Preparation of Metal Powders by Sodium Reduction - Advances in

Preparation of Metal Powders by Sodium Reduction T. P. WHALEY

Downloaded by UNIV OF SYDNEY on May 5, 2015 | http://pubs.acs.org Publication Date: January 1, 1957 | doi: 10.1021/ba-1957-0019.ch014

Ethyl Corp., Baton Rouge, La.

Finely divided iron, nickel, cobalt, manganese, cadmium, zinc, tin, silver, and copper were prepared by reaction of sodium dispersions with hydrocarbon suspensions or ether solutions of their metal halides. The initiation, or threshold, temperature for the reaction differed with the various metal halides, some of the metals being reduced at room temperature or lower, and others requiring temperatures as high as 300° to 350°C. before reduction took place. The metal powders were separated from the by-product sodium chloride by leaching with deaerated water and drying the products under vacuum. Particle sizes were so small that centrifugation was necessary to separate the metal powders from the suspending liquids. Only the iron powder was pyrophoric at room temperature, the other metals requiring elevated temperatures for combustion in air. Quantitative analytical results are available for only the iron powder; all other metal powders were evaluated only qualitatively or semiquantitatively by x-ray diffraction. Evaluation showed the iron powder to have an average particle size of less than 1 micron.

L ess t h a n 20 years after the isolation of s o d i u m m e t a l b y S i r H u m p h r e y D a v y , Oersted discovered the first p r a c t i c a l use for the m e t a l — r e d u c i n g a l u m i n u m chloride to produce a l u m i n u m . I n i t i a l l y , the a l u m i n u m a n d s o d i u m industries were so completely interdependent as to be considered as one. W h e n C h a r l e s H a l l r e v o l u t i o n i z e d the a l u m i n u m i n d u s t r y w i t h his historic w o r k o n electrolysis, s o d i u m was relegated to a m i n o r role i n the field of m e t a l p r o d u c t i o n a n d large scale usage was confined to other areas. R e s e a r c h o n s o d i u m r e d u c t i o n of m e t a l halides has c o n t i n u e d t h r o u g h the years, however, a n d recent c o m m e r c i a l d e v e l o p ments attest to the success of this w o r k . M o s t of the metals a n d metalloids h a v e b e e n isolated f r o m t h e i r halides b y reaction w i t h s o d i u m . I n fact, s o d i u m r e d u c t i o n of m e t a l halides c a n be considered almost a u n i v e r s a l m e t h o d f o r p r e p a r i n g metals o r metalloids i n a n elemental state. I n m a n y cases, u n f a v o r a b l e economics prevents c o m m e r c i a l i z a t i o n of p r o c esses i n v o l v i n g r e d u c t i o n of a m e t a l h a l i d e w i t h a n active m e t a l , w h e n c o m p a r e d w i t h processes that i n v o l v e electrolysis, c a r b o n r e d u c t i o n , o r h y d r o g e n r e d u c t i o n , a n d active m e t a l r e d u c t i o n must be relegated to the role of a l a b o r a t o r y p r o c e d u r e . 129

In HANDLING AND USES OF THE ALKALI METALS; Advances in Chemistry; American Chemical Society: Washington, DC, 1957.


