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The hydrogen precipitation of vanadium from aqueous solutions O'Brien, Robert Neville 1952

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TIB HYDROGEN PRECIPITATION OP VANADIUM FROM AQJMXiJS SOLUTIONS  by ROBERT NEVILLE O'BRIEN  A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n the Department o f Mining and Metallurgy at the University o f B r i t i s h Columbia  We accept this thesis as conforming to the standard required from candidates f o r the degree of "Master of Applied Science"  Members of the Department of Mining and Metallurgy THE UNIVERSITY OF BRITISH COLUMBIA SEPTEMBER, 1952  Acknowledgment The author i s g r a t e f u l f o r the f i n a n c i a l assistance  o f the National Research Council of Canada  which made t h i s work possible. The author i s likewise g r a t e f u l f o r the assistance  and encouragement of the s t a f f of the  Department o f Mining and Metallurgy especially DroJ.HalpeBa whose d i r e c t i o n was invaluable  to t h i s investigation'.  Table of Contents Page ABSTRACT  1  INTRODUCTION  3  A*  Chemistry of Vanadium  3  B«  Extraction of Vanadium from i t s Ores  6  Co  Scope of Present Research  7  EXPERIMENTAL  10  A*  Materials  B»  Equipment  C,  Procedures  10 '  11 12  (i) Analytical  12  ( i i ) P r e c i p i t a t i o n Experiments  12  RESULTS AND DISCUSSION  14  A*  Nature of the Reaction  14  B.  P r e c i p i t a t i o n Rate Curves  18  Ct  Reproduceability of Results  21  D.  E f f e c t of Vanadium Concentration  23  E«  E f f e c t of Catalyst  24  P.  E f f e c t of Hydrogen Pressure  30  G«  E f f e c t of Temperature  35  H.  E f f e c t of Solution Volume  35  I.  E f f e c t of Solution Composition  40  CONCLUSIONS A.  Summary o f Kinetic Data  B.  Mechanism of Reaction  C.  Suggestions f o r Further Research  APPENDIX A. Summary o f K i n e t i c Measurements APPENDIX B> Details o f Typical P r e c i p i t a t i o n Experiments REFERENCES  LIST OF ILLUSTRATIONS Page Figure 1.  Typical Bate Curves f o r the P r e c i p i t a t i o n of  19  Vanadium from Carbonate Solutions., Figure 2.  E f f e c t of I n i t i a l Vanadium Concentration on  20  the Rate of P r e c i p i t a t i o n * Figure 3.  Bate Curves f o r the p r e c i p i t a t i o n of Vanadium  25  i n the Presence of Different Amounts of Nickel Figure 4.  E f f e c t of Nickel Catalyst Concentration on the  27  Bate of P r e c i p i t a t i o n of Vanadium. Figure 5*  E f f e c t of Recycling Catalyst on the Bate of  29  Precipitation. Figure 6.  Bate Curves f o r the P r e c i p i t a t i o n of Vanadium  31  under Different Pressures of Hydrogen Figure 7»  E f f e c t of Hydrogen Pressure on the Bateof  32  P r e c i p i t a t i o n of Vanadium. Bate versus Pressure. Figure 8.  E f f e c t of Hydrogen Pressure on the Bate of  33  P r e c i p i t a t i o n of Vanadium. Bate versus Square Boot of Pressure. Figure 9«  Bate Curves f o r the P r e c i p i t a t i o n of Vanadium at Different  Temperatures.  36  Page Figure 10.  Arrhenius Plot Showing the E f f e c t of  38  Temperature on the Bate o f P r e c i p i t a t i o n of Vanadium. Figure 11.  E f f e c t of Solution Volume on the Bate of P r e c i p i t -  39  a t i o n of Vanadium. Figure 12.  Hate Curves f o r the P r e c i p i t a t i o n of Vanadium from Different  Solutions.  41  ABSTRACTo  It has been found that sodium vanadate i n aqueous solutions, can he reduced by gaseous hydrogen, at temperatures of 3 0 0 «. l+0Q°F and i n the presence of a n i c k e l t  c a t a l y s t , and p r e c i p i t a t e d as an oxide of lower valence. The following reaction has been proposed: 2 TO3- -t 2H  2  —»  V 0 2  3  •+- 20BT +- HgO  The k i n e t i c s of t h i s reaction have been examined, and the following e f f e c t s observed. ( l ) Two d i s t i n c t stages of reduction are involved.In the o v e r a l l reaction, V03~is f i r s t reduced to a  1.  soluble tetravalent ion, believed to be  followed by f u r t h e r  reduction to the t r i v a l e n t oxide which i s p r e c i p i t a t e d .  The  * l a t t e r stage i s rate c o n t r o l l i n g * (2) The k i n e t i c s of the reaction are of zero order, the rate being independent of the  initially  concentration  of vanadium down to a value of one gram per l i t r e .  Below t h i s  *  value the k i n e t i c s become f i r s t  order.  (3) The rate i s d i r e c t l y proportional to the amount of n i c k e l c a t a l y s t present. (4) The rate i s proportional to the square root „ of the pressure of hydrogen. (5) The a c t i v a t i o n energy f o r the reaction i s  7850 c a l o r i e s per mole. The mechanism of the reaction i s discussed i n the l i g h t of these k i n e t i c r e s u l t s and a rate c o n t r o l l i n g step i s proposed consisting of a reaction between VgOip ions.and H atoms, both adsorbed on the surface of the n i c k e l .  Such a  mechanism i s consistent with a l l the k i n e t i c observations.  INTRODUCTION  A  Chemistry o f Vanadium  Vanadium which f i n d s e x t e n s i v e use today, p a r t i c u l a r l y as an a l l o y i n g c o n s t i t u e n t o f s t e e l s ,  i s one o f  the' more r e c e n t metals to have been d i s c o v e r e d and examined. A l t h o u g h r e f e r e n c e s to vanadium can lie found i n the l i t e r a t u r e as e a r l y as 1801. ( l ) , there i s good reason to doubt t h a t the m a t e r i a l s r e f e r r e d to were a c t u a l l y  the metal i t s e l f .  Like  uranium, vanadium i s h i g h l y e l e c t r o p o s i t i v e , and i t s oxides are  reduced to the metal o n l y w i t h g r e a t d i f f i c u l t y .  I t thus  appears that the m a t e r i a l which E e r s e l i u s examined i n 1831 and  /  i d e n t i f i e d with the element vanadium, was a c t u a l l y an oxide of In 1 8 6 7 , Rosiae ( 2 ) , f i n a l l y i s o l a t e d and character-  the metal.  ised the pure metal and established i t s place i n Period IV,Group 5A of the periodic table.  The atomic weight of the element i s  i t s atomic number i s 2 3 , and the electron configuration  50.95.  i s 1 § , 2 § , 2p , 3 B , 3 ? , 4 s , 3 a , ( 3 ) . 2  6  2  2  6  2  3  It appears that i n  addition to the three 3d electrons, the ks electrons i n the ground state can be excited (probably as valence electrons.  to a 3d l e v e l ) and function  The element thus forms compounds i n which  i t exhibits valencies ranging from 2 to 5 «  As expected from the  electron configuration the valencies of 3 and 5 are,the most important. Because of this wide range of valencies, and the fact that compounds of vanadium undergo substantial hydrolysis and polymerization* i n solution, the chemistry of vanadium i s probably more complex than that of any other element. attempt i s made below to review  An  the most important aspects of  this chemistry insofar as i t has been established and applies to the present (i)  investigation ( 4 , 5 ) .  