Chemisorption of ethyl xanthate on silver—gold alloys

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 83 (1994) l-7 0921-7757/94/$07.00 0 1994 ~ Elsevier Science B.V. All rights reserved.

Chemisorption

of ethyl xanthate on silver-gold alloys

R. Woods”,“, C.I. Basiliob, D.S. Kimb, R.-H. Yoonb “CSIRO Division of Mineral Products, P.O. Box 124, Port Melbourne, Vie. 3207, Australia bVirginia Centerfor Coal and Minerals Processing, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA (Received

21 January

1992; accepted

19 August

1993)

Abstract The characteristics of the interaction of ethyl xanthate with silver-gold alloys have been compared with those of the pure component metals. Voltammograms for silver and silver-gold alloys (50: 50 and 20: 80 (wt.%)) in the presence of ethyl xanthate show that xanthate is chemisorbed on silver sites in the alloy surfaces at potentials below those at which silver ethyl xanthate is formed. The coverage of chemisorbed xanthate was determined for the alloys as a function of potential and found to fit the Frumkin isotherm previously derived for silver, when the decreased activity of silver in the alloys was taken into account. The potentials at which finite contact angles developed correlated with those for the onset of chemisorption. It is concluded that the presence of silver in native gold will render gold-bearing ores easier to float with ethyl xanthate as collector. Key words: Chemisorption;

Ethyl xanthate;

Frumkin

isotherm;

Silver-gold

alloys; Voltammetry

Based on ex situ FTIR

Introduction Studies of the interaction of ethyl xanthate with gold, silver and different compositions of silvergold alloys have been reported recently [ 11. The application of Fourier transform infrared

spectroscopy,

the previ-

ous investigations [l] also provided evidence for the formation of “unknown” surface species at low potentials. A recent study [2] using voltammetry and in situ FTIR spectroscopy showed that ethyl xanthate is chemisorbed in the potential region preceding silver ethyl xanthate formation. The

(FTIR) spectroscopy, cyclic voltammetry and X-ray photoelectron spectroscopy showed that diethyl dixanthogen and silver ethyl xanthate were the respective products of anodic oxidation at gold and silver electrodes. On the Ag-Au alloys, the formation of both these species was demonstrated, with silver xanthate being formed at potentials below that for dixanthogen. The potential of the onset of silver xanthate formation for 50: 50 and 20: 80 ( wt.%) Ag-Au alloys was found to be about 50 mV and 100 mV, respectively, more positive than the value for the pure metal.

present authors have characterized the chemisorption of ethyl xanthate on silver [3] and shown that the equilibrium coverage of chemisorbed xanthate obeys a Frumkin isotherm similar to that found for copper and chalcocite surfaces [ 41. As pointed out previously [ 11, gold in its ores commonly contains silver, and hence it is important to characterize the adsorption behavior of collectors on Ag-Au alloys, rather than on the pure metal, to identify optimum flotation conditions. Since chemisorbed xanthate species, in

*Corresponding

addition to metal xanthates, are hydrophobic [3,4], it is essential

author.

SSDZ 0927-7757(93)02649-Y

known to be to define the

R. Woods et al./Colloids

2

conditions under which xanthate chemisorbs on Ag-Au alloys in addition to those under which silver xanthate

and dixanthogen

Surfaces

a Hewlett-Packard

The

Electrodes Ag-Au

alloy

1-7

were recorded

on

7015B X-Y recorder.

electrodes

were

prepared by encapsulation into epoxy resin. The alloys were 50: 50 and 20: 80 (wt.%) Ag-Au gold respectively. They were provided by Kultakeskus Oy (Gold Centre) in Finland and were made by a double-melting homogenization procedure using 99.99% pure metals. The composition was verified by the Technical Research Centre of Finland using scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS) and fire assay methods. The gold and silver samples (99.9985%; Puratronic) were provided by Aesar (USA). The exposed surface areas of the Ag, Au, 50 : 50 Ag-Au and 20 : 80 AggAu electrodes were 33 mm’, 36 mm’, 30 mm2 and 25 mm2 respectively. A fresh electrode surface was generated before each run by polishing with alumina and washing three times with hot ethanol in an ultrasonic bath.

