631

Bioleaching of mineral ores in a suspended solid bubble column

J. Garcia Ochoa a, S. Poncin a, D. Morin b and G. Wild a

a Laboratoire des Sciences du Genie Chimique CNRS ENSIC, B.P. 451, F - 54001 Nancy Cedex (France)

b Bureau de Recherches Geologiques et Minirres. B.P. 6009, F - 45060 Orleans Cedex 2 (France)

Bioleaching of precious metals contained in pyrite and/or arseniopyrite is usually done in huge aerated stirred tank reactors. In this work, the possibility is investigated to use slurry bubble columns with high solid loading in order to limit the reactor volume and decreasing the water consumption which can be of great importance from an environmental point of view. In order to investigate the feasibility of bioleaching in slurry bubble columns, two long duration bioleaching experiments have been performed in a bubble column (ID 0.1 m; height: 2.0 m) with a pyrite load of 33% (W/W), simultaneously with determination of hydrodynamics and gas-liquid mass transfer characteristics. One of the conclusions of this work is that oxygen mass transfer limitations play an important role in bioleaching in suspended solid bubble columns, even in the small scale equipment tested here. It is also shown that higher solid loads may be used in this kind of equipment than in the traditional stirred tank reactor.

1. INTRODUCTION

Many precious metals (e.g. gold, antimony and cobalt) are present in nature as traces in mineral ores containing mainly pyrite and arseniopyrite. Bioleaching is an attractive means of separating the metals from the pyrite. During bioleaching, acidophilic micro-organisms attack the iron sulfide and use the oxygen of air to oxidize the sulfide to sulfuric acid. The design and development of bioleach reactors has been the subject of several recent studies (Batty and Post, 1998, Dew et al., 1997, Schultz and Buisman, 1998). In industrial mineral ore treatment, bioleaching of sulfides is usually implemented in very large mechanically agitated reactors. Suspension of the finely ground pyrite particles and aeration to ensure the gas liquid mass transfer rates allowing bioleaching to take place, require considerable amounts of mechanical energy. In addition, in this type of reactors, generally a maximum solid loading of 20 % is used, since several factors such as oxygen requirements, nutrient availability and effect of shear and turbulence generated by agitation, could limit the bioleaching efficiency as reported by Bailey and Hansford (1993) and Morin (1995). The use of slurry bubble columns may be an alternative to overcome this limitation. In fact, in such reactors, shear and turbulence are usually smaller and the solid concentration in a bubble column can be more homogeneous than in an agitated reactor.

However, the design of such a reactor requires the knowledge of the hydrodynamic behaviour (critical gas and/or liquid velocity for complete suspension of the particles, hold-up

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and mixing of phases), of mass transfer (volumetric gas-liquid mass transfer coefficient kLa) and of heat transfer (heat transfer coefficient h to a tube) parameters. Unfortunately, there are no data available with such untypical solid systems (polydisperse high density particles). Furthermore, as reported by Charinpanitkul et al. (1993), contradictory results have been reported concerning the effect of small solid particles on the gas holdup and the volumetric gas-liquid mass transfer coefficient, although no bubble disintegration is expected for the concerned particle diameters. The gas distributor, the particle diameter and the solid concentration may also play an important role, even if these aspects have not oRen been investigated up to now.

In order to investigate the feasibility of bioleaching in slurry bubble columns, two long duration bioleaching experiments have been performed with a pyrite load of 30%. During the pyrite oxidation reaction, the evolution of gas holdup and volumetric gas-liquid mass transfer have been measured in order to estimate the oxygen uptake rate.

2. EXPERIMENTAL

Experiments are carried out in a cylindrical glass column shown schematically in Figure 1. The column (1) has an inner diameter of 0.1 m and a total height of 2 m. Pressure sensors (2) and sampling taps (3) are positioned at regular intervals along the column wall. The water or slurry is pumped through the system by a peristaltic pump (4). The flow is regulated by using a valved by-pass line and measured with an electromagnetic flow-meter (5). After passing through the column, the liquid retums to the feed tank (6). The gas used is air, its flow rate is measured by a rotameter (9). The air and the liquid are introduced at the bottom of the column. A ring sparger with 62 orifices of 0.7 mm diameter is used; the solid is supported by a perforated plate with orifices of 1 mm diameter.

