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Bioleaching of phosphorus from rock phosphate containing pyrites by Acidithiobacillus ferrooxidans
Minerals Engineering 19 (2006) 979–981 This article is also available online at: www.elsevier.com/locate/mineng
Technical note
Bioleaching of phosphorus from rock phosphate containing pyrites by Acidithiobacillus ferrooxidans R. Chi *, C. Xiao, H. Gao Wuhan Institute of Chemical Technology, Hubei Key Laboratory of Novel Chemical Reactor and Green Chemical Technology, Wuhan, Hubei 430073, PR China Received 12 April 2005; accepted 5 October 2005 Available online 18 November 2005
Abstract Pyrites can be oxidized by the bacterium Acidithiobacillus ferrooxidans (At. f.), producing H2SO4 and FeSO4. Rock phosphate is dissolved by H2SO4, forming soluble phosphorus. Fe2+ in FeSO4 is oxidized to Fe3+, producing energy to sustain the growth of At. f. The effects of four factors (rock phosphate dosage, pyrite dosage, culture temperature and time) on the fraction of phosphorous leached were investigated. It is suggested that the optimal conditions are as follows: rock phosphate dosage 1 g/L, pyrite dosage 30 g/L, culture temperature 30 °C, culture time 84 h. The fraction of phosphorous leached is up to 11.8%. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Bio-leaching; Rock phosphate; Pyrites
1. Introduction The bio-leaching of ores has been widely developed recently, and is successfully applied to copper sulfide ores, uranium ores, and for the pre-oxidation of refractory gold ores, etc. (Tang et al., 2000). Currently, the bioleaching of rock phosphate has been little reported, although several reports have indicated that some microorganisms are capable of solubilizing insoluble inorganic phosphate compounds (Goldstein, 1986; Bosecker, 1997). Since these microorganisms are heterotrophic, it is generally accepted that the major mechanism of mineral phosphate leaching is by the action of organic acids synthesized by the solubilization of organic carbon sources in soil microorganisms (Leyval and Berthelin, 1989; Salih et al., 1989; Halder et al., 1990), and the fraction of phosphorous leached from rock phosphate is relatively slow (Zhang et al., 2000). The bacterium Acidithiobacillus ferrooxidans (At. f.) is commonly used in bio-leaching, and recently in the biobeneficiation of sulfide minerals such as pyrites (Kelly *
Corresponding author. Tel./fax: +86 27 87194500. E-mail address: [email protected] (R. Chi).
0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.10.003
and Wood, 2000). Being an autotroph it utilizes atmospheric CO2 as its sole carbon source (Silver, 1970). Its energy is obtained from chemical reactions, oxidizing minerals or inorganic solutes such as S0 and FeS2 rather than organic carbon compounds, and producing inorganic acids such as H2SO4. Its ability to oxidize Fe2+ to Fe3+ in acidic solution is well known. Although bio-leaching of low-grade sulfide minerals by At. f. is successfully applied in mineral processing (Kawatra and Eisele, 1999; Somasundaran et al., 2000), the use of At. f. for leaching soluble phosphorus from rock phosphate has not been reported. In this work, an attempt is made to use At. f. as a bio-oxidation agent of pyrites, and to use the H2SO4 generated to dissolve the rock phosphate. 2. Materials and methods 2.1. The strain and rock phosphate The strain of At. f. was kindly supplied by the Central South University (Changsha, PR China). Rock phosphate (mostly calcium phosphate of particle size about 0.07– 0.10 mm) came from Baokang phosphorite (Hubei, PR
R. Chi et al. / Minerals Engineering 19 (2006) 979–981
2.2. Culture medium At. f. was grown in a modified 9 K medium: 3 g/l (NH4)2SO4, 0.5 g/l MgSO4 Æ 7 H2O, 0.5 g/l K2HPO4, 0.1 g/l KCl and 0.01 g/l Ca (NO3)2, 44.2 g/l FeSO4 Æ 7 H2O. pH was adjusted to 2.5 with 1:1 H2SO4. 2.3. Experimental and analytical methods A proper volume of inocula of active At. f. cells was inoculated into 30 ml of 9 K medium in a 100 ml flask, with some rock phosphate and pyrites, in a rotary shaker at 160 r/min. The medium was centrifuged at 9000 r/min for 15 min, the supernatant was collected and the content of P2O5 was determined using the phosphomolybate method with a UV–Vis 8500 spectrophotometer at 420 nm. 3. Results and discussion Based on Section 2.3, equal volumes of culture medium in five flasks were inoculated with 10 ml solution of At. f. cells. The effects of dosage of rock phosphate, dosage of pyrites, culture temperature and time on the fraction of phosphorous leached were determined. After 48 h (96 h in Section 3.4), the fraction of phosphorous leached in each flask was examined. The experiments were repeated three times and the results are shown in Figs. 1–4. 3.1. Effect of the dosage of rock phosphate on the fraction of phosphorous leached
8.0
Fraction of phosphorous leached (%)
China), with CaO and P2O5 contents of 49.8% and 27.5%, respectively. The content of S in the pyrites (particle size about 0.07–0.01 mm) was 48%.