Voltammetry A conventional three-electrode system was used for the voltammetric experiments and the cell solution was deoxygenated with high purity nitrogen. All experiments were carried out with an electrolyte of pH 9.2 (0.05 mol dme3 Na2B,0,) containing 0 or lop3 mol dme3 potassium ethyl xanthate. The electrode potential was controlled with a Pine Instrument model RDE4 potentiostat/ galvanostat and a PAR model 175 programmer. The reference electrode was a silver/silver chloride electrode in 4 mol dmp3 potassium chloride. All potentials are given on the standard hydrogen electrode (SHE) scale, taking the potential of the silver/silver chloride electrode to be 0.20 V against

coverage

coverage

determined

and

83 (1994)

are formed.

Experimental

gold

Eng. Aspects

the SHE [ 51. Voltammograms

Chemisorption

Silver,

A: Physicochem.

from

of chemisorbed measurements

xanthate of the

was charge

passed in cathodically stripping the chemisorbed layer. For these determinations, the electrode was held at a selected potential for 1 min. The current flowing was negligible after this time, indicating that equilibrium had been established. A negativegoing scan was then applied at 20 mV s-i and the cathodic charge passed between the starting potential and the lower limit integrated. The xanthate desorption charge was taken as the difference between the charges passed on scans from the same potential run in the presence and absence of xanthate. This procedure was previously applied to copper and chalcocite surfaces with ethyl xanthate as collector [ 41. Contact angle measurements Contact angle measurements were made on silver, gold and the Ag-Au alloy electrodes under potential control using a modified electrochemical cell. The potential was first held at -0.5 V to remove any oxidized species from the electrode surface. It was then increased to the selected value and the contact angle was measured after 2 min using a Rame Hart Model 100 goniometer. The potential was returned to -0.5 V prior to the next experiment. Results and discussion Voltammetry Figure l(a) presents voltammograms for silver and 50 : 50 and 20 : 80 Ag-Au gold alloy electrodes in pH 9.2 solution containing 1O-3 mol dmm3 ethyl xanthate. The main feature on each positive-going scan is a peak which appears at positive potentials. It has been demonstrated by FTIR spectroscopy

R. Woods et al./Colloids I

Surfaces A: Physicochem. I

I

I

I

Eng. Aspects 83 (1994)

I-7

an anodic

1

100

3

current

flows

owing

to dixanthogen

formation as found by previous workers [ 1,6]. As reported previously [ 11, the formation of silver xanthate in Fig. l(a) commences at about 0.05 V and 0.1 V more positive for the 50: 50 and

50

20 : 80 alloys, respectively,

than for pure silver. This

reflects a decrease in the activity of silver when alloyed with gold. The differences in the Gibbs free energy of formation (dG”) between silver in the pure metal and in the 50 : 50 and 20: 80 alloys, calculated from the thermodynamic data collated by White et al. [7], are -3.54 kJ (g atom))’ and - 11.2 kJ (g atom))’ respectively. Thus the shifts in potential of the silver/silver xanthate couple are expected to be 0.037 V and 0.116 V for the respective alloys. The shifts observed in the voltammogram of 0.05 V and 0.10 V are in agreement with

-150

I

1

0.6

I

-0.4

Potential

(a)

I

I

0.2

0.4

I

-0.2

0

(V

vs

these values. The voltammograms in Fig. 1 (a) are also characterized by a prewave. In the case of pure silver, the prewave has been identified with the underpotential deposition of a chemisorbed monolayer of ethyl xanthate by the charge transfer process [3]

SHE)

C,H,0CS;-+(C2H50CS2)ads+e-

ci_,o-0.6

-0.6

Fig. 1. taining 50: 50 (curve

-0.4

Potential

(b)

-0.2

0.0

(V vs

0.2

0.4

0.6

SHE)

Voltammograms at 50 mV sC1 in pH 9.2 solution con10e3 mol dme3 ethyl xanthate for (a) silver (curve l), wt.% Ag-Au alloy (curve 2), 20: 80 wt.% Ag-Au alloy 3) and for (b) gold.

