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Aqueous oxidation of galena Andersen, John Enevold 1951

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AQUEOUS OXIDATION OF GALENA hy JOHN ENEVOLD ANDERSEN  A THESIS SUBMITTED IN PARTIAL FULFILMENT THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED  OF  SCIENCE  i n t h e Department of Mining  and M e t a l l u r g y  We a c c e p t t h i s t h e s i s a s c o n f o r m i n g t o t h e standard r e q u i r e d from c a n d i d a t e s f o r t h e degree o f MASTER OF APPLIED SCIENCE  • • _ ,» < .1  iMcar.wwi  . . . . . .  Members o f t h e Department o f Mining  THE UNIVERSITY  and M e t a l l u r g y  OF BRITISH COLUMBIA  April,  1951  Abstract  To complement the data, on the aqueous o x i d a t i o n of s u l p h i d e m i n e r a l s , t h a t i s , treatment of s u l p h i d e s i n aqueous s o l u t i o n a t e l e v a t e d temperatures under oxygen p r e s s u r e , a study was undertaken of the aqueous o x i d a t i o n of galena i n sodium h y d r o x i d e s o l u t i o n .  I t wa-s hoped by  the study of t h e k i n e t i c s of t h e r e a c t i o n t o l e a r n something of t h e mechanism i n v o l v e d . The r e a c t i o n was f o l l o w e d by means o f a cathoderay p o l a r o g r a p h .  A c r y s t a l of galena was mounted i n a s m a l l  a u t o c l a v e equipped w i t h s u i t a b l e e l e c t r o d e s , and d u r i n g the course of t h e r e a c t i o n photographs were taken at i n t e r v a l s of the c u r r e n t - v o l t a g e c u r v e s .  From the h e i g h t of  the l e a d wave r e l a t i v e c o n c e n t r a t i o n s c o u l d be recorded, and a f t e r a s u i t a b l e time the run was stopped and t h e s o l u t i o n assayed t o g i v e a b s o l u t e values t o t h e wave h e i g h t s . The v a r i a b l e s of oxygen p a r t i a l pressure  agitation,  sodium h y d r o x i d e c o n c e n t r a t i o n , temperature, s i l i c a t e - i o n , and e l e c t r i c a l p o t e n t i a l were i n v e s t i g a t e d . t h a t the polarograph  I t was found  under these c o n d i t i o n s gave r e p r o -  ducible r e s u l t s . On t h e b a s i s of t h e experimental  r e s u l t s three  a l t e r n a t i v e mechanisms f o r the r e a c t i o n were proposed.  One  of these was t o o i n d e f i n i t e t o t r e a t q u a n t i t a t i v e l y , but the other two were examined by c a l c u l a t i o n s u s i n g t h e theory  of a b s o l u t e r e a c t i o n r a t e s .  One was found t o g i v e a g r e e -  ment i n r a t e w i t h experiment, a model i n which d e s o r p t i o n accompanied by h y d r a t i o n and i o n i z a t i o n was the r a t e controlling  step.  The e x p e r i m e n t a l  r e s u l t s were reviewed' i n the  l i g h t of t h i s mechanism and appeared t o show no s e r i o u s c o n t r a d i c t i o n s , so t h i s model i s put forward as a p o s t u l a t e of the r e a c t i o n of galena w i t h oxygen i n sodium h y d r o x i d e solution.  Acknowledgement  The a u t h o r I s g r a t e f u l f o r f i n a n c i a l summer o f 195°  a i d i n the  f r o m t h e C o n s o l i d a t e d M i n i n g and S m e l t i n g  Company of Canada, L i m i t e d , and f o r t h e Cominco F e l l o w s h i p from  the same company f o r t h e r e g u l a r  Without  session  of  1950-51*  such a i d t h i s work c o u l d not have been c a r r i e d o u t . Profound  g r a t i t u d e i s a l s o e x p r e s s e d to t h e  staff  of t h e Department o f M i n i n g and M e t a l l u r g y f o r t h e i r c o operation,  encouragement, and a d v i c e .  debted p a r t i c u l a r l y who  was  most h e l p f u l  research,  t o A s s o c i a t e P r o f e s s o r W. i n directing  rates.  M.  Armstrong  t h e b e g i n n i n g of t h i s  and t o Dr. J . Halpe-rn who  of r e a c t i o n  The a u t h o r i s i n -  guided the c a l c u l a t i o n s  Table  of  Contents Page  Introduction  1  Equipment  3  The  Cathode-Ray P o l a r o g r a p h  3  The  Autoclave  5  The  Electrodes  9  Theory  9  of Operation of the Polarograph  Experimental  11  Materials  11  Experimental Procedure  12  1  Rates  2 Rates 3  on C r y s t a l s  12  on P u l p s  lk  Qualitative Investigation Applied  o f the E f f e c t of  lb  Fields  15  Results Products  of t h e P r e s s u r e O x i d a t i o n of  i n Sodium H y d r o x i d e The  Galena 15  Solution  15  Wave Forms  The R a t e C u r v e s  17  Appearance o f t h e C r y s t a l a f t e r O x i d a t i o n  21  of V a r i a b l e s  23  1  Oxygen P a r t i a l P r e s s u r e  23  2  Agitation  Investigation  (Rate  of Flow of Medium P a s t  Particle)  the 2k  3  C o n c e n t r a t i o n o f Sodium H y d r o x i d e  25  k  Temperature  26  5  E f f e c t of S i l i c a t e - I o n  28  6 Effect  of S u b s t i t u t i n g  Sodium A c e t a t e f o r  30  Sodium H y d r o x i d e 7 Effect  Page  3O  of E l e c t r i c a l P o t e n t i a l  32  D i s c u s s i o n of R e s u l t s  32  Numerical Values Mechanism of t h e R e a c t i o n i n 0.5  Normal Sodium  Hydroxide  33  Mechanism I  35  Mechanism I I  36  Mechanism I I I  39  C o n s i d e r a t i o n o f Mechanism I I w i t h R e s p e c t t o 39  the Remaining V a r i a b l e s 1 Agitation  and C o n c e n t r a t i o n o f Sodium H y d r o x i d e  2 Effect  of S i l i c a t e - I o n  3 Effect  of S u b s t i t u t i n g  4-0 Sodium A c e t a t e f o r  Sodium H y d r o x i d e 4 Effect  39  o f Change i n S u r f a c e A r e a  4-0' 4-1 41  Conclusions Appendix A  4-4-  Appendix B  4-5  Appendix C  4-7 50  Bibliography Illustrations Figure 1  Power S u p p l y  4-  Figure 2  Other C i r c u i t s  4-  Figure 3  E l e c t r o n i c Equipment  6  F i g u r e 4-  A u t o c l a v e and A c c e s s o r i e s  6  Figure 5  P h o t o g r a p h showing A u t o c l a v e Assembly, H e a t i n g and S t i r r i n g Mechanisms  Figure 6  Polarogram  8 16  Page Figure  7  R a t e Curve f o r a C r y s t a l  18  Figure  8  Rate C u r v e f o r a C r y s t a l  18  Figure  9  Rate  20  Figure  10  Photomicrograph o f Galena C r y s t a l a f t e r A t t a c h i n 0.5N C a u s t i c a t 1.5Ooc f o r 200 m i n u t e s  22  Photomicrograph of Galena C r y s t a l a f t e r A t t a c k i n 0.5N C a u s t i c a t 175°C f o r 132 m i n u t e s  22  Figure  11  Curve f o r a P u l p  Figure  12  Rate v s . N o r m a l i t y  Figure  13  Arrhenius  Plot  KaOH  2? 29  I  AQUEOUS OXIDATION OF GALENA  Introduction  The  first  mention of p r e s s u r e  o x i d a t i o n of s u l -  phides  i n aqueous s o l u t i o n  i n technical literature  Patent  D.R.P. N r . 524, 353,  April  16,  1931,  F a r b e n i n d u s t r i e Akt.-Ges. i n F r a n k f u r t . cess f o r the p r o d u c t i o n of metal  i s German  awarded t o I . G-. T h i s was f o r a p r o -  s u l p h a t e s from metal  sul-  p h i d e s by o x i d a t i o n i n aqueous s o l u t i o n by t h e use o f oxygen or ly,  an o x y g e n - c o n t a i n i n g  gas.  however, and was used  aration of H S 2  The r e a c t i o n was known p r e v i o u s -  i n a c y c l i c process  from i l l u m i n a t i n g . g a s .  was awarded H u n g a r i a n  Patent  I n 1939  Dr. A Bognar  122479 f o r t h e t r e a t m e n t o f  o r e s and f l u e d u s t s a t h i g h t e m p e r a t u r e s  1  f o r t h e sep-  and p r e s s u r e s I n t h e  2 p r e s e n c e o f some l i q u i d water, of  w i t h o r w i t h o u t the  addition  s u l p h u r or s u l p h u r - h e a r i n g m a t e r i a l , to form w a t e r - s o l u b l e  s u l p h a t e s of m e t a l s .  I n t h e same y e a r T r o n e v , Bondin,  and  1 7 Zviagincev the  '  oxidation  tions  published of zinc  two p a p e r s i n w h i c h t h e y d e s c r i b e  and c o p p e r s u l p h i d e s i n a l k a l i n e  solu-  and i n water, u s i n g h i g h p r e s s u r e a i r a t t e m p e r a t u r e s  up t o 250°C. At  the U n i v e r s i t y  of B r i t i s h  pressure  o x i d a t i o n h a s been done by R.  Congreve  , R.  the  B.  p a p e r by J .  M c l n t o ah5, F.  Stenhouse  and J .  F.  C o l u m b i a work on  Carter3, w. Stenhouse^.  i s the only  K.  Of t h e s e ,  one which d e a l s  clusively  w i t h t h e mechanism and f u n d a m e n t a l r e a c t i o n s  process.  Mr.  Stenhouse  oxidized pyrite  A.  ex-  of the  i n sodium h y d r o x i d e  s o l u t i o n and measured t h e r a t e o f t h e r e a c t i o n by t h e r a t e 1 T r o n e v , V., and B o n d i n , S., O s i d a t i o n o f Z i n c S u l p h i d e and T r a n s f e r e n c e o f Z i n c i n t o Aqueous o r A l k a l i n e S o l u t i o n u n d e r A i r P r e s s u r e , Comptes Rendus Acad, o f S c i e n c e U.R.S.S. 23, pp. 5^1-3, Moscow, 1939. 2 Z v i a g i n c e v and Tronev, V., O x i d a t i o n of Cupper S u l p h i d e and T r a n s f e r e n c e o f Copper i n t o Aqueous S o l u t i o n under A i r P r e s s u r e , Comptes Rendus Acad, o f S c i e n c e U.R.S.S. 23, pp. 54-3-4, Moscow, 1939. 3 C a r t e r , R., I n f l u e n c e o f R o a s t i n g Temperature on G o l d R e c o v e r y from a R e f r a c t o r y G o l d Ore, M. A. Sc. T h e s i s , U n i v e r s i t y o f B r i t i s h Columbia, 194-9. 4- Congreve, W. K. A., Use o f H i g h P r e s s u r e Oxygen i n Ext r a c t i o n M e t a l l u r g y , R e p o r t t o t h e R e s e a r c h Committee, The U n i v e r s i t y o f B r i t i s h Columbia, 194-9. 