Microbially induced flotation and flocculation of pyrite and sphalerite

Colloids and Surfaces B: Biointerfaces 36 (2004) 91–99

Microbially induced flotation and flocculation of pyrite and sphalerite Partha Patra, K.A. Natarajan∗ Department of Metallurgy, Indian Institute of Science, Bangalore 560012, India Received 24 December 2003; accepted 11 May 2004

Abstract Cells of Paenibacillus polymyxa and their metabolite products were successfully utilized to achieve selective separation of sphalerite from pyrite, through microbially induced flocculation and flotation. Adsorption studies and electrokinetic investigations were carried out to understand the changes in the surface chemistry of bacterial cells and the minerals after mutual interaction. Possible mechanisms in microbially induced flotation and flocculation are outlined. © 2004 Elsevier B.V. All rights reserved. Keywords: Paenibacillus polymyxa; Sphalerite; Pyrite; Flocculation; Flotation; Surface chemistry

1. Introduction The beneficiation of complex base-metal sulphide ores is generally based on the selective production of zinc, lead and copper concentrates from which the respective metals are extracted by metallurgical processes. The exhaustion of available mineral resources will probably lead to the search for more advanced solution to the problem of beneficiation of some refractory ores, where conventional flotation or flocculation yields poor results [1]. Lead–iron and zinc–iron ratios in lead–zinc ores are higher than copper–iron ratio in copper ores which are undesirable and uneconomical from a mineral processing point of view. On the other hand, complete elimination of iron sulfides (pyrite, pyrrhotite) from zinc concentrates is economically attractive from the angle of subsequent smelting [2]. Many processes and plant practices have been known to overcome the problem of high levels of iron in the zinc concentrate, such as: (a) pyrite depression at higher alkaline pH [3] and (b) use of thiocarbonate or thiourea collectors as more selective collectors against pyrite producing a better separa-

∗

Corresponding author. Tel.: +91 80 23600120; fax: +91 80 23600472. E-mail address: [email protected] (K.A. Natarajan).

0927-7765/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2004.05.010

tion of chalcopyrite or copper activated sphalerite from pyrite in alkaline and neutral pH conditions [4]. Utility of microorganisms in mineral beneficiation has been recently understood. Considering the above scenario, a system of sphalerite along with pyrite was chosen in the present study in order to separate sphalerite from a binary mixture of sphalerite and pyrite. Natarajan et al. [5–7] have recently carried out beneficiation studies on bauxite and iron ores using Paenibacillus polymyxa. P. polymyxa is a Gram-positive, neutrophilic, periflagellated heterotroph indigenously associated with many mineral deposits. Exo-polysaccharides, proteins and organic acids such as oxalic acid, formic acid and acetic acid are the principal components of the metabolic product obtained from P. polymyxa [8]. In order to utilize the beneficial properties of the microorganisms in microbe–mineral interaction, surface chemical studies of both bacteria and minerals along with the mutual attachment behavior among them becomes imperative. The presented investigation deals with microbially induced flotation and flocculation to separate sphalerite from a binary mixture of sphalerite and pyrite. Selective flocculation procedure was successfully used to remove fine sphalerite from a binary mixture. Micro flotation studies were also carried out for selective separation of sphalerite and pyrite. Surface chemistry of minerals before and after mu-

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tual interaction was evaluated through adsorption studies and zeta-potential measurements.

