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Electrochemical study of pyrrhotite 1971

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AN ELECTROCHEMICAL STUDY OF PYRRHOTITE BY KYOSUKE JIBIKI B.Eng. Hokkaido U n i v e r s i t y , 1966. M. Eng. Hokkaido University, 1968. A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of METALLURGY We accept t h i s thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA February, 1971 «4 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced deg ree at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t he L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r ee t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t he Head o f my Depar tment o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l no t be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Depar tment o f Metallurgy The U n i v e r s i t y o f B r i t i s h C o l u m b i a V a n c o u v e r 8 , Canada Date A p r i l 28, 1971 - i i - ABSTRACT The rest p o t e n t i a l of sulphide electrodes was examined from both thermodynamic and k i n e t i c aspects. The k i n e t i c aspect has been found to be necessary for the i n t e r p r e t a t i o n of the establishment of the mixed p o t e n t i a l i n polyelectrode system, to which most sulphide systems belong. The p y r r h o t i t e electrode system was studied by measuring the rest p o t e n t i a l while changing the concentrations of ferrous ion, hydrogen ion and hydrogen sulphide i n the e l e c t r o l y t e , and the composition of py r r h o t i t e . A mixed p o t e n t i a l of p y r r h o t i t e consisting of the reaction S (in pyrrhotite) + 2H+ + 2e —*- H^S as a cathodic process and the reaction [ [ Fe (in pyrrhotite) —*• Fe + 2e as an anodic process accounts for the dependence of the rest p o t e n t i a l on those i o n i c species i n the e l e c t r o l y t e and the composition of py r r h o t i t e electrodes. - i i i - ACKNOWLEDGEMENT The author wishes to express his gratitude to Dr. E. Peters f o r his continuing guidance and i n t e r e s t i n t h i s project. His thanks i s extended to fellow graduate students and the t e c h n i c a l s t a f f of the Department of Metallurgy for t h e i r h e l p f u l discussions and assistance. F i n a n c i a l support from the National Research Council of Canada i n the form of a Research Assistantship i s g r a t e f u l l y acknowledged. - i v - TABLE OF CONTENTS Page I. INTRODUCTION 1 I I . SIGNIFICANCE OF THE PRESENT WORK 3 I I I . A REVIEW OF THE LITERATURE 4 (1) Description of Ferrous Sulphide 4 1) Fe-S phase diagram 4 2) Thermodynamic diagrams g 3) pH-potential diagrams f o r i r o n sulphides ... -̂0 3-1) Equilibrium pH-potential diagram 10 3-2) Meta-stable pH-potential diagram 10 (2) Electrochemical Study of P y r r h o t i t e 13 (3) Leaching of Pyr r h o t i t e 14 IV. THE METAL SULPHIDE ELECTRODE 19 (1) Thermodynamic Aspect 19 (2) K i n e t i c Aspect 2 1 V. EXPERIMENTAL 2 8 Cl) Materials 2 8 (2) X-Ray Analysis of Pyrrhotites 33 (3) Sulphide Electrode 33 (4) E l e c t r o l y t i c C e l l 37 (5) Reagents 37 (6) Experimental Procedure 37 VI. RESULTS AND DISCUSSION 41 Cl) E f f e c t of Ferrous Ion Concentration 41 C2) E f f e c t of pH 43 - v - Page (3) E f f e c t of Hydrogen Sulphide 43 (4) E f f e c t of Non-Stoichiometry of P y r r h o t i t e ...... 47 (5) E f f e c t of Residual Impurity i n the E l e c t r o l y t e . 52 (6) Interpretation of the Measured Rest P o t e n t i a l .. 54 (7) Galvanic and P o l a r i z a t i o n E f f e c t on the Hydrogen Sulphide Evolution 62 (8) Electrochemical Mechanism of Leaching Reactions. 64 VII. CONCLUSIONS 71 VIII. SUGGESTIONS FOR FUTURE WORK 73 APPENDIX 74 A. Measurement of the Equilibrium Pressure of H^S on Py r r h o t i t e 74 (1) Introduction 74 (2) Experimental 76 (3) Results and Discussion 78 B. Table VI. Dependence of the rest p o t e n t i a l on ferrous i on concentration at pH = 2.8, 25°C 82 C. Table VII. Dependence of the rest p o t e n t i a l on PH at [Fe++] = 0.01 M, 25°C 83 D. Table VIII. V a r i a t i o n i n the re s t p o t e n t i a l with change i n composition of p y r r h o t i t e at 25°C, pH=.3, [Fe"1-1"] = 0.01 M. . .. 84 REFERENCES 85 - v i - LIST OF TABLES Table Page I Measured potentials of the c e l l (-)FeS/FeSO^.x M/KC1/ Calomel R.E. (+) at 18°C 13 II. Comparison of the rest p o t e n t i a l s measured with the mounted electrode and the powder electrode 35 I I I . Comparison of the rest p o t e n t i a l s measured with and without reduction of e l e c t r o l y t e 52 IV. Equilibrium constants of M 2^S + 2H + = H 2S(aq) + 2/nM n + for various sulphides at 25°C 59 V. Values of P u _, [Fe"*"1"], pH and K" 78 r i ^ b VI. Dependence of the rest p o t e n t i a l on ferrous ion con- centration at pH = 2.8, 25°C 82 VII. Dependence of the rest p o t e n t i a l on pH at [Fe*"1"] = 0.01 M, 25°C . 83 VIII. V a r i a t i o n i n the rest p o t e n t i a l with change i n comp- I [ o s i t i o n of pyr r h o t i t e at 25°C, pH * 3, [Fe ] = - 0.01 M 84 - v i i - LIST OF FIGURES Figure Page 1 Tentative diagram of Fe-S system at low temperatures. 5 2 D e t a i l s of the Fe-S diagram i n the v i c i n i t y of t r o i l i t e and p y r r h o t i t e 6 3 Schematic diagram of a c t i v i t y of sulphur i n the Fe-S system 9 4 Potential-pH diagram for stable Fe-S systems at 25°C. n 5 Potential-pH diagram for metastable Fe-S systems at 25°C under conditions where p y r i t e i s not formed. 12 6 Schematic diagram for current-density p o t e n t i a l r e l a t i o n s h i p of sulphide electrode i n acid region... 25 7 Total vapour pressure of sulphur between 120 and 450°C 31 8 V a r i a t i o n i n Fe content with d i f f e r e n t sulphur bath temperatures 32 9 X-ray d i f f r a c t i o n patterns of various i r o n sulphides using Co-K r a d i a t i o n 34 a 10 Iron sulphide electrodes (a) mounted; (b) powder 36 11 Sketch of e l e c t r o l y t i c c e l l 38 12 V a r i a t i o n i n the rest p o t e n t i a l with time 40 13 Dependence of the rest p o t e n t i a l on ferrous ion concentration 42 14a Dependence of the rest p o t e n t i a l on pH (a); pH-dependence of the r e v e r s i b l e p o t e n t i a l for [S] + 2H + + 2e ^ H 2S calculated from the Nernst equation 44 14b Dependence of the rest p o t e n t i a l on pH 45 14c Dependence of the rest p o t e n t i a l on pH 46 15a Rest p o t e n t i a l changes i n d i f f e r e n t atmosphere 48 15b Rest p o t e n t i a l changes i n d i f f e r e n t atmosphere 49 16 V a r i a t i o n i n the rest p o t e n t i a l with change i n composition of pyr r h o t i t e 51 - v i i i - Figure Page 17 Sketch of the c e l l for reduction of the e l e c t r o l y t e 53 18 Experimental v a r i a t i o n i n the rest p o t e n t i a l of galena for low (Pb ) at pH = 0 55 19 Current-density p o t e n t i a l r e l a t i o n s h i p s for the c e l l (a) pyrrhotite|X-FeS0 4, y-H 2S0 4|S.H.E. (25°C) (b) p y r r h o t i t e s | H 2 S 0 4 , He or H2S|S.H.E. (25°C) 57 20 Current-density p o t e n t i a l r e l a t i o n s h i p s for the c e l l d i f f e r e n t pyrrhotites|FeS0 4, H 2S0 4|S.H.E. (25°C) ... 63 21 V a r i a t i o n i n H2S evolution rate with a galvanic contact and anodization of p y r r h o t i t e 65 22 V a r i a t i o n i n H 2S evolution rate with change i n p o t e n t i a l of p y r r h o t i t e electrode 66 23 I l l u s t r a t i o n of the form of sulphur during o x i d i z i n g leaching of sulphide minerals 70 24 Schematic i l l u s t r a t i o n of the equipment for H 2S • pressure measurement 77 25 Increase i n H 2S pressure with time 79 26 Dependence of K" on pH 81 / I. INTRODUCTION The electrochemical properties of sulphide minerals i n aqueous so l u t i o n , which were f i r s t examined i n the l a s t century, have been studied mainly by geologists who determined electrode p o t e n t i a l s of many sulphide minerals. From a p r a c t i c a l point of view these works succeeded i n arranging sulphides i n a serie s of p o t e n t i a l s analogous to the electrochemical series of metals. However, the po t e n t i a l s of sulphides measured were poorly reproducible and inconsistent with the values calculated from thermochemical data, and i n f a c t , the meaning of electrode p o t e n t i a l s of sulphides i s not p r e c i s e l y understood today. Meanwhile, these electrochemical properties of sulphides have been u t i l i z e d i n the study of various hydrometallurgical processes, e.g. e l e c t r o l y s i s , leaching and f l o t a t i o n . Anodic e l e c t r o l y s i s of sulphides y i e l d s metal ions and elementary sulphur or sulphate ions as products. The e l e c t r o l y s i s of n i c k e l matte i s already commercialized, and known as the Hybinette process. Galvanic action may occur between p a r t i c l e s of d i f f e r e n t sulphides i n a s l u r r y , analogous to galvanic corrosion between d i f f e r e n t metals; t h i s was f i r s t noticed during geologic studies of mineral deposits. Some attempts were made to int e r p r e t leaching reactions of sulphides as electrochemical processes, s i m i l a r to the corrosion process - 2 - of metals which i s now reasonably well understood. The a p p l i c a t i o n of pH-potential diagrams to the sulphide systems by various workers y i e l d s much information of a thermodynamic kind. However, a l l of the works undertaken to date f a i l e d to show the experimental v a l i d i t y of the pH-potential diagram. One of the p o s s i b i l i t i e s f o r t h i s i s that k i n e t i c considerations were l a r g e l y ignored. A f u l l understanding of the electrochemical k i n e t i c behaviour of sulphides i s necessary before the properties of systems studied over a short time i n t e r v a l (shorter than geol o g i c a l time ) can be completely understood for extractive metallurgy purposes. Thus some processes that come to equilibrium over a period of years, may be saf e l y ignored i n determining a useful diagram for extraction purposes, but then the diagram i s one which may contain metastable phases, thermodynamically speaking, which do not react appreciably i n allowed periods of time. I I . SIGNIFICANCE OF THE PRESENT WORK The present work was undertaken to o b t a i n the systematic measurement of the r e s t p o t e n t i a l s of sulphides which were i n t e r p r e t e d i n terms of e l e c t r o c h e m i c a l k i n e t i c s r a t h e r than f i n a l thermodynamic e q u i l i b r i a . In a d d i t i o n to these measurements, an attempt was made to o b t a i n thermodynamic data of sulphides which were necessary f o r the more q u a n t i t a t i v e i n t e r p r e t a t i o n of e l e c t r o c h e m i c a l data. P y r r h o t i t e , Fe^_^S (a << 1), was chosen as the sulphide i n which the present work was undertaken. Although p y r r h o t i t e i s not as important as p y r i t e f o r m e t a l l u r g i c a l purposes because of i t s r a r e r occurrence i n n a t u r a l ores, i t i s o f t e n accompanied by n i c k e l sulphide ores and the e l e c t r o c h e m i c a l behaviour seems to be c l o s e l y r e l a t e d to that of p y r i t e . Only a few works have been reported on e l e c t r o c h e m i c a l s t u d i e s of p y r r h o t i t e . P y r r h o t i t e appears i n a n o n - s t o i c h i o m e t r i c compound w i t h a wide range of Fe:S r a t i o forming an i r o n d e f i c i e n t l a t t i c e , t h i s non-stoichiometry of p y r r h o t i t e may be expected to have an e f f e c t on the e l e c t r o c h e m i c a l behaviour of the m i n e r a l s . I I I . A REVIEW OF THE LITERATURE (1) Description of Py r r h o t i t e 1) The Fe-S phase diagram Although the Fe-S binary system has been studied i n f a i r d e t a i l from liquidus temperatures down to about 300°C, the low temperature phase r e l a t i o n s h i p s are le s s well understood because of d i f f i c u l t y with sluggish reaction rates. Nevertheless, i n F i g . 