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Evaluation of processing options to avoid the passivation of chalcopyrite
International Journal of Mineral Processing 125 (2013) 1–4
Contents lists available at ScienceDirect
International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro
Evaluation of processing options to avoid the passivation of chalcopyrite Gianfranco Debernardi a,⁎, Juan Carlos Gentina a, Pablo Albistur b, Gino Slanzi b a b
Escuela de Ingeniería Bioquímica, Pontificia Universidad Católica de Valparaíso, Chile. Avenida Brasil 2950, Valparaíso, Chile Codelco división Andina, Santa Teresa 513, Los Andes, Chile
a r t i c l e
i n f o
Article history: Received 25 June 2013 Received in revised form 6 August 2013 Accepted 2 September 2013 Available online 17 September 2013 Keywords: Chalcopyrite passivation Oxidative leaching Elemental sulfur layer Jarosite layer Polysulfides
a b s t r a c t The feasibility of two options to avoid the passivation of a chalcopyrite concentrate was evaluated in an oxidative process by ferric ion at 30 °C. These options attempt to reduce the formation of a layer of iron precipitates, and sequentially inoculate the bacterium Acidithiobacillus thiooxidans to consume the elemental sulfur layer, without modifying the oxidizing power of the leaching solution. QEMSCAN analysis revealed a reduction in ferric compound precipitation over the mineral surface from 32% to 2% by mass by controlling the formation of precipitates. However, the increase in the rate of copper extraction was insignificant, increasing this only from 3.8% to 4.0% in 30 days. The observed precipitates were mostly of type Fesulfates, with minor appearance of jarosite. In neither case, QEMSCAN analysis revealed formation of elemental sulfur on the mineral. Consequently, the inoculation of sulfur oxidizing bacterium A. thiooxidans in a controlled precipitation system produced no improvement in the extraction of copper. The leaching behavior fitted with the model of SCM (shrinking core model), indicating the formation of a layer of undetected compounds which exerts an effect on the diffusion of the leaching agent. Therefore, none of the options taken to avoid the passivation of chalcopyrite were successful, indicating that passivation occurs even in the absence of iron precipitation and the presence of sulfur oxidizing bacteria. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Chalcopyrite passivation has been widely investigated, with no scientific consensus about its causes and how to avoid it (Watling, 2006; Klauber, 2008; Debernardi and Carlesi, 2013). To date only three possible causes for this phenomenon have been identified (i) the formation of an iron precipitate layer that blocks the contact between chalcopyrite and the leaching solution (Stott et al., 2000; Córdoba et al., 2008), (ii) the formation of an elemental sulfur layer, which reduces the diffusion of the leaching agent to the mineral reacting surface (Carneiro and Leão, 2007; Munoz et al., 1979), and (iii) the formation of an intermediate layer during the dissolution reaction of chalcopyrite, such as sulfides, disulfides or polysulfide compounds (referred to as polysulfide layer in this paper), which have slow dissolution kinetics and hence controls the net rate of the process (Klauber, 2008; Harmer et al., 2006; Parker et al., 2008). The removal of the iron precipitate layer, once it has formed, has not been successful (Stott et al., 2000). This happens because amorphous iron precipitates undergo a crystallization process to minerals that are practically insoluble in oxidizing conditions, such as jarosite. The increase in porosity of the elemental sulfur layer has been reported as the cause of increased copper extraction rates found when leaching with chloride-based solutions (Carneiro and Leão, 2007; Dutrizac, ⁎ Corresponding author at: Gómez Carreño 336, Recreo Alto, Viña del Mar, Chile. Tel.: +56 9 77073926. E-mail address: [email protected] (G. Debernardi). 0301-7516/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.minpro.2013.09.001
1990). This higher porosity diminishes the diffusional barrier created by accumulated sulfur to the flow of leaching agents. The sulfur oxidizing bacteria Acidithiobacillus thiooxidans is capable of oxidizing elemental sulfur to sulfuric acid (Blight et al., 2009), whereby it is expected that their presence can decrease the effects of this layer on chalcopyrite leaching rate. In heap leaching, it is often difficult to control the conditions of the leaching solution to avoid the formation of iron precipitates and to provide good conditions for bacterial activity. Thus, the objective of this study was to evaluate the importance of controlling iron precipitation and the presence of sulfur-oxidizing bacteria in solution, on the rate of copper extraction from chalcopyrite at 30 °C with a leaching solution representative of industrial operations.
