61

The design of bioreactors

G. Rossi

Diparnmento Geoingegneria e Tecnologie ambientali, Universit~ degli Studi Piazza d'Armi, 19, 09123 Cagliari, Italia

A survey of the literature on biohydrometallurgical topics revealed that the papers devoted to bioreactors amount to less than 5 % of the total and refers mainly to the analysis of the performance and to design guidelines of stirred tank and of bubble column machines, the so-called Pachuca tanks. These machines can be defined conventional in the sense that they have been borrowed from chemical engineering and hydrometallurgy and adapted to the requirements of

biohydrometallurgical processes. However, past experience has shown that these types of reactors do not fully match the very particular conditions that exist in biohydrometallurgical systems that, quite correctly have been qualified as "hybrid", owing to their specificity since they are characterized by many of the features of hydrometaUurgical operations and of biological conversions. Literature data and the author's personal experience demonstrate that one of the present major drawbacks of these reactors is the power requirement, that seriously affects the competitivity of biohydrometallurgy with pyrometallurgy. The factors affecting the performance of biohydrometallurgical reactors are discussed with special reference to the process parameters and an analysis of the conditions to be satisfied by an ideal bioreactor is carried out. In the light of these considerations, the reactors currently operating in commercial plants are examined. The new prospects opened up by recent developments are finally discussed and, also on the grounds of experience recently gained on a laboratory scale, the potentials of machines tailor-designed for the conditions reigning in biohydrometallurgical systems are outlined.

1. INTRODUCTION

Compared to the great effort devoted worldwide to the biology and physiology of microorganisms and to biosolubilization kinetics in the light of microbe/minerals interactions. where the influence of reactors has unfortunately been overlooked - the published results of investigations on reactors suitable to biohydrometallurgical processes only represent a small minority. In fact, out of the total number of papers published in the volumes of Symposia Proceedings and in the journals over the years, not even 5% have been devoted to bioreactor technology.

Most of these papers provide very good design guidelines, and an indication of the excellent cultural and practical background of the authors in chemical engineering. However, they are

based on the implicit assumption that the microflora is kind of a biological catalyzer, whose

62

overall performance is only moderately dependent on the operating characteristics of the machines where the process is carried out. In my mind this is probably the Achille's heel of this approach.

Hence, the reason why I accepted to give a talk on the subject of biohydrometallurgical reactor design was the desire to discuss the state of the art and to point to the need for further research aimed at providing our technology with suitable machines where the potential of biohydrometallurgy can be fully exploited.

2. EXPECTED PERFORMANCE OF BIOHYDROMETALLURGICAL REACTORS

Biohydrometallurgical processes are very attractive insofar as they present few environmental hazards. However, they are still a long way from being able to compete with pyrometallurgical and pressure leaching processes mainly because of the unsatisfactory all- round performance of the reactors, the devices where the process is carried out.

The performance of a reactor is considered economically convenient when - tbr comparable qualitative and quantitative characteristics of the end product - the incidence of the investment and operation costs on the unit product - in our case the tonne of metal solubilized and recovered - is reasonably lower than the expected returns and, in any case, is lower than that of other processes.

The parameters for assessing the biohydrometaUurgical performance of a reactor for a given production are (i) tank size related to the dry mineral throughput, (ii) total power requirements, i.e. the power for mixing and aeration, referred to the unit mass of metal recovered in unit time in bioleaching or to the unit mass of sulphur removed in unit time in biodesulphurization, (iii) the chemical compounds added to the aqueous phase as nutrients tbr the microflora or as pH modifiers, (iv) plant attendance and supervision, (v) effluents purification and (vi) maintenance. The first parameter affects investment costs, the other five determine operating costs. These costs are of the same order of magnitude (1-3).

