handling and uses of the alkali metals - American Chemical Society

Present and Potential Uses of Sodium in Metallurgy W. J . KROLL P. O. Box 808

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: January 1, 1957 | doi: 10.1021/ba-1957-0019.ch015

Corvallis, Oregon

The high intermetallic affinity of sodium for certain elements such as sulfur, arsenic, antimony, and bismuth suggests its use as a cleanser for raw metals. Its high halogen affinity recommends it as a reducing agent for producing pure metals and alloys. Alloy equilibria between sodium and fused halides in the presence of a metallic solvent are described. Sodium alloys with other alkali and alkaline earth metals are obtained with sodium and the corresponding fused halides. Reactions of calcium carbide with sodium chloride or fluoride liberate sodium. In the presence of a m e t a l s o l v e n t , a l l o y s a r e formed. The reduction of titanium tetrachloride and zirconium tetrachloride with sodium is discussed and a comparison is made with magnesium reduction.

S o D I U M has, u p to n o w , been rather neglected b y the metallurgist despite the fact that it is a cheap r e d u c i n g a n d cleansing agent. A certain prejudice of u n safety has developed about this metal, w h i c h reacts b r i s k l y w i t h water a n d b u r n s spectacularly, w h e n i g n i t e d i n air. Those w h o have w o r k e d w i t h it find it pleasant to handle, especially because of its easy f u s i b i l i t y (melting point 97.8° C ) , w h i c h permits p i p i n g it to the location w h e r e it is to be used. T o f o r m a n idea whether, or where, to e m p l o y s o d i u m i n m e t a l l u r g y it is necessary to look over the p h y s i c a l a n d c h e m i c a l properties of the element a n d of its compounds a n d alloys, a n d to compare t h e m w i t h those of possible equivalents. F o r this purpose T a b l e s I, II, a n d III h a v e been p r e p a r e d . F u r t h e r m o r e , price should be considered. T a b l e I shows the m e l t i n g points, b o i l i n g points, a n d v a p o r pressure of s o d i u m a n d some of its more c o m m o n compounds (68, 79). T h e heats of f o r m a t i o n of intermetallic compounds of s o d i u m w i t h various other elements a n d those of some other metals are g i v e n i n T a b l e II. T a b l e III c a n be used to compare the heats of f o r m a t i o n of other inorganic compounds, a n d to see whether s o d i u m c o u l d be used i n specific cases, either as a r e d u c i n g agent or as a cleanser. T h e v a p o r pressure of s o d i u m m e t a l is h i g h , c o m p a r e d w i t h that of m a g n e s i u m or c a l c i u m , w h i c h m a y be a n advantage or, more often, a disadvantage. It c a n easily be purified b y v a c u u m distillation b e l o w 500° C , at w h i c h t e m p e r a ture its v a p o r pressure is sufficient for a r a p i d evaporation o n a n i n d u s t r i a l scale. In reductions, w h e r e m u c h heat m a y be evolved, sodium's l o w b o i l i n g point of 880° C . m a y be troublesome. T h e w e a k o x y g e n affinity of s o d i u m c o m p a r e d w i t h that of m a g n e s i u m or c a l c i u m indicates that it has b u t f e w chances of b e i n g used 138

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

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Table I. Physical Properties of Sodium and Sodium Compounds Designation

M e l t i n g Point, °C.

Na

B o i l i n g Point, °C.

97.8

882.9

NaCl

798.0

1412.0

NaF Na S NasO Na 0

988.0 980.0 920.0 675.0

1600.0 1600.0

2

2

2

1705.0

V a p o r Pressure, Mm. Hg °C. 0.1 1.0 2.4 10.0 87.0

356 437 747 950 1426


Table II. Heats of Formation of Intermetallic Compounds Compared with Sodium Intermetallics (Kilocalaries per Mole) Sodium NaHg NaPb NaBi NaSb NaSn Na S 2

Calcium

20.0 11.6 15.5 16.0 12.0 88.5

Magnesium

CaPb 11.6 C a B i 126.0 CasSb 174.0 CaSn 12.0 CaS 111.3 3

Mg Pb MgsBi Mg Sb Mg Sn MgS 2

2

2

2

3

2

2

12.6 16.0 30.5 12.2 84.2

TABLE III. Heats of Formation of Various Inorganic Compounds Compared with Sodium (Kilocalories per Mole) Sodium NaF NaCl Na 0 Na S

136.0 98.3 99.5 88.5

2

2

Cr Q 2

2

Potassium K F 134.5 K C 1 104.4 K 0 86.0 K S 95.0 2

Lithium LiF LiCl Li 0 2

145.6 97.7 142.0

2

Calcium CaF CaCl CaO CaS 2

2

290.2 190.6 151.7 114.3

Magnesium MgF MgCl MgO MgS 2

2

263.0 153.2 146.1 84.2

Beryllium BeF BeCl BeO BeS

227.0 112.6 147.3 56.1

2

2

268.0; F e O 64.4; AhOs 399.0; A 1 F 323.0; F e S 22.8; M n S 44.4; T i C U 181.4; Z r C k 230.0 3

for oxide reductions or deoxidations. T h e l o w o x y g e n affinity appears f r o m the fact that s o d i u m h y d r i d e , a w e a k c o m p o u n d of s o d i u m used for descaling i r o n , does not reduce c h r o m i u m oxide (1). H o w e v e r , s o d i u m reduces i r o n oxide b e l o w 1200° C . at atmospheric pressure. B u n t z e l (10) states that i r o n reduces s o d i u m oxide at 550 ° C , tungsten at 550 ° C , m o l y b d e n u m at 600 ° C , n i c k e l a n d cobalt above 1000 ° C . at ambient pressure. S o d i u m c a n be used to deoxidize the surface of i r o n before a p p l y i n g a coating of some other metal. O r the s o d i u m c a n be e m b o d i e d i n the m e t a l to be plated o n — f o r instance, i n the f o r m of a l o w - s o d i u m lead, zinc, or t i n a l l o y (21, 24). T h e weak o x y g e n affinity was w e l c o m e i n the e a r l y days w h e n s o d i u m was r e d u c e d f r o m its oxide compounds such as N a O H 0 ( h y d r o x i d e ) or N a O C C > 2 (carbonate) w i t h i r o n or c a r b o n (26). 2

2

2

T h e outstanding affinity of s o d i u m for halogens opens b r o a d perspectives for the metal. Its affinity for chlorine a n d fluorine is almost e q u a l to that of c a l c i u m for these elements, a n d higher t h a n that of m a g n e s i u m a n d silicon. H o w ever, s o d i u m c h l o r i d e c a n be r e d u c e d i n d i r e c t l y b y silicon i n the presence of c a l c i u m oxide (49). T h i s latter liberates some s o d i u m oxide w i t h the f o r m a t i o n of c a l c i u m c h l o r i d e a n d the f o r m e r is r e d u c e d easily b y silicon w i t h p r o d u c t i o n of c a l c i u m silicate (2 C a O S i O > ) a n d free s o d i u m . T h e reaction takes place at about 800° C . i n vacuo a n d this process was used for a time on a n i n d u s t r i a l scale i n the U n i t e d States. T h e s o d i u m affinity of certain metals a n d metalloids such as lead, b i s m u t h , a n t i m o n y , arsenic, a n d s u l f u r is considerable, but it u s u a l l y does not exceed that of m a g n e s i u m or c a l c i u m for the same elements. T h e h i g h v o l a t i l i t y of s o d i u m m a y be the cause of reaction e q u i l i b r i a , w h i c h c a n be d r i v e n one w a y or the other, d e p e n d i n g on the e x t e r n a l pressure. T h u s s o d i u m can be p r o d u c e d b y r e d u c i n g the sulfide (10) i n vacuo w i t h i r o n at 1250° C , b u t b e l o w this temperature i r o n sulfide is desulfurized b y s o d i u m . T h e desulfurization of fused cast i r o n w i t h soda ash is essentially based on this reaction. S t e e l is not d e s u l f u r i z e d w i t h s o d i u m . A s s o d i u m does not f o r m a n y stable n i t r i d e , w h i c h is an advantage over c a l c i u m , this gas is used as a protective atmosphere for the metal. T h i s inertness also eliminates the r i s k of i n t r o d u c i n g nitrogen as an i m p u r i t y i n the r e d u c e d m e t a l .


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S o d i u m is insoluble i n m o l t e n copper, m a g n e s i u m , i r o n a n d i r o n group metals, tungsten, m o l y b d e n u m , v a n a d i u m , t a n t a l u m , n i o b i u m , t i t a n i u m , z i r c o n i u m , a n d silicon, a n d is o n l y v e r y s l i g h t l y soluble i n a l u m i n u m . T h i s is a considerable advantage w h e n these metals are r e d u c e d w i t h sodium, a n excess of w h i c h w i l l not contaminate the element p r o d u c e d . I n h i g h temperature r e d u c t i o n of h a l o g e n ides w i t h s o d i u m one has to consider the fact that this m e t a l is soluble to some extent i n fused s o d i u m chloride, a n d that this solubility increases r a p i d l y w i t h the temperature. L o r e n z (56) showed that the chloride dissolves 4.2% s o d i u m at its m e l t i n g point, a n d 15 to 2 0 % at 850° C . S o m e other chlorides, w h e n added, increase a n d others decrease the solubility. C u b i c i o t t i (69) studied such m e t a l salt alloys. S o d i u m solutions or dispersions i n the chloride f o r m e d d u r i n g the r e d u c t i o n m a y seriously interfere w i t h the r e d u c t i o n . A s to prices, a comparison o n a n equivalent basis between s o d i u m a n d m a g n e s i u m shows that at 16.5 cents p e r p o u n d f o r s o d i u m a n d 32.5 cents p e r p o u n d for m a g n e s i u m i n carloads, a n d ton lots, respectively, s o d i u m is just as expensive as m a g n e s i u m i n the U n i t e d States. T h i s price situation m a y e v e n be m o r e f a v o r a b l e f o r s o d i u m , i f its f u l l substitution f r o m the technological angle is possible, because of process cost savings of v a r i o u s kinds, l i k e easy r e m o v a l of the c o m p o u n d f o r m e d , for instance, b y wet methods.