2. Experimental procedures 2.1. Mineral concentrate A chalcopyrite concentrate obtained from Codelco's Andina division was used. This concentrate was dry sieved, later ET-sieved and finally fluidized within a reactor to remove all fine particles not retained at the operating flow rate. The recovered sample was washed with acetone, ethanol and distilled water, and then dried overnight in oven at 60 °C. The particle size distribution was 90% over 40 μm, and 5% above 100 μm. The concentrate contained 11.26% of pyrite, and the copper distribution was 98% chalcopyrite, 0.92% covellite, 0.62% bornite, 0.36%
2
G. Debernardi et al. / International Journal of Mineral Processing 125 (2013) 1–4
enargite, 0.12% chalcocite, 0.04% tennantite/tetrahedrite, and 0.01% copper oxide. 2.2. Bacterial culture A. thiooxidans ATCC 19377 was cultured in a medium adapted from Kim et al. (2002), referred to as SJ medium, and it contained (NH4) H2PO4 2.6 g/L, MgSO4·7H2O 0.2 g/L, KCl 0.1 g/L. The propagation was carried out in a 2 L reactor at 30 °C, with 0.5% of elemental sulfur, 1 vvm aeration, 500 rpm and pH controlled at 1.8. The sulfur was gravity settled by stopping agitation and airflow, and the supernatant was centrifuged at 15,000 ×g for cell concentration. 2.3. System description and experimental development The chalcopyrite concentrate was fluidized by the leaching solution flow in a fluidized bed reactor (FBR) with a decanter in its upper part, for retain by gravity mineral particles inside the reactor. The leaching solution was fed from a stirred tank reactor (STR) to the FBR, and subsequently recirculated to the STR (Fig. 1). The pulp density in the FBR was 40%, with a residence time of 0.42 min. Considering the whole system volume, the pulp density was 3.5%. The experimental method is based in direct comparison of copper extraction rate from batches in the presence or absence of iron precipitates and/or elemental sulfur. The batch “Control”, is performed in abiotic media, in the presence of 9 K medium, which is known for to generate high iron precipitation, but without ferrous sulfate. The composition of this medium was (NH4)2SO4 3.0 g/L, K2HPO4 0.5 g/L, MgSO4·7H2O 0.5 g/L, KCl 0.1 g/L and Ca(NO3)2 0.01 g/L. The second batch, called “No-Fe pp”, is analogous to batch “Control”, but changing the 9 K medium to SJ medium. SJ medium was confirmed to present low amounts of iron precipitation (data not shown). The third batch, called “Biotic”, is analogous to batch “No-Fe-pp”, but the bacterium A. thiooxidans is inoculated at zero time, to consume the elemental sulfur layer formed during chalcopyrite dissolution. This bacterium has no direct oxidant effect on chalcopyrite (Bevilaqua et al., 2002). This experimental scheme permits to evaluate the net effect of iron precipitate layer and/or elemental sulfur on chalcopyrite extraction rate. In all batches the conditions of the leaching solution were controlled and adjusted in the STR to maintain the intrinsic chalcopyrite dissolution kinetics. The parameters chosen were 5 g/L of ferric ion and less than 0.1 g/L ferrous ion (Eh N 500 mV Ag/AgCl), pH 1.8–2.0, and 30 °C. These bioleaching parameters are representative of industrial conditions (Watling, 2006). The total iron concentration was kept constant by adding a ferrous sulfate solution and H2O2 was used to oxidize ferrous ion. pH was controlled with concentrated sulfuric acid and sodium carbonate solutions as necessary. The temperature was set to 30 °C
by a bath-circulator, and an oxygen saturation of about 95% was kept by feeding air at 1 L/min to the STR. To avoid the presence of iron-oxidizing bacteria, the mineral was heat sterilized in autoclave (Brickett et al., 1995). The reactors and solutions used were sterilized properly. 2.4. Analytical methods Liquid samples were taken from the STR and centrifuged for 10 min at 10,000 ×g to remove precipitates. Ferric ion and total iron were analyzed by the sulfosalicylic acid method (Karamanev et al., 2002), ferrous ion by the modified o-phenanthroline method (Herrera et al., 1989) and total copper by the modified bicinchoninic acid method (Debernardi et al., 2012). Eh and pH were measured in the STR. The bacterial count was performed with non-centrifuged samples. Solid samples were collected using a small sterile spoon introduced at the top of FBR to the reaction zone, removing close 2 g of mineral. The sample was suspended in acidified water (pH 2.0) and gravity settled, discarding the supernatant. The process was repeated 2 times. Finally the sample was dried in oven at 60 °C. QEMSCAN analysis was used to identify mineralogical species and particle size distribution in solid samples. This technology confines mineral particles in a briquette, and then the briquette is polished. Mineral species larger than 3 μm depth occurring in the border of the polished particles can be detected. The equipment used was a third generation QEMSCAN, with a SEM Zeiss EVO-50, and 4 EDS detectors. 3. Results 3.1. Effect of iron precipitates on chalcopyrite leaching Leaching conditions were successfully controlled, achieving at all times a ferric ion concentration between 4 and 5 g/L, Eh N 500 mV, and pH in the range 1.8–2.0 (Fig. 2). Rapid precipitation, mainly in 9 K medium, forced the iron additions to maintain the defined conditions, as shown in Fig. 4. Analysis of the solid samples (Fig. 3) in “Control” batch shows an increased average particle size of 50 to 80 μm, due to iron precipitation over particles, mainly Fe-sulfates, which reach up to 30% mineral mass. Crystalline precipitates as jarosite only reached 5% mineral mass. By contrast, in “No Fe-pp” batch there was no increase in particle size, precipitating 2% Fe-sulfates and less than 0.1% of jarosite. This indicates that despite the iron precipitation of 7 g/L “No-Fe-pp” batch (Fig. 4) this was not deposited on the mineral surface, accomplishing the objective of a chalcopyrite leaching without coating the mineral by iron precipitates.
