HydrometaUurgy, 24 (1990) 373-392 373 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

Characterization and electrochemical analysis of gold cementation from alkaline cyanide solution by suspended zinc particles

J.D. MILLER, R.Y. WAN and J.R. PARGA

Department o[ MetaUurgy and Metallurgical Engineering, University o[ Utah, Salt Lake City, UT 84112 (U.S.A.)

(Received September 20, 1988; revised and accepted August 14, 1989)

ABSTRACT

Miller, J.D., Wan, R.Y. and Parga, J.R., 1990. Characterization and electrochemical analysis of gold cementation from alkaline cyanide solution by suspended zinc particles. Hydrometallurgy, 24: 373-392.

Recent research efforts on gold cementation from alkaline cyanide solutions have provided fur- ther information on the nature of cementation reactions. The experimental work reported herein includes the kinetics of gold cementation by suspended zinc particles and electrochemical mea- surements using rotating zinc disk electrodes. The reaction which was usually limited by film diffusion was studied as a function of such variables as gold concentration, temperature, sodium cyanide concentration and lead nitrate addition using suspended zinc particles. The critical effect of the cyanide concentration on reaction kinetics was observed. Of particular significance was a particle aggregation phenomenon which involved the rapid coalescence and growth of reacting zinc particles (45 ¢tm) to aggregates exceeding several millimeters in size. Evidence of zinc pas- sivation and anodic closure were found from electrochemical measurements and support the anal- ysis of the suspended particle reaction kinetics.

INTRODUCTION

Zinc-dust cementation for gold and silver recovery, also known as the Mer- rill-Crowe Process, is one of the most common contact-reduction processes in hydrometallurgy. The efficiency of the reaction, which was commercially de- veloped by Merrill in 1897, was improved by Crowe in 1918 using a vacuum deaerator to remove oxygen prior to zinc addition. The reaction can be ex- tremely efficient but is sensitive to suspended solids in the leach solution, and thus solution clarification is required before cementation. Advantages of the Merrill-Crowe Process are fast reaction rates, low gold inventories, and ease of operation. However, leach liquor clarification, zinc cost, and reaction sen- sitivity to impurities of the leach liquor can make zinc-dust cementation tech-

0304-386X/90/$03.50 © 1990 Elsevier Science Publishers B.V.

374 J.D. MILLER ET AL.

nology less competitive with other process technologies, particularly carbon adsorption [ 1 ].

The Merrill-Crowe Process is generallly used for gold precipitation from dilute aurocyanide solutions and is favored especially for gold cyanide solu- tions containing a high silver concentration. Recently, M I N T E K researchers suggested that zinc cementat ion should be considered as a process alternative to electrowinning for direct t rea tment of carbon eluates in the CIP process [ 2 ]. At present, in South Africa, one operating CIP plant uses cementat ion for direct t rea tment of carbon eluate, and in the U.S. two other plants now use the zinc-dust cementat ion process in this way.

Although cementat ion of precious metals from cyanide solution is used ex- tensively in industry, most of the early articles published on the cementat ion of gold dealt only with plant practice. Research studies on gold cementat ion have been confined to the study of reaction kinetics with rotating disks or cylinders. Nicol et al. [3] reported the results of an electrochemical study re- garding the kinetics and mechanisms of gold cementat ion on a rotating-disk electrode. The effect of various factors including cyanide concentration, pH, agitation, addition of lead, and the presence of various impurities on both the anodic zinc half-cell reaction and the cathodic half-cell reaction for the reduc- tion of the aurocyanide anion were investigated. It was reported that the ad- dition of 3 mg l - 1 lead to a gold cyanide solution results in an increase in the current density for the gold reduction half-cell reaction. Some interesting re- sults regarding the influence of various metal ions on the rate of cementat ion were reported. Barin et al. [4] studied the kinetics of gold cementat ion from alkaline cyanide solution using a zinc-coated gold cylinder. They concluded that gold cementat ion takes place with intermediate Zn(OH)2 formation at ( C N ) < 3.5 X 10 -;~ M. The reaction rate increased with increasing KCN con- centration but increased only slightly for cyanide concentrations exceeding 3.5 X 10 -:~ M. This cyanide concentration is referred to as the critical cyanide concentration, at which the zinc consumption is minimum. Paul et al. [ 2 ] stud- ied the electrochemical reactions involved in the cementat ion of gold by zinc from concentrated aurocyanide electrolytes using rotating zinc electrodes. The morphologies of the gold deposits on the zinc electrodes obtained for two dif- ferent gold concentrations were reported to differ significantly. A porous layer of gold with well-defined crystals was obtained from the electrolyte containing 80 mg l - 1 Au, whereas a compact, nonporous layer was obtained from the so- lution with a gold content of 640 mg l - 1. The presence of the compact layer of gold could certainly account for a decrease in reaction rate due to anodic clo- sure at the zinc surface.

