- Email: [email protected]
Kinetics of copper dissolution in the purification of molybdenite concentrates by sulfidation and leaching
Hydrometallurgy 137 (2013) 78–83
Contents lists available at SciVerse ScienceDirect
Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Kinetics of copper dissolution in the purification of molybdenite concentrates by sulfidation and leaching Rafael Padilla ⁎, Hugo Letelier, Maria C. Ruiz Department of Metallurgical Engineering, University of Concepcion, Edmundo Larenas 285, Concepcion 4070371, Chile
a r t i c l e
i n f o
Article history: Received 26 February 2013 Received in revised form 9 May 2013 Accepted 25 May 2013 Available online 31 May 2013 Keywords: Molybdenite purification Sulfidation leaching Chalcopyrite Shrinking core kinetics
a b s t r a c t Molybdenum is found in low quantities in copper porphyries as the mineral molybdenite. In the processing of these ores, a bulk copper-molybdenum concentrate is produced, from which molybdenite is separated by differential flotation. However, complete separation of molybdenum from copper by flotation is difficult, and molybdenum concentrates produced by this route require a further step of chemical purification to reduce their copper content. This paper is concerned with a novel process of chemical purification of molybdenite concentrate containing 3.4% Cu and 3.04% Fe where chalcopyrite is the main impurity. The process involves a sulfidation of the molybdenite concentrate with S2(g) at 380 °C followed by a leaching stage with H2SO4–NaCl–O2. The leaching of the sulfidized molybdenite concentrate for 90 min at 100 °C dissolved 96% of the copper which yielded a final concentrate with less than 0.2% Cu. Molybdenite dissolution was negligible in this leaching media, and less than 20% Fe was dissolved. The kinetics of copper dissolution from the sulfidized concentrate was analyzed using the shrinking core model for chemical reaction control. Activation energy of 52.5 kJ/mol was calculated for the temperature range of 70 °C–100 °C. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Molybdenum is a valuable by-product in the processing of porphyry copper deposits, where the molybdenum is present in low quantities in the form of molybdenite (MoS2). In the conventional processing of copper sulfide ores by froth flotation, most of the molybdenite in the ore floats along with the copper sulfides, and thus a bulk copper-molybdenum concentrate is produced. Afterwards, the bulk copper–molybdenum concentrate is treated by differential flotation to obtain a rougher molybdenum concentrate, which requires several successive cleaning steps to produce a final molybdenum concentrate. However, it is widely recognized that flotation alone cannot produce economically a high grade molybdenite concentrate, i.e., with less than 0.5% Cu. Therefore, the final flotation molybdenite concentrates must be further purified by leaching to lower the level of impurities. The copper content in the molybdenite concentrates is of primary concern because the steel industry is the major consumer of the molybdenum produced from molybdenite. Therefore, the molybdenum compounds produced for the steel industry should contain very little copper to minimize the deleterious effect of this impurity on the physical properties of the resulting alloyed steels (Kim et al., 2008). Common methods of chemical purification of molybdenite concentrates used by the copper industry include ferric chloride leaching, hydrochloric acid leaching and cyanide leaching (Dorfler and Laferty, ⁎ Corresponding author. Tel.: +56 41 2203666; fax: +56 41 2243418. E-mail address: [email protected] (R. Padilla). 0304-386X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hydromet.2013.05.012
1981; Gupta, 1992). In practice, these processes have shown some operational problems related to severe corrosion and/or environmental pollution problems. Due to these problems, various other leaching processes have been studied in an attempt to overcome the shortcomings of the industrial processes (Askari Zamani et al., 2006; Prasad et al., 2007; Ruiz and Padilla, 1998). This investigation focused on an alternative method for removing copper from molybdenite concentrates when the copper impurity is in the form of refractory chalcopyrite. The process consists of a sulfidation step to transform the chalcopyrite impurity into a mixture of covellite and pyrite, followed by a leaching step to dissolve the covellite. Because covellite is a much more reactive mineral than the original chalcopyrite, the leaching step of the sulfidized concentrate can be carried out under less aggressive conditions than those required for chalcopyrite dissolution, which would result in a reduction of molybdenum dissolution in the process and simpler recovery of the copper from the leaching solution. The reaction of chalcopyrite with elemental sulfur to transform the chalcopyrite into more reactive sulfides at temperatures in the range of 325 °C–500 °C has been investigated previously as an alternative route to treat the refractory chalcopyrite mineral (Warren et al., 1968); Subramanian and Kanduth, 1973; Demopoulos and Distin, 1983; Adam and Neumeier, 1987; Neumeier and Adam, 1990; Adam et al., 1994; Padilla et al., 2003a, 2003b). From these studies, it has become clear that when chalcopyrite was heated in the presence of gaseous sulfur a rapid transformation of its mineralogy occurred forming separate phases of covellite and pyrite at temperatures below 400 °C (Adam and Neumeier, 1987; Neumeier and
R. Padilla et al. / Hydrometallurgy 137 (2013) 78–83
79
Adam, 1990; Padilla et al., 2003a, 2003b) according to the following reaction: CuFeS2 þ 0:5S2 ðgÞ ¼ CuS þ FeS2
ð1Þ
On the other hand, if the sulfidation were carried out at higher temperatures, the mineral phases produced were idaite (Cu5FeS6) and pyrite. Acid leaching studies of sulfidized chalcopyrite were carried out by using different oxidizing agents such as oxygen under pressure (Padilla et al., 2007; Subramanian and Kanduth, 1973; Warren et al., 1968), ferric sulfate (Subramanian and Kanduth, 1973), and cupric and ferric chlorides (Demopoulos and Distin, 1983; Jolly and Neumeier, 1991). In ambient pressure leaching, Padilla et al. (2003c) investigated the use of H2SO4–NaCl–O2 media to dissolve the copper from sulfidized chalcopyrite. It was reported that the presence of chloride ions in the solution accelerated significantly the rate of reaction in comparison to pure sulfuric acid media, thus allowing the effective dissolution of the copper at atmospheric pressure according to the following reaction:
Fig. 1. Micrograph (200×) of the molybdenite concentrate sample showing several chalcopyrite particles (yellow color) of different sizes. The elongated particles (light to dark gray color) are molybdenite.