5 M c i n t o s h , R. B., R e c o v e r y o f C o b a l t from T a y l o r Gem Ore by Aqueous O x i d a t i o n , M. A. Sc. T h e s i s , U n i v e r s i t y of B r i t i s h Columbia, 1950. 6 Stenhouse, J . F., Humid O x i d a t i o n o f P y r i t e , M. A. T h e s i s , U n i v e r s i t y of B r i t i s h Columbia, 1950.  Sc.  3 of consumption of oxygen.  T h i s r e a c t i o n i n v o l v e d the forma-  t i o n of a c o a t i n g of the o x i d e s of i r o n on the f i n e p a r t i c l e s of p y r i t e . A f t e r s t u d y i n g the p r e v i o u s work i n t h i s f i e l d , i t was d e c i d e d t h a t the mechanism of o x i d a t i o n where a l l p r o d u c t s were s o l u b l e i n the medium had not been adequately i n vestigated.  The p r e s s u r e o x i d a t i o n of g a l e n a i n sodium  h y d r o x i d e s o l u t i o n appeared t o o f f e r a s u i t a b l e r e a c t i o n f o r study.  A method of f o l l o w i n g the r e a c t i o n without removing  m a t e r i a l from the a u t o c l a v e was f e l t t o be d e s i r a b l e .  For  t h i s purpose the p o l a r o g r a p h seemed t o o f f e r good hope of success.  A f t e r c o n s i d e r i n g the d i f f e r e n t m o d i f i c a t i o n s of  t h i s d e v i c e , the model b u i l t by J . E. B. Randies? was t o be most s u i t a b l e .  A copy of h i s instrument was  thought  built  and a f t e r some i n i t i a l d i f f i c u l t i e s was made t o g i v e r e p r o ducible r e s u l t s .  Equipment  The Cathode-Ray P o l a r o g r a p h The c i r c u i t of the cathode-ray polarograph used i n t h i s r e s e a r c h i s g i v e n i n F i g u r e s 1 and 2, page 4. c o n s i s t s of s i x u n i t s :  It  a low v o l t a g e e l e c t r o n i c a l l y smoothed  power s u p p l y , a source of i n c r e a s i n g p o t e n t i a l , which a l s o i n c l u d e s the current-measuring c i r c u i t ; a s y n c h r o n i z i n g u n i t 7 R a n d i e s , J . E. B., A Cathode-Ray P o l a r o g r a p h , T r a n s a c t i o n s of the Faraday Society, #44,1948, pp. 322-327.  4  1  „.A  P O W E R  P A C K  Figure 1 Power Supply  POTENTIAL SWEEP UNITl H .„„ V  Figure 2 Other Circuits  AMPLIFIERS  5 which s t a r t s and stops the sweep a f t e r the a p p r o p r i a t e time i n t e r v a l s ; a d i r e c t c u r r e n t a m p l i f i e r t o a m p l i f y the h o r i z o n t a l v o l t a g e s t o g i v e the h o r i z o n t a l sweep; a d i r e c t c u r r e n t a m p l i f i e r t o a m p l i f y the v e r t i c a l ( c u r r e n t ) s i g n a l s ; and a cathode-ray tube w i t h i t s h i g h - v o l t a g e power supply f o r the v i s u a l p r e s e n t a t i o n .  The  cathode-ray tube used  was  i n c o r p o r a t e d i n an. o x c l l l o s c o p e , T r i p l e t Model 34-40 ( f i v e i n c h ) , whose c h a s s i s was grounded to the c h a s s i s housing r e s t of the e l e c t r o n i c equipment. a m p l i f i e r s was  output of the D.  C.  connected d i r e c t l y to the d e f l e c t i o n p l a t e s  of the cathode-ray t u b e . The  The  the  The whole i s shown i n F i g . 3, page  Autoclave The a u t o c l a v e i s i l l u s t r a t e d i n F i g . 4-, page 6.  A l l s u r f a c e s exposed to the s o l u t i o n are s t a i n l e s s  steel,  w i t h the e x c e p t i o n of the s t i r r i n g mechanism and the thermometer w e l l .  Magnetic m a t e r i a l was necessary  so m i l d s t e e l was  f o r the  stirrer  used i n the absence of c o n v e n i e n t l y a v a i l -  a b l e magnetic s t a i n l e s s .  The  thermometer w e l l was an emer-  gency r e p a i r a f t e r a p r e v i o u s arrangement f a i l e d , and as i t served the purpose i t was not r e p l a c e d .  I t c o n s i s t e d of a  p i e c e of o r d i n a r y i r o n pipe welded over at the end.  Various  m a t e r i a l s were t r i e d f o r use as a l i n e r — g l a s s , I n c o n e l , cop p e r , and type 304- s t a i n l e s s s t e e l . the s t a i n l e s s s t e e l was  On a short-term b a s i s  the only s a t i s f a c t o r y m a t e r i a l .  The  g l a s s d i s s o l v e d at an e x c e s s i v e r a t e , the I n c o n e l apparently d i s p l a c e d l e a d from the s o l u t i o n , and the copper d i s s o l v e d and r e p r e c i p i t a t e d as a c u p r i c hydroxide  suspension.  The h e a t i n g and s t i r r i n g d e v i c e s are shown i n  Figure 3 E l e c t r o n i c Equipment Heavy  Pt Wir»  *0D Class  Tube  Triermomet.r  Sparkplug  Vffl  Silver So/eer Joins Short Brass  forateit Pt 0isc Gasket o.eoy Pt Wire.  Thermomtttr  WetJ  Wooas Metal Stainless 5tee) Strap Ga/ena  Crystal  Aoueous Sa/uttoij Stainless Steel imer  'liindum Cement S»«et Steel  Figure 4 Autoclave and A c c e s s o r i e s  7 Fig.  5,  shaping  page 8. coiled  Heat i s s u p p l i e d from  an element made by  nichrome w i r e a b o u t a form  of the  same  diameter  as t h e body of the a u t o c l a v e , p l a s t e r i n g w i t h alundum cement, baking,  and  setting  the r e s u l t i n g  the form, i n a m e t a l was to  then  can.  The  cylinder,  space  c h i n k e d xtfith more alundum.  g i v e 500  a t 110  watts  c o n t r o l l e d by temperature  volts.  a v a r i a c and  of the  contents  thermometer w h i c h was  The  a f t e r r e m o v a l of  between can and  cylinder  The  designed  element was  a c t u a l heat  approximated  100  watts.  of t h e a u t o c l a v e was  inserted  supplied  i n i t s w e l l and  was  The  read o f f the connected  t h e r m a l l y w i t h m o l t e n Wood's m e t a l . The ing  contents  rotat-  a 2 . 5 - i n c h A l n i c o magnet j u s t below the bomb, which moved  the c y l i n d r i c a l  rotor  the r o t o r t o r e d u c e of  of t h e a u t o c l a v e were s t i r r e d by  on the i n s i d e .  i t s bearing surface.  t h e magnet c o u l d be v a r i e d  pulleys  on t h e b e l t  drive,  sistance controlling Oxygen and bottles,  A  r i m was The  from 80 t o 250  speed RPM  and by v a r y i n g t h e  the v a r i a b l e  speed  turned  onto  of  by  rotation  changing  series re-  motor.  o t h e r g a s e s were s u p p l i e d from  commercial  r e g u l a t e d by t h e u s u a l diaphragm v a l v e s , and  the  p r e s s u r e s r e a d o f f the Bourdon gauges i n c o r p o r a t e d i n t o r e g u l a t o r assemblies. gave f l e x i b i l i t y  A p i e c e of h i g h - p r e s s u r e rubber  to t h e gas  connections,  and  the hose  a shut-off valve  a l l o w e d t h e a u t o c l a v e t o be d i s c o n n e c t e d w h i l e a t e l e v a t e d temperatures. The  a u t o c l a v e had  r e s t r i c t e d the t e m p e r a t u r e  been t e s t e d t o 200 p . s . i .  This  and p r e s s u r e r a n g e s i n v e s t i g a t e d .  9 The  Electrodes The  e l e c t r o d e s were made as f o l l o w s ;  m i c r o - e l e c t r o d e was made by f u s i n g wire The  and w e l d i n g  t h e end of a p i e c e o f f i n e  t h e o t h e r end t o a c o a r s e r p l a t i n u m  heavy w i r e was f u s e d i n t o a p i e c e o f p y r e x  tubing, less  which was h e l d i n a r u b b e r  s t e e l pipe plug.  unpolarized foil,  packing  wire.  capillary  under a s t a i n -  T h i s i s a l l shown i n F i g . 4.  The  e l e c t r o d e was a p i e c e of p e r f o r a t e d p l a t i n u m  w h i c h was b r a z e d t o a s h o r t r o d c o n n e c t e d  centre  The p l a t i n u m  to the  e l e c t r o d e o f a Champion VR-1 model a i r c r a f t  spark-  plug. Theory of Operation The ray  of the Polarograph  v o l t a g e o f the p o t e n t i a l  polarograph  i n c r e a s e s a t a r a t e of a p p r o x i m a t e l y one  v o l t p e r second.  T h i s i s about  as i n t h e n o r m a l p o l a r o g r a p h . is  sweep o f t h e c a t h o d e -  one h u n d r e d t i m e s as f a s t The r e s u l t  t o change t h e law o f o p e r a t i o n from  of t h i s h i g h r a t e  dependency on t h e  o  laws o f s p h e r i c a l d e p l e t i o n from theory  diffusion  very t h i n  0  t o dependency on t h e r a t e o f  l a y e r s about t h e e l e c t r o d e .  o f o p e r a t i o n i s e x p l a i n e d by R a n d i e s ^ ,  out t h e s o l u t i o n diffusion, methods.  who worked  of the d i f f e r e n t i a l equation f o r plane  which t h i s process Mathematical  complicated  The  approximates,  d i f f i c u l t i e s here  by n u m e r i c a l  a r e caused  e q u a t i o n f o r t h e boundary c o n d i t i o n s .  by t h e The  8 K o l t h o f f a n d Lingane, Polarography^ I n t e r s c i e n c e P u b l i s h e r s Inc., New York, 1941. 9 R a n d i e s , J . E . B., The C u r r e n t V o l t a g e C u r v e s Transa c t i o n s o f t h e Faraday Society, #44, 1948, pp. 327-338.  10 results 1.  of t h e use o f t h i s The u s u a l current  2.  r a p i d sweep a r e ;  sharp r i s e s  regions  I f the point  a s s o c i a t e d with  a r e r e p l a c e d by c u r r e n t  f o r the beginning  taken a t a voltage  state diffusion,  r i s e f o r t h e succeeding preceding  current  3. Peak h e i g h t , the  the current  value. a f u n c t i o n of a l l  f o rdiffusion  v e n t i o n a l polarography, to the rate  reaction i s  r e a c t i o n i s independent o f  i n a d d i t i o n to being  quantities listed  maxima.  