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Electrochemistry of pyrite and other sulfides in acid oxygen pressure leaching Bailey, Leonard Keith 1977

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ELECTROCHEMISTRY OF PYRITE AND OTHER SULFIDES IN ACID OXYGEN PRESSURE LEACHING by LEONARD KEITH BAILEY B.S. U n i v e r s i t y of Utah M.A. Sc. U n i v e r s i t y of B r i t i s h Columbia A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n the Department of Metallurgy We accept t h i s t h e s i s as conforming to the r e q u i r e d standard The U n i v e r s i t y of B r i t i s h Columbia February, 1977 <§) Leonard K e i t h B a i l e y , 1977 In present ing th is thes is in p a r t i a l fu l f i lment of the requirements for an advanced degree at the Un ivers i ty of B r i t i s h Columbia, I agree that the L ibrary sha l l make it f ree ly ava i l ab le for reference and study. I fur ther agree that permission for extensive copying of th is thes is for scho la r ly purposes may be granted by the Head of my Department or by h is representat ives . It is understood that copying or pub l i ca t ion of th is thes is fo r f inanc ia l gain sha l l not be allowed without my wri t ten permission. Department of The Univers i ty of B r i t i s h Columbia 2075 Wesbrook P l a c e Vancouver, Canada V6T 1W5 Date ABSTRACT The oxygen pressure l e a c h i n g of p y r i t e has been s t u d i e d i n s u l f u r i c and p e r c h l o r i c a c i d using an oxygen-18 t r a c e r technique. The r e s u l t s are con s i s t e n t w i t h an e n t i r e l y e l e c t r o c h e m i c a l mechanism. The l e a c h i n g p o t e n t i a l of a p y r i t e pulp has been measured as 0.699V SHET*(110°C, IM H 2S0 4, 176 p s i 0 2) and p o t e n t i o s t a t i c experiments at that p o t e n t i a l have y i e l d e d the same r e a c t i o n product r a t i o s as observed i n oxygen pressure l e a c h i n g . The r a t i o of s u l f a t e to elemental s u l f u r produced during p y r i t e l e a c h i n g has been found to be a f u n c t i o n of the le a c h i n g mixed p o t e n t i a l . The y i e l d of s u l f a t e i s increased w i t h i n c r e a s i n g p o t e n t i a l u n t i l a l l the mineral s u l f u r i s converted to the s u l f a t e form at p o t e n t i a l s above 1.0V. A mechanistic model of p y r i t e l e a c h i n g has been advanced, which includes the e l e c t r o c h e m i c a l formation of a p r o t e c t i v e s u l f u r f i l m as i t s b a s i s . The theory has been supported by p o l a r i z a t i o n s t u d i e s i n combination w i t h Eh-pH diagrams. 18 C h a l c o p y r i t e l e a c h i n g has been s t u d i e d using the same 0 technique. Again the r e s u l t s are c o n s i s t e n t w i t h an e l e c t r o c h e m i c a l mechanism. The r a t i o of the s u l f a t e to elemental s u l f u r i n the r e a c t i o n products has been observed to increase w i t h higher oxygen pressures. The mixed p o t e n t i a l of l e a c h i n g i s als o increased at higher pressure i n d i c a t i n g that the same type of mechanism observed i n the p y r i t e work i s o p e r a t i o n a l f o r c h a l c o p y r i t e . Molybdenite l e a c h i n g has al s o been discussed i n terms of the e l e c t r o -chemical model derived f o r p y r i t e w i t h good r e s u l t s , a n d the mechanism ther e f o r e appears to have a p p l i c a t i o n s i n many,if not m o s t , s u l f i d e systems. * The term SHET r e f e r s t o the hydrogen e l e c t r o d e at the ope r a t i n g temperature f o r the run. (In t h i s case 110°C). i i ACKNOWLEDGEMENT The author wishes to thank the s t a f f and students of the Department of Met a l l u r g y f o r t h e i r help i n b r i n g i n g t h i s work to completion. In p a r t i c u l a r , Dr. E. Peters i s recognized since h i s open door p o l i c y was of great help both i n s e t t i n g up the experimental program and i n a n a l y z i n g the r e s u l t s . The Department of Chemistry Mass Spectrometry group i s a l s o 18 appreciated f o r t h e i r time i n performing analyses on the 0 samples. TABLE OF CONTENTS i i i Page INTRODUCTION 1 PART A FeS 2 6 I General 1. Previous Work 6 2. Thermodynamics 12 3. S t r u c t u r e 16 4. Scope of t h i s Work 18 18 I I 0 Tracer Tests on P y r i t e 1. I n t r o d u c t i o n .. 19 2. Experimental 20 2.1 M a t e r i a l s 20 2.2 Apparatus .. 20 2.3 Procedure 24 3. Results and D i s c u s s i o n 27 I I I E l e c t r o c h e m i s t r y of P y r i t e Leaching 1. I n t r o d u c t i o n 35 2. Experimental .. .. .. 35 2.1 M a t e r i a l s 35 2.2 Apparatus 36 a. Pressure Vessel 36 b. The Test C e l l 39 c. Reference E l e c t r o d e 40 d. S u l f i d e M i n e r a l E l e c t r o d e s 44 i v Page e. Instrumentation .. 45 f. A n a l y s i s 48 2.3 Schematic of C i r c u i t s 48 3. Results and D i s c u s s i o n 50 3.1 Mixed P o t e n t i a l Measurement 50 3.2 Anodic D i s s o l u t i o n 52 3.3 An E l e c t r o c h e m i c a l Mechanism 54 a. General 54 b. Elemental S u l f u r Y i e l d .. .. 55 c. Rest P o t e n t i a l 57 d. A p p l i c a b i l i t y of Model to A c i d Consumption Data 60 e. Mathematical Expression of the E l e c t r o -chemical Mechanism 64 3.4 P o l a r i z a t i o n Studies on FeS^ 82 a. S u l f u r Layer Studies 82 b. E f f e c t of Scan Speed 87 c. E f f e c t of Temperature 92 d. E f f e c t of Oxygen 96 +3 +2 3.5 Examination of E° f o r the Fe /Fe Couple as a Function of Temperature 99 a. Procedure 99 b. R e s u l t s and D i s c u s s i o n 100 PART B CuFeS 2 and MoS 2 103 I General 1. I n t r o d u c t i o n and Previous Work 103 Page I I O"1"" Tracer Tests on C h a l c o p y r i t e 1. Experimental 108 2. Results and D i s c u s s i o n 108 I I I Measurement of Mixed P o t e n t i a l s during Leaching of CuFeS 2 and MoS 2 1. Experimental ..; 113 2. Results and D i s c u s s i o n 113 PART C General I A p p l i c a t i o n of t h i s Work to S u l f i d e M i n e r a l s i n General.. 120 I I Conclusions 121 I I I Suggested Future Work .. 122 APPENDICES ' | A The Debye Hiickel Temperature C o r r e c t i o n C o e f f i c i e n t . . . . 124 B I d e n t i f i c a t i o n of S u l f u r Product on Anodized P y r i t e .. .. 127 C Comparison of Molar Volumes of P y r i t e and Elemental S u l f u r 128 D A c t i v a t i o n P o l a r i z a t i o n 129 E Geometrical A n a l y s i s of T a f e l Curves f o r Determining b^.. 132 F Elemental S u l f u r P r o t e c t i o n of Molybdenite along the Basal Plane 135 REFERENCES .. .. -v i LIST OF FIGURES Figure Number Page 1 Eh-pH Diagram f o r the Fe-S-H 20 System. 25°, a_ +2 = 10~3M, a.. c = 10~-LM, other species u n i t *e H 2 b a q a c t i v i t y .. . . 13 2 Eh-pH Diagram f o r the Fe-S-H 20 System. 100°C, aH2S = other species u n i t a c t i v i t y 14 3 P y r i t e P o l a r i z a t i o n Curves - e l e c t r o c h e m i c a l model to e x p l a i n observed r e s t p o t e n t i a l 15 4 P y r i t e C r y s t a l S t r u c t u r e . A f t e r Dana^ 3 0^ 17 5 Oxygen-18 e l e c t r o l y s i s and p r e s s u r i z a t i o n system .. 23 6 Pressure V e s s e l f o r E l e c t r o c h e m i c a l Tests 37 7 Photograph of Pressure V e s s e l and Test C e l l .... 38 8 T y p i c a l C e l l C o n f i g u r a t i o n 41 9 P o t e n t i a l of the Ag/AgCl el e c t r o d e as a f u n c t i o n of Temperature 43 10 El e c t r o d e Design f o r Massive Specimens 46 11 E l e c t r o d e Design f o r Powder Specimens 47 (44) 12 Schematic of E l e c t r i c a l C i r c u i t . A f t e r Peters .. 49 13 P y r i t e Mixed P o t e n t i a l as a Function of Oxygen P a r t i a l Pressure - IM HC10. 51 4 14 E l e c t r o c h e m i c a l Model - E f f e c t of Reaction Products on the P y r i t e Surface. 56 15 E f f e c t of Cathodic Excursions on the Rest P o t e n t i a l of P y r i t e 58 16 Eh-pH Diagram f o r the Fe-S-^O System d e t a i l i n g S u l f u r S t a b i l i t y - 25°C, a„ + 2 = 1 0 _ 3 M , a H „ = 1 0 _ 1 M • e u 2 aq other species u n i t a c t i v i t y 59 I v i i F igure Number Page 17 A c i d Consumption vs. l o g ( i n i t i a l a c i d concentration) .. 62 18 Log (Rate of FeS„ d i s s o l u t i o n ) vs. l o g (P. ), IM H oS0, , 110°C 7 7 2 68 2 4 19 Anodic P o l a r i z a t i o n Curve f o r FeS„ at 110°C, IM HCIO^, lOOmV/min. , 50 p s i He 69 20 Schematic Showing two Components of Anodic P o l a r i z a t i o n Curve 70 21 Cathodic P o l a r i z a t i o n Curves f o r FeS at 110°C, IM HCIO^, lOOmV/min. , 50 p s i 0 2 and He 72 22 Cathodic Curves f o r Oxygen Reduction at 110°C, 1MHC10., lOOmV/min 73 4 23 Movement of Charged P a r t i c l e over Double Layer Energy B a r r i e r 77 24 Langmuir Adsorption Model - B a i l e y Data - 1/Rate vs. 1/P_ 79 : °2 25 Cathodic P o l a r i z a t i o n Curve on P y r i t e - 110°C, IM H 2S0 4, lOOmV/min., 50 p s i He .. 83 26 P o t e n t i a l vs. Current - 110°C, IM H SO,, lOOmV/min. 50 p s i He 84 27 Eh-pH Diagram f o r the Fe-S-H 20 System D e t a i l i n g S u l f u r S t a b i l i t y - 100°C, aFe+2 = 10" 3M, a • _ = 10~lM other species u n i t a c t i v i t y 86 H 2 aq 28 Cathodic P o l a r i z a t i o n Curve on FeS 2 - 50°C, IM H 2S0^, lmV/min., deoxidized N 2 88 29 Cathodic P o l a r i z a t i o n Curve on FeS 2 - 50°C, IM H 2S0^, 100mV/min., deoxidized N 2 89 30 Constant P o t e n t i a l - Current Density vs. Time .... 90 31 Cathodic P o l a r i z a t i o n Curve on FeS2» 25°C, IM ^SO.^, lOOmV/min. and 'steady s t a t e ' p o i n t s ... 91 Figure Number Page 32 P o t e n t i a l vs. Current on a P y r i t e Powder as a f u n c t i o n of Temperature - lOOmV/min. IM HC10. 93 4 33 Anodic P y r i t e P o l a r i z a t i o n Curve - Temperature Dependence 94 34 Cathodic P y r i t e P o l a r i z a t i o n Curves - Temperature Dependence 95 35 P o t e n t i a l vs. Recovery time a f t e r a Cathodic Scan as a Function of the Oxygen P a r t i a l Pressure .... 97 36 F e + 3 / F e + 2 couple P o t e n t i a l vs. T°C 101 37 E° f o r the F e + 3 / F e + 2 couple vs. T° C 102 38 Eh-pH Diagram f o r the Mo-S-H20 System, 25°C a l l species u n i t a c t i v i t y 105 39 Eh-pH Diagram f o r the Cu-Fe-S-H 0 System, 25°C, a C u + 2 = 0.01 M, a l l other species 0.1 M 107 40 Mixed P o t e n t i a l of CuFeS vs. Oxygen P a r t i a l Pressure, IM HC10, 114 4 41 Current Density vs. Temperature f o r CuFeS„; 0.1M H„ S0 4, 700mV SHET 7 7 115 42 Mixed P o t e n t i a l of MoS vs. Oxygen P a r t i a l Pressure 25°C, IM HC10 4 .... 7 117 43 Diagramatic Representation of E l e c t r o c h e m i c a l Mechanism i n MoS^ Leaching 118 D - 1 T y p i c a l T a f e l Curve Form 131 E - 1 Geometrical Argument f o r Determining a value of b 2 1 3 3 i x LIST OF TABLES Table Page I R e s i s t i v i t i e s of S u l f i d e s and Metals -I I Open C i r u c i t P o t e n t i a l s f o r S u l f i d e M i n e r a l s 4 I I I A n a l y s i s of S u l l i v a n P y r i t e . . 21 IV Species Analyzed by Mass Spectrometry 28 V Background C o r r e c t i o n Factors f o r D i f f e r e n t SO^ Mass Numbers 30 VI Experimental Data - Peak Heights 31 18 VI I 0 Y i e l d s i n S u l f a t e O x idation Products 32 V I I I E f f e c t of P o t e n t i a l on S u l f a t e Formation 53 +3 -2 IX Fe and SO, Y i e l d at Various I n i t i a l A c i d i t i e s .... 61 4 X A n a l y s i s of: Preleached Phoenix CuFeS^ 109 18 XI CuFeS 2 0 Tracer Data 110 18 X I I CuFeS 2 0 Tracer Results I l l A-1 Debye Hiickel Temperature C o r r e c t i o n Data . 125 1 INTRODUCTION The e l e c t r o c h e m i s t r y of s u l f i d e minerals has been s t u d i e d from a number of viewpoints. Thermodynamicists have constructed Eh - p H ^ diagrams o u t l i n i n g s t a b i l i t y c o n d i t i o n s f o r v a r i o u s mineral systems; process o r i e n t e d s t u d i e s have been made i n areas such as d i r e c t r e f i n i n g of n i c k e l (2) mattes ; and t h i r d l y , k i n e t i c s t u d i e s on l e a c h i n g of s u l f i d e s are beginning to invoke e l e c t r o c h e m i c a l mechanisms to e x p l a i n the observed r e s u l t s . The work reported i n t h i s t h e s i s stems mainly from the t h i r d area, but r e l i e s h e a v i l y on thermodynamics f o r support. The goal of the study i s to provide a b a s i s of understanding which can a l l o w process o r i e n t e d work to be more e f f e c t i v e . Several papers have been w r i t t e n d e a l i n g w i t h the general e l e c t r o -chemistry of s u l f i d e m i n e r a l s P ' ^ ' " ' ' ^ K o c h ^ d e t a i l s some of the e l e c t r o -chemical c h a r a c t e r i s t i c s of s u l f i d e s , such as t h e i r r e s i s t i v i t y , i n h i s review. —8 —7 Many minerals show r e s i s t i v i t y comparable to metals, 10 -10 Qm, and most -3 f i t i n t o the range of semiconductors (e.g. germanium, ^ 10 fim) . Table I , a f t e r K e l l e r i n c l u d e s values f o r s e v e r a l s u l f i d e minerals. With such evidence, i t may be expected that some charge t r a n s f e r processes have an e f f e c t on mineral d i s s o l u t i o n . (4) The general anodic r e a c t i o n , discussed by Habashi , f o r mineral decomposition i s : MSy >- M4"11 + yS° + ne" (1) This may be accompanied by a number of cathodic r e a c t i o n s ; f o r example the reduction of molecular oxygen: 0 2 + 4H + + 4e~' > 2H 20 (2) Cathodic m i n e r a l decomposition i s a t t r i b u t e d to the r e a c t i o n : MS^+ -ne" — M ° + 4 S " 2 (3) 0 2 Table I R e s i s t i v i t i e s of S u l f i d e s and Metals - A f t e r K e l l e r M a t e r i a l Common Name R e s i s t i v i t y , fim NiS M i l l e r i t e 2-4 x 10" 7 Cu.FeS, J 4 Born i t e 1.6-6000 x 10" 6 (Fe, N i ) 9 S g P e n t l a n d i t e 1-11 x 10~ 6 F e 7 S 8 P y r r h o t i t e 2-160 x 10" 6 CuS C o v e l l i t e 0.3-83 x 10~ 6 CuFeS 2 C h a l c o p y r i t e 150-9000 x 10~ 6 PbS Galena 6.8 x 10" 6 FeS 2 P y r i t e 1.2-600 x 10~ 3 Ag 2S A r g e n t i t e 1.5-2.0 x 10~ 3 ZnS S p h a l e r i t e 2.7 x 10~ 3 to 1.2 x 10 4 B i 2 S 3 B i s m u t h i n i t e 3-570 MnS 2 Haverite 10-20 Cu Copper 1.67 x 10~ 8 j Fe Iron 9.71 x 10~ 8 3 which i s followed i n a c i d s o l u t i o n by the combination of s u l f i d e ions w i t h hydrogen ions to form hydrogen s u l f i d e : S" 2 + 2H + —»• H 2S (4) M i n e r a l d i s s o l u t i o n by the above r e a c t i o n s serves as a b a s i s f o r s e v e r a l i n d u s t r i a l proposals discussed by Habashi but such simple r e a c t i o n s do not t e l l the complete s t o r y . Majima and P e t e r s ^ discussed a g a l v a n i c e f f e c t i n s u l f i d e l e a c h i n g by which minerals w i t h h i g h open c i r c u i t p o t e n t i a l s (see Table I I a f t e r (8) Rachenberg ) were protected when contacted w i t h other minerals having l e s s noble p o t e n t i a l s . The d i s s o l u t i o n r a t e s on the l e s s noble m a t e r i a l s were correspondingly higher when two minerals were i n contact than i f they were leached s e p a r a t e l y . This e f f e c t has been observed by a number of authors on systems ranging from s u l f i d e combinations to a r s e n i d e - s u l f i d e m i x t u r e s ^ ' . Another area of c o m p l i c a t i o n concerns the p a r t i t i o n i n g of mi n e r a l s u l f u r between the elemental form and s u l f a t e . Majima and Peters i n d i s c u s s i n g oxygen pressure l e a c h i n g , c i t e studies^""""'''' "*"2^ on galena (PbS) , s p h a l e r i t e (ZnS), and p y r r h o t i t e (FeS) at temperatures below about 110°C showing almost q u a n t i t a t i v e conversion of the m i n e r a l s u l f u r t o the elemental form by equation (1). The cathodic r e a c t i o n i s the re d u c t i o n of oxygen (eq. ( 2 ) ) . In the case of p y r i t e (7eS^)t and to a l e s s e r extent copper s u l f i d e s however, the y i e l d s of elemental s u l f u r are much lower. Not more than 50% of the s u l f u r i n p y r i t e has been observed to end up i n the elemental form. The r e s t goes i n t o s o l u t i o n as s u l f a t e , S0^ , the copper minerals sometimes (13) y i e l d elemental s u l f u r but have a l s o been found to produce c o v e l l i t e from the r e d u c t i o n of c u p r i c ions by s u l f u r as w e l l as to form s u l f a t e . * Other s o l u b l e s u l f u r species i . e . S^O^ , S0^ , s x ^ 2 » a n c^ H 2 S ^° n o t r e m a i n i n a c i d s o l u t i o n s under o x i d i z i n g c o n d i t i o n s . 4 Table I I Open C i r c u i t P o t e n t i a l s f o r S u l f i d e M i n e r a l s  A f t e r Rachenberg pH 4 PH 9 M i n e r a l Eh (m V.S.H.E.) M i n e r a l Eh (m V.S.H.E.) P y r i t e 658 Ma r c a s i t e 646 Marcasite 634 P y r i t e 540 C h a l c o p y r i t e 558 B o r n i t e 453 S p h a l e r i t e 459 C h a l c o p y r i t e 434 C o v e l l i t e 448 C o v e l l i t e 434 B o r n i t e 416-448 C h a l c o c i t e 416 Galena 395 Molybdenite 411 A r g e n t i t e 276 Galena 328 S t i b n i t e 120 S p h a l e r i t e 181 Molybdenite 109 S t i b n i t e 96 5 Reaction mechanisms proposed to explain the observed results have not been totally successful. In the case of pyrite, most of the proposals do not include electrochemical steps. As pyrite exhibits some of the most unusual behaviour with respect to sulfur partitioning i t is of interest to study the system further. 6 P A R T A FeS 2 I. GENERAL 1. Previous Work P y r i t e i s the most abundant of the s u l f i d e minerals and w h i l e i t i s g e n e r a l l y considered as a gangue m a t e r i a l , i t has been s t u d i e d e x t e n s i v e l y by a number of authors. The e a r l y work centered around hi g h temperature o x i d a t i o n f o r producing s u l f u r i c a c i d v i a c a t a l y t i c conversion (14) of S0 2 . As by-product a c i d production increased however, w i t h growing smelting c a p a c i t y f o r s u l f i d e metals, the demand f o r p y r i t e r o a s t i n g processes d e c l i n e d . The 1950's saw emphasis placed on the aqueous h i g h pressure o x i d a t i o n of p y r i t e as a source of s u l f u r i c a c i d i n pressure l e a c h i n g processes f o r non-ferrous metals(1^,16)^ M O S t i n v e s t i g a t i o n s at t h i s time, d e a l t w i t h k i n e t i c s of a c i d production. Several mechanistic models were advanced but no c o n c l u s i v e evidence was presented. W a r r e n ^ ^ \ i n studying the p y r i t e system at pressures from 25 to 180 p s i 0 2 and temperatures from 130 to 210°C concluded that the d i s s o l u t i o n r e a c t i o n i n v o l v e d d i s s o c i a t e d oxygen, w i t h a / I O N measured a c t i v a t i o n energy of 20 kcal/mole. McKay and Halpern s t u d i e d the system, f i n d i n g a f i r s t order dependence of the l e a c h i n g r a t e on oxygen p a r t i a l pressure from 0 to 60 p s i 0 2 and 100 to 130°C. The a c t i v a t i o n energy was measured as 13 kcal/mole and a mechanism i n v o l v i n g chemisorption of an oxygen molecule on the surface followed by a t t a c k of a second molecule was proposed. The o v e r a l l equation suggested was: slow F e S 2 - 0 2 + 0 2 (aq) • FeS 2«20 2 FeS0 4 + S° (5) E f f e c t s of v a r i o u s parameters such as temperature, a c i d i t y , pressure, surface area and other ions on the amount of s u l f a t e and elemental s u l f u r produced 7 were a l s o noted. These changes were explained by proposing v a r y i n g r a t e s of elemental s u l f u r o x i d a t i o n to s u l f a t e f o r each c o n d i t i o n . (19) Gerlach, Hahne, and Pawlek made a s i m i l a r study over more extended ranges of the v a r i a b l e s temperature, pressure, and a c i d i t y w i t h e s s e n t i a l l y the same conclusions. The f i r s t order oxygen pressure dependence however, d i d not appear to extend above 90-100 p s i 0^. The a c t i v a t i o n energy was reported as 13.1 kcal/mole. Kim and C h o i ^ 2 ^ i n examining the p y r i t e system at pressures up to 2 284 p s i 0^ (20 Kg/cm ), concluded that over the 0 to 156 p s i 0^ range the rat e was f i r s t order w i t h respect to pressure. The data p o i n t at 284 p s i 0^ however, was found to be f a r above the f i r s t order l i n e i n d i c a t i n g e i t h e r a s h i f t to a higher order mechanism or an e r r o r i n the experimental r e s u l t s . The a c t i v a t i o n energy was determined to be 10.1 kcal/mole over the 70 to 150°C temperature range. In c o n t r a s t to the work of McKay, Kim found no e f f e c t of i n i t i a l a c i d i t y on the l e a c h i n g r a t e . (21) Mathews and Robins s t u d i e d the o x i d a t i o n of " c o a l " p y r i t e at temperatures up to 70°C and pressures of < 15 p s i 0^. They concluded t h a t t h e i r r a t e data j u s t i f i e d acceptance of the f i r s t order oxygen pressure dependence found by McKay. As more work has been done on the p y r i t e system the number of proposed d i s s o l u t i o n mechanisms has increased. Proposals range from the (22) simple molecular model of McKay through the work of Peters and Majima who p o s t u l a t e d c o n t r i b u t i o n s from both molecular, s t r i c t l y chemical r e a c t i o n s , and e l e c t r o c h e m i c a l sources. The e l e c t r o c h e m i c a l approach was used to e x p l a i n (13) the phenomena of p y r i t e p r o t e c t i o n during l e a c h i n g of mixed s u l f i d e s . The p r e f e r e n t i a l d i s s o l u t i o n of other minerals such as s p h a l e r i t e , c o v e l l i t e and 8 galena was thought to r e s u l t from g a l v a n i c p r o t e c t i o n of the p y r i t e . The Peters-Majima work i n v e s t i g a t e d both anodic and cathodic p y r i t e p o l a r i z a t i o n but i t has been most noted f o r a t e s t i n which a p y r i t e specimen was anodized at 1.05VSHE* and the r e a c t i o n products analyzed. No elemental s u l f u r was observed. A charge balance coupled w i t h i r o n and s u l f a t e analyses showed good agreement w i t h the equation: FeS 2 + 8H 20 • F e + 3 + 2 S 0 4 ~ 2 + 16H + + 15e" (6) The absence of elemental s u l f u r formation l e d to a general c o n c l u s i o n that anodic c o r r o s i o n of p y r i t e produced only s u l f a t e as a product and hence,that a second molecular or chemical r e a c t i o n path was necessary to create the s u l f u r found i n l e a c h i n g t e s t s . Among the chemical a l t e r n a t i v e s a system i n v o l v i n g (24) t h i o s u l p h a t e was proposed by Majima and Peters . The o v e r a l l s t o i c h i o m e t r y of the r e a c t i o n was given as: FeS 2 + 20 2- >• F e + 2 + S 0 4 ~ 2 + S° (7) The proposed mechanism f o r the r e a c t i o n was: 0 2 (aq) — * 2(0) ads (8) 3(0) ads •+-. FeS 2 (FeS 0 ) ads (9) ( F e S 2 ° 3 ) a d s + 2 H F e + H 2 S 0 T + S ° (10) * H 2 S 0 3 + ^°2 *" H S 0 4 + H (11) In the paper, a value of 0.81V SHE has been reported. Further t e s t i n g done xn the present research program has shown that t h i s value could not be reproduced. A f t e r c o n s u l t a t i o n w i t h Dr. Peters i t has been surmised that the reported value should be 0.81V vs the standard calomel e l e c t r o d e i . e . 