2. Materials and methods 2.1. Minerals Hand-picked highly pure mineral samples of pyrite and sphalerite were obtained from Alminrock, Indscer Fabriks, Bangalore, India. Chemical, X-ray and mineralogical analyses were carried out to ascertain the purity of the minerals. The purity of minerals was ascertained as pyrite 99.9% and sphalerite 99.6%. The above samples were ground in a porcelain ball mill, sieved and fractioned to obtain different size fractions (−105, +74 ␮m). The −37 ␮m fraction was further ground and a sample of −3 ␮m size fraction was obtained by sedimentation. Particle size analysis determined from a Malvern Zetasizer gave an average particle size of ∼5 ␮m for pyrite, and ∼3 ␮m for sphalerite. These size fractions were used for the electro-kinetic and adsorption studies. The surface areas of the samples used for adsorption were determined by BET nitrogen-specific surface area method. The surface area of the minerals from the above method is 1.045 m2 /g for pyrite and 1.939 m2 /g. 2.2. Bacterial culture Strains of P. polymyxa used in this study were obtained from National Collection of Industrial Microorganisms, National Chemical Laboratory, Pine, India. They were subcultured in the laboratory using Bromfield medium [9]. Potassium nitrate was used to maintain the ionic strength, while nitric acid and potassium hydroxide were used as pH modifiers. All reagents used in the present studies were of analytical grade. Deionised double distilled water with a specific conductivity of <1.5 ␮mho was used in all the tests. The bacteria were cultured by inoculating 10 ml of pure strain of the bacterial cells to the Bromfield medium. This was incubated at 30 ◦ C on a Remi rotary shaker maintained at 240 rpm. A Petroff-Hausser counter under a Leitz phase contrast microscope (Laborlux K Wild MPS 12) was used to determine the bacterial cell count. The change in pH was monitored at regular time intervals (30 min) using a Systronics digital pH meter. 2.3. Preparation of cell-free metabolite The fully grown bacterial culture (48 h) was centrifuged (Sorvall RC-5B) at 10 000 rpm for 15 min at 5 ◦ C. The supernatant was decanted and filtered through sterile Millipore (0.2 ␮m) filter paper to remove all insoluble materials and any bacterial cells still left out. The cell pellet was washed with deionised double distilled water and again centrifuged. This process was repeated twice to obtain pure cell pellet.

2.4. Isolation of extracellular bacterial protein (EBP) from culture supernatant One liter of batch culture of P. polymyxa obtained after a growth period of 48 h was centrifuged. The supernatant was filtered through sterile Millipore (0.2 ␮m) filter paper. Analytical grade, extra pure and fine powdered ammonium sulfate was added slowly to a saturation level of 90% (600.16 g/l) in cold condition (4 ◦ C) with constant shaking. The solution was allowed to stay under refrigeration for 12 h at 4 ◦ C. The protein precipitate was dissolved in a minimum volume of 1 M Tris hydrochloride buffer of pH 7. It was dialysed against the same buffer for over 18 h at 4 ◦ C. The precipitate, which was formed during dialysis, was removed by centrifugation and discarded. The clear supernatant was lyophilised, weighed and kept at 4 ◦ C for further use [10]. 2.5. Isolation of extracellular polysaccharide (ECP) from culture supernatant One liter of completely grown (48 h) bacterial culture of P. polymyxa was centrifuged to remove cells. The supernatant containing the ECP was filtered through sterile Millipore filter paper. It was then lyophilised using Virtis Freezemobile 12EL lyophiliser at −80 ◦ C and at a vacuum of 100 mTorr. The dehydrated solid substance was dissolved in 10 ml of distilled millipore water and cooled to below 10 ◦ C. Twenty milliliters of double distilled ethanol was added to precipitate ECP from other components of the bacterial supernatant. It was then kept stationary for 8 h at 4 ◦ C in refrigerator. The precipitate was washed with double distilled water. This ethanol precipitation was repeated two or three times more for further purification to separate polysaccharide. This polysaccharide solution was dialysed with double distilled water. Before dialysis tubes were boiled in a solution containing 0.01 M EDTA and 2% sodium bicarbonate for 10–15 min in water bath. After dialysis ECP was stored at low temperature [11]. The concentration of ECP utilized was determined by the phenol–sulphuric acid method [12]. 2.6. Adsorption studies For adsorption tests, 1 g each of the individual mineral powder (∼5 ␮m for pyrite, and ∼3 ␮m for sphalerite; average size >80%) was taken and pulped in 100 ml of 10−3 M KNO3 solution at the desired pH maintaining a known concentration of bacterial cells in 250 ml Erlenmeyer flasks. Five-micrometer particles of pyrite and 3 ␮m particles of sphalerite were used in adsorption studies. The suspension was agitated for 15 min on a Remi orbital shaking incubator at 250 rpm and 30 ◦ C. After equilibration, the slurry pH was again recorded. The suspension was then centrifuged at 2000 rpm for 5 min to separate the mineral particles containing the cells. The supernatant solution containing the

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unadsorbed cells was further filtered through Whatman No. 42 filter paper and the residual cell population in the supernatant estimated.

