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Eh measurements in sulphide mineral slurries
International Journal o f Mineral Processing, 13 (1984) 29--42
29
Elsevier Science Publishers B.V., Amsterdam --Printed in The Netherlands
Eh M E A S U R E M E N T S IN S U L P H I D E M I N E R A L S L U R R I E S
D.A.J. RAND and R. WOODS CSIRO Division o f Mineral Chemistry, P.O. Box 124, Port Melbourne, Vic. 3207 (Australia)
(Received April 25, 1983; revised and accepted August 1, 1983)
ABSTRACT Rand, D.A.J. and Woods, R., 1984. Eh measurements in sulphide mineral slurries. Int. J. Miner. Process., 13: 29--42. Problems in utilizing open-circuit potential (Eh) values as a measurement of the redox conditions in metallurgical systems are explored. Factors which determine the Eh when more than one redox couple is present have been illustrated by studies with platinum, gold and galena indicator electrodes in iron/oxygen and xanthate/oxygen systems. Potential measurements in galena slurries have demonstrated that smooth noble-metal electrodes are sensitive to the conditions at the mineral particle/solution interface.
INTRODUCTION T h e separation o f mineral sulphides f r o m their ores b y i n t e r a c t i o n with thiol collectors in the process o f f r o t h f l o t a t i o n has b e e n e x a m i n e d f r o m an e l e c t r o c h e m i c a l v i e w p o i n t (Granville et al., 1 9 7 2 ; Woods, 1972 and 1 9 7 6 ; G u t i e r r e z , 1 9 7 3 ; H e y e s and Trahar, 1 9 7 9 ; R i c h a r d s o n et al., 1 9 8 4 ; Walker et al., 1984). I n t e r a c t i o n o f the c o l l e c t o r species with the mineral surface is c o n s i d e r e d t o take place via c o u p l e d charge-transfer processes involving (a) a n o d i c o x i d a t i o n o f the c o l l e c t o r to f o r m a h y d r o p h o b i c species, and (b) c a t h o d i c r e d u c t i o n o f o x y g e n . T h e p o t e n t i a l d i f f e r e n c e across the mineral/solution i n t e r f a c e is the i m p o r t a n t f a c t o r in d e t e r m i n i n g t h e rate o f such reactions and h e n c e the f l o t a t i o n b e h a v i o u r e x h i b i t e d b y the mineral. T h e p o t e n t i a l at a solid/solution interface is d e t e r m i n e d b y t h e presence o f oxidizing and r e d u c i n g species in solution. In o r d e r to m o n i t o r r e d o x properties, it is c u s t o m a r y t o measure the p o t e n t i a l o f an i n d i c a t o r e l e c t r o d e placed in the solution. A n o b l e metal {usually p l a t i n u m ) is generally used f o r this p u r p o s e because such metals have a high resistance to corrosion. When r e p o r t e d against the standard h y d r o g e n e l e c t r o d e (SHE), the r e d o x p o t e n t i a l is r e f e r r e d t o as the " E h " . Eh m e a s u r e m e n t s have been m a d e in industrial f l o t a t i o n plants ( W o o d c o c k and Jones, 1 9 7 0 a , b ; L e o n o v and Badenikov, 1 9 7 3 ; Natarajan and Iwasaki, 1 9 7 3 ) , and the attractive p r o p o s i t i o n t h a t the p o t e n t i a l m a y provide an
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© 1984 Elsevier Science Publishers B.V.
30 efficient and convenient parameter for the automatic control of flotation circuits has been discussed with some confidence. However, whereas physical control of flotation plants is becoming fairly commonplace {McKee et al., 1976), control systems making use of chemical and electrochemical variables are still in the early stages of development. This paper discusses problems in interpreting Eh data obtained from measurements made in mineral-sulphide flotation plants. EXPERIMENTAL Materials
Analar sodium tetraborate was recrystallized from doubly distilled water. Potassium ethyl dithiocarbonate (xanthate) was prepared from ethanolic potassium hydroxide and carbon disulphide (Vogel, 1956). Xanthate that had come into contact with air was purified by washing with diethyl ether, and then recrystallized from acetone by addition of an equal amount of ether. Experiments with xanthate solutions were performed in 0.05 M sodium tetraborate of pH 9.2. These solutions were prepared daily and stored under nitrogen. The nitrogen was purified from traces of oxygen by passage over BTS copper catalyst (supplied by Badische Anilin- und SodaFabrik AG, Ludwigshafen am Rhein, Germany) and through a liquid-air trap. Solutions containing iron(II)/iron(III) couples were prepared by dilution of a stock solution (0.1 M AnalaR FeSO4 + 0.05 M Fe2(SO4)3 in 1 M H2SO4) with 1 M H2SO4. Electrodes
Indicator electrodes for Eh measurements were constructed from noble metals and galena mineral. Metal electrodes each consisted of a 4 × 5 X 0.1 mm piece of foil which was spot welded to a short length of 28-gauge wire of the same metal. Platinum-black electrodes were composed of Teflonbonded platinum pressed on to a tantalum mesh (4 X 5 mm) which was spot welded to platinum wire. This form of platinum has a surface roughness factor (RF) of ~ 2 0 0 0 ; RF is defined as the ratio of the real electrode surface area to the geometric surface area. Galena electrodes were prepared from cubes of 2--4 mm edge cut from massive natural specimens of high quality supplied by the Zinc Corporation mine, Broken Hill, NSW. Electrical connection was made b y attachment of tinned copper wire to the back surface of the mineral specimen by means of silver-epoxy conducting cement (Epo-Tek 410E, E p o x y Technology Inc., Watertown, Mass., USA). The specimen was then arranged centrally in the lower end of a short length ( ~ 2 0 cm) of 6 mm acrylic tubing and encapsulated in epoxy resin (Araldite D). The contact wire protruded from
31 the other end o f the tube and was fixed in position by a plug of e p o x y resin. The working surface of the electrode was prepared by grinding on successively finer grades of silicon carbide paper down to 600 grade. The galena slurry em pl oye d in laboratory experiments consisted of particles of - 2 0 0 mesh prepared by grinding high-quality, massive galena mined by the Zinc Corporation at Broken Hill, NSW. Silica particles used in slurries were purified by t r e a t m e n t with hot hydrochloric acid solution to remove traces of iron. Electrochemical measurements
Noble-metal indicator electrodes were cleaned for application in flotation plants by t r e a t m e n t with chromic acid and for laboratory studies by using an electrochemical method. The latter procedure was carried out in a conventional, 3-compartment, glass cell. T r e a t m e n t consisted of subjecting each electrode to triangular potential cycles in 1 M H2SO4 until a voltammogram of constant shape was obtained. The electrode potential was controlled by a p o ten tio s t at programmed with a sweep generator; bot h instruments were designed and constructed in these laboratories. In order to ensure that the galena indicator electrode had a fresh surface before each laboratory or flotation-plant experiment, the electrode was re-ground on 600-grade silicon carbide paper between each run. Potentials were measured against a standard calomel reference electrode and are reported as Eh, i.e., potentials on the SHE scale. T he potential o f the calomel electrode is 0.244 V relative to the SHE (Bates, 1964). In iron and oxygen systems, current-potential curves were obtained using a potential sweep rate of 4 mV s -1 and were recorded on a HewlettPackard 7004 X-Y recorder. In laboratory studies of flotation systems, open-circuit potential measurements were made using a one-litre, cylindrical, flat-bottomed, glass cell fitted with a large diameter flat-flange joint. A glass lid was clamped to this joint and had tapered sockets to a c c o m m o d a t e four indicator electrodes, the reference electrode, a glass stirrer, and gas inlet and outlet tubes. T he shaft of the stirrer was m o u n t e d vertically and rotated freely in a Teflon bearing which was housed in the central socket of the lid. The stirrer was driven by a small m o t o r via a flexible rubber coupling. The end of the stirrer consisted of two glass paddles arranged at 180 ° to each other. T he cell was water-jacketted for t em pe r at ur e control and all experiments were carried o u t at 25°C. Open-circuit potentials were recorded using a PDP-8/E computer-based instrumentation system. For studies in industrial flotation pulps, a Perspex holder which could a c c o m m o d a t e several electrodes was m o u n t e d on a central aluminium rod. This rod was clamped at 90 ° to a second rod secured across the flotation cell a b o u t 50 cm above the head of the froth. The clamping position was adjusted so that the reference and indicator electrodes were immersed
32 side-by-side in the mineral pulp at a depth of 10 cm below the froth layer. This depth ensured that the electrical circuit through the solution phase was maintained and that the indicator electrodes were in contact with particles consisting predominantly of the mineral being floated. Potentials were 0isplayed on a Siemens Type 192 × 288 6-channel recorder situated next to the flotation cell. Since this recorder had a relatively low input impedance, a voltage follower was used in order to avoid loading of the electrodes. RESULTS AND DISCUSSION Reversible and mixed potentials
It is generally assumed that the oxidizing or reducing conditions in a metallurgical system can be represented by a redox potential and that this potential can be measured by introducing an inert indicator electrode into the system. This view is realistic when a reversible redox couple is dominant. In this case, the measured potential is the reversible potential of the couple as given by the Nernst equation: E R = E ° + (RT/nF)ln(r.
