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2 Hydrometallurgy 93 (2008) Contents lists available at ScienceDirect Hydrometallurgy journal homepage: Leaching of chalcopyrite with ferric ion. Part I: General aspects E.M. Córdoba a, J.A. Muñoz b, M.L. Blázquez b, F. González b, A. Ballester b, a Escuela de Ingeniería Metalúrgica y Ciencia de los Materiales, Facultad de Ingenierías Físico-Químicas, Universidad Industrial de Santander, Bucaramanga, Colombia b Departamento de Ciencia de Materiales e Ingeniería Metalúrgica, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Madrid, Spain ARTICLE INFO ABSTRACT Available online 2 May 2008 Keywords: Chalcopyrite Leaching Ferric iron Mechanism This paper presents a review of the literature on chalcopyrite leaching with ferric sulphate in acid medium. The effects of several parameters (ferric salt anion, oxidant concentration, ph and temperature) are examined and possible explanations are offered for the passivation of this sulphide during dissolution. The main theories related with chalcopyrite passivation point to the formation of a diffusion layer surrounding the chalcopyrite during dissolution, consisting of: bimetallic sulphide, copper polysulphide with a deficit of iron with respect to chalcopyrite, and elemental sulphur. Recent studies suggest that ferric ion plays two important and opposite roles in this process: as a mineral oxidizing agent and as the agent responsible for chalcopyrite passivation Elsevier B.V. All rights reserved. 1. Introduction 2. Chalcopyrite crystal structure Chalcopyrite, (CuFeS 2 ), is the most abundant copper mineral in nature (Dutrizac, 1978) counting for about 70% of copper reserves in the world (Rivadeneira, 2006). In metallurgical applications it is mainly subjected to pyrometallurgical treatment after concentration by a flotation process. Hydrometallurgy, as an alternative to pyrometallurgy, presents important advantages such as the possibility of treating lowgrade ores (increasingly more abundant in the case of copper) and easier control of waste, with the attendant benefits to the environment. At present, approximately 18% of world copper production is treated by hydrometallurgy (Bravo, 2006). The process involves static leaching in heaps followed by solvent extraction and electrolytic precipitation of copper. Despite that, the only existing industrial processes for the treatment of chalcopyrite concentrates at present are pyrometallurgical. Chalcopyrite is highly refractory under hydrometallurgical conditions, due to surface transformations which render products very stable under oxidizing conditions (Burkin, 1969). Nevertheless, different hydrometallurgical processes have been investigated on both laboratory and pilot scales in attempts to implement these technologies at industrial level. Any proposal for an economically viable hydrometallurgical process for the treatment of chalcopyrite should take into account basic studies that elucidate the chemical and electrochemical aspects governing leaching. This review presents information on the fundamental aspects of chalcopyrite leaching and the problems associated with low copper recoveries from this mineral. The chemical formula that best describes chalcopyrite is Cu + Fe 3+ S 2 2 (De Filippo et al., 1988; Boekema et al., 2004; Mikhlin et al., 2004). The crystal structure consists in a relatively simple tetragonal lattice, close to cubic (Betejtin, 1977; Sand et al., 2001), with each sulphur ion surrounded by four metal ions of copper and iron located on tetrahedron angles and in a certain order in each plane (Fig. 1). In the first and fifth planes, corresponding to the lower and upper faces of the tetragonal prism, Fe ions are located at the square angles and Cu ions in the middle. In the third plane, at the prism centre, the order is the reverse. In the second and fourth planes, two Cu ions bond two Fe ions, but there are iron ions underneath copper cations in the second plane, and vice versa in the fourth plane. Corresponding author. Tel.: ; fax: address: ambape@quim.ucm.es (A. Ballester). Fig. 1. Chalcopyrite crystal lattice (Betejtin, 1977) X/$ see front matter 2008 Elsevier B.V. All rights reserved. doi: /j.hydromet

