Phase relations in fluoroborate systems—I

J. inorg, nucl.Chem.. 1971,Vol. 33, pp. 337 to 343. PergamonPress. Printedin Great Britain

PHASE

RELATIONS

IN

FLUOROBORATE

SYSTEMS-I

M A T E R I A L P R E P A R A T I O N A N D THE SYSTEMS N a F - N a B F t A N D KF-KBF4*t C J. B A R T O N , L. O. G I L P A T R I C K , J. A. B O R N M A N N $ , H. H. S T O N E , T. N. M c V A Y § and H E R B E R T I N S L E Y § Reactor Chemistry Division, Oak Ridge National Laboratory, O a k Ridge, T e n n . 37830

(Received 22 June 1970)

Abstract-High-purity NaBF4 and KBF4 having higher melting points than any previously reported were prepared. These preparations were used in a reinvestigation of phase relations in the systems N a F - N a B F 4 and KF-KBF4 by means of differential thermal analysis (D.T.A.) and gradient quenching techniques, Both were found to be simple eutectic systems with eutectics melting at 3 8 4 ± 2°C for the NaF-NaBF4 system and 460 ± 2°C for the KF-KBF4 system. INTRODUCTION

INTERES'r in low-melting, low-cost molten salts for use as the coolant in moltensalt breeder reactors prompted reexamination of phase relations in alkali fluoroborate systems. In addition to their cost and melting point advantages, the possibility of removing BF3 from fuel salt accidentally contaminated by coolant leakage in the heat exchanger is also an attractive feature of alkali fluoroborate systems. The system NaBF4-KBF4 was studied earlier at this laboratory [1] but renewed interest was focused initially on the N a F - N a B F 4 system which was reported by Selivanov and Stender[2] to have a eutectic composition melting at 304°C. These investigators also presented a phase diagram for the system K F KBE4 as did Pawlenko[3] who investigated the ternary system K B F 4 - K F KBF3OH. Some earlier unpublished studies[4] at this Laboratory of the N a F - N a B F 4 system gave a value of 382°C for the eutectic temperature, in good agreement with the results of the investigation reported here. Varying melting points reported in the literature for the compounds NaBF4 and KBF4 indicated the need for pure compounds and for care in heating them to prevent hydrolysis or loss of BF3. We will cover in this paper our experience in preparing NaBF4 and KBF4 and the results of our investigation of phase relations in the systems N a F - N a B F 4 and KF-KBF4. Subsequent papers will cover our study of the ternary system N a F - K F - B F 3 and other fluoroborate systems. *Research sponsored by the U.S. A t o m i c Energy C o m m i s s i o n under contract with U n i o n Carbide Corporation. ?Presented in part before the Inorganic Chemistry Division at the 155th National Meeting of the A m e r i c a n Chemical Society, San Francisco, California (April 1968). :]:Research participant from Lindenwood College, St. Charles, Mo. §Consultant. 1. R. E. Moore, J. G. Surak and W. R. G r i m e s , Phase Diagrams of Nuclear Reactor Materials (Edited by R. E. T h o m a ) , O R N L-2548, p. 25. (Nov. 6, 1959). 2. V . G . Selivanov and V. V. Stender, J. inorg. Chem. USSR 111,447 (1958). 3. S. Pawlenko, Z. anorg, allg. Chem. 336, 172-78 (1965). 4. Roy E. T h o m a and Gordon Hebert, U.S. Pat. 3,448,054 (June 3, 1969). 337

338

C . J . BARTON et al.

