Dissolution kinetics of gold in iodide solutions

Hydrometallurgy, 27 ( 1991 ) 4 7 - 6 2 Elsevier Science Publishers B.V., Amsterdam

47

Dissolution kinetics of gold in iodide solutions P.H. Qi and J.B. Hiskey Department of Materials Science and Engineering, University of A rizona, Tucson, A Z 88 721, USA (Received April 9, 1990; revised version accepted December 3, 1990)

ABSTRACT Qi, P.H. and Hiskey, J.B., 1991. Dissolution kinetics of gold in iodide solutions. Hydrometallurgy, 27: 47-62.

Of the halogens, the gold iodide complexes are the most stable in aqueous solutions. A series of e,xperiments were performed to investigate the kinetics and mechanism of the leaching reaction between gold and iodide. Using a rotating disk technique, the effects of rotation speed, iodide and iodine concentration, temperature, pH and the presence of different electrolytes were measured. Oxygen and hydrogen peroxide were also examined as oxidants in the iodide system. A first order reaction rate was found with respect to I~- and half order reaction rate with respect to I - . A comparison of gold leaching between iodide and cyanide is also presented, in which a rate of about 2.6 X 10 9 m o l / c m 2 sec for 10-2 M Nal and 5 X 10-3 M 12 w a s obtained. This value is close to that for typical cyanidation.

INTRODUCTION

Over the last few years there has been an intense effort to identify lixiviants other than cyanide for gold and silver leaching. Although traditional cyanidation remains the overwhelming choice for treating free milling gold and silver ores, there are certain classes of ores and concentrates that are considered refractory. The inability of conventional cyanidation to treat these materials effectively has prompted the search for more powerful lixiviants. In addition, because of its toxicity and the problems of waste disposal management, the use of cyanide has been of some environmental concern. Hiskey and Atluri [ 1 ], recently reviewed the dissolution chemistry of gold and silver in different lixiviants. The halides were shown to have special properties for the processing of gold and silver ores and concentrates. Gold forms both Au (I) and Au (III) complexes with chloride, bromide and iodide depending on the solution chemistry conditions. In aqueous solution, the stability of the gold-halide complexes increases in the order C1- < Br- < I- which shows a preference for gold to bond with large polarizable ligands. Finkelstein and Hancock [ 2 ] have analyzed the stability of various gold complexes in

0304-386X/91/$03.50

© 1991 - - Elsevier Science Publishers B.V.

48

P.H. QI AND J.B. HISKEY

terms of standard potential diagrams for aurous and auric complexes. They demonstrated that with C1- the stable species is the AuC12 complex. On the other hand, with I - the preferred species is the AuI2 complex. A borderline situation appears to exist with Br-. This behavior reflects the fact that Au ( I ) has a greater affinity for soft polarizable ligands than does Au (III). Bromide and iodide also exhibit the ability to form polyhalide complexes [3,4]. This characteristic promotes the dissolution of Br2m and I 2 ~ in aqueous solution. Iodine reacts with iodide ion as follows:

Iz(~) =I2(aq)

( 1)

I2(aq ) + I - = I y

(2)

The triiodide ion can serve as an oxidant for gold leaching according to the following electrochemical reaction: Au+2I-~Auly

+ e - anodic

I!~ + 2 e - - - , 3 I - cathodic 2Au + I - + I~- ~ 2AuI£ overall

(3) (4) (5)

McGrew et al. [ 5 ] were among the first to propose the use of an electrolyte containing I - to leach gold ores. An iodine lixiviant is added to an ore containing iodine reducing components. As iodide ion increases in the lixiviant, it is possible with continued addition of iodide to achieve the desired concentration for leaching gold. This is made possible by the complexation of I2 with I - to form the polyiodide species. They report that the initial solubility of iodine is about 1.2X 10 -3 M and that the desired concentration of about 1.2 X 10-2 M could be attained by the recycling solution. In a column experiment, 80% of the gold in a marcasite ore was recovered by iodine leaching. It should be noted that gold did not start to dissolve until a sufficient concentration of iodine species remained in solution. The fundamental kinetics of the iodide leaching of gold have been examined using a rotating disk technique and are reported in this paper. The most important kinetic parameters have been identified and a rate expression is proposed. Electrochemical measurements for the gold in different halides with special emphasis on iodide will be presented in a separate paper [ 12 ]. EXPERIMENTAL PROCEDURE