3 82 E.M. Córdoba et al. / Hydrometallurgy 93 (2008) Although chalcopyrite presents a crystal structure essentially characterized by covalent metal sulphide bonds, its semiconducting properties indicate that not all bonding electrons correspond to simple covalent crystals (Forward and Warren, 1960). The substitution of copper and iron by other metal atoms in the crystal lattice of natural chalcopyrites leads to the formation of n- and p-type semiconductor structures. Fig. 2 shows a simplified energy band diagram for n-type semiconductor chalcopyrite with an energy gap around 0.6 ev (Torma, 1991). While the electronic character of the lower part of the conduction band is given by Fe 3d orbitals, the upper part of the valence band is given by Cu 3d and S 3p orbitals. 3. Chemical leaching of chalcopyrite with ferric ion The leaching of copper from sulphide ore bodies occurs naturally, releasing copper and iron. In ancient times, copper was recovered by cementation with metal iron and iron in the form of ferrous sulphate by evaporation of the solution. The first reference to the application of ferric acid sulphate solutions to the leaching of copper minerals is dated 1752, at Río Tinto (Huelva, Spain). At that time, mineral heaps were irrigated with acid solutions and copper recovered by cementation with metal iron. Nevertheless, the development of chalcopyrite leaching processes using ferric solutions has been limited, chiefly because the slow kinetics makes for an incomplete mineral attack. More recently, most research on chalcopyrite dissolution has been directed at elucidating reaction mechanisms and improving the leaching rate with ferric sulphate Thermodynamics and stoichiometry of the chalcopyrite dissolution The Pourbaix diagram for the CuFeS 2 H 2 O system (Fig. 3) (Garrels and Christ, 1965) shows that the dissolution of chalcopyrite in acid medium takes place through a solid transformation in different intermediate sulphides (Cu 5 FeS 4, CuS, Cu 2 S), increasingly richer in copper (Biswas and Davenport, 1976). According to this diagram, a ph lower than 4 and an oxidizing redox potential higher than +0.4 V is required to dissolve copper from chalcopyrite. These conditions are achieved using oxidizing agents, the most common being ferric ion as a sulphate or chloride. Chalcopyrite dissolves in the presence of ferric ion according to the following reactions (Dutrizac and MacDonald, 1974): CuFeS 2 þ 4Fe 3þ Cu 2þ þ 5Fe 2þ þ 2S- CuFeS 2 þ 4Fe 3þ þ 3O 2 þ 2H 2 O Cu 2þ þ 5Fe 2þ þ 2H 2 SO 4 Several researchers have reported the stoichiometry of chalcopyrite leaching. Chronologically, Traill and McClelland (1926) were the first to study the dissolution of chalcopyrite with concentrated ferric chloride (70 g/l) at high temperature (95 C); they obtained a copper yield of 90% ð1þ ð2þ Fig. 3. Pourbaix diagram for the CuFeS 2 H 2 O system at 25 C (Garrels and Christ 1965). after 5 h of leaching and only 60 70% of Fe, probably because it hydrolysed and precipitated during the process. In that study, the recovery of elemental sulphur was as low as 5%, confirming that the reaction is stoichiometric (2). In 1933, Sullivan investigated dissolution of chalcopyrite concentrates in both ferric chloride and ferric sulphate, concluding that around 75% of chalcopyrite dissolves according to reaction (1) and the remaining by reaction (2). The lower rate of sulphate formation was attributed to the presence of dissolved oxygen more than to the action of ferric ion. Ichikuni (1960) established that ferric sulphate dissolved chalcopyrite according to reaction (1); he found only slight deviations of the Cu/Fe molar ratio in solution during the initial stage of leaching with ferric chloride and concluded that there was a preferential dissolution of Fe from the chalcopyrite lattice. Dutrizac et al. (1969) studied the dissolution process of chalcopyrite-sinterized discs in acid ferric sulphate. The stoichiometry of chalcopyrite dissolution was described by reaction (1) and the rate-controlling step was assumed to be the transport of ferric sulphate through a sulphur layer, in constant growth, surrounding the chalcopyrite surface. Then, in 1971, Haver and Wong (1971) proposed a third