Sodium as a Metal Cleanser A cleanser m a y o n l y neutralize a n i m p u r i t y as does, for instance, manganese i n steel d e s u l f u r i z i n g , b y t y i n g it u p as a c o m p o u n d , w h i c h remains entrapped i n the m a t r i x as a less obnoxious i n c l u s i o n ; or the c o m p o u n d f o r m e d m a y be separated m e c h a n i c a l l y f r o m the fused m a t r i x i n solid f o r m . S u c h l i q u a t i o n methods are based o n the fact that the i m p u r i t y is less a n d less soluble i n the b a t h w i t h d r o p p i n g temperature a n d c a n be e l i m i n a t e d as a solid c o m p o u n d . L i q u a t i o n c a n be p e r f o r m e d i n a large kettle at d r o p p i n g temperatures, as, for instance, i n the desilverization of l e a d w i t h zinc. T h e more or less pasty scum, consisting of the h i g h e r m e l t i n g a n d lighter i m p u r i t y c o m p o u n d , absorbed i n l i q u i d base metal, is s k i m m e d off. O r the l i q u a t i o n takes place i n special furnaces on a n i n c l i n e d sole at r i s i n g temperatures, w h e r e b y the more fusible base m e t a l or eutectics are separated f r o m the Haertling, w h i c h is the c o m p o u n d wetted w i t h m a t r i x . S e p a r a t i o n b y l i q u i d l a y e r f o r m a t i o n — i . e . , i n the f u l l y fused state — a f t e r a d d i t i o n of the cleanser is a n i d e a l b u t exceptional case, w h i c h happens w i t h s o d i u m . L i q u a t i o n methods i n v o l v i n g solid i m p u r i t y compounds are used m a i n l y to separate s m a l l quantities of impurities, since losses b y e n t r a p p e d base m e t a l w o u l d become p r o h i b i t i v e , i f the quantity of m a t r i x - e n t r a p p i n g s c u m was increased. S e p a r a t i o n methods i n v o l v i n g m u t u a l l y insoluble l i q u i d l a y e r s are most suitable f o r the e l i m i n a t i o n of large quantities of i m p u r i t i e s , w i t h b u t s m a l l losses of the base m e t a l . A n e u t r a l i z a t i o n of i m p u r i t i e s is p r o b a b l y what takes place w h e n brass a n d r e d brass are treated w i t h s o d i u m . T h e cleanser is u s u a l l y i n t r o d u c e d as a z i n c s o d i u m master a l l o y . Z i n c dissolves not m o r e t h a n 2.8% s o d i u m at 557° C . a n d this alloy, w h e n solid, is constituted of z i n c - s o d i u m c o m p o u n d i n a zinc m a t r i x . A n action of s o d i u m o n sulfides, antimonides, a n d arsenides, even i f these i m purities w e r e tied u p w i t h zinc, c o u l d be expected i n these copper alloys. S o d i u m has been suggested as a desulfurizer for cast i r o n a n d shooting it into the m e t a l b a t h has been proposed. In another proposition the s o d i u m is w e i g h e d d o w n w i t h lead w i t h i n a n i r o n shell w i t h a perforated bottom, w h i c h is l o w e r e d into the fused cast i r o n . E v i d e n t l y s o d i u m - l e a d alloy w o u l d p e r f o r m as w e l l . T h e d e a n t i m o n i z i n g a n d dearsenizing of lead, t i n solder, a n t i m o n i a l copper, a n d speiss, a possibility w h i c h K r o l l m e n t i o n e d (43) 25 years ago, have f o u n d their w a y i n m e t a l l u r g y to o n l y a l i m i t e d degree. T h e m a i n reason is that the s o d i u m arsenide f o r m e d is one of the most dangerous poisons k n o w n o w i n g to the f o r m a t i o n of arsine w h e n exposed to d a m p a i r . Stibine has not been f o u n d i n the gas escaping w h e n s o d i u m antimonide is decomposed w i t h water. T h e m a i n effort of later inventors was directed towards r e d u c i n g the arsine h a z a r d . H a n a k (29) adds s o d i u m h y d r o x i d e after i n t r o d u c i n g his s o d i u m into i m p u r e l e a d to oxidize i n situ the arsenide a n d antimonide f o r m e d . R e h n s (70) operates u n d e r


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h e a v y oils a n d grease i n s o d i u m d e a n t i m o n i z i n g of t i n solder, a n d b u r n s the carbonaceous dross f o r m e d separately i n a n o x i d i z i n g atmosphere. I n one t y p i c a l case (43) a t i n solder w i t h 1.25% a n t i m o n y , 1% copper, a n d traces of arsenic was treated w i t h 0.55% s o d i u m w i t h addition of 0.05% p e t r o l a t u m . T h e output of s o d i u m - f r e e solder was 79.3% a n d the m e t a l contained 0.25% a n t i m o n y . T h i s d e a n t i m o n i z i n g was considered sufficient. T h e r e was a dross representing 4.9% of the input, w i t h 17.0% a n t i m o n y , besides the s o d i u m s c u m corresponding to the balance. C o p p e r a n d s o d i u m were r e m o v e d f r o m the solder together w i t h sulfur. K r o l l used s o d i u m i n 1922 (43) to deantimonize a 12% a n t i m o n i a l lead, w h i c h also contained 0.3% arsenic a n d e m p l o y e d the same process to eliminate 8% arsenic contained i n a r a w t i n , the f o r m e r w i t h a 15-ton charge, the latter w i t h 150 k g . I n b o t h cases, a b l a c k , r e a d i l y fusible l i q u i d separated f r o m the m e t a l (melting point of s o d i u m a n t i m o n i d e is 465 °C.) ; the l i q u i d was spooned off w i t h great precautions a n d decomposed separately w i t h superheated steam. P o l i n g w i t h steam e l i m i n a t e d the last traces of arsenic, a n t i m o n y , a n d s o d i u m f r o m the metal. S u c h a scheme m i g h t w e l l a p p l y also to the treatment of arsenical speiss, if a suitable process a n d equipment for the safe treatment of the arsenide c o u l d be devised. T h e r e is no reason to believe that this w o u l d be a v e r y difficult task, because at present, dangerous gases s u c h as h y d r o g e n c y a n i d e gas are h a n d l e d safely. A r s i n e c a n be c r a c k e d to m e t a l a n d h y d r o g e n above 500° C . a n d it c a n be b u r n e d w i t h excess a i r to arsenic oxide. T i n a n d noble metals are not affected b y s o d i u m (66) f o u n d that s o d i u m c a n be used to help r e m o v e of l e a d a n d that a dross, c o n t a i n i n g the copper arsenic, c a n be f o r m e d w h e n s o d i u m is i n t r o d u c e d metals are not affected i n the absence of zinc.

i n l e a d alloys (43). P e n a r o y a zinc after the desilverization together w i t h a n t i m o n y a n d into l e a d b u l l i o n . T h e noble

T i n solder is c o m m o n l y deantimonized a n d dearsenized w i t h a l u m i n u m (63), a m e t h o d w h i c h the author also r e c o m m e n d e d i n 1922 (43). T h e danger f r o m the angle of arsine poisoning is here less t h a n w i t h sodium, b u t m a n y fatal cases have been r e p o r t e d i n a l u m i n u m treatment of i m p u r e solder. F r o m the m e t a l l u r g i c a l point of v i e w a l u m i n u m offers here slight advantages, i n so f a r as the losses of e n t r a p p e d base m e t a l are l o w , the q u a n t i t y of dross b e i n g s m a l l because of the l o w a n t i m o n y content. A l s o , a l u m i n u m is s l i g h t l y easier to h a n d l e i n the l i m i t e d quantities i n v o l v e d . T h e affinity of s o d i u m for b i s m u t h is f a i r l y h i g h , so that s o d i u m c a n be considered for the d e b i s m u t h i z i n g of l e a d (43). E x p e r i e n c e has shown, h o w e v e r , that s o d i u m alone does not l o w e r the b i s m u t h content of l e a d as f a r as does potassium, m a g n e s i u m , c a l c i u m , a c o m b i n a t i o n of c a l c i u m a n d m a g n e s i u m , o r a m i x t u r e of s o d i u m plus s o d i u m h y d r o x i d e , the latter b e i n g most efficient (7, 8, 15). S o d i u m c a n also be i n t r o d u c e d into the a l l o y b y w a y of a s o d i u m h y d r o x i d e — c a l c i u m carbide flux, i n w h i c h the carbide acts as a r e d u c i n g agent (15) f o r sodium. K r o l l used s o d i u m also to deantimonize a 2 0 % c o p p e r - a n t i m o n y a l l o y (43). S i n c e copper melts at a rather h i g h temperature, above the b o i l i n g point of s o d i u m , difficulties w i t h b u r n i n g s o d i u m a n d w i t h refractories are encountered. S o d i u m , w h e n added to alloys containing copper, i r o n , noble metals, a n t i m o n y , arsenic, a n d sulfur, o r s e l e n i u m a n d t e l l u r i u m , m a k e s a c l e a n cut, w h e r e b y the five latter elements are separated as a l i q u i d slag s w i m m i n g o n the surface of the base metals. F o r d e s u l f u r i z i n g , s o d i u m m i g h t be too expensive, b u t f o r dearsenizing it m i g h t be considered, i f the arsine p r o b l e m c a n be solved.

