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Reductive precipitation of molybdenum oxides for recovery of molybdenum from hypochlorite leach solutions 1979

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REDUCTIVE PRECIPITATION 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 C o l u m b i a , 1972 M.L.S., The U n i v e r s i t y o f B r i t i s h C o l u m b i a , 1974 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCE i n 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 c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA September 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 degree a t the U n i v e r s i t y of B r i t i s h Columbia, I agree 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 s t u d y . I f u r t h e r agree t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y purposes may be g r a n t e d by the Head o f my Department o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r 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 w r i t t e n p e r m i s s i o n . Department o f 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 Vancouver, Canada V6T 1W5 October 5,1979 Date - ( i i ) - ABSTRACT The f e a s i b i l i t y 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 o x i d e s as t h e molybdenum 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 r o u g h e r 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 . Hydrogen gas a t e l e v a t e d t e m p e r a t u r e and p r e s s u r e and h y d r a z i n e a t moderate t e m p e r a t u r e and a t m o s p h e r i c p r e s s u r e were used as r e d u c t a n t s . R e d u c t i o n was p e r f o r m e d on s o l u t i o n s c o n t a i n i n g 5 t o 17 g/1 Mo as sodium m o l y b d a t e . Hydrogen r e d u c t i o n was s u c c e s s f u l o n l y i n t h e p r e s e n c e o f a P t c a t a l y s t , t e m p e r a t u r e = 200°C, p r e s s u r e = 30 atm o f H. , and i n i t i a l a c i d i f i c a t i o n t o pH = 2. Ten h o u r s was r e q u i r e d t o o b t a i n 90% r e c o v e r y o f molybdenum as MoC^. R e d u c t i o n w i t h h y d r a z i n e y i e l d e d an MoO(OH) p r e c i p i t a t e w i t h 90% 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 a t 50°C, 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. P r e c i p i t a t i o n under 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 o f 3 M NaCl gave o n l y 70% r e c o v e r y i n 4 ho u r s and 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 NaCl i s e x p l a i n e d i n terms o f s t a b i l i z a t i o n o f mixed 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 N a C l . - ( i i i ) - TABLE OF CONTENTS Page ABSTRACT i i TABLE OF CONTENTS i i i LIST OF TABLES i v LIST OF FIGURES v ACKNOWLEDGEMENTS v i 1. INTRODUCTION 1 2. LITERATURE REVIEW 11 2.1 Aqueous C h e m i s t r y o f Molybdenum 11 V I 2.1.1 Mo 11 2.1.2 Reduced S p e c i e s and P r e c i p i t a t e s 13 2.1.2.1 Molybdenum B l u e 14 2.1.2.2 Mo V 18 2.1.2.3 M o I V 21 2.1.2.4 M o 1 1 1 22 2.1.3 Summary 2 3 2.2 R e d u c t i o n w i t h Hydrogen and Carbon Monoxide 25 2.3 R e d u c t i o n w i t h H y d r a z i n e 33 2.4 R e d u c t i o n w i t h S 0 2 and H 2S 43 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 R e d u c t i o n w i t h H y d r a z i n e 54 6. DISCUSSION 74 7. CONCLUSION 80 8. REFERENCES 82 - ( 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 fo 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 cons tants ob ta ined fo r d i f f e r e n t va lues o f i n i t i a l hydraz ine t o molybdenum r o l e r a t i o - (v) - LIST OF FIGURES Page 1. Proposed f lowsheets 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 molybdeni te c o n t a i n i n g concen t ra t e s . 3 2. P r i n c i p a l ope ra t ions and products i n p r o c e s s i n g o f molybdeni te concen t ra t e s . 7 3. P o t e n t i a l - p H e q u i l i b r i u m diagram fo r the system Mo-H 0 a t 25°C. 8 4. Predominance area diagram for Mô "*" i n 3 N N a C l . 12 5. Reduct ion o f ammonium paramolybdate s o l u t i o n s by hydrogen i n the presence o f a c o l l o i d a l p a l l a d i u m c a t a l y s t . 26 6. Reduct ion o f sodium molybdate s o l u t i o n s by hydrogen at 200°C and 40 atm H . 28 7. 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 pH, and hydrogen pressure 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 8. Reduct ion 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 P t c l a d niobium mesh. 52 9. E f f e c t o f sampling technique on c o n c e n t r a t i o n o f molybdenum remaining 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 concen t r a t i on -5 g / 1 , and i n i t i a l hydraz ine to molybdenum mole r a t i o - 4 : 1 . 55 10. Comparison o f r e s u l t s ob ta ined 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 ob ta ined by p rev ious d i l u t i o n o f samples w i t h an equal volume o f c o l d water . 57 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 t ime a t 50°C w i t h pH = 4 . 5 , i n i t i a l molyb- 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 hydraz ine to molyb- denum mole r a t i o s 4 : l . 58 12. 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 t ime a t 50°C w i t h pH = 4 . 5 , i n i t i a l molyb- 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 hydraz ine to molybdenum = 2 : 1 . 59 - (v i ) - 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 fo 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 hydraz ine to 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 hydraz ine to 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 of 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 fo 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 i n i t i a l mole r a t i o o f hydraz ine to molybdenum =4:1. 16. E f f e c t of a d d i t i o n of 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 hydraz ine to molybdenum mole r a t i o - 4 : 1 . 17. 1.5 order i n molybdenum p l o t s fo 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 . 18. Order i n N^H^ based on 1.5 order i n molybdenum. 19. E f f e c t o f temperature on r a t e assuming 1.5 order i n molybde num for 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 hydraz ine to molybdenum - 4 : 1 . 20. Ar rhen ius p l o t based on 1.5 order i n molybdenum. 21. Thermogravimetric weight l o s s curve fo 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. Concent ra t ions o f hydraz ine and molybdenum remaining i n s o l u t i o n vs t ime 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 hydraz ine to molybdenum mole r a t i o - 4 : 1 . - ( v i i ) ; - ACKNOWLEDGEMENT I would l i k e to express my g r a t i t u d e to Dr . Ian H . Warren f o r h i s enthusiasm and suggest ions 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 support i n the form o f a S t e l c o Research F e l l o w s h i p . Las t but not l e a s t , I would l i k e t o express my a p p r e c i a t i o n f o r the he lp and adv ice 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 , and f e l l o w students i n the Department o f M e t a l l u r g y . - 1 - „1 . INTRODUCTION Conven t iona l technology fo 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 byproduct from p r o c e s s i n g o f porphyry copper ores i s based on a bu lk or rougher f l o t a t i o n fo r the recovery o f a copper sulf ide-molybdenum s u l f i d e concen t ra te . Th 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 to p r o - duce separate copper and molybdenum s u l f i d e concen t ra t e s . The molybdenum concent ra te i s then c leaned by f u r t h e r f l o t a t i o n steps to o b t a i n a market- ab le M0S2 p roduc t . Accord ing to da ta p u b l i s h e d by Sutulov"*" f o r f i f t y f l o t a t i o n p l a n t s t r e a t i n g such ores the mean recovery o f molybdenum i n the copper rougher concent ra te i s 64% w h i l e the mean o v e r a l l recovery 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 concentra tes c o u l d be an economica l ly feavourable a l t e r n a t i v e to fu r the r f l o t a t i o n s teps i f the molybdenum recovery from the rougher con- cen t ra t e s approached 100% and i f the r es idue was s u i t a b l e fo r p r o d u c t i o n o f a copper concent ra te by f l o t a t i o n . They proposed a sodium h y p o c h l o r i t e l each f o r t reatment o f rougher concent ra tes t ha t i n v o l v e d o n - s i t e e l e c t r o - l y t i c gene ra t ion o f sodium h y p o c h l o r i t e u s i n g commerc ia l ly a v a i l a b l e hypo- c h l o r i t e c e l l s . A s i m i l a r approach has been proposed by workers a t the 3—8 U . S . Bureau o f Mines (USBM). In the USBM process h y p o c h l o r i t e l e a c h i n g would be a p p l i e d t o the o f f -g rade molybdenum s u l f i d e concent ra te 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 concen t ra t e . Th i s p roces s , then , would r ep l ace o n l y the c l e a n i n g stages o f p r o d u c t i o n o f an MoS~ - 2 - concent ra te w h i l e the process proposed by Warren e t a l . would a l s o r e p l a c e the d i f f e r e n t i a l f l o t a t i o n s t ep . A fu r the r d i f f e r e n c e between the two p roposa l s i s t h a t the USBM process i n v o l v e s gene ra t ion o f sodium hypoch lo 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 of the concent ra te as i t i s pumped through a s p e c i a l l y designed e l e c t r o l y t i c c e l l . The USBM process has been p i l o t e d on concent ra tes c o n t a i n i n g 16 to 35% Mo and 6 to 15% Cu. Warren e t a l . have designed t h e i r process to t r e a t concent ra tes c o n t a i n i n g approximate ly 0.3% Mo and 12% Cu. The b a s i c f l ow sheets fo r both processes are compared i n f i g u r e 1. Both p r o - cesses use Na^CO^ f o r pH adjustment. Warren e t a l . propose to 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 process operates a t pH 5.5 to 7. The l e a c h temperature fo r both processes i s 45 - 50° 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 represented by the equa t ion : 2 - - 2 - 9 OCl + MoS 2 + 60H — •+ Mo0 4 + 9C1 + 2S0 4 + 3 ^ 0 which shows t ha t 1 kg o f NaOCl should be consumed to b r i n g 0.143 kg o f Mo i n t o s o l u t i o n . Accord ing to Warren e t a l . modern h y p o c h l o r i t e genera tors r e q u i r e 3.52 kwh per kg NaOCl. I t f o l l o w s tha t i n t h e i r process energy g consumption should be 24.5 kwh per kg Mo. In a recen t p i l o t p l a n t study the USBM process r e q u i r e d 19.8 to 28.6 kwh per kg Mo to t r e a t concent ra tes c o n t a i n i n g 16 to 35% Mo and 6 to 15% Cu. The molybdenum recovery decreased from 93 to 97% to o n l y 75% when the copper content i nc r ea sed to 15% and the power consumption co r r e spond ing ly inc reased to the 28.6 kwh v a l u e . The decrease i n molybdenum recovery was a t t r i b u t e d t o the format ion o f i n s o l u - b l e copper molybdates . Curren t r esea rch i n t h i s l a b o r a t o r y has shown, however, t ha t the e f f e c t o f copper mine ra l s on the molybdenum recovery i s Cu-Mo ROUGHER CONCENTRATE residue to copper flotat ion copper product Mo product •< No 2 S0 4 LEACH I No CO, < 2 3 COPPER REMOVAL NaOCl recycle ACIDIFICATION H 2 S 0 4 Mo RECOVERY EVAPORATION NaOCl REGENERATION WARREN et al F i g u r e 1: Proposed f lowsheets f o r h y p o c h l o r i t e OFF-GRADE MOLYBDENITE CONCENTRATE Na 2C0 3 brin e recycle Na 2 C0 3 ELECTRO- OX I DATION SO. ACI DI FICATION SULFATE REMOVAL N a 2 S 0 4 SOLVENT EXT RACTION CARBON ADSORPTION CRYSTALLIZATION f ( N H 4 ) 6 M o 7 0 2 4 - 4 H 2 0 U S B M PROCESS l e a c h i n g o f molybdeni te c o n t a i n i n g concen t r a t e s . RESIDUE j to waste NH 4Re0 4 solution - 4 - more complex than t h i s . A t the pH o f the USBM l e a c h , f o r example, i t i s l i k e l y t ha t s i g n i f i c a n t decomposi t ion o f NaOCl to NaClO^ o c c u r s . S ince NaClO^ does not o x i d i z e MoS^ a t t h i s pH such decomposi t ion reduces the energy e f f i c i e n c y o f the l e a c h . The use o f carbonate fo r pH c o n t r o l has a l s o been 9 shown to be an impor tant 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 area i s c o n t i n u i n g . On the b a s i s o f the a v a i l a b l e i n - fo rmat ion , then , i t seems t ha t the process proposed by Warren e t a l . o f f e r s a h igher recovery 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 roces s . Warren et a l . c a l c u l a t e d the approximate compos i t ion o f the l e a c h s o l u t i o n t ha 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 rocess to a rougher concent ra te c o n t a i n i n g 0.3% Mo. Table I compares t h i s w i t h the compos i t ion r e s u l t i n g from a p p l i c a t i o n o f the USBM process to a molybdeni te concen t ra t e . I t can be seen t ha t the two s o l u t i o n s are s i m i l a r . I t would thus be p o s s i b l e to apply 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 sequence f o r recovery o f molybdenum and rhenium to Warren e t a l . ' s p roces s . A disadvantage o f the USBM procedure however, i s the n e c e s s i t y to a c i d i f y 4 the s o l u t i o n to pH< 2 fo r molybdenum e x t r a c t i o n by a t e r t i a r y amine. A process capable 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 fo r a h y p o c h l o r i t e l each ope ra t i ng a t pH = 9. A l t e r n a t i v e means o f molybdenum recovery have been suggested. These 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 tempera- tu re and pressure to produce i n s o l u b l e MoO^® ^ and r e d u c t i o n w i t h i r o n fo l lowed 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)^.''" 