[oxidized]/~ [reduced] )
(1)
The same applies when there are a number of couples present in equilibrium because each couple will have the same reversible potential expressed by eq. (1). However, many systems of metallurgical interest do not fall into this category, and the potential measured by the indicator electrode is not a reversible potential b u t rather a "mixed potential". A mixed potential arises when there are two (or more) redox couples present in the system and they are not in equilibrium, i.e., for the couples Redl/Oxl and Red2/Ox2: [Oxl] E°Red'/Oxl + nRlTF i n -[Red1]
¢
RT [Ox2] E°Red~/Ox2 + n ~ I n -[Red2] -
(2)
Thus, the redox condition in solution is not characterized by a unique Eh value, but rather it is represented by two potentials, i.e., those of the two redox couples present (Fig. 1). When an indicator electrode is introduced into the solution, its electrons can exchange with the two separate redox couples and the measured potential will reach a value, lying between the reversible potentials of the two couples where the c o m p o n e n t anodic and cathodic processes proceed at equal and opposite rates. This is illustrated in Fig. 1. The mixed-potential concept can be illustrated by measurements of potential in solutions containing an iron(II)/iron(III) couple in the presence of oxygen. We have made such measurements in 1 M H2SO4 solutions containing equal concentrations of the two iron species, but differing in total iron concentration.
33
Redl..-,.- Oxl + nle-
-~ .
/(
ER
Red,/Ox,/ i
o
g~
Redz .-,.. Ox2 + nz e -
7
ER
Red2/Ox2~
pot
~.cz o
Oxt+nle----,- Red I
Oxz+nze-----Red 2
Fig. 1. Schematic representation of a mixed-potential system. I Platinum I 1.0
~.
•
+
a
I
I
a
a
I
1
o++
o u.l 0.8
o,+
I i0-6
I
I I 1 I i0-~ i0 -4 i0-3 lO-Z Fe (1"1") and Fe (111) concentration / M
I i0 -~
Fig. 2. Variation of electrode potential with iron concentration [Fe(II)] = [Fe(III)] prepared from 0.1 M FeSO, + 0.05 M F%(SO4) 3 in 1 M H2SO +. Platinum (~) and gold (o) electrodes in absence of oxygen; platinum (=) and gold electrodes (a) in presence of oxygen. Line 1 = reversible potential of iron(II)/iron(III) system; lines 2 , 3 = v a l u e s deduced from rate measurements (Fig. 2) for gold and platinum, respectively.
The experimental p o i n t s in Fig. 2 s h o w the potentials measured by s m o o t h platinum and gold indicator electrodes for a range o f iron concentrations b o t h in the absence and in the presence o f o x y g e n . In oxygen-free solution, i.e., w h e n o n l y the iron(II)/iron(III) redox c o u p l e is present, the experimental potentials are close to the reversible p o t e n t i a l o f the iron system (line 1) for all c o n c e n t r a t i o n s investigated. N o t e that this reversible potential is n o t the standard potential of the Fe2+/Fe 3+ couple, but is a
34 formal potential representing equal concentrations of Fe(II) and Fe(III) in a 1 M H2SO4 solution. When oxygen is also present in the solution, the experimental potentials (Fig. 2) differ from the value for the reversible iron system. The departure becomes more pronounced as the iron concentration is decreased and is greater for platinum than for gold. Natarajan and Iwasaki (1974), in discussing the influence of various factors on the potential of platinum electrodes in the presence of redox couples, concluded that platinum was not an inert electrode as had c o m m o n l y been thought. We believe this viewpoint to be misleading since the term " n o t inert" is generally taken to mean that the material corrodes under the experimental conditions. For the system in question, the departure of the potential from that of the reversible iron couple does not arise from corrosion, but is a consequence of the system not being at equilibrium. The iron(II)/iron(III) and O J H 2 0 couples have different reversible potentials and the potential of the electrode will be determined, not by solution equilibria, but by the rates of electrode reactions at its surface. It will be a mixed potential. Mixed potentials in the same system can be different for different indicator electrodes because the rate of an electrode reaction is dependent on the nature of the electrode surface. The difference will be appreciable when one of the processes is electrocatalytic. Such a process involves the participation of species adsorbed on the electrode surface; the reaction rate will depend on the ability to adsorb the relevant species and hence can vary significantly from one electrode surface to another. This is the case for oxygen reduction and the electrode potential of the iron/oxygen system under investigation must be expected to be different for different electrode materials. Figure 2 shows that the Eh measured by platinum differs from that measured by gold and that the departure from the iron(II)/iron(III) potential is greater for the more catalytically active platinum. The mixed potential can be calculated from the rates of the individual electrode reactions taking part. Current-potential curves for oxygen reduction on platinum and gold were determined in 1 M H2SO4 solution. Similar curves for the oxidation of Fe(II) and the reduction of Fe(III) were determined in oxygen-free 1 M H2SO4 solution containing 10 -1 M of both Fe(II) and Fe(III). The current-potential relationships at other iron concentrations were computed by assuming the current at each potential to be linearly dependent on reactant concentration. The current-potential curves for the iron and oxygen systems are shown in Fig. 3 in the form of an Evans diagram. Such diagrams were developed by Evans (1963) to represent the component electrochemical reactions in corrosion systems. The currents are given in logarithmic form and both the anodic and cathodic currents are presented on the same side of the potential axis. Thus, the mixed potential will be given by the intersection of the anodic and cathodic current curves.
35 I
'
IEI
'
I0-z
I / ~
i0_3 ~
L.// ..Z,,,~
I
'
"
.