The experimental method employed for the leaching tests utilized the rotating disk technique. Two grams of high purity gold powder, which was obtained from D.F. Goldsmith Chemical and Metal Corp., was used to make the gold disk. The flat disk was formed in a 1.27 cm diameter pressure mold at a total pressure of 137/895 kPa (20 000 p.s.i. ). The calculated density of

49

DISSOLUTION KINETICS OF GOLD 1N IODIDE SOLUTIONS

the pellet was approximately 17.6 g / c m 3 (compared with the theoretical density for gold of 19.3 g / c m 3). The disk was then sealed in a Teflon holder which could be screwed on the analytical rotator (Model ASR2 of Pine Instrument Co. ) so that only one surface of the disk was exposed to the solution. The surface of the gold disk was carefully polished on 600 grit silicon carbide paper and thoroughly rinsed prior to each test. All chemicals used in this research were of reagent grade quality. Distilled, deionized water was used to prepare the required leaching solution. Pure oxygen was used in the test requiring oxygen sparging. To study the effect of temperature, a constant temperature water bath/circulator was employed with circulated water through a jacketed reaction flask. The leaching reaction was carried out in a 500 ml round-bottomed reaction flask with three angled necks. Sample were withdrawn at regular intervals for analysis. These samples were analyzed using a Perkin-Elmer (model 2380) atomic absorption spectrophotometer. The leaching experiments were carried out under the following conditions, which (except for p H ) were kept constant (unless being studied): Sok[tion: Temperature: Rotating speed: Time: pH:

1X 10 - 2 M Nal and 5 X 10 - 3 M 12 room temperature (23 ° C ) 500 rpm 6h 4-6 (natural pH )

A schematic diagram of the apparatus is presented in Fig. 1. RESULTS AND DISCUSSION

As previously stated, dissolution of gold in iodide proceeds by the following reaction:

I I I I I I

*m

Jacket IReact o r ' ~ .

Weter -1 Constent Temperaturel Both 8~ Circulator

I I I I I I

[

Fig. 1. Schematic representation o f the reaction apparatus.

50

P.H. QI AND J.B. HISKEY

2 A u + I - + I ~ = 2AuI~-

(5)

As shown, iodide and iodine are consumed during the course of this reaction. The dissolution experiments were carried out with negligible changes in the bulk concentration of iodide and iodine. As predicated from the general reaction, the amount of gold dissolved from a rotating disk of constant area should vary as a linear function of time. Linear rates having the units (~tmol/ cm 2 h) were determined from the slopes of these plots.

I2/I7 solution chemistry Iodine is generally viewed as dissolving in aqueous solution by the formation of polyiodide complexes. Spectrophotometric studies have suggested the following iodine/iodide complexation equilibria [ 6 ]: 12(aq) + I - ~ - I 3

(2)

I-+Is~I

(6)

2-

12 +Ig- ~-I~-

(7)

213 ~ 12

(8)