1.05V SHE. 9 Equation (9) was suggested as the r a t e c o n t r o l l i n g step g i v i n g the r a t e equation: d FeS --dT^ = K A F e S 2 6 <12> Where 0 i s the f r a c t i o n a l s u rface coverage of adsorbed oxygen. Suggestion of an adsorption isotherm where the surface coverage would increase l i n e a r l y at low oxygen pressure and s h i f t to a lower order w i t h i n c r e a s i n g pressure q u a l i t a t i v e l y explained the r e s u l t s of Warren, McKay and Pawlek. (25) A more recent study of p y r i t e pressure l e a c h i n g was made by B a i l e y By u t i l i z i n g oxygen p a r t i a l pressures up to 976 p s i i n the l e a c h , the d i s s o l u t i o n r e a c t i o n r a t e was found to be c o n s i s t e n t w i t h a Langmuir adsorption isotherm, which has the form: K 3 V (13) R a t e = K C o 2 = ~TTTP7 2 where C i s the number of surface s i t e s of which C are occupied by oxygen, °2 P i s the p a r t i a l pressure of oxygen, K i s a r a t e constant and "a" i s a °2 constant which c h a r a c t e r i z e s the adsorption-desorption e q u i l i b r i u m on the surface. The study a l s o showed that i n i t i a l a c i d c o n c e n t r a t i o n a f f e c t s the composition of the products. Examination of the o v e r a l l equation f o r p y r i t e d i s s o l u t i o n : FeS 2 + (h + |y + ^ x ) 0 2 + (2 + x-2y)H + • ( l - x ) F e + 2 + x F e + 3 + (2-y)S° + y S 0 4 " 2 + (1-y + Hx)H 20 , (14) y i e l d e d values f o r the constants x and y under v a r i o u s c o n d i t i o n s . At low a c i d c o n c e n t r a t i o n s , below ,. 0.4 M^SO^, p y r i t e o x i d i z e d to produce a net excess of s u l f u r i c a c i d , w h i l e at higher i n i t i a l concentrations the net change i n a c i d c o n c e n t r a t i o n was negative over the course of a run. Higher 10 concentration r e s u l t e d i n increased y i e l d s of elemental s u l f u r w h i l e low a c i d l e v e l s enhanced the formation of more s u l f a t e . The a c t i v a t i o n energy was found to be 12.2±0.7 kcal/mole. This value was used i n making a c a l c u l a t i o n of the a c t i v a t i o n entropy of the r e a c t i o n . Applying the E y r i n g equation of absolute r e a c t i o n r a t e theory f o r the unimolecular heterogeneous system, Rate Constant, K = C. K B T AS^/R AH^/RT (15) °2 ~Y~ e e and assuming t o t a l coverage of the surface by oxygen, the a c t i v a t i o n entropy AS^ was c a l c u l a t e d as -12.7 e.u. ± 4 e.u. This i s based on a r a t e f o r t o t a l surface coverage, ( i n f i n i t e p r e s s u r e ) , e x t r a p o l a t e d from a 1/rate vs 1/P^ . Langmiiir p l o t f o r 110°C, lMH^SO^ c o n d i t i o n s and assuming AH^ to be equal to the Arrhenius a c t i v a t i o n energy. The ± 4 e.u. e r r o r bar assumes the o v e r a l l approximations to be c o r r e c t to w i t h i n one order of magnitude. Since d i r e c t chemisorption of a gas or d i l u t e s o l u t e r e s u l t s i n a negative entropy change of 30 to 40 entropy u n i t s , the c a l c u l a t e d value supports the view that chemisorption of oxygen on the p y r i t e surface i s not the r a t e determining step i n the d i s s o l u t i o n r e a c t i o n . I t was proposed that the - 12.7 e.u. value represented a change from a weakly held oxygen molecule on the p y r i t e s u r f a c e to a more t i g h t l y s t r u c t u r e d c o n d i t i o n such as the oxygen i n the s u l f a t e i o n . This i n t e r p r e t a t i o n i s more s o p h i s t i c a t e d than the simple chemisorption theory of McKay but f a i l s to provide a c l e a r answer as to the r e a l r e a c t i o n mechanism. Furthermore, such a model i m p l i c i t l y i n v o l v e s oxygen atom t r a n s f e r to s u l f a t e . In a d d i t i o n to the work by Peters and Majima, other authors have been i n t e r e s t e d i n the e l e c t r o c h e m i c a l behaviour of p y r i t e . Abramov s t u d i e d the p o t e n t i a l of p y r i t e s u r faces i n v a r i o u s media. He i n t e r p r e t e d h i s r e s u l t s as 11 i n d i c a t i n g a r a t e dependence on d i s s o l v e d oxygen w i t h p o s s i b l e involvement of hydrogen peroxide i n the mechanism. He a l s o concluded that elemental s u l f u r could be formed e l e c t r o c h e m i c a l l y only i n very a c i d i c reducing systems. (27) Sato advanced the case f o r e l e c t r o c h e m i c a l s u l f u r formation i n h i s p o t e n t i a l - p H study of s e v e r a l s u l f i d e minerals i n c l u d i n g p y r i t e . Unfortunately he d i d not experimentally i d e n t i f y the s u l f u r species. Instead, the r e a c t i o n : FeS 2 ---> F e + 2 + S 2 + 2e~ (16) was proposed, and f r e e energy data f o r the molecular s u l f u r species c i t e d to e x p l a i n the p o t e n t i a l observed. (28) Nagai and K i u c h i enhanced the c r e d i b i l i t y of an e l e c t r o c h e m i c a l theory f o r p y r i t e d i s s o l u t i o n and disproved the Abramov reducing c o n d i t i o n c o n c l u s i o n when i n 1975, they showed that elemental s u l f u r could be produced from the mineral by passage of a current. From t h e i r s t u d i e s between 145° and 175°C they proposed a d i s s o l u t i o n mechanism which produced mainly elemental s u l f u r by the r e a c t i o n : FeS 2 — • F e + 2 + 2S° + 2e~ (17) An amount of HSO^ was a l s o produced (dependent on the current density) by the r e a c t i o n : FeS 2 + 8H 20 >• F e + 2 + 2HS0 4~ + 14H + + 14e~ (18.) I t was suggested that p y r i t e d i s s o l v e d p r i m a r i l y by equation (17) and that the s u l f u r thus formed was then o x i d i z e d to s u l f a t e to give the product composition found i n pressure l e a c h i n g t e s t s . U n f o r t u n a t e l y no c o r r e l a t i o n between the el e c t r o c h e m i c a l r e s u l t s and the l e a c h i n g t e s t s was s u c c e s s f u l . The elemental s u l f u r y i e l d s were observed to be much higher (up to 81%S°) i n el e c t r o c h e m i c a l t e s t s than under pressure l e a c h i n g c o n d i t i o n s . 12 2. Thermodynamics The thermodynamics of the p y r i t e system at 25 and 100°C are shown on Eh-pH diagrams (Figures 1 and 2) constructed using the equations of (29) B i e r n a t and Robins . The concentrations were chosen to represent the system e a r l y i n a l e a c h i n g process before the i r o n c o n c e n t r a t i o n has a chance to b u i l d up. According to the thermodynamics, the p o t e n t i a l at which p y r i t e i s i n e q u i l i b r i u m w i t h ferrous i o n , the r e s t p o t e n t i a l , should be about 0.35V at 25°C. Experimental r e s u l t s however, have g e n e r a l l y given higher values. (22) The r e s t p o t e n t i a l has been measured as about 0.62V SHE J i n lMHClO^. This d i f f e r e n c e can be expl a i n e d i n terms of a mixed p o t e n t i a l argument as shown i n (22) Figure 3. The anodic and cathodic p o l a r i z a t i o n curves can be viewed as coupled r e a c t i o n s o p e r a t i n g at steady s t a t e w i t h a current d e n s i t y equal to that at the cross over point of the dashed l i n e s . 13 Figure 1. Eh-pH Diagram f o r the Fe-S-H 20 System. 25°, a -!-2 = 10 M, aH San = 1 0 ' a 1 1 o t n e r species u n i t a c t i v i t y f UJ X in JC - 0 . 5 - I PH Figure 2. F/h-pH Diagram f o r the Fe-S-H 0 System. 100°C, a +2= 10 M, a -1 ^ e H Saq = 10 M, other species u n i t a c t i v i t y . 1 5 Figure 3. Pyrite Polarization Curves - electrochemical model to explain observed rest potential. 16 3. Str u c t u r e P y r i t e has a s t r u c t u r e s i m i l a r to that of NaCl. The i r o n atoms take the sodium p o s i t i o n s and S^ groups replace the c h l o r i n e . The molecules are o r i e n t e d so as to leave no d i s t o r t i o n i n any c r y s t a l d i r e c t i o n (see Figure 4 a f t e r Dana^ ^ ) . • (31 32) Studies on p y r i t e surfaces by Nagai and K i u c h i ' have shown that d i s s o l u t i o n on a macro s c a l e i s f a i r l y uniform w h i l e examination using a scanning e l e c t r o n microscope shows formation of "pyramidal etch h i l l s " and/or "bamboo l e a f etch f i g u r e s " depending on the experimental c o n d i t i o n s . A c t i v e d i s s o l u t i o n takes place l e a v i n g pyramidal h i l l s w i t h the {1,1,1} planes as faces i n the f i r s t case, w h i l e no favored plane has been i d e n t i f i e d i n the bamboo l e a f etch f i g u r e system. Figure 4. P y r i t e C r y s t a l S t r u c t u r e , A f t e r Dana^ 3 0^ 18 4. The Scope of This Work The present study was undertaken i n an e f f o r t to provide d e f i n i t e evidence d i s t i n g u i s h i n g between p o s s i b l e atom t r a n s f e r mechanisms f o r p y r i t e d i s s o l u t i o n and e l e c t r o c h e m i c a l processes. The evidence r e s u l t s from oxygen-18 t r a c e r t e s t s on p y r i t e pulps during oxygen pressure l e a c h i n g . Further c o n f i r m a t i o n of the t r a c e r r e s u l t s was a f f e c t e d by p o t e n t i o s t a t i c experiments on both pulps and massive specimens. With the d i s s o l u t i o n mechanism f o r p y r i t e f i r m l y e s t a b l i s h e d , i t was decided to extend the study to other s u l f i d e s i n the hope of f i n d i n g a g e n e r a l l y a p p l i c a b l e mechanism. C h a l c o p y r i t e was chosen f o r f u r t h e r t r a c e r t e s t s and both c h a l c o p y r i t e and molybdenite were examined e l e c t r o c h e m i c a l l y during l e a c h i n g t e s t s . R e sults of these t e s t s j u s t i f y the extension of the p y r i t e mechanism to at l e a s t these two minerals studied^and perhaps even f u r t h e r , to general s u l f i d e l e a c h i n g , to the extent that s u l f a t e i s formed. 19 I I . OXYGEN-18 TRACER TESTS ON PYRITE 1. I n t r o d u c t i o n As noted p r e v i o u s l y , the work done by e a r l i e r authors does not allow any conclusions to be drawn regarding the f a t e of the reacted oxygen i n terms of a mechanistic model f o r pressure l e a c h i n g t e s t s . The a b i l i t y to d i s t i n g u i s h between oxygen i n s u l f a t e formed i n an atom t r a n s f e r process versus that i n s u l f a t e produced e l e c t r o c h e m i c a l l y , allows d e f i n i t e c h a r a c t e r i z a t i o n of the r e a c t i o n mechanism. For example, the oxygen which i s adsorbed on the p y r i t e surface may react w i t h e l e c t r o n s o n l y , ( s u p p l i e d from anodic d i s s o l u t i o n of p y r i t e ) i n a cathodic r e a c t i o n , v i z . 0 2(ads) + 4H + + 4e~ — + 2H 20 (2) In such an e l e c t r o c h e m i c a l system the s u l f a t e formed i n the anodic r e a c t i o n , FeS 2 + 8H 20 • F e + 3 + 2 S 0 4 " 2 + 16H + + 15e" (6) contains oxygen from water, r a t h e r than from the high pressure gas phase. A l t e r n a t i v e l y the adsorbed oxygen may react i n a f u l l y molecular path, an atom (18) t r a n s f e r r e a c t i o n , such as proposed by McKay and Halpern : 20„(ads) + FeS. ( s u b s t r a t e ) — > • F e + 2 + SO ~ 2 + S°* (7) I l 4 In t h i s case the product s u l f a t e contains oxygen from the gas phase. A t h i r d s i t u a t i o n i n v o l v i n g a mixed r e a c t i o n that i s p a r t l y atom t r a n s f e r and p a r t i a l l y e l e c t r o c h e m i c a l i s a l s o p o s s i b l e . While r e a c t i o n ( 7 ) makes more sense i n t u i t i v e l y , i t f a i l s to account f o r the wide v a r i a t i o n of s u l f a t e y i e l d s a c t u a l l y observed, or f o r the g a l v a n i c * This i s not the only p o s s i b l e atom t r a n s f e r r e a c t i o n . A process i n v o l v i n g t h i o s u l f a t e has been described by equations 8 to 11 and other routes are p o s s i b l e . 20 p r o t e c t i o n of p y r i t e during the l e a c h i n g of mixed s u l f i d e s . To r e s o l v e the question, a s e r i e s of t e s t s were performed using oxygen-18 (a n o n - r a d i o a c t i v e isotope) as a t r a c e r . The measurements r e q u i r e i d e n t i f i c a t i o n of oxygen-18 by mass spectrometry. 2. Experimental 2.1 M a t e r i a l s The p y r i t e used i n t h i s study was obtained from the S u l l i v a n Mine of Cominco Mines L t d . The chemical a n a l y s i s i s shown i n Table I I I . X-ray d i f f r a c t i o n was used to check the p y r i t i c c r y s t a l s t r u c t u r e and no other major phases were evident. The m a t e r i a l was crushed by hand using a mortar and p e s t l e , s i z e d (+150 -200 mesh), washed to remove any f i n e s , and a i r d r i e d . S u f f i c i e n t m a t e r i a l was crushed i n i t i a l l y f o r a l l the t e s t s conducted. 18 Oxygen-18 gas was obtained by e l e c t r o l y s i s of enriched water (4.27%0 ) which was purchased through M i l e s L a b o r a t o r i e s Inc. The low enrichment was 18 determined by economic c o n s t r a i n t s ; the concentration of 0 being as low as p o s s i b l e w h i l e r e t a i n i n g the a b i l i t y to d i s t i n g u i s h between molecular and e l e c t r o c h e m i c a l mechanisms. D i s t i l l e d water was used throughout the t e s t s and a l l other chemicals employed were reagent grade. 2.2 Apparatus Most t e s t s were made under constant volume c o n d i t i o n s i n a 106.5 ml zirconium shaking autoclave w i t h 15 ml of separate ( i . e . room temperature) gas r e s e r v o i r space. Zirconium tubing and f i t t i n g s were used wherever the hot Table I I I A n a l y s i s of S u l l i v a n P y r i t e S u l l i v a n P y r i t e T h e o r e t i c a l FeS 2 %Fe 45.86%* 46.6% %S 51.65 53.4 S/Fe Mole r a t i o 1.97 2.00 % S i 0.2 -%Cu 0.03 -The t o t a l a n a l y s i s shown adds up to only 97.74% the r e s t of the m a t e r i a l i s made up of traces of Mg, Pb, As, Sn and oxygen. 22 leach s o l u t i o n would be i n contact w i t h a metal surface. A t o t a l l y enclosed t e f l o n gasket i n a Bridgeman - type s e a l provided the c l o s u r e . Shaking was provided h o r i z o n t a l l y at a r a t e of 288 - 1.5 i n c h , strokes per minute. The c y l i n d r i c a l autoclave was t i l t e d at a 45° angle to give b e t t e r a g i t a t i o n and to f a c i l i t a t e sample removal. Temperature was c o n t r o l l e d to w i t h i n ±h°C by a Yellow Springs temp-erature c o n t r o l l e r coupled w i t h a v a r i a c . The temperature was measured by a thermistor probe l o c a t e d i n a w e l l at the center of the autoclave. Oxygen consumption throughout the course of a run was measured by a pressure transducer,monitored by a s t r i p chart recorder. As a check on the oxygen consumption measurements, the autoclave was p e r i o d i c a l l y sealed without a mineral charge, heated, and p r e s s u r i z e d . I f a f t e r s i x hours no pressure drop was detected, the runs made p r i o r to the t e s t were judged acceptable. 18 Production of oxygen-18 gas was achieved by e l e c t r o l y s i s of 0 enriched water. The equipment i s shown diagrammatically i n Figure 5. The e l e c t r o l y z i n g equipment cons i s t e d . o f a 100 ml pyrex "u" tube equipped w i t h two 18 p l a t i n i z e d platinum e l e c t r o d e s . Before e l e c t r o l y s i s , the 0 enriched water was mixed w i t h s u l f u r i c a c i d (to 200 g/1 H^SO^) to improve c o n d u c t i v i t y . When connected to the power supply (Hewlett Packard model 6203B) , oxygen was evolved at the anode and hydrogen at the cathode. I t was necessary to place a f l o w i n g water c o o l i n g bath ground the "u" tube to d i s s i p a t e the heat generated during e l e c t r o l y s i s . The oxygen produced was c o l l e c t e d i n an i n v e r t e d 6 % h o l d i n g f l a s k . I n i t i a l l y the f l a s k was f i l l e d w i t h water which was d i s p l a c e d by the oxygen and flowed i n t o another 6 £ overflow f l a s k . A f l o a t v a l v e on the hydrogen s i d e of the e l e c t r o l y s i s tube maintained the solution at a reasonable K3 • • • . U > F i g u r e 5. Oxygen-18 e l e c t r o l y s i s and p r e s s u r i z a t i o n system. 24 l e v e l and kept the backpressure of the system from blowing the water out of the "u" tube thus stopping the e l e c t r o l y s i s . When s u f f i c i e n t oxygen had been produced make a run, the gas was d r i e d i n two stages ( d r i e r i t e - c o l d trap) and l i q u i f i e d i n a s t a i n l e s s s t e e l s p i r a l made of 1/8 i n c h o.d. autoclave pressure tubing. The s p i r a l had a volume of s l i g h t l y over 6 ml. L i q u i f i c a t i o n was achieved by immersing the s p i r a l i n a Dewar f l a s k of l i q u i d n i t r o g e n (b.p.-195.8°C) which condensed the oxygen (b.p.-183.0°C). Water from the overflow f l a s k was siphoned back i n t o the ho l d i n g f l a s k g i v i n g a good i n d i c a t i o n of how much gas was l i q u i f i e d . V a l v i n g o f f the s p i r a l c o n t a i n i n g the l i q u i f i e d oxygen and connecting i t to the autoclave system, followed by removal of the Dewar f l a s k p r e s s u r i z e d the e n t i r e u n i t to 1000 p s i ( l e s s i f a l l 6 I of oxygen gas had not been l i q u i f i e d ) . 