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io 0

FRACTIONAL CRYSTALLIZATION REACTIONS IN THF,SYSTEM LiF-BeFz-ThF4 1

R. E. Thoma and J. E. Ricci

i 'f

ABSTRACT Equilibrium and non-equilibrium crystallization reactions in the system LiF-BeFz-ThF4 are analyzed in relation to their potential application to molten salt reactor fuel reprocessing. Heterogeneous equilibria in the

\*#

temperature range from the liquidus at 59OoC to the solidus at 35OoC are described qmtitatively and in detail by means of ten typical isothermal sections and by three temperature-composition sections. The implications of metastable fractionation'4n this temperature interval are discussed as a possible feed control step in reductive extraction reprocessing of molten salt breeder reactor fuels.

NOTICE This document contains information of a preliminory nature and was prepared primarily for internal use a t the Oak Ridge National Laboratory. I t i s subject to revision or correction and therefore does not represent o final report.

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CONTENTS

Abstract

Page 1

...........'.*"'. ..".'*"

Introduction

................

Liquid-Solid Phase Reactions I n The System LiF-BeFa-ThFL ,

. .

. -

P o t e n t i a l Application of F r a c t i o n a l C z y s t a l l i z a t i o n I n Chemical Reprocessing

.........................

INTRODUCTION The ORNL Molten Salt Reactor Program is devoted to the development of molten salt breeder reactors which employ mixtures of molten fluorides as core fluids. Until recently, the most promising approach to the development of molten salt breeder reactors appeared to be a tworegion reactor with fissile and fertile materials in separate fuel and blanket streams. Thorium would be carried in the blanket salt, in a salt stream which would consist of a 7LiF-E3eF2-ThF4 mixture. Advances in chemical reprocessing have provided evidence recently that

3Pa and

possibly the rare earth fission products can be separated from mixed thorium-uranium salt by reductive extraction methods employing liquid bismuth.

This development, along with other design developments, makes

possible a single-fluid breeder reactor, one which has greater simplicity and reliability than the two-fluid reactor. The fuel for the single fluid reactor would be composed of 7LiF, BeF2, ThF4, and 233UF4, and might be expected to contain

12 mole $ ThF4. Optimization of the 7LiF

and BeF2 concentrations is not complete, because the trade-off values of several significant factors have not yet been established. These include selection limitations imposed by the equilibrium phase behavior of the LiF-BeF2-ThF4 system (233uF4concentration will be only 0.2 mole

$, and is therefore of little consequence in this connection), physical properties such as viscosity, vapor pressure, thermal conductivity, and the relations of LiF-BeFz-ThFb composition to the development of chemical processes for removal of protactinium and the lanthanides. Effective separation of the rare earth fission products from fluoride salt streams which contain thorium fluoride is the keystone to development of semi-continuous reprocessing in single-fluid molten salt reactors. Several methods for reprocessing spent LiF-BeF2-ThF4-UF4 fuels are currently under investigation. The method which is regarded as most tractable for engj-neeringdevelopment involves the selective chemical reduction of the var'ious components into liquid bismuth solutions at about 6OO0C, utilizing multistage countercurrent extraction operations. The

current status of engineering development of this process has been described by Whatley et al.'

The initial steps remove uranium and

protactinium by reductive extraction.

A strong incentive then exists to

remove the rare-earth fission products from the remaining salt. The most nearly feasible approach to this separation seems to be their extraction