7.5 7.0 6.5
Rock phospate: 10g/l Temperature: 30˚C Initial pH: 2.5
6.0 5.5 5.0 0
10
20
30
40
50
Dosage of pyrites (g/l) Fig. 2. Effect of pyrites dosage on the fraction of phosphorous leached (means ± SE, n = 3).
Fraction of phosphorous leached (%)
980
9
8
7
Rock phosphate: 10g/l Pyrites: 10g/l Initial pH: 2.5
6
5 10
Fig. 1 shows that, with increasing dosage of rock phosphate, the fraction of phosphorous leached decreases shar-
20
30
40
Culture temperature Fig. 3. Effect of culture temperature on the fraction of phosphorous leached (means ± SE, n = 3).
Fraction of phosphorous leached (%)
14 12
Pyrites: 10g/l Temperature: 30˚C Initial pH: 2.5
10 8
ply, this reaction being controlled by the dynamics of bioleaching. Theoretically, in a certain range, the higher the dosage of rock phosphate the easier is the leaching reaction and so more P2O5 transforms, but the fraction of phosphorous leached decreases. This indicates that the fraction of phosphorous which can be leached is extremely limited. 3.2. Effect of the dosage of pyrites on the fraction of phosphorous leached
6 4 2 0
10
20
30
40
50
Dosage of rock phosphate (g/L) Fig. 1. Effect of rock phosphate dosage on the fraction of phosphorous leached (means ± SE, n = 3).
As can be seen from Fig. 2, the fraction of phosphorous leached increases gradually with increasing dosage of pyrites, indicating that abundant pyrites can supply enough energy for At. f. and produce H2SO4 to transform rock phosphate to soluble phosphorus. However, when the dosage of pyrites exceeds 30 g/l, the fraction of phosphorous leached begins to decrease, implying that excessive pyrites
R. Chi et al. / Minerals Engineering 19 (2006) 979–981
H2 SO4 + Ca3 (PO4 )2 !3CaSO4 + 2H3 PO4
10
Fraction of phosphorous leached (%)
981
9
When pyrites act as substrates, At. f. are used to leach the soluble phosphorus which can be absorbed by plants, so this is feasible and significant work. However, further study is still needed in order to successfully apply it.
8 7 6 5
Rock phosphate: 10g/l Pyrites: 10g/l Temperature: 30˚C Initial pH: 2.5
4 3 2 1 0 0
20
40
60
80
100
Culture time (h) Fig. 4. Effect of culture time on the fraction of phosphorous leached (means ± SE, n = 3).
