- Email: [email protected]
Acid equilibrium during bioleaching of alkaline low-grade sulfide copper ore
RARE METALS Vol. 25, No. 6, Dee 2006, p . 680
Acid equilibrium during bioleaching of alkaline low-grade sulfide copper ore WEN Jiankang, RUAN Renman, YAO Guocheng, and SONG Yongsheng General Research Institutefor Nonferrous Metals, Beding 100088, China (Received ux)5-12-10}
Abstract: This article reports the study on acid equilibrium during bioleaching of alkaline low-grade copper sulfide ore. Adding auxiliary agents 1' (sulfur) and 2' (pyrite) makes bacterial leaching of copper and acid production canied out si-
multaneously because the auxiliary agents can be oxidized by bacteria and the oxidation products involve acid. The acid required for dissolving alkaline gangue during bacterial leaching is produced, and acid equilibrium is reached during the ore bio-leaching. The recovery of copper reaches more than 95%. Key words: bioleaching; alkaline low-grade copper sulfide ore; acid equilibrium; process mineralogy; auxiliary agent
1. Introduction
'
Biohydrometallurgyis a (green) process flow that is used for the direct extraction of metals from ores using natural components such as water, air, and microbes under normal temperature and pressure conditions; it has many advantages including short technological process, simple equipments, reduced water consumption, lower capital and operating costs, and environmental-friendly character [1-31, and is especially suitable for processing low-grade ores and the mineral resources in remote areas [4]. Since the end of the 1980s, the application of microbial hydrometallurgy has been growing steadily, and the bio-extraction for metals such as copper, uranium, gold, nickel, and cobalt has also been successfully carried out, thereby leading to industrialization. With the considerable decrease in high-grade mineral resources and the regulations for environment protection becoming increasingly strict, biohydrometallurgical technology has been generally regarded as one of the most competitive technology for humans to alleviate the problems arising due to resource shortage and environmental pollution [4]. At present, all the commercial biohydrometallurgical technologies are used under acidic conditions Corresponding author: WEN Jiankang
(PH I2.5) IS]; however, some mineral resources contain large amounts of alkaline gangue, the treatment of which requires a large amount of acid. This results in an increase in the pH value beyond acidic conditions, which is pH required for bacterial growth, and thus the leaching process will not proceed [6]. For example, the sulfur content of some alkaline low-grade copper sulfide ores such as limonite, pyrite, and chalcocite is only 0.57%, and the content of pyrite for producing acid by bacterial oxidization is only 0.75%. A large amount of pyrite is oxidized into limonite, which consumes l&ge amount of acid; meanwhile, a large amount of acid-soluble carbonate mineral (calcite) comprises about 18.30%of the total amount of minerals. Under acidic conditions, bioleaching of this ore may be carried out, with a large amount of acid beiig consumed. With regard to the acidic conditions for bacterial growth, pyrite oxidation by bacteria does not provide sufficient amounts of acid for bacterial growth. When biohydrometallurgicalprocess is used for treating minerals with high content of alkaline gangue, the common problems encountered by all the researchers in this field are: how to maintain acidic conditions required for bacterial growth and how to solve the problems of acid equilibrium and
E-mail: [email protected]
Wen J.K. et aL,Acid equilibrium during bioleaching of alkaline low-grade sulfide copper ore
low recovery for low-grade complex sulfide minerals in bioleaching, which is also the focus of this article.
2. Properties of test materials Table 1 provides the results of the multielement chemical analysis of the ore, and Table 2 lists the mineral components and their relative contents.
681
The ore is in the highly oxidized state. Little amount of ore contains pyrite which exists in the form of rubble. The pyrite in most of the lumps has been transformed into limonite except little amount of the pyrite which contains net or nervation chalcopyrite. The surface and the body of the copper mineral have been fully covered or transformed into malachite and bluestone.
Table 1. Results of multielementchemical analysis of the ore Element
cu
AS!
S
TFe
Se
AU
Pb
zn
Content
0.65%
40.0dt
0.479%
3.81%
0.001%
4 . 1d t
0.02%
0.06%
Element
Sb
SiOz
A1203
CaO
Na20
Content
0.01%
55.14%
8.10%
11.60%
MgO 3.16%
K20 2.00%
As 0.002%
1.60%
Table 2. Mineral componentsand their relative contents
I
Minerals
Relative content I %
0.65
Pyrite (marcasite)
0.75
0.10
Limonite (hematite)
1.95
Minerals
Relative content / %
Chalcocite Bornite Covellite
0.05
Quartz
39.20
Blue chalcocite
Feldspar
27.00
Malachite
chlorite Calcite, dolomite
8.20 18.30
Sericite
1.00
Clay minerals etc
2.50
Bluestone Chalcopyrite
0.005
The ore consists of many copper minerals. Its main component is chalmcite, and its other important 'komponentsinclude bornite, covellite, malachite, a small amount of blue chalcocite, bluestone, and little amounts of chalcopyrite and native copper. Chalcocite mainly exists in a compact-massive or compact-grained form. Chalcocite often combines with secondary copper sulfide ores and copper oxide ores. The combination of these minerals results in the formation of copper mineral aggregates. These aggregates are distributed among rubbles. Graphic bomite and silty covellite penetrate into come particles, and their surface is frequently covered with emerald malachite powder and bluestone. Copper ore aggregation dominates in rich copper ores. As the major ore, chalcocite is covered with copper OXide ores and includes other fine-grained secondary
copper ores, but in lean ores, chalcocite is dispersed in the form of fine grains within gangue or limonite. A large amount of carbonate and limonite are widely distributed in the ore and are closely related to the copper minerals. The coarse copper minerals often combine with limonite carbonate to form colloidal rubble, whereas frne copper minerals are directly mixed in carbonate and not easy to be released. Based on the properties of the ore mentioned above, in bioleaching, the content of pyrite that is oxidized for acid production is very less. With bacterial oxidation and leaching, the main copper mineral of chalcocite may also consume acid. Moreover, the amounts of carbonate and limonite in the ore are relatively high, and both may be dissolved under acidic conditions, resulting in the consumption of a
RARE METALS, Vol. 25, No. 6, Dec 2006
682
large amount of acid. Therefore, when bacteria are used for leaching this ore, equilibriumis not reached between the amount of acid produced and that of acid consumed, and the amount of acid consumed is greater than that of acid produced.
