Galvanic Leaching of Chalcopyrite in Atmospheric Pressure and

The chalcopyrite and pyrite ore samples were collected from Mazraeh copper mine located in East Azerbaijan of Iran. The samples were crushed and groun...
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Ind. Eng. Chem. Res. 2010, 49, 5997–6002

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Galvanic Leaching of Chalcopyrite in Atmospheric Pressure and Sulfate Media: Kinetic and Surface Studies S. M. Javad Koleini,*,† M. Jafarian,‡ M. Abdollahy,† and V. Aghazadeh† Mineral Processing Department, Tarbiat Modares UniVersity, Tehran, Iran, Chemistry Department, Kajhe Nasir Tosi UniVersity, Tehran, Iran, Mineral Processing Engineering Department, Tarbiat Modares UniVersity, Tehran, Iran, and Faculty of Engineering, Tarbiat Modares UniVersity, Tehran, Iran

Chalcopyrite leaching rate in atmospheric pressure and in sulfate media with ferric ion is low because of passive layer formation around chalcopyrite particles. In this investigation pyrite is used as a catalyst of chalcopyrite leaching. Because of galvanic interaction between chalcopyrite and pyrite, the chalcopyrite leaching rate is increased. Effects of parameters such as stirring speed, pyrite to chalcopyrite ratio, solution potential, temperature, and initial acid concentration were investigated. Results showed that maximum copper recovery (more than 95%) is obtained in less than 24 h, stirring speed of 1150 rpm, pyrite to chalcopyrite ratio 4, solution potential 410 mV, temperature 85 °C, and initial sulfuric acid concentration 45 g/L. Also, kinetic investigation showed that chalcopyrite dissolution in the presence of pyrite followed the shrinking core model and the reaction was controlled by the surface reaction. Activation energy (Ea) was calculated at 77.79 kJ/ mol. Surface studies showed that in the absence of pyrite, the surface of chalcopyrite particles was covered by passive layer. 1. Introduction Chalcopyrite with crystalline structure, a semiconductor with chemical formulation Cu+1Fe+3S2, is the most important and abundant copper mineral in the world.1-4 At present, 70% copper in the world is produced from this mineral after a flotation process followed by a pyrometallurgical method.2,5-7 Because of environmental aspects and the high cost of copper production by pyrometallurgical methods, the application of hydrometallurgical methods have been increased.6,8,9 Chalcopyrite is highly refractory under hydrometallurgical conditions due to the surface transformation which renders products very stable under oxidation conditions,10-12 and thus, different oxidants and leaching media for leaching of chalcopyrite are used.9 Sulfuric acid and hydrochloric acid, ferric sulfate, and ferric chloride are the most common media and oxidants, respectively.6,8 Because of simple chemistry, low cost, no corrosion behavior, and use of custom solvent extraction and electrowinning, sulfuric acid with ferric sulfate is the suitable media for leaching of chalcopyrite.14-16 The low rate of chalcopyrite leaching in this media is the most important problem. Most of the researchers believe that the cause is passive layer formation around the surface of chalcopyrite particles. Despite this finding, there is controversy surrounding the composition of this layer among researchers. Nevertheless, low porosity and low electrical conductivity have been accepted by almost all researchers.5 Formation of elemental sulfur,3,17-21 formation of intermediate components such as bornite and covellite,22 formation of a copper-rich polysulfide layer,3,20,23 and formation of an iron component such as jarosite10,13,24-26 are the most important theories to explain the low dissolution rate of chalcopyrite in sulfate media with ferric ion. High-pressure processing, bioleaching, ultrafine grinding, and adding additives such as silver, chloride ions, and activated * Corresponding author. E-mail: [email protected]). † Mineral Processing Department, Tarbiat Modares University, Tehran, Iran. ‡ Chemistry Department, Kajhe Nasir Tosi University, Tehran, Iran.

carbon for enhancing the leaching of chalcopyrite in sulfate media are used. However, there are some disadvantages with them. In this research, pyrite as a catalyst for enhancing the chalcopyrite leaching in sulfate media with ferric sulfate was used. Chalcopyrite dissolution in sulfate media with ferric sulfate is an electrochemical reaction so that, overall, anodic and cathodic half cell reactions can be written as eqs 1, 2, and 3.2,6,26,27 CuFeS2 + 4Fe+3 f Cu+2 + 5Fe+2 + 2S