may inhibit the contact of the rock phosphate and solvent, thus restricting the phosphorous leached. 3.3. Effect of culture temperature on the fraction of phosphorous leached Fig. 3 shows that the fraction of phosphorous leached is dissimilar under different culture temperatures. The optimum culture temperature is 30 °C. However, it can also be seen from Fig. 3 that when the culture temperature is too high, the leaching process is inhibited. When the culture temperature changes from 30 to 44 °C, the fraction of phosphorous leached sharply decreases. 3.4. Effect of culture time on the fraction of phosphorous leached As can be seen from Fig. 4, the fraction of phosphorous leached varies with culture time. With increasing culture time, the fraction of phosphorous leached increases rapidly. After 84 h, the fraction of phosphorous leached hardly increases. At. f. can oxidize pyrites to form H2SO4 and FeSO4 when water and the atmosphere are present, and Fe2+ is further oxidized to Fe3+. Under the action of At. f., the Fe3+ can oxidize pyrites to sulfur, and then the sulfur can also be oxidized to H2SO4. Finally, the pyrites are oxidized to H2SO4 and FeSO4, which act as a solvent and energy source for the bio-leaching process. The reactions are as follows 2FeS2 + 7O2 + 2H2 O !2H2 SO4 + 2FeSO4 2H2 SO4 + 4FeSO4 + O2 !2Fe2 (SO4 )3 + 2H2 O FeS2 + Fe2 (SO4 )3 !3FeSO4 + 2S 2S+3O2 + 2H2 O !2H2 SO4
4. Conclusions Pyrites are oxidized by At. f., and produce H2SO4 and FeSO4. The rock phosphate is dissolved by H2SO4, forming soluble phosphorus compounds. Fe2+ from the FeSO4 is oxidized to Fe3+, producing energy for the growth of At. f. In this process acidic conditions are formed in the culture medium, as H2SO4 is produced in the reaction. This benefits the growth of the At. f., aiding the continuous oxidation of pyrites and the leaching of the phosphorus from the rock phosphate. Acknowledgements Financial support for this work by the Key Project Foundation of the Education Department(Z200515002) in Hubei Province, PR China, Hubei Key Laboratory of Novel Chemical Reactor and Green Chemical Technology, PR China, is gratefully acknowledged. References Bosecker, K., 1997. Biotransformation: metal solubilization by microorganisms. FEMS Microbiol. Rev. 20, 591–604. Goldstein, A.H., 1986. Bacterial solubilization of mineral phosphates: historical perspective and future prospects. Am. J. Altern. Agri. 1, 51– 57. Halder, A.K., Mishra, A.K., Bhattacharyya, P., Chakrabartty, P.K., 1990. Solubilization of rock phosphate by Rhizobium and Bradyrhizobium. J. Gen. Appl. Microbiol. 36, 81–92. Kawatra, S.K., Eisele, T.C., 1999. Depression of pyrites by yeast and bacteria. Miner. Metall. Process. 16 (4), 1–5. Kelly, D.P., Wood, A.P., 2000. Reclassification of some species of Thiobacillus to the newly designated genera Acidithiobacillus gen. nov., Halothiobacillus gen. nov. and Thermithiobacillus gen. nov. Int. J. Syst. Evol. Microbiol. 50, 511–516. Leyval, C., Berthelin, J., 1989. Interaction between Laccaria laccata, Agrobacterium radiobacter and beech roots: influence on P, K, Mg and Fe movilization from minerals and plant growth. Plant Soil 117, 103– 110. Salih, H.M., Yahya, A.Y., Abdul-Rahem, A.M., Munam, B.H., 1989. Availability of phosphorus in a calcareous soil treated with rock phosphate or superphosphate as affected by phosphate dissolving fungi. Plant Soil 120, 181–185. Silver, M., 1970. Oxidation of elemental sulphur and sulphur compounds and CO2 fixation by Ferrobacillus ferrooxidans (Thiobacillus ferrooxidans). Can. J. Microbiol. 16, 845–849. Somasundaran, P.K., Deo, N., Natarajan, K.A., 2000. Utility of bioreagents in mineral processing. Miner. Metall. Process. 17 (2), 112–115. Tang, Y., Gui, B.W., Liu, Q.J., 2000. Experimental study on bacterial transformation. Non-Ferrous Min. Metall. 16 (6), 23–28 (In Chinese). Zhang, Y.K., Wang, A., Chen, M.C., 2000. Fundamental research on the dissolution of phosphate rock by microorganisms. Multipurpose Utilization of Mineral Resource 12 (6), 32–34 (in Chinese).
