HANDLING AND USES OF THE ALKALI METALS

The Manufacture of Potassium and NaK C. B. J A C K S O N and R. C. WERNER

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.ch018

Mine Safety Appliances Co., Callery, Pa.

Metallic potassium and sodium-potassium alloys (NaK) are manufactured by the reaction of high temperature sodium at atmospheric pressure with molten potassium chloride. Early operations of a batch process have been succeeded by a continuous one in which either pure potassium or sodium-potassium alloy of any desired composition can be produced. Molten potassium chloride is introduced into a packed column and brought in contact with ascending sodium vapors in a reaction zone to produce an equilibrium vapor of sodium and potassium. A fractionating column above the reaction zone separates the lighter boiling potassium to any degree of purity desired. The sodium chloride formed is continuously withdrawn from below the reaction zone.

S o m e 20 years ago, the N a v a l Research L a b o r a t o r y , i n search for a r e a d y s u p p l y of o x y g e n f o r self-contained b r e a t h i n g apparatus, investigated the use of potass i u m superoxide. T e c h n i c a l l y , a l l signs seemed to point to metallic s o d i u m for the p r o d u c t i o n of potassium f r o m its compounds as a step i n the p r o d u c t i o n of potassium s u p e r oxide. S o d i u m is c o m m e r c i a l l y p r e p a r e d b y the electrolysis (1) of m o l t e n s o d i u m chloride to w h i c h c a l c i u m c h l o r i d e has been added to l o w e r the m e l t i n g point. T h e analogous process c o u l d not be used for potassium p r o d u c t i o n (7) because the potassium w i l l attack the graphite electrodes a n d because of the danger of explosion due to potassium c a r b o n y l sometimes f o r m e d i n the process. R a t h e r t h a n w o r k o n alternate electrodes of other m a t e r i a l , a t h e r m o c h e m i c a l process was developed, u s i n g the r e d u c t i o n of a potassium salt b y sodium. O t h e r p r o cesses (4) w e r e investigated b y K r a u s . R i n c k (6), f r o m comprehensive investigations, described the e q u i l i b r i a b e t w e e n m o l t e n salts a n d metals i n the a l k a l i a n d a l k a l i n e e a r t h groups. P o t a s s i u m chloride was selected for the t h e r m o c h e m i c a l reduction o n the basis of a v a i l a b i l i t y a n d price p e r unit of contained potassium.

Batch Process T h e i n i t i a l process was a batch reaction of s o d i u m w i t h potassium chloride to produce N a K , w h i c h was later fractionated b y distillation to produce p u r e potass i u m . F i g u r e 1 is a schematic d i a g r a m s h o w i n g the gas-fired reaction vessel, the a i r - c o o l e d condenser, the salt trap, a n d the s o d i u m - p o t a s s i u m collector. T h e 169

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

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v e r t i c a l neck of the reaction vessel was fitted w i t h a c h a r g i n g p l u g . T h e reaction vessel, neck a n d condenser line were of A I S I T y p e 316 stainless steel, the r e m a i n der of the system b e i n g m i l d steel. T h e fractional distillation equipment s h o w n i n F i g u r e 2 consisted of a gasfired still pot, a 14-foot distillation c o l u m n , a n a i r - c o o l e d condenser, a collector for the potassium, a n d a collector for sodium. T h e c o l u m n was p a c k e d w i t h 0.25i n c h diameter stainless steel R a s c h i g rings contained between perforated baskets. T h e components a n d connecting p i p i n g are of A I S I T y p e 316 stainless steel.


D u r i n g operation 150 pounds of 1500°F. m o l t e n potassium chloride was charged to the preheated reaction vessel, followed b y 23 pounds of b r i c k s o d i u m to give a 2 to 1 mole ratio of potassium chloride to sodium. N i t r o g e n gas was i n t r o d u c e d t h r o u g h a l i n e o n the p l u g to keep air f r o m entering the system. A s f o u n d b y R i n c k , the e q u i l i b r i u m N a - f K C 1 ^± Κ + N a C l was established r a p i d l y , even though a salt a n d a m e t a l l a y e r were formed. T h i s attainment of e q u i l i b r i u m was p r o b a b l y aided b y the p a r t i a l m u t u a l s o l u b i l i t y of one phase into the other, w h i c h is appreciable at the h i g h temperatures e m p l o y e d . W i t h continued heating of the reaction vessel, the more volatile metallic phase distilled f r o m the salt phase. T h e r e was considerable v a p o r condensation

Figure 1. Sodium-potassium

Figure 2.

