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Pressure leaching of sulfidized chalcopyrite in sulfuric acid–oxygen media
Hydrometallurgy 86 (2007) 80 – 88 www.elsevier.com/locate/hydromet
Pressure leaching of sulfidized chalcopyrite in sulfuric acid–oxygen media R. Padilla ⁎, D. Vega, M.C. Ruiz Department of Metallurgical Engineering, University of Concepción, Edmundo Larenas 285, Concepción, Chile Received 1 June 2006; received in revised form 2 October 2006; accepted 29 October 2006 Available online 9 January 2007
Abstract The recovery of copper from chalcopyrite concentrates by leaching is difficult due to the slow dissolution kinetics of this mineral in most leaching media. However, recovery of copper from sulfidized chalcopyrite (a mixture of CuS and FeS2) by leaching is faster and could be selective depending on the leaching media. In this paper, the result of an investigation on the H2SO4–O2 pressure leaching of sulfidized chalcopyrite is presented. The variables considered in the study were stirring speed, concentration of sulfuric acid, temperature, and partial pressure of oxygen. The experimental data indicated that stirring speed over 500 rpm and sulfuric acid concentration over 0.1 M had very little effect on the leaching rate. An increase in temperature from 90 to 108 °C increased both copper and iron dissolution; however, further increase to 120 °C affected negatively the copper dissolution. Oxygen partial pressure was found to be the main variable that controls the copper/iron selectivity of the leaching. An increase in oxygen partial pressure increased significantly the rate of copper dissolution but deteriorated the copper/iron selectivity. The analysis for elemental sulfur of the solid leach residues indicated that most of the copper sulfide sulfur in the sulfidized concentrate oxidized to elemental sulfur. © 2006 Elsevier B.V. All rights reserved. Keywords: Pressure leaching; Covellite; Sulfidation; Chalcopyrite
1. Introduction Among the copper sulfides used to produce metallic copper, chalcopyrite (CuFeS2) is the predominant mineral. The concentrates of chalcopyrite are generally treated by pyrometallurgical methods of smelting and converting processes to win the copper metal. However, this high temperature route has always been plagued with environmental issues concerning pollution not only with SO2 gas emissions but also with emissions of toxic
⁎ Corresponding author. E-mail address: [email protected] (R. Padilla). 0304-386X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2006.10.006
compounds of arsenic, antimony, and other heavy metals. The alternative hydrometallurgical routes of direct leaching of chalcopyrite have had limited industrial application, mainly because the kinetics of chalcopyrite dissolution is extremely slow even in very strong oxidizing leaching media. Additionally, a selective extraction of copper over iron by leaching chalcopyrite in acidic media is not feasible since for every copper ion dissolved one iron ion passes also into solution. Furthermore, the precipitation of iron from chalcopyrite leaching solutions and the subsequent disposal of the precipitate poses a problem since the precipitates are potential contaminants and usually require expensive confining.
R. Padilla et al. / Hydrometallurgy 86 (2007) 80–88
1.1. Sulfidation of chalcopyrite In recent investigation from this laboratory (Padilla et al., 2003a,b), it was determined that chalcopyrite reacts with gaseous sulfur at moderate temperatures to transform its mineralogy to various simple sulfides which can be leached more easily than chalcopyrite. The reaction products found were mixtures of covellite and pyrite or idaite and pyrite depending on the sulfidation temperature. For the temperature range of 325 to 400 °C, the transformation of chalcopyrite proceeds as follows: CuFeS2 ðsÞ þ 0:5S2 ðgÞ ¼ CuSðsÞ þ FeS2 ðsÞ
ð1Þ
where the reaction product is a mixture of simple sulfide phases covellite and pyrite. Above 400 °C, the chalcopyrite transformation proceeds according to: 5CuFeS2 þ 2S2 ðgÞ ¼ Cu5 FeS6 ðsÞ þ 4FeS2 ðsÞ
ð2Þ
where the reaction product is a mixture of idaite and pyrite phases. Reaction (1) is the most interesting and desirable reaction for copper recovery since the chalcopyrite transforms into separated copper and iron sulfides. Therefore, in leaching a sulfidized chalcopyrite concentrate, the extraction of copper should be faster than from the nonsulfidized chalcopyrite because covellite is more reactive than chalcopyrite and it could also be selective because copper and iron are in separate phases. 2. Leaching of sulfidized chalcopyrite Few studies have been conducted on the leaching of chalcopyrite heat-treated with elemental sulfur. At ambient pressures, Subramanian and Kanduth (1973) conducted a limited study on the copper extraction from activated chalcopyrite concentrate (chalcopyrite concentrate reacted with liquid sulfur) by using oxygen, ferric sulfate or manganese dioxide as oxidant in sulfuric acid solutions. From their studies, they concluded that in the ferric sulfate leach of the activated (sulfidized) chalcopyrite a large concentration of ferric sulfate (near saturation) was necessary to obtain a rapid dissolution and high extraction of copper, while in the manganese dioxide leaching a large excess of MnO2 (200% excess) was necessary to obtain about 86% of copper extraction in 4 h. In these studies, the copper dissolution was not selective since the iron was co-dissolved in the order of about 40%. Recently, Padilla et al. (2003c) studied the leaching of sulfidized chalcopyrite also at ambient pressures in
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H2SO4–NaCl–O2. These investigators found that the copper dissolution in that media was rapid and reported copper extractions over 90% in less than 90 min at about 100 °C. Among the variables studied, they found that temperature and chloride ion concentration were the variables that influence most on the copper dissolution while the iron dissolution was reported as less than 6% in all tested conditions. Although the rate of copper dissolution in the sulfate–chloride–oxygen media is rapid and the iron dissolution is low, the presence of chloride ions complicates the solution purification before electrowinning of copper. This complication brings the need to study alternative methods to dissolve the copper from the sulfidized chalcopyrite. The leaching with oxygen overpressures is another alternative to accelerate the dissolution of metals from sulfides in sulfate media. On this matter, Warren et al. (1968) pretreated chalcopyrite with 10% elemental sulfur at 475 °C in a steel retort and leached the resulting product (sulfidized chalcopyrite) in sulfuric acid (one mol of acid per mole of copper) at 90 °C and 482 kPa oxygen partial pressure. They reported 90% copper and 27% iron extraction in 1 h of leaching. Subramanian and Kanduth (1973) conducted also a limited pressure leaching experiments of chalcopyrite concentrates treated with liquid sulfur. They concluded that the optimum leaching conditions for copper dissolution were 110 °C and 346 kPa oxygen. At lower temperatures, the rate of dissolution was poor and at higher temperatures, (128 °C) the reaction was slow presumably due to blockage by liquid sulfur film on the particle surface. In both studies, the iron dissolution accelerated when most of the copper was extracted. 3. Leaching chemistry of sulfidized chalcopyrite The sulfidation product obtained at 375 °C in this study is a mixture of covellite and pyrite, thus the following discussion will be concerned only with these two sulfides. The dissolution of covellite in sulfuric acid–oxygen system can proceed with the production of elemental sulfur or sulfate according to the following reactions (Cheng and Lawson, 1991; Lotens and Wesker, 1987). CuS þ 0:5O2 þ 2Hþ →Cu2þ þ So þ H2 O
ð3Þ
CuS þ 2O2 →Cu2þ þ SO2− 4
ð4Þ
We can notice that reaction (3) consumes acid and reaction (4) does not.
