SO4)

F4 1500 g/t (ZnSO4) F7 200 g/t (Dextrin) F3 60 g/t ( Danafloat) + 10 g/t ( KAX)

F6 60 g/t ( Danafloat) + 15 g/t ( KAX) + 1500 g/t (ZnSO4) + 300 g/t (NaHSO3) + 200 g/t (Dextrin)

2.5 Analysis of flotation product

X-ray fluorescence (XRF) equipment at the Boliden AB company in Skellefteå was used to analyse the flotation products. The samples were analyzed by PANalytical AXIOS XRF Spectrometer. The samples were ground in a laboratory mill kind Herzog in 45 seconds. Steel rings that shaped briquettes were filled with ground material and compressed using brikett hydraulic press with a strength between 11 and 12 tonnes. The briquettes were cleared also not provide non compacted material in the spectrometer during analysis. Finished pellets were placed in the spectrometer to analyze. The device was programmed by the computer and in this case chosen references for the investigation of copper and zinc concentrates.

2.6 Diffuse reflectance FTIR measurement

The Bruker FTIR spectroscope model IFS 66 v/s was used for FTIR-DRIFT analysis. Solid fractions of dried ground products were subjected to record infrared spectra. Diffuse reflectance infrared Fourier transform (DRIFT) method was used in measurement with 2.8 wt% concentration of mineral sample in potassium bromide (KBr) matrix. Typical measurement time was about 1 min (256 scans) at a resolution of 4 cm-1.

3 Results and discussion

3.1 Effect of depressant addition during grinding

3.1.1 Dissolved oxygen and formation of H2O2

Fig. 2 Shows the effect of depressant addition during grinding, independently and in combination, on the amount of H2O2 formation and the quantity of dissolved oxygen in the pulp liquid before and after grinding. Dissolved oxygen in the pulp liquid decreased after grinding in all the cases of depressant addition, either dextrin (test F7) or ZnSO4 (test F4) independently or in combination of dextrin and ZnSO4 (test F18) or all the three depressants including NaHSO3 together. In general, the decrease in dissolved oxygen after grinding is related to its consumption in the formation of H2O2. The decrease in the formation of H2O2 with the addition of depressants during grinding could also be correlated to a decrease in pulp pH (Fig. 3b) since a decrease in pulp pH value, the formation of hydrogen peroxide is shown to decrease (Ikumapayi, et al., 2012). The amount of H2O2 generated is seen to be at the same level in the presence of dextrin whether it is added in the grinding stage (test F7) or in the

6 pulp after grinding (test F1'). The variation in H2O2 amount is seen mostly due to ZnSO4 and

NaHSO3 addition during the grinding. This result preclude any dextrin depression effect on pyrite since pyrite surfaces are the most effective in the generation of H2O2. Cohn et al. (2006) showed that ferrous iron associated with pyrite in the presence of dissolved molecular • í oxygen, can form superoxide anion (O2 ) (eq. 1), which further reacts with ferrous iron to form H2O2 (eq. 2).

2+ 3+ • ௅ Fe (pyrite) + O2ĺ Fe (pyrite) + (O2 ) (1)

2+ • ௅ + 3+ Fe (pyrite) + (O2 ) + 2H ĺ Fe (pyrite) + H2O2 (2)

10 0.8

8 0.6

6 (ppm) 2 0.4 (mM) 2 O 2

4 H disolved O

O2 (before grinding) 0.2 2 O2 (after grinding) H2O2 (mM)

0 0 F7 F4 F18 F2 F1' Test no. as shown in Table 1

Fig. 2. Effect of addition of depressants on H2O2 concentration and dissolved oxygen.