1 the phase re l a t i o n s h i p s of the system at low temperatures are shown based on phases observed i n nature and on a l i m i t e d number of laboratory studies."'" According to F i g . 1 i t can be seen that at low temperature the phase r e l a t i o n s h i p s are more complicated than those at high temperature. On cooling to 320 + 5°C, the high temperature hexagonal p y r r h o t i t e passes through an inversion to low temperature hexagonal p y r r h o t i t e . The temperature of t h i s inversion does not seem to be influenced by the composition of the p y r r h o t i t e . Further cooling near the FeS composition leads to a second inve r s i o n at 139°C and the t r o i l i t e phase becomes stable. I t i s noted i n F i g . 1 that the t r o i l i t e s t a b i l i t y i s very r e s t r i c t e d both i n regard to composition, which cannot deviate from the stoichiometric FeS and i n regard to temperature. In f a c t , a recent work done by R. Yund 2 and H. H a l l showed that t r o i l i t e appears to be r e s t r i c t e d to the - 5 - 600H Hex.= Hexagonal Mono. = Monoclinic H.T. » High temperature L.T. ? Low » Po.s Pyrrhotite Py,= Pyrite 500. 400. O o az 3 < or iii CL U J 300 200. 100 TroUite Hex. H.T. Po. Hex. L.T. Po. Tr. Hex. Po oIL , 50 Hex. H.T. R>. + Py. Hex. L.T. Po. + Py. 310 rMono. Po. -Hex. L.T. Po. + Mono. Po Mono. Po. + Py. Mono. Po. +• Fe3S4 Fe3S4 + Py. Fe^Q ' Fe3S4 40 ATOMIC PERCENT Fe py. + liquid Sulphur Py. + Mono. S 114.5 102 Py. + ortho- rhombic S FeS2 30 Figure 1. Tentative diagram of Fe-S system at low temperatures. - 6 - 140. 1204 O I O O J o it! ? 80 £ 6 0j 4 0 l CCD Troilite + I ron Troilite + I H e x . L T. CL| Pyrrhotite Troilite He x. L .T Pyrrhotite 50 49 48 47 ATOMIC PERCENT Fe F i g u r e 2. D e t a i l s o f t h e Fe-S d i a g r a m i n t h e v i c i n i t y o f t r o i l i t e and p y r r h o t i t e . stoichiometric FeS, shown i n F i g . 2. The univariant f i e l d e x i s t i n g below 139°C between t r o i l i t e and the low temperature hexagonal p y r r h o t i t e increases s i g n i f i c a n t l y i n width with decreasing temperature. These two phases commonly co-exist i n many ores. a A monoclinic p y r r h o t i t e was f i r s t found i n a number of Swedish ores. I t has gradually become apparent that t h i s mineral i s quite common i n ore deposits. Monoclinic p y r r h o t i t e has been synthesized i n the pure Fe-S system, being stable below 310°C i n the presence of sulphur vapour. In Fi g . 1 i t i s t e n t a t i v e l y shown as a stable phase below 310°C. The compositions of numerous monoclinic p y r r h o t i t e s have been found to vary only s l i g h t l y ; the range i s 46.45 to 46.70 atomic percent Fe. Monoclinic p y r r h o t i t e and low-temperature hexagonal p y r r h o t i t e form a common assemblage i n natural ores as evidenced by X-ray powder d i f f r a c t i o n studies. Monoclinic pyrrhotite-marcasite assemblages are found to be quite common i n ores. A phase with Fe^S^ composition and rhombohedral structure was f i r s t reported and named smythite i n 1957. However, i t s s t a b i l i t y region i s not confirmed yet, so i n F i g . 1 a breakdown of the Fe^S^ compound i s indicated t e n t a t i v e l y at about 100°C. In natural ores p y r i t e as well as marcasite i s very common as the ir o n disulphide phase. However, the pyrite-marcasite r e l a t i o n has been a puzzle f o r many years. In composition there i s a difference between the two minerals, indicated by numerous experiments, i . e . the orthorhombic marcasite contains le s s sulphur than the cubic p y r i t e which i s e s s e n t i a l l y stoichiometric FeS.. This account for the fac t - 8 - that, when marcasite i s heated with elemental sulphur under confining pressure i t converts to p y r i t e i n a matter of days, at temperatures even as low as 150°C, forming a p y r i t e rim around i n d i v i d u a l marcasite grains. Several other studies on these minerals i n d i c a t e that hydrogen apparently plays an important r o l e i n the formation of marcasite because both marcasite and p y r i t e form i n Fe-S-O-H experiments but p y r i t e only forms i n Fe-S-0 experiments. Although more extensive studies need to be done, i t now appears that the H-S bond may s t a b i l i z e the marcasite structure. For these reasons marcasite i s not shown as a phase i n the pure Fe-S system. 2) Thermodynamic diagrams Thermodynamic considerations y i e l d information on the stable phases i n the Fe-S system to appear i n selected environments. Because sulphur as one of components of the Fe-S system i s a very active element, i t s a c t i v i t y i n the environment determines the phase to be s t a b i l i z e d . To date, although many studies have been made on the thermodyanmic properties of the Fe-S system at high temperatures, studies at low temperatures are not a v a i l a b l e . However, the data at high temperature can be extrapolated to approximate the thermodynamic properties of sulphides at low temperature. The sulphur a c t i v i t i e s can be calculated as a function of composition across the composition ranges, shown i n the phase diagram of F i g . 1, by * using the e x i s t i n g thermodynamic data. In F i g , 3 the sulphur a c t i v i t i e s In t h i s work, unless otherwise state, a l l thermodynamic data were obtained from "Oxidation P o t e n t i a l s " by Latimer. - 9 - 400 °C FeS RS2 -20-L r- r — I - J 50 55 60 65 ATOMIC PERCENT of S Figure 3. Schematic diagram of a c t i v i t y of sulphur i n the Fe-S system. - 10 - were calculated at 400°C, and are shown schematically f o r lower temperature regions corresponding to d i f f e r e n t phase r e l a t i o n s h i p s . 3) pH-Potential diagrams f o r i r o n sulphide The pH-potential diagram shows a stable region of each phase i n the Fe-S system i n aqueous environments from a thermodynamic point of view. 3-1) Equilibrium pH-potential diagram Fi g . 4 shows the pH-potential diagram i n acid regions made by 4 -3 H. Majima. The ferrous ion concentration i s 1 M and 10 M to make the diagram more applicable to p r a c t i c a l considerations, concentrations of other solutes are 1 M. In t h i s f i g u r e , the p y r r h o t i t e domain i s a small region, compared with p y r i t e . 3-2) Meta-stable pH-potential diagram In p r a c t i c e the py r r h o t i t e phase p e r s i s t s at pot e n t i a l s and pH's where p y r i t e i s stable because of the extremely slow formation of p y r i t e from p y r r h o t i t e . Sulphur that i s l e f t on p y r r h o t i t e during oxidation does not react i n laboratory times with unreacted p y r r h o t i t e to form p y r i t e . In F i g . 5 with t h i s consideration the meta-stable pH-potential 4 diagram for p y r r h o t i t e i n acid regions i s shown by H. Majima. This diagram i s applicable only to iron-saturated p y r r h o t i t e , which i s stoichiometric p y r r h o t i t e , because the free enthalpy value used i s of pyrr h o t i t e saturated with i r o n . For non-stoichiometric p y r r h o t i t e , i f thermodynamic data are a v a i l a b l e , the'same metal-stable diagrams can be drawn. The domain for p y r r h o t i t e of composition Fe^ g^S i s drawn to indicate the change i n s t a b i l i t y due to compositional changes - 11 - LU X CO to "o > UJ r-o CL 0 . 8 H r: 0.447 -0.4H Fe + H2S i -2 0 2 PH 4 I M (Fe2+) „ „ _ ,0-3M(Fe2 +) Figure 4. Potential-pH diagram f o r stable Fe-S systems at 25°C. - 12 - 1 r 1 1 -2 0 2 4 P H I M (Fe2"1") IO"3M(Fe2 +) Figure 5. Potential-pH diagram for metastable Fe-S systems at 25°C under conditions where p y r i t e i s not formed. - 13 - of t h i s non-stoichiometric compound. The composition Fe. Q^S i s near that which corresponds to equilibrium with p y r i t e , i . e . , monoclinic p y r r h o t i t e , Fe^S 0. / o (2) Electrochemical Study of Pyr r h o t i t e An early electrochemical study i n p y r r h o t i t e was made by K.E. Wrabetz,"* as a part of extensive contributions to electrochemical studies of sulphides by both himself and his co-workers. In t h i s study the synthesized p y r r h o t i t e was used to investigate the e f f e c t of ferrous ion concentration on the electrode p o t e n t i a l . The data are shown- i n Table I. Table I. Measured p o t e n t i a l of the c e l l (-)FeS/FeS0. 