Based on these results, Padilla et al. (2013) conducted a preliminary evaluation of the applicability of the sulfidation and leaching process as a way to remove the copper from molybdenite concentrates obtained in the treatment of porphyry copper ores containing molybdenum. Considering the previous statements, the main objective of this research was to further assess the technical feasibility of this process to selectively remove copper from molybdenite concentrates and to determine the kinetic parameters of copper removal in the leaching of sulfidized molybdenite concentrates with H2SO4–NaCl–O2 at atmospheric pressure.
flow. Once the set temperature was reached, a glass boat containing about 5 g of the molybdenite concentrate and a separate glass boat containing elemental sulfur were introduced in the furnace tube to the region of constant temperature, and after closing the tube, the nitrogen flow was shut. At the end of the experiment, the system was flushed with nitrogen and the reacted sample was withdrawn and cooled under a slow flow of nitrogen. When the sample was cold, it was weighed and stored. Inasmuch as the molybdenite particles did not react with the gaseous sulfur and remained unchanged, the extent of sulfidation of the concentrate was calculated from the weight gain of the samples according to the stoichiometry of reaction (1), assuming that all the copper in the molybdenite concentrate was present as chalcopyrite.
2. Experimental work
2.3. Leaching methodology
2.1. Materials
The leaching of the sulfidized concentrate was carried out at ambient pressure in H2SO4–NaCl–O2. The leaching reactor was a glass vessel equipped with a variable speed mechanical stirrer, a water condenser, a system for oxygen injection into the solution and a solution sampling device. The procedure consisted of heating 1 L of the leaching solution to a desired temperature in a slow flow of oxygen. When the temperature reached the set value, a sample of 3 g of sulfidized sample was introduced rapidly in the reactor, and the agitation and flow of oxygen were adjusted to the determined values for the experiment. During the experiments, various solution samples were withdrawn at time intervals, and when the reaction time was completed, the solution was filtered. The sampled solutions were analyzed by atomic absorption spectroscopy.
þ
−
CuS þ 1=2O2 þ 2H þ Cl →Cu
2þ
o
−
þ S þ H2 O þ Cl
ð2Þ
The molybdenite concentrate used in this research was obtained from the flotation plant of El Teniente Division of CODELCO, Chile. This concentrate sample was a fine material (100%–38 μm), and it contained a substantial amount of organic flotation reagents on the particle's surface; therefore, the concentrate was firstly heat treated in nitrogen atmosphere to remove the organic reagents, and afterwards, it was washed in dilute sulfuric acid solution to remove the surface oxides that may have formed during the handling of the sample. The molybdenite concentrate thus prepared was analyzed for Cu and Fe. The results indicated that the concentrate contained 3.4% Cu and 3.04% Fe. The microscopic analysis showed that the concentrate contained 83.4% MoS2 (which represents 50% Mo), and the major copper mineral present was chalcopyrite with very small amounts of chalcocite, covellite, and bornite. The X-ray diffraction analysis (XRD) also showed that molybdenite and chalcopyrite were the main components of the concentrate. Fig. 1 shows a micrograph of the molybdenite concentrate, where several chalcopyrite particles of different sizes were identified among the prevalent molybdenite particles. 2.2. Sulfidation methodology The sulfidation of the molybdenite concentrate was the first step in the purification process studied, and it was carried out according to the methodology described by Padilla et al. (2003a). The method consisted basically of contacting a sample of molybdenite concentrate with gaseous sulfur in a horizontal tube furnace at temperatures below 400 °C. The procedure followed for each experiment consisted of heating the reaction tube to the set temperature under slow N2
3. Results 3.1. Sulfidation of the molybdenite concentrate The sulfidation results obtained at 360 °C and 380 °C are presented in Table 1, where it can be observed that the conversion Table 1 Chalcopyrite sulfidation in the molybdenite concentrate. Time (min)
Conversion (%) 360 °C
Conversion (%) 380 °C
20 40 60 70 80
42.9 76.7 91.7 94.7 96.5
50.5 82.2 95.1 96.8 97.8
80
R. Padilla et al. / Hydrometallurgy 137 (2013) 78–83
of chalcopyrite in sulfidation at 380 °C was higher than 95% after 60 min. Therefore, for the subsequent leaching experiments, the molybdenite concentrate was sulfidized at 380 °C and 60 min. The sulfidation product was characterized by XRD spectroscopy, and the result showed strong diffraction peaks for molybdenite and peaks for covellite and pyrite. This result was confirmed by microscopic analysis as illustrated in Fig. 2, where we can observe clearly that the chalcopyrite originally present in the concentrate was transformed into covellite and pyrite. As can be seen in Fig. 2, a large part of the covellite formed by sulfidation is located on the surface of the newly formed porous pyrite. This covellite product did not form a compact protective layer over the formed pyrite center as was observed earlier by Padilla et al. (2003b). The formation of this discontinuous layer of covellite in the present research could be due to the small size of the original chalcopyrite particles in the molybdenite concentrate, i.e., due to their high surface to mass ratio. A significant portion of the covellite was also observed inside the porous structure of the pyrite. On the other hand, the MoS2 particles in the concentrate remained unchanged during the sulfidation stage. 3.2. Leaching of the sulfidized concentrate As already discussed, the copper mineral species to be dissolved in the leaching is covellite, which should be much more amenable to leaching than the original chalcopyrite. The effect of the main leaching variables on the dissolution rate of copper from the sulfidized concentrate was studied. In some cases, the extent of the dissolution of iron and molybdenum was also determined. Preliminary experiments were carried out using stirring speeds in the range of 200–700 rpm. The results are presented in Table 2, which shows that stirring speeds over 400 rpm affects little the copper dissolution from the sulfidized concentrate, indicating that a proper suspension of the particles occurs for stirring speeds over 400 rpm. Thus, a stirring of 620 rpm was used in the subsequent experiments to assure independency of this variable. 3.3. Effect of the oxygen flow rate The effect of the oxygen flow rate on the copper dissolution from the molybdenite concentrate was studied at 100 °C and 60 min in a solution containing NaCl and H2SO4 in concentration of 1.0 M and 0.6 M, respectively, with stirring speed of 620 rpm. The copper dissolutions data obtained for oxygen flow rates in the range of 0.1–1.6 L/min are shown in Fig. 3. It can be observed in Fig. 3 that oxygen flow rates over about 0.4 L/min did not affect the dissolution
Fig. 2. Micrographs of molybdenite concentrate sample sulfidized at 380 °C for 60 min, showing transformed chalcopyrite particles (porous, blue-light yellow color particles). Other particles are molybdenite (grey color) and original pyrite (light solid yellow color).