o f t h e sweep i s  where t h e p r e c e d i n g  c o n t r o l l e d by s t e a d y  the  diffusion  current  i s also directly  i n con-  proportional  of change o f v o l t a g e .  4. The h a l f - w a v e p o t e n t i a l , b a s i s polarography,  o f i d e n t i f i c a t i o n by  c o r r e s p o n d s t o t h e c u r r e n t maximum.  5. The peak h e i g h t  i s p r o p o r t i o n a l t o t h e r a t e o f an  electrode process.  F o r a very  slow p r o c e s s  there  w i l l be a slow r i s e a n d no maximum i n t h e m a t h e m a t i c a l sense. T h e r e a r e no p a r t i c u l a r d i s a d v a n t a g e s f o r a n a l y t i c a l work i n such a d e v i c e ,  and f o r f o l l o w i n g r a p i d r e a c t i o n s t h e  advantages a r e obvious. Page" , who b u i l t 1-0  and  They were r e a l i z e d by Snowdon and  a more r e f i n e d machine than R a n d i e s '  used i t t o f o l l o w very  decomposition of organic  r a p i d r e a c t i o n s i n s y n t h e s i s and  compounds.  Because o f t h e n a t u r e o f t h e p r o c e s s  being  10 Snowdon, F. 0., and Page, H. T., A Cathode-Ray P o l a r o g r a p h , A n a l y t i c a l C h e m i s t r y , V o l . 22, No. 8, August, 1950, pp.969-80.  11 i n v e s t i g a t e d , which only takes p l a c e at h i g h temperatures and p r e s s u r e s , the u s u a l dropping mercury cathode and  calomel  h a l f c e l l f o r the anode and r e f e r e n c e , p o t e n t i a l c o u l d not he used, o r , at l e a s t , not e a s i l y .  I t has been s t a t e d i n  R a n d i e s ' papers and o t h e r s t h a t r e a c t i o n s at s o l i d e l e c t r o d e s w i l l g i v e c o n s i s t e n t a n a l y t i c a l r e s u l t s , although some of the s i m p l i f y i n g assumptions used i n the mathematical d e r i v a t i o n s of the p o l a r o g r a p h i c c u r r e n t curves are no longer t r u e .  C o n s i s t e n t r e s u l t s were obtained i n t h i s  application. The requirement f o r the u n p o l a r i z e d e l e c t r o d e i s t h a t i t have a l a r g e area compared t o the m i c r o - e l e c t r o d e , and t h a t i t have a steady, known h a l f - c e l l p o t e n t i a l , v a r y ing  l i t t l e with current.  The  second c r i t e r i o n i s probably  not met by a sheet of smooth p l a t i n u m , but i f the p o t e n t i a l i s reasonably  steady, a g i v e n i o n can be i d e n t i f i e d by the  running of s u i t a b l e standards under the same c o n d i t i o n s . At e l e v a t e d temperatures and p r e s s u r e s the half-wave v o l t a g e s are not known anyway.  There was  l i t t l e f l u c t u a t i o n of the  p o t e n t i a l s of l e a d and. oxygen waves under l i k e c o n d i t i o n s .  Experimental  Materials "Bakers Analyzed"  chemicals and d i s t i l l e d water  were used throughout f o r making up medium. Two  types of galena were used, a ground  12 c o n c e n t r a t e from t h e C o n s o l i d a t e d M i n i n g and S m e l t i n g Company of Canada, and handpicked c r y s t a l s from Violamac Mines (B.C.) L i m i t e d a t Sandon, B. C. The a n a l y s e s a r e g i v e n below; C. M. & S. Concentrate OG-S 740 Ag  Pb  94.7 o z / t o n  82.7$  Zn  Fe  1.6$  1.0$  Spectrographs Analysis Low  Trace  Ca A l S i Mg Sb Mn Cu  Cd As B Sn Ge V T i Ba S r B i  The a n a l y s i s of t h r e e c r y s t a l s of t h e same type as those used f o r the r e s e a r c h , from Violamac Mines, gave the f o l l o w i n g a n a l y s i s ; Ag  126 o z / t o n  Au  0.02 oz/ton  Spectrographic Analysis Sn Sb Zn  0.2$ 0.06 O.O3  Si) Fe ) Mg )  l e s s than 0.05$  As ) The r e s u l t s of the s p e c t r o g r a p h i c a n a l y s i s a r e from the P r o v i n c i a l Assay O f f i c e , V i c t o r i a , B. C., who k i n d l y d i d t h i s work. E x p e r i m e n t a l Procedure 1_ Rates on C r y s t a l s A s l a b o f galena c r y s t a l measuring approximately 0.2 by 0.3 by 0.5 i n c h e s was broken from a l a r g e r p i e c e . The s u r f a c e s were ground f l a t and p a r a l l e l to the 100 axes on No. 2 emery, the c r y s t a l was measured by a micrometer, washed i n c o l d , running water, and p l a c e d i n a s t a i n l e s s  13 s t e e l wire h o l d e r placed  i n F i g . 5.  as I l l u s t r a t e d  i n i t s p o s i t i o n i n the l i n e r ,  155  The h o l d e r was  ml. o f medium added  t o b r i n g t h e l i q u i d up t o t h e r e q u i r e d l e v e l , ped  the r o t o r drop-  i n , and, i n l a t e r e x p e r i m e n t s , a few m l . o f water were  introduced  between t h e l i n e r and t h e body  of the a u t o c l a v e .  A f t e r w a s h i n g t h e l i d and e l e c t r o d e s t h e l i d was b o l t e d i n t o position,  the autoclave  the  gas l i n e  and  gas p r e s s u r e  connected.  caution against valve  placed  turned  i n the heating  The s h u t - o f f v a l v e was then on a g a i n s t  a g i t a t o r s t a r t e d , and t h e p o l a r g r a p h  the  autoclave  zero.  was up t o t e m p e r a t u r e ,  allowing  on.  When  t h e t o p v a l v e was opened  the s o l u t i o n .  each time  T h i s was t a k e n as time  s h u t t i n g o f f t h e a g i t a t i o n and  t h e s o l u t i o n t o come t o r e s t .  connected to the polarograph  A f t e r about 180 and  switched  P h o t o g r a p h s were t a k e n o f t h e c a t h o d e - r a y tube f a c e a t  15-minute i n t e r v a l s ,  only  i f the shut-off  The h e a t was t h e n s w i t c h e d on,  the  oxygen t o r e a c h  closed  i t . (This s t e p was a p r e -  steam g e t t i n g i n t o t h e l i n e  d i d not c l o s e p r o p e r l y . )  to allow  j a c k e t , and  taking a reading.  m i n u t e s t h e t o p v a l v e was c l o s e d , t h e h e a t  oxygen t u r n e d  tap.  while  The e l e c t r o d e s were  o f f and t h e a u t o c l a v e  T h i s was t a k e n a s t  f i n £ )  - , L  and f i v e  quenched under a m i n u t e s were added  f o r r e a c t i o n o c c u r r i n g a f t e r t h e a p p a r a t u s h a d been shut The with  autoclave  washings p u t a s i d e  g i v e n i n A p p e n d i x A.  off.  was opened when c o o l and t h e s o l u t i o n f o r assay.  The a s s a y p r o c e d u r e i s  The c r y s t a l was t h e n examined u n d e r t h e  m i c r o s c o p e and sometimes p h o t o g r a p h e d . The curves  photographic  p l a t e s showing t h e c u r r e n t - v o l t a g e  were t h e n d e v e l o p e d and the peak h e i g h t s  measured,  14 c o r r e c t e d , and p l o t t e d . to t  f l n a l  The r a t e curve was e x t r a p o l a t e d  p l u s f i v e minutes, and the o r d i n a t e a t t h i s p o i n t  corresponded t o the t o t a l l e a d content of the s o l u t i o n . The p o l a r o g r a p h  was s e t w h i l e t a k i n g the f i r s t  r e a d i n g and the adjustments were not changed d u r i n g the run. The r e s u l t s were c o n s i s t e n t a few anomalously h i g h r a t e s .  w i t h i n 5$, except f o r  These were c o n s i d e r e d t o be  due t o s p a l l i n g of s m a l l fragments of g a l e n a , which would be ground under the r o t o r and g i v e an i n c r e a s e i n s u r f a c e area. 2_ Rates on P u l p s When a r a t e d e t e r m i n a t i o n was made on ground g a l e n a , the procedure was e x a c t l y the same except t h a t a weighed amount of p u l p was p l a c e d i n the l i n e r i n s t e a d of a crystal. 3 Q u a l i t a t i v e I n v e s t i g a t i o n of the E f f e c t of Applied Fields When i t was d e s i r e d t o i n v e s t i g a t e the e f f e c t of an a p p l i e d f i e l d , a s m a l l galena c r y s t a l was set I n s o l d e r i n a cup d r i l l e d i n t o the end of a brass r o d .  The other end  of the brass r o d was welded t o the centre e l e c t r o d e of a model a i r c r a f t s p a r k p l u g , and a l l metal and c r y s t a l except for  a s m a l l window t o a 100 plane was p a i n t e d with a p l a s t i c -  base v a r n i s h .  The c l o s e d a u t o c l a v e c o n t a i n i n g medium was  then brought up t o b o i l i n g - p o i n t , the sparkplug  screwed i n t o  p o s i t i o n so t h a t the c r y s t a l was below the l e v e l of t h e  15 l i q u i d , ' o x y g e n p r e s s u r e a p p l i e d , and EMF's measured by a vacuum-tube v o l t m e t e r .  Sometimes the e l e c t r o d e s were c o n -  nected t o the p e r i o d i c v o l t a g e s from the p o l a r o g r a p h i c apparatus and t h e wave forms  observed.  Results  P r o d u c t s o f t h e P r e s s u r e O x i d a t i o n o f Galena i n Sodium Hydroxide S o l u t i o n A n a l y s i s showed the products of the r e a c t i o n c o u l d be c o n s i d e r e d t o be sodium plurnbite and sodium s u l p h a t e , o r , more c o r r e c t l y , s u l p h a t e - i o n and p l u m b i t e - i o n .  No  h i g h e r valence forms of l e a d and no lower valence forms of s u l p h u r , f o r example, p o l y t h i o n a t e s , were found. p a r e n t l y t h e s u l p h u r atom was o x i d i z e d completely  Ap-  before  i t had t r a v e l l e d f a r i n t o t h e s o l u t i o n . The Wave Forms The wave forms f o r t h e p l a t i n u m m i c r o - e l e c t r o d e were a l l s i m i l a r t o the example shown i n F i g . 6, page 16. These were the r e s u l t s of employing the m i c r o e l e c t r o d e as a cathode.  