[l] that for all three surfaces this peak arises from the formation of silver ethyl xanthate: Ag + C,H,OCS;

-+C2H,0CS2Ag

+ e-

(1)

Voltammograms for gold in the same solution (Fig. 1 (b)) show no features arising from xanthate oxidation below about 0.2 V; above this potential

(2)

The prewave for the alloy electrodes can confidently be assigned to the same process. The charge associated with the prewave for the three electrodes was found to be approximately proportional to the silver content of the surface. This indicates that the chemisorption of ethyl xanthate in the prewave potential region occurs only on silver sites in the alloy surfaces. Adsorption

isotherms

The charge associated with the chemisorption of ethyl xanthate on the alloy electrodes was determined as a function of potential from the charge passed in cathodically stripping the layer from the surface after holding the electrodes at each selected potential. Figure 2 presents representative cathodic stripping voltammograms for the 20 : 80 alloy in the absence and presence of lop3 mol dme3 ethyl xanthate. The starting potentials correspond to

R. Woods et al.JColloids

4

Surfaces A: Physicochem.

Eng. Aspects 83 (1994 ) 1-7

This ratio is very close to the ratio of the atomic fractions of silver in the three respective surfaces, i.e. 1:0.65: 0.31. This confirms that xanthate adsorbs

only on silver sites in the alloy surfaces in

the prewave

potential

region.

With the procedure adopted for determining coverage, a monolayer on the alloys is defined as the occupation of each available silver site in the surface. -0.5

-0.4

-0.3

Potential

-0.2

(V

-0.1

vs

0.0

0.1

SHE)

Fig. 2. Cathodic stripping voltammograms for the 20: 80 wt.% Ag-Au alloy at 20 mV s-l in pH 9.2 solution containing 10e3 mol dmm3 ethyl xanthate recorded after stepping the potential to the selected value and holding for 1 min.

values at which the fractional surface coverage is about 0.1, 0.45 and 1.0 respectively. In order to convert the stripping charge to fractional surface coverage, it is necessary to know the charge corresponding to a monolayer. As adopted previously for silver [ 31, and both copper and chalcocite [4], the assumption was made that the coverage of chemisorbed xanthate reaches a monolayer before silver xanthate develops. Since no silver xanthate can form at potentials below the reversible value of the silver/silver xanthate couple, the charge corresponding to stripping the layer at a potential just below the reversible value was taken to represent a monolayer. At all other potentials, surface coverage was taken as the ratio of the measured

charge

to that determined

The coverages

for the 50: 50 and 20: 80

Ag-Au alloys are presented as a function of potential in Fig. 3. Coverage data for a silver electrode in pH 6.8 and 9.2 solutions containing 10e2, 10-3, 10-4, and lop5 mol dmm3 ethyl xanthate were found [3] to fit a Frumkin isotherm of the form [9] [0/( 1 - /3)] exp g0 = Ku, exp(yFE/RT)

(3)

where 0 is the fractional surface coverage, g and K are constants, aA is the activity of the adsorbate in solution, y is the electrosorption valency, and the other terms have their usual meaning. The term go arises from variation of the free energy of adsorption with coverage, through either hetero-

for the

monolayer. An ethyl xanthate radical occupies 0.29 nm2 [S] and hence the charge corresponding to the deposition of a close-packed ethyl xanthate monolayer on a smooth surface is 55 uC cm-2. The monolayer charge for the silver electrode was found to be 68 uC cmm2, which agrees with the calculated value, assuming a surface roughness factor of about 1.2. Such a roughness factor is reasonable for a polished surface. The monolayer charges found for the 50 : 50 and 20 : 80 alloys were 44 uC cm-’ and 23 uC cmm2 respectively. Thus the respective monolayer charges for the silver and the two alloy electrodes are in the ratio 1 : 0.65 : 0.34.

