Microbially enhanced thiourea leaching of gold and silver from lead-zinc sulphide flotation tailings

Microbially enhanced thiourea leaching of gold and silver from lead-zinc sulphide flotation tailings

Hydrometallurgy, 25 (1990) 51-60 51 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands Microbially enhanced thiourea leachi...

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Hydrometallurgy, 25 (1990) 51-60

51

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Microbially enhanced thiourea leaching of gold and silver from lead-zinc sulphide flotation tailings D.S.R. Murthy National Metallurgical Laboratory, Council of Scient~[~c and Industrial Research, Jamshedpur 831 O07 (India) (Received March 21, 1989; revised and accepted October 16, 1989 )

ABSTRACT Murlhy, D.S.R., 1990. Microbially enhanced thiourea leaching of gold and silver from lead-zinc sulphide flotation railings. Hydrornetallurgy, 25:51-60. The influence of bioleaching in improving the gold and silver recoveries from lead-zinc sulphide flotation railings has been investigated. Thiourea was employed as the lixiviant for the extraction of gold and silver in place of the conventional cyanide. Direct thiourea leaching has yielded gold and silver recoveries of 23% and 45% whereas thiourea leaching of the bacterial leach residues has resulted in enhanced gold and silver recoveries of 92% and 78%, respectively. The tolerance of the bacterium Thiobacillus ferro-oxidans to the thiourea s o l u t i o n s w a s examined and some inhibition shown. Finally, the significance of combining biological pre-oxidation and thiourea leaching for the extraction of gold and silver from low-grade sulphide resources is discussed.

INTRODUCTION

In the case of refractory sulphidic gold and silver ores, whether high or low grade, which do not respond to straight leaching, a pre-oxidation step often becomes necessary [ 1 ]. Although roasting has been the traditional oxidative technique, various other methods [2,3,4 ] such as chemical oxidation, pressure oxidation and biological oxidation have been suggested. Out of all these techniques, bacterial oxidation has special significance when dealing with lowgrade ores. The oxidation of metal sulphides by Tkiobacillusferro-oxidans is well documented [ 5,6 ]. Currently bacterial leaching processes are being successfully used in commercial heap and dump leaching operations for the extraction of copper. In times ahead, bacterial leaching processes may be employed for the extraction of metals from sulphide concentrates as well [6,7]. It has been suggested [ 8 ] that bacterial oxidation processes could be employed to re0304-386X/90/$03.50

© 1990 Elsevier Science Publishers B.V.

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D.S.R. MURTHY

move unwanted and detrimental metals prior to extraction of metals of interest. In gold and silver bearing sulphide minerals, where these precious metals are finely disseminated in the matrices of pyrite, arsenopyrite, chalcopyrite etc., biological pre-oxidation may be very effective in releasing these precious metals by dissolving, albeit in part, the base metal sulphides. The oxidation of base metal minerals can be achieved in indirect and direct ways of bacterial metabolism. In the indirect way of bacterial action, pyrite is initially oxidised by the Thiabacillusferra-axidans to produce ferric sulphate and sulphuric acid. bacteria

2FeS2 +7.502 + H 2 0

,Fe2(SO4)3 +H2SO4

( 1)

The ferric sulphate thus produced oxidises the other metal sulphides: bacteria

2FeAsS+Fe2(SO4)3+602 + 4 H 2 0

, 2H3AsO4 +4FeSO4 +S

(2)

The ferrous sulphate and sulphur are further oxidised to ferric sulphate and sulphuric acid by the bacteria: bacteria

2FeSO4 +0.502 +H2SO4 St

1.502 +H20

'

Fe2(SO4)3 + H 2 0

bacteria , H2SO 4

(3)

(4)

and the iron redox cycle is repeated. In the direct mechanism, the metal sulphides are oxidised to metal sulphates in the presence of Thiobacillusferro-oxidans: bacteria

4FeAsS+ 1402 +2H2804 + 4 H 2 0 -

2CuFeS2 +8.502 +H2504

, 4H3AsO4 +2Fe2(504)3

bacteria ' 2Cu504 + Fe2(504)3 +H20

(5) (6)

