Electrochemical study of the pyrite-oxygen-xanthate system

International Journal of Mineral Processing, 1 ( 1974) 135--140 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

ELECTROCHEMICAL STUDY OF THE PYRITE--OXYGEN--XANTHATE SYSTEM

A.H. USUL and R. TOLUN*

METU Department of Chemistry, Ankara (Turkey) (Accepted for publication October 17, 1973)

ABSTRACT Usul, A.H. and Tolun, R., 1973. Electrochemical study of the pyrite--oxygen--xanthate system. Int. J. Miner. Process., 1: 135--140. Polarograms have been determined with pyrite electrode in a buffer solution at pH 9.1, with and without potassium ethyl xanthate and dissolved air, and the resulting oxidation-reduction processes interpreted. It has been observed that the addition of dixanthogen in a nitrogenated solution does not produce any hydrophobicity at the surface of pyrite electrode and does not alter the rest potential and cathodic polarogram. The experimental observations and thermodynamic interpretations provide some support for the production of dixanthogen, developing the first hydrophobic layer, directly at the surface of the pyrite by the action of oxygen of air. INTRODUCTION

Dixanthogen as the product providing a hydrophobic coating at the surface of pyrite during flotation has been identified by means of i.r. and u.v. spectrography, (Fuerstenau et al., 1968; Majima and Takeda, 1968) and polarization experiments conducted on pyrite supported the presence of dixanthogen. As the rest potential measured in a xanthate solution coincided with the redox potential of the xanthate--dixanthogen couple, Majima and Takeda (1968) proposed the following equation to explain the adsorption mechanism on pyrite: 1 / 2 0 2 (ad) + 2 X - +H20-+ X2 (ad) + 2 O H -

(1)

Klymowsky and Salman (1970) studying the adsorption of xanthate in deoxygenated, aerated and oxygenated media, supported the mechanism proposed by Fuerstenau et al. (1968): 2 X- + 2Fe (OH)3 + 6H÷ -+ X2 + 2Fe 2+ 6 H20

*Present address: Marmara Scientific and Industrial Research Institute, P.O.Box 21, Gebze - Kocaeli, Turkey.

(2)

136

and stated that dixanthogen thus produced then becomes chemisorbed on the fresh pyrite surface. Excess oxygen was, however, harmful owing to accelerated precipitation of ferric hydroxide. If this view is correct, dixanthogen added to the solution in a deoxygenated medium should adsorb onto pyrite. This study was undertaken to elucidate the apparent disagreement between the proposed mechanisms. EXPERIMENTAL

The apparatus described by Tolun and Kitchener (1963--1964) was adopted for experimental work. The pyrite electrode was prepared from freshly broken pieces a b o u t 1 cm in size by sealing with an epoxy-resin adhesive to the end of a glass tube. Polished pyrite specimens when examined under the reflected--light microscope were found free from contaminating minerals. A 2 X 1 0 - ~ M borax solution was prepared with boiled, distilled water and used for the preparation of all solutions. The pH of these solutions was found to be 9.1. Potassium ethyl xanthate was purified by dissolution in acetone and reprecipitation with pure ether. Oxygen--free nitrogen, after being washed in a pyrogallol solution and passed through a bottle filled with glas~ wool, was used to deoxygenate the medium. Electrode rest potential and polarograms were determined manually with a regular agitation produced by bubbles of gas. During the bubble--contact tests, the agitation was stopped. A Beckman H--3 type pH meter with an internal resistance of 1012 ohms was used to determine the potentials. RESULTS AND DISCUSSION

Pyrite electrodes in buffer solution In Fig.1 the polarograms of pyrite electrode are presented. The rest potential of the electrode in nitrogenated solutions was - 0.060 V versus SCE (saturated calomel electrode). This is close to - 0.070 V which was given by Majima and Takeda. In aerated solution the potential was shifted to 0.030 V by the cathodic effect of oxygen of air (mixed potential). The cathodic polarogram indicated no appreciable reduction in nitrogenated solution. In aerated solution, however, the effect of oxygen was observed although it was quite feeble in comparison with that observed with a platinum electrode. The anodic curves are similar for nitrogenated and aerated solutions. Up to 0.300 V pyrite remained quite resistant to oxidation. At higher potentials the rate of oxidation increased rapidly and a brown ferric hydroxide film developed at the surface of the electrode. U n d o u b t e d l y the sulphur species are also oxidized at this region up to sulphate ion, preventing the passivation of the pyrite surface.