Table 2 Effect of stirring speed on copper dissolution. Stirring speed (rpm)
Copper dissolution (%)
200 300 400 620 700
71.49 82.75 85.72 86.01 84.56
of copper from the molybdenite concentrate. This indicates that over 0.4 L/min the mass transfer from the gas to the liquid will not be rate controlling. Thus, 1 L/min was used in all subsequent experiments. 3.4. Effect of the concentration of sulfuric acid To study the effect of the concentration of sulfuric acid on the copper dissolution, several experiments were carried out using leaching solutions containing H2SO4 in the molar range of 0.1–0.6. The results are shown in Fig. 4, where one can see that for sulfuric acid concentrations over 0.4 M, the copper dissolution rate was nearly independent on the acid concentration, while below 0.4 M, the concentration of sulfuric acid has a clear effect on the rate of copper dissolution. It can also be observed in Fig. 4 that in all the range studied the concentration of sulfuric acid did not affect the dissolution of iron and in 2 h of leaching about 20% of the iron in the concentrate was dissolved. The subsequent leaching experiments were conducted using a molar concentration of sulfuric acid equal to 0.6 to assure that the copper dissolution was independent of this variable. 3.5. Effect of the concentration of sodium chloride Fig. 5 shows the effect of sodium chloride concentration on the rate of copper dissolution. It can be observed in Fig. 5 that in leaching the sulfidized concentrate with only sulfuric acid–oxygen the dissolution of copper is low even at 2 h of leaching time. However, when sodium chloride is added to the leaching media, the dissolution rate of copper improves drastically. It is also interesting to note that an increment in the concentration of sodium chloride over 0.6 M NaCl is not necessary since over this concentration value the copper dissolution rate does not improve any longer. 3.6. Effect of the temperature on the copper dissolution Leaching experiments were carried out in the temperature range of 70 °C–100 °C. The other leaching variables were kept constant at
Fig. 3. Effect of oxygen flow rate on copper dissolution from sulfidized molybdenite concentrate for the conditions shown in the figure and agitation of 620 rpm.
R. Padilla et al. / Hydrometallurgy 137 (2013) 78–83
Fig. 4. Effect of sulfuric acid on the copper dissolution from sulfidized molybdenite concentrate. Conditions 0.6 M NaCl, 100 °C, and 600 rpm, 1 L/min O2.
0.6 M H2SO4, 1 M NaCl, 1 L/min O2, and 620 rpm. These variable values fall in the range in which they have little effect on the leaching rate. The results are presented in Fig. 6, where it can be observed that the temperature has an important effect on the copper dissolution rate. As seen in Fig. 6, leaching at 100 °C dissolved as much as twice copper from the sulfidized molybdenite concentrate than a leaching at 70 °C. This large effect of temperature on the rate of copper dissolution suggests that the dissolution reaction is controlled by chemical reaction.
81
Fig. 6. Dissolution of copper from sulfidized molybdenite concentrates in H2SO4–NaCl–O2 solution for the temperature range of 70 °C–100 °C. Leaching conditions were 0.6 M H2SO4, 1 M NaCl, 1 L/min O2, 620 rpm.
shrinking core model with chemical reaction control, which in its integrated form is given by the following equation (Levenspiel, 1998; Wadsworth, 1979) 1=3
1−ð1−XÞ
¼ kt
ð3Þ
Because of the fact that the leaching experiments in this research were carried out at high stirring speeds and high oxygen flow rates, mass transfer effects were not considered to be a limiting factor. Thus, it can be assumed that the controlling step of the overall leaching rate of the covellite particles is a surface chemical reaction, as indicated before on discussing the large effect of temperature on the copper dissolution rate. Regarding the physical structure of the pyrite and covellite phases formed in the sulfidation stage (shown in Fig. 2), it is clear that the leaching reagents could easily diffuse through the porous pyrite particles; consequently, the entire surface of the covellite particles, regardless of their location, would be exposed to the leaching reagents. Taking this into consideration, the dissolution rate of the covellite would be represented well by a
In Eq. (3), k is a global rate constant and X is the fraction of covellite reacted. Thus, a plot of the left hand side of Eq. (3) versus t should result in a straight line. In Fig. 7, we can observe that the plot of 1 − (1 − X)1/3 vs. t for the copper dissolution data shown in Fig. 6 does display a straight line behavior confirming the adequacy of the kinetic Eq. (3). The values of the global kinetic constants obtained from Fig. 7 were used to draw an Arrhenius plot, as shown in Fig. 8. As can be seen in Fig. 8, the rate constant show a linear dependency with the temperature, and the activation energy value calculated from the slope of the line was 52.5 kJ/mol. This activation energy value is typical for a process controlled by a surface chemical reaction. The use of the shrinking core kinetic model allows for the determination of the dependence of the rate of copper dissolution on the sulfuric acid concentration, and the chloride ions concentration from the experimental data shown in Figs. 4 and 5. Thus, plots of ln(k) versus ln[H2SO4] and ln(k) versus ln[NaCl] were drawn, which are shown in Figs. 9 and 10, respectively. As can be seen, both plots show excellent linear behavior. The value of the slope in Fig. 9 indicates that
Fig. 5. Effect of the concentration of NaCl on the dissolution of copper from a sulfidized molybdenite concentrate. Leaching conditions: 100 °C, 0.6 M H2SO4, 620 rpm, 1 L/min O2.
Fig. 7. Chemical control kinetics for copper dissolution from molybdenite concentrate.
3.7. Kinetics of copper dissolution from the sulfidized concentrate
82
R. Padilla et al. / Hydrometallurgy 137 (2013) 78–83
Fig. 8. Arrhenius plot for the dissolution of copper from sulfidized molybdenite concentrates with H2SO4–NaCl–O2, for the temperature range of 70 °C–100 °C.
Fig. 10. Rate constant as a function of the chloride ions in solution.
the rate of copper dissolution is of 1/4 order with respect to the concentration of sulfuric acid, for concentrations up to 0.4 M. On the other hand, the slope in Fig. 10 indicated a 0.9 dependency of the rate with respect to the concentration of chloride ions in the solution for concentrations up to 0.6 M. Over the indicated concentration limits, both variables have no further effect on the copper dissolution rate.
electrowinning technology to produce copper cathodes, and the solution could be recycled to leaching without further treatment. In the case of plants that use chloride ions in their heap leaching operations, it could be more convenient to mix the solutions generated in the molybdenite purification process with the heap leaching solutions to be treated together.
