The role of iron-hydroxy precipitates in the passivation of chalcopyrite during bioleaching

Minerals Engineering, Vol. 13. No. 10 l, pp. 1117-1127, 2000

Pergamon 0892-6875(00)00095-9

© 2000 Published by Elsevier Science Ltd All rights reserved 0892-6875/00/$ - see front matter

THE ROLE OF IRON-HYDROXY PRECIPITATES IN THE PASSIVATION OF CHALCOPYRITE DURING BIOLEACHING*

M . B . S T O T T ¶, H . R . W A T L I N G §, P.D. F R A N Z M A N N *

a n d D. S U T T O N ¶

¶ Department of Microbiology, University of Western Australia, Nedlands, WA 6907, Australia § A. J. Parker Cooperative Research Centre for Hydrometallurgy, CSIRO Minerals, PO Box 90, Bentley, WA 6982, Australia. E-mail [email protected] I" Centre for Groundwater Studies, CSIRO Land and Water, Underwood Avenue, Floreat Park, WA 6014, Australia (Received 21 April 2000; accepted 27 June 2000)

ABSTRACT Tl~e bioleaching of chalcopyrite in an acidic sulphate nutrient medium was investigated using Sulfobacillus thermosulfidooxidans, a moderately thermophilic iron- and sulphur oxidising bacterium. Copper release to solution was initially rapid but this slowed significantly after about 50 hours. The decrease in chalcopyrite dissolution rate coincided with significant precipitation of jarosite on the mineral surface. Cultures of the moderately thermophilic acidophilic bacteria Acidimicrobium ferrooxidans, Sulfobacillus acidophilus and Sulfobacillus thermosulfidooxidans were grown in anaerobic media containing chalcopyrite passivated by jarosite. The moderate thermophiles used the ferric ion in the jarositic surface precipflate as a terminal electron acceptor in place of oxygen in the anoxic environment. Despite extensive bioreduction of the iron-hydroxy precipitates, it was found that the jarosite was not completely removed and that subsequent biooxidation of the treated concentrate achieved no significant increases in copper release compared with concentrate that had not been subjected to prior biooxidation or bioreduction. © 2000 Published by Elsevier Science Ltd. All rights reserved.

Keywords Bioleaching; sulphide ores; hydrometallurgy

INTRODUCTION Chalcopyrite (CuFeSz), is both the most abundant and the most refractory of the copper sulphides. It is processed almost entirely by concentration and smelting, which achieves up to 99% copper recovery but generates sulphur dioxide, a toxic emission that is increasingly the target of regulatory legislation. For this reason, leaching and bioleaching technologies are the focus of much processing research. The (bio)heap and dump leaching of copper oxide and secondary copper sulphide ores are proven processes that account for about 25% of annual copper production. Copper recoveries of between 70 and 90% can be achieved in 150-210 days (Montealagre and Bustos, 1991). However, copper recoveries are much lower when chalcopyritic ores are leached (Carvallo and Montoya, 1995).

* Presented at Hydromet 2000, Adelaide, Australia, April 2000

1117

M.B. Stottet al.

1118

In laboratory tests, acidophilic chemolithotrophs such as Thiobacillus ferrooxidans and SulJbbacillus thermosulfidooxidans typically release only 10-25% of the available copper in chalcopyrite via the aerobic oxidation of ferrous iron and sulphur (Eqs. 1-3) before copper release slows significantly or ceases. It is thought that (i) the oxidation of sulphide to elemental sulphur forming a layer on the chalcopyrite particles, (ii) the generation of a polysulphide layer, and/or (iii) the precipitation of a layer ofjarosite (as little as 1 ~tm thickness), may hinder greater copper extraction by restricting the flow of bacteria, nutrients, oxidants and reaction products to and from the mineral surface (Boon et al., 1993; Hackl et al., 1995). These effects are loosely termed "passivation". For leaches of extended duration, the reaction products of ferric ion hydrolysis (e.g. iron hydroxides, oxyhydroxides and jarosite (KFe3(SOa)z(OH)6), Eq. 4) are readily observed. 4Fe 2+ + 02 + 4H + --) 4Fe 3+ + 2H20