3.1.2 Eh and pH of the pulp liquid

Fig. 3a shows the effect of depressant addition during grinding on Eh of the pulp liquid before and after grinding. The tests clearly show that the addition of depressants during grinding increased the Eh value since the pH is seen to decrease (Fig. 3b). The increase in Eh value is relative to the reference value when the depressants are added during flotation after grinding. However, the difference between Eh before and after grinding when all depresants added to mill (test F2) is more than when all depressants added after grinding (test F1'). This is due to

7 more generation of H2O2 in F1' than F2 (Fig. 2). Fig. 3b shows the effect of depressant addition during grinding on pH of the pulp liquid before and after grinding exhibiting an increase in pH value after grinding. Since the measurements of the pulp liquid before grinding is recorded immediately after water addition, hardly any H2O2 formation is expected and the changes in Eh and pH values suggest that the H2O2 generated during grinding interact with surface species.

500 12 a) b)

10 400

8 300

6 pH 200 Eh (mV, Eh (mV, SHE) 4 Eh (before grinding) pH (before grinding) 100 Eh (after grinding) 2 pH (after grinding)

0 0 F7 F4 F18 F2 F1' F7 F4 F18 F2 F1' Test no. as shown in Table 1 Test no. as shown in Table 1

Fig. 3. Effect of Addition of depressants on a) pH and b) Eh before and after grinding

3.1.3 Flotation tests

Table 2 shows the chemical analysis of all flotation tests products that are discussed in this paper. Fig. 4a, b displays that the Cu-Pb concentrate with addition of depressant independently in test F4 (ZnSO4) or in combination in tests F18 (ZnSO4+Dextrin) and F2

(ZnSO4+Dextrin+NaHSO3) during grinding, the grade of Cu and Pb decreased due to a decrease in H2O2 formation in the pulp liquid as shown Fig. 1. The H2O2 is a strong depressant for pyrite (Hu, et al., 2009) and a decrease in its amount may decrease selectivity between metal sulphides and ferrous sulphide in flotation. Fig. 4a shows that the addition of dextrin (test F7) during grinding has produced more Cu and Pb grade and recovery than when it added after grinding (test F1') since F7 test conditions generated more H2O2 than test F1' (Fig.

1). Fig. 4c shows that addition of depressant in tests F2, F4 and F18 during grinding the recovery of Fe (pyrite) increased due to a decrease in H2O2 formation in the pulp liquid as

8 shown Fig. 1 but in test F7, the recovery of Fe (pyrite) decreased due to a increase in H2O2 formation in the pulp liquid. It is thus clear that the generated H2O2 is functioning as a strong depressant for pyrite.

Tabel 2. Analysis of flotation product

b) 16 a) 16

12 12 F4 F7

F7 F2 8 F1' 8 F1'

F2 F18 4

F18 Concentrate grad (%) F4

Concentrate grad (%) 4

0 0 20 40 60 80 100 20 40 60 80 100 Cumulative recovery (%) Cumulative recovery (%)

c) 100%

80%

60%

wt % 40%

20%

0% F18F7F4F2F1'Feed Gangue 58.71 52.77 60.18 54.46 56.04 80.79 FeAsS 0.13 0.15 0.13 0.16 0.14 0.11 FeS2 9.78 8.33 8.70 9.81 8.27 9.78 PbS 2.17 2.52 2.09 2.50 2.44 0.60 (Zn,Fe)S 15.72 18.92 15.46 17.40 17.48 5.28 CuFeS2 13.50 17.31 13.43 15.67 15.62 3.43

Fig. 4. Effect of depressant addition during grinding on recovery and grade of a) Cu in Cu-Pb conc. b) Pb in Cu-Pb conc. c) mineral recovery in Cu-Pb concentrate.

