Oxidation of lead sulphide in molten PbCl2+KCl: Electrolysis and visible spectrophotometry

J. Electroanal. Chem., 105 (1979) 143--148

143

© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

O X I D A T I O N O F L E A D S U L P H I D E IN M O L T E N PbC12 + KCl: ELECTROLYSIS AND VISIBLE SPECTROPHOTOMETRY

A. DE GUIBERT, V. PLICHON and J. BADOZ-LAMBLING Laboratoire de Chimie Analytique, E.S.P.C.I., 10 rue Vauquelin, F75231-Paris Cedex 05 (France)

(Received 18th December 1978; in revised form 27th March 1979)

ABSTRACT Controlled potential electrolyses of lead sulphide solutions, coupled with visible absorption spectrophotometry have shown: (a) the exchange of two moles of electrons per mole of sulphide ion, whatever the electrolysis potential is on the two oxidation steps of PbS; (b) the formation of an intermediate polysulphide during electrolyses on the first step.

INTRODUCTION Electrolysis o f metallic sulphides dissolved in the c o r r e s p o n d i n g m o l t e n chlorides is an attractive process f o r m e t a l p r e p a r a t i o n , with regard to t h e eliminat i o n o f sulphur d i o x i d e emissions p r o d u c e d b y o t h e r processes such as pyrometallurgy. In this general field, the case o f lead sulphide is an interesting o n e and has been already studied on several occasions [ 1 - - 1 1 ] . T h e m a j o r i t y o f these investigations were p e r f o r m e d with the aim o f improving the yield Of electrolyses [ 4 - - 6 , 8 , 9 ] ; t h e y have s h o w n t h a t the main difficulties o c c u r at the a n o d i c side w h e r e s u l p h u r is recovered. T h e m e c h a n i s m o f t h e a n o d i c reactions is n o t well k n o w n and has b e e n the subject o f r e c e n t studies in PbC12 + NaC1 [9--11] or PbC12 + KC1 [ 1 2 , 1 3 ] . T h e following results were o b t a i n e d . (1) In PbC12 + KC1, PbS oxidizes in t w o steps: v o l t a m m e t r i c curves o f lead sulphide solutions show the existence o f t w o o x i d a t i o n waves [ 12]. (2) A t t h e t i m e scale (less t h a n o n e second) o f c h r o n o p o t e n t i o m e t r i c [11] or c h r o n o a m p e r o m e t r i c [ 13] m e a s u r e m e n t s , it has b e e n s h o w n t h a t the f i r s t step is a t w o - e l e c t r o n t r a n s f e r c o n t r o l l e d b y the diffusion o f sulphide ion. T h e prod u c t i o n o f sulphur o n the e l e c t r o d e is f o l l o w e d u p b y a r e a c t i o n in the m e l t [11]. In PbC12 + KC1, t h e s e c o n d step appears c o m p l e x and has n o t b e e n comp l e t e l y e l u c i d a t e d [ 13]. In PbC12 + KC1, if the same reactions o c c u r at the time scale o f electrolyses, we w o u l d e x p e c t an exchange o f t w o electrons and a mass-transfer c o n t r o l l e d electrolysis o n the first step. On the s e c o n d step, a c o m p l e x m e c h a n i s m is e x p e c t e d , perhaps w i t h the e x c h a n g e o f m o r e t h a n t w o electrons. E x p e r i m e n t a l results are f o u n d in c o m p l e t e c o n t r a d i c t i o n with these forecasts.

144

EXPERIMENTAL

(1) Experiments performed without spectrophotometric measurements are carried out in the melt previously used [12] of composition PbCI: 77 + KC1 23 (mol. %) which corresponds to the greatest solubility of lead sulphide [4]. Spectrophotometric experiments are carried out either in the same melt or in PbC12 52 + KC1 48 (mol. %) where the lead chloride absorbancy is lower in the range 400--500 nm. Electrolysis results do n o t depend on the melt composition. (2) Cell and electrodes. Fibre optics enable us to obtain the absorption spectra of species inside the electrolytic cell. The spectroelectrochemical cell has been described previously [12]. The melt is contained in Pyrex crucibles. Electrolyses are performed on graphite rods (Le Carbone Lorraine, spectroscopically pure, 6 mm diam.). The electrode area is about 2.2 cm 2. When electrolyses are carried out without spectrophotometric measurements, the melt is contained in a glassy carbon crucible which is used as the working electrode. In this way we obtain electrodes of greater area than the graphite rods. The cathodic compartment is separated from the bulk of the melt by a glass frit of medium porosity. Concentrations of solutes have been calculated with the values of melt densities measured by Boardman et al. [14] (d = 4.35 and 3.58 g cm -3 for the melts PbC12 + KC1 77--23 and 52--48, respectively). RESULTS

