USNRDL-TR-847 I8 April 1965

ION-EXCHANGE PROCESSES BETWEEN IMMISCIBLE MOLTEN PHASES

by R . C. Sche id t E. C. Freiling

U.S. N A V A L R A D I O L O G I C A L D E F E N S E L A B O R A T O R Y

S A M F R A N C I S C O C A L I F O R N I A 9 4 1 3 5

PHYSICAL C-STRY BRANCH E. C. Freiling, Head

CHEMICAL TECHNOLOGY DIVISION R. Cole, Head

ADMINISTRATNE INFORMATION

The work requested is part of a project sponsored by the Atomic Energy Commission under Contract No. AT-(49-2)-=67.

We are grateful to Messrs. R o b e r t Cochran and Robert Brawnlee for experimental assist- ance,

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ABSTRACT

T O test the hypothesis t h a t cat ion d i s t r ibu t ion i n (alkali oxide)- (boron oxide) - ( a lka l i hal ide) systems occurs by ion-exchange processes, some d e f i n i t i v e experiments were devised and carr ied out. ments consisted of equi l ibra t ing Na22-labelled sodium borate with K C 1 a t 83OOc. Higher concentrations of K+ than C1' i n the borate phase and Na' than BO2 i n t h e s a l t phase established the operation of cation- exchange processes i n the immiscible molten system.

The experi-

i

SUMMAR'Y

If Na22-labelled sodium borate i s equi l ibrated w i t h K C 1 a t 83OoC, it is found t h a t t h e concentration of t h e foreign cat ion always exceeded the concentration of t h e foreign anion i n the immiscible phase being con- sidered, i .e . , i n the oxide phase the concentration of K+ exceeds t h a t of C1' while i n the sal t phase Na' exceeds t h e concentration of Bo:. This observation confirms the existence of ion-exchange processes pre- viously postulated t o occur i n molten a l k a l i hal ide-alkal i borate systems.

ii

BACKGROUND

2 Stglhane’ and Dunicz and Scheidt have studied the misc ib i l i t y of molten a l k a l i borates with molten a l k a l i hal ides (except f luor ides) i n t h e region 800-9800~. (boron oxide) show misc ib i l i ty gaps i n t h i s temperature range a t low alkali-oxide contents. s t ruc tures f o r a l k a l i borate glasses which s t rongly resemble those of synthet ic , organic ion exchangers. Specif ical ly , they a r e visualized as mobile cations, a t t r ac t ed by coulombic forces t o 1ocal.izations of negative charge, the l a t t e r being part of a th ree dimensional network of covalently bonded atoms.

The ternary systems ( a l k a l i oxide) - ( a l k a l i hal ide) - Biscoe and Warren,3 among others, have postulated

4 Adam and Quan and Krogh-Moe’ have fu r the r assumed t h a t such s t ruc- t u re s p e r s i s t when the glasses a re molten. Therefore, current models of l iquid borate s t ruc ture suggest that the immiscible phases of a l k a l i borate a re e s sen t i a l ly molten ion-exchangers i n contact with molten e lec t ro ly tes . This in te rpre ta t ion i s supported by the d i s t r ibu t ion co- e f f i c i e n t s of a l k a l i , a lkal ine-ear th , and rare-ear th cations reported by Rowell.6 Rowell’s r e s u l t s show that, i n ce r t a in regions o f a l k a l i oxide content, d i s t r ibu t ion coef f ic ien ts resemble those obtained between Dmex-50 and d i l u t e HC1, not only i n order of s e l e c t i v i t y but a l s o i n magnitude.

This report first describes an ion-exchange mechanism f o r t h e d i s - t r i b u t i o n of cations i n these systems. It then presents the r a t iona le f o r t e s t i n g the mechanism, and f i n a l l y , presents da ta t o support the proposed in te rpre ta t ion .

PROPOSED MECHANISM

Figure 1 shows a typ ica l mi sc ib i l i t y diagram f o r the systems i n question, plot ted i n terms of components of t he type B20 , MB02 and MX (M=alkali metal, X=C1, Br or I). The important fea ture af t h i s diagram

1

(K, Na) BO,

i

NRDL 14-65

.

( K , Na) Cl

Fig. 1 Compositions Tested for Ion-Exchange Processes (830'~)

2

f o r t he subject at hand i s that t h e misc ib i l i t y gap fa l l s i n t o two regions and this i s indicated by both the behavior of t he t i e l i n e s and the shape of t he curve.

