Bioleaching kinetics of copper from copper smelters dust

Journal of Industrial and Engineering Chemistry 17 (2011) 29–35 Contents lists available at ScienceDirect Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec Bioleaching kinetics of copper from copper smelters dust Fereshteh Bakhtiari a,*, Hossein Atashi b, Mortaza Zivdar b, Seyedali Seyedbagheri c, Mohammad Hassan Fazaelipoor a a Department of Chemical Engineering, Faculty of Engineering, Shahid Bahonar University of Kerman, P.B. 76175-133 Kerman, Iran Department of Chemical Engineering, Faculty of Engineering, University of Sistan and Baluchestan, Zahedan 98164, Iran c Hydrometallurgy Research Group, R&D Center, Sarcheshmeh Copper Complex, Rafsanjan, Iran b A R T I C L E I N F O Article history: Received 2 December 2009 Accepted 10 February 2010 Available online 8 October 2010 Keywords: Bioleaching Copper flue dust Kinetics Shrinking core model A B S T R A C T The smelting factory of Sarcheshmeh Copper Complex in Iran produces about 50 tons per day of copper dust containing 36% Cu, 22.2% Fe and 12.2% S. The dust is currently recycled to the smelters. This method is not desirable in terms of operation, and energy consumption. A mixed culture of mesophilic bacteria was used to examine the bioleaching of copper from the dust. The effect of various parameters such as pulp density, nutrients, temperature, and the amount of pyrite added to the bioleaching media were examined in the dust bioleaching tests. It was shown that the bacteria contributed effectively in the leaching of copper from the dust. The collected data showed that at pH 1.8 and the pulp density less than 7%, the dissolution of copper followed shrinking core kinetic model and the process was limited by diffusion of lixiviant. With the pulp density of 7%, however, the process showed to be reaction limited. ß 2010 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. 1. Introduction Bioleaching is an efficient, simple and eco-friendly process for metal extraction from ores and concentrates as compared to other conventional processes. Bioleaching is used essentially for the recovery of gold, copper, cobalt, nickel, zinc and uranium [1,2]. Other suitable materials for bioleaching process are: reverberatory and converter furnaces slag and dust as well as flotation tailings [3–7]. About 50 tons per day of copper dust containing 36% Cu, 22.2% Fe and 12.2% S is generated as a byproduct in the smelting furnaces of Sarcheshmeh Copper Plant, Iran. The dust is recycled to the smelters which reduces their efficiencies and increases the burden on the unit as well as the required energy for smelting process; furthermore, it damages the refractory bricks. Previous studies have shown that the copper dusts of Sarcheshmeh smelting factory mostly contain the secondary copper sulfide minerals [4–6], which are more suitable than chalcopyrite in bioleaching [8,9]. Different types of microorganisms are capable of attacking sulfide ores and concentrates. The most common mesophilic bacteria present in sulfide leaching are the iron-and sulfur-oxidizing Acidithiobacillus ferrooxidans, the sulfur-oxidizing Acidithiobacillus thiooxidans, Acidithiobacillus caldus and the iron-oxidizing Leptospirillum ferrooxidans and Leptospirillum ferriphilum [2,8,10–13]. The role * Corresponding author. Tel.: +98 341 211 8298; fax: +98 341 211 8298. E-mail address: [email protected] (F. Bakhtiari). of microorganisms is to generate the leaching chemical and to create the space in which the leaching reactions take place. The bioleaching microorganisms need