(I) Electrolyses without spectrophotometry Electrolyses are performed on the plateau of the first or the second oxidation waves of PbS. The results show in both cases four characteristic c o m m o n features: (a) an exchange of 1.8 to 1.9 mole of electrons per mole of sulphide ion is involved until the electroactive species have totally disappeared (Table 1); (b) the two oxidation waves disappear at the same time; (c) n o reduction wave appears; (d) sulphur is recovered on the lid of the cell. On the contrary, different results are obtained on the two waves for the following features: (a) shape of the curves current vs. time; (b) growth of bubbles on the electrode. When electrolyses are performed on the first oxidation step, the current does not follow an exponential decay (Fig. 1). At the beginning of electrolyses, the current increases until the exchange of about 0.15 mole of electron per mole of sulphide. The current decrease is then very slow. When electrolyses are performed on the second step, the current decrease is much nearer to an exponential one (Fig. 2). Bubbles arise from the electrode, which never happened on the first step.

145 TABLE 1 Controlled potential electrolysis of PbS Electrolysis potential/V

Anode

Electrons exchanged per mole of PbS

PbS initial concentration/ mol kg -1

First wave

0.720 0.720 0.800 0.800 0.930 0.688 0.705

Graphite Graphite Graphite Graphite Graphite Glassy carbon crucible

rod rod rod rod rod

1.87 1.89 1.98 1.98 1.61 1.81 1.82

9.7 9.7 9.7 1.45 9.7 5.09 4.85

X 10 -2 × 10 -2 × 10 -2 × 10 -1 × 10 -2 × 10 -2 × 10 -2

Second wave

1.17 1.26 1.035 1.108 1.150

Graphite rod Graphite rod Glassy carbon crucible

1.67 1.83 1.90 1.62 1.77

1.45 9.7 5.20 5.35 4.94

× 10 -1 × 10 -2 X 1 0 -2

X 10 -2 × 10 -2

(II) Electrolyses coupled with spectrophotometry F i g u r e 3 d i s p l a y s t h e v i s i b l e a b s o r p t i o n s p e c t r a o f PbC12 5 2 + KC1 4 8 ( m o l %) ( c u r v e 1) a n d P b S d i s s o l v e d i n t h e m e l t . T h e s p e c t r u m o f PbC12 + KC1 is i n fair a g r e e m e n t w i t h t h a t a l r e a d y p u b l i s h e d b y S u n d h e i m a n d G r e e n b e r g [ 15] f o r t h e e q u i m o l a r m i x t u r e a t 5 7 5 ° C. T h e s p e c t r u m o f P b S a p p e a r s as a b r o a d edge,

1 c= ~ 0.5

i0.! mo= 50

100

150 time

200

/ rain

250

300

20

40 time

/ rain

60

80

Fig. 1. Typical current decrease during electrolysis on the first oxidation step of PbS, at E --+0.720 V vs. Pb/Pb(II). Graphite electrode area about 2.2 cm 2. Fig. 2. Typical current decrease during electrolysis on the second oxidation step of PbS, at E = +1.150 V vs. Pb/Pb(II). Anode, glassy carbon crucible (area about 30 cm2).