(1)

Anodic half-cell reaction: chalcopyrite oxidation CuFeS2 f Cu+2 + Fe+2 + 2S + 4e-

(2)

Cathodic half-cell reaction: reduction of ferric ion 4Fe+3 + 4e- f 4Fe+2

(3)

In the absence of pyrite both anodic and cathodic half-cell reactions take place on the chalcopyrite surface. Researchers claim that the slow dissolution rate of chalcopyrite is due to the anodic half cell reaction. Tshilombo et al.28 claim that the dissolution rate of chalcopyrite is slow because of the slow cathodic half-cell reaction on the surface of chalcopyrite. Therefore, if there is suitable surface for a higher rate cathodic half-cell reaction, the dissolution rate of chalcopyrite will be increased. In the presence of pyrite, the cathodic reaction takes place on the surface of pyrite rapidly, and the dissolution of chalcopyrite will be increased. The rest potential of chalcopyrite and pyrite in 1 N sulfuric acid concentration at temperature 20 °C is 0.52 and 0.63 V, respectively, relative to the standard hydrogen electrode (SHE). Thus, these two minerals can form a galvanic cell in solution.20 Therefore, a cathodic reaction and an anodic reaction take place on the surface of pyrite and chalcopyrite, respectively. The effects of parameters such as stirring speed, pyrite to chalcopyrite ratio, solution potential, temperature, and initial

10.1021/ie100017u  2010 American Chemical Society Published on Web 06/09/2010

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Table 1. Mineralogical Compositions of Chalcopyrite and Pyrite Samples by XRD sample

chalcopyrite (%)

pyrite (%)

clinochlore (%)

sch spy

97 6

3 92

2

Table 2. Chemical Composition of Concentrated Chalcopyrite and Pyrite Samples by XRF sample

Cu (%)

Fe (%)

S (%)

SiO2 (%)

Al2O3 (%)

CaO (%)

Mo (%)

LOI (%)

sch spy

31.61 4.46

31.06 45.96

29.17 32.07

3.31 2.11

1.85 1.48

0.55 -

tr -

2.45 13.92

acid concentration were investigated. Kinetic investigation with the assumption that the leaching of chalcopyrite in ferric sulfate media in the presence of pyrite meets the shrinking core model was used for the calculation of activation energy (Ea). Surface studies were also investigated by scanning electron microscope/ energy-dispersive X-ray spectroscopy analysis (SEM-EDX). 2. Materials and Methods 2.1. Materials Characterization. The chalcopyrite and pyrite ore samples were collected from Mazraeh copper mine located in East Azerbaijan of Iran. The samples were crushed and ground to the required particle size. These prepared samples were concentrated by the flotation method to produce chalcopyrite (sch) and pyrite (spy) samples. Characterizations of concentrated samples by XRD and XRF are shown in Tables 1 and 2, respectively. The size fraction of -75 + 53 µm was prepared by wet sieving for further experiments. In the leaching experiments, sulfuric acid with a purity of approximately 98%, ferrous heptahydrate sulfate with a purity of 100%, and ferric sulfate (Fe2(SO4)3 · H2O) with a purity of 76-82% were used. All of these chemical materials were made by Merck & Company. Distilled water was used for the leaching experiments. 2.2. Leaching Apparatus and Experimental Procedure. Leaching experiments were performed in a 1.5-L glassy reactor setup in a thermostatically controlled water bath, equipped with a digitally controlled thermometer (within (2 °C). For minimizing liquid loss when the system was heated, a reflux condenser was mounted on the top of the cell. The leaching system is shown in Figure 1. Solution potential was initially controlled by adjusting the ferric to ferrous ion ratio. To control the solution potential, oxygen gas was injected inside the reactor continuously. The solution potential of the leaching reactor was controlled manually within (25 mV. At desired time intervals, 5 mL of solution was withdrawn by syringe for copper analysis by AAS. Solution potential was measured by ohm meter model 827 pH Lab. All of the reported potentials in this article are referred to Ag/AgCl. In all experiments solid weight percent, total iron concentration, and the amount of chalcopyrite were 2.5%, 5 g/L and 5 g, respectively. In calculating the leaching efficiency of copper, a volume correction formula (eq 4) was used to consider the volume losses due to sampling.29 i-1

i-1

(V Xm,i )