to be adapted to acidic conditions, the presence of certain heavy metals, and possibly a wide range of inorganic ions [14]. Several important parameters including temperature, pH, nutrients, pulp density, sulfide minerals, O2 and CO2, and metal toxicity affect bioleaching of copper [15]. A. thiooxidans can oxidize sparingly soluble sulfides (such as wurtzite), but not the sulfide, which are totally insoluble (such as covellite), except when iron is present [11]. The microbial oxidation of Fe2+ increases the Fe3+/Fe2+ ratio, and so the redox potential. When the redox potential is low and more Fe2+ is in solution, A. ferrooxidans would predominate, because this organism has a faster growth rate and will build up a larger number of cells in the system. However, as the redox potential increases due to a higher Fe3+/Fe2+ ratio, L. ferrooxidans would predominate, because this organism has a higher affinity for Fe2+ than does A. ferrooxidans. A. ferrooxidans is also more sensitive to inhibition due to high concentrations of Fe3+ in solution [10]. Gomez et al. [16] related the pulp density of a Cu–Zn–Fe complex sulfide ore to the extraction rate of metals by a Michaelis– Menten type equation. They also showed that the rate of copper and zinc bioleaching were controlled by the chemical reaction. Mehta et al. [17] showed that an indirect mechanism was involved in the bioleaching of a converter slag which was mostly an oxidic/ silicious material. Rodriguez et al. [18] studied the chalcopyrite bioleaching mechanism at low and high temperatures. They concluded that the bioleaching of chalcopyrite was possibly the 1226-086X/$ – see front matter ß 2010 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2010.10.005 30 F. Bakhtiari et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 29–35 Table 1 Chemical and mineralogical composition of the mixed dust. Element/mineral Cu (total) Cua Fe S Cu2S CuS CuFeS2 Cu5FeS4 Cu–N FeS2 Fe2O3 Fe3O4 Gangue Weight (%) Estimated portion of total Cu 35.8 12.9 15.3 12.2 18.6 61.6 1.7 4.7 2.0 2.9 2.6 6.8 5.8 24 0.3 0.9 14.8 53.3 a Acid soluble copper. Table 2 Chemical and mineralogical composition of the pre-leached mixed dust. Element/mineral Cu (total) Cua Fe S Cu2S CuS CuFeS2 Cu5FeS4 Cu–N FeS2 Fe2O3 Fe3O4 Gangue Weight (%) Estimated portion of total Cu 35.1 4.2 22.2 5.9 24.4 80.9 1.4 3.9 2.7 3.9 3.5 9.2 0.5 2.1 1.9 3.7 19.8 42.1 a Acid soluble copper. mixed to obtain a representative sample. Table 1 shows the Chemical and mineralogical characteristics of the sample. The main copper sulfide minerals in the dust were chalcocite 18.6%, bornite 2.6%, chalcopyrite 2% and covellite 1.7%. Screen size analysis showed that about 70% of particles were finer than 80 m (d70 = 80). Because of the substantial amount of acid soluble copper portion (13%), a pre-leaching test of 10% (w/v) pulp density was carried out in rolling bottles with sulfuric acid solution with pH = 1.5. It resulted in the dissolution of 80% of the acid soluble copper after 120 min. Chemical and mineralogical analysis of the pre-leached dust is in Table 2. simultaneous bioleaching of the pyretic phase of mineral, by an indirect mechanism via thiosulfate, and the indirect bioleaching of chalcopyrite, probably by a mechanism by the aid of polysulfide and elemental sulfur. Mehta et al. [19] showed that the biodissolution of the metals from Indian Ocean nodules in the temperature range of 293–308 K at a pH of 2 and 5% pulp density with the particle size of 300 to 75 mm followed shrinking core kinetic model. The purpose of this research was the assessment of bioleaching for copper extraction from the smelting furnaces dust using a mixed native culture of A. ferrooxidans, A. thiooxidans and L. ferrooxidans. To that end, different variables were studied (pulp densities, nutrients, temperature, and the amount of pyrite added to the bioleaching media). Furthermore, we obtained the kinetic of bioleaching as a function of pulp density. 