Fig. 5a shows the results of Zn concentrate with depressant addition during grinding. It can be seen that addition of ZnSO4 (test F4) during grinding decreased the Zn grade and recovery relative to when all the depressants are added after grinding (test F1') due to decreasing H2O2 formation in pulp liquid as shown in Fig. 1. This is in agreement with other studies where it was observed that H2O2 increases sphalerite recovery flotation (Javadi & Hanumantha Rao, 2013) and zinc sulphate depresses sphalerite (Cao & Liu, 2006; El-Shall, et al., 2000). Fig. 5a also shows that the addition of all the depressants (test F2) during grinding increased the Zn grade and recovery relative to when all the depressants added after grinding. This could be

10 due to the effect of depressants during grinding where sodium bisulphide depresses galena, pyrite and sphalerite (Forssberg, et al., 1993) and dextrine depresses galena and sphalerite (Houot & Duhamet, 1992) and zinc sulphate depresses sphalerite (Cao & Liu, 2006; El-Shall, et al., 2000). Fig. 5b shows addtion of depressant durning grinding increased the pyrite amount in Zn concentrate due to decreasing H2O2 formation as shown Fig. 1 and it is clear that H2O2 is a strong depressant for pyrite.

a) 100% 30 b) F2 80% F7 F1' 60% F18 20 F4 wt % 40%

20%

Concentrate grad (%) 10 F18 F7 F4 F2 F1' Feed Gangue 59.56 56.31 57.34 52.17 56.78 80.79 FeAsS 0.26 0.31 0.32 0.34 0.32 0.11 FeS2 32.91 35.03 35.08 36.45 34.38 9.78 PbS 0.21 0.24 0.29 0.26 0.28 0.60 0 0 10203040(Zn,Fe)S 6.68 7.72 6.56 10.31 7.84 5.28 Cumulative recovery (%) CuFeS2 0.38 0.39 0.40 0.48 0.41 3.43

Fig. 5. Effect of depressant addition on Zn concentrate: a) Zn grade vs. recovery b) mineral recovery in Zn concentrate.

The Cu-Pb concentrate results in Fig. 6a reveal that the addition of depressants during grinding (tests F2, F4 and F18 except test F7), flotation selectivity of Cu-Pb minerals against

Fe (pyrite) decreased due to decreasing of H2O2 formation in these tests conditions while the

F7 test conditions increases the generation of H2O2 as shown in Fig. 1. This suggests that the

H2O2 depresses pyrite (Hu, et al., 2009).

Fig. 6b shows the same test results of Fig. 6a but the selectivity of Cu-Pb concentrate against Zn. The results demonstrate that the addition of depressants during grinding have a better depression of Zn mineral compared to the reference test where the depressants are added only during the flotation stage. Adding depressant during grinding where freshly broken mineral surfaces are produced, the depressant effect is seen to be significant. Among all the depressants, dextrin is more effective when added during grinding. Yelloji Rao and Natarajan

(1988) observed that adding reagents to the mill minimized or often eliminated the effects of the galvanic interaction.

F2 a) b) 80 F18 80 F4 F1' 60 F7 60

40 40

20 20 Cumulative Cumulative Fe recovery(%) Cumulative Cumulative Zn recovery(%)

0 0 50 60 70 80 90 100 50 60 70 80 90 100 Cumulative Cu recovery (%) Cumulative Cu recovery (%)

Fig. 6. Effect of depressant addition during grinding on Cu-Pb concentrate: a) selectivity against pyrite b) selectivity against Zn.

The Zn concentrate results in Fig. 7 shows that when all the three depressants (F2) added together during grinding produced more Zn recovery with less Fe content concentrate compared to reference test where the depressants are added during the flotation stage. The effect of ZnSO4 depressant addition during grinding is found to be less effective relative to the normal reference flotation test.

50 F7 F2 F1' 40 F18 F4 30

10 Cumulative Cumulative Zn recovery(%)

0 0 204060 Cumulative Fe recovery (%) Fig. 7. Effect of depressant addition during grinding on Zn concentrate and its selectivity against pyrite.