In addition to these, Sillen and Martell [ 4 ] have reported the possible presence of I7 and I2-. Unfortunately, thermodynamic data are extremely scarce for the polyiodide complexes. Latimer [3 ] gives an equilibrium constant of K 2 = 7 . 1 4 X 10 2 for eq. (2) and it is possible to calculate a value of K== 1.21 from the data provided by Sillen and Martell [ 4 ]. From these considerations, the solubility of iodine should depend on the aqueous concentration of iodide. Silcock [ 7 ] reports data for the solubility of iodine at 25°C as a function of NaI concentration. The solubility of iodine varies linearly with sodium iodide concentration up to 2 × 10 -2 M NaI. The highest concentration of iodide used in this investigation was 1 × 10 -2 M. At this level, the m a x i m u m solubility of iodine is approximately 6 X 1 0 - 3 M 12. The distribution of iodine species plays an important role in this system. As shown above, there are a number of polyiodides that may exist in an aqueous solution of iodine. The formation of these follows the general equilibria: I2(aq ) +I~-_ 2 ~ I ~

Where x is an odd number. Alternatively:

(9)

DISSOLUTION KINETICSOFGOLDINIODIDESOLUTIONS i-+I~:_

~ 2~Ix,

51 (10)

where x' is an even number. It can be seen that the formation of the monovalent polyiodide complexes ( I x ) involves reactions with I2(aq ) while formation of the divalent polyiodide (I27) complexes requires I - . With available t h e r m o d y n a m i c data it is possible to calculate distribution of species relationship for I - , I2(aq), and 12- . Other species can be considered negligible according to the mass action consideration and due to the fact that, in our study, dilute concentrations o f both I - and I2 were utilized. The distribution of I2(aq) and I~- as a function of iodide concentration is shown in Fig. 2. At 1 X 10-2 M I - , the triiodide species is the p r e d o m i n a n t form in aqueous solution.

Effect of rotating speed The gold dissolution rate was determined as a function of disk rotating speed in solution containing 1 X 10 - 2 M I - and 5 X 10 -3 M I 2. Linear dissolution rates were observed for rotating speeds from 400 to 1800 rpm as shown in Fig. 3. Levich [8 ] derived the following mass flux expression for hydrodynamic convective diffusion in liquids:

( 11)

j=O.62D2/3 v-l~6 601/2 Co

Where: j = t h e mass flux ( m o l / c m 2 sec); D = the diffusion coefficient (cm2/sec ); v = the kinematic viscosity (cm2/sec); co = the rotating speed ( r a d / s e c ) ; Co = the concentration o f bulk solution ( m o l / c m 3 ). The above equation shows that the diffusional flux to the surface of the disk

10

08......... 06

"'"',,

020.4/ P " ' , 0 0 5

4

-3

-2

". . . . . . -I

io9 [i-]

Fig. 2. Distribution oflz(aq) and I~- as a function of iodide concentration.

52

P.H. QI AND J.B. HISKEY 100

f~ ~:

9O

• a • A

80

7O 25 E 6O

1800 rpm 1200 1000 800

/

600

•

o

/

~oo

/

/,-

/

•

/

[]

•

~1 50

g

2 o

~o

40 50

:~ 20 10 0 1

0

2

3 4 Time (hrs)

5

6

Fig. 3. Gold dissolution at various disk rotation speeds using 10 - 2 M NaI and 5 X 10 - 3 M 12. 20

l x 1 0 - 2 M Nal ~ x_

16

5x10 -3

M

12

o

-6 E

,u 4~

~

4 0 0

2

4

6

8 ~/2

10 12 (s ~/2 )

14

16

Fig. 4. Gold dissolution rate plotted against the square root of disk rotation speed. is proportional to the 2 / 3 power of the diffusion coefficient D, to the ( - 1/ 6 ) power of the kinematic viscosity, and to the 1/2 power of the disk rotating speed. Figure 4 represents a plot o f the dissolution rate ( R ) versus the square root of disk rotating speed (co). The linear relationship shown in Fig. 4 agrees favorably with the diffusion theory o f Levich and suggests that the rate is controlled by a diffusional process. However, at the concentrations of iodine and iodide employed, the rate is significantly lower than predicted by eq. ( 11 ) for typical values of D.