still

Fractionating column


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a n d r e f l u x i n g i n the uninsulated neck, the o v e r - a l l effect b e i n g better t h a n a o n e plate distillation separation between the potassium a n d the s o d i u m . R i n c k h a d s h o w n that the static e q u i l i b r i u m obtained w i t h the i n i t i a l m o l e ratio of reactants w o u l d give a m e t a l l i c phase containing 34 atom % potassium. T h e distillate a c t u a l l y contained 47 atom % potassium, w i t h the balance b e i n g p r i m a r i l y s o d i u m . S o m e of the salt phase distilled off at the 1540°F. distillation temperature, g i v i n g a condensate w h i c h was a m i x t u r e of s o d i u m - p o t a s s i u m alloy a n d s o d i u m a n d potassium chlorides. T h i s condensate collected i n the salt trap, w h e r e most of the salts d r o p p e d out of solution at the lower temperature (about 3 0 0 ° F . ) , the r e m a i n i n g s o d i u m - p o t a s s i u m alloy r u n n i n g over into the collector. T h e next step i n p r o d u c i n g p u r e potassium was to separate the s o d i u m - p o t a s s i u m alloy into its components b y fractional distillation at atmospheric pressure. S o d i u m - p o t a s s i u m a l l o y was f e d into the center of the c o l u m n , w h e r e potassium was t a k e n off o v e r h e a d at 1400°F. a n d s o d i u m collected i n the 1650°F. still pot. A d e q u a t e reflux was obtained s i m p l y b y adjusting the heights of the insulation o n the c o l u m n to change the amount of radiant cooling. V a p o r - l i q u i d e q u i l i b r i a for m i x t u r e s of a l k a l i metals have been r e p o r t e d (5). A c c u m u l a t e d s o d i u m was r e m o v e d f r o m the still pot p e r i o d i c a l l y to a n evacuated collector for r e - u s e i n the reaction step, w h e n the frozen s o d i u m p l u g was m e l t e d d u r i n g the heating of the 0.5-inch T y p e 316 stainless steel d r a i n line. T h e pipe temperature rose f r o m 210° to 1600 °F. i n a matter of a few seconds. T h e r e were considerable w r i t h i n g a n d m o v e m e n t of the pipe b u t no ruptures or other serious incidents d u r i n g years of operation of three of these units. T h i s operation is a s t r i k i n g i l l u s t r a t i o n of t h e r m a l shock a n d t h e r m a l stress fatigue withstood b y the stainless steel. T h i s two-step process p r o d u c e d considerable tonnages of metallic potassium at a p r i c e f a r b e l o w that for w h i c h it h a d f o r m e r l y sold. B y c o m b i n i n g the t w o operations into a continuous process (3) the price was l o w e r e d still f u r t h e r .

Continuous Process F i g u r e 3 depicts d i a g r a m m a t i c a l l y the equipment used i n the continuous process. It consists of four basic components, a l l constructed of T y p e 316 stainless steel: a furnace a n d boiler tubes for v a p o r i z i n g sodium, a c o l u m n w i t h the l o w e r p o r t i o n acting as a reaction a n d s t r i p p i n g section a n d the u p p e r p o r t i o n as a fractionating section, a salt feed a n d drainage system, a n d a condensing system. T h e energy requirements of the process are s u p p l i e d f r o m a gas-fired furnace i n w h i c h b o i l e r tubes of 3 - i n c h pipe c o n t a i n i n g s o d i u m are heated b o t h b y c o n vection a n d r a d i a t i o n . T h e boiler tubes are of a n opened h a i r p i n type w e l d e d to the c o l u m n at sufficient angles ( a p p r o x i m a t e l y 3 ° ) to p e r m i t n a t u r a l c i r c u l a t i o n of the s o d i u m . T h e c o l u m n is 18 inches i n diameter b y 21 feet i n length, fabricated f r o m 0.25i n c h plates r o l l e d a n d b u t t - w e l d e d w i t h 720 inches of w e l d . M o l t e n potassium chloride is i n t r o d u c e d into the c o l u m n t h r o u g h a trap. T h e 6-foot fractionating section of the c o l u m n is e q u i p p e d w i t h a 6 - i n c h v a p o r take-off line w h i c h serves as a condenser. E l e c t r o m a g n e t i c alternating c u r r e n t conduction p u m p s are used for reflux feed to the top of the c o l u m n a n d continuous s o d i u m feed to the bottom of the c o l u m n . D u r i n g operation there is continuous i n t r o d u c t i o n of the r a w materials, m o l t e n s o d i u m a n d m o l t e n potassium chloride, to the c o l u m n . T h e s o d i u m is v a p o r i z e d i n the b o i l e r tubes a n d ascends the c o l u m n , c o m i n g into contact w i t h the descending l i q u i d potassium chloride to establish the e q u i l i b r i u m N a + K C 1 ?± Κ + N a C l , r e s u l t i n g i n b o t h s o d i u m a n d potassium vapors. P o t a s s i u m is s e p a r a t e d b y fractionation i n the u p p e r section of the c o l u m n , condensed, a n d c o l lected. S o d i u m chloride is continuously r e m o v e d a n d discarded. T h e c o l u m n is operated slightly above atmospheric pressure t h r o u g h use of traps a n d b y i n t r o d u c i n g nitrogen into the s o d i u m l i q u i d a n d v a p o r phases a n d the m o l t e n s o d i u m c h l o r i d e phase i n the bottom of the c o l u m n . T h i s arrangement serves as a l i q u i d l e v e l device w h e n m e a s u r i n g the n i t r o g e n pressures r e q u i r e d to m a i n t a i n a c o n stant flow t h r o u g h these lines.