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A D V A N C E S IN

CHEMISTRY SERIES

W h e n the need for certain p h y s i c a l or c h e m i c a l properties of a g i v e n m a t e r i a l dictates that economics must be considered secondary to p r o d u c t quality, however, active m e t a l r e d u c t i o n of halides m a y be considered o n its c h e m i c a l merits rather t h a n on its a b i l i t y to compete w i t h i n h e r e n t l y less costly processes. S u c h has been the case w i t h t i t a n i u m , z i r c o n i u m , a n d other metals that r e q u i r e stringent control of i m p u r i t i e s for c o m m e r c i a l u t i l i t y . S i m i l a r l y , atomization of massive m e t a l , electrodeposition, c a r b o n y l decomposition, a n d h y d r o g e n r e d u c t i o n of oxides are less costly t h a n s o d i u m r e d u c t i o n of halides for the p r e p a r a t i o n of n o n r e f r a c t o r y m e t a l powders for most applications. E a c h m e t h o d has its o w n limitations, h o w ever, w h i c h m a y dictate against its use for p r e p a r i n g m e t a l powders f o r specific applications. F o r example, the l o w e r l i m i t of particle size r e p o r t e d for standard c a r b o n y l i r o n p o w d e r is about 3 m i c r o n s (3). T h i s is m o r e t h a n adequate for e l e c tronic core applications but is not sufficiently fine for p e r m a n e n t magnet f a b r i c a t i o n (4). N i c k e l p o w d e r is reported (8) to possess r e d u c e d catalytic activity w h e n p r e p a r e d above 300 ° C . H i g h p u r i t y is a prerequisite for m a n y p o w d e r m e t a l l u r g i c a l uses a n d cannot be obtained b y some of the r e d u c t i o n procedures; i n the p r e p a r a t i o n of i r o n powder, for example, the p r o d u c t r e s u l t i n g f r o m the r e d u c t i o n of ores — as contrasted w i t h scale or synthetic oxides — is too i m p u r e for f a b r i c a t i o n into magnetic coils (2). T h e research project described i n the f o l l o w i n g paragraphs was initiated i n an attempt to prepare superfine i r o n p o w d e r for permanent magnet application. R e s e a r c h i n F r a n c e b y the Société d ' E l e c t r o - C h i m i e d'Electro-Métallurgie et les Aciéries E l e c t r i q u e s d ' U g i n e h a d s h o w n that p e r m a n e n t magnets fabricated f r o m superfine i r o n powder were comparable i n q u a l i t y to A l n i c o magnets if the particle size of the i r o n p o w d e r were sufficiently s m a l l (11). Inasmuch as superfine i r o n p o w d e r c o u l d not be p r o d u c e d b y c o n v e n t i o n a l methods, it appeared that a process i n v o l v i n g s o d i u m r e d u c t i o n of f e r r i c (or ferrous) halides m i g h t have c o m m e r c i a l m e r i t if it c o u l d produce superfine particles, a n d r a w m a t e r i a l costs w e r e not prohibitive. T h e first attempts (5) to reduce m e t a l salts w i t h s o d i u m at l o w temperatures were made b y researchers w o r k i n g w i t h solutions of s o d i u m i n l i q u i d a m m o n i a . In 1925 K r a u s a n d K u r t z (7) showed that l i q u i d a m m o n i a solutions of s o d i u m c o u l d be used to reduce halides of metals that f o r m alloys w i t h s o d i u m . O p e r a t i n g at temperatures below the b o i l i n g point of the a m m o n i a solutions they succeeded i n r e d u c i n g the halides of m e r c u r y , c a d m i u m , zinc, tin, lead, a n t i m o n y , b i s m u t h , a n d t h a l l i u m , and, b y u s i n g an excess of sodium, concomitantly p r o d u c e d s o d i u m alloys of these metals. K r a u s a n d K u r t z postulated mechanisms for the reactions a n d showed that m a n y of the alloys f o r m e d were unstable i n l i q u i d a m m o n i a — i.e., they disproportionated into free s o d i u m a n d l o w e r s o d i u m alloys. T h e o n l y other k n o w n w o r k along these lines was reported b y Scott a n d W a l k e r (10). T h e reaction described t h e r e i n involves the r e d u c t i o n of m e t a l halides w i t h the h i g h l y colored a d d i t i o n compounds (9) w h i c h are f o r m e d w h e n s o d i u m m e t a l is added to p o l y n u c l e a r h y d r o c a r b o n s i n certain " a c t i v e e t h e r " m e d i a such as 1,2-dimethoxyethane a n d d i m e t h y l ether. F o r example, s o d i u m reacts w i t h a solution of naphthalene i n 1,2-dimethoxyethane to produce a d a r k green solution w h i c h contains the s o d i u m a d d i t i o n c o m p o u n d of naphthalene, h a v i n g the e m p i r i c a l f o r m u l a C i o H N a . If n i c k e l c h l o r i d e is added to the colored solution, the m e t a l is reduced to the elemental state as a finely d i v i d e d n i c k e l p o w d e r w i t h a n average particle size of less t h a n 20 microns. S i m i l a r l y , i r o n , cobalt, a n d other m e t a l powders can be p r o d u c e d b y this method. 8

2

A n analysis of the reduction procedures just described shows two important facts that are c o m m o n to b o t h : T h e s o d i u m is i n a state of m a x i m u m dispersion, a dispersion of atomic dimensions, a n d the c h e m i c a l f o r m of the s o d i u m is different f r o m the consolidated m e t a l itself. If l o w temperature reaction w i t h m e t a l halides requires that the c h e m i c a l f o r m of s o d i u m be altered, there c a n be little hope that s o d i u m m e t a l as such w i l l show the same r e a c t i v i t y ; this i m p l i e s that the free energy change for the reaction i n v o l v i n g massive s o d i u m is not favorable. T h i s is not the situation, h o w e v e r ; t h e r m o c h e m i c a l calculations (1) indicate a favorable free energy change of such m a g n i t u d e that most m e t a l halides should react s p o n taneously w i t h s o d i u m m e t a l at r o o m temperature. A s this is not the case, one


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must conclude that the basis for the l o w temperature reaction w i t h m e t a l halides must l i e i n the altered p h y s i c a l , rather t h a n c h e m i c a l , f o r m of the s o d i u m . O n e is thus l e d d i r e c t l y to the conclusion that the p r o b l e m is one of kinetics r a t h e r than t h e r m o d y n a m i c s a n d that the r e a c t i v i t y at l o w temperatures is d u e l a r g e l y to the h i g h degree of dispersion of the sodium. If this conclusion is v a l i d , a s o d i u m dispersion s h o u l d a p p r o a c h the degree of r e a c t i v i t y t o w a r d m e t a l halides that is s h o w n b y s o d i u m a d d i t i o n compounds or l i q u i d a m m o n i a solutions of s o d i u m .


Experimental PRELIMINARY. T h e i n i t i a l experiments were little more t h a n beaker tests i n a nitrogen b o x w h e r e i n a f e w grams of s o d i u m dispersion i n toluene were added to about 100 m l . of 1,2-dimethoxyethane, f o l l o w e d b y the a d d i t i o n of about 3 grams of a n h y d r o u s f e r r i c chloride. A vigorous exothermic reaction took place at r o o m temperature a n d the color changed f r o m g r a y to b l a c k . W h e n the o r d e r of a d d i t i o n was changed so that a n h y d r o u s f e r r i c chloride was added d i r e c t l y to the d i m e t h o x y ether of ethylene g l y c o l , a n orange solution r e s u l t e d ; ferric chloride was f o u n d later to be a p p r e c i a b l y soluble i n several ethers, i n c l u d i n g 1,2-dimethoxyethane. A d d i t i o n of a s o d i u m dispersion i n toluene caused a color change f r o m orange to g r a y - b l a c k a n d concomitant b u b b l i n g as the exothermic reaction took place. I s o p r o p y l alcohol was added to destroy a n y r e s i d u a l active sodium, a n d water was added to dissolve the b y - p r o d u c t salts. A b l a c k residue r e m a i n i n g i n the beaker after décantation of the aqueous solution d r i e d o n a clay plate to a b l a c k p o w d e r . A s the b l a c k p o w d e r was exposed to air, the color changed to green a n d e v e n t u a l l y to rust; this was interpreted as b e i n g the n o r m a l oxidative sequence of i r o n , Fe° -> F e -> F e . + +