Production of Alloys with Sodium as a Reducing Agent A s s o d i u m c a n be used to eliminate i m p u r i t i e s contained i n metals, one c o u l d consider it as a means for p r o d u c i n g w a n t e d compounds a n d alloys. T h i s field of a p p l i c a t i o n is r a t h e r large a n d the f o l l o w i n g cases of a l l o y p r o d u c t i o n , m a i n l y based o n the strong halogen affinity of this m e t a l , m a y be e x a m i n e d : a l k a l i n e earth m e t a l alloys, c a l c i u m s o d i u m alloys a n d c a l c i u m m e t a l d e r i v e d f r o m t h e m , a n d s o d i u m alloys, especially those w i t h potassium, w h e n c a l c i u m , m a g n e s i u m , c e r i u m , a l u m i n u m , u r a n i u m , t h o r i u m , a n d t i t a n i u m , f o r instance, are i n t r o d u c e d into a base m e t a l .


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B a r i u m - l e a d alloys were made a century ago b y C a r o n (13), w h o r e d u c e d b a r i u m chloride w i t h s o d i u m lead. K r o l l (37) used the C a r o n process for m a n y years to m a k e thousands of tons of l e a d - c a l c i u m , sold as b e a r i n g alloy to replace t i n B a b b i t t m e t a l i n G e r m a n y after W o r l d W a r I. T h e p r o c e d u r e is v e r y s i m p l e : L e a d is fused i n a kettle a n d m o l t e n c a l c i u m chloride is p o u r e d o n top of it. S o d i u m is then injected into the l e a d b a t h — f o r instance, b y means of the pot s h o w n i n F i g u r e 1—by c a r b o n dioxide pressure. T h e s o d i u m c a n also be i n t r o d u c e d ingotwise, sticking it t h r o u g h the fused salts into the l e a d ; the w o r k m a n is protected b y a steel cover plate p u t over the kettle. T h e s o d i u m is stuck t h r o u g h a hole i n this cover, t h r o u g h the salts into the lead. I n this case it must show a clean surface, since otherwise light explosions m a y occur because of the moisture content of the oxide crust. W i t h injection, the reaction is instantaneous a n d u p to 120 k g . of s o d i u m c a n be made to react i n 2 minutes to a n e q u i l i b r i u m point according to the equation (35, 55): 2 Na/Pb + CaCl

2

-

Ca/Pb + 2 NaCl

(1)

B y r e n e w a l of the c a l c i u m c h l o r i d e layer, the r e s i d u a l s o d i u m c a n be r e m o v e d to a n y desired degree, b u t at the expense of large quantities of c a l c i u m chloride. T h i s second c a l c i u m chloride w a s h is used i n the next batch w i t h s o d i u m - r i c h lead. H o w e v e r , the recovery of c a l c i u m f r o m its chloride, w h e n p r o d u c i n g a c a l c i u m - l e a d alloy w i t h 3% c a l c i u m a n d 0.2% sodium, is o n l y 33%, a n d 2 5 % c a l c i u m chloride, calculated o n the lead i n p u t has to be p u t to w o r k . If s o d i u m is tolerated i n l a r g e r quantities i n the alloy obtained, the c a l c i u m r e c o v e r y is better a n d efficiencies higher t h a n 7 5 % c a n be expected. T h e s o d i u m r e c o v e r y as s o d i u m plus c a l c i u m exceeds u s u a l l y 90%. T h e salts c a n be r e c l a i m e d b y leaching, b u t this does not p a y . T h e p r o d u c t i o n of c a l c i u m - b a r i u m - l e a d b e a r i n g alloy was based o n this process i n G e r m a n y . S u c h a n alloy c o u l d be obtained b y this procedure i n a few hours, w h i l e it took almost a week to m a k e the same

Figure 1.

Sodium injector


143

KROLL—SODIUM IN METALLURGY

alloy b y fusion electrolysis (59), the method used i n the U n i t e d States i n W o r l d W a r I. Regardless of the s i m p l i c i t y of the c h e m i c a l process, it was r e p l a c e d b y a still cheaper one, suggested b y the author i n 1927 (38) b y using c a l c i u m carbide as a reagent to produce s o d i u m - c a l c i u m alloys of lead. A s this operation is based essentially o n the a b o v e - m e n t i o n e d e q u i l i b r i u m , the reaction of s o d i u m w i t h c a l c i u m chloride ( E q u a t i o n 1) is discussed below. W h e n l e a d is heated w i t h c a l c i u m carbide at 1200° C . i n a n inert atmosphere (38) such as h y d r o g e n , kerosene, or noble gases, the carbide is b r o k e n u p a n d the v e r y exothermic lead c o m p o u n d P b C a is f o r m e d according to the r e l a t i o n : :!


CaC

2

+ 3 Pb = Pb.Ca + 2 C

(2)

H i g h temperatures a n d inert gases are expensive. M u c h lower temperatures — f o r instance, 850° C . — c a n be used w h e n operating i n a flux such as c a l c i u m c h l o r i d e - s o d i u m chloride (7). In such a flux, containing s o d i u m chloride, the carbide liberates s o d i u m b y the w e l l k n o w n reaction: CaC

2

+ 2 NaCl =

2 Na +

CaCL +

2 C

(3)

T h i s equation has been r e c o m m e n d e d for p r o d u c i n g s o d i u m i n vacuo (28). E q u a tions 2 a n d 3 f u r n i s h a l l the elements needed to operate the e q u i l i b r i u m E q u a t i o n 1, since s o d i u m is liberated, w h i c h reduces the c a l c i u m chloride present i n the flux or f o r m e d f r o m s o d i u m chloride a n d carbide. T h i s process is easily p u t to practice, b y m e l t i n g lead i n a kettle, a d d i n g fused c a l c i u m c h l o r i d e - s o d i u m chloride, i n t r o d u c i n g pea size, p r e f e r a b l y h i g h grade c a l c i u m carbide, a n d s t i r r i n g the latter into the b a t h w i t h a m e c h a n i c a l m i x e r at 850° C . T h e flux has to take u p the c a r b o n liberated f r o m the carbide as w e l l as its c a l c i u m oxide content. It must r e m a i n fluid throughout the r u n to cover the alloy. Otherwise lead a n d c a l c i u m losses m a y occur. I n this w a y l e a d - c a l c i u m is p r o d u c e d b y the ton for d e b i s m u t h i z i n g l e a d at less t h a n 40 cents a p o u n d of c a l c i u m contained. W h e n a l u m i n u m is added i n this process, alloys w i t h 5% c a l c i u m can be m a d e (77). S i m i l a r l y t i n - c a l c i u m alloys c a n be p r o d u c e d . S o d i u m r e d u c t i o n c a n be a p p l i e d here quite as w e l l a n d alloys of c a l c i u m w i t h a n t i m o n y , b i s m u t h , c a d m i u m , or zinc have been p r e p a r e d b y straight s o d i u m r e d u c t i o n of c a l c i u m chloride (35). C a l c i u m a n d s o d i u m are f u l l y miscible at 1200° C . bttt b e l o w this t e m p e r a ture a gap of m i s c i b i l i t y exists. T h e r e is a eutectic at 700° C . w i t h 7% s o d i u m a n d at this temperature 14% c a l c i u m is soluble i n s o d i u m m e t a l (74). S u c h alloys are obtained according to the e q u i l i b r i u m reaction: CaCl

2

+ 3 Na -

Ca/Na + 2 NaCl

(4)

In this operation one c a n produce, for instance, a c a l c i u m alloy w i t h 17% s o d i u m at 850° C . w h i c h stays i n e q u i l i b r i u m w i t h a salt b a t h containing 71% c a l c i u m chloride. B y treating this alloy w i t h large quantities of pure fused c a l c i u m chloride, the excess s o d i u m c a n be washed out completely, b u t at a price. T h i s process, w h i c h yields a n i t r o g e n - f r e e c a l c i u m , lends itself to a c o n tinuous countercurrent operation i n a f u l l y l i q u i d m e d i u m . E x c e s s s o d i u m chloride, w h i c h is m u c h m o r e volatile t h a n c a l c i u m chloride, could be r e m o v e d b y v a c u u m evaporation. H o w e v e r , it w o u l d seem m o r e convenient to m a k e c a l c i u m f r o m the 25 to 65% c a l c i u m - s o d i u m alloy obtained as a b y - p r o d u c t b y E q u a t i o n 4 i n the fusion electrolysis of s o d i u m b y the D o w n s process, i n w h i c h a s o d i u m - c a l c i u m chloride electrolyte is used. S o d i u m contained i n this alloy can be e l i m i n a t e d b y dissolution i n e t h y l alcohol (12, 14), w h i c h yields n i t r o g e n free c a l c i u m flakes, contaminated o n l y w i t h a few tenths of a p e r cent chloride a n d a few per cent c a l c i u m oxide. S u c h a n alloy could also be separated b y filtering (19) o r v a c u u m distillation (3) at elevated temperature, a n d , w h e n a heated condenser is used, sodium, s o d i u m chloride, a n d a n o x i d e - f r e e c a l c i u m c a n be collected separately (3). A t present p u r e redistilled c a l c i u m sells at about $3.50 a p o u n d . T h e profit m a r g i n i n the process described is large a n d it w o u l d i n v i t e f u r t h e r investigation of this case, i f there was a m a r k e t for a somewhat cheaper, h i g h p u r i t y c a l c i u m . T h i s metal is used for the p r o d u c t i o n of u r a n i u m m e t a l b y reduction of u r a n i u m tetrafluoride u n d e r argon (16).