The beneficial effects of bacterial pre-oxidation in improving the gold and silver recoveries by cyanidation have been reported by Robinson [9] and Fridman and Savari [ 10] without revealing any experimental details. Lawrence and Bruynesteyn [ 3 ] have reported that it has been possible to remove nearly 87% of the pyrite by biological pre-oxidation from a refractory goldbearing sulphide concentrate. Following the pyrite removal, it has been possible to achieve enhanced gold and silver recoveries of 90% and 98%, respectively, in a subsequent cyanidation process. In the present study, the flotation railings were initially oxidised through bacterial action and later, the bacterial leach residues were subjected to leaching with thiourea, which is a very promising and non-toxic lixiviant for gold and silver. Thiourea [ 11,12 ], as opposed to alkaline-based leaching agents, forms gold and silver complexes ofch as hydrogen peroxide, ferric iron, formamidine disulfide, oxygen etc. The dissolution of gold and silver in acidic

M1CROBIALLY ENHANCED THIOUREA LEACHING OF GOLD AND SILVER

53

thiourea solutions in the presence of ferric iron oxidant may be represented as:

A u + F e 3+ + 2 C S ( N H 2 ) 2 =Au[CS(NH2)2]~- + F e 2+

(7)

A u + Fe3+ + 3CS (NH2)2 = A u [CS(NH2)2 ] ~- + F e 2+

(8)

The rationale of the present study was to investigate the applicability of bacterial technology as well as thiourea leaching for the extraction of gold and silver from low-grade lead-zinc sulphide flotation tailings. An attempt was also made to assess the tolerance of Thiobacillusferro-oxidans to the thiourea solutions using Warburg respirometry. EXPERIMENTAL

Materials The gold- and silver-bearing material used in this investigation was a leadzinc sulphide flotation tailing from the Pecos Mine in San Migual County, New Mexico, which analysed 1.75 g / t gold, 22.5 g / t silver, 0.44% copper, 0.68% zinc, 0.54% lead, 12.60% iron and 10.20% sulphur. Reagent grade thiourea (Fisher Scientific), oxone ( o x i d a n t ) ( D u p o n t de Nemours), methyl isobutyl ketone (Aldrich) and H z S O 4 w e r e used in the present work. A strain of Thiobacillus ferro-oxidans maintained routinely on 9 K nutrient solution [ 13 ] was used as an inoculum in the bacterial-leaching experiments.

Thiourea leaching Direct thiourea leaching of the tailings was carried out in suitable glass reactors equipped with mechanical agitators and pH controls. All the experiments were carried out at 35 °C on 25 g scale at a pulp density of 25% solids. pH was varied between 2.3 and 1.3. The strength of the thiourea solution was 0.5 M. The concentration of the oxidant, oxone, was varied between 0 and 8.33 g/l. M a x i m u m leaching time was 4 h. Leach solutions were withdrawn at predetermined time intervals and analysed for gold and silver.

Bacterial leaching All the bacterial-leaching experiments were carried out in 250-ml Erlenmeyer flasks charged with 25 g of the flotation tailings, 70 ml of the iron-free

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D.S.R. M U R T H Y

9 K nutrient medium and 5 ml of iron-grown Thiobacillusferro-oxidans. These suspensions were incubated at pH 2.3, 35°C and 250 rev. min-1 on a New Brunswick Scientific incubator rotary shaker. Leaching experiments were carried out for different durations varying from 1 to 30 days. During bacterial leaching 1 ml of the leach liquor was withdrawn at regular intervals, diluted and analysed for dissolved copper, zinc and iron to verify the bacterial activity. At the end of the bacterial-leaching stage, required amounts of thiourea and oxone were added directly to the bacterial-leach suspensions and further leaching was carried out for 4 h.

Warburg respirometry Harvesting of the cells Thiobacillusferro-oxidans cells were grown on ferrous iron in the presence of a 9 K nutrient medium at pH 2.3 in an aerated tank. After about 80% of the iron was oxidised, the solution was decanted to allow the suspended matter to settle. Bacterial cells were collected from the decanted solution by centrifugation [ 14] (model Rc 2-B super-speed centrifuge supplied by Sorval Inc., New Town, Conn). The solutions were first centrifuged at 500 rev. m i n and the supernatant fluid containing bacteria was carefully removed so as not to disturb the sediment. This clear fluid was then centrifuged at 18000 rev. min -~ . The packed cells were re-suspended in a basal salts medium at pH 2.3 in a proportion producing 10% (wet wt./vol.) suspension and were used in the Warburg respirometry experiments.

