Exploring the possibilities of biological beneficiation of...

Delvasto, P., Ballester, A., Muñoz, J.A., González, F., Blázquez, M. L., García-Balboa, C., 2005. Exploring the possibilities of biological beneficiation of iron-ores: The phosphorus problem. In: Proceedings of the 15th Steelmaking Conference, 5th Ironmaking Conference & 1st Environment and Recycling Symposium IAS (CD-ROM). Argentinean Steelmaking Institute (IAS). San Nicolás, Buenos Aires, Argentina, November 7-10, 2005. pp 71-82

Exploring the possibilities of biological beneficiation of iron-ores: The phosphorus problem

P. Delvasto1, A. Ballester1, J.A. Muñoz1, F. González1, M.L. Blázquez1, C. García-Balboa2

1 Biohidrometallurgy Research Group. Department of Materials Science and Metallurgical Engineering.

Complutense University. 28040 Madrid. Spain. 2 Department of Industrial Technology. Alfonso X “el Sabio” University. Villanueva de la Cañada, Madrid, Spain

Synopsis In past decades, preferences of international iron-ore trade markets have moved to low-phosphorus iron ores because of the well-known detrimental effects of phosphorus on steel and other iron-related materials [1, 2]. This fact has made that in most places only low-phosphorus ore is exploited making premium grade resources more scarce and leaving mines enriched in high-phosphorus ore [2]. Although it is a well established fact that these type of ores can be treated by roasting and acid-leaching processes [2, 3], traditionally low prices of iron ores make these processes not attractive from the economical standpoint. Moreover, such processes imply practices that, if not carefully considered, can be potentially deleterious for the environment. Biotechnology can bring a solution to overcome the afore mentioned problems. Since phosphorus is a major nutrient for most living forms, biological extraction of phosphorus from iron ores can become reality. Little attention has been paid to biological dephosphorization or iron ores. Some works [4, 5] have dealt with the use of fungal metabolites to attain dephosphorization, however not very high yields have been reported. More recently, phosphorus-containing blast furnace slag has been successfully leached using heterotrophic bacteria [6]. The main drawback of these investigations relies on the fact that species used for beneficiation were not related to the ore being treated. In biological beneficiation processes it is always recommendable to employ indigenous species to reduce environmental impacts. In this work, authors report the successful activation of the indigenous microflora associated to a rejected high-phosphorus iron ore from a Brazilian mine. By using the shake-flask technique, samples of the ore were amended with glucose-containing liquid medium to promote the growth of microorganisms naturally present in the ore. It was found that microflora was complex and in some cases its growth was led by fungi while in others biomass was predominantly composed by bacteria. The microflora was screened to detect species able to solubilize phosphorus using standard microbiological techniques. In such a way bacterial species of genus Burkholderia and fungus of genus Aspergillus showing phosphorus solubilization were isolated from ore microflora. It was found that in bacterial-led experiments dephosphorization attained was around 10% while in fungi-led experiments dephosphorization of the ore reached 20%. The results are discussed and an analysis of the use of the shake-flask technique in this research is criticized. Fore-coming research is also outlined.

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Delvasto, P., Ballester, A., Muñoz, J.A., González, F., Blázquez, M. L., García-Balboa, C., 2005. Exploring the possibilities of biological beneficiation of iron-ores: The phosphorus problem. In: Proceedings of the 15th Steelmaking Conference, 5th Ironmaking Conference & 1st Environment and Recycling Symposium IAS (CD-ROM). Argentinean Steelmaking Institute (IAS). San Nicolás, Buenos Aires, Argentina, November 7-10, 2005. pp 71-82 1. Literature review Two major routes have been traditionally proposed for the dephosphorization of iron ores: physical and hydrometallurgical routes [1]. Choosing one or another will strongly depend on the characteristics of the ore as well as the degree and type of association between iron minerals and phosphorus. When phosphorus is present as a product of primary mineralization, i.e. occluded phosphates in an iron oxide matrix, dephosphorization can be attained through a combination of physical processes such as mechanical liberation and flotation [1]. However this is not always the case. Some Iron ores are composed by secondary minerals produced through weathering of primary rocks. This type of ores exhibit such an intimate relationship between P and Fe-oxides that it does not result easy to elucidate the real nature and degree of association between both phases. For this type of ores, the hydrometallurgical route has been the usual approach [1, 2, 9]. Acid or alkaline leaching of previously roasted high phosphorus iron ores has been described in several research papers as well as patents, as shown on table 1. As seen on table 1, dephosphorization yields using hydrometallurgical routes range between 60% and 97%.

