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Microorganisms as chemical reagents: The hematite system
Minerals Engineering, Vol. 5, Nos. 3-5, pp. 547-556, 1992
0892-6875/92 $5.00+0.00 © 1992 Pergamon Press plc
Printed in Great Britain
MICROORGANISMS AS CHEMICAL REAGENTS: THE HEMATITE SYSTEM
J. DUBEL §, R. W. SMITHI, M. MISRAt and S. CHENt § Department of Chemical Engineering, Technical University of Karlsruhe, Karlsruhe, Germany t Department of Chemical and Metallurgical Engineering, Mackay School of Mines, University of Nevada, Reno, Nevada, USA
ABSTRACT Microorganisms can function as flocculating agents, flotation modifiers and flotation collectors. Thus, they can take the place of chemical reagents in mineral processing and hydrometallurgy operations and, therefore, can be themselves considered as reagents. For example, a microorganism should function somewhat as a chemical flocculant if it is to some degree hydrophobic and can attach itself in some manner, such as though electrostatic interaction, to the fine mineral particles to be flocculated. In addition, hydrophobic microorganisms should render a hydrophilic mineral surface somewhat hydrophobic i f they attach themselves to the mineral surface. A previously nonfloatable mineral can in this manner be rendered floatable. The present paper investigates the principles of the use of a hydrophobic, highly negatively charged, bacterium, Mvcobacterium Dhlei. as a flocculating agent for finely divided hematite suspensions and as a flotation collector for hematite. Keywords Microorganisms, minerals, flocculation, flotation. INTRODUCTION Previous work [1-5] has shown that Mycobacterium phlei is both highly hydrophobic (contact angle 65-70 °) and highly negatively charged with an isoelectric point (iep) at about pH 2.0 - 2.5. It should, thus, readily adsorb onto many hydrophilic mineral surfaces if the mineral is of low negative, neutral or positive charge. The adhesion could also be promoted by a hydrophobic mineral surface [1,2]. Where both the organism and the mineral are negatively charged the adhesion can be explained based on DLVO theory [1-5]. Experimental work on the flocculation of phosphate slimes, dolomite, clay minerals, and hematite has been performed. Some of this work has been reported on elsewhere [4,5]. It was found that M. phlei functions as an excellent flocculant for a dolomitic phosphate slime and hematite but does not function well as a flocculant for mineral suspensions where large quantities of the solids are finer in size than the approximately 1/~m size of the bacterium. Since the bacterium has an rod/ellipsoid/coccal shape (the organism is pleomorphic and can assume different forms depending on growth conditions), rather than a long chain structure such as possessed by synthetic polymeric flocculants, certain of its flocculation characteristics are markedly different from those of the polymeric flocculants. While both the bacterium and long chain polymers in part flocculate mineral particles by bridging between mineral particles, the entwining characteristics of the long chain molecules are absent in the case of M. phleL Thus, hydrophobic interactions between adhering microorganisms on adjacent solid particles are of greater importance in the formation of 547
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mineral aggregations. One consequence of this mechanism of aggregation is that the aggregations are often very "tight" and dense and contain little water. If the minerals being flocculated are, however, themselves somewhat hydrophobic (such as, for example, many coal minerals) the aggregations may not be so dense [4,5]. That very dense aggregations are formed in the flocculation of hematite by the microorganism is particularly evident. It is also likely that a reduction of positive electrostatic charge on the mineral by interaction with the negatively charged microorganism cells contributes to the aggregation of the hematite particles. Since M. phlei is a highly hydrophobic and negatively charged organism it should readily adhere to a more positively charged solid such as hematite over a wide pH range. It, thus, should also render the mineral floatable over this wide pH range. EXPERIMENTAL The Mycobacterium phlei used in the experimentation was grown from cultures obtained from Carolina Biological Supply Company. Several different media were used initially to culture the bacterium. The medium selected to culture the bacteria for experimentation was one suggested by Pratt [6] and Guirard and Snell [7] because of its simplicity and the rapid growth of the organism in the medium. The composition of the medium was (in gin/din 3 water): glucose 10, beef extract 1, yeast extract 1 and enzyme-hydrolysed casein 1. Culturing was done in 250 ml flasks continuously shaken in a water shaker bath held at 35 °C. HCI and NaOH used for pH adjustment in various studies was of reagent grade. The hematite used was a specular hematite from Republic, Michigan. Spectrographic analysis of the mineral indicated that it was of high purity [8]. It was prepared for experimentation by first crushing to - 2.36 mm followed by washing in hydrochloric acid solution. The material was then ground in a porcelain mortar and pestle