Very little research has been reported for suspended particle kinetics. Con- sequently, such a detailed fundamental study of gold cementat ion from alka- line cyanide solutions was undertaken. The experimental work includes kinet- ics for suspended zinc particles and electrochemical measurements with

(IOIJD CEMENTATION FROM CYANIDE SOLUTION BY ZINC PARTICLES :~75

rotating-disk electrodes. The study also includes a systematic SEM examina- tion of the deposit structures obtained under various experimental conditions.

EXPERIMENTAL

The batch reactor, suspended particle cementation experiments were done in four-necked 1-1 glass reaction cells supported in a constant-temperature bath. A condenser, stirrer, nitrogen-dispersion tube, and sampling device were placed into the reactor through the opening in the lid. In all studies, purified nitrogen was passed through 500 ml of solution, via a dispersion tube, before and during the experiment to maintain an oxygen-free environment. Prior to the beginning of an experiment the solution was adjusted to the desired pH and temperature, and the nitrogen gas was bubbled for at least 15 min. The particulate sample of zinc ( 1 g) was washed with acetone and cleaned with acid to remove any surface oxide film before being introduced into the reaction flask. In order to follow the course of the cementation reaction, solution ali- quots of 5 ml were taken periodically and analyzed for gold with a direct-cou- pled plasma spectrophotometer. After each run, the reaction product was fil- tered, washed with deionized water, rinsed with acetone, and dried in a vacuum dessicator. The surface deposits were studied using a scanning electron micro- scope, Hitachi S-530.

Standard electrochemical measurements were carried out with rotating-disk electrodes. A platinum electrode was used as the counter electrode. IR drops in the system were minimized by positioning a Luggin capillary close to the surface of the disk electrode and by maintaining a certain concentration of background electrolyte. Potentiodynamic polarization experiments were de- signed to understand the fundamentals of each of the half-cell reactions and the effects of various experimental variables. The scan was typically 2 mV s- '. The apparatus employed to carry out the potential scan was a EG&G PAR Model 173 Potentiostat/Galvanostat used in conjunction with a PAR Model 175 Programmer and a Model 376 Logarithmic Current Converter. Mixed po- tential measurements versus time were carried out to analyze the A,,/Aa ratio during the cementation reaction. All mixed potentials were measured relative to a saturated calomel electrode using a high-impedance voltmeter.

BATCH REACTION WITH SUSPENDED PARTICLES

Variables such as noble metal ion concentration, temperature, sodium cya- nide concentration, and lead addition were studied using suspended zinc par- ticles. The rate of the cementation reaction is expected to be a function of the surface area, and consequently the cementation reaction kinetics were studied using zinc particles of different size. Experimental result are discussed in terms

376 J.D. MILLER ET AL.

of the kinetic features of the reaction as influenced by surface deposit characteristics.

First-order reaction kinetics

The cementation reaction kinetics were studied using zinc particles of dif- ferent size. As expected, the results were found to conform to first-order reac- tion kinetics with over 99% removal of gold from the gold cyanide solution, as shown in Fig. 1. The micrograph of the reacted zinc particle shown in Fig. 1 reveals the nature of the gold deposit. Further examination of the cementation reaction product showed that the gold formed a uniform layer around the zinc particle, apparently with sufficient porosity to sustain the first-order kinetics.

Table I lists the values of the reaction velocity constants for the cementation

1 0 °

w

o

z _o lo 1 r,-

k- Z UJ (J Z (3 (J

o. \ \ Z 2 0 0 X 1 5 0 UJ :E r~

-2 lO 4 o o x 2 7 o 2 7 o x 2 o o

i i I J J i l J t I r

2 4 6 8 10 12 14 16 18 20 22

T I M E , rn in

Fig. 1. First-order plots showing the rate of removal of gold from cyanide solution by suspended zinc particles of various size.