Table 1. Summary of conditions for several chemical treatments for dephosphorization of iron ores.

Ore composition

Roasting conditions

Roasting additives Leaching agent

Maximum removal of

P (%) Reference

Iron oxide (unspecified )

900 ºC 2 hr. LiCl Sulfuric Acid 96.4 [7]


900 ºC 2 hr. CaCl2 Nitric Acid 96.6 [7]


900 ºC 2 hr. BaCl2

Hydrochloric Acid 85.7 [7]

Hematite Goethite.

1200 ºC 1 hr. none Sulfuric Acid 68.0 [8]

Hematite Goethite

1100 ºC 1 hr. none Sodium

hydroxide 63.0 [8]

Magnetite none none Nitric Acid 75.0 [3] Magnetite Hematite Goethite

1200 ºC 1 hr. none Hydrofluoric

Acid 72.7 [9]

Magnetite Hematite Goethite

1200 ºC 1 hr. none Hydrochloric

Acid 61.8 [9]

Hematite Goethite

1250 ºC hours none Sulfuric Acid 68.7 [2]

Although this kind of processes have demonstrated high efficiency, they involve energy consuming steps as well as handling of highly dangerous substances. In addition, iron ore is a commodity of traditional low price, so the cost of additional facilities for

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Delvasto, P., Ballester, A., Muñoz, J.A., González, F., Blázquez, M. L., García-Balboa, C., 2005. Exploring the possibilities of biological beneficiation of iron-ores: The phosphorus problem. In: Proceedings of the 15th Steelmaking Conference, 5th Ironmaking Conference & 1st Environment and Recycling Symposium IAS (CD-ROM). Argentinean Steelmaking Institute (IAS). San Nicolás, Buenos Aires, Argentina, November 7-10, 2005. pp 71-82 dephosphorization in iron ore processing plants is not always justified, neither from the economical nor the environmental standpoint. This fact has made researchers to turn their attention to biological dephosphorization processes. P is a limiting nutrient for all living forms. It is a well documented fact that, under starvation conditions, microorganisms can utilize phosphorus from mineral sources such as feldspars and phosphatic rocks [10]. In fact, it has been determined that phosphorus containing minerals can be biologically solubilized easier than non-phosphorus-containing minerals [10]. This ability has been extensively investigated and applied in soil science and agriculture [11], because of its implications on the fertility of soils. The accumulation of these evidences reveals that microbiological activity can be useful to remove phosphorus from iron ores. Despite of it, few works can be found in the literature regarding to this field. An interesting work by Parks et al. [5] characterized all metabolic products generated by a fungus, Penicillium sp., while growing in contact with a high phosphorus iron ore concentrate. These researchers found that this fungus was able to produce itaconic and oxalic acids when consuming glucose as carbon source. The dephosphorization degree reported in this work reached 20%. They also tried mixtures of fungal metabolic products with HCl, finding that dephosphorization could be increased up to 50%. Buis [4], studied the dephosphorization capacity of several fungi including: Paxillus involutus, Hebeloma crustiniforme, Thelephora terrestris and Laccaria bicolor. Although the fungi studied by this researcher were able to solubilize hidroxylapatite, a heavily insoluble phosporus compound, the solubilization of phosphorus from iron ores was so poor, reaching around 1%. More recently, some researchers from India [6] have used a soil bacterium, Frateuria aurentia, to solubilize phosphorus from a LD slag (30%Ca, 20%Fe, 5% Mg, 1.4%P), a steelmaking byproduct. They report that this bacterium can solubilize between 72% and 90% of the P present in this iron slag, making this material able to be re-circulated, as fluxing material, into the blast furnace operations. 2. Materials and methods 2.1 Sample characterization The high-P iron ore employed throughout the study was the rejected fraction from a magnetic separation process. The origin of the ore is the “Jangada” mine in the region of Minas Gerais, Brazil. This ore underwent a crushing process so its size ranged between 7.0 mm and 0.03 mm with an average size of 2.3 mm. X-ray diffraction analyses revealed hematite and quartz as main mineralogical species present in the ore, with some goethite also present. X-ray fluorescence elemental chemical analysis is shown on table 2.