I
'
I(a)
(t --
~
~
~
/ 0
2
reduction on
" " ~ platinum
(C)
\
Cd}
X~O,~ongold \\(el reduction
\
\ I0-7
\ \
(f)
I0 -a
I0 -s
_
0.6
0.7
I
0.8 Eh/volt
I
I
0.9
I
I
I.O
Fig. 3. Evans diagram for the iron/oxygen system. Solid lines are reactions of dissolved iron on platinum and gold for [Fe(II)] = [Fe(III)] at given concentrations in 1 M H2SO,; (a) + (f): 10 -~ to 10 -6 M in multiples of 10 -1 M. Dashed lines are for oxygen reduction of platinum and gold in oxygen-saturated 1 M H2SO4. T h e o x i d a t i o n o f iron(II) and the r e d u c t i o n o f iron(III) reactions, being simple e l e c t r o n exchange processes, have practically the same rates o n p l a t i n u m and gold surfaces. With increase in p o t e n t i a l f r o m the reversible value (0.68 V), it is seen (Fig. 3) t h a t the c u r r e n t increases logarithmically in w h a t is t e r m e d the Tafel region. In this region, the rate o f o x i d a t i o n o f Fe(II) is d e t e r m i n e d b y the rate o f e l e c t r o n transfer b e t w e e n the dissolved iron species and the e l e c t r o d e (i.e., " a c t i v a t i o n c o n t r o l " ) . As the p o t e n t i a l is increased f u r t h e r , the logarithmic relationship no longer holds because the rate o f t r a n s p o r t o f the Fe(II) species t o t h e e l e c t r o d e surface is n o t sufficient to maintain such a r e a c t i o n rate. Eventually, the c u r r e n t b e c o m e s limited b y the rate o f mass t r a n s p o r t o f Fe(II) t o the e l e c t r o d e surface (i.e., " m a s s - t r a n s p o r t c o n t r o l " ) and h e n c e no longer increases with potential. A similar sequence o f events occurs f o r the r e d u c t i o n o f Fe(III) with decrease in p o t e n t i a l f r o m the reversible value. In c o n t r a s t t o the iron processes, the r e d u c t i o n o f o x y g e n is m u c h faster on p l a t i n u m t h a n on gold. In the range 0.9--1.0 V, the r e a c t i o n o n p l a t i n u m is activation c o n t r o l l e d , whereas below 0.8 V it is limited b y mass-transport c o n t r o l . T h e decrease in activity at ~ 1 . 0 V is d u e to the a d s o r p t i o n o f o x y g e n o n the p l a t i n u m surface (Damjanovic and Bockris, 1966). T h e rate o f o x y g e n r e d u c t i o n o n gold is activation c o n t r o l l e d over t h e entire p o t e n t i a l range o f Fig. 3. T h e sum o f the rates o f a n o d i c reactions m u s t be equal to t h a t o f the rates o f t h e c a t h o d i c reactions at the m i x e d p o t e n t i a l , since the n e t c u r r e n t flow is zero. Ignoring the back reactions, which have negligible rates at
36 potentials removed from the reversible values, the experimental Eh should have a value at which the oxygen and iron(II) lines intersect in Fig. 3. Lines 2 and 3 in Fig. 2 represent the mixed potentials derived from the Evans diagram plots. It can be seen that there is satisfactory agreement between the Eh ~'alues derived from the kinetic data and those measured experimentally. The mixed potential will also be influenced by other factors which affect the rate of each participating reaction. Stirring the solution will enhance the rates of mass-transport-controlled, but not activation-controlled, processes. For example, in oxygen-saturated solutions dilute in iron, the measured potential decreases significantly on stirring because iron(II) oxidation is mass-transport-controlled and oxygen reduction is activation-controlled. Adsorption of impurities decreases the rate of an electrocatalytic process much more than that of a non-catalytic reaction. Over extended periods, the potentials in the presence of oxygen decrease from the values presented in Fig. 2 towards the redox potential of the iron system, due to inhibition of oxygen reduction by adsorbed impurities. Natarajan and Iwasaki (1975) found that the potential of a platinum indicator electrode in a solution that was 10 -2 M in iron(II) and iron(III) differed significantly from the reversible potential of the iron couple after application of the electrode in solutions containing H2S. This behaviour was due to the establishment of a mixed potential involving anodic oxidation of sulphur present on the electrode surface and cathodic reduction of iron(III). The presence of sulphur inhibited the latter process and hence the mixed potential was sustained. The above observation demonstrates the need for indicator electrodes to be cleaned before transfer from one system to another. Although electrochemical activation by anodic/cathodic treatment is the most rigorous m e t h o d for cleaning noble-metal electrodes, dipping in chromic acid provides an effective and rapid means for obtaining clean, reproducible surfaces.
Mixed potentials in xanthate solutions The solution phase of flotation systems contains a number of electrochemically active species such as collectors, dissolved oxygen, and modifying reagents. As in the case of the iron/oxygen system discussed above, the Eh measured by an indicator electrode placed in the flotation solution will be determined by the rates of the anodic and cathodic processes occurring at the electrode surface. Eh measurements on four different indicator electrodes in air-saturated 0.05 M sodium tetraborate solution (pH 9.2) containing different amounts of potassium ethyl xanthate are shown in Fig. 4. It can be seen that for a given xanthate concentration, there is a significant difference in the potentials measured by two different types of platinum electrode (cf. Fig. 4 for platinum black and smooth platinum). This identifies a further factor which can
37 I
I
t
I
I
1
0.6
~ 0.4
black
~ Platinum
o
0.2
0.0
I I
I
I0 I00 I00( Xanthote concentration/ppm
Fig. 4. Eh measurements with different indicator electrodes in 0.05 M sodium tetraborate solution (pH 9.1) containing different amounts of potassium ethyl dithiocarbonate (xanthate).
/ / / ~ ~ -
i//~ ~ P)-block/
i
~
X2
-Pt Eh
--
Pt , ~ / / ~ t ~
J=
pf black
02~ 0 H-
i...~i,~hEx-]4
ehaviour I
L0.
[x-I
F behoviour
Fig. 5. Schematic representation of the dependence of Eh on the electrode surface roughness in a mixed-potential xanthate/oxygen system.
influence the measured Eh in a mixed-potential system, namely, change in electrode surface roughness. Figure 5 illustrates schematically the influence of reactant concentrations and electrode roughness on current-potential curves and the resulting mixed potentials. The rate of an activation-controlled process is proportional to the real surface area of the electrode, because electron transfer can occur at each site at the solid/solution interface. On the other hand, the rate of
38 a mass-transport-controlled process is dependent on the flux of the reactant in the solution; this flux is not affected by the microstructure of the electrode surface and hence the reaction rate is proportional to the geometrical (or projected) surface area. At low collector concentrations, the electrochemical oxidation of xanthate is mass-transport-controlled and, therefore, the rate is unaffected by changes in the surface roughness of the indicator electrode. On the other hand, the electrochemical reduction of oxygen is activation-controlled and has an enhanced rate on a platinum-black electrode because this electrode has a greater real surface area than smooth platinum. Therefore, at low collector concentrations, the measured Eh obtained with the platinum-black electrode is more positive than that given by the smoothplatinum electrode. The converse situation arises when the xanthate concentration is greater than that of dissolved oxygen. Under these conditions, oxygen reduction is mass-transport-controlled whereas xanthate oxidation is activation-controlled. Hence, increase in the surface roughness of the indicator electrode results in a negative shift in the measured Eh. The Eh data presented in Fig. 4 show that at low xanthate concentrations, the smooth-gold electrode yields lower potentials than those obtained with the smooth-platinum electrode. The explanation for this difference in potential is the same as that given above for the iron/oxygen system, namely, a lower electrocatalytic activity of gold for oxygen reduction compared to that of platinum. The activity of galena for oxygen reduction is less than that of the noble metals (Biegler et al., 1977; Rand, 1977). This in part accounts for the lower Eh values observed with galena (Fig. 4). A further factor contributing to this difference in potential is that oxidation of xanthate on galena occurs at a more negative potential than on noble metals. This shift in the xanthate oxidation potential arises from the formation of different reaction products: dixanthogen is produced on noble metals whereas chemisorbed xanthate and lead xanthate are additional products on galena (Woods, 1976). Eh measurements in galena slurries The above discussions have explored the role played by dissolved electrochemically active species in the measurement of Eh. In order to interpret Eh behaviour in a total flotation system, the influence exerted by the presence of mineral particles must also be considered. To achieve this objective, studies were made of the effect of the addition of mineral particles on the Eh measured by different indicator electrodes in cells containing air-saturated xanthate solutions. Typical results for galena slurries are given in Fig. 6. Prior to addition of mineral to the cell, the indicator electrodes were held in stirred solution until each respective potential reached a steady value. Note that with mechanical stirring, it is difficult to reproduce exactly mass-transport conditions. Thus, for a given electrode, differences in Eh are observed between experimental runs. How-