3. Test method and instruments 3.1. Test method The specified quantity of ore powder and auxiliary agent are weighed and transferred into a 300-ml Erlenmeyer flask containing deionized water according to the specified pulp density. 10% diluted weak sulfuric acid solution is used to adjust the acidity to the required pH value and to maintain it constant. After adding bacterial suspension, the Erlenmeyer flask is agitated by placing it on a shaking table for vibrating, and the residue is leached at constant air temperature. During leaching, pH value of the slurry and the potential are measured once a day. After leaching is completed, the leached residues are filtered, washed, and dried. The chemical analysis of the leached residues and lixivium, respectively, conf m s the recovery and the acid equilibriumresults.
3.2. Test instruments and applications The test instruments and their applications are as follows: (1) thermostatic oscillator, for culturing bacteria and for leaching; (2) the pH meter, for detecting the pH value of the leaching slurry; (3) dissolved oxygen meter, for detecting the amount of dissolved oxygen in the leaching solution; (4) potentiometer, for testing the potential of the slurry, and the potential measured is a value relative to the standard calomel electrode (vs. SCE); (5) microscope, for detecting the activity of bacteria adsorbing on the surface of the leaching solution and mineral powder; (6) atomic absorption spectrum analyzer, for analyzing the content of metal elements in lixivium and leached residues; (7) spectral photometer, for detecting the activity of bacteria in lixivium and leached residues and for analyzing the content of metal element in the leaching solution and in the leached residues.
4. Theoretical calculation of the acid equilibrium With regard to the main acid-consuming minerals such as calcite and hematite in ore, the amount of acid consumed may be calculated based on the chemical reaction equilibrium theory; the details are shown below: (1) Calcite, CaC03 + HZSO4 = CaS04+ H20 + CO?. (2) Hematite, 2Fe03.3H20 + 6H2S04 = 2F@(so4), + The amount of acid consumed by calcite in ore per ton is 0.17934 t sulfuric acid; and the amount of acid consumed by hematite in ore per ton is 0.003 t sulfuric acid. At the same time, F@(SO&generated from the hematite dissolved by acid may be used as an oxidizer for bioleaching, and the ore per ton may produce 0.004 t Fe2(S04),which is directly used for oxidizing and leaching chalcocite and bomite in ore. The leaching principle is shown below: (3) Chalcocite, Cu2S+ 2Fe2(S04)3= 2CuS04 + 4FeS04 + So, 2s'
+ 302+ 2H20
> 2H2S04.
(4) Bomite, Cu5FeS4+ 6Fe2(S04),= 5CuS04+ 13FeS04+ 4S0, 2s' +302+2H20 bacteria > 2H2S04. According to the reaction Eqs. (3) and (4), Fez (S04h generated by dissolution of the hematite by acid is sufficient for oxidizing and leaching chalcocite and bornite. The production of sulfuric acid from both the ores per ton is 0.30 kg and 0.07 kg, respectively. During bacterial leaching, the main acid-producing mineral is pyrite; its reaction equations for bioleaching and acid-production are as below:
(5) 2FeS2 + 7 0 2 2H2S04.
+ 2H20
bacteria
> 2FeSO4 +
(6) 4FeS04 + 0 2 + 2H2S04 bacteria > 2Fe2 ( so & + 2H20. From reaction Eqs. (5) and (6), it is found that sulfuric acid produced from bacterial leaching of ore
Wen J.K. et al., Acid equilibrium during bioleaching of alkaline low-grade sulfide copper ore
per ton is 0.3 kg. Thus, in theory, the amount of sulfuric acid consumed for bacterial leaching of ore per ton is 181.67kg. During the bioleaching process, the pH value of the leaching system is maintained at 1.7-2.50, and the actually measured amount of sulfuric acid consumed for ore per ton is 182.00 kg, which is consistent with the result of the chemical calculations.
Test results and analysis of acid equilibrium process 5.