3.8. Selectivity in leaching 4. Conclusions Fig. 11 compares the dissolution of copper, iron and molybdenum from the sulfidized concentrate in the optimum conditions for covellite dissolution. We observe in Fig. 11 that the sulfuric acid–sodium chloride–oxygen leaching media is very effective and selective for the dissolution of copper over molybdenum, which shows a negligible dissolution. On the other hand, iron is dissolved only partially in this leaching media. Nevertheless, the process studied allows a very effective elimination of copper from molybdenite concentrates without dissolving molybdenum. Final molybdenite concentrate with less than 0.2% Cu could be obtained by this method, as shown in Fig. 12, where the copper and iron content in the leaching residue is shown for two leaching conditions. This result corroborates the technical feasibility of removing copper selectively from molybdenite concentrates when the copper impurity is present as chalcopyrite. Is should be point out that the leaching solutions produced by this method could be treated by the conventional solvent extraction and
The proposed method of sulfidation and leaching can be used effectively for the removal of copper from molybdenite concentrates when the copper impurity is in the form of refractory chalcopyrite mineral, which is the case for the majority of Chilean molybdenite concentrates. The leaching of the sulfidized molybdenite concentrate in sulfate– chloride–oxygen media for 90 min at 100 °C dissolved 96% of the copper which yielded a final concentrate with less than 0.2% Cu. Under these leaching conditions, the molybdenum dissolution was negligible and iron dissolution was lower than 20%. The kinetics of the covellite dissolution from the sulfidized molybdenite concentrates was analyzed by using the shrinking core model for chemical reaction control 1 − (1 − X)1/3 = kt. The calculated activation energy was 52.5 kJ/mol for the range of 70 °C–100 °C. The global rate constant was found to be of 1/4 order with respect to H2SO4 up to 0.4 M, and 0.9 order with respect to NaCl up to 0.6 M.
Fig. 9. Dissolution rate constant as a function of the concentration of sulfuric acid.
Fig. 11. Copper, iron, and molybdenum dissolution from sulfidized molybdenite concentrate in the optimum leaching conditions of 100 °C, 0.6 M H2SO4, 0.6 M NaCl, 620 rpm, 1 L/min O2.
R. Padilla et al. / Hydrometallurgy 137 (2013) 78–83
83
References
Fig. 12. Copper and iron elimination from molybdenite concentrate by sulfidation and leaching of the sulfidized concentrate in H2SO4–NaCl–O2 media.
For higher concentrations the rate constant was independent of both variables. Finally, from the view point of practical application, the sulfidation and leaching method for molybdenite concentrate purification would alleviate some of the difficulties, concerning pollution and corrosion, encountered in the traditional processes used for copper elimination from molybdenite concentrate. In addition, the copper in the leaching solution could be conveniently recovered by solvent extraction and electrowinning technology, allowing the recycle of the solution to the leaching operation.
Acknowledgments The authors would like to thank the National Fund for Scientific and Technological Development (FONDECYT) of Chile for the financial support of this study through project no. 1110590.
Adam, M.J., Neumeier, L.A., 1987. Partitioning of chalcopyrite by sulfidation. In: Vassiliou, A.H., Hausen, D.M., Carson, D.J.T. (Eds.), Process Mineralogy VII. Metall, Soc. AIME, pp. 511–523. Adam, M.J., Swallow, K.A., Stephenson, J.B., 1994. In: Petruk, W., Rule, A.R. (Eds.), Process Mineralogy XII. The Minerals, Metals & Materials Society, pp. 287–296. Askari Zamani, M.A., Vaghar, R., Oliazadeh, M., 2006. Selective copper dissolution during bioleaching of molybdenite concentrates. Int. J. Miner. Process. 81, 105–112. Demopoulos, G.P., Distin, P.A., 1983. Ferric chloride leaching of sulphidized chalcopyrite. Hydrometallurgy 10, 111–122. Dorfler, R.R., Laferty, J.M., 1981. Review of molybdenum recovery processes. JOM 5, 48–54. Gupta, C.K., 1992. Extractive Metallurgy of Molybdenum, first ed. CRC Press, Boca Raton. Jolly III, F.A., Neumeier, L.A., 1991. Leaching sulfidation-partitioned chalcopyrite to selectively recover copper. Report of Investigations.U.S. Bureau of Mines 9343. Kim, B.S., Jha, M.K., Jeong, J., Lee, J.C., 2008. Leaching of impurities for the up-gradation of molybdenum oxide and cementation of copper by scrap iron. Int. J. Miner. Process. 88, 7–12. Levenspiel, O., 1998. Chemical Reaction Engineering. John Wiley & Sons, New York. Neumeier, L.A., Adam, M.J., 1990. In: Petruk, W., Hagni, R.D. (Eds.), Process Mineralogy IX. The Minerals, Metals & Materials Society, pp. 311–322. Padilla, R., Rodriguez, M., Ruiz, M.C., 2003a. Sulfidation of chalcopyrite with elemental sulfur. Metall. Mater. Trans. B 34, 15–23. Padilla, R., Olivares, E., Ruiz, M.C., Sohn, H.Y., 2003b. Kinetics of the sulfidation of chalcopyrite with gaseous sulfur. Metall. Mater. Trans. B 34, 61–68. Padilla, R., Zambrano, P., Ruiz, M.C., 2003c. Leaching of sulfidized chalcopyrite with H2SO4–NaCl–O2. Metall. Mater. Trans. B 34, 153–159. Padilla, R., Vega, D., Ruiz, M.C., 2007. Pressure leahing of sulfidizedChalcopyrite in sulfuricacid-oxygen media. Hydrometallurgy 86, 80–88. Padilla, R., Letelier, H., Ruiz, M.C., 2013. Copper removal form molybdenite by sulfidation-leaching process. In: Zhang, L., Allanore, A., Wang, C., Yurko, J.A., Crapps, J. (Eds.), Materials Processing Fundamentals. John Wiley & Sons, Inc., Hoboken, NJ, USA. http://dx.doi.org/10.1002/9781118662199.ch24. Prasad, P.M., Balasubramanian, K., Mukkopadyay, R., 2007. Separation chemistry in the refining of an off-grade molybdenite concentrate by leaching with an acid mix. Miner. Metal Proc. 24 (2), 97–104. Ruiz, M.C., Padilla, R., 1998. Copper removal from molybdenite concentrates by sodium dichromate leaching. Hydrometallurgy 48, 313–325. Subramanian, K.N., Kanduth, H., 1973. Activation and leaching of chalcopyrite concentrates. CIM Bull. 66, 88–91. Wadsworth, M.E., 1979. Hydrometallurgical processes. In: Sohn, H.Y., Wadsworth, M.E. (Eds.), Rate Processes of Extractive Metallurgy. Plenum Press, New York, pp. 133–153. Warren, H.I., Vizsolyi, A., Forward, F.A., 1968. The pretreatment and leaching of chalcopyrite. CIM Bull. 5, 637–640.
