Electrochemical aspects of zinc sulphide leaching by Thiobacillus ferrooxidans

Hydrometallurgy, 33 (1993) 137-152

137

Elsevier Science Publishers B.V., Amsterdam

Electrochemical aspects of zinc sulphide leaching

by Thiobacillus ferrooxidans W.K. ChoP, A.E. T o r m a b, R.W. Ohline c and E. Ghali d aDepartment of Materials and Metallurgical Engineering, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA D1NEL, EG & EG, Idaho Inc., Idaho Falls, 1D 83415, USA CDepartment of Chemistry, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA dDepartment of Mining and Metallurgy, Laval University, Quebec City, Quebec, Canada

(Received May 7, 1990; revised version accepted July 8, 1992)

ABSTRACT Choi, W.K., Torma, A.E., Ohline, R.W. and Ghali, E., 1993. Electrochemical aspects of zinc sulphide leaching by Thiobacillusferrooxidans. Hydrometallurgy, 33:137-152. It is proposed that the bioleachability of a zinc sulfide flotation concentrate is influenced by the electrochemical behavior of ZnS. The overall reaction of bio-oxidation: ZnS+ bacteria 202 , ZnSO4 was found to involve a number of intermediate electrochemical reactions, as indicated by the cyclic voltammetric measurements using carbon paste-ZnS working electrodes. The solubilization of ZnS during leaching is solid-state diffusion controlled, as indicated by chronopotentiometric and chronoamperometric measurements. The activation energy of the solid-state-diffusion process was determined by Sand's method to be 21.3 kJ.

INTRODUCTION Z i n c is a v e r y i m p o r t a n t metal. T h e d e m a n d for it is c o n t i n u o u s l y increasing, especially in the galvanizing, c h e m i c a l , agricultural, r u b b e r a n d p a i n t industries. T h e m o s t c o m m o n zinc sulfide m i n e r a l is sphalerite, which is f o u n d o f t e n in a s s o c i a t i o n with iron, lead, c o p p e r a n d c a d m i u m sulfides. Z i n c rec o v e r y f r o m the a b o v e resources b y b i o h y d r o m e t a l l u r g i c a l m e t h o d s has b e e n s t u d i e d b y m a n y investigators [ 1-5 ] a n d the overall r e a c t i o n is given by: ZnS + 2 02 bacte~aznsO4

( 1)

Correspondence to: E. Ghali, Universit6 Laval, Facult6 des Sciences et G6nie, D6partement de Mines et M6tallurgie, Cit6 Universitaire, Qu6bec, Que. G 1K7P4, Canada.

0304-386X/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.

138

W.K.CHOIET AL.

E h - p H diagrams provide information about the stable species that result during the leaching process. However, the E h - p H diagrams do not provide information on intermediate reactions which may occur during the leach process; only the stable initial reactants and final products are shown in these diagrams. The intermediate reactions often control the rate of the overall extractions. These intermediates can be assessed using cyclic voltammetric techniques [ 5,6 ]. When zinc sulfide is finely disseminated with pyrite, zinc dissolution is accelerated by galvanic interactions [7,8 ] in which, ZnS acts as the anode and is preferentially oxidized, while pyrite essentially acts as the cathode, where oxygen is reduced to water. However, a small amount of pyrite may also be oxidized to ferric sulfate and sulfuric acid as shown by the following equation [91: 2 FeS2+7½ O2-k-H20

bacteria

, Fe2(504)3 +H2504

(2)

Ferric sulfate is known to be an oxidizing agent for zinc sulfide [ 10] which contributes to the bioleaching process: ZnS + Fe2 ( 5 0 4 ) 3 -+ Z n S O 4 q- 2 F e S O 4 -b S °

( 3)

Ferrous sulfate and elemental sulfur formed in eq. (3) are oxidized by the bacteria to ferric sulfate and sulfuric acid [ 11 ]: 2 FeSO4 -I-H2 SO4 + 1 02 bacter!aFe2 (SO4) 3 + H20

(4)

So+ 1½ 02 + H2 obactefi'aH2 SO4

(5)