6 As desc r ibed i n the l i t e r a t u r e , however, bo th o f the processes operate 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 rou tes to molybdenum recovery from - 5 - Warren et a l . USBM H 2 0 1100 kg 900 kg NaCl 170 kg 100 kg N a 2 S 0 4 68 kg 37 kg Na 2 Mo0 4 10 kg 4 - 43 kg Cu v a r i a b l e NaOCl„ 3 n i l 5 - 13 kg NaCIO „ 4 5 - 8 kg .14 - .72 kg Re .001 - .050 kg pH 9 5 - 7 Table I 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 - 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 ope ra t ions i n v o l v e d 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 roduc t . In a d d i t i o n the a b i l i t y o f a g iven process to recover rhenium c o u l d be a d e c i d i n g f a c t o r i n cases where the concent ra te to be t r e a t e d conta ined 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 concent ra tes are roas ted to 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 t h i s product i s the p r o d u c t i o n o f c a l c i u m molybdate, molybdic ox ide b r i q u e t s , o r ferromolybdenum fo r a l l o y s t e e l p r o d u c t i o n . T e c h n i c a l grade MoO^ can a l s o be fu r t he r p u r i f i e d by s u b l i m a t i o n to y i e l d pure MoO^ or by hydro- m e t a l l u r g i c a l means to produce ammonium molybdate or sodium molybdate (Figure 2 ) . A h y d r o m e t a l l u r g i c a l t reatment o f MoS^ c o n t a i n i n g concentra tes should a t l e a s t be capable o f p roduc ing a product o f equal q u a l i t y to : t e c h n i - c a l grade MoO^. By way o f example Table I I shows 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 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, or ammonium molybdate product o f h igh p u r i t y d i r e c t l y from an o f f -g rade or rougher concent ra te c o u l d , however, e l i m i n a t e not o n l y r o a s t i n g but a l s o s u b l i m a t i o n or o ther subsequent p u r i f i c a t i o n s t eps . The a b i l i t y to produce such a product cou ld 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 Pourbaix (Figure 3) i s r e l a t i v e l y s imple and i n d i c a t e s tha t r e d u c t i v e 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 over a wide range o f pH. Seve ra l reduc ing agents might be cons ide red . These i n c l u d e E^t SC^, H^S, CO, i r o n and h y d r a z i n e . The gaseous reagents and hydraz ine are a t t r a c t i v e because they o f f e r the p o s s i b i l i t y o f p roduc ing a h igh p u r i t y p roduc t . MoS. concentrate 2 i ROAST T _*-MoS 2 I u b r i cants t—~ calcium molybdate \ oxide brique t s t e c h n i c a l grade M o o ^ r f e r r o - molybdenu m DISSOLUTION 8 CRYSTALLIZATION ; sodium molybdate } f ? : } ceramics ferti l izer chemicals direct addition to ste el SUBLIMATION pure MoOj DISSOLUTION 8 CRYSTALLIZATION i ammonium molybdate pigments chemicals coat ings 1 T chemical ca ta lys reagent REDUCTION F i q u r e 2; molybdenum metal P r i n c i p a l o p e r a t i o n s and produc ts i n p roces s ing o f molybdeni te concen t ra tes . - 8 - - 9 - OXIDE Range % T y p i c a l % Guaranteed % Mo 57.0 - 62.0 59.0 57.0 min . Cu 0.05 - 0.10 0.075 0.10 max. S 0.03 - 0.10 0.06 0.10 max. P — 0.01 0.05 max. Pb 0.015 - 0.050 0.025 0.05 max. B i 0.02 - 0.04 0.030 0.05 max. wo 3 0.030 0.16 max. s i o 2 5.0 - 15.0 8.0 15.0 max. Fe 0.020 - 0.45 0.35 — CaO 0.050 - 0.120 0.07 — OXIDE BRIQUETTES (PITCH) Range % T y p i c a l g, "o Guaranteed % Mo 50.6 - 54.0 53.0 51.6 min . C 10.0 - 15.0 11.00 12.0 approx. Cu 0.05 - 0.15 0.075 0.15 max. S 0.09 - 0.15 0.11 0.15 max. P — 0.01 0.05 max. B i 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 - Despi te the apparent s i m p l i c i t y o f the Pourba ix diagram, however, the aqueous chemis t ry o f molybdenum i s complex. There i s , i n f a c t , a r i c h chemis t ry o f po lymer ic molybdates and reduced molybdenum spec ies t ha t has r e s u l t e d i n an ex tens ive and o f t en c o n t r a d i c t o r y l i t e r a t u r e . A review o f t h i s l i t e r a t u r e i s necessary as a b a s i s fo r c o n s i d e r i n g r e d u c t i v e p r e c i p i t a t i o n as a means o f molybdenum recovery from a l k a l i n e s o l u t i o n s . - 11 - 2. LITERATURE REVIEW 2.1 Aqueous Chemistry o f Molybdenum VI 2 .1 .1 Mo 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 2- pH 's g rea te r 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 ions po lymer ize consuming H + to produce i sopolymolybdates such as the paramolybdate i o n Mo^O ^ . S e v e r a l i sopolymolybdates have been repor ted and the l i t e r a t u r e has been reviewed 19 by Sasak i and S i l l e n . They c h a r a c t e r i z e d i sopolymolybdates i n terms o f t h e i r a c i d i t y , z , de f ined as moles H + bound per mole Mo. Thus paramolyb- date formed accord ing to the equa t ion , 8 H + + 7Mo0 4 2 ~ = H 8 ( M o 0 4 ) ? 6 ~ = Mo O ^ 6 " , can be represented by z = 8/7 = 1.14. I t i s g e n e r a l l y accepted tha t paramolybdate i s the f i r s t spec ies 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 tha t i t i s the on ly 20-22 i s o p o l y a n i o n , o r at l e a s t by f a r predominant, up to z = 1.14 (pH - 5 ) . The nature o f the polymolybdates formed on fu r the r a c i d i f i c a t i o n must be regarded as u n c e r t a i n s ince there i s some disagreement i n the 19 23-25 l i t e r a t u r e . ' F igu re 4 shows a predominance area diagram cons t ruc ted 22 19 by Baes who accepted Sasak i and S i l l e n ' s sequence o f i sopo lymolybda tes . For a c i d i f i c a t i o n up to z = 1.5 i t has been shown tha t i sopo lymo lyb - date e q u i l i b r i a are almost ins tantaneous f o r moderate molybdenum concen t ra - t i o n s . For z S- 1.5, however, where p o l y m e r i z a t i o n proceeds beyond the 24 heptamer or octamer s tage , Aveston e t a l . noted tha t the e q u i l i b r i a are o n l y s l o w l y e s t a b l i s h e d .  - 13 - The f o r m a t i o n o f i s o p o l y m o l y b d a t e s i s s i g n i f i c a n t f o r r e c o v e r y o f molybdenum from p r o c e s s s o l u t i o n s because 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 p o l y m e r i z e d molybdenum s p e c i e s . 30 C h a r i o t has s t a t e d , f o r example, t h a t monomeric mo l y b d a t e i o n i s r e d u c e d i n f i n i t e l y s l o w l y i n a l k a l i n e s o l u t i o n . P o l a r o g r a p h i c s t u d i e s have t e n d e d V I . . , . ^ ^ ,.31-33 t o c o n f i r m t h i s w i t h Mo g i v i n g no r e d u c t i o n waves f o r pH > 5 S i m i l a r l y t h e n e c e s s i t y f o r a c i d i f i c a t i o n 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 s p e c i e s i n v o l v e s i s o p o l y - m o l y b d a t e s o r even 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 e f f i c i e n t 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 s p e c i e s a r e i s o p o l y m o l y b d a t e s o r 34 c a t i o n s . 2.1.2 Reduced S p e c i e s and P r e c i p i t a t e s F o r l a c k o f thermodynamic d a t a P o u r b a i x was a b l e t o c o n s i d e r o n l y 3+ Mo, Mo , and M0O2 i n h i s d i a g r a m . The predominance a r e a o f t h e mixed- v a l e n t molybdenum b l u e compounds c o u l d o n l y be shown a p p r o x i m a t e l y based on q u a l i t a t i v e o b s e r v a t i o n s . I t i s now w e l l r e c o g n i z e d , however, t h a t V IV I I I Mo , Mo , and Mo s p e c i e s can be p r e p a r e d and a r e s t a b l e i n aqueous 35 s o l u t i o n . H y d r a t e d o x i d e s o f each o f t h e s e v a l e n c e s t a t e s can be p r e - c i p i t a t e d i n a p p r o p r i a t e pH i n t e r v a l s . Some p r o g r e s s has a l s o been made 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 b l u e s . V I L i k e Mo t h e r e d u c e d molybdenum a q u o - i o n s show a t e n d e n c y towards p o l y m e r i z a t i o n . B o t h Mo11"1" and M o I V can fo r m d i m e r s ^ and Mo V has been r e p o r t e d t o form d i m e r s , t e t r a m e r s , and h i g h e r p o l y m e r s . The molybdenum b l u e s a r e u n d o u b t e d l y p o l y m e r i c s i n c e t h e i r f o r m a t i o n 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 . The reduced molybdenum spec ies are of i n t e r e s t i n the present study and the chemis t ry o f each i s b r i e f l y reviewed i n the f o l l o w i n g s e c t i o n s . 2 . 1 . 2 . 1 Molybdenum Blue 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 to 0 y i e l d s more or l e s s i n t e n s e l y co loured b lue s o l u t i o n s . I f r e d u c t i o n proceeds beyond the optimum fo r format ion o f the b lue the c o l o u r d e n s i t y decreases 37 and the c o l o u r may change from b lue to green or brown. There have been many attempts to c h a r a c t e r i z e the b lue c o l l o i d a l p r e c i p i t a t e s t ha t can be j * i - , 38-42 n n 38 • . prepared from such s o l u t i o n s . Glemser and Lu tz and Sacconi and . . 39 C m i concluded tha t molybdenum blue was n e i t h e r a unique compound nor d i d i t represent 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. Th i s c o n c l u s i o n seems to have been g e n e r a l l y accepted and i s s t a t e d i n s e v e r a l works on 20,21,43 44 i n o r g a n i c chemis t ry . On the o ther hand Weiser has po in t ed out t ha t the evidence f o r the ex i s t ence o f d i f f e r e n t compounds was based 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 o f the same order o f magnitude as the exper imenta l e r ro r s i nhe ren t i n a n a l y z i n g a c o l l o i d a l mass. 41 A r n o l d and Walker 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 lue i n t o bu tano l and de te rmin ing the mean o x i d a t i o n s t a t e of molybdenum by p o t e n t i o m e t r i c t i t r a t i o n . They ob ta ined the formula 40 MOg0^7 which agreed w i t h the e a r l i e r r e s u l t s o f Treadwel l and Schaeppi . 42 Ostrowetsky s t ud i ed the format ion of molybdenum blue by mix ing V VI s o l u t i o n s .of Mo 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 . T o t a l molybdenum i n s o l u t i o n was a l s o v a r i e d . The format ion o f the b lue was ana lyzed by spectrophotometry, by i s o l a t i o n o f i t s rub id ium s a l t , and by - 15 - p o t e n t i o m e t r y . 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 t h e b l u e gave t h e same r e s u l t s . V VI 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 were pH = 1.22, Mo /Mo = 0.5, and [Mo], , = 0.015 M. The b l u e s p e c i e s was f o r m u l a t e d as t o t a l Mo, V O Mo. VI 18 The c o r r e s p o n d i n g a c i d , H Mo 0 , a g r e e d w i t h t h e r e s u l t s o f A r n o l d and 2. b i o Walker and T r e a d w e l l and S c h a e p p i . Decrease o f t o t a l molybdenum c o n c e n t r a - t i o n i n h i b i t e d f o r m a t i o n o f t h e b l u e . I f a s o l u t i o n o f t h e 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 entioned above, was t i t r a t e d w i t h NaOH th e s o l u t i o n became 2-g r e e n , t h e n brown. The brown s o l u t i o n was f o u n d t o c o r r e s p o n d t o HMo.O o 18 and i t s f o r m a t i o n from t h e b l u e w r i t t e n as V Mo, Mo, VI °18 2- + \ MoY 0 „ 4 + + 80H~ 2 4 8 Mo. VI Mo. V ° 1 8 H 2- + 2H 20 V V I The brown 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 d i r e c t l y by m i x i n g Mo and Mo Maximum y i e l d was o b t a i n e d a t pH = 2.7. A t h i r d m ixed compound, a l s o brown, was f o u n d t o be formed a t V V VI pH = 3 t o 4 and Mo /Mo = 2 . 0 . I t was f o r m u l a t e d as Mo. Mo, V I ° 1 7 H F o r pH > 4, however, i t s c o n c e n t r a t i o n d e c r e a s e d and a t pH = 4.5 a p r e c i p i - t a t e o f MoO(OH)^ was o b s e r v e d . The work o f O s t r o w e t s k y p e r m i t s a l o g i c a l i n t e r p r e t a t i o n o f t h e c o l o u r changes o b s e r v e d on 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 range 0 t o 4.5. F o r pH < 0 t h e b l u e compound c a n n o t form and no c o l o u r change i s - 16 - observed u n t i l the Mo stage o r lower i s a t t a i n e d . For pH between 0 and 4 the f i r s t c o l o u r t o appear i s b lue f o r low va lues o f M o V / M o V I . As 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 M o V / M o V I spec ies form. The s o l u t i o n p r e c i p i t a t e s MoO(OH) as the Mo V stage i s approached more c l o s e l y . 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 spec ies 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 . In a d d i t i o n the mixed v a l e n t spec ies can c o e x i s t i n v a r i o u s p ropor - VI V 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 excess Mo o r Mo depending on the p a r t i c u l a r c o n d i t i o n s . Th i s p robably 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 ob ta ined by d i f f e r e n t workers u s i n g d i f f e r e n t 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 Os t rowetsky ' s work was p rov ided by F i l i p p o v 45 and Nuger who observed tha t the cha rac t e r and i n t e n s i t y o f the molybdenum b lue spectrum observed d u r i n g r e d u c t i o n o f molybdic a c i d by hydraz in ium c h l o r i d e v a r i e d acco rd ing to thepH a t which the r e d u c t i o n was performed. The maximum absorbence o b s e r v e d by F i l i p p o v and Nuger occur red a t pH = 1.31 which corresponds to the optimum pH f o r format ion o f the b lue compound proposed by Ostrowetsky. As the pH o f r e d u c t i o n was i nc rea sed the absorb- ence due to format ion o f the b lue decreased. Unfo r tuna te ly F i l i p p o v and Nuger d i d not r eco rd the spec t r a i n the r e g i o n o f absorbence o f the two brown spec ies so i t i s not p o s s i b l e to t e l l i f the absorbences due to these two spec ies i nc reased as would be expected i f the o v e r a l l degree o f reduc- t i o n ob ta ined was independent o f pH. In the case o f F i l i p p o v and Nuger ' 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 fo r pH > 4 no r e d u c t i o n was observed. 