Contents lists available at SciVerse ScienceDirect
Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet
Kinetics of copper dissolution in the purification of molybdenite concentrates by sulfidation and leaching Rafael Padilla ⁎, Hugo Letelier, Maria C. Ruiz Department of Metallurgical Engineering, University of Concepcion, Edmundo Larenas 285, Concepcion 4070371, Chile
a r t i c l e
i n f o
Article history: Received 26 February 2013 Received in revised form 9 May 2013 Accepted 25 May 2013 Available online 31 May 2013 Keywords: Molybdenite purification Sulfidation leaching Chalcopyrite Shrinking core kinetics
a b s t r a c t Molybdenum is found in low quantities in copper porphyries as the mineral molybdenite. In the processing of these ores, a bulk copper-molybdenum concentrate is produced, from which molybdenite is separated by differential flotation. However, complete separation of molybdenum from copper by flotation is difficult, and molybdenum concentrates produced by this route require a further step of chemical purification to reduce their copper content. This paper is concerned with a novel process of chemical purification of molybdenite concentrate containing 3.4% Cu and 3.04% Fe where chalcopyrite is the main impurity. The process involves a sulfidation of the molybdenite concentrate with S2(g) at 380 °C followed by a leaching stage with H2SO4–NaCl–O2. The leaching of the sulfidized molybdenite concentrate for 90 min at 100 °C dissolved 96% of the copper which yielded a final concentrate with less than 0.2% Cu. Molybdenite dissolution was negligible in this leaching media, and less than 20% Fe was dissolved. The kinetics of copper dissolution from the sulfidized concentrate was analyzed using the shrinking core model for chemical reaction control. Activation energy of 52.5 kJ/mol was calculated for the temperature range of 70 °C–100 °C. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Molybdenum is a valuable by-product in the processing of porphyry copper deposits, where the molybdenum is present in low quantities in the form of molybdenite (MoS2). In the conventional processing of copper sulfide ores by froth flotation, most of the molybdenite in the ore floats along with the copper sulfides, and thus a bulk copper-molybdenum concentrate is produced. Afterwards, the bulk copper–molybdenum concentrate is treated by differential flotation to obtain a rougher molybdenum concentrate, which requires several successive cleaning steps to produce a final molybdenum concentrate. However, it is widely recognized that flotation alone cannot produce economically a high grade molybdenite concentrate, i.e., with less than 0.5% Cu. Therefore, the final flotation molybdenite concentrates must be further purified by leaching to lower the level of impurities. The copper content in the molybdenite concentrates is of primary concern because the steel industry is the major consumer of the molybdenum produced from molybdenite. Therefore, the molybdenum compounds produced for the steel industry should contain very little copper to minimize the deleterious effect of this impurity on the physical properties of the resulting alloyed steels (Kim et al., 2008). Common methods of chemical purification of molybdenite concentrates used by the copper industry include ferric chloride leaching, hydrochloric acid leaching and cyanide leaching (Dorfler and Laferty, ⁎ Corresponding author. Tel.: +56 41 2203666; fax: +56 41 2243418. E-mail address: [email protected] (R. Padilla). 0304-386X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.hydromet.2013.05.012
1981; Gupta, 1992). In practice, these processes have shown some operational problems related to severe corrosion and/or environmental pollution problems. Due to these problems, various other leaching processes have been studied in an attempt to overcome the shortcomings of the industrial processes (Askari Zamani et al., 2006; Prasad et al., 2007; Ruiz and Padilla, 1998). This investigation focused on an alternative method for removing copper from molybdenite concentrates when the copper impurity is in the form of refractory chalcopyrite. The process consists of a sulfidation step to transform the chalcopyrite impurity into a mixture of covellite and pyrite, followed by a leaching step to dissolve the covellite. Because covellite is a much more reactive mineral than the original chalcopyrite, the leaching step of the sulfidized concentrate can be carried out under less aggressive conditions than those required for chalcopyrite dissolution, which would result in a reduction of molybdenum dissolution in the process and simpler recovery of the copper from the leaching solution. The reaction of chalcopyrite with elemental sulfur to transform the chalcopyrite into more reactive sulfides at temperatures in the range of 325 °C–500 °C has been investigated previously as an alternative route to treat the refractory chalcopyrite mineral (Warren et al., 1968); Subramanian and Kanduth, 1973; Demopoulos and Distin, 1983; Adam and Neumeier, 1987; Neumeier and Adam, 1990; Adam et al., 1994; Padilla et al., 2003a, 2003b). From these studies, it has become clear that when chalcopyrite was heated in the presence of gaseous sulfur a rapid transformation of its mineralogy occurred forming separate phases of covellite and pyrite at temperatures below 400 °C (Adam and Neumeier, 1987; Neumeier and
R. Padilla et al. / Hydrometallurgy 137 (2013) 78–83
79
Adam, 1990; Padilla et al., 2003a, 2003b) according to the following reaction: CuFeS2 þ 0:5S2 ðgÞ ¼ CuS þ FeS2
ð1Þ
On the other hand, if the sulfidation were carried out at higher temperatures, the mineral phases produced were idaite (Cu5FeS6) and pyrite. Acid leaching studies of sulfidized chalcopyrite were carried out by using different oxidizing agents such as oxygen under pressure (Padilla et al., 2007; Subramanian and Kanduth, 1973; Warren et al., 1968), ferric sulfate (Subramanian and Kanduth, 1973), and cupric and ferric chlorides (Demopoulos and Distin, 1983; Jolly and Neumeier, 1991). In ambient pressure leaching, Padilla et al. (2003c) investigated the use of H2SO4–NaCl–O2 media to dissolve the copper from sulfidized chalcopyrite. It was reported that the presence of chloride ions in the solution accelerated significantly the rate of reaction in comparison to pure sulfuric acid media, thus allowing the effective dissolution of the copper at atmospheric pressure according to the following reaction:
Fig. 1. Micrograph (200×) of the molybdenite concentrate sample showing several chalcopyrite particles (yellow color) of different sizes. The elongated particles (light to dark gray color) are molybdenite.