reaction to describe chalcopyrite dissolution with FeCl 3 : CuFeS 2 þ 3FeCl 3 FeCl 3 CuCl þ 4FeCl 2 þ 2S- ð3þ These authors found that around 70% of the sulphur was oxidized to elemental sulphur and the remainder to sulphate, and they proposed that copper was dissolved in the monovalent form (Cu + ). Dutrizac (1989) established that during chalcopyrite leaching with ferric sulphate at 95 C the sulphide sulphur was practically all oxidized to S (94%). More recently, in 1995, Hackl et al. (1995) found that during leaching of chalcopyrite with ferric sulphate at high temperature and an oxygen pressure of 0.69 MPa, the oxidation of sulphide sulphur to sulphate increased from 28 to 100% when the temperature rose from 110 to 200 C. This was associated with an increase in oxygen consumption. These results indicate that although both reactions (1) and (2) can take place during the leaching of chalcopyrite, the formation of sulphate is limited by the availability of oxygen in the leaching medium. Nevertheless, recent studies have demonstrated that chalcopyrite transforms into intermediate phases and so reactions (1) and (2) must be considered as overall reactions of the chalcopyrite dissolution process Effect of ferric salt anion Fig. 2. The energy band diagram for chalcopyrite (Torma, 1991). Sullivan concluded that ferric chloride is a better chalcopyrite leaching agent than ferric sulphate and that the effect was more

4 E.M. Córdoba et al. / Hydrometallurgy 93 (2008) authors estimated that the concentration of Fe 3+ and FeHSO 4 2+ ionic species increased when the total iron concentration increased, up to a limiting value of 0.1 M. Above this value, the most important species was FeSO 4 +. They concluded that the Fe 3+ and FeHSO 4 2+ ionic species are responsible for chalcopyrite dissolution in sulphate medium Effect of ph Fig. 4. Influence of ph on chalcopyrite leaching at 68 C and 0.5% PD (100 ml of leaching solution and 0.5 g of mineral). marked at high temperatures. Dutrizac et al. (1969) found that, in a temperature range between 50 and 100 C, the chalcopyrite leaching rate was linear in chloride medium and parabolic in sulphate medium. Two years later, Dutrizac and MacDonald demonstrated that the addition of chloride ion sped up chalcopyrite dissolution with ferric sulphate at temperatures higher than 50 C (Dutrizac and MacDonald, 1971). At lower temperatures there was no effect. Majima et al. (1985) established that the chalcopyrite leaching rate was approximately one order of magnitude lower with ferric sulphate than with ferric chloride. This agrees with the observation made by these authors that the elemental sulphur formed during leaching was more porous with ferric chloride than with ferric sulphate. These researches support the idea that ferric chloride has greater oxidizing power than ferric sulphate during chalcopyrite leaching. However, several factors restrict the application of ferric chloride on an industrial scale, such as: - The affinity of this ion towards many elements complicates the process of separating copper from the solution. - Chloride solutions are extremely corrosive. - Hydrochloric acid is more expensive than sulphuric acid. The last factor is of prime importance since pyrometallurgical extraction of copper from chalcopyrite, as in other industries, entails the recovery of large amounts of SO 2 to achieve sulphuric acid production at very low cost Effect of ferric ion concentration Different researchers have pointed out that the chalcopyrite dissolution rate is strongly affected by ferric ion concentration, but only at low concentrations. At high concentrations, the effect is negligible. Sullivan (1933), Linge (1976), Dutrizac et al. (1969), Muñoz et al. (1979) and Kametani and Aoki (1985), among others, found no clear kinetic effect when the ferric ion concentration exceeded 0.01 M. A few authors have observed a relationship of dependence between chalcopyrite dissolution rate and ferric ion concentration. Hirato et al. (1987) found that chalcopyrite dissolution improved when the concentration of ferric sulphate was increased from to 0.1 M. These ph also affects chalcopyrite dissolution. According to Dutrizac et al. (1969), the acid prevents