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Study of the anodic polarization mechanism of A1₂O₃-Na₃A1F₆ and A1₂O₃-Na₃A1F₆-LiF electrolytes using… Izard, John William 1974

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A STUDY OF THE ANODIC POLARIZATION MECHANISM , : L 2 0 3 ~ N a 3 A 1 F 6 a n d A l 2 0 3 - W a 3 A 1 F 6 - L : L F ELECTROLYTES USING A ROTATING CYLINDRICAL CARBON ANODE by JOHN WILLIAM IZARD B.A.Sc.(Hons.) U n i v e r s i t y of B r i t i s h Columbia, Vancouver, 1971 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department of M e t a l l u r g y We accept t h i s t h e s i s as conforming t o the requ i r e d standard THE UNIVERSITY OF'BRITISH COLUMBIA A p r i l , l97Jx In presenting t h i s t h e s i s in p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree that the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r reference and study. I f u r t h e r agree that permission for extensive copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the Head of my Department or by h i s representatives. It i s understood that copying or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n permission. Department of eAx?30ujo\KH The U n i v e r s i t y of B r i t i s h Columbia Vancouver 8, Canada - i i -ACKNOWLEDGEMENT The author g r e a t l y appreciates the advice and add given by Prof e s s o r A. M i t c h e l l . In a d d i t i o n the author i s very g r a t e f u l to the support given to him by other f a c u l t y and graduate students. The author i s indebted t o Mr. J . Walker f o r the help i n b u i l d i n g the experimental equipment. The a s s i s t a n c e o f the N a t u r a l Research C o u n c i l f o r p r o v i d i n g funds through operating Grant No. A-l+528 i s g r a t e f u l l y acknowledged. - i i i -ABSTRACT The anodic p o l a r i z a t i o n o f a r o t a t i n g c y l i n d r i c a l carbon anode i n AlgO^-Na^AlFg and AlgO^-Na^AlF^-Lif e l e c t r o l y t e s has been examined. The anodic p o l a r i z a t i o n has been measured at temperatures o f 8Uo°G, 990°C, 1020°C. and 10Uo°C. and at 2 \rt.% and 5 vt.% alumina concentra-t i o n s . An expression has been derived t o ex p l a i n the experimental T a f e l slopes of 0.07 ± 0.02 V o l t s found at the current d e n s i t i e s o f 0.2 - 1.0 A./cm.^. The d i f f u s i o n p o l a r i z a t i o n has been estimated by measuring the p o l a r i z a t i o n decreases found on r o t a t i n g the anode at speeds up to U900 r.p.ia.. D i f f u s i o n p o l a r i z a t i o n accounts f o r l e s s than 10$ of the t o t a l anodic p o l a r i z a t i o n f o r anodic current d e n s i t i e s l e s s than 0.25 A./cm. . A semi-empirical expression has been derived f o r the d i f f u s i o n - p o l a r i z a t i o n as a f u n c t i o n of the anodic current d e n s i t y and the alumina concentration f o r e l e c t r o l y t e s operated at great e r than Uo°C above t h e i r l i q u i d u s temperatures. - i v -TABLE OF COI'JTENTS TITLE PAGE. i ACKNOWLEDGEMENT i i ABSTRACT i i i TABLE OF CONTENTS i v LIST OF FIGURES v i i LIST OF TABLES. x LIST OF SYMBOLS x i 1. INTRODUCTION 1 1.1. The H a l l - H e r a u l t Process 1 1.1.A. Current E f f i c i e n c y 1 1.1.B. Ohmic P o t e n t i a l Drop h 1.1. C. Busbar & Contact P o t e n t i a l 10 l . l . D . Back .Electromotive Force 10 1.1. D.l Cathode Reaction .... 10 l . l . D . 2 Decomposition Potential.- 11 1.1. D. 3 Anode Reaction 13 1 • ^  P o l a r i z a t i o n 13 1.2. A. D i f f u s i o n Overvoltage lh . I.2.B. Charge Transfer Overvoltage 17 I.2.C. Reaction P o l a r i z a t i o n 18 - V -Page 1.2. D. Methods o f Determining E l e c t r o d e Reaction Mechanisms 19 1.3 Anode Reaction 22 1.3. A. Gases evolved at the Anode 22 I .3.B. Anodic P o l a r i z a t i o n 23 I.3.C. Anode E f f e c t 27 I .3.D. Anode Mechanism 31 l.h Conclusions 1*2 1. 5 Research Program h 5 2. EXPERIMENTAL TECHNIQUE U8 2 .1 Equipment Design 1+8 2.2 Reagents 59 2 .3 Measurement Technique 50 3. RESULTS 63 3.1 P o l a r i z a t i o n Measurements 53 3.1.A. 2 vt.% Alumina - 93 wt.$ C r y o l i t e . . . ,<3 3.I.B. 5 vt.% Alumina - 95 vt.% C r y o l i t e 63 3 .l.C. 5 vt.% Alumina - 20 wt.? Lithium F l u o r i d e -75 vt.% C r y o l i t e 66 3.2 D i f f u s i o n Overvoltase Measurements 71 U. DISCUSSION 81* - v i -Page h.l Accuracy of Results $k h.2 P o l a r i z a t i o n Experiments 90 U.3 D i f f u s i o n Overvoltage Experiments 9H k.k Comparison w i t h P u b l i s h e d Results ;j 02 U.5 Anode Mechanism. 10k 5. CONCLUSIONS 108 6. SUGGESTIONS FOR FUTURE WORK I l l APPENDIX I . V a r i a t i o n o f E l e c t r o d e P o t e n t i a l as a Function of the Anode Gas Composition 115 APPENDIX I I . D i f f u s i o n Overvoltage 1 2 i APPENDIX I I I . Charge Transfer Overvoltage 1 2 6 APPENDIX IV. Reaction Overvoltage 3_32 APPENDIX V. The Faradaic Impedance f o r each type of Overvoltage 1^2 REFERENCES ^ - v i i -LIST OF FIGURES Figure Page 1 The s p e c i f i c c o n d u c t i v i t y o f i>Ta0AlFY-MA melts at innnop j o x 1000°C. _ 2 The d e n s i t y of Na3AlFg-?.4Ax melts at 1000°C 7 3 The i n t e r f a c i a l t e n s i o n (electrolyte/aluminum) of Na 3AlF 6-MA x melts at 1000°C ' 8 . k The v i s c o s i t y of Na^AlF^-MA^. melts at 1000°C. 5 Freezing p o i n t depression of molten c r y o l i t e w i t h d i f f e r e n t a d d i t i v e s q o 6 Summary of publ i s h e d anodic p o l a r i z a t i o n T a f e l p l o t s f o r ?5 c r y o l i t e based melts oc 7 V a r i a t i o n o f anodic p o l a r i z a t i o n as a f u n c t i o n o f alumina content 26 8 Faradaic impedance, p y r o l y t i c graphite cut p e r p e n d i c u l a r t o the b a s a l plane at 8, 20 and ho mA./cm.^ 28 9 Faradaic impedance, v i t r e o t B carbon at 8 and 20 mA./cm.2... 28 10 Faradaic impedance, baked carbon 8, 20 and ^0 mA./cm.^.... 29 11 R e l a t i o n s h i p between c.c.d. and the alumina content 29 1 2 Anodic overvoltage vs current d e n s i t y at 1000°C 35 13 Exchange current d e n s i t y vs a l u m i n a c o n c e n t r a t i o n 30 ik Faradaic impedance f o r d i f f u s i o n p o l a r i z a t i o n h i 15 Faradaic impedance f o r charge t r a n s f e r p o l a r i z a t i o n k i 16 Faradaic impedance f o r heterogeneous r e a c t i o n p o l a r i z a t i o n L i 17 Schematic of experimental apparatus 50 18 Schematic of (COg, C 0/C/Al 2 0 3-Ka 3AlFg) reference e l e c t r o d e assembly. 53 19 Schematic of ( A l / l f a ^ A l F ^ - A l g O ^ a t i ) reference electrode assembly 53 - v i i i -Figure Page 20 Furnace temperature p r o f i l e 55 21 Schematic o f e l e c t r i c a l measuring c i r c u i t f o r measuring anodic p o l a r i z a t i o n 56 22 O s c i l l o s c o p e t r a c e of the time constant of the equipment.. 5b 23 V a r i a t i o n o f p o t e n t i a l between the anode and refe r e n c e e l e c t r o d e w i t h time Cl 2k T a f e l p o l a r i z a t i o n p l o t of run 19 at 1010°C. — 2 vt.% A 1 2 ° 3 " 9 8 **** W a 3 A 1 F 6 61* 25 T a f e l p o l a r i z a t i o n p l o t o f run 22 at 10l+0°C. — 2 vt.% A 1 2 ° 3 ~ 9 8 v r t ,'° Na^AlFfc 65 26 T a f e l p o l a r i z a i t o n p l o t of run 23 at 990°C. — 5 vt.% A 1 2 ° 3 " 9 5 N a 3 A 1 F 6 6 ? 27 T a f e l p o l a r i z a t i o n p l o t o f run 10 at 1025°C. — 5 vt.% M2°3 - 95 vt.% TTa 3AlFg 68 28 T a f e l p o l a r i z a t i o n p l o t o f run 26 at 10k0°C. — 5 vt.% A 1 2 0 3 - 95 '--Tt.% Ha^AlFg 69 29 T a f e l p o l a r i z a t i o n p l o t of ran 2k at 990°C. — 5 vt.% A 1 2 0 3 - 95 vt.% Na 3 A l F 6 70 30 T a f e l p o l a r i z a t i o n p l o t of runs 27 and 29 at 31*0°C. — 5 w t . ^ A l 2 0 3 - 20 \rt.% L i F - 75 vt.% Na-jAlFg 72 31 T a f e l p o l a r i z a t i o n p l o t of run 31 at 990°C. — 5 vt.% A 1 2 0 3 - 20 vt.% L i F - 75 vt.% Na 3AlPg 73 32 P o l a r i z a t i o n decrease vs. Re .for run 19 at 1010°C. —' 2 vt.% A 1 2 0 3 - 90 vt.% N a 3 A l F g 7)4 33 P o l a r i z a t i o n decrease vs Re f o r run 22 at 10VO°C. — 2 vt.% A 1 2 0 3 - 98 vt.% ? I a 3 A l F 6 75 3^ P o l a r i z a t i o n decrease vs Re f o r runs 23 and. 2^ at Q90°C — 5 vt.%. A 1 2 0 3 - 95 vt.% N a 3 A l F 6 78 35 P o l a r i z a t i o n decrease vs Re f o r run 26 at 10i+0°C. — 5 vt.% - 95 vt.% r-!a3AlFg 79 36 P o l a r i z a t i o n decrease vs Re f o r runs 27 and 29 at 8k0°C. — 5 vt.% A 1 2 0 3 - 20 vt.% L i F - 75 vt.% N a ^ l F g 80 - IX Figure Page 37 1+0 Ohraic r e s i s t a n c e vs. anodic current d e n s i t y f o r run 27 8 l 33 Ohmic r e s i s t a n c e vs. anodic current d e n s i t y f o r run 3 0 . . . 39 Photograph of experimental c e l l and as s o c i a t e d equipment. 82 85 ' cm., O s c i l l o s c o p e t r a c e of p o t e n t i a l vs. time - 0 .60 A./cm 0.2 V./cm., 0.5 msec./cm 87 hi O s c i l l o s c o p e tra.ce of p o t e n t i a l vs. time - 0.09 A. /cm. c , 0.1V./em., 0 .5 .msec./cm 83 h2 D i f f u s i o n overvoltage vs. anodic current d e n s i t y 97 h3 O s c i l l o s c o p e t r a c e of p o t e n t i a l r i s e t o a quasi-steady s t a t e value a f t e r the c e l l c i r c u i t i s c l o s e d - O.'+O A. /cm. , 0.2 V./cm., 5.0 sec./cm 100 kh S u p e r p o s i t i o n of p o t e n t i a l decay curves at 0.09 A./cm, f o r r o t a t i o n speeds ( l e f t t o r i g h t ) of 3000, 1000 and 100 r.p.m 101 U5 Summary of published anodic T a f e l p o l a r i z a t i o n p l o t s and the extreme T a f e l p o l a r i z a t i o n p l o t s of toe present work (Runs 22 and 29). . 103 h6 Proposed anode and c e l l design 112 i*7 Schematic r e p r e s e n t a t i o n of the e f f e c t of the compact double l a y e r p o t e n t i a l on the anodic and cathodic a c t i v a t i o n energies f o r a net cathodic current at the el e c t r o d e " 127 - X -LIST OF TABLES Taple Page I Observed d e p o s i t i o n p o t e n t i a l i n mV. of 5 AlgO^ i n LiF- N a j A l F ^ " m e l t s . 12 I I P r o p e r t i e s o f G r a p h i t i t e A by Carbon Carborundum Corp lip I I I E l e c t r o l y t e compositions and temperatures 5? IV Summary of d i f f u s i o n overvoltage experiments 76 - x i -LIST OF SYMBOLS Symbol Q.L A c t i v i t y of s p e c i e s " i 1 1 Ci. Concentration of species " i " Cj. Bulk c o n c e n t r a t i o n of species' 1!" C s Capacitance of a.c. impedance X) D i f f u s i o n c o e f f i c i e n t Average diameter of bubble on desorption £ 0 E q u i l i b r i u m p o t e n t i a l E l e c t r o d e reference p o t e n t i a l at current d e n s i t y " ! " £ (Vi E l e c t r o d e reference p o t e n t i a l at zero current 3* Faraday equivalent V, Current d e n s i t y b 0 Charge t r a n s f e r exchange current d e n s i t y l r L i m i t i n g heterogeneous r e a c t i o n current d e n s i t y L i m i t i n g d i f f u s i o n current d e n s i t y K. Reaction r a t e constant Ke^. E q u i l i b r i u m constant Distance from edge of e l e c t r o d e . Y\ E l e c t r o d e r e a c t i o n valence -p Chemical r e a c t i o n order T*-. Pra,ndtl No. "Re.. Reynolds No. - x i i -Symbol "R.s Real term of a.c. impedance T Temperature °C. or °K. t Time \j^o V e l o c i t y adjacent to surface o f r o t a t i n g e l e c t r o d e uo Angular y e l o c i t y •x Chemical species Y Reaction order f o r d e s o r p t i o n of C0 2 "Z, Impedance JL Charge t r a n s f e r valence oc Charge t r a n s f e r c o e f f i c i e n t P Surface coverage f r a c t i o n o f the e l e c t r o d e Nernst d i f f u s i o n l a y e r t h i c k n e s s f o r t u r b u l e n t f l u i d flow o ? P e n e t r a t i o n Model's Nernst d i f f u s i o n l a y e r t h i c k n e s s <5 Nernst d i f f u s i o n l a y e r t h i c k n e s s f o r a gas e v o l v i n g e l e c t r o d e Overvoltage or p o l a r i z a t i o n -n D i f f u s i o n overvoltage Charge t r a n s f e r overvoltage Reaction p o l a r i z a t i o n or overvoltage Q x| Surface coverage f r a c t i o n of species" x' B x | E q u i l i b r i u m surface coverage f r a c t i o n o f species x -n_ Resistance V Kinematic v i s c o s i t y %. S t o i c h i o m e t r i c f a c t o r of species S i i n o v e r a l l e l e c t r o d e r e a c t i o n V Rate of gas e v o l u t i o n - XI11- -Symbol [ 1 In metal nhase L ] In e l e c t r o l y t e nhase 1. INTRODUCTION 1 . 1 The H a l l - H e r a u l t Process Aluminum metal i s produced i n d u s t r i a l l y by the e l e c t r o l y t i c r e d u c t i o n of alumina. The H a l l - H e r a u l t process accounts f o r over 95% of the t o t a l production. In t h i s process alumina p u r i f i e d by the Bayer process, i s d i s s o l v e d i n a fused s a l t composed p r i m a r i l y o f sodium and aluminum f l u o r i d e s . Calcium and l i t h i u m f l u o r i d e s may be 1 present between 5 wt.% and 10 vt.% . The anode i s carbon w h i l e the cathode i s a molten pad of aluminum. The anode i s consumed by the process w h i l e the cathode i s produced. The commercial H a l l - H e r a u l t process r e q u i r e s 6 . 0 - 9 . 0 kwhr. per pound o f aluminum produced which i s at l e a s t four times the t h e o r e t i c a l power r e q u i r e d t o reduce alumina. F i v e f a c t o r s c o n t r i b u t e t o t h i s i n e f f i c i e n c y . These are:-poor current e f f i c i e n c y , ohmic power l o s s i n the e l e c t r o l y t e , ohmic power l o s s i n the busbars and c e l l c o n t a c t s , the anodic and cathodic p o l a r i -z a t i o n and anode e f f e c t s . 1.1.A Current E f f i c i e n c y The current or f a r a d a i c e f f i c i e n c y i n i n d u s t r i a l c e l l s v a r i e s from 80 - 92%. Four sourses account f o r current e f f i c i e n c y l o s s e s . - 2 -These are:- r e o x i d a t i o n o f s o l u b l e subvalent species of aluminum and sodium i n the e l e c t r o l y t e , r e o x i d a t i o n of metal d r o p l e t s swept away from the metal pad, evapouration o f subvalent species and short c i r c u i t i n g between the anode and the cathode. The r e o x i d a t i o n o f the subvalent species o f sodium and aluminum i s considered the most important. The most commonly p o s t u l a t e d chemical e q u i l i b r i a f o r the formation o f these subvalent m e t a l l i c 2 3 k species are: ' ' 2 JAl] + A l 3 + Metal [Xl] + 6lJa+ Metal [Al] + 3Na+ Metal J3Na] + {3Na+j These ions are generated at the aluminum p a d - e l e c t r o l y t e i n t e r f a c e and t r a n s p o r t e d by convection to the v i c i n i t y o f the anode where they 2 react p r i m a r i l y w i t h the carbon d i o x i d e gas bubbles. Thonsta.d has measured the s o l u b i l i t i e s of r'Al+" and "Na +" as f u n c t i o n s o f temperature and " c r y o l i t e r a t i o " (weight r a t i o o f NaF:AlF ). The s o l u b i l i t y o f the aluminum subvalent ions- v a r i e d 10% f o r c r y o l i t e r a t i o s between 1.0 and 3.5- The sodium subvalent species v a r i e d from 0.7 - 0.05 vt.% f o r same c r y o l i t e r a t i o s . S i m i l a r i l y the temperature e f f e c t on the s o l u b i l i t y o f subvalent sodium was much greater than t h a t f o r aluminum. Thonstad and Solbu have measured the { E l e c t r o l y t e ] ^ {3A1+] ( l ) ^ E l e c t r o l y t e " ^ ^ [3Na 2+J+ JA1 3 + J (2) J E l e c t r o l y t e ^ ^ (a1 3 + ) + |3Ua] (3) ^ J3Na 2 +^ (k) - 3 -r a t e of o x i d a t i o n of an aluminum pool which was covered by a c r y o l i t e -alumina fused s a l t . Carbon d i o x i d e was bubbled through the s a l t at v a r y i n g flow r a t e s . The r a t e of o x i d a t i o n was found to be d i r e c t l y p r o p o r t i o n a l to the area of the metal-fused s a l t i n t e r f a c e and a l s o increased w i t h i n c r e a s i n g carbon d i o x i d e f l o w r a t e s up to a l i m i t i n g r a t e . Gjerstad- 1 has a l s o found the r a t e o f o x i d a t i o n i s p r o p o r t i o n a l t o r a t e o f How of carbon d i o x i d e . Grjotheim measured convection by r a d i o a c t i v e t r a c e r technique i n i n d u s t r i a l c e l l s and determined that metal and e l e c t r o l y t e had average v e l o c i t i e s o f 10 - 20 cm./sec. w i t h peak v e l o c i t i e s of 100 cm./sec. Grjotheim^ noted t h a t anode— cathode distance does a f f e c t i n d u s t r i a l c e l l s current e f f i c i e n c y . I n the i n d u s t r i a l c e l l laminar flow i s present except at the ends where v o r t i c e s are found and d i r e c t l y under the anode where v i o l e n t mixing r e s u l t s from the gas e v o l u t i o n . Under the anodes the gas e v o l u t i o n gives r i s e t o r a p i d mixing o f gases and e l e c t r o l y t e and i f the anode-cathode d i s t a n c e i s too small then the i n t e r f a c i a l area between the aluminum pad and the e l e c t r o l y t e w i l l be g r e a t l y enlarged. Good c e l l current e f f i c i e n c y depends on a s i g n i f i c a n t d i f f e r e n c e i n the d e n s i t i e s of the' aluminum and the e l e c t r o l y t e . As the d e n s i t y d i f f e r e n c e increases the i n t e r f a c i a l area decreases, reducing the r a t e of formation o f the subvalent m e t a l l i c i o n s . The l o s s o f current e f f i c i e n c y r e s u l t i n g from metal d r o p l e t s being swept up i n t o the e l e c t r o l y t e w i l l a l s o decrease as the d e n s i t y d i f f e r e n c e i n c r e a s e s . These metal d r o p l e t s are swept away from the metal pool becausethe - k -c i r c u l a t i o n p a t t e r n s of the e l e c t r o l y t e and the metal pad are d i f f e r e n t . The e l e c t r o l y t e c i r c u l a t i o n i s caused by the anode gasses and thermal convection. The metal c i r c u l a t i o n r e s u l t s from the electromagnetic fo r c e s present i n the metal poo l . •7 Haupin' a n a l i z e d the "metal m i s t " observed above aluminum c e l l s . The "mist" contained sodium, aluminum monofluoride and sodium t e t r a f l u o r a l u m i n a t e but would o n l y form i n the presence of moisture. Haupin's evapouration r a t e s as a f u n c t i o n o f the c r y o l i t e r a t i o were p r o p o r t i o n a l to the t o t a l s o l u b i l i t i e s o f "A1+" and "Na 2 +" found by Thonstad 2. 1.1.B Ohmic P o t e n t i a l Drop In the modern aluminum r e d u c t i o n p l a n t s c e l l c u r r e n t s are o f t e n as h i g h as 150,000 Amperes^. The voltage drop i n the e l e c t r o l y t e w i l l vary from 1.3 v o l t s to 2.3 v o l t s depending on what the process c o n t r o l w i l l a l l o w f o r a minimum anode to cathode d i s t a n c e . T h i s represents 35% to k-5% of the t o t a l power consumed by the H a l l - H e r a u l t process. The power l o s s heats the c e l l and l i m i t s the maximum c e l l c u r r e n t . As the c e l l temperature increases the current e f f i c i e n c y decreases and e l e c t r o l y t e l o s s e s increase. I f the c e l l temperature increases to the point where the " f r e e z e " , f r o z e n e l e c t r o l y t e adjacent t o the carbon cathode l i n i n g , melts then the cathode l i n i n g l i f e w i l l be g r e a t l y reduced. Most i n d u s t r i a l c e l l s axe operated w i t h i n 30°C of the l i q u i d u s temperature o f the e l e c t r o l y t e . The r e d u c t i o n of the v o l t a g e drop across the e l e c t r o l y t e may be - 5 -accomplished i n two ways. The f i r s t i s t o reduce the r e s i s t i v i t y of the e l e c t r o l y t e and maintain the same anode t o cathode d i s t a n c e . The second approach i s t o deduce the anode t o cathode d i s t a n c e hut t h i s r e q u i r e s g r e a t e r process c o n t r o l which may not be p o s s i b l e . The e l e c t r o l y t e r e s i s t i v i t y may be changed by the a d d i t i o n o f va r i o u s h a l i d e s a l t s . These h a l i d e s a l t s must a l s o not adversely a f f e c t the e l e c t r o l y t e ' s p h y s i c a l and chemical p r o p e r t i e s . The important p r o p e r t i e s a r e : - d e n s i t y , e l e c t r i c a l c o n d u c t i v i t y , r a t e o f s o l u t i o n and s o l u b i l i t y of alumina, l i q u i d u s temperature, b a s i c i t y , surface t e n s i o n between aluminum and carbon and surface t e n s i o n between aluminum and the e l e c t r o l y t e . The l e s s important p r o p e r t i e s are:- v i s c o s i t y , surface t e n s i o n between anode carbon and e l e c t r o l y t e and e l e c t r o l y t e vapour pressure. At 1000°C. the d e n s i t i e s o f aluminum and c r y o l i t e are 2.29 g/c.c. and 2.11 g/c,c. r e s p e c t i v e l y . Any s a l t which incr e a s e s the e l e c t r o l y t e ' s d e n s i t y w i l l e i t h e r reduce current e f f i c i e n c y or make process c o n t r o l more d i f f i c u l t . The r a t e of s o l u t i o n and the s o l u b i l i t y of alumina i s very c r i t i c a l t o process c o n t r o l . Most h a l i d e s a l t a d d i t i v e s reduce the s o l u b i l i t y o f alumina and r e q u i r e g r e a t e r process c o n t r o l as a r e s u l t . A l l h a l i d e s a l t a d d i t i v e s lower the l i q u i d u s temperature and t h i s i s favourable. Lower temperatures reduce the r a t e o f formation of the subvalent species [Alf] and reduce e l e c t r o l y t e evapouration l o s s e s . The surface t e n s i o n between aluminum and carbon a f f e c t s the r a t e at which the carbon l i n i n g i s destroyed. As the c e l l temperatures vary the aluminum and sludge - 6 -adjacent t o the carbon w i l l f r e e z e and melt y i e l d i n g a freeze-thaw e f f e c t . S i m i l a r i l y the degree of w e t t i n g of the cathode carbon s i d e -w a l l by e l e c t r o l y t e when i t i s l i q u i d w i l l a l s o a f f e c t the degree of e r o s i o n o c c u r r i n g during temperature v a r i a t i o n s . The b a s i c i t y of the e l e c t r o l y t e g r e a t l y a f f e c t s the p u r i t y of the metal pad. The a c t i v i t y of sodium i n the aluminum pad increases r a p i d l y f o r c r y o l i t e r a t i o s greater than 1.20:1 by wei g h t 8 . While the surface t e n s i o n between the e l e c t r o l y t e and the carbon anode does i n f l u e n c e the current d e n s i t y at the onset o f the anode e f f e c t the alumina c o n c e n t r a t i o n i s much more important. Figures 1 - 5 summarize the e f f e c t t h a t a d d i t i v e s have on the p r o p e r t i e s of cryolite"*". L i t h i u m f l u o r i d e and sodium c h l o r i d e appear to be the best a d d i t i v e s . Sodium c h l o r i d e has the u n d e s i r a b l e property i n th a t i t s presence g r e a t l y increases the melt's b a s i c i t y w i t h the subsequent r e d u c t i o n i n metal p u r i t y . L i t h i u m f l u o r i d e i s expensive and does not a f f e c t the e l e c t r o l y t e p r o p e r t i e s t o as favourable degree as sodium c h l o r i d e . L i t h i u m f l u o r i d e has been found to increase c e l l current e f f i c i e n c y . 