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The effects of Pb-Zn flotation reagents on the bioleaching process by mesophilic bacteria
International Journal of Mineral Processing 143 (2015) 80–86
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The effects of Pb-Zn flotation reagents on the bioleaching process by mesophilic bacteria Reza Dehghan ⁎, Mahdie Dianati School of Mining and Metallurgical Engineering, Yazd University, Yazd, Iran
a r t i c l e
i n f o
Article history: Received 24 July 2014 Received in revised form 6 July 2015 Accepted 9 September 2015 Available online 12 September 2015 Keywords: Bioleaching Mesophilic bacteria Flotation chemicals Sphalerite
a b s t r a c t The flotation reagents absorbed on the mineral surfaces may have some positive or negative effects on the bacteria involving in the bioleaching process; however a few researches have paid attention to these effects. In this research, the effects of the galena and sphalerite flotation reagents on the activity of mesophilic bacteria were investigated. The bioleaching experiments were carried out using three microbial species including Acidithiobacillus Ferrooxidans (AF), Acidithiobacillus Thiooxidans (AT), Leptosprillum Ferrooxidans (LF), and also the mixed culture of these mesophilic microorganisms. The flotation reagents were used exactly in similar concentrations and in the same way as applied in the industrial lead-zinc flotation plants. Moreover, the effects of two xanthate collectors of potassium ethyl xanthate (KEX) and potassium amyl xanthate (KAX) were separately studied on the bioleaching of zinc sulphide by AF bacteria. The zinc dissolution rate under bioleaching process was increased due to the cumulative effects of the flotation reagents at the concentration ranges needed to float both galena and sphalerite minerals. However, such a positive effects of the reagents was more pronounced on the mixed-culture of bacteria compared to that of single species. The maximum zinc extraction was 97% after 30 days using the mixed-culture bacteria in the presence of chemical flotation reagents while it was only 74% in the absence of the flotation reagents. It should be noted that the corresponding percentages of zinc extraction for AF bacteria were 86% and 64% and for the LF bacteria were 70% and 61%, respectively. An increase in the bioleaching rate was also observed when the KAX collector was added into the solid sample instead of adding to the solution of cell culture media. The zinc extraction was increased from 64% to 74% when the solid sample was conditioned by adding 350 g/t of KAX to the solid sample before bioleaching. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Over thirteen million tons of zinc metal is produced annually worldwide and the demand has increased significantly since 2005. During the last 25–30 years, the zinc extraction from the sulphide concentrates has moved away from pyrometallurgy to hydrometallurgy (Giaveno et al., 2007; Rodríguez et al., 2003). The potential benefits of commercial bioleaching of zinc sulphide ores are significant in the exploitation of low-grade ores that are difficult to process using conventional technologies. In addition, sphalerite bioleaching when compared to the traditional process of Roast-Leach-Electrowinning (RLE), has the advantages that it does not require roasting, sulfuric acid plants and washing of the gaseous effluents (Harvey et al., 2002). Differential flotation is the most suitable technique for the production of galena and sphalerite concentrates. However, flotation of these ores and the disposal of the resulting tailings pose a significant risk to the surrounding environment. Most of the flotation tailings have been left without any management in lead-zinc mines. The improper ⁎ Corresponding author. E-mail address: [email protected] (R. Dehghan).
http://dx.doi.org/10.1016/j.minpro.2015.09.007 0301-7516/© 2015 Elsevier B.V. All rights reserved.
management of such tailings in the past resulted in the migration of heavy metals to the surrounding environment, causing the contamination of the soil and groundwater. Therefore, the presence of toxic heavy metals in flotation tailings can cause a lot of serious environmental problems. In order to resolve the above problems, it is of crucial importance to develop a suitable and economical process for the recovery of valuable metals and removal of heavy metals from flotation tailings (Liu et al., 2007). Bioleaching, as a relatively novel process for the extraction of metals from flotation concentrates and tailings seems to be a promising option for such purposes. In the flotation process for sulphide lead-zinc ores, at the first step, sphalerite is depressed while the galena is floated and then sphalerite is activated and floated. Therefore, galena collector, sphalerite depressant, pH adjustment agent, sphalerite collector and flotation frother are the main chemical reagents required in this process. In cases where the mineral concentrates are subsequently processed by roasting, residual flotation reagents associated with them are of little or no importance. However, with increasing application of bioprocessing of metal sulphides, the possible interactions between the bacteria and such reagents must be studied. According to some researches (Okibe and Johnson, 2002; Dopson et al., 2006; Huerta et al.,
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Table 1 Flotation chemicals of sulphide lead-zinc flotation used in the conditioning of the bioleaching feed samples. Chemical reagent
Role of the reagent
Consumption (gr/ton)
Conditioning time (min.)
pulp pH
Gasoline Potassium Ethyl Xanthate Ferrous Sulfate Potassium Amyl Xanthate Copper sulfate
Shale flotation Galena collector Sphalerite depressant Sphalerite collector Sphalerite activator
5 350 350 325 1000
8 10 10 10 10
5.5 8.5 8.5 9.5 9.5
1995; Dong and Lin, 2012), mineral-associated flotation reagents could have a significant deleterious impact on the oxidation of concentrates, via inhibiting the activities of bioleaching microorganisms. Their impact would be accentuated by the fact that bioleaching microorganisms tend to bind specifically to the surfaces of sulfide minerals like the flotation chemicals (Okibe and Johnson, 2002). The stimulative or inhibitive effects of flotation reagents on the bioleaching systems have been reported in a few researches. The effect of chemicals used in preparation of mineral concentrates and subsequent extraction of metals to the thermophilic, acidophilic microorganism Sulfolobus Metallicus has been reported by Dopson et al. (2006). According to their results, potassium amyl xanthate (KAX) was the flotation chemical with the least negative effect on bioleaching by thermophilic bacteria, so that an increase in the bioleaching rate of chalcopyrite has been observed in the presence of this collector. The possible reason for such a positive effect has been attributed to the solubilization of elemental sulfur which formed some passivation layers on the surface of minerals. They also reported that the effect of flotation chemicals on bioleaching depends on conditions and time frame of the experiments due to their mode of toxicity and stability in acidic pH. On the other hand, some flotation collectors such as ethyl xanthate, isopropyl xanthate, butyl xanthate, iso-amyl xanthate and butylamine have shown some negative effects on bioleaching process and growth of bacteria (Okibe and Johnson, 2002; Huerta et al., 1995; Dong and Lin, 2012; Tuovinen, 1978). Torma et al. (1976) have demonstrated that the chalcopyrite oxidation ability of Thiobacillus Ferrooxidans is considerably reduced in presence of the surface active agents, such as Tween 20, 40, 60 and 80 (Torma et al., 1976). In their results, the diminishing of the surface tension of culture medium and the oxygen concentration at saturation by the surface active agents have been reported as the main reasons for the limitation in the bacterial activity. However, both Tween-80 in concentrations lower than 10-8 g/L and sodium isobutyl xanthate in concentrations of 10- 4 to 10-8 g/L have been reported to have a positive effect on the growth of microorganisms but inhibit their activity and were even harmful at higher concentrations (Zhang et al., 2008). The toxicity of metal extraction and flotation chemicals has become increasingly important as much more concentrates are being treated by bioleaching microorganisms. This is especially important for chemicals that attach to the mineral surface which directly interact with microorganisms and form a biofilm on the mineral during bioleaching (Rohwerder et al., 2003).