to the desired size. The electrokinetic measurements were made using a Komline-Sanderson zeta reader in a conventional manner. A 2 ml sample of the bacteria or hematite solution was diluted with distilled water just prior to measurement and the pH adjusted with either HC1 or NaOH. The flotation tests were performed in a modified Hallimond tube with a long flotation column. Before flotation a 1 gm, - 20 or - 53, + 20/~m, sample of hematite was reacted for 10 min with a M. phlei suspension and placed in the Hallimond tube. Flotation was then started and carried on for 3 or 10 min depending on the test. Contact angle measurements were made on the hematite surface in the presence of M. phlei. The measurements were made by the sessile drop method in which a drop of M. phlei aqueous suspension at a particular pH value and microorganism concentration was placed on a smooth, clean hematite surface. A Rame-Hart model A-100 contact angle goniometer was used in the measurements. Sedimentation measurements were made by first mixing 1000 ml of the suspensions to be studied in controlled stirring vessels (HS-4 stirrer, Phipps and Bird). To provide good mixing, bacteria were added to a pH adjusted hematite suspension (-20 #m hematite) at an impeller speed of 250 rpm. After 2 min stirring the stirrer speed was reduced to 30 rpm for 5 min for floe growth. The suspension was transferred to a modified Andreasen pipette. Samples were taken at a 4 cm height from the bottom of the pipette. The solid concentration was determined gravimetrically. Filtration studies were performed using the pressure filtration device depicted in figure 1. A paper filter was placed in the position noted (held in place by a plate with holes in it). The lower part of the apparatus was screwed upward to bring the apparatus together. The suspension to be studied was placed in the upper part of the device and the top sealed. The proper pressure was then applied and the results determined as a function of time, tl. through measuring the weight of filtrate produced with time. The light was simply a small
M i c r o o r g a n i s m s as chemical reagents
549
bulb that could be inserted into the top of the device (after removing the pressure and the top seal). The purpose was to observe the character of the filter cake and when its formation was complete. Subsequent to the final formation of the filter cake, pressure was reintroduced to the vessel and the pressure dewatering of the cake determined as a function of time, t z.
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Fig.l Pressure filter apparatus. EXPERIMENTAL RESULTS Figure 2 illustrates the zeta potential of M. phlei and hematite as a function of pH. It is to be noted that the zeta potential of M. phlei is much more negative than that of hematite, the isoelectric point (iep) of the former being at about pH 2 and of hematite at about pH 5.5. It should be noted, however, that iep's and points of zero charge (pzc's) of various hematites have been measured from about pH 3 to pH 8+. Nevertheless, the present data indicate that there should be a strong coulombic interaction between M. phlei and hematite. The light micrograph of figure 3 from Smith et al. [4] confirms the strong interaction between the organism and the mineral leading to the obvious attachment of the two entities to each other and the aggregation of the hematite particles. Figure 4 from Smith et al. [4] shows sedimentation data on fine (- 20 #m) hematite in the presence and absence of M. phlei. It can be readily seen that the settling is much more rapid in the presence of M. phlei, the material substantially settled after 4 rain while the same amount of settling was not achieved after 30 rain in the absence of the organism. As a follow up on this experimentation the hematite in the presence and absence of M. phlei was subjected to pressure filtration (2 bar pressure). Initial suspension volume was 100 ml and initial solids concentration was 18 wt %. Figure 5 shows results obtained of filtrate volume (V F) as a function of time (tl). The results show the increased filterability of the hematite after treatment with the microorganism.
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Fig.3 Photo micrographs of fine hematite in the presence and absence of Mycobacterium phlei; from Smith et al. [4].
Microorganisms as chemical reagents
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Fig.4 Sedimentation of a 2 wt. % suspension of hematite in the presence and absence of 10.2 mg Mycobacterium phlei/kg solids, % solids measurement made I0 cm down in an Andreasen pipette; from Smith et al. [4].
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[sec] Fig.5 Filtrate volume recovered as a function of time after application of pressure to pressure filtration of hematite suspensions in the presence and absence of Mycobacterium phlei, 2 bar pressure. Figure 6 from Smith and Misra [5] shows the Hallimond tube flotation of - 20/~m hematite as a function of M. phlei concentration at pH 7 + 0.5 and a 10 min flotation time. It was noted from visual observation that the drop in flotation at the higher organism concentration is apparently due to the very large hematite floes formed that can not be levitated by the gas bubbles.