The bioleaching of zinc sulfide according to reaction ( 1 ) represents the direct mode of bacterial action [ 12 ], while the oxidation by ferric sulfate, as shown in reaction (3), gives the indirect mode of bacterial attack [ 13 ]. The role of bacteria in the latter leaching mechanism is to reoxidize continuously ferrous to ferric ion and elemental sulfur to sulfuric acid. In general, the leaching kinetics of metal sulfides are often described by methods which are based on metal corrosion. However, most naturally occurring metal sulfides are semiconductors [ 14 ] rather than conductors. Therefore, a better approach to the kinetic description of metal sulfide leach phenomena could be achieved by using models that are based on the solid-state properties of semiconductor minerals. Zinc sulfide often occurs in a tetrahedron structure. It has two polytype modifications: cubic (sphalerite or zinc blende) and hexagonal (wurtzite). Sphalerite is a diamagnetic semiconductor that has a large energy gap (3.63.9 eV) [ 15 ], very low conductivity ( 10- t2 ~ - ~ m - ' [ 16 ] ) and a relatively low solubility (Ksp= 10- 24 ) [ 14 ]. The naturally occurring ZnS mineral has

ELECTROCHEMICAL ASPECTS OF ZnS LEACHING

139

many crystal defects and zinc can also be replaced by other metal ions. Therefore, zinc sulfide can occur in different forms and types (the n form, having excessive negative charges, and p type semiconductor, having excessive positive charge). The present study describes the intermediate reactions involved in the bacterial leaching of a zinc sulfide concentrate in sulfuric acid, which were investigated by electrochemical techniques, such as cyclic voltammetry, chronoamperometry and chronopotentiometry. MATERIALS AND METHODS

Bacteria A culture of Thiobacillusferrooxidans used in this investigation was originally isolated from copper mine drainage waters and adapted to the zinc sulfide concentrate [ 17 ]. It was routinely maintained in our laboratories on a modified 9K nutrient medium [ 18 ] in which the ferrous sulfate energy source was replaced by a ZnS concentrate. When bacterial growth reached the late logarithmic phase, a portion of the leach suspension was used as experimental inoculum or stored at 4 °C to maintain the stock culture. The stock culture was renewed on a monthly basis. A typical SEM picture of the leaching bacteria is shown in Fig. 1.

Fig. 1. Scanning electron microscope (SEM) characterization of Thiobacillusferrooxidans at 25,00 magnification.

140

W.K. CHO1 ET AL.

ZnS flotation concentrate The ZnS flotation concentrate was obtained from Cominco Ltd., Trail, B.C. It was ground to a particle size of less than - 4 0 0 mesh (37 ~tm). Analysis showed that it contained 53.7% zinc, 32.1% total sulfur, 4.6% iron, 7.8% lead and 1.8% other minor concentrations of base metals.

Bacterial leaching experiments This series of experiments was carried out in 250 c m 3 Erlenmeyer flasks containing 1.50-18.75 g zinc sulfate concentrate, 70 cm 3 iron-free nutrient m e d i u m (pH = 2.3 ) and 3 cm 3 inoculum of T.ferrooxidans. In the sterile controls, 5 cm 3 of a 2% thymol in methanol solution was added to the leach suspension instead of the bacterial inoculum. The flasks were incubated at 35 ° C using an incubator shaker agitated at a constant 250 rpm in presence of air [ 19 ]. At predetermined time intervals, the water lost was compensated for with distilled water, and if the pH had increased to above a level of 2.5, it was adjusted to pH 2.3 with 1 N sulfuric acid. A 1 cm 3 volume of liquid sample was removed periodically from the flasks and analyzed for dissolved zinc content by atomic absorption spectrophotometry (AAS). The 1 c m 3 samples were replaced with an equal volume of iron-free nutrient medium.