∑

νi)CM,i +

∑νC

i M,i

i)1

i)1

m(cM /100)

(4)

where XM,i is the Cu extraction corresponding to sample i, V is the initial volume of the leaching solution in the reactor (mL), νi the volume of sample i withdrawn from the reactor (mL),

Figure 1. Schematic view of the leaching apparatus. 1 - Digital electrically heated water bath, 2 - leaching reactor, 3 - lid made of PTEF, 4 - sampling system, 5 - Eh electrode, 6 - mechanical impeller made of stainless steel, 7 - reflux condenser, 8 - sparger made of stainless steel, 9 - flow meter, and 10 - oxygen capsule. Table 3. Parameters Values for the Leaching of Chalcopyrite parameters

values

stirring speed (rpm) pyrite/chalcopyrite mass ratio solution potential (mV Ag/AgCl) temperature (°C) acid concentration (g/L)

850, 1150, 1450 1, 2, 3, 4, 5 357, 410, 446 48, 68, 85 15, 30, 45

CM,i the Cu concentration in sample i (mg/L), m the initial mass of the chalcopyrite in grams added into the reactor and CM the initial Cu percentage in the chalcopyrite. Parameters and their values for leaching of chalcopyrite in the presence of pyrite were selected as shown in Table 3. 2.3. Surface Study. Surface studies were carried out to investigate particles surface of chalcopyrite and pyrite particles surface properties during the leaching by SEM-EDX XL30 model manufactured by Philips Company, the Netherlands, and the sputter coater was SCDOOS model manufactured by BalTec Company, Switzerland. 3. Results and Discussion 3.1. Effect of Parameters. 3.1.1. Effect of Stirring Speed. Three different stirring speeds (850, 1150, and 1450 rpm) were used. Other conditions were: solution potential 410 mV, temperature 68 °C, chalcopyrite and pyrite particle size +53-75, initial sulfuric acid concentration 15 g/L and pyrite to chalcopyrite ratio 2.The effect of stirring speed on the dissolution of chalcopyrite is shown in Figure 2. According to the results, stirring speed has no significant effect on the dissolution of chalcopyrite; thus, the dissolution reaction rate is not controlled via mass transfer in fluid film. However, the stirring speed is needed to keep the pulp in suspended form.18 Therefore, the stirring speed of 1150 rpm was used to run all experiments. 3.1.2. Effect of Pyrite to Chalcopyrite (Py/Cp) Ratio. Five different Py/Cp weight ratios (1, 2, 3, 4, and 5) were used with solution potential 410 mV, temperature 68 °C, chalcopyrite and pyrite particle size +53-75, initial sulfuric acid concentrate 15 g/L, and stirring speed 1150 rpm. Results are presented in Figure 3 As can be seen, as the Py/Cp ratio is increased, the copper recovery is increased. An increase of the Py/Cp ratio to

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Figure 2. Effect of stirring speed on the copper recovery. Figure 5. Effect of temperature on the copper recovery.

Figure 6. Effect of initial acid concentration on the copper recovery. Figure 3. Effect of the Py/Cp ratio on the copper recovery.

Figure 4. Effect of solution potential on the copper recovery.

