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Bioleaching of copper and cobalt arsenic-bearing ores: A chemical and mineralogical study
397
Bioleaching o f copper mineralogical study
and
cobalt
arsenic-bearing
ores:
A
chemical
and
J.V. Wiertz a, R. Lunarb, H. Maturana c and B. Escobar a aDept. de Ingenieria de Minas - Universidad de Chile, Chile. bDept. de Cristalografia - Universidad Complutense de Madrid, Spain. CDept. de Ingenieria de Minas - Unversidad de la Serena, Chile.
Arsenic is a major impurity present in numerous sulfide deposits. Two types of problems are associated with the presence of this element: metallurgical and environmental problems. Both enargite (Cu3AsS4)and cobalt bearing sulfides ((Co,Fe)(As, Sh) contain arsenic as a main component. They are present in important Chilean deposits and have been the object of very scarce studies. This work is a multidisciplinary study that includes chemical, biological and mineralogical aspects. Both minerals could present important variations in their chemical composition and belong to isomorphe series. Enargite forms part of the enargite-luzonite-famatinite series and the cobalt bearing phase to a complex family of sulfides that includes arsenopyrite (FeAsS), 1611ingite (FeAs2) and glaucodot ((Fe, Co)AsS). Both minerals also contain variable amounts of minor elements like Sb, Bi and Ag in the case of enargite and like Ni, Pb and other elements for the cobalt bearing sulfides. The presence of these elements affects significantly the crystal structure and the defect density and consequently modifies the susceptibility of the minerals to leaching and bioleaching processes. Depending on the control stage of the leaching process, these changes in the structure and composition of the mineral can modify the global kinetics of the leaching, inducing changes in the chemical reaction, its kinetics and the extension and composition of the passivating layer or enhancing local attack on crystal disturbed zones. Furthermore, these elements are generally toxic for microorganisms and their dissolution could inhibit bacterial activity in the bioleaching process, particularly in a concentrate leaching process. A bacterial adaptation and a control of dissolved impurities would then be required. Small changes in the mineral composition and structure can then explain the great differences observed in the result of a same process applied to minerals that were expected to be similar but indeed have different origin and composition. This work demonstrates and illustrates the importance of a complete mineralogical characterization of the ores and concentrates at both macroscopic and microscopic scales.
398 I. INTRODUCTION During the last ten years, the presence of arsenic as a major impurity in base metal sulfide ores has been the object of growing concern. The presence of arsenic results in two types of problems. On the one hand, arsenic produces metallurgical problems, making difficult the metal extraction and the recovery of a final product of high purity. On the other hand, arsenic that is thought to be a highly toxic contaminant also results in environmental problems due to its atmospheric release and possible water contamination associated to the processing of arsenic bearing ores and concentrates. In this context, leaching processes are regarded as attractive options for the treatment of such kind of ores. Among those processes, bacterial leaching in acid medium has been successfully applied in copper metallurgy and a new bioleaching process has been recently developed for cobalt recovery. Nevertheless, the processing of complex arsenic bearing ores and concentrates requires a good understanding of the mechanisms involved previous to the design and development of an adapted sequence of process units. In particular, a complete chemical and mineralogical characterization of the ore is essential from both biological and metallurgical points of view in order to determine the possible inhibition problems and the requirement for effluent treatment processes. In the present work, this approach will be illustrated and discussed with two examples. The first case is a bacterial leaching study of pure enargite and the second is related with the design of a leaching process for a complex cobalt concentrate. Enargite (Cu3AsS4) is a common sulfide present in numerous porphyry copper deposits of the North of Chile and Peru. It belongs to the isomorphe series enargite-luzonite-famatinite [ 1, 2]. The crystal structure of enargite is an orthorhombic space group Pmn2I [3] wich allows substitution of As by Sb up to 6% in weight. Only few studies have been carried out on leaching and bioleaching of enargite [4-6]. Enargite has shown to be a mineral strongly refractory to leaching in sulfuric acid medium. A passivation behavior of the sulfide dissolution has been observed in both leaching and bioleaching processes [6]. Cobalt sulfides ((Co,Fe)(As, S)2) exist in several locations in the world, associated with cobaltiferous pyrrothite ores containing arsenic as a main component, and represent an important potential reserve of cobalt although they are not yet exploited due to metallurgical and environmental limitations. Their composition vary in a wide range from arsenopyrite (FeAsS), usually containing small amount of cobalt, to glaucodot ((Co,Fe)AsS) with a cobalt/iron ratio higher than 1/2. Substitution of sulfur by arsenic like in 1611ingite (FeAs2) represents another extreme phase of the series. Both enargite and cobalt bearing sulfides also contain variable amounts of other elements like Sb, Bi and Ag in the case of enargite and like Ni, Pb and Cu and other elements in the cobalt sulfides. We will analyze and discuss how the presence of such impurities could be determinant for the dissolution behavior of the mineral in both chemical and biological leaching.