2.7. Electrokinetic measurements Zeta-potentials of pyrite and sphalerite suspensions before and after interaction with the bacterial cells were measured as a function of pH using a Malvern Zetasizer 3000 instrument. One gram of the mineral sample (∼5 ␮m for pyrite, and ∼3 ␮m for sphalerite; average particle size >80%) was interacted with 5×109 cells/ml at pH 7 and at a temperature of 30 ◦ C for different time intervals. Five-micrometer particles of pyrite and 3 ␮m particles of sphalerite were used in electrokinetic measurements. After interaction, the mineral particles were separated by centrifugation followed by filtration and the mineral surface was washed 2–3 times to remove any entrapped bacterial cells. The mineral particles were conditioned in 10−3 M KNO3 solution at the required pH in the range of 2–12 for 30 min prior to zeta-potential measurements.

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2.10. Microflotation tests Initially 1 g of desired mineral (+75–105 ␮m) was pulped in 100 ml of deionised double distilled water in a conical flask containing 5 × 108 cells/ml of bacterial cells at neutral pH. The flask was incubated on a rotary shaker for 30 min. After interaction with the bacterial cells, the mineral particles were separated through decantation of the supernatant. Mineral particles remaining at the bottom was filtered through Whatmann 42 filter paper followed by washing with deionised double distilled water to remove further attached bacterial cells on the mineral surface. The conditioned minerals were transferred to a modified Hallimond tube [13]. Flotation of the mineral was carried out using nitrogen at a flow rate of 40 ml/min for 3 min. The settled and floated fractions were separated, dried and weighed. The minerals were conditioned with collector like potassium isopropyl xanthate (PIPX) along with activator as CuSO4 (1 × 10−6 M) and the flotation behavior studied. Effect of the sequence of addition of the collector as well as bacterial cells or bioreagents on the flotation behavior of the minerals was also separately studied. 2.11. Selective microflotation tests

2.8. Flocculation tests For the flocculation tests 1 g of the mineral sample (∼5 ␮m size for pyrite, and 3 ␮m size for sphalerite) was added to 100 ml of bacterial cell suspension in a measuring cylinder. The bacterial cells were obtained for the purpose after centrifugation followed by washing the cells in deionised double distilled water. Cell count was adjusted before adding to the mineral mixture. The bacterial cell count was maintained at 5 × 108 cells/ml. The cylinder was tumbled 10 times and kept still for 3 min. The supernatant (80 ml) was carefully decanted, filtered and weighted. Experiments were carried out at different ranges of pH (3–12) and time.

For this investigation 1 g each of the minerals (1:1, wt.%) with a particle size in the range of 75–105 ␮m was pulped in 200 ml of desired solution. First the minerals were interacted with the bacterial cells (5 × 108 cells/ml) for 15–20 min in a magnetic stirrer. The mineral mixture sample was then decanted and suspended in desired pH and flotation studies carried out as outlined in the previous section. Floated pyrite and sphalerite fractions were analysed through ICP spectroscopy and the percent recoveries calculated.

3. Results and discussions 3.1. Adsorption

2.9. Selective flocculation tests For these experiments, 2.5 g of the mineral mixture (d50 < 5 ␮m) was taken in a stoppered cylinder and thoroughly mixed with 100 ml of cell suspension. The flocculation test was performed as mentioned above and the supernatant decanted, filtered and weighed as before. Desliming was done four more times. Desliming refers to a process of agitation of the fine particles in the aqueous medium so as to separate colloidal submicron size fines as a suspension, yielding a settled portion containing the desired size particles. In each case, 80 ml of cell suspension, bacterial protein (EBP) or extra-cellular bacterial polysaccharides (ECP) was added each time. Amounts of pyrite and sphalerite in the settled and dispersed fractions were determined through chemical analysis using ICP spectroscopy.

