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Bioleaching — Our experience
hydrometallugy ELSEVIER
Hydrometallurgy 38 (1995) 99-109
Bioleaching -
our experience
K.K. Dwivedy, AK. Mathur” Atomic Minerals Division, Department of Atomic Energy. Begumpet, Hyderabad 500016, India
Received 18 April 1994; revised version accepted 28 May 1994
Abstract Extraction of metals by the application of bioleaching methods holds great promise. Various physicochemical parameters, such as the biomass availability of CO*, particle size, mineralogy of the
ore, and pulp density, influence the bioleaching rates. The pyrite associated with uranium and other sulphide minerals is oxidised to acid ferric sulphate by the Ferrobacillus group of bacteria. Ferric iron is an excellent oxidant for tetravalent uranium, making acidic ferric sulphate an ideal lixiviant for uranium minerals. Isolation and identification of this group of bacteria from different types of uranium ores has been carried out and used for bioleaching studies. Microbes have also been used for bioremoval of radium and manganese from effluents of a uranium mill, and sulphide from Heavy Water plant effluent. Studies on the precipitation and biosorption of uranium from aqueous solutions in the laboratory have been carried out. The data generated and published elsewhere are reviewed.
1. Introduction Extraction of metals by the application of bioleaching methods holds great promise and will have an important role to play in the changing scenario of raw material requirements: ever-decreasing grades of highly disseminated types of mineralisation and a lack of amenability to physical beneficiation. This emerging science will meet the growing challenge. Whereas centuries have to pass by before a real breakthrough is achieved in other fields of scientific endeavour, biotechnology, which has come on the scene only recently, has made enormous progress and one can say that this science has come of age [ 11. The prime objective of biomineral engineering has been the enhancement of metal recoveries from ores of different types and grades. Treatment of effluents from the mineral industry, with incidental recovery of some metal values, constitutes an equally important area of biotechnology. Live and immobilised micro-organisms have a very important role in mineral engineering [ 21. Biological leaching of mono- and multiple-sulphides is an * Corresponding author. 0304-386X/95/$09.50
0 1995 Elsevier Science B.V. All rights reserved
SSD10304-386X(94)00034-2
K.K. Dwivedy, A.K. Mathur / Hydrometallurgy 38 (1995) 99-109
100
established commercial process. The use of micro-organisms for the removal of metal values will shortly be commercialised. The flotation of minerals, the alteration of flotation characteristics of minerals, the flocculation of ferric slime and phosphatic slime removal of cyanide and other toxic chemicals discharged from mineral engineering operations are potential areas where significant advances in the application of micro-organisms are likely to have commercial viability. Genetically engineered micro-organisms and the development of improved strains will enhance the overall applicability of biotechnology to the mineral industry. The idea of using heterotrophic bacteria and fungi in leaching is receiving attention at present. The biofixed beads used by the Bureau of Mines, USA, for treatment of solid and liquid effluents from the mineral engineering industry are also worthy of note. The results of the earlier work carried out on conventional and bacterial leaching studies for the recovery of uranium from Indian uraniferous ores, bioleaching of arsenopyrite for the recovery of gold, the study of bioremoval of uranium from uraniferous leach and barren liquors, and the treatment of Heavy Water plant effluents is briefly reviewed here [ 3-71.
2. Role of metallurgical mineralogy
The technical characteristics of mineral beneficiation and the economic feasibility of extracting the metal from an ore is mainly influenced by the rock type and its mineralogy and texture. The factors that are influenced by the mineralogy of an ore are: ( 1) the degree of comminution required for effective liberation of the desired mineral; (2) applicability of physical beneficiation techniques for upgrading the ore; (3) the nature and quantity of lixiviant to be used; (4) leach liquor characteristics; (5) residue mineralogy [ 81. Secondary uranium minerals are easily solubilised in both acid and alkali carbonate media. Primary uranium minerals, such as uraninite or coffinite, will need an initial oxidation level change before uranium metal can be solubilised. Chemically refractory minerals such as brannerite and davidite, will need severe leaching conditions and bioprocessing is not applicable. Although most of the uranium minerals are associated with acidic and the inter mediate group of rocks, a considerable amount of uranium is found in limestones, carbonatites, calcretes and phosphorites. Uranium extraction as a primary product from the last four examples listed, by direct acidulation is not economically feasible. Fungal and other microbial derivatives from heterotrophs can be effective at neutral pH for extracting uranium from calcareous types of ores. Oxidation of UOz (uraninite) to its hexavalent form UOi+ is accomplished generated: 2FeS0,
in the presence of ferric sulphate. In bioleaching
+H,SO,
+OSO, +Fe,(SO,),
+H,O
ferric sulphate is bacterially
K. K. Dwivedy, A. K. Mathur / Hydrometallurgy 38 (I 995) 99-l 09
101
3. Case studies 3.1. Microbes in uranium deposit formation 3.1 .I. Udaisagar, Udaipur district, Rajasthan, India
The Aravalli supergroup of metasedimentary rocks, consisting of conglomerates, phyllites, impure limestones, carbonaceous phyllites and ferrugeneous breccia, is intruded by granite. Uranium mineralisation is confined to the kaolinised and brecciated fault zone within the depth range of 25-45 m. Detailed study of microflora in the mineralised portion and the host rock has been carried out. Prolific growth of anaerobes, Desulfovibrio desulfuricans, was found in the samples collected from the fault zone and the host rocks, indicating a higher reducing environment. The host rock carbonaceous phyllite showed a significant growth of the oxidising chemoautotroph Thiobacillus ferrooxidans. Anaerobes were essentially absent (Table 1) . It can be easily deduced that the chemoautotrophs in the host rocks help in mobilising uranium, while the reducing conditions created by the anaerobes in the clay-rich zone have led to uranium precipitation and formation of the deposit [ 51. 3.1.2. Astotha, Hamirpur district, Himachal Pradesh, India
Significant uranium anomalies are noted at the contact zone between the Middle and Upper Siwalik in the Astotha area, Hamirpur district, Himachal Pradesh. The abundant presence of pyrite, considered to be of biogenic origin, has been noted. The biogenic pyrite under ground water conditions, oxidises to give rise to sulphuric acid. This oxidation is brought about by the Thiobacillus group of bacteria. In the presence of calcium carbonate this acidic condition leads to the formation of gypsum, carbonic acid and bicarbonates. ( 1) Reaction near pyrite and calcite contact: Table I Data on rock and soil samples Sample Rock type No. 1
Carbonaceous
pH
mV
rH,
e&OS
U,Oa
S
so:-
(%‘o)
(o/o)
(%)
(%I
Oxidising bacteria
Reducing bacteria
phyllite
8.2
- 200 23.3
0.004
0.003
n.d.
n.d.