39 ,
i
I
Platinum black
I
(a) GalIena sllurry
0.25 Platinum
0.20 0.15 OJO .,= 0.25 w
(b) Silica + Galena slurr Platinum black
0.20 Platinum 0.15
OJO ~Galena 0.05
I
-20
I
L
1
I
I
I
0
20
40
60
80
I00
Time from addition of solids/rain Fig. 6. Eh measurements w i t h different indicator electrodes in 0.05 M sodium tetraborate solution (pH 9.2) containing 100 p p m potassium ethyl diothiocarbonate (xanthate) before and after the a d d i t i o n of: (a) galena (23 w / w %); and (b) silica (29 w / w %) + galena (0.6
w/w %). ever, within a single run, all electrodes in the cell experience the same constant stirring conditions. Addition of galena to form a slurry will result in a change in the composition of the solution due to the interaction of both xanthate and oxygen with the galena particles. This is reflected by a small shift in the Eh measured by the platinum-black and galena electrodes. The Eh values measured by smooth-platinum and gold electrodes exhibited a much greater fall on the addition of the galena particles and then approached potentials within 20 mV of that displayed by the galena electrode (Fig. 6a). This suggests that smooth noble-metal electrodes act as slurry electrodes and respond to the potential of the sulphide particles. The difference in potential between the smooth noble-metal and galena electrodes is small compared with the potential range over which galena floats in the presence of ethyl xanthate (Guy and Trahar, 1982). The potential of the platinum-black electrode does not take on the character of the galena slurry because of its high surface roughness factor -the electrode has a network of pores which have a large surface area in contact with the solution phase but only a small projected area for interaction with mineral particles. Hence, the electrode potential is still determined by reactions at the platinum/solution interface. This slurry electrode behaviour is also evident in pulps containing small
40 quantities of galena, even when a large excess of inert material, such as silica, is present (Fig. 6b). The potential changes observed in Fig. 6b are due entirely to the presence of a galena c o m p o n e n t in the slurry since Eh values were found to be unaffected when silica alone was added to the solution. Studies were also carried out in the lead rougher circuit of the flotation plant operated by the Zinc Corporation at Broken Hill, NSW. During the test runs, the residual ethyl xanthate concentration in the flotation cells was ~ 1 0 ppm, measured by a CSIRO-designed on-stream xanthate monitor* (Sullivan and Woodcock, 1973); the solution pH was ~ 9 . As found in the above laboratory experiments, plant measurements showed that the potential of a smooth-platinum electrode (20-h average: 0.172 V) followed closely that of a galena electrode (20-h average: 0.164 V), whereas the potential exhibited by a platinum-black electrode was over 100 mV more positive (20-h average: 0.310 V). CONCLUSIONS In this paper, we have demonstrated how Eh depends on the nature of the indicator electrode used to measure this parameter. For well-poised systems in which a single redox couple is dominant, the Eh value is unaffected by the choice of electrode. In mixed-potential systems, on the other hand, different electrode materials can yield different Eh values. In these systems, the potential is not determined solely by thermodynamics but also by the kinetics of reactions at the electrode/solution interface. Thus, significantly different Eh values are exhibited by platinum-black, smooth-platinum, gold and galena electrodes in solutions containing xanthate and oxygen. In flotation systems, the relevant Eh is that established at the mineral/ solution interface. This suggests that an electrode constructed from the mineral being concentrated should be the most appropriate for Eh measurement. However, since oxidized surface layers form on mineral sulphide electrodes through reaction with dissolved species in the flotation pulp, such electrodes could rapidly reach a condition where they no longer respond rapidly to changes in the pulp environment. It has been shown that smoothplatinum and gold electrodes assume Eh values close to those exhibited by freshly prepared galena when immersed in galena slurries, particularly at high pulp densities. Since these electrodes are influenced b y the conditions established at the mineral-particle/solution interface, they are suitable for monitoring Eh in galena flotation processes. It would be of interest to investigate whether such electrodes are applicable to all flotation systems. Smooth platinum has been the conventional electrode material used for measuring Eh. However, close inspection of the experimental results ob*This instrument is available commercially from the Australian Mineral Development Laboratories, Flemington Street, Frewville, South Australia 5063.
41
tained from laboratory studies (Fig. 6) suggests that gold could well be a better material since it responds more rapidly to changes in slurry conditions and approaches the mineral electrode potential more closely. ACKNOWLEDGEMENTS
The authors are indebted to Mr. M. Moignard for assistance with the experimental work and to the Zinc Corporation (Broken Hill, N.S.W.) for permission to carry out studies in their lead flotation plant.
REFERENCES Bates, R.G., 1964. Determination of pH. John Wiley, New York, N.Y., p. 278. Biegler, T., Rand, D.A.J. and Woods, R., 1977. Oxygen reduction on sulphide minerals. In: J.O'M. Bockris, D.A.J. Rand and B.J. Welch (Editors), Trends in Electrochemistry. Plenum Press, New York, N.Y., p. 291. Damjanovic, A. and Bockris, J.O'M., 1966. The rate constants for oxygen dissolution on bare and oxide-covered platinum. Electrochim. Acta, 11: 376. Evans, U.R., 1963. An Introduction to Metallic Corrosion. Arnold, London, Ch. III. Granville, A., Finkelstein, N.P. and Allison, S.A., 1972. Review of reactions in the flotation system galena-xanthate-oxygen. Inst. Min. Metall. Trans., Sect. C, 81 : 1. Gutierrez, C., 1973. The mechanism of flotation of galena by xanthates. Miner. Sci. Eng., 5: 108. Guy, P.J. and Trahar, W.J., 1982. The influence of grinding and flotation environments on the laboratory batch flotation of galena. 56th Colloid and Surface Science Symposium, Am. Chem. Soc., Blacksburg, Virginia. Heyes, G.W. and Trahar, W.J., 1979. Oxidation-reduction effects in the flotation of chalcocite and cuprite. Int. J. Miner. Process., 6 : 229. Leonov, S.B. and Badenikov, V.Ya., 1973. Redox potential of flotation pulp as a generalized and controllable parameter of flotation. Fiz.-Khim. I. Tekhnol. Issled. Protsessov Pererabotki Polezn. Iskop., 12. McKee, D.J., Fewings, J.H., Manlapig, E.V. and Lynch, A.J., 1976. Computer control of chalcopyrite flotation at Mount Isa Mines Limited. In: M.C. Fuerstenau (Editt)r), Flotation. A.M. Gaudin Memorial Volume, Vol. 2, AIME, New York, N.Y., p. 994. Natarajan, K.A. and Iwasaki, I., 1973. Practical implications of Eh measurements in sulfide flotation circuits. Trans. AIME, 254: 323. Natarajan, K.A. and Iwasaki, I., 1974. Eh measurements in hydrometallurgical systems. Miner. Sci. Eng., 6: 35. Natarajan, K.A. and Iwasaki, I., 1975. Adsorption mechanism of sulphides at a platinumsolution interface. AIChE Symp. Ser., 71(150): 148. Rand, D.A.J., 1977. Oxygen reduction on sulphide minerals. Part III. Comparison of activities of various copper, iron, lead and nickel mineral electrodes. J. Electroanal. Chem., 83: 19. Richardson, P.E., Stout III, J.V., Proctor, C.L. and Walker, G.W., 1984. Electrochemical flotation of sulfides: chalcocite-ethylxanthate interactions. Int. J. Miner. Process., 12: 73--93. Sullivan, J.V. and Woodcock, J.T., 1973. A simple on-stream xanthate monitor. Proc. Australas. Inst. Min. MetaU., 248 : 1. Vogel, A.I., 1956. Textbook of Practical Organic Chemistry. Longmans, London, p. 499.
42 Walker, G.W., Stout III, J.V. and Richardson, P.E., 1984. Electrochemical flotation of sulfides: reactions of chalcocite in aqueous solution. Int. J. Miner. Process., 12: 55--72. Woodcock, J.T. and Jones, M.H., 1970a. Oxygen concentrations, redox potentials, xanthate residuals, and other parameters in flotation plant pulps. In: M.J. Jones (Editor), Mineral Processing and Extractive Metallurgy. Inst. Min. Metall., London, p. 439. Woodcock, J.T. and Jones, M.H., 1970b. Chemical environment in Australian lead-zinc flotation plant pulps. I. pH, redox potentials, and oxygen concentrations. Proc. Australas. Inst. Min. MetaU., 235 : 45. Woods, R., 1972. Electrochemistry of sulphide flotation. Proc. Australas. Inst. Min. Metall., 241: 53. Woods, R., 1976. Electrochemistry of sulfide flotation. In: M.C. Fuerstenau (Editor), Flotation. A.M. Gaudin Memorial Volume, Vol. 1. AIME, New York, N.Y., p. 298.