The current research has been performed with the purpose of assessing the effects of Pb-Zn flotation reagents on the activity and ability of mesophilic bacteria under zinc bioleaching process. Therefore, the overall effects of the conventional flotation reagents under use in leadzinc flotation plants were investigated on the activity of three mesophilic bacteria including Acidithiobacillus Ferrooxidans (AF), Leptosprilium Ferrooxidans (LF) and Acidithiobacillus Thiooxidans (TT). The concentrations and the mode of addition of the reagents were exactly the same as applied in the sulphide lead-zinc flotation circuits. The individual effect of the addition of two conventional flotation collectors of potassium amyl xanthate (KAX) and potassium ethyl xanthate (KEX) on Acidithiobacillus Ferrooxidans was also evaluated. Moreover, the effect of the addition of the collector into solid sample instead of cell culture media solution was studied. 2. Materials and methods 2.1. Ore sample and flotation chemicals A lead-zinc ore sample containing 10.17% Zn and 12% Fe supplied by Kooshk lead-zinc mine and used in the bioleaching experiments. According to the results of X-ray diffraction (XRD) analysis, galena and sphalerite were the major lead and zinc minerals, respectively and the main gangue minerals were pyrite, gypsum, quartz and clay minerals. The ore sample was crushed in laboratory crusher and consequently in laboratory ball mill to achieve the particle size (d80) of minus 106 μm. Then, 500 gr of the mill discharge was used in the conditioning stage in a 3-liter Denver laboratory flotation cell. The conditioning slurry with a solid percent of 40% was prepared with the flotation reagents listed in Table 1. The role of each reagent, its concentration, the conditioning time and the pulp pH at different stages of the conditioning process are also presented in Table 1. Lime was used in order to adjust the pH of the pulp. After addition of the flotation chemicals, the slurry was dried in the laboratory ambient via sun-drying and without any filtration. Therefore, all of the flotation reagents were remained in the solid sample. Consequently, 5 gr of the representative sample of the dried chemicallytreated materials was used for each of the bioleaching experiments. Similar bioleaching experiments were also performed using 5 gr of the fresh solid sample without addition of any flotation reagent. 2.2. Bioleaching experiments
Table 2 Bioleaching experiments examining the overall effect of chemicals and types of bacteria. Test number
code
Bacteria
Type of solid sample
Medium
1 2 3 4 5 6 7 8 9 10
1 2 1 2 1 2 1 2 1 1
AF AF LF LF AT AT Mixed Mixed AF AT
Chemically treated Fresh feed Chemically treated Fresh feed Chemically treated Fresh feed Chemically treated Fresh feed Chemically treated Chemically treated
9 k + FeSO4 9 k + FeSO4 9 k + FeSO4 9 k + FeSO4 9k+S 9k+S 9 k + FeSO4 + S 9 k + FeSO4 + S 9 k + FeSO4 9k+S
Bioleaching experiments were carried out using three strains of mesophilic bacteria including Acidithiobacillus Ferrooxidans (AF), Leptosprilium Ferrooxidans (LF), Acidithiobacillus Thiooxidans (AT) and the mixed-culture of these bacteria in volume proportions of 40%, 20% and 40% respectively. Bioleaching experiments were carried out in 500 mL flasks, containing 90 mL of 9 k medium and 10 mL of inocula. The initial cell density of the inoculated solution was 1.0 × 108 cell.mL-1 and the bacteria were previously adapted with the solid sample. In adaptation stage, the bacteria were gradually acclimatized themselves to different concentrations of the solid sample. The isolates were adapted and grown with gradually increased concentrations of the solid in 100 mL of the media. In the first stage of adaptation only 0.1 gr of the solid sample were used and after growth of the cells, the
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Table 3 Bioleaching experiments comparing the effect of KAX and KEX and their mode of addition. Test number
code
Bacteria
Type of solid sample
medium
11 12 13
KAX KEX KAX Added to Solid
AF AF AF
Fresh feed Fresh feed Fresh feed
9 k + FeSO4+ KAX 9 k + FeSO4 + KEX 9 k + FeSO4
solid content were increased to 0.5, 1, 1.5, 2, 4, 6, 8 and 10 gr. In bioleaching experiments, the solid percent was 5% (wt/vol). Flasks were maintained at 34 °C in an incubator shaker having the speed of 175 rpm. The bioleaching experiments were continued for 30 days and pH value, redox potential and zinc concentration in the bioleaching solution were determined at certain intervals. The loss of water due to evaporation was compensated by distilled water. The solution samples were analyzed for Zn using atomic absorption spectroscopy model Analytik Jena novAA350. Sodium benzoate solution was added to the medium to inhibit bacterial growth in control bioleaching tests. The operating conditions in the bioleaching experiments are presented in Tables 2 and 3. As can be seen in these tables, the experiments were carried out in two different groups. In the first group (No. 1-10) the behavior of the bioleaching of chemically treated samples were compared with the fresh feed samples. In Table 2, code 1 and code 2 are used for the bioleaching experiments in the presence and absence of flotation chemicals, respectively. Experiments No. 1, 3 5, 7, 9, and 10 were carried out on the ore sample that was previously treated with flotation chemicals and the experiments No. 2, 4, 6 and 8 were on the fresh feed samples without any chemical treatment. It should be emphasized that only in bioleaching experiments No. 9 and 10 the slurry pH has been controlled on 1.8 during 30 days period of the test and sulfuric acid solution was used for this purpose. In other experiments, pulp pH was monitored and recorded without adjustment. The effects of two xanthate collectors of potassium amyl xanthate (KAX) and potassium ethyl xanthate (KEX) were compared on the bioleaching of zinc sulphide by AF bacteria in the second group of experiments (No 11-13). It is also clear in Table 3 that the solid samples used in these three tests were the fresh feed that only 325 g/t of KAX or KEX collectors was added. Such a dosage of collectors was equal to the concentration of 1.625 × 10-2 g/L in solution media. It should be noted that these two collectors were also used in combination with the other flotation reagents in chemical treatment of the solid samples for the first group of experiments (Table 1). However, their effects in the absence of other chemicals were evaluated, when they were added into the solutions of cell culture media in experiments No. 11 and 12. Experiment
No. 13 in Table 3 was also performed to study the effect of addition of KAX into the solid sample instead of into the solution of cell culture. For this purpose, the slurry containing 40% solid was prepared, its pH was increased to 9.5 by the addition of lime and consequently the required amounts of KAX was added. Afterwards, the slurry was dried by sun-drying and then 5 gr. of the dried solid was used in this bioleaching experiment. 3. Results and discussion The variations of pH during the bioleaching process with different bacteria in the presence and in the absence of the flotation chemicals are compared in Fig. 1. According to Fig. 1a and b there has been an initial increase in the slurry pH for the bioleaching experiments by AF and LF before day 5. Oxidation of ferrous iron to ferric iron by the bacteria according to the reaction 1 and/or the dissolution of the acid consuming minerals at the initial stages of bioleaching have been the reason for such an increase in pH profiles. However, at pH values higher than 2, the ferric iron may have been precipitated as ferric hydroxide and/or jarosites that resulted in the formation of hydrogen ions as shown in reaction 2, and consequently the pH of the medium was lowered. In addition to ferrous oxidation, AF bacteria are also capable to oxidize elemental sulfur to sulfate according to reaction 3 and to decrease the pH of the medium. 2Fe2þ þ 2Hþ þ 1
2
O2 →2Fe3þ þ H2 O
Fe3þ þ 6H2 O→2 FeðOHÞ3 þ 2Hþ
S þ H2 O þ 3
2
þ O2 →SO2− 4 þ 2H
ð1Þ ð2Þ ð3Þ
According to Fig. 1, the pH of the culture medium after 30 days has been decreased to 1.82 and 1.97 for AF bacteria in the presence or absence of flotation reagents, respectively. The corresponding values of the pH have been 1.91 and 1.98 for LF bacteria. Unlike to AF and LF
Fig. 1. pH variations during 30 days of bioleaching with a) AF, b) LF, c) AT and d) Mixed-culture bacteria (code 1 in presence and code 2 in the absence of flotation chemicals).
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Fig. 2. Growth curves of bacteria during 30 days period of bioleaching with a) AF, b) LF, c) AT and d) Mixed-culture bacteria (code 1 in presence and code 2 in the absence of flotation chemicals).
bacteria that have been able to maintain the acidic pH in the reaction, pH was increased to 6.91 and 4.92 in the experiments by AT bacteria in the presence and in the absence of flotation chemicals, respectively. Such an increase in the pH that is mainly attributed to the dissolution of acid-consuming minerals inhibited the bacterial growth and activity. This result is also shown in Fig. 2c when the cell counts of AT were compared with other species. The maximum cell count achieved for AT bacteria was 1 × 108 cell/mL, while the corresponding values were 5 × 108 cell/mL and 1.4 × 109 cell/mL for LF and AF bacteria, respectively. Different strains of mesophilic bacteria have definitely different sensitivities to the pH fluctuation in their cultures. The results of the bioleaching experiments with pH control showed that the different pH profile for AT bacteria was mainly attributed to the more sensitivity of this strain to acidity of the environment. Consequently relatively similar results were observed for AT bacteria in the presence and absence of the flotation chemicals. Ultimate zinc extraction in the bioleaching experiments after 30 days in the presence and absence of the flotation chemicals are also compared in Table 4. Accordingly, zinc extraction values achieved by AT bacteria were much lower than the other bacteria and only 14.8% and 9.5% of zinc was extracted in the presence and absence of flotation reagents respectively after 30 days. This result suggested that AT bacteria were more sensitive to the slurry pH and pH should be precisely controlled in case of bioleaching using these bacteria. Preliminary studies on the growth curves of the mesophilic bacteria under study showed that the lag phase of AT bacteria was more than 10 days, while the lag phases were 5 days and 6 days for AF and LF bacteria, respectively. Therefore, the different behavior of AT bacteria in comparison with two other species may be attributed to such a difference in lag phases. Obviously, more acid consuming minerals were dissolved due to the more delays in the growth of AT bacteria and so the solution pH was increased to the level that inhibited the bacterial growth.