Pentavalent Vanadium Vanadium i s readily oxidised to the pentavalent  state, and i t s pentavalent  compounds are thermodynamically more  stable than those of anyoother valency. :  from the well known oxide ^0^.  These compounds derive  This oxide i s amphoteric and  i s soluble i n both a c i d and a l k a l i n e solutions, but insoluble  i n neutral solutions.  In a l k a l i n e solutions, the oxide i s con-  verted to vanadates, of which several are known, the p r i n c i p a l ones "being meta-, ortho- and pyrovanadates having the forms MVO3, M 3 V U J ; and M2V2O7, respectively, and corresponding to d i f f e r e n t pH ranges.  The orthovanadate ion 70^-, appears to be stable at  high pH values (>12).  In the pH range 12 to 10.6 i t i s converted  to the pyrovanadate ion, V20"p , which predominates down to the pH =  of 9»0.  Below pH  a  9. metavanadates ions of the form VO3"", and  polymerized ions of the form V ^ 0 ^ , etc., are obtained, the =  degree of polymerization increasing as the pH i s lowered.  It  i s l i k e l y that a l l these ions co-exist i n any given solution, and there i s p r a c t i c a l l y no information about the  thermodynamics  or the chemistry of the more complex forms. (ii)  Tetravalent Vanadium The compounds of tetravalent vanadium derives from  the oxide VOj, or as i t i s more generally regarded, V20^.  Very  l i t t l e i s known about this oxide or i t s solutions as these compounds are unstable and readily oxidized to the pentavalent state by atmospheric oxygen.  V2O4 i s also reported as being  amphoteric, d i s s o l v i n g i n a c i d and a l k a l i n e solutions to form vanadites of the form M Vi|0c;. 2  These s a l t s are very soluble i n  water giving r i s e to brown solutions. (iii)  Trivalent Vanadium The compounds of t r i v a l e n t vanadium derive', from  the o x i d e , V2O3, a b l a c k powder o f d i s t i n c t l y b a s i c c h a r a c t e r , i n s o l u b l e i n a l k a l i s but r e a d i l y s o l u b l e i n acids,,  1  :  Very  little j  i s known o f the s o l u t i o n chemistry o f t h i s compound, (iv)  D i v a l e n t Vanadium The oxide VO and o t h e r compounds o f the vandous  s e r i e s a r e so u n s t a b l e t h a t they cannot be i s o l a t e d i n the p r e s e n c e of water o r oxygen. I t s h o u l d a l s o be noted t h a t i n a d d i t i o n to the simple stoichiometric o x i d e s , v'.jO-j, V 0 ^ and V 0 ^ r e f e r r e d t o above 2  2  vandium and oxygen farm a continuous s e r i e s o f compounds o f intermediate composition.  These a r e sometimes c o n s i d e r e d to  be s o l u t i o n s o f the elementary  o x i d e s , a l t h o u g h such a  i" c l a s s i f i c a t i o n appears  to be a r b i t r a r y .  B. E x t r a c t i o n o f Vanadium from i t s Ores. Vanadium g e n e r a l l y occurs i n nature i n t h e form o f p e n t a v a l e n t compounds, the most common m i n e r a l s b e i n g vanad i n i t e , Pb^PbClJVO^; p a t r o n i t e ,  (A.%V)4(3103)  2  V 5> r o s c o e l i t e , HgK (M ,Ee) S  2  and c a r n o t i t e K ° * 3 W Q  2  2  • 2°5* $2$' V  8  C O M M O N  procedures f o r e x t r a c t i n g vanadium from these o r e s g e n e r a l l y i n v o l v e r o a s t i n g o r f u s i n g the o r e w i t h a n a l k a l i such as sodium carbonate, f o l l o w e d by l e a c h i n g .  With some o r e s , n o t a b l y  c a r n o t i t e , l e a c h i n g a l o n e w i t h a carbonate s o l u t i o n c a n b e s  used.(6) When e i t h e r o f these procedures a r e used' the  vanadium i s o b t a i n e d as a sod'ium vanadate a sodium carbonate s o l u t i o n . i n alkaline solutions,  salt, generally i n  S i n c e most vanadates a r e s o l u b l e  i t has always been found n e c e s s a r y t o  n e u t r a l i z e the s o l u t i o n s by a d d i t i o n o f a c i d to a pH o f about 6 i n o r d e r to p r e c i p i t a t e the vanadium as h y d r a t e d V 2 O 5 , r e c o v e r i t from the l e a c h s o l u t i o n .  and  Such a procedure i s c  o b v i o u s l y w a s t e f u l o f reagents ( i e EfaoCC^ &nd B^IQii.) and i s t h e r e f o r e not r e a d i l y a p p l i c a b l e to ores o f low  grade.  C. Scope o f the P r e s e n t R e s e a r c h .  An 'improvement over t h i s method o f p r e c i p i t a t i o n appeared  to be a f f o r d e d by a xreantion d i s c o v e r e d i n t h i s  o r a t o r y , whereby vanadium was  lab-  p r e c i p i t a t e d by r e d u c i n g i t  w i t h gaseous hydrogen from the p e n t a v a l e n t s t a t e to a lower v a l e n c e s t a t e , a p p a r e n t l y i n s o l u b l e i n a l k a l i n e  solutions.  •This procedure i s a m o d i f i c a t i o n o f the method which has been used f o r p r e c i p i t a t i n g nickel,, copper and c o b a l t from ammonia leach solutions.  However whereas w i t h n i c k e l ,  c o b a l t the metal i t s e l f hydrogen  copper  i s produced and p r e c i p i t a t e d  and on  r e d u c t i o n , i n the case o f vanadium which i s more  electropositive,  r e d u c t i o n w i l l not p r o c e e d as f a r as the  m e t a l , and i n s t e a d a lower oxide i s p r e c i p i t a t e d . a d d i t i o n . i t was  In  found that the presence o f a c a t a l y s t  "flffettallic n i c k e l was  such as  necessary f o r the r e d u c t i o n o f vanadium  to proceed w i t h measureable  r a t e s under moderate c o n d i t i o n s o f  8.  temperature and  pressure.  Because t h i s reaction makes use of the p r i n c i p l e of a l t e r i n g only the metal which i t i s desired to p r e c i p i t a t e , rather than the whole s o l u t i o n , during the course of precipita» t i o n , i t o f f e r s obvious advantages over the e x i s t i n g methods of p r e c i p i t a t i n g vanadium from a l k a l i n e leach solutions by neutralizations  The e s s e n t i a l character of the leach solutions  i s not a l t e r e d during p r e c i p i t a t i o n and the solutions can be recycled f o r further leaching*  Such a c y c l i c process, permitting  re-use of leach solutions with low reagent consumption i s e s s e n t i a l i n t r e a t i n g low grade ores. The present research, i n which the k i n e t i c s of the reduction and p r e c i p i t a t i o n of vanadium by hydrogen were i n v e s t i gated, was undertaken with a view to obtaining information about the chemistry and mechanism of the reaction, to determine quantitatively the variables upon which the rate depends, and to evaluate i t s scope and possible applications.  Experiments  were c a r r i e d out i n a high pressure autoclave i n which the temperature and pressure of hydrogen could be controlled* P r e c i p i t a t i o n rate curves were determined by periodic sampling and analysis of the s o l u t i o n .  Variables investigated included  s o l u t i o n composition, vanadium concentration,  c a t a l y s t con-  centration, temperature, and hydrogen pressure.  