4 •XM/KCl/Calomel R.E at 18°C. [Fe"1"1"] M E (mV) vs S.H.E. [Fe"1"1"] M E (mV) vs S.H.E. 0.358 396 0.0075 394 0.138 387 0.0020 402 0.042 394 0.00046 399 0.020 406 <average> 397 As seen i n Table I, i t was concluded that the p o t e n t i a l did not depend on ferrous ion concentration i n range of 0.0004 ^ 0.3 M. Further, using the c e l l (-)FeS/FeS0 4, 0.1 M. H 2S0 4, 0.1 M/KCl/Calomel R.E. (+) the po t e n t i a l measured showed -0.40 ̂  -0.45 V(S.H.E.) for the synthesized - 14 - py r r h o t i t e and +0.51 V(S.H.E.) for the natural p y r r h o t i t e . There was a large d i f f e r e n c e i n p o t e n t i a l between those p y r r h o t i t e s . This diff e r e n c e i n p o t e n t i a l was not interpreted i n his work. Then, M. Sato published a systematic work i n several sulphides that was undertaken to measure the rest p o t e n t i a l of sulphides i n changing the pH, and the concentrations of corresponding metal ions and sulphide ions i n the e l e c t r o l y t e . Unfortunately the p o t e n t i a l measurement of p y r r h o t i t e i n the acid region f a i l e d because of i t s poor r e p r o d u c i b i l i t y caused by the formation of hydrogen sulphide and ferrous ions through the action of acids. Nevertheless, the data i n basic regions showed that the p o t e n t i a l f o r the natural p y r r h o t i t e was about 600 mV higher than that f o r the synthesized p y r r h o t i t e . Recently S. Venkatachalam and R. Mallikarjunan'' showed the independence of the p o t e n t i a l of the p r e c i p i t a t e d ferrous sulphide on ferrous ion concentration i n the range of 0.001 ^ 0.5 M i n ferrous ammonium sulphate s o l u t i o n . For the general electrochemical behaviour of p y r r h o t i t e i n acid regions i t can be described that the rest p o t e n t i a l i s not influenced by ferrous ion concentration i n the e l e c t r o l y t e and the rest p o t e n t i a l of natural p y r r h o t i t e i s more noble than that of synthesized p y r r h o t i t e . (3) Leaching of Py r r h o t i t e It i s well known that p y r r h o t i t e e a s i l y dissolves into acid s o l u t i o n forming ferrous ions and hydrogen sulphide as reaction products according to the following equation, - 15 - FeS + 2H + --—* Fe"1"4" + H 2S (+) ( I I I - l ) This occurs i n Kipp's generators to produce hydrogen sulphide i n standard chemical laboratories. g H.A. Pohl proposed the following mechanism of hydrogen sulphide evolution from the p r e c i p i t a t e d ferrous sulphide i n a c i d , FeS + H + • Fe4"*" + HS (III-2) HS + H + • H 2S (+) (III-3) accounting for the fact that FeS, CdS and ZnS dissolve i n k i n e t i c a l l y f i r s t order reactions with respect to the concentration of hydrogen ion, which suggests the step of (III-2) as a rate-determining reaction. In industry the hydrogen sulphide formed from p y r r h o t i t e can be of i n t e r e s t to produce elemental sulphur as a commercially valuable product by the oxidation process, i . e . H 2S + l / 2 0 2 y H 20 + S° (III-4) When py r r h o t i t e i s d i r e c t l y oxidized i n aqueous media by oxygen, the following stoichiometry of reaction i s established, 4FeS + 30 2 > 2 F e 2 ° 3 + 4 ? (IH-5) The mechanism of the oxidation process of p y r r h o t i t e i s not f u l l y - 16 - understood yet. 9 K.W. Downes and R.W. Bruce c a r r i e d out the oxidation of py r r h o t i t e at 110-125°C under high oxygen pressure. In autoclave experiments no elemental sulphur was observed except when the pH of the s o l u t i o n reached about 1.5. The evolution of hydrogen sulphide, when py r r h o t i t e i s added to autoclave l i q u o r at room temperature, has been noticed. These facts lead to the postulation of the following mechanism of reaction; water and pyr r h o t i t e i n the autoclave react f i r s t forming ferrous sulphate, FeS + 20„ y FeSO. (III-6) 2 4 This sulphate i s oxidized to f e r r i c sulphate, 6FeS0 4 + 1 l / 2 0 2 > lle^SO^^ + ^2°3 (III-7) This f e r r i c sulphate being unstable i n neutral water hydrolyses to f e r r i c oxide and sulphuric a c i d , F e 2 ( S 0 4 ) 3 + 3H20 • ?e2°3 + 3 H 2 S ° 4 CIH-8) The sulphuric acid then dissolves p y r r h o t i t e to form H^S and ferrous sulphate, FeS •+ H oS0. v FeSO. + H„S CUI - 9 ) 2 4 4 2 As a following step, H^S is oxidized by f e r r i c sulphate or oxygen to form elemental sulphur, H 2S + F e 2 ( S 0 4 ) 3 • 2FeS0 4 + ^SO^ + S° (111-10) H 2S + l / 2 0 2 y H 20 + S° (III-4) J. Gerlach, H. Hahne and F. Pawlek"^ studied the k i n e t i c s of the oxygen pressure leaching of p y r r h o t i t e . Sulphur, hydrogen sulphide and sulphate were detected as reaction products of sulphur during leaching, then, as a mechanism of reaction the following steps were proposed. FeS + 2H + H 2S + Fe"^1" ( I I I - l ) H 2S + l / 2 0 2 • S° + H 20 (III-4) H 2S + 20 2 • S0 4 + 2H + ( I I I - l l ) H 2S + 2Fe y S + 2Fe + 2H (111-12) H2S + 8 F e + + + + 4H 20 y S0 4 + ZYtt* + 10H + (111-13) Also, elemental sulphur reacts with oxygen to form sulphate ion, S° + 3/202 + H 20 y S0 4 + 2H + (111-14) - 18 - The oxidation of ferrous to f e r r i c ion by oxygen occurs r e l a t i v e l y slowly i n sulphuric acid media, so the reactions (111-12) and (111-13) seem les s s i g n i f i c a n t , but the reaction ( I I I - l ) i s predominant because most of the sulphur was found as elemental sulphur (more than 70%). IV. THE METAL SULPHIDE ELECTRODE (1) Thermodynamic Aspect A metal sulphide electrode consists of two components and therefore i t s equilibrium p o t e n t i a l can be described i n terms of eit h e r of i t s components, i . e . according to the Nernst equation f o r equilibrium between metal i n the sulphide and metal cation i n the e l e c t r o l y t e , M^aq) + ne ^—»• M° ( i n sulphide) v v° ^ 2 . 3 R T V**" ,_„ E = E + — - — log (IV-1) M M nF B a M O and f o r equilibrium between sulphur i n the sulphide and hydrogen sulphide, S° ( i n sulphide) + 2H + + 2e H 2S (aq) 2 v T 7 O , 2.3RT a H+ * a S ° ES - ES + "IF— l o g I^T ( I V " 2 ) where E° and E° are the standard electrode p o t e n t i a l s . M S When equilibrium i s reached between the electrode and the e l e c t r o l y t e , i . e . the sulphide i s i n a t o t a l s o l u b i l i t y equilibrium with the e l e c t r o l y t e , the value of the p o t e n t i a l i s the same i n both cases, because the electrode can exert only one p o t e n t i a l . Therefore, according to equations (IV-1) and (IV-2), - 20 - 2 * w ° 4 . 2-3RT , V * F o . 2.3RT _ V ' a S ° E = E M + "nT" l o g 7 ~ = ES + ~~2F l o g ( I V _ 3 ) N L ? A M O b Z i aH 2S i s obtained. A basic thermodynamic property of the metal sulphide, ^2/n^l+ ( a << 1), i s that the free enthalpy of formation of the sulphide phase defines the r e l a t i o n s h i p between metal and sulphur a c t i v i t i e s , i . e . 2/nM° + (1 + <x)S° ^ M 2 / nS 1 - + a (IV-4) A F ( 4 ) = -2.3RT l o g [ a M s 7 ^ - - g o 2/n l+a - 2.3RT log a ^ n . a ^ (IV-5) a,. W h e r e X/ s 1 + = 1 2/n l + a For a s o l u b i l i t y equilibrium M 0 / S-, + 2H + >- 2/nM n + + H_S + a S ° (IV-6) z/n l+a •« z . 2/n a 2/n a V+ ' S ' as° V+ ' *H S " as° K ( 6 ) - 2 — - 2 ( I V " 7 ) \ , S 1 + * V - a H + 2/n l+a i s obtained, where i s the equilibrium constant for (IV-6) and a^ = 1 as mentioned before. According to (IV-2) and (IV-7), the M2/n bl+a following equation can be obtained: I? w ° _L 2.3RT 1 r 2/n l+a ,„ , , T T 7 O N ES = ES + I f - l o g [aMn+ * V / K ( 6 ) J ( I V " 8 ) - 21 - From t h i s equation and (IV-5) v y° 2.3RT 2.3RT 2/n , 1 , _o 0 . 2/n Es = ES " ~^2F~ 1 0 8 K ( 6 ) + - 2 F ~ l o g V + + 2F ( F ( 4 ) _ 2 ' 3 R T l o g V > „o 2.3RT . „ " ( 4 ) , 2.3RT . 2/n 2.3RT 1 2/n = ES ' ~2F l o g K ( 6 ) + —W + ~2T- l o g *U*+ ~ ~2F l o g V AF° _,o 2.3RT (4) , 2.3RT . , . . , T T 7 n N = ES " ~ 2 F — l o g K ( 6 ) + " 2 F — + " n F - L O 8 ( V + / a M o ) ( I V _ 9 ) i s obtained. In (IV-9) AF° _o 2.3RT . _ . a (4) n ES " -JT- l 0 g K ( 6 ) + ~~2F - M therefore, E S = ; E M + — l 0 g i ^ This equation shows the v a l i d i t y of (IV-3). (2) K i n e t i c Aspect The r e v e r s i b l e p o t e n t i a l of sulphide electrodes which can be calculated from thermochemical data using the Nernst equation does not always agree with that obtained i n measurements. This arises because most sulphide electrode systems belong to a polyelectrode system where a k i n e t i c consideration i s necessary to i n t e r p r e t the p o t e n t i a l of the sulphide electrode. In the acid region the possible electrochemical reactions i n t h i s polyelectrode system include; 1. Oxidation of metal i n the sulphide to metal cations M°(in sulphide) • M n + + ne; i al - 22 - 2. Oxidation of sulphur i n the sulphide to sulphate ions, S° ( i n sulphide) + 4 H 2 0 • S0 4 + 8H + + 6e; i a 2 3. Reduction of sulphur i n the sulphide to hydrogen sulphide, S° (in sulphide) + 2H + + 2e *• H 2S; i c 3 4. Reduction of hydrogen ions into hydrogen molecules, 2H + + 2e y H„; i . 2 c4 5. Reduction of corresponding metal ions, which are added to e l e c t r o l y t e , into metal, M n + + 2e • M°; i _ c5 6. Oxidation of hydrogen sulphide which i s dissolved i n the e l e c t r o l y t e , into elemental sulphur, H.S y S° + 2H + + 2e; i , 2 ab These reduction and oxidation, i . e . cathodic and anodic processes can occur simultaneously but s t a t i s t i c a l l y independent of one another. The rate of each reaction, i . e . current density, i or i , can c a be described by the following equations, according to electrochemical k i n e t i c s . - 59 - Table IV. Equilibrium constants of + 2H + = H 2S(aq) + 2/-n^+ for various sulphides at 25°C 2/n . 2 = V + • aH 2S / aH+ Sulphide log K Sulphide log K MnS 8.0 CdS - 6.14 FeS . 2.55 PbS - 7.10 CoS -0.33 CuS -15.0 NiS (y) -6.69 Cu 2S -18.9 ZnS (Spal) -4.12 Ag 2S -15.58 (Wurt) -1.80 HgS -32.3 According to t h i s equation, although g(aq') l s dependent on ap e++ -2 2 i n the case when ape-H- i s 10 or less than i t , the a c t i v i t y of -2 aqueous hydrogen sulphide at equilibrium i s more than 3.55 x 10 which suggests the continuous evolution of hydrogen sulphide into the He gas atmosphere which i s used i n t h i s work. On the other hand, Table IV states that CuS, Cu 2S, Ag 2S and PbS have extremely small values of K. This i s associated with n e g l i g i b l e H 2S evolution and therefore promises the p o s s i b i l i t y of measuring the r e v e r s i b l e p o t e n t i a l of the respective sulphides,and indeed these have been experimentally 1 ,1. • A 6 > 1 6 obtained. When the rest p o t e n t i a l of pyr r h o t i t e i s co n t r o l l e d by the reactions ++ + of [Fe] -y Fe + 2e as an anodic process and [S] + 2H + 2e •+ R^S as a cathodic process,the current density of each reaction can be equated according to Equations (IV-10) and (IV-11); for the anodic process - 23 - for anodic current density, n r r . a_ ... a exp {• 2 P Za.F RT e } (IV-10) for cathodic current density, i =-ZFk a" c c 1 V w • • • • 3. 6 X p -ZBF RT (IV-11) where k k are reaction rate constants f o r anodic and cathodic reactions, r e s p e c t i v e l y : a , a are a c t i v i t i e s of reactants of anodic P q and cathodic reactions, r e s p e c t i v e l y ; m, n, r, u, v, w, are orders of anodic and cathodic reactions with respect to each reactant; Z i s the number of electrons involved i n each reaction; a, 0 are t r a n s f e r c o e f f i c i e n t s for anodic and cathodic reactions, r e s p e c t i v e l y ; F i s " t h e Faraday constant; and e i s the p o t e n t i a l of the sulphide electrode. According to these equations, the rate of each reaction w i l l be governed by the e l e c t r i c a l p o t e n t i a l at the sulphide electrode, rate constant, a c t i v i t i e s of reactants and the transfer c o e f f i c i e n t . I f a steady state i s established and there i s no external disturbance, the sum of cathodic reaction rates w i l l be equal to the sum of anodic reaction rates; Z i = c Z i a (IV-12) Considering the topography of the electrode surface i n the poly- electrode system, i t i s not necessary to have separate macroscopic areas - 24 - which are e x c l u s i v e l y cathodic or anodic on a sulphide electrode, e i t h e r operationally or conceptually. Any one s i t e may be anodic during one instant of time and cathodic during another instant, and anodic and cathodic processes can occur simultaneously on atomically . adjacent s i t e s . This homogeneous surface condition w i l l become e s p e c i a l l y important when we consider current density instead of current i n a quantitative understanding of the electrochemical behaviour of the electrode. Here, i t i s h e l p f u l to use a current-density p o t e n t i a l diagram i n order to understand better the current-density p o t e n t i a l r e l a t i o n s h i p i n the polyelectrode system. In F i g . 