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Reductive precipitation of molybdenum oxides for recovery of molybdenum from hypochlorite leach solutions Reid, Duncan Craig 1979

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REDUCTIVE P R E C I P I T A T I O N OF MOLYBDENUM OXIDES FOR RECOVERY OF MOLYBDENUM FROM HYPOCHLORITE LEACH SOLUTIONS  by  DUNCAN CRAIG REID B . A . S c , The U n i v e r s i t y o f B r i t i s h M.L.S., The U n i v e r s i t y o f B r i t i s h  Columbia, Columbia,  1972 1974  A THESIS SUBMITTED I N P A R T I A L F U L F I L M E N T OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF A P P L I E D SCIENCE  in  THE FACULTY OF GRADUATE STUDIES (Department o f M e t a l l u r g i c a l E n g i n e e r i n g )  We a c c e p t t h i s  t h e s i s as conforming  to the required  standard  THE U N I V E R S I T Y OF B R I T I S H COLUMBIA S e p t e m b e r 1979 (c)  Duncan C r a i g  R e i d , 1979  In p r e s e n t i n g  t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r  an advanced d e g r e e a t the U n i v e r s i t y of B r i t i s h C o l u m b i a , I a g r e e t h a t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and I f u r t h e r agree that permission f o r s c h o l a r l y p u r p o s e s may by h i s r e p r e s e n t a t i v e s .  for extensive  study.  copying of this thesis  be g r a n t e d by the Head o f my Department o r It i s understood that copying or p u b l i c a t i o n  o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my written  permission.  Department of  Metallurgy  The U n i v e r s i t y o f B r i t i s h Columbia 2075 wesbrook P l a c e V a n c o u v e r , Canada V6T 1W5 Date  October 5,1979  -  (ii)-  ABSTRACT  The  feasibility  o f r e d u c t i v e p r e c i p i t a t i o n o f molybdenum  oxides  as t h e m o l y b d e n u m r e c o v e r y s t a g e o f a h y p o c h l o r i t e l e a c h o f Cu-Mo c o n c e n t r a t e s has been i n v e s t i g a t e d . and  p r e s s u r e and h y d r a z i n e  were used as r e d u c t a n t s .  a t moderate temperature Reduction  5 t o 17 g/1 Mo a s s o d i u m m o l y b d a t e . only i n the presence  at  a n MoO(OH)  acidification  pressure  temperature t o pH = 2.  = 200°C, p r e s s u r e = T e n h o u r s was r e q u i r e d  Reduction  with  hydrazine  p r e c i p i t a t e w i t h 9 0 % r e c o v e r y o b t a i n e d i n 40 m i n u t e s  50°C, pH = 4.5, a n d i n i t i a l  4:1.  and atmospheric  H y d r o g e n r e d u c t i o n was s u c c e s s f u l  o b t a i n 9 0 % r e c o v e r y o f m o l y b d e n u m a s MoC^.  yielded  temperature  was p e r f o r m e d o n s o l u t i o n s c o n t a i n i n g  of a Pt catalyst,  30 a t m o f H. , a n d i n i t i a l to  Hydrogen gas a t e l e v a t e d  rougher  mole r a t i o o f h y d r a z i n e  t o molybdenum o f  P r e c i p i t a t i o n u n d e r t h e same c o n d i t i o n s i n t h e p r e s e n c e  of 3 M  N a C l g a v e o n l y 7 0 % r e c o v e r y i n 4 h o u r s a n d t h e p r e c i p i t a t e c o n t a i n e d 3.3% sodium.  The e f f e c t o f N a C l i s e x p l a i n e d i n t e r m s o f s t a b i l i z a t i o n o f  m i x e d v a l e n t i o n i c molybdenum s p e c i e s i n t h e p r e s e n c e  o f NaCl.  -  ( i i i )-  TABLE OF CONTENTS Page  ABSTRACT TABLE OF CONTENTS  i  L I S T OF TABLES  i  i  i i v  L I S T OF FIGURES  v  ACKNOWLEDGEMENTS  v i  1.  INTRODUCTION  2.  LITERATURE REVIEW  11  2.1  Aqueous C h e m i s t r y o f Molybdenum VI Mo  11  2.1.1 2.1.2  1  Reduced S p e c i e s and P r e c i p i t a t e s  11 13  2.1.2.1  Molybdenum B l u e  14  2.1.2.2  Mo  18  2.1.2.3  Mo  I V  21  2.1.2.4  Mo  1 1 1  22  2.1.3  ,  i  V  Summary  23  2.2  R e d u c t i o n w i t h Hydrogen and Carbon Monoxide  25  2.3  Reduction with Hydrazine  33  2.4  Reduction with S0  2.5  Summary  44  3.  SCOPE OF PRESENT WORK  45  4.  EXPERIMENTAL  46  5.  RESULTS  51  5.1  Hydrogen R e d u c t i o n  51  5.2  Reduction with Hydrazine  54  6.  DISCUSSION  74  7.  CONCLUSION  80  8.  REFERENCES  82  2  and H S 2  43  -  (iv)  -  LIST OF TABLES  Comparison o f l e a c h l i q u o r s o l u t i o n s f o r molybdenum recovery T e c h n i c a l grade MoO^ s p e c i f i c a t i o n s f o r Endako Mines Rate c o n s t a n t s o b t a i n e d f o r d i f f e r e n t h y d r a z i n e t o molybdenum r o l e r a t i o  values of  initial  -  (v)  -  LIST OF FIGURES Page 1. 2. 3.  Proposed f l o w s h e e t s f o r h y p o c h l o r i t e l e a c h i n g o f m o l y b d e n i t e containing concentrates.  3  P r i n c i p a l o p e r a t i o n s and p r o d u c t s i n p r o c e s s i n g o f m o l y b d e n i t e concentrates.  7  P o t e n t i a l - p H e q u i l i b r i u m diagram f o r the system Mo-H 0 a t 25°C.  8  4.  Predominance a r e a diagram f o r Mo^"*" i n 3 N N a C l .  5.  Reduction o f the presence Reduction of and 40 atm H  6. 7.  8.  9.  10.  11.  12.  ammonium paramolybdate s o l u t i o n s by hydrogen i n of a c o l l o i d a l palladium c a t a l y s t . sodium molybdate s o l u t i o n s by hydrogen a t 200°C .  12  26 28  E f f e c t o f v a r i o u s c a t a l y s t s , i n i t i a l p H , and hydrogen p r e s s u r e on hydrogen r e d u c t i o n o f sodium molybdate s o l u t i o n s a t 200°C.  30  R e d u c t i o n o f 17 g/1 sodium molybdate s o l u t i o n by hydrogen a t 200°C and 30 atm i n the presence of Pt c l a d niobium mesh.  52  E f f e c t o f s a m p l i n g t e c h n i q u e on c o n c e n t r a t i o n o f molybdenum r e m a i n i n g i n s o l u t i o n vs time a t 50°C w i t h pH = 4 . 5 , i n i t i a l molybdenum c o n c e n t r a t i o n -5 g / 1 , and i n i t i a l h y d r a z i n e t o molybdenum mole r a t i o - 4 : 1 .  55  Comparison o f r e s u l t s o b t a i n e d by d i r e c t f i l t r a t i o n o f s l u r r y samples w i t h those o b t a i n e d by p r e v i o u s d i l u t i o n o f samples w i t h an e q u a l volume o f c o l d w a t e r .  57  D i s t r i b u t i o n o f molybdenum between s o l u t i o n and p r e c i p i t a t e s as a f u n c t i o n o f time a t 50°C w i t h pH = 4 . 5 , i n i t i a l m o l y b denum c o n c e n t r a t i o n -5 g / 1 , and i n i t i a l h y d r a z i n e to m o l y b denum mole r a t i o s 4 : l .  58  D i s t r i b u t i o n o f molybdenum between s o l u t i o n and p r e c i p i t a t e s as a f u n c t i o n o f time a t 50°C w i t h pH = 4 . 5 , i n i t i a l m o l y b denum c o n c e n t r a t i o n -5 g / 1 , and i n i t i a l mole r a t i o o f h y d r a z i n e to molybdenum = 2 : 1 .  59  -  (vi)  -  LIST OF FIGURES (Continued)  13.  E f f e c t o f temperature on r a t e o f p r e c i p i t a t i o n f o r pH = 4.5 i n i t i a l molybdenum c o n c e n t r a t i o n -5 g / 1 , and i n i t i a l mole r a t i o o f h y d r a z i n e t o molybdenum - 4 : 1 .  14.  E f f e c t o f i n i t i a l mole r a t i o o f h y d r a z i n e t o molybdenum on r a t e o f p r e c i p i t a t i o n a t 50°C w i t h pH = 4.5 and i n i t i a l c o n c e n t r a t i o n o f molybdenum -5 g / 1 .  15.  E f f e c t o f 3 M NaCl on r a t e o f p r e c i p i t a t i o n a t 50°C f o r pH = 4 . 5 , i n i t i a l molybdenum c o n c e n t r a t i o n -5 g / 1 , and i n i t i a l mole r a t i o o f h y d r a z i n e to molybdenum =4:1.  i  16.  E f f e c t o f a d d i t i o n o f 0.55 g/1 Cu as copper s u l f a t e on the r a t e o f p r e c i p i t a t i o n o f molybdenum a t 50°C f o r pH = 4 . 5 , i n i t i a l molybdenum c o n c e n t r a t i o n =5 g / 1 , and i n i t i a l h y d r a z i n e t o molybdenum mole r a t i o -4:1.  17.  1.5 o r d e r i n molybdenum p l o t s f o r T = 5 0 ° C , pH = 4 . 5 , and i n i t i a l molybdenum c o n c e n t r a t i o n =5 g / 1 .  18.  Order i n N^H^ based on 1.5 o r d e r i n molybdenum.  19.  E f f e c t o f temperature on r a t e assuming 1.5 o r d e r i n molybde num f o r pH = 4 . 5 , i n i t i a l molybdenum c o n c e n t r a t i o n -4 g / 1 , and i n i t i a l mole r a t i o o f h y d r a z i n e t o molybdenum - 4 : 1 .  20.  A r r h e n i u s p l o t based on 1.5 o r d e r i n molybdenum.  21.  T h e r m o g r a v i m e t r i c w e i g h t l o s s c u r v e f o r brown p r e c i p i t a t e produced by r e d u c t i o n w i t h h y d r a z i n e .  22.  C o n c e n t r a t i o n s o f h y d r a z i n e and molybdenum r e m a i n i n g i n s o l u t i o n vs time a t 50°C w i t h pH = 4 . 5 , i n i t i a l molybdenum c o n c e n t r a t i o n =5 g / 1 , and i n i t i a l h y d r a z i n e t o molybdenum mole r a t i o - 4 : 1 .  -  (vii);-  ACKNOWLEDGEMENT  I would l i k e t o express my g r a t i t u d e t o D r . Ian H . Warren f o r h i s enthusiasm and s u g g e s t i o n s throughout the course o f t h i s work.  I am  a l s o g r a t e f u l t o the S t e e l Company o f Canada f o r s u p p o r t i n the form o f a S t e l c o Research F e l l o w s h i p . L a s t b u t not l e a s t ,  I would l i k e t o e x p r e s s my a p p r e c i a t i o n  the h e l p and a d v i c e I r e c e i v e d from the F a c u l t y , t e c h n i c a l s t a f f , f e l l o w s t u d e n t s i n the Department o f M e t a l l u r g y .  for and  - 1 -  „1.  INTRODUCTION  C o n v e n t i o n a l t e c h n o l o g y f o r the p r o d u c t i o n o f M0S2  C  O  N  C  E  N  T  R  A  T  E  S  as a b y p r o d u c t from p r o c e s s i n g o f p o r p h y r y copper o r e s i s based on a b u l k o r rougher  f l o t a t i o n f o r the r e c o v e r y o f a copper  s u l f i d e concentrate.  sulfide-molybdenum  T h i s i s f o l l o w e d by d i f f e r e n t i a l f l o t a t i o n t o p r o -  duce s e p a r a t e copper and molybdenum s u l f i d e c o n c e n t r a t e s . c o n c e n t r a t e i s then c l e a n e d by f u r t h e r a b l e M0S2 p r o d u c t . flotation plants  The molybdenum  f l o t a t i o n steps to o b t a i n a market-  A c c o r d i n g t o d a t a p u b l i s h e d by Sutulov"*" f o r  fifty  t r e a t i n g such o r e s the mean r e c o v e r y o f molybdenum i n the  copper rougher c o n c e n t r a t e i s 64% w h i l e the mean o v e r a l l r e c o v e r y i s o n l y 2 50%.  Warren e t a l .  have shown t h a t s e l e c t i v e l e a c h i n g o f molybdenum  from rougher c o n c e n t r a t e s c o u l d be an e c o n o m i c a l l y f e a v o u r a b l e to further  alternative  f l o t a t i o n s t e p s i f the molybdenum r e c o v e r y from the rougher  c e n t r a t e s approached 100% and i f the r e s i d u e was s u i t a b l e o f a copper c o n c e n t r a t e by f l o t a t i o n .  con-  for production  They proposed a sodium h y p o c h l o r i t e  l e a c h f o r t r e a t m e n t o f rougher c o n c e n t r a t e s t h a t i n v o l v e d o n - s i t e  electro-  l y t i c g e n e r a t i o n o f sodium h y p o c h l o r i t e u s i n g c o m m e r c i a l l y a v a i l a b l e h y p o chlorite cells.  A s i m i l a r approach has been proposed by workers a t 3—8  U . S . Bureau o f Mines (USBM). would be a p p l i e d t o the o f f - g r a d e  the  I n t h e USBM p r o c e s s h y p o c h l o r i t e l e a c h i n g molybdenum s u l f i d e c o n c e n t r a t e produced  by d i f f e r e n t i a l f l o t a t i o n o f the i n i t i a l orugher c o n c e n t r a t e .  This process,  t h e n , would r e p l a c e o n l y t h e c l e a n i n g s t a g e s o f p r o d u c t i o n o f an MoS~  - 2 -  concentrate  w h i l e the p r o c e s s proposed by Warren e t a l . would a l s o  the d i f f e r e n t i a l f l o t a t i o n s t e p .  A further  replace  d i f f e r e n c e between the two  p r o p o s a l s i s t h a t the USBM p r o c e s s i n v o l v e s g e n e r a t i o n o f sodium h y p o c h l o r i t e " i n - s i t u " by e l e c t r o l y s i s o f a b r i n e s l u r r y o f the c o n c e n t r a t e  as  it  i s pumped t h r o u g h a s p e c i a l l y d e s i g n e d e l e c t r o l y t i c c e l l . The USBM p r o c e s s has been p i l o t e d on c o n c e n t r a t e s t o 35% Mo and 6 t o 15% C u . t r e a t concentrates  c o n t a i n i n g 16  Warren e t a l . have d e s i g n e d t h e i r p r o c e s s  c o n t a i n i n g a p p r o x i m a t e l y 0.3% Mo and 12% C u .  b a s i c f l o w sheets f o r b o t h p r o c e s s e s c e s s e s use Na^CO^ f o r pH adjustment.  are compared i n f i g u r e 1.  The Both p r o -  Warren e t a l . propose t o c a r r y out  l e a c h i n g a t pH 9 w h i l e the USBM p r o c e s s o p e r a t e s a t pH 5.5 t o 7. l e a c h temperature f o r b o t h p r o c e s s e s  + MoS  2  + 60H  The  i s 45 - 5 0 ° C .  The s t o i c h i o m e t r y o f the l e a c h can be r e p r e s e n t e d  9 OCl  to  — •+ Mo0  24  by the  equation:  2+ 9C1 + 2 S 0 + 3^0 4  w h i c h shows t h a t 1 kg o f NaOCl s h o u l d be consumed t o b r i n g 0.143 kg o f Mo into solution.  A c c o r d i n g t o Warren e t a l . modern h y p o c h l o r i t e g e n e r a t o r s  r e q u i r e 3.52 kwh p e r kg NaOCl.  I t follows that i n t h e i r process  energy g  consumption s h o u l d be 24.5 kwh p e r kg Mo.  In a recent p i l o t plant  the USBM p r o c e s s r e q u i r e d 19.8 t o 28.6 kwh p e r kg Mo t o t r e a t c o n t a i n i n g 16 t o 35% Mo and 6 t o 15% C u .  study  concentrates  The molybdenum r e c o v e r y  decreased  from 93 t o 97% t o o n l y 75% when the copper c o n t e n t i n c r e a s e d t o 15% and the power consumption c o r r e s p o n d i n g l y i n c r e a s e d t o the 28.6 kwh v a l u e . d e c r e a s e i n molybdenum r e c o v e r y was a t t r i b u t e d ble  copper m o l y b d a t e s .  to the formation o f  The insolu-  C u r r e n t r e s e a r c h i n t h i s l a b o r a t o r y has shown,  however, t h a t the e f f e c t o f copper m i n e r a l s on the molybdenum r e c o v e r y i s  Cu-Mo ROUGHER CONCENTRATE OFF-GRADE MOLYBDENITE CONCENTRATE  No CO,  residue to copper flotat ion  LEACH  <  2  3  Na C0 2  I  copper product  3  ELECTROOX I DATION  NaOCl recycle  COPPER REMOVAL  j  brin e recycle H S0 2  to waste SO. ACI DI FICATION  4  ACIDIFICATION Na C0 2  Mo product •<  3  SULFATE REMOVAL 2  2  SOLVENT EXT RACTION  Mo RECOVERY Na S0  No S0  NH Re0  CARBON ADSORPTION  4  4  solution  EVAPORATION 4  CRYSTALLIZATION NaOCl REGENERATION WARREN et al  F i g u r e 1:  RESIDUE  Proposed f l o w s h e e t s  (NH ) Mo 0 4  6  7  USBM  f o r h y p o c h l o r i t e leaching o f molybdenite c o n t a i n i n g  f 2 4  -4H 0 2  PROCESS concentrates.  4  - 4 -  more complex than t h i s .  A t the pH o f t h e USBM l e a c h , f o r example, i t  l i k e l y t h a t s i g n i f i c a n t d e c o m p o s i t i o n o f NaOCl t o NaClO^ o c c u r s .  is  Since  NaClO^ does n o t o x i d i z e MoS^ a t t h i s pH such d e c o m p o s i t i o n reduces the e f f i c i e n c y o f the l e a c h .  energy  The use o f c a r b o n a t e f o r pH c o n t r o l has a l s o been 9  shown t o be an i m p o r t a n t f a c t o r i n i n c r e a s i n g molybdenum r e c o v e r y . Research i n t h i s a r e a i s c o n t i n u i n g . formation, then,  On the b a s i s o f the a v a i l a b l e i n -  i t seems t h a t t h e p r o c e s s proposed by Warren e t a l .  offers  a h i g h e r r e c o v e r y o f molybdenum w i t h a b e t t e r l e a c h i n g energy e f f i c i e n c y than the USBM p r o c e s s . Warren et a l . c a l c u l a t e d the approximate c o m p o s i t i o n o f the  leach  s o l u t i o n t h a t would r e s u l t on a p p l i c a t i o n o f t h e i r p r o c e s s t o a rougher c o n c e n t r a t e c o n t a i n i n g 0.3% Mo.  T a b l e I compares t h i s w i t h the c o m p o s i t i o n  r e s u l t i n g from a p p l i c a t i o n o f t h e USBM p r o c e s s t o a m o l y b d e n i t e I t can be seen t h a t the two s o l u t i o n s a r e s i m i l a r .  I t would t h u s be  p o s s i b l e t o a p p l y the USBM s o l v e n t e x t r a c t i o n - c a r b o n a d s o r p t i o n for  concentrate.  sequence  r e c o v e r y o f molybdenum and rhenium t o Warren e t a l . ' s p r o c e s s .  disadvantage  o f the USBM p r o c e d u r e however, i s the n e c e s s i t y t o  the s o l u t i o n t o pH< 2 f o r molybdenum e x t r a c t i o n by a t e r t i a r y  A  acidify 4  amine.  A p r o c e s s c a p a b l e o f r e c o v e r i n g molybdenum from a l k a l i n e s o l u t i o n s would be p r e f e r a b l e  f o r a h y p o c h l o r i t e l e a c h o p e r a t i n g a t pH = 9.  