9 - 7 -Addition^ ( Mole % ) Figure k. The v i s c o s i t y o f Na^AlF^-MA.^ melts at 1000°C. - 9 -1020 1000 u o 980 LU \ \\\\N\ I RATI 960 \ \ \ V \ 2 L U a. LU 940 \ \ \ ^ N a C I 92 0 \ \ AIF 3 M g F 2 900 \ B e F 2 BaCI 2 1 1 1 1 1 • ' • • 0 10 20 30 40 50 Add i t ions ( Mo|e % ) Figure 5. Freezing p o i n t depression of molten c r y o l i t e w i t h d i f f e r e n t a d d i t i v e s . - 10 -1.1.C Busbar and Contact P o t e n t i a l s The power l o s s e s i n the i n d u s t r i a l c e l l busbar system have been optimized by busbar design and s i z e . The v o l t a g e drop due t o c e l l connections i s approximately 0.15 - 0.30 v o l t s . The l e n g t h of the busbars i n a p o t l i n e as w e l l as the current they c a r r y make i t un-economic t o reduce t h i s l o s s any f u r t h e r . The contact r e s i s t a n c e between the aluminum pad and the carbon l i n i n g i s normally l e s s than 0.3 v o l t s . I f the l a y e r of alumina between the aluminum pad and the l i n i n g i s deep and hard then much higher p o t e n t i a l drops w i l l occur. This "sludging e f f e c t " depends on c e l l d e s i g n , process c o n t r o l and the form of the alumina being fed t o the c e l l s . 1•1• D Back E l e c t r o m o t i v e Force The back emf of the c e l l s i n the p o t l i n e i s composed o f the decomposition p o t e n t i a l , the anodic p o l a r i z a t i o n and the cathodic p o l a r i z a t i o n . 1.1.D.l Cathode Reaction The r e d u c t i o n of aluminum and sodium occurs at the cathode. In e l e c t r o l y t e s c o n t a i n i n g c r y o l i t e - a l u m i n a - l i t h i u m f l u o r i d e the decomposition p o t e n t i a l s f o r a platinum oxygen-evolving anode vary from 2.0 - 2.3 v o l t s f o r both aluminum and sodium. L i t h i u m has a decomposition p o t e n t i a l o f - 11 -0.2 v o l t s higher than aluminum as shown i n Table 1.^ The cathodic overvoltage measured i n l a b o r a t o r y c e l l s under c o n d i t i o n s s i m i l a r t o i n d u s t r i a l o p e r a t i o n has been found t o be i n the range of 0.05 - 0 .2 v o l t s . 1 0 The d i f f e r e n c e i n r e d u c t i o n p o t e n t i a l s of aluminum and sodium i s 0.1 - 0 .3 v o l t s . This d i f f e r e n c e increases as the c r y o l i t e r a t i o i s decreased so that most c e l l s operate at c r y o l i t e r a t i o s o f 1.25 - 1 .35 . 1.1.D.2 Decomposition P o t e n t i a l s The decomposition p o t e n t i a l o f alumina i n c r y o l i t e depends p r i m a r i l y on the anode r e a c t i o n , although the c r y o l i t e r a t i o w i l l e f f e c t the r e d u c t i o n p o t e n t i a l f o r aluminum and sodium.. The decomposition p o t e n t i a l f o r c r y o l i t e saturated i n alumina i s 2.19 v o l t s at 1000°C. I f a carbon anode i s used the decomposition p o t e n t i a l depends on the off-gas composition as f o l l o w s : E = 1.15 a. + 1.0U l - x + 0.125 X l o g x - 0.125 1+x . - x+1 J (1+x) where: Ex x l - x Decomposition p o t e n t i a l at gas composition "x." P a r t i a l pressure of C0 2 evolved i n atmosphere. = P a r t i a l pressure of CO evolved i n atmospheres, assumes - u n i t a c t i v i t y of AlgO^, A l - t o t a l pressure = 1 atmosphere - 12 -TABLE I OBSERVED DEPOSITION POTENTIAL IN raV. OF 5 VT.% A 1 2 0 3 IN LiF-Na 3A.lF 6 MELTS Composition D e p o s i t i o n p o t e n t i a l s , mV. vs 0~ e l e c t r o d e A l Na+ Li+ 95# Na 3AlFg % A 1 2 0 3 2065. 2320. 5$ L i F 90$ Na 3AlFg 5% A 1 2 0 3 2080. 2280. 2390. 10? L i F 85$ Na^AlFg 5# A 1 2 0 3 2105. 2270. 2385. 1% L i F 80? N a 3 A l F 6 ' , 5# A 1 2 0 3 2130. • 2265. 2 3 7 0 . - 13 -The d e r i v a t i o n of equation (5) i s given i n Appendix I and i s based on a paper by Thonstad et a l . ^ - This equation agrees w i t h the experimental data obtained by Thonstad. 1.1.D.3 Anode Reaction At present there i s c o n f l i c t i n g ideas as t o the mechanism and magnitude of the anodic p o l a r i z a t i o n . This r e a c t i o n w i l l be t r e a t e d i n d e t a i l a f t e r c o n s i d e r i n g p o l a r i z a t i o n mechanisms. 1.2 P o l a r i z a t i o n V e t t e r d e f i n e s overvoltage as the d i f f e r e n c e i n the p o t e n t i a l o f an e l e c t r o d e when a current f l o w s , t o the e q u i l i b r i u m p o t e n t i a l when no current f l o w s . This assumes t h a t the zero current p o t e n t i a l i n v o l v e s the same el e c t r o d e r e a c t i o n . P o l a r i z a t i o n i s de f i n e d as the d i f f e r e n c e i n p o t e n t i a l of the e l e c t r o d e when current flows to the mixed p o t e n t i a l of s e v e r a l e l e c t r o d e r e a c t i o n s o c c u r r i n g at zero c u r r e n t . = E - Eo = overvoltage (6) "V^  = E ( i ) - E(o) = p o l a r i z a t i o n (7) V e t t e r defines anodic c u r r e n t s as p o s i t i v e , generating a p o s i t i v e overvoltage at the anode s i m i l a r i l y cathodic currents are negative generating negative overvoltages at the cathode. The o v e r a l l e l e c t r o d e r e a c t i o n i s composed of the f o l l o w i n g p o s s i b l e p a r t i a l r e a c t i o n s : -- Ik -t r a n s p o r t o f r e a c t a n t s and products t o or from the e l e c t r o d e s u r f a c e , t r a n s p o r t o f charge c a r r i e r s across the e l e c t r i c a l double l a y e r , a chemical r e a c t i o n o c c u r r i n g e i t h e r homogeneously i n the e l e c t r o l y t e w i t h i n the d i f f u s i o n l a y e r or heterogeheously a t the e l e c t r o d e ' s surface and a c r y s t a l l i z a t i o n r e a c t i o n whereby the r e a c t i n g atoms are incorpo r a t e d or removed from the e l e c t r o d e ' s l a t t i c e . For the d i s c u s s i o n of these p a r t i a l e l e c t r o d e r e a c t i o n s i n i t i a l l y assume t h a t the e q u i l i b r i u m p o t e n t i a l and the zero current p o t e n t i a l are the same, i . e . : - the d i f f e r e n c e i n p o t e n t i a l w i l l be an overvoltage. 1.2.A D i f f u s i o n Overvoltage The d i f f u s i o n overvoltage i s the change i n the e l e c t r o d e p o t e n t i a l a r i s i n g from the change i n con c e n t r a t i o n o f the e l e c t r o d e r e a c t a n t s or products as a r e s u l t o f current flow. From Appendix I I the d i f f u s i o n overvoltage can be expressed i n terms of concentrations o r current d e n s i t i e s . In both expressions a l a r g e i n d i f f e r e n t e l e c t r o l y t e i s present and there i s no preceding chemical e q u i l i b r i u m . In the aluminum r e d u c t i o n c e l l the sodium and f l u o r i d e ions are the 13 charge c a r r i e r s . The e l e c t r i c a l m i g r a t i o n e f f e c t o f the oxygen-ion-complexes can be considered to be n e g l i g i b l e . The d i s s o c i a t i o n of the (8) (9) - 15 -complex aluminum - oxygen anions i s considered to be r a p i d and not 11+ r a t e determining t h e r e f o r e chemical homogeneous e q u i l i b r i a e f f e c t s may be assumed t o be n e g l i g i b l e . The s i z e of the d i f f u s i o n overvoltage depends on the c o n c e n t r a t i o n d i f f e r e n c e across the "Nernst" d i f f u s i o n l a y e r . The c o n c e n t r a t i o n d i f f e r e n c e depends on the width o f the Nernst d i f f u s i o n l a y e r and the e l e c t r o d e current d e n s i t y . The width of the Nernst d i f f u s i o n l a y e r f o r e l e c t r o l y t e f l o w i n g across the e l e c t r o d e i s a f u n c t i o n o f the r a t e of f l o w , the v i s c o s i t y and the d i f f u s i o n c o e f f i c i e n t . The d i f f u s i o n l a y e r t h i c k n e s s f o r t u r b u l e n t f l o w i s given by V e t t e r t o be:-I S « JL - U U T D — JL R e Pr (10) where - S = Nernst d i f f u s i o n l a y e r t h i c k n e s s f o r t u r b u l e n t flow X = d i s t a n c e from the edge of the e l e c t r o d e T^C = v e l o s i t y adjacent t o the surface o f the r o t a t i n g e l e c t r o d e ^ = Kinematic v i s c o s i t y D = D i f f u s i o n c o e f f i c i e n t Re = Reynolds No. P r = Pran&L No. The d i f f u s i o n l a y e r t h i c k n e s s i n (10) i s defined f o r a f l o w i n g e l e c t r o l y t e across the e l e c t r o d e surface. The v i s c o s i t y of c r y o l i t e - a l u m i n a melts has 15 - 17 been measured by s e v e r a l workers. The v a r i a t i o n of v i s c o s i t y as a -16-f u n c t i o n of temperature i s sm a l l . Nishihana"^ found t h a t v i s c o s i t y decreased l e s s than 10% f o r a temperature v a r i a t i o n from 1030°C t o 1070°C f o r both 2 vt.% and 5 vt.% alumina - c r y o l i t e melts. Only f o r high alumina concentrations (greater than 10 vt.%) d i d the v i s c o s i t y decrease s i g n i f i c a n t l y f o r temperature i n c r e a s e s . Vayna 1^. determined t h a t , c r y o l i t e melts c o n t a i n i n g O.vt.% - 10. vt.% alumina showed very small v i s c o s i t y i ncreases at temperatures very c l o s e t o t h e i r l i q u i d u s temperatures. Janssen'^''^ has measured d i f f u s i o n l a y e r thicknesses f o r gas ev o l v i n g e l e c t r o d e s . A l l work i n t h i s f i e l d has been c a r r i e d out at low temperatures i n aqueous s o l u t i o n s i n order t h a t the gas bubble e v o l u t i o n could be f i l m e d . A wide v a r i e t y o f l o g $/ l o g i slopes have heen determined. At higher current d e n s i t i e s the slope increases i n absolute value. A recent t h e o r e t i c a l attempt to e x p l a i n the v a r i a t i o n of the Nernst d i f f u s i o n l a y e r t h i c k n e s s as a f u n c t i o n of the r a t e of gas 20 e v o l u t i o n has been made by I b l et a l . The e l e c t r o l y t e mixing was considered to be caused by the bubble forming at the e l e c t r o d e surface and pushing the e l e c t r o l y t e away. On detachment from the e l e c t r o d e surface the e l e c t r o l y t e sweeps i n behind the bubble. I b l formulated a pe n e t r a t i o n model on t h i s p r i n c i p l e and derived the f o l l o w i n g equation: \Z "XT ( I - p ) (11) - 17 -where - Sp = P e n e t r a t i o n models Nernst d i f f u s i o n l a y e r t h i c k n e s s D = D i f f u s i o n c o e f f i c i e n t of r e a c t i n g species d = Average diameter of bubble on d e s o r p t i o n b v = r a t e of gas e v o l u t i o n p = Surface coverage f u n c t i o n of the e l e c t r o d e by gas bubbles T h i s equation ( l l ) p r e d i c t s the slope of a l o g £p vs Log i p l o t to be(- 0.5). Janssen*" 9 found a slope near(- 0.3)at low c u r r e n t d e n s i t i e s and as h i g h a s ( - 0.9)at current d e n s i t i e s greater than 30 mA/cm^. Janssen considers t h a t the d i f f u s i o n l a y e r t h i c k n e s s at low c u r r e n t d e n s i t i e s i s caused by e l e c t r o l y t e f l o w adjacent t o the e l e c t r o d e due t o I b l ' s p e n e t r a t i o n mechanism. At higher current d e n s i t i e s the bubbles on the e l e c t r o d e surface coalesce r e s u l t i n g i n extreme mixing. Films-*-^ of the bubbles c o a l e s c i n g showed t h a t the bubbles v i b r a t e d very s t r o n g l y on c o a l e s c i n g . These bubbles rose v e r t i c a l l y from the surface of a h o r i z o n t a l e l e c t r o d e i n s t e a d of being swept i n the d i r e c t i o n o f the e l e c t r o l y t e flow as was the case f o r the non-coalescing bubble e v o l u t i o n . 1.2.B Charge Transfer Overvoltage The r e s i s t a n c e t o t r a n s f e r across the Helmholtz double l a y e r by the e l e c t r o d e r e a c t a n t s or products i s c a l l e d Charge Transfer overvoltage. The a c t i v a t i o n energies increase i n the d i r e c t i o n of the e l e c t r o d e ' s current as shown i n Figure (-U?) of Appendix I I I . The equation r e l a t i n g the e l e c t r o d e ' s (12) T 18 -current d e n s i t y t o pure charge t r a n s f e r overvoltage i s known as the. Butler-Volmer e q u a t i o n x , i . e . I- ) R T 1 RT -I At higher overvoltages t h i s equation may be approximated by: ( i ) Anodic Overvoltages y > Q L _ L 0 e-xp ocZ.'3'n \ OR n _ RT U j . _ R T \n. i. (13) ( i i ) Cathodic Overvoltages T\. t < $ c o i r — — j l* o-«o*3- (Psyo-1 I f the anode r e a c t i o n c o n s i s t e d o f two successive c h a r g e - t r a n s f e r r e a c t i o n s w i t h an intermediate e l e c t r o d e species produced and consumed, the charge t r a n s f e r overvoltage i s given as derived i n Appendix I I I . L - 2 i-o, [ exp ocR 3 -1* e * p - « g f t i ] (15) e For l a r g e anodic overvoltage t h i s reduces t o : L = 2 u R <*pf«*i 3nt1 (16) 1 RT J where: » . j g h [ L ^ ] 1.2.C Reaction Overvoltage Reaction overvoltage i s caused by a slow chemical r e a c t i o n . The species being consumed or produced by t h i s r e a c t i o n a f f e c t s the -19-e q u i l i b r i u m p o t e n t i a l of the a s s o c i a t e d charger t r a n s f e r r e a c t i o n . From Appendix IV the r e a c t i o n overvoltage i s given by: where: S = Species which accumulates or i s depleted as a r e s u l t of the slow chemical r e a c t i o n . n = Charge t r a n s f e r valence of the p a r t i a l e l e c t r o d e r e a c t i o n which precedes or f o l l o w s the slow chemical r e a c t i o n . The r e a c t i o n overvoltage may a l s o be expressed i n terms of current d e n s i t y . From Appendix IV , the r e a c t i o n o v e r v o l t a g e , i s : "•"U = T R T In [ i - _L ] (18) pn. 3» L I P where: i ^ = L i m i t i n g hetergeneous r e a c t i o n current d e n s i t y P = Order of r e a c t i o n f o r the slow chemical r e a c t i o n T = S t o i c h i o m e t r i c f a c t o r of the a s s o c i a t e d p a r t i a l e l e c t r o d e r e a c t i o n . 1.2.D Methods f o r Determining E l e c t r o d e Reaction Mechanisms In order t o determine the r e a c t i o n mechanism at an e l e c t r o d e the v a r i a t i o n of the e l e c t r o d e p o t e n t i a l at d i f f e r e n t current d e n s i t i e s must be known. The T a f e l plot* p o l a r i z a t i o n vs l o g current d e n s i t y , i s the best i n i t i a l approach. From t h i s p l o t the e l e c t r o d e r e a c t i o n may be p a r t i a l l y determined. I f the slope changes with i n c r e a s i n g - 20 -current d e n s i t y t h i s i n d i c a t e s t h a t the r e a c t i o n mechanism a l s o has changed. I f the slope decreases at higher c u r r e n t d e n s i t i e s then a second e l e c t r o d e r e a c t i o n i n v o l v i n g d i f f e r e n t r e a c t a n t s and products may be o c c u r r i n g simultaneously w i t h the r e a c t i o n which was predominant at lower current d e n s i t i e s . Two approaches may be employed to measure overvoltages as a f u n c t i o n o f cur r e n t d e n s i t y . The d i r e c t measurement approach measures the e l e c t r o c h e m i c a l c e l l v o l t a g e s as a f u n c t i o n o f the current d e n s i t y . The other e l e c t r o d e i n v o l v e d i n the r e a c t i o n has a very l a r g e surface area so t h a t i t s current d e n s i t y i s low enough to be considered n o n p o l a r i z a b l e at the c e l l c u r r e n t s . The el e c t r o d e overvoltage i s then obtained by s u b t r a c t i n g the ohmic drop. When a reference e l e c t r o d e i s used i t must have a s t a b l e p o t e n t i a l which does not vary w i t h c e l l c urrent or time due t o c o r r o s i o n by the e l e c t r o l y t e . I n a d d i t i o n the reference e l e c t r o d e must be c l o s e t o the e l e c t r o d e t o reduce the r e s i s t i v e drop between i t and the e l e c t r o d e . While the el e c t r o d e must be c l o s e t o reduce t h i s r e s i s t a n c e drop i t must not s h i e l d the e l e c t r o d e surface otherwise the current d e n s i t y at the surface c l o s e s t t o the reference e l e c t r o d e w i l l be l e s s . x The d i r e c t method allows accurate t o t a l overvoltage measurements. I f the overvoltage i s f l u c t u a t i n g due t o a gas being evolved at the el e c t r o d e ' s surface then t h i s method w i l l a l l o w accurate time-averaged values. The weakness of the method i s t h a t the e l e c t r o l y t e r e s i s t a n c e must be constant. With gas e v o l v i n g e l e c t r o d e s the e l e c t r o l y t e c o n d u c t i v i t y g r e a t l y decreases near the e l e c t r o d e . The e f f e c t i v e ' r e s i s t a n c e at higher current d e n s i t i e s can o n l y be estimated. - 21 -The second approach i s to measure overvoltages i n s t a n t a n e o u s l y a f t e r the current i s a l t e r e d . I f steady-state has been reached and the e l e c t r i c a l c i r c u i t i s broken then the r e s i s t a n c e part disappears instantaneously. While t h i s method e l i m i n a t e s the r e s i s t a n c e term of the overvoltage the p o t e n t i a l decay curve i s a negative e x p o t e n t i a l . As a r e s u l t of the shape of the decay curve very ra.pid measuring i n s t r u -ments are needed to record the p o t e n t i a l . O s c i l l o s c o p e s and synchroscopes w i t h time bases of l e s s than 1. p.sec. should be used. This r e q u i r e s an e l e c t r o n i c s w i t c h i n g c i r c u i t which has a p e r f e c t l y 22 matched impedance power sourse. Takaha.shi & Amada developed a measuring c e l l u s i n g an i n t e r r u p t o r which was capable of measuring overvoltages at a time base of l e s s than 1. usee. I t only took them four years t o f u l l y develope t h e i r technique! The measurement of d i f f u s i o n overvoltage may be achieved by r o t a t i n g e l e c t r o d e s or chronopotentiometry. The r o t a t i n g e l e c t r o d e s measure the overvolta.ge decrease as a f u n c t i o n of r o t a t i o n . The r o t a t i n g d i s c e l e c t r o d e i s the most commonly used since the v e l o c i t y i s almost constant across the surface. U n f o r t u n a t e l y f o r a 'gas e v o l v i n g e l e c t r o d e , the gas becomes trapped under the d i s c and r e s u l t s i n very l a r g e o s c i l l a t i o n s i n the e l e c t r o d e p o t e n t i a l . The c y l i n d r i c a l e l e c trode does not have the gas entrapment problem but the v e l o c i t y p r o f i l e has not been a c c u r a t e l y determined. I f the c y l i n d r i c a l , r o t a t i o n i s not p e r f e c t l y t r u e then c a v i t a t i o n w i l l occur at the e l e c t r o d e surface and t h i s w i l l not only a f f e c t the d i f f u s i o n l a y e r but a l s o - 22-a f f e c t the desorption o f the gaseous product. Chronopotiometry measures the l i m i t i n g d i f f u s i o n c i i r r e n t s by the a p p l i c a t i o n of l a r g e c u r r e n t s and measuring the time constant. V e t t e r gives a very d e t a i l e d a n a l y s i s based on the Sand Equation. The f a r a d a i c impedance o f an e l e c t r o d e r e a c t i o n w i l l v a r y w i t h the d i f f e r e n t types o f e l e c t r o d e p o l a r i z a t i o n . This method enables the p o l a r i z a t i o n type t o be determined but does not y i e l d the type or number of species t a k i n g p a r t i n the r e a c t i o n , v i z - Faradaic impedance w i l l determine i f the p o l a r i z a t i o n i s ch a r g e - t r a n s f e r o r not but i t w i l l not measure how many e l e c t r o n s are i n v o l v e d . Most experimental measurements are c a r r i e d out at low current d e n s i t i e s where there i s a l i n e a r r e l a t i o n s h i p between p o l a r i z a t i o n and current d e n s i t y . Vetter 1'^ gives a d e t a i l e d d e s c r i p t i o n o f the theory i n v o l v e d . 