Electrochemical measurements The 200 cm 3 electrochemical cell was equipped with a ZnS-graphite paste working electrode, platinum inert (counter) electrode and saturated calomel (reference) electrode, E h - p H meters, temperature controller, potentiostat and x-y recorder. The ZnS (flotation concentrate )-graphite paste electrode is shown in Fig. 2. The carbon-paraffin oil mixture in the open end of the electrode was composed of 5 g graphite and 2 c m 3 paraffin oil, upon which was put a homogenized layer of a paste composed of 1 g ZnS, 4 g graphite powder and 2 cm 3 paraffin oil. The cell was charged with 75 cm 3 leach suspension (inoculated or sterile ), a potential was applied between the working and reference electrodes and swept at 100 mV s-1 from negative to positive potentials, and the complete cycle reversed. The corresponding cell current between working electrode and counter electrode was recorded [ 20,21 ]. Figure 3 shows a typical cyclic voltammetric curve (Fig. 3b) together with the sweep curves (potential (E) versus time). During the oxidation sweep, the generalized reduced species, R, was oxidized to O and electrons are released. The associated peak current was registered. The half peak current was then estimated and the corresponding half peak potential read from the potential scale (Ep/2). If this is measured against a reference electrode (SCE), then first this

ELECTROCHEMICAL ASPECTS OF ZnS LEACHING

141

RUBBER

GI..ASSTUBE~ [

RUBBER

\

TEFLON

\

CARBON + PARAFFIN OIL

I~- 0.6 -H

CPEE ; CARBON + PARAFFIN OIL + ZINC SULFIDE

Fig. 2. Diagram of the ZnS-carbon paste electrode.

\,, Time

(A)

(+) ic

Ep/~ vs.SCEc°nv:~Eo/2vs.SHE =E°catc. 03)

Fig. 3. Typical sweep and cyclic voltammetric curves.

value is converted to a potential versus standard hydrogen electrode. That potential is approximately equal to the standard electrode potential (E°), which can be calculated from thermodynamic data by:

EO R T =~-ff l n g e q

(6)

where R = the universal gas constant; T = absolute temperature; n = number of electrons involved in the half cell reaction; F = t h e Faraday constant; Keq = the equilibrium constant. Keq can be calculated from the standard free energy (AG °) of the reaction: 0

0

z~GReaction-~- ~z~G Products- - ~-~/JaReactants 0

(7 )

The free energy change of products and reactants are available from thermo-

142

W.K. CHOI ET AL.

dynamic tables. The equilibrium constant can then be assessed from the expression: 0

AGReaction -~

-RT

I n Keq

(8)

If Keq is known, the standard half cell potential (E °) can be calculated from eq. (6). In chronopotentiometry, a constant current is applied to the working and counter electrodes and the half cell potential against the reference electrode is measured as a function of time [20,21 ]. In chronoamperometric measurements a constant potential is applied to the electrodes, and the cell current is measured as a function of time [20]. RESULTS A N D D I S C U S S I O N

Bioleaching of ZnS-concentrate The efficiency of bacterial leaching is shown in Fig. 4, where inoculated experiments and sterile control experiments are compared. The difference in efficiency increases enormously as a function of time. For example, the zinc extraction in an inoculated solution was two times that of the sterile solution after approximately 100 h and became eight times after 800 h. There are almost two slopes for the curves of the inoculated solution and one slope for the sterile solution. The initial continuous rise in zinc leaching for the inoculated solution could correspond to the additional leaching of zinc by ferric sulfate, the bacterial oxidation of sulfur to sulfuric acid and the regeneration 9S8070-

60 g 50

4o ~z ~o 2O Sterile

o 100

200

300

400

Time

500

600

700

800

(h)

Fig. 4. Extraction of zinc from a 20% pulp density suspension in the presence and absence of Thiobacillusferrooxidans at 35 ° C and 2 50 rpm.