hydrolysis and precipitation of ferric salts, since proton attack is negligible. Some researchers (Lu et al., 2000; Antonijević and Bogdanović, 2004) have reported poor results for chalcopyrite dissolution at low ph (b1.0). The first authors reported that the precipitation of ferric ion as jarosite is possible even at ph 0.9, which shows how readily ferric salts hydrolyse. Antonijević and Bogdanović found that at ph lower than 0.5, the chalcopyrite surface lacks iron and this provokes its passivation. Our results (Fig. 4) show that, in a ph range of 0.5 to 2.0, although increased ph favours hydrolysis and precipitation of the oxidant, dissolution of chalcopyrite with ferric sulphate diminishes with decreasing ph. This is understandable if we consider the speciation diagrams for Fe(III) in sulphate medium. These show that the species responsible for oxidation of the chalcopyrite is not Fe 3+ proper, but probably Fe(SO 4 ) 2, since that is the only species to show an increase of concentration in the given ph range Effect of temperature Table 1 shows the values of activation energy reported by different researchers during chalcopyrite leaching with ferric ion. It also indicates the kinetic control proposed in each study. The high values of activation energy found by different authors demonstrate the need for high temperatures in order to break down bonds in the chalcopyrite crystal lattice. Our results (Fig. 5) show that the effect of temperature on chalcopyrite chemical leaching is very pronounced in the range between 35 and 68 C. In that range, copper extraction increased from b3% to N80% after 13 days of leaching and an activation energy of kj/mol was determined, which is appreciably higher than that reported by other researchers (71 88 kj/mol) in the range of temperature between 50 and 94 C using a sulphate medium (Table 1). Thus, the differences observed are presumably due to the different range of temperature tested. These results agree with the theoretical analysis of Hiskey (1993) who concluded that due to n-type semiconduction of chalcopyrite, the transport of electrons through vacancies is minimum. The first stage in chalcopyrite oxidation, consumes vacants, which favours the transport of electrons through the crystal lattice. Therefore, the heat consumed in the displacement of ions from the bulk to the surface eliminates vacancies at the surface and promotes the transport of electrons. This means chemical mechanisms control chalcopyrite dissolution; however, as noted earlier, diffusion of the oxidant is also very important in this process. We would note that there are clear differences when using one or other type of medium (sulphate or chloride). The activation energies Table 1 Effect of temperature on the kinetics during the chalcopyrite leaching with ferric ion Medium Temperature ( C) Activation energy (kj/mol) Kinetic control Bibliographic reference Ferric sulphate Parabolic kinetics Diffusion control Dutrizac et al., Parabolic kinetics Electrochemical control Muñoz et al., Parabolic-linear kinetics Chemical control Hirato et al., Parabolic-linear kinetics Chemical control Córdoba et al., in press Ferric chloride Linear kinetics Electrochemical control Hirato et al., Linear kinetics Electrochemical control Majima et al., Linear kinetics Diffusion control Dutrizac, 1978

5 84 E.M. Córdoba et al. / Hydrometallurgy 93 (2008) Fig. 5. Influence of temperature on chalcopyrite leaching. are higher in sulphate than in chloride medium, which is consistent with the lower oxidizing power of ferric sulphate (Section 3.2). 4. Passivation of chalcopyrite during leaching with ferric ion After almost a century of research into the mechanisms of chalcopyrite dissolution in ferric medium, there is consensus with respect to the formation of a passivating layer on the surface which slows down the oxidation reaction. But in spite of that, the nature of this film is still unknown, although it has been postulated that it must have low porosity and/or be a bad electric conductor. Following is an account of the principal theories on this topic. In 1969, Burkin (1969) established that the diffusion film surrounding the chalcopyrite during leaching with Fe 3+ formed by a