GOLI) CEMENTATION FROM CYANIDE SOLUTION BY ZINC PARTICLES 377

TABLE 1

Comparison of experimental reaction velocity constants with calculated mass-transfer coeffi- cients for suspended particles in the Au (CN)~/Zn system NaCN = 10 -2 M, pH = 10.5, stirring speed= 1000 rpm, Au = 10 mg l- ~ (5 × 10 5 M)

Particle size Experimental Calculated (mesh) reaction velocity mass-transfer

constant coefficient (cm s-X) (cm s 1)

70×100 1.7×10 2 1.28X 10-'~ 100×150 1.7X10 -2 1.36X10 ~ 150X 200 1.8X 10 -2 1.60X 10 2 200X270 1.9X10 ~ 1.64X 10 -2 270X400 1.9X10 2 2.15X10 2

of gold by zinc dust which were de t e rmined by using the geometr ic mean size for each par t ic le size interval . F rom these data, it is ev ident t h a t the cemen- t a t ion of gold by zinc dust conforms to wel l -behaved f i rs t -order reac t ion ki- netics. F u r t h e r analysis of these rate da ta by compar i son with the expec ted magni tude of suspended-par t ic le mass - t r ans fe r coeff icients suppor t s the hy- pothes is t ha t the reac t ion is l imited by t r a n s p o r t in the aqueous phase. T h e mass - t r ans fe r coeff ic ient for suspended spherical par t ic les can be pred ic ted from the Har r io t t corre la t ion [5], which involves reactor design, st irr ing speed, and power input , and is expressed by the agi ta t ion in tensi ty , I:

I=ND~/:~ (DI/T) (T/Z) 1/;~ (1)

where: N = s t i r r ing speed ( m i n - 1 ); DI = impeller d iamete r ( inches) ; T = t an k d i ame te r ( inches) ; and Z = suspens ion dep th ( inches) .

T h e reac t ion veloci ty cons tan t s were found to be similar to the expec ted mass - t r ans fe r coeff icients unde r these condi t ions as shown in Table 1. Th is ag reemen t demons t r a t e s tha t , for low initial gold concen t ra t ions , the cemen- t a t ion reac t ion exhibi ts the p roper dependence on par t ic le size (surface a rea ) , and t ha t the reac t ion is cont ro l led by a mass - t r ans fe r bounda ry - l aye r diffu- s ion- l imited process.

Temperature effect and aggregation phenomenon

T h e t e m p e r a t u r e coeff ic ient of the reac t ion rate, or the ac t iva t ion energy, can provide useful i n fo rma t ion regarding rate control . Prev ious studies [4] wi th a ro ta t ing cyl inder have repor ted an appa ren t ac t iva t ion energy of 3.1 kcal m o l - 1 ( 13.0 kJ m o l - 1 ) in the t e m p e r a t u r e range be tween 283-313 ° K, for an init ial Au (CN)~- concen t r a t i on of 2 X 10 5 M and a free cyanide concen-

378 J.D. MILLER ET At.

tration of 5× 10 -'~ M. It was concluded that under these circumstances the rate is controlled by film diffusion of the Au (CN)~ anion. In this current study with suspended particles, the cementation kinetics for gold were studied at different temperatures for two different initial gold concentrations. The Ar- rhenius plots for low, 10 mg 1-1 ( 5 X 1 0 - 5 M), and high, 85 mg 1-1 (4.3X10-4M), initial concentrations of gold are shown in Fig. 2, where the reaction velocity data (calculated from the first-order rate equation ) are plot- ted versus reciprocal temperature on a semilog plot.

Examination of the data suggests that at higher temperatures well-behaved film diffusion-controlled first-order reaction kinetics are observed. Generally, ~;he reaction velocity constants are independent of initial gold concentration and an activation energy of about 3 kcal mol-1 (12.6 kJ mo1-1 ) is calculated. As the temperature decreases, deviation from expected behavior occurs, and a

T E M P E R A T U R E , ° C

6 0 5 0 4 0 3 0 2 0 10 0 l I I I I I I

. 05 - -

(o (D

(9

'~ ~a .=33.5 KJ /mo I

<

0 .01 m _.1 I..LI

;> 85 mg/ I Z " \ 0 ",, F-- o .oo5 'Q

LU 10 mg/I n"

I I I I I I I 2.9 3.0 3.1 3.2 3 .3 3 .4 3.5 3.6 3.7

1 / T °K ' l . X l O 3

Fig . 2. Arrhenius plots for the cementat ion of gold by zinc dust particles at two different concen- trations of gold, 5 × 10 - 5 M a n d 4 . 3 X 10 4 M .