The adsorption behavior of bacterial cells onto sphalerite and pyrite as a function of time, pH and equilibrium concentration was initially established and the results are portrayed in Figs. 1–3. Kinetics of bacterial cell adsorption was obtained by estimating the adsorption density of cells onto the mineral surface as a function of time. Adsorption behavior as a function of time was investigated in the pH range of 8–9 in 10−3 M KNO3 electrolyte solution. The initial cell concentration before adsorption was 5 × 109 cells/ml. Fig. 1 shows that the adsorption density of bacterial cells onto pyrite was 1 × 109 cells/m2 at 15 min and 4 × 108 cells/m2 in 15 min in case of sphalerite. This suggests that the adsorption of bacterial cells onto pyrite is high compared to that onto sphalerite. All subsequent adsorption experiments were carried out at

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polysaccharides and proteins [14]. Various charged functional groups present on the cell surface regulates this adsorption by formation of various cationic and anionic groups. From Fig. 3(a) it is evident that the adsorption density of cells onto pyrite steadily increases with increasing equilibrium cell concentration for all pH values studied (pH 9). From Fig. 3(b) it is observed that the number of cells adsorbed onto sphalerite increases with equilibrium cell concentration. But it is observed that the adsorption density for the bacterial cells onto sphalerite is less at higher alkaline pH (pH 9). However, it subsequently attains a plateau at all the pH values studied. As observed earlier, adsorption density for bacterial cells are relatively higher on pyrite than on sphalerite.

Fig. 1. Adsorption density of P. polymyxa cells onto pyrite and sphalerite with respect to time.

15 min equilibrium time since a both pyrite and sphalerite reached saturation in adsorption density within that period. The adsorption of cells onto minerals as a function of pH is given in Fig. 2. It was observed that there was a remarkable decrease in adsorption density of the cells in the alkaline pH range for sphalerite. Pyrite was found to have higher adsorption density compared to sphalerite in the acidic region. This can be attributed to comparatively unstable zinc hydroxide on sphalerite surface. Since both the minerals have negative charge on its surface the possibility of electrostatic forces contributing to attachment of cells (bacterial cell surface is negative) is nullified. Hence a physiochemical phenomenon controls the attachment of cells onto mineral surface. Cell surface comprises of both

Fig. 2. Adsorption density of P. polymyxa on to pyrite and sphalerite with respect to pH.

Fig. 3. Adsorption isotherms of bacterial cells onto (a) pyrite and (b) sphalerite.

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3.2. Electrokinetic studies A knowledge on the changes in the surface chemistry of the minerals after interaction with bacterial cells is essential to establish the possibility of selective separation of pyrite from a mixture of sphalerite and pyrite. Zeta-potentials were measured to assess the changes in the surface chemical characteristics of minerals before and after bacterial interaction. The cell density used for these experiments was 1 × 107 cells/ml and the interaction time was varied from 1 to 24 h (1, 12, and 24 h). Surface chemical changes on mineral surfaces after interaction with bacterial cells are illustrated in Fig. 4(a) and (b). Zeta-potential of pyrite as a function of pH before and after interaction with bacterial cells is shown in Fig. 4(a). There is neither a significant change in surface chemical charge or in IEP. The unreacted pure pyrite is observed to have an IEP in the pH range of 1.5–2.5. At higher cell concentrations (higher than 1 × 107 cells/ml), pyrite gets flocculated in less than 5 min. So in this experiment pyrite was

Fig. 4. Zeta-potential of minerals: (a) pyrite and (b) sphalerite after interaction with cells of P. polymyxa at different pH.