On the basis of the theoretical analysis mentioned above, for treatment using the biohydrometallurgical process, this ore must be. added along with extra sulfuric acid or with acid produced from bacterial oxidation. Leaching and acid production can be carried out simultaneously by the addition of the
683
acid-producing substances and the bacterial oxidation. Given the actual conditions of this ore, the latter, i.e. acid produced by bacterial oxidation, is optimum for acid equilibrium test during bacteria leaching. Both acid producing materials (auxiliary agent 1' and 2') have been studied in detail. 5.1. Muence of the dosage of auxiliary agent 1'
on copper recovery and the amount of acid produced by bacterial oxidation Table 3 provides the dosage of auxiliary agent 1' and the test results. Fig. 1 shows the variation of the slurry pH value and the redox potential during the leaching process. Test conditions: slurry concentration lo%, bacteria inoculation lo%, leaching time 18 d, leaching temperature 30-32"C, and rotating speed of shaking table 150 r/min.
Table 3. Test results of the copper leaching rate and the amonnt of the produced acid for a d r y agent 1' Auxiliary agent 1' dosage I %
2.5
5.o
9.5
Initial pH value
1.72
1.70
1.70
1.70
Final pH value
1.63
1.20
1 .oo
0.76
Initial potential I mV vs. SCE
341
345
337
339
Final potential I mV vs. SCE
570
514
557
559
Oxidationrate of Auxiliary agent 1' I %
96.08
73.88
70.20
68.52
Amount of sulfuric acid produced by ore per ton I kg
73.56 93.08
113.12 91.47
214.90 91.32
314.76 91.61
Copper leaching rate I % 700 W
2 600
--
I# leachingads dosage 2 5% I# leaclung aids dosage 5.0% + I# leachmgads dosage 9 S% + Itfleachnraidsdosaee 135%
d
>
$ 500 -. 3
.3
c-'
400 300;
'
4
I
I
I
'
I
I
8 12 16 Leaching time / d
'
I 0.6 20
Fig. 1. Innuence of the dosage of auxiliary agent 1' on the pH value of the slurry and the redox potential during the leaching process
From Table 3 and Fig. 1, it can be found that after
13.5
bioleaching for 18 d, addition of auxiliary agent 1' may not influence the efficiency of leaching copper from ore. When the dosage of auxiliary agent 1' is 2.5%, its rate of oxidation may reach up to 96.03%. By increasing its dosage, the bacterial oxidation rate with respect to the auxiliary agent 1' decreases to some extent. On the other hand, when the dosage of auxiliary agent 1' reaches 5%-13.5%, the oxidation rate with respect to auxiliary agent 1' may decrease to 70%, which is caused by the lower oxidation rate of bacteria in auxiliary agent 1' and the longer oxidizing period required. During bacterial leaching, with increase in the oxidization of bacteria in auxiliary agent 1' and with the improvement in the oxidation rate, the amount of acid produced is greater than that consumed; although the pH value of the leach-
RARE METALS, VoL 25,No. 6, Dec 2006
684
leaching time, the bacterial oxidation rate with respect to auxiliary agent 1' may be increased accordingly. Moreover, after the leaching time is increased to over 15 d, the bacterial oxidation rate with respect 5.2. Influence of leaching time on the oxidation to auxiliary agent 1' increases rapidly. M e n the rate of auxiliary agent 1' and the copper leaching leaching time is 30 d, the bacterial oxidation rate rate with respect to auxiliary agent 1' reaches up to 94.56%, whereas the copper recovery reaches up to Table 4 provides the leaching time and the test 95%, and the amount of acid produced reaches up to results, and Fig. 2 shows the variation in pH value of 144.70 kg; if the consumption of acid for bacterial the slurry and the redox potential during the leaching leaching of ore completely satisfies the requirement, process. Test conditions: dosage of auxiliary agent needs to be not less than 5.9% auxiliary agent 1' 5.9%, pulp density lo%, bacterial inoculation lo%, leaching temperature 30-32"C, and rotating added. Therefore, the amount of acid produced and the amount of acid consumed during the bacterial speed of shaking table 150 r/min. leaching process reaches equilibrium. Table 4 and Fig. 2 indicate that by prolonging the Table 4. Test results of the oxidation rate and the copper recovery of auxiliary agent 1#
ing system decreased continuously, it did not influence the redox potential of the slurry and the copper recovery rate.
Leaching time / d Initial pH value Final pH value Initial potential / mV vs. SCE Final potential / mV vs. SCE Oxidationrate of auxiliary agent 1' / % Amount of sulfuric acid produced by ore per ton / kg Copper leaching rate / % _ .600, 2.0 1.8
z.
1.6 - * pH '*\ Potential
400;
zg
+-
350/ 350 ,
,
,
1.4
8
7 T,
\. - 1.2
Leaching time / d Fig. 2. Relationships of the pH value of the slurry and the redox potential to the leaching time.