For brevity, w e w i l l c a l l t h e region of law alkali metaborate con- t e n t region A. This region extends over borate compositions ranging from pure boric oxide t o approximately M O*5B203. I n t h i s region there is appreciable a l k a l i hal ide so lub i l i t y fi .e., e l ec t ro ly t e penetration) i n t h e oxide (exchanger) phase. alkali hal ide corner show t h a t t h e so lub i l i t y of t h e oxide (exchanger) i n t h e hal ide phase (co-phase) i s s l igh t . of t he soluble species i n the sodium chloride system t o be B2O3 and the s o l u b i l i t y t o be 0.04 mole $.6

The convergence of the t i e l i n e s a t the

R o w e l 1 determined the formula

Region B extends from the M20*5B20

by the sharply increasing borate so lub i l i t y . found NaBO2 dissolving a t compositions more basic than M ~ O O ~ B ~ O

M20 3B2O3 6

composition t o the p l a i t point. It shows increasing e l ec t ro ly t e penetra 2 ion but i s characterized b e t t e r

polyborate species, such a s MB305, a t compositions more basic t 2 an

I n t h e NaCl system Rowel1 and

I n view of these f a c t s the most reasonable mechanism by which ion- exchange processes can proceed between the oxide and ha l ide phases i n region A appears t o be t h a t shown i n Fig. 2. Here t h e convention i s used t o indicate an oxygen atom shared between two boron atoms, only one of which i s indicated. The f igure i s otherwise self-explanatory. I n region B t h e f igure should be modified t o show the presence of (M+) - Bo2(B203); ion pa i rs and s imi la r s t ruc tures which a r i s e from the break- down of t h e borate s t ruc ture and d issoc ia te i n the hal ide phase.

RATIONAG3

The foregoing considerations ind ica te ce r t a in differences from the familiar acpeous ion exchange system: there is no neut ra l phase corres- ponding t o water (o r organic solvent); there a re no conditions of negl i - g ib l e e l ec t ro ly t e penetration and under most conditions penetration is high; a t a l l compositions, there i s a sensible exchanger so lub i l i t y , and a t some compositions t h i s s o l u b i l i t y i s appreciable. I n view of these differences, it i s w o r t b h i l e c la r i fy ing what w e mean by "ion exchange" i n general before proceeding.

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OXIDE PHASE SALT PHASE

M+ X'

Rapid exchange equili brium

. , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I 1 controlled

t l M+ X- ion pairs dissolved in oxide phase equilibrium

-+ Slow, diffusion

Fig. 2 Systems a t Low Alkali Oxide Concentrations.

Postulated Behavior of (Boron Oxide) - ( A l k a l i Oxide) - ( A l k a l i Halide)

4

There i s no de f in i t i on which has y e t received universal acceptance by workers i n t h e f i e ld . appear t o represent what workers i n t h e f i e l d mean i n t h e i r correspond- ence :

However, the following set of def in i t ions

Ion-exchange mechanism: A mechanism which produces a heterogeneous, ion ic metathesis.

Ion-exchange process: A chemical process which proceeds by an ion-exchange mechanism.

Ion-exchanger process: A chemical process carr ied out by an ion exchanger(e.g., ca ta lys i s , ion exclusion, s i t e sharing, e tc . , including ion-exchange processes).

This de f in i t i on of ion-exchange mechanism is operational and appl i - cable t o any heterogeneous system.

Returning now t o Fig. 2, suppose a so lu te hal ide NX i s introduced i n t o the sa l t phase. and the MX dissolved

This c& d i s t r i b u t e i t s e l f between t h e sal t phase i n t h e oxide phase according t o t h e e q i l i b r i u m .

where a ind ica tes thermodynamic a c t i v i t y and subscripts 0 and S ind ica te the oxxde and salt phases respectively. occurring, t h e concentration C&+ of equal t o or less than - the concentration of X'. However, i f ion ex-

If t h i s were the onlyprocess i n the oxide phase would be c l ea r ly

change occurs there i s % he addi t iona l equilibrium

This ecpilibrium permits addi t iona l N+ t o be concentrated i n t h e oxide phase and the poss ib i l i t y that t h e oxide phase concentration of N+ w i l l exceed tha t of X'. Therefore, a su f f i c i en t , but not necessary, condition f o r proving the occurrence of an ion-exchange process i s tha t