ELECTROCHEMICAL ASPECTS OF ZnS LEACHING

143

of ferric sulfate. The second slope may correspond to the optimum conditions of zinc leaching due to the direct and indirect modes of bacterial leaching. Electrochemical measurements Using the cyclic voltammetric experimental setup and the carbon paste electrodes, a series of potentiodynamic anodic and cathodic polarization curves were measured (in the range of applied potentials - 1.0 to + 1.4 V ) in the presence and absence of T. ferrooxidans. The results are shown in Fig. 5. In the initial portion of the anodic potentiodynamic curves, the current density measured in the presence of bacteria was the highest. In the cathodic potentiodynamic curves, the current density in the presence of bacteria was the lowest. This phenomenon is in accordance with the fact that the bacteria are efficient only under oxidizing conditions, which prevail only under anodic potentiodynamic conditions. No data are available from literature for comparison. The results obtained in the cyclic voltammetric measurements are summarized in Fig. 6. The relatively higher peaks obtained from the inoculated runs are due to the small amounts of zinc transferred into the leach suspension with the bacterial inoculum. Since several oxidation peaks (B, C, D) are shown on the cyclic voltammetric curves, the zinc sulfide oxidation is more complex than suggested by eq. (4). In fact, the oxidation and reduction peaks are the result of the following electro-chemical reactions:

I#"

a, a': carbon paste (sterile) b, I~ : ZnS(20 %)-carbon paste / (sterile) .'"t " .... ' "" c, c • ZnS(20 %)-carbon paste ..' / " ~'.\ :'. " (inoculated) .." / •

~ ~: .~

"..,

..

~', ", k{-.. I "..

0.1,

t

."

\\ ...,.,y

/,

,'!°'

',\" "... i7 -~

44

r-.-'..""

/¢

•

001

/

-/ ....-/:

o'n

o14

is

1~2

E (V) Fig. 5. P o t e n t i o d y n a m i c anodic (a, b, c) and cathodic (a', b', c' ) curves measured in nutrient m e d i u m at p H = 2 . 3 and 21 °C.

144

W.K. CHOI ET AL.

BH

2~'

'E ?

/q* Sterile

< E

0.0'

Inoculated

C

~--'~B'/~/B

fi:!i/"::::' '~ ' /

A

-l.0,

-4.0"

~

-1,o

D

E~ J

E (V) vs. SHE

0,0

-,u -to

-0",5

o,,s 0~0

,,0 o'5

E (V) vs. SCE Fig. 6. Cyclic voltammograms taken in the initial phase of ZnS leaching in the presence and absence of Thiobacillusferrooxidans using 20% ZnS-carbon paste electrodes (first cycle). The CV measurements were made in stagnant leach suspensions at pH 2.3, 35°C and sweep rate of 100 mV s - t . The potential scale is given against both standard hydrogen electrode (SHE) and saturated calomel electrode (SCE).

cYCLES 2.0, ¢-I

LE t~

~0~"

-l.0-

-~ -~0

E (V) vs. SHE

a,~

45

oio

a5

tl o'5

E (V) vs. SCE Fig. 7. The influence of time on the cyclic voltammogram of a 20% ZnS-carbon paste electrode in the presence of Thiobacillusferrooxidans at 21 ° C and a sweep rate of 100 mV s - ~.

145

E L E C T R O C H E M I C A L ASPECTS O F ZnS L E A C H I N G

BN

4 ~ . , . !!

l :

(~/~)

0.00 2.

3

9.25 16.50

I, ~!

2..._ e':~I! zo.

q

- .:~1

,~

.

...

,,,

::.:.,:..:.:._:..:-...

<~' L o~

-2.0-

A!i'

-4.0-

tl

i i

-p

-u.

-1~

E (V) vs. S H E

~

43

a,5

o~o

o'.5

E ( V ) vs. S C E Fig. 8. T h e effect o f zinc ion c o n c e n t r a t i o n s o n t h e cyclic v o l t a m m o g r a m o f a 20% Z n S - c a r b o n paste electrode in the p r e s e n c e o f Thiobacillusferrooxidans at 21 ° C a n d a s w e e p rate o f 100 m V S-1.