1.3 Anode Reaction 1.3.A Gases Evolved at the Anode At the anode i n the c r y o l i t e - a l u m i n a c e l l , the three p o s s i b l e gases which c o u l d be evolved are carbon monoxide, carbon d i o x i d e and carbon t e t r a f l u o r i d e . In normal operation of an aluminum r e d u c t i o n c e l l at an anode current d e n s i t y of 0.6 - 1.1 A/cm? the anode gas e l e c t r o l y t i c a l l y evolved i s 100% carbon d i o x i d e . Carbon monoxide-is formed by the r e a c t i o n o f the anode gas e i t h e r w i t h carbon o r w i t h subvalent species present i n the bath. During an anode e f f e c t the - 23 -ele c t r o d e p o t e n t i a l i s high enough due t o the d i f f u s i o n overvoltage t h a t carbon t e t r a f l u o r i d e 2 3 > 2 ^ i s formed. At 1000°C the c e l l p o t e n t i a l s f o r carbon monoxide and carbon d i o x i d e e v o l u t i o n are 2-': A 1 2 0 3 + 3/2 C = 2 Al° + 3/2 C0 2 E°i 000°C = 1 , 1 6 V o l t S (19) A l o 0 , + 3C = 2A1° + 3C0 E° „ = 1.03 v o l t s 2 3 1000°C (20) For e l e c t r o d e s at 1000°C. w i t h c o e v o l u t i o n of carbon monoxide and carbon d i o x i d e the c e l l p o t e n t i a l i s given by equation (5). While carbon monoxide i s the thermodynamically favoured product i t i s not evolved at current d e n s i t i e s greater than 0.2 amp/cm^. Thonstad found t h a t the Bordouard r e a c t i o n would not occur at the anode i f the 2 current d e n s i t y was g r e a t e r than 0.05 - 0.10 A,/qm.« He reasoned t h a t as the current d e n s i t y i n c r e a s e d , the surface coverage by C-0 chemisorbed species would occupy a l l the f r e e s i t e s where the Bordouard r e a c t i o n could occur. As carbon d i o x i d e s a t i s f i e s a l l the bonds of the carbon and oxygen atoms the desorption of carbon d i o x i d e should have a lower a c t i v a t i o n energy than f o r carbon monoxide. 1.3.B Anodic P o l a r i z a t i o n The anodic p o l a r i z a t i o n of carbon and platinum anodes i n the 11 22 27 28 2° c r y o l i t e - a l u m i n a melts has been measured by many worker s . ^ ' ^ ' . ^ ' ' " P o l a r i z a t i o n and overvoltage as a f u n c t i o n of current d e n s i t y are shown f o r carbon anodes i n Figure 6. From Figure 6 i t i s apparent t h a t t here i s a v i d e v a r i a t i o n of both T a f e l slopes and magnitudes of the over-2k-p o t e n t i a l s . Experimenters u s i n g the d i r e c t measurement technique 28 ?Q 11 27 31 are Welch , Welch & Richards y , Thonstad , , Haupin and 33 Drossbach . Experimenters employing the g a l v a n o s t a t i e i n d i r e c t method are P i o n t e l l i 3 0 , P a u n o v i c 3 \ D r o s s b a c h 3 8 and T a k a h a s h i 2 2 . E l e c t r o l y t e s were u s u a l l y c r y o l i t e s aturated i n alumina. Some melts 28 20 33 contained e i t h e r c alcium f l u o r i d e > 7 or l i t h i u m f l u o r i d e . Temperatures f o r the e l e c t r o l y s i s v a r i e d from 870°C 3 3 t o 1050°C 3°. The a f f e c t of the alumina c o n c e n t r a t i o n i n the e l e c t r o l y t e on the anode p o l a r i z a t i o n has been measured by s e v e r a l workers?^'29*30,31 27 Thonstad 1 summarized the published data and t h i s i s shown i n Figure 7 , f o r anode current d e n s i t i e s c l o s e t o aluminum r e d u c t i o n c e l l o p e r a t i n g c o n d i t i o n s . As would be expected from the v a r i a t i o n i n the T a f e l p l o t s , the anodic p o l a r i z a t i o n at constant current d e n s i t y and v a r i a b l e alumina c o n c e n t r a t i o n a l s o has wide v a r i a t i o n s . A general trend i s that f o r concentrations g r e a t e r than k.vt.% alumina the p o l a r i z a t i o n i s almost constant,at l e s s than 1 vt.% alumina the p o l a r i z a t i o n increases r a p i d l y w i t h the decreasing alumina content. Anodic p o l a r i z a t i o n curves have been measured f o r d i f f e r e n t types of carbon anodes. T h o n s t a d ^ measured anodic p o l a r i z a t i o n f o r carbon anodes w i t h d i f f e r e n t r e a c t i v i t y . Fe 0CL , Ua oC0 owere added t o the 2 3 and 2 3 anodes t o increase the r e a c t i v i t y and H^BOg was added t o lower the r e a c t i v i t y . The p o l a r i z a t i o n curves f o r the d i f f e r e n t types of carbon showed a very s l i g h t decrease i n overvoltage as the carbon r e a c t i v i t y increased. L a t e r work by Thonstad 2^ found t h a t the T a f e l slope was Ref. 11 - Thonstad & Hove Anodic Current Density - Anrr>s./cr. figure 6. Su-roary of -published anodic p o l a r i z a t i o n Tafel nlots for cryolite based melts. - 26 -o ' 1 I 0 k . 8 12 \rt.% Al 20 O j Figure 7. Variation of anodic polarization as a function of alumina content. Anodic -polarization vs. concentration of A1?0 2Q 2 I Richards and Welch ' =0.5 A/cm. II Pointelli et al 3° = O.h A/cm.2 I l l Thonstad 2 7 = 0.5 A/cm.2 IV Vetyukov and Baraka 2 7 = 0.5 A/cm/' V Haupin1*0 = 0.62 A/cm.2 - 27-0.22V f o r p y r o l y t i c g r a p h i t e and 0.23 - 0 . 2 8 V f o r r e g u l a r g r a p h i t e and baked carbon. In both sets o f experiments Thonstad employed the d i r e c t method technique f o r measuring overvoltage. Welch and R i c h a r d s 2 9 measured the e f f e c t o f e l e c t r o l y t e composition and carbon monoxide t o carbon d i o x i d e r a t i o at the e l e c t r o d e on the anodic p o l a r i z a t i o n . The a d d i t i o n s o f LigAIFg and CaF 2 showed no change i n p o l a r i z a t i o n w i t h i n the experimental l i m i t s of accuracy. The carbon dioxide-carbon monoxide r a t i o of the impressed gas only a f f e c t e d the values of the exchange current d e n s i t y . At higher current d e n s i t i e s the e f f e c t was n e g l i g i b l e . As a consequence t o the wide v a r i e t y of p o l a r i z a t i o n curves and accompanying mechanisms, Thonstad 3^ a n d Drossbach 3^ have a n a l i z e d the anode r e a c t i o n by measuring the e l e c t r o d e impedance as a f u n c t i o n of frequency. Figures 8, 9 and 10 show the v a r i a t i o n s found by Thonstad35 o f the r e s i s t i v e and c a p a c i t i v e components of the f a r a d a i c impedance as a f u n c t i o n of the frequency of the superinposed a.c. current f o r p y r o l y t i c g r a p h i t e , v i t r e o u s carbon and baked carbon. Thonstad has used a.c. cu r r e n t s of only 2. mamp. 1.3.C Anode E f f e c t In the e l e c t r o l y s i s of alumina i f the concentration of alumina i s low or the anode cur r e n t d e n s i t y i s v e r y high an "anode e f f e c t " w i l l occur. An anode e f f e c t i s c h a r a c t e r i z e d by a r a p i d i n c r e a s e i n anodic - elu -1 p o l a r i z a t i o n and the e v o l u t i o n of carbon t e t r a f l u o r i d e gas. I f the anodic current d e n s i t y i s maintained during an anode e f f e c t , the anodic p o l a r i z a t i o n may reach as high as lUO. v o l t s . A r c i n g across the f i l m surrounding the e l e c t r o d e w i l l occur at 1^0. v o l t s . T h o n s t a d 3 7 found that i n baths c o n t a i n i n g l e s s than 0.75% d i s s o l v e d alumina t h a t a s m a l l p o t e n t i a l jump preceeded the anode e f f e c t . In baths w i t h higher alumina concentrations only a small change i n the slope of the p o t e n t i o s t a t i c v o l t a g e - current curve would precede the anode e f f e c t . E a r l i e r work by Thonstad- 3 using chronopotentiometry found t h a t the c r i t i c a l current d e n s i t y a t which the anode e f f e c t would take p l a c e was a l i n e a r f u n c t i o n o f the alumina c o n c e n t r a t i o n . E a r l i e r p o t e n t i o s t a t i c work by Thonstad 3^ showed th a t the c r i t i c a l c u r r e n t d e n s i t y was l i n e a r l y p r o p o r t i o n a l to the alumina c o n c e n t r a t i o n . Figure 11 i s a summary by Thonstad 3^ of the published p o t e n t i o s t a t i c c r i t i c a l current d e n s i t i e s . In a l l curves there i s a change i n slope at ~ l - k . v t . % alumina c o n c e n t r a t i o n . A l l t h e o r i e s as to the cause of the anode e f f e c t consider t h a t d i f f u s i o n overvoltage i s present before the anode e f f e c t . P i o n t e l l i 3 0 measured the c r i t i c a l current d e n s i t i e s ( C C D . ) as a f u n c t i o n of e l e c t r o d e shape and alumina c o n c e n t r a t i o n . The C C D . always in c r e a s e d w i t h i n c r e a s i n g alumina c o n c e n t r a t i o n . The e l e c t r o d e shape g r e a t l y a f f e c t e d the C C D . , E l e c t r o d e shapes a l l o w i n g easy escape f o r anode gases had much higher C C D . ' s than those which tended to t r a p the gases. F i o n t e l l i a l s o found t h a t lower temperatures and bath a c i d i t y tend t o lower the C C D . Thonstad's t h e o r y 3 7 i s t h a t the d i f f u s i o n overvoltage i n c r e a s e s t i l l the e l e c t r o d e p o t e n t i a l f o r carbon - 31 -t e t r a f l u o r i d e i s reached. At t h i s stage the e v o l u t i o n of the carbon t e t r a f l u o r i d e d r a m a t i c a l l y increases the wetting angle of the e l e c t r o l y t e . Since the e l e c t r o l y t e w i l l not wet the e l e c t r o d e , an i n s u l a t i n g gas f i l m adheres to the e l e c t r o d e . Thonstad found t h a t the coulombic charge passed before the anode e f f e c t was not enough t o produce the gaseous f i l m . . He considered the anode e f f e c t t o be the r e s u l t of a high concentration overvoltage due t o the d e p l e t i o n of oxygen ions followed by the c o - e v o l u t i o n of carbon t e t r a f l u o r i d e . The carbon t e t r a f l u o r i d e formation i n h i b i t s the oxygen e v o l u t i o n as carbon d i o x i d e . At present the cause of the blockage of the surface during the carbon t e t r a f l u o r i d e formation i s unknown. I t i s p o s t u l a t e d that e i t h e r a surface compound of carbon and f l u o r i n e or an absorbed l a y e r of fLuorine gas i s preventing the carbon d i o x i d e formation. 1.3.D Anode Mechanism Many i n v e s t i g a t o r s have attempted t o e x p l a i n the anode p o l a r i z a t i o n mechanism. Haupin^O applying a g a l v a n o s t a t i c technique but without a reference e l e c t r o d e determined t h a t the p o l a r i z a t i o n was composed of two p a r t s . The i n i t i a l p a r t of the p o t e n t i a l decay curve disappeared i n l e s s than 1 msec, and the second p a r t took s e v e r a l minutes. Haupin measured p o l a r i z a t i o n at high current d e n s i t i e s (up t o 3.7 A/cm?). The alumina concentration d i d a f f e c t the p o l a r i z a t i o n e s p e c i a l l y at current d e n s i t i e s above 1.2 A/cm?. P o l a r i z a t i o n at a l l current d e n s i t i e s - 32 ~ increased as the alumina c o n c e n t r a t i o n decreased. A g i t a t i o n of the anode by a 30° r o t a r y o s c i l l a t i o n reduced the p o l a r i z a t i o n s i g n i f i c a n t l y f o r anodic currents above 0 .6 kjcm? i n a bath c o n t a i n i n g 5.2% AlgOg. Welch and R i c h a r d s 2 ^ and W e l c h 2 0 measured anodic p o l a r i z a t i o n u s i n g a (CO2, CO/C/AI2O3 - Na^AIF^) reference e l e c t r o d e by a steady-state p o t e n t i a l approach. The anodic p o l a r i z a -t i o n was obtained by s u b t r a c t i n g the "ohmic" drop between the anode and the reference e l e c t r o d e from the measured p o t e n t i a l between these 29 two e l e c t r o d e s . Welch & Richards considered a n e g l i g i b l e surface coverage of any C-0 species and a second order r e a c t i o n of these C-0 species to produce the carbon d i o x i d e gas, i . e . : 0 2 ~ + C C*0 + 2Q~ (21) 2C*0 = (2* - 1) C + C 0 2 (22) I f ( 2L) i s r a t e c o n t r o l l i n g and the p o l a r i z a t i o n i s l a r g e then: i = z 3 - K c o z _ exp (23) RT~ and T a f e l slope i s given by: ^ 0 . — 2.303 R T (2k) >^ log i- 20C At 1010°C. t h i s would have a T a f e l slope of 0.126/oc . Welch chose oc to be O.k so th a t h i s T a f e l slope o f 0.33 would be approximated. P i o n t e l l i 3 0 a p p l i e d a g a l v a n o s t a t i c technique s i m i l a r to th a t used by Haupin'* 0 but at lower current d e n s i t i e s . He found the p o l a r i z a t i o n decay curves to be composed of f a s t and slow decaying p a r t s . P i o n t e l l i used e i t h e r an (Al/lfe^AIFg - AlgO-^sat)) reference e l e c t r o d e or a - 33 " (C0 2, CO/C/AI 20 3-Ua 3AIFg) reference e l e c t r o d e . At current d e n s i t i e s up to 0 .6 A./cm.2, the r e s i s t a n c e p o t e n t i a l drop on breaking the c e l l current supply remained ohmic. P i o n t e l l i ' s p o l a r i z a t i o n curves Ul appear very p r e c i s e and the author claimed the aluminum reference e l e c t r o d e gave very reproducable r e s u l t s being r e v e r s i b l e up t o q u i t e l a r g e current d e n s i t i e s . Takahashi and Amada a l s o used the g a l v a n o s t a t i c t r a n s i e n t approach. They compared t h e i r r e s u l t s to Welch and R i c h a r d s 2 9 a n d found t h a t w h i l e t h e i r steady-state measurements were i n good agreement t h e i r g a l v a n o s t a t i c r e s u l t s were d i f f e r e n t . In a d d i t i o n they found t h a t the r e s i s t a n c e of the gas-evolved f i l m adjacent t o the e l e c t r o d e was f a r from being ohmic. At an anodic current d e n s i t y of l.OA./cm.^ the r e s i s t i v i t y o f the gas-evolved f i l m was greater than 7 times the normal r e s i s t i v i t y of the e l e c t r o l y t e . Paunovic a l s o a p p l i e d the g a l v a n o s t a t i c approach and found T a f e l slopes i n c l o s e agreement with P i o n t e l l i , i . e . at current d e n s i t i e s l e s s than 0.1 A./cm.2 the slope i s 0.290V. and above 0.1 - 0.2 A./c.m.2 the slope i s O.lUO - 0.150V.. Takaheshi's r e s u l t s above O.OlA./cm.^ had a slope of 0.15V.. Only Paunovic p o s t u l a t e d an anode mechanism. He considered t h a t the anode r e a c t i o n mechanism co u l d be represented as: O2" + C = CO/ + 2e- (25) CO/ + 0 2~ = C 0 2 + 2e- (26) - 3k -Paunovic i n i t i a l l y considered t h a t (26) i s at e q u i l i b r i u m and t h a t (25) may be represented by: I = a 3- K, [ O*" ] [ C ] exp[ocn.3 T> 1 L R T J or ( i f oc =0.5) the t a f e l slope i s : (27) _ 3.. 3 0 3 R T _ 2 . 3 Q 3 R T 2.00 3- 3-(28) At higher current d e n s i t i e s Paunovic considered (26) as the r a t e determining step. 'col [ O*- ] e*p [ ^ ] (29) Paunovic a p p l i e d an approximation f o r 9 C 0[ , the surface coverage f r a c t i o n of CO, and obtained the expression: •RT (30) or — 2 . 5 R T ^ I03 L 3 ? The weakness of Paunovic's approach i s that the T a f e l slope decreases from 0.290V. to 0.130V. w i t h i n c r e a s i n g current d e n s i t y . Thonstad has s t u d i e d the anode p o l a r i z a t i o n f o r carbon and platinum anodes. The anodic overvoltage curve f o r a platinum anode - 3$ -32 i s given i n Figure 12 . Thonstad p o s t u l a t e d t h a t the overvoltage i s due t o two successive charge t r a n s f e r r e a c t i o n s , i . e . : ( l - x ) (3-x) 9 . AIOFx = AIFx + 0 2~ (31) I 0 2- = 0/ + 2er (32) I I 0/+02" = 0 2/ + 2e" (33) 0 2/ = 0 2 f (3U) 12 V e t t e r d e f i n e s the c u r r e n t - o v e r v o l t a g e r e l a t i o n s h i p f o r these two successive charge t r a n s f e r r e a c t i o n s by: R T J d ~ , (35) where: L° = Exchange current d e n s i t y f o r (32) L\ = Exchange current d e n s i t y f o r (33) ^\ = Charge t r a n s f e r overvoltage o^x ( ocji ss Symmetry f a c t o r s f o r I and I I . I f '£fl I _j. then two d i f f e r e n t T a f e l slopes w i l l occur i n the anodic overvoltage curve. I f the c h a r g e - t r a n s f e r overvoltage l i e s w i t h i n the range . (, ~ « r ! < < f W» i i then the current-overvoltage curve i s given hy: w i t h a T a f e l slope (35) S > r l — 2.3Q3 KT - 3 6 -Figure 12. Anodic overvoltage vs current d e n s i t y at 1000 C. f o r a platinum anode i n Na_AlFg - A l p 0 _ melts - 37 ~ L • O 2 h i cxp [ a. 3- •>-[ (38) w i t h a T a f e l slope — 2 13£3_RJ (39) The d e r i v a t i o n of equations (35) - (39) i s given i n Appendix I I I . . Thonstad by co n s i d e r i n g the symmetry f a c t o r s , oc^ , cc„ , t o be equal t o 0.50, found t h a t t h i s y i e l d e d good agreement between h i s experimental r e s u l t s shown i n Figure 12 and- equations (37) and (39). The d e v i a t i o n from the second T a f e l l i n e above » 1.0 A./cm. c o u l d be a t t r i b u t e d t o three p o s s i b l e f a c t o r s . Resistance e s t i m a t i o n e r r o r could occur at higher current d e n s i t i e s s i n c e Thorstad used a. steady-state method. Reaction overvoltage could be present f o r the desor p t i o n o f 0g}. F i n a l l y at current d e n s i t i e s of 3.5 A./ C n u 2 the platinum anode was observed t o r e p i d l y corrode due t o anodic d i s s o l u t i o n . Comparing f i g u r e s 6 and 12 shows t h a t the anodic p o l a r i z a t i o n f o r carbon anodes i s more than double the anodic overvoltage f o r platinum anodes. Thonstad has measured anodic p o l a r i z a t i o n of carbon anodes w i t h the steady-state method. He f o u n d^ ' ^ 7 t h a t carbon r e a c t i v i t y does a f f e c t the anodic p o l a r i z a t i o n e s p e c i a l l y the pure p y r o l y t i c g r a p h i t e i n comparison to a l l other carbonds. In the current d e n s i t y range o f 0.02 - 1.0 A./cm. , p y r o l y t i c g r a p h i t e r e g a r d l e s s of o r i e n t a t i o n had T a f e l slopes of 0.21 - 0.23V.. The other types of carbon had T a f e l slopes o f 0.20 - 0.33 without good r e p r o d u c a b i l i t y . H o r i z o n t a l e l e c t r o d e s tended to have l a r g e r T a f e l slopes than v e r t i c a l e l e c t r o d e s . This could be explained as an e r r o r i n determining r e s i s t a n c e p o t e n t i a l drop si n c e the c o n d u c t i v i t y o f the e l e c t r o l y t e i s g r e a t l y lowered adjacent t o the anode at higher current d e n s i t i e s on account o f the .gas e v o l u t i o n . Thonstad d i d measure c e l l r e s i s t a n c e between the anode and reference e l e c t r o d e by a.c. impedance up to 0.15 A./cm. • Above t h i s anodic current d e n s i t y the c e l l p o t e n t i a l f l u c t u a t i o n s due t o gas e v o l u t i o n made measurement by t h i s method impossible. In a d d i t i o n to measuring the e f f e c t o f carbon r e a c t i v i t y T h o n s t a d ^ attempted t o measure the cathodic l i m i t i n g current d e n s i t y or exchange current d e n s i t y , c r . From V e t t e r ^ the c o n c e n t r a t i o n dependence of l i m i t i n g r e a c t i o n current d e n s i t y i s determined by the r e a c t i o n order of the r a t e determining step, i . e . : ^ log \jr p> ^ l°9 c ^ o 3 (h0) where: P = Chemical r e a c t i o n order of the r a t e determining chemical r e a c t i o n L r = Exchange current d e n s i t y f o r r a t e determining step Exchange current d e n s i t i e s of the v a r i o u s types of carbon anodes as measured from low current d e n s i t y t e s t s v a r i e d from .02k - .0026 A./cm. The v a r i a t i o n of the exchange current d e n s i t y w i t h alumina i s shown i n Figure 13. The slope of the l o g exchange current d e n s i t y - l o g alumina - ko -c o n c e n t r a t i o n curve was found by Thonstad to be 0.52 - 0.59. The accuracy o f these exchange current d e n s i t y measurements a t low c u r r e n t d e n s i t i e s i s at l e a s t questionable. Thonstad c a l c u l a t e d the r e a c t i o n order o f the slow chemical r e a c t i o n from the T a f e l slope. V e t t e r 1 2 g i v e s an expression assuming l a r g e current d e n s i t i e s , low surface coverage w i t h Langmuir behaviour, which r e l a t e s the T a f e l slope t o the r e a c t i o n order of the slow chemical step, i . e . : Cvv)P 0 / y r ^ p ^ > By u s i n g the T a f e l slope of 0.22V. f o r p y r o l y t i c g r a p h i t e and assuming n/V=2 then the r e a c t i o n order P w i l l be 0.57- I f Thonstad had chosen a T a f e l slope of 0.33V. i n s t e a d then P would be » 0 . 