29
Elsevier Science Publishers B.V., Amsterdam --Printed in The Netherlands
Eh M E A S U R E M E N T S IN S U L P H I D E M I N E R A L S L U R R I E S
D.A.J. RAND and R. WOODS CSIRO Division o f Mineral Chemistry, P.O. Box 124, Port Melbourne, Vic. 3207 (Australia)
(Received April 25, 1983; revised and accepted August 1, 1983)
ABSTRACT Rand, D.A.J. and Woods, R., 1984. Eh measurements in sulphide mineral slurries. Int. J. Miner. Process., 13: 29--42. Problems in utilizing open-circuit potential (Eh) values as a measurement of the redox conditions in metallurgical systems are explored. Factors which determine the Eh when more than one redox couple is present have been illustrated by studies with platinum, gold and galena indicator electrodes in iron/oxygen and xanthate/oxygen systems. Potential measurements in galena slurries have demonstrated that smooth noble-metal electrodes are sensitive to the conditions at the mineral particle/solution interface.
INTRODUCTION T h e separation o f mineral sulphides f r o m their ores b y i n t e r a c t i o n with thiol collectors in the process o f f r o t h f l o t a t i o n has b e e n e x a m i n e d f r o m an e l e c t r o c h e m i c a l v i e w p o i n t (Granville et al., 1 9 7 2 ; Woods, 1972 and 1 9 7 6 ; G u t i e r r e z , 1 9 7 3 ; H e y e s and Trahar, 1 9 7 9 ; R i c h a r d s o n et al., 1 9 8 4 ; Walker et al., 1984). I n t e r a c t i o n o f the c o l l e c t o r species with the mineral surface is c o n s i d e r e d t o take place via c o u p l e d charge-transfer processes involving (a) a n o d i c o x i d a t i o n o f the c o l l e c t o r to f o r m a h y d r o p h o b i c species, and (b) c a t h o d i c r e d u c t i o n o f o x y g e n . T h e p o t e n t i a l d i f f e r e n c e across the mineral/solution i n t e r f a c e is the i m p o r t a n t f a c t o r in d e t e r m i n i n g t h e rate o f such reactions and h e n c e the f l o t a t i o n b e h a v i o u r e x h i b i t e d b y the mineral. T h e p o t e n t i a l at a solid/solution interface is d e t e r m i n e d b y t h e presence o f oxidizing and r e d u c i n g species in solution. In o r d e r to m o n i t o r r e d o x properties, it is c u s t o m a r y t o measure the p o t e n t i a l o f an i n d i c a t o r e l e c t r o d e placed in the solution. A n o b l e metal {usually p l a t i n u m ) is generally used f o r this p u r p o s e because such metals have a high resistance to corrosion. When r e p o r t e d against the standard h y d r o g e n e l e c t r o d e (SHE), the r e d o x p o t e n t i a l is r e f e r r e d t o as the " E h " . Eh m e a s u r e m e n t s have been m a d e in industrial f l o t a t i o n plants ( W o o d c o c k and Jones, 1 9 7 0 a , b ; L e o n o v and Badenikov, 1 9 7 3 ; Natarajan and Iwasaki, 1 9 7 3 ) , and the attractive p r o p o s i t i o n t h a t the p o t e n t i a l m a y provide an
0301-7516/84/$03.00
© 1984 Elsevier Science Publishers B.V.
30 efficient and convenient parameter for the automatic control of flotation circuits has been discussed with some confidence. However, whereas physical control of flotation plants is becoming fairly commonplace {McKee et al., 1976), control systems making use of chemical and electrochemical variables are still in the early stages of development. This paper discusses problems in interpreting Eh data obtained from measurements made in mineral-sulphide flotation plants. EXPERIMENTAL Materials
Analar sodium tetraborate was recrystallized from doubly distilled water. Potassium ethyl dithiocarbonate (xanthate) was prepared from ethanolic potassium hydroxide and carbon disulphide (Vogel, 1956). Xanthate that had come into contact with air was purified by washing with diethyl ether, and then recrystallized from acetone by addition of an equal amount of ether. Experiments with xanthate solutions were performed in 0.05 M sodium tetraborate of pH 9.2. These solutions were prepared daily and stored under nitrogen. The nitrogen was purified from traces of oxygen by passage over BTS copper catalyst (supplied by Badische Anilin- und SodaFabrik AG, Ludwigshafen am Rhein, Germany) and through a liquid-air trap. Solutions containing iron(II)/iron(III) couples were prepared by dilution of a stock solution (0.1 M AnalaR FeSO4 + 0.05 M Fe2(SO4)3 in 1 M H2SO4) with 1 M H2SO4. Electrodes
Indicator electrodes for Eh measurements were constructed from noble metals and galena mineral. Metal electrodes each consisted of a 4 × 5 X 0.1 mm piece of foil which was spot welded to a short length of 28-gauge wire of the same metal. Platinum-black electrodes were composed of Teflonbonded platinum pressed on to a tantalum mesh (4 X 5 mm) which was spot welded to platinum wire. This form of platinum has a surface roughness factor (RF) of ~ 2 0 0 0 ; RF is defined as the ratio of the real electrode surface area to the geometric surface area. Galena electrodes were prepared from cubes of 2--4 mm edge cut from massive natural specimens of high quality supplied by the Zinc Corporation mine, Broken Hill, NSW. Electrical connection was made b y attachment of tinned copper wire to the back surface of the mineral specimen by means of silver-epoxy conducting cement (Epo-Tek 410E, E p o x y Technology Inc., Watertown, Mass., USA). The specimen was then arranged centrally in the lower end of a short length ( ~ 2 0 cm) of 6 mm acrylic tubing and encapsulated in epoxy resin (Araldite D). The contact wire protruded from
31 the other end o f the tube and was fixed in position by a plug of e p o x y resin. The working surface of the electrode was prepared by grinding on successively finer grades of silicon carbide paper down to 600 grade. The galena slurry em pl oye d in laboratory experiments consisted of particles of - 2 0 0 mesh prepared by grinding high-quality, massive galena mined by the Zinc Corporation at Broken Hill, NSW. Silica particles used in slurries were purified by t r e a t m e n t with hot hydrochloric acid solution to remove traces of iron. Electrochemical measurements
Noble-metal indicator electrodes were cleaned for application in flotation plants by t r e a t m e n t with chromic acid and for laboratory studies by using an electrochemical method. The latter procedure was carried out in a conventional, 3-compartment, glass cell. T r e a t m e n t consisted of subjecting each electrode to triangular potential cycles in 1 M H2SO4 until a voltammogram of constant shape was obtained. The electrode potential was controlled by a p o ten tio s t at programmed with a sweep generator; bot h instruments were designed and constructed in these laboratories. In order to ensure that the galena indicator electrode had a fresh surface before each laboratory or flotation-plant experiment, the electrode was re-ground on 600-grade silicon carbide paper between each run. Potentials were measured against a standard calomel reference electrode and are reported as Eh, i.e., potentials on the SHE scale. T he potential o f the calomel electrode is 0.244 V relative to the SHE (Bates, 1964). In iron and oxygen systems, current-potential curves were obtained using a potential sweep rate of 4 mV s -1 and were recorded on a HewlettPackard 7004 X-Y recorder. In laboratory studies of flotation systems, open-circuit potential measurements were made using a one-litre, cylindrical, flat-bottomed, glass cell fitted with a large diameter flat-flange joint. A glass lid was clamped to this joint and had tapered sockets to a c c o m m o d a t e four indicator electrodes, the reference electrode, a glass stirrer, and gas inlet and outlet tubes. T he shaft of the stirrer was m o u n t e d vertically and rotated freely in a Teflon bearing which was housed in the central socket of the lid. The stirrer was driven by a small m o t o r via a flexible rubber coupling. The end of the stirrer consisted of two glass paddles arranged at 180 ° to each other. T he cell was water-jacketted for t em pe r at ur e control and all experiments were carried o u t at 25°C. Open-circuit potentials were recorded using a PDP-8/E computer-based instrumentation system. For studies in industrial flotation pulps, a Perspex holder which could a c c o m m o d a t e several electrodes was m o u n t e d on a central aluminium rod. This rod was clamped at 90 ° to a second rod secured across the flotation cell a b o u t 50 cm above the head of the froth. The clamping position was adjusted so that the reference and indicator electrodes were immersed
32 side-by-side in the mineral pulp at a depth of 10 cm below the froth layer. This depth ensured that the electrical circuit through the solution phase was maintained and that the indicator electrodes were in contact with particles consisting predominantly of the mineral being floated. Potentials were 0isplayed on a Siemens Type 192 × 288 6-channel recorder situated next to the flotation cell. Since this recorder had a relatively low input impedance, a voltage follower was used in order to avoid loading of the electrodes. RESULTS AND DISCUSSION Reversible and mixed potentials
It is generally assumed that the oxidizing or reducing conditions in a metallurgical system can be represented by a redox potential and that this potential can be measured by introducing an inert indicator electrode into the system. This view is realistic when a reversible redox couple is dominant. In this case, the measured potential is the reversible potential of the couple as given by the Nernst equation: E R = E ° + (RT/nF)ln(r.