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Table 1 Chemical analysis of the chalcopyrite concentrates
4. Experimental work
Size fraction
Cu, %
Fe, %
S, %
4.1. Materials and procedure
Concentrate 1 106/75 μm 75/53 μm 53/45 μm 45/38 μm
32.1 32.4 32.2 34.2
28.6 30.5 31.3 28.3
34.8 33.8 34.6 35.7
Concentrate 2 53/45 μm
27.1
27.8
35.8
The experimental study was carried out using chalcopyrite concentrates from Andina Mine of CODELCO, Chile. These concentrates were products from flotation operations of copper sulfide minerals, which were further classified by size. Table 1 shows the chemical analysis of the two chalcopyrite concentrates used in this study, and Table 2 shows the mineralogical composition of both concentrates. As we can see in Table 2, concentrate 2 has lower content of chalcopyrite but it contains more pyrite and molybdenite than concentrate 1.
Similarly, the dissolution of pyrite has been typically represented by the following reactions (Lotens and Wesker, 1987; Long and Dixon, 2004). FeS2 þ 3:5O2 þ H2 O→Fe2þ þ 2Hþ þ 2SO2− 4
ð5Þ
4.2. Sulfidation of chalcopyrite
o FeS2 þ 2O2 →Fe2þ þ SO2− 4 þS
ð6Þ
The chalcopyrite concentrates were subjected to the sulfidizing process according to the methodology described by Padilla et al. (2003a). The procedure consisted mainly in contacting the chalcopyrite concentrate with sulfur gas at 375–400C °C for certain periods of time. In this investigation, 375 °C was chosen for the sulfidation reaction and the results are shown in Fig. 1, where the chalcopyrite conversion data is depicted as a function of time at this temperature. As we can see, after 90 min of reaction the chalcopyrite conversion was 97%. This conversion was calculated based on experimental weight gain of the chalcopyrite sample and the theoretical weight gain according to reaction (1), assuming all the copper in the concentrate is present as chalcopyrite. The morphology of the sulfidized chalcopyrite can be seen in Fig. 2, which shows a
Reaction (3) is the most desirable for the dissolution of copper from the sulfidized chalcopyrite because the sulfur in the covellite passes into the solid residues as elemental sulfur. However, in practice, the proportion of sulfur and sulfate formed will depend on the leaching conditions, mainly on the temperature. The leaching of CuS according to reactions (3) or (4) is very slow at atmospheric conditions (Cheng and Lawson, 1991); therefore, high temperatures and oxygen overpressures are required for the copper dissolution to occur at fast rates in sulfuric acid media. Considering the preceding discussion, the present investigation focused on the study of the pressure leaching of the sulfidized chalcopyrite concentrate in H2SO4–O2 media. The main objective was to determine the optimum conditions to extract the copper rapidly and selectively from the sulfidized chalcopyrite concentrate. Table 2 Mineralogical composition of chalcopyrite concentrates Species
Concentrate 1 Weight percent, %
Concentrate 2 Weight percent, %
Chalcopyrite Pyrite Bornite Chalcocite Covellite Tetrahedrite Molybdenite Sphalerite Enargite Magnetite Gangue
94.24 2.82 0.78 – 0.56 0.51 0.17 0.08 0.01 – 0.83
80.41 10.88 0.29 0.15 0.80 0.69 1.08 – 0.15 0.69 4.86
Fig. 1. Sulfidation of chalcopyrite concentrate 2 with gaseous sulfur at 375 °C as a function of time.
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analyzed for Cu and Fe and some solid residues were observed by microprobe and light microscopy. Additionally, some of the solid residues were also analyzed for elemental sulfur by the Soxhlet extraction method using carbon disulfide as the solvent for sulfur. 5. Results The main variables studied were the degree of agitation, concentration of sulfuric acid in the leaching solution, partial pressure of oxygen (304 to 1520 kPa), temperature (90 to 120 °C), and time of reaction. The results obtained are summarized in the following. Fig. 2. Light micrograph of sulfidized chalcopyrite particles showing the covellite phase surrounding the porous pyrite phase. Concentrate 2, particle size 53/45 μm.