GOI,I) CEMENTATION FROM CYANIDE SOLUTION BY ZINC PARTICLES 379

higher activation energy of about 8 kcal mo1-1 (33.5 kJ mo1-1) is found at lower temperatures depending on the initial concentration. In the case of 85 mg l- 1 (4.3 X 10- 4 M) gold, this transition seems to happen at 35 ° C, while for 10 mg l- 1 (5 × 10 -~ M), the transition occurs at a lower temperature of 23 ° C. These results might suggest a change in rate control at lower temperature, a phenomenon which has been observed previously in several different cemen- tation systems [6]. In some cases, this transition has been related to surface deposit effects and in fact does not represent a change in rate control. Such appears to be the case for the gold cyanide/zinc system.

In this study, a change in the morphology of the gold deposit with change in temperature was observed. Figure 3 shows the deposit at 23 ° C and 66 ° C from the more concentrated gold solution, 85 mg l- 1 Au (4.3 × 10 4 M). At the lower temperature, a smooth deposit is produced whereas at the higher temperature the deposit morphology changes significantly, becoming quite botryoidal and rough in appearance. However, the increase in surface are (due to roughness) seems, in this case, to have an insignificant effect on the reaction rate for sus- pended particles. The reaction velocity constants do not exceed the predicted values. Two-stage or enhanced reaction kinetics could not be observed for the first-order reaction plots, as is frequently found in rotating-disk or rotating- cylinder studies [6]. It seems that the surface roughness factor is of minor importance for suspended particles [6 ], unlike the situation for rotating disks or rotating cylinders. Such a finding is not surprising, because, in the case of the rotating-disk and rotating-cylinder studies, the surface velocity takes on a finite value of substantial magnitude (the slip velocity is significant), and con- sequently distortion of the boundary layer and/or microstreaming at the sur- face leads to increased mass transfer and an increase in the reaction velocity constant. For the suspended particle, however, the slip velocity relative to the liquid phase is small. Consequently, the nature of the surface deposit has little impact on the reaction velocity constant. Distortion of the boundary layer or microstreaming would not be expected, and any increase in the reaction veloc- ity should be of minor importance. Similar results have been observed for the Ag(CN)~-/Zn system [7].

The unexpected or unusual decrease in reaction velocity constant at low temperature is mostly due to a reduction in surface area because of particle aggregation, an interesting phenomenon which has been observed and char- acterized during these experiments. Under certain circumstances, low temper- ature and/or more concentrated gold solutions, particle aggregation occurs. That is, during the first three minutes of the reaction, the initially dispersed 45-~m zinc particles aggregate, and the originally opaque suspension becomes clear. In many instances, the aggregates were found to attach to the reactor wall and/or the gas sparger and become immobilized. In this regard, the re- duction in the reaction velocity constant is not surprising due to a reduction in reaction area and an increase in the effective diffusion distance. An SEM

~'~80 J,D. MILLER ET AI,.

Fig. 3. SEM micrographs showing the gold deposit at two different temperatures. Magnification 1000 × .

micrograph of such an aggregate is shown in Fig. 4. These aggregates have considerable stability when wet and a brilliant gold color when formed at lower temperature.

Anodic closure

During the cementation reaction, the anodic zinc surface may be closed by an adherent gold layer or may be passivated by an oxide film at low cyanide concentrations.

GOLD CEMENTATION FROM CYANIDE SOLUTION BY ZINC PARTICLES 381

Fig. 4. SEM micrographs showing aggregates of gold-coated zinc particles at high gold concentra- lion ( lOOmgl l ) a n d 2 3 ~ C .

Adherent gold layer Theoretically, if the reaction kinetics are limited by mass-transfer, bound-

ary-layer diffusion-control, the reaction velocity constant should be indepen- dent of the initial gold concentration. However, the experimental results in- dicate that the cementation reaction rate decreases with increasing gold concentration. Table 2 lists the reaction velocity constants (taken from Fig. 5 ) for different initial concentrations of gold. The apparent deviation from the first-order reaction kinetics might suggest a change in reaction mechanism, but it is doubtful that such an explanation can be supported.