+
_
6 16 19 38 49 52
Soil Soil Carbonaceousphyllite Calcareous breccia Carbonaceous phyllite Calcareous phyllite
8.5 6.6 8.2 10.2 9.0 9.3
-245 -300 -265 - 100 - 260 -280
25.5 23.5 25.4 23.8 27.0 26.5
0.0003 0.011 0.010 0.002 0.004 0.002
0.0002 0.0115 0.009 0.0011 0.003 0.002
n.d. n.d. n.d. nil traces nil
n.d. n.d. nd. traces 0.11 nil
n.d. n.d. nd. + -
n.d. n.d. n.d.
66 67 68 69
Limestone Ore Sulphurous ore Ore
8.6 3.8 2.6 4.0
- 120 -380 -480 - 380
21.3 20.7 21.7 21.7
0.002 0.043 0.044 0.043
0.001 0.053 0.059 0.053
nd. 0.92 3.10 1.55
nd. 0.18 2.97 0.20
+ + +
+ + + +
n.d. = not determined; - = absent; + = present. rH, = negative logarithm of the pressure in the atmosphere of the hydrogen in solution e&O, = equwalent U,O,.
+ _
K. K. Dwivedy, A.K. Mathur / Hydrometalkrgy 38 (1995) 99-109
102
Fe& +3.50,
+HZO+FeSO,
CaCO:, +H,SO,
+H,SO,
+H,O-+CaSO,
(2)
.H,O+HCO;
+H+
(3)
(2) Reaction near uraninite: UO;+
+2H20+OH-
-+U(OH);
(4)
Formation andfixation of UOz: These reactions result in a continuous change in the redox and pH conditions in the permeable sandstone. This causes dissolution and hydrolysis of uraninite and pyrite, forming uranium hydroxide and uranium silicate. These secondary uranium minerals are adsorbed on the clay. H2S emission is noted in certain sections of the deposits and species of Desulfovibrio desulfuricans have been isolated from the organicmatter-rich samples of this area: 2(C20H2104)
+UO;+
-+ (CZ~HZOO&UO;+
+H,
(5)
Organic matter: 5H2 +2H2S0,
-+H2S+H2S03
+5H20
(6)
Autotrophs are abundant in certain sections of deposits while heterotrophs, Bacilli sp., mostly predominate. The conflicting conditions created by microbes has led, it is believed, to the mobilisation and redeposition of uranium [ 71. 3.2. Bacterial leaching studies 3.2.1. Keruadungri, Singhbhum district, Bihar, India schist of Keruadungri ore analysed 0.035% U308, 13% The uraniferous quartz-chlorite Fe and 0.05% sulphur. The Ferrobacillus group of bacteria native to the ore was isolated and identified. Microbial leaching using 9k nutrients and Thiobacillus ferrooxidans yielded a leachability of 92.8% over 10 weeks. The leachability recorded in the control experiments was only 62.5% (Table 2, Fig. 1) . It was observed that the indigenous microbial population with suitable nutrients was sufficient for producing the required redox potential for uranium leaching from the ore [ 31. 3.2.2. Narwapahar, Singhbhum district, Bihar, India Isolation and identification of autotrophs from mine water samples and a quartz chlorite schist sample from Narwapahar analysing 0.03% U,Os were carried out. The profuse presence of Thiobacillusferrooxidans was observed in both water and soil samples. Bacterial oxidation of iron was more effective in the pH range of 1.8-2.0, while purely chemical oxidation with MnO, was effective at pH 1.8. The role of different nutrients in the normal 9k nutrients was also studied. A perceptible fall in uranium leachability was recorded when magnesium sulphate, calcium nitrate and ammonium sulphate were not added in the medium, indicating poor growth of Thiobacillus ferrooxidans, Fig. 2, [ 61. 3.2.3. Copperfiotation tailings, Singhbhum district, Bihar, India The close spatial association of copper and uranium mineralisation in the Singhbhum shear zone is well known [ 91. The copper concentration plant at Mosabani, STB, Bihar, generates flotation tailings with uranium oxide ranging from 0.007% to 0.014% U308. Some
K. K. Dwivedy, A. K. Mathur / Hydrometallurgy
38 (1995) 99-109
103
Table 2 Heap and bacterial leaching of Keradungri ore Exp. Head No. assay U@, (%)
2 weeks
Reagents used (kg/t) H,.SO, Pyrite
L
4 weeks
Pop/ml
1 2 3 4 5 6 7
0.034 0.035 0.035 0.036 0.045 0.039 0.042
4.70 3.93 4.46 3.93 4.23 3.02 6.67
4.9 28.9 15.2 30.2 21.7 4.4 32.5
Nil Nil Nil 5 5 10 5
Pop/ml
L
(%o) Nil 1.25X104 2.5X ld 7.5X105 10.0X106 Nil Nil
6 weeks
Pop/ml
L
(%I
(%I
23.2 70.1 34.9 65.6 52.5 31.3 51.3
Nil 42.0 2.5X1@ 85.7 1.25X lo6 81.5 6.25X20’78.0 5.OXld 67.1 Nil 56.0 Nil 59.3
Nil 5.0X105 1.25X lo6 6.25X1@ 5.0X105 Nil Nil
8 weeks
10 weeks
L
L
Pop/ml
Pop/ml
(%)
(%)
55.9 91.0 88.3 83.3 72.6 73.4 65.7
Nil 62.5 Nil 1.0X106 92.8 2.5X1@ 2.5~ lad 91.6 7.5~ 10s 7.5X106 85.3 2.5X1@ 7.5X10’ 75.0 2.5X1@ Nil 81.4 Nil Nil 68.3 Nil
Fe = 13.1%; S = 0.05%; size = 4 mm; stack height = 35 cm; T.f. pop/ml = Thiobacillusferrooxidans cell count per/ml; L = leachability.
sulphides are also associated; part of the uranium occurs as inclusions in the phylliosilicates, as fine disseminations, and is not amenable to gravity concentration. Recoveries of uranium on a shaking table vary between 25% and 45%. The feasibility of uranium extraction by direct leaching of the flotation tailings was studied. Percolation bacterial leaching using biogenically produced ferric sulphate was tried. The experiments were set up under ambient
100
__-/
I/
i 10 0
/I / //
_A,--,,, //
_A
-
0
1
2
3
4
Time Fig. 1. Keruadungti
ore: leachability
5
!
6
,
7
,
8
in weeks curves with time. at pH 2.