A l t e r n a t i v e means o f molybdenum r e c o v e r y have been s u g g e s t e d . i n c l u d e r e d u c t i o n o f molybdate s o l u t i o n s w i t h hydrogen a t e l e v a t e d t u r e and p r e s s u r e t o produce i n s o l u b l e MoO^®  ^  These tempera-  and r e d u c t i o n w i t h i r o n  f o l l o w e d by n e u t r a l i z a t i o n t o p r e c i p i t a t e MoCOH)^.''" As d e s c r i b e d i n the 6  literature,  however, b o t h o f the p r o c e s s e s o p e r a t e i n a c i d s o l u t i o n .  The f e a s i b i l i t y o f a l t e r n a t i v e  r o u t e s t o molybdenum r e c o v e r y from  - 5 -  Warren et  al.  USBM  H 0  1100 kg  900 kg  NaCl  170 kg  100 kg  68 kg  37 kg  10 kg  4 - 43 kg  2  Na S0 2  4  Na Mo0 2  4  Cu  variable  NaOCl„ 3  nil  NaCIO „ 4  5 - 8 kg  5 - 13 kg .14 - .72 kg  Re pH  Table I  .001 - .050 kg 9  5 - 7  Comparison o f l e a c h l i q u o r f o r molybdenum  recovery  solutions  - 6-  l e a c h s o l u t i o n s depends on the economics o f the u n i t o p e r a t i o n s  involved  and on the m a r k e t a b i l i t y o f the r e s u l t i n g p r o d u c t .  the  In a d d i t i o n  a b i l i t y o f a g i v e n p r o c e s s t o r e c o v e r rhenium c o u l d be a d e c i d i n g  factor  i n cases where the c o n c e n t r a t e t o be t r e a t e d c o n t a i n e d a s i g n i f i c a n t amount o f rhenium. C o n v e n t i o n a l l y almost a l l molybdenum s u l f i d e c o n c e n t r a t e s roasted  t o produce a t e c h n i c a l grade MoO^.  The p r i n c i p a l use o f  p r o d u c t i s the p r o d u c t i o n o f c a l c i u m molybdate, m o l y b d i c o x i d e o r ferromolybdenum f o r a l l o y s t e e l p r o d u c t i o n . a l s o be f u r t h e r  are this  briquets,  T e c h n i c a l grade MoO^ can  p u r i f i e d by s u b l i m a t i o n t o y i e l d pure MoO^ o r by h y d r o -  m e t a l l u r g i c a l means t o produce ammonium molybdate o r sodium molybdate (Figure 2 ) .  A h y d r o m e t a l l u r g i c a l t r e a t m e n t o f MoS^ c o n t a i n i n g c o n c e n t r a t e s  s h o u l d a t l e a s t be c a p a b l e o f p r o d u c i n g a p r o d u c t o f e q u a l q u a l i t y t o : t e c h n i c a l grade MoO^. specifications  By way o f example T a b l e I I shows t e c h n i c a l grade MoO^  f o r Endako Mines L t d . H y d r o m e t a l l u r g i c a l p r o d u c t i o n o f a  molybdenum o x i d e , sodium molybdate, p u r i t y d i r e c t l y from an o f f - g r a d e  o r ammonium molybdate p r o d u c t o f h i g h  o r rougher c o n c e n t r a t e c o u l d ,  however,  e l i m i n a t e n o t o n l y r o a s t i n g b u t a l s o s u b l i m a t i o n o r o t h e r subsequent p u r i f i c a t i o n steps.  The a b i l i t y t o produce such a p r o d u c t c o u l d i n f l u e n c e  the economic f a v o u r a b i l i t y o f a h y d r o m e t a l l u r g i c a l r o u t e . The p o t e n t i a l - p H diagram f o r the molybdenum water system p u b l i s h e d 18 by P o u r b a i x  ( F i g u r e 3) i s r e l a t i v e l y s i m p l e and i n d i c a t e s  that  p r e c i p i t a t i o n o f MoO^ c o u l d be p o s s i b l e o v e r a wide range o f p H . r e d u c i n g agents might be c o n s i d e r e d . and h y d r a z i n e .  reductive Several  These i n c l u d e E^t S C ^ , H^S, CO, i r o n  The gaseous r e a g e n t s and h y d r a z i n e are a t t r a c t i v e  they o f f e r the p o s s i b i l i t y o f p r o d u c i n g a h i g h p u r i t y  product.  because  MoS. concentrate  _*-MoS I u b r i cants 2  i  2  ROAST  technical  \  t—~  calcium molybdate  oxide brique t s  DISSOLUTION 8  r  T  ferromolybdenu m  grade M o o ^ }  ceramics  SUBLIMATION  ;  pigments  chemicals  ?  chemicals  }  direct addition to ste el  DISSOLUTION  i  8  pure  MoOj  coatings  1  ammonium molybdate  T  chemical reagent  REDUCTION molybdenum metal F i q u r e 2;  :  CRYSTALLIZATION  CRYSTALLIZATION sodium molybdate  f  fertilizer  P r i n c i p a l o p e r a t i o n s and p r o d u c t s i n p r o c e s s i n g o f m o l y b d e n i t e c o n c e n t r a t e s .  catalys  - 8 -  - 9 -  OXIDE Range % Mo  57.0  Typical %  - 62.0  59.0  Guaranteed % 57.0 m i n .  Cu  0.05 - 0.10  0.075  0.10 max.  S  0.03 - 0.10  0.06  0.10 max.  0.01  0.05 max.  P  —  Pb  0.015 - 0.050  0.025  0.05 max.  Bi  0.02 - 0.04  0.030  0.05 max.  0.030  0.16 max.  wo  3  sio  5.0 - 15.0  8.0  Fe  0.020 - 0.45  0.35  —  CaO  0.050 - 0.120  0.07  —  2  15.0  max.  OXIDE BRIQUETTES (PITCH) Range %  Typical  g, "o  Guaranteed %  Mo  50.6 - 5 4 . 0  53.0  51.6 m i n .  C  10.0  11.00  12.0  - 15.0  approx.  Cu  0.05 - 0.15  0.075  0.15 max.  S  0.09 -  0.11  0.15 max.  0.01  0.05 max.  P  0.15  —  Bi  0.02 - 0.04  0.03  —  Fe  0.20 - 0.40  0.29  —  Pb  0.015 - 0.050  0.025  —  Table I I  T e c h n i c a l grade Mo0_ s p e c i f i c a t i o n s f o r Endako Mines  - 10 -  D e s p i t e the apparent s i m p l i c i t y o f the P o u r b a i x d i a g r a m , the aqueous c h e m i s t r y o f molybdenum i s complex.  however,  There i s , i n f a c t ,  a  r i c h c h e m i s t r y o f p o l y m e r i c molybdates and reduced molybdenum s p e c i e s t h a t has r e s u l t e d i n an e x t e n s i v e and o f t e n c o n t r a d i c t o r y l i t e r a t u r e . review of t h i s l i t e r a t u r e  i s necessary  as a b a s i s f o r c o n s i d e r i n g  A  reductive  p r e c i p i t a t i o n as a means o f molybdenum r e c o v e r y from a l k a l i n e s o l u t i o n s .  - 11 -  2. 2.1 2.1.1  Mo  LITERATURE REVIEW  Aqueous C h e m i s t r y o f Molybdenum  VI  VI A h y p o c h l o r i t e l e a c h o f MoS^ produces Mo i n s o l u t i o n which f o r 2pH's  greater  than about s i x e x i s t s as the monomeric molybdate i o n MoO^  On a c i d i f i c a t i o n ,  however, molybdate i o n s p o l y m e r i z e consuming H  produce i s o p o l y m o l y b d a t e s such as the paramolybdate i o n Mo^O ^  +  .  to Several  i s o p o l y m o l y b d a t e s have been r e p o r t e d and the l i t e r a t u r e has been r e v i e w e d 19 by S a s a k i and S i l l e n .  They c h a r a c t e r i z e d i s o p o l y m o l y b d a t e s i n terms o f  t h e i r a c i d i t y , z , d e f i n e d as moles H date  formed a c c o r d i n g t o the 8H + 7Mo0 ~ = +  2  4  +  bound p e r mole Mo.  equation, H (Mo0 ) ~ 6  8  4  ?  =  Thus p a r a m o l y b -  Mo O ^ " , 6  can be r e p r e s e n t e d by z = 8/7 = 1.14. I t i s g e n e r a l l y a c c e p t e d t h a t paramolybdate i s the f i r s t  species  to form on a c i d i f i c a t i o n o f molybdate s o l u t i o n s and t h a t i t i s the o n l y i s o p o l y a n i o n , o r a t l e a s t by f a r p r e d o m i n a n t , up t o z = 1.14 (pH - 5 ) .  20-22  The n a t u r e o f the p o l y m o l y b d a t e s formed on f u r t h e r  must be r e g a r d e d as u n c e r t a i n s i n c e t h e r e i s some disagreement  acidification i n the  19 23-25 literature. ' F i g u r e 4 shows a predominance a r e a diagram c o n s t r u c t e d 22 19 by Baes who a c c e p t e d S a s a k i and S i l l e n ' s sequence o f i s o p o l y m o l y b d a t e s . For a c i d i f i c a t i o n up t o z = 1.5 i t has been shown t h a t i s o p o l y m o l y b date e q u i l i b r i a are a l m o s t i n s t a n t a n e o u s tions.  F o r z S- 1.5,  f o r moderate molybdenum c o n c e n t r a -  however, where p o l y m e r i z a t i o n proceeds beyond the 24  heptamer o r octamer s t a g e , A v e s t o n e t a l . only slowly  established.  noted t h a t the e q u i l i b r i a  are  - 13 -  The  formation  o f isopolymolybdates  i ssignificant  for recovery of  molybdenum f r o m p r o c e s s s o l u t i o n s b e c a u s e b o t h s o l v e n t e x t r a c t i o n and r e c d u c t i o n r e a c t i o n s appear t o i n v o l v e o n l y polymerized  molybdenum  species.  30 Chariot  has s t a t e d , f o r example, t h a t monomeric m o l y b d a t e i o n i s r e d u c e d  infinitely  slowly i n a l k a l i n e solution.  to confirm  t h i s w i t h Mo  VI  Polarographic  s t u d i e s have t e n d e d  . . , . ^ ^ ,.31-33 g i v i n g no r e d u c t i o n w a v e s f o r pH > 5  Similarly the necessity for acidification  t o pH < 2 f o r s o l v e n t e x t r a c t i o n  i n t h e USBM p r o c e s s i n d i c a t e s t h a t t h e e x t r a c t e d  species  involves  m o l y b d a t e s o r e v e n c a t i o n i c molybdenum s p e c i e s .  A r e v i e w o f molybdenum  s o l v e n t e x t r a c t i o n by Z e l i k m a n c o n f i r m s  t h a t e x t r a c t i o n i s most  i n a c i d s o l u t i o n and t h a t t h e e x t r a c t e d  species  isopoly-  efficient  are isopolymolybdates  or  34 cations. 2.1.2  Reduced S p e c i e s For  and P r e c i p i t a t e s  l a c k o f thermodynamic data  Pourbaix  was a b l e  to consider  only  3+ Mo, Mo  , a n d M0O2 i n h i s d i a g r a m .  v a l e n t molybdenum b l u e  The p r e d o m i n a n c e a r e a  o f t h e mixed-  compounds c o u l d o n l y b e shown a p p r o x i m a t e l y  on q u a l i t a t i v e o b s e r v a t i o n s . V IV III Mo , Mo , a n d Mo species 35  c a n be p r e p a r e d and a r e s t a b l e i n aqueous  solution.  o f each o f these valence  Hydrated oxides  c i p i t a t e d i n appropriate  I t i s now w e l l r e c o g n i z e d ,  pH i n t e r v a l s .  Some p r o g r e s s  i n t h e c h a r a c t e r i z a t i o n o f t h e molybdenum  however,  based that  s t a t e s c a n be p r e h a s a l s o b e e n made  blues.  VI L i k e Mo polymerization. reported blues  t h e r e d u c e d m o l y b d e n u m a q u o - i o n s show a t e n d e n c y B o t h Mo " " a n d M o 11  1  t o form dimers, tetramers,  a r e undoubtedly polymeric  I V  c a n form d i m e r s ^ and Mo and h i g h e r  since their  polymers.  formation  V  towards  has been  The m o l y b d e n u m  a t a g i v e n pH i s  - I n -  dependent on molybdenum c o n c e n t r a t i o n . are o f i n t e r e s t  i n the p r e s e n t study and the c h e m i s t r y o f each i s b r i e f l y  r e v i e w e d i n the f o l l o w i n g 2.1.2.1  The reduced molybdenum s p e c i e s  sections.  Molybdenum B l u e M i l d r e d u c t i o n o f molybdate s o l u t i o n s i n the pH range 4 t o 0 y i e l d s  more o r l e s s i n t e n s e l y c o l o u r e d b l u e s o l u t i o n s .  If reduction  proceeds  beyond the optimum f o r f o r m a t i o n o f the b l u e the c o l o u r d e n s i t y d e c r e a s e s and the c o l o u r may change from b l u e t o green o r brown. many a t t e m p t s t o c h a r a c t e r i z e  There have been  the b l u e c o l l o i d a l p r e c i p i t a t e s  j * i, 38-42 p r e p a r e d from such s o l u t i o n s . .  37  38 Glemser and L u t z n  n  t h a t can be  • . and S a c c o n i and  . 39  Cmi  c o n c l u d e d t h a t molybdenum b l u e was n e i t h e r a unique compound nor  d i d i t r e p r e s e n t a d e f i n i t e o x i d a t i o n s t a t e o f molybdenum.  This conclusion  seems t o have been g e n e r a l l y a c c e p t e d and i s s t a t e d i n s e v e r a l works on 20,21,43 44 inorganic chemistry. On the o t h e r hand W e i s e r has p o i n t e d out t h a t the e v i d e n c e f o r the e x i s t e n c e o f d i f f e r e n t l a r g e l y on a n a l y t i c a l d i f f e r e n c e s  compounds was based  o f the same o r d e r o f magnitude as  experimental errors inherent i n analyzing a c o l l o i d a l 41 A r n o l d and Walker  the  mass.  avoided a n a l y z i n g a c o l l o i d a l p r e c i p i t a t e  by  e x t r a c t i n g the b l u e i n t o b u t a n o l and d e t e r m i n i n g the mean o x i d a t i o n s t a t e o f molybdenum by p o t e n t i o m e t r i c  titration.  MOg0^ w h i c h agreed w i t h the e a r l i e r r e s u l t s 42 7  Ostrowetsky V s o l u t i o n s .of Mo  They o b t a i n e d the  formula  o f T r e a d w e l l and S c h a e p p i .  40  s t u d i e d the f o r m a t i o n o f molybdenum b l u e by m i x i n g VI  and Mo  i n v a r y i n g r a t i o s and a t v a r y i n g p H ' s .  molybdenum i n s o l u t i o n was a l s o v a r i e d . a n a l y z e d by s p e c t r o p h o t o m e t r y ,  Total  The f o r m a t i o n o f the b l u e was  by i s o l a t i o n o f i t s r u b i d i u m s a l t ,  and by  - 15  potentiometry.  -  E l e c t r o l y t i c p r e p a r a t i o n o f the b l u e gave the  Optimum c o n d i t i o n s f o r f o r m a t i o n o f t h e b l u e w e r e pH and  [Mo], , = 0.015 total  M.  The  blue  s p e c i e s was  = 1.22,  formulated  same Mo  V  results. VI  /Mo  as  O VI Mo.  The  corresponding  a c i d , H Mo 2.  W a l k e r and  Treadwell  tion inhibited  and  0  bio  , agreed  Schaeppi.  formation of the  =  0.5,  V Mo, 18  w i t h the r e s u l t s of Arnold  D e c r e a s e o f t o t a l molybdenum  and  concentra-  blue.  I f a s o l u t i o n o f the b l u e , under t h e optimum c o n d i t i o n s f o r i t s f o r m a t i o n m e n t i o n e d a b o v e , was green, and  t h e n brown.  The  t i t r a t e d w i t h NaOH t h e  b r o w n s o l u t i o n was  i t s f o r m a t i o n from the b l u e w r i t t e n  found  to correspond  VI Mo,  +  °18  \ MoY 0 „ 2 4 8  o  +  80H~  V°18 Mo.  +  H  2H 0 2  V The  b r o w n s o l u t i o n c o u l d a l s o be p r e p a r e d  Maximum y i e l d was  pH  o b t a i n e d a t pH  =  d i r e c t l y b y m i x i n g Mo  VI and  VI Mo,  > 4,  however, i t s c o n c e n t r a t i o n decreased  t a t e o f MoO(OH)^ was The  work o f O s t r o w e t s k y  F o r pH  and  a t pH  °17  at  H  = 4.5  a  precipi-  observed.  c o l o u r changes observed 0 t o 4.5.  Mo  2.7.  A t h i r d m i x eV d compound, a l s o b r o w n , was f o u n d t o be f o r m e d VI = 3 t o 4 a n d Mo /Mo =2.0. I t was f o r m u l a t e d a s Mo.V  F o r pH  218  2-  VI Mo. 4 +  t o HMo.O  as  2-  V Mo,  s o l u t i o n became  permits  a logical  interpretation of  the  o n r e d u c t i o n o f m o l y b d a t e s o l u t i o n s i n t h e pH  < 0 t h e b l u e compound c a n n o t f o r m a n d  no  range  c o l o u r change i s  - 16 -  o b s e r v e d u n t i l the Mo  stage o r l o w e r i s a t t a i n e d .  F o r pH between 0 and  4 the f i r s t c o l o u r t o appear i s b l u e f o r low v a l u e s o f M o / M o . V  As  V I  r e d u c t i o n proceeds the s o l u t i o n becomes green o r brown as the two brown Mo /Mo V  species form.  V I  The s o l u t i o n p r e c i p i t a t e s MoO(OH)  i s approached more c l o s e l y .  as the M o stage V  I f a s o l u t i o n c o n t a i n i n g any o f the mixed  v a l e n t s p e c i e s i s n e u t r a l i z e d MoO(OH)  i s p r e c i p i t a t e d and molybdates  are  formed i n s o l u t i o n . I n a d d i t i o n t h e mixed v a l e n t s p e c i e s can c o e x i s t i n v a r i o u s p r o p o r VI t i o n s i n s o l u t i o n w i t h v a r y i n g amounts o f e x c e s s Mo the p a r t i c u l a r c o n d i t i o n s .  V o r Mo  depending on  T h i s p r o b a b l y e x p l a i n s the range o f mean o x i -  d a t i o n s t a t e s f o r molybdenum o b t a i n e d by d i f f e r e n t workers u s i n g  different  means o f p r e p a r a t i o n o f the c o l l o i d a l b l u e . P a r t i a l c o n f i r m a t i o n o f O s t r o w e t s k y ' s work was p r o v i d e d by F i l i p p o v 45 and Nuger  who o b s e r v e d t h a t the c h a r a c t e r and i n t e n s i t y o f t h e molybdenum  b l u e spectrum o b s e r v e d d u r i n g r e d u c t i o n o f m o l y b d i c a c i d by h y d r a z i n i u m c h l o r i d e v a r i e d a c c o r d i n g t o t h e p H a t w h i c h the r e d u c t i o n was p e r f o r m e d . The maximum absorbence o b s e r v e d  by F i l i p p o v and Nuger o c c u r r e d a t pH = 1.31  which c o r r e s p o n d s t o the optimum pH f o r f o r m a t i o n o f the b l u e compound proposed by O s t r o w e t s k y .  As the pH o f r e d u c t i o n was i n c r e a s e d the  ence due t o f o r m a t i o n o f the b l u e d e c r e a s e d .  absorb-  U n f o r t u n a t e l y F i l i p p o v and  Nuger d i d n o t r e c o r d the s p e c t r a i n the r e g i o n o f absorbence o f the two brown s p e c i e s so i t i s n o t p o s s i b l e t o t e l l i f the absorbences  due t o t h e s e  two s p e c i e s i n c r e a s e d as would be e x p e c t e d i f the o v e r a l l degree o f r e d u c t i o n o b t a i n e d was independent o f p H .  I n the case o f F i l i p p o v and N u g e r ' s  work i t i s l i k e l y t h a t the degree o f r e d u c t i o n d i d depend on pH s i n c e f o r pH > 4 no r e d u c t i o n was o b s e r v e d .  