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 - r e s u l t o f a k i n e t i c e f f e c t s i n c e , as w i l l be shown l a t e r , hydraz ine i n V s u f f i c i e n t excess can reduce molybdate almost q u a n t i t a t i v e l y to Mo fo r pH between 4.5 and 5 .0 . Ostrowetsky proposed t ha t s i nce the mixed v a l e n t spec ies were hexamers i t was l i k e l y t h a t they 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 ha t molybdenum b lues were i s o s t r u c t u r a l w i t h the molybdate spec ies from which they 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,_ (where R = N H . , Rb ) , can be 2 b 4 46 prepared i n a c i d s o l u t i o n by s tandard procedures . 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 hydro lyze r a p i d l y to g ive a brown c o l o u r fo r [Mo] > 0.1 M which becomes amber or 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 Accord ing to M e l l o r K lason ob ta ined MoOtOH)^ by adding 3 moles o f ammonia to 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 tha t i f excess ammonia was used the p r e c i p i t a t e was p a r t i a l l y decomposed and the f i l t e r e d s o l u t i o n conta ined M o V I . The anhydrous oxide was ob ta ined by s e v e r a l workers by hea t ing 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 streams. 48 Simon and Souchay performed a d e t a i l e d spec t rophotomet r ic study o f V the h y d r o l y s i s and concluded tha t below 2 M HC1 and 3 M H^SO^ Mo was not complexed by the anion o f the a c i d used. T i t r a t i o n o f (NH^)^MoOCl^ w i t h NaOH suggested the h y d r o l y s i s product immediately before p r e c i p i t a t i o n c o u l d be formulated 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 - t i o n . 49 Ardon and P e r n i c k confirmed t h i s work conc lud ing tha t the predomi- nant Mo V spec ies i n d i l u t e HC1, HCIO^, and o ther ac ids i s a b i n u c l e a r c a t i o n w i t h charge 2+ and i s not coord ina ted to c h l o r i d e . 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 behaviour 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^ 6 proposed tha t fo r [M0V] > 10 2 M the predominant spec ies i s a te t ramer which t ransforms i n t o (Mo0 2 + ) on d i l u t i o n . They a l s o r epor ted tha t i f a s o l u t i o n o f Mo V was n e u t r a l i z e d to 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 - 19 - p r e c i p i t a t e d w h i l e ( M o 0 2 + ) 2 and ( M o 0 2 + ) ^ c o e x i s t e d i n s o l u t i o n w i t h a p r e v i o u s l y unknown c h e s t n u t c o l o u r e d s p e c i e s . The c h e s t n u t s p e c i e s c o u l d be removed from s o l u t i o n on an a n i o n exchange r e s i n . I t was found t h a t i t was h i g h l y p o l y m e r i z e d . 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 t h a t s m a l l c o n c e n t r a t i o n s o f i t s u p p r e s s e d t h e p o l a r o g r a p h i c maximum ob- s e r v e d f o r M o V I i n 2 N H C l . V i o s s a t and 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 had an o v e r a l l n e g a t i v e c h a r g e r e s u l t - i n g from s t r o n g l y a d s o r b e d c h l o r i d e i o n s . O n ly 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 Mo V appear t o have been p u b l i s h e d . They a r e i n f a i r agreement w i t h each o t h e r . K a t s o b a s h v i l i e t a l . p r o d u c e d Mo V i n HCl 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 . The r e s u l t - i n g s o l u t i o n s c o n t a i n e d 0.021 M Mo and were 0.03 N i n A l C l ^ . T i t r a t i o n w i t h 0.02 N NaOH showed p r e c i p i t a t i o n was co m p l e t e between pH 6 and 6.5. Molyb- denum r e m a i n i n g 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 , however, t h a t between pH 8 and 10 h y d r o x y l i o n s were a d s o r b e d by t h e p r e c i p i t a t e and t h e molybdenum c o n c e n t r a - V I t i o n i n s o l u t i o n i n c r e a s e d . They found t h a t Mo was d i s s o l v i n g from t h e p r e c i p i t a t e so t h e phenomenon was n o t s i m p l y a q u e s t i o n o f i n c r e a s i n g s o l u - V V I b i l i t y o f Mo a t a h i g h e r pH. Mo a l s o appeared i n s o l u t i o n i f NH^ was used f o r n e u t r a l i z a t i o n . The amount a p p e a r i n g i n c r e a s e d w i t h t i m e and t e m p e r a t u r e o f s t a n d i n g f o r a g i v e n pH and w i t h i n c r e a s i n g pH f o r c o n s t a n t V I t i m e and t e m p e r a t u r e . I t was n o t e d t h a t p l o t s o f Mo a p p e a r i n g v s ti m e f o r c o n s t a n t pH's between 6.5 and 9 had 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 t h a t a t l e a s t p a r t o f t h e d i s s o l u t i o n o c c u r r e d 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 - i z a t i o n . Souchay e t a l . 5 " ' " n e u t r a l i z e d a l i q u o t s o f 0.03 M Mo V s o l u t i o n s i n H C l - 20 - w i t h v a r y i n g amounts of NaOH, a g i t a t e d them fo r 10 minutes , and separated 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 remaining i n s o l u t i o n was determined p o l a r o g r a p h i c a l l y i n 6 N HCl a f t e r o x i d a t i o n to M o V I w i t h C e ^ + . Sodium and c h l o r i d e ions i n the p r e c i p i - t a t e were determined a f t e r 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 i n d i c a t e d V the p r e c i p i t a t i o n o f Mo was complete fo r pH > 6 but t ha t the consumption of NaOH exceeded the one e q u i v a l e n t expected from the equat ion ( M o 0 2 ) 4 + 40H~ + 4H 2 0 = 4MoO(OH) 3 I t was concluded t ha t the p r e c i p i t a t e ac ted as an i o n exchanger absorb ing N a + and r e l e a s i n g H + . The p ickup o f sodium inc rea sed when NaCl was added to the s o l u t i o n s . Souchay e t a l . a l s o observed 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 a f t e r a s o l u t i o n o f pH = 12.65 was a g i t a t e d fo r s e v e r a l hours . I t was found by po l a rog raph i c a n a l y s i s t ha t a f t e r such d i s s o l u t i o n the p r e c i p i t a t e IV VI conta ined 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 smuta t ion o f Mo accord ing t o : „ V . IV VI 2Mo • Mo + Mo In 5 N NaOH complete d i smu ta t i on was almost ins tantaneous y i e l d i n g a p r e c i p i t a t e i n which the mole r a t i o o f molybdenum to sodium was one to one. The r e a c t i o n proposed was: + 2- 2MoO(OH)3 + 30H + Na • MoO + HNaMoO^ + 4H 2 0 which i n v o l v e d format ion o f an i n s o l u b l e molybdi te i d e n t i f i e d i n a s i m i l a r IV 51 study o f the p r e c i p i t a t i o n o f Mo - 21 - IV 2 . 1 . 2 . 3 Mo IV The behaviour o f Mo i n aqueous s o l u t i o n has been a matter o f con- t r o v e r s y . Accord ing to M e l l o r s e v e r a l workers r epor ted o b t a i n i n g v a r i o u s hydrates 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 . Con- V v e r s e l y , K lason mainta ined these p r e c i p i t a t e s were probably impure Mo products and hydrated ox ides o f M o I V d i d not e x i s t . Th i s o p i n i o n was r e i n f o r c e d by Haight and coworkers who concluded , 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 of the r e d u c t i o n o f M o V I by I I 53 IV Sn , t h a t Mo was uns tab le i n aqueous s o l u t i o n and d i s p r o p o r t i o n a t e d I I I V i n t o Mo and Mo . 54 Guibe and Souchay, however, had p r e v i o u s l y demonstrated the e x i s t - ence o f M o I V u s i n g po la rography . Souchay, C a d i o t , and Duhameaux"^ subse- IV quen t ly n e u t a l i z e d a Mo s o l u t i o n to p r e c i p i t a t e Mo0(0H) 2 and formulated IV + the predominant Mo i o n before 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 ryoscopy . Recent ly C h a l i l p o y i l and Anson repor ted t ha 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 behaviour 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 ud i ed the p r e c i p i t a t i o n o f Ho^. IV Va ry ing amounts of NaOH were used to 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 eady desc r ibed fo r t h e i r work on M o V . P r e c i p i - t a t i o n began a t pH = 1.5 and was complete a t pH = 2 . 8 . Sodium and c h l o r i d e i ons 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 water . The product was ana lyzed as Mo0 2 • 2 H 2 0 . 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 to g ive the spectrum and polarogram c h a r a c t e r i s t i c o f Mo . A f t e r d r y i n g a t 130°C, however, i t took on a g r a i n y appearance and cou ld no longer - 22 - be d i s s o l v e d i n a c i d . 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 an e x c e s s o f NaOH g i v i n g a f i n a l pH o f 11.5 t h e p r o d u c t was an 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 . The r e a c t i o n p r o p o s e d was MoO(OH) 2 + MOH = Mo0 3HM + + + where M = Na , L i , K . 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 r e d i s s o l v e d t o IV 2- g i v e Mo i n s o l u t i o n . The i o n MoO^ was p r o p o s e d by a n a l o g y w i t h molyb- d a t e . 59,60 Lagrange and Schwmg o b t a i n e d an Mo0 2 • 2H 20 p r e c i p i t a t e by e l e c t r o l y s i s o f 0.125 M Na^oO^ s o l u t i o n s on a mercury cathode a t -0.6 V (vs SHE) and pH 5 - 6. The p r e c i p i t a t e . o n l y formed f o r pH > 2. A t l o w e r pH molybdenum b l u e was formed and no d e p o s i t i o n o c c u r r e d . The p r e c i p i t a t e gave t h e ASTM d i f f r a c t i o n p a t t e r n f o r Mo0 2 a f t e r d r y i n g a t 500°C i n n i t r o - gen. L a m a c h e 6 1 f o u n d t h a t e l e c t r o l y s i s a t -.29 t o -.38 V (vs SHE) i n an a c e t i c b u f f e r o f pH 4.6 gave M o 1 1 1 i n s o l u t i o n and a p r e c i p i t a t e c o n t a i n i n g V IV e q u a l amounts o f Mo and Mo 2.1.2.4 M o 1 1 1 M o 1 1 1 can 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 w i t h Cd, Zn, Hg o r by e l e c t r o l y s i s . M o 1 1 1 d e p o s i t s have been formed by e l e c t r o - l y t i c r e d u c t i o n i n a l k a l i n e o r n e u t r a l s o l u t i o n b u t t h e r a t e o f d e p o s i t i o n 62-66 i s s l o w . No d e t a i l e d 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 1 1 1 t o y i e l d M o ( 0 H ) 3 - 23 - have been made. Mo(OH).^ and the anhydrous ox ide 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 to 47 Mo(OH)^ by p r e p a r i n g enough o f the d e p o s i t fo r a n a l y s i s . M e l l o r r epor ted t ha 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 i n a c i d s . 64 VI Watt and Davies ob ta ined anhydrous M020^ by r e d u c t i o n o f Mo ox ide w i t h s o l u t i o n s o f potass ium i n l i q u i d ammonia. They r epor t ed tha t Mo^O^ d i s s o l v e s r e a d i l y i n 6 N HC1. MO2O2 was conver ted 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 25°C. Mo(OH) prepared from Mo^O^ was found to have the same p r o p e r t i e s as Mo(OH)^ p r e - pared by e l e c t r o l y s i s of M o V I s o l u t i o n s a t pH 3.1 t o 4 . 4 . N e i t h e r Mo(OH)^ nor 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 . On hea t i ng to 325°C i n an i n e r t a tmosphere , however, Mo(OH)^ gave d i f f r a c t i o n pa t t e rn s f o r Mo and Mo02 w h i l e Mo^Oj s t i l l gave no p a t t e r n . Messner and Z i m m e r l y 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 to 3.5 and p r e c i p i t a t e d Mo(0H) 3 by n e u t r a l i z a t i o n t o pH 3.6 to 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 con ta ined 2.12% Fe and 0.14% Cu. 2 .1 .3 Summary The p reced ing rev iew o f aqueous molybdenum chemis t ry suggests tha 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 products i n c l u d i n g mixed v a l e n t p r e c i p i t a t e s . I t a l s o i l l u s t r a t e s the complex nature o f molybdenum spec ies i n aqueous s o l u t i o n . Which aqueous spec ies o r p r e c i p i t a t e predomi- nates i n a g iven s i t u a t i o n i s l i k e l y to be dependent on t o t a l molybdenum c o n c e n t r a t i o n , pH, and p o t e n t i a l . Furthermore as these c o n d i t i o n s change du r ing a r e a c t i o n the predominant spec ies and hence r e a c t i o n path might be - 24 - expected to change. A review 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 r e d u c t i o n w i t h hydraz ine serves to i l l u s t r a t e these p o i n t s . - 25 - 2.2 Reduct ion 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 ^ s t ud i ed r e d u c t i o n o f ammonium paramolybdate s o l u t i o n s by hydrogen i n the presence o f a c o l l o i d a l p a l l a d i u m c a t a l y s t . F igu re 5 shows percent r e d u c t i o n to M o I V ( c a l c u l a t e d on the b a s i s o f H^ consumed) 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 out a t 30°C and atmospheric p r e s su re . The stage was a t t a i n e d a f t e r two days a t which p o i n t the r e a c t i o n s topped. Reduct ion was cont inued by hea t ing t o 50° to 60°C and a p p l y i n g a s l i g h t overpressure o f hydrogen. The r e a c t i o n proceeded s l o w l y and stopped again a f t e r three days when the H 2 consumption corresponded to M o 1 1 1 . The end s o l u t i o n was y e l l o w and the f i n e b l a c k p r e c i p i t a t e formed was very d i f f i c u l t to d i s s o l v e i n c o l d or hot concent ra ted HC1 or H 2 S 0 4 . The s m a l l q u a n t i t y t ha 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 s l u r r y formed a t the Mo stage gave a weight be- tween Mo(OH)^ and Mo0(0H) 2 when d r i e d i n vacuo wi thou t h e a t i n g . When d r i e d w i t h m i l d hea t ing the weight approximated M o 0 2 . The p r e c i p i t a t e was ana lyzed f o r molybdenum a f t e r d i s s o l u t i o n i n aqua r e g i a . These r e s u l t s seem to be i n accord w i t h those o f Souchay e t al.^"*" who found tha 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 2 *nH 2 0 w i t h n ve ry n e a r l y 2 . P a a l and But tne r d i d not ana lyze the p r e c i p i t a t e cor responding to the M o 1 1 1 s tage . The d i f f i c u l t s o l u b i l i t y i n , and the red c o l o u r imparted t o , concentra ted a c i d i s i n accord w i t h the r epor t ed p r o p e r t i e s of Mo(OH)^. The y e l l o w c o l o u r o f the end s o l u t i o n c o u l d have been tha t o f the Mo"^ +(H„0). 