2.2. Microorganisms Previously isolated native species of A. ferrooxidans, A. thiooxidans and L. ferrooxidans from Sarcheshmeh Copper Mine [20] were used. The isolates were separately grown in 9K medium [21], containing 44.2 g/L ferrous sulfate (pH = 1.8) for A. ferrooxidans, 40 g/L ferrous sulfate (pH = 1.7) for L. ferrooxidans and 10 g/L elemental sulfur (pH = 2) for A. thiooxidans. Cultures were incubated at 32 8C in a temperature-controlled orbital shaker at 150 rpm. The bioleaching shake flask tests were performed with a mixed culture of the above isolates. The isolates were grown 2. Experimental 2.1. Sample preparation The substrate was a sample of smelters copper dust from the smelting factory of Sarcheshmeh Copper Complex, Iran. Samples were collected on different days during the smelting process and Table 3 Test conditions in biological leaching experiments. Variable factor Test no. Flask no. Pulp density (%) Pyrite addition (%) Nutrient medium Temp. (8C) Inoculation (%) Pulp density 1 1 2 1 2 1 2 1 2 3 4 1 2 3 4 1 2 3 1 2 3 4 1 2 3 4 2 – 32 4 – 7 – 2 3 – 0.2 1.0 – – 0.2 1.0 – – 0.2 – – 3 – 9K Dist. watera 9K Dist. water 9K Dist. water 9K 9K 9K Dist. water 9K 9K 9K Dist. water 9K 9K Dist. water 9K Norris M Dist. water 9K Norris M Dist. water 20 – 25 – 40 – 20 20 20 – 25 25 25 – 40 40 – 20 20 20 – 20 20 20 – 2 3 Pyrite addition 4 5 6 Nutrient medium 7 Temp. (8C) 8 a Distilled water (control). 4 7 32 32 32 32 32 32 36 [(Fig._2)TD$IG] 31 F. Bakhtiari et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 29–35 Table 4 Nutrient media used in dust bioleaching tests. Nutrient (g/L) [(Fig._1)TD$IG] 9K [21] M [22] Norris [23] MgSO47H2O K2HPO4 KCl Ca(NO3)2H2O 3 1 0.2 0.5 0.24 0.2 0.63 0.5 0.2 0.1 0.05 – 0.014 – – a 120 100 (NH4)2SO4 2% solid 2% (control) 4% solid 4% (control) 80 1/V (L.h/g) Medium a 7% solid 7% (control) 60 40 y = 1890.8x + 2.7052 2 R = 0.997 Redox Potential (mV) 800 20 700 0 600 0 400 0.03 0.04 0.05 0.06 b1000 300 800 5 10 15 20 25 30 35 1/V (L.h/g) 0 Time (days) b Cu Concentration (g/L) 0.02 1/S (L/g) 200 2% solid 2% (control) 4% solid 4% (control) 7% solid 7% (control) 20 18 16 14 12 10 8 6 4 2 0 600 400 y = 15099x + 77.443 2 R = 0.9852 200 0 0 0.01 0.02 0.03 0.04 0.05 0.06 1/S (L/g) Fig. 2. Plot of Michaelis–Menten for the (a) copper dissolution, (b) iron dissolution. 0 5 10 15 20 25 30 35 Time (days) c 2% solid 2% (control) 4% solid 4% (control) 7% solid 7% (control) 100 Cu Extraction (%) 0.01 500 80 60 40 20 0 0 5 10 15 20 25 30 35 Time (days) Fig. 1. Influence of pulp density on the (a) redox potential, (b) Cu concentration, (c) Cu extraction. separately, and mixture of the three cultures was used as the bioleaching solution. The cultures were as the exponential phase of growth when they were mixed. The volume percentages of the cultures in the final solution were 40% for A. ferrooxidans, 40% for L. ferrooxidans and 20% for A. thiooxidans. and bacterial solutions incubated in a Kohner orbital shaker incubator at 150 rpm. For all experiments the initial pH was adjusted to 1.8. The pH was measured by a pH meter and maintained at 1.8 using concentrated sulfuric acid. Oxidation– reduction potential (ORP) of all the flasks was measured as an