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interacted with low concentration of cells and hence the effect on the surface charge was negligible even after longer periods of bacterial interaction. Zeta-potential of sphalerite as a function of pH before and after interaction with bacterial cells is shown in Fig. 4(b). In this case also, no significant change in surface chemical charge or in IEP was observed due to bacterial interaction. The unreacted pure sphalerite is observed to have an IEP at pH 2.5. At higher cell concentrations (higher than 1 × 107 cells/ml), sphalerite gets flocculated in less than 5 min through the acidic pH range. So in this experiment sphalerite was reacted with lower cell concentration and hence the effect on the surface charge was negligible even after longer interaction periods. In the alkaline pH range due to formation of unstable Zn(OH)2 the effect of cell attachment on the mineral surface becomes uncertain. 3.3. Flocculation tests The settling behavior of pyrite and sphalerite mineral fines were established under different experimental conditions such as in presence of bacterial cells, EBP, ECP and both as a function of time and pH. The settling behavior of pyrite and sphalerite was studied with respect to pH. From Fig. 5(a) it can be observed that in presence of bacterial cells about 90% and 85% of pyrite particles settled at pH 8 and 9, respectively. Whereas in case of sphalerite, percent settling decreased to 25% and 15% at pH 8 and 9. Settling behavior was also studied in presence of EBP. From Fig. 5(b) it can be observed that pyrite settled in presence of EBP (100 mg/g) was 80% and 79% at pH 8 and 9, respectively. But it is interesting to note that sphalerite settled in presence of EBP (100 mg/g) was less than 10% both at pH 8 and 9. All further flocculation studies were carried out in the pH range of 8–9. Fig. 6(a) shows the settling behavior of minerals in the presence and absence of bacterial cells as a function of time in the pH range of 8–9. It is observed that percent pyrite settled in absence of bacterial cells increased from 25% at 5 min to 70% in 30 min. In presence of bacterial cells percent pyrite settled was increased from 90% at 5 min to 96% in 30 min. For sphalerite the amount settled in the absence of cells was 25% at 5 min, which increased to 40% in 20–30 min. In presence of cells the amount of sphalerite settled was 10% at 5 min and increased to less than 20% at 30 min. This indicates a significant change in surface chemistry from surface hydrophobicity point of view on sphalerite upon interaction with bacterial cells. The mineral surface becomes hydrophopic and gets dispersed in the solution in the pH range of 8.5–9. Since bacterial cell wall comprises of EBP (extracellular bacterial protein) and ECP (extracellular bacterial polysaccharides) the above-mentioned settling behavior was also studied in the presence of such bioreagents. From Fig. 6(b) it can be observed that the amount of pyrite settled was enhanced from 20% at 5 min to 70% in 30 min in absence of

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Fig. 5. Settling of pyrite and sphalerite in presence of (a) bacterial cells, (b) EBP with respect to pH.

bioprotein and to 95% in presence of EBP. In presence of EBP the amount of sphalerite settled was 10% at 5 min and increased to only 15% at 30 min. Settling behavior was also studied in presence of ECP. From Fig. 6(c) it can be observed that in presence of ECP the amount of pyrite settled increased from 12% at 5 min to 40% at 30 min. Similar studies with sphalerite indicated that the settling of sphalerite increased from 15% at 5 min to 70% at 30 min. The presence of ECP enhanced the flocculation of both pyrite and sphalerite, with the effect on sphalerite significantly higher than that on pyrite. From the above discussions it can be concluded that the rapid flocculating behavior of pyrite in the pH range of 8–9 with bacterial cells was mainly due the EBP. Similarly enhanced dispersion of sphalerite with cells was due to extracellular bacterial protein only. Hence selective separation of sphalerite from a mixture of pyrite and sphalerite was attempted in presence of bacterial cells and EBP.

Fig. 6. Settling of pyrite and sphalerite in presence of (a) bacterial cells, (b) EBP and (c) ECP and with respect to time at pH 8.5–9.