Peak A:

Z n S + 2 H + + 2e---,Zn°+ H2S

E°= -0.89 V

(9)

Ep/2= - 1.04 V vs. S C E = - 0 . 8 0 V vs. SHE

(1o)

Zn 2+ + 2e--,Zn ° Ep/2= - 1.04 V vs. S C E = - 0 . 8 0 V vs. SHE Peak B':

Z n ° + H 2 S ~ Z n S + 2H + + 2 e

E°= -0.89 W

( 11 )

Ep/2= - 0 . 7 5 V vs. S C E = - 0 . 5 1 V vs. SHE Peak B":

Zn°--,Zn 2÷ + 2 e

E°= -0.76 V

(12)

Ep/2= - 0 . 7 4 V vs. S C E = - 0 . 5 0 V vs. SHE Peak B " :

H2S+S2-~2S°+2H+

+4e

E°=-0.17V

(13)

Ep/2= - 0 . 4 3 V vs. S C E = - 0 . 1 9 V vs. SHE Peak C:

Z n S ~ Z n 2+ + S ° + 2 e

E°=0.26 V

Ep/2= - 0 . 0 2 V vs. S C E = 0 . 2 2 V vs. SHE

(14)

146

W.K. CHOIET AL.

20

T- . . . . j , , /-,1

3-~

_,--7~,I

OJ'

". . . . . . . . .

.~

/://f

c

/.JZ/~"

" ....

•

~.:."

Fe- / F e (both in g / 1 ) 1 : 0.0/0.0 2 : 0.0/ 9.0 3 : 0.8/ 8.2 4 : 3.7/ 5.3

/ -2.0' ]

,/ E (V) vs. S H E

-0.5

0.0 -05 E (V) vs. S C E

-1.0

0.5 0.0

LO 05

Fig. 9. The effect o f ferric/ferrous ion ratio on the cyclic voltammogram of a 20% Z n S - c a r b o n paste electrode in the presence o f Thiobacillusferrooxidans at 21 ° C and a sweep rate of 100 mV S-I"

c J " (g/I) 1 : 0.0

2S' 1-

B

/~

2:0.8 3:1.3

A,~

[

4 " 28

3~-H-. 2 "b~" .¢ \""-'~"//-

¢J

1

/

k.

-2.5, E (V) vs. S H E

-l~o

-o~5

o~o

o~5

1:o

E (V) vs. SCE Fig. 10. The effect of cupric ion concentration on the cyclic voltammogram of a 20% ZnScarbon paste electrode in the presence o f 100 mV s -~.

P e a k D:

S°-I- 4 H 2 0 ~

Thiobacillusferrooxidans at

SO 2- +8H+

+6e

E p / 2 - - - - 0 . 5 0 V vs. S C E = 0 . 7 4

P e a k E:

SO 2- + 8 H + + 6 e ~ S ° + 4 H 2 0

21 ° C and a sweep rate of

E°=0.36 V

(15)

V vs. S H E E°=0.36 V

(16)

ELECTROCHEMICAL ASPECTS OF ZnS LEACHING

147

(A)

1400 1200

i =50

1000

>

800 600

,L~

400-

U A ' c m -2

A'.

2000-

~o 2o ao go io

0

do io do

9o

t [see] ~1~"~ 0.8.

~

0.6.

Inoculated

"~ 0.4. 0.2.

0,0'

AA. ~ ' ~ - ~ t ' I t • =

0.00

0.04

o.b8

Sterile

o.'12

o.'~6

o.2c

i [mA. cm2] Fig. 11. ( A ) A typical c h r o n o p o t e n t i o m e t r i c curve o f 20% Z n S - c a r b o n paste electrode in 9K n u t r i e n t m e d i u m in the p r e s e n c e o f Thiobacillusferrooxidans at 21 ° C. ( B ) A s u m m a r y o f the • t e r m s o f plots o f . tr 1 / 2 V ersus i for the i n o c u l a t e d a n d sterile runs. c h r o n o a m p e r o m e t r i c data in