indicator of bacterial growth. Some samples were periodically taken from each flask to analyze copper and iron tenors by Atomic Absorption Spectroscopy. Evaporation and sampling losses were compensated by the addition of distilled water and 9K nutrient medium, respectively. After the completion of the tests, the solid residue were washed and dried to determine the copper and iron content. One factor at a time procedure was used to investigate the effect of pulp density, nutrient medium, temperature and the amount of pyrite added to the bioleaching media on the bioleaching (Table 3). The composition of the nutrient media is presented in Table 4 [21– 23]. The results of each series of the experiments were compared with the control samples containing 10% (v/v) of thymol in methanol. Table 5 Values of maximum rate of dissolution, Vm, and kinetic constant Ks, for copper and iron bioleaching. Element 2.3. Bioleaching experiments All bioleaching tests were performed in 1000 ml Erlenmeyer containing 2–7% (w/v) of the pre-leached dust, 400 ml of nutrient Cu Fe Constant Vm (mg/L h) Ks (g/L) 369.66 12.9 698.95 194.97 [(Fig._3)TD$IG] 32 F. Bakhtiari et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 29–35 0.6 0.6 2% solid 3% solid 0.5 1-(1-XCu)1/3 1-(1-XCu)1/3 0.5 0.4 0.3 0.2 0.4 0.3 0.2 y = 0.0242x y = 0.0253x 2 2 R = 0.5852 0.1 R = 0.8852 0.1 0 0 0 5 10 15 20 0 25 5 10 Time (days) 20 25 Time (days) 0.5 0.6 7% solid 4% solid 0.4 1-(1-XCu)1/3 0.5 1-(1-XCu)1/3 15 0.4 0.3 0.2 0.3 0.2 y = 0.0127x y = 0.0189x 2 2 0.1 R = 0.9879 0.1 R = 0.9888 0 0 0 5 10 15 20 25 30 0 5 10 Time (days) [(Fig._4)TD$IG] 15 20 25 30 35 40 Time (days) Fig. 3. Chemical kinetics control model for biodissolution of copper from copper dust. 1.2 1.4 2% solid 3% solid 1.2 1 0.8 0.8 XCu XCu 1 0.6 0.6 0.4 0.4 y = 0.0504x y = 0.0497x 2 2 0.2 R = 0.1804 0.2 R = 0.6974 0 0 0 5 10 15 20 0 25 5 10 15 20 25 Time (days) Time (days) 1 1.2 7% solid 4% solid 1 0.8 0.8 XCu XCu 0.6 0.6 0.4 y = 0.0259x 0.4 y = 0.0349x 2 2 0.2 R = 0.9132 0.2 R = 0.9656 0 0 0 5 10 15 20 Time (days) 25 30 0 5 10 15 20 25 Time (days) Fig. 4. Liquid film diffusion control model for biodissolution of copper from copper dust. 30 35 40 33 F. Bakhtiari et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 29–35 Bioleaching of copper dust was initially performed to study the feasibility of the process for the pulp densities of 2, 4, 6, 8, 10, 12, 14% (w/v). The results showed, except in the flask with pulp density of 2%, no growth in the other flasks (not shown). It revealed the possibility of the bioleaching of the copper dust by these microorganisms, but it was necessary to adapt the bacteria to the higher concentrations of the dust gradually. The inoculum of each test was prepared by culturing the bacteria of previous test which adapted with lower ferrous sulfate and elemental sulfur. In this paper only the results of 2, 4 and 7% (w/v) solid–liquid ratio were presented (tests 1–3). 3. Results and discussion 3.2. Bioleaching experiments Fig. 1 presents the effect of different pulp densities on the redox potential, copper concentration and copper recovery. The results of increasing the pulp density showed an increase in the lag phase of bacterial growth as well as the increasing of acid consumption, toxicity of metal ions, copper concentration, and shear stress. They, in turn, resulted in the decrease of the ORP and copper recovery. According to the copper recovery curves, maximum copper extraction from the control and the biological conditions for the pulp densities of 2%, 3%, 4%, and 7%, were found as 42.2% and 