2 6 58 monomer. The abrupt decrease 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 to - 26 - F i g u r e 5: Reduct ion o f ammonium paramolybdate s o l u t i o n s by hydrogen i n the presence o f a c o l l o i d a l p a l l a d i u m c a t a l y s t . - 27 - depo lymer i za t i on o f molybdate o c c u r r i n g when r e d u c t i o n has consumed s u f f i - c i e n t a c i d . For example the r e d u c t i o n can be w r i t t e n : Mo 7 C> 2 4 6 ~ + 7H 2 + 6 H + + 4H 2 0 = 7Mo(OH) 4 from which i t f o l l o w s tha t r e d u c t i o n consumes a c i d . The paramolybdate i t s e l f bu f fe r s the r e a c t i o n i n the pH range 5 to 6 by i t s own d i s s o c i a t i o n acco rd ing t o : 6- 2- + M o 7 0 2 4 + 4H 2 0 = 7Mo0 4 + 8H . I t can be shown tha t a f t e r 57% o f the t o t a l molybdenum has been reduced VI 2-to Mo(OH) 4 the remaining Mo e x i s t s on ly as Mo0 4 . In t h i s p a r t i c u l a r system the pH i s then h e l d between 9 and 10 by the N H 4 + / N H ^ b u f f e r . S ince 2- l t seems apparent t ha t MoC>4 i s reduced o n l y ve ry s l o w l y the r e a c t i o n proceeds to comple t ion a t a ve ry slow r a t e . I t was found tha 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 comple t e ly . Lyapina and Zelikmann"*"^ s t ud i ed 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° to 200°C and 10 to 60 atm hydrogen p r e s su re . F igu re 6 shows t h e i r r e s u l t s 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 atm and 200°C. The recommended optimum pH was 2 which corresponds to com- 2- p l e t e a c i d i f i c a t i o n o f MoC>4 . Th i s i n i t i a l pH assures the r e a c t i o n i s buf fe red between pH 5 and 6 up to comple t ion . I t should be noted , however, t ha t i n t h i s work i t was found tha t the s o l u t i o n s con ta ined i r o n . Thus f o r lower pH ' 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 a u t o c l a v e . In subsequent runs a quar tz l i n e r was used but i n these runs v a r i o u s c a t a l y s t s were a l s o added. The product 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 rev ious 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 igu re 7 curve a and F igu re 7 curve b , r e s p e c t i v e l y , fo r i n i t a l pH = 3, 40 atm H 2 , and 200°C. Mo0 2 was added i n 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 amount t h e o r e t i c a l l y necessary fo r the r e a c t i o n 2- + Mo + 2Mo0 4 + 4H = 3Mo0 2 + 2H 2 0 I t was proposed t ha 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 which served as cen t res 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 2 from p rev ious r educ t ions was not an e f f i c i e n t c a t a l y s t . F i g u r e 7 curve c shows the r e s u l t s fo r no added c a t a l y s t ( 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 ob ta ined i n four hours u s ing 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" 2 , and 200°C (Figure 7 curve d ) . Under these c o n d i t i o n s decreas ing the i n i t i a l molybdenum c o n c e n t r a t i o n from 44 g/1 to 5 g/1 decreased the time r e q u i r e d fo r complete r e d u c t i o n from 4 hours to l e s s than % hour . In no case was complete r e d u c t i o n ob ta ined w i t h an i n i t i a l pH > 2 . Th i s i s i n accord w i t h the assumption tha t polymolybdates are necessary f o r an app rec i ab l e r a t e o f r e d u c t i o n . 34 In a l a t e r rev iew 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 necessary m e t a l l i c Mo, and 40 g/1 molybdenum tha t the r a t e of r e d u c t i o n was l i n e a r l y dependent on /PH 2 . He concluded tha 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 tep i n atomic form. 12 Sobol s t ud i ed r e d u c t i o n by both H 0 and CO at e l eva ted temperature 30 - ioor (a) initial pH=3, 4 0 a t m H 2 ~ slurry catalys t (b) initial pH=3, 40 atm H 2 metal l ic Mo catalyst (c) initial pH=3, 40 otm H 2 no c a t a l y s t added (d) initial pH = 2 , 6 0 atm H 2 m e t a l l i c Mo catalyst T IME (hour*) 10 F i g u r e 7: 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 pH, and hydrogen p ressure 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. - 31 - and p re s su re . Us ing CO at 80 - 85 atm pressure 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 ob ta ined i n t imes approaching one day. Th i s was a t t r i b u t e d to the b u f f e r i n g a c t i o n o f formic a c i d produced acco rd ing 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 to 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 to pH = 2 i n agreement w i t h Zel ikman and L y a p i n a . I t was concluded, however, t h a t the k i n e t i c s o f gaseous reduc- 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^ pu lp produced by hydro- gen r e d u c t i o n o f molybdate s o l u t i o n s was found to have no s i g n i f i c a n t c a t a l y t i c e f f e c t . S o b o l , t h e r e f o r e , proposed the use of m e t a l l i c molyb- denum as reduc tan t and h i s fu r the r work d i d not cons ide r gaseous r e d u c t i o n . Kunda and R u d y k 1 5 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 fo r molybdenum recovery 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 ^ ) 2 S ° 4 ' a n d mole r a t i o f ree NH^/Mo = 2. Under these 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 ob ta ined i n l e s s than o n e - h a l f hour . The r e s u l t i n g oxide 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 but 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 fo 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 sh molybdate s o l u t i o n s . From the s o l u - t i o n compos i t ion g iven i t i s apparent t ha 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 the N H 4 + / N H 3 b u f f e r . The f a c t tha t r e d u c t i o n went r a p i d l y t o comple t ion i n these 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 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 ^ e q u i - l i b r i a . No data i s a v a i l a b l e fo r the polymolybdates but u s i n g free energy 69 data from Barner and Scheuerman's c o m p i l a t i o n i t can be shown tha t the pH o f the N H 4 + / N H 3 b u f f e r f a l l s from 9.27 a t 25°C to 5.75 a t 200°C. Presumably t h i s f a l l was s u f f i c i e n t to ensure tha t the system was buf fe red i n a pH range where polymolybdates cou ld e x i s t . - 32 - The ox ide product ana lyzed 60 - 65% Mo, 4 - 5 % N H 3 , and 0.5% S compared to 59% Mo f o r Mo0(0H) 3 , 75% fo r M o 0 2 , and 65% fo r Mo(0H) 3 - S o l i d s t a t e hydrogen r e d u c t i o n o f t h i s product gave 99.9% molybdenum m e t a l . The p r e c i s e nature o f the ox ide product was not determined. X- ray d i f f r a c t i o n was not mentioned. They d i d r epo r t t ha 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 those l i s t e d above in te rmedia te steps were observed; 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 idue which l a t e r agglomerated i n t o b l a c k , oval -shaped p a r t i c l e s o f lower molybdenum o x i d e . I t may be supposed t ha t the brown re s idue was Mo0(0H) 3 and i t i s p o s s i b l e t ha t i n t h i s case IV complete r e d u c t i o n to the Mo s t a t e d i d not p roceed . I t was observed i n the present study t h a t under some c o n d i t i o n s Mo0(0H) 3 p r e c i p i t a t e d from molybdenum blue 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 the r than i t s 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 to recover molybdenum from 1.5 - 3.0 g/1 molybdenum and 60 - 120 g/1 s u l f a t e s o l u t i o n s a c i d i f i e d to pH = 2 . Reduct ion was c a r r i e d out a t 180°C and 20 - 25 atm i n a f low through r e a c t o r w i t h a r e s idence t ime o f 4% hours . The s o l u t i o n e x i t i n g the r e d u c t i o n v e s s e l was coo led to 80°C and expanded to 0.1 atm before f i l t r a t i o n to recover the molybdenum p roduc t . The f i l t r a t e conta ined 0.05 g/1 Mo thus molybdenum recovery was 97%. The a n a l y s i s of the product was not g i v e n . No c a t a l y s i s was mentioned. These r e s u l t s are c o n s i d e r a b l y b e t t e r than those ob ta ined by Zelikmann and Lyapina (Figure 7) a t a h igher temperature and hydrogen p r e s su re . S ince Wagenmann d i d not 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 i t i s not p o s s i b l e to determine whether the w a l l s o f the r e a c t o r p l ayed a r o l e i n the r e d u c t i o n . - 33 - 2.3 Reduct ion w i t h Hydrazine The behaviour o f h y d r a z i n e , N^H^ , as a r educ ing agent has been the subjec t o f much work, the genera l aim o f which was to determine the reasons 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 ox idan t s and e x p e r i - mental c o n d i t i o n s . Hydrazine was shown to r eac t acco rd ing to two l i m i t i n g r e a c t i o n s : * N 2 H 5 + = N 2 + 5 H + + 4e~ N H + = JjN. + NH + 2H + + e~ Z b Z j which cou ld o f t en occur 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 l i m i t s . Some o x i d i z i n g agents added s l o w l y to b o i l i n g h i g h l y a c i d hydraz ine 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 hydrazo ic a c i d , HN^. 70 Browne and S h e t t e r l y , i n summarizing the r e s u l t s o f a s e r i e s o f i n v e s t i g a t i o n s , r ecogn ized three c l a s s e s o f o x i d i z i n g agents fo r hydraz ine i n hot a c i d s o l u t i o n : c l a s s a c l a s s b c l a s s c produce f a i r l y l a r g e l i t t l e o r no HN^ l i t t l e or no amounts o f NH^ and HN 3 but much NH^ H N 3 o r N H 3 H 2 ° 2 KMnO 4 K I 0 3 KC10„ 4 Mn0 2 HgO K 2 S 2 ° 8 F e 2 ° 3 Hgci 2 *Most o f the work to be d i scussed was performed i n s o l u t i o n s o f pH < 7 where the protonated form o f h y d r a z i n e , N„H + , predominates . - 34 - They s t a t e d t h a t so many d i f f e r e n t f a c t o r s i n f l u e n c e d t h e c o u r s e o f t h e 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 t o e s t a b l i s h a s i m p l e 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 a g e n t s and t h e i r a b i l i t y t o produce HN^, NH^, o r . 71 Cuy e t 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 2 H 5 + • N 2 H 3 + 2 H + + e" f o l l o w e d by one o f t h e two r e a c t i o n s : N 2 H 3 • NH 3 + N NH 3 + 2N H >• N H 2NH + N2 2 3 4 6 3 I f e i t h e r o f t h e s e two subsequent r e a c t i o n s o c c u r r e d f a s t e r t h a n f u r t h e r o x i d a t i o n o f N 2H" 3 a l i m i t i n g 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 h y d r a z i n e ought t o o b t a i n . 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 one e q u i v a l e n t p e r mole a mixed s t o i c h i o m e t r y ( i . e . between 1 and 4 e q u i v a l e n t s p e r mole o f h y d r a z i n e ) was e x p l a i n e d i n terms o f t h e g e n e r a - t i o n o f a n o t h e r o x i d i z i n g a g e n t as an i n t e r m e d i a t e u 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: 2- + . 4e~ _ Cr„0_ + N-HV > N„ + HCr»O c + 2H„0 2 1 2 5 2 2 5 2 + 2e~ 3+ + HCr O + 2N H * 2Cr + 2NH. + N_ + 5H„0 2 5 2 5 4 2 2 + 2-g i v m g t h e o v e r a l l s t o i c h i o m e t r y o f 1.5 moles K̂ H,- p e r mole o f Cr^O^ An 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 as moles o f - 35 - e l e c t r o n s / m o l e o f h y d r a z i n e ) t o i n c r e a s e i n a l k a l i n e s o l u t i o n was t h o u g h t due t o a h i g h e r r a t e o f f u r t h e r o x i d a t i o n o f i n t e r m e d i a t e i n a l k a l i n e s o l u t i o n s as opposed t o a h i g h e r r a t e o f i t s d e c o m p o s i t i o n i n a c i d s o l u t i o n s . 72 K i r k and Browne l i k e w i s e p r o p o s e d d i v i s i o n o f o x i d i z i n g a g e n t s 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. Those t h a t a c c e p t more t h a n one 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 - a t o r s , t r i d e l e c t r o n a t o r s , e t c . W i t h m o n o d e l e c t r o n a t o r s c o mplete o x i d a t i o n t o n i t r o g e n c o u l d be o b t a i n e d b u t i n c a s e s where 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 was ammonia formed a c c o r d i n g t o : N_H_ + >• N„EL + 2 H + + e~ (slow) Zo 2 3 2 N H — — > N .H N„ + 2NH_ ( f a s t ) 2 3 4 6 2 3 W i t h d i d e l e c t r o n a t o r s c o m p l e t e o x i d a t i o n t o n i t r o g e n was p r e d o m i n a n t b u t f o r i n c o m p l e t e o x i d a t i o n ammonia and a s m a l l q u a n t i t y o f h y d r a z o i c a c i d were formed. The r e a c t i o n sequence i n t h i s c a s e i n v o l v e d i n i t i a l f o r m a t i o n o f ^2^2 a c c o r d l n 9 t o : N n H c + • N„H_ + 3H + + 2e (slow) z b I 2 f o l l o w e d by a s e r i e s o f f a s t s u b sequent r e a c t i o n s t o p r o d u c e o r HN^ and NH . 3 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 r e d u c t i o n i n more t h a n two s t a g e s (complex d e l e c t r o n a t o r s ) 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- and d i d e l e c t r o n a t o r s , f o r example: - 36 - 3+ - 2+ VO + e = VO 3+ - + VO + 2e = VO 73 Higg inson e t a l . proposed tha 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 those reagents which o x i d i z e d hydraz ine a t room temperature i n a c i d s o l u t i o n to produce N 2 o n l y . 