+ + +

SODIUM-ETHER COMPLEX. D u r i n g the early e x p e r i m e n t a l w o r k the r e d u c i n g agent was thought to be a n active s o d i u m - e t h e r c o m p l e x (13), w h i c h appeared to f o r m w h e n dispersed s o d i u m was added to c o m m e r c i a l - g r a d e 1,2-dimethoxyethane. W h e n a s o d i u m dispersion was added to the latter m a t e r i a l , a white flocculent precipitate f o r m e d w h i c h e x h i b i t e d properties s i m i l a r to m e t a l l i c s o d i u m i n that it reacted w i t h alcohols a n d water w i t h l i b e r a t i o n of h y d r o g e n gas, yet a p p a r e n t l y reverted to s o d i u m m e t a l w h e n isolated f r o m the excess ether. T h e precipitate c o u l d not be isolated for c h e m i c a l analysis, a n d for l a c k of a better n a m e the m a t e r i a l was t e r m e d the s o d i u m - e t h e r complex. T h e m a t e r i a l appeared to reduce f e r r i c chloride r e a d i l y a n d most of the earlier runs were c a r r i e d out i n 1,2dimethoxyethane m e d i u m . F u r t h e r e x p e r i m e n t a t i o n showed that no complex f o r m e d w h e n dispersed s o d i u m was added to d i m e t h o x y e t h a n e that h a d been c a r e f u l l y d r i e d a n d p u r i f i e d b y distillation f r o m a s o d i u m - n a p h t h a l e n e a d d i t i o n c o m p o u n d , y e t f e r r i c chloride was r e d u c e d almost as r e a d i l y as w h e n the s o - c a l l e d c o m p l e x h a d been p r e f o r m e d (12). T h i s was t a k e n as evidence that the ether was acting also as a solvent f o r the f e r r i c chloride a n d that the complex must have been dependent on i m p u r i t i e s i n the c o m m e r c i a l - g r a d e ether, w h i c h either reacted w i t h s o d i u m or acted as a catalyst for c o m p l e x formation. Nevertheless, the ether was used as the m e d i u m for a l l i r o n p o w d e r runs because of its solvent action f o r f e r r i c chloride a n d the smaller particle sizes w h i c h were obtained as a result of its use. A t y p i c a l f e r r i c chloride r e d u c t i o n

FERRIC CHLORIDE REDUCTION. FeCl

3

+

3 N a -> F e +

3NaCl

was c a r r i e d out i n a 1-liter 3-necked flask e q u i p p e d w i t h a H e r s h b e r g stirrer t h r o u g h a s t i r r i n g g l a n d i n the center neck, a w a t e r - c o o l e d reflux condenser i n one side neck, a d r o p p i n g f u n n e l a n d n i t r o g e n inlet i n the other side neck. A thermometer was suspended b y a w i r e t h r o u g h the bore of the reflux condenser. A d r y ice b a t h was used to cool the reaction vessel d u r i n g some of the runs, a n d a n i t r o g e n purge m a i n t a i n e d a n inert atmosphere i n the reaction vessel. T h e e a r l y r u n s were p l a g u e d b y difficulties encountered i n both the r e d u c t i o n a n d isolation steps, p a r t i c u l a r l y the latter. S e v e r a l methods of m i x i n g the r e actants were investigated before the best m e t h o d was established; these i n c l u d e d


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ADVANCES IN CHEMISTRY SERIES