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Table 2. Main components (w%) of the iron ore, as obtained by elemental X-ray fluorescence chemical analysis.

Fe O Si Al Mn P Cr Ti

55,27 33,30 9,21 1,58 0,24 0,26 0,059 0,038

2.2 Isolation of the microorganisms present in the ore Since this work is focused on the phosphorus problem, microbiological techniques were used to screen microorganisms naturally present in the ore exhibiting phosphate solubilization capacity. The problem was approached applying procedures usually described for the isolation of phosphate solubilizing microorganisms (hereafter PSM) from soils [11]. Cells and spores were detached from ore surface by shaking at 150 rpm a mixture of 5 g of fresh mineral and 100 ml of sterile distilled water for 24 hours. Supernatant was serial diluted in sterile distilled water and suitable aliquots spread over Petri dishes containing a differential growth media for PSM. The growth media composition is shown on table 3.

Table 3. Composition (per liter of distilled water) of the PSM differential media employed in this study.

Component gr/l

Glucose 10.0

Ca3(PO4)2 2.5

Agar-Agar 20.0

Nutrient salts solution (ml)* 50.0

*Per liter: MgCl2·6H2O, 100 g; MgSO4·7H2O, 5 g; KCl, 4 g; (NH4)SO4, 2 g.

This isolation technique is a standard plate assay method for isolation of PSM based on the formation of a solubilized halo around the colonies of microorganisms capable to solubilize a calcium phosphate insoluble compound [12]. Inoculated plates were incubated at 30 ºC for at least 12 days. To obtain pure PSM strains, colonies were collected using a loop and streaked upon fresh medium plates. The isolates were then sent to specialized laboratories for its characterization by molecular –genetic techniques. 2.3 Shake-flask leaching experiments Two types of shake-flasks leaching tests were carried out: Bioactivation of the ore by supplying a nutrient solution directly to the fresh ore and leaching of the previously sterilized ore using a pure PSM isolate.

