thiourea solution, chalcopyrite and pyrite increased the rate of dissolution when ... cyanide solutions when both electrodes were in the same container.
Minerals Engineering, Vol. 3, No. 6, pp. 5 8 9 - 5 9 7 , 1990
0892-6875/90 $3.00 + 00 © 1990 Pergamon Press plc
Printed in Great Britain
GALVANIC INTERACTIONS DURING THE DISSOLUTION OF GOLD IN CYANIDE AND THIOUREA SOLUTIONS J.S.J. VAN DEVENTERI, M.A. REUTER, L. LORENZEN and P.J. HOFF
Department of Metallurgical Engineering, University of Stellenbosch, Stellenbosch 7600, South Africa t Author for correspondence (Received 15 November 1989; accepted after revision 1 March 1990)
ABSTRACT Galvanic interactions between gold and associated substances are amongst the many factors that could influence the rate of dissolution of gold in a cyanide or an acidic thiourea solution. Some of these galvanic influences were investigated here by short-circuiting two rotating disc electrodes either in the same container or in two separate containers linked by a salt bridge. In a cyanide solution it was established that the largest decrease in the rate of gold dissolution was obtained when gold was in contact with copper, chalcopyrite and pyrite in the same container. On the other hand, in an acidic thiourea solution, chalcopyrite and pyrite increased the rate of dissolution when both electrodes were in the same container. This was ascribed to sulphur species in solution that inhibited the degradation of thiourea to formamidine disulphide, one of the by-products of the dissolution of gold in thiourea. Galena produced a very marked increase in the rate of dissolution of gold in cyanide solutions when both electrodes were in the same container. This could be ascribed to the action of Pb(ll) ions on the surface of the gold. However, galena had little or no effect on the rate of dissolution of gold when in the same container in an acidic thiourea medium.
Keywords Leaching; gold; thiourea; cyanide; galvanic interaction
INTRODUCTION During the leaching of gold in cyanide and thiourea media, gold is oxidized anodically to the aurous state. Subsequently, gold goes into solution either as the aurocyanide complex [1,2] Au(CN)2" or the aurothiourea complex [3-5], Au[CS(NH2)2]2 ÷ respectively. However, this is where the similarity between these electrochemical reactions ends. Sufficient evidence exists for the cyanide medium to suggest that dissolved oxygen is reduced cathodically first to hydrogen peroxide and then to the hydroxyl ion [1,2]. For the acidic thiourea system it is generally accepted that the oxidizing agents Fe(III), O 2 and H~O 2 oxidize not only the gold, but also the thiourea to form formamidine disulphide [3-5], which may also take part in the oxidation [3]. Schulze [4] showed that when the formamidine was selectively reduced back to thiourea in order to maintain the oxidizing part of thiourea to about 50% of the initial concentration of thiourea, rapid dissolution of the gold could be maintained. The potential for using thiourea as a replacement for cyanide in the leaching of gold, was demonstrated by Chen et al. [6]. For the thiourea solution it was shown that the rate of 589
590
J.S.J.
VAN DEVENTER et al.
dissolution of gold was 12 times as high, and silver 10.8 times as high as that for the corresponding cyanide solutions. Gold is closely associated with a variety of conductive minerals in the ore. These include amongst others pyrite, chalcopyrite, galena, pyrrhotite, arsenopyrite, haematite and sphalerite. During the milling of the ore, some of the free gold may rub off not only onto these minerals, but also onto steel which is introduced during blasting and milling. Various authors [7-9] have discussed the passivation of gold as a function of these minerals which are associated with gold. The method proposed by Van Deventer and Lorenzen [7] to investigate this phenomenon, differed from previous attempts in that both the gold and metal or mineral were in direct contact with each other and acted together as the working electrode, either in the same or in separate containers. Previous attempts [8,9] focused mainly on surface effects and the influence of dissolved minerals on the rate of dissolution. The approach used by Van Deventer and Lorenzen [7] is also followed in this paper. It is the objective of this paper to compare the leaching behaviour of gold, in contact with various metals and minerals, in solutions of cyanide and thiourea. EXPERIMENTAL In all experiments the working electrode consisted of a gold electrode in electrical contact with the mineral or metal electrode, the behaviour of which was being investigated. All electrodes were of the rotating disc type to ensure reproducibility. The dissolution of a gold electrode in electrical contact with another electrode may be influenced by galvanic interaction, the species dissolved from the other electrode, or both effects. These dissolved species could either produce a passive film on the gold electrode, or could enhance the dissolution by another mechanism. If only the galvanic interaction was being investigated, the electrodes were placed in separate containers as shown in Figure 1. If the combined effects of galvanic interaction and dissolved species were being investigated, the electrodes were placed in the same container, as shown in Figure 2.