E0/2=0.20 V vs. SCE=0.44 V vs. SHE When, in the inoculated experiments, the cyclic voltammograms were repeated several times during the first 5 rain of leaching, the peak currents decreased, as shown in Fig. 7. The decrease in the peak currents corresponds to the disappearance of active sites from the surface of the ZnS-carbon paste electrode. As the time of leaching progressed, larger concentrations of zinc, iron and copper were solubilized. The increase in the concentration of zinc had an influence on peaks B', B", B'" and C (Fig. 8 ). The current increase indicates that the electrode is more exposed to the solution and corresponds to a more important attack (Fig. 8). The shift of the peak potential to more positive values can be caused by the increase in the reaction products. Ferric and ferrous ions concentrations especially influenced peaks B", C and D (Fig. 9). Peak B" can reflect not only zinc but also iron dissolution. The latter became distinct at high concentrations of Fe3+/Fe 2+ (curve 4). It can also be observed that, when the ferric ion concentration was increased, the currents of peaks C and D were increased enormously, due to the accelerated oxidation of sulfur ions to sulfur or sulfate. The increase in the cupric ion concentration (Fig. 10 ) also influenced the shape of the cyclovoltammogram, indicating the

148

W . K . C H O 1 E T AL.

0"251

(A)I

0.201 ,.~

0.154 500 m V

0.10..I 0.054 0.00 4

1'0

2'0

3"0 4.0 t [sec]

0.6~..~ ~.

'E 4

5.0

6"0 70

Inoculated ...... Sterile

(B) mV

0.4-

900

........ • X

o.2- X XX

.,~a

XX

0.0,,

0

...Z .

1"0 2.0

....

X Z

.....

3"0 4"0 s.0 t [sec]

X

700 5O0 900 X 700

X

X

500

e"0 70

Fig. 12. (A) A typical chronoamperometric curve using a 20% ZnS-carbon paste electrode in 9K nutrient medium in the presence and absence of Thiobacillusferrooxidans at 21 ° C. (B) A summary of data in terms of plots o f i z 1/2 versus t.

possibility of high overpotentials of the existing peaks or new peaks associated with other sulfides. In the presence of Cu 2÷ ions the surface of ZnS can be converted to CuS or Cu2S [ 22 ]. The nature of zinc sulfide leaching and whether it is chemically or diffusion controlled, was studied by chronopotentiometric and c h r o n o a m p e r o m e t r i c techniques in the presence and absence of bacteria. The chronopotentiometric experiments were carried out in the range of constant currents of 0.0250.190 m A cm -2. A typical chronopotentiometric curve is shown in Fig. 11A, indicating the determination of transition time r. F r o m Fig. 11 B it can be seen that, at higher than 0.16 m A c m - 2 current densities, the i z 1/2 term becomes independent of further increase in i. This indicates that ZnS dissolution is controlled by solid-state diffusion. A similar statement has been m a d e [23] in relation to the anodic treatment of copper sulfides. The solid-state diffusion associated with the zinc sulfide concentrate leaching has been verified by c h r o n o a m p e r o m e t r i c measurements. These experiments were carried out at constant cell potentials at 500, 700 and 900 m V by observing the change

149

E L E C T R O C H E M I C A L ASPECTS O F ZnS L E A C H I N G

°'61(A )

O3) .,. a

~mm

- I ~~

m)~XX XX NX

• ×mXo e * ~ *~*

o.4

'"b

f.'d

++ + ,,,,,

=~

.

~E

e

7 0.0

13.3-

~

o.oo o.bs o.1o o.;s 0.20 o.?s

i [ma. cm=]

12.3.

11.3. 2.8

31o ]03/T

312

3.~

[K -z ]

Fig. 13. A plots of iz 1/2 as a function of i. ( B ) Arrhenius type plot of In ( iT ~/2 ) versus 1/ T. Data are from chronopotentiometric measurements using 20% ZnS-carbon paste electrode in 9K nutrient sterile leach medium at various temperatures.

t

S°

S°

H2S + Zz2*

ff

> S(~f

? "ii

Zff +S °

'//////P/7////2~ (A) S t e r i l e

(B) B a c t e r l a

Fig. 14. Schematic presentation of ZnS leaching in acid leach suspensions (A) in the absence and (B) in the presence of bacteria.