The r e s u l t s  of the research are presented and discussed i n the present thesis*  EXPERIMENTAL  A.Materials. ( i ) Ammonium Me tavanadate, C P . Grade, was supplied by Brickman and Co., Montreal, Que. and used i n making up the solutions. ( i i ) Nickel Powder, C P . Grade was supplied by Eimer and Amend.  This powder was used as catalyst i n the p r e c i p i t a t i o n  experiments.  Surface area measurements by the B.E.T. method,  kindly carried out by the National Research Council, Ottaw , a  gave an apparent surface area of 0,72 square metres per gram of  nickel.  ( i i i ) Hydrogen and N i t r o g e n Gas, were o f commercial grade and s u p p l i e d i n c y l i n d e r s "by Canadian L i q u i d A i r Co. (iv)  Sodium Carbonate, = "bicarbonate, s u l p h a t e , h y d r o x i d e ,  hydrogen p e r o x i d e and o t h e r c h e m i c a l s used i n p r e p a r i n g and i n the a n a l y t i c a l p r o c e d u r e s were o f C P . (v)  solutions  Grade*  D i s t i l l e d Water was used i n the p r e p a r a t i o n o f a l l  solutions. B.  Equipment* ( i ) Autoclave P r e c i p i t a t i o n experiments were c a r r i e d out i n  a h i g h p r e s s u r e a u t o c l a v e manufactured by A u t o c l a v e E n g i n e e r s I n c o r p o r a t e d , and d e s i g n e d f o r p r e s s u r e s up to 2000 p s i .  The  i n s i d e dimensions o f the v e s s e l were 5 i n c h e s , diameter by h e i g h t , c o r r e s p o n d i n g to a volume o f about one g a l l o n . a u t o c l a v e ^ was  14"  The  equipped w i t h an e x t e r n a l l y d r i v e n s t i r r e r ,  and  thermocouple w e l l , coToling c o i l and sampling tube, a l l connected through the l i d .  P r e s s u r e was measured by a s t a n d a r d gauge.  The a u t o c l a v e was heated e l e c t r i c a l l y The temperature was  heaters.  r e c o r d e d and c o n t r o l l e d by a Leeds and  Northrop Micromax u n i t to (ii)  by e x t e r n a l s t r i p  within±5°E.  Spectrophotometer: A Beckman Model DU Spectrophotometer was used i n  measuring a b s o r p t i o n s p e c t r a and i n c a r r y i n g out determinations. ( i i i ) pH  Meter  vanadium  12  pH measurements and p o t e n t i o m e t r i c were made w i t h a Beckman Model H-2  titrations  pH meter.  C. P r o c e d u r e s (i) Analytical Vanadium was  determined  spectophotometrically  u s i n g the H SO^ - H2O2 method, as g i v e n by S a n d e l l (7). 2  a 5 cc a l i q u o t o f s o l u t i o n b e i n g a n a l y z e d jf> HgOjpand 30 up  to 100  cc o f 10$  cc and  wave l e n g t h o f 460  HgSOij,,  The  To  were added 1 cc o f  r e s u l t a n t mixture was  made  the i n t e n s i t y o f the c o l o u r measured a t a m^vwith a Beckman spectrophotometer.  vanadium c o n c e n t r a t i o n was  The  c a l c u l a t e d from a s u i t a b l e c a l i - '  b r a t i o n graph. . Carbonate.,., b i c a r b o n a t e  and  hydroxyl  i o n concen-  t r a t i o n s were determined by the u s u a l p o t e n t i o m e t r i c prodedures w i t h s t a n d a r d HG1  titration  (8)»  ( i i ) P r e c i p i t a t i o n Experiments A standard prepared  s o l u t i o n o f sodium vanadate  by d i s o l v i n g a weighed amount o f  ffi^VO^in  and' b o i l i n g to d r i v e o f f ammonia i n accordance w i t h following  was  5$  NagCO^  the  equation:  2NB4VO3 + Na2C03 —»> 2 KaV0 -+ M3 3  A l i q u o t s o f t h i s s o l u t i o n were d i l u t e d c o n c e n t r a t i o n o f vanadium and  + H2O + CP2"U)*  to g i v e the d e s i r e d  used i n the  precipitation  13 experiments. The o f s o l u t i o n and placed  charge c o n s i s t i n g , g e n e r a l l y ,  a desired quantity  i n the a u t o c l a v e ,  n i t r o g e n and  which was  heated to the d e s i r e d  charge was  stirred continually.  litres  of n i c k e l c a t a l y s t sealed,  and  A  desired  maintained while  Samples o f the  mixture  were withdrawn a t measured time i n t e r v a l s , g e n e r a l l y 10 minutes.centrifuged vanadium, and  the  i n some i n s t a n c e s generally  n i c k e l and  s o l u t i o n s were a n a l y z e d , f o r f o r CO  =  and  OEP.  The  of  precipitated vanadium,  reaction  and  was  c o n t i n u e d u n t i l s o l u t i o n a n a l y s i s showed t h a t  p r e c i p i t a t i o n was p o i n t s had  to remove the  was  flushed with  temperature.  p r e s s u r e of hydrogen v/as t h e n i n t r o d u c e d the  of 2  complete, o r u n t i l a s u f f i c i e n t  number o f  been o b t a i n e d from which to determine the  of p r e c i p i t a t i o n .  rate  A f t e r each experiment the a u t o c l a v e  c l e a n e d w i t h hydrogen p e r o x i d e to remove any  was  vanadium  p r e c i p i t a t e d on the w a l l s o r i n t e r n a l p a r t s of. the  vessel.  14  RESULTS AMD  A.  Nature o f the  DISCUSSION"  Reaction  Most o f the experiments were made u s i n g s o l u t i o n c o n t a i n i n g 50 as Na VOg.  gm/l  o f NagCO-jand 2 gm/l  Sodium carbonate was  s e l e c t e d as  the  medium, s i n c e i t d i s s o l v e s vanadium r e a d i l y and to work w i t h ,  being  o f Vanadium standard i s convenient  l e s s c o r r o s i v e then most a l k a l i n e media*  I n a d d i t i o n , s i n c e most l e a c h i n g p r o c e s s e s f o r vanadium the use  a  of sodium carbonate s o l u t i o n s , the  results  obtained  by u s i n g t h i s medium, c o u l d be more r e a d i l y e v a l u a t e d o f the proposed a p p l i c a t i o n s .  involve  i n terms  15  The f i r s t effect observed i n reacting such a s o l u t i o n with hydrogen i n the presence of n i c k e l powder at  temperatures of about 300°F, was a marked colour change.  The vanadate solutions, i n i t i a l l y colourless, always turned a deep red brown before p r e c i p i t a t i o n commenced.  Following  this colour change vanadium was p r e c i p i t a t e d as a s o l i d ranging from brown to black i n colour.  As the p r e c i p i t a t i o n  proceeded the i n t e n s i t y of the s o l u t i o n colour decreased, remaining proportional to the vanadium concentration.  The  changes i n the appearance of the mixture during the course of p r e c i p i t a t i o n are shown i n Appendix B. These observations already suggest that the p r e c i p i t a t i o n of vanadium involves two consecutive reactions, as follows: (1) The reduction of vanadate to a soluble i o n of lower valence, (2) the further reduction of t h i s ion to an insoluble compound of s t i l l lower valence, General considerations of the chemistry  of  vanadium would suggest that the intermediate soluble compound trivalento  The following equations are proporsed as being  i n conformity with t h i s scheme, 2V03^rH2 —=>  7 G - -j H 0 2  5  2  (2)  followed by 7  2°5  %  ^>  2 0 H  t 2°3 V  W  The  o v e r a l l r e a c t i o n o b t a i n e d by combining these e q u a t i o n s ,  27O3" + H2 2  —»  V2O3 +  20H" *+ H2O  (4)  An attempt to i d e n t i f y the i n t e r m e d i a t e compound by i t s a b s o r p t i o n spectrum, which was found  to have a peak a t 3150  t h e r e was  Angstroms, was  and  u n s u c c e s s f u l as literature.  