8
6
1.
SALT BRIDGE
2.
SATrKNO2-SOLUTION
3.
CALOMEL ELECTRODE
4.
PLATINUM ELECTRODE
5.
AERATORS
6.
KCN*SOLUTION
7.
SAT~KNO2
8.
CAPILLARIES
9.
Au
10.
MINERAL/METAL
11.
SINTERED GLASS DISC
Fig.1 Experimental equipment with the electrodes in separate containers The circuits in Figures 1 and 2 were completed by a platinum electrode and a calomel reference electrode, both of which were brought into close contact with the working electrode by a Luggin capillary.' These circuits were used for electrochemical measurements as described by Lorenzen [10] and Hoff [11]. Before each, e x p e r i m e n t the working electrodes were prepared by polishing them with a diamond paste and subsequently etching
G a l v a n i c interactions d u r i n g g o l d dissolution
591
them in a 10% ammonium persulphate solution. Finally the electrodes were conditioned for 3 minutes in a solution containing either 10% KCN or 10% thiourea, whichever was applicable.
1.
STANDARDCALOMEL ELECTRODE
2.
PLATINUM ELECTRODE
3.
GOLD DISC
4.
MINEI~L/METAL DISC
$.
THERMOMETER
6.
AERATOR
7.
KCN-SOLUTION
7
Fig.2 Experimental equipment with the electrodes in the same container The physical and chemical conditions within the containers are summarised as follows:
Cyanide solution: [KCN] = 0.2 g/l; pH = 10.3 - 10.5; [02] = 8.2 mg/l; Temperature = 20°C; Rotational speed of electrodes ffi 100 rpm
Thiourea solution: [Fe2(SO4)3] = 0.01 mol/l; [H2SO4] = 0.1 tool/l; [CS(NH2)2] = 0.I tool/l; Temperature = 20°C; Rotational speed of electrodes = 750 rpm The mass of gold that dissolved was monitored by taking samples at intervals of 30 minutes and analysing them with an atomic absorption spectrophotometer. RESULTS AND DISCUSSION From the analyses of solution samples the average rate R of dissolution could be calculated by using the equation: n
R ffi 1 n 6t
E Di/i i=l
where: D i ffi cumulative gold dissolved at time (i St) in (mgAu/cm2). In this study n was 4 and 6t = 30 rain. By using this equation and the rate of dissolution of gold alone as the reference, the entries in Table 1 could be calculated. The effect that each of the six metals and minerals in contact with the gold electrode exerted on t h e rate of dissolution, will be discussed below under separate headings.
VAN DEVENTERet al.
J, S. J.
592
TABLE 1 A comparison of the relative dissolution rates of gold in contact with the various metals and minerals In cyanide and thiourea solutions Thiourea
Cyanide
Mineral or metal in c o n t a c t wlth gold
Gold Copper Chalcopyrite Galena Haematite Mild steel Pyrite
One Cell
Two Cells
Cell
100 16 34 154 91 59 40
100 21 51 70 91 71 48
i00 21 156 96 80 5 151
TWO Cells
One
i00 98 95 76 95 83 96
The gold electrode (99.9% pure) used in cyanide solutions had an area of 3.8 cm 2, while that used in the thiourea solutions had an area of 1.29 cm 2. The areas of the electrodes in galvanic contact with the gold electrode are given in the captions of Figures 3 to 8, which depict the dissolution curves. In these Figures, the curve for the same cell is indicated by 1, and the curve for separate cells is indicated by 2. 0.7
20 - Thiourea
-Cyanide
18-
0.6
~
O
A
u
160.5
E
04
14-
-
~
C
u
2
o~ Q
.