bimetallic sulphide with chemical and structural properties different from the original raw material, but with the same semiconducting properties. These intermediate products are the result of solid state transformations that favour solubilization of cations from the crystal lattice at different rates. Therefore, the model implies that the reaction takes place preferentially at the interface, within the thickness of a reaction front. Later, Ammou-Chokroum et al. (1977) suggested that the rate of the dissolution process is controlled by the formation and evolution of a compact diffusion layer of a low-solubility copper polysulphide, less reactive than the original chalcopyrite and with less iron. The thickness of this film is the result of two opposite reactions (Fig. 6): 1) formation of a passive layer, C p, because solid state diffusion is slower in copper than in iron, which obeys a parabolic law; and 2) linear dissolution of the passive layer, producing an outer film of porous sulphur with no controlling effect. In 1979, Muñoz et al. (1979) postulated that the limiting step in chalcopyrite dissolution with ferric ion is the transport of electrons, necessary to reduce the ferric ion in the solid liquid interface, through an insulating film of elemental sulphur (Fig. 7). According to this Fig. 7. Model of chalcopyrite oxidation by Fe 3+ with formation of a low porous layer of elemental sulphur (after Muñoz et al., 1979). model, the elemental sulphur affects both anodic and cathodic semireactions, limiting the overall redox reaction of chalcopyrite dissolution. This model has been ratified by Dutrizac (1989) and Majima et al. (1985). In 1995, Hackl et al. (1995) proposed a model mixture of chemical and diffusion control to explain the leaching and passivation of chalcopyrite in sulphate medium. This model complements the model of Ammou-Chokroum et al. (1977). Initially, iron dissolves preferentially to copper forming an intermediate disulphide phase, Cu 1 x Fe 1 y S 2,where ynnx and x+y 1: CuFeS 2 Cu 1 x Fe 1 y S 2 þ xcu 2þ þ yfe 2þ þ 2ðx þ yþe ; ynnx; x þ y 1 In the second stage: Cu 1 x Fe 1 y S 2 Cu 1 x z S 2 þ zcu 2þ þð1 yþfe 2þ þ 2ðz þ 1 yþe which is slower than the previous stage, the disulphide dissolves to form a copper polysulphide, Cu 1 x z S 2 or CuS n, where n =2/(1 x z). This polysulphide is probably responsible for chalcopyrite passivation. Thus, the rate-controlling step consists of a very slow decomposition of copper polysulphide to cupric ions and porous elemental sulphur, with no effect on chalcopyrite passivation: Cu 1 x z S 2 ð1 x zþcu 2þ þ 2S- þ 2ð1 x zþe This model greatly depends on temperature, since the polysulphide dissolution rate increases with temperature. Chalcopyrite passivation does not occur at 200 C. In a recent work, Hiroyoshi et al. (2001) studied the effect of ferrous ion on chalcopyrite oxidation with ferric ions in sulphuric acid and in the absence of oxygen. They discovered that in the presence of high concentrations of cupric ion, the ferrous ion has a very positive effect on copper extraction and the kinetics were controlled by the ratio of Fe 3+ / Fe 2+ concentrations or by the redox potential of the solution. However, the ferrous ion adversely affected chalcopyrite oxidation at low cupric ion concentrations. To account for these results, they proposed a reaction model involving incomplete reduction of chalcopyrite, according to the following reaction: ð4þ ð5þ ð6þ CuFeS 2 þ 3Cu 2þ þ 3Fe 2þ 2Cu 2 S þ 4Fe 3þ ð7þ This reaction is followed by oxidation of Cu 2 S (more sensitive to leaching than chalcopyrite) by ferric ion: Cu 2 S þ 4Fe 3þ 2Cu 2þ þ S- þ 4Fe 2þ ð8þ Fig. 6. Model of chalcopyrite oxidation by Fe 3+ with formation of an inner layer of polysulphide (C p ) and outer layer of porous sulphur (after Ammou-Chokroum et al., 1977). The two reactions (7) and (8) give the overall reaction (1), which traditionally describes the leaching of chalcopyrite in ferric medium. According to this model, the rate-controlling step is the initial reduction of chalcopyrite. This only happens when the redox potential of the solution is below a critical value, which in turn is a function of the concentration of ferrous and cupric ions.