(bacterially mediated)

(2)

Cu FeS~ + 4Fe 3+ --) 5Fe 2+ + Cu 2+ + 2S ° 2S ° + 2H20 + 302--) 2S042 + 4H +

(1)

(bacterially mediated)

3Fe 3+ + 2SO42- + 6H20 -) Fe3(SO4)2(OH)6 + 6H*

(3) (4)

The bacterial reduction of iron-based minerals and precipitates, especially in pH-neutral, anoxic environments, has been documented (Lovley and Phillips, 1986; Lovley, 1995). When exposed to oxygenlimiting conditions or when easily oxidisable energy sources have been exhausted, iron-reducing bacteria will use ferric ion as the terminal electron acceptor while obtaining energy from other inorganic or organic energy sources (Lovley, 1995). However, there are only a few reported examples of iron-reduction in acidic conditions. Z ferrooxidans (Sand, 1989), Thiobacillus thiooxidans and Sulfolobus acidocaldarius (Brock and Gustafson, 1976) and members of the genus Acidiphilium (Johnson et al., 1993) are capable of iron(lII) reduction. More recently, Bridge and Johnson (1998) found that several moderately thermophilic ironoxidising bacteria (representative strains of S. thermosulfidooxidans, Sulfobacillus acidophilus and Acidimicrobiumferrooxidans) will not only reduce ferric ion in anoxic environments, but also several ironbased minerals. S. acidophilus, in particular, was found to be capable of reducing jarosite, ferric hydroxide and goethite when cultured with glycerol as the energy and carbon source. The aims of this study were to examine the role ofjarosite precipitation in the bioleaching of chalcopyrite, to determine whether the moderate thermophiles, S. thermosulfidooxidans, S. acidophilus and A. ferrooxidans could remove the iron-hydroxy precipitates from the chalcopyrite surface, via the reduction of iron(III), and whether the copper leaching rates were restored to their original values by this treatment.

MATERIALS AND METHODS Microorganisms

Sulfobacillus thermosulfidooxidans DSM 9293 v was grown in the nutrient and medium conditions stipulated by the German Collection of Microorganism and Cell Cultures (DSMZ) [i.e., (NH4)2SO4 (3g/L); MgSO4.7H20 (0.5g/L); K2HPO4 (0.5 g/L); KCI (0.1 g/L); Ca(NO3)2 (0.01 g/L); pH 2.0 (H2SO4)] unless otherwise stated. Sulfobacillus acidophilus DSM 10332 v and A cidimicrobium ferrooxidans DSM 10331 v were grown in the nutrient and medium conditions stipulated by the DSMZ [i.e., MgSO4.7H20 (0.5 g/L); (NH4)2SO4 (0.4 g/L); KzHPO4 (0.2 g/L); KCI (0.1 g/L); pH 2.0 (H2SO4)]. Ferrous ions (44.2 g FeSO4/L) and 0.02% yeast extract were added aseptically (after passage through a 0.22 ~trn pore size membrane) to both media. The ferrous ions were used as a bacterial energy source.

Chalcopyrite A chalcopyrite concentrate consisting of 64% chalcopyrite, 6.6% pyrite (FeS2), 3.3% pyrrhotite (FevSs) and 25% quartz (Si02) was prepared by hand-picking a crushed sample of massive chalcopyrite. When ground, this material had a particle size range of 249-0.16 ~m with a mean size of 27 ~tm (Pso 66 ~m), and a surface area of 0.36 m2/g (5-point N2 BET analysis).

Role of iron-hydroxyprecipitates in passivationof chalcopyriteduring bioleaching

1119

Experimental A three-stage experiment was devised, which included: (i) an initial biooxidation stage involving the leaching of chalcopyrite and jarosite formation (Table 1, A1-6, B1-4), (ii) a bioreduction of Fe(III) stage (jarosite dissolution) (Table 1, A 1-4, B 1-2) and (iii) a second biooxidation stage (leaching of chalcopyrite and jarosite formation) (Table 1, AI, 3, 5 and 7, B1, 3 and 5). The necessary non-inoculated controls were run in parallel.