600
10
550
Eh
30°°
pH
pH, FeT (g/L)
8 7
500
6 450
5 4
400
3 2
350
Potential (mV Ag/AgCl)
9
1 300
0 0
5
10
15
20
25
30
time (d) Fluidized bed reactor (FBR)
Stirred tank reactor (STR)
Fig. 1. Experimental setup and control parameters.
Fig. 2. Behavior of the leaching solution for batches “Control” and “No Fe-pp”. (Circles: pH, triangles: total iron, rhombus: redox potential. Control batch: close symbols; No Fe-pp batch: open symbols).
G. Debernardi et al. / International Journal of Mineral Processing 125 (2013) 1–4
40
4.5
90
35
4.0
30
70 60
25
50
20
40
15
30
10
Mass percent, %
Particle size (µm)
80
Copper extraction (%)
100
20 10 0 0
10
20
3
3.5 3.0 2.5 2.0 1.5 1.0
5
0.5
0
0.0
30
0
10
time (d)
20
30
time (d)
Fig. 3. Particle size and percent of iron compound precipitates on mineral particles in batches “Control” and “No Fe-pp”. (Circles: particle size, triangles: Fe-sulfate content, rhombus: jarosite content. Control batch: close symbols; No Fe-pp batch: open symbols).
Fig. 5. Kinetics of copper extraction obtained in batch “Control” and the lot “No Fe-pp”. (●: Control, ○: No Fe-pp).
The copper extraction is presented in Fig. 5. The rapid initial leaching corresponds to secondary sulfides, chalcocite and covellite, which constitute approximately 1% copper. From this moment the leaching is chalcopyrite based. No dissolution of enargite occurs. After 30 days there is an insignificant difference in the extraction of copper from both batches (3.8% vs. 4.0%), and the data fit successfully to model SCM solution (Table 1).
identified on the surface of chalcopyrite. This result has been found in samples of the “Control” and “No Fe-pp” batches (data not shown).
3.2. Effect of elemental sulfur formation in the leaching of chalcopyrite The difference between the “Biotic” and “No Fe-pp” batches is the inoculation of A. thiooxidans to the startup of the “Biotic” batch. Cell adhesion was quantified as cell disappearance from the leaching solution. Adherence was completed within the first 4 h, reaching a concentration of 8 × 109 cells/g. This value is higher than 1.5 × 109 cells/g reported by Harneit et al. (2006) as maximum cell adhesion on a chalcopyrite concentrate (size 50–100 μm). At regular intervals, liquid samples from STR were inoculated into flasks with commercial elemental sulfur, presenting sulfur oxidizing activity in all cases. The results of batch “Biotic” were similar to the “No Fe-pp” batch, regarding the control of the leaching solution (Fig. 2), and the formation of iron precipitates (Fig. 3). The copper extraction kinetics (Fig. 6) did not show any difference compared to that observed in batch “No Fe-pp”, even the slope obtained by fitting the data to the SCM model (Table 1) is similar. This is consistent with the analysis of solid samples, in which elemental sulfur is not
12
Iron precipitates, g/L
10 8 6
4. Discussion The kinetics of copper extraction were not affected by the processing options evaluated. The poor results are of the order of those reported by Peacey et al. (2003), who with a chalcopyrite concentrate of size − 100# + 200# at 20 °C extracted 2% copper in 43 d. Table 1 shows the kinetic constants obtained by fitting copper extractions to the SCM model according to Eq. (1), where X represents the percent of copper extraction, t the reaction time, and k the kinetic constant. The proper fitting of the results indicates the presence of a passive layer that affects the diffusion of the oxidizing agent to the chalcopyrite surface in all batches. 2=3
1−2=3X−ð1−X Þ
¼ k t:
ð1Þ
The large decrease in the formation of Fe-sulfate precipitates among batches “Control” and “No Fe-pp” indicates that the cause of passivation is not related to these compounds. Nevertheless, the low amounts of jarosite, a high crystallinity compound, found in “Control” batch (b5%), does not permit to evaluate its passivating effect in larger quantities. In neither batch elemental sulfur was detected by the mineralogical analysis. It is possible that this layer was thinner than 3 μm, impeding its detection by QEMSCAN, or that it could be volatilized by the vacuum environment where the QEMSCAN briquettes are sealed (Klauber, 2008). In either case, there is a passivating layer which cannot be consumed by A. thiooxidans. Recently, Blight et al. (2009) reported that A. thiooxidans is able to consume orthorhombic elemental sulfur (commercial type sulfur), but does not consume polymeric elemental sulfur. It has not been demonstrated that the elemental sulfur formed on chalcopyrite is of orthorhombic nature. Instead, Parker et al. (2008) detected by Raman spectroscopy an inactive sulfur