The l a t t e r o v s e r v a t i o n i s l i k e l y  the  - 17 -  result of a k i n e t i c effect  s i n c e , as w i l l be shown l a t e r ,  hydrazine i n V  sufficient  e x c e s s can reduce molybdate a l m o s t q u a n t i t a t i v e l y t o Mo  for  pH between 4 . 5 and 5 . 0 . Ostrowetsky proposed t h a t s i n c e the mixed v a l e n t s p e c i e s were hexamers i t was l i k e l y t h a t t h e y were formed by r e d u c t i o n o f hexameric molybdate s p e c i e s .  F i l i p p o v and Nuger a l s o assumed t h a t molybdenum b l u e s  were i s o s t r u c t u r a l w i t h the molybdate s p e c i e s from which t h e y were formed.  - 18 -  2.1.2.2  Mo  V  V Mo and i t s c h l o r i d e s a l t s ,  R_Mo0Cl,_ 2  b  + + (where R = N H . , Rb ) , can be 4  46 prepared  i n a c i d s o l u t i o n by s t a n d a r d p r o c e d u r e s .  On n e u t r a l i z a t i o n  or  d i l u t i o n the green c h l o r i d e s o l u t i o n s h y d r o l y z e r a p i d l y t o g i v e a brown colour for  [Mo] > 0.1 M which becomes amber o r y e l l o w on i n c r e a s i n g d i l u t i o n .  Complete n e u t r a l i z a t i o n y i e l d s a brown Mo^ p r e c i p i t a t e . 47 According to Mellor  K l a s o n o b t a i n e d MoOtOH)^ by a d d i n g 3 moles o f  ammonia t o a s o l u t i o n c o n t a i n i n g one mole o f  (NH^)^MoOCl^ w h i l e Debray found  t h a t i f excess ammonia was used the p r e c i p i t a t e the f i l t e r e d s o l u t i o n c o n t a i n e d M o . V I  was p a r t i a l l y decomposed  and  The anhydrous o x i d e was o b t a i n e d by  s e v e r a l workers by h e a t i n g the p r e c i p i t a t e i n vacuo o r i n i n e r t gas s t r e a m s . 48 Simon and Souchay performed a d e t a i l e d s p e c t r o p h o t o m e t r i c study o f V the h y d r o l y s i s and c o n c l u d e d t h a t below 2 M HC1 and 3 M H^SO^ Mo was not complexed by t h e a n i o n o f the a c i d u s e d .  T i t r a t i o n of  (NH^)^MoOCl^ w i t h  NaOH suggested the h y d r o l y s i s p r o d u c t i m m e d i a t e l y b e f o r e p r e c i p i t a t i o n c o u l d be f o r m u l a t e d as  (HoO^)^  where x i n d i c a t e d an unknown degree o f p o l y m e r i z a -  tion . 49 Ardon and P e r n i c k nant Mo s p e c i e s V  c o n f i r m e d t h i s work c o n c l u d i n g t h a t the p r e d o m i -  i n d i l u t e HC1, HCIO^, and o t h e r a c i d s i s a b i n u c l e a r  w i t h charge 2+ and i s n o t c o o r d i n a t e d t o c h l o r i d e .  cation  T h e i r work i n v o l v e d  a n a l y s i s o f i t s e l u t i o n b e h a v i o u r from a c a t i o n exchange column and c r y o s copy o f a 0.02 M s o l u t i o n i n e u t e c t i c  HC10„. 4  Subsequently V i o s s a t and Lamache^ the predominant dilution.  species  6  proposed t h a t f o r  i s a tetramer which transforms  They a l s o r e p o r t e d  into  [M0 ] > 10 V  (Mo0 ) +  2  pH = 1.2 and heated a t 80°C a s i g n i f i c a n t f r a c t i o n o f the molybdenum  M  on  t h a t i f a s o l u t i o n o f Mo was n e u t r a l i z e d V  2  to  - 19 -  precipitated while  (Mo0  + 2  )  p r e v i o u s l y unknown c h e s t n u t be  and ( M o 0 ) ^ c o e x i s t e d i n s o l u t i o n w i t h a +  2  2  coloured  removed from s o l u t i o n o n an a n i o n  i t was h i g h l y p o l y m e r i z e d . that small concentrations served  for Mo  V I  species.  The c h e s t n u t  exchange r e s i n .  species  could  I t was f o u n d  that  One m a n i f e s t a t i o n o f t h i s p o l y m e r i z a t i o n was o f i t suppressed the polarographic  i n 2 N HCl.  maximum o b -  V i o s s a t a n d Lamache p r o p o s e d t h a t t h e c h e s t n u t  V Mo  s p e c i e s was a c t u a l l y c a t i o n i c b u t h a d a n o v e r a l l n e g a t i v e  ing  from s t r o n g l y adsorbed c h l o r i d e i o n s . Only two s t u d i e s o f t h e p r e c i p i t a t i o n o f M o  published.  They a r e i n f a i r  et a l . produced Mo ing 0.02  V  V  charge  appear t o have been  agreement w i t h each o t h e r .  Katsobashvili  i n H C l s o l u t i o n u s i n g z i n c as a r e d u c t a n t .  solutions contained  0.021 M Mo a n d w e r e 0.03 N i n A l C l ^ .  The r e s u l t Titration  N NaOH showed p r e c i p i t a t i o n was c o m p l e t e b e t w e e n pH 6 a n d 6.5.  denum r e m a i n i n g  result-  with  Molyb-  i n s o l u t i o n was beyond t h e l i m i t o f s e n s i t i v i t y o f c o l o r i -  m e t r i c a n a l y s i s . I t was o b s e r v e d , h o w e v e r , t h a t b e t w e e n pH 8 a n d 10 h y d r o x y l i o n s were a d s o r b e d b y t h e p r e c i p i t a t e a n d t h e molybdenum c o n c e n t r a tion i n solution increased. precipitate b i l i t y o f Mo  T h e y f o u n d t h a t Mo  s o t h e phenomenon was n o t s i m p l y V  a t a h i g h e r pH.  used f o r n e u t r a l i z a t i o n . temperature o f standing  VI  was d i s s o l v i n g f r o m t h e  a question o f increasing solu-  VI Mo a l s o a p p e a r e d i n s o l u t i o n i f NH^ was  T h e amount a p p e a r i n g  i n c r e a s e d w i t h time and  f o r a g i v e n pH a n d w i t h i n c r e a s i n g pH f o r  constant  VI time and temperature. for  constant  I t was n o t e d t h a t p l o t s o f Mo  appearing  vs time  p H ' s b e t w e e n 6.5 a n d 9 h a d p o s i t i v e i n t e r c e p t s i n d i c a t i n g  at least part of the dissolution occurred  that  a l m o s t i n s t a n t a n e o u s l y on n e u t r a l -  ization. S o u c h a y e t a l . " ' " n e u t r a l i z e d a l i q u o t s o f 0.03 M M o 5  V  solutions i n HCl  - 20 -  w i t h v a r y i n g amounts o f NaOH, a g i t a t e d them f o r 10 m i n u t e s , and s e p a r a t e d the p r e c i p i t a t e by f i l t r a t i o n .  The pH o f the f i l t r a t e was measured and  V Mo  r e m a i n i n g i n s o l u t i o n was d e t e r m i n e d p o l a r o g r a p h i c a l l y i n 6 N H C l  after  oxidation to M o  V I  with Ce^ . +  t a t e were determined a f t e r V  Sodium and c h l o r i d e i o n s i n the  d i s s o l v i n g i t i n 3 N HNO^.  The r e s u l t s  precipiindicated  the p r e c i p i t a t i o n o f Mo was complete f o r pH > 6 but t h a t the consumption o f NaOH exceeded the one e q u i v a l e n t expected from the e q u a t i o n (Mo0 ) + 40H~ + 4 H 0 = 4MoO(OH) 2  4  2  3  I t was c o n c l u d e d t h a t the p r e c i p i t a t e a c t e d as an i o n exchanger a b s o r b i n g Na  +  and r e l e a s i n g H .  The p i c k u p o f sodium i n c r e a s e d when NaCl was added  +  t o the s o l u t i o n s . Souchay e t a l . a l s o o b s e r v e d d i s s o l u t i o n o f the p r e c i p i t a t e but o n l y after  a s o l u t i o n o f pH = 12.65 was a g i t a t e d f o r s e v e r a l h o u r s .  found by p o l a r o g r a p h i c a n a l y s i s t h a t a f t e r  I t was  such d i s s o l u t i o n the  precipitate  IV VI c o n t a i n e d a mole o f Mo f o r each mole o f Mo appearing i n s o l u t i o n .  The  VI V appearance o f Mo thus r e s u l t e d from the d i s m u t a t i o n o f Mo a c c o r d i n g t o : „ V 2Mo  . IV • Mo  VI + Mo  I n 5 N NaOH complete d i s m u t a t i o n was a l m o s t i n s t a n t a n e o u s y i e l d i n g a p r e c i p i t a t e i n w h i c h the mole r a t i o o f molybdenum t o sodium was one to one.  The r e a c t i o n proposed was:  2MoO(OH)  3  +  30H  +  + Na  •  2MoO  +  HNaMoO^  +  4H 0 2  which i n v o l v e d f o r m a t i o n o f an i n s o l u b l e m o l y b d i t e i d e n t i f i e d i n a s i m i l a r study o f the p r e c i p i t a t i o n o f Mo  IV 51  - 21 -  2.1.2.3  Mo  IV  IV The b e h a v i o u r o f Mo i n aqueous s o l u t i o n has been a m a t t e r o f c o n troversy.  A c c o r d i n g to M e l l o r s e v e r a l workers r e p o r t e d o b t a i n i n g v a r i o u s  h y d r a t e s o f MoC^ by n e u t r a l i z a t i o n o f reduced molybdate s o l u t i o n s .  ConV  v e r s e l y , Klason maintained these p r e c i p i t a t e s p r o d u c t s and h y d r a t e d o x i d e s o f M o  I V  were p r o b a b l y impure Mo  d i d not e x i s t .  T h i s o p i n i o n was  r e i n f o r c e d by H a i g h t and coworkers who c o n c l u d e d , on the b a s i s o f p o l a r o graphic work^  2  and a n a l y s i s o f the k i n e t i c s o f the r e d u c t i o n o f M o  I I 53 IV Sn , t h a t Mo was u n s t a b l e III V i n t o Mo and Mo . 54 Guibe and Souchay, ence o f M o  i n aqueous s o l u t i o n and  V I  by  disproportionated  however, had p r e v i o u s l y demonstrated  the  exist-  using polarography. Souchay, C a d i o t , and Duhameaux"^ s u b s e IV q u e n t l y n e u t a l i z e d a Mo s o l u t i o n t o p r e c i p i t a t e Mo0(0H) and f o r m u l a t e d IV + the predominant Mo i o n b e f o r e p r e c i p i t a t i o n as MoO(OH) . IV Ardon and coworkers suggested Mo was a c t u a l l y d i m e r i c based on 56 57 58 i o n exchange experiments and c r y o s c o p y . R e c e n t l y C h a l i l p o y i l and Anson I V  2  r e p o r t e d t h a t a d i m e r i c assignment  i s c o n s i s t e n t w i t h the b e h a v i o u r o f M o  I V  d u r i n g e l e c t r o c h e m i c a l o x i d a t i o n and r e d u c t i o n . Souchay, C a d i o t , and Viossat"*^ s t u d i e d the p r e c i p i t a t i o n o f  Ho^. IV  V a r y i n g amounts o f NaOH were used t o n e u t r a l i z e a l i q u o t s o f 0.03 M Mo i n the same procedure  as a l r e a d y d e s c r i b e d f o r t h e i r work on M o . V  t a t i o n began a t pH = 1.5 and was complete a t pH = 2 . 8 .  Precipi-  Sodium and c h l o r i d e  i o n s c o u l d be e l i m i n a t e d from the p r e c i p i t a t e by washing w i t h w a t e r . p r o d u c t was a n a l y z e d as Mo0  2  • 2H 0. 2  The  I t c o u l d be e a s i l y d i s s o l v e d i n 2 N IV  HC1 t o g i v e the spectrum and p o l a r o g r a m c h a r a c t e r i s t i c  o f Mo  .  After  d r y i n g a t 1 3 0 ° C , however, i t took on a g r a i n y appearance and c o u l d no l o n g e r  - 22 -  be  dissolved  i n acid.  I n s o l u b i l i t y i n a c i d i s a c h a r a c t e r i s t i c o f anhydrous  47  Mo0 2  I f n e u t r a l i z a t i o n was c a r r i e d o u t w i t h  a n e x c e s s o f NaOH g i v i n g  f i n a l pH o f 11.5 t h e p r o d u c t was a n i n s o l u b l e m o n o a l k a l i n e m o l y b d i t e . reaction  a The  p r o p o s e d was MoO(OH)  + + + w h e r e M = Na , L i , K .  2  +  MOH  =  3  I n 5 N NaOH t h e m o l y b d i t e p a r t i a l l y  IV g i v e Mo  Mo0 HM  redissolved  to  2i n solution.  The i o n MoO^  was p r o p o s e d b y a n a l o g y w i t h  molyb-  date . 59,60 Lagrange and Schwmg  o b t a i n e d an Mo0  e l e c t r o l y s i s o f 0.125 M N a ^ o O ^ s o l u t i o n s ( v s SHE) a n d pH 5 - 6. pH  • 2H 0 p r e c i p i t a t e by 2  o n a m e r c u r y c a t h o d e a t -0.6 V  The p r e c i p i t a t e . o n l y  f o r m e d f o r pH > 2.  m o l y b d e n u m b l u e was f o r m e d a n d no d e p o s i t i o n  g a v e t h e ASTM d i f f r a c t i o n p a t t e r n  2  f o r Mo0  2  occurred.  after drying  A t lower  The p r e c i p i t a t e a t 500°C i n n i t r o -  gen. Lamache  6 1  found that  e l e c t r o l y s i s a t -.29 t o -.38 V ( v s SHE) i n a n  acetic buffer  o f pH 4.6 g a v e M o V IV e q u a l a m o u n t s o f Mo a n d Mo 2.1.2.4  Mo Mo  Hg  is  1 1 1  c a n be p r o d u c e d i n a c i d s o l u t i o n by r e d u c t i o n  reduction  slow.  i n s o l u t i o n and a p r e c i p i t a t e  containing  1 1 1  o r by e l e c t r o l y s i s . M o  lytic  1 1 1  1  1  1  with  Cd, Zn,  d e p o s i t s have been formed by e l e c t r o -  i n alkaline or neutral  solution but the rate o f  deposition  62-66  No d e t a i l e d  studies  of the precipitation of Mo  1 1 1  t o y i e l d Mo(0H)  3  - 23 -  have been made.  Mo(OH).^ and the anhydrous o x i d e are p o o r l y c h a r a c t e r i z e d .  62 Smith  v e r i f i e d t h a t e l e c t r o l y s i s i n a l k a l i n e and n e u t r a l s o l u t i o n s l e d t o 47 Mo(OH)^ by p r e p a r i n g enough o f the d e p o s i t f o r a n a l y s i s . M e l l o r reported t h a t Mo(OH)^ d i s s o l v e s o n l y w i t h d i f f i c u l t y Watt and D a v i e s  64  i n acids.  VI o b t a i n e d anhydrous M020^ by r e d u c t i o n o f Mo  o x i d e w i t h s o l u t i o n s o f p o t a s s i u m i n l i q u i d ammonia. Mo^O^ d i s s o l v e s r e a d i l y i n 6 N HC1.  They r e p o r t e d  MO2O2 was c o n v e r t e d to i n s o l u b l e  b l a c k Mo(OH)^ by a g i t a t i o n i n water f o r 15 minutes a t 2 5 ° C . p r e p a r e d from Mo^O^ was found t o have the same p r o p e r t i e s p a r e d by e l e c t r o l y s i s o f M o  s o l u t i o n s a t pH 3.1 t o 4 . 4 .  V I  n o r Mo^o^ gave X - r a y d i f f r a c t i o n p a t t e r n s . i n e r t atmosphere, Mo0 w h i l e Mo^Oj 2  that  Mo(OH)  as Mo(OH)^ p r e N e i t h e r Mo(OH)^  On h e a t i n g t o 325°C i n an  however, Mo(OH)^ gave d i f f r a c t i o n p a t t e r n s f o r Mo and still  gave no p a t t e r n .  Messner and Z i m m e r l y 3.5 and p r e c i p i t a t e d Mo(0H)  3  1 6  reduced M o  V I  s o l u t i o n s w i t h i r o n a t pH 1 t o  by n e u t r a l i z a t i o n t o pH 3.6 t o 4 . 5 .  The MoO^  produced by r o a s t i n g the p r e c i p i t a t e c o n t a i n e d 2.12% Fe and 0.14% C u .  2.1.3  Summary The p r e c e d i n g r e v i e w o f aqueous molybdenum c h e m i s t r y s u g g e s t s t h a t  r e d u c t i v e p r e c i p i t a t i o n c o u l d l e a d t o v a r i o u s p r o d u c t s i n c l u d i n g mixed valent precipitates.  I t a l s o i l l u s t r a t e s the complex n a t u r e o f molybdenum  s p e c i e s i n aqueous s o l u t i o n .  Which aqueous s p e c i e s o r p r e c i p i t a t e p r e d o m i -  n a t e s i n a g i v e n s i t u a t i o n i s l i k e l y t o be dependent on t o t a l molybdenum c o n c e n t r a t i o n , p H , and p o t e n t i a l .  Furthermore as t h e s e c o n d i t i o n s  change  d u r i n g a r e a c t i o n the predominant s p e c i e s and hence r e a c t i o n p a t h might be  -  expected t o change.  24  -  A r e v i e w o f the l i t e r a t u r e  on hydrogen r e d u c t i o n and  reduction w i t h hydrazine serves to i l l u s t r a t e these p o i n t s .  - 25 -  2.2  R e d u c t i o n w i t h Hydrogen and Carbon Monoxide  P a a l and B r u n j e s ^ and P a a l and B u t t n e r ^ ammonium  paramolybdate  palladium catalyst.  s o l u t i o n s by hydrogen i n the p r e s e n c e o f a c o l l o i d a l  F i g u r e 5 shows p e r c e n t r e d u c t i o n to M o  on the b a s i s o f H^ consumed)  The  stage was  two days a t w h i c h p o i n t the r e a c t i o n s t o p p e d .  The r e a c t i o n proceeded s l o w l y and stopped a g a i n a f t e r 2  consumption corresponded t o M o  fine black precipitate concentrated  1 1 1  .  2  4  o f hydrogen.  t h r e e days when the  The end s o l u t i o n was y e l l o w and  formed was v e r y d i f f i c u l t  HC1 o r H S 0 .  attained  R e d u c t i o n was c o n t i n u e d  by h e a t i n g t o 50° to 60°C and a p p l y i n g a s l i g h t o v e r p r e s s u r e  H  (calculated  I V  vs time f o r the i n i t i a l p e r i o d o f r e d u c t i o n  c a r r i e d o u t a t 30°C and a t m o s p h e r i c p r e s s u r e . after  studied reduction of  the  t o d i s s o l v e i n c o l d o r hot  The s m a l l q u a n t i t y t h a t d i d d i s s o l v e gave a  l i g h t red s o l u t i o n . IV The brown b l a c k  slurry  tween Mo(OH)^ and Mo0(0H)  2  formed a t the Mo  stage gave a w e i g h t b e -  when d r i e d i n vacuo w i t h o u t h e a t i n g .  When d r i e d  w i t h m i l d h e a t i n g the w e i g h t approximated M o 0 .  The p r e c i p i t a t e was a n a l y z e d  f o r molybdenum a f t e r  These r e s u l t s  2  d i s s o l u t i o n i n aqua r e g i a .  seem t o be i n  a c c o r d w i t h those o f Souchay e t al.^"*" who found t h a t n e u t r a l i z a t i o n o f M o  I V  y i e l d e d Mo0 *nH 0 w i t h n very n e a r l y 2. 2  2  P a a l and B u t t n e r d i d not a n a l y z e the p r e c i p i t a t e Mo  1 1 1  stage.  concentrated  The d i f f i c u l t  corresponding to  s o l u b i l i t y i n , and the r e d c o l o u r i m p a r t e d  a c i d i s i n a c c o r d w i t h the r e p o r t e d p r o p e r t i e s  the  to,  o f Mo(OH)^.  The y e l l o w c o l o u r o f t h e end s o l u t i o n c o u l d have been t h a t o f the Mo"^ (H„0). 