2.3 Procedure To produce the oxygen f o r a run, the h o l d i n g f l a s k was f i r s t evacuated w i t h a vacuum pump. Slowly i n t r o d u c i n g water i n t o the f l a s k from the overflow v e s s e l allowed any d i s s o l v e d oxygen to be drawn o f f by the pump. When the hold i n g f l a s k was f i l l e d w i t h water the pump was shut o f f , the val v e to the e l e c t r o l y s i s tube opened, and e l e c t r o l y s i s s t a r t e d . A f t e r approximately 24 hours of e l e c t r o l y s i S j p r o d u c i n g about 6 I of oxygen,the f l a s k was sealed o f f . At t h i s p o i n t the autoclave was charged w i t h 5 grams of mineral and 50 ml of the leach s o l u t i o n . The autoclave top was then sealed and shaking, and h e a t i n g begun. The system reached operating temperature i n 10 to 15 minutes during which time the oxygen i n the h o l d i n g f l a s k was l i q u i f i e d i n the s p i r a l . To condense the oxygen i t was f i r s t necessary to evacuate the sampling, drying,and condensing tubes to remove a l l n i t r o g e n . A flewar 25 f l a s k of l i q u i d n i t r o g e n was then placed around the s p i r a l and the v a l v e to the h o l d i n g f l a s k opened. The d r i e r i t e column and the a c e t o n e - l i q u i d c o l d trap were necessary to stop water vapor from e n t e r i n g the s p i r a l where i c e c r y s t a l s would b l o c k the tubing. When the 6 £ of gas had been l i q u i f i e d , the s p i r a l was valved o f f . As the autoclave reached operating temperature the s p i r a l was connected and the Dewar f l a s k c a r e f u l l y removed. V a p o r i z a t i o n of the l i q u i d oxygen r e s u l t e d i n p r e s s u r i z a t i o n of the autoclave which was then valved o f f . The remaining h i g h pressure oxygen i n the s p i r a l was r e l i q u i f i e d w i t h the Dewar of l i q u i d n i t r o g e n , the v a l v e to the h o l d i n g and sample f l a s k s opened, and the oxygen c a r e f u l l y allowed to v o l a t i l i z e . V a l v i n g o f f the h o l d i n g and sample f l a s k s then returned the system to e l e c t r o l y s i n g c o n d i t i o n s . Samples of the input gas f o r each run were analysed using the MS-9 mass spectrometer (Associated E l e c t r i c a l I n d u s t r i e s ) of the UBC Department of Chemistry to determine the oxygen-18 content. The consumption of oxygen i n the autoclave, by the m i n e r a l , was monitored continuously v i a a pressure transducer. At the end of the run the autoclave was cooled r a p i d l y (10-15 min) by a f l o w i n g water c o o l i n g c o i l to ^60°C and the contents removed by s u c t i o n . The residue was f i l t e r e d and a i r d r i e d w h i l e the remaining s o l u t i o n was d i v i d e d i n t o two p a r t s . One was used f o r i r o n analyses (25) to compare the oxygen-18 t e s t s w i t h e a r l i e r work , and the other was s t r i p p e d of s u l f a t e by a d d i t i o n s of P b ( C l O ^ ^ which formed an i n s o l u b l e PbSO^ p r e c i p i t a t e . 18 To analyze the product s u l f a t e f o r 0 i t was necessary to convert the s u l f a t e a to a gaseous product. This was done oy r e a c t i n g the p r e c i p i t a t e with^weighed amount of t e s t lead to give s u l f u r d i o x i d e gas, v i z . PbSO, + Pb h - ^ t 2Pb0 + S0 o (19) 4 z 26 The conversion r e a c t i o n was c a r r i e d out i n an evacuated vycor tube connected to a gas sample b o t t l e . The tube was heated by a hand t o r c h . This procedure produced an adequate amount of SC^ gas f o r high r e s o l u t i o n mass spectrometry. Results from the MS - 9 were i n the form of i n t e n s i t y peaks on a s t r i p chart. These were measured and the composition of the SO^ w i t h respect to 0^ and 0"^  c a l c u l a t e d . 27 3. Results and D i s c u s s i o n In a l l , a t o t a l of 11 s u c c e s s f u l runs were made, two as standards using oxygen e l e c t r o l y z e d from d i s t i l l e d water and nine u s i n g oxygen-18 18 enrichment. The l a s t run i n the s e r i e s was a reverse t r a c e r t e s t w i t h 0 water and commercial c y l i n d e r oxygen. The oxygen consumption curves and 18 i r o n analyses on s o l u t i o n and residues show no e f f e c t due to 0 enrichment or production of oxygen from water,when compared w i t h previous work using oxygen from commercial c y l i n d e r s . S u l f a t e samples from each of the runs were converted to SC^ and analyzed f o r sulfur-oxygen species at s i x mass numbers. The species examined and t h e i r r e s p e c t i v e masses are shown on Table VI. An i n d i c a t i o n of the r e l a t i v e abundance of each species under normal and enriched c o n d i t i o n s i s 34 shown i n the l a s t two columns. The S components were i n c l u d e d i n the 34 a n a l y s i s s i n c e the n a t u r a l abundance of S i s 4.22% of a l l s u l f u r , more than 32 18 33 s u f f i c i e n t to mask the S -0 species i f not accounted f o r . The S species was not considered as i t does not i n t e r f e r e w i t h the even mass numbers; 64,66, 68 and 70. C o n t r i b u t i o n s due to 0 ^ could conct&vably a f f e c t the r e s u l t s at mass numbers 66 and 68 but the c o n c e n t r a t i o n of 0"^ was l e s s than 0.2% i n the 18 enriched water (< 0.04% n a t u r a l abundance) a f a c t o r of 20 l e s s than the 0 . The assumption that any.effect i s n e g l i g i b l e seems j u s t i f i e d . I n i t i a l concentrations of oxygen-18 i n the autoclave were analyzed 18 as 4.3 to 3.8%0 . I t i s i n t e r e s t i n g to note that s i n c e the i n i t i a l concent-18 r a t i o n of the enriched water before d i l u t i o n w i t h a c i d was 4.27%0 , the concentration i n the gas has been increased. This can be p a r t i a l l y explained by the l i m i t e d a n a l y t i c a l accuracy (± ^8%) but may a l s o r e s u l t from an isotope 28 Table IV Species Analyzed by Mass Spectrometry Gas Mass Number R e l a t i v e Maximum Peak Heights 0.042 S 3 4 0.002 0 1 8 0.042 S 3 4 0.04 0 1 8 s 3 2 o f 63.9619 0.9542 0.8829 s 3 4 o f 65.9577 0.0418 0.0387 s 3 2 o 1 6 o 1 8 65.9661 0.0019 0.0368 s 3 4 o 1 6 o 1 8 67.9619 8.3 x 10"5 1.6 x 10~3 s 3 2 o f 67.9704 3.8 x 10"6 1.5 x 10~3 s 3 4 o f 69.9662 3.5 x 10~6 6.7 x 10~5 29 e f f e c t during the e l e c t r o l y s i s r e a c t i o n through which the formation of 0 1 8 species ( 0 1 6 0 1 8 and 0 1 8 ) i s favored r e l a t i v e to the O^6. A second c o n t r i b u t i o n to the v a r i a t i o n of i n i t i a l oxygen-18 concentration stems from the a d d i t i o n of f r e s h s o l u t i o n to the e l e c t r o l y s i s tube to replace the water decomposed. The s l i g h t d i f f e r e n c e s i n i n i t i a l gas composition had no detec t a b l e e f f e c t on the t e s t r e s u l t s . Readings were taken from the mass spectrometer v i a a fou r channel recorder (1,3,10 and 30 times the source s i g n a l ) by measuring peak heights to the nearest 0.5 mm. The numbers thus obtained were then m u l t i p l i e d by a background c o r r e c t i o n f a c t o r , which was determined f o r each mass number by comparing a background scan, taken immediately p r i o r to each sample scan, w i t h the a c t u a l sample scan. A set of t y p i c a l background f a c t o r ranges i s shown i n Table V. The a c t u a l peak height data, f o r a l l the t e s t s , c o r r e c t e d f o r background, are shown i n Table VI. The e r r o r range f o r the t e s t s was 34 c a l c u l a t e d on the ba s i s of the n a t u r a l abundance of S which i s constant at 32 16 3A 16 3'-f 4.22%. Peak heights f o r S 0 and S 0 were used to c a l c u l a t e S percentages fo r a l l samples. These values gave an average of 4.19 w i t h a standard d e v i a t i o n of ± 0.27%. The accuracy to which a l l peak heights i n the sample scans could be measured i s the same (0.5 mm) f o r both the la r g e and s m a l l peaks. The e r r o r range of ±0.27% t h e r e f o r e , a p p l i e s to the e n t i r e t e s t i n g sequence. Table V I I shows the summary of a l l r e s u l t s obtained i n the study. The f i r s t column d e t a i l s the i n i t i a l c o n d itions f o r each run. Except as noted i n runs 8 and 11 a l l experiments were conducted using 5g of p y r i t e i n 50 ml of s o l u t i o n f o r s i x hours. Run 8 was made without a mineral charge to determine i f any exchange took place between the molecular oxygen i n the gas phase and the oxygen t i e d up as s u l f a t e i n the a c i d . No i n f l u e n c e of the gas on the oxygen 18 i n the s u l f a t e was noted. Run 11, the reverse t r a c e r t e s t u s i n g 0 enriched s o l u t i o n and commercial c y l i n d e r oxygen, was made as a check on the t e s t i n g procedure. Table V Background Correction Factors for Different S0 2 Mass Numbers Peak Mass Numbers Background Correction Factor 63.96 0.99 - 0.98 65.96 0.96 - 0.84 65.96 0.96 - 0.84 67.96 0.25 - 0.00 67.97 0.25 - 0.00 69.97 0.10-0.00 Table VI Experimental Data  Peak Heights in mm Corrected for Background Run 63.96S 3 202 6 65.96S 3 402 6 6 5 . 9 7 S 3 2 0 1 6 0 1 8 67. 9 6 S 3 4 0 1 6 0 1 8 67.97S 3 202 8 6 9 . 9 7 S 3 4 0 2 8 1 1019 48 4.5 .3 .3 .3 2 i n s u f f i c i e n t gas to complete a n a l y s i s 3 270 12.6 2 0 0 0 4 343 15 3 0 0 .5 5 431 19.3 .3 0 0 0 6 461 19.3 0 0 0 0 7 276 11.2 0 0 0 0 8 . 547 . 22.2 0 0 0 0 9 370 17.1 0 0 0 0 10 355 15 2.2 0 0 0 11 450.5 22.0 8.3 0 0 0 Second Set of Analyses 2 1210 54 3 2 0 2 3 990 47 2 0 0 .6 6 985 38.6 4.2 0 1.8 0 7 1099 44 5.5 0 .5 0 8 1280 53 3 0 .4 0 10 1107 54 2 0 0 0 Table V I I 18 0 Y i e l d s i n S u l f a t e Oxidation Products I n i t i a l Conditions F i n a l I n i t i a l Gas % 0 1 8 %0 x 8maximum %0 18 Average, Atom Pressure// (S0 4 determ) i n S0= e r r o r * * * Tran.% 1 2* 1. 976psi,110°C,1.0MH2S0,, no 0 1 8 enrichment 432 p s i 0.24 0.22 0.28 - 0.2810.27 2. 976psi,110°C,1.0MH SO,, no 0 1 8 enrichment 436 0.25 0.23 - 0.35 0.35±0.27 3. 976psi,110°C,1.0MH2S04 437 4.23 2.12 0.35 0.16 0.26+0.27 3±14% 4. 976psi,110°C,0.4MH„S0, 2 4 382 3.82 2.77 0.55 - 0.55±0.27 13H1% 5. 476psi,110°C,1.0MH SO, 2 4 98 4.32 1.68 0.03 - 0.03±0.27 -12118% 6. 176psi,110°C,1.0MHoS0, z 4 2 3.99 1.13 0.00 0.37 0.1810.27 -3130% . 7. 976psi,85°C,1.0MKLS0, I 4 707 4.01 1.24 0.00 0.28 0.14±0.27 -6126% 8. 176psi,110°C,1.0MH SO , no m i n e r a l 176 3.87 - 0.00 0.14 0.07±0.27 _ 9. 820psi,110°C,1.0MHC10, 4 312 4.01 4.01 0.00 0.0010.27 -517% 10. 326psi,110°C,1.0MNa0H 89 3.79 3.79 0.30 0.09 0.2010.27 018% 11. 976psi,110°C,38ml 1.18M 976 1.58 1.10 0.86 0.8610.27 27130% H 2S0 4,8gFeS 2** i n H 20 # Constant volume experiments * Repeat a n a l y s i s w i t h lower background and higher gain; r e s u l t s are probably more accurate ** Constant pressure reverse t r a c e r t e s t , enriched 0 1 8 s o l u t i o n , c y l i n d e r gas. *** Standard Deviation. 33 Column two i n the t a b l e l i s t s the f i n a l pressure of the autoclave f o r the various constant volume t e s t s . The constant volume c o n d i t i o n was imposed; (25) f i r s t to i n s u r e c o n t i n u i t y between t h i s work and previous t e s t i n g ; and second to avoid the need f o r i n c o r p o r a t i n g a second pressure v e s s e l to serve as a gas r e s e r v o i r ,and a gas r e g u l a t o r i n t o the system, both of which would have increased the system volume r e q u i r i n g l a r g e r q u a n t i t i e s of oxygen per run. Column three shows the con c e n t r a t i o n of oxygen-18 i n the input gas. 18 Runs 1 and 2 were made using a c i d i f i e d d i s t i l l e d water. The minor 0 enrichment shown ( n a t u r a l abundance 0.204%) i s due to the isotope e f f e c t mentioned e a r l i e r . 18 Column four l i s t s c a l c u l a t e d values f o r the percentage of 0 i n the f i n a l s u l f a t e assuming a completely molecular or atom t r a n s f e r mechanism, i . e . 16 18 s u l f a t e formed from p y r i t e s u l f u r i s assumed to have the same 0 - 0 18 composition as the input gas. For run 11 the column represents the maximum 0 l e v e l f o r an e l e c t r o c h e m i c a l mechanism. This c a l c u l a t e d composition takes i n t o account any s u l f a t e i n the system i n i t i a l l y as H^SO^; weighing the s u l f a t e produced from the p y r i t e against the q u a n t i t y already i n the system. The 18 18 percentage of 0 i n the s u l f a t e f o r the 0 gas t e s t s , assuming a completely e l e c t r o c h e m i c a l mechanism, i s the n a t u r a l abundauce, 0.204%, s i n c e the oxygen i n t h i s case comes from water r a t h e r than from the high pressure gas.(Equation ( 2 ) ) . Over the course of a run an e l e c t r o c h e m i c a l mechanism would produce 18 some 0 water, the q u a n t i t y however i s very s m a l l when compared to the amount of n a t u r a l water i n the system and the e f f e c t can be neglected. 18 Column seven of the t a b l e l i s t s the percentage of 0 a c t u a l l y found i n the s u l f a t e samples. Values f o r s e v e r a l of the t e s t s are reported as an average of two mass spectrometer analyses. Column eig h t i n t e r p r e t s the observed .18 . 0 concentration as the p o s s i b l e extent of a molecular r e a c t i o n mechanism, 34 using the equation: 18 — 18 Measured %0 i n SO^ = X(%0 f o r complete molecular r e a c t i o n ) + 18 (1-X)(%0 f o r e l e c t r o c h e m i c a l r e a c t i o n , 0.204%) (20) where X i s the molecular f r a c t i o n of the o v e r a l l p y r i t e d i s s o l u t i o n r e a c t i o n . The a b i l i t y of the t r a c e r technique to d i s t i n g u i s h between mechanisms i s confirmed by the reverse t r a c e r t e s t . Though the e r r o r range i n the l a s t columns of the t a b l e i s r a t h e r l a r g e due to the low t r a c e r enrichment, even 18 w i t h these l i m i t a t i o n s the r e s u l t s are c o n c l u s i v e . The f a c t that a l l 0 gas t e s t s show sm a l l or even zero values f o r molecular mechanism c o n t r i b u t i o n s , i n d i c a t e s t h a t the p y r i t e d i s s o l u t i o n r e a c t i o n , under oxygen pressure l e a c h i n g , c o n d i t i o n s , i s probably t o t a l l y e l e c t r o c h e m i c a l . In no case i s the e r r o r range l a r g e enough to j u s t i f y an o x i d a t i o n mechanism which i s more than 18 27% molecular f o r any of the 0 gas t e s t s . On the other hand, an e n t i r e l y e l e c t r o c h e m i c a l model f i t s a l l the r e s u l t s . The p e r c h l o r i c a c i d run (no.9) i s p a r t i c u l a r l y important s i n c e an atom t r a n s f e r c o n t r i b u t i o n would have been observable i f i t had exceeded about 2% of the t o t a l r e a c t i o n . In IM NaOH the d i s s o l u t i o n mechanism a l s o appears to be e l e c t r o c h e m i c a l . While the s i n g l e t e s t (no.10) i s not c o n c l u s i v e i t does appear to j u s t i f y f u r t h e r e l e c t r o c h e m i c a l s t u d i e s on the higher pH d i s s o l u t i o n process. 35 I I I . ELECTROCHEMISTRY OF PYRITE PRESSURE LEACHING 1. I n t r o d u c t i o n The work presented i n the previous s e c t i o n leads d i r e c t l y to a study of p y r i t e e l e c t r o c h e m i s t r y . With the knowledge that the d i s s o l u t i o n r e a c t i o n during pressure l e a c h i n g i s e l e c t r o c h e m i c a l , i t i s necessary to d i s c a r d or r e v i s e most of the t h e o r i e s proposed by others. Even the work of Nagai and (28) K i u c h i which proposed an anodic d i s s o l u t i o n path f o l l o w e d by o x i d a t i o n of elemental s u l f u r to s u l f a t e , must be i n t e r p r e t e d to i n c l u d e an e l e c t r o -chemical route f o r s u l f a t e formation. This study was undertaken to i d e n t i f y the p r e c i s e e l e c t r o c h e m i c a l mechanism of p y r i t e d i s s o l u t i o n and to confirm such a mechanism by comparison w i t h pressure l e a c h i n g t e s t s . Along w i t h the mechanistic arguments, the behaviour of p y r i t e under anodic and cathodic p o l a r i z a t i o n was of i n t e r e s t at temperatures up to 130°C. Extension of the p y r i t e experiments to other s u l f i d e minerals such as molybdenite and c h a l c o p y r i t e was made i n the hope of p r o v i d i n g a s o l i d b a s i s f o r a general understanding of s u l f i d e m i n e r a l chemistry during o x i d a t i o n processes which lead to the formation of s u l f a t e . 2. Experimental 2.1 M a t e r i a l s P y r i t e powders prepared as described e a r l i e r were used along w i t h massive p y r i t e specimens. The massive m a t e r i a l s were part of the o r i g i n a l sample before crushing and g r i n d i n g and th e r e f o r e had the same composition as the powder. In attempting to get good mass balances i n the experimental program, i t was found that the p y r i t e contained s m a l l traces of FeS and F e n 0 o . 36 These traces d i d not a f f e c t the p o t e n t i a l s observed but d i d cause e r r o r s i n the mass balance. To e l i m i n a t e t h i s source of e r r o r the p y r i t e powders f o r the t e s t s were pre-leached f o r f i v e minutes i n b o i l i n g 6N HCI, which removed the i m p u r i t i e s . A f t e r such a pre-leach the mass balances gave good r e s u l t s and observed p o t e n t i a l s were unchanged. A l l chemicals were reagent grade and d i s t i l l e d water was used throughout the t e s t s . 2.2 Apparatus 2.2 a. Pressure V e s s e l A l a r g e c y l i n d r i c a l autoclave, 18" high x 12" i . d . , was used f o r a l l runs r e q u i r i n g pressures above 1 atm. The s h e l l was constructed by (33) Jones of 3/8" t h i c k type 304 s t a i n l e s s single-welded p l a t e f o r the w a l l s , and h" type 304 p l a t e f o r the end caps backed by a d d i t i o n a l 1" t h i c k s t e e l p l a t e s f o r e x t r a r i g i d i t y . Eight 7/8" diameter b o l t s passing through the end p l a t e s h e l d the u n i t together. Seals at the top and bottom were a f f e c t e d by seating rubber 0-rings i n grooves machined i n e i t h e r end of the c y l i n d r i c a l s h e l l w a l l . The v e s s e l was designed f o r use at 250 p s i w i t h a s a f e t y f a c t o r of f i v e . Figures 6 and 7 show the autoclave and accompanying t e s t c e l l . Removing the nuts from the bottom of the b o l t s allowed the w a l l s , b o l t s , and top cap to be r a i s e d by an overhead h o i s t . T h i s gave f r e e access t o the bottom p l a t e where a l l equipment and e l e c t r i c a l connections were placed. Three conax pressure s e a l s mounted i n the bottom p l a t e s allowed the e l e c t r i c a l connections to pass through. The autoclave was equipped w i t h two 3/4" sapphire windows placed 37 BOLTS (8) 7/8" O.D. V////////TTTT/////T7 SAPPHIRE WINDOW (2) BUBBLING INLET a PRESSURE INLET 18 12' >///// CON AX SEALS (3) / / / /-TYPE 304 /• S.S. SHELL / / / / J ~ V / Z Z 7 T 7 (tt/' in Figure 6. Pressure Vessel for Electrochemical Tests. Figure 7. Photograph of Pressure Vessel and Test C e l l . 39 approximately h a l f way up the w a l l s f o r viewing the t e s t c e l l i n s i d e . The windows were spaced at an angle of 120° from each other which allowed a l i g h t to be shone through one, w h i l e observations were made through the other. Such observations were important i n experiments where gases were bubbled through a s o l u t i o n w i t h the autoclave at high pressure. Four valved openings were a v a i l a b l e on the autoclave. Two were used f o r gas i n l e t s , one f o r general p r e s s u r i z a t i o n and the other f o r bubbling s e l e c t e d gases through the c e l l s o l u t i o n ; one was used as an e x i t to avoid b u i l d i n g up excessive pressures i n the system; and the f o u r t h , l o c a t e d i n the base p l a t e , was used as a d r a i n i n case of s p i l l s or other d i f f i c u l t i e s . In a d d i t i o n to the four valved openings, a pressure r e l e a s e u n i t was a l s o i n s t a l l e d and set to r e l e a s e i f the system pressure exceeded the 250 p s i l i m i t . Pressures i n the autoclave were monitored v i a a gauge on the top i n l e t pipe. A l l gauges on the output stages of the gas r e g u l a t o r s used were checked w i t h the autoclave gauge to i n s u r e proper c a l i b r a t i o n . T h i s was p a r t i c u l a r l y necessary f o r bubbling gases through the t e s t c e l l . 