I f the m i c r o e l e c t r o d e was made anodic no c u r -  r e n t maxima were observed except f o r t h e e v o l u t i o n of oxygen, which o c c u r r e d at a very low p o t e n t i a l . Q u a n t i t a t i v e r e s u l t s were a l l based on the waves f o r the r e d u c t i o n of oxygen and of l e a d - i o n t o m e t a l l i c lead.  There was no s h i f t i n t h e half-wave v o l t a g e s f o r  these r e a c t i o n s  as might have been expected, due t o p o i s o n -  i n g o f the p l a t i n u m anode, or because of the b u i l d - u p of  ]  Figure 6 Polarogram Photograph showing, from l e f t to right, a lead oxidation wave, oxygen reduction wave, and lead reduction wave.  17 l e a d on the cathode.  Wave h e i g h t s were measured by drawing  tangents as d e s c r i b e d i n K o l t h o f f and L i n g a n e , f o r although the maximum was  easy t o r e a d , the beginning of the r i s e  was not so d e f i n i t e .  The v e r t i c a l d i s t a n c e from the  tangent t o the maximum was c a l l e d the wave h e i g h t . l i n e s on the photographic  lower As  the  p l a t e were of a p p r e c i a b l e t h i c k -  ness, the c e n t r e of t h i s l i n e was  taken as the t r u e v a l u e .  Because of the r a p i d a t t a c k of sodium hydroxide on g l a s s , i t was necessary  to use a wire d i p p i n g i n t o the  s o l u t i o n as the m i c r o - e l e c t r o d e .  As c u r r e n t i s a f u n c t i o n  of s u r f a c e a r e a , t h i s made c u r r e n t readings very to changes i n l e v e l of the s o l u t i o n .  For t h i s reason,  because no o t h e r s u i t a b l e substance was used as a r e f e r e n c e standard.  and  found, oxygen was  I t was c o n s i d e r e d i n the  case of measuring r a t e s of o x i d a t i o n of galena t h a t the r e a c t i o n was  sensitive  crystals  slow enough t h a t the surface l a y e r s  of s o l u t i o n would always be s a t u r a t e d w i t h oxygen, a f t e r the f i r s t few m i n u t e s .  A f t e r water was  i n t r o d u c e d between  l i n e r and a u t o c l a v e , the changes i n l e v e l became almost insignificant. The wave forms of the r e a c t i o n at a galena, e l e c t r o d e were much more complica.ted and no attempt  was  made t o get q u a n t i t a t i v e i n f o r m a t i o n from t h e s e . The Rate Curves The r a t e curves f o r c r y s t a l s were a l l of the form shown i n F i g u r e s 7 and 8, page 18.  The  i n i t i a l toe  of the curve i s caused by d i f f u s i o n of oxygen i n t o the l i q u i d g r a d u a l l y r e a c h i n g e q u i l i b r i u m , at which time some  I 0030 0 5  N  N»_OH  300* F  136  PS.G  100  50 TIME  MINUTES  Figure 7 Rate  Curve f o r a C r y s t a l  Figure 8 Rate Curve f o r a C r y s t a l  19 other step c o n t r o l s the o v e r - a l l r a t e . t h a t the medium was may  I t i s considered  approaching s a t u r a t i o n w i t h oxygen.  It  he noted i n the two examples given t h a t s a t u r a t i o n was  much more r a p i d w i t h more c o n c e n t r a t e d  sodium h y d r o x i d e .  T h i s can be e x p l a i n e d by two c o n t r i b u t i n g factors*. i s l e s s soluble i n concentrated r a t e i s slower.  oxygen  c a u s t i c , and the r e a c t i o n  I t w i l l be noted t h a t the curves are e s s e n -  t i a l l y s t r a i g h t a f t e r the i n i t i a l t o e .  T h i s would seem to  mean t h a t the p r o d u c t s , at l e a s t i n these low  concentrations,  have no e f f e c t on the r e a c t i o n , and t h a t the e f f e c t i v e s u r face area remains c o n s t a n t , or t h a t these e f f e c t s e x a c t l y cancel.  The r e a c t i o n was always slow enough and the time  short enough t h a t the o v e r a l l s i z e of the c r y s t a l was much changed.  There was  to p i t t i n g e f f e c t s .  The  the i n i t i a l p e r i o d was  not  an i n c r e a s e i n a r e a , however, due s t r a i g h t n e s s of the l i n e a f t e r  checked by stopping the r e a c t i o n  a f t e r s h o r t e r p e r i o d s than u s u a l and seeing i f the concent r a t i o n - t i m e r e l a t i o n s h i p was  linear.  be so w i t h i n the experimental  error.  This was  The r a t e curve f o r ground pulps was the shape shown i n F i g . 9, page 20.  found to  found t o have  I t appears  reasonable  c o n s i d e r i n g the r e d u c t i o n i n area a.s the pulp i s consumed. There appears t o be no i n i t i a t i o n p e r i o d h e r e , no doubt due to  the f i n e p a r t i c l e s approaching the oxygen-rich  i n t e r f a c e i n the e a r l y s t a g e s .  gas-liquid  With incomplete r e a c t i o n s  the r e s i d u e c o n s i s t e d e n t i r e l y of c o a r s e r p a r t i c l e s ,  no  doubt due t o the e f f e c t mentioned above, and perhaps due i n p a r t t o the h i g h e r surface energies of very f i n e p a r t i c l e s .  20  21 A l l r a t e r e l a t i o n s h i p s were worked out on  single crystals.  difficult  sampling  There a r e two  from  dangers i n t h i s :  p r o b l e m i s c r e a t e d and  data a  l a c k o f homogeneity  i n a l a r g e s l a b might p r o d u c e e f f e c t s which would not be i n a ground p u l p .  Nevertheless,  as i t was  the r a t e s of f l o w of medium p a s t the  effects  of a t t r i t i o n ,  single crystals.  No  s l a b s o f galena, was  the  mon  boundaries, formed by no  i t was  lines,  Oxidation  covered  with  the  some s o r t  fine pits  o f 111  surface  was  of b l o c k  of v a r y i n g  planes,  and  size,  there  was  p a r t i c u l a r l y poor during  the  of change of phase i n the surfa.ce.  As  a g i t a ; t i o n was  c o o l i n g p e r i o d , w h i l e r e a c t i o n was t h e r e was  individual  t h r e e a t t r i b u t e s i n com-  apparently  the i n t e r s e c t i o n  evidence  on  r e g a r d l e s s o f r a t e of r e a c t i o n or  c o n c e n t r a t i o n , had  etched  done  found.  when v i e w e d under the m i c r o s c o p e :  c r o s s e d by  to a v o i d  a l m o s t a l l t h e work was  d i f f e r e n c e i n r a t e between  crystals,  sodium h y d r o x i d e  d e s i r e d t o measure  s u r f a c e and  A p p e a r a n c e . o f the C r y s t a l a f t e r All  met  some a c c u m u l a t i o n  high caustic concentrations. a t 93°C, the  surface  still  of p r o d u c t , I n one  showed a web  taking place, particularly in  case,  i n 0,5  of the y e l l o w  ( h y d r a t e d P b O ) a l o n g t h e r i d g e s between these f o r t u n a t e l y t h i s was  not p h o t o g r a p h e d .  The  N NaOH product  pits.  Un-  c o a t i n g s were  only l o o s e l y adherent. Two 10 the  p h o t o m i c r o g r a p h s are g i v e n on page 22.  shows t h e boundary l i n e s or g r o o v e s and pits.  F i g . 11  Fig.  shows  Figure  10  35x  Photomicrograph of Galena C r y s t a l C a u s t i c a t 1500c f o r 200 minutes  a f t e r Attack  mmr • • *r F i g u r e 11  i n 0.5  '•«».  500x  Photomicrograph of Galena C r y s t a l C a u s t i c a t 175°C f o r 132 m i n u t e s  a f t e r Attack  in  0.5N  N  23 I n v e s t i g a t i o n of V a r i a b l e s The f o l l o w i n g v a r i a b l e s were i n v e s t i g a t e d w i t h r e s p e c t t o rea.ction r a t e s of s i n g l e c r y s t a l s o f g a l e n a , measured as r a t e of p r o d u c t i o n of l e a d - i o n : 1. Oxygen p a r t i a l p r e s s u r e . 2. A g i t a t i o n , i . e. r a t e of f l o w of medium past the p a r ticle. 3. C o n c e n t r a t i o n of sodium h y d r o x i d e . 4. Temperature. 5. E f f e c t of s i l i c a t e - i o n . 6. E f f e c t of s u b s t i t u t i n g sodium a c e t a t e f o r sodium hydroxide. ? . E f f e c t of e l e c t r i c a l p o t e n t i a l ( q u a l i t a t i v e l y  only).  1 Oxygen P a r t i a l P r e s s u r e The e f f e c t o f oxygen p a r t i a l p r e s s u r e was i n v e s t i gated i n 0.5 N NaOH a t 123°C i n the range zero t o 11 a t mospheres.  The r e s u l t s are t a b u l a t e d below:  p atm.  p2  Rate, ^ x moles cm ~sec  0  0  0  1.97  1.41  5.64  IO.96  1  Rate £52 1 -1 1 moles cn , s e c - x a t m 2  4.1 x 10-°  2.38  5.77 x 10~ 10.08 x 10~9  3.31  13.14 x 10-9  4.0 x 10-9  9  4.2 x  10-9  — where P i s the p a r t i a l pressure of oxygen i n atmospheres. I t was noted t h a t a t zero p a r t i a l pressure o f o x  ygen  ( a c t u a l l y not q u i t e zero because commercial n i t r o g e n  was  used) not only c o u l d no l e a d or sulphur be d e t e c t e d I n  the s o l u t i o n , but t h e surface of a t t a c k a f t e r  3 hours  showed no t r a c e o f any k i n d  at 123°C.  An attempt was a l s o made a t t h i s time t o i n v e s t i g a t e the r o a s t r e a c t i o n .  The c r y s t a l was heated under  n i t r o g e n p r e s s u r e w i t h sodium plumbite s o l u t i o n a t 123°C f o r 3 hours.  A g a i n no r e a c t i o n appeared t o take p l a c e , the  s u r f a c e was unchanged, and t h e r e was no i n c r e a s e i n l e a d content of the s o l u t i o n . 2 A g i t a t i o n (Rate of Flow of Medium P a s t the Particle) Rate of f l o w past the p a r t i c l e were c a l c u l a t e d from t h e speed of r o t a t i o n of the magnet.  