2 6 58 monomer. The a b r u p t d e c r e a s e i n the r a t e o f r e a c t i o n can be a t t r i b u t e d t o +  - 26 -  F i g u r e 5:  R e d u c t i o n o f ammonium p a r a m o l y b d a t e s o l u t i o n s by hydrogen i n the p r e s e n c e o f a c o l l o i d a l palladium catalyst.  - 27 -  d e p o l y m e r i z a t i o n o f molybdate o c c u r r i n g when r e d u c t i o n has consumed cient acid.  suffi-  F o r example the r e d u c t i o n can be w r i t t e n : Mo C> ~  +  6  7  24  7H  +  2  6H  +  +  4H 0 2  =  7Mo(OH)  from w h i c h i t f o l l o w s t h a t r e d u c t i o n consumes a c i d . i t s e l f buffers  4  The paramolybdate  the r e a c t i o n i n the pH range 5 t o 6 by i t s own d i s s o c i a t i o n  according to:  Mo 0 7  62 4  +  4H 0  I t can be shown t h a t a f t e r t o Mo(OH)  4  =  2  7Mo0  24  + 8H .  +  57% o f the t o t a l molybdenum has been reduced  VI 2the r e m a i n i n g Mo e x i s t s o n l y as Mo0 .  In t h i s  4  particular  system the pH i s then h e l d between 9 and 10 by the N H / N H ^ b u f f e r .  Since  +  4  2l t seems apparent t h a t MoC>  i s reduced o n l y v e r y s l o w l y the  4  reaction  proceeds t o c o m p l e t i o n a t a v e r y s l o w r a t e . I t was found t h a t the p r e c i p i t a t e s  formed absorbed the p a l l a d i u m  c a t a l y s t almost c o m p l e t e l y . L y a p i n a and Zelikmann"*"^ s t u d i e d hydrogen r e d u c t i o n o f 0.05 M sodium molybdate s o l u t i o n s a t 100° t o 200°C and 10 t o 60 atm hydrogen F i g u r e 6 shows t h e i r r e s u l t s atm and 2 0 0 ° C .  pressure.  f o r s o l u t i o n s o f i n i t i a l pH 2 and 7 a t 40  The recommended optimum pH was 2 w h i c h c o r r e s p o n d s t o com2-  p l e t e a c i d i f i c a t i o n o f MoC>  4  .  T h i s i n i t i a l pH a s s u r e s the r e a c t i o n  b u f f e r e d between pH 5 and 6 up t o c o m p l e t i o n .  I t s h o u l d be n o t e d ,  is  however,  t h a t i n t h i s work i t was found t h a t the s o l u t i o n s c o n t a i n e d i r o n .  Thus  f o r lower p H ' s some o f the r e d u c t i o n was performed by the w a l l s o f  the  autoclave.  I n subsequent r u n s a q u a r t z l i n e r was used but i n t h e s e runs  v a r i o u s c a t a l y s t s were a l s o added.  The p r o d u c t o f r e d u c t i o n was i d e n t i f i e d  - 28 -  - 29 -  as MoC>2 by X - r a y d i f f r a c t i o n and a n a l y s i s f o r molybdenum. The e f f e c t o f MoO^ s l u r r y from a p r e v i o u s r e d u c t i o n and m e t a l l i c molybdenum as c a t a l y s t s  i s shown i n F i g u r e 7 c u r v e a and F i g u r e 7 curve b ,  r e s p e c t i v e l y , f o r i n i t a l pH = 3, 40 atm H , and 2 0 0 ° C .  Mo0 was added i n  2  2  a c o n c e n t r a t i o n o f 67 g/1 and m e t a l l i c molybdenum as 7% o f the t h e o r e t i c a l l y n e c e s s a r y f o r the  Mo  +  2Mo0  24  amount  reaction  +  + 4H  =  3Mo0  2  +  2H 0 2  I t was proposed t h a t the r e a c t i o n o f m e t a l l i c molybdenum w i t h the s o l u t i o n formed " a c t i v e " Mo0  2  p a r t i c l e s w h i c h s e r v e d as c e n t r e s o f c r y s t a l l i z a t i o n .  I t i s apparent from F i g u r e 7 t h a t Mo0 efficient catalyst  catalyst.  2  from p r e v i o u s r e d u c t i o n s was not an  F i g u r e 7 c u r v e c shows the r e s u l t s  f o r no added  ( a l s o , however, w i t h no l i n e r ) .  Complete r e d u c t i o n was o b t a i n e d i n f o u r hours u s i n g m e t a l l i c molybdenum as a c a t a l y s t and o p e r a t i n g w i t h i n i t i a l pH = 2, 60 atm H" , and 2  200°C ( F i g u r e 7 curve d ) .  Under t h e s e c o n d i t i o n s d e c r e a s i n g the  molybdenum c o n c e n t r a t i o n from 44 g/1 t o 5 g/1 d e c r e a s e d f o r complete r e d u c t i o n from 4 hours t o l e s s t h a n % h o u r . complete r e d u c t i o n o b t a i n e d w i t h an i n i t i a l pH > 2 . the assumption t h a t p o l y m o l y b d a t e s are n e c e s s a r y of  initial  the time r e q u i r e d I n no case was  This i s i n accord with  f o r an a p p r e c i a b l e  rate  reduction. 34 I n a l a t e r r e v i e w Zelikmann  noted f o r 200°C, 7% o f s t o i c h i o m e t r i -  c a l l y n e c e s s a r y m e t a l l i c Mo, and 40 g/1 molybdenum t h a t the r a t e o f r e d u c t i o n was l i n e a r l y dependent on / P H . 2  He c o n c l u d e d t h a t hydrogen p a r t i c i p a t e d i n  the r a t e c o n t r o l l i n g s t e p i n atomic form. 12 Sobol  s t u d i e d r e d u c t i o n by b o t h H  0  and CO a t e l e v a t e d temperature  30 -  ioor  (a)  initial ~  pH=3, 4 0 a t m H slurry catalys t  (b) initial pH=3, 40 atm H metallic Mo catalyst (c) initial pH=3, 40 otm H no c a t a l y s t added  2  2  2  ( d ) initial pH = 2 , 6 0 atm H metallic  TIME (hour*) F i g u r e 7:  Mo  2  catalyst  10  E f f e c t o f v a r i o u s c a t a l y s t s , i n i t i a l p H , and hydrogen p r e s s u r e on hydrogen r e d u c t i o n o f sodium molybdate s o l u t i o n s at 200°C.  - 31 -  and p r e s s u r e .  U s i n g CO a t 80 - 85 atm p r e s s u r e and 200 - 220°C temperature  almost complete r e d u c t i o n o f molybdate c o u l d be o b t a i n e d i n t i m e s one day.  T h i s was a t t r i b u t e d  approaching  t o the b u f f e r i n g a c t i o n o f f o r m i c a c i d produced  a c c o r d i n g to the r e a c t i o n CO + E^O = HCOOH.  Hydrogen r e d u c t i o n was found  t o r e q u i r e p r e l i m i n a r y a c i d i f i c a t i o n t o pH = 2 i n agreement w i t h Z e l i k m a n and L y a p i n a .  I t was c o n c l u d e d , however, t h a t the k i n e t i c s o f gaseous r e d u c -  t i o n were slow and c a t a l y s i s was r e q u i r e d .  An MoO^ p u l p produced by h y d r o -  gen r e d u c t i o n o f molybdate s o l u t i o n s was found t o have no s i g n i f i c a n t catalytic effect.  Sobol, therefore,  denum as r e d u c t a n t and h i s f u r t h e r Kunda and R u d y k  1 5  proposed the use o f m e t a l l i c m o l y b work d i d not c o n s i d e r gaseous r e d u c t i o n .  proposed hydrogen r e d u c t i o n a t 180°C and 23 atm  H^ i n the presence o f 0.025 g/1 P d C ^ c a t a l y s t  f o r molybdenum r e c o v e r y from  s o l u t i o n s c o n t a i n i n g about 1 M molybdenum, 1.54 M ( N H ^ ) ° 4 ' S  a  n  d  2  f r e e NH^/Mo = 2 .  mole r a t i o  Under t h e s e c o n d i t i o n s e s s e n t i a l l y complete r e d u c t i o n was  o b t a i n e d i n l e s s than o n e - h a l f h o u r .  The r e s u l t i n g o x i d e powder  absorbed  the p a l l a d i u m c h l o r i d e c a t a l y s t b u t r e t a i n e d enough c a t a l y t i c a c t i v i t y f o r use i n two " d e n s i f i c a t i o n s " w i t h f r e s h molybdate s o l u t i o n s .  From the  solu-  t i o n c o m p o s i t i o n g i v e n i t i s apparent t h a t the hydrogen i o n a c t i v i t y i n t h i s system was c o n t r o l l e d by t h e N H / N H +  4  3  buffer.  The f a c t t h a t  reduction  went r a p i d l y t o c o m p l e t i o n i n t h e s e c o n d i t i o n s can perhaps be e x p l a i n e d by the e f f e c t libria.  o f temperature on the molybdate p o l y m e r i z a t i o n and N H ^ / N H ^  No d a t a i s a v a i l a b l e f o r the p o l y m o l y b d a t e s but u s i n g f r e e  equi-  energy  69 d a t a from B a r n e r and Scheuerman's pH o f the N H / N H +  4  3  buffer  Presumably t h i s f a l l  c o m p i l a t i o n i t can be shown t h a t  f a l l s from 9.27 a t 25°C t o 5.75 a t  200°C.  was s u f f i c i e n t t o ensure t h a t the system was  i n a pH range where p o l y m o l y b d a t e s c o u l d e x i s t .  the  buffered  - 32 -  The o x i d e p r o d u c t a n a l y z e d 60 - 65% Mo, 4 - 5 % N H , and 0.5% S 3  compared t o 59% Mo f o r M o 0 ( 0 H ) , 75% f o r M o 0 , and 65% f o r M o ( 0 H ) 3  2  Solid  3  s t a t e hydrogen r e d u c t i o n o f t h i s p r o d u c t gave 99.9% molybdenum m e t a l . The p r e c i s e n a t u r e o f the o x i d e p r o d u c t was not d e t e r m i n e d . d i f f r a c t i o n was not m e n t i o n e d .  They d i d r e p o r t  t h a t when r e d u c t i o n was  c a r r i e d out under m i l d e r c o n d i t i o n s than t h o s e l i s t e d above s t e p s were o b s e r v e d ;  X-ray  intermediate  the s o l u t i o n became marine b l u e , then molybdenum  p r e c i p i t a t e d as a brown amorphous r e s i d u e w h i c h l a t e r agglomerated  into  b l a c k , o v a l - s h a p e d p a r t i c l e s o f lower molybdenum o x i d e .  supposed  t h a t the brown r e s i d u e was Mo0(0H)  3  I t may be  and i t i s p o s s i b l e t h a t i n t h i s  case  IV complete r e d u c t i o n t o the Mo  s t a t e d i d not p r o c e e d .  t h e p r e s e n t study t h a t under some c o n d i t i o n s Mo0(0H)  3  I t was observed i n precipitated  molybdenum b l u e s o l u t i o n s had a b l u e - b l a c k c o l o u r r a t h e r than i t s  from customary  brown. 14 Wagenmann  patented a process  i n v o l v i n g hydrogen r e d u c t i o n  r e c o v e r molybdenum from 1.5 - 3.0 g/1 molybdenum and 60 - 120 g/1 s o l u t i o n s a c i d i f i e d t o pH = 2 . 20 - 25 atm The s o l u t i o n  to sulfate  R e d u c t i o n was c a r r i e d out a t 180°C and  i n a f l o w t h r o u g h r e a c t o r w i t h a r e s i d e n c e time o f 4% h o u r s . e x i t i n g the r e d u c t i o n v e s s e l was c o o l e d t o 80°C and expanded  t o 0.1 atm b e f o r e  f i l t r a t i o n t o r e c o v e r the molybdenum p r o d u c t .  The  filtrate  c o n t a i n e d 0.05 g/1 Mo thus molybdenum r e c o v e r y was 97%.  The a n a l y s i s o f  the p r o d u c t was n o t g i v e n .  These r e s u l t s  No c a t a l y s i s was mentioned.  are  c o n s i d e r a b l y b e t t e r than t h o s e o b t a i n e d by Zelikmann and L y a p i n a ( F i g u r e 7) a t a h i g h e r temperature and hydrogen p r e s s u r e . d e s c r i b e the c o n s t r u c t i o n o f h i s r e a c t o r  S i n c e Wagenmann d i d not  i t i s not p o s s i b l e t o  whether the w a l l s o f t h e r e a c t o r p l a y e d a r o l e i n t h e  determine  reduction.  - 33 -  2.3  Reduction w i t h Hydrazine  The b e h a v i o u r o f h y d r a z i n e , N ^ H ^ , as a r e d u c i n g agent has been s u b j e c t o f much work, the g e n e r a l aim o f which was t o determine t h e for  the wide v a r i a t i o n o f s t o i c h i o m e t r y w i t h d i f f e r e n t  mental c o n d i t i o n s .  the  reasons  o x i d a n t s and e x p e r i -  H y d r a z i n e was shown t o r e a c t a c c o r d i n g t o two l i m i t i n g  reactions:*  N H 2  + 5  N H  +  Z b  =  N  +  =  JjN.  2  Z  5H  +  +  +  NH  j  4e~  +  2H  +  +  e~  w h i c h c o u l d o f t e n o c c u r i n p a r a l l e l g i v i n g s t o i c h i o m e t r i e s between the two limits.  Some o x i d i z i n g agents added s l o w l y t o b o i l i n g h i g h l y a c i d h y d r a z i n e  s o l u t i o n s a l s o gave a s i g n i f i c a n t y i e l d o f h y d r a z o i c a c i d , H N ^ . 70 Browne and S h e t t e r l y , investigations,  i n summarizing the r e s u l t s o f a s e r i e s o f  r e c o g n i z e d t h r e e c l a s s e s o f o x i d i z i n g agents f o r h y d r a z i n e  i n hot a c i d s o l u t i o n : class a produce f a i r l y l a r g e amounts o f NH^ and H N  H  2°2  KC10„ 4 K  2 2°8 S  class b  3  class c  l i t t l e o r no HN^ but much NH^  l i t t l e o r no HN or NH  KMnO  KI0  Mn0 F e  4  3  3  3  HgO  2  Hgci  2°3  2  *Most o f the work t o be d i s c u s s e d was performed i n s o l u t i o n s o f pH < 7 where the p r o t o n a t e d form o f h y d r a z i n e , N„H , p r e d o m i n a t e s . +  -  34  They s t a t e d t h a t s o many d i f f e r e n t r e a c t i o n s t h a t i t was i m p o s s i b l e  -  f a c t o r s i n f l u e n c e d t h e course  of the  t o e s t a b l i s h a simple r e l a t i o n s h i p  between t h e p o t e n t i a l o f t h e v a r i o u s o x i d i z i n g agents and t h e i r a b i l i t y t o p r o d u c e HN^, NH^, o r  .  71 Cuy  et a l .  summarized a f u r t h e r s e r i e s o f i n v e s t i g a t i o n s by sug-  g e s t i n g t h e r e were two c l a s s e s o f o x i d i z i n g a g e n t s .  Those u n d e r g o i n g a  change o f one e q u i v a l e n t p e r mole r e a c t e d a c c o r d i n g t o :  N H 2  •  + 5  N H 2  +  3  2H  +  +  e"  f o l l o w e d b y one o f t h e two r e a c t i o n s :  N H 2  • NH  3  2N H  o x i d a t i o n o f H" 2  hydrazine  3  NH 2NH  4 6  +  3  +  3  N2  two s u b s e q u e n t r e a c t i o n s o c c u r r e d a limiting  N  N  >• N H  2 3  I f e i t h e r o f these  +  3  ought t o o b t a i n .  f a s t e r than f u r t h e r  s t o i c h i o m e t r y o f one e q u i v a l e n t p e r mole o f F o r o x i d i z i n g a g e n t s u n d e r g o i n g a change o f  more t h a n o n e e q u i v a l e n t p e r m o l e a m i x e d s t o i c h i o m e t r y 4 e q u i v a l e n t s p e r mole o f hydrazine)  was e x p l a i n e d  t i o n o f a n o t h e r o x i d i z i n g agent a s an i n t e r m e d i a t e  ( i . e . between 1 and  i n terms o f t h e generau n d e r g o i n g o n l y one  e q u i v a l e n t r e d u c t i o n , f o r example: 2Cr„0_ 2  + . 4e~ +  1  2  5  givmg  the overall An  O 2 5  +  +  HCr»O  2 +  HCr  _ > N„  N-HV 2N H 2 5  2e~  2 3+  * 2Cr  +  c  +  2H„0  5 2NH. 4  2 +  +  N_ 2  +  5H„0 2  + 2s t o i c h i o m e t r y o f 1.5 m o l e s K^H,- p e r m o l e o f Cr^O^  o b s e r v e d t e n d e n c y o f t h e s t o i c h i o m e t r y , R, ( d e f i n e d a s m o l e s o f  - 35  electrons/mole  -  of hydrazine) to increase  due t o a h i g h e r  i n a l k a l i n e s o l u t i o n was  rate of further oxidation of intermediate  s o l u t i o n s as opposed t o a h i g h e r  thought  i n alkaline  rate of i t s decomposition i n acid solutions.  72 K i r k and Browne  l i k e w i s e proposed d i v i s i o n of o x i d i z i n g agents  i n t o two c l a s s e s : 1.  Those t h a t a c c e p t o n l y one e l e c t r o n p e r " a c t i v e " u n i t (atom, i o n , m o l e c u l e ) were termed m o n o d e l e c t r o n a t o r s ;  2.  T h o s e t h a t a c c e p t more t h a n o n e e l e c t r o n p e r " a c t i v e " u n i t were termed p o l y d e l e c t r o n a t o r s , i . e . d i d e l e c t r o n ators, tridelectronators, etc.  With monodelectronators complete o x i d a t i o n t o n i t r o g e n obtained  >• N„EL 2 3  +  Zo  2NH 2 3  ——>  +  N .H 4 6  2H  +  N„ 2  +  e~  (slow)  +  2NH_ 3  (fast)  With d i d e l e c t r o n a t o r s complete o x i d a t i o n t o n i t r o g e n f o r i n c o m p l e t e o x i d a t i o n ammonia  were formed. a  c  c  o  r  d  l  n  9  t  o  was  predominant  of hydrazoic initial  acid  formation  :  n  z  followed  and a s m a l l q u a n t i t y  The r e a c t i o n s e q u e n c e i n t h i s c a s e i n v o l v e d  N H  and  was  formed a c c o r d i n g t o :  N_H_  o f ^2^2  be  b u t i n c a s e s w h e r e R was l e s s t h a n f o u r t h e s o l e b y p r o d u c t  ammonia  but  could  + c  •  b  N„H_ I 2  +  3H  +  +  2e  by a s e r i e s o f f a s t subsequent r e a c t i o n s  (slow)  t o produce  or  HN^  NH . 3 O x i d i z i n g agents undergoing reduction  (complex d e l e c t r o n a t o r s )  i n more t h a n two  stages  c o u l d m a n i f e s t t h e c h a r a c t e r i s t i c s o f mono- a n d  d i d e l e c t r o n a t o r s , f o r example:  - 36 -  VO VO  3+ 3+  -  +  e  +  2e  = -  VO  2+  + VO  =  73 Higginson et a l .  proposed t h a t mono- and d i d e l e c t r o n a t o r s  be  d i s t i n g u i s h e d by the f o l l o w i n g c r i t e r i o n : 1. D i d e l e c t r o n a t o r s were t h o s e r e a g e n t s w h i c h o x i d i z e d h y d r a z i n e a t room temperature i n a c i d s o l u t i o n t o produce N o n l y . 2  2.  M o n o d e l e c t r o n a t o r s were those r e a g e n t s w h i c h o x i d i z e d h y d r a z i n e t o NH^" " and N under t h e same c o n d i t i o n s as above w i t h the r a t i o o f N H ^ " " / ^ depending on r e l a t i v e i n i t i a l c o n c e n t r a t i o n s o f h y d r a z i n e and o x i d a n t . 1  2  1  S i m p l e mechanisms f o r mono- and d i d e l e c t r o n a t i o n were  didelectronation N H 2  monodelectronation  +  N  5  2 5 H  2  2  + 3H 2e  N  2  +  le~  2e~ N H  proposed:  N  dimerization  + 4H  2N H 2  2 3 H  le N H  3  2  2  fast fast  4 6  2e  fast N  2  + 2NH  N  3  predominant reaction i n acid solution The s t o i c h i o m e t r y  f o r .monodelectronators  2  + 4H  predominant reaction i n alkaline solution  c o u l d l i e between R = 1 and R = 4  depending on t h e r e l a t i v e r a t e s o f the two subsequent r e a c t i o n s o f  N 2  H . 