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Fig.6 Hallimond tube flotation recovery of - 20/~m hematite as a function of Mycobacterium phlei concentration, pH 7 -+ 0.5, 10 min flotation time; from Smith and Misra [5]. When - 53, + 20 #m hematite was studied and a flotation time of 3 min used the data depicted in figure 7 were obtained. The figure shows the flotation recovery of hematite as a function of organism concentration, again at pH 7 +_0.5. The same limit on flotation with increasing M. phlei concentration was not observed in this case, probably because of the larger size of the hematite and consequent fewer hematite particles per unit weight of hematite.
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Fig.7 HaUimond tube flotation recovery of - 53, + 20 #m hematite as a function of Mycobacterium phlei concentration, pH 7 +_ 0.5, 3 rain flotation time. Figure 8 illustrates the contact angle on hematite as a function of M. phlei concentration at p H 7 + 0.5. Contact angle increases with increasing organism concentration up to about 40 ppm M. phlei. Above this concentration the contact angle on hematite remains sensibly constant at about 46 ° at least up to an organism concentration of 145 ppm.
Microorganisms as chemical reagents
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Concentration of M__.phlei (ppm) Fig.8 Contact angle on hematite in the hematite - Mycobacterium phlei air system as a function of M. phlei concentration, pH 7 ± 0.5. Figure 9 depicts the flotation of - 53, + 20 #m hematite as a function of pH in the presence of 75 ppm M. phlei (3 min flotation time). It is to be noted that flotation is greatest at about pH 2.5 and drops from about 78% at this oH value to about 50 % near pH 4. The recovery then remains essentially constant as a function of pH to a pH value more basic than pH 10. .
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F l o c c u l a t i o n and filtration
Figures 3 and 4 clearly demonstrate that such an adhesion takes place resulting in aggregation of the hematite particles and enhanced settling of a fine hematite suspension. There appears to b e , from observation of the micrographs of figure 3, bridging of particles by the bacteria cells. The aggregation of the particles after accumulation of the bacterium must be through the hydrophobic interaction between the now hydrophobic bacterial surface of the hematite particles plus reduction in the magnitude of charge on the hematite. The bacterial aggregation of the particles also enhances the filtration of a fine hematite suspensions as is demonstrated by the data of figure 5. In the figure the dashed line represents the approximate completion of a filter cake on the filter (cake height 4 mm). Considering the apparatus used (figure 1), by means of the applied pressure difference the filtrate is forced through the apparatus and the solids are retained on the filter paper, in the position noted, to form a filter cake. It can be seen, then, that the time required for cake formation under conditions studied is reduced from about 6 1/3 min to 4 rain when the microorganism is present. Upon completion of the cake, drying was continued by retaining (or reintroducing) the 2 bar pressure in the system. It was then possible to determine the residual amount of moisture in the cake as a function of time. Figure i0 illustrates the type of data obtained of weight percentage residual moisture remaining (RF) as a function of time after the start of the pressure cake dewatering operation (t2). The presence of the microorganism obviously also enhances this operation. It is also to be noted that the minimum dewatering possible with or without M. phlei is to about 18 wt % residual moisture.