12--
0.5
/ /
~ Au-Cu2
//I . / /
,o
o Au-Cu I 6
o Au- Cui
6-
0.2 0.1
:50
0
60
90
120
I
150 0
15
50
i
45
? 60
Time (mini
l 75
~ 90
I 105
? 120
i 135
I 150
Time (mini
Fig.3 Dissolution rate of gold in contact with copper (electrode area = 3.8 cm 2) in (a) cyanide (b) thiourea 0.7
30 - Thiourea
- Cyanide
27-
0,6
24-
0.5
A
a
18
[ g5
0.3
l:~ 12 ~
0.2 0.1
21
%
E 0.4 .~
7 "
9
rltte 2 n Au-choLcopyrit.e I
" l
30
I
60
I
90
Time (mini
I
120
6 5
_
/o"
/
/
= Au-chatcopyrit,e 2
1
150
30
60
90
120
Time (mini
Fig.4 Dissolution rate of gold in contact with chalcopyrite (electrode,area ffi 3.3 cm 2) in (a) cyanide (b) thiourea
I 15o
593
G a l v a n i c interactions d u r i n g g o l d d i s s o l u t i o n
0.7
30--
- cyon~
27--
0.6
Thiourea
24--
0.5
21
E 0.4 .~
18
15 E 0.3 Q
9
0.2
0.1
a Au-pyrite I
I
30
I
I
60 90 Time (rain)
3
I
120
_ ~
I
~
I
150 0
30
a Au-pyrite 2 0 Au-pyritel
i
I
60 90 Time (rain)
L
120
I
150
Fig.5 Dissolution rate of gold in contact with pyrite (electrode area ffi 1.89 cm 2) in (a) cyanide (b) thiourea
I - Cyanide 0.9 - ~
20
B
Tt.ourea
~ 18
0.8 --
IE
o.7
14
E 0.6 v
0.4
a
~
E
0.3
6
// ~
0.2 0.1
~
a Au-goLeno 2 0 Au-gaLena I
I
o
30
I
I
60 90 Time (rain)
I
120
4 _ 2i
...j~
I
I 50
150 (
o Au-gaLena 2 A Au-gaLena I I I 60 90 Time (min)
I 120
I
150
Fig .6 Dissolution rate o f gold in contact with galena (electrode area = 3.36 cm 2) in (a) cyanide (b) thiourea
0.7 B
Thiourea
Cyanide
0.6
IE
0.5
t4
~
0.4 i
0.3
v
O
8
6
0.2
4 _ ~ 2
a Au-haemaUte 2 n Au-haematRe I
o.l 0
I0
I
30
I
60 Time
I 90 (mini
I 120
I
150 0
~ i
j~'J~
30
. Au-haematite 2 o Au-haematite I
I
60 Time
I (minl
90
i
120
I
150
Fig.7 Dissolution rate of gold in contact with haematite (electrode area = 2.6 c m 2) in (a) cyanide (b) thiourea From Figures 3a to 8a it is evident that the dissolution curves for gold in cyanide all have a convex shape, which suggests that significant passivation occurred. Even the initial rates of dissolution were much lower than that predicted only from boundary layer theory for the rotating disc electrode. Although Figures 3b to 8b show some scattering in the data owing to inaccuracies in the analyses, it can be seen that the gold disc in thiourea did not become passivated in the case of two cells. As expected, the rate of leaching in thiourea was much faster than that in cyanide.
594
J.S.J. VANDEVENTER I
2o _- Cyanide
et al.
B
Thiourea
ta
0.9 O,e _
~
16 14 12
03 0.6
0.5 E ~ 0.4 Q 0.3 0.2 / / / 0.1
~' ; u - F e o Au-Fe I 30
I 60
I 90
a 8 6 4
2 I
2 [ 120
~
~
~.,//
. Au-Fe2 =Au-Fel I
I 150 0
T i m e (rain)
150 T i m e (rain)
Fig.8 Dissolution rate of gold in contact with mild steel (electrode area = 3.8 cm z) in (a) cyanide (b) thiourea Copper in Contact with Gold In these experiments pure metallic copper was placed in contact with gold.