I U

Figure 1. Experimental apparatus.

(1) Column (2) Pressure sensor and/or

oxygen probe (3) Sampling taps (4) Peristaltic pump (5) Flow-meter (6) Feed tank (7) Coil (8) Thermostated bath (9) Rotameter

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The concentration, size distribution and properties of pyrite used are shown in Table 1.

Table 1. CI mcentration, size distribution and proper

Concentration (% w/w)

33

Ps

(kg/m 3)

4482

dp (gtm)

77

:ies of pyrite particles. Size distribution

!% 10-

! 0 I

1,32 9,48 68,3 492

The bacterial population originates from mine samples provided by the Bureau des Recherches G6ologiques et Mini&es (BRGM). Collinet-Latil (1989) isolated strains of Thiobacillus ferrooxidans and Thiobacillus thiooxidans from this population. In order to adapt the inoculum to the pyrite used in this work, the micro-organisms were subcultured several times on this substrate. The nutritive medium used for bacteria growth was the OK modified medium used by Collinet-Latil (1989). The standard composition of this medium was (NH4)2SO4 3.7 kg m3; n3PO4 (85% w/w) 0.8kg m3; KOH 0.5kg m-3; MgSOa-7H20 0.5 kg m 3. The substrate used in these experiences consists in pyrite (FeS2) particles (mean dp = 77 lam).

The operating conditions for the bioleaching experiments are presented in the table 2. During the bioleaching reaction, free bacteria in solution were counted using a Thoma counting cell under an optical microscope (xl00). The dissolved oxygen concentration was measured regularly by a Clark probe at the inlet of the column.

Table 2. Operating conditions for the bioleaching experiments

Experiment U s (m/s) UL (m/s) pH regulation by: T (K) kLa ~ (s -t)

BIO-1 0.015 0.0035 NaOH (4N) 305.7 2.72x10 -2

BIO-2 0.023 0.0035 CaCO3 (100 kg/m 3 ) 305.7 3.68x10 2

The evolution of the particle size distribution during bioleaching experiments has been obtained by sampling of solids in the column and analysing by laser diffraction (Malvern Instruments SB.0C).

The overall gas hold-up is obtained by measuring the gas volume in the column after simultaneous shutdown of gas and liquid feeds.

The oxygen uptake rate (OUR) has been estimated by measuring the evolution of oxygen concentration in the pulp after shutdown of the gas feed. A plug flow model is assumed for the liquid phase, due to the small values of liquid dispersion coefficient obtained in absence of gas. The mass balance equation for the liquid phase is as follows:

c~C c~C OUR - -uL c~z ~ (1)

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The liquid superficial velocity of liquid is taken in this equation since during the oxygen uptake rate determination, no gas is present in column.

For the resolution of this equation, at beginning of OUR experiment, the oxygen concentration axial profile in the column is the one obtained in steady state operation:

' + E L ~ z 2 -UL "~Z t = 0 t=0

kLa(HP - CI OUR �9 "Jl- ~ -- - gL e VR ~L

- 0 (2)

The volumetric gas-liquid mass transfer coefficient (kLa) is determined by the dynamic physical absorption/desorption technique. Experimental details have been discussed previously by Garcia-Ochoa et al. (1997). During the determination of the volumetric gas-liquid mass transfer coefficient, the oxygen concentration in pulp is measured at five axial positions along the column and at the entrance of the liquid (below the gas sparger) by Clark probes connected to a data acquisition system. The time constants of the probes are taken into account by deconvolution with the impulse response of the probes. A plug flow model with axial dispersion has been used for the liquid phase. The oxygen uptake ratio has been taken into account in the mass balance equation for the liquid:

0C0t ,o3C~_z oz02C kLaIHP- CI OUR - -UL + EL-Z-T + ~ - - E;L e VR ~L

(3)

For the estimation of OUR and kLa values, a simultaneous resolution of equations (1) and (3) was made by adjusting equation (1) to the oxygen concentration profile obtained atter shutdown of gas feed and equation (3) to the profile obtained after exchange from air to nitrogen.