No allowance was  made f o r drag o f t h e w a l l s of the v e s s e l , and t h e r a t e  cal-  c u l a t e d f o r the middle of t h e "leading-edge" of the c r y s t a l was  t a k e n as t h e average. No d i f f e r e n c e i n s p e c i f i c r a t e was d e t e c t i b l e i n  0.5N NaOH s o l u t i o n a t 150°C under 5«64 atmospheres p a r t i a l p r e s s u r e of oxygen when the "average" r a t e of flow of s o l u t i o n past t h e c r y s t a l was v a r i e d from 9 cm/sec t o 48 cm/sec.  Under the same c o n d i t i o n s of temperature and  p r e s s u r e , the r a t e s i n 3N and 6N NaOH were dependent on a g i t a t i o n , as shown i n the t a b l e below: Rate (moles cm-2sec~:Latm~2') x V e l o c i t y of Medium  C o n c e n t r a t i o n of NaOH 0.5N 3N .. 6N  cm s e c " 1  6.17 2.44 0.53  48 cm s e c " 1  6.17 3-^7 2.01  9  lO4^  25 To r e l a t e the v e l o c i t i e s t o something more f a m i l i a r , the f r e e s e t t l i n g v e l o c i t i e s of galena i n c o l d w a t e r 1 1 are g i v e n below f o r a few screen s i z e s , w i t h approximate Reynold's numbers: Screen S i z e Openings/inch  Settling Velocity cm/sec  Reynold's Number  100  6.72  9.9  !50  5.27  5.5  200  2.86  2.1  325  .85  0.37  The Reynold's numbers f o r the f l o w v e l o c i t i e s a s s o c i a t e d w i t h the r e a c t i o n r a t e s above va.ry from 1480 85.  to  I t would appear from these numbers t h a t the i n f l u e n c e  of a g i t a t i o n becomes a p p r e c i a b l e as soon as f l o w , by the Reynold's c r i t e r i o n , becomes v i s c o u s .  From the Reynold's  numbers f o r ground pulps i t would appear t h a t t h i s  situation  would always o b t a i n , and t h a t i n c r e a s e d a g i t a t i o n would g i v e Increases i n r a t e s of r e a c t i o n .  T h i s would be  inde-  pendent of any a e r a t i o n e f f e c t s . _2 C o n c e n t r a t i o n of Sodium Hydroxide The  e f f e c t of c o n c e n t r a t i o n of sodium hydroxide  on the s o l u b i l i t y of oxygen i n aqueous s o l u t i o n s at 25°C i s g i v e n by G e f f c k e n 1 2 .  The r a t e of the r e a c t i o n i s known  11 R i c h a r d s , R. H. and L o c k e , C. E., Textbook of Ore D r e s s i n g , T h i r d E d i t i o n , 4th i m p r e s s i o n , McGraw-Hill Book Co. I n c . , New'York and London, 1940. 12 Geffcken G., B e l t r a g e zur Kenntnis der L o s l i c h k e i t b e e i n f l u s s u n g , Z. f . Phys. Chemie 49, 257-302 1904.  from experiment to v a r y w i t h the square r o o t of the p a r t i a l pressure  of oxygen, and t h e r e f o r e , assuming oxygen obeys  Henry's Law,  the r e a c t i o n r a t e should vary w i t h the square  r o o t of the oxygen c o n c e n t r a t i o n i n s o l u t i o n . at constant p a r t i a l pressure  Therefore,  of oxygen the r a t e should  vary  as the square r o o t of L, the Ostwald s o l u b i l i t y , which i s d e f i n e d as c o n c e n t r a t i o n of gas i n the l i q u i d d i v i d e d by c o n c e n t r a t i o n i n the gas phase. J-  I n F i g . 12, page 27,  and K, the s p e c i f i c r e -  a c t i o n r a t e , are p l o t t e d i n a r b i t r a r y u n i t s a g a i n s t n o r m a l i t y Isk and K are so a d j u s t e d that they  of sodium h y d r o x i d e .  have the same o r d i n a t e at 0.5  N NaOH.  for different f l u i d velocities.  Curves are p l o t t e d  These curves would seem  t o i n d i c a t e t h a t the e f f e c t s of sodium hydroxide a r e , f i r s t , t o reduce the s o l u b i l i t y of oxygen i n the s o l u t i o n ,  and,  second, to i n c r e a s e the v i s c o s i t y of the l i q u i d to cause d i f f u s i o n through the f i l m s around the p a r t i c l e to c o n t r o l the r a t e . I t i s r e a l i z e d there i s an e x t r a p o l a t i o n here from 25°C t o 150°C.  The assumption i m p l i e d i s that the  v a r i a t i o n of oxygen s o l u b i l i t y w i t h sodium hydroxide conc e n t r a t i o n f o l l o w s the same shaped curve at 150°C as i t does at 25°C.  I t might be expected that the curve would  be somewhat f l a t t e r at the h i g h temperatures, thus showing an even g r e a t e r f i l m  effect.  4 Temperature The  e f f e c t s of temperature were i n v e s t i g a t e d i n  NORMALITY  Figure  N t O H  12  Rate v s . Normality NaOH P l o t of L"2 and Reaction Rate i n a r b i t r a r y u n i t s against normality of sodium hydroxide. Temperature 150°C, pressure 5.64 Atmospheres.  28  0.5  N NaOH s o l u t i o n under an oxygen p a r t i a l p r e s s u r e o f  atmospheres,  i n the range 93°C t o 175°C.  The A r r h e n i u s p l o t The  i s shown i n F i g . 13,  s l o p e c o r r e s p o n d s t o an a p p a r e n t a c t i v a t i o n  6820 c a l o r i e s p e r mole. of  5.64  I t i s known t h a t t h e  page  29.  energy of  solubilities  g a s e s i n l i q u i d s d e c r e a s e w i t h t e m p e r a t u r e , so i t was  wondered- i f t h e r e  s h o u l d be a c o r r e c t i o n f o r t h i s  However, when l o g L was  effect.  p l o t t e d against r e c i p r o c r l  t u r e , u s i n g v a l u e s from I n t e r n a t i o n a l C r i t i c a l  tempera-  Tables"^,  and f r o m t h e Handbook of C h e m i s t r y and P h y s i c s 1 ^ f o r s o l u b i lity  i n w a t e r , i t was  very f l a t  seen t h a t the s l o p e o f the curve  i n t h e b e g i n n i n g of the range  i n v e s t i g a t e d by the  a u t h o r , w i t h d e c r e a s i n g s l o p e a p p a r e n t l y the t r e n d . c o r r e c t i o n a t t h e low t e m p e r a t u r e end o f t h e r e a c t i o n plot  o n l y amounts t o 60  n o t known what e f f e c t  was  c a l o r i e s p e r mole.  sodium h y d r o x i d e has  The rate  Again, i t i s on the  solubility-  temperature c u r v e . J5 E f f e c t  of S i l i c a t e - i o n  Stenhouse1^ effect  on t h e o x i d a t i o n o f p y r i t e .  some s o r t In  found t h a t s i l i c a had a T h i s was  of p o i s o n i n g of t h e s u r f a c e w i t h  fluid velocities  o f 48cm/sec, 5.64  pronounced a t t r i b u t e d to  silicate-ion.  atm. p a r t i a l p r e s s u r e  13 I n t e r n a t i o n a l C r i t i c a l T a b l e s o f N u m e r i c a l D a t a , McG-rawH i l l Book Co. Inc., New York, 1 9 2 6 . 14 Handbook o f C h e m i s t r y and P h y s i c s 30th E d i t i o n , Rubber P u b l i s h i n g Co., C l e v e l a n d , Ohio, 1946. 15  Stenhouse, J . F . ,  ibid.  Chemical  -86  ,-d.e  2J2  24  2.8  T  1000  Figure 13 Arrhenlus Plot Log K (in moles cm ^ sec - atm2) versus 1 T  of oxygen, at a temperature of in  0.5 N NaOH, 0.015  122°C,  i t was found that  Molar s i l i c a t e - i o n had very  little  e f f e c t , and O.O3 Molar s i l i c a t e - i o n gave a reduction i n r a t e of approximately 10$.  I t was n o t i c e d , however, that at  p o i n t s of low turbulence where a l i t t l e  oxide coating might  be expected to accumulate, a f i n e - g r a i n e d b l a c k , adherent coating formed. ion  has l i t t l e  I t was therefore deduced that  silicate-  a f f i n i t y f o r sulphide surfaces, but i s  r e a d i l y chemisorbed to oxide surfaces, probably i n the case of p y r i t e s e a l i n g them against d i f f u s i o n of i o n s . This would be s i m i l a r t o surface r e a c t i o n s occurring i n flotation. _6 E f f e c t of S u b s t i t u t i n g Sodium Acetate  f o r Sodium  Hydroxide When sodium acetate was s u b s t i t u t e d f o r sodium hydroxide, the medium at  122°C  attacked the mild s t e e l i n  the autoclave and even removed the adherent oxide coating from the s t a i n l e s s s t e e l . in  The r e s u l t was such a reduction  oxygen concentration of the s o l u t i o n that there was no  r e a c t i o n with the galena.  Shortage of time prevented t r e a t -  ment i n g l a s s c o n t a i n e r s . 2 E f f e c t of E l e c t r i c a l P o t e n t i a l Various measurements of the p o t e n t i a l of galena under pressure were made, but unfortunately no s o l u t i o n was found f o r the problem of a reference e l e c t r o d e . ments were against b r i g h t platinum P o t e n t i a l d i f f e r e n c e s at  100°C  in  A l l measure-  i n the same s o l u t i o n .  1N  NaOH s o l u t i o n a p p r o x i -  mated 0.5 v o l t s , the galena being negative.  The emf  31 appeared t o be s u b s t a n t i a l l y independent of oxygen p r e s s u r e , as v o l t a g e s of t h e same order were measured i n s o l u t i o n s from which the oxygen had been removed by b o i l i n g . Using a vacuum tube v o l t m e t e r and p l a t i n u m probes, and r e v e r s i n g probes f r e q u e n t l y , an apparent emf i n the o r d e r of 10 m i l l i v o l t s was found between a 111 plane and a 100 plane of a galena c r y s t a l i n a s o l u t i o n exposed t o the a i r a t 50°C.  The 111 plane was r e l a t i v e l y p o s i t i v e .  When p o s i t i v e p o t e n t i a l s from the p o l a r o g r a p h i c apparatus were a p p l i e d t o galena c r y s t a l s , a c o a t i n g , a p p a r e n t l y PbO, was formed on the exposed face and the c u r rent tended t o d r o p .  When negative p o t e n t i a l s were a p p l i e d ,  t h e r e was i n c r e a s e d e t c h i n g of the surface and on the p l a s t i c v a r n i s h around the c r y s t a l a d e p o s i t of PbO was l e f t i n a c i r c u l a r p a t t e r n w i t h the c r y s t a l the c e n t r e .  