(cN+)o> (cx-)o

The poss ib i l i t y that exchange occur i n the sal t phase subsequent t o d isso lu t ion of oxide as MBo2 leads t o t h e same r e su l t s . phase consis ts of pure NX, s imi la r considerations lead t o t h e a l t e rna t ive condition

If t h e hal ide

5

The experiments described here t e s t t he presence of these r e l a t ion - ships a t various points i n regions A and B by equi l ibra t ing molten phases of composition NagO.x B 0 ' w i t h molten K C 1 a t 8 3 0 " ~ . 2 3

ExmmNTAL

Equil ibrat ion charges were prepared from reagent grade chemicals. Sodium borate glasses of known composition were label led with known quant i t ies of sodim-22, t h e pur i ty of which had been ver i f ied by gamma- ray spectrometry. Labelled glasses were dehydrated with a Meker burner. Before equi l ibrat ion, they were fu r the r dried f o r 15 minutes a t WOoC and then equi l ibrated with potassium chloride a t 830 f 5OC. The overa l l K C 1 composition was 54 mole % i n each case. Ecpilibr&ions were carried out i n graphi te crucibles under a dry argon atmosphere w i t h mechanical s t i r r i n g . Equi l ibrat ions las ted 95 minutes. The crucible was then r e - moved f r o m t h e furnace and cooled rapidly i n a j e t of cold air. Crucibles were sawed open and severa l samples of each phase were removed f o r analy- sis. Samples were analyzed i n t r i p l i c a t e . Alkal i oxide was determined by t i t r a t i o n with 0.1 N hydrochloric acid t o the methyl-red end point. Boron oxide was d e t e d n e d by conversion t o mannitoboric acid and subse- quent t i t r a t i o n with 0.1 N sodium hydroxide t o the phenolphthalein end point. by t h e Volhard method. mercuric n i t r a t e and a mixed indicator (bromophenol blue, diphenyl car- bazone and xylene cyanole FF i n 95 % ethanol) gave more accurate r e su l t s . Sodium was determined radiometrically and potassium was calculated by difference.

Samples of high sglt concentration were analyzed f o r chloride For low sal t concentrations, a method using

RESULTS AND DISCUSSION

Figure 1 shows the region of the misc ib i l i t y diagram where each r u n was carr ied out.

6

Figure 3 shows t h e values of C&+ and C i n t h e oxide phase for each run. ( regardless of associated cat ion) t h a t would be present i n t h e oxide phase were no chlor ide present. This i s a convenient measure of posi- t i o n i n t h e misc ib i l i t y diagram f o r comparing r e s u l t s between systems containing d i f f e r e n t a l k a l i halides. The higher value of C&+ i n every case c l ea r ly established the operation of cation-exchange processes.

The abscissa i n t h i s f igure i s @?-mole f r ac t ion of BO:

The r e s u l t s of Fig. 4 confirm the above conclusion. This f i g u r e presents t h e complementary data f o r sa+ and 5 0 : i n t h e s a l t phase. Again t h e concentration of t h e fore ign ca t ion exceeds t h a t of t he foreign anion f o r every case.

Final ly , it is of i n t e r e s t t o compare these da t a with Rowell's r e s u l t s f o r t h e d i s t r i b u t i o n of t r a c e r Rb+ and Cs+ i n t he system Nag0 - N a C l - B20 i n region A, t h e 2 arger ca t ion always showed t h e g rea t e r preference f o r the oxide phase. t he l a rge r ca t ion showed t h e g rea t e r preference for t h e s a l t phase. Figure 5 shows t h e d i s t r i b u t i o n da ta i n t h e present case p lo t ted as

a t 830Oc. Rowell found that, for Na+, Rb' and Cs+

This s i t u a t i o n was exact ly reversed i n region B: here

f o r Na' and k?. appears t o be coincidental.

The occurrence of t h i s reversal a t t h e region boundary

The f i g u r e shows t h e same qualitative behavior as t h a t found by T h a t is, R o w e l l i n s p i t e of t h e increased loading of the exchanger.

t h e l a rge r ca t ion had t h e g rea t e r a f f i n i t y f o r t h e oxide phase i n region A, but t h e l e s s e r a f f i n i t y i n region B. The occurrence of t h e reversal a t t h e region boundary appears t o be coincidental .