Values of the liquid dispersion coefficients EL determined experimentally in absence of reaction are used. These values have been obtained by the tracer pulse input technique, injecting a NaC1 concentrated solution and then measuring the tracer concentration at the inlet and at the outlet of the reactor.

During bioleaching, Fe § ions are released into the liquid. The liquid phase iron concentration has been measured by atomic absorption spectrometry. The precipitate formed during the bioleaching reaction has been analysed by X-ray diffraction technique and by dissolution with HCI (SN) for the determination ofFe m iron in the precipitate and with a HNO3 solution for the determination of total iron in the precipitate. The conversion rate has been estimated by assuming the following stoichiometry for the pyrite oxidation:

5 1 bact. Fe3+ F e S 2 + ~ O2 -]- 2 H 2 0 > + 2 S O 4 2 + I-I* (4)

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3. RESULTS

The conversion rates obtained in the bioleaching experiments are presented in Table 3. Although the pyrite oxidation yield is approximately the same in both tests, the total duration ofbioleaching reaction was reduced by a factor 2 in the BIO-2 test. This time reduction is due, first to the increase of carbon dioxide in solution (by acid decomposition of CaCO3), as reported by Torma et al. (1972) and Liu et al. (1988), but also to the increase of the volumetric gas-liquid mass transfer coefficient by increase of gas velocity (initial kLa varies between 2.72x10 "2 and 3.68x10 "2 s'l).

Table 3. Principal results obtained in the bioleaching experiments

Experiment Total reaction Conversion Maximal bacteria time (h) rate population (celi/ml)

BIO- 1 1025 77.4 1.86x101~ BIO-2 453 79.5 1.59x101~

In order to analyse the importance of oxygen transfer during bioleaching reaction, the evolution of the oxygen concentration in the pulp and of the volumetric gas liquid mass transfer coefficient during the reaction have been examined.

The oxygen concentration in the pulp can indicate a limitation of oxygen during the bioleaching reaction. This parameter has been widely studied by many authors (e.g. Myerson, 1981; Liu et al., 1988; Chapman et a1.,1993). In literature, the minimum oxygen concentration to be attained in pulp varies between 10 "4 and 10 -3 kg/m 3, depending on the quality and quantity of substrate, the agitation-aeration conditions and the system temperature.

Even if generally bioleaching experiments reported in literature show that, at the small solid concentrations used up to now (typically performed in stirred tank reactors), the gas-liquid mass transfer is not limiting, this is not valid for slurry bubble columns with high solid loading. The bioleaching experiments performed here show a limitation of the biochemical reaction due to the oxygen transfer during the reaction (Figure 2). In two bioleaching experiments the minimal oxygen concentration in the column during the reaction was 5x10 4 kg/m 3. The increase of the oxygen transfer rate (by increase of kLa) between the BIO-1 et BIO-2 experiments is not sufficient to avoid the oxygen transfer limitation (indicated by the very low dissolved oxygen concentration observed when the cell concentration is at its highest). However, this increase of the oxygen transfer rate induces an increase of the bioleaching rate, reducing the total duration of the bioleaching reaction. Figure 3 shows the evolution of the average gas holdup during the bioleaching reaction. The gas holdup obtained in absence of bacteria is approximately the same as the one measured at the beginning of reaction and no influence of pH has been observed. A slight decrease of the gas holdup in the column has been observed during the first 150 hours of reaction, after that, the average gas holdup is more or less constant in time. The diminution of gas holdup is probably caused by a decrease of bubble volume by oxygen consumption. In fact, for the maximum oxygen uptake rate (Figure 3), a reduction of 7% of gas holdup along the column has been estimated, considering a linear diminution of gas volume with oxygen consumption. Experimentally, a reduction of 5% of gas holdup for the maximum uptake rate has been obtained.