3  - 37 -  S i n c e i n a l k a l i n e s o l u t i o n s even m o n o d e l e c t r o n a t o r s  produce  nitrogen  q u a n t i t a t i v e l y t h e e f f e c t o f i n c r e a s i n g pH was seen as d e c r e a s i n g the  rate  o f d i m e r i z a t i o n o f N^H^ r e l a t i v e to i t s f u r t h e r o x i d a t i o n . 74 S u b s e q u e n t l y H i g g i n s o n and S u t t o n performed an i s o t o p i c study 15 i n v o l v i n g the o x i d a t i o n o f N e n r i c h e d h y d r a z i n e by excess o f v a r i o u s o x i d i z i n g agents.  The i s o t o p i c d i s t r i b u t i o n o f the r e s u l t i n g p r o d u c t s  confirmed the v a l i d i t y o f the s i m p l e mechanisms p r o p o s e d .  Cahn and  75 Powell  i n d e p e n d e n t l y performed a s i m i l a r i s o t o p i c s t u d y w i t h the same  results. I n a subsequent r e v i e w H i g g i n s o n " ^ noted t h a t f o r o x i d i z i n g agents i n g e n e r a l , y i e l d s o f ammonia r e l a t i v e t o h y d r a z i n e consumed are v e r y much smaller i n a l k a l i n e , neutral,  and weakly a c i d s o l u t i o n than the y i e l d s  o b t a i n e d by u s i n g m o n o d e l e c t r o n a t o r s  i n s o l u t i o n s w i t h pH < 3.  Thus i n  a l k a l i n e s o l u t i o n few, i f any, o x i d a n t s g i v e v a l u e s g r e a t e r than 0.1 f o r moles NH_/mole N _ H . whereas f o r pH < 3 most m o n o d e l e c t r o n a t o r s g i v e mole 3 2 4 NH^/mole  1 0.75.  Use o f t h e  equations: 4  moles NH^ moles ^2^4  moles NH^ moles N_H . 2 4  4 moles +  +  moles N^H^  moles N  =  R  4 2  moles INLH,, 2 4  4 (where  i s d e f i n e d as  due t o 4 e q u i v a l e n t r e d u c t i o n )  c o r r e s p o n d s t o R 2 3.7 i n a l k a l i n e s o l u t i o n and R £ 1.75  shows t h a t  this  f o r pH ^ 3.  H i g g i n s o n f e l t i t was l i k e l y t h e change from low t o h i g h s t o i c h i o m e t r y o c c u r r e d i n t h e pH range 3 t o 5 based on the o b s e r v a t i o n t h a t a t pH = 2  - 38  -  most monodelectronators gave R < 1.75  w h i l e i t was  observed  the monodelectronator f e r r i c y a n i d e r e a c t e d w i t h R = 4.  t h a t a t pH  = 6  T h i s p r e d i c t i o n was  v e r i f i e d f o r o x i d a t i o n w i t h manganese t r i s p y r o p h o s p h a t e .  For d i d e l e c t r o n a -  t o r s H i g g i n s o n proposed the change t o h i g h s t o i c h i o m e t r y o c c u r r e d f o r pH  > 0. Browne and  s i o n s o f MoO^ slow and  Shetterly  i n v e s t i g a t e d the r e a c t i o n o f aqueous suspen-  7 0  with hydrazine.  some ammonia was  w i t h excess MoO^  In a l k a l i n e s o l u t i o n s the r e a c t i o n was  produced.  In b o i l i n g  very  sulfuric acid solutions  a p p r e c i a b l e amounts o f ammonia and  some HN^  were formed.  72 On  the b a s i s o f t h i s work K i r k and Browne  c o n s i d e r e d molybdate i o n s as  didelectronators. Jakob and Kozlowski  77  z i n e p r a c t i c a l l y completely Mo  V  found to  t h a t molybdate s o l u t i o n s o x i d i z e d  -  They were a b l e t o p r e p a r e  compounds by r e d u c t i o n i n weakly a c i d s o l u t i o n s .  hydraVI mixed Mo /  In s l i g h t l y more a c i d V  s o l u t i o n s molybdenum b l u e was  formed w h i l e a t s t i l l  produced d i r e c t l y w i t h o u tV i n tVI e r m e d i a t e p r o d u c t s .V Mo^O ed p r e p a r a t i o n o f the Mo /Mo compound NH  h i g h e r a c i d i t y Mo  In pVI a r t i c u l a r they r e p o r t Mo O^ from a s o l u t i o n  (OH),  c o n t a i n i n g 0.317  M molybdenum as ammonium paramolybdate and  T h i s compound has the same Mo /Mo V  VI  was  0.06  M  N  H 2  4*  r a t i o as the brown s p e c i e s r e p o r t e d  by  42 Ostrowetsky  between pH  3 and  4.  In f a c t i f the formula  g i v e n by  Jakob  and Kozlowski i s r e w r i Vt t e n to c o n t a i n s i x atoms o f molybdenum and water i s removed i t becomes Mo, 17 Mo, and,  i f i t i s assumed to be p r o t o n a t e d  i n solution,  - 39 -  V Mo . 4  O H  which i s the c o m p o s i t i o n found by O s t r o w e t s k y . H o l t j e and Geyer  31  and Rao and Suryanarayama  78  r e p o r t e d t h a t under  no c o n d i t i o n s does h y d r a z i n e reduce molybdate s o l u t i o n s beyond the Mo state.  V  I t was thus recommended as a method f o r the p r e p a r a t i o n o f M o  V  VI stock s o l u t i o n s .  The l a t t e r a u t h o r s reduced 0.4220 moles o f Mo  HC1 on t h e b o i l i n g water b a t h w i t h 0.154 moles o f h y d r a z i n e .  in 1 N  They were  i n t e r e s t e d o n l y i n o b t a i n i n g q u a n t i t a t i v e y i e l d o f M o so d i d not V  the excess h y d r a z i n e . R - 2.74.  These r e s u l t s ,  however, i n d i c a t e t h e y o b t a i n e d  Since, according to Higginson, a monodelectronator  approach R = 1 under such c o n d i t i o n s t h e s e r e s u l t s a  determine  should VI  suggest Mo  behaved  as  didelectronator. 45 F i l l i p o v and Nuger  r e p o r t e d t h a t a t pH > 4 , 0.008 M molybdate  s o l u t i o n s were n o t reduced by 0.02 t o 0.04 M h y d r a z i n e .  A t pH 1.3 t o  1.4  o n l y molybdenum b l u e was formed. 79 Ostrowetsky and B r i n o n  s t u d i e d r e d u c t i o n o f molybdate s o l u t i o n s by  h y d r a z i n e i n 0.1 t o 10 N a c i d s o l u t i o n s .  I n t h i s range o f a c i d i t y mixed  M o / M o " compounds d i d n o t form and the o n l y p r o d u c t was M o . V  V]  V  The r a t e o f  o f the r e a c t i o n i n c r e a s e d w i t h i n c r e a s i n g temperature and d e c r e a s e d increasing a c i d i t y .  with  D e t a i l e d i n v e s t i g a t i o n s were made i n 2 N HC1 a t 0°C  where t h e r e a c t i o n was s l o w enough t o f o l l o w w i t h o u t d i f f i c u l t y .  Spot  t e s t s showed t h a t the s o l u t i o n s c o n t a i n e d h y d r o x y l a m i n e and p o l a r o g r a p h y i n 4 N NaOH a l l o w e d d e t e r m i n a t i o n o f the t o t a l c o n c e n t r a t i o n o f ^ H ^ + + NH^OH .  VI V Mo and Mo were determined by p o l a r o g r a p h y i n 2 N HC1.  - 40 -  It  was f o u n d t h a t t h e t o t a l c o n c e n t r a t i o n o f N„ H„ + NH OH: d i d .not c h a n g e 2 4 3 V  d u r i n g t h e r e a c t i o n a n d t h a t a p l o t o f Mo  vs i n i t i a l  mole r a t i o  [N^H^]/  [Mo] showed a b r e a k a t m o l e r a t i o 0.5. The r e a c t i o n p r o p o s e d was + + 2e~ , V „ + N-H.. + HMo_0. (Mo 0 ) „ + 2NH O H Z  o  The s p e c i e s HMo Og  +  2  Z b  Z  Z  o  3  80 had been p r o p o s e d by Chauveau e t a l . as the predomi-  VI n a n t Mo  species i n 2 N HCl. I n t h e same c o n d i t i o n s o f t e m p e r a t u r e a n d a c i d i t y ,  found t h a t f o r t h e s t o i c h i o m e t r i c r a t i o of  M t o 0.24 M c h a n g e d t h e r e a c t i o n s o t h a t N  f i e d products  resulted.  or  2  from  V I  as w e l l as o t h e r u n i d e n t i -  The r e a c t i o n s p r o p o s e d i n t h i s c a s e w e r e :  2HMo_0 *  +  N_H_ Z i>  H Mo 0 _ Z 4 13  +  N H  Z b  equation)  concentration of Mo  V I  2  0.02  It  ( a c c o r d i n g t o t h e above  [N H<_ ] / [ M o ] = 0.5 a n i n c r e a s e o f i n i t i a l +  h o w e v e r , i t was  +  +  Z o  + +  H  =  +  3H  +  Mo 0 4 B y l  =  Mo 0 4  4 + 0  4  +  Q  o  +  N_  +  N  Z  2  +  3H„0  +  5H 0  Z  Z  was r e a s o n e d t h a t t h e c h a n g e i n t h e r e a c t i o n was d u e t o a c h a n g e i n t h e  degree o f p o l y m e r i z a t i o n o f e i t h e r M o  V I  o r Mo . V  C h a u v e a u e t al.^° h a d  r e p o r t e d a n e q u i l i b r i u m b e t w e e n HMo„0 * a n d H Mo.O., _ w h i l e 2 6 2 4 13 V i o s s a t a n d Lamache"^  i n d i c a t e d t h a t t e t r a m e r i c Mo  with a dimeric  At a given a c i d i t y ,  form.  t i o n o f molybdenum w o u l d f a v o u r  V  t h e work o f  could exist  i nequilibrium  t h e r e f o r e , an i n c r e a s e i n concentra-  t h e tetracondensed  f o r m s o f t h e Mo " V  and Mo  C  V  s p e c i e s and hence a f o u r e q u i v a l e n t o x i d a t i o n o f h y d r a z i n e . Ostrowetsky and B r i n o n ' s  oxidation o f hydrazine  work a p p e a r s t o be t h e o n l y r e p o r t o f 81  t o hydroxylamine.  of o x i d a t i o n o f hydrazine  Audrieth  a n d Ogg  i ntheir  s t a t e d t h a t no e v i d e n c e had been r e p o r t e d  s t r a t e t h a t hydroxylamine i s an o x i d a t i o n product  o f hydrazine.  review  t o demon-  - 41 -  82  Huang a n d S p e n c e  a t 70°C i n p h o s p h a t e b u f f e r s o f pH 1.2 t o 3.2. N^Hj.  +  t o Mo  VI  VI a n d Mo  investigated the reaction o f hydrazine  was m a i n t a i n e d a t 0.5 a n d Mo  VI  The i n i t i a l m o l e r a t i o o f  was v a r i e d b e t w e e n 5 x 1 0  -4  M  -4 and  7 x 10  M.  The h y d r a z i n e  r e a c t i o n was f i r s t with  order  was o x i d i z e d q u a n t i t a t i v e l y t o  i n each r e a c t a n t .  i n c r e a s i n g pH a n d e x h i b i t e d a n o r d e r  t h a t t h e f r a c t i o n a l dependence on H  +  The r a t e o f r e a c t i o n i nH  could  o f 0.25.  +  I t was s u g g e s t e d  VI  t i o n o f Mo +  increased  be due t o changes i n p o l y m e r i z a -  VI  ment o f H  and t h e  w i t h pH, i o n i z a t i o n o f a m o n o m e r i c Mo i n a rate c o n t r o l l i n g step.  by mass s p e c t r o m e t r y a n d t r a p p i n g stiochiometry  N  H 2  2  w  a  ^  S  species,  e t e c t e c  with unsaturated  ^  qualitatively  acids.  and t h e p r e s e n c e o f ^H,, i n d i c a t e d M o  V I  or involve-  The o b s e r v e d  behaved as a d i d e l e c -  tronator. 83 Nusgra and Sinha 0.01 and  M Mo  V I  reacted  ^ 2 ^ 2 depending  o  o b s e r v e d t h a t i n 1 N H^SO^ 0.025 M h y d r a z i n e  to yield N n  t  n  e  quantitatively.  r e l a t i v e amount o f M o  V I  They d e t e c t e d  both  and  N H 2  added t o h y d r a z i n e  3  solu-  tions . The  preceding  to nitrogen with  summary i n d i c a t e s t h a t M o  R - 4.  This  didelectronation to yield N H 2  delectronation  steps.  V I  tends t o o x i d i z e  has been i n t e r p r e t e d i n terms o f predominant 2  as a f i r s t  Near-quantitative  step  a n d a l s o b y s e r i e s o f mono-  oxidation to N  2  by one e l e c t r o n  however, appears t o be u n l i k e l y except i n a l k a l i n e s o l u t i o n . steps  r e q u i r e t h a t t h e molybdate species  are monomeric.  steps,  Two e l e c t r o n  i n v o l v e d be r e d u c e d t o M o  Mechanisms b a s e d o n t h e assumed c h e m i s t r y  however, be open t o q u e s t i o n  hydrazine  of Mo  I V  I V  i f they  must,  u n t i l more i s known a b o u t t h e c h e m i s t r y  o f Mo . I V  VI Since  Mo  t e n d s t o p o l y m e r i z e i t seems r e a s o n a b l e t o assume t h a t t h e n a t u r e  o f t h e r e a c t i o n c o u l d depend on t h e p a r t i c u l a r M o  V I  species  involved.  - 42  As h a s b e e n s e e n t h i s  concept  was  -  u s e d by O s t r o w e t s k y  and  Brinon to explain  a change i n t h e n a t u r e o f t h e r e a c t i o n w i t h i n c r e a s i n g molybdenum tion i n acid  solution.  T h e r e h a v e b e e n no  u n d e r c o n d i t i o n o f c o n c e n t r a t i o n and nate  d e t a i l e d s t u d i e s o f the r e a c t i o n  where m o l y b d a t e p o l y a n i o n s  i n s o l u t i o n a n d w h e r e f o r m a t i o n o f m i x e d Mo /Mo^^~  expected  V  to occur.  The  work o f J a k o b and  t h a t f o u r e l e c t r o n s t o i c h i o m e t r y and Mo  pH  V . VI /Mo species are  t o be  expected.  concentra-  predomi-  species might  K o z l o w s k i does, however,  at least intermediate formation  be  suggest of  - 43 -  2.4  R e d u c t i o n w i t h SO  and H S  31 H o l t j e and Geyer r e p o r t e d t h a t SC^ reduces weakly a c i d molybdate s o l u t i o n s t o g i v e l i g h t green o r b l u e s o l u t i o n s i n w h i c h o n l y 0.5% o f the V molybdenum i s p r e s e n t as Mo . 88 reaction occurred.  Wardlaw  In more c o n c e n t r a t e d a c i d no n o t i c e a b l e made s i m i l a r o b s e r v a t i o n s f o r b o t h SC^ and I ^ S .  Thus a t b e s t i t appears t h a t n e i t h e r o f t h e s e r e d u c t a n t s can take molybdate p a s t the mixed v a l e n t molybdenum b l u e  stage.  - 44  2.5  The  work r e p o r t e d  i n the  t a t i o n o f molybdenum o x i d e s  -  Summary  literature  indicates that reductive  w i t h hydrogen or hydrazine  feasible.  Hydrogen and  hydrazine,  reductants  thus they o f f e r the p o t e n t i a l of producing  Most o f the work r e p o r t e d , and  as  the a c t i o n of these reductants  on  m i g h t be t e c h n i c a l l y  a four electron reductant,  however, has  precipi-  are  a pure oxide  clean product.  been performed i n a c i d s o l u t i o n s  n e u t r a l or a l k a l i n e molybdate s o l u t i o n s  i s not w e l l documented. I n t h e n e u t r a l pH d a t e i o n s m i g h t be VI v a l e n t Mo could  range i t i s apparent t h a t p o l y m e r i z a t i o n  a f a c t o r i n any  reduction  reaction.  of  molyb-  In a d d i t i o n mixed-  V /Mo  compounds m i g h t be  i n f l u e n c e the  t i o n with hydrazine d e p e n d i n g on pH I t has  course of the the  expected to form i n t h i s reduction reaction.  r e a c t i o n m i g h t be  r a n g e and  In the  case of  these reduc-  able to f o l l o w d i f f e r e n t paths  and  degree of p o l y m e r i z a t i o n of molybdate. Ill IV V b e e n s e e n t h a t Mo , Mo , and Mo hydrated oxides  as m i x e d v a l e n t o x i d e s also a p o s s i b i l i t y  can  be  t h a t t h e Mo  precipitated in neutral solution. V  p r e c i p i t a t e may  a c t as  an  ion  as  well  There i s  exchanger.  - 45 -  3.  SCOPE OF PRESENT WORK  The p r e s e n t work was undertaken t o i n v e s t i g a t e the use o f hydrogen and h y d r a z i n e f o r r e d u c t i v e p r e c i p i t a t i o n o f molybdenum o x i d e s from s o l u t i o n s produced by a sodium h y p o c h l o r i t e l e a c h o f Cu-Mo rougher c o n c e n t r a t e s .  The  s p e c i f i c o b j e c t i v e s were t o d e f i n e the pH range i n w h i c h r e d u c t i o n c o u l d be a c h i e v e d , the k i n e t i c s o f the r e d u c t i o n r e a c t i o n s ,  and the n a t u r e o f  the  oxide products. The p r o c e d u r e adopted was f i r s t t o determine the b e h a v i o u r o f r e a c t i o n s u s i n g sodium molybdate s o l u t i o n s then t o i n v e s t i g a t e the  the  effects  o f o t h e r components o f the a c t u a l l e a c h s o l u t i o n s , i n p a r t i c u l a r copper and sodium c h l o r i d e .  - 46 -  4.  EXPERIMENTAL  Molybdate s o l u t i o n s f o r r e d u c t i o n by b o t h hydrogen and h y d r a z i n e were made up w i t h M a l l i n c k r o d t a n a l y t i c a l r e a g e n t sodium molybdate (Na2MoO^"2H2O) .  The m a n u f a c t u r e r ' s assay was 99.5% sodium molybdate  minimum and the c h e m i c a l was used w i t h o u t f u r t h e r  purification.  molybdate s o l u t i o n s were s t a n d a r d i z e d by flame atomic a b s o r p t i o n  The spectro-  photometry and g r a v i m e t r i c a n a l y s i s f o r molybdenum. Mo^ s t o c k s o l u t i o n s f o r p o l a r o g r a p h i c c a l i b r a t i o n c u r v e s and n e u t r a l i z a t i o n experiments were produced by r e d u c i n g 3.5 N HC1 sodium molybdate s o l u t i o n s by s h a k i n g i n a f l a s k w i t h m e t a l l i c m e r c u r y .  The s o l u t i o n s were 46 i  s t a n d a r d i z e d by t i t r a t i o n w i t h e e r i e s u l f a t e  u s i n g f e r r o i n as an i n d i c a t o r .  '  H y d r a z i n e s o l u t i o n s were made up by d i l u t i o n o f BDH 99-100% h y d r a z i n e "hydrate o r Eastman Kodak 64% h y d r a z i n e i n d i s t i l l e d w a t e r .  The h y d r a z i n e 85  s o l u t i o n s were s t a n d a r d i z e d by p o t a s s i u m i o d a t e t i t r a t i o n i n 5 N HC1. Commercial tank hydrogen was used f o r hydrogen r e d u c t i o n  experiments.  Hydrogen r e d u c t i o n was performed i n a P a r r 2 l i t r e a u t o c l a v e . g l a s s l i n e r was used f o r a l l r u n s . s o l u t i o n were t i t a n i u m .  