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Delvasto, P., Ballester, A., Muñoz, J.A., González, F., Blázquez, M. L., García-Balboa, C., 2005. Exploring the possibilities of biological beneficiation of iron-ores: The phosphorus problem. In: Proceedings of the 15th Steelmaking Conference, 5th Ironmaking Conference & 1st Environment and Recycling Symposium IAS (CD-ROM). Argentinean Steelmaking Institute (IAS). San Nicolás, Buenos Aires, Argentina, November 7-10, 2005. pp 71-82 In the first case 10 g of fresh ore with a particle size of 2 mm was added under sterile conditions into Pyrex bottles containing 100 ml of sterile glucose-based nutrient broth (per liter: glucose 10 g and 50 ml of nutrient salts solution). Up to 18 replicates of this experiment were performed and run over 70 days. pH of the broth was regularly measured. At the end of the experiment, the mineral was washed with a 5% sodium hypochlorite solution under shaking to separate the biomass, washed repeatedly with distilled water and pulverized in a ball mill. Remaining P content was determined by X-ray fluorescence spectroscopy (XRF). In the second case, 0.2 mm ore was sterilized by autoclaving to prevent interference with other microorganisms present in the ore. Using the same procedure described above, 7.5 g of the ore was added to 150 ml of glucose nutrient broth. Then the reactor was inoculated with 0.2 ml of an Aspergillus niger spore suspension. This fungus was isolated from the ore, as shown on section 3.1. The process was monitored by measuring pH as well as iron content of the leaching liquor, determined by atomic absorption spectroscopy. At the end of experiments the ore was treated similarly as for the bioactivation experiments and the final P content in the ore was determined using XRF as well. In this case all experiments were prepared by triplicate. 3. Results and discussion 3.1 Bioactivation of ore microflora As a first approach, the microflora naturally associated to the ore was activated by means of the addition of a phosphorus-free glucose-based nutrient liquid. The aim of this experiment was to force microflora to utilize the phosphorus present in the ore. The experiment revealed the complexity of the ore microflora. Several repetitions of this experiment were performed, and it was found that in 31% of the cases bacterial populations dominated exclusively the microcosms, 56% of the microcosms were dominated exclusively by fungi and 13% of the microcosms exhibited a mixed behavior. Figure 1 (a) depicts the typical in-flask and in-plate features of a microcosm dominated by fungi while figure 1(b) shows the same in the case of bacterial domination. Bacteria-dominated microcosms became turbid after 12 days of culturing. Fungi-dominated microcosms, on the other hand, generated clear supernatants and abundant fungal filaments growing intermixed with the ore particles. These two types of behavior also showed physiochemical differences, as depicted in figure 2, where the pH evolution throughout the entire experiment is shown. It can be seen that within the first 5 days of the experiment both kind of microcosms have a similar behavior. After this time, bacterial-led microcosms increased the pH values above the initial pH of the system, reaching a maximum at day 12, coinciding with the beginning of the turbidity in the supernatant. Fungi-led experiments exhibited a pH drop until day 12, for later increase softly but always below the initial pH value. At the end of the experiment, day 70th, the ore was separated and analyzed for P.

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Figure 1. Bioactivation of the iron ore with a glucose-based medium. The complexity of the ore microflora led to two main types of microbiological behaviors. In (a) Fungi as predominant species and (b) Bacteria as predominant species.

1.00

2.00

3.00

4.00

5.00

6.00

7.00

0 20 40 60 8

Time, days

Ave

rage

pH

0

Bacteria

Fungi

Figure 2. Evolution of pH in bioactivation experiments. The difference exhibited between bacterial led experiments and fungi led experiments is shown.

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Delvasto, P., Ballester, A., Muñoz, J.A., González, F., Blázquez, M. L., García-Balboa, C., 2005. Exploring the possibilities of biological beneficiation of iron-ores: The phosphorus problem. In: Proceedings of the 15th Steelmaking Conference, 5th Ironmaking Conference & 1st Environment and Recycling Symposium IAS (CD-ROM). Argentinean Steelmaking Institute (IAS). San Nicolás, Buenos Aires, Argentina, November 7-10, 2005. pp 71-82 The percentage of dephosphorization attained in the case of bacteria-led experiments was 10% while in the case of fungi-led experiments increased up to 20%. This evidences the importance of acidification for the dephosphorization process. Despite it was not measured, visual inspection of the cultures indicates that produced fungal biomass was higher than bacterial biomass. Since P is a structural component of living cells, it can be hypothesized that fungi-led microcosms can incorporate more of the mobilized P. To elucidate how this complex microflora was composed and select those microorganisms with the higher capacity to solubilize mineral phosphates, a microbiological screening procedure was performed. 3.2 Isolation of phosphate solubilizing microorganisms from the ore Phosphate solubilizing microorganisms (PSM) is a generic classification that includes several bacteria and fungi that are able to mobilize phosphorus from hardly soluble phosphorus sources, like mineral phosphates. Using a standard technique, based on the in-plate solubilization of calcium phosphate, it was found that the iron ore analyzed in this work had up to five PSM species among its indigenous microflora. The identified species are shown on table 4. In figure 3, the halo formation is depicted for three of the isolates.