S i m i l a r l y , attempts to i d e n t i f y the p r e c i p i t a t e by d i f f r a c t i o n measurements were a l s o u n s u c c e s s f u l o  adium compound, but  coloured  determined  no r e c o r d o f a s i m i l a r spectrum i n the  p a t t e r n s d i d not correspond  is  x^ray The  observed  w i t h those r e c o r d e d f o r any  van-  i t should be p o i n t e d out t h a t such i n -  f o r m a t i o n about vanadium o x i d e s , whose complexity  has a l r e a d y  b  been r e f e r r e d to i s very meagre. • •No  i n f o r m a t i o n about the r e a c t i o n was' 6btained 5  by attempts to measure the o x i d a t i o n p o t e n t i a l o f samples o f the s o l u t i o n , withdrawn a t d i f f e r e n t times d u r i n g course  of p r e c i p i t a t i o n *  to be i r r e p r o d u c i b l e and  The measured p o t e n t i a l s were  found  to v a r y c o n s i d e r a b l y w i t h time and  w i t h atmospheric exposure o f the s o l u t i o n s * significant  the  To  obtain  i n f o r m a t i o n a l o n g these l i n e s i t would appear  to be necessary  to i n c o r p o r a t e an e l e c t r o d e i n t o the  auto-  c l a v e assembly and measure the a c t u a l p o t e n t i a l changes occuning  i n the s o l u t i o n i n the a u t o c l a v e d u r i n g  precipitation.  The  r e s u l t s o f such measurement would be o f g r e a t  interest  and  should throw c o n s i d e r a b l e l i g h t on the n a t u r e  and  thermodynamics of the r e a c t i o n s accompanying the r e d u c t i o n and p r e c i p a t i o n o f vanadium*  However a p p r e c i a b l e e x p e r i m e n t a l  d i f f i c u l t i e s a r e i n v o l v e d , the s o l u t i o n o f which was the scope of the p r e s e n t  investigation*  Some support f o r the suggested  reaction  o b t a i n e d from measurements o f the amount o f OH" precipitation*  I t was  found t h a t one 0H~*  every atom o f V p r e c i p i t a t e d *  was  formed d u r i n g  i o n was  produced  for  T h i s i s i n agreement w i t h  e q u a t i o n 4, and r u l e s out the p o s s i b i l i t y being a vanadite s a l t  beyond  of the p r e c i p i t a t e  r a t h e r t h a n an oxide»  F u r t h e r support f o r e q u a t i o n 4 i s f u r n i s h e d by a v a i l a b l e thermodynamic i n f o r m a t i o n * of  r e a c t i o n f o r the r e d u c t i o n o f V0  lower o x i d e s are l i s t e d below*  The  3  The  s t a n d a r d heats  by hydrogen t o v a r i o u s c a l c u l a t i o n s are  based  on d a t a from Bichowsky and R o s s i n i ( 9 ) * 2703"+  H2  2VO3"" - 2 2 H  2YO3- ~  H  ~*  7204 +-20H**(5);AH°* 23 Keal/mole v  2°3 -  V °2 2  w  H  2°  - 20lT ( 4 ) ; A H ° * 7 Keal/mole  2 2° H  "  2 0 K  *  ( 7 ) ; A H * 74 0  Zeal/mole  Of the t h r e e p o s s i b l e r e a c t i o n s l i s t e d , e q u a t i o n 4, c o r r e s ^ ponding  to the f o r m a t i o n o f ^2^3appears to be thermodyna-*  m i c a l l y the most l i k e l y one h a v i n g the lowest heat o f r e a c t i o n * The o t h e r r e a c t i o n s i n v o l v i n g the f o r m a t i o n o f YgO^pT Y2O2, would r e q u i r e e x c e s s i v e e n e r g i e s and are t h e r e f o r e thermoe  d y n a m i c a l l y u n l i k e l y , even a t the temperatures  and  pressures  a t which the p r e c i p i t a t i o n experiments were c a r r i e d out*. The and  r e a c t i o n s r e p r e s e n t e d "by e q u a t i o n s  k a r e thus c o n s i s t e n t w i t h the changes observed  p r e c i p i t a t i o n , and w i t h the g e n e r a l f e a t u r e s o f the o f vanadium, o u t l i n e d e a r l i e r .  2,  3  during chemistry  However, i t s h o u l d be empha-  s i z e d t h a t the e q u a t i o n s are not c o n s i d e r e d to be  accurate  i n d e t a i l , but o n l y to r e p r e s e n t the g e n e r a l changes i n v a l e n c e and likely  in solubility  involved.  The a c t u a l r e a c t i o n i s  to be more complex i n v o l v i n g s i m u l t a n e o u s l y s e v e r a l  forms o f each i o n , d i f f e r i n g i n degree o f h y d r o l y s i s and p o l y m e r i z a t i o n , as w e l l as a mixture f i e a t i o n s and h y d r a t e s B. P r e c i p i t a t i o n Bate  o f s e v e r a l oxide modi-  ahs- t h e i n f i n a l p r o d u c t , Curves.  T y p i c a l p r e c i p i t a t i o n r a t e curves d e p i c t i n g the course o f p r e c i p i t a t i o n o f vanadium from a sodium car.* bonate s o l u t i o n as a f u n c t i o n o f time, a r e shown i n F i g u r e The  i n i t i a l p a r t o f the r a t e c u r v e s , sometimes preceded  2.  by  a s h o r t lindanetion p e r i o d d u r i n g which the c o l o u r change i n the s o l u t i o n o c c u r s , i s always l i n e a r doxvn to a concent r a t i o n o f vanadium of about 1 gm/l.  T h i s i s r e f e r r e d to  as the zero o r d e r r e g i o n . .As the c o n c e n t r a t i o n o f vanadium i n s o l u t i o n , i s reduced v/l,  f u r t h e r by p r e c i p i t a t i o n , below 1 gnv  the r a t e o f p r e c i p i t a t i o n f a l l s  off progressively,  • f i g u r e 1.  T y p i c a l Bate Curves f o r the P r e c i p i t a t i o n o f vanadium  from Carbonate s o l u t i o n s *  ioo  0 TIME— MINUTES  m  VANADIUM-GRAMS PER LITRE  21  f o l l o w i n g an e x p o n e n t i a l  or f i r s t  order r e l a t i o n i i . e *  o f p r e c i p i t a t i o n i n t h i s r e g i o n i s approximately to the  concentration  the  rate  proportional  o f vanadium remaining i n s o l u t i o n *  All  the r a t e curves o b t a i n e d w i t h sodium carbonate s o l u t i o n s were e s s e n t i a l l y o f t h i s form. C.  Reproducibility  An  example of the r e p r o d u c i b i l i t y o f  the  p r e c i p i t a t i o n r e s u l t s o b t a i n e d i n d i f f e r e n t experiments under the  same c o n d i t i o n s  i s shown i n F i g u r e  are  s h i f t e d s l i g h t l y , due  vanadium c o n c e n t r a t i o n but a r e  and  are l i s t e d  the average r a t e i s £  rate  curves  initial  i n the p r e c i p i t a t i o n s t a r t i n g time,  slopes  i n slope*  o f the  i n Table I« 3-7$  and  zero o r d e r  Precipitation regions  10$*  The  The  mean d e v i a t i o n  considered  reproducibility i s believed  the p r e c i s i o n w i t h which the  the a u t o c l a v e c o u l d be  from  the maximum d e v i a t i o n ^ 11»7$*  On 'the whole the measured r a t e s are  by  two  r a t e curves f o r a group o f experiments made under i d e n t i c a l  conditions,  v/i t h i n  The  to d i f f e r e n c e s i n the  e s s e n t i a l l y s i m i l a r i n shape and  r a t e s , c a l c u l a t e d from the o f the  1.  