6 the possible reactions, l ) - 6 ) are schematically p l o t t e d . The locations of each l i n e depends on the parameters such as k , k , a , a . Also the slope of each l i n e depends r a c p q on the value of a or 0 and Z. The e f f e c t of concentration p o l a r i z a t i o n which w i l l be s i g n i f i c a n t at high current-density i n t h i s diagram i s not accounted for i n order to s i m p l i f y the discussion. In t h i s sulphide polyelectrode system where each possible reaction i s independent and a l t e r n a t i v e , the p o t e n t i a l of the electrode i s l a r g e l y determined by the coupled reactions which have the highest current-density. The highest possible current d e n s i t i e s , i ^ ^ and i ^ a r e shown i n F i g . 6, as the i n t e r s e c t i o n s of cathodic and anodic l i n e s coordinated with the potentials of the electrode E^^ and E ^ - S t r i c t l y speaking, t h i s i n t e r p r e t a t i o n f or the p o t e n t i a l from the i n t e r s e c t i o n of both l i n e s i s not correct, because equation (IV-12) can not be s a t i s f i e d at the i n t e r s e c t i o n . However, i f the other minute current densities at the p o t e n t i a l E m or E n 9 were neglected, equation (IV-12) y i e l d s  - 26 - i = i (IV-13) a c When both of the coupled reactions determining the p o t e n t i a l of the sulphide are i d e n t i c a l , as the example shown i n F i g . 6, the p o t e n t i a l i s c a l l e d the r e v e r s i b l e or equilibrium p o t e n t i a l . In eit h e r of these cases, the r e v e r s i b l e p o t e n t i a l f o r the metal-metal cation equilibrium, from (IV-10), (IV-11), (IV-13) and Z = n m rnaF _ , • , n£F _ , x01 = xa = _ 1 c = n F k a V 6 X P {~RT E 0 1 } = n F k c V + e x p { " "RT E 0 1 } (IV-14) i s obtained. Equation (IV-14) y i e l d s RT „ k c aMn+ J01 (a+B)nF k a * a M O Here, a+g = 1 i n the case when anodic and cathodic processes are i d e n t i c a l and k /k = K which i s the equilibrium constant for M n + + C SL ne -—>- M°. Therefore, -«- RT „ v . RT „ E n i = — An K + — In 01 nF nF a M O „o . 2.3RT . ,T_. . „ = EM + ~~~o~F~ ~^yp~ ( I V _ 1 5 ) This equation i s i d e n t i c a l with the Nernst equation derived i n the section on thermodynamic considerations. In the same manner for the sulphur-hydrogen sulphide equilibrium the r e v e r s i b l e p o t e n t i a l - 27 - 02 = E, + 2.3RT 2F log 3 H+ (IV-16) is derived. V. EXPERIMENTAL (1) Materials Natural and synthesized py r r h o t i t e s were used i n t h i s experiment. Natural minerals were obtained from the S u l l i v a n Mine i n Kimberley, B.C. and the Chichibu Mine i n Japan. Microscopic observations did not show any other phase except p y r r h o t i t e . Natural py r r h o t i t e s are i n a more stable state thermodynamically than synthesized p y r r h o t i t e s . However, they i n v a r i a b l y contain impurity elements such as N i , Co, Cu, As etc. For experimental purposes i t i s very d i f f i c u l t to obtain py r r h o t i t e s having a systematically varying range of composition. The synthesized pyrrhotites are required e s p e c i a l l y to examine the Fe:S composition r a t i o . Three methods were used i n t h i s work to synthesize p y r r h o t i t e s . Method I. Sulphur lump c r y s t a l s (chemical pure) and i r o n wire (99.9%, 0.022 cm diameter) r e s p e c t i v e l y were weighed out to correspond to an appropriate composition of p y r r h o t i t e , then placed together into a 96% s i l i c a glass ("Vycor") tube, 0.8 cm outer-diameter, which was evacuated and sealed. The Vycor tube was placed i n the furnace, heated to 500°C for one day and to 700°C for two days, then furnace cooled to room temperature. In each Vycor tube about one gram pyrrhot i t e was produced. At 700°C the sulphur decomposition pressure e q u i l i b r a t e d - 29 - with p y r r h o t i t e was n e g l i g i b l y small up to the composition of the p y r r h o t i t e of 48 atomic percent Fe, i . e . less than 0.1 atm, so i t was assumed that a l l sulphur put into the Vycor tube reacted with i r o n . However, below about 48 atomic percent Fe the sulphur decomposition pressure of p y r r h o t i t e can not be neglected i n the material balance of sulphur. Therefore t h i s method was not used for preparing p y r r h o t i t e material of le s s than 48 atomic percent Fe. Method I I . The "Dew point method"; the apparatus for t h i s method consisted of a Vycor tube, 1.5 cm outer-diameter and 30 cm length, i n which sheet i r o n (99.99%) and sulphur were placed at each end, then t h i s tube was evacuated, sealed and placed i n furnace system which consists of two separately heated zones. On the i r o n side the temperature was kept constant at 700°C, while on the sulphur side the temperature was adjusted i n the range 110°C to 450°C to e s t a b l i s h a chosen p a r t i a l pressure of sulphur. In t h i s method, by weighing the i r o n samples before and a f t e r each run the i r o n content i n sulphide can be calculated. Thus the determination of Fe content i n the p y r r h o t i t e does not contain any error due to incomplete reaction of sulphur. However, when the i r o n being sulphidized was kept at 700°C, a sulphur temperature between 110°C to 450°C was too high to synthesize the p y r r h o t i t e containing more than 48 atomic percent Fe. Thus methods I and II i n combination permitted the synthesis of pyrrhotites with wide range of composition. In t h i s method each run took 4 days to complete s u l p h i d i z a t i o n of sheet i r o n (0.04 cm thickness) and to homogenize the r e s u l t i n g p y r r h o t i t e . - 30 - The vapour pressure of sulphur i s known from the work of W. West and 13 A; Menzies, shown i n F i g . 7. I t was assumed that by steady state conditions the t o t a l sulphur pressures at both ends i n the Vycor tube were equal but not the p a r t i a l pressures of the d i f f e r e n t molecular species. The vapour density of the gas increased markedly from the hot zone at 700°C, where the gas consisted of mainly molecules, to the colder part held i n the range 110-450°C, where i t consisted of Sg, S^, S^, S,., S^, S^ and molecules. In F i g . 8 the v a r i a t i o n i n Fe content with d i f f e r e n t sulphur bath temperatures i s shown. Pyrrhotite made at 700°C was f a i r l y massive and could be used f o r electrodes i n electrochemical studies. Method I I I . This method can be c a l l e d the "Melt method". Iron powder, 99% pu r i t y , and sulphur powder were mixed i n a weight r a t i o of 1:1; then th i s mixture was gradually heated i n a graphite c r u c i b l e to 700°C at which temperature i t was held f or 5 hours. A f t e r that the temperature was increased to 1250°C, above the melting point (1190°C) of FeS, where molten FeS was kept for a ha l f an hour, then cooled to 750°C, from t h i s temperature the sample was cooled to room temperature over a period of 10 days. A l l processes of heating, melting and cooling were undertaken i n an i n e r t atmosphere of He flow. This p y r r h o t i t e was supposed to be i r o n saturated or le s s excess sulphur p y r r h o t i t e . This technique was e s s e n t i a l for the production of lumps of p y r r h o t i t e . Besides p y r r h o t i t e s , other materials occasionally used were p y r i t e , i r o n powder and cha l c o c i t e . The source of p y r i t e was not known, chalcocite was from Montana, U.S.A. Iron powder used was of 99% p u r i t y . -5-J . 1 • . • : 100 200 300 400 TEMPERATURE °C Figure 7. To t a l vapour pressure of sulphur between 120 and 450 CC. CD 47- ,§) U, Z U J ce U J C L O § 45" o 100 200 300 400 SULPHUR BATH TEMPERATURE °C Figure 8. V a r i a t i o n i n Fe content with d i f f e r e n t sulphur bath temperatures. - 33 - (2) X-Ray Analysis of Pyrrhotites Pyrrhotites used i n t h i s study were examined by X-ray d i f f r a c t i o n to i d e n t i f y the phases present. A Debye-Scherrer camera was used to take the powder d i f f r a c t i o n patterns with a CoK X-ray tube. In F i g . 9 a the X-ray d i f f r a c t i o n patterns are presented. ASTM cards for p y r r h o t i t e , p y r i t e , and marcasite are included on l i n e s 1, 10 and 11, r e s p e c t i v e l y for comparison with r e s u l t s obtained. According to these data, i t can be concluded that pyrrhotites synthesized by methods described corresponded to p y r r h o t i t e , while neither p y r i t e nor marcasite were present. The d i f f r a c t i o n l i n e represented by (102), which has the highest i n t e n s i t y , changed s l i g h t l y i n p o s i t i o n due to the extent 14 of non-stoichiometry of p y r r h o t i t e as reported by M. Haraldsen. However, data obtained i n t h i s work were too scattered to e s t a b l i s h a r e l i a b l e r e l a t i o n s h i p . In patterns 12 and 13, the powder pyrr h o t i t e s before and a f t e r the rest p o t e n t i a l measurement were examined to check the p o s s i b i l i t y of a phase change; however, X-ray pictures indicated that no such phase change occurred, because both X-ray patterns were e s s e n t i a l l y i d e n t i c a l . The phase r e l a t i o n s h i p s described e a r l i e r was not apparent i n t h i s X-ray study. (3) Sulphide Electrodes In t h i s study two kinds of sulphide electrodes were used. One of them was made i n the following way; a mineral plaque was mounted i n s e l f - s e t t i n g p l a s t i c r e s i n , "Koldmount", with the two f l a t sides free from r e s i n . A mercury contacting column containing a copper lead wire was formed i n a r e s i n mount on one of the free sides of the electrode, which j_L LA J » — M — I Sullivan Po. j j I I - i Synthesized Po.50at%Fe ± J I L i • i i i_ J j L _ j I — A - J_J I I i i » . i n i i.» 1111 • • j l • 49.67 = •48.72 = •47.72 = -46.39 = 45.42 * • Pyrite ASTM Py. I • » • IM, . ASTM S Marcasite I 45.36 = 8 -mm m before run after run 30 60 90 120 2 6 Figure 9. X-ray d i f f r a c t i o n patterns of various i r o n sulphides using Co-K. r a d i a t i o n . a - 35 - remained i s o l a t e d from the s o l u t i o n . The other free side then became the active electrode surface i n contact with the e l e c t r o l y t e . A drawing of t h i s "mounted electrode" i s shown i n F i g . 10(a). The other electrode consisted of sulphide powder f l o a t i n g on a mercury pool. A glass U-tube was f i l l e d with mercury on one side of which the sulphide powder was f l o a t e d . A copper lead wire entered the mercury from the other side for an e l e c t r i c a l connection. This electrode, c a l l e d a "powder electrode", i s shown i n F i g . 