micrograph of the sulfidized material obtained with a light microscope. In this micrograph, we can observe that the covellite phase was formed in the outer surface of the particle. The mechanism of formation of covellite during sulfidation involves a solid state diffusion of copper towards the external surface of the particle as reported by Padilla et al. (2003b). We observe also that the pyrite phase constitutes the inner part of the particles and it is extremely porous due to the large difference in molar volume between the pyrite (23.9 mL) and the chalcopyrite (43.7 mL) from which it forms. 4.3. Leaching experiments The leaching experiments of the sulfidized chalcopyrite were conducted in a 2 L Parr autoclave made of alloy 20 Cb. This autoclave was equipped with a heating mantle, a PID temperature controller, a variable speed stirrer and an internally mounted cooling coil. The stirring system had two axial impellers with 6 blades with 45° downdraft mounted 4.5 cm apart. All the experiments were carried out batch-wise using 2 g of sulfidized chalcopyrite, 1000 ml of leaching solution (various concentrations of H2SO4) and pure oxygen. The procedure consisted on heating the solution to 95 °C; once at this temperature, the solid sample was added and further heated to the temperature of the experiment. At the set temperature, the oxygen was admitted and the partial pressure of oxygen was adjusted to the desired value, which was maintained constant for the duration of the experiment. At the end of the experiment, the oxygen was shut down, the autoclave was rapidly water-cooled, and the solution was filtered for collecting the solid residues. The solution was
5.1. Effect of agitation The effect of the extent of agitation of the leaching pulp on the copper dissolution from the sulfidized chalcopyrite was studied under an oxygen partial pressure of 1013 kPa The result is shown in Fig. 3 where it can be seen that the stirring speed has an important effect on the dissolution of copper up to about 500 rpm. The stirring speed was maintained at 500 rpm which was considered to be appropriate to eliminate the effect of this variable. 5.2. Effect of sulfuric acid The effect of sulfuric acid on the leaching of sulfidized chalcopyrite was studied in the range 0.005 M to 0.2 M. Fig. 4 shows the dissolution of copper and iron as a function of concentration of sulfuric acid in the leaching solution. We can observe in this figure that the acid concentration affects the dissolution of copper up to about 0.1 M H2SO4. This result suggests
Fig. 3. Effect of agitation on the extraction of copper from sulfidized chalcopyrite. Concentrate 1, and size fraction 75/53 μm.
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Fig. 4. Effect of sulfuric acid concentration on the copper and iron dissolution from the sulfidized chalcopyrite. Concentrate 1, size fraction 75/53 μm.
Fig. 6. Effect of temperature on the copper dissolution from sulfidized chalcopyrite; conditions: 0.2 M H2SO4, 1013 kPa PO2, and 500 rpm. Concentrate 1, size fraction 53/45 μm.
that the dissolution of copper occurs primarily through reaction (3) with the production of elemental sulfur. This conclusion was confirmed by sulfur analysis of the residues of leaching, as discussed later. In the case of iron, the sulfuric acid concentration in the range studied does not have an effect on the dissolution.
1013 kPa. We can observe in Fig. 5 that the dissolution rate of concentrate 2 is affected little by changes in the temperature from 90 to 108 °C This result suggests that in this range of temperature, a diffusion process controls the overall dissolution reaction of copper. The same result is obtained in leaching concentrate 1 at oxygen pressure of 1013 kPa, except that the dissolution rate of copper is higher in this case due to the higher partial pressure of oxygen, as seen in Fig. 6. We can also see in these figures that at 120 °C, the dissolution of copper is lower than that obtained at the other temperatures. This evident drop of the copper dissolution at 120 °C is due to the formation of a liquid sulfur layer which covers the particle's surface thus inhibiting further dissolution of copper. This passivation effect was also observed by Subramanian and Kanduth (1973) during the pressure dissolution of activated (sulfidized) chalcopyrite concentrate. The iron dissolution from the sulfidized chalcopyrite can be seen in Fig. 7. We observe that temperature does have a pronounced effect on iron dissolution. Increasing temperature increases the iron dissolution. We can clearly see that the passivation effect of 120 °C on the copper dissolution cannot be observed in the case of iron.
5.3. Effect of temperature Sulfidized chalcopyrite samples of both concentrates 1 and 2 were leached at temperatures varying from 90 to 120 °C to determine the effect of temperature on the copper and iron dissolution. The results for the copper dissolution are shown in Fig. 5 for experiments carried out with oxygen partial pressure of 507 kPa and in Fig. 6 for the case of partial pressure of oxygen equal to
5.4. Effect of oxygen overpressure
Fig. 5. Effect of temperature on the copper dissolution from sulfidized chalcopyrite. 0.2 M H2SO4, 507 kPa PO2, and 500 rpm, concentrate 2, size fraction 53/45 μm.
The effect of partial pressure of oxygen on the copper and iron dissolution was studied using concentrates 1 and 2. Although both concentrates differ in copper and iron content, it was determined that the fraction of copper and iron extracted from each concentrate was the same under the same experimental conditions. The results can be seen in Fig. 8 for copper and Fig. 9 for
R. Padilla et al. / Hydrometallurgy 86 (2007) 80–88
Fig. 7. Effect of temperature on the iron dissolution from sulfidized chalcopyrite at 1013 kPa of oxygen overpressure for concentrate 1 of size fraction 53/45 μm.
iron dissolution. In Fig. 8, we can observe that the rate of copper dissolution depends significantly on the partial pressure of oxygen in the range 304 to1520 kPa. Increasing the partial pressure of oxygen 304 to 1013 kPa, a significant increase in the rate of copper dissolution is obtained. However, a further increase in the oxygen pressure from 1013 to 1520 kPa does not produce a substantial increase in the dissolution rate. The dissolution of iron also depends strongly on the partial pressure of oxygen as can be seen in Fig. 9, where we can observe that the dissolution of iron occurs at a very low rate at 304 kPa of oxygen. However, at 1013 kPa the rate of iron dissolution is fast and keeps
Fig. 8. Effect of partial pressure of oxygen on the dissolution of copper from sulfidized chalcopyrite. Conditions: 100 °C, 0.2 M H2SO4, 500 rpm. Concentrates 1 of size fraction 53/45 μm for PO2 = 507, 1013 and 1520 kPa, and concentrate 2 of size fraction 53/45 μm for PO2 = 304 kPa.
85
Fig. 9. Effect of partial pressure of oxygen on the dissolution of iron from sulfidized chalcopyrite. Conditions: 100 °C, 0.2 M H2SO4, 500 rpm. Concentrates 1 of size fraction 53/45 μm for PO2 = 304, 1013, and 1520 kPa, and concentrate 2 of size fraction 53/45 μm for PO2 = 507 kPa.
increasing with increasing partial pressure of oxygen up to 1520 kPa. Consequently, according to these data for a selective dissolution of copper from sulfidized chalcopyrite the leaching should be carried out at low pressures, around 304 kPa. 5.5. Effect of particle size on the copper dissolution The effect of particle size on the copper dissolution is shown in Fig. 10. As we can see in this figure, the rate of reaction increases as the size of the particles decreases. This effect is due to the increase in the interfacial area of reaction as the solid particles become smaller. However,
Fig. 10. Effect of particle size of chalcopyrite on the copper dissolution from the sulfidized concentrate in 0.2 M sulfuric acid, 100 °C, and 1013 kPa of partial pressure of oxygen for concentrate 1.