into bismuth

even though the recycle volumes of extractant are

marginally acceptable. The efficiency of this separation step would be greatly enhanced if the concentration of the rare earths in the salt mixture were increased by at least tenfold, and if the residual salt solutions were of a much lower concentration of thorium fluoride. That the LiF-BeF2-ThF4phase diagram4 shows the occurrence of low melting mixtures of low thorium fluoride content which are producible from MSBR single-core fluids by metastable crystallization has suggested the possibility that non-equilibrium fractionation reactions might be exploited as a feed control step in the reductive extraction process. Because of its relative complexity, the unpublished version of the LiF-BeFa-ThF4 phase diagram may experience less frequent or less effective application in molten salt reactor technology than is warranted by the developments cited above. We therefore describe in this report further detailed aspects of equilibrium and non-equilibrium behavior in the system. LIQUID-SOLID PHASE REACTIONS IN TKE SYSTEM LiF-BeFz-ThF4 Methods for interpreting polythermal and isothermal phase diagrams are described extensively in an earlier report5 where the phase relationships in a number of fluoride systems were analyzed in detail. Interpretation of the equilibrium'behavior in the system LiF-BeF2-ThF44 (Figure 1) is somewhat more complex than for the systems analyzed because of the occurrence of an unusual s o l i d solution which is produced as the compound 3LiF.ThF4 crystallizes from LiF-BeFz-ThF4 melts. The crystal phase of nominal composition, 3LiFeThF4, precipitates as a ternary solid solution which, at its maximum in composition variability (near the solidus), is described by a composition triangle with apices at LiF-ThF4 (75-25 mole $), LiF-BeFz-ThF4 (58-16-26mole $), and LiF-BeF2-ThF4 (59-20-21 mole $,).

Two substitution models may provide an explanation for the

single phase solid solution area:

+ for a Li

(1) a substitution of one Be2+ ion

ion with the simultaneous formation of a Th

vacancy for every

four Be2+ ions substituted for Li ions to provide electroneutrality and

2+ + ( 2 ) substitution of a single Be ion for a Li ion with the simultaneous

formation of a Li vacancy. Model (1) would afford a solid solution

ORNL-LR-DWG 37420AR6

ThG 44t4

TEMPERATURE IN OC COMPOSITION IN mole To

3LiF.1

526 BeF2

548 P 458

E i60

Fig. 1. Polythermal Projection of the LiF-EkF,-ThF4 Equilibrium Phase Diagram

limit in good agreement with the leg of the triangular area with the lesser ThF4 content whereas model (2) would give a line extending from 3LiFaThF4 toward BeFz-ThF4 (60-40 mole

4).

This is a limiting line

which permits considerably higher ThF4 content than that found experimentally. Accordingly, it appears that both models are simultaneously applicable for the crystallization behavior of 3LiF*ThF4 as it zqystallizes from LiF-BeFz-ThF4 melts.

Once the crystal structure of 3LiFeThF4 has 6

been established (a study of the structure is currently in progress ) it w i l l be possible to appraise the validity of these models. Application of ternary phase diagrams to technology often requires a knowledge of the identi-tliesand compositions of the various phases in equilibrium at specific temperatures. Such information is represented by equilibrium phase diagrams. Typically, phase diagrams of ternary systems are presented as projections of temperature-composition prisms on their basal planes. When such schematic representation includes liquidus temperatures, equilibrium crystallization and melting reactions can be described in a quantitative manner. Here, the use of isothermal sections is often valuable, particularly if the phase diagram is complex. The chief feature of the isothermal section is that it provides information both about the identity and relative masses of coexisting phases. The crystallization behavior of the 3LiF.ThF4 ternary solid solution determines the composition sequence as LiF-BeFz-ThF4 melts are cooled. A series of equilibrium isotherms is shown in Figs. 2 to 11, which describe a11 the equilibrium reactions in the temperature interval from 59OoC to 35OoC, i .e,, the Liquidus-solidus interval of chief relevance to the compositions which are likely to have application in molten salt reactor technology, and in which all 3LiFsThF4 solid solution meltingfreezing reactions occur. Within this interval all the solid phases

of the system are involved, The equilibrium behavior of chief importance to us is described further by the temperature-composition sections, 3LiF-ThFq-2LiFaBeF2,LiF.ThF4-2LiF-BeF2,and LiF*2ThF4-2LiF*BeF2,shown in Figs. 12-14 (schematic, not to scale).