2.2 b. The Test C e l l Many of the i n i t i a l experiments were performed i n l a r g e H c e l l s constructed by j o i n i n g two 1 l i t e r B e r z e l i u s beakers w i t h a 30 mm o.d. tube, c o n t a i n i n g a f r i t t e d g l a s s d i s k . As the experimental program proceeded however, i t was found that the l a r g e volume of the H c e l l was a disadvantage i n a n a l y z i n g very d i l u t e s o l u t i o n s . Simply reducing the volume of the H c e l l 40 by using smaller beakers d i d not prove p r a c t i c a l due to the s i z e and number of e l e c t r o d e s necessary f o r the t e s t s , so a compromise was reached. A one l i t e r B e r z e l i u s beaker was used and the a u x i l i a r y e l e c t r o d e was l o c a t e d i n a 30 mm tube w i t h a f r i t t e d g l a s s end. The concern over the l o c a t i o n of the a u x i l i a r y e l e c t r o d e i n e l e c t r o c h e m i c a l t e s t s , i s to minimize i o n i c t r a n s p o r t of m a t e r i a l s from the anode to the cathode and v i c e v e r s a . In the case of i r o n i n s o l u t i o n f o r example, f e r r i c i o n may be generated at an anode, transported to the cathode by i o n i c movement, and reduced to the f e r r o u s s t a t e . The f e r r o u s i o n may then migrate back to the anode where i t i s r e o x i d i z e d to f e r r i c i o n . Any such process w i l l consume e l e c t r o n s which would normally be used i n the t e s t r e a c t i o n being observed and w i l l thus obscure the r e s u l t s . A l l e l e c t r o d e s and other equipment i n the t e s t c e l l were supported by a t i g h t f i t t i n g machined t e f l o n c e l l top. In a t y p i c a l run, the c e l l would i n c l u d e : a m i n e r a l working e l e c t r o d e , a reference e l e c t r o d e i n a Luggin C a p i l l a r y , a platinum gauze a u x i l i a r y e l e ctrode i n a f r i t t e d g l a s s tube, a gas d i s p e r s i o n tube, a 410 watt Glo-quartz heater, and a t h e r m i s t o r probe f o r the temperature c o n t r o l l e r . Magnetic s t i r r i n g was used i n a l l t e s t s . F i g u r e 8 shows the t y p i c a l c e l l c o n f i g u r a t i o n . 2.2 c. Reference E l e c t r o d e The Ag/AgCl system was used as the reference e l e c t r o d e i n a l l runs. (34) E l e c t r o d e s were e a s i l y constructed using the "thermal" method of Rule and LaMer A 1/8" o.d. s p i r a l of 0.015" or 0.020" platinum w i r e , which had been cleaned i n hot n i t r i c a c i d then washed, was coated w i t h a paste made of seven p a r t s 41 CURRENT LEAD OUTLET INLET POTENTIAL LEAD REE i. THER-fln am. HEATER AUXILLARY ELECTRODE III! IIII •III II I I • I I I • I I I I I I I I I I I I I • I I I I I I •d-LJ Figure 8. Typical Cell Configuration. 42 s i l v e r oxide (AgO) and one part s i l v e r c h l o r a t e (AgClO^) i n water. The s p i r a l was then f i r e d i n an e l e c t r i c furnace at 650°C f o r f i v e minutes. This procedure was repeated twice f o r a t o t a l of three f i r i n g s . S e v e r a l such elec t r o d e s gave p o t e n t i a l s w i t h i n 0.25 mv. The goal of the e l e c t r o c h e m i c a l work was to study s u l f i d e minerals under a c i d c o n d i t i o n s . The major work was centered on IM s u l f u r i c a c i d , H^SO^, or a n e a r l y equivalent IM HC10^. To minimize the l i q u i d j u n c t i o n p o t e n t i a l between the reference e l e c t r o d e s o l u t i o n i n the Luggin c a p i l l a r y and the t e s t s o l u t i o n , i t was decided to use IM a c i d i n the reference e l e c t r o d e plus an appropriate c h l o r i d e concentration to prevent d i s s o l u t i o n of the Ag/AgCl mixture. I n i t i a l l y 0.01M CI was chosen but l a t e r 0.1M CI was s u b s t i t u t e d . In both cases, the reference e l e c t r o d e was c a l i b r a t e d at 25°C versus a standard hydrogen e l e c t r o d e (SHE). The c a l i b r a t i o n allowed the a c t i v i t y c o e f f i c i e n t f o r c h l o r i d e i o n to be determined as 0.798. For c a l i b r a t i o n of the el e c t r o d e at higher temperatures, the measured a c t i v i t y c o e f f i c i e n t was incorporated i n t o a Nernst equation using E° values from the (35) work of Greeley . The Greeley study r e p o r t s values f o r the Ag/AgCl system against the hydrogen e l e c t r o d e at the same temperature as a f u n c t i o n temperature. Figure 9 shows the temperature e f f e c t on the Ag/AgCl reference e l e c t r o d e p o t e n t i a l at 0.01 and 0.1M HCI. The upper curve i n each case incorporates a Debye-Huckel c o r r e c t i o n f o r temperature on the c h l o r i d e (36) a c t i v i t y as explained by B o c k r i s and Reddy (see Appendix A f o r more d e t a i l e d d i s c u s s i o n ) and i s the source of temperature-potential i n f o r m a t i o n f o r a l l t e s t s conducted. The p o t e n t i a l s reported are i n terms of the hydrogen el e c t r o d e p o t e n t i a l at the same temperature denoted by SHET.The measured Ag/AgCl p o t e n t i a l s were found .to be v i r t u a l l y i d e n t i c a l i n both s u l f a t e and pe r c h l o r a t e s o l u t i o n s . 0 . 3 5 0 F T E M P E R A T U R E ( ° C ) Figure 9. Potential of the Ag/AgCl Electrode, as a Function o f Temperature. 44 Throughout the experimental program,at the c o n c l u s i o n of a run^ the reference e l e c t r o d e was removed from the c e l l , s t i l l i n the Luggin c a p i l l a r y , and checked against a second Ag/AgCl e l e c t r o d e i n f r e s h s o l u t i o n . This allowed an e s t i m a t i o n of the d r i f t over the course of a run which was l e s s than 5 mV i n most cases. O v e r a l l the r e s u l t s of the t e s t i n g program are b e l i e v e d to l i e w i t h i n a p r e c i s i o n of± 5mV. 2.2 d. S u l f i d e M i n e r a l Electrodes (33) In h i s work on c h a l c o p y r i t e , Jones t r i e d a number of e l e c t r o d e designs. He s e t t l e d on an epoxy encased mineral sample exposed to the s o l u t i o n on one face and connected to the e l e c t r o n i c equipment by two platinum leads which were press f i t i n t o holes d r i l l e d i n the back of the s u l f i d e before being sealed i n the epoxy. A s i m i l a r c o n f i g u r a t i o n was t r i e d f o r p y r i t e at the s t a r t of the present work but ran i n t o two major d i f f i c u l t i e s . F i r s t , p y r i t e i s a hard b r i t t l e mineral which made d r i l l i n g holes d i f f i c u l t , and second p y r i t e e x h i b i t s an appreciable point contact r e s i s t a n c e . I t was the second problem which mandated a new method f o r a t t a c h i n g the platinum leads to the sample sin c e the r e s i s t a n c e s upwards of 50 ft caused l a r g e voltage e r r o r s even at very low c u r r e n t s . The f i n a l s o l i d e l e c t r o d e design i n v o l v e d s p u t t e r i n g a t h i n l a y e r of gold over the back of a p y r i t e specimen approximately 0.5 cm t h i c k . During the s p u t t e r i n g operation p a r t of the mineral surface was masked to give two separate f i e l d s of gold covered p y r i t e . Two t e f l o n covered platinum leads w i t h s m a l l pieces of platinum f o i l attached (^  2 mm square, impact j o i n e d ) were then cemented to the gold covered p y r i t e w i t h a conductive s i l v e r based epoxy (Aremco-Bond 525) . One l e a d was used f o r p o t e n t i a l measure-ment while the other c a r r i e d the current. A f t e r c u r i n g at 140°C f o r 2 hours 45 the sample was then encased i n a 5:1 mixture of EPON 828 and S h e l l Curing Agent Z. Further c u r i n g at 80°C f o r four hours produced a strong c h e m i c a l l y i n e r t epoxy s h e l l . The face of the m i n e r a l was then exposed by g r i n d i n g to 240 g r i t paper. Figure 10 shows the e l e c t r o d e design. Resistance of p y r i t e samples prepared using t h i s method i s very low. Tests i n s o l u t i o n p l o t t i n g p o t e n t i a l versus current y i e l d s values of about 0.5 Q f o r the r e s i s t a n c e . This i s a reasonable v a l u e s i n c e most work was done at c u r r e n t s of l e s s than 10 mA. The IR e r r o r i s then l e s s than 5 mV. For t e s t s on p y r i t e p a r t i c l e l e a c h i n g , a t e f l o n and platinum holder was designed (see Figure 11). A sample of p y r i t e powder was placed i n a shallow platinum c r u c i b l e and a platinum gauze was secured on top by a t e f l o n r i n g assembly. One lead was provided f o r measuring or applying p o t e n t i a l s . 2.2 e. Instrumentation The p o t e n t i o s t a t used throughout t h i s work was a Wenking 70HV1/90 w i t h a one amp output at a maximum of 90V through the c i r c u i t . P o t e n t i a l s were measured w i t h a Wenking PPT69 p r e c i s i o n potentiometer. Scanning experiments were conducted using a Wenking SMP66 step motor potentiometer. The number of coulombs passed i n an experiment was monitored by a V e r i t e c h coulometer and current or p o t e n t i a l was recorded by a Sargent SR recorder. Heating of the experimental c e l l was done by a 410 watt "Glo-Quartz" imersion heater c o n t r o l l e d through a thermistor by a YSI Model 71 temperature c o n t r o l l e r . A v a r i a c i n s e r i e s w i t h the c o n t r o l l e r l i m i t e d the output of the heater to minimize temperature f l u c t u a t i o n s . Figure 10. E l e c t r o d e Design f o r Massive Specimens. Pt L E A D P Y R E X T U B E Pt G A U Z E 1 \ t U~ ~"(~~~~ U M E F L O N H O L D E R Pt C R U C I B L E Figure 11. Electrode Design for Powder Specimens. 48 2.2 f. A n a l y s i s Iron a n a l y s i s was performed using an atomic abs o r p t i o n spectrophoto-meter. S u l f a t e determinations were made by the standard barium c h l o r i d e p r e c i p i t a t i o n technique. 2.3 Schematic of C i r c u i t s The o v e r a l l schematic f o r a four lead e l e c t r o c h e m i c a l t e s t i n g c i r c u i t i s shown i n Figure Li. A four lead system i s used to reduce IR drop through the sample i n the p o t e n t i a l measuring c i r c u i t . Since the 13 r e s i s t a n c e of the voltmeter i s on the order of 10 fi, current f l o w i n g through the measuring c i r c u i t w i l l be very s m a l l . U n f o r t u n a t e l y the separation between the current c a r r y i n g and p o t e n t i a l measuring c i r c u i t s i s not complete and a f r a c t i o n , 3, of the t o t a l IR, through the specimen i s seen by the voltmeter. The system a l s o removes any c o n t r i b u t i o n due to the semi-conductor impedence from the sensed p o t e n t i a l . Throughout the t e s t i n g program a n . e f f o r t was made t o e l i m i n a t e s t r a y i n t e r f e r e n c e which might obscure t e s t r e s u l t s . The autoclave s h e l l was grounded and t e s t s of the e f f e c t of the imersion heater and magnetic s t i r r e r s on the p y r i t e e l e c t r o d e s were made using an o s c i l l o s c o p e . Induced AC r i p p l e on the measuring c i r c u i t was found to be l e s s than 1.5mV peak to peak i n a l l cases, and u s u a l l y l e s s than 0.5 mV. 49 E L E C T R O C H E M I C A L C E L L A Amme te r B D i r e c t Cu r r e n t S o u r c e C Po te n tio sta tic C o n t r o l l e r D D. C. C u r r ent Contr ol L Luggin C a p i l l a r y R R e f e r e n c e E l e c t r o d e S Switch U A u x i l i a r y E l e c t r o d e and (optional) container V High Impedence V o l t m e t e r W W orking E l e c t r o d e (Sulphide mounted in epoxy) •B H I B Rn I A / W W W V H l -vvwyW Z 2 R| Z ( •u R II I w E Q U I V A L E N T C I R C U I T E g Voltage of D. C. Source E/\ P o t e n t i a l of R e f e r e n c e E l e c t r o d e Eyy P o t e n t i a l of Working " ; i E l e c t r o d e EJJ Potential of A u x i l i a r y E l e c t r o d e Z| Impedence of E l e c t r o d e -E l e c t r o l y t e Interface Zg Impedence of Semiconducting Connection to C i r c u i t Rj R e s i s t a n c e of E l e c t r o d e /S F r a c t i o n of IR^ i n c o r p o r a t e d into voltage m easurement R^ R e s i s t a n c e of E l e c t r o l y t e Q F r a c t i o n of IR^ i n c o r p o r a t e d into voltage measurement R « R e s i s t a n c e of Insulating f i l m Figure 12. Schematic of E l e c t r i c a l C i r c u i t . A f t e r Peters 50 3. R e s u l t s and D i s c u s s i o n 3.1 Mixed P o t e n t i a l Measurement The oxygen-18 t r a c e r work on p y r i t e shows that an e l e c t r o c h e m i c a l mechanism i s o p e r a t i o n a l i n the d i s s o l u t i o n r e a c t i o n . The f i r s t step necessary i n the d e t a i l e d study of such a mechanism i s to examine the mixed p o t e n t i a l during oxygen pressure l e a c h i n g . A three or four e l e c t r o d e c e l l c o n f i g u r a t i o n was chosen which in c l u d e d : two m i n e r a l e l e c t r o d e s , one massive and the other a powder i n the platinum h o l d e r , the Ag/AgCl reference e l e c t r o d e , and a platinum gauze which was omitted i n some runs. The platinum gauze e l e c t r o d e was included to measure the p o t e n t i a l of the f e r r i c - f e r r o u s couple and thus the r a t i o of the two i o n i c i r o n species. The c e l l w i t h the v a r i o u s e l e c t r o d e s was f i l l e d w i t h the e l e c t r o l y t e (IM HC10 4)at the s t a r t of a t e s t and helium was bubbled c o n s t a n t l y w h i l e the system was brought up to temperature and pressure. Once the d e s i r e d experimental c o n d i t i o n s were reached,the helium flow was replaced by oxygen and the r e s u l t i n g mixed p o t e n t i a l s recorded. Figure 13 shows the e f f e c t of oxygen pressure on the mixed p o t e n t i a l at 85°,110°,and 130°C. The mineral e l e c t r o d e s both gave the same p o t e n t i a l w i t h i n 2 or 3 mV. i n each case. The upper curve i n the f i g u r e shows the p o t e n t i a l of the f e r r i c - f e r r o u s couple at 110°C. The d i f f e r e n c e between the mineral p o t e n t i a l and that observed on the platinum sensing e l e c t r o d e i s a t t r i b u t e d to homogeneous o x i d a t i o n of f e r r o u s i o n t o f e r r i c by molecular oxygen i n the s o l u t i o n . I n a l l cases:, an i n c r e a s e i n the p a r t i a l pressure of oxygen on the , system r a i s e s the mixed p o t e n t i a l f o r d i s s o l u t i o n . I t i s a l s o noted that the slope of the mixed p o t e n t i a l vs oxygen p a r t i a l pressure curve increases w i t h 51 0.9 _ 0 . 8 t-LU X (O >-0.7 UJ 0.6h 0.5 • • • T ? • e e o • © o - ° ° • • e • o ° * I IO°CFe+ 3 /Fe +2 e • ) e I30°C FeS 2 o I I0°C FeS 2 i o 8 5 ° C F e S 2 » • » • 50 100 150 200 250 OXYGEN PARTIAL PRESSURE (PSI) Figure 13. P y r i t e Mixed P o t e n t i a l as a Function of Oxvgen Partial Pressure 1 M HC10,. 52 i n c r e a s i n g temperature. The s i g n i f i c a n c e of t h i s e f f e c t w i l l be discussed i n terms of an e l e c t r o c h e m i c a l model i n a l a t e r s e c t i o n . 3.2 Anodic D i s s o l u t i o n With the oxygen-18 t r a c e r work and the evidence that the mixed p o t e n t i a l of l e a c h i n g p y r i t e i s a f u n c t i o n of the oxygen p a r t i a l p r e s s u r e ) an e l e c t r o c h e m i c a l model f o r the d i s s o l u t i o n r e a c t i o n seemed promising. U n f o r t u n a t e l y , no previous author had been able to c o r r e l a t e e l e c t r o c h e m i c a l s t u d i e s w i t h a c t u a l pressure l e a c h i n g r e s u l t s . The s u l f a t e and elemental s u l f u r forming processes had been observed s e p a r a t e l y (eq. 6 and 17) but the composition produced i n l e a c h i n g w i t h oxygen had never been d u p l i c a t e d . To o b t a i n c o n c l u s i v e proof of an anodic d i s s o l u t i o n mechanism a s e r i e s of t e s t s wo?. performed i n which the p y r i t e l e a c h i n g p o t e n t i a l was p o t e n t i o s t a t i c a l l y c o n t r o l l e d . In such a t e s t , the e l e c t r o n s from the anodic r e a c t i o n are removed v i a an e x t e r n a l c i r c u i t r a t h e r than r e a c t i n g on the p y r i t e surface i n a cathodic r e a c t i o n such as the r e d u c t i o n of oxygen. P y r i t e powder samples(0.5g, -200 +270 mesh) were anodized at 110°C f o r a s u f f i c i e n t time to produce analyzable samples f o r i r o n and s u l f a t e (up to 24 hours depending on the p o t e n t i a l ) . The charge passed through the powder was a l s o recorded. The experimental c o n d i t i o n s were kept the same as those f o r the mixed p o t e n t i a l measurements w i t h the exception that helium was c o n s t a n t l y bubbled i n s t e a d of oxygen. Results of the t e s t s are shown i n Table V I I I . At 0.699V the production of s u l f a t e i s estimated to be 50 to 60% depending on the choice of data; 50% from coulombs passed per equivalent of i r o n found, and 60% from s u l f a t e found per equivalent of i r o n . These values compare f a v o r a b l y w i t h a 57% s u l f a t e (25) value observed by the author i n a previous work on oxygen pressure l e a c h i n g 53 Table V I I I E f f e c t of P o t e n t i a l on S u l f a t e Formation 0.699 Potential(SHET) - V o l t 1.000 0.800 0.900 T o t a l S u l f a t e gm .0764 .0684 .0907 .361 moles 7.91x10" -4. 7.12xl0~ 4 9.44x10" •4 3.75x10" -3 T o t a l Iron gm .0373 .0315 .039 .116 moles 6.68x10" -4 5.64xl0" 4 6.98x10" •4 2.08x10" •3 Run Duration,hr. 24.0 4.7 5.0 27.7 T o t a l charge Coulombs 517 529 707 2694 equiv. 5.36x10" •3 5.48xl0~ 3 7.33x10" 3 2.79x10" 2 Charge/mole Fe 8.02 9.72 10.50 13.4 % S u l f a t e Y i e l d ( c a l c ) 50.2% 56.0% 62.5 86.7 S u l f a t e / i r o n ( m o l a r r a t i o ) 1.192 1.262 1.352 1.803 % S u l f a t e Yield(meas) 59.6% 63.1% 67.6% 90.2% 54 at 110°C and 176 p s i 0^- At p o t e n t i a l s higher than 0.699V the t e s t s show i n c r e a s i n g production of s u l f a t e . At 1.000V e s s e n t i a l l y a l l the p y r i t e s u l f u r i s r e a c t i n g to produce s u l f a t e by equation (6). While elemental s u l f u r was not analyzed q u a n t i t a t i v e l y i t s e x i s t e n c e was confirmed by X-ray d i f f r a c t i o n on the t e s t r e s i d u e s . The d i f f r a c t i o n p a t t e r n i s reported i n Appendix B. 3.3 An E l e c t r o c h e m i c a l Mechanism 3.3 a. General I t appears that the p a r t i t i o n i n g of p y r i t e s u l f u r between s u l f a t e and elemental s u l f u r i s a r e s u l t of two competing anodic r e a c t i o n s shown by equations (17) and (18), the r a t e s of which are i n t u r n c o n t r o l l e d by the p y r i t e p o t e n t i a l . This can e x p l a i n the wide v a r i a t i o n of r e a c t i o n products observed by other i n v e s t i g a t o r s w i t h changes i n reactant concentrations and the i d e n t i t y of the oxidant. In oxygen pressure l e a c h i n g , the i n i t i a l cathodic r e a c t i o n i s the redu c t i o n of oxygen (equation ( 2 ) ) . As p y r i t e i s d i s s o l v e d , the concentration of i r o n species i n s o l u t i o n b u i l d s up, and may complicate matters. The i r o n going i n t o s o l u t i o n has a f i x e d f e r r i c / f e r r o u s r a t i o c o n t r o l l e d by the mixed p o t e n t i a l of the leach. Once i n s o l u t i o n however, the homogeneous o x i d a t i o n of fe r r o u s to f e r r i c by oxygen begins to take p l a c e : F e + 2 + H + + V0 2 - — ' F e + 3 + ^ 0 (21) The f e r r i c i o n formed may then migrate back to the m i n e r a l s u r f a c e and take part i n the p y r i t e d i s s o l u t i o n i n the cathodic r e a c t i o n : F e + 3 + e~ — • F e + 2 (22) 55 I f the i r o n c o n c e n t r a t i o n i n s o l u t i o n i s h i g h , the cathodic r e d u c t i o n to the ferrous s t a t e may a c t u a l l y r a i s e the mixed p o t e n t i a l f o r le a c h i n g and increase the r e a c t i o n r a t e , a l t e r i n g the products a c c o r d i n g l y . 