3 9 . The author f e e l s the T a f e l slope of 0.22V. chosen by Thonstad was to ensure a r e a c t i o n order between 0.5 - 0.8 which are the orders o f r e a c t i o n hp found f o r combustion of carbon i n oxygen gas. ^  Thonstad studied the anodic p o l a r i z a t i o n on carbon by a.c. impedance measurements. The types of carbon anodes examined'were: non-porous ( p y r o l y t i c g raphite and v i t r e o u s carbon), r e g u l a r graphites and baked carbon. Figures lk, 15 and l 6 are a summary of the v a r i a t i o n o f the impedance as a f u n c t i o n of the frequency of a.c. c u r r e n t f o r charge t r a n s f e r , d i f f u s i o n and r e a c t i o n p o l a r i z a t i o n s . Comparing Thonstad's r e s u l t s f o r p y r o l y t i c g r a p h i t e , v i t r e o u s carbon and CHB gra p h i t e shown i n F i g u r e s , 8 , 9, and 10 w i t h the t h e o r e t i c a l p o l a r i z a t i o n curves i t i s apparent t h a t the p o l a r i z a t i o n i s pre-- kl -- -dominantly r e a c t i o n p o l a r i z a t i o n . At low current d e n s i t i e s a l l f a r a d a i c impedance curves have an i n c r e a s i n g r e s i s t i v e term at low frequencies i n d i c a t i n g d i f f u s i o n p o l a r i z a t i o n . T his d i f f u s i o n p o l a r i z a t i o n occurs at low current d e n s i t i e s where the r e a c t i o n takes p l a c e s e l e c t i v e l y on the surface. I f some of the more r e a c t i v e p a r t s occur i n the pores then d i f f u s i o n p o l a r i z a t i o n may r e s u l t from d e p l e t i o n of the oxygen anion complexes i n the e l e c t r o l y t e i n s i d e the pore or the escape of the anode r e a c t i o n products, CO and COg. Thonstad found t h a t the v i t r e o u s carbon measurements had poor r e p r o d u c a b i l i t y but d i d appear t o have a constant r e s i s t i v e component at h i g h frequencies which i s i n d i c a t i v e o f charge t r a n s f e r p o l a r i z a -t i o n . The c a p a c i t i v e component of the f a r a d a i c impedance f o r v i t r e o u s carbon and to a l e s s e r extent CHB g r a p h i t e had a maxima a\ « 2 r>z . The reason f o r t h i s maxima i s not understood. l.h Conclusions The mechanism of the anode r e a c t i o n i n c r y o l i t e - a l u m i n a melts i s not w e l l understood. Published p o l a r i z a t i o n measurements vary i n magnitude up t o 100$ at a l l current d e n s i t i e s . ' Faradaic impedence measurements by Thonstad 3-* and Drossbach3° have poor r e p r o d u c a b i l i t y . The anode r e a c t i o n mechanism i s very complex. The anodes are e i t h e r g r a p h i t e or carbon which are both inhomogeneous and porous. At current d e n s i t i e s below the anode e f f e c t the r e a c t i o n product may be e i t h e r carbon d i o x i d e or carbon monoxide. The anode gases are t r a n s p o r t e d away from - 1*3 ~ the e l e c t r o d e as hubbies. The form o f these bubbles v a r i e s w i t h t h e curre n t d e n s i t y and the w e t t a b i l i t y of the anode by the e l e c t r o l y t e . The bubble formation i s q u i t e s i m i l a r t o th a t of s o l u t i o n b o i l i n g . When the surface coverage by bubbles being formed i s l a r g e the average uncovered surface area becomes a f u n c t i o n of time and current d e n s i t y . At current d e n s i t i e s greater than 0.10 A./cm. 2 the anode p o t e n t i a l becomes a f u n c t i o n of time and shape of the anode. The ease by which the bubbles escape from the surface o f the anode g r e a t l y i n f l u e n c e s the e f f e c t i v e r e s i s t i v i t y of the e l e c t r o l y t e adjacent t o the anode. As a r e s u l t methods which do not d i r e c t l y e l i m i n a t e the r e s i s t i v e component of the anode - reference e l e c t r o d e p o t e n t i a l are subject t o l a r g e e r r o r s at higher current d e n s i t i e s . The anode p o l a r i z a t i o n i s p r i m a r i l y r e a c t i o n p o l a r i z a t i o n w i t h d i f f u s i o n p o l a r i z a t i o n o c c u r r i n g at very high and very low current d e n s i t i e s . The anode mechanism at current d e n s i t i e s greater than 0.10 A./cm. 2 i s probably one of the f o l l o w i n g forms;35 I A I 0 F x ( 1 " x ^ = A I F x ^ 3 " x ^ + 0 2- Fast 0 2 - = 0/ + 2e~ Fast 0/+XC = CxO/ Fast Cx0/+0 2 _ = C0 2/ + ( x - l ) C + 2e~ Fast C0 2/ = C0 2 ( t ) Slow (1*2) Or I I A I O F x ^ 1 " ^ = AIF x ( 3 - x ) + 0 2-0 2" 0/ + 2e-0/+xC = CxO | 02~+CxO/ = Cx0?0/+2e-. CxO.O/ = C0 2/+(x-l)C C0 2/ = C 0 2 (G) (1+3) I f mechanism I i s c o r r e c t then the slow chemical r e a c t i o n i s the desorp t i o n o f C0 2. I f mechanism I I i s c o r r e c t then the slow chemical r e a c t i o n c o u l d be e i t h e r the de s o r p t i o n of C 0 2 or the formation of absorbed C0 2. Thonstad favours mechanism I I i n order to account f o r the second maxima found i n the capacative component o f the f a r a d a i c impedance measurements. The d i f f u s i o n p o l a r i z a t i o n i s s i g n i f i c a n t at low and high current d e n s i t i e s but not i n between. At low current d e n s i t i e s the d i f f u s i o n p o l a r i z a t i o n was detected by Thonstad 3^. When the curr e n t d e n s i t y i s small the anode r e a c t i o n w i l l occur at the more r e a c t i v e s i t e s on the anode. I f some of these more r e a c t i v e s i t e s are i n s i d e pores then the d i f f u s i o n of e i t h e r the oxygen anions or the gaseous products may be p a r t i a l l y rate-determining. Carbon monoxide i s the thermodynamically favoured anode product but expe r i m e n t a l l y at current d e n s i t i e s greater than 0.10 A./cm. ^ carbon d i o x i d e i s the only gas e l e c t r o l y t i c a l l y e v o l v e d . 2 ^ At lower current d e n s i t i e s both gases may be simultaneously evolved. I t i s p o s s i b l e f o r the carbon monoxide t o be evolved i n s i d e the pores where the surface coverage i s low w h i l e carbon d i o x i d e i s evolved on the outer surface where the surface coverage i s too high f o r carbon monoxide e v o l u t i o n . At high c u r r e n t d e n s i t i e s the t r a n s p o r t of oxygen anions t o the anode becomes p a r t i a l l y r a t e determining. As the " h5 ~ c r i t i c a l current d e n s i t y i s approached the anode p o l a r i z a t i o n i ncreases very r a p i d l y due t o t h i s d i f f u s i o n p o l a r i z a t i o n . The measurement of the d i f f u s i o n p o l a r i z a t i o n as a f u n c t i o n of current d e n s i t y i s very d i f f i c u l t since the Nernst d i f f u s i o n l a y e r t h i c k n e s s i s also a f u n c t i o n of current d e n s i t y and anode shape. This p a r t i a l l y e x p l a i n s the d i f f e r e n c e s i n T a f e l slopes found by Thonstad 27. f o r h o r i z o n t a l and v e r t i c a l e l e c t r o d e s . Thonstad considered a pre-c o n d i t i o n i n g of the anode e i t h e r by low cur r e n t d e n s i t i e s f o r long times or by high c u r r e n t d e n s i t i e s f o r very short times a n e c e s s i t y i f reproducable r e s u l t s were to be a t t a i n e d . P r e - c o n d i t i o n i n g reduces the number of s i t e s o f high r e a c t i v i t y which a f f e c t the p o l a r i z a t i o n curves at lower current d e n s i t i e s . 1.5 Research Program At present the anode r e a c t i o n mechanism i s assumed t o c o n t a i n a slow chemical r e a c t i o n , w i t h no d e f i n i t i v e evidence o f the magnitude of the d i f f u s i o n a f f e c t s beyond the inferences drawn i n the preceding s e c t i o n s . At very high current d e n s i t i e s greater than l.O.A./om. 2 d i f f u s i o n p o l a r i z a t i o n i s considered, t o be s i g n i f i c a n t . Only Haupin' 4 0 has attempted to measure d i f f u s i o n p o l a r i z a t i o n a t high current d e n s i t i e s . His r e s u l t s are questionable s i n c e no allowance i s made f o r the e l e c t r o l y t e r e s i s t i v i t y change adjacent to the anode. The conce n t r a t i o n of d i s s o l v e d alumina on the anode p o l a r i z a t i o n has been measured by s e v e r a l workers but the p o l a r i z a t i o n curves show very l a r g e v a r i a t i o n s . The g a l v a n o s t a t i c method allows measurement of _ h6 _ the anode p o l a r i z a t i o n w h i l e e l i m i n a t i n g the r e s i s t i v e p a r t of the e l e c t r o l y t e . The proposed research i s t o measure the anode p o l a r i z a t i o n and to evaluate the s i g n i f i c a n c e of the d i f f u s i o n p o l a r i z a t i o n as a f u n c t i o n o f current d e n s i t y . The g a l v a n o s t a t i c approach i s t o be ap p l i e d to a c y l i n d r i c a l anode which may be r o t a t e d a t speeds up t o 5000 r.p.m. . From the change i n the anodic p o l a r i z a t i o n as a f u n c t i o n of r o t a t i o n speed the t o t a l d i f f u s i o n component i s t o be e x t r a p o l a t e d . This w i l l a l s o a l l o w determination of the sum of the r e a c t i o n and charge t r a n s f e r p o l a r i z a i t o n s as a f u n c t i o n of current d e n s i t i e s . The e f f e c t of v a r i o u s alumina concentrations on the d i f f u s i o n p o l a r i z a t i o n w i l l be examined as a f u n c t i o n o f cu r r e n t d e n s i t y and tenperature. L i t h i u m appears t o be a favourable a d d i t i v e t o the e l e c t r o l y t e . Since an e l e c t r o l y t e c o n t a i n i n g 20% (wt.) L i f - 5% A I 2 0 3 - l^&^kTF^ has a m e l t i n g p o i n t o f ~ 8 2 0°C.^, l i t h i u m e l e c t r o l y t e s w i l l be employed t o measure the temperature e f f e c t on t o t a l and d i f f u s i o n p o l a r i z a t i o n s . From Appendix I the d i f f e r e n c e i n decomposition p o t e n t i a l s f o r carbon monoxide and carbon d i o x i d e e v o l u t i o n i s hi mV. at 840°C. compared t o l l l m V . a t 990°C. I f carbon monoxide i s evolved only at very low current d e n s i t i e s then the shape of the anodic p o l a r i z a t i o n curves should change w i t h temperature. The curve corresponding t o carbon d i o x i d e e v o l u t i o n w i l l become predominant at lower current d e n s i t i e s at lower temperatures. The e f f e c t of l i t h i u m - 47 -on the anodic p o l a r i z a t i o n curves i s expected t o he small since Grjotheim-'- shows t h a t l i t h i u m does not a f f e c t the w e t t i n g angel of c r y o l i t e on carbon i n c r y o l i t e melts and hence does not a f f e c t the anodic d e s o r p t i o n process. - 1+8 -2. EXPERIMENTAL TECHNIQUE 2.1 Equipment Design The experimental equipment was designed t o measure anodic p o l a r i z a t i o n curves. D i f f u s i o n p o l a r i z a t i o n was t o be determined by the p o l a r i z a t i o n decrease w i t h r o t a t i o n o f the anode. I n order t o estimate t h i s d i f f u s i o n p o l a r i z a t i o n component the anode was r o t a t e d at speeds up t o l+,900 r.p.m. . The experimental c e l l i s shown i n Figure 17. The anode, K, was made of G r a p h i t i t e A by Carbon Carborundum Corp. G r a p h i t i t e A has good thermal s t a b i l i t y and s t r e n g t h at high temperatures. Table 2 l i s t s i t s p r o p e r t i e s . The graphite surface was machined before most but not a l l experimental runs. Boron n i t r i d e i n s u l a t o r s , J , ensured a constant anode surface area at a l l t i m e s , i . e . constant exposed area of carbon on a macroscale , The BN i n s u l a t o r s a l s o allowed the carbon e l e c t r o d e t o be w e l l below the e l e c t r o l y t e s u r f ace. The anode and the BN i n s u l a t o r s were threaded on t o a 1/1+" diameter molybdenum rod, C, which could be r o t a t e d by an e l e c t r i c motor, A. The molybdenum rod was t r u e t o w i t h i n 0.001" throughout i t s l e n g t h . The r o d , C, was supported by the furnace tube coverplate assembly through three bearings. The anode l e a d , 5, from the power supply was e l e c t r i c a l l y connected to the molybdenum rod by a mercury p o o l , B. - U9 -TABLE 2 PROPERTIES OF GRAPHITITE A BY CARBON CARBORUNDUM Property 70°F. 1+200°F. S p e c i f i c G r a v i t y 1.88 1.87 T e n s i l e Strength - K s i 2.9 5.6 Compressive Strength - K s i 11.0 19.0 Traverse Strength - K s i 5.0 9.0 Modiolus of E l a s t i c i t y x 10^ 18 27 Thermal Expansion 9 27 E l e c t r i c R e s i s t i v i t y x 10~5 ohm - i n 35 1+0 Thermal C o n d u i t i v i t y BTU ft.°F. h r ; 92 9 Ash Content % ,0k ,0k - 50 -Figure 1 7 . Schematic of experimental apparatus. - 51-The e l e c t r i c motor was e l e c t r i c a l l y i n s u l a t e d from the molybdenum rod by a p l a s t i c connector sleeve. The e l e c t r i c motor r o t a t e d the anode up t o i|,900 r.p.m. although some v i b r a t i o n and "whipping" normally occurred at speeds of g r e a t e r than 2,000 r.p.m.. When a new anode and a BN i n s u l a t o r were f i r s t employed speeds of 3,000 r.p.m. could be reached w i t h only small v i b r a t i o n s . Anode diameters of 0.50 in.and heights of 0.20 i n . were used. The diameter of the anode before and a f t e r each experimental run was measured. The mean macroscopic surface area was chosen t o c a l c u l a t e the anodic current d e n s i t i e s . The cathode, V, was a g r a p h i t e c r u c i b l e and no aluminum was added at any time. This c r u c i b l e having inner dimensions o f 2.0 i n c h diameter and 3.0 i n c h height had a base and a side w a l l t h i c k n e s s of 0.15 i n . and 0.20 i n . r e s p e c t i v e l y . The c r u c i b l e was supported by a carbon pedestool 8.0 i n . l o n g . The pedestool a l s o acted as the e l e c t r i c a l connection to the cathode. I t was screwed onto the base p l a t e and t h i s was connected t o the negative l e a d , R, from the D.C. power supply. A termocouple, Q, was p o s i t i o n e d j u s t below the base of the c r u c i b l e . Two d i f f e r e n t types of reference electrodes were used t o measure anodic p o l a r i z a t i o n . The carbon (COg, CO/C/AIgOj - Na3AIFg) reference e l e c t r o d e and the aluminum (Al/Na-^AIFg - AlgO^ s a t ) reference electrode were employed. The carbon reference e l e c t r o d e as shown i n Figure 18 c o n s i s t e d o f a carbon ( G r a p h i t i t e A) e l e c t r o d e p a r t i a l l y - 52-immersed i n the e l e c t r o l y t e and separated from the c e l l e l e c t r o l y t e by a porous boron n i t r i d e sheath. The base of the sheath was i n e a r l i e r experiments l e s s than .OkO inches t h i c k . G r j o t h e i m ^ 3 determined t h a t i f the w a l l t h i c k n e s s of a s i n t e r e d BN tube was l e s s than .060 inches the w a l l acted as a porous membrane. An aluminum reference e l e c t r o d e a l s o had a BN w a l l t h i c k n e s s of l e s s than .060 inches. The aluminum reference e l e c t r o d e assembly i s shown i n Figure 19. In l a t e r experimental work the carbon reference e l e c t r o d e had a BN sheath w i t h a 0,10 i n c h hole i n i t s base. The BN sheaths f o r both the aluminum and carbon reference e l e c t r o d e s were threaded on t o a s t e e l tube, L. The reference e l e c t r o d e l e a d , N, was i n s u l a t e d from the s t e e l tube by an alumina sheath, M. This was necessary i n order to e l e c t r i c a l l y i n s u l a t e the anode from the reference e l e c t r o d e except through the e l e c t r o l y t e . The c e l l had temperature and atmospheric c o n t r o l . I t was enclosed i n an Inconel 600 furnace tube 3.1/8 i n . O.D. x 2k i n . l e n g t h x 1/8" t h i c k n e s s . A s t a i n l e s s s t e e l p l a t e , D, r e s t e d on s i l i c o n e rubber s e a l s , H, on the top of the furnace tube. This p l a t e , D, was water cooled and had three holes through i t . Two of which were f o r the reference e l e c t r o d e l e a d and the anode s h a f t . The t h i r d hole could be used to take temperature measurements or observe the melt. At the base of the furnace tube was a s t a i n l e s s s t e e l p l a t e . This was connected t o the negative D.C. power l e a d , R, and was e l e c t r i c a l l y - 53 -Alumina sheath O.OoO" I.D. Mild S t e e l sheath 5 / l 6 " O.D. Ma Qr. \i wire . 0 2 0 " 0 Boro n nitride 0 . 5 0 " O.D. Graphitite A reference electrode Na 3AlFg - A1 2 0 3 SAT 0 . 0 5 0 ' Figure 18. Schematic o f ( C 0 2 j C 0 / C / A 1 o 0 - Na_AlF6) reference electrode assemblv- "~ J Alumina sheath . 0 6 0 " I.D. Mild steel sheath 5/lC" O.D. b'.'.. sheath 0 . 5 0 " O.D. Na 3AlF 6 - A1 2 0 3 SAT. Mo or W wire . 0 4 0 " 0 Al (k-9's pure) reference electrodi 0 . 0 6 0 " Figure 19. Schematic of (Al/Na 3AlF 6 ~ A1 20 3 Sat.) refe assembly. ;rence electrode _ 5h-i n s u l a t e d from the i n c o n e l furnace tube by a s i l i c o n e s e a l , H. In order t o p r o t e c t the s i l i c o n e s e a l t h i s p l a t e was water cooled. A gas i n l e t tube, T, passed through the base p l a t e - a l l o w i n g the furnace to have any atmosphere desired'. I n a l l experiments argon gas was blown i n t o the furnace tube i n order to o b t a i n an i n e r t atmosphere. Normal argon flow r a t e s were approximately 700 s.cc./rain. . The experimental furnace had Kanthal r e s i s t a n c e heating elements. Two 3.75 i n . I.D. x 12 i n . Kanthal heating elements enabled the experimental c e l l t o be operated at temperatures up t o 1050°C. without d i f f i c u l t y . These heating elements were i n s u l a t e d by low thermal c o n d u c t i v i t y f i r e c l a y b r i c k s . A temperature p r o f i l e of the furnace i s shown i n Figure 20. The furnace temperature was c o n t r o l l e d i n i t i a l l e d by two v a r i a c s connected to the heating elements and l a t e r by a c o n t r o l l e r which was connected t o the Chromel-Alumel thermocouple, Q. The c o n t r o l l e r enabled the temperature t o be maintained w i t h i n a 15°C. range. The e l e c t r i c a l c i r c u i t used t o measure anodic p o l a r i z a t i o n was simple. A schematic of the c i r c u i t i s given i n Figure 21. The power supply was a Hewlett Packard 6203B w i t h a maximum l o a d of 3 Amperes. The e l e c t r i c a l c e l l was i n p a r a l l e l w i t h a 25-n.- 50 watt r e s i s t o r i n order to a i d the response on the D.C. u n i t on opening or c l o s i n g the c i r c u i t . The c i r c u i t was opened or c l o s e d by a double-throw k n i f e switch operated manually. Touching t h i s k n i f e s w i t c h was a s p r i n g - 55 -O \r 0 200 )+00 600 . 800 1000 1200 Temperature i n s i d e furnace tube °C. - Figure 20. Furnace temperature p r o f i l e - 56 -D.C. nower supplv double throw k n i f e switch Anode Reference electrode Cathode, Figure 21. Schematic o f e l e c t r i c a l measuring c i r c u i t f o r measuring .anod 1c no 1 a r i z at i on. - 57 -loaded switch used as an e x t e r n a l t r i g g e r f o r the o s c i l l o s c o p e . The p o s i t i v e l e a d from the k n i f e switch was connected to the s t a i n l e s s s t e e l p l a t e s at the bottom o f the mercury p o o l . The negative l e a d was connected t o the base p l a t e of the cathode assembly. The anode and reference electrodes ha.d s h i e l d e d leads t o the o s c i l l o s c o p e . The o s c i l l o s c o p e was a Tekronix type 561* storage o s c i l l o s c o p e w i t h a type 3A3 dual t r a c e d i f f e r e n t i a l a m p l i f i e r and a type 3B1 time base. The 3A3 a m p l i f i e r has c a l i b r a t e d a ccuracies t o l e s s than 3% e r r o r . Further c a l i b r a t i o n i s p o s s i b l e f o r each channel and s e n s i t i v i t y . The time base 3B1 was accurate to l e s s than 5/« e r r o r . I n order t o t e s t the c i r c u i t response a 0.35-O. r e s i s t o r r e p l a c e d the e l e c t r o l y t i c c e l l . As shown by Figure 22 the response t o zero c e l l p o t e n t i a l a f t e r opening the c i r c u i t i s l e s s than 5msec. . In order to avoid A.C. p i c k up from v a r i o u s sourses w i t h i n the l a b o r a t o r y the furnace s t e e l c a s i n g was grounded i n a d d i t i o n t o tho o s c i l l o s c o p e l e a d s . I t was e s s e n t i a l t h a t the Inconel 600 furnace tube was not e l e c t r i c a l l y connected at e i t h e r top or bottom, thus the s i l i c o n e s e a l s were used. I f e i t h e r end was not f u l l y i n s u l a t e d A.C. pi c k up on the o s c i l l o s c o p e screen was observed. The D.C. power supply was not grounded due t o A.C. pick-up i n the e l e c t r i c a l ground. The o s c i l l o s c o p e and furnace cas i n g were grounded to a waterpipe. As a r e s u l t the e l e c t r o l y t i c c e l l was f l o a t i n g w i t h respect to the o s c i l l o s c o p e ground. - 56 -Figure 2 2 . Oscilloscope trace of the time constant of the equipment, for a 0 . 3 5 - O . resistance in place of the electrolytic c e l l . - 0 . 2 volts - en. - 1 , 0 . 0 5 sec-cm.~1 - 59 -2.2 Reagents The reagents used i n the e l e c t r o l y t i c c e l l were c h e m i c a l l y pure grades. The alumina was 99.9% Al^Oy The e l e c t r o l y t e s employed are given i n Table I I I . E l e c t r o l y t e Composition Temperatures 98^Na.AlF. - 2% A1„0_ 1010°C, 1050°C 9 5&*a 3AlF 6 - 5% A 1 2 0 3 990 , 1050 75-^ Na A 1 F 6 - 2.0% LiF - 5% A l g 0 3 3ko , 990 The c r y o l i t e s always had a 1.50 '• 1.0 weight r a t i o . S y n t h e t i c c r y o l i t e was t r i e d , i n Runs 1 - XV and n a t u r a l c r y o l i t e i n runs XVI - XXXI. I f the carbon reference e l e c t r o d e was employed w i t h a 3%' sheath having a porous base then the reference e l e c t r o d e s o l u t i o n vas n a t u r a l c r y o l i t e s aturated i n alumina. When the aluminum reference e l e c t r o d e was used the aluminum was 99.9% A l . and the reference e l e c t r o d e ' s e l e c t r o l y t e -contained n a t u r a l c r y o l i t e saturated i n alumina. 