53. Influence of the dosage of auxihy agent 2' on the copper leaching rate and the amount of acid produced Table 5 provides data on the dosage of auxiliary agent 2' and the test results. Fig. 3 shows the pH
22_
7
14
18
2.01
2.02
2.01
2.02
2.02
1.86
1.67
1.63
1.53
1.18
343
340
337
339
335
~
_
30 _
510
525
537
567
550
42.56
62.77
74.02
82.89
94.56
65.17
96.11
113.35
126.90
144.70
85.79
90.20
91.92
92.78
95.00
values of the slurry and the variation of the redox potential during the leaching process. Test conditions: pulp density lo%, bacterial inoculation lo%, leaching temperature 30-32"C, rotating speed of shaking table 150 rpm, and leaching period 15 d. Table 5 and Fig. 3 show that the addition of auxiliary agent 2' can not only accelerate copper leaching but can also speed up the bacteria oxidizing rate of auxiliary agent 2'. When the dosage of auxiliary agent 2' is more than 4%, after bacterial leaching for 15 d, the copper recovery may be more than 94%, and the pH value of the leaching system shows a decreasing trend. The more the dosage of auxiliary agent 2' added, the better is recovery of copper. Moreover, the amount of acid produced may be increased. However, since the time for the leaching test is less, with the increasing dosage of auxiliary agent 2', the bacterial oxidation rate with respect to the auxiliary agent 2' becomes lower. E the leaching
Wen J.K. et d.,Acid equilibrium during bioleaching of alkaline low-grade sulfide copper ore
time of bacteria is prolonged, auxiliary agent 2' will be completely oxidized. When the dosage of auxiliary agent 2' is 11.89%, the problem of acid consumption for bacterial leaching of copper ore with
685
18.30% calcite and 1.95% hematite may be completely solved and equilibrium is reached between acid production and acid consumption.
Table 5. Test results of the copper leaching rate and the produced acid for arudliary agent Dosage of auxiliaryagent 2' I %
2
4
6
8
Initial pH value Final pH value Initial potential I mV vs. SCE Final potential I mV vs. SCE Oxidation rate of auxiliary agent 2' / % Amount of sulfuric acid produced by ore per ton I kg
1.84
1.85
1.84
1.84
1.94
1.67
1.58
1.51
345
350
35 1
355
526
528
528
534
79.28
74.9
71.55
68.50
24.00
45.00
65.70
83.90
93.84
94.07
94.78
95.35
Copper leaching rate I %
5.4. Influence of ore size on copper recovery
2.1 2.0 1.9 $ 1.8 1.7
a
E
1.6 1.5
0
8 12 Leaching time I d
4
16
Fig. 3. Jnfluence of the dosage of arudliary agent 2' on
the pH values of the slurry and the redox potential.
The results of the bacterial leaching experiment of ores in diverse sizes are shown in Table 6. Experimental condition: pulp density lo%, auxiliary agent 1' dosage 5.9%, auxiliary agent 2' dosage 4.0%, inoculation amount lo%, leaching perid 15 d, leaching temperature 30-32"C,and rotating speed of shaking table 150 r/min From Table 6, it is seen that the ore size is an important factor in bacterial leaching. With the decrease in ore size, both the leaching rate and the recovery of copper increase.
Table 6. Experimental results of the influence of ore size on the copper recovery Ore size
-2 mm 100.00%
-74 pm 46.45%
Copper recovery (adding auxiliary agent 1)' I % Copper recovery (adding auxiliary agent 2"> I %
65.39
75.12
91.35
92.50
66.26
76.78
94.15
94.56
6. Conclusions (1) The test results of the ore components show that the content of pyrite oxidized for producing acid during bacterial leaching is very less. With the oxidation of main copper mineral of chalcocite by bacteria for leaching, the acid will be consumed. Moreover, the contents of carbonate and limonite in ore are high, and both components may be dissolved
-74 pn 87.60% - 4 5 p 96.50%
under acidic conditions, thereby consuming a large amount of acid. Therefore, when bacteria are used for leaching this kind of ore, the amount of acid consumed is greater than that of acid produced. (2) Based on the properties of ore for testing, the main acid-consuming minerals are calcite and hematite. After calculating in accordance with the chemical reaction equilibrium theory, the amount of sulfuric acid consumed for this alkaline low-grade copper sulfide ore per ton is 0.18167 t. During the bioleach-
RARE METALS, VoE. 2 5 No. 6,Dec 2006
686
bioleaching process, to maintain the pH value of the leaching system at 1.70-2.50, the amount of sulfuric acid actually measured for ore per ton is 0.182 t. (3) Leaching and acid production can be carried out simultaneously by the addition of auxiliary agents 1' and 2'. Therefore the acid required for the bacterial leaching of ore is produced and acid equilibrium during the bioleaching process is achieved. The added dosages of auxiliary agents 1' and 2' are 5.9% and 1139%respectively, both with the copper leaching rate more than 95%.
References Corode. L. Brierley and Tames. A. Brierley, Copper bioleaching: state-of-the-art, [in] Proceeding of the Copper99-Cober99 International Conference, USA, 1999: 59. Brierley, C.L. and Brierley, J.A., Present and future
commercial applications of biohydrometallurgy. [in] Amils, R. and Ballester, A. 4 s . Biohydrometallurgy and the Environment Toward the Miming of the 21st Century,PartA, Elsevier, Amsterdam, 1999: 81. Douglas. E. Rawlings, Biomining: Theory, Microbes and Industrial Processes. Springer Verlag, Berlin, 1997: 29. Wang D.Z., Ruan R.M.,Dong Q.H., et al., Study and progress of mineral bioextraction technologies. [in] Paper Collection on Technical Communication and Seminar for Copper-Nickel Hydrometallurgy, The Nonferrous Society of China, Beijing, 2001: 1. Karavaiko G.I., Rossi G.and Agate D.A., et al., Biogeotechnology of Metals Manual, Center for Intemational Projects GKNT, Moscow, 1988: 47. Wen J.K., Rum R.M., and Sun X.N., Study on bioleaching for Jinchuan low-grade nickel ores, Min. Metall., 2002,11(1):55.