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Fig. 11. Backscattered electron image (BEI) of a residue obtained in leaching sulfidized concentrate sample in 0.2 M H2SO4, 120 °C, 507 kPa of oxygen overpressure, and 60 min of leaching time. In the image: Cv is covellite (light color phase) and Py is pyrite (dark color, porous phase).
since the stirring speed for the experiments was 500 rpm, the large particles (106/75 μm size fraction) may not be fully suspended, which may explain in part the lowest dissolution rate shown in the figure. 5.6. Characterization of the residues from leaching Fig. 11 shows a backscattered electron image (BEI) of a partially reacted sample obtained in leaching at 120 °C, partial pressure of oxygen 507 kPa, for 60 min of leaching time. We can observe that the physical aspect of the particles did not change very much as compared to the non-leached sulfidized particles. The dense layer of covellite surrounding the porous pyrite can still be seen clearly in the image because the corresponding copper dissolution was only 51%. On the other hand, Fig. 12(a) shows a BEI image of a residue which was obtained in leaching at 100 °C for 240 min under oxygen overpressure of 507 kPa, with copper dissolution of 94%. Fig. 12(b), (c) and (d) show X-ray spectra for copper, iron, and sulfur respectively in the sample. The morphology and constituent phases of the residue can be seen clearly in this figure. In the BEI image (a), we can only see the porous pyrite phase as expected since for 94% conversion, the external covellite phase has all been dissolved. We can also see that the small amount of undissolved covellite is located in the interior pores of the pyrite as indicated by the copper content found in the spectrum (b). Image (a) also shows two primary (dense) pyrite particles which were
present in the original chalcopyrite concentrate and a metallic iron phase, which is a constituent of the used resin sample holder. Images (b) through (d) essentially confirm that the reacted particle does not show a covellite phase surrounding the porous pyrite phase which indicates almost complete copper dissolution as evidenced by spectrum (b). The sulfur spectrum shown in Fig. 12(d) does not show a coherent elemental sulfur layer surrounding the particle, it shows only the sulfur corresponding to the pyrite. To verify that the dissolution of copper from sulfidized chalcopyrite occurs through reaction (3), the solid residues from some of the experiments were analyzed for elemental sulfur by Soxhlet extraction method as mentioned earlier. The results are shown in Fig. 13, where the fraction of elemental sulfur produced from the sulfidized chalcopyrite is shown as a function of dissolved copper. We can observe in this figure that about 70% of the reacted sulfide sulfur is present in the solid residue as elemental sulfur. These data indicate that the pressure leaching of sulfidized chalcopyrite in sulfuric acid–oxygen for the conditions studied proceeds mainly through reaction (3) and to a lesser extent through reaction (4). In the overall framework of treating chalcopyrite concentrates by sulfidation and leaching, the leaching of the sulfidized concentrate in autoclave in sulfuric acid – oxygen system is one alternative that effectively extracts copper from the sulfidized concentrate at 100 °C and 1013 kPa. Even though the copper/iron selectivity was not good at these conditions, the pressure leaching of sulfidized chalcopyrite is a viable process to produce pregnant solutions that can be inserted in the conventional industrial lines of SX purification and EW. 6. Conclusions From the results presented the following can be concluded: – Sulfuric acid concentration in the solution over 0.1 M had very little effect on the leaching of sulfidized chalcopyrite in sulfuric acid–oxygen in autoclave. – An increase in leaching temperature from 90 to 108 °C, increased the copper and iron dissolution. Increasing the temperature further to 120 °C has deleterious effect on the copper dissolution. – An increase in the partial pressure of oxygen from 304 to 1520 kPa increased significantly the copper and iron dissolution. The oxygen partial pressure is the variable that controls the selectivity of Cu/Fe dissolution.
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Fig. 12. (a) Backscattered electron image (BEI) showing porous pyrite particle in the residue and primary pyrite from the chalcopyrite concentrate; (b) X-ray spectrum for copper; (c) X-ray spectrum for iron; (d) X-ray spectrum for sulfur.
– The pressure leaching in sulfuric acid–oxygen media is a viable method to extract copper from sulfidized chalcopyrite concentrates producing solutions to be treated by SX and EW. – Selective copper dissolution from sulfidized chalcopyrite can be obtained at 100 °C and oxygen overpressures of about 304 kPa in less than 3 h of residence time. However, for longer times, iron extraction increases rapidly and selectivity decreases. Acknowledgements
Fig. 13. Fraction of elemental sulfur produced in the pressure leaching of sulfidized chalcopyrite at 108 °C, 507 kPa of oxygen partial pressure.
The authors acknowledge The National Fund for Scientific and Technological Development (FONDECYT) of Chile for the financial support of this study through Project No. 1050948 and the Research Council
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of the University of Concepción through Project No. DI 203.095.081-1.0. References Cheng, C., Lawson, Y., 1991. The kinetics of leaching covellite in acidic oxygenated sulphate-chloride solutions. Hydrometallurgy 27, 269–284. Long, H., Dixon, D.G., 2004. Pressure oxidation of pyrite in sulfuric acid media: A kinetic study. Hydrometallurgy 73, 335–349. Lotens, J.P., Wesker, E., 1987. The behaviour of sulphur in the oxidative leaching of sulphidic minerals. Hydrometallurgy 18, 39–54.
Padilla, R., Rodríguez, M., Ruiz, M.C., 2003a. Sulfidation of chalcopyrite with elemental sulfur. Metall. Mater. Trans. 34B, 15–23. Padilla, R., Olivares, E., Ruiz, M.C., 2003b. Kinetic of the sulfidation of chalcopyrite with gaseous sulfur. Metall. Mater. Trans. 34B, 61–68. Padilla, R., Zambrano, P., Ruiz, M.C., 2003c. Leaching of sulfidized chalcopyrite with H2SO4–NaCl–O2. Metall. Mater. Trans., B, Proc. Metall. Mater. Proc. Sci. 34B, 153–159. Subramanian, K.N., Kanduth, H., 1973. Activation and leaching of chalcopyrite concentrates. CIM Bull. 66, 88–91. Warren, H.I., Vizsolyi, A., Forward, F.A., 1968. The pretreatment and leaching of chalcopyrite. CIM Bull. 5, 637–640.
