Elsevier

Minerals Engineering

Volume 24, Issue 15, December 2011, Pages 1687-1693
Minerals Engineering

The effect of surface oxidation of copper sulfide minerals on clay slime coating in flotation

https://doi.org/10.1016/j.mineng.2011.09.007Get rights and content

Abstract

The industry is well aware of the difficulty in treating copper ores in the presence of clay minerals. In this study, the effect of bentonite on the flotation of chalcopyrite (a primary copper sulfide mineral) and chalcocite (a secondary copper sulfide mineral) was investigated in terms of surface coating. Based on the flotation of copper ores containing both chalcopyrite and chalcocite, the flotation of chalcopyrite and chalcocite single minerals in the presence and absence of bentonite, and the zeta potential measurement of chalcopyrite, chalcocite and bentonite, it was found that the oxidation of chalcopyrite and chalcocite had a different effect on their interaction with bentonite. Under the normal grinding and flotation condition, significant oxidation occurred on the surface of chalcocite which was electrostatically attractive to bentonite resulting in bentonite slime coating and the depressed flotation of chalcocite. The reduction of oxidation on chalcocite could mitigate bentonite slime coating due to electrostatic repulsion between unoxidized chalcocite and bentonite. Unlike chalcocite, chalcopyrite with and without surface oxidation exhibited an electrostatic repulsion to bentonite. Its flotation was less affected by bentonite slimes.

Graphical abstract

The oxidation of chalcopyrite and chalcocite under the normal grinding and flotation condition alters the electrical properties on the surfaces and their interaction with bentonite slimes.

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Highlights

► The flotation of chalcocite is more affected by bentonite than the flotation of chalcopyrite. ► Chalcocite is more oxidized than chalcopyrite under the normal grinding and flotation condition. ► An electrostatic attraction between the oxidized chalcocite and bentonite promotes the slime coating. ► Bentonite has less effect on the flotation of chalcopyrite with and without strong oxidation.

Introduction

Copper orebodies are formed when geothermal solutions (superheated under pressure) bring copper dissolved from deep underground to cool near surface environments where the copper and associated metals precipitate as minerals in veins and disseminations within the rock (Sillitoe and Petersen, 1996). Copper is usually deposited as copper sulfide minerals or in some environments as native copper metal. The most common copper minerals in the primary hydrothermal zone are chalcopyrite (CuFeS2) and bornite (Cu5FeS4). During millions of years the mineral deposit may be exposed to oxygen by airpenetration, or by oxygen rich water flowing over it. This oxidation alters the mineralogy, replacing the copper and iron sulfides with carbonates and oxides as the sulfur is oxidized to soluble sulfate and carried away in acid solution (Sillitoe and Petersen, 1996). The most common copper minerals in the oxidized zone are azurite (Cu3(CO3)2(OH)2), cuprite (Cu2O), malachite (Cu2CO3(OH)2) and tenorite (CuO), etc. Beneath the oxidized zone, some dissolved copper is precipitated as secondary or supergene copper minerals. This enriches the sulfides, making a secondary enrichment, or transitional zone. The secondary enrichment replaces iron in the minerals with more copper, further enriching the ore (Sillitoe and Petersen, 1996). The most common copper minerals in the secondary enrichment zone are chalcocite (Cu2S) and covellite (CuS). To recover primary and secondary copper sulfide minerals from ores, froth flotation which exploits the difference in surface wettability is normally used to separate them from other gangue minerals. Extensive studies have been conducted to understand their oxidation and flotation behavior.

Like other sulfide minerals, the surface of copper sulfide minerals is reactive and starts to oxidize as soon as they are in contact with air. Mild oxidation results in a surface that is rich in polysulfides with some metal hydroxides present at the surface, due to the dissolution of metal ions from the surface and near surface layers, as observed in ambient air, acidic and alkaline conditions by X-ray Photoelectron Spectroscopy (XPS) (Buckley and Woods, 1983, Buckley and Woods, 1984). Extensive oxidation results in high quantities of metal hydroxides on the mineral surface (Senior and Trahar, 1991). The oxidation of chalcopyrite is slow in general. A number of studies shows that a small amount of copper ions are dissolved from chalcopyrite even with oxygen purging (Fairthorne et al., 1997, Lascelles and Finch, 2002). Electrochemical studies indicate that the initial product of chalcopyrite oxidation may be chalcocite or covellite (Arce and Gonzalez, 2002, Lazaro et al., 1995). The oxidation of secondary copper sulfides are faster than chalcopyrite. By EDTA extraction, Lascelles and Finch (2002) found that chalcocite produced about 50 times more copper ions than chalcopyrite at the same size fraction (150/212 μm). Fullston et al. (1999) measured the zeta potential of the copper sulfide minerals including chalcocite, covellite, chalcopyrite, bornite, enargite and tennantite as a function of pH and the oxidation condition. They found that the change in zeta potential was governed by a copper hydroxide layer covering a metal-deficient sulfur-rich surface and with the extent of this copper hydroxide coverage increasing with the oxidation condition. They also demonstrated that among these copper sulfide minerals examined, chalcopyrite was the most electrochemically noble while chalcocite was the most electrochemically active in terms of surface oxidation.