a d d i t i o n of a s o d i u m dispersion to a h y d r o c a r b o n suspension of finely d i v i d e d a n h y d r o u s f e r r i c chloride, a d d i t i o n of solid a n h y d r o u s f e r r i c chloride to a s o d i u m dispersion, addition of a s o d i u m dispersion to a 1,2-dimethoxyethane f o l l o w e d b y a d d i t i o n of solid a n h y d r o u s f e r r i c chloride, a d d i t i o n of a s o d i u m dispersion to a n ether solution of a n h y d r o u s f e r r i c chloride, a n d addition of a n ether solution of a n h y d r o u s f e r r i c c h l o r i d e to a s o d i u m dispersion. T h e latter m e t h o d w a s f o u n d to produce a smoother a n d m o r e easily controlled reaction w i t h the most c o n sistent results, a n d was followed i n most of the later f e r r i c chloride reductions. A different procedure w a s used i n the r u n s w h e r e a n elevated ( > 1 0 0 ° C . ) t e m perature was r e q u i r e d to initiate the reaction. T h e temperature requirements of the r e d u c t i o n depended l a r g e l y o n the reaction m e d i u m . W h e n a n ether solution of f e r r i c chloride w a s used, the reaction proceeded at a l o w e r temperature t h a n w h e n a h y d r o c a r b o n suspension of f e r r i c chloride w a s used. I n a s m u c h as a l o w e r reaction temperature w a s considered effective i n p r o d u c i n g s m a l l particle size, most i r o n p o w d e r r u n s were c a r r i e d out i n a n ether m e d i u m . A l t h o u g h a f e w successful reductions were m a d e i n e t h y l ether, the heat of r e a c t i o n was so great that good temperature c o n t r o l w a s difficult a n d the h i g h e r b o i l i n g 1,2-dimethoxyethane w a s used i n a l l the later r u n s . P r o b a b l y the most significant modification of the m e t h o d w a s developed i n the isolation step. I n a l l the e a r l y experiments a great deal of difficulty w a s encountered i n the attempts to separate the finely d i v i d e d i r o n f r o m the b y product s o d i u m chloride. W h e n o r d i n a r y distilled water w a s added to the reaction mass i n o r d e r to l e a c h the s o d i u m chloride, the finely d i v i d e d i r o n o x i d i z e d before it c o u l d be isolated, even w h e n the operation w a s c a r r i e d out i n a n i t r o g e n b o x . Reasoning that the oxidation m a y h a v e been d u e to dissolved o x y g e n i n the distilled water, a l l water used i n the l e a c h i n g operation w a s b o i l e d v i g o r o u s l y to dispel a n y dissolved o r t r a p p e d a i r . T h i s practice a v o i d e d e x c e s s i v e product o x i d a t i o n d u r i n g the leaching step a n d p e r m i t t e d successful sequestration. S u b sequent leaching o n a l l samples w a s c a r r i e d out w i t h deaerated water u n t i l the w a s h water gave o n l y a v e r y w e a k silver nitrate test f o r c h l o r i d e i o n . C o m p l e t e chloride r e m o v a l w a s deleterious to the isolation, however, because t h e particle size of the product was so s m a l l that a n u n b r e a k a b l e c o l l o i d a l suspension f o r m e d u p o n complete r e m o v a l of the electrolyte. O n several occasions a trace of a m m o n i u m chloride w a s added to the c o l l o i d a l i r o n suspension i n order to p e r m i t product separation b y centrifugation. TYPICAL IRON POWDER RUN. T o the reaction vessel were added 83 grams of a 4 3 % s o d i u m dispersion i n toluene a n d 2 grams of s o d i u m methoxide catalyst. T h e stirrer w a s inserted t h r o u g h the s t i r r i n g g l a n d a n d a solution of 75 grams of a n h y d r o u s ferric chloride i n 1,2-dimethoxyethane was a d d e d to the d r o p p i n g f u n n e l . T h e ethereal solution of f e r r i c c h l o r i d e was a d d e d dropwise into the agitated s o d i u m dispersion at a rate w h i c h p e r m i t t e d a 20° to 30 ° C . temperature control. A f t e r a l l of the ferric chloride solution h a d been added, the reaction was p e r m i t t e d to continue for about 1 hour, at the e n d of w h i c h time the reaction was complete. T o the b l a c k reaction mass was added sufficient m e t h a n o l to react w i t h the excess sodium, a n d then distilled water w h i c h h a d been degassed b y b o i l i n g . T h e product was a b l a c k colloidal suspension w h i c h c o u l d be b r o k e n b y continued centrifugation into a b l a c k solid a n d clear supernatant l i q u i d . T h e supernatant l i q u i d was decanted i n a nitrogen b o x a n d the product again m i x e d w i t h degassed water. T h e aqueous w a s h i n g - c e n t r i f u g a t i o n cycle was continued u n t i l a l l soluble impurities were r e m o v e d ; the final t w o washes were made w i t h a n h y d r o u s m e t h a n o l a n d e t h y l ether, respectively. V a c u u m d r y i n g p r o d u c e d a quantitative y i e l d of a finely d i v i d e d b l a c k p o w d e r w h i c h was p y r o p h o r i c i n a i r at r o o m temperature. PRODUCT EVALUATION. Iron powders p r e p a r e d b y the described p r o c e d u r e were evaluated b y S - K - C Research Associates, Paterson, N . J . , f o r particle size, p u r i t y , a n d magnetic properties. T h e particle size was d e t e r m i n e d v i s u a l l y u n d e r a m i c r o scope. P u r i t y was d e t e r m i n e d b y weight loss after heating i n h y d r o g e n at 400°, 650°, a n d 1120°C. to remove, i n order, volatile l i q u i d s , o x y g e n f r o m i r o n oxide, a n d s o d i u m chloride. M a g n e t i c properties were d e t e r m i n e d b y d r y - m i x i n g the sample w i t h a zinc stéarate b i n d e r , pressing into cores, a n d testing b y comparison


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W H A L E Y — M E T A L POWDERS BY SODIUM REDUCTION

w i t h a standard i r o n p o w d e r used i n v e r y h i g h f r e q u e n c y electronic core a p p l i cations w h e r e a h i g h Q v a l u e is r e q u i r e d . Results of a t y p i c a l e v a l u a t i o n are as follows: P a r t i c l e size.

A v e r a g e 0.50 to 0.75 m i c r o n .

A p p a r e n t density. 0.47 g r a m - p e r - c c . f o r a i r - d r i e d p o w d e r . 0.82 g r a m - p e r - c c . for h y d r o g e n - d r i e d p o w d e r at 650 ° C . C u m u l a t i v e weight loss i n h y d r o g e n . 16.1% at 4 0 0 ° C . 44.8% at 6 5 0 ° C . 49.3% 1120°C. M a g n e t i c properties. S a m p l e (after h y d r o g e n d r y i n g at 650°C.) showed 70 to 8 0 % of Q v a l u e a n d 50 to 6 5 % p e r m e a b i l i t y , at 25 to 30 megacycles, of reference standard carbonyl powder. A s i n d i c a t e d b y the weight losses i n h y d r o g e n , the samples w e r e i m p u r e ; nevertheless, the evaluation data were sufficient to indicate that the p a r t i c l e size was s m a l l a n d that the magnetic properties w e r e r e m a r k a b l e i n v i e w of the l o w p u r i t y of samples. E x t r a p o l a t i o n of the data d e r i v e d f r o m the i m p u r e samples predicts a Q v a l u e f o r the p u r e m a t e r i a l of 110 to 130% of the v a l u e s h o w n b y the reference s t a n d a r d c a r b o n y l powder.