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Fig.l 0 Residual moisture in the hematite filter cake as a function of time after start of dewatering of the cake in the presence and absence of Mycobacteriurn phlei, - 20 #m hematite, 2 bar pressure. Considering figure 5 again, the data can be rearranged making use of the following expressions [9]: t l / V F = (slope x VF) + intercept or
t l / V F = {#lcr¢/(2A2Zxp)}VF + intercept
(1)
(2)
where: t 1 is time after application of pressure, V F is filtrate volume, # is viscosity of water, r e is filter cake resistance, A is filter area, Ap is pressure drop across the filter and
Microorganisms as chemical reagents
=
555
(ms/mF)p 1 / [(1-¢)Ps-¢(ms/mF)Pl]
where: ¢ is the porosity of the cake, PF is the density of water, Ps is the density of the solids, m s is the mass of the solids and m E is the mass of the filtrate. These expressions indicate that if t l / V F is plotted as a function of V F the slope of the straight line curve generated should yield the expression from which filter cake resistance (re) can be calculated:
slope = #~r c / (2A2Ap) Thus, the curves of figure 11 were plotted from the data obtained. It can be noted that straight lines are generated from which the slopes can be calculated and the filtration resistance, re, determined. For the non-flocculated hematite in the absence of M. phlei the filtration resistance was calculated to be about 9.4 x 1014 m "2 while for the flocculated hematite in the presence of M. phlei the value was determined to be 5.3 x 1014 m "z, a reduction of about 44%. 4
3
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40
60
80
100
Vr [mi] Fig.l I Time (after initial application of pressure)/filtrate volume recovered (tl/V F) as a function of filtrate volume recovered (VF) in the presence and absence of Mycobacterium phlei. The slopes are proportional to the filtration resistance (re). Flotation
Since electrophoretic mobility measurements indicate that the hematite is much less negatively charged than M. phlei and that the organism has a hydrophobic surface it is not surprising that it readily functions as a collector for hematite. Figure 8 shows that, indeed, finite contact angles are obtained on hematite in a hematite - M. phlei suspension - air system. Further, as shown by the data of figure 7, when the hematite particles are larger than 20/~m flotation recovery as a function of organism concentration in a Hallimond tube roughly corresponds to contact angle as a function of concentration (at pH 7 ± 0.5). When the hematite particles are very small (- 20 #m) and, thus, numerous it is also not surprising that too many organisms can decrease flotation through the formation of hematite aggregations too large to be levitated by Hallimond tube air bubbles (see figure 6). The relative insensitivity of flotation with pH at pH values greater than pH 2.5 is indicative of insensitivity of bacterial adhesion to hematite with pH. Perhaps the phenomenon is due to a competition between charge interaction and hydrophobic interactions as a function of
556
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pH. The greater flotation at pH 2.5 may be due to greater adhesion of uncharged hydrophobic bacteria to highly charged hematite particles. Perhaps, at higher pH values the organisms become more negatively charged and the hematite particles become less positively charged maintaining approximately the same adhesion and, thus, flotation. CONCLUSIONS From the work reported on here the following conclusions can be stated: 1. 2. 3. 4. 5.
The bacterium M. phlei, a hydrophobic, highly negatively charged organism, readily accumulates on and adheres to hematite over a wide range of pH values. The organism readily causes the aggregation of finely divided hematite particles. It, thus, functions as a flocculating agent for hematite much as do chemical flocculants. The aggregation greatly enhances the settling of aqueous hematite suspensions. The aggregation also substantially enhances the filterability of such suspensions. The organism functions as a flotation collector for hematite. Best flotation under the conditions studied is at about pH 2.5. At more basic pH values, using the organism as collector, flotation is rather insensitive to pH.
R E F E R E N C E S
.
van Loosdrecht, M.C.M., Bacterial Adhesion, Doctoral Dissertation, The Agricultural University, Wagenhingen, The Netherlands, (1988).
.
van Loosdrecht, M.C.M., Lyklema, J., Norde, W., Schraa, G. & Zehnder, A.J.B., The Role of Bacterial Cell Wall Hydrophobicity in Adhesion, Appl. Environ. Microbiol., 53, 1893-1897, (1987).
.
van Loosdrecht, M.C.M., Lyklema, J., Norde, W., Schraa, G. & Zehnder, A.J.B., Eleetrophoretic Mobility and Hydrophobicity as a Measure to Predict the Initial Steps of Bacterial Adhesion, Appl. Environ. Microbiol., 53, 1898-1901, (1987).
.
.
Smith, R.W., Misra, M. & Dubel, J., Mineral Bioprocessing and the Future, Minerals Engineering, 4 (7-11 ), 1127-1141, ( 1991 ). Smith, R.W. & Misra, M., Mineral Bioprocessing, an Overview, to be published in:
Mineral Bioprocessing, eds. R. W. Smith and M. Misra, The Minerals, Metals and Materials Society, paper presented at the Engineering Foundation Conference: Mineral Bioprocessing, June 16-21, 1991. .
Pratt, D., Nutrition of Mycobacterium phlei, I. Requirements for Rapid Growth, Journ. Bacteriol., 64,651-657, (1952).
.
Guirard, B.M. & Snell, E.E., Biochemical Factors in Growth, Manual of Methods for General Bacteriology, editor in chief P. Gerhardt, American Society for Microbiology, 79-111, (1981).
.
.
Rajala, J.A. & Smith, R.W., Ionic Strength, Collector Chain Length and Temperature Interactions in Alkyl Sulfate Flotation of Hematite, Trans. SME/AIME, 274, 19471953, (1984). El-Shall, H.E., Beneficiation Research - Clay Disposal, an Overview, Unpublished paper, Florida Institute of Phosphate Research, Bartow, Florida.