Based on these results, Padilla et al. (2013) conducted a preliminary evaluation of the applicability of the sulfidation and leaching process as a way to remove the copper from molybdenite concentrates obtained in the treatment of porphyry copper ores containing molybdenum. Considering the previous statements, the main objective of this research was to further assess the technical feasibility of this process to selectively remove copper from molybdenite concentrates and to determine the kinetic parameters of copper removal in the leaching of sulfidized molybdenite concentrates with H2SO4–NaCl–O2 at atmospheric pressure.
flow. Once the set temperature was reached, a glass boat containing about 5 g of the molybdenite concentrate and a separate glass boat containing elemental sulfur were introduced in the furnace tube to the region of constant temperature, and after closing the tube, the nitrogen flow was shut. At the end of the experiment, the system was flushed with nitrogen and the reacted sample was withdrawn and cooled under a slow flow of nitrogen. When the sample was cold, it was weighed and stored. Inasmuch as the molybdenite particles did not react with the gaseous sulfur and remained unchanged, the extent of sulfidation of the concentrate was calculated from the weight gain of the samples according to the stoichiometry of reaction (1), assuming that all the copper in the molybdenite concentrate was present as chalcopyrite.
2. Experimental work
2.3. Leaching methodology
2.1. Materials
The leaching of the sulfidized concentrate was carried out at ambient pressure in H2SO4–NaCl–O2. The leaching reactor was a glass vessel equipped with a variable speed mechanical stirrer, a water condenser, a system for oxygen injection into the solution and a solution sampling device. The procedure consisted of heating 1 L of the leaching solution to a desired temperature in a slow flow of oxygen. When the temperature reached the set value, a sample of 3 g of sulfidized sample was introduced rapidly in the reactor, and the agitation and flow of oxygen were adjusted to the determined values for the experiment. During the experiments, various solution samples were withdrawn at time intervals, and when the reaction time was completed, the solution was filtered. The sampled solutions were analyzed by atomic absorption spectroscopy.
þ
−
CuS þ 1=2O2 þ 2H þ Cl →Cu
2þ
o
−
þ S þ H2 O þ Cl
ð2Þ
The molybdenite concentrate used in this research was obtained from the flotation plant of El Teniente Division of CODELCO, Chile. This concentrate sample was a fine material (100%–38 μm), and it contained a substantial amount of organic flotation reagents on the particle's surface; therefore, the concentrate was firstly heat treated in nitrogen atmosphere to remove the organic reagents, and afterwards, it was washed in dilute sulfuric acid solution to remove the surface oxides that may have formed during the handling of the sample. The molybdenite concentrate thus prepared was analyzed for Cu and Fe. The results indicated that the concentrate contained 3.4% Cu and 3.04% Fe. The microscopic analysis showed that the concentrate contained 83.4% MoS2 (which represents 50% Mo), and the major copper mineral present was chalcopyrite with very small amounts of chalcocite, covellite, and bornite. The X-ray diffraction analysis (XRD) also showed that molybdenite and chalcopyrite were the main components of the concentrate. Fig. 1 shows a micrograph of the molybdenite concentrate, where several chalcopyrite particles of different sizes were identified among the prevalent molybdenite particles. 2.2. Sulfidation methodology The sulfidation of the molybdenite concentrate was the first step in the purification process studied, and it was carried out according to the methodology described by Padilla et al. (2003a). The method consisted basically of contacting a sample of molybdenite concentrate with gaseous sulfur in a horizontal tube furnace at temperatures below 400 °C. The procedure followed for each experiment consisted of heating the reaction tube to the set temperature under slow N2
3. Results 3.1. Sulfidation of the molybdenite concentrate The sulfidation results obtained at 360 °C and 380 °C are presented in Table 1, where it can be observed that the conversion Table 1 Chalcopyrite sulfidation in the molybdenite concentrate. Time (min)
Conversion (%) 360 °C
Conversion (%) 380 °C
20 40 60 70 80
42.9 76.7 91.7 94.7 96.5
50.5 82.2 95.1 96.8 97.8
80
R. Padilla et al. / Hydrometallurgy 137 (2013) 78–83
of chalcopyrite in sulfidation at 380 °C was higher than 95% after 60 min. Therefore, for the subsequent leaching experiments, the molybdenite concentrate was sulfidized at 380 °C and 60 min. The sulfidation product was characterized by XRD spectroscopy, and the result showed strong diffraction peaks for molybdenite and peaks for covellite and pyrite. This result was confirmed by microscopic analysis as illustrated in Fig. 2, where we can observe clearly that the chalcopyrite originally present in the concentrate was transformed into covellite and pyrite. As can be seen in Fig. 2, a large part of the covellite formed by sulfidation is located on the surface of the newly formed porous pyrite. This covellite product did not form a compact protective layer over the formed pyrite center as was observed earlier by Padilla et al. (2003b). The formation of this discontinuous layer of covellite in the present research could be due to the small size of the original chalcopyrite particles in the molybdenite concentrate, i.e., due to their high surface to mass ratio. A significant portion of the covellite was also observed inside the porous structure of the pyrite. On the other hand, the MoS2 particles in the concentrate remained unchanged during the sulfidation stage. 3.2. Leaching of the sulfidized concentrate As already discussed, the copper mineral species to be dissolved in the leaching is covellite, which should be much more amenable to leaching than the original chalcopyrite. The effect of the main leaching variables on the dissolution rate of copper from the sulfidized concentrate was studied. In some cases, the extent of the dissolution of iron and molybdenum was also determined. Preliminary experiments were carried out using stirring speeds in the range of 200–700 rpm. The results are presented in Table 2, which shows that stirring speeds over 400 rpm affects little the copper dissolution from the sulfidized concentrate, indicating that a proper suspension of the particles occurs for stirring speeds over 400 rpm. Thus, a stirring of 620 rpm was used in the subsequent experiments to assure independency of this variable. 3.3. Effect of the oxygen flow rate The effect of the oxygen flow rate on the copper dissolution from the molybdenite concentrate was studied at 100 °C and 60 min in a solution containing NaCl and H2SO4 in concentration of 1.0 M and 0.6 M, respectively, with stirring speed of 620 rpm. The copper dissolutions data obtained for oxygen flow rates in the range of 0.1–1.6 L/min are shown in Fig. 3. It can be observed in Fig. 3 that oxygen flow rates over about 0.4 L/min did not affect the dissolution
Fig. 2. Micrographs of molybdenite concentrate sample sulfidized at 380 °C for 60 min, showing transformed chalcopyrite particles (porous, blue-light yellow color particles). Other particles are molybdenite (grey color) and original pyrite (light solid yellow color).