Technical note
Bioleaching of phosphorus from rock phosphate containing pyrites by Acidithiobacillus ferrooxidans R. Chi *, C. Xiao, H. Gao Wuhan Institute of Chemical Technology, Hubei Key Laboratory of Novel Chemical Reactor and Green Chemical Technology, Wuhan, Hubei 430073, PR China Received 12 April 2005; accepted 5 October 2005 Available online 18 November 2005
Abstract Pyrites can be oxidized by the bacterium Acidithiobacillus ferrooxidans (At. f.), producing H2SO4 and FeSO4. Rock phosphate is dissolved by H2SO4, forming soluble phosphorus. Fe2+ in FeSO4 is oxidized to Fe3+, producing energy to sustain the growth of At. f. The effects of four factors (rock phosphate dosage, pyrite dosage, culture temperature and time) on the fraction of phosphorous leached were investigated. It is suggested that the optimal conditions are as follows: rock phosphate dosage 1 g/L, pyrite dosage 30 g/L, culture temperature 30 °C, culture time 84 h. The fraction of phosphorous leached is up to 11.8%. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Bio-leaching; Rock phosphate; Pyrites
1. Introduction The bio-leaching of ores has been widely developed recently, and is successfully applied to copper sulfide ores, uranium ores, and for the pre-oxidation of refractory gold ores, etc. (Tang et al., 2000). Currently, the bioleaching of rock phosphate has been little reported, although several reports have indicated that some microorganisms are capable of solubilizing insoluble inorganic phosphate compounds (Goldstein, 1986; Bosecker, 1997). Since these microorganisms are heterotrophic, it is generally accepted that the major mechanism of mineral phosphate leaching is by the action of organic acids synthesized by the solubilization of organic carbon sources in soil microorganisms (Leyval and Berthelin, 1989; Salih et al., 1989; Halder et al., 1990), and the fraction of phosphorous leached from rock phosphate is relatively slow (Zhang et al., 2000). The bacterium Acidithiobacillus ferrooxidans (At. f.) is commonly used in bio-leaching, and recently in the biobeneficiation of sulfide minerals such as pyrites (Kelly *
Corresponding author. Tel./fax: +86 27 87194500. E-mail address: [email protected] (R. Chi).
0892-6875/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.10.003
and Wood, 2000). Being an autotroph it utilizes atmospheric CO2 as its sole carbon source (Silver, 1970). Its energy is obtained from chemical reactions, oxidizing minerals or inorganic solutes such as S0 and FeS2 rather than organic carbon compounds, and producing inorganic acids such as H2SO4. Its ability to oxidize Fe2+ to Fe3+ in acidic solution is well known. Although bio-leaching of low-grade sulfide minerals by At. f. is successfully applied in mineral processing (Kawatra and Eisele, 1999; Somasundaran et al., 2000), the use of At. f. for leaching soluble phosphorus from rock phosphate has not been reported. In this work, an attempt is made to use At. f. as a bio-oxidation agent of pyrites, and to use the H2SO4 generated to dissolve the rock phosphate. 2. Materials and methods 2.1. The strain and rock phosphate The strain of At. f. was kindly supplied by the Central South University (Changsha, PR China). Rock phosphate (mostly calcium phosphate of particle size about 0.07– 0.10 mm) came from Baokang phosphorite (Hubei, PR
R. Chi et al. / Minerals Engineering 19 (2006) 979–981
2.2. Culture medium At. f. was grown in a modified 9 K medium: 3 g/l (NH4)2SO4, 0.5 g/l MgSO4 Æ 7 H2O, 0.5 g/l K2HPO4, 0.1 g/l KCl and 0.01 g/l Ca (NO3)2, 44.2 g/l FeSO4 Æ 7 H2O. pH was adjusted to 2.5 with 1:1 H2SO4. 2.3. Experimental and analytical methods A proper volume of inocula of active At. f. cells was inoculated into 30 ml of 9 K medium in a 100 ml flask, with some rock phosphate and pyrites, in a rotary shaker at 160 r/min. The medium was centrifuged at 9000 r/min for 15 min, the supernatant was collected and the content of P2O5 was determined using the phosphomolybate method with a UV–Vis 8500 spectrophotometer at 420 nm. 3. Results and discussion Based on Section 2.3, equal volumes of culture medium in five flasks were inoculated with 10 ml solution of At. f. cells. The effects of dosage of rock phosphate, dosage of pyrites, culture temperature and time on the fraction of phosphorous leached were determined. After 48 h (96 h in Section 3.4), the fraction of phosphorous leached in each flask was examined. The experiments were repeated three times and the results are shown in Figs. 1–4. 3.1. Effect of the dosage of rock phosphate on the fraction of phosphorous leached
8.0
Fraction of phosphorous leached (%)
China), with CaO and P2O5 contents of 49.8% and 27.5%, respectively. The content of S in the pyrites (particle size about 0.07–0.01 mm) was 48%.