382 J.D. MILLER ET AL

TABLE 2

Comparison of the reaction velocity constants for different concentra t ions of gold. NaCN = 10- : M, pH = 10.5, st irr ing speed = 1000 rpm, 23 °C

Initial gold Reaction velocity Layer thickness concentra t ion constants calculated (mgl l) (cm s - l ) (pm)

1.7 2.20X 10 2 0.002 10.6 1.90X 10 2 0.015 66.0 1.30 X 10 e 0.080

100.0 0.94X 10 ~ 0.127

o

o

z o_ I,-

er k.,

Z .1 UJ 10 0 Z 0 0

(D

UJ ,,.J Z 0

Z w

10 0

0 2 4 6 8 10

T I M E , min

Fig. 5. First-order plots showing removal of gold from cyanide solution for various init ial concen- t ra t ions of gold.

GOLD CEMENTATION FROM CYANIDE SOLUTION BY ZINC PARTICLES 383

The results presented may be interpreted in several ways. First, particle ag- gregation occurs significantly at lower temperatures and/or high gold concen- trations ( > 50 mg l - 1 ) as was observed during these experiments. This aggre- gation phenomenon definitely reduces the reaction surface area and increases the effective diffusion distance, leading to a decrease in the apparent reaction velocity constant.

Second, the presence of the gold layer itself might cause the reaction velocity constant to decrease. At a low gold concentration, 1.7 mg 1-1, the reaction

Fig. 6. SEM micrographs of cross-sections of gold-coated zinc particle as prepared from alkaline gold cyanide solution. Excess zinc remains inside the particle. Magnification: (a) 3000X; (b) lO,O00X.

'384 J.D. MILLER ET AL.

velocity constant was 2.2× 10 -2 cm s-1, and an increase in the initial gold concentration to 100 mg l-1 caused the first-order reaction velocity constant to decrease by about a factor of two, as presented in Table 2. If an increase of the gold layer thickness accounted for the decrease in cementation rate, the kinetics would not be expected to be linear on a first-order plot but, rather, would have a continual decrease in slope. Such is not the case, as shown in Fig. 5.

Of course in the extreme case, the gold layer on an individual zinc particle could become significant and lead to anodic closure. Paul and Howarth [2] found that for a high concentration of gold (640 mg l- 1) the presence of the compact layer of gold caused passivation of the zinc surface. When the results in Table 2 are considered together with electrochemical measurements as we will discuss later in this paper, the possibility of anodic closure should be con- sidered. Even at a relatively high CN- concentration (10 -2 M) it was found that the cathodic to anodic area ratio can vary by a factor of two as determined from electrochemical measurements and analysis (see Fig. 10). Further, when the initial gold concentration is increased, the gold deposit layer was clearly observed by SEM analysis. Figure 6 shows the SEM micrographs of the cross- sections of 30-~m zinc particles (0.1 g) after 3 days of cementation in 500 ml of solution with an initial gold concentration of 1000 mg 1-1 and 0.1 M in sodium cyanide. Examination of these micrographs indicates that the gold layer which formed on the zinc particles was sufficiently strong to maintain struc- tural integrity during the anodic dissolution of zinc. Finally, it should be pointed out that rate control by mass transfer boundary layer diffusion can still pre- dominate during anodic closure as can be established from electrochemical theory [2 ].

Hydroxide passivation It has been known for a long time that the formation of a "white precipitate"

of hydrated zinc oxide inhibits the cementation rate of gold and silver with zinc dust and is detrimental in plant operation. Barin et al. [3] found that, for [ C N ] - < 3 . 5 × 1 0 -:~ M, gold cementation takes place with intermediate Zn (OH)2 formation, and that above a critical CN- concentration the influ- ence of hydroxyl ions in gold cementation disappears; i.e., Zn(CN)24- may be formed directly during the cementation reaction. Nicol et al. [3] have also explained that the lower rates obtained for gold cementation at lower concen- trations of sodium cyanide are associated with the dissolution of zinc under conditions during which the rate is controlled by the diffusion of free cyanide to the zinc surface and concluded that this effect can be expected to be more pronounced if a heavy gold deposit is formed on the surface of the zinc, thereby restricting the diffusion to take place within the open pores of the deposit.