[oxidized]/~ [reduced] )
(1)
The same applies when there are a number of couples present in equilibrium because each couple will have the same reversible potential expressed by eq. (1). However, many systems of metallurgical interest do not fall into this category, and the potential measured by the indicator electrode is not a reversible potential b u t rather a "mixed potential". A mixed potential arises when there are two (or more) redox couples present in the system and they are not in equilibrium, i.e., for the couples Redl/Oxl and Red2/Ox2: [Oxl] E°Red'/Oxl + nRlTF i n -[Red1]
¢
RT [Ox2] E°Red~/Ox2 + n ~ I n -[Red2] -
(2)
Thus, the redox condition in solution is not characterized by a unique Eh value, but rather it is represented by two potentials, i.e., those of the two redox couples present (Fig. 1). When an indicator electrode is introduced into the solution, its electrons can exchange with the two separate redox couples and the measured potential will reach a value, lying between the reversible potentials of the two couples where the c o m p o n e n t anodic and cathodic processes proceed at equal and opposite rates. This is illustrated in Fig. 1. The mixed-potential concept can be illustrated by measurements of potential in solutions containing an iron(II)/iron(III) couple in the presence of oxygen. We have made such measurements in 1 M H2SO4 solutions containing equal concentrations of the two iron species, but differing in total iron concentration.
33
Redl..-,.- Oxl + nle-
-~ .
/(
ER
Red,/Ox,/ i
o
g~
Redz .-,.. Ox2 + nz e -
7
ER
Red2/Ox2~
pot
~.cz o
Oxt+nle----,- Red I
Oxz+nze-----Red 2
Fig. 1. Schematic representation of a mixed-potential system. I Platinum I 1.0
~.
•
+
a
I
I
a
a
I
1
o++
o u.l 0.8
o,+
I i0-6
I
I I 1 I i0-~ i0 -4 i0-3 lO-Z Fe (1"1") and Fe (111) concentration / M
I i0 -~
Fig. 2. Variation of electrode potential with iron concentration [Fe(II)] = [Fe(III)] prepared from 0.1 M FeSO, + 0.05 M F%(SO4) 3 in 1 M H2SO +. Platinum (~) and gold (o) electrodes in absence of oxygen; platinum (=) and gold electrodes (a) in presence of oxygen. Line 1 = reversible potential of iron(II)/iron(III) system; lines 2 , 3 = v a l u e s deduced from rate measurements (Fig. 2) for gold and platinum, respectively.
The experimental p o i n t s in Fig. 2 s h o w the potentials measured by s m o o t h platinum and gold indicator electrodes for a range o f iron concentrations b o t h in the absence and in the presence o f o x y g e n . In oxygen-free solution, i.e., w h e n o n l y the iron(II)/iron(III) redox c o u p l e is present, the experimental potentials are close to the reversible p o t e n t i a l o f the iron system (line 1) for all c o n c e n t r a t i o n s investigated. N o t e that this reversible potential is n o t the standard potential of the Fe2+/Fe 3+ couple, but is a
34 formal potential representing equal concentrations of Fe(II) and Fe(III) in a 1 M H2SO4 solution. When oxygen is also present in the solution, the experimental potentials (Fig. 2) differ from the value for the reversible iron system. The departure becomes more pronounced as the iron concentration is decreased and is greater for platinum than for gold. Natarajan and Iwasaki (1974), in discussing the influence of various factors on the potential of platinum electrodes in the presence of redox couples, concluded that platinum was not an inert electrode as had c o m m o n l y been thought. We believe this viewpoint to be misleading since the term " n o t inert" is generally taken to mean that the material corrodes under the experimental conditions. For the system in question, the departure of the potential from that of the reversible iron couple does not arise from corrosion, but is a consequence of the system not being at equilibrium. The iron(II)/iron(III) and O J H 2 0 couples have different reversible potentials and the potential of the electrode will be determined, not by solution equilibria, but by the rates of electrode reactions at its surface. It will be a mixed potential. Mixed potentials in the same system can be different for different indicator electrodes because the rate of an electrode reaction is dependent on the nature of the electrode surface. The difference will be appreciable when one of the processes is electrocatalytic. Such a process involves the participation of species adsorbed on the electrode surface; the reaction rate will depend on the ability to adsorb the relevant species and hence can vary significantly from one electrode surface to another. This is the case for oxygen reduction and the electrode potential of the iron/oxygen system under investigation must be expected to be different for different electrode materials. Figure 2 shows that the Eh measured by platinum differs from that measured by gold and that the departure from the iron(II)/iron(III) potential is greater for the more catalytically active platinum. The mixed potential can be calculated from the rates of the individual electrode reactions taking part. Current-potential curves for oxygen reduction on platinum and gold were determined in 1 M H2SO4 solution. Similar curves for the oxidation of Fe(II) and the reduction of Fe(III) were determined in oxygen-free 1 M H2SO4 solution containing 10 -1 M of both Fe(II) and Fe(III). The current-potential relationships at other iron concentrations were computed by assuming the current at each potential to be linearly dependent on reactant concentration. The current-potential curves for the iron and oxygen systems are shown in Fig. 3 in the form of an Evans diagram. Such diagrams were developed by Evans (1963) to represent the component electrochemical reactions in corrosion systems. The currents are given in logarithmic form and both the anodic and cathodic currents are presented on the same side of the potential axis. Thus, the mixed potential will be given by the intersection of the anodic and cathodic current curves.
35 I
'
IEI
'
I0-z
I / ~
i0_3 ~
L.// ..Z,,,~
I
'
"
.