0892-6875/92 $5.00+0.00 © 1992 Pergamon Press plc
Printed in Great Britain
MICROORGANISMS AS CHEMICAL REAGENTS: THE HEMATITE SYSTEM
J. DUBEL §, R. W. SMITHI, M. MISRAt and S. CHENt § Department of Chemical Engineering, Technical University of Karlsruhe, Karlsruhe, Germany t Department of Chemical and Metallurgical Engineering, Mackay School of Mines, University of Nevada, Reno, Nevada, USA
ABSTRACT Microorganisms can function as flocculating agents, flotation modifiers and flotation collectors. Thus, they can take the place of chemical reagents in mineral processing and hydrometallurgy operations and, therefore, can be themselves considered as reagents. For example, a microorganism should function somewhat as a chemical flocculant if it is to some degree hydrophobic and can attach itself in some manner, such as though electrostatic interaction, to the fine mineral particles to be flocculated. In addition, hydrophobic microorganisms should render a hydrophilic mineral surface somewhat hydrophobic i f they attach themselves to the mineral surface. A previously nonfloatable mineral can in this manner be rendered floatable. The present paper investigates the principles of the use of a hydrophobic, highly negatively charged, bacterium, Mvcobacterium Dhlei. as a flocculating agent for finely divided hematite suspensions and as a flotation collector for hematite. Keywords Microorganisms, minerals, flocculation, flotation. INTRODUCTION Previous work [1-5] has shown that Mycobacterium phlei is both highly hydrophobic (contact angle 65-70 °) and highly negatively charged with an isoelectric point (iep) at about pH 2.0 - 2.5. It should, thus, readily adsorb onto many hydrophilic mineral surfaces if the mineral is of low negative, neutral or positive charge. The adhesion could also be promoted by a hydrophobic mineral surface [1,2]. Where both the organism and the mineral are negatively charged the adhesion can be explained based on DLVO theory [1-5]. Experimental work on the flocculation of phosphate slimes, dolomite, clay minerals, and hematite has been performed. Some of this work has been reported on elsewhere [4,5]. It was found that M. phlei functions as an excellent flocculant for a dolomitic phosphate slime and hematite but does not function well as a flocculant for mineral suspensions where large quantities of the solids are finer in size than the approximately 1/~m size of the bacterium. Since the bacterium has an rod/ellipsoid/coccal shape (the organism is pleomorphic and can assume different forms depending on growth conditions), rather than a long chain structure such as possessed by synthetic polymeric flocculants, certain of its flocculation characteristics are markedly different from those of the polymeric flocculants. While both the bacterium and long chain polymers in part flocculate mineral particles by bridging between mineral particles, the entwining characteristics of the long chain molecules are absent in the case of M. phleL Thus, hydrophobic interactions between adhering microorganisms on adjacent solid particles are of greater importance in the formation of 547
548
J. DunE[. et al.
mineral aggregations. One consequence of this mechanism of aggregation is that the aggregations are often very "tight" and dense and contain little water. If the minerals being flocculated are, however, themselves somewhat hydrophobic (such as, for example, many coal minerals) the aggregations may not be so dense [4,5]. That very dense aggregations are formed in the flocculation of hematite by the microorganism is particularly evident. It is also likely that a reduction of positive electrostatic charge on the mineral by interaction with the negatively charged microorganism cells contributes to the aggregation of the hematite particles. Since M. phlei is a highly hydrophobic and negatively charged organism it should readily adhere to a more positively charged solid such as hematite over a wide pH range. It, thus, should also render the mineral floatable over this wide pH range. EXPERIMENTAL The Mycobacterium phlei used in the experimentation was grown from cultures obtained from Carolina Biological Supply Company. Several different media were used initially to culture the bacterium. The medium selected to culture the bacteria for experimentation was one suggested by Pratt [6] and Guirard and Snell [7] because of its simplicity and the rapid growth of the organism in the medium. The composition of the medium was (in gin/din 3 water): glucose 10, beef extract 1, yeast extract 1 and enzyme-hydrolysed casein 1. Culturing was done in 250 ml flasks continuously shaken in a water shaker bath held at 35 °C. HCI and NaOH used for pH adjustment in various studies was of reagent grade. The hematite used was a specular hematite from Republic, Michigan. Spectrographic analysis of the mineral indicated that it was of high purity [8]. It was prepared for experimentation by first crushing to - 2.36 mm followed by washing in hydrochloric acid solution. The material was then ground in a porcelain mortar and pestle to the desired size. The electrokinetic measurements were made using a Komline-Sanderson zeta reader in a conventional manner. A 2 ml sample of the bacteria or hematite solution was