Table 2 Effect of stirring speed on copper dissolution. Stirring speed (rpm)
Copper dissolution (%)
200 300 400 620 700
71.49 82.75 85.72 86.01 84.56
of copper from the molybdenite concentrate. This indicates that over 0.4 L/min the mass transfer from the gas to the liquid will not be rate controlling. Thus, 1 L/min was used in all subsequent experiments. 3.4. Effect of the concentration of sulfuric acid To study the effect of the concentration of sulfuric acid on the copper dissolution, several experiments were carried out using leaching solutions containing H2SO4 in the molar range of 0.1–0.6. The results are shown in Fig. 4, where one can see that for sulfuric acid concentrations over 0.4 M, the copper dissolution rate was nearly independent on the acid concentration, while below 0.4 M, the concentration of sulfuric acid has a clear effect on the rate of copper dissolution. It can also be observed in Fig. 4 that in all the range studied the concentration of sulfuric acid did not affect the dissolution of iron and in 2 h of leaching about 20% of the iron in the concentrate was dissolved. The subsequent leaching experiments were conducted using a molar concentration of sulfuric acid equal to 0.6 to assure that the copper dissolution was independent of this variable. 3.5. Effect of the concentration of sodium chloride Fig. 5 shows the effect of sodium chloride concentration on the rate of copper dissolution. It can be observed in Fig. 5 that in leaching the sulfidized concentrate with only sulfuric acid–oxygen the dissolution of copper is low even at 2 h of leaching time. However, when sodium chloride is added to the leaching media, the dissolution rate of copper improves drastically. It is also interesting to note that an increment in the concentration of sodium chloride over 0.6 M NaCl is not necessary since over this concentration value the copper dissolution rate does not improve any longer. 3.6. Effect of the temperature on the copper dissolution Leaching experiments were carried out in the temperature range of 70 °C–100 °C. The other leaching variables were kept constant at
Fig. 3. Effect of oxygen flow rate on copper dissolution from sulfidized molybdenite concentrate for the conditions shown in the figure and agitation of 620 rpm.
R. Padilla et al. / Hydrometallurgy 137 (2013) 78–83
Fig. 4. Effect of sulfuric acid on the copper dissolution from sulfidized molybdenite concentrate. Conditions 0.6 M NaCl, 100 °C, and 600 rpm, 1 L/min O2.
0.6 M H2SO4, 1 M NaCl, 1 L/min O2, and 620 rpm. These variable values fall in the range in which they have little effect on the leaching rate. The results are presented in Fig. 6, where it can be observed that the temperature has an important effect on the copper dissolution rate. As seen in Fig. 6, leaching at 100 °C dissolved as much as twice copper from the sulfidized molybdenite concentrate than a leaching at 70 °C. This large effect of temperature on the rate of copper dissolution suggests that the dissolution reaction is controlled by chemical reaction.
81
Fig. 6. Dissolution of copper from sulfidized molybdenite concentrates in H2SO4–NaCl–O2 solution for the temperature range of 70 °C–100 °C. Leaching conditions were 0.6 M H2SO4, 1 M NaCl, 1 L/min O2, 620 rpm.
shrinking core model with chemical reaction control, which in its integrated form is given by the following equation (Levenspiel, 1998; Wadsworth, 1979) 1=3
1−ð1−XÞ
¼ kt
ð3Þ
Because of the fact that the leaching experiments in this research were carried out at high stirring speeds and high oxygen flow rates, mass transfer effects were not considered to be a limiting factor. Thus, it can be assumed that the controlling step of the overall leaching rate of the covellite particles is a surface chemical reaction, as indicated before on discussing the large effect of temperature on the copper dissolution rate. Regarding the physical structure of the pyrite and covellite phases formed in the sulfidation stage (shown in Fig. 2), it is clear that the leaching reagents could easily diffuse through the porous pyrite particles; consequently, the entire surface of the covellite particles, regardless of their location, would be exposed to the leaching reagents. Taking this into consideration, the dissolution rate of the covellite would be represented well by a
In Eq. (3), k is a global rate constant and X is the fraction of covellite reacted. Thus, a plot of the left hand side of Eq. (3) versus t should result in a straight line. In Fig. 7, we can observe that the plot of 1 − (1 − X)1/3 vs. t for the copper dissolution data shown in Fig. 6 does display a straight line behavior confirming the adequacy of the kinetic Eq. (3). The values of the global kinetic constants obtained from Fig. 7 were used to draw an Arrhenius plot, as shown in Fig. 8. As can be seen in Fig. 8, the rate constant show a linear dependency with the temperature, and the activation energy value calculated from the slope of the line was 52.5 kJ/mol. This activation energy value is typical for a process controlled by a surface chemical reaction. The use of the shrinking core kinetic model allows for the determination of the dependence of the rate of copper dissolution on the sulfuric acid concentration, and the chloride ions concentration from the experimental data shown in Figs. 4 and 5. Thus, plots of ln(k) versus ln[H2SO4] and ln(k) versus ln[NaCl] were drawn, which are shown in Figs. 9 and 10, respectively. As can be seen, both plots show excellent linear behavior. The value of the slope in Fig. 9 indicates that
Fig. 5. Effect of the concentration of NaCl on the dissolution of copper from a sulfidized molybdenite concentrate. Leaching conditions: 100 °C, 0.6 M H2SO4, 620 rpm, 1 L/min O2.
Fig. 7. Chemical control kinetics for copper dissolution from molybdenite concentrate.
3.7. Kinetics of copper dissolution from the sulfidized concentrate
82
R. Padilla et al. / Hydrometallurgy 137 (2013) 78–83
Fig. 8. Arrhenius plot for the dissolution of copper from sulfidized molybdenite concentrates with H2SO4–NaCl–O2, for the temperature range of 70 °C–100 °C.
Fig. 10. Rate constant as a function of the chloride ions in solution.
the rate of copper dissolution is of 1/4 order with respect to the concentration of sulfuric acid, for concentrations up to 0.4 M. On the other hand, the slope in Fig. 10 indicated a 0.9 dependency of the rate with respect to the concentration of chloride ions in the solution for concentrations up to 0.6 M. Over the indicated concentration limits, both variables have no further effect on the copper dissolution rate.
electrowinning technology to produce copper cathodes, and the solution could be recycled to leaching without further treatment. In the case of plants that use chloride ions in their heap leaching operations, it could be more convenient to mix the solutions generated in the molybdenite purification process with the heap leaching solutions to be treated together.