6 E.M. Córdoba et al. / Hydrometallurgy 93 (2008) Nicol and Lazaro (2003) propose a different mechanism to explain the chalcopyrite dissolution at low redox potentials. At potentials lower than 0.4 V, chalcopyrite could be reduced by proton attack: CuFeS 2 þ 4H þ Cu 2þ þ Fe 2þ þ 2H 2 S ð9þ The next step would be the oxidation of hydrogen sulphide by ferric ions: 2H 2 S þ 4Fe 3þ 2S- þ 4Fe 2þ þ 4H þ ð10þ The overall reaction of this mechanism is again reaction (1). In this model, the rate-controlling step is electrochemical. The rate of reaction (10) decreases when the redox potential of the chalcopyrite surface increases from 0.5 to 0.7 V. Various different studies on chalcopyrite leaching with ferric ion accept the theory that there is a diffusion barrier between the leaching solution and the chalcopyrite that slows down the dissolution rate. However, the nature of this barrier is still in debate since the mechanism of chalcopyrite dissolution by Fe 3+ has not yet been elucidated. Córdoba (2005) reports clear evidence that chalcopyrite passivation is related to high redox potentials or high Fe 3+ /Fe 2+ ratios. Ferric/ ferrous sulphate leaching solutions tend quickly to reach a chemical equilibrium where activities of both ions are equal. This is associated with a critical potential of approximately 450 mv vs Ag/AgCl. Then, when the initial redox potential is very high, that tendency to equilibrium favours rapid precipitation of ferric ion as jarosite (Fig. 8) and consequently chalcopyrite passivation. The evidence cited above suggests that passivation of chalcopyrite during leaching is chiefly the result of hydrolysis and precipitation of Fe 3+ rather than partial transformation of the chalcopyrite surface. 5. Silver-catalyzed chalcopyrite leaching A way to improve the chalcopyrite dissolution rate is by adding Ag +. In 1976, Pawlek (1976) studied the effect of Ag + on the leaching of chalcopyrite at 110 C and 100 kpa. After 30 min, chalcopyrite dissolution was complete in the presence of silver compared with 40% copper dissolution in the absence of silver. In 1979, Miller and Portillo (1979) proposed a model to interpret the catalytic effect of silver ions in the leaching of chalcopyrite with sulphuric acid/ferric sulphate solutions. The authors suggested that in the absence of silver ions, a dense elemental sulphur layer forms on the mineral surface, acting as a diffusion barrier and delaying the oxidative leaching of chalcopyrite with ferric ions. Conversely, in the presence Fig. 9. Electrochemical characterization of a massive chalcopyrite electrode at 35 C: a) anodic polarization and b) cyclic voltammetry. of silver ions the leaching of chalcopyrite takes place according to the following reactions: CuFeS 2 þ 4Ag þ Cu 2þ þ Fe 2þ þ 2Ag 2 S Ag 2 S þ 2Fe 3þ 2Ag þ þ 2Fe 2þ þ S- ð11þ ð12þ The rate of copper extraction is faster in the presence of silver ions since the product layer is formed by a mixture of sulphur and Ag 2 S, which is porous and does not exert a barrier effect on the chalcopyrite dissolution. Price and Warren (1986) reported that the elemental sulphur obtained under silver catalytic conditions, besides being more porous, presented higher electrical conductivity and hence facilitated the transport of electrons through the chalcopyrite surface. The model proposed by Miller and Portillo has since been confirmed by Burkin (1982) and Price and Warren (1986), who detected the formation of silver sulphide on the chalcopyrite surface. At the same time, some authors (Burkin, 1982) have pointed out that silver can substitute copper in the crystal lattice of copper sulphides, given the chemical similarities between copper and silver. With an excess of Ag, the reaction product on the mineral surface could contain silver and silver sulphate. This is undesirable because of the amount of silver that may be consumed and lost before the objective is achieved i.e., to diminish the formation of elemental sulphur on the chalcopyrite surface. An excess of silver can also facilitate the sequestering of silver by jarosite (Carranza et al., 1997; Bolorunduro et al., 2003). The formation of silver jarosite may be represented by reaction: Ag þ þ 3Fe 3þ þ 2SO 2 4 þ 6H 2O AgFe 3 ðso 4 Þ 2 ðohþ 6 þ 6H þ ð13þ 6. Electrochemical characterization and passivation of chalcopyrite Fig. 8. SEM micrograph of a leaching residue at E Initial =600 mv. Fig 9a and b shows the electrochemical