at

REDUCTION OF OTHER METAL HALIDES. T h e r e d u c t i o n of several other m e t a l h a l ides w i t h s o d i u m dispersions p r o v e d v e r y successful, a l t h o u g h h i g h e r t e m p e r a tures were r e q u i r e d f o r the r e d u c t i o n of some of the metals. I n most instances, r e duction d i d not occur u n t i l a specific " t h r e s h o l d , " o r " t r i g g e r , " temperature was r e a c h e d ; n o attempt was made to determine w h e t h e r this p h e n o m e n o n was due to a potential b a r r i e r w h i c h r e q u i r e d a h i g h activation energy to overcome, o r to some other t h e r m o d y n a m i c a n d / o r k i n e t i c properties of the system. Attempts to correlate free energy data w i t h t h r e s h o l d temperatures w e r e unsuccessful. T h e necessity f o r h i g h e r temperatures to reduce some of the metals bears out the contention that the r e d u c t i o n of m e t a l halides w i t h s o d i u m is m o r e dependent u p o n the kinetic aspects of the reaction t h a n o n t h e r m o d y n a m i c s . A s is pointed out b y K r o l l ( 6 ) : " T h e most i m p o r t a n t fact, left out e n t i r e l y b y the t h e r m o chemists, w h i c h is, however, essential to the metallurgist, is the 'ignition' t e m perature o r the k n o w l e d g e of rates f o r a g i v e n t e m p e r a t u r e . " T h e i n i t i a l attempts to reduce other m e t a l halides w i t h s o d i u m were m a d e i n beaker tests a n d were c r u d e ; nevertheless, the results of these tests p e r m i t t e d a p r e l i m i n a r y e v a l u a t i o n of the s o d i u m dispersion m e t h o d as it applies to l o w temperature r e d u c t i o n of m e t a l halides other t h a n i r o n . T w o systems w e r e i n vestigated: a n h y d r o u s m e t a l h a l i d e a d d e d to a s o d i u m dispersion i n toluene at r o o m temperature a n d at 80 ° C , a n d s o d i u m dispersion a d d e d to a large excess of 1,2-dimethoxyethane (ether c o m p l e x ) f o l l o w e d b y a d d i t i o n of the a n h y d r o u s m e t a l h a l i d e at r o o m temperature. Results of these b e a k e r tests w e r e as follows (12): Metal Halide SnI* HgCl AgBr Hgl2 CuCb NiCb MnBr2 T1C1 Agi CdBr ZnCh CrCls PbCla 2

2

Sodium Dispersion 80 ° C . 20° C . Not reduced Reduced Not reduced Reduced Not reduced Reduced Not reduced Reduced Not reduced Reduced Not reduced Not reduced Not reduced Not reduced Not reduced Not reduced Not reduced Reduced Not reduced Reduced Not reduced Reduced* Not reduced Not reduced Not reduced Not reduced 51

a

S o d i u m - E t h e r , 20' Reduced Reduced Reduced Reduced Reduced* N o t reduced Not reduced Not reduced Reduced Reduced* Reduced* Not reduced Not reduced

* P a r t i a l reaction o c c u r r e d u n d e r these conditions, b u t subsequent experimentation showed that m u c h higher temperatures were r e q u i r e d f o r complete reduction.

These tests w e r e crude a n d d i d not establish positively w h i c h m e t a l halides were r e a d i l y r e d u c i b l e at lower temperatures, b u t they d i d give a n i n d i c a t i o n of the metals that showed promise i n this direction. NICKEL. Its c h e m i c a l s i m i l a r i t y to i r o n , together w i t h its potential usefulness i n finely d i v i d e d f o r m , dictated that n i c k e l b e investigated as a p r o d u c t to b e p r e p a r e d b y a s o d i u m route. A f t e r several unsuccessful attempts to p r e p a r e n i c k e l


ADVANCES IN CHEMISTRY SERIES

134

p o w d e r b y s o d i u m reduction of n i c k e l (II) chloride, it appeared that neither the s o d i u m - e t h e r system n o r a s o d i u m dispersion was capable of effecting the r e d u c tion u n d e r the conditions used. H o w e v e r , because t h e r m o d y n a m i c calculations predicted that the reaction s h o u l d proceed spontaneously, a search was m a d e for the reaction conditions w h i c h were most favorable. T w o variables w e r e selected as b e i n g most w o r t h y of investigation: Increased agitation to p r o v i d e m a x i m u m surface area a n d reactant contact, and increased temperatures.