I
'
I(a)
(t --
~
~
~
/ 0
2
reduction on
" " ~ platinum
(C)
\
Cd}
X~O,~ongold \\(el reduction
\
\ I0-7
\ \
(f)
I0 -a
I0 -s
_
0.6
0.7
I
0.8 Eh/volt
I
I
0.9
I
I
I.O
Fig. 3. Evans diagram for the iron/oxygen system. Solid lines are reactions of dissolved iron on platinum and gold for [Fe(II)] = [Fe(III)] at given concentrations in 1 M H2SO,; (a) + (f): 10 -~ to 10 -6 M in multiples of 10 -1 M. Dashed lines are for oxygen reduction of platinum and gold in oxygen-saturated 1 M H2SO4. T h e o x i d a t i o n o f iron(II) and the r e d u c t i o n o f iron(III) reactions, being simple e l e c t r o n exchange processes, have practically the same rates o n p l a t i n u m and gold surfaces. With increase in p o t e n t i a l f r o m the reversible value (0.68 V), it is seen (Fig. 3) t h a t the c u r r e n t increases logarithmically in w h a t is t e r m e d the Tafel region. In this region, the rate o f o x i d a t i o n o f Fe(II) is d e t e r m i n e d b y the rate o f e l e c t r o n transfer b e t w e e n the dissolved iron species and the e l e c t r o d e (i.e., " a c t i v a t i o n c o n t r o l " ) . As the p o t e n t i a l is increased f u r t h e r , the logarithmic relationship no longer holds because the rate o f t r a n s p o r t o f the Fe(II) species t o t h e e l e c t r o d e surface is n o t sufficient to maintain such a r e a c t i o n rate. Eventually, the c u r r e n t b e c o m e s limited b y the rate o f mass t r a n s p o r t o f Fe(II) t o the e l e c t r o d e surface (i.e., " m a s s - t r a n s p o r t c o n t r o l " ) and h e n c e no longer increases with potential. A similar sequence o f events occurs f o r the r e d u c t i o n o f Fe(III) with decrease in p o t e n t i a l f r o m the reversible value. In c o n t r a s t t o the iron processes, the r e d u c t i o n o f o x y g e n is m u c h faster on p l a t i n u m t h a n on gold. In the range 0.9--1.0 V, the r e a c t i o n o n p l a t i n u m is activation c o n t r o l l e d , whereas below 0.8 V it is limited b y mass-transport c o n t r o l . T h e decrease in activity at ~ 1 . 0 V is d u e to the a d s o r p t i o n o f o x y g e n o n the p l a t i n u m surface (Damjanovic and Bockris, 1966). T h e rate o f o x y g e n r e d u c t i o n o n gold is activation c o n t r o l l e d over t h e entire p o t e n t i a l range o f Fig. 3. T h e sum o f the rates o f a n o d i c reactions m u s t be equal to t h a t o f the rates o f t h e c a t h o d i c reactions at the m i x e d p o t e n t i a l , since the n e t c u r r e n t flow is zero. Ignoring the back reactions, which have negligible rates at
36 potentials removed from the reversible values, the experimental Eh should have a value at which the oxygen and iron(II) lines intersect in Fig. 3. Lines 2 and 3 in Fig. 2 represent the mixed potentials derived from the Evans diagram plots. It can be seen that there is satisfactory agreement between the Eh ~'alues derived from the kinetic data and those measured experimentally. The mixed potential will also be influenced by other factors which affect the rate of each participating reaction. Stirring the solution will enhance the rates of mass-transport-controlled, but not activation-controlled, processes. For example, in oxygen-saturated solutions dilute in iron, the measured potential decreases significantly on stirring because iron(II) oxidation is mass-transport-controlled and oxygen reduction is activation-controlled. Adsorption of impurities decreases the rate of an electrocatalytic process much more than that of a non-catalytic reaction. Over extended periods, the potentials in the presence of oxygen decrease from the values presented in Fig. 2 towards the redox potential of the iron system, due to inhibition of oxygen reduction by adsorbed impurities. Natarajan and Iwasaki (1975) found that the potential of a platinum indicator electrode in a solution that was 10 -2 M in iron(II) and iron(III) differed significantly from the reversible potential of the iron couple after application of the electrode in solutions containing H2S. This behaviour was due to the establishment of a mixed potential involving anodic oxidation of sulphur present on the electrode surface and cathodic reduction of iron(III). The presence of sulphur inhibited the latter process and hence the mixed potential was sustained. The above observation demonstrates the need for indicator electrodes to be cleaned before transfer from one system to another. Although electrochemical activation by anodic/cathodic treatment is the most rigorous m e t h o d for cleaning noble-metal electrodes, dipping in chromic acid provides an effective and rapid means for obtaining clean, reproducible surfaces.
Mixed potentials in xanthate solutions The solution phase of flotation systems contains a number of electrochemically active species such as collectors, dissolved oxygen, and modifying reagents. As in the case of the iron/oxygen system discussed above, the Eh measured by an indicator electrode placed in the flotation solution will be determined by the rates of the anodic and cathodic processes occurring at the electrode surface. Eh measurements on four different indicator electrodes in air-saturated 0.05 M sodium tetraborate solution (pH 9.2) containing different amounts of potassium ethyl xanthate are shown in Fig. 4. It can be seen that for a given xanthate concentration, there is a significant difference in the potentials measured by two different types of platinum electrode (cf. Fig. 4 for platinum black and smooth platinum). This identifies a further factor which can
37 I
I
t
I
I
1
0.6
~ 0.4
black
~ Platinum
o
0.2
0.0
I I
I
I0 I00 I00( Xanthote concentration/ppm
Fig. 4. Eh measurements with different indicator electrodes in 0.05 M sodium tetraborate solution (pH 9.1) containing different amounts of potassium ethyl dithiocarbonate (xanthate).
/ / / ~ ~ -
i//~ ~ P)-block/
i
~
X2
-Pt Eh
--
Pt , ~ / / ~ t ~
J=
pf black
02~ 0 H-
i...~i,~hEx-]4
ehaviour I
L0.
[x-I
F behoviour
Fig. 5. Schematic representation of the dependence of Eh on the electrode surface roughness in a mixed-potential xanthate/oxygen system.
influence the measured Eh in a mixed-potential system, namely, change in electrode surface roughness. Figure 5 illustrates schematically the influence of reactant concentrations and electrode roughness on current-potential curves and the resulting mixed potentials. The rate of an activation-controlled process is proportional to the real surface area of the electrode, because electron transfer can occur at each site at the solid/solution interface. On the other hand, the rate of
38 a mass-transport-controlled process is dependent on the flux of the reactant in the solution; this flux is not affected by the microstructure of the electrode surface and hence the reaction rate is proportional to the geometrical (or projected) surface area. At low collector concentrations, the electrochemical oxidation of xanthate is mass-transport-controlled and, therefore, the rate is unaffected by changes in the surface roughness of the indicator electrode. On the other hand, the electrochemical reduction of oxygen is activation-controlled and has an enhanced rate on a platinum-black electrode because this electrode has a greater real surface area than smooth platinum. Therefore, at low collector concentrations, the measured Eh obtained with the platinum-black electrode is more positive than that given by the smoothplatinum electrode. The converse situation arises when the xanthate concentration is greater than that of dissolved oxygen. Under these conditions, oxygen reduction is mass-transport-controlled whereas xanthate oxidation is activation-controlled. Hence, increase in the surface roughness of the indicator electrode results in a negative shift in the measured Eh. The Eh data presented in Fig. 4 show that at low xanthate concentrations, the smooth-gold electrode yields lower potentials than those obtained with the smooth-platinum electrode. The explanation for this difference in potential is the same as that given above for the iron/oxygen system, namely, a lower electrocatalytic activity of gold for oxygen reduction compared to that of platinum. The activity of galena for oxygen reduction is less than that of the noble metals (Biegler et al., 1977; Rand, 1977). This in part accounts for the lower Eh values observed with galena (Fig. 4). A further factor contributing to this difference in potential is that oxidation of xanthate on galena occurs at a more negative potential than on noble metals. This shift in the xanthate oxidation potential arises from the formation of different reaction products: dixanthogen is produced on noble metals whereas chemisorbed xanthate and lead xanthate are additional products on galena (Woods, 1976). Eh measurements in galena slurries The above discussions have explored the role played by dissolved electrochemically active species in the measurement of Eh. In order to interpret Eh behaviour in a total flotation system, the influence exerted by the presence of mineral particles must also be considered. To achieve this objective, studies were made of the effect of the addition of mineral particles on the Eh measured by different indicator electrodes in cells containing air-saturated xanthate solutions. Typical results for galena slurries are given in Fig. 6. Prior to addition of mineral to the cell, the indicator electrodes were held in stirred solution until each respective potential reached a steady value. Note that with mechanical stirring, it is difficult to reproduce exactly mass-transport conditions. Thus, for a given electrode, differences in Eh are observed between experimental runs. How-