10(b). Mercury i s known as a thermodynamically noble metal whose standard s i n g l e electrode p o t e n t i a l i s +0.789 v o l t s and has a very high hydrogen overyoltage : -13 -11 associated with a very low exchange current density, i = 10 -10 2 15 A/cm . In addition, p y r r h o t i t e w i l l not react with mercury because of more negative standard free enthalpy of formation of sulphides for FeS, -23.32 Kcal/mole, than that for HgS, -11.05 Kcal/mole. These factors were considered i n the experiments comparing the rest p o t e n t i a l s when measured with the mounted electrodes as compared to the powder electrode. In Table II the r e s u l t s for both natural p y r r h o t i t e and chalcocite are shown. Table I I . Comparison of the r e s t p o t e n t i a l measured with the mounted electrode and the powder electrode. Chichibu p y r r h o t i t e , 25°C pH = 2.85, [Fe*4-] = 0.01 M Chalcocite, 25°C pH = 1.35 [Cu"4"1"] = 0.1 M mounted electrode mounted electrode +161 mV (S.H.E.) +143 +433 mV (S.H.E.) powder electrode +186 mV (S.H.E.) +124 powder electrode +439 mA (S.H.E.) - 36 - •Copper lead wire —Glass tube Mercury rResin Sulphide pSulphide powder Mercury (a) Figure 10. Iron sulphide electrodes Ca) mounted; (b) powder. According to these data, the powder electrode i s s u i t a b l e f o r the sulphide electrode, although the values appear more scattered with the powder electrode. (4) E l e c t r o l y t i c C e l l The rubber bung acting as the top of the c e l l contained a gas disperser, the sulphide electrode, a Pt-counter electrode, a Luggin c a p i l l a r y with reference electrode, and a gas outlet tube, and was f i t t e d to a 400-ml ( t a l l style) beaker. Usually 250 ml of e l e c t r o l y t e was placed i n the beaker and agitated m i l d l y with a magnetic s t i r r e r . In F i g . 11 the sketch f o r the c e l l i s shown. P o t e n t i a l measurements were made with the KC1 saturated calomel electrode as a reference associated with a Luggin c a p i l l a r y . The end of the gas outlet tube was water sealed, so the small p o s i t i v e pressure i n the c e l l caused by the water seal resulted i n an improved contact of the sulphide with mercury on which the sulphide powder was f l o a t e d . (5) Reagents The e l e c t r o l y t e s o l u t i o n consisted of 1 M Na_S0, as a buffer, H„S0 2 4 2 • for pH control of the s o l u t i o n and FeSO^ of the desired ferrous ion concentration s o l u t i o n . Helium and hydrogen sulphide gas used were d i r e c t l y passed from both gas cylinders without p u r i f i c a t i o n . (6) Experimental Procedure Before each run the e l e c t r o l y t e was deoxygenated by bubbling helium - 38 - I 2 4 5 I- Gas out-let tube 2. Pt counter electrode 3. Gas disperser 4. Sulphide electrode 5. Luggin capillary 6. Stirrer magnet Figure 11. Sketch of electrolytic c e l l . - 39 - gas through i t for at l e a s t two hours. Then an electrode, e i t h e r the mounted electrode, which was f i r s t polished on emery paper, or the powder electrode, which was f i r s t ground i n a ceramic mortar under methanol, was immersed into the e l e c t r o l y t e . After a c e r t a i n period both electrodes were cathodized for 30 minutes at around -400 mV. This cathodic excursion could not be expected to change the composition of p y r r h o t i t e , because the current d i d not exceed about 1 coulomb. Afte r the cathodic excursion the rest p o t e n t i a l was read at i n t e r v a l s u n t i l a stable p o t e n t i a l value was obtained, i . e . , 1-5 days. To read p o t e n t i a l and to p o l a r i z e the electrode, a Wenking Standard Potentiostat Model 68 TS10 was used. F i g . 12 shows the r e l a t i o n s h i p s of p o t e n t i a l with time during the measurement. A l l p o t e n t i a l s were measured against the KCl-saturated calomel electrode, which was taken to be +0.241 v o l t s r e l a t i v e to the standard hydrogen electrode at room temperature, and the p o t e n t i a l s are reported on the standard p o t e n t i a l scale i n t h i s work. The temperature of the s o l u t i o n was not e s p e c i a l l y measured and con t r o l l e d i n room temperature experiments. Before and a f t e r each run the e l e c t r o l y t e was usually analysed to determine pH and ferrous ion concentration. However, i n most cases no s i g n i f i c a n t changes i n these values were observed. REST POTENTIAL : Volts (SHE) - 0*7 - VI. RESULTS AND DISCUSSION (1) E f f e c t of Ferrous Ion Concentration The ferrous ion e f f e c t on the rest p o t e n t i a l was investigated i n the range of concentration of 0.001-0.1 M obtained by addition of FeSO^.7H20 at approximate pH 2.8 with He bubbling. The ferrous ion concentration was checked before and a f t e r each run. However, i n most cases no s i g n i f i c a n t change was detected. F i g . 13 shows data obtained for four d i f f e r e n t stoichiometries of p y r r h o t i t e . The ferrous ion e f f e c t on the rest p o t e n t i a l i s obscure because of scattered data; nevertheless no e f f e c t of ferrous ion may be seen for the l i m i t i n g compositions of 46.2 and 50 atomic percent of p y r r h o t i t e s . The experiment i n which ferrous ion was increased to 0.01 M from 0.001 M a f t e r the measurement of the rest p o t e n t i a l i n 0.001 M showed no change i n the p o t e n t i a l , as indicated by arrows i n F i g . 13. It may be concluded that ferrous ion does not a f f e c t the rest p o t e n t i a l s i g n i f i c a n t l y . This i s supported by K.E. Wrabetz"* and S. Venkatachalam et a l . ^ who found no e f f e c t of ferrous ion on the rest p o t e n t i a l of the p y r r h o t i t e . At higher concentrations of ferrous ion than 0.1 M at about 2.8 of pH a p r e c i p i t a t e formed i n the e l e c t r o l y t e , so such concentrations were not used. - 42 - 0.2 LU X if) o > 0- -0.2- < Z LU O CL -0.4- LU (46.20 at%Fe) J JL o o c g b - - " - - ^ 8 (49.26 at%Fe ) (49.86 at%Fe) A (50 at%Fe) •2-}h-» 1 0M(Fe*+) pH^2.8 3 -2 -I LOG. ( Fe24) : M Figure 13. Dependence of the rest potential on ferrous ion concentration. - 43 - (2) E f f e c t of pH In changing pH by addition of sulphuric acid the rest p o t e n t i a l of pyrrhotites was measured i n the presence of ferrous ion i n the e l e c t r o l y t e . The measurements were made on four d i f f e r e n t s t o i c h i o - metrics of the py r r h o t i t e . F i g . 14(a)-(c) present the data obtained. It i s clear that the rest p o t e n t i a l decreases sharply as pH increases. The dependence of pH ranged from about -150 to -350 mV/pH. From these dependences of the rest p o t e n t i a l on ferrous ion and pH i t i s evident that equilibrium between i r o n i n sulphide and ferrous ion i n the elec t r o l y t e i s not established at le a s t i n these ranges. I f equilibrium were reached, the p o t e n t i a l would depend on the ferrous ion concentra- t i o n and would not depend on pH, according to the Nernst equation (IV-1). (3) E f f e c t of Hydrogen Sulphide The next experiment was ca r r i e d out with hydrogen sulphide bubbled through the e l e c t r o l y t e . Since mercury reacts with hydrogen sulphide to form HgS, the powder electrode could not be used i n t h i s experiment, and only the mounted electrode was used. I n i t i a l l y the rest p o t e n t i a l was measured i n a helium atmosphere, then ^ S was introduced and the rest p o t e n t i a l was again measured at a sui t a b l e i n t e r v a l . Results are shown i n F i g . 15(a) and (b). These data were obtained at pH = 3.01 without ferrous ion i n the e l e c t r o l y t e . There are sharp drops i n the p o t e n t i a l f or the natural Chichibu p y r r h o t i t e and the synthesized 47.49 atomic percent Fe p y r r h o t i t e . However, no change i n p o t e n t i a l was found for the 50 atomic percent synthesized - 44 - 0.2- \ ts  (S H E ) o- \ : V o l V P O T E N T IA L -0.2" \ ( a ) \ \ \ R E ST  -0.4- 46.20 at%Fe (Fe 2 4> O.OIM r 2 • i 3 4 PH Figure 14a. Dependence of the rest p o t e n t i a l on pH Ca); pH-dependence of the r e v e r s i b l e p o t e n t i a l for IS] + 2H + + 2e t H 0 S calculated from the Nernst equation. - 45 - 0.2 -0.6-J r — , , r I 2 3 4 PH Figure 14b. Dependence of the r e s t p o t e n t i a l on pH. - 46 - 0.2H Figure 14c. Dependence of the rest p o t e n t i a l on pH. - 47 - py r r h o t i t e even a f t e r l^S introduction. In these experiments before RyS bubbling the outlet gas from the c e l l f o r the 50 atomic percent Fe p y r r h o t i t e contained B^S (as detected by smell), but no E^S was detected from the c e l l s containing the other p y r r h o t i t e s . A l l exhaust gases were passed through a s o l u t i o n containing 1 M Cd ions or Ag ions, and i n a l l cases yellowish CdS or brown Ag^S p r e c i p i t a t e s formed, although the p r e c i p i t a t i o n rate was much greater for the 50 atomic percent synthesized p y r r h o t i t e . According to t h i s experiment, i t i s pos s i b l e to make the following conclusion; for the natural and 47.49 atomic percent Fe synthesized pyrrhotites the e f f e c t of hydrogen sulphide on the p o t e n t i a l i s large because of a low hydrogen sulphide evolution rate from the electrodes. On the other hand, for the 50 atomic percent Fe p y r r h o t i t e the e f f e c t of hydrogen sulphide i s not detectable because of a high i n i t i a l rate of hydrogen sulphide evolution from the electrode. (4). E f f e c t of Non-Stoichiometry of Py r r h o t i t e The a c t i v i t i e s of sulphur and i r o n i n the p y r r h o t i t e as w e l l as the a c t i v i t i e s of ions i n the e l e c t r o l y t e can a f f e c t the rest p o t e n t i a l , according to the Nernst equation (IV-1) and (IV-2). In t h i s work the rest p o t e n t i a l was measured with d i f f e r e n t I | compositions of p y r r h o t i t e at pH = 3 and [Fe ] = 0.01 M. The * The e f f e c t of a c t i v i t y of the components i n a sing l e phase-two component electrode has been ignored i n most published works on sulphide electrochemistry. These a c t i v i t i e s are very s e n s i t i v e to composition i n the s i n g l e phase region, and as a r e s u l t cause d r a s t i c changes i n the p o t e n t i a l when composition i s changed. - 48 - so Too T IME : Hrs Figure 15a. Rest p o t e n t i a l changes i n d i f f e r e n t atmosphere. - 49 - T I M E : Hrs Figure 15b. Rest potential changes i n different atmosphere. - 50 - r e s u l t s are shown i n F i g . 16. From these data, the rest p o t e n t i a l varies through a wide range between -350 and +150 mV as the Fe content changes from 50 to 46 atomic percent. In F i g . 16 the p o t e n t i a l measured for i r o n powder on a mercury pool was shown as point (A). Point (B) i n F i g . 16 shows the rest p o t e n t i a l of the p y r r h o t i t e of 52.8, atomic percent Fe and containing two phases; Fe and FeS. Point (C) i n F i g . 