A m i x t u r e of 12 grams of a n h y d r o u s n i c k e l (II) chloride, 5 grams of dispersed s o d i u m a n d about 100 m l . of 1,2-dimethoxyethane was charged to a creased stirring vessel a n d agitated w i t h a P r e m i e r Dispersator. A s the h i g h - s p e e d s t i r r i n g c o n tinued, the temperature rose to about 72 °C. b u t no visible sign of reaction was n o t e d ; t h e temperature rise w a s p r e s u m a b l y d u e to f r i c t i o n a l heat developed d u r i n g agitation r a t h e r t h a n to a n y heat of reaction. N o n i c k e l p o w d e r was isolated f r o m the reaction mass. A second r u n was made w i t h a s o d i u m dispersion i n C a r n a t i o n o i l (a h i g h b o i l i n g white o i l f r o m L . S o n n e b o r n S o n s ) , 5 grams of dispersed s o d i u m i n the h y d r o c a r b o n , a n d 12 grams of a n h y d r o u s n i c k e l (I) chloride charged to a 1-liter t h r e e - n e c k e d flask. T o this m i x t u r e was a d d e d 1 g r a m of s o d i u m methylate as catalyst. T h e m i x t u r e was heated to 240 ° C . w i t h vigorous agitation b y means of a P r e m i e r Dispersator a n d these conditions were m a i n t a i n e d f o r 1 h o u r . D u r i n g this time the color of the m i x t u r e changed to b r o w n but there was n o other visible sign of reaction. T h e excess s o d i u m was k i l l e d w i t h m e t h a n o l a n d the reaction mass washed several times w i t h water to r e m o v e a n y s o d i u m chloride, s o d i u m methylate, a n d unreacted n i c k e l chloride. A f t e r the aqueous washes, the product was washed successively w i t h alcohol a n d ether, d r i e d , a n d submitted for analysis b y x - r a y diffraction. T h e x - r a y diffraction showed o n l y a diffuse p a t t e r n w i t h h i g h b a c k g r o u n d a n d the pattern appeared to be entirely amorphous. A t h i r d r u n was made, using 12 grams of a n h y d r o u s n i c k e l (II) chloride a n d 8 grams of massive sodium. T h i s m i x t u r e w a s added to a 300-ml. t h r e e - n e c k e d flask containing 100 m l . of C a r n a t i o n o i l . T h e m i x t u r e was heated to the m e l t i n g point of s o d i u m a n d t h e n agitated v i g o r o u s l y w i t h the P r e m i e r Dispersator as the heating continued. B y the time the temperature reached 210°, the m i x t u r e was a definite g r a y color a n d between 250° a n d 290 ° C . the reaction m i x t u r e t u r n e d black, s i m i l a r to the color of the i r o n p o w d e r runs. T h e m i x t u r e was p e r m i t t e d to cool, m e t h a n o l was added to k i l l the excess sodium, a n d the p r o d u c t was washed several times w i t h water to r e m o v e s o d i u m chloride, s o d i u m methoxide, a n d a n y unreacted n i c k e l (II) chloride. T h e product was t h e n washed successively w i t h alcohol a n d ether, d r i e d u n d e r v a c u u m , a n d submitted for analysis b y x - r a y diffraction. T h i s time x - r a y diffraction showed a v e r y h i g h percentage of n i c k e l powder, estimated c o n s e r v a t i v e l y to be over 75 to 8 0 % . T h e sample was not p y r o p h o r i c at r o o m temperature, but b u r n e d at elevated temperatures. It thus appeared that other m e t a l powders, w h i c h c o u l d not be p r e p a r e d b y r e d u c t i o n w i t h s o d i u m dispersions at l o w e r temperatures, m i g h t w e l l be p r e p a r e d b y r e d u c t i o n w i t h s o d i u m dispersions at elevated temperatures. T h i s , of course, p r e c l u d e d the use of 1,2-dimethoxyethane as a reaction m e d i u m i n reactions that r e q u i r e d temperatures i n excess of 8 4 ° C , its b o i l i n g point. COBALT. T h e last m e m b e r of the ferrous m e t a l group (iron, n i c k e l , a n d cobalt) b e h a v e d s i m i l a r l y to n i c k e l i n that a h i g h e r temperature was r e q u i r e d f o r the r e action of s o d i u m dispersion w i t h a n h y d r o u s cobalt (II) chloride, CoCL +

2 N a -> C o +

2NaCl

the most interesting feature of the reaction b e i n g the v e r y sharp " t r i g g e r " t e m perature at w h i c h the reaction s u d d e n l y takes place. T w e l v e grams of a n h y d r o u s cobalt (II) chloride a n d 7 grams of s o d i u m were added to a p p r o x i m a t e l y 60 m l . of C a r n a t i o n o i l i n a 300-ml. t h r e e - n e c k e d creased flask a n d heated u n d e r nitrogen without agitation to about 100 °C. A t this point the s o d i u m was m o l t e n a n d the fluidized reactants w e r e agitated as the temperature increased. N o reaction was apparent u n t i l a temperature of 3 2 5 ° C . was r e a c h e d ; at this point, a l t h o u g h the heating m a n t l e w a s disconnected, the temperature began i n c r e a s i n g at a rate of about 10 ° C . p e r m i n u t e a n d the contents of the flask t u r n e d b l a c k . T h e h i g h l y


WHALEY—METAL POWDERS BY SODIUM REDUCTION

135

exothermic reaction continued u n t i l a temperature of a p p r o x i m a t e l y 375°C. was reached a n d most of the reaction m e d i u m h a d v a p o r i z e d . T h e contents of the flask w e r e cooled a n d treated w i t h m e t h a n o l to k i l l the excess sodium, after w h i c h the soluble s o d i u m salts w e r e leached w i t h b o i l e d water a n d the m e t a l p o w d e r was dried under vacuum. A n a l y s i s b y x - r a y diffraction showed the strongest reflections for t h e h e x agonal c l o s e - p a c k e d cobalt m e t a l w i t h no other components present. T h e cobalt p o w d e r was not p y r o p h o r i c at r o o m temperature, b u t b u r n e d w i t h a n orange flame w h e n p l a c e d o n a hot plate. LEAD.