EXPERIMENTAL Preparation o f materials The potassium fluoride used in this investigation was optical grade scrap material obtained from the Harshaw Chemical Company. Because of its hygroscopic nature, the large pieces containing initially less than 500 ppm of water were ground to pass a 120 mesh screen in a dry box filled with helium containing less than 10 ppm of water. The ground material was sealed in glass bottles. Purified sodium fluoride was supplied by another investigator at this Laboratory (S. Cantor). He prepared it by melting reagent grade NaF, cooling it slowly, and hand selecting clear pieces from the crushed melt. Potassium tetrafiuoroborate was prepared by the following procedure. 350 g of technical grade KBF4 (General Chemical) was mixed with 5 1. of 2.5 M H F in a polypropylene beaker which was then heated to about 90°C on a steam bath, with stirring. The hot solution was vacuum filtered through filter paper supported by a submerged polyethylene Buchner funnel. The clear solution was allowed to cool overnight and the resulting crystals were separated from the supernatant liquid by filtration, washed with cold water followed by ethanol and dried in air. The crystals were ground to pass a 120-mesh sieve, redried at 110°C, and cooled to room temperature in a vacuum desiccator. An analysis of this material, designated Stock XVII, is shown in Table 1. It melted quite sharply at 570-+ I°C as compared to the highest previously reported literature value of 552°C[1] and the unpublished value[4] of 566°C. A number of methods have been used to prepare sodium tetrafluoroborate, but only two will be described in detail. The aqueous method differed from that used to prepare pure KBF4. Commercial NaBF~ (Harshaw Chemical Company) was dissolved in a minimum amount of distilled water and filtered to remove insoluble impurities such as CaF2 and PbFC1. The filtered solution was made 0-12 M in HF, and evaporated to about 50 per cent of its original volume, where crystals started to appear. It was then cooled to room temperature and allowed to stand overnight. The crystals, about 3-5 mm in length, were filtered, washed with a little cold water, and air dried at room temperature and then ground to pass a 120-mesh sieve and redried at 110 °. This material was stored in glass bottles after cooling to room temperature in a vacuum desiccator. Analysis of a typical batch, designated Stock V-3, is shown in Table 1. Although the melting point of this material (405+ l°C) was higher than any value previously reported for this compound (previous literature high, 368°C[2] and the earlier O R N L figure of 398°C[4]) the differential thermal analysis melting curve displayed a lack of sharpness characteristic of impure compounds, in contrast to the melting behavior of the recrystallized KBF4. An attempt to improve the purity by recrystallization of NaBF4 from the melt by slowly passing a tube filled with the material through a temperature gradient failed to raise the melting point or to produce any significant improvement in the sharpness of the DTA melting curve. A composition (Stock IV, Table 1) prepared by reacting anhydrous N a F with BF3 at 300 to 600°C proved to be nonstoichiometric (excess NaF) and it melted at about 381°C. Table 1. Analysis of sodium and potassium tetrafluoroborate preparations Salt

Stock No.

KBE4 KBF4 KBF4 NaBF4 NaBF4 NaBF4 NaBF4 NaBF4 NaBF4 NaBF4 NaBF4 NaBF4

XVII XXVI Theoretical IV V-3 XV VII XXII XXIII XXX XXXII Theoretical

Na

wt. % K B 31-20

8.33 8.46 31-05 8.59 21.7 9-33 20-1 9-61 20.7 9.76 21.8 9.62 20.6 9.83 20.2 9-71 19.6 9.63 20.4 9.68 20.94 9.85

0.04

M.P. (°C ± 2°)

Method of prep.

<0.01 0.04

570

Recryst. HF NaBF4 + KHF2

0.23 0.08 0.03 0.02 0.03 0.06

381 405 403 406 405 408 406

NaF + BF3 Recryst. H F Recryst. HF Commercial BF~+ HF Dry B F 3 + H F Dry BF~ + H F Dry Con. H F