Acid equilibrium during bioleaching of alkaline low-grade sulfide copper ore WEN Jiankang, RUAN Renman, YAO Guocheng, and SONG Yongsheng General Research Institutefor Nonferrous Metals, Beding 100088, China (Received ux)5-12-10}
Abstract: This article reports the study on acid equilibrium during bioleaching of alkaline low-grade copper sulfide ore. Adding auxiliary agents 1' (sulfur) and 2' (pyrite) makes bacterial leaching of copper and acid production canied out si-
multaneously because the auxiliary agents can be oxidized by bacteria and the oxidation products involve acid. The acid required for dissolving alkaline gangue during bacterial leaching is produced, and acid equilibrium is reached during the ore bio-leaching. The recovery of copper reaches more than 95%. Key words: bioleaching; alkaline low-grade copper sulfide ore; acid equilibrium; process mineralogy; auxiliary agent
1. Introduction
'
Biohydrometallurgyis a (green) process flow that is used for the direct extraction of metals from ores using natural components such as water, air, and microbes under normal temperature and pressure conditions; it has many advantages including short technological process, simple equipments, reduced water consumption, lower capital and operating costs, and environmental-friendly character [1-31, and is especially suitable for processing low-grade ores and the mineral resources in remote areas [4]. Since the end of the 1980s, the application of microbial hydrometallurgy has been growing steadily, and the bio-extraction for metals such as copper, uranium, gold, nickel, and cobalt has also been successfully carried out, thereby leading to industrialization. With the considerable decrease in high-grade mineral resources and the regulations for environment protection becoming increasingly strict, biohydrometallurgical technology has been generally regarded as one of the most competitive technology for humans to alleviate the problems arising due to resource shortage and environmental pollution [4]. At present, all the commercial biohydrometallurgical technologies are used under acidic conditions Corresponding author: WEN Jiankang
(PH I2.5) IS]; however, some mineral resources contain large amounts of alkaline gangue, the treatment of which requires a large amount of acid. This results in an increase in the pH value beyond acidic conditions, which is pH required for bacterial growth, and thus the leaching process will not proceed [6]. For example, the sulfur content of some alkaline low-grade copper sulfide ores such as limonite, pyrite, and chalcocite is only 0.57%, and the content of pyrite for producing acid by bacterial oxidization is only 0.75%. A large amount of pyrite is oxidized into limonite, which consumes l&ge amount of acid; meanwhile, a large amount of acid-soluble carbonate mineral (calcite) comprises about 18.30%of the total amount of minerals. Under acidic conditions, bioleaching of this ore may be carried out, with a large amount of acid beiig consumed. With regard to the acidic conditions for bacterial growth, pyrite oxidation by bacteria does not provide sufficient amounts of acid for bacterial growth. When biohydrometallurgicalprocess is used for treating minerals with high content of alkaline gangue, the common problems encountered by all the researchers in this field are: how to maintain acidic conditions required for bacterial growth and how to solve the problems of acid equilibrium and
E-mail: [email protected]
Wen J.K. et aL,Acid equilibrium during bioleaching of alkaline low-grade sulfide copper ore
low recovery for low-grade complex sulfide minerals in bioleaching, which is also the focus of this article.
2. Properties of test materials Table 1 provides the results of the multielement chemical analysis of the ore, and Table 2 lists the mineral components and their relative contents.
681
The ore is in the highly oxidized state. Little amount of ore contains pyrite which exists in the form of rubble. The pyrite in most of the lumps has been transformed into limonite except little amount of the pyrite which contains net or nervation chalcopyrite. The surface and the body of the copper mineral have been fully covered or transformed into malachite and bluestone.
Table 1. Results of multielementchemical analysis of the ore Element
cu
AS!
S
TFe
Se
AU
Pb
zn
Content
0.65%
40.0dt
0.479%
3.81%
0.001%
4 . 1d t
0.02%
0.06%
Element
Sb
SiOz
A1203
CaO
Na20
Content
0.01%
55.14%
8.10%
11.60%
MgO 3.16%
K20 2.00%
As 0.002%
1.60%
Table 2. Mineral componentsand their relative contents
I
Minerals
Relative content I %
0.65
Pyrite (marcasite)
0.75
0.10
Limonite (hematite)
1.95
Minerals
Relative content / %
Chalcocite Bornite Covellite
0.05
Quartz
39.20
Blue chalcocite
Feldspar
27.00
Malachite
chlorite Calcite, dolomite
8.20 18.30
Sericite
1.00
Clay minerals etc
2.50
Bluestone Chalcopyrite
0.005
The ore consists of many copper minerals. Its main component is chalmcite, and its other important 'komponentsinclude bornite, covellite, malachite, a small amount of blue chalcocite, bluestone, and little amounts of chalcopyrite and native copper. Chalcocite mainly exists in a compact-massive or compact-grained form. Chalcocite often combines with secondary copper sulfide ores and copper oxide ores. The combination of these minerals results in the formation of copper mineral aggregates. These aggregates are distributed among rubbles. Graphic bomite and silty covellite penetrate into come particles, and their surface is frequently covered with emerald malachite powder and bluestone. Copper ore aggregation dominates in rich copper ores. As the major ore, chalcocite is covered with copper OXide ores and includes other fine-grained secondary
copper ores, but in lean ores, chalcocite is dispersed in the form of fine grains within gangue or limonite. A large amount of carbonate and limonite are widely distributed in the ore and are closely related to the copper minerals. The coarse copper minerals often combine with limonite carbonate to form colloidal rubble, whereas frne copper minerals are directly mixed in carbonate and not easy to be released. Based on the properties of the ore mentioned above, in bioleaching, the content of pyrite that is oxidized for acid production is very less. With bacterial oxidation and leaching, the main copper mineral of chalcocite may also consume acid. Moreover, the amounts of carbonate and limonite in the ore are relatively high, and both may be dissolved under acidic conditions, resulting in the consumption of a
RARE METALS, Vol. 25, No. 6, Dec 2006
682
large amount of acid. Therefore, when bacteria are used for leaching this ore, equilibriumis not reached between the amount of acid produced and that of acid consumed, and the amount of acid consumed is greater than that of acid produced.