3. THE FACTORS AFFECTING BIOHYDROMETALLURGICAL REACTOR PERFORMANCE

Biohydromellurgical processes take place in three-phase systems, consisting of (i) an aqueous phase, that is a solution of salts providing the nutrients for a microflora which acts as a biological catalyzer of the metal sulphides oxidation processes, (ii) a solid phase, consisting of the finely ground ore which contains a mixture of waste rock and metal values combined with sulfur to form sulphides, and (iii) a gaseous phase consisting of a mixture of atmospheric oxygen and carbon dioxide. The aqueous phase is the suspending medium where several elementary processes occur: (a) the growth of microorgasnisms, (b) the encounter of solid particles with microorganisms, (c) the encounter of solid particles with chemically active molecules, (d) the release of metal ions, (e) the uniform distribution and effective dissolution of oxygen and carbon dioxide. The solid phase is the energy source for microbial biosynthesis i.e. for microbial growth and continuously releases metal and sulphur ions in oxidized tbrm. The gaseous phase supplies the oxygen required for the oxidation processes as well as the carbon dioxide that the microflora uses for its biosynthesis (4).

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4. THE GENERAL DESIGN PROCEDURE OF A BIOHYDROMETALLURGICAL REACTOR

The design procedure shown in the block diagram of Figure 1 is based on the well documented expected performance of a biohydrometallurgical reactor.

The inputs are represented by the "production" and "kinetic data". At this juncture it should be stressed that in effect, the real independent outputs are the production data, insofar as the kinetic data are strongly dependent upon reactor type and environment. This has been highlighted in recent papers (5-6) and justifies the "feedback" line linking the "Type of reactor" box to the "Kinetic data" box. In fact, the microflora is more than a simple catalyzer. In conventional chemical engineering processes the amount of catalyzer does not vary with time, whereas in biohydrometallurgical processes the microbial population - whose growth has been described as a "pseudo autocatalytic process" - may vary with time depending on the conditions reigning in the bioreactor. The faster the microbial growth kinetics, the faster the oxidation and solubilization process. The most important factors influencing microbial growth are oxygen availability, determined by its mass transfer coefficient and substrate availability, that depends upon the exposed mineral surface, both achieved by adequate mixing. The most significant factors adversely affecting microbial growth are the shear stresses within the suspension, the accumulation of metabolites, incorrect temperature and hydrogen ion concentration of the aqueous phase and the release of toxic substances by the surfaces (minerals and equipment) with which the suspension comes into contact.

The reactors most commonly employed in biohydrometaUurgical processes are the Stirred Tank Reactor (STP`), and the AirLift Reactor (ALP.) (Pachuca tank) (7).

Other reactors have been proposed recently but are dealt within a separate section as they can be regarded as typical examples of possible developments of bioreactors tailor-made for biohydrometallurgy. As a specific design procedure has to be followed for each type of reactor, in the following sections the main types of reactors will be considered separately.

Some time ago it was suggested that the plug-flow reactor is the most suitable to the type of reactions occurring in metal sulphide bioleaching (4, 8-10). However, on account of the almost unsormountable practical problems that arise with this type of reactor, it has to be simulated by a cascade consisting of a suitable number of vessel reactors. It has been shown that at least six vessels are required (4,11) as a smaller number results in short-circuiting of the suspension with loss of effectiveness.

4.1 The stirred tank reactor (STR) This reactor, borrowed from chemical engineering, has been given priority since the early

days of biohometallurgy, in spite of the evident drawbacks that have emerged in biohydrometaUurgical applications. A very interesting technico-economic analysis, carried out by one of the European parmers in the coal biodepyritization pilot plant project at Porto Torres (Sardinia, Italy), funded by the Commission of European Communities, produced evidence that, as far as the achievement of the desired levels of mass transfer, mixing and suspension is concerned, the STR performs better than the Pachuca tank (12). The experimental data reported by Acevedo et al. also show the STR to be superior in this sense (13).

64

�9 - - - = 1 , |

~ =~ _

~ ' ~ ~ -- e t - - ' ~ I ' -

1

e ~, �9 -'2 -~| " "

u . = I

Figure 1 - Logical diagram for the design of a biohydrometallurgical reactor.

65

The most important role is played by the impeller, which as to accomplish three major tasks: solids suspension, mixing and dissolution of the required atmospheric oxygen into the aqueous phase, maximizing the interfacial area between the gaseous and aqueous phases. Initially and for many years the Rushton-type turbine was the most widely used impeller for these reactors, but latterly the curved blade, axial flow impeller has been shown to outperform the Rushton turbine as it requires less power for achieving the same performance and induces smaller shear stresses induced in the suspensions (14, 15). Chemical engineering has provided some correlations that help in establishing, as a first approximation, the machine's characteristic parameters also with reference to the different types of agitators.