TABLE 1 Experimental regime ( • non-inoculated controls) Biooxidation (50°C)

Bioreduction (45°C)

Biooxidation (50°C)

A1

S. thermosulfidooxidans

S. thermosulfidooxidans S. acidophilus A. ferrooxidans

S. thermosulfidooxidans

A2

S. thermosulfidooxidans

•

A3

S. thermosulfidooxidans

S. thermosulfidooxidans S. acidophilus A. ferrooxidans . S. acidophilus ,4. ferrooxidans

A4

S. thermosulfidooxidans

S. acidophilus A. ferrooxidans

•

A5

S. thermosulfidooxidans

•

S. thermosulfidooxidans

A6

S. thermosulfidooxidans , •

• •

• S. thermosulfidooxidans

B1 B2

S. thermosulfidooxidans

S. thermosulfidooxidans

S. thermosulfidooxidans

S. thermosulfidooxidans

S. thermosulfidooxidans

•

B3

S. thermosulfidooxidans

•

S. thermosulfidooxidans

B4

S. thermosulfidooxidans •

• •

• S. thermosulfidooxidans

A7

S. thermosulfidooxidans

A8

B5 B6

Stage 1, biooxidation The prepared chalcopyrite concentrate was substituted for the ferrous ions as the energy source in the initial biooxidation stage. The concentrate was autoclaved (121 °C, 220 kPa) under nitrogen for 20 minutes prior to use. Sulfobacillus thermosulfidooxidans was cultured in an environmental incubator (50°C, 160rpm) in 250 mL Erlenmeyer flasks with a chalcopyrite concentrate pulp density of 3.5%. Biooxidation was terminated after 600 hours, at which time the leaching rate had slowed. The leached solid residues were filtered under vacuum through a 0.22 [am pore size membrane, washed with distilled water, and dried in an oven at 30 °C for 24 hours. A subsample of the dried residues was retained for analysis. Stage 2, bioreduction During the bioreduction stage, the nutrient medium (see above) for S. acidophilus DSM 10332 v and A. ferrooxidans DSM 10331 v was used for all bacterial species. Either two or three bacterial strains were used in the bioreduction stage (Table 1, A1-4). The selected microorganisms were inoculated into 500 mL airtight Schott bottles containing - 8 g of the chalcopyrite residue from the biooxidation stage (autoclaved again under N2) and 200 mL of anaerobic nutrient medium that contained 10raM glycerol (Bridge and Johnson, 1998). The glycerol was provided as an energy and carbon source for the bacteria. The bottles

l l20

M.B. Stottet al.

were incubated at 45 °C in an environmental incubator. The bioreduction stage was terminated when the rate of ferrous ion generation declined. The solid residues were filtered, washed and dried, as above. Stage 3, biooxidation

The second biooxidation stage was carried out as for the initial biooxidation stage but using the residues (autoclaved under nitrogen) from bioreduction (Stage 2). Ferric ion oxidation o f massive chalcopyrite

In an independent test, a small piece of massive chalcopyrite was subjected to 0.1 M ferric ion in sulphate medium at 65 °C for 24 hours with stirring. The chalcopyrite surface became coated in an iron hydroxy precipitate. A second ferric ion leach was conducted in sulphate medium using the coated specimen and a second, untreated piece of massive chalcopyrite to compare the effect of the precipitate on the copper extraction rate. The pieces were split and the cross sections examined using SEM.

Sampling and analysis The pH and suspension potential were measured periodically throughout the experiment. Ferrous ion, copper and (total) iron concentrations in solution were determined after centrifugation to separate the solid material. Ferrous ion concentrations in solution were determined by spectrophotometry using the method of Wilson (1960). The total iron and copper concentrations in solution were measured using Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). Residues sampled throughout the experiment were dried (30°C) and characterised using X-ray diffraction (XRD) and scanning electron microscopy/elemental analysis (SEM-EDS). Particle size distributions in residues from the initial biooxidation and the bioreduction stages were determined by laser diffraction (Malvern Mastersizer) following sonication at 60 Hz for 20 minutes (Sony Clean).