2. MATERIAL AND METHODS 2.1. Mineral and concentrate samples The Department of Geology, University of Chile, provided the enargite sample used in this study. Chemical and mineralogical analysis confirmed the high purity of the sample. Only small amounts of quartz, pyrite and chalcopyrite were observed in the selected mineral
399 particles. The mineral was ground and separated into different granulometric fractions. The chemical analysis of the ground fraction-100 +150 mesh (-147 +104 lam) used in the leaching study showed the following metal content: Cu: 46.2%; As: 16.3%; Fe: 0.55%. A copper-cobalt sulfide concentrate was prepared from a complex Cu-Co ore with high content of As from Martincito Mine, located North of La Serena. The chemical composition of the concentrate is: Cu: 13.5%; Co: 0.74%; Fe: 30.9%; As: 8.4%. The main minerals are: pyrrhotite, chalcopyrite, a cobalt sulfide phase (intermediate between arsenopyrite, glaucodot and 1611ingite) and iron oxides (magnetite, hematite and goethite).
2.2. Leaching conditions Enargite leaching experiments were performed in 250 ml Erlenmeyer flasks using 5 g of mineral sample -100 +150 mesh (-147 +104 lum) in 100 ml of MC medium pH 1.6 [6] as leaching solution (5% pulp density). The flasks were shaken on a rotary shaker and maintained at 30~ Chemical leaching experiments were performed in MC medium pH 1.6 without iron or with 3.0 g Fe3+/1 added as ferric sulfate. Bacterial leaching experiments were performed under the same conditions but with 4 x 10 6 viable bacteria initially added to the flasks as inoculum. Each experiment reported was performed in duplicate. Cu-Co concentrate samples (1 g) were leached under different conditions in 100 ml MC medium pH 1.6. Chemical leaching experiments were performed with and without initial addition of 3.0 g Fe3+/l added as ferric sulfate. Bacterial leaching experiments were performed under the same conditions with and without initial addition of ferric sulfate.
2.3. Analysis For enargite leaching experiments, the flasks were periodically analyzed for pH and Eh; ferrous and total iron were analyzed by the o-phenanthroline colorimetric method [7, 8]. Total arsenic and copper concentrations in the leaching solution were determined by atomic absorption spectrophotometry. Arsenic(m) was determined by titration [9]. Total bacteria concentration was determined microscopically by direct counting in a Petroff-Hauser chamber. Viable cells were determined by plating in solid agarose medium [10].
2.4. Electron microscope Scanning Electron Microscope (SEM) analysis were carried out with a microscope JEOLJSM-6400 with a resolution of 35A and equipped with BSE detector and XEDS micro-analysis system.
2.5. Microprobe analysis Chemical composition of enargite sample was obtained by electron-microprobe analysis (EPMA) with a JEOL Superprobe JXA 8900-M analyzer.