137 /u.amp 600 500 Cathodic 400

H2 evolution

-

300200

/7

-

,',/

100 -~1.0

I

+0,8 ,

-~0.6

+0.4

~_~__~4~',

+0,2

'

,

-0.2.

/Fe(O'H)3 Formation

-0.4.

-0.8

-1.0

%2 volt vs SCE

2003oo400-

• • • ......

500 Anod~c

-0.4

6o0

Fig.1. Polarograms at pyrite electrode, pH

Borax Borax

solution (air-suturoted) solution(air-free)

•

= 9.1.

The above observations and the rest potentials indicate that ferric hydroxide should be present at the surface of the pyrite electrode even in nitrogenated solution, and hence should be more or less in equilibrium with the first oxidation step of pyrite. FeS~ -~ Fe 2÷ + 2S + 2 e-

E~ = 0.424 V

Fe(OH)3 + 3 H ÷ + e --~ F e 2 + + 3 H 2 0

E~=l.057V

2Fe (OH)3 +FeS2 + 6 H ÷ - ~ 3 F e 2 + + 2 S + 6 H 2 0 Eh = 0 . 6 3 5 - 0.059 pH

(3) (4) K=1021"4

(5) (6)

At pH 9.1 the potential becomes - 0 . 1 4 2 V versus SCE. The measured potential in nitrogenated solution was, however, 82 mV higher because of the slowness of the reaction 3. According to eq. 6 a potential decrease of 59 mV/pH is in good agreement with the experimental data cited in the literature (Sato, 1960; Mukai and Wakamatsu, 1962; Natarajan and Iwasaki, 1972). Pyrite e l e c t r o d e in xanthate solution

I'n Fig.2 the polarograms of the pyrite electrode in a nitrogenated or aerated buffer solution containing potassium ethyl xanthate (KEX) at 1 0 - 3 M concentration are presented. In nitrogenated solution the potential was lowered to - 0 . 1 0 1 V versus SCE, which is almost the same as Majima and Takeda's value. As this potential is

138

/~amp 50O Cathodic

H2 evo [ut ion

400 300

i

2O0 .1.0

.0.8

.0.6

I

l

I

.0.4 I

.0.2

X2 i No stic king reductl~fd- ' stick~n~/__~)"'__'_

100

i

-

i

I

i

-02

-0.4

-0.6

9

. -

o _o-~

i

_ _ _ o

-0.8

~ o

-1.0

.'/: sfi~kih~ ,oZ~e°~ . . . . . . . ~00

//

400

.~

T

--O--

7/

Anod,'c

~

-

O --

~

1

-1,2 volt -1.4 vs SCE

Borax solution ,10~ KEX ( a i r - s a t u r a t e d ) -3 Borax solution *IOM KEX(air-free )

600 700

Fig.2. Polarog~ams at p y r i t e e l e c t r o d e , pH = 9.1.

slightly higher than that of the xanthate--dixanthogen couple: 2 X- = X2 + 2 e-

Eh = - 0.081 - 0.059 log (X-)

(X-) = 10-3M

E = - 0.144 V versus SCE

(7)

a feeble redox reaction might occur between ferric hydroxide and xanthate ion, coupling eq. 7 and 4. No appreciable reduction was observed, however, during cathodic polarization. Slight anodic polarization indicated the formation of dixanthogen at the surface of pyrite and bubble contact could be obtained. At potentials higher than 0.300 V, the formation of ferric hydroxide and the inhibition of bubble contact were observed. This result accords with that of Klymowsky and Salman (1970) in an oxygen-saturated medium. In aerated solutions, the potential rose to -0.061 V and bubble contact could be obtained at this potential, indicating a hydrophobic coating with dixanthogen. The cathodic effect of the oxygen of air was acting against the anodic effect of xanthate and hence a mixed potential was created which was much higher than that of the redox potential of the xanthate--dixanthogen couple. Taking into account that oxygen of air increases the electrode potential by 90 mV and observing that this difference corresponds to 1.5 pH units in eq.6, it could easily be deduced that beyond the critical pH 10.5, the electrode potential of pyrite would be lower than that necessary to produce dixanthogen even in aerated solution.