Cyanide solution Gold in contact with a copper electrode in either one or two containers produced a marked decrease in the rate of dissolution in Fig. 3(a). The fact that film formation did not play a significant role when the two electrodes were in separate containers, provides some evidence that galvanic interaction played the major role in retarding the rate of dissolution. The slightly lower rate of dissolution when both electrodes were in the same container points to a layer of a copper cyanide complex which possibly aggravated the passivation during the later stages of the reaction.
Thiourea solution Whereas cyanide produced a large decrease in the rate of dissolution for the same as well as for separate cells, Fig. 3(b) shows that the effect of copper in a thiourea solution was varied. If both electrodes were in the same container the gold was almost completely passivated. This was due to a brownish layer which covered the gold electrode. On the other hand, very little galvanic interaction was exhibited when the two electrodes were in two different containers. Chalcopyrite or Pyrite in Contact with Gold The chalcopyrite used for the electrode contained Zn as the principal impurity, with traces of Mo, Ba, Mn, Ni, Se, Rb, Sr, Pb and Co also present. The major impurity in the pyrite electrode was Cu, while traces of Co, Zn, Se, Rb, Mo and Pb were also present. Figures 4 and 5 indicate that the electrodes of chalcopyrite and pyrite reacted almost identically.
Cyanide solution Figures 4(a) and 5(a) reveal that galvanic interaction between gold and chalcopyrite or pyrite decreased the rate of dissolution significantly in cyanide medium. When the electrodes were in the same container, the rate was decreased even further owing to a layer of unidentified cyanide complexes which gradually caused an additional passivation of the gold electrode.
Thiourea solution When the electrodes were in separate containers, the rate of dissolution in thiourea decreased only marginally, as is shown in Figures 4(b) and 5(b). This means that galvanic
Galvanic interactions during gold dissolution
595
influences were more pronounced in cyanide than in thiourea. However, when both electrodes were in the same container, a large increase in the rate was produced. This could be explained in terms of the regeneration of thiourea from formamidine disulphide by sulphur species dissolved from the sulphide mineral, and is in accordance with the results produced by Schulze [4]. Galena In Contact with Gold
The galena electrode contained Cu, Fe, Se and Zr as minor impurities with traces of Zn, Au and Ag.
Cyanide solution As indicated in Fig. 6(a), galvanic interaction decreased the rate of dissolution of gold by 30% when the electrodes were in separate containers. On the other hand, when the electrodes were in the same container the rate increased by 54%. This is attributed to the lead ions in solution which increased the rate of gold dissolution, a phenomenon investigated by Fink [8]. This phenomenon could be attributed to an alteration of the surface character of the gold due to the alloying of the Pb(II) ions with the displaced metals.
Thiourea solution A decrease of 24% in the dissolution rate was observed in Fig. 6(b) when the two electrodes were in separate containers, which indicates considerable galvanic interaction. The increased rate of dissolution when the electrodes were in one container, compared with the case of separate containers, is not that straightforward to explain. It was possible that sulphur species regenerated thiourea from formamidine disulphide, as was the case for chalcopyrite and pyrite in thiourea. This effect however, was not as marked as in the case of ehalcopyrite. A precipitate covered the galena electrode during the later stages of dissolution, which probably decreased the concentration of sulphur species that could be used for regeneration. It could also have been possible that the Pb(II) ions in solution acted in the same way as for galena in cyanide. Haematite in Contact with Gold
Cyanide solution Haematite is virtually insoluble in cyanide [2]. Fig. 7(a) reveals that the dissolution curve of gold with both electrodes in the same container was identical to the curve for the electrodes in separate containers. This indicated that film formation on the gold was not significant, which supports the view of Habashi [2]. Hence, it could be concluded that only galvanic interaction played a role in retarding the dissolution rate. This effect, however, was not very significant, which means that haematite had very little effect on the dissolution rate of gold in cyanide.
Thiourea solution The rate of dissolution did not change noticeably when the electrodes were in two containers, as shown in Fig. 7(b). This indicates that galvanic interaction was insignificant, and even less than in the ease of cyanide. Whereas the haematite was not readily soluble in cyanide, it seemed that haematite dissolved to such an extent in thiourea that the dissolution of gold was affected adversely when the electrodes were in the same container. Mild Steel in Contact with Gold
Most of the equipment such as grinding mills used to treat ores are made of steel, which could rub off on the gold.