16 shows the p o t e n t i a l for the mixture of i r o n powder and the i r o n saturated p y r r h o t i t e powder. Point (D) shows the r e s t p o t e n t i a l measured for p y r i t e and Points (E) show the rest p o t e n t i a l s of natural p y r r h o t i t e specimens from Chichibu. Generally natural sulphides have more p o s i t i v e p o t e n t i a l s than synthesized sulphides. P y r r h o t i t e conforms to t h i s generalization. Most sulphides tend towards a non-stoichiometry containing excess sulphur which i s more stable under an o x i d i z i n g atmosphere. Therefore, natural purrhotite which has been formed at high sulphur a c t i v i t i e s and l a t e r exposed to o x i d i z i n g atmospheres w i l l always show a more p o s i t i v e p o t e n t i a l than synthesized p y r r h o t i t e formed at high i r o n a c t i v i t i e s . If these rest p o t e n t i a l s measured corresponded to r e v e r s i b l e p o t e n t i a l s , the a c t i v i t y of each component, i . e . , sulphur and i r o n , could be calculated, according to the Nernst equation and the Gibbs- Duhem r e l a t i o n s h i p . However, the p o s s i b i l i t y of measuring the r e v e r s i b l e p o t e n t i a l has already been shown to be poor, and so a c t i v i t i e s were not calculated from the rest p o t e n t i a l s . - 51 - 0.4 D 0.2H LU X CO co O > - J < LU I- O CL •OA -0.21 r- CO LU 0C -0.4H C loA 100^ B 8 %f/0 o / 9Qf 8 ° / 9§ O O o / 1/ A, Iron B, 52.8 at%Fe C, A + B D, Pyrite E, Natural Pyrrhotite (Fe2+)=0.0l M PH =. 3 -tt r 53 50 48 46 ATOMIC PERCENT Fe Figure 16. V a r i a t i o n i n the rest p o t e n t i a l with change i n composition of p y r r h o t i t e . - 52 - (5) E f f e c t of Residual Impurity i n the E l e c t r o l y t e In electrochemical experiments i t i s known that the r e s i d u a l oxidant or reductant i n the e l e c t r o l y t e sometimes plays an important role to determine the p o t e n t i a l of the electrode even when very d i l u t e . In t h i s experiment, possible oxidants are f e r r i c ion and oxygen gas. The former can come from the ferrous sulphate reagent and the l a t t e r can scarcely be avoided from the atmosphere even with He gas bubbling. The experiment was ca r r i e d out i n the c e l l shown i n F i g . 17 i n order to check the e f f e c t of r e s i d u a l oxidants i n the e l e c t r o l y t e , i f they e x i s t , -on the rest p o t e n t i a l . I f oxidants e x i s t i n the e l e c t r o l y t e , they can be reduced on the Pt wire cathode during e l e c t r o l y s i s . An anode compartment i s i s o l a t e d from the e l e c t r o l y t e with a c a p i l l a r y tube to prevent the migration of oxidant species formed on the anode into the bulk of e l e c t r o l y t e . In Table III data obtained i n t h i s c e l l are compared with those measured i n the ordinary c e l l . The cathodization of e l e c t r o l y t e was continued during the rest p o t e n t i a l measurement. Table I I I . Comparison of the re s t p o t e n t i a l s measured with and without reduction of e l e c t r o l y t e reduction of e l e c t r o l y t e measured po t e n t i a l s (mV) with +101, +91 without +139, +147, +91, +80, +150, +60, +50 [Fe**] = 0.01 M, pH = 2.80 cathode p o t e n t i a l = -259 mV for 46.2 at % Fe py r r h o t i t e . - 53 - 2 3 1. Gas out-let & Anode compartment 2. Pt cathode 3. Gas bubbler 4. Sulphide electrode 5. Luggin capillary 6. Stirrer magnet Figure 17. Sketch of the c e l l for reduction of the e l e c t r o l y t e . - 54 - Before and a f t e r the run, the pH was checked, but no change i n pH was detected. According to Table I I I , i t may be seen that there i s no s i g n i f i c a n t change i n the p o t e n t i a l . Therefore, i t may be concluded that either there i s no oxidant i n the e l e c t r o l y t e or such oxidants as e x i s t do not take part i n the p o t e n t i a l determing reaction. (6) Interpretation of the Measured Rest P o t e n t i a l The behaviour of the rest p o t e n t i a l of p y r r h o t i t e can be described as follows: 1) The p o t e n t i a l does not depend on the ferrous ion concentration. 2) In the presence of ferrous ion i n the e l e c t r o l y t e the p o t e n t i a l decreases as pH increases. 3) The H^S e f f e c t on the r e s t p o t e n t i a l i s not consistent for pyrrhotites of a l l compositions, that i s , H^S a f f e c t s the p o t e n t i a l for the pyrrhotites containing excess sulphur, but has no e f f e c t on the stoichiometric p y r r h o t i t e . 4) The e f f e c t of non-stoichiometry of pyrrhotites on the p o t e n t i a l i s s u b s t a n t i a l , i . e . as the excess sulphur content i n pyrrhotites increases the p o t e n t i a l s h i f t s towards more noble values. In this respect, the p y r r h o t i t e electrode i s d i f f e r e n t i n character i n the f i r s t three points mentioned above from sulphide electrodes, i . e . for Cu-S, Pb-S and Ag-S systems a metal ion concentra- t i o n dependence was always obtained and the observed p o t e n t i a l was con- s i s t e n t with an equilibrium between metal ions i n the e l e c t r o l y t e and metal i n the sulphide phase; also H^S i n the e l e c t r o l y t e was apparently i n equilibrium with sulphur i n the sulphide. The fourth point 16 above i s s i m i l a r to observations by J. Brodie. His measurements are  reproduced i n Fi g . 18. The curve i n F i g . 18 was obtained by the following method; a galena electrode was cathodized with a current 2 density 1 mA/cm i n 1 M HCIO^ so l u t i o n for 1 hr, then anodized i n 2 fr e s h l y deoxygenated 1 M HCTO^ s o l u t i o n with a current density 1 mA/cm . During anodization the current was interrupted for the measurement of the rest p o t e n t i a l a f t e r successive short periods. Meanwhile, the e l e c t r o l y t e was sampled f o r Pb ion analysis. The curve s i g n i f i e s that galena saturated with Pb metal by cathodization was gradually changed i n composition from metal-rich to sulphur-rich by anodization, equilibrium between the electrode and the e l e c t r o l y t e being reached to e s t a b l i s h the p o t e n t i a l of the galena electrode. From t h i s curve the e f f e c t of composition of galena on the rest p o t e n t i a l i s seen, although a quantitative r e l a t i o n s h i p showing the precise stoichiometry range could not be obtained. The behaviour of the py r r h o t i t e electrode w i l l be interpreted schematically on the current-density p o t e n t i a l diagram introduced e a r l i e r . During the following i n t e r p r e t a t i o n i t i s assumed that concentration p o l a r i z a t i o n w i l l not appear and the k i n e t i c parameters, i.e.. ka, kc, a and g, remain constant as p o t e n t i a l changed. In other words l i n e a r r e l a t i o n s h i p s of logarithm-current-density vs po t e n t i a l are maintained. Although these conditions seem to be over s i m p l i f i e d , i t i s easier to understand the sulphide electrode when these assumptions are made. (6-1) I n i t i a l l y , l e t us consider the e f f e c t s of ferrous ion and hydrogen ion. In F i g . 19(a) the current-density p o t e n t i a l r e l a t i o n s h i p s for [Fe] -> Fe4"1" + 2e,* [S] + 2H + + 2e -> H2S,* and Fe*4" + 2e •+ Fe are From here on, iro n and sulphur i n sulphide phase are expressed as [Fe] and [S;] , respectively. POTENTIAL : Volts (SHE) Figure 19. Current-density p o t e n t i a l r e l a t i o n s h i p s f o r the c e l l Ca) p y r r h o t i t e |X-FeS04> y-H^SO^|S.H.E. C25°C) (b) pyrrhotites |H O S0 / | } He or H0S|s.H.E. (25°C). - 58 - schematically described. In F i g . 19(a), the l i n e of the cathodic reaction of ferrous ion which i s added 0.1 M as a maximum to the e l e c t r o l y t e i n order to obtain the r e v e r s i b l e p o t e n t i a l of Fe + 2e [Fe] i s located below the l i n e for the cathodic reaction of [S] + 2H + + 2e -*- H^S. Therefore, the p o t e n t i a l determining coupled reactions consist of the anodic reaction of [Fe] ->• Fe + 2e and the cathodic reaction of [S] + 2H + + 2e R^S i n the region where the experiment was undertaken. In th i s case the re s t p o t e n t i a l of p y r r h o t i t e does not depend on the ferrous ion concentration but i t depends on pH because the l i n e for the cathodic process i s a function of hydrogen ion concentration. When the pH i s decreased, i . e . hydrogen ion concentration i s increased, the l i n e f or the cathodic reaction s h i f t s upwards l i f t i n g the i n t e r s e c t i o n with the anodic l i n e as a r e s u l t . This causes the increase i n p o t e n t i a l as pH decreases. The detection of hydrogen sulphide evolution from the c e l l supports the p o s s i b i l i t y of [S] + 2H + + 2e -> Yl^S as a cathodic process of the potential-determining reactions. A thermodynamic consideration for the reaction of FeS + 2H~*~ —»• Fe + H2S suggests the p o s s i b i l i t y of [S] + 2H + 2e -> H 2S as a cathodic process i n the potential-determining reactions of p y r r h o t i t e . In Table IV the equilibrium constants of + 2H + = H 2S(aq) + 2/nM n + 3 which are calculated from the free enthalpy data from the Latimer are presented.. Using the value K for p y r r h o t i t e , when pH i s 3, a . . can be calculated i n the following way; = 3.55 x 10 -4 1 (VI-1) 3H 2S(aq) x - 60 - a n d for the cathodic process *c " ~ 2 F k c a s V *** <" ^ <VI"3> At the equipotential on the pyr r h o t i t e electrode and i n the assumption of a steady state, from (VI-2) and (VI-3) the equation \ " - c = 2 F k a a F e *** ̂  - 2 F k c a s V ^ { ~ ^ < V I" 4 ) i s obtained, where E i s the p o t e n t i a l of p y r r h o t i t e . Equation (VI-4) y i e l d s for the pH-dependence of the p o t e n t i a l , E = - ^ 0.059(PH) - | ^ f _ l o g ^ (VI-5) c s According to data of the dependence of the res t p o t e n t i a l on pH i n t h i s work, i . e . (-150) to (-350) mV/pH, the sum of the transfer c o e f f i c i e n t s f o r the cathodic and anodic reactions w i l l be predicted to be less than unity. (6-2) Secondly, the e f f e c t of H^S on the res t p o t e n t i a l i s discussed i n the same manner using the current-density p o t e n t i a l diagram shown i n F i g . 19(b). In F i g . 19(b), the possible electrochemical I j i _ I processes are; [Fe] -> Fe + 2e, [S] + 2H + 2e H 2S and H2S -> 2H + S + 2e. - 6 1 - Before H^S bubbling, the rest p o t e n t i a l f or the 5 0 atomic percent Fe p y r r h o t i t e and natural or 4 7 . 4 9 atomic percent Fe py r r h o t i t e s are shown E Q ^ and E Q 2 (EQ2 > ^ 0 1 ^ ' w k i c n a r e determined by the coupled ++ + reactions of [Fe] -*- Fe + 2e as an anodic process and [ S ] + 2H + 2e ->• H^S as a cathodic process. Then, by H^S bubbling through the e l e c t r o l y t e the l i n e of the anodic reaction of H^S -»• 2H + + S + 2e appears on the diagram. As a r e s u l t , when t h i s new anodic l i n e i s lower than the anodic l i n e of [Fe] -> Fe + 2e, as i s the case for the 5 0 atomic percent Fe p y r r h o t i t e , the coupled reactions determining the rest p o t e n t i a l are s t i l l [Fe] -> Fe + 2e and [ S ] + 2H + 2e -> H 2 S . However, for the natural and 4 7 . 