The maximum percentage of zinc extractions that has been achieved using the mixed-culture bacteria may be attributed to the fact that more kinds of enzymes are exist in the mixed-culture medium. Each enzymes digest very specific food. Thus increasing the diversity of enzymes increased the compounds digesting, so bacterium can use many kinds of foods (Donati and Sand, 2007). The consideration of the composition of flotation chemicals under use in this study may help to understand the possible reasons for the stimulating effects of flotation reagents on mesophilic bacteria. Three organic reagents (including xanthate collectors and gasoline) and two inorganic salts were added into the solid samples before bioleaching (Table 1). According to Puhakka and Tuovinen, the pure culture of Acidithiobacillus Ferrooxidans and the mixed cultures of acidophilic bacteria required a low concentration of organic carbon for the biological leaching of sulphide ore substrate (Puhakka and Tuovinen, 1987). The bacterially influenced acid degradation of both amyl and ethyl xanthate was also reported by Pacholewska et al. when the bacteria AT were cultured in the presence of these collectors (Pacholewska et al., 2008). Ferric sulfate, as the sphalerite depressant in galena flotation, is an energy source for iron-oxidizing bacteria. The concentration of copper ion in the culture media, due to the addition of copper sulfate, can be calculated as 20 mg/L that will not be poisonous for bacteria. Also of importance is the fact that he mesophilic bacteria under study was easily used in the bioleaching of copper ore and could tolerate high concentrations of the copper ion. However, complete inactivation of the metabolism of Acidithiobacillus Thiooxidans was reported by
Table 4 Comparison of the ultimate Zn recovery after 30 days of bioleaching by different bacteria species in the presence and absence of flotation chemicals. Type of bacteria
Acidithiobacillus Ferrooxidans Leptosprilium Ferrooxidans Acidithiobacillus Thiooxidans Mixed- culture
Ultimate Zn extraction after 30 days (%) In presence of flotation reagents
In absence of flotation reagents
86 72 14.8 97
64 61 9.5 74
Fig. 3. Zn extraction curves in bioleaching experiments (code 1 in presence and code 2 in the absence of flotation chemicals).
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Table 5 Zinc extraction equations fitted for calculating the area between two curves of code 1 and 2. Bacteria
Code
Equation
R-squared
AF
Code 1 Code 2 Code 1 Code 2 Code 1 Code 2
0.17 + 6.05X − 0.5X2 + 0.02X3 − 2.9 × 10−4X4 − 2.4 × 10−6X5 0.02 + 5.8X − 0.87X2 + 0.07X3 − 2.2 × 10−3X4 − 2.4 × 10−5X5 0.1 + 3.5X − 0.6X2 + 0.05X3 − 1.7 × 10−3X4 + 2.1 × 10−5X5 0.14 + 1.3X − 0.08X2 + 2.9X3 − 5.4 × 10−5X4 − 5.1 × 10−7X5 0.5 + 5.33X + 0.05X2 − 0.03X3 + 1.4 × 10−3X4 − 2.3 × 10−5X5 − 0.17 + 8.6X − 1.32X2 + 0.11X3 − 4.1 × 10−3X4 + 5.3 × 10−5X5
0.997 0.984 0.984 0.974 0.991 0.997
AT Mixed-culture
Pacholewska et al., when the flotation modifier of Cu(II) ions were used (Pacholewska et al., 2008). From a practical point of view, it is more interesting to consider dissolution rates in relation to the bacterial kinetics. Therefore, the progresses of zinc extraction with time in the bioleaching experiments are compared in Fig. 3. This figure clearly proves that not only ultimate recovery of zinc has increased in the presence of chemicals, but also the dissolution rate was significantly increased. To evaluate and compare the effect of flotation chemicals on the activity of any kind of bacteria (AF, AT and Mixed), the area between two curves of zinc extraction (Code 1 and Code 2) for each species was calculated by fitting a five order equation. Equations fitted to the zinc extraction in six bioleaching experiments are presented in Table 5. This approach is proposed and used for the first time in this paper for the analysis of the kinetics behavior of the bioleaching processes. The technique has the advantage that the effects of the process variables on the kinetics during the reaction progress are very clearly illustrated based on the size and shape of the area between curves. Moreover, the zinc extraction curves and the area between them are presented in Figs. 4 to 6 for different bacterial species. As can be seen the area between these curves that shows the degree of influence of flotation chemicals on sphalerite bioleaching were 344.16, 318.82 and 450.16 for the bioleaching experiments by AF, AT and mixed-culture, respectively. Comparing the values of the area between extraction curves in Figs. 4 and 5, it could be concluded that the degree of the influence of flotation chemicals on AT bacteria was also comparable with that of AF. However, the lower Zn extraction of the former was attributed to its more sensitivity to pH control during bioleaching. Therefore, it can also be appreciated that all of the bacteria responded well to the addition of flotation chemicals. Obviously, the effect of flotation reagents on the mixed-culture was more pronounced than the other single species. In summary, it could be concluded that the positive influence of the flotation chemicals on the activity and the ability of the bacteria was increased with the progress of the bioleaching process. This result suggests that the flotation chemicals have had the most influence on the kinetics of zinc bioleaching in the final stages of the process. The future studies will clarify their possible roles to change the properties of the passivation layers that are assumed to be formed on the surface of sphalerite particles during bioleaching (Rodríguez
et al., 2003). These results are consistent when the cell count profiles in Fig. 2 to be compared with the Zn extraction curves in Fig. 3. Adjustment of pH during the bioleaching experiments showed a positive influence on the growth of AF and AT bacteria and on the kinetics of Zn dissolution. The kinetic curves of zinc extraction for these two strains of bacteria are compared in Fig. 7. These two strains of mesophiles have definitely different sensitivities to the pH fluctuation in their cultures. It can be seen that, the ultimate Zn recovery by AT bacteria was increased from 14.8% without adjustment of pH to 91.8% with pH adjustment. This figure also shows the positive effect of pH adjustment on AF bacteria, so that the final zinc recovery was increased from 86% without adjustment of pH to 96.4% when the pH was adjusted. Such a big influence of pH adjustment may be, to some extent, due to the lime addition during the chemical treatment of the solid samples. Moreover, controlling the pH during the bioleaching experiment means the addition of more acids into the bioleaching reactor. Therefore, one result of the pH adjustment is to promote the growth of acidophilic bacteria, via preventing of the undesirable increase in pH, and consequently to improve their activities. Addition of more acid may also be in the favor of more rapid degradation of the xanthate collectors. The effective acid-degradation of the flotation collectors, mainly xanthates, in the tailing dams of the flotation plants is well documented (Pacholewska et al., 2008). The first three points in Fig. 7 shows more Zn extraction by AF bacteria in comparison to AT that was explained due to the more lag time in the growth of latter species. In previous sections the cumulative effects of the Zn-Pb flotation chemicals on the sphalerite bioleaching were discussed. However, the effect of the organic flotation collectors on the bioleaching was also reported by some previous researches (Dong and Lin, 2012; Tuovinen, 1978; Torma et al., 1976; Zhang et al., 2008). The effect of flotation collectors on bioleaching depends on the concentration that the experiments were carried out. It has previously been observed that amyl xanthate was much less toxic than ethyl xanthate (Valdivia and Chaves, 2001) and also the positive effect of potassium amyl xanthate on the bioleaching rate was shown by Dopson et.al.. Xanthate flotation compounds have been previously added to the cultures of Acidthiobacillus Ferrooxidans in leaching chalcopyrite, inhibited the bacterial growth and caused the lag phases to increase 5-11 times longer (Loon and Madgwick, 1995). Therefore, for comparing the effects
Fig. 4. The area enclosed between two biological Zn extraction curves by AF bacteria (code 1 in presence and code 2 in the absence of flotation chemicals).
Fig. 5. The area enclosed between two biological Zn extraction curves by AT bacteria (code 1 in presence and code 2 in the absence of flotation chemicals).
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Fig. 6. The area enclosed between two biological Zn extraction curves by Mixed-culture bacteria (code 1 in presence and code 2 in the absence of flotation chemicals).
of two conventional flotation collectors including potassium amyl xanthate (KAX) and potassium ethyl xanthate (KEX) in the concentration of 325 g/t on the rate of zinc extraction, they were added into the solutions of cell culture media in two bioleaching experiments and the results are presented in Fig. 8. According to this figure, the addition of 1.625 × 10-2 g/L KAX into the solution of cell media has increased the zinc extraction by AF bacteria from 64% to 71% after 30 days. Meanwhile, in the presence of the same concentration of KEX, 63% of zinc was extracted after 30 days that was equal to the Zn extraction without addition of any collector. On the contrary to earlier findings (Huerta et al., 1995; Dong and Lin, 2012; Tuovinen, 1978), two xanthate collectors used in this study have improved the growth and activity of the bacteria and an inconsiderable change was observed in the lag phase of the bacterial growth when the flotation collectors were added. Pacholewska et al. have also demonstrated that the ethyl and amyl xanthates may improve the metabolic activity of the Acidithiobacillus Thiooxidans. In this study we observed such a stimulating effect of amyl and ethyl xanthate for both the mesophilic bacteria of Acidthiobacillus Ferrooxidans and Acidithiobacillus Ferrooxidans. Higher rates of zinc extraction can be observed in Fig. 8 in the first 20 days of the bioleaching when the collectors were added. Comparing the curves in Fig. 8, it is obvious that the addition of the flotation collectors has shown a positive influence on the initial rates of Zn bioleaching and the vertical distances between curves were decreased after 15 days. In Fig. 8 the results of a control bioleaching experiments is also presented. This experiment has been carried out in sterile condition by addition of the sodium benzoate (1%) into the solution and obviously shows that the Zn extraction achieved in different experiments are due to the bacterial activities. In comparison with the results from KEX, higher Zn extraction was more easily observed in the presence of KAX in the last days of the bioleaching. On the basis of this result, that is consistent with findings
Fig. 7. Effects of medium pH adjustment on the bioleaching performance by AT and AF, in the presence of flotation chemicals.