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0.008

0.00( L

0.004 [O51

0.007 (kg/n

0 ~ -'~

Figure 2.

a) BIO-1 b) BIO-2

20 0.008, Icell]~ xl 20 [cell] xl 0 .9

_ ~ (cell/ml)-15 0.0061 15

- t .10 0.00 ' 10

-5 0 . 0 0 ~

. . . . 10 0 100 200 300 400 5()0 0 200 400 600 800 10000-

t (hours) t(hours)

Evolution with time of oxygen concentration at column inlet and of bacteria concentration in column.

0.08

0.06

0.04

0.02

~g ~ 8gO

U 0 U v u I O

0 0 100 200 300 400 500

t (hours) Evolution Figure 3. with time

average gas holdup during the BIO-2 experiment.

0.0012

~ 0.0009 E

~0.0006 I D

~0.0003

�9 , , , ,

0 100 200 300 400 500 t (hours)

of Figure 4. Evolution with time of oxygen uptake rate during the BIO-2 experiment.

However, after 150 hours of reaction, the oxygen uptake rate decreases slightly and no increase of gas holdup is observed after this time. This phenomenon can be related to the iron hydroxide precipitation observed after 150 hours of reaction as shown in Figure 4. The diminution of ionic strength caused by this precipitation induces an increase of bubble coalescence by reduction of repulsive forces generated by the electrically charged liquid surface around the bubbles as reported by Chang et al. (1986).

The reduction of gas holdup by increase of bubble coalescence may compensate the increase of gas volume by diminution of oxygen consumption and no significant difference in gas holdup is observed. The precipitate formation may also play a role by modification of solid properties.

637

2.5 en E .~ 2 '5 .~ 1.5

~ 1 . , .

=~ 0.5 j,-

F e

0 100 200 300 400 500 t (hours)

Figure 5. Evolution with time of dissolved iron concentration during the BIO-2 experiment.

kLa (s-')

0.04

0.03

0.02

0.01

/ k~a ~

~ . . . . . O . O ~ O ~ _ . _ Q O 0 0 0 0

0 0 0 0

0 100 200 300 400 500 t (hours)

Figure 6. Evolution with time of kLa during the BIO-2 experiment.

While the gas holdup was approximately the same in absence of bacteria as at beginning of reaction, the volumetric gas-liquid mass transfer obtained in absence of bacteria is smaller than the one obtained after addition of bacteria and before the beginning of reaction (Figure 5). The difference of volumetric gas liquid mass transfer is probably caused by the pH diminution inducing an increase of ionic strength. This increase of ionic strength is weak and no influence on gas holdup is observed. In fact, at slight variations of ionic strength, the inhibition of bubble coalescence is less effective for larger bubbles, since the inertia of larger bubbles may predominate compared with the repulsive force between the bubbles in the bed, as explained by Kim and Kang (1997). In this case, the volume occupied by gas varies only slightly (gg constant), but the interfacial area of bubbles increase (kLa increases).

During the reaction, the volumetric gas-liquid mass transfer coefficient follows the same trend as the gas holdup: a decrease of this parameter has been observed during the first 150 hours, after that, the mass transfer coefficient is approximately constant in time. The variation of kLa is more important than the variation of gas holdup. This trend has been observed previously in the three phase system in absence of reaction with pyrite and glass beads as solids (Garcia-Ochoa et al., 1997).

On the other hand, a size reduction of particles is observed between 100 and 250 hours of reaction. This reduction is due on one hand to the corrosion by bacterial attack of pyrite particles and on the other hand to the formation of precipitate. As reported by Mustin et al. (1992), the pore formation in pyrite has been observed by scanning electron microscopy. The principal cause of the decrease of the mean particle diameter is the precipitate formation. As shown in Figure 6, the volume percentage of solids having a diameter smaller than 10 ~tm increased considerably after 100 hours of reaction. It is interesting to observe that, after 100 hours of reaction, two peaks in the size distribution of particles appear, with maxima at 76 et 2.6 lam. The peak at 76 ~tm is always observed with this kind of pyrite, but its amplitude decreases with time.