Effect of the iodine concentration The effect of iodine concentration on gold dissolution in solutions containing 1 X l0 -2 M I - was examined for I2 additions ranging from 1 × 10 -4 to 5 X 1 0 - 3 M. Gold dissolution as a function of I2 concentrations is depicted in Fig. 5. These data display excellent first order kinetics and indicate increasing rate with increasing iodine concentration. As explained earlier, the concentra-

53

DISSOLUTION KINETICS O F G O L D IN IODIDE S O L U T I O N S

70 cq

lx10 - 2 M Nol

6O

I~2 5x10 - 3 M

E

-~ 5o

~ 40

~

A

2.5x10 .3

~ 3o o

~

2o

2:

lo

.s 1o_4 ~ o ¢~ 4 ~' - / /~ ~ " ~ =

lx10 - 4

0

0

1

2

,3

4

5

6

7

9

Time (hrs) Fig. 5. Effect o f I2 c o n c e n t r a t i o n o n the d i s s o l u t i o n o f gold in 10-2 M NaI. TABLE 1 Dislribution of iodine species as a function of I2 Nal (Mt

I2~s) (M)

[I3] (M)

[1 ] (M)

[12] (M)

[I 2- ] (M)

Rate* R

10 2 10 -2 10 2 10 -2 10 -2

5 )< 10 3 2.5X10 3 1 )<10 -3 5 x l 0 -4 1 )<10 -4

4.02X 10 -3 2.13)<10 3 8.73)<10 -4 4.42)<10 -4 9.35)<10 -5

5 . 9 4 × 10 -3 7.86)<10-3 9.13)<10 -3 9.56)<10 -3 9.91)<10 -3

9.48)< 10 -4 3.80)<10-4 1.34)<10 -4 6.48)<10 -5 1.32)<10 -5

1.96)< 10 -5 5.49)<10-6 9.22)<10 -7 2.36)<10 -7 1.06)<10 -8

8.71 5.58 4.04 2.10 0.47

*Dissolution rate (lamol/cm 2 h).

tion of iodine was limited by its solubility in sodium iodide. Furthermore, the complexation equilibrium must be considered in determining the distribution of oxidant species. The concentration of i o d i d e / i o d i n e species is given in "Fable 1 for various concentrations of I2(s) added to a solution containing 10 - 2 M NaI. Also provided in Table 1 are dissolution rates as a function of 12 concentration. The concentration of I ; is approximately five times greater than I2(aq) for these conditions. A reaction order plot was determined for I ; for initial iodine concentrations of 1 X 10 -3, 5 X 1 0 - 4 and 1 X 1 0 - 4 M. Iodide concentration remains essentially constant in this range. This plot is shown in Fig. 6 and yields a slope of 0.96. A first order dependence on I~- is indicated by these results. It should be noted that a first order dependence on I2(aq~ is also obtained for the reaction under these conditions. In some early experiments, the effect o f oxidants other than iodine was investigated using a stationary disk and a stirring bar for agitation. Oxygen and hydrogen peroxide are compared (in Fig. 7 ) with different iodine concentrations in solutions containing 1 × 10-2 M NaI. These results show that almost no gold was dissolved using either a 101.33 kPa ( 1 arm ) Po2 sparge or 2 × 10 - 2

P.H. QI AND J.B. HISKEY

54 1.0 0.7 ~6 0.4 (~0.96

-~ 0.1 0.2 -0.5 -4.5

-4.0

-3.5

-2.5

-3.0

log [ 13 ] F i g . 6. R e a c t i o n

110 1 O0 90

o r d e r p l o t f o r I~-.

lx10 -2 M Nal

/ a

• 2×1o - 2 M lx10 - 2

E E

70 60 50 40 30

• 5x10_ 3

20 10 0

2x10 -3 • 1

2

5

4

5

6

H20~ or 7

8

9

10

1

Time ( h r s )

Fig. 7. Effect of I2 concentration on the dissolution of gold from a stationary disk using a stirring bar for agitation. M hydrogen peroxide solution. Unlike conventional cyanidation, which is characterized by relatively low oxidation potentials and the use of air in the oxidation process, leaching gold in iodide requires more aggressive oxidants. Even at low concentrations of iodine measurable levels of gold are found to dissolve. It should be noted that the experiments employing 1 X 10 -2 and 2 X 10- 2 M 12 were above the solubility limit of iodine at 1 X 10- 2 M NaI. The lower rate observed for 5 X 10- 3 M 12 in Fig. 7 as compared to that in Fig. 5 is attributed to the decreased hydrodynamic efficiency of the system using the stirring bar.