Pressure leaching of sulfidized chalcopyrite in sulfuric acid–oxygen media R. Padilla ⁎, D. Vega, M.C. Ruiz Department of Metallurgical Engineering, University of Concepción, Edmundo Larenas 285, Concepción, Chile Received 1 June 2006; received in revised form 2 October 2006; accepted 29 October 2006 Available online 9 January 2007
Abstract The recovery of copper from chalcopyrite concentrates by leaching is difficult due to the slow dissolution kinetics of this mineral in most leaching media. However, recovery of copper from sulfidized chalcopyrite (a mixture of CuS and FeS2) by leaching is faster and could be selective depending on the leaching media. In this paper, the result of an investigation on the H2SO4–O2 pressure leaching of sulfidized chalcopyrite is presented. The variables considered in the study were stirring speed, concentration of sulfuric acid, temperature, and partial pressure of oxygen. The experimental data indicated that stirring speed over 500 rpm and sulfuric acid concentration over 0.1 M had very little effect on the leaching rate. An increase in temperature from 90 to 108 °C increased both copper and iron dissolution; however, further increase to 120 °C affected negatively the copper dissolution. Oxygen partial pressure was found to be the main variable that controls the copper/iron selectivity of the leaching. An increase in oxygen partial pressure increased significantly the rate of copper dissolution but deteriorated the copper/iron selectivity. The analysis for elemental sulfur of the solid leach residues indicated that most of the copper sulfide sulfur in the sulfidized concentrate oxidized to elemental sulfur. © 2006 Elsevier B.V. All rights reserved. Keywords: Pressure leaching; Covellite; Sulfidation; Chalcopyrite
1. Introduction Among the copper sulfides used to produce metallic copper, chalcopyrite (CuFeS2) is the predominant mineral. The concentrates of chalcopyrite are generally treated by pyrometallurgical methods of smelting and converting processes to win the copper metal. However, this high temperature route has always been plagued with environmental issues concerning pollution not only with SO2 gas emissions but also with emissions of toxic
⁎ Corresponding author. E-mail address: [email protected] (R. Padilla). 0304-386X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2006.10.006
compounds of arsenic, antimony, and other heavy metals. The alternative hydrometallurgical routes of direct leaching of chalcopyrite have had limited industrial application, mainly because the kinetics of chalcopyrite dissolution is extremely slow even in very strong oxidizing leaching media. Additionally, a selective extraction of copper over iron by leaching chalcopyrite in acidic media is not feasible since for every copper ion dissolved one iron ion passes also into solution. Furthermore, the precipitation of iron from chalcopyrite leaching solutions and the subsequent disposal of the precipitate poses a problem since the precipitates are potential contaminants and usually require expensive confining.
R. Padilla et al. / Hydrometallurgy 86 (2007) 80–88
1.1. Sulfidation of chalcopyrite In recent investigation from this laboratory (Padilla et al., 2003a,b), it was determined that chalcopyrite reacts with gaseous sulfur at moderate temperatures to transform its mineralogy to various simple sulfides which can be leached more easily than chalcopyrite. The reaction products found were mixtures of covellite and pyrite or idaite and pyrite depending on the sulfidation temperature. For the temperature range of 325 to 400 °C, the transformation of chalcopyrite proceeds as follows: CuFeS2 ðsÞ þ 0:5S2 ðgÞ ¼ CuSðsÞ þ FeS2 ðsÞ
ð1Þ
where the reaction product is a mixture of simple sulfide phases covellite and pyrite. Above 400 °C, the chalcopyrite transformation proceeds according to: 5CuFeS2 þ 2S2 ðgÞ ¼ Cu5 FeS6 ðsÞ þ 4FeS2 ðsÞ
ð2Þ
where the reaction product is a mixture of idaite and pyrite phases. Reaction (1) is the most interesting and desirable reaction for copper recovery since the chalcopyrite transforms into separated copper and iron sulfides. Therefore, in leaching a sulfidized chalcopyrite concentrate, the extraction of copper should be faster than from the nonsulfidized chalcopyrite because covellite is more reactive than chalcopyrite and it could also be selective because copper and iron are in separate phases. 2. Leaching of sulfidized chalcopyrite Few studies have been conducted on the leaching of chalcopyrite heat-treated with elemental sulfur. At ambient pressures, Subramanian and Kanduth (1973) conducted a limited study on the copper extraction from activated chalcopyrite concentrate (chalcopyrite concentrate reacted with liquid sulfur) by using oxygen, ferric sulfate or manganese dioxide as oxidant in sulfuric acid solutions. From their studies, they concluded that in the ferric sulfate leach of the activated (sulfidized) chalcopyrite a large concentration of ferric sulfate (near saturation) was necessary to obtain a rapid dissolution and high extraction of copper, while in the manganese dioxide leaching a large excess of MnO2 (200% excess) was necessary to obtain about 86% of copper extraction in 4 h. In these studies, the copper dissolution was not selective since the iron was co-dissolved in the order of about 40%. Recently, Padilla et al. (2003c) studied the leaching of sulfidized chalcopyrite also at ambient pressures in
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H2SO4–NaCl–O2. These investigators found that the copper dissolution in that media was rapid and reported copper extractions over 90% in less than 90 min at about 100 °C. Among the variables studied, they found that temperature and chloride ion concentration were the variables that influence most on the copper dissolution while the iron dissolution was reported as less than 6% in all tested conditions. Although the rate of copper dissolution in the sulfate–chloride–oxygen media is rapid and the iron dissolution is low, the presence of chloride ions complicates the solution purification before electrowinning of copper. This complication brings the need to study alternative methods to dissolve the copper from the sulfidized chalcopyrite. The leaching with oxygen overpressures is another alternative to accelerate the dissolution of metals from sulfides in sulfate media. On this matter, Warren et al. (1968) pretreated chalcopyrite with 10% elemental sulfur at 475 °C in a steel retort and leached the resulting product (sulfidized chalcopyrite) in sulfuric acid (one mol of acid per mole of copper) at 90 °C and 482 kPa oxygen partial pressure. They reported 90% copper and 27% iron extraction in 1 h of leaching. Subramanian and Kanduth (1973) conducted also a limited pressure leaching experiments of chalcopyrite concentrates treated with liquid sulfur. They concluded that the optimum leaching conditions for copper dissolution were 110 °C and 346 kPa oxygen. At lower temperatures, the rate of dissolution was poor and at higher temperatures, (128 °C) the reaction was slow presumably due to blockage by liquid sulfur film on the particle surface. In both studies, the iron dissolution accelerated when most of the copper was extracted. 3. Leaching chemistry of sulfidized chalcopyrite The sulfidation product obtained at 375 °C in this study is a mixture of covellite and pyrite, thus the following discussion will be concerned only with these two sulfides. The dissolution of covellite in sulfuric acid–oxygen system can proceed with the production of elemental sulfur or sulfate according to the following reactions (Cheng and Lawson, 1991; Lotens and Wesker, 1987). CuS þ 0:5O2 þ 2Hþ →Cu2þ þ So þ H2 O
ð3Þ
CuS þ 2O2 →Cu2þ þ SO2− 4
ð4Þ
We can notice that reaction (3) consumes acid and reaction (4) does not.