,
9
,
10
K. K. Dwivedy, A. K. Mathur / Hydrometallurgy 38 (1995) 99-109
104
90 7 80 _--Control
70 60 50 40 30 20 10 “0
-
10
30
20
40
Days Fig. 2. Leaching of Natwapahar
ore.
temperature conditions, pH was maintained at 1.6-l .8 by the addition of dilute sulphuric acid. A uranium leachability of 64% was obtained in percolation leaching experiments in 35 days and 67% uranium leachability was recorded in pneumatic agitation leaching experiments within 6 h [ lo]. 3.2.4. Heap leaching at Begin& Chhinjra, Himachal Pradesh, India; (feed assay varying between 0.042% and 0.23% lJ,O,) The uraninite ore of Chhinjra occurs as orthoquartzite, with veins and veinlets of pitchblende and minor copper-iron sulphides. Uranium extraction by heap leaching, using dilute sulphuric acid as leachant and manganese dioxide as oxidant, was carried out. The redox build-up was not satisfactory and leaching rates were slow. To overcome this, a part of the 9k nutrient in the form of commercial grade ferrous sulphate was added to the stack and the native bacteria were allowed to grow. A rapid build-up of ferric sulphate was noticed and nearly 84% of uranium could be leached. The majority of the unleached uranium was accounted for by the presence of refractory minerals, such as brannerite and zircon, which are not leachable under normal heap leaching conditions, either by chemical- or bio-oxidation [ 4,111. 3.2.5. Domiasiat, Meghalaya, India The upper Cretaceous-lower Mahadek sandstone possesses significant uranium mineralisation. Petrographically, the lower Mahadek sandstone is a feldspathic quartz arenite with organic matter, biogenic pyrite, lesser amounts of carbonate, fine quartz and an illite-
K.K. Dwivedy, A.K. Mathur/HydrometaElurgy 38 (1995) 99-109
105
Table 3 Bioleaching of gold bearing arsenopyrite from Bharat gold mines Sl. No.
Description of Head assay gold Treatment Degradation conditions sample (ppm) Duration pH H SO c:g/t; (days) Arsenopyrite concentrate Arsenopyrite concentrate
50.6
Flotation concentrate
11.00
36
T.f. Control T.f. Control T.f. Control T.f. Control T.f. Control T.f. Control FeCl, (@lo kg/t)
4 4 4 4 6 6 8 8 6 6 8 8 0.5
2.0 2.0 1so 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50
19.6 25.0 17.0 16.5 23.04 24.60 30.60 32.80 23.50 63.50 49.40 70.40 45.00
Gold in residue Gold leachability (%I fppm)
30.80 30.80 15.99 17.06 6.40 10.08 3.30 6.48 2.01 3.17 0.80 1.oo 1.40
39.10 39.10 55.60 52.60 82.20 72.00 90.80 82.00 81.70 65.70 92.90 87.30 87.30
T.f. = Thiobacillusferroonidans cell count = 6.0 X 105;ppm = parts per million.
smectite-kaolinite matrix [ 121. Uranium occurs as free uraninite, associated with organic matter in an adsorbed state, and also as fine uraninite crystals within carbonaceous matter. The average grade of the deposit is 0.1% UsOs values. A significant aspect of this deposit is the presence of water-soluble uranium compounds. In certain cases, up to 54% of uranium and 23 gm kg-’ of SO:- ion could be extracted with tap water [ 131. The indigenous microflora found is a species of Bacillus sp., Desulfovibrio sp. (organic-matter-rich samples). The chemo-autotroph Thiobacillus ferrooxidans is absent in most of the samples from this area. The deposit will be exploited by open cast mining. The over-burden low grade uranium ore, analysing 0.01-0.03% U30s could be taken for bacterial leaching/heap leaching using biogenically produced ferric sulphate as oxidant augmented with dilute sulphuric acid, the leachant.
3.2.6. Bharat Gold Mines, Kolar, Karnataka, India Auriferrous sulphides ores (iron pyrite and arseno-pyrite) from Bharat Gold Mines, Kolar, contain finely disseminated gold and other precious metals. Under normal leaching conditions this disseminated gold remains unattacked by cyanide or thiourea solutions. Oxidative removal of the sulphides helps in the exposure of gold, occurring as inclusions in pyrite or arsenopyrite. Bioleaching of the sulphides coatings for 24 h using an adapted culture of Thiobacillus ferrooxidans was carried out. On cyanidation of the pretreated mineral a recovery of 90% gold values could be achieved (Table 3). In this way indirect leaching of gold using bacteria was successful and beneficial [ 141.
106
K.K. Dwivedy, A.K. Mathur/Hydrometallurgy
38 (1995) 99-109
Table 4 MI?+ and *16Raprecipitation by Arthrobactersp. in Jaduguda barren liquor Experiment Durationof EndpH No. experiment of the liquor (h)
Weightof Mn*+ 226Ra the residue Initial Final Precipitation Initial Final concentration concentration (%) concentration (pci I-‘) (8) (pci 1-l) (mg I-‘) (mg I-‘)
I
60
2 3 4
240 170 170
7.08 7.12 7.90 6.50
0.047 0.12 0.10 0.005
22.5 22.5 6.0 2.5
9.0 2.5 nil nil
5( control)
240
7.26
-
22.5
22.5
60 92.5 100 100
Precipitation (%)
5.5f1.7 5.5* 1.7 2.8* 1.0 2.4* I.4
3.11t1.3 b.d.1. b.d.1.
44 95 95
b.d.1.
95
5.5f1.7
5.5f1.7
-
Initial pH of the liquor 7.9. b.d.1. = below detection limit; Nutrient: yeast extract (0.005%) + sucrose (0.005%). Detection limit: 50 m Bq. I-’ at 20 confidence level.
3.3. Treatment of efJluents 3.3.1. Uranium mill efJluents from Jaduguda, Bihar, India The acidic liquid effluents from a uranium mill are generally neutralised with lime and
the overflow waters from the tailings pond at the discharge point, containing Mn2+ ( > 10 mg 1-l) and 226Ra ( > 3.0 pci 1-l)) need further treatment before disposal to the public utility. Arthrobacter sp. was used for precipitation of Mn as hydrous oxide of manganese (MnO, +nH*O) . 226Raprecipitates along with manganese. The effluent after bacterial treatment consisted of Mn’+ and 226Rawithin the range of permissible concentration as per the I.C.R.P. (International Committee of Radiological Protection) and M.P.C. (Maximum Permissible Concentration) standards (Mn’+ < 0.5 mg I- ‘, 226Ra< 3.0 pci I- ‘) . About 92% of Mn2+ and more than 95% of 226Racould be precipitated using Arthrobacter sp. (Table4) [15]. 3.3.2. Precipitation of uranium by microbes Species of Bacillus subtilis, Bacillus pumilus and Aspergillus fumigatus, isolated from the ores, were grown in leach liquors containing uranium along with nutrients. Uranium precipitation up to 29% by Bacillus species and 98.6% by fungus Aspergillusfumigatus within 60 days was observed (Tables 5 and 6) [ 161. 3.3.3. Biosorption of uraniumfrom a aqueous solutions by yeast Yeast, Saccharomyces ceriuisiae, isolated in pure form was used as pre-cultured biomass for the absorption of uranium from leach liquor, ion exchange eluate, barren liquor and synthetic solutions. As much as 90% of the uranium contained in uranium sulphate liquors and ion exchange eluates could be fixed on yeast cells in just 1 h. Desorption of uranium from the yeast cells is effected by 0.1 M ammonium bicarbonate solution. The biomass could be recycled (Table 7) [ 171. 3.3.4. Treatment of H2S efluentfrom Heavy Water plant, Kota, Rajasthan, India During Heavy Water production, an H$S-D$ mixture is used as an intermediary. The aqueous effluent from this industry becomes enriched in dissolved H,S. The permissible limits for discharge of the effluent containing soluble sulphides is 0.5-l .Omg/l. H,S could
K. K. Dwivedy, A. K. Mathur / Hydrometallurgy 38 (1995) 99-109
107
Table 5 Uranium precipitation with heterotmphs Test No.