Hence, for the impeller speed, Njs, necessary to satisfy the just-suspended condition for solid particles in the vessel, "Zwietering's criterion" (16 ) is usually adopted, for which I prefer the tbllowing expression

N2"D Pl 0.2 a �9 �9 = k " �9 B 0"13

v g hp (1)

Reynolds Number Froude's Number

that highlights the relationship existing between Reynolds' number, Froude number, the ratios of the impeller diameter to the particle diameter and of the tank diameter to the impeller diamater and the percentage ratio "B", between liquid and solid masses in the suspension (17). Since all the quantities in brackets only refer to the physical system, they can be symbolized by a single dimensional settling parameter Y, while the impeller is characterized by its dimensioness value S (17), where S = k(T/D) ~, and k and a depend on impeller type and relative blade height.

Finally, the criterion can be written:

Nj~ = S. Y.D - o . 8 5 (2)

When designing the Porto Torres reactors I used the following correlation, proposed by Nienow (18):

4 t3 0.5 TO.25 NCD = " - ' - ~ G " (3)

for cross-checking the results obtained with Zwietering's criterion. It was no surprise that the values obtained were quite different, as Table 1 shows.

The optimum speed, measured over year's pilot plant operation, was 5.65 rad.s 1 (54 r.p.m.), 34 % higher than Zwietering's correlation and 43% lower than Nienow's (19).

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Table 1 Impeller speed calculated usin~ Zwieterin~'s and Nienow's correlations.

Rushton turbine Zwietering's correlation Nienow' s correlation

diameter m r a d ' s 1 m ' s 1 r.p.m, rad.s "1 m.s 1 r.p.m.

1,00 2,59 2,59 25,00 4,45 4,45 42,50

0,67 3,65 2,44 35,00 10,00 6,68 95,60

For impeller power prediction, we can use either Mills et al.'s correlation (20): /14925 _ 1

V 0.10 - 0.0018"e V 0"4627 "G (4)

or van't Riet's correlation (21):

P _ (kLg) 1"4286 1

V O. 0001 17 0.2857 "G (5)

For a 6 flat-blade Rushton disc impeller, Neale and Pinches (22) report the following application ofvan't Riet's equation:

(_~) 0.52 ~r0.24 kLa = 0.0069 " G (6)

and for the BX04 Impeller:

(~-~/0.79 kLa:O.O084 V~ 58 (7)

It goes without saying that adequate experimentation is required. Both correlations require the tank volume and the air superficial velocity to be known. The

useful tank volume is calculated by multiplying the suspension volumetric flow rate, Vp, by the residence time, 0~p; the latter, in turn, is derived by setting the desired percent sulphide removal, Ap, and introducing this condition into the equation:

0 A p = l n ( 1 - Ap) - K ' (8)

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which was derived for pyrite (23), under the assumption that its oxidation reaction is first order, by integrating the following equation

d eS ] dt - _K~eS2] (9)

The kinetic coefficient, Kc, is obtained empyrically, usually from bench scale tests carried out either in agitated Erlenmeyer flasks or in STR's whose carefully optimized performance is taken as the absolute optimum for pyrite oxidation kinetics. It is reasonable to assume that it is precisely this passage that is the pitfall behind the design procedure; in effect, this assumption equates to stating that the pyrite oxidation kinetics in the STR's are the best achievable and are intrinsic of the biooxidation process..

Thus, the Kc adopted for the Porto Torres plant bioreactors was 1.2 �9 10 2 h 1 (23), but the value calculated from the results of one year's plant operation turned out to be considerably higher, 1,53 �9 10 "2 h "1 (24).

Tests carried out using bioreactors of new concept operated under the same conditions as the STR's (25) yielded much higher Kc.