7

NRDL 14-65

I I

P - 10 L POTASSIUM E

I- /

E

+ Fig. 3 Concentrations of K and C1- Ions i n t h e Oxide Phase (830'~)

8

NRDL 14-65 IO

PLAIT

a

a

f

k-

m

0.1 a LL I- z

z 0

r BORATE ION CONCENTRATION :.”’

0.01 d

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

(cBo-) /[(c,o-90+ (CS l o ] 2 0 2 3

4- Fig. 4 Concentrations of Na and BO; Ions in the Salt Phase ( 8 3 0 ’ ~ )

9

1 .o

n Y

0.1

0.01

,

NRDL

I I I I I I I I I -

-

-Region A d b - Region B -d 0.2 0.4 0.6 0.8 1 .o

(CBOz-)O ’ [‘CBOF)O -k (cB$s)O]

+ + Fig. 5 Distribution of K and Na Ions Between Immiscible Phases in the System (K, Na) B02-B203-(K, Na) C1 at 830’~.

10

0 1.

2.

B. L. Stalhane, Z Electrochem., 35; 486 (1929); - 36, 404 (1930).

B. L. Dunicz and R. C . Scheidt, "Solute Distribution i n the Na20- B203-NaCl System. 111. Immiscibility Diagrams of Molten Sodium Halide-Sodium Polyborates . " USNRDL-TR-752, 22 May 1964 . J. Biscoe and B. E. Warren, J. Am. Ceram. SOC., 21, 287 (1938). 3.

4. C. E. Adams and J. T. b a n , "Vapor Pressures i n the Liquid System Rb20- +O uS&L-m-566, 6 June 1962, t o be submitted t o J. Phys. Chem.

J. Krogh-Moe, Phys. -IC and Chem. - of Glasses, 3, 101 (1962).

Derived Thermodynamic Data and a Structural Interpretation."

5.

6. M. Rowell, "Solute Distribution i n the Na20-B20 -NaC1 System. I. Alkali Metals." USNRDL-TR-~~~, 9 October 1962; $1, Alkaline Earths and Rare Earths, USNRDL-TR-760, 12 June 1964, t o be submitted t o J. Inorg. Chem.

11

r

0 J

F

4 V

Chemistry

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Chief, Bureau of Ships (Codes 320-364A) Chief, Bureau of Ships (Code 210L) Director, Naval Research Laboratory Chief of NavaUResearch (Code 422) CO, Office of Naval Research, FPO, New York Supt . , Naval Postgraduate School, Monterey Commander, Mare Island Naval Snipyard (Library)

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1 O R I G I N A T I N G A C T I V I T Y (Corporate author) 2 a R E P O R T S E C U R I T Y C L A S S I F I C A T I O N

U. S. Naval Radiological Defense Laboratory UNC LASS D I E D San Francisco, Cal i forn ia 94135 2 6 G R O U P

None 3 R E P O R T T I T L E

ION-EXCHANGE PROCESSES BEWEEN IMMISCIBLE MOLTEN PHASES

L. D E S C R I P T I V E N O T E S (Type of report and inclusive detee)

j . AUTHOR(S1 (Lest name, ffraf name, inltiel)

Scheidt, Ronald C. F re i l ing , Edward C.

i. R E P 0 R T D A T E 7 a . T O T A L N O . O F P A G E S 7 6 . N O . O F R E F S

25 June 1965 17 6 le. C O N T R A C T O R G R A N T N O . 9 8 . O R I G I N A T O R ' S R E P O R T NUMSER(S)

USNRDL-TR-847 Contract No. AT-(49-2)-1167 b. P R O J E C T N O .

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3. A B S T R A C T

To tes t t h e hypothesis t h a t ca t ion d i s t r ibu t ion i n ( a l k a l i oxide)-(boron oxide)-(alkal i halide) systems occurs by ion-exchange processes, some def in i t ive experiments were devised and c k r i e d out. b ra t ing Na22-labelled sodium borate with K C 1 a t 830%. of K+ than C1' i n the borate phase and Na' than BG i n t h e sal t phase es tab l i shed t h e operation of cation-exchange processes i n t h e immiscible molten system.

The experiments consis ted of equ i l i - Higher concentrations

UNC LASS D I E D Security Classification

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R O L E W T

Fused sal t Molten salt ion exchange Immiscibility diagram phase equilibrium Ion distribution

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