146

"o

.-%.s

~0.5

0 400

500 wavelength / nm

600

0

0'.1

sulphide

0~2

0~3

concentration /mol

110:4 -

Fig. 3. Optical d e n s i t y vs. PbS c o n c e n t r a t i o n in PbC12 77 + KC1 23 (mol. %) light pathlength: 0.3 cm. PbS c o n c e n t r a t i o n ( m o l - 1 ) : curve 1, 0; curve 2, 0 . 0 3 4 ; curve 3, 0 . 0 7 4 5 ; c u r v e 4, 0 . 1 4 4 ; curve 5, 0 . 2 8 0 ; curve 6, 0.454. Fig. 4. A b s o r b a n c y at ~. = 5 0 0 n m vs PbS c o n c e n t r a t i o n ; p a t h - l e n g t h , 0.3 cm.

without any m a x i m u m in the visible wavelength range. The UV maximum is hidden by the absorbancy of the solvent. The optical density is linear vs. lead sulphide concentration (Fig. 4), even at relatively high concentrations of sulphide ion. We have followed during electrolyses the decrease of the optical density at a fixed wavelength. On the first wave, the optical density is almost constant at the beginning of electrolyses (Fig. 5), showing the existence of at least one intermediate species which absorbs in the same wavelength range as the sulphide ion. After the

~O.S

¢~ 0.5 1

5 o

moles electrons per mole of PbS

0 ~ moles electrons per mole

of

PbS

2

Fig. 5. T y p i c a l decrease o f t h e optical density at k = 5 0 0 n m d u r i n g electrolysis o n the first wave, in PbC12 52 + KC1 48 (tool. %); p a t h - l e n g t h , 0.45 c m ; PbS initial c o n c e n t r a t i o n , 0.157 tool 1-1 . Fig. 6. T y p i c a l decrease o f t h e optical density at k = 510 n m d u r i n g electrolysis o n the s e c o n d wave, in PbC12 77 + KC1 23 (tool. %); p a t h - l e n g t h , 0.3 c m ; PbS initial c o n c e n t r a t i o n , 0 . 4 2 1 m o l 1-1 .

147

exchange of 1.3 mole of electrons, the optical density decreases linearly until the complete disappearance of the intermediate species which can be oxidized at the same potential. The junction between the two parts of the curve occurs between 0.9 and 1.3 mole of electrons. On the second wave, the decrease of the optical density is always linear since the beginning of electrolyses (Fig. 6). DISCUSSION

A first view on electrolyses results yields to an apparent contradiction with experiments performed at the time scale of chronoamperometry.

(a) First oxidation step At the time scale of chronoamperometry, the first step is a rapid two-electron transfer limited by the diffusion of sulphide ion [13]. The reaction at the electrode can be written: S -2 ~- S

+

2e-

At the time scale of electrolyses, we know now that the reaction is complicated by the production of an intermediate polysulphide, as proposed in PbC12 + NaC1 [ 11]. The apparent contradiction between short and long time results rules out assuming that the polysulphide formation is a slow reaction outside the diffusion layer. The nature of the polysulphide cannot be known either by visible spectrophotometry (the polysulphide absorbs in the same wavelength range as the sulphide ion), or by electrochemistry (no reduction wave has ever been noticed within the electroactivity range). General knowledge on polysulphides yields information on the degrees of polysulfuration that we can expect. It is well known that low degree polysulphides absorb in the visible range [16]. In LiC1 + KC1 eutectic, the presence of polysulphides is attested by blue colour of "dissolved sulphur" [17,18] and by electrochemical measurements [19--22]; S~ and S~ in equilibrium with their dimers have been proposed to be at the origin of the blue colour [17,18]. By comparison, we can deduce that low degree polysulphides, absorbing only in the UV range, are present in PbC12 + KC1. So a possible mechanism for the whole reaction can be: S:- -~ S + 2e2 - with S + nS 2--~ Sn+l

n = 2 or 3

The polysulphide can be oxidized at the same potential as the sulphide.

(b) Second oxidation step At short times, the second step is a complex one: a primary exchange of more than two electrons per mole of sulphide leads to a cationic species of sulphur. However, at the time scale of electrolyses, it is a simple one, without intermediate formation of polysulphides, according to: S(--II) -+ S (s)

148

ACKNOWLEDGEMENT

This work has been supported in part by the C.N.R.S. (LA-28: Chimie Analytique G~n~rale, and ATP: Epargne d'~nergie dans les operations chimiques industrielles). REFERENCES

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