4 Table 1 Fitting of copper extractions to the SCM model.
2 0 0
10
20
30
time (d) Fig. 4. Total iron precipitation in batches “Control” and “No Fe-pp”. (●: Control, ○: No Fe-pp).
Batch
Kinetic constant (d−1)
R2
Control No Fe-pp Biotic
2.98E−6 3.49E−6 3.28E−6
0.968 0.966 0.981
4
G. Debernardi et al. / International Journal of Mineral Processing 125 (2013) 1–4
4.5
References
Copper extraction (%)
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0
10
20
30
time (d) Fig. 6. Copper extraction in batches “No Fe-pp” and “Biótic”. (●:No Fe-pp, ○: Biotic).
layer on leached chalcopyrite that under photodecomposition yields a polymeric sulfur phase. Consequently, if there is any elemental sulfur passivating layer over chalcopyrite, this is not of orthorhombic type. Polymeric elemental sulfur tends to decompose to orthorhombic elemental sulfur slowly with time and temperature (Steudel and Eckert, 2003; Parker et al., 2008). It is possible that the apparent lack of passivation of chalcopyrite at temperatures above 60 °C (Watling, 2006) be a result of natural changes in the sulfur layer towards orthorhombic sulfur. 5. Conclusion • The dissolution of chalcopyrite in oxidizing medium occurs according to the SCM model, indicating the presence of a layer that retards the particle reactivity as the dissolution progresses. • The formation of amorphous iron precipitates, as Fe-sulfates, on chalcopyrite particles is not a cause of mineral passivation. It was not possible to evaluate the effect of highly crystalline precipitates as jarosite. • The presence of sulfur-oxidizing bacteria, specifically A. thiooxidans, does not avoid the passivation of chalcopyrite, even in surfaces free of precipitates. • It was not possible to identify a chalcopyrite passivating layer through its detection by QEMSCAN technology.
Bevilaqua, D., Leite, A.L.L.C., García Jr., O., Tuovinen, O.H., 2002. Oxidation of chalcopyrite by Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans in shake flasks. Process Biochem. 38, 587–592. Brickett, L.A., Hammack, R.W., Edenborn, H.M., 1995. Comparison of methods used to inhibit bacterial activity in sulphide ore bioleaching studies. Hydrometallurgy 39, 293–305. Blight, K.R., Candy, R.M., Ralph, D.E., 2009. The preferential oxidation of orthorhombic sulfur during batch culture. Hydrometallurgy 99, 100–104. Carneiro, M.F.C., Leão, V.A., 2007. The role of sodium chloride on surface properties of chalcopyrite leached with ferric sulphate. Hydrometallurgy 87, 73–79. Córdoba, E.M., Muñoz, J.A., Blázquez, M.L., González, F., Ballester, A., 2008. Leaching of chalcopyrite with ferric ion. Part II: effect of redox potential. Hydrometallurgy 93, 88–96. Debernardi, G., Carlesi, C., 2013. Chemical-electrochemical approaches to the study passivation of chalcopyrite. Miner. Process. Extr. Metall. Rev. 34, 10–41. Debernardi, G., Landaeta, R., Carlesi, C., 2012. Improvement and discussion of the bicinchoninic acid method for copper determination in bacterial leachates and acid mine drainage. Lat. Am. Appl. Res. 42, 127–130. Dutrizac, J.E., 1990. Elemental sulphur formation during the ferric chloride leaching of chalcopyrite. Hydrometallurgy 23, 153–176. Harmer, S., Thomas, J.E., Fornasiero, D., Gerson, A.R., 2006. The evolution of surface layers formed during chalcopyrite leaching. Geochim. Cosmochim. Acta 70, 4392–4402. Harneit, K., Göksel, A., Kock, D., Klock, J.H., Gehrke, T., Sand, W., 2006. Adhesion to metal sulfide surfaces by cells of Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum ferrooxidans. Hydrometallurgy 83, 245–254. Herrera, L., Ruiz, P., Aquillon, J.C., Fehrmann, A., 1989. A new spectrophotometric method for the determination of ferrous iron in the presence of ferric iron. J. Chem. Technol. Biotechnol. 44, 171–181. Karamanev, D.G., Nikolov, L.N., Mamtarkova, V., 2002. Rapid simultaneous quantitative determination of ferric and ferrous ions in drainage waters and similar solutions. Miner. Eng. 15, 341–346. Kim, T.W., Kim, C.J., Chang, Y.K., Ryu, H.W., Cho, K., 2002. Development of an optimal medium for continuous ferrous iron oxidation by immobilized Acidithiobacillus ferroxidans cells. Biotechnol. Prog. 18, 752–759. Klauber, C., 2008. A critical review of the surface chemistry of acidic ferric sulphate dissolution of chalcopyrite with regards to hindered dissolution. Int. J. Miner. Process. 