A definite threshold temperature for the r e d u c t i o n of lead (II) chloride


PbCl

2

+

2 N a -> P b +

2NaCl

was discovered d u r i n g the course of two l e a d p o w d e r r u n s . T h e first r u n consisted of h e a t i n g 13.5 grams o f l e a d chloride a n d 2.6 grams of s o d i u m i n 50 m l . of m i n e r a l oil without agitation u n t i l the s o d i u m melted, then heating u n d e r n i t r o g e n w i t h h i g h speed agitation u n t i l reaction o c c u r r e d . N o visible signs of reaction were evident u n t i l the temperature reached 220 ° C , at w h i c h point a r a p i d exothermic reaction suddenly caused a temperature j u m p to 275°C. w i t h a concomitant change an color of the reaction mass f r o m light g r a y to black. T h e contents of the flask w e r e treated w i t h m e t h a n o l to k i l l a n y excess sodium. T h e second r u n consisted of a d d i n g 3.5 grams of sodium, 12.5 grams of lead (II) chloride, a n d 0.2 g r a m of oleic acid to 60 m l . of m i n e r a l o i l , heating u n d e r nitrogen u n t i l the s o d i u m melted, then heating w i t h h i g h - s p e e d agitation u n d e r n i t r o g e n u n t i l reaction o c c u r r e d . T h e reaction m i x t u r e began to change color — f r o m gray to b l a c k — at about 150 ° C . a n d the heat of reaction caused the temperature to increase u n t i l a temperature of 275 °C. was reached. A t this temperature the reaction mass was b l a c k a n d no further reaction appeared to take place. A f t e r 15 minutes at 275 ° C , the contents of the flask w e r e p e r m i t t e d to cool to r o o m temperature a n d then treated w i t h m e t h a n o l to k i l l a n y excess s o d i u m . A q u e o u s l e a c h i n g f o l l o w e d b y acetone w a s h i n g a n d v a c u u m d r y i n g p r o d u c e d a p o w d e r w h i c h was f o u n d b y x - r a y diffraction to c o n t a i n about 5 0 % l e a d m e t a l powder, a large percentage of both r e d a n d y e l l o w l e a d oxide, a n d lesser amounts of l e a d oxides, chlorides, a n d oxychlorides. T h e particle size was estimated to be greater t h a n 1000 A . MANGANESE. T h r e e r u n s o n the r e d u c t i o n of manganese (II) chloride w i t h d i s persed s o d i u m ( M n C l + 2 N a — » M n + 2 N a C l ) w e r e sufficient to indicate that the r e d u c t i o n proceeds satisfactorily b u t that sequestering is difficult i n v i e w of the r e activity of finely d i v i d e d manganese w i t h water. A n h y d r o u s manganese (II) c h l o r ide (24.8 grams) was m i x e d w i t h m i n e r a l o i l a n d g r o u n d i n a m o r t a r to a fine paste. T h e paste was then a d d e d to a 300-ml. creased t h r e e - n e c k e d flask, covered w i t h m i n e r a l o i l , a n d m i x e d w i t h 8.2 grams of s o d i u m (10% less t h a n the stoichiometric a m o u n t ) . T h e m i x t u r e was heated without agitation u n t i l the s o d i u m melted, at w h i c h point the fluid system was agitated v i g o r o u s l y w i t h a c r u c i f o r m stirrer. A reaction appeared to b e g i n at about 250 ° C . a n d a p p a r e n t l y was complete at about 325 ° C ; f u r t h e r heating to 340 ° C . p r o d u c e d no visible effects. A second r u n w i t h 27.5 grams of a n h y d r o u s manganese (II) chloride a n d 8.6 grams of s o d i u m proceeded i n a n identical m a n n e r , except that the reaction started at a l o w e r t e m perature. A t h i r d r u n , made u n d e r s i m i l a r conditions w i t h 12.5 grams of m a n g a nese (II) chloride a n d 5 grams of sodium, gave no i n d i c a t i o n of reaction u n t i l a temperature of about 295 ° C . was reached. T h e reaction was complete at 323 ° C . 2

M a n g a n e s e p o w d e r was n e v e r isolated f r o m these r u n s because of the r e activity of the r e d u c t i o n product w i t h m e t h a n o l a n d / o r w a t e r ; consequently, data on p u r i t y a n d y i e l d are not available for this system. CADMIUM. T h e p r e l i m i n a r y e x p l o r a t o r y w o r k o n the r e d u c t i o n of c a d m i u m halides ( C d B r + 2 N a -> C d + 2 N a B r ) indicated that some r e d u c t i o n apparently o c c u r r e d at 20 ° C . w h e n the s o d i u m - e t h e r system was used, b u t that n o r e d u c t i o n took place w h e n a s o d i u m dispersion was used, even at 80 ° C . I n order to gain f u r t h e r i n f o r m a t i o n o n this system, a m i x t u r e of 16 grams of a n h y d r o u s c a d m i u m b r o m i d e , 3 grams of sodium, a n d 0.5 g r a m of potassium was a d d e d to 125 m l . of 2


136

A D V A N C E S IN CHEMISTRY SERIES

C a r n a t i o n o i l i n a 300-ml. t h r e e - n e c k e d flask a n d r a p i d l y agitated at 110°C. u n t i l the s o d i u m a n d potassium appeared to be finely d i v i d e d . T h e heating a n d agitation w e r e c o n t i n u e d a n d at 140 ° C . the m i x t u r e changed i n color f r o m g r a y to b l a c k . A t 3 0 0 ° C , a temperature j u m p of 2 2 ° C . took place a n d the temperature was m a i n tained at 300° to 340°C. f o r 20 minutes. T h i s temperature j u m p was s i m i l a r to the sudden temperature rises noted i n the r e d u c t i o n of other m e t a l halides. T h e excess s o d i u m was k i l l e d w i t h m e t h a n o l a n d c r y s t a l l i n e c a d m i u m was a p p a r e n t l y isolated. N o quantitative analysis of the product was made, b u t x - r a y diffraction confirmed the presence of c a d m i u m m e t a l .