90.1% in 23 days, 45.9% and 83.1% in 20 days, 45.2% and 89.2% in 29 days, and 43.4% and 81.9% in 35 days, respectively. A Michaelis–Menten type equation was used to relate the pulp density to the bioleaching rate. The equation has been given as: Samples of mesophile bacteria containing A. ferrooxidans, A. thiooxidans and L. ferrooxidans separated from acid mine drainage, were isolated. The ability of the mixed culture isolates to grow on different concentration of copper dust was evaluated. The culture grew well on the copper dust at low pulp densities (below 2%), but no growth was observed at a solid concentration of 5%, because of sudden increasing of the pulp and copper content. Therefore, the solid concentrations were gradually increased and the culture tolerated 7% (w/v) pre-leached dust and 20 g/L of copper in solution after a two-month period. Microscopic observation of bacteria in solution showed that the proportion of L. ferrooxidans had improved with increasing the solid concentrations. V¼ V m :S ð K S þ SÞ 0.16 3% solid 2% solid 0.16 1-2/3XCu-(1-XCu)2/3 1-2/3XCu-(1-XCu)2/3 (1) where V is the extraction rate of metals, Vm is the maximum metal extraction rate, S is the pulp density and Ks is the Michaelis’ constant. This constant gives an idea about the affinity of the bacteria to the mineral substrate. In Fig. 2a and b plots of 1/V versus 1/S for copper and iron dissolutions are shown, respectively. From these representations the kinetic parameters Vm and Ks for copper and iron dissolutions can be obtained. Their values are summarized in Table 5. These results confirmed that iron dissolved better than copper from the copper flue dust, because Vm,Fe is 28 times higher than Vm,Cu. The higher value of Ks,Cu than Ks,Fe might indicate a preference of the microorganisms for the copper from the dust. 0.2 0.12 0.08 y = 0.0084x 2 R = 0.9349 0.04 0 0.12 0.08 y = 0.0072x 2 R = 0.9962 0.04 0 0 5 10 15 20 25 0 5 10 Time (days) 15 20 25 Time (days) 0.2 0.16 4% solid 7% solid 1-2/3XCu-(1-XCu)2/3 0.16 1-2/3XCu-(1-XCu)2/3 [(Fig._5)TD$IG] 3.1. Isolation and adaptation of the culture 0.12 0.08 y = 0.006x 2 R = 0.9917 0.04 0 0.12 0.08 y = 0.0035x 2 R = 0.9037 0.04 0 0 5 10 15 20 Time (days) 25 30 0 5 10 15 20 25 Time (days) Fig. 5. Ash layer diffusion control model for biodissolution of copper from copper dust. 30 35 40 [(Fig._6)TD$IG] 34 F. Bakhtiari et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 29–35 (2) ¼ kobs: t where XCu is the copper recovery at time t and kobs. is the observed rate constant [17,20]. The rate controlling factor is the diffusion of the lixviant via ferric sulfate and sulfuric acid through the passive layer such as jarosite which was identified in the XRD of the leached residue. The plot of [1  (1  XCu)1/3] versus time (Fig. 3) for the pulp density 7% showed the applicability of the chemicalcontrolled shrinking core model for the copper extraction. Bioleaching of the dust with a mixed culture of mesophile bacteria is governed by the bacterial oxidation (Eqs. (3)–(10)) providing intermediate products such as sulfurous acid from sulfur and ferrous iron from covellite, chalcocite, chalcopyrite, bornite and pyrite. CuS þ 2O2 ! Cu2þ þ SO4 2 (3) Cu2 S þ 0:5O2 þ 2Hþ ! CuS þ Cu2þ þ H2 O (4) Cu2 S þ 2:5O2 þ 2Hþ ! 2Cu2þ þ SO4 2 þ H2 O (5) Cu5 FeS4 þ 9O2 þ 4Hþ ! 