diluted with distilled water just prior to measurement and the pH adjusted with either HC1 or NaOH. The flotation tests were performed in a modified Hallimond tube with a long flotation column. Before flotation a 1 gm, - 20 or - 53, + 20/~m, sample of hematite was reacted for 10 min with a M. phlei suspension and placed in the Hallimond tube. Flotation was then started and carried on for 3 or 10 min depending on the test. Contact angle measurements were made on the hematite surface in the presence of M. phlei. The measurements were made by the sessile drop method in which a drop of M. phlei aqueous suspension at a particular pH value and microorganism concentration was placed on a smooth, clean hematite surface. A Rame-Hart model A-100 contact angle goniometer was used in the measurements. Sedimentation measurements were made by first mixing 1000 ml of the suspensions to be studied in controlled stirring vessels (HS-4 stirrer, Phipps and Bird). To provide good mixing, bacteria were added to a pH adjusted hematite suspension (-20 #m hematite) at an impeller speed of 250 rpm. After 2 min stirring the stirrer speed was reduced to 30 rpm for 5 min for floe growth. The suspension was transferred to a modified Andreasen pipette. Samples were taken at a 4 cm height from the bottom of the pipette. The solid concentration was determined gravimetrically. Filtration studies were performed using the pressure filtration device depicted in figure 1. A paper filter was placed in the position noted (held in place by a plate with holes in it). The lower part of the apparatus was screwed upward to bring the apparatus together. The suspension to be studied was placed in the upper part of the device and the top sealed. The proper pressure was then applied and the results determined as a function of time, tl. through measuring the weight of filtrate produced with time. The light was simply a small
M i c r o o r g a n i s m s as chemical reagents
549
bulb that could be inserted into the top of the device (after removing the pressure and the top seal). The purpose was to observe the character of the filter cake and when its formation was complete. Subsequent to the final formation of the filter cake, pressure was reintroduced to the vessel and the pressure dewatering of the cake determined as a function of time, t z.
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Fig.l Pressure filter apparatus. EXPERIMENTAL RESULTS Figure 2 illustrates the zeta potential of M. phlei and hematite as a function of pH. It is to be noted that the zeta potential of M. phlei is much more negative than that of hematite, the isoelectric point (iep) of the former being at about pH 2 and of hematite at about pH 5.5. It should be noted, however, that iep's and points of zero charge (pzc's) of various hematites have been measured from about pH 3 to pH 8+. Nevertheless, the present data indicate that there should be a strong coulombic interaction between M. phlei and hematite. The light micrograph of figure 3 from Smith et al. [4] confirms the strong interaction between the organism and the mineral leading to the obvious attachment of the two entities to each other and the aggregation of the hematite particles. Figure 4 from Smith et al. [4] shows sedimentation data on fine (- 20 #m) hematite in the presence and absence of M. phlei. It can be readily seen that the settling is much more rapid in the presence of M. phlei, the material substantially settled after 4 rain while the same amount of settling was not achieved after 30 rain in the absence of the organism. As a follow up on this experimentation the hematite in the presence and absence of M. phlei was subjected to pressure filtration (2 bar pressure). Initial suspension volume was 100 ml and initial solids concentration was 18 wt %. Figure 5 shows results obtained of filtrate volume (V F) as a function of time (tl). The results show the increased filterability of the hematite after treatment with the microorganism.
550
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HEMATITE+ M. phlei
NO BACTERIA
Fig.3 Photo micrographs of fine hematite in the presence and absence of Mycobacterium phlei; from Smith et al. [4].
Microorganisms as chemical reagents
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Hematite
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5
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Time [min]
Fig.4 Sedimentation of a 2 wt. % suspension of hematite in the presence and absence of 10.2 mg Mycobacterium phlei/kg solids, % solids measurement made I0 cm down in an Andreasen pipette; from Smith et al. [4].
100
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[sec] Fig.5 Filtrate volume recovered as a function of time after application of pressure to pressure filtration of hematite suspensions in the presence and absence of Mycobacterium phlei, 2 bar pressure. Figure 6 from Smith and Misra [5] shows the Hallimond tube flotation of - 20/~m hematite as a function of M. phlei concentration at pH 7 + 0.5 and a 10 min flotation time. It was noted from visual observation that the drop in flotation at the higher organism concentration is apparently due to the very large hematite floes formed that can not be levitated by the gas bubbles.