Discussion T h e r e d u c t i o n of m e t a l halides w i t h s o d i u m m e t a l has b e e n p r a c t i c e d for m a n y years, b u t s u c h reductions were c a r r i e d out at r e l a t i v e l y h i g h temperatures, despite the fact that t h e r m o d y n a m i c considerations p r e d i c t a spontaneous reaction at r o o m temperature for most of the s o d i u m - m e t a l h a l i d e systems. T h i s w o u l d seem to indicate that a compromise between t h e r m o d y n a m i c a n d k i n e t i c aspects of the p r o b l e m must be met — i.e., the v e l o c i t y of the reaction must be increased w i t h out r e v e r s i n g the d i r e c t i o n of the free e n e r g y change a n d p r e f e r a b l y w i t h o u t a p p r e c i a b l y r e d u c i n g the m a g n i t u d e of the free energy change. T h e s o d i u m d i s persion m e t h o d f o r r e d u c i n g m e t a l halides was investigated a n d subsequently d e v e l o p e d as a means f o r i n c r e a s i n g reaction v e l o c i t y b y i n c r e a s i n g the contact surface area of the reactants. Ideally, the r e d u c t i o n w o u l d be conducted i n a homogeneous nonaqueous system w i t h b o t h s o d i u m a n d the m e t a l h a l i d e i n s o l u t i o n ; u n f o r t u n a t e l y , the n u m b e r of nonaqueous solvents for s o d i u m m e t a l or for i n d i v i d u a l m e t a l halides is e x t r e m e l y s m a l l a n d perhaps o n l y one solvent for both s o d i u m m e t a l a n d c e r t a i n specific m e t a l halides is k n o w n . T h i s solvent is l i q u i d a m m o n i a , a n d the use of s o d i u m - i n - l i q u i d - a m m o n i a as a r e d u c i n g system f o r certain m e t a l halides is w e l l k n o w n . T h e p r i n c i p a l d r a w b a c k is the i n a b i l i t y to go to h i g h e r temperatures, i f necessary, because of the l o w b o i l i n g point of the solvent. T h e i n i t i a l tests of the s o d i u m d i s p e r s i o n - m e t a l h a l i d e system w e r e m a d e w i t h f e r r i c chloride, fortunately, a n d w i t h a nonaqueous solvent i n w h i c h f e r r i c chloride was soluble. T h u s , f e r r i c c h l o r i d e was present as a solution a n d , conse quently, presented m a x i m u m surface for contact w i t h the s o d i u m particles. T h i s feature, c o u p l e d w i t h the l o w e r activation energy requirements, p e r m i t t e d the reaction to proceed at temperatures w e l l b e l o w r o o m temperature a n d established the o p e r a b i l i t y of the m e t h o d . T h e success of the i n i t i a l (ferric c h l o r i d e ) tests lent encouragement to tests o n other m e t a l systems a n d p r o m p t e d c o n t i n u e d investigations w h e n the i n i t i a l r u n s at l o w e r temperatures f a i l e d . T h e d i s c o v e r y of the threshold, o r trigger, temperature for n i c k e l (II) c h l o r i d e r e d u c t i o n p a v e d the w a y f o r successful r e d u c t i o n of other m e t a l halides s u c h as manganese (II) chloride, cobalt (II) c h l o r i d e , a n d c a d m i u m b r o m i d e . T h e resistance to r e d u c t i o n of some of the m e t a l halides [ v a n a d i u m (II) c h l o r ide, z i r c o n i u m ( I V ) c h l o r i d e , a n d t i t a n i u m ( I V ) chloride] p r o b a b l y indicates that h i g h e r t h r e s h o l d temperatures exist f o r these compounds t h a n was possible w i t h the dispersing m e d i a a v a i l a b l e f o r studies m a d e d u r i n g this p r o g r a m . T h e r m o d y n a m i c s favors the s o d i u m r e d u c t i o n of v a n a d i u m (II) c h l o r i d e , z i r c o n i u m ( I V ) chloride, o r t i t a n i u m ( I V ) c h l o r i d e i n the 298° to 6 0 0 ° C . range as m u c h , i f not more, t h a n other m e t a l halides successfully r e d u c e d d u r i n g this p r o g r a m ; i t w o u l d seem, therefore, that m o r e e n e r g y is r e q u i r e d f o r these reactions to p r o c e e d at a n acceptable rate a n d that h i g h e r temperatures w i l l p r o v i d e the necessary activation energy. T w o other factors m a y b e a r o n the situation: T h e surface area of the s o d i u m i n s o d i u m dispersions m a y n o t be great e n o u g h to c i r c u m v e n t t h e n e e d f o r h i g h e r t h r e s h o l d temperatures, a n d the h y d r o c a r b o n m e d i a m a y exert a d a m p e n i n g influence b y r e d u c i n g the i n t i m a c y of the contact b e t w e e n s o d i u m a n d the m e t a l halide.

Literature Cited (1) (2)

Glassner, Α . , A r g o n n e N a t i o n a l L a b o r a t o r y , R e p t . 5107, 5-21 (1953). G o e t z e l , C . G. " T r e a t i s e o n P o w d e r M e t a l l u r g y , " v o l . I, p p . 167-72, Interscience, N e w Y o r k , 1949.


W H A L E Y — M E T A L POWDERS BY SODIUM REDUCTION (3) (4) (5) (6) (7) (8)

Ibid., p. 182. Ibid., v o l . II, p p . 245-7, 267-9, 1949. J o a n n i s , Α . , Compt. rend. 113, 795 (1891). K r o l l , W . J., Metal Ind. (London) 41, 243 (Sept. 26, 1952). K r a u s , C . Α . , K u r t z , H . F . , J. Am. Chem. Soc. 47, 43-60 (1925). M e l l o r , " C o m p r e h e n s i v e Treatise o n Inorganic a n d T h e o r e t i c a l C h e m i s t r y , " v o l . XV, p. 47, L o n g m a n s , L o n d o n , 1923. Scott, N. D . (to Ε. I. d u P o n t de N e m o u r s & C o . ) , U . S . Patent 2,027,000 ( J a n . 7, 1936). Scott, N. D . , W a l k e r , J. F., Ibid., 2,177,412 (Oct. 24, 1939). W e i l , L., Proc. Intern. Conference Powder Metallurgy, G r a z , J u l y 12-17, 1948. W h a l e y , T . P . (to E t h y l C o r p . ) , U . S . Patent 2,687,951 ( A u g . 31, 1954). Ibid., 2,716,057 ( A u g . 23, 1955).


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