temperature and  c o n t r o l l e d , by the  reproducible  to  to be l i m i t e d pressure i n  sampling  and  a n a l y t i c a l p r o c e d u r e s , by v a r i a t i o n s i n the d i s p e r s a l of c a t a l y s t caused by  changing s t l m i n g  of v a r i a b l e impurities  r a t e and  i n the system.  The  by  the  rates of  presence catalytic  22:  TABLE I.  Reproducibility of Bate Measurements. I n i t i a l 7 Concentration  ^2 3 C0  Experiment No.  Concentration  - 2.0 gm/l. -  5°  gm/l  Hickel Catalyst  - 10  Temperature  - 300°E.  gm/l  Bate of P r e c i p i t a t i o n " 7 Concentration Deviation from a t Break i n Curve (gm7/l/Min) Mean Rate ( gm/l)  7-60  o.oi35  - 0.7 $  1.05  7-35  0.0140  2.9  0.85  7-59  0.0149  7-47  0.0120  « 11.7  Average  0.0136  3*7$  9.6  r e a c t i o n s a r e known to be s e n s i t i v e to such i m p u r i t i e s . . D« E f f e c t o f Yanadium C o n c e n t r a t i o n .  Rate curves d e p i c t i n g the p r e c i p i t a t i o n of vanadium from a s e r i e s o f s o l u t i o n s , d i f f e r i n g i n vanadium c o n c e n t r a t i o n , are shown i n F i g u r e 2*  initial  The  rates  c a l c u l a t e d from the s l o p e s o f the zero o r d e r r e g i o n s a r e summarized i n Table I I ,  The  f o l l o w i n g e f f e c t s should  be  noted* 1„ The The  initial  rate curves are a l l e s s e n t i a l y  r a t e s range from.0,0135 to 0,0153 gm V / l / m i n .  the v a r i a t i o n s i s c o n s i d e r e d to be wi'thinn the e r r o r and no concluded  parallel.  systematic trend i s r e f l e c t e d *  experimental  Fromthis  t h a t the k i n e t i c s i n t h i s r e g i o n a r e t r u l y  o r d e r , the r a t e b e i n g independent o f the i n i t i a l c o n c e n t r a t i o n , as w e l l as o f the changing  but  i t is zero  vanadium  concentration of  vanadium d u r i n g p r e c i p i t a t i o n . Z.t The  t r a n s i t i o n from zero o r d e r to  o r d e r k i n e t i c s always occurs a t approximately vanadium c o n c e n t r a t i o n o f about one is  important  first  the same  gram p e r l i t r e *  as i t shows t h a t the change i s not  This  r e l a t e d to  the time of p r e c i p i t a t i o n , to the amount o f vanadium p r e c i p i t a t e d , or to the amount of 013" produced i n accordance e q u a t i o n 4, a l l o f which vary w i t h the i n i t i a l concentration.  vanadium  with  2k  E.Effect  of  Catalyst.  Hate c u r v e s d e p i c t i n g  the p r e c i p i t a t i o n o f  vanadium w i t h d i f f e r e n t amounts o f n i c k e l c a t a l y s t p r e s e n t are  shown i n F i g u r e 3»  The r e s u l t s a r e summarized i n  F i g u r e k and Table II»  The r a t e o f p r e c i p i t a t i o n i s seen  to be d i r e c t l y p r o p o r t i o n a l showing was  that  the r e a c t i o n  to the amount o f c a t a l y s t p r e s e n t  is entirely catalytic*  reaction  observed i n the absence o f n i c k e l . An experiment was  the  No  made to determine whether  a c t i v i t y o f the c a t a l y s t or the p r o p e r t i e s  o f the s o l u t i o n .  were a l t e r e d d u r i n g the course o f p r e c i p i t a t i o n *  After  c o m p l e t i o n o f a p r e c i p i t a t i o n experiment the a u t o c l a v e was opened, and without removing more vanadium was t i o n repeated.  dissolved  the  The  the p r e c i p i t a -  curve i s compared to the  two curves are seen to be  s i m i l a r i n shape, except f o r a f l u c t u a t i o n i n  second r a t e curve which appears to have been caused by  a f l u c t u a t i o n i n the s t i r r i n g experiments  The  respectively  f o r the two  rate  or p r e c i p i t a t e ,  i n the s o l u t i o n and  The r e s u l t i n g r a t e  o r i g i n a l one i n F i g u r e 5» essentially  the c a t a l y s t  temperature or p r e s s u r e d u r i n g the  i n i t i a l rates, a r e  curves shows that  0,0135 and 0,0132 grm v / l / m i n  experiments.  The s i m i l a r i t y o f the  two  the f a l l o f f i n r a t e o f p r e c i p i t a t i o n  toward the end o f each experiment, i s not caused by a change i n the a c t i v i t y o f the c a t a l y s t o r by a change i n the  properties  255 Figure 3*  Bate Curves f o r the P r e c i p i t a t i o n o f Vana&itaia i n the Presence o f D i f f erent Amounts of N i c k e l .  0  TIME- MINUTES  26  TABLE I I . E f f e c t of I n i t i a l Vanadium  Concentration  on the Hate of P r e c i p i t a t i o n NagCO-j Concentration - 50 gm/l Nickel Catalyst  - 10 gm/l  Hydrogen Pressure  - 300 p s i  Temperature  -  300°P.  I n i t i a l V Concentration - as given "below. Eperiment No.  I n i t i a l Vanadium gm/l  Rate of P r e c i p i t a t i o n Gm. V/l/min  V-57  1*1  Continually decreasing  V-60  2,02  0.0135  V-65  3.9  0.0153  V-72  5.0  0.0148  f i g u r e 4.  E f f e c t o f K i c k e l Catalyst 0 oncentration on the Eat© ©f P r e c i p i t a t ion of Yanadium.  28  TABLE IIIo E f f e c t of Catalyst on the Bate of P r e c i p i t a t i o n of Vanadium I n i t i a l V Concentration - 2.0 NagCO^Concentration  - 50 gm/l  Nickel C a t l y s t  - as given "below  Hydrogen Pressure  - 300 p s i '  Temperature  -  a  Eperlment Ho.  gm/l  Nickel Catalyst gm/l  300°F  Bate o f P r e c i p i t a t i o n Bate gm/v/l/min Catalyst  5  0.0067  0.00134  V-60  10  0.0135  0.00135  V-69  15  0.0205  0.00137  V-63  20  0.0265  0.00133  Average  0.00135  30  o f the  s o l u t i o n other  which i t has  already  than the  concentration  o f vanadium, to  been r e l a t e d .  Jm. E f f e c t o f Evdroaen P r e s s u r e  The  e f f e c t of hydrogen p r e s s u r e  p r e c i p i t a t i o n o f vanadium was  pounds per  i n Figure pressure The  6, and  square i n c h .  the  investigated i n a series  of experiments i n which the p r e s s u r e to 400  on  was  The  v a r i e d from  r a t e curves a r e  100 shown  the v a r i a t i o n o f the r a t e w i t h hydrogen  i s summarized i n F i g u r e s  r a t e i s seen to be  r o o t o f the p r e s s u r e  7 and  8 and  i n Table  d i r e c t l y p r o p o r t i o n a l to the  o f hydrogen, s u g g e s t i n g  IV.  square  t h a t the  hydrogen  p a r t i c i p a t i n g i n t t h e r a t e c o n t r o l l i n g step i s d i s s o c i a t e d , i n accordance w i t h the f o l l o w i n g e q u i l i b r i u m ,  H2  —>  2 H  00  ±  K  (7)  0  0  *  2  )  (  9  )  T h i s d i s s o c i a t i o n p r o b a b l y o c c u r s on the n i c k e l s u r f a c e , is generally  considered  to be  responsible  f o r the  and  catalytic  a c t i v i t y of n i c k e l i n hydrogenation r e a c t i o n s . Varying  the hydrogen p r e s s u r e  had  no  the shape o f the r a t e curves or on the c o n c e n t r a t i o n vanadium which the was  t r a n s i t i o n from zero  observed to o c c u r *  to f i r s t  e f f e c t on of  order k i n e t i c s  31  -figure 6. Effect of P resuuxej Bate Curves for the Precipitation of Vanadium tinder Different Pressures of Hydrogen,  i  I  '  0  100  TIME- MINUTES  Figure 7.  