RESULTS AND DISCUSSION The initial oxidative aerobic leach, using S. thermosulfidooxidans, released approximately 1600 mg/L (22%) of the copper from chalcopyrite in 500 hours (Figure 1). The experiments were terminated once the copper leaching rate had slowed. In the non-inoculated controls (Table 1, A7-8, B5-6), the ferrous ion, total iron and copper concentrations did not increase above 180 mg/L (-2%) over the incubation period. The pH of the control flasks increased from pH 2.0 to 2.4. Extensive jarosite formation was detected using scanning electron microscopy (Figure 2) and confirmed via analysis of XRD patterns (Figure 3). The precipitation of iron at approximately 50 hours post-inoculation (Figure 1) corresponded to the retardation of the copper leaching rate. At the same time, the ferrous ion concentration decreased from 60 mg/L to less than 30 mg/L. The solution pH, which had been increasing steadily, also decreased from this time. The decrease in pH is indicative of a change in bacterial activity from predominantly ferrous ion oxidation, an acid consuming reaction (Eq. 1) to sulphur oxidation, an acid generating reaction (Eq. 3). This change from ferrous ion oxidation to sulphur oxidation is consistent with the hypothesis that the iron-hydroxy precipitates hinder chalcopyrite leaching by restricting the mass transfer of ions to solution (Cu 2+, Fe 2+) and by preventing bacterial and iron(III) access to the mineral sulphide surface (Boon et al., 1993). Independent tests, conducted in ferric sulphate medium, show that the jarosite precipitate can completely cover the chalcopyrite surface and prevent further leaching (Figure 4).

Role of iron-hydroxy precipitates in passivation of chalcopyrite during bioleaching

1121

2000

2.5

1500

2

~ E 1000 8 8 "

O

pH 1.5

500

0

1 0

100

200

300

400

500

Time (h) Fig.1 Stage 1: Biooxidation (Table 1, A1-6). Concentrations of iron (I--1) and copper (11) leached from chalcopyrite by S. thermosulfidooxidans. (A) represents ferrous ion concentration and (0) solution pH.

Fig.2 SEM image o f iron hydroxy precipitates on the chalcopyritic ore. EDS analysis of the precipitate was consistent with jarosite.

(5)

Fes(SO4)a(OH)6- + 6H + ~ 3Fe 3+ + 2SO42- + 6H20 4Fe s+ + organic compound + 2H20--) 4Fe 2+ + 4H + + organic compound + S ° --) HzS + C02 Cu 2+ + HS- "-) CuS(s) + H +

CO 2

(bacterially mediated)

(6)

(bacterially mediated)

(7) (8)

1122

M 13. Stott et al.

8000

booxidaion

6000

A

"~4000 2000

/ ; r

]

~I:,

/VIi blo.. nj~//,,/.,,, ] /// ~ reaMCTIO

chalcopyrite

~ -peak £pre-bioleach

0 32.5

33

33.5

34

34.5

35

2-theta Fig.3 X-ray diffraction pattern of chalcopyrite concentrate, bio-oxidised residue and the residue after bioreduction (stage 2). The 2-theta peaks on the concentrate reflect pyrite (33.3 °, 34.9 °) and chalcopyrite (34.2°). The peaks at 33.4 ° and 33.8 ° reflect jarosite in the bioleach and bioreduction XRD patterns.

A

2.0

B

.J 1.5

._.

g

1.o

.m

0

0.5 0.0

1,~

0

/

I

20

40

Time (h)

Fig.4 Restriction of copper leaching from massive chalcopyrite after surface precipitation ofjarosite. The micrograph (A) shows the jarosite layer up to 50 pm thick that prevented copper release (B) in a 0.1 M Fe2(SO4)3 solution (11). The clean massive chalcopyrite control is represented by (E]). In the bioreduction stage, iron(IIl) in the precipitate was reduced in the flasks inoculated with