[II

0 H

ORNL-DWG 68-f4678R

TEMPERATURE IN OC COMPOSITION IN mole % '

LiF- 2ThF4 LiF ThG 3LiF ThF4 ss

T = 57OoC

LiF

P 458

F i g . 3.

E 360

Isothermal Section of the System LiF-BeF2-ThF4 at 570OC.

ORNL-

3LiF

DWG

68-

11679R

LiF ThF4 TEMPERATURE IN “C COMPOSITION IN mole % LiF-k

\ 2LiF

Li BeF,

3LiF

848

Fig.

\\\\\

ThF,

2LiF

Isothermal

BeF2<001450 P 458

Section

4007 E 360

400

450

500

of the System LiF-BeFz-'J!hFk at 562'C.

,E 526 BeF2 555

ORNL-DWG 68-11677R2

LiF* 2ThFq

-l TEMPERATURE IN OC COMPOSITION IN mole YO

3LiF

2LiF BeF,

1 BeF2 555

Fig. 5.

Isothermal Section of the System LiF-BeFz-ThF4 at 49OOC.

ORNL- DWG 68 1 1680R2

TEMPERATURE IN O C COMPOSITION IN mole '70

BeF2

555 Fig. 6 .

Isothermal Section of the System LiF-BeFZ-ThFb at 457OC.

ORNL-DWG 68-11675R

ThF, 1111

TEMPERATURE IN O C COMPOSITION IN mole %

LiF

BeF,

2LiF *BeF2’

848 Fig. 7 .

Isothermal Section of t h e System LiF-BeFz-ThF,:

555

a t 447OC.

T h h tftl

ORNL- DWG 6 9 5297

TEMPERATURE IN OC COMPOSITION IN mole Ye

44OOC

LiF 848

2LiF BeF2’

F i g . 8.

Isothermal Section of t h e System LiF-BeFZ-ThF4 a t U O O C .

BeF

55g

ORNL-DWG 68-14674

TEMPERATURE IN OC COMPOSITION IN mole Yo

LiF 2ThF4

LiF 848

2LiF BeFz Fig. 9.

Isothermal Section of the System LiF-BeFz-ThF4 at 43OoC.

BeF2 555

ORNL-DWG

Thb

LiF

68-If673

1111

2ThFq

LiF 848

2LiF

Fig.

BeF,’

10. Isothermal

BeF2 555

500

Section

of the System LiF-BeFz-ThF4

at 43O'C.

ORNL-DWG

68-11672

TEMPERATURE IN OC COMPOSITION IN mole %

LiF 848

8eF2 555

Fig.

11. Isothermal

Section

of the System LiF-BeF2-ThF4

at 350째C.

The isothermal sections included in Figs. 2 to 11 are drawn to scale and represent the experimental results which were the basis of the 4 previously published phase diagram. Composition-temperature relations in the LiF-BeFZ-ThFL system for LiF concentrations greater than 50 mole

$, are shown in detail in Fig. 15. The straight lines appearing in Figs. 2 to 11 are tie-lines (or "conodes") connecting two phases which are in equilibrium. In Fig. 16, point P, as a point on such a tie-line joining points b and z, represents a mixture of the phases (or compositions) b and z with the mole fraction of b equal to the ratio of line lengths zP/zb. In the case of a mixture of three phases, such as the points a, b, c making up the total composition at point P (Fig. 16), the relative amounts of the phases a, b, c making up P may be determined as follows, w i t h t h e three fractions defined as x of a, y of b, 1-x-y of c. Then: (1) Graphically: extend the line bP to fix the point z on the line ac. Then y = zP/zb, and x = (zc/ac) (1-y) (2)

Analytically: let the fractions of the components A and B

at each of the four points (a, b, c, P) be Aa

% Ac

A.~7

' P *

Then by similar triangles, we have

Then BZ =

Hence

Then

(3%

= (Ba

+ BAz

+ p%

- CXAa -

CXAa

B-a:

y = B - B

%m d x =

A A

A c

- A

- y).