144

A D V A N C E S IN

CHEMISTRY SERIES

S o d i u m - p o t a s s i u m alloys, proposed as heat exchangers i n atomic reactions (58), c a n be made b y s i m i l a r methods. T w o operable w a y s are open: a n e x change of s o d i u m m e t a l w i t h fused potassium h y d r o x i d e (34, 52) a n d the e q u i l i b r i u m between s o d i u m m e t a l a n d potassium chloride (27). T h e relations can be w r i t t e n as follows: 2 Na +

KOH -


2 Na +

Na/K +

KC1 — N a / K +

NaOH

(5)

NaCl

(6)

In the latter case, fluoride (73), b r o m i d e (71), or iodide (72) can be s u b s t i tuted for the chloride but difficulties are caused b y the f u s i b i l i t y of the fluorides a n d the l o w b o i l i n g point of s o d i u m . E x c h a n g e s s i m i l a r to those s h o w n i n E q u a t i o n s 5 a n d 6 can be p e r f o r m e d w i t h the corresponding r u b i d i u m a n d c e s i u m salts. A c c o r d i n g to R i n c k (75) w h o studied E q u a t i o n 6, a n alloy of s o d i u m w i t h 16.1% potassium is i n e q u i l i b r i u m w i t h a fused salt containing 61.2% potassium chloride, a n d the r e m a i n d e r s o d i u m chloride, a m i x t u r e that corresponds almost to the eutectic, that melts at 650° C . E v i d e n t l y , as shown i n the previous case, the salt m i x t u r e s h o u l d be reprocessed to r e c l a i m at least the expensive potassium chloride. T h i s m i g h t h a v e to be done b y aqueous methods, since the b o i l i n g points of b o t h salts are f a i r l y close together, so that h i g h temperature v a c u u m e v a p o r a tion m i g h t not give a c l e a n separation. M a g n e s i u m - c a l c i u m alloys w i t h about 25% c a l c i u m , used as master alloys for c a l c i u m i n t r o d u c t i o n i n m a g n e s i u m , c a n be m a d e b y s o d i u m r e d u c t i o n of fused c a l c i u m c h l o r i d e i n the presence of m a g n e s i u m u n d e r a protecting a t mosphere according to: 2 Na +

CaCl

2

+

Mg =

Mg/Ca +

2 NaCl

(7)

F l u o r i d e s m a y also be present (9). A r g o n is preferable to h y d r o g e n as a blanket, since c a l c i u m h y d r i d e f o r m a t i o n c a n thus be a v o i d e d . In one t y p i c a l case p e r f o r m e d on a l a b o r a t o r y scale, 5.25 k g . of c a l c i u m chloride, 1.5 k g . of s o d i u m , a n d 1.87 k g . of m a g n e s i u m w e r e fused together at 850° C . u n d e r a r g o n i n a n i r o n pot. T h e m e t a l recovered after cooling was a m a g n e s i u m alloy w i t h 25.1% c a l c i u m a n d about 1% s o d i u m . T h i s latter i m p u r i t y is undesired, as it i m p a i r s the corrosion properties of m a g n e s i u m . It c o u l d be e l i m i n a t e d b y v a c u u m d i s t i l l a tion or b y reaction w i t h m o r e fused c a l c i u m chloride. It is the s o d i u m w h i c h reduces c a l c i u m chloride a n d the c a l c i u m so p r o d u c e d is passed on to the m a g n e s i u m w i t h w h i c h it alloys. I n the case presented, the r e c o v e r y of s o d i u m (as c a l c i u m ) amounted to 43%, a n d 4 0 % of the c a l c i u m contained i n the c h l o r i d e was extracted. B u t there was a loss of 10% of the m a g n e s i u m present. S o d i u m c a l c i u m alloys, obtained, for instance, i n D o w n ' s cells, c a n be split i n a flux w i t h m a g n e s i u m to f o r m free s o d i u m a n d c a l c i u m - m a g n e s i u m alloys. It is k n o w n that c a l c i u m h y d r i d e dissolves i n a l l proportions i n c a l c i u m chloride. S i n c e s o d i u m reduces the latter salt to c a l c i u m metal, if this r e d u c t i o n was p e r f o r m e d i n the presence of h y d r o g e n , one c o u l d expect f o r m a t i o n of c a l c i u m h y d r i d e w h i c h w o u l d disappear i n the flux, i n w h i c h it is completely soluble. T h i s is i n d e e d w h a t takes place, a n d A l e x a n d e r (2, 86) proposes this m e t h o d to produce the h y d r i d e d salt first, w h i c h then reacts w i t h t i t a n i u m c h l o r i d e w i t h l i b e r a t i o n of h y d r o g e n a n d t i t a n i u m . T h e large b u l k of salts takes care of the heat dissipation p r o b l e m . H o w e v e r , this proposition has various drawbacks. N a t u r a l l y the c a l c i u m c h l o r i d e used must be free of oxide, w h i c h entails purification before u s i n g it. T h i s also concerns the h y d r o g e n i n t r o d u c e d i n this cycle. N e i t h e r the chloride n o r the h y d r o g e n contributes d i r e c t l y i n the t i t a n i u m reduction. B o t h are lost after the operation. P e r h a p s a s m a l l p a r t of the h y d r o g e n contributes to the r e d u c t i o n b y b r e a k i n g d o w n some t i t a n i u m tetrachloride to l o w e r v a l e n c y chloride w i t h l i b e r a t i o n of h y d r o g e n chloride. T h i s is a n u n w a n t e d b y - p r o d u c t w h i c h w o u l d h a v e to be processed f u r t h e r . C a l c i u m h y d r i d e is an energetic r e d u c i n g agent a n d the fact that it can be p r o d u c e d cheaply w i t h i n a n a l k a l i n e earth halogenide flux b y r e d u c t i o n of the latter u n d e r h y d r o g e n , w h e n s o d i u m is used as a r e d u c i n g agent, s h o u l d find some other interesting applications. T h e r e are m a n y other metals that c o u l d be r e d u c e d f r o m their halogenides b y s o d i u m a n d f u r t h e r a l l o y e d w i t h a base metal, either present d u r i n g the r e -



KROLL—SODIUM

IN

145

METALLURGY

duction, o r brought i n contact later w i t h the fused reaction cake. F o r instance, b e r y l l i u m chloride, dissolved i n s o d i u m c h l o r i d e as a c a r r i e r salt to lower evaporation losses of the former, c a n react w i t h s o d i u m i n the presence of a l u m i n u m , b y preference i n a n inert atmosphere. O r l i t h i u m chloride is m e l t e d w i t h a l u m i n u m , a n d m i x e d w i t h s o d i u m ; or c e r i u m chloride a n d s o d i u m chloride react i n a s i m i l a r w a y w i t h a l u m i n u m a n d s o d i u m . I n a l l these cases alloys of a l u m i n u m w i t h u p to 10% b e r y l l i u m , l i t h i u m , or c e r i u m c a n easily be made. T i t a n i u m , z i r c o n i u m , u r a n i u m , a n d b o r o n alloys w i t h a l u m i n u m a n d w i t h other base metals c a n be obtained i n the same w a y . Because of the l o w b o i l i n g point of s o d i u m , difficulties m a y arise w h e n metals of h i g h m e l t i n g point are r e d u c e d , especially i f the solvent m e t a l itself is not r e a d i l y fusible or becomes h a r d e r to melt because of a l l o y i n g . I n this case the halogenide m a y first be r e d u c e d alone w i t h s o d i u m , below the b o i l i n g point of the latter, a n d the c o m m i n u t e d solid base m e t a l added to the reacted fused m i x t u r e a n d melted. I r o n - t i t a n i u m a n d i r o n - z i r c o n i u m c a n be made i n this w a y . F o r the p r o d u c t i o n of c o p p e r - t i t a n i u m W a r t m a n (87) uses a m a g n e s i u m - c o p p e r alloy as a r e d u c i n g agent for t i t a n i u m chloride. S o d i u m , w h i c h does not alloy w i t h copper, c o u l d be used as a substitute for m a g n e s i u m i n this operation. I n the r e d u c t i o n of halogenides w i t h s o d i u m a basic mistake is f r e q u e n t l y c o m m i t t e d b y o v e r l o o k i n g the fact that chlorides, w h e n exposed i n the hot or fused state to a i r , are o x i d i z e d b y a switch of o x y g e n a n d chlorine, or b y direct reaction w i t h the moisture of the a i r or w i t h the one contained i n the salt itself (40). T h e fused chloride must be t h o r o u g h l y deoxidized before the r e d u c t i o n (83), for instance, b y passing h y d r o g e n chloride gas or a chlorinated h y d r o c a r b o n c o m p o u n d t h r o u g h the bath. O n l y recently has it been possible to reduce c e r i u m chloride, starting w i t h a f u l l y o x i d e - f r e e salt. O x i d e , w h e n present, inhibits coalescing of the m e t a l particles, a n d at best a b l a c k m e t a l powder is obtained. FLUORIDES. U p to n o w fluorides have not been especially stressed i n this report as salts that c o u l d be used f o r a r e d u c t i o n w i t h sodium. A comparison of their merits w i t h the chlorides w o u l d be of interest. S o d i u m fluoride melts at 993° C . — i . e . , 113° C . above the b o i l i n g point of s o d i u m m e t a l — w h i c h complicates the use of the latter as a r e d u c i n g agent for fluorides. F l u x i n g m i x t u r e s w o u l d h a v e to be used, a n d as such, additions of potassium fluoride, s o d i u m chloride, a n d potassium chloride have been suggested. Eutectics w i t h s o d i u m fluoride a n d s o d i u m chloride are f o r m e d at 675° C . a n d 35 mole % of the latter, at 700° C . a n d 60 mole % potassium chloride, the rest b e i n g s o d i u m fluoride. A eutectic exists i n the system s o d i u m fluoride-potassium fluoride at 60 mole % a n d 699° C . E a s y fusibility s h o u l d p r e v a i l a l l over the reduction, i n the b e g i n n i n g to a v o i d s o d i u m evaporation, a n d at the e n d to m a k e sure that the difficultly fusible s o d i u m fluoride does not envelop the m e t a l particles a n d i n h i b i t coalescing.