Conditions
Time
End composition of leach liquor
&OS precipitation
(days)
(%o) PH
Bacillus
I
subtilis
10 30 60 10 30 60
+o.l% yeast extract +O.l% Mannitol Bacilluspumilus
2
+0.146 yeast extract +O.l% Mannitol Initial leach liquor composition:U,0p=3.65
2.68 2.78 2.86 2.42 2.12 2.86
Eh
WA
(mV)
(g/U
-480 - 320 -300 -480 -320 -290
3.29 3.16 2.59 3.29 3.24 2.60
9.86 13.67 29.04 9.86 11.23 28.76
g/l; Fe2+ =0.168 g/l: Fe’+ = 0.84Og/l; pH =2.00, redoxpotential=
-500 mV.
Table 6 Precipitation with fungi Test conditions
Time (days)
Initial leach liquor composition
pH
Leach Liquor + Czapeck’s nutrients 2. Leach Liquor + Czapeck’s nutrients 3A. Leach liquor + 1% peptone + 1% glucose 3D. Control I.
7.
Leach liquor + 1% glucose
Eh
Final composition
pH
Eh
Calcined mould grade
U308 (%I
pas (%)
U,O* (%)
(mV)
U@, (g/t)
pa5 (mg/U
60
1.44 -400
10.9
80.0
1.68 -220
5.5
0.02
46.7
-
60
1.56
10.0
3.90 -140
0.023
0.02
98.6
-
15
1.80 -390
0.573 253.1
2.24 -310
0.274
-
52.1
-
30 15 30 15 30
1.80 -390 0.573 1.80 -390 0.573 1.80 -400 0.937 --
2.48 1.80 1.60 1.90 2.00
0.156 0.566 0.572 0.790 0.643
188
72.7
25.7
-
15.7 31.4
-
1.05
253.1 253.1 ?
(mV)
-290 -380 -380 -360 -300
U@s (g/l)
Precipitation
p,4 (mg/l)
-
50.0
25.0
Table 7 Biosorption of uranium by yeast (Saccharomyces cervisiae) NO.
1. 2. 3. 4. 5. 6.
Experimental solution
Eluate Eluate ElIlate Leach liquor Uranyl sulphate solution Uranyl sulphate solution
Uranium concentration
Contact tie
Biosorption
(h)
(%)
Initial
Final
@pm)
(ppm)
64.5 159.0 68.5 129.0 14.9
9.8 30.0 0.7 68.5 1.5
1 2 2 2 I
88.0 81.0 99.0 47.0 90.0
16.4
0.7
2
96.0
be oxidised to a level below detection limits ( < 20 pgll H,S) within 6 h using Thiobacillus The oxidation of H$ by bacteria formed colloidal sulphur floccules which settled at the bottom of the tanks (Table 8) [ 181
ferrooxidans.
108
Table 8 Experiments Exp. No.
K.K. Dwivedy, A.K. Mathur/Hydrometallurgy
on effluents from Heavy Water plant, Kota Treatment
Initial observation PH
1. 2. 3. 4.
38 (1995) 99-109
KE+Tf KE+Tf KE+Tf KE+Tf
5.40 5.62 5.32 5.60
Cells (counts/ml) 2.8 2.8 2.8 2.8
x x x x
lo4 10J IO4 IO4
Observation Sulphide
pH
(ppm) 13.0 11.6 12.8 13.0
5.48 5.50 5.30 5.60
after 6 h
Cells (counts/ml) 6.8 6.8 6.8 6.8
x x x x
lo5 lo5 lo5 IO5
Sulphide (ppm) < < < <
20 20 20 20
ppb ppb ppb ppb
K.E. = Kota effluent; T.f. = Thiobacillusferrooxidans.
4. Conclusion
The future for biomineral engineering is bright, although a word of caution is felt necessary. It should be remembered that bioprocessing in the mineral industry is not a magic wand but is a supplementary tool to conventional mineral engineering and metallurgical processes. Biomineral processing is appropriate for treating very low grade ores and, if properly developed, biomineral processing will be in a position to reap all the metal wealth present in the oceans, both in water and sediments, an area where conventional technology is just not available.
Acknowledgement
Devoted work carried out by colleagues in the Mineral Technology Laboratory is thankfully acknowledged.
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G., An assessment
of the current Indian Scenario in biotechnology.
Current Sci. 60 ( 1991):
[ 21 Smith, R.W. and Mishra, M., Recent developments in the bioprocessing of minerals. Miner. Process. Extract. Metnll. Rev., 12 (1993): 37-60. 131 Dwivedy, K.K., Bhurat, M.C. and Jayaram, K.M.V., Heap and bacterial leaching tests on sample of low grade uranium ore from Keruadungri, Singhbhum, Bihar, NML Tech. J., 14 ( 1972): 72-75. 141 Jayaram, K.M.V. and Kulshresta, S.C., Some results of lixivation tests on bulk uraniferous quartzite ore under field conditions at Chhinjra, Kulu district, H.P. INSA, 36 (1970): 291-297. [ 5 ] Jayaram, K.M.V., Dwivedy, K.K., Bhurat, M.C. and Kulshresta, SC., A study on the influence of microflora on the genesis of uranium occurence at Udaisagar, Udaipur district, Rajasthan. In: Formation of Uranium Ore Deposits, IAEA, Vienna (1974), pp. 89-98. 161 Jayaram, K.M.V., Dwivedy, K.K., Deshpande, AS. and Ramachar, T.M., Studies on the recovery of uranium from low-grade ores in India. In: Uranium Ore Processing, IAEA, Vienna (1976), pp. 155-170. 171 Mathur, A.K., Sankaran, R.N., Dwivedy, K.K. and Jaysram, K.M.V., Role of microbes in metal mobilisation in Siwalik sediments of Astotha area, H.P. In: A.K. Sinha (Editor), Contemporary Geoscientific Researches in Himalaya, Vol. 2 (1982), pp. 123-128.