Effect of iodide concentration The effect of iodide concentration on gold dissolution was investigated for a fixed iodine concentration. In this series of experiments, NaI additions

D I S S O L U T I O N K I N E T I C S O F G O L D IN I O D I D E S O L U T I O N S

55

ranging from 10 . 3 to 10 . 2 M were used. Gold dissolution as a function of NaI concentrations is shown in Fig. 8. The rate was observed to increase with increasing NaI concentration. The concentration of iodide/iodine species is given in Table 2 for the conditions employed in this series of tests. A reaction order plot is shown for I - in Fig. 9. The slope of this line is 0.52 which indicates a half-order dependence on I - for the leaching reaction.

Effi'cts of electrolytes (;old dissolution using 1 × 10 . 2 NaI and 5 × 10 . 3 M 12 w a s examined with different concentration o f sodium sulfate and sodium chloride. These results are summarized in Table 3. The rate of gold dissolution is relatively insensitive to sodium sulfate concentration in the range 1 × 10 - 3 to 1 × 10-1 M. However, there is a noticeable decrease in the rate constant when the sodium sulfate concentration is increased to 5 × 10-1 M. The data for sodium chloride indicate similar behavior. The decrease in the gold dissolution rate at high concentrations o f both sodium sulfate and sodium chloride can be ex40

y-

l x 1 0 - 5 M b2


30

E

Nal lx10

2 M

5x10_3

20 o m m k~

2x10 5

10

o Ix10

0

1

2

3

4

5

6

7

8

9

1

3

0

Time (hrs) Fig. 8. Effect o f NaI concentration on the dissolution o f gold in the presence o f 10-3 M 12. TABLE 2 Distribution of iodine species as a function of Nal Nal (M)

I2(s) (M)

[lj- ] (M)

IxIO 2 5 X 1 0 -3 2 X I 0 -3 l X I 0 -3

10-3

10 -3 10 .3 10 .3

8.73X 7.62X 5.35X 3.48X

*Dissolution rate ( g m o l / c m 2 h).

[I- ] (M) 10 -4 10 -4 10 -4 10 -4

9.13X 4.24X 1.46X 6.52X

10 -3 10 -3 10 -3 10 -4

[12] (M)

[I62- ] (M)

1.34X 10 4 2.52X 10 -4 5.13X10 -4 7.48X 10 -4

9.22 × 7.03× 3.46× 1.47X

Rate* R 10 -7

10 7 10 7 10 7

3.94 2.86 1.51

1.02

P.H. QI AND J.B. HISKEY

56 ~.0 - -

0.5 ct c~

2 0.(3

0.5 -3.5

-3.0

2.5

-2.0

-1.5

log [q- ] Fig. 9. Reaction order plot for I - . TABLE 3

Gold dissolution rate in the presence of salts Na2SO4

Rate

NaCI

Rale

IM )

( ~tmol/cm 2 h )

(M)

( I~mol/cm 2 h )

, X 10 - 3 5×10 3 1×10 2 5 X 10 - 2 l × 1 0 -~ 5×10 i

10.68 10.85 9.85 9.09 9.31 7.14

I X 10 -3 5Xl0 3 1×10 -2 5)< 10 - 2 1 × 1 0 -~ 5×10 i

10.02 11.05 9.80 9.93

11.1l 7.06

plained in terms of possible activity coefficient effects and the fact that iodine solubility decreases in the presence of these salts [7 ].