86, 1–17. Munoz, P.B., Miller, J.D., Wadsworth, M.E., 1979. Reaction mechanism for the acid ferric sulphate leaching of chalcopyrite. Metall. Mater. Trans. B 10, 149–158. Parker, G., Hope, G., Woods, R., 2008. Gold-enhanced Raman observation of chalcopyrite leaching. Colloids Surf. A 325, 132–140. Peacey, J., Guo, X.J., Robles, E., 2003. Copper hydrometallurgy—current status, preliminary economics, future directions and positioning versus smelting. In: Riveros, P.A., Dixon, D., Dreisinger, D.B., Menacho, J. (Eds.), Copper-Cobre 2003. Volume VI—Hydrometallurgy of Copper (Book 1). Santiago de Chile, pp. 205–222. Steudel, R., Eckert, B., 2003. Solid sulfur allotropes. In: Steudel, R. (Ed.), Topics in Current Chemistry: Elemental Sulfur and Sulfur-Rich Compounds I. Springer, pp. 1–79. Stott, M.B., Watling, H.R., Franzmann, P.D., Sutton, D., 2000. Role of iron-hydroxy precipitates in the passivation of chalcopyrite during bioleaching. Miner. Eng. 13, 1117–1127. Watling, H.R., 2006. The bioleaching of sulphide minerals with emphasis on copper sulphides: a review. Hydrometallurgy 84, 81–108.
Contents lists available at ScienceDirect
International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro
Evaluation of processing options to avoid the passivation of chalcopyrite Gianfranco Debernardi a,⁎, Juan Carlos Gentina a, Pablo Albistur b, Gino Slanzi b a b
Escuela de Ingeniería Bioquímica, Pontificia Universidad Católica de Valparaíso, Chile. Avenida Brasil 2950, Valparaíso, Chile Codelco división Andina, Santa Teresa 513, Los Andes, Chile
a r t i c l e
i n f o
Article history: Received 25 June 2013 Received in revised form 6 August 2013 Accepted 2 September 2013 Available online 17 September 2013 Keywords: Chalcopyrite passivation Oxidative leaching Elemental sulfur layer Jarosite layer Polysulfides
a b s t r a c t The feasibility of two options to avoid the passivation of a chalcopyrite concentrate was evaluated in an oxidative process by ferric ion at 30 °C. These options attempt to reduce the formation of a layer of iron precipitates, and sequentially inoculate the bacterium Acidithiobacillus thiooxidans to consume the elemental sulfur layer, without modifying the oxidizing power of the leaching solution. QEMSCAN analysis revealed a reduction in ferric compound precipitation over the mineral surface from 32% to 2% by mass by controlling the formation of precipitates. However, the increase in the rate of copper extraction was insignificant, increasing this only from 3.8% to 4.0% in 30 days. The observed precipitates were mostly of type Fesulfates, with minor appearance of jarosite. In neither case, QEMSCAN analysis revealed formation of elemental sulfur on the mineral. Consequently, the inoculation of sulfur oxidizing bacterium A. thiooxidans in a controlled precipitation system produced no improvement in the extraction of copper. The leaching behavior fitted with the model of SCM (shrinking core model), indicating the formation of a layer of undetected compounds which exerts an effect on the diffusion of the leaching agent. Therefore, none of the options taken to avoid the passivation of chalcopyrite were successful, indicating that passivation occurs even in the absence of iron precipitation and the presence of sulfur oxidizing bacteria. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Chalcopyrite passivation has been widely investigated, with no scientific consensus about its causes and how to avoid it (Watling, 2006; Klauber, 2008; Debernardi and Carlesi, 2013). To date only three possible causes for this phenomenon have been identified (i) the formation of an iron precipitate layer that blocks the contact between chalcopyrite and the leaching solution (Stott et al., 2000; Córdoba et al., 2008), (ii) the formation of an elemental sulfur layer, which reduces the diffusion of the leaching agent to the mineral reacting surface (Carneiro and Leão, 2007; Munoz et al., 1979), and (iii) the formation of an intermediate layer during the dissolution reaction of chalcopyrite, such as sulfides, disulfides or polysulfide compounds (referred to as polysulfide layer in this paper), which have slow dissolution kinetics and hence controls the net rate of the process (Klauber, 2008; Harmer et al., 2006; Parker et al., 2008). The removal of the iron precipitate layer, once it has formed, has not been successful (Stott et al., 2000). This happens because amorphous iron precipitates undergo a crystallization process to minerals that are practically insoluble in oxidizing conditions, such as jarosite. The increase in porosity of the elemental sulfur layer has been reported as the cause of increased copper extraction rates found when leaching with chloride-based solutions (Carneiro and Leão, 2007; Dutrizac, ⁎ Corresponding author at: Gómez Carreño 336, Recreo Alto, Viña del Mar, Chile. Tel.: +56 9 77073926. E-mail address: [email protected] (G. Debernardi). 0301-7516/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.minpro.2013.09.001