>4 has no significant effect in copper recovery. This phenomenon has been confirmed by other researchers.30,31 3.1.3. Effect of Solution Potential. Figure 4 gives copper recovery as a function of the solution potential at a temperature of 68 °C, chalcopyrite and pyrite particle size +53-75 µm, initial sulfuric acid concentration 15 g/L, Py/Cp ratio 4, and stirring speed 1150 rpm. According to the experimental results, the solution potential has a significant effect on copper recovery. Maximum copper recovery was achieved in 410 mV. The ferric to ferrous ion ratios were adjusted at 0.01, 0.1, and 1 in 68 °C, and the solution potentials in these ratios were 357, 410, and 446 mV, respectively. Adjusting the optimum solution potential is very important because of the pyrite oxidation phenomenon. Although the oxidation of pyrite is dependent on parameters such as temperature, particle size, leaching time, pulp concentration, and pH, it seems pyrite oxidation takes place in a solution potential >700 mV against SHE.20,32 3.1.4. Effect of Temperature. The effect of temperature on copper recovery was investigated at various temperatures (48, 68, and 85 °C) under solution potential of 410 mV, chalcopyrite

and pyrite particle size +53-75 µm, initial sulfuric acid concentration 15 g/L, Py/Cp ratio 4, and stirring speed 1150 rpm. Figure 5 shows that the temperature has significant effect on the copper recovery. Results of other studies33 show that a 2-fold increase in reaction rates is noted for each 10 °C rise in temperature. The results of an investigation by Hirato et al.17 in 1987 for the second stage of chalcopyrite leaching show that, when the temperature increases, the dissolution of chalcopyrite increases very rapidly. Because of the decrease in the rest potential of chalcopyrite with increase of temperature, this mineral at high temperature is more active, and the dissolution rate of chalcopyrite will be increased.20 3.1.5. Effect of Initial Acid Concentration. The effect of initial acid concentration on copper recovery was investigated at various initial acid concentrations (15, 30, and 45 g/L) at solution potential 410 mV, temperature 85 °C, chalcopyrite and pyrite particle size +53-75 µm, Py/Cp ratio 4, and stirring speed 1150 rpm. Figure 6 shows that the initial acid concentration has little effect on the copper recovery. Results of other studies18 showed that an initial acid concentration of 0.1 M) is needed for prevention of hydrolysis and precipitation of ferric ion. Results of this investigation have shown that in a high initial acid concentration, the recovery of copper is high. Tshilombo et al.20 claimed that this phenomenon is related to the increasing galvanic effect and not to precipitation of the iron component. 3.2. Kinetic Analysis. For solid-fluid reactions, the shrinking core model and the shrinking particle model are used redundantly. The most common models considered in the leaching are illustrated in Figure 7.34,35 It was claimed that, because of passive layer formation with low porosity during chalcopyrite dissolution in ferric sulfate media, kinetic models a and c (Figure 7) cannot be considered

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Figure 7. Schematics of different mechanisms of leaching.

Figure 9. Variation of 1 - (1 - X)1/3 with time at various temperatures.

Figure 8. Model of chalcopyrite oxidation by Fe3+ with formation of a low porosity layer of elemental sulfur (after Mun˜oz et al., 197915).

for chalcopyrite dissolution. This phenomena is clearly shown in Figure 8.5 A heterogeneous reaction (solid-liquid reaction) can be considered as the following: aAfluid + bBparticle f product

Figure 10. Variation of 1 - 2/3X - (1 - X)2/3 with time at various temperatures.

(5)

Chalcopyrite dissolution mathematical models with the assumption of a shrinking core/constant particle model can be written as follows.36,37 If the reaction rate is controlled by diffusion through the product layer, the model can be expressed by eq 6. However, if the reaction rate is controlled by a surface reaction, eq 7 can be employed. kcMBCA 2 1 - x - (1 - X)2/3 ) t ) kdt 3 Fbar20 1 - (1 - X)

1/3

kcMBCA ) t ) krt Fbar0

(6) Figure 11. Arrhenius plot of reaction rate against temperature.