3. Test method and instruments 3.1. Test method The specified quantity of ore powder and auxiliary agent are weighed and transferred into a 300-ml Erlenmeyer flask containing deionized water according to the specified pulp density. 10% diluted weak sulfuric acid solution is used to adjust the acidity to the required pH value and to maintain it constant. After adding bacterial suspension, the Erlenmeyer flask is agitated by placing it on a shaking table for vibrating, and the residue is leached at constant air temperature. During leaching, pH value of the slurry and the potential are measured once a day. After leaching is completed, the leached residues are filtered, washed, and dried. The chemical analysis of the leached residues and lixivium, respectively, conf m s the recovery and the acid equilibriumresults.
3.2. Test instruments and applications The test instruments and their applications are as follows: (1) thermostatic oscillator, for culturing bacteria and for leaching; (2) the pH meter, for detecting the pH value of the leaching slurry; (3) dissolved oxygen meter, for detecting the amount of dissolved oxygen in the leaching solution; (4) potentiometer, for testing the potential of the slurry, and the potential measured is a value relative to the standard calomel electrode (vs. SCE); (5) microscope, for detecting the activity of bacteria adsorbing on the surface of the leaching solution and mineral powder; (6) atomic absorption spectrum analyzer, for analyzing the content of metal elements in lixivium and leached residues; (7) spectral photometer, for detecting the activity of bacteria in lixivium and leached residues and for analyzing the content of metal element in the leaching solution and in the leached residues.
4. Theoretical calculation of the acid equilibrium With regard to the main acid-consuming minerals such as calcite and hematite in ore, the amount of acid consumed may be calculated based on the chemical reaction equilibrium theory; the details are shown below: (1) Calcite, CaC03 + HZSO4 = CaS04+ H20 + CO?. (2) Hematite, 2Fe03.3H20 + 6H2S04 = 2F@(so4), + The amount of acid consumed by calcite in ore per ton is 0.17934 t sulfuric acid; and the amount of acid consumed by hematite in ore per ton is 0.003 t sulfuric acid. At the same time, F@(SO&generated from the hematite dissolved by acid may be used as an oxidizer for bioleaching, and the ore per ton may produce 0.004 t Fe2(S04),which is directly used for oxidizing and leaching chalcocite and bomite in ore. The leaching principle is shown below: (3) Chalcocite, Cu2S+ 2Fe2(S04)3= 2CuS04 + 4FeS04 + So, 2s'
+ 302+ 2H20
> 2H2S04.
(4) Bomite, Cu5FeS4+ 6Fe2(S04),= 5CuS04+ 13FeS04+ 4S0, 2s' +302+2H20 bacteria > 2H2S04. According to the reaction Eqs. (3) and (4), Fez (S04h generated by dissolution of the hematite by acid is sufficient for oxidizing and leaching chalcocite and bornite. The production of sulfuric acid from both the ores per ton is 0.30 kg and 0.07 kg, respectively. During bacterial leaching, the main acid-producing mineral is pyrite; its reaction equations for bioleaching and acid-production are as below:
(5) 2FeS2 + 7 0 2 2H2S04.
+ 2H20
bacteria
> 2FeSO4 +
(6) 4FeS04 + 0 2 + 2H2S04 bacteria > 2Fe2 ( so & + 2H20. From reaction Eqs. (5) and (6), it is found that sulfuric acid produced from bacterial leaching of ore
Wen J.K. et al., Acid equilibrium during bioleaching of alkaline low-grade sulfide copper ore
per ton is 0.3 kg. Thus, in theory, the amount of sulfuric acid consumed for bacterial leaching of ore per ton is 181.67kg. During the bioleaching process, the pH value of the leaching system is maintained at 1.7-2.50, and the actually measured amount of sulfuric acid consumed for ore per ton is 182.00 kg, which is consistent with the result of the chemical calculations.
Test results and analysis of acid equilibrium process 5.