3. RESULTS AND DISCUSSION
3.1. Leaching experiments Enargite is a sulfide very refractory to the leaching process at 30~ in sulfuric acid medium in the presence of ferric ion. A passivation behavior has been observed as evidenced by the decrease of leaching rate shown in Figure 1. In the presence of bacteria, leaching rate was higher but a similar decrease of the dissolution rate was observed. In all cases, copper and
400 12 10
d
o~
'1
8
4
~-
I
I
I
0
200
400
600
Time - - m - - F e r r i c Leaching
-=
8OO
(hours)
Bacterial Leach
,~
Ferric Bact Leach
Figure 1 Chemical and bacterial leaching of enargite arsenic dissolution rate were similar, suggesting that no selective leaching process is taking place. Leaching experiments on Cu-Co concentrate show significant cobalt dissolution under the different conditions (see Table 1). Cobalt extraction was very high (more than 90%) in both bacterial leaching experiments, with or without initial ferric ion. Moreover, the dissolution seems to be very selective and only a small fraction of copper is leached, especially in bacterial leaching without initial ferric ion (exp. 1). Without bacteria, the cobalt dissolution was lower while copper dissolution increased. Surprisingly, the highest copper dissolution was obtained in the chemical leaching without ferric ion (exp. 4). This suggests that copper dissolution is prevented by the activation of a galvanic interaction when the redox potential of the solution is high, as in bacterial leaching and in chemical ferric leaching. Table 1 Results of Cu-Co concentrate leachin ,.xperiment Experiment pH Bact. Inoc. Fe3~ Metal extraction (%) N~ (ml) (g/l) Cu Co 5.2 94.4 1.6 2.5 1.6
3.2.
3
1.6
4
1.6
2.5
3
Solid Cu content (%) Initial Final 13.75 27.74
8.7
49.7
13.75
20.92
14.8
99.4
13.75
20.56
40.8
43.2
13.75
17.98
Electron microscopy
Enargite residues leached under different conditions were observed and analyzed by Scanning Electron Microscope (SEM). Elemental sulfur that forms locally on enargite surface during the chemical ferric leaching
401
Figure 3" SEM microphotography of a leached crystal of enargite (bacterial ferric leaching).
402 does not form a continuous layer but appears as small crystal agglomerates (Figure 2). This was previously reported by Dutrizac in the leaching of chalcopyrite [ 11 ]. The enargite residues from bacterial and ferric-bacterial leaching experiments present evidence of a stronger attack. Oriented structures of attack on the surface of enargite crystals can be observed. During the ferric-bacterial leaching, the dissolution is more important and some enargite crystals are strongly attacked (Figure 3). In this case, no elemental sulfur is observed on the surface of leached crystals. Microscopic observation of leached residues of Cu-Co concentrate showed an extensive leaching of pyrrhotite and cobalt bearing phases while chalcopyrite was only partially attacked. Ferric arsenate precipitates were also identified. 3.3.
Microprobe analysis
Microprobe analysis were also performed to determine variations in the chemical composition of the minerals and to detect the presence of minor elements and its possible relation with the observed behavior and patterns of chemical and biological leaching. Tables 2 and 3 show the results of microprobe analysis of unleached enargite and cobalt mineral samples, respectively. Table 2 Selected micro 9robe anal, ,sis of unleached enar Sample - Cu As Sb S Fe Bi Enargite* 48.40 19.04 32.56 En4-1 49.29 18.02 0.67 3i.01 0.00 0.08 En4-2 49.18 18.22 0.62 31.20 0.02 0.14 En4-3 48.76 17.85 0.46 32.52 0.00 0.08 En4-4 49.05 17.94 0.42 32.27 0.00 0.10 En2-5 48.82 19.38 0.49 30.55 0.00 0.22 En2-6 47.76 17.68 0.98 33.34 0.14 0.17 En2-7 47.33 17.73 1.46 32.31 0.26 0.11 En2-8 47.61 18.11 0.80 32.56 0.16 0.19 En2b-9** 48.25 18.58 1.29 30.42 0.01 0.13 En2b-10** 48.43 18.38 1.27 30.47 0.01 0.17 * Stoechiometric composition of enargite ** En2b-9 and En2b-10 correspond to a fractured zone in a single enargite crystal
Ag 0.03 0.01 0.01 0.02 0.04 0.01 0.02 0.01 0.51 0.56
Total 100.00
99.10 99.39 99.38 99.80 99.50 99.90 99.22 99.44 99.19 99.29
As shown in Table 2, the composition of the analyzed samples differs slightly from the expected stoechiometric composition of enargite, with a lower arsenic content and variable amounts of antimony, bismuth and silver. Analysis of the fractured zone of an enargite crystal (samples En2b-9 and En2b-10) show a significantly higher silver concentration and a lower sulfur content. A good correlation has been observed between the presence of such type of crystal defect and strongly attacked areas during chemical and bacterial leaching. No significant differences have been observed in the chemical composition of leached particles of enargite, suggesting that no selective metal dissolution is taking place during the chemical and biological leaching of enargite. However, significant amounts of elemental sulfur have been observed on the surface of partially reacted particles of enargite leached under chemical conditions, without bacteria.