39 ,
i
I
Platinum black
I
(a) GalIena sllurry
0.25 Platinum
0.20 0.15 OJO .,= 0.25 w
(b) Silica + Galena slurr Platinum black
0.20 Platinum 0.15
OJO ~Galena 0.05
I
-20
I
L
1
I
I
I
0
20
40
60
80
I00
Time from addition of solids/rain Fig. 6. Eh measurements w i t h different indicator electrodes in 0.05 M sodium tetraborate solution (pH 9.2) containing 100 p p m potassium ethyl diothiocarbonate (xanthate) before and after the a d d i t i o n of: (a) galena (23 w / w %); and (b) silica (29 w / w %) + galena (0.6
w/w %). ever, within a single run, all electrodes in the cell experience the same constant stirring conditions. Addition of galena to form a slurry will result in a change in the composition of the solution due to the interaction of both xanthate and oxygen with the galena particles. This is reflected by a small shift in the Eh measured by the platinum-black and galena electrodes. The Eh values measured by smooth-platinum and gold electrodes exhibited a much greater fall on the addition of the galena particles and then approached potentials within 20 mV of that displayed by the galena electrode (Fig. 6a). This suggests that smooth noble-metal electrodes act as slurry electrodes and respond to the potential of the sulphide particles. The difference in potential between the smooth noble-metal and galena electrodes is small compared with the potential range over which galena floats in the presence of ethyl xanthate (Guy and Trahar, 1982). The potential of the platinum-black electrode does not take on the character of the galena slurry because of its high surface roughness factor -the electrode has a network of pores which have a large surface area in contact with the solution phase but only a small projected area for interaction with mineral particles. Hence, the electrode potential is still determined by reactions at the platinum/solution interface. This slurry electrode behaviour is also evident in pulps containing small
40 quantities of galena, even when a large excess of inert material, such as silica, is present (Fig. 6b). The potential changes observed in Fig. 6b are due entirely to the presence of a galena c o m p o n e n t in the slurry since Eh values were found to be unaffected when silica alone was added to the solution. Studies were also carried out in the lead rougher circuit of the flotation plant operated by the Zinc Corporation at Broken Hill, NSW. During the test runs, the residual ethyl xanthate concentration in the flotation cells was ~ 1 0 ppm, measured by a CSIRO-designed on-stream xanthate monitor* (Sullivan and Woodcock, 1973); the solution pH was ~ 9 . As found in the above laboratory experiments, plant measurements showed that the potential of a smooth-platinum electrode (20-h average: 0.172 V) followed closely that of a galena electrode (20-h average: 0.164 V), whereas the potential exhibited by a platinum-black electrode was over 100 mV more positive (20-h average: 0.310 V). CONCLUSIONS In this paper, we have demonstrated how Eh depends on the nature of the indicator electrode used to measure this parameter. For well-poised systems in which a single redox couple is dominant, the Eh value is unaffected by the choice of electrode. In mixed-potential systems, on the other hand, different electrode materials can yield different Eh values. In these systems, the potential is not determined solely by thermodynamics but also by the kinetics of reactions at the electrode/solution interface. Thus, significantly different Eh values are exhibited by platinum-black, smooth-platinum, gold and galena electrodes in solutions containing xanthate and oxygen. In flotation systems, the relevant Eh is that established at the mineral/ solution interface. This suggests that an electrode constructed from the mineral being concentrated should be the most appropriate for Eh measurement. However, since oxidized surface layers form on mineral sulphide electrodes through reaction with dissolved species in the flotation pulp, such electrodes could rapidly reach a condition where they no longer respond rapidly to changes in the pulp environment. It has been shown that smoothplatinum and gold electrodes assume Eh values close to those exhibited by freshly prepared galena when immersed in galena slurries, particularly at high pulp densities. Since these electrodes are influenced b y the conditions established at the mineral-particle/solution interface, they are suitable for monitoring Eh in galena flotation processes. It would be of interest to investigate whether such electrodes are applicable to all flotation systems. Smooth platinum has been the conventional electrode material used for measuring Eh. However, close inspection of the experimental results ob*This instrument is available commercially from the Australian Mineral Development Laboratories, Flemington Street, Frewville, South Australia 5063.
41
tained from laboratory studies (Fig. 6) suggests that gold could well be a better material since it responds more rapidly to changes in slurry conditions and approaches the mineral electrode potential more closely. ACKNOWLEDGEMENTS
The authors are indebted to Mr. M. Moignard for assistance with the experimental work and to the Zinc Corporation (Broken Hill, N.S.W.) for permission to carry out studies in their lead flotation plant.
REFERENCES Bates, R.G., 1964. Determination of pH. John Wiley, New York, N.Y., p. 278. Biegler, T., Rand, D.A.J. and Woods, R., 1977. Oxygen reduction on sulphide minerals. In: J.O'M. Bockris, D.A.J. Rand and B.J. Welch (Editors), Trends in Electrochemistry. Plenum Press, New York, N.Y., p. 291. Damjanovic, A. and Bockris, J.O'M., 1966. The rate constants for oxygen dissolution on bare and oxide-covered platinum. Electrochim. Acta, 11: 376. Evans, U.R., 1963. An Introduction to Metallic Corrosion. Arnold, London, Ch. III. Granville, A., Finkelstein, N.P. and Allison, S.A., 1972. Review of reactions in the flotation system galena-xanthate-oxygen. Inst. Min. Metall. Trans., Sect. C, 81 : 1. Gutierrez, C., 1973. The mechanism of flotation of galena by xanthates. Miner. Sci. Eng., 5: 108. Guy, P.J. and Trahar, W.J., 1982. The influence of grinding and flotation environments on the laboratory batch flotation of galena. 56th Colloid and Surface Science Symposium, Am. Chem. Soc., Blacksburg, Virginia. Heyes, G.W. and Trahar, W.J., 1979. Oxidation-reduction effects in the flotation of chalcocite and cuprite. Int. J. Miner. Process., 6 : 229. Leonov, S.B. and Badenikov, V.Ya., 1973. Redox potential of flotation pulp as a generalized and controllable parameter of flotation. Fiz.-Khim. I. Tekhnol. Issled. Protsessov Pererabotki Polezn. Iskop., 12. McKee, D.J., Fewings, J.H., Manlapig, E.V. and Lynch, A.J., 1976. Computer control of chalcopyrite flotation at Mount Isa Mines Limited. In: M.C. Fuerstenau (Editt)r), Flotation. A.M. Gaudin Memorial Volume, Vol. 2, AIME, New York, N.Y., p. 994. Natarajan, K.A. and Iwasaki, I., 1973. Practical implications of Eh measurements in sulfide flotation circuits. Trans. AIME, 254: 323. Natarajan, K.A. and Iwasaki, I., 1974. Eh measurements in hydrometallurgical systems. Miner. Sci. Eng., 6: 35. Natarajan, K.A. and Iwasaki, I., 1975. Adsorption mechanism of sulphides at a platinumsolution interface. AIChE Symp. Ser., 71(150): 148. Rand, D.A.J., 1977. Oxygen reduction on sulphide minerals. Part III. Comparison of activities of various copper, iron, lead and nickel mineral electrodes. J. Electroanal. Chem., 83: 19. Richardson, P.E., Stout III, J.V., Proctor, C.L. and Walker, G.W., 1984. Electrochemical flotation of sulfides: chalcocite-ethylxanthate interactions. Int. J. Miner. Process., 12: 73--93. Sullivan, J.V. and Woodcock, J.T., 1973. A simple on-stream xanthate monitor. Proc. Australas. Inst. Min. MetaU., 248 : 1. Vogel, A.I., 1956. Textbook of Practical Organic Chemistry. Longmans, London, p. 499.
42 Walker, G.W., Stout III, J.V. and Richardson, P.E., 1984. Electrochemical flotation of sulfides: reactions of chalcocite in aqueous solution. Int. J. Miner. Process., 12: 55--72. Woodcock, J.T. and Jones, M.H., 1970a. Oxygen concentrations, redox potentials, xanthate residuals, and other parameters in flotation plant pulps. In: M.J. Jones (Editor), Mineral Processing and Extractive Metallurgy. Inst. Min. Metall., London, p. 439. Woodcock, J.T. and Jones, M.H., 1970b. Chemical environment in Australian lead-zinc flotation plant pulps. I. pH, redox potentials, and oxygen concentrations. Proc. Australas. Inst. Min. MetaU., 235 : 45. Woods, R., 1972. Electrochemistry of sulphide flotation. Proc. Australas. Inst. Min. Metall., 241: 53. Woods, R., 1976. Electrochemistry of sulfide flotation. In: M.C. Fuerstenau (Editor), Flotation. A.M. Gaudin Memorial Volume, Vol. 1. AIME, New York, N.Y., p. 298.