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Table 1 Chemical analysis of the chalcopyrite concentrates
4. Experimental work
Size fraction
Cu, %
Fe, %
S, %
4.1. Materials and procedure
Concentrate 1 106/75 μm 75/53 μm 53/45 μm 45/38 μm
32.1 32.4 32.2 34.2
28.6 30.5 31.3 28.3
34.8 33.8 34.6 35.7
Concentrate 2 53/45 μm
27.1
27.8
35.8
The experimental study was carried out using chalcopyrite concentrates from Andina Mine of CODELCO, Chile. These concentrates were products from flotation operations of copper sulfide minerals, which were further classified by size. Table 1 shows the chemical analysis of the two chalcopyrite concentrates used in this study, and Table 2 shows the mineralogical composition of both concentrates. As we can see in Table 2, concentrate 2 has lower content of chalcopyrite but it contains more pyrite and molybdenite than concentrate 1.
Similarly, the dissolution of pyrite has been typically represented by the following reactions (Lotens and Wesker, 1987; Long and Dixon, 2004). FeS2 þ 3:5O2 þ H2 O→Fe2þ þ 2Hþ þ 2SO2− 4
ð5Þ
4.2. Sulfidation of chalcopyrite
o FeS2 þ 2O2 →Fe2þ þ SO2− 4 þS
ð6Þ
The chalcopyrite concentrates were subjected to the sulfidizing process according to the methodology described by Padilla et al. (2003a). The procedure consisted mainly in contacting the chalcopyrite concentrate with sulfur gas at 375–400C °C for certain periods of time. In this investigation, 375 °C was chosen for the sulfidation reaction and the results are shown in Fig. 1, where the chalcopyrite conversion data is depicted as a function of time at this temperature. As we can see, after 90 min of reaction the chalcopyrite conversion was 97%. This conversion was calculated based on experimental weight gain of the chalcopyrite sample and the theoretical weight gain according to reaction (1), assuming all the copper in the concentrate is present as chalcopyrite. The morphology of the sulfidized chalcopyrite can be seen in Fig. 2, which shows a
Reaction (3) is the most desirable for the dissolution of copper from the sulfidized chalcopyrite because the sulfur in the covellite passes into the solid residues as elemental sulfur. However, in practice, the proportion of sulfur and sulfate formed will depend on the leaching conditions, mainly on the temperature. The leaching of CuS according to reactions (3) or (4) is very slow at atmospheric conditions (Cheng and Lawson, 1991); therefore, high temperatures and oxygen overpressures are required for the copper dissolution to occur at fast rates in sulfuric acid media. Considering the preceding discussion, the present investigation focused on the study of the pressure leaching of the sulfidized chalcopyrite concentrate in H2SO4–O2 media. The main objective was to determine the optimum conditions to extract the copper rapidly and selectively from the sulfidized chalcopyrite concentrate. Table 2 Mineralogical composition of chalcopyrite concentrates Species
Concentrate 1 Weight percent, %
Concentrate 2 Weight percent, %
Chalcopyrite Pyrite Bornite Chalcocite Covellite Tetrahedrite Molybdenite Sphalerite Enargite Magnetite Gangue
94.24 2.82 0.78 – 0.56 0.51 0.17 0.08 0.01 – 0.83
80.41 10.88 0.29 0.15 0.80 0.69 1.08 – 0.15 0.69 4.86
Fig. 1. Sulfidation of chalcopyrite concentrate 2 with gaseous sulfur at 375 °C as a function of time.
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analyzed for Cu and Fe and some solid residues were observed by microprobe and light microscopy. Additionally, some of the solid residues were also analyzed for elemental sulfur by the Soxhlet extraction method using carbon disulfide as the solvent for sulfur. 5. Results The main variables studied were the degree of agitation, concentration of sulfuric acid in the leaching solution, partial pressure of oxygen (304 to 1520 kPa), temperature (90 to 120 °C), and time of reaction. The results obtained are summarized in the following. Fig. 2. Light micrograph of sulfidized chalcopyrite particles showing the covellite phase surrounding the porous pyrite phase. Concentrate 2, particle size 53/45 μm.