Fig. 8. The sphalerite dissolution curves by AF bacteria in the presence and absence KAX and KEX collectors.
of Dopson et al. (2006), it may be concluded that potassium amyl xanthate, in the concentration under study, is less toxic than potassium ethyl xanthate for AF bacteria. It is possible that the reduced toxicity of amyl xanthates could be due to either their larger size as neutral species of ethyl xanthates will cross the cellular membrane more rapidly, or precipitation of less soluble amyl xanthides with toxic metals present in the leach solution. Xanthates decompose very fast in acidic solutions (Reactions 4 and 5; R is a non-polar group, and X is the xanthate), with approximately 50% decomposition at room temperature after 1 min at pH 2 RX þ Hþ →RXH
ð4Þ
RXH→ROH þ CS2
ð5Þ
One decomposition product is CS2 that, amongst other uses, is used commercially for solubilization of sulfur. The effect of the xanthate collectors on the bioleaching systems may also be in relation to the mechanisms governing the dissolution reaction of the sulphide mineral. It was observed that the leaching of sphalerite is controlled by a layer of elemental sulphur formed on the surface, which controls diffusion. However, Rodriguez et al. (2003) have reported that the elemental sulfur formed on the sphalerite surface was not readily soluble with CS2 (Rodríguez et al., 2003). They concluded that insolubility of the sulfur may be due to the amorphous and plastic like
Fig. 9. The effect of KAX collector on biological zinc extraction by AF bacteria depending on the absorbing on the mineral surface or the addition into the solution of cell culture.
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morphology of the product layer that is formed under bioleaching condition. They also observed that pyrite was completely dissolved under bioleaching process when the elemental sulfur was removed using CS2. Therefore, with higher concentrations of xanthates increased leaching rates may be due to removal of sulfur passivation layers that may build up on the mineral surface during bioleaching. It is of importance to mention that pyrite was the major gangue mineral present in the lead-zinc ore used in this study. Dissolution of pyrite under bioleaching release Fe2 + that stimulates the bacteria for sphalerite bioleaching via indirect mechanism. The higher Zn extractions in the presence of KAX in the last days of bioleaching may be due to the effect of the decomposition products of this collector. However, to confirm this result, the analysis of the species on the mineral surface is recommended for the future studies. Escobar et al. (2009) reported that isopropyl-xanthate at concentration of 50 μg/mL did not affect the mechanism of bioleaching by contact of S. metallicus on the sulphide (direct mechanism), however it strongly affected the indirect mechanism (Escobar et al., 2009). In order to compare the effect of the addition of KAX collector into the solid sample instead of into the solution of cell culture, Zn bioleaching graphs by AF bacteria are presented in Fig. 9. The results indicated that the extraction rate was markedly increased when KAX was initially absorbed into the surface of the solid ore particles. In Fig. 9, the dissolution rate of Zn was stopped after 28 days in the bioleaching experiment without the flotation collector, while the reasonable dissolution rate could be observed at the last days of bioleaching when the collector was previously absorbed on the particles. As a result of this research, flotation reagents have great impact on bacterial leaching process. One reason is that flotation reagents and their decomposition products will change the surface propoerties of minerals such as wettability; on the other hand, flotatio reagent can affect the growth and activity of leaching bacteria. 4. Conclusions On the basis of the experimental results of zinc sulphide bioleaching in the presence and absence of lead-zinc flotation reagents the following conclusions can be highlighted: 1- The composition of the flotation reagents as applied in an industrial flotation circuit of sulphide lead-zinc ore have shown the positive effects on the growth of mesophilic bacteria and on the performance of Zn bioleaching using these bacteria. However, the bacterial activity and the process performance were sensitive to the pH adjustment, depending of the type of the bacteria involved. Without controlling the solution pH, the ultimate zinc recovery achieved after 30 days in the presence of flotation chemicals by the mixed species, AF, LF and AT bacteria were 97%, 86%, 70% and 14.8% respectively. The corresponding zinc extraction values in the absence of flotation chemicals have been 74%, 64%, 61% and 9.5%, respectively. 2- The higher Zn dissolution rate and the higher overall zinc recovery have been achieved in the bioleaching experiments by the mixedculture bacteria when compared to those of the other single species under study. 3- The area between Zn extraction curves have been used for evaluating the effects of operating conditions on the bioleaching rate. Higher reaction rates were observed in the bioleaching experiments using the mixed-culture bacteria. The bioleaching rate was also improved when the potassium amyl xanthate (KAX) was absorbed on the surface of the ore particles before bioleaching in comparison with the case that KAX was added into the solution of culture media.
4- AT bacteria was more sensitive to the pH of the bioleaching solution and zinc extraction was increased from 14.8% to 91.8% after 30 days by adjustment of pH value on 1.8. In the bioleaching experiments using these bacteria without pH adjustment, the pH values were increased to 6.41 and 4.92 in 30 days in the presence and absence of flotation chemicals, respectively. However, pH adjustment was less critical for AF bacteria and increased the zinc extraction from 86% to 96.4%. 5- According to the results of previous researches, flotation reagents may have a positive or negative effect on the bioleaching process depending on their concentration levels and other process conditions. However, the results of this research showed that the bioleaching process can be effectively used for zinc extraction from the sulphide zinc ore polluted with the flotation reagents in concentrations required for lead-zinc flotation in commercial scale. This means that the bioleaching could be a promising option for zinc extraction from the flotation tailings. References Donati, E.R., Sand, W., 2007. Microbial processing of metal sulfides. Springer, pp. 103–119. Dong, Yingbo, Lin, Hai, 2012. Influences of Flotation reagents on bioleaching of chalcopyrite by Acidithiobacillus Ferrooxidans. Miner. Eng. 32, 27–29. Dopson, Mark, Sundkvist, Jan-Eric, Börje, Lindström E., 2006. Toxicity of metal extraction and flotation chemicals to Sulfolobus Metallicus and chalcopyrite bioleaching. Hydrometallurgy 81, 205–213. Escobar, B., Quiroz, L., Vargas, T., 2009. Effect of flotation and solvent extraction reagents on the bioleaching of a copper concentrate with Sulfolobus Metallicus. Adv. Mater. Res. 71–73, 421–424. Giaveno, A., Lavalle, L., Chiacchiarini, P., Donati, E., 2007. Bioleaching of zinc from lowgrade complex sulfide ores in an airlift by isolated leptospirillum ferrooxidans. Hydrometallurgy 89, 117–126. Harvey, T.J., Van Der Merwe, W., Afewu, K., 2002. The application of the GeoBiotics GEOCOAT® biooxidation technology for the treatment of sphalerite at Kumba resources Rosh Pinah mine. Miner. Eng. 15, 823–829. Huerta, G., Escobar, B., Rubio, J., Badilla-Ohlbaum, R., 1995. Adverse effect of surface-active reagents on the bioleaching of pyrite and chalcopyrite by Thiobacillus Ferrooxidans. World J. Microbiol. Biotechnol. 11 (5), 599–600. Liu, Yun-Guo, Zhou, Ming, Zeng, Guang-Ming, Li, Xin, Xu, Wei-Hua, Fan, Ting, 2007. Effect of solids concentration on removal of heavy metals from mine tailings via bioleaching. J. Hazard. Mater. 141, 202–208. Loon, H.Y., Madgwick, J., 1995. The effect of xanthate flotation reagents on bacterial leaching of chalcopyrite by Thiobacillus Ferrooxidans. Biotechnol. Lett. 17 (9), 997–1000. Okibe, N., Johnson, D.B., 2002. Toxicity of flotation reagents to moderately thermophilic bioleaching microorganisms. Biotechnol. Lett. 24, 2011–2016. Pacholewska, M., Cwalina, B., Steindor, K., 2008. The influence of flotation reagents on sulfur-oxidizing bacteria Acidithiobacillus Thiooxidans. Physicochem. Probl. Miner. Process. 42, 37–46. Puhakka, J., Tuovinen, O.H., 1987. Effect of organic compounds on the microbiological leaching of a complex sulphde ore materil. MICERN J. 3, 429–436. Rodríguez, Y., Ballester, A., Blázquez, L., González, F., Muñoz, J., 2003. New information on the sphalerite bioleaching mechanism at low and high temperature. Hydrometallurgy 71, 57–66. Rohwerder, T., Gehrke, T., Kinzler, K., Sand, W., 2003. Bioleaching review part A: progress in bioleaching: fundamentals and mechanisms of bacterial metal sulfide oxidation. Appl. Microbiol. Biotechnol. 63 (3), 239–248. Torma, A.E., Gabra, G.G., Guay, R., Silver, M., 1976. Effect of surface active agents on the oxidation of chalcopyrite by Thiobacillus Ferrooxidans. Hydrometallurgy 1, 301–309. Tuovinen, O.H., 1978. Inhibition of Thiobacillus Ferrooxidans by mineral flotation reagents. Eur. J. Appl. Microbiol. Biotechnol. 5, 301–304. Valdivia, D.N.U., Chaves, A.P., 2001. Influence of flotation compounds on the bio-leaching process using Thiobacillus Ferrooxidans. International Biohydrometallurgy symposium. Elsevier, Amsterdam, Ouro Petro, pp. 159–166. Zhang, Chenggui, Xia, Jinlan, Zhang, Ruiyong, Peng, Anan, Nie, Zhenyuan, Qiu, Guanzhou, 2008. Comparative study on effects of Tween-80 and Sodium Isobutyl-Xanthate on the growth and sulfur-oxidizing activities of Acidithiobacillus albertensis BY-05. Trans. Nonferrous Metals Soc. China 18, 1003–1007.