behaviour of an electrode of massive chalcopyrite at 35 C. It is accepted that during anodic polarization of chalcopyrite from its rest potential, there are normally two different electrochemical responses (Biegler and Horne, 1985; Warren et al., 1985; Price and Warren, 1986; Gómez et al., 1996). The first peak, between the rest potential and approximately +740 mv vs. Ag/AgCl, corresponds to the passive zone or prewave, and the second peak, at higher potentials, is known as the transpassive zone. In the passive zone, the chalcopyrite surface transforms into a copper-rich sulphide (CuS) and S because of the preferential dissolution of iron from chalcopyrite. The formation of covellite in this zone has been justified by reaction (14) (Jones and Peters, 1976), reaction (15) (Biegler and Swift, 1979; Biegler and Horne, 1985) or by the transformation of chalcopyrite in an intermediate phase (bornite)

7 86 E.M. Córdoba et al. / Hydrometallurgy 93 (2008) (reaction (16)) followed by oxidation to non-stoichiometric covellite (reaction (17)) (Warren et al., 1985): CuFeS 2 þ 2H 2 O 1=2CuS þ 1=2CuSO 4 þ Fe 2þ þ S- þ 4H þ þ 6e CuFeS 2 0:75CuS þ 0:25Cu 2þ þ Fe 2þ þ 1:25S- þ 2:5e CuFeS 2 Cu 1 x Fe 1 y S 2 z þ xcu 2þ þ yfe 2þ þ zs- þ 2ðx þ yþe ð14þ ð15þ ð16þ Cu 1 x Fe 1 y S 2 z ð2 zþcus non stoichiom: þð1 yþcu 2þ þð1 yþfe 2þ þ 2ð1 yþe ; where ynx ð17þ Whether or not the reactions by which CuS is formed are stoichiometric, the different authors agree that this copper sulphide is responsible for chalcopyrite passivation during oxidative dissolution. In the transpassive zone, the passive layer breaks down and massive chalcopyrite dissolution takes place through reactions (18) and (19) (Ammou-Chokroum et al., 1981; Yin et al., 1995, Gómez et al., 1996) or through reaction (20): CuFeS 2 Cu 2þ þ Fe 3þ þ 2S- þ 5e ð18þ CuFeS 2 þ 8H 2 O Cu 2þ þ Fe 3þ þ 2SO 2 4 þ 16Hþ þ 17e ð19þ CuFeS 2 þ 2H 2 O Cu 2þ þ Fe 2þ þ 3=2S- þ 1=2H 2 SO 4 þ 3H þ þ 7e ð20þ In the anodic polarization curve (Fig. 9a), starting from the rest potential (+230 mv), the two mentioned zones are clearly appreciable. The passivation zone, with two electrochemical responses, A1 and A2, indicates that chalcopyrite dissolves in this low-potential zone through reactions (16) and (17). In the cathodic branch (Fig. 9b), when the scan direction shifts at +800 mv, current density drops sharply to zero and practically remains there over a wide range of potentials. Finally, from 150 to 500 mv, there is a large cathodic response (peak C). It is in this zone that the reduction of chalcopyrite and covellite takes place, through the following reactions: 2CuFeS 2 þ 6H þ þ 2e Cu 2 S þ 2Fe 2þ þ 3H 2 S ð21þ 2CuS þ 2H þ þ 2e Cu 2 S þ H 2 S ð22þ When the scan direction shifts to anodic values, the current density again increases, showing two new anodic electrochemical responses at approximately +310 mv (D) and +570 mv (E). According to Biegler and Horne (1985), peak D reflects the oxidation of reduction products formed during the cathodic scan, Cu 2 S or non-stoichiometric sulphides such as Cu 8 S 5,Cu 7 S 4, etc., as follows: Cu 2 S CuS þ Cu 2þ þ 2e ð23þ Finally, the anodic peak E may be associated with the prewave reaction (passive zone at low potential, peak A) of chalcopyrite oxidation to CuS and S (reactions (14) to (17)). This peak could further be related to the oxidation reaction of ferrous ion: Fe 2þ Fe 3þ þ e ð24þ and later precipitation of the ferric ion as proposed by Tshilombo and Dixon (2003). These authors observed the formation of ferric hydroxides during the electrochemical oxidation of chalcopyrite at high potential and temperature and concluded that they are formed by chemical precipitation of ferric ion generated by oxidation of ferrous ion. 7. Conclusions While the refractoriness of chalcopyrite to chemical attack is very much a matter of debate, it is unanimously agreed that the leaching conditions must be oxidizing. The presence of ferric ion always plays a central role since it is a by-product of chalcopyrite dissolution irrespective of the leaching conditions tested. The presence of ferric iron in solution affects the process in two ways: It favours leaching of the mineral when the ferric concentration is suitable. It causes passivation of the mineral surface and protects it from attack through the formation of intermediate compounds when the ferric concentration is high. 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