F u s i b i l i t y is frequently achieved w i t h various a l k a l i fluo salts w h i c h c a n mostly be obtained b y aqueous methods. S u c h salts are, for instance, 2 N a F - S i F ; 2 N a F - T i F ; N a F - B F ; 2 KF-TaFr,; 2 N a F - B e F ; 3 N a F - A l F ; and 2 N a F - Z r F . A l l are unstable at elevated temperature w h e r e b y the b o n d w i t h the a l k a l i fluoride is r u p t u r e d . T h e t h e r m a l stability increases i n the g i v e n order f r o m the left to the right. S o d i u m silicofluoride is v e r y unstable a n d the silicon t e t r a fluoride liberated b y t h e r m a l dissociation c a n be used as a fluorizing reagent. A l l these fluo salts h y d r o l y z e at least somewhat, w h e n d r y i n g , a n d they react also w i t h moisture f r o m the air, w h e n hot, w h e r e b y o x y g e n is introduced. 4

4

3

2

3

4

A n o t h e r point of v i e w w h e n r e d u c i n g fluorides w i t h s o d i u m concerns the means of separating the salt f r o m the cake. V a c u u m separation at elevated temperature, w h i l e possible, w o u l d have to take place above 1200° C , w h i c h rules out the use of cheap i r o n v a c u u m retorts. A q u e o u s methods have the disadvantage that s o d i u m fluoride is b u t slightly soluble i n water (about 4% at r o o m t e m p e r a t u r e ) , b u t this m i g h t be adequate for p r a c t i c a l purposes.

Production of Elements by Reduction of Fluorides and Chlorides with Sodium T a b l e I V shows w h i c h of the more active elements have been r e d u c e d f r o m their fluorides or chlorides w i t h s o d i u m as reported i n the literature. Sometimes, w h e n the affinities are close, e q u i l i b r i a develop a n d alloys w i t h s o d i u m are


146

ADVANCES

f o r m e d as described above. produced.

IN CHEMISTRY SERIES

I n one case, only a subsalt or a metal-salt alloy is

FLUORIDE REDUCTIONS. A s p l a i n fluorides are rather difficult to m a k e , a l k a l i fluo salts, obtained f r o m aqueous solutions, are c o m m o n l y p r e f e r r e d . T h e f o l l o w i n g have been suggested for the p r o d u c t i o n of various elements b y s o d i u m or potassium r e d u c t i o n : K B F (81); K T a F (5); N a S i F (85); K T i F (88); a n d Na-jZrFe (67). T h e p o t a s s i u m - b e a r i n g salts f o r m w i t h excess s o d i u m the a b o v e mentioned r e a d i l y fusible a n d p y r o p h o r i c s o d i u m - p o t a s s i u m alloys, w h i c h are somewhat d i s t u r b i n g o n leaching of the cake. 4

2

T

2

e

2

e

Table IV. Sodium Reductions of Halogenides Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: January 1, 1957 | doi: 10.1021/ba-1957-0019.ch015

Halogenide Element Produced Al Be Β Ba Ca Ce Cr Κ Li Mg Mn Rb Si Sr Ta Th Ti U V Zr Χ. Ο. S. E.

Fluoride Ε Ε Χ Ο ο

Chloride X X X s Ε Χ χ Ε Ε Χ Χ Ε Χ Ε

Used Double fluor: Ε Ε Χ

Ε Ε Ε

Χ Χ Χ χ χ

Ε Χ Ο

Χ

χ χ

χ χ

χ

χ

Χ

Complete reduction to metal. N o reduction. Subsalt f o r m e d . Equilibrium.

T h e best described case is the one of potassium t a n t a l u m fluoride reduction. T h i s salt is c u r r e n t l y obtained i n the M a r i g n a c separation, f r o m the potassium c o l u m b i u m oxyfluoride, b y crystallization. T h e f u l l y d r i e d salt is m i x e d w i t h clean s o d i u m chips, pressed l i g h t l y i n a steel b o m b w h i c h is sealed a n d heated e x t e r n a l l y w i t h gas. A f t e r the flash the cooled b o m b is d r i l l e d out a n d the cake is treated w i t h alcohol, water, a n d various acids. T h e powder obtained is processed to compact m e t a l b y pressing i n bars a n d h i g h v a c u u m sintering. T h e e q u i l i b r i u m between s o d i u m a n d a l u m i n u m fluoride has been studied b y J a n d e r (33), w h o reports that the reduction proceeds u p to 1090° C . w i t h liberation of a l u m i n u m , according to: A1F + 3 N a - 3 N a F + A l (8) A b o v e this temperature the reaction reverses a n d sodium is produced, a method that has been patented b y Specketer (82). I n vacuo, s o d i u m c a n be obtained at a still lower temperature. A c c o r d i n g to a patent of E l e c t r o c h i m i e U g i n e (18) even i r o n c a n be used to reduce sodium fluoride i n vacuo. G r u n e r t (25) has shown that the cryolite complex is r e d u c e d b y a l u m i n u m a n d s o d i u m a n d is obtained at 1000° C . or above, as indicated b y the e q u i l i b r i u m : 3

A l F - 3 N a F + A l — 2 A1F + 3 N a (9) T h i s proves that i n the presence of cryolite s o d i u m is nobler t h a n a l u m i n u m , b u t only above 1000° C . W h i l e w i t h a l u m i n u m fluoride, sodium liberates a l u m i n u m u p to 1090° C , it cannot reduce cryolite above 1000° C , because the b o n d between a l k a l i fluoride and a l u m i n u m fluoride must be b r o k e n . E q u a t i o n 8, w h e n reversed, is used for the beneficiation of 12% eutectic a l u m i n u m alloys, i n w h i c h a little sodium, liberated b y reaction of a l u m i n u m w i t h s o d i u m fluoride, refines the silicon grains. S i m i l a r results are obtained directly b y a d d i n g s m a l l amounts of s o d i u m to the alloy. 3

3

Conditions l i k e those described i n E q u a t i o n 9 exist also i n the reduction of b e r y l l i u m fluoride o r its double fluoride (90) w i t h sodium, according to: 2 BeF

2

+

2 Na -

BeF -2 N a F + Be 2


(10)

KROLL—SODIUM IN

147

METALLURGY

T h i s reaction comes to a stop w i t h the f o r m a t i o n of the stable complex b e r y l l i u m f l u o r i d e - s o d i u m difluoride ( B e F - 2 N a F ) a n d o n l y a part of the b e r y l l i u m c a n thus be obtained as m e t a l (78). A n a l o g o u s conditions have been described b y L e b e a u i n the fusion electrolysis of b e r y l l i u m f l u o r i d e - m o n o s o d i u m f l u o r i d e (51, 78), w h i c h stops w h e n the b a t h reaches the composition of b e r y l l i u m f l u o r i d e s o d i u m difluoride.


2

T h e r e d u c t i o n of the fluotitanates a n d fluozirconates of a l k a l i metals w i t h s o d i u m has been t r i e d b y various authors (67, 88) w i t h o n l y fair success. U s u a l l y the m e t a l so p r o d u c e d is b l a c k a n d h e a v i l y oxidized. T h e r e is no reason to believe that the r e d u c t i o n of these fluo salts w i t h s o d i u m s h o u l d give poorer results t h a n the fusion electrolysis, w h i c h starts f r o m the same salts. M o s t of the trouble m a y be attributed to the f o l l o w i n g causes: T h e b a t h has not been f u l l y deoxidized before the reduction, the s o d i u m contained oxide, the reaction proceeded too fast, thus y i e l d i n g too fine a powder, or the m e t a l crystals obtained were depassivated b y h y d r o l y s i s of fluorides a n d were attacked b y water on leaching. T h e remedies can be d e r i v e d f r o m these r e m a r k s . S p e c i a l attention s h o u l d be devoted to slow g r o w t h of large crystals, less subject to c h e m i c a l attack w h e n leaching, thus r e p r o d u c i n g the conditions of the fusion electrolysis, w h e r e the dendrites are g i v e n time to grow. A s i n the electrolysis, a container p r o b l e m also arises i n s o d i u m reduction, especially on account of the necessity of t h o r o u g h l y d e o x i d i z i n g the bath, for instance, w i t h h y d r o g e n fluoride, a n d graphite m i g h t w e l l be the o n l y m a t e r i a l of construction that c o u l d be used without r i s k of contamination. It is questionable whether the fluorides c o u l d seriously compete w i t h the chlorides i n s o d i u m reduction, since fluorine costs on the basis of the atomic weights about 3.5 times as m u c h as chlorine (41). A l l fluorine originates f r o m c a l c i u m fluoride or fluorapatite, f r o m w h i c h it is d r i v e n out as h y d r o g e n fluoride b y s u l f u r i c acid. T h i s is an expensive m e t h o d of p r o d u c t i o n for g y p s u m . R e c l a i m i n g waste fluorides f r o m s o d i u m reductions, such as s o d i u m fluoride, p o tassium fluoride or m a g n e s i u m fluoride w i t h sulfuric acid creates other problems. CHLORIDE REDUCTIONS. T h e r e d u c t i o n of m e t a l chlorides w i t h s o d i u m was practiced b y o l d - t i m e metallurgists such as B e r z e l i u s (6). T h e classical a l u m i n u m r e d u c t i o n m e t h o d of W o e h l e r , i n d u s t r i a l i z e d b y St. C l a i r e D e v i l l e (76), m a y be m e n t i o n e d as one w a y of operating. T h e volatile a l u m i n u m chloride (boiling point 175° C.) was stabilized i n a c a r r i e r salt, s o d i u m chloride. Bussy's (11) r e d u c t i o n m e t h o d for m a g n e s i u m chloride b y a l k a l i metals was used i n E n g l a n d d u r i n g W o r l d W a r I to produce m a g n e s i u m (80). W e m p e (89) proposes the r e duction of b e r y l l i u m chloride w i t h s o d i u m u n d e r a n inert gas. T h i s m e t h o d w o u l d be p r a c t i c a l , if some of the c o m m o n mistakes, especially those concerning a not f u l l y deoxidized bath, were t a k e n care of. A n o x i d i z e d b e r y l l i u m chloride bath produces fine m e t a l crystals, that are m o r e susceptible to attack b y water, w h e n the cake is leached, especially i n the presence of chlorine ions. T o raise the temperature after the r e d u c t i o n to the m e l t i n g point of b e r y l l i u m i n v i e w of coalescing the metal, as W e m p e suggests, presumes that a container m a t e r i a l c o u l d be f o u n d that resists the c h e m i c a l attack b y hot s o d i u m a n d that w i l l not alloy w i t h b e r y l l i u m at the h i g h temperature i n v o l v e d (melting point of b e r y l l i u m =: 1283° C ) . O n e of the difficulties of t r y i n g to reduce gaseous b e r y l l i u m chloride, f o l l o w i n g the principles used successfully for the p r o d u c t i o n of z i r c o n i u m f r o m z i r c o n i u m tetrachloride w i t h m a g n e s i u m as a r e d u c i n g agent, is the fact that b e r y l l i u m chloride melts at 405° C. a n d boils at 492° C . Condensation on a hot core to compact the chloride, a n d its r e - e v a p o r a t i o n f r o m this core, are difficult because the chloride drips off i n a temperature i n t e r v a l , between m e l t i n g and b o i l i n g point. F u r t h e r m o r e , the v e r y light m e t a l flakes obtained float to the surface of the s o d i u m chloride b a t h a n d b l o c k the access of the chloride to the sodium, w h i c h makes the use of a m i x e r i m p e r a t i v e . Nevertheless, b e r y l l i u m of h i g h q u a l i t y c o u l d c e r t a i n l y be obtained f r o m its f u l l y deoxidized chloride, either directly or i n a c a r r i e r salt, after these difficulties h a d been overcome. B e r y l l i u m chloride is rather difficult to produce a n d to p u r i f y . Its hygroscopicity makes h a n d l i n g of this salt uncomfortable. T h e p r e s e n t - d a y r e d u c t i o n of b e r y l l i u m fluoride w i t h m a g n e s i u m i n t r o d u c e d i n 1929 (39), gives satisfaction. American