+ CXAz

19 ORNL

- DWG

- 5294

573

565

LIQ

P 458O

P, 444O -3Tss

L3Tss+LiF + L2B

3 LiF. ThF4 (L3T) Fig. 12. The Section 3LiF*ThF4-2LiF-BeFz

20 -.

ORNL-DWG 69-5295

LT = L i F ThF4 LT2 = LiF * 2ThF4 LT4 = LiF 4ThF4 L2B = PLiF * BcF,

P-762O

L la

LIQ+LT2

P-597O

LIQ t LT

L I Q t LT + L,T,,

LIQtLiF

P-458O + LIO t L iF + L2B P-4440 P-4330

Fig. 13. The Section LiF*ThF,!,-2LiF*BeF2

OANL- DWG 69

- 5296

LT = L iF . ThF4 LT2= LiF. 2ThF4 LT4= LiF. 4ThF4 L2B=2Li F Be F2

?-897'

?-762a

LIQ+ LT2

/LIQ+ LiF P-458O

P-4480

ZLiF.BeF2

(12b)

Fig. 14. The SecCion LiF.2ThF4-2LiF.BeF2

ORNL-DWB (18-942A

TEYPERATUIKS IN *C COMPOSITION IN mob X

Fig. 15. Phase Diagram of the LiF-BeFz-ThFq System for Compositions 50 - 100 mole $ LiF.

ORNL DWG. 69-7559

c Fig. 16. Schematic Drawing for Use in Calculating Relative Fractions of Coexisting Phases at Point P.

POTENTIAL APPLICATION OF FRACTIONAL CRYSTALLIZATION

IN CHEMICAL REPROCESSING Under equilibrium conditions, the crystallization end-point in three component systems such as in the LiF-BeFz-ThF4 system, depends on the "compatibility" or three solid phase triangles of the equilibrium diagram. As an example, compositions in the triangle LiF 2LiF eBeF2 have their crystallization end-point at the 444'C reaction point. As noted previously:

3LiF.ThF4peritectic

dynamic crystallization of

LiF-BeFz-ThF4 mixtures does not follow the equilibrium crystallization diagram exactly; instead, non-equilibrium crystallization proceeds characteristically by sub-cooling (i.e., delayed crystallization under dynamic cooling), and by incomplete recombination of liquid and solid phases at the peritectic reaction points. Thus, liquids are produced from mixtures which are of interest to us, primarily those containing high concentrations of LiF, which are richer in BeF2 than their equilibrium counterparts, and which crystallize as described by the lower melting areas of the phase diagram. The consequence of nonequilibrium fractionation is thus to produce liquid residues which are lower in ThF4 content than at equilibrium.

Let us examine the difference between equilibrium and non-equilibrium crystallization behavfor of a liquid composition that would partially typify the reactions of MSBR salts. Suppose the composition c, LiFBeF2-ThF4 (63-32-5 mole @, undergoes equilibrium crystallization. On complete solidification, the frozen salt w i l l consist of the three crystalline phases, 3LiFaThF4 ss, 2LiFoBeF2 and LiF.2ThF4 in proportions given by the position of point c in the corresponding triangle of Figs. 9, 10, and 11. For non-equilibrium crystallization this triangle has no significance. The non-equilibrium process consists of four consecutive steps, seen on the basis of the following diagram:

LiF

Step (1): freezing starts at 446OC for composition c, and the liquid travels on the solid solution liquidus surface to reach curve PI - P3 at some point R (at 44OoC), while precipitating some solid

solution of composition between a and b, say a' as average. Step (2):

liquid travels on curve Pl-P3, to reach P3 (433OC),

while precipitating a mixture of solia solution (of composition between b and s, say b' as average) and 2LIFoBeF2. Step ( 3 ) : liquid travels on curve P3-E, to reach E(356') precipitating mixture of LiF02ThF4 + 2LiFoBeF2. Step ( 4 ) : liquid at E(356') 2LiFaBeF2

while

freezes to mixture of LiF-2ThF4 +

BeF2.