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International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro
The effects of Pb-Zn flotation reagents on the bioleaching process by mesophilic bacteria Reza Dehghan ⁎, Mahdie Dianati School of Mining and Metallurgical Engineering, Yazd University, Yazd, Iran
a r t i c l e
i n f o
Article history: Received 24 July 2014 Received in revised form 6 July 2015 Accepted 9 September 2015 Available online 12 September 2015 Keywords: Bioleaching Mesophilic bacteria Flotation chemicals Sphalerite
a b s t r a c t The flotation reagents absorbed on the mineral surfaces may have some positive or negative effects on the bacteria involving in the bioleaching process; however a few researches have paid attention to these effects. In this research, the effects of the galena and sphalerite flotation reagents on the activity of mesophilic bacteria were investigated. The bioleaching experiments were carried out using three microbial species including Acidithiobacillus Ferrooxidans (AF), Acidithiobacillus Thiooxidans (AT), Leptosprillum Ferrooxidans (LF), and also the mixed culture of these mesophilic microorganisms. The flotation reagents were used exactly in similar concentrations and in the same way as applied in the industrial lead-zinc flotation plants. Moreover, the effects of two xanthate collectors of potassium ethyl xanthate (KEX) and potassium amyl xanthate (KAX) were separately studied on the bioleaching of zinc sulphide by AF bacteria. The zinc dissolution rate under bioleaching process was increased due to the cumulative effects of the flotation reagents at the concentration ranges needed to float both galena and sphalerite minerals. However, such a positive effects of the reagents was more pronounced on the mixed-culture of bacteria compared to that of single species. The maximum zinc extraction was 97% after 30 days using the mixed-culture bacteria in the presence of chemical flotation reagents while it was only 74% in the absence of the flotation reagents. It should be noted that the corresponding percentages of zinc extraction for AF bacteria were 86% and 64% and for the LF bacteria were 70% and 61%, respectively. An increase in the bioleaching rate was also observed when the KAX collector was added into the solid sample instead of adding to the solution of cell culture media. The zinc extraction was increased from 64% to 74% when the solid sample was conditioned by adding 350 g/t of KAX to the solid sample before bioleaching. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Over thirteen million tons of zinc metal is produced annually worldwide and the demand has increased significantly since 2005. During the last 25–30 years, the zinc extraction from the sulphide concentrates has moved away from pyrometallurgy to hydrometallurgy (Giaveno et al., 2007; Rodríguez et al., 2003). The potential benefits of commercial bioleaching of zinc sulphide ores are significant in the exploitation of low-grade ores that are difficult to process using conventional technologies. In addition, sphalerite bioleaching when compared to the traditional process of Roast-Leach-Electrowinning (RLE), has the advantages that it does not require roasting, sulfuric acid plants and washing of the gaseous effluents (Harvey et al., 2002). Differential flotation is the most suitable technique for the production of galena and sphalerite concentrates. However, flotation of these ores and the disposal of the resulting tailings pose a significant risk to the surrounding environment. Most of the flotation tailings have been left without any management in lead-zinc mines. The improper ⁎ Corresponding author. E-mail address: [email protected] (R. Dehghan).
http://dx.doi.org/10.1016/j.minpro.2015.09.007 0301-7516/© 2015 Elsevier B.V. All rights reserved.
management of such tailings in the past resulted in the migration of heavy metals to the surrounding environment, causing the contamination of the soil and groundwater. Therefore, the presence of toxic heavy metals in flotation tailings can cause a lot of serious environmental problems. In order to resolve the above problems, it is of crucial importance to develop a suitable and economical process for the recovery of valuable metals and removal of heavy metals from flotation tailings (Liu et al., 2007). Bioleaching, as a relatively novel process for the extraction of metals from flotation concentrates and tailings seems to be a promising option for such purposes. In the flotation process for sulphide lead-zinc ores, at the first step, sphalerite is depressed while the galena is floated and then sphalerite is activated and floated. Therefore, galena collector, sphalerite depressant, pH adjustment agent, sphalerite collector and flotation frother are the main chemical reagents required in this process. In cases where the mineral concentrates are subsequently processed by roasting, residual flotation reagents associated with them are of little or no importance. However, with increasing application of bioprocessing of metal sulphides, the possible interactions between the bacteria and such reagents must be studied. According to some researches (Okibe and Johnson, 2002; Dopson et al., 2006; Huerta et al.,
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Table 1 Flotation chemicals of sulphide lead-zinc flotation used in the conditioning of the bioleaching feed samples. Chemical reagent
Role of the reagent
Consumption (gr/ton)
Conditioning time (min.)
pulp pH
Gasoline Potassium Ethyl Xanthate Ferrous Sulfate Potassium Amyl Xanthate Copper sulfate
Shale flotation Galena collector Sphalerite depressant Sphalerite collector Sphalerite activator
5 350 350 325 1000
8 10 10 10 10
5.5 8.5 8.5 9.5 9.5
1995; Dong and Lin, 2012), mineral-associated flotation reagents could have a significant deleterious impact on the oxidation of concentrates, via inhibiting the activities of bioleaching microorganisms. Their impact would be accentuated by the fact that bioleaching microorganisms tend to bind specifically to the surfaces of sulfide minerals like the flotation chemicals (Okibe and Johnson, 2002). The stimulative or inhibitive effects of flotation reagents on the bioleaching systems have been reported in a few researches. The effect of chemicals used in preparation of mineral concentrates and subsequent extraction of metals to the thermophilic, acidophilic microorganism Sulfolobus Metallicus has been reported by Dopson et al. (2006). According to their results, potassium amyl xanthate (KAX) was the flotation chemical with the least negative effect on bioleaching by thermophilic bacteria, so that an increase in the bioleaching rate of chalcopyrite has been observed in the presence of this collector. The possible reason for such a positive effect has been attributed to the solubilization of elemental sulfur which formed some passivation layers on the surface of minerals. They also reported that the effect of flotation chemicals on bioleaching depends on conditions and time frame of the experiments due to their mode of toxicity and stability in acidic pH. On the other hand, some flotation collectors such as ethyl xanthate, isopropyl xanthate, butyl xanthate, iso-amyl xanthate and butylamine have shown some negative effects on bioleaching process and growth of bacteria (Okibe and Johnson, 2002; Huerta et al., 1995; Dong and Lin, 2012; Tuovinen, 1978). Torma et al. (1976) have demonstrated that the chalcopyrite oxidation ability of Thiobacillus Ferrooxidans is considerably reduced in presence of the surface active agents, such as Tween 20, 40, 60 and 80 (Torma et al., 1976). In their results, the diminishing of the surface tension of culture medium and the oxygen concentration at saturation by the surface active agents have been reported as the main reasons for the limitation in the bacterial activity. However, both Tween-80 in concentrations lower than 10-8 g/L and sodium isobutyl xanthate in concentrations of 10- 4 to 10-8 g/L have been reported to have a positive effect on the growth of microorganisms but inhibit their activity and were even harmful at higher concentrations (Zhang et al., 2008). The toxicity of metal extraction and flotation chemicals has become increasingly important as much more concentrates are being treated by bioleaching microorganisms. This is especially important for chemicals that attach to the mineral surface which directly interact with microorganisms and form a biofilm on the mineral during bioleaching (Rohwerder et al., 2003).