403 Table 3 Selected micro 3robe anabrsis of unleached Sample (~o Fe As 17 8,12 26,66 49,00 20 10,84 1 8 , 3 4 69,57 21 7,76 26,04 53,06 22 10,80 1 8 , 6 0 68,37 23 7,19 27,46 47,93 24 7,25 27,21 49,26 28 7,17 27,41 48,72 29 8,04 26,64 48,34
cobalt mineral sam S Cu 16,42 0,00 1,01 0,07 13,65 0,07 1,02 0,05 16,77 0,04 16,08 0,00 16,21 0,05 15,83 0,02
Bi 0,08 0,00 0,03 0,08 0,00 0,08 0,10 0,05
Pb 0,07 0,02 0,02 0,00 0,00 0,02 0,03 0,00
Total 100,35 99,85 100,65 98,97: 99,421 99,91 99,70 98,94
The composition of cobalt bearing mineral samples (Table 3) shows variations in a wide range. When cobalt concentration decreases, iron concentration increases, suggesting a replacement of cobalt atoms by iron atoms. A same behavior can be observed between sulfur and arsenic atoms. In Table 4, the same results are expressed in molar fractions, assuming that the sum of cobalt, iron, arsenic and sulfur molar fractions is one. As it can be seen from these results, the sum of cobalt and iron fractions as well as the sum of arsenic and sulfur fractions are quite constant and the global composition is close to (Co,Fe)(As, S)2. Table 4 Molar composition of selected sam_~les of cobalt mineral (molar fraction) Sample Co As Fe Co+Fe As+S B A 17 0,08 0,27 0,37 0,29 0,35 0,65 20 0,12 0,22 0,63 0,02 0,35 0,65 21 0,08 0,41 0,25 0,65 0,27 0,35 22 0,13 0,23 0,62 0,02 0,35 0,65 23 0,07 0,28 0,36 0,29 0,35 0,65 24 0,07 0,28 0,37 0,28 0,34 0,66 28 0,07 0,28 0,37 0,29 0,35 0,65 29 0,08 0,27 0,37 0,28 0,35 0,65 98* 0,05 0,26 0,35 0,34 0,31 100" 0,06 0,24 0,34 0,36 0,30 * Samples 98-100 correspond to partially leached mineral particles
0,69 0,70
A/B 0,53 0,53 0,53 0,55 0,53 0,53 0,53 0,54 0,45 0,43
In the same Table 4, the last 2 lines correspond to partially leached particles selected from the solid residues of the bacterial leaching experiments. The composition of these particles is different from the initial mineral, showing a lower cobalt concentration as well as a lower (Co+Fe) to (As+S) ratio. This suggests a selective dissolution of cobalt resulting in a mineral phase which chemical composition is close to the composition of arsenopyrite (FeAsS).
404
4. CONCLUSIONS Chemical and bacterial leaching of metal sulfides is strongly controlled by mineralogical factors. The presence of impurities and defects in the crystal structure affects the leaching rate and extend. On the one hand, the mineral attack is not homogeneous and proceeds preferentially through specific crystal structures and defects. On the other hand the presence of impurities dissolved during the leaching process could affect the bacterial activity. It is of the main interest to proceed to an extensive characterization of the different minerals in order to better identify the possible control steps in the leaching process.
ACKNOWLEDGEMENTS This work was supported by the Spanish International Cooperation Agency through the Scientific Cooperation Program.