Acidimicrobium sp. and both Sulfobacillus spp. (Table 1, A1-2), generating 950 mg/L of ferrous ions in the leach solution in 700 hours (Eqs. 5 and 6). In contrast, when only two bacterial strains were present (Table 1, A3-4), it took a further 1200 hours to generate the same concentration of ferrous ions (Figure 5). During the same period, sulphur was reduced to hydrogen sulphide (Eq. 7) (malodorous) and all the soluble copper was precipitated as insoluble copper sulphide (Eq. 8). SEM-EDS and XRD characterisation of leach residues indicated a significant loss of jarosite as a function of time (Figure 3). As the presence of S. thermosulfidooxidans was the only difference between the two flasks, it was concluded that this strain was

Role of iron-hydroxyprecipitatesin passivationof chalcopyriteduringbioleaching

1123

the main contributor to the accelerated rate of ferrous ion production and to the reduction of sulphur to sulphide, with the consequent precipitation of copper sulphide (Figure 5).

1200

A

75

900 .o_ =

B

50

600

d

300

o

25

o 0

1000

2000

0

Time (h)

450

1000

2000

Time (h)

C

3.5

~-~400 350

3

aoo

2.s

"" 250 200 150

2

D

1.5 0

1000 Time (h)

2000

0

1000

2000

Time (h)

Fig.5 Bioreduction of iron-hydroxy precipitate on bioleached chalcopyrite ore. Flasks were inoculated with either S. thermosulfidooxidans, S. acidophilus and A ferrooxidans (o) (Table 1, A1, A2) or with S. acidophilus and A. ferrooxidans ( . ) (Table 1, A3, A4). A, concentration of ferrous ion in solution; B, total copper in solution; C, suspension potential (vs Ag/AgCI); D, pH. On the re-introduction of aerobic conditions, S. thermosulfidooxidans leached a further 12% of the copper in 300 hours from the chalcopyrite residues from the bioreduction-stage flasks with three bacterial species. Thus the combined total copper leached during Stage 1 (22% at 500 h) and Stage 3 (12% at 300 h) was approximately 34%. This represented a 4% increase over the control (residues obtained from noninoculated, bioreduction flasks). Flasks inoculated with only S. acidophilus and A. ferrooxidans in the bioreduction stage released approximately the same concentration of copper as the control (Figure 6). The 4% increase in copper recovery achieved during re-oxidation, together with rapid ferrous ion production during the bioreduction stage indicated that S. thermosulfidooxidans dominated ferric ion reduction during Stage 2. For this reason, the experiment was repeated using S. thermosulfidooxidans as the sole ferric-reducing bacterial strain (Table 1, B1-6). In this second experiment, Sulfobacillus thermosulfidooxidans also released -22% copper from the concentrate over 500 hours of oxidation. Significant quantities ofjarosite were precipitated, followed by a subsequent reduction in the copper leaching rate. In the bioreduction stage, S. thermosulfidooxidans reduced the iron(Ill) in the iron-hydroxy precipitate to ferrous ions (-930-950 mg/L in 700 hours, equivalent to -70 mg jarosite/g oxidation residue). It is estimated from the difference in copper and iron concentrations in leach solutions, and allowing for the complete dissolution of the pyrrhotite fraction (XRD analysis), that -100 mg jarosite/g oxidation residue was produced during the initial oxidation of chalcopyrite. This corresponded to the reduction of 71% of the iron(Ill) making up the jarosite precipitate.

1124

M.B. Stott et al.

Analysis of the XRD pattern (Figure 3) confirmed that not all of the jarosite was dissolved (reduced). This result is less than that reported by Johnson et al. (1998), who used a representative strain of S. acidophilus to solubilise in the order of 100% ofjarosite synthesised from a culture of T. ferrooxidans grown with ferrous sulphate nutrient solution.

2000 ~Em1500 1000 500 0 0

200

400 600 Time (h)

800

1000

Fig.6 Biooxidation using S. thermosulfidooxidans showing copper release from the bioreduced residue. Symbols refer to the bioreduction stage where a,, • , and <> represent the control (Table 1, A6), bioreduction with S. acidophilus and A. ferrooxidans (Table 1, A3), and bioreduction with S. thermosulfidooxidans, S. acidophilus and A. ferrooxidans (Table 1, A 1), respectively. Particle size analysis of slurry samples from the bioreduction stage (Table 1, B1) revealed that the mean particle diameter decreased with time (Figure 7), indicating that iron(Ill) in the iron-hydroxy compounds

¢-

6

._o .Q ° m

4

° m

c~ t~

o IU

ft.