2.3 Measurement Technique The measurement of anodic p o l a r i z a t i o n T a f e l curves i n c r y o l i t e -alumina melts has been e i t h e r by steady-state methods or .qalvonostatic pulse methods. The l a t t e r method was chosen since the r e s i s t a n c e o f - 60 -the e l e c t r o l y t e between the anode and reference e l e c t r o d e does not have t o be known i n order t o determine the anode p o l a r i z a t i o n . This i s very c r i t i c a l f o r r o t a t i n g e l e c t r o d e experiments where the r e s i s t a n c e between the contacts t o and.from the mercury pool vary. The s i n g l e pulse g a l v a n o s t a t i c method determines the p o l a r i z a t i o n at the anode immediately a f t e r the c e l l c i r c u i t i s opened. Figure 23 shows the anode-reference e l e c t r o d e p o t e n t i a l as a f u n c t i o n of time. On c l o s i n g the c e l l c i r c u i t the p o t e n t i a l i n s t a n t a n e o u s l y jumps by the r e s i s t i v e component between the anode and the reference e l e c t r o d e . A f t e r l e s s than 5 seconds the anode-reference e l e c t r o d e p o t e n t i a l w i l l have reached a qausi-steady s t a t e c o n d i t i o n . The carbon anode i n the c r y o l i t e - a l u m i n a melts has a c y c l i c p o t e n t i a l on account of the c o n s t a n t l y changing surface area of the anode due t o the gas e v o l u t i o n . The gas e v o l u t i o n a l s o has a time dependant e f f e c t on the e l e c t r o l y t e r e s i s t a n c e very c l o s e t o the anode. On opening the c e l l c i r c u i t the p o t e n t i a l drop i n the e l e c t r o l y t e i n s t a n t l y vanishes l e a v i n g the decaying p o l a r i z a t i o n p o t e n t i a l . Since the decaying p o l a r i z a t i o n p o t e n t i a l i s a negative exponential f u n c t i o n of time, the t r a c e speed of the p o t e n t i a l measuring instrument must be very f a s t . For best r e s u l t s a storage o s c i l l o s c o p e or syncroscope should be used having an e l e c t r o n i c t r a c e t r i g g e r so t h a t t r a c e speeds of 1 yUsec./cmv can be achieved. Since no e l e c t r o n i c t r a c e t r i g g e r was a v a i l a b l e a manual switch was used enabling t r a c e speeds of 0. 5 rnsec. / c r n T h i s gave good r e s u l t s except a higher c u r r e n t s where the r e s i s t i v e drop - 61 -Figure 2 3 . Variation of potential between the anode and reference electrode with time. - 62-was l a r g e . At higher current d e n s i t i e s the r e s i s t i v e drop was as l a r g e or l a r g e r than the p o l a r i z a t i o n r e s i s t a n c e . In order t o o b t a i n a time-averaged p o l a r i z a t i o n values each measurement was repeated 5 - 1 0 times. The d i f f u s i o n p o l a r i z a t i o n was determined by r o t a t i n g the anode at d i f f e r e n t speeds. The anode r o t a t i o n speeds were g e n e r a l l y 100, 500, 1000, 1500, 2000 and 3000 r.p.m.. The decrease i n p o l a r i z a t i o n as a f u n c t i o n of the Reynolds number was determined before each set of p o l a r i z a t i o n measurements at a f i x e d c u r r e n t d e n s i t y and anode r o t a t i o n speed a higher current d e n s i t y pulse was a p p l i e d f o r up t o 30 seconds. At current d e n s i t i e s of l e s s than 0.20 A./cm. t h i s p o l a r i z i n g pulse was e s s e n t i a l . Without i t . the anode p o l a r i z a t i o n would be low i n i t i a l l y and slo w l y r i s e t o the value obtained when a p o l a r i z i n g pulse was used. In the high speed anode r o t a t i o n t e s t s the p o l a r i z i n g pulse was necessary i f the r e s u l t s obtained were t o be at a l l meaningful. At the c o n c l u s i o n of each group of anode r o t a t i o n speeds ( i . e . 100, 500, 1000, 1500, 2000, 3000 r.p.m.) at constant current d e n s i t y a set of t e s t s were .car r i e d out at 100 r.p.m'. t o check w i t h the f i r s t set of 100 r.p.m. t e s t s . - 63 -3. RESULTS 3*1 P o l a r i z a t i o n Measurements The anodic p o l a r i z a t i o n curves f o r the carbon anodes i n c r y o l i t e -based melts were measured. Three supporting e l e c t r o l y t e compositions were used: 2 vt.% alumina - 98 vt.fn c r y o l i t e , 5 wt.$ alumina - 95 vt.%> c r y o l i t e and 5 vt.% alumina - 20 vt.% l i t h i u m f l u o r i d e - 75 vt.% c r y o l i t e . 3 - 1 - A 2 wt.^ Alumina - 98 vt.% C r y o l i t e Anodic p o l a r i z a t i o n curves f o r the 2 vt.% alumina melts were c a r r i e d at melt temperatures of 1010°C- and 10U0°C. Fi g u r e s 2h and 25 show the T a f e l p l o t s of anodic p o l a r i z a t i o n a t these temperatures. These temperatures are «=< 12°C- and s=»i+2°C. above t h e l i q u i d u s temperature. The p o l a r i z a t i o n curves s t a r t t o l e v e l o f f above 0.2 A . / c m . 2 current d e n s i t y . In Figure 2h at current d e n s i t i e s 0.0^ - 0 . 2 A . / c m 2 . the T a f e l curve has a slope of +0.35 i .05 v o l t s . At current d e n s i t i e s l e s s than 0.0k A./cm?, the i n i t i a l c o n d i t i o n of the anode i s important. In Figure 25 a t r a n s i t i o n appears t o occur from 0.08 - 0.20 A./cm. Above 0.20 A./cm?, the T a f e l slope f a l l s to «s 0.08 v o l t s s i m i l a r t o Figure 2h. 3.1.B 5 vt.% Alumina - 95 vt.% C r y o l i t e The anodic p o l a r i z a t i o n curves f o r 5 vt.fo alumina - 95 vt.% O 800 700 600 500 400 £ 300 o c cr 200 100 020 , 0 3 0 050 . . 0 7 0 .100 .150 .200 Periodic Current "JSensiTY R.-C.tr>"' 500 . 7 0 0 1 .000 Figure 2 5 . T a f e l p o l a r i z a t i o n p l o t of run 22 at 1040°C - 2 vt% A l 0 3 - 98 vt.% Na-^AlFg - 66 -c r y o l i t e melts were measured at melt temperatures o f 990°C., 1025°C. and 10l*0°C. . These temperatures are ^ 10°C. , as h5°C. and «=s60°C. r e s p e c t i v e l y above the l i q u i d u s temperature. Figures 2 6 , 2 7 and 2 8 show the T a f e l p l o t s of these p o l a r i z a t i o n curves. In a d d i t i o n F i g u r e 2 9 shows the T a f e l p l o t f o r the same c o n d i t i o n s as Figure 2 6 . A l l T a f e l p l o t s e x h i b i t a change i n slope between 0.10 - 0.20 A./ c m 2. Above 0.20 A./ en?, a l l T a f e l p l o t s have a slope l e s s than 0.10 v o l t s . At current d e n s i t i e s l e s s than 0.10 A./ Cn?. Figures 26 and 29 have d i f f e r e n t slopes. The anode was unused at the s t a r t of the measurements of run 23 shown i n Figure 2 6 but a l l measurements were preceeded by a short but l a r g e p o l a r i z i n g c i i r r e n t . In a d d i t i o n both the e l e c t r o l y t e and the anode were unused at the beginning of run23 i n Figure 2 6 but were reused i n run 2h shown i n Figure 2 9 . The range o f current d e n s i t i e s at which anodic p o l a r i z a t i o n measurements were taken was 0 . 0 0 9 - 0.90 A./en?. . 3.1.C 5 vt.% Alumina - 20 vt.% L i t h i u m F l u o r i d e - 7 5 vt.% C r y o l i t e (Natural) The l i t h i u m f l u o r i d e - c r y o l i t e based melts had anodic p o l a r i z a t i o n measurements taken at 8^0°C. and 990°C. . The l i q u i d u s temperature f o r 5 vt.% A 1 2 0 3 - 20 vt.% L i F - 7 5 vt.% N a 3 A l F 6 i s 8 2 0 ° C . 1 0 . F i g u r e 30 shows the p o l a r i z a t i o n curves f o r Runs 27 and 29 both of which were at 8!+0°C. . The i n i t i a l p o l a r i z a t i o n curve f o r Run 27 i s lower than i n the l a t e r measurements. Both p o l a r i z a t i o n curves have a charge p o l a r i z a t i o n - 67 -; 8 Q 0 F 700[ 5 0 0 h i .0 0 400h o '•6 200 | -c cr IliiHt-i-l+HJ+Hlf-i-i-ttj^  jj±f .tilt .... . . I ! j .1 : i i.-l! .i-i-i-i < f > - ~ ' i H — t ~ ~ H - 4 4 30 j 40 50j | 70 "0 81 H ' i t H t H itiij ._; [_„ L... i i r I i. i . • iqo i3o ibo 3pQ SPQ MM mil tH-H* t m '.'t'i Plnodi'c. C u r r e n t H>e.nsi"t\) _ 1 igure d pa T a f e l p o l a r i z a t i o n p l o t of run 2k at 9 9 0°C- 5 vt.% A l o 0 o - 95 vt.JJ Na 3AlFg mP\ - cm. -2°3 —j o - 71 -p o t e n t i a l at the lower c u r r e n t d e n s i t i e s o f .01 - 0.03 A./cm? Both curves appear to l e v e l o f f above the 0.10 - 0.20 A . / c m r current density-range. Figure 31 shows the p o l a r i z a t i o n curve f o r Run 31 at 990°C. . Th i s p o l a r i z a t i o n curve l e v e l s o f f above 0.09 - 0.20 A./cmr current d e n s i t y . The T a f e l slope o f the p o l a r i z a t i o n curve i s much steeper a t lower c u r r e n t d e n s i t i e s being +0.1+5 - 0.05 v o l t s between 0.02 -0.08 A./cm? The slope i n Figure 30 between 0.02 - 0.08 A./cm? i s o n l y + 0.21 ± 0.05 v o l t s . Figures 30 and 31 show t h a t the p o l a r i z a t i o n i s lower on the f i r s t group of measurements i n which the current d e n s i t y i s i ncreased. Further measurements as the current d e n s i t y i s increased and decreased have g e n e r a l l y a higher p o l a r i z a t i o n at lower current d e n s i t i e s and are reproducable to w i t h i n 50 mV.. 3.2 D i f f u s i o n Overvoltage Measurements D i f f u s i o n overvoltages were measured i n d i r e c t l y . The d i f f u s i o n overvoltage was estimated by the decrease i n p o l a r i z a t i o n as a f u n c t i o n o f the anode r o t a t i o n speed or the Reynolds number at the e l e c t r o d e ' s surface. D i f f u s i o n overvolta.ges were estimated f o r the e l e c t r o l y t e s and temperatures l i s t e d i n Table IV. Figures 32 and 33 show the p o l a r i z a t i o n decrease as a f u n c t i o n of the Reynolds number f o r 2 \rt.% alumina - 98 wt.JS c r y o l i t e melts. Although the r e s u l t s are not very reproducable Run 19 i n Figure 32 d i d have a greater p o l a r i z a t i o n decrease w i t h anode r o t a t i o n than d i d Run 22 i n Figure 33. Rung 19 and 22 were operated at °»12°C and S5l+2°C r e s p e c t i v e l y above the l i q u i d u s temperature. W - 1 I 111 •-iA--A LO 10 Figure 31. 20 hO 50 bO 60 100 150 200 3 0 ° hOO f-lnod'ic CorranV 1>&nsV+y _ m A.- C t n - J 600 Goo 1000 T a f e l p o l a r i z a t i o n p l o t of run 31 at 990 C - 5 vt.£ A l p 0 3 - 20 \rt.% L i f - 75 wt.2 I l a ^ l F ^ - Ik -- 75 -300 6 7 0 y 5 6 7 3 p 1000 1500 2000 10,000 Figure 33. P o l a r i z a t i o n decrease vs Re f o r run 22 at 1040°C -2- yt-.# A1 20 3 - 98 vt.% I-Ia3AlFg. - 76 -TABLE IV SUMMARY OF DIFFUSION OVERVOLTAGE EXPERIMENTS Run E l e c t r o l y t e Temperature, T, T - T Li q u i d u s 19 2% AlgO - 9&% Na 3AlFg 101'0°C. 12°C. 22 2% A 1 2 0 3 - 9855 Na^AlFg 1040°C U2°C. 23, 2k 5% A 1 2 0 3 - 95% Na 3AlFg 990°C 10°C. 26 5% A 1 2 0 3 - 95% Na 3AlFg 1040°C. 60°C. 27, 29 5% A1 20 - 20% L i f - 15% 0 5 • H a 3 A 1 F ( 5 3U0 C. 20°C. - 77'-Figures 31 and 32 show the p o l a r i z a t i o n decrease f o r 5 vt.% alumina - 95 wt.'» c r y o l i t e melts as a f u n c t i o n of the Reynolds number f o r temperatures 10°C. and 60°C. above the l i q u i d u s . These curves have very poor r e p r o d u c a b i l i t y but again show a smaller decrease i n p o l a r i z a t i o n f o r the same current d e n s i t y but at a. higher melt temperature. The p o l a r i z a t i o n decreases at low current d e n s i t i e s tended to be l a r g e and have very poor r e p r o d u c a b i l i t y . F o l l o w i n g the high speed anode r o t a t i o n t e s t s at low current d e n s i t i e s , one set of t e s t s at 100 r.p.m. was c a r r i e d out. The magnitude of the p o l a r i z a t i o n was found to be l e s s than i n the e a r l i e r t e s t at 100 r.p.m.. At higher current d e n s i t i e s of 0.25 A./cm.2 the p o l a r i z a t i o n decrease i s approximately zero f o r Reynolds numbers less than 1500. At lower current d e n s i t i e s t h i s i n i t i a l p l a t e a u i s not observed. At current d e n s i t i e s l e s s than 0.08 A./cm/- the p o l a r i z a t i o n decreases have very p o o r l y r e p r o d u c a b i l i t y . Figure 36 shows the p o l a r i z a t i o n decrease f o r the l i t h i u m f l u o r i d e - c r y o l i t e melts at 840°C. While the r e p r o d u c a b i l i t y was f a i r the p o l a r i z a t i o n decreases were s i m i l a r t o F i g u r e s 32 and 3^ i n which the melts were c l o s e to t h e i r l i q u i d u s temperatures. The c e l l r e s i s t i v e p o t e n t i a l drop found on onening the c i r c u i t was measured. Unfor t u n a t e l y the r e s i s t a n c e of the mercury pool contact was not very constant. The r e s i s t a n c e o f t e n decreased a f t e r s e v e r a l current p u l s e s had been passed through the c e l l . P l o t s of the c e l l r e s i s t i v e p o t e n t i a l drop are shown i n Figures 37 and 3-J. At lower - 78 -- 70 -- 80 -300 1*00 500 600 700 8 9 1000 1500 2000 3000 1*000 5 6 7 8 Q 10 000 "Re. Figure 36. P o l a r i z a t i o n decrease vs, Re f o r runs 27 and 29 at 81*0°C. - 5. vt.% A 1 2 0 3 - 20.vt.% L i F - 75 vt.%' N a , A l F g . -Figure 37. Ohmic r e s i s t a n c e vs. anodic current d e n s i t y !for run 27. - 82 -hO 80 120 160 200 240 320 360 flnod.'e. Co rrcrvV "DcnsiTY _ m A. .cm".1 Figure 38. Ohmic r e s i s t a n c e vs. anodic current d e n s i t y f o r run 30. - «3 -2 current d e n s i t i e s ( l e s s than 0 . 3 A./cm. ) the r e s i s t a n c e appears constant. Above 0.50 A./cm. the r e s i s t a n c e appears t o decrease but t h i s i s probably o c c u r r i n g between the s t a i n l e s s s t e e l contact p l a t e s and the mercury pool . Anode - cathode r e s i s t a n c e s measured u s i n g an A.C. 1 khz r e s i s t a n c e meter v a r i e d between 0.20 - 1.50 ohms p r i m a r i l y depending on the p u r i t y of the mercury and the degree of c o r r o s i o n the s t a i n l e s s s t e e l contact p l a t e s and the molybdenum shaft had experienced. The d i f f u s i o n overvoltage appears t o be sm a l l . At low current d e n s i t i e s the anode r o t a t i o n speed g r e a t l y a f f e c t s the anode p o l a r i z a t i o n but the r e p r o d u c a b i l i t y i s very poor. Low anode r o t a t i o n speed t e s t s f o l l o w i n g the high speed t e s t s at low current d e n s i t i e s have very low p o l a r i z a t i o n values which are almost equal t o the high speed t e s t s values i f no p r e p o l a r i z i n g pulse i s employed. I f the anode v i b r a t e s a t higher anode r o t a t i o n speeds the p o l a r i z a t i o n decreases r a p i d l y . This decrease w i l l occur f o r c o n t i n u i n g t e s t s at the same current d e n s i t y and anode r o t a t i o n speed. Only Run 22 was not l i m i t e d i n anode r o t a t i o n speed on account o f anode v i b r a t i o n . Above 0.10 A./cm. the anode v i b r a t i o n d i d a f f e c t r e p r o d u c a b i l i t y but only s l i g h t l y . The anode p o l a r i z a t i o n v a r i a t i o n w i t h time at 500 - 1000 r.p.m. was much l e s s than at 100 r.p.m. a l l o w i n g greater r e p r o d u c a b i l i t y f o r current d e n s i t i e s g reater than 0.10 A./cm. . k. D I S C U S S I O N ^•1 Accuracy of R e s u l t s The determination of the p o l a r i z a t i o n as a f u n c t i o n of current d e n s i t y and anode r o t a t i o n speed r e q u i r e d s e v e r a l measurements. When an. instrument i s used to re c o r d a value two types of measuring e r r o r s occur. Systematic e r r o r s are thos which determine the "mechanical" accuracy of the measuring u n i t w h i le random e r r o r s are determined by " v i s u a l " accuracy by which an instrument can be read. Figure 39 i s a photograph of the experimental c e l l and a s s o c i a t e d measuring equipment. The systematic e r r o r s o c c u r r i n g during the p o l a r i z a t i o n measurements were l a r g e . The ammeter was accurate t o ±10%. The p o l a r i z a t i o n was measured employing an o s c i l l o s c o p e which had a systematic e r r o r o f l e s s than ±3% of the f u l l s c a l e reading. This e r r o r was caused by voltage f l u c t u a t i o n s i n the o s c i l l o s c o p e ' s power supply. The c e l l temperature f l u c t u a t e d ±10°C. and ± 7 . 5°C f o r furnace temperature s e t t i n g s of 840°C. and 990°C. r e s p e c t i v e l y . The gas e v o l u t i o n from the anode at current d e n s i t i e s greater than 0.10 A./cm. was i r r e g u l a r r e s u l t i n g i n c e l l v o l t age f l u c t u a t i o n s of up to ±10%. The area o f the anode surface f r e e o f absorbed gases changed r a p i d l y w i t h time causing these c e l l v o l t a g e f l u c t u a t i o n s . In a d d i t i o n Figure 39. Photograph of experimental c e l l and associated equipment. - 86 -the e l e c t r o l y t e r e s i s t i v i t y c l o s e t o the anode a l s o v a r i e d w i t h time. The motor f o r r o t a t i n g the anode was never c a l i b r a t e d hut i t was assumed to have been accurate t o w i t h i n ±5% systematic e r r o r . The random e r r o r s which occurred d u r i n g the p o l a r i z a t i o n measurements were l a r g e at higher current d e n s i t i e s . The o s c i l l o s c o p e reading of the p o l a r i z a t i o n t r a c e had the l a r g e s t random e r r o r . At low current d e n s i t i e s the t r a c e could be read t o a c c u r a c i e s of ±5mV. and ± lOmV. f o r o s c i l l o s c o p e v o l t a g e s e t t i n g s of 0 . 1 volts/cm. and 0 . 2 v o l t s / cm. r e s p e c t i v e l y . At l a r g e r current d e n s i t i e s where the anode-reference el e c t r o d e I.R. p o t e n t i a l drop was g r e a t e r than 2 0 0 mV., the accuracy as t o where the p o l a r i z a t i o n decay curve s t a r t e d a f t e r the c e l l c i r c u i t was opened could only be determined t o the nearest ±60 mV. f o r the 0 . 2 volts/cm. voltage s e t t i n g . Figures ho and hi are photographs of the o s c i l l o s c o p e p o t e n t i a l vs time t r a c e s . These f i g u r e s show the d i f f i c u l t y i n determining the top of the p o l a r i z a t i o n decay curve. The c e l l current could be read t o ±5mA./cm. F i n a l l y the speed at which the anode motor was r o t a t i n g c o u l d be measured to • ±50 r.p.m1. The determination of the peak of the anode p o l a r i z a t i o n decay curve was the most c r i t i c a l measurement. In order to raduce the random e r r o r s a s s o c i a t e d w i t h t h i s measurement each p o l a r i z a t i o n measurement at f i x e d anodic current d e n s i t y and anode r o t a t i o n speed was repeated 5 or more times. The r e p e t i t i o n of t e s t s a l s o enabled a time averaged p o l a r i z a t i o n - 87 -Figure ko. Oscilloscope trace of potential vs. tine - 0.60 A./cm.2, 0.2 V./cm., 0.5 msec./cm. x - 89-value t o be obtained at the higher current d e n s i t i e s where the c e l l p o t e n t i a l v a r i e d w i t h time. C e l l temperatures were recorded w i t h each p o l a r i z a t i o n reading. From each set of t e s t s the temperature e f f e c t on the readings was estimated, summed and averaged. This c o r r e c t i o n f a c t o r was then a p p l i e d t o a l l the r e s u l t s . At 8U0°C. the experimental runs averaged •» -5 mV. per 10°C. temperature i n c r e a s e . At 990°C. the experimentals runs not c o n t a i n i n g l i t h i u m f l u o r i d e averaged a i -Ik mV. per 10°C. temperature i n c r e a s e . The p o l a r i z a t i o n measurements a f t e r being adjusted f o r the temperature v a r i a t i o n were t r e a t e d as a normal d i s t r i b u t i o n to determine the mean value and a standard d e v i a t i o n f o r each set of t e s t s at the same current and anode r o t a t i o n speed. The standard d e v i a t i o n was normally l e s s than 10 mV. but c o u l d be as high as 25 mV. . The high values of the standard d e v i a t i o n c a l c u l a t i o n s normally occurred at low anodic current d e n s i t i e s or high anode r o t a t i o n speeds. The anode r o t a t i o n t e s t s at the same current d e n s i t y had f a i r r e p r o d u c a b i l i t y . The standard d e v i a t i o n was approximately 10 mV. but i n one group o f t e s t s the maximum p o l a r i z a t i o n decrease w i t h anode r o t a t i o n was only 7 mV. . C o n t r a s t i n g t h i s , the p o l a r i z a t i o n decreases would be l a r g e i f c a v i t a t i o n occurred a t the anode's surface. I f the boron n i t r i d e and anode carbon were not t i g h t l y f i t t e d onto the anode's - 90-molybdenum shaft then the shaft would v i b r a t e at high speeds causing c a v i t a t i o n . T his c a v i t a t i o n would provide e a s i e r desorption o f carbon monoxide and carbon d i o x i d e bubbles and would p h y s i c a l l y erode the anode's surface. The p h y s i c a l e r o s i o n of the anode surface would uncover the more h i g h l y r e a c t i v e s i t e s and e f f e c t i v e l y i n c r e a s e the r e a c t i v i t y of the anode's surface. At cur r e n t d e n s i t i e s l e s s than 0.08 A./cm.2 the p o l a r i z a t i o n decreases were very l a r g e at high anode r o t a t i o n speeds. These p o l a r i z a t i o n decreases were i n v e r s e l y r e l a t e d t o the current d e n s i t y . k.2 P o l a r i z a t i o n Experiments In the anode r e a c t i o n i n c r y o l i t e - a l u m i n a based melts the anode gas could be e i t h e r carbon d i o x i d e or carbon monoxide at anode current d e n s i t i e s below the anode e f f e c t . Most researchers agree t h a t above '-^0.10 A./cm. the anode gas e l e c t r o l y t i c a l l y evolved i s 100> carbon d i o x i d e . T h o n s t a d ^ found t h a t the Boudouard r e a c t i o n would not occur above 0.10 A./cm. and f e e l s t h a t carbon monoxide may never be e l e c t r o l y t i c a l l y evolved at any current d e n s i t y . Appendix I shows tha t carbon monoxide i s the thermodynamically favoured e l e c t r o l y t i c product. At 81+0°C. the d i f f e r e n c e i n the decomposition p o t e n t i a l s of carbon monoxide and carbon d i o x i d e i s o n l y ^1 mV. w h i l e i t i s 111 mV. at 990°C. The melts c o n t a i n i n g l i t h i u m f l u o r i d e were operated at those two temperatures to determine i f carbon monoxide i s evolved only at the higher temperatures. Figure 30 (at current d e n s i t i e s - 91 -of 0.01 - 0.03 A./cm.