REFERENCES
1. Posfai, M. and Sundberg, M. American Mineralogist 83 (1998) 365 2. Posfai, M. and Buseck, P.R. American Mineralogist 83 (1998) 373 3. Henao, J.A., Dedelgado, G.D., Delgado, J.M., Castrillo, F.J. and Odreman, O. Material Research Bulletin, 29 (1994) 1121 4. Ehrlich, H. Economic Geology 59 (1964) 1306. 5. Hao, P.L., Chang, W.S. and Wang, N.M. Microbiological leaching of copper from a low grade enargite, Proc. IV IFS Ferment. Technol. Today (1972) 509. 6. Escobar, B., Huenupi, E. and Wiertz J.V.,. Biotech. Letters, 19 (8) (1997) 719 7. Muir, M.K. and Anderson, T., Metall. Trans B., 8B (1977) 517 8. Herrera, L., Ruiz, P., Aguill6n, J.C. and Fehrmann, A., J. Chem. Tech. Biotechnol. 44 (1989) 171 9. Vogel, A. Quimica Analitica Cuantitativa. (Spanish). Editorial Kapelusz. Argentina 1960 10. Harrison, A. P., Jr. Annu. Rev. Microbiol., 38 (1984) 265 11. Dutrizac, J.E. Hydrometallurgy, 23 (1990) 153.
Bioleaching o f copper mineralogical study
and
cobalt
arsenic-bearing
ores:
A
chemical
and
J.V. Wiertz a, R. Lunarb, H. Maturana c and B. Escobar a aDept. de Ingenieria de Minas - Universidad de Chile, Chile. bDept. de Cristalografia - Universidad Complutense de Madrid, Spain. CDept. de Ingenieria de Minas - Unversidad de la Serena, Chile.
Arsenic is a major impurity present in numerous sulfide deposits. Two types of problems are associated with the presence of this element: metallurgical and environmental problems. Both enargite (Cu3AsS4)and cobalt bearing sulfides ((Co,Fe)(As, Sh) contain arsenic as a main component. They are present in important Chilean deposits and have been the object of very scarce studies. This work is a multidisciplinary study that includes chemical, biological and mineralogical aspects. Both minerals could present important variations in their chemical composition and belong to isomorphe series. Enargite forms part of the enargite-luzonite-famatinite series and the cobalt bearing phase to a complex family of sulfides that includes arsenopyrite (FeAsS), 1611ingite (FeAs2) and glaucodot ((Fe, Co)AsS). Both minerals also contain variable amounts of minor elements like Sb, Bi and Ag in the case of enargite and like Ni, Pb and other elements for the cobalt bearing sulfides. The presence of these elements affects significantly the crystal structure and the defect density and consequently modifies the susceptibility of the minerals to leaching and bioleaching processes. Depending on the control stage of the leaching process, these changes in the structure and composition of the mineral can modify the global kinetics of the leaching, inducing changes in the chemical reaction, its kinetics and the extension and composition of the passivating layer or enhancing local attack on crystal disturbed zones. Furthermore, these elements are generally toxic for microorganisms and their dissolution could inhibit bacterial activity in the bioleaching process, particularly in a concentrate leaching process. A bacterial adaptation and a control of dissolved impurities would then be required. Small changes in the mineral composition and structure can then explain the great differences observed in the result of a same process applied to minerals that were expected to be similar but indeed have different origin and composition. This work demonstrates and illustrates the importance of a complete mineralogical characterization of the ores and concentrates at both macroscopic and microscopic scales.
398 I. INTRODUCTION During the last ten years, the presence of arsenic as a major impurity in base metal sulfide ores has been the object of growing concern. The presence of arsenic results in two types of problems. On the one hand, arsenic produces metallurgical problems, making difficult the metal extraction and the recovery of a final product of high purity. On the other hand, arsenic that is thought to be a highly toxic contaminant also results in environmental problems due to its atmospheric release and possible water contamination associated to the processing of arsenic bearing ores and concentrates. In this context, leaching processes are regarded as attractive options for the treatment of such kind of ores. Among those processes, bacterial leaching in acid medium has been successfully applied in copper metallurgy and a new bioleaching process has been recently developed for cobalt recovery. Nevertheless, the processing of complex arsenic bearing ores and concentrates requires a good understanding of the mechanisms involved previous to the design and development of an adapted sequence of process units. In particular, a complete chemical and mineralogical characterization of the ore is essential from both biological and metallurgical points of view in order to determine the possible