In effect, the dependence of Kc on several factors is well documented though never explicitly stated. Of these, solids concentration plays a major role in STR's and in Pachuca tanks. The limit of about 20% solids concentration for metals sulphides bioleaching in STR' s was experimentally ascertained by several researchers (26) as long as thirty years ago and today has become an accepted rule for commercial plants, as shown by Table 2. This limitation is the major drawback of STR's, since it affects both investment costs (size of the machinery) and operating costs (power and maintenance).

4.2 The Pachuca tank Several researchers (4, 27-30) have developed correlations for predicting power requirements

and oxygen mass transfer coefficients of this type of reactor. A great deal of work has been done by the Delft University school. After Bos et al. (31), for large Pachucas the following simple correlation holds:

kza = 0.6"V~ (10)

whereas Boon et al. (30) proposed the following empirical correlation:

kLa = (229"10-2Qa) ~

that, combined with Lamont's power dissipation law (27) yields:

F l ,

(P/V in Wm 3)

(11)

(12)

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According to Meester (1992), an equation describing the correlation among the variables influencing mixing can be derived if we assume that mixing takes place in two steps, and on this there is general consensus. On a large scale the flow of the reactor contents produces mixing by convection, while further mixing takes place by diffusion on smaller scale. The state of mixing that thus is attained .by turbulent flow is determined by the size of the smallest eddies in the flow pattern, for which the following definition is proposed by Hinze (32):

3

le= Pm"

(13)

For suspensions, Zwietering's correlation is valid also for Pachucas, within the above mentioned limits.

The bioleaching performance of Pachuca tanks also rapidly decreases when the oxidizable solids concentration rises beyond 20% (26) with the same implications, as far as metal sulphide solubilization rate constant is concerned, already discussed for the STR' s.

4.3 The importance of solids concentration All endeavours to employ STR's and Pachuca tanks for bioleaching metal sulphide suspensions

with oxidizable solids concentrations higher than about 20% have so far failed. The reasons for this limitation have been the subject of much speculation, extensively discussed

in earlier reviews (4,7,33,35). The problem has been approached from the strictly abiotic viewpoint in chemical engineering journals with several interesting suggestions. However, only recently has some light finally begun to be shed on these intriguing issues thanks to the contributions of South African and Australian researchers (36-37).

Evidence was provided by Ragusa that mechanical and brutal detachment of microorganisms from the mineral surfaces to which they adhere during bioleaching - such as that produced by shear stresses acting within the suspension - causes irreversible damage to microbial cells that, despite still being viable, lose their ability to adhere and oxidize the minerals.

Moreover, Hansford and Bailey (33-34, 37 ) produced evidence that the determining factor in bioleaching is the proportion of oxidizable solids, i.e. the oxygen-consuming solid fraction in the suspension, not simply the solids concentration. Those solids that are inert to oxidation only slightly affect the process. Under the above assumptions, the presumed threshold value of about 20% solids concentration would mean, quite simply, that the oxygen mass transfer coefficient of the STR's and Pachucas currently used in biohydrometallurgy does not provide, in the best operating conditions, enough oxygen for oxidizing larger amounts of metal sulphides.

Any attempt to enhance reactor performance by increasing aeration inevitably results in increased agitation and greater shear stresses within the suspension and possibly waste of injected air due to partial flooding. Greater damage to microorganisms and insufficient oxygen mass transfer are probably the reasons behind what may be defined the "20% threshold solids concentration".

The experimental results of bench scale and pilot plant operation for coal biodepyritization support these conclusions. At Deutsche Montan Technologie (DMT) in Essen, Germany, and at Delft University (The Netherlands) bench scale tests were carried with Pachuca tanks that were successfully operated up to 40% coal (assaying 2% pyritic sulphur) (45- 46); the 8-m 3 STR's of

69

the Porto Torres pilot plant performed very well with a 40% solids suspension of the same coal (26).

In this perspective, the difficulties in developing a satisfactory conventional bioreactor design are obvious and probably unsurmountable.