5Cu2þ þ Fe2þ þ 4SO4 2 þ 2H2 O (6) þ 2H 200 þ 5 10 15 20 25 30 35 40 45 50 55 Time (days) b 2%solid, no pyrite 4%solid, no pyrite 7%solid, no pyrite 2%solid, 0.2% pyrite 4%solid, 0.2% pyrite 7%solid, 0.2% pyrite 2%solid, 1% pyrite 4%solid, 1% pyrite 7%solid (control) 20 18 16 14 12 10 8 6 4 2 0 5 10 15 20 25 30 35 40 45 50 55 Time (days) c (10) A typical source of energy is the oxidation of a mineral such as pyrite. So, pyrite was used in order to study its potential effect on the enhancement of copper recovery. Results showed that the addition of pyrite to the pulp densities of more than 2% did not have a considerable effect on the ORP, copper concentration and recovery (Fig. 6). Pyrite addition to the higher pulp density slurries had a reverse effect on copper recovery because of the high shear stress, toxicity of metal ions and precipitation of jarosite. This precipitate may cover the dust particles surface, resulting in a decrease in the oxidation of these substrates by the microorganisms. Maximum copper extraction was 89.2% after 23 days 2% pulp density with 0.2% pyrite addition. The high concentration of pyrite seems to have an inhibitory effect. This is in agreement with the results obtained in previous investigations for bioleaching of synthetic chalcocite and covellite by A. ferrooxidans [24]. The effect of nutrient medium on the ORP, copper concentration and extraction are shown in Fig. 7. The results show that Norris medium, although increased the bacterial lag phase, represents the highest recovery of copper. Furthermore, it contains the least amount of the nutrients, which is very important from the economical point of view. Moreover, because of the low concentration of sulfate, ammonium and potassium ions, the precipitation of jarosites reduced. Therefore, Norris is the most suitable culture medium in copper dust bioleaching process. At Cu Extraction (%) (8) (9) þ 4SO4 300 2%solid, no pyrite 4%solid, no pyrite 7%solid, no pyrite 2%solid, 0.2%pyrite 4%solid, 0.2%pyrite 7%solid, 0.2%pyrite 2%solid, 1%pyrite 4%solid, 1%pyrite 7%solid (control) 90 2CuFeS2 þ 8:5O2 þ 2Hþ ! 2Cu2þ þ 2Fe3þ þ 4SO4 2 þ H2 OðIndirectÞ 2FeS2 þ 7:5O2 þ H2 O ! 2Fe 400 100 (7) 2 500 0 2Cu5 FeS4 þ 18:5O2 þ 10Hþ ! 10Cu2þ þ 2Fe3þ þ 8SO4 2 þ 5H2 O 3þ 2%solid, 1%pyrite 4%solid, 1%pyrite 7% solid (control) 600 0 CuFeS2 þ O2 þ 4Hþ ! Cu2þ þ Fe2þ þ 2S0 þ 2H2 OðDirectÞ 2%solid, 0.2%pyrite 4%solid, 0.2%pyrite 7%solid, 0.2%pyrite 700 Redox Potential (mV) 12 3X Cu  ð1  X Cu Þ2=3 2%solid, no pyrite 4%solid, no pyrite 7%solid, no pyrite a Cu Concentration (g/L) To relate the pulp density to the metal extraction, which directly depends on the bacterial activity, a kinetics study has been performed. According to the bioleaching data, various kinetic models such as chemical, liquid film and ash diffusion control were examined (Figs. 3–5). The kinetic data for the pulp densities of 2%, 3% and 4%, showed a better fit to the diffusion controlled shrinking core model which is given by Eq. (2) (Fig. 5): 80 70 60 50 40 30 20 10 0 0 5 10 15 20 25 30 35 40 45 50 55 Time (days) Fig. 6. Influence of pulp density and pyrite addition on the (a) redox potential, (b) Cu concentration, (c) Cu extraction. optimum condition, maximum copper recovery was 84.1% after 20 days compared with control medium of 45.9%. To investigate the effect of temperature on the copper recovery, an experiment was performed at 36 8C with 3% pulp density. No ORP increase was observed at 36 8C in all media while the copper extraction in the bacterial media was more than the control medium. It seems that the microorganisms dissolved the copper without ORP increase. Maximum copper extraction was 73.8% after 25 days in 3% pulp density and 9K medium at 36 8C compared with 83.1% after 20 days at 32 8C. As a result, the mixed culture was performed better at 32 8C for bioleaching