^) has a higher p o l a r i z a t i o n curve than does Figure 31 f o r melts at 990°C. In Figure 30, at 340°C., the T a f e l slope from 0.02 - 0.10 A./cm.2 i s +0.21 ±0.05 v o l t s w h i l e i n Figure 31 at 990°C. the T a f e l slope i n t h i s current d e n s i t y range i s +0.45 ±0.05 v o l t s . Comparison of the magnitude of the pola.riza.tion at these two temperatures shows the p o l a r i z a t i o n i s l e s s at 840°C but t h i s d i f f e r e n c e i s n e g l i g i b l e i n comparison t o the accuracy of the measurements as discussed below. In the s i n g l e pulse g a l v a n o s t a t i c method f o r measuring p o l a r i z a -t i o n the zero-current p o t e n t i a l value must remain constant w i t h time. The anode p o l a r i z a t i o n w i t h respect t o the reference e l e c t r o d e p o t e n t i a l w i l l equal the absolute anodic p o l a r i z a t i o n o n ly i f the reference electrode p o t e n t i a l remains constant. The e l e c t r i c a l ground i n the l a b o r a t o r y was a poor one on account of the a.c. i n t e r f e r e n c e i n th a t ground from the other equipment operating i n the l a b o r a t o r y . The anode, cathode and reference e l e c t r o d e were not grounded. The furnace and o s c i l l o s c o p e were grounded t o a water pipe and t h i s ground removed the a.c. i n t e r f e r e n c e found e a r l i e r on the o s c i l l o s c o p e when the e l e c t r i c a l ground was used. I n the e a r l y experimental runs the reference e l e c t r o d e had a carbon d i o x i d e atmosphere. T h i s atmosphere h e a v i l y o x i d i z e d the c r u c i b l e and the reference e l e c t r o d e . Since the removal of t h i s impressed atmosphere d i d not appear to a f f e c t the base p o t e n t i a l p o s i t i o n on the o s c i l l o -scope i t was discontinued. U n f o r t u n a t e l y the l a c k of an impressed atmosphere - 9 2 -r e s u l t s i n a f a i r l y s t a b l e but unreproducable anode-reference e l e c t r o d e zero current p o t e n t i a l . The zero c u r r e n t base p o t e n t i a l must be known since the time r e q u i r e d f o r the anode-reference e l e c t r o d e p o t e n t i a l t o be l e s s than 30 mV. of the o r i g i n a l base p o t e n t i a l i s very long a f t e r the c e l l c i r c u i t has been broken. The anode-reference e l e c t r o d e base p o t e n t i a l s would normally vary during each run ±20 mV. w i t h respect t o the furnace's water pipe ground but these base p o t e n t i a l s v a r i e d up t o 35 mV. between experimental runs. The o s c i l l o s c o p e ' s water pipe ground base p o t e n t i a l would move ±15 mV on the o s c i l l o s c o p e screen. This would, be caused by voltage f l u c t u a t i o n s t o the o s c i l l o s c o p e ' s a m p l i f i e r . The aluminum reference electrode was t r i e d i n runs #19 - 21 i n c l u s i v e but i n only Run 19 was the c o r r o s i o n o f the molybdenum or tungsten l e a d wires small enough f o r the reference e l e c t r o d e to have a s t a b l e p o t e n t i a l . The 2 vt.% alumina - 98 wt.# c r y o l i t e melt experimental runs were c a r r i e d out t o determine the v a r i a t i o n o f the d i f f u s i o n component of the anodic p o l a r i z a t i o n as a f u n c t i o n of temperature. The T a f e l slopes at current d e n s i t i e s g r e a t e r than 0.20 A./cm. do not appear t o decrease as much as the 5 vt.% alumina - 95 vt.%> c r y o l i t e melts. Figure 2k of Run 19 at 1010°C. shows a p o l a r i z a t i o n curve which does not l e v e l o f f at higher current d e n s i t i e s and whose p o l a r i z a t i o n values are l a r g e . At greater than 0.25 A./cm. - the T a f e l slope does not appear to decrease. At lower current d e n s i t i e s Run 19 appears t o - 93 " have a steep T a f e l slope o f +0.35 ±.05 v o l t s . Run 22 at 10U0°C. has a pl a t e a u followed by a very steep T a f e l slope and then above 0.20A./en/ a lower slope. The d i f f e r e n c e between the two p o l a r i z a t i o n curves o f 2 Runs 19 and 22 below 0.20 A./cm. current d e n s i t y i s p r i m a r i l y i n the 2 0.03 - 0.06 A./cm. current d e n s i t y ra,.nge. P a r t of t h i s d i f f e r e n c e i n p o l a r i z a t i o n could be a t t r i b u t a b l e t o d i f f u s i o n . In the f a r a d a i c 35 impedance measurements by Thonstad d i f f u s i o n p o l a r i z a t i o n was found at low current d e n s i t i e s . The d i f f u s i o n p o l a r i z a t i o n was assumed to occur i n the pores of the carbon e l e c t r o d e . The higher p o l a r i z a t i o n found i n Run 19 could be a t t r i b u t e d t o t h i s d i f f u s i o n p o l a r i z a t i o n . A second f a c t o r i s t h a t a t 1040°C. the decomposition p o t e n t i a l f o r carbon monoxide e v o l u t i o n i s 11 mV. lower than at 1010°C. . The 5 wt.% Alumina - 95 wt.% C r y o l i t e melt experimental runs were c a r r i e d out to determine the e f f e c t o f temperature and composition on the anodic p o l a r i z a t i o n . The temperatures were chosen c l o s e t o the l i q u i d u s temperature so tha.t the v i s c o s i t y would vary and the d i f f u s i o n p o l a r i z a t i o n would be much l a r g e r f o r runs at 990°C. than f o r runs at 1025°C. and 1040°C. Above 0.20 A./cm.2 the T a f e l slopes are + 0.07 ±.02 v o l t s and do not a.ppear t o i n c r e a s e i n slope at current d e n s i t i e s 2 2 of g r e a t e r than 0 .5 A./cm. Below 0.10 A./cm. the T a f e l slopes have two forms s i m i l a r t o the 2 wt.% alumina - 98 wt.% c r y o l i t e experimental runs. The exception i s Run 2k shown i n Figure 29. At 990°.Run 2k has a lower p o l a r i z a t i o n than Run 23 shown, i n F i g u r e 26 even though both melts were at the same temperature. In Run 2k the same melt and anode - 9k-were reused from Run 23. I t i s p o s s i b l e t h a t t h i s melt was contaminated w i t h i m p u r i t i e s . In Runs 23 and 26 the baths were prepared from n a t u r a l c r y o l i t e and alumina. At the end of each run the i n c o n e l furnace tube was covered, on the i n s i d e near i t s top w i t h condensed bath vapours. On c o o l i n g these condensed fumes scaled o f f and f e l l i n t o the melt. Some c o r r o s i o n of the i n c o n e l tube was observed i n t h i s r e g i o n . k.3 D i f f u s i o n P o l a r i z a t i o n Measurements The d i f f u s i o n p o l a r i z a t i o n at the anode i n c r y o l i t e - a l u m i n a based melts w i l l be a f u n c t i o n o f the anodic current d e n s i t y , bulk alumina co n c e n t r a t i o n and the e f f e c t i v e t h i c k n e s s o f the Nernst d i f f u s i o n l a y e r . From the t h e o r e t i c a l approach i n Appendix I I the d i f f u s i o n p o l a r i z a t i o n i s given by: ^ = E I r la [ i + I r Si (kk) where: Sj ~ E f f e c t i v e Nernst d i f f u s i o n l a y e r t h i c k n e s s at the gas e v o l v i n g e l e c t r o d e and. i s a f u n c t i o n o f anodic current d e n s i t y , i . Tj = S t o i c h i o m e t r i c f a c t o r f o r the oxygen i o n s . Cj = Bulk concentration of oxygen i o n s . J3 = D i f f u s i o n c o e f f i c i e n t f o r oxygen ions i n the e l e c t r o l y t e . ~ 95 ~ At higher current d e n s i t i e s where the d i f f u s i o n p o l a r i z a t i o n is greater than 100 mV. then {hh) may be approximated by: 3 c j J ( ] '5) In the current d e n s i t y range where (1*5) holds, the T a f e l slope f o r the d i f f u s i o n p o l a r i z a t i o n i s given bv: ^) \o$i "^"3" ln-'3' D U3 o (h5) 19 Janssen measured e f f e c t i v e Nernst d i f f u s i o n l a y e r t h i c k n e s s e s f o r a gas e v o l v i n g e l e c t r o d e i n aqueous s o l u t i o n s . At current d e n s i t i e s greater than 30 mA./cm.^ the d i f f u s i o n l a y e r t h i c k n e s s was found t o be a f u n c t i o n of current d e n s i t y . The slope of the l o g - l o g p l o t s of the e f f e c t i v e Nernst d i f f u s i o n l a y e r thickness versus e l e c t r o d e current d e n s i t y depended on whether the bubbles of the gas bein.? evolved coalesced or not. I f the bubbles coalesced the slope would be approximately -0.9 but i f the bubbles d i d not coalesce the slope would only be approximately - 0.3. Oxygen bubbles were found t o coalesce i n both a c i d and b a s i c aqueous s o l u t i o n s . I f the carbon d i o x i d e bubbles evolved at the anode i n c r y o l i t e - a l u m i n a melts do coalesce then ('*6) becomes: -i.s .. . - 9 6 " Figure k2 i s the plot, of the d i f f u s i o n p o l a r i z a t i o n a." determined i n Appendix I I . I t i s based on a c a l c u l a t i o n combining Jans sen' e m p i r i c a l r e l a t i o n s h i p f o r the Nernst d i f f u s i o n l a y e r t h i c k n e s s and the c r i t i c a l current d e n s i t y as a f u n c t i o n o f alumina concentra-t i o n as found by Thonstad"', Since Janssen c a r r i e d out h i s experimental work u s i n g agueous s o l u t i o n s h i s r e l a t i o n s h i p f o r the Nernst d i f f u s i o n l a y e r t h i c k n e s s may only be app l i e d to c e l l s having e l e c t r o l y t e s w i t h s i m i l a r hydrodynamic p r o p e r t i e s . The d i f f u s i o n p o l a r i z a t i o n measurements i n the -present work were taken at anodic current d e n s i t i e s up to 250 mA./cm. The experimental runs operated at over Uo°C. above t h e i r l i q u i d u s .temperature had v i s c o s i t i e s and d i f f u s i o n c o e f f i c i e n t s s i m i l a r t o aqueous s o l u t i o n s . The d i f f u s i o n p o l a r i z a t i o n measurements f o r these runs at anodic current d e n s i t i e s between 100-250 raA./cm. were i n reasonable agreement w i t h Figure h2. The lower temperature experimental runs were operated c l o s e to the e l e c t r o l y t e ' s l i q u i d u s temperatures and the v i s c o s i t i e s were l a r g e r . The d i f f u s i o n p o l a r i z a t i o n values f o r the 100-250 mA./cm.2 anodic current d e n s i t y range were l a r g e r than those values shown i n Figure k2. At low anodic current d e n s i t i e s the p o l a r i z a t i o n decreases found when the anode was r o t a t e d at high speeds were l a r g e and had very poor r e p r o d u c a b i l i t y . The p o l a r i z a t i o n decreases due t o anodic r o t a t i o n increased i n magnitude as the current density was decreased from 80 mA./cm.2 t o 31 mA./cm.2 There are two p o s s i b l e explanations f o r these r e s u l t s . At the high r o t a t i o n speeds extreme mixing c o n d i t i o n s or even c a v i t a t i o n w i l l occur at the anode i f the anode v i b r a t e s . The - 98 -v i o l e n t mixing or c a v i t a t i o n aids i n the desorption of the carbon d i o x i d e bubbles and reduce the r e a c t i o n p o l a r i z a t i o n . An a l t e r n a t e p o s s i b i l i t y i s that the c a v i t a t i o n at the anode surface w i l l be excessive enough to al l o w CO e v o l u t i o n . At these low current d e n s i t i e s the f r a c t i o n of the anode surface covered by absorbed gases i s s t i l l low so th a t t h i s carbon monoxide desorption r e a c t i o n may occur. The anode r o t a t i o n measurements had poor r e p r o d u c a b i l i t y s i n c e the p o l a r i z a t i o n decreases w i t h r o t a t i o n were s m a l l . At low current d e n s i t i e s the r e p r o d u c a b i l i t y was always poor. At high current d e n s i t i e s the p o l a r i z a t i o n measurements' ac c u r a c i e s were l i m i t e d on account of the time v a r i a n t p o l a r i z a t i o n r e s u l t i n g from the anode gas e v o l u t i o n . At anode r o t a t i o n speeds up to 1000 r.p.m. and f o r current d e n s i t i e s between 100 - 250 mA./cm.2 the p o l a r i z a t i o n measurements were more reproducable. At these higher current d e n s i t i e s no p o l a r i z a t i o n decrease was observed f o r anode r o t a t i o n speeds up to 500 r.p.m. . The temperature e f f e c t on the p o l a r i z a t i o n decrease w i t h r o t a t i o n was s i g n i f i c a n t . Both the 2 wt.% and the 5 wt.% alumina melts had l a r g e r p o l a r i z a t i o n decreases w i t h r o t a t i o n f o r the melts operated c l o s e t o t h e i r l i q u i d u s temperatures. Measurements of v i s c o s i t y ^ - IT of c r y o l i t e - a l u m i n a based melts show l i t t l e v a r i a t i o n w i t h temperature although few measurements have been taken at l e s s than 10°C. above the - 99 -l i q u i d u s temperature. The coalescence o f the carbon dioxid.e bubbles desorbing from the anode surface may a l s o be a f f e c t e d by the low temperatures. At current d e n s i t i e s greater than 70 mA./cm.2 the 2 \rt.% alumina melts have a greater p o l a r i z a t i o n decrease w i t h r o t a t i o n than do the 5 wt.$ alumina melts. Below 70 mA./cm.2 the r e p r o d u c a b i l i t y of the r e s u l t s i s too poor t o compare. The p o t e n t i a l r i s e and p o t e n t i a l decay curves were photographed. Figure 1+3 shows the r a p i d p o t e n t i a l r i s e time f o r th- 3 anode-reference electrode p o t e n t i a l to reach steady-state at an anodic current d e n s i t y of 1+00 mA./cm.2. The r i s e time f o r current d e n s i t i e s greater than 100 mA./cm. was reduced when the anode was pre-c o n d i t i o n e d . An unused anode when subjected to a high current d e n s i t y pulse w i l l r i s e r a p i d l y s i m i l a r to Figure 1+3 but at a l a t e r time i t w i l l i n c r e a s e i t s p o l a r i z a t i o n i n an unreproducable way. In Figure 1+1+ the p o t e n t i a l p decay curves are shown f o r an anodic current d e n s i t y of 90 mA./cm.." ana anode r o t a t i o n speeds o f 100, 1000 and 3000 r.p.m.. A l l p o t e n t i a l decay curves have a very s i m i l a r shape but the .1000 and 3000 r.p.m. curves approach the zero current p o t e n t i a l more r a p i d l y than the 100 r.p.m. curve. The p o l a r i z a t i o n decrease below the 100 r.p.m. p o l a r i z a t i o n value was 30 mV. and 1+0 :nV. f o r the anode r o t a t i o n speeds of 1000 and 3000 r.p.m. r e s p e c t i v e l y . Since the p o l a r i z a t i o n decrease i s very small i t i s not s u r p r i s i n g t h a t these p o t e n t i a l decay curves have a very s i m i l a r shape. Figure 43. Oscilloscope trace of potentii.1 rise to a quasi-3teady state value after the c e l l circuit is closed - 0.40 A./cm.2, 0.2 V./c 5.0 sec./cm. _ 102_ ^•^ Comparison w i t h Published L i t e r a t u r e The published l i t e r a t u r e i s f a r from unanimous about the mechanism of the anode r e a c t i o n or even the shape o f the T a f e l p o l a r i z a t i o n curve. Figure k^ shows the pub l i s h e d r e s u l t s f o r the anode p o l a r i z a t i o n . The experimental r e s u l t s of Run 29 and Run 22 are shown as w e l l . The p o l a r i z a t i o n curve f o r Run 29 i s s i m i l a r t o P i o n t e l l i ' s r e s u l t 3 0 but the p o l a r i z a t i o n values are l a r g e r . The p o l a r i z a t i o n curve f o r Run 22 i s not s i m i l a r t o any of the published p o l a r i z a t i o n curves. The p o l a r i z a t i o n curves f o r Runs 29 and 22 were p l o t t e d t o show the extremes o f the present experimental r e s u l t s . Most o f the p o l a r i z a t i o n curves (Figures 2k - 31) are s i m i l a r i n shape t o P i o n t e l l i and Takahashi . Both researchers employed the i n d i r e c t method.of the g a l v a n o s t a t i c pulse technique. The other published p o l a r i z a t i o n curves were found by the d i r e c t method a f t e r s u b t r a c t i n g the r e s i s t i v e component. Although the r e s i s t a n c e of the r e s i s t i v e p o l a r i z a t i o n was not found t o increase w i t h i n c r e a s i n g current d e n s i t y as found by Takahashi the r e s i s t a n c e of the mercury pool was hot c o n s i s t e n t enough to draw any co n c l u s i o n s . The experimental T a f e l slopes found f o r the 5 vt.% alumina melt runs at current d e n s i t i e s greater than 0.2 A./cm.2 were s i m i l a r t o P i o n t e l l i ' s being 0.07 ±0.02 v o l t s . The experimental T a f e l p l o t s a l l had a change i n the slope of the T a f e l curve i n the current d e n s i t y range o f 0.09 - 0.20 A./cm.2 as d i d P i o n t e l l i ' s r e s u l t s . Ref. 11 Thonstad & Rove Takahashi & Amada 3oc4-Ref. 27 - Thonstad Ref. 28 - Welch I Ref. 29 - 'Welch & Richards _Ref. 30 - P i o n t e l l i , Mazza & P e d e r f e r r i Haupin F i r s t number = wt.% A I 2 O 3 Second number = c r y o l i t e w t . r a t i o 60oL Ref. 31 -Ref. 1+0 - Haunin 1+00J-200-h 1 .05 . 0 6 . 0 7 . 0 8 .10 .20 .30 Anodic Current Density - Amns./cm.' 1 1 1 1 r ho .50 . 6 0 . 7 0 . 8 0 1 Fi frure k 5. Summary of published anodic Tafel polarization Plots and the extreme Tafel polarization plots of the present work (Runs 22 and 29) . - 104 -There are few p u b l i c a t i o n s on the determination of the hO d i f f u s i o n p o l a r i z a t i o n . .iaupin measured the p o l a r i z a t i o n by-i n t e r r u p t i n g the c e l l current supply and. e s t i m a t i n g the c e l l p o l a r i z a t i o n from, the p o t e n t i a l decay curves. He d i d not use a reference e l e c t r o d e . He measured the e f f e c t of a g i t a t i o n on the c e l l p o l a r i z a t i o n by o s c i l l a t i n g the anode using a simple harmonic o s c i l l a t i o n of the anode through 30° at 6.7 hz.. H i s r e s u l t s showed a very small p o l a r i z a t i o n decrease below 0.7 A./cm. . In the present experimental work much greater mixing has been used, but at much lower current d e n s i t i e s . The p o l a r i z a t i o n decrease at current d e n s i t i e s o l e s s than 0.08 A./cm." i s probably due to changes i n the r e a c t i o n p o l a r i z a t i o n r a t h e r than d i f f u s i o n p o l a r i z a t i o n . The p o l a r i z a t i o n decreases at current d e n s i t i e s greater than 0.1 A./cm.2 were small agreeing w i t h Haupin's r e s u l t s . The maximum curr e n t d e n s i t y at which the r o t a t i o n t e s t s were t r i e d was 0 .25 A./cm. . Higher current d e n s i t i e s were l i m i t e d by experimental design. h.5 Anode Mechanism The mechanism of the anode p o l a r i z a t i o n i n the aluminum c e l l i s not f u l l y understood. Most researchers consider that the r e a c t i o n p p o l a r i z a t i o n i s dominant from current d e n s i t i e s o f 0.10-1.0 A./cm. Below 0.10 A./cm.2 there i s no c o n c l u s i v e evidence t h a t the gas evolved i s e i t h e r carbon d i o x i d e or carbon monoxide or both. A n a l y s i s - 105 -of the o f f gases from c e l l s operated below 0.10 A./cm.2 i s hindered by the slow r a t e of formation and the Boudouard r e a c t i o n . Above important and i s the reason f o r the anode p o l a r i z a t i o n becoming l a r g e enough f o r the c o e v o l u t i o n of carbon t e t r a f l u o r i d e and s i m i l a r gases. Chronopotentiometrie st u d i e s have shown th a t the anode e f f e c t caused by the c o e v o l u t i o n of these gases occurs at current d e n s i t i e s p r o p o r t i o n a l to the alumina content of the bath. In the present experimental work the anode p o l a r i z a t i o n was examined i n two ways. By measuring anodic p o l a r i z a t i o n T a f e l p l o t s f o r a temperature v a r i a t i o n of 150°C. f o r the same bath composition or200°C f o r l i t h i u m and n o n - l i t h i u m melts i t was attempted t o determine the e f f e c t of the d i f f e r e n c e i n the carbon d i o x i d e and carbon monoxide e q u i l i b r i u m p o t e n t i a l s . The second approach was t o determine the d i f f u s i o n component of the p o l a r i z a t i o n as a f u n c t i o n of current d e n s i t y . The d i f f u s i o n component above 0.10 A./cm. was small so the p o l a r i z a t i o n curve's T a f e l slope was a n a l i z e d to determine an anode mechanism. The anode r e a c t i o n mechanism considered was th a t used by Thonstad . 