F

H20

O

59.8 60.4 60.35 68.4 67.8 69.5 69-3 68.8 70.0 69-2 68.4 69-22

0.007 <0.05 0.23 0.01 0.01 0.21 <0.005 0.01 0.04 0-05

0-02

Phase relations in fluoroborate s y s t e m s - I

339

Our highest-purity NaBF 4 (Stock XXIII, Fable I) was prepared by heating N aBF4, recrystallized from aqueous HF, to 500°(2 and bubbling a mixture of BF:~, HF, and He (2 moles BF:Jmole HF) through the melt for 3 hr. The melt was then cooled to 425°( and the gas treatment was continued for 2 more hr. It was then allowed to cool to room temperature under an atmosphere of helium and BF:~ and ground in a helium-filled dry box to pass a 120-mesh sieve. This composition had a melting point of 4 0 8+- I°C and the sharpness of the DTA melting curve approached, but did not quite match. that of our best KBF~. Analysis of a hu-ge (24001b) batch of NaBF4 supplied by a commercial producer (Harsha,a Chemical ('ompany) is also shown in Table 1 (Stock VII). Te('hnique,~

Weighed quantities of the previously described preparation~ (Stock XVII K B F~ and Stock XX 111 or V-3 NaBF4) totaling 10-20 g for each composition, were placed in screw capped glass bottles and mixed on a tumbler overnight. The gradien! quenching technique used in these investigations was described in an earlier publication from this laboratory [5]. ['he principal modifications of the published technique Ihat we made for the present studies were to evacuate the loaded quench tube at about 100°C to 1 x l0 :' tort- for 30 rain and to seal the tube under vacuum. After the tubes were equilibrated at the required temperature for lengths of time vmying from 3 to l i) days, they were quenched and the individual tube segments were opened for examination with a polarizing microscope. An approximate temperature for phase boundaries and the phases present were determined by this method. 3½g quantities of the same preparations used for the gradient quench experiments were loaded into small (0.375 in. o.d. x 2.25 in. long) nickel tubes fitted with thermocouple wells, which were then pumped down to a vacuum of about I × 10 5 torr at 100°C and welded shut. The DTA equipment and details of the technique used with it were described in another paper[6]. The DTA data used in constructing the phase diagrams were averages of three to five values obtained by cycling the samples through the temperature ranges of interest. In general, thermal effects observed on heating were considered to give the most reliable values because of the prevalence of under-cooling effects in the cooling curves. This technique gave more reliable and accurate temperature indications than the quench technique for this relatively dynamic system. RESUI/IS

The system K F - K B F j Our proposed diagram for the system KB-KBF4 shown in Fig. I was constructed from DTA and quenching data in Table 2. Our value for the eutectic composition, 74.5-+ 1 mole of KBF 4, agrees quite well with that reported by Pawlenko[3] but our eutectic melting point, 460_+2°C is significantly higher than Pawlenko's (441°C). The liquidus values found by the quenching technique, not shown in Fig. 1, were consistently lower than the DTA data but we believe the DTA data to be more reliable. Agreement between DTA and quenching values for the euctectic temperature was satisfactory. The accuracy of the quenching iiquidus value depends on the ability of the petrographer to detect the first appearance of the well-crystallized primary phase in the quenched samples. In the systems considered here this determination was made difficult by our inability to quench liquids to glasses, which resulted in microcrystallization of samples quenched from above the liquidus. Our inability to prevent the inversion of the high-temperature crystalline forms of NaBF4 and KBF4 to their low-temperature forms during quenching also made observations difficult. The average difference between DTA and quenching values for the liquidus was 5. ( . J. Barton et al., J. A m. ('cram. Soc. 41.63 (1958). 6. 1,. O. Gilpatrick et al., Thermal Analysis Vol (1), In~trumentation. Orj,,anic Materials and Polymers (Edited by R. F. Schwenker. Jr. and P. D. (tam. p. 85-96. Academic Press, New York (1969)

340

C.J. BARTON et al. 900

,~857oc

I

I

I

L

I

t

[

LIQUID

800 700

~

l

I

I

I" KF-r LIQUID

L.,GHTEMPERA%REFORM) | ~

, +LIQUID X

, --

600

~: 5 0 0 400 [----~--KF

+KBF4 (HIGH-TEMPERATURE FORM) F ~ - - -

500

t

2 0 0 [- -

• LIQUIDUS

(LOW-TEMPERATURE FORM)