of the copper dust. In all cases, due to the presence of acid consuming compounds, especially, limestone, changes of pH were high in the early stages of the tests. As expected, increasing the pulp density increased the acid consumption, while pyrite addition did not have substantial effect on the amount of acid consumption. Therefore, pyrite did not play an important role in the acid production in the copper dust bioleaching. The acid consumption in 9K medium was the same as Norris medium and more than M medium, while the control [(Fig._7)TD$IG] F. Bakhtiari et al. / Journal of Industrial and Engineering Chemistry 17 (2011) 29–35 a 9K, 32°C 9K, 38°C Norris,32°C Norris, 38°C M, 32°C M, 38°C Control, 32°C Control, 38°C Redox Potential (mV) 700 600 500 400 300 200 0 5 10 15 20 25 Time (days) b 9K, 32°C 9K, 38°C Norris, 32°C Norris, 38°C M, 32°C M, 38°C Control, 32°C Control, 38°C Cu Concentration (g/L) 8 7 pyrite is acid producing, the process of copper dust bioleaching was acid consuming. The apparent lack of a universal mechanism to explain the bioleaching of different copper sulfide minerals in the copper dust warrants fundamental studies of biological and electrochemical mechanisms to understand optimized conditions and to develop models for bioleaching process. The bioleaching data from copper smelters dust followed the diffusion controlled shrinking core kinetic model at the temperature 32 8C and up to 4% of solid concentration. Higher pulp density about 7% may interfere with the mass transfer of oxygen and carbon dioxide. Therefore, the kinetic model may change to a combination of diffusion and reaction controlled mechanisms. It is possible that this limitation can be overcome with an increase in aeration and agitation in a laboratory reactor [4,5]. Overall, the results showed the possibility of copper extraction from Sarcheshmeh pre-leached copper dust using a mixed native mesophile bacteria, and it could be an alternative and promising process to cope with the problem of dust accumulation in the plant. 6 5 Acknowledgments 4 3 This paper is published with the permission of National Iranian Copper Industries Company. The assistance from the staff of the R&D Division of National Iranian Copper Industries Company is gratefully acknowledged. 2 1 0 0 5 10 15 20 25 Time (days) c 9K, 32°C 9K, 38°C Norris, 32°C Norris, 38°C References M, 32°C M, 38°C Control, 32°C Control, 38°C 100 90 Cu Extraction (%) 35 80 [1] [2] [3] [4] [5] 70 60 [6] 50 [7] 40 30 [8] 20 10 [9] 0 0 5 10 15 20 25 Time (days) Fig. 7. Influence of nutrient medium and temperature on the (a) redox potential, (b) Cu concentration, (c) Cu extraction, (3% pulp density). medium consumed less acid. Therefore, when the microorganisms performed well, the acid consumption was increased and vice versa. [10] [11] [12] [13] [14] [15] 4. Conclusions In the bioleaching tests of copper dust samples, increasing the pulp density caused an increase in the adaptation time of the microorganisms and a decrease in the final copper recovery. Norris nutrient medium was the most suitable medium because of higher copper extraction and lower concentrations of basal salts. Pyrite addition and higher temperatures did not have a significant effect on the ORP and copper recovery. Despite the fact, that oxidation of sulfides in copper concentrate containing substantial amount of [16] [17] [18] [19] [20] [21] [22] [23] [24] F. Acevedo, EJB 3 (3) (2000) 184. D.E. Rawlings, J. Ind. Microbiol. Biotechnol. 20 (1998) 268. F. Acevedo, J.C. Gentina, J. 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