3.3 b. Elemental S u l f u r Y i e l d The formation of elemental s u l f u r has not been observed to exceed 50% i n oxygen pressure l e a c h i n g . When the molar volumes of p y r i t e and elemental s u l f u r are compared, the reason becomes more obvious. At 110°C the P i l l i n g -Bedworth r a t i o (volume of s o l i d products/volume of r e a c t i n g mineral) becomes greater than 1.0 i f l e s s than about 30% of the p y r i t i c s u l f u r i s o x i d i z e d to s u l f a t e ( C a l c u l a t i o n s f o r t h i s f i g u r e are shown i n Appendix C). That i s , i f more than about 70% of the p y r i t e s u l f u r stays on the mi n e r a l surface i n the elemental form^a p r o t e c t i v e l a y e r w i l l be formed. For l e a c h i n g c o n d i t i o n s which produce more than 30% s u l f a t e from the p y r i t i c s u l f u r the products of the r e a c t i o n w i l l have voids or pores which allow f u r t h e r d i s s o l u t i o n to proceed. From t h i s argument i t i s evident that changes i n the mixed p o t e n t i a l of l e a c h i n g w i l l have a marked e f f e c t on the r a t e of r e a c t i o n by c o n t r o l l i n g the product composition. Leaching at p o t e n t i a l s near 1.0V produces almost e n t i r e l y s u l f a t e which keeps the mi n e r a l surface c l e a r f o r f u r t h e r r e a c t i o n . At lower p o t e n t i a l s however, the product s u l f u r covers a p o r t i o n of the d i s s o l u t i o n s i t e s , slowing the r a t e . The extreme case of complete p r o t e c t i o n of the m i n e r a l s u r f a c e by elemental s u l f u r , f o r c e s the d i s s o l u t i o n of p y r i t e to be c o n t r o l l e d by d i f f u s i o n of the i r o n species through the s u l f u r l a y e r . The three cases are shown d i a g r a m a t i c a l l y i n Figure 14. No Elemental S u l f u r Y i e l d B A R E M I N E R A L < 70% Elemental S u l f u r Y i e l d > 70% Elemental S u l f u r Y i e l d Figure 14 E f f e c t of Products on the P y r i t e Surface. 57 3.3 c. Rest P o t e n t i a l As mentioned e a r l i e r i n the text, the r e s t p o t e n t i a l of p y r i t e i s thought to be the r e s u l t , o f a mixed p o t e n t i a l e f f e c t . Extension of the s u l f u r f i l m concept j u s t described helps to c l a r i f y the s i t u a t i o n . The measured values of the r e s t p o t e n t i a l f a l l i n a range where the m i n e r a l w i l l be r e a c t i n g t o produce predominantly elemental s u l f u r . Thus, a slow r e a c t i o n of p y r i t e accompanied by d i f f u s i o n of f e r r o u s i o n through the s u l f u r l a y e r corresponds w e l l w i t h the observed p o t e n t i a l value. (22) The work of Peters and Majima i n combination w i t h the Eh-pH diagram f o r p y r i t e gives support to the s u l f u r f i l m concept. Figure 15 taken from the Peters work shows the p y r i t e r e s t p o t e n t i a l observed a f t e r making "cathodic excursions" to v a r i o u s p o t e n t i a l s , then a l l o w i n g the e l e c t r o d e to stand f o r 10 minutes. The scans were made at 240 mV/min., i n a 25°C s o l u t i o n c o n s t a n t l y bubbled w i t h helium. I t i s i n t e r e s t i n g to note that excursions past about 0.1V r e s u l t i n l a r g e negative changes i n the r e s t p o t e n t i a l . The r e s t p o t e n t i a l value d e c l i n e s u n t i l i t reaches about 0.25V. I f the Eh-pH diagram i s examined i n c l u d i n g the S ^ ^ S e q u i l i b r i u m l i n e (see F i g ure 16), the p o i n t at which elemental s u l f u r becomes unstable w i t h respect to H^S i s about 0.15 V at pH 0. I t i s reasonable to assume, t h e r e f o r e , that the p a s s i v i t y of p y r i t e i s due to a p r o t e c t i v e f i l m of elemental s u l f u r which d i s s o l v e s c a t h o d i c a l l y v i a the r e a c t i o n : S° + 2H + + 2e~ — • H 2S (23) when the p o t e n t i a l f a l l s below 0.15V. S t r i p p i n g o f f the f i l m would leave an a c t i v e surface w i t h a p o t e n t i a l c l o s e r to the p r e d i c t e d thermodynamic r e s t p o t e n t i a l . In the region between 0.15 and 0.02V, the f i l m w i l l d i s s o l v e but 58 59 Figure 16. Eh-pH Diagram fog the Fe-S-H 0 System d e t a i l i n g S u l f u r S t a b i l i t y . 25°C, a F e+2= 10 » a H s =10 M, other species u n i t a c t i v i t y . . 2 aq 60 p y r i t e remains s t a b l e . The thickness of such a s u l f u r f i l m would only need to be a few atomic distances to b lock the s urface and change the r e s t p o t e n t i a l . I f the f i l m d i d not exceed about 10 atomic l a y e r s , i t could remain e l e c t r i c a l l y conductive. The f i l m might even reach a steady s t a t e where formation of s u l f u r i s balanced by r e a c t i o n s forming s u l f a t e such as: S° + 4H 20 » 2HS0~ + 6H + + 4e~ (24) The h i g h p y r i t e r e s t p o t e n t i a l may be a combination of the two anodic r e a c t i o n s , one producing and the other consuming s u l f u r coupled to a c a thodic r e a c t i o n such as oxygen r e d u c t i o n (eq.(2)). 3.3 d. A p p l i c a b i l i t y of the E l e c t r o c h e m i c a l Model to A c i d Consumption Data (25) In e a r l i e r work by B a i l e y , i t was found that the i n i t i a l a c i d c o ncentration i n l e a c h i n g t e s t s played a l a r g e r o l e i n determining the f i n a l d i s t r i b u t i o n of p y r i t i c s u l f u r between elemental s u l f u r and s u l f a t e . . I t was found that i n d i l u t e s u l f u r i c a c i d p y r i t e r e a c t s to produce more a c i d w h i l e at high i n i t i a l concentrations a c i d i s consumed i n the l e a c h i n g process. In performing the oxygen-18 t r a c e r t e s t s a d d i t i o n a l data on the subject became a v a i l a b l e . These data are reported along w i t h the older m a t e r i a l i n Table IX and F i g u r e l 7 . The a c i d consumption i s reported as: 2 + x-2y from equation (14): FeS 2 + (h + |y + -|x)0 2 +(2 + x - 2y)H + • ( l - x ) F e + + + x F e + 3 + (2 - y)S° + ySO^ + (1 - y + -|x)H20 (14) Since the value of x i s n e a r l y 1.0 we can s i m p l i f y the a c i d r e l a t i o n s h i p to (3-2y)H . S o l v i n g f o r the value of y at which no hydrogen i o n i s consumed or produced, y = 1.5, i . e . the r e a c t i o n produces no net change i n a c i d i f 75% of the p y r i t e s u l f u r i s o x i d i z e d to s u l f a t e . I n t e r p o l a t i o n of the data i n Table IX shows that the value of y i s 1.5 at about 0.17M a c i d concentration. 61 Table IX +3 -2 Fe and SO^ Y i e l d s at Various I n i t i a l A c i d i t i e s I n i t i a l A c i d Concentration X %S0. 4 y %Fe ex t r a c t e d 0.01 MH„S0. 2 4 0.925 88.9 1.778 66.16* 0.1 0.896 80.0 1.599 77.88* 0.2 0.923 72.1 1.443 88.04 0.4 0.911 68.9 1.377 91.23 1.0 0.898 66.9 1.338 83.72 3.0 0.894 63.7 1.273 69.77 * 5 hr runs as compared to normal 6 hours. +3 x = f r a c t i o n of Fe present as Fe . y = moles of SO. per mole FeS decomposed. 0.4 -0.81 1 1 1 «— - 2 - 1 0 I L O G (INITIAL H-rSOA. M ) Figure 17 Acid Consumption vs. l o g ( i n i t i a l acid concentration) 63 In terms of the e l e c t r o c h e m i c a l model, equation (14) can be understood as the combination of an a n o d i c r e a c t i o n : FeS 2 + 4yH 20—»• F e + 3 + (2-y)S° + ySO^ + 8yH + + (3+6y)e~ (24) and a cathodic r e a c t i o n : (0.75 + 1.5y)0 2 + (3+6y)H + + (3+6y)e~—• (1.5+3y)H 20 (25) assuming that x has the value of 1.0. Breaking the a c i d consumption mechanism down f u r t h e r r e q u i r e s examining the s u l f a t e and elemental s u l f u r producing r e a c t i o n s . As shown above, a c i d consumption i s zero at about 75% s u l f a t e y i e l d from p y r i t i c s u l f u r . To produce 75% s u l f a t e , i n t e r p o l a t i o n of Table V I I I shows that a p o t e n t i a l of greater than 0.9V i s re q u i r e d . The p y r i t e d i s s o l u t i o n r e a c t i o n must, t h e r e f o r e , be w r i t t e n to produce f e r r i c i o n s . Assuming one mole of p y r i t e i s being d i s s o l v e d , one f o u r t h w i l l react to form elemental s u l f u r and three f o u r t h s to form s u l f a t e . The anodic r e a c t i o n s w i t h the accompanying cathodic processes are: 1/4 FeS2->- 1/4 F e + 3 + 1/2 S° + 3/4 e" (26) 3/16 0 2 + 3/4 H + + 3/4 e" —*• 3/8 H 20 (27) f o r elemental s u l f u r , and: 3/4 FeS 2 + 6H 20 —»• 3/4 F e + 3 + 3/2 S0^ + 12H + + |^ e" (28) II 0 2 + 45/4 H + + 45/4 e" —*• 45/8 H 20 (29) f o r s u l f a t e production. The net consumption of 3/4 H + i n the elemental s u l f u r r e a c t i o n s i s balanced by the net production of 3/4 H + i n the s u l f a t e r e a c t i o n s . The proposed r e a c t i o n mechanism i s thus capable of e x p l a i n i n g the observed leach i n g r e s u l t s . For cases where the i n i t i a l a c i d c o n c e n t r a t i o n i s low, the mixed p o t e n t i a l f o r l e a c h i n g r e s u l t s i n a dominance of the s u l f a t e forming r e a c t i o n s , y i e l d i n g a net increase i n a c i d c o n c e n t r a t i o n . The reverse e f f e c t , 64 reducing the a c i d l e v e l i s observed f o r s u l f u r p r o d u c t i o n , and occurs at high i n i t i a l a c i d l e v e l s . Since the r a t e s of r e a c t i o n are r e l a t i v e l y slow i n p y r i t e d i s s o l u t i o n the a c i d c o n c e n t r a t i o n at the surface w i l l not vary s i g n i f i c a n t l y . The hydrogen i o n m o b i l i t y i s so much greater than any other i o n i c species that development of an a c i d concentration gradient would r e q u i r e a much l a r g e r r e a c t i o n r a t e . 3.3 e. Mathematical Expression of the E l e c t r o c h e m i c a l Mechanism An o v e r a l l mechanism based on p y r i t e e l e c t r o c h e m i s t r y has been presented that i s c o n s i s t e n t w i t h present and previous experimental work. There are a t o t a l of four a p p l i c a b l e equations: (a) FeS 2 + 4yH 20 — • F e + 2 + yS0~ 2+ (2-y)S° + 8 y H + + (2+6y)e~* (30) (b) Cathodic r e d u c t i o n of oxidants: 0 2 + 4H + + 4 e ~ — • 2H 20 (2) +3 +2 Fe + e — • Fe (22) (c) Homogenous r e a c t i o n : 4 F e + 2 + 0 2 + 4H + — • 2^0 + 4 F e + 3 (31) In the absence of d i s s o l v e d i r o n , i . e . i n i t i a l c o n d i t i o n s , only equations(30) and (2) need to be considered. Equation (22) would i n c r e a s e the t o t a l r a t e of Equation (30) by r a i s i n g the mixed p o t e n t i a l on the p y r i t e * The occuirance of f e r r i c as a product i n r e a c t i o n (30) can i n f a c t be incorporated i n t o r e a c t i o n (31) w i t h the r e c o g n i t i o n that t h i s i s a heteugeneous c o n t r i b u t i o n to the otherwise homogeneous r e a c t i o n . 65 s u r f a c e . * Equation (30) can, i n p r i n c i p l e , be subdivided according to the f i n a l s u l f u r form, i . e . I | _ FeS 2 —»• Fe + 2S° + .2e (30a) FeS 2 + 8H 20 — • Fe 4 4" + 2S0^ + 16H + + 14e~ (30b) However, as shown e a r l i e r equation (30a) would lead to an increase i n volume of s o l i d s when more than about 70% of the p y r i t e reacted v i a that route. The increased volume would i n h i b i t f u r t h e r r e a c t i o n . Rather than attempting to model t h i s s i t u a t i o n i t seems more reasonable t o confine the i n v e s t i g a t i o n to more usu a l l e a c h i n g c o n d i t i o n s where l e s s than 70% of the s u l f u r i s going to the elemental form. Under such c o n d i t i o n s we can recombine equations (30a) and (30b) i n t o equation (30) and make y(> 0.3), the parameter i d e n t i f y i n g the number of moles of s u l f a t e y i e l d e d from one mole p y r i t e , a fu n c t i o n of p o t e n t i a l (or more d i r e c t l y , of current d e n s i t y ) . Reactions (30) and (2) can be sc h e m a t i c a l l y w r i t t e n i n the form of T a f e l equations, assuming p o l a r i z a t i o n to be a c t i v a t i o n determined:** E30 - E30 = a30 + b30 1 0 8 So ( 3 2 ) E , - E\ = a 2 - b 2 l o g i 2 (33) r r where E^Q and E 2 are r e v e r s i b l e p o t e n t i a l s of the s u b s c r i p t equations, a^Q* a 2 ' ^30 a n (^ ^ 2 a r e ^ a ^ e ^ P a r a m e t e r s > a n < i ^30' "*"2 a r e n e t c u r r e n t d e n s i t i e s at p o t e n t i a l s E^Q> E 2 r e s p e c t i v e l y . . The negative s i g n i n equation(33) r e s u l t s from a s s i g n i n g b 2 a p o s i t i v e value. These equations can be combined i f no other e l e c t r i c a l or e l e c t r o c h e m i c a l processes are operative by making m 30 2 c 30 2 * The le a c h i n g r a t e on p y r i t e d i s p e r s i o n s at constant pressure does not drop o f f w i t h decreasing s i z e ( i n c r e a s i n g time) u n t i l the mi n e r a l i s more than 80% decomposed. Thus c o n t r i b u t i o n from Fe+++ may be the o f f s e t t i n g e f f e c t a r i s i n g from decreasing p y r i t e surface area w i t h time. ** For a b r i e f d e s c r i p t i o n of a c t i v a t i o n p o l a r i z a t i o n see Appendix D. 66 108 ^ =4 " " a 3 0 ] " t 2 [~\ + E2 + a 2 ] ( 3 4 ) ^ ^ ( W <35) The net cathodic (oxygen reduction) r e a c t i o n i s then found by s u b s t i t u t i n g equation (35) i n t o equation (34), v i z : l QS K = b-Tb^ [ " E30 + E 2 " a30 + al) ( 3 6 ) The oxygen pressure a f f e c t s the mixed p o t e n t i a l E^, as shown i n Figure 13. This e f f e c t must be exerted through the T a f e l term a 2 of equation (36). I t i s necessary to w r i t e i n more fundamental terms f o r t h i s purpose, and t h i s can be done by assuming the T a f e l equation to be an e m p i r i c a l form of the Butler-Volmer equation, which should be, f o r oxygen discharge: Log i = l o g 1° + l o g f + n 2 3 2 F (37) 2 2 °2 2T3RT where 1% i s the exchange current d e n s i t y at standard c o n d i t i o n s , f- i s the 2. U2 surface a c t i v i t y of oxygen, 3^ 1 S a symmetry f a c t o r f o r the p o t e n t i a l energy r b a r r i e r , F i s the Faraday, and n 0 i s the o v e r p o t e n t i a l (n = - i n equation (33)). By analogy w i t h equation (33), i . e . a 2 n2 l o g i = " — (38) 2 b 2 b 2 -2.3RT w h e r e b 2 = —&^r The negative s i g n r e s u l t i n g from the p o s i t i v e b 2 convention. %t i s evident that a 2 = b 2 l o g i° + b 2 l o g f Q (39) S u b s t i t u t i n g t h i s i n t o equation(36)leads to the oxygen dependence on r a t e : l o g r a t e = l o g = b ~ | ^ + E* - a 3 Q + b 2 l o g i° + b £ l o g f^  (40) 67 Combining the constants E^ Q, E^, a 3 Q and b 2 l o g i° together, we o b t a i n the l o g - l o g r e l a t i o n s h i p l o g r a t e = D + u \ , l o g f 0 ? <41> b 2 + b30 2 ^ e r e D = E 2 " E30 + b 2 l o g *2 ' a30 b30 + b 2 I f the form of f_ i s l i n e a r w i t h oxygen p a r t i a l p r essure, the l o g - l o g 2 b 2 p l o t should have a slope of — - — . In f a c t , the p l o t i s curved, as b l l b30 shown i n F i g . 18*. The high pressure slope i s c l o s e to i n d i c a t i n g that b^ and b ^ may be approximately equal i n t h i s region. A l t e r n a t i v e l y , i f i s s u b s t a n t i a l l y l a r g e r than b^Qj a higher slope would r e s u l t , and the curvature to a lower slope may r e f l e c t a n o n - l i n e a r i t y of f w i t h oxygen u 2 pressure. To o b t a i n b^ and b^ values p o l a r i z a t i o n curves were drawn from e x p e r i -mental measurements. Figure 19 shows the anodic curve f o r p y r i t e at 110°C i n l M H C l O ^ . The p o t e n t i a l scan was made at 100 mV/min under 50 p s i He. The value f o r b^ Q taken from t h i s curve i s 0.133 which compares f a v o r a b l y w i t h a v a l u e of 0.10 (37) obtained by B i e g l e r at 25°C. In a c t u a l f a c t the curve of Figure 19 may i n c l u d e p o r t i o n s of two l i n e s f o r the two p y r i t e d i s s o l u t i o n r e a c t i o n s , (Figure 20), however, the e r r o r i n v o l v e d i n assuming one slope i s s m a l l . Figure 21 shows the cathodic curve under the same c o n d i t i o n s as the anodic scan and a l s o the e f f e c t of 50 p s i oxygen. As shown i n the f i g u r e , there are two curves under the helium atmosphere which may be i n c l u d e d i n the a n a l y s i s . The lower helium curve which curves o f f to very s m a l l current values * Low pressure f i r s t order dependence on pressure, as observed i n o l d e r i n v e s t i g a t i o n s , j u s t i f i e s e x t r a p o l a t i o n to a low pressure slope c l o s e to u n i t y on t h i s p l o t . The high pressure data are from previous work by the present author(25), 68 _ ^ * * I i , i i i • ' 1.0 1.4 1.8 2.2 2.6 3.0 L O G ( P , PSI) 2 Figure 18 Log (Rate of FeS„ d i s s o l u t i o n ) vs. l o g (P ), IM HoS0,, 110°C 2 I k 69 L O G ( i , fnA / C I Y T ) Figure 19 Anodic P o l a r i z a t i o n Curve f o r FeS at 110°C, IM HC10 4 > 100mV/min., 50 p s i He 70 L O G i Figure 20 Schematic Showing two Components of Anodic P o l a r i z a t i o n Curve 71 at about 0.16V was produced by scanning upward on the p o t e n t i a l s c a l e a f t e r completing a downward scan. This procedure should r e s u l t i n a completely c l e a n mineral s urface. The upper helium curve, which e s s e n t i a l l y p a r a l l e l s the oxygen curve was produced by scanning to -0.3 v o l t s to remove any 1.surface f i l m , t u r n i n g o f f the p o t e n t i o s t a t to a l l o w the p o t e n t i a l to d r i f t upward, then making a p o t e n t i a l scan downward. The d i f f e r e n c e between the curves may be due to formation of a surface f i l m . Such a f i l m could be formed e i t h e r by r e a c t i o n of the s m a l l oxygen content of the helium gas d i s s o l v i n g p y r i t e to produce s u l f u r or by l ^ S , formed by cathodic p y r i t e d i s s o l u t i o n , r e a c t i n g on the surface to form elemental s u l f u r and hydrogen i o n s . The type of surface that a c t u a l l y e x i s t s on the cathodic p y r i t e s i t e s during l e a c h i n g i s open to question. As the matter has not been r e s o l v e d , both curves are presented. To generate a value of the cathodic T a f e l slope b^ i t i s necessary to subtract the values f o r current d e n s i t y under helium from those under oxygen i n Figure 21. Figure 22 shows the curves f o r both types of helium scan substracted from the oxygen values. The slope of t h i s curve corresponds to the value of b^. The d i f f e r e n c e between the two curves i s minimal due to the l o g a r i t h m i c s c a l e . The i d e n t i f i c a t i o n of the proper T a f e l s l o p e from the f i g u r e i s d i f f i c u l t . At best, the curves give a g u i d e l i n e as to the range of values f o r b^. A p p l i c a t i o n of equation (41) to the p y r i t e d i s s o l u t i o n system under a c t u a l l e a c h i n g c o n d i t i o n s r e q u i r e s that the values of b^Q and b^ be v a l i d at the l e a c h i n g mixed p o t e n t i a l s . This c o n d i t i o n i s e a s i l y met f o r b^Q but the c o r r e c t value f o r b^ i s u n c e r t a i n . I t i s not p o s s i b l e to extend the cathodic curves to higher p o t e n t i a l s due to complications from other anodic r e a c t i o n s . The l i m i t i n g values from Figure 22 show that b 0 may be greater I i ; i i , i i i _ - 3 - 2 - 1 0 I 2 3 L O G ( i , m A / C M 2 ) Figure 21 Cathodic P o l a r i z a t i o n Curves f o r FeS at 110°C, IM HC10 / | 5 lOOmV/min. ,50psi 0 9 and He I I 0 1 2 L O G ( i , m A / C M 2 ) Figure 2 2 Cathodic Curves f o r Oxygen Reduction at 1 1 0 ° C , IM H C 1 0 , lOOmV/min. 74 ; (3 7) than 0.092 and l e s s than 1.875. B i e g l e r s r e s u l t s at 25°C show a value of 0.13. Another means of e s t i m a t i n g b^ must be found t h e r e f o r e , to complete the a n a l y s i s . I f the f u n c t i o n f- i s assumed to be l i n e a r over a range of pressures, °2 a value of the o v e r a l l constant 30 2 can be obtained by measuring the change b 3 0 + b 2 i n p o t e n t i a l of a l e a c h i n g p y r i t e pulp as the oxygen pressure i s changed. The l i n e a r assumption appears to be j u s t i f i e d at pressures under 200 p s i 0^ where stu d i e s have shown n e a r l y f i r s t order oxygen pressure dependence. At 110°C Figure 13 shows a 40 mV increase i n p o t e n t i a l w i t h an increase i n pressure from 77 to 227 p s i 0^. A geometrical a n a l y s i s of the t a f e l curves f o r anodic and cathodic r e a c t i o n s (see Appendix E f o r d e t a i l s ) combined w i t h the assumption that a_ = K P. r e s u l t s i n the equation: E 2 - E 1 = b30 b 2 (42) P 2 " b 3 0 + b 2 i 2 l o g - r — \ 1 2 I n s e r t i n g the values f o r AE, P , P. and s e t t i n g b„_, = 0.133 as determined U2 V2 30 e a r l i e r r e s u l t s i n a value of b^ = 0.25. Another value f o r b^ can be obtained using equation (41) w i t