Qmntities involved for 1 mole of starting composition c :

S t e p (1): t o f i x point

draw s t r a i g h t l i n e a ' c and extend it t o curve PI-P,,

. Moles of l i q u i d r e a c h i n g a =

ml;

a'a Moles of ThF4 p r e c i p i t a t e d ( i n s t e p 1, o r between 446 and 440') =

i n which x

S t e p (2):

xa ' (1-ml)

= p1,

mole f r a c t i o n of ThF4 a t a ' , e t c .

d r a w s t r a i g h t l i n e b'-H, and extend s t r a i g h t l i n e JP3

back t o f i x p o i n t y on l i n e b'-H:

Moles of liquid reaching P3 =

YP3

(ml) = m2;

Moles of ThF4 precipitated (in step 2, o r between 440'

-- % ' yH (ml-mz) b'H

and 433')

= p2.

Step ( 3 ) : draw straight line DH and extend straight line P@ back to fix point z on the l i n e DH:

LiF 2LiF.BeF2 (=HI

Moles of liquid reaching E =

ZP $ (m2)

m3;

Moles ThF4 precipitated (in step 3 , or between 433' =

zH -32 (-) DH

and 356')

(m2-rn3) = p3,

since xD = 2/3. Step ( 4 ) : moles ThF4 precipitated in this step (at 356')

(PI + p2 + p3). Thus, given the original composition c on the phase diagram as we = xC

have it, one can make estimates regarding what happens in steps (1) and (2),

and these estimates fix what happens in steps (3) and ( 4 ) , for the

limit of non-equilibrium behavior. This means a process in which there is never any interaction between precipitated solid and solution. Actual behavior w i l l of course be somewhere between this and the equilibrium process. Since non-equilibrium fractionation of LiF-BeF2-ThF4 melts produces final liquids which are low in thorium, and since the concentrations of rare earths in the solutions are expected to be about

20 ppm at the time when fuel processing is economically mandatory, one might anticipate that a semi-zone refining step might w e l l produce and transport liquids of low thorium concentration and containing a relatively high concentration of rare earths (the solubility of the lanthanide trifluorides in any of the melts one might encounter is almost certainly to be at least 200 ppm at the low temperatures which would be present in this part of the feeder apparatus),

The efficiency of this concentration step could possibly be impaired seriously if the rare earth trifluorides either formed intermediate compounds (such

compounds are formed only for the lanthanides of = 6 3 ) which interacted with the crystallizing phases or otherwise formed solid solutions with 8 any'of the crystallizing phases. The structure of 2LiF.BeF2 and LiFeThF?

are known and believed to be incapable of serving as solid

state hosts for the rare earth fluorides. The 3LiFeThF4 solid solution is an unknown factor in this consideration and could conceivably act as a solvent for lanthanide ions. This possibility a$ well as the possibility that LiF92ThF4 might also serve as a solid state solvent for lanthanide ions could be examined easily through a small scale laboratory program.

Consultant, Department of Chemistry, New York University, University Heights, New York.

M. E. Whatley et al., Nuclear Applications, (in press).

3. J. H. Shaffer and D. M. Moulton, Reductive Extraction Processing of MSBR Fuels, in Reactor Chemistry Division Annual Report for Period Ending February 28, 1968, ORNL-4396. 4.

R. E. Thoma, H. Insley, H. A. Friedman, and C. F. Weaver, J. Phys. Chem. 64, 865 (1960).

5. J. E. Ricci, Guide to the Phase Diagrams of the Fluoride Systems, ORNL-2396, 1958. 6. G. D. Brunton, ORNL, Unpublished work, 1969.

7. C. F. Weaver, R. E. Thoma, H. Insley, and H. A. Friedman, Phase Equilibria in Molten Salt Breeder Reactor Fuels, I. The System LiF-EkF2-UF4-ThF4, ORNL-2896, December, 1960.

8. J. H. Burns and E. K. Gordon, Acta Cryst. 20, 135 (1966). 9. G. D. Brunton,

21, 814 (1964).

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