P. Patra, K.A. Natarajan / Colloids and Surfaces B: Biointerfaces 36 (2004) 91–99 Table 1 Selective recovery of pyrite from a mixture of pyrite and sphalerite (1:1, wt.%) in presence of cells (5×108 cells/ml) of P. polymyxa Number of desliming stages (3 min each)

1 2 3 4 5

Sphalerite (cumulative) removal (%) at different pH 8

9

20.6 49.8 64.8 75.8 84.4

51.2 66.7 72.3 79.8 87.2

3.4. Selective flocculation tests From the results of individual flocculation tests it is clear that sphalerite can be separated from pyrite after interaction with cells of P. polymyxa or bioproteins. Selective flocculation tests were carried out at pH 8–9 using a mixture of sphalerite and pyrite (1:1, wt.%). Five desliming stages of 3 min duration each were used. The wt.% separation is shown in Table 1. At pH 8–9 and in presence of bacterial cells, sphalerite separation was observed to be 84.4% and 87.2% at pH 8 and 9, respectively. Subsequent investigations were also carried out in presence of EBP. From Table 2 it can be observed that in presence of EBP (100 mg/g) percent sphalerite separation from a mixture of 1:1, wt.% was 94.2% and 96% at pH 8 and 9, respectively. It becomes evident that the proteinaceous material available on the surface of cells acts as a bio-flocculant for pyrite, facilitating its rapid settling and as a dispersant for sphalerite. Such a preferential behavior of EBP can be attributed to numerous active functional groups available with the protein structure in EBP.

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can be seen that the flotation recovery of pyrite decreased to 25–30% without any collector and to 10–15% in presence of bacterial cells. This implies that bacterial interaction confers surface hydrophilicity to pyrite leading to significant decrease in flotability. Pyrite exhibited more than 90% flotability at about neutral pH range in presence of 1 × 10−4 M PIPX and 1 × 10−6 M CuSO4 . It is interesting to note that the flotability of pyrite decreased to 25–30% after interaction with bacterial cells irrespective of the sequence of addition of PIPX along with bacterial cells. This indicates that the mineral surface shows higher affinity for bacterial cells than to PIPX. As could be seen from Fig. 7(b), sphalerite readily floats in the presents of xanthate collector. In the absence of collector addition, interaction with bacterial cells inhibited sphalerite flotation. However, if the collector was also added simultaneously with the presence of bacterial cells, flotability of sphalerite could be enhanced, especially beyond neutral pH.

3.5. Microflotation tests The beneficial effects of bacterial reagents in mineral flocculation with respect to modification of surface chemistry can be extended to flotation separation also. Prior to flotation, the minerals were interacted with bacterial cells. Flotation tests were also conducted by sequential addition of potassium isopropyl xanthate (PIPX) before and after interaction with bacterial cells. From the results in Fig. 7(a) it Table 2 Selective recovery of sphalerite from a mixture of pyrite and sphalerite (1:1, wt.%) in presence of EBP (50 mg/g) isolated from supernatant of pure culture of P. polymyxa Number of desliming stages (2 min each)

1 2 3 4 5

Sphalerite (cumulative) removal (%) at different pH 8

9

42.2 62 76.8 88.4 94.2

45 56.7 70.9 86 96

Fig. 7. Effect of pH on flotation of (a) pyrite and (b) sphalerite, interaction with bacterial cells and collector potassium isopropyl xanthate (PIPX).

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Table 3 Selective flotation on a mixture of pyrite and sphalerite using P. polymyxa cells and PIPX as collector reagents at pH 8–9 Experimental conditions

Cell concentration

PIPX concentration

Float (%)

Sink (%)

Float (%)

Flotation with prior interaction with cells followed by treatment with PIPX (1 × 10−6 M CuSO4 ) when both the minerals are present

2 × 109 cells/ml

1 × 10−3 M

56

44

94

5.5

5 × 10−4 M

49.2

50

94.2

5.5

1× Flotation with prior interaction with PIPX (1 × 10−6 M CuSO4 ) followed by interaction with cells when both the minerals are present

2 × 109 cells/ml

Flotation with prior interaction with cells followed by treatment with PIPX (1 × 10−6 M CuSO4 ) when both the minerals were interacted separately

2×

Flotation with prior interaction with PIPX (1 × 10−6 M CuSO4 ) followed by interaction with cells when both the minerals were interacted separately