micrograph of the sulfidized material obtained with a light microscope. In this micrograph, we can observe that the covellite phase was formed in the outer surface of the particle. The mechanism of formation of covellite during sulfidation involves a solid state diffusion of copper towards the external surface of the particle as reported by Padilla et al. (2003b). We observe also that the pyrite phase constitutes the inner part of the particles and it is extremely porous due to the large difference in molar volume between the pyrite (23.9 mL) and the chalcopyrite (43.7 mL) from which it forms. 4.3. Leaching experiments The leaching experiments of the sulfidized chalcopyrite were conducted in a 2 L Parr autoclave made of alloy 20 Cb. This autoclave was equipped with a heating mantle, a PID temperature controller, a variable speed stirrer and an internally mounted cooling coil. The stirring system had two axial impellers with 6 blades with 45° downdraft mounted 4.5 cm apart. All the experiments were carried out batch-wise using 2 g of sulfidized chalcopyrite, 1000 ml of leaching solution (various concentrations of H2SO4) and pure oxygen. The procedure consisted on heating the solution to 95 °C; once at this temperature, the solid sample was added and further heated to the temperature of the experiment. At the set temperature, the oxygen was admitted and the partial pressure of oxygen was adjusted to the desired value, which was maintained constant for the duration of the experiment. At the end of the experiment, the oxygen was shut down, the autoclave was rapidly water-cooled, and the solution was filtered for collecting the solid residues. The solution was
5.1. Effect of agitation The effect of the extent of agitation of the leaching pulp on the copper dissolution from the sulfidized chalcopyrite was studied under an oxygen partial pressure of 1013 kPa The result is shown in Fig. 3 where it can be seen that the stirring speed has an important effect on the dissolution of copper up to about 500 rpm. The stirring speed was maintained at 500 rpm which was considered to be appropriate to eliminate the effect of this variable. 5.2. Effect of sulfuric acid The effect of sulfuric acid on the leaching of sulfidized chalcopyrite was studied in the range 0.005 M to 0.2 M. Fig. 4 shows the dissolution of copper and iron as a function of concentration of sulfuric acid in the leaching solution. We can observe in this figure that the acid concentration affects the dissolution of copper up to about 0.1 M H2SO4. This result suggests
Fig. 3. Effect of agitation on the extraction of copper from sulfidized chalcopyrite. Concentrate 1, and size fraction 75/53 μm.
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Fig. 4. Effect of sulfuric acid concentration on the copper and iron dissolution from the sulfidized chalcopyrite. Concentrate 1, size fraction 75/53 μm.
Fig. 6. Effect of temperature on the copper dissolution from sulfidized chalcopyrite; conditions: 0.2 M H2SO4, 1013 kPa PO2, and 500 rpm. Concentrate 1, size fraction 53/45 μm.
that the dissolution of copper occurs primarily through reaction (3) with the production of elemental sulfur. This conclusion was confirmed by sulfur analysis of the residues of leaching, as discussed later. In the case of iron, the sulfuric acid concentration in the range studied does not have an effect on the dissolution.
1013 kPa. We can observe in Fig. 5 that the dissolution rate of concentrate 2 is affected little by changes in the temperature from 90 to 108 °C This result suggests that in this range of temperature, a diffusion process controls the overall dissolution reaction of copper. The same result is obtained in leaching concentrate 1 at oxygen pressure of 1013 kPa, except that the dissolution rate of copper is higher in this case due to the higher partial pressure of oxygen, as seen in Fig. 6. We can also see in these figures that at 120 °C, the dissolution of copper is lower than that obtained at the other temperatures. This evident drop of the copper dissolution at 120 °C is due to the formation of a liquid sulfur layer which covers the particle's surface thus inhibiting further dissolution of copper. This passivation effect was also observed by Subramanian and Kanduth (1973) during the pressure dissolution of activated (sulfidized) chalcopyrite concentrate. The iron dissolution from the sulfidized chalcopyrite can be seen in Fig. 7. We observe that temperature does have a pronounced effect on iron dissolution. Increasing temperature increases the iron dissolution. We can clearly see that the passivation effect of 120 °C on the copper dissolution cannot be observed in the case of iron.
5.3. Effect of temperature Sulfidized chalcopyrite samples of both concentrates 1 and 2 were leached at temperatures varying from 90 to 120 °C to determine the effect of temperature on the copper and iron dissolution. The results for the copper dissolution are shown in Fig. 5 for experiments carried out with oxygen partial pressure of 507 kPa and in Fig. 6 for the case of partial pressure of oxygen equal to
5.4. Effect of oxygen overpressure
Fig. 5. Effect of temperature on the copper dissolution from sulfidized chalcopyrite. 0.2 M H2SO4, 507 kPa PO2, and 500 rpm, concentrate 2, size fraction 53/45 μm.
The effect of partial pressure of oxygen on the copper and iron dissolution was studied using concentrates 1 and 2. Although both concentrates differ in copper and iron content, it was determined that the fraction of copper and iron extracted from each concentrate was the same under the same experimental conditions. The results can be seen in Fig. 8 for copper and Fig. 9 for
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Fig. 7. Effect of temperature on the iron dissolution from sulfidized chalcopyrite at 1013 kPa of oxygen overpressure for concentrate 1 of size fraction 53/45 μm.
iron dissolution. In Fig. 8, we can observe that the rate of copper dissolution depends significantly on the partial pressure of oxygen in the range 304 to1520 kPa. Increasing the partial pressure of oxygen 304 to 1013 kPa, a significant increase in the rate of copper dissolution is obtained. However, a further increase in the oxygen pressure from 1013 to 1520 kPa does not produce a substantial increase in the dissolution rate. The dissolution of iron also depends strongly on the partial pressure of oxygen as can be seen in Fig. 9, where we can observe that the dissolution of iron occurs at a very low rate at 304 kPa of oxygen. However, at 1013 kPa the rate of iron dissolution is fast and keeps
Fig. 8. Effect of partial pressure of oxygen on the dissolution of copper from sulfidized chalcopyrite. Conditions: 100 °C, 0.2 M H2SO4, 500 rpm. Concentrates 1 of size fraction 53/45 μm for PO2 = 507, 1013 and 1520 kPa, and concentrate 2 of size fraction 53/45 μm for PO2 = 304 kPa.
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Fig. 9. Effect of partial pressure of oxygen on the dissolution of iron from sulfidized chalcopyrite. Conditions: 100 °C, 0.2 M H2SO4, 500 rpm. Concentrates 1 of size fraction 53/45 μm for PO2 = 304, 1013, and 1520 kPa, and concentrate 2 of size fraction 53/45 μm for PO2 = 507 kPa.
increasing with increasing partial pressure of oxygen up to 1520 kPa. Consequently, according to these data for a selective dissolution of copper from sulfidized chalcopyrite the leaching should be carried out at low pressures, around 304 kPa. 5.5. Effect of particle size on the copper dissolution The effect of particle size on the copper dissolution is shown in Fig. 10. As we can see in this figure, the rate of reaction increases as the size of the particles decreases. This effect is due to the increase in the interfacial area of reaction as the solid particles become smaller. However,
Fig. 10. Effect of particle size of chalcopyrite on the copper dissolution from the sulfidized concentrate in 0.2 M sulfuric acid, 100 °C, and 1013 kPa of partial pressure of oxygen for concentrate 1.