0.01

0.1

1

10

100

1000

Particle size (pro) Fig.7 The reduction in particle size during bioreduction with S. thermosulfidooxidans after 0 hours (11) and 1172 hours ([3). was being reduced and the precipitates solubilised. On this occasion, however, the soluble copper was only partially precipitated (Figure 8), hydrogen sulphide was not detected and the redox potential decreased rapidly from 440 mV (Ag/AgCI) and stabilised at - 300 mV (Ag/AgC1). Comparison of the results obtained from the two bioreduction experiments leads to the conclusion that A. ferrooxidans and/or S. acidophilus participate in reactions that result in the observed differences, particularly the generation of hydrogen sulphide.

Role of iron-hydroxyprecipitatesin passivationof chalcopyriteduringbioleaching

100

A

1125

B

500

80

400 20 0 0

,

,

200

400

300 60O

Time (h)

0

200

400

600

Time (h)

Fig.8 Bioreduction of iron-hydroxy precipitate on bioleached chalcopyrite ore. A. total copper concentration in solution; B. suspension potential (Ag/AgCI). ( , ) represents flasks inoculated with S. thermosulfidooxidans (Table 1, B1) and (<>)the non-inoculated control (Table 1, B3). The re-oxidation of the bioreduced concentrate (Table 1, B1) released approximately 840 mg/L of copper into solution in 420 hours (Figure 9). This is approximately the same concentration of copper as that released in the control from the bioreduction stage (Table 1, B3). Copper leaching rates for chalcopyrite pre- and post-bioreduction (Table 1, A1, B1) are compared with those obtained for a bioreduced, non-inoculated control (Table 1, B3) (Figure 9). When freshly prepared chalcopyrite concentrate is bioleached using S. thermosulfidooxidans, the initial copper release rate is 8.9 mg CuZ+/L/h. This slows to - 2.4 mg CuZ+/L/h after 190 hours. Bioleaching of the residues obtained from the bioreduction stages (Table l, A3, BI) releases copper at rates o f - 2.6 mg CuZ+/L/h. Hence, the leaching rate is the same as that obtained for passivated chalcopyrite (coated in jarosite as a result of the initial biooxidation), - 2.2 mg Cu2+/L/h.

1600 ~E~1200 800 400 0 0

I

1O0

200 300 Time(h)

400

500

Fig.9 The biooxidation of chalcopyrite residues with S. thermosulfidooxidans. Copper released during biooxidation of the initial chalcopyrite concentrate (II,) is compared with that of the postbioreduction with three moderate thermophiles in the initial experiment (A) (Table 1, A 1) and postbioreduction by S. thermosulfidooxidans (11) in the second experiment (Table 1, B 1). (12]) represents the non-inoculated control from the bioreduction stage (Table 1, B3). It is evident that, in spite of the significant dissolution of the jarosite that coated the chalcopyrite surface after oxidation, the remaining jarosite (-30%) prevented the resumption of non-restricted leaching. The reduction in the rate of ferrous production (Figure 5) indicated that further incubation (to 1700 hours) failed to remove this residual jarosite from the chalcopyrite surface [confirmed with XRD pattern analysis (Figure 3)]. This result suggested that the residual precipitate was strongly bound to the chalcopyrite surface. Significant quantities of iron-hydroxy precipitates were reduced by S. thermosulfidooxidans and by the combined activity of three moderate thermophiles. However, despite the losses of ferric iron precipitates

1126

M.B. Stottet al.

from the surface of the chalcopyrite, bioreduction did not increase the copper extraction significantly compared with non-treated controls. XRD patterns of the bioreduced residues (Figure 3) continued to exhibit the peaks characteristic ofjarosite, despite being subject to extended incubation. This suggests that a longer leaching time is required to remove the precipitate, or more likely that the jarosite is so strongly attached to the chalcopyrite surface that bacteria would be unable to remove it to the point of regenerating the fresh sulphide surface. As a result, the application of a three-stage oxidation /reduction /oxidation microbial process would not be expected to overcome passivation and the resultant poor copper yields from chalcopyrite during bioleaching.