1990). This higher porosity diminishes the diffusional barrier created by accumulated sulfur to the flow of leaching agents. The sulfur oxidizing bacteria Acidithiobacillus thiooxidans is capable of oxidizing elemental sulfur to sulfuric acid (Blight et al., 2009), whereby it is expected that their presence can decrease the effects of this layer on chalcopyrite leaching rate. In heap leaching, it is often difficult to control the conditions of the leaching solution to avoid the formation of iron precipitates and to provide good conditions for bacterial activity. Thus, the objective of this study was to evaluate the importance of controlling iron precipitation and the presence of sulfur-oxidizing bacteria in solution, on the rate of copper extraction from chalcopyrite at 30 °C with a leaching solution representative of industrial operations.
2. Experimental procedures 2.1. Mineral concentrate A chalcopyrite concentrate obtained from Codelco's Andina division was used. This concentrate was dry sieved, later ET-sieved and finally fluidized within a reactor to remove all fine particles not retained at the operating flow rate. The recovered sample was washed with acetone, ethanol and distilled water, and then dried overnight in oven at 60 °C. The particle size distribution was 90% over 40 μm, and 5% above 100 μm. The concentrate contained 11.26% of pyrite, and the copper distribution was 98% chalcopyrite, 0.92% covellite, 0.62% bornite, 0.36%
2
G. Debernardi et al. / International Journal of Mineral Processing 125 (2013) 1–4
enargite, 0.12% chalcocite, 0.04% tennantite/tetrahedrite, and 0.01% copper oxide. 2.2. Bacterial culture A. thiooxidans ATCC 19377 was cultured in a medium adapted from Kim et al. (2002), referred to as SJ medium, and it contained (NH4) H2PO4 2.6 g/L, MgSO4·7H2O 0.2 g/L, KCl 0.1 g/L. The propagation was carried out in a 2 L reactor at 30 °C, with 0.5% of elemental sulfur, 1 vvm aeration, 500 rpm and pH controlled at 1.8. The sulfur was gravity settled by stopping agitation and airflow, and the supernatant was centrifuged at 15,000 ×g for cell concentration. 2.3. System description and experimental development The chalcopyrite concentrate was fluidized by the leaching solution flow in a fluidized bed reactor (FBR) with a decanter in its upper part, for retain by gravity mineral particles inside the reactor. The leaching solution was fed from a stirred tank reactor (STR) to the FBR, and subsequently recirculated to the STR (Fig. 1). The pulp density in the FBR was 40%, with a residence time of 0.42 min. Considering the whole system volume, the pulp density was 3.5%. The experimental method is based in direct comparison of copper extraction rate from batches in the presence or absence of iron precipitates and/or elemental sulfur. The batch “Control”, is performed in abiotic media, in the presence of 9 K medium, which is known for to generate high iron precipitation, but without ferrous sulfate. The composition of this medium was (NH4)2SO4 3.0 g/L, K2HPO4 0.5 g/L, MgSO4·7H2O 0.5 g/L, KCl 0.1 g/L and Ca(NO3)2 0.01 g/L. The second batch, called “No-Fe pp”, is analogous to batch “Control”, but changing the 9 K medium to SJ medium. SJ medium was confirmed to present low amounts of iron precipitation (data not shown). The third batch, called “Biotic”, is analogous to batch “No-Fe-pp”, but the bacterium A. thiooxidans is inoculated at zero time, to consume the elemental sulfur layer formed during chalcopyrite dissolution. This bacterium has no direct oxidant effect on chalcopyrite (Bevilaqua et al., 2002). This experimental scheme permits to evaluate the net effect of iron precipitate layer and/or elemental sulfur on chalcopyrite extraction rate. In all batches the conditions of the leaching solution were controlled and adjusted in the STR to maintain the intrinsic chalcopyrite dissolution kinetics. The parameters chosen were 5 g/L of ferric ion and less than 0.1 g/L ferrous ion (Eh N 500 mV Ag/AgCl), pH 1.8–2.0, and 30 °C. These bioleaching parameters are representative of industrial conditions (Watling, 2006). The total iron concentration was kept constant by adding a ferrous sulfate solution and H2O2 was used to oxidize ferrous ion. pH was controlled with concentrated sulfuric acid and sodium carbonate solutions as necessary. The temperature was set to 30 °C