A l l p a r t s o f the bomb c o n t a c t i n g  The r e s i s t a n c e  c o n t r o l l e d by a Y e l l o w s p r i n g s Instrument C o n t r o l l e r Model 71 and a V a r i a c .  A the  h e a t e r s u r r o u n d i n g the bomb was Company Thermistemp  Temperature  The V a r i a c was s e t t o g i v e the a p p r o x i -  mate t e m p e r a t u r e d e s i r e d and the Thermistemp m a i n t a i n e d the d e s i r e d  tempera-  t u r e by o p e r a t i n g a r e l a y w h i c h reduced the power i n p u t to the h e a t e r by about 15% i n the c o o l i n g c y c l e s .  I t was found t h a t on p r e s s u r i z i n g the bomb  w i t h hydrogen t h e r e was a s i g n i f i c a n t t e m p e r a t u r e i n c r e a s e t o 30 atm H  (e.g. pressurizing  a t 180°C l e d t o a t e m p e r a t u r e i n c r e a s e o f around 3 0 ° C ) .  To  - 47 -  reduce t h i s e f f e c t a d d i t i o n a l c o o l i n g was p r o v i d e d by a j e t o f compressed a i r o r n i t r o g e n i n t r o d u c e d a t the base o f the bomb.  The gas f l o w was c o n -  t r o l l e d by a s o l e n o i d v a l v e which opened on the c o o l i n g c y c l e o f Thermistemp.  the  I n a l l runs s t i r r i n g was m a i n t a i n e d a t 600 rpm.  I n a t y p i c a l r u n one l i t r e o f sodium molybdate s o l u t i o n a d j u s t e d  to  the d e s i r e d pH was added t o the g l a s s l i n e r and the bomb was s e a l e d . N i t r o g e n was bubbled through the s o l u t i o n f o r about f i v e minutes and the bomb was p r e s s u r i z e d t o 50 p s i g w i t h n i t r o g e n .  The bomb was brought r a p i d l y t o  j u s t below the d e s i r e d temperature u s i n g f u l l power i n p u t t o the  furnace.  The V a r i a c s e t t i n g was then reduced and the temperature o f the r u n c o n t r o l l e d by the Thermistemp. t o the bomb.  Once the temperature was s t a b l e hydrogen was a d m i t t e d  The hydrogen p r e s s u r e was i n c r e a s e d t o t h a t d e s i r e d and m a i n -  t a i n e d throughout the r u n .  The v a r i a t i o n i n temperature d u r i n g a r u n was  ±5°C and p r e s s u r e ±5 p s i g . Samples were t a k e n a t a p p r o p r i a t e  t i m e s throughout  each r u n .  sampling system was c l e a r e d b e f o r e each sample by d i s c h a r g i n g a t 20 ml o f s o l u t i o n .  A g i t a t i o n was m a i n t a i n e d d u r i n g s a m p l i n g .  The  least  Samples were  c o o l e d e i t h e r by s t a n d i n g a t room temperature o r h o l d i n g under c o l d  tap  water. A f t e r each r u n the a p p a r a t u s was c l e a n e d by r u n n i n g f o r one hour 100°C w i t h 50% n i t r i c a c i d i n the g l a s s  at  liner.  H y d r a z i n e experiments were performed i n a 420 ml p y r e x v e s s e l . The r e a c t o r was m a i n t a i n e d a t the d e s i r e d temperature by immersion i n a water b a t h .  Temperature c o n t r o l was ± 0 . 3 ° C .  I n a t y p i c a l r u n 200 ml o f sodium molybdate s o l u t i o n a d j u s t e d d e s i r e d pH w i t h HCl was brought t o temperature i n the r e a c t o r .  to  An e q u a l  the  - 48 -  volume o f h y d r a z i n e s o l u t i o n a d j u s t e d t o the same pH was p r e h e a t e d i n a separate f l a s k .  A t z e r o time the h y d r a z i n e s o l u t i o n was added to the m o l y b -  date s o l u t i o n . S t i r r i n g was m a i n t a i n e d a t 300 rpm f o r each r u n .  The pH  was c o n t r o l l e d by a d d i t i o n s o f 1:1 HC1 as r e q u i r e d d u r i n g the r u n . Samples were t a k e n by p i p e t t i n g 15 t o 30 ml o f the r e a c t i n g s l u r r y from the r e a c t o r .  V a r i o u s methods were used t o f r e e z e the r e a c t i o n to p e r m i t  d e t e r m i n a t i o n o f the c o n c e n t r a t i o n s o f molybdenum and h y d r a z i n e as  functions  of t i m e . S i n c e p r e l i m i n a r y runs had shown t h a t the r e d u c t i o n r e a c t i o n d i d n o t o c c u r above pH 6.5 the f i r s t method adopted was t o d i s c h a r g e the sample i n t o a c e n t r i f u g e tube c o n t a i n i n g enough c o n c e n t r a t e d NaOH t o r a i s e the pH t o V between 8 and 12.  I t was hoped t h i s would a l s o p r e c i p i t a t e a l l o f the Mo  formed. The second method used was t o d i s c h a r g e the sample i n t o a p o r c e l a i n filter  c r u c i b l e and c o l l e c t the f i l t r a t e i n a c e n t r i f u g e tube a g a i n c o n -  t a i n i n g enough NaOH t o r a i s e the pH t o 8 t o 12. gave s i g n i f i c a n t l y d i f f e r e n t  T h i s method o f a n a l y s i s  results.  The t h i r d p r o c e d u r e was t o d i s c h a r g e the sample i n t o a s m a l l beaker and q u i c k l y t i t r a t e i t w i t h 0.1 t o 1 N NaOH t o o b t a i n a p a r t i c u l a r p H . The r e s u l t i n g s l u r r y was then f i l t e r e d and the f i l t r a t e a n a l y z e d f o r molybdenum and h y d r a z i n e . A f o u r t h p r o c e d u r e was s i m p l y t o d i s c h a r g e the sample i n t o a beaker c o n t a i n i n g an e q u a l volume o f water a t about 0 ° C .  P r e l i m i n a r y runs had shown  the r e a c t i o n was v e r y slow a t room temperature so i t was f e l t t h i s p r o c e d u r e would p e r m i t time f o r subsequent The f i f t h  filtration.  t e c h n i q u e used was t o d i s c h a r g e the sample i n t o a  filter  - 49 -  c r u c i b l e then d i l u t e the f i l t r a t e t o a c o n c e n t r a t i o n s u i t a b l e f o r t i o n o f molybdenum by atomic a b s o r p t i o n  determina-  (-10 u g / m l ) .  A few runs were performed t o f o l l o w the e v o l u t i o n o f n i t r o g e n gas during reaction.  The r e a c t i o n was c a r r i e d out i n a s e a l e d f l a s k and the  evolved c o l l e c t e d over mercury. ture.  These runs were c a r r i e d o u t a t room tempera-  To b e g i n a r u n a h y d r a z i n e s o l u t i o n a d j u s t e d  a molybdate s o l u t i o n o f the same pH i n the f l a s k . s t o p p e r e d and s t i r r i n g begun.  gas  t o pH = 4.5 was added t o The f l a s k was q u i c k l y  The pH was not c o n t r o l l e d d u r i n g these r u n s .  Molybdenum i n s o l u t i o n was determined b o t h by flame atomic a b s o r p t i o n spectrophotometry  u s i n g a P e r k i n Elmer Model 306 s p e c t r o p h o t o m e t e r  p o l a r o g r a p h y u s i n g a Sargent Model XXI p o l a r o g r a p h .  and by  Solutions for p o l a r -  ography were d i l u t e d t o c o n t a i n between 0.05 and 0.15 g/1 Mo w h i l e those for  atomic a b s o r p t i o n were d i l u t e d to c o n t a i n between 0.01 and 0.04 g/1 Mo.  D i l u t i o n f o r atomic a b s o r p t i o n was performed w i t h a 10% A l C l ^ ,  5% NH^Cl  86 s o l u t i o n which was found by Ismay  to eliminate interferences.  was performed i n 2 N H C l where the waves o f M o and M o V  are  V I  Polarography  distinct.  7 9  Atomic a b s o r p t i o n gave t o t a l molybdenum r e g a r d l e s s o f the p r o p o r t i o n s V VI o f Mo and Mo i n the sample and h y d r a z i n e d i d not i n t e r f e r e . Polarography VI V o f Mo /Mo m i x t u r e s i n the p r e s e n c e o f h y d r a z i n e was u n s u i t a b l e f o r s e p a r a t e V VI d e t e r m x n a t i o n o f Mo amd Mo because a t the a c i d i t y r e q u r i e d (2 N HCl) the r e d u c t i o n o f M o by h y d r a z i n e proceeded a t a r a t e s u f f i c i e n t t o make V I  V the d e t e r m i n a t i o n o f Mo determined.  of doubtful accuracy.  T o t a l molybdenum c o u l d be  Atomic a b s o r p t i o n and p o l a r o g r a p h y agreed w i t h i n 2%.  H y d r a z i n e i n sample f i l t r a t e s was determined by t i t r a t i o n w i t h 0.1 N KIO-j i n 5 N H C l . The end p o i n t was marked by the d i s a p p e a r a n c e i o d i n e c o l o u r from a C C l ^ l a y e r i n the t i t r a t i o n f l a s k . ^  5  o f the p i n k  Ammonia,  - 50 -  h y d r o x y l a m i n e , and Mo  d i d not i n t e r f e r e .  When a p p r e c i a b l e h y d r o x y l a m i n e  was p r e s e n t i n the t i t r a t i o n s o l u t i o n , however, a f a i n t p i n k c o l o u r r e t u r n e d V t o the C C l ^ l a y e r on s t a n d i n g f o r a few h o u r s .  Mo  did interfere  i n the  t i t r a t i o n b u t i n g e n e r a l the amount o f M o p r e s e n t i n the r e a c t i o n V  filtrates  was s m a l l enough t o r e n d e r a c o r r e c t i o n u n n e c e s s a r y . Ammonia i n sample f i l t r a t e s was determined by a method d e s c r i b e d by 87 D e V r i e s and G a n t z . viously described. Na^SO^.  H y d r a z i n e was f i r s t o x i d i z e d by 0.4 N KIO^ as p r e The I  +  and excess 10^  were reduced to I  by excess  The excess s u l f i t e was then removed by b u b b l i n g a i r f o r 15 m i n u t e s .  Ammonia was then determined by d i s t i l l a t i o n i n t o 0.1 N HC1 a f t e r making the solution basic.  Hydroxylamine d i d not  interfere.  P r e c i p i t a t e s produced by h y d r a z i n e r e d u c t i o n were a n a l y z e d f o r t o t a l 89 molybdenum g r a v i m e t r i c a l l y by p r e c i p i t a t i o n o f molybdenum as PbMoO^. The mean o x i d a t i o n s t a t e o f molybdenum was determined by o x i d a t i o n w i t h excess e e r i e s u l f a t e and back t i t r a t i o n w i t h f e r r o u s ammonium s u l f a t e u s i n g f e r r o i n 46 as an i n d i c a t o r . analyzer.  Water was d e t e r m i n e d u s i n g a Dupont 950 t h e r m o g r a v i m e t r i c  C h l o r i n e was determined by d i s s o l v i n g a sample o f the  precipitate  i n 3 M HNO^, a d d i t i o n o f excess AgNO^, and back t i t r a t i o n w i t h H C l u s i n g a s i l v e r e l e c t r o d e and a p o t a s s i u m s u l f a t e r e f e r e n c e  electrode."^  Sodium was  determined by flame e m i s s i o n p h o t o m e t r y . P r e c i p i t a t e s r e s u l t i n g from hydrogen r e d u c t i o n were c h a r a c t e r i z e d by X - r a y d i f f r a c t i o n and t h e r m o g r a v i m e t r i c a n a l y s i s .  - 51 -  5. 5.1  RESULTS  Hydrogen R e d u c t i o n  Hydrogen r e d u c t i o n experiments were performed a t 200° t o 220°C w i t h 30 atm p r e s s u r e o f  .  The i n i t i a l c o n c e n t r a t i o n o f molybdenum and  the i n i t i a l pH were 17 g/1 and 2, r e s p e c t i v e l y .  F o r t i m e s up t o 7 hours  r e d u c t i o n proceeded o n l y t o the molybdenum b l u e s t a g e and no p r e c i p i t a t i o n occurred. An experiment was then performed i n t h e presence o f a p l a t i n u m c l a d expanded n i o b i u m mesh.  I n t h i s case the s o l u t i o n passed through the m o l y b -  denum b l u e stage i n 30 minutes and l a t e r samples were o n l y s l i g h t l y c o l o u r e d . F i g u r e 8 shows a p l o t o f p e r c e n t r e d u c t i o n vs t i m e .  The f i n a l pH was 8.  The b l a c k p r e c i p i t a t e formed o n l y s l i g h t l y p l a t e d the apparatus and was easily filterable. 5-0452 f o r Mo0 • 2  anhydrous.  I t gave an X - r a y d i f f r a c t i o n p a t t e r n  i d e n t i c a l t o ASTM  T h e r m o g r a v i m e t r i c a n a l y s i s showed the p r e c i p i t a t e was  Experiments w i t h the mesh were n o t c o n t i n u e d because the n i o b i u m  s u b s t r a t e s u f f e r e d extreme hydrogen damage and broke under i t s own w e i g h t . The r u n w i t h the mesh r e s u l t e d i n some p r e c i p i t a t i o n on the and o t h e r f i x t u r e s as w e l l as the g l a s s l i n e r .  A further  stirrer  run performed  w i t h o u t c l e a n i n g the a p p a r a t u s r e s u l t e d i n f o r m a t i o n o f a b l u e p r e c i p i t a t e g i v i n g an X - r a y powder d i f f r a c t i o n p a t t e r n  i n d i c a t i n g the presence o f Mo0  2  90 and MoO^ (ASTM 21-569) .  Ceric sulfate t i t r a t i o n  i n d i c a t e d the  o x i d a t i o n s t a t e o f molybdenum i n t h i s p r e c i p i t a t e was 5 . 5 3 . amount o f p r e c i p i t a t e was formed a f t e r  12 h o u r s .  average  Only a s m a l l  The dark b l u e s o l u t i o n  c o n t a i n e d 16 g/1 molybdenum w i t h an average o x i d a t i o n s t a t e o f 5 . 6 7 . The a p p a r a t u s was then c l e a n e d t o remove a l l d e p o s i t s and the r u n was  - 52 -  20  L_  0  • 5  L 10 TIME (hours)  F i g u r e 8:  R e d u c t i o n o f 17 g/1 sodium molybdate s o l u t i o n by hydrogen a t 200°C and 30 atm H i n the p r e s e n c e o f P t c l a d n i o b i u m mesh. 2  - 53 -  repeated.  Almost no r e d u c t i o n o c c u r r e d .  The s o l u t i o n was c o l o u r e d l i g h t  b l u e and t h e r e was no p r e c i p i t a t e . Three runs were performed i n 3 M NaCl to s i m u l a t e s o l u t i o n s from the proposed sodium h y p o c h l o r i t e l e a c h .  M i n i m a l r e d u c t i o n was o b t a i n e d even  when u s i n g m e t a l l i c molybdenum powder as a c a t a l y s t . Hydrogen r e d u c t i o n experiments were d i s c o n t i n u e d when i t was o b s e r v e d t h a t the l i q u i d condensing between t h e g l a s s l i n e r and the bomb s e v e r e l y c o r r o d e d the t i t a n i u m bomb.  - 54 -  5.2  Reduction with Hydrazine  P r e l i m i n a r y runs were c a r r i e d out a t 50°C u s i n g 5 g/1 molybdenum s o l u t i o n s and v a r i o u s h y d r a z i n e c o n c e n t r a t i o n s .  F o r pH ~ 5 r e a c t i o n  o c c u r r e d a l m o s t i m m e d i a t e l y on m i x i n g the r e a c t a n t s . went a s e r i e s o f c o l o u r changes. f i n a l l y an opaque brown.  concentration,  f i r s t became b l u e , then g r e e n ,  A brown p r e c i p i t a t e  but not during the i n i t i a l blue The e f f e c t  It  The s o l u t i o n u n d e r -  formed d u r i n g the green  stage  one.  o f pH was i n v e s t i g a t e d  a t 50°C, 5 g/1 i n i t i a l molybdenum  and an i n i t i a l h y d r a z i n e t o molybdenum mole r a t i o o f 4 : 1 .  F o r pH > 6.5 no r e a c t i o n o c c u r r e d .  As the pH was d e c r e a s e d below 6.5  r a t e o f p r e c i p i t a t i o n i n c r e a s e d but f o r pH < 3 the n a t u r e o f the changed.  and  the  reaction  The i n i t i a l b l u e c o l o u r remained t h r o u g h o u t the r e a c t i o n and  rate of p r e c i p i t a t i o n decreased.  The p r e c i p i t a t e  the  formed was b l u e r a t h e r  than brown. More d e t a i l e d i n v e s t i g a t i o n s were performed a t pH = 4 . 5 , 4:1 i n i t i a l mole r a t i o o f h y d r a z i n e t o molybdenum. a convenient r e a c t i o n r a t e for a n a l y s i s .  50°C and  These c o n d i t i o n s gave  The r e s u l t s o b t a i n e d ,  however,  were  found t o depend m a r k e l y on the s a m p l i n g method employed. F i g u r e 9 c u r v e a i s a p l o t o f the c o n c e n t r a t i o n  o f molybdenum r e m a i n -  i n g i n s o l u t i o n vs time o b t a i n e d by n e u t r a l i z i n g 20 ml samples o f the  re-  a c t i n g s l u r r y w i t h 8 N NaOH and then c e n t r i f u g i n g t o o b t a i n a s u p e r n a t a n t s o l u t i o n for a n a l y s i s .  Curves b t h r o u g h e were o b t a i n e d by n e u t r a l i z i n g  20 ml samples o f the s l u r r y by t i t r a t i o n t o the p H ' s i n d i c a t e d f o l l o w e d by f i l t r a t i o n through p o r c e l a i n f i l t e r c r u c i b l e s .  Curve f was o b t a i n e d by  d i s c h a r g i n g the sample o f s l u r r y i n t o 20 ml o f c o l d water f o l l o w e d by  - 55 -  I 0 F i g u r e 9:  I  30  I  I  60 90 TIME (minutes)  L_  120  E f f e c t o f s a m p l i n g t e c h n i q u e on c o n c e n t r a t i o n o f molybdenum r e m a i n i n g i n s o l u t i o n vs t i m e a t 50°C w i t h pH = 4 . 5 , i n i t i a l molybdenum c o n c e n t r a t i o n =5 g / 1 , and i n i t i a l h y d r a z i n e t o molybdenum mole r a t i o =4:1. "  - 56 -  filtration. Figure direct  10 c o m p a r e s t h e r e s u l t s o b t a i n e d  filtration  charging  the latter  direct filtration for  o f t h e s l u r r y samples w i t h those  t h e s l u r r y i n t o an e q u a l  represent  technique.  The t r i a n g l e s r e p r e s e n t  results  of the slurry  1 m l o f 8 N NaOH.  The o p e n  methods gave  points  similar  i n solution. i n t o c e n t r i f u g e tubes  p r e c i p i t a t e on mixing  with the caustic.  