E f f e c t o f Hydrogen Pressure on the Bate of P r e c i p i t a t i o n of Yanadium 1 ate versus Pressure  IB Figure 8.  Effect of Hydrogen Pressure on the Bate of Precipitation of Vanadium Bate versus Square Boot of Pressure.  34:-  X&BLE IVo E f f e c t of Rvdrogen Pressure on the Rate o f P r e c i p i t a t i o n of Vanadium. I n i t i a l V Concentration - 2.0 gm/l  Experiment Hbe  NagCO^Concentration  - 50 gm/l  Nickel Catalyst  - 10 gm/l  Hydrogen Pressure  — as given "below  Temperature  - 300°P.  Hydrogen Pressure psi  Rate o f P r e c i p i t a t i o n Rate Rate 2. gm V/l/min Pressure (Pressure)  V-44  100  0.00750  0.000075  0.00075  V-62  200  0.0110  0.000055  0.00078  V«60  300  **0.0135  0.000045  0.00078  V«6l  400  *>Qpl56  0.000039  0.00078  Average  0.00077  Gr«-Effect o f  Temperature.  The e f f e c t o f temperature on the p r e c i p i t a t i o n o f vanadium was  i n v e s t i g a t e d i n a s e r i e s o f experiments' i n  which the temperature was v a r i e d from 300  to 400° P.  The  rate  curves a r e shown i n ' F i g u r e 9 and the r e s u l t s a r e summarised i n Table V and F i g u r e 10.  A good ArrheuiuB p l o t was o b t a i n e d ;  the a c t i v a t i o n energy, E , c a l c u l a t e d from the s l o p e o f t h i s p l o t i s 7850, c a l / m o l e .  Many heterogeneous  a c t i v a t i o n energies of this order.  As the temperature  i n c r e a s e d , a p r o g r e s s i v e tendency was from zero to f i r s t  r e a c t i o n s have  noted f o r the  was  transition  o r d e r k i n e t i c s to o c c u r a t h i g h e r c o n c e n t r a ^  t i o n s o f vanadium but the r e l a t i o n c o u l d not be  determined  accurately* H s E f f e c t o f Volume*  The e f f e c t o f changing the volume o f the s o l u t i o n on the r a t e o f p r e c i p i t a t i o n i s shown i n F i g u r e 11* \ When the volume was f  i n c r e a s e d from 2 to 3 l i t r e s  (with a  c o r r e s p o n d i n g i n c r e a s e i n the amount o f c a t a l y s t ) , t h e  rate  curve remained e s s e n t i a l l y unchanged, except that a  small  decrease i n the rate from 0*0135.to 0*0114 gm v/l/min  was  noted.  This i s believed to be due  to the tendency of the  heavy n i c k e l c a t a l y s t to settle,thus f a i l i n g to remain uniformly  dispersed  throughout the l a r g e r volume of s o l u t i o n . •  36 Figure 9.  Bate Curves f o r the P r e c i p i t a t i o n of Vanadium at Different  TIME- MINUTES  Temperatures.  37  TABLE V, E f f e c t of Temperature on the Bate of P r e c i p i t a t i o n of Vanadium^ I n i t i a l V Concentration - 2*0  gm/l  NagCO-j Concentration  - 50 gm/l  Nickel Catalyst  - 10  Hydrogen Pressure  - 300. p s i .  Temperature  - as given below*  gm/l  Bate of P r e c i p i t a t i o n (gm v/l/min)  Experiment No.  Temperature^  V-60  300  0.0135  V-53  325  0.0196  V-79  350  0.0250  T-80  375  d.0315  V-77  400  0.0395  38  39 Figure 11.  Effect of Solution volume on the Bate of Precipitation of Vanadium.  I. E f f e c t o f Solution Composition.  A series of experiments were made to examine  i  the e f f e c t of varying the composition of the s o l u t i o n on the p r e c i p i t a t i o n of vanadium.  Sodium vanadate was dissolved  i n water as well as i n solutions of sodium carbonate,sodium hydroxide, sodium sulphate and sodium bicarbonate.  Comparative  rate curves are shown i n Figure 12 and the r e s u l t s are summarized i n Table VI. Although the rate curves f o r the various s o l u tions showed s i g n i f i c a n t differences both i n shape and slope, the r e s u l t s are d i f f i c u l t to interpret and the variations bear no d e f i n i t e r e l a t i o n to either the pH or the e l e c t r o l y t e content of the s o l u t i o n .  Thus the rates were lowest i n  BaOH solutions ( a 5$ BaOH solution f a i l e d to give p r e c i p i t a tion) and highest i n KaHCO^ solution.  Water and Na S©^ s o l u -  tions,with s t i l l lower pH values, gave intermediate Ha2  c o  2  rates.  3 and Na2 SO4 with widely d i f f e r i n g pH values, hado  nearly i d e n t i c a l rate curves.  The shapes of the rate curves  also varied s i g n i f i c a n t l y , the curves f o r NaH CO-j and water being l i n e a r down to very low vanadium concentrations,  while  that f o r NaOH showing a very marked curvature from the s t a r t . There i s some suggestion that the reaction i t s e l f may not be i d e n t i c a l i n a l l the solutions as evidenced by the f a c t  til Figure 12. Bate Curvee f o r the P r e c i p i t a t i o n o f VamAtua from. Different S o l * utlone*  IOO  0  TIME-MINUTES  TABLE VI. E f f e c t o f Solution Composition on the P r e c i p i t a t i o n of Vanadium. I n i t i a l V Concentration - 2.0 gm/l  Experiment No.  Nickel Catalyst  - 10  Hydrogen Pressure  - 300 p s i  Temperature  - 300  Solution  - as given below.  Composition  Solution Composition Concentration Salt  -  gm/l  °7.  Bate of P r e c i p i t a t i o n gm V/l/min  V-73  None  V-60  NagCOj  50  V-91  BaHC0  50 gm/l  0.0520  V-75  Na S0^  50  8.0160  V-93  NaOH  20 gm/l  Continually decreasing  V-92  NaOH  50 gm/l  No P r e c i p i t a t i o n  2  3  gm/l  gm/l  0,0220  0.0135  that the intermediate colours of the solutions d i f f e r .  This  i s shown i n Appendix B. A further factor which i s known to "be a f f e c t e d by changes i n the nature and concentration i s the s o l u b i l i t y of hydrogen.  of s a l t s ,  It i s l i k e l y that t h i s  accounts i n part f o r the observed variations i n the r a t e . However i t appears that other factors are also involved and i t would c l e a r l y be of interest to investigate .theergactton i n solutions other than carbonate.  CONCLUSIONS  At SW»mrY Qf K i n e t i c Pate* It has teen found that sodium vanadate In carbonate solutions Is reduced by hydrogen gas a t temperatures o f the order o f 300°F, and p r e c i p i t i t a t e d as an oxide of lower valence, i n accordance with a reaction which i s believed to proceed as follows:  2  2 Hg  —* *2°3  +  2 0 f f  * + ^jP  She k i n e t i c s of t h i s p r e c i p i t a t i o n reaction have been examined, and the r e s u l t s described and discussed In the preceding  1'.  sections*  She following effects have been observed, (1) She precipitation reaction appears to  involve two distinct stages of reduction,  YO3*  is first  reduced to a soluble tetravalent vanadium lon, followed by the further reduction of this lon to the trivalent oxide which is precipitated. The f i r s t reaction i s fast and the second or slow stage appears to control the rate of the precipitation process. (2) She kinetics of the reaction are i n i t i a l l y of zero order, the rate being independent of the  concentration  of vanadium In the solution, down to a concentration of about one gram per l i t r e .  