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Figure 3.

Continuous process for potassium

If suitable contact of the l i q u i d potassium chloride a n d gaseous s o d i u m is m a i n t a i n e d , a l l the potassium c a n be extracted f r o m the potassium chloride. P r o p e r reflux adjustment assists i n m a x i m u m utilizations of the potassium c h l o r ide, a n d separation of potassium a n d s o d i u m i n the fractionating section. Reflux ratios a r e easily r e g u l a t e d b y a d j u s t i n g the electromagnetic p u m p w h i c h r e t u r n s a p o r t i o n of the product to the c o l u m n . P o t a s s i u m of 9 9 . 5 + % p u r i t y c a n r o u t i n e l y be p r o d u c e d at a rate of a p p r o x i m a t e l y 200 pounds p e r h o u r . A n y desired m i x t u r e of s o d i u m a n d potassium c a n be p r o d u c e d b y c o n t r o l l i n g the c o l u m n operation.

Results I n both processes it was f o u n d that threaded pipe a n d flange joints are u n satisfactory at temperatures above 500 °F. a n d that the a l l - w e l d e d construction of the e q u i p m e n t must be of good q u a l i t y . Contact w i t h s o d i u m i n the temperature range of 1400° to 1700°F. has p r o v e d to be the best test of w e l d i n g quality. S l a g inclusions t h r o u g h a w e l d are i n v a r i a b l y dissolved, r e s u l t i n g i n a leak. L e a k s experienced over the years have been s m a l l a n d i n a l l cases the equipment c o u l d be cooled before serious damage was done. In e v e r y case b u t one, the leaks have been i n welds w i t h the cause attributable to poor w e l d i n g as evidenced b y slag inclusions o r microfissures i n the a s - w e l d e d condition. In the one exception, e x a m i n a t i o n r e v e a l e d that the plate was grossly contaminated w i t h scale w h e n it was r o l l e d . A t no time has there been a n y i n d i c a t i o n of a reaction between s o d i u m , s o d i u m - p o t a s s i u m alloy, or potassium m e t a l w i t h the nitrogen cover gas, even t h o u g h n i t r o g e n is i n constant contact w i t h the a l k a l i metals f r o m 1700°F. d o w n to r o o m temperature. N o special precautions have been taken r e g a r d i n g the p u r i t y of the gas; however, i f large amounts of o x y g e n or water v a p o r entered the system, corrosion w o u l d be increased. W i t h off a n d o n reflux r e t u r n passing t h r o u g h a distributor above the p a c k i n g , it was f o u n d that the distributor w o u l d go to pieces w i t h i n the r e l a t i v e l y short p e r i o d of a w e e k or so. T h i s was u n d o u b t e d l y d u e to repetitious t h e r m a l shocks caused b y r a p i d c o o l i n g f r o m a n i n i t i a l temperature o f 1400 °F. to a l i q u i d potass i u m reflux temperature of 400 °F. It is estimated that several thousand cycles of these transients resulted i n almost complete f a i l u r e of the T y p e 316 stainless steel distributor head. N a t u r a l convection t h r o u g h the boiler tubes, along w i t h some ebullition, was sufficient to produce a heat flux of 15,000 B . t . u . p e r h o u r p e r square foot. T u b e life averaged 1000 hours (2).


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Literature Cited (1) (2)


(3) (4) (5) (6) (7)

D o w n s , J . C., U. S. Patent 1,501,756 (1924). J a c k s o n , C . B . , "Liquid Metals H a n d b o o k , S o d i u m - N a K S u p p l e m e n t , " U . S. G o v e r n m e n t P r i n t i n g Office, Washington, D . C . , 1955. J a c k s o n , C . B., W e r n e r , R. C . , U. S. Patent 2,480,655 (1949). K r a u s , C . Α . , N a v a l Research L a b o r a t o r y Rept. P-2011 (1942). N a v a l Research L a b o r a t o r y Rept. P-2958 (1941). R i n c k , E., Ann. chim. 18, 397 (1932). Smatko, J. S., FIAT F i n a l Report 695, N o . PB-23646, H o b a r t P u b l i s h i n g C o . , Washington, D . C . (1946).


HANDLING AND USES OF THE ALKALI METALS

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