Table 4. PSM isolated from the iron ore microflora

Type Identified species Halo generation

Bacterium Burkholderia sp.

+

Bacterium Clavibacter xyli +/- Bacterium Burkholderia

caribiensis +

Bacterium Burkholderia cepacia

+

Filamentous fungus Aspergillus niger

+

It must be pointed out that several other unidentified species, especially fungi (4 more) were isolated, however were not selected because of their poor or inexistent solubilization ability. It has been reported [13] that Burkholderia species produce some anti-fungal compounds. This could explain why in bacteria-led microcosms fungi are not present. On the other hand, if fungi are able to develop faster, the acidification of the liquid media, due to fungal growth, will suppress bacterial growth. An intermediate growth condition would explain the mixed behavior exhibited by 13% of the microcosms. Experiments made in liquid medium (data not shown) showed that the best phosphorus solubilizer was the isolated fungal strain Aspergillus niger. This isolate was chosen for a more complete experiment of dephosphorization of the ore.

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Figure 3. Phosphate solubilization halo (dark zones) in plate cultures of some PSM isolated from iron ore. (a) Bacterial colonies of Burkholderia sp. and (b) Burkholderia caribiensis. (c) Fungal colonies of Aspergillus niger. Scale bar: 1 mm.

3.3 Dephosphorization of the ore using isolate Aspergillus niger Due to the complexity derived from the multiple interactions between the different species present in the ore microflora, it was decided to investigate the iron ore dephosphorization using a pure isolate. In this case Aspergillus niger was chosen. It is well known the ability of this microorganism for generating different leaching substances including citric, oxalic and gluconic acid [14]. This property has been successfully used in heterotrophic leaching and beneficiation of different minerals including Ni and Co laterites, clays, Cu minerals and silicates [14]. In figure 4 can be observed that A. niger is able to mobilize slightly more than 30% of the phosphorus originally present in the ore in 21 days of treatment when using glucose as carbon source. Since there is no other P source in the nutrient solution, A. niger is obligated to scavenge the P present in the ore to continue growing. That explains the semi-sigmoidal trend exhibited by the dephosphorization percentage as a function of time. This is in agreement with the evolution of pH, figure 5, where the initial value of 5.6 rapidly decreases to a value close to 3 for then getting stabilized around 2.6. These pH values are somehow lower than those reached in the fungi-led bioactivation experiments, figure 2. This might be due to the competition established among all the fungal species present in the ore microflora. If a extremely effective acid-producing species like A. niger have a sharing-free access to nutrients, its metabolism will be evolve into an optimum condition that permits a higher acidification of the surrounding media, and as a consequence, a higher mobilization and uptake of the P occluded into the ore. An important factor that should be addressed is the leaching of the ore itself. This factor is quantified by the iron that goes into solution, expressed as percentage of iron losses in

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Delvasto, P., Ballester, A., Muñoz, J.A., González, F., Blázquez, M. L., García-Balboa, C., 2005. Exploring the possibilities of biological beneficiation of iron-ores: The phosphorus problem. In: Proceedings of the 15th Steelmaking Conference, 5th Ironmaking Conference & 1st Environment and Recycling Symposium IAS (CD-ROM). Argentinean Steelmaking Institute (IAS). San Nicolás, Buenos Aires, Argentina, November 7-10, 2005. pp 71-82 figure 5. In a hypothetical biobeneficiation process of iron ore, what is important is to leach away the impurities present in the ore, P in this particular case, with a minimum loss of the metal value of the ore i.e. iron.

0

5

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25

30

35

0 5 10 15 20 25

Time, days

Dep

hosp

horiz

atio

n ef

ficie

ncy

([ %

P in

itial

- %

P fi

nal]

/ % P

initi

al)*

100

Figure 4. Dephosphorization of iron ore (particle size 0.2 mm; initial P content 0.26 w%) with an isolate of Aspergillus niger using the shake flask technique. Pulp density 5%. The growth medium used was a phosphorus-free, glucose-based nutrient solution.