Figure 7 shows the sodium cyanide concentration effect. For high concen- trations of free cyanide, 10 -1 M and 10 -2 M, well-behaved first-order rate

GOLD CEMENTATION FROM CYANIDE SOLUTION BY ZINC PARTICLES 385

1.0

0.5 0 ~ ' ~ Na CN, M

\ \ \ E 3 [] 10. 3 o\~\~ ~ lo-~ 0.2 0 10 -~

o.1 \,,,\ ~ o \

.05 ~ EJ . . . . . 0 \Lx

Z ~ \\\\

LU \\

S .02

~ A 0 \ AulCN] 2 5 x lO-4M

. o, \ , \ pH 10.5

Z i.g \

~, .005 0 \

\\\

.002 \ \ \\ LX~

I I ; I '~ I t 410 I 10 20 30

TIME, min Fig. 7. Effect of sodium cyanide on the cementation of gold at p H = 10.5 and rpm = 1000.

kinetics were observed. For a low concentration of cyanide, 10 -~ M, an inhib- iting effect on the reaction rate was observed, apparently due to the formation of zinc hydroxide at the surface of the zinc particle, thus protecting the zinc from the alkaline cyanide solution.

The results shown in Fig. 8 indicate that the passivated surface can be ac- tivated on addition of cyanide because the rate increased immediately when the free cyanide concentration was raised from this initial low level of 10 - s M to 10 -2 M after the reaction had stopped. Also note that the rate is almost the same as it was prior to passivation. These results indicate that increasing the free cyanide concentration removes the zinc hydroxide which had formed on the surface of the zinc particle, restoring electrical continuity between anodic and cathodic half-cells, which leads to the original mass transfer-limited ce- mentation rate. Figure 8 also shows the scanning electron micrographs of the

386 J.D. MILLER ET AI,.

1.0

0.5

\ O

' ~ 0.2 • - \

[3

~ 0.1 NaCN 10-3M

Z

0 05 , n ~ Z \ 0 0 \ or) i

iii \ ~Z .02 O

Z w ~ .01 D

. 0 0 5

. 0 0 2

10 20 30 40 5 0 60 T I M E , min

Fig. 8. Effect of increased free cyanide concentration by injection after passivation.

gold deposited on zinc particles when passivation occurs and after the addition of free cyanide when the original rate is restored. From these SEM micro- graphs, it can be seen that when the reaction stopped there were some empty places on the zinc surface where zinc hydroxide appears to have formed and blocked the anodic sites. When the free cyanide was increased, the zinc hy- droxide, no longer being stable, was dissolved. Gold cementation was then pos- sible, and the passivation areas originally present became covered with a layer of gold completely enveloping the zinc particles (anodic closure), but with sufficient porosity to sustain the first order reaction.

GOLD CEMENTATION FROM CYANIDE SOLUTION BY ZINC PARTICLES 387

Effect of lead addition

Finally it was found that, at low cyanide concentration, 10 -:3 M, gold ce- mentat ion is terminated apparently due to zinc hydroxide formation at the zinc surface. This inhibition effect that was found at low concentrations of free cyanide was eliminated by the addition of lead to the alkaline cyanide gold

,oO

Au 5X104 M \

'\[~ pH 10.5

k \

\

i 1 o_ 1 6 - - b .

" ~ [ ~ Pb NONE Z ~

O Z O U 0'3 U') U,,I _1 z O

z ul

o

Pb lOmg/I

0 6 12 18 24 30 T I M E ,rain

Fig. 9. Effec t of lead add i t i on on t he c e m e n t a t i o n of gold a t 10 -~* M s o d i u m cyanide .

388 J.D. MILLER ET AL.

solution. The results are shown in Fig. 9. The presence of lead in the system and its fixation at the zinc surface may reduce the surface concentration of zinc in the anodic interfacial region and decrease proton reduction, thus re- ducing the tendency for zinc hydroxide to form. This analysis is supported by the electrochemical measurements and will be discussed later.

E L E C T R O C H E M I C A L C O N S I D E R A T I O N S

The cementation reaction is a metal displacement reaction which is similar in many ways to electrochemical corrosion reactions. In this regard, the ce- mentat ion reaction can be analyzed in terms of the respective half cells; the cathodic reduction of the aurocyanide anion,

A u ( C N ) ~ + e = A u + 2 C N - (2)

which is coupled to the anodic dissolution of the zinc,

Z n + 4 C N - = Z n ( C N ) ] +2e (3)

A common electrochemical technique to analyze the mechanistic features of leaching, corrosion, and cementation reactions is the Evans diagram approach [6]. In essence, the diagrams are simply the superposition of the polarization curves for the cathodic and anodic half-cell reactions. The mixed potential Em is established at the point of intersection of the anodic and cathodic polariza- tion curves. The current at that point is a measure of the expected reaction rate, assuming the polarization curves to be independent of each other.