3.2 Effect of collector addition during grinding

3.2.1 Dissolved oxygen and H2O2 formation in the pulp liquid

Fig. 8 Shows the effect of both collector and depressant addition during grinding on formation of H2O2 and dissolved oxygen in the pulp liquid. Test F6 (collector and depressant) results show that more production of H2O2 and lower dissolved oxygen content compared to the reference flotation test F1'. The lower dissolved oxygen could be due to the oxygen consumption in the generation of hydrogen peoxide as described above in Part 3.1.1.

10 1

8 0.8

6 0.6 (ppm) 2 (mM) 2 O 2

4 0.4 H disolved O

2 O2 (before grinding) 0.2 O2 (after grinding) H2O2 (mM)

0 0 F3 F6 F1' Test no. as shown in Table 1

Fig. 8. Effect of collector and depressant addition on H2O2 concentration and dissolved oxygen in the pulp liquid.

3.2.2 Measurement of Eh and pH

Fig. 9a shows the effect of collector and depressant addition during grinding on Eh of pulp liquid before and after grinding. The Eh value of the pulp liquid in test F6 is higher than the reference test (F1') corresponding to higher H2O2 formation (Fig. 8). The lower pH value of pulp liquid in test F6 (Fig. 9b) correlates to the test conditions of higher H2O2 amount.

300 a) 12 b)

250 10

200 8

6 150 pH Eh (mV, Eh (mV, SHE) 100 Eh (before grinding) 4 pH (before grinding) Eh (after grinding) pH (after grinding) 50 2

0 0 F3 F6 F1' F3 F6 F1' Test no. as shown in Table 1 Test no. as shown in Table 1

Fig. 9. Effect of collector and depressant addition on a) Eh and b) pH in pulp liquid before and after grinding.

3.2.3 Flotation tests

Fig. 10a shows that Cu grade and recovery in Cu-Pb concentrate improved significantly with the collector (test F3) and in combination with depressant addition (test F6) during grinding. These results are in agreement with the studies reported where the addition of collector in mills minimizes galvanic interaction and improve chalcopyrite flotation (Yelloji Rao & Natarajan, 1988). More Cu grade and recovery when added depressant and collector together

(test F6) compared to only collector addition (test F3) correspond to higher H2O2 formation in pulp liquid (Fig. 8). Fig. 10b shows that the Pb grade and recovery in Cu-Pb concentrate decreased in tests F3 and F6 due to increasing H2O2 amounts in pulp liquid (Fig. 8) and that

H2O2 depresses galena flotation (Hu, et al., 2009). Fig. 10c shows that the addition of collector and depressant during grinding on pyrite depression is considerable. This agrees with other study where H2O2 is a strong depressant for pyrite (Hu, et al., 2009).

20 a) 20 b)

15 15

F6 F1' 10 10 F3 F3

F1' F6 5 5 Concentrate grad (%) Concentrate grad (%) 0 0 10 20 30 40 50 60 70 80 90 100 10 20 30 40 50 60 70 80 90 100 cumulative recovery (%) cumulative recovery (%)

C) 100%

80%

60%

wt % 40%

20%

0% F6 F3 F1' Feed Gangue 46.94 51.42 56.04 80.79 FeAsS 0.14 0.15 0.14 0.11 FeS2 7.19 8.23 8.27 9.78 PbS 2.73 2.91 2.44 0.60 (Zn,Fe)S 17.39 18.05 17.48 5.28 CuFeS2 25.60 19.24 15.62 3.43

Fig. 10. Effect of collector and depressant addition during grinding on recovery and grade of (a) Cu in Cu-Pb conc. (b) Pb in Cu-Pb conc. (c) minerals in Cu-Pb concentrate.

Fig. 11a shows the results of Zn concentrate that addition of collector (test F3) alone and collector and depressant (test F6) together significantly increased Zn grade and recovery relative to when collector added in flotation stage. Better results in test F6 than test F3 correlate to higher H2O2 formation (Fig. 8) in pulp liquid. H2O2 increases Zn recovery by effectively depressing pyrite has been reported (Javadi & Hanumantha Rao, 2013; Javadi Nooshabadi & Hanumantha Rao, 2013a). Fig. 11b shows that addition of collector and depressant (test F6) decreased flotation recovery of Fe due to increasing H2O2 amount (Fig. 8) which is a strong depressant for pyrite (Hu, et al., 2009).