2 . Monodelect ronators were those reagents which o x i d i z e d hydraz ine to NH^"1" and N 2 under the 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 ^ " 1 " / ^ depending on r e l a t i v e i n i t i a l concen t r a t i ons o f hydraz ine and o x i d a n t . Simple mechanisms f o r mono- and d i d e l e c t r o n a t i o n were proposed: d i d e l e c t r o n a t i o n N 2 H 5 + 2e~ N 2 H 2 + 3H 2e N 2 + 4H monodelec t ronat ion N 2 H 5 + l e ~ N 2 H 3 d i m e r i z a t i o n 2 N 2 H 3 f a s t 4 6 f a s t N 2 + 2NH 3 l e N 2 H 2 f a s t 2e N 2 + 4H predominant r e a c t i o n i n a c i d s o l u t i o n predominant r e a c t i o n i n a l k a l i n e s o l u t i o n The s t o i c h i o m e t r y for .monodelectronators c o u l d l i e between R = 1 and R = 4 depending on the 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 - Since i n a l k a l i n e s o l u t i o n s even monodelectronators produce n i t r o g e n q u a n t i t a t i v e l y the e f f e c t of i n c r e a s i n g pH was seen as decreas ing the r a t e 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 Subsequently Higg inson and Sut ton 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 en r i ched hydraz ine 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 products confirmed the v a l i d i t y o f the s imple mechanisms proposed. Cahn and 75 P o w e l l independent ly performed a s i m i l a r i s o t o p i c s tudy w i t h the same r e s u l t s . In a subsequent rev iew Higg inson"^ noted t ha t fo 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 to hydraz ine consumed are ve ry much s m a l l e r i n a l k a l i n e , n e u t r a l , and weakly a c i d s o l u t i o n than the y i e l d s ob ta ined by u s i n g monodelectronators 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, ox idan t s g ive va lues g rea te r than 0.1 f o r moles NH_/mole N_H. whereas fo r pH < 3 most monodelectronators g ive mole 3 2 4 NH^/mole 1 0 .75 . Use o f the equa t ions : 4 moles NH^ 4 moles + = R moles ^2^4 moles N^H^ 4 moles NH^ moles N 2 + moles N_H . moles INLH,, 2 4 2 4 4 (where i s de f ined as due t o 4 e q u i v a l e n t reduc t ion) shows tha t t h i s corresponds to 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 fo r pH ^ 3. Higg inson f e l t i t was l i k e l y the change from low to h i g h s t o i c h i o m e t r y occur red i n the pH range 3 to 5 based on the o b s e r v a t i o n tha t a t pH = 2 - 38 - most monodelectronators gave R < 1.75 while i t was observed that at pH = 6 the monodelectronator f e r r i c y a n i d e reacted with R = 4. This p r e d i c t i o n was v e r i f i e d for oxidation with manganese trispyrophosphate. For didelectrona- tors Higginson proposed the change to high stoichiometry occurred f o r pH > 0. Browne and S h e t t e r l y 7 0 investigated the reaction of aqueous suspen- sions of MoO^ with hydrazine. In a l k a l i n e solutions the reaction was very slow and some ammonia was produced. In b o i l i n g s u l f u r i c acid solutions with excess MoO^ appreciable amounts of ammonia and some HN^ were formed. 72 On the basis of t h i s work Kirk and Browne considered molybdate ions as didelectronators. 77 Jakob and Kozlowski found that molybdate solutions oxidized hydra- VI zine p r a c t i c a l l y completely to - They were able to prepare mixed Mo / Mo V compounds by reduction i n weakly acid s olutions. In s l i g h t l y more acid V solutions molybdenum blue was formed while at s t i l l higher a c i d i t y Mo was produced d i r e c t l y without intermediate products. In p a r t i c u l a r they report- from a s o l u t i o n V VI ed preparation of the Mo /Mo compound NH V VI Mo^O Mo O^ (OH), containing 0.317 M molybdenum as ammonium paramolybdate and 0.06 M N 2 H 4 * This compound has the same Mo V/Mo V I r a t i o as the brown species reported by 42 Ostrowetsky between pH 3 and 4. In f a c t i f the formula given by Jakob and Kozlowski i s rewritten to contain s i x atoms of molybdenum and water i s removed i t becomes Mo, Mo, V 17 and, i f i t i s assumed to be protonated i n s o l u t i o n , - 39 - V Mo . 4 O H which i s the compos i t ion found by Ostrowetsky. 31 78 H o l t j e and Geyer and Rao and Suryanarayama repor t ed tha t under V no c o n d i t i o n s does hydraz ine reduce molybdate s o l u t i o n s beyond the Mo s t a t e . 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 Mo V VI s tock s o l u t i o n s . The l a t t e r authors reduced 0.4220 moles of Mo i n 1 N HC1 on the b o i l i n g water bath w i t h 0.154 moles o f h y d r a z i n e . They were i n t e r e s t e d on ly 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 Mo V so d i d not determine the excess h y d r a z i n e . These r e s u l t s , however, i n d i c a t e they ob ta ined R - 2 .74 . S i n c e , a cco rd ing to H i g g i n s o n , a monodelectronator should VI approach R = 1 under such c o n d i t i o n s these r e s u l t s suggest Mo behaved as a d i d e l e c t r o n a t o r . 45 F i l l i p o v and Nuger r epor ted t h a t a t pH > 4, 0.008 M molybdate s o l u t i o n s were not reduced by 0.02 to 0.04 M h y d r a z i n e . At pH 1.3 to 1.4 o n l y molybdenum blue 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 hydraz ine i n 0.1 to 10 N a c i d s o l u t i o n s . In t h i s range o f a c i d i t y mixed Mo V /Mo V ] " compounds d i d not form and the o n l y product was M o V . The r a t e o f o f the r e a c t i o n inc reased w i t h i n c r e a s i n g temperature and decreased w i t h i n c r e a s i n g a c i d i t y . 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 at 0°C where the r e a c t i o n was slow enough to f o l l o w wi thou 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 con ta ined hydroxylamine and polarography i n 4 N NaOH a l lowed de t e rmina t ion o f the t o t a l c o n c e n t r a t i o n o f ^ H ^ + + VI V NH^OH . Mo and Mo were determined by polarography i n 2 N HC1. - 40 - I t was found 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 change 2 4 3 V d u r i n g t h e r e a c t i o n and 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 br e a k a t mole 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 o ) „ + 2NH O H Z o Z b Z Z 3 + 80 The s p e c i e s HMo 2Og had been p r o p o s e d by Chauveau e t a l . as t h e p r e d o m i - VI n a n t Mo s p e c i e s i n 2 N H C l . 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 and a c i d i t y , however, i t was 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 ( a c c o r d i n g t o t h e above e q u a t i o n ) o f [N2H<_ + ] / [Mo V I] = 0.5 an i n c r e a s e o f i n i t i a l c o n c e n t r a t i o n o f M o V I from 0.02 M t o 0.24 M changed t h e r e a c t i o n so t h a t N 2 as w e l l as o t h e r u n i d e n t i - f i e d p r o d u c t s r e s u l t e d . 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 were: 2HMo_0 * + N_H_ + + H + = M o y l 0 0 4 + + N_ + 3H„0 Z b Z i> 4 B Z Z o r H Mo 0 _ + N H + + 3H + = Mo 0 Q 4 + + N + 5H 0 Z 4 13 Z o 4 o 2 Z I t was r e a s o n e d t h a t t h e change i n t h e r e a c t i o n was due t o a change 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. Chauveau e t al.^° had r e p o r t e d an e q u i l i b r i u m between HMo„0 * and H Mo.O., _ w h i l e t h e work o f 2 6 2 4 13 V i o s s a t and 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 V c o u l d e x i s t i n e q u i l i b r i u m w i t h a d i m e r i c form. A t a g i v e n a c i d i t y , t h e r e f o r e , an i n c r e a s e i n c o n c e n t r a - t i o n o f molybdenum would f a v o u r t h e t e t r a c o n d e n s e d forms o f t h e MoV"C and Mo 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 . O s t r o w e t s k y and B r i n o n ' s work appears t o be t h e o n l y r e p o r t o f 81 o x i d a t i o n o f h y d r a z i n e t o h y d r o x y l a m i n e . A u d r i e t h and Ogg i n t h e i r r e v i e w o f o x i d a t i o n o f h y d r a z i n e 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 t o demon- s t r a t e t h a t h y d r o x y l a m i n e i s an o x i d a t i o n p r o d u c t o f h y d r a z i n e . - 41 - 82 V I Huang and Spence i n v e s t i g a t e d t h e r e a c t i o n o f h y d r a z i n e and Mo a t 70°C i n phosphate b u f f e r s o f pH 1.2 t o 3.2. The i n i t i a l mole r a t i o o f + VI VI -4 N^Hj. t o Mo was m a i n t a i n e d a t 0.5 and Mo was v a r i e d between 5 x 1 0 M -4 and 7 x 10 M. The h y d r a z i n e 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 and t h e r e a c t i o n was f i r s t o r d e r i n each r e a c t a n t . The r a t e o f 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 pH and e x h i b i t e d an o r d e r i n H + o f 0.25. I t was s u g g e s t e d t h a t t h e f r a c t i o n a l dependence on H + c o u l d be due t o changes i n p o l y m e r i z a - VI V I t i o n o f Mo w i t h pH, i o n i z a t i o n o f a monomeric Mo s p e c i e s , o r i n v o l v e - ment o f H + i n a r a t e c o n t r o l l i n g s t e p . N 2 H 2 w a S ^ e t e c t e c ^ q u a l i t a t i v e l y by mass s p e c t r o m e t r y and t r a p p i n g w i t h u n s a t u r a t e d a c i d s . The o b s e r v e d s t i o c h i o m e t r y 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 behaved as a d i d e l e c - t r o n a t o r . 8 3 Nusgra and S i n h a 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 and 0.01 M M o V I r e a c t e d t o y i e l d N q u a n t i t a t i v e l y . They d e t e c t e d b o t h N 2 H 3 and ^ 2 ^ 2 d e p e n d i n g o n t n e r e l a t i v e amount o f M o V I added t o h y d r a z i n e s o l u - t i o n s . The p r e c e d i n g summary i n d i c a t e s t h a t M o V I t e n d s t o o x i d i z e h y d r a z i n e t o n i t r o g e n w i t h R - 4. T h i s has been i n t e r p r e t e d i n terms o f pr e d o m i n a n t d i d e l e c t r o n a t i o n t o y i e l d N 2 H 2 as a f i r s t s t e p and a l s o by s e r i e s o f mono- d e l e c t r o n a t i o n s t e p s . N e a r - q u a n t i t a t i v e o x i d a t i o n t o N 2 by one e l e c t r o n s t e p s , however, appears t o be u n l i k e l y e x c e p t i n a l k a l i n e s o l u t i o n . Two e l e c t r o n s t e p s r e q u i r e t h a t t h e molybdate s p e c i e s i n v o l v e d be r e d u c e d t o M o I V i f t h e y a r e monomeric. Mechanisms based on t h e assumed c h e m i s t r y o f M o I V must, however, be open t o q u e s t i o n u n t i l more i s known about t h e c h e m i s t r y o f Mo I V. VI S i n c e 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 s p e c i e s i n v o l v e d . - 42 - As has been seen t h i s c o n c e p t was used by O s t r o w e t s k y and B r i n o n t o e x p l a i n 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 c o n c e n t r a - t i o n i n a c i d s o l u t i o n . There have been no d e t a i l e d s t u d i e s o f t h e r e a c t i o n under 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 pH where mo l y b d a t e p o l y a n i o n s p r e d o m i - n a t e i n s o l u t i o n and where f o r m a t i o n o f mixed MoV/Mo^^~ s p e c i e s m i g h t be e x p e c t e d t o o c c u r . The work o f J a k o b and K o z l o w s k i does, however, s u g g e s t 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 a t l e a s t i n t e r m e d i a t e f o r m a t i o n o f V . VI Mo /Mo s p e c i e s a r e t o be e x p e c t e d . - 43 - 2.4 Reduct ion w i t h SO and H S 31 H o l t j e and Geyer repor ted t ha t SC^ reduces weakly a c i d molybdate s o l u t i o n s to g ive l i g h t green o r b lue s o l u t i o n s i n which on ly 0.5% o f the V molybdenum i s present as Mo . In more concent ra ted a c i d no n o t i c e a b l e 88 r e a c t i o n o c c u r r e d . Wardlaw made s i m i l a r obse rva t ions f o r both SC^ and I ^ S . Thus a t bes t i t appears t ha t n e i t h e r o f these reductants can take molybdate pas t the mixed v a l e n t molybdenum blue s tage . - 44 - 2.5 Summary The work r e p o r t e d i n t h e l i t e r a t u r e i n d i c a t e 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 o f molybdenum o x i d e s w i t h hydrogen o r h y d r a z i n e m i g h t be t e c h n i c a l l y f e a s i b l e . Hydrogen and h y d r a z i n e , as a f o u r e l e c t r o n r e d u c t a n t , a r e c l e a n r e d u c t a n t s t h u s t h e y o f f e r t h e p o t e n t i a l o f p r o d u c i n g a p u r e o x i d e p r o d u c t . Most o f t h e work r e p o r t e d , however, has been p e r f o r m e d i n a c i d s o l u t i o n s and t h e a c t i o n o f t h e s e r e d u c t a n t s on n e u t r a l o r a l k a l i n e m olybdate s o l u t i o n s i s n o t w e l l documented. I n t h e n e u t r a l pH range i t i s a p p a r e n t t h a t p o l y m e r i z a t i o n o f molyb- d a t e i o n s m i g h t be a f a c t o r i n any r e d u c t i o n r e a c t i o n . I n a d d i t i o n mixed- VI V v a l e n t Mo /Mo compounds m i g h t be e x p e c t e d t o form i n t h i s range and t h e s e c o u l d i n f l u e n c e t h e c o u r s e o f t h e r e d u c t i o n r e a c t i o n . I n t h e c a s e o f r e d u c - t i o n w i t h h y d r a z i n e t h e r e a c t i o n m i g h t be a b l e t o f o l l o w d i f f e r e n t p a t h s d e p e n d i n g on pH and degree o f p o l y m e r i z a t i o n o f m o l y b d a t e . I l l IV V I t has been seen t h a t Mo , Mo , and Mo h y d r a t e d o x i d e s as w e l l as mixed v a l e n t o x i d e s can be p r e c i p i t a t e d i n n e u t r a l s o l u t i o n . There i s a l s o a p o s s i b i l i t y t h a t t h e Mo V p r e c i p i t a t e may a c t as an i o n e xchanger. - 45 - 3. SCOPE OF PRESENT WORK The present work was undertaken to i n v e s t i g a t e the use o f hydrogen and hydraz ine fo 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 oxides 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 each o f Cu-Mo rougher concen 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 to def ine the pH range i n which r e d u c t i o n cou ld be ach ieved , the k i n e t i c s of the r e d u c t i o n r e a c t i o n s , and the nature o f the oxide p r o d u c t s . The procedure adopted was f i r s t to determine the behaviour o f the 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 to i n v e s t i g a t e the e f f e c t s o f o ther 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 fo r r e d u c t i o n by both hydrogen and hydraz ine 