Manometric technique The conventional manometric technique described by Umbreit et al. [ 15 ] was employed with 20-ml Warburg flasks, activated by a model 15-AD-8 mechanism manufactured by Precision Scientific Company, Chicago, Ill. Each reaction flask contained a total volume of 2.8 ml solution consisting of 2.5 ml 9 K medium (pH 2.3) and 0.3 ml of 10% wt./vol, bacterial suspension and 200 mg of flotation railings. The centre well contained 0.2 ml of a 20% KOH solution. These experiments were carried out at 35 °C and at a speed of 120 strokes per minute over a period of 2.5 h. After 15 min of equilibration, the reaction was started by tipping the cell suspension from the side arm into the main compartment.

Analytical methods All the analyses were carried out on a Perkin-Elmer model 703 atomic absorption spectrophotometer. While estimating gold, it was first concentrated into methyl isobutyl ketone [ 16 ] and then analysed.

MICROBIALLY ENHANCED THIOUREA LEACHING OF GOLD AND SILVER

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RESULTS AND DISCUSSION

T he results o b t a i n e d on the direct t h i o u r e a leaching o f the flotation tailings at p H 2.3 an d 1.3 are shown in Tables 1 and 2. As can be seen from these results, the presence o f the o x i d a n t has a decisive effect in i m p r o v i n g the gold and silver recoveries. Further, decreasing the p H has a beneficial effect on the silver extraction, whereas the gold extraction is not affected. G o l d and silver extractions in the presence o f the o x i d a n t ( o x o n e ) at p H 2.3 were 23% and 36%, respectively, whereas at p H 1.3, the silver extraction has increased to 45%, while the gold extraction r e m a i n e d constant at 23%. T h i o u r e a concentrations higher th a n 0.5 M and also oxone c o n c e n t r a t i o n s b e y o n d 3.3 3 g/1 did not show any further i m p r o v e m e n t in the extraction o f gold and silver. T h e dissolutions o f co pper and zinc during bacterial leaching (Figs. 1, 2) have been observed to be 67% and 32%, respectively, over a period o f 16 days. TABLE 1 Results obtained on the extraction of gold and silver in direct thiourea leaching at pH 2.3 Leaching time (h)

% gold and silver extracted without oxone

0.5 1.0 2.0 3.0 4.0

with oxone

Au

Ag

Au

Ag

5.6 6.6 9.9 13.2 16.5

14.4 21.0 26.4 28.2 31.2

8.8 14.2 19.2 22.4 23.0

20.0 29.6 34.0 35.0 36.1

TABLE 2 Results obtained on the extraction of gold and silver in direct thriourea leaching at pH 1.3 Leaching time (h)

% gold and silver extracted without oxone

0.5 1.0 2.0 3.0 4.0

with oxone

Au

Ag

Au

Ag

5.6 6.5 10.0 13.4 16.7

17.6 25.8 35.3 39.0 40.0

9.0 14.0 19.4 22.7 23.0

24.5 33.0 40.0 43.0 45.0

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D.S.R. MURTHY

I00, •

INOCULATED

0

STERILE CONTROL

8Co ,.i I,-

E6o & °° 40

20

I 0

I

4

t

8 12 TIME ( d )

I 16

20

Fig. 1. P e r c e n t c o p p e r s o l u b i l i z e d vs. t i m e in b i o l e a c h i n g o f l e a d - z i n c f l o t a t i o n railings. Ioo

@ 0

BC

INOCULATED STERILE CONTROL

o

~6o x~J (..) z N40

20--

0

14

8 12 TIME ( d )

16

20

Fig. 2. Percent zinc solubilized vs. t i m e in bioleaching o f l e a d - z i n c flotation tailings.