inhibition problems and the requirement for effluent treatment processes. In the present work, this approach will be illustrated and discussed with two examples. The first case is a bacterial leaching study of pure enargite and the second is related with the design of a leaching process for a complex cobalt concentrate. Enargite (Cu3AsS4) is a common sulfide present in numerous porphyry copper deposits of the North of Chile and Peru. It belongs to the isomorphe series enargite-luzonite-famatinite [ 1, 2]. The crystal structure of enargite is an orthorhombic space group Pmn2I [3] wich allows substitution of As by Sb up to 6% in weight. Only few studies have been carried out on leaching and bioleaching of enargite [4-6]. Enargite has shown to be a mineral strongly refractory to leaching in sulfuric acid medium. A passivation behavior of the sulfide dissolution has been observed in both leaching and bioleaching processes [6]. Cobalt sulfides ((Co,Fe)(As, S)2) exist in several locations in the world, associated with cobaltiferous pyrrothite ores containing arsenic as a main component, and represent an important potential reserve of cobalt although they are not yet exploited due to metallurgical and environmental limitations. Their composition vary in a wide range from arsenopyrite (FeAsS), usually containing small amount of cobalt, to glaucodot ((Co,Fe)AsS) with a cobalt/iron ratio higher than 1/2. Substitution of sulfur by arsenic like in 1611ingite (FeAs2) represents another extreme phase of the series. Both enargite and cobalt bearing sulfides also contain variable amounts of other elements like Sb, Bi and Ag in the case of enargite and like Ni, Pb and Cu and other elements in the cobalt sulfides. We will analyze and discuss how the presence of such impurities could be determinant for the dissolution behavior of the mineral in both chemical and biological leaching.
2. MATERIAL AND METHODS 2.1. Mineral and concentrate samples The Department of Geology, University of Chile, provided the enargite sample used in this study. Chemical and mineralogical analysis confirmed the high purity of the sample. Only small amounts of quartz, pyrite and chalcopyrite were observed in the selected mineral
399 particles. The mineral was ground and separated into different granulometric fractions. The chemical analysis of the ground fraction-100 +150 mesh (-147 +104 lam) used in the leaching study showed the following metal content: Cu: 46.2%; As: 16.3%; Fe: 0.55%. A copper-cobalt sulfide concentrate was prepared from a complex Cu-Co ore with high content of As from Martincito Mine, located North of La Serena. The chemical composition of the concentrate is: Cu: 13.5%; Co: 0.74%; Fe: 30.9%; As: 8.4%. The main minerals are: pyrrhotite, chalcopyrite, a cobalt sulfide phase (intermediate between arsenopyrite, glaucodot and 1611ingite) and iron oxides (magnetite, hematite and goethite).
2.2. Leaching conditions Enargite leaching experiments were performed in 250 ml Erlenmeyer flasks using 5 g of mineral sample -100 +150 mesh (-147 +104 lum) in 100 ml of MC medium pH 1.6 [6] as leaching solution (5% pulp density). The flasks were shaken on a rotary shaker and maintained at 30~ Chemical leaching experiments were performed in MC medium pH 1.6 without iron or with 3.0 g Fe3+/1 added as ferric sulfate. Bacterial leaching experiments were performed under the same conditions but with 4 x 10 6 viable bacteria initially added to the flasks as inoculum. Each experiment reported was performed in duplicate. Cu-Co concentrate samples (1 g) were leached under different conditions in 100 ml MC medium pH 1.6. Chemical leaching experiments were performed with and without initial addition of 3.0 g Fe3+/l added as ferric sulfate. Bacterial leaching experiments were performed under the same conditions with and without initial addition of ferric sulfate.
2.3. Analysis For enargite leaching experiments, the flasks were periodically analyzed for pH and Eh; ferrous and total iron were analyzed by the o-phenanthroline colorimetric method [7, 8]. Total arsenic and copper concentrations in the leaching solution were determined by atomic absorption spectrophotometry. Arsenic(m) was determined by titration [9]. Total bacteria concentration was determined microscopically by direct counting in a Petroff-Hauser chamber. Viable cells were determined by plating in solid agarose medium [10].
2.4. Electron microscope Scanning Electron Microscope (SEM) analysis were carried out with a microscope JEOLJSM-6400 with a resolution of 35A and equipped with BSE detector and XEDS micro-analysis system.
2.5. Microprobe analysis Chemical composition of enargite sample was obtained by electron-microprobe analysis (EPMA) with a JEOL Superprobe JXA 8900-M analyzer.