On the basis of the theoretical analysis mentioned above, for treatment using the biohydrometallurgical process, this ore must be. added along with extra sulfuric acid or with acid produced from bacterial oxidation. Leaching and acid production can be carried out simultaneously by the addition of the
683
acid-producing substances and the bacterial oxidation. Given the actual conditions of this ore, the latter, i.e. acid produced by bacterial oxidation, is optimum for acid equilibrium test during bacteria leaching. Both acid producing materials (auxiliary agent 1' and 2') have been studied in detail. 5.1. Muence of the dosage of auxiliary agent 1'
on copper recovery and the amount of acid produced by bacterial oxidation Table 3 provides the dosage of auxiliary agent 1' and the test results. Fig. 1 shows the variation of the slurry pH value and the redox potential during the leaching process. Test conditions: slurry concentration lo%, bacteria inoculation lo%, leaching time 18 d, leaching temperature 30-32"C, and rotating speed of shaking table 150 r/min.
Table 3. Test results of the copper leaching rate and the amonnt of the produced acid for a d r y agent 1' Auxiliary agent 1' dosage I %
2.5
5.o
9.5
Initial pH value
1.72
1.70
1.70
1.70
Final pH value
1.63
1.20
1 .oo
0.76
Initial potential I mV vs. SCE
341
345
337
339
Final potential I mV vs. SCE
570
514
557
559
Oxidationrate of Auxiliary agent 1' I %
96.08
73.88
70.20
68.52
Amount of sulfuric acid produced by ore per ton I kg
73.56 93.08
113.12 91.47
214.90 91.32
314.76 91.61
Copper leaching rate I % 700 W
2 600
--
I# leachingads dosage 2 5% I# leaclung aids dosage 5.0% + I# leachmgads dosage 9 S% + Itfleachnraidsdosaee 135%
d
>
$ 500 -. 3
.3
c-'
400 300;
'
4
I
I
I
'
I
I
8 12 16 Leaching time / d
'
I 0.6 20
Fig. 1. Innuence of the dosage of auxiliary agent 1' on the pH value of the slurry and the redox potential during the leaching process
From Table 3 and Fig. 1, it can be found that after
13.5
bioleaching for 18 d, addition of auxiliary agent 1' may not influence the efficiency of leaching copper from ore. When the dosage of auxiliary agent 1' is 2.5%, its rate of oxidation may reach up to 96.03%. By increasing its dosage, the bacterial oxidation rate with respect to the auxiliary agent 1' decreases to some extent. On the other hand, when the dosage of auxiliary agent 1' reaches 5%-13.5%, the oxidation rate with respect to auxiliary agent 1' may decrease to 70%, which is caused by the lower oxidation rate of bacteria in auxiliary agent 1' and the longer oxidizing period required. During bacterial leaching, with increase in the oxidization of bacteria in auxiliary agent 1' and with the improvement in the oxidation rate, the amount of acid produced is greater than that consumed; although the pH value of the leach-
RARE METALS, VoL 25,No. 6, Dec 2006
684
leaching time, the bacterial oxidation rate with respect to auxiliary agent 1' may be increased accordingly. Moreover, after the leaching time is increased to over 15 d, the bacterial oxidation rate with respect 5.2. Influence of leaching time on the oxidation to auxiliary agent 1' increases rapidly. M e n the rate of auxiliary agent 1' and the copper leaching leaching time is 30 d, the bacterial oxidation rate rate with respect to auxiliary agent 1' reaches up to 94.56%, whereas the copper recovery reaches up to Table 4 provides the leaching time and the test 95%, and the amount of acid produced reaches up to results, and Fig. 2 shows the variation in pH value of 144.70 kg; if the consumption of acid for bacterial the slurry and the redox potential during the leaching leaching of ore completely satisfies the requirement, process. Test conditions: dosage of auxiliary agent needs to be not less than 5.9% auxiliary agent 1' 5.9%, pulp density lo%, bacterial inoculation lo%, leaching temperature 30-32"C, and rotating added. Therefore, the amount of acid produced and the amount of acid consumed during the bacterial speed of shaking table 150 r/min. leaching process reaches equilibrium. Table 4 and Fig. 2 indicate that by prolonging the Table 4. Test results of the oxidation rate and the copper recovery of auxiliary agent 1#
ing system decreased continuously, it did not influence the redox potential of the slurry and the copper recovery rate.
Leaching time / d Initial pH value Final pH value Initial potential / mV vs. SCE Final potential / mV vs. SCE Oxidationrate of auxiliary agent 1' / % Amount of sulfuric acid produced by ore per ton / kg Copper leaching rate / % _ .600, 2.0 1.8
z.
1.6 - * pH '*\ Potential
400;
zg
+-
350/ 350 ,
,
,
1.4
8
7 T,
\. - 1.2
Leaching time / d Fig. 2. Relationships of the pH value of the slurry and the redox potential to the leaching time.