The mixed potential during a cementation reaction can be used to analyze the system with respect to the relative importance of the anodic and cathodic half-cell reaction areas [8]. For a rotating-disk electrode, if the cathodic re- action is mass-transfer controlled, then the cathodic current can be expressed a s

i~. = - 0 . 6 2 n c F D 2/;~ u -1/6 0) 1/2 ( C - C s ) (4)

where ic = limiting current density, amp cm -2 n¢. = valence of the ion being discharged, F = Faraday constant, amp s tool -1, D = diffusion coefficient, cm 2 s - ~, v = kinematic viscosity, cm 2 s - 1, (o = angular velocity, rad s - 1, C = bulk concentration, mol cm -~, and Cs = surface concentration, mol cm -3

For the case of a fast surface reaction, Cs = 0, the anodic half-cell reaction, if the reaction is in the Tafel region, is:

GOLD CEMENTATION FROM CYANIDE SOLUTION BY ZINC PARTICLES 3 8 9

t t E = E ° - (1--~-a)~a F lnia'°-t- (1-~a)~/~,F lnia (5)

where (~a=transfer coefficient of the anodic half-cell reaction and i~,,, = exchange current density. At the mixed potential in, ix = ia = -- i¢~.

Thus Eq. 5 can be expressed as,

E=Ea- (1-C~a)t/~f) ln ia,o-- (1---~) t /~F

where C~a is the transfer coefficient of the anodic half-cell reaction and i .... is the current density.

For cementa t ion reactions, we can use the Wagner-Traud Theory of corro- sion [8], which assumes tha t the surface area available for each half-cell re- action is the same, and equal to the total area of metal in contact with the solution, However, in certain circumstances, the anodic zinc surface may be passivated by an oxide film at low cyanide concentrat ions or may be covered by an adherent gold layer at high gold concentrations. Thus, it is expected tha t the cathodic and anodie surface areas will not remain the same during the course of the reaction, and, because of this possibility, changes in cathodic and anodic surface areas should be taken into consideration,

i~A, = -icA,, (7)

o r

n c . i~ = ~,. (8)

Aa

Then, subst i tut ion of Eq. 8 into Eq. 5 and sett ing E as E ' in order to take into consideration the relative change in anodic area and cathodic area, gives

RT RT RT A~. E '=E°- {(X-~ai~ /~F} l n i a . o - - { 1 - - ~ a } l n i ~ + { ( 1 - ~ ) ~ / ~ F } l nA a

(9)

Combining Eqs. 6 and 9 then provides an expression relating the change in the mixed potential E - E ' to the relative change of the cathodic and anodic half- cell areas:

RT E' -E= { ( I_~)~I~F} ln ~ (10)

Here E is the mixed potent ial at the init iat ion of the cementa t ion reaction,

390 J.D. MILLER ET AI,.

] I i J i I i I

CN-IO-3-MM] A u ( C N ) 2 I Z n SYSTEM

10

_o 8 I--

<~ 6 A¢ _ (1 - n l n F )] cr~ A a - e X p ~ ( E c o r r ( t ) - E c o r r ( O )

4

2

• C N - I O - 2 M ]

I J I i - - ~ . . . . i _ ~ I

1 2 3 4 5 6 7 8

T IME, r a i n

Fig. 10. Plots of the calculated ratio A¢/Aa in the Au(CN )~/Zn metal displacement reaction.

I ; I I L

40

. / "

0 30 / / ' " / li'"

20 .-

/ . • CN-IO 3 M -'" Au 5X10 4 M

- ~ Pb 2mg/I / i f 10 /E l l • Pb NONE

J - - I i I J 2.5 7.5 10 12.5

TI/~E , r a i n s

Fig. 11. Plots of the calculated ratio AJA, in the Au(CN)~-/Zn metal displacement reaction showing the effect of lead addition•

a s s u m i n g A a = Ac, and E' is the mixed potential for the cementation reaction at time t. From Eq. 10, the significance of the anodic closure during the ce- mentat ion reaction of Au (CN)2 - /Zn can be determined by the change of the mixed potential during the course of the reaction. Eq. 10 enables us to deter- mine the ratio Ac/A a during the cementation reaction,

GOLD CEMENTATION FROM CYANIDE SOLUTION BY ZINC PARTICLES 391

d~a =exp {E(t) - E ( 0 ) } (11)

The variation of the A c / A a ratio for the Au (CN) ~ / Z n reaction with a rotating- disk geometry at low and high concentrations of cyanide is shown in Fig. 10.