Chemical Library

Society

In HANDLING 1AND ALKALI METALS; 1 5 5 USES 1 6 t h OF St..THEN.W. Advances in Chemistry; W aAmerican s h i n g t o Chemical n . D . C . Society: 2 0 0 3 6Washington, DC, 1957.


148

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IN CHEMISTRY SERIES

T h e r e d u c t i o n of z i r c o n i u m tetrachloride i n a c a r r i e r salt w i t h s o d i u m as a r e d u c i n g agent m a y be e x a m i n e d next. A g a i n , as described before, complete deoxidation of the b a t h before r e d u c t i o n is the essential condition f o r success, if ductile m e t a l is w a n t e d . Z i r c o n i u m tetrachloride is soluble i n s o d i u m chloride or potassium c h l o r i d e (48) a n d a salt w i t h about 2 5 % z i r c o n i u m tetrachloride c a n be m e l t e d without excessive z i r c o n i u m chloride losses. S u c h a b a t h c a n e v e n be obtained f r o m p o w d e r y c o m m e r c i a l z i r c o n i u m suicide a n d i r o n d i c h l o r i d e , w h i c h react w h e n heated a n d d e l i v e r a stream of z i r c o n i u m tetrachloride, contaminated w i t h some t i t a n i u m tetrachloride a n d silicon tetrachloride. T h e gas so p r o d u c e d c a n be condensed i n a fused salt b a t h s u c h as potassium c h l o r i d e - s o d i u m chloride, i n w h i c h o n l y the z i r c o n i u m tetrachloride dissolves (47). T o obtain a l o w oxide m e t a l after r e d u c t i o n w i t h s o d i u m , the conditions f o r fluo salt deoxidation must be observed. T h i s process of z i r c o n i u m p r o d u c t i o n has n o special interest, except for obtaining p o w d e r f o r getter purposes. A c a r r i e r salt, w h i c h m i g h t introduce oxide, is not wanted, as the reaction itself liberates s o d i u m chloride. A G e r m a n patent (20) claims the r e d u c t i o n of t i t a n i u m tetrachloride i n the presence of potassium chloride w i t h s o d i u m as a reagent. A t the temperature i n d i c a t e d i n the specification t i t a n i u m tetrachloride is little or not at a l l soluble i n potassium c h l o r i d e . T h e observations m a d e above as to the nuisance of a c a r r i e r salt c o n c e r n this case as w e l l . H o w e v e r , w i t h suitable precautions this process m a y y i e l d a ductile m e t a l . T h e p r o d u c t i o n of z i r c o n i u m f r o m its chloride i n the absence of a c a r r i e r salt, w i t h s o d i u m as a reagent, involves m o r e complications t h a n m a g n e s i u m r e d u c t i o n . Those t y p i c a l of z i r c o n i u m are discussed h e r e ; the ones c o n c e r n i n g t i t a n i u m w i l l be e x a m i n e d later. Replacement of part of the m a g n e s i u m used i n the r e d u c t i o n of z i r c o n i u m tetrachloride w i t h s o d i u m has been a c h i e v e d successfully b u t w i t h o u t special advantage (49). S i n c e s o d i u m is m o r e active t h a n m a g n e s i u m , it reacts first, thus f o r m i n g the m o r e difficultly fusible s o d i u m chloride ( m e l t i n g point of s o d i u m •chloride 800° C ; of m a g n e s i u m c h l o r i d e 720° C ) . T h e advantage of the r e a d i l y f u s i b l e m a g n e s i u m c h l o r i d e - s o d i u m chloride eutectic appears therefore o n l y at the e n d of the operation, w h e n the m a g n e s i u m also participates i n the r e d u c t i o n . T h e reaction of z i r c o n i u m tetrachloride w i t h clean o x i d e - f r e e s o d i u m , w h e n the procedure e m p l o y e d i n m a g n e s i u m r e d u c t i o n is used, y i e l d s a sponge of good q u a l i t y . H o w e v e r , one of the m a i n d r a w b a c k s i n v o l v e d i n this substitution is the fact that l e a c h i n g of the sponge, the o n l y advantage d e r i v e d f r o m the use of sodium, is r i s k y o n account of the explosion h a z a r d a l w a y s p r e s e n t , when m o r e or less finely d i v i d e d z i r c o n i u m is b r o u g h t i n contact w i t h w a t e r (50). T h i s is confirmed b y recent accidents. T h e alternative of v a c u u m distillation at a n elevated temperature to separate the s o d i u m c h l o r i d e f r o m the sponge, introduces another element of i n s e c u r i t y — f i r e h a z a r d because of t h i n crusts of p y r o p h o r i c s o d i u m , condensed i n the cooler parts of the retort. T h i s adds to the trouble encountered w i t h the p y r o p h o r i c properties of z i r c o n i u m itself.

Sodium Reduction of Titanium Halogenides T i t a n i u m fluoride melts at 400° C . u n d e r pressure a n d sublimes at 284° C . T h i s p r o p e r t y w o u l d suggest its r e d u c t i o n w i t h s o d i u m b y the method n o w used for the p r o d u c t i o n of z i r c o n i u m — i . e . , gas phase r e d u c t i o n (42). H o w e v e r , the s o d i u m fluoride p r o d u c e d melts at 988° C . or 108° C . above the b o i l i n g point of sodium, a n d solid crusts of s o d i u m fluoride m i g h t s u r r o u n d the r e d u c i n g agent. A t this temperature i r o n w o u l d also react w i t h the t i t a n i u m obtained. T h i s eliminates the s o d i u m r e d u c t i o n of the fluoride at atmospheric pressure, b u t b o m b r e d u c t i o n m i g h t w o r k especially after a d d i t i o n of zinc fluoride as indicated b y S p e d d i n g (57, 83) f o r the r e d u c t i o n of z i r c o n i u m tetrafluoride w i t h c a l c i u m . E s s e n t i a l l y this m e t h o d solves the problems of the b o m b l i n i n g , i n this case c a l c i u m fluoride; a n d of t i t a n i u m reaction w i t h the i r o n w a l l . T h i s c o m p o u n d , because of its l o w m e l t i n g point (1300° C ) , w h i c h is m a n y h u n d r e d degrees b e l o w that of either t i t a n i u m o r z i r c o n i u m , w o u l d m e l t i f it h a d to c o n t a i n the p u r e fused metals. H o w e v e r , the zinc alloys w i t h 20 to 3 0 % zinc melt b e l o w 1300° C , thus m a k i n g possible the use of c a l c i u m fluoride as a l i n e r f o r the bombs.


