May, 1953
DIAGRAMS OF THE SYSTEM URANIUM HEXAFLUORIDE-HYDROGEN FLUORIDE
viscosity of the oils under test, the 1/8 h.p. motor was replaced with 1 / 3 ~ h.p. Bodine Type NSE-11R,115V AC-DC, 0.43 ampere, 5000 r.p.m., 10:1 gear reduction. While not highly precise, the method serves for a rough check of viscosity. Table A is a listing of anode load current for torques of 640,1600 and 2400 g. cm. over the speed range 25 to 500 r.p.m. Table B shows the stability of the no-load current over a period of 1.5 hours for the same speed range. Electronic Multimeter.-Because of the high conductance of the carbon blacks used it was found more convenient to measure the d.c. resistance than dielectric loss factor and this was done with a general purpose multimeter. This instrument is similar to many commercial test instruments but has a range of 0.3 ohm to 50,000 megohms in resistance measurement, from 5 X microampere to 5.0 microamperes for current measurement, and from 0.5 to 500 volts for potential measurements. An additional voltage range of 0.5 volt is provided which draws less than 5 X
TABLE A THYRATRON ANODELOAD CURRENT, MA. Speed, r.p.m.
25 50 100 200 300 400 500
Torque, g. cm. 640 1600 2400
16.0 16.0 14.5 15.3 14.5 14.0 13.3
40.0 40.0 39.8 37.5 35.3 35.0 34.8
60 61.5 58.5 56.5 52.5
... ...
54 1
TABLE B THYRATRON NO-LOAD CURRENT, MA. Speed, r.p.m.
30
Time, min.
25 50 100 200 300 400 500
26.0 29.5 30.5 33.8 36.5 40.0 42.8
60
90
27.0 28.0 31.0 33.8
...
25.0 26.8 28.0 31.5 35.5
37.0 40'.8
...
...
10-12 ampere in current for use in electrokinetic and in glass-electrode potential measurements.
FREEZING POINT DIAGRAM AND LIQUID-LIQUID SOLUBILITIES OF THE SYSTEM URANIUM HEXAFLUORIDE-HYDROGEN FLUORIDE' BY GENEP. RUTLEDGE, ROGER L. JARRYAND WALLACE DAVIS,JR. Carbide and Carbon Chemicals Company, Union Carbide and Carbon Corporation, K-26 Laboratory Division, Oak Ridge, Tennessee Received January IO, 1063
The complete freezing point diagram for the system uranium hexafluoride-hydrogen fluoride has been determined. In addition, liquid-liquid solubilities of these compounds have been measured up to the consolute temperature of 101'. A miscibility gap starts at 61.2' and extends over the composition range 10 to 80 formula % UFe. The eutectic temperature of this binary system is -85". There is no indication of compound formation.
Hydrogen fluoride is a frequent, and sometimes a major, impurity in uranium hexafluoride, either as a result of the method of preparing the latter or as a result of its reaction with water. As described by Katz and Rabinowitch,2 much effort has been devoted to studies of methods of removing hydrogen fluoride from uranium hexafluoride and t o the use of getters that would keep the latter free of the former. These studies, as well as others devoted to methods of analysis of hydrogen fluoride impurity12 were not designed to provide complete knowledge of phase equilibria in the binary system uranium hexafluoride-hydrogen fluoride. The present paper is the first of two designed to present solid-liquid, liquid-liquid and liquid-vapor equilibria of this system over the full composition range and over the temperature range -85" (i.e., the eutectic temperature) t o 105" ( i e . , above the consolute temperature of 101"). Solid-liquid and liquid-liquid equilibria are presented below. Since previous work on this system has been fragmentary only a few comparisons with the present work are justified.
Experimental Apparatus.-Three types of apparatus have been used in this study: (1) filter cell; (2)freezing point cell; (3)transparent Fluorothene tube. The filter cell was u;ed to measure solubilities at temperatures of -5 to 25 , in which range thermal effects in the freezing point cell could not be measured. The freezing point cell (Fig. l), developed from (1) This document is based on work performed for the Atomic Energy Commission by Union Carbide and Carbon Corporation a t Oak Ridge, Tennessee. Presented at the 123rd National Meeting of the American Chemical Society, Lo8 Angeles, Calif., March 13-19.1953, (2) J. J. Katz and E. Rabinowitch, "The Chemistry of Uranium," Part I. "The Element, Ita Binary and Related Compounds," MoGrawHill Book Co., Inc., New York, N. Y., 1951.