MICROBIALLYENHANCEDTHIOUREALEACHINGOF GOLDAND SILVER

57

TABLE 3

Gold and silver extractions obtained in thiourea leaching of the bacterial leach residues at pH 2.3 Duration of the bacterial leach (days)

0 10 15 25 30

Duration of thiourea leach (h)

4 4 4 4 4

% gold and silver extracted without oxone

with oxone

Au

Ag

Au

Ag

16.5 41.2 43.6 43.6 43.6

31.2 50.0 52.4 52.4 52.4

23.0 62.0 65.5 66.0 66.0

36.1 52.7 55.6 56.0 56.0

TABLE 4

Gold and silver extractions obtained in thiourea leaching of the bacterial leach residues at pH 1.3 Duration of the bacterial leach (days)

0 10 15 25 30

Duration of thiourea leach (h)

4 4 4 4 4

% gold and silver extracted without oxone

with oxone

Au

Ag

Au

Ag

16.7 35.0 39.5 49.0 50.6

40.0 49.0 52.0 58.8 60.8

23.0 60.0 69.1 87.1 92.3

45.0 58.2 63.4 74.0 78.4

Prolonging the bacterial leach up-to 30 days had no further effect as far as the solubilisation of copper and zinc was concerned. The gold and silver extractions obtained after the thiourea leaching of the bacterial leach residues at different pH values are shown in Tables 3 and 4. As can be seen from these results, the maximum gold and silver extractions in the thiourea leaching step at pH 2.3 were 66% and 56%, respectively. However, if the thiourea leaching was carried out at a lower pH of 1.3, higher gold and silver extractions of respectively 92% and 78% were obtained. This may perhaps be attributed to the fact that during bacterial leaching, jarosite forms a coating on the otherwise liberated precious metal particles. When thiourea leaching is carried out at the lower pH of 1.3, the jarosite coating is minimised, thereby permitting enhanced extractions of gold and silver. Based on

58

D.S.R. MURTHY GOLD AND SILVER CONTAINING SULPHIDE ORE/TAILINGS

NUTRIENTS

H2SO4

~[

pH z 2.3

I SOLID /

~,

SOLUTION RECYCLE

[

BtOLEACHING

F

BACTERIA

LIO.UID SEPARATION

Li THIOUREA LEACHING I 0.5 M THIOUREA,WITH I

i

l SOLID /LIQUID I SEPARATION

OXONE

.

pH -- 1.3

LEACH RESIDUE TO WASTE

1 i PREONANT SOLOT,ON I ADSORPTION OF GOLD AND BAI~RENSOLUTION SILVER ON ACTIVATED CHARCOAL

RECYCLE

DESORPTION OF GOLD AND SILVER FROM THE ACTIVATED CHARCOAL

SILVER BY ELECTROLYSIS

Fig, 3. Conceptual flow-sheet for the extraction of gold and silver using biological prc-oxidation followed by thiourea leaching. 500 /"

THIOUREA CONCENTRATIONS: 0'000 M

A

-

400-

B

O'O05M

C

O'OIOM

o',ooM

D E

o'

,r" ~

ooM

~ 300 ~

D

~200 :D z

x

I00

0

i BO

40 TIME

J 120

I 150

( rain )

Fig. 4. Oxygen up-take vs. time in Warburg respirometry experiments at different thiourea concentrations.

MICROBIALLY ENHANCED THIOUREA LEACHING OF GOLD AND SILVER

59

these studies, a conceptual flow sheet has been suggested, which is shown in Fig. 3. The results obtained on the Warburg respirometer (Fig. 4) indicated that there was considerable oxygen uptake by the bacteria only up to thiourea concentrations of 0.1 M. At higher thiourea concentrations, the oxygen uptake sharply declined showing decreased bacterial activity. CONCLUSIONS

The experimental results obtained in this study have revealed that biological pre-oxidation of the lead-zinc flotation tailings has been very effective in enhancing the gold and silver recoveries in the subsequent thiourea leaching. Thiourea leaching of the bacterial leach residues has yielded gold and silver recoveries of 92% and 78%, respectively, as compared to 23% and 45% in direct thiourea leaching of the flotation railings. It would be a very attractive proposition to carry out bacterial as well as thiourea leaching simultaneously. However, the Warburg respirometry experiments have revealed that the bacteria may not be active at thiourea concentrations higher than 0.1 M. In times ahead, with the development of bacterial strains which can withstand higher concentrations of thiourea, even solution mining of gold and silver may become a commercial reality (such a possibility is less attractive with the conventional cyanide leachant because of groundwater pollution). ACKNOWLEDGEMENTS