The current research has been performed with the purpose of assessing the effects of Pb-Zn flotation reagents on the activity and ability of mesophilic bacteria under zinc bioleaching process. Therefore, the overall effects of the conventional flotation reagents under use in leadzinc flotation plants were investigated on the activity of three mesophilic bacteria including Acidithiobacillus Ferrooxidans (AF), Leptosprilium Ferrooxidans (LF) and Acidithiobacillus Thiooxidans (TT). The concentrations and the mode of addition of the reagents were exactly the same as applied in the sulphide lead-zinc flotation circuits. The individual effect of the addition of two conventional flotation collectors of potassium amyl xanthate (KAX) and potassium ethyl xanthate (KEX) on Acidithiobacillus Ferrooxidans was also evaluated. Moreover, the effect of the addition of the collector into solid sample instead of cell culture media solution was studied. 2. Materials and methods 2.1. Ore sample and flotation chemicals A lead-zinc ore sample containing 10.17% Zn and 12% Fe supplied by Kooshk lead-zinc mine and used in the bioleaching experiments. According to the results of X-ray diffraction (XRD) analysis, galena and sphalerite were the major lead and zinc minerals, respectively and the main gangue minerals were pyrite, gypsum, quartz and clay minerals. The ore sample was crushed in laboratory crusher and consequently in laboratory ball mill to achieve the particle size (d80) of minus 106 μm. Then, 500 gr of the mill discharge was used in the conditioning stage in a 3-liter Denver laboratory flotation cell. The conditioning slurry with a solid percent of 40% was prepared with the flotation reagents listed in Table 1. The role of each reagent, its concentration, the conditioning time and the pulp pH at different stages of the conditioning process are also presented in Table 1. Lime was used in order to adjust the pH of the pulp. After addition of the flotation chemicals, the slurry was dried in the laboratory ambient via sun-drying and without any filtration. Therefore, all of the flotation reagents were remained in the solid sample. Consequently, 5 gr of the representative sample of the dried chemicallytreated materials was used for each of the bioleaching experiments. Similar bioleaching experiments were also performed using 5 gr of the fresh solid sample without addition of any flotation reagent. 2.2. Bioleaching experiments
Table 2 Bioleaching experiments examining the overall effect of chemicals and types of bacteria. Test number
code
Bacteria
Type of solid sample
Medium
1 2 3 4 5 6 7 8 9 10
1 2 1 2 1 2 1 2 1 1
AF AF LF LF AT AT Mixed Mixed AF AT
Chemically treated Fresh feed Chemically treated Fresh feed Chemically treated Fresh feed Chemically treated Fresh feed Chemically treated Chemically treated
9 k + FeSO4 9 k + FeSO4 9 k + FeSO4 9 k + FeSO4 9k+S 9k+S 9 k + FeSO4 + S 9 k + FeSO4 + S 9 k + FeSO4 9k+S
Bioleaching experiments were carried out using three strains of mesophilic bacteria including Acidithiobacillus Ferrooxidans (AF), Leptosprilium Ferrooxidans (LF), Acidithiobacillus Thiooxidans (AT) and the mixed-culture of these bacteria in volume proportions of 40%, 20% and 40% respectively. Bioleaching experiments were carried out in 500 mL flasks, containing 90 mL of 9 k medium and 10 mL of inocula. The initial cell density of the inoculated solution was 1.0 × 108 cell.mL-1 and the bacteria were previously adapted with the solid sample. In adaptation stage, the bacteria were gradually acclimatized themselves to different concentrations of the solid sample. The isolates were adapted and grown with gradually increased concentrations of the solid in 100 mL of the media. In the first stage of adaptation only 0.1 gr of the solid sample were used and after growth of the cells, the
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Table 3 Bioleaching experiments comparing the effect of KAX and KEX and their mode of addition. Test number
code
Bacteria
Type of solid sample
medium
11 12 13
KAX KEX KAX Added to Solid
AF AF AF
Fresh feed Fresh feed Fresh feed
9 k + FeSO4+ KAX 9 k + FeSO4 + KEX 9 k + FeSO4
solid content were increased to 0.5, 1, 1.5, 2, 4, 6, 8 and 10 gr. In bioleaching experiments, the solid percent was 5% (wt/vol). Flasks were maintained at 34 °C in an incubator shaker having the speed of 175 rpm. The bioleaching experiments were continued for 30 days and pH value, redox potential and zinc concentration in the bioleaching solution were determined at certain intervals. The loss of water due to evaporation was compensated by distilled water. The solution samples were analyzed for Zn using atomic absorption spectroscopy model Analytik Jena novAA350. Sodium benzoate solution was added to the medium to inhibit bacterial growth in control bioleaching tests. The operating conditions in the bioleaching experiments are presented in Tables 2 and 3. As can be seen in these tables, the experiments were carried out in two different groups. In the first group (No. 1-10) the behavior of the bioleaching of chemically treated samples were compared with the fresh feed samples. In Table 2, code 1 and code 2 are used for the bioleaching experiments in the presence and absence of flotation chemicals, respectively. Experiments No. 1, 3 5, 7, 9, and 10 were carried out on the ore sample that was previously treated with flotation chemicals and the experiments No. 2, 4, 6 and 8 were on the fresh feed samples without any chemical treatment. It should be emphasized that only in bioleaching experiments No. 9 and 10 the slurry pH has been controlled on 1.8 during 30 days period of the test and sulfuric acid solution was used for this purpose. In other experiments, pulp pH was monitored and recorded without adjustment. The effects of two xanthate collectors of potassium amyl xanthate (KAX) and potassium ethyl xanthate (KEX) were compared on the bioleaching of zinc sulphide by AF bacteria in the second group of experiments (No 11-13). It is also clear in Table 3 that the solid samples used in these three tests were the fresh feed that only 325 g/t of KAX or KEX collectors was added. Such a dosage of collectors was equal to the concentration of 1.625 × 10-2 g/L in solution media. It should be noted that these two collectors were also used in combination with the other flotation reagents in chemical treatment of the solid samples for the first group of experiments (Table 1). However, their effects in the absence of other chemicals were evaluated, when they were added into the solutions of cell culture media in experiments No. 11 and 12. Experiment
No. 13 in Table 3 was also performed to study the effect of addition of KAX into the solid sample instead of into the solution of cell culture. For this purpose, the slurry containing 40% solid was prepared, its pH was increased to 9.5 by the addition of lime and consequently the required amounts of KAX was added. Afterwards, the slurry was dried by sun-drying and then 5 gr. of the dried solid was used in this bioleaching experiment. 3. Results and discussion The variations of pH during the bioleaching process with different bacteria in the presence and in the absence of the flotation chemicals are compared in Fig. 1. According to Fig. 1a and b there has been an initial increase in the slurry pH for the bioleaching experiments by AF and LF before day 5. Oxidation of ferrous iron to ferric iron by the bacteria according to the reaction 1 and/or the dissolution of the acid consuming minerals at the initial stages of bioleaching have been the reason for such an increase in pH profiles. However, at pH values higher than 2, the ferric iron may have been precipitated as ferric hydroxide and/or jarosites that resulted in the formation of hydrogen ions as shown in reaction 2, and consequently the pH of the medium was lowered. In addition to ferrous oxidation, AF bacteria are also capable to oxidize elemental sulfur to sulfate according to reaction 3 and to decrease the pH of the medium. 2Fe2þ þ 2Hþ þ 1
2
O2 →2Fe3þ þ H2 O
Fe3þ þ 6H2 O→2 FeðOHÞ3 þ 2Hþ
S þ H2 O þ 3
2
þ O2 →SO2− 4 þ 2H
ð1Þ ð2Þ ð3Þ
According to Fig. 1, the pH of the culture medium after 30 days has been decreased to 1.82 and 1.97 for AF bacteria in the presence or absence of flotation reagents, respectively. The corresponding values of the pH have been 1.91 and 1.98 for LF bacteria. Unlike to AF and LF
Fig. 1. pH variations during 30 days of bioleaching with a) AF, b) LF, c) AT and d) Mixed-culture bacteria (code 1 in presence and code 2 in the absence of flotation chemicals).
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Fig. 2. Growth curves of bacteria during 30 days period of bioleaching with a) AF, b) LF, c) AT and d) Mixed-culture bacteria (code 1 in presence and code 2 in the absence of flotation chemicals).
bacteria that have been able to maintain the acidic pH in the reaction, pH was increased to 6.91 and 4.92 in the experiments by AT bacteria in the presence and in the absence of flotation chemicals, respectively. Such an increase in the pH that is mainly attributed to the dissolution of acid-consuming minerals inhibited the bacterial growth and activity. This result is also shown in Fig. 2c when the cell counts of AT were compared with other species. The maximum cell count achieved for AT bacteria was 1 × 108 cell/mL, while the corresponding values were 5 × 108 cell/mL and 1.4 × 109 cell/mL for LF and AF bacteria, respectively. Different strains of mesophilic bacteria have definitely different sensitivities to the pH fluctuation in their cultures. The results of the bioleaching experiments with pH control showed that the different pH profile for AT bacteria was mainly attributed to the more sensitivity of this strain to acidity of the environment. Consequently relatively similar results were observed for AT bacteria in the presence and absence of the flotation chemicals. Ultimate zinc extraction in the bioleaching experiments after 30 days in the presence and absence of the flotation chemicals are also compared in Table 4. Accordingly, zinc extraction values achieved by AT bacteria were much lower than the other bacteria and only 14.8% and 9.5% of zinc was extracted in the presence and absence of flotation reagents respectively after 30 days. This result suggested that AT bacteria were more sensitive to the slurry pH and pH should be precisely controlled in case of bioleaching using these bacteria. Preliminary studies on the growth curves of the mesophilic bacteria under study showed that the lag phase of AT bacteria was more than 10 days, while the lag phases were 5 days and 6 days for AF and LF bacteria, respectively. Therefore, the different behavior of AT bacteria in comparison with two other species may be attributed to such a difference in lag phases. Obviously, more acid consuming minerals were dissolved due to the more delays in the growth of AT bacteria and so the solution pH was increased to the level that inhibited the bacterial growth.
The maximum percentage of zinc extractions that has been achieved using the mixed-culture bacteria may be attributed to the fact that more kinds of enzymes are exist in the mixed-culture medium. Each enzymes digest very specific food. Thus increasing the diversity of enzymes increased the compounds digesting, so bacterium can use many kinds of foods (Donati and Sand, 2007). The consideration of the composition of flotation chemicals under use in this study may help to understand the possible reasons for the stimulating effects of flotation reagents on mesophilic bacteria. Three organic reagents (including xanthate collectors and gasoline) and two inorganic salts were added into the solid samples before bioleaching (Table 1). According to Puhakka and Tuovinen, the pure culture of Acidithiobacillus Ferrooxidans and the mixed cultures of acidophilic bacteria required a low concentration of organic carbon for the biological leaching of sulphide ore substrate (Puhakka and Tuovinen, 1987). The bacterially influenced acid degradation of both amyl and ethyl xanthate was also reported by Pacholewska et al. when the bacteria AT were cultured in the presence of these collectors (Pacholewska et al., 2008). Ferric sulfate, as the sphalerite depressant in galena flotation, is an energy source for iron-oxidizing bacteria. The concentration of copper ion in the culture media, due to the addition of copper sulfate, can be calculated as 20 mg/L that will not be poisonous for bacteria. Also of importance is the fact that he mesophilic bacteria under study was easily used in the bioleaching of copper ore and could tolerate high concentrations of the copper ion. However, complete inactivation of the metabolism of Acidithiobacillus Thiooxidans was reported by
Table 4 Comparison of the ultimate Zn recovery after 30 days of bioleaching by different bacteria species in the presence and absence of flotation chemicals. Type of bacteria
Acidithiobacillus Ferrooxidans Leptosprilium Ferrooxidans Acidithiobacillus Thiooxidans Mixed- culture
Ultimate Zn extraction after 30 days (%) In presence of flotation reagents
In absence of flotation reagents
86 72 14.8 97
64 61 9.5 74
Fig. 3. Zn extraction curves in bioleaching experiments (code 1 in presence and code 2 in the absence of flotation chemicals).