modifications of apparatus of Skaus by W. S. Wendolkowski and E. J. Barber of this Laboratory, was used to determine the freezing point diagram of solutions containing 7 to 100 formula % UFe. The Fluorothene tube used in investigations of liquid-liquid equilibrium was a test-tube type unit, connected to a metal valve through flare fittings and fitted with a thermocouple well extending nearly to its base. This type of tube was also used with solutions containing 2 to 10 formula % ' UF6, a range overlapped by the other two units. The filter cell was composed of two Fluorothene tubes connected by Crane HGP valves through - a -porous nickel filter disc. Materials.-Uranium hexafluoride used in this work contained less than 0.015 weight % impurity. The vapor ressure4 at various temperatures was determined frequentry to detect any leaks or volatile impurities. The middle fraction of a commercial tank of hydrogen fluoride was used. This fraction was further purified by the method described by Jarry and Davis.' The resulting hydrogen fluoride contained 0.04 k 0.02 mole % impurit,y as determhed by the rocedure of partial melting in an adiabatic calorimeter.B routine check on the dryness of hydrogen fluoride was obtained by condensing some of this material into a Fluorothene tube and then adding a small quantity of uranium hexafluoride. The presence of even minute quantities of water resulted in the formation of a yellow precipitate. Procedure.-One of the tubes of the filter cell assembly was charged by condensation with a synthetic mixture of hydrogen fluoride and uranium hexafluoride, containing an excess of the latter. The unit was mounted in a low temperature thermostat and rotated overnight at a controlled temperature. An average of several Beckman thermometer readings, taken the last hour before filtering, was used as a measure of the solution temperature. Fluctuations in temAfter perature in the thermostat were less than 10.05'. stopping the thermostat rotor so that the charged tube was
1
(3) E. L. Skau, Proc. Am. Acad. Arts Sci., 67, 551 (1933). (4) G. D. Oliver, H. T. Milton and J. W. Grisard, J . Am. Chsm. SOC.,76, in press (1953). (5) R. L. Jarry and W. Davis, Jr., TRISJOURNAL,67, in press (1953). (6) J. W. Grisard, H. A. Bernhardt and G. D. Oliver, J . Am. Chem. SOC.,78, 5725 (1951).
GENEP. RUTLEDGE, ROGERL. JARRY AND WALLACE DAVIS,JR.
542
n --I
I
I I
I
I
I I
4 I
I I
I I
I
I
I
I
I I I
I
I I
I I
L,
I I J
__ ________ _______
Fig. 1.-Freezing point cell with auxiliary equipment: A freezing point cell; B, solenoid; C, copper shield; D, cell thermocouple; E, shield thermocouple; F, air thermostat; G, glass dewar; H, nichrome heater coil; I, metal dewar; J, glass wool insulation; K, stopcock leading to vacuum pump; L, cell anchor; M, plastic lids; N, firebrick shield base; 0, U F ~+ HF p, piston of stirrer; Q, nickel rod piston stops.
EXPXRIMENTAL DATAFOR Formula % UFa
0.00 .16 .24 .27 .32 .41 .45 .48 ,781 .98 1.593 3.04 3.93 4.20 5.39 6.24 6.67 6.90 7.95 8.32 8.51 9.45 10.28
THE
t,
oc.
in an inverted position (the empty one therefore being upright), the valve on this tube was opened to allow solution to pass through the filter disk and into the empty tube. This filtrate was weighed and then analvzed for uranium. The freezing point cell (Fig. 1) was loaded under vacuum conditions with uranium hexafluoride and the weight determined, using an analytical balance, with an accuracy of f 2 mg. Enough hydrogen fluoride was then added, by liquid nitrogen condensation, to give the desired synt,hetic mixture. The melting point of a sample of carefully purified uranium hexafluoride was used as a primary standard after the absolute correction of the coppoer-constantan thermocouple was found to be only about 0.1 This fixed point temperature was measured by Oliver, Milton and Grisard4 as 64.02 i 0.05'. Above 7 formula % ' UFe all freezing temDerature measurements were within 5 -degrees of this-melting temperature. 1 H The melting point of hydrogen fluoride at -83.6' was used as a calibration point for measurements of the uranium hexafluoride-hydrogen fluoride eutectic temperature, which was less than two degrees from this melting temperature. White double and K-2 potentiometers were used in conjunction with the freezing point cell. I n order,to determine the temperature a t which two liquid layers disappear, forming one homogeneous solution, it was necessary to use a transparent Fluorothene tube. This tube was partially filled with weighed quantities of the two chemicals and heated in an air thermostat, with manual shaking of the mixture, until the two layers disappeared. Temperature was measured by use of a calibrated copperconstantan thermocouple and a K-2 potentiometer. The major source of error arose from the difficulty of seeing exactly when the two layers disappeared or reappeared. Errors involved in this method amount to as much as f 2 " . I
.
Results and Discussion I and are The data Obtained are given in plotted in Fig. 2. The inset in Fig. 2 gives in more detail the region of low hydrogen fluoride concentration which is of value for analytical determinations. It should be kept in mind that all of this work was done in the absence of foreign gases and
TABLE I FREEZING POINTDIAGRAM AND LIQUID-LIQUID SOLUBILITIES OF HEXAFLUORIDE-HYDROGEN FLUORIDE= Miscibility
Eutectic
Vol. 57
Freezing
oc. -83.6 5.0 5.1 5.0 5.2 - 5.0 0.0 - 0.1 5.0 25.0 26.5 44.3 50 52 55 55 59.12 t,
gap t, O C .