(7)

where X is the fraction reacted, kc is the kinetic constant, MB is the molecular weight of the solid, CA is the concentration of the dissolved lixiviant, A is the bulk of the solution, a is the stoichiometric coefficient of the reagent in the leaching reaction, r0 is the initial radius of the solid particle, t is reaction time, D is the diffusion coefficient in the porous product layer, Fb is density of the solid, and kd and kr are rate constants. It can be concluded from eq 1 that, if the acid concentration is high enough, it will have little effect on the leaching rate of chalcopyrite. Thus, the acid concentration of the solution was controlled to prevent ferric ion as well as other ions of interest from hydrolysis and precipitation. The results from this investigation have also shown that the sulfuric acid concentration has no significant effect on the leaching rate of chalcopyrite. Equations 6 and 7 were applied to the results obtained from each temperature value. Figures 9 and 10 present the data plots according to chemical reaction control and diffusion control

processes, respectively. The results revealed that the values for the correlation coefficient(R2) in Figure 9 are closer to one than those in Figure 10. Therefore, eq 7 was found to be well fitted by the data. The results indicate that the linear relationship between 1 - (1 - X)1/3 and leaching time (t) is significant and suggests that the leaching rate of chalcopyrite is controlled by the surface reaction. Arrhenius equation (kr ) A exp (-Ea/RT)) is used for calculation of activation energy. Where A is frequency factor, Ea is activation energy of the reaction, R is universal gas constant and T is absolute temperature. The apparent rate constants (kr) were calculated as slopes of the straight lines. Using the apparent rate constants obtained by application of eq 7, the Arrhenius plot (ln kr vs 1/T) was made (Figure 11), and the activation energy was calculated as 77.79 kJ/mol. This value clearly confirms that this process is most likely controlled by a surface reaction.

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Figure 12. SEM micrograph of leaching residue of chalcopyrite alone at an initial solution potential of 410 mV.

Figure 14. SEM micrograph of leaching residue of chalcopyrite in the presence of pyrite at a solution potential of 460 mV.

Figure 13. SEM micrograph of leaching residue of chalcopyrite in the presence of pyrite at a solution potential of 410 mV.

chalcopyrite because of the difference between rest potentials of these minerals. Parameters such as stirring speed, Py/Cp ratio, solution potential, temperature, and initial acid concentration were investigated. Stirring speed and initial acid concentration have no significant effect on the copper recovery, but the Py/ Cp ratio, solution potential, and temperature have significant effects on copper recovery. Maximum copper recovery (more than 95%) was obtained at a stirring speed 1150 rpm, Py/Cp ratio of 4, solution potential of 410 mV, temperature of 85 °C, and initial sulfuric acid concentration of 45 g/L for less than 24 h. Kinetic investigation showed that chalcopyrite dissolution in the presence of pyrite meets the shrinking core model, and the reaction is controlled by the surface reaction. Activation energy (Ea) was calculated at 77.79 kJ/mol. Surface studies also show that, in the absence of pyrite, a passive layer is formed; but in the presence of pyrite, the layer has no negative effect on the chalcopyrite leaching.

4. SEM-EDX Investigation

Acknowledgment

For investigation of changes on chalcopyrite and pyrite surfaces in different conditions of leaching, surface studies were done by SEM-EDX. Figure 12 shows the chalcopyrite surface after leaching of chalcopyrite in the absence of pyrite. As shown in this figure the chalcopyrite surface is covered by elemental sulfur shown by EDX analysis. Figure 13 shows the chalcopyrite surface after leaching of chalcopyrite in the presence of pyrite at a solution potential of 410 mV. As shown in this figure, the chalcopyrite surface is covered by elemental sulfur, but the surface of pyrite is clean. Figure 14 shows the chalcopyrite surface after leaching of chalcopyrite in the presence of pyrite at 446 mV. As shown, the chalcopyrite surface is covered by elemental sulfur too. It seems that the surface of pyrite is somehow covered by a layer. It can be concluded that in the presence of pyrite, on the surface of chalcopyrite a sulfur layer is formed, but because of the high copper recovery this layer does not prevent the leaching of chalcopyrite. Therefore, in the presence of pyrite, permeability and the porosity of the sulfur layer formed on the chalcopyrite surface are high. Also, results of kinetic studies show that chalcopyrite dissolution in the presence of pyrite is controlled by chemical reaction.

We thank Tarbiat Modares University and Sarcheshmeh Copper Complex for their supports.

5. Conclusions According to the results obtained, pyrite has a catalytic effect on the chalcopyrite leaching by forming a galvanic cell with

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ReceiVed for reView January 4, 2010 ReVised manuscript receiVed April 29, 2010 Accepted May 11, 2010 IE100017U