h the assumption that f„ = K P . °2 °2 b 2 l o g l c • D + b 3 ( ) + b 2 l o g K P 0 2 ( 4 3 ) separating the l a s t term i n t o : b 2 b 2 l o g r a t e = l o g ± c = D + ^ — — l o g K + b ^ + ^ l o g P ^ b 2 = D' + Z TT~ l o g P n (44) b 1 0 + b 2 ° 2 y i e l d s a form where a measured r a t e law can be s u b s t i t u t e d to c a l c u l a t e 75 b values. Mathews and Robins i n t h e i r work, described the p y r i t e d i s s o l u t i o n r a t e dependence on oxygen p a r t i a l pressure as: A p i Rate = K(P. ) ' o x M/min. (45) at 25°C. In l o g a r i t h m i c form, l o g (rate) = l o g K + 0.81 l o g P (46) 2 S e t t i n g the slopes i n equations (44) and (46) equal, y i e l d s • •••• = 0.81. b30 + b 2 Assuming the value of b^ n to be 0.13, gives b^ = 0.55. The c a l c u l a t e d values f o r of 0.25 and 0.55 are both s i g n i f i c a n t l y l a r g e r than the slope of the anodic t a f e l curve, b^ n = 0.133. With such b^ values the slope term of equation (41) i s c a l c u l a t e d as 0.66 to 0.81. While the 0.66 s l o p e , i f a p p l i e d to the data i n Figure 18 does make the curve l i n e a r , the reported f i r s t order oxygen pressure dependence by McKay, Pawlek, and Kim tend to weight the argument i n favour of the l a r g e r slope and hence higher b^ value. The assumption that the r e a c t i o n i s n e a r l y f i r s t order at lower oxygen p a r t i a l pressures, introduces the c o m p l i c a t i o n of a d e c l i n i n g r a t e at h i g h pressures. There are at l e a s t two p o s s i b l e explanations f o r such behaviour. F i r s t , an adsorption c o n t r o l l e d l i m i t i n g of the cathodic r e a c t i o n r a t e ; and second, a change i n the cathodic T a f e l slope, b^, w i t h i n c r e a s i n g pressure. To separate the two p o s s i b l e e x p l a n t i o n s , i t i s necessary to examine the r e a c t i o n path of molecular oxygen as i t approaches the cathodic mineral surface. Again i n c o n s i d e r i n g the p o s s i b l e mechanisms f o r oxygen t r a n s f e r , there are at l e a s t two a l t e r n a t i v e s . For example, the r a t e determining step could be the t r a n s f e r of a proton to an adsorbed oxygen molecule from a nearby water molecule. Such a mechanism could be described by the equations: slow H 20 + 0 2 (ads) + e~ • H0 2 (ads) + OH (47) 76 OH + H + (aq) H O (48) H0 2 (ads) + 3H + + 3e" -£f2-t 2H 0 (49) The Butler-Volmer equation upon which the e l e c t r o c h e m i c a l model i s based, c a l l s f o r the movement of a charged species over an energy b a r r i e r as shown i n Figure 23. A proton moving toward the surface would be a s s i s t e d i n c l i m b i n g the b a r r i e r by the n e g a t i v e l y charged e l e c t r i c f i e l d emanating from the mineral or retarded by a p o s i t i v e f i e l d . The mechanism would leave the b^ value a constant and the oxygen on the surface would be c o n t r o l l e d by an adsorption isotherm. One of the most simple adsorption r e l a t i o n s h i p s i s the Langmuir adsorption isotherm which has the form: a P 0 aP 0 C 6 = iTap- » C 0 2 = l + a P ~ <50> u2 °2 where a and C are constants and 8 i s the f r a c t i o n of the surface covered by oxygen. The adsorption mechanism e x p l a i n s the curvature of r a t e data by the f a c t that once a mineral surface i s completely covered by adsorbed oxygen, f u r t h e r increases i n the oxygen p a r t i a l pressure w i l l not i n c r e a s e the coverage and consequently the r a t e and mixed p o t e n t i a l w i l l not change. At lower oxygen pressures, where aP n <<1, the isotherm y i e l d s a f i r s t order pressure dependence. When the isotherm i s included i n equation 41 the equation becomes: b a P o 2 2 C51) l o g r a t e - l o g ± c = D + + l o g - — -j(J z u 2 (25) Previous work by B a i l e y has been i n t e r p r e t e d i n terms of the Langmiur isotherm and values f o r the r a t e along w i t h the constants a and C at 110°C and i n f i n i t e 77 M O V E M E N T TOWARD E L E C T R O D E D I S T A N C E F R O M 1 E L E C T R O D E Figure 23 Movement of Charged Particle over Double Layer Energy Barrier 78 pressure have been obtained by e x t r a p o l a t i o n . Figure 24 shows the i n i t i a l r ate data at various pressures obtained i n the e a r l i e r study p l o t t e d i n the 1/Rate vs l / P ^ form. A s i m i l a r p l o t reported i n the former study has been u 2 found to r e q u i r e c o r r e c t i o n . * The 1/Rate vs 1/P^ form i s a more convenient 2 r e p r e s e n t a t i o n of the o v e r a l l Langmiur equation: R a t e = K C n - K a P ° 2 C ( 1 3 ) 1+aP i n v e r t i n g both sides g i v e s : l + a P 0 2 °2 Rate K aP„ C ^ 5 2' ) 2 This equation i s of the form y = MX + b. The p l o t of r - ^ — vs gives Rate P 0 1 1 °2 a slope of ----- and an i n t e r c e p t of — . The values f o r these constants (from Figure 24) are 1.45 x 1 0 6 p s i r , h ^ and 1.94 x 1 0 3 b r o , r e s p e c t i v e l y . cm FeS 2 cm FeS 2 v J 1 -4 KC can be evaluated t h e r e f o r e as 94 xIQ3 = 5.15 x 10 cm/hr. This i s the l i m i t i n g r a t e f o r t o t a l p y r i t e surface coverage by adsorbed oxygen. The le 1 1 -7 c m F e S 2 term can be t r e a t e d s i m i l a r l y g i v i n g KaC = . . _ _ nt: = 6.9 x 10 — — KaC J 0 1.45x10° hr p s i The "a" value can t h e r e f o r e be obtained by d i v i d i n g KaC by KC; a = 1.34 x -3 10 p s i . S t r i c t l y speaking, the above a n a l y s i s i s c o r r e c t only i f the e l e c t r o -chemical r e a c t i o n i s under completely cathodic c o n t r o l . In the case of p y r i t e there i s some anodic p o l a r i z a t i o n but i t i s neglected i n the a n a l y s i s . The constant term "D" i n equation (51) can a l s o be evaluated using the e x t r a p o l a t e d i n f i n i t e pressure data. As the pressure of oxygen increases aP toward i n f i n i t y the value of 0 2 goes to 1.0 and the e n t i r e second term 1 + a P°2 i n the equation becomes zero. Therefore at i n f i n i t e pressure: lo g r a t e = l o g i = D (53) * The p l o t was a c t u a l l y a r a t e of oxygen consumption r a t h e r than p y r i t e d i s s o l u t i o n and was not i n terms of i n i t i a l r ates but r a t h e r of intermediate r a t e s . 80 -4 D then, equals l o g r a t e = l o g KC = l o g (5.15 x 10 cm/hr). I n s e r t i n g the values of a and D i n t o equation (51) g i v e s : _, b a ^ x l O ^ s i " 1 ^ ft... lo g (rate) = l o g i = log(5.15 x 10 V v + h l o 8 ^ II C 30 2 1+(1.33x10 p s i ) P n °2 when the r a t e i s expressed i n cm/hr and the pressure i s i n p s i . Combining terms and t a k i n g a n t i l o g s y i e l d s : . ^ ^-3^-1 \ b 2 Rate = i = 5.15x10, ^ ~ ^0 + b 2 C 1+(1.34x10 p s i )P. / \ u2 ' b2 The above a n a l y s i s assumes of course that : ;—:— has a value c l o s e to one. b 3 0 + b 2 C a l c u l a t i o n s using t h i s model must a l s o take i n t o account the f a c t that the o r i g i n a l r a t e data were obtained from l e a c h i n g p y r i t e powders. The c a l c u l a t i o n s i n v o l v i n g surface areas were estimated using a simple s p h e r i c a l shape f a c t o r and hence the r e s u l t s are probably good only to order of magnitude values due to surface roughness and shape e f f e c t s . The second approach to e x p l a i n i n g the curvature of Figure 18 i s to b 2 assume that the value of — — T V — changes as a f u n c t i o n of oxygen p a r t i a l b 3 Q + b 2 pressure. Such an assumption req u i r e s that the charged species moving over the energy b a r r i e r of Figure 23 not be a simple i o n but r a t h e r an e n t i t y capable of a l t e r i n g i t s charge. I t i s p o s s i b l e f o r a molecule approaching the surface to be a f f e c t e d by the e l e c t r o s t a t i c charge and to form a d i p o l e . The charge of such a d i p o l e would be dependent on the st r e n g t h and charge of the e l e c t r i c f i e l d on the mineral surface. The oxygen may form a s m a l l d i p o l e due to the two unpaired e l e c t r o n s i n i t s s t r u c t u r e . As the molecule approaches a n e g a t i v e l y charged surface the e l e c t r o n s would r e p o s i t i o n 81 themselves f u r t h e r away from the surface than the c e n t r a l atoms causing a sma l l charge separation. Changes i n the p o t e n t i a l of the surface caused by increased oxygen pressure would increase the d i p o l e s e p a r a t i o n . The value of b 2 would be increased by such a separation and thus could account f o r the observed curvature i n the rate p l o t . This approach f o r e x p l a i n i n g the rate data has a number of u n c e r t a i n t i e s . Among others, the common assumption that the p o t e n t i a l between a mineral surface and the Outer Helmhotz Plane (OHP) v a r i e s l i n e a r l y must be discarded. I f the e l e c t r i c f i e l d was l i n e a r no motion of a charged d i p o l e would r e s u l t . On a molecular s c a l e the n o n - l i n e a r i t y i s not d i f f i c u l t to accept. The sur f a c e i s covered w i t h a number of adsorbed species and wh i l e the average p o t e n t i a l of the region may vary l i n e a r l y i t i s probable that on a l o c a l b a s i s there i s a considerable v a r i a t i o n . The second mechanism i s at best an i n t e r i m s o l u t i o n to the t o t a l e x planation of the v a r i a t i o n of r a t e vs oxygen pressure. While <tn oxygen d i p o l e may account f o r part or even a l l of the curvature observed i n the l o g ra t e vs l o g P n p l o t (Figure 18), an adsorption isotherm w i l l e v e n t u a l l y take 2 over as the pressure i s r a i s e d s i n c e there i s a f i n i t e surface f o r oxygen adsorption. Once the surface i s f u l l y covered, no f u r t h e r i n c r e a s e i n pressure w i l l have an e f f e c t on the r e a c t i o n r a t e . Since the oxygen d i p o l e argument req u i r e s such a l a r g e number of assumptions and i s n u m e r i c a l l y d i f f i c u l t to t r e a t , the adsorption isotherm mechanism i s probably the best explanation of the observed r e s u l t s . Further d e t a i l e d study of the mechanism w i l l be necessary to d e f i n i t e l y confirm the choice. 82 3.4 P o l a r i z a t i o n Studies on P y r i t e 3.4 a. S u l f u r Layer Studies The use of p o l a r i z a t i o n curves f o r determining the T a f e l parameters of the p y r i t e system has already been discussed i n the previous s e c t i o n . There, the slopes provided i n f o r m a t i o n on the anodic and cathodic d i s s o l u t i o n r e a c t i o n s assuming that the surface had been c l e a r e d of any i n t e r f e r i n g species. Figures 19 and 21 were both obtained a f t e r p r e l i m i n a r y cathodic pretreatment to d i s s o l v e away the surface l a y e r s of p y r i t e and s u l f u r by the r e a c t i o n s : FeS 2 + 4H + + 2e~ •*• Fe** + 2H 2S (56) and S° + 2H + + 2e~ H 2S (23) A c l o s e r examination of the p o l a r i z a t i o n curves f o r p y r i t e i s h e l p f u l i n confirming the a p p l i c a b i l i t y of the e l e c t r o c h e m i c a l mechanism discussed e a r l i e r . One of the key p o i n t s of the proposed mechanism i s the coverage of the p y r i t e surface by a s u l f u r f i l m . As evidence of such a f i l m , the work of Peters and Majima was c i t e d . Cathodic " e x c u r s i o n s " on p y r i t e specimens reduced the measured r e s t p o t e n t i a l of the m i n e r a l , presumably by removing a s u l f u r l a y e r . Figure 25 shows a cathodic p o l a r i z a t i o n scan on a p y r i t e sample at 110°C. The scan was made 2 minutes f o l l o w i n g an anodic sweep of the mi n e r a l . Helium was bubbled c o n s t a n t l y through the IM H 2S0^ s o l u t i o n . I t i s important to note the l a r g e h y s t e r e s i s between the two curves. Current values f o r the i n i t i a l sweep downward are much higher than f o r the upward sweep. The r e s t p o t e n t i a l , where the net current i s zero has a l s o s h i f t e d downward as a r e s u l t of the scan. Figure 26 shows a p o r t i o n of 83 Figure 25 Cathodic P o l a r i z a t i o n Curve on P y r i t e -IM H 2S0 4, lOOmV/min., 50 p s i He 110°C, 84 0.3 \ 0 2.0 4 .0 6.0 8.0 10.0 12.0 i ( m A / C M ^ Figure 26 P o t e n t i a l vs. Current lOOmV/min. 50 p s i He - 110°C, IM H oS0., 2 4 85 the same cathodic scan data p l o t t e d i n a p o t e n t i a l vs current form. This form shows the d i f f e r e n c e between the upward and downward sweeps more c l e a r l y and a l s o allows c a l c u l a t i o n s to be made on the number of coulombs passed i n the region between 0.3 and -0.2 v o l t s where the proposed s u l f u r f i l m would be d i s s o l v i n g . The scan r a t e i n the t e s t was 100 mV/min. The 0.2 to -0.2 v o l t region t h e r e f o r e represents about 4 minutes. G r a p h i c a l l y i n t e g r a t i n g the area under the two curves and t a k i n g the d i f f e r e n c e r e s u l t s 3 i , 17.4 mA min 1.05x10 mA sec , , , 2 i n a value or 2 » 2 ' o r 1'05 couL/cm , a s u l f u r cm cm f i l m one monolayer t h i c k , r e q u i r e s two equivalents of e l e c t r o n s per mole of S* -4 to d i s s o l v e , and would take about 3.5x10 coul, f o r removal. This assumes a smooth s u r f a c e , where i n a c t u a l f a c t there may be a high roughness f a c t o r . The observed f i l m may be q u i t e t h i c k as a r e s u l t of the a n o d i z a t i o n treatment given to the sample p r i o r to c a t h o d i z a t i o n . Attempts to study f i l m s made wi t h l e s s severe anodic treatments were f r u s t r a t e d by the l a c k of p r e c i s i o n i n current d e n s i t y measurement. This area w i l l r e q u i r e f u t u r e study using other techniques. Examination of Figure 26 a l s o allows good c o r r e l a t i o n w i t h the Eh-pH diagram f o r the system, Figure 27. On the downward sweep the current i s very s m a l l u n t i l the p o t e n t i a l reaches about 0.3V. The Eh-pH diagram, i n t u r n , p r e d i c t s that both p y r i t e and elemental s u l f u r should be s t a b l e between 0.36 and 0.23 V. In the range 0.23 to 0.09V the diagram shows p y r i t e to be s t a b l e while elemental s u l f u r i s reduced to H^S. This range i s a l s o evident i n Figure 26 where the current i n the upward sweep goes to n e g l i g i b l e values when the p o t e n t i a l r i s e s above 0.05V. Below 0.09V both p y r i t e and elemental s u l f u r are unstable and react to form H 2S which accounts f o r th le 86 0.5 p 0.0 LU X . LU -0.5 .31 Fe^H^S Fe°, H^ S -I Fe + 2 , HSO: S°AND FeS^UNS TABLE S°AND FeSgSTABLE S° UNSTABLE FeS 2 STABLE S° AND FeS 2 UNSTABLE 0 p H FeS 2 FeS Figure 27 Eh-pH Diagram f o r the Fe-S-H 20 System D e t a i l i n g S u l f u r S t a b i l i t y - 100°C, a- + 9 = 10~ 3M, a u _ = 10 _ 1M, t e A H2 baq other species u n i t a c t i v i t y 87 modest current observed i n the upward cathodic sweep at negative p o t e n t i a l s . The c o r r e l a t i o n of the experimental data w i t h the diagram i s not exact. The decomposition of p y r i t e and s u l f u r i s seen to begin at s l i g h t l y lower p o t e n t i a l s than those i n d i c a t e d on the diagram. This i s e a s i l y e xplained by the f a c t that the Eh-pH diagram i s constructed f o r e q u i l i b r i u m c o n d i t i o n s , w h i l e the r e a l processes r e q u i r e an overvoltage as a d r i v i n g f o r c e . 3.4. b. E f f e c t of Scan Speed The ra t e of scanning has a marked e f f e c t on both cathodic and anodic p o l a r i z a t i o n curves. On the cathodic side, the d i s s o l u t i o n of a s u l f u r f i l m may cover a much l a r g e r p o t e n t i a l range i f the scan ra t e i s increased. A given amount of current must be passed to remove the l a y e r regardless of how f a s t the p o t e n t i a l i s being changed. Slower scan r a t e s produce data c l o s e r to the values p r e d i c t e d by thermodynamic e q u i l i b r i u m . Figures 28 and 29 show scans at 1 and 100 mV/min, r e s p e c t i v e l y . The scans were made at 50°C i n IM ^SO^. In general, the current d e n s i t y values are lower, at a given p o t e n t i a l , f o r the slower r a t e . Since the observed current d e n s i t i e s are a f u n c t i o n of the p o t e n t i a l scan r a t e i t i s sometimes u s e f u l to compare a p o l a r i z a t i o n curve generated by a p o t e n t i a l scan technique w i t h a steady s t a t e curve obtained by s e t t i n g a p o t e n t i a l on the p o t e n t i o s t a t , w a i t i n g u n t i l the current d e n s i t y reaches a steady s t a t e , then s h i f t i n g to another p o t e n t i a l . In a c t u a l i t y an i n f i n i t e time might be r e q u i r e d to reach a true steady s t a t e , however, a reasonable value can u s u a l l y be obtained a f t e r s e v e r a l (< 20) minutes. Figure 30 shows the current density decay as a f u n c t i o n of time at 25° f o r s e v e r a l p o t e n t i a l s . Figure 31 shows a t y p i c a l _ Q 4 I I 1 1 1 1 1 - 3 - 2 - 1 0 I 2 3 L O G ( i , m A / C M 2 ) Figure 28 Cathodic P o l a r i z a t i o n Curve on FeS - 50°C, IM H 9S0 / | 5 lmV/min., deoxidized N„ Figure 29 Cathodic P o l a r i z a t i o n Curve on FeS - 50°C, IM H SO , lOOmV/min., deoxidized Wl 9 0 0 .08 h TIME (MIN.) Figure 30 Constant Potential - Current Density vs. Time —100 mV/MIN. o STEADY STATE _Q_ -3 -2 -I 0 LOG ( i ,mA/CM 2 ) Figure 31 Cathodic P o l a r i z a t i o n Curve on FeS , 25°C, IM H^SO^, lOOmV/min. and 'steady s t a t e ' poin 92 100 mV/min cathodic scan compared w i t h s e v e r a l steady s t a t e values at 25°C. The r a p i d scan obviously obscures some of the f i n e r d e t a i l s shown by the steady s t a t e p o i n t s . There are a l s o other considerations however which favor the use of the p o t e n t i a l scan technique. These i n c l u d e : a) the time required to complete a steady s t a t e curve, b) the p o s s i b l e changes i n the mineral surface during the long steady s t a t e t e s t s (at p o t e n t i a l s where mineral d i s s o l u t i o n i s r a p i d an e l e c t r o d e may be destroyed i n a s i n g l e run), and c) d i f f i c u l t i e s i n keeping the reference e l e c t r o d e operating f o r extended periods ( p a r t i c u l a r l y at high temperature and pressure where the c e l l i s not a c c e s s i b l e ) . S i m i l a r arguments apply f o r anodic p o l a r i z a t i o n as f o r the cathodic work already discussed. The anodic formation of a s u l f u r f i l m w i l l r e q u i r e passage of at l e a s t a minimum charge. The scan r a t e t h e r e f o r e , may r e s u l t i n a s h i f t of the observed curve away from the r e a l current p o t e n t i a l r e l a t i o n s h i p . 3.4. c. E f f e c t of Temperature The e f f e c t of i n c r e a s i n g temperature on the p o l a r i z a t i o n curves f o r p y r i t e i s mainly to increase the current d e n s i t y at a given p o t e n t i a l . This i s shown e f f e c t i v e l y i n Figure 3 2 where the current i s p l o t t e d vs p o t e n t i a l f o r a powdered sample at s e v e r a l temperatures. The anodic scans shown were made at 100 mV/min under a helium atmosphere using the platinum powder holder. Figure 33 shows anodic p o l a r i z a t i o n curves on massive specimens at various temperatures. Again the current d e n s i t i e s are higher f o r a given p o t e n t i a l . Figure 34 shows a s i m i l a r e f f e c t on cathodic 1.2 25°C I /2 HR. 25°C 50°C 1/2 HR. IAFTER INITIAL INITIAL AFTER25°C 50°C I/2HR. AFTER IIO°C 8 0 ° C 1/2 HR. AFTER 50° C Figure 32. Potential vs. Current on a Pyrite Powder as a Function of Temperature 1 0 0 mV/Min. 1 M H C 1 0 , . . 4 9 4 0.4h 0.2" ' • -> • ' 1 - 3 - 2 - 1 0 I 2 3 LOG ( i .mA/CM 2 ) Figure 33. Anodic Pyrite Polarization Curve - Temperature Dependence. 95 0.2 b 3 L O G ( i . m A / C M 2 ) Figure 34. Cathodic Pyrite Polarization Curves - Temperature Dependence. 96 p o l a r i z a t i o n . 