Below this value the rate decreases  following approximately a f i r s t order relation. (3) She rate of precipitation i s directly proportional to the amount of nickel catalyst present, confirming the catalytic nature of the reaction.  She activity of the  catalyst apparently remains unchanged during the course of the reaction. (4) She rate of precipitation i s proportional to the square root of the pressure of hydrogen gas and consequently to the square root of the concentration of hydrogen In the solution. Shis indicates that the hydrogen p a r t i c i pating In the .rate controlling step i s i n a dissociated state. (5) She reaction was found to have an activation energy of 7850 calories per mole. She rate doubles with every  46 increase i n temperature of about 60°F i n the range 300 «• 400°F, (6) In accordance with these kinetic results, the rate of precipitation of vanadium i n the zero order region is expressed by the following relation:  - dCvJ m E Org Ql2$ where  (IO)  (J) i s the concentration of vanadium i n the solution  CJiJ i s the surface area of nickel catalyst per unit volume of solution (kg) i s the pressure of hydrogen and  E i s the rate constant of the reaction and i s given by the following expressions I t A  where  e - 7850/ET  (u)  A i s the frequency factor* At 300°F, 300 p s l H ^pressure, and i n the  presence of 10 gm/l nickel catalyst (corresponding to a sur4 9* face area of 7*2 x 10 cur) the rate of precipitation i s 1*35 x lO^gm 7 / 1 / min or 4>4 x 10"^moles 7 / l / sec* constant, k, has the value  The rate  9  E m U35 x 1 0 * mole 7 cm* atm*^" sec" 2  1 1  and  A « 1*4 x IO" mole 7 cm** atm"£" sec" 7  2  1  1  Be Mechanism of the Reaction, Since the f i r s t stage i n the reaction i s fast, the kinetic results can provide no information about i t s  4?  mechanism*  However some conclusions may he drawn regarding  the mechanism o f the second or rate c o n t r o l l i n g step i n the p r e c i p i t a t i o n process, which Aas been postulated to proceed as follows: V  2  0  5  %  E, ^  7 G -t-2QH2  3  It has been shown that the r e a c t i o n i s heterogenous, occur— ing  on the surface o f the n i c k e l catalyst*  General consid-  erations o f the k i n e t i c s of heterogenous reactions, show that the following stages must be Involved ( l l ) * ( i ) D i f f u s i o n o f the reactants i n s o l u t i o n to the catalyst surface* ( i i ) Adsorption o f the reactants on the surface* (111) Reaction on the surface* (iv)  Desorptlon of the products i n t o the s o l u -  t i o n and d i f f u s i o n away from the surface* The k i n e t i c r e s u l t s indicate that s t e p ( l ) above i s not r a t e - c o n t r o l l i n g , a t least i n the i n i t i a l zero order region o f the reaction*  The rate o f d i f f u s i o n of a d i s s o l v e d  reactant to the c a t a l y s t surface i s proportional to I t s concentration i n the s o l u t i o n (by Pick's Law),  and a d i f f u s i o n  c o n t r o l l e d reaction rate would therefore be of f i r s t with respect to one o f the reactants*  order  The observed k i n e t i c s  of p r e c i p i t a t i o n , however, were o f zero order with respect  48  to the concentration o f vanadium and o f one h a l f order with respect to the concentration of hydrogen*  This would appear  to r u l e out the p o s s i b i l i t y o f the d i f f u s i o n step being rate-controlling*  The observed a c t i v a t i o n energy o f 7850  c a l o r i e s per mole i s also considerably i n excess o f that to be expected f o r a d i f f u s i o n process In aqueous s o l u t i o n * S i m i l a r l y i t Is u n l i k e l y that step ( i v ) i n the above sequence, involving the removal o f the r e a c t i o n products from the catalyst surface i s rate c o n t r o l l i n g , as the k i n e t i c s o f t h i s step would be Independent of the pressure or concentration o f hydrogen* E i t h e r o f the remaining stages i n the r e a c t i o n sequence, the adsosptlon o f the reactants o r the r e a c t i o n at the surface may be r a t e - c o n t r o l l i n g *  Both appear to be  consistent with the observed reaction k i n e t i c s * The f i r s t p o s s i b i l i t y i s that the adsorption o f hydrogen i s rate c o n t r o l l i n g *  I t i s known that the rate  of adsorption o f hydrogen on n i c k e l i s proportional to the square root of i t s pressure* kinetics*  This agrees with the observed  At high concentration o f vanadium, the adsorption  o f vanadium i s more |»pid than that o f hydrogen and the rate of r e a c t i o n i s independent of the vanadium concentration* This corresponds to the zero order region*  At lower concen-  t r a t i o n s o f vanadium i t s adsorption may become slower than  49 that of hydrogen and control the rate o f p r e c i p i t a t i o n *  $M<*  would account f o r the observed t r a n s i t i o n from zero to f i r s t order k i n e t i c s at low vanadium concentrations*  However I t  would he expected that the vanadium concentration a t which the t r a n s i t i o n occurs, would Increase with the pressure o f hydrogen*  She r e s u l t s f a i l e d to confirm t h i s * The most l i k e l y r a t e - c o n t r o l l i n g process  thus appears to he step ( i l l ) i n the reaction sequence i s the surface r e a c t i o n i t s e l f *  This Implies that the  adsorption of both vanadium and hydrogen on the n i c k e l surface i s rapid , the adsorption e q u i l i b r i a being maintained throughout the reaction*  The rate o f r e a c t i o n i n this  case, would be proportional to the concentrations of hydrogen atoms and vanadium ions adsorbed on the surface*  She observed  r e a c t i o n order suggests that the surface i s only sparsely covered with hydrogen, so that the concentration of adsorbed hydrogen i s proportional to the square root of the gas pre** ssure*  At the same time, the adsorption of vanadium Ions appears  to increase with the vanadium i n s o l u t i o n up to a concentration o f about one gram o f vanadium per l i t r e , when the surface becomes saturated, and the concentration o f adsorbed vanadium remains constant*  Shis would account f o r the zero order r e a c t i o n k i n e t i c s  and the t r a n s i t i o n to f i r s t order k i n e t i c s a t a given value o f the vanadium concentration*  I t i s a l s o consistent with the  50 observation that t h i s value tends to i n c rease with temperature, since i t i s known that adsorption generally decreases with r i s i n g temperatures, and higher concentrations of vanadium i n s o l u t i o n would be required to saturate the surface. While t h i s explanation of the observed k i n e t i c s , and the assumptions on which I t i s based are reaeeinable, they do not exclude the p o s s i b i l i t y of other with the experimental data*  mechamsasconsistent  Further v e r i f i c a t i o n of the nature  o f the rate c o n t r o l l i n g step by a p p l i c a t i o n of the absolute r e a c t i o n rate t h e o r y ( l l ) , which permits values of the frequency f a c t o r to be calculated and compared with the experimental value, was considered impossible i n this case, i n view of the uncert a i n t i e s surrounding the nature of the reaction and the i d e n t i f i c a t i o n of the reactants and products involved* ff* Suflfiqstftons faff Further ffesear^h.  