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 reagent sodium molybdate (Na2MoO^"2H2O) . The manufacturer 's assay was 99.5% sodium molybdate minimum and the chemical was used wi thout fu r the r p u r i f i c a t i o n . The molybdate s o l u t i o n s were s t andard ized by flame atomic abso rp t ion s p e c t r o - 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 tock 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 curves and n e u t r a l - i z a t i o n experiments were produced by reduc ing 3.5 N HC1 sodium molybdate s o l u t i o n s by shaking i n a f l a s k w i t h m e t a l l i c mercury. The s o l u t i o n s were 46 i s t andard ized 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 ing f e r r o i n as an i n d i c a t o r . ' Hydrazine s o l u t i o n s were made up by d i l u t i o n of BDH 99-100% hydraz ine "hydrate or Eastman Kodak 64% hydraz ine i n d i s t i l l e d water . The hydraz ine 85 s o l u t i o n s were s t andard ized by potass ium ioda te 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 exper iments . Hydrogen r e d u c t i o n was performed i n a Pa r r 2 l i t r e a u t o c l a v e . A g l a s s l i n e r was used f o r a l l runs . A l l p a r t s o f the bomb c o n t a c t i n g the s o l u t i o n were t i t a n i u m . The r e s i s t a n c e heater sur rounding the bomb was 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 Company Thermistemp Temperature C o n t r o l l e r Model 71 and a V a r i a c . The V a r i a c was se t to g ive the a p p r o x i - mate temperature d e s i r e d and the Thermistemp mainta ined the d e s i r e d tempera- tu re by o p e r a t i n g a r e l a y which reduced the power i npu t to the heater by about 15% i n the c o o l i n g c y c l e s . I t was found tha t on p r e s s u r i z i n g the bomb w i t h hydrogen there was a s i g n i f i c a n t temperature inc rease ( e . g . p r e s s u r i z i n g t o 30 atm H at 180°C l e d to a temperature i nc r ea se 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 rov ided by a j e t of compressed a i r o r n i t r o g e n in t roduced a t the base o f the bomb. The gas f low was con- t r o l l e d by a s o l e n o i d va lve which opened on the c o o l i n g c y c l e o f the Thermistemp. In a l l runs s t i r r i n g was mainta ined a t 600 rpm. In a t y p i c a l run one l i t r e o f sodium molybdate s o l u t i o n adjusted to the d e s i r e d pH was added to 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 fo r about f i v e minutes and the bomb was p r e s s u r i z e d to 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 to j u s t below the d e s i r e d temperature u s ing f u l l power inpu t to 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 run c o n t r o l l e d by the Thermistemp. Once the temperature was s t a b l e hydrogen was admit ted to the bomb. The hydrogen pressure was inc reased to tha t d e s i r e d and main- 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 run was ±5°C and pressure ±5 p s i g . Samples were taken a t appropr i a t e t imes throughout each r u n . The sampling system was c l e a r e d before each sample by d i s c h a r g i n g a t l e a s t 20 ml o f s o l u t i o n . A g i t a t i o n was mainta ined du r ing sampl ing . Samples were coo led e i t h e r by s tanding a t room temperature or h o l d i n g under c o l d tap water . A f t e r each run the apparatus was c leaned by running f o r one hour a t 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 l i n e r . Hydrazine experiments were performed i n a 420 ml pyrex v e s s e l . The r e a c t o r was mainta ined 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 . In a t y p i c a l run 200 ml of sodium molybdate s o l u t i o n adjus ted to the d e s i r e d pH w i t h HCl was brought to temperature i n the r e a c t o r . An equal - 48 - volume o f hydraz ine s o l u t i o n adjusted to the same pH was preheated i n a separate f l a s k . At zero time the hydraz ine s o l u t i o n was added to the molyb- date s o l u t i o n . S t i r r i n g was main ta ined a t 300 rpm for 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 du r ing the r u n . Samples were taken by p i p e t t i n g 15 to 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 . Var ious methods were used to f reeze the r e a c t i o n to permi t de t e rmina t ion of the concen t r a t i ons of molybdenum and hydraz ine as func t ions of t ime . S ince p r e l i m i n a r y runs had shown tha t the r e d u c t i o n r e a c t i o n d i d not occur above pH 6.5 the f i r s t method adopted was to d i scharge 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 concent ra ted NaOH to r a i s e the pH to 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 to d i scharge the sample i n t o a p o r c e l a i n f i l t e r 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 again con- t a i n i n g enough NaOH to r a i s e the pH to 8 to 12. This method of a n a l y s i s gave s i g n i f i c a n t l y d i f f e r e n t r e s u l t s . The t h i r d procedure was to d i scharge 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 to 1 N NaOH to o b t a i n a p a r t i c u l a r pH. 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 ana lyzed fo r molyb- denum and h y d r a z i n e . A f o u r t h procedure was s imply to d i scharge the sample i n t o a beaker c o n t a i n i n g an equal 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 ve ry slow at room temperature so i t was f e l t t h i s procedure would permi t time fo r subsequent f i l t r a t i o n . The f i f t h technique used was to d i scharge the sample i n t o a f i l t e r - 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 to a c o n c e n t r a t i o n s u i t a b l e fo r de termina- t i o n o f molybdenum by atomic abso rp t ion (-10 u g / m l ) . A few runs were performed to 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 du r ing r e a c t i o n . The r e a c t i o n was c a r r i e d out i n a sea led f l a s k and the gas evolved c o l l e c t e d over mercury. These runs were c a r r i e d out a t room tempera- t u r e . To beg in a run a hydraz ine s o l u t i o n adjus ted to pH = 4.5 was added to a molybdate s o l u t i o n o f the same pH i n the f l a s k . The f l a s k was q u i c k l y stoppered and s t i r r i n g begun. The pH was not c o n t r o l l e d d u r i n g these runs . Molybdenum i n s o l u t i o n was determined both by flame atomic abso rp t ion spectrophotometry u s i n g a P e r k i n Elmer Model 306 spectrophotometer and by polarography u s i n g a Sargent Model XXI po la rog raph . S o l u t i o n s fo r p o l a r - ography were d i l u t e d to c o n t a i n between 0.05 and 0.15 g/1 Mo w h i l e those f o r atomic abso rp t ion 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 fo 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 e l i m i n a t e i n t e r f e r e n c e s . Polarography was performed i n 2 N HCl where the waves o f Mo V and M o V I are d i s t i n c t . 7 9 Atomic abso rp t ion gave t o t a l molybdenum rega rd 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 hydraz ine d i d not i n t e r f e r e . Polarography VI V o f Mo /Mo mix tures i n the presence o f hydraz ine was u n s u i t a b l e fo r separate V VI determxnat ion 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 V I by hydraz ine proceeded a t a r a t e s u f f i c i e n t to make V the de te rmina t ion o f Mo o f doub t fu l accuracy . T o t a l molybdenum cou ld be determined. Atomic a b s o r p t i o n and polarography agreed w i t h i n 2%. Hydrazine 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 disappearance of the p ink i o d i n e co lou 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 Ammonia, - 50 - hydroxylamine, and Mo d i d not i n t e r f e r e . When app rec i ab l e hydroxylamine was presen 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 ink c o l o u r re turned V to the C C l ^ l a y e r on s tand ing for a few hours . Mo d i d i n t e r f e r e i n the t i t r a t i o n but i n genera l the amount o f Mo V p resent i n the r e a c t i o n f i l t r a t e s was sma l l enough to render a c o r r e c t i o n unnecessary. Ammonia i n sample f i l t r a t e s was determined by a method desc r ibed by 8 7 DeVries and Gantz . Hydrazine was f i r s t o x i d i z e d by 0.4 N KIO^ as p r e - v i o u s l y d e s c r i b e d . The I + and excess 10^ were reduced to I by excess Na^SO^. The excess s u l f i t e was then removed by bubb l ing a i r fo r 15 minutes . 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 s o l u t i o n b a s i c . Hydroxylamine d i d not i n t e r f e r e . P r e c i p i t a t e s produced by hydraz ine r e d u c t i o n were ana lyzed fo 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 rous ammonium s u l f a t e u s ing f e r r o i n 46 as an i n d i c a t o r . Water was determined u s i n g a Dupont 950 thermogravimet r ic a n a l y z e r . 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 p r e c i p i t a t e i n 3 M HNO^, a d d i t i o n of excess AgNO^, and back t i t r a t i o n w i t h HCl u s ing a s i l v e r e l ec t rode and a potass ium s u l f a t e reference e l e c t r o d e . " ^ Sodium was determined by flame emiss ion photometry. 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- ray d i f f r a c t i o n and thermogravimetr ic a n a l y s i s . - 51 - 5. RESULTS 5.1 Hydrogen Reduct ion Hydrogen r e d u c t i o n experiments were performed a t 200° to 220°C w i t h 30 atm pressure o f . The i n i t i a l c o n c e n t r a t i o n of 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 . For t imes up to 7 hours r educ t ion proceeded on ly t o the molybdenum blue stage and no p r e c i p i t a t i o n o c c u r r e d . An experiment was then performed i n the presence o f a p l a t i num c l a d expanded niobium mesh. In t h i s case the s o l u t i o n passed through the molyb- denum b lue stage i n 30 minutes and l a t e r samples were on ly 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 percent r e d u c t i o n vs t ime . 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 on ly s l i g h t l y p l a t e d the apparatus and was e a s i l y f i l t e r a b l e . I t gave an X- ray 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 to ASTM 5-0452 fo r Mo0 2 • Thermogravimetric a n a l y s i s showed the p r e c i p i t a t e was anhydrous. Experiments w i t h the mesh were not cont inued because the niobium subs t ra te suf fe red extreme hydrogen damage and broke under i t s own weigh t . The run 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 s t i r r e r and other 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 fu r the r run performed wi thou t c l e a n i n g the apparatus r e s u l t e d i n format ion o f a b lue 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) . C e r i c s u l f a t e t i t r a t i o n i n d i c a t e d the average 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 .53 . Only a sma l l amount o f p r e c i p i t a t e was formed a f t e r 12 hours . The dark b lue s o l u t i o n conta ined 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 .67 . The apparatus was then c leaned to remove a l l depos i t s and the run was - 52 - 2 0 L_ • L 0 5 10 T I M E (hours ) F i g u r e 8: Reduct ion of 17 g/1 sodium molybdate s o l u t i o n by hydrogen a t 200°C and 30 atm H 2 i n the presence o f P t c l a d n iobium mesh. - 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 co loured l i g h t b lue and there was no p r e c i p i t a t e . Three runs were performed i n 3 M NaCl to s imula te 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 . Min imal r e d u c t i o n was ob ta ined 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 observed t ha t the l i q u i d condensing between the g l a s s l i n e r and the bomb seve re ly corroded the t i t a n i u m bomb. - 54 - 5.2 Reduct ion w i t h 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 us ing 5 g/1 molybdenum s o l u t i o n s and v a r i o u s hydraz ine c o n c e n t r a t i o n s . For pH ~ 5 r e a c t i o n occur red almost immediately on mix ing the r e a c t a n t s . The s o l u t i o n under- went a s e r i e s o f c o l o u r changes. I t f i r s t became b l u e , then green, and f i n a l l y an opaque brown. A brown p r e c i p i t a t e formed du r ing the green stage but not du r ing the i n i t i a l b lue one. The e f f e c t 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 c o n c e n t r a t i o n , and an i n i t i a l hydraz ine t o molybdenum mole r a t i o o f 4 : 1 . For pH > 6.5 no r e a c t i o n o c c u r r e d . As the pH was decreased below 6.5 the r a t e o f p r e c i p i t a t i o n inc reased but fo r pH < 3 the nature o f the r e a c t i o n changed. The i n i t i a l b lue c o l o u r remained throughout the r e a c t i o n and the r a t e o f 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 formed was b lue 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 , 50°C and 4:1 i n i t i a l mole r a t i o o f hydraz ine t o molybdenum. These c o n d i t i o n s gave a convenient r e a c t i o n r a t e f o r a n a l y s i s . The r e s u l t s ob ta ined , however, were found to depend markely on the sampling method employed. F igu re 9 curve 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 remain- i n g i n s o l u t i o n vs time ob ta ined by n e u t r a l i z i n g 20 ml samples o f the r e - 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 to o b t a i n a supernatant s o l u t i o n fo r a n a l y s i s . Curves b through e were ob ta ined 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 to the pH ' 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 ob ta ined 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 I I I L _ 0 30 60 9 0 120 TIME (minutes) F i g u r e 9: E f f e c t o f sampl ing technique on c o n c e n t r a t i o n o f molybdenum remain ing i n s o l u t i o n vs t ime 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 hydraz ine to molybdenum mole r a t i o =4:1. " - 56 - f i l t r a t i o n . F i g u r e 10 compares t h e r e s u l t s o b t a i n e d u s i n g methods i n v o l v i n g d i r e c t f i l t r a t i o n o f t h e 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 f i r s t d i s - c h a r g i n g t h e s l u r r y i n t o an e q u a l volume o f c o l d w a t e r . The open c i r c l e s r e p r e s e n t t h e l a t t e r t e c h n i q u e . The t r i a n g l e s r e p r e s e n t p o i n 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 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 f o 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. The c r o s s e s r e p r e s e n t p o i n 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 t h e s l u r r y samples i n t o c e n t r i f u g e t u b e s c o n t a i n i n g 1 ml o f 8 N NaOH. A l l t h r e e o f t h e s e methods gave s i m i l a r r e s u l t s f o r molybdenum r e m a i n i n g i n s o l u t i o n . When s a m p l i n g was p e r f o r m e d by f i l t r a t i o n i n t o c e n t r i f u g e t u b e s c o n - t a i n i n g 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 brown f i l t r a t e formed a brown o r b l u e - g r e e n p r e c i p i t a t e on m i x i n g w i t h t h e c a u s t i c . F i g u r e 11 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 on f i l t e r i n g 25 ml samples o f s l u r r y t h r o u g h a p o r c e l a i n f i l t e r c r u c i b l e , t h e d r y w e i g h t o f t h e 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 t u b e , and t h e 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 i n the c e n t r i f u g e d f i l t r a t e . F i g u r e 12 shows t h e r e s u l t s o b t a i n e d under t h e same c o n d i t i o n s (50°C, pH = 4 . 