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Fig. 11. Backscattered electron image (BEI) of a residue obtained in leaching sulfidized concentrate sample in 0.2 M H2SO4, 120 °C, 507 kPa of oxygen overpressure, and 60 min of leaching time. In the image: Cv is covellite (light color phase) and Py is pyrite (dark color, porous phase).
since the stirring speed for the experiments was 500 rpm, the large particles (106/75 μm size fraction) may not be fully suspended, which may explain in part the lowest dissolution rate shown in the figure. 5.6. Characterization of the residues from leaching Fig. 11 shows a backscattered electron image (BEI) of a partially reacted sample obtained in leaching at 120 °C, partial pressure of oxygen 507 kPa, for 60 min of leaching time. We can observe that the physical aspect of the particles did not change very much as compared to the non-leached sulfidized particles. The dense layer of covellite surrounding the porous pyrite can still be seen clearly in the image because the corresponding copper dissolution was only 51%. On the other hand, Fig. 12(a) shows a BEI image of a residue which was obtained in leaching at 100 °C for 240 min under oxygen overpressure of 507 kPa, with copper dissolution of 94%. Fig. 12(b), (c) and (d) show X-ray spectra for copper, iron, and sulfur respectively in the sample. The morphology and constituent phases of the residue can be seen clearly in this figure. In the BEI image (a), we can only see the porous pyrite phase as expected since for 94% conversion, the external covellite phase has all been dissolved. We can also see that the small amount of undissolved covellite is located in the interior pores of the pyrite as indicated by the copper content found in the spectrum (b). Image (a) also shows two primary (dense) pyrite particles which were
present in the original chalcopyrite concentrate and a metallic iron phase, which is a constituent of the used resin sample holder. Images (b) through (d) essentially confirm that the reacted particle does not show a covellite phase surrounding the porous pyrite phase which indicates almost complete copper dissolution as evidenced by spectrum (b). The sulfur spectrum shown in Fig. 12(d) does not show a coherent elemental sulfur layer surrounding the particle, it shows only the sulfur corresponding to the pyrite. To verify that the dissolution of copper from sulfidized chalcopyrite occurs through reaction (3), the solid residues from some of the experiments were analyzed for elemental sulfur by Soxhlet extraction method as mentioned earlier. The results are shown in Fig. 13, where the fraction of elemental sulfur produced from the sulfidized chalcopyrite is shown as a function of dissolved copper. We can observe in this figure that about 70% of the reacted sulfide sulfur is present in the solid residue as elemental sulfur. These data indicate that the pressure leaching of sulfidized chalcopyrite in sulfuric acid–oxygen for the conditions studied proceeds mainly through reaction (3) and to a lesser extent through reaction (4). In the overall framework of treating chalcopyrite concentrates by sulfidation and leaching, the leaching of the sulfidized concentrate in autoclave in sulfuric acid – oxygen system is one alternative that effectively extracts copper from the sulfidized concentrate at 100 °C and 1013 kPa. Even though the copper/iron selectivity was not good at these conditions, the pressure leaching of sulfidized chalcopyrite is a viable process to produce pregnant solutions that can be inserted in the conventional industrial lines of SX purification and EW. 6. Conclusions From the results presented the following can be concluded: – Sulfuric acid concentration in the solution over 0.1 M had very little effect on the leaching of sulfidized chalcopyrite in sulfuric acid–oxygen in autoclave. – An increase in leaching temperature from 90 to 108 °C, increased the copper and iron dissolution. Increasing the temperature further to 120 °C has deleterious effect on the copper dissolution. – An increase in the partial pressure of oxygen from 304 to 1520 kPa increased significantly the copper and iron dissolution. The oxygen partial pressure is the variable that controls the selectivity of Cu/Fe dissolution.
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Fig. 12. (a) Backscattered electron image (BEI) showing porous pyrite particle in the residue and primary pyrite from the chalcopyrite concentrate; (b) X-ray spectrum for copper; (c) X-ray spectrum for iron; (d) X-ray spectrum for sulfur.
– The pressure leaching in sulfuric acid–oxygen media is a viable method to extract copper from sulfidized chalcopyrite concentrates producing solutions to be treated by SX and EW. – Selective copper dissolution from sulfidized chalcopyrite can be obtained at 100 °C and oxygen overpressures of about 304 kPa in less than 3 h of residence time. However, for longer times, iron extraction increases rapidly and selectivity decreases. Acknowledgements
Fig. 13. Fraction of elemental sulfur produced in the pressure leaching of sulfidized chalcopyrite at 108 °C, 507 kPa of oxygen partial pressure.
The authors acknowledge The National Fund for Scientific and Technological Development (FONDECYT) of Chile for the financial support of this study through Project No. 1050948 and the Research Council
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of the University of Concepción through Project No. DI 203.095.081-1.0. References Cheng, C., Lawson, Y., 1991. The kinetics of leaching covellite in acidic oxygenated sulphate-chloride solutions. Hydrometallurgy 27, 269–284. Long, H., Dixon, D.G., 2004. Pressure oxidation of pyrite in sulfuric acid media: A kinetic study. Hydrometallurgy 73, 335–349. Lotens, J.P., Wesker, E., 1987. The behaviour of sulphur in the oxidative leaching of sulphidic minerals. Hydrometallurgy 18, 39–54.
Padilla, R., Rodríguez, M., Ruiz, M.C., 2003a. Sulfidation of chalcopyrite with elemental sulfur. Metall. Mater. Trans. 34B, 15–23. Padilla, R., Olivares, E., Ruiz, M.C., 2003b. Kinetic of the sulfidation of chalcopyrite with gaseous sulfur. Metall. Mater. Trans. 34B, 61–68. Padilla, R., Zambrano, P., Ruiz, M.C., 2003c. Leaching of sulfidized chalcopyrite with H2SO4–NaCl–O2. Metall. Mater. Trans., B, Proc. Metall. Mater. Proc. Sci. 34B, 153–159. Subramanian, K.N., Kanduth, H., 1973. Activation and leaching of chalcopyrite concentrates. CIM Bull. 66, 88–91. Warren, H.I., Vizsolyi, A., Forward, F.A., 1968. The pretreatment and leaching of chalcopyrite. CIM Bull. 5, 637–640.