0.00

1.00

2.00

3.00

4.00

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6.00

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8.00

9.00

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rage

pH

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0.900

1.000

Iron

loss

es, w

%

Average pH

Iron losses, w%

Figure 5. Evolution of pH and iron losses during dephosphorization of iron ore (particle size 0.2 mm initial P content 0.26 w%) with an isolate of Aspergillus niger using the shake flask technique. Pulp density 5%. The growth medium used was a phosphorus-free, glucose-based nutrient solution.

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Delvasto, P., Ballester, A., Muñoz, J.A., González, F., Blázquez, M. L., García-Balboa, C., 2005. Exploring the possibilities of biological beneficiation of iron-ores: The phosphorus problem. In: Proceedings of the 15th Steelmaking Conference, 5th Ironmaking Conference & 1st Environment and Recycling Symposium IAS (CD-ROM). Argentinean Steelmaking Institute (IAS). San Nicolás, Buenos Aires, Argentina, November 7-10, 2005. pp 71-82 It is seen that iron losses increase as the time proceeds. Moreover, as iron losses increase, dephosphorization increases as well. This is a natural consequence of the fungal growth and its strategy to scavenge such an important and limiting nutrient as P is. The fungal colonization of the ore can be observed in the scanning electron microscope (SEM) image shown in figure 6. Fungal filaments, made up of fungal cells, colonize the ore surface and producing organic acids which in turn dissolve those ore phases on which phosphorus is present and not readily available to sustain fungal growth. Once these phases are attacked, P is liberated and then utilized by the fungal cells. This ore attack is indicated by the solubilization of the iron in the ore. So, to attain dephosphorization, some of the iron associated with phosphorus, or enclosing phosphatic phases, should be dissolved.

Figure 6. SEM image of Aspergillus niger filaments colonizing particles of iron ore. Some works indicate that phosphorus can be present in iron ores associated with iron oxy-hydroxide phases such as goethite [9]. Although the ore employed in this investigation has not been characterized to that extent, goethite was found to be present in the ore in little quantities. Since this phase is less stable, in the physicochemical sense, when compared to hematite the main iron phase present in the ore, the dissolution of the P-bearing goethite possibly present in the ore may explain the little iron losses found, which in any case, were not higher than 1% during the period of time studied. 4. Concluding remarks In this work, dephosphorization of iron ores using microorganisms has been proven to be possible. Bioactivation of the ore revealed the complexity of the microflora associated to the studied iron ore and conducted to two different microbiological behaviors: one supported by bacterial growth and other supported by fungal growth. Microbiological screening procedures were employed to detect those species capable to solubilize mineral phosphates. Among the isolated species, one fungus of species Aspergillus niger was