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Fig.6 Hallimond tube flotation recovery of - 20/~m hematite as a function of Mycobacterium phlei concentration, pH 7 -+ 0.5, 10 min flotation time; from Smith and Misra [5]. When - 53, + 20 #m hematite was studied and a flotation time of 3 min used the data depicted in figure 7 were obtained. The figure shows the flotation recovery of hematite as a function of organism concentration, again at pH 7 +_0.5. The same limit on flotation with increasing M. phlei concentration was not observed in this case, probably because of the larger size of the hematite and consequent fewer hematite particles per unit weight of hematite.
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Fig.7 HaUimond tube flotation recovery of - 53, + 20 #m hematite as a function of Mycobacterium phlei concentration, pH 7 +_ 0.5, 3 rain flotation time. Figure 8 illustrates the contact angle on hematite as a function of M. phlei concentration at p H 7 + 0.5. Contact angle increases with increasing organism concentration up to about 40 ppm M. phlei. Above this concentration the contact angle on hematite remains sensibly constant at about 46 ° at least up to an organism concentration of 145 ppm.
Microorganisms as chemical reagents
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Concentration of M__.phlei (ppm) Fig.8 Contact angle on hematite in the hematite - Mycobacterium phlei air system as a function of M. phlei concentration, pH 7 ± 0.5. Figure 9 depicts the flotation of - 53, + 20 #m hematite as a function of pH in the presence of 75 ppm M. phlei (3 min flotation time). It is to be noted that flotation is greatest at about pH 2.5 and drops from about 78% at this oH value to about 50 % near pH 4. The recovery then remains essentially constant as a function of pH to a pH value more basic than pH 10. .
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F l o c c u l a t i o n and filtration
Figures 3 and 4 clearly demonstrate that such an adhesion takes place resulting in aggregation of the hematite particles and enhanced settling of a fine hematite suspension. There appears to b e , from observation of the micrographs of figure 3, bridging of particles by the bacteria cells. The aggregation of the particles after accumulation of the bacterium must be through the hydrophobic interaction between the now hydrophobic bacterial surface of the hematite particles plus reduction in the magnitude of charge on the hematite. The bacterial aggregation of the particles also enhances the filtration of a fine hematite suspensions as is demonstrated by the data of figure 5. In the figure the dashed line represents the approximate completion of a filter cake on the filter (cake height 4 mm). Considering the apparatus used (figure 1), by means of the applied pressure difference the filtrate is forced through the apparatus and the solids are retained on the filter paper, in the position noted, to form a filter cake. It can be seen, then, that the time required for cake formation under conditions studied is reduced from about 6 1/3 min to 4 rain when the microorganism is present. Upon completion of the cake, drying was continued by retaining (or reintroducing) the 2 bar pressure in the system. It was then possible to determine the residual amount of moisture in the cake as a function of time. Figure i0 illustrates the type of data obtained of weight percentage residual moisture remaining (RF) as a function of time after the start of the pressure cake dewatering operation (t2). The presence of the microorganism obviously also enhances this operation. It is also to be noted that the minimum dewatering possible with or without M. phlei is to about 18 wt % residual moisture.