7.5 7.0 6.5
Rock phospate: 10g/l Temperature: 30˚C Initial pH: 2.5
6.0 5.5 5.0 0
10
20
30
40
50
Dosage of pyrites (g/l) Fig. 2. Effect of pyrites dosage on the fraction of phosphorous leached (means ± SE, n = 3).
Fraction of phosphorous leached (%)
980
9
8
7
Rock phosphate: 10g/l Pyrites: 10g/l Initial pH: 2.5
6
5 10
Fig. 1 shows that, with increasing dosage of rock phosphate, the fraction of phosphorous leached decreases shar-
20
30
40
Culture temperature Fig. 3. Effect of culture temperature on the fraction of phosphorous leached (means ± SE, n = 3).
Fraction of phosphorous leached (%)
14 12
Pyrites: 10g/l Temperature: 30˚C Initial pH: 2.5
10 8
ply, this reaction being controlled by the dynamics of bioleaching. Theoretically, in a certain range, the higher the dosage of rock phosphate the easier is the leaching reaction and so more P2O5 transforms, but the fraction of phosphorous leached decreases. This indicates that the fraction of phosphorous which can be leached is extremely limited. 3.2. Effect of the dosage of pyrites on the fraction of phosphorous leached
6 4 2 0
10
20
30
40
50
Dosage of rock phosphate (g/L) Fig. 1. Effect of rock phosphate dosage on the fraction of phosphorous leached (means ± SE, n = 3).
As can be seen from Fig. 2, the fraction of phosphorous leached increases gradually with increasing dosage of pyrites, indicating that abundant pyrites can supply enough energy for At. f. and produce H2SO4 to transform rock phosphate to soluble phosphorus. However, when the dosage of pyrites exceeds 30 g/l, the fraction of phosphorous leached begins to decrease, implying that excessive pyrites
R. Chi et al. / Minerals Engineering 19 (2006) 979–981
H2 SO4 + Ca3 (PO4 )2 !3CaSO4 + 2H3 PO4
10
Fraction of phosphorous leached (%)
981
9
When pyrites act as substrates, At. f. are used to leach the soluble phosphorus which can be absorbed by plants, so this is feasible and significant work. However, further study is still needed in order to successfully apply it.
8 7 6 5
Rock phosphate: 10g/l Pyrites: 10g/l Temperature: 30˚C Initial pH: 2.5
4 3 2 1 0 0
20
40
60
80
100
Culture time (h) Fig. 4. Effect of culture time on the fraction of phosphorous leached (means ± SE, n = 3).