K.K. Dwivedy, A.K. Mathur /Hydromefallurgy 3R (1995) 99-109
109
[ 81 International Atomic Energy Agency, Significance of mineralogy in the development of flowsheets for processing of uranium ores. IAEA, Vienna, Techn. Rep. Ser., 196 (1980): 6. [9] Bhola, K.L., Uranium deposits in Singhbhum and their development for use in nuclear power programme. Proc. Nat. Sci. Acad., 37A (1971). [ lo] Muralikrishna, N. and Bhurat, M.C., Recovery of uraniumby bacterial leaching from copper flotation tailings, Mosabani, Singhbhum district, Bihar. Int. Symp. Importance of Biotechnology, Andhra Univ., Waltair, A.P. (1993), pp. 92-95. [ 111 Sankaran, R.N., Sah, V.N., Singh, T.P. and Dwivedy, K.K., Heap leaching on uranium ores at Baginda, Himachal Pradesh. A case study of effective pollution control. Explor. Res. Atom Minerals, 1 ( 1988): 165167. [ 121 Shivananda, S.R., Dwivedy, K.K. and Ravi Kaul, Characterisation of extractable uranium in Domiasiat sandstone, West Khasi Hills district, Meghalaya, J. Geol. Sot. Ind., 40 ( 1992): 76-78. [ 131 Dhanaraju, R., Panneer Selvam, A. and Vimave, S.N., Characterisation of the Upper Cretaceous lower Mahadek sandstone and its uranium mineralisation in the Domiasiat-Gomaghat-Polengshakap area, Meghalaya. India. Explor. Res. Atom. Minerals, 2 (1989): l-27. [ 14 1Bhurat, M.C., Dwivedy, K.K., Muralikrishna, N. and Krishna Murthy, V., Leaching of gold pyrite tailings from Bharat Gold Mines Ltd., India, with Thiobacillus ferrooxidans. In: Nat. Symp. Appl. Geomicrobiol., Dept. Geology and Microbiology, Jaihind College, India ( 1991): 62-68. [ 15] Mathur, A.K. and Dwivedy, K.K., A biogenic approach to the treatment of uranium mill effluents. Uranium 4 (1988): 385-394. [ 161 Bhurat, M.C., Jayaram, K.M.V., Mathur, A.K. and Dwivedy, K.K., Studies on the microbial precipitation of uranium. Ind. J. Microbial., 19 (1979): 158-162. [ 171 Mathur, A.K., Muralikrishna, N., Krishna Murthy, V. and Sankaran, R.N., Biosorption of uranium by yeast. In: Proc. Symp. Uran. Tech. BARC, Bombay (1989). pp: 374-887. [ 181 Bhurat, MC., Dwivedy, K.K., Jayaram, K.M.V., Muralikrishna, N. and Krishna Murthy, V., Biological treatment of H$ effluents from Heavy Water Plant, Kota, India. In. Nat. Symp. Appl. Geomicrobiol. Dept. Geology and Microbiology, Jaihind College, Dhule, India ( 1991) , pp. 15l-l 54.
Hydrometallurgy 38 (1995) 99-109
Bioleaching -
our experience
K.K. Dwivedy, AK. Mathur” Atomic Minerals Division, Department of Atomic Energy. Begumpet, Hyderabad 500016, India
Received 18 April 1994; revised version accepted 28 May 1994
Abstract Extraction of metals by the application of bioleaching methods holds great promise. Various physicochemical parameters, such as the biomass availability of CO*, particle size, mineralogy of the
ore, and pulp density, influence the bioleaching rates. The pyrite associated with uranium and other sulphide minerals is oxidised to acid ferric sulphate by the Ferrobacillus group of bacteria. Ferric iron is an excellent oxidant for tetravalent uranium, making acidic ferric sulphate an ideal lixiviant for uranium minerals. Isolation and identification of this group of bacteria from different types of uranium ores has been carried out and used for bioleaching studies. Microbes have also been used for bioremoval of radium and manganese from effluents of a uranium mill, and sulphide from Heavy Water plant effluent. Studies on the precipitation and biosorption of uranium from aqueous solutions in the laboratory have been carried out. The data generated and published elsewhere are reviewed.
1. Introduction Extraction of metals by the application of bioleaching methods holds great promise and will have an important role to play in the changing scenario of raw material requirements: ever-decreasing grades of highly disseminated types of mineralisation and a lack of amenability to physical beneficiation. This emerging science will meet the growing challenge. Whereas centuries have to pass by before a real breakthrough is achieved in other fields of scientific endeavour, biotechnology, which has come on the scene only recently, has made enormous progress and one can say that this science has come of age [ 11. The prime objective of biomineral engineering has been the enhancement of metal recoveries from ores of different types and grades. Treatment of effluents from the mineral industry, with incidental recovery of some metal values, constitutes an equally important area of biotechnology. Live and immobilised micro-organisms have a very important role in mineral engineering [ 21. Biological leaching of mono- and multiple-sulphides is an * Corresponding author. 0304-386X/95/$09.50
0 1995 Elsevier Science B.V. All rights reserved
SSD10304-386X(94)00034-2
K.K. Dwivedy, A.K. Mathur / Hydrometallurgy 38 (1995) 99-109
100
established commercial process. The use of micro-organisms for the removal of metal values will shortly be commercialised. The flotation of minerals, the alteration of flotation characteristics of minerals, the flocculation of ferric slime and phosphatic slime removal of cyanide and other toxic chemicals discharged from mineral engineering operations are potential areas where significant advances in the application of micro-organisms are likely to have commercial viability. Genetically engineered micro-organisms and the development of improved strains will enhance the overall applicability of biotechnology to the mineral industry. The idea of using heterotrophic bacteria and fungi in leaching is receiving attention at present. The biofixed beads used by the Bureau of Mines, USA, for treatment of solid and liquid effluents from the mineral engineering industry are also worthy of note. The results of the earlier work carried out on conventional and bacterial leaching studies for the recovery of uranium from Indian uraniferous ores, bioleaching of arsenopyrite for the recovery of gold, the study of bioremoval of uranium from uraniferous leach and barren liquors, and the treatment of Heavy Water plant effluents is briefly reviewed here [ 3-71.