638

60

40 _

20

p �9 o 0

(l~m) �9 ~ �9

_ _/l~ine particles (dp> 10grn~

i I i I

0 100 200 300 400

t (hours) 500

Figure 7. Evolution with time of mean particle size and of volume percentage of fine particles (dp<10~tm) during the BIO-2 experiment.

60

40

It is important to note that the decrease of the mean particle diameter by pyrite oxidation does not induce a priori a decrease of gas holdup or volumetric gas liquid mass transfer coefficient, as was observed with glass beads (Charinpanitkul et al., 1993; Garcia-Ochoa et al., 1997). As in absence of reaction, the gas holdup and the volumetric gas liquid mass transfer coefficient of pyrite particles are approximately the same as those obtained with the fraction of particles having a diameter smaller than 35 lam. During the bioleaching reaction, this fraction of solid is always present in pyrite, the percentage of fine particles of pyrite in the solid increases with time.

In order to compare the performance of bioleaching in the bubble column with those obtained in a stirred tank and in air-lift reactors, our results have been compared to those obtained by d'Hugues (1996) and Haddadin (1995) (Table 4).

Although the conversion rate obtained in this study is close to the one obtained in the stirred thank reactor used by d'Hugues (1996), the total duration of reaction in the slurry bubble column is smaller. A reduction of the total bioleaching duration implies a reduction of operating cost of pyrite oxidation.

It is important to note that the pyrite used by d'Hugues (1996) and Haddadin (1995) was cobaltiferous pyrite while the pyrite used in this study is ferrous pyrite. Normally, cobaltiferous pyrite is easier attacked by Thiobacillus ferrooxidans. A better performance should be obtained with the slurry bubble column withcobaltiferous pyrite. Supplementary experiments should be performed in a slurry bubble column using ores of interesr in industry,in order to make a more significant comparison of our results with literature results.

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Table 4. Comparison of performances of some industrial and laboratory pyrite bioleaching reactors

d'Hugues (1996) Haddadin (1995) BIO-2

Reactor type Stirred tank reactor Airlift reactor Slurry bubble column VR (m 3) 0.022 0.016 0.030 Pyrite used Cobaltiferous pyrite Cobaltiferous pyrite Ferrous pyrite Solid uptake (W/W) 0.30 0.06 0.33 pH 1.7 1.7 1.5 pH regulation by CaCO3 CaCO3 CaCO3 Air enriched with CO2 (1%) Yes Yes No [bacteria]m~, (cell/ml) 1.25xl 01~ 3.0xl 0 9 1.6xl 01~ kLa ~ (s "1) 2.48xl 0 .2 not reported 3.68xl 0 .2 OURm,x/VR (mol/m 3s) 4.9 lxl 0 .3 not reported 4.72xl 0 .2 Total duration (h) 725 600 450 Conversion rate (%) 79 46 79

4. CONCLUSIONS

In order to analyse the feasibility of pyrite bioleaching in a slurry bubble column, two long duration bioleaching experiments are presented. A conversion rate close to 80 % is obtained; the total reaction time decreases by increasing the volumetric gas-liquid mass transfer (i.e. by increasing the gas velocity). In spite of this increase of gas velocity, a limitation of oxygen transfer (small dissolved oxygen concentration) is observed.

During the bioleaching reaction, the gas holdup and the volumetric gas liquid mass transfer coefficient decrease during the first 150 hours, after that, they stay constant. This diminution is caused by a reduction of bubble volume due to the oxygen consumption by reaction, and by a decrease of ionic strength caused by the iron hydroxide precipitation after 150 hours of reaction. The decrease of ionic strength may increase the bubble coalescence by reduction of repulsive forces generated by the electrical charged liquid surface around the bubbles.

Regardless of the conversion rate obtained in this work, an a priori design of a large scale bioleaching slurry bubble column with high solid loading is not yet possible. Supplementary bioleaching experiments will have to be realised in larger scale equipment (for which oxygen depletion is still more likely to happen) in order to determine the optimal operation conditions from the study of hydrodynamics and mass transfer during the reaction so as to overcome the oxygen limitation and decrease further the duration of reaction (or increase the bioleaching rate). However, it is already clear, that the use of slurry bubble columns can lead to higher solid loadings than the usual stirred tank reactors. This can lead to smaller water consumptions, which can be of great importance from an environmental point of view.