5. THE NEW DEVELOPMENTS IN BIOHYDROMETALLURGICALREACTOR DESIGN

The growing awareness of the inadequacy of STR' s and of ALR' s to cope with the problems inherent in biohydrometallurgical processing owing to the conflicting requirements to enhance agitation, for Oxygen Transfer Rate (OTR) increase, and the one hand, and of a quiescent environment with negligible shear stresses on the other, has prompted the search for new types of reactors where these conditions are fulfilled to as great an extent as possible.

I will now attempt to review these new developments, and I wish to apologize for having

Coal, recycled water, tines & bacteria

Baffles

Porous pipe

"-Air

Figure 2 - Aerated through bioreactor (After Andrews et al., 49).

involuntarily overlooked those that escaped to my literature search. I think that it is fair to mention first the contributions by Andrews et a1.(47- 48) who - with a

view to developing a reactor suitable for coal biodesulphurization - already in the late eighties focussed their attention on the features that this new reactor should have had. Based on the consideration that the relatively low value of coal called for low biodesulphurization plant investment and operating costs, Andrews found, when investigating the kinetics of pyrite bioleaching, that small reactor volumes, high solids concentrations of the suspensions and the largest possible active microbial populations were the objectives to be pursued. As for microbial populations, Andrews committed himself to giving the size of the most suitable microbial population: 10 TM bacteria per cubic centimetret A figure that then would have probably been considered almost science fiction.

5.1 The aerated trough bioreactor (49) On these grounds, Andrews and his research team developed what they called "aerated

trough bioreacto~'.The device basically consists of a long rectangular tank with V-shaped bottom, along which a perforated pipe runs that acts as an air sparger (Fig. 2). The reactor is structurally

70

identical to the Callow-type pneumatic flotation cell (50). Its most significant feature resides in the fact that it operates in a manner very similar to the plug flow reactor, hence complying with the kinetics of pyrite bioleaching (9).

As far as aeration is concerned, it is doubtful that there was any improvement in OTR, since the drawbacks of pneumatic reactors, like the Pachucas, do not seem to have been overcome. In effect, the kLa in water was 0.070 s 1, whereas it decreased to 0.012 s 1 in a 45% solids concentration pulp, i.e. by about one order of magnitude (Andrews, 1990): this suggests that aeration was unsatisfactory.

5.2 The Low Energy Bioreactor (51) Almost contemporaneously to Andrews' aerated trough a research team of the CRA Company,

active in Australia, made a successful attempt to solve the problem of maximizing OTR without inducing excessively large shear stresses in the suspension.. The device they developed, called a "Low Energy Bioreactor", basically consists of three components (Fig. 3):0) a tank, where bioleaching is carried out; the "off-the-bottom" condition for the solids being satisfied by an agitator, operating inside of a draft tube, most likely for reducing the shear stresses; (ii) an aeration device where the pulp is very energetically aerated by means of a Venturi pipe; (iii) a pump for pulp recirculation.

This device represents a major step forward in the development of a tailor-made bioreactor for biohydrometallurgical processes; certainly as far as power requirements are concerned, as emphasized by the authors. The resort to separate aeration is also a significant feature, insofar as it permits the pulp to be aerated as intensively as needed.

Flow direction

Annulus /

/

I

T

Draft tuoe

Venturi

Aerator pump

Figure 3 - Low energy bioreactor, a~er (51),

71

O~

O~

O~

�9 o,-~

O~ o,..~

�9

O~ o,-~

�9

o o e ~

cO O~ c ~ ~f

~4

(D

O O~ ~n v

Z �9

(D

t ~

F-,

�9 @2

e ~ O

~ o~ v ~

O O o o O

0 0 oO 0

O O O O o o ~'~ o o

X X X

~,1 ('-,I

u.1

O O O O O

[ ' L ' v v (/ ') , ~ v v

o

.o

O

@2

A.J .,..d

O

E

O

o II

O

O

O ~a

O

A.J

O

o II

O

72

However, the suspension is subjected to violent turbulence and shear during its residence inside the aerator.

Further doubts remain as to the shear stresses (i) generated within the pulp by the impeller, (ii) produced inside of the annulus where, similarly to what happens in the Pachuca tank, the pulp flows upwards and (iii) induced in the pulp by the pump, by means of which the pulp is repeatedly circulated.