I f a coating  e x i s t e d on the c r y s t a l , i t was very r a p i d l y removed. The hydrogen overvoltage on galena appeared t o be h i g h . In 3 N II a OH s o l u t i o n , at room temperature, e l e c t r o l y s i s of a galena c r y s t a l a t 1.5 v o l t s w i t h galena the cathode and w i t h an i n e r t anode produced H 2 S and a c o a t i n g of spongy l e a d on the cathode. In view o f the i n c r e a s e d r a t e of r e a c t i o n w i t h moderate negative p o t e n t i a l s a p p l i e d t o the c r y s t a l , there was some worry that the contact between s t a i n l e s s s t e e l and galena would produce a c e l l which would produce f i e l d s i n the s o l u t i o n and d i s t o r t the r e s u l t s .  Two experiments were  done t o q u a l i t a t i v e l y i n v e s t i g a t e t h i s p o s s i b i l i t y .  In  32 each, fragments of the same c r y s t a l were o x i d i z e d side by s i d e on a s t a i n l e s s s t e e l r a c k .  I n the f i r s t experiment,  one c r y s t a l was i n s u l a t e d w i t h a t h i c k c o a t i n g o f Garbocoat, a p l a s t i c v a r n i s h , and the o t h e r made f a i r c o n t a c t w i t h the  stainless steel surface.  I n the second experiment,  one c r y s t a l was i n s u l a t e d by b e i n g p l a c e d i n an alundum b o a t , and the other i n c o n t a c t w i t h c l e a n s t a i n l e s s  steel.  In both cases o x i d a t i o n was f o r a r e a l t i v e l y short t i m e , so t h a t d i f f e r e n c e s i n r a t e should be more e a s i l y observable under t h e microscope. difference i n rate.  I n n e i t h e r case was t h e r e any apparent  A f t e r the f i r s t few experiments the  s t a i n l e s s s t e e l was covered w i t h a t h i c k adherent c o a t i n g of For  o x i d e , which s h o u l d have behaved as an i n s u l a t i n g l a y e r . these reasons i t i s not b e l i e v e d t h a t the p h y s i c a l s i t u a -  t i o n of t h e c r y s t a l had an a p p r e c i a b l e e f f e c t on the r a t e .  D i s c u s s i o n of R e s u l t s  Numerical Values The n u m e r i c a l v a l u e s of such q u a n t i t i e s as a c t i v a t i o n energy are c o n s i d e r e d t o be c o r r e c t w i t h i n a few percent under the c o n d i t i o n s of the experiment.  I f c o n t r o l i s by  some other mechanism i n another experiment, then of course there w i l l be no r e l a t i o n s h i p between the two v a l u e s . P a r t i c l e s i z e w i l l have e f f e c t s a l s o .  Probably the g r e a t e s t  discrepancies i n extrapolating t h i s data to reactions with  33 suspended pulps would be caused by the i n c r e a s e d a c t i v i t y of v e r y f i n e p a r t i c l e s . S u r p r i s i n g l y enough, i n these experiments d i d not seem t o i n t r o d u c e any a p p r e c i a b l e e r r o r .  sampling  Apparently  the h a l f - d o z e n c r y s t a l s used, which were a l l from Violamac, and a l l s e l e c t e d by t h e i r r e g u l a r shape, absence of t w i n s , and s t r a i g h t n e s s of axes, were very s i m i l a r i n chemical properties.  From a l l c r y s t a l s used there were a few very  h i g h r e s u l t s which were considered t o be due to fragments f a l l i n g under the r o t o r and b e i n g ground up t o give a l a r g e increase i n surface area. In an i n d u s t r i a l a p p l i c a t i o n , c o n t r o l by d i f f u s i o n through the g a s - l i q u i d i n t e r f a c e would be e a s i l y p o s s i b l e w i t h f i n e p a r t i c l e s and inadequate a e r a t i o n .  Even more  l i k e l y , i n f a i r l y strong c a u s t i c (which would be necessary i n a batch p r o c e s s ) would be c o n t r o l by d i f f u s i o n through the f i l m s at the s o l i d - l i q u i d i n t e r f a c e .  Motion of the  p a r t i c l e would, be c o n t r o l l e d by the laws of v i s c o u s f l o w , and i t would not be p o s s i b l e t o get out of t h i s range. As was  seen on page 2 4 t h i s would mean c o n t r o l by d i f f u -  s i o n through the f i l m about the p a r t i c l e . Mechanism of the R e a c t i o n i n 0.5  Normal Sodium Hydroxide  A c c o r d i n g t o E y r i n g 1 ^ a r e a c t i o n of a gas at a s u r f a c e may be separated i n t o f i v e s t e p s , the slowest of 16 G l a s s t o n e , S., L a i d l e r , K., and E y r i n g , H., The Theory of Rate P r o c e s s e s , McC-raw-Hill Book Go. Inc., New York and London, 1 9 4 1 .  3  which w i l l " determine the r a t e of the o v e r - a l l p r o v e s s .  4  The  steps are: 1. Transport of the r e a c t a n t s to the s u r f a c e . 2. A d s o r p t i o n of the r e a c t a n t s . 3. R e a c t i o n on the s u r f a c e 4. D e s o r p t l o n of the p r o d u c t s . 5.  Transport of the products from the s u r f a c e .  Using t h i s . c l a s s i f i c a t i o n f o r the pressure o x i d a t i o n of g a l e n a , the experiments w i t h a g i t a t i o n i n d i c a t e that c o n t r o l must be by steps 2, 3, or 4.  I t i s a l s o known t h a t  the r a t e i s proportiona.1 to the square root of the oxygen p a . r t i a l p r e s s u r e , t h a t a n e g a t i v e p o t e n t i a l on the g a l e n a i n c r e a s e s the r a t e , and t h a t the a c t i v a t i o n energy i s low. Stenhouse 1 ? found i n the o x i d a t i o n of p y r i t e t h a t the r a t e of o x i d a t i o n of i r o n depended on the square root of the p a r t i a l pressure of oxygen, but t h a t at low pressures the r a t e of o x i d a t i o n of sulphur depended on the f i r s t power of the p a r t i a l pressure of oxygen.  I n other work of the  department on aqueous o x i d a t i o n , p o l y t h i o n a t e s formed c o u l d be e x p l a i n e d by the i n i t i a l f o r m a t i o n of S 0 2 , which would g i v e dependency on the f i r s t power of the oxygen p a r t i a l p r e s s u r e , i f t h i s were the r a t e - c o n t r o l l i n g s t e p . pH, the o x i d a t i o n of sulphur i s slow or incomplete  At low and  a p p a r e n t l y sulphur i s l i b e r a t e d to form g l o b u l e s of the element1^.  I n Mr. Stenhouse's work, and  17 Stenhouse, J . F., 18 Mcintosh, R. B.,  ibid. ibid.  g e n e r a l l y observed  i n aqueous o x i d a t i o n , i t i s found t h a t t h e a c t i v a t i o n energy for  t h e o x i d a t i o n of metals i s lower than t h a t f o r s u l p h u r .  For these reasons the r a t e - c o n t r o l l i n g step i n t h i s r e a c t i o n , where no new phase appeared a t the "boundary between galena a.nd s o l u t i o n , w i l l be c o n s i d e r e d t o i n v o l v e only the o x i d a t i o n of l e a d . For t h e r a t e to be p r o p o r t i o n a l t o the square r o o t of the p a r t i a l p r e s s u r e of oxygen there are two p o s s i b i l i t i e s f o r the a c t u a l mechanism: 1. A d s o r p t i o n i s r a p i d but d i s s o c i a t i o n i s slow. 2. The r e a c t i o n i n v o l v i n g d i s s o c i a t i o n i s r a p i d , but the f r a c t i o n of t h e s i t e s i n v o l v e d i s s m a l l .  This f o l -  lows from the Langmuir isotherm; i  = p2  x  a  1 - x where x i s the f r a c t i o n of the s i t e s Involved a i s a constant p i s the oxygen p a r t i a l p r e s s u r e when x i s small and 1 - x approaches 1. Mechanism I For the f i r s t mechanism, c o n s i d e r t h e r e a c t i o n : i  0  2  + Pb? gives  (OPb 2 )#  where P b 2 r e p r e s e n t s two adjacent s i t e s and ( 0 P b 2 ) ^ the a c t i v a t e d complex.  The r a t e equation f o r t h i s r e a c t i o n i s :  r a t e ( i n molecules cm"2 s e c " 1 ) =,. c |  c  _kT  t#  exp.-H#/RT  36 where" c Q  i s the c o n c e n t r a t i o n of oxygen i n s o l u t i o n  C p k i s the c o n c e n t r a t i o n of s i t e s k i s Boltzmann's constant h i s Planck's  constant  T i s the a b s o l u t e temperature f7  a n d f  ^, fo 2 >  pt) are t h e p a r t i t i o n f u n c t i o n s o f  the a c t i v a t e d complex, of oxygen i n s o l u t i o n , and of the s i t e s , r e s p e c t i v e l y , f o r a standard s t a t e of one molecule per cm3 o r cm 2 R i s t h e gas constant H# i s the enthalpy o f a c t i v a t i o n The r a t e on the b a s i s of t h i s mechanism has been worked out, u s i n g a v a i l a b l e d a t a , and g i v e s a value which i s 23,50° times as f a s t as the experimental v a l u e .  The  c a l c u l a t i o n s a r e g i v e n i n Appendix B. Mechanism I I I n the second case above, the r a t e c o n t r o l l i n g step i s one which f o l l o w s c h e m i s o r p t i o n , and i t was p o s t u l a t e d t h a t i t was d e s o r p t i o n accompanied by h y d r a t i o n of PbO, t o g i v e a f t e r i o n i z a t i o n HPbO" i o n .  This would e x p l a i n the  e f f e c t of p o t e n t i a l as a i d i n g i n d e s o r p t i o n , by r e p e l l i n g the a n i o n . PbO  The r a t e c o n t r o l l i n g step would be: + H 0  (Pb0.H0-...H+)#  —^  9  s £ and the r a t e equation would be: r a t e = c P b 0 c H 0 kT h  f# f  Pb0  f  exp. -H#/RT H2°  37 where"  c  H2Q  i s the c o n c e n t r a t i o n of water  c  i s  Pb0  t5ie  c o n c e n t r a t i o n of PbO s i t e s  fti o i s the c o n c e n t r a t i o n p a r t i t i o n f u n c t i o n of 2 l i q u i d water and t h e other symbols have the same s i g n i f i c a n c e as p r e v i o u s l y By s u b s t i t u t i o n of q u a n t i t i e s from the e q u i l i b r i a :  °2  °?  =  gas  solution  and Pb + -|02  =  P^O  the r a t e equation may be w r i t t e n Rate = c f  c„. c,_ M ^  -rr  rarjj  f# ?  =f  exp. - ( H - H  - H^)/RT  where Hj_ i s the enthalpy of s o l u t i o n of oxygen i n the aqueous medium Hg i s the enthalpy of the chemisorption of atomic oxygen i s the enthalpy of a c t i v a t i o n The term (Hj. + Ez + H"^) + RT i s equal t o the experimental v a l u e of a c t i v a t i o n energy, E.  Ej_ i s negative and i s of the order of 100 c a l o r i e s per mole, by c a l c u l a t i o n from data i n I n t e r n a t i o n a l  Critical  Tables. H 2 corresponds t o d i s s o c i a t i o n and chemisorption of oxygen t o a metal s u r f a c e . slightly  exothermic. This means t h a t W  &  The r e a c t i o n i s probably  -*  r  equals E + 100_- H ? , and  38 t h e r e f o r e t h a t H#  may  be c o n s i d e r a b l y l a r g e r than E.  On c a l c u l a t i n g the r a t e from the above e x p r e s s i o n , s u r p r i s i n g l y good agreement w i t h experiment was The  achieved.  c a l c u l a t i o n s are given i n Appendix C. By t h i s  model, the a c t i v a t i o n energy f o r the  d i s s o c i a t i o n and a d s o r p t i o n of oxygen to the surface must be low.  I t has been found i n the a d s o r p t i o n of  hydrogen  on n i c k e l or p l a t i n u m , a c t i v a t i o n energies of the order of 5 k i l o c a l o r - i e s were observed. P o l a n y i 1 ^ e x p l a i n s t h i s by c o n s i d e r i n g the metal surface t o be h i g h l y degenerate, thus rounding o f f the peak corresponding  to the a c t i v a t i o n energy  and r e d u c i n g i t s v a l u e . There i s a p o s s i b i l i t y t h a t p a r t of the e f f e c t of p o t e n t i a l may  be i n i n c r e a s i n g the hydrogen-ion c o n c e n t r a t i o n .  I t I s known from G-effcken's data that h y d r o x y l has a, d e p r e s s i n g e f f e c t on s o l u b i l i t y of oxygen i n water, but t h a t hydrogeni o n has l i t t l e  effect.  These e f f e c t s c o u l d be evaluated  experimentally  by measuring the a c t i v a t i o n energy w i t h a p p l i e d p o t e n t i a l . I f there were no change i n a c t i v a t i o n energy, then the e f f e c t would only be on the temperature independent term, through i n c r e a s e i n oxygen c o n c e n t r a t i o n ; whereas i f the e f f e c t i s on d e s o r p t i o n there would be a decrease i n the experimental  a c t i v a t i o n energy.  19 P o l a n g i , M., C a t a l y t i c A c t i v a t i o n of Hydrogen, The S c i e n t i f i c J o u r n a l of the Royal C o l l e g e of Science, V o l . V I I , 193?-  39 Mechanism I I I A third, p o s s i b i l i t y  i s a r e a c t i o n analagous t o  the c o r r o s i o n of a metal or a l l o y , w i t h the metal t e n d i n g t o r e a c t w i t h hydrogen-Ion and being d e p o l a r i z e d by the d i s s o l v e d oxygen.  That i s , I n terms of c e l l s , the presence of the  oxygen-hydroxyl h a l f - c e l l i s necessary t o r a i s e the emf sufficiently  to cause r e a c t i o n .  The k i n e t i c s of such a  p i c t u r e i s not c l e a r enough to attempt e v a l u a t i o n of s u l p h i d e o x i d a t i o n on these terms. I t i s r e a l i z e d t h a t these three mechanisms are by no means e x h a u s t i v e , that there are other  possibilities.  However, Mechanism I I does e x p l a i n the observed e f f e c t s , and the r a t e deduced from i t by the theory of absolute r e a c t i o n r a t e s checks wi t h the observed r a t e .  I t remains t o  c o n s i d e r Mechanism I I w i t h regard to v a r i a b l e s not i n c l u d e d i n the r e s t r i c t e d s i t u a t i o n t r e a t e d above. C o n s i d e r a t i o n of Mechanism I I w i t h Respect t o the Remaining V a r i a b l e s 1 A g i t a t i o n and C o n c e n t r a t i o n of Sodium Hydroxide These v a r i a b l e s appear i n s e p a r a b l e and w i l l be treated  together. I t i s observed that the r a t e f a l l s w i t h c a u s t i c  c o n c e n t r a t i o n i n a manner that appears to roughly  conform  to the square r o o t of the f a l l i n g oxygen s o l u b i l i t y .  The  g e n e r a l e f f e c t of c a u s t i c c o n c e n t r a t i o n on the r a t e of the r e a c t i o n would be by displacement of the e q u i l i b r i u m : °?  "gas  =  °2  solution  40 to the l e f t .  Although the a c t i v i t y of oxygen can be s a i d  to remain the same when the temperature and pressure o f the gas a r e t h e same, but c a u s t i c c o n c e n t r a t i o n d i f f e r e n t , n e v e r t h e l e s s the change i n the a c t i v i t y c o e f f i c i e n t of the oxygen would be r e f l e c t e d by a change i n t h e a c t i v i t y c o e f f i c i e n t of PbO o r a c t i v a t e d complex i n t h e same d i r e c t i o n . T h i s would have the r e s u l t of making t h e r a t e p r o p o r t i o n a l t o c o n c e n t r a t i o n r a t h e r than a c t i v i t y . There i s a f u r t h e r c o m p l i c a t i o n of v i s c o s i t y h e r e , however, and r e a c t i o n r a t e s are seen to vary w i t h a g i t a t i o n . T h i s i s e x p l a i n e d by the mechanical p i c t u r e of stagnant f i l m s about p a r t i c l e s moving r e l a t i v e t o a f l u i d .  The f i l m  resist-  ance i s considered to be h i g h enough that i t takes c o n t r o l of the r e a c t i o n , by v i r t u e of being the slowest of the f i v e steps l i s t e d above.  I t i s not known c o n c l u s i v e l y whether  c o n t r o l i s by d i f f u s i o n of r e a c t a n t s i n or of products out from t h e i n t e r f a c e . 2 E f f e c t of S i l i c a t e - I o n S i l i c a t e - i o n a p p a r e n t l y rea.cts w i t h macroscopic c o a t i n g s of oxide but not w i t h the oxygen chemisorbed t o the s u r f a c e .  I t i s not understood why t h i s should be so,  u n l e s s r e a c t i o n w i t h s i l i c a t e - i o n i s dependent on h y d r a t i o n . _3_ E f f e c t of S u b s t i t u t i n g Sodium Acetate f o r Sodium Hydroxide The r e s u l t s of t h i s s u b s t i t u t i o n were a b o r t i v e , as s t a t e d p r e v i o u s l y .  4 E f f e c t of Change i n Surface Area Although not considered as a v a r i a b l e , the s u r face a r e a was noted to change, mainly by formation of e t c h marks c o n s i s t i n g of i n t e r s e c t i n g 111 p l a n e s . t h i s has no e f f e c t on the r a t e . 1. The  This may  Apparently  be f o r two reasons  s u r f a c e becomes covered w i t h p i t s i n the  initial  stages of the r e a c t i o n , but t h i s i s concealed by the build.-up of oxygen c o n c e n t r a t i o n , marked by the toe of the r a t e c u r v e . 2 . The  r a t e s on 111 planes are slower by a f a c t o r which  compensates e x a c t l y f o r the i n c r e a s e i n a r e a . P i t s may  be caused by d i f f e r e n c e s i n a c t i v i t y  between d i f f e r e n t p a r t s of the c r y s t a l .  Etchmarks are a  well-known phenomenon, used i n geology f o r symmetry d e t e r m i n a t i o n s on m i n e r a l c r y s t a l s .  Conclusions  The use of a cathode-ray  polarograph  method  of measuring the r a t e of o x i d a t i o n of galena immersed i n a s o l u t i o n , under oxygen p r e s s u r e , i s s a t i s f a c t o r y f o r measuri n g the r a t e of s o l u t i o n of l e a d .  I t g i v e s no i n d i c a t i o n  of the r a t e of o x i d a t i o n of sulphur, but as no  separate  phase appeared and no p o l y t h i o n a t e s were d e t e c t e d , undoubtedl y the r a t e i n terms of formation of sulphate would have been i d e n t i c a l w i t h t h a t i n terms of s o l u t i o n of l e a d .  42  T h i s technique could be a p p l i e d p r o f i t a b l y to  the  measurement of r a t e s of c o r r o s i o n under extreme c o n d i t i o n s of temperature and p r e s s u r e of a l l m e t a l s , a l l o y s , or m i n e r a l s where a s o l u b l e , r e d u c i b l e or o x i d i z a b l e product i s formed.  I t i s r e s t r i c t e d i n t h a t i t cannot be used i n  v e r y c o n c e n t r a t e d s o l u t i o n s of the i o n or molecule being measured, and i n t e r f e r e n c e by i m p u r i t i e s may it  a l s o render  inoperative. I n any  commercial a p p l i c a t i o n of aqueous o x i d a -  t i o n , a cathode-ray p o l a r o g r a p h would be a very u s e f u l cont r o l instrument f o r measuring c o n c e n t r a t i o n solution. operation  of oxygen i n  Such i n f o r m a t i o n might be v i t a l to e f f i c i e n t of the  process.  Even i n f u s e d - s a l t e l e c t r o l y s i s one  can see  possible  uses f o r a device p a t t e r n e d on t h i s , i n e i t h e r r e s e a r c h plant  or  operation. I t i s not p o s s i b l e from the experimental r e s u l t s  here o b t a i n e d t o decide whether r a t e of o x i d a t i o n of l e a d or of sulphur c o n t r o l s the o v e r - a l l r a t e .  From other con-  s i d e r a t i o n s i t i s thought t h a t l e a d i s i n v o l v e d i n the r a t e controlling  step.  I n h i g h c a u s t i c c o n c e n t r a t i o n s , r a t e i s found to be c o n t r o l l e d by d i f f u s i o n through stagnant f i l m s about the particle. Under r e a c t i o n c o n t r o l , r a t e of s o l u t i o n of l e a d i s p r o p o r t i o n a l to the square root" of the oxygen p a r t i a l sure.  The  a c t i v a t i o n energy i s 6820 c a l o r i e s per mole.  presA negative  p o t e n t i a l a p p l i e d t o the c r y s t a l profoundly i n c r e a s e d the rate.  