ACKNOWLEDGEMENTS

The authors want to thank the Consejo Nacional de Ciencia y Tecnologia of Mexico for the Ph.D. grant to J. Garcia-Ochoa

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NOTATION

C dp

EL

He kLa kLa ~ OUR P T t

U U w

VR Y z

Oxygen concentration in the liquid bulk, mol/m 3 Particle diameter, lam Liquid-phase dispersion coefficient, m2/s Henry constant, (m 3 Pa/mol) Volumetric gas-liquid mass transfer coefficient, s 1 Volumetric gas liquid mass transfer coefficient in absence of reaction, s 1 Oxygen uptake rate, mol/s Total pressure, Pa Temperature, K Time, s Superficial velocity, m/s Interstitial velocity, m/s Reactor volume, m 3 Oxygen mole fraction in the gas Axial position in the column, m

Greek letters Average phase holdup

Subscripts L s

g

Liquid phase Solid phase Gas phase

R E F E R E N C E S

- Bailey, A. D. and Hansford, G. S. Biotechnology and Bioengineering, 42 (1993) 1164.

- Batty, J. and Post, T. Randol Copper Hydromet Roundtable'98, Vancouver, Canada (November 1998).

- Chang, S. K., Kang, Y. and Kim, S. D., Journal of Chemical Engineering of Japan, 19 (1986) 524.

- Chapman, J. T., Marchant, P. B., Lawrence, R. W. et Knopp, R., FEMS Microbiology Review, 11 (1993)243.

- Charinpanitkul, T., Tsusumi, A. and Yoshida, K., Journal of Chemical Engineering of Japan, 26(4) (1993) 440.

- Collinet-Latil, M.N. Lixiviation bacterienne par Thiobacillus ferrooxydans et ThiobaciUus thiooxydans d'un concentre de flottation arseno pyriteux aurifere. (Refractaire /t la cyanuration directe), Ph.D. Thesis, University of Provence, Marseille, France (1989).

- Dew, D. W. Lawson, E. N. and Broadhurst, J. L Rawlings, D. E. (5Ed.) Biomining: Theory, Microbes and Industrial Processes, Springer, Berlin and Landes Bioscience, Austin TX, 1997.

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- d'Hugues, P. Biolixiviation d'une pyrite cobaltifere - Optimisation des conditions de culture et bilan gazeux, Ph.D.Thesis, University of Provence Aix-Marseille I., France (1996).

- Garcia-Ochoa, J., Khalfet, R., Poncin, S. and Wild, G. Chemical Engineering Science, 52(21- 22) (1997) 3827.

- Haddadin, J. Etudes cinetiques et microbiologiques de la lixiviation bact~rienne en rracteurs: Effet de diff&ents param~tres physico-chimiques, d~veloppement d'un proc~d~ en r~act~urs air-lift et lit fluidis~ et application a l'extraction de rantimoine, Ph.D. Thesis, Institut National Polytechnique de Lorraine, Nancy, France (1995).

- Kim, S. D. and Kang, Y., Chemical Engineering Science, 52(21-22) (1997) 3639.

- Liu, M.S., Branion, R.M.R. and Duncan, D.W., The Canadian Journal of Chemical Engineering, 66 (1988) 445.

- Morin, D. Bacterial leaching of refractory gold sulfide ores, dans Bioextraction and Biodeterioration of metals, Scheiner, B. J., Doyle, F. M. and Kawatra, S. K. (Eds.), University Press, Cambridge, UK, 1995, 25.

- Mustin, C., de Donato, Ph. and Berthelin, Biotechnology and Bioengineering, 39 (1992) 1121.

- Myerson, A. S., Biotechnology and Biongineering, 23 (1981) 1413.

- Schultz, C. E. and Buisman, C. J. N. Randol Copper Hydromet Roundtable'98, Vancouver, Canada (November 1998).

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