-0.4

-0.3

-0.2

Potential

-0.1

0

0.1

(V VI SHE)

Fig. 3. Potential dependence of the fractional coverage of chemisorbed xanthate in solutions at pH 9.2 containing 10m3 mol dme3 ethyl xanthate. Solid lines are isotherms for silver (curve 1) from Ref. 3; calculated for 50:50 wt.% Ag-Au alloy (curve 2) and 20: 80 wt.% Ag-Au alloy (curve 3). Data points are experimental fractional coverages for the 50: 50 (0) and 20 : 80 alloys (A).

R. Woods et al./Colloids Surfaces A: Physicochem. Eng. Aspects 83 (1994)

geneity adsorbed

of the

surface

molecules.

reflects changes

or interactions

The electrosorption

in the double

1-7

between valency

layer associated

y

with

the adsorption process, in addition to the number of electrons involved in the charge transfer reaction. The isotherm for ethyl xanthate chemisorption on a silver electrode at 25°C was found to be [0/( 1 - e)] exp 48 =4.25 x lo7 [X-l

exp(E/0.023) (4)

where [X-l is the xanthate ion concentration. The isotherm derived for 10m3 mol dmm3 ethyl xanthate is presented in Fig. 3 (curve 1). Analysis of the chemisorption of xanthate on silver sites in the Ag-Au alloy surfaces must take into account the diminished activity of silver. Thus the isotherm is expected to be of the form [0/( 1 - 19)]exp g0 = Kax-a,,

exp(yFE/RT)

[0/( 1 - 0)] exp 48 x 107a,,[X-]

exp(E/0.023)

-0.2

-0.1

Potsntlal

0

0.1

0.2

0.3

(V vs SHE)

Fig. 4. Dependence of contact angle on potential in pH 9.2 solution containing 10e3 mol dmm3 ethyl xanthate for silver (curve l), 50:50 wt.% Ag-Au alloy (curve 2), 20:80 wt.% Ag-Au alloy (curve 3) and gold (curve 4).

(5)

where ax- and aAg are the activities of xanthate in solution and silver in the alloy surface respectively. Therefore the isotherms for the alloys should be

=4.25

-0.3

(6)

Values of silver activity for the alloys can be derived from the thermodynamic data collated by White et al. [7]. For the 50 : 50 and 20 : 80 Ag-Au alloys, aAg is calculated to be 0.24 and 9.3 x lop3 respectively. Curves 2 and 3 in Fig. 3 are the isotherms for the respective alloys obtained by inserting these values in Eq. (6). It can be seen that the coverage data fit the derived isotherms within experimental error. Surface hydrophobicity Figure 4 presents measurements of the contact angle established for silver, gold and 50: 50 and 20: 80 Ag-Au alloy electrodes in pH 9.2 solution containing 10e3 mol dme3 ethyl xanthate after polarization at different potentials. It can be seen that each electrode surface is hydrophilic at suffi-

ciently negative potentials but becomes hydrophobic at a critical potential. Comparison of Figs. 3 and 4 shows that this critical value corresponds to the commencement of ethyl xanthate chemisorption and that the contact angle increases with increase in potential in a similar manner to chemisorption coverage. Thus it is clear that the chemisorption of ethyl xanthate results in a hydrophobic surface and should render silver and Ag-Au alloy particles floatable. The development of a silver ethyl xanthate phase is not necessary for flotation to occur. For the silver and the 50: 50 alloy, the maximum contact angle is close to the value considered by Sutherland and Wark [lo] to be characteristic of ethyl xanthate for any surface “that has responded fully to the collector”. The maximum angle developed by the 20: 80 alloy in the prewave potential region is significantly less, being about 50”. This can be explained by a lower coverage of xanthate on the alloy surface. Although monolayer coverage is reached with respect to silver atoms, only 20% of metal sites on the surface are bonded to ethyl xanthate. The remaining 80% of surface sites constitute gold atoms which are not hydrophobic in

R. Woods et al./Colloids

6

Surfaces A: Physicochem.