2. Role of metallurgical mineralogy
The technical characteristics of mineral beneficiation and the economic feasibility of extracting the metal from an ore is mainly influenced by the rock type and its mineralogy and texture. The factors that are influenced by the mineralogy of an ore are: ( 1) the degree of comminution required for effective liberation of the desired mineral; (2) applicability of physical beneficiation techniques for upgrading the ore; (3) the nature and quantity of lixiviant to be used; (4) leach liquor characteristics; (5) residue mineralogy [ 81. Secondary uranium minerals are easily solubilised in both acid and alkali carbonate media. Primary uranium minerals, such as uraninite or coffinite, will need an initial oxidation level change before uranium metal can be solubilised. Chemically refractory minerals such as brannerite and davidite, will need severe leaching conditions and bioprocessing is not applicable. Although most of the uranium minerals are associated with acidic and the inter mediate group of rocks, a considerable amount of uranium is found in limestones, carbonatites, calcretes and phosphorites. Uranium extraction as a primary product from the last four examples listed, by direct acidulation is not economically feasible. Fungal and other microbial derivatives from heterotrophs can be effective at neutral pH for extracting uranium from calcareous types of ores. Oxidation of UOz (uraninite) to its hexavalent form UOi+ is accomplished generated: 2FeS0,
in the presence of ferric sulphate. In bioleaching
+H,SO,
+OSO, +Fe,(SO,),
+H,O
ferric sulphate is bacterially
K. K. Dwivedy, A. K. Mathur / Hydrometallurgy 38 (I 995) 99-l 09
101
3. Case studies 3.1. Microbes in uranium deposit formation 3.1 .I. Udaisagar, Udaipur district, Rajasthan, India
The Aravalli supergroup of metasedimentary rocks, consisting of conglomerates, phyllites, impure limestones, carbonaceous phyllites and ferrugeneous breccia, is intruded by granite. Uranium mineralisation is confined to the kaolinised and brecciated fault zone within the depth range of 25-45 m. Detailed study of microflora in the mineralised portion and the host rock has been carried out. Prolific growth of anaerobes, Desulfovibrio desulfuricans, was found in the samples collected from the fault zone and the host rocks, indicating a higher reducing environment. The host rock carbonaceous phyllite showed a significant growth of the oxidising chemoautotroph Thiobacillus ferrooxidans. Anaerobes were essentially absent (Table 1) . It can be easily deduced that the chemoautotrophs in the host rocks help in mobilising uranium, while the reducing conditions created by the anaerobes in the clay-rich zone have led to uranium precipitation and formation of the deposit [ 51. 3.1.2. Astotha, Hamirpur district, Himachal Pradesh, India
Significant uranium anomalies are noted at the contact zone between the Middle and Upper Siwalik in the Astotha area, Hamirpur district, Himachal Pradesh. The abundant presence of pyrite, considered to be of biogenic origin, has been noted. The biogenic pyrite under ground water conditions, oxidises to give rise to sulphuric acid. This oxidation is brought about by the Thiobacillus group of bacteria. In the presence of calcium carbonate this acidic condition leads to the formation of gypsum, carbonic acid and bicarbonates. ( 1) Reaction near pyrite and calcite contact: Table I Data on rock and soil samples Sample Rock type No. 1
Carbonaceous
pH
mV
rH,
e&OS
U,Oa
S
so:-
(%‘o)
(o/o)
(%)
(%I
Oxidising bacteria
Reducing bacteria
phyllite
8.2
- 200 23.3
0.004
0.003
n.d.
n.d.
+
_
6 16 19 38 49 52
Soil Soil Carbonaceousphyllite Calcareous breccia Carbonaceous phyllite Calcareous phyllite
8.5 6.6 8.2 10.2 9.0 9.3
-245 -300 -265 - 100 - 260 -280
25.5 23.5 25.4 23.8 27.0 26.5
0.0003 0.011 0.010 0.002 0.004 0.002
0.0002 0.0115 0.009 0.0011 0.003 0.002
n.d. n.d. n.d. nil traces nil
n.d. n.d. nd. traces 0.11 nil
n.d. n.d. nd. + -
n.d. n.d. n.d.
66 67 68 69
Limestone Ore Sulphurous ore Ore
8.6 3.8 2.6 4.0
- 120 -380 -480 - 380
21.3 20.7 21.7 21.7
0.002 0.043 0.044 0.043
0.001 0.053 0.059 0.053
nd. 0.92 3.10 1.55
nd. 0.18 2.97 0.20
+ + +
+ + + +
n.d. = not determined; - = absent; + = present. rH, = negative logarithm of the pressure in the atmosphere of the hydrogen in solution e&O, = equwalent U,O,.
+ _
K. K. Dwivedy, A.K. Mathur / Hydrometalkrgy 38 (1995) 99-109
102
Fe& +3.50,
+HZO+FeSO,
CaCO:, +H,SO,
+H,SO,
+H,O-+CaSO,
(2)
.H,O+HCO;
+H+
(3)
(2) Reaction near uraninite: UO;+
+2H20+OH-
-+U(OH);
(4)
Formation andfixation of UOz: These reactions result in a continuous change in the redox and pH conditions in the permeable sandstone. This causes dissolution and hydrolysis of uraninite and pyrite, forming uranium hydroxide and uranium silicate. These secondary uranium minerals are adsorbed on the clay. H2S emission is noted in certain sections of the deposits and species of Desulfovibrio desulfuricans have been isolated from the organicmatter-rich samples of this area: 2(C20H2104)
+UO;+
-+ (CZ~HZOO&UO;+
+H,
(5)
Organic matter: 5H2 +2H2S0,
-+H2S+H2S03
+5H20
(6)
Autotrophs are abundant in certain sections of deposits while heterotrophs, Bacilli sp., mostly predominate. The conflicting conditions created by microbes has led, it is believed, to the mobilisation and redeposition of uranium [ 71. 3.2. Bacterial leaching studies 3.2.1. Keruadungri, Singhbhum district, Bihar, India schist of Keruadungri ore analysed 0.035% U308, 13% The uraniferous quartz-chlorite Fe and 0.05% sulphur. The Ferrobacillus group of bacteria native to the ore was isolated and identified. Microbial leaching using 9k nutrients and Thiobacillus ferrooxidans yielded a leachability of 92.8% over 10 weeks. The leachability recorded in the control experiments was only 62.5% (Table 2, Fig. 1) . It was observed that the indigenous microbial population with suitable nutrients was sufficient for producing the required redox potential for uranium leaching from the ore [ 31. 3.2.2. Narwapahar, Singhbhum district, Bihar, India Isolation and identification of autotrophs from mine water samples and a quartz chlorite schist sample from Narwapahar analysing 0.03% U,Os were carried out. The profuse presence of Thiobacillusferrooxidans was observed in both water and soil samples. Bacterial oxidation of iron was more effective in the pH range of 1.8-2.0, while purely chemical oxidation with MnO, was effective at pH 1.8. The role of different nutrients in the normal 9k nutrients was also studied. A perceptible fall in uranium leachability was recorded when magnesium sulphate, calcium nitrate and ammonium sulphate were not added in the medium, indicating poor growth of Thiobacillus ferrooxidans, Fig. 2, [ 61. 3.2.3. Copperfiotation tailings, Singhbhum district, Bihar, India The close spatial association of copper and uranium mineralisation in the Singhbhum shear zone is well known [ 91. The copper concentration plant at Mosabani, STB, Bihar, generates flotation tailings with uranium oxide ranging from 0.007% to 0.014% U308. Some
K. K. Dwivedy, A. K. Mathur / Hydrometallurgy
38 (1995) 99-109
103
Table 2 Heap and bacterial leaching of Keradungri ore Exp. Head No. assay U@, (%)
2 weeks
Reagents used (kg/t) H,.SO, Pyrite
L
4 weeks
Pop/ml
1 2 3 4 5 6 7
0.034 0.035 0.035 0.036 0.045 0.039 0.042
4.70 3.93 4.46 3.93 4.23 3.02 6.67
4.9 28.9 15.2 30.2 21.7 4.4 32.5
Nil Nil Nil 5 5 10 5
Pop/ml
L
(%o) Nil 1.25X104 2.5X ld 7.5X105 10.0X106 Nil Nil
6 weeks
Pop/ml
L
(%I
(%I
23.2 70.1 34.9 65.6 52.5 31.3 51.3
Nil 42.0 2.5X1@ 85.7 1.25X lo6 81.5 6.25X20’78.0 5.OXld 67.1 Nil 56.0 Nil 59.3
Nil 5.0X105 1.25X lo6 6.25X1@ 5.0X105 Nil Nil
8 weeks
10 weeks
L
L
Pop/ml
Pop/ml
(%)
(%)
55.9 91.0 88.3 83.3 72.6 73.4 65.7
Nil 62.5 Nil 1.0X106 92.8 2.5X1@ 2.5~ lad 91.6 7.5~ 10s 7.5X106 85.3 2.5X1@ 7.5X10’ 75.0 2.5X1@ Nil 81.4 Nil Nil 68.3 Nil
Fe = 13.1%; S = 0.05%; size = 4 mm; stack height = 35 cm; T.f. pop/ml = Thiobacillusferrooxidans cell count per/ml; L = leachability.