The flotation of copper sulfide minerals is intimately associated with their surface oxidation. The collectorless flotation of copper sulfide minerals is attributed to polysulfides on the surface as a result of mild surface oxidation (Lekki and Drzymala, 1990, Ekmekçi and Demirel, 1997). Metal hydroxides on the mineral surface decrease the flotation of copper sulfide minerals even in the presence of collector (Senior and Trahar, 1991). Barzyk et al. (1981) identified that chalcocite surface oxidation had a strong effect on both xanthate adsorption and chalcocite floatability, and the most oxidized chalcocite sample required 100 times more collector consumption to obtain the same flotation results than the least oxidized sample. There is a strong relationship between Cu recovery and the electrochemical potential in flotation. High chalcopyrite, bornite or chalcocite flotation recovery may be obtained at well controlled electrochemical potentials (Tolley et al., 1996).

The flotation of copper sulfide minerals is complicated by the presence of clay minerals. The industry is well aware of the difficulty in treating weathered copper ores containing clay minerals. Currently, the only way to treat this type of ores is to blend them at a small proportion with normal ores. Limited studies have been conducted to understand the role of clay minerals in copper flotation. Clay minerals are anisotropic and hydrated phyllosilicates with unit cells comprising a layer of one alumina octahedral sheet and either one (such as kaolinite) or two (such as smectite) silica tetrahedral sheets (Brigatti et al., 2006). A single particle may comprise several layers stacked on top of one another. Due to isomorphous substitution (e.g., Al(III) replacing Si(IV) in the SiO2 layer), the basal faces carry a constant negative charge which is pH independent (Luckham and Rossi, 1999, Zhao et al., 2008). At the edges of the layers, the tetrahedral silica sheets and the octahedral alumina sheets have broken primary bonds. The electrical charge of the edge, arising from hydrolysis reactions from broken Al–O and Si–O bonds, is pH dependent. The edges of clay particles are positively charged in the neutral and acid pH ranges depending on the type of clay minerals (Swartzen-Allen and Egon, 1974). The anisotropic charges on edges and basal faces allow clay slime coating on the surface of a range of minerals. It has been well documented that clay slime coating occurs on galena, coal and bitumen surfaces through the electrostatic attraction, reduces surface hydrophobicity and then depresses the flotation significantly (Gaudin et al., 1960, Arnold and Aplan, 1986, Liu et al., 2005a, Liu et al., 2005b).

It is generally observed that when clay minerals are placed in a solution where the pH and/or charged ions, reagents or other minerals (e.g., iron oxides) promote clay particle aggregation, these structures result in high viscosity (Luckham and Rossi, 1999). There is also evidence that the propensity towards high viscosity is markedly enhanced as particle size decreases (Tu et al., 2005). Clay minerals are naturally fine-grained with particles of colloidal size (Schoonheydt and Johnston, 2006, Kotlyar et al., 1996). Therefore, a relatively low concentration of clay minerals, in the range of 5–12 wt.% may be sufficient to cause high viscosity (Tu et al., 2005). The effect of viscosity itself, potentially caused by clay mineral aggregate structures, on mineral flotation has not been adequately examined (Schubert, 2008).

In this paper, the effect of clay slime coating on the flotation of primary (chalcopyrite) and secondary (chalcocite) copper sulfide minerals is explored. The effect of viscosity itself, caused by clay mineral aggregate structures, on the flotation of copper sulfide minerals was reported previously (Peng et al., 2010).

Section snippets

Materials and reagents

An underground copper ore and an open pit copper ore were crushed to a size of −2.36 mm before grinding and flotation. The mineral composition of the two ores analyzed by X-ray Diffraction (XRD) is shown in Table 1. The two ores contain both chalcopyrite and chalcocite. The main copper mineral in the underground ore is chalcopyrite, 85% of the copper minerals. The main copper mineral in the open pit ore is chalcocite, 67% of the copper minerals. Another distinct difference in the two ores is the

Flotation of copper ores

Fig. 1 shows the flotation of underground and open pit copper ores with 100 g/t collector. As can be seen, Cu recovery from the flotation of the underground ore was normal, about 87% at the completion of 8 min flotation. The mass pull of the concentrate and water recovery in the end of flotation (not shown in Fig. 1) were 3.5% and 9.8%, respectively. However, Cu recovery from the flotation of the open pit ore was low, only 56% in the end of flotation although the mass pull of the concentrate and

Discussion

The flotation of base metal sulfide minerals is always associated with surface oxidation. Surface oxidation not only modifies surface hydrophobicity, but also changes the electrical property on the surface and therefore the interaction with gangue minerals. The oxidation of base metal sulfide minerals produces hydrophobic metal-deficient sulfur or polysulfides, and also hydrophilic oxidation products like S2O32-,SO32-,SO42- and metal hydroxides (Buckley and Woods, 1984, Guy and Trahar, 1984).

Conclusion

The flotation of chalcocite in the presence of bentonite is low. One of the contributing factors is the strong oxidation of chalcocite under the normal grinding and flotation condition and the alteration of the electrical property of the chalcocite surface. An electrostatic attraction between the oxidized chalcocite and bentonite promotes bentonite slime coating resulting in the depressed chalcocite flotation. Unlike chalcocite, chalcopyrite with and without surface oxidation exhibited an

Acknowledgment

The first author gratefully acknowledges financial support of this work from the New Staff Grant at the University of Queensland.

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