by a bath-circulator, and an oxygen saturation of about 95% was kept by feeding air at 1 L/min to the STR. To avoid the presence of iron-oxidizing bacteria, the mineral was heat sterilized in autoclave (Brickett et al., 1995). The reactors and solutions used were sterilized properly. 2.4. Analytical methods Liquid samples were taken from the STR and centrifuged for 10 min at 10,000 ×g to remove precipitates. Ferric ion and total iron were analyzed by the sulfosalicylic acid method (Karamanev et al., 2002), ferrous ion by the modified o-phenanthroline method (Herrera et al., 1989) and total copper by the modified bicinchoninic acid method (Debernardi et al., 2012). Eh and pH were measured in the STR. The bacterial count was performed with non-centrifuged samples. Solid samples were collected using a small sterile spoon introduced at the top of FBR to the reaction zone, removing close 2 g of mineral. The sample was suspended in acidified water (pH 2.0) and gravity settled, discarding the supernatant. The process was repeated 2 times. Finally the sample was dried in oven at 60 °C. QEMSCAN analysis was used to identify mineralogical species and particle size distribution in solid samples. This technology confines mineral particles in a briquette, and then the briquette is polished. Mineral species larger than 3 μm depth occurring in the border of the polished particles can be detected. The equipment used was a third generation QEMSCAN, with a SEM Zeiss EVO-50, and 4 EDS detectors. 3. Results 3.1. Effect of iron precipitates on chalcopyrite leaching Leaching conditions were successfully controlled, achieving at all times a ferric ion concentration between 4 and 5 g/L, Eh N 500 mV, and pH in the range 1.8–2.0 (Fig. 2). Rapid precipitation, mainly in 9 K medium, forced the iron additions to maintain the defined conditions, as shown in Fig. 4. Analysis of the solid samples (Fig. 3) in “Control” batch shows an increased average particle size of 50 to 80 μm, due to iron precipitation over particles, mainly Fe-sulfates, which reach up to 30% mineral mass. Crystalline precipitates as jarosite only reached 5% mineral mass. By contrast, in “No Fe-pp” batch there was no increase in particle size, precipitating 2% Fe-sulfates and less than 0.1% of jarosite. This indicates that despite the iron precipitation of 7 g/L “No-Fe-pp” batch (Fig. 4) this was not deposited on the mineral surface, accomplishing the objective of a chalcopyrite leaching without coating the mineral by iron precipitates.
600
10
550
Eh
30°°
pH
pH, FeT (g/L)
8 7
500
6 450
5 4
400
3 2
350
Potential (mV Ag/AgCl)
9
1 300
0 0
5
10
15
20
25
30
time (d) Fluidized bed reactor (FBR)
Stirred tank reactor (STR)
Fig. 1. Experimental setup and control parameters.
Fig. 2. Behavior of the leaching solution for batches “Control” and “No Fe-pp”. (Circles: pH, triangles: total iron, rhombus: redox potential. Control batch: close symbols; No Fe-pp batch: open symbols).
G. Debernardi et al. / International Journal of Mineral Processing 125 (2013) 1–4
40
4.5
90
35
4.0
30
70 60
25
50
20
40
15
30
10
Mass percent, %
Particle size (µm)
80
Copper extraction (%)
100
20 10 0 0
10
20
3
3.5 3.0 2.5 2.0 1.5 1.0
5
0.5
0
0.0
30
0
10
time (d)
20
30
time (d)
Fig. 3. Particle size and percent of iron compound precipitates on mineral particles in batches “Control” and “No Fe-pp”. (Circles: particle size, triangles: Fe-sulfate content, rhombus: jarosite content. Control batch: close symbols; No Fe-pp batch: open symbols).
Fig. 5. Kinetics of copper extraction obtained in batch “Control” and the lot “No Fe-pp”. (●: Control, ○: No Fe-pp).
The copper extraction is presented in Fig. 5. The rapid initial leaching corresponds to secondary sulfides, chalcocite and covellite, which constitute approximately 1% copper. From this moment the leaching is chalcopyrite based. No dissolution of enargite occurs. After 30 days there is an insignificant difference in the extraction of copper from both batches (3.8% vs. 4.0%), and the data fit successfully to model SCM solution (Table 1).
identified on the surface of chalcopyrite. This result has been found in samples of the “Control” and “No Fe-pp” batches (data not shown).