I00 0

i

0 KF

t0

20

50

40 50 60 KBF4 (mole %)

70

80

90

1 O0 KBF4

Fig. 1. The system KF-KBF4. Table 2. Differential thermal analysis and quenching data for KF-KBF4 mixtures

Composition (mole%KBF4) 10 25 30 33-3 40 50 60 70 80 85 90 95 100

Liquidus

Thermal effects (°C) Eutectic Inversion

828 778

459

286

745 715 654 589 505 489 512 534 551 570

462

285

462 463 461 457 460 459 456

285 284 281 282 282 281 283

Other

346 345 346 345 345 345 345,414

Quenching data Liquidus Eutectic Temp. (°C) Phase Temp. (°C) 811_+5 748 _+4 724--+4 7 !2 _+2 679 ___3 624 ± 3 552 ___3 479 -+ 2 485 _ 2 504 _+2 522 _+2 536_+2

KF KF KF KF KF KF KF KF KBF4 KBF4 KBF4 KBF4

460 ___2 460±2 454 __+2 460___2 455 ___3

30°C for c o m p o s i t i o n s in w h i c h K F was the p r i m a r y p h a s e as c o m p a r e d to 10° w h e r e K B F 4 w a s the p r i m a r y phase. T h e p h a s e t r a n s f o r m a t i o n o f K B F 4 at 283 -----2°C was u n a f f e c t e d b y the p r e s e n c e of K F s h o w i n g the a b s e n c e of significant solid s o l u t i o n of K F in this c o m p o u n d . T h e size o f the t h e r m a l effect at the i n v e r s i o n t e m p e r a t u r e was c o m p a r a b l e to that at the e u t e c t i c t e m p e r a t u r e , as in the N a F - N a B F 4 s y s t e m . A small b u t r e p r o d u c i b l e t h e r m a l effect at 345°C s h o w n in T a b l e 2 is p o s s i b l y

Phase relations in fluoroborate s y s t e m s - 1

341

due to the presence of a trace of KBF3OH which is reported by Pawlenko [3] to have a melting point of 332°C. We found[7] a liquidus temperature of 355°C for the purest KBF3OH that we have made to date. Pawlenko also reported that KBF3OH forms a eutectic with KBF4 melting at 307°C. We have not investigated this system. It should be noted that the 345°C effect was consistently exothermic in the heating cycle as well as during cooling. We do not have an explanation for this odd behavior. We found no thermal effects nor any microscopic evidence in support of the two compounds in this system (KF.KBF~ and KF-2KBF4) postulated by Pawlenko[3]. We believe that KF and KBF4 form a simple eutectic system as shown in Fig. 1.

The system NaF-NaBF4 Compositions covering the range 22.3-96.1 mole % NaBF4 were studied by DTA and several compositions were examined by use of the quenching technique. The data shown in Fig. 2, based on DTA and quenching data in Table 3, indicates that this is a simple eutectic system with a eutectic composition containing 92-*-1 mole % NaBF4 which melts at 384+_2°C. The liquidus values obtained by quenching the mixtures containing 50 and 60 mole % NaBF4 fell below the line based on DTA data but, for the reasons discussed in the preceding section, we accept the DTA data. The liquidus line in the high NaF region is in good agreement with data obtained from vapor pressure determinations[8]. The data in Fig. 2 show that the previously published diagram for this system[2] is grossly in error and that the earlier O R N L value[4] of 382°C for the eutectic Ioo0

•~ 900

I

8,O0

-

-

!

i"

i'

q ~ ~

,

,

'

i --

U ~ 700

:'

NaF + LIQUID

~ 600

i

Ld 5 0 0 p-

"

"'°u'°US¸

]

1

SOLIDUS CRYSTAL INVERSION-~

:J

[

i

I

't

LIQUID

*

-,, I

-

~

i

/

N°BF4 '--O°°C (H,GH-TEM~ERATURE FORM) \

-

+LIQUID

-

_

~'