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Table 5 Zinc extraction equations fitted for calculating the area between two curves of code 1 and 2. Bacteria
Code
Equation
R-squared
AF
Code 1 Code 2 Code 1 Code 2 Code 1 Code 2
0.17 + 6.05X − 0.5X2 + 0.02X3 − 2.9 × 10−4X4 − 2.4 × 10−6X5 0.02 + 5.8X − 0.87X2 + 0.07X3 − 2.2 × 10−3X4 − 2.4 × 10−5X5 0.1 + 3.5X − 0.6X2 + 0.05X3 − 1.7 × 10−3X4 + 2.1 × 10−5X5 0.14 + 1.3X − 0.08X2 + 2.9X3 − 5.4 × 10−5X4 − 5.1 × 10−7X5 0.5 + 5.33X + 0.05X2 − 0.03X3 + 1.4 × 10−3X4 − 2.3 × 10−5X5 − 0.17 + 8.6X − 1.32X2 + 0.11X3 − 4.1 × 10−3X4 + 5.3 × 10−5X5
0.997 0.984 0.984 0.974 0.991 0.997
AT Mixed-culture
Pacholewska et al., when the flotation modifier of Cu(II) ions were used (Pacholewska et al., 2008). From a practical point of view, it is more interesting to consider dissolution rates in relation to the bacterial kinetics. Therefore, the progresses of zinc extraction with time in the bioleaching experiments are compared in Fig. 3. This figure clearly proves that not only ultimate recovery of zinc has increased in the presence of chemicals, but also the dissolution rate was significantly increased. To evaluate and compare the effect of flotation chemicals on the activity of any kind of bacteria (AF, AT and Mixed), the area between two curves of zinc extraction (Code 1 and Code 2) for each species was calculated by fitting a five order equation. Equations fitted to the zinc extraction in six bioleaching experiments are presented in Table 5. This approach is proposed and used for the first time in this paper for the analysis of the kinetics behavior of the bioleaching processes. The technique has the advantage that the effects of the process variables on the kinetics during the reaction progress are very clearly illustrated based on the size and shape of the area between curves. Moreover, the zinc extraction curves and the area between them are presented in Figs. 4 to 6 for different bacterial species. As can be seen the area between these curves that shows the degree of influence of flotation chemicals on sphalerite bioleaching were 344.16, 318.82 and 450.16 for the bioleaching experiments by AF, AT and mixed-culture, respectively. Comparing the values of the area between extraction curves in Figs. 4 and 5, it could be concluded that the degree of the influence of flotation chemicals on AT bacteria was also comparable with that of AF. However, the lower Zn extraction of the former was attributed to its more sensitivity to pH control during bioleaching. Therefore, it can also be appreciated that all of the bacteria responded well to the addition of flotation chemicals. Obviously, the effect of flotation reagents on the mixed-culture was more pronounced than the other single species. In summary, it could be concluded that the positive influence of the flotation chemicals on the activity and the ability of the bacteria was increased with the progress of the bioleaching process. This result suggests that the flotation chemicals have had the most influence on the kinetics of zinc bioleaching in the final stages of the process. The future studies will clarify their possible roles to change the properties of the passivation layers that are assumed to be formed on the surface of sphalerite particles during bioleaching (Rodríguez
et al., 2003). These results are consistent when the cell count profiles in Fig. 2 to be compared with the Zn extraction curves in Fig. 3. Adjustment of pH during the bioleaching experiments showed a positive influence on the growth of AF and AT bacteria and on the kinetics of Zn dissolution. The kinetic curves of zinc extraction for these two strains of bacteria are compared in Fig. 7. These two strains of mesophiles have definitely different sensitivities to the pH fluctuation in their cultures. It can be seen that, the ultimate Zn recovery by AT bacteria was increased from 14.8% without adjustment of pH to 91.8% with pH adjustment. This figure also shows the positive effect of pH adjustment on AF bacteria, so that the final zinc recovery was increased from 86% without adjustment of pH to 96.4% when the pH was adjusted. Such a big influence of pH adjustment may be, to some extent, due to the lime addition during the chemical treatment of the solid samples. Moreover, controlling the pH during the bioleaching experiment means the addition of more acids into the bioleaching reactor. Therefore, one result of the pH adjustment is to promote the growth of acidophilic bacteria, via preventing of the undesirable increase in pH, and consequently to improve their activities. Addition of more acid may also be in the favor of more rapid degradation of the xanthate collectors. The effective acid-degradation of the flotation collectors, mainly xanthates, in the tailing dams of the flotation plants is well documented (Pacholewska et al., 2008). The first three points in Fig. 7 shows more Zn extraction by AF bacteria in comparison to AT that was explained due to the more lag time in the growth of latter species. In previous sections the cumulative effects of the Zn-Pb flotation chemicals on the sphalerite bioleaching were discussed. However, the effect of the organic flotation collectors on the bioleaching was also reported by some previous researches (Dong and Lin, 2012; Tuovinen, 1978; Torma et al., 1976; Zhang et al., 2008). The effect of flotation collectors on bioleaching depends on the concentration that the experiments were carried out. It has previously been observed that amyl xanthate was much less toxic than ethyl xanthate (Valdivia and Chaves, 2001) and also the positive effect of potassium amyl xanthate on the bioleaching rate was shown by Dopson et.al.. Xanthate flotation compounds have been previously added to the cultures of Acidthiobacillus Ferrooxidans in leaching chalcopyrite, inhibited the bacterial growth and caused the lag phases to increase 5-11 times longer (Loon and Madgwick, 1995). Therefore, for comparing the effects
Fig. 4. The area enclosed between two biological Zn extraction curves by AF bacteria (code 1 in presence and code 2 in the absence of flotation chemicals).
Fig. 5. The area enclosed between two biological Zn extraction curves by AT bacteria (code 1 in presence and code 2 in the absence of flotation chemicals).
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Fig. 6. The area enclosed between two biological Zn extraction curves by Mixed-culture bacteria (code 1 in presence and code 2 in the absence of flotation chemicals).
of two conventional flotation collectors including potassium amyl xanthate (KAX) and potassium ethyl xanthate (KEX) in the concentration of 325 g/t on the rate of zinc extraction, they were added into the solutions of cell culture media in two bioleaching experiments and the results are presented in Fig. 8. According to this figure, the addition of 1.625 × 10-2 g/L KAX into the solution of cell media has increased the zinc extraction by AF bacteria from 64% to 71% after 30 days. Meanwhile, in the presence of the same concentration of KEX, 63% of zinc was extracted after 30 days that was equal to the Zn extraction without addition of any collector. On the contrary to earlier findings (Huerta et al., 1995; Dong and Lin, 2012; Tuovinen, 1978), two xanthate collectors used in this study have improved the growth and activity of the bacteria and an inconsiderable change was observed in the lag phase of the bacterial growth when the flotation collectors were added. Pacholewska et al. have also demonstrated that the ethyl and amyl xanthates may improve the metabolic activity of the Acidithiobacillus Thiooxidans. In this study we observed such a stimulating effect of amyl and ethyl xanthate for both the mesophilic bacteria of Acidthiobacillus Ferrooxidans and Acidithiobacillus Ferrooxidans. Higher rates of zinc extraction can be observed in Fig. 8 in the first 20 days of the bioleaching when the collectors were added. Comparing the curves in Fig. 8, it is obvious that the addition of the flotation collectors has shown a positive influence on the initial rates of Zn bioleaching and the vertical distances between curves were decreased after 15 days. In Fig. 8 the results of a control bioleaching experiments is also presented. This experiment has been carried out in sterile condition by addition of the sodium benzoate (1%) into the solution and obviously shows that the Zn extraction achieved in different experiments are due to the bacterial activities. In comparison with the results from KEX, higher Zn extraction was more easily observed in the presence of KAX in the last days of the bioleaching. On the basis of this result, that is consistent with findings
Fig. 7. Effects of medium pH adjustment on the bioleaching performance by AT and AF, in the presence of flotation chemicals.