100% 50 a) b) 80% F6 40 60% F3

F1' 40% 30 wt % 20%

20 0% F6 F3 F1' Feed Concentrate grad (%) Gangue 47.96 54.94 56.78 80.79 10 FeAsS 0.38 0.35 0.32 0.11 FeS2 34.20 32.23 34.38 9.78 0 PbS 1.02 0.47 0.28 0.60 0 204060 (Zn,Fe)S 15.86 11.56 7.84 5.28 Cumulative recovery (%) CuFeS2 0.58 0.46 0.41 3.43 Fig. 11. Effect of collector and depressant addition on Zn concentrate: a) grade and recovery of Zn b) Fe recovery.

Fig. 12 shows the results of Cu-Pb concentrate where the collector (test F3) and collector and depressant together (test F6) are added during grinding. It can be seen that the Fe content (Fig. 12a) and Zn content (Fig. 12b) decreased in the Cu-Pb concentrate illustrating a better depression of pyrite and sphalerite.

a) b) 80 80

60 60 F1' F1' 40 F3 40 F3 F6 F6

20 20 cumulative Zn recovery(%) Zn cumulative cumulative Fe recovery(%) Fe cumulative

0 0 50 60 70 80 90 100 50 60 70 80 90 100 cumulative Cu recovery (%) cumulative Cu recovery (%) Fig. 12. Effect of collector (F3) and collector and depressant (F6) addition during grinding on Cu-Pb concentrate: a) selectivity against pyrite b) selectivity against Zn.

Fig. 13 shows the results of Zn concentrate and the addition of reagents during grinding increased the Zn recovery while maintaining pyrite depression at the same level.

100

60 F6

F3 40 F1'

20 cumulative Zn recovery(%) Zn cumulative

0 0204060 cumulative Fe recovery (%) Fig. 13. Effect of collector (F3) and depressant and collector (F6) addition during grinding on Zn concentrate and its selectivity against pyrite.

3.3 Diffuse reflectance FTIR studies

Solid fractions of grinding products were subjected to FTIR-DRIFT analysis which were conducted on dried samples. Fig. 14. shows that the spectrum with the addition of depressant (test F2) during grinding has the lowest intensity of absorption peaks while the addition of depressant and collector (test F6) together has the highest intensity peaks. Increasing of intensities in test F6 samples corresponds to increasing level of H2O2 formation in the pulp liquid. The bands at 666, 1160, 1453 cm-1 are S–O vibrations in sulfate, and the band at 985 cm-1 can be assigned to S=O stretching in sulfite and sulfate (Smith, 1999; Socrates, 2001).

985.00 778.01 665.69 3567.92 1452.80 1160.26 1.2 1.0 0.8 0.6 Kubelka Munk 0.4 0.2

3500 3000 2500 2000 1500 1000 Wavenumber cm-1 Fig. 14. DRIFT spectra of corresponding ore solid residue separated from solution after grinding for 60 minutes in tap water (blue line: F6; green line: F1`; pink line: F2)..

4 Conclusions

The following conclusions can be derived based on the results presented in this paper:

1. The addition of depressants during sulphide ore grinding decreases the formation of

H2O2 in pulp liquid and the flotation results are not promising. However, better flotation results are obtained when dextrin depressant is used during grinding.

2. The addition of collector during sulphide ore grinding increases the formation H2O2 in pulp liquid and metal suphides grade and recovery improved. 3. The addition of collector and depressant during sulphide ore grinding increases the

formation H2O2 in pulp liquid and metal suphides grade and recovery significantly improved. 4. The use of reagents during grinding has beneficial effects on flotation grade and recovery of metal sulphides.

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.

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