shows t h e d r y w e i g h t o f p r e c i p i t a t e r e t a i n e d o n f i l t e r i n g s l u r r y through a p o r c e l a i n f i l t e r  (50°C,  u n d e r t h e same c o n d i t i o n s  denum c o n c e n t r a t i o n = 5 g / 1 )  formed  Figure  11  25 m l s a m p l e s o f  and t h e c o n c e n t r a t i o n o f molybdenum  i n s o l u t i o n i n the centrifuged f i l t r a t e .  r e s u l t s obtained  con-  c r u c i b l e , the d r y weight o f the p r e c i p i -  formed i n t h e c e n t r i f u g e tube,  remaining  b u t w i t h a 2:1  Figure pH = 4 . 5 ,  initial  12 shows t h e initial  molyb-  mole r a t i o o f h y d r a z i n e  molybdenum i n s t e a d o f 4 : 1 . Since  t h e above s a m p l i n g  m e t h o d was r a t h e r t e d i o u s  i t was  decided  use t h e s i m p l e r p r o c e d u r e o f d i r e c t a n a l y s i s o f t h e sample f i l t r a t e f o r  molybdenum r e m a i n i n g  i n s o l u t i o n i n order  temperature and i n i t i a l  hydrazine  t a t i o n r e a c t i o n . . The i n i t i a l pH  represent  samples i n t o c e n t r i f u g e tubes  was p e r f o r m e d b y f i l t r a t i o n  a brown o r b l u e - g r e e n  to  circles  8 N NaOH i t was o b s e r v e d t h a t t h e c l e a r l i g h t b r o w n f i l t r a t e  taining  to  dis-  p o i n t s o b t a i n e d by  The c r o s s e s  A l l three o f these  f o r molybdenum r e m a i n i n g When s a m p l i n g  tate  by f i r s t  o f t h e s l u r r y and d i l u t i o n o f an a l i q u o t o f t h e f i l t r a t e  by d i r e c t f i l t r a t i o n  containing  obtained  volume o f c o l d w a t e r .  a t o m i c a b s o r p t i o n a n a l y s i s o f molybdenum.  obtained  u s i n g methods i n v o l v i n g  = 4.5  f o r each run.  to investigate the effects of  c o n c e n t r a t i o n on t h e r a t e o f t h e p r e c i p i -  m o l y b d e n u m c o n c e n t r a t i o n was ~5  Figures  13 a n d 14 show t h e r e s u l t s  g/1 and t h e  obtained.  - 57 -  pH = 4.5 T=50°C inilibl Mo = 5 g/1 initial N H = 6 . 6 g / l 2  4  o d i s c h a r g e into cold water then f i l t r a t i o n a d i r e c t filtration into c a u s t i c A direct  filtration then d i l u t i o n  20 T I M E (minutes) Figure  10:  30  Comparison o f r e s u l t s o b t a i n e d by d i r e c t f i l t r a t i o n o f s l u r r y samples w i t h t h o s e o b t a i n e d by p r e v i o u s d i l u t i o n o f samples w i t h an e q u a l volume o f c o l d wa'ter.  - 58 -  T I M E (minuteS' ) F i g u r e 11:  D i s t r i b u t i o n o f molybdenum between s o l u t i o n and p r e c i p i t a t e s as a f u n c t i o n o f time a t 50° C w i t h pH=4.5, i n i t i a l molybdenum c o n c e n t r a t i o n =5 g/1, and i n i t i a l h y d r a z i n e t o molybdenum mole r a t i o =4:1. *  - 59 -  0 F i g u r e 12:  20  40 TIME (minutes)  60  D i s t r i b u t i o n o f molybdenum between s o l u t i o n and p r e c i p i t a t e s as a f u n c t i o n o f time a t 50°C w i t h pH = 4 . 5 , i n i t i a l m o l y b denum c o n c e n t r a t i o n =5 g / 1 , and i n i t i a l mole r a t i o o f h y d r a z i n e t o molybdenum - 2 : 1 .  - 60  -  - 61 -  0 F i g u r e 14:  10  20 T I M E (minutes)  30  E f f e c t o f i n i t i a l mole r a t i o o f h y d r a z i n e t o molybdenum on r a t e o f p r e c i p i t a t i o n a t 50°C w i t h pH = 4.5 and i n i t i a l c o n c e n t r a t i o n o f molybdenum =5 g/1.  - 62 -  I n o r d e r t o s i m u l a t e the c o n d i t i o n s e x p e c t e d i n t r e a t i n g  solutions  from t h e sodium h y p o c h l o r i t e l e a c h a r u n was c a r r i e d o u t i n 3 M N a C l . F i g u r e 15 compares the r e s u l t s o f t h i s r u n w i t h one performed under  the  same c o n d i t i o n s b u t w i t h o u t NaCl a d d i t i o n . The e f f e c t o f copper on the r e a c t i o n i s shown i n F i g u r e 1 6 . presence o f copper d i d n o t a f f e c t  The  the r a t e o f p r e c i p i t a t i o n o f molybdenum.  Some o f the copper was a l s o p r e c i p i t a t e d .  The p r e c i p i t a t e i t s e l f  changed  from the normal brown c o l o u r t o deep b l u e on d r y i n g f o r a few minutes i n air. I t can be seen from F i g u r e s 14 and 15 t h a t f o r low i n i t i a l mole r a t i o s o f h y d r a z i n e t o molybdenum and i n the p r e s e n c e o f 3 M NaCl t h e r e was a s i g n i f i c a n t t i m e b e f o r e p r e c i p i t a t i o n began.  I n each c a s e ,  however,  r e a c t i o n began i m m e d i a t e l y on m i x i n g the r e a c t a n t s as e v i d e n c e d by the appearance o f the c o l o u r s c h a r a c t e r i s t i c o f the mixed v a l e n t molybdenum blue species.  I n the extreme case o f a 1:1 i n i t i a l mole r a t i o o f r e a c t a n t s  no p r e c i p i t a t e was formed i n 1 hour a t 50°C and pH 4 . 5 even though some r e d u c t i o n had o c c u r r e d . The a p p a r e n t r e a c t i o n o r d e r i n molybdenum was 1.5 as shown i n F i g u r e 17 w h i c h was p l o t t e d u s i n g the d a t a o f F i g u r e 1 4 .  The s l o p e s o f  the l i n e s i n F i g u r e 17 were used t o determine the o r d e r i n ^ H ^ . 18 i s a p l o t o f I n s l o p e v s I n [N H ] • 2  Figure  From F i g u r e 18 the o r d e r i n N H 2  4  1 5 was 1.64.  W r i t i n g the p r e c i p i t a t i o n r e a c t i o n as -d[Mo]  =  k[Mo]  1.64 [N^H^] "  a l l o w e d d e t e r m i n a t i o n o f the r a t e c o n s t a n t k from the  of F i g u r e 17.  slopes  The v a l u e s o f k so o b t a i n e d are g i v e n i n T a b l e I I I .  The v a l u e o b t a i n e d f o r i n i t i a l h y d r a z i n e c o n c e n t r a t i o n = 0.522 m o l e / 1  - 63 -  5.oh 60---  4.0 3 M  No Cl  3.0  <  or Z LU O  o u  2 .0  o  2  no NoCI 1.0  -L  3 4 TIME (hours) F i g u r e 15:  E f f e c t o f 3 M NaCl on r a t e o f p r e c i p i t a t i o n a t 50°C f o r pH = 4.5, i n i t i a l molybdenum c o n c e n t r a t i o n =5 g/1, and i n i t i a l mole r a t i o o f h y d r a z i n e t o molybdenum - 4:1.  - 64 -  1  0 F i g u r e 16:  i  20  i  40 TIME ( m i n u t e s )  :  i _  60  E f f e c t o f a d d i t i o n o f 0.55 g/1 Cu as copper s u l f a t e on the r a t e o f p r e c i p i t a t i o n o f molybdenum a t 50°C f o r pH = 4 . 5 , i n i t i a l molybdenum c o n c e n t r a t i o n =5 g/1," and i n i t i a l h y d r a z i n e t o molybdenum mole r a t i o - 4:1.  - 65 -  0 F i g u r e 17:  10  20 TIM E (minutes)  30  1.5 o r d e r i n molybdenum p l o t s f o r T = 50°C, pH = 4.5, and i n i t i a l molybdenum c o n c e n t r a t i o n =5 g/1.  - 66 -  —1  -1.8  LN  1  I  I  -1.4 I N I T I A L HYDRAZINE  I  I  -1.0 -0.6 CONCENTRATION  (mole/1) F i g u r e 18:  Order i n N H  L_  based on 1.5 o r d e r i n molybdenum.  - 67 -  i n i t i a l hydrazine to molybdenum mole r a t i o  3:1  3.35  4:1  3.55  5.5:1  3.32  7:1  3.07  Table I I I  Rate Constants Obtained of  (10:1  , -2.14 2.14 k mole 1  I n i t i a l Hydrazine  . -1 mm  for Different  Values  t o Molybdenum Mole R a t i o  i n i t i a l mole r a t i o o f h y d r a z i n e t o molybdenum) has n o t been i n c l u d e d  because t h e r e a c t i o n was more r a p i d than c o u l d be f o l l o w e d a c c u r a t e l y w i t h the sampling procedure The temperature  used.  dependence d a t a o f F i g u r e 13 i s p l o t t e d i n F i g u r e 19  assuming 1.5 o r d e r i n molybdenum c o n c e n t r a t i o n .  In each case pH = 4.5,  i n i t i a l molybdenum c o n c e n t r a t i o n =5 g/1, and i n i t i a l mole r a t i o o f h y d r a z i n e to  molybdenum = 4:1.  F i g u r e 20 i s an A r r h e n i u s p l o t based on the s l o p e s o f  the l i n e s from F i g u r e 19.  The a c t i v a t i o n energy  o b t a i n e d i s 14.1 kcal/mole.  G r a v i m e t r i c a n a l y s i s o f t h e brown p r e c i p i t a t e a f t e r d r y i n g o v e r n i g h t in  an evacuated  MoO(OH)^58.9%).  d e s s i c a t o r y i e l d e d 57.5% and 56.5% molybdenum  (expected f o r  C e r i c s u l f a t e t i t r a t i o n s assuming a l l molybdenum was  V p r e s e n t as Mo The  y i e l d e d 54.0% and 52.0% molybdenum.  thermogravimetric  weight l o s s curve o b t a i n e d i n a h e l i u m atmos-  phere i s shown i n F i g u r e 21. to  16.57% expected  The observed weight l o s s was 15.8%, compared  f o r l o s s o f water by MoO(OH) .  - 68 -  0  20  F i g u r e 19:  40  60 80 T I M E (minutes)  100  E f f e c t o f t e m p e r a t u r e on r a t e assuming 1.5 o r d e r i n molybdenum f o r pH = 4 . 5 , i n i t i a l molybdenum c o n c e n t r a t i o n =4 g / 1 , and i n i t i a l mole r a t i o o f h y d r a z i n e t o molybdenum = 4 : 1 .  I  I  0  F i g u r e 21:  I  200  I  J  I  I  400 600 TEMPERATURE (°C)  L_  1——  8 00  T h e r m o g r a v i m e t r i c w e i g h t l o s s curve f o r brown p r e c i p i t a t e produced by reduction with hydrazine.  - 71 -  P r e c i p i t a t e formed a t pH = 4.5 and 50°C was found t o c o n t a i n ± 0.09 p e r c e n t sodium. w i t h NaOH a f t e r  N e u t r a l i z a t i o n o f the r e a c t o r  0.42  s l u r r y t o pH = 7  c o m p l e t i o n o f a run a t pH = 4.5 and 50°C gave a p r e c i p i -  t a t e c o n t a i n i n g 0.61 ± 0.03 p e r c e n t sodium w h i l e n e u t r a l i z a t i o n t o pH = 8 r e s u l t e d i n 1 p e r c e n t sodium c o n t e n t .  P r e c i p i t a t e formed a t 50°C and  pH = 4.5 i n 3 M NaCl s o l u t i o n c o n t a i n e d 3.3 p e r c e n t sodium. i o n was found i n any o f the  No c h l o r i d e  precipitates.  F i g u r e 22 shows the c o n c e n t r a t i o n s  o f molybdenum and h y d r a z i n e  r e m a i n i n g i n s o l u t i o n vs time a t 50°C w i t h pH = 4.5 and i n i t i a l mole r a t i o o f h y d r a z i n e t o molybdenum o f 4 : 1 . t r a t i o n remained c o n s t a n t a f t e r continued to f a l l .  A l t h o u g h the h y d r a z i n e c o n c e n -  about one hour the molybdenum c o n c e n t r a t i o n  A f t e r s t a n d i n g o v e r n i g h t a t room temperature the m o l y b -  denum c o n c e n t r a t i o n i n s o l u t i o n reached 0.053 g / 1 .  T h i s compares  to  0.041 g/1 o b t a i n e d by n e u t r a l i z i n g a s t o c k M o s o l u t i o n t o pH = 4 . 5  at  V  50°C.  S i m i l a r b e h a v i o u r was observed i n o t h e r  runs.  T h i s b e h a v i o u r i n t r o d u c e d some u n c e r t a i n t y the s t o i c h i o m e t r y o f t h e r e a c t i o n .  i n t o the computation o f  The method f i n a l l y adopted was s i m p l y  to use the h y d r a z i n e c o n c e n t r a t i o n r e m a i n i n g a t the end o f each r u n when the molybdenum c o n c e n t r a t i o n was reduced t o l e s s than 0.4 g / 1 . r e d u c t i o n o f M o ^ t o Mo^ was assumed even though, as w i l l be  Complete  discussed, VI  V  the molybdenum r e m a i n i n g i n s o l u t i o n was p r e s e n t as mixed v a l e n t Mo /Mo species.  S i n c e v e r y l i t t l e molybdenum a c t u a l l y remained i n s o l u t i o n t h i s  procedure d i d n o t make much d i f f e r e n c e  t o the s t o i c h i o m e t r y o b t a i n e d .  The  average s t o i c h i o m e t r y based on n i n e runs was 1.55 moles molybdenum reduced p e r mole h y d r a z i n e consumed.  The extreme v a l u e s were 1.47 and  T h i s s t o i c h i o m e t r y suggested  1.77.  t h a t molybdenum was a c t i n g as a  I  0  i  I  I  1  2  3  :  J  4  TIME (hours) F i g u r e 22:  C o n c e n t r a t i o n s o f h y d r a z i n e and molybdenum r e m a i n i n g i n s o l u t i o n vs time at 50°C w i t h pH = 4.5, i n i t i a l molybdenum c o n c e n t r a t i o n =5 g/1, and i n i t i a l h y d r a z i n e t o molybdenum mole r a t i o =4:1.  1  5  - 73 -  monodelectronator.  Based on the l i t e r a t u r e  the e x p e c t e d r e a c t i o n  from m o n o d e l e c t r o n a t i o n o f h y d r a z i n e are n i t r o g e n and ammonia.  products  To v e r i f y  the s t o i c h i o m e t r y a r u n was performed i n which the e v o l v e d gas was c o l l e c t e d and the f i l t r a t e a n a l y z e d f o r h y d r a z i n e , molybdenum, and ammonia. volume o f s o l u t i o n used was 200 ml atm 6.95 x 10 -3 g e n e r a t i n g 1.616 x 10  moles o f n i t r o g e n .  -3  moles N H 2  4  The  were consumed  Only a t r a c e o f ammonia was  found and the p i n k c o l o u r d i d n o t r e t u r n t o the C C l ^ l a y e r i n the h y d r a z i n e t i t r a t i o n s o l u t i o n on s t a n d i n g .  Gas chromatography o f the c o l l e c t e d gas  d i d n o t d e t e c t hydrogen. In e v e r y r u n the end f i l t r a t e was c o l o u r e d i n d i c a t i n g the presence o f mixed v a l e n t M o / M o V  V I  HCl and i n 5 N NaOH.  species.  T h i s was v e r i f i e d by p o l a r o g r a p h y i n 2 N  I n 2 N H C l the waves o f M o and Mo " were d i s t i n c t V  V]  but q u a n t i t a t i v e d e t e r m i n a t i o n o f the r a t i o o f M o t o M o V  V I  was not p o s s i b l e  because i n 2 N H C l the excess h y d r a z i n e p r e s e n t reduced the r e m a i n i n g M o at a s i g n i f i c a n t r a t e .  Polarography i n 2 N H C l , therefore,  V I  s e r v e d o n l y to  VI c o n f i r m t h a t Mo  was i n d e e d p r e s e n t i n c o l o u r e d f i l t r a t e s .  Polarography  i n 5 N NaOH made use o f the o b s e r v a t i o n by Souchay e t a l . " ' " t h a t M o 5  dismutes i n t o M o  V I  and Mo" ' i n such s o l u t i o n s . I V  Since M o  V I  V  does not g i v e  any p o l a r o g r a p h i c wave i n b a s i c s o l u t i o n the o b s e r v a t i o n o f a r e d u c t i o n wave i n d i c a t e d the presence o f M o  I V  r e s u l t i n g from the d i s m u t a t i o n .  This  t e c h n i q u e showed t h a t the M o o b s e r v e d on p o l a r o g r a p h y i n 2 N H C l was not V  due s i m p l y t o the r e d u c t i o n o f M o  V I  by h y d r a z i n e i n 2 N H C l .  i n 5 N NaOH d i d not prove s u i t a b l e f o r q u a n t i t a t i v e  Polarography  determination of Mo .  The h e i g h t o f the Mo ~ wave o b s e r v e d was s e n s i t i v e t o time before IV  V  polar-  ography and the s o l u t i o n s tended t o drop out a g r e y - g r e e n p r e c i p i t a t e on standing.  - 74 -  6.  DISCUSSION  The hydrogen r e d u c t i o n experiments c o n f i r m e d the importance o f heterogeneous c a t a l y s i s and pH i n a t t a i n m e n t o f r e a s o n a b l e tion.  rate of reduc-  The o n l y r u n i n w h i c h s i g n i f i c a n t r e d u c t i o n was o b t a i n e d was t h a t  performed i n the presence o f the p l a t i n u m c l a d n i o b i u m mesh c a t a l y s t w i t h a s o l u t i o n a c i d i f i e d t o pH = 2 .  The r a t e o f r e d u c t i o n was s i m i l a r t o t h a t  o b t a i n e d by L y a p i n a and Zelikman"*"^ u s i n g m e t a l l i c molybdenum as a (Figure 7 curve d ) .  catalyst  Even i n t h i s r u n , however, complete p r e c i p i t a t i o n o f  molybdenum was n o t o b t a i n e d .  The f a i l u r e t o o b t a i n complete r e d u c t i o n was  p r o b a b l y due t o the i n c r e a s e o f pH t o 8 and d e p o l y m e r i z a t i o n o f the m o l y b denum r e m a i n i n g i n s o l u t i o n .  T h i s d e p o l y m e r i z a t i o n would r e s u l t i n a 68  d e c r e a s e o f the r a t e o f r e d u c t i o n l i k e t h a t found by P a a l and B u t t n e r and shown i n F i g u r e 5 . i n the l i t e r a t u r e  The r e s u l t s o f the p r e s e n t study and those  c a s t some doubt on Wagenmann's c l a i m s .  the w a l l s o f h i s a p p a r a t u s were i n v o l v e d i n the S a t i s f a c t o r y performance  reported  I t seems p r o b a b l e  reaction.  o f hydrogen r e d u c t i o n would thus  s u f f i c i e n t a c i d i f i c a t i o n to maintain p o l y m e r i z a t i o n of M o  V I  require  species.  In a  b a t c h p r o c e s s t h i s c o u l d be a c c o m p l i s h e d by i n i t i a l a d d i t i o n o f a c i d beyond the H^MoO^ p o i n t o r by pH c o n t r o l d u r i n g the r e d u c t i o n .  In a d d i t i o n a  c a t a l y s t and an a u t o c l a v e m a t e r i a l r e s i s t a n t t o the c o n d i t i o n s  encountered  i n hydrogen r e d u c t i o n o f h i g h temperature c h l o r i d e s o l u t i o n s would be required. The experiments u s i n g h y d r a z i n e as a r e d u c t a n t a g a i n r e f l e c t importance o f p o l y m e r i c molybdenum s p e c i e s , num c o n c e n t r a t i o n ,  the  and hence pH and t o t a l molybde-  i n the r e d u c t i o n and p r e c i p i t a t i o n r e a c t i o n s .  