4 9 atomic percent Fe p y r r h o t i t e s , the anodic l i n e of the reaction H2S-»- 2H + S + 2e i s over the [Fe] Fe + 2e anodic reaction l i n e , so a new p o t e n t i a l i s established, as determined by the coupled reactions of H 2 S -> 2H + + S + 2e and [ S ] + 2H + + 2e UyS. This new p o t e n t i a l E Q 3 > shown i n F i g . 1 9 ( b ) , must be more negative than E Q 2 , having a higher current density i ^ ^ than 1 Q 2 > If equilibrium between sulphur i n sulphide and sulphur deposited from the anodic reaction of hydrogen sulphide i s established, the equilibrium p o t e n t i a l for [S] + 2H + + 2e t R^S can be obtained. ( 6 - 3 ) T h i r d l y , the behaviour of the non-stoichiometry of pyrrhotites w i l l be interpreted using the current-density p o t e n t i a l diagram. The observations on the e f f e c t of the non-stoichiometry are these; when the excess sulphur i n the p y r r h o t i t e increases, the rest p o t e n t i a l increases and the rate of the hydrogen sulphide evolution decreases. In consideration of these f a c t s , the current-density - 62 - p o t e n t i a l r e l a t i o n s h i p i s shown i n F i g . 20, as a function of the non- stoichiometry of p y r r h o t i t e . When the content of excess sulphur i n the pyrrhotite increases, the l i n e s f o r the cathodic reaction of [S] + 2H + + 2e -»• H^S and the anodic reaction of [Fe] -> Fe + 2e s h i f t towards more noble values, because the a c t i v i t y of sulphur increases, while the a c t i v i t y of i r o n decreases with increase i n excess sulphur i n the pyrr h o t i t e . As a r e s u l t the rest p o t e n t i a l shown as an i n t e r s e c t i o n of the cathodic and anodic l i n e s moves towards more noble p o t e n t i a l s and the value of the exchange current density decreases when the excess sulphur content increases. (7) Galvanic and P o l a r i z a t i o n E f f e c t on the Hydrogen Sulphide Evolution The current-density p o t e n t i a l r e l a t i o n s h i p for the cathodic reaction of [S]+2H + + 2e -> H 2S predicts that the rate of the hydrogen sulphide evolution must be affected i n the following manner; a) anodic p o l a r i z a t i o n of the p y r r h o t i t e electrode should reduce the rate of hydrogen sulphide evolution, b) cathodic p o l a r i z a t i o n should accelerate hydrogen sulphide evolution. In consequence, when py r r h o t i t e e l e c t r i c a l l y contacts a material which i s lower i n p o t e n t i a l than the p y r r h o t i t e , H2S evolution from the p y r r h o t i t e should be accelerated. Two attempts were ca r r i e d out to test these p r e d i c t i o n s . To test for a galvanic e f f e c t two kinds of p y r r h o t i t e , one of stoichiometric composition and the other containing excess sulphur were immersed i n a c e l l and the e l e c t r i c a l lead wires from both specimens were shorted. The p o l a r i z a t i o n e f f e c t on the hydrogen sulphide evolution was studied with an electrode of stoichiometric composition. The rate of hydrogen - 63 - (S)+2H+2e-^H 2S i 1 - 0 . 5 0 P O T E N T I A L : Volts (SHE) Figure 20. Current-density p o t e n t i a l r e l a t i o n s h i p s f o r the c e l l d i f f e r e n t pyrrhotites , | FeSO,., H SOjS.H.E. (25°C). - 64 - sulphide evolution was measured i n the following way; He c a r r i e r gas from the c e l l was bubbled through the s o l u t i o n contained 1 M of CdCNO^)^ to c o l l e c t H^S gas i n the form of CdS p r e c i p i t a t e . At 1 or 1/2 hour i n t e r v a l s the CdS p r e c i p i t a t e was f i l t e r e d i n a Gooch c r u c i b l e , washed, dissolved into 1:1 hydrochloric a c i d , and analysed f o r Cd with an atomic adsorption spectrometer. In F i g . 21 and 22, the r e s u l t s are shown. A galvanic e f f e c t on the H^S evolution i s obvious, and the anodization of p y r r h o t i t e decreases the ̂ S evolution and the cathodization increases the ^ S evolution rate. Using an H^S evolution rate of 2.7 x 10 ^ mol/hr under open c i r c u i t conditions i n a s o l u t i o n of pH = 2.65 and an 2 electrode surface area of 2.92 cm , the d i s s o l u t i o n equivalent current -4 2 density was calculated to be 5.0 x 10 A/cm . This value i s much —8 2 larger than the value of 10 A/cm for an i r o n electrode i n 1 M FeSO^ s o l u t i o n obtained by Roiter et al.'*"'' This supports the view that [ | the r e v e r s i b l e reaction for Fe + 2e -*• [Fe] i n the stoichiometric p y r r h o t i t e , where a ^ = 1, can not be a p o t e n t i a l determining reaction at t h i s pH. (8) Electrochemical Mechanism of Leaching Reactions When the leaching process proceeds i n an o x i d i z i n g atmosphere, the cathodic reduction of oxidants becomes part of the sulphide electrode system. For example, when p y r r h o t i t e i s leached with oxygen as a oxidant, the possible electrochemical reactions include the following; 1) oxidation of i r o n i n py r r h o t i t e into ferrous ion [Fe] ->- Fe 4 4" + 2e • e-FH*—G *-FM<—A—^-R i i » i i i i i i i i < i i * 5 10 15 TIME : Hrs Figure 21. Variation in Ĥ S evolution rate with a galvanic contact and anodization of pyrrhotite.  - 67 - 2) oxidation of sulphur i n py r r h o t i t e i n t o sulphate ion [S] + 4H.0 + S0. = + 8H + + 6e 2 4 3) reduction of sulphur i n pyr r h o t i t e into hydrogen sulphide [S] + 2H + + 2e -y 4) reduction of oxygen gas on the sulphide surface, 0 2 + 4H + + 4e -y 2H20 5) oxidation of ferrous ion i n so l u t i o n into f e r r i c ion on the sulphide surface _ ++ ^ 4-H- , Fe -> Fe + e 6) reduction of f e r r i c ion i n so l u t i o n into ferrous ion on the sulphide surface Fe + e -> Fe 7) oxidation of hydrogen sulphide i n s o l u t i o n i n t o elemental sulphur on the sulphide surface H2S -y 2H + + S + 2e These a l l possible reactions must be considered, and some of these reactions, i . e . 4), 5), 6) and 7) can combine as homogeneous electron transfer reactions occurring remote from the sulphide surface, which complicates the ;system s t i l l more. However, the reaction p o t e n t i a l on the sulphide surface i s determined by coupling of the p a r t i c u l a r cathodic and anodic reactions which lead to a maximum exchange current density i n the system. As a r e s u l t the reaction rates, i . e . the current d e n s i t i e s , for the slower reactions must be con t r o l l e d by thi s p o t e n t i a l . In a p r a c t i c a l case the concentration p o l a r i z a t i o n e f f e c t must also be considered. I f t h i s e f f e c t e x i s t s , the rates of effected - 68 - reactions are determined by d i f f u s i o n a l parameters rather than by the electrode p o t e n t i a l . Therefore, the analysis of the process of leaching becomes very complicated. Besides t h i s , an electrochemical study of oxidants must encounter experimental d i f f i c u l t i e s , because these oxidants often react with the electrode changing i t s surface condition and, leading to data that are poorly reproducible. Nevertheless, i n the a p p l i c a t i o n of electrochemical mechanisms to leaching processes, the form of sulphur as a reaction product can be anticipated; a) If the p o t e n t i a l on the sulphide surface during the leaching i s so low that the hydrogen sulphide evolution may occur, the sulphur product i s hydrogen sulphide, or when hydrogen sulphide can be oxidized i n the s o l u t i o n homogeneously as a sequential process i t causes the formation of elemental sulphur or sulphate ion. b) If the p o t e n t i a l on the sulphide i s between that of hydrogen sulphide evolution and that of oxidation to sulphate ion, elemental sulphur w i l l remain l i k e an anode slime a f t e r the d i s s o l u t i o n of metal from the sulphide l a t t i c e . This sulphur i s p a r t i c u l a r l y r e s i s t a n t to further oxidation, once i t has r e c r y s t a l l i z e d from the i n i t i a l skeleton form representing the sulphur l a t t i c e of the mineral. c) If the p o t e n t i a l at the sulphide surface i s so high that sulphate can be formed, sulphur i n the sulphide may dissolve i n the form of sulphate ion, accompanying metal d i s s o l u t i o n . * ' 18 16 These p o t e n t i a l s , 0.81 V for p y r i t e and about 1 V for galena were found. - 69 - In t h i s case sulphur i s obtained as sulphate or a mixture of elemental sulphur and sulphate. This behaviour of sulphur from sulphides i n an o x i d i z i n g leaching process i s i l l u s t r a t e d i n F i g . 23. S u l p h u r i n S u l p h i d e I I i o ^Reaction at the sulphide surface >• Reaction remote from the sulphide surface Figure 23. I l l u s t r a t i o n of the form of sulphur during o x i d i z i n g leaching of sulphide minerals. VII. CONCLUSIONS (1) The rest p o t e n t i a l of py r r h o t i t e was independent of the ferrous ion concentration i n the e l e c t r o l y t e i n the range of 0.001 M - 0.1 M. (2) The rest p o t e n t i a l of pyr r h o t i t e was dependent on pH i n the range 2 to 4 even i n the presence of ferrous ion i n the e l e c t r o l y t e . (3) The H^S i n the e l e c t r o l y t e affected the rest p o t e n t i a l of pyrr h o t i t e containing excess sulphur by reducing the p o t e n t i a l , but did not have an e f f e c t on the rest p o t e n t i a l of stoichiometric p y r r h o t i t e . (4) The e f f e c t of non-stoichiometry of py r r h o t i t e on the rest p o t e n t i a l was s u b s t a n t i a l . Excess sulphur i n pyr r h o t i t e increased the rest p o t e n t i a l . (5) A mixed p o t e n t i a l of pyr r h o t i t e consisting of the reaction S° i n py r r h o t i t e + 2H+ + 2e —»- H 2S as a cathodic process and the reaction o -H-Fe i n pyr r h o t i t e —*• Fe + 2e as an anodic process accounts f o r the character of py r r h o t i t e electrodes described above. - 72 - (6) The h y d r o g e n s u l p h i d e e v o l u t i o n f r o m p y r r h o t i t e by a c i d i f i - c a t i o n may be e x p l a i n e d as an e l e c t r o c h e m i c a l e f f e c t , i . e . , a g a l v a n i c c o n t a c t o f p y r r h o t i t e w i t h a s u b s t a n c e o f d i f f e r e n t p o t e n t i a l imposes a p o l a r i z a t i o n on p y r r h o t i t e t h a t e i t h e r a c c e n t u a t e s o r s u p p r e s s e s H-S e v o l u t i o n . VIII. SUGGESTIONS FOR FUTURE WORK (1) The rest p o t e n t i a l s measured were very scattered. This scatter must be corrected or accounted for so as to i n t e r p r e t the data q u a n t i t a t i v e l y . (2) The p o l a r i z a t i o n studies of p y r r h o t i t e electrodes are necessary for discussion i n more d e t a i l . However, i t must be considered that the system of