3.4. d. E f f e c t of Oxygen The e f f e c t of oxygen pressure on the mixed p o t e n t i a l of p y r i t e l e a c h i n g has been discussed p r e v i o u s l y . That d i s c u s s i o n however, d e a l t w i t h r e l a t i v e l y high pressures. I t i s i n t e r e s t i n g to examine the e f f e c t of oxygen under very low p a r t i a l pressure c o n d i t i o n s . Reference was made e a r l i e r to "cathodic e x c u r s i o n " s t u d i e s performed by Peters and Majima which showed that the r e s t p o t e n t i a l of p y r i t e could be depressed by making cathodic sweeps to negative p o t e n t i a l s . In the present work such experiments were c a r r i e d a step f u r t h e r by i n t r o d u c i n g oxygen to the system a f t e r the r e s t p o t e n t i a l had been lowered by the cathodic sweep. I f pure oxygen was bubbled, the r e s t p o t e n t i a l returned to the o r i g i n a l h i g h value (about 0.62V) very r a p i d l y . Bubbling w i t h a gas having a very low oxygen con c e n t r a t i o n caused the r e t u r n to the higher p o t e n t i a l to be slower. Figure 35 shows the p o t e n t i a l vs.the recovery time p l o t t e d f o r a number of d i f f e r e n t , gases. The runs l a b e l l e d : c a t a l y t i c a l l y deoxidized r e f e r to the use of an R3-11 d e o x i d i z i n g c a t a l y s t obtained from BASF (Badische A n i l i n - and Soda-Fabrik AG). The gases w i t h the lowest oxygen content r e s u l t i n the longest recovery times. I t i s a l s o i n t e r e s t i n g to note that the p o t e n t i a l can be reduced from the high value by bubbling an i n e r t gas as shown by the top curve i n the f i g u r e . These r e s u l t s support the b e l i e f t h a t oxygen i s not chemisorbed on the p y r i t e surface but r a t h e r , p h y s i c a l l y adsorbed s i n c e lowering the oxygen p a r t i a l pressure i n s o l u t i o n has so great an e f f e c t . In terms of the s u l f u r f i l m theory proposed e a r l i e r , the lowering of the mixed p o t e n t i a l i s c o n s i s t e n t • N 2 , 0 2 AT 10 MIN., He AT 18 MIN. v He, NO BUBBLING 0 TO 10 MIN. o He, CATALYTICALLY DEOXIDIZED • H 4 , CATALYTICALLY DEOXIDIZED \ i • i i • • • • i i i i i i i i i i 1 1 i i ic } 'O 2 4 6 8 10 12 14 16 18 20 22 6 0 RECOVERY TIME (MIN., A F T E R S C A N TO -0.3 V SHET) re 35. Potential vs. Recovery Time After a Cathodic Scan, as a Function of the Oxygen Partial Pressure. with removal of the film. This might be due to insufficient oxygen renew the film as i t is consumed by chemical or removed by physical processes. 99 +3 +2 3.5 Examination of E° for the Fe /Fe Couple as  a Function of Temperature Over the course of this work i t became necessary to know whether the pyrite dissolution reaction was producing mainly fe r r i c or ferrous ions at a number of potentials and temperatures. The basic equation for the couple i s : „ I I I — I I Fe + e — • Fe (22) which results in a Nernst equation of the form: 2 3 PT A F E E = E o + 2 J L R T l Q g ^ ( 5 ? ) The E° value has been reported at 25° under a number of solution conditions (0.679V in 0.5 M H 2S0 4 > 0.747 in IM HC104, 0.77 in IM HCl). The purpose of this study was to determine values of E° at higher temperatures in IM H SO,. 2 4 3.5. a. Procedure Three standard solutions were made having known ferric/ferrous ratios; 10.8, 1.08, and 0.108. The ratios were confirmed using the standard eerie sulfate titration for ferric with 1, 10 orthophenanthroline as an indicator. A test c e l l was then f i l l e d and placed in the autoclave. The system was pressurized with 200 psi helium. The c e l l contained two electrodes; the reference and a platinum gauge for sensing the potential of the f e r r i c -ferrous couple. Experimental runs were started at about 20°C before the heater was turned on. The potential vs temperature curve was recorded up to 110°C. then held at that temperature for 10 minutes. After the 10 minute hold, the 10.8 ratio run was heated to 150°C. The heater was then shut off the potential recorded as the temperature decreased. 100 3.5. b. Results and D i s c u s s i o n Data from the t e s t s are shown i n g r a p h i c a l form i n Figure 36. P o t e n t i a l s at each temperature were c a l c u l a t e d using the reference c a l i b r a t i o n shown e a r l i e r . The f i g u r e shows very l i t t l e h y s t e r e s i s i n the measurements i n d i c a t i n g that the c o n s t a n t l y bubbling helium prevented o x i d a t i o n of the ferrous i o n . A n a l y s i s f o l l o w i n g each run a l s o confirmed the r a t i o s t a b i l i t y . The E° p o t e n t i a l at various temperatures were c a l c u l a t e d by applying the Nernst equation at each concentration r a t i o , then t a k i n g an average of the three values. F i g u r e 37 shows the average E° values as a f u n c t i o n of temperature. The p o i n t s at 130 and 150°C are not average values and are t h e r e f o r e l e s s r e l i a b l e than the other p o i n t s . The value of E° at 25°C i s 0.684 which i s at the lower end of the p r e v i o u s l y reported value range. The 110°C E° p o t e n t i a l i s 0.724. This i s s i g n i f i c a n t as the measured mixed p o t e n t i a l f o r oxygen pressure l e a c h i n g at 176 p s i 0^ and 110°C i s about 0.700 v o l t s . The p y r i t e r e a c t i o n products at t h i s p o t e n t i a l t h e r e f o r e should be n e a r l y evenly d i v i d e d between f e r r i c and ferrous ions p r i o r to f u r t h e r homogeneous o x i d a t i o n of ferrous by oxygen. 101 0.82 0.78 0.74 \-LU X CO O.tO LU 0 .66 0.6 8 oo o o o 0 0 Fe* 3 /Fe + 2 = 10.8 o o <9 o o no Fe r j /Fe^= 1.08 oo 00 8 00 o o o ° o ccP Fe* 3 /Fe f 2 = 0.108 6 0 100 140 T E M P E R A T U R E ° C +3 +2 Figure 36. Fe /Fe Couple, P o t e n t i a l vs. Temperature, °C. 102 0.74 0 . 6 8 ' 1 • 1 • 1 • 1 1 1 ' 1 • 1 20 4 0 6 0 8 0 100 120 140 T E M P E R A T U R E t ° C Figure 37. E° for the Fe /Fe Couple vs. Temperature, °C. 103 P A R T B CuFeS 2 and MoS 2 I. GENERAL 1. I n t r o d u c t i o n and Previous Work C h a l c o p y r i t e , CuFeS 2, and molybdenite, MoS 2 each have c h a r a c t e r i s t i c s s i m i l a r to p y r i t e . C h a l c o p y r i t e i s known to y i e l d both s u l f a t e and elemental s u l f u r i n a c i d d i s s o l u t i o n , though the f r a c t i o n of the mineral s u l f u r which ends up i n the elemental form i s u s u a l l y higher than i n the case of p y r i t e (38 39) (40) ' . Molybdenite i s a l s o known to produce elemental s u l f u r and has the same r a t i o of metal to s u l f u r as p y r i t e . The work reported i n t h i s t h e s i s on c h a l c o p y r i t e and molybdenite i s not intended to completely describe e i t h e r system. Rather, the study was performed to see i f the general e l e c t r o c h e m i c a l behaviour e s t a b l i s h e d f o r p y r i t e could be extended to the other minerals. C h a l c o p y r i t e , being the most common copper m i n e r a l , has recei v e d much a t t e n t i o n i n l e a c h i n g s t u d i e s . Several l e a c h i n g agents have been t r i e d , i n c l u d i n g : f e r r i c c h l o r i d e , ferrous s u l f a t e , oxygen i n a c i d s o l u t i o n s and (39 40 42) (41) others ' ' . A study by Yu, Hansen, and Wadsworth i n v e s t i g a t e d c h a l c o p y r i t e d i s s o l u t i o n k i n e t i c s , i n a c i d oxygen pressure l e a c h i n g , at temperatures between 125 and 175°C. Oxygen pressures of from 75 to 400 p s i 0 2 were stud i e d w i t h a c i d concentrations ranging from 0.2 to 0.5 NH^O^. Some of the conclusions of the study were: a) that the oxygen pressure dependence of the d i s s o l u t i o n r a t e followed a Langmuir adsorption isotherm, b) that chemical r e a c t i o n s r a t h e r than charge t r a n s f e r processes were r a t e c o n t r o l l i n g even though the d i s s o l u t i o n was thought to be e l e c t r o c h e m i c a l , and c) that no voltage dependence was observed. The s u l f a t e y i e l d s i n the study were 104 very high (over 96% of the mineral s u l f u r ) and increased w i t h i n c r e a s i n g temperature. Corresponding y i e l d s of s u l f a t e f o r p y r i t e have been shown to r e q u i r e l e a c h i n g p o t e n t i a l s i n the range of 1.0V. To t e s t the v a l i d i t y of a more general a p p l i c a t i o n of the p y r i t e r e s u l t s a program was undertaken to v e r i f y the e l e c t r o c h e m i c a l nature of the c h a l c o p y r i t e d i s s o l u t i o n r e a c t i o n and to make p o t e n t i a l measurements on l e a c h i n g pulps at various oxygen pressures. The l e a c h i n g of both c h a l c o p y r i t e and molybdenite i n h y p o c h l o r i t e (42) s o l u t i o n s has been s t u d i e d r e c e n t l y by Ismay . He concluded that the h y p o c h l o r i t e l i x i v i a n t could be a p p l i e d to s e l e c t i v e e x t r a c t i o n of molybdenum from copper rougher concentrates. The molybdenum e x t r a c t i o n was complete i n 30 minutes at 30°C and pH 9 w i t h copper e x t r a c t i o n of only 0.6%. The e f f e c t i v e n e s s of the h y p o c h l o r i t e leach on molybdenite leads t o the question acid of an e l e c t r o c h e m i c a l mechanism f o r d i s s o l u t i o n . Hypochlor^aS^is a very / / o N strong oxidant w i t h an E° value of about 1.6V . In an e l e c t r o c h e m i c a l system, a high cathodic p o t e n t i a l would lead to a high mixed p o t e n t i a l and corresponding high d i s s o l u t i o n r a t e . The examination of the e l e c t r o c h e m i c a l argument i s even more s i g n i f i c a n t when i t i s r e a l i z e d that molybdenite / / O N leaches more s l o w l y i n n i t r i c a c i d s o l u t i o n s (E° = IV ) and at a n e g l i g i b l e r a t e i n f e r r i c s o l u t i o n s (E° 'v 0.7V). Proposal of an e l e c t r o c h e m i c a l mechanism f o r molybdenite s i m i l a r to that of p y r i t e seems j u s t i f i e d due. to the many s i m i l a r i t i e s between the two minerals. The Eh-pH diagram f o r molybdenite (Figure 38 a f t e r M a j i m a ) has the same form as the p y r i t e diagram (Figure 16). The s u l f u r r a t i o of the two minerals i s a l s o the same, two moles of s u l f u r per mole of metal. Applying the same molar volume argument to molybdenite, as used p r e v i o u s l y on p y r i t e , shows the mineral 105 106 surface to be m a r g i n a l l y unprotected. With complete conversion of mi n e r a l s u l f u r to the rhombic form, the surface would be about 92% covered. The monoclinic and amorphous forms would r e s u l t i n 98 and 99% coverage, r e s p e c t i v e l y . The c r y s t a l s t r u c t u r e of molybdenite complicates the a n a l y s i s , however, s i n c e the s t r u c t u r e i s not c l o s e packed. Molybdenite f r a c t u r e s i n t o f l a k e s which expose p r i m a r i l y the b a s a l plane to the environment. Reaction on the b a s a l plane e f f e c t i v e l y increases the d e n s i t y of the l a t t i c e plane seen by the s u l f u r (see Appendix F) which r e s u l t s i n p r o t e c t i o n of the surface by elemental s u l f u r i f more than about 45% of the molybdenite s u l f u r i s converted to the elemental form. This p r o t e c t i o n a p p l i e s only on the b a s a l plane but s i n c e (45) t h i s plane i s dominant i n f r a c t u r e d p a r t i c l e s m i neral d i s s o l u t i o n w i l l be slow. The non-close packed planes w i l l be l e s s protected than the o v e r a l l d e n s i t y c a l c u l a t i o n i n d i c a t e s , but si n c e t h i s type of surface i s smaller than the b a s a l plane areas, a s u l f u r f i l m p r o t e c t i o n i s dominant. An e l e c t r o c h e m i c a l mechanism very s i m i l a r to that of p y r i t e may t h e r e f o r e be a p p l i c a b l e f o r molybdenite. The Eh-pH diagram f o r c h a l c o p y r i t e (Figure 39 a f t e r Peters i s much more complicated than the diagrams f o r p y r i t e or molybdenite. This makes i n t e r p r e t a t i o n of p o s s i b l e e l e c t r o c h e m i c a l r e a c t i o n s more d i f f i c u l t . Conversion of c h a l c o p y r i t e to other s u l f i d e s such as c h a l c o c i t e or p y r i t e , i n a d d i t i o n to the aqueous sp e c i e s , i s thermodynamically p o s s i b l e though the i r r e v e r s i b i l i t y of such s o l i d - s o l i d r e a c t i o n s i s high. A d e t a i l e d study of the e n t i r e system, which i s beyond the scope of t h i s work, w i l l be necessary f o r a complete understanding. 107 Figure 39. Eh-pH Diagram for the Cu-Fe-S-H20 System, 25°C, a C u + 2 = a l l other species 0.1 M. 0.01 M, 108 I I . 0~" TRACER TESTS ON CHALCOPYRITE 1. Experimental The same techniques were used i n the c h a l c o p y r i t e t e s t s as reported e a r l i e r f o r p y r i t e , w i t h a few minor exceptions. To ensure c o n t i n u i t y w i t h the Yu work, most of the t e s t s were run at constant oxygen pressure. This was f e a s i b l e s i n c e the maximum pressure r e q u i r e d was only 400 p s i 0^' A 66 ml gas r e s e r v o i r replaced the 15 ml chamber used p r e v i o u s l y and a pressure r e g u l a t o r was i n s e r t e d i n the system between 1 8 the r e s e r v o i r and the autoclave. The same 0 production and l i q u i f i c a t i o n technique was used to p r e s s u r i z e the gas r e s e r v o i r to 1000 p s i 0^ as reported e a r l i e r . Oxygen consumption was recorded by f o l l o w i n g the decrease i n pressure of the gas r e s e r v o i r as oxygen was su p p l i e d to the autoclave v i a the r e g u l a t o r . As i n the p y r i t e study, the t e s t s were conducted using 5g of powdered mineral i n 50 ml of a c i d s o l u t i o n . The c h a l c o p y r i t e used was obtained from the Phoenix mine and had an a n a l y s i s shown i n Table X. As i n the Yu study, i m p u r i t i e s had been decreased by a pre-leach i n b o i l i n g IM HCIO^ f o r 5 minutes. The residue was washed with water and e t h y l a l c o h o l then a i r d r i e d . Most t e s t s were performed using p e r c h l o r i c a c i d r a t h e r than s u l f u r i c to minimize i n t e r f e r e n c e from the a c i d s u l f a t e . The same procedures f o r preparing SO^ samples, a n a l y s i s , and i n t e r p r e t a t i o n of data were followed as i n the p y r i t e study. 2. Results and D i s c u s s i o n F i v e s u c c e s s f u l runs were made using oxygen-18 enrichment. Four were at constant pressure c o n d i t i o n s and the f i f t h was a constant volume t e s t performed w i t h the o r i g i n a l p y r i t e s e r i e s . The peak height data f o r the study i s shown i n Table XI. Table X I I shows the t e s t r e s u l t s f o r Table X A n a l y s i s of Pre-Leached Phoenix CuFeS A c t u a l T h e o r e t i c a l CuFeS 2 Cu 29.5% 34.6% Fe 30.1 30.4 Table XI 18 CuFeS 20 Tracer Data Peak Heights i n mm, Corrected f o r Background Run 63.96 S320f 65.96 S 3 ^ 6 65.97 S 3 2 0 1 6 0 1 8 67.96 S3W8 67.97 S 3 2 0 2 8 69.97 S 3 4 0 f 1 335.0 14.8 1.0 0 0 0 2 388.6 18.1 1.0 0 0 0 3 356.4 16.2 1.0 0 0 0 4 534.6 24.4 3.4 0 0 0 5 412 18.2 4.5 0 0 0 Table X I I 18 CuFeS„ 0 Tracer R e s u l t s I n i t i a l Conditions F i n a l Pressure F i n a l SO, 4 I Q I n i t i a l Gas %0 18 %0 maximum (S0 4 determ) 1 Pi — %0 i n SO, 4 t e r r o r ** % Atom Transfer (1) 110 p s i 0 2 >130°C,1MHC10 4 110 p s i 0 2 37.2% •I Q 4.79%0 1 Q 4.79%0 0.14±0.27 -1.4+5.9% (2) 110 p s i 02,160°C,1MHC104 110 36.7 3.51 3.51 0.12±0.27 -2.5±8.1% (3) 310 p s i 02,160°C,1MHC104 310 51,9 3.88 3.88 0.13+0.27 -2.0±7.3% (4) 300 p s i 0 2,H0°C,1MHC10 4 300 27.4 4.09 4.09 0.30±0.27 2.5±6.9% (5) 400 p s i 0 ,110°C, 1.0MH.S0. * 2 4 240 - 3.98 0.92 0.52±0.27 44±38% * Constant volume experiment, m a t e r i a l not pre-leached. ** Standard D e v i a t i o n 112 determining molecular r e a c t i o n c o n t r i b u t i o n . The f i r s t four runs, which cover the range of c o n d i t i o n s s t u d i e d by Yu, are completely c o n s i s t e n t w i t h a t o t a l l y e l e c t r o c h e m i c a l r e a c t i o n process. The f i f t h t e s t suggests an increased c o n t r i b u t i o n from an atom t r a n s f e r (completely chemical) route but i t must be noted that the p r e c i s i o n of these r e s u l t s i s s u b s t a n t i a l l y lowered by the presence of s u l f a t e from the i n i t i a l a c i d . Contrary to the work of Yu, the f r a c t i o n of the m i n e r a l s u l f u r which ended up as s u l f a t e was low i n the t e s t s . Column three of Table X I I shows the values obtained by a n a l y s i s of s u l f a t e formed versus the q u a n t i t y of mineral d i s s o l v e d . The r e s u l t s were a l s o checked using the oxygen consumption data. The higher temperature and pressure run, Cp - 3, shows the l a r g e s t s u l f a t e y i e l d . Lowering temperature or pressure leads to greater formation of elemental s u l f u r at the expense of s u l f a t e . O v e r a l l , the t e s t s support a t o t a l l y e l e c t r o c h e m i c a l d i s s o l u t i o n f o r c h a l c o p y r i t e . This i s not s u r p r i z i n g s i n c e s t u d i e s i n which the mineral r e a c t s c a t h o d i c a l l y ( r a t h e r than a n o d i c a l l y i n oxygen pressure leaching) have v e r i f i e d the operation of g a l v a n i c couples. Hiskey and Wadsworth^ 4 <^ , f o r example, showed that l e a c h i n g of c h a l c o p y r i t e was a c c e l e r a t e d by the a d d i t i o n of m e t a l l i c copper v i a g a l v a n i c conversion of c h a l c o p y r i t e to c h a l c o c i t e (Cu^S). The next step i n understanding the oxygen pressure l e a c h i n g r e a c t i o n s i s to examine the mixed p o t e n t i a l s f o r l e a c h i n g as a f u n c t i o n of the oxygen p a r t i a l pressure. 113 I I I . MEASUREMENTS OF LEACHING MIXED POTENTIALS ON CuFeS 2 AND MoS 2 1. Experimental The same platinum holder was used as i n the p y r i t e work. About 0.5g of the mineral t e s t e d (-200 +270 mesh, preleached CuFeS^ or hand picked molybdenite f l a k e s * a n a l y z i n g 99.8% MoS 2) was placed i n the holder and the p o t e n t i a l d i f f e r e n c e between the mineral and the Ag/AgCl reference e l e c t r o d e was measured as a f u n c t i o n of oxygen p a r t i a l pressure. 2. Results and D i s c u s s i o n F i g u r e 40 shows the mixed p o t e n t i a l of c h a l c o p y r i t e l e a c h i n g vs oxygen p a r t i a l pressure at 25 and 110°C i n lMHClO^. The f i r s t f eature observed i n the f i g u r e i s the decrease i n the mixed p o t e n t i a l w i t h i n c r e a s i n g temperature. This i s c o n s i s t e n t w i t h the work of Jones on c h a l c o p y r i t e i n which he examined the le a c h i n g r a t e (current density) as a f u n c t i o n of temperature. Figure 41 ( a f t e r Jones) shows that at a constant p o t e n t i a l of 0.7V SHE the r a t e of r e a c t i o n at temperatures l e s s than about 60°C i s very s m a l l . The f a c t that at higher temperature the mixed p o t e n t i a l of c h a l c o p y r i t e l e a c h i n g i s lower than the corresponding p y r i t e p o t e n t i a l i s a l s o b e l i e v e d to account f o r the d i f f e r e n c e s i n mineral d i s s o l u t i o n rates observed i n f e r r i c c h l o r i d e and other s o l u t i o n s w i t h changing temperature. The second important observation i s the increase i n mixed p o t e n t i a l w i t h i n c r e a s i n g oxygen pressure. This e f f e c t , coupled w i t h the s u l f a t e y i e l d data i n the previous s e c t i o n shows a p o t e n t i a l dependence of the s u l f a t e formation r e a c t i o n s i m i l a r to that observed f o r p y r i t e . This e f f e c t i s contrary to the work of Yu. Attempts to extend the 110°C data to higher oxygen pressures were f r u s t r a t e d . The r a p i d d i s s o l u t i o n of the * The molybdenite f l a k e s were picked from a sample c o n t a i n i n g molybdenite surrounded by quartz. 114 115 T E M P E R A T U R E (°C) gure 41. Current Density vs. Temperature f o r CuFeS ; 0.1 M H SO , 7 0 0 mV. S H E T . 