sulphides are also associated; part of the uranium occurs as inclusions in the phylliosilicates, as fine disseminations, and is not amenable to gravity concentration. Recoveries of uranium on a shaking table vary between 25% and 45%. The feasibility of uranium extraction by direct leaching of the flotation tailings was studied. Percolation bacterial leaching using biogenically produced ferric sulphate was tried. The experiments were set up under ambient
100
__-/
I/
i 10 0
/I / //
_A,--,,, //
_A
-
0
1
2
3
4
Time Fig. 1. Keruadungti
ore: leachability
5
!
6
,
7
,
8
in weeks curves with time. at pH 2.
,
9
,
10
K. K. Dwivedy, A. K. Mathur / Hydrometallurgy 38 (1995) 99-109
104
90 7 80 _--Control
70 60 50 40 30 20 10 “0
-
10
30
20
40
Days Fig. 2. Leaching of Natwapahar
ore.
temperature conditions, pH was maintained at 1.6-l .8 by the addition of dilute sulphuric acid. A uranium leachability of 64% was obtained in percolation leaching experiments in 35 days and 67% uranium leachability was recorded in pneumatic agitation leaching experiments within 6 h [ lo]. 3.2.4. Heap leaching at Begin& Chhinjra, Himachal Pradesh, India; (feed assay varying between 0.042% and 0.23% lJ,O,) The uraninite ore of Chhinjra occurs as orthoquartzite, with veins and veinlets of pitchblende and minor copper-iron sulphides. Uranium extraction by heap leaching, using dilute sulphuric acid as leachant and manganese dioxide as oxidant, was carried out. The redox build-up was not satisfactory and leaching rates were slow. To overcome this, a part of the 9k nutrient in the form of commercial grade ferrous sulphate was added to the stack and the native bacteria were allowed to grow. A rapid build-up of ferric sulphate was noticed and nearly 84% of uranium could be leached. The majority of the unleached uranium was accounted for by the presence of refractory minerals, such as brannerite and zircon, which are not leachable under normal heap leaching conditions, either by chemical- or bio-oxidation [ 4,111. 3.2.5. Domiasiat, Meghalaya, India The upper Cretaceous-lower Mahadek sandstone possesses significant uranium mineralisation. Petrographically, the lower Mahadek sandstone is a feldspathic quartz arenite with organic matter, biogenic pyrite, lesser amounts of carbonate, fine quartz and an illite-
K.K. Dwivedy, A.K. Mathur/HydrometaElurgy 38 (1995) 99-109
105
Table 3 Bioleaching of gold bearing arsenopyrite from Bharat gold mines Sl. No.
Description of Head assay gold Treatment Degradation conditions sample (ppm) Duration pH H SO c:g/t; (days) Arsenopyrite concentrate Arsenopyrite concentrate
50.6
Flotation concentrate
11.00
36
T.f. Control T.f. Control T.f. Control T.f. Control T.f. Control T.f. Control FeCl, (@lo kg/t)
4 4 4 4 6 6 8 8 6 6 8 8 0.5
2.0 2.0 1so 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50 1.50
19.6 25.0 17.0 16.5 23.04 24.60 30.60 32.80 23.50 63.50 49.40 70.40 45.00
Gold in residue Gold leachability (%I fppm)
30.80 30.80 15.99 17.06 6.40 10.08 3.30 6.48 2.01 3.17 0.80 1.oo 1.40
39.10 39.10 55.60 52.60 82.20 72.00 90.80 82.00 81.70 65.70 92.90 87.30 87.30
T.f. = Thiobacillusferroonidans cell count = 6.0 X 105;ppm = parts per million.
smectite-kaolinite matrix [ 121. Uranium occurs as free uraninite, associated with organic matter in an adsorbed state, and also as fine uraninite crystals within carbonaceous matter. The average grade of the deposit is 0.1% UsOs values. A significant aspect of this deposit is the presence of water-soluble uranium compounds. In certain cases, up to 54% of uranium and 23 gm kg-’ of SO:- ion could be extracted with tap water [ 131. The indigenous microflora found is a species of Bacillus sp., Desulfovibrio sp. (organic-matter-rich samples). The chemo-autotroph Thiobacillus ferrooxidans is absent in most of the samples from this area. The deposit will be exploited by open cast mining. The over-burden low grade uranium ore, analysing 0.01-0.03% U30s could be taken for bacterial leaching/heap leaching using biogenically produced ferric sulphate as oxidant augmented with dilute sulphuric acid, the leachant.
3.2.6. Bharat Gold Mines, Kolar, Karnataka, India Auriferrous sulphides ores (iron pyrite and arseno-pyrite) from Bharat Gold Mines, Kolar, contain finely disseminated gold and other precious metals. Under normal leaching conditions this disseminated gold remains unattacked by cyanide or thiourea solutions. Oxidative removal of the sulphides helps in the exposure of gold, occurring as inclusions in pyrite or arsenopyrite. Bioleaching of the sulphides coatings for 24 h using an adapted culture of Thiobacillus ferrooxidans was carried out. On cyanidation of the pretreated mineral a recovery of 90% gold values could be achieved (Table 3). In this way indirect leaching of gold using bacteria was successful and beneficial [ 141.
106
K.K. Dwivedy, A.K. Mathur/Hydrometallurgy
38 (1995) 99-109
Table 4 MI?+ and *16Raprecipitation by Arthrobactersp. in Jaduguda barren liquor Experiment Durationof EndpH No. experiment of the liquor (h)
Weightof Mn*+ 226Ra the residue Initial Final Precipitation Initial Final concentration concentration (%) concentration (pci I-‘) (8) (pci 1-l) (mg I-‘) (mg I-‘)
I
60
2 3 4
240 170 170
7.08 7.12 7.90 6.50
0.047 0.12 0.10 0.005
22.5 22.5 6.0 2.5
9.0 2.5 nil nil
5( control)
240
7.26
-
22.5
22.5
60 92.5 100 100
Precipitation (%)
5.5f1.7 5.5* 1.7 2.8* 1.0 2.4* I.4
3.11t1.3 b.d.1. b.d.1.