5 , i n i t i a l molyb- denum c o n c e n t r a t i o n = 5 g /1) b u t w i t h a 2:1 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 i n s t e a d o f 4 : 1 . S i n c e t h e above s a m p l i n g method was r a t h e r t e d i o u s i t was d e c i d e d t o 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 o r d e r t o i n v e s t i g a t e t h e e f f e c t s o f t e m p e r a t u r e and 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 on t h e r a t e o f t h e p r e c i p i - t a t i o n r e a c t i o n . . The i n i t i a l molybdenum c o n c e n t r a t i o n was ~5 g / 1 and t h e pH = 4 .5 f o r e ach r u n . F i g u r e s 13 and 14 show t h e r e s u l t s o b t a i n e d . - 57 - pH = 4.5 T = 5 0 ° C ini l ibl Mo = 5 g/1 in i t ia l N 2 H 4 = 6 . 6 g / l 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 rect f i l t rat ion into caust ic A d i rect filtration then d i lu t ion 2 0 T IME (minutes) 3 0 F i g u r e 10: Comparison of r e s u l t s ob ta ined 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 ob t a ined by p r ev ious d i l u t i o n o f samples w i t h an equal volume o f c o l d wa'ter. - 58 - T I M E (minuteS' ) Figure 11: D i s t r i b u t i o n of 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 function of time at 50° C with pH=4.5, i n i t i a l molybdenum concentra- t i o n =5 g/1, and i n i t i a l hydrazine to molybdenum mole r a t i o =4:1. * - 59 - 0 2 0 4 0 6 0 TIME (minutes) F i g u r e 12: 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 t ime a t 50°C w i t h pH = 4 . 5 , i n i t i a l molyb- 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 hydraz ine t o molybdenum - 2 : 1 . - 60 - - 61 - 0 10 2 0 3 0 T I M E (minutes) Figure 14: E f f e c t of i n i t i a l mole r a t i o of hydrazine to molybdenum on rate of p r e c i p i t a t i o n at 50°C with pH = 4.5 and i n i t i a l concentration of molybdenum =5 g/1. - 62 - In o rder to s imula te the c o n d i t i o n s expected i n t r e a t i n g s o l u t i o n s from the sodium h y p o c h l o r i t e l e a c h a run was c a r r i e d out i n 3 M N a C l . F igu re 15 compares the r e s u l t s o f t h i s run w i t h one performed under the same c o n d i t i o n s but w i thou 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 igu re 16. The presence o f copper d i d not a f f e c t 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 to deep b lue on d r y i n g f o r a few minutes i n a i r . I t can be seen from F i g u r e s 14 and 15 t ha t f o r low i n i t i a l mole r a t i o s o f hydraz ine t o molybdenum and i n the presence o f 3 M NaCl there was a s i g n i f i c a n t t ime before p r e c i p i t a t i o n began. In each case , however, r e a c t i o n began immediate ly on mix ing the r eac tan t s as evidenced 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 b lue s p e c i e s . In the extreme case o f a 1:1 i n i t i a l mole r a t i o o f r eac tan 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 apparent r e a c t i o n o rder i n molybdenum was 1.5 as shown i n F i g u r e 17 which was p l o t t e d u s i n g the data o f F i g u r e 14. The s lopes o f the l i n e s i n F i g u r e 17 were used to determine the o rder i n ^ H ^ . F i g u r e 18 i s a p l o t o f In s lope vs In [N 2H ] • From F i g u r e 18 the order i n N 2 H 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 lowed de te rmina t ion o f the r a t e cons tant k from the s lopes o f F i g u r e 17. The va lues o f k so ob ta ined are g i v e n i n Table I I I . The va lue ob ta ined f o r i n i t i a l hydraz ine c o n c e n t r a t i o n = 0.522 mole/1 5.oh 60--- - 63 - 4.0 3 M No Cl 3.0 < or Z LU O o u o 2 2 .0 no NoCI 1.0 -L 3 4 TIME (hours) Figure 15: E f f e c t of 3 M NaCl on rate of p r e c i p i t a t i o n at 50°C for pH = 4.5, i n i t i a l molybdenum concentra- t i o n =5 g/1, and i n i t i a l mole r a t i o of hydrazine to molybdenum - 4:1. - 64 - 1 i i : i _ 0 2 0 4 0 6 0 TIME ( m i n u t e s ) F i g u r e 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 fo r pH = 4 . 5 , i n i t i a l molybde- num c o n c e n t r a t i o n =5 g/1," and i n i t i a l hydraz ine to molybdenum mole r a t i o - 4:1. - 65 - 0 10 20 3 0 TIM E (minutes) Figure 17: 1.5 order 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 concentration =5 g/1. - 66 - —1 1 I I I I L _ -1.8 -1.4 -1.0 -0.6 LN INITIAL HYDRAZINE C O N C E N T R A T I O N (mole/1) F i g u r e 18: Order i n N H based on 1.5 o rde r i n molybdenum. - 67 - i n i t i a l hydrazine to molybdenum mole r a t i o , -2.14 2.14 . -1 k mole 1 mm 3:1 3.35 4:1 3.55 5.5:1 3.32 7:1 3.07 Table III Rate Constants Obtained f o r D i f f e r e n t Values of I n i t i a l Hydrazine to Molybdenum Mole Ratio (10:1 i n i t i a l mole r a t i o of hydrazine to molybdenum) has not been included because the reaction was more rapid than could be followed accurately with the sampling procedure used. The temperature dependence data of Figure 13 i s p l o t t e d i n Figure 19 assuming 1.5 order i n molybdenum concentration. In each case pH = 4.5, i n i t i a l molybdenum concentration =5 g/1, and i n i t i a l mole r a t i o of hydrazine to molybdenum = 4:1. Figure 20 i s an Arrhenius p l o t based on the slopes of the l i n e s from Figure 19. The a c t i v a t i o n energy obtained i s 14.1 kcal/mole. Gravimetric analysis of the brown p r e c i p i t a t e a f t e r drying overnight i n an evacuated dessicator yielded 57.5% and 56.5% molybdenum (expected for MoO(OH)^58.9%). Ceric s u l f a t e t i t r a t i o n s assuming a l l molybdenum was V present as Mo yielded 54.0% and 52.0% molybdenum. The thermogravimetric weight loss curve obtained i n a helium atmos- phere i s shown i n Figure 21. The observed weight loss was 15.8%, compared to 16.57% expected for loss of water by MoO(OH) . - 68 - 0 2 0 4 0 6 0 8 0 100 T I M E (minutes) F i g u r e 19: E f f e c t o f temperature on r a t e assuming 1.5 o rder 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 hydraz ine to molybdenum =4:1.  I I I I J I I L_ 1 — — 0 2 0 0 4 0 0 6 0 0 8 00 TEMPERATURE (°C) F i g u r e 21: Thermogravimet r ic weight l o s s curve 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 . - 71 - P r e c i p i t a t e formed at pH = 4.5 and 50°C was found to c o n t a i n 0.42 ± 0.09 percent sodium. N e u t r a l i z a t i o n o f the r e a c t o r s l u r r y to pH = 7 w i t h NaOH a f t e r comple t ion 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 percent sodium w h i l e n e u t r a l i z a t i o n to pH = 8 r e s u l t e d i n 1 percent sodium conten 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 con ta ined 3.3 percent sodium. No c h l o r i d e i o n was found i n any o f the p r e c i p i t a t e s . F igu re 22 shows the concen t r a t i ons of molybdenum and hydraz ine remaining 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 of hydraz ine t o molybdenum o f 4 : 1 . Al though the hydraz ine concen- t r a t i o n remained cons tant a f t e r about one hour the molybdenum c o n c e n t r a t i o n cont inued to f a l l . A f t e r s t and ing ove rn igh t a t room temperature the molyb- 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 . This compares to 0.041 g/1 ob ta ined by n e u t r a l i z i n g a s tock Mo V s o l u t i o n to pH = 4 .5 a t 50°C. S i m i l a r behaviour was observed i n o ther runs . This behaviour in t roduced some u n c e r t a i n t y i n t o the computation o f the s t o i c h i o m e t r y o f the r e a c t i o n . The method f i n a l l y adopted was s imply to use the hydraz ine c o n c e n t r a t i o n remaining a t the end o f each run when the molybdenum c o n c e n t r a t i o n was reduced to l e s s than 0.4 g / 1 . Complete r e d u c t i o n o f M o ^ to Mo^ was assumed even though, as w i l l be d i s c u s s e d , VI V the molybdenum remaining i n s o l u t i o n was present as mixed v a l e n t Mo /Mo s p e c i e s . Since ve ry 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 not make much d i f f e r e n c e to 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 nine runs was 1.55 moles molybdenum reduced per mole hydraz ine consumed. The extreme va lues were 1.47 and 1.77. Th i s s t o i c h i o m e t r y suggested tha t molybdenum was a c t i n g as a I i I I : J 1 0 1 2 3 4 5 TIME (hours) Concentrations of hydrazine and molybdenum remaining i n solution vs time at 50°C with pH = 4.5, i n i t i a l molybdenum concentration =5 g/1, and i n i t i a l hydrazine to molybdenum mole r a t i o =4:1. Figure 22: - 73 - monodelect ronator . Based on the l i t e r a t u r e the expected r e a c t i o n products from monodelec t ronat ion o f hydraz ine are n i t r o g e n and ammonia. To v e r i f y the s t o i c h i o m e t r y a run was performed i n which the evolved gas was c o l l e c t e d and the f i l t r a t e analyzed fo r h y d r a z i n e , molybdenum, and ammonia. The -3 volume o f s o l u t i o n used was 200 ml atm 6.95 x 10 moles N 2 H 4 were consumed -3 genera t ing 1.616 x 10 moles o f n i t r o g e n . Only a t r a c e of ammonia was found and the p ink c o l o u r d i d not r e t u r n to the C C l ^ l a y e r i n the hydraz ine 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 not de t ec t hydrogen. In every run the end f i l t r a t e was co lou red i n d i c a t i n g the presence o f mixed v a l e n t M o V / M o V I s p e c i e s . Th i s was v e r i f i e d by polarography i n 2 N HCl and i n 5 N NaOH. In 2 N HCl the waves o f Mo V and Mo V ]" were d i s t i n c t but q u a n t i t a t i v e de t e rmina t ion o f the r a t i o o f Mo V to M o V I was not p o s s i b l e because i n 2 N HCl the excess hydraz ine p resen t reduced the remaining M o V I 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 , t h e r e f o r e , served o n l y to VI conf i rm t h a t Mo was indeed presen t i n co loured f i l t r a t e s . Polarography i n 5 N NaOH made use o f the obse rva t i on by Souchay e t a l . 5 " ' " t ha t Mo V dismutes i n t o M o V I and Mo" I'V i n such s o l u t i o n s . S ince M o V I does not g ive 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 technique showed tha t the Mo V observed on polarography i n 2 N HCl was not due s imply to the r e d u c t i o n o f M o V I by hydraz ine i n 2 N H C l . Polarography i n 5 N NaOH d i d not prove s u i t a b l e fo r q u a n t i t a t i v e de te rmina t ion o f M o V . The h e i g h t o f the Mo I V ~ wave observed was sens i t ive to time before p o l a r - ography and the s o l u t i o n s tended to drop out a grey-green p r e c i p i t a t e on s t a n d i n g . - 74 - 6. DISCUSSION The hydrogen r e d u c t i o n experiments confirmed the importance o f heterogeneous c a t a l y s i s and pH i n a t ta inment o f reasonable r a t e o f reduc- t i o n . The on ly run i n which s i g n i f i c a n t r e d u c t i o n was ob ta ined was tha t performed i n the presence o f the p l a t i num c l a d niobium 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 to 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 to t ha t ob ta ined by Lyapina and Zelikman"*"^ u s i n g m e t a l l i c molybdenum as a c a t a l y s t (F igure 7 curve d ) . Even i n t h i s run , however, complete p r e c i p i t a t i o n o f molybdenum was not ob t a ined . The f a i l u r e to o b t a i n complete r e d u c t i o n was probably due t o the inc rease o f pH to 8 and d e p o l y m e r i z a t i o n o f the molyb- denum remaining i n s o l u t i o n . Th i s depo lymer i za t i on would r e s u l t i n a 68 decrease of the r a t e o f r e d u c t i o n l i k e t ha t found by P a a l and Bu t tne r and shown i n F igu re 5 . The r e s u l t s o f the present study and those repor ted i n the l i t e r a t u r e cas t some doubt on Wagenmann's c l a i m s . I t seems probable the w a l l s o f h i s apparatus were i n v o l v e d i n the r e a c t i o n . S a t i s f a c t o r y performance o f hydrogen r e d u c t i o n would thus r e q u i r e s u f f i c i e n t a c i d i f i c a t i o n to ma in t a in p o l y m e r i z a t i o n o f M o V I s p e c i e s . In a batch process t h i s c o u l d be accomplished 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 or 