3.2. Effect of elemental sulfur formation in the leaching of chalcopyrite The difference between the “Biotic” and “No Fe-pp” batches is the inoculation of A. thiooxidans to the startup of the “Biotic” batch. Cell adhesion was quantified as cell disappearance from the leaching solution. Adherence was completed within the first 4 h, reaching a concentration of 8 × 109 cells/g. This value is higher than 1.5 × 109 cells/g reported by Harneit et al. (2006) as maximum cell adhesion on a chalcopyrite concentrate (size 50–100 μm). At regular intervals, liquid samples from STR were inoculated into flasks with commercial elemental sulfur, presenting sulfur oxidizing activity in all cases. The results of batch “Biotic” were similar to the “No Fe-pp” batch, regarding the control of the leaching solution (Fig. 2), and the formation of iron precipitates (Fig. 3). The copper extraction kinetics (Fig. 6) did not show any difference compared to that observed in batch “No Fe-pp”, even the slope obtained by fitting the data to the SCM model (Table 1) is similar. This is consistent with the analysis of solid samples, in which elemental sulfur is not
12
Iron precipitates, g/L
10 8 6
4. Discussion The kinetics of copper extraction were not affected by the processing options evaluated. The poor results are of the order of those reported by Peacey et al. (2003), who with a chalcopyrite concentrate of size − 100# + 200# at 20 °C extracted 2% copper in 43 d. Table 1 shows the kinetic constants obtained by fitting copper extractions to the SCM model according to Eq. (1), where X represents the percent of copper extraction, t the reaction time, and k the kinetic constant. The proper fitting of the results indicates the presence of a passive layer that affects the diffusion of the oxidizing agent to the chalcopyrite surface in all batches. 2=3
1−2=3X−ð1−X Þ
¼ k t:
ð1Þ
The large decrease in the formation of Fe-sulfate precipitates among batches “Control” and “No Fe-pp” indicates that the cause of passivation is not related to these compounds. Nevertheless, the low amounts of jarosite, a high crystallinity compound, found in “Control” batch (b5%), does not permit to evaluate its passivating effect in larger quantities. In neither batch elemental sulfur was detected by the mineralogical analysis. It is possible that this layer was thinner than 3 μm, impeding its detection by QEMSCAN, or that it could be volatilized by the vacuum environment where the QEMSCAN briquettes are sealed (Klauber, 2008). In either case, there is a passivating layer which cannot be consumed by A. thiooxidans. Recently, Blight et al. (2009) reported that A. thiooxidans is able to consume orthorhombic elemental sulfur (commercial type sulfur), but does not consume polymeric elemental sulfur. It has not been demonstrated that the elemental sulfur formed on chalcopyrite is of orthorhombic nature. Instead, Parker et al. (2008) detected by Raman spectroscopy an inactive sulfur
4 Table 1 Fitting of copper extractions to the SCM model.
2 0 0
10
20
30
time (d) Fig. 4. Total iron precipitation in batches “Control” and “No Fe-pp”. (●: Control, ○: No Fe-pp).
Batch
Kinetic constant (d−1)
R2
Control No Fe-pp Biotic
2.98E−6 3.49E−6 3.28E−6
0.968 0.966 0.981
4
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4.5
References
Copper extraction (%)
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0
10
20
30
time (d) Fig. 6. Copper extraction in batches “No Fe-pp” and “Biótic”. (●:No Fe-pp, ○: Biotic).
layer on leached chalcopyrite that under photodecomposition yields a polymeric sulfur phase. Consequently, if there is any elemental sulfur passivating layer over chalcopyrite, this is not of orthorhombic type. Polymeric elemental sulfur tends to decompose to orthorhombic elemental sulfur slowly with time and temperature (Steudel and Eckert, 2003; Parker et al., 2008). It is possible that the apparent lack of passivation of chalcopyrite at temperatures above 60 °C (Watling, 2006) be a result of natural changes in the sulfur layer towards orthorhombic sulfur. 5. Conclusion • The dissolution of chalcopyrite in oxidizing medium occurs according to the SCM model, indicating the presence of a layer that retards the particle reactivity as the dissolution progresses. • The formation of amorphous iron precipitates, as Fe-sulfates, on chalcopyrite particles is not a cause of mineral passivation. It was not possible to evaluate the effect of highly crystalline precipitates as jarosite. • The presence of sulfur-oxidizing bacteria, specifically A. thiooxidans, does not avoid the passivation of chalcopyrite, even in surfaces free of precipitates. • It was not possible to identify a chalcopyrite passivating layer through its detection by QEMSCAN technology.
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