The author wishes to express his gratitude to Dr. Arpad E. Torma, Professor, Department of Metallurgical and Materials Engineering, New Mexico Institute of Mining and Technology, U.S.A. for his keen interest in this investigation and for providing financial support and the laboratory facilities. This work was supported in part by the U.S. Department of the Interior, Bureau of Mines Grant No. G: 1144135. The opinions expressed in this article are entirely those of the author and not necessarily of the funding agency. REFERENCES 1 McQuiston, J.R. and Shoemaker, R.S., 1975. Gold and Silver Cyanidation Plant Practice. Soc. Min. Eng., AIME, Hoboken, N.J., Vol. 1, 187 pp. 2 Eisele, J.A., Colombo, A.F. and McClelland, 1984. Recovery of gold and silver from ores by hydrometallurgical processing. In: V. Kudryk, D.A. Corrigan and W.W. Liang (Editors), Precious Metals, Conf. Proc. TMS-AIME, Warrendale, Penn., pp. 387-402. 3 Lawrence, R.W. and Bruynesteyn, A., 1983. Biological preoxidation to enhance gold and silver recovery from refractory pyritic ores and concentrates. Can. Min. Metall. CIM Bull., 76(857): 107-110. 4 Roman, M.G.S., Berezowsky and Robert Weir, D., 1984. Pressure oxidation pretreatment of refractory gold ores. Paper presented at the 7th Annual Symposium on Uranium and Precious Metals, Lakewood, Colo., August 22-24, 1983. In: Practical Hydromet '83, SMEAIME, New York, N.Y., pp. 101-104.

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5 Brierley, C.L., 1982. Microbiological mining. Sci. Am., 247 (2): 44. 6 McElroy, R.O. and Bruynesteyn, A., 1978. Continuous biological leaching of chalcopyrite concentrates, demonstration and economic analysis. In: L.E. Murr, A.E. Torma and J.A. Brierley (Editors), Metallurgical Applications of Bacterial Leaching and Related Microbiological Phenomena. Academic Press, New York, N.Y., pp. 441-462. 7 Pinches, A., 1975. Bacterial leaching of an arsenic bearing sulphide concentrate. In: A.R. Burkin (Editor), Leaching and Reduction in Hydrometallurgy. Inst. Min. Metall., London, pp. 28-35. 8 Torma, A.E., 1978. Complex lead sulfide concentrate leaching by microorganisms. In: L.E. Murr, A.E. Torma and J.A. Brierley (Editors), Metallurgical Applications of Bacterial Leaching and Related Microbiological Phenomena. Academic Press, New York, N.Y., pp. 375-387. 9 Robinson, P.C., 1983. Mineralogy and treatment of refractory gold ores from the Porgera deposit, Papua New Guina. Trans. Inst. Min. Metall., Sect. C, 92: 83-89. 10 Fridman, I.D. and Savari, E.E., 1983. Treating carbon containing Ag, Au, As concentrates. World Min., 36(7): 45. 11 Brent Hiskey, J., 1984. Thiourea leaching of gold and silver. Paper presented at the 7th Annual Symposium on Uranium and Precious Metals, Lakewood, Colo., Aug. 22-24, 1983. In: Practical Hydromet '83. SME-AIME, New York, N.Y., pp. 95-99. 12 Moussoulos, L., Potamianos, N. and Kontopoulos, A., 1984. Recovery of gold and silver from arseniferous pyrite cinders by acidic thiourea leaching. Paper presented at the AIME Annual Meeting, Los Angeles, Feb. 27-29, 1984. In: Precious Metals, Conf. Proc., TMSAIME, Warrendale, Penn., pp. 323-335. 13 Silverman, M.P. and Lundgren, D.G., 1959. Studies on the chemeautothropic iron bacterium Ferrobacillusferro-oxidans, 1. An improved medium and a harvesting procedure for securing high cell yields. J. Bacteriol., 77: 642-647. 14 Torma, A.E., 1976. Biodegradation of chalcopyrite. In: J.M. Sharpley and A.M. Kaplan (Editors), Proceedings of the 3rd Int. Biodegradation Symposium. Applied Science Publishers Ltd., London, pp. 937-946. 15 Umbreit, W.W., Burris, R.H. and Stauffer, J.F., 1972. Manometric and Biochemical Techniques. Burgess Publishing Company, Minneapolis, 99 pp. 16 Perkin Elmer, 1976. Analytical Methods for Atomic Absorption Spectrometry. Norwalk, CT, GC 1-2.