139

The effect of dixanthogen To elucidate whether dixanthogen was formed by the mediation of ferric species in solution or directly at the surface by the effect of oxygen of air, dixanthogen previously prepared as an emulsion in a nitrogenated buffer solution was added to the nitrogenated 1 0 - 3 M xanthate solution as a small quantity just sufficient to produce a slight turbidity. No change in the potential of the pyrite electrode was observed and no bubble contact developed. The cathodic polarization curve Fig.3 was exactly the same as that in Fig. 2 (xanthate in nitrogenated solution). The observations mentioned above indicate that dixanthogen, even if previously formed in solution, cannot adsorb onto a hydrophilic surface of pyrite and convert it into a hydrophobic one. Yticesoy and Yarar (1974) measured the zeta--potentials of dixanthogen at various pH values and found a potential of -100 mV for a medium similar to ours. Pyrite being also negatively charged in basic media (Fuerstenau et al., 1968), physical adsorption of dixanthogen on pyrite would not be expected. Hence the hydrophobicity created in alkaline pH by the action of oxygen was the result of an increase in the electrochemical potential of pyrite to allow the oxidation of xanthate ions directly at the surface. In this way, eq.1 proposed by Majima and Takeda was supported.

600 Cathodic

500

/

4O0 300 200

qf e'

g

100 I

-02

i

1

I

i

-OA

- 0.6

-0.8

- tO

~5¢E

2OO Anodi¢ 4OO

v

- Iv.20tt

.....

Borax sotution÷lO~ KEX + Xt(dispersed ) (air-free)

Fig.3. Polarograms at pyrite electrode, pH = 9.1.

CONCLUSIONS

Electrochemical study of the pyrite--xanthate--oxygen system and the related thermodynamic equilibria indicate that at pH 9.1 the rest potential

140

of pyrite is near that of the xanthate--dixanthogen couple at a xanthate concentration of 10-3 M. The very feeble interaction, indicated by a small decrease of the electrode potential of pyrite in the presence of xanthate ions in nitrogenated solution, does not provide a floatable condition. In the presence of the oxygen of air or by anodic polarization an increase of the electrochemical potential (mixed potential) provides a suitable catalytic surface for the formation of dixanthogen directly on pyrite, rendering it hydrophobic. Excess anodic oxidation, however, leading to the formation of ferric hydroxide, inhibits bubble contact. Dixanthogen, that could also be formed by mediation of ferric species in solution, does not adsorb on a pyrite surface in a nitrogenated basic medium. This result is also consistent with recent data on the zeta--potential of dixanthogen when compared with that of pyrite.

ACKNOWLEDGMENT

The authors wish to thank Dr. J.A. Kitchener, Department of Mining and Mineral Technology, Imperial College, London, for valuable discussion during the preparation of this paper.

REFERENCES Fuerstenau, M.C., Kuhn, M.C. and Elgillani, D.A., 1968. The role of dixanthogen in xanthate flotation of pyrite. Trans. AIME, Soc. Min. Eng. 241: 148--156. Klymowsky, I.B. and Salman, T., 1970. The role of oxygen in xanthate flotation of galena, pyrite and chalcopyrite. Trans. Can. Min. Metal. Bull. 73: 147--152. Majima, H. and Takeda, M., 1968. Electrochemical studies of the xanthate--dixanthogen system on pyrite. Trans. AIME, Soc. Min. Eng. 241: 431--436. Mukai, S. and Wakamatsu, T., 1962. Electrochemical study on flotation. Mem. Fac. Eng. Kyoto Univ., XXIV--4: 140--189. Natarajan, K.A. and Iwasaki, I., 1972. E h - - pH response of noble metal and sulfide mineral electrodes. Trans. AIME, Soc. Min. Eng. 252: 437--439. Sato, M., 1960. Oxidation of sulfide ore bodies. II. Oxidation mechanisms on sulfide minerals at 25°C. Econ. Geol. 55: 1202--1231. Tolun, R. and Kitchener, J.A., 1963--1964. Electrochemical study of the g a l e n a xanthate -- oxygen flotation system. Trans. Inst. Min. Metal., 72: 313--322. Yt~cesoy, A. and Yarar, B., 1974. Zeta potential measurement in the galena -- xanthate -oxygen flotation system. Trans. Inst. Min. Metal., in press.