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 au toc lave m a t e r i a l r e s i s t a n t to 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 igh temperature c h l o r i d e s o l u t i o n s would be r e q u i r e d . The experiments u s i n g hydraz ine as a reductan t aga in r e f l e c t the importance o f po lymer ic molybdenum s p e c i e s , and hence pH and t o t a l molybde- num c o n c e n t r a t i o n , 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 monomeric M o V I p r e d o m i n a t e s and no r e d u c t i o n i s o b s e r v e d . On d e c r e a s i n g pH r e d u c t i o n o c c u r s and t h e r a t e o f p r e c i p i t a t i o n r e a c h e s a maximum a t about pH = 4.5. T h i s can be a t t r i b u t e d t o f o r m a t i o n o f r e d u c - i b l e p o l y m o l y b d a t e s . Below 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 d e c r e a s e s because i n t h i s pH range i n t e r m e d i a t e mixed v a l e n t Mo^/Mo^ compounds a r e s t a b l e . A t pH = 4.5 t h e sequence o f c o l o u r change o b s e r v e d d u r i n g r e d u c t i o n 37 i s c o n s i s t e n t w i t h t h a t d e s c r i b e d i n t h e l i t e r a t u r e and i n t e r p r e t e d by 42 O s t r o w e t s k y . Thus on 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 r e a c t i o n p r o d u c t s a r e c o l o u r e d mixed v a l e n t Mo V I/Mo V s p e c i e s . A t pH = 4.5, however, t h e s e s p e c i e s a r e n o t s t a b l e and may be r e a c t i v e towards f u r t h e r r e d u c t i o n t o y i e l d a Mo V p r e c i p i t a t e o r t e n d t o h y d r o l y s e y i e l d i n g a Mo V p r e c i p i t a t e and M o V I i n s o l u t i o n . A t pH = 3 where mixed Mo V/Mo V I s p e c i e s 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 d e c r e a s e s . The c u r v e s o f F i g u r e s 11 and 12 i l l u s t r a t e q u a l i t a t i v e l y t h e i m p o r t - ance o f mixed v a l e n t s p e c i e s i n t h e p r e c i p i t a t i o n r e a c t i o n . The c u r v e s showing t h e w e i g h t 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 t u b e s r e p r e s e n t Mo V p r e s e n t i n mixed v a l e n t s p e c i e s . That i s , when t h e sample f i l t r a t e m i xed 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 t u b e s t h e mixed v a l e n t s p e c i e s were made u n s t a b l e by t h e i n c r e a s e o f pH and decomposed t o y i e l d Mo V p r e c i p - VI , i t a t e and Mo 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 enough t o cause some d i s p r o p o r t i o n a t i o n o f Mo V t o g i v e M o I V 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 s o l u t i o n . 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 q u a l i t a t i v e v a l u e o n l y . ) I t c an be seen i n F i g u r e s 11 and 12 t h a t t h e maximum r a t e o f p r e c i p i - t a t i o n a p p r o x i m a t e l y 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 - v a l e n t s p e c i e s . Th i s suggests t ha t p r e c i p i t a t i o n occurs through fu r the r r e d u c t i o n o f these spec ies to y i e l d a Mo V p r e c i p i t a t e . Polarography indeed confirmed t ha t the s l i g h t l y co lou red s o l u t i o n s at the end o f each V VI VI run conta ined both Mo and Mo . Complete r e d u c t i o n o f Mo i s never ob ta ined because as the t o t a l molybdenum i n s o l u t i o n reaches a low enough va lue the e q u i l i b r i u m between po lymer i c and monomeric M o V I ensures tha t 2- a 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 remaining i n s o l u t i o n ob ta ined i n -4 t h i s work was 0.053 g/1 (5.5 x 10 M) . This i s o f the same order as the 2- 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 po lymer ic molybdate spec ies a t pH = 4.5 (Figure 4 ) . The format ion o f mixed v a l e n t spec ies a l s o e x p l a i n s the o b s e r v a t i o n o f a s i g n i f i c a n t time l a g between beg inn ing o f r e d u c t i o n and beg inn ing of p r e c i p i t a t i o n fo r low i n i t i a l mole r a t i o s o f hydraz ine to 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, 15) . In the f i r s t two cases no p r e c i p i t a t i o n occurs u n t i l r e d u c t i o n has proceeded f a r enough to y i e l d an apprec iab le 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 tha t i s u l t i m a t e l y reduced to y i e l d Mo V p r e c i p i t a t e . Lower mole r a t i o s of r eac tan t s and lower temperatures de lay the a t ta inment of t h i s c o n c e n t r a t i o n . The e f f e c t o f NaCl may be due e i t h e r to s t a b i l i z a t i o n o f mixed v a l e n t spec ies o f lower degrees o f r e d u c t i o n than the spec ies t ha t i s r e d u c i b l e V to Mo , o r to 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 spec ies may a l s o be the reason f o r the behaviour shown i n F igu re 22 where r e d u c t i o n , as represented by the curve of hydraz ine c o n c e n t r a t i o n vs t ime , s tops w h i l e p r e c i p i t a t i o n V o f Mo con t inues . Presumably t h i s i s due to the slow decomposi t ion o f - 77 - VI V V VI mixed v a l e n t Mo /Mo spec ies to g ive Mo p r e c i p i t a t e and Mo i n s o l u t i o n . As has been mentioned, 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 resen 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 present as i r r e d u c i b l e 2- monomeric MoO^ so consumption o f hydraz ine through fu r the 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 of sampling procedure on the curves o f molybdenum vs time as shown i n F igu res 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 to 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 curves o f F i g u r e 9 were generated by n e u t r a l i z i n g s l u r r y samples before f i l t r a t i o n . 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 inc reased the amount o f molybdenum found i n the f i l t r a t e f o r any g iven t ime . Obv ious ly d i s p r o - p o r t i o n a t i o n occur red 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 K a t s o b a s h v i l i and the h igher the pH the g rea te r the extent o f d i s p r o p o r - t i o n a t i o n . I t i s c l e a r t ha 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 before n e u t r a l i z a t i o n gave r e s u l t s i n d i s t i n g u i s h a b l e from those ob ta ined 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 ha t the k i n e t i c r e s u l t s ob ta ined can be q u a l i t a t i v e l y VI V IV exp la ined i n terms o f the known chemis t ry o f Mo , Mo , and Mo . Q u a n t i - 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 , however, because o f the complex i ty o f the phenomena i n v o l v e d . The e m p i r i c a l r a t e law ob t a ined , for example, was based s imply on de te rmin ing t o t a l molybdenum remaining i n s o l u t i o n a f t e r f i l t r a t i o n o f s l u r r y samples. I t i s o f f r a c t i o n a l order i n both r eac tan t s and r e f l e c t s the behaviour a t one pH o n l y . The o v e r a l l process 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 probably i n v o l v e s a combinat ion o f consecu t ive and p a r a l l e l r e a c t i o n s as w e l l as the complex e q u i l i b r i a a t tendant w i t h the - 78 - involvement o f po lymer ic 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 ob ta ined o f 14.1 k c a l / m o l e represents tha 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 roces s . I t does i n d i c a t e a t l e a s t tha t chemica l p r o - cesses 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 thermogravimet r ic 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 ha t on s torage 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 o x i d i z e d 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 . The d i f f e r e n c e o f the g r a v i m e t r i c and thermogravimet r ic r e s u l t s from the expected va lues i s p robably due to v a r i a t i o n s o f the s torage and d r y i n g procedure before weighing samples f o r a n a l y s i s . S ince no c h l o r i d e i o n was found i n the p r e c i p i t a t e s analyzed i t i s concluded tha t the sodium presen t was the r e s u l t o f the i o n exchange process desc r ibed by Souchay e t a l . 5 " ' " The inc rease o f sodium content f o r p r e c i p i t a t i o n c a r r i e d out i n 3 M NaCl i s c o n s i s t e n t w i t h t h i s i n t e r p r e t a t i o n . The observed 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 i n terms o f the p r o - d u c t i o n o f n i t r o g e n and ammonia acco rd ing to the r e a c t i o n N 2 H 4 »• JjN + NH 3 + e~ (1) and the p r o d u c t i o n o f hydroxylamine acco rd ing to the r e a c t i o n 2H 2 0 + N 2 H" 4 >• 2NH2OH + 2 H + + 2e~ (2) -3 I t was found tha t c o n s u m p t i o n o f 6.95 x 10 moles o f hydraz ine generated 1.616 x 10 ^ moles o f n i t r o g e n . Assuming a l l the n i t r o g e n was generated accord ing to equat ion 1 and tha t the balance o f the hydraz ine r eac ted a c c o r d i n g to equat ion 2 y i e l d s the f o l l o w i n g r e s u l t s : - 79 - NH 3 produced = 3.232 x 10 moles -3 NH^OH produced = 7.436 x 10 moles -3 e l e c t r o n s generated = 10.668 x 10 moles The s t o i c h i o m e t r y w i t h respec t to r e d u c t i o n o f M o V I to Mo V ob ta ined i s 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 ince the volume o f s o l u t i o n used was 200 ml the r e s u l t a n t concen t r a t ions of ammonia and hydroxylamine would be 0.016 M and 0.037 M r e s p e c t i v e l y . Both o f these concen t r a t ions are sma l l enough to escape d e t e c t i o n by the a n a l y t i c a l techniques used. The s t o i c h i o m e t r y observed i n the p resen t work i s lower than tha t 79 7 7 8 2 8 3 repor ted i n the l i t e r a t u r e which v a r i e s between 2 and 4 . ' ' None of the r e s u l t s r epor ted i n the l i t e r a t u r e , however, were ob ta ined under c o n d i - t i o n s o f pH, c o n c e n t r a t i o n , and mole r a t i o o f r eac tan t s s i m i l a r to. those o f the present work. Given the v a r i a b l e nature o f the proposed hydraz ine 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 spec ies a c t i n g as ox idan 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 ob ta ined under one set o f c o n d i t i o n s to those ob ta ined under another . There does not seem to be any reason not t o suppose t ha t hydraz ine cou ld r e a c t to produce hydroxylamine i n a 2 e l e c t r o n path 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 . I t 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 products 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 present s tudy , however, the impor tant f a c t i s t ha t the economica l ly favourable 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 ob ta ined i n the c o n d i t i o n s o f i n t e r e s t . - 80 - 7. CONCLUSION I t i s apparent t ha 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 hydraz ine i s an i d e a l method for recovery o f molybdenum from h y p o c h l o r i t e l e a c h s o l u t i o n s . Both reductants 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 to be t r e a t e d so there i s no advantage to be gained over so lven t e x t r a c t i o n i n t h i s r e s p e c t . Hydrogen r e d u c t i o n can o n l y be c a r r i e d out a t a reasonable 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 s tandpoin t and an e f f i c i e n t c a t a l y s t i s r e q u i r e d . Reduct ion w i t h hydraz ine 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 r e q u i r e - ment o f an a p p r e c i a b l e excess of hydraz ine over the s t o i c h i o m e t r i c amount to o b t a i n a reasonable r a t e of p r e c i p i t a t i o n . The p r e c i p i t a t e produced by hydraz ine r e d u c t i o n w i l l c o n t a i n on the order 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 or i t w i l l contaminate the p r e c i p i t a t e . 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 decreased i n the 3 M Nadexpec t ed i n the l e a c h s o l u t i o n . The bar ren s o l u t i o n from p r e c i p i t a t i o n by hydraz ine would c o n t a i n a t l e a s t 0.05 g/1 molybdenum p lu s excess h y d r a z i n e . Recycle o f t h i s s o l u t i o n fo r^hypoch lor i t e r egene ra t ion would r e q u i r e i n v e s t i - g a t i o n o f the e f f e c t o f hydraz ine and molybdate on the e l e c t r o l y t i c process employed. Reduct ion w i t h hydraz ine 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 imple r e a c t o r . One way to perform the r e a c t i o n would be to add hydraz ine and a c i d to 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 then d i scharge the r e a c t i n g mix ture to a t h i c k e n e r s i z e d to g ive a s u i t a b l e res idence t ime . P r e c i p i t a t e c o u l d be d i scharged as a s l u r r y and the over f low 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 scha rge s l u r r y - 81 - cou ld be f i l t e r e d fo r p r e c i p i t a t e r e c o v e r y . I t i s c l e a r t ha t the cos t o f hydraz ine i n r e l a t i o n to the p r i c e of molybdenum and the m a r k e t a b i l i t y o f an MoO(OH) product contaminated w i t h sodium are the d e c i d i n g f a c t o r s i n the f e a s i b i l i t y o f u s ing hydraz ine f o r molybdenum r e c o v e r y . On the b a s i s o f t h i s study recovery o f 1 kg o f molyb- denum conta ined i n MoO(OH)3 would r e q u i r e consumption o f about 1.2 kg o f hydraz ine t o o b t a i n reasonable k i n e t i c s and completeness o f p r e c i p i t a t i o n . At the cu r r en t p r i c e fo r hydraz ine o f $3 .52/kg i n tank car l o t s i t would c o s t $4.22 fo r hydraz ine per kg o f molybdenum as MoO(OH) . Molybdenum ox ide i s c u r r e n t l y s e l l i n g f o r approximate ly $20/kg con ta ined molybdenum. 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