~i

400

300

. 245oc

20O NoF

NoF+NoBF4 (HIGH-TEMPERATURE FORM) i NaP + NaBF4 (LOW-TEMPERATURE FORM) 20

40

60

80

NoBF4. ( mole % )

Fig. 2. The system NaF-NaBF4. 7. C.J. Barton and L. O. Gilpatrick, ORNL-4344, p. 156 (Feb. 1969). 8. S. Cantor, Oak Ridge National Laboratory. Private communication.

i

NaBF 4

C. J. B A R T O N et al.

342

Table 3. Differential thermal analysis and quenching data for NaF-NaBF4 mixtures

Composition Thermal effects (°C) (Mole % NaBF4) Liquidus Eutectic Inversion 22.3 30 40 43.3 50 63 70 80 82.1 88 90 90.5 92 92.5 94 95 96-1

100

937 906 858 813 741 676 538 469 391

384 384 382

384 380 381 384 385 381

385

381

395 408

384

Quenching data Liquidus Eutectic Temp. (°C) Phase Temp. (°C)

247 246 246

240

753±4

NaF

380±2 383 ± 1

555 ± 2

NaF

378±2

>420

NaF

381 ± 1°

408 ± 2 383 ± 1 391 ± 2 392±2 404----- 1 402±3

NaF NaBF4 NaBF4 NaBF4 NaBF4 NaBF4

383 ± 1 379 ± 1 380 ± 2

244

383 ± 1

244

melting point was very nearly correct, but that the eutectic composition ( - 9 5 mole % NaBF4) was somewhat misplaced. The crystal inversion at 243 ± I°C was found to be quite energetic. The size of the DTA peak at the inversion temperature was approximately one-half that observed at the eutectic temperature but we did not attempt to measure the magnitude of heat effect. Calorimetric measurements of the heat content of NaBF4 and KBF4 have been reported by Dworkin [9]. The inversion temperature appeared to be independent of composition. Several effects could contribute to the low melting points of NaBF4 and KBF4 previously reported by other investigators [2, 3] and observed in our own studies. Nonstoichiometry (excess NaF or KF) can lower the melting point as inspection of Fig. 1 or Fig. 2 shows. Obviously any impurities such as CaF2 or PbFCI that are soluble in liquid NaBF4 or KBF4 would lower their melting point. Possibly a more important effect is that of hydroxy fluoroborates. The melting behavior of NaBF3OH and mixtures of this compound with NaBF4 remain to be investigated because we have been unable thus far to produce pure NaBF3OH. However, we noted an endothermic effect at 305 ± 3°C on heating some NaBF4-NaF mixtures. This effect was quite large in some of the earlier mixtures prepared from NaBF4 that had a higher oxygen content than the Stock V-3 or Stock X X I I 1 material. Some of these early preparations showed a small thermal effect in this temperature range on the first heating curve but not on subsequent heating or cooling curves. We believe that this thermal effect is most likely caused by the oxygen present. Due to the fact that the 305 ° effect observed during the heating cycle in this system was endothermic while the 345 ° effect observed in the KBF4-KF 9. A . S . Dworkin, O R N L - 4 3 4 4 , p. 157 (Feb. 1969).

Phase relations in fluoroborate systems - 1

343

system was exothermic, it is not clear whether the two effects are analogous. We did not conduct a systematic search for the 305°C thermal effect with the preparations recorded in Table 3. The hydroxyfluoroborate compounds are said to be formed by hydrolysis of the tetrafluoroborates[10] and it is apparent from Pawlenko's work[l] that the presence of any significant quantity of hydroxyfluoroborates would lower the melting point of tetrafluoroborate preparations. The small size of the extraneous thermal effects noted with our better fluoroborate preparations leads us to believe that the amounts of oxygen impurity in our preparations were not large enough to affect liquidus and solidus temperatures significantly. 10. H. S. Booth and D. R. Martin, Boron Jrifluoride and Its' Derivatives pp. 90-100. Wiley. New York ( 1949L