3. RESULTS AND DISCUSSION
3.1. Leaching experiments Enargite is a sulfide very refractory to the leaching process at 30~ in sulfuric acid medium in the presence of ferric ion. A passivation behavior has been observed as evidenced by the decrease of leaching rate shown in Figure 1. In the presence of bacteria, leaching rate was higher but a similar decrease of the dissolution rate was observed. In all cases, copper and
400 12 10
d
o~
'1
8
4
~-
I
I
I
0
200
400
600
Time - - m - - F e r r i c Leaching
-=
8OO
(hours)
Bacterial Leach
,~
Ferric Bact Leach
Figure 1 Chemical and bacterial leaching of enargite arsenic dissolution rate were similar, suggesting that no selective leaching process is taking place. Leaching experiments on Cu-Co concentrate show significant cobalt dissolution under the different conditions (see Table 1). Cobalt extraction was very high (more than 90%) in both bacterial leaching experiments, with or without initial ferric ion. Moreover, the dissolution seems to be very selective and only a small fraction of copper is leached, especially in bacterial leaching without initial ferric ion (exp. 1). Without bacteria, the cobalt dissolution was lower while copper dissolution increased. Surprisingly, the highest copper dissolution was obtained in the chemical leaching without ferric ion (exp. 4). This suggests that copper dissolution is prevented by the activation of a galvanic interaction when the redox potential of the solution is high, as in bacterial leaching and in chemical ferric leaching. Table 1 Results of Cu-Co concentrate leachin ,.xperiment Experiment pH Bact. Inoc. Fe3~ Metal extraction (%) N~ (ml) (g/l) Cu Co 5.2 94.4 1.6 2.5 1.6
3.2.
3
1.6
4
1.6
2.5
3
Solid Cu content (%) Initial Final 13.75 27.74
8.7
49.7
13.75
20.92
14.8
99.4
13.75
20.56
40.8
43.2
13.75
17.98
Electron microscopy
Enargite residues leached under different conditions were observed and analyzed by Scanning Electron Microscope (SEM). Elemental sulfur that forms locally on enargite surface during the chemical ferric leaching
401
Figure 3" SEM microphotography of a leached crystal of enargite (bacterial ferric leaching).
402 does not form a continuous layer but appears as small crystal agglomerates (Figure 2). This was previously reported by Dutrizac in the leaching of chalcopyrite [ 11 ]. The enargite residues from bacterial and ferric-bacterial leaching experiments present evidence of a stronger attack. Oriented structures of attack on the surface of enargite crystals can be observed. During the ferric-bacterial leaching, the dissolution is more important and some enargite crystals are strongly attacked (Figure 3). In this case, no elemental sulfur is observed on the surface of leached crystals. Microscopic observation of leached residues of Cu-Co concentrate showed an extensive leaching of pyrrhotite and cobalt bearing phases while chalcopyrite was only partially attacked. Ferric arsenate precipitates were also identified. 3.3.
Microprobe analysis
Microprobe analysis were also performed to determine variations in the chemical composition of the minerals and to detect the presence of minor elements and its possible relation with the observed behavior and patterns of chemical and biological leaching. Tables 2 and 3 show the results of microprobe analysis of unleached enargite and cobalt mineral samples, respectively. Table 2 Selected micro 9robe anal, ,sis of unleached enar Sample - Cu As Sb S Fe Bi Enargite* 48.40 19.04 32.56 En4-1 49.29 18.02 0.67 3i.01 0.00 0.08 En4-2 49.18 18.22 0.62 31.20 0.02 0.14 En4-3 48.76 17.85 0.46 32.52 0.00 0.08 En4-4 49.05 17.94 0.42 32.27 0.00 0.10 En2-5 48.82 19.38 0.49 30.55 0.00 0.22 En2-6 47.76 17.68 0.98 33.34 0.14 0.17 En2-7 47.33 17.73 1.46 32.31 0.26 0.11 En2-8 47.61 18.11 0.80 32.56 0.16 0.19 En2b-9** 48.25 18.58 1.29 30.42 0.01 0.13 En2b-10** 48.43 18.38 1.27 30.47 0.01 0.17 * Stoechiometric composition of enargite ** En2b-9 and En2b-10 correspond to a fractured zone in a single enargite crystal
Ag 0.03 0.01 0.01 0.02 0.04 0.01 0.02 0.01 0.51 0.56
Total 100.00
99.10 99.39 99.38 99.80 99.50 99.90 99.22 99.44 99.19 99.29
As shown in Table 2, the composition of the analyzed samples differs slightly from the expected stoechiometric composition of enargite, with a lower arsenic content and variable amounts of antimony, bismuth and silver. Analysis of the fractured zone of an enargite crystal (samples En2b-9 and En2b-10) show a significantly higher silver concentration and a lower sulfur content. A good correlation has been observed between the presence of such type of crystal defect and strongly attacked areas during chemical and bacterial leaching. No significant differences have been observed in the chemical composition of leached particles of enargite, suggesting that no selective metal dissolution is taking place during the chemical and biological leaching of enargite. However, significant amounts of elemental sulfur have been observed on the surface of partially reacted particles of enargite leached under chemical conditions, without bacteria.