Eng. Aspects 83 (1994 ) l-7

Conclusions

‘OOiO

Voltammograms

for 50: 50 and

20: 80 (wt.%)

Ag-Au alloys in a pH 9.2 solution containing xanthate are, like that of silver, characterized chemisorption

prewave

occurring

at

ethyl by a

potentials

lower than that for the formation of silver ethyl xanthate. The charge associated with the prewave indicates -0.4

-0.3

Potential

-0.2

-0.1

0

(V vs Er(X’/X2))

Fig. 5. Potential at which ethyl xanthate has a fractional chemisorption coverage of 0.1 of silver sites in Ag-Au alloy surfaces, with respect to the reversible potential of the xanthate/ dixanthogen couple.

xanthate solutions at potentials below the region where dixanthogen is formed [ 111. The development of a finite contact angle for gold occurs at the potential at which ethyl xanthate is known to oxidize to dixanthogen [ 1,6,11]. It can be seen from Fig. 4 that the two alloys become hydrophobic at potentials substantially more negative than that of gold, the difference being about 0.45 and about 0.4 for the 50: 50 and 20: 80 alloys respectively. The potential at which ethyl xanthate chemisorbs on Ag-Au alloys can be calculated for all alloy compositions from Eq. (6) and the thermodynamic data of White et al. [ 71. Figure 5 presents the dependence on alloy composition of the potential at which the fractional coverage of surface silver sites is 0.1, calculated in this way. Potentials in Fig. 5 are referred to the reversible potential of the xanthatejdixanthogen couple, and hence to the potential at which gold becomes hydrophobic. It can be seen from Fig. 5 that the presence of a few per cent of silver shifts the potential at which a hydrophobic xanthate is formed on gold by over 0.2 V. As the silver content increases, the potential for the onset of xanthate adsorption becomes more negative. Thus the presence of silver in native gold particles should result in the gold ore being much easier to float with ethyl xanthate as the collector.

that xanthate

chemisorbs

only on silver

sites in the alloy surface. The potential at which silver ethyl xanthate develops for the alloy electrodes is shifted to higher potentials compared with that for the pure metal owing to the diminished activity of silver. The potential dependence of the coverage of chemisorbed xanthate on the 50 : 50 and 20: 80 alloys was found to obey the Frumkin isotherm previously derived for silver when it was modified to account for the known change in activity of silver when alloyed with gold. The development of finite contact angles on silver and the alloys correlates with chemisorption of xanthate. Gold only becomes hydrophobic at much higher potentials, where dixanthogen is formed. Consideration of the variation with composition of the potential at which ethyl xanthate is chemisorbed on Ag-Au alloy surfaces indicates that the presence of silver in native gold particles should result in the gold ore being much easier to float with ethyl xanthate as the collector.

Acknowledgment The authors are indebted to Dr. J.O. Leppinen of the Technical Research Centre, Outokumpu, Finland, for providing the analyses of the alloy samples.

References 1 2

J.O. Leppinen, R.-H. Yoon and J.A. Mielcrarski, Colloids Surfaces, 61 (1991) 189. P. Talonen, J. Rastas and J. Leppinen, Surf. Interface Anal., 17 (1991) 669.

R. Woods et al./Colloids Surfaces A: Physicochem. Eng. Aspects 83 (1994) 3 4 5 6 7

R. Woods, C.I. Basilio, D.S. Kim and R.H. Yoon, J. Electroanal. Chem., 328 (1992) 179. R. Woods, C.A. Young and R.H. Yoon, Int. J. Miner. Process., 30 (1990) 17. R.G. Bates, Determination of pH, Theory and Practice, Wiley, New York, 1964. R. Woods, J. Phys. Chem., 75 (1971) 354. J.L. White, R.L. Orr and R. Hultgren, Acta Metall., 5 (1957) 747.

8 9

10 11

l-7

7

A.M. Gaudin and G.S. Preller, Trans. AIME, 169 (1946) 248. J.W. Schultze, in S. Bruckenstein, J.D.E. McIntyre, B. Miller and E. Yeager (Eds.), Proc. 3rd Symp. Electrode Processes, 1979, The Electrochemical Society, Princeton, NJ, 1979, pp. 1677189. K.L. Sutherland and I.W. Wark, Principles of Flotation, Aust. IMM, Melbourne, Australia, 1955, p. 97. J.R. Gardner and R. Woods, Aust. J. Chem., 27 (1974) 2139.