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Delvasto, P., Ballester, A., Muñoz, J.A., González, F., Blázquez, M. L., García-Balboa, C., 2005. Exploring the possibilities of biological beneficiation of iron-ores: The phosphorus problem. In: Proceedings of the 15th Steelmaking Conference, 5th Ironmaking Conference & 1st Environment and Recycling Symposium IAS (CD-ROM). Argentinean Steelmaking Institute (IAS). San Nicolás, Buenos Aires, Argentina, November 7-10, 2005. pp 71-82 used for dephosphorization of iron ore with promising results. A maximum dephosphorization degree of 30% was reached. It is, however lower than chemical based dephosphorization processes. More research should be conducted to improve the culturing conditions in order to increase the dephosphorization percentage attained and justify a possible industrial application of these findings. Acknowledgements Iron ore samples were kindly supplied by Professor Armando Corrêa de Araújo (Universidade Federal de Minas Gerais) and Minerações Brasileiras Reunidas (MBR). Financial support, in the form of Doctoral Scholarship, is also acknowledged by one of the authors (P. Delvasto) from the National Found for Science, Technology and Innovation of Venezuela (FONACIT) and the Simón Bolívar University of Venezuela. References [1] Kokal, H. (1990) “The origins of phosphorus in ironmaking raw materials and methods of removal- A review” 63th annual meeting, Minnesota section, AIME. January 1990. pp 225-257. [2] Cheng, C., Misra, V., Clough, J., Mun, R. (1999) “Dephosphorisation of western Australian iron ore by hydrometallurgical process” Minerals Engineering, 12 No. 9. pp 1083-1092. [3] Muhammed, M., Zhang, Y. (1989) “A hydrometallurgical process for the dephosphorization of iron ore” Hydrometallurgy 21. pp 277-292. [4] Buis, P. (1995) “ Bioremediation techniques for the removal of phosphorus from iron ore” PhD Dissertation thesis. Mining engineering. Michigan Technological University. (1995) [5] Parks, E., Olson, G., Brinckman, F.; Baldi, F. (1990) “Characterization by HPLC of the solubilization of phosphorus in iron ore by a fungus” J. Ind. Microbiol. 5. pp. 183-190 [6] Pradhan, N., Das, B., Acharya, S., Kar, R., Sukla, L., Misra V.(2004) “Removal of phosphorus from LD slag using heterotrophic bacteria”. Minerals and Metallurgical Processing 21 No. 3. pp 149-152 [7] Feld, I., Franklin, T., Lampking, M. (1968) “Process for removing phosphorus from iron ores” U.S. Patent No. 3,402,041. Sept. 17, 1968. [8] Gooden, J.; Walker, W.; Allen, R. (1974) “ADEMPHOS – A chemical process for dephosphorisation of iron ore”. National chemical engineering conference 1974. Process industries in Australia – Impact and growth. Surfers Paradise, Queensland, Australia, July 10 to 12th, 1974. pp 38-49. [9] Araujo, A.; Fonseca, D.; Souza, C. (1994) “Hydrometallurgical routes for the reduction of phosphorus in iron ores”. A. Sutulov Memorial Volume. Vol. III. Chemical metallurgy. IV Meeting of the Southern Hemisphere on Mineral Technology. Edited by I. Wilkomirsky, M. Sánchez and C. Hecker, Universidad de Concepción. Concepción-Chile. pp 83-92

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Delvasto, P., Ballester, A., Muñoz, J.A., González, F., Blázquez, M. L., García-Balboa, C., 2005. Exploring the possibilities of biological beneficiation of iron-ores: The phosphorus problem. In: Proceedings of the 15th Steelmaking Conference, 5th Ironmaking Conference & 1st Environment and Recycling Symposium IAS (CD-ROM). Argentinean Steelmaking Institute (IAS). San Nicolás, Buenos Aires, Argentina, November 7-10, 2005. pp 71-82 [10] Banfield, J., Barker, W., Welch, S., Taunton, A. (1999) “Biological impact on mineral dissolution: Application of the lichen model to understanding mineral weathering in the rhizosphere”. Proceedings of the national Academy of Sciences, USA. Vol. 96, pp 3404-3411. [11] Kucey, R.M.N. (1983) “Phosphate-solubilizing bacteria and fungi in various cultivated and virgin Alberta soils”. Can. J. Soil Sci. 63. pp 671-678 [12] Nautiyal, C.S. (1999) “An efficient microbiological growth medium for screening phosphate solubilizing microorganisms” FEMS microbiology letters, Vol. 170. pp 265-270 [13] Ravnskov, S., Larsen, J., Jakobsen, I. (2002) “Phosphorus uptake of an arbuscular mycorrhizal fungus is not effected by the biocontrol bacterium Burkholderia cepacia” Soil Biology and Biochemistry. Vol. 34. pp 1875-1881. [14] Jain, N., Sharma, D. (2004) “Biohydrometallurgy for nonsulfidic minerals-A review” Gemicrobiology Journal, vol. 21. pp 134-144

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