sulphide electrodes i s more complicated than that of metal electrodes, so the p o l a r i z a t i o n curves obtained may involve those of more than one reaction. In addition, during the p o l a r i z a t i o n of a sulphide such as p y r r h o t i t e which e x i s t i n large non-stoichiometric ranges, the composition can change, and t h i s i s an e s s e n t i a l problem. Composition changes must be avoided f o r meaningful measurements. (3) The study of n i c k e l - i r o n sulphide minerals, i . e . pentlandite, can be undertaken i n a meaningful way, only when the p y r r h o t i t e mineral i s w e l l understood. - 74 - APPENDIX A. Measurement of the Equilibrium Pressure of ^ S on P y r r h o t i t e (1) Introduction When there i s no net current, because of a balance between anodic and cathodic processes, the o v e r a l l reaction can also be considered as i f i t were a straightforward chemical reaction with c h a r a c t e r i s t i c k i n e t i c s and equilibrium. In the following experiments a measurement of the a c t i v i t y of FeS i n py r r h o t i t e was attempted on the basis of chemical equilibrium with acid s o l u t i o n s . The data on a c t i v i t i e s of FeS i n the non- stoichiometric sulphide are useful f o r describing thermodynamic functions across the composition range. For the measurement of the a c t i v i t y i n sulphides a conventional method i s to measure the e q u i l i - brium sulphur vapour pressure over the sulphide,• e i t h e r with hydrogen- hydrogen sulphide gas mixtures or with sulphur gas i n an i n e r t c a r r i e r gas. However, t h i s method can not be applied at low temperatures because of unmeasurably small equilibrium pressures of sulphur over sulphides and probably very slow e q u i l i b r a t i o n rates. Therefore, i n t h i s work a measurement of a c t i v i t y was attempted u t i l i z i n g a reaction of the sulphide with an aqueous so l u t i o n . P y r r h o t i t e may be considered as a binary compound of the components FeS and S. The component of FeS w i l l react with hydrogen ion i n the so l u t i o n according to the following equation: FeS ( i n pyrrhotite) + 2H Fe + H 2S(aq) (1) forming ferrous ion and hydrogen sulphide. For t h i s equation, the equilibrium constant can be expressed i n the following manner; " W " " ^ ( a q ) K = ^ (2) aFeS aH+ The hydrogen sulphide i n the s o l u t i o n w i l l e q u i l i b r a t e with hydrogen sulphide i n gaseous phase; H 2S(aq) = H 2S (gas) (3) K ( 3 ) = 2 3H 2S(aq) So, f i n a l l y Equation (2) y i e l d s [ F e + + ] P H g K K / O N • = K- = K ' (5) ( 3 ) • « w ! where a-p£-H- a n d a H + are assumed to be equivalent to concentrations of each ion. According to Equation (5), when [Fe ] and [H ] are known, the a c t i v i t y of FeS can be determined i n measuring the pressure of H 2S eq u i l i b r a t e d with the system a f t e r f i x i n g a standard state. Consequently, the a c t i v i t y of S i n the FeS-S binary system can be calculated from the a c t i v i t y data of FeS and the composition of p y r r h o t i t e , using the Gibbs-Duhem i n t e g r a t i o n method for the binary system; - 76 - £n a_ = /. N. FeS d£n a. FeS (6) where N„ and N FeS are mole f r a c t i o n of S and FeS, r e s p e c t i v e l y . (2) Experimental Most thermodynamic studies of sulphides done i n aqueous systems at low temperatures have encountered experimental d i f f i c u l t i e s because of sluggish reaction rates and very small d i f f u s i v i t i e s i n s o l i d state. In t h i s work, a s p e c i a l l y designed b a l l m i l l was used so as to obtain an equilibrium as soon as possible and avoid a heterogenity i n composition of p y r r h o t i t e from the surface to the bulk. The b a l l m i l l was f i l l e d to 2/3 of i t s capacity with s o l u t i o n containing ^SO^ and FeSO^, (100 ml volume). 10 gms of powdered py r r h o t i t e was sealed into a pyrex glass tube and put i n the b a l l m i l l to avoid a reaction before the system was deoxygenated. Af t e r the whole system was deoxygenated by depressurizing and f i l l e d with nitrogen gas at atmospheric pressure, a h o r i z o n t a l shaking action of the b a l l m i l l was started. The pressure of H^S was measured with an Hg or o i l manometer at c e r t a i n i n t e r v a l s . After a stable H^S pressure was measured, which usually took 5-10 hrs, the s o l u t i o n was analysed for ferrous ion and pH. The experimental system i s i l l u s t r a t e d i n F i g . 24. S u l l i v a n p y r r h o t i t e . Porcelain ball mill Sampling tube. Shaking table To air To air V T o vac. p-To qas cylinder Water trap Hg Manometer Oil Manometer Figure 24. Schematic i l l u s t r a t i o n of the equipment for H^S pressure measurement. - 78 - (3) Results and Discussion In F i g . 25, the v a r i a t i o n i n R^S pressure with time i s plo t t e d . In t h i s experiment the i n i t i a l s o l u t i o n contained only 3 cc/1 R^SO^. According to these curves, the increases i n H^S pressure are fa s t at the beginning of each run and gradually decreased to a stable pressure. Table V shows the data on H^S pressure, ferrous ion concentration and pH at equilibrium f o r i n i t i a l solutions free of ferrous ion and containing 3 cc/1 H^SO^, and for 1 M of ferrous ion and 6 cc/1 H^SO^, respe c t i v e l y . In the same table, from these data the values of K" = ++ + 2 K'a_ n = [Fe ]P T T „/[H ] are calculated and presented. FeS H.S ^ 3 6 The K" values obtained are scattered i n the range from 10 to 10 . Table V. Values of P u _, [Fe ], pH and K". S u l l i v a n powder p y r r h o t i t e at 25°C. I n i t i a l s o l u t i o n PH 2S < C I " H g > [ F e " ] gr/1 : pH K" atm/M 3 cc/1 H 2S0 4 10.55 4.18 3.05 1.33 X i o 4 4 no FeSO. 4 8.45 3.30 3.25 2.08 X 10 7.15 5.64 4.32 4.27 X i o 6 6.85 4.70 3.06 1.02 X 10 4 6.35 57.00 2.45 6.77 X i o 3 6 cc/1 H 2S0 4 17.40 53.74 2.10 3.50 X i o 3 8.62 58.17 2.76 4.55 X i o 4 1 M FeSO. 4 27.25 58.22 2.87 2.06 X i o 5 24.36 56.84 3.08 4.72 X i o 5 18.54 57.22 3.06 1.89 X i o 5 27.06 59.22 2.91 2.50 X i o 5 22.42 58.06 2.95 2.44 X i o 5 •' 27.74 59.28 2.78 1.34 X i o 5 15.30 57.67 2.41 1.13 X i o 4  - 80 - In F i g . 26 these values of K" are plotted as a function of pH. According to F i g . 26, i t i s seen that the values of K" depend on pH and vary with the i n i t i a l s o l u t i o n used. The K" values obtained i n the i n i t i a l s o l u t i o n of 1 M FeSO. and 6 cc/1 H„S0, are larger than 4 2 4 those obtained i n the i n i t i a l s o l u t i o n of 3 cc/1 ^SO^ and no-FeSO^. The dependence of K" on pH was not expected. Also the wide v a r i a t i o n i n pH of the f i n a l solutions was not reasonable when the same i n i t i a l solutions were used. The reason for these unexpected r e s u l t s may be that there i s a problem i n sampling the f i n a l s o l u t i o n . Usually the f i n a l s o l u t i o n was drained through the sampling tube a f t e r leading nitrogen gas into the b a l l m i l l , then the s o l u t i o n was f i l t e r e d i n a Gooch c r u c i b l e , with an asbestos f i l t e r base. During t h i s f i l t e r i n g process of 2-5 minutes the reaction between acid and p y r r h o t i t e suspended i n the s o l u t i o n sampled would be possible, because t h i s f i l t e r a t i o n was conducted under a hydrogen sulphide-free atmosphere with evacuation. If t h i s reaction occurred, the concentrations of hydrogen ion and ferrous ion i n the f i l t r a t e s o l u t i o n would d i f f e r from those i n the s o l u t i o n sampled, and vary with d i f f e r e n t periods i n which the sampled sol u t i o n was exposed to a free-hydrogen sulphide atmosphere. g Furthermore, high d i s s o l u t i o n rates of ferrous sulphide and zinc 19 sulphide i n acid solutions are reported. The more accurate r e s u l t s to be obtained,•the avoidance of possible reaction between py r r h o t i t e and acid during .the sampling process i s necessary. Figure 26. Dependence of K" on pH. - 82 - B. Table VI. Dependence of the re s t p o t e n t i a l on ferrous ion concentration at pH = 2.8 at 25°C (for F i g . 13). [Fe 4 4"] M Fe content 0.001 M i n p y r r h o t i t e 0.01 M 0.1 M mV mV mV 46.2 at % Fe +055 , +046 +060 , +150 +151 +053 , +046 +050 , +147 +135 +139 +120 +091 +095 +080 +035 49.26 at % Fe -045 +016 -055 +009 -059 49.86 at % Fe -159 -099 -029 -129 -069 -179 -189 50. at % Fe no [Fe**] -411 -420 -425 -440 - 83 - Table VII. Dependence of the rest p o t e n t i a l on pH, at 25°C, [Fe"1"1"] = 0.01 M (for F i g . 14) Fe content pH the p o t e n t i a l Fe content pH the p o t e n t i a l i n p y r r h o t i t e (mV) i n p y r r h o t i t e (mV) at % Fe at % Fe 46.2 2.8 +150 +147 +139 +091 +080 +060 +050 49.86 2.8 3.8 -099 -129 -179 -189 -339 -429 49.26 3.8 1.8 2.0 2.8 -109 -259 +041 000 -045 -055 -059 50.24 2.0 2.8 +051 +041 +021 -130 -279 -285 3.8 -239 -274 D. Table VIII. - 84 - Variation in the rest potential with change in composition of pyrrhotite at 25°C, pH ̂  3 and [Fe"1-1"] = 0.01 M (for Fig. 16) Fe content the potential Fe content the potential Fe content the potential at % Fe ' mV at % Fe mV at % Fe mV 46.02 46.20 46.43 46.80 46.95 50.24 +116 +111 +091 +071 +061 +150 +147 +139 +091 +080. +060 +050 +116 +071 +116 +060 +038 +033 +126 +121 -139 -279 -285 47.26 47.30 47.48 47.57 47.72 48.41 + 75 + 70 + 60 + 43 + 41 + 91 + 41 + 91 + 61 + 51 + 41 + 36 + 33 -005 -010 -040 -139 48.72 49.16 49.26 49.39 49.67 +015 -027 -035 -025 -030 -035 -038 -215 -279 -045 -055 -059 -010 -030 -099 -129 -179 -189 (A) iron -434 powder (B) 52.8 -354 at % Fe (C) (A) + (B) -419. (D) prite +347 (E) Chichibu +186 pyrrhotite +161 (natural) +143 +124 - 85 - REFERENCES • 1. Kullerud, G. Research i n Geochemistry, Volume I I . 2. Yund, R. and H a l l , H. Mat. Res. B u l l . 3, 779 (1968). 3. Latimer, W.M. Oxidation p o t e n t i a l s , Prentice H a l l Inc. 4. Majima, H. unpublished work. 5. Wrabetz, K.E. Z e i t . fur Elektrochem. 60, 722 (1956). 6. Sato, M. Economic Geology, 55, 1202 (1960). 7. Venkatachalam, S. and Mallikarjunan, R. Trans, of Inst, of Min. and Met. 79, C181 (1970). 8. Pohl, H.A. J . Amer. Chem. Soc. 76, 2182 (1954). 9. Downes, K.W. and Bruce, R.W. Trans. CIMM 58, 77 (1955). 10. Gerlach, J . , Hahne, H. and Pawlek, F. Erzmetall. 18, 73 (1965). 11. Vetter, K. Electrochemical K i n e t i c s , Academic Press. 12. Conway, B.E. Theory and P r i n c i p l e s of Electrode Processes, The Ronald Press Co. 13. West, W.A. and Menzies, A.W.C. J. of Phys. Chem. 33, 1880 (1929). 14. Haraldsen, H. Z e i t . anorg. und allgem. Chem. 246, 169 (1941). 15. Conway, B.E. Electrochemical Data, E l s e v i e r Publishing Co. 16. Brodie, J . Thesis of M.A.Sc. U.B.C. (1969). 17. Roiter, Acta Physicochim, 10, 389 (1939). 18. Peters, E. and Majima,.H. Canadian Met. Quarterly 7, 111 (1968). 19. Romankiw, L.T., De Bruyn, P.L. Unit Process i n Hydrometallurgy, Group A, p. 45.

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