2 × 109 cells/ml

109

cells/ml

3.6. Selective microflotation tests A mixture (1:1, wt.%) of pyrite and sphalerite was taken and treated with the bacterial cells for 1 h at pH range of 8–9. The mixture was separated from the interacting solution and treated with the collector for 15 min and the mixture was floated. The individual minerals were also interacted separately with the above-mentioned reagents and were mixed just before floating in a Hallimond tube. Flotation results are illustrated in Table 3. It can be observed that when the mixture was initially treated with bacterial cells with subsequent conditioning with PIPX maximum flotation recovery of sphalerite, was 86.3% with 9.2% pyrite at a cell concentration of 2 × 109 cells/ml and PIPX concentration of 3 × 10−4 M. When the mineral-mixtures were initially treated with the collector reagent followed by bacterial interaction maximum recovery of sphalerite was 86.9% with 21.4% of pyrite at a cell concentration of 2 × 109 cells/ml and PIPX concentration of 3 × 10−4 M. It can also be observed that with initial treatment with bacterial cells followed by PIPX conditioning, the recovery of sphalerite could be improved to about 88% with almost complete depression of pyrite. Thus selective removal of pyrite from sphalerite could be achieved.Even though microbe–mineral interaction periods reported above were of the order of 15–60 min, shorter durations of 5–15 min were also tried to achieve similar results. Since microbially induced flotation and flocculation of minerals is mainly dependent on changes in the

10−4

Pyrite

Sphalerite Sink (%)

M

7.1

92.4

81

18.4

3 × 10−4 M

9.2

89.9

86.3

13

88.4

11

1 × 10−3 M

89

10.5

5 × 10−4 M

67.4

33

87

13

1 × 10−4 M

16.4

83.6

83.22

12.8

3 × 10−4 M

21.4

78

86.9

13

3×

10−4

M

4.6

95.2

87.4

12

3 × 10−4 M

13.4

86.1

77.6

22.4

surface chemistry of the interacted minerals, the reaction rates are relatively faster and the time period required to bring about selective mineral separation through subsequent flotation/flocculation would be very small, a few minutes. Thus microbially induced beneficiation is a faster process compared to bioleaching which is relatively slower since mineral biodissolution is involved. Such microbial processes are ideally suited to the beneficiation of complex sulfides such as the ores containing copper–lead–iron–zinc or copper–nickel–cobalt–molybdenum, since selectivity in separation could readily be achieved.

4. Conclusions The following major conclusions could be drawn based on this study: 1. Significantly higher adsorption of cells of P. polymyxa onto pyrite compared to sphalerite was observed beyond neutral pH range. 2. Flocculation of pyrite and enhanced dispersion of sphalerite were observed in the pH range of 8–9 after interaction with either bacterial cells or extracellular bioproteins. 3. Pyrite upon interaction with bacterial cells was found to become more hydrophilic whereas sphalerite surfaces became hydrophobic at near neutral pH conditions. Hence microbially assisted flotation with xanthate was

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successful in the selective removal of pyrite from a binary mixture of pyrite and sphalerite.

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[6] N. Deo, K.A. Natarajan, Miner. Eng. 10 (1997) 1339. [7] N. Deo, K.A. Natarajan, Int. J. Miner. Process. 55 (1998) 41. [8] S.C. Prescott, C.G. Dunn, Industrial Microbiology, CBS Publishers, New Delhi, 1987. [9] S.M. Bromfield, J. Gen. Microbiol. 11 (1954) 1. [10] M.P. Deutscher, Methods in Enzymology Guide to Protein Purification, Academic Press, New York, 1990, p. 182. [11] D.T. Plummer, An Introduction to Practical Biochemistry, vol. II, McGraw-Hill, London, 1978, p. 66. [12] M. Dubois, K.A. Gilles, J.K. Hamilton, P.A. Rebers, F. Smith, Anal. Chem. 28 (1956) 350. [13] D.W. Fuerstenau, P.H. Metzger, G.D. Seele, Eng. Mining J. 158 (1957) 93. [14] L.M. Prescott, J.P. Harley, D.A. Klein, Microbiology, Brown, Dubuque, IA, 1993.