Effect of temperature Two sets of tests were performed to determine the influence of temperature on the gold dissolution rate. One set was carried out using 5 X 10- 3 M I2 and the other using 1 × 10- 3 M I2. The concentration of Nal was fixed at 1 X 10- 2 M. Temperatures ranging from 10 to 35 °C were investigated. Owing to the volatility and thermal instability of iodide, experiments at temperatures higher than 35 °C were not performed. The results are summarized in Figs. 10 and 11 for 5 × 10- 3 and 1 X 10- 3 M I2, respectively. The initial kinetic data obtained after 2 h or less resulted in good first-order rate plots, yielding an intercept closer to the origin. The rate constants for the initial stages of the reaction are shown in Table 4. Initial kinetic values are plotted according to Arrhenius' law in Fig. 12. The activation energies under these conditions are 31.6 kJ/mol for 5 × 10 -3 M I 2 and 34.4 kJ/mol for 1 × 10 -3 MI2. The high

57

DISSOLUTION KINETICS OF GOLD IN IODIDE SOLUTIONS 120 • 55 °c

/

•EE O

190

6o 4o

k~ 20

1

2

5

4

5

6

7

Time (hrs)

Fig.. 10. Effect o f t e m p e r a t u r e o n t h e d i s s o l u t i o n o f gold in 10 2 M N a I a n d 5 X 1 0 - 3 M I2.

70 /~

• 35 °c

oq 6o E ~. 5o

~ 40 ~ 3o ©

Q 10 0

I

2

3

4

5

6

7

Time (hrs)

Fig. 1 I. Effect o f t e m p e r a t u r e o n t h e d i s s o l u t i o n o f gold in 1 0 - 2 ,~r N a l a n d 1 X l 0 - 3 M I2. TABLE 4 Initial dissolution rate o f gold as a f u n c t i o n o f temperature

Iodine

D i s s o l u t i o n rate ( g m o l / c m 2 h ) at:

(M) 5x10 lxl0

3 3

10oc

15oC

20oc

25oc

30oc

35oC

6.41 3.31

8.10 5.04

8.75 6.41

11.40 7.97

15.26 9.28

18.49 !1.04

activation energy supports the possibility of an electrochemical process controlling the reaction rate. It is recognized that the equilibrium constant for the iodide/iodine speciation (eq. ( 2 ) ) is a function of temperature. Sillen and Martell [4] reported the', values for eq. (2) given in Table 5. All conditions for the temperature experiments were maintained constant

58

P.H. QI AND J.B. HISKEY

3.5 "C" x- 3.0 ~'~,~o

= 31.6 kJ/tool

-~2.5 E "~ 2.0 Eo = 34.4-kJ/mol

~

5x10-3 M 12

--~ 1.5 • lx10 -3 M 12 1.0 3.2

3.3

3.4

3.5

3.6

3.7

l/T, * 10-3 (K-1) Fig. 12. Arrhenius plot of the leaching of gold in 10-2 M NaI solution containing 5 × 10-3 and 1 × 1 0 3MI2. TABLE 5 Values of the equilibrium constant for iodide/iodine speciation--eq. (2)--from [4] Temperature

log K

('~c) 15 20 25 30 35

2.97 2.92 2.86 2.80 2.74

(i.e. stirring, initial NaI and I 2 concentrations, and pH), however, the concentrations of I - and Iy varied slightly with temperature. Calculation of the distribution of these species as a function of temperature and normalization of the reaction rate with regards to I - and I~- resulted in activation energies of 35.6 and 32.7 k J / m o l for 1X 10 - 3 and 5 × 1 0 - 2 M I2, respectively. This difference is not considered significant in terms of mechanistic effects.