Above  - 75 -  pH  =6.5 m o n o m e r i c M o  decreasing  V I  p r e d o m i n a t e s a n d no r e d u c t i o n  pH r e d u c t i o n o c c u r s  maximum a t a b o u t pH = 4.5. i b l e polymolybdates.  i s observed.  On  and the r a t e o f p r e c i p i t a t i o n reaches a  T h i s c a n be a t t r i b u t e d t o f o r m a t i o n  B e l o w pH -4.5 t h e r a t e o f p r e c i p i t a t i o n  b e c a u s e i n t h i s pH r a n g e i n t e r m e d i a t e  o f reduc-  decreases  m i x e d v a l e n t M o ^ / M o ^ compounds a r e  stable. A t pH = 4.5 t h e s e q u e n c e o f c o l o u r c h a n g e o b s e r v e d d u r i n g  reduction  37 i s consistent with that described  i n the literature  and i n t e r p r e t e d by  42 Ostrowetsky.  Thus o n r e d u c t i o n o f p o l y m o l y b d a t e s t h e i n i t i a l  products are coloured  mixed v a l e n t  however, these s p e c i e s reduction  V I  V I  V  p r e c i p i t a t e o r tend  i n solution.  curves o f Figures  ance o f mixed v a l e n t  species.  A t pH = 4.5,  species  to hydrolyse  y i e l d i n g a Mo  A t pH = 3 w h e r e m i x e d M o / M o V  a r e more s t a b l e t h e r a t e o f p r e c i p i t a t i o n The  V  a r e n o t s t a b l e a n d may b e r e a c t i v e t o w a r d s f u r t h e r  t o y i e l d a Mo  p r e c i p i t a t e and M o  Mo /Mo  reaction  V I  V  present  i n mixed v a l e n t  11 a n d 12 i l l u s t r a t e q u a l i t a t i v e l y i n the precipitation  species.  reaction.  the import-  The c u r v e s  T h a t i s , when t h e s a m p l e  VI i t a t e a n d Mo  by t h e i n c r e a s e  represent  filtrate  mixed w i t h t h e c a u s t i c i n t h e c e n t r i f u g e tubes t h e mixed v a l e n t w e r e made u n s t a b l e  species  decreases.  showing t h e weight o f p r e c i p i t a t e formed i n t h e c e n t r i f u g e tubes Mo  species  o f pH a n d d e c o m p o s e d t o y i e l d M o  , i n s o l u t i o n . (The pH i n f a c t was l i k e l y h i g h  c a u s e some d i s p r o p o r t i o n a t i o n o f M o  V  V  t o give Mo  I V  V  precip-  enough t o  i n t h e p r e c i p i t a t e and  VI a d d i t i o n a l Mo  i n solution.  q u a l i t a t i v e value  T h i s phenomenon r e n d e r e d t h e r e s u l t s o f  only.)  I t c a n be seen i n F i g u r e s t a t i o n approximately  11 a n d 12 t h a t t h e maximum r a t e o f p r e c i p i -  c o i n c i d e s w i t h t h e maximum c o n c e n t r a t i o n  o f mixed  - 76 -  valent species.  T h i s s u g g e s t s t h a t p r e c i p i t a t i o n o c c u r s through  r e d u c t i o n o f these s p e c i e s t o y i e l d a Mo p r e c i p i t a t e . V  further  Polarography  i n d e e d c o n f i r m e d t h a t the s l i g h t l y c o l o u r e d s o l u t i o n s a t the end o f each run  V VI c o n t a i n e d b o t h Mo and Mo .  VI Complete r e d u c t i o n o f Mo is  never  o b t a i n e d because as the t o t a l molybdenum i n s o l u t i o n r e a c h e s a low enough v a l u e t h e e q u i l i b r i u m between p o l y m e r i c and monomeric M o  V I  ensures t h a t  2a r e s i d u a l c o n c e n t r a t i o n o f i r r e d u c i b l e MoO^ always e x i s t s .  The minimum  r e s i d u a l c o n c e n t r a t i o n o f molybdenum r e m a i n i n g i n s o l u t i o n o b t a i n e d i n -4  t h i s work was 0.053 g/1 2-  (5.5 x 10  c o n c e n t r a t i o n o f MoO^  i n e q u i l i b r i u m w i t h p o l y m e r i c molybdate s p e c i e s  pH = 4.5  M).  T h i s i s o f the same o r d e r as  the at  (Figure 4 ) .  The f o r m a t i o n o f mixed v a l e n t s p e c i e s a l s o e x p l a i n s the  observation  o f a s i g n i f i c a n t time l a g between b e g i n n i n g o f r e d u c t i o n and b e g i n n i n g o f p r e c i p i t a t i o n f o r low i n i t i a l mole r a t i o s o f h y d r a z i n e t o molybdenum and a t low temperature and i n the presence o f 3 M N a C l . ( F i g u r e s 13, 14, 1 5 ) . I n the f i r s t two cases no p r e c i p i t a t i o n o c c u r s u n t i l r e d u c t i o n has proceeded  far  enough t o y i e l d an a p p r e c i a b l e c o n c e n t r a t i o n o f the mixed v a l e n t i o n t h a t i s u l t i m a t e l y reduced to y i e l d M o p r e c i p i t a t e . V  Lower mole r a t i o s o f  r e a c t a n t s and lower temperatures d e l a y the a t t a i n m e n t  of t h i s  concentration.  The e f f e c t o f NaCl may be due e i t h e r t o s t a b i l i z a t i o n o f mixed v a l e n t s p e c i e s o f l o w e r degrees o f r e d u c t i o n t h a n the s p e c i e s t h a t i s r e d u c i b l e V t o Mo , o r t o s t a b i l i z a t i o n o f t h i s i o n i t s e l f . V VI The p r o p e r t i e s o f mixed v a l e n t Mo /Mo s p e c i e s may a l s o be the r e a s o n f o r t h e b e h a v i o u r shown i n F i g u r e 22 where r e d u c t i o n , as r e p r e s e n t e d by the c u r v e o f h y d r a z i n e c o n c e n t r a t i o n vs t i m e , s t o p s w h i l e p r e c i p i t a t i o n V o f Mo c o n t i n u e s .  Presumably t h i s i s due t o the slow d e c o m p o s i t i o n o f  - 77 -  VI V V mixed v a l e n t Mo /Mo s p e c i e s t o g i v e Mo p r e c i p i t a t e As has been mentioned,  VI and Mo in solution.  f o r the low t o t a l c o n c e n t r a t i o n o f molybdenum VI  p r e s e n t , a s i g n i f i c a n t f r a c t i o n o f t h i s Mo  would be p r e s e n t as i r r e d u c i b l e  2monomeric MoO^  so consumption o f h y d r a z i n e through f u r t h e r  r e d u c t i o n would  be n e g l i g i b l e . The e f f e c t  o f s a m p l i n g p r o c e d u r e on the c u r v e s o f molybdenum vs time  as shown i n F i g u r e s 9 and 10 can be i n t e r p r e t e d i n terms o f the a b i l i t y o f V IV VI Mo t o d i s p r o p o r t i o n a t e i n t o i n s o l u b l e Mo and s o l u b l e Mo . The c u r v e s o f F i g u r e 9 were g e n e r a t e d by n e u t r a l i z i n g s l u r r y samples b e f o r e  filtration.  As can be seen i n c r e a s i n g the pH o f n e u t r a l i z a t i o n i n c r e a s e d the  amount  o f molybdenum found i n the f i l t r a t e  f o r any g i v e n t i m e .  Obviously dispro-  p o r t i o n a t i o n o c c u r r e d a t the i n s t a n t o f n e u t r a l i z a t i o n as observed by 50 Katsobashvili tionation.  and the h i g h e r the pH the g r e a t e r the e x t e n t o f d i s p r o p o r -  I t i s c l e a r t h a t the p r e c i p i t a t e  i t s e l f was d i s p r o p o r t i o n a t i n g  because f i l t r a t i o n b e f o r e n e u t r a l i z a t i o n gave r e s u l t s  indistinguishable  from those o b t a i n e d w i t h no n e u t r a l i z a t i o n f o l l o w e d by f i l t r a t i o n  (Figure  10) . I t i s apparent t h a t the k i n e t i c r e s u l t s o b t a i n e d can be q u a l i t a t i v e l y VI V IV e x p l a i n e d i n terms o f the known c h e m i s t r y o f Mo t a t i v e i n v e s t i g a t i o n would be d i f f i c u l t , o f t h e phenomena i n v o l v e d .  , Mo , and Mo  .  Quanti-  however, because o f the c o m p l e x i t y  The e m p i r i c a l r a t e law o b t a i n e d ,  for  example,  was based s i m p l y on d e t e r m i n i n g t o t a l molybdenum r e m a i n i n g i n s o l u t i o n after  f i l t r a t i o n o f s l u r r y samples.  r e a c t a n t s and r e f l e c t s  I t i s of f r a c t i o n a l order i n both  the b e h a v i o u r a t one pH o n l y .  The o v e r a l l p r o c e s s  o f r e d u c t i o n and p r e c i p i t a t i o n p r o b a b l y i n v o l v e s a c o m b i n a t i o n o f and p a r a l l e l r e a c t i o n s  consecutive  as w e l l as the complex e q u i l i b r i a a t t e n d a n t w i t h the  - 78 -  i n v o l v e m e n t o f p o l y m e r i c molybdate and mixed v a l e n t s p e c i e s .  The a c t i v a t i o n  energy o b t a i n e d o f 14.1 k c a l / m o l e r e p r e s e n t s t h a t o f the o v e r a l l r e d u c t i o n and p r e c i p i t a t i o n p r o c e s s .  I t does i n d i c a t e a t l e a s t t h a t c h e m i c a l p r o -  c e s s e s are r a t e c o n t r o l l i n g .  )  The g r a v i m e t r i c , o x i d i m e t r i c , and t h e r m o g r a v i m e t r i c r e s u l t s are i n i  f a i r agreement w i t h those expected i f the Mo p r e c i p i t a t e wa's MoCKOH)^. I t was found t h a t on s t o r a g e i n a d e s s i c a t o r the p r e c i p i t a t e s l i g h t l y and t h i s may e x p l a i n the low o x i d i m e t r i c r e s u l t s . o f t h e g r a v i m e t r i c and t h e r m o g r a v i m e t r i c r e s u l t s  oxidized The d i f f e r e n c e  from the expected  i s p r o b a b l y due t o v a r i a t i o n s o f the s t o r a g e and d r y i n g p r o c e d u r e w e i g h i n g samples f o r a n a l y s i s . precipitates  values before  S i n c e no c h l o r i d e i o n was found i n the  a n a l y z e d i t i s c o n c l u d e d t h a t the sodium p r e s e n t was the  r e s u l t o f t h e i o n exchange p r o c e s s d e s c r i b e d by Souchay e t a l . " ' " 5  The  i n c r e a s e o f sodium c o n t e n t f o r p r e c i p i t a t i o n c a r r i e d o u t i n 3 M NaCl i s consistent with this  interpretation.  The o b s e r v e d s t o i c h i o m e t r y can be i n t e r p r e t e d d u c t i o n o f n i t r o g e n and ammonia a c c o r d i n g t o the N H 2  4  »• JjN  +  NH  3  +  reaction e~  (1)  and the p r o d u c t i o n o f h y d r o x y l a m i n e a c c o r d i n g t o the 2H 0 + N H" 2  2  4  >• 2NH OH 2  +  2H  +  i n terms o f the p r o -  +  reaction  2e~  (2)  -3 I t was found t h a t c o n s u m p t i o n o f 6.95 x 10 1.616 x 10 ^ moles o f n i t r o g e n .  moles o f h y d r a z i n e  Assuming a l l the n i t r o g e n was  a c c o r d i n g to e q u a t i o n 1 and t h a t the b a l a n c e o f the h y d r a z i n e a c c o r d i n g t o e q u a t i o n 2 y i e l d s the f o l l o w i n g  results:  generated  generated reacted  - 79 -  NH  3  produced  =  NH^OH produced  3.232 x 10  =  7.436 x 10  e l e c t r o n s generated The s t o i c h i o m e t r y w i t h r e s p e c t  =  moles -3  moles  10.668 x 10  to reduction of M o  V I  -3  moles  to Mo obtained  is  V  thus 1.53 which i s i n good agreement w i t h the average s t o i c h i o m e t r y found o f 1.55.  S i n c e the volume o f s o l u t i o n used was 200 ml the  resultant  concentrations  o f ammonia and h y d r o x y l a m i n e would be 0.016 M and 0.037 M  respectively.  Both o f t h e s e c o n c e n t r a t i o n s  d e t e c t i o n by the a n a l y t i c a l t e c h n i q u e s  a r e s m a l l enough t o escape  used.  The s t o i c h i o m e t r y o b s e r v e d i n the p r e s e n t work i s l o w e r t h a n t h a t 79 r e p o r t e d i n the l i t e r a t u r e which v a r i e s between 2 the r e s u l t s  r e p o r t e d i n the l i t e r a t u r e ,  t i o n s of pH, c o n c e n t r a t i o n , o f the p r e s e n t work.  778283 and 4 .  '  '  None o f  however, were o b t a i n e d under  condi-  and mole r a t i o o f r e a c t a n t s s i m i l a r to. t h o s e  G i v e n the v a r i a b l e n a t u r e o f the proposed h y d r a z i n e  o x i d a t i o n r e a c t i o n s and the p o s s i b i l i t y o f d i f f e r e n t molybdenum s p e c i e s a c t i n g as o x i d a n t s i t i s d i f f i c u l t  to r e l a t e  r e s u l t s o b t a i n e d under one  o f c o n d i t i o n s t o t h o s e o b t a i n e d under a n o t h e r .  set  There does not seem t o be  any r e a s o n not t o suppose t h a t h y d r a z i n e c o u l d r e a c t to produce h y d r o x y l a m i n e i n a 2 e l e c t r o n p a t h and ammonia and n i t r o g e n i n a 1 e l e c t r o n p a t h .  It  thus seems l i k e l y t h a t one o f the u n i d e n t i f i e d r e a c t i o n p r o d u c t s mentioned by Ostrowetsky and B r i n o n was ammonia.  From the p o i n t o f view o f  the  p r e s e n t s t u d y , however, the i m p o r t a n t f a c t i s t h a t the e c o n o m i c a l l y f a v o u r a b l e 4 e l e c t r o n s t o i c h i o m e t r y i s not o b t a i n e d i n the c o n d i t i o n s o f interest.  - 80 -  7.  CONCLUSION  I t i s apparent t h a t n e i t h e r hydrogen r e d u c t i o n nor r e d u c t i o n w i t h h y d r a z i n e i s an i d e a l method f o r r e c o v e r y o f molybdenum from h y p o c h l o r i t e leach solutions.  B o t h r e d u c t a n t s r e q u i r e a c i d i f i c a t i o n o f the s o l u t i o n t o  be t r e a t e d so t h e r e i s no advantage t o be g a i n e d o v e r s o l v e n t in this respect.  Hydrogen r e d u c t i o n can o n l y be c a r r i e d o u t a t a  r a t e i n r a t h e r severe c o n d i t i o n s from a m a t e r i a l s catalyst is  extraction  standpoint  reasonable  and an e f f i c i e n t  required.  R e d u c t i o n w i t h h y d r a z i n e i n v o l v e s a low s t o i c h i o m e t r y p l u s the  require-  ment o f an a p p r e c i a b l e excess o f h y d r a z i n e o v e r the s t o i c h i o m e t r i c amount o b t a i n a reasonable  rate of p r e c i p i t a t i o n .  to  The p r e c i p i t a t e produced by  h y d r a z i n e r e d u c t i o n w i l l c o n t a i n on the o r d e r o f 3% sodium and copper must be e l i m i n a t e d from the l e a c h s o l u t i o n o r i t w i l l contaminate  the  In a d d i t i o n the r a t e o f p r e c i p i t a t i o n i s s i g n i f i c a n t l y d e c r e a s e d N a d e x p e c t e d i n the l e a c h s o l u t i o n . by h y d r a z i n e would c o n t a i n a t l e a s t  i n the 3 M  The b a r r e n s o l u t i o n from p r e c i p i t a t i o n 0.05 g/1 molybdenum p l u s excess  Recycle of t h i s s o l u t i o n for^hypochlorite regeneration g a t i o n o f the e f f e c t  precipitate.  would r e q u i r e  o f h y d r a z i n e and molybdate on the e l e c t r o l y t i c  hydrazine. investiprocess  employed. R e d u c t i o n w i t h h y d r a z i n e c o u l d , however, be c a r r i e d out a t a r e l a t i v e l y low temperature and i n a s i m p l e r e a c t o r .  One way t o perform the  reaction  would be t o add h y d r a z i n e and a c i d t o the l e a c h s o l u t i o n i n a s m a l l s t i r r e d tank and t h e n d i s c h a r g e the r e a c t i n g m i x t u r e t o a t h i c k e n e r s i z e d t o g i v e a suitable residence  time.  P r e c i p i t a t e c o u l d be d i s c h a r g e d as a s l u r r y and  the o v e r f l o w r e c y c l e d f o r h y p o c h l o r i t e r e g e n e r a t i o n .  The d i s c h a r g e  slurry  - 81 -  c o u l d be f i l t e r e d f o r p r e c i p i t a t e  recovery.  I t i s c l e a r t h a t the c o s t o f h y d r a z i n e i n r e l a t i o n to the p r i c e o f molybdenum and the m a r k e t a b i l i t y o f an MoO(OH) sodium are t h e d e c i d i n g f a c t o r s molybdenum r e c o v e r y .  p r o d u c t contaminated w i t h  i n the f e a s i b i l i t y o f u s i n g h y d r a z i n e  for  On the b a s i s o f t h i s s t u d y r e c o v e r y o f 1 kg o f m o l y b -  denum c o n t a i n e d i n MoO(OH) would r e q u i r e consumption o f about 1.2 kg o f 3  hydrazine to o b t a i n reasonable  k i n e t i c s and completeness  of p r e c i p i t a t i o n .  A t the c u r r e n t p r i c e f o r h y d r a z i n e o f $ 3 . 5 2 / k g i n tank c a r l o t s i t would c o s t $4.22 f o r h y d r a z i n e p e r k g o f molybdenum as MoO(OH)  .  Molybdenum  o x i d e i s c u r r e n t l y s e l l i n g f o r a p p r o x i m a t e l y $20/kg c o n t a i n e d molybdenum. A t the c u r r e n t p r i c e o f $20/kg Mo as m o l y b d i c a c i d and $ 3 . 5 2 / k g h y d r a z i n e i n tank c a r l o t s i t would c o s t $4.22 f o r h y d r a z i n e p e r kg Mo c o n t a i n e d i n MoO(OH) .  - 82 -  8.  REFERENCES  1.  S u t o l o v , A l e x a n d e r . Copper p o r p h y r i e s . S a l t Lake C i t y , U t a h , U n i v e r s i t y o f Utah P r i n t i n g S e r v i c e s , 1974.  2.  Warren, I . H . e t a l . Canadian I n s t i t u t e o f M i n i n g and M e t a l l u r g y . CIM annual volume, 1977, p . 1 1 .  3.  L i n d s t r o m , R . D . and B . J . S c h e i n e r . i n v e s t i g a t i o n s 7802, 1974.  4.  F i s c h e r , D . D . et a l . 8088, 1975.  5.  B a r r , D.S. et a l . p . 303, 1975.  6.  Ibid.  7.  Scheiner, B . J . et a l . 8145, 1976.  8.  S c h e i n e r , B . J . e t a l . 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