in the reaction currents, i, as a function of time (t). A typical chronoamperometric curve is shown in Fig. 12A. Also, the composed values were plotted against time (Fig. 12B). As can be seen, the ir 1/2 term becomes independent of time above 18 s. This behavior is typical for reactions which are controlled by solid-state diffusion. Similar findings have been reported for copper sulfide-carbon paste measurements [23,5]. Furthermore, if the chronopotentiometric measurements are carried out at different temperatures, then the activation energy, dEa of solid-state diffusion can be calculated according to the Sand's approach [23 ]. Figure 13A is a plot of iT 1/2 versus i

iT1/2

150

W.K, CHOI ET AL.

at temperatures varying from 21 ° to 73 ° C. The iz 1/2 terms are constant above about 0.16 mA cm -2. These values have been used to calculate dE,. Since:

izl/2 = ½z~l/2nFD l/2C

(17)

where n = the number of electrons transferred in the electrochemical reaction; F = Faraday's constant; C = t h e concentration of zinc sulfide concentrate; D = t h e solid-state diffusion coefficient, which is defined as: D =Doexp (-XlEa/RT)

( 18 )

where R = t h e gas constant; T--the absolute temperature. Combining eqs. ( 17 ) and ( 18 ) results in the following expression:

(i.f 1/2= ½n~/2nFC[Doex p ( - D E , / R T ) ],/2) 2

(19)

Taking the natural logarithm of eq. ( 19 ) we have: In ( i 2~.)

=

In ( zc n 2F 2009 C 2 / 4 ) - AEa/R T

( 20 )

A plot o f l n ( i 2 r ) versus 1/Tgives a straight line (as shown in Fig. 13B) with a slope equal to -AEa/R, from which the apparent activation energy has been calculated to be xlEa= 21.3 kJ. This relatively low value of AEa suggests that the zinc sulfide concentrate oxidation is controlled by solid-state diffusion, which is in excellent agreement with the interpretation of the data in Figs. 11 and 12. The mechanism of zinc extraction from the sulfide flotation concentrate by Thiobacillusferrooxidans can thus be explained by the semiconductor structure of zinc sulfide. In the interface between the solid semiconductor ZnS and acidic bioleaching solution, there are many ionic species which have different energy levels [24]. When the free energy of the electrons in redox system (102/O2-- and Fe3+/Fe z+ ), Ef~redox), of the nutrient bioleaching solution is higher than the Fermi energy level, Ef in the solid ZnS semiconductor, then ZnS will be decomposed. The process will continue until equilibrium is reached, when Ef=Et~reOox). The relative position of Ef(redox) of the leach system to the decomposition energy level, Ed, of ZnS provides information about the stability of ZnS. Figure 14 illustrates zinc sulfide leaching in the presence and absence of micro-organisms. It is clear that the bacteria can operate on leaching in direct a n d / o r indirect modes. These are demonstrated by the increasing slope of zinc leaching as a function of time and by the increase in oxidative peaks to sulfur and sulfates in the cyclovoltammograms obtained in presence of Fe 3+ ions. CONCLUSIONS

The following can summarize the essential conclusions in this study: (1) Zinc extraction in an inoculated solution (Thiobacillusferrooxidans)

ELECTROCHEMICAL ASPECTS OF ZnS LEACHING

151

can accelerate zinc leaching from zinc concentrate by up to eight times over the sterile control. (2) The effect of bacteria on zinc leaching can be explained as the direct oxidation of the concentrate, the oxidation of sulfur to sulfuric acid and the regeneration of the oxidant. ( 3 ) Cyclovoltammograms showed that the oxidation of part of the zinc sulfide to sulfate occurs through the oxidation of sulfur to sulfate. (4) Potentiodynamic curves show that bacteria have no direct influence on the cathodic reaction. (5) Chronopotentiometric and chronoamperometric techniques, together with the calculated low activation energy, confirm that the oxidation reaction is under solid-state diffusion control at high leaching rates. ACKNOWLEDGEMENTS

The present study was supported in part by a U.S. Department of the Interior Grant G 1194135. The opinions expressed in this article are those of the authors and not necessarily those of the funding agency.

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