I t i s not known whether t h i s i n c r e a s e i n r a t e i s due  to r e p u l s i o n of HPbCf or to the i n c r e a s e d s o l u b i l i t y of oxygen i n the absence of h y d r o x y l - i o n .  The  former appears  t o be most p r o b a b l e . Three p o s s i b l e mechanisms f o r the r e a c t i o n were suggested. Mechanism I , c o n t r o l by the d i s s o c i a t i o n c f oxygen at the surface was  found to give too h i g h a c a l c u l a t e d r a t e .  Mechanism I I I , modelled on a c o r r o s i o n p i c t u r e , was  not  v e r i f i e d as no method of c a l c u l a t i o n from fundamental data appeared to be  available.  I n Mechanism I I the r a t e - c o n t r o l l i n g step c o n s i s t ed of d e s o r p t i o n w i t h h y d r a t i o n .  The r a t e c a l c u l a t e d on  t h i s b a s i s from fundamental d a t a using the v a l u e of a c t i v a t i o n energy was sec""! Under  k x lO 1 -^ molecules  c e r t a i n c o n d i t i o n s , compared t o an  v a l u e of 2 . 5 5 x l O - ^ . check.  experimental  This was  cm - 2  experimental  considered an e x c e l l e n t  I t i s r e a l i z e d the u n c e r t a i n t i e s i n the p a r t i t i o n  f u n c t i o n of the a c t i v a t e d s t a t e , and i n the number of s i t e s a v a i l a b l e f o r r e a c t i o n , are such t h a t so c l o s e an agreement must be c o n s i d e r e d a c o i n c i d e n c e . No experimental data c o n f l i c t e d w i t h the p i c t u r e from Mechanism I I , although i t i s of course p o s s i b l e that there i s an e q u a l l y p l a u s i b l e , a l t e r n a t i v e model which would a l s o f i t the  observations.  44  Appendix A  Assay of S o l u t i o n by t h e Molybdate Method (Gravimetric) The b a s i c s o l u t i o n was made j u s t a c i d t o l i t m u s w i t h n i t r i c a c i d , then s l i g h t l y b a s i c by the a d d i t i o n o f ammonium a c e t a t e — u s u a l l y about f i v e grams. was  then heated t o b o i l i n g and f i l t e r e d .  The s o l u t i o n  To the b o i l i n g  f i l t r a t e was added 40 m l . o f ammonium molybdate s o l u t i o n f o r each 0.1 g of l e a d p r e s e n t .  A f t e r b o i l i n g a few  minutes the s o l u t i o n was a l l o w e d t o stand, then f i l t e r e d and' washed w i t h hot water containing 2% ammonium n i t r a t e . The r e s i d u e was i g n i t e d at d u l l r e d h e a t , c o o l e d , and weighed as PbMoO^. PbMoO^ x 0.5643 = Pb Ammonium Molybdate S o l u t i o n D i s s o l v e 4 g of the s a l t p e r l i t r e of water p l u s 10 ml a c e t i c a c i d .  4-5 Appendix B  C a l c u l a t i o n of Rate from Mechanism I From the r e a c t i o n : P b 2 + -f02 g i v e s (OPb 2 the r a t e equation i s g i v e n by: cm"2  r a t e (molecules  s e c " 1 ) = c^  Ct31  _ kT  f#  exo. -H# /RT ( l )  The s o l u t i o n of oxygen i s c o n s i d e r e d t o reach e q u i l i b r i u m . Then, Cone, of oxygen i n l i q u i d = u2 Cone, of oxygen i n gas phase f  n u  ( l i q u i d ) exp. -H-./RT (gas)  2  where f r e p r e s e n t s the designated  partition  functions H-^ i s the enthalpy of s o l u t i o n S u b s t i t u t i n g f o r C Q i n ( l ) , and c a n c e l l i n g f rate =  c£  £  *_r _ £ ! L  w  . -Ctl> +  Q  liquid,  H~)/*T  E x p e r i m e n t a l r e s u l t s are c o n v e n i e n t l y a v a i l a b l e f o r 4-00°K. In t h i s a n a l y s i s c o n c e n t r a t i o n p a r t i t i o n f u n c t i o n s based on a standard s t a t e of one molecule per square c e n t i metre or per cubic centimetre were used.  Choice of t h i s  standard s t a t e was p u r e l y a matter of convenience.  The  r e s u l t i n g p a r t i t i o n f u n c t i o n s are very l a r g e numbers b e cause of t h i s c h o i c e . The c o n c e n t r a t i o n p a r t i t i o n f u n c t i o n of gaseous oxygen i s :  46 T r a n s l a t i o n . . . (2 7r mkT)  3/2  = 2.73 x 10  Rotation  (8 ^ I k T ) = 190 h2  Vibration  approximates 1 where I i s t h e moment of i n e r t i a of the oxygen molecule m i s the mass of the oxygen molecule the- other symbols have t h e u s u a l s i g n i f i -  Then f  cance, = 5.2 x 10 28  Q  2 gas c un C  = 6.02 x 10 2 3 x 273 - I.83 x 1 0 1 9 molecules cm"3 2 gas 22400 4~00  Pb  =  7  ' '* "  x  tf  t  2  k, Boltzmann 1 s constant = I.38 x l O - ^ erg d e g " 1 h , P l a n c k ' s constant = 6.62 x 10~2< erg se c e i s the base f o r n a t u r a l l o g a r i t h m s , 2.72 -1 E i s the experimental a c t i v a t i o n energy, 6820 c a l mol  frf = f p ^ and both approximate 1 On s u b s t i t u t i n g these n u m e r i c a l v a l u e s , r a t e = z.?z *  x  * 7 x /o x /.za x-/<?  * /  y  &  = 6.0 x l O 1 ^ molecules cm""2 The observed r a t e i s 4.1 x 10"9 x 6. 02 x 1023 = 2.55 x 101-5 molecules cm - 2 I t i s seen t h a t t h e c a l c u l a t e d r a t e i s 23,500 times t o o large.  47  Appendix G  C a l c u l a t i o n of Rate from Mechanism I I I f the r a t e - c o n t r o l l i n g step i s d e s o r p t i o n of the PbO, w i t h accompanying  hydration:  PbO + H 2 0 g i v e s (PbO.HO-.. .H+ )# The f o l l o w i n g r e a c t i o n s w i l l have reached e q u i l i b r i u m : gas  solution  (l)  and i  - k ) o  As i n Appendix B  + Pb = PbO  c0  =  e  W**  As  cph£>  r e g a r d s (2)  ^  =  f ^  -H,/RT-  3 "> : -^ a-^ * »U fa  ^  *^  ~  " 3- /* r~  ff  where H£ i s the enthalpy of r e a c t i o n  c„e  Then And r a t e =  = c„ c£  c„t0  /*r e„6  kT  -4~T  £  o r , e x p r e s s i n g e n t h a l p i e s as an experimental  **s>.v**)/«T  activation  energy,  I t i s seen that t h i s expression d i f f e r s from equation  (2) of Appendix B by a f a c t o r  .  Ctr A = 6 x 107 3 molecules cm~3 H 2° IS 2  C a l c u l a t i o n of C o n c e n t r a t i o n P a r t i t i o n F u n c t i o n of L i q u i d Water SH  =16.75 E.U. m o l e - 1  a  (Hougan and Watson) 2 ^ 1 E.U. mole- 1  Change i n  = 2.^8  Then S „ „  = 19-1 E.U. mole" 1  (Kelley)21  To change from the standard s t a t e of l i q u i d water t o a standard s t a t e of one molecule per cm3, d i l u t e the water I d e a l l y t o one molecule per cm3. S = R I n 6 x 1 0 2 3 = IO3 E.U. mole- 1 IB Then the entropy i n a s t a t e of one molecule per cm3 = 122.1 f  2  =  e  E.U. mole" 1  122/ * - = 5 x 10 26  -/ Rate from Appendix B = 6 x 1 0 1 ? molecules cnrd- fee.  Rate f o r t h i s model = 6 x l O 1 ^ x 6 x 10 2 3 IB 4 x 1015  x  1  .=  5 x 10 2 6 molcules c i i W  which a p p r o x i m a t e l y checks w i t h the experimental value of 2.55 x l O 1 ^ . There are two u n c e r t a i n t i e s i n t h i s c a l c u l a t i o n of the r a t e .  Cpt), "the c o n c e n t r a t i o n of Pb s i t e s a v a i l a b l e  f o r r e a c t i o n , may be i n c r e a s e d by roughening of the surfa.ce, or reduced by p o i s o n i n g ,  the p a r t i t i o n f u n c t i o n of the  20 Hougan, 0. A., and Watson, K. M., Chemical Process P r i n c i p l e s , P a r t I I , John Wiley & Sons Inc., New York, 194721 K e l l e y , K. K., C o n t r i b u t i o n s to the Data on T h e o r e t i c a l M e t a l l u r g y X, B u l l e t i n 476, U n i t e d States Covernment P r i n t i n g O f f i c e , Washington, 1949.  a c t i v a t e d c o m p l e x , may have a v i b r a t i o n p a r t i t i o n f u n c t i o n o f some m a g n i t u d e ,  b u t p r o b a b l y not g r e a t e r  than  10.  50 Bibliography  G a r r e t , A. B>, V e l l e n g a , S., and Fontana, C M., The S o l u b i l i t y of Red, Y e l l o w and Black Lead Oxides and Hydrated Lead Oxide i n A l k a l i n e S o l u t i o n s , The Character of the Lead-Bearing I o n , J o u r n a l of the American Chemical S o c i e t j 61, 1939, P. 36?. G i l b e r t , P . T., The C o r r o s i o n o f Zinc and Zinc-Coated S t e e l i n Hot Waters, P i t t s b u r g I n t e r n a t i o n a l Conference on Surface R e a c t i o n s , C o r r o s i o n P u b l i s h i n g Company, P i t t s b u r e Pa., 1948. G l a s s t o n e , S., L a i d l e r , K. J . , and E y r i n g , H., A p p l i c a t i o n of the Theory of Absolute R e a c t i o n Rates t o Overvoltage, J o u r n a l of Chemical P h y s i c s , 7, pp.IO53-IO65. L a t i m e r , M. L., The O x i d a t i o n States of the Elements and t h e i r P o t e n t i a l s i n Aqueous S o l u t i o n s , P r e n t i c e - H a . i l I n c . , New Y o r k , I938. L e i d h e i s e r , H., and Gwathmey, A. T., The I n f l u e n c e o f C r y s t a l Face on the E l e c t r o c h e m i c a l P r o p e r t i e s of a S i n g l e C r y s t a l of Copper, T r a n s a c t i o n s E l e c t r o c h e m i c a l S o c i e t y , 91, 1947, P 95. Markhan, A., and Kobe, K., S o l u b i l i t y of Gases i n L i q u i d s , Chemical Reviews, 28, 1941, p. 519. M e l l o r , J . ¥ . , A Comprehensive T r e a t i s e on I n o r g a n i c and T h e o r e t i c a l Chemistry, V o l . V I I , Longmans Green & Co. . L t d . , London, 1927. P a u l i n g , L., The Nature of the Chemical Bond, C o r n e l l U n i v e r s i t y P r e s s , I t h a c a , New York, 194-8. S c o t t , W. W., Standard Methods of Chemical A n a l y s i s , V o l . I , D.'Van Nostrand Co. I n c . , New York, 1939. U h l i g H. H., The C o r r o s i o n Handbook, Sponsored by the E l e c t r o c h e m i c a l S o c i e t y I n c . , John Wiley & Sons I n c . , New Y o r k , 1948.  

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