403 Table 3 Selected micro 3robe anabrsis of unleached Sample (~o Fe As 17 8,12 26,66 49,00 20 10,84 1 8 , 3 4 69,57 21 7,76 26,04 53,06 22 10,80 1 8 , 6 0 68,37 23 7,19 27,46 47,93 24 7,25 27,21 49,26 28 7,17 27,41 48,72 29 8,04 26,64 48,34
cobalt mineral sam S Cu 16,42 0,00 1,01 0,07 13,65 0,07 1,02 0,05 16,77 0,04 16,08 0,00 16,21 0,05 15,83 0,02
Bi 0,08 0,00 0,03 0,08 0,00 0,08 0,10 0,05
Pb 0,07 0,02 0,02 0,00 0,00 0,02 0,03 0,00
Total 100,35 99,85 100,65 98,97: 99,421 99,91 99,70 98,94
The composition of cobalt bearing mineral samples (Table 3) shows variations in a wide range. When cobalt concentration decreases, iron concentration increases, suggesting a replacement of cobalt atoms by iron atoms. A same behavior can be observed between sulfur and arsenic atoms. In Table 4, the same results are expressed in molar fractions, assuming that the sum of cobalt, iron, arsenic and sulfur molar fractions is one. As it can be seen from these results, the sum of cobalt and iron fractions as well as the sum of arsenic and sulfur fractions are quite constant and the global composition is close to (Co,Fe)(As, S)2. Table 4 Molar composition of selected sam_~les of cobalt mineral (molar fraction) Sample Co As Fe Co+Fe As+S B A 17 0,08 0,27 0,37 0,29 0,35 0,65 20 0,12 0,22 0,63 0,02 0,35 0,65 21 0,08 0,41 0,25 0,65 0,27 0,35 22 0,13 0,23 0,62 0,02 0,35 0,65 23 0,07 0,28 0,36 0,29 0,35 0,65 24 0,07 0,28 0,37 0,28 0,34 0,66 28 0,07 0,28 0,37 0,29 0,35 0,65 29 0,08 0,27 0,37 0,28 0,35 0,65 98* 0,05 0,26 0,35 0,34 0,31 100" 0,06 0,24 0,34 0,36 0,30 * Samples 98-100 correspond to partially leached mineral particles
0,69 0,70
A/B 0,53 0,53 0,53 0,55 0,53 0,53 0,53 0,54 0,45 0,43
In the same Table 4, the last 2 lines correspond to partially leached particles selected from the solid residues of the bacterial leaching experiments. The composition of these particles is different from the initial mineral, showing a lower cobalt concentration as well as a lower (Co+Fe) to (As+S) ratio. This suggests a selective dissolution of cobalt resulting in a mineral phase which chemical composition is close to the composition of arsenopyrite (FeAsS).
404
4. CONCLUSIONS Chemical and bacterial leaching of metal sulfides is strongly controlled by mineralogical factors. The presence of impurities and defects in the crystal structure affects the leaching rate and extend. On the one hand, the mineral attack is not homogeneous and proceeds preferentially through specific crystal structures and defects. On the other hand the presence of impurities dissolved during the leaching process could affect the bacterial activity. It is of the main interest to proceed to an extensive characterization of the different minerals in order to better identify the possible control steps in the leaching process.
ACKNOWLEDGEMENTS This work was supported by the Spanish International Cooperation Agency through the Scientific Cooperation Program.
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