3.8. Selectivity in leaching 4. Conclusions Fig. 11 compares the dissolution of copper, iron and molybdenum from the sulfidized concentrate in the optimum conditions for covellite dissolution. We observe in Fig. 11 that the sulfuric acid–sodium chloride–oxygen leaching media is very effective and selective for the dissolution of copper over molybdenum, which shows a negligible dissolution. On the other hand, iron is dissolved only partially in this leaching media. Nevertheless, the process studied allows a very effective elimination of copper from molybdenite concentrates without dissolving molybdenum. Final molybdenite concentrate with less than 0.2% Cu could be obtained by this method, as shown in Fig. 12, where the copper and iron content in the leaching residue is shown for two leaching conditions. This result corroborates the technical feasibility of removing copper selectively from molybdenite concentrates when the copper impurity is present as chalcopyrite. Is should be point out that the leaching solutions produced by this method could be treated by the conventional solvent extraction and
The proposed method of sulfidation and leaching can be used effectively for the removal of copper from molybdenite concentrates when the copper impurity is in the form of refractory chalcopyrite mineral, which is the case for the majority of Chilean molybdenite concentrates. The leaching of the sulfidized molybdenite concentrate in sulfate– chloride–oxygen media for 90 min at 100 °C dissolved 96% of the copper which yielded a final concentrate with less than 0.2% Cu. Under these leaching conditions, the molybdenum dissolution was negligible and iron dissolution was lower than 20%. The kinetics of the covellite dissolution from the sulfidized molybdenite concentrates was analyzed by using the shrinking core model for chemical reaction control 1 − (1 − X)1/3 = kt. The calculated activation energy was 52.5 kJ/mol for the range of 70 °C–100 °C. The global rate constant was found to be of 1/4 order with respect to H2SO4 up to 0.4 M, and 0.9 order with respect to NaCl up to 0.6 M.
Fig. 9. Dissolution rate constant as a function of the concentration of sulfuric acid.
Fig. 11. Copper, iron, and molybdenum dissolution from sulfidized molybdenite concentrate in the optimum leaching conditions of 100 °C, 0.6 M H2SO4, 0.6 M NaCl, 620 rpm, 1 L/min O2.
R. Padilla et al. / Hydrometallurgy 137 (2013) 78–83
83
References
Fig. 12. Copper and iron elimination from molybdenite concentrate by sulfidation and leaching of the sulfidized concentrate in H2SO4–NaCl–O2 media.
For higher concentrations the rate constant was independent of both variables. Finally, from the view point of practical application, the sulfidation and leaching method for molybdenite concentrate purification would alleviate some of the difficulties, concerning pollution and corrosion, encountered in the traditional processes used for copper elimination from molybdenite concentrate. In addition, the copper in the leaching solution could be conveniently recovered by solvent extraction and electrowinning technology, allowing the recycle of the solution to the leaching operation.
Acknowledgments The authors would like to thank the National Fund for Scientific and Technological Development (FONDECYT) of Chile for the financial support of this study through project no. 1110590.
Adam, M.J., Neumeier, L.A., 1987. Partitioning of chalcopyrite by sulfidation. In: Vassiliou, A.H., Hausen, D.M., Carson, D.J.T. (Eds.), Process Mineralogy VII. Metall, Soc. AIME, pp. 511–523. Adam, M.J., Swallow, K.A., Stephenson, J.B., 1994. In: Petruk, W., Rule, A.R. (Eds.), Process Mineralogy XII. The Minerals, Metals & Materials Society, pp. 287–296. Askari Zamani, M.A., Vaghar, R., Oliazadeh, M., 2006. Selective copper dissolution during bioleaching of molybdenite concentrates. Int. J. Miner. Process. 81, 105–112. Demopoulos, G.P., Distin, P.A., 1983. Ferric chloride leaching of sulphidized chalcopyrite. Hydrometallurgy 10, 111–122. Dorfler, R.R., Laferty, J.M., 1981. Review of molybdenum recovery processes. JOM 5, 48–54. Gupta, C.K., 1992. Extractive Metallurgy of Molybdenum, first ed. CRC Press, Boca Raton. Jolly III, F.A., Neumeier, L.A., 1991. Leaching sulfidation-partitioned chalcopyrite to selectively recover copper. Report of Investigations.U.S. Bureau of Mines 9343. Kim, B.S., Jha, M.K., Jeong, J., Lee, J.C., 2008. Leaching of impurities for the up-gradation of molybdenum oxide and cementation of copper by scrap iron. Int. J. Miner. Process. 88, 7–12. Levenspiel, O., 1998. Chemical Reaction Engineering. John Wiley & Sons, New York. Neumeier, L.A., Adam, M.J., 1990. In: Petruk, W., Hagni, R.D. (Eds.), Process Mineralogy IX. The Minerals, Metals & Materials Society, pp. 311–322. Padilla, R., Rodriguez, M., Ruiz, M.C., 2003a. Sulfidation of chalcopyrite with elemental sulfur. Metall. Mater. Trans. B 34, 15–23. Padilla, R., Olivares, E., Ruiz, M.C., Sohn, H.Y., 2003b. Kinetics of the sulfidation of chalcopyrite with gaseous sulfur. Metall. Mater. Trans. B 34, 61–68. Padilla, R., Zambrano, P., Ruiz, M.C., 2003c. Leaching of sulfidized chalcopyrite with H2SO4–NaCl–O2. Metall. Mater. Trans. B 34, 153–159. Padilla, R., Vega, D., Ruiz, M.C., 2007. Pressure leahing of sulfidizedChalcopyrite in sulfuricacid-oxygen media. Hydrometallurgy 86, 80–88. Padilla, R., Letelier, H., Ruiz, M.C., 2013. Copper removal form molybdenite by sulfidation-leaching process. In: Zhang, L., Allanore, A., Wang, C., Yurko, J.A., Crapps, J. (Eds.), Materials Processing Fundamentals. John Wiley & Sons, Inc., Hoboken, NJ, USA. http://dx.doi.org/10.1002/9781118662199.ch24. Prasad, P.M., Balasubramanian, K., Mukkopadyay, R., 2007. Separation chemistry in the refining of an off-grade molybdenite concentrate by leaching with an acid mix. Miner. Metal Proc. 24 (2), 97–104. Ruiz, M.C., Padilla, R., 1998. Copper removal from molybdenite concentrates by sodium dichromate leaching. Hydrometallurgy 48, 313–325. Subramanian, K.N., Kanduth, H., 1973. Activation and leaching of chalcopyrite concentrates. CIM Bull. 66, 88–91. Wadsworth, M.E., 1979. Hydrometallurgical processes. In: Sohn, H.Y., Wadsworth, M.E. (Eds.), Rate Processes of Extractive Metallurgy. Plenum Press, New York, pp. 133–153. Warren, H.I., Vizsolyi, A., Forward, F.A., 1968. The pretreatment and leaching of chalcopyrite. CIM Bull. 5, 637–640.