-
-84.18 58 60.25 59 60.55 69
THE
SYSTEMURANIUM Miscibility
Formula % UFe
10.38 11.92 12.20 16.29 19.85 23.39 24.52 28.58 29.13 39.38 49.97 53.14 53.53 55.02 55.40 56,77 57.09 58.45 61.35 62.03 64,49 65.21 66.70
Eutectic t,
oc.
Freezing t,
oc.
gap t,
oc.
69 60.96 78 83 87.9 90.3 61.16 97.2 93.2 98.8+ 100.5 99.9 -85.13 61.14 95.7 61.25 95 98 90.5 93.5 87.5 -85.06 83
DIAGRAMS OF THE SYSTEM URANIUM HEXAFLUORIDE-HYDROGEN FLUORIDE
May, 1953
543
TABLE I (Continued) Eutectic
Formula,
% UFO
t,
Miscibility gap t , “C.
Freezing
oc.
t,
oc.
84.0 79.5
67.13 69.35 70.44 72.54 72.96 81.56 82.03 83.06 87.73 90.46
Eutectic
Formula % UFs
t,
91.09 94.15 95.00 95.77 90. 47 96.47 98.45 99.63 100.00
61.18 61.25 76 61.32 -85.10 -84.99 61.56 61.95
Miscibility gap
Freezing
oc.
t,
oc.
1,
“C.
62.3 62.45 -85.10 62.51 62.39 62.53 63.09 63.76 64.02
Experimental information on the individual measurements may be obtained by using this table with Fig. 1
TWO IMMISCIBLE SOLUTIONS
I
I
I
70
so
I 0
I IO
I
I
20
80
I
I
30
I
1
I
10061
90
FORMULA PERCENT UF6
%LID HF t S L I D UFs
I
I
I
40 50 60 COMPOSITION, FORMULA PERCENT U g
,
70
I
I
80
I
I
I
SO
e,
Fig. 2.-Phase diagram of the binary system: UF6-HF. visual observation of disappearance of two liquid layers; 0,visual observation of precipitation of solid UFB; A, filter cell; 0, freezing point cell-K-2 Potentiometer; 0 freezing point cell-White Potentiometer; f , rupture of Fluorothene tube.
that pressures were not atmospheric, but the normal vapor pressures of the binary mixtures a t given temperatures. I t should be noted that data are presented in terms of formula %; the term “mole yo” caiiiiot be used since the variation of the degree of polymerization of hydrogen fluoride in these solutions was not determined. In addition, differences in liquid and vapor compositions necessitated correction of some synthetic composition values. These corrections were importaiit, in the present work only in
the region 80 to 100 formula yoUFs and mere made by use of data soon to be published.’ Katz and Rabinowitch2 have reported, from Manhattan Project literature, a cryoscopic constant of 0.065’ per 0.01 mole 7 0 hydrogen fluoride, as contrasted with an ideal value of 0.0839’. Within experimental error, the present data indicate ideal solubility from 0-2.5 formula % hydrogen fluoride. From recent vapor pressure data4on uranium hexaI
(7) R. L. Jariy, F. D. Rosen, C. F. Hale and W. Davis, (Jr., Tms J ~ ~u npi ~ I blinhnd. ~ ~ t ,
544
GENEP. RUTLEDGE, ROGERL. JARRY AND WALLACE DAVIS,JR.
fluoride, a cryoscopic constant of 0.0828' per 0.01 formula % hydrogen fluoride may be calculated. In contrast t o earlier statements12it should be noted from Fig. 2 that hydrogen fluoride is very soluble in uranium hexafluoride, to the extent of about 20 formula yo a t 61.2' (at one end of the miscibility gap), or even 90 formula % (if uranium hexafluoride is considered as solvent a t the other end of the miscibility gap). As the solution temperature is increased to 101' complete miscibility is obtained. Calculations of activity coefficients indicate large positive deviations from ideality as the concentration of hydrogen fluoride increases above 2.5 formula %. Below 61.2' solubilities are much lower than ideal values, evidently as a result of large difference in the "cohesive energy densities" of the two compounds. Above the four phase invariant temperature of 61.2' uranium hexafluoride and hydrogen fluoride exhibit a miscibility gap with a consolute temperature of about 101'. At about 100' and, therefore, over 10 atmospheres pressure
VOl. 57
Fluorothene tubes begin to get soft; in two experiments they ruptured. As a result few measurements were made above 98". Although no experiment was performed with a mixture having the maximum temperature of incomplete miscibility, it is probable that the interpolated value of the consolute temperature 101" is accurate within *2". Compositions of the two immiscible solutions at 61.2' are approximately 10 and 80 formula % uranium hexafluoride. Measurements over the composition range 6.9 to 95 formula % uranium hexafluoride have shown a single binary eutectic a t -85'. Acknowledgments.-Gratef ul acknowledgment is given to E. J. Barber for his many helpful comments and suggestions during the course of this work, to G. D. Oliver and J. W. Grisard for determining the purity and triple point of hydrogen fluoride, and to W. S. Wendolkowski for loan and aid in use of the White double potentiometer and auxiliary apparatus.
r