1.0 A./cm. the d i f f u s i o n p o l a r i z a t i o n becomes i n c r e a s i n g l y 0| + x C C x 0 I + o2-y co 2 o (D) (C) (3) (A) (hi) - 106 -Step (D) i s considered to be r a t e determining. From Appendix IV an expression i s derived f o r the t o t a l r e a c t i o n p o l a r i z a t i o n assuming t h a t the surface coverage for. 0 , CxO , C0 2 are f u n c t i o n s of the p o l a r i z a t i o n . Reaction (B) i s assumed t o be f a s t and i n e q u i l i b r i u m and the p a r t i a l pressure of the CO evolved i s a p p r o x i -mately constant. The derived expression f o r the r e a c t i o n p o l a r i z a -t i o n i s : V ' 4?Y J e ^ L Sol (i-zelJ i i i n . o where: E q u i l i b r i u m p o t e n t i a l f o r CO e v o l u t i o n a t zero current flow * £<t) = E q u i l i b r i u m p o t e n t i a l f o r C0 o e v o l u t i o n w i t h current flow l = Current d e n s i t y K-D = Rate constant f o r adsorption of 00^ Y = Reaction order f o r de s o r p t i o n of CO^ —5° ^co 2 = P a r t i a l pressure o f CQ^ adjacent t o the anode 6 0j = Surface coverage of o| at current d e n s i t y v, S O | = T o t a l surface coverage o f absorbed species a t current d e n s i t y AT I = Surface coverage of species A at zero current flow At present the values f o r surface coverage of the absorbed species are not known as f u n c t i o n s of current d e n s i t y . • I f p a r t s I I and. I l l are - 107 -assumed t o vary only to a small degree with current d e n s i t y then the 2.3RT T a f e l slope i s ^ hjY. The r e a c t i o n order f o r carbon monoxide e v o l u t i o n from carbon anodes i n a i r has been found t o be between 0.5 - 0 . 8 . 1 + 2 At 1000°C. u s i n g only p a r t I the T a f e l slopes are 0.03 v o l t s and 0.13 v o l t s f o r r e a c t i o n r a t e s of 0 .8 and 0.5 r e s p e c t i v e l y . For high surface coverage by absorbed species I I I w i l l become s i g n i f i c a n t although the Q and the (l-SQ .^) terms w i l l tend to counteract each other. The experimental r e s u l t s have T a f e l slopes o f 0.07 ±.02 v o l t s f o r current d e n s i t i e s above 0.20 A./cm.2 which i s i n reasonable agreement. Janss e n 1 8 ' 1 ? has studied the surface coverage of e l e c t r o d e s as a f u n c t i o n o f t h e i r current d e n s i t y . At present only aqueous s o l u t i o n s have been examined using high speed photography. Although i t w i l l be very d i f f i c u l t t o determine the surface concentration of the intermediate species the time averaged t o t a l surface coverage should be p o s s i b l e . - 108 -5. CONCLUSIONS 5 • P o l a r i z a t i o n Tests 1. The accuracy o f the present experimental r e s u l t s i s u s e f u l f o r q u a l i t a t i v e and se m i - q u a n t i t a t i v e a n a l y s i s . 2. The l i t h i u m f l u o r i d e melts showed two s l i g h t l y d i f f e r e n t T a f e l p l o t s f o r runs operated at 8k0°C. and 990°C. . The 8l40°C. runs had a l a r g e r p o l a r i z a t i o n at lower anodic current d e n s i t i e s than d i d the 990°C. runs. This i s i n d i c a t i v e t h a t carbon monoxide i s evolved at higher current d e n s i t i e s at 990°C. where i t i s more thermodynamically favoured than carbon d i o x i d e e v o l u t i o n . 3. The T a f e l p l o t s o f the 2 vt.% and 5 vt.% alumina e l e c t r o l y t e s showed t h a t the slopes f o r the 2 vt.% alumina runs were g r e a t e r than f o r the 5 vt.% alumina runs at higher anodic current d e n s i t i e s . The d i f f e r e n c e i n the T a f e l slopes i s small but f i n i t e i n d i c a t i n g a small concentration p o l a r i z a t i o n . h. The T a f e l p l o t s are s i m i l a r to those found by P i o n t e l l i , Takahashi and Paunovic. A l l experimental T a f e l p l o t s show th a t the curves have a break i n t h e i r slope between 90 and 200 mA./cm. . In a l l runs the T a f e l slope decreases w i t h i n c r e a s i n g current d e n s i t y . -109 ~ 5. A c a l c u l a t i o n has been made f o r the anodic p o l a r i z a t i o n based on r e a c t i o n p o l a r i z a t i o n . The T a f e l slope f o r the 5 wt.% alumina melts i s i n agreement w i t h the r e a c t i o n p o l a r i z a t i o n cancellation. 5 ' 2 D i f f u s i o n P o l a r i z a t i o n Tests 1. The accuracy of the present experimental approach i s adequate f o r q u a l i t a t i v e and s e m i - q u a n t i t a t i v e a n a l y s i s . 2. D i f f u s i o n p o l a r i z a t i o n represents l e s s than 10$ of the t o t a l p o l a r i z a t i o n f o r anodic current d e n s i t i e s l e s s than'250 raA'./cm. . At greater than 2 wt.i alumina e l e c t r o l y t e s or low current d e n s i t i e s the d i f f u s i o n p o l a r i z a t i o n i s s m a l l . 3. D i f f u s i o n p o l a r i z a t i o n increases r a p i d l y as the c e l l temperature approaches to w i t h i n 20°C of the e l e c t r o l y t e s l i q u i d u s temperature. In c r e a s i n g the c e l l temperature from <=^12°C. to «42°C. above the l i q u i d u s temperature, f o r a 2 wt.% alumina melt has approximately an equal e f f e c t on reducing the d i f f u s i o n p o l a r i z a t i o n as changing to a 5 wt.% alumina melt ana maintaining a temperature s=slO°C. .above the l i q u i d u s temperature. k. The p o t e n t i a l r i s e time f o r the anodic p o l a r i z a t i o n to reach a quasi-steady s t a t e a f t e r a c i r c u i t i s closed i s l e s s than 5 seconds i f the anode has been"' preconditioned. I f no p r e c o n d i t i o n i n g the r i s e time w i l l be greater unless the current density i s greater than 100 m-A./cm/. - 110 -5. Anode r o t a t i o n atlow current d e n s i t i e s r e s u l t s i n an unstable p o l a r i z a t i o n . This i n s t a b i l i t y i s caused by the v i o l e n t e l e c t r o l y t e mixing at i t s surface. The unstable and reduced p o l a r i z a t i o n could be due to f a s t e r and e a s i e r d e s o r p t i o n of carbon d i o x i d e or carbon monoxide e v o l u t i o n . 6. At l a r g e r current d e n s i t i e s g r e a t e r than 100 mA./cm.2 the p o l a r i z a t i o n measurements are s t e a d i e r and more reproducable at an anode r o t a t i o n speed o f 1000 r.p.m. than at 100 r.p.m.. This increased re p r o d u c s . b i l i t y a r i s e s from greater ease of carbon dio x i d e desorption. 7. A c a l c u l a t i o n has been made to estimate the d i f f u s i o n p o l a r i z a t i o n as a f u n c t i o n of the alumina c o n c e n t r a t i o n and the anodic current i n d e n s i t y . This c a l c u l a t i o n i s based on the works o f Janssen and 39 Thonstad . The d i f f u s i o n overvoltages found f o r melts operated greater than 20°C. above the l i q u i d u s i s i n agreement with the 2 c a l c u l a t e d values f o r current d e n s i t i e s greater than 0.10 A./cm. . 8. The r o t a t i o n o f the anode w i l l reduce the e f f e c t i v e Nernst d i f f u s i o n l a y e r t h i c k n e s s s i m i l a r t o f l u i d flow across a p l a t e . From equation (10) f o r t u r b u l e n t flow across a f l a t p l a t e the e f f e c t i v e d i f f u s i o n l a y e r t h i c k n e s s was p r o p o r t i o n a l t o the f l u i d flow r a t e t o the power o f (-0.9). I t i s expected t h a t a s i m i l a r r e l a t i o n s h i p i s present r e l a t i n g anode r o t a t i o n speed t o an e f f e c t i v e Nernst d i f f u s i o n l a y e r t h i c k n e s s . Anode r o t a t i o n speeds o f 3000 - 5000 r.p.m. should reduce the d i f f u s i o n l a y e r t h i c k n e s s and d i f f u s i o n overvoltage by grea t e r than twenty times. The determination o f the d i f f u s i o n overvoltage from the p o l a r i z a t i o n decrease - 110 A w i t h r o t a t i o n should be accurate t o w i t h i n 5% i n the absence of any other changing e l e c t r o d e r e a c t i o n parameter. ~ 111 " 6. SUGGESTIONS FOR FUTURE WORK ' °•1 P o l a r i z a t i o n Measuring Apparatus . The p o l a r i z a t i o n measuring apparatus should have the f o l l o w i n g f e a t u r e s : 1. E l e c t r o n i c p o t e n t i a l measuring equipment should he capable of measuring p o t e n t i a l decay curves at speeds of 1 ^.sec/cm.. An e l e c t r o n i c t r i g g e r c i r c u i t would have t o be attached t o the o s c i l l o s c o p e used t o record the p o t e n t i a l decay curves. 2. The reference e l e c t r o d e should be a (COg, C0/C/Al o0 3 - Na-^AlFg) electrode which has an impressed carbon d i o x i d e atmosphere. While the reference e l e c t r o d e should have the carbon d i o x i d e atmosphere the carbon c r u c i b l e should be s h i e l d e d from t h i s atmosphere t o prevent c o r r o s i o n . This problem can be overcome by having an i n c o n e l tube f i t over the reference e l e c t r o d e . This i n c o n e l tube would extend i n t o the e l e c t r o l y t e and prevent the carbon d i o x i d e gases from reaching the c r u c i b l e . 3. The furnace should be l a r g e r t o accomodate a l a r g e r c r u c i b l e . I n a d d i t i o n the furnace should have a l a r g e r constant temperature zone and a b e t t e r temperature c o n t r o l l e r . The c o n t r o l l e r should be able t o hold the furnace temper at lire t o w i t h i n ±2°C. - 112 -CO, Pt. or Mo tube 5: lead-Inconel tube nranhite reference electrod graphite cathode L7 o o o O O °| -electrical insulator Anode shaft insulators J Electrolyte • p-r*)r}ii.te anode Figure k6. Proposed anode and c e l l design. - 113 -h. The anode should be b u i l t so t h a t as i t wears i t s r e p r o d u c a b i l i t y does not d e t e r i o r a t e as a r e s u l t of gas impingement. One p o s s i b l e design i s an i n v e r t e d d i s c e l e c t r o d e as shown i n Figure kS. 5. The mercury pool contact between the anode and the power supply must be improved or replaced. P o s s i b l e improvements are l a r g e r contact p l a t e s and a permanently sealed mercury p o o l . An a l t e r n a t e moving contact c o u l d be made u s i n g a set of brushes. °"•2 Experiments to be Considered Future experimental work using the g a l v a n o s t a t i c method depends on the above improvements. I f the above improvements give acceptable r e p r o d u c a b i l i t y then the f o l l o w i n g experiments would be of va l u e : 1. R e p e t i t i o n o f the p o l a r i z a t i o n measurements f o r the l i t h i u m f l u o r i d e - a l u m i n a - c r y o l i t e melts at 840°C. and 990°C. to determine i f carbon monoxide i s ever evolved. 2. R e p e t i t i o n of the anode r o t a t i o n experiments i n the curr e n t d e n s i t y range 0.10 - 3.0 A./cm. . The d i f f u s i o n p o l a r i z a t i o n r e s u l t s should be reproducable i n t h i s current d e n s i t y range. The v a r i a t i o n o f the d i f f u s i o n overvoltage w i t h temperature and current d e n s i t y should be compared t o the c a l c u l a t i o n based on Janssen's"^ and Thonstad 1 s"*' work. Most i n d u s t r i a l , c e l l s operate w i t h i n 30°C. of the l i q u i d u s - 114 -temperature o f the e l e c t r o l y t e so t h a t t h i s temperature range should d e f i n i t e l y he examined. 3. S e l e c t an oxide-fused s a l t system which has a r e l a t i v e l y low l i q u i d u s temperature and i s transparent. Employ a carbon anode and examine the e v o l u t i o n o f the anode gases u s i n g high speed photograph} t o determine the e f f e c t s of v i s c o s i t y on the c o a l e s c i n g on the anode gas bubbles. - 115 " APPENDIX I ELECTRODE POTENTIAL VS. ANODE GAS COMPOSITION The thermodynamic c e l l p o t e n t i a l depends on the anodic and cathodic products. I n the aluminum r e d u c t i o n c e l l most e l e c t r o l y t e s have a c r y o l i t e r a t i o o f l e s s than 1.35. The metal, pad has a very low conce n t r a t i o n of sodium so that the thermodynamic c e l l p o t e n t i a l depends on the anode gas composition. 1. Platinum Anode When a platinum anode i s used f o r a c r y o l i t e - a l u m i n a e l e c t r o l y t e the anode gas i s oxygen. The c e l l p o t e n t i a l i s given by: Q > 3' " T o I s (A.I-1) The standard s t a t e p o t e n t i a l s depend on the temperature and the f r e e energy o f formation of alumina at the p a r t i c u l a r temperature. The range of standard c e l l p o t e n t i a l s f o r oxygen e v o l v i n g anodes at 8!+0°C. , 990°C. and 1050°C. i s (25): At 81+0°C. E(0) = 2.29 V o l t s At 990°C. E(0) = 2.20 V o l t s At 1050°C E(0) = 2.17 V o l t s (A.1-2) - 116 . 2. Carbon Anode When a carbon anode i s used the gaseous products w i l l be e i t h e r carbon monoxide or carbon d i o x i d e . For a c e l l w i t h an e l e c t r o l y t e saturated i n c»c-alumina and an aluminum cathode the decomposition p o t e n t i a l s are as f o l l o w s : A. Carbon monoxide e v o l u t i o n may be represented by the c e l l r e a c t i o n s . A1 20 (oc) = 2 A l + 3/2 0 2 3/2.0 2 + 3C = 3C0 (A.1-3) The r e d u c t i o n i n the thermodynamic c e l l p o t e n t i a l by carbon monoxide e v o l u t i o n i s equal t o : 840°C. = -l.OQO V o l t s 990°C. = -1.153 V o l t s 1050°C. = -1.185 V o l t s (A.1-4) Net c e l l thermodynamic p o t e n t i a l s f o r carbon monoxide e v o l u t i o n are: 840°C. £ ( o V £ U = 1.20 V o l t s 990°C. £ C o ) - Z'ao = 1.04 V o l t s 1050°C. £ ( o ) - e ° o o = 0.98 V o l t s (A. 1-5) - 117 B. Carbon d i o x i d e e v o l u t i o n may be represented by the c e l l , r e a c t i o n : A 12 ° 3 («) = 2A1 + 3/2.0 2 3/2.0 2 + 3/2C. = .3/2. C0 o (A. 1-5) The r e d u c t i o n i n the thermodynamic c e l l p o t e n t i a l by carbon d i o x i d e e v o l u t i o n i s equal t o : 840°C. £co a= - 1 . 0 4 9 990°C. £°ooa= -1 .049 1050°C. E%o = - 1 . 049 (A.1-7) Net c e l l thermodynamic p o t e n t i a l s f o r carbon monoxide e v o l u t j :-lon are: 84o°C. Z(o)-Z°c^ = 1.24 V o l t s 9 9 0°C gCo)-£°oo a = 1.15 V o l t s 1050°C. £ < o ) - E'o^ = 1.12 V o l t s (A. 1-8) The d i f f e r e n c e i n decomposition p o t e n t i a l s f o r c e l l s producing carbon monoxide and carbon d i o x i d e i s : -3 4 o ° c ^ _ g ^ = k l m V o l t g 990°C. €°c« - E°ooj, = i n m V o l t s 1050°C. S0^ _ = • 136 m V o l t s (A. 1-9) - 118 -3• C e l l s e v o l v i n g both carbon monoxide and carbon tiioxide simultaneously A d e r i v a t i o n o f an equation v h i c h determines the e q u i l i b r i u m c e l l p o t e n t i a l as a f u n c t i o n of the anode gas composition i s given below: Let: a = Faradaic f r a c t i o n of the anode r e a c t i o n i n which carbon dio x i d e i s generated. b = Faradaic f r a c t i o n o f the anode r e a c t i o n i n which carbon monoxide i s generated. CO^ = Mole f r a c t i o n of vapour pressure f o r CO^ = x CO = Mole f r a c t i o n of vapour pressure f o r CO = l - x C e l l r e a c t i o n may be expressed as: o- RUQ3 •+ 3 / 2 o, C 2.CX.RI + 3 / 2 a. COj. (A.I-10) b RUO s + 3 b C ^ 2 b fll + 3 b CO ( A . I - l l ) Combining (A.I-10) and ( A . I - l l ) y i e l d s : f a + b ) flla03 + 3 ^ 4 . b ) c 5 * 2 (a+b) Rl JL <x COg, -4- 3 b C O (A.1-12) Case 1 - The external_pres sure i s v a r i a b l e a.-*- b = 1 Wence x = o~ OR (A.I-13a) b » Jtx. • (A.I-.13b) i+oc - 119-Expressing (A.1-13) and A.1-12) i n terms of x y i e l d s : L'+ J^ [n^ J irrsU (A. i ( A . I - l l i ) The f r e e energy a s s o c i a t e d w i t h the carbon d i o x i d e e v o l u t i o n as i n equation (A.I - 1 0 ) i s : L J (A.1-15) S i m i l a r i t y f o r carbon monoxide e v o l u t i o n : b AQcx. = b k G ' ^ + trr In. [Pco] 3*" (A.I -16) From (A.I . 1 5)and (A.I -16) then equation (A.I-lU) may be expressed i n terms of free energy as: or [%.']H Expressing (A.I-17) i n terms of c e l l p o t e n t i a l s : r - •!-»£=* (A .1-17) 3 6 ^ ~^fT i g T + ^ ^ f e l ( A . I - 1 6 ) 120 -Case 2 - sxternal_pj^^3_ure i s 1 atmosphere Hence: a a. + 3 t = i z o r : <X = 2. x 3 b = I • Applying these r e l a t i o n s h i p s to(A.1-12) y i e l d s : (A.1-19) Expressing (A.1-19) i n terms of f r e e energy y i e l d s : R T In. [Pcoa,]X [-peer**"0 (A.1-20) or i n terms o f decomposition p o t e n t i a l s _ » -+- "^T In. [ f c 31 a f x + i 0 [ l ^ (A.1-21) At 990°C. usi n g Janaf t a b l e s 2 5 f o r (A.1-21); (A.1-22) - 121 -APPENDIX I I DIFFUSION OVERVOLTAGE A. Nernst d i f f u s i o n l a y e r i s not a f u n c t i o n o f ele c t r o d e current density. The general o v e r a l l e l e c t r o d e r e a c t i o n i s given by: - v", S, -+-<-"»*) S 8 -r- - : =^s= Y% Sx * S y -i- ne.- ( A . I I - l ) Where: V\ = ~j - S t o i c h i o m e t r i c f a c t o r f o r species j s j = Species r e a c t i n g or being generated by the ele c t r o d e r e a c t i o n For the general e l e c t r o d e r e a c t i o n i n ( A . I I - l ) the e q u i l i b r i u m p o t e n t i a l i s determined by the Nernst equation: 3 0 (A.II - 2 ) Where: E.« = E q u i l i b r i u m c e l l p o t e n t i a l f o r a l l r e a c t a n t s and products at u n i t a c t i v i t y = A c t i v i t y of species away from electrode surface D i f f u s i o n overvoltage r e s u l t s when the r a t e o f consumption or production of the ele c t r o d e species exceeds the r a t e of t r a n s p o r t to or from the electrode surface. The surface concentration of the el e c t r o d e species w i l l then be a f u n c t i o n of current d e n s i t y and time. The d i f f u s i o n overvoltage i s defined as the change i n the c e l l p o t e n t i a l r e s u l t i n g from the change i n a c t i v i t y of the absorbing or desorbing electrode i n i t i a l and f i n a l products, i . e . : - 122 ~ (A.II-3) For steady s t a t e c o n d i t i o n s and a p p l y i n g a l i n e a r c o n c e n t r a t i o n gradient approximation the a c t i v i t y gradient adjacent t o the e l e c t r o d e surface i s given by: <£, ' (A.II-H) Where: * * " CLj = A c t i v i t y of species J adjacent to the e l e c t r o d e surface _ " .» CLj = A c t i v i t y of species J outside the Hernst d i f f u s i o n l a y e r «Sj = t h i c k n e s s of the Nernst d i f f u s i o n l a y e r I f a l a r g e i n d i f f e r e n t e l e c t r o l y t e i s present then the m i g r a t i o n of the r e a c t i n g and produced species due to the e l e c t r i c f i e l d w i l l be n e g l i g i b l e . Low a c t i v i t i e s o f r e a c t i n g or produced species w i l l normally obey Henry's law so that concentrations may be used i n s t e a d of a c t i v i t i e s : •> (A.II-5 ) I f the r a t e determining species i s s. react ant then i t s c o n c e n t r a t i o n at the e l e c t r o d e surface decreases w i t h i n c r e a s i n g current d e n s i t y . A. l i m i t i n g d i f f u s i o n c u r r e n t , ^ , i s reached when the surface - 123-concentration goes to zero. 3 (A.II - 6 ) From ( A . I I - 3 ) - ( A . I I - 6 ) an expression f o r the d i f f u s i o n overvoltage as a f u n c t i o n of the e l e c t r o d e current d e n s i t y and l i m i t i n g d i f f u s i o n current d e n s i t y i s obtained: ^3- J I L u J ( A . I l - 7 ) B. Nernst d i f f u s i o n l a y e r i s a f u n c t i o n of e l e c t r o d e current d e n s i t y . At e l e c t r o d e s which evolve gaseous products the desorption of the gases g r e a t l y increases the mixing of the e l e c t r o l y t e adjacent t o the e l e c t r o d e surface. The Nernst d i f f u s i o n l a y e r t h i c k n e s s becomes a f u n c t i o n o f the current d e n s i t y and the form of the bubbles of the evolved gases. I f the bubble coalesce then the mixing of the e l e c t r o l y t e adjacent t o the e l e c t r o d e surface increases g r e a t l y . From (A.II-3) the d i f f u s i o n overvoltage i s : 3^- J (A.II-3) From (A.II - 5 ) expressing F i c k s 1st law of d i f f u s i o n , t h e surface concentration i s given by: TV. 5 .D ( A . I I - 8 ) - 12k ~ Combining (A.II - 3 ) and A.II - 8 ) the d i f f u s i o n overvoltage i s given by: ^ ( A . n _ 9 ) For l a r g e d i f f u s i o n overvoltages (greater than 60 mV. at 1000°C): \ ^ i r r |„ [J r . In [_Vj ] "RT w In [ <S, 1 The T a f e l slope f o r l a r g e r d i f f u s i o n overvoltages becomes (A.11-10) ^ ^ 5 1 ^ T ~ (A. I l - l l ) 19 Janssen has measured the v a r i a t i o n o f an e f f e c t i v e Nernst d i f f u s i o n l a y e r t h i c k n e s s as a f u n c t i o n o f the e l e c t r o d e current d e n s i t y and gas evolved. For c o a l e s c i n g oxygen bubbles the slope of a l o g - l o g p l o t f o r d i f f u s i o n l a y e r t h i c k n e s s vs el e c t r o d e current d e n s i t y was found to be -0.9 f o r current d e n s i t i e s greater than 30 milliamp/cm. <~. Thonstad has measured the c r i t i c a l current d e n s i t i e s f o r c r y o l i t e melts. From these two sets of experimental data the d i f f u s i o n overvoltage as a f u n c t i o n of anodic current d e n s i t y i n c r y o l i t e - a l u m i n a based melts has been determined as f o l l o w s : 39 From Thonstad's work the c r i t i c a l current d e n s i t y i s given by: CCD. = 2.9 + 1.9 (wt.jS alumina) (A.11-12) - 125 -The d i f f u s i o n l a y e r t h i c k n e s s at the CCD. i s given by: Jc.C.a>. (A.11-13) The d i f f u s i o n l a y e r t h i c k n e s s at any other current d e n s i t y i i given bv: Sj c. CD, JCc.T>. 