149



T h e c o m p o u n d z i r c o n i u m - z i n c melts i n c o n g r u e n t l y at 1050° C . (57). N a t u r a l l y the zinc contained i n this alloy must be r e m o v e d i n a second step, w h i c h is p e r f o r m e d after c r u s h i n g , b y distillation i n a h i g h temperature v a c u u m furnace. T h e last f e w per cent of zinc a r e e l i m i n a t e d b y arc m e l t i n g the metal. Z i n c , w h i c h is released at least t e m p o r a r i l y i n the r e d u c t i o n of zinc fluoride, boils at 907° C , a f e w degrees h i g h e r t h a n sodium, a n d the latter m i g h t be substituted as a r e d u c i n g agent for the expensive c a l c i u m . S u c h b o m b methods, o n account of the h i g h pressure i n v o l v e d , do not seem to be adaptable to a large tonnage p r o d u c t i o n , even w h e n cheap tetrachloride is used instead of tetrafluoride, w i t h w a t e r - c o o l e d , triggered bombs. T h e y are, h o w ever, the classical ones. N i l s o n a n d Petterson (62) used this type of r e d u c t i o n w i t h t i t a n i u m tetrafluoride a n d s o d i u m i n 1887. T h e i r m e t a l was brittle. H u n t e r (31) i n 1910 repeated these experiments w i t h s i m i l a r e q u i p m e n t a n d greater care a n d h a d m o r e l u c k . H i s m e t a l was at least m a l l e a b l e i n the hot state. C o m p l e t e d u c t i l i t y at r o o m temperature was obtained b y the same m e t h o d i n 1914 b y L e l y a n d H a m b u r g e r (53). P e r h a p s a h i g h oxide content of the s o d i u m interfered w i t h N i l s o n ' s e x p e r i m e n t . T h i s m e t a l dissolves m o r e t h a n 0.5% oxide (30, 57) a n d certainly the s o d i u m made b y the electrolysis of s o d i u m h y d r o x i d e i n olden times contained m o r e o x y g e n t h a n the one obtained today f r o m D o w n ' s cells, w h i c h use a s o d i u m c h l o r i d e - c a l c i u m chloride electrolyte. T h e basic i d e a of the p r e s e n t - d a y s o d i u m or m a g n e s i u m r e d u c t i o n of t i t a n i u m (44, 45) a n d z i r c o n i u m tetrachlorides, w h i c h m a d e this m e t h o d p r a c t i c a l , is not to m i x the ingredients a l l at once a n d flash t h e m , p r o d u c i n g h i g h pressures, b u t to release the heat p r o d u c e d gently a n d progressively b y a d d i n g the reagents as needed. A n efficient b r a k e i n case of a r u n a w a y is p r o v i d e d for b y the use of a noble gas, added w h e n necessary, w h i l e the pressure remains substantially that of the atmosphere. I n r e d u c i n g t i t a n i u m chlorides (tetra, t r i - , a n d d i - ) the solubility of these salts i n the s o d i u m chloride p r o d u c e d is of great interest. T i t a n i u m tetrachloride seems to be b u t little soluble i n s o d i u m c h l o r i d e . E h r l i c h (17) has s h o w n that the t r i v a l e n t salt, N a T i C l e , w i t h 59.2% t i t a n i u m t r i c h l o r i d e melts at 554° C . b u t starts dissociating at 600° C . T h e potassium salt, K T i C l , w h i c h is slightly m o r e stable, melts at 609° C . a n d starts decomposing at 750° C . T h e t h e r m a l d i s s o c i a tion of these complexes is r a t h e r complicated a n d a d i v a l e n t salt is p r o d u c e d as w e l l as t i t a n i u m tetrachloride. Indications i n a patent (36) refer to still l o w e r m e l t i n g points. A salt containing 26.5% t i t a n i u m d i c h l o r i d e , 34.5% t i t a n i u m trichloride, a n d 3 9 % s o d i u m c h l o r i d e is said to m e l t at 79° C ? T h i s i n f o r m a t i o n , as w e l l as a statement b y E h r l i c h , shows that i n the r e d u c t i o n of t i t a n i u m tetrachloride a t w o - s t e p process is possible (36), b y w h i c h a l o w m e l t i n g b a t h o f s o d i u m c h l o r i d e - t i t a n i u m t r i c h l o r i d e - t i t a n i u m d i c h l o r i d e is made, for instance, b y p a r t i a l r e d u c t i o n of t i t a n i u m tetrachloride w i t h h y d r o g e n , t i t a n i u m , m a g n e s i u m or sodium, a n d the r e d u c t i o n is t h e n b r o u g h t to the e n d point i n a second operation w i t h m o r e r e d u c i n g agent, at a temperature exceeding the m e l t i n g point of s o d i u m c h l o r i d e . 3

5

F r o m the point of v i e w of the p h y s i c a l properties of s o d i u m a n d s o d i u m c h l o r i d e there a r e three temperature areas w i t h i n w h i c h r e d u c t i o n of t i t a n i u m tetrachloride to m e t a l appears possible (46): (1) b e l o w the m e l t i n g point of s o d i u m c h l o r i d e — i . e . , 800° C ; (2) between this m e l t i n g point a n d the b o i l i n g point of s o d i u m o r 880° C ; a n d (3) above this b o i l i n g point b u t b e l o w the t e m perature at w h i c h t i t a n i u m reacts w i t h i r o n , w h i c h is about 975° C . T h e first case has been studied b y U . S . I n d u s t r i a l C h e m i c a l s C o . a n d N a t i o n a l D i s t i l l e r s C h e m i c a l C o . , a n d a description of their m e t h o d of u s i n g " h i g h surface s o d i u m " i n t i t a n i u m tetrachloride r e d u c t i o n is g i v e n i n a p a m p h l e t (84). I m p e r i a l C h e m i c a l Industries obtained patents (32, 64) o n a s i m i l a r process i n w h i c h l o w temperature r e d u c t i o n is f o l l o w e d b y a heating above the m e l t i n g point of s o d i u m chloride, to separate most of this salt b y décantation or tapping, a n d to coalesce the m e t a l particles to be leached later. I n the process described b y U . S. I n d u s t r i a l C h e m i c a l s C o . , a s o d i u m f i l m is p r o d u c e d o n solid s o d i u m chloride u n d e r argon, b y m i x i n g this salt w i t h m o l t e n s o d i u m at temperatures b e l o w 500° C . T h e c h l o r i d e picks u p f r o m 2 to 10% s o d i u m as a film. T h i s m e t a l reacts w i t h


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the instilled t i t a n i u m tetrachloride a n d forms a finely d i v i d e d , p y r o p h o r i c m e t a l powder. T h i s m a t e r i a l is suitable as a catalyst, but on leaching it y i e l d s o n l y o x i d i z e d metal. C o n s o l i d a t i o n of the m e t a l crystals either b y m e l t i n g the salt a n d separating it b y v a c u u m d i s t i l l i n g f r o m the cake, or b y heating the latter above 800° C. before leaching, is necessary. T h e r e d u c t i o n must take place u n d e r specific conditions. F i r s t , to dissipate the heat, m i x i n g is necessary; the reaction temperature must be controlled a n d kept l o w enough so that no agglomerating of the s o d i u m chloride takes place. F u r t h e r m o r e , there must always be a n excess of sodium, because fusion occurs due to the a b o v e - m e n t i o n e d l o w - m e l t i n g m i x tures of s o d i u m chloride w i t h l o w e r t i t a n i u m chlorides. T h i s w a y of operating seems usable, but it is d o u b t f u l what advantages, besides c o n t r o l l e d heat d i s s i p a tion, it w o u l d offer over a r e d u c t i o n above the m e l t i n g point of s o d i u m chloride, a n d below the b o i l i n g point of s o d i u m . T h i s second m e t h o d of r e d u c i n g is p r o b a b l y the most c o m m o n l y used. Since the temperature m a r g i n between the m e l t i n g point of s o d i u m chloride a n d the b o i l i n g point of s o d i u m is o n l y 80° C , sharp temperature control is needed. T h i s means that heat i n p u t a n d dissipation must be c a r e f u l l y correlated. C o n t r o l of the heat i n p u t concerns the furnace, w h i c h c a n easily be regulated, a n d the heat of reaction, w h i c h depends on the amount of s o d i u m a n d t i t a n i u m tetrachloride brought to reaction i n the u n i t of time. T h e chloride can easily be metered, but reliable m e a s u r i n g a n d p u m p i n g devices for s o d i u m are still f a m i l i a r to o n l y a few. If both reagents are used together, the m e t e r i n g devices for b o t h must be correlated to take care of the stoichiometric proportions. Injection of s o d i u m a n d i n s t i l l i n g of t i t a n i u m c h l o r i d e presume that the problems of p l u g g i n g u p of the nozzles can be satisfactorily solved. T h e s o d i u m inlet m a y f o u l u p o n account of gaseous t i t a n i u m tetrachloride i n the reactor atmosphere, reacting w i t h the s o d i u m at the injection point; the inlet for the chloride m a y become obstructed because of the h i g h v a p o r pressure of s o d i u m at the operation temperature, w h i c h fills the r o o m as a gas w h e n e v e r t i t a n i u m c h l o r i d e becomes deficient. W h e n a l l of the s o d i u m needed is i n t r o d u c e d at once, before the operation is started, the c o n t r o l of the reagent extends o n l y to the chloride, w h i c h can easily be checked. N o b l e gas is used to stop a r u n a w a y , as i n m a g n e s i u m r e d u c t i o n . H e a t dissipation m a y be controlled b y air cooling of the reactor shell, c i r c u l a t i o n a n d cooling w i t h the noble gas, a n d e v e n t u a l l y w i t h the s o d i u m chloride p r o d u c e d . If a l i n e r is used inside the reactor, heat dissipation becomes aleatory because of the gas gap between the c r u c i b l e a n d the shell. W h e n the noble gas is c i r c u l a t e d a n d cooled i n v i e w of heat control, it might be loaded u p w i t h s o d i u m vapor, as proposed b y L e v y (54) i n m a g n e s i u m r e duction. S u c h a gas phase r e d u c t i o n process is a v a r i a t i o n of the t h i r d possibility m e n t i o n e d a b o v e — u s i n g s o d i u m . L e v y ' s proposition is of interest, because it w o u l d p e r m i t operation b e l o w the b o i l i n g point of sodium, but, on account of the l o w p a r t i a l pressure a n d s m a l l concentration of the r e d u c i n g agent i n the noble gas, large v o l u m e s of the latter have to be p u m p e d a r o u n d . It w o u l d t h e r e fore seem that the gas phase r e d u c t i o n c o u l d be more efficiently conducted above the b o i l i n g point of sodium, a n d b e l o w the reaction temperature of t i t a n i u m w i t h i r o n , w h i c h is a p p r o x i m a t e l y 975° C . T h i s temperature m a r g i n c o u l d be extended u p w a r d s , if t i t a n i u m c o u l d be used as a l i n e r i n the reaction r o o m . T h i s depends o n w h e t h e r the m e t a l w o u l d resist the action of t i t a n i u m tetrachloride at h i g h temperature, w h e r e it m i g h t be c o r r o d e d rather q u i c k l y b y f o r m a t i o n of l o w e r chlorides. G a s phase r e d u c t i o n offers the possibility of p r o d u c i n g highest p u r i t y metal, because the r e d u c i n g agent is purified b y evaporation just before it reacts. S o d i u m must be purified at least b y filtration to eliminate the oxide before b e i n g used (30). D e o x i d a t i o n b y passing it over c a l c i u m turnings at 500° C , or b y distillation, is advisable. S o m e variations of the s o d i u m r e d u c t i o n process m a y be considered. I m p e r i a l C h e m i c a l Industries use a n a l l o y of potassium a n d s o d i u m as a r e d u c i n g agent (60). T h i s alloy, w h i c h c a n be p r o d u c e d b y c h e m i c a l methods as s h o w n above a n d perhaps b y fusion electrolysis (4), is a l i q u i d at r o o m temperature i n the composition range between 10 a n d 60% sodium, a n d it ignites spontaneously w h e n exposed to the atmosphere. T h e temperature m a r g i n i n the r e d u c t i o n w i t h