Effect of pH The effect o f p H on gold dissolution using 1 X 10 - 2 M N a I and 1 × 10 - 3 M I2 with 1 × 10-1 M Na2SO4 was examined in the pH range 2-10. The initial pH of this solution was approximately 8.20. Sodium hydroxide and sulfuric acid were used to adjust the pH. As shown in Fig. 13, the rate of dissolution of gold with I - / I 2 is relatively insensitive to pH over this range.

Comparison with cyanidation Gold leaching using iodide and cyanide are compared in Fig. 14, in which

DISSOLUTION KINETICS OF GOLD IN IODIDE SOLUTIONS

"C"

59

lx10 - 2 M Nal lxlQ - 3 M 12 lx10 -1 M No2SO4

6

5 E

o

4

o

o

u

4

5

o

o

3

2 0

1

2

,5

6

7

8

9

10 11 12

pH

F i g . 13. E f f e c t

of pH

on the dissolution

of gold

i n 1 0 - 2 M N a I , l 0 - 3 M 12 a n d 1 0 - ] M N a 2 S O 4 .

8O 04 E E

70

~o m

,'?

5O 40 30

k5 20

~2.7x10- 5 M KCN

/

60

5x10 - 5 M 12

/

~ i 1

i 2

1 lx10 -'5 M 12

3

i 4

i 5

6

i 7

i 8

9

10

Time (hrs)

Fig. 14. Comparisonof iodide and cyanide leachingof gold for differentI2 concentrations. the leaching conditions are listed separately and the data for cyanide leaching is selected for Kudryk and Kellogg [ 9 ]. Nearly the same gold dissolution rate was achieved in the solutions containing 1 × 10 - 2 M NaI and 5 × 10 -3 M I2 as in alkaline cyanide solution containing > 2.7 × 10- 3 M KCN with air as an oxidant. This suggests that iodide may adequately serve as a substitute for cyanide in the leaching of gold.

Kinetic considerations The rate controlling step for the dissolution of gold in iodide solutions is postulated to involve a mixed kinetic regime similar to that involved in the cyanidation o f gold [ l 0] and silver [ 11 ]. The gold dissolution rate follows a linear relationship with the square root of the disk rotation speed, which may indicate that diffusion of reactants through the liquid boundary layer to the gold surface may control the rate. However, from this observation alone it is

60

P.H. QI AND J.B. HISKEY

not possible to eliminate mixed kinetics (i.e., diffusion plus charge transfer) from being rate controlling. As demonstrated by the fundamental electrochemical studies by Kudryk and Kellogg [ 9 ] for the cyanidation of gold and by Hiskey and Sanchez [ 11 ], for the cyanidation of silver, the mixed kinetic region exhibits anodic dissolution rates that show a linear relationship to the square root of rotation speed. Iodine solubility was considered in determining the effects of iodine and iodide on the dissolution of gold. Iodine remained soluble for all the data at various I2 concentrations; the rate of gold dissolution increased with increasing I2 under these conditions such that: Rvc [I£ ]

The triiodide complex had the highest concentration of iodine capable of oxidizing gold and the rate was found to have a first order dependence on this species. A first order dependence was also observed for aqueous iodine (I2¢aq)). For a constant total iodine concentration, the rate increased uniformly with increasing iodide concentration according to the following relationship: Roc [ I - ] 1/2 The half-order dependence on iodide suggests that a process other than strict diffusion is controlling the rate of gold dissolution. As shown in Table 2, the predominant iodine species shift from aqueous iodine to the triiodide species as the NaI increases from 1 X 10-3 M. However, if the reaction order with respect to [ I - ] is determined for the narrow range of NaI where [I3] predominates (i.e. NaI at 1 X 10 - 2 and 5X 10 -3 M) and is essentially constant, the reaction order is 0.42 with respect to [I- ]. On the other hand, if [ I 2 ( a q ) ] predominates (i.e., NaI at 2X 10 - 3 and 1 X 10 - 3 M ) the reaction order is 0.49 with respect to [ I - ]. Under conditions where the triiodide complex prevails (i.e. high [I- ] ) the rate should vary according to: R = k e -Ea/RT [I~- ] [I- ]1/2

(12)

If the following conditions are chosen, [NaI] = 10 .2 M and [I21 =

10-3M

TABLE 6 C o m p a r i s o n o f predicted a n d e x p e r i m e m a l rates ( R ) ( g m o l / c m 2 h ) Nal

12

Predicted

Experimental

(M)

(M)

R

R

10 2 10 .2 10 -2

1X 10 -3 5 × 10 . 4 1 × 10 . 4

4.02 2.08 0.45

4.04/3.94 2.10 0.47

DISSOLUTION KINETICS OF GOLD IN IODIDE SOLUTIONS

61

where Ea = 34.4 kJ/mol, it is found that the predicted rates are very close to the experimental ones shown in Table 6. The results from this work suggest that a rate-controlling process other than simple diffusion may control the dissolution of gold in I - / I 2 solutions. Analysi,; of the electrochemistry of the gold-iodide system will be presented in a later paper [ 12 ]. CONCLUSIONS

From the results above, we can draw the following conclusions: ( 1 ) Gold dissolution in the presence of iodide using iodine as an oxidant proceeds by the following reaction: 2 A u + I - +I~- = 2 A u I y (2) Dissolution of gold in I - / I 2 solutions follows first order kinetics. (3) The first order rate is proportional to the square root of the disk rotation speed. (4) The reaction is first order with respect to the triiodide complex and half order with respect to iodide. ( 5 ) Oxidants such as oxygen and hydrogen peroxide have almost no effect on the gold dissolution with iodide. (6) Gold dissolution is insensitive to the presence of Na2SO4 and NaC1 and is also insensitive to pH over the range pH 2-10. ( 7 ) The rate of gold dissolution in relatively dilute I2/I- solution ( 5 × 10- 3 MI2 and 10 -2 M N a I ) is about the same as that for cyanide (2.7× 10 3 M KCN and air). (8) The experimental activation energy averages about 33 k J / m o l under the conditions studied. ACKNOWLEDGEMENTS

This research project was financed by the Arizona Mining and Mineral Resources Research Institute under U.S. Bureau of Mines Grant Number G1184104. This support is gratefully acknowledged.

REFERENCES 1 Hiskey, J.B. and Atluri, V.P., Miner. Proc. Extr. Metall. Rev., 4 ( 1988): 95-134. 2 Finkelstein, N.P. and Hancock, R.D., Gold Bull., 7 ( 1974): 72-77. 3 Latimer, W.M., The Oxidation States of the Elements and Their Potentials in Aqueous Solution. Prentice-Hall, Englewood Cliffs, N.J., 2nd Ed. (1952). 4 Sillen, L.G. and Martell, A.E., Stability Constants of Metal Ion Complexes. Chemical Soc., London (1964). 5 McGrew, K.J. and Murphy, J.W., US Pat. 4,557,759 (1985).

62

P.H. Q1 AND J.B. HISKEY

6 Sano, T., Hori, H., Yamamoto, M. and Yasunaga, T., Bull. Chem. Soc. Jpn., 2 (1984): 575-576. 7 Silcock, H.L., Solubilities of Inorganic Compounds. Vol. 3, Part 1. Pergamon, Oxford (1979). 8 Levich, V,G., Physicochemical Hydrodynamics. Prentice-Hall, Englewood Cliffs, N.J. (1962). 9 Kudryk, V. and Kellogg, H.H., J. Metals, 5 (1954): 541-548. 10 Wadsworth, M.E., Min. Eng., 6 ( 1985): 557-562. 11 Hiskey, J.B. and Sanchez, V.M., J. Appl. Electrochem., 20 (1990): 479-487. 12 Qi, P.H. and Hiskey, J.B., Electrochemical behaviour of gold in iodide solutions. Hydrometallurgy ( 1991 ) (submitted).