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Fig.l 0 Residual moisture in the hematite filter cake as a function of time after start of dewatering of the cake in the presence and absence of Mycobacteriurn phlei, - 20 #m hematite, 2 bar pressure. Considering figure 5 again, the data can be rearranged making use of the following expressions [9]: t l / V F = (slope x VF) + intercept or
t l / V F = {#lcr¢/(2A2Zxp)}VF + intercept
(1)
(2)
where: t 1 is time after application of pressure, V F is filtrate volume, # is viscosity of water, r e is filter cake resistance, A is filter area, Ap is pressure drop across the filter and
Microorganisms as chemical reagents
=
555
(ms/mF)p 1 / [(1-¢)Ps-¢(ms/mF)Pl]
where: ¢ is the porosity of the cake, PF is the density of water, Ps is the density of the solids, m s is the mass of the solids and m E is the mass of the filtrate. These expressions indicate that if t l / V F is plotted as a function of V F the slope of the straight line curve generated should yield the expression from which filter cake resistance (re) can be calculated:
slope = #~r c / (2A2Ap) Thus, the curves of figure 11 were plotted from the data obtained. It can be noted that straight lines are generated from which the slopes can be calculated and the filtration resistance, re, determined. For the non-flocculated hematite in the absence of M. phlei the filtration resistance was calculated to be about 9.4 x 1014 m "2 while for the flocculated hematite in the presence of M. phlei the value was determined to be 5.3 x 1014 m "z, a reduction of about 44%. 4
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Vr [mi] Fig.l I Time (after initial application of pressure)/filtrate volume recovered (tl/V F) as a function of filtrate volume recovered (VF) in the presence and absence of Mycobacterium phlei. The slopes are proportional to the filtration resistance (re). Flotation
Since electrophoretic mobility measurements indicate that the hematite is much less negatively charged than M. phlei and that the organism has a hydrophobic surface it is not surprising that it readily functions as a collector for hematite. Figure 8 shows that, indeed, finite contact angles are obtained on hematite in a hematite - M. phlei suspension - air system. Further, as shown by the data of figure 7, when the hematite particles are larger than 20/~m flotation recovery as a function of organism concentration in a Hallimond tube roughly corresponds to contact angle as a function of concentration (at pH 7 ± 0.5). When the hematite particles are very small (- 20 #m) and, thus, numerous it is also not surprising that too many organisms can decrease flotation through the formation of hematite aggregations too large to be levitated by Hallimond tube air bubbles (see figure 6). The relative insensitivity of flotation with pH at pH values greater than pH 2.5 is indicative of insensitivity of bacterial adhesion to hematite with pH. Perhaps the phenomenon is due to a competition between charge interaction and hydrophobic interactions as a function of
556
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pH. The greater flotation at pH 2.5 may be due to greater adhesion of uncharged hydrophobic bacteria to highly charged hematite particles. Perhaps, at higher pH values the organisms become more negatively charged and the hematite particles become less positively charged maintaining approximately the same adhesion and, thus, flotation. CONCLUSIONS From the work reported on here the following conclusions can be stated: 1. 2. 3. 4. 5.
The bacterium M. phlei, a hydrophobic, highly negatively charged organism, readily accumulates on and adheres to hematite over a wide range of pH values. The organism readily causes the aggregation of finely divided hematite particles. It, thus, functions as a flocculating agent for hematite much as do chemical flocculants. The aggregation greatly enhances the settling of aqueous hematite suspensions. The aggregation also substantially enhances the filterability of such suspensions. The organism functions as a flotation collector for hematite. Best flotation under the conditions studied is at about pH 2.5. At more basic pH values, using the organism as collector, flotation is rather insensitive to pH.
R E F E R E N C E S
.
van Loosdrecht, M.C.M., Bacterial Adhesion, Doctoral Dissertation, The Agricultural University, Wagenhingen, The Netherlands, (1988).
.
van Loosdrecht, M.C.M., Lyklema, J., Norde, W., Schraa, G. & Zehnder, A.J.B., The Role of Bacterial Cell Wall Hydrophobicity in Adhesion, Appl. Environ. Microbiol., 53, 1893-1897, (1987).
.
van Loosdrecht, M.C.M., Lyklema, J., Norde, W., Schraa, G. & Zehnder, A.J.B., Eleetrophoretic Mobility and Hydrophobicity as a Measure to Predict the Initial Steps of Bacterial Adhesion, Appl. Environ. Microbiol., 53, 1898-1901, (1987).
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.
Smith, R.W., Misra, M. & Dubel, J., Mineral Bioprocessing and the Future, Minerals Engineering, 4 (7-11 ), 1127-1141, ( 1991 ). Smith, R.W. & Misra, M., Mineral Bioprocessing, an Overview, to be published in:
Mineral Bioprocessing, eds. R. W. Smith and M. Misra, The Minerals, Metals and Materials Society, paper presented at the Engineering Foundation Conference: Mineral Bioprocessing, June 16-21, 1991. .
Pratt, D., Nutrition of Mycobacterium phlei, I. Requirements for Rapid Growth, Journ. Bacteriol., 64,651-657, (1952).
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Guirard, B.M. & Snell, E.E., Biochemical Factors in Growth, Manual of Methods for General Bacteriology, editor in chief P. Gerhardt, American Society for Microbiology, 79-111, (1981).
.
.
Rajala, J.A. & Smith, R.W., Ionic Strength, Collector Chain Length and Temperature Interactions in Alkyl Sulfate Flotation of Hematite, Trans. SME/AIME, 274, 19471953, (1984). El-Shall, H.E., Beneficiation Research - Clay Disposal, an Overview, Unpublished paper, Florida Institute of Phosphate Research, Bartow, Florida.