151


Table V. Comparison of Sodium ai id Magnesium as Reducing Agents for Titanium Tetrachloride SODIUM

MAGNESIUM

H i g h activity, so that excess needed for c o m plete reduction c a n be h e l d below 2%

Excess of at least 15% needed, mostly récupérable w h e n chloride is separated f r o m sponge b y v a c u u m distillation, b u t lost i n leaching E a s y surface cleaning of ingots b y p i c k l i n g


Purification of sodium for oxide r e m o v a l b y filtration or distillation, necessary a n d not too easy E n e r g y requirements i n electrolytic sodium p r o d u c t i o n £ higher t h a n those for m a g n e s i u m . H e a t evolution i n r e d u c t i o n à higher t h a n w i t h magnesium, w h i c h creates heat dissipation problems T e m p e r a t u r e m a r g i n for r e d u c t i o n small Waste N a C l obtained i n leaching or after t a p p i n g is of l o w value Reaction temperature c o n t r o l difficult o n a c count of l o w m a r g i n i n the r e d u c t i o n N a C l p r o d u c e d is not hydroscopic S o d i u m does not alloy w i t h t i t a n i u m

L e a c h i n g of sponge w i t h m i n i m u m of acid is possible, as excess of free sodium is s m a l l R e m o v a l of sponge f r o m reactor is easier, as it is impregnated b y little sodium, a soft metal

L e a c h i n g is cheaper t h a n v a c u u m distillation, w h i c h c o u l d be applied N o excess sodium p r o d u c t i o n capacity is a v a i l able i n U . S . to recycle N a C l or m a k e u p for s o d i u m losses A f t e r leaching, sponge picks u p only little h y d r o g e n , as only s m a l l quantities of a c i d are used

O x y g e n p i c k e d u p d u r i n g leaching of sponge increases its hardness L e a c h e d sponge h a s l o w e r d e n s i t y t h a n v a c u u m - d i s t i l l e d metal, c a n more r e a d i l y be f o r m e d to electrodes f o r a r c m e l t i n g Condensate of N a C l deposited i n a r c m e l t i n g furnace is not hygroscopic a n d less p y r o p h o r i c o n account of l o w s o d i u m content Expenses for sodium as r e d u c i n g agent, o n basis of U . S . prices a n d v a l e n c y , p e r p o u n d of t i t a n i u m p r o d u c e d , i d e n t i c a l w i t h those of m a g n e s i u m . R e c y c l i n g of s o d i u m chloride p r o d u c e d , t h r o u g h fusion electrolysis cell, offers no appreciable p r i c e saving

P o w e r consumption i n p r o d u c i n g m e t a l a n d heat evolution i n its use h lower t h a n w i t h sodium L a r g e temperature m a r g i n for the r e d u c t i o n and lower m e l t i n g point of the M g C k p r o d u c e d compared w i t h N a C l A n h y d r o u s M g C b obtained b y t a p p i n g a n d v a c u u m distillation is valuable, c a n be r e c y c l e d i n a fusion electrolysis, l o w e r i n g cost of r e d u c i n g agent b y à M u c h larger temperature m a r g i n makes t e m perature control easy Deliquescent MgCL?, left b e h i n d i n sponge, m a y cause moisture p i c k u p o n storing a n d spattering i n m e l t i n g M a g n e s i u m alloys w i t h t i t a n i u m a n d lowers corrosion resistance. Some remains i n m e t a l after leaching, w h i c h m a y cause oxidation on storing. V a c u u m distillation removes magnesium If leaching of sponge is chosen, large q u a n t i ties of acid are r e q u i r e d to dissolve more t h a n 15% excess m a g n e s i u m present. E n trapped chloride is lost b y leaching H e a v y lathe is r e q u i r e d to remove sponge a n d m a k e suitable chips f o r leaching. Sponge is impregnated w i t h at least 15% of m e c h a n i cally strong m a g n e s i u m . If sponge is treated directly b y v a c u u m distillation, r e m o v a l after this operation is even more difficult because it is sintered V a c u u m distillation produces a denser sponge of lower hardness t h a n leached one Excess electrolysis capacity available for r e c y c l i n g anhydrous M g C b . T h e r m a l m a g n e s i u m r e d u c t i o n capacity obtainable to m a k e u p for magnesium losses If l e a c h i n g of the sponge is chosen, this is h e a v i l y loaded u p w i t h h y d r o g e n because of large amount of m a g n e s i u m removed. L e a c h e d sponge retains at least 0.2% M g . V a c u u m distilled sponge is p r a c t i c a l l y free of h y d r o g e n a n d m a g n e s i u m M e t a l of hardness below 105 B r i n e l l c a n be obtained b y v a c u u m distillation of sponge V a c u u m - d i s t i l l e d sponge is v e r y dense a n d m o r e difficult to f o r m to consumable e l e c trodes t h a n leached one T r a s h condensate i n a r c furnace is h y g r o scopic because of presence of M g C b , a n d rather p y r o p h o r i c w i t h leached sponge w h i c h contains a p p r e c i a b l e amounts of magnesium P r i c e advantage f o r m a g n e s i u m a m o u n t i n g to up to one t h i r d over sodium, c a n be e x pected w h e n r e c y c l i n g a n h y d r o u s m a g n e s i u m chloride o n a sufficiently large scale t h r o u g h fusion electrolysis cell, w h i c h also yields a usable chlorine

this alloy c a n be increased b y 150° C . w h e n operating so as to obtain the eutectic salt m i x t u r e , w h i c h melts at 650° C . T h e r e arises the p r o b l e m of potassium c h l o r i d e - s o d i u m chloride separation, mentioned above. I n p l a i n s o d i u m r e d u c tion the I m p e r i a l C h e m i c a l Industries suggests leaching excess s o d i u m a n d the s o d i u m chloride p r o d u c e d w i t h l i q u i d a m m o n i a (61 ), w h i c h dissolves both. It is questionable whether this m e t h o d is usable, as a n y l o w e r t i t a n i u m chloride left b e h i n d i n the cake w i l l l e a d to f o r m a t i o n of n i t r i d e d compounds, w h i c h w i l l introduce n i t r o g e n i n the sponge. R e d u c t i o n i n t w o steps, b e l o w a n d above the


ADVANCES

152

IN CHEMISTRY SERIES


m e l t i n g point of s o d i u m chloride, offers the advantage of heat control b y fusion of this salt. K i n g s b u r y (36) a n d Glasser (22) first p r o d u c e d the above described r e a d i l y fusible l o w e r chloride m i x t u r e w i t h s o d i u m chloride, a n d used this salt, after solidification i n a second r e d u c t i o n step w i t h excess s o d i u m . I m p e r i a l C h e m i c a l s Industries use almost the same process b u t p r o d u c e m e t a l a n d c o n tinue the operation i n the same heat at temperatures above 800° C . to coalesce the m e t a l particles. A n u m b e r of patents refer to the use of s o d i u m a m a l g a m as a cheap l o w temperature r e d u c i n g agent. G l a s s e r a n d H a m p e l (23) a n d I m p e r i a l C h e m i c a l s Industries (65) indicate h o w to use the a m a l g a m . T h e m a i n d r a w b a c k s of this m e t h o d a r e the h e a l t h h a z a r d w i t h hot m e r c u r y , the p y r o p h o r i c properties of the i n t e r m e d i a r y products, a n d the necessity of u p g r a d i n g the a m a l g a m before its use, b y v a c u u m distillation a n d e l i m i n a t i o n of excess m e r c u r y . T h i s proposition has been discussed i n m o r e detail elsewhere (42). T h e reader w o u l d , of course, l i k e to k n o w w h a t advantages s o d i u m w o u l d offer w h e n substituted f o r m a g n e s i u m as a r e d u c i n g agent for t i t a n i u m chloride. T o facilitate s u r v e y i n g this r a t h e r c o m p l e x question, T a b l e V shows the a d vantages a n d disadvantages of b o t h reagents. T a b l e V appears rather disconcerting because, for a cost calculation, the price of the r e d u c i n g agents has to be balanced against technological advantages d e r i v e d f r o m their use. These advantages cannot easily be expressed i n m o n e y v a l u e a n d they c a n be estimated correctly o n l y b y those w h o operate t h e plants. It w o u l d seem, however, that the p r i c e p a i d f o r the r e d u c i n g agents does not, at present, p l a y a n y i m p o r t a n t part i n the p r o d u c t i o n cost of the sponge, as the a n h y d r o u s t i t a n i u m chloride is the biggest p r o d u c t i o n cost item, representing about 4 8 % of the total expense, c o m p a r e d w i t h 9 to 10% for the s o d i u m or m a g n e s i u m . T h a t leaves b u t little m a r g i n for possible savings, i n switching f r o m m a g n e s i u m to sodium, w h i l e there is p l e n t y of r o o m for l o w e r i n g the p r o d u c t i o n cost of t i t a n i u m tetrachloride. T h i s m a y be different later, w h e n the price of the chloride has h i t its lowest l e v e l a n d competition imposes savings i n a l l phases of the process.

Conclusions T h i s report has s h o w n w h e r e s o d i u m is b e i n g used or m i g h t be e m p l o y e d i n m e t a l l u r g y , a n d w h e r e it s h o u l d p l a y a m u c h larger part t h a n at present. H a n d l i n g difficulties are not serious. T h e development w o r k done i n atomic energy w i t h s o d i u m as a heat transfer agent, w i t h purification a n d p u m p i n g methods w i l l ease the task of those w h o consider the use of s o d i u m as a cleanser i n m e t a l p u r i f i c a t i o n or as a r e d u c i n g agent i n m e t a l w i n n i n g .

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handling and uses of the alkali metals - American Chemical Society

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