may inhibit the contact of the rock phosphate and solvent, thus restricting the phosphorous leached. 3.3. Effect of culture temperature on the fraction of phosphorous leached Fig. 3 shows that the fraction of phosphorous leached is dissimilar under different culture temperatures. The optimum culture temperature is 30 °C. However, it can also be seen from Fig. 3 that when the culture temperature is too high, the leaching process is inhibited. When the culture temperature changes from 30 to 44 °C, the fraction of phosphorous leached sharply decreases. 3.4. Effect of culture time on the fraction of phosphorous leached As can be seen from Fig. 4, the fraction of phosphorous leached varies with culture time. With increasing culture time, the fraction of phosphorous leached increases rapidly. After 84 h, the fraction of phosphorous leached hardly increases. At. f. can oxidize pyrites to form H2SO4 and FeSO4 when water and the atmosphere are present, and Fe2+ is further oxidized to Fe3+. Under the action of At. f., the Fe3+ can oxidize pyrites to sulfur, and then the sulfur can also be oxidized to H2SO4. Finally, the pyrites are oxidized to H2SO4 and FeSO4, which act as a solvent and energy source for the bio-leaching process. The reactions are as follows 2FeS2 + 7O2 + 2H2 O !2H2 SO4 + 2FeSO4 2H2 SO4 + 4FeSO4 + O2 !2Fe2 (SO4 )3 + 2H2 O FeS2 + Fe2 (SO4 )3 !3FeSO4 + 2S 2S+3O2 + 2H2 O !2H2 SO4
4. Conclusions Pyrites are oxidized by At. f., and produce H2SO4 and FeSO4. The rock phosphate is dissolved by H2SO4, forming soluble phosphorus compounds. Fe2+ from the FeSO4 is oxidized to Fe3+, producing energy for the growth of At. f. In this process acidic conditions are formed in the culture medium, as H2SO4 is produced in the reaction. This benefits the growth of the At. f., aiding the continuous oxidation of pyrites and the leaching of the phosphorus from the rock phosphate. Acknowledgements Financial support for this work by the Key Project Foundation of the Education Department(Z200515002) in Hubei Province, PR China, Hubei Key Laboratory of Novel Chemical Reactor and Green Chemical Technology, PR China, is gratefully acknowledged. References Bosecker, K., 1997. Biotransformation: metal solubilization by microorganisms. FEMS Microbiol. Rev. 20, 591–604. Goldstein, A.H., 1986. Bacterial solubilization of mineral phosphates: historical perspective and future prospects. Am. J. Altern. Agri. 1, 51– 57. Halder, A.K., Mishra, A.K., Bhattacharyya, P., Chakrabartty, P.K., 1990. Solubilization of rock phosphate by Rhizobium and Bradyrhizobium. J. Gen. Appl. Microbiol. 36, 81–92. Kawatra, S.K., Eisele, T.C., 1999. Depression of pyrites by yeast and bacteria. Miner. Metall. Process. 16 (4), 1–5. Kelly, D.P., Wood, A.P., 2000. Reclassification of some species of Thiobacillus to the newly designated genera Acidithiobacillus gen. nov., Halothiobacillus gen. nov. and Thermithiobacillus gen. nov. Int. J. Syst. Evol. Microbiol. 50, 511–516. Leyval, C., Berthelin, J., 1989. Interaction between Laccaria laccata, Agrobacterium radiobacter and beech roots: influence on P, K, Mg and Fe movilization from minerals and plant growth. Plant Soil 117, 103– 110. Salih, H.M., Yahya, A.Y., Abdul-Rahem, A.M., Munam, B.H., 1989. Availability of phosphorus in a calcareous soil treated with rock phosphate or superphosphate as affected by phosphate dissolving fungi. Plant Soil 120, 181–185. Silver, M., 1970. Oxidation of elemental sulphur and sulphur compounds and CO2 fixation by Ferrobacillus ferrooxidans (Thiobacillus ferrooxidans). Can. J. Microbiol. 16, 845–849. Somasundaran, P.K., Deo, N., Natarajan, K.A., 2000. Utility of bioreagents in mineral processing. Miner. Metall. Process. 17 (2), 112–115. Tang, Y., Gui, B.W., Liu, Q.J., 2000. Experimental study on bacterial transformation. Non-Ferrous Min. Metall. 16 (6), 23–28 (In Chinese). Zhang, Y.K., Wang, A., Chen, M.C., 2000. Fundamental research on the dissolution of phosphate rock by microorganisms. Multipurpose Utilization of Mineral Resource 12 (6), 32–34 (in Chinese).