She research described and discussed i n t h i s t h e s i s , has shown that vanadium can be reduced by gaseous hydrogen i n the presence of a suitable c a t a l y s t , and thereby p r e c i p i t a t e d from a variety of a l k a l i n e solutions*  She general k i n e t i c  features of t h i s p r e c i p i t a t i o n process have been elucidated and i t has been shown how variables*  the rate depends on a number of Important  However the d e t a i l e d course of the reaction, and the  nature of the reactants, intermediates and products Involved have not been f u l l y determined; nor has the mechanism or rate c o n t r o l l i n g step been d e f i n i t e l y established*  She study must therefore be  53.  considered to be of a preliminary nature only and It i s suggested that i t would be of interest to continue It along the following lines* (l) To attempt, by means of more refined physical and chemical measurements (i»e* pH, potentials, absorption  spectra,  x-ray diffraction ;k.) on the solution and precipitates, to identify the reacting ions and the products i n order to establish the exact nature of the reaction* (2) To Investigate the adsorption characteristics of each of the species involved i n the reaction, i n order to obtain information which would be useful In interpreting the kinetic results* (3) To investigate quantitatively the effect of further variables on the kinetics of the reaction*  The  following' would be of particular Interest: pH(using buffered solution); ionic strength, potentials; different catalysts* (4) Using more precise analytical methods, to investigate more fully the kinetics of the reaction at lower vanadium concentrations i.e» i n the f i r s t order region below one gram of vanadium per l i t r e *  The kinetic measurement i n  the present study were largely concerned with the zero order region at higher vanadium concentrations* (5) So extend the investigations to solutions other than carbonate in order to establish the nature and kinetics of the reactions occuring i n different solutions*  5&  (6) fo carry out studies on the chemistry of vanadium and i t s compounds p a r t i c u l a r l y those of lower valence* E x i s t i n g knowledge i n t h i s f i e l d i s very meagre*  53  APPENDIX A Summary of Elastic Measurements* Experiment Solution No*  Initial Nickel 7 Cone Catalyst (gm/l) (gm/l)  Hydrogen Temp Initial Pressure Precipitation (psl) 9f (gm7/l/min)  7-35  5$ BagCO^  2*0  10  300  7-44  5$ NagCO-j  1.98  10  100  300  .0075  7-47  5# Ba C0  3  1*8?  10  300  300  •0120  7-53  5% Na C0  3  1.96  10  300  325  .0196  v-57  5$ Ba C0  3  1.1  10  300  300  V-59  555 BagCOj  2.13  10  300  300  continually decreasing .0149  7-60  5$ BagCO-j  2*02  10  300  300  •0135  7-61  5£ Na C0  1.97  10  400  300  .0156  7-62  5$ Hs^CC^  2*0  10  200  300  •0110  7-63  5# EkgCOj  1*98  20  300  300  •0265  7-64  5^ lagCOj  1.97  5  300  300  .0067  7-65  5$ Na C0  3.9  10  300  300  .0153  7-69  5$ NagCO-j  2.04  15  300  300  •0205  7-72  5# SagC0  5.0  10  300  300  •0143  V-73  2*04  10  300  300  •0220  7-75  S 0 2 5$ Ba S0^  2.07  10  300  300  •0160  V-77  5# Na C03  2*08  10  300  400  .0395  7-79  Na C0-j  2.08  10  300  350  .0250  7-80  5$ Na C0  2.02  10  300  375  .0315  2  2  2  2  2  3  3  3  2  2  2  2  3  300  •0140  5* Experiment Solution Ho* 7-81  5$ JfagCo^  7-82  Initial V Cone (gm/l)  Hieke1 Hydrogen Temp I n i t i a l Catalyst Pressure Precipitation (gm/l) (psi) °F (gm V/l/min)  2*0  10  300  300  •0135  2*0  10  300  300  •0220  7-87  5i BagCOj  1*94  10  300  300  •0114  7-88  5$ Ba2C0  2*11  10  300  300  .0132  7-91  5# BaHCOj  2*0  10  300  300  •0520  7-92  5$ BaOH  1*94  10  300  300  0  V-93  2$ HaOH  2.21  10  300  300  3  continual1/ decreasing  APPENDIX  £  Details of Typical P r e c i p i t a t i o n  Experiments.  P or the purpose of indentifying  the centrifuge  tubes i n the above photograph a numbering system from l e f t to right w i l l he used f o r both the upper and lower rows.  The  tubes and the samples i n them w i l l be referred to as U - l to U-14 f o r the upper row and L - l to 1-14 f o r the lower row. The straight-walled  test tubes w i l l be i d e n t i f i e d only by p o s i t i o n .  56 She color picture was taken of samples (after centrlfuging) of Bun 7-81; a typical standard run In 5$ Ba200jsolution, 300° F, 300 psi hydrogen pressure vith 10 gms/lit re 51 catalyst, and Bun 7-82 performed under the same conditions except i n water solution to show the difference In color change during the reaction due to solution composion  japle No.  Time Taken Minutes  Concentration Oms.y/lltre  U-l  0  2.6  U-2  16  1.90  U-3  30  1.6?  U-4  47i  1.45  U-5  60  1.30  U-6  72^  1.15  U-7  80  1.04  U-8  90  .95  U-9  100  .85  U-10  116  • 72  U-ll  136  .53  U-12  140  .50  u-13  150  .43  U-14  160  .38  57  Sample No.  Time Taken. Minutes  Concentration Gms. V / l i t r e  L~l  0  2.0  L~2  15  2.0  L-3  31  2.0  50  2.0  1-5  60  1.95  L-6  70  1.70  L-7  80  1.49  L-8  92£  1.15  1-9  102^  .82  L-10  ne  .37  L-ll  132£  .08  L~3£  140  •05  L-13  150  .05  L»14  160  .©5  The f i r s t test tube i n the top row from the left is sample U-4 as diluted and treated for analysis.  The  second test tube i n the upper row is sample TJ-14 similarly prepared.  The firsttest tube from the left i n the bottom row  is a 15 ml. aliquot of L - l plus 2 ml. of 3$ HgOg.  The second  is the solution that was diluted to make up the charge used i n these runs.  Its concentration is 50 gms. V / l i t r e at pH 7»1»  BIBLIOGHAPHY li)  von Huobolt, A . , Gehlen*s Journal, 2.695, (1804)  2)  Ho8Coe, H . E . , Philosophical Transactions of the loyal Society  3)  1, (1868)  Bice, O.K., Electronic Structure and Chemical Bonding. McGrav H i l l , (1940)  4)  Sidgvick, XT. 7., The Chemical Elements and Their Compounds Oxford, (1950)  5)  Mellor, J.W., A Somprehenslve Treatise on Inorganic and Theoretical Chemistry, 7ol IX Longmans Green, (1922)  6)  Llddell, D.M., Handbook of Honferrous Metallurgy, 7ol. 2 McGrav H i l l , (1945)  7)  Sandell, S . B . , Colorimetrie Determinations of Traces of Metals, Interscience (1950)  8)  Eblthoff, I . M . , and Sandell, E . B . , Quantitative  Inorganic  Analysis, MacMillan, (1943) 9)  Bichovsky, F.B. and Rossini, P.D., Thermochemistry of Thermochemical Substances, Beinhold Publishing Co. (1940)  10) Seidell, A., Solubilities of Inorganic compounds, Van Uostrand (1940)  -  11) Glasstone, g., Laidler, E . J . and Byring, H . , Theory of Bate Processes, McGrav H i l l (1943)  

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