I was not able to find data concerning the kLa of this device in the literature. It is a well known fact that air flowrate alone is not sufficient, insofar as it does not provide a measure of the air actually dissolved into the pulp, as demonstrated experimentally by Andrews and Quintana (49). Neither could I find data concerning cell growth, cell numbers and residence times. Furthermore, the inventors claim that their machine was developed for processing low- grade ores (such as run-of-mine) obtaining iron leaching rates from pyrite of about 6 g.dmS.day 1

However, this performance is not exceptional since in the STR's of the Porto Torres pilot plant the research team of the CEC project observed iron leaching rates from pyrite in coal, hence comparable to a low-grade run-of-mine ore, as high as 9,2 g.dm3.day 1 .This result suggests that the objective of achieving an atmospheric oxygen mass transfer coefficient high enough to promote a drastic increase in microbial population and finally in pyrite solubiliztion kinetics has not been attained.

5.3 The Falling Laminar Liquid Film principle It has long been recognized (52) that mass transfer across gas-liquid free surfaces plays a very

D D D D z'

/" / / i" / /" /

./ . / / ./' i i i' I

i" i' / ' i ,, a ,i i" i' /

T

k_ P

Figure 4 -Deltt Inclined Plate Bioreactor. A = agitator; D = head tanks of inclined plates; Sp = inclined plates for slurry cascading; S = slurry; T = main tank; P = pumping device.

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important role in several natural and technological processes, ranging from oxygen supply and CO2 removal from animal cells and cultures to aeration of the culture media of shaken flasks or STR's, wastewater treatment and industrial microbial processes, like wine-vinegar manufacture and respiration of aerobic life occurring near the sea or lake surfaces (4, 53). A free-falling laminar liquid film poured into a pool of the same liquid can be designed in such a way as to cause very slight shear stresses in its bulk or in the liquid layers located near to the surface of the pool. So, it is only natural to consider investigating the potentials of a three-phase biohydrometallurgical reactor designed in such a way as to utilize, for atmospheric oxygen transfer, the properties of falling laminar liquid films. Based on this concept two independent research teams, operating at the Universities of Cagliari (Italy) and Delft (The Netherlands) endeavoured to develop this new type of bioreactor.

5.4 The Delft Inclined Plate (DIP) Bioreactor (54) This device is schematically shown in Figure 4. It consists of a main tank, T, containing the

slurry s: the solids are kept in suspension by an inclined agitator, A; a pumping device, P, draws the suspension from the main tank and conveys it, through a distributing system, to the head

800 -

700 -.

r~

6 0 0 -

5 0 0 - I:1

4 0 0 - tD

O

3 0 0 -

2 0 0 -

[" 1 0 0 - r~ r~

Conven t i ona l b io reac tors B io reac to r s for

b i o h y d r o m e t a l l u r g y

I I I I I I . . . . I I I I I l I I I I I

0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 1 4 1 5 1 6 1 7

Bioreactors

Figure 5 - Mass transfer coefficients for atmospheric oxygen in water for the most common bioreactors. 1: plunging jet reactor (57); 2: deep jet fermenter (57); 3: mechanically stirred tank (57); 4: axial flow turbines (57); 5: multiple Rushton turbines (57); 6: single Rushton turbine (57); 7: simple bubble reactor (57); 8: deep shaft areator (57); 9: air-lift Pachuca-type reator (29); 10: vortex agitation reactor (57); 11: Biorotor (56); 12: low energy bioreactor (51); 13: aerated trough bioreactor (48); 14: Delft Inclined Plate Bioreactor (54); 15: Biorotor (58); 16: aerated trough bioreactor (measured in a 45% solids suspension(48).

tanks, D, that feed the inclined plates, Ps; the suspension flows down these slopes cataracting into

74

the main tank. According to the inventors, atmospheric oxygen transfer is produced by the sheets of suspension plunging into the liquid surface of the main tank and entraining air bubbles. The magnitude of oygen transfer is adjusted by setting the length and slope of the inclined plates and of the flee-fall height as well as of the suspension flow rate.