44 95 95
b.d.1.
95
5.5f1.7
5.5f1.7
-
Initial pH of the liquor 7.9. b.d.1. = below detection limit; Nutrient: yeast extract (0.005%) + sucrose (0.005%). Detection limit: 50 m Bq. I-’ at 20 confidence level.
3.3. Treatment of efJluents 3.3.1. Uranium mill efJluents from Jaduguda, Bihar, India The acidic liquid effluents from a uranium mill are generally neutralised with lime and
the overflow waters from the tailings pond at the discharge point, containing Mn2+ ( > 10 mg 1-l) and 226Ra ( > 3.0 pci 1-l)) need further treatment before disposal to the public utility. Arthrobacter sp. was used for precipitation of Mn as hydrous oxide of manganese (MnO, +nH*O) . 226Raprecipitates along with manganese. The effluent after bacterial treatment consisted of Mn’+ and 226Rawithin the range of permissible concentration as per the I.C.R.P. (International Committee of Radiological Protection) and M.P.C. (Maximum Permissible Concentration) standards (Mn’+ < 0.5 mg I- ‘, 226Ra< 3.0 pci I- ‘) . About 92% of Mn2+ and more than 95% of 226Racould be precipitated using Arthrobacter sp. (Table4) [15]. 3.3.2. Precipitation of uranium by microbes Species of Bacillus subtilis, Bacillus pumilus and Aspergillus fumigatus, isolated from the ores, were grown in leach liquors containing uranium along with nutrients. Uranium precipitation up to 29% by Bacillus species and 98.6% by fungus Aspergillusfumigatus within 60 days was observed (Tables 5 and 6) [ 161. 3.3.3. Biosorption of uraniumfrom a aqueous solutions by yeast Yeast, Saccharomyces ceriuisiae, isolated in pure form was used as pre-cultured biomass for the absorption of uranium from leach liquor, ion exchange eluate, barren liquor and synthetic solutions. As much as 90% of the uranium contained in uranium sulphate liquors and ion exchange eluates could be fixed on yeast cells in just 1 h. Desorption of uranium from the yeast cells is effected by 0.1 M ammonium bicarbonate solution. The biomass could be recycled (Table 7) [ 171. 3.3.4. Treatment of H2S efluentfrom Heavy Water plant, Kota, Rajasthan, India During Heavy Water production, an H$S-D$ mixture is used as an intermediary. The aqueous effluent from this industry becomes enriched in dissolved H,S. The permissible limits for discharge of the effluent containing soluble sulphides is 0.5-l .Omg/l. H,S could
K. K. Dwivedy, A. K. Mathur / Hydrometallurgy 38 (1995) 99-109
107
Table 5 Uranium precipitation with heterotmphs Test No.
Conditions
Time
End composition of leach liquor
&OS precipitation
(days)
(%o) PH
Bacillus
I
subtilis
10 30 60 10 30 60
+o.l% yeast extract +O.l% Mannitol Bacilluspumilus
2
+0.146 yeast extract +O.l% Mannitol Initial leach liquor composition:U,0p=3.65
2.68 2.78 2.86 2.42 2.12 2.86
Eh
WA
(mV)
(g/U
-480 - 320 -300 -480 -320 -290
3.29 3.16 2.59 3.29 3.24 2.60
9.86 13.67 29.04 9.86 11.23 28.76
g/l; Fe2+ =0.168 g/l: Fe’+ = 0.84Og/l; pH =2.00, redoxpotential=
-500 mV.
Table 6 Precipitation with fungi Test conditions
Time (days)
Initial leach liquor composition
pH
Leach Liquor + Czapeck’s nutrients 2. Leach Liquor + Czapeck’s nutrients 3A. Leach liquor + 1% peptone + 1% glucose 3D. Control I.
7.
Leach liquor + 1% glucose
Eh
Final composition
pH
Eh
Calcined mould grade
U308 (%I
pas (%)
U,O* (%)
(mV)
U@, (g/t)
pa5 (mg/U
60
1.44 -400
10.9
80.0
1.68 -220
5.5
0.02
46.7
-
60
1.56
10.0
3.90 -140
0.023
0.02
98.6
-
15
1.80 -390
0.573 253.1
2.24 -310
0.274
-
52.1
-
30 15 30 15 30
1.80 -390 0.573 1.80 -390 0.573 1.80 -400 0.937 --
2.48 1.80 1.60 1.90 2.00
0.156 0.566 0.572 0.790 0.643
188
72.7
25.7
-
15.7 31.4
-
1.05
253.1 253.1 ?
(mV)
-290 -380 -380 -360 -300
U@s (g/l)
Precipitation
p,4 (mg/l)
-
50.0
25.0
Table 7 Biosorption of uranium by yeast (Saccharomyces cervisiae) NO.
1. 2. 3. 4. 5. 6.
Experimental solution
Eluate Eluate ElIlate Leach liquor Uranyl sulphate solution Uranyl sulphate solution
Uranium concentration
Contact tie
Biosorption
(h)
(%)
Initial
Final
@pm)
(ppm)
64.5 159.0 68.5 129.0 14.9
9.8 30.0 0.7 68.5 1.5
1 2 2 2 I
88.0 81.0 99.0 47.0 90.0
16.4
0.7
2
96.0
be oxidised to a level below detection limits ( < 20 pgll H,S) within 6 h using Thiobacillus The oxidation of H$ by bacteria formed colloidal sulphur floccules which settled at the bottom of the tanks (Table 8) [ 181
ferrooxidans.
108
Table 8 Experiments Exp. No.
K.K. Dwivedy, A.K. Mathur/Hydrometallurgy
on effluents from Heavy Water plant, Kota Treatment
Initial observation PH
1. 2. 3. 4.
38 (1995) 99-109
KE+Tf KE+Tf KE+Tf KE+Tf
5.40 5.62 5.32 5.60
Cells (counts/ml) 2.8 2.8 2.8 2.8
x x x x
lo4 10J IO4 IO4
Observation Sulphide
pH
(ppm) 13.0 11.6 12.8 13.0
5.48 5.50 5.30 5.60
after 6 h
Cells (counts/ml) 6.8 6.8 6.8 6.8
x x x x
lo5 lo5 lo5 IO5
Sulphide (ppm) < < < <
20 20 20 20
ppb ppb ppb ppb
K.E. = Kota effluent; T.f. = Thiobacillusferrooxidans.
4. Conclusion
The future for biomineral engineering is bright, although a word of caution is felt necessary. It should be remembered that bioprocessing in the mineral industry is not a magic wand but is a supplementary tool to conventional mineral engineering and metallurgical processes. Biomineral processing is appropriate for treating very low grade ores and, if properly developed, biomineral processing will be in a position to reap all the metal wealth present in the oceans, both in water and sediments, an area where conventional technology is just not available.
Acknowledgement
Devoted work carried out by colleagues in the Mineral Technology Laboratory is thankfully acknowledged.
References [I] Padmanabhan, 510-573.
G., An assessment
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