116 mineral was accompanied by a r a p i d l y changing p o t e n t i a l which made measure-ments of l i t t l e v alue. Figure 42 shows the mixed p o t e n t i a l of molybdenite vs oxygen p a r t i a l pressure at 25°C. The p o t e n t i a l s , are higher than f o r e i t h e r p y r i t e or c h a l c o p y r i t e under s i m i l a r c o n d i t i o n s . This observation i s co n s i s t e n t w i t h an e l e c t r o c h e m i c a l mechanism f o r molybdenite as w e l l as f o r c h a l c o p y r i t e and p y r i t e . As a f u r t h e r t e s t on molybdenite, a volume of n i t r i c a c i d was added to the t e s t c e l l to b r i n g the a c i d c o n c e n t r a t i o n to IM HNO^ i n a d d i t i o n to the 1MHC10, already present. The p o t e n t i a l of the mineral rose from 0.480V to 4 0.788V w i t h the i n t r o d u c t i o n of the n i t r i c a c i d to the helium bubbled c e l l at 25°C. A s i m i l a r t e s t at 85°C showed a r i s e from 0.507 to 0.732V w i t h the a c i d a d d i t i o n . Stopping the helium flow a f t e r the a c i d a d d i t i o n f u r t h e r increased the p o t e n t i a l to 0.938V at 25°C. This i s thought to r e s u l t from the decomposition of n i t r i c a c i d to n i t r o u s a c i d and NO gas. The r e a c t i o n s (47) i n v o l v e d are discussed by Peters as: HN0 3 + 2H + + 2e~ f l ^ W HN02 + H 20 (58) HN02 + H + e 1 Z T N 0 ( g ) + H 20 (59) and 3HN0. moderate R + 2 N Q + R Q ( 6 0 )  2 . • + 3 (g) 2 Stopping the helium flow allows the concentration of NO i n s o l u t i o n to b u i l d up thereby generating more HN02 by equation (59). The cathodic r e a c t i o n (59) i s known to have a higher exchange current d e n s i t y than equation (58) and w i l l t h e r e f o r e increase the mixed p o t e n t i a l on the miner a l . A diagramatic r e p r e s e n t a t i o n of the e f f e c t i s shown i n Figure 43. 117 L O G i Figure 43. Diagramatic Representation of Electrochemical Mechanism in MoS„ Leaching. 119 The results presented, though limited in number, are sufficient to justify further work on the chalcopyrite and molybdenite systems. It is not the aim of this work to do a comprehensive study on either mineral but rather to show that electrochemistry may play a major role in their dissolution chemistry. 120 P A R T C I. A p p l i c a t i o n of This Work to S u l f i d e M i n e r a l s i n General The work presented on p y r i t e has produced techniques and t h e o r i e s which may be extended to other s u l f i d e minerals such as: galena (PbS) , c h a l c o c i t e (CuS), c o v e l l i t e (CuS), and others. Using a platinum e l e c t r o d e to sense the l e a c h i n g p o t e n t i a l s of a pulp has been shown to be s u c c e s s f u l w i t h p y r i t e , c h a l c o p y r i t e , and molybdenite. Tests have shown that f o r s t i r r e d pulps, i t i s not even necessary to enclose the mineral i n a holder. A platinum gauze suspended i n the pulp w i l l sense the p o t e n t i a l on the impinging p a r t i c l e s . Such an e l e c t r o d e could be used to i n v e s t i g a t e a p o s s i b l e e l e c t r o c h e m i c a l r o l e i n many h y d r o m e t a l l u r g i c a l operations. Systems i n which the p a r t i t i o n i n g of mineral s u l f u r , between elemental s u l f u r and s u l f a t e , i s known to occur, may b e n e f i t from the competing r e a c t i o n model proposed f o r p y r i t e . C h a l c o p y r i t e l e a c h i n g has already been shown to be a f f e c t e d by p o t e n t i a l changes and p o t e n t i o s t a t i c c o n t r o l of r e a c t i o n s e i t h e r by e x t e r n a l e l e c t r o n i c s or by a d d i t i o n of reagents could provide the exact s u l f a t e / s u l f u r r a t i o d e s i r e d to optimize p l a n t economics. C o n t r o l of the lea c h i n g p o t e n t i a l might a l s o add to the s e l e c t i v i t y of the op e r a t i o n , i n d i s t i n g u i s h i n g between s e v e r a l minerals. The use of Eh-pH diagrams has proven e f f e c t i v e i n e x p l a i n i n g the operating r e a c t i o n s on a p y r i t e e l e c t r o d e as a f u n c t i o n of p o t e n t i a l . This may be extended to s u l f i d e s i n general and. coupled w i t h the s t u d i e s i n d i c a t e d abovejmay lead to increased a b i l i t y to p r e d i c t the e f f e c t s of pH, other m i n e r a l s , reagents e t c . on any given m a t e r i a l . a 121 I I . . CONCLUSIONS 1. The a c i d oxygen pressure l e a c h i n g of p y r i t e and c h a l c o p y r i t e have been 18 stu d i e d using an 0 t r a c e r technique. The d i s s o l u t i o n of both minerals has been found to be c o n s i s t e n t w i t h a t o t a l l y e l e c t r o c h e m i c a l mechanism as opposed to an atom t r a n s f e r or chemical process. 2. P o t e n t i o s t a t i c c o n t r o l i n the anodic d i s s o l u t i o n of p y r i t e shows the p a r t i t i o n i n g of m i n e r a l s u l f u r between s u l f a t e and elemental s u l f u r to be a f u n c t i o n of the mixed p o t e n t i a l of le a c h i n g . Q u a n t i t a t i v e y i e l d s of s u l f a t e are produced i f the mixed p o t e n t i a l i s above about 1.0V SHE. A s i m i l a r e f f e c t i s seen i n the le a c h i n g of c h a l c o p y r i t e . 3. I t i s p o s s i b l e to sense mineral p o t e n t i a l s by c o n t a c t i n g a mineral w i t h a platinum e l e c t r o d e during l e a c h i n g . The p o t e n t i a l s thus observed on p y r i t e are almost e x a c t l y (± ^ 3mV) the same as those on massive mineral e l e c t r o d e s under the same lea c h i n g c o n d i t i o n s . 4. The high mixed p o t e n t i a l of p y r i t e can be a t t r i b u t e d to a l a y e r of elemental s u l f u r on the mineral suface. This l a y e r decomposes to hydrogen s u l f i d e when the mineral p o t e n t i a l i s made more negative than ^ 0.25V (110°C pH 0) r e s u l t i n g i n a mixed p o t e n t i a l close to the thermodynamically p r e d i c t e d value. 122 I I I . SUGGESTED FUTURE WORK 1. P y r i t e The data obtained i n t h i s study have allowed a model to be developed f o r the p y r i t e d i s s o l u t i o n mechanism. This model must be t e s t e d and r e f i n e d i n f u r t h e r work. The concept of a p r o t e c t i v e s u l f u r f i l m i s b a s i c to the proposed model and must be examined. One of the p o s s i b l e t e s t s i n v o l v e s c y c l i c voltammetry where p o t e n t i a l i s scanned very r a p i d l y f i r s t c a t h o d i c a l l y then a n o d i c a l l y . By f o l l o w i n g the current vs p o t e n t i a l r e l a t i o n s h i p on an o s c i l l o s c o p e a record could be made of the r a p i d sweeps. A f t e r any surface i m p u r i t i e s have been:removed, the c y c l i c t r a c e s should reach a steady s t a t e which could then be analyzed. By i n t e g r a t i n g the current vs time p l o t f o r both anodic and cathodic sweeps a comparison of the charge necessary f o r forming or removing a surface l a y e r could be made. I f the f i l m i s elemental s u l f u r , the a n a l y s i s would show the charge passed i n the cathodic sweep to be twice that passed i n the anodic sweep. This i s a r e s u l t of the s u l f u r forming and consuming r e a c t i o n s : FeS 2 — y Fe 2(S° + 2H + + 2e + 2S° + 2e (17) (23) Further work on the e l e c t r o c h e m i c a l d i s s o l u t i o n model i s a l s o necessary to d i s t i n g u i s h between the adsorption isotherm and the oxygen d i p o l e mechanisms. This may n e c e s s i t a t e going to much higher pressures than the 976 p s i 0 p r e v i o u s l y s t u d i e d . 123 2. E l e c t r o c h e m i c a l Sensing Through the course of t h i s work i t has been found that a platinum e l e c t r o d e i n contact w i t h e i t h e r p y r i t e , c h a l c o p y r i t e , or molybdenite takes on the p o t e n t i a l of the l e a c h i n g m i n e r a l . Platinum e l e c t r o d e s not i n contact w i t h minerals have been used to sense the p o t e n t i a l s of i o n couples such as f e r r i c - f e r r o u s i n s o l u t i o n . I f , as suggested by t h i s work, mineral systems have an e l e c t r o c h e m i c a l mechanism, i t may be p o s s i b l e to c o n t r o l l e a c h i n g r e a c t i o n s by monitoring p o t e n t i a l sensing e l e c t r o d e s . I f , f o r example, i t i s known that a mineral leaches when the p o t e n t i a l i s above a c r i t i c a l l e v e l , a d d i t i o n agents could be c o n t r o l l e d by the sensing e l e c t r o d e to keep the p o t e n t i a l s s u i t a b l y high. 3. Extension of the E l e c t r o c h e m i c a l Mechanism to other M i n e r a l s The present work has confirmed e l e c t r o c h e m i c a l mechanisms f o r p y r i t e and c h a l c o p y r i t e . I t has a l s o discussed the l e a c h i n g of molybdenite assuming the r e a c t i o n s to be e l e c t r o c h e m i c a l l y c o n t r o l l e d . Since r e s t p o t e n t i a l measurements have been made on many minerals (Table I I ) and g a l v a n i c i n t e r a c t i o n s observed i n a number of systems, i t i s not unreasonable to expect that e l e c t r o c h e m i s t r y plays a major r o l e i n a wide v a r i e t y of mineral r e a c t i o n s . Future work must be done to i d e n t i f y such systems w i t h the hope that the e l e c t r o c h e m i c a l means of mineral s e p a r a t i o n and p u r i f i c a t i o n may be found. 124 APPENDIX A The Debye-Hiickel Temperature Correction C o e f f i c i e n t The Debye-Huckel assumption, that i n d i l u t e solutions, the behaviour of an ion i s c o n t r o l l e d simply by e l e c t r o s t a t i c i n t e r a c t i o n proves a mathematical basis for c a l c u l a t i n g a c t i v i t y c o e f f i c i e n t s . The Debye-Huckel l i m i t i n g law for the mean i o n i c a c t i v i t y c o e f f i c i e n t i s : log f ± = A ( Z + Z J I * - (A-l) Where f i s the mean i o n i c a c t i v i t y c o e f f i c i e n t , A i s a constant including several e l e c t r o s t a t i c parameters, Z + and Z are the charges on the cations and anions of a m a t e r i a l , and I i s the i o n i c strength (I = \ E c^Z_^, c_^  = concentration i n moles per l i t e r ) . A treatment of the work involved i n solvating an ion and i n changing the concentration of the solvent water i s (37) shown by Bockris and.Reddy . The equation for the a c t i v i t y c o e f f i c i e n t i s : * 2.303 RT log f + , , = 2 - 3 0 3 R T A / " - 2.303 RT ^ a ±(exp) i + Ba ^ 7 n + n + 2.303 RT log ( A _ 2 ) ° n + n - n . w h A,B and a are empirical constants;n, n^ and n are concentrations i n moles per l i t e r of e l e c t r o l y l e i n water, water i n the primary hydration sheath, and water i t s e l f . Obtaining values for a l l the constants at various temperatures (from Bockins and Reddy) allowed s u b s t i t u t i o n into the equation and yielded a mean a c t i v i t y c o e f f i c i e n t as a function of temperature. Values of the constants and the a c t i v i t y c o e f f i c i e n t s are shown i n Table AI. F i n a l l y , the a c t i v i t y c o e f f i c i e n t s obtained were applied to the Nernst equation for c o r r e c t i o n of * The equation of Bockris has been modified to include 2.3 RT factors on the f i r s t two terms. This was done to correct an apparent typographical error i n the Bockris work. The factor was included i n the d e r i v a t i o n but was omitted i n the f i n a l equation form. Table A l Debye Huckel Temperature C o r r e c t i o n Data _g a = 5 x 10 cm a = 0.97 w n h = 8 n = 1 n = 53 w Temp A 10" 8B f + IM • • f + 1.1 M 20 0.5070 0.3282 0.964 0.957 40 0.5262 0.3323 0.951 0.944 60 0.5494 0.3371 0.935 0.929 80 0.5767 0.3426 0.918 0.912 100 0.6086 0.3488 0.899 0.891 126 the reference p o t e n t i a l data as shown i n Figure 9. The a p p l i c i a b i l i t y of the c a l c u l a t i o n s i n IM e l e c t r o l y t e appears to be reasonable. The l a s t two terms i n equation (A-2) compensate f o r the concentration of the s o l u t i o n . In NaCl s o l u t i o n s , the a c t i v i t i e s p r e d i c t e d by the theory are a p p l i c a b l e up to 5 moles per l i t e r . 127 APPENDIX B I d e n t i f i c a t i o n of S u l f u r Product on Anodized P y r i t e Line Observed d Spacing S u l f u r * d S p a c i n g ( 4 8 ) 1 3.83 3.85 2 3.43 3.44 3 3.20 3.21 4 2.83 2.84 5 2.62 2.62 6 2.50 2.50 7 2.37 2.37 8 2.28 2.29 9 2.11 2.11 The above l i n e s were observed i n a d d i t i o n to the l i n e s normally (49) a s s o ciated w i t h p y r i t e as compared w i t h a p y r i t e standard f i l m p r i o r to the c a l c u l a t i o n of d values. A l l p y r i t e l i n e s i n the Swanson p a t t e r n appeared on the standard f i l m which was a ca r r y over from previous work (25) by B a i l e y ^ . . * Orthorhombic S u l f u r . 128 APPENDIX C Comparison of Molar Volumes of P y r i t e and Elemental S u l f u r Density of FeS 2 - 5.0 g/cm3 ( 3 0 ) 3 5.0 g/cm _ FeSo, 3 im no / — i — = 0.0417 mole ^/cm 119.98 g/mole 3 Density of Mon o c l i n i c S u l f u r - 1.96 g/cm - the monoclinic form i s s t a b l e from 95.5° to the melting p o i n t at 119°C^ 5 0^ 3 1.96 g/cm n mole S° , 3 a — — 0.0611 m/cm 32.064 g/mole S° m For 1.0 mole of p y r i t e we have 2.0 moles of p y r i t i c s u l f u r . Therefore the p y r i t e has a volume of (1.0 mole FeS 2) mole FeS„ = 23.981 cm 0.0417 ^ — -cm I f a l l the p y r i t i c s u l f u r was converted to the monoclinic form the volume would be (2.0 mole S) 3 : ; — - — — = 32.733 cm n mole S, 3 0.0611 /cm For a P i l l i n g - B e d w o r t h r a t i o of 1.0, the f r a c t i o n of the p y r i t i c s u l f u r which must be o x i d i z e d to the s u l f a t e form i s : 32.733 - 23.981 . _ „ 32~733 = 0*^67 or about 27%. In the case of lower temperature work w i t h a rhombic s u l f u r product the s u l f a t e percentage f o r a u n i t y P i l l i n g - B e d w o r t h r a t i o i s 22.6%. Above the melti n g p o i n t the den s i t y of s u l f u r i s a f u n c t i o n of 3 temperature. At 130°C the d e n s i t y i s 1.795 g/cm which r e s u l t s i n a completely p r o t e c t i v e l a y e r at l e s s than about 33% o x i d a t i o n of p y r i t i c s u l f u r to s u l f a t e . 129 APPENDIX D  A c t i v a t i o n P o l a r i z a t i o n P o l a r i z a t i o n , u s u a l l y denoted by the symbol n, i s simply a d e v i a t i o n from the r e v e r s i b l e thermodynamic p o t e n t i a l of a system. The r e v e r s i b l e p o t e n t i a l i s given by the Nernst equation f o r the r e a c t i o n : aA + bB + mH+ + ne" —> cC + dD (D-l) as: a b m a, a„ a o , 2.3 RT "A aB H+ E = E° + l o g , Rev nF c d aC a D a b a. a„ A B u 0 2.3 RT m „ . 2.3 RT . J r . v = E° - pH + — l o g c d (D-2) F n nF a c ^ where E° = ~ ^ r ~ , R i s a constant (8.32 ^  Coul/o^ m 0 ] _ e ) ^  p ^ s t ^ e F a r a c j a y (23,060 c o u l / e q u i v a l e n t ) T i s the temperature (°K), and n i s the number of el e c t r o n s t r a n s f e r r e d . To describe an e l e c t r o c h e m i c a l process, not at e q u i l i b r i u m , i n v o l v i n cathodic, v i z : A + e~ + B (D-3) and anodic: B -> A + e~ (D-4) r e a c t i o n s , the Butler-Volmer equation has been developed. I t has the form: . = ^ [ e(l-B)nF/RT _ e-3nF/RT] ( D _ 5 ) 2 where i i s the current deeflity ( i . e . mA/cm ), i i s the e q u i l i b r i u m or exchang current d e n s i t y , 6 i s a symmetry f a c t o r d e a l i n g w i t h the e l e c t r i c a l double l a y e r , and n i s the departure of the p o t e n t i a l from e q u i l i b r i u m or over-p o t e n t i a l . A s i m p l i f i c a t i o n of the Butler-Volmer equation can be made i f n 130 i s greater than about 0.1V since the second term i n the brackets of equation (D-5) i s then much smaller than the f i r s t ; i = i e ( 1 - ^ F / R T (D-6) o P u t t i n g t h i s equation i n t o logarithmic form and s o l v i n g f o r n y i e l d s : - 2.303 RT . . . . 2.303 RT . . . . n = (1 - B)F l o g \ + (1 - B)F l o g 1 ' • ( D _ 7 ) R e c a l l i n g that the overvoltage, n, i s simply the d i f f e r e n c e between the a c t u a l and r e v e r s i b l e p o t e n t i a l s , and combining the constant terms give the form: E - E R e v = a + b l o g i (D-8) , -2.303 RT . . . , , 2.303 RT _ , . where a = Q _ l o g i Q and b = ^ _ . The constant, b, i s r e f e r r e d to as the T a f e l slope i n r e c o g n i t i o n of T a f e l who was the f i r s t to p u b l i s h measurements showing such behaviour. Figure D-l shows the t y p i c a l form of a T a f e l p l o t . E x t r a p o l a t i o n of the T a f e l slope back to the r e v e r s i b l e p o t e n t i a l gives a value f o r i Q , the exchange current d e n s i t y . Figure D-l. Typical Tafel Curve Form. 132 APPENDIX E Geometrical A n a l y s i s of T a f e l Curves f o r Determining t>2 A t y p i c a l p a i r of T a f e l curves are shown i n Figure E - l . In the l i n e a r r e g i o n , the cathodic curve has a slope of " " t ^ - The anodic slope i s b^Q. I f the cathodic curve i s s h i f t e d upward by i n c r e a s i n g the p a r t i a l 1 2 pressure of oxygen, from P to P„ , the increase expected i n the current "2 2. d e n s i t y or r a t e , l o g i , i s C i f the p o t e n t i a l remains unchanged. C = l o g ± 2 - l o g ± 1 ( E - l ) Since the a c t i v i t y of oxygen on the surface a. i s p r o p o r t i o n a l to the o 2 current i C = l o g a 2 2 - l o g a ^ (E-2) The a c t i v i t y i s defined as a f u n c t i o n of the oxygen p a r t i a l pressure. Therefore C = l o g P ^ + l o g F ^ - l o g P ^ - i o g F J 9 (E-3) From Figure E - l we obt a i n the r e l a t i o n s h i p s : a = — and (E-4) 30 E 2 - E 1 c " a = — b ~ — (E-5) S o l v i n g f o r C y i e l d s : c . e 2 - E l + E 2 - E 1 , (E2 - E l ) ( b 3 0 + b 2 ) ( E 6 ) ^ b 3 0 b 2 and f i n a l l y : E 2 - E 1 _ b30 b2 (E-7) C b30 + b 2 133 L O G i Figure E- l . Geometrical Argument for Determining a Value of b 134 With the assumption that a = KP , the l o g f terms i n equa'tion (E-3) cancel out. S u b s t i t u t i n g f o r C t h e r e f o r e r e s u l t s i n : E 2 - E 1 E 2 - E 1 = b 3 0 b 2 (E-8) l o g P 2 - l o g p l P 2 b30 + b2 °2 °2 l o g _2l P o 2 135 APPENDIX F Elemental S u l f u r P r o t e c t i o n of Molybdenite along the B a s a l Plane Molybdenite has a laye r e d form i n which planes of a hexagonally close packed s t r u c t u r e are separated from each other by a f a i r l y l a r g e di s t a n c e . The C/a r a t i o f o r molybdenite i s reported as 3.89^ 4 9^ or 3.90^ 5 1^ (52) w h i l e f o r a completely c l o s e packed system the r a t i o i s 1.632 . The clo s e packed d i r e c t i o n determines the b a s a l plane. Since there i s a l a r g e separation between the clo s e packed planes, g r i n d i n g the mineral r e s u l t s i n cleavage along those planes exposing the c l o s e packed surfaces. Reaction of the mineral on the exposed surface t h e r e f o r e i s eq u i v a l e n t t o a c l o s e c r y s t a l f o r the f i r s t l a y e r and approaches the average m i n e r a l packing arrangement w i t h f u r t h e r p e n e t r a t i o n . At the f i r s t l a y e r , the de n s i t y of the mineral would be a f a c t o r of 3.90/1.632 = 2.39 times that of the o v e r a l l molybdenite c r y s t a l . I f only the f i r s t l a y e r d i s s o l v e d , the P i l l i n g - B e d w o r t h r a t i o would be greater than 1.0 f o r more than about 45% of the mineral s u l f u r converted to the elemental form. 136 REFERENCES 1. G a r r e l s , R. and C h r i s t , C : S o l u t i o n s , M i n e r a l s and E q u i l i b r i a , Harper and Row 1965. 2. Renzoni, L.S., McQuire, R.C. and Barkev, W.V.: J . M e t a l s , 10, p.414, (1958). 3. Koch, D.F.A.: Modern Aspects of E l e c t r o c h e m i s t r y , V o l . _10 Plenum, p.211, (1975). 4. Habashi, F.: Minerals S c i . Engng., p. 3, (1970). 5. Majima, H. and P e t e r s , E.: V I I I I n t e r n a t i o n M i n e r a l Processing Congress, Leningrad Paper E - l (1968). 6. Majima, H. and P e t e r s , E.: Trans AIME V o l . 236, p.1409 (1966). 7. K e l l e r , G.V. and F r i s c h k n e c h t , F.C.: E l e c t r i c a l Methods i n Geophysical Pr o s p e c t i n g , Pergamon Press, London, p.5 (1966). 8. Rachenberg, H.: Nenes Jahrb. 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Swanson e t . a l . : NBS C i r c u l a r 539, V o l . V, (1955). 50. The S u l f u r Data Book, McGraw - H i l l , (1954). 51. Donnay e t . a l . : C r y s t a l Data Determinative Tables 2nd E d i t i o n . 52. Bragg, W.L.: The C r y s t a l l i n e S t a t e , London, B e l l and Sons, V o l . 1, p.145, (1949). 53 Majima, H.; P r i v a t e Communication. 

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