The device has only been tested, as far as I know, with plain water and the authors claim lower power requirements and higher kLa' s than STR' s and Pachucas. The highest kLa value in water is reported to be 0, 0384 s 1. i.e. 138.24 h 1, hence very close to the kLa of the STR (Figure 5). The most attractive feature of the device seems to be the low power demand and, possibly, the mild agitation required for mixing. However, the presence of the agitator, A, and of the pumping device still appears to involve some shear stresses within the suspension, thus partially offsetting the advantage of less violent aeration.

As far as I know, no testing on mineral suspensions has been carried out so far with the DIP, hence it is not possible to properly assess its potential for biohydrometallurgy.. As far as the mode of oxygen transfer is concerned, the contribution of the contact of the relatively large surface area of the cataracting water sheet with the atmosphere should not be neglected, although very little is known on this topic (52, 55), and it warrants further investigation.

5.5 The revolving drum bioreactor "Biorotor" (56) From the very first endeavours to transfer biohydrometallurgy from bench to pilot to

commercial scale I became aware that the Achille's heel of this new, promising biotechnlogy was the inadequacy of the reactors used (7).My previous experience as a superintendent and then designer of mineral dressing plants was very helpful: I recalled that the sink-and-float drums were very effective for gravity separation of ores their operation being very quiet and smoot. The only drawback was that, in certain conditions, the entrainment of air caused by part of the suspension plunging into the pool that formed in the lower half of the drum produced a certain instability.

In the sink-and-float drum this was an undesirable effect: the reverse would have been true had it been intended to mix the air with the suspension.

The first prototype of the revolving drum bioreactor was developed in 1991, but it was only in 1993 that the final version of what was baptized "Biorotor" was thoroughly tested, first with plain water and finally with pure pyrite.

An isometric view of the device is shown in Fig.6: it has been exhaustively described in earlier papers, which the interested reader should consult for more details. In short, the reactor consists of a cylindrical barrel whose in.er wall is fitted with regularly spaced lifters L. Each lifter forms, with the inner wall of the barrel, a sort of tray where the suspension collects. The suspension and the air plus carbon dioxide mixture are conveyed through a pipe T fitted into one front head M of the barrel (the "feed head"); a pipe fitted into the opposite head is the exhaust outlet.

As the barrel revolves on its rollers R, the suspension is lifted upwards and when the tray reaches the top position it is discharged as a thin cataracting film with length equal to height of the cylindrical barrel.

Oxygen mass transfer most occurs during the cataracting and when the free falling film plunges into the pool of suspension in the lower part of the barrel.

This mode of operation ensures the satisfaction of all the prerequisites listed above for the ideal biohydrometallurgical reactor: (i)mixing is complete without any "dead volumes"; (ii) the only shear stresses induced into the suspension are limited to the relative motion of the plunging film with respect to the suspension in the pool: for a 2-meter diameter barrel, this relative speed

75

is lower than V/2.g.Hf -- x/2.9.81.1.4 = 5.24 m.s-1 (the free-fall height, He, being 0.7D) and the suspension drops into the upper few millimetres of the top layer, (iii) a value of kLa of at least one order of magnitude larger than that of an optimized STR (Figure 5).

The most interesting feature of this bioreactor is, however, that the OTR actually matches the kLa " i.e. the oxygen is made available to the microflora that can attain much higher growth

Figure 6 - Isometric view of Biorotor.

kinetics than those currently reported in the literature. Tests carried out with a 30% solids suspension, the solids being museum-grade pyrite, yielded solubilization rates as high as about 600 g.m3.h 1. This means that the rate constant is at least one order of magnitude higher than the value considered until now as the maximum obtainable.

Hence, the Biorotor exploits, at a very high level, the potentials of the microflora for enhancing pyrite solubilization.

Figure 7 gives the values of kLa corresponding to the range of rotation speeds considered as most suitable for biohydrometallurgical purposes (56). It seems rather trivial, but some remarks that have appeared in the literature seem to justify our pointing out that at rotation speeds higher than to - (0.5.g.D) ~ ("critical speed", corresponding to centrifugation of the suspension) the machine does not work. For the Biorotor prototype, which has a diameter of 0.3 metres, this critical speed is 7.62 rad 's 1 (corresponding to 72.8 r.p.m). Investigations carried out on a refractory gold-beating complex sulphides concentrate with a conceptually similar machine (58) confirmed the superior performance of Biorotor, although it was only operated at 0.16 rad .s 1 . In effect, extrapolating the lower branch of the diagram of Figure 7 yields the same values for kLa.