It can be seen that, for a low concentration of sodium cyanide ( 10 -3 M), the anodic area decreases due to the formation of zinc hydroxide. These electro- chemical measurements agree with the previous results for suspended parti- cles, when inhibition of the cementation reaction occurred at low concentra- tion of sodium cyanide. See Figs. 7 and 8.

To further examine the electrochemical nature of this system, the mixed potential measurements were used to test the effect of lead addition. Results shown in Fig. 11 indicate that, for a low concentration of sodium cyanide (10- '~ M) and without lead, the anodic area suddenly decreases after several minutes' reaction. However, in the presence of lead, the relative decrease in the anodic area is not as significant.

S U M M A R Y

Experiment results indicate that gold cementation from alkaline cyanide solutions by suspended zinc particles conforms to well-behaved first-order re- action kinetics under certain circumstances. Under these conditions, experi- mental reaction velocity constants correspond to the expected magnitude of the limiting mass-transfer coefficients and support the position that the ce- mentation reaction is limited by mass transfer of the aurocyanide species to the reaction surface. The Arrhenius plots for the Au (CN)~/Zn cementation reaction support the notion of mass-transfer rate control at high temperature with an activation energy of about 3 kcal mol-1 (12.55 kJ mol-1).

Although there is some change in gold deposit morphology, especially at a higher temperature, unlike rotating-disk or rotating-cylinder systems, this phenomenon seems to have little effect on reaction kinetics for suspended par- ticles. Of greater significance is the particle aggregation phenomenon. At low temperatures and high initial gold concentrations, particle aggregation occurs, which accounts for an increase in the apparent activation energy. The aggre- gates were found to form from reacting 45-/tm zinc particles during the initial stage of reaction and to reach a size of several millimeters in diameter.

Cyanide concentration is critical in the cementation reaction. Low concen- trations of free cyanide lead to passivation of the reaction due to the formation of zinc hydroxide at the zinc particle surface. Generally, an increase in the concentration of free cyanide eliminates the passivation in a reversible fash- ion. Also, the addition of lead ion reduces the tendency for zinc hydroxide passivation.

392 J.D. MILLER ET AL.

E l e c t r o c h e m i c a l m e a s u r e m e n t s s u p p o r t t h e a n a l y s i s o f t h e s u s p e n d e d p a r -

t i c l e r e a c t i o n k i n e t i c s . E v i d e n c e for z inc h y d r o x i d e p a s s i v a t i o n a n d a n o d i c c lo-

s u r e we re f o u n d f r o m m i x e d p o t e n t i a l m e a s u r e m e n t s a t a r e a c t i n g r o t a t i n g z inc

d i s k e l e c t r o d e .

REFERENCES

1 Mooiman, M.B., Miller, J.D., Hiskey, J.B. and Hendriksz, A.R., 1983. Comparison of process alternatives for gold recovery from cyanide leach solutions. In: J.B. Hiskey (Editor), Au and Ag Heap and Dump Leaching Practice. AIME, New York, N.Y., pp. 93-107.

2 Paul, R.L. and Howarth, D., 1986. Cementation of gold onto zinc from concentrated auro- cyanide electrolytes. In: Gold 100, Proc. Int. Conf. Gold. 2, Johannesburg, pp. 157-171.

3 Nicol, M.J., Schalch, E. and Balestra, P., 1979. A modern study of the kinetics and mecha- nism of the cementation of gold. J. S. Afr. Inst. Min. Metall., 79:191 198.

4 Barin, I., Barth, H. and Yaman, A., 1980. Electrochemical investigations of the kinetics of gold cementation by zinc from cyanide solutions. Erzmetallurgy, 33: 399-403.

5 Harriott, P., 1962. Mass transfer to particles: Part 1. Suspended in agitated tanks. J. Am. Inst. Chem. Eng., 8 ( 1 ): 93 102.

6 Miller, J.D., 1981. Solution concentration and purification. In: John K. Tien and John F. Elliot (Editors), Metallurgical Treatises. AIME, New York, N.Y., pp. 95-116.

7 Parga, J.R. Wan, R.Y. and Miller, J.D., 1988. Zinc-dust cementation of silver from alkaline cyanide solutions - - Analysis of plant data. Miner. Metall. Proc. 8:170 176.

S Power, G.P. and Ritchie, I.M., 1981. Mixed potential measurements in the elucidation of corrosion mechanisms. Electrochim. Acta, 26: 1078-1981.