1 (A.11-14) The con c e n t r a t i o n d i f f e r e n c e across the Nernst d i f f u s i o n l a y e r i s then given by: A c = I • V; £• ——=—J • _J.C "3* D: (A.11-15) The concentration at the el e c t r o d e surface i s the d i f f e r e n c e between C j and AC so that the d i f f u s i o n overvoltage i s given by ( A . I I - 3 ) . The d i f f u s i o n overvoltage values determined by t h i s i t e r a t i v e procedure are shown i n Figure 42. - 1 2 6 -APPENDIX I I I CHARGE TRANSFER OVERVOLTAGE Charge t r a n s f e r overvoltage r e s u l t s from the r e s i s t a n c e of the e l e c t r i c a l double l a y e r to the t r a n s p o r t of electrode species across i t . The e l e c t r o d e species i n order t o cross the double l a y e r must overcome the .activation energies f o r the anodic and cathodic r e a c t i o n s . These a c t i v a t i o n energies depend on the p o t e n t i a l d i f f e r e n c e across 12 the double l a y e r . This i s shown s c h e m a t i c a l l y i n Figure (kj) The dependence of the anodic and cathodic a c t i v a t i o n energies on the p o t e n t i a l o f the double l a y e r can be approximately described by: - - <=*• i t 2F* £»4>c ( A . I I I - l ) = o^- - f ( A . I I I - 2 ) Using reference e l e c t r o d e s the a c t i v a t i o n energies can be expressed i n terms of el e c t r o d e p o t e n t i a l . = - - e ^ ^ k - S ^ (A.III-3) = -+ n - * ) z 5 (t-w- S } (A.III - 4 ) Where:^E ^ £ = Are the zero current a c t i v a t i o n energies r e l a t i v e ; t o the reference e l e c t r o d e £ u — £ = P o t e n t i a l across the d i f f u s e double l a y e r 127 -Figure 47. Schematic representation of the effect of the compact double layer potential on the anodic and cathodic activation energies for a net cathodic current at the electrode. 64>c = Potential difference across double layer due to current i . Chemical anodic activation cnerry Anodic activation energv Chemical cathodic activation energ. Cathodic activation energy - 128 -In the case of an i n d i f f e r e n t e l e c t r o l y t e the anodic and cathodic currents f o r a metal-ion e l e c t r o d e are: = K + C - C L icvr The net current i s given hv: ( A . I I I -( A . I I I -1£T 12.T S: Where u c = Exchange current d e n s i t y at e q u i l i b r i u m . At high charge t r a n s f e r overvoltages ( A . I I I - 7 ) can be approximated bv: ( i ) Large anodic overvoltages >^ 3 J «*- ^ = - "K.T In t 0 -f- ^ ^ V t ocZ3- (A. in-:": ( i i ) Larse cathodic overvoltages \y\ I K T > o • I £ 3 C = - Co txp [ _ Q-oQ 2-3" \ t 1 OR.. ^ = -RT k Uo — "RT ' \rt |b| ^ J ^ G-o0£3 Cl-oe»3 ( A . I I I - 9 In the case of charge t r a n s f e r overvoltage w i t h a. sequence of s e v e r a l d i f f e r e n t charge t r a n s f e r r e a c t i o n s s e v e r a l inte.rmed.iate species w i l l 12 be present. The d e r i v a t i o n by V e t t e r assumes the f o l l o w i n g : 1. Pure charge t r a n s f e r overvoltage - i . e . concentrations of the i n i t i a l and f i n a l o x i d i z e d and reduced, species are not f u n c t i o n s - 129 ~ of the current d e n s i t y . 2. The intermediate species' concentrations are fu n c t i o n s of the current and overvoltage and are small so that the--' a.re produced and consumed 100/! e l e c t r o c h e m i c a l l y . 3. The supporting e l e c t r o l y t e i s i n d i f f e r e n t . k. For two consecutive charge-transfer r e a c t i o n s the r e a c t i o n mechanizm i s : S r ^ S m c.i>. - i ( A . I I I - 1 0 ) S m So rx 2. ( A . I I I - 1 1 } Current d e n s i t i e s are f o r each step: (A.111-12) •RT ( A . I I T - 1 3 ) Exchange current d e n s i t i e s ; K T^T * L V ? J ( A . I I I - l U ) ° t "RT - 0 - * ) ' (A.II.T-15) - 130 _ Combining (A.III - 7 ) and (A.III - 1 2)-(A.III - 1 5 ) and e l i m i n a t i n g cm, and noting::.; E - E« the f o l l o w i n g s o l u t i o n h o l d s : ( i ) Anodic overvoltages ~*~ !-* expf- ( i o c 0-°< R)3 , r l I La *• T?-r J (A.III-16) f o r l a r g e anodic overvoltages n » RJ In. u»t then (A.III - 1 6 ) becomes: I - 2. Ua (A.III - 1 7 ) A l s o i f CK^> i°0 then f o r charge t r a n s f e r overvoltages i n range: _ « n « V 2 ^ L n3« tit I t the f o l l o w i n g s i m p l i f i c a t i o n s are p o s s i b l e : e x p f - ag r , ^ w 0 1 -•- Lo, <2-xp hence the overvoltage - current r e l a t i o n s h i p i s : * T L -RT J ( i i ) S i m i l a r i l y f o r cathodic overvoltages: (A.III-18) exp I 4- CXTe. - Ot, ) 3 « (A.III - 1 9 ) - 131 -f o r l a r g e cathodic overvoltages X » * I In. n3» Lo 2, t Q exp£- Ojj*KJ?Ly'^t (A.III-20) I f the i n t e r c e p t s of the anodic and cathodic vT_t v s l o g i curves (at r^j* o ) are not equal i n magnitude then t h i s may be considered as a c r i t e r i o n f o r the presence of two successive charge t r a n s f e r r e a c t i o n s assuming e l e c t r o d e r e v e r s a b i l i t y . i . e . Anodic i n t e r c e p t = Cathodic i n t e r c e p t = In. 2. t°K - 132-APPENDIX IV REACTION OVERVOLTAGE I f a chemical r e a c t i o n r a t e f o r a r e a c t i o n o c c u r r i n g at an electrode i s not a f u n c t i o n of the el e c t r o d e p o t e n t i a l , the r e s u l t i n g o v e r p o t e n t i a l i s defined as a r e a c t i o n overvoltage. The ele c t r o d e r e a c t i o n may he considered as a p a r t i a l electrode- r e a c t i o n at e q u i l i b r i u m and a slow r a t e determining chemical r e a c t i o n . I f the slow chemical r e a c t i o n f o l l o w s the charge t r a n s f e r r e a c t i o n then the c o n c e n t r a t i o n o f the r a t e determining intermediate species, 3, being consumed increases w i t h i n c r e a s i n g current d e n s i t y . Since S i s generated by the charge t r a n s f e r r e a c t i o n the Nernst p o t e n t i a l of tha t r e a c t i o n w i l l vary w i t h the con c e n t r a t i o n of S. i . e : (A.IV-1) Where: £ = Ele c t r o d e p o t e n t i a l at current d e n s i t y ( i ) So = Zero current e l e c t r o d e p o t e n t i a l Q.sio = A c t i v i t y of rate-determining species S at current d e n s i t y ( i ) Clj-co) = Concentration of species S at current d e n s i t y = 0 The r e a c t i o n overvoltage i s given by: X — " (A.IV-2) - 133 -In the case of pure r e a c t i o n overvoltage the components i n the o v e r a l l e l e c t r o d e r e a c t i o n remain almost constant even during current flow. I f the r a t e determining r e a c t i o n i s homogeneous, the o v e r a l l r e actants and products w i l l he almost constant i f t h e i r c o n c e n t r a t i o n s are very l a r g e compared t o the species being consumed by the r a t e determining chemical r e a c t i o n . I f the r a t e determining r e a c t i o n i s heterogeneous, there w i l l be no d i r e c t l y a s s o c i a t e d d i f f u s i o n overvoltage since the r e a c t i o n occurs on an absorption l a y e r at the e l e c t r o d e ' s surface. Consider an e l e c t r o d e mechanism as f o l l o w s f o r a metal/ion e l e c t r o d e : P a r t i a l E l e c t r o d e Reaction: (A.IV -3) Rate determining step: Ar3' S 3| — tfs S s (A.IV-k) O v e r a l l r e a c t i o n : V;S m + \SX — % % S s + a e.- (A.IV-5) The r a t e of consumption of r a t e determining species i s : & -1 (A.IV -6) - 134 -I f the a c t i v i t y o f 3^ i s independent o f the conce n t r a t i o n or a c t i v i t y os the r a t e of r e a c t i o n becomes: At e q u i l i b r i u m *v=o hence: (A.IV -7) r - i * (A.IV-8) Where: Ok i = |= E q u i l i b r i u m a c t i v i t y of S^ ^ 3 = S t o i c h i o m e t r i c f a c t o r = r e a c t i o n order The equation f o r the consumption o f species S becomes XT = na0 [ [Og»> L a H The r e a c t i o n overvoltage then becomes: (A.IV - 9 ) a s 3 | (A.IV-10) Where: r i _ = Valence o f n a r t i a l e l e c t r o d e r e a c t i o n The r a t e of r e a c t i o n as a f u n c t i o n of current d e n s i t y i s : ( A . I V - l l ) The maximum r a t e of consumption o f S occurs at the l i m i t i n g heterogeneous r e a c t i o n current d e n s i t y ( b r ) . L 3 H r3' (A.IV-12) - 135 -The r e a c t i o n overvoltage may now be expressed as a f u n c t i o n of the current d e n s i t y instead, of a c t i v i t i e s - i . e . : T-| = Vi|X I n J l V + u]ri\ _ V^KT U [. - I ] (A.IV-13) I f the r a t e determining step i n (A.IV-4) i s c o r r e c t V3 i s equal to the r e a c t i o n order whereas Y5 i s r e l a t e d t o "n" by (A.IV-3). For l a r g e current d e n s i t i e s such that then (A.IV-13) mav be ac.proxima.ted as _^ In I L I V R » " 3 ' ( A . T V - l U ) Assumptions of t h i s d e r i v a t i o n are: 1. Surface coverage of S 3 should be « 1 f o r (A.IV-IO) t o be exact. 2. ^ " s 5 w a' s not a f u n c t i o n o f ( i ) . A c t u a l l y Q-Ss w i l l vary i t higher current d e n s i t i e s i f i t i s evolved. 3. Rate of r e a c t i o n v w i l l be a f u n c t i o n of the surface covera.ge f r a c t i o n of ( • At higher current d e n s i t i e s 0s 3| w i l l approach 1. » i . VI.1 Surface Coverage Factor Very l i t t l e experimental work has been done t o measure the v a r i a t i o n - 136 -of surface coverage of an el e c t r o d e surface as a f u n c t i o n o f e l e c t r o d e current d e n s i t y . An intermediate species which i s absorbed on the surface o f an el e c t r o d e can only be analysed w h i l e the e l e c t r o d e r e a c t i o n i s proceeding. The f o l l o w i n g d e r i v a t i o n of the e f f e c t of the intermediate absorbed species on the T a f e l p o l a r i z a t i o n i s given. In of order t o o b t a i n a b e t t e r understanding/reaction p o l a r i z a t i o n one of 35 the two most commonly p o s t u l a t e d r e a c t i o n mechanizms i s chosen. IV.2 Ef f e c t o f S i g n i f i c a n t Surface Absorption of Intermediate Snecies During Reaction P o l a r i z a t i o n Reaction Mechanism: o*- ^= o I -(- z.eT (A.IV - 1 5 ) o | x C C-*Ol (A.IV-lfa) G-xOl -+- O2"- CO* . | -+ 2<2T (A.IV - 1 7 ) ] C 0 2 | = 3 C 0 2 t (A.IV--]8) Assumptions: 1. The c o n c e n t r a t i o n 3 o f O l , CxO|, COp| are f u n c t i o n s of current d e n s i t y 2. No d i f f u s i o n overvoltage - i . e . ^ - Q V =5^  f (current d e n s i t y ) ~ P c o 2 ?£• f (current d e n s i t y ) - 137 -3. Reaction (A.IV-16) i s f a s t and not r a t e determining. h. Reaction (A.IV-18) i s slow and r a t e determining. Reaction (A.IV-15) z L "S- J 1 - R T J ( A . I '•/-19) Exchange current d e n s i t y i f (A.IV-15) i s not r a t e determining at current d e n s i t y = t i s given by: z " ~R~r J L " I S T — : • J R T (A.IV-20) Reaction (A.IV-15) i s i n a q u a s i - e q u i l i b r i u m s t a t e except t h a t as the ele c t r o d e p o t e n t i a l changes w i t h i n c r e a s i n g current d e n s i t y so does the surface coverage of the el e c t r o d e by 0|, CxOl and CO,|. E q u i l i b r i u m p o t e n t i a l (U» o ) i s determined by equating both parts i n (A.IV-20) equal to l 0 i . e . "K", a Q i - [ 1 - 2: 6] &xp [ 2. oc 3" V] = • e G | ^p[-Q-^)23-£°l •RrT L T ? T (A.IV-21) s o l v i n g f o r £ y i e l d s change i n p o t e n t i a l f o l l o w i n g flow of current i s given by: (A. IV-22] (A.IV-23) - 138 _ Al s o from expression f o r L©, (A.IV-20), and expressing (A.IV-20) i n terms of zero current v a l u e s : Combining (A.IV-24) and (A.IV-23) y i e l d s : 1 i - s e - J r 2 oc T2T S o l S i m i l a r i l y using RES of (A.IV-20) and (A.IV-23): to, (L**) = | i - t e \ i — 2 9" Combining (A.IV-25) and A.IV-26) y i e l d s : (A.IV-2U) (A.IV-25) (A.IV-26) G a l ^ 1 (A.IV-27) Reaction (2) at thermodynamic e q u i l i b r i u m Reaction (3) k=i<3 e C 3 l 0 | [ a 0 z - . e, 6oC Q C ' O - Z J Q ) CxOI (A.IV-28) (A.IV-29) - 139 -Exchange current density i s given by: i - o ^ K j G c * o l • C L 0 « . exp = K 3 Qcoz\ ' 6 - 2 ^ - a c ' w [ - 0 - y 3 - 6 ° ] (A.IV-30) S o l v i n g (A.IV-30) f o r ^ y i e l d s : V<3 . ^CQ Z| -6-s6v O f r 0 1 (A.IV-31) and change i n el e c t r o d e p o t e n t i a l w i t h current flow i s given by: • R T *3 ©CyO| 2 -(A.IV-32) Expressing the exchange current d e n s i t y (when L±?o ) i n terms of the exchange current d e n s i t y at zero current and the changes i n c o n c e n t r a t i o n of the adsorbed r e a c t i o n species y i e l d s : L * fc3c»OI and s i m i l a r i l y f o r the r e d u c t i o n r e a c t i o n : (A.IV-33) O c x o | Combining (A.IV-33) and (A.IV-34) y i e l d s : (A.IV-34) (A.IV-35) - iho -Reaction (A.IV-18) Rate determining stent In. T Q &oa| 1 ^3" L fy° J v CO. I At e q u i l i b r i u m r a t e of r e a c t i o n *u i s = © th e r e f o r e [ V - [ F o o J - [ s At higher current d e n s i t i e s : (A.IV-36) ( A . I V - 3 7 ) ( A . I V - 3 8 ) 4 5 <=°z\ t h e r e f o r e ( A . I V - 3 9 ) (A . IV-I tO) Therefore r a t i o of surface coverages / ©co* at higher current d e n s i t i e s i s given by: — 1 ''Y .1 = f-k-s • J<_ J L4 k . CO, (A.IV-41) Hence overvoltage i s given by the f o l l o w i n g f o r r e a c t i o n (A.IV-18) alone: — ~R" ( A . I V - 4 2 ) I n c l u d i n g changes i n the surface coverage i n r e a c t i o n s (A.IV-15) and (A.IV-17): & £°. = £° CC *o> _ £* u = «) & £° = -gr- k f Q.i . (\-se0>) 1 »f L eji" o"^e>J (A.IV-33) - R T In. Z-3-Q-=°2i . C>- a s ) . (A.IV-32) t h e r e f o r e t o t a l r e a c t i o n p o l a r i z a t i o n becomes: (A.IV-43) or r-i = Tg,-r \ a o (A.IV-MO Unknown values are: - unknown fu n c t i o n s o f current d e n s i t y - not known - Ih2 -APPENDIX V THE FARADAIC IMPEDANCE FOR EACH TYPE OF OVERVOLTAGE V. 1 Charge Trails.fer Overvoltage 13 V e t t e r defines charge t r a n s f e r r e s i s t a n c e as the d e r i v a t i v e o f 13 the charge t r a n s f e r p o l a r i z a t i o n by the current d e n s i t y . V e t t e r determines t h a t the charge t r a n s f e r r e s i s t a n c e i s given by: (A.V-1) Where: = Charge t r a n s f e r r e s i s t a n c e Z = Charge t r a n s f e r valence Lo = Exchange current d e n s i t y This value f o r the charge t r a n s f e r r e s i s t a n c e i s f o r s n a i l changes i n charge t r a n s f e r p o l a r i z a t i o n . The c a p a c i t i v e reactance i s determined from the instantaneous change i n p o l a r i z a t i o n f o r a small change i n c u r r e n t , i . e . : (A.V-2) - 1 4 3 -The equivalent c i r c u i t diagram f o r an e l e c t r o d e w i t h pure charge-t r a n s f e r p o l a r i z a t i o n i s a r e s i s t o r , T? r, i n p a r a l l e l w i t h the c a p a c i t o r C-j, . The expression f o r the charge t r a n s f e r r e s i s t a n c e i s only v a l i d at small changes i n p o l a r i z a t i o n w h i l e the c a p a c i t i v e term i s v a l i d at a l l p o l a r i z a t i o n changes. ^•2 D i f f u s i o n Overvoltage Impedence 13 Vetter . has de r i v e d an expression f o r p o l a r i z a t i o n r e s i s t a n c e and. c a p a c i t i v e reactance as a f u n c t i o n of the superimposed a l t e r n a t i v e current on the d i r e c t c e l l c u rrent. The A.C. causes a damped con c e n t r a t i o n wave i n the e l e c t r o l y t e . . The c o n c e n t r a t i o n v a r i a t i o n i s determined from, the a p p l i c a t i o n of F i c k s laws 1 and 2 for d i f f u s i o n and. Faradays law. The p o l a r i z a t i o n l a g s the current change by 4 5 ° , therefore the r e s i s t i v e and c a p a c i t i v e components are equal. Using, a s e r i e s 13 connection V e t t e r " shows the impedence components are determined by: • R T (A.V-3) (A.V-1: / 2 ~ - 141+ _ Diffusion pola.riza.tion resistance Diffusion polarization capacitance Diffusion polarization impedence Charge transfer valence Bulk concentration of the rate determining species Frequency of superimposed A.C. Stoichiometric factor for rate determining species V.3 Reaction JPolarization Impedance (Meterogcr.eous React ion) 13 The reaction polarization impedance as derived hy Vetter varies with current density. The pha.se shift between current and voltage increases with increasing frequencies. At high frequencies of the superimposed alternating current the rate determining intermediate species w i l l be completely depleted and the polarization w i l l he caused hy a damped concentration wave. As a result the phase shift w i l l approach 45° and the resistive and capacitive components w i l l be approximately equal. Vetter derived expressions for the resistive and capacitive components of the impedance in terms of A.C. frequency, chemical reaction order and. concentration, i.e.: Where: = o. = ZJ = t = 1 „ "RT y 2 w C | - £ K (A.V-7) k. and c (A.V-6) - 1 4 5 . Where: n. = Charge t r a n s f e r valence of p a r t i a l e l e c t r o d e r e a c t i o n C = Concentration of intermediate species f o r d i r e c t current to c e l l = Rate of formation of species 3 i n r a t e determining r e a c t i on. P = Reaction order f o r consumption o f o i n r a t e determin-i n g r e a c t i o n At low A.C. frequencies the ohmic impedance approaches zero. At high frequencies (A.V-6) and (A.V-7) approximately equal (A.V-3) and (A.V-4) which i s the case of d i f f u s i o n p o l a r i z a t i o n . The equation d e r i v e d by V e t t e r f o r r e a c t i o n p o l a r i z a t i o n impedance are based on a s i n g l e slow r a t e determining heterogeneous r e a c t i o n . - lhC -REFERENCES 1. Grjotheim, K., Holm, J.L., Krohn, C., t % t i a s o v s k y , K., Kemisk T i d s k r i f t ( i 9 6 0 ) 7 9 , ( 1 0 ) , 5 4 7 - 5 6 6 . 2 . Thonstad, J . , Canadian J . of Chemistry ( 1965) 1+3, 3429-34.32. 3. G j e r s t a d , S., Welch, B.J., J . of E l e c t r o c h e m i c a l Soc. ( 1 9 6 4 ) m 5 ( 8 ) , 9 7 6 - 9 8 0 . 4 . Frank, W.B., J . of E l e c t r o c h e m i c a l Soc. ( 1965) 1 1 2 , ( 6 ) , 6 4 9 - 6 5 0 . 5 . Thonstad, J . , Solbu, A., Trans. AIME ( 1 9 6 8 ) , 24 2, 301-306. 6. Grjotheim, K., Krohn, C., Noeumann, R., I t f r k l e p , K. , Met. Trans. (1970) 1, ( 2 ) , 3133-3141. 7. Haupin, W.E., J . of E l e c t r o c h e m i c a l Soc. ( i 9 6 0 ) 107, ( 3 ) , 2 3 2 - 2 3 6 . 8 . M i t c h e l l , J.C., Samis, C.S., Trans. A.I.M.E. ( 1969) 2 4 5 , ( 6 ) , 1 2 2 7 - 1 2 3 4 . 9 . Grjotheim, K., Matiasovsky, K., Malinovsky, M., E l e c t r o c h i m i c a Acta ( 1 9 7 0 ) 1 5 , 2 5 9 - 2 6 9 . 1 0 . P i o n t e l l i , R., from Proceedings of 1 s t A u s t r a l i a n Conference of E l e c t r o c h e m i s t r y . Pergamon Press ( 1 9 6 4 ) , 932 - 9 3 8 . 1 1 . Thonstad, J . , Hove, E. , Canadian J . of Chemistry ( 1 9 6 4 ) 4_2, 1 5 4 2 - 1 5 5 0 . 1 2 . V e t t e r , K.J., E l e c t r o c h e m i c a l K i n e t i c s , Academic P r e s s , New York ( 1 9 6 7 ) . 13. T u a l , A., R o l i n , M., E l e c t r o c h i m i c a Acta ( 1972) 17, 2 2 7 7 - 2 2 9 1 . 14. Grjotheim, IC. , Holm, J.L. , Krohn, C. , Thonstad, J . , Recent progress i n the theory of Aluminum E l e c t r o l y s i s from Selected Topics i n High Temperature Chemistry. U n i v e r s i t e t s f o r l a g e t , Oslo. ( i 9 6 0 ) . 15. Vayna, A., .Alumina ( 1 9 5 0 ) 1 9 , 133-145. 1 6 . Margulescu, I.G., Zuca, 3., Acad. Rep. P o p u l a i r e Romine S t u d i i C e r c e t a r i Chim (1961) <?, ( i ) , 5 5 - 6 1 . " -17. Nishihana, K., Matsumuka, Y., Komatsu, K., Noguchi, M., Kyoto Diagaku Suiyokai S u i y o k a i - s h i 0-964)15, ( 6 ) , 311 -315 . - 1.1*7 -18. Janssen, L.J.J. , Hoogland, J.G. , Electrochimica Acta (1970) 15_, 1013-1022. 19. Janssen, L.J.J., Hoogland, J.G., Electrochimica Acta (1973) l S , 543-550. 20. Ibl, N. , Venczel, J. , Metalloberflache (1970) 2k, 365. 21. Sundheim, B.R., J. of Electrochemical Soc. (1968) 115 ( 2 ) , 158-160. 22. Takahashi, M. , Amada, Y. , Denki Kogaku (196U) 32, ( 2 ) , 133-140. 23. Holliday, R.D., Henry, J.L., Ind. & Eng. Chera. (1959) 51, (10) 1289-1292. 2k. Pearson, T.G., Waddington, J., Disc, of the Faradav Soc. (1947) 1 , 307-319. 25. J.A.N.A.F. Thermochemical Data i 9 6 0 . 26. Thonstad, J., J. of Electrochemical Soc. (1964) 111, ( 8 ) , 959-965. 27- Thonstad, J., Electrochimica Acta (1970) 15, 1569-1580. 28. Welch, B.J., Proc. Aust. I.M.M. (1965) 214, 1-19. 29. Welch, B.J. , Richards, N.E., Anodic Overpotentials in the Electrolysis of Alumina from Extractive Metallurgy o f Aluminium (1962) 2_, 15-30. 30. P i o n t e l l i , R., Mazza, B. Pedeferri, P., Electrochimica Acta (1965) .10, 111 7-1124. 31. Haupin, W.E., J. Metals (1971) 1 0 , k6-ko. 32. Thonstad, J., Electrochimica Acta (1968) 13 . 1 , 449-456. 33. Drossbach, P., Hashino, T., J. of Electrochemical Soc. of Janan (1965) 3 3 , 101-130. 34. Paunovic, M., Electrochimica Hetallorum (1968) 3 , ( 4 ) , 373-375-35. Thonstad, J., Electrochimica Acta (1970) 15, 1581-1595. 36. Drossbach, P., Hashino, T., J. of Electrochemical Soc. of Japa: (196k) 3 3 , (4), 229-246. n 37. Thonstad, J., Nordmo, F., Vee, K., Electrochimica Acta (1973) 18 ( 1 ) , 27-32. 38. Thonstad, J. , Electrochimica Acta (1969) l U , 127-13-4. 39. Thonstad, J., Electrochimica Acta (1967) 12, 1219-1226. - 148 -Haupin, W.E., J . of El e c t r o c h e m i c a l Soc. (1956) 103, ( 3 ) , 174-173. P i o n t e l l i , R., A t t i Accad. N a z i . L i n c e i Rend., Classe 3 c i . ^is. , Mat. e. Nat. (1959) 26, 18-20. Thonstad, J . , E l e c t r o c h i m i c a Acta (1970) 15 , 1569-1580 References 23 and 24. Grjotheisa, K., Matiasovsky, K. , I'lyhre-Anderson, 8. , 0 r e , M.A. , E l e c t r o c h i m i c a Acta (1968) 13 . 1 , 91-99. 

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