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CONCLUSIONS

The experience gained over more than thirty years of laboratory research and commercial operation clearly points out the undeniable limitations of the reactors used so far for metal sulphide minerals bioleaching.

"7

d

500

450 -

400 -

350 -

300 -

250 -

200 -

150 -

1 0 0 -

5 0 -

0 i 0,00 0,50

I I I I I I

1,00 1,50 2,00 2,50 3,00 3,50 4,00

Rotation speed, rad s "1

Figure 7 - Plot OfkLa vs. rotation speed for Biorotor (56).

These limitations are related to the intrinsic structure and mode of operation of these machines (insufficient mass transfer coefficient, high shear stresses induced into the mineral suspensions, to mention just the most significant)as well as to poor accuracy of the design

formulae. The new devices proposed over the last decade are a clear demonstration that the designers

have achieved a full understanding of the specific features that should characterize the biohydrometallurgical reactors: namely, effective Oxygen Transfer Rate, effective but mild mixing and, consequently, low retention times and low specific power requirements.

The experience gained with the latest developments, the revolving drum bioreactor, has clearly demonstrated that with an efficient reactor microbial growth can also be optimized.

77

Most of these encouraging results have, however, been obtained empirically: more theoretical research is needed so as to develop reliable design and scale-up procedures based on sound theoretical foundations.

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List of Symbols (Units consistent with respective formulae)

a - -

C s a t =

dv = g =

k L = kLa = kq =

Po =

Ms " -

B =

n a =

n L =

n T =

H = K c =

N = Nj~ = N C D =

QI = Qa = p =

P G =

Ps =

PT = PT = P / V - S = S h =

V =

interfacial area, m E "m 3, saturated oxygen concentration relative to the sparged gas composition, bubble diameter, m, acceleration of gravity, m's 2, or gravitational conversion factor, kg.ml.Nl.s, overall liquid phase mass transfer coefficient, m.s 1, volumetric mass transfer in a slurry, s i, dimensionless empirical constant, atmospheric air pressure, solids settling velocity, m.s 1, distance from impeller midplane to tank bottom, m, impeller diameter, m, diffusivity of oxygen in aqueous solution, m 2 " s "1,

tank diameter, m, suspension interface height above vessel bottom, m, kinetic coefficient for pyrite solubilization, rotational frequency of the impeller, rotational frequency of impeller for just suspended solids, rotatonal frequency of impeller at which the gas is just dispersed throghout the vessel, volumetric rate at which the liquid is pumped through the impeller, m3.s 1, volumetric gas flowrate, m3.s -~, power to produce suspension to height H, kW, agitation shaft power (gassed), kW, power to get off-bottom particle motion, kW, total power input (agitator power + bubble expansion power), kW, total power, kW.m 3, power dissipation per reactor volume, kW.m 3, impeller dimensionless parameter in Zwietering's correlation, liquid phase Sherwood number, dimensionless, liquid volume plus particle volume below air-liquid interface, m 3,

80

V G - -

W p =

y =

E =

__

0Ap=

laf =

~G =

~L =

PL =

Pm =

Ps = Ap = Ap =

~v =

air superficial velocity based on tank cross section (gas flow rate divided by cross sectional area of the tank) m .1, suspension volumetric flow rate, m3"s "1,

dimensional settling parameter in Zwietering correlation, volume fraction of liquid in the suspension (volume concentration of solids) dimensionless, volume percent of suspended solids, residence time of suspension in reactor, s, intrinsic fluid phase viscosity, Pa's, gas viscosity, kg'ml's q, liquid viscosity, kg.mq's 1, liquid density, kg'm 3, slurry density below slurry-liquid interface, kg.m 3, particle density, kg'm 3, desired sulphide percent removal, density difference, gas-liquid, k g ' m "3.

volumetric air flow rate, m3"s q