CONCLUSIONS The passivation of chalcopyrite during bioleaching is partly due to the precipitation of iron-hydroxy compounds, particularly jarosite, on the mineral surface. [The possible contribution of sulphur precipitation, or of the formation of a polysulphide layer at the chalcopyrite surface, to passivation cannot be discounted but has not been addressed in this paper.] The jarosite layer that is formed around the mineral particles prevents bacterial access to the mineral surface and restricts the mass transfer of oxidant to the surface and leachate from the surface. Moderately thermophilic acidophiles can reduce the ferric ion in jarosite and solubilise the precipitate (up to 70%). However, the partial removal of this layer via the bioreduction of the iron-hydroxy precipitates that coat the surface does not significantly increase the rate at which copper is subsequently leached. This indicates that a relatively thin surface coating of precipitated jarosite is sufficient to diminish the copper leaching rate significantly. Thus, a strategy to reduce or prevent jarosite formation, rather than to remediate the iron-based precipitate post-biooxidation, is probably required to overcome the limitation that passivation places on copper recovery during the bioleaching of chalcopyrite using moderately thermophilic bacteria.

ACKNOWLEDGEMENTS The authors wish to thank B. Clark for her technical assistance and M. Houchin for constructive criticism of the manuscript. M. Stott acknowledges, with thanks, the award of a CSIRO post-graduate scholarship. The financial support of the Australian Federal Government to the A. J. Parker Cooperative Research Centre is gratefully acknowledged.

REFERENCES

Boon, M. and Heijnen, J.J., Mechanisms and rate limiting steps in bioleaching of sphalerite, chalcopyrite and pyrite with Thiobacillusferrooxidans. In Biohydrometallurgical Technologies Volume I, ed. A.E. Torma, J.E Wey and V.1. Lakshmanan. TMS, Pennsylvania, 1993, pp217-235. Bridge, T. and Johnson, D., Reduction of soluble iron and reductive dissolution of ferric iron-containing minerals by moderately thermophilic iron-oxidising bacteria. Applied and Environmental Microbiology, 1998, 64, 2181-2186. Brock, T.D. and Gustafson, S., Ferric iron reduction by sulfur- and iron-oxidising bacteria. Applied and Environmental Microbiology, 1976, 32, 567-571. Cavallo, J. and Montoya, R., Hydrometallurgical study of the tailings dams, Division El Teniente, Codelco, Chile. In Copper '95---Cobre '95: Proc. Int. Conference (Santiago, Chile) Volume 111- Eleetorefining and Hydrometallurgy of Copper, ed. W.C. Cooper, D.B. Dreisinger, J.E. Dutrizac, H. Hein and G. Ugate, The Metallurgical Society of CIM, Montreal, 1995, pp795-806. Hackl, R.P., Dreisinger, D.B., Peters, E. and King, J.A., Passivation of chalcopyrite during oxidative leaching in sulfate media. Hydrometallurgy, 1995, 39, 25-48. Johnson, D.B., Ghauri, M.A. and S. McGinness, S., Biogeochemical cycling of iron and sulphur in leaching environments. FEMS Microbiology Reviews, 1993, 11, 63-70. Lovley, D.R., Microbial reduction of iron, manganese and other metals. Advances in Agronomy, 1995, 54, 175-231. Lovley, D. R. and Phillips, E.J., Organic matter mineralisation with reduction of ferric iron in anaerobic sediments. Applied and Environmental Microbiology, 1986, 51,683-689.

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Montealegre, R. Bustos, S., Industrial application of the bacterial thin layer process (BTL). In Bioleaching from Molecular Biology to Industrial Applications (Santiago, Chile, 1989), ed. R. Badilla, T. Vargas and L. Herrera. Universitaria, Santiago, Chile, 1991, pp95-106. Sand, W., Ferric iron reduction by Thiobacillusferrooxidans at extremely low pH-values. Biogeochemistry, 1989, 7, 195-201. Wilson, A.D., The micro determination of ferrous iron in silicate minerals by a volumetric and colorimetric method. Analyst (London), 1960, 85, 823-827.

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