53. Influence of the dosage of auxihy agent 2' on the copper leaching rate and the amount of acid produced Table 5 provides data on the dosage of auxiliary agent 2' and the test results. Fig. 3 shows the pH
22_
7
14
18
2.01
2.02
2.01
2.02
2.02
1.86
1.67
1.63
1.53
1.18
343
340
337
339
335
~
_
30 _
510
525
537
567
550
42.56
62.77
74.02
82.89
94.56
65.17
96.11
113.35
126.90
144.70
85.79
90.20
91.92
92.78
95.00
values of the slurry and the variation of the redox potential during the leaching process. Test conditions: pulp density lo%, bacterial inoculation lo%, leaching temperature 30-32"C, rotating speed of shaking table 150 rpm, and leaching period 15 d. Table 5 and Fig. 3 show that the addition of auxiliary agent 2' can not only accelerate copper leaching but can also speed up the bacteria oxidizing rate of auxiliary agent 2'. When the dosage of auxiliary agent 2' is more than 4%, after bacterial leaching for 15 d, the copper recovery may be more than 94%, and the pH value of the leaching system shows a decreasing trend. The more the dosage of auxiliary agent 2' added, the better is recovery of copper. Moreover, the amount of acid produced may be increased. However, since the time for the leaching test is less, with the increasing dosage of auxiliary agent 2', the bacterial oxidation rate with respect to the auxiliary agent 2' becomes lower. E the leaching
Wen J.K. et d.,Acid equilibrium during bioleaching of alkaline low-grade sulfide copper ore
time of bacteria is prolonged, auxiliary agent 2' will be completely oxidized. When the dosage of auxiliary agent 2' is 11.89%, the problem of acid consumption for bacterial leaching of copper ore with
685
18.30% calcite and 1.95% hematite may be completely solved and equilibrium is reached between acid production and acid consumption.
Table 5. Test results of the copper leaching rate and the produced acid for arudliary agent Dosage of auxiliaryagent 2' I %
2
4
6
8
Initial pH value Final pH value Initial potential I mV vs. SCE Final potential I mV vs. SCE Oxidation rate of auxiliary agent 2' / % Amount of sulfuric acid produced by ore per ton I kg
1.84
1.85
1.84
1.84
1.94
1.67
1.58
1.51
345
350
35 1
355
526
528
528
534
79.28
74.9
71.55
68.50
24.00
45.00
65.70
83.90
93.84
94.07
94.78
95.35
Copper leaching rate I %
5.4. Influence of ore size on copper recovery
2.1 2.0 1.9 $ 1.8 1.7
a
E
1.6 1.5
0
8 12 Leaching time I d
4
16
Fig. 3. Jnfluence of the dosage of arudliary agent 2' on
the pH values of the slurry and the redox potential.
The results of the bacterial leaching experiment of ores in diverse sizes are shown in Table 6. Experimental condition: pulp density lo%, auxiliary agent 1' dosage 5.9%, auxiliary agent 2' dosage 4.0%, inoculation amount lo%, leaching perid 15 d, leaching temperature 30-32"C,and rotating speed of shaking table 150 r/min From Table 6, it is seen that the ore size is an important factor in bacterial leaching. With the decrease in ore size, both the leaching rate and the recovery of copper increase.
Table 6. Experimental results of the influence of ore size on the copper recovery Ore size
-2 mm 100.00%
-74 pm 46.45%
Copper recovery (adding auxiliary agent 1)' I % Copper recovery (adding auxiliary agent 2"> I %
65.39
75.12
91.35
92.50
66.26
76.78
94.15
94.56
6. Conclusions (1) The test results of the ore components show that the content of pyrite oxidized for producing acid during bacterial leaching is very less. With the oxidation of main copper mineral of chalcocite by bacteria for leaching, the acid will be consumed. Moreover, the contents of carbonate and limonite in ore are high, and both components may be dissolved
-74 pn 87.60% - 4 5 p 96.50%
under acidic conditions, thereby consuming a large amount of acid. Therefore, when bacteria are used for leaching this kind of ore, the amount of acid consumed is greater than that of acid produced. (2) Based on the properties of ore for testing, the main acid-consuming minerals are calcite and hematite. After calculating in accordance with the chemical reaction equilibrium theory, the amount of sulfuric acid consumed for this alkaline low-grade copper sulfide ore per ton is 0.18167 t. During the bioleach-
RARE METALS, VoE. 2 5 No. 6,Dec 2006
686
bioleaching process, to maintain the pH value of the leaching system at 1.70-2.50, the amount of sulfuric acid actually measured for ore per ton is 0.182 t. (3) Leaching and acid production can be carried out simultaneously by the addition of auxiliary agents 1' and 2'. Therefore the acid required for the bacterial leaching of ore is produced and acid equilibrium during the bioleaching process is achieved. The added dosages of auxiliary agents 1' and 2' are 5.9% and 1139%respectively, both with the copper leaching rate more than 95%.
References Corode. L. Brierley and Tames. A. Brierley, Copper bioleaching: state-of-the-art, [in] Proceeding of the Copper99-Cober99 International Conference, USA, 1999: 59. Brierley, C.L. and Brierley, J.A., Present and future
commercial applications of biohydrometallurgy. [in] Amils, R. and Ballester, A. 4 s . Biohydrometallurgy and the Environment Toward the Miming of the 21st Century,PartA, Elsevier, Amsterdam, 1999: 81. Douglas. E. Rawlings, Biomining: Theory, Microbes and Industrial Processes. Springer Verlag, Berlin, 1997: 29. Wang D.Z., Ruan R.M.,Dong Q.H., et al., Study and progress of mineral bioextraction technologies. [in] Paper Collection on Technical Communication and Seminar for Copper-Nickel Hydrometallurgy, The Nonferrous Society of China, Beijing, 2001: 1. Karavaiko G.I., Rossi G.and Agate D.A., et al., Biogeotechnology of Metals Manual, Center for Intemational Projects GKNT, Moscow, 1988: 47. Wen J.K., Rum R.M., and Sun X.N., Study on bioleaching for Jinchuan low-grade nickel ores, Min. Metall., 2002,11(1):55.