Fig. 8. The sphalerite dissolution curves by AF bacteria in the presence and absence KAX and KEX collectors.
of Dopson et al. (2006), it may be concluded that potassium amyl xanthate, in the concentration under study, is less toxic than potassium ethyl xanthate for AF bacteria. It is possible that the reduced toxicity of amyl xanthates could be due to either their larger size as neutral species of ethyl xanthates will cross the cellular membrane more rapidly, or precipitation of less soluble amyl xanthides with toxic metals present in the leach solution. Xanthates decompose very fast in acidic solutions (Reactions 4 and 5; R is a non-polar group, and X is the xanthate), with approximately 50% decomposition at room temperature after 1 min at pH 2 RX þ Hþ →RXH
ð4Þ
RXH→ROH þ CS2
ð5Þ
One decomposition product is CS2 that, amongst other uses, is used commercially for solubilization of sulfur. The effect of the xanthate collectors on the bioleaching systems may also be in relation to the mechanisms governing the dissolution reaction of the sulphide mineral. It was observed that the leaching of sphalerite is controlled by a layer of elemental sulphur formed on the surface, which controls diffusion. However, Rodriguez et al. (2003) have reported that the elemental sulfur formed on the sphalerite surface was not readily soluble with CS2 (Rodríguez et al., 2003). They concluded that insolubility of the sulfur may be due to the amorphous and plastic like
Fig. 9. The effect of KAX collector on biological zinc extraction by AF bacteria depending on the absorbing on the mineral surface or the addition into the solution of cell culture.
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morphology of the product layer that is formed under bioleaching condition. They also observed that pyrite was completely dissolved under bioleaching process when the elemental sulfur was removed using CS2. Therefore, with higher concentrations of xanthates increased leaching rates may be due to removal of sulfur passivation layers that may build up on the mineral surface during bioleaching. It is of importance to mention that pyrite was the major gangue mineral present in the lead-zinc ore used in this study. Dissolution of pyrite under bioleaching release Fe2 + that stimulates the bacteria for sphalerite bioleaching via indirect mechanism. The higher Zn extractions in the presence of KAX in the last days of bioleaching may be due to the effect of the decomposition products of this collector. However, to confirm this result, the analysis of the species on the mineral surface is recommended for the future studies. Escobar et al. (2009) reported that isopropyl-xanthate at concentration of 50 μg/mL did not affect the mechanism of bioleaching by contact of S. metallicus on the sulphide (direct mechanism), however it strongly affected the indirect mechanism (Escobar et al., 2009). In order to compare the effect of the addition of KAX collector into the solid sample instead of into the solution of cell culture, Zn bioleaching graphs by AF bacteria are presented in Fig. 9. The results indicated that the extraction rate was markedly increased when KAX was initially absorbed into the surface of the solid ore particles. In Fig. 9, the dissolution rate of Zn was stopped after 28 days in the bioleaching experiment without the flotation collector, while the reasonable dissolution rate could be observed at the last days of bioleaching when the collector was previously absorbed on the particles. As a result of this research, flotation reagents have great impact on bacterial leaching process. One reason is that flotation reagents and their decomposition products will change the surface propoerties of minerals such as wettability; on the other hand, flotatio reagent can affect the growth and activity of leaching bacteria. 4. Conclusions On the basis of the experimental results of zinc sulphide bioleaching in the presence and absence of lead-zinc flotation reagents the following conclusions can be highlighted: 1- The composition of the flotation reagents as applied in an industrial flotation circuit of sulphide lead-zinc ore have shown the positive effects on the growth of mesophilic bacteria and on the performance of Zn bioleaching using these bacteria. However, the bacterial activity and the process performance were sensitive to the pH adjustment, depending of the type of the bacteria involved. Without controlling the solution pH, the ultimate zinc recovery achieved after 30 days in the presence of flotation chemicals by the mixed species, AF, LF and AT bacteria were 97%, 86%, 70% and 14.8% respectively. The corresponding zinc extraction values in the absence of flotation chemicals have been 74%, 64%, 61% and 9.5%, respectively. 2- The higher Zn dissolution rate and the higher overall zinc recovery have been achieved in the bioleaching experiments by the mixedculture bacteria when compared to those of the other single species under study. 3- The area between Zn extraction curves have been used for evaluating the effects of operating conditions on the bioleaching rate. Higher reaction rates were observed in the bioleaching experiments using the mixed-culture bacteria. The bioleaching rate was also improved when the potassium amyl xanthate (KAX) was absorbed on the surface of the ore particles before bioleaching in comparison with the case that KAX was added into the solution of culture media.
4- AT bacteria was more sensitive to the pH of the bioleaching solution and zinc extraction was increased from 14.8% to 91.8% after 30 days by adjustment of pH value on 1.8. In the bioleaching experiments using these bacteria without pH adjustment, the pH values were increased to 6.41 and 4.92 in 30 days in the presence and absence of flotation chemicals, respectively. However, pH adjustment was less critical for AF bacteria and increased the zinc extraction from 86% to 96.4%. 5- According to the results of previous researches, flotation reagents may have a positive or negative effect on the bioleaching process depending on their concentration levels and other process conditions. However, the results of this research showed that the bioleaching process can be effectively used for zinc extraction from the sulphide zinc ore polluted with the flotation reagents in concentrations required for lead-zinc flotation in commercial scale. This means that the bioleaching could be a promising option for zinc extraction from the flotation tailings. References Donati, E.R., Sand, W., 2007. Microbial processing of metal sulfides. Springer, pp. 103–119. Dong, Yingbo, Lin, Hai, 2012. Influences of Flotation reagents on bioleaching of chalcopyrite by Acidithiobacillus Ferrooxidans. Miner. Eng. 32, 27–29. Dopson, Mark, Sundkvist, Jan-Eric, Börje, Lindström E., 2006. Toxicity of metal extraction and flotation chemicals to Sulfolobus Metallicus and chalcopyrite bioleaching. Hydrometallurgy 81, 205–213. Escobar, B., Quiroz, L., Vargas, T., 2009. Effect of flotation and solvent extraction reagents on the bioleaching of a copper concentrate with Sulfolobus Metallicus. Adv. Mater. Res. 71–73, 421–424. Giaveno, A., Lavalle, L., Chiacchiarini, P., Donati, E., 2007. Bioleaching of zinc from lowgrade complex sulfide ores in an airlift by isolated leptospirillum ferrooxidans. Hydrometallurgy 89, 117–126. Harvey, T.J., Van Der Merwe, W., Afewu, K., 2002. The application of the GeoBiotics GEOCOAT® biooxidation technology for the treatment of sphalerite at Kumba resources Rosh Pinah mine. Miner. Eng. 15, 823–829. Huerta, G., Escobar, B., Rubio, J., Badilla-Ohlbaum, R., 1995. Adverse effect of surface-active reagents on the bioleaching of pyrite and chalcopyrite by Thiobacillus Ferrooxidans. World J. Microbiol. Biotechnol. 11 (5), 599–600. Liu, Yun-Guo, Zhou, Ming, Zeng, Guang-Ming, Li, Xin, Xu, Wei-Hua, Fan, Ting, 2007. Effect of solids concentration on removal of heavy metals from mine tailings via bioleaching. J. Hazard. Mater. 141, 202–208. Loon, H.Y., Madgwick, J., 1995. The effect of xanthate flotation reagents on bacterial leaching of chalcopyrite by Thiobacillus Ferrooxidans. Biotechnol. Lett. 17 (9), 997–1000. Okibe, N., Johnson, D.B., 2002. Toxicity of flotation reagents to moderately thermophilic bioleaching microorganisms. Biotechnol. Lett. 24, 2011–2016. Pacholewska, M., Cwalina, B., Steindor, K., 2008. The influence of flotation reagents on sulfur-oxidizing bacteria Acidithiobacillus Thiooxidans. Physicochem. Probl. Miner. Process. 42, 37–46. Puhakka, J., Tuovinen, O.H., 1987. Effect of organic compounds on the microbiological leaching of a complex sulphde ore materil. MICERN J. 3, 429–436. Rodríguez, Y., Ballester, A., Blázquez, L., González, F., Muñoz, J., 2003. New information on the sphalerite bioleaching mechanism at low and high temperature. Hydrometallurgy 71, 57–66. Rohwerder, T., Gehrke, T., Kinzler, K., Sand, W., 2003. Bioleaching review part A: progress in bioleaching: fundamentals and mechanisms of bacterial metal sulfide oxidation. Appl. Microbiol. Biotechnol. 63 (3), 239–248. Torma, A.E., Gabra, G.G., Guay, R., Silver, M., 1976. Effect of surface active agents on the oxidation of chalcopyrite by Thiobacillus Ferrooxidans. Hydrometallurgy 1, 301–309. Tuovinen, O.H., 1978. Inhibition of Thiobacillus Ferrooxidans by mineral flotation reagents. Eur. J. Appl. Microbiol. Biotechnol. 5, 301–304. Valdivia, D.N.U., Chaves, A.P., 2001. Influence of flotation compounds on the bio-leaching process using Thiobacillus Ferrooxidans. International Biohydrometallurgy symposium. Elsevier, Amsterdam, Ouro Petro, pp. 159–166. Zhang, Chenggui, Xia, Jinlan, Zhang, Ruiyong, Peng, Anan, Nie, Zhenyuan, Qiu, Guanzhou, 2008. Comparative study on effects of Tween-80 and Sodium Isobutyl-Xanthate on the growth and sulfur-oxidizing activities of Acidithiobacillus albertensis BY-05. Trans. Nonferrous Metals Soc. China 18, 1003–1007.