596
J.S.J.
VAN DEVENTER et al.
Cyanide solution It is clear from Fig. 8(a) that the rate of dissolution decreased when gold was in contact with mild steel. Galvanic interaction decreased the rate of dissolution by 29%. The additional decrease in the rate of dissolution when the electrodes were in the same container could be attributed to a greyish layer of iron cyanide which accumulated on the gold surface, hence forming an additional passivating layer.
Thiourea solution When the electrodes were placed in the same container in thiourea, the rate of dissolution decreased drastically, as illustrated in Fig. 8(b). Since galvanic influence appeared to be less significant, the decrease could be attributed mainly to a light brown film (iron thiourea complex) that covered the gold electrode. CONCLUSIONS (1)
As could be expected, the specific dissolution rate of gold on its own in thiourea was significantly higher than that in cyanide.
(2)
The gold electrode on its own in cyanide passivated gradually, while the acidic thiourea prevented such behaviour.
(3)
The rate of dissolution of gold in contact with copper, steel, pyrite and chalcopyrite in cyanide was inhibited significantly due to the galvanic interaction between these substances and gold.
(4)
Additional film formation in cyanide decreased the rate even further, but not to such a large extent as the galvanic interaction. Haematite had very little effect on the rate of dissolution in cyanide.
(5)
A reasonable degree of galvanic interaction between gold and galena in cyanide in separate cells inhibited the rate of dissolution. However, when the gold and galena electrodes were in the same container, the dissolution rate was enhanced due to the action of Pb(II) ions on the surface of the gold.
(6)
Galena and steel showed some galvanic interaction, whereas the other minerals showed very little interaction in thiourea. The large decrease in the rate of dissolution when steel or copper was in contact with gold in thiourea in the same container was caused mainly by film formation.
(7)
The increase in the rate of dissolution when gold was in contact with pyrite or chalcopyrite in thiourea was caused probably by the ability of liberated sulphur species to regenerate thiourea.
(8)
Similarly, galena in contact with gold in thiourea probably also liberated sulphur species. This enhanced the rate of dissolution in the early stages of the reaction, but gradually formed a film on the gold, which inhibited the rate at a later stage. ACKNOWLEDGEMENT
Gratitude is expressed to the Anglo American Corporation of S.A., who sponsored part of this project. The technical assistance provided by Mr J.M. Barnard and Mr W.P. van Reenen is gratefully appreciated.
Galvanic interactionsduring gold dissolution
597
REFERENCES II
Cathro K.J. and Koch D.F.A.J. Electrochem. Soc. 111 (12), 1416 (1964).
2.
Habashi F. Bulletin 59, 42 p. Bureau of Mines and Geology, State of Montana (1967).
3.
Groenewald T. J.S. Afr. Inst. Min. Metall. 77 (11), 217 (1977).
4.
Schulze R.G.J. Metals. 36 (6), 62 (1984).
5.
Yen W.T. and Wyslouzil D.M. Gold 100: Proc. of an Inter. Con. on Gold, Vol. 2, Extr. Metall. of Gold, p. 579. C.E. Fivaz and R.P. King (eds.). S. Afr. Inst. Min. Metall., Johannesburg. (1986).
.
7. .
Chen C.K., Lung T.N. and Wan C.C. Hydrometallurgy, 5, 207 (1980). Van Deventer J.S.J. and Lorenzen L. Separation Processes in Hydrometallurgy, p. 49. G.A. Davies (ed.). Ellis Horwood Ltd, Chichester, England. (1987). Fink C.G. and Putnam G.L. Min. Eng.. Trans. AIME, 187 (9), 952 (1950).
9.
Mrkusic P.D.P. Report 24., 111 p. National Institute for Metallurgy, Johannesburg. (1970).
10.
Lorenzen L. M.Eng. Thesis, 248 p. University of Stellenbosch, S. Africa. (1984).
11.
Hoff P.J. Final year project, 132 p. University of Stellenbosch, S. Africa. (1987).
