hydrometallurgy Hydrometallurgy 47 (1997) 37-45
A case of ferrous sulfate addition enhancing chalcopyrite leaching Naoki Hiroyoshi
*, Masahiko Hirota, Tsuyoshi Hirajima, Masami Tsunekawa
Faculry of Engineering, Hokkaido University, Kita 13, Nishi 8, Sapporo 060, Japan
Received 8 January 1997; accepted 22 April 1997
Abstract It is generally accepted that ferric ions as an oxidant are effective for leaching chalcopyrite but ferrous ions contribute to the leaching only as a source of ferric ions. However, this paper found that several chalcopyrite samples were more effectively leached in ferrous sulfate solution than in ferric sulfate solution. Chalcopyrite samples from four different sources were leached in 0.1 mol dm-3 sulfuric acid solution containing 0.1 mol dm -3 ferrous sulfate or ferric sulfate at 303 K in air for 168 h. Three samples were more effectively leached in the ferrous sulfate solution than in the ferric sulfate solution. Especially, with the sample from the Akenobe mines, Hyogo, Japan, the amount of copper extracted with ferrous sulfate was about five times larger than that with ferric sulfate. By using the Akenobe sample, leaching experiments and oxygen consumption measurements were carried out under various conditions. The amount of extracted copper increased markedly with increasing ferrous sulfate addition and decreasing pH. During the leaching experiments, most of the soluble iron was present in the ferrous form. By adding ferrous sulfate. proton consumption increased. The mole ratio of elemental sulfur to extracted copper was about 2. When the leach solution was purged with nitrogen, the amount of copper extracted was negligible even with ferrous sulfate. By adding ferrous sulfate, dissolved oxygen consumption on the sample surface increased. From these results, it was concluded that ferrous ions enhance the following reaction for the Akenobe sample: CuFeS, + 0, i- 4H+= CL?+ + Fe’++ 2s” + 2H,O. The importance of this effect in the bacterial leaching of chalcopyrite is discussed. 6 1997 Elsevier Science B.V.
* Corresponding author. Fax: + 8 1-l 1-7166175; e-mail: [email protected]
0304-386X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO304-386X(97)00032-7
N. Hiroyoshi et al. / Hydrometallurgy 47 (1997) 37-45
1. Introduction Recently, dump and heap leaching have been widely used to recover copper from low grade ores which would otherwise be wasted. Copper oxides are easy to leach in sulfuric acid solution and the leaching rate of sulfides such as covellite (CuS) and chalcocite (Cu,S) are relatively high in the presence of iron oxidizing bacteria 111. But the leaching rate of chalcopyrite (CuFeS,), a major source of copper, is very slow [ll. To establish the optimum conditions for copper recovery, the factors influencing the leaching rate of chalcopyrite must be determined. In acidic solutions, chalcopyrite is oxidized by ferric ions and dissolved oxygen to release copper ions. As ferrous ions are also extracted from chalcopyrite during the process, much attention has been paid to the role of ferrous ions in chalcopyrite leaching. Ferrous ions are rapidly oxidized to ferric ions in the presence of iron oxidizing bacteria such as Thiobacillus ferrooxidans , making the role of ferrous ions a source of the oxidant ferric ions. It has also been reported that ferrous ions suppress chalcopyrite oxidation by ferric ions [3,4]. Dutrizac et al.  mentioned that the reaction rate is controlled by the diffusion of ferrous ions through a surface sulfur layer produced during the leaching at high ferric concentrations. Hirato et al.  reported that the oxidation of ferrous to ferric ions occurs on the chalcopyrite surface in sulfuric acid solution and that this causes the slow kinetics. From these studies, it appears that ferrous ions contribute to the leaching only as a source of ferric ions and their concentration must be minimized to improve the leaching rate of chalcopyrite. However, there is a report  which suggests that ferrous ions contribute to copper extraction from chalcopyrite. Kametani and Aoki  established the effect of suspension potential on leaching of chalcopyrite in mixed solutions of ferrous and ferric sulfate and reported that there is a critical potential at which the leaching rate is maximum and that the rate suddenly decreases above this potential. As the suspension potential increases with increasing ferric to ferrous ratio, these results indicate that the leaching rate is faster with an optimum concentration of ferrous ions than without ferrous ions. In this paper, chalcopyrite samples from four sources were leached in ferrous sulfate and ferric sulfate solutions to compare the effects of ferric and ferrous ions on copper extraction and it was found that ferrous sulfate was more effective in extracting copper from three samples. Copper extraction was most strongly enhanced in ferrous sulfate solution with the sample from Akenobe mines. Leaching experiments and oxygen consumption measurements were performed under various conditions with this sample and it was established that ferrous sulfate enhances chalcopyrite oxidation by dissolved oxygen.
2. Methods 2.1. Ore samples and reagents Chalcopyrite samples from four different sources were used. Two samples were concentrates from Akenobe mines, Hyogo, Japan and from Zhezkent, Kazakhstan; the
N. Hiroyoshi et al. / Hydrometallurgy Table I Results of analysis Source
Akenobe Zhezkent Unknown Ohmine
on the chalcopyrite
28.70 20.66 33.32 29.30
25.42 26.67 29.63 35.90
5.81 6.5 1 0.20 < 0.01
3.44 2.83 < 0.01 < 0.01
0.09 0.28 0.80
a Determined by XRD. b Mean ~01% Heywood diameter determined L Not detected.
Other minerals a
m.d (tom6 m “1
ZnS ZnS, FeS,, PbS N.D. ’ N.D. ’
8.0 25.5 5.3 4.5
by image analysis.
others were a massive ore from Ohmine mines, Iwate, Japan and one of unknown origin. Since surfactans affect chalcopyrite leaching [6,7], the concentrate was washed with I mol dm-’ HClO, to decompose xanthate adsorbed on the surface, then washed with distilled water and vacuum dried at 303 K. The absence of xanthate on the mineral surface was confirmed by Fourier transform infrared spectroscopy. The massive chalcopyrite was ground with an agate mortar and the -200 mesh size portion was used. The chemical composition, minerals and mean diameter of the samples are listed in Table I. Leaching solutions were prepared with reagent grade H2 SO,, FeSO, .7H,O, Fe2(S0,j3 nH,O, and distilled-deionized water. 2.2. Leuching
Leaching experiments were carried out in a 50 cm3 Erlenmeyer flask containing 10 cm3 of the leaching solution and 0.1 g of chalcopyrite, or in a 500 cm” Erlenmeyer flask containing 200 cm3 of the leaching solution and 2.0 g of chalcopyrite. The experiments were carried out in a capped flask with a gas permeable plug (silica plug) reciprocally shaken in a thermostatic water bath at 303 K in air. The supernatant samples for the chemical analysis were obtained by filtration and centrifuging. Metal ion concentrations were determined by atomic absorption spectroscopy and ferrous ions were determined by the o-phenanthroline method; the solution pH was measured. The amount of elemental sulfur produced was determined as follows: the leaching residue was collected by filter paper (Toyo Co., No. 5A), then dried at room temperature in a glove box purged with nitrogen. Elemental sulfur in the residue was extracted into benzene by a Soxlet extractor for 3 h. The concentration of elemental sulfur in the benzene was determined from the light absorption at 360 nm. 2.-q. Oxygen consumption
A YSI model 5300 biological oxygen monitor was used to measure dissolved oxygen consumption. A glass chamber capped with a gas permeable plug containing 1.0 g of chalcopyrite, 3 cm3 of the leaching solution and a magnetic stirring bar was shaken for
N. Hiroyoshi et al. / Hydrometallurgy 47 (1997) 37-45
? 1200 E m qot)o--
Fig. 1. Amount of extracted copper with four chalcopyrite samples leached for 168 h in 0.1 mol dm-’ acid solutions containing 0.1 mol dme3 ferrous ions or 0.1 mol drn-’ ferric ions.
24 h in the manner described in the leaching experiments and the chamber was moved to the monitor. A dissolved oxygen electrode was inserted into the suspension and the chamber was closed to air by a Lucite plunger. Then the dissolved oxygen concentration was measured for 2 h with stirring at 303 K.
3. Results and discussion The effects of ferrous sulfate and ferric sulfate additions on copper extraction were compared for the four chalcopyrite samples. As shown in Fig. 1, for the Zhezkent sample, the amount of extracted copper for 168 h is slightly larger in the ferric sulfate solution than that in the ferrous sulfate solution. With the other samples, however, the amounts of extracted copper for 168 h were larger when ferrous sulfate was added. Especially with the Akenobe sample, the amount of extracted copper was about 5 times larger in ferrous solution than in ferric solution. Using me Akenobe sample, leaching experiments and oxygen consumption measurements were performed under various conditions. Fig. 2 shows the effect of ferrous
~‘~‘~‘J’~‘( 0.02 FeSO,
Fig. 2. Effect of ferrous concentration flask.
on the amount of extracted
copper for 168 h; initial pH 1.8, 50 cm3
N. Hiroyoshi et al./Hydrometallurgy
E ul 800 F
47 (1997) 37-45 ,
0.04 mol dm3 Fe” NO ADDITIVES
Fig. 3. Effect of initial pH on the amount of extracted copper for 168 h with or wlthout ferrous sulfate: 50 cm” flask.
sulfate concentration on the amount of extracted copper for 168 h at an initial pH 1.8. The amounts of extracted copper increased considerably with increasing ferrous sulfate concentration. Fig. 3 shows the effect of the initial pH on the amount of extracted copper for 168 h. The amount of extracted copper depended strongly on pH and increased with decreasing pH when ferrous sulfate was added. When the experiment was carried out with 0.04 mol dmm3 ferrous sulfate at the initial pH 1.8 for 168 h, 0.77 X 10m3 mol elemental sulfur was detected for 1.0 g of the sample and this value was about 2 times the amount of extracted copper, 0.41 X 10m3 mol. Figs. 4-6 show the results of the leaching experiments with 0 and 0.04 mol dm-’ ferrous sulfate at an initial pH of 0.98 as a function of time. The results of the leaching experiments with 0.2 mol dme3 ferric sulfate and with 0.04 mol dm-3 ferrous sulfate purged with nitrogen are also shown in Fig. 4. Fig. 4 confirms that the amount of extracted copper is larger with ferrous sulfate than with ferric sulfate. Even with ferrous sulfate, copper extraction was negligible when the solution was purged with nitrogen. This indicates that ferrous ions do not leach chalcopyrite and that oxygen is needed to extract copper from chalcopyrite.
1000 % *
??0.04 mol dmJ Fe’*, AIR ??0.20 mol dm3 Fe’*, AIR .A NO ADDITIVES, AIR 0 0.04 mol dmJ Fe”, N1
lJJ 200 0’ 0 100
TIME I h Fig. 4. Amount of extracted copper as a function initial pH 0.98; 500 cm-? flask.
of time under aerobic (air) and anaerobic
IV. Hiroyoshi et al./ Hydrametallurgy 47 (1997) 37-45
100 TIME I h
Fig. 5. Effect of ferrous sulfate addition on the pH of the leach solution; 500 crnm3 flask.
In acidic solution, following reaction: CuFeS, In addition ions: 4Fe2++
+ 0, + 4H+ = Cu*+ + Fe*+ + 2s’ + 2H,O to this reaction,
0, = 4Fe”++
occurs in the presence
If ferric ions are produced by the reaction in Eq. (2) chalcopyrite oxidation ions (Eq. (3)) would be taking place in addition to the reaction in Eq. (1): CuFeS,
+ 4Fe3+ = Cu*+ + 5Fe2+ + 2s’
As shown in Fig. 5, the pH increases with time with ferrous sulfate addition, indicating that protons are consumed during the leaching by reactions in Eq. (1) and/or Eq. (2). As shown in Fig. 6, most of the soluble iron was present in the ferrous form and the ferric concentration was negligible. This result allows two interpretations: (a) ferrous . negligibly slow, or (b) ferric ions produced by the reaction in Eq. oxidation (Eq. (2)) IS (2) are rapidly consumed by the reaction in Eq. (3). If (a) is the case, chalcopyrite is
0.04 mol dme3Fe*‘, AIR
,” 3000 p S 2000 5 i
NO ADDITIVES, AIR-
TIME I h Fig. 6. Concentrations of total Fe (solid symbols) and Fe’+ (open symbols) in the leach solutions without ferrous sulfate as a function of time. Initial pH 0.98; 500 cm-’ flask.
N. Hiroyoshi et al. / Hydrometallurgy 47 (19971 37-45
5 G 0 ”
50.0 NO ADDITIVES
0 >” “0 2 E
TIME I h
Fig. 7. Effect of ferrous sulfate addition on dissolved oxygen consumption
leached by the reaction in Eq. (1). If (b) is the case, the reaction in Eq. (3) is related to copper extraction in addition to the reaction in Eq. (1). However, it should be noted that the reaction in Eq. (2) and the reaction in Eq. (3) take place in series and their rates are the same, since the rate of the reaction in Eq. (3) is determined by ferric supply according to the reaction in Eq. (2). Taking this into consideration, the total reaction is equivalent to the reaction in Eq. (1) even in the case of (b). Therefore we may conclude that the total reaction is described by the reaction in Eq. (1) in both cases. As the total equation is Eq. (1) and copper extraction was enhanced by adding ferrous sulfate (Figs. 2 and 4), the consumption rate of dissolved oxygen should increase by the addition of ferrous sulfate. To confirm this and to obtain more detailed information, oxygen consumption measurements were carried out in chalcopyrite suspensions with 0 and 0.04 mol dm-” ferrous sulfate at an initial pH of I .8, shaken for 24 h. As shown in Fig. 7, straight lines were obtained for the relationship between time and the logarithm of dissolved oxygen concentration. Therefore, the kinetic equation is d[O, l/dt
= - Lt&
The total rate constant, k,,,,, was determined as 1.09 hP ’ with 0.04 mol dm ’ ferrous sulfate and about three times larger than without ferrous sulfate (0.36 hh’ 1. In the presence of ferrous ions, dissolved oxygen may be consumed by ferrous oxidation (Eq. (2)) in the liquid phase and/or on chalcopyrite surface. To evaluate the rate of the reaction in Eq. (2) in the liquid phase, oxygen consumption was measured in the filtrate obtained from the chalcopyrite suspension with 0.04 mol dme3 ferrous sulfate shaken for 24 h, filtration was conducted with a membrane filter (pore size: 0.2 pm>. In the filtrate, the rate constant was negligible (0.07 hh’ ), indicating that oxygen was mainly consumed on the chalcopyrite sample. From these results, the following two models may be considered: (Al Ferrous ions catalyze the direct oxidation of chalcopyrite by dissolved oxygen according to the reaction in Eq. (1). The reactions in Eqs. (2) and (3) do not directly relate to the enhancement in copper extraction by ferrous addition. (B) On the chalcopyrite surface, ferrous oxidation by dissolved oxygen (Eq. (2)) is faster than in the liquid phase. Ferric ions produced on the surface oxidize chalcopyrite according to the reaction in Eq. (3) and they are simultaneously reduced to ferrous ions.
N. Hiroyoshi et al. / Hydrometallurgy 47 (1997) 37-45
In this model, ferrous and ferric ions adsorbed on the chalcopyrite surface mediate the electron transfer from chalcopyrite to dissolved oxygen. In both models, the total reaction is described by the reaction in Eq. (1). Model (B) seems to be adequate to explain the increase in copper extraction with increasing ferrous concentration, since the amount of ferrous ions adsorbed on chalcopyrite surface (the mediator for the electron transfer) would increase with increasing ferrous concentration. However, it is difficult to explain why chalcopyrite is more effectively leached in ferrous solution than in ferric solution, as ferric ions are the oxidant directly reacting with chalcopyrite in model (B). Model (A) does not contradict the result that chalcopyrite was more effectively leached in ferrous sulfate solution than in ferric sulfate solution since ferrous ions are a catalyst for the reaction in Eq. (1) and their role is independent of the reactions in Eqs. (2) and (3) in this model. As shown in Fig. 1, however, the effect of ferrous ions on copper extraction apparently depends on the chalcopyrite samples. This may indicate that impurities and other minerals in the chalcopyrite samples influence the chalcopyrite leaching in ferrous solution and further study is needed to establish these details of the leaching. The findings shown here are important in considering the role of iron oxidizing bacteria such as T. ferrooxiduns on chalcopyrite leaching. T. ferrooxidans catalyzes the ferrous oxidation (Eq. (2)) and it is generally accepted that the bacteria enhance chalcopyrite leaching. This understanding is based on the premise that ferric ions accelerate chalcopyrite leaching but that ferrous ions do not contribute to the leaching. However, as shown in Figs. 1 and 4, ferric solutions do not always leach chalcopyrite better than ferrous solutions. It must, therefore, be considered that the bacteria may suppress copper extraction for the chalcopyrite which is leached more effectively in ferrous solution than in ferric solution. The authors performed the bacterial leaching of chalcopyrite by T. ,ferrooxiduns using the Akenobe samples and confirmed that the amount of extracted copper decreased with increasing inoculated cell numbers when 0.04 mol dmP3 ferrous sulfate was added to the leach solution .
4. Conclusions In this study, chalcopyrite samples from four sources were leached in ferrous sulfate and ferric sulfate solution, and it was found that three samples were more effectively leached in ferrous sulfate solution. Especially, the sample from Akenobe mines showed a remarkable enhancement in copper extraction. Further leaching experiments and dissolved oxygen consumption measurements were carried out under various conditions with this sample. The amount of extracted copper increased markedly with increasing ferrous sulfate addition and with decreasing pH. Elemental sulfur was detected as a reaction product and the mole ratio of the sulfur to copper extracted was about 2. Soluble iron was dominantly present in ferrous form and protons were consumed during the leaching. The oxygen consumption of the sample was enhanced by adding ferrous sulfate. Based on these results, it was concluded that ferrous ions enhance the following reaction: CuFeS,
+ 0, + 4H+=
CL?+ + Fe”
+ 2s’ + 2H,O
N